Photoinduced Ligand Redistribution Chemistry of Quadruply Bonded Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) Complexes

The quadruply bonded metal-metal complexes cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) (R(3) = Et(3), Me(3), Me(2)Ph, MePh(2); 6-mhp = 2-hydroxy-6-methylpyridinato) photoreact when their solutions are irradiated with visible and near-UV light. The primary photoprocess leads to the ligand redistribution products Mo(2)Cl(3)(6-mhp)(PR(3))(3) and Mo(2)Cl(6-mhp)(3)(PR(3)). In THF at room temperature, these photoproducts are stable and over time they back-react completely to the starting material. Photolysis of cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) in DMF results in the same products; however, Mo(2)Cl(3)(6-mhp)(PR(3))(3) rapidly decomposes, leaving Mo(2)Cl(6-mhp)(3)(PR(3)) as the only isolable photoproduct. Conversely, when the reaction is carried out in benzene, Mo(2)Cl(6-mhp)(3)(PR(3)) undergoes a slow secondary photoreaction and Mo(2)Cl(3)(6-mhp)(PR(3))(3) is the photoproduct that is isolated. At a given wavelength, the photolysis quantum yield (Phi(p)) increases along the solvent series C(6)H(6) < THF < DMF (Phi(p)(405) = 0.00042, 0.00064, and 0.00097, respectively, for cis-Mo(2)Cl(2)(6-mhp)(2)(PMe(2)Ph)(2)). For a given solvent, Phi(p) increases with decreasing excitation wavelength (Phi(p)(546) = 0.00012, Phi(p)(436) = 0.00035, Phi(p)(405) = 0.00042, Phi(p)(366) = 0.0022, and Phi(p)(313) = 0.0079 in C(6)H(6)). This wavelength dependence of the photoreaction quantum yield in conjunction with the excitation spectrum establishes that the photoreaction does not originate from the lowest energy deltadelta excited state, which possesses a long lifetime and an appreciable emission quantum yield in C(6)H(6), CH(2)Cl(2), THF, and DMF. The photochemistry is instead derived from higher energy excited states with the maximum photoreactivity observed for excitation wavelengths coinciding with absorption features previously assigned to ligand-to-metal charge transfer transitions.