Dihydridoiridium diolefin complexes as intermediates in homogeneous hydrogenation

Abstract

Two isomeric series of dihydrido-diolefin complex cations, cis- and cis- trans- [IrH 2(cod)L 2]PF 6 (cod = 1,5-cyclooctadiene; L = tertiary phosphine)have been observed directly by PMR spectroscopy and in some cases have been isolated as crystalline solids. They are prepared by H 2 addition to a diolefin complex (unsaturated route) or by diolefin addition to a hydrido complex (hydride route), respectively. These complexes appear to be important intermediates in the catalytic hydrogenation of (cod) by the catalysts [Ir(cod)L 2]PF 6. The cis- isomers transfer hydrogen to the coordinated (cod) much more rapidly than the cis, trans isomers; the hydrogen transfer to the olefin seems to require a coplanar M(CC)H system. H 2 addition to [Ir(cod) 2] + at −80° C gives [IrH 2(cod) 2] +; electron-withdrawing substituents therefore do not deactivate the metal center with respect to oxidative addition.