Effect of solvent binding on UV-vis spectra and redox potentials of octaethylporphyrins containing first-row transition metal ions

Abstract

The UV-visible spectra and oxidation/reduction potentials for six octaethylporphyrins with first-row transition metal ions were measured in four non-aqueous solvents and then analyzed as a function of the Gutmann solvent parameters, donor number (DN) or acceptor number (AN). The utilized solvents were dichloromethane ( CH2Cl2 ), N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and pyridine (py). The investigated porphyrins were (OEP)M, where OEP = the dianion of octaethylporphyrin and M = CoII, NiII, CuII, ZnII, FeIIICl or MnIIICl . UV-visible spectra upon solvent (S) binding to give (OEP)M(S) or [ (OEP)M(S)2]+Cl- were monitored during a titration of the porphyrin in CH2Cl2 and formation constants ( logβ n, n = 1 or 2) for the ligand addition reactions were determined using standard equations. Five-coordinate (OEP)CoII(S) and (OEP)ZnII(S) are generated in CH2Cl2 solutions containing DMF, DMSO or py while six-coordinate [ (OEP)Mn(S)2]+Cl and [ (OEP)Fe(S)2]+Cl- are formed under the same solution conditions. The magnitudes of the solvent binding constants are discussed in terms of both the solvent donor/acceptor properties and the electronegativity (EN) or stability index (SI) of the porphyrin central metal ion. A comparison between solvent binding constants measured in this study for (OEP)M and literature data for (TPP)M with the same central metal ions is presented.