Photochemistry of Os(dmpe)2H2:: Matrix, transient solution, and NMR studies of 16-electron Os(dmpe)2 (dmpe = Me2PCH2CH2PMe2)

The complex Os(dmpe)(2)H-2 (dmpe =Me2PCH2CH2PMe2) was synthesized by reaction of trans-Os(dmpe)(2)Cl-2 with sodium under hydrogen and was shown to consist of cis and trans isomers in a ratio of ca. 60:1. UV irradiation in argon and methane matrices yielded Os(dmpe)(2) with a characteristic multiband UV-vis spectrum; the lowest energy transition occurs close to 800 nm. In CO-doped matrices, Os(dmpe)(2)(CO) was generated. Laser flash photolysis experiments (266 nm excitation) demonstrated that the same species was produced in cyclohexane solutions of Os(dmpe)(2)H-2 at room temperature. The initial photoproduct, Os(dmpe)(2), is formed within 60 ns of the laser flash. This species reacts with hydrogen and with CO with rate constants exceeding 10(9) dm(3) mol(-1) s(-1), indicating that there is essentially no barrier to its reaction with these substrates. Rate constants for reaction with ethene, triethylsilane, and nitrogen are all close to 10(8) dm(3) mol(-1) s(-1). Comparison of the UV-vis spectrum of Os(dmpe)(2) with Ru-0, Rh+, and Ir+ analogues indicates that Os(dmpe)(2) has a structure close to square planar (local Du, symmetry). The products of photochemical reaction of Os(dmpe)(2)H-2 in solution were also studied by NMR spectroscopy. Reaction with benzene yields cis-Os(dmpe)(2)(Ph)H, while reaction with ethene yields cis- and trans-Os(dmpe)(2)(CH=CH2)H.