Abstracts

ARTICLE

Template synthesis of symmetrical transition metal dithiocarbamates

Khwaja S. Siddiqi^{I, *} * e-mail: khwajas_siddiqi@yahoo.co.in ; Shahab A. A. Nami^I; Lutfullah^I; Yonas Chebude^II

^IDepartment of Chemistry, Aligarh Muslim University, Aligarh-202002, India

^IIDepartment of Chemistry, Addis Ababa University, Addis Ababa-1176, Ethiopia

ABSTRACT

Novel transition metal dithiocarbamates of the type [M₂(etdtc)₂] and [M¹(etdtc)Cl]₂ where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II), M¹ = Cr(III), Fe(III) and etdtc = (S₄N₄C₁₁H₁₈ ) have been synthesized by the use of self-assembly techniques. The metal dithiocarbamates were prepared from the primary amine, CS₂ and metal chloride. They have been characterized by spectroscopic, TGA/DSC, magnetic susceptibility and conductivity data. The complexes, [Mn₂(etdtc)₂], [Fe₂(etdtc)₂], [Co₂(etdtc)₂], [Zn₂(etdtc)₂], [Cd₂(etdtc)₂] and [Hg₂(etdtc)₂] have been suggested to be tetrahedral while [Ni₂(etdtc)₂] and [Cu₂(etdtc)₂] have square-planar geometry. [Cr(etdtc)Cl]₂ and [Fe(etdtc)Cl]₂ have chlorine bridged distorted-octahedral geometry. It has been observed that the dithiocarbamato moiety is symmetrically bonded in all the cases. A three stage TGA profile is observed for all the complexes leading to the formation of respective metal sulfide as the end-product. The molar conductance of 10^-3 mol L^-1 solution of the complexes measured in DMSO is indicative of their non-ionic nature.

Keywords: dithiocarbamates, transition metals, thermal analysis, symmetrical bonding

RESUMO

Novos ditiocarbamatos de metais de transição do tipo [M₂(etdtc)₂] e [M¹(etdtc)Cl]₂ [M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) e Hg(II), M¹ = Cr(III), Fe(III) e etdtc = (S₄N₄C₁₁H₁₈ )] foram sintetizados por meio de técnicas de automontagem. Os ditiocarbamatos metálicos foram preparados a partir da amina primária, CS₂ e cloreto metálico. Foram caracterizados por técnicas espectroscópicas, TGA/DSC, susceptibilidade magnética e dados de condutividade. Sugere-se que os complexos [Mn₂(etdtc)₂], [Fe₂(etdtc)₂], [Co₂(etdtc)₂], [Zn₂(etdtc)₂], [Cd₂(etdtc)₂] e [Hg₂(etdtc)₂] tenham geometria tetraédrica, e [Ni₂(etdtc)₂] e [Cu₂(etdtc)₂], quadrática plana. Os complexos [Cr(etdtc)Cl]₂ e [Fe(etdtc)Cl]₂ contêm ligantes cloro em ponte e possuem geometria octaédrica distorcida. Em todos os casos, o ligante ditiocarbamato encontra-se ligado simetricamente. No perfil da curva TG são observados três estágios para todos os complexos, levando à formação dos respectivos sulfetos metálicos como produtos finais. A condutância molar de soluções dos complexos em DMSO de concentração 10^-3 mol L^-1 confirma sua natureza não iônica.

Introduction

Dithiocarbamates are used to stabilize a wide range of oxidation states of different metal ions.¹ A large number of compounds are known where CS₂ binds in h¹-end on, h²-side or in h³-coordination modes.^2-4 The major advantage of using the small bite-angle of dithiocarbamato moiety as a stabilizing chelating ligand, is its unique property to remain intact under a variety of reaction conditions.⁵

Nowadays copper(II) dithiocarbamate is successfully used as single source precursor for the growth of semiconducting copper sulfide thin films.⁶ The iron(II), iron(III) dithiocarbamates have been studied for their spin-crossover phenomenon,⁷ radical traps for NO,⁸ and as antioxidants and pro-oxidants in biological systems.⁹ Diethyl dithiocarbamates are also known to inhibit the activity of Cu/Zn-superoxidedismutase (SOD) through the withdrawal of copper from the protein both in vivo and in vitro.¹⁰ Some dialkyl-substituted dithiocarbamates have proved to be an efficient anti-alkylating, anti-HIV and froth-floatation agents.¹¹ The optical and electrochemical properties of dithiocarbamates can effectively be used to construct sensors for guest molecules and macromolecules.^12-14

