Many biological processes, from metabolic C–H activation to water oxidation, involve the concerted transfer of both a proton and an electron (CPET). There has recently been interest in how the kinetics of CPET reactions can be affected by the energies of proton transfer ΔGPT and of electron transfer ΔGET. Many reports have implicated ΔGPT and/or ΔGET in specific systems and rationalized this dependence with imbalanced transition states or similar concepts. Other reports have called into question whether it is theoretically justified to invoke an imbalanced transition state, and by extension trends with ΔGPT and ΔGET, in a CPET reaction. We find that ΔGPT and ΔGET have a statistically significant effect on the rates of CPET reactivity between a broad set of metal oxo complexes and C–H bonds. To better understand the theoretical basis of these trends, we probed the transition state of imbalanced C–H activation with variable temperature kinetic isotope effect measurements. These revealed temperature insensitive KIEs, which cannot be explained with the semiclassical transition state theory underlying DFT analysis and imbalanced transition states – the proton transfer must be treated quantum mechanically. We additionally present a model system with which reconciles the concepts of imbalanced transition state effects with quantum mechanical proton transfer, demonstrating that the effect of ΔGPT and ΔGET on CPET reactions are theoretically justified all the same.

Experiment and theory support the broad influence of ΔGPT and ΔGET on concerted proton-electron transfer

ACS Spring 2022

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