Transition-metal complexes of pyrrole pigments. Part 20. Redox behaviour of chromium complexes with macrocyclic tetrapyrroles

Abstract

The redox chemistry of oxo (7,8,12,13-tetraethyl-2,3,17,18-tetramethylcorrolato)chromium(V), [Cr(O)(tetmc)], and the (5,10,15,20-tetraphenylporphinato) chromium(III) complexes, [Cr(tpp)X](X = OMe or Cl), was investigated by means of cyclic voltammetry and controlled-potential electrolysis in the presence of tetrabutylammonium perchlorate at 25 ± 2 °C. Dichloromethane and NN-dimethylformamide were used as solvents for [Cr(O)(tetmc)] and [Cr(tpp)X] respectively. The compound [Cr(O)(tetmc)] underwent metal reduction more readily than [Mo(O)(tetmc)]. This was attributed to an extensive interaction between chromium d_xy and ligand orbitals which tends to reduce the d-electron density at the chromium site (nephelauxetic effect) and to raise the d_xy level. As for the tpp complexes, the first reduction took place at the ligand (tpp) site for [Cr(tpp)(OMe)] but at the metal site for [Cr(tpp)Cl]. The less ready reduction of the former complex at the metal site was attributed to effective covalent character of the Cr–OMe bond. Redox behaviour of the tpp complexes was discussed on the basis of the energy-level diagram of d orbitals under C_4v co-ordination symmetry and π-orbital interactions between chromium and tpp.