Transition-metal complexes of pyrrole pigments. 18. Redox behaviors of oxomolybdenum(V) complexes formed with macrocyclic tetrapyrroles

Article abstract of DOI:10.1021/ic50221a060

The redox chemistry of (2,3,17,18-tetramethyl-7,8,12,13-tetraethylcorrolato)oxomolybdenum(V) [Mo(O)(MEC)] and (5,10,15,20-tetraphenylporphinato)oxomolybdenum(V) complexes [Mo(O)(TPP)(X), X = MeO, AcO, and Cl] was investigated in dichloromethane by means of cyclic voltammetry and controlled-potential electrolysis. One-electron oxidation and reduction of Mo(O)(MEC) at Mo^V were observed at +0.70 and -0.72 V vs. SCE, respectively. Such oxidation and reduction potentials for Mo(O)(TPP)(X) were very dependent on the nature of axial ligand X and consequently on the covalent character of the Mo^V-X bond: one-electron reduction becomes less facile as the covalent character increases and reaches the value of that in Mo(O)(MEC) for X = MeO. The TPP complexes were much more resistant to oxidation of Mo^V than the MEC complex. Two successive reductions of TPP were observed for Mo(O)(TPP)(X) at -1.1 and -1.5 V vs. SCE while no ligand reduction was detected for Mo(O)(MEC) in the cathodic region up to -2.0 V vs. SCE. On the basis of complete redox schemes for Mo(O)(MEC) and Mo(O)(TPP)(X), correlations between redox properties and ligand structures have been discussed. Coordination equilibria for reactions of Mo(O)(TPP)(MeO) with AcO^-, Cl^-, and ClO₄^- were investigated in dichloromethane, and the chloro complex was found to exist as a dimer while the others are monomers in solution.