Free Energies of Proton-Coupled Electron Transfer\nReagents and Their Applications

We\npresent an update and revision to our 2010 review on the topic\nof proton-coupled electron transfer (PCET) reagent thermochemistry.\nOver the past decade, the data and thermochemical formalisms presented\nin that review have been of value to multiple fields. Concurrently,\nthere have been advances in the thermochemical cycles and experimental\nmethods used to measure these values. This Review (i) summarizes those\nadvancements, (ii) corrects systematic errors in our prior review\nthat shifted many of the absolute values in the tabulated data, (iii)\nprovides updated tables of thermochemical values, and (iv) discusses\nnew conclusions and opportunities from the assembled data and associated\ntechniques. We advocate for updated thermochemical cycles that provide\ngreater clarity and reduce experimental barriers to the calculation\nand measurement of Gibbs free energies for the conversion of X to\nXHn in PCET reactions. In particular,\nwe demonstrate the utility and generality of reporting potentials\nof hydrogenation, E°(V vs H2), in\nalmost any solvent and how these values are connected to more widely\nreported bond dissociation free energies (BDFEs). The tabulated data\ndemonstrate that E°(V vs H2) and\nBDFEs are generally insensitive to the nature of the solvent and,\nin some cases, even to the phase (gas versus solution). This Review\nalso presents introductions to several emerging fields in PCET thermochemistry\nto give readers windows into the diversity of research being performed.\nSome of the next frontiers in this rapidly growing field are coordination-induced\nbond weakening, PCET in novel solvent environments, and reactions\nat material interfaces.