coordination chemistry of ligands with electron …coordination chemistry of ligands with electron...
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COORDINATION CHEMISTRY OF LIGANDS WITH ELECTRON RICH N, O OR S
BITING CENTRE
ABSTRACT
THESIS SUBMITTED FOR THE AWARD OF THE DEGREE OF
Boctor of Pljilosoptip IN
CHEMISTRY
BY
S A R V E N O R A KUMAR
DEPARTMENT OF CHEMISTRY
ALIGARH MUSLIM UNIVERSITY
ALIGARH ( INDIA)
2007
ABSTRACT
The work embodied in the thesis is the description of efficient synthetic procedure
for obtaining transition metal complexes of novel polyaza macrocyclic moieties bearing
tetraamide /tetrapeptide functions and tridentate [N,N,N] dipodal chelating ligands
containing two imidazoline or benzimidazoie ring systems bonded to a secondary amine
skeleton. The binding characteristic and reactivities of the new diversified class of
ligands towards metal ions have been carefully examined and the probable mechanisms
for the formation of the final products have been discussed.
• Chapter 1 is a brief survey of the various known macrocyclic compounds, their
coordination chemistry, importance as material uses and as models systems to
understand the various biological phenomena.
• Chapter 2 deals the svntheses of transition metal encapsulated complexes of a
symmetrical class of 30-membered [Nio] macrocycle, 3,6,9,12,20,23,26,29,35,36-
Decaaza-tricyclo[29.3.Ll'^''^]hexatriaconta-l(34),14,16,18(36),31(35),32-
hexaene-2,13,19,30-tetraone, (L ' ) , 24-membered [Ng] macrocycle,
3,6,9,17,20,23,29,30-Octaaza-tricydo[23.3. l.l'''^^]triaconta-
l(28),ll,13,15(30),25(29),26-hexaene-2,10,16,24-tetra-one {l}) and 20-
membered [NfJ macrocycle. 3,7,15,19,25,26-Hexaaza -tricyclo [19.
3.Lf'"]hexacosa-l(24),9,ll,13(26),2l(25),22-hexaene-2,8,14,20-tetraone, (L^),
emplo)ing the metal template procedure. The complexes of the ligand L ' were
prepared emplo\ing a two step single pot template procedure. In the first step.
triethylenetetraamine and the metal salts (1:1 mole ratio) were reacted to get
triethylenetetraamine metal complexes stabilized in the solution. In second step,
this reaction mixture was further reacted with equimolar amount of pyridine-2,6-
dicarboxylic acid at room temperature, which has afforded stable solid products,
usually after an over night stirring of the reaction mixture. Almost, all the
complexes exhibited solubility in water only, therefore, the various physico-
chemical studies and spectral investigations have been performed as an aqueous
solution. Most of the complexes either melt sharply or get decomposed between
280 - 300 °C suggesting formation of a monomeric species.
The complexes of L and L were prepared employing a similar two step single
pot template synthetic procedure. First step involved an in-situ formation of a
metal ion chelate complex from reaction of either N -(2-Amino-ethyl)-ethane-l,2-
diamine (i.e. diethylene triamine) or 1,3-diamino propane with metal chlorides in
2:1 mole ratio in ethanol. In second step the reaction mixture containing metal
diamine chelate was reacted with 2 mole equivalent (with respect to metal ion) of
pyridine-2,6-dicarboxylic acid at room temperature. This has analogously
afforded stable solid products, usually after over night stirring of the reaction
mixture. Here too. the complexes (9 - 24) were fairly soluble in water only,
therefore, physico-chemcial measurement, electronic, ESI mass and NMR
spectral studies etc. have been performed in aqueous medium. FT-IR. 'H-and ' T
-NK4R spectra of the final products suggest that the complexes are formed via a
metal ion assisted cyclic condensation mechanism. The analytical and Electron
Spray Ionization (ESI)-mass spectral data are consistent with the formation of
I l l
homo-bimetallic complexes [M^L'CinKn = 4, M = Cr, Co, Ni, Cu, Zn, Cd, Hg; n
= 6. M = Fe) and mono-nuclear complexes [MLCIn](L = L^ or L"*), (n= 4, M = Cr,
Co, Ni, Cu, Zn, Cd, Hg; n = 6, M = Fe). Magnetic moment, EPR and iigand field
spectral studies of these complexes suggest that the metal ions in the complexes,
are, in general, in high-spin state and acquire a hexa-coordinate geometry. This
has been verified form the molecular model computation to get optimum energy
plots for the perspective geometry of the complex molecules. The important bond
lengths and bond angles have also been computed and discussed.
In chapters, preparation of novel dipodal ligands, bis(4,5-Dihydro-lH-imidazol-
2-ylmethyl)amine (Im) and bis(lH-benzoimidazol-2-yImethyl)amine (BIm)
have been described. The ligands were obtained from condensation of
iminodiacetic acids with diamines viz. 1,2-diaminoethane and 1,2-diaminobenzene
in the presence of orthophosphoric acid as catalyst under melt conditions at
moderately high temperature (-ISO^C) in a sealed tube. The final products are
obtained in a fairly good yields (>60%), if the reactions are carried out in presence
of a few drops of ortho-phosphoric acid. The presence of ortho-phosphoric acid,
probably acts as a catalyst of this condensation reaction. The ligands (Im) and
(BIm) have been characterized using usual physico-chemical and spectroscopic
methods and their reactivates as chelating agent towards transition metal ions have
also been investigated. FT-IR. 'H- and ' C - NMR spectral data for (Im) and
(BIm) indicate the presence of two imidazoline rings in (Im) and two
benzimidazole rings in (BIm) bound to a secondary amine function via (CH2)
IV
skeletons. The analytical data and Fast Atom Bombardment (FAB)-mass spectral
studies of (Im) and (BIm) support their proposed molecular formulae. They
exhibit considerable reactivity towards Cr"" . Ni'^. Co"~. Cu"" and Fe ^ ions giving
corresponding comple.xes in good yields. The complexes have been thoroughly
characterized on the basis of elemental analyses, magnetic susceptibility
measurements. FT-IR, 'H- , ' C - NMR, electronic ligand field and EPR spectral
studies. The spectral data indicate formation of coordinatively unsaturated five -
coordinate geometries for Cr'^, Ni" , Co" and Cu ^ which can further add up
efficiently a strong a - donor like, 1,10-phenonthroline, to attain a coordinatively
saturated he.xa-coordinate geometry. This has been verified form molecular model
computations, which indicate that the ligands (Im) and (BIm) belong to a class of
dipodal tridentate [N.N.N] chelating ligand.
• In chapter 4, the results of the ^ Fe -Mossbauer Spectral Studies of Fe^ -
Complexes i.e., [Fe2L'Cl4]Cl2 (2), [FeL^CbJCl (10), [FeL^CyCl (18),
[Fe(Im)Cl3] (26) and [Fe(BIm)Cl3] (31) have been discussed to augment the
stereo-chemical informations obtained form other spectral techniques discussed in
chapter 2 and 3.
• The electro - chemical behaviour of these complexes in solution have been
studied using cyclic voltammetic and conductometric techniques.
Thermodynamic first ionic association constant (Kj) as well as the corresponding
ice energy (AG) of the metal encapsulated macrocyclic comple.xes ( 3 - 5 and 9 - 28) in
water and (29 - 34) in DM SO have also estimated by carrying out the conductomeric
studies at RT.
The electro - chemical behaviour of the newly systhesized macrocylic
complexes described in chapter 2 i.e. [M2L CI4] [M = Co (3), Ni (4), Cu (5)],
[Fe.L'CUJCb (2), [ML2CI2] [M = Cr (9). Co (11), Ni (12), Cu (13)], [FeL^CyCl (10),
[ML^Ch] [M - Cr (17), Co (19), Ni (20), Cu (21)], [FeL^Cl2]CI (18) as well as the metal
complexes containing dipodal ligands described in chapter 3 i.e. [M(Iin)Cl2] [M = Cr
(25), Co (27), Cu (29)], [Fe(Im)Cl3] (26), [M(BIm)Cl2] [M = Cr (30), Co (32), Ni (33),
Cu (34)] and [Fe(BIm)Cl3] (31) have been studied in water using tetrabutylammonium
perchlorate as a supporting electrolyte, Ag/AgCl as a reference electrode in the potential
range + 1.0 to - 1.0 V at 0.05, 0.10 and 0.20 vs"' scan rates. The CV studies indicate the
formation of quasi-reversible redox couples i.e. M"", M'"'", Fe'"''" and Fe'^"", which
suggest that the present ligand systems are capable to stabilize the species containing low
(i.e. +1) as well as high (i.e. +3) oxidation states of the metal ions (in addition to the
normal i.e. +2 oxidation state) in the solution.
.ffl.e^'" »
•i,&
COORDINATION CHEMISTRY OF LIGANDS WITH ELECTRON RICH N, O OR S
BITING CENTRE
THESIS SUBMJTTED FOR THE AWARD OF THE DEGBEE OF
Boctor of $l)ilQsopl)p IN
CHEMISTRY
BY
SARVENDRA KUMAR
DEPARTMENT OF CHEMISTRY
ALIGARH MUSLIM UNIVERSITY
ALIGARH (INDIA)
2007
<C X '%
y
T659]
(Dedicated to my Loving Mom
DEPARTMENT OF CHEMISTRY :\LIGARH MUSLIM UNIVERSITY A L I G A R H — 202 002
Phones CExt
' ilnt Ext. (0571)270351;
''ISO. 3351
Dated. %C'/)-'Oy
Certificate
Certified that the worh^ embodied in this thesis entitled "Coordination
Cfiemistry ofLigands 'With electron ^fi % O orS (Biting Centre" is the resuCt of the
original research carried out under my supervision By Mr. Sarvendra %umar and is
suitaSk for submission for the award of (ph.<D. degree ofjiCigarh 9dusCim Vniversity,
jAligarh, India.
((Prbf ZafarMmadSiddiqi) (Ph\(D., jAvJ{Te[[ow (germanyj S.'T'.Ji. TetCow (Japan) U^emher, %'Y.Jl.Sc. (U.S.Jl.J 'Member, J^.j4.j4.Sc. (U.S./.)
ACKNOWLEDGEMENTS first of a[[, I tfian({jllmigfity for giving me the courage andaSiCity to start and finish
this thesis.
There are a number of peopk who have heCped me during the course of my research;
hoivever, it is impossiSk to record a[[ of them here. I hope that they accept my sincere thanhs
and appreciation. I find inyseff deepfy indebted to some peopk that I have to mention their
names.
[ wouCd fil^ to eyy)ress my deep gratitude and thanks to my supervisor (Prof. Zafar
jUtmad Siddiqi, whose encouragement, schoCastic guidance, vafuaSfe advice, constructive
comments and suggestions for the improvement of the thesis have affSeen of inestimaSCe vaCue
for the preparation of the thesis in its present form. I am greatCy indebted to him.
I especiaffy thanh^ to (Director, (Prof.Jijay gupta, iVC Indore, and (Dr. V. (^ l^edy
for the Te-MossSauer spectraCstudies.
Thanks are aCso due to (Prof. % J. Singh, (Department of (Physics, Jifigarh MusCim
Vrnversity, J^figarh, who afways hefpedme in T^%spectra[studies whenever required.
'My speciaC thanks and appreciation go to V(^C for granting me a scholarship to carry
on my higher studies and in return Tm so gratefuCfor their hefp, support and concern during
my studies.
Special thanks are afso due to my coCCeagues (Dr. %aresh %. (paC, !Mohd. XfiaCid Siddiqi,
Mohd. Shahuf, Mrs. ShaSana 'Koor, Mr. Vtswa (prakash jArya for their company and
encouragement. _A deep sense of appreciation to my Mom, Sisters and friend, her face kept
encouraging me attthe tune.
Last but not the kast, my greatest debt of gratitude and appreciation is to my family,
whose help, courage and support are behind my achievement. I feef obliged to mention my
nephew/niece (Diksha Chaudhary, (Priya ^wat, jirun Cfiaudhary, JiBhay <l(flwat and
jiShishek^ Sikfitwar, who have shown a great sense of responsibility, understanding and
courage.
{Sarvendra %umar)
ABSTRACT i-v
Chapter 1: An Overview of Polyaza [Nx] Macrocycles: Scope and Prospects I
ol'Macrocyclic Moieties as Encapsulating/Chelating Ligands
Chapter 2: Synthesis of Homo-bimetallic Complexes of a Di-nucleating [Nio] 27
Macrocyclic Ligand and Mono-nuclear Complexes of [Nf,] and
[Ng] Macrocyclic Ligands Possessing Four Carbonyl
Functions
2A: Template Syntheses of Homo-bimetallic Complexes Incorporating 31
the Macrocyclic Ligand, 3,6,9,12,20,23, 26,29,35,36-
Decaazatricyclof29.3.1. l"'-'^Jhexatriaconta-l(34), 14,16,18(36)
31 (35),32-hexaene-2,13,19,30-ietraone (L') Using Cr'^ Fe^^
Co' \ Ni"", Cu'^, Zn'^, Cd'^ and Hg"" Ions as Templating Agents
2B: Template Syntheses of Mono-metallic Complexes Incorporating 48
the Macrocyclic Ligand, 3,6,9,17,20,23,29,30-Octaaza-tricyclo
123.3.1.1"'-]triaconta-l(28),ll,l3,15(30),25(29),26-hexaene-2,
10,16,24-tetraone {L') and 3,7,15,19,25,26-Hexaaza-tncyclo
[19.3.1. f''^]hexacosa-l(24),9,ll,13(26),21(25),22-hexaene-2,8,
14,20-tetraone (L) Using Cr-^ Fe^', Co'', Kf\ Cu'\ Zn""", Cd"'
and Hg" Ions as Templating Agents
Chapter 3: Synthesis and Spectral Studies of A Novel (midazoh'ne derivative, 76
Bis(4,5-Dihydro-lFl-imidazol-2-ylmethyl)amine (Im) and
Benzimidazole derivative, Bis(lH-benzimidazol-2-
ylmethyl)amine (BIm) and Their Transition Metal Complexes
Chapter 4: ''Fe -Mossbauer Spectral Studies of Fe^^-Complexes, 118
[Fe2L'Cl4]Cl2 (2), [FeL^CbjCl (10), [FeL^Cl.jCl (18),
[Fe(Im)CI:,] (26) and [Fe(BIm)Cl3] (31)
Chapters: Cyclic-Voltammetric (CV) and Conductometric Investigation on 136
the Solution of the Present Homo - bimetallic and Mono -
nuclear Complexes
List of Publications:
1. Spectral and electro-chemical characterization of transition metal complexes of a modified [NcJ macrocycle. A mimic to cyclic hexapeptide. Zafar Ahmad Siddiqi, Mohd. Khalid and S. Kumar, Tran. Metal Chem., 32, 913-921 (2007).
2. Spectral and electrochemical characterization of bimetallic complexes of a novel 32-membered iwsymmetrical [NjjJ macrocycle. Zafar Ahmad siddiqi, M. Mansoob Khan, Mohd. Khalid and Sarvendra Kumar, Tran. Metal Chem., 32, 927-935 (2007).
3. An 18-membered diniicleating Unsymmetrical [NmJ macrocycle and Its Bimetallic Complexes M.^LChJClO^) (M = Co, Ni, Cii, Zn, Cd or Hg), Z. A. Siddiqi, R. Arif, S. Kumar and Md. Khalid, / . Coord. Chem. (2007) in press.
4. Mossbauer and electronic spectral characterization of homo-bimetallic Feflll) complexes of wviymmetrical [N/oJ and [Nji] macrocyclic ligands. Zafar Ahmad Siddiqi, Razia arif. Sarvendra Kumar and Mohd. Khalid, Spectrochimica Acta: A. (2007) in press.
5. .4 novel his-imidazole acting as a dipodal tridentate chelating ligand towards Co'' and Ni~' ions, Zafar Ahmad Siddiqi, Sarvendra Kumar, Mohd. Khalid, South Af. J. of Chem., (Communicated).
ABSTRACT
The work embodied in the thesis is the description of efficient synthetic procedure
for obtaining transition metal complexes of novel polyaza macrocyciic moieties bearing
tetraamide /tetrapeptide functions and tridentate [N,N,N] dipodal chelating ligands
containing two imidazoline or benzimidazole ring systems bonded to a secondary amine
skeleton. The binding characteristic and reactivities of the new diversified class of
ligands towards metal ions have been carefully examined and the probable mechanisms
for the formation of the final products have been discussed.
• Chapter 1 is a brief survey of the various known macrocyciic compounds, their
coordination chemistry, importance as material uses and as models systems to
understand the various biological phenomena.
• Chapter 2 deals the syntheses of transition metal encapsulated complexes of a
symmetrical class of 30-meiTibered [Nio] macrocycle, 3,6,9,12,20,23,26,29,35,36-
Decaaza-tncyclol29.3.Ll'^'^^]hexainaconta-l(34),14,16,18(36),31(35),32-
hexaene-2,13,19,30-tetraone, (L*), 24-membered [Ns] macrocycle,
3,6,9,17,20,23,29,30-Octaaza-trlcyclof23.3.1.j"'^Jtnaconta-
l(28),ll,13,15(30),25(29),26-hexaene-2,10,16,24-tetra-one {l}) and 20-
membered [N6] macrocycle, 3,7,15,19,25,26-Hexaaza -tricyclo [19.
3.1.1'^''^Jhexacosa-l(24),9,ll,13(26),21(25),22-hexaene-2,8,14,20-tetraone, (L^),
employing the metal template procedure. The complexes of the ligand L were
prepared employing a two step single pot template procedure. In the first step,
triethylenetetraamine and the metal salts (1:1 mole ratio) were reacted to get
triethylenetetraamine metal complexes stabilized in the solution, hi second step,
this reaction mixture was further reacted with equimolar amount of pyridine-2,6-
dicarboxylic acid at room temperature, which has afforded stable solid products,
usually after an over night stirring of the reaction mixture. Almost, all the
complexes exhibited solubility in water only, therefore, the various physico-
chemical studies and spectral investigations have been performed as an aqueous
solution. Most of the complexes either melt sharply or get decomposed between
280 - 300 °C suggesting formation of a monomeric species.
The complexes of L^ and L^ were prepared employing a similar two step single
pot template synthetic procedure. First step involved an in-situ formation of a
metal ion chelate complex from reaction of either N'-(2-Amino-ethyl)-ethane-1,2-
diamine (i.e. diethylene triamine) or 1,3-diamino propane with metal chlorides in
2:1 mole ratio in ethanol. In second step the reaction mixture containing metal
diamine chelate was reacted with 2 mole equivalent (with respect to metal ion) of
pyridine-2,6-dicarboxylic acid at room temperature. This has analogously
afforded stable solid products, usually after over night stirring of the reaction
mixture. Here too, the complexes (9 - 24) were fairly soluble in water only,
therefore, physico-chemcial measurement, electronic, ESI mass and NMR
spectral studies etc. have been performed in aqueous medium. FT-IR,'H-and'T
-NMR spectra of the final products suggest that the complexes are formed via a
metal ion assisted cyclic condensation mechanism. The analytical and Electron
Spray Ionization (ESI)-mass spectral data are consistent with the formation of
I l l
homo-bimetallic complexes [M2L'Cln](n = 4, M = Cr, Co, Ni, Cu, Zn, Cd, Hg; n
= 6, M = Fe) and mono-nuclear complexes [MLCIn](L = L^ or L^), (n= 4, M = Cr,
Co, Ni, Cu, Zn, Cd, Hg; n = 6, M = Fe). Magnetic moment, EPR and ligand field
spectral studies of these complexes suggest that the metal ions in the complexes,
are, in general, in high-spin state and acquire a hexa-coordinate geometry. This
has been verified form the molecular model computation to get optimum energy
plots for the perspective geometry of the complex molecules. The important bond
lengths and bond angles have also been computed and discussed.
In chapter 3, preparation of novel dipodal ligands, bis(4,5-Dihydro-lH-imidazol-
2-ylmethyI)amine (Ini) and bis(lH-benzoimidazol-2-ylmethyl)aniine (Blm)
have been described. The ligands were obtained from condensation of
iminodiacetic acids with diamines viz. 1,2-diaminoethane and 1,2-diaminobenzene
in the presence of orthophosphoric acid as catalyst under meU conditions at
moderately high temperature (~180°C) in a sealed tube. The final products are
obtained in a fairly good yields (>60%), if the reaccions are carried out in presence
of a few drops of ortho-phosphoric acid. The presence of ortho-phosphoric acid,
probably acts as a catalyst of this condensation reaction. The ligands (Im) and
(Blm) have been characterized using usual physico-chemical and spectroscopic
methods and their reactivates as chelating agent towards transition metal ions have
also been investigated. FT-IR, 'H- and ' C - NMR spectral data for (Im) and
(Blm) indicate the presence of two imidazoline rings in (Im) and two
benzimidazole rings in (Blm) bound to a secondary amine function via (CH2)
IV
skeletons. The analytical data and Fast Atom Bombardment (FAB)-mass spectral
studies of (Im) and (Elm) support their proposed molecular formulae. They
exhibit considerable reactivity towards Cr"", Ni" , Co'", Cu" and Fe ions giving
corresponding complexes in good yields. The complexes have been thoroughly
characterized on the basis of elemental analyses, magnetic susceptibility
measurements. FT-IR, 'H- , ' C - IMMR, electronic ligand field and EPR spectral
studies. The spectral data indicate formation of coordinatively unsaturated five -
coordinate geometries for Cr'^, Ni'^, Co"" and Cu * which can further add up
efficiently a strong a - donor like, 1,10-phenonthroline, to attain a coordinatively
saturated hexa-coordinate geometry. This has been verified form molecular model
computations, which indicate that the ligands (Im) and (Blm) belong to a class of
dipodal tridentate [N,N,N] chelating ligand.
• In chapter 4, the results of the ' Fe -Mossbauer Spectral Studies of Fe -
Compiexes i.e., [Fe.L'CUJCij (2), [FeL^CyCl (10), [FeL^CyCI (18),
[Fe(Im)Cl3] (26) and [Fe(BIm)Cl3] (31) have been discussed to augment the
stereo-chemical informations obtained form other spectral techniques discussed in
chapter 2 and 3.
• The electro - chemical behaviour of these complexes in solution have been
studied using cyclic voltammetic and conductometric techniques.
Thermodynamic first ionic association constant (Ki) as well as the corresponding
free energy (AG) of the metal encapsulated macrocyclic complexes ( 3 - 5 and 9 - 28) in
water and (29 - 34) in DMSO have also estimated by carrying out the conductomeric
studies at RT.
The electro ~ chemical behaviour of the newly systhesized macrocylic
complexes described in chapter 2 i.e. [M2L'Cl4] [M = Co (3), Ni (4), Cu (5)],
[Fe.L'CUlCb (2), [ML2CI2] [M = Cr (9), Co (11), Ni (12), Cu (13)], [FeL^ChlCl (10),
[ML^Cb] [M = Cr (17), Co (19), Ni (20), Cu (21)]. [FeL^Cl2]Cl (18) as well as the metal
complexes containing dipodal ligands described in chapter 3 i.e. [M(Im)Cl2] [M = Cr
(25). Co (27), Cu (29)], [Fe(Ini)Cl3] (26), [M(BIm)Cl2] [M = Cr (30), Co (32), Ni (33),
Cu (34)] and [Fe(BItn)Cl3] (31) have been studied in water using tetrabutylammonium
perchlorate as a supporting electrolyte, Ag/AgCl as a reference electrode in the potential
range + 1.0 to - 1.0 V at 0.05, 0.10 and 0.20 vs"' scan rates. The CV studies indicate the
formation of quasi-reversible redox couples i.e. M " ' , M""", Fe'"' " and Fe'^"", which
suggest that the present ligand systems are capable to stabilize the species containing low
(i.e. +1) as well as high (i.e. +3) oxidation states of the metal ions (in addition to the
normal i.e. +2 oxidation state) in the solution.
CHapter 1
An Overview of Polyaza [N ] Macrocycles: Scope and Prospects
of Macrocyclic Moieties as Encapsulating/
Chelating Ligands
Cdapler 1 \
The cyclic compounds with nine or more members including all the hetero atoms
and possessed with three or more electron rich donor atoms have been termed as
macrocycle systems [1]. These donor atoms are usually positioned so that upon
coordination, preferably five or six membered chelate rings are formed with the metal
ion. The interests in the chemistry of macrocyclic compounds have increased rapidly
during the last 2 - 3 decades or so because of their many biological and industrial
applications. Henceforth, macrocyclic ligands can be placed under the class of
polydcntate ligands containing donor atoms either incorporated in or less commonly
attached to the cyclic backbone. In other words, multidentate macrocyclic ligands are
cyclic molecules consisting of an organic framework interspersed with heteroatoms,
which are capable of interacting with a variety of species. Macropolycyclic ligands are a
three - dimensional extension of macromonocycles, in which more than one macrocycle
is incorporated in the same molecule. Macrocyclic and macropolycyclic molecules
display unique and excited chemistries in that they can function as receptors for
substrates of widely differing physical and chemical properties and upon complexation
can drastically alter these properties. Selective substrate recognition, stable complex
formation, transport capabilities and catalysis are examples of the wide-ranging
properties of these molecules. A review on the coordination chemistry of macrocyclic
and macropolycyclic ligands cannot be comprehensive in a few pages, however, some
important findings in this field are outlined here.
Terminology:
Useful definitions of terms occurring frequently in dealing with this chemistry are
presented below.
1. A complex is an entity consisting of two or more molecular species, which
interact in such a manner that they are being held together in a physically
characterisable structural relationship.
2. Receptor and substrate are terms describing the species involved in complex
formation. The terms receptor and substrate usually imply that the complex
formed has the uell ~ defined structural and chemical properties of a supra-
Coordination ckemistry ofCigands vAtfi ekctwn ricH % O orS Siting centre"
<Doctora[thesis suSmittedtoJiRgorfiMustim 'University, I^(DIJ4
chapter 1 2
molecule, as in biological receptor - substrate associations.
3. Most and guest are terms covering ail kinds of inter-molecular associations from
the well-defined supra-molecules to loosely packed solid-state inclusion
compounds.
4. Homo-nuclear refers to multi-site binding in which identical substrates are bound.
5. Hetero-nuclear refers to multi-site binding in which different substrates are
bound.
6. Monotopic refers to receptors which possess only one binding subunit.
7. Ditopic or polylopic receptors are those which possess two or more binding
subunits.
8. A cascade complex is one in which an additional substrate may be included
between metal cations (or eventually anions) in a ditopic or polytopic ligand.
9. Macrocyclic and macropolycyclic effects designate the great thermodynamic
stability of macrocylic ligand complexes compared to non-macrocyclic analogues.
Common nomenclature has developed over the years, in particular for coronand
and cryptand ligands. The crown-type ligands are commonly named according to the total
number o\^ atoms in the macrocyclic ring enclosed in brackets and preceding the
classification (crown), followed by the number of oxygens.
The number of donor atoms in the macrocycle and the imposed degree of rigidity
influence the nature of cavity. While a rigid framework results in a preformed cavity,
flexibility allows latent cavity formation. The selectiveties observed for the crown ethers
and cryptands in the complexation of the alkali and alkaline earth metal ions are closely
related to cavity size, although in exceedingly large cavities selectivity may become lost
due to a preponderance of flexibility [2]. If a substrate is too small for a given ligand
cavity, the resulting complex will be destabilized by substrate-receptor repulsions and
ligand deformation. On the other hand, for a substrate that is too large for a macrocycle.
destabilized complexes will result due to poor ligand-substrate binding contact or
unfavourable ligand deformation in order to achieve binding contact. This influence of
Coordination ckemistry ofCigands •with electron rich % O orS Biting centre" iDoctoraftkesis suSmittecftoJiRgarfi'MusRm Vniversity, I'MDlyi
chapter 1 3
cavity size also pertains to cyHndrical cavities, for which the length of chain substrates
compared to cyhndricai macrocyciic cavity size has been found to be distinctly correlated
to substrate selectivity and complex stability [3-5].
The incorporation of chiral units within the macrocyciic skeleton is an important
route to the design of macrocyciic receptors capable of enantiomeric or chiral substrate
recognition. In order to achieve this goal, the receptor must display fine structures as a
result of chiral barriers, which then allows diastereomeric receptor-substrate interactions,
in addition to maintaining the desired cavity properties [6-8].
The coordination chemistry of multidentate ligands has been a field of intensive
research amongst inorganic chemists and bioinorganic chemists over the past decades.
There is no dearth of literature on the synthetic structural or functional aspects of such
ligand. Naturally, occurring macrocyciic complexes like vitamin B12, metalloporphyrins,
chlorophyll and the industrially important metal-phthalocyanine have been studied for
many years [1.9-12]. Synthetic ring complexes which copy [13] aspects of these naturally
occurring complicated macrocyciic ring systems are known. Investigations of such
compounds find an analogy with the natural systems, and were the main goal of early
stage research. Although, the results obtained do not always closely parallel those in
nature, the biochemical role of metal ions in the natural system is better understood
through these synthetic models. At present the emphasis of research is ranged over the
whole spectrum of chemistry. The wide spread interests in these molecules are due to
their unique and exciting chemistries in that they can serve as receptors for the metal
ions, molecular cations, neutral molecules or molecular ions of widely differing physical
and chemical properties. Moreover, metal ions complexation can drastically alter these
properties. They possess several desirable properties such as selective substrate
recognition, stable complex formation, transport capabilities and catalysis [14].
\n general, macrocyciic ligands arc organic heterocyclic compounds containing
between 10 to 30 atoms in a ring. They have an internal hydrophilic cavity formed by the
donor atoms and an external hydrophobic cavity formed with the external hydrophobic
framework made up of chains. An interesting feature of macrocyciic compounds is that
the design and syntheses with varying ring sizes and donor sites, with specific properties
'Coordination chemistry offigands with ekct'-on rich % O orS Siting centre" <Doctorai thesis submitted to /'.Hoarfi 'Musftm 'University, UNOIJi
chapter 1
can be achieved with relative ease. Extensive series of macrocyclic ligands have been
prepared and studied, which are classified into various subdivisions [14]. lUPAC
nomenclature of these compounds is cumbersome and not illustrative. Simple but not
unequivocally defined notations have been suggested for certain types of macrocycles.
However, sophisticated compounds when represented by their structural formulae
provide a clear picture of the macrocyles.
Macrocyclic ligands have been prepared using conventional organic synthesis as
well as employing in-situ procedures involving cyclization in the presence of a metal ion
[15-18], In some reactions the presence of a metal ion is required. The metal ion is said to
act as a template and such reactions have been termed as metal assisted template
synthesis. Schiff base condensation between a carbonyl compound and an organic amine
in presence of a metal ion [19,20] to yield an imine linkage has led to the synthesis of
many aza macrocycle complexes as illustrated below in Figure 1.1.
R /
Me NO;,
^N
C \ N
/ M
k / N \
N
\
y
CH.O
RNH, M(en)2
M=Ni-^ Cu-"
CH.O
r^tNO,
^N N.
k / N \ ^
N
\ NOj
Figure 1.1
The crown polyethers are examples of macroclycles which have been prepared
[21 -23] mainly by the direct synthesis as shown in Figure 1.2 a. Mixed oxa - thia crowns
are obtained [24-26] from oligo (ethylene glycol) dichloride reactions with dithiol
{Figure 1.2 h).
OH
OH
CI o " \ CI
(0)
'Coordination chemistry offigands tvitfi ekuron ricfi % O or.S biting centre" (Doctoraltfie-sis submitted to j\p£arfi'Mus[im Vniwrsity, IHi'DIJ^
CHapter 1
O V V \ l ^HS' SH
(b)
Figure 1.2
Ether - ester macrocycles are derived [27,28] from acid cliloride and oligo
(ethylene glycol) (Figure J.3).
HO O 0 • \
OH
CI CI 0
0 ' \ ^ . ^ "0
Figure 1.3
The main target in macrocyclic design is to synthesize macrocycles, which are
able to discriminate among the different metal cations. Many factors influencing the
seleciivities of macrocycles for cations have been determined. These include macrocyclic
cavity dimensions, shape and topology, substituent effect, conformational flexibility or
rigidity and donor atom type, number and arrangement [29-32]. For the first row
transition metal ions the consecutive increments in radii are not large so it becomes
difficult to effect discrimination solely on the cation-cavity best-fit. The other features
involved are
• The natural order of stability constants.
• The metal ion-ligand donor compatibility derived from hard and soft acid and
base character.
• The preferentially site symmetry required [33] by the metal ion.
The match between the cation and the macrocyclic cavity diameter is especially
visible in small cryptands and other preorganized macrocycles, such as calixarences and
Coordinatum chemistry ofCigands with electron rich % O orS Siting centre"
doctoral thesu suhmittedto J^Rgarh 'Musfim XJniversity, I!^'<DIJi
chapter i 6
spherands. In these cases, size selectivity goes together with lack of flexibility of the ring,
which is too rigid to undergo conformational changes upon complexation. The influence
of the cavity shape is envisaged in some calixarences which exhibit very high
•"coordination geometn' selectivity" specially toward IJO " [34]. Small cryptands and
other reorganized rigid type macrocycles. discriminate between cations that are either
smaller or larger than the one with the optimum size i.e "peak selectivity". Macrocycles
of flexible cavity type, such as larger crown ethers and cryptands, discriminate
principally among smaller cations i.e ''plateau selectivity"[35].
Fenton and co-workers [33,36,37] have investigated the design and synthesis of
oxaaza macrocyclic ligands with varying ring sizes and flexibilities including both weak
and strong donor atoms in varied donor sets and sequences in order to defme the
principles underlying transition metal selectivity by the macrocyclic ligands. It was
realized that discrimination for different metal ions to be structurally or stereochemically
based. This behavioural trend has been well documented through the work of Tasker and
co-workers [38.39]. Lindoy and co-workers [40] have developed the design strategies
for new macrocyclic ligand systems, which are able to recognize particular transition and
post-transition metal ions. Differences in log k values were used both as a monitor and
control of the ligand synthesis. Starting from a particular macrocyclic ligand which gives
rise to discrimination, the related derivatives can be synthesized. This is done through a
systematic tuning-up process by employing a typical three-dimensional structural matrix
procedure. The macrocyclic ring substitution is achieved along another direction, and
var}ing the donor atom type along the third dimension. This matrix procedure [41]
enables the effects of incremental structural variations on log k difference to be followed.
flie metal complexes usually are synthesized by the reaction of the required
metal ions with the preformed ligands. However, there are potential disadvantages, if this
method is adopted for the preparation of metal complexes of preformed macrocycles. The
s\nthesis of a macrocycle in the free form (non-template) results in a low yield of the
desired product with a number of side reactions in which polymerization predominates.
In order to circumvent this problem the ring closure step in the synthesis may be carried
out under conditions of high dilution [4?] or a rigid group be introduced to restrict
Coordination chemistry qfCigands with eUctron rich % O orS Siting centre" (Donnraf thesis sufmittedto AUifarh 'Musfm 'University, [9iOIJi
CHapter 1 7
rotation in the open - chain precursors [43-45], thereby, facilitating cyclization. An
effective method for the synthesis of macrocyclic complexes involves an in-situ approach
wherein the presence of metal ion in the cyclization reaction enhances the yield of the
cyclic product. The metal ion play an important role in directing the steric course of the
reaction and this effect is termed as "metal template effect" [46]. The metal ion may
direct the condensation preferentially to cyclic rather than polymeric products (the kinetic
template effect) or stabilize the macrocycle once formed (the thermodynamic template
effect). In another attempts the presence of mineral acids like HCI, HNO3 or HCIO4 in
place of metal ions [46] has also facilitated the cyclization step by minimizing the
unwanted side reactions.
Substitution of the coordinated metal ion by other metal ions, which are not
effective as templates, has also been achieved by the transmetalation (metal exchange)
reaction. In this way a wide range of mono- and di- nuclear complexes [44] have been
prepared. A metal ion which can not serve as a template for a particular macrocycle can
effectively coordinate to form stable complexes if reacted with the free macrocycles. For
the large Schiff base macrocycles, the transition metal ions often are ineffective as
templates. Consequently, the kinetic stability of the metal ions present in the macrocyclic
complexes of the s- and p- block cation some times enable the generation of
corresponding transition and inner- transition metal complexes by transmetallation with a
second reactions. On treating the kinetically labile complexes with a second metal ion the
liberated macrocycle is captured and stabilized by coordination to the new metal ion
before decomposition. In this way a range of Ni"' and Cu" complexes {Figure 1.5) of
macrocyces have been obtained which are not accessible by the direct template methods
[47-57].
The transmetallation is the resultant of the stability difference of the parent
complex and the complex of the transmentallating ion. Thus, for the transmetallation to
be feasible the stability of the complex of the transmetallating ion should be greater than
that of the parent complex. Transmetallation has been exploited [47-50] to synthesize a
range of di-nuclear complexes of 2:2 macrocycles from the corresponding mononuclear
complexes. However, in some cases transmentaliation is accompanied by a change
' CoorSnation chemutry oftigands with ekctron ricli%OorS Siting centre"
Ooctorolthesis suSmittedtoMujarh ^usdm 'University, FJ^TOjj^
Cfiapter 1
Figure 1.4
in geometry of the resulting complex. In some transmetaliation reactions new types of
mcrocyclic ligands arising from ring expansion or contraction are also obtained which
depends on the demands and the size of the transmetallating ions [47,58,59].
The kinetic lability of the lanthanide complexes has also been exploited to
generate the corresponding complexes of the transition metals. For examples, reactions of
the lanthanum, (ill) complexes of 1.5 as well as of 1.6 with Cu"^ yield di-nuclear copper
(II) complexes depending on the reaction conditions [60] as shown in the following
equations.
