Manuscript accepted on : November 04, 2008
Published online on: 28-12-2008
Comparative study on the IR spectra of some transition metal dithiocarbamates
Mohammad Tarique1 and Mohammad Aslam1*
Department of Chemistry, G.F.(P.G.) College, Shahjahanpur India.
ABSTRACT: This study explores how the changes of the electron density due to the substitution of the various substituents on the nitrogen atom of the various amines manifest themselves in the shifts of principal absorption bands attributed to the n(C-N), n(C-S) and n(M-S) stretching vibrations.
KEYWORDS: R spectra; transition metal dithiocarbamates
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Introduction
Extensive studies have been reported on the mode of bonding of dithiocarbamate ligands to the metal.1-3 The different resonating structures of the dithiocarbamate moiety can be represented as:
The extent to which these resonance forms contribute to the other structure and this effect on the physical and chemical properties have been extensively studied. It was suggested that the resonance form (c) contributes to the structure to a considerable extent, but resonance forms (a) and (b) too contributing equally to the structure provided dipole moment studies were also taken into consideration.4 It was also pointed out that the contribution of the resonance form (c) is greater than that of the others to the total structure of dithiocarbamato complexes and it increases appreciably in n-alkyl derivatives.5
Extending our work6,7 on transition metal dithiocarbamates, we are reporting here the comparative study on the IR spectra of some transition metal dithiocarbamates.
Expermental
All amines viz. dimethylamine, diethylamine, dipropylamine, diisopropylamine, diisobutylamine, piperidine; carbon disulphide, sodium hydroxide, salts of nickel, copper and zinc (all E. Merck) were used as such. Solvents (all BDH) were purified by standard methods8 before use.
In the present work, the replacement reaction method9 was adopted for the synthesis of all the complexes studied. This method involves replacement reaction using the sodium salt of the dithiocarbamate with metal salt.
2(R2NCS2)Na + MX2à M(R2NCS2)2 + 2NaX
The infrared spectra of the prepared sodium dithiocarbamates as well as metal dithiocarbamates were scanned by Nujol technique in the region 4000-200 cm-1 on Perkin Elmer Model 1620 Fourier-Transform Infrared (FT-IR) spectrophotometer by Jamia Millia Islamia University, New Delhi, India.
Results and Discussion
The major interest in the preparation of these transition metal dithiocarbamates was to study the effect of the attachment of dithiocarbamate ligand to 3d-transition metal in presence of different organic substituents (R). Dithiocarbamates, general formula M(R2NCS2)2, possess a lone electron pair at the nitrogen atom, that brings about conjugation with CSS group. Thus the electron density at the C-N, C-S and M-S bonds can be affected by the replacement of hydrocarbon chains.
The Tables-1 and 2 present the IR spectral data of sodium dithiocarbamates and dithiocarbamate chelates respectively. Table-3 presents data on the C-N, C-S and M-S average bond lengths found in literature.10-18
Table 1: IR Spectral data of sodium dithiocarbamates.
Compound | ν(C-N) | ν(C-S) |
Na[Me2 dtc] | 1490 | 989 |
Na[Et2 dtc] | 1480 | 990 |
Na[Pr2 dtc] | 1470 | 980 |
Na[iPr2 dtc] | 1440 | 945 |
Na[iBu2 dtc] | 1470 | 980 |
Na[Pip dtc] | 1475 | 980 |
Table 2: IR Spectral data of Ni(II),Cu(II) and Zn(II) dithiocarbamates of the type ML2.
