Electrocatalytic metal hydride generation using CPET mediators

Transition metal hydrides (M-H) are ubiquitous intermediates in a wide range of enzymatic processes and catalytic reactions, playing a central role in H+/H2 interconversion1, the reduction of CO2 to formic acid (HCOOH)2 and in hydrogenation reactions. The facile formation of M-H is a critical challenge to address to further improve the energy efficiency of these reactions. Specifically, the easy electrochemical generation of M-H using mild proton sources is key to enable high selectivity versus competitive CO and H2 formation in the CO2 electroreduction to HCOOH, the highest value-added CO2 reduction product3. Here we introduce a strategy for electrocatalytic M-H generation using concerted proton–electron transfer (CPET) mediators. As a proof of principle, the combination of a series of CPET mediators with the CO2 electroreduction catalyst [MnI(bpy)(CO)3Br] (bpy = 2,2′-bipyridine) was investigated, probing the reversal of the product selectivity from CO to HCOOH to evaluate the efficiency of the manganese hydride (Mn-H) generation step. We demonstrate the formation of the Mn-H species by in situ spectroscopic techniques and determine the thermodynamic boundary conditions for this mechanism to occur. A synthetic iron–sulfur cluster is identified as the best CPET mediator for the system, enabling the preparation of a benchmark catalytic system for HCOOH generation.