Metal Ion-/Proton-Coupled\nElectron Transfer (MPCET)\non <i>ortho</i>-Quinone

Quinol/quinone equilibria are ubiquitous in nature and\nfind multiple\ntechnological applications, most recently in electrical charge storage.\nMuch research has been devoted to proton-coupled electron transfer\n(PCET) in such systems and to bidentate complexation of <i>ortho</i>-quinol (catechol) ligands with multivalent metal ions but rarely\nto the interplay of these two reactions. Here, we investigate the\nimpact of a redox-inactive metal ion, as a complexing and charge-compensating\nagent, on redox processes of catechol in aqueous solutions, that is,\nin the presence of proton equilibria. We pay separate attention to\ntheir thermodynamics and kinetics, which can be regulated by the pH\nand buffer capacity. As the proton buffer concentration decreases,\nproton equilibria during catechol PCET are slower to establish, thus\nkinetically prioritizing the participation of the metal ion rather\nthan the proton in the redox charge compensation. Making use of this\nkinetic interplay can be a general strategy to conceive organic battery\ncathodes for proton-free metal-ion aqueous batteries.