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Polyhedron 27 (2008) 593–601

Syntheses, characterization and X-ray crystal structures of Co(III)and Mn(II) complexes of pyrimidine derived Schiff base ligands

Somnath Roy a, Tarak Nath Mandal a, Anil Kumar Barik b, Samik Gupta a,Ray J. Butcher c, Munirathinam Nethaji d, Susanta Kumar Kar a,*

a Department of Chemistry, University College of Science, 92, A.P.C. Road, Kolkata 700 009, Indiab Department of Chemistry, St. Paul’s C.M. College, 33/1, Raja Rammohan Roy Sarani, Kolkata 700 009, India

c Department of Chemistry, Howard University, 2400 Sixth Street, N.W., Washington, DC 200 59, USAd Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India

Received 21 August 2007; accepted 15 October 2007Available online 26 November 2007

Abstract

Two pyrimidine based NNS tridentate Schiff base ligands S-methyl-3-((2-S-methyl-6-methyl-4-pyrimidyl)methyl)dithiocarbazate[HL1] and S-benzyl-3-((2-S-methyl-6-methyl-4-pyrimidyl)methyl)dithiocarbazate [HL2] have been synthesised by 1:1 condensation of2-S-methylmercapto-6-methylpyrimidine-4-carbaldehyde and S-methyl/S-benzyl dithiocarbazate. One Co(III) and one Mn(II) complexof HL1 and one Mn(II) complex of HL2 have been prepared and characterized by elemental analyses, molar conductivities, magneticsusceptibilities and spectroscopic studies. All the bis-chelate complexes have a distorted octahedral arrangement with an N4S2 chromo-phore around the central metal ion. Each ligand molecule binds the metal ion using pyrimidyl nitrogen, azomethine nitrogen and thethiolato sulfur atoms. In the free ligand moieties, the pyrimidine nitrogen atoms, azomethine nitrogen atoms and thione sulfur atomsare in EEE orientation to each other. During chelation, all the donor sites of the ligands are reoriented to ZEZ configuration in orderto facilitate the chelation process. In all the complexes, the respective ligand molecule functions as the monoanionic tridentate one. Allcomplexes were analyzed by single crystal X-ray diffraction and significant differences concerning the distortion from octahedral geom-etry of the coordination environment were observed.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Cobalt(III) and manganese(II) complexes; Pyrimidine derived dithiocarbazate ligands; X-ray structures; Spectroscopy

1. Introduction

Thiosemicarbazones of S-alkyl dithiocarbazate are oneof the most important classes of NNS donor ligands, whichhave considerable pharmacological interest due to their sig-nificant antibacterial, antimalarial, antileprotic and anti-cancer activities [1–14]. Moreover, metal complexes ofpyrimidine derived thiosemicarbazones have been exten-sively studied in recent years because of presence of thepyrimidine ring which is an integral part of the active sites

0277-5387/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2007.10.012

* Corresponding author. Tel.: +91 033 24322936; fax: +91 03323519755.

E-mail address: skkar_cu@yahoo.co.in (S.K. Kar).

of several enzymes [15–22]. Most of the metal complexes ofthiosemicarbazones particularly with copper, cobalt, plati-num and palladium show marked and diverse biologicalactivity [23–28]. However, manganese is also an essentialtrace element, forming the active sites of a number ofmetalloproteins [29]. In these metalloproteins, manganesedoes exist in any of the five oxidation states or in mixedvalence states. The most important role of manganese isin the oxidation of water in green plant photosynthesiswhere its presence in photosystem II is essential [30,31].Manganese complexes exhibit interest due to their catalyticanti-oxidant activity, in blocking oxidant trace in vitro [32].Synthesis, characterization, structure and reactivity studiesof pyrimidine derived NNS donor ligands lead to valuable

