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

Selective two-electron reduction of dioxygen (O ) to hydrogen peroxide (H O ) has been achieved by two saddle-distorted N,N'-dimethylated porphyrin isomers, an N21,N'22-dimethylated porphyrin (anti-Me P) and an N21,N'23-dimethylated porphyrin (syn-Me P) 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 by anti-Me P with higher turnover number (TON=250 for 30 min) than that by syn-Me P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti-Me Iph or syn-Me Iph) by spectroscopic measurements. The rate-determining step in the catalytic ORRs was concluded to be proton-coupled electron-transfer reduction of O with isophlorins based on kinetic analysis. The ORR rate by anti-Me Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn-Me Iph by external protons was observed. The different mechanisms in the O reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O with inner NH protons of the isophlorins.