Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications

Abstract

We present an update and revision to our 2010 review on the topic of protoncoupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XH n in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H 2 ), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H 2 ) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

“… 6 The position of HAT equilibria can be expressed as the difference in the bond dissociation free energies (BDFEs) of the two bonds to hydrogen that are undergoing exchange. 22 To be energetically favorable, the BDFE of the bond broken in the reactants must be lower (weaker) than the BDFE of the bond formed in the products. This simple requirement presents a significant challenge to HAT activation of many common organic functional groups, which would require either the cleavage of exceptionally strong bonds, or the formation of unusually weak bonds, to hydrogen.…”

“…However, metal hydrides, which are often used as catalytic H • donors, generally have M–H BDFEs ≥ 50 kcal mol –1 as complexes with lower bond strengths tend to spontaneously evolve H 2 . 22 Conceivably, this requirement can be overcome when an unfavorable HAT is then coupled with a fast and irreversible exothermic downhill reaction of the generated radical, but this is limited to specific circumstances. In light of these limitations, general methods for the direct HAT activation of many common functional groups have yet to be reported ( Figure 1 C).…”

“…For a further discussion on the derivation of this equation, see Warren, Tronic, and Mayer. 22 [Recently, the literature accepted values of C g have been revised. Many of the works discussed in this Review used the previous standard values of C g in solving the Bordwell equation for the calculation of both E–H bond BDFEs and reagent pair effective BDFEs.…”

“…In this Review we discuss the conclusions of the original research works without correction but make this note such that readers are aware of this recent revision. See reference ( 22 )b for a discussion.] Oxidant and base combinations may act as formal H • acceptors, while reductant and acid combinations may act as formal H • donors.…”

“… 16 , 29 , 30 The abundance of p K a , E °, and BDFE data available in the literature facilitates the rational design of new PCET reagent combinations. We would direct the reader to the useful literature on excited-state redox potentials, 45 , 46 redox potentials in organic solvents, 21 , 22 , 39 , 47 and p K a scales in organic solvents. 22 , 48 , 49 Examples of both excited-state and ground-state MS-PCET reagent pairs and the corresponding effective BDFE values are given in Figure 3 .…”