594 S. Roy et al. / Polyhedron 27 (2008) 593–601

information toward understanding the functions ofdifferent enzymes at the molecular level [15]. Attentionhas been given towards the chemistry of Co(III) andMn(II) complexes with pyrimidine derived thiosemicarba-zones primarily because of their coordination mode, novelelectrochemical and electronic properties as well as theirbiological importance [33–37]. Although there has beenconsiderable interest in transition metal complexes derivedfrom pyridine and pyrazole containing heterocyclic thio-semicarbazones but a scanty attention has been centeredtowards the coordinating properties of pyrimidine derivedthiosemicarbazones. In continuation of our previous work[38], here we report the synthesis and structural character-ization of Co(III) and Mn(II) complexes of S-methyl-3-((2-S-methyl-6-methyl-4-pyrimidyl)methyl)dithiocarbazate[HL1] and Mn(II) complex of S-benzyl-3-((2-S-methyl-6-methyl-4-pyrimidyl)methyl)dithiocarbazate [HL2]. Thecrystal and molecular structures, room temperature mag-netic properties, electrochemical properties and electronicspectra of [Co(L1)2]ClO4 Æ H2O, [Mn(L1)2] and [Mn(L2)2]complexes have been investigated.

2. Experimental

2.1. Materials

Ethyldiethoxyacetate were purchased from AldrichChemical Company, USA and were used as received. Allother solvents and reagents were obtained from commer-cial sources and purified by standard procedures [39].S-methy/S-benzyl dithiocarbazate and 2-S-methylmerca-pto-6-methylpyrimidine-4-carbaldehyde were synthesizedfollowing the reported methods [40,41].

Caution! Since perchlorate compounds in presence oforganic ligands are explosive, only a small amount of thematerial should be prepared and it should be handled withcare.

2.2. Physical measurement

Elemental analyses [carbon, hydrogen, nitrogen and sul-fur] of 1 and 2 complexes were determined with a Perkin–Elmer CHNS/O analyzer 2400 at the Indian Associationfor the Cultivation of Science, Kolkata. The molar conduc-tance values of the complexes were measured in purifiedmethanolic solution with a Systronic model 304 digital con-ductivity meter. The electronic spectra of the complexes inpurified acetonitrile solution were recorded on a Hitachimodel U-3501 spectrophotometer. IR spectra [KBr pellet,4000–400 cm�1] were recorded on a Perkin–Elmer model883 infrared spectrophotometer. Room temperature mag-netic susceptibilities were measured with a PAR 155 vibrat-ing sample magnetometer. Cyclic voltammetry was carriedout using Sycopel model AEW2 1820 F/S instrument. Themeasurements were performed at 300 K in acetonitrilesolution containing 0.2 (M) TEAP as the supporting elec-trolyte and 10�3–10�4 (M) of the Co(III) and Mn(II)

complexes, deoxygenated by bubbling with nitrogen. Theworking, counter and reference electrodes used were a plat-inum wire, a platinum coil and an SCE.

2.3. Preparation of the ligands

S-methyl-3-((2-S-methyl-6-methyl-4-pyrimidyl)methyl)dith-iocarbazate [HL1] and S-benzyl-3-((2-S-methyl-6-methyl-4-pyrimidyl)methyl)dithiocarbazate [HL2] were synthesizedfollowing the method as reported earlier [38]. The twoligands HL1 and HL2 were synthesized in a similar wayby refluxing a methanolic solution (10 cm3) of 2-S-methyl-mercapto-6-methylpyrimidine-4-carbaldehyde (0.168 g,1 mmol) with S-methyl dithiocarbazate (0.122 g, 1 mmol)and S-benzyl dithiocarbazate (0.198 g, 1 mmol), respec-tively, also taken in methanol (10 cm3). Reflux was contin-ued for ca. 2 h at water bath temperature during which asolid microcrystalline compound separated. It was filteredoff, washed several times with cold methanol and dried invacuo over fused CaCl2. Single crystals of HL1 and HL2,suitable for X-ray diffraction were obtained by slow evap-oration of the respective methanolic solution of the ligand.

HL1: pale yellow; yield, 0.217 g (80%); m.p. (�C) 186, m/z 272 (M+, 100%). Anal. Calc. for C9H12N4S3: C, 39.70; H,4.41; N, 20.58; S, 35.29. Found: C, 39.83; H, 4.72; N, 20.52;S, 35.23%.

HL2: yellow; yield, 0.278 g (80%); m.p. (�C) 175, m/z 348(M+, 100%). Anal. Calc. for C15H16N4S3: C, 51.72; H,4.59; N, 16.09; S, 27.05. Found: C, 51.84; H, 4.89; N,16.01; S, 27.51%.

