Reaction of tris ( bipyridine ) " ruthenium ( III ) with hydroxide and its application in a solar energy storage system ( dioxygen production / water oxidation / dihydrogen formation / hydroxyl radicals / photosynthesis ) CAROL CREUTZ

Irradiation of 'Ru(bipy)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)33+. The latter may be reduced by hydroxide Ru(bipy 33+ + OH = Ru(bipy)32+ + '402 + Y/2H20 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)33+-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 kobsd = (1.41 : 0.04) X 10-4 sec1 (250, ionic strength IA = 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 kobsd = ka[OH+ kb[OH-2 sec1; ka = 148 M-1 sec1 and kb = 138 MA sec1 (250, IA = 1.00 M, sodium sulfate). For the ka term, the activation parameters are AH: = 15.3 i 1.0 kcal mol' and ASt = 7 + 3 cal deg-' mol' (1 cal = 4.184 J). An intermediate species (Xmax 800 nm) forms at the same rate as ruthenium(II) in this hydroxide range. It disappears with kobsd = 1.2 + 1.1 X 102 [OH-] sec-1 at 250. Similarly absorbing (Xma. 750 to 800 nm) species are generated by the addition of hydroxyl radical to M(bipy)s2+/3+ [M = Fe(II), Os(II), Ru(II), Cr(III), Ru(III)J 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 For reduction of Ru(bipy)33+ by hydrogen peroxide ruthenium(II) production is first order with kobsd = k4HO2-] + kd[H202 where kc = 5.4 X 107 M-1 sect and kd = 8.3 M-l sect (25°, IS = 1.00 M, pH 3.5 to 9.7). This reaction produces dioxygen in 83 L 15% yield at pH 6.8 and in 1.0 N sulfuric acid. The photodecomposition of water into its elements is one of the most attractive means of storing solar energy. This decomposition process may be broken down conceptually into three steps: light absorption, water oxidation, and water reduction. Certain transition metal complexes, by virtue of their high absorption in the visible region and their facility in undergoing oxidation-reduction reactions, should be capable of mediating some or all of these processes. Tris(bipyridine)ruthenium (and related) complexes possess particularly desirable qualities in this regard. Irradiation of Ru(bipy)32+ (bipy = 2,2'-bipyridine) with light below 560 nm results in the formation of a relatively long-lived [lifetime r = 0.6 ;sec in water at 250 (1)] charge-transfer excited state [*Ru(bipy)32+] (2, 3) potentially capable of reducing water to dihydrogen (1), Eq. 1. *Ru(bipy)32+ + H20 = Ru(bipy)33+ +