Speciation of a water-soluble chromium porphyrin by spectral and electrochemical methods

A water-soluble chromium porphyrin, [CrIIITF4TMAP]5+ (chromium(III) meso-tetrakis(2,3,5,6-tetrafluoro-N,N,N-trimethyl-4-anilinium)yl) porphyrin) was synthesized and the spectral and electrochemical properties were examined. In aqueous solution, [CrIIITF4TMAP]5+ exhibits two pKas at 7.04 and 10.07. The cyclic voltammogram showed that a reversible reduction wave appeared at −0.84 V in pH 5.0 buffer solution, which is assigned as the CrIII/II redox couple. The oxidation formal potentials were obtained by spectroelectrochemistry. With suitable control of the oxidation potentials and solution conditions, the high valent oxo-CrIV and oxo-CrV porphyrins were obtained. The ligand trans to the oxygen atom is either H2O or OH−, depending on the pH of the solution. In this paper, oxo-CrVTF 4TMAP is thus the first reported water-soluble CrV porphyrin that can be generated electrochemically and chemically. Electrogenerated oxo-CrV porphyrin species catalyzed the oxidation of cyclopent-2-ene-1-acetic acid to cyclopent-2-ene-4-one-1-acetic acid in the presence of dioxygen and returned to oxo-CrIV porphyrin, which is not reactive toward the substrate.