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

By Wataru Suzuki (423403), Hiroaki Kotani (1415575), Tomoya Ishizuka (1415599) and Takahiko Kojima (1415593)


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

Topics: Biophysics, Biochemistry, Physiology, Pharmacology, Evolutionary Biology, Cancer, Biological Sciences not elsewhere classified, Chemical Sciences not elsewhere classified, Physical Sciences not elsewhere classified, two-point hydrogen-bonding adduct, isophlorin, nitrogen atoms, 1 H NMR, N H protons, form hydrogen bonds, isophlorin-O, UV, dimethylated, Gibbs activation energy, O 2, porphyrin, O 2 conversions, H 2 O 2, DFT, interconversion, porphyrin-H
Year: 2019
DOI identifier: 10.1021/jacs.9b01038.s001
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