Generation and reactivity of phosphoryl-substituted cobalt(III) isoporphyrins: Their relevance as oxidizing agents

Abstract

Chemical generation of high-oxidized species of phosphoryl-substituted Co(III)-tetraphenylporphyrinates in the reaction with peroxides are studied applying spectral and electrochemical methods. We determine the kinetic regularities and describe the possible mechanism of formation of the active species. The coordination capacity of cobalt porphyrinates is reported to be responsible for accessibility of high-oxidized intermediates afforded when tBuOOH expose. The complex-peroxide systems are studied using UV–visible, IR spectroscopy, mass-spectometry and cyclic voltammetry. Modificating the macroheterocyclic ligand in the complexes through the introduction of phosphoryl and methyl substituents, followed by conformational distortions of the ligand, as well as varying peroxide concentration are shown to affect the mechanism of generation and the identity of active species. The electrochemical and spectral studies are complemented with a quantum-chemical simulation. An comparative analysis of the energy state of the frontier molecular orbitals of the titled complexes and their oxidized species are performed to understand the distinctive properties of the phosphoryl-substituted cobalt isoporphyrins for the first time. Studied cobalt(III) isoporphyrins are found to have a lower HOMO-LUMO energy gap than parent complexes, which accounts for the appearance of UV–visible spectra at lower energy regions, their ease of reduction and possibility to be used as effective oxidants. The examined highly reactive species are capable of oxidizing β-carotene and methylene blue. The findings contribute to providing additional information about the structure−property relationship to predict the reactivity of the metalloporphyrin key intermediates for catalysis.