Hydrogen production through water splitting is one of the most promising solutions for the storage of renewable energy. [NiFe] hydrogenases are organometallic enzymes containing nickel and iron centres that catalyse hydrogen evolution with performances that rival those of platinum. These enzymes provide inspiration for the design of new molecular catalysts that do not require precious metals. However, all heterodinuclear NiFe models reported so far do not reproduce the Ni-centred reactivity found at the active site of [NiFe] hydrogenases. Here, we report a structural and functional NiFe mimic that displays reactivity at the Ni site. This is shown by the detection of two catalytic intermediates that reproduce structural and electronic features of the Ni-L and Ni-R states of the enzyme during catalytic turnover. Under electrocatalytic conditions, this mimic displays high rates for H evolution (second-order rate constant of 2.5 × 10 M s; turnover frequency of 250 s at 10 mM H concentration) from mildly acidic solutions.

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.