First-Principles Insights into Proton-Coupled Electron Transfer versus Hydrogen Evolution Reaction Selectivity from a Base-Appended Cobaltocene Mediator.

Performing selective proton-coupled electron transfer (PCET) to substrates such as N2, CO2, and unsaturated organic molecules under electrochemical conditions requires the suppression of the competing hydrogen evolution reaction (HER). To address this challenge, our laboratory previously demonstrated a PCET mediator strategy using a dimethylaniline-appended cobaltocene complex, [(CpCoCpNMe2)H]+, which performs selective reductive chemistry while suppressing the HER. However, the origin of the suppressed, yet still observable, HER has not been thoroughly established. In this work, we perform density functional theory (DFT) calculations to elucidate the HER mechanism involving this redox mediator and to provide atomistic insights into the bifurcation between the PCET and HER pathways. We find that protonation of the aniline moiety to form [CpCoCpNMe2H]+ is more favorable, both kinetically and thermodynamically, than formation of the ring-protonated species [(CpCo(Cp-H)NMe2)]+. Furthermore, PCET to acetophenone is energetically more favorable via [CpCoCpNMe2H]+ than via [(CpCo(Cp-H)NMe2)]+1/0. In contrast, the most favorable HER pathway involves the ring-protonated Co(I) species. These results offer mechanistic insights into HER versus PCET bifurcation and establish guiding principles for designing PCET mediators for selective electroreductive transformations.