Activation of Small Molecules Mediated by Photoredox reactions of Metal Complexes

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Tri-mu-oxobis(tetraphenylporphinatoniobium(V)), complex 1, was prepared. Coordination behaviors of alcohols, phenols and nitrogen bases at the axial sites of 1 were followed by means of visible absorption spectroscopy. Alcohols and phenols coordinated to 1 at the axial site and resulted mononuclear complexes, [Nb(tpp)(O)(L)(HL)], 2. Catechol acted as a bidentate ligand and afforded a complex, [Nb(tpp)(O)(Hcat)], 3.2,6-di-t-butylphenol and nitrogen bases did not coordinate to 1.The complex 1 in benzene was reduced upon anaerobic irradiation with visible light in the presence of monodentate ligands such as alcohols or phenols. Aerobic irradiation of 1 gave superoxide complexes in the presence of phenols, while no such complexes were detected upon aerobic irradiation in the presence of alcohols. The complex 3 did not react upon irradiation under the both aerobic and anaerobic conditions. The complex 1 in benzene without addition of ligands gave a superoxide complex upon aerobic irradiation, but it was not reduced upon anaerobic irradiation conditions.A benzene solution of 1 showed a time resolved ESR signal due to a triplet biradical after pulsed irradiation under anaerobic conditions. The triplet species activated the coexisting molecular oxygen under aerobic conditions. The complex 2 gave corresponding phenoxyl radical upon anaerobic pulsed irradiation. The Additives which did not coordinated to 1 did not give transient radical species upon anaerobic irradiation. The activation of the additives coordinating to 1 was initiated by homolysis of the axial coordination bond.