EtOH [La(L)(N03)3] + 2 Cu(CI04)2.6H20-
iLa(L)(N03)3] + 6 Cu(C104)2.6H20-
where L - 7.5 or 7.6.
-*- [Cu(L)(OH)2](C104).3H20 + La(N03)3
EtOH [Cu(L)(^-OH)2J(C104)3 + La(N03)3
. . - ^ •
• ^ . ^ '
OH
Figure 1.5 Figure 1.6
"C.oordinatmn chemistry offigandi imth electron rich K 0 orS Siting centre"
'DnctoraftH.esis suSmittedto ARflarh 'MvsRm Vniversity, I'MDIA
chapter 7 9
The kinetic lability of the metals present in the generation of macrocyclic complexes
derived from the use of alkaline earth and main group template agents has enabled the
generation of the correponding transition metal complexes through transmetallation
reaction [47,48,50,51,57]. This approach has been particularly successful when applied to
the generation of dinuclear copper(ll) complexes of tetraimine Schiff base macrocycles,
used as speculative models for the bimetallobiosites in cupro-proteins such as
haemocyanin and tryosinase [61]. The size of the cation used as template has proved to
be of importance in directing the synthetic pathway in the Schiff base sustems, shown by
the scheme in Figure 1.7.
Of the alkaline earth cations only magnesium generates the pentadentate "1:1"
macrocycle but it is ineffective in generating the hexadentate "1:1" macrocycle which is
readily synthesized in the presence of the large cations ca"" , sr" , ba'^ and Pb'' . These
cations, however, generate the "T.T macrocycle derived from the components giving the
"1:1" macrocycle with magnesium [50,51.55]. .A. further size related effect is metal -
induced ring contraction. If the metal ion is too small for the macrocyclic cavity and there
is a fuctional group (=NH or -OH) available for addition to the imine bond then this can
add to produce a smaller, more accommodating cavity for the available metal ions
[62,63]. Most of the early work featured the use of transition metal ions in the template
synthesis of tetradentate macrocycles. The directional influence of the orthogonal d-
orbital is also instrumental in guiding the s>'nthetic pathway. The last two decade has
seen an extension of this technique, which was further extended to include the use of
organo-transition metal derivatives to generate iridentate cyclononane complexes [64,65].
The smaller Schiff base macrocycles have been termed "1:1" macrocycles. The metal
complexes of "2:2" macrocycles may be mono- or di- nuclear in nature.
Macrocycles with pendant groups represented a class of ligands (deliberately
synthesis) to achieve metal ion discrimination. Macrocycles with pendant groups bearing
neutral oxygen donors have beoi synthesized in abundance mainly due to the synthetic
simplicity [66-72J. The hydrox) ethyl pendant groups produce a marked decrease in
complex stability for small metai ions and a moderate increase in complex stability for
the large Pb"* ion and so. This is in accordance with the rule regarding the effect of
'Coordination cHemLstry of (igand's with election rich % 0 orS diting centre" (DocloraCthesis submitted toMfiarfi 'Muslim University, TMDI^
Cha-pter 1
M-
Mg-
H,N
Pb-'
Figure 1.7 : Schiffbase macrocycle sunthesis in the presence of non-transition metal
neutral oxygen donors on the complex stability in relation to metal ion size. This lead to a
simple generalization that in order to increase the selectivity of macrocycles for large
metal ions, all that is necessary is to add groups bearing neutral oxygen donors [73]. The
synthesis of highly functionalized macrocyclic receptors is an important initial step in the
investigation of molecular recognition properties of these large ring compounds [74].
Condensation reactions between a diamine and ethylene diamine tetraacetic acid
(EDTA) dianhydride or diethylenetriaminepentaacetic acid (DTPA) dianhydride give
dioxopolyazacyclo alkanes with differing ring size and with a different number of
pendant carboxymethyl groups [74]. The use of 1.3-diaminopropane as a diamine gives
13-membered [75] macroclycles {Figure 1.8). These macrocyclic ligands form ionic
metal chelates with bivalent and trivalent metal ions respectively. The partial double-
bond character of the C - N bond in an amide group decreases the flexibility of the
macrocyclic ring and defines the conformation of the ligands. When an additional
Coordination chemistry of[igands with electron rich % O orS biting centre"
'Doctoralthesis suhmittedto JiRgarh 'Muslim "University, IWil^
chapter 1 II
functional group is introduced in the new series of macrocycles, the resulting
macrocycles are expected to have a higher steric constraint.
oc-/
-NH
-COOH
-NH
OC- -COOH
Figure 1.8
A great deal of interest has been directed to synthesize functionally modified
macrocycles. Modified ligands. achieved by variation of the heteroatoms or ring
substituents as well as ring size can greatly influence observed selectively patterns. The
ability of certain molecules to bind specifically a closely related species in biological
systems is of great significance. Macrocyclic polyamines are one among such class of
compounds that have been structurally modified to develop ligands with specific
properties.
The secondary amine donors in macrocyclic polyamines can synthetically be
substituted with amides or with other heteroatoms, which may also act as characteristic
donors. These simple structural modifications would dramatically alter the complexing
behaviour [76]. An amide group offers two potential binding atoms, the oxygen and
nitrogen for complexation of protons and metal ions. They are planar with 40% double -
bond character in carbon - nitrogen bond and strongly favours the trans form as shown
below:
H
-N
R
0 \
Figure 1.9
Coordination chemistry of ligands vAth electron ricfi!N,0 orS Biting centre"
(Doctoralthesis sudrmttedto jACu^arh 'Muslim University, l'M)IJi
Cfiapterl 12
A free or unconnected amide is a weak coordinating group due to the weakly
basic amide oxygen atom and weak acidity of hydrogen. With such weakly basic 0 -
atoms (pKa = I) [77,78] strong metal complexation will not occur at that site. The metal
ion interaction with the neutral nitrogen atom also provides only the weak complexes. On
the other hand, substitution of nitrogen bound hydrogen by a metal ion should create a
very strong bond. However, the very weak acidity of hydrogen (pKa = 15) [79,80]
implies that alkali and alkaline earth metal ions will not effect its removal. Transition
metal ions promise to be more effective in substituting nitrogen bound amide hydrogen
but they suffer metal ion hydrolysis and precipitation in neutral and basic solutions [81].
Therefore, metal ion must be capable of substituting a nitrogen bound amide hydrogen.
To do so in neutral solutions, metal ions require an effective anchor (primary ligating
sites) to inhibit metal ion hydrolysis [81].
The synthesis of functionalized macarocycle is an important step in the
investigation of molecular recognition properties of large ring compounds [82].
Functionalized polyazamacrocycles with pendant arms or exocyclic substituents are
reported [83], which exhibit flexibility and possess big cavity sizes to effectively
encapsulate large cations or metal ions. The ability of certain molecules to bind
specifically, a closely related species in biological system is of great significance.
Macropolyamines are one among such class of compounds that have been structurally
modified to develop ligands with specific properties. Kimura and Kodama have reported
detailed studies of macrocyclic polyamines with one-, two- and three- substituted amide
groups [84] {Figure 1.10). These classes of compounds are known as macrocyclic
oxopolyamines. Their structures bear the dual features of macrocyclic polyamines and
oligopeptides. As polyamines donor ligands, they have affinities for a broad range of
heavy metal ions and transition metal ions. Certain macrocyclic polyamines can enclose
alkali and alkaline earth metal ions [85.86],
Macrocycle polyamines containing 1. 2 and 3 carbonyl functions whose structure
bear dual feature of macrocyclic polyamines and oligopeptides show more selectivity as
compared to the carbonyl free systems [84]. Until early I970's macrocyclic polyamines
(e.g. cyclam) had been used mostly as chelating agents for transition metal ions of basic
Coordination chemistry ofCigands -witfi ekctron rich % 0 orS Siting centre"
''Doctoral thesis submitted to JiRgarfi Micsfim University, I!K<DI_^
Cftapter 1 13
coordination chemistry [87.88]. They possess some common properties as those of
nitrogen-containing bifunctional molecules (F/gM/-e/.//, fl-r/) such as porphyrins [87],
,NH
NH
HN.
HN
X - Y = Z - H2 X = 0, Y = Z = H2 X = Y =0, Z - H2 X = Y = Z =0
Figure 1.10
peptides (e.g. Gly-Gly-His) [89,90] or biogenic polyamines (e.g. spermine) and even
more variety of functions [91,92].
,NH HN.,
NH HN
cyclam Porphyrin Gly-Gly-His Spermine
(a) (b) (c) (d)
Figure 1.11
Highly functionalized macrocycles can be designed by considering the properties
of macrocyclic polyamines, which arise mainly from the composite nitrogen donors and
their basicities [76], for example very distorted metal complexes could be constructed
due to extraordinary macrocyclic stabilities. There are reports [93,94] that metal ions
bigger than the expected cavity size of the macrocycles lie out of the macrocyclic basal
plane and chelation/coordination is achieved by distorting the geometry. Reaction
intermediates or reaction transition states that are extremely reactive molecules may also
"Coordination chemistry ofCigands with electron ricfi%0 orS Siting antre"
(Doctoral thesis submitted to JiRgarh Muslim University, HMOIJi
Cftapter 1 14
be designed. In other words, new metal catalysts of metallo-enzyme models may be
easily tailored from the basic macroyclic structures [76].
A free amide is a weak coordinating group due to weakly basic amide oxygen
atom and weak acidity of hydrogen. This results in weak complexation at that site. On the
other hand substitution of nitrogen bound hydrogen by a metal ion should create a very
strong bond. However, the very weak acidity of hydrogen (pKa = 15) [79,80] implies that
alkali and alkaline earth metal ion will not effect its removal. Transition metals are more
effective as compared to alkali and alkaline earth metals but suffer metal ion hydrolysis
and precipitation in neutral and basic solutions [81]. To avoid this hydrolysis, an effective
anchor ligand (Primary Ligating site) bound to metal is required to inhibit metal ion
hydrolysis [81]. If a macrocycle does not contain amide bonds, amine groups within the
ring or terminal groups, drastic condition are needed for complex formation.
,NH HN,
NH HN
,NH HN,
NH HN NH
,NH HN,
HN
Cyclam Monoxocyclam Dioxocyclam
(a) (b) (c)
Figure 1.12
Polyamine macroycles possess cavity capable of providing a favourable
environment for transition metal ions [95]. The strength of the ion binding is determined
by ion size, macrocyclic cavity size, and ligand conformation [96,97], Typically the 14-
membered tetraamine macrocycles. cyclam, monooxocyclam and dioxocyclam (mono-
and di- amide macrocycles) [Figure 1.12, a-c) incorporate metal ions into their cavities
and form a stable square-planar complex with several configuration [97,98] like that
known for porphyrins and corrins. Macrocyclic oxopolyamines are unique metal
chelators, their structure bears dual features of macroyclic polyamines and oligo-peptides
"Coordination cfiemistry ofRgands unth electron rich%0 orS Siting centre'
(Doctoralthesis suSmittedto Mujarh 'Muslim University, miDIJl
Cfiapter 1 15
[99-102]. The oxopolyamines owing to their important biological functions and some
unusual properties have been extensively studied and structural features are well
recognized [85.99-102]. The two amido groups in macrocyclic dioxotetraamines are
equivalent when coordinated to 3d metal ion, the amido groups get deprotonated
simultaneously [85] as the presence of a non-deprotonated or a singly deprotonated
complexes is unlikely [102].
Certain macrocyclic polyamines can enclose alkali and alkaline earth metal ions
[85.86]. In contrast, oligopeptides such as triglycine {Figure 1.13 a-b) and tetraglycine
{Figure 1.13 c) complex with a very limited number of metal ions i.e. Cu" , Ni , Co
and Pd" and the resulting complex dissociate easy and fast [103,104].
Complexation studies [89,90,102-107] with the variety of macroyclic
oxopolyamines in reference to oxo free polyamines have led to the conclusion that the
macroyclic oxo-tetraamine, in general, are more selective than oxo free systems in
interaction with metal ions.
J J
,NH HN. .NH HN. ~* >
NH2 H2N
X=Y=H2, 2.3, 2-tet
X = Y = 0, dioxo (2.3,2) tet.
NHj HO
Triglyane
(b) Figure 1.13
NH
NH,
HN..
CH,COOH
Tetraglyine
(c)
A combination of amide groups and soft donors (e.g. sulfur donors) in the
macroyclic skeleton accommodates only nobel metal ions, Pt" and Pd'^, but not the
common transition metal ions [108,109]. The stability of the complexes formed varies
with the ring sizes, which are more stable than the corresponding peptide complexes. The
thermodynamic stability of the macroyclic system is suggested to result from the unusual
slow dissociation (or substitution) rates. It is well recognized that organic amide group
stabilize high oxidation states of metal ions when coordinated with the deprotonated
"Coordination cfiemistry ofRgands udtH electron ricli%0 orS Siting centre"
T)octora[thesis suSmittedtoJllyarii yAusCm University, HKOIA
CHapter 1
nitrogens. The Cu" and Ni'^ macroyclic complexes, in general, are kinetically more
stable and hence their lives are longer than peptide complexes [84]. A fundamental
knowledge of oxo-(mono/di) macrocylic complexes have been found applicable in the
oxygenase model [110] and the superoxide dismutase model [111,112]. Their distinctive
properties have found wider scope of chemical and biochemical applications in fields
such as selective metal ion transport, as redox enzymes models and stabilization of
unusual oxidation states of metal ions [113]. Subsequent deprotonation of the amide
protons give the stable macroyclic complexes [114], Although mono-, di- and tri-amide
macrocycles derived form cyclam are now well known, however, those bearing
exclusively amide donor groups (i.e. tetraamine macrocycles) are quite rare. The
possibility of effecting metal insertions into polyamide macroycles free of any accessible
donor groups is usually, difficult and this has been one of the reasons hindering the
development of this area.
Margerum and Rybka have reported [114,115] the detailed study of a 1-1-
macroyclic tetrapeptide complexation {Figure 1.14, a) with Cu' . This important
contribution demonstrated that metal insertion to give a tetraamido-N complex is possible
for a macroyclic tetraamide. In this system metal insertion was performed in the presence
of aqueous sodium hydroxide using freshly precipitated Cu(0H)2. Similar approach
adopted by Collins and co-workers for effecting metal insertion into macroyclic
tetraamides {Figure 1.14 b,c) reported by them was not successfull for any of the first
row metals from Chromium to Copper [116]. They have reported the reactions of these
macrocycles with the divalent metals including Chromium [117], manganese [118,119],
Iron [120], Cobalt [12IJ, nickel [122] and copper [123]. The key features oftheir method
included the use of an anhydrous solvent (THF), low temperature when bases strong
enough to decompose THF are employed, strong bases to deprotonate the ligand prior to
metal addition and the use of divalent transition metal salts which have some solubility in
THF [116]. The tetraamido-N ligands are strongly donating upon tetra-deprotonation and
resistant to oxidative destruction. The isolation and characterization of several higher
oxidation states metal complexes implied that the macroycles possess the property of
being compatible with strongly oxidizing coordination environments. Such ligands would
Coordination chemistry ofCigands unth electron ricii%0 orS Biting antre"
<Doctora[ thesis submitted to JiCigarh ^MusRm 'University, ^(DIJA
chapter 1 17
allow the isolation and characterization of the reactive intermediates formed at the
reaction transition states in the homogeneous catalytic oxidation processes [118,123].
Metal complexes containing saturated polyaza macrocyciic ligands bearing
pendant donors functions, often N-carboxymethyl groups, appear in a wide range of
applications, including MRl imaging agents and other therapeutic agents requiring
NH
NH
HN,
HN
(cO
,bV
(b)
Figure 1.14
Q NH HN
y^NH HN
(c)
bifunctional chelates [124]. Pendant - arm chelating macrocycles also form a class of
ligand intermediate between simple macrocycie and fully encaplsuiating ligands with a
three ~ dimensional framework [125]. The pendant - arm macrocycles are usually
synthesized prior to coordination. Fully alkylated ligands, which possess only tertiary
nitrogen can be relatively straight forward synthesized while partially alkylated
macrocycles which contain both secondary and tertiary nitrogen are more difficult to
achieve [126]. The low yields and the formation of mixture of partially alkylated product
are main problems in the synthesis of such compounds in some cases mixture of isomers
and indeed some geometric some isomer has to be inaccessible [127].
The use of macrocyciic ligands for selective metal complex formation has
received considerable attentions over the last many years [128]. Synthetic macrocyciic
ligands are useful models for the study of biologically important metallo-proteins or
enzymes. The availability of a number of structural derivatives and the ability to complex
a large number of metal ions in different oxidation states have made this class of ligands
'Coordination cHemistry offigands with electron ricli%0 orS Siting centre"
Ooctordtfusis suSmittedtoJiligarfi MiuGm Vmxvrsity, HKDIJi
CHapter 1
attractive and a potential subject to further investigations. The template method [19] has
been exploited extensively for isolating metal inclusion derivatives. However, metal ion
free macrocyclic ligand may be obtained [129] in fair yields employing cyclo-
condensation reactions in the absence of metal ions as templating agent. In non-template
procedures, cyclization is facilitated only under high dilution conditions, and in some
cases the presence of H" ions is necessary for the precipitation of stable salts of the
macrocyclic ligand.
Several synthetic approaches have been proposed to design discrete polynuclear
complexes. One of them consists of the ingenious use of compartmental ligands, which
are organic molecules able to hold together two or more metal ions. The Schiff bases
derived from 2,6-difrormyl-4-methylphenol (Robson-type ligands) and from 3-
formylsalicyclic acid are among the most popular ligands belonging to this family
[130,131]. These ligands are especially appropriate to generate either homo-binuclear
complexes, symmetrical or un-symmetrical or hetero-binuclear complexes. The ligands
{Figure 1.15 a and Figure 1.15 b), which were obtained starting from 3-
formylsalicysclic acid [131] usually produced binuclear complexes in which the ligand
(a) (b)
Figure 1.15
(b) is a compartmental acyclic side-off ligand possessing dissimilar compartments. The
latter is excellent ligand for stepwise synthesis of heterobinuclear complexes [132]. The
Coordimtion chemistry of ligands with electron ricfi% 0 orS Siting centre"
(DoctoraltHesis suBmitteiftoJiligarlt 'Muslim Vniversity, I!N''DIJi
Cfiapter 1
unsymmetrical tetradentate schiff - base ligands derived from 3-formylsalicyclic acid
{Figure 1.16) has been shown to be suitable precursor for the design of homo- and
hetero- trinuclear systems.
NH;
CiiClj
HCl
-H,0
CU(C104)2
NajCO;
Na^CO:,
HCl
N 0 0 \ _ /
A 0 0 N-
Cu(CI04)2 NajCOj
\/\J AAJ
O O N
Figure 1.16
Coordination chemistry ofCigands with ekctron rich % O crS Siting centre"
(Doctoralthesis suSmittedtoJ^Rqarfi iMusfim University, IJiOIJi
chapter 1 20
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"Coordination chemistry offigands xvitk electron ricH % O orS Siting centre"
(DoctoraltluSIS subrnil'udtoARgdrh 'Muslim 'University, Il^<DlJi
CHapter ] 2 1
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chapter 1 24
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chapter J 25
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chapter] 26
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"coordination chemistn ofCigands wit ft electron rich % O or.S Biting centre' <Doctora[tfiesis su6mittedto Jidgarf. Wnslm Vniversity, PJ^OIJI
Cfiapter 2
Synthesis of Homo-bimetallic Complexes of a Di-nucleating
[Nio] Macrocyclic Ligand and Mono-nuclear Complexes
of [Ng] and [Ng] Macrocyclic Ligands
Possessing Four Carbonyl
Functions
27
Introduction
Metal inclusion compounds containing saturated polyaza macrocyclic ligands
bearing bifunctional pendant armed donors [1] like N-carboxymethyl groups, appear in a
wide range of applications, including MRI imaging agents and other therapeutic agents.
A number of complexes with pendant armed macrocyclic ligands having diverse
coordinating behaviours with different metal ions are known [2-5]. Kimura and
Kodama have reported detailed studies of macrocyclic polyamines with one-, two- and
three- substituted amide groups [6].
Importance of the non-template procedure for isolating metal ion free [7] polyaza
macrocycles have also been emphasized by some authors. In non-template procedures
cyclization process is often facilitated by the presence of mineral acid producing salt of
the desired macrocycle out of the solution. Studies in functionalized polyaza macrocycles
with suitable pendant arms [8,9] or exocyclic substituents that can provide additional
coordination sites and a big flexible cavity size of the ring have also been an area of
current research interest. Macrocyclic polyamines containing 2 and 3 carbonyl function
[6] whose structure bear the dual features of macrocyclic polyamines and oligopeptides,
generally, show more selectivity in binding metal ions as compared to carbonyl free
systems. Furthermore, like oligopeptide, the macrocyclic polyamides encapsulate metal
ions, preferably in their higher oxidation states via deprotonation of the amide nitrogens
in alkaline medium. The preferential chelation (or stabilization) of higher oxidation state
of the metal ions compared to its normal +2 oxidation state is the consequence of
shrinking of the cavity size due to deprotonation in an alkaline medium prior to the
encapsulation [6].
It is known [10] that ligand cyclization has a pronounced effect on the complexes
which too depend on the size of the macrocycles. A variety of tetra-aza [N4] with
different exocyclic substituents, which encapsulate metal ions have been reported [8,9]
using template method. Recently, a variety of pendant arms have been introduced [11] as
endocyclic substituents in the ring with a view to obtain a bigger cavity size for
accommodating divalent ions in the ring system. Some workers have emphasized
macrocycles with additional pendant arms to make available extra biting sites with a view "Coordination cHemistry ofRgands with electron rich %OorS Siting centre"
(DoctoraCthesis suSmittedtoJlRgarh MusRm "University, I^fDI^
28
to expand not only the cavity size but also the number of donor groups available to bind
metal ions through exocyclic or endocyclic way in the ring system.
It was observed that in reference to oxo - free polyamines the macrocycles with
amide functions are more selective in binding (encapsulating) metal ions. The stability of
the complexes formed varies with carbonyl functions whose structure mimics synthetic
peptides such as tripeptide [6,12] are reported to exhibit stereo-chemical rigidity towards
Cu(II) and Ni(II) ions. Furthermore, the cavity size of such macrocycles is small
encouraging the stabilization of only higher +3 oxidation state of metal ions. Earliest
reports [12] using such type of macrocycles have indicated that the peptide nitrogen, even
that in synthetic ones, do not participate in coordination to metal ions.
The linear dioxotetraamine like ligand forms the neutral amide ionized complex
[Cu(L-2H)] in aqueous alkaline medium (pH==8) [12]. Tri-peptide such as
glycylglycylglycine [13,14] or glycylglycyl histidine show a similar mode of
coordination to Cu through the two terminal donors and the two deprotonated amide
nitrogens in similar aqueous alkaline medium. It has been shown [12] that doubly
deprotonated dioxotetraamine complexes are stabilized by ligand cyclization and of the
13- and 15- membered rings, the 14- membered ring show the most profound effect.
Similar macrocyclic effects were reported [15] for unsubstituted tetraamine [N4]
macrocyclic complexes which normally result in octahedral coordination [16]. The
modified tetraamines with two carbonyl functions, whose structure mimics tripeptides
such as glycylglycylclycine, result in square planar complexes with Cu " and Ni " , as a
consequence of rigid planarity caused by the dissociation of two amide protons (at pH ca.
8).
Siddiqi and co-workers have recently designed [17,18] novel [N4] macrocycles
with four endocyclic carbonyl functions generating a cyclic tetraamide/tetrapeptide with
flexible cavities capable to accommodate metal ions even in their lower oxidation states,
e.g. Co(I) and Cu(I). Here the metal ions are chelated through the amide nitrogen as the
most accessible sites rather than the carbonyl functions as reported [6,16,19,20] earlier
for the tripeptide suytems. This feature led the design and synthesis of metal ion free
macrocycles bearing cyclic hexaamide functions [21]. The coordination complexes of "Coordination chemistry ofRgamfs with ekctron rich % O orS Siting centre"
OoctoraCthesis suSmittedto JlRgarh MusRm University, HN^IJi
29
these macrocycles with 3d metal ions and platinum group metals have been thoroughly
studied in this laboratory [22,23].
In this chapter synthesis and characterization of transition metal encapsulated
complexes of a 30-membered [Nio] macrocycle, 3,6,9,12,20,23,26,29,35,36-Decaaza-
tricydo[29.3Jj'''''']hexatriaconta-l(34),14,16,lS(36),31(35),32-hexaene-2,13,19,30-
tetraone, a 24-membered [Ng] macrocycle, 3,6,9,17,20,23,29,30-Octaaza-
tricydol23.3.U"'^^]triaconta-l(28),ll,13,15(30),25(29),26-hexaene-2,10,16,24-tetra-
one and a 20-membered [Ng] macrocycle, 3,7,15,19,25,26-Hexaazatricyclo [19.3.
l.f''^Jhexacosa-l(24),9,ll,13(26),21(25),22-hexaene-2,8,14,20-tetraone, which are
symmetrical class of macrocycles have been discussed. The macrocyclic ligands possess
four amide link and provide potentially equivalent metal binding sites. The complexes
were prepared using metal template procedure.
"Coordination cftemistry qfSgands xvith eUaron rick % O orS Biting centre' (Doctora[thesis suSmittedtoARgath MusRm "University, IWDI^
2A
Template Syntheses of Homo-bimetallic Complexes
Incorporating the Macrocyclic Ligand, 3,6,9,12,20,23,
26,29,35,36-Decaazatricyclo[29.3.Lf^'^^]hexatriaconta-l(34),
14,16,18(36),31(35),32-hexaene-2,13,19,30-tetraone (L^) Using
Cr^^ Fe^^ Co^^ Ni ^ Cu^^ Zn^^ Cd " and Hg ^ Ions as
Templating Agents
30
EXPERIMENTAL
Materials:-
The metal salts viz. anhydrous chromium(II) chloride, anhydrous iron(III)
chloride (both C. D. H., India), cobalt(II) chloride hexahydrate, nickel(II) chloride
hexahydrate, copper(II) chloride dihydrate, anhydrous zinc(II) chloride, cadmium(II)
chloride dihydrate and anhydrous mercury(II) chloride (all B. D. H. India), were
commercially pure samples used as received. The reagents pyridine-2,6-dicarboxylic
acid, triethylenetetraamine, diethyenetriamine and 1,3-diaminopropane (all E. Merck,
Germany) were used as received. Solvents like methanol, ethanol were purified while
acetone, dichloromethane, ether, chloroform, n-hexane, tetrahydrofuran, acetonitrile and
dimethylsulphoxide (all s. d. fine, L R grade) were used without ftirther purification.
Insirumentation:-
FT-IR spectra of compounds were recorded as KBr disc on Perkin Elmer Model
spectrum GX spectrophotometer. ' H - and '^C-NMR spectra of compounds dissolved in
deuterated water (D2O) were recorded on a Brucker DRC-300 spectrometer using SiMe4
(TMS) as external standard. Electronic spectra and conductivities of 10' M solution in
water were recorded on a Cintra-5GBS UV-Visible spectrophotometer and Systronics-
305 digital conductivity bridge, respectively, at room temperature. The electrospray mass
spectra (ESI) were recorded on a MICROMASS QUATTRO II triple quadrupole mass
spectrometer. The samples (Dissolved in water) were introduced into the ESI source
through a syringe pump at the rate of 5 mL per minute. The ESI capillary were set at 3.5
kv and the cone voltage was 40 V. The X-band EPR spectrum of the polycrystalline
samples was recorded on Jeol-RE series Model JES-RE3X spectrometer calibrated with
diphenylpicrylhydrazy (dpph: g = 2.003) at room temperature. Magnetic susceptibility
was estimated at room temperature using Faraday balance calibrated with Hg[Co(NCS)4].
Results of the Microanalyses were obtained from Micro-Analytical Laboratory of Central
Drug Research Institute (CDRI), Lucknow, India.
"Coordination chemistiy ofBgands witk etearon ricH % O orS Siting centre ' OoctoraC thesis suSmittecfto^Bgatii !MusGm University, I!M)IJl
2A 31
SYNTHESES OF THE MACROCYCLIC COMPLEXES
Synthesis of homo-bimetallic complex [CriL CI4] (1):-
Triethylenetetraamine (1.0 mL, 5.0 mmol) was dropped in the ethanolic solution
of anhydrous CrCl2 (0.62 g, 5 mmol) with stirring at room temperature in an inert N2
atmosphere. The reaction mixture was stirred for 30 minutes until stable green coloured
solution was formed. An ethanolic solution (40 mL) of pyridine-2,6-dicarboxylic acid
(PDA) (0.9 g, 5.0 mmol) was slowly added drop-wise in the reaction mixture with
stirring. A grey coloured precipitate was produced after the complete addition of PDA,
the stirring was continued for additional 24 h. The gray coloured mass was filtered off,
washed with water followed by ethanol and dried under vacuum. [Grey colour, m.p.
289°C dec, yield 0.411 g].
Anal. Calcd. for C26H38Nio04.Cr2Cl4: C, 39.09; H, 4.76; N, 17.54. Found for (1): C,
39.12; H, 4.72; N, 17.57. Molar Conductance, Am (in water): 132.4 ohm''cm^mole''.
FT-IR (as KBr discs, cm-'): 3108bw (NH), 1621s, 1563s, 1272s (Amide I, II, III), 1614s
(C=N), 1433s, 773s, 722s (C=C, ring), 424m (M-N).
The mother liquor was evaporated to dryness on a water bath, which provided
only the un-reacted pyridine-2,6-dicarboxylic acid and chromium chloride characterized
from the m.p. and IR studies.
Synthesis of homo-bimetallic complex [Fe2L CI4JCI2 (2):-
Anhydrous FeCb (0.8 g, 5 mmol) dissolved in 20 mL ethanol was added dropwise
to an ethanolic solution of triethylenetetraamine (1.0 mL, 5.0 mmol) at room temperature
with continuous stirring. The brown coloured suspension formed at the initial stage of the
reaction disappeared when the reaction mixture was stirred for about 30 minutes giving a
stable brown coloured clear solution. Pyridine-2,6-dicarboxylic acid (0.9 g, 5.0 mmol)
dissolved in 40 mL ethanol was dropped to the reaction mixture with stirring which
produced orange precipitate, which was filtered off, washed with ethanol and then dried
under vaccum affording brown coloured product, [m.p. >300°C, yield 1.346 g]
"Coordination chemistry qfSgands mtH ekuron ricli%OorS Sitir^ centre" <Doctora[thesis suSmitudtoM^arh Muslim "University, IM)IJl
2A 32
Anal calcd for C26H38Nio04.Fe2Cl6: C, 35.49; H, 4.32; N, 15.92. Found for (2): C, 35.42;
H, 4.29; N, 15.93. Molar Conductance, Am (in water): 220.2 ohm''cm'^mole''.
ESI-Mass spectrum: [FeiL'Clg + H]"" m/z = 877 (40%), [FezL'CU - H]* m/z = 875 (28%),
[FczL'Cle + 2H]+ m/z = 878 (20%), [Fe2L'Cl4+3H]^ m/z = 809 (40%), [Fe2L'Cl3-2H]^
m/z 769 (20%), [Fe2L'Cl3+4H]^ m/z = 775 (100%), [FejL'CbJ^m/z = 701 (18%), [Fe2L'-
2H]^ m/z = 664 (35%) [FeL' + H]^ m/z = 611 (35%), [FeL' - 3H]^ m/z = 608 (20%), [L'-
2H]^ m/z = 552 (100), [L'-4H]^m/z = 550 (93%), [L ' -O] m/z = 538 (76%), [L^-20] m/z
= 522 (55%), [ L ' - 2 0 + 2 H ] m/z 524 (98%), [ L ' - 3 0 ] m/z = 506 (18%), [ L ' - 4 0 ] m/z = 494
(43%)). [spectrum contained the relevant isotopic mass peak for all the metal and chlorine
nuclei, however, positions for the strongest peak have been recorded here]
FT-IR (as KBr discs, cm'): 3058bm (NH), 1648s, 1530s, 1273s (Amide I, II, III), 1594s
(C=N), 1428s, 760s, 739s (C-C, ring), 423m (M -N) .
Synthesis of homo-bimetallic complex [Co 2L CI J (3):-
An ethanoic solution of C0CI2.6H2O (1.2 g, 5.0 mmol) was reacted with
triethylenetetraamine (1.0 mL, 5.0 mmol) as above. The reaction mixture was stirred for
30 minutes which formed a stable violet-red coloured solution then it was reacted with
pyridine-2,6-dicarboxylic acid (0.9 g, 5.0 mmol) in the manner stated above. The brick-
red coloured precipitate produced was filtered off, washed with water followed with
ethanol then dried under vacuum to give brick-red product, [m.p. 180°C dec, yield 0.92
g]
Anal calcd for C26H38N10O4.C02CI4: C, 38.42; H, 4.46; N, 17.24. Found for (3): C, 38.38;
H, 4.48; N, 17.19. Molar Conductance, Am (in water): 141.0 ohm''cm^mole''.
FT-IR (as KBr discs, cm"'): 3090bm (NH), 1621s, 1571s, 1285s (Amide I, II, III), 1603s
(C=N), 1427s, 768m, 720s (C=C, ring), 420m (M - N).
Synthesis of homo-bimetallic complex [Ni2L CI4] (4):-
An ethanoic solution of NiCl2.6H20 (1.2 g, 5.0 mmol) was reacted with
triethylenetetraamine (1.0 mL, 5.0 mmol) at room temperature. The blue colour solution
'Coor£nation chemistry ofGgands with ekctron rich % 0 orS Biting antre' (Doctoral thesis submitted to Rgarh MusRm Vniversity, I!M)IA
2A
formed was further reacted with ethanoUc solution of pyridine-2,6-dicarboxylic acid (0.9
g, 5.0 mmol) as above. The blue colour of the reaction mixture turned into a light grey
colour, which has finally afforded light-grey coloured product in quantitative yield.
[Grey colour, m.p. 230 °C, yield 1.812 g]
Anal calcd for C26H38N,o04.Ni2Cl4: C, 38.51; H, 4.69; N, 17.28. Found for (4): C, 38.47;
H, 4.72; N, 17.31. Molar Conductance, Am (in water): 115.2 ohm''cm^mole''.
ESJ-Mass spectrum: [KnVCh+nf m/z = 741 (90%), [Ni2L'Cl+H]^ m/z = 706 (9%),
[NijL'+H]^ m/z = 671 (8%) , [Ni(TETA)Cl - H]^ m/z = 203 (100%), [Ni(TETA)+H]^
m/z = 205 (40%) and [TETA+H]^ m/z = 147 (8%).
FT-IR (as KBr discs, cm"'): 3128bm (NH), 1630s, 1569s, 1280s (Amide I, II, III), 1593s
(C=N), 1429s, 768m, 725s (C=C, ring), 425m (M -N).
Synthesis of homo-bimetallic complex [CU2L CI4] (5):-
The stable reaction mixture of CUCI2.2H2O (0.9 g, 5.0 mmol) and
triethylenetetraamine (1.0 mL, 5.0 mmol) taken in 20 mL ethanol was further reacted
with an ethanolic solution of pyridine-2,6-dicarboxylic acid (0.9 g, 5.0 mmol) at room
temperature in an identical manner as above. The whole reaction mixture was further
stirred over-night at RT which has produced blue coloured precipitate, [m.p. 196°C, yield
2.45 g].
Anal calcd for C26H38N,o04.Cu2Cl4: C, 38.04; H, 4.63; N, 17.07. Found for (5): C, 38.01;
H, 4.65; N, 17.01. Molar Conductance, Am (in water): 109.5 ohm"'cm^mole''.
FT-IR (as KBr discs, cm''): 3193bm (NH), 1628s, 1563s, 1273s (Amide I, II, III), 1595s
(C=N), 1428s, 769m, 723s (C-C, ring), 418m (M -N) .
Synthesis of homo-bimetallic complex [Zn2L^ CI4] (6):-
An ethanolic solution (40 mL) of pyridine-2,6-dicarboxylic acid (0.9 g, 5.0 mmol)
was dropped to the magnetically stirred reaction mixture of triethylenetetraamine (1.0
mL, 5.0 mmol) and ZnCb (0.7 g, 5.0 mmol) taken in 20 mL ethanol at RT. The pinkish-
white coloured precipitate formed was filtered off, washed with water followed by
"Coordination chemistry ofEgamis •with electron rich%0 orS Siting centre" iDoctordthesis suSmitted'toJiGgaTh MvsRm Vniversity, I9fDIJl
2A 34
ethanol, dried under vacuum, dessiccator. [Pinkish-white colour, m.p. 262°C dec, yield
0.97 g].
Anal calcd for C26H38N10O4.CU2CI4: C, 37.95; H, 4.62; N, 17.03. Found for (6): C, 37.89;
H, 4.62; N, 17.05. Molar Conductance, Am (in water): 145.5 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm"'): 3069bm (NH), 1620s, 1572s, 1284s (Amide I, II, III), 1580s
(C=N), 1426s, 780m, 724s (C=C, ring), 423m (m -N) .
Synthesis of homo-bimetallic complex [Cd2L CI4] (7):-
A similar procedure as stated above for the preparation of [Zn(L')Cl2] was
followed here too. The white precipitate formed was filtered off, washed with water
followed by ethanol then dried under vacuum. [White colour, m.p. > 300°C, yield 0.37 g]
Anal calcd for C26H38Nio04.Cd2Cl4: C, 33.83; H, 4.12; N; 15.18. Found for (7): C, 33.79;
H, 4.10; N, 15.20. Molar Conductance, Am (in water): 135.2 ohm"'cm^mole''.
