Hydrosilylation of Terminal Alkynes Catalyzed by a ONO-Pincer Iridium(III) Hydride Compound: Mechanistic Insights into the Hydrosilylation and Dehydrogenative Silylation Catalysis

The catalytic activity in the hydrosilylation of terminal alkynes by the unsaturated hydrido iridium(III) compound [IrH(kappa(3)-hqca)(coe)] (1), which contains the rigid asymmetrical dianionic ONO pincer ligand 8-oxidoquinoline-2-carboxylate, has been studied. A range of aliphatic and aromatic 1-alkynes has been efficiently reduced using various hydrosilanes. Hydrosilylation of the linear 1-alkynes hex-1-yne and oct-1-yne gives a good selectivity toward the beta-(Z)-vinylsilane product, while for the bulkier t-Bu-C CH a reverse selectivity toward the beta-(E)-vinylsilane and significant amounts of alkene, from a competitive dehydrogenative silylation, has been observed. Compound 1, unreactive toward silanes, reacts with a range of terminal alkynes RC CH, affording the unsaturated eta(1)-alkenyl complexes [Ir(kappa(3)-hqca)(E-CH=CHR)(coe)] in good yield. These species are able to coordinate monodentate neutral ligands such as PPh3 and pyridine, or CO in a reversible way, to yield octahedral derivatives. Further mechanistic aspects of the hydrosilylation process have been studied by DFT calculations. The catalytic cycle passes through Ir(III) species with an iridacyclopropene (eta(2)-vinylsilane) complex as the key intermediate. It has been found that this species may lead both to the dehydrogenative silylation products, via a beta-elimination process, and to a hydrosilylation cycle. The beta-elimination path has a higher activation energy than hydrosilylation. On the other hand, the selectivity to the vinylsilane hydrosilylation products can be accounted for by the different activation energies involved in the attack of a silane molecule at two different faces of the iridacyclopropene ring to give eta(1)-vinylsilane complexes with either an E or Z configuration. Finally, proton transfer from a eta(2)-silane to a eta(1)-vinylsilane ligand results in the formation of the corresponding beta-(Z)- and beta-(E)-vinylsilane isomers, respectively.