Synthesis, Characterization Studies and Bond Valence Sum (BVS) analysis on 2:1 Adducts involving Zinc(II)dithiocarbamates and 4,4’-Bipyridine

Introduction

Zinc and cadmium dithiocarbamate complexes have continued to attract attention in recent years on account of their industrial applications^1,2 and biological profiles.^3,4 Dithiocarbamates are a family of compounds with diverse biological activities, suggesting that it may have multiple, clinical uses. Zinc and cadmium dithiocarbamates are used as single source precursor molecules to grow ZnS and CdS nanostructures respectively, for applications such as solar cells, photoconductors, lasers, optical waveguides, and non-linear integrated optical  devices.^5-7 Zinc dithiocarbamates and their adducts with primary and secondary amines have previously been prepared and some of these complexes are very active accelerators for the vulcanization of rubber by sulphur⁸ and the low temperature vulcanisation of latex.⁹

Tetrahedral complexes of divalent zinc and cadmium are known to expand their coordination shells by adding neutral nitrogenous ligand.^10,11 The additional ligand shows its steric and electronic effects structurally on the complex formed. Synthesis, spectral, thermal, cyclicvoltammetry and single crystal X-ray structural studies on monomeric mixed ligand complexes involving bis(dithiocarbamato)zinc(II) and nitrogenous bases such as 2,2’-bipyridine, 1,10-phenanthroline, tetramethylethylenediamine (TMED),were reported extensively.^12-18 All the adducts are containing discrete molecular units with ZnS₄N₂  chromophore in a distorted octahedral geometry. X-ray photoelectron spectroscopy (XPS), Cyclic voltammetry, C¹³ and H¹NMR studies showed the increased electron density on metal centre on adduct formation. The crystal structures of dimeric zinc complexes involving zinc dithiocarbamates and 4,4’-bipyridine were reported from our laboratory.^15,16  X-ray structural studies  show that   the zinc ion is five coordinated with a ZnS₄N coordination environment in the dimeric complexes.

Since the crystal structures of some of the  4,4’-bipyridine adducts  were reported earlier,^15,19 in this paper, we report the results of spectral, thermal and electrochemical  characterization studies of the  adducts. In addition the results of the bond valence sum (BVS) calculations are also reported.

Experimental

Preparation of the Adducts     

Preparation of  [Zn₂(dtc)₄(4,4’–bipy)] (dtc^–= pipdtc^–, dnpdtc^–, dedtc^– )

The adducts were   prepared by adding hot solutions of 4,4’–bipyridine (156mg, 1mmol) in chloroform to the hot solutions of respective parent [Zn(dtc)₂]₂ (2mmol) complex in chloroform. The resulting solution was left for evaporation at room temperature. Yellow precipitate of the adduct separated out and analysed to the proposed formula. [Zn₂(pipdtc)₄(4,4’–bipy)]: ν_max = 1480 (C-N), 975 (C-S); (Found: C, 43.7; H, 5.0; N, 8.9%. Calc. for C₃₄H₄₈Zn₂N₆S₈: C, 44.0; H, 5.2; N, 9.1 %); [Zn₂(dnpdtc)₄(4,4’–bipy)]:  ν_max = 1476 (C-N), 977 (C-S); (Found: C, 45.7; H, 6.3; N, 8.4%. Calc. for C₃₈H₆₄Zn₂N₆S₈: C, 46.0; H, 6.5; N, 8.5 %); [Zn₂(dedtc)₄(4,4’–bipy)]:  ν_max = 1491 (C-N), 994 (C-S); (Found: C, 40.8; H, 5.4; N, 9.4%. Calc. for C₃₀H₄₈Zn₂N₆S₈: C, 41.0; H, 5.5; N, 9.6 %);

Preparation of [Zn₂(dtc)₄(4,4′-bipy)](dtc^–= nmedtc^–, deadtc^–).

The  adducts were prepared by the methods reported earlier [15]. A hot ethanolic solution of [Zn(dtc)₂]₂ (430mg(deadtc^–) or 325mg(nmedtc^–),  2mmol) was added to a hot solution of 4,4’-bipyridine (156 mg, 1mmol) in ethanol. The resulting yellow solution was left for evaporation at room temperature. After two days yellow crystals separated out.

Preparation of [Zn₂(dmdtc)₄(4,4′-bipy)].

