Photophysical and photochemical behavior of tetrahydridobis(bis)1,2-diphenylphosphino(ethane)-molybdenum and -tungsten: optical emission and photoreduction of alkenes

The complexes H4M(DPPE)2 and D4M(DPPE)2 (M = Mo, W; DPPE = bis(1,2-diphenylphosphino)ethane) exhibit emission of visible light upon photoexcitation at 770K in 2-methyltetrahydrofuran. The emission lifetime at 770K for the W species (approx. 13 μs) is shorter than for the Mo species (about 90 μs) and is independent of whether the substance is 1H or 2H. The shorter lifetime for the W species suggests an H-N2 antibonding emissive triplet state, since the solution photochemistry at 2980K is dominated by H2 loss. While the lifetime of the 1H species (0.21). The data suggest that 2H diminishes nonradiative decay from the singlet excited state reached by direct absorption. This diminished nonradiative decay, yielding the emissive triplet by intersystem crossing with higher efficiency. Irradiation of H4M(DPPE)2 in the presence of an alkene (e.g., 1-pentene, cis-2-pentene, 3,3-dimethyl-1-pentene, cyclopentene) results a stoichiometric reduction to the alkane. Irradiation of H4M(DPPE)2 in the presence of alkene and 10 psi of H2 yields photoassisted alkane formation, yielding many alkane molecules per H4M(DPPE)2 initially present. Photoreduction of 1-pentene or cis-2-pentene occurs without detectable isomerization to cis- and trans-2-pentene or 1- and trans--2-pentene, respectively, an unprecedented result, photoassisted alkene reactions using polyhydride precursors