Journal of Molecular Structure 1032 (2013) 185–194

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Journal of Molecular Structure

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Cu(II) complexes of monobasic bi- or tridentate (NO, NNO) azo dye ligands:Synthesis, characterization, and interaction with Cu-nanoparticles

Mohamed Gaber a,⇑, Yusif S. El-Sayed a,b, Kamal El-Baradie a,c, Rowaida M. Fahmy a

a Chemistry Department, Faculty of Science, Tanta University, 31527 Tanta, Egyptb Prepared Basic Science, Deanery of Academic Services, Taibah University, Al-Maddinah, Saudi Arabiac Faculty of Science and Arts at Al-Rass, Qassim, Saudi Arabia

h i g h l i g h t s

" Characterization of Cu(II) azo dyes containing the triazol and thiadiazole moieties." Prepared spherical copper nanoparticles were characterized using UV–Vis spectroscopy and TEM." Formation of surface complex between azo-dye ligands and colloidal copper nanoparticles." The stability constant of the copper nanoparticles complexes is higher compared with the corresponding bulk ones.

a r t i c l e i n f o

Article history:Received 21 April 2012Received in revised form 12 July 2012Accepted 12 July 2012Available online 1 August 2012

Keywords:Azo dyesTriazolesThiadiazolesCopper complexesThermal studiesCopper nanoparticles

0022-2860/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.molstruc.2012.07.019

⇑ Corresponding author. Tel.: +20 403350570; fax:E-mail address: mabuelazm@hotmail.com (M. Gab

a b s t r a c t

A series of copper(II) azo complexes having the formula [CuL1–4(nH2O)]�OAc�xH2O where (n = 1–2) and(x = 0–1) have been synthesized using azo dyes containing the triazol and thiadiazole moieties. The azod-yes and their metal complexes were characterized by elemental analysis, molar conductance, IR, elec-tronic, mass, ESR spectra, magnetic moment measurements, and thermal analyses. IR spectra showedthat the ligands having triazole moiety were coordinated with the copper(II) ion in a tridentate mannerwith ONN donor sites of the naphthyl OH, N-atoms of azo group, and triazole moiety while azodyes hav-ing thiadiazole moiety were coordinated with the copper(II) ion in a bidentate manner with ON donorsites of the naphthyl OH and the N-atom of the group. The thermodynamic activation parameters suchas DE�, DH�, DS�, and DG� were calculated from the TG curves. Prepared spherical copper nanoparticleswere characterized using UV–Vis spectroscopy and transition electron microscope (TEM). The spectraldata showed the formation of surface complex between azo-dye ligands and colloidal copper nanoparti-cles through (AOH) anchoring group. The stability constant of the prepared copper nanoparticles com-plexes is higher compared with the corresponding bulk ones due to the larger surface area of coppernanoparticles.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction and antimycotic [11], and powerful antifungal agents [12]. On

Azo compounds containing a heterocyclic moiety and their me-tal complexes play an important role in many practical applica-tions for molecular memory storages, nonlinear optical elements,and printing system [1–3]. Also, the importance of the heterocyclicazo dyes and their metal complexes may stem from its analyticalapplications [4,5] and biological activity [6]. Nowadays, muchattention has been focused on 1,2,4-triazole and 1,3,4-thiadiazolederivatives as a very important class of nitrogen-containing aro-matic heterocyclic compounds, which have attracted a great dealof interest due to their diverse biological activities such as antimi-crobial [7], antitumor [8], antibacterial [9,10], anti-inflammatory

ll rights reserved.

+20 403350804.er).

the other hand, 1,3,4-thiadiazole derivatives are another importantclass of heterocyclic, which display a broad spectrum of biocidalactivities possibly due to the presence of toxophoric ANACASmoiety [13].

The metal complexes of 1,2,4-triazole or 1,3,4-thiadiazole deriv-atives are of current interest especially because of its relevance tometal interactions with biological molecules [9,10,14]. Coordina-tion chemistry of nanoparticles [15] is a new area of investigationin modern chemistry. Nanoparticles are giant pseudomoleculeswith complex internal structures; they mostly consist of the coreand the shell and often contain surface functional groups. Coppernanoparticles have attracted increased attention because of theirlow cost (in contrast to Au and Ag) and novel optical, catalytic,mechanical, electrical, magnetic, and heat conduction properties,which are different from those of bulk metals [16].

186 M. Gaber et al. / Journal of Molecular Structure 1032 (2013) 185–194

As part of our continuous work on hetero azo dyes and theirmetal complexes [17], we described here the synthesis andcharacterization of Cu(II) complexes with four azo dyes derivedfrom 1H-[1,2,4]triazol-3-ylamine, 5-methylsulfanyl-1H-[1,2,4]tria-zol-3-ylamine, [1,3,4]thiadiazole-2-ylamine, 5-amino [1,3,4]thia-diazole-2-thiol, and b-naphthole as well as the favorableconditions for complex formation, stoichiometry, and spectropho-tometric determination of Cu(II) ion. Also, the interest is focused onthe study of the chelating properties of the investigated azo dyesbased on elemental analyses, magnetic moments, spectral methods(IR, UV–Vis, and ESR), and conductance as well as the thermalbehavior. Since the interaction between metallic nanoparticlesand organic ligands attracted our attention [15,18], the interactionbetween the investigated azo dyes and colloidal solution of Cunanoparticles was also studied.

2. Experimental

2.1. Materials and analyses

All the reagents and solvents were of reagent–grade quality andused as received. 1H-[1,2,4]triazol-3-ylamine, 5-methylsulfanyl-1H-[1,2,4]triazol-3-ylamine, [1,3,4]thiadiazole-2-ylamine, 5-ami-no-[1,3,4]thiadiazole-2-thiol, cetyltrimethylammoniumbromide(CTAB), and b-naphthole were purchased from Sigma–Aldrich.

