Dioxygen/Hydrogen Peroxide Interconversion Using Redox Couples of Saddle-Distorted Porphyrins and Isophlorins

Interconversion between dioxygen (O₂) and hydrogen peroxide (H₂O₂) has attracted much interest because of the growing importance of H₂O₂ as an energy source. There are many reports on O₂ conversions to H₂O₂; however, no example has been reported on O₂/H₂O₂ interconversion. Herein, we describe successful achievement of a reversible O₂/H₂O₂ conversion based on an N21, N23-dimethylated saddle-distorted porphyrin and the corresponding two-electron-reduced porphyrin (isophlorin) for the first time. The isophlorin could react with O₂ to afford the corresponding porphyrin and H₂O₂; conversely, the porphyrin also reacted with excess H₂O₂ to reproduce the corresponding isophlorin and O₂. The isophlorin-O₂/porphyrin-H₂O₂ interconversion was repeatedly proceeded by alternate bubbling of Ar or O₂, although no reversible conversion was observed in the case of an N21, N22-dimethylated porphyrin as a structural isomer. Such a drastic change of the reversibility was derived from the directions of inner N H protons in hydrogen-bond formation of the isophlorin core with O₂ as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with H₂O₂. The intriguing isophlorin-O₂/porphyrin-H₂O₂ interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O₂/porphyrin-H₂O₂ states and the Gibbs activation energy of the interconversion. On the basis of the kinetic and thermodynamic analysis on the isophlorin-O₂/porphyrin-H₂O₂ interconversion using ¹H NMR and UV-vis spectroscopies and DFT calculations, we propose the formation of a two-point hydrogen-bonding adduct between the N21, N23-dimethylated porphyrin and H₂O₂ as an intermediate.