2.4. Preparation of the complexes

In continuation of our previous work which describedthe preparation and structural studies of Ni(II) complexof HL1 and Co(III) and Fe(III) complexes of HL2, herethree complexation reactions of these ligands (HL1 andHL2) have been investigated with Co(III) and Mn(II) ofHL1 and Mn(II) of HL2. All the complexes [1–3] wereobtained from suitable reaction mixtures of the respectiveligand and the corresponding hydrated metal salts, takenin 2:1 molar proportion in suitable solvent. The oxidationto Co(III) occurs during preparation of the complex.Although the mechanism of this oxidation is not provenwe can safely assume that the starting cobalt(II) salt pre-sumably undergoes aerial oxidation in presence of theligand in methanolic solution.

2.4.1. [Co(L1)2]ClO4 Æ H2O (1)

A methanolic solution (5 cm3) of Co(ClO4)2 Æ 6H2O(0.0915 g, 0.25 mmol) was added dropwise to a solutionof HL1 (0.136 g, 0.5 mmol) in the same solvent (15 cm3)with constant stirring. The resulting pink coloured solutionwas further stirred for ca. 15 min and then refluxed at waterbath temperature for ca. 2 h. The solution was cooled toroom temperature and filtered. The filtrate was kept forslow evaporation. After two days, X-ray quality crystals

S. Roy et al. / Polyhedron 27 (2008) 593–601 595

of 1 were isolated. Yield, 0.45 g (70%). Anal. Calc. forC18H22.5ClCoN8O4.25S6 (1): C, 30.61; H, 3.19; N, 15.87;S, 27.21. Found: C, 30.69; H, 3.25; N, 15.85; S, 27.20%.IR: m (cm�1) 2925(mNH); 1596(mC@Npym); 1510(mCH@N);1267(mC@S); 1079(mN–N); 741(mC@S); UV–Vis, kmax: 343,413, 711 nm. DRS, kmax: 436, 517, 703 nm. kM (MeOH,X�1 cm2 mol�1): 93. leff (at 298 K): 0.00 B.M.

2.4.2. [Mn(L1)2] (2)

HL1 (0.136 g, 0.5 mmol) was dissolved in chloroform(20 cm3). Solid Mn(ClO4)2 Æ 6H2O (0.362 g, 1 mmol) wasadded to the above solution with constant stirring using amagnetic stirrer and few drops of methanol was added to dis-solve Mn(ClO4)2 Æ 6H2O. To it two drops of triethyl aminewere added. The resulting red coloured solution was stirredfor an additional 2 h. The solution was filtered and the fil-trate was kept for slow evaporation. After two days, a redcoloured sticky mass was obtained. This was dried over fusedCaCl2 and dissolved in purified CH3CN. It was filtered andthe filtrate was kept for slow evaporation. After three days,single crystals, suitable for X-ray diffraction were separated.Yield, 0.16 g (60%). Anal. Calc. for C18H22MnN8S6 (2): C,36.12; H, 3.68; N, 18.74; S, 32.12. Found: C, 36.20; H,3.72; N, 18.72; S, 32.15%. IR: m (cm�1) 2920(mNH);1582(mC@Npym); 1508(mCH@N); 1268(mC@S); 1074(mN–N);749(mC@S). UV–Vis, kmax: 425 nm. kM (MeOH, X�1 cm2

mol�1): 20. leff (at 298 K): 5.82 B.M.

2.4.3. [Mn(L2)2] (3)

Complex 3 was prepared using the same procedure andreaction stoichiometry as for 2. The colour of the resultingreaction mixture was red. Yield, 0.16 g (60%). Anal. Calc.for C30H30MnN8S6 (3): C, 48.00; H, 4.00; N, 14.93; S,25.60. Found: C, 48.15; H, 4.12; N, 14.91; S, 25.57%. IR:m (cm�1) 2929(mNH); 1575(mC@Npym); 1512(mCH@N); 1334,1267(mC@S); 1072(mN–N); 745(mC@S). UV–Vis, kmax: 352,428 nm. kM (MeOH, X�1 cm2 mol�1): 25. leff (at 298 K):5.85 B.M.

