Dihydride transfer: a bimolecular mechanism in the isomerization of cis-dihydridobromo(carbonyl)[bis(diphenylphosphino)ethane]iridium, IrH2Br(CO)(dppe)

Abstract

Abstract : The oxidative addition of H2 to IrBr(CO) (doppe), 2, (dppe - 1,2-bis(diphenyphosphino)ethane) yields a kinetic dihydride species, 3, which then isomerizes to a more stable isomer 4. This isomerizatoin of 3 to 4 has been studied kinetically as a function of initial H2 pressure. Two pathways are operative at ambient temperature. The first is a reductive elimation/oxidative addition sequence which is first order in complex, while the second is a bimolecular pathway involving dihydride transfer from 3 to 20 to produce 4 and regenerate 2. The dihydride transfer pathway is second order in complex and becomes the dominant isomerizatoin mechanism when less than one equivalent of H2 relative to 2 has been added to the system. All of the kinetic data have been fit to a complete rate law which leads to a bimolecular rate constant for dihydride transfer of 0.21/M.min. Below -20C, the dhydride transfer pathway for isomerization is the only one operating.