Tuning the Thermochemistry and Reactivity of a Series of Cu-Based 4H/4e Electron-Coupled-Proton Buffers.

Electron-coupled-proton buffers (ECPBs) store and deliver protons and electrons in a reversible fashion. We have recently reported an ECPB based on Cu and a redox-active ligand that promoted 4H/4e reversible transformations ( , 144, 16905). Herein, we report a series of Cu-based ECPBs in which the ability of these to accept and/or donate H equivalents can be tuned via ligand modification. The thermochemistry of the 4H/4e ECPB equilibrium was determined using open-circuit potential measurements. The reactivity of the ECPBs against proton-coupled electron transfer (PCET) reagents was also analyzed, and the results obtained were rationalized based on the thermochemical parameters. Experimental and computational analysis of the thermochemistry of the H/e transfers involved in the 4H/4e ECPB transformations found substantial differences between the stepwise (namely, BDFE, BDFE, BDFE, and BDFE) and average bond dissociation free energy values (BDFE). Our analysis suggests that this "redox unleveling" is critical to promoting the disproportionation and ligand-exchange reactions involved in the 4H/4e ECPB equilibria. The difference in BDFE within the series of Cu-based ECPBs was found to arise from a substantial change in the redox potential () upon modification of the ligand scaffold, which is not fully compensated for by a change in the acidity/basicity (p), suggesting "thermochemical decompensation".