FT-IR (as KBr discs, cm''): 3170bm (NH), 1621s, 1569s, 1278s (Amide I, II, III), 1603s
(C=N), 1448s, 773m, 722s (C=C, ring), 431m (M -N) .
Synthesis of homo-bimetallic complex [Hg2L CI4] (8):-
HgCla (1.35 g, 5.0 mmol) was reacted with triethylene tetraamine (1.0 mL, 5.0
mmol) in ethanol. The reaction mixture was stirred for 30 minutes to get a stable milky
coloured colloidal suspension at room temperature. Pyridine-2,6-dicarboxylic acid (0.9 g,
5.0 mmol) in 40 mL was added in this reaction mixture with stirring, which was further
stirred over night at room temperature. The stable white precipitate formed was filtered
off, washed with water followed by ethanol and dried under vacuum, [white colour, m.p.
162''C dec, yield 0.66 g].
Anal calcd for C26H38N,o04.Hg2Cl4: C, 28.41; H, 3.46; N, 12.75. Found for (8): C, 28.45;
H, 3.49; N, 12.72. Molar Conductance, Am (in water): 104.3 ohm"'cm^mole''.
FT-IR (as KBr discs, cm"'): 3098bm (NH), 1623s, 1571s, 1274s (Amide I, II, III), 1603s
(C=N), 1435s, 769s, 724s (C=C, ring), 433m (M -N).
"Coordination cHemistry ofGgands witH e&anm Ticli%0 orS Siting centre" <Doctorat thesis submitted to l^arh MusRm University, IWDIA
2^ 35
RESULTS AND DISCUSSION
The complexes (1-8) were prepared employing a two step single pot template
procedure. In the first step, triethylenetetraamine and the metal salts (1:1 mole ratio) were
reacted to get a triethylenetetraamine metal complexes stabilized in the solution. In
second step, this reaction mixture was further reacted with equimolar amount of pyridine-
2,6-dicarboxylic acid at room temperature, which has afforded stable solid products,
usually after an over night stirring of the reaction mixture. Almost, all the complexes
exhibited solubility in water only, therefore, the various physico-chemical studies and
spectral investigations have been performed as an aqueous solution. Most of the
complexes either melt sharply or get decomposed between 280 - 300 °C suggesting
formation of a monomeric species.
Analytical and ESI-mass spectral studies:-
Analytical data (Experimental section) of complexes (1 - 8) agreed with the
molecular formula C26H38N10O4.M2CI4 [M = Cr " (1), Co^^ (3), Ni ^ (4), Cu ^ (5), Zn ^
(6), Cd "" (7) or Hg "" (8)] and C26H38Nio04.Fe2Cl6 (2) formulated as MJL 'CU and
Fe2L'Cl5, respectively.
ESI-mass spectral data of the complexes Fe2L'Cl6 (2) and Ni2L'Cl4 (4) confirm
the probable stoichiometry of the complexes indicated by their analytical data. The
Electron Spray Ionization (ESI) spectrum of the complex FeaL'Cle (2) exhibited various
important bunch of peaks (pertinent with the isotope abundance ratio). The peaks
positions (m/z value) having the highest intensity have been considered because the metal
as well as the chlorine nuclei are well known to exhibit the isotopic peaks in varying
intensities. The positions of the peaks were consistent with the formation of molecular
ions [Fe2L'Cl6+H]^ m/z = 877 (40% abundance), [Fe2L'Cl6+2H]^ m/z = 878 (20%
abundance), [Fe2L'Cl6 - H]"" m/z = 875 (28% abundance). These molecular ion peaks
further undergo step wise dissociation formation the various volatile fragmentation peaks
such as [Fe2L'Cl4+3H]^ m/z = 809 (40% abundance), [Fe2L'Cl3-2H]^ m/z = 769 (20%
"Coor£nation cHemistry ofGgaiuts witH etectron ricfi%0 orS Biting centre" <Doctora[thesis suBmittedto AEgarh Muslim "University, I!M)IJl
2A 36
abundance), [Fe2L'Cl3+4Hf m/z - 775 (100% abundance), [FcsL'CbJ^m/z = 701 (18%
abundance), [Fe2L'-2H]* m/z = 664 (35% abundance) [FeL' + H]^ m/z - 611 (35%
abundance), [FeL* - 3H]* m/z 608 (20% abundance) as well as a number of endocyclic
fragmentation products from the ligand moiety. Some of the important peaks are as
following [L ' -2H]^ m/z = 552 (100% abundance), [L'-4H]^m/z = 550 (93% abundance),
[L ' -O] m/z = 538 (76% abundance), [ L ' - 2 0 ] m/z = 522 (55% abundance), [ L ' - 2 0 + 2 H ]
m/z 524 (98% abundance), [ L ' - 3 0 ] m/z = 506 (18% abundance) and [ L ' - 4 0 ] m/z ^ 494
(43%) abundance) etc.. The spectrum also exhibited peaks due to decomposition of
macrocyclic ligand moiety into the constituent fragments like pyridine-2,6-dicarboxylic
acid (PDA) [PDA - H]^ m/z 166 (30% abundance) and triehylenetetraamine (TETA)
[TETA - 2H]^ m/z = 130 (15% abundance). In view of the present ESI data it is
reasonable to conclude that the complex is dinuclear in nature i.e. contained two Fe
nuclei encapsulated/chelated in the macrocyclic cavities of the ligand moiety,
3A9,12M23a6,29,35,36-Decaaza-tricyclo[29JJA^'''^^]hexatriaconta-l(34),14,16,18
(36),31(35),32-hexaene-2,13,19,30-tetraone, ( L ' ) . The appearance of molecular ion
[FeL' - 2H]" m/z = 664 (35% abundance) suggest that the dinuclear species is quite stable
and volatile enough to get recorded under the ESI condition. However, this dinuclear
species further undergoes cleavage to produce a mononuclear species [FeL'-3H]* m/z =
608 (20% abundance).
The ESI-mass spectrum of the complex Ni2L'Cl4 (4) exhibits a number of peaks
(Experimental section), which can be reasonably assigned to the formation of various
volatile species which are due to complex molecular ion and the step-wise fragmentation
the of exocyclic as well as the endocyclic ligands from the complex molecular ion. The
appearance of a weak intensity peak at m/z = 813 (-10%) abundance) is consistent with
the formation of the molecular ion [Ni2L'Cl4+3H] , which has low abundance probably
due to a low volatility. However, the peak observed at m/z = 741 is assignable to the
species [Ni2L Cb+H]^ formed due to thermal cleavage of exocyclic ligand has a
relatively higher volatility (90% abundance). The other fragmentations generated due to
cleavage of exocyclic ligands are [Ni2L'CI+H]' m/z = 706 and [Ni2L'+H]' m/z = 671
which also show a very low intensity (<10% abundance) probably due either to a very
"Coordination cfiemistry ofSgands with ekctron ricli%0 orS Biting centre" OoctoraC thesis si^itted to AUgarh Muslim "University, I^^IJl
2^ 37
low volatility or a low thermal stability undergoing further endocyclic cleavages to give
species like [Ni(TETA)Cl - H]^ m/z = 203 (100% abundance), [Ni(TETA)+H]^ m/z =
205 (40% abundance) and [TETA+H]^ m/z = 147 (8% abundance).
The present ESI data of the complexes (2) and (4) therefore confirm that the
macrocyclic ligand L ' is dinuclear in nature encapsulating two metal ions resulting
homo-bimetallic complexes, which show a considerable stability under the ESI-Mass
condition.
FT-IR spectral studies:-
FT-IR spectra of the complexes (1 - 8) provide valuable information regarding
the nature of fundamental group attached to the metal ions. The spectra did not show the
strong absorption bands expected in the region 1700 - 1800 cm' and 3300 - 3500 cm"
characteristic for v(C=0) and v(0 - H) stretching vibrations, respectively, reported [24]
for the carboxylic (CO - OH) group. However, the spectra contained (Experimental
section) absorption bands assignable to the v(N - H) and amide group frequencies (i.e.
amide I, amide II and amide III). The spectra also contained bands characteristic of
v(C=N) and v(C=C) stretching vibrations indicating the presence of pyridine moiety in
the complex molecule. Henceforth, the disappearance of the band characteristic of
v(COOH) group with the concomitant appearance of new bands due to amide group
vibrations supported the cyclic condensation process in the presence of metal ions as
tempelating agent according to the mechanism shown in Figure 2A.1, 2A.2. The
appearance of a medium intensity band in the region 418 - 433 cm"' is due to M - N
bond stretching vibration indicating the chelation / encapsulation of the metal ions
through the aza biting sites of the amine as well as the amide functions of the macrocyclic
ligand.
'H- and '^C- NMR spectral studies:-
The proton NMR spectra of the complexes (1, 3, 4) recorded in D2O exhibited
(Table 2A.1) two separate categories of well defined aromatic and aliphatic protons
resonance signals. The aromatic set is in the region i.e. 8.5 - 8.9 ppm as multiplet (m, "Coordination cfiemistry ofRgands ivitH ekaron rich % O orS Siting untre '
Ooctorafttiesis suSmitted'toJiSgad MusRm Vniversity, imiljl
2^ 38
6H). The aliphatic (-CH2-) set of protons appear as three well separated resonance signals
as triplet in the regions 2.65 - 2.74 ppm (t, 8H), 2.80 - 2.94 ppm (t, 8H) and 3.05 - 3.14
ppm (t, 8H). The spectra however, could not show a signal characteristic of the NH
proton either of the amide group or of the secondary amine group. It is probably due to
the presence of a fast proton exchange with the solvent through a hydrogen bonding
(Inter- or Intra-molecular) [25]. ' C-NMR spectra of the complexes (1,3,4) exhibited a
number of resonance lines which are assignable to ' C resonances of C=C (138 - 155
ppm), CO -NH (168 - 172 ppm) as well as characteristic of the aliphatic carbon atoms in
the 42 - 55 ppm regions {Jable 2A.2).
Conductivity measurements:-
The magnitude of the experimentally measured molar conductivities
(Experimental section) of an aqueous (10" M) solution of the compounds (1,3 - 8) are in
the 100 - 145 ohm"'cm^mole"' region, respectively. The magnitude of Am indicate that
these complexes behave [26] as 1:1 electrolyte (c.f Am = 100 - 200 ohm"'cm^mole"' for
1:1 electrolyte in water). The conductivity data for (1, 3-8) therefore indicate that the
complexes get ionized in water generating [27] an aquated complex cation
[M2L'C13(H20)]^ and the counter chloride ion as following:
[MiL'Ck] + H2O ^ =^[M2L'CkH20J^+ Cr
[M = Cr ^ (1), Co^^ (3), Ni ^ (4), Cu ^ (5), Zn ^ (6), Cd ^ (7) or Hg^^ (8)]
However, Am of the complex (2) (Am - 220.2 ohm''cm^mole"' ) indicate that it
behaved as 1:2 electrolyte (c.f Am = 201 - 300 ohm"'cm^mole"' for 1:2 electrolyte in
water) in solution.
Magnetic moment, EPR and electronic (ligand field) spectral studies:-
The observed |ieff. value of the homo-bimetallic complex [Cr2L'Cl4] (1) is 6.9 BM
(or lOeff, per Cr" = 3.45 BM) at room temperature. The magnitude is lower compared to
the theoretically calculated spin-only (|Xs 0 = 8.9 BM) moment for four unpaired electrons
[28,29] on each Cr " ion in a binuclear Cr " - Cr ^ complex having two magnetically non-"Coonfination chemistry ofRgands with efeanm rich%OorS Biting centre"
<DoctoraCthesis suBmitud to ARgarh MusRm University, IWDIA
2A 39
interacting spin-systems [(Scr " - Scr' ) = 2,2] in the absence of any exchange interaction.
This decrease in magnetic moment is not an unusual observation for dinuclear complexes
because intra-molecular anti-ferromagnetic exchange interaction usually exist in many
dinuclear complexes containing paramagnetic metal ions [29,30]. The presence of inter-
molecular anti-ferromagnetic interaction may not be ruled out. The existence of any of
the above described spin-exchange phenomena considerably lowers the experimentally
observed magnitude of the magnetic moment. The electronic spectrum of an aqueous
solution of (1) exhibited the strong absorption bands {Table 2A.3) in the u.v. region
characteristic of ligand TT- TC* and metal-*—ligand (M<—L) charge transfer transitions. In
the visible region a weak asymmetrical broad band at 570 nm (17,540 cm"') due to ligand
field ^T2g*-^Eg transition was also observed. The observed asymmetry in the absorption
band may arise due to the presence of the dynamic John-Teller distortion [31] geometry
of the molecule. The room temperature X-band EPR spectrum of the microcrystalline
sample of the complex exhibited only one broad signal with gav. = 1.94. The nature of the
spectrum did not change its shape even at liquid nitrogen temperature (LNT). This
unusual broadening is the effect of dipolar interactions of the unpaired spins present in a
highly paramagnetic species [32].
The jieff. Value of the homo-bimetallic complex [Fe2L'Cl6] (2) are considerably
lower than that of the theoretically calculated spin-only moment for two Fe " nuclei with
un-coupled spins (Spe * - Spe ^ = 5/2,5/2) in a high-spin {t2g, Cg) configuration [33] for
each of the Fe"* ions. A strong anti-ferromagnetic coupling between the two adjacent
Fe"* centers encapsulated in the macrocyclic cavity can efficiently reduce the observed
l eff value {Table 2A.3) similar to that reported [34,35] for various dinuclear Fe "
complexes having a distorted octahedral geometry. The electronic spectrum consisted of
a strong broad absorption band at 372 nm (26,880 cm'') characteristic of metal-f-ligand
(M<—L) charge transfer transition. The asymmetrical bands observed in the visible region
{Table 2A.3) are due to the ligand field (d - d) transitions [31].
The observed i eff Value for [Co2L'Cl4] (3) ( eff = 7.1 BM or |ieff = 3.6 BM per
Co ion) is comparable to the theoretically expected spin-only moment \i^o. 6.92 BM
expected for two magnetically independent Co^^ nuclei (Sco^ - Sco ^ = 3/2, 3/2) with "Coordination chemistry ofGgamCs with ekctron rich%0 orS Biting untre"
(Doctoral thesis suSmitud to JlRgarh MusGm Vniversity, lOmiJi
2A 40
three unpaired spin (3 UPE) on each Co " ion in the homo-bimetallic complex (3). The
presence of spin-orbit coupling may have some contribution to the observed Heff. value of
Co^^ complexes. This is not an unusual observation in various mononuclear as well as di-
nuclear Co^* complexes with hard donor centres [36]. The X - band EPR spectrum of the
poly crystalline sample of the complex, however, did not show a reasonably good looking
resonance signal even to a temperature down to liquid nitrogen temperature (LNT). This
is probably due to the presence of a very high spin-state of the species (total six unpaired
spin). This indicates a high spin configuration of {t2g, e/) for both Co " nuclei in the
present homo-nuclear bimetallic complex (3). The electronic spectrum of the complex
exhibited two sets of weak ligand field (d - d) bands along with two strong bands due to
the ligand (7i->7r*) and charge transfer (M<—L) transitions {Table 2A.3). The observed
weak intensity ligand field bands with their assignments are summarized in Table 2A.3.
The present homo-bimetallic complex [NiiL^CU] (4) is paramagnetic in nature
which ruled out the possibility of a square-planar geometry around Ni " ion which
requires a diamagnetism of the molecule. However, eff. value for the complex (Heff, = 4.8
BM or 2.4 BM per Ni " ion) is comparable to that reported [36] for various hexa-
coordinate homo-bimetallic Ni " complexes (SNJ^^ - '&m^ = 1,1) where the orbital
contribution to the magnetic moment is nearly negligible. The position of the ligand field
bands with their assignments [31] are indicated in Table 2A.3, which are characteristic of
an octahedral environment around each Ni ion.
The electronic (lignad field) spectrum of the complex [Cu2L'Cl4] (5) show a
broad ligand field band, which could be due to ill defined doublet at 370 nm (27,027 cm'
) and 610 nm (16,339 cm"'). The ligand asymmetry due to weak axial coordinations from
counter CI" anions relative to the strong basal coordinations from aza groups of the
macrocycle would also be responsible for a gross distortion from the perfect Oh
symmetry. Splitting of energy level Eg into ^Big, ^Aig and of T2g into B2g, Eg are
dependant on the extent of the tetragonal distortion [22]. The resulting energy levers are
in the order Big<^Aig<'B2g<^Eg, which may provide three allowed transitions. The
observed ligand field band (Table 2A.3) are assignable to double ^Eg<-^Big and
"Coordination chemistry qfCxgands with ekaron rich%0 orS Siting centre' (DoctoraC thesis suSmittedtoASgarh MusRm "University, I9/iDIJi
2Ji 41
^B2g<-^Big (also considered as dx^-y^ <- dz^ and dx^-y^^dxy,yz) transiton, similar to that
reported [37,38] for homo-binuclear Cu ^ complexes like [Cu2L4(DMSO)2] (L =
tomoltin, i-buprofen and naproxen). The X-band EPR spectrum of polycrystalline sample
of complex (5) recorded at room temperature shows an isotropic resonance signal at giso.
= 2.02.
In view of the present spectroscopic data it is reasonable to suggest that the ligand
3A9,12,20,23,26,29,35,36-Decaazairicyclof29JJ.l^'''^''jhexairiaconta-l(34),14,16,18
(36),31 (35),32-hexaene-2,13,19,30-tetraone (l)) encapsulates two metal ions in its
cavity to result homo-bimetallic complexes (1 - 8) such that of the metal ions, Cr " , Fe " ,
Co^ , Ni + and Cu * acquire hexa-coordination. The homo-bimetallic complexes retained
its identity ever under ESI-mass spectrum condition. The molecular geometries around
the metal ions have been further ascertained form the molecular model computations
which have provided the most probable optimum energy plots for the molecules. The
structural parameters like important bond lengths and bond angles have also be computed
employing the well established available software as following:-
Molecular modeling:-
The molecular model calculations based on the CSChem-3D MOP AC have been
employed [39,40] to solve the molecular structures and to get the various bond lengths
and bond angles in the molecules. The plots indicate that the ligand ( L ' ) has eight
potential aza donors to provide two set of [N,N,N,N] tetradentate biting sites for each
metal ions. The computed bond lengths and bond angles have been displayed in Table
2A.4. The mechanical adjustments via augmented mechanical field were used to draw the
optimum minimum energy plot as shown in Figure 2A.3 and 2A.4 for the geometry of
the metal complexes [M2L'Cl4] [M - Cr ^ (1), Co^^ (3), Ni ^ (4), Cu^^ (5), Zn^^ (6), Cd ^
(7) or Hg " (8)] and [Fe2L*Cl4]Cl2 (2). It is apparent that the metal ions acquire a hexa -
coordinate geometry.
"Coordination cRemistty qfSgands with ekctron nch!N,0 orS Siting centre' 'Doctoraltfiesis suBmittecftoASgaHi Muslim University, I^/^IJi
2A 42
Scheme I
-NH NH2
+ MCI2
Pyridine,2,6-dicarboxylic acid 1 mole ration
V^
-4H7O
.--"H CI
,NK
NH
,V^N ^
,M.
CI
/ NH
CI
NH
HN,
HN
.M
V>o
NH^ / [ \ NH'
CI
V ^ Figure 2A.1: Proposed mechanism for the formation of the complexes [M2L'Cl4]
[M = Cr'^ (1), Co ^ (3), Ni "" (4), Cu ^ (5), Zn^^ (6), Cd "" (7) or Hg^* (8)]
'CoonRnation chemistry ofSgands witH electron rich % O orS Siting centre " OoctoraCthesis suSmittedto^Rgarfi MusRm University, HMDIA
2A 43
Scheme 2
-4H2O
^ ^ ^
Figure 2A.2: Proposed mechanism for the formation of the complex [Fe2L^ CI4f* (2). "Coordination cfiemistty ofSgands tvith electron rick % O orS Siting centre '
(DoctoraltHesis suSmittedtoJiRgarA 9iluslim Vnivenity, I:M)IJI
iA 44
Figure 2A3: Perspective coordination geometry around M^'^ ion in [M2(V)Cl4j(M Ci^\ Co^^, Nt, Ct^^, Zi?\ Ctf\ Hi^) • C; • H; • N; • M ; t CI; • O
Figure2A.4:Perspective coordination geometry around F^* ion in Fe^L')Cl4f^ • C; • H; t N ; • Fe ; S CI; • O
'Coontmation c6emistty of Bgcmds with ekctrimiidi% O or S Btiing centre' (DoctordtiesissuSmtttaftoASg"^ MusGm Vmversity, I9fiDIJi
ZA 45
lexes. Table 2A.1: ' H - N M R spectral data of the homo-bimetallic compk
Complexes Pyridine ring CONH-CH2 - CONH-CH2-CH2 NH-CH2-CH2-NH
"^ r 8.52-8.76 5 2.65 5 (t, 8H, 2.88 5 (t, 8H, 3.09 5 (t, 8H, (m, 8H) JH-H = 7.2cps) JH-H = 7.3CPS) JH-H = 6.8CPS)
2 8.58-8.818 2.69 5(t, 8H, 2.80 5(t, 8H, 3.05 5(t, 8H, (m, 8H) JH-H = 7.4 ops) JH_H = 6.9 cps) JH-H = 7.1 cps)
3 8.72-8.89 5 2.74 8(t,8H, 2.86 5(t, 8H, 3.14 5(t, 8H, (m, 8H) JH-H = 7.3 cps) JH-H = 7.1 cps) JH-H = 7.0 cps)
4 8.68-8.85 5 2.68 5(t, 8H, 2.94 5(t, 8H, 3.12 5(t,8H, (m, 8H) JH-H = 7.4 cps) JH-H = 7.2 cps) JH-H = 6.9 cps)
5 8.63 - 8.85 5 2.71 5 (t, 8H, 2.91 5 (t, 8H, 3.06 5 (t, 8H, (m, 8H) JH-H = 7.2 cps) JH-H = 6.8 cps) JH-H = 6.8 cps)
Table 2A.2: '^C- NMR spectral data of the homo-bimetailic complexes
Complexes Pyridine ring CO-NH CONH- CONH-CH2- NH-CH2-CH2-
CH2 CH2 NH
1 125.1-140.2 5 169.2 5 42.3 6 49.3 6 52.4 5
2 128.2-144.6 5 170.4 5 44.5 5 50.15 53.3 5
3 126.7-148.7 8 168.3 6 43.6 8 49.7 6 54.5 6
4 128.1-151.9 6 171.8 6 46.3 5 50.8 6 53.7 6
5 127.3-146.7 6 170.9 5 44.9 8 49.9 6 52.9 5
"Coordination chemistiy ofRgands witfi ekctnm ricli%OorS Biting antn " (Doctoralthesis suSmittetftoJiSgarfi MusRm Vniversity, I'N^Dl^
2A 46
Table 2A.3: i eff. and ligand field spectral data of the complexes.
Complexes
1
2
3
4
5
Jieff.
(MB)
6.9
3.5
7.1
4.8
1.8
Hcff.(per
M"*)
3.54
1.75
3.6
2.4
0.9
Bands
nm (cm'*)
215(46,510)
285 (35,080)
570(17,540)
372 (26,880)
386 (25,900)
465(21,500)
623 (16,050)
210(47,620)
280(35,710)
480 (20,830)
690 (14,490)
210(47,620)
270 (37,030)
370 (27,020)
590 (16,950)
680 (14,700)
210(47,620)
270 (37,030)
370 (27,020)
610(16,390)
Transitions
CT
T2g<- Aig
rT2g, T]gJ<r- A]g
CT
%g(P)^%g (F)
%g(F)^%g (F)
CT
%(P) ^%g
%(F) ^%g
%(F) ^%g
CT
Fg *- Bjg
B2g^ Big
EPR
(giso.)
1.94
2.02
'Coordination cHemistty ofGgands witH ekctTon ricH% 0 orS Siting centne' (DoctoraC thesis submitted to Ggarh MusGm Vniversity, IJ{DIA
i o o o oo
00 00
0^ 00
r-- OS o o o oo
o o o 00
u
o o
C3
o e o
x: o "^ u >-,
"M
c 03 T3 C O
_o -o c a
o
< CO
x: +-» W) c TD C O ^ T3 -4—*
£ o o
t: o
-C5
E5
I
33
u
K U I
u
z I
o u en
"a B o U
00 ON
00
O O o 00
o o o OS
IT) 00
o o o OS
o o o 00
o o o 0\
0\ OS 00
OS
' OS r--00 OS Os 00
o o o OS
OS 00
o d OS
OS
OS 00
00 OS
o o d OS
Os 0\ 00
00
ON
o o d OS
00 m 00 00 oo
00
as 00
OS
00 OS
00 OS OS oo
OS 00 Os 00
Os OS
00 00
ON 00
rr V
o
(N
O
(N
O in
CM
O
(N
O
(N
O
(N
O
rsi
o 00 <N
iO ^ ^ CN in so C> ,-H ^
— rsi <N
NO ^ *0 VD vo vo \o '^ ^ '^ 't ^ '^ rf '^ ^ '^ '* rt '^ Tt-
' - H i r i r < - i > i O > O r ^ ' ^ o O O N O o r ~ - ( N m ^ o o O s N O N q v - i N o o r ^ i r n
I NO vo NO NO NO \0 NO NO
1-H *N ro in vo 00
1,
^^s
2B
Template Syntheses of Mono-metallic Complexes
Incorporating the Macrocyclic Ligand, 3,6,9,17,20,23,29,30-
Octaaza-tricyclo[23.3.1.l"'^^ltriaconta-l(28),ll,13,15(30),
25(29),26-hexaene-2,10,16,24-tetraone (L ) and 3,7,15,19,25,26-
Hexaaza-tricyclo [19.3.1.1 ' ]hexacosa-
l(24),9,ll,13(26),21(25),22-hexaene-2,8,14,20-tetraone (L )
Using Cr^^ Fe^\ Co^^ Ni ^ Cu^ Zn\ Cd ^ and Hg ^ Ions as
Templating Agents
2(B 48
SYNTHESIS OF MACROCYCLIC COMPLEXES
Synthesis offCrL^Clz] (9):-
The ethanolic solution of diethylenetriamine (0.62 rtiL, 5.0 mmol) was added
drop-wise to the ethanolic solution ofCrCh (0.31 g, 2.5 mmol) in an inert N2 atmosphere
with stirring at room temperature. The reaction mixture was stirred for 2 h until a stable
green coloured solution was obtained. Pyridine-2,6-dicarboxylic acid (0.9 g, 5.0 mmol)
dissolved in 40 mL ethanol was then dropped in the reaction mixture with stirring at RT.
The light purple coloured solution formed was stirred continuously for 24 h which has
produced purple coloured precipitate. The precipitate was filtered off, washed with water
followed by ethanol and dried under vacuum. [Purple colour, m.p. 289°C dec, yield 0.41
g]
Anal. Calcd for C22H28N804.CrCl2: C, 44.67; H, 4.73; N, 19.95. Found for (9): C, 44.69;
H, 4.69; N, 19.99. Molar conductance. Am (in water): 111.0 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm"'): 3077bm (NH), 1639s, 1581s, 1264s (Amide I, II, III), 1607s
(C=N), 1439s, 766s, 724s (C=C, ring), 430m (M-N).
Synthesis oflFeL^ClJCl (10):-
Anhydrous FeCl3 (0.4 g, 2.5 mmol) was reacted with diethylenetriamine (0.62
mL, 5.0 mmol) in ethanol which gave a reddish brown solution at room temperature. The
solution of pyridine-2,6-dicarboxylic acid (0.9 g, 5.0 mmol) in 40 mL ethanol was added
drop-wise with stirring in the above reaction mixture at room temperature. The solution
turned brownish which afforded brown precipitate after over-night stirring. The
precipitate was filtered off, washed first with water then with ethanol, dried under
vacuum. [Brown colour, m.p. 209°C, yield 1.16 g].
Anal. Calcd for C22H28N804.FeCl3: C, 41.80; H, 4.44; N, 17.78. Found for (10): C, 41.82; I 9 I
H, 4.40; N, 17.75. Molar conductance, Am (in water): 78.5 ohm' cm mole .
ESI-mass spectrum: [FeL^CIj]" m/z = 629 (<I0%), [FeL^Cb]^ m/z = 594 (<10%),
[FeL^Cl]'' m/z = 559 (<10%), [FeLY m/z = 524 (<10%), [Fe(LV2)Cl+H]^ m/z = 324
"Coordination chemistry ofRgands witH ekctron ricli%OorS Biting centre ' <Doctora(thesis suBmittedto ARgath MusRm Vniversity, I9{1)IJl
2(8 49
(24%), [Fe(L^/2)+4H]^ m/z = 294 (18%), [Fe(NHCOC6H3CONH)2+2H]^ m/z = 288
(24%), [L^]^ m/z - 468 (<10%), [DETA+H]^ m/z = 104 (100%), [NH2-CH2-CH2-NH-
CH2-CH2 - H]^ m/z = 87 (25%), [Fe(NH2-CH2-CH2-NH-CH2-CH2-NH2)2-4H]^ m/z =
262 (<10%), [NH2-CH2-CH2-NH2]* m/z = 58 (10%), [C6H3N(COOH)2]^ m/z = 167
(25%).
FT-IR (as KBr discs, cm"'): 3084bm (NH), 1642s, 1567s, 1273s (Amide I, II, III), 1590s
(C=N), 1441s, 769s, 730s (C=C, ring), 439m ( M - N ) .
Synthesis offCoL^ChJ (11):-
The reaction mixture obtained after reacting an ethanoic solution of C0CI2.6H2O
(0.6 g, 2.5 mmol) with diethylenetriamine (0.62 mL, 5.0 mmol) at RT was stirred for Y2 h
to get a stable solution. It was further reacted with ethanolic solution of pyridine-2,6-
dicarboxylic acid (0.9 g, 5.0 mmol). The colour of the reaction mixture changed to
blackish - yellow. It was further stirred for 2 h to get buff coloured precipitate, which was
filtered off, washed with ethanol and then dried under vacuum. [Buff colour, m.p. 225°C,
yield 0.85 g].
Anal. Calcd for C22H28N8O4.C0CI2: C, 44.15; H, 4.68; N, 18.73. Found for (11): C,
44.20; H, 4.72; N, 18.69. Molar conductance, Am (in water): 127.7 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm''): 3073bm (NH), 1627s, 1539s, 1291s (Amide I, II, III), 1582s
(C=N), 1420s, 769s, 748s (C=C, ring), 431m ( M - N ) .
Synthesis offML^ChJ (12):-
It was prepared in an identical manner as stated above for the complex (11). The
yellowish green coloured reaction mixture obtained after addition of PDA was stirred
over-night which produced green coloured precipitate. It was filtered off, washed with
ethanol and dried under vacuum. [Green colour, m.p. 238°C, yield 1.74 g]
Anal. Calcd for C22H28N804.NiCl2: C, 44.17; H, 4.68; N, 18.74. Found for (12): C, 44.12;
H, 4.69; N, 18.69. Molar conductance, (in water): 93.6 ohm''cm^mole"'.
ESI-Mass spectrum: [NiL'Cb]^ m/z - 596 (20%), [NiL^Cl - H]^ m/z = 560 (<10%),
[NiLY m/z = 525 (28%), [Ni(L^/2)Cl2 - H]"" m/z = 361 (>10%), [Ni(L^/2)Cl + H]^ m/z =
"Coordination chemistry ofRgands with ekctron rich%0 orS Siting centn" <Doctora[thesis suBmitted'toAligarh Muslim "University, IIM)IJl
2<B 50
328 (12%), [NiL'/2 + 2H]^ m/z = 294 (15%), [Ni(DETA)2+H]+ m/z = 265 (25%), [L ]
m/z = 468 (<10%), [DETA + H]^ m/z = 104 (100%).
FT-IR (as KBr discs, cm''): 3098bm (NH), 1622s, 1572s, 1274s (Amide 1, II, III), 1592s
(C=N), 1429s, 765s, 733s (C=C, ring), 434m (M -N) .
Synthesis offCuL^CIzJ (13):-
An identical procedure for the preparation of this complex was adopted as
described above. This has resulting in a quantitative yield of the final product. [Blue
colour, m.p. 230°C, yield 1.346 g]
Anal. Calcd for C22H28N8O4.CUCI2: C, 43.81; H, 4.64; N, 18.58. Found for (13): C,
43.79; H, 4.66; N, 18.62. Molar conductance, Am (in water): 75.7 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm"'): 3151bm (NH), 1637s, 1588s, 1271s (Amide I, II, III), 1617s
(C=N), 1428s, 773s, 730s (C=C, ring), 436m (M -N) .
Synthesis offZnL^ChJ (14), [CdL^Ch] (15) and [HgL^ChJ (16):-
A general procedure for the synthesis of these complexes were used. The
ethanolic solution of diethylenetriamine (0.62 mL, 5.0 mmol) was first reacted with
equimolar solution of the metal chloride at room temperature. The stable reaction mixture
was then reacted with pyridine-2,6-dicarboxylic acid (0.9 g, 5.0 mmol) dissolved in 40
mL ethanol, which with continuously stirred over night to afford quantitative amount
precipitate which were filtered off, washed and dried as above.
[(14) Light pink colour, m.p. 260°C, yield 0.99 g]
Anal. Calcd for C22H28N804.ZnCl2: C, 43.68; H, 4.63; N. 18.53. Found for (14): C, 43.72;
H, 4.68; N, 18.52. Molar conductance, Am (in water): 85.0 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm"') (14): 3101bm (NH), 1619s, 1581s, 1277s (Amide I, II, III),
1592s (C=N), 1426s, 771s, 742s (C=C, ring), 436m (M-N).
[(15) Pink colour, m.p. 252°C dec, yield 1.310 g]
Anal. Calcd for C22H28N804.CdC]2: C, 40.53; H, 4.29; N, 17.19. Found for (15): C,
40.56; H, 4.32; N, 17.24. Molar conductance. Am (in water): 74.1 ohm"'cm^mole"'.
"Coordination chemistry ofRgands witH ekctron ricli%0 orS Siting antre" (Doctoralthesis suBmittedtoJiligarh MusRm Vniversity, I!M)IJi
2(B 51
FT-IR (as KBr discs, cm"') (15): 3098bm (NH), 1639s, 1569s, 1282s (Amide I, II, III),
1614s (C=N), 1426s, 769s, 726s (C=C, ring), 418m ( M - N ) .
[(16) white colour, m.p. 234°C d e c , yield 0.736 g]
Anal. Calcd for C22H2gN804.HgCl2: C, 35.69; H, 3.78; N, 15.14. Found for (16): C,
35.72; H, 3.76; N, 15.16. Molar conductance, Am (in water): 220.4 ohm"'cm^mole' ' .
FT-IR (as KBr discs, cm"') (16): 3065bs (NH), 1641s, 1577s, 1266s (Amide I, II, III),
1622s (C=N), 1422s, 755s, 724s (C=C, ring), 426m ( M - N ) .
Synthesis offCrL^ClJ (17):-
A mixture containing 1,3-diaminopropane (0.42 mL, 5.0 mmol) and CrCb (0.31
g, 2.5 mmol) in ethanol was stirred for 30 minute under an inert N2 atmosphere at room
temperature to get a stable coloured solution. An ethanolic solution of pyridine-2,6-
dicarboxylic acid (0.9 g, 5.0 mmol) was added in this reaction mixture with stirring
giving a grey coloured solution which was stirred continuously for 24 h. The gray
coloured precipitate formed was filtered off, washed with ethanol and dried under
vacuum. [Gray colour, m.p. 228°C, yield 1.03 g]
Anal. Calcd for C2oH22N604.CrCl2: C. 45.03; H, 4.13; N, 15.76. Found for (17): C, 44.99,
H, 4.17; N, 15.81. Am (in water): 120.2 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm"'): 3076bw (NH), 1633s, 1576s, 1288s (Amide 1, II, III), 1629s
(C=N), 1466s, 754s, 701s (C=C, ring), 431m ( M - N ) .
Synthesis offFeL^ChJO (18):-
The first step was to obtain a stable orange coloured reaction mixture of FeCla
(0.4 g, 2.5 mmol) with 1,3-diaminopropane (0.42 mL, 5.0 mmol) in 40 mL ethanol. In
the second step an ethanolic solution of pyridine-2,6-dicarboxylic acid (0.9 g, 5.0 mmol)
was reacted with the above reaction mixture, which produced yellow solution. The
yellow coloured precipitate produced after over night stirring, was filtered off, washed
with water followed by ethanol then dried under vacuum. [Yellow colour, m.p. 222°C,
yield 1.672 g],
"Coordination chemistry ofUgamts witH electron rich % 0 orS 6iting centre" OoctoraCthesis suSmittetftoJlligarh MusRm Vniversity, I^fiDIJi
2(8 52
Anal. Calcd for C2oH22N604.FeCl3: C, 41.93; H, 3.84; N, 14.67. Found for (18): C, 41.97;
H, 3.86; N, 14.65. Am (in water): 150.9 ohm"'cm^mole-'.
ESI-Mass spectrum: [FeL^Cb]^ m/z 571 (24%), [FeL^C^]^ m/z = 536 (12%), [FeL^Cl]^
m/z = 501 (15%), [FeL^ + Hf m/z = 467 (30%), [Fe(LV2)Cl2f m/z = 331 (<10%),
[Fe(LV2)Cl]^ m/z = 261 (20%), [Fe(LV2)]^ m/z = 296 (<10%), [Fe(DPA)]^ m/z = 204
(40%), [L+H]^ m/z - 411 (45%), [DAP + H]* m/z = 75 (98%), [C5H3N(COOH)2] m/z =
167(20%).
