Hydride-Free Hydrogenation: Unraveling the Mechanism of Electrocatalytic Alkyne Semihydrogenation by Nickel-Bipyridine Complexes.

Hydrogenation reactions of carbon-carbon unsaturated bonds are central in synthetic chemistry. Efficient catalysis of these reactions classically recourses to heterogeneous or homogeneous transition-metal species. Whether thermal or electrochemical, C-C multiple bond catalytic hydrogenations commonly involve metal hydrides as key intermediates. Here, we report that the electrocatalytic alkyne semihydrogenation by molecular Ni bipyridine complexes proceeds without the mediation of a hydride intermediate. Through a combined experimental and theoretical investigation, we disclose a mechanism that primarily involves a nickelacyclopropene resting state upon alkyne binding to a low-valent Ni(0) species. A following sequence of protonation and electron transfer steps via Ni(II) and Ni(I) vinyl intermediates then leads to olefin release in an overall ECEC-type pattern as the most favored pathway. Our results also evidence that pathways involving hydride intermediates are strongly disfavored, which in turn promotes high semihydrogenation selectivity by avoiding competing hydrogen evolution. While bypassing catalytically competent hydrides, this type of mechanism still retains inner-metal-sphere characteristics with the formation of organometallic intermediates, often essential to control regio- or stereoselectivity. We think that this approach to electrocatalytic reductions of unsaturated organic groups can open new paradigms for hydrogenation or hydroelementation reactions.