Five- and six-coordinate ruthenium(II) porphyrin tetiaryphosphine complexes, and their reactions with dioxygen via inner- and outer-sphere mechanisms

Abstract

The 5-coordinate ruthenium(II) octaethylporphyrin complex Ru(OEP)(PPh 3) has been prepared by reduction of Ru(OEP)(PPh 3)Br using zinc amalgam. Both the Ru(OEP)(PPh 3) 3 complexes (n = 1,2) undergo reaction in toluene with O 2 to generate OPPh 3, RuO 2, and the parent porphyrin H 2(OEP); trace water and the μ-oxo dimer [Ru(OEP)(OH)] 2O are implicated in the oxidation reaction, which is considered to be initiated by coordination of O 2 to Ru(OEP)(PPh 3). In contrast, a catalytic O 2-oxidation of excess PPh 3 to the oxide probably goes via an initial outer-sphere reaction with Ru(OEP)(PPh 3) 2, that generates superoxides and Ru(III), both detectable by ESR; the superoxide is believed to be stabilized via portion addition as HO 2· that subsequently disproportionates to O 2 and H 2O 2. PPh 3 is oxidized by the peroxide, and during a reduction step that regenerates the Ru(II) catalyst from Ru(III).