Acid Strength Effects on 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 dicationic
organocatalyst (Ph2Phen2+) for the ORR. 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, a kinetically relevant peroxo dimer species forms with
all acids. However, under spectrochemical conditions, strong acids
diminish the kinetic contribution of this dimer to the observed rate
due to lower catalyst concentrations, whereas weaker acids were still
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 reaction tuning analogous to transition
metal-based systems.