Rhenium in Homogeneous Catalysis: [ReBrH(NO)(labile ligand) (large-bite-angle diphosphine)] Complexes as Highly Active Catalysts in Olefin Hydrogenations

The reaction of [ReBr2(MeCN)(NO)(P boolean AND P)] (P boolean AND P = 1,1'-bisdiphenylphosphinoferrocene (dppfc) (la), 1,1'-bisriiisopropylphosphinopherrocene (diprpfc) (1b), 2,2'-bis(diphenylphosphino)diphenyi ether (dpephos) (1c), 10,11-dihydro-4,5bis(diphenylphosphino)dibenzo[bf]oxepine (homoxantphos) (1d) and 4,6-bis(diphenylphosphino)-10,10-dimethylphenoxasilin (Sixantphos) (1e)) led in the presence of HSiEt3 and ethylene to formation of the ethylene hydride complexes [ReBrH(eta(2)-C2H4)(NO)(P boolean AND P)] (3a,b,d), the MeCN ethyl complex [ReBr(Et)(MeCN)(NO)(dpephos)] (5c) and two ortho-metalated stereoisomers [ReBr(eta(2)-C2H4)(NO)(eta(3)-o-C6H4-Sixantphos)] 8e(up) and 8e(down). The complexes 3a,b,d, and 5c and the isomers of 8e showed high catalytic activity (TOFs ranging from 22 to 4870 h(-1), TONs up to 24000) in the hydrogenation of monosubstituted olefins. For 8e(down) and 8e(up) a remarkable functional group tolerance and catalyst stability were noticed. Kinetic experiments revealed k(obs) to be first order in c(cat) and c(H-2) and zeroth order in c(olefin). Mechanistic studies and DFT calculations suggest the catalysis to proceed along an Osborn-type catalytic cycle with olefin before H2 addition. The unsaturated key intermediates [ReBrH(NO)(P boolean AND P)] (2a-e) could be intercepted with MeCN as [ReBrH(MeCN)(NO)(P boolean AND P)] (10a-d) complexes or isolated as dimeric mu(2)-(H)(2) complexes [{ReBr(mu(2)-H)(NO)(P boolean AND P)}(2)] (9b and 9e). Variation of the bidentate ligand demonstrated a crucial influence of the (large)-bite-angle on the catalytic performance and reactivity of 3a,b,d, 5c, and 8e.