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) x 10(-4) sec(-1) (25 degrees , ionic strength mu = 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 degrees , mu = 1.00 M, sodium sulfate). For the k(a) term, the activation parameters are DeltaH(double dagger) = 15.3 +/- 1.0 kcal mol(-1) and DeltaS(double dagger) = 7 +/- 3 cal deg(-1) mol(-1) (1 cal = 4.184 J). An intermediate species (lambda(max) 800 nm) forms at the same rate as ruthenium(II) in this hydroxide range. It disappears with k(obsd) = 1.2 + 1.1 x 10(2) [OH(-)] sec(-1) at 25 degrees . Similarly absorbing (lambda(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 x 10(7) M(-1) sec(-1) and k(d) = 8.3 M(-1) sec(-1) (25 degrees , mu = 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.