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.