Photophysical and Photochemical Behaviour of Tetrahydridobis(bis(1,2-diphenylphosphino)ethane)molybdenum and -tungsten: Optical Emission and Photoreduction of Alkenes

Article abstract of DOI:10.1021/ja00390a023

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 77 K in 2-methyltetrahydrofuran.The emission lifetime at 77 K for the W species (ca.13 μs) is shorter than for the Mo species (ca.90 μs) and is independent of whether the substance is the 1H or 2H species.The shorter lifetime for the W species is consistent with an emissive triplet state that is antibonding with respect to M-H2 interactions, since the solution photochemistry at 298 K is dominanted by H2 loss.While the lifetime of the 1H and 2H species is the same, the quantum yield of emission for M = Mo is higher for the 2H species (0.28) than for the 1H species (0.21).The data are consistent with an effect from 2H that diminished nonradiative decay from the singlet excitet state reached by direct absorption.This diminished nonradiative decay allows intersystem crossing to be more competitive, yielding the emissive triplet with higher efficiency.Irradiation of the H4M(DPPE)2 species in the presence of an alkene (e.g., 1-pentene, cis-2-pentene, 3,3-dimethyl-1-pentene, cyclopentene) results in stoichiometric reduction to form an alkane; i.e., each H4M(DPPE)2 molecule yields two molecules of alkane.Irradiation of H4M(DPPE)2 in the presence of alkene and 10 psi of H2 yielsd photoassisted alkane formation, yielding many alkane molecules per H4M(DPPE)2 initially present, as illistrated with the reduction of 1-pentene.The intriguing finding concerning the photoreduction of 1-pentene or cis-2-pentene is that the reduction occurs without detectable isomerization to cis- and trans-2-pentene or 1- and trans-2-pentene, respectively, an unprecedented results for such photoassisted alkene reduction using polyhydride precursors.