2.5. Single crystal X-ray crystallography

Relevant crystallographic data are summarized in Table1. The intensity data for the ligand HL1 and the complexes1, 2 and 3 were collected on a Bruker SMART CCD dif-fractometer, using graphite monochromated Mo Ka radia-tion [k = 0.71073 A] in the x and u scan mode and x scanmode at 100(2) K, 296(2) K, 293(2) K and 273(2) K forHL1, 1, 2 and 3, respectively. Accurate unit cell parametersand orientation matrices for data collection were obtainedfrom least-squares refinement using the programs SMART

and SAINT [42] and the data were corrected for absorptionusing SADABS [42]. All structures were solved using SHELXTL

and the SHELX-97 packages of programmes [42] i.e. SHELXS-97 for the solution of the structure and refined by full-matrix least-squares technique based on F2 [SHELXL-97].Positions of H-atoms were treated as riding on their parentatoms.

3. Results and discussion

3.1. Syntheses

These newly formed three complexes (1–3) were charac-terized by elemental analyses, solution electrical conductiv-ity, magnetic susceptibility, IR, UV–Vis spectra andelectrochemical property. The results are consistent withthe proposed mononuclear formulation. The complexeswere found to be fairly soluble in most of the commonorganic solvents. Molar conductivity value of 1 is in accordwith 1:1 electrolyte behavior but the molar conductivityvalues of 2 and 3 show non-electrolytic behavior in nature.The complex 1 is diamagnetic in nature. The effective mag-netic moment values (5.82 B.M. for 2 and 5.85 B.M. for 3)suggest the high spin nature of the paramagnetic complexeswith five unpaired electrons.

3.2. Description of crystal structures

Structural representation of the ligand HL1 and that ofthe complexes 1, 2 and 3 with atom numbering schemes areshown in Figs. 1–4, respectively. Selected metrical parame-ters are listed in Table 2. The unit cell of both HL1 and 1

comprises of two molecules and those of both 2 and 3 com-prise of eight molecules. A packing diagram of 2 and 3 areshown in Figs. 5 and 6, respectively.

3.2.1. Structural description of HL1

The structure of HL1 (Fig. 1) shows that the neutral tri-dentate ligand has a EEE configuration based on the C(6)–C(7), N(2)–C(7) and N(4)–C(8) bonds for the donor centersnitrogen, nitrogen and sulfur, respectively. But, on coordi-nation to the metal ion, the ligand undergoes structuralreorientation about the C(6)–C(7), N(2)–C(7) and N(4)–C(8) changes from EEE to ZEZ configuration. This sug-gests that a possible rotation about the azomethine doublebond on coordination to the metal ion and this ZEZ con-figuration is necessary for its participation as a tridentateNNS chelate ligand. From the analysis of the bond lengths(Table 2) of HL1, we may conclude that the ligand exists inthione form in its solid state but in presence of metal ions,the ligand behaves as a monoanionic tridentate one in itsthiol tautomer with concomitant deprotonation of the thiolhydrogen. The tridentate ligand is nearly planar whichis evident from the analysis of torsion angles of HL1

(Table 3).

3.2.2. Structural description of 1The complex 1 contains [Co(L1)2]+ cation and a ClO4

�

anion. The single crystal X-ray diffraction studies show adistorted octahedral geometry for the compound. A listof selected bond parameters and angles of 1 is presentedin Table 2. The Co(III) center is coordinated in an N4S2

meridional manner by two monodeprotonated ligand moi-eties using pairs of cis-pyrimidyl-N, trans-azomethine-Nand cis-thiolate-S atoms and form four five membered

Table 1Experimental data for crystallographic analysis

Compound HL1 1 2 3

Empirical formula C9H12N4S3 C18H22.5ClCoN8O4.25S6 C18H22MnN8S6 C30H30MnN8S6

Formula weight 272.44 705.68 597.74 374.96Temperature (K) 100(2) 296(2) 293(2) 273(2)Wavelength (A) 0.71073 0.71073 0.71073 0.71073Crystal system triclinic triclinic monoclinic monoclinicSpace group P�1 P�1 C2/c C2/cUnit cell dimensions

a (A) 7.2222(4) 8.9950(18) 16.261(5) 22.449(10)b (A) 9.3683(6) 12.560(2) 12.510(4) 14.372(6)c (A) 10.3845(6) 13.426(3) 26.392(12) 12.328(5)a (�) 68.4650(10) 78.834(3)b (�) 72.4370(10) 88.597(3) 99.132(7) 111.395(7)c (�) 89.5870(10) 75.617(3)

