Photochemistry of Molybdenum(V) Tetraphenylporphyrin Studied by Laser Flash Photolysis. Light-Induced Homolysis of the Mo-O Bond of Oxoalkoxo- and Oxo(nitrito)molybdenum(V) Tetraphenylporphyrin

Article abstract of DOI:10.1021/j100180a035

Photolysis of oxoethoxomolybdenum(V) tetraphenylporphyrin (O=Mo^V(TPP=OC2H5) in a degassed mixture of 9:1 toluene-ethanol gives rise to the homolysis of the Mo-OC2H5 bond with the yield of 0.03 to produce oxomolybdenum(IV) tetraphenylporphyrin (O=Mo^IVTPP as a stable product.Oxo(nitrito)molybdenum(V) tetraphenylporphyrin (O=Mo^V(TPP)ONO) in toluene also undergoes photochemical cleavage of the Mo-ONO bond with the yield 0.28 to give O=Mo^IVTPP and NO2, which recombine to regenerate O=Mo^V(TPP)ONO.The quantum yield measurements in the temperature range 300-77 K have shown that the activation energies for the photocleavage of the Mo-OC2H5 and Mo-ONO bonds were 5.4 and 8.0 kcal mol^-1, respectively.The lowest excited state of O=Mo^V(TPP)OC2H5, presumably ascribable to the tripquartet state, is detected by the laser photolysis at 77 K.Because of the fact that the quantum yield for the photocleavage of the Mo-OC2H5 bond upon Q-band irradiation is very low in comparison with those obtained upon irradiation of the Soret and CT bands, it is concluded that the lowest excited state is not responsible for the photocleavage.The photoproduct O=Mo^IVTPP is found to react with oxygen, resulting in formation of the dioxygen adduct.The adduct gradually changes to the μ-oxo dimer VTPP<2O.The structure of the dioxygen adduct is discussed on the basis of optical and ESR measurements.