Exploring the bimodal nature of a nickel-based catalytic system for the hydrogenation of alkenes and polycyclic aromatic hydrocarbons

Abstract

We communicate the syntheses, characterization, and catalytic application of a series of NHC-tagged nickel(II) half sandwich complexes (NHC = N-heterocyclic carbene). The described piano stool compounds function as precatalysts in the homogeneous hydrogenation of CC bonds whereby the catalytic protocol is simple, robust, and additive-free. Moreover, one can opt between two modes both of which display distinguished catalytic features that can be exploited to achieve chemo- and regioselectivity. Mechanistic insights are provided with emphasis on the elucidation of the actual catalytically active species and, for the first time, we report on the formation of a hydride-bridged, dinuclear Ni cluster which contains a peculiar, six-membered diaza-nickelacycle. This compound resulted from oxidative addition of an NHC-related C–N bond onto a Ni moiety and this process is likely to represent a general deactivation mode for pertinent nickel catalysts.