FT-IR (as KBr discs, cm"'): 305 lbs (NH), 1660s, 1589s, 1250s (Amide I, II, III), 1648s
(C=N), 1460s, 755s, 703s (C=C, ring), 415m (M-N).
Synthesis offCoL^ClJ (19), [NiL^ClJ (20) andfCuL^ChJ (21):-
These complexes have been prepared following the procedure stated above.
[(19), purple colour, m.p. 198°C dec, yield 0.601 g]
Anal. Calcd for C20H22N6O4.C0CI2: C, 44.45; H, 4.07; N, 5.56. Found for (19): C, 44.40;
H, 4.11; N, 5.59. Am (in water): 116.9 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm"'): 3194bm (NH), 1622s, 1577s, 1286s (Amide I, II, III), 1589s
(C=N), 1448s, 762s, 724s (C=C, ring), 432m (M-N).
[(20), Green colour, m.p. > 300°C, yield 1.60 g]
Anal. Calcd for C2oH22N604.NiCl2: C, 44.46; H, 4.07; N, 15.57. Found for (20): C, 44.50;
H, 4.04; N, 15.58. Am (in water): 97.7 ohm"'cm^mole"'.
ESI-Mass spectrum: [NiL^Cl2]'' m/z = 538 (15%), [NiL^Cl]^ m/z = 503 (20%), [ N I L Y
m/z = 468 (25%), [Ni(LV2)Cl2]^ m/z = 333 (<10%), \H\{L^I2)C\t m/z = 298 (20%),
[Ni(LV2)]^ m/z = 263 (<10%), [Ni(DAP)]^ m/z - 207 (40%), [L]^ m/z = 410 (55%).
FT-IR (as KBr discs, cm"'): 3105bw (NH), 1635s, 1572s, 1281s (Amide I, II, III), 1611s
(C=N), 1418s, 772s, 734s (C-C, ring), 433m (M-N).
[(21), Blue colour, m.p. 268''C, yield 1.62 g]
Anal. Calcd for C20H22N6O4.CUCI2: C, 44.07; H, 4.04; N, 15.42. Found for (21): C,
44.11;H, 4.02; N, 15.44. Am (in water): 114.1 ohm"'cmWle"'.
"Coordination cHemistry ofGgands •witR ekctron rich % O orS biting centre" (Doctora[thesis suSmitted to Rgarh MusGm "University, I!M)IA
2<B 53
FT-IR (as KBr discs, cm"'): 3032bw (NH), 1634s, 1576s, 1276s (Amide I, II, III), 16.4s
(C=N), 1423s, 771s, 729s (C-C, ring), 418m (M -N).
Synthesis oflZnL^ClJ (22), [CdL^ClJ (23) and [HgL^ChJ (24):-
The reaction mixture containing equimolar rations of 1,3-diaminopropane (5.0
mmol) and metal chloride (2.5 mmol) in 20 mL ethanol was reacted with pyridine-2,6-
dicarboxylic acid (5.0 mmol) taken in 40 mL ethanol at room temperature with stirring.
The resulting solution was stirred over night which produced solid mass which was
filtered off, washed as above and dried in vacuuo.
[(22), light pink colour, m.p. 250°C dec, yield 1.146 g]
Anal. Calcd for C2oH22N604.ZnCl2: C, 43.93; H, 4.02; N, 15.37. Found for (22): C, 43.96;
H, 4.00; N, 15.42. Am (in water) (22): 140.3 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm"') (22): 3019bm (NH), 1634s, 1584s, 1279s (Amide I, 11, III),
1623s (C=N), 1426s, 769s, 730s (C=C, ring), 438m (M -N) .
[(23), White colour, m.p. 276°C dec, yield 1.03 g]
Anal. Calcd for C2oH22N604.CdCl2: C, 40.44; H, 3.70; N, 14.15. Found for (23): C,
40.41; H, 3.68; N, 14.19. Am (in water) (23): 107.5 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm"') (23): 3096bw (NH, 1651s, 1569s, 1279s (Amide I, II, III),
1609s (C=N), 1427s, 760s, 726s (C=C, ring), 418m (M -N) .
[(24), White colour, m.p. 240°C dec, yield 1.64 g]
Anal. Calcd for C2oH22N604.HgCl2: C, 35.21; H, 3.23; N, 12.32. Found for (24): C,
35.19; H, 3.20; N, 12.35. Am (in water) (24): 162.2 ohm"'cm^mole"'.
FT-IR (as KBr discs, cm"') (24): 3082bw (NH), 1622s, 1564s, 1266s (Amide I, II, III),
1603s (C=N), 1428s, 755s, 723s (C=C, ring), 418m (M-N).
"Coordination chemistry qfUgands witH electron rich % O orS Siting centre' OoctoraCthesis suhmittedto JiRgarh Musfim Vniversity, I!MDI^
2(8 54
RESULTS AND DISCUSSION
The complexes (9 - 24) were prepared employing a two step single pot template
synthetic procedure. First step, involved in-situ formation of a metal ion chelate complex
from reaction of either N'-(2-Amino-ethyl)-ethane-l,2-diamine (i.e. diethylene triamine)
or 1,3-diamino propane with metal chlorides in 2:1 mole ratio in ethanol. In second step
the reaction mixture was reacted with 2 mole equivalent (with respect to metal ion) of
pyridine-2,6-dicarboxylic acid at room temperature, which has afforded stable solid
products, usually after over night stirring of the reaction mixture. The complexes (9 - 24)
were fairly soluble in water only, therefore, their molar conductivity measurement,
electronic, ESI mass and NMR spectral studies etc. have been performed in aqueous
medium.
Analytical data, Conductivity measurements and ESI-mass spectral studies:-
Analytical data (Experimental section) of the complexes (9 - 24) agreed with the
molecular formulae C22H28N8O4.MCI2, [M = Cr ^ (9), Co^^ (11), Ni^^ (12), Cu "" (13),
Zn^'' (14), Cd^^ (15) or Hg^^ (16)] or C22H2gN804.FeCl3 (10) and C20H22N6O4.MCI2, [M =
Cr ^ (17), Co^^ (19), Ni^^ (20), Cu^^ (21), Zn^+ (22), Cd^^ (23) or Hg^^ (24)] or
C2oH22N604.FeCl3 (18), which are consistent with the formulations MLCI2 (L = L ' or L^)
and FeLCb (L = L ' or L^), respectively. The molecular formulae of the complexes have
been further verified from Electron Spray Ionization (ESI) mass spectral studies of the
complexes (10), (12), (18) and (20).
The experimentally measured molar conductivities of the aqueous solutions (10'^
M) of the complexes (9 - 24) (Experimental section) are higher than those normally
observed [26] for the non-electrolytes but are at the border line of the 1:1 electrolytes in
aqueous medium [26]. The observed molar conductance (Am) data therefore indicate that
the complexes behave as pseudo 1:1 electrolytes in water. This is probably due to the
existence of a ligand displacement reaction [27] of the complexes with solvent (water)
molecule releasing the exocyclic ligand (chlorine) form the complex moieties. This
bahaviour of water is not un-usual owing to its polar nature which may act as potent
"Coordination cfiemistry ofRgands with electron ricH %OorS Siting antre' OoctoraftHesis suSmitted toAligirH Muslim Vniversity, HMUJi
^s^v:. donor ligand. This may result in the correspondmg ^ aquated complex cation
[M(L)(H20)C1]C1 (L = L or L ) according to the foilowinggeneraJT equilibrium jedction.
[M(L)Cl2] + H2O • ' • [M(L)(H20)C1]^ + Cf (L = Vlsri^"
(M = Cr^^ Co^\ Ni^^ Cu^^ Zx^\ Cd ^ or Hg^ )
The ESI mass spectra of the complexes FeL^Cls (10), [NiL^Cb] (12), [FeL^Cb]
(18) and [NiL CI2] (20) exhibited peaks from weak (10-20 % abundance) to very strong
(-100% abundance) assignable to the complex molecular ion fragmentation products
formed due to step-wise cleavage of the exocyclic ligands as well as species generated
form the endocyclic fragmentations of the coordinated macrocyclic ligand. The various
peaks due to molecular as well as of its step-wise fragmentations usually contained
equally spaced bunch of peaks of varying intensities (abundance) due to the isotopes of
metal and of chlorine nuclei. The peak (s) of highest intensity amongst the individual
bunches have been taken as the most probable m/z values of the fragments containing
these nuclei. For the complex (12) a weak intensity peak assignable to the molecular ion
[FeL^Cla]^ appears at m/z 629 (<10% abundance) and that of the corresponding
fragments due to step wise cleavage of exocyclic ligands [FeL^Ch]^ and [FeL^Cl]^
appear at m/z = 594 (<10% abundance) and m/z - 559 (<10% abundance), respectively,
(Experimental section). The peak m/z = 468 is due to the metal ion free macrocyclic
ligand [L ]" . The various fragmentations of the free macrocyclic ligands were also
formed. The mechanism of the fragmentations of the complex molecular ion, the metal
ion free macrocyclic ligand and its possible fragmentations have been shown in Figure
2B.1. The low volatility of the various species formed in the present Electron Spray
Ionization (ESI) mass spectral conditions is responsible for their weak intensities (low
abundance).
The behaviour of the complex FeL CI3 (18) under ESI conations have been found
nearly identical to that for the complex (10). The spectrum contained weak intensity
peaks due to the molecular ion [FeL^Clj]^ at m/z = 571 and its various fragments viz.
[FeL Cb]"* , [FeL^Cl]" and [FeL^]^ (Experimental section). Peaks assignable to the metal
"Coordination cHemistry ofBgands xmtii electron rich % 0 orS Biting centre' (Doctordthesis suSmittedtoMigarh OdusCvm "University, IJ{<DIA
2(8 56
ion free macrocyclic ligand [L ]" and its various fragments were also indicated. The
meciianism of the fragmentation has been shown in Figure 2B.2.
The ESI-mass spectra of NiL^Cb (12) and NiL^C^ (20) are comparable to each
other exhibiting the molecular ions [NiL^Cb]^ at m/z - 596 and [NiL^Cb]^ at m/z = 538
as well as their corresponding fragments due to the step wise cleavage of the exocyclic
lignads etc. (Experimental section).
The present ESI-mass data indicate the formation of molecular ions [FeLCh]"^ (L
= \} or L^), [NiLCb]^ (L = \} or L^) as well as the corresponding species [FeL]"^ (L = L^
or L^) and [NiL]^ (L = L^ or L^) as stable entities due to complete cleavage of the
exocyclic chloride ligand from the molecular moieties. These observations strongly
support that the ligands are quite strong to bind the metal ion under normal condition,
which however, under harsh ESI condition can produce a low amount of the metal ion
free macrocyclic ligand (L or L ).
FT-IR spectral studies:-
FT-IR studies of the mono-nuclear complexes (9 - 24) provide valuable
information regarding the nature of fundamental group attached to the metal ions. The
appearance of new bands in the region 1619 - 1651 cm"' (amide I), 1539 - 1589 cm''
(amide II) and 1250 - 1291 cm"' (amide III) with the concomitant disappearance of
stretching frequencies characteristic of COOH group [24] support the metal ion template
cyclo-condensation process according to the mechanism shown in Figures 2B.3 - 2B.6
contained the additional band due to characteristic of pyridine ring i.e. v(C=N), v(C=C)
stretching vibrations (Experimental section). The appearance of a medium intensity band
in the region 418 - 438 cm"' is due to M - N bond stretching vibration indicating the
chelation / encapsulation of the metal ion through the aza biting sites.
'H- and '^C- NMR spectral studies:-
' H - NMR spectra of the complexes (9-12) recorded in D2O did not show the
resonance signal of amide (CO - NH) protons. The presence of the strong hydrogen
bonding (intra-molecular and inter-molecular way) as well as the fast exchange with the
"Coordination cHemistty ofRgands with electron rich % O orS Siting centre " (DoctoraCthesis suBmittedto^Rgarh CHusRm Vniversity, I!N<DIA
2(B 57
solvent molecules are often responsible for [25] the disappearance of a signal due to
amide functions. However, the spectra contained three sets of well defined resonance
signals {Table 2A.1) in 8.52 - 8.95 8 (m, 6h), 3.14 - 3.22 5 (t, 8H) and 2.85 - 2.92 5 (t,
8H) regions {Table 2B.1). The first set is a group of multiplet assignable to the aromatic
protons resonances while the remaining two sets of triplets are due to - C H 2 - protons
resonance of (-CH2 - N H C O - ) and (-CH2 - N H - ) funcfions, respectively.
The nature of ' H - N M R spectra of the complexes (17 - 20) were identical to that
above described complexes (9 - 12). The positions with assignments of the observed
resonance signals have been summarized in Table 2B.1.
'^C - NMR spectra of the complexes (9 - 12, 17 - 20) exhibited four sets of '^C
resonance signals whose positions and assignments are summarized in Table 2B.2.
Magnetic moment, EPR and electronic (ligand field) spectral studies:-
The approximate geometry around the metal ions in the metal encapsulated
complexes of the macrocyclic ligands \} and L^ have been ascertained from the magnetic
susceptibility, Ligand field and EPR spectral studies, as following:-
The observed magnetic moment, for the complexes [CrL CI2] (9) and [CrL CI2]
(17) are |aeff. = 5.0 BM and ^eff. = 5.2 BM, respecfively. The values are in the range
reported [33] for a mono-nuclear Cr " complexes with four unpaired electrons (d'' system)
in an octahedral environment of weak ligand field having t2g^,eg' configuration. The
presence of spin-orbit (S - L) coupling interaction may also contribute to the observed
magnetic moment of the molecule. The electronic spectra of (9) and (17) exhibited two
well resolved low intensity band {Table 2B.3) due to ligand field (d - d) transitions in
addition to strong ligand n - * TT* and M<—L charge transfer (CT) transifions in (UV)
region of the spectra. The ^D ground electronic state of the free Cr " usually splits in a
weak octahedral environment to ^Eg as ground state and ^T2g as the excited state. This
should normally show a broad absorption band [31] characteristic of T2g-«— Eg transition.
However, the ligand asymmetry affects the octahedral ligand field around the metal ions
offsetting the orbital degeneracy. This may cause splitting [31] of the (d - d) transitions
into two weak intensity bands characterized as ^T2g <— ^Big and ^Ajg <— ^Big. The
"Coordination chemistry ofdgaiuis with electron rich % 0 orS Biting antre" ^DoctoraCthesis su6mitted'toJlSgarh Muslim University, HN^IA
2(8 58
complex moieties have four basal nitrogens of the amide (or peptide moiety) coordinated
with the Cr ^ ions and the exocyclic ligand CI" ion occupies at the axial positions of the
octahedral geometry {Figure 2B.3) resulting in an asymmetry. This split component does
not arise from spin - orbit coupling which often splits the ligand field band, but its
magnitude is very small (A~ 1,000-2,000 cm'') [31,41].
The X - band EPR spectra of the micro - crystalline samples of the complex (9)
and (17) exhibited a broad isotopic signals at giso = 1.96 and L95, respectively. The broad
nature of the signals observed for d"* systems are in general due to the dipolar interaction
of the poly-electronic un-paired spins [42].
The magnetic moments of [FeL^Cl2]Cl (10) and [FeL^Cl2]Cl (18) are ^eff. s 1.50
BM {Table 2B.3) indicating a low-spin state of Fe " ion. The magnitude is slightly lower
than the theoretical spin - only moment (|xs.o. = 1.73 BM) for one unpaired spin (S - VT)
on the metal ion. However, the magnitude is comparable [31] with complexes having
incompletely quenched orbital moment in t2g ground state (i.e. ' Tig term) of Fe " (d
system). Low-spin Fe ^ complexes are known to exist [41] but are not very common as
such complexes are produced from ligands with interaction ligand field strengths. The
presence of strong antiferromagnetic interactions (intermolecular or intramolecular) is
known to lower the magnetic susceptibilities of the various Fe " complexes [41]
therefore, it presence may not be ruled out. The electronic spectra of these complexes
exhibited weak intensity broad absorption bands around 475 nm and 650 nm for (10) and
480 nm and 620 nm for (18) arise due to the spin-allowed transition [31] from the doublet
Eg ground electronic state {Table 2B.3). The observed two bands are the split component
of the allowed band due to the presence of ligand asymmetry in the molecule.
The observed magnetic moments for the complexes [CoL CI2] (11) and [CoL CI2]
(19) (laeff -3 .0 BM) are smaller than that known for high-spin Co " complexes having
three unpaired spins on Co " ion in an octahedral weak ligand field (high-spin state). The
ground state term for Co^^ ion in a weak octahedral crystal field is ''Tig (F). The magnetic
properties of d system with 'T' ground electronic terms are critically affected by two
factors. First is the presence of low symmetry ligand field component and the second is
the t2g electron delocalization (i.e. orbital moment contribution). In general, the orbital
"Coordination cRemistry of ligands witH electron rich % O orS Biting centre ° (Doctoralthesis suSmittedtoJiligaTh Muslim "University, I^WIJi
2<B 59
moment contribution to the magnetic moments enhances the magnitude of the observed
moment compared to the calculated spin-only value {\iso = 3.9 BM). However, presence
of low symmetry ligand field created by a departure for the perfect octahedral geometry
(i.e. Oh symmetry) of the complexes has a pronounced effect. The observed ^eff. value
approaches near the spin-only ( iso) value when contribution form ligand asymmetry
becomes larger compared to that of the spin-orbit coupling constant. This effect is a
consequence of further quenching of orbital angular momentum associated with the
ground state (tjg) levels. The magnitude of magnetic moment for spin-paired d system
with one unpaired electron (S = Vi) is expected to be -1.8 BM such systems have Eg
ground state term [42] but with a considerable orbital contribution to the magnetic
moment. The magnitude (jxeff = 1.8 BM) for Co " in low - spin state ( Eg ground state)
in much lower than that observed for the complexes (11) and (19) henceforth, ruled out
such a possibility. However, under such circumstances the possibility that the complex is
some where in between a high - spin and a low - spin state i.e. exhibiting an equilibrium
state, high-spin-" ^ low-spin (spin-crossover point) may not be rule out [33]. The
electronic spectra of (11) and (19) are identical exhibiting two well resolved strong
absorption band in the UV region and three relatively weak absorption band in two
visible region of the spectrum (Table 2B.3). The low energy weak intensity bands are
characteristic of the ligand field (d - d) transitions for Co^^ in an octahedral environment.
The ground state free ion "F term of Co " ion (d or t2g ,eg ) splits in octahedral ligand
field giving ''Tig (F) as the ground electronic state. The observed d - d bands are assigned
to spin - allowed transitions %g (F) ^ ^T,g (F) and ^T,g (P) ^T,g (F) {Table 2B.3).
The paramagnetic nature of the complexes [NiL^Cl2] (12) and [NiL^Cl2] (20)
ruled out the square - planar geometry. However, the observed |a,eff value {Table 2B.3)
is comparable to that reported [33] for Ni * complexes with d configuration in an
octahedral geometry. The electronic spectra for these complexes exhibited three well
resolved absorpfion maxima in the low energy region assignable to ligand field (d - d)
transition for the complexes having octahedral geometry. The low energy ligand field
band in these complexes arise due to the spin allowed excitations form A2g (F) ground
state to T2g (F) and ^Tig (F) and ^Tig (P) excited states {Table 2B.3). The spectra also
"Coordination chemistry ofRgands with electron rich % O orS Siting centn" Ooctoraf thesis suSmitteiftoAdgarh MusRm Vniversity, HN'OIJi
2(B 60
contained bands in the near UV region (iiigh energy region) characteristic of the n —»• TC*
transition of the ligand as well as M •<— L charge transfer excitation.
The electronic spectra of the complexes (13) and (21) exhibit a weak broad band
spanning 500 - 800 nm (20,000 - 12,500 cm"') due to d - d excitation, ^T2g • - \ of
Cu " ion (d^ configuration). However, the magnetic susceptibility of the complexes (^leff.
~ 0.9 BM) is very small compared to the calculated spin - only value for one unpaired
spin (fj-so. = 1.83 BM) of the d configuration. This lowering of magnetic moment could
be due to the presence of a strong antiferromagnetic interaction [43] similar to that
observed for Cu(CH3COO)2 [32]. The X - band EPR spectra of the micro - crystalline
samples of the complex (13) and (21) exhibited a broad isotopic signals at giso = 2.09 and
2.13, respectively.
Molecular modeling:-
The molecular model calculations [39,40] based on the CSChem-3D MOPAC
have been employed to solve the minimum energy plots for the molecular structures of
the complexes and to get the various bond lengths and bond angles in the molecules. The
plot shows that ligands (L ) and (L ) provide four potential aza sites to bind metal ions in
the complexes moieties. The computed bond lengths and bond angles have been
displayed in Tables 2B.4 and 2B.5. The mechanical adjustments via augmented
mechanical filled were used to draw the optimum minimum energy plot for complexes
[MLCI2] (M - Cr^^ Co^^ Ni^^ Cu^^ Zn^^ Cd^^ or Hg^*) as well as [FeLCijJCI (L = L '
and L^) , which indicate that the metal ions acquire a hexa - coordinate geometry as
shown in Figures2B.7-2B.10.
'Coordination cdemistry offigam^ with ekctron ricH% O orS Siting centre" <DoctoTa[thesis suSmittedto^Rgarh 9iusRm "University, I!N<DIJi
2<B 61
m/z = 468(<10%)
' r OH OH
m/z=167(25%)
V - N H O
H -N-
a I ,0 NH—«;
Fe.
CI
-N-
NH-< O
CI
m/z = 629(<10%)
-CI
[FeL^Clz]"" m/z = 594(<10%)
•CI
[FeL^CIl m/z = 559{<10%)
m/z = 324 (24%)
-CI
FeL2/2+4H]+ m/z = 294 (18%)
-CI
m/z = 524(<10%)
H -N-
NH, NH,
m/z =104 (100%)
[NHCHjCHzNH]" m/z58(10%)
-1 +
NH. ,NH
HN' ~NH
H -N-
m/z = 262(<10%)
_ +
+ 2H
^—NH HN—'o
m/z = 388 (24%)
Figure 2B.1. Mechanism for fragmentations oflFeL^ChJCl under ESI-mass spectral condition
"Coordination cHemistty ofRgands witH electron rich % O orS Siting centre" (DoctoraCthesis su6mittedtoJiBgaTii MusRm University, I^<DIJi
2® 62
[L'+H]+ m/z = 411(45%)
~1° OH
+
-,
+H
NH2 NH2
_
m/z=167(20%) m/z = 75 (98%)
-NH NH-
f N Fe N. />
W / \ W / -NH NH-
0' I CI I O
CI
m/z = 571(<10%)
•CI
[FeLXl2]'' m/z = 536 (12%)
[FeL^ m/z = 5
1
FeL'/2H m/z = 331
-CI
CI]'' 01(15%)
(<10%)
-CI rppf 3, 111+
ra/z = 467 (30%)
r r^ 1 1
H2N NH2 \ /
Fe / \
H2N NH2
m/z = 204 (40%)
-CI
FeLV2+Cl]'' m/z = 296 (20%)
•CI
[FiLVl]"-
9—NH HN
O^—NH HN—^
m/z = 387 (28%)
m/z=261(<10%)
Figure2B.2. Mechanism for the fragmentations of[FeL^Cl2jCl under ESI-mass spectral condition.
"Coordination cHemtstry ofRgands with electron rich % O orS Biting centre" Ooctoraf thesis suBmittetftoJiligarh MusRm University, IJ<1)IA
2® 63
H -N-
-NH-Ethanol NH.
MCI, NH, NH, Stirring for 30 min.
A''-(2-Amino-ethyl)-ethane-l,2-diamine
2 mole of pyridyne-2,6-dicarboxylic acid 1:2 mole ratio
-4H2O
CI
- M ,
,NH
/ W NH
N
NH
o
H -N-
Cl
.M^
CI
H - N -
NH
N
NH
W /
o
Figure 2B.3: Proposed mechanism for formation of the complexes [ML^Ch] [M = C/^ (9), Co^^ (11), Ni^^ (12), Cit^ (13), Zn^^ (14), Ccf^ (IS) or Hg^^ (16)]
"Coordination chemistry ofGgands witk ekctron rich % 0 orS Siting antn" (Doctoralthesis suBmittedto Rgarh MusRm "University, HN^IJi
2(B 64
H -N-
+ FeClj
NH, NH,
A' -(2-Amino-ethyl)-ethane-1,2-diamine
^
OH H—NH
OH H--NH
H -N-
Cl
Fe.
CI
pyridyne-2,6-dicarboxylic acid
1:2 mole ratio
NH- -H HO-
NH- H HO-
\ /
H -vl-
-4H2O
\
-NH
NH O
H -N-
Cl
Fe.
CI
H - N -
NH-.0
N
NH O
W /
Figure 2B.4: Proposed mechanism for formation of the complex [FeL^Chf (10).
"Coorcfination cRemistiy ofRgands witk electron ricH % 0 orS Biting centre" (Doctoralthesis suSmittedto JiGgarh Huston "University, /5V®/JI
2® 65
CI
HN NH
+ MCI, NH, NH,
HN
M
CI
NH
pyridyne-2,6-dicarboxylic acid
1:2 mole ratio
Q V
\
•OH H-CI
-NH NH- H HO
•OH H--NH
M
CI
NH- HO-0
-4H2O
0 V
0
-NH CI
M
NH-
-NH NH-
Cl
,0
Figure 2B.5: Proposed mechanism for formation of the complexes [ML^Ch] [M = Cr^^ (17), Co^^ (19), Nf^ (20), Cu^^ (21), Zn^^ (22), C / ^ (23) or Hg^^ (24)]
"Coordination cHemistty qffigands witH electron rich % O orS Siting centre' (DoctoraftHesis suSmittedtoMw^ MusCvm University, I!M)IJi
2® 66
NHo NH
pyridyne-2,6-dicarboxylic acid
1:2 mole ratio
-4H2O
O, V Cl
-NH NH
Fe
-NH NH
N>
O CI A O
Figure 2B.6: Proposed mechanism for formation of the complex [FeL^Ch/^ (18).
"Coordination cHemistry qfdgands with electron ricfi% O orS Biting centre' (Doctoralthesis suhmittedtoJiSgarh MusRm Vniversity, I9fiDIJl
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Figure 2B.7; Perspective coordination geometry around M^^ ion in Cf^\ Co^\ Nf\ Cu^*, Zn^\ C(f\ Hg^^) # C; • H; • N • M'^; t CI or • 0
Figure2B.8: Perspective coordination geometry around Fe^^ ion in #C; •H;#N#M2^;tClor»0
"Coordiiiation chemistry ofEgands with ekctron rich % O orS Siting centre" <Doctoraf thesis suSmttetCto^Ggarh Muslim "Oniversity, I!NU)IJi
2(8 68
Figure 2B.9: Perspective coordination geometry aroundM ion in [M2(V)Cl4](M Ci^\ Co^\ Ni^\ Cii\ Zn, Cd^\ Hg^*) 9 C; • H; • N • M^ ; €* CI or • 0
j+ Figure2B.10: Perspective coordination geometry around Fe ion in IFe(L')a2t 9 C; • H; • N • M^ ; % CI or • 0
'Coordination chemistry ofGgands witfi eCectron Tick % O orS Siting centre" (DoctoraC thesis submitted to Sqarh MusRm "University, IW)I^
2(B 69
Table 2B.1: H-NMR spectral data of the monometallic complexes.
Complexes Pyridine ring
9
CONH-C^ - N H - C ^
10
11
12
17
18
19
20
8.52-8.76 8 3.18 5(t, 8H, 2.89 5 (t, 8H, JH-H (m, 6H) JH-H = 6.9 cps) = 7.3 cps)
8.62-8.89 5 3.22 5(t,8H, 2.86 5 (t, 8H, JH-H (m, 6H) JH-H = 7.2 cps) = 7.1 cps)
8.59-8.72 5 3.14 5(t, 8H, 2.91 5 (t, 8H, JH-H (m, 6H) JH- H = 6.7 cps) =6.9 cps)
8.86-8.95 5 3.17 8(t, 8H, 2.92 5 (t, 8H, JH-H (m, 6H) JH-H ==7.0 cps) =7.1 cps)
8.75 - 8.89 5 2.98 5 (t, 8H, (m, 6H) JH-H = 6.8 cps)
8.70-8.86 8 3.12 5(t, 8H, (m, 6H) JH-H = 7.1 cps)
8.78 - 8.92 8 2.92 5 (t, 8H, (m, 6H) JH-H = 6.8 cps)
8.73 - 8.89 8 3.07 5 (t, 8H, (m,6H) JH-H = 7.0 cps)
C - C ^ - C
1.87 8 (m,4H)
1.80 8 (m,4H)
1.89 8 (m,4H)
1.92 8 (m,4H)
13 Table 2B.2: C- NMR spectra! data of the monometallic complexes
Complexes
9
10
11
12
17
18
19
20
Pyridine ring
138.2-142.5
140.6-152.3
137.2-144.3
141.8-151.8
120.8-140.3
124.2-146.8
129.6-150.2
122.4-143.7
CO-NH
168.2
169.8
172.0
169.3
168.7
171.4
168.0
169.6
CONH-
CH2
45.4
45.7
45.3
45.6
40.3
40.7
40.2
40.4
CONH-CH2-
CH: !
50.6
51.2
50.8
50.5
-
-
-
-
NH-CH2-CH2-
NH
-
-
-
-
32.8
33.1
33.4
32.9
"Coordination chemistry ofRgands with electron rich % 0 orS Siting antre' (Doctoral thesis suhmittetftoJiGgaTh iMusRm University, HN^IA
2(3 70
Table 2B.3. eff and ligand field spectral data of the complexes.
Complexes 9
10
11
12
13 17
18
19
20
21
V-ttt. (MB) 5.0
1.54
3.0
3.25
0.8 5.2
1.50
3.1
3.30
0.84
Bands, nm 320 350 432 572 328 372 475 650 325 358 470 620 324 360 428 498 630 780 325 365 428 574 330 368 480 620 324 353 470 632 322 365 430 490 640 780
cm'' 31,250 28,570 23,150 17,480 30,485 26,880 21,050 15,380 30,770 27,930 21,270 16,130 30,860 27,770 23,360 20,080 15,870 12,820 30,770 28,090 23,360 17,420 30,300 27,170 20,800 16,130 30,860 28,330 21,270 15,820 31,050 27,390 23,250 20,400 15,625 12,820
Transitions «—>;r* CT B2g *- Big Aig<— Big
n—*n* CT Eg ^ T2g
2 77 ^ 2rp
tg^ l2g n—>7t*
CT
'Tig(P) ^ 'Tig (F) %g(F) ^ %g(F) n—^n* CT yig(P) - %g 'Tig(F) ^ %g %g(F) Y %g Tig-^ Eg
n-^n* CT B2g Big Aig-^ Big
n—^K*
CT Eg ^ T2g
2r^ , 2rp tg<r- l2g
n—*n* CT %g(P) ^ 'Tig (F) %(F) ^ %g(F) n-^K* CT 'Tig(P) ^ %g %(F) ^ %g %g(F) ^ %g ^T2g ^ ^Eg
EPR(gi,o.) 1.96
2.09 1.95
2.13
"Coordination chemistry ofRgands with electron rich%0 orS Siting antre" (DottoraCthesis suSmitteiftoASgaHi MusRm Vniversity, HN^IA
-o c IS 00
=+1 o C/5 (U X
s o o
"o 3 C o c o £
-o
C 03
-o c o X)
c C3
<
c
•a c o x> T3
3 Q.
O O
C
t; o D.
u
V
I
K ^
u I
33 U
u I
O
u
o u
O)
X
"E o U
o o o 00
ON
ON 00 0 \
ON
0 \ 0\
o\
ON 00 ON
ON
ON
o o o 00
ON
ON 00
O
o d 00
OS
ON oo
o
ON
00 ON
ON 00
ON 00
ON
ON 00 ON 00
O NO
ON
ON 00
NO
ON
ON
ON ON
ON 00
ON 00
O o d ON
O O
d 00
ON ON 00
00 ON
ON
ON 00
00 0^ O N
o o d ON
o in
ON 00 ON 00
O
00 ON
ON 00
O
ON 00 ON 00
ON
ON 00
CNI CN (N (N CN
NO NO
00 00
NO •r-i 00
NO 'rr 00
NO m 00
o o
NO NO (N in ON —
o i~( r^ (
ON O N
ON 00
O O d 00
ON
ON OO
o 00
NO NO
(N CN
m '^ r--
m vn T^
NO 00 r~-
ro i n "*
00
o 00
m ^n ^
en (N 00
? l o
'*
r-r--i >
(T) m '^
o >n r--
ro i n "
(N irt 00
ro i r i
'^
t~-r< r-~
(^ <j~i
-*
N O N O N O N O N O N O N O N O
IT) ^
I
-a c
X
_u
S o o
"o 3 C O c o
0) (U
BO (U TD V — ^
c C3 -o c o -a
00
+-» W) c —
C O x> -o a; •»-»
3 D,
S o u •4-»
c
o
E
V
K Z
Z
u I
M ^
u I K u
O u E/1
"a S o U
(N ^ o\ 00
o r o\ 00
m o\ ON 00
" 00 0^ 00
m o\ 0^ 00
t~ 00 ON 00
Tt
r-ON 00
c ON Os 00
in
ON
ON
ON 00
ON 00 0\
ON oo ON 00
ON >n
00 ON
OO NO ON
ON
ON
ON 00 lO ON
ON ON
in ON ON 00
ON
ON 00
00 ON OO
00 ON 00
ON
ON 00
o o o o o o o r-- NO in -"d- ^ -* r--—; '-; —; r-; -; CN rr CN (N (N rj CN r-i tN
NO in 00
NO ^ 00
NO
m 00 NO CN 00
CN '-< CN '- NO NO
00 OO in
ON NO NO
« ON O (N r< (N
o o o 00 o
o o ON
00 ON
ON 00
ON 00 ON
00 ON 00
NO 00 00 C~~
ON ON 00
ON 00 ON
>n OO O N 00
ON ON
00 ON 00
00 ON
O
o o ON
en
ON
CO ON ON OO
NO 00 ON
00 ON OO
O 00 -^ CN
NO NO >0 NO rf (N in NO 00 ON -H —
ON —' 00 O in NO
ON
r-cn
NO ON ro
O • *
TT
00 00 cn
r-r--cn
00 00 r<-)
00 NO
m
OO C3N cn
N O N O N O N O N O N O ^ N O
CN
I
2(B 73
References
1. R.B. Lauffer, Chem. Rev., 87, 701 (1987); D. Parker, Chem. Rev., 19, 271 (1990).
2. H. Sigei, R.B. Martin, Chem. Rev., 82, 385 (1982).
3. R.L. Dutta, A.K. Sarkar, J. Inorg. Nucl. Chem., 43, 57 91981).
4. I.S. Ahuja, R. SriRamula, R. Singh, Indian J. Chem., 19 A, 909 (1980).
5. R.C. Aggrawal, N.K. Singh, R.P. Sing, Inorg. Chem., 20, 2794 (1981).
6. E. Kimura, J. Coord. Chem., 15, 1 (1986).
7. C.A. David, P.A. Duckworth, L.F. Lindoy, W.E. Moody, Aust. J. Chem., 48, 1819
(1995) and ref. cited therein.
8. M.B. Inoue, R.E. Navarro, M. Inoue, Q. Fernando, Inorg. Chem., 34 ,6074 (1995).
9. X.H. Bu, X.C. Cao, D.L. Au, R.H. Zhang, C. Thomas, E. Kimura, J. Chem. Soc.,
Dahon Trans, A33(\99S).
10. V. Alexander, Chem. Rev., 95 ,276 (1995) and ref. cited therein.
11. J. Costa, R. Delgado, Melocarmo, Figueira, R.T. Henriques, M. Teixeira, J. Chem.
Soc., Dalton Tram, 65 (1997).
12. M. Kodama, E. Kimura, J. Chem. Soc., Dalton Trans, 325 (1979).
13. M.K. Kim, A.E. Martell, J. Am. Chem. Soc, 88, 714 (1966).
14. A.P. Brunetti, M.C. Lim, G.H. Nancollas, J. Am. Chem. Soc, 90, 5120 (1968).
15. F.P. Hinz, D.W. Margerum, J. Am. Chem. Soc, 96, 4993 (1974).
16. M. Kodama, E. Kimura, J. Chem. Soc, Dalton Trans, 694 (1981) and ref. cited
therein.
17. Z.A. Siddiqi, V.J. Mathew, Polyhedron, 13, 799 (1994).
18. Z.A. Siddiqi, V. J. Mathew, M.M. Khan, Main Group Metal Chem., 24, 857 (2002).
19. M. Kodama, E. Kimura, J. Chem. S o c , Dalton Trans, 327 (1980).
20. M. Kodama, T. Yatsunami, E. Kimura, J. Chem. Soc, Dalton Trans, 1783 (1979).
21 . Z.A. Siddiqi, S.M. Shadab, Indian J. Chem., 43 A, 2274 (2004).
22. Z.A. Siddiqi, M.M. Khan, M. Khalid, S. Kumar, Trans. Metal Chem., 32, 927 (2007).
23. Z.A. Siddiqi, Mohd. Khalid, S. Kumar, Trans. Metal Chem., 32, 913 (2007).
Coordination chemistry ofGgands with eteUnm rich !K,OOTS biting centre ' OoctoraCthesis suBmittedto^Rgarh MusRm "University, IWiIJi
2<B 74
24. K. Nakamoto, In Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Wiley-Interscience, New York Publication, (1886), pp.
