[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)₂Cl₂]Cl (1) as catalyst, [Ru(bpy)₃]Cl₂ (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, Na₃[Rh^I(dpm)₃Cl] 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 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 H₂-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)₂Cl₂] ⁺ 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 H₂ production.