L |
NiL2
ν(C-N) ν(C-S) ν(M-S) |
CuL2
ν(C-N) ν(C-S) ν(M-S) |
ZnL2
ν(C-N) ν(C-S) ν(M-S) |
[Me2 dtc]– | 1532 974 384 | 1528 976 352 | 1525 975 379 |
[Et2 dtc]– | 1522 993 387 | 1508 995 356 | 1505 995 380,400 |
[Pr2 dtc]– | 1516 976 385 | 1505 985 354 | 1501 976 375,385 |
[iPr2 dtc]– | 1503 945 402 | 1495 943 376 | 1485 945 395, 410 |
[iBu2 dtc]– | 1508 985 391 | 1505 985 386 | 1485 987 390 |
[Pip dtc]– | 1518 1000 388 | 1505 996 398 | 1490 985 399 |
The n(C-N) band position was seen to be affected mainly by the nature of central atom and the character of the group attached to the nitrogen atom. On the other hand the structure of the coordination sphere did not affect appreciably the positions of the absorption bands pursued, it influenced the band shape in the region 440-350 cm-1. The n(M-S) absorption bands comprised vibrations of all M-S bonds and were also affected by vibrations of the remaining parts of the molecule.
The effect of the substituents bonded at the nitrogen atom on the electron density distribution at the C-N, C-S and M-S bonds could be best studied on the nickel bis(dithiocarbamate) chelates, where the inner coordination sphere was the same in all cases, viz. planar (the nickel atom was in the centre of symmetry of the basic cell).
If a methyl group is substituted for hydrogen at the nitrogen atom, the wave number of the n(C-N) band increases substantially,4,19-21 due to the hyperconjugation effect of the methyl group. As seen from the Table-2, lengthening of the alkyl chain was accompanied by a decrease of the n(C-N) band wave number; the lowest values were observed for Ni(Bu2 dtc)2, Cu(Pr2 dtc)2 and Zn(Pr2 dtc)2; further lengthening of the chain did not lead to appreciable changes. Taking into account that the lengthening of the alkyl chain on from Ni(Et2 dtc)2 to Ni(Pr2 dtc)2 was not accompanied by a change in the C-N bond length, the gradual lowering of the n(C-N) vibrational frequency with lengthening alkyl chain, occurring in spite of the increasing inductive effect of the alkyl groups, could be attributed to the growing mass of the latter. This was in accordance with the observations22 concerning the proportions of influence of the kinematic and electronic factors on the vibrations of bonds in dithiocarbamates. The frequencies of the chelates derived from piperidine and their C-N interatomic distances were comparable with alkyl groups possessing the same number of carbon atoms.
The dependence of the n(C-S) vibrational frequency on the length of the alkyl chain at the nitrogen atom was somewhat complex. The lowering of the n(C-S) frequencies with the lengthening alkyl chain was ascribed to the growing mass of the alkyl group. The irregularities in the wave numbers were associated with their spatial arrangement.
As evident from the Table-3, the C-N and C-S bond lengths of the nickel chelates practically remained constant with changing alkyl substituent at the nitrogen atom, which indicates that the +I effect of the alkyl groups practically did not appear at these bonds. However in Ni(iPr2 dtc)2 the electron density at the M-S bond was enhanced and the bond was appreciably shortened, which could be explained by combination of the hyperconjugation and inductive effects of the isopropyl groups and conjugation of the electronic system of the substance, enabling charge transfer from the alkyl groups as far as the M-S bonds. This view was also supported by the n(M-S) vibrational wave numbers of Ni(iPr2 dtc)2, Cu(iPr2 dtc)2 and Zn(iPr2 dtc)2, which were 17, 22 and 20 cm-1 respectively, higher than those of the respective n-derivatives.
Table 3: Average bond lengths (pm) of C-N, C-S and M-S bonds in some Ni(II), Cu(II) and Zn(II) dithiocarbamates.