2.2. Physical measurements

IR spectra were recorded on a range 400–4000 cm�1 Perkin El-mer 1430 Recording Infrared Spectrophotometer as potassium bro-mide discs. UV–Vis absorption spectra were recorded in the range200–700 nm on a Shimadzu 50 UV–visible spectrophotometer. Ele-mental analysis (C, H, and N) was carried out using Perkin–Elmermodel 2400 automated analyzer. The copper ion content wasdetermined by Perkin–Elmer 2380 atomic absorption spectropho-tometer. The conductance measurements for the prepared solidCu(II) azo complexes were recorded with the aid of Hana model1331 conductometer. The room temperature magnetic susceptibil-ities were determined on Sherwood Scientific Magnetic Suscepti-bility Balance using Hg[Co(SCN)4] as calibrant. The mass spectrawere recorded using Shimadzu Qp-2010 plus. The thermal stabilitywas examined using Shimadzu TG-50 thermal analyzer, whichused to record simultaneously TG and DTG curves. The measure-ments were carried out in a dynamic nitrogen atmosphere with aheating rate of 10 �C min�1 in the temperature range 25–800 �Cusing platinum crucibles. The morphology of the prepared coppernanoparticles was investigated using transmission electron micro-scope model JEOL-JEM-100SX. The particle size and size distribu-tions were obtained by image analyses.

2.3. Synthesis of the azo dye ligands (HL1–4)

Each of 2-amino-1,3,4-thiadiazole (0.101 g, 1 mmol), 2-amino-5-mercapto-1,3,4-thiadiazole (0.133 g, 1 mmol), 3-amino-1H-1,2,4-triazole (0.84 g, 1 mmol), and 3-amino-5-methylmercapto-1H-1,2,4-triazole (0.130 g, 1 mmol) was dissolved in a suitable vol-ume of water containing 2–2.5 ml of hydrochloric acid and diazo-tized below 5 �C with sodium nitrite (0.069 g, 1 mmol) in coldwater (5 ml). The resulting diazonium salt solution was coupledwith b-naphthole (0.144 g, 1 mmol) dissolved in 30 ml sodiumhydroxide solution (30%) and cooled to �5 to 0 �C in an ice-bath.The precipitated colored solids were filtered with suction, washedseveral times with bidistilled water, purified by further recrystal-lized, and finally dried in desiccator over anhydrous CaCl2 to givethe pure ligands. The purity of the azo dyes (Fig. 1) was first

checked by the m.p constancy, elemental analysis, and finally byspectral methods.

(L1: m.p = 184–186 �C, Analysis: Found C(60.21); H(3.81);N(29.4)%, calcd. for C12H9N5O, C(60.25); H(3.79); N(29.27)%, IR:1467 cm�1(N@N); 1659 cm�1(C@N); 3122 cm�1(NH), color: Or-ange, Yield: 95%, kmax = 450 nm).

(L2: m.p = 233–235 �C, Analysis: Found C(54.7); H(3.89);N(24.5)%, calcd. for C13H11N5OS, C(54.72); H(3.90); N(24.55)%, IR:1490 cm�1(N@N); 1675 cm�1(C@N); 3167 cm�1(NH), color: Red-dish Orange, Yield: 92%, kmax = 464 nm).

(L3: m.p = 175–177 �C, Analysis: Found C(56.2); H(3.12);N(21.89)%, calcd. for C12H8N4OS, C(56.24); H(3.15); N(21.86)%, IR:1459 cm�1(N@N); 1593 cm�1(C@N); 626 cm�1(CS), color: FaintBrown, Yield: 82%, kmax = 530 nm.

(L4: m.p = 216–218 �C, Analysis: Found C(50); H(2.73);N(19.4)%, calcd. for C12H8N4OS2, C(49.98); H(2.8); N(19.43)%, IR:1460 cm�1(N@N); 1598 cm�1(C@N); (2633) cm�1SH; 653 cm�1

(CS), color: Dark Pink, Yield: 94%, kmax = 500 nm.

2.4. Synthesis of solid complexes

The solid azo-copper complexes were prepared in molar ratio1:1(M:L) by adding of 1 mmol of copper(II) acetate in 50 ml hotethanolic solution to (1 mmol) of the ligands in 50 ml of hot etha-nol dropwise under vigorous stirring, whereupon a suspension ofthe copper(II) complexes results. The reaction mixture was re-fluxed for several hours using TLC plates to detect the end of thereaction. The precipitated solids were collected by filtration,washed with double distilled water, and then dried in desiccatorover anhydrous CaCl2 to obtain the copper(II) azo-complexes. Spe-cific details for each complex are given below in Table 1.

2.5. Synthesis of copper nanoparticles

Reaction was done in clean dry 125-ml flask open to the air. In atypical reaction, a 10-ml volume of 0.003 M copper nitrate preparedin IPA solution is added drop wise (about 1 drop/s) to 0.090 M ofCTAB/IPA solution. The reaction mixture was stirred vigorously ona magnetic stir plate. The solution turned violet after the additionof 2 ml of copper nitrate, darker violet after addition of 5 ml coppernitrate, and dark violet when the entire volume of copper nitrate wasadded. The appearance of a violet color indicated the presence ofcopper nanoparticles. The plasmon absorbance at 560 nm waspresent in a spectrum taken within minutes after the color appeared.The product, stored in a closed vial, was stable for a week. After1 week, the violet color and plasmon band started to vanish [19].

3. Results and discussion

3.1. Electronic absorption spectra of the azo-ligands and theircopper(II) azo-complexes

The electronic absorption spectra of the azo ligands and theircopper(II) complexes in methanol (D = 33) are given in Fig. 2 forL1 and its Cu(II) complex as example. The values of the chemicalshift were determined by taking the difference between the absorp-tion maximum of the azo-copper complexes with corresponding li-gand. The electronic absorption spectra of the azo-ligands (L1–4]within the wavelength range 200–700 nm showed three mainabsorption bands except L2 that exhibited only two bands; the firstband appeared within the range 210–270 nm was assigned to themoderate energy p–p� electronic transition within the phenylmoiety representing the (1La ?

1A) state, whereas the second bandobserved in the range 290–330 nm is attributed to low energy p–p�

electronic transition of the heterocyclic moiety and phenyl ring

N

N

N

R2

H

N

N

OH

R2=H, L1, (E)-1-((1H-1,2,4-triazol-3-yl)diazenyl)naphthalen-2-ol.

R2=SCH3, L2, (E)-1-((5-(methylthio)-1H-1,2,4-triazol-3-yl)diazenyl)naphthalen-2-ol.

NN

S

N

N

OH

R1

R1=H, L3, (E)-1-((1,3,4-thiadiazol-2-yl)diazenyl)naphthalen-2-ol.

R1=SH, L4, (E)-1-((5-mercapto-1,3,4-thiadiazol-2-yl)diazenyl)naphthalen-2-ol.

Fig. 1. Structure of azo-dye ligands (L1–4).

300 400 500 600 7000.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

[Cu+2]= 6x10-5 Mol.L-1

[Cu+2]=10-5 Mol.L-1

Abs

orba

nce

Wavelength (nm)

Fig. 2. Absorption spectra of azo ligand (L2) and its Cu(II) complex in methanol.

