Synthesis, characterization and electrochemical investigation of cobalt(III) complexes of 3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioxime with sulfur ligands in the axial sites

Abstract

Cobalt(III) complexes of the tetraaza quadridentate chelate, 3,8-dimethyl-5,6-benzo-4,7-diazadeca- 3,7-diene-2.9-dione dioxime with a sulfur ligand such as 2-mercaptoethanol, ethanethiol, thiourea, thiocyanate and thiophenol in one of the axial coordination sites of cobalt have been synthesized to study the lability of the CoS bond and the reactivity of the lower valent cobalt ion. These complexes have been characterized by C, H, N, Co analysis, conductivity measurements, magnetic susceptibility and IR, UVVis and 1H NMR spectroscopy. The thiocyanato and 2-mercaptoethanolato complexes exhibit pseudooctahedral geometry whereas the others undergo tetragonal distortion. All these complexes exhibit two polarographic one electron reduction waves corresponding to the reductions, Co(III) to Co(II) and Co(Il) to Co(I). The cyclic voltammograms on a GC disk electrode consist of two cathodic waves corresponding to the reductions, Co(III) to Co(II) and Co(Il) to Co(I) and two anodic waves corresponding to the oxidations, Co(I) to Co(II) and Co(II)to Co(III). Both Co(III)/Co(II) and Co(II)/Co(I) redox couples are irreversible as evidenced by the Δ E p and i pa/ i pc values. The i pa/ i pc ratio of the Co(II)/Co(I) couple is 0.5 indicating a 50% Co(I) yield. This suggests the involvement of a coupled chemical reaction such as an ECE mechanism. The polarographic and cyclic voltammetric E 1 2 values lie at less negative potentials when compared to that of the complexes of nitrogenous bases despite the fact that the field strength, Dq z , of the axial sulfur ligands are comparable to that of the nitrogenous bases. This indicates that the CoS bond is thermodynamically less stable than the CoN bond. Electrochemical evidence is sought for the π-back bonding tendency of sulfur ligands.