Electrochemical Strategy for Proton Relay Installation Enhances the Activity of a Hydrogen Evolution Electrocatalyst

A new method to install a proton relay that enhances the reactivity near an active catalytic site for H₂ production is reported, afforded by the electrochemical reduction and protonation of one of the ligands in the paddlewheel Rh₂(II,II) hydrogen evolution complex, cis-[Rh₂(DPhF)₂(bncn)₂]²⁺ (Rh-bncn; DPhF = N,N'-diphenylformamidinate, bncn = benzo[c]cinnoline). An electrochemical reversible prewave is observed for Rh-bncn at potentials more positive than the first bncn-centered reduction couple in the presence of strong acids, observed at -0.72 V vs Fc^+/0 (Fc = ferrocene) in the cyclic voltammograms (CVs) in DMF (0.1 M TBAPF₆). The origin of this prewave is shown to arise from a precatalytic transformation that originates from a concerted proton-electron transfer (CPET) event occurring at one of the bridging bncn ligands. Through electrochemical analysis, CV simulations, and electronic structure calculations, a reaction mechanism is elucidated. In this system, the electrochemically formed N-H bond on the reduced bncn ligand serves as a proton relay in the H₂ formation reaction through a cooperative interligand pathway involving one of the bridging DPhF ligands after a second reduction step, accessible at approximately -1.15 V vs Fc^+/0. Since calculations show that hydrogen evolution takes place at the bridging ligands and does not involve the dirhodium core, it is predicted that more abundant metal centers can be incorporated into this ligand scaffold, leading to new candidates for electrocatalytic hydrogen reduction. As such, this work delineates a new design strategy to incorporate proton relays in molecular bimetallic hydrogen evolution electrocatalysts to achieve higher efficiency.