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.