Table 1Elemental analyses and physical properties of the prepared Cu(II)azo-complexes.

Complex formula/(mol.wt.) Color Content% (calcd.)/found K (X�1 cm2 mol�1) leff B.M.

%C %H %N %Cu

[CuL1(H2O)]�OAc/378.85 Reddish violet (44.38)/44.3 (3.46)/3.51 (18.48)/18.4 (16.77)/16.51 92 1.74[CuL2(H2O)]�OAc/424.9 Bluish violet (42.4)/42.1 (3.6)/3.47 (16.5)/16.34 (14.95)/14.8 88 1.77[CuL3(2H2O)]�OAc/413.89 Reddish violet (40.63)/40.5 (3.41)/3.26 (13.54)/13.3 (15.35)/15.15 78 1.83[CuL4(2H2O)]�OAc.H2O/463.97 Reddish violet (36.2)/36 (3.5)/3.38 (12.1)/11.91 (13.7)/13.51 89 1.80

M. Gaber et al. / Journal of Molecular Structure 1032 (2013) 185–194 187

corresponding to the(1Lb ? 1A) state. These two bands are nearlyunaffected both by the nature of the substituent and by the typeof solvent used. The third band appearing in the range 450–530 nm corresponding to n–p� electronic transition involving thewhole electron system and charge transfer interaction within themolecules. The spectra of the copper(II)-azo complexes (C1–C4)showed that the main peak of free ligands was shifted to higherwavelengths at 530, 560, 570, and 590 nm, respectively, relativeto their copper free ligands indicating a bathochromic shift due toLMCT between azo ligands and copper(II) ion. This also indicatedthat the central copper(II) ion has influence on the absorption bandsof the complexes according to modern molecular orbital theory.Any factor, such as the electron negativity, the radius, and elec-tronic configuration of the metal(II) ion, that can influence the elec-tronic density of a conjugated system must result in the differenceof the absorption band.

3.2. Mass spectroscopy

The mass spectrum of L1 showed the molecular ion peak [M]+ atm/z = (239, 39%) equivalent to its molecular mass attributed to

188 M. Gaber et al. / Journal of Molecular Structure 1032 (2013) 185–194

C12H9N5O. The molecular ion [M+] undergoes fragmentations in dif-ferent positions. The base peak appeared at m/z = (114, 100%),which pointed to molecular formula C9H6. The peaks at m/z = 222,143, 142, and 96 corresponded to the other fragmentation. For L2,the presence of peak [M]+ at m/z = (285, 27%) pointed to molecularmass of the formula C13H11N5OS, which gave base peak atm/z = (267, 100%) resulting from loss of hydroxyl radical followedby hydrogen radical, respectively. The base peak undergoes frag-mentation by loosing C10H6 to give intense peak at m/z = (141,28%), which loose N2 followed by hydrogen radical to give anotherintense peak at m/z = (114, 57%) corresponding to C3H4N3S. For L3,the molecular ion peak [M]+ at m/z = (256, 12%) corresponded tothe formula C12H8N4OS. This ligand is fairly abundant as comparedto the previous ones. Great abundant base peak at m/z = (113, 100%)confirmed the break of bond between naphthalene ring and azogroup. Finally, L4 with empirical formula C12H8N4OS2 represented[M+] at m/z = 288 corresponded to this molecular mass. This peakalso was the base peak with relative abundance =100%. The mainfragmentation path was the break of bond between naphthalenemoiety and nitrogen of azo group forming fragment ion at m/z=145(20%) which confirmed by the appearance of peak at m/z=143(31%) corresponded to C10H7O formula. The fragment ion atm/z =145(20%) may lose nitrogen gas followed by SH group formingfragment ion at m/z = 84(46%) attributed to C2N2S formula.

3.3. Complexes formation of the Cu(II) in solution

The composition of the complexes in solution was establishedusing the mole ratio, MRM [20], and continuous variation methods,CVM [21]. The results indicated that the Cu(II)azo-complexes (1–4)were formed in solution in the molar ratio 1:1 (M/L). The condi-tional formation constants calculated using Harvey and Manningequation applying the data obtained from MRM and CVM werelisted in Table 2. The free energy (DG) of formation of Cu(II)azo-complexes was also calculated. The stoichiometry of the Cu(II)azo-complexes was determined by conductometric titrationswhere the data obtained confirm the formation of 1:1 (M/L)complexes. The increase in conductance indicated that the reactionbetween the Cu(II) ion and azo dyes occurs via the formation of acovalent linkage with the oxygen atom of the OH group. The limits

Table 2Spectrophotometric data, stability constants, and analytical data (molar absorptivity(e), specific absorptivity (a), Sandell sensitivity (S), standard deviation (SD), correla-tion coefficient (CC), and obeyance to Beer’s law range for Cu(II) with L1–4.

Data L1 L2 L3 L4

kmax (nm) 530 560 570 590Beer’s range (ppm) 0–1.91 0–3.81 0–3.81 0–3.81Ringbom range (ppm) �0.09 to 0.28 0.26–

0.580.18–0.58 0.21–0.58

CC 0.99974 0.9960 0.9956 0.9957SD 0.00425 0.02709 0.02666 0.01862e (l mol�1 cm�1) 20.3 � 103 12 � 103 10.4 � 103 10.8 � 103

a (ml g�1 cm�1) 0.319 0.188 0.163 0.170S (lg cm�2) 0.003 0.005 0.006 0.0058MRM 1:1 1:1 1:1 1:1CVM 1:1 1:1 1:1 1:1CM 1:1 1:1 1:1 1:1Logbn 5.45 5.14 4.97 6.54�DG� 7427 7008 6776 8911Y � 10�5 5 5 5 5X � 10�5 5 5 5 5K (bulk-copper) 34,560 22,231 12,046 7564K (nano-copper) 37E+8 127E+8 28E+8 79E+8

MRM, Mole ratio method; CVM, continuous variation method; CM, conductometricmethod; bn, stability constant of complex; DG�, Gibbs free energy; X, concentrationof prepared sample (mol L�1); ‘‘EDTA-titration’’; Y, concentration of sample(mol L�1) calculated experimentally. ‘‘EDTA-titration’’.

of validity to Beer’s law, molar extinction coefficient, specificabsorptivity, standard deviation, and correlation coefficient werecalculated. The results of the spectrophotometric titration of Cu(II)ion with EDTA using the azo-dye ligands (L1–4) indicated that theapplied ligands can be used as indicators for the spectrophotomet-ric titration of the Cu(II) with EDTA. The values collected in Table 2confirmed the possible application of the present spectrophoto-metric method for the determination of Cu(II) ion. Ringbom [22]concentration range was also determined, and the results are listedin Table 2. The obeyence to Beer’s law and Ringbom plot is shownin Fig. 3.

