A Mechanistic Dichotomy in Two-Electron Reduction of Dioxygen Catalyzed byN,N'-Dimethylated Porphyrin Isomers

Selective two-electron reduction of dioxygen (O-2) to hydrogen peroxide (H2O2) has been achieved by two saddle-distortedN,N'-dimethylated porphyrin isomers, anN21,N'22-dimethylated porphyrin (anti-Me2P) and anN21,N'23-dimethylated porphyrin (syn-Me2P) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved byanti-Me(2)Pwith higher turnover number (TON=250 for 30 min) than that bysyn-Me2P(TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti-Me(2)Iphorsyn-Me(2)Iph) by spectroscopic measurements. The rate-determining step in the catalytic ORRs was concluded to be proton-coupled electron-transfer reduction of O(2)with isophlorins based on kinetic analysis. The ORR rate byanti-Me(2)Iphwas accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate withsyn-Me(2)Iphby external protons was observed. The different mechanisms in the O(2)reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O(2)with inner NHprotons of the isophlorins.