Mechanism of the Formation of Dihydrogen from the Photoinduced Reactions of Poly(pyridine)ruthenium(II) and Poly(pyridine)rhodium(III) Complexes

Article abstract of DOI:10.1021/ja00392a022

The irradiation of Ru(bpy)3²⁺, Rh(bpy)3³⁺, and triethanolamine (TEOA) solutions 10⁹ 8, 25 deg C) with 450 +/- 20-nm light yields rhodium (I) (Φ = 0.13 +/- 0.02) (units for Φ in mol einstein^-1 throughout the paper) and dihydrogen (Φ = 0.11 +/- 0.02) in the absence and presence of platinum, respectively.A detailed mechanistic scheme has been deduced from the results of continuous- and flash-photolysis experiments: light absorption by Ru(bpy)3²⁺ gives the excited state (*)Ru(bpy)3²⁺ which is oxidized by Rh(bpy)3³⁺ (k = 3.9*10⁸ M^-1 s^-1) yielding Ru(bpy)3³⁺ and Rh(bpy)3²⁺ with a cage escape yield of 0.15 +/- 0.03.Back-reaction of Ru(bpy)3³⁺ with Rh(bpy)3²⁺ (k = 3*10⁹ M^-1 s^-1) is prevented by reduction of Ru(bpy)3³⁺ by TEOA (k = 0.2*10⁸ M^-1 s^-1) The oxidized TEOA radical so generated undergoes a TEOA-promoted rearrangement (k = 0.3*10⁷ M^-1 s^-1) to a reducing radical.The latter reduces Rh(bpy)3²⁺ so that the net yield for Rh(bpy)3²⁺ formation is 0.3 +/- 0.1.Rate-determining loss of bpy from Rh(bpy)3²⁺ (k = 1.0 +/-0.5 s^-1) is followed by rapid reduction of Rh(bpy)2²⁺ by Rh(bpy)3²⁺ (k = 0.3* 10⁹ M^-1 s^-1) giving Rh(bpy)3³⁺ and Rh(I).In the presence of platinum, H2 is formed at the expense of Rh(I); catalyzed reaction of Rh(II) with water occurs before disproportionation to Rh(I) can take place.The H2 quantum yield in this system is limited only by the cage escape of the primary products, the homogeneous and heterogeneous dark reactions being very efficient.In the course of this study the electrochemistry of Rh(bpy)3³⁺, Rh(phen)3³⁺, and Rh(bpy)2(OH)2⁺ in aqueous solution was investigated, and the quenching of Ru(bpy)3²⁺ emission by these Rh(III) complexes was characterized.