Photochemical generation and kinetic studies of a putative porphyrin-ruthenium(v)-oxo species

Abstract

Photo-disproportionation of a bis-porphyrin-diruthenium(IV) μ-oxo dimer gave a porphyrin-ruthenium(III) species and a putative porphyrin-ruthenium(V)-oxo species that can be detected and studied in real time via laser flash photolysis methods. As determined by its spectral and kinetic behavior, the same oxo transient was also formed by photolysis of a porphyrin-ruthenium(III) N-oxide adduct. Second-order rate constants for reactions with several substrates at 22 °C were determined; representative values of rate constants were k_ox = 6.6 × 10³ M⁻¹ s⁻¹ for diphenylmethanol, k_ox = 2.5 × 10³ M⁻¹ s⁻¹ for styrene, and k_ox = 1.8 × 10³ M⁻¹ s⁻¹ for cyclohexene. The putative porphyrin-ruthenium(V)-oxo transient reacted 5–6 orders of magnitude faster than the corresponding trans-dioxoruthenium(VI) porphyrins, and the rate constants obtained in this work were similar to those of the corrole-iron(V)-oxo derivative. The high reactivity for the photochemically generated ruthenium-oxo species in comparison to other porphyrin-metal-oxo intermediates suggests that it is a true ruthenium(V)-oxo species.