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

Interconversion between dioxygen (O 2 ) and hydrogen peroxide (H 2 O 2 ) has attracted much interest because of the growing importance of H 2 O 2 as an energy source. There are many reports on O 2 conversions to H 2 O 2; however, no example has been reported on O 2 /H 2 O 2 interconversion. Herein, we describe successful achievement of a reversible O 2 /H 2 O 2 conversion based on an N 21, N 23-dimethylated saddle-distorted porphyrin and the corresponding two-electron-reduced porphyrin (isophlorin) for the first time. The isophlorin could react with O 2 to afford the corresponding porphyrin and H 2 O 2; conversely, the porphyrin also reacted with excess H 2 O 2 to reproduce the corresponding isophlorin and O 2 . The isophlorin-O 2 /porphyrin-H 2 O 2 interconversion was repeatedly proceeded by alternate bubbling of Ar or O 2, although no reversible conversion was observed in the case of an N 21, N 22-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 O 2 as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with H 2 O 2 . The intriguing isophlorin-O 2 /porphyrin-H 2 O 2 interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O 2 /porphyrin-H 2 O 2 states and the Gibbs activation energy of the interconversion. On the basis of the kinetic and thermodynamic analysis on the isophlorin-O 2 /porphyrin-H 2 O 2 interconversion using 1 H NMR and UV–vis spectroscopies and DFT calculations, we propose the formation of a two-point hydrogen-bonding adduct between the N 21, N 23-dimethylated porphyrin and H 2 O 2 as an intermediate.