In this paper, we report a convenient one-pot synthesis of the transition metal dithiocarbamates via template procedure. In the present work, a condensation reaction was carried out between acetylacetone, ethylenediamine, carbon disulfide in a single step leading to the formation of a ring like complex (Figure 1 and Figure 2).

Experimental

Hydrated metal chlorides (BDH) acetylacetone, carbon disulfide (Merck) and ethylenediamine (Ranbaxy) were used as received. Methanol was used after distillation. Elemental analyses (C, H, N and S) were carried out with a Flash EA-1112 Analyser, CE Instrument. IR spectra (4000-400 cm^-1) were recorded on a FT-IR Perkin-Elmer Spectrum BX as KBr disc while the 600-200 cm^-1 range were scanned on CsI on Nexus FT-IR Thermo Nicolet, Madison Wisconsin. The conductivity measurements were carried out with a CM-82T Elico conductivity bridge in DMSO. The electronic spectra were recorded on a Cintra 5GBC spectrophotometer in DMSO. Magnetic susceptibility measurements were done with a Sherwood Scientific MSB Auto at room temperature. TGA/DSC was performed with a Universal V3.8 B TA SDT Q600 Build 51 Thermal Analyzer under nitrogen atmosphere using alumina powder as reference material. The heating rate was maintained at 10 °C min^-1. The metal contents were estimated by complexometric titrations,¹⁵ while the chlorine was determined gravimetrically as AgCl in case of Cr(III) and Fe(III) complexes.¹⁶

Synthesis of the complexes

In 40 mL methanolic solution of acetylacetone (20.0 mmol, 2.05 mL) neat ethylenediamine (40.0 mmol, 2.67 mL), carbon disulfide (40.0 mmol, 2.41 mL) and hydrated metal chloride (20.0 mmol) dissolved in 20 mL methanol were added simultaneously in a single pot to obtain a brisk precipitation of the metal dithiocarbamate in moderate yields. The precipitate was filtered, thoroughly washed with methanol and dried over calcium chloride in vacuo.

Results and Discussion

The microanalytical data (Table 1) correspond to the composition [M₂(etdtc)₂] and [M¹(etdtc)Cl]₂ where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II), M¹ = Cr(III), Fe(III) and etdtc = (S₄N₄C₁₁H₁₈ ) as shown in Figures 1 and 2. The complexes are stable to moisture and heat. The complexes are amorphous powder, soluble in DMSO only. The molar conductance of 10^-3 mol L^-1 solutions of the complexes in DMSO are indicative of their non-electrolytic nature.¹⁷

IR spectra

In metal dithiocarbamato complexes the region 950-1050 cm^-1 is considered characteristic of the nature of binding of the dithiocarbamato moiety. According to the criterion of Bonati and Ugo,¹⁸ the presence of a solitary band in the above region is due to symmetrical bidentate coordination of the dithiocarbamato group while the splitting of this band within a narrow range of 20 cm^-1 is due to the unsymmetrical monodentate nature of the dithiocarbamato group shown below:

In the present study, a single sharp band of high intensity has been observed in all the complexes in 1017-1036 cm^-1 range, suggesting symmetrical bidentate binding of the dithiocarbamato moiety.¹⁹ It has been shown from the crystal and molecular structure of [NiCl(tzdtc)(PPh₃)]0.5CHCl₃ that the C-N and C-S bond distances are shorter than the corresponding C-N and C-S distances in pyridine and thiophene, respectively which is due to the delocalization of electrons,²⁰ over the entire NCS₂ group after chelation:

Consequently the C-N and C-S stretching frequencies of the thioureide band also changes (Table 2) showing partial double bond character of S₂CN. It has been shown from a study of alkyl dithiocarbamates that the lengthening of the alkyl chain is accompanied by a decrease in n(C–N) although the n(C–S) remains unaltered as it has been found to be insensitive to the nature of alkyl substituents.²¹

