Breaking Bonds with Electrons and Protons. Models and Examples

Besides its theoretical interest, the attention currently aroused by proton-coupled electron transfers (PCET reactions) has two main motives. One is a better understanding of biological processes in which PCET reactions are involved, Photosystem II as well as a myriad of other natural systems. The other is directed toward synthetic processes, many of which are related to global energy challenges. Until recently, the analyses of the mechanism and reactivity of PCET reactions have focused on outersphere transfers, those in which no bond between heavy atoms (all atoms with the exception of H) is concomitantly formed or broken. Conversely, reactions in which electron transfer triggers the breaking of a heavy-atom bond with no proton transfer have been extensively analyzed, both theoretically and experimentally. In both cases, strategies have been developed to distinguish between stepwise and concerted pathways. In each case, kinetic models have been devised, allowing the relation between activation and thermodynamic driving force to be established by means of parameters pertaining to the initial and final state. Although many natural and artificial processes include electron transfer, proton transfer, and heavy-atom bond breaking (/formation), no means were offered until recently to analyze the mechanism of such reactions, notably to establish the degree of concertedness of the three constitutive events. Likewise, no kinetic models were available to describe reactions where the three events are concerted. In this Account, we discuss the strategies to distinguish stepwise, partially concerted (when two of the three events are concerted), and totally concerted pathways in these reactions that include electron transfer, proton transfer, and heavy-atom bond breaking. These mechanism analysis methods are illustrated and validated by three examples. First we describe the electrochemical cleavage of an O-O bond in an aliphatic peroxide molecule with a pendant carboxylic acid group that can serve as proton donor for electron transfer and bond breaking. In the second example, we examine the breaking of one of the C-O bonds of CO2 within a multistep process where the reduction of CO2 into CO is catalyzed by an electrogenerated iron(0) porphyrin in the presence of various Brönsted acids. In this case, an intramolecular electron transfer triggers proton transfer and bond cleavage. In the first two examples, all three events are concerted. The third example also involves catalysis. It describes the cleavage of a cobalt-carbon bond in the reduction of chloroacetonitrile catalyzed by an electrogenerated cobalt(I) porphyrin. It illustrates the rather common case where the intermediate formed by the reaction of a transition metal complex with the substrate has to be cleaved to close the catalytic cycle. In the first two examples, all three events are concerted, whereas, in the last case, a partially concerted pathway takes place: proton transfer and bond-breaking (Co-C cleavage) are concerted after an initial electron transfer step. The all-concerted cases require a model that connects the kinetics to the thermodynamic driving force of the reaction. Starting from previous models of outersphere electron transfer, concerted proton-electron transfer, and concerted dissociative electron transfer, we describe a model for all-concerted proton-electron-bond breaking reactions. These pathways skip the high-energy intermediates that occur in stepwise pathways, but could introduce kinetic penalties. The all-concerted model allows one to assess these penalties and the way in which they can be fought by the supplement of driving force offered by concerted proton transfer.