Reaction of tris(bipyridine)ruthenium(III) with hydroxide and its application in a solar energy storage system

Abstract

Irradiation of Ru(bipy) 3 2+ (bipy = 2,2′-bipyridine) with light below 560 nm results in the formation of a charge-transfer excited state potentially capable of reducing water to dihydrogen with concomitant production of Ru(bipy) 3 3+ . The latter may be reduced by hydroxide [Formula: see text] to form dioxygen and regenerate the starting complex. The use of these reactions in a cell designed to bring about the photochemical decomposition of water is proposed. The stoichiometry, kinetics, and mechanism of the Ru(bipy) 3 3+ -hydroxide reaction have been investigated by conventional and stopped-flow spectrophotometry. The dioxygen yield is a sharp function of pH, attaining its maximum value (about 80%) at pH 9. At low pH (3 and 4.8) the production of ruthenium(II) is first order with k obsd = (1.41 ± 0.04) × 10 -4 sec -1 (25°, ionic strength μ = 1.00 M with sodium sulfate). In the intermediate pH range (7.9-10.0) complex kinetics are observed. In the hydroxide range 0.01-0.50 M, ruthenium(II) production is predominantly first order with k obsd = k a [OH - ] + k b [OH - ] 2 sec -1 ; k a = 148 M -1 sec -1 and k b = 138 M -2 sec -1 (25°, μ = 1.00 M, sodium sulfate). For the k a term, the activation parameters are ΔH ‡ = 15.3 ± 1.0 kcal mol -1 and ΔS ‡ = 7 ± 3 cal deg -1 mol -1 (1 cal = 4.184 J). An intermediate species (λ max 800 nm) forms at the same rate as ruthenium(II) in this hydroxide range. It disappears with k obsd = 1.2 + 1.1 × 10 2 [OH - ] sec -1 at 25°. Similarly absorbing (λ max 750 to 800 nm) species are generated by the addition of hydroxyl radical to M(bipy) 3 2+/3+ [M = Fe(II), Os(II), Ru(II), Cr(III), Ru(III)] in pulse radiolysis experiments. The kinetics above pH 7 are described in terms of rate-determining nucleophilic attack by hydroxide on the bound bipyridine ring. The hydroxide adduct so generated is tentatively identified with that observed in the pulse radiolysis experiments with Ru(bipy) 3 2+ . For reduction of Ru(bipy) 3 3+ by hydrogen peroxide ruthenium(II) production is first order with k obsd = k c [HO 2 - ] + k d [H 2 O 2 ] where k c = 5.4 × 10 7 M -1 sec -1 and k d = 8.3 M -1 sec -1 (25°, μ = 1.00 M, pH 3.5 to 9.7). This reaction produces dioxygen in 83 ± 15% yield at pH 6.8 and in 1.0 N sulfuric acid.