[Zn₂(dmdtc)₄(4,4′-bipy)] complex was prepared by the earlier method reported [12]. It was prepared by adding a hot solution of 4,4’-bipyridine (156mg, 1mmol) in benzene to a hot solution of [Zn(dmdtc)₂]₂ (2mmol) in benzene. The resulting yellow solution was left for evaporation at room temperature. After two days yellow precipitate of the adduct separated out and analysed to the proposed formula [Zn₂(dmdtc)₄(4,4′-bipy)].

Analytical and Physical Measurement

All the reagents and solvents employed were commercially available analytical grade materials, used as supplied, without further purification. IR spectra were recorded on a JASCO IR – 700 spectrophotometer (range  4000 – 400 cm^-1) as KBr pellets. JASCO UVIDEC – 340 double beam spectrophotometer was used for recording the electronic spectra of the complexes. The spectra were recorded in alcohol, benzene and chloroform. BAS electrochemical analyzer was used for recording the cyclic voltammograms of the complexes. Working electrodes were hanging mercury drop electrode (HMDE) or platinum electrode, counter electrode was platinum wire and reference electrode was Ag/AgCl. The solvent, dichloromethane was purified by distillation methods. Supporting electrolyte was tetrabutylammonium perchlorate (0.1M). Experimental solution was thermostated at 25 ± 1°C and oxygen free atmosphere was provided by bubbling purified nitrogen through the solution.STA 1500 PL and Perkin – Elmer TGA7 Thermal Sciences instrument were used for the thermogravimetric analysis. The heating rate of the furnace was fixed at 10°C per minute. About 5mg of the sample was taken in porcelain crucible for each thermogravimetric experiment.

Results and Discussion

Infrared and Electronic Spectra 

Important absorptions in the dithiocarbamate complexes are due to the nC–N and nC–S stretching modes. nC–N has been used as a measure of the contribution of the thioureide group to the structure of the dithiocarbamate. The polar nC = N⁺   appears at an intermediate value between the two extremes of (1250 – 1350 cm^-1)  and (1650 – 1690 cm^-1).^20,21  In this study nC–N for different [Zn(dtc)₂]₂ complexes appear in the region of 1485 – 1519 cm^-1. In the case of adducts the nC–N values shift to lower wave number compared to the [Zn(dtc)₂]₂ complexes. The shift of  nC–N to lower wave number in the case of adducts is due to the change in coordination geometry from tetrahedral to tetragonal pyramid.²² This observation is an indication of increase of electron density on zinc due to the adduct formation. The nC–S bands appear around 1000 cm^-1 without splitting.²³ The thioureide nC–N of the dithiocarbamates are well differentiated from those of 4,4′-bipyridine nC–N bands by comparison with their IR spectra.

The adducts are all faint to intense yellow colour. The electronic spectra of the adducts show transitions only due to charge transfer. The d–d transitions are absent as Zn(II) is d¹⁰ ion. The ligand transitions of the dithiocarbamate and the charge transfer transition in the adducts are observed in the region 260-320nm.

Cyclic Voltammetric Studies

The cyclicvoltammograms on the parent zinc dithiocarbamates and  their 4,4’-bipyridine adducts were recorded in CH₂Cl₂ with TBAP as the supporting electrolyte. Cyclic voltammetric studies on simple [Zn(dtc)₂]₂ complexes at HMDE have been reported earlier.²⁴ A two electron reduction process with the formation of zinc amalgam (Zn(Hg)) has been proposed.

The CV responses for [Zn(nmedtc)₂]₂ ^_  1.025 V  corresponds to the process in Eq.(1). Oxidation of  Zn(Hg) in the reverse scan    was observed at ^_  0.400 V. The process envisaged is:

 

The reduction potential at ^_ 1.500V is far higher in magnitude when compared to the parent [Zn(nmedtc)₂]₂ which is a two electron reduction process. ^24,25 The observation

clear indication of the increased electron density on zinc in the adduct. The redox    couple, ^_ 0.925/^_0.675 was observed only when HMDE was used and was absent when a Pt electrode was used.

Table1: Voltammetric data for the Zinc complexes  Click here to View table

 

The involvement of an electroactive species produced as result of the interaction of mercury with the complex is envisaged.^26,27 Nevertheless, the stoichiometry could not be established. Formation of a bimetallic complex involving dithiocarbamate is due to the inherent property of mercury to react with the  complexes.^28-30 The most important observation in the present study is the increased electron density on zinc during adduct formation. For other 4,4’-bipyridine adducts, also, a similar conclusion is arrived at in the present study. The cyclic voltammetry data  are given in Table 1.

