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

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  • A Mechanistic Dichotomy in Two‐Electron Reduction of Dioxygen Catalyzed by <i>N</i>,<i>N</i>’‐Dimethylated Porphyrin Isomers

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Abstract

<jats:title>Abstract</jats:title><jats:p>Selective two‐electron reduction of dioxygen (O<jats:sub>2</jats:sub>) to hydrogen peroxide (H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>) has been achieved by two saddle‐distorted <jats:italic>N</jats:italic>,<jats:italic>N</jats:italic>’‐dimethylated porphyrin isomers, an <jats:italic>N</jats:italic>21,<jats:italic>N</jats:italic>’22‐dimethylated porphyrin (<jats:italic><jats:bold>anti</jats:bold></jats:italic><jats:bold>‐Me<jats:sub>2</jats:sub>P</jats:bold>) and an <jats:italic>N</jats:italic>21,<jats:italic>N</jats:italic>’23‐dimethylated porphyrin (<jats:italic><jats:bold>syn</jats:bold></jats:italic><jats:bold>‐Me<jats:sub>2</jats:sub>P</jats:bold>) 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 <jats:italic><jats:bold>anti</jats:bold></jats:italic><jats:bold>‐Me<jats:sub>2</jats:sub>P</jats:bold> with higher turnover number (TON=250 for 30 min) than that by <jats:italic><jats:bold>syn</jats:bold></jats:italic><jats:bold>‐Me<jats:sub>2</jats:sub>P</jats:bold> (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (<jats:italic><jats:bold>anti</jats:bold></jats:italic><jats:bold>‐Me<jats:sub>2</jats:sub>Iph</jats:bold> or <jats:italic><jats:bold>syn</jats:bold></jats:italic><jats:bold>‐Me<jats:sub>2</jats:sub>Iph</jats:bold>) by spectroscopic measurements. The rate‐determining step in the catalytic ORRs was concluded to be proton‐coupled electron‐transfer reduction of O<jats:sub>2</jats:sub> with isophlorins based on kinetic analysis. The ORR rate by <jats:italic><jats:bold>anti</jats:bold></jats:italic><jats:bold>‐Me<jats:sub>2</jats:sub>Iph</jats:bold> 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 <jats:italic><jats:bold>syn</jats:bold></jats:italic><jats:bold>‐Me<jats:sub>2</jats:sub>Iph</jats:bold> by external protons was observed. The different mechanisms in the O<jats:sub>2</jats:sub> reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O<jats:sub>2</jats:sub> with inner N<jats:italic>H</jats:italic> protons of the isophlorins.</jats:p>

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