The n(M-S),²² occurring in the far-IR region depends on the nature of the metal ion and the substituents attached with the sulfur. We have observed medium intensity n(M-S) in the 527-442 cm^-1 range which is in agreement with the observations made by Sonbati.²³

On the basis of normal coordinate analysis of non-planar halogen bridged molecules it has been found that the bridging n(M-X) are 40-45% lower than the terminal n(M-X).^24-26 The four absorption bands found for terminal M-X bonds have higher frequencies while the bridging M-Cl are relatively weaker and appear in lower wave number region. In the case of [Fe(etdtc)Cl]₂ complex, two medium and two strong n(M-X) have been observed in 248-331 cm^-1 region which is probably due to the bridging M-Cl group. The magnetic moment value is not very conclusive (3.67 B.M.) but it may be due to an equilibrium between high-spin and low-spin Fe(III).²⁷ Moreover, a halogen bridged structure of this compound would be more stable with respect to non-halogen bridged five coordinate Fe(III).

For a triply bridged Cr₂Cl₉^3- there are twelve infra red active bands above 200 wave number but only two terminal n(Cr-Cl) are observed. Terminal Cr-Cl bonds are stronger than the bridging ones as has been observed in the case of Fe-complex. In Cr₂Cl₉^3- there are two strong Cr-Cl terminal bonds while the weaker ones have been ascribed to bridging Cr-Cl bonds.^28,29 However, we have observed two medium intensity bands in the case of [Cr(etdtc)Cl]₂ which are assigned to bridging n(Cr-Cl) (Table 2).

Electronic spectra and magnetic moments

The octahedral Cr(III) ion is known to exhibit two prominent bands at 17,000 and 24,000 cm^-1 besides a very weak charge transfer as a shoulder, approximately at 37,000 cm^-1. The other weak absorptions are insignificant as they are spin forbidden. We have observed three bands in the case of [Cr(etdtc)Cl]₂ and assigned to the transitions from ⁴A_2g ground term to three excited quartet terms (Table 3). The magnetic moment of 3.74 B.M. calculated for three unpaired electrons and the d-d bands are suggestive of an octahedral geometry for Cr(III) in the [Cr(etdtc)Cl]₂ complex.³⁰ The Racah parameters calculated are indicative of considerable metal-ligand bond character.

A variety of Fe(III) dithiocarbamato complexes have been reported. In the case of trisdialkyl dithiocarbamate Fe(III) complexes a trigonally distorted octahedral structure has been shown.³¹ However, when one or two dithiocarbamate groups are substituted by other bidentate ligands the extent of distortion increases.

It has been shown by Hoskins and White,³² from the molecular and crystal structure of monochlorobisdiethyldithiocarbamate Fe(III) that it has a square-pyramidal structure. It is monomeric in solid state with a magnetic moment value of 3.9 B.M. In the present complex, [Fe(etdtc)Cl]₂, each Fe(III) atom is surrounded by four sulfur atoms and one free chlorine. The proximity of the chlorine may be a reason for chlorine bridged octahedral structure. A value of 3.67 B.M. has been found at room temperature for the complex, [Fe(etdtc)Cl]₂ which may be regarded as a population-weighted-average,²⁷ due to the spin-cross between high (µ_eff = 5.9 B.M.) and low-spin (µ_eff = 1.88 B.M.). Three intense bands at 27,548; 24,509 and 19,032 cm^-1 further supports the proposed octahedral geometry for [Fe(etdtc)Cl]₂ complex.

The characteristic colour of tetrahedral Mn(II) complexes is generally yellow or yellowish green as opposed to octahedral Mn(II) complexes. [Mn₂(etdtc)₂] exhibits yellowish green colour in that absorbs in the visible region at 24,630 cm^-1 assigned to ⁴T₁¬ ⁶A₁ transition. In addition, it also shows two charge transfer bands at 32,051 and 30,674 cm^-1. The magnetic moment of Mn(II) in [Mn₂(etdtc)₂] complex (5.72 B.M.) is close to the calculated spin only value indicating a tetrahedral arrangement of Mn(II) ion.³³

The tetrahedral high-spin Fe(II) complex shows one spin allowed d-d transition in the visible region at 21052 cm^-1 corresponding to ⁵E ¬ ⁵T₂ transition. The magnetic moment of tetrahedral Fe(II) complexes lies between 5.0-5.5 B.M. corresponding to four unpaired electrons while we have obtained a value of 5.05 B.M. which is consistent with the tetrahedral nature,³⁴ of Fe(II) ion in [Fe₂(etdtc)₂].

