Thermodynamic fitness of an Ru^III(EDTA)/Ru^V=O(EDTA) couple in the oxygen atom transfer reactions

Abstract

The complex [Ru^III(EDTA-H)(H₂O)]1 (EDTA-H = protonated ethylenediaminetetraacetic acid) catalyzes the epoxidation of cyclohexene and the oxidation of PPh₃ to OPPh₃ with molecular oxygen or NaOC1 as the oxidant. In both the cases, the active catalytic species is the [Ru^v=O(EDTA)]⁻ ion 2̲ characterized by elemental analyses, UV-vis, infrared spectra and cyclic voltammograms. When oxygen is used as an oxidant in the reaction, oxidation of the substrates proceeds through a μ-peroxo-Ru(IV) intermediate [Rusi^IV(EDTA)(S)]₂ O²⁻₂3̲ (S=olefin, PPh₃) which undergoes a fast homolytic cleavage of the O-O bond to form [Ru^v=O(EDTA)(S)]4̲ and a rate determining transfer of O atom to S to form SO and1. The cleavage of the O-O bond in 3 and O-atom transfer to S proceed in a concerted step. With NaOCl as an oxidant (excess), oxidation of 1̲ to 2̲ proceeds through a unimolecular decomposition of the [Ru^III(EDTA)(OCl)]²⁻ intermediate. The transfer of the O-atom from 2̲ to S proceeds by an associative (Ia) pathway involving a concerted cleavage of the Ru=O bond and formation of an SO bond. The free energy (ΔG) values for various steps are computed and the thermodynamic fitness of 2̲ as an oxidant discussed.