Transition-Metal Complexes of Pyrrole Pigments. 18. Redox Behaviors of Oxomolybdenum(V) Complexes Formed with Macrocyclic Tetrapyrroles

The redox chemistry of (2,3,17,18-tetramethyl-7,8,12,13-tetraethylcorrolato)oxomolybdenum(V) [Mo(0)(MEC)] and (5,10,15,20-tetraphenylporphinato)oxomolybdenum(V) complexes [Mo(0)(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(0)(MEC) at Mov were observed at +0.70 and -0.72 V vs. SCE, respectively. Such oxidation and reduction potentials for Mo(0)(TPP)(X) were very dependent on the nature of axial ligand X and consequently on the covalent character of the Mov-X bond: one-electron reduction becomes less facile as the covalent character increases and reaches the value of that in Mo(0)(MEC) for X = MeO. The TPP complexes were much more resistant to oxidation of Mov than the MEC complex. Two successive reductions of TPP were observed for Mo(0)(TPP)(X) at -1.1 and -1.5 V vs. SCE while no ligand reduction was detected for Mo(0)(MEC) in the cathodic region up to -2.0 V vs. SCE. On the basis of complete redox schemes for Mo(0)(MEC) and Mo(0)(TPP)(X), correlations between redox properties and ligand structures have been discussed. Coordination equilibria for reactions of Mo(0)(TPP)(MeO) with AcO”, Cl”, and C104” were investigated in dichloromethane, and the chloro complex was found to exist as a dimer while the others are monomers in solution.