Volume (A3) 618.84(6) 1441.0(5) 5301(3) 3703(3)z 2 1 8 8Dcalc (Mg m�3) 1.462 1.627 1.498 1.345Absorption coefficient (mm�1) 0.577 1.165 0.994 0.727F(000) 284 721 2456 1548Crystal size (mm) 0.30 · 0.55 · 0.65 0.03 · 0.21 · 0.28 0.29 · 0.32 · 0.34 0.29 · 0.32 · 0.35h Range (�) for data collection 2.35–30.53 2.52–29.25 2.50–27.00 1.7–27.5Index ranges �10 6 h 6 10,

�13 6 k 6 13,�14 6 l 6 14

�12 6 h 6 11,�17 6 k 6 17,�18 6 l 6 18

�20 6 h 6 21,�15 6 k 6 10,�32 6 l 6 33

�25 6 h 6 28,�18 6 k 6 19,�16 6 l 6 16

Goodness-of-fit on F2 1.083 0.983 0.986 0.835Completeness to h = 25.00� (%) 99.9 98.4 100 100Independent reflections [Rint] 3744 [0.0130] 8037 [0.0676] 5811 [0.1526] 4309 [0.138]Absorption correction multi-scan multi-scan empirical empiricalMaximum and minimum

transmissionnone 0.7362 and 0.9659 0.7273 and 0.7620 0.7829 and 0.8174

Refinement method full-matrix least-squareson F2

full-matrix least-squares onF2

full-matrix least-squareson F2

full-matrix least-squareson F2

Data/restraints/parameters 3744/0/148 8037/3/358 5811/0/304 4309/0/206Reflections collected 7479 16175 10916 15295Final R indices [I > 2r(I)] R1 = 0.0301,

wR2 = 0.0820R1 = 0.0567,wR2 = 0.1429

R1 = 0.0859,wR2 = 0.1843

R1 = 0.0827,wR2 = 0.2099

R Indices (all data) R1 = 0.0314,wR2 = 0.0833

R1 = 0.0971,wR2 = 0.1609

R1 = 0.1728,wR2 = 0.2245

R1 = 0.2040,wR2 = 0.2955

Largest difference in peak and hole(e A�3)

0.508 and �0.222 1.235 and �0.995 0.779 and �1.097 0.75 and �0.81

Fig. 1. Structural representation and atom numbering scheme of HL1.

596 S. Roy et al. / Polyhedron 27 (2008) 593–601

chelate rings. The meridional configurations of the NNStridentate thiosemicarbaone ligands around the metal ionhave been found in other bis-ligated metal chelates[43,44]. Consequently, the two pyrimidyl nitrogen atoms[N(1A) and N(1B)] and two thiolato sulfur atoms [S(1A)

and S(1B)] are positioned in a square plane. In 1, the atomsdefining the equatorial plane lie �0.257, 0.256, �0.236 and0.236 A, respectively, are out of the least squares planethrough them. The axial positions are occupied by thetwo remaining azomethine nitrogen atoms [N(2A) andN(2B)] which is evident from the N(2A)–M–N(2B) angle[169.60�]. From the bond angles Table 2, it is observed thatthe coordination geometry is quite far from a perfect octa-hedron due to the steric interactions and asymmetric nat-ure of the ligand molecules. The maximum distortionsfrom the ideal octahedral geometry occur for N(1A)–Co–N(2A) [81.43�] and N(1B)–Co–N(2B) [81.36�] angles inCo(III) complex [38]. The dihedral angle formed by themean planes of the bicyclic chelate systems of each of theligands is 88.78�. The C(8)–S(1) [A and B] bond distancesappear to be longer than those reported for free thiosemi-carbazone such as 1.678(2) A in 4-formylpyridinethio-

Fig. 2. Structural representation and atom numbering scheme of cationiccomplex 1.

Fig. 3. Structural representation and atom numbering scheme of 2.

Fig. 4. Structural representation and atom numbering scheme of 3.

