Iridium Oxidation States in Catalytic Hydrogenation Intermediates

The known iridium(III) hydrides [IrH₂(NCMe)₃(PiPr₃)]BF₄, [IrH(h³-C₃H₅)(NCMe)₂(PiPr₃)]BF₄, [IrH(E-CH=CHPh)(NCMe)₃(PiPr₃)]BF₄ and [IrH{C(Ph)=CH₂}(NCMe)₃(PiPr₃)]BF₄, derived from the Crabtree-type catalyst precursor [Ir(cod)(NCMe)(PiPr₃)]BF₄, have been investigated in reactions with substrates typically involved in homogeneous catalytic hydrogenations. New complexes such as the iridium(I) tris-ethylene species [Ir(NCMe)(h²-C₂H₄)₃(PiPr₃)]BF₄, the products of diphenylacetylene insertion [IrH{Z-C(Ph)=CHPh}(NCMe)₃(PiPr₃)]BF₄ and [Ir(η³-C₃H₅){Z-C(Ph)=CHPh}(NCMe)₂(PiPr₃)]BF₄, and the derivatives of the latter [Ir(k²O-acac)(η³-C₃H₅){Z-C(Ph)=CHPh}(PiPr₃)] and [Ir{k²C-C₆H₄-2-E-(CH=CPh)}(NCMe)₃(PiPr₃)]BF₄, have been characterized. The set of experimental observations suggests that iridium(I) species, though accessible, are unlikely hydrogenation intermediates. On the basis of deuteration experiments, a new hydride-alkenyl to carbene tautomerization is proposed.

homogeneous catalysis; hydrogenation; iridium catalysts; mechanism