Circumventing Kinetic Barriers to Metal Hydride Formation with Metal-Ligand Cooperativity

We report the two-electron, one-proton mechanism of cobalt hydride formation for the conversion of [Co^IIICp(P^Ph₂N^Bn₂)(CH₃CN)]²⁺ to [HCo^IIICp(P^Ph₂N^Bn₂)]⁺. This complex catalytically converts CO₂ to formate under CO₂ reduction conditions, with hydride formation as a key elementary step. Through a combination of electrochemical measurements, digital simulations, theoretical calculations, and additional mechanistic and thermochemical studies, we outline the explicit role of the P^Ph₂N^Bn₂ ligand in the proton-coupled electron transfer (PCET) reactivity that leads to hydride formation. We reveal three unique PCET mechanisms, and we show that the amine on the P^Ph₂N^Bn₂ ligand serves as a kinetically accessible protonation site en route to the thermodynamically favored cobalt hydride. Cyclic voltammograms recorded with proton sources that span a wide range of pK_a values show four distinct regimes where the mechanism changes as a function of acid strength, acid concentration, and timescale between electrochemical steps. Peak shift analysis was used to determine proton transfer rate constants where applicable. This work highlights the astute choices that must be made when designing catalytic systems, including the basicity and kinetic accessibility of protonation sites, acid strength, acid concentration, and timescale between electron transfer steps, to maximize catalyst stability and efficiency.