An iridium–SPO complex as bifunctional catalyst for the highly selective hydrogenation of aldehydes

Abstract

A secondary phosphine oxide (SPO) ligand (tert-butyl(phenyl)phosphine oxide) was employed to generate an Ir–SPO complex which shows a particular ability to activate dihydrogen under mild conditions without the help of an external base or additive. Such an iridium(I) complex serves as a precursor for homogeneous catalysis since under H₂ it is converted to a mixture of several iridium(III) hydride species that are the active catalysts. This system was found to be a highly active catalyst for the hydrogenation of substituted aldehydes, giving very high conversions and chemoselectivities for a wide range of substrates. The SPO ligand presumably plays a key role in the catalytic process through heterolytic cleavage of H₂ by metal–ligand cooperation. In addition, an exhaustive characterization of the different iridium hydride species was performed by 1D and 2D NMR spectroscopy. The oxidative addition of H₂ to the Ir(I)–SPO complex is highly stereoselective, as all generated Ir(III) hydrides are homochiral. Finally, the crystal structure, as determined by X-ray diffraction, of a dinuclear iridium(III) hydride complex is described.