Thermogravimetric Analysis

The temperature range and percentage weight losses of the decomposition reactions are given in Table 2. The TG curves obtained for  all the  [Zn₂(dtc)₄ (4,4’-bipy)](where = dmdtc^–,pipdtc^– deadtc^– , dedtc^– and nmedtc^–) adducts show a single stage weight loss, which ultimately leads to the formation of ZnS around 600°C. In the above mentioned complexes, the loss of 4,4’-bipyridine is not observed as an independent step. In contrast, a well defined step for the loss of 2,2’-bipyridine ligand is observed at 190°C in [Zn(dmdtc)₂(2,2’-bipy]. But thermal decomposition of the dithiocarbamate moiety leads to the formation of ZnS completely around 600°C without any intermediate step for the formation of Zn(SCN)_2. The above comparison shows that, the 4,4’-bipyridine adducts are thermally more stable  than the 2,2’-analogues probably  because of the dimeric nature. In addition, the TG analysis support the proposed molecular formulae of the complexes. Thermograms of  [Zn₂(dmdtc)₄(4,4′-bipy)] and  [Zn(dmdtc)₂(2,2′-bipy)] are given in Fig.1. Dimeric structure of  [Zn₂(pipdtc)₄(4,4′-bipy)] is given in Fig.2

Table2: Thermogravimetric data  Click here to View table

 

Figure1: Thermograms of (a) [Zn2(dmdtc)4(4,4′-bipy)] and (b) [Zn(dmdtc)2(2,2′-bipy)]   Click here to View figure

 

Figure2: Dimeric structure of  [Zn2(pipdtc)4(4,4′-bipy)]  Click here to View figure

 

Bond Valence Sum (BVS) Analysis

The bond valence sum method is popular method in coordination chemistry to estimate the oxidation states of atoms.  The chemist wishing to estimate an unknown bond length in a molecule or crystal is confronted with intimidtating array of covalent radii, ionic radii and metallic radii etc., from which to choose.³¹ The bond valence method,^32,33 has recently had considerable success in predicting and interpreting bond lengths in ‘ionic solids’. As it can be applied to estimate the bond lengths, vice-versa the sum of these bond lengths should give information about the valence of the central ion.

atoms i and j is defined so that the sum of all the bond valences from a given atom i with valence V_i obeys    Σ v_ij = V_i

where:

v_ij = exp[(R_ij – d_ij/B],

R_ij = r_i +r _j – [r_ir _j (Öc_i-Öc_j)²] / [ c_ir_i+c_jr_j]

and B is a universal constant equal to 0.37. The BVS values obtained for  a number of dimeric complexes whose  crystal structures have been reported from our laboratory and a few others reported from other laboratories, with two different sets of R_ij values are presented in Table 3. Examination of the results clearly show the BVS values to be close to ‘2’, which is equivalent to the formal oxidation state of zinc in the  complexes. Change in coordination number and change in coordination environment  around the zinc ion have adjusted themselves in such a way that the valency of the central ion is satisfied.

Table3: BVS values for the complexes  Click here to View table

 

Conclusion

Results of the analysis of the  4,4’-bipyridine adducts  using IR, UV-Visible, CV, TG techniques and BVS analysis are presented in this paper. The thioureide band(νC-N) in the IR spectra of the complexes shifts to lower wave number. The observation is in line with the expected increase in electron density on the zinc ion. Electronic spectra of the adducts show weak bands in the region 260 – 320nm only due to charge transfer. The increased electron density on zinc in the adducts is well supported by the cyclic voltammetric studies. Thermal studies showed that the 4,4’-bipyridine adducts are thermally more stable  than the 2,2’-analogues probably  because of the dimeric nature. The  BVS analysis  has confirmed the valency of the central metal to be 2.0 as expected.

