Acid Strength Effects on Off-Cycle Dimerization During Metal-Free Catalytic Dioxygen Reduction

Development of earth-abundant catalysts for the reduction of dioxygen (ORR) is essential for the development of alternative industrial processes and energy sources. Here, we report a transition metal-free diium organocatalyst (Ph2Phen2+) for the ORR via an outer-sphere mechanism. The ORR performance of this compound was studied in acetonitrile solution under both electrochemical conditions and spectrochemical conditions, using halogenated acetic acid derivatives spanning a pKa range of 12.65 to 20.3. Interestingly, it was found that under electrochemical conditions, an off-cycle dimer species forms via an inner-sphere reaction due to the relatively high concentration of Ph2Phen•+ in the reaction-diffusion layer. However, under spectrochemical conditions, strong acids were able to rapidly protonate O2•– en route to disproportionation, avoiding the off-cycle dimeric species, whereas weaker acids were found to be rate-limited by the dimer equilibrium. Together, these results provide insight into the mechanisms of ORR by organic-based, metal-free catalysts, suggesting that balancing redox activity and unsaturated character of these molecules with respect to the pKa of intermediates can enable tuning analogous to transition metal-based systems.