Acid Strength Effects\non Dimerization during Metal-Free\nCatalytic Dioxygen Reduction

Development of earth-abundant catalysts for the reduction\nof dioxygen\n(ORR) is essential for the development of alternative industrial processes\nand energy sources. Here, we report a transition metal-free dicationic\norganocatalyst (<b>Ph</b><sub><b>2</b></sub><b>Phen</b><sup><b>2+</b></sup>) for the ORR. The ORR performance of this\ncompound was studied in acetonitrile solution under both electrochemical\nconditions and spectrochemical conditions, using halogenated acetic\nacid derivatives spanning a p<i>K</i><sub>a</sub> range\nof 12.65 to 20.3. Interestingly, it was found that under electrochemical\nconditions, a kinetically relevant peroxo dimer species forms with\nall acids. However, under spectrochemical conditions, strong acids\ndiminish the kinetic contribution of this dimer to the observed rate\ndue to lower catalyst concentrations, whereas weaker acids were still\nrate-limited by the dimer equilibrium. Together, these results provide\ninsight into the mechanisms of ORR by organic-based, metal-free catalysts,\nsuggesting that balancing redox activity and unsaturated character\nof these molecules with respect to the p<i>K</i><sub>a</sub> of intermediates can enable reaction tuning analogous to transition\nmetal-based systems.