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 <i>ortho</i>-phenylenediamine (opda) ligands that perform stepwise and reversible 6H⁺/6e^- transformations. Four of the Fe complexes involved in the PCET transformation (namely ^<b>X</b><b>6</b>^<b>2+</b>, ^<b>X</b><b>8H</b>_<b>2</b>^<b>2+</b>, ^<b>X</b><b>10H</b>_<b>4</b>^<b>2+</b> and ^<b>X</b><b>12H</b>_<b>6</b>^<b>2+</b>) were structurally and/or spectroscopically characterized. The reductive protonation of ^<b>X</b><b>6</b>^<b>2+</b> to ^<b>X</b><b>12H</b>_<b>6</b>^<b>2+</b> and the oxidative deprotonation of ^<b>X</b><b>12H</b>_<b>6</b>^<b>2+</b> to ^<b>X</b><b>6</b>^<b>2+</b> were carried out using PCET reagents, which indicate that these 6H⁺/6e^- transformations occurred in a 2H⁺/2e^- fashion, accumulating the intermediate species ^<b>X</b><b>8H</b>_<b>2</b>^<b>2+</b>and ^<b>X</b><b>10H</b>_<b>4</b>^<b>2+</b>. 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 (BDFE_avg) of the 2H⁺/2e^- reductive protonation of ^<b>X</b><b>6</b>^<b>2+</b> to ^<b>X</b><b>8H</b>_<b>2</b>^<b>2+</b> was substantially higher than the BDFE_avg of the 6H⁺/6e^- conversion of ^<b>X</b><b>6</b>^<b>2+</b> to ^<b>X</b><b>12H</b>_<b>6</b>^<b>2+</b>. We also show that the BDFE_avg 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.