Table 2Selected bond distances (A) and angles (�) involving HL1, 1, 2 and 3 (hereM stands for the corresponding metal ion)

HL1 1 2 3

Bond distances (A)

S2–C8 1.6595(12) M–S1A 2.2195(12) 2.519(3) 2.555(2)N2–C7 1.2792(16) M–S1B 2.2196(11) 2.546(2) 2.555(2)C6–C7 1.4666(16) M–N1A 2.021(3) 2.307(5) 2.332(5)N4–C8 1.3478(16) M–N1B 2.033(3) 2.382(5) 2.332(5)

M–N2A 1.896(3) 2.234(5) 2.233(5)M–N2B 1.899(3) 2.250(5) 2.233(5)S1A–C8A 1.737(4) 1.727(8) 1.717(6)S1B–C8B 1.709(4) 1.694(8) 1.717(6)N2A–C7A 1.284(5) 1.289(9) 1.279(9)N2B–C7B 1.288(5) 1.258(9) 1.279(9)N4A–C8A 1.295(5) 1.303(8) 1.314(8)N4B–C8B 1.315(5) 1.321(9) 1.314(8)

1 2 3

Bond angles (�)

S1A–M–S1B 90.99(4) 100.59(8) 104.90(8)S1A–M–N1A 166.54(8) 146.72(14) 145.92(12)S1A–M–N1B 89.88(8) 94.42(13) 89.81(12)S1A–M–N2A 85.21(9) 75.44(14) 74.91(13)S1A–M–N2B 87.58(9) 93.69(14) 101.49(13)S1B–M–N1A 89.91(8) 92.95(14) 89.81(12)S1B–M–N1B 166.72(8) 143.33(13) 145.92(12)S1B–M–N2A 87.20(8) 103.07(14) 101.49(13)S1B–M–N2B 85.44(8) 75.05(14) 74.91(13)N1A–M–N1B 92.32(11) 92.43(18) 94.82(16)N1A–M–N2A 81.43(11) 71.92(18) 72.07(16)N1A–M–N2B 105.88(11) 119.25(18) 112.07(17)N1B–M–N2A 106.08(11) 113.07(18) 112.07(17)N1B–M–N2B 81.36(11) 70.72(18) 72.07(16)N2A–M–N2B 169.60(11) 168.56(19) 174.25(17)

S. Roy et al. / Polyhedron 27 (2008) 593–601 597

semicarbazone [45] and 1.684(4) A in 2-keto-3-ethoxybu-tyraldehyde-bis(thiosemicarbazone) [46].

3.2.3. Structural descriptions of 2 and 3Structural representations of [Mn(Ln)2] (where n = 1

and 2) with atom numbering schemes are shown in Figs.3 and 4, respectively. The anionic ligands HL1 and HL2

act as tridentate ones which coordinate to manganese(II)through the pyrimidyl nitrogen N(1), azomethine nitrogenN(2) and thiolato sulfur S(1) atoms with ZEZ configura-tion based on the C(6)–C(7), N(2)–C(7) and N(4)–C(8)bonds for the NNS donor set. Like 1, the manganeseatoms (in 2 and 3) are also coordinated in a distorted octa-hedral arrangement in a trans-N(2)-cis-N(1)-cis-S(1) fash-ion. Of these sulfur involving rings are somewhat planarand slightly puckered, while the other two are almostplanar. In complexes 2 and 3, the atoms defining the equa-

torial plane [N(1) and S(1)] (A and B) lie �0.796, 0.787,�0.623 and 0.664 A in 2 and �0.776, 0.776, �0.616 and0.616 A in 3 are out of the least squares plane throughthem. The two thiolato sulfur atoms are observed to bedeviated farthest from the central Mn(II) atoms, at

Fig. 5. Packing diagram of 2 along c axis.

Fig. 6. Packing diagram of 3 along c axis.

