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Dioxygen/Hydrogen Peroxide Interconversion Using Redox Couples of Saddle-Distorted Porphyrins and Isophlorins
journal contribution
posted on 2019-03-18, 00:00 authored by Wataru Suzuki, Hiroaki Kotani, Tomoya Ishizuka, Takahiko KojimaInterconversion 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 NH 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.