Compound | C-N Bond | C-S Bond | M-S bond |
Ni[Et2 dtc]2 | 133 | 170.7 | 2201 |
Ni[Pr2 dtc]2 | 133 | 170.8 | 220.3 |
Ni[iPr2 dtc]2 | 133 | 170.5 | 218.1 |
Ni[Pip dtc]2 | 132 | 167.8 | 220.6 |
Cu[Me2 dtc]2 | 131 | 172.1 | 231.05 |
Cu[Et2 dtc]2 | 134 | 171.7 | 242.1 |
Cu[Pr2 dtc]2 | 133 | 172.2 | 242.8 |
Cu[Pip dtc]2 | 133 | 172.5 | 229.5 |
Zn[Me2 dtc]2 | 134.7 | 172.1 | 245.3 |
Zn[Et2 dtc]2 | 135.7 | 172.7 | 246.5 |
In the case of sodium dithiocarbamates, the electronic effects were affected by the presence of the lone electron pairs at the sulphur atom, which prevented delocalization of the nitrogen electrons towards the –CS2 group, because the negative charge at the sulphur atom acted against the electron donor tendency of the alkyl groups. The hyperconjugation effect and the kinematic factors affected the n(C-N) frequencies similarly as in the dithiocarbamate chelates, but the n(C-N) bands appeared at lower wave numbers because of the lower p– share in the C-N bond. The mutually close positions of the n(C-S) bands of the sodium dithiocarbamates and metal dithiocarbamates implied that the effect of the M-S bond on the n(C-S) vibrational frequencies was not very marked.
Conclusion
With respect to all these experimental facts we could conclude that the position of the absorption bands of the dithiocarbamates is affected mainly by the nature of the central atom and the character of the group bonded to the nitrogen atom, influencing the charge density distribution at the C-N and M-S bonds. The structure of the coordination sphere affects the M-S and C-N bonds which however manifests itself only in the IR band shape, the n(C-N), n(C-S) and n(M-S) band positions remaining unchanged.
Acknowledgement
We are grateful to the Principal and the Chairman, Department of Chemistry, G.F.(P.G.) College, Shahjahanpur(UP), India for providing the necessary infrastructure for this work.
References
- K.M. Yusuf, K.M. Basheer and M. Gopalan; Polyhedron, 2, 839 (1983).
- Sharma, G.S. Sodhi and N.K. Kaushik; Bull. Chim. Soc., France, 1-52 (1983).
- Fabretti, F. Forgheiri, P. Giusti and G. Tosi; Inorg. Chim. Acta, 86, 127 (1984).
- Chatt, L.A. Duncanson and L.M. Venanzi; Nature, 177, 1042 (1956).
- Nakamoto, J. Fujita, R.A. Condrate and Y. Morimoto; J. Chem. Phys., 39, 423 (1963).
- Tarique and M. Aslam; Orient. J. Chem., 24(1), 267 (2008).
- Tarique and M. Aslam; Bioscience, Biotech. Res. Asia, 5(1), 355 (2008).
- I. Vogel; A Text Book of Practical Organic Chemistry,(ELBS and Longmans, London) (1968).
- S. Siddiqi, P. Khan, S. Khan M.R.H. Siddiqi and S.A.A. Zaidi; Croatica Acta, 55(3), 271 (1982).
- Bonamico, G. Dessy, C. Mariani, A. Vaciago and L. Zambonelli; Acta. Crystallgr., 19, 619, 886, 898 (1965).
- Peyronel, A. Pignedoli and L. Antolini; Acta Crystallogr., B 28, 3596 (1972).
- W.C. Newman and A.N. White; J. Chem. Soc., Dalton Trans., 2238 (1972).
- Kettman, J. Garaj and S. Kudela; Coll. Czechoslov. Chem. Commun., 43, 1204 (1978).
- S. Field and F.W.B. Einstein; Acta Crystallogr., B 30, 2928 (1974).
- F. Willa and W.E. Hatfield; Inorg. Chem., 10, 2038 (1971).
- Peyronel and A. Pignedoli; Acta Crystallogr., 23, 398 (1967).
- Kettman, J. Garaj and S. Kudela; Coll. Czechoslov. Chem. Commun., 42, 402 (1977).
- P. Klug; Acta Crystallogr., 21, 536 (1966).
- V. Melnikova and A. T. Pilipenko; Ukr. Khim. Zh., 36, 671 (1970).
- V. Melnikova; Zh. Prikl. Spektrosk., 12, 1041 (1970).
- V. Melnikova and A. T. Pilipenko; Ukr. Khim. Zh., 35, 951 (1969).
- A. Brown, W.K. Glass and M.A. Burke; Spectrochim. Acta, A 32, 137 (1975).
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