Quinones participate in diverse electron transfer and proton-coupled electron transfer processes in chemistry and biology. To understand the relationship between these redox processes, an experimental study was carried out to probe the 1 e– and 2 e–/2 H+ reduction potentials of a number of common quinones. The results reveal a non-linear correlation between the 1 e– and 2 e–/2 H+ reduction potentials. This unexpected observation prompted a computational study of 134 different quinones, probing their 1 e– reduction potentials, pKa values, and 2 e–/2 H+ reduction potentials. The density functional theory calculations reveal an approximately linear correlation between these three properties and an effective Hammett constant associated with the quinone substituent(s). However, deviations from this linear scaling relationship are evident for quinones that feature intramolecular hydrogen bonding in the hydroquinone, halogen substituents, charged substituents, and/or sterically bulky substituents. These results, particularly the different substituent effects on the 1 e– versus 2 e–/2 H+ reduction potentials, have important implications for designing quinones with tailored redox properties.

Macrocyclic metal complexes and p-benzoquinones are commonly used as co-catalytic redox mediators in aerobic oxidation reactions. In an effort to gain insight into the mechanism and energetic efficiency of these reactions, we investigated Co(salophen)-catalyzed aerobic oxidation of p-hydroquinone. Kinetic and spectroscopic data suggest that the catalyst resting-state consists of an equilibrium between a Co(II)(salophen) complex, a Co(III)-superoxide adduct, and a hydrogen-bonded adduct between the hydroquinone and the Co(III)-O2 species. The kinetic data, together with density functional theory computational results, reveal that the turnover-limiting step involves proton-coupled electron transfer from a semi-hydroquinone species and a Co(III)-hydroperoxide intermediate. Additional experimental and computational data suggest that a coordinated H2O2 intermediate oxidizes a second equivalent of hydroquinone. Collectively, the results show how Co(salophen) and p-hydroquinone operate synergistically to mediate O2 reduction and generate the reactive p-benzoquinone co-catalyst.

The molecular cobalt complex, Co(salophen), and para-hydroquinone (HQ) serve as effective cocatalysts for the electrochemical reduction of O to water. Mechanistic studies reveal redox cooperativity between Co(salophen) and HQ. HQ serves as an electron-proton transfer mediator (EPTM) that enables electrochemical O reduction at higher potentials and with faster rates than is observed with Co(salophen) alone. Replacement of HQ with the higher-potential EPTM, 2-chloro-HQ, allows for faster O reduction rates at higher applied potential. These results demonstrate a unique strategy to achieve improved performance with molecular electrocatalyst systems.

Mediated fuel cells are electrochemical devices that produce power in a manner similar to that of conventional proton exchange membrane fuel cells (PEMFCs). They differ from PEMFCs in their use of redox mediators dissolved in liquid electrolyte to conduct oxidation of the fuel or reduction of the oxidant, typically O 2 , in bulk solution. The mediators transport electrons (and often protons) between the electrode and the catalysts or chemical reagents in solution. This strategy can help overcome many of the challenges associated with conventional fuel cells, including managing complex multiphase reactions (as in O 2 reduction) or the use of challenging or heterogeneous fuels, such as hydrocarbons, polyols, and biomass. Mediators are also commonly used in enzymatic fuel cells, where direct electron transfer from the electrode to the enzymatic active site can be slow. This review provides a comprehensive survey of historical and recent mediated fuel cell efforts, including applications using chemical and enzymatic catalysts.

A soluble, bis-ketiminate-ligated Co complex [Co(NO)] was recently shown to catalyze selective reduction of O to HO with an overpotential as low as 90 mV. Here we report experimental and computational mechanistic studies of the Co(NO)-catalyzed O reduction reaction (ORR) with decamethylferrocene (Fc*) as the reductant in the presence of AcOH in MeOH. Analysis of the Co/O binding stoichiometry and kinetic studies support an O reduction pathway involving a mononuclear cobalt species. The catalytic rate exhibits a first-order kinetic dependence on [Co(NO)] and [AcOH], but no dependence on [Fc*] or [O]. Differential pulse voltammetry and computational studies support Co-hydroperoxide as the catalyst resting state and protonation of this species as the rate-limiting step of the catalytic reaction. These results contrast previous mechanisms proposed for other Co-catalyzed ORR systems, which commonly feature rate-limiting protonation of a Co-superoxide adduct earlier in the catalytic cycle. Computational studies show that protonation is strongly favored at the proximal oxygen of the Co(OOH) species, accounting for the high selectivity for formation of hydrogen peroxide. Further analysis shows that a weak dependence of the ORR rate on the p K values of the protonated Co(OOH) species across a series of Co(NO) catalysts provides a rationale for the unusually low overpotential observed for O reduction to HO.