598 S. Roy et al. / Polyhedron 27 (2008) 593–601

distances of 2.519(3) A [Mn–S(1A)] and 2.546(2) A [Mn–S(1B)] for 2 and 2.555(2) A [Mn–S(1)] (A and B) for 3,while the Mn–N(1) and Mn–N(2) bonds are observed atan average distances of 2.33 A and 2.23 A, respectively.The Mn–Nazomethine, Mn–Npym and Mn–S bond lengthsare comparable well with the previously reported manga-nese(II) complexes [47,48]. These comparatively largerbond lengths are indicative of weak bonding of the ligandto the metal. The coordination process lengthens theC(8A)–S(1A) bond by 0.068 A, C(8B)–S(1B) by 0.035 Afor 2 and C(8)–S(1) (A and B) bond by 0.061 A for 3.The decrease in bond length of N(4A)–C(8A), N(4B)–C(8B) by 0.045 and 0.027 A for 2 and N(4)–C(8), N(4)–

Table 3Selected torsion angles (�) involving HL1, 1, 2 and 3 torsion angles (�)

HL1

N1–C6–C7–N3 �175.62(10) N1A–C6A–C7A–NN3–N4–C8–S2 175.95(8) N2A–N4A–C8A–S

N1B–C6B–C7B–N2N2B–N4B–C8B–S1

C(8) (A and B) by 0.036 A for 3, respectively, on coordina-tion confirms the deprotonation after enolization [49]. TheS–Mn–S angles are much greater than 90� [100.59(8)� for 2

and 104.90(8)� for 3]; it is closer to the tetrahedral valueand demonstrates that the two ligands exert significant ste-ric effects on each other. The –S–CH2–Ph in 3 exertinggreater steric congestion over –S–CH3 in 2, registers higherS–Mn–S angle in 3. The negative charge of themonoanionic ligands is delocalised over the mainbackbone of the respective ligand and the increased S–Cbond distances are consistent with the single bond charac-ter, while the imine C–N distances and both thioamide C–N distances (2 and 3) indicate considerable double bondcharacter. In both the cases, the tridentate ligands arenearly planar. The displacement from coplanarity is indi-cated by the dihedral angle between the pyrimidyl ringand the plane defined by the five membered chelate ringsMn–S–C–N–N, 0.94� and 2.34� for 2 and 7.87� for 3.The pyrimidine ring thus makes an extended coplanar sys-tem with the two chelate rings. This extended coplanar ringsystem of the two ligands (HL1 and HL2) face each otherorthogonally. This is supported from the analysis of theirdihedral angles [88.52� for 2 and 81.66� for 3].

4. Characterization of the complex species

All the three complexes gave satisfactory C, H, N and Sanalyses. Spectral properties of these complexes aredescribed below in details.

4.1. IR spectra of the complexes

A comparative study of the IR spectral data of thereported complexes with those of the uncomplexed ligandgives supportive information regarding bonding sites ofthe ligand molecules. The Schiff base contains a protonadjacent to the thiocarbonyl group and consequently canexhibit thione–thiol tautomerism. The IR spectra of theligands, however, display no band at ca. 2600 cm�1 corre-sponding to mS–H [50] indicating that the ligands remainas a thione tautomer. The ligand (HL1) when dissolved inmethanol in presence of Co(II) salts, quickly converts tothe corresponding ‘thiol’ form with the concomitant for-mation of cobalt(III) complexes of the deprotonated mer-captide form of the ligand.

Some characteristic and recognizable spectral changeshave been noticed in the IR spectra of the complexes ascompared to that of the free ligands and the data can be

1 2 3

2A 0.8(5) 4.1(9) 1.0(9)1A �0.1(5) 0.3(9) �5.0(8)B �2.3(5) 8.1(10) 1.0(9)B �0.3(4) �4.0(10) �5.0(8)

Fig. 7. Cyclic voltammogram of 2. Scan rate 50 mV/s.

Fig. 8. Cyclic voltammogram of 3. Scan rate 50 mV/s.

S. Roy et al. / Polyhedron 27 (2008) 593–601 599

successfully utilized for an additional support of the bond-ing mode. The corresponding spectral data of the twoligands are described in our earlier report [38]. A red shiftof m(C@N) from 1511 cm�1 (in HL1) and 1523 cm�1 (in HL2)to 1510 cm�1 (in 1), 1508 cm�1 (in 2), 1512 cm�1 (in 3) isconsistent with coordination of azomethine nitrogen [51].The presence of a band at ca. 550 cm�1is also assignableto (mM–N) [38] (here M stands for the corresponding metalion). A blue shift of m(C@Npym) from 1570 cm�1 in theuncomplexed ligand toward ca. 1584 cm�1 in all the com-plexes is also consistent with coordination of pyrimidinenitrogen [51]. For all the complexes, the thioamide bandat ca. 775 cm�1 in the free ligand owing to m(C@S) undergoesa red-shift by ca. 30 cm�1, pointing to the sulfur atom ofthiol as a possible coordination site [51]. The low frequencyband appearing in the region at ca. 315–320 cm�1, 340–360 cm�1 and 490–515 cm�1 in all the three complexesare assigned as mM–N(pym), mM–S and mM–N(azomethine), respec-tively [52].