191.
25. R.S. Drago,"Physical Methods in Inorganic Chemistry", Reinhold Publishing
Corporation, New York Ltd. London Champman & Hall, affiliated East-West Press
Pvt. New Delhi (1965), pp. 279.
26. J. Liu, Y. Masuda, E. Sekido, Bull. Chem. Soc. Jpn., 63, 2516 (1990).
27. S. Flangan, J. Dong, K. Haller, S. Wang, W.R. Scheidt, R.A. Scott, T.R. Wed, D.M.
Stanbury, L.J. Wilson, J. Am. Chem. Soc, 119, 8857 (1997).
28. C.W. Yan, T.L. Yan, Y.Z. Chum, S.G. Hua, Synth. React. Met.-Org. Chem., 34, 929
(2004).
29. Y.T. Li, C.W. Yan, S.H. Maio, D.Z. Lio, Polyhedron, 15, 2491 (1998).
30. S.L. Lambert, C.L. Sapiro, R.R. Gange, D.N. Hendrickson, Inorg. Chem., 21, 68
(1982).
31.A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd Edn. Elsevier, Amsterdam
(1984).
32. T. Kurisaki, T. Yamaguchi, M. Fujiwara, R. Kiraly, H.J. Wakita, J. Chem. Soc,
Dalton Trans., 3121 (1996) and references cited therein.
33. J. Lewis, R.G. Wilkins, Modern Coordination Chemistry, Interscience Publicsher,
New York, (1960).
34. W.H. Armstrong, A. Spool, G.C. Papaefthymion, R.B. Frankel, S.J. Lippard, J. Am.
Chem. Soc, 106,3653(1984).
35. K. Wieghardt, K. Pohl, W. Gebert: Angew, Chem. Int. Ed Engl., 22, 727 (1983).
36. F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Wiley Eastern Ltd., N.
Delhi, (1976), pp. 652.
37. L. Dubicki, R. L. Martin, Inorg Chem., 5, 2203 (1966).
38. CD. Samara, P.D. Jannakoudakis, D.P. Kessissoglou, G.E. Manoussakis, D.
Mentzafos, A. Terzis, J. Chem. Sac, Dalton Trans., 3259 (1992).
39. J.J.P. Steward, J. Comp Chem., 10 (2), 221 (1989).
Coordination cHemistry ofRgamCs mtti electron rich % O orS Siting centre' (Doctoratthesis suSmittedto Adgarfi 9dusRm University, HN'DI^
2(8 75
40. D. Hinks, J. Lye, H.S. Freeman, "-Computer AidedDyestuffDesign", pp. 74, Book of
Paper- AATCC, International Conference and Exhibition, Atlanta, Georgia (1995).
41. B.N. Figgis, J. Lewis, F.E. Mabb, J. Chem. Soc, (A), 422 (1966).
42. Z.A. Siddiqi, M. Khalid, M.M. Khan, Polish J. Chem., 80, 377 (2006).
43. B.N. Figgis, ''Introduction to LigandFields" Wiley Eastern, New Delhi (1976).
Coordination chemistry ofRgands with electron rich %OorS Siting centre" (Doctoraf thesis suBmittecftoJlBgarh MusGm University, HN^IJi
CUcipter 3
Synthesis and Spectral Studies of A Novel Imidazoline
derivative, Bis(4,5-Dihydro-lH-imidazol-2-ylmethyl)amine (Im)
and Benzimidazole derivative, Bis(lH-benzimidazol-2-
ylmethyl)amine (BIm) and Their
Transition Metal Complexes
CHapterS 76
INTRODUCTION
Imidazole also termed as 1,3-diazole is a planer, five membered ring system,
contains three carbon atoms with two hetero nitrogen atoms at 1- and 3-position of the
ring. It is well known [1] that imidazole ring is a part of the of the histidine moiety, which
functions as a potential chelating agents towards transition metals ions in an array of
biologically important molecules including iron-porphyrin system, vitamin-Bn and its
derivatives as well as several metallo-protein. Benzimidazole which is a substituted
analogue of the imidazole ring also behaves as a good chelating agent towards transition
as well as rare earth metal ions [2].
The histidine unit in the globin molecule is believed to form a bond with the hemin
molecule, which is between the imidazole nitrogen atom and iron atom of hemin. One of
the most important characteristics of the hemoglobin (ferrohemoglobin) is its ability to
react reversibly with oxygen as in lungs where the partial pressure of O2 is high without
changing the valency or oxidation state of iron. The ionic linkage between the imidazole
nitrogen and the hemin iron which exists itself in hemoglobin changes to a covalent bond
during oxygenation [1] process.
It is necessary to discuss briefly, the structural features associated with the
imidazole ring. The parent imidazole falls in the class of the aromatic heterocycles. Its
unique structural features are conveniently discussed with reference to pyridine and
pyrrole ring, imidazole is structurally related to both. It is well established that aromaticity
in completely conjugated monocyclic system requires a planar array of atoms with
(4n+2)7t electrons [3]. The possibility for aromaticity in imidazole can then be recognized
if imidazole is considered to be constructed from a trigonal nitrogen with two electrons in
the unhybridized p-orbital (N-1, pyrrole nitrogen), a trigonal nitrogen with a lone pair in a
hybrid orbital and a single electron in the p-orbital (N-3, pyridine nitrogen) and three
trigonal carbons each with one electron in a p-orbital {Figure 3.1).
"Coorifination cHemistty ofGgands •witH ekaron Ticfi%0 orS Biting autre" (Doctoratthesis suSmittedto^SgarH MusCim University, HN^DIJl
chapters 77
The pyridine nitrogen (N-3) display functional negative a and n electronic charges
indicating that this nitrogen donate substantial fractional electronic charge to the 71-system
but withdraw an even grater amount of charge from the a orbitals so that the net result is a
gain of the electronic charge at the pyrrole N-1 nitrogen of the imidazole. The carbon
atoms at position 4 and 5 of imidazole are o acceptors. All the carbon atoms are weak %
acceptors, but polarization of the o electrons dominates the overall charge distribution.
A crucial structural features with respect to the coordination site of imidazole is
classified when the aromatic nature of the molecule is recognized. There is only one lone
pair of electrons at N-3. The 7i-electron of the N-1 are part of the aromatic sextet. Bonding
of proton or metal ion at N-1 is expected to be very unfavorable since the aromaticity of
the ring is thereby compromised. The lower basicity of the N-1 towards metal ions is
probably, because of the additional electron withdrawing effect of the pyridine nitrogen. It
is concluded that the neutral imidazole molecule presents only a single energetically
favourable coordinating site for Lewis Acids or metal ions through the unshared pair on
N-3. Structures {Figures 3.2 and 3.3) must be of very high energy.
Figure 3.2 Figure 3.3
'Coordination ckmistTy qfSgamCs witH electron ricA^OorS Siting centre" <Doctora[thesis suSmitud to ABgarH MiuRm University, /WD/J?
Ctiapter 3 78
The transfer of the N-1 proton to N-3 would be very favourable energetically, and species
such as 3.2 and 3.3 would not be expected to be observable. The structures
of the protonated and metal ion complexes of imidazole are the aromatic cations as
represented by Figures 3.4 and3.5. The distinction between N-1 and N-3 nitrogens is lost
in the protonated imidazolium cation. This cation becomes symmetrical with equivalent
C-4 and C-5 protons, which give a single resonance peak [4] in ' H - N M R spectrum.
Anionic imidazole generated, in alkaline solution is also aromatic, possesses two
equivalent sites for coordination and acts as a potential bridging ligands as shown in
Figure 3.6.
r\ H N
\ ^
N — M n+
Figure 3.4 Figure 3.5
2M"+ + N ^ ^ N
r\ I — M — N ^ N - M —
(2n-ir
Figure 3.6
There are reports [5,6], which describe relative electrophilic reactivities of various
ring sites in heterocyclic compounds possessing aromatic character. The reactivities
usually depend upon both the nature of substrate and the nature of electrophiles. For
imidazole ring 4(5) positions are set to be more reactive than 2-position [7] e.g. in
iodination reaction, while the reverse is true for deuteration [8]. It is true, imidazole exists
'Coordination cHemistry ofRgands -with ekuron ricfi%0 orS Siting centre' OoaoraCtResissuBmittedtoJiSgaHi MiuRm Vniversity, I!K1>IA
CHapterS 79
in conjugated acid (Figure 3.8) or conjugated base (Figure 3.9) form in addition to that of
the neutral forms (Figure 3.7).
H ,H K
0\ (0 IQ N—H
H H H
Figuere 3.7 Figure 3.8 Figure 3.9
Imidazole and also benzimidazole are amphoteric in nature, behave as moderately strong
organic bases capable of accepting protons at N-3, as well as very weak acids capable to
lose a proton from N-1. (Figures 3.10 and 3.11).
-H H-N. <!J M - N
H
ImH^
N • H—N
H
Im
Figure 3.10
Im-
H
N
BzlmH"
-H"
Bzim
-H"
BzIm"
Figure 3.11
Imidazole molecule usually functions as a ligand via the unshared pair of electron
on N-3 although, structures involving bonding with "pyrrole" nitrogen have been
'Coordination cfiemistry ofdgands with ekctron rich % O orS biting antre" (Doctoraf thesis suBmitted to ASgarh iMusGm University, HN'DI^
Cfiapter3 80
proposed [9] or considered as reaction intermediate [10]. These seem to be fairly wide
spread acceptance of possible two bonding modes [11,12] as a ligand. There is no
authenticated case of such bonding and it is priori, quite unlikely because an "unshared
pair" does not exist at pyrrole nitrogen as the electrons are delocalized throughout the n-
system.
It has been reported [13] that deuteraed reaction of imidazole by D2O proceeds
through a mechanism in which proton at 2- position is more reactive leading to an ylide
intermediate formation. The imidazole series include a number of natural and synthetic
compounds, which possess marked physiological activities. Imidazole itself has little
physiological action and is relatively non-toxic [14]. However its substituted derivatives
like 4(5)-methylimidazole is more toxic [15] and is reported [16] to exert a slight
hypertensive action. A brief description of the various medicinal uses of imidazole
derivatives has been summarized in the following few paragraphs.
The imidazole system occurs [17,18] in the essential a-amino acid, histidine
(Figure 3.12), which play a part in enzyme catalysis of hydrolysis. It is also a part [18] of
the related harmone histidine {Figure 3.13).
Figure 3.12 Figure 3.13
A survey of literature reveals that imidazole ring system is also involved in many chemo-
therapeutically useful compounds [18], as in metro-imidazole {Figure 3.14), which is
active against a very wide range of micro-organism. One of its most important uses
is in treatment of amoebic dysentery. 4,5-dimethylimidazole, 4-chloroimidazole,
"Coordination cRemistry ofRgands with electron ricli%0 orS Siting centre" (Doctoral tHesis submitted to^Rgarfi MuiGm University, HN'DIA
CHapterS
OoN
HOH2CH2C
Figure 3.14
4-aminoniethyl and N-substituted 4-aminomethylimidazoles cause vaso-constriction and
increase blood pressure [19]. l-decyl-2-methylimidazole is said to be a typical anaesthetic
[20]. It has also been shown that 2-thio-4-aminomethylimidazole possesses some insulin
like action in diabetes [21], 4,5(P-aminoethyl)imidazole stimulates smooth muscle [22]
and exerts a profound effect on the mammalian circulatory system. It is a strong gastric
stimulatant causing increased gastric secretion. It also increases the blood sugar content
[23]. Release of histamine or a histamin like substance is believed to be responsible for
certain allergic manifestation in human body, including bronchial asthma and urticarial
eruptions [24]. It therefore, appears that extent and direction of physiological effects are
largely controlled by the nature of substituents, present on the imidazole ring. Substituted
benzimidazole have been used as herbicides, fungicide and the antihelminthics e.g.
thiabendazole {Figure 3.15, R=H) and cambendazole (Figure 3.15, R=NHC02Pr') are of
proven efficiency both for human and veterinary use.
Figure 3.15
There are also some fungicides known among the 2-aminoimidazoles. A variety of
metal complexes of imidazolthiol are reported to exhibit fungicidal and insecticidal
'Coordination chemistry qfGganJs with electron rich % O or3 Biting centre" (Doctoralthesis suBmittecftoJt^arh MusRm Vniversity, HS^IJl
CHapterS 82
behaviour, however in some cases the antioxidant and heat resistant property [25-28] have
also been noted. The possibility of the use of the imidazole complexes as anti-tumour drug
has also been reported [29].
Coordination chemistry of Imidazole and its derivatives are well studied to yield
coordination compounds [3,20-34] with several metal ions. It may coordinate metal ions
through lone pairs at either N-3 or N-I or both. Though in majority of the cases
coordination is reported via N-3 but there are several examples, which favour [35,36] N-I
coordination. Furthermore, N-I hydrogen possesses acidic character [37] (pKa=I4) and
therefore imidazole or its derivatives can coordinate in the anionic form too. The
coordination compounds with this ligand can be divided into two classes:-
(a) Adducts of general formula [MXn(IZH)m] with X ~ also coordinated to M""*
ions.
(b) Solvents of general formula [M(IZH)ni"" ](X)""with counter anions present only
in second coordination sphere of the M"' ion.
A number of adducts of the type (a) are reported [31,32,38-40] which include
[MCl4(IZH)2] (M = Sn, Ti, Zr, Hf, Th), [MXaCIZH)] (M = Mn, Ni, Cu, and X = CI, Br, I),
[MX2(IZH)2] (M = Mn, Ni, Cu, Zn and X - CI, Br, I), [M(N03)2(IZH)2] (M = Co, Cu and
Zn) and [Mn(CNS)2(IZH)2] etc.
The solvates of the type (b) are also known [31,41-43] but their numbers are
relatively small. Some of these are: [M(IZH)6X2] (M=Mn, Co, Ni and X= CI, Br, I),
[M(IZH)6(N03)] (M=Mn, Co, Ni, Zn), [M(IZH)4(N03)] (M=Cu, Zn) [M(IZH)6(CNS)2)],
[Zn(IZH)6Cl2] and [Ag (IZH)2N03] etc. In basic media the conjugate base of imidazole
(Iz) is formed which also functions as a ligand [41,44-46]. Dipositive metal ions form
covalent complexes of stochiometry [M(IZ)2] (M==Fe, Co, Ni, Cu and Zn). The
coordination around Zn (II) and Co(II) in [M(IZ)2] is tetrahedral [41] while a distorted
square-planar arrangement has been reported [41,47] for Cu(II) and Ni(II) imidazolates.
Furthermore three different form [48] of [Cu(IZ)2] have been described which differ in
colour and also in magnetic behaviour, however their detailed structures are unknown.
"Coordination cRemistry ofGgands witA electron ricfi%0 orS Siting centre" <DoctoraCthesis suSmittedtoJiEgarfi MusQm Vniversity, I!M)IA
chapters 83
The imidazolate salt of Cu(I) has also been reported [49], for which a polymeric bridged
structure {Figure 3.16) has been indicated i.e.
Figure 3 J6
The recent recognition of the importance of metallo-mesogens i.e. liquid crystals
containing metal ions, for development of advanced materials [50] with new electronics,
optical and magnetic properties [51] has encouraged coordination chemists to design new
lipophilic rod like or disc like complexes exhibiting mesomorphic behaviour [52]. A large
variety of bidentate chelating units have been successfully used for the design of metallo-
mesogenes [52]. It has been shown that metal complexes with thermotropic calamitic [53]
and discotic [54] mesogenic ligand containing linear bidentate 2,2-bipyridine unit [55]
display only poor [56] or no [57] mesogenic behaviour. The closely related tridentate
receptor 2,2; 6,2 terpyridine and other tridentate binding units [58] were essentially
ignored in this field [52] as well as Claude Piguet and co-worker shown that related
tridentate ligands (L) and their derivatives {Figures 3.17 and 3.18) are particularly well
suited to produce lanthanide building blocks [Ln(L')(N03)3] (i = 3-10) [59] and
[Ln(L')3](C104)3 (i = 1-10) [60], exhibiting predetermined structural, thermodynamic,
magnetic and spectroscopic characteristic which might induce fascinating new properties
in advanced materials.
The Philippe's benzimidazole synthesis [61], which utilizes condensation of
dinitrile with dimines at high temperature in presence of a catalyst was considered an
established synthetic procedure. This procedure to synthesize 2-phenylbenzimidazole
"CoorSnation chemistry ofdgands with electron rich % O orS Siting autre" (DoctoraCthesis suSmittecCtoJlEgarh MusRm University, 19^1^
chapters 84
CO
R-
, . ^ ^
f^ N
CO
R
Figure 3.17
R = OEt L, R = NEtj Lj
Ri R2
H Me
H Et
H Oct
H Bz(0Me)2
H Et
Phe Bz(0Me)2
p-PheN02
p-PheNEt2
R3
H
H
H
H
Me
H
)J
J?
L
L3
L4
L5
L6
L7
L8
L9
LIO
% ^
Figure 3.18
produced only a trace yield [62] of the desired product. However, it was obtained in high
yield if condensation is carried out in a sealed tube at 180° C in the presence of aq. HCl. A
few other 2-arylbenzimidazole has been obtained in pair to fair yield by this method [63].
It was reported [64] the o-phenylenediamine is the to reacted less readily with carboxylic
acids in presence of aq. HCl as a catalyst. Hein and co-worker [65] found that
"Coordination cRemistry qfUgands with etectron ricHlH.O orS Siting centre' (Doctoralthesis suBmittedtoA^arh !Mus[im Vniversity, IWDIJl
Cfiapter3 85
polyphosphoric acid is a higlily effective, convenient and very general catalyst for
promoting condensations of this type and for promoting similar condensation leading to
the benzoxazole and benzothiazole series. This also serves [65] as a suitable solvent for
the affective condensation reaction of carboxylic acids, amides, esters or even nitriles with
aromatic diamines, o-aminophenols and o-aminothiophenols. A much better method to get
such tripode ligand with imidazoline rings is reported to be the direct fusion of
nitrilotriacetonitrile or nitrilotriacetic acid with o-phenylenediamine at high temperature,
followed by recrystallization from methanol. Thompson and co-worker [66] have used
identical synthetic procedure for preparation of a novel system namely tris(2-
benzylimidazylmethyl)amine (NTB), which has the right geometrical features to be a
stereo-chemically selective 'tripode' ligand. The molecule contains three benzimidzole
rings attached to the backbone of an alkyl amine skelton (Figure 3.19). The ligand NTB
acts as a tetradentate ligand in an approximately Csv-symmetry capable of forcing metal
ions to adopt predominantly five coordinate trigonal-bipyremidal structures. The resulting
Co(II) and Zn(II) complexes contain mixed stereochemistry e.g. five coordinate trional-
bipyremidal cation M(NTB)X] and four coordinate tetrahedral anions [MX4] " (M=Co,
Zn; X=C1, Br, NCS).
It was therefore thought worthwhile to design a ligand system analogous to NTB
which may instead specifically provide a tridentate coordinating site in Cav-symmerty
Figure 3.19
"Coordination chemistry qfUganJs with ekctron rich%OorS hiting centre" (Doctoralthesis suSmitted to Rgarh Muslim Vniversity, IWDIJi
chapters 86
capable of forcing metal ion to result in coordinatively saturated structural features of the
resulting complexes. In this chapter preparation of novel of ligands, bis(4,5-Dihydro-lH-
imidazol-2-ylmethyI)amine (Im) and bis(lH-benzoiinidazol-2-yImethyl)amine (BIm)
have been described. The ligands were obtained from condensation of iminodiacetic acids
with diamines viz. 1,2-diaminoethane and 1,2-diaminobenzene .in the presence of
orthophosphoric acid as catalyst at moderately high temperature (~180°C) in a sealed tube.
The ligands have been characterized using usual physico-chemical and spectroscopic
methods and its reactivity as chelating agent towards transition metal ions have also been
investigated.
'Coordination chemistry of ligands with electron rich % O orS Biting centre' iDoctoraf thesis submitted to Rgarh MusEm "University, I!N<DIA
CHapterS 87
EXPERIMENTAL
Materials:-
The metal salts viz. anhydrous chromium(II) chloride, anhydrous iron(III) chloride
(both C. D. H., India), cobalt(II) chloride hexahydrate, nickel(II) chloride hexahydrate,
copper(II) chloride dihydrate (all B. D. H. India), were all commercially pure samples
used after recrystallization. Iminodiacetic acid (E. merck, Germany), 1,2-diaminoethane,
1,2-diaminobezene (E. merck) and 1,10-phenontroline (Aldrich) were used as received.
Instrumentation:-
I.R. spectra were recorded as KBr discs on Interspec-2020 (spectrolab, UK), 'H-
and '^C- NMR spectra were recorded in deuterated methanol using tetramethyl silane
(SiMe4) as an internal reference on Bruker DRX300 spectrometer at room temperature.
Electronic spectra of 1x10' M solution in H2O were recorded on Elico SL-159 UV-
Visible spectrophotometer while electrical conductivity of 1x10" M solution were
recorded in DMSO or water on Elico CM-180 conductivity Meter Bridge. Positive ion
FAB-mass spectra were recorded on JEOL SX-102/DA-6000 mass spectrometer at lOKV
accelerating voltage using m-nitrobenzyl alcohol (NBA) as matrix and argon as the FAB
gas. The matrix peaks were indicated at m/z = 136, 137, 154, 289, 307. The resuUs of the
Elemental Analyses for C, H and N were obtained directly from Central Drug Research
Institute, Lucknow, India, which were measured on Heracus-carlo Erba 1108 instrument.
Preparation of bis(4,5-Dihydro-lH-imidazol-2-ylmethyl)amine, (Im) :-
Powdered iminodiacetic acid (8.0 g, 60.0 mmol) was mixed thoroughly with 1,2-
diaminoethane (8.0 mL, 120.0 mmol) along with a few drops of o-phosphoric acid in a
hard pyrex tube, which was sealed then heated to a melt at 180°C for about 2 h in an oil
bath. The reaction mixture was brought to room temperature, which gave a yellow
coloured wax like product. The yellow wax was extracted with 100 mL methanol to give
a yellow solution. It was filtered off, the residue was washed with methanol and
discarded. The methanol extract was dried under vacuum giving a yellow colour wax
"Coordination chemistry ofGgands •with electron rich % O orS Biting centre" (DoctoraCthem suSmittedto^Rgarh MvsGm Vniversity, I9^IA
CfiapterS 88
type product, which changed to a hard mass when kept for about a 6 weeks in a
desiccator over CaCb- The hard mass was crushed to a fine powder and was
recrystallized from methanol.
[yellow colour, m.p. 142°C, yield 8.6 g, 79%]
Anal. calc. for CsHijNs: C, 53.03; H, 8.28; N, 38.67, Found for (Im) C, 53.03; H, 8.28;
N, 38;67.
FAB - Mass (m-nitrobenzyialcoho NBA, matrix): [Im-2H+Bz3]"^ (m/z = 333), [Im
+2H+Bz4]^ (m/z = 472), [Im +2H+Bz5]^ (m/z - 490), [Im-CH2CH2+ Bzj] (m/z = 307),
[Im-CH2CH2 +2H+Bz4]^ (m/z = 444), [Im-NH.CH2CH2-H+ Bz,]"^ (m/z = 273), [Im-
C.NH.N.CH2CH2+ BZ4] (m/z = 413), [Im-C2H4(NH)N.C.CH2.NH] (m/z = 218); [c.f
Fragments of NBA matrix, Bz,=136, Bz2=137, Bz3=154, Bz4=289 and Bz5=307].
IR (as KBr Discs, cm"'): 3282s (NH), 1545m, 1629s (C=N).
UV - visible (CH3OH) 390 nm (G - 1.5 x 10 LmoF^cm"')
' H - N M R (CD3OD, ppm): 3.32 5 (t, 4H, JR-H ^ 5.6 cps, -CHz-im), 3.41 8 (t, 4H, JH-H = 5.8
cps, -CH2-im), 4.50 5 (s, 4H, -CH2-).
' C-NMR (CD3OD, ppm): 161.4 8 (C=N)in,,53.3 8 (-CH2 -N) , 34.5 8, 52.4 8 (-CH2-)in,.
Preparation ofbis(lH-benzoimidazol-2-ylmethyl)amine, (Blm):-
Iminodiacetic acid (8.0 g, 60.0 mmol) and 1,2-diaminobenzene (13.0 g, 120.0
mmol) were ground and thoroughly mixed in a mortar as a fine homogeneous mixture.
This mixture was taken in a pyrex tube along-with a few drops of o-phosphoric acid. It
was sealed then heated at 190°C for 2 h immersed in an oil bath. The reaction mixture
was then brought to room temperature and the solid mass was then crushed to fine
powder. It was then extracted in a magnetically agitated 500 mL of methanol which was
later refluxed for ~1 h. The extract was filtered off while hot and the residue was
discarded. It was evaporated to dryness under vacuum on a steam bath to get
microcrystalline solid. The solid was recrystallized from methanol,
[pink colour, m.p. 160°C, yield 10.80g, -65%].
Anal. calc. for CigHisNj: C, 69.31; H, 5.41; N, 25.27. Found for (BIm): C, 69.34; H,
5.29; N, 25.29.
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chapters 89
FAB - Mass of BIm (m-nitrobenzylalcoho NBA, matrix): [BIm +H]^ (m/z = 278), [BIm
-C6H4(NH)NC] (m/z - 160), [BIm-C6H4(NH)N.C.CH2] (m/z = 146), [Blm-
C6H4(NH)N.C.CH2.NH] (m/z =131), [BIm-C6H4(NH)N.C.CH2.NH +H]^ (m/z = 132),
[C6H4(NH)N +3H]'' (m/z = 108), [C6H4(NH)N +2H+ Bzj]^ (m/z = 261), [BIm + Bzj]
(m/z-431).
IR (as KBr Discs, cm-'): 3182bm (NH), 1584m, 1630s (C=N), 1501s, 1436m, 733vs
(ring).
UV - visible (CH3OH) 390 nm (G = 1.5 x 10 Lmol'cm-')
^H-NMR (CD3OD, ppm): 7.22 5 (m, 4H, ring proton), 7.53 8 (m, 4H, ring protons),
4.16 5(s,4H,-CH2-).
' C-NMR (CD3OD, ppm): 140.5 8 (C=N),n,, 115-140 8 (C=C)ar, 40.7 8 (-CH2 -N) .
PREPARATION OF METAL COMPLEXES
Synthesis offCr(Im)ChJ, (25):-
Anhydrous chromium(Il) chloride (1.0 g, 4.0 mmol) dissolved in 10 mL methanol
was added drop wise under inert N2 atmosphere to a magnetically stirred solution of the
ligand (Im) (0.72 g, 4mmol) in 20 mL methanol at room temperature. The yellow colour
of the ligand solution started changing, initially a green colour and finally a brownish-
green colour solution was formed after the metal salt solution has been completely added.
The dark green precipitate appeared after an over night stirring of this reaction mixture at
room temperature. The precipitate was filtered off, washed with methanol and dried under
o
vacuo, [dark green colour, m.p. 144 C dec, 0.45 g]
Anal. Calc. for CsHisNsCrCb: C, 31.16; H, 4.87; N, 22.72. Found for (25): C, 31.20; H,
4.85; N, 22.69. Molar conductance. Am (in water): 63.0 ohm^'cm^mor'.
IR (as KBr Discs, cm"'): 3240s (NH), 1559s, 1669s (C=N), 421m (M-N).
Synthesis of[Fe(Im)Cl3j,(26):-
To a magnetically stirred solution of the ligand, (Im) (0.90 g, 5.0 mmol) in 60
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chapter 3 90
mL methanol was dropped a methanolic solution (10 mL) of anhydrous ferric chloride
(0.64 g, 4.0 mmol) with stirring at room temperature. Reddish-yellow colour solution was
formed after complete addition of the metal salt. The reaction mixture was stirred for ~2
days at room temperature which gave yellow colour precipitate. The precipitate was
filtered off, washed with 3 portions of 2 mL methanol and dried in vacuo to give
microcrystalline product. [Yellow colour, m.p. 186°C, yield 0.61 g]
Anal. Calc. for CieHisNsFeCh: C, 27.94; H, 4.36; N, 20.37. Found for (26): C, 27.84; H,
3.31; N, 20.45. Molar conductance. Am (in water): 82.5 ohm'^cm^mof'.
IR (as KBr Discs, cm^'): 3233s (NH), 1577s, 1622s (C-N), 41 Im (M - N).
Synthesis of fCo(Im)ChJ, (27):-
The ligand, (Im), (0.72 g, 4.0 mmol) was reacted with C0CI2.6H2O (0.98 g, 4.0
mmol) in methanol in an analogous manner as described above. The reaction mixture was
stirred overnight at room temperature, which produced microcrystalline solid. This o
product was isolated in the manner as above. [Blue colour, m.p. 106 C dec, yield 0.48 g]
Anal. Calc. for CgHisNsCoCb: C, 30.86; H, 4.82; N, 22.50. Found for (27): C, 30.84; H,
4.79; N, 22.51. Molar conductance, Am (in wate): 55.8 ohm''cm^mor'
IR (as KBr Discs, cm"'): 3264s (NH), 1563s, 1627s (C=N), 412m (M -N) .
' H - N M R (CD3OD, ppm): 3.45 8 (t, 4H, JH-H = 5.9 cps, -CHz-im), 3.68 8 (t, 4H, JH-H = 5.7
cps, -CH2-™), 4.68 8 (s, 4H, -CH2-).
Synthesis offNiflmjChJ, (28):-
It was prepared in the manner as described above by reacting methanolic
solution of the ligand (0.72 g, 4.0 mmol) with NiCl2.6H20 (0.98 g, 4.0 mmol) which o
finally afforded microcrystalline product (28). [Coffee colour, m.p. 220 C dec, yield 0.87
g]
Anal. Calc for CgHisNsNiClj: C, 30.90; H, 4.83; N, 22.53. Found for (28): C, 30.93; H,
4.82; N, 22.57. Molar conductance, Am (in water): 61.4 ohm"'cm^mor'.
IR (as KBr Discs, cm"'): 3240s (NH), 1571s, 1636s (C-N), 413m (M-N).
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chapters 91
' H - N M R (CD3OD, ppm): 3.42 8 (t, 4H, JH-H = 5.7 cps, -CHz-i, ), 3.63 5 (t, 4H, JH-H = 5.5
cps,-CH2-i,.),4.616(s,4H,-CH2-).
Synthesis of [Cu(Im)Cl2], (29):-
The ligand (0.72 g, 4.0 mmol) was reacted with CUCI2.2H2O (0.68 g, 4.0
mmol) in the same manner as described above. The green colour precipitate produced
initially on addition a few drops of the metal salt solution disappeared when all of the
metal chloride solution was added. The reaction mixture was then stirred overnight at
room temperature, which gave blue colour precipitate. The solid mass was immediately 0
filtered off, washed with methanol and dried in vacuo. [(29) blue colour, m.p. 210 C,
yield 0.43 g]
Anal. Calc. for CgHisNsCuCb: C, 30.42; H, 4.75; N, 22.18. Found for (29): C, 30.45; H,
4.73; N, 22.15. Molar conductance, Am (in water): 96.1 ohm"'cm^mor'
IR (as KBr Discs, cm"'): 3246s (NH), 1584s, 1623s (C=N), 423m (M-N).
' H - N M R (CD3OD, ppm): 3.39 8 (t, 4H, JH-H = 5.8 cps, -CHs-im), 3.67 8 (t, 4H, JH-H = 5.9
cps,-CH2-™),4.72 8(s,4H,-CH2-)
Synthesis of the [Cr(BIm)Cl2j, (30):-
Anhydrous chromium(II) chloride (1.0 g, 4.0 mmol) dissolved in 10 ml methanol
was added dropwise under inert N2 atmosphere to a magnetically stirred solution of the
ligand (Blm) (1.30 g, 4.0 mmol) in 60ml methanol at room temperature. The brownish-
green colour reaction mixture was stirred overnight at room temperature and then refluxed
for 6 h. It was then concentrated on a steam bath to 1/3' '' of the original volume and
brought to room temperature. The concentrated reaction mixture was then poured in a
large excess of diethyl ether (~250ml) which has resulted in the formation of brown colour
precipitate. The precipitate was immediately filtered off, washed with ether then dried
under vacuum, [brown, m.p. 284°C dec, yield 0.86 g]
Anal. Calc. for dgHisNsCrCb: C, 48.13; H, 3.76; N, 17.54. Found for (30): C, 48.02; H,
3.54; N, 17.24. Molar conductance, Am (in DMSO): 27.1 ohm"'cm^mor'.
"Coordination cRemistry ofEgands with electron ricli%OorS Siting centre" <Doctora[ thesis submitted to M^arh !MusRm University, HWDIJl
Cfiapter3 92
IR (as KBr Discs, cm"'): 3050bm (NH), 1590m, 1629s (C=N), 1509m, 1447m, 758vs
(C=C, ring), 419m (M-N).
Synthesis of[Fe(BIm)Cl3],(31):-
A methanolic solution (10 mL) of anhydrous ferric chloride (0.64 g, 4.0 mmol)
was dropped to a magnetically stirred solution of the ligand, BIm (1.30 g, 4.0 mmol) in 60
mL methanol solution with stirring at room temperature. Reddish-yellow colour solution
was formed after complete addition of the metal salt. The reaction mixture was stirred for
~2 days at room temperature which gave yellow coloured precipitate. The precipitate was
filtered off, washed with 3 portions of 2 mL methanol and dried in vacuo. [Yellow colour,
m.p. 162°C, yield 0.30g, -20%]. The mother liquor was concentrated and then poured in a
large excess (~250ml) of diethyl ether, which gave a second crop of the product. The
precipitate was filtered off, washed with ether then vacuum dried. The colour, melting
point and analytical data of the 2nd fraction was identical to that of the first fraction.
Anal. Calc. for CeHisNsFeCh: C, 47.66; H, 3.72; N, 17.37. Found for (31): C, 47.23; H,
3.64; N, 17.24. Molar conductance. Am (in DMSO): 24.5 ohm"'cm^mol"'.
IR (as KBr Discs, cm''): 3210vs (NH), 1599m, 1619s (C=N), 1530s, 1458m, 752bs (C-C,
ring), 422m (M-N).
Synthesis of/CofBImJChJ, (32:-)
The ligand, (BIm) (0.98g, 4mmol) in 60 mL methanol and C0CI2.6H2O (0.98 g,
4.0 mmol) in 10 mL methanol were reacted together in an analogous manner as described
above. The reaction mixture was stirred overnight at room temperature and then refluxed
for 6 h. It was concentrated on a steam bath, cooled and poured in a large excess (250 mL)
of diethyl ether, which resulted in the formation of brown precipitate. The precipitate was
immediately filtered off, washed with ether and dried in vacuo, [m.p. 228°C dec, yield
0.93]. The filterate and washings, however, did not yield any second fraction.
Anal. Calc. for CeHisNsCoClz: C, 47.30; H, 3.69; N, 17.24. Found for (31): C, 47.23; H,
3.64; N, 17.04. Molar conductance. Am (in DMSO): 30.24 ohm"'cm^mor'
IR (as KBr Discs, cm"'): 3108bm(NH), 1588m, 1619s (C=N), 1524s, 1459m, 741vs
"Coordination chemistry qfSgands witk ekaron ricfi%0 orS Biting antre" 'DoctoraC thesis suBmitudto JURgarii MusCim Vniversity, im>IJi
chapters 93
(C=C, ring), 431m (M-N).
' H - N M R (CD3OD, ppm)
aromatic ring protons), 4.27 5 (s, 4H, -CH2-).
' H - N M R (CD3OD, ppm): 7.46 5 (m, 4H, aromatic ring proton), 7.82 8 (m, 4H,
Synthesis offNiflBrnjChJ, (33):-
It was prepared in the same manner as described above by reacting methanolic
solution of the ligand (1.30 g, 4.0 mmol) with nick;el(II)chloride hexahydrate (0.97 g, 4.0
mmoi) also taken in 10 mL methanol. The reaction mixture was refluxed then
concentrated on a steam bath to \/3'^ of its volume, cooled and then poured in large excess
(250 mL) of diethyl ether. Brown colour precipitation formed were filtered off, washed
with ether and then dried in vacuo. [Brown colour, m.p. >300°C, yield 0.40g]. Anal. Calc.
for CisHisNsNiCb : C, 47.33; H, 3.69; N, 17.25. Found for (33): C, 46.98; H, 3.58; N,
16.88. Molar conductance. Am (in DMSO): 37.24 ohm''cm^mor'
IR (as KBr Discs, cm'): 3200bm (NH), 1576m, 1622s (C=N), 1505s, 1454m, 743vs
(C=C, ring), 426m (M-N).
^H-NMR (CD3OD, ppm): 7.42 6 (m, 4H, aromatic ring proton), 7.76 5 (m, 4H,
aromatic ring protons), 4.28 6 (s, 4H, -CH2-).