For the calculation of the formation constant of the Cu(II)-azocomplexes (1–4), we selected the absorption peak with the longerwavelength, because this wavelength shows the maximum spec-tral variation with changes in the concentration of Cu(II). The Bene-si–Hildebrand (B–H) equation [23] was employed to determine theformation constant of azo-complexes with 1:1 (donor–acceptor)stoichiometry, for cells with 1 cm optical path length is:

½L0�A� A0

¼ 1eML � eL

þ 1ðeML � eLÞK½M0�

ð1Þ

where A0 and A are absorbance of the free azo-dye solutions and thecorresponding copper complexes, respectively, at a given wave-length, eL and eML, are the molar extinction coefficients of the azo-dyes and their copper complexes, respectively, [L0] is the initial con-centration of azo-dyes, and finally [M0] is the copper ion concentra-tion. Eq. (1) is valid under the condition for 1:1 donor–acceptorcomplexes. A straight line if only a 1:1 complex is formed, while amixture of 1:1 and 1:2 complexes in a system would lead to a curve,and we must use another equation to obtain a linear plot.

Formation constant (K) values for the complexes of azo-dyeswith Cu(II) ion are calculated from the plots of [L0]/(A � A0) vs. 1/[M0] Fig. 4, and the values are listed in Table 2. The results obtainedrevealed that the complexes of copper(II) with triazole azo ligandsare more stable than those of thiadiazole azo ligands due to the for-mation of two rings give stability larger than one ring, which gavea great evidence to the proposal structure of the preparedcomplexes.

3.4. Characterization of the solid copper complexes

The elemental analyses listed in Table 1 confirmed the sug-gested formulas [CuL1–4(nH2O)]�AcO�xH2O, where n = 1 for C1, C2;n = 2 for C3, C4, and x = 0 for C1–C3; x = 1 for C4. The copper com-plexes are insoluble in most organic solvents but soluble in DMFand DMSO. The molar conductivity values listed in Table 1 are inthe range of 1:1 electrolyte [24].

3.4.1. IR spectraEvidence for complex formation was obtained by comparing the

infrared spectra of the free azo dyes and complexes. The most char-acteristic bands and their assignments are given in Table 3. Thebands within the range 1459–1490 cm�1 belonging to t(N@N)stretching were shifted to lower frequencies in the range 1424–1452 cm�1 supporting the participation of N@N in coordinationmode, which is supported by the appearance of new bands withinthe range 432–455 cm�1 due to t(CuAN) bond. The IR spectra of L1

and L2 revealed a medium band at 1659 and 1675 cm�1, respec-tively, due to t(C@N) of the triazole nitrogen. This band was shiftedto lower frequency in the complexes indicating the participation ofazomethine nitrogen in the coordination to the Cu(II) ion. A broadband at 3440 cm�1 showed the vibrational mode of t(OAH) due tothe presence of lattice water in complex 4. The appearance ofbands at 3348–3349, 1730–1695, 838–824, and 613–572 cm�1 inthe spectra of all complexes were attributed to the t(OAH),

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8(a)L4L3L2

L1A

bsor

banc

e

[Cu+2] in ppm

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

20

30

40

50

60

70

80(b)L3L4

L2L1

Tra

ncm

itta

nce

(%)

Log [Cu+2] in ppm

Fig. 3. (a) Obeyance Beer law for complexes of Cu(II) with (L1–4). (b) Ringbom plots for complexes of Cu(II) with (L1–4).

30000 60000 90000 120000 150000 180000 2100000.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007 L4

L3

L2 L1

[L0]

/ (A

-A0)

1 / [M0]

Fig. 4. Linear dependence of [L0]/(A � A0) on the reciprocal concentration of Cu(II)coordinated with (L1–4) according to Benesi and Hildebrand equation.

Table 3IR spectra (cm �1), electronic, and ESR spectral parameters.

Parameter Complex 1 Complex 2 Complex 3 Complex 4

kmax (cm�1) 15151.5 17,857 18,868 17,54422,727 22,727 25,000 22,727

g11 2.2443 2.249 2.24 2.25g\ 2.06 2.06 2.05 2.06gav 2.121 2.123 2.113 2.1233G 4.07 4.15 4.80 4.17KII 0.744 0.815 0.823 0.81K\ 0.8899 0.8899 0.849 0.8898N@N 1452 1424 1439 1452C@N 1594 1597 1593 1598MAO 517 521 540 528MAN 446 448 432 455CAS – 650 630 660CAO 1318 1323 1334 1372OH 3446 3444 3424 3440

M. Gaber et al. / Journal of Molecular Structure 1032 (2013) 185–194 189

d(H2O), qr(H2O) and qw(H2O) vibration of the coordinated water,respectively. The phenolic t(CAO) band appeared in the region1254–1279 cm�1 in the free ligands L1–4 were appeared (as med-ium to high intensity band) in the region 1318–1372 cm�1in com-plexes 1–4, supporting the formation of CuAO bonds viadeprotonation. The appearance of t(CuAO) at 517–540 cm�1 indi-cated the chelation through oxygen atom of the phenolic group.

The above results together with the elemental analyses indi-cated that the triazole azo coordinated with the Cu(II) ion via thenitrogen atoms of azo group and triazole moiety, as well as thephenolic oxygen, that is, the ligands L1 and L2 behave as monobasictridentate ligands. For the thiadiazole azo, the coordination sitesare the nitrogen of azo group and oxygen atom of phenolic group,respectively, that is, the ligands L3and L4 behave as monobasicbidentate ligands, Scheme 1.

3.4.2. Thermogravimetric analysesThe Cu(II)azo-complexes (C1–C4) were thermally investigating

and plausible degradation had done. Table 4 shows the tempera-ture range, weight loss, and the supposed fragments for all thestudied Cu(II)azo-complexes.

