[Rh-III(dmbpy)(2)Cl-2](+) 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*-(H2O)]SO4 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 rhodiumbased 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 TONh(-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)(H2O)](2+), which is the key intermediate for the H-2 production.