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(κ3-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 β-(Z)-vinylsilane product, while for the
bulkier t-Bu-CCH a reverse selectivity toward
the β-(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
η1-alkenyl complexes [Ir­(κ3-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 (η2-vinylsilane)
complex as the key intermediate. It has been found that this species
may lead both to the dehydrogenative silylation products, via a β-elimination
process, and to a hydrosilylation cycle. The β-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 η1-vinylsilane complexes with either
an E or Z configuration. Finally,
proton transfer from a η2-silane to a η1-vinylsilane ligand results in the formation of the corresponding
β-(Z)- and β-(E)-vinylsilane
isomers, respectively.