Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

The proton-coupled redox chemistry has been studied with the use of thermochemical data-reduction potentials, pKa values, and bond dissociation free energies and enthalpies. The square schemes are particularly valuable in analyzing mechanistic pathways for H-transfers. A detailed knowledge of all of the microscopic steps is important to understand PCET process. The redox potentials are typically determined electrochemically. The average of the anodic and cathodic peaks in the cyclic voltammogram, E1/2, is typically used as a good measure of the thermodynamic potential. The use of bond dissociation free energies (BDFE) rather than bond dissociation enthalpies (BDE) is especially important for transition metal complexes because they can have large entropic contributions for a PCET reaction. The concerted proton-electron transfer (CPET) reactivity in solution should be analyzed with solution BDFE, but common tabulations of bond strengths are gas-phase BDE.