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

Hydrogenation reactionsof carbon-carbon unsaturated bondsare central in synthetic chemistry. Efficient catalysis of these reactionsclassically recourses to heterogeneous or homogeneous transition-metalspecies. Whether thermal or electrochemical, C-C multiple bondcatalytic hydrogenations commonly involve metal hydrides as key intermediates.Here, we report that the electrocatalytic alkyne semihydrogenationby molecular Ni bipyridine complexes proceeds without the mediationof a hydride intermediate. Through a combined experimental and theoreticalinvestigation, we disclose a mechanism that primarily involves a nickelacyclopropeneresting state upon alkyne binding to a low-valent Ni(0) species. Afollowing sequence of protonation and electron transfer steps via Ni(II) and Ni(I) vinyl intermediates then leads to olefinrelease in an overall ECEC-type pattern as the most favored pathway.Our results also evidence that pathways involving hydride intermediatesare strongly disfavored, which in turn promotes high semihydrogenationselectivity by avoiding competing hydrogen evolution. While bypassingcatalytically competent hydrides, this type of mechanism still retainsinner-metal-sphere characteristics with the formation of organometallicintermediates, often essential to control regio- or stereoselectivity.We think that this approach to electrocatalytic reductions of unsaturatedorganic groups can open new paradigms for hydrogenation or hydroelementationreactions.