4.2. UV–Vis and DRS of the complexes

The DRS of 1 exhibits a broad band at ca. 436 nm withtwo distinct shoulders at low energy side (at ca. 517 nm andca. 703 nm). The broad band is due to the S! Co(III)charge transfer transition as is also evident from the solu-tion spectral study. The other two bands may be assignedas 1A1g! 1T1g and 1A1g! 3T2g transitions, respectively,[53]. In methanolic solution, it displays S! Co(III) chargetransfer transition (LMCT) at 413 nm with one d–d bandat 711 nm. The intense band at ca. 343 nm is due to theintraligand CT band.

The UV–Vis spectral data of HL1 and HL2 have alreadybeen reported in our previous communication. The com-plex 2 in CH3CN displays the absorption band at425 nm, which is assigned as the intraligand CT band(n! p* transition of imine portion of the TSC moiety)[54]. The intense band of the complex 3 in CH3CN areexhibited at 428 and 352 nm which are due to n! p* tran-sition of imine portion of the TSC moiety and p! p* tran-sition of the aromatic ring, respectively [55]. Theabsorption of the organic ligand tailing into the visibleregion obscure the very weak d–d absorption bands ofthe manganese(II) complexes [55].

5. Electrochemistry

Figs. 7 and 8 show the cyclic voltametric profile of thecomplexes 2 and 3, respectively. Cyclic voltametric testson the complexes reveal the presence of an irreversible,anodic process (Ep = +0.79 V for 2 and +0.82 V for 3)and quasi-reversible reduction (E1/2 = �1.06 V for 2 and�1.04 V for 3), which regenerates in the reverse scan ofthe oxidation of the original complexes. On this basis, theoxidation may be attributed to the Mn(II)/Mn(III) couple,where the Mn(III) complex after its formation, being quiteunstable in the original geometry, evolves to an unidenti-

fied differently coordinated species (fragmentation cannot be ruled out), which, however upon reduction regener-ates the Mn(II) complex. Similar type of observation wasrecorded by Godbole et al. [56] in their [Mn(mesalim)2Cl]complex (Hmesalim is methyl salicylimidate) where manga-nese being in originally in +III stable oxidation state iselectrochemically oxidized to unstable Mn(IV) species.From the comparison of E1/2 values of Mn(III)/Mn(II)couple for 2 and 3, we may state that E1/2 for 3 being com-paratively of higher value and hence HL2 stabilizes Mn(II)more than HL1.

6. Concluding remarks

We have prepared Co(III) and Mn(II) complexes of HL1

and a Mn(II) complex of HL2 where HL1 and HL2 arepyrimidine derived similar Schiff base ligands. In all thecomplexes, the ligands HL1 and HL2 behave as the mono-anionic NNS tridentate one through deprotonation of oneproton from their corresponding thiol tautomer. Each ofthem forms bis-chelate with the metal ions having distorted

600 S. Roy et al. / Polyhedron 27 (2008) 593–601

octahedral geometry. The deprotonation accompaniedwith tautomerization produces a delocalised ring systemwith overall intermediate interatomic bond distances onthe ligand network. Cyclic voltametric studies show an irre-versible anodic process and a quasi-reversible reduction.

Acknowledgement

Financial support [01(1916)/04/EMR-II] from Councilof Scientific and Industrial Research (CSIR), New Delhi,India, is gratefully acknowledged.

Appendix A. Supplementary material

CCDC 657938, 657939, 657940 and 657941 contain thesupplementary crystallographic data for HL1, 1, 2 and 3.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from theCambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; ore-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-ciated with this article can be found, in the online version,at doi:10.1016/j.poly.2007.10.012.

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