The thermogram of C1 is taken as a representative example. Thiscomplex [CuL1(H2O)]. OAc is thermally decomposes within temper-ature range 25–800 �C at successive three degradation steps. Thefirst step within temperature range 42.4–344.5 �C with an estimatedmass loss of 20.3% (calcd. 19.4%) corresponding to the loss of coordi-nated water and acetate molecules. The activation energy of thisstep is 72.1 kJ mol�1. The second degradation step was the loss ofN2 gas within the temperature range 344.5–376.9 �C with an esti-mated mass loss of 9.3% (calcd. 10.2%). The activation energy of thisstep is 395.3 kJ mol�1. The third degradation step that is responsiblyaccounted for the pyrolysis of L1 molecule with a final copper oxideresidue was the loss of hetero ring and C10H6 within the temperaturerange 376.9–603.4 �C with an estimated mass loss of 46.01% (calcd.45.3%). The activation energy of this step is 89.4 kJ mol�1.

The thermodynamic and kinetic parameters for the thermaldegradation steps have been studied employing the Coats–Redfern[25] and Horowitz–Metzger [26] equations.

Coats–Redfern equation:

For n ¼ 1: log� logð1� aÞ

T2

� �¼ log

ARbEa

� �1� 2RT

Ea

� �� Ea

2:303RT

� �ð2Þ

For n–1: log1� ð1� aÞ1�n

T2ð1� n2Þ

" #¼ log

ARbEa

1� 2RTEa

� �� Ea

2:303RT

� �

ð3Þ

where b is the heating rate, a is the fraction decomposed, A isArrhenius pre-exponential, R is the universal gas constant, T isthe absolute temperature, and Ea is the activation energy. The term(AR/bEa) (1 � 2RT/Ea) is practically constant and the value of(1 � 2RT/Ea) � 1. Hence, for n = 1, a plot of log (�log(1 � a)/T2) vs.

O

NN

NHN

N

R2

Cu

H2O

.CH3CO2

+

.XH2O

NS

N

R1

NN

CuO

H2O

H2O.CH3COO .XH2O

R2 = H, X = 0, Complex 1 R1 = H, X = 0, Complex 3 R2 = SCH3, X = 0, Complex 2 R1 = SH, X = 1, Complex 4

Scheme 1. Structure of complexes (C1–C4).

Table 4Thermo analytical results (TGA and DrTGA) of Cu(II) complexes (C1–C4).

Complexes (TGA) DrTGA (�C)

Step Temperature (�C) wt.% loss (calcd.)/found Assignment

C1 1st 42.4–344.5 (20.3)/19.4 Loss of (coordinated water, acetate) 2982nd 344.5–376.9 (9.3)/10.2 Loss of N2" 3563rd 376.9–603.4 (46.01)/45.3 Loss of (hetero ring, C10H6) 445

C2 1st 63.9–308 (37.7)/38.3 Loss of (coordinated water, hetero ring, N2")2nd 308–577.2 (51.07)/51.25 Loss of (C10H6, acetate) 472

C3 1st 67.3–189.2 (4.55)/4.53 Loss of coordinated water 1322nd 189.2–393 (22.49)/22.61 Loss hetero ring 3403rd 393–565.5 (52.64)/52.78 Loss of (C10H6, N2") 4674th 565.5–661 (21.6)/21.59 Loss of acetate 611

C4 1st 41.84–168.47 (4.01)/4.15 Loss of lattice water 852nd 168.47–197.94 (4.21)/4.19 Loss of coordinated water 1873rd 197.94–450 (28.58)/28.79 Loss of hetero ring 2424th 450–705.7 (55.69)/55.55 Loss of (C10H6, acetate, N2") 486

190 M. Gaber et al. / Journal of Molecular Structure 1032 (2013) 185–194

1000/T gave a straight line with a slope equal (�Ea/2.303R) inwhich activation energy can be calculated and an intercept equallog(AR/bEa) in which Arrhenius pre-exponential factor can be alsocalculated. For n – 1, a plot of log(1 � (1 � a)1�n/T2(1 � n2)) vs.1000/T gave a straight line with a slope equal (�Ea/2.303R) andan intercept equal log(AR/bEa), also DE and A were calculated.See Fig. 5a

1.36 1.44 1.52 1.60 1.68 1.76

-17.5

-17.0

-16.5

-16.0

-15.5

-15.0

-14.5(a)

1000/T K-1

Y

step1 step2 step3

Fig. 5. (a) Plotting Y = log(�log (1 � a)/T2) vs. 1000/T(k�1) for n = 1 and Y = log(1 � (1 �complex 1 using Coats–Redfern equation. (b) Plotting Y = log(�log(1 � a)/T2) vs. h for n =of complex 1 using Horowitz–Metzger equation.

Horowitz–Metzgar equation:

For n ¼ 1: log½� logð1� aÞ� ¼ Eah

RT2s

ð4Þ

For n–1: log1� ð1�1Þ1�n

1� n

" #¼ log

Ab

RT2s

E

!� hEa

RTsð5Þ

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5(b)

Y

step1 step2 step3

-8 -6 -4 -2 0 2 4 6 8

a)1�n/T2(1 � n2)) vs. 1000/T(k�1) for n – 1 for the different decomposition steps of1 and Y = log(1 � (1 � a)1–n/T2(1 � n2)) vs. h for n – 1 different decomposition steps

M. Gaber et al. / Journal of Molecular Structure 1032 (2013) 185–194 191

where h = T � Ts (Ts is the DTG peak temperature, T the temper-ature corresponding to weight loss).

In this method, a straight line should be observed between theleft hand sides of Eqs. (4) and (5) vs. h with a slope of Ea=RT2

s .Fig. 5b shows the Horowitz–Metzger plots for the copper com-plexes under study.

The values of activation enthalpy (DH�), the activation entropy(DS�), and the free energy of activation (DG�) are given in Table 5using the previous two equations where:

DH ¼ E� RT

DS ¼ R ln½Ah=KT�

DG ¼ DH � TDS

From the results, the following remarks can be pointed out:

1. The high values of the energy of activation of the complexesrevealed the high stability of the investigating complexes dueto their covalent character.

2. The positive sign of DG� for the Cu(II) complexes under investi-gation revealed that the free energy of the final residue is higherthan that of the initial compound and all the decompositionsteps are non-spontaneous processes.

3. The values of DG� increased for the subsequent decompositionstages of a given complex. Increasing the values of DG�of agiven complex reflected that the rate of removal of the subse-quent ligand was lower than that of the precedent ligand. Thisis due to increasing in the values of TDS from one step toanother, which overrides the values of DH�.

