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

Abstract

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 (BDFE_avg) of the 2H⁺/2e^– reductive protonation of ^X6²⁺ to ^X8H₂²⁺ was substantially higher than the BDFE_avg of the 6H⁺/6e^– conversion of ^X6²⁺ to ^X12H₆²⁺. 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.