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A Mechanistic Dichotomy in Two‐Electron Reduction of Dioxygen Catalyzed by N , N ’‐Dimethylated Porphyrin Isomers
Author(s) -
Suzuki Wataru,
Kotani Hiroaki,
Ishizuka Tomoya,
Kojima Takahiko
Publication year - 2020
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.202000942
Subject(s) - porphyrin , chemistry , catalysis , electron transfer , acetonitrile , medicinal chemistry , hydrogen peroxide , ferrocene , photochemistry , proton , electrochemistry , organic chemistry , electrode , physics , quantum mechanics
Selective two‐electron reduction of dioxygen (O 2 ) to hydrogen peroxide (H 2 O 2 ) has been achieved by two saddle‐distorted N , N ’‐dimethylated porphyrin isomers, an N 21, N ’22‐dimethylated porphyrin ( anti ‐Me 2 P ) and an N 21, N ’23‐dimethylated porphyrin ( syn ‐Me 2 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 2 P with higher turnover number (TON=250 for 30 min) than that by syn ‐Me 2 P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins ( anti ‐Me 2 Iph or syn ‐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 by anti ‐Me 2 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 2 Iph by 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 N H protons of the isophlorins.