Synthesis offCuflBmJChJ, (34:-)
The ligand (1.30 g, 4.0 mmol) was reacted with CUCI2.2H2O (0.72 g, 4.0 mmol)
in the same manner as described above. The green colour precipitate produced on
addition of initial drops of the metal salt solution disappeared on stirring at room
temperature. The reaction mixture was then stirred overnight at room temperature. Green
color precipitate formed was filtered off, washed with methanol then dried in vacuo. [1^'
crop, m.p. 202°C (dec), yield 0.27g]. The mother liquor was refluxed for 6h then
concentrated on a steam bath. The concentrated solution was cooled, poured to an excess
of diethyl ether (~250ml), which has resulted in the formation of green colour solid
material. The solid mass was immediately filtered off, washed with ether and dried in
vacuo. [2"'' crop, m.p. 202°C, yield 0.23g]. Both the crops exhibit similar appearance and
melting points.
"CooTtfinatum chemistry ofSgands witH ekctnm rick % 0 orS Siting centre" (DoctoraltHesissuSmittedtoJiRgatfi!MusGm "University, HhPDI^
chapter 3 94
Anal. Calc. for CieHisNsCrCb: C, 40.49; H, 3.16; N, 14.76. Found for (34): C, 39.79; H,
3.06; N, 13.94. Molar conductance, Am (in DMSO): 14.27 ohm"Vm^mor'
IR (as KBr Discs, cm"'): 3116bm (NH), 1599m, 1616s (C=N), 1525s, 1457m, 734vs
(C=C, ring), 423m (M - N).
^H-NMR (CD3OD, ppm): 7.48 5 (m, 4H, aromatic ring proton), 7.92 8 (m, 4H,
arornatic ring protons), 4.32 5 (s, A\i, -CH2-).
Reaction of [MfBImjChJ with 1,10-phenonthroline (Phen):- Isolation of the adducts
[M(BIm)(Phen)a]a [M = Co (35) or Ni (36)J
The complex [Co(BIm)Cl2] (27) (0.4 g, 1.0 mmol) or [Ni(BIm)Cl2] (28) (0.4 g,
1.0 mmol) was added in small batches to a hot vigorously stirred methanolic solution of
1,10-phenonthroline (0.27 g, 1.0 mmol), which give a dark coloured clear solution. The
reaction mixture was refluxed for 3h then brought to RT. It afforded fine powdery mass,
which was filtered off, washed with methanol and dried under vacuum.
[(35) dark brown, m.p. 300°C (dec); (36) brown, m.p. >300°C].
Anal. calc. for C0C28H23N7CI2: C,57.24; H, 3.91; N, 16.69. Found for (35) C, 57.27; H,
3.89; N, 16.73. Molar conductance, Am (in DMSO): 65.0 ohm''cm^mor'.
IR (as KBr Discs, cm"') (35): 3098bm (NH), 1584m, 1620b (C=N), 1504m, 1450m, 760vs
(C=C,C-C),424m(M-N).
Anal. calc. for NiC2gH23N7Cl2: C,57.26; H, 3.92; N, 16.70. Found for (36) C, 57.23; H,
3.89; N, 16.72. Molar conductance. Am (in DMSO): 58.0 ohm 'cm^mol"'.
IR (as KBr Discs, cm"') (36): 3084bm (NH), 1589m, 1636s (C-N), 1620b, 1512m,
1456m, 768vs (C=C, C - C), 427m (M-N).
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CfiapterS 95
RESULTS AND DISCUSSION
The ligand, bis(4,5-dihydro-lH-imidazol-2-ylmethyl)amine (Im) was obtained
employing the cyclic condensation reaction of iminodiacetic acid, NH-(CH2COOH)2 with
1,2-diaminoethane in presence of ortho-phosphoric acid under melt condition (~190°C) in
closed reaction vessel. The substituted analogue, bis(lH-benzimidazol-2ylmethyl)amine
(Blm) was also prepared following identical procedure. The final products were obtained
in a fairly good yield (>60%), if the reactions were carried out in presence of a few drops
of ortho-phosphoric acid. The presence of ortho-phosphoric acid, probably acts as a
catalyst of this condensation reaction. It is known that reaction of 1,2-diaminobenzene
with R-X (X = CN, COOH, CONH2; R = alkyl or aryl group) proceed [65] via a ring
closure path catalyzed by the mineral acids like con. HCl, HNO3 or poly-phosphoric acid.
These condensation reactions in the presence of mineral acids in general produce
benzimidazole derivatives as the ultimate products via a cyclization mechanism. The
cyclisation process was found less effective for reactions when carboxylic acid was used,
which however, becomes more facile and effective in presence [65] of a few drops of
poly-phosphoric acid. Reactions of 1,2-diaminobenzene with either nitrilotriacetonitrile,
N-(CH2CN)3 or Nitrilotriacetic acid, N-(CH2COOH)3 under melt conditions (200-210°C)
is reported [66] to follow an identical ring closure mechanism to produce tris(2-
benzimidazylmethyl)amine or tris(lH-benzimidaz-2-ylmehtyl)amine in fair yield (~50%).
The present reactions of iminodiacetic acid with 1,2-diaminoethane as well as of
iminodiacetic acid with 1,2-diaminobenzene (1:2 mole ratio) under identical conditions
also follow the analogous ring closure mechanism which are catalyzed by the presence of
ortho-phosphoric acid to afford (Im) and (Blm), respectively, as the ultimate products in
good yields (>60%, Experimental Section). The physico-chemical, spectroscopic
investigations and molecular model computations (vide infra) support the proposed
mechanism (Figure 3.20) for the formation of the products (Im) and (Blm) which
contain, respectively, two imidazoline and benzimidazole rings connected to a secondary
amine function via methylene (-CH2-) skeltons or linkages.
'Coordination chemistry ofRgands tvith ekaron rich % O orS Biting centre " (DoctoraCthesis suSmitUtttoJiSgaHi MusRm Vniversity, I^<DIJi
CHaperS 96
Analytical and FAB-mass spectral data:-
Analytical data of (Im) and (BIm) (Experimental section) are consistent with the
molecular formulae CgHisNs and C16H15N5, respectively. FAB - mass spectrum of (Im)
recorded in NBA matrix (Experimental section) failed to indicate any of the possible
molecular ions peaks i.e. [(Im)+2H]\ [(Im)+H]^ [(Im)]^ [(Im)-H]^ or [(Im>-2H]*
expected at m/z = 183, 182, 181, 180 or 179. Possibly the molecular ion has either a low
volatility to get recorded or has a low thermal stability under the FAB conditions, which
undegoes further cleavage. This is quite possible that the molecular ion is a highly reactive
species that combines with the matrix (NBA) fragments to generate addition products. It is
interesting to note that the spectrum contained prominent peaks (-70% abundance) for the
addition products at m/z = 333, 472 and 490 assignable to [(Iin)-2H+Bz3]^
[(Im)+2H+Bz4]"^ and [(Im)+2H+Bz5]' , respectively, (where BZ3, BZ4 and Bzs are the
matrix fragments, NBA, Experimental section). These observationss support that unlike
parent molecular ions their addition products with the various fragments of the matrix are
volatile enough to give the corresponding peaks with fair abundance (-70%).
The FAB-Mass spectrum of (BIm) under identical experimental condition,
however, exhibited (experimental section) an intense peak (99% abundance) at m/z = 278
consistent with the formation of the molecular [(Im)-H]"^. The spectra of both, (Im) as
well as (BIm) contained various peaks arising from the thermal fragmentations of the
molecular ions and of their addition products with the matrix fragments. The present FAB-
Mass spectral data, therefore, support the molecular formulae of the ligand molecules i.e.
(Im) and (BIm) as indicated from the analytical data (Experimental section).
Electronic andIR spectral data:-
The electronic spectrum of (Im) and (BIm) recorded in CH3OH exhibited a strong
band (G = 1.5 x 10 Lmof'cm"') at ?. = 390 nm (25,640 cm"') and 398 nm (25,130 cm"'),
respectively. This strong absorption band is attributed [67] to the electronic transition from
filled n - orbital (HOMO) to empty ji* orbital (LUMO) (i.e. TT^TI* transition) of (C=N)
bond.
"Coordiimtion cHemistry ofSgands with electron rich % O orS Siting autre" OoctoraCthesis suhmitted to Rgarh iMusdm "University, I3W)IJi
CfiapterS 97
FT-IR spectra of (Im) and (BIm) recorded as KBr pellets exhibited an absorption
frequency (Experimental section) assignable to v(N-H) stretching vibration [15,68]. The
spectra did not show the strong broad absorption bands characteristic of carboxylic group
of iminodiacetic acid expected in 1700 - 1800 cm"' and 3300 - 3500 cm"' region.
Appearance of two well resolved sharp and strong bands (Experimental section) in 1500 -
1620 cm"' region characteristic of v(C=N) stretching vibrations [66] confirm that the
condensation process is followed by the ring closure steps (Figure 3.20) to result in the
cyclized product (Im) or (BIm).
'H-NMR and '^C-NMR spectral data:-
' H - N M R spectrum of (Im) recorded in CD3OD exhibits two well resolved
symmetrical triplets at 3.32 8 (t, 4H, JH-H = 5.6 cps) and 3.41 8 (t, 4H, JH-H = 5.8 cps) due
to imidazoline ring methylene protons in addition to a singlet (experimental section) at
4.50 5 (s, 4H) due to the skeleton methylene protons (Experimental section) observed
attached to the secondary amine function. The spectrum of (BIm) recorded in CD3OD,
also exhibited a singlet for the skeleton -CH2- protons along with two well resolved
equal intensity multiplets in the down field side centered at 7.53 8 (m, 4H) and 7.22 8 (m,
4H) characteristic of the aromatic protons. The resonance signals in down field side
basically appear as a symmetrical set of peaks typical of the AA\ BB' aromatic protons.
The symmetrical nature of these resonance peaks due to aromatic protons in (BIm) are
consistent with the presence of a C2 symmetry axis and a Ov symmetry mirror plane in
(BIm) moiety. The symmetry elements apparently bisect the individual unit in
benzimidazole unit of the ligand (BIm). Resonance signals characterisfic of the NH
protons of the secondary amine as well as pyrolic groups of imidazoline and
benzimidazole, however, were not observed in the spectra. This may probably be due to
the presence of a fast proton exchange which a well known effect to cause the
disappearance of such resonance siganls. ' C-NMR spectra of (Im) and of (BIm) recorded
in CD3OD contained '^C-resonance signals (Experimental section) attributable to iminic
(C=N), methylene (CH2) and aromatic (C=C) groups [69,70] in the moiety.
"Coor£nation cRemistry ofRgands with electron rich %OOTS Siting centre' <Doctora[ thesis submitted to AUsarh Muslim University, I!NU)IJi
Cfiapter3 98
Reactivity of the (Im) and (Blm) towards metallic substrates: Isolation and
characterization of metal complexes.
The ligands (Im) and (Blm) exhibited considerable reactivity with metal chloride
MCI3 (M = Fe) and MCI2 (M = Cr, Co, Ni or Cu) producing air and moisture stable
compounds with stochiometries [MLCI2] [L = (Im), M = Cr (25), Co (27), Ni (28), Cu
(29); L = (Blm), M = Cr (30), Co (32), Ni (33), Cu (34)] and [FeLCU] [L = (Im) (26);
(Blm) (31)]. The observed molar conductances (Am) of the complexes (25 - 29) (Am <
100 ohm"'cm^mor') and of (30 - 34) (Am < 50 ohm''cm^mor') measured in a high
dielectric constant ionizing solvent water (G = 78) and DMSO (€ = 46) indicate that all
the complexes behave as a non-electrolytes [71,72] in solution. These informations
support the proposed stoichiometrices of the complexes as [MLCI2] [L = (Im), M = Cr
(25), Co (27), Ni (28), Cu (29); L - (Blm), M = Cr (30), Co (32), Ni (33), Cu (34)] and
[FeLCb] [L = (Im) (26); (Blm) (31)].
IR spectral characterization of the complexes:-
IR spectra of the complexes (Experimental section) exhibited fundamental v(N-H),
v(C=H), v(C=C) stretching vibrations [73] characteristic of (Im) and (Blm) ligand
moieties, which are consequently coordinated to the metal ions. A considerable negative
shift (Av = 50-100 cm'') in the v(N—H) and a small positive shift in v(C=N) stretching
vibrations have been observed in the complex relative to the free un-coordinated ligands.
This is due to the involvement of both the secondary amine function as well as the pyridyl
nitrogens of the imidazoline or benzimidazole [2] of the (Im) and (Blm) to the metal ion.
The characteristic of ring vibrations v(C-H) and v{C-C) were observed at the appropriate
positions. A medium intensity band observed in the region 415-430cm"' is due to M— N
bond stretching vibration [68], which confirm that the metal ions are bonded with nitrogen
donor sites.
Magnetic susceptibility measurements, Electronic (ligand field) andEPR spectral data:-
The observed )ieff Value of complexes [Cr(Im)Cl2] (25) and [Cr(BIm)Cl2] (30)
"CoorcRnation chemistry ofGgands with electron rich % O orS Biting centre' Ooctoraf thesis suSmittedto^Rgarh MusRm "University, HffiDIJi
CfiapterS 99
{Table 3.1) are comparable to the calculated spin-only moment (lOso) required for four
unpaired electrons (i.e. 4 UPE) on metal ion. This is consistent with a high-spin state (d'*,
hg^ eg' configuration) of the Cr " ion. The ligand field spectra (Table 3.1) show a broad
asymmetrical band in at around ~17,500 cm'' region arising from the ^T2g*—^Eg transition
[73].
The \Jk{f Value of the complexes [Fe(Iin)Cl3] (26) and [Fe(BIin)Cl3] (31) (Table
3.1) are higher than range of the theoretical estimated spin-only moment ( so) for five
unpaired electrons (i.e. 5 UPE). However, the magnitude is within the range reported for
various high-spin Fe^* complexes in octahedral geometry. This indicates that the Fe ^ ion
acquires a high-spin d (tag, Cg) configuration in the present complexes. Contribution
from diamagnetic correction of 7r-ring system (imidazoline^enzimidazole) and the
electronic rich [74] donor centres (pyridyl and secondary amine) of the ligand moiety
may contribute the to the observed Heir value. The presence of temperature independent
ferromagnetic interaction in neighbouring Fe ^ nuclei, which usually enhances the
magnetic moment, may not be ruled out. The electronic spectrum of these complexes
exhibit well defined ligand field (d - d) transifion bands in addition to the metal •<— ligand
(M-<—L) charge transfer band. The position and intensities of these bands are comparable
[73] with that known for octahedral high-spin Fe ^ complexes. The positions and
assignments of the ligand field bands are summarized in Table 3.1.
The Co^^ complexes i.e. [Co(Im)Cl2] (27) and [Co(BIni)Cl2] (32) exhibited three
ligand field (d - d) bands, whose energies as well as intensities are comparable with those
reported [73] for various Co ^ complexes having five-coordinate trigonal-bipyramidal
geometry. Five-coordinate Co complexes of the type [C0L2X] [where L = diphenyl(o-
diphenylarisophenyl)phosphine or diphenyl(o-methylsilinomethyl)phosphine, X = CI, Br
or I] having trigonal-bipyramidal geometry are reported [75] to exhibit three well defined
absorption bands in the visible region below 600 nm (16,670 cm"') similar to that
observed in the present case. These absorption bands are characteristic of the transitions
^E'(F)*-U'2(F), %'(P)^U'2(F) and ^E" (P)^U'2(F) in increasing order of energy
[Table 3.1). Calculafions based on crystal field model as well as angular overlap
"Coordination cfumistry ofSgands with electron rich % O orS Siting centre" (Doctoralthesis suSmittedtoJiGgaTh Muslim Vniversity, I9ifiDIJl
CfiapterS 100
approach [76] carried out for five coordinate chromophores in a pseudo trigonal-
bipyramidal environment agree well for Csv symmetry of the molecule. However,
presence of spin-orbit coupling [73] in d' (Co^*) system may disturb the probable overall
Csv symmetry of the molecule. The observed room temperature magnetic moment ([leff.
= 4.5 BM) indicates a high-spin state of Co " in ion (27) and (32). The presence of high-
spin state of 3d metal ions is not an un-usual phenomenon particularly for Co" (d
system) complexes with ligand having hard donor sites. This phenomenon is more
pronounced, specifically, for transition metal complexes with tripodal [N,N,N] chelating
agent [66,73].
The complexes [Ni(Im)Cl2] (28) and [Ni(BIm)Cl2] (33) are paramagnetic {Table
3.1), which rules out the possibility of a diamagnetic four coordinate square-planar
geometry around Ni ^ ions. The observed magnetic moment (fiefr. - 2.5 BM) is
considerably lower in magnitude compared to the calculated spin-only moment (jiso) for
two unpaired spins (S = 1) in either four coordinate tetrahedral or six coordinate
octahedral stereochemistry around the Ni " [77]. The five coordinate Ni " complexes are
reported to occur in both high-spin (S = 1) as well as low-spin (S = 0) state. There are
numerous examples for trigonal bipyramidal and square-pyramidal geometries known
[78]. Most of these complex contain a tripodal ligands. The symmetry of such complexes
with few a exception are not exactly perfect DBH symmetry of a true trigonal bipyramid,
although they are usually called five coordinate trigonal-bipyramidal complexes. For the
high-spin (S = I) complex, [Ni(Me6tren)Br]* the replacement of the N - donor amine
ligand by P, As, S or Se etc. cause not only a large ligand field splitting but also stabilizes
the low-spin configuration [79] of the metal ion. The magnitude of the observed |4eff is
between the high-spin and the low-spin values. This arises when the ligand field strength
is nearly equal to the mean electronic pairing energy for five-coordinate geometry. This
indicates the presence of a spin state equilibrium i.e. High-spin — — Low-spin.
However, the electronic ligand field (d-d) spectrum contained two bands, ~ 450 nm
(-22,220 cm"') and -650 nm (-15,380 cm''), which are very similar to that reported [73]
"Coonfimtion chemistry ofSgands xviti e&arm ricli%0 orS Biting centre" (DoctoraCthesis suSmituitoMtsarH Muslim University, HWDIJi
Cfuipter3 101
for high-spin five coordinate Ni' ' complex [Ni(Me6tren)X]*, having a pseudo trigonal-
pyramidal geometry around the metal ion.
The observed magnetic moment (jj ff = 2.0 BM) for the complexes [Cu(Iin)Cl2]
(29) and [Cu(BIm)Cl2] (34) indicate presence of one un-paired spin (S = I/2) on the metal
ion having d (i.e. r^/, e/) configuration. The electronic spectrum contained broad bands
centered at -630 nm (~15,800cm'') and at -560 nm (-18,000 cm'* ) which is apparently
a split component of the d - d transition ( r?g*— E^ arising from the presence of the
dynamic John- Teller distortion in the molecule. The ligand asymmetry arising due to a
weak axial coordination from the counter (CI) anions relative to the strong basal [N,N,N]
coordination (in five coordinate geometry) would also be responsible for a distortion
from the perfect Oh symmetry. Splitting of the energy levels Eg into Big, Aig and of T2g
into ^B2g, ^Eg are dependant on the extent of tetragonal distortion [80,81]. The resulting
energy levels are in the order ^Bjg < ^Aig < ^B2g < ^Eg, which may provide three
allowed transitions ^Eg<-'^Big, ^B2g<-^Big and ^Aig<-^B}g in the decreasing order
of energies. The observed ligand field bands {Table 3.1) are assignable to
^Eg<-^Big and ^B2g<-'^Big (or dx2-y2<-dz2 and dx2-y2<-dxy,yz) transitions.
The X-bands EPR spectra of (29) and (34) are nearly identical {Table 3.1) the
spectra exhibit an anisotropy giving two resonance signals i.e. (gy = 2.17, gx= 2.05) for
(29) and (g|| = 2.16, gi =2.04) for (34). The observed g|| > gi indicate the presence of an
axial symmetry with ^Big {dx^-y^) ground electronic state of Cu " ion. In an axial
symmetry, gy and givalues for d system having B]g ground electronic state [80] are
given by the expression.
g|,= 2 - 8 K | | ' ^ E{'B2g^'B,g)
gi = 2 - 2Ki^ X E{^Eg<f-^B,g)
Where Ks are orbital reduction factors and X is spin-orbital coupling constant of
Cu " ion {X = 829 cm""'). The parameter G, which measures the exchange
coupling effects, is related to the experimentally observable quantities. g||, gi, Ki_
K|| and energies of ligand field transitions by the following expressions:
"Coordination cRemistry ofRgands with eleanm rich %OOTS Biting centre ' OoctoraC thesis submitted to Rgarh MusEm "University, IJiOIA
CfiapterS 102
g|l~2 4K|| E{'B2,^'BJ,)
G = = .
g i - 2 Ki' E{'E,<-'B,,)
The evaluated magnitude of G has been used [82] as a criterion for measuring the
extent of exchange coupling effects on local Cu ^ environment in the complexes and also
the extent of anisotropy in the g - values. The magnitude of G < 4.0 suggests the presence
of a considerable exchange coupling [80-83] as well as anisotropy in the present Cu ^
complexes (29) and (34).
All the complexes with (Im) and (BIm) usually adopt a five coordinate geometry
which is apparently a coordinatively un-saturated species. They can expand coordination
number to attain a stable hexa-coordination if reacted with strong o- donors or chelating
agents with this view the complexes [Co(BIm)Cl2] (32) and [Ni(BIni)Cl2] (33) were
reacted with a strong o-chelating agent, 1,10-phenantholine (Phen) in 1:1 mole ratios.
This has afforded corresponding stable adduct complexes (35) and (36), respectively.
Both the adduct complexes were of different colour and melting points compared to the
corresponding parent complex (32) and (33), respectively. Analytical data (Experimental
section) are consistent with the molecular formulae [C28H23N7.MCI2] [M = Co (35) or
(36)]. The observed magnitudes of the molar conductivities of their 10" M solution in
DMSO [Am= 65 Scm^mof' (35), Am= 58 Scm^mol"' (36)] are comparable to that of a 1:1
electrolyte [72]. The adducts therefore, ionize in DMSO to generate the corresponding
complex cation [M(BIm)(Phen)Cl]' and the counter CI' ion in the solution. This is
consistent with the stoichiometry of these adduct complexes as [M(BIni)(Phen)Cl]Cl [M
= Co (35), Ni (36)]. IR spectra of (35) and (36) show frequencies characteristic of the
coordinated ligand (BIm) moiety. The broad feature of the additional band observed at
1620 cm"' is characteristic of the presence of coordinated 1,10-phenonthroline [84] in the
molecular moiety. The observed magnetic moments, |ieff = 5.2 BM for (35) is very close
to i so value for three unpaired spins (S = 3/2) for a high-spin d (or t2ge^) configuration
of the metal ion in the complex. Similarly, ^eff of (36) (fxeff. = 4.8 BM) is also
comparable to the spin-only moment of Ni ^ with d (or t2geg^) configuration having
"Coordination chemistry of^aiufs with electron rich % O orS Biting centre" OoctoraC thesis suBmitted to JiRgarh MusSm University, IWilJl
chapters 103
some orbital-moment contribution (S = 1) in a hexa-coordinate environment of tlie ligand
[77]. The electronic spectra of (35) and (36) exhibited a weak broad absorption band
centered at 510 nm (19,610 cm"') and two weak intensity bands at 425 nm (23,530 cm'')
and 675 nm (14,800 cm"') for (35) and (36), respectively, characteristic [73] of the ligand
field d-d transitions in an octahedral environment around Co {t2g eg ) and Ni {t2g eg)
complexes. These informations support that the Co' ^ and Ni " ions in presence of
additional strong a-donor ligand 1,10-phenonthroline results in adduct complexes
[Co(BIm)(Phen)Cl]Cl (35) and [Ni(BIm)(Phen)Cl]Cl (36) where the metal ions (Co^^
Ni ) acquire a coordinatively saturated hexa-coordinate [79] octahedral geometry.
Molecular Modeling:-
The molecular model calculations based on CSChein-3D-M0PAC were
employed [85,86] to solve molecular structures and to get the various bond lengths and
bond angles in the molecules similar to that already discussed in chapter 2. The ligands
(Im) and (BIm) provide three aza potential sites [N,N,N] to bind the metal ions. The
computed bond lengths and bond angles in the ligands as well as the metal complexes
have been displayed in Tables 3.2 and 3.8. The mechanical adjustments via augmented
mechanical fields were used to draw the optimized minimum energy plot {Figure 3.21 -
3.27) for the geometry of the ligands (Im) and (BIm) as well as their metal complexes.
The perspective view of (Im) and (BIm) {Figure 3.21 and 3.24) indicate that the two
imidazoline and benzimidazole pyridyl (sp -hybrid) and the secondary amine (sp -hybrid)
nitrogens are not coplanar to form a perfect triangular [N,N,N] basal plane. However, the
coordination or chelation of metal ions by these ligands appear to be very effective to
bring the tridentate [N,N,N] biting sites to form a near trigonal basal plane to resuh in the
five coordinate trigonal-bipyramidal coordination geometry around the metal ions {Figure
3.22 and 3.25). The various bond lengths and bond angles for the optimum structural plots
of the ligand as well as the metal complexes obtained form the semi-empirical calculations
are summarized in Table 3.2 and 3.3. The plot for the complexes [MLCI2] [L = (Im), M =
Cr (25), Co (27), Ni (28), Cu (29); L - (BIm), M = Cr (30), Co (32), Ni (33), Cu (34)]
indicate a distorted trigonasl-pyramidal geometry around metal ions. However, for the
"Coordination chemistry ofRgands with ekaron rich %OorS Biting centre' <DoctoraC thesis st^mitted to Sgarh MusRm University, I!N^I^
chapters 104
complexes [FeLC^] [L ~ (Im) (26); (BIm) (31)] an octahedral geometry around metal
Fe^* ion has been indicated. The MOPAC plots for the adduct complexes
[Co(BIm)(Phen)Cl]Cl (35) and [Ni(BIm)(Phen)Cl]Cl (36) provide an octahedral
geometry around the metal ions {Figure 3.27).
'Coordination cHemistry ofRgands witt electron rich % O orS biting centre" <Doctora[thesis suSmittedto^Rgarii 94usCm "University, NWIJi
CfiapterS 105
Scheme:-
Figure 3.20: Mechanism of the formation of ligands (Im) and (BIm).
'Coordination cfiemistry ofdgands witH electron ricfi% 0 orS Biting centre' (Doctoralthesis suSmittedtoJi^aHi MusSm University, HffiDIA
copters 106
yH^
^
Figure 3.21: Perspective view of the ligand (Im).
#2+ Figure 3.22: Perspective coordination geometry around M ion in lM(Im)Cl2] • C; • H; • N; • M^ ; # CI.
"Coordination cdemistry ofBgands witA ekctnm ricA % O orS Biting centre' (Doctoral thesis submitted to Rf^arh 9AusGm Vniversity, lyfiDI^
CHapterJ 107
Figure 3.23: Perspective coordination geometry around Fe^* ion in fFeflmJChJCl
• C; • H; • N; • Fe %- • CI
'Coordination cHemistiy ofGgands with electron rich % O orS Biting centre' (Doctoral thesis sudmittedtoJiSgarh !MusGm Vniversity, HN'DJJi
Cliapter3 108
Figure 3.24. Perspective perspective view of the ligand (BIm).
Figure 3.25. Perspective view of [M(BIm)2Cl2] complexes.
#C; •H;#N;#M2";ta.
'Coor£nation ctiemistry cfSgands witH electron ricH % O orS Siting centre" (Doctor(^ thesis submitted to A^afh!MusRm University, J9fiDI^
CfiapterJ 109
Figure 3.26: Perspective coordination geometry around Fe ion in [Fe(BIm)Cl3]
t2+ Figure 3.27: Perspective coordination geometry around M ion in
lM(BIm)(Phen)Cl]Cl (M = Co^^ andNf^) # ; • H ; # N ; # M 2 ^ ; # C 1 .
'Coordination cfiemistry ofBgands witfi ekctron rich % O orS Biting centre' (Doctoral thesis submitted to Ggark Muslim University, I!M)IA
Chapters 110
Table 3.1:- |j.eff (BM) and electronic ligand field spectral data of the compounds with
their assignment.
Compounds
25
26 •
27
28
29
30
31
32
33
34
Meff(BM)
4.6
5.9
3.0
2.5
1.9
4.8
6.1
3.5
2.8
2.0
v(nin)
569
498
507
645
493
552
630
440
660
582
632
580
482
503
640
490
550
620
455
630
545
630
Band positions
V (cm'')
17,574
20,000
19,700
15,500
20,283
18,115
15,873
22,727
15,151
17,182
15,822
17,240
20,740
19,880
15,625
20,408
18,181
16,129
21,978
15,873
18,348
15,873
Band Assignments
'^2g ^ ^g
%^%(4G)^'AJ,
%,(G)^%,
''T2g,''T,g(G)<^ '^Aig
^E" (PJ^U'^fF)
%'(P)^U'2,(F)
'E'(F)^U'2,(F),
'£"•^'4'
'E'^'A'
B,g
Eg*- Big
T2g <- Eg
%/Eg(4G)^'A,g
%g(G)^%g
•*T2g,''Tig(G)<^ Uig
'E" (P)^U'2,(F)
%'(P)^U'2,(F)
'E'(F)^'A'2.(F),
'E"^'A'
'E'^'A'
B2g<r- Big
Eg*- Big
EPR signal
gii g-i-
-
-
-
-
2.17 2.05
-
-
-
2.16 2.04
"Coordination chemistry ofRgands with electron rich % O orS Siting centre" (Doctordthesis suSmitudtoJiGgarh MusRm "University, ZW®/
CfiapterS 11
Table 3.2: Important calculated bond lengths (A") and bond angles (degree) in the (Im)
and their complexes.
Bond length
d(C-Na„,)
d(C=Nin,)
d(C-N™)
d(M-Nan,)
d(M-N™)
d(M-Cl)
Bond angles
(C-Na^-C)
(C-C=N,,)
(C-C-N,,)
(N™-M-N.„,)
(Nam-M-Nin,)
(Nan,-M-Cl)
(N™-M-C1)
(Cl-M-Ci)
(Im)
1.49
1.29
1.37
-
-
-
111.73
123.31
124.08
-
-
-
-
-
25
1.50
1.26
1.37
1.77
1.74
2.28
115.46
114.95
130.59
101.95
89.53
104.45
85.07,
156.60
78.81
26
L52
1.26
1.37
1.82
1.846
2.160
118.6
117.5
138.8
90.0
90.0
89.6
89.4,
179.2
89.3
27
1.49
1.27
1.37
1.80
1.75
2.28
113.47
117.69
128.20
118.09
85.98
97.70
94.07,
121.95
82.26
28
1.56
1.28
1.39
1.79
1.74
2.21
120.84
117.09
129.43
100.28
93.04
109.79
87.47,
145.58
81.24
29
1.56
1.28
1.39
1.83
1.77
2.35
120.16
116.83
128.73
119.66
87.38
99.28
91.20,
123.42
77.64
am.=amine nitrogen, Im.=Imidazole nitrogen.
"Coordination cHemistty ofBgands witi electron rich % O orS Biting centre" <Doctora(thesis suSmittetftoJlGgarh iMuslim University, ly/tDJJi
CHapterS 12
Table 3.3: Important calculated bond lengths (A°) and bond angles (degree) in the (Blm)
and their complexes.
Bond length
d(C-Na„)
d(C=N™)
d(C-N,n,)
d(M-Na.)
d(M-N™)
d(M-Cl)
d(M-Nphen)
Bond angles
(C-Nan,-C)
(C-C=Ni™)
(C-C-Ni,)
(Ni„-M-Ni^)
(Nan,-M-Ni„)
(Na„-M-Cl)
(Ni„-M-Cl)
(Cl-M-Cl)
(Cl-M-Nphen)
(Nim-M- Nphen)
(Blm)
1.47
1.35
1.35
-
-
-
-
111.1
125.1
123.2
-
-
-
-
-
-
-
30
1.80
1.31
1.34
1.80
1.74
2.28
-
113.9
116.5
132.2
96.0
87.0
108.3
84.8,
163.4
80.4
-
~
31
149.6
1.31
1.34
1.82
1.76
2.20
115.2
115.5
131.4
89.8
89.7
89.6
89.8,
180.2
89.7
32
1.50
1.31
1.34
1.80
1.74
2.28
-
113.2
116.8
132.0
98.5
86.9
120.1
85.1,
155.6
80.3
-
"
33
1.50
1:31
1.34
1.80
1.74
2.20
-
113.4
116.4
132.3
97.9
86.6
125.9
84.9,
160.2
79.8
-
~
34
1.50
1.31
1.34
1.80
1.74
2.20
-
113.6
116.6
132.2
97.6
86.9
121.2
84.7,
159.2
80.2
-
"
35
1.48
1.32
1.34
1.84
1.76
2.20
1.80
115.2
115.1
131.4
123.3
79.3
146.6
92.2
-
83.7,
105.8
77.6,
155.7
36
1.49
1.32
1.34
1.81
L76
2.20 '
1.79
112.6
116.5
131.2
163.7
82.2
89.2
89.3
-
85.7,
166.2
84.6,
100.3
am.=amine nitrogen, Im.=Imidazole nitrogen, Phen. = 1,10-phenonthroline nitrogen.
"Coordination chemistry ofBgands with electron rich % O orS Biting centre' (Doaoraf thesis suBmittecftoJiGgarh MusRm University, I!N>DIJi
CHapterS 113
REFERENCE
1. R. J. Sundbug, R. B. Martin, Chemical review, 74, No.-4 (1974).
2. J. Reedijk, Reel. Trav. Chem, 88, 1451 (1969) and references cited therein.
3. Coryell, L. Pauling, J. Bio. Chem. 132, 769 (1940).
4. G. M. Badger, ''Aromatic character and aromaticity" Cambridge University Press,
London, (1969), pp. 1-37..
5. W. Adam, A. Grimison, Terahedron. 22, 835 (1965).
6. J. H. Ridd, ''Physical Methods in Heterocyclic Chemistry.^' Vol. I, A. R. Katritsky,
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Academic Press, New York. (1963), vol. I.
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143 (1996); L. Douce, R. Ziessel, R. Seghrouchin, A. Skoulios, E. Campillos, R.
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(1995).
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'Coorcftnation cfiemistry of Glands wtk ekctron ricfi % O orS Biting uittre' OoctordtHesis suSmitted to JiRgarfi MusRm University, I!M>IJi
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Dalton Trans., 3727 (1996) and references cited therein.
'Coordination cRemistty ofSgands -with eUaron ricfi%0 orS Biting centre' (Doaord thesis submitted to Migarh MusRm "University, IM)IJl
chapters 117
82.1.M. Proctor, B.J. Hathaway, P. Nicholls, J. Chem. Soc, (A) 1678 (1968).
83. F. Lions, I.G. Dane, J. Lewis, J. Chem. Soc. (A) 565 (1967).
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Paper- AATCC, International Conference and Exhibition, Atlanta, Georgia (1995).
"Coordination chemistry ofGgands witH elearon Ticli%0 orS Biting centn' <Doctora[thesis suSmitteSto^Rgarh MusGm University, JWDIJi
CHaper 4
Fe - Mossbauer Spectral studies of Fe -complexes
[Fe2L^Cl4]Cl2 (2), [FeL^CyCl (10), [FeL^ChjCl (18),
[Fe(Im)Cl3] (26) and [Fe(BIm)Cl3] (31)
Cfiapter4
INRTRODUCTION
The importance of the phenomenon to chemists emerged out form the observation
that the positions of the Mossbauer resonance peaks are a function of the electron density
around the Mossbauer nucleus. Mossbauer resonance phenomenon is the emission and
absorption of y-rays without the loss of energy due to recoil of the nucleus and without
the thermal broadening [1]. It was realized that the Mossbauer resonance line is
extremely narrow and allows hyperfme interactions to be evaluated in a rather straight
forward way, all disciplines in natural science enjoyed a boom in the applications of this
effect. The positions of the resonance were found to depend on the electron density
around the Mossbauer nucleus.
Mossbauer spectroscopy has been used as a key to unlock some physical,
chemical and biological phenomena. It has also been used as a guide for finding new
ways solving applied scientific and technical problems. The continuing rapid growth in
the number of publications can be illustrated by comparing the size of the annual
Mossbauer effect data indices. In Mossbauer spectrum, we try to know the physics of the
hyperfine interactions and relevant parameters. The chemically interesting informations
are measured in terms of isomer shifts and quadrupole splitting. These informations
provide knowledge about the bonding and structural properties of the molecules
containing Mossbauer nuclei.
ELEMENTRY PRINCIPLE
Resonance scattering of gamma rays:-
A stationary decaying nucleus must recoil to conserve linear momentum. The
conservation of energy requirement is that the emission energy would not be same as the
transition energy. Similarly, a free stationary nucleus cannot absorb the radiation, emitted
by a source having exactly the same nucleus transition, due to the energy loss in the
recoil process. On the other hand, if the recoil energy loss of the nucleus is eliminated,
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Cfiapter4 119
i.e. the energy of the incoming photon is exactly equal to the nuclear transition energy,
then it can absorbed to give resonance [2].
On the basis of kinematic relation, it can be shown that the recoil energy, ER is
given by En = (1/2) Mv = Ey/2Mc , where M is the mass of the recoiling nucleus and c
is the velocity of light. Such a recoil energy loss is small, for a gamma ray of 100 KeV,
with a nucleus of mass number 100, it is only 5x10 ' eV. This energy loss does become
significant when it is compared with the inherent width of the gamma radiation, which
arises from the finite life time of the excited state. The uncertainty in energy, defined as
the width Y of the gamma radiation and the uncertainty in the mean life time t of the
nucleus are related by the Heisenberg uncertainty relation as below:
AE.At = hor TsT^h.
or r=0.693 h/tii2, (r = In2x tm)
For h/2= lO 's, the line width is F- 4.6 x 10" eV, which is very smaller than the energy
lost in the nuclear recoil (« 5.0 xlO" eV). As, a result, the gamma-ray emission line does
not overlap with the absorption line and thus the nuclear resonace absorption is not
observable.