References

1. Nieuwenhuizen J., Ehless A. W., Haashoot J. G.,,  Janse S. R.,  Reedijk J., Baerends J.,  J. Am. Chem. Soc., 1999; 121: 163.
2. McCleverty J. A., Gill S., J. Chem. Soc., Dalton Trans., 1982; 493.
3. Casas J. S., Sanchez A.,  Bravo J., Garcia-Fontan S., Castellano E. E., Jones M. M.,  Inorg. Chim. Acta, 1989; 158: 119.
4. Jones M. M., Jones S. G., Inorg. Chim. Acta, 1983; 79: 288.
5.  Das S. K., Morris G. C., J. Appl. Phys., 1993; 73: 782.
6.  Keitoku S., Ezumi E., Sol. Energy Mater. Sol. Cells, 1994; 35: 299.
7.  Seo K. W., Yoon S. H., Lee S. S.,  I.W. Shim I. W., Bull.  Korean Chem. Soc., 2005; 26: 1582.
8. Higgins G. M. C., Saville B., J. Chem. Soc., 1963; 2812.
9. Mc Cleverty J. A., Gill S., J. Chem. Soc., Dalton   Trans., 1982, 493.
10. Klug H. P.,  Acta Crystallogr., 1996; 21: 536.
11. Fraser K. A., Harding M. M.,  Acta Crystallogr., 1967; 22: 75.
12. Manohar A., Venkatachalam V.,  Thirumaran S., Ramalingam K., Bocelli G., Cantoni A., J. Chem. Crystallogr., 1998; 28: 86.
13. Thirumaran S., Ramalingam K., Bocelli G., Cantoni A.,  Polyhedron, 1999; 18: 925.
14. Ramalingam K., Uma S., Rizzoli C., Marimuthu G.,  J. Coord. Chem., 2010; 63:  4123.
15.  Manohar A., Ramalingam K., Bocelli G., Righi L., Inorg. Chim. Acta, 2001; 314: 177.
16.  Marimuthu G., Ramalingam K., Rizzoli C., Polyhedron, 2010; 29: 1555.
17.  Srinivasan N., Thirumaran S., Ciattini S., J Mol. Struct., 2009; 936: 234.
18.  Arul Prakasam B., Ramalingam K., Bocelli G., Cantoni A., Polyhedron,  2007;  26: 4489.
19. Lai C. S., Tiekink E. R. T.,  Appl. Organometal. Chem., 2003; 17: 253.
20. R. Baggio, A. Frigerio, E.B.  Halac, D. Vega and M.  Perec, J. Chem. Soc. Dalton  Trans.,  1992, 549.
21. Aravamudan G., Brown D. H.,  Venkappayya D., J. Chem. Soc.(A), 1971; 2744.
22.  Hassaan A. M. A., Soliman E. M., El-Roudi A. M., Polyhedron,  1989; 8: 925.
23.  Bonati F., Ugo R.,  J. Organometal. Chem., 1967; 10: 257.
24.  Bond A. M., Hollen-Kamp A. F,  Inorg. Chem., 1990; 29: 284.
25.  Budniknov G. K., Ulakhovich N. A.,  Zh. Obshch. Khim., 1976; 46: 1129.
26.  Aggarwal R. C., Singh B., Singh M. K.,  J. Indian Chem. Soc., 1982; 59: 269.
27.  Aggarwal R. C., Singh B., Singh M. K., Indian J. Chem., 1993; 22: 533.
28.  Bond A.M., Thackery J. R., Casey A. T., Inorg. Chem., 1973; 12:  887.
29.  Joris S. J., Aspila K. I., Chakraparti C. L.,  Anal. Chem., 1969; 41: 1441.
30.  Randle T. H., Cardwell T. J., Magee R. J.,  Aust. J. Chem., 1975; 28: 21.
31.  Pauling L., The Nature of Chemical Bonding, 3rd edn., Cornell Ithaca, New  York, USA, 1960.
32. Brown I.D.,  Structure and Bonding in Crystals, M. O’ Keeffe  A. Navrotsky, Eds., Vol. 2 Academic, New York, 1981.
33.  Keeffe M. O.,  Structure and Bonding, 1989; 71: 162.
34.  Summers S. P., Abbound K. A., Farrah S. R.,  Palenik G. J., Inorg. Chem., 1994;  33: 88.
35. Keeffe  M. O.,  Brese N. E., J. Am. Chem. Soc., 1991; 113: 3226.
36. Brese N. E., Keeffe M. O.,  Acta Crystallogr. Sec.B, 1991; 47: 192.
37. Ray N. K., Samuels L., Parr N. G.,  J. Chem. Phys. 1979; 70: 3680.
38. Brown I.D., Altermatt D.,  Acta Crystallogr., 1985; B41: 244.

---

This work is licensed under a Creative Commons Attribution 4.0 International License.