4. The negative values of DS� indicated a more order activatedcomplex than reactant and/or the reaction is slow.

5. The positive values of DH� means that the decomposition pro-cesses are endothermic.

Table 5Thermodynamic parameters of the thermal decomposition of complexes using Coats–Red

Complexes Step Method n CC Thermodynamic

E� (kJ mol�1)

C1 1st CR 1 0.98608 72.08867HM 2 0.992609 86.96874

2nd CR 1 0.98627 395.2789HM 2 0.98633 410.9222

3rd CR 1 0.991 89.41051HM 0 0.98304 115.4456

C2 1st CR 1 0.97686 45.3733HM 2 0.980794 51.63

2nd CR 1 0.98615 71.85215HM 2 0.98794 84.57605

C3 1st CR 1 0.94292 25.96638HM 2 0.96113 32.65213

2nd CR 1 0.99833 21.68282HM 0 0.99495 30.10538

3rd CR 1 0.96839 295.6282HM 2 0.97042 316.503

4th CR 1 0.96154 492.9411HM 2 0.97476 528.0858

C4 1st CR 1 0.99604 11.81103HM 2 0.99556 18.36376

2nd CR 1 0.99539 144.8052HM 2 0.99467 155.9365

3rd CR 1 0.96649 243.2623HM 2 0.98579 255.4438

4th CR 0 0.99764 213.7702HM 0 0.99809 219.997

CC: Correlation coefficient.

6. The values of the kinetic parameters obtained from the approx-imation method (Horowitz–Metzgar) were larger than thosecalculated from the integral method (Coats–Redfern), Table 5.This is due to the different mathematical treatment of theobtained data.

3.4.3. Mechanistic aspectsThe assignment of the mechanism of thermal decomposition is

based on assumption that the form of g(a) depends on the reactionmechanism. Nine forms of g(a) suggested by Satava [27] were usedto enunciate the mechanism of thermal decomposition in eachstage using the method proposed by the integral method (Coats–Redfern) according to equation:

ln½gðaÞ=T2� ¼ lnðAR=bEÞ � E=RT

The different symbols have their conventional meaning: whereb is the heating rate, a is the fraction decomposed, A is Arrheniuspre-exponential, R is the universal gas constant, T is the absolutetemperature, g(a) is the kinetic model, and E is the activation en-ergy. The quantity log[g(a)/T2] appearing in various equations isplotted as a function of (1/T). The correlation coefficient for allthese nine forms was calculated and the form of g(a) for whichthe correlation has a maximum value is chosen as the mechanismof reaction. The results are listed in Table S1

For Cu(II) complex, C1, the TGA process was repeated at differ-ent heating rates, namely 5, 10, and 15 �C /min. Three or two sep-arated and well defined steps for the thermal decompositionreaction are observed at heating rate 10 and 15 �C/min or 5 �C/min, respectively. Analysis of the decomposition is carried outaccording to Coats–Redfern treatment. The values of the activationenergy (E) and the Arrhenius pre-exponential (A) were calculatedfrom the slop and the intercept, Table S2. The values of E are ofthe same order for the specific step (steps I and III) of the thermalanalysis at different heating rate. Also, the results exhibited that

fern (CR) and Horowitz–Metzger (HM) methods.

parameters

H� (kJ mol�1) A (S�1) �DS� (J mol�1 K�1) DG� (kJ mol�1)

67.29898 68776.98 0.166093 162.98582.17904 3120944.8 0.126061 154.8026390.0657 9.00E�21 0.64315 793.3472405.709 5.30E+32 0.375372 641.082283.26447 105119.8 0.164639 204.972109.2996 11128094 0.117564 196.207442.35864 324169.5 0.149354 96.5142348.61531 82588.99 0.152408 103.878566.25163 3134052 0.13564 157.622278.97553 27403.59 0.166729 191.2887

22.78862 55546018 0.107027 63.6962229.47437 179.9404 0.203803 107.371417.19573 1.96E+09 0.080288 60.5272625.6183 3.054965 0.240558 155.4478289.9619 5.77E�11 0.456111 600.8087310.8367 9.63E+22 0.188205 439.1055485.8642 4.22E�11 0.594551 991.9471521.0089 1.24E+31 0.34163 811.8052

8.591039 2.59E+09 0.07519 37.7120315.14377 0.893 0.248025 111.2033140.9733 6.43E�05 0.33709 296.3386152.1046 9.27E+15 0.057137 178.4394239.3022 1.20E�14 0.5236 488.7024251.4837 1.52E+26 0.252419 371.7153207.5325 0.006204 0.303152 434.9795213.7752 2.94E+13 0.005273 217.7214

Fig. 6. TEM image of prepared copper nanoparticles, direct mgn.: 10,000�.

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

[Cu0]=2.54x10-10 Mol.L-1

[Cu0]=8.46x10-11 Mol.L-1

Abs

orba

nce

Wavelength (nm)

Fig. 7. Absorption spectra of azo ligand (L2) on adding different concentrations ofcolloidal copper nanoparticles. As seen saturation occurs above[Cu0] = 4.23 � 10�10.

192 M. Gaber et al. / Journal of Molecular Structure 1032 (2013) 185–194

the values of the thermodynamic functions of activation of thethermal decomposition increased with the heating rate except stepII.

3.4.4. Electronic spectra and magnetic moments measurementsThe significant electronic absorption bands of Cu(II)azo-com-

plexes (C1–C4) recorded as Nujol mull are given in Table 3. Themagnetic moment of the Cu(II) complexes Table 1 suggestedsquare planar coordination for complexes C1, C2, C3, and C4, respec-tively [28]. The electronic spectra of the Cu(II) complexes in thesolid state exhibited two bands within the range 15,152–17,857and 22,727–25,000 cm�1, which are in position typically foundfor a square planar geometry and may be assigned to 2B1g ? 2E1gand 2B1g ? 2Eg, respectively [29].

3.4.5. ESR spectraThe ESR spectra of the solid Cu(II)-azo complexes (C1–C4) were

recorded at room temperature (25 �C) and are presented in Table 3,Figs. S1 and S2. The ESR spectra of complexes are characteristic of amonomeric configuration and having an axial symmetry type ofdx2–y2 ground state, which is most common for Cu(II) complexes[30]. The ESR spectra of Cu(II)-azo complexes exhibited two g val-ues. The g-values of all complexes have a positive contributionfrom the value of the free electron (2.0023) due to the measurablecovalent character in the bonding between the azo and metal ions[31]. The g11 < 2.3 indicating that the metal ligand bonding in thecomplex is regarded as covalent [32]. Also, the g values are relatedby the expression [30] G = (g11 � 2)/(g\ � 2). If the value of G > 4,the exchange interaction is negligible, whereas if G < 4, a consider-able exchange interaction is indicated in the solid complexes. TheG values in Cu(II)-azo complexes are higher than 4, indicatingthat there is no interaction between Cu centers. The g-values ofCu(II)-azo complexes with a 2B1g ground state (g11 > g\) may beexpressed [33] by:

k211 ¼ ðg11 � 2:0023ÞD2=8k

k2? ¼ ðg? � 2:0023ÞD1=2k

k2 ¼ ½k211 þ 2k2

?�=3

where k211 and k2

? are the parallel and perpendicular compo-nents, respectively, of the orbital reduction factor k, k is thespin–orbit coupling constant (�828 cm�1) for free copper, D1 andD2 are the electronic transition 2B1g ?

