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

Article abstract of DOI:10.1021/ja00297a021

The oxidative addition of H2 to the iridium(I) chelates IrX(CO)(dppe)(n+) (n=0; X=Cl, Br, I, CN, H; n=1, X=PPh3; dppe=1,2-bis(diphenylphosphino)ethane) proceeds with >99percent stereoselectivity 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_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 the orthorhombic space group P2₁2₁2¹ with unit cell parameters a=12.291(3) Angstroem, b=17.349(4) Angstroem, c=12.189(3) Angstroem, V=2599 Angstroem³, and Z=4.The structure refined to a conventional R factor of 0.035.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^-1), and the mechanism likely involves a two-step H2 reductive 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.For all of the complexes studied, the stereochemistry of hydrogen oxidative addition is the same, and the observed stereoselectivity is dictated by electronic differences between CO and X ligands.Possible explanations for the observed stereoselectivity are discussed in relation to current theories on the intimate mechanism of H2 oxidative addition.