Mechanism of the formation of dihydrogen from the photoinduced reactions of tris(bipyridine)ruthenium(II) with tris(bipyridine)rhodium(III)

Abstract

Visible light irradiation of aqueous solutions of Ru(bpy)/sub 3//sup 2 +/, Rh(bpy)/sub 3//sup 3 +/, triethanolamine TEOA/TE OAH/sup +/ (pH 8.1), and K/sub 2/PtCl/sub 4/ (or K/sub 2/PtCl/sub 6/) yields dihydrogen with a quantum yield of up to 0.11 +- 0.01 mol einstein/sup -1/ depending on the conditions used. The dihydrogen is the end product of a sequence of electron-transfer reactions. Ru(bpy)/sub 3//sup 2 +/ formed by visible light absorption is oxidized by Rh(bpy)/sub 3//sup 3 +/ to produce Rh(bpy)/sub 3//sup 2 +/ and Ru(bpy)/sub 3//sup 3 +/. The latter is rapidly reduced by TEOA. In the absence of platinum, the rhodium(II) disproportionates to give rhodium(III) and rhodium(I); in the presence of platinum, the rhodium(II) may either disproportionate or yield dihydrogen. Observations on the photochemical and dark reactions in this system are reported. The rhodium system is more expensive than that based on methylviologen and is subject to efficiency-lowering side reactions. 1 figure. (DP)