Impact of Pendant Amine Basicity on Electrochemically-Promoted Cobalt Hydride Formation: Kinetic and Mechanistic Analysis

We report the role of pendant amine basicity on the proton-coupled electron transfer (PCET) reactivity for the conversion of [CoIIICp(PPh2NR2)(CH3CN)]2+ complexes to [HCoIIICp(PPh2NR2)]+, which is a key transformation involved in catalytic CO2 conversion to formate and in H2 evolution. Three complexes were studied, where the amine substituent (R) varies from benzyl, methoxyphenyl, or phenyl. In previous work on the benzyl system, we showed that the amine on the PPh2NBn2 ligand serves as a kinetically accessible protonation site and enables three participating hydride formation mechanisms. In this work, a combination of electrochemical measurements and theoretical calculations were used to show that the electronic donation at the pendant amine influences the accessible PCET mechanism and proton transfer kinetics related to cobalt hydride formation under analogous reaction conditions. Notably, the amine with the most electron-donating substituent correlates to the lowest barrier for amine protonation, and specific cobalt hydride formation mechanisms can be shut off for the amine with the least electron-donating substituent. The mechanistic and kinetic changes upon modulation of the amine substituent have great implications for overall catalytic efficiency and selectivity, especially to generate the cobalt hydride intermediate involved in selective CO2 reduction to formate. This work shows how to exploit kinetic basicity using ligand-cooperative design to facilitate PCET reactions involved in energy related transformations.