The energy distribution due to Doppler motion can be estimated from the ideal
gas theory. The mean translational energy per degree of freedom of an atom in a gas at a
temperature T is given by
Ek = 'A MV\ = 'A keT
Where ks is Boltzmann constant, T absolute temperature and Vx is the mean square
velocity of the atoms in the direcfion of motivation. The mean of the Doppler effect (ED)
for the randomly moving atoms gives an estimate of the Doppler broadening (DT ) of the
energy distribution i.e.
_ _ j^
DT = ED= Mvivly = ^lE^Mv^ = 2^E^E^
The Doppler broadening is proportional to the square root of the product of thermal and
recoil energies. Thus, the y-ray distribution is displaced by ER and broadened by twice the
geometric mean of recoil energy and average thermal energy.
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Cfiapter4 120
Recoil free fraction ofMossbauer radiation:-
For nuclei bound in solids, tiie recoil energy and momentum is shared by whole
mass of the crystal and thus the energy loss becomes negligible. When the recoil energy
is above the threshold displacement energy (15 to 30 e O the atom will be dislodged from
its lattice sites. If ER is larger than the characteristic of the lattice vibration (Phonon
energy) but less than the displacement energy, the atom will still remain at its lattice site
and this energy gets dissipated by heating the crystal. If the recoil energy is less than the
phonon energy (~10" eV), lattice being the quantized system can not be excited in an
arbitrary fashion, the transition energy is radiated intact (Zero - phonon transition). It can
be shown quantum mechanically that for an Einstein solid characterized by 3N vibration
modes (N being the number of atoms in solid) each having the same frequency o , the
mean energy transferred to the lattice per emission is exactly the free - atom recoil
energy [3]. Neglecting the infrequent of multiphonon process, the mean recoil energy of
single phonon process becomes
ER ER = (1 -f) h CD whenf= 1
/ = the recoil energy and only those events five rise to the Mossbauer effect.
The Mossbauer instrumentation involves:
(a)- Single line source
(b)- Absorber or sample preparation
(c)- Detector
(d)- Mossbauer drive
(e)- Data acquisition and
(f)- Cryogenics with temperature controller and other external perturbations like
hydrostatic pressure, magnetic field, etc..
The source used in the Mossbauer experiments is not an ordinary radioactive
source. There are a number of criteria to be taken into account in the preparation of
source. These are several different nuclear transitions leading to the excited nuclear level
of practical interest. One should preferentially choose the one, which leads to the highest
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Cliapter4 121
intensity of the Mossbauer quanta and has the longest half-life of the precursor nuclei.
The gamma ray energy spectrum of the source should be as simple as possible to avoid
background counts. Furthermore, the chemical composition of the source material should
be such as to produce high-spin emission line as intense as possible [3] (Figure 4.1).
Cn-aiin days)
0.16%
complex
3C+
1/2+
Fe-57
EC 99.8%
706.4 keV
366.8 keV
136.3 keV
14.4 keV
Okev
Figure 4.1. Decay scheme of Fe' 57
The effective thickness of the absorber is defined as tA = aodAnafAa,, where QQ is
the maximum absorption cross-section, dA the physical thickness of the absorber in cm,
Ha the number of atoms/cm^ of the particular element, fA the recoil-free fraction of the
absorbing material and aa the isotropic abundance of resonance isotope. If the thickness
tA»l , the absorber line shapes get broadened with the large deviation from Lorenzian
shape. However, if tA«l the line shapes could be approximated to an ideal Lorenzian
line. Therefore, the best compromise in the preparation of the absorber is to have
thickness around ts~\.
In Mossbauer spectroscopy three types of detectors are generally used for the
detecting the low energy y-rays; (a)- proportional counters, (b)- scintillation counters
and (c)-solid state detectors. The scintillation detectors are frequently used for the y-ray
with energies in the range of 50-150 keV. The energy resolution with scintillation
detectors is very poor. Below 40keV, the gas filled proportional counters is generally
used because it has very good resolution in the energy range 1 -20 keV. However, it has
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Cfiapter4 122
low electron efficiency. The solid-state detectors like lithium-drifted germanium give
well-resolved energy spectrum.
The most convenient velocity modulation technique is based on the Doppler
effect. The purpose of motion device is to provide energy modulation to examine a region
of the spectrum very near to the unperturbed energy of the gamma ray. Most of the
velocity drives used fall into two categories. These are constant velocity drive and
acceleration drive types. In the constant velocity drive [4,5] systems the Mossbauer
spectra are recorded by counting the transmitted y-ray photons for a fixed period of time
at each velocity. Several constant velocity spectrometers have been developed on various
principles such as mechanical, electro-mechanical, hydraulic, etc. However, control
velocity mode is essential for the purpose of dual modulation technique. In the constant
acceleration mode [6,7] the source or absorber is made to pass through all the velocity
intervals required until the spectrum is recorded in a short interval of time -50 ms. The
process is repeated until the spectrum is obtained to required statistical accuracy. This
method necessitates the use of a multi-channel analyzer (MCA) or a data storage device
like a small electronic computer. The MCA can be used in pulse height analysis (PHA)
modulation mode or multi channel scaling (MCS) mode (time mode). Each channel
corresponds to a particular velocity interval. In PHA mode the instantaneous velocity
signal obtained from the pick up coil is used to modulate the pulses from the single
channel analyzer.
In the MCS time mode each channel of the analyzer is opened/closed sequentially
at a clock-controlled rate. An external oscillator, whose frequency is n (number of
channel) times the frequency of the driving voltage, scans through the channel of the
analyzer and the periodic velocity of the source or the absorber is kept in
synchronization. The total time spent per channel can be set very accurately. If the
velocity is a linear function of time, then a linear relationship between the channel
number and the velocity can be achieved. However, since source velocity is not directly
measured, knowledge of the velocity waveform calibration is required in order to
calibrate the velocity scale, which is generally achieved from the standard known
transition spectrum.
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Cfiaj>ter4 123
The Debye-waller factors increase with the decrease of temperature. Mossbauer
studies are specifically for those transitions for which f-factor at room temperature is very
small. Many of the substances may show magnetic ordering at low temperature and there
by the low temperature facilities are useful for investigation of relaxation effects and the
isomer shift precisely etc.
Mossbauer effect and its Importance: -
When the source of radioactive parent isotope decays to the excited state of the
Mossbauer isotope the subsequent radiation emission gives rise to the gamma radiation
from the parent nuclide. When the source is embedded in a solid matrix, the gamma
transition may be of recoilless process while decaying to the ground state. The
transmitted intensity of this radiation through the resonance absorber is measured as a
function of the gamma ray energy, which can be varied by Doppler shifts being achieved
through moving the source, mechanically, relative to the absorber, i.e. AE ofv) = Ey(l +
v/c), where v is the relative velocity between the source and absorber. The Mossbauer
spectrum is obtained by recording the intensity of transmitted gamma rays versus the
Doppler velocity of source.
Mossbuaer spectroscopy has a number of advantages over other methods because
it is more selective and it singles out a specific isotope such as Fe etc. Properties of the
surrounding environment can be studied, although indirectly, via hyperfine interactions
[8]. Mossbuaer spectroscopy can be used as an analytical tool to determine the total
amount of a given isotope, to check the in homogeneity of the sample or to identify the
various components it may contain [9]. The line width of Fe is 4.5 x 10" ev and the
quality factor Q = A E / Ey = 4.5 x 10" / 14.4 x 10' gives a high sensitivity of about 3
parts in 10'\ A few other isotopes known for their high selectivity are Ta'*' (Q = 10'*)
and Zn (Q = 2 x 10 ) but they are too difficult to perform experiments [10] with. Thus,
the Mossbauer resonance furnishes important information about the very small
perturbations.
The limitations of the method are obvious, too. The number of useful isotopes is
relatively small. In biological studies, in particular, the only nuclei of immediate
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chapter 4 124
significance are l' ^ and l' . Otlier limitations are the fixed energy resolution determines
by the nuclear lifetime and the fact that only immediate environment of the Mossbauer
isotopes can be studied. A further serious restriction is that the system must be in the
solid form.
Hyperfme interaction in the Mossbauer effect [3]:-
In the Mossbauer effect, the line width of the nuclear transition is small and the
resonant absorption is extremely sensitive to energy variation of the gamma radiation.
Therefore, the minute interaction between the nucleus and the orbital electrons, which are
not observed generally by other methods, can easily be observed in the Mossbauer effect.
It is thus the influence of the electronic environment on the nuclear gamma transition,
both in emission and absorption, which determines the hyperfine structure of the
Mossbauer radiation.
The principal interactions in the Mossbauer effect concerned to chemists are; -
•* Nuclear Isomer - Shift
• Nuclear Quadrupoie Interaction
• Nuclear Magnetic Hyperfine Interaction
Isomer-Shift [W]:-
The nuclear electric charge is extended in space and is surrounded by the orbital
electronic cloud. We know that the s-electrons are nearest to the nucleus. Therefore, the
electrostatic interaction is difficult for point charges and extended charge distributions. A
charge in the s-electron density will result in an altered coulombic interaction, which
produces a shift of the nuclear energy levels. This effect could be called as the 'electric
monopole interaction'. The effect depends on the difference in the nuclear radii of the
ground (Rgd) and isomeric excited (Rex) states and is known as the 'isomer shift'.
The energy of the gamma ray represents the different electrostatic energy of the
nucleus in two different states of excitation, differing only in nuclear radius. The effect of
the electric monopole interaction is to shift nucleus levels with out lifting the spin
'Coordination cHemistry ofGgands with ekctfon rich % O orS Biting centre" (Doctoralthesis suSnitted to Rgarh Mus&n University, ISN^DIA
Cfiapter4 125
degeneracy. These shifts are very small compared to the total energy of the gamma ray
and generally of the order 10''^ Ef.
Quadrupole interaction [11].'-
In the case if isomer-shift the effect of the electrostatic interaction between
nuclear
and electronic charge is derived by assuming the nucleus to be spherical and the charge
density to be uniformed. If these conditions are relaxed, other effects appear which are in
fact higher order terms in the multipole expansion of the electrostatic interaction. These
terms do not shift the nuclear levels, they split them, i.e. they lift all or part of their (21+1)
fold degeneracy (where I is the nuclear spin quantum number).
The second non-vanishing term of the electrostatic interaction of a nucleus with
its surrounding electronic charge is the quadrupole coupling. This is the result of the
interaction of the nuclear quadrupole moment with gradient of the electric field due to
other charges in the crystal etc. The nuclear quadrupole moment reflects the deviation of
the nucleus from spherical symmetry. An oblate (flattened) nucleus has a negative
quadrupole moment while a prolate (elongated) one has a positive moment. Nuclei whose
spin is zero or Vi (I = 0 or Vi) are spherically symmetric and have a zero quadrupole (AEQ
= 0) moment. Thus the ground state of ^^Fe with 7 = 1 / 2 cannot exhibit nuclear
quadrupole splitting. However, the excited state has / = 3/2 giving the quadrupole
splitting of this energy level.
M6SSBAUER INSTRUMENTATION
The instrumentation required for the measurement of Mossbauer effect is
fundamentally that encountered in y -ray spectroscopy. The Mossbauer experiment can be
performed either in transmission geometry or in scattering mode. In both the methods y-
ray photons are either transmitted through the absorber or scattered by the scatterer,
which are counted as a function of the relative velocity between source and absorber.
However, the former technique (transmission geometry) is usually employed. The
physical configuration in transmission geometry is shown in Figure 4.2.
'Coordimtion ciemistiy ofGganJs with ekctron rich % O orS Biting untn' QoctoraCthesis suhmittedto^Ggarh MusGm Vniversity, HffiDlH
chapter 4 126
Drive
Velocity Servo Meter
i Absorber
Detector
irr
Signal Ge net ate r
PC with MCA Card
s> Pre-Amp
\ p ^
\ 7
Signal Channel analyser
Figure 4.2. Block diagram of a typical Mossbauer Spectroscopy
An accurate measurement of the relative velocity scale between source and
absorber is required. For this purpose the spectrum of a-Fe is used for the velocity
calibration [12]. The conversion has been adopted so that positive velocity is defined as
the motion moving away from each other. In plotting a Mossbauer spectrum, for the sake
of convenience, the measured counts of y-radiation in usually normalized to the off-
resonance count rate resulting in relative transmission.
Data processing:-
The raw experimental data acquired as a function of velocity is analysed to
evaluate nuclear hyperfine parameters like, isomer shift, quadrupole interaction and
internal magnetic moment, in addition to other parameters. The velocity scale calibration
was carried out using a-iron and stainless steel absorbers as standards. Various curve
fitting like NORMOS, Moss Win, Recoil, etc., were used.
"Coordination chemistry ofGgands with ekctron rich !M,OorS Biting centre" (Doctoratthesis suhmittedto JiGgarh MusHm "University, I9^IJi
chapter 4 127
EXPERIMENTAL
Method:-
Required instrument for the measurement was designed at UGC-DAE
Consortium for Scientific Research, Indore center. The Mossbauer measurements were
made with a standard PC-based system operating in the constant acceleration mode. The
velocity drive was calibrated using 57-Co source using a 25 |am thick natural iron foil as
an absorber. The sextet of a-iron was useful in calibrating the center of gravity of the
spectrum and velocity calibration constant, which are essential for the analysis of the
spectrum. The velocity calibration was carried out using the ^^Fe-Mossbauer
spectroscopy of a - iron. The experiment was performed at room temperature about ~300
K when the absorber was kept stationary and the source device was moving with a
constant velocity (lOmm/sec). All the spectra were taken without an applied magnetic
field. The spectrum was fitted with NORMOS program 1990 for its solution.
"Coordination chemistiy ofSgands witH ekctnm rich%OorS Biting centre' <Doctora[tliesissu6mitudtoJiGgaHiMuslm Vniversity, HMDIJl
chapter 4 128
RESULT AND DISCUSSION
The FT-IR spectral informations coupled with the Mossbauer spectral
investigations of Fe " complexes can be used to establish the molecularity of Fe"^-
complexes. The important Mossbauer parameters which include chemical shift (8),
quadrupole splitting (AEQ), line-width, and line-width rations (%) usually provide
additional evidences to support the molecularity assignments as following:
The representative Mossbauer spectra of the complexes [Fe2L'Cl4]Cl2 (2),
[FeL^CbjCl (10), [FeL^Cl2]Cl (18), [Fe(Im)Cl3] (26) and [Fe(BIm)Cl3] (31) recorded at
300 K are shown in Figures 4.1 -4.5 and the various Mossbauer parameters obtained
from the computations of the spectral data have been summarized in Table 4.1. Least
square fitting employing Lorenzian line shape was used for the spectral plot which
clearly indicated that the spectra contained only one type quadrupole split doublet. This is
consistent with the presence of an identical oxidation state of the Fe nuclei in the
present complexes. The magnitudes of the isomer shifts (6) for the complexes {Table 4.1)
and the observed quadrupole splitting parameters (AEQ) are in the range reported [3] for
the various high-spin state Fe ^ nuclei (t2g'',eg) in hexa-coordinate environment of the
ligands. This indicates that the present complexes contain Iron in the high-spin (S = 5/2)
rather than the low-spin (S = 3/2) state. The quadrupole doublet in each case has nearly
the same areas and half widths. The peak height ratio (HWh/HWi) for the homo-
bimetallic complex [Fe2L'Cl4]Cl2 (2) is higher than that of the mono-nuclear complexes
[FeL^CyCl (10), [FeL^CbJCl (18), [Fe(Im)Cl3] (26) and [Fe(BIm)Cl3] (31). ft is
interesting to note that the magnitude of the peak height ratio (HWh/HWi) for the
complex (2) is characteristic of the dimeric species like (Fe-Salen.Cl)2, (Fe-
Salen)2MeN02, (Fe-Salen)2CH3CN and (Fe-Salen.Cl)2.CHCl3 etc. [13]. However, the
observed peak height ratios (HWh/HWi = 0.8) for the complexes (10), (18), (26) and (31)
are comparable to that reported [14] for the monomeric Fe "*" complexes (Fe-
salen.Cl)MeCN for a given Fe salen an spin state. The magnitude of isomers shifts for
Fe " complexes usually decrease in the order: eight-coordinate > six-coordinate > four-
coordinate [14]. The magnitude of 5 for six-coordinate and four-coordinate complexes
'Coordination cHemistty ofEgands with ekctron ricfi%0 orS Siting untre" <Doctora[tfiesis suSmitUcfto^Bgad Muslim University, HN^IA
Cliapter4 129
are nearly 0.6 and 0.4 mms'', respectively [15]. However, the isomer shift for the present
six-coordinate Fe ^ complexes are in the range 0.4 - 0.5 mms"'. It is well known that as
the extent of distortion from octahedral symmetry increases the isomer shift for six-
coordinate Fe ^ containing compounds deceases and attains a magnitude near to that of
the four-coordinate geometry (8 ~ 0.4 mms"'). There is a difference in quadrupole
splitting for the mono-nuclear species compared to that for the dinuclear complexes.
Usually AEQ is high for the dinuclear or dimeric species compared to that for the
monomeric (mononuclear) complexes [13,3]. The observed peak height ratio (HWh/HW|)
and the magnitude of quadruple splitting (AEQ) for the present complexes therefore,
indicates a di-nuclear nature for [Fe2L'Cl4]Cl2 (2) and a mono-nuclear nature for the
complexes [FeL^ClzJCl (10), [FeL^CyCl (18), [Fe(Im)Cl3] (26) and [Fe(BIm)Cl3] (31).
The observed nuclear excitations is from high energy state (I = ± 3/2) to the low
energy state (I = ± 1/2) of the Iron nucleus. The spectra did not show any magnetic
splitting on application of an external magnetic field. Henceforth, the effective internal
magnetic field at the Mossbaure nuclei is either absent or it is too low to show any
significant interaction and the spin state remain doubly degenerate (i.e. presence of
Kramer's degeneracy). The quadrupole splitting in high-spin Fe(III) complexes with
t2g Cg ground electronic configuration arises mainly due to the presence of asymmetry m
the ligand field. Henceforth, the presence of quadrupole doublets in each case confirm the
presence of ligand field asymmetry. This phenomenon is more pronounced for bimetallic
Fe^^ complex [13] to give a relatively high magnitude of AEQ for complex (2). However,
the nearly symmetric nature of the quadrupole doublets rules out the presence of not only
the fluctuating electric or magnetic field near Fe(III) nuclei but also the possibility of a
spin-lattice relaxation.
Asymmetry in the shapes rather than in the areas of quadrupole doublet
components can arise from paramagnetic relaxation in the ion association with
Mossbauer nucleus. Treating the effects of the hyperfine interactions as an effective
internal magnetic field, Blum [16] showed that asymmetric quadrupole doublets are
possible when the ionic fluctuating rate is comparable with the precessional frequency of
the nucleus in this internal field. For metal ions having S ground term (i.e. high-spin
"Coordination cfiemistiy ofSgands with electron rich % O orS Biting centre" <Doaora[thesis suSmittetftoJiGgaHi MusRm "University, HN^IJi
Cliapter4 130
Fe " ) the fluctuating field is mainly due to spin-spin relaxations i.e. spin-lattice relaxation
can be neglected [17,18]. The presence of magnetic exchange i.e. intra-molecular anti-
ferromagnetic interaction between Fe ^ ions in the dinuclear complex (2) and inter-
molecular anti-ferromagnetic interaction between the neighbouring Fe " ion in mono
nuclear complexes (10), (18), (26) and (31) may enhance the mechanism of the spin-spin
relaxation in these molecules.
The peak position (5) is in the positive side of the zero velocity indicating that the
s-electron density on the Fe(III) nuclei in the complexes [3,17] is smaller relafive to that
on the source ^ Fe nucleus. However, it is in the negative direction (or smaller) compared
to that reported [19] for mononuclear Fe(III)-[N4] macrocyclic complexes. This suggests
that the present macrocyclic ligands behave as a a-donor, which probably enhances the
effective s-electron density at the Fe(III) nuclei. The d-electrons on the metal ions can
also influence the magnitude of s-electron density at the nucleus through s-electrons
screening. The unsaturation in the macrocyclic moiety [19] due to the presence of iminic
(C=N) bonds, which enhances the 7i-accepting behaviour of the ligands through a d -p t
back bonding may also affect the magnitude of isomer shift (6).
'Coor£nation chemistry qfSgands witH electron ricfi%OorS Biting centre' (Doctoralthesis suBmitud to Rgarh Muslim "University, UMilJi
Cfiapter4 13]
1.005
1 000
c o w 03
E c 03
H <1i >
0.995
0.990
0.985
0 980
0.975
0.970
velocity(mm/s)
Figure 4.3. Mossbauer spectrum of the complex [Fe2L'CUJCh (2)
1,002
1.001
1.000
c o 'w w
1 CO
ro 0 999
>
^ ^ ^ ^ ^ i ^ % p
j5 0 998-
0,997 -
0 996-
0 995
0
o o Q o
6 .<Sj&iigfe«ee
%*^%° 0 ^ " ' °
-10 10
Velocity (mm/s)
F/gMrg 4.4. Mossbauer spectrum of the complex [FeL^ChJCl (10)
"Coordination chemistry ofSgands wkH e&ctron ricfi%OorS Siting antre" <I>octora[tliesissu6mittedtoA^arhiMusBm University, im>IA
Cfuipter4 132
1010-
1 005-
g 1.000-CO
m 0.995 c (0
ro
0.990
^ 0.985-
0.980 •
0.975 • -10
O (ft 5 <3 O ^$^ pOoO
-5 0
velocity(mm/s)
10
Figure 4.5. Mossbauer spectrum of the complex [FeL^Cl2]Cl (18)
005-
1.000
g 0.995
i 0 990 c CO
0,985 -H (D >
0) 0,980
0.975
0 970.
oo 8
-10
w^im s^:^Mm O O CD o o o ° ooo o
~1 10
velocity{mm/s)
Figure 4. Z Mossbauer spectrum of the complex [Fe(Im) ChJCl (26)
'Coordination chemistry ofKgands with electron rich%0 orS Siting centre" (Doctoralthesis suSmittecfto^Egarh Musim Vnivenity, IM)IA
Cliapter4 133
1.006 •
1.004 •
-10 -5 10
Velocity (mm/s)
Figure 4.7. Mossbauer spectrum of the complex [Fe(BIm) ChJCl (31)
"Coordination cfiemistiy ofGgamCs with eCearon ricli%0 orS Siting centre" (Doctoralthesis suhmittedto ARgath MiuCm Vniversity, I9^IJl
^
C3
0^
00 00 00
o
C/2
C3 0)
(N 00
o
\o
o o o d -H
t- i2
o o
m o o o d
00 oo VO m o o d
en o o o d -H
(N
o d
o o p d
00
o d
O N o o O d -H
o o
-t-»
o
ON
ON
in
00
in <N ON
d
O N OO
IS
a.
3
a on :0
[in
c
O
• 4 - *
I
C
00
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00
m
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UJ
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a
'— 'ui
s s
c w <
VO 0 0
i n
^ ^
IS
<u
s o
E
CO
(U X
"a S o U
«n m tN (N
q d -H
in in 00 (N
O d 4
00
in
d
o ON
en d
m ON 00 O O
d -H
O
<N (U
en o OS in
q d -H
O
cn
q d -H
(N ON
m en
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rsi 00
o d -H
o r—1 (N
u s ^ (U
UH
o 1-H \ w ^
O 1
r
u •o ^ OJ
U H
00 l>
CN
m
d
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(N cn in (N
NO tN O O d -H
m <N (N O
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m o VO o (N «n
00 CN VO 00 t--
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en in ON (N m o
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en CN o o d -H
CN m NO in en
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CN en Tt-00 o O d -H
ON o m 00 ' VO
o " o o in
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Chapter 4 135
REFERENCE
1. R.L. Mossbauaer, Z Physik. 151, 124 (1958).
2. K.G. Malmforms, ''Resonance Scatting of Gamma - rays" in Beta - Gamma ray
spectroscopy edited by K. Siegbahn, North Holland Amsterdam (1964).
3. N.N. Greenwood, T.C. Gibbs, "Mossbauer Specteoscopy", Chapman and Hall
Ltd., London (1976).
4. H. Frauenfelder, ''Mossbauer Effects'", W.A. Benjamin Inc., New York (1962).
5. Mossbaer Effect Methodology, V-1, ed. LJ. Gruverman, Plenum Press, New York
(1965).
6. E. Kankeleit, Rev. Sci. Instr., 35, 194 (1964).
7. K. Rama Reddy, Ph.D. thesis, Department of Physics, Carnegie Institute of
Technology, Pittsburg, Pennsylvania (1964).
8. R.V. ?o\xndi, phys. Rev., 79, 685 (1950).
9. I.I. Gruverman, Mossbauer Effect Methodology, vol. I and VII, Plenum Press,
New York.
10. D.D. Graining, D.E. Nagle, D.R.F. Cocharan, Phys. Rev. Lett, 4, 561 (1960).
ll.G.K. Wertheim, Mossbauer "Effect, Principal and application" Academic Press
INC. (London) Ltd. (1968), pp. 49.
12. U. Gonser (ed), ''Mossbauer Spectroscopy I and IF Topic Appl. Phys. Vol. 5 and
Topic Current Phys., vol. 25, Springer, Berlin, Heidelberg, (1975) and (1981).
13. G.M. Bancroft, A.G. Maddock, R.P. Randl, J. Chem. Soc. (A), 2939 (1986).
14. G.M. Bancroft, R.G. Burns, A.G. Maddock, Geochim. Cosmochim. Acta, 31,
1967(1967).
15. G.M. Bancroft, A.G. Maddock, W.K. Ong, R.H. Prince, J. Chem. Soc, 723
(1966).
16. M. Blume, Phys. Rev. Lett., 14, 96 (1965).
17. M. Blume, J. A. Tjon, Phys. Rev., 165, 446 (1968).
18. M. Blume, Phys. Rev. Lett., 18, 305 (1967).
19. H.S. Mainford, D.B. Mac-Queen, A. Ali, J.W. Otros, M. Calvin, R.B. frankel,
L.O. Spreer. Inorg. Chem. 33, 1748 (1994).
"Coordination chemistry of Glands with electron rich % O orS Siting centre" (Doctoral thesis suBmitted to JlUgath VtiusGm University, lyfDIJi
CHapter 5
Cyclic-Voltammetric (CV) and Conductometric Investigation
on the Solution of the Present Homo - Bimetallic and Mono -
Nuclear Complexes
CHapterS 136
INTRODUCTION
Conductometric Studies:-
The informations regarding the selectivity and the ionic associations in metal ion
encapsulated macrocyclic moieties can be obtained through investigations of the
electrical conducting behaviour of these complexes in suitable solvents. These
informations have been utilized.by some workers [1] to explore the suitability of
macrocyclic complexes to ion-selective electrodes (ISE). The ionic transport behaviour
associated in a polymeric membrane of ISE depend largely on the selectivity as well as
the stability of such macrocyclic complexes. The electrical conducting property is known
to be strongly affected by the solvent and contacts of complex cation or counter anions
[2].
The ionic association constants of macrocyclic complexes have been determined
after carrying out a detailed conductometric studies [3,4]. Jenkins and Monk [5]
suggested that conductance measurements are also applicable for determining the
formation constants of the complexes. Katayama and Tamamush [6,7] determined the
ion-pair formation constants of tris(ethylenediamine)cobalt(III) complexes with maleate
and fumarate bases using continuous variation method form conductometric studies.
Conductometric studies for the ionic association constant of cobalt (II), Nickel (II),
copper (II) and zinc (II) perchlorates in methanol-ethylene glycol mixtures have been
reported by Doe and Kitagawa [8]. Koryta and Dvorak [9] have introduced a method
for the conductometric determination of dissociation constants of transition metal
complexes. The effective size of complex cation is one of the various factors, which can
influence the extent to which ions associate in solution. It has been shown that the
dissociation constants increase with increasing size of the organic ions [5].
In this chapter, conductometric measurements in non-aqueous and aqueous
system has been made and the data were treated employing a modified Onsagar limiting
equation of the Fuoss and Edelson method [10]. The thermodynamic ionic association
constants of the encapsulated M " and M^ complexes of the present macrocyclic and the
dipodal tridentate ligands have been determined. The corresponding free energies (AG) of
the first association reaction of these complexes have also been calculated.
"Coordination cHemistty ofSgands witH e&ctron ticR % 0 orS Biting centre' (Doctoralthesis suSmittecf to ARgarh MusRm "University, IM3IJi
chapters 137
Another important and the most versatile electrical technique is the cyclic
voltameteric studies. This methodology is widely used to study the ligating or
encapsulating behaviour of macrocyclic ligands for the divers various field sizes of the
metal ions in their different oxidation states. The technique of cyclic voltammetry (CV)
involves cycling the potential of an electrode, which is immersed in an unstirred solution
and measuring the resulting current. The effectiveness of CV results from its capability
for rapidly observing the redox behaviour over a wide potential range. Cyclic
voltammetry provides a method to study the behaviour of an electroactive species
generated during the forward scan and then probing its fate with the reverse scan and the
subsequent cycles, all in a matter of seconds or even less. The time scale of the
experiment is adjustable over several orders of magnitude by changing the potential scan
rate enabling some assessment of the rates of various redox reactions.
The important parameters of cyclic voltammogram are the magnitude of the
anodic peak current (ipa) and the cathodic peak current (ipc), the anodic peak potential
(Epa) and the cathodic peak potential (Epc).
The formal reversible redox potential (E°]/2) for a reversible couple is centered
between Epa and Epc and its magnitude is evaluated as
^ - Epa + Epc E 1/2 (1)
The number of electrons (n) transferred in the electrode reaction for a reversible
couple can be determined from the separation between the peak potentials, which at 25''C
is give as
AE = Epa~Epc= (2) n
For a reversible redox couple the values for ip and ip* should be of comparable magnitude
i.e. ip' /ip'' = 1. The peak current for a reversible system increases with V'" ' and is directly
proportional to concentration. However, the ratio of peak current can be significantly
influenced by chemical reactions coupled to the electrode process. A faster or rapid scan
rate allows less time for the chemical reaction to occur between the positive and negative
"Coonfination cHemistry ofRgands witH ekctron ricH^OorS Siting centre' 'DoctoraltHesis suSmittedtoARgaHi MusSm Vniversity, lOWIJi
CfiapterS 138
scans. In fact, for sufficiently fast scan rates the effect of the chemical reaction becomes
negligible and iVi' c approaches unity.
Electrochemical irreversibility is caused by slow exchange of the redox species
with the working electrode and is characterized by a separation of peak potentials greater
than indicated by equation 2. In this chapter results of cyclic voltametric studies on the
present complexes and the mechanisms of the electrode reactions have been discussed.
EXPERIMENTAL
Physical MeasuremenU-
Conductivity measurements were carried out at a frequency of 1 KHz on a
Systronic Conductivity Bridge at room temperature. The cell content was thermostated at
25 ± 0.5 °C and the cell constant of the conductivity cell (1.00 cm'') was standardized by
using aqueous 10" M NaCl as standard solution. The concentration ranges of the
complexes studies were from 10" to lO'^mol/liter.
Cyclic voltametric measurements were recorded on a Princeton Applied research
Model 263 A Potemtiostate/Galvanostate electrochemical analyzer. Double distilled
water and high purity DMSO were employed for the CV studies. An aqueous 0.4 M
tetraethylammoniumperchlorate solution was used as a supporting electrolyte. A three
electrode configuration was used which comprised of a glassy carbon working electrode,
a Ft wire as auxiliary electrode and Ag/AgCl as the reference electrode. All experiments
were carried out at room temperature.
"Coordination cHemistry ofRgands witH electron rich%0 orS biting centre" <Doctora[thesis suhmitudtoM^arh tMusRm Vniversity, I3^IJl
ClmpterS 139
RESULTS AND DISCUSSION
Evaluation of First Ionic Association Constant (Ki) and the related Thermodynamic
parameters for the complexes:-
Thermodynamic first ionic association constant (Ki) as well as the corresponding
free energy {AG) of the metal encapsulated macrocyclic complexes ( 3 - 5 and 9 - 28) in
water and (29 - 34) in DMSO have also estimated by carrying out the conductomeric
studies at RT. Conductometric data were treated according to modified Onsagar limiting
equation employing Fuoss and Edelson method [10]. The Onsagar limiting equation at
low concentration range [10,11] of the electrolytes is given by the relation.
A = A"-S.Cj^ ....(1)
Here, A is the equivalent conductance at each appropriate concentration Co, A° is
the limiting equivalent conductance and S represents slope of the plot of yl vs. Co • The
consecutive association equilibria of the complexes in solution can be represented as
following:
A^^ + X - AX" and
AX" +X ^ AX2 (2)
Where, A^^ indicates the corresponding complex cations and X' is the counter anion. The
corresponding association constant Ki and K2 are given by
Kj=fArjfJ[A''j[XJf2,f- (4)
K2 = [AXJf [AT] [XJff- (5)
Where^2+,/+ , / and/_ are the activity coefficients ofA^'^, AX^, AX2 and X' respectively.
In general K2«<Ki and its concentration is very low in comparison to Ki, henceforth,
the ionic equilibrium represented by equation (4) is only considered. The Onsagar
limiting law then simplifies as
AF^A"- DKi/A"
where, D = C/'AF(AF - AV2)
and F = [(l- dCo"^)'' + (A" - Xo-)/2A]/[l + (A" > A^^2A°] (6)
"Coordination cdemistry ofEgands witH ekaron ricH % 0 orS Siting centre" 'DoctoraftHesis suBmittedtoARgarii MusRm University, HN'DIJl
CdapUrS 140
Here, F is a function whicli corrects tlie conductance ratio, A/A°, for the effect of
interionic forces on the mobility. The parameter 6 is the ratio of slope (S) and limiting
equivalent conductance of the complexes cation and A^- is the limiting equivalent
conductance of A~ The ionic activity coefficient of ^^ 2+ has been estimated using the
following modified Debye-Huckel equation:
. -logf2^ = 250.3Z\7t3Co/^'^ (7)
where ZB = 2 (effective charge on the complexes)
and s= dielectric constant of the solvent DMSO (£•^46) and H2O {e= 78.06)
The experimental equivalent conductivity data of each complexes and the
estimated several parameters like limiting equivalent conductance (yl"), the activity
coefficient (/2+) and D for the complexes ( 3 - 6 and 9 - 28) measured in water and (29 -
34) measured in DMSO at room temperature have been summarized in Tables 5.1, 5.3,
5.5, 5.7 and 5.9 . Figure 5.1-5.5 represents the plots of yl verses Cg'^'^ for the various
complexes according to equation (1). The limiting equivalent conductance, A° value was
obtained form extrapolating the plot of A vs Cj^^. The Onsagar slope (S) for solutions
has been determined as the slope of the plots of A vs Co'^'^ for complexes (3) (Figure
5.1).
The AF vs D were estimated by using equation (6) and were plotted for each
complexes as typified by Figure 5.2 for the complex (3). The slope of the plot of AF vs
D has provided the magnitude of (Ki/yl°) according to equation (6) which ultimately
resulted in the magnitude of Ki summarized in Table 5.2, 5.4, 5.6, 5.8 and 5.10.
The limiting equivalent ionic conductance Ao+ of the macrocylic complex cations
has been determined according to Kohlrausch law [12] as following.
A = /I0+ + / lo-
or(Jio+=A" +Xo-) (8)
"Coordination cfumistiy ofGgands with electron rich%0 orS Siting centre" (Doctoraf thesis suSmittedtoACtgarh !MusRm "University, IM)IA
CfiapterS 141
The free energy changes due to first ionic association reactions were also
evaluated using the thermodynamic relationship i.e.
AG=-RTInK, (9)
and magnitudes for the various complexes are shown in Table 5.2, 5.4, 5.6, 5.8 and 5.10.
It can reasonably be seen that the magnitudes of ionic associate constant (Ki) for
homo-bimetallic complexes of the macrocycles L* are in general very small (~ 1 - 1.5
times) compared to that of the mononuclear macrocyclic complexes of L ' and I? ligands.
This indicates that the tendency of ion-pair formation in the solution of bimetallic
complexes (3,4,6) is higher than those exhibited by the corresponding mono-nuclear
complexes (9 -15). Furthermore, the magnitudes of Ki for the mono-nuclear complexes of
L'' are comparable to that of the corresponding mono-nuclear complexes of L .
However, some gradation in the magnitude of Ki within the series has been observed. It
has been found that for metal ions having d"*, d , d , d configurations, the magnitude of
K] for [ML^Cb] (n=2, M = Cr, Co, Ni; n=3, M = Fe) is higher compared to that for
[ML^Cb] (n=2, M - Cr, Co, Ni; n=3, M = Fe). This suggests that for the latter case ionic
association or extent of ion - pair formation is more. However, metal ions with d and d'°
configuration (i.e. for Cu " , Zn " , Cd " ) the order is reversed.
The magnitude of Ki for complexes with the dipodal tridentate ligands (Im) are
comparable to that of corresponding complexes of its substituted analogue (BIm). Here
too, some gradation in magnitude of ionic association (Ki) do exist within the series. It is
higher for Cr " , Fe^^ and Cu " (i.e. d"*, d and d , respectively) while for Co " (d ) and
Ni ^ (d ) the order is reversed for complexes with (BIm) ligand.
"Coordination chemistry of ligands witli electron ricli%0 orS biting centre" (DoctoraltHesis suBmittedto^Bgarii iMuslim "University, I!N>DIJl
chapters 142
Table 5.1: Several Parameters of metal complexes in H2O at room temperature
Comp.
