Oxidation of copper(II) hydroxyporphyrin (oxophlorin); oxidative ring opening and formation of an ester-linked, dinuclear copper complex

Abstract

Addition of copper(II) acetate to octaethyl(meso-hydroxy)porphyrin (H2OEPOH) in THF under a dinitrogen atmosphere produces CuII(OEPOH), which has been isolated as red crystals and shown to have a meso-hydroxyporphyrin structure by UV-vis and EPR spectroscopy. CuII(OEPOH) undergoes oxidation by dioxygen to form a novel dinuclear copper complex {CuII(OEPOC(O)OEB)CuII}. This dinuclear complex is composed of a copper(II) meso-substituted porphyrin portion (with a planar CuN4 unit), which is attached through an ester linkage to a helical copper(II) tetrapyrrole that has been ring-opened through oxidative cleavage of a second molecule of CuII(OEPOH). The oxidative ring-opening reaction resembles that of natural heme catabolism but is arrested at a stage where the oxidized meso-carbon is still appended to the helical open tetrapyrrole. A mechanism is suggested for this process.Key words: copper porphyrin, crystal structure, porphyrin oxidation, linear tetrapyrrole, hydroxy-porphyrin, oxophlorin.