Coordination and Dehydrogenation of Diphosphine–Borane Ph<sub>2</sub>PCH<sub>2</sub>PPh<sub>2</sub>·BH<sub>3</sub> at a Heterometallic MoRe Center to Give an Agostic Boryl-Bridged Derivative
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The coordination chemistry of the title diphosphine–borane adduct at heterometallic MoRe centers was examined through its reactions with the hydride complex [MoReCp(μ-H)(μ-PCy2)(CO)5(NCMe)] (Cp = η5-C5H5). The latter reacted rapidly with stoichiometric amounts of dppm·BH3 (dppm = Ph2PCH2PPh2) in refluxing toluene solution, with displacement of the nitrile ligand, to give [MoReCp(μ-H)(μ-PCy2)(CO)5(κ1P-dppm·BH3)], with a P-bound diphosphine–borane ligand arranged trans to the PCy2 group. Decarbonylation of the latter complex was accomplished rapidly upon irradiation with visible-UV light in toluene solution at 263 K, to give the agostic derivative [MoReCp(μ-H)(μ-PCy2)(CO)4(κ1P,η2-dppm·BH3)] as major product (Mo–Re = 3.2075(5) Å), along with small amounts of the diphosphine-bridged complex [MoReCp(μ-H)(μ-PCy2)(CO)4(μ-dppm)]. Extended photolysis of the agostic complex at 288 K promoted an unprecedented dehydrogenation process involving the borane group and the hydride ligand, to give the diphosphine–boryl complex [MoReCp(μ-η2:κ2P,B-H2B·dppm)(μ-PCy2)(CO)4] (Mo–Re = 3.075(1) Å). The latter displayed a boryl ligand in a novel bridging coordination mode, it being σ-bound to one of the metal atoms (B–Re = 2.38(2) Å) while interacting with the second metal atom via a strong side-on tricentric B–H–M interaction (B–Mo = 2.31(1); H–Mo = 1.9(1) Å). The overall dehydrogenation process was endergonic by 43 kJ/mol, according to density functional theory calculations.
