Kinetics and mechanism of the photooxidation of bis(bipyridine)dichlororuthenium(II) and the photoreduction of bis(bipyridine)dichlororuthenium(III) in chloroform

Abstract

In air-saturated CHCl3 irradiation of [Ru(bpy)2Cl2]0/+ (bpy = 2,2′-bipyridine) with the full UV output of a 200-W mercury lamp converted the Ru(II) complex completely to Ru(III) at or below 58°C, and converted the Ru(III) complex completely to Ru(II) at or above 60°C. No thermal reaction occurred in either direction. The photooxidation takes place through a radical chain mechanism initiated by absorption of light by chloroform followed by C-Cl bond homolysis. This leads to the formation of trichloromethylperoxy radicals, each of which can cause the oxidation of two molecules of [Ru(bpy)2Cl2]. The mechanism proposed is consistent with the experimental rate law, d[Ru(III)]/dt = a{flo[Ru(II)]}1/2. The photoreduction in deoxygenated solution takes place through direct excitation of [Ru(bpy)2Cl2]+, which then oxidizes Cl- to Cl atoms. Because of its short lifetime, only the Ru(III) complexes that are ion-paired with chloride ion can react.