Steroselective Oxidative Addition of Hydrogen to Iridium(I) Complexes. Kinetic Control Based on Ligand Electronic Effects

摘要

The oxidative addition of H2 to the iridium(I) chelates IrX(CO) (dppe) (X(-) = Cl(-), Br(-), I(-), Cn(-), H(-), PPh3; dppe = 1, 2-bis-(diphenylphosphino)ethane) proceeds with > 99% steroselectivity to yield a cis dihydride product with one hydride trans to P(dppe) and the other hydride trans to CO. For X = Cl, Br, and I, the kinetic product of formula IrH2X(CO)(dppe) equilibrates with a more stable cis isomer which has one hydride trans to P and the other trans to X (K sub eq = 41, 35, and 13, respectively). The stereochemical assignments based on chemical shifts of the hydride ligands are confirmed by single-crystal X-ray diffraction analysis of the thermodynamic isomer for X = Br. The complex IrH2Br(CO)(dppe) crystallizes in an orthorhombic space group. The isomerization reaction between the two dihydride isomers has been studied mechanistically in acetone and benzene solvents. In acetone, the isomerization of IrH2Br(CO)(dppe) proceeds with first-order kinetics in iridium complex (k = 0.011/min), and the mechanism likely involves a two-step H2 recuctive elimination/oxidative addition process. For X = CN, the kinetic dihydride isomer is the most stable isomer, but it does thermally equilibrate with two other isomers. For X(-) = H(-) or PPh3, only a single isomer is observed and it appears to be the most stable isomer. The stereoselectivity of D2 oxidative addition for X = H is established by generating the reactive species IrH(CO)(dppe) in situ by dehydrohalogenation of IrH2Cl(CO)(dppe) under D2.