Reactivity of Ru(bpy) +3 towards the radicals originating from the scavenging of hydrogen atoms and hydroxyl radicals by methanol, ethanol, propan-2-ol, tert-butyl alcohol and formate ions in aqueous solution: a pulse radiolytic study

Abstract

Ru(bpy)⁺₃, where Ru(bpy)⁺₃=[Ru^II(bpy)₂(bpy˙^–)]⁺ and bpy = 2,2′-bipyridine, generated in nominally neutral aqueous solution via reduction of Ru(bpy)²⁺₃ by e^–_aq, reacts [k=(1.9 ± 0.5)× 10⁹ dm³ mol^–1 s^–1] with the radicals R˙[R˙=˙CH₂OH, CH₃ĊHOH, (CH₃)₂ĊOH, and CO^˙–₂] originating from the scavenging of H˙ and ˙OH by CH₃OH, CH₃CH₂OH, (CH₃)₂CHOH and HCO^–₂, respectively, and with the radical ˙CH₂C(CH₃)₂OH originating from the scavenging of ˙OH by (CH₃)₃COH. The products of these reactions, which are believed to proceed via addition of R˙ to coordinated bpy˙^– to give [Ru^II(bpy)₂(bpyR^–)]⁺, show an absorption in the 390 nm region (ε= 13 000 ± 1500 dm³ mol^–1 cm^–1). A subsequent decay of the absorbance at 390 nm follows first-order kinetics with k_obs=(a+b[H₂PO^–₄]) s^–1; the values of a and b, which are related to the rate constants for proton transfer from H₂O and H₂PO^–₄, respectively, to [Ru^II(bpy)₂(bpyR^–)]⁺ to give [Ru^II(bpy)₂(bpyHR)]²⁺, have been determined for each R˙. In alkaline solution, the formation of [Ru^II(bpy)₂(bpyR^–)]⁺ is observed only when R˙=˙CH₂C(CH₃)₂OH and CO^˙–₂ owing to the fact that the basic forms of the α-hydroxylalkyl radicals, i.e. CH₂O˙^–, CH₃CHO˙^– and (CH₃)₂CO˙^– are rapidly scavenged (k= 1.6 × 10⁹, 3.6 × 10⁹ and 3.1 × 10⁹ dm³ mol^–1 s^–1, respectively, at pH 13 and µ= 0.1 mol dm^–3) by Ru(bpy)²⁺₃ to give Ru(bpy)⁺₃.