2Eg and 2B1g ?2B2g, respec-

tively. The calculated values of k211; k

2? indicated that k11 < k\, which

is a good evidence for the assumed 2B1g ground state. The lowervalues of k than unity are indicative of their covalent nature, whichin agreement with the conclusion obtained from the values of g11.Hathaway [34] has pointed out that for pure r-bonding,k11 � k\ � 0.77 and for in-plane p bonding, k11 < k\; while forout-of-plane p bonding, k11 > k\. In all complexes, k11 < k\, whichindicated the presence of significant in-plane p bonding.

3.5. Coordination with copper nanoparticles

Classic coordination chemistry studies quite a definite range ofproblems such as complexing metal ion, its electronic structure,coordination numbers, geometry (spatial arrangement of valenceorbitals), size, and effective charge, ligands, their charges, chargedistribution, orbitals fit to bonding to a metal ion, steric demandsof the ligand, and chelating effect, metal–ligand interaction bondtypes, and charge distribution along the metal–ligand bond. Allthese problems also arise, to some extent or other, when the objectof coordination is not a separate metal ion or several combinedions (atoms) but a spheroidal particle comprising 103 to 105 atoms.

Clearly, only its surface atoms (ions) will interact with an environ-ment (ligands), not the entire particle. Generally, nanoparticle–or-ganic ligand binding mechanism involves the same basicinteractions as in classic complexation of metal ions: electrostaticattraction between ions, donor–acceptor interactions between n-donating sites and vacant orbitals of the metal, and covalent bond-ing. However, dispersion interactions and spatial screening of thenanoparticle surface are much more important here.

The chemical reaction is the reduction of copper(II) nitrate withisopropyl alcohol:

Cu2þ þ CH3—CHðOHÞ—CH3 ! Cu0 þ ðCH3Þ2C ¼ Oþ 2Hþ

The method produces 20 nm particles size as determined fromTEM Fig. 6. The plasmon absorbance at 560 nm.

From the absorption spectra of azo-ligand in the presence andabsence of colloidal Cu0 nanoparticles at different concentrationsFig. 7; we observed that upon increasing the concentration of col-loidal Cu0 nanoparticles, the absorption of azo-ligands (L1–4) de-creases regularly with appearance of new peak due to theadsorption of azo-ligand on the surface of colloidal Cu0 nanoparti-cles leading to formation of surface complex to a limits in whichthe Cu0 surface is saturated with ligand molecules and up to this

Fig. 8. TEM images of the complex of copper nanoparticles with (L4) direct mgn.:40,000�. As seen copper nanoparticles in the center and ligand molecules surroundit.

N

N

N

R1

H

N

N

O

Cu0

-

(a) R1=H, Complex 1; R1 = SCH3, Complex 2.

N

S

Cu0

-

(c

Scheme 2. The mode of interaction between (a) L1,2, (

M. Gaber et al. / Journal of Molecular Structure 1032 (2013) 185–194 193

concentration there is no observed increase in absorption spectra,which were confirmed by TEM image Fig. 8.

The equilibrium for the complex formation between azo-li-gands and colloidal Cu0 nanoparticles can be given by equation(6), where Kapp represents the apparent association constant seeScheme 2

Azo-ligandþ Cu0nanoparticles$ Azo-ligand � � �Cu0nanoparticles

Kapp ¼½Azo-ligand � � �Cu0nanoparticles�½Azo-ligand� � ½Cu0nanoparticles�

ð6Þ

The change in intensity of the absorption peak as a result of theformation of the surface complex as shown was utilized to obtainKapp according to Benesi and Hildebrand [23].

Aobs ¼ ð1� aÞC0eazo-ligand þ aC0ecomplex ð7Þ

where Aobs is the observed absorbance of the solution containingdifferent concentrations of colloidal Cu0 nanoparticles, a is the de-gree of association between azo-ligand and Cu0 nanoparticles,eazo-ligand and ecomplex are the molar extinction coefficients at the de-fined wavelength of azo-ligand and the formed complex, respec-tively, in water Eq. (7) can be expressed as Eq. (8), where A0 andAc are the absorbance of azo-ligand and the complex respectivelywith a concentration of C0:

NN

SN

N

O

R2

-Cu0

(b) R2=H, Complex 3.

N

SN

N

O-

Cu0

) Complex 4.

b) L3, and (c) L4 with the surface of colloidal Cu0.

0.00E+000 8.00E+009 1.60E+010 2.40E+0100.0000

0.0001

0.0002

0.0003

0.0004

0.0005

L1

L2

L4

L3

1 / (

Aob

s-A0)

1 / [Cu0]

Fig. 9. Linear dependence of 1/(Aobs � A0) on the reciprocal concentration ofcolloidal copper nanoparticles coordinated with (L1–4) according to Benesi andHildebrand equation.

194 M. Gaber et al. / Journal of Molecular Structure 1032 (2013) 185–194

Aobs ¼ ð1� aÞA0 þ aAc ð8Þ

At relatively high Cu0 nanoparticles concentrations, a can beequated to:

ðKapp½Cu0 nanoparticles�Þ=ð1þ Kapp½Cu0nanoparticles�Þ: ð9Þ

In this case, Eq. (8) can be changed to (10):

1Aobs � A0

¼ 1Ac � A0

þ 1KappðAc � A0Þ½Cu0nanoparticles�

ð10Þ

The enhancement of absorbance was due to absorption of thesurface complex, that is, donor–acceptor behavior [35] based onthe good linear relationship between 1/(Aobs � A0) vs. reciprocalconcentration of colloidal Cu0 nanoparticles (stock solu-tion = 8.46 � 10�9 mole L�1) with a slope equal to 1/Kapp(Ac � A0)and intercept equal to 1/(Ac � A0) see Fig. 9, Table 2 showed the va-lue of the apparent association constant (Kapp) that determinesfrom this plot. Further, the reason for the highest apparent associ-ation constant of Cu0 nanoparticles compared to that bulk coppermay be due to the larger surface area of the Cu0 nanoparticles.

