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

Dithiocarbamates are a class of metal-chelating compounds with various applications in medicine. They have been used for the treatment of bacterial and fungal infections, possible treatment of AIDS, and most recently cancer. Dithiocarbamates are well-known compounds to bind strongly and selectively to many metal ions.Dithiocarbamate ligands readily form chelates with metal ions through its two donor sulphur atoms.A new series ofplumbum(II) complexes with mixed ligands, dithiocarbamate, and 1,10-phenanthroline were successfully synthesised using in situ method. Microelemental analysis data of the complexes were in agreement with the general formula, Pb[S 2 CNR’R”] 3 phen (R’ = ethyl, R” = butyl, cyclohexyl; phen = 1,10-phenanthroline). Infrared spectra of the complexes showed that the thioureide (C-N) band was in the region 1450–1500 cm-1. The unsplitting band of (C-S) in the region 930–1000 cm-1indicated the bidentate nature of the chelated dithiocarbamato ligands. The 13C NMR chemical shift of the carbon atom of the N-CS 2 group appeared in the range of 202–207 ppm.


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 actinide elements (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 anisobidentatemodesare 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 1950s, 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

Synthesis of dithiocarbamate compounds Synthesis of Plumbum(II) N-alkyl-Nethyldithiocarbamate 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-Nethyldithiocarbamate (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 SpectrumGX spectrophotometer; using KBr discs for the wavenumber range between 4000 and 400 cm -1 and nujol in polyethylene tablets for the wave number range between 400 and 250 cm -1 .The 13 C NMR spectra were recorded using a Jeol JNM-LA 400 spectrometer withTMS as the internal standard.

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   2.All the compounds were nonhygroscopicand stable at room temperature.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.

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 (C-N) and (C-S) stretching modes (Geeta and Thirumaran 2008).The (C-N) was used as a measure of the contribution of the thioureide group to the structure of the dithiocarbamate.The (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  ).The thioureide band was well differentiated from the ring frequencies associated with N-donors, which were observed in the 1600-1500 cm -1 .The (C-S) bands appear around 990 cm -1 in all the complexes without any splitting, supporting the bidentate coordination of the dithiocarbamate moiety (Manohar et al. 1998).Figure 1(a-b) shows the IR spectra of compound 1 and 3.
The 13 C NMR spectra data of compound 1 to 4 are shown in Table 34.The assignments of -CSS group in the investigated compounds were straight ward as observed in the range of 201.99-207.08 ppm, indicating the coordination of sulphur to the lead atom.The N 13 CS 2 chemical shift was the most important signal in the 13 C NMR spectra for the dithiocarbamate compounds.The 13 C NMR spectra of compound 3 and 4 were observed at 207.08 and 202.09 ppm, respectively, which shifted to downfield compared to the parent compound (compound 1 and 2).The shift could be explained by an increase of -bond order in the whole NCS 2 moiety.This means that the chelation of the bidentate

CONCLUSION
The parent plumbum(II) dithiocarbamate and their nitrogenous adduct have been successfully synthesised and characterised.The IR spectra data of the comppunds showed that the thioureide band values were lower than the values observed for the parent dithiocarbamate compounds.A reduction in thioureide strectcing frequency was due to the increase in coordination around the cadmium ion and the resultant increase in electron density.