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.