Abstract
Irradiation of Ru(bipy)32+ (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 [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)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) × 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 kobsd = ka[OH-] + kb[OH-]2 sec-1; ka = 148 M-1 sec-1 and kb = 138 M-2 sec-1 (25°, μ = 1.00 M, sodium sulfate). For the ka 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 (λmax 800 nm) forms at the same rate as ruthenium(II) in this hydroxide range. It disappears with kobsd = 1.2 + 1.1 × 102 [OH-] sec-1 at 25°. Similarly absorbing (λmax 750 to 800 nm) species are generated by the addition of hydroxyl radical to M(bipy)32+/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)32+.
For reduction of Ru(bipy)33+ by hydrogen peroxide ruthenium(II) production is first order with kobsd = kc[HO2-] + kd[H2O2] where kc = 5.4 × 107 M-1 sec-1 and kd = 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.
Keywords: dioxygen production, water oxidation, dihydrogen formation, hydroxyl radicals, photosynthesis
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Selected References
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