Photochemistry of Os(dmpe)₂H₂:  Matrix, Transient Solution, and NMR Studies of 16-Electron Os(dmpe)₂(dmpe = Me₂PCH₂CH₂PMe₂)

Abstract

The complex Os(dmpe) 2 H 2 (dmpe = Me 2 PCH 2 CH 2 PMe 2 ) 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 D 2 h 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 CH 2 )H.