Electrochemical and spectroscopic characterisation of ferrocenethiosemicarbazone ligand and copper complexes

Abstract

Electrochemical and spectral behavior of two thiosemicarbazone ligands, containing ferrocene as a substituent and their two copper complexes; as well as a ligand with no ferrocene substituent and its copper complex were studied using cyclic voltammetry, ultraviolet-visible (UV/Vis) spectroscopy and infrared (IR) spectroscopy. The ligands included: acetylferrocenyl-4-phenylthiosemicarbazone (Dan 1), acetylpyridine-2-thiophenecarboxylsemicarbazone (Dan 5), acetylferrocenyl-2- thiosemicarboxyl-semicarbazone (Dan 11). Their respective copper complexes were Dan 2, Dan 6 and Dan 12. Ferrocene and Potasium ferrocyanide were used as standards and they gave reversible voltamograms of one electron transfer due to Fe+2IFe+3 redox couple and diffusion coefficient of2.8x 10 -5± 0.08cm2s-' and 7.0 x 10 -6 ± 0.03cm2s-', respectively, which was comparable to the literature value. Preliminary work also involved characterization of a Nitrosyl ligand 4-phenacetylidene-2,2,5,5-tetramethyl-3-imidazolidin-I-oxyl nitroxyl, which was done in order to ensure that the electrochemical procedure followed was appropriate for the current research wok. It was possible to reproduce the voltammetric behavior of the nitrosyl ligand. All the Thiosemicarbazone ligands gave well defined cyclic voltamograms. Dan 1 showed a reversible voltammogram at a scan rate of 0.030v/s, where the ratio of anodic current and cathodic current was approximately unity, thus confirming reversibility. Its cathodic to anodic peak potential difference (i'1E) increased with scan rate which is characteristic of slow electron transfer. Dan 2 which was the copper complex of Dan 1 gave a reduction and oxidation peak at -0.494 V and -0.21 V Vs SCE reference electrode, respectively. The peak ratio was greater than unity which is common in transition metal complex due to shift in the geometry of coordination sphere. The ratio decreased with the scan rate and came closer to unity at higher scan rates, which suggested a chemical reaction follow up, showing that the oxidation product was not stable. This phenomenon was observed in all other copper complexes, but their peak potentials differed depending on the side chains of the thiosemicarbazone ligand. Dan 5 exhibited a one electron transfer reversible voltamogram where the current ratio (iPa/ipc )was approximately unity and i\E was 0.084± 0.01 V. Dan 11 also gave a one electron electron transfer at a scan rate of 0.025 Vis. i\E was 0.065± 0.01 versus SCE.For the three ligands and the three metal complexes UV/visible spectral gave a weak absorption peak at 450nm and this was attributed to weakly allowed asymmetrical d-d transition for the metal in the ligand and the two metals in the complex. Intense separate peaks of shorter wave lengths (Amax) were observed for all compounds which are characteristics of charge transfer absorption peaks. When the ligand (Dan 1) was complexed with copper to form Dan 2 UV-Vis exhibited blue shift, i.e., the absorption peak of the complex shifted to shorter wavelength, suggesting transfer of electron from the ligand into the d orbitals of the metal (ligandmetal charge transfer), which is predominant if complexes have ligands with relatively high energy lone pairs (example Sulphur) or if the metal has low lying empty d orbitals. When the ligand (Dan 5) was complexed with copper to form Dan 6 UV -Vis exhibited red shift suggesting metal-ligand charge transfer, similar to what was observed for Dan 11 and 12. IR spectrophotometry confirmed the bidentate character of Dan 1 and Dan 11 ligands for the copper complexes. C=N band of the thiosemicarbazone which appeared at 1601cm-1 in Dan 1 shifted towards lower wave number in Dan 2, indicating coordination via nitrogen to the copper metal. The band at Sl Scm', corresponding to C=S was shifted towards lower wave numbers when complexed with copper metal, which indicates presence of thione sulphur coordination. Other characteristic bands observed were 1650cm-1 for C=O, 3367cm-1 for N-H andl041cm-1 for N=N. The above Thiosemicarbazone (Dan 1, Dan 2, Dan 5, Dan 11 and Dan 12) are known to exhibit bioactivity properties from previous research work hence the recommendation for further work to correlate the present research results to the bioactivity of the compound. -