Stepwise 6H⁺/6e– Electron-Coupled Proton Buffers Based on Fe and Redox-Active Ligands

Herein, we report electron-coupled-proton buffers (ECPBs) based on Fe and redox-active ortho-phenylenediamine (opda) ligands that perform stepwise and reversible 6H+/6e– transformations. Four of the Fe complexes involved in the PCET transformation (namely X62+, X8H22+, X10H42+ and X12H62+) were structurally and/or spectroscopically characterized. The reductive protonation of X62+ to X12H62+ and the oxidative deprotonation of X12H62+ to X62+ were carried out using PCET reagents, which indicate that these 6H+/6e– transformations occurred in a 2H+/2e– fashion, accumulating the intermediate species X8H22+and X10H42+. The thermochemistry of the 2H+/2e– and overall 6H+/6e– transformations was studied by open-circuit potential measurements and comproportionation reactions. Interestingly, the Fe-based ECPBs depicted redox unleveling, in which the average bond dissociation free energy (BDFEavg) of the 2H+/2e– reductive protonation of X62+ to X8H22+ was substantially higher than the BDFEavg of the 6H+/6e– conversion of X62+ to X12H62+. We also show that the BDFEavg of the PCET transformations involving the Fe system bearing unsubstituted opda are higher than the systems bound by 4,5-Me2-opda and 4,5-(MeO)2-opda, a manifestation of redox decompensation. The capability of the Fe-based ECPBs to accept and donate H-atom equivalents, as well as their ability to dehydrogenate organic substrates using O2 as oxidant in a decoupled fashion, was also evaluated.