3
4
5
6
Concentration (mol L')
1x10-'
2.5x10"
3.5x10"
5x10-'
7x10-'
10x10-
1x10-"
2.5x10"'
3.5x10-'
SxlO"'
TxlO-'
10x10-'
1x10-'
2.5x10-'
3.5x10-'
5x10"'
7x10-'
10x10"'
1x10'
2.5x10-'
3.5x10-'
5x10-'
A (ohm'cm' mot')
170.4
158.0
156.7
155.3
146.5
141.0
150.0
144.2
130.0
128.0
127.0
115.2
195.0
160.6
139.8
124.6
119.3
109.5
184.5
173.0
157.0
153.4
A" (ohm'cni mot')
176.0
168.0
184.0
201.5
f2+
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
F
0.9611
0.9549
0.9449
0.9334
0.9312
0.9252
0.9581
0.9340
0.9456
0.9311
0.9130
0.9182
0.8977
0.9004
0.9195
0.9331
0.9241
0.9277
0.9574
0.9421
0.9554
0.9443
D
0.1891
0.8655
1.3003
1.9814
3.2325
4.9584
0.3298
1.0290
1.7450
2.6028
3.6380
5.4923
0.1479
1.3011
2.2418
3.4660
4.8942
6.9874
0.4147
1.4334
2.4295
3.6113
"Coorcfmation ckemistry ofRgands witk ekcttxm rich % O orS Siting centre" <Doctoraftliesis submitted to Rgarfi MusRm Vniversity, I!N>DIA
CfiapterS 143
7x10"* 150.0 0.8599 0.9320 5.1920
lOxlO"* 145.4 0.8346 0.9198 7.5633
Table 5.2: Ionic association constant {K{) (L mor'), free energy {AG) (KJ moV') and
limiting equivalent conductance of cations (?^+) in complexes.
Compd,
3
4
5
6
K,
701.34
766.76
2189.60
1137.62
AG
-16.23
-16.45
-19.05
-17.42
A<o+
99.65
91.65
107.65
125.15
Table 5.3: Several Parameters of metal complexes in H2O at room temperature
Comp.
9
10
11
Concentration (mol L"')
1x10-'
2.5x10-'
3.5x10-^
5x10-
7x10-'
10x10"'
1x10-'
2.5x10-'
3.5x10-'
5x10-'
7x10-'
lOxlQ-'
1x10-'
2.5x10-'
A (ohm'cm' mof')
148.5
139.6
132.9
125.8
116.0
111.0
109.0
101.4
96.8
86.6
81.7
78.5
193.5
168.2
A° (ohm''cm' mot')
165.0
123.0
212.0
f2+
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
F
0.9472
0.9233
0.9275
0.9123
0.9141
0.9019
0.9324
0.9028
0.8922
0.8930
0.8825
0.8634
0.9384
0.9375
D
0.3233
1.0629
1.6181
2.5377
3.7636
5.4216
0.2051
0.6576
0.9950
1.5544
2.2090
3.1237
0.5215
1.9558
"Coordination chemistry of Elands xvith ekaron rich % 0 orS Siting antre" (DoUoraC thesis suBmittedtoJiSgarh Muslim Vniversity, IJ^IJl
CHapterS 144
12 1x10"* 111.0 120.6
13 1x10"* 102.0 117.0
3.5x10'*
5x10'*
7x10"*
10x10*
1x10"*
2.5x10"*
3.5x10"^
5x10"*
7x10""
10x10"^
1x10""
2.5x10"*
3.5x10"*
5x10"^
7x10-*
10x10"^
1x10"*
2.5x10"*
3.5x10"*
5x10"*
7x10"^
10x10"*
1x10"*
2.5x10'*
3.5x10*
5x10'*
159.8
144.5
134.3
127.7
111.0
104.8
102.6
101.4
96.5
93.6
102.0
92.0
85.3
79.9
77.7
75.7
129.0
119.8
114.5
105.6
97.1
85.0
93.5
88.6
85.0
81.1
14 1x10"* 129.0 150.0
15 1x10"* 93.5 103.0
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.9349
0.9474
0.9530
0.9447
0.9530
0.9314
0.9200
0.9064
0.8976
0.8811
0.9179
0.8915
0.8791
0.8667
0.8492
0.8244
0.9375
0.9098
0.9008
0.8996
0.8982
0.9126
0.9377
0.9068
0.8950
0.8810
2.9417
4.5252
6.4724
9.1989
0.1480
0.5124
0.7780
1.1606
1.7716
2.6244
0.2067
0.6552
0.9908
1.4554
2.0262
2.8434
0.3319
1.0206
1.5200
2.3046
3.2959
4.6891
0.1268
0.4157
0.6442
0.9922
'Coordination cHemistry ofSgamCs witfi ekctrm ricR %OOTS Biting centre ' OoaoraCtHesis suSmitted'toJlSgarA MusCm University, IWDIJi
CHapterS 145
7x10-^ 77.8 0.8599 0.8650 1.4463
lOxlQ-'* 74.1 0.8346 0.8467 2.1081
Table 5.4: Ionic association constant {Kj) (L mor'), free energy (AG) (KJ mor') and
limiting equivalent conductance of cations (Ao+) in complexes.
Compd.
9
10
11
12
13
14
15
K,
1275.45
1420.20
1397.12
1050.37
1308.66
1438.80
1236.75
AG
-17.71
-17.94
-17.93
-17.23
-17.77
-17.01
-17.63
A.0+
88.65
46.65
135.65
44.25
40.65
76.65
26.65
Table 5.5: Several Parameters of metal complexes in H2O at room temperature
Concentration A A° f2+ F D ( m o l L * ) (ohm'cn^ mor') (ohm'cm'mor')
xlO"' 151.0 163.0
2.5x10-^ 142.2
3.5x10"'' 138.5
5x10-^ 135.1
7x10-^ 131.4
10x10"^ 120.2
18 1x10"" 176.0 191.0
2.5x10"" 169.6
3.5x10"" 164.3
5x10'" 161.5
7x10"" 152.9
10x10'" 150.9
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
144.78
134.08
129.54
124.90
120.02
110.19
0.9681
0.9511
0.9467
0.9361
0.9349
0.9226
0.2491
0.8855
1.3632
2.0945
3.1050
4.8566
0.3316
1.0937
1.7346
2.6171
4.1350
6.0157
"Coordination chemistry qfSgands with tkctron rich !K,Oor3 Biting autre" tDoctoraf thesis suBmitted to Rgarh Muslim Vniversity, I!N^I^
chapters 146
19 1x10"^ 133.0 140.0
20 1x10-^ 153.0 172.0
1x10"^
2.5x10"^
3.5x10-^
5x10-^
7x10-'
10x10-'
1x10-"
2.5x10-'
3.5x10-^
5x10-'
7x10-'
10x10^
1x10"'
2.5x10"'
3.5x10-'
5x10-'
7x10-'
10x10"'
1x10-'
2.5x10-'
3.5x10-'
5x10"'
7x10-'
10x10-'
1x10-'
133.0
128.2
124.9
122.5
118.3
116.9
153.0
134.0
123.9
120.2
111.5
97.7
142.0
135.2
131.5
125.1
118.6
114.1
162.0
156.3
150.2
149.1
143.5
140.3
131.0
21 1x10"^ 142.0 156.0
22 1x10-^ 162.0 171.5
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9678
0.9527
0.9485
0.9383
0.9341
0.9172
0.9300
0.9188
0.9207
0.9060
0.9052
0.9237
0.9519
0.9339
0.9252
0.9198
0.9137
0.9010
0.9707
0.9565
0.9560
0.9446
0.9405
0.9290
0.1371
0.4981
0.8022
1.2678
1.9621
2.9333
0.3992
1.3745
2.0783
3.0243
4.3175
6.1576
0.2659
0.8575
1.3139
2.0730
3.1073
4.5639
0.2115
0.7502
1.2604
1.9006
2.9683
4.4767
23 1x10-^ 131.0 141.0 0.9445 0.9550 0.1877
"Coordinatwn cRemmry ofSgands witH electTon ricli%OorS Biting centre ' (Doctoral thesis suBmitUtftoJiGgari MusGm Vniversity, IT^DIA
CfiapterS 147
2.5x10"^
3.5x10'
5x10'"
7x10-'
10x10-'
24 IxlQ-' 195.5 210.5
2.5x10-'
S.SxlO"'
5x10-'
7x10-
10x10-'
124.6
119.9
115.4
112.3
107.5
195.5
187.2
179.8
175.1
166.2
162.2
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.9305
0.9356
0.9286
0.9203
0.9082
0.8965
0.9661
0.9504
0.9485
0.9407
0.9400
0.8346
0.6503
1.0386
1.6263
2.3944
3.5891
0.3856
1.3234
2.1433
3.3191
5.1041
7.5030
Table 5.6: Ionic association constant {Kj) (L mor'), free energy {AG) (KJ mor') and
limiting equivalent conductance of cations (^+) in complexes.
Compd.
17
18
19
20
21
22
23
24
K,
1121.78
1006.55
1013.75
1464.97
1138.92
1014.55
1119.69
1071.55
AG
-17.39
-17.15
-17.14
-18.05
-17.43
-17.14
-17.38
-17.28
A,o+
88.65
114.65
63.65
95.65
79.65
95.15
64.65
34.15
Table 5.7: Several Parameters of metal complexes in H2O at room temperature
Comp. Concentration A A" h^ F D ( m o l L ' ) (ohm'cm'mot') (ohm'cm'mor')
25 1x10-' 79.0 86.0 0.9445 0.9305 0.0867
2.5x10-' 74.0 0.9135 0.8958 0.2983
"Coordination cHemistry ofRgawCs with electron ricH %OorS Siting centre " (DoctoraCthesis suBmittecftoJiligarA MusRm "University, IM)IJi
CHapterS 148
26 1x10- 121.5 138.5
27 1x10" 88.0 103.0
3.5x10''*
5x10"^
7x10-"
10x10-^
IxlO-'
2.5x10"^
3.5x10-^
5x10"^
7x10-"
10x10"^
IxlO-'
2.5x10"^
3.5x10"^
5x10"^
7x10"*
10x10-^
1x10-'
2.5x10-'
3.5x10-'
5x10-^
7x10-^
10x10"'
1x10-'
2.5x10-'
3.5x10-'
5x10-'
71.5
69.0
66.8
63.0
121.5
107.8
103.9
99.7
89.6
82.5
88.0
79.0
71.6
66.7
62.6
55.8
87.0
79.2
75.1
71.3
65.1
61.4
146.5
133.2
127.1
116.8
28 1x10-" 87.0 97.5
29 1x10-' 146.5 167.5
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8599
0.8346
0.9445
0.9135
0.8986
0.8803
0.8798
0.8619
0.8407
0.8181
0.9335
0.9174
0.9061
0.8927
0.8964
0.8915
0.9047
0.8660
0.8573
0.8429
0.8257
0.8135
0.9165
0.8811
0.8665
0.8481
0.8344
0.8139
0.9387
0.9205
0.9118
0.9136
0.4569
0.6944
1.0087
1.4823
0.2663
0.8945
1.3134
1.9389
2.8129
3.9870
0.1758
0.5404
0.8034
1.1576
1.5964
2.1824
0.1337
0.4417
0.6636
0.9855
1.4118
1.9822
0.3894
1.2567
1.8809
2.8554
"Coordination chemistry ofCiganis %vitli etectnm ricfi%0 orS Biting centre' <Doctora[ thesis suSmittetftoAdgarA Muslim University, HMDIA
chapters 149
7x10"^ llO.l 0.8599 0.9121 4.0545
10x10-^ 96.1 0.8346 0.9249 5.8269
Table 5.8: Ionic association constant (Kj) (L mor'), free energy {AG) (KJ mor') and
limiting equivalent conductance of cations {Ao+) in complexes.
Compd.
25
26
27
28
29
Ki
1226.22
1399.07
1669.83
15.03.00
1396.91
AG
-17.68
-17.96
-18.37
-18.37
-17.93
^ 0 +
9.65
62.15
26.65
21.15
19.15
Table 5.9: Several Parameters of metal complexes in DMSO at room temperature
Comp, Concentration A A" f2+ F D (mol L ') (ohm'cm'mot') (ohm'cn^mot')
30 1x10-^ 41.0 47.0 0.9330 0.9811 0.0254
2.5x10-^ 36.8 0.8963 0.9953 0.0852
3.5x10"^ 33.1 0.8784 1.0340 0.1343
5x10-^ 32.2 0.8568 1.0300 0.1963
7x10-^ 30.1 0.8328 1.0500 0.2835
10x10'^ 27.1 0.8033 1.0940 0.4131
31 1x10"^ 40.0 47.5 0.9330 0.9879 0.0294
2.5x10"^ 34.4 0.8963 1.0223 0.0972
3.5x10-^ 33.5 0.8784 1.0189 0.1402
5x10"^ 31.5 0.8568 1.0330 0.2085
7x10"^ 27.4 0.8328 1.0840 0.3080
10x10"* 24.5 0.8033 1.1480 0.4377
32 IxlO-'' 56.4 68.0 0.9330 0.8895 0.0834
'Coordtnation chemistry of Brands witfi electron rich %OorS Biting centre " <Doctordthesis suBmitteiftoJiGgarh MusRm Vniversity, IM)IA
chapters 150
33
34
2.5x10'^
3.5x10"^
5x10"^
7x10-"
10x10-^
1x10-'
2.5x10"^
3.5x10-^
5x10-"
7x10-"
10x10'^
1x10-'
2.5x10-^
3.5x10-^
5x10"^
7x10-'
lOxlO-"
49.7
44.2
37.8
34.0
30.2
66.7
59.1
53.0
46.6
42.3
37.2
30.1
25.4
22.8
20.7
17.9
14.3
79.5
41.5
0.8963
0.8784
0.8568
0.8328
0.8033
0.9330
0.8963
0.8784
0.8568
0.8328
0.8033
0.9330
0.8963
0.8784
0.8568
0.8328
0.8033
0.8865
0.8385
0.8366
0.8263
0.8185
0.8634
0.8041
0.7804
0.7569
0.7277
0.6989
1.0915
1.1686
1.2400
1.3036
1.4252
1.6766
0.2442
0.3525
0.4928
0.6540
0.8598
0.1177
0.3404
0.4849
0.6685
0.8748
1.1180
0.0265
0.0782
0.1150
0.1675
0.2372
0.3377
Table 5.10: - Ionic association constant (A"/) (L mof'), free energy (AG) (KJ moF') and
limiting equivalent conductance of cations f' +j in complexes.
Compd. Ki AG X,o+
31
32
33
34
1180.66
1244.98
2215.35
2482.93
1113.39
-17.52
-17.65
-19.07
-19.36
-17.37
-29.35
-28.85
-8.35
3.15
-34.85
"Coordination chemistry ofRgamfs witi electnm rich % 0 orS Siting centre' (DoctoraCthesis suBmittedto^Rgarh MusRm University, I9PDIA
CHapterS 15]
175
140-
Figure 5.1. Plotting of A vs. Cj^ for fCo2L'Cl4j (3)
Figure 5.2. Plotting ofAF vs. D for fCo2L'CI4J (3)
"Coordination cHemistry ofSgands -with ebaron ric6%0 orS biting centre" <Doctora[thesis suBmittedto JiGgarh MusGm Vniversity, I!H1)I^
CfiapterS 152
CYCLIC VOLTAMMETRY
The electro-chemical redox behaviour of the present complexes in solutions
employing Cyclic Voltammetric (CV) studies: Estimation of Cathodic Potential
(Ep'), Anodic Potential (Ep") and Half Wave Potential (E°i/2)
The electro - .chemical behaviour of metal complexes described in chapter 2 i.e.
[MSL'CU] [M = Co (3), Ni (4), Cu (5)], [Fe2L'Cl4]Cl2 (2), [ML^Cb] [M = Cr (9), Co
(11), Ni (12), Cu (13)], [FeL^CyCl (10), [ML^Cb] [M = Cr (17), Co (19), Ni (20), Cu
(21)], [FeL^CliJCl (18) as well as of the metal complexes containing dipodal ligands
described in chapter 3 i.e. [M(Im)Cl2J [M = Cr (25), Co (27), Cu (29)], [Fe(Im)Cl3]
(26), [M(BIm)Cl2] [M = Cr (30), Co (32), Ni (33), Cu (34)] and [Fe(BIm)Cl3] (31) have
been studied in water using tetrabutylammonium perchlorate as a supporting electrolyte,
Ag/AgCl as a reference electrode in the potential range + 1.0 to ~ 1.0 V at 0.05, O.IO and
0.20 vs"' scan rates. The electro-chemical data for the complexes have been summarized
in Table 5.11 and the representative typical cyclic voltammograms which, have been
generated through computer stimulation are shown in Figures 5.3 - 5.20.
The cyclic voltammogram (CV) of the homo - bimetallic complex [Fe2L^Cl4]Cl2
(2) recorded at 0.05 and 0.10 vs"' scan rates {Figure 5.3) are nearly identical. This
contained two peaks, a weak intensity at Ep* - - 0.377 V and another relatively strong
intensity at Ep* - +0.69 V. The reverse cycle exhibited two peaks at Ep - - 0.412 V and
+0.80 V. The reverse sweep however, indicated the formation of a quasi - reversible
redox wave at E°i/2 = +0.78 V. The observed peak separation, AE = 0.165 V and the
cathodic to anodic current ratio, \pl\^ = 1.0 of the redox wave satisfy the requirements
for the reversibly of the redox reactions. The position of the reversible redox - wave (E°i/2
= + 0.78 V) is apparently oxidative in nature i.e. Fe(III) - Fe(IV), consistent with
the formation of the redox couple Fe'^^" in solution. It is reasonable to suggest [14,15]
that a 2e- equivalent one step redox process involving le- oxidation at each metal ion
occurs under the electro - chemical condition according to the following redox reaction
fPgiiiLiFeiii] .""'" - Fei^L^Fe'V] + 2e-
"Coordination cHemistry ofGgands witH electron ricli% O orS Biting centre" (DoUoraCthesis suSmittedto JiRgarli !MusRm Vniversity, I9^IJl
chapters 153
The additional irreversible cathodic wave appearing at Ep* = -0.412 V is indicative of a
one step 2e~ equivalent reduction as shown below.
[Fe'"L'Fe'"] + 2e- . - [Fe^L^Fe"]
The cyclic voltammogram {Figure 5.4) of the complex [CUIL'CU] (5) studied at
0.10 vs"' scan rate is comparable to that reported for the various homo - bimetallic Cu(II)
complexes [16] and is typical of bimetallic the Cu(II) complexes. It shows two
irreversible cathodic waves at Ep* = - 0.1 V and - 0.4 V and an unusually strong
irreversible anodic peak at + 0.3 V. However, none of these waves could generate a
reversible or quasi - reversible redox cycles. The presences of two consecutive cathodic
waves are consistent with the existence of two steps one electron reduction processes as
shown below.
ICU"L^CU"] + 1 e" • [CU'L'CU"]
[CU'L^CU"] + 1 e-— • [CU'L^CU^]
The appearance of the unusually strong irreversible anodic peak is indicative [17-19] of
the formation of an electro - chemically active species which is usually matellic copper
(Cu"), deposited near the cathode through a one step two electron process i.e.
[CU'L'CU'] + 2 e" • Cu" + [CU"L'] + L '
However, the possibility of a fast chemical disproportionation rection [16,17] may not be
ruled out.
The cyclic voltammograms {Figure 5.5) for the complex [Co2L'Cl4] (3) recorded
at 0.05 vs"' and 0.10 vs"' scan rates were identical which contained only one strong
cathodic peak. This is assignable to a one step 2 electron equivalent irreversible reduction
process. The unusual strong intensity of the peak is probably due to the presence of a
chemical dispraportionation process [16,17], which is coupled with the irreversible
reduction as following.
[Co"L^Co"] + 2 e- • [CO'L'CO']
2[CO'L'CO'] • Co" + [CO"L*] + L
"Coorcfination chemistry ofRgands with electron rich % O orS Siting untre" OoctoraCthesis suSmittedto^Sgarh MusSm University, H^PDIA
CfuipterS 154
An identical beliaviour was observed for the homo - bimetallic Ni(II) complex
[Ni2L'Cl4] (4) indicating an identical electro - chemical behaviour for this complex
{Figure 5.6).
The cyclic voltammograms {Figure 5.7) for the complexes [FeL^Cl2]Cl (10) and
[FeL^CbJCl (18) recorded at 0.10 vs' scan rate are nearly identical suggesting a similar
electro - chemical behaviour of these complexes in solution. The voltammograms
indicated the formation of a flattened quasi-reversible [14,15] redox wave with peak
separation AE = 0.10 V, peak height ratio ipVip'' = 1 and the half wave potential E°i/2 = +
0.16 V and + 0.15 V for the complexes (10) and (18), respectively {Table 5.11). The E\a
{Table 5.11) observed for both the complexes are comparable to that reported for various
the Fe(III)-porphyrine complexes which undergo electro-reductive alkylation giving a
quasi - reversible redox couple Fe^^^" at E°i/2 = + 0.20 V [20]. In the present complexes
the reversible redox wave is oxidative in nature consistent with the redox process, Fe(III)
' Fe(IV) giving the formation of the redox couple [14,15] Fe'^''" in the solution.
Furthermore, the voltammograms contained two additional well resolved cathodic peaks
at Ep' - 0.37 V and - 0.540 V {Table 5.11) for the complex (10) and only one cathodic
peak at Ep' =- 0.526 V for the complex (18). The presence of two additional cathodic
waves for (10) is indicative of an irreversible two step one electron reduction process as
given below.
[Fe"'L^] + 1 e- »- [Fe"L^] (Ep' = - 0.37 V)
[Fe"L^] + 1 e- »- [Fe'L^] (Ep'= = - 0.54 V)
while, formation of only one irreversible reduction wave for the complex (18) is
probably due to a one step irreversible two electron process i.e.
[Fe"'L^] + 2e- »- [Fe^L ]
The cyclic voltammograms for the complexes [Co(L)Cl2] [L = \} (11), L'' (19)]
{Figure 5.8) and [Ni(L)Cl2] [L - V (12), L^ (20)] {Figure 5.9) exhibited nearly identical
features. However, the voltammograms do not show an apparent well defined quasi -
reversible redox waves at slow as well as at fast scan rates. The observed peak positions
{Table 5.11) for the cathodic and the anodic waves {Figure 5.8) are consistent with the
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existence of an irreversible electro - chemical one electron reduction and one electron
oxidation processes, respectively, as represented below.
At Cathodic
[CoV] + e- ^[Co^L^]
[CO"L^] + e" • [Co'L^]
[NiV] + e- • [ N i V ]
[NiV] + e- •[Ni'L^]
At Anode
[CO"L^] • [CO'"L^] + e"
[CO"L^] * - [CO'"L^] + e-
[Ni"L^] *• [Ni"'L^] + e-
[Ni"L^] —*• [Ni"'L^] + e"
The analogous Cr(II) complexes [Cr(L)Cl2] [L = l} (9), L^ (17)] also exhibited
irreversible nature of the electro - chemical oxidation and reduction processes {Figure
5.10) consistent with the following reactions.
At Cathode
[CrV] + e- • [ C r V ]
At Anode
[Cr"L^] • [Cr"'L^] + e"
The cyclic voltammograms for the complex [Cu(L^)Cl2] (13) {Figure 5.11)
indicate an ill-defined quasi - reversible redox wave (peak separation, AE ~ 0.12 V and
peak height ratio ip7ip < 1.0) with the half wave potential, E°i/2 = +0.78 V. This is
apparently oxidative in nature i.e [CU"L^] - . ' [CU'L^], consistent with the formation
of Cu"'' ' redox couple [21] in the solution. The additional cathodic peak observed at Ep*
= - 0.351 V is due to electro - chemical irreversible reduction [Cu L ] + e"
— • [ C u L ]. However, for the complex [Cu(L )Cl2] (21) an interesting feature
for the electro - chemical redox process has been observed as shown in Figure 5.11. The
voltammogram recorded at 0.10 vs'' contain a flattened quasi - reversible redox wave at
"Coordiitation cfUmistry ofGgands witH ekctron rich % 0 orS Siting centre' OoctoraftHesis suSmitteif to A%irfi Muslim Vniversity, HJWIA
CtiapterS 156
E°,/2 = - 0.093 V (peak separation, AE = 0.009 V and peak height raio ipVip" <1). The
formation this redox wave is interestingly reductive in nature [22] consistent with
formation of Cu' redox couple through the redox process.
+ e" [ C U " L ^ ] ^ = ^ [ C U V ]
The voltammograms contained an additional unusually strong anodic peak at Ep = +
0.323 V indicating the presence of an electrode active species i.e. deposition of copper
(Cu") at that electrode. In some Cu(II) complexes the appearance of unusual strong
anodic peak is explained in terms of the presence of a chemical dispropornation reaction
[16,17] as below.
2[CU'L'] • Cu° + [CU"L^] + L^
The cyclic voltammograms {Figure 5.12) of the complex [Fe(Ini)Cl3] (26) were
recorded at 0.05, 0.10 and 0.20 vs'' scan rates. The voltammogram at 0.05 vs'' exhibited
a strong cathodic wave at Ep* = - 0.191 V and a weak anodic wave at Ep == + 0.617 V.
However, the reverse cycle exhibited only one wave at + 0.621 V, which gets coupled
with the former peak to result in the formation of a quasi - reversible redox wave with
E°i/2 = - 0.40 V. The quasi - reversible wave is apparently reductive in nature giving the II -f-fe", ,
reversible redox reaction [Fe (Im)] -——r— [Fe (Im)] consistent with the formation of - e ' the redox couple Fe"' in solution.
It is interesting to note that as the scan rate is increased from 0.05 vs" to 0.10 and
0.20 vs'', the voltammograms showed an additional anodic wave at Ep" = + 0.04 V which
gets coupled to generate an additional flattened quasi - reversible redox wave. This is
apparently, oxidative in nature giving the reversible redox reaction, [Fe"'(Im)] •« . ^
[Fe^'(Im)] consistent with the formation of Fe''*'' " redox couple [14,15] in the solution.
The voltammogram given consequently show the existence of the generation of both,
oxidative i.e. Fe' ' " as well as the reductive i.e. Fe"'' ' redox couples in the solution.
The complex [Cr(Im)Cl2] (25) recorded at 0.05 vs'' scan rate contained {Figure
5.13) only one irreversible cathodic wave. This behaviour of the cyclic voltammogram
did not change when recorded at a relatively higher scan rate (0.1 vs''). This indicates
'Coordination chemistry ofSgamCs with electron rich % O orS Biting centre' (DoctoraCthesis suBmittedtoJlSgarh OtlusCim University, HN^DIA
CfiapterS 157
that the complex undergoes a chemical disproportion process [14,15] in the solution. An
identical chemical - disproportionation has also observed for the Co(II) complex
[Co(Im)Cl2] (27) (Figure 5.14).
The electro - chemical behaviour of the complex [Cu(Ini)Cl2] (29) was studied at
two different scan rates 0.05 and 0.10 vs"' as shown in Figure 5.15. At 0.05 vs"' scan
rate, the voltammogram contained a quasi - reversible wave at E°\a = + 0.21 V, which is
oxidative in nature i.e. [Cu"(Im)] ".^g.. - [Cu"'(Im)] giving the formation of Cu"'^'
redox couple in solution. However, when the scan rate was increased to 0.10 vs"' the
voltammogram contained an improved quasi - reversible redox wave at E°i/2 = +0.301 V.
The presence of additional unusually strong anodic wave at Ep* = + 0.240 V here too,
suggest the presence of electro - deposition of Cu" as well as the chemical disproportion
process.
The cyclic voltammograms {Figures 5.16 - 5.20) for the analogous ligand
bis(lH-benzimidazol-2-ylmethyl)amine (BIm) with the metal ion Cr " , Fe^^ Co " , Ni "
and Cu . However, indicated the formation of only irreversible oxidation and reduction
process in the solution. For Cu(II) complex the a chemical disproportion to give the
formation of electro active species [Cu°] was also indicated.
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! \
- 0 6 - 0 5 - 0 4 - 0 3 - O Z - 0 1 OO 0 1 0 2 0 3 0-4 O S 0 6 0 7 0 8 0 9 1 0 1 1 Potantlal (V)
Figure 5.3. Cyclic voltammograms of the complex [Fe2L'Cl4]Cl2 (2) at 0.05 ( 0.10 ( ) vs"'scan rates.
-)and
-2.0E-OS
Figure 5.4. Cyclic voltammogram of the complex [CuaL'CU] (3) at 0.10 vs'' scan rate.
"Coordination chemistry ofBgands •with electron rich % O orS biting centre" (Doctoralthesis suBmittedto ^Bgarh MusRm "University, IJWIJi
chapter 5 159
-0:2 0.0 0.2 Potential (V>
Figure 5.5. Cyclic voltammograms of the complex [C02L CI4] (3) at 0.05 ( ) and 0.10 ( ) vs''scan rates.
I I \
•2 0E-O5 -1.0 -o.( -o.e -0.-4 -o^ 0.0 0.2 0.4 o.Q o.e 1.0
Potential <V)
Figure 5.6. Cyclic voltammograms of the complex [Ni2L'Cl4] (3) at 0.05 ( ) and 0.10 ( ) vs'' scan rates.
"Coordination chemistry ofBgarufs witfi electron rich %OorS Biting centre" (DoctoraC thesis suhmittetfto^Ggarh MusSm "University, 19^1^
chapters
r%
V • " v
Potential (V)
Figure 5.7. Cyclic voltammograms of the complexes [FeL CI4] (10) ( — ) and [FeL^CU] (18) ( ) at 0.10 vs' scan rate.
o.OE+00
-2.0E-O5
-0.-4 -0.2 0.0 0.2 Potential (V>
0 6 0 8 1 0
Figure 5.8. Cyclic voltammograms of the complexes [NiL CI4] (11) ( ) and [NiL^Cl4](19) ( ) at 0.10 vs"' scan rate.
'Coordinatimcfiemistryaflig(mdstiHtAdeawnTich%OorS6i^^ (Doctofd thesis sttSmittedtoASgarfi MusBm Vmversity, I^i^IJl
CHapterS 161
-0.6 -0.4 -0.2 O.O 0-2 O.A 0.6 0.8 Potential (V>
Figure 5.9. Cyclic voltammograms of the complexes [CoiL CI4] (12) ( ) and [CoL^Cl4](20) ( ) at 0.10 vs'' scan rate.
^ 1.0E-05 *
-5.0E-0S - . , , -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0-1 0-2 0.3 0 4 0.5 0.6 0-7 0.8 0_9 1.0
Potential (V)
Figure 5.10. Cyclic voltammograms of the complexes [CrL CI4] (9) ( ) and [CrL^Cl4](17) ( ) at 0.10 vs"' scan rate.
'Coordination chemistry ofGgands witfi ekctron rich % O orS biting centre" (DoctoraC thesis submitted to Bgarh Muslim "University, I!M)IJi
CfiapterS 162
-1 o -OS -o.e - 0 . 4 - 0 - 2 O.O 0 . 2 O.A P o t e n t i a l fV>
0 ,6 0 . 8 1.0 1.2
Figure 5.11. Cyclic voltammograms of the complexes [CuL CI4] (13) ( ) and [CuL^Cl4](21) ( ) at 0.10 vs"' scan rate.
^ 3.0E-05
-3.0E-05
-4 OE-OS
-0-4 -0,2 0 0 0-2 Potential (V)
O.A O.e o.a 1 0
Figure 5.12. Cyclic voltammograms of the complex [Fe(Im)Cl3] (26) at 0.05 ( ) , 0.10 ( ) and 0.20 ( ) vs'' scan rates.
'CoonBnatUm diendstty ofSgands with etectrm ricA % O orS Biting centre' (DoctoraCttiesis suSmitteitoMgasfi MusGm Vrnversity, I^NDl^
CHapterS 163
s. I ? 2.0E-05I
-2.0E-0S' -1 .0 -0 .8 -o a -0.4 -0.2 0.0 0.2
PoterrtiMl (V) 0.8 1 0
Figure 5.13. Cyclic voltammogram of the complex [Cr(Iin)Cl2] (25) at 0.05 vs"' scan rate
I 3
-2 .0E-05t -1.0 -0.8 -O 6 -0.4 -0.2 0.0 0.2 0.4
Potential (V) 0.6 OS 1 0
Figure 5.14. Cyclic voltammogram of the complex [Co(Im)Cl2] (27) at 0.05 vs scan rate
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chapters 164
-O.e -0 .6 -0--4 -0.2 D.O 0.2 Potontieil (V)
0 .6 0 .8 1.0
Figure 5.15. Cyclic voltammogra of the complex [Cu(Im)Cl2] (29) at 0.05 ( ) and 0.10 ( ) vs' scan rate
s .oE-oef !
•4 .oe-oe '
3 .0E-oe t
2 . 0 E - o e |
1 .0E-06 •
O.OE+Ool
I - 1 . 0 6 - 0 6 |
-a.oE-oei I
- 3 - 0 E - 0 6 '
-4.0E-06I
-S .0E-06 t
i -e .0E-06
-7.oe.-oei
- a .OE-oe ' - 1 . 0 - 0 . 8 -o .e -0..4 - 0 . 2 O.O 0 . 2 0.-4 O B O S 1.0
Potent ia l ( V )
Figure 5.16. Cyclic voltammogram of [Cr(BIm)Cl2](30) at 0.10 vs"' scan rate
'Coordination chemistry cfBgands ivitt electron rich % O orS Biting centre" (DoaoraC thesis submitted to JiBgarh MusBm "University, lOWlJl
CfiapterS 165
2.0E-05r
\
\
-2 .0E-05I -1 .0 -O.B -0.8 -0.-4 -0.2 0.0 0.2
PotantM (V) O.* 0.6 0.8 1.0
Figure 5.17. Cyclic voltammogram of [Fe(BIm)Cl3](31) at 0.10 vs' scan rate
-1.0E-06
. 1 . 0 -0 .8 - 0 . 6 -0 .4 -0 .2 0 .0 0 .2 O.A Potential <V)
0-6 0.8 1.0
Figure 5.18. Cyclic voltammogram of [Co(BIm)C12](32) at 0.10 vs"' scan rate
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CfiapterS 166
9 . 0 E - 0 6
a.oE-osS-
7 . 0 E - 0 6
6 .OE-06
s.oE-oei
•4.0E-06J
3 . 0 E - 0 6
2 . 0 E - 0 6 f
1 -0E-06
0 . 0 E » 0 0
- 1 . 0 E - 0 6
- 2 . 0 E - 0 6
- 3 . 0 E - 0 6
- 4 . 0 E - 0 6
- 5 . 0 E - 0 6
- 6 . 0 E - 0 6
- 7 . 0 E - 0 6
- a . 0 E - 0 6
- S . 0 E - 0 6
-1 .OE-0S
- 1 . 1 E - 0 5 ' --1 .0
\ \ \
o .4 o.a Potential (V)
Figure 5.19. Cyclic voltammogram of [Ni (BIm)Cl2](33) at 0.10 vs'' scan rate
- 0 . 2 O.O 0 . 2 Potont la l ( V )
Figure 5.20. Cyclic voltammogram of [Cu(BIm)Cl2](34) at 0.10 vs'' scan rate
"Coordination cHemistry ofGgamfs with electron rich % O orS Siting centre' (DoctoraCthesis suhmitudto^Rgarh Muslim "University, I!N>DIA
CfiapterS 167
Table 5.11. Electrochemical data of 10" M solution of complexes recorded in water at
room temperature.
Compounds
[FesL'CUlCla
[CUZL'CU]
[COJL 'CU]
[NizL'CU]
[FeL^CyCl
[FeL^CyCl
[NiL'Cb]
[NiL^Cb] [CoL^Cy [CoL^Cb] [CuL^Cy
[CuL'ClJ
[Fe(Im)Cl3]
[Cr(Im)Cl2]
[Co(lm)Cl2]
Code
(2)
(5)
(3)
(4)
(10)
(18)
(11)
(19) (12) (20) (13)
(21)
(26)
(25)
(27)
E ^ (Volts)
-0.377^ +0.696 X -0.412 y +0.69 y -O.lOy -0.40 y -0.49 X -0.43 y -0.53 X -0.54 y -0.54 y -0.37 y
+0.031 y -0.536y +0.059y -0.306y -0.573y -0.354y -0.247 y -0.521 y -0.35 y
+0.726 y -0.088 y
-0.191 X
-0.175y + 0.04 y -0.15U +0.04, -0.406 X
-0.529x
1^ 1/2
{Volts)
-+0.78 X
-+ 0.78 y
-
------
+0.16y -
+0.15y
-+ 0.78 y -0.093 y
-0.40 X
-0.41 y
-0.42,
E ; (Volts)
-+0.88 X
-
+0.85V +0.30
--
+0.81 X +0.81 y
--
+ .28 y -
+ 0.240 y
-0.859y -0.097 y +0.323 y -0.624 X +0.617x -0.641 y +0.669 y -C,697, +0.665,
X, y and z indicate the 0.05, 0.10 and 0.20 vs" scan rates.
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CHapterS 168
REFERENCES
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CfiapUrS 169
21. B. Cervera, J.L. Sanz, M.J. Ibanez, G. Vila, F. LLOret, M. Julve, R. Ruiz, X.
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"Cooriination cHemistty ofSgands with ekctron Ticfi%0 orS Siting centre" (Doctoralthesis suhmittedto^^aih Muslim "University, UNiDIJl