Synthesis and Characterisation of Phenanthroline Adducts of Pb(II) Complexes of BisN-alkyl-N-ethyldithiocarbamates

Introduction
Dithiocarbamates are highly versatile monoanionic chelating ligands that form stable complexes with all the transition elements and the
majority of main group elements, lanthanide and actinideelements(Hogarth 2012). Dithiocarbamates are prepared from primary or secondary amines.Depending upon the nature of the cation, dithiocarbamates can show good solubility in water or organic solvents. Dithiocarbamates are lipophilic and generally bind to metals in a symmetrical chelate fashion.There are also other coordination modes, but monodentate and anisobidentate modes are the two most prevalent ones.

Dithiocarbamates stabilise metals in a wide range of oxidation states, this being attributed to the existence of soft dithiocarbamate and hard thioureide resonance forms, the latter formally resulting from delocalisation of the nitrogen lone pair onto the sulphurs, and consequently their complexes tend to have a rich electrochemistry (Onwudiwe 2014). Tetraethyl thiuramdisulphide (disulfiram or antabuse) has been used as a drug since the 1950 s, but it is only recently that dithiocarbamate complexes have been explored within the medicinal domain. Over the past two decades, anticancer activity of gold and copper complexes has been noted.Technetium and copper complexes have also been used in PET imaging. Copper complexes have also been investigated as SOD inhibitors. Other than that, dithiocarbamates have been used to treat acute cadmium poisoning.

Tetrahedral metal dithiocarbamate complexes are known to expand their coordination numbers by adding neutral nitogen ligands. The
crystal structure on mixed ligand complexes involving cadmium dithiocarbamate with nitrogen bidentate ligand, 1,10-phenantrhroline has been reported (Lai et al. 2003; Ali et al. 2009).All the adducts contain discrete molecular units with CdS 4N 2 chromophore in a distorted octahedral geometry.

In our previous work, we have reported several compounds and adduct of dithiocarbamate complexes (Awang et al. 2011; Awang et al. 2010). As an extension of this research, here we report the synthesis of two new Pb(II) dithiocarbamate compounds, which arePb[S 2 CN(C 2 H 5 )(C 4 H 9 )] 2 (compound 1) and Pb[S 2 CN(C 2 H 5 )(C 6 H 11 )] 2 (compound 2). We also report their adducts, which are Cd[S2CN((C2H5)(C4H9)]]2(phen) (compound 3) and Cd[S2CN(C2H5)(C6H11)]2(phen) (compound 4), together with their elemental analysis and spectroscopic characterisation.

Materials and Method

Reagents

Secondary amines (N-butyl-N-ethylamine and N-cyclohexyl-N-ethylamine) and ethanol (95%) were obtained from Fluka Chemical. Carbon disulphide and methanol (99.5%) were from Ajax Chemical Ltd., and dichloromethane(99.5%) from R & M Chemical (99.5%). PbCl 2 chloride was purchased from Merck. All the chemicals were used as supplied without purification.

Synthesis of dithiocarbamate compounds Synthesis of Plumbum(II) N-alkyl-N- ethyldithiocarbamate Compounds (Compound 1 and 2)

Compound 1 and 2 were synthesised by the method given by Thirumaran et al. (1998) with some modifications.Carbon disulphide (30 mmol) in ethanol was slowly added to an ethanolic solution of N-alkyl-N-ethylamine (30 mmol). The reaction temperature was controlled under 4°C. The mixture was stirred about 1h,and a pale yellow solution formed.After that, 30 mmol of plumbum(II) chloride in ethanol solution was added, and the yellowish precipitate was obtained. The precipitate was filtered and washed with cold ethanol. The reaction scheme for the above preparation is given in Figure 1.

Synthesis of Plumbum(II) N-alkyl-N- ethyldithiocarbamate (Phen) Compounds
(Compound 3 and 4)

Compound 3 and 4 were prepared by adding equimolar of compound 1 or compound 2 and bidentate nitrogen ligand (1,1-phenanthroline)
in the mixture of ethanol and chloroform solutions. The resulting mixture was stirred for 1 h and allowed to evaporate at room temperature. The pale yellow precipitate formed and was washed with cold ethanol and dried in vacuo. The reaction scheme to synthesise compounds 3 and 4 is shown in Figure 2.

Physical Measurements

Elemental analysis (CHNS) was carried out on a CHNS/O Fison Model EA 1108 elemental analyser. Melting points were determined using an Electrothermal 9100 instrument.The infrared spectra were recorded on a Perkin Elmer FT-IR Model Spectrum GX spectrophotometer; using KBr discs for the wave number range between 4000 and 400 cm-1 and nujol in polyethylene tablets for the wave number range between 400 and 250 cm-1.

The 13C NMR spectra werere corded using a Jeol JNM-LA 400 spectrometer with TMS as the internal standard.

Results and Discussion

Physical and Elemental Analysis

The synthesised compounds were solid and stable in air. The elemental analysis dan melting point data for the title compounds are given in Table 1.Elemental analysis was done to compare the observed values with predicted values of percentage of the carbon, hydrogen, nitrogen, and sulphur. The elemental analysis data were in good agreement with the expected values. These data showed that the chemical formula for the compound 1 and 2 was Pb[S2CN(CH2CH3)(R)]2 (R = C6H11 or C4H9 ), whereas for compound 3 and 4 was Pb[S2CN(CH2CH3 )(R)]2.C12H8 N2 (R = C6 H11 or C4H9). The proposed structures for compound 1 and 2 are shown in Figure 2.All the compounds were Nonygroscopic and stable at room temperature.

Table1: Results of elemental analysis of compound 1 to 4  Click here to View table

 

Table2: Infrared spectra data of important bands for compound 1 to 4  Click here to View table

 

These compounds were insoluble or sparingly soluble in most common organic solvent and quite soluble in chloroform.They had sharp melting point. The small range of their melting point showed that these compounds had a good purity (Awang et al. 2010).

Infrared Spectroscopy

The characteristic infrared absorption frequencies (cm-1) and assignment of important absorption bands of the compound 1 to 4 are listed
in Table 2. The significant absorptions in the dithiocarbamate complexes mainly are due to the v(C- N) and v(C-S)stretching modes (Geeta and Thirumaran 2008). The v(C- N)was used as a measure of the contribution of the thioureide group to the structure of the dithiocarbamate. Thev(C- N) appeared at an intermediate value between the two extremes of 1250–1350 cm-1) and (1650–1690 cm-
1) (Baggio et al. 1992). IR spectra of the complexes showed that the shift of the thioureide bands to lower frequencies compared to the parent bisdithiocarbamate was due to the increase in coordination number around the metal ion (Awang

Table3: 13C NMR spectra data (δ, ppm) for compound 1 to 4  Click here to View table

 

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