In situ Fourier transform infrared spectroelectrochemical studies of the oxidation of some octaethylporphyrin complexes

The oxidations of OEPNi(II), OEPCu(II), OEPFe(III)Cl, OEPMn(III)ClO4, OEPAg(II), OEPCo(II), and OEPZn(II) (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin dianion) have been studied by in situ FTIR reflectance spectroelectrochemistry in dibromomethane. One-electron oxidation of the porphyrin ring to produce a π-cation radical results in the appearance of a strong new absorbance band near 1550 cm−1. Other features typical of π-cation radical formation are a strong absorbance increase near 1470 cm−1, and absorbance decreases near 1270 and 1018 cm−1. Oxidation of the metal centre, as observed for OEPCo(II) in the presence of CH3CN, and for OEPAg(II), results in considerably less perturbation of the vibrational spectra of the complexes. For OEPCo(II) in dry dibromomethane, initial oxidation at the porphyrin ligand is followed by a second oxidation at the metal centre. This order is reversed when acetonitrile is added to solution. Production of porphyrin π-dications was observed for OEPCu(II) and OEPNi(II). This results in a shift of the 1550 cm−1 π-cation radical marker band to higher energy, and a strong absorbance increase near 1380 cm−1. Unusual spectral changes observed on one-electron oxidation of OEPZn(II) are suggested to result from the formation of a dimer.