[Rh(III)(dmbpy)2Cl2]+ as a highly efficient catalyst for visible-light-driven hydrogen production in pure water: comparison with other rhodium catalysts.

We report a very efficient homogeneous system for the visible-light-driven hydrogen production in pure aqueous solution at room temperature. This comprises [Rh(III) (dmbpy)(2)Cl(2)]Cl (1) as catalyst, [Ru(bpy)(3)]Cl(2) (PS1) as photosensitizer, and ascorbate as sacrificial electron donor. Comparative studies in aqueous solutions also performed with other known rhodium catalysts, or with an iridium photosensitizer, show that 1) the PS1/1/ascorbate/ascorbic acid system is by far the most active rhodium-based homogeneous photocatalytic system for hydrogen production in a purely aqueous medium when compared to the previously reported rhodium catalysts, Na(3)[Rh(I) (dpm)(3)Cl] and [Rh(III)(bpy)Cp*(H(2)O)]SO(4) and 2) the system is less efficient when [Ir(III) (ppy)(2)(bpy)]Cl(PS2) is used as photosensitizer. Because catalyst 1 is the most efficient rhodium-based H(2)-evolving catalyst in water, the performance limits of this complex were further investigated by varying the PS1/1 ratio at pH 4.0. Under optimal conditions, the system gives up to 1010 turnovers versus the catalyst with an initial turnover frequency as high as 857 TON h(-1). Nanosecond transient absorption spectroscopy measurements show that the initial step of the photocatalytic H(2)-evolution mechanism is a reductive quenching of the PS1 excited state by ascorbate, leading to the reduced form of PS1, which is then able to reduce [Rh(III)(dmbpy)(2)Cl(2)](+) to [Rh(I)(dmbpy)(2)](+). This reduced species can react with protons to yield the hydride [Rh(III)(H)(dmbpy)(2)(H(2)O)](2+), which is the key intermediate for the H(2) production.