Synthesis of Triazolylidene Nickel Complexes and Their Catalytic Application in Selective Aldehyde Hydrosilylation

A set
of nickel­(II) Cp complexes with triazolylidenes containing
various different wingtip groups (aryl, alkyl, pyridyl, CH2OH) have been synthesized by direct metalation of the corresponding
triazolium salt with nickelocene. Distinct effects of the substituents
on electronic and steric properties of the formed complexes are demonstrated
by NMR spectroscopy and single-crystal structural analysis, as well
as by the catalytic activity of the complexes in the hydrosilylation
of aldehydes. While all complexes display appreciable catalytic performance,
the cyclometalated pyridyl-functionalized triazolylidene nickel complex 2e is highly active and reaches turnover frequencies of >20 000
h–1 with good catalyst stability (full conversion
at 0.05 mol % catalyst, maximum turnovers of ∼6000). A variety
of functional groups on the benzaldehyde are tolerated. Ketones are
not converted, which identifies complex 2e as a highly
selective precatalyst for the hydrosilylation of aldehydes in the
presence of ketones.