Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile

Homogeneous electron transfer reactions of the Cu(II) complexes of 5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) with various oxidizing reagents were spectrophotometrically investigated in acetonitrile. The reaction products were confirmed to be the π-cation radicals of the corresponding Cu(II)-porphyrin complexes on the basis of the electronic spectra and the redox potentials of the complexes. The rate of the electron transfer reaction between the Cu(II)-porphyrin complex and solvated Cu²⁺ was determined as a function of the water concentration under the pseudo first-order conditions where Cu²⁺ is in large excess over the Cu(II)-porphyrin complex. The decrease in the pseudo first-order rate constant with increasing the water concentration was attributed to the stepwise displacement of acetonitrile in [Cu(AN)₆]²⁺ (AN = acetonitrile) by water, and it was concluded that only the Cu²⁺ species fully solvated by acetonitrile, [Cu(AN)₆]²⁺, possesses sufficiently high redox potential for the oxidation of Cu(II)-OEP and Cu(II)TPP. The reactions of the Cu(II)-porphyrin complexes with other oxidizing reagents such as [Ni(tacn)₂]³⁺ (tacn = 1,4,7-triazacyclononane) and [Ru(bpy)₃]³⁺ (bpy = 2,2′-bipyridine) were too fast to be followed by a conventional stopped-flow technique. Marcus cross relation for the outer-sphere electron transfer reaction was used to estimate the rate constants of the electron self-exchange reaction between Cu(II)-porphyrin and its π-cation radical: log(k/M^-1 s^-1) = 9.5 ± 0.5 for TPP and log(k/M ^-1 s^-1) = 11.1 ±0.5 for OEP at 25.0 °C. Such large electron self-exchange rate constants are typical for the porphyrin-centered redox reactions for which very small inner- and outer-sphere reorganization energies are required.