4. Conclusion

A series of coordinated Cu(II) azodyes complexes have beensynthesized by the reaction of Cu(II) acetate with some heterocy-clic azo-compounds derived from triazole and thiadiazole deriva-tives. The IR spectra showed that the ligands having triazolemoiety are coordinated with the copper(II) ion in a tridentate man-ner with ONN donor sites while azodyes having thiadiazole moietyare coordinated with the copper(II) ion in a bidentate manner withON donor sites. From the electronic, ESR spectra, conductance,magnetic moments and thermal analyses measurements, thestructure of the complexes is given in Scheme 1. The reaction be-tween the azodyes and Cu(II) ion are also studied spectrophotom-etry in solution. The stoichiometry and stability constants aredetermined. The results indicate that the investigated azodyes

can be used as an indicator for spectrophotometric determinationof Cu(II) ion. The interaction between azodyes under investigationand the Cu nanoparticles is considered. The spectral data showedthe formation of surface complex between azo-dye ligands and col-loidal copper nanoparticles through (AOH) anchoring group. Thestability constant of the prepared copper nanoparticles complexeshas been calculated.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.molstruc.2012.07.019.

References

[1] Z. Chen, F. Huang, Y. Wu, D. Gu, F. Gan, Inorg. Chem. Commun 9 (2006) 21–24.[2] X.Y. Li, Y.Q. Wu, D.D. Gu, F.X. Gan, Mater. Sci. Eng. B 158 (2009) 53–57.[3] X. Li, Y. Wu, D. Gu, F. Gan, Dyes Pigments 86 (2010) 182–189.[4] A.M. Khedr, M. Gaber, R.M. Issa, H. Erten, Dyes Pigments 67 (2005) 117–126.[5] A.K. Mahapatra, S.K. Manna, P. Sahoo, Talanta 85 (2011) 2673–2680.[6] N.M. Rageh, Spectrochim. Acta A 60 (2004) 1917–1924;

M. Tuncel, H. Kahyaoglu, M. Cakir, Trans. Met. Chem. 33 (2008) 605–613.[7] A. Demirbas, D. Sahin, N. Demirbas, S. Karaoglu, Eur. J. Med. Chem. 44 (2009)

2896–2903.[8] K.S. Bhat, B. Poojary, D.J. Prasad, P. Naik, B.S. Holla, Eur. J. Med. Chem. 44 (2009)

5066–5070.[9] G.B. Bagihalli, P.G. Avaji, S.A. Patil, P.S. Badami, Eur. J. Med. Chem. 43 (2008)

2639–2649.[10] Z.H. Chohan, S.H. Sumrra, M.H. Youssoufi, T.B. Hadda, Eur. J. Med. Chem. 45

(2010) 2739–2747.[11] K. Zamani, K. Faghihi, T. Tofighiet, M.R. Shariatzadeh, Turk. J. Chem. 28 (2004)

95–100.[12] E.R. Fernandez, J.L. Manzano, J.J. Benito, R. Hermosa, E. Monte, J.J. Criado, J.

Inorg. Biochem. 99 (2005) 1558–1572.[13] A. Mavrova, D. Wesselinova, Y.A. Tsenov, P. Denkova, Eur. J. Med. Chem. 44

(2009) 63–69.[14] E.E. Chufan, S.G. Granda, M.R. Diaz, J. Borras, J.C. Pedregosa, J. Coord. Chem. 54

(2001) 303–312.[15] N. El-Wakiel, Y.S. El-Sayed, M. Gaber, J. Mol. Struct. 1001 (2011) 1–11.[16] B. Lee, Y. Kim, S. Yang, I. Jeong, J. Moon, Curr. Appl. Phys. 9 (2009) 157–160.[17] M. Gaber, A.M. Hassanein, A.A. Lotfalla, J. Mol. Struct. 875 (2008) 322–328, and

references therein; A.M. Khedr, M.Gaber, E.H. Abd El-Zaher, Chinese J. Chem.29 (2011) 2421–2427, and references therein.

[18] A.S. Al-Kady, M. Gaber, M. Hussein, E. Ebeid, J. Phys. Chem. A 113 (2009) 9474–9484;Eur. J. Pharm. Biopharm. 77 (2011) 66–74.

[19] A.A. Athawale, P. Prachi, K. Kumar, M. Majumdar, Mater. Chem. Phys. 91 (2005)507–512.

[20] J.H. Yoe, A.L. Jones, Ind. Eng. Chem. Analyst. Ed. 16 (1944) 111–115.[21] G.H. Ayers, B.D. Narang, Anal. Chim. Acta 24 (1961) 241–249.[22] A. Ringbom, Z. Anal. Chem. 115 (1939) 332–343.[23] H.A. Benesi, J.H. Hildbrand, J. Am. Chem. Soc. 71 (1949) 2703–2707.[24] W.J. Jeary, Coord. Chem. Rev. 7 (1971) 81–122.[25] A.W. Coats, J.P. Redfern, Nature 201 (1964) 68–69.[26] H.H. Horowitz, G. Metzger, Anal. Chem. 35 (1963) 1464–1468.[27] V. Satava, Thermochim. Acta 2 (1971) 423–428.[28] B.S. Garg, R.K. Sharma, E. Kundra, Trans. Met. Chem. 30 (2005) 552–559.[29] A.A. El-Asmy, G.A. Al-Hazmi, Spectrochim. Acta A 71 (2009) 1885–1890.[30] M.J. Bew, B.J. Hathaway, R.J. Fereday, J. Chem. Soc. Dalton Trans (1972) 1229–

1237;B.J. Hathaway, D.E. Billing, Coord. Chem. Rev. 5 (1970) 143–207.

[31] I. Fidone, K.H. Stevens, Proc. Phys. Soc. 73 (1959) 116–117.[32] A.D. Kivelson, R. Neiman, J. Chem. Phys. 35 (1961) 149–155;

J.R. Wasson, C. Trapp, J. Phys. Chem. 73 (1969) 3763–3772.[33] C.J. Balhausen, An Introduction to Ligand Field Theory, Mc Graw-Hill, New

York, 1962. p. 134.[34] B.J. Hathaway, Struct. Bond. 14 (1973) 49–67.[35] A. Kathiravan, R. Renganathan, J. Colloid Interface Sci. 331 (2009) 401–407.