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

Interconversion between dioxygen (O) and hydrogen peroxide (HO) has attracted much interest because of the growing importance of HO as an energy source. There are many reports on O conversions to HO; however, no example has been reported on O/HO interconversion. Herein, we describe successful achievement of a reversible O/HO 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 O to afford the corresponding porphyrin and HO; conversely, the porphyrin also reacted with excess HO to reproduce the corresponding isophlorin and O. The isophlorin-O/porphyrin-HO interconversion was repeatedly proceeded by alternate bubbling of Ar or O, 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 O as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with HO. The intriguing isophlorin-O/porphyrin-HO interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O/porphyrin-HO states and the Gibbs activation energy of the interconversion. On the basis of the kinetic and thermodynamic analysis on the isophlorin-O/porphyrin-HO interconversion using H 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 HO as an intermediate.