For the tetrahedral Co(II) complexes two bands are observed at 15,600 and 22,471 cm^-1 corresponding to ⁴T₂(F) ¬ ⁴A₂(F) and ⁴T₁(F) ¬ ⁴A₂(F) transitions, respectively, in addition to a charge transfer at 27,000 cm^-1. The magnetic moment of tetrahedral,³⁵ Co(II) complexes lies in the range 4.2 to 4.7 B.M. while for octahedral Co(II) complexes it falls between 4.4 to 5.5 B.M. The observed value in the present case is 4.38 B.M. The ligand field spectrum and magnetic moment value suggest a tetrahedral environment around Co(II) ion.

An important feature of Ni-S₄ systems is the presence of high intensity absorption bands, which is also the case in present investigation. Two d-d transitions are observed at 15,503 and 23,696 cm^-1 corresponding to the ¹A_2g¬ ¹A_1g and ¹B_1g¬ ¹A_1g transitions, respectively which support a square-planar structure for the Ni(II) complex.³⁶

The green Cu(II) complex shows two d-d absorption bands at 18,384 and 13,330 cm^-1 corresponding to ²A_1g¬ ²B_1g and ²E_g¬ ²B_1g transitions, respectively and a charge transfer band at 24,570 cm^-1. It is well known that for square-planar Cu(II) complexes the magnetic moment value falls in 1.82-1.86 B.M. while for tetrahedral Cu(II) ion, the µ_eff value is slightly higher (1.92-2.00 B.M.).³⁷ A value of 1.83 B.M. for Cu(II) ion has been observed which is well within the expected region found for square-planar Cu(II) complexes.

TGA/DSC

The observed percentage weight loss corresponding to various steps in all the thermograms were compared (Table 4) with those calculated on the basis of possible decompositions of the expelled groups.³⁸ Generally the dithiocarbamato complexes on pyrolysis either volatilize leaving behind negligible amount of residue or decompose to yield respective metal sulfide.³⁹ In the present study the TGA profile consists of three well defined stages. The first stage starts above 100 °C implying that that there is neither water nor solvent molecule attached to the complex. The first degradation step starts at 145 °C and nearly 9% of weight loss is observed corresponding to two methyl groups as methane (Calc. 8.2%) and it continues upto a temperature range of 190 °C. The second stage of decomposition starts from 200 °C and continues upto 550 °C involving decomposition of remaining organic moiety which constitutes approximately 65% (Calc. 64.2%) of mass of the complex leaving behind the respective metal sulfide.⁴⁰ Finally, the thermogram showed a straight line which did not change even on heating upto 700 °C indicating that there is no further change. Hence, respective metal sulfide is the end product of all the complexes.

The DSC profile exhibits enthalpic changes from endothermal and exothermal bands and peaks. In the present case the DSC peaks correlate well with the TGA data. A sharp endothermic peak is observed in the temperature range of 150-200 °C due to the liberation of methane while a broad exothermic peak is obtained in between 300-400 °C due to the pyrolysis of the whole organic moiety. However, there is no well defined exotherm or endotherm corresponding to the last process of formation of metal sulfide.

Conclusions

The above work describes a convenient high yield template synthesis of ring type dithiocarbamates which can act as models to study the magnetic interaction between metal ions, and also mimic naturally occurring macrocyclic ligands. Furthermore, they can successfully be used as models for protein-metal binding sites in biological systems

Acknowledgments

One of the authors (SAAN) thanks the CSIR, New Delhi for financial assistance through grant No. 01(1783)/02/EMR-II. Finally, authors thank the referees and the editor for their constructive comments.

Received: May 3, 2005

Published on the web: December 15, 2005

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