Radiation-induced electron-transfer processes in Ru(II)—Diimine complexes in aqueous solution. An application of Marcus electron-transfer theory

Abstract

The rate constants for the reduction of RuL 2+ (where RuL 2+ represents ten complexes of the general formula Ru(bpy) 3− m− z(bpm) m (bpz) 2+ z , bpy = 2,2′-bipyridine, bpm = 2,2′-bipyridine, bpz = 2,2′-bipyrazine, m and z = 0, 1, 2, 3 and m + z /z/ 3) to RuL +/sd by CO -· 2 radicals and the rate constants for the reactions between the couples RuL 2+ RuL +· or RuL 2+, H + RuLH 2+· and the couple MV 2+/MV 2+/MV +· (MV 2+ = 1,1′-dimethyl-4,4′-bipyridinium dication) have been measured in aqueous solution using pulse radiolysis methods. The kinetic data, interpreted with use of Marcus electron-transfer theory, have generated the following parameters: for the reaction CO -· 2 + RuL 2+ → CO 2 + RuL +·, the value of the electronic transmission coefficient k is 1 and that of the intrinsic barrier Δ G ≠(0) is 57.3 kJ mol -1. For the CO 2 CO −· 2 self-exchange electron-transfer reaction, values of ΔG ≠(0)=97.0 kJ mol −1 and k se = 1.4 × 10 -31 mol -1 s -1 have been derived. For the electron-transfer reactions involving the MV 2+ MV +· and RuL 2+ RuL +· or RuL 2+, H + RuLH 2+· couples, values of Δ ≠(0)=20.5 and 32.2 kJ mol -1 were obtained in alkaline and acid solution, respectively, by assuming adiabatic behaviour, i.e. k = 1; the difference between these two values is attributed to an additional intrinsic barrier associated with the protonation/deprotonation process.