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 (J. Am. Chem. Soc. 2022, 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, BDFE₁, BDFE₂, BDFE₃, and BDFE₄) and average bond dissociation free energy values (BDFE_avg.). 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_avg. within the series of Cu-based ECPBs was found to arise from a substantial change in the redox potential (E_1/2) upon modification of the ligand scaffold, which is not fully compensated for by a change in the acidity/basicity (pK_a), suggesting "thermochemical decompensation".