Transition-Metal Complexes of Pyrrole Pigments. XXII. Photoredox Behavior of Oxo(tetra-p-tolylporphinato)molybdenum(V) Complexes

Abstract

Abstract Photochemical reduction of oxomolybdenum(V) complexes of 5,10,15,20-tetra-p-tolylporphine, [Mov(X)(ttp)], was investigated under irradiation with the visible light in benzene, where X stands for an axial ligand placed trans to the oxo group. Irradiation of [MovO(X)(ttp)] resulted in a homolytic cleavage of the Mo–X bond to afford [MoIVO(ttp)] and a radical species generated from the axial ligand (X). The reduction rate was controlled by the nature of the Mo–X bond, and a complex with less ionic character of the axial bond was more reactive in the homolysis process. The bulkiness of the axial alkoxo ligands may provide two different effects which are opposite to each other on the reactivity: (i) The repulsive interaction between an axial ligand and the equatorial ttp accelerates the homolysis of the Mo–X bond; (ii) the diffusion rate of the radical species, generated by the photochemical cleavage of the axial coordinate bond, from the coordination sphere is progressively retarded as the bulkiness of the alkoxo ligand increases. The second effect seems to be more pronounced for the present photochemical reaction. The photochemical reduction of [MoVO(X)(ttp)] is essentially composed of two sequential steps, photochemical cleavage of the Mo–X bond and diffusion of the radical species from the coordination sphere. Reoxidation of [MoIVO(ttp)] under aerobic irradiation conditions afforded [MoVO(OH)(ttp)] and [(MoVO(ttp))2O] via the postulated intermediate formation of [MoV(OO·)(ttp)].