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

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.