Facile aerial oxidation of a porphyrin. Part 3. Some metal complexes of meso-tetrakis-(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin

Abstract

Metal complexes of the title porphyrin were prepared and their behaviour in basified dichloromethane solution examined using u.v.–visible and e.s.r. spectroscopy. The free base, Zn^II, Fe^III, and Co^II complexes undergo drastic colour changes with rapid and irreversible aerial oxidation. Acidification does not regenerate the porphyrin spectrum, but shaking with aqueous metabisulphite or dithionite does. The V^IVO, Mn^III, Cu^II, Sn^IV, Pd^II, and Pt^II complexes also change colour but their metalloporphyrin u.v.–visible spectra largely return on acidification. Some deterioration over time in the returned B band intensity is noted, which is greatest for the Ni^II complex and is most likely due to autoxidation. E.s.r. spectra, arising from oxidation or autoxidation of the complexes to radical species, were observed for the Mn^III, Ni^II, Zn^II, and Sn^IV complexes, the latter in neutral dichloromethane. This gave a triplet e.s.r. spectrum that decayed with second-order kinetics. Addition of base to the Sn^IV complex quenched the e.s.r. spectrum. An explanation is offered in which the metal modulates electron transfer onto the macrocycle, from the phenoxy meso-substituents, via back-bonding and changes in porphyrin conformation.