No abstract

Homogeneous systems for light-driven reduction of protons to H(2) typically suffer from short lifetimes because of decomposition of the light-absorbing molecule. We report a robust and highly active system for solar hydrogen generation in water that uses CdSe nanocrystals capped with dihydrolipoic acid (DHLA) as the light absorber and a soluble Ni(2+)-DHLA catalyst for proton reduction with ascorbic acid as an electron donor at pH = 4.5, which gives >600,000 turnovers. Under appropriate conditions, the precious-metal-free system has undiminished activity for at least 360 hours under illumination at 520 nanometers and achieves quantum yields in water of over 36%.

The complex [Co(bdt)(2)](-) (where bdt = 1,2-benzenedithiolate) is an active catalyst for the visible light driven reduction of protons from water when employed with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial electron donor. At pH 4.0, the system exhibits very high activity, achieving >2700 turnovers with respect to catalyst and an initial turnover rate of 880 mol H(2)/mol catalyst/h. The same complex is also an active electrocatalyst for proton reduction in 1:1 CH(3)CN/H(2)O in the presence of weak acids, with the onset of a catalytic wave at the reversible redox couple of -1.01 V vs Fc(+)/Fc. The cobalt-dithiolene complex [Co(bdt)(2)](-) thus represents a highly active catalyst for both the electrocatalytic and photocatalytic reduction of protons in aqueous solutions.

The splitting of water through artificial photosynthesis (AP) is a key transformation toward the conversion of solar energy into stored chemical potential in the form of fuel and oxidizer.[1] For water splitting, the reductive side of the reaction involves the light-driven conversion of aqueous protons into H 2 . To perform this half-reaction, photocatalytic systems typically consist of a catalyst, photosensitizer (PS), and sacrificial electron donor.[2] Recent studies on noblemetal-based [3] and noble-metal-free [4] homogeneous systems for light-driven hydrogen production have shown high activity. However, significant problems in the noble-metalfree molecularly based systems include relatively low catalyst turnover numbers (TON < 500 mole H 2 /mole catalyst) for hydrogen formation, and photodecomposition of the systems within a few hours. For most organic dye based systems that have recently been reported, the photochemical quenching step of the excited-state dye (PS*) is reductive, thus leading to unstable PS À radical anions that undergo decomposition.[4b]Thus, the development of more active catalysts, specifically ones that quench PS* oxidatively, would be of great value for obtaining long-lived homogeneous AP systems. Herein, we describe a new homogenous catalyst for H 2 production that has both high activity and the ability to oxidatively quench PS*, thus leading to a much longer system lifetime. Nickel cathodes are used in commercial electrolyzers, suggesting that nickel may be a worthwhile basis for homogeneous catalysts as well.[5] Nickel thiolate complexes have received special attention in recent years because sulfurligated nickel complexes mimic the [Fe-Ni]-hydrogenase active site, [6] and dimeric metal complexes based on nickel thiolate hydrides have been shown to be catalytically active for proton reduction.[7] DuBois and co-workers have also shown that mononuclear nickel(II) bis(diphosphine) complexes are effective catalysts for electrochemical hydrogen generation.[8] While photocatalytic hydrogen generation from the nickel-phosphine complexes is long-lived, the activity of the photocatalytic system with the nickel phosphine catalyst is low, with a turnover frequency (TOF) of approximately 20 equivalents of H 2 per hour.[9] Related nickel(II) complexes containing pyridine-2-thiolate ligands have been known for over two decades, [10] but their catalytic properties for proton reduction have not been reported. In the present study, the complex [Ni(pyS) 3 ] À (1 À ; pyS = pyridine-2-thiolate) is found to have impressive catalytic activity for the photocatalytic production of H 2 in a homogeneous system with fluorescein (Fl) as the PS and triethylamine (TEA) as the sacrificial electron donor.Photolysis of a solution of Fl and 1 À in EtOH/H 2 O (1:1) using a green-light-emitting diode (LED) (l = 520 nm, 0.12 W) at 15 8C results in H 2 generation which was monitored in real time by the pressure change in the reaction vessel, and quantified at the end of the photolysis by GC analysis of the headspace gase...

A series of mononuclear nickel(II) thiolate complexes (Et4N)Ni(X-pyS)3 (Et4N = tetraethylammonium; X = 5-H (1a), 5-Cl (1b), 5-CF3 (1c), 6-CH3 (1d); pyS = pyridine-2-thiolate), Ni(pySH)4(NO3)2 (2), (Et4N)Ni(4,6-Y2-pymS)3 (Y = H (3a), CH3 (3b); pymS = pyrimidine-2-thiolate), and Ni(4,4'-Z-2,2'-bpy)(pyS)2 (Z = H (4a), CH3 (4b), OCH3 (4c); bpy = bipyridine) have been synthesized in high yield and characterized. X-ray diffraction studies show that 2 is square planar, while the other complexes possess tris-chelated distorted-octahedral geometries. All of the complexes are active catalysts for both the photocatalytic and electrocatalytic production of hydrogen in 1/1 EtOH/H2O. When coupled with fluorescein (Fl) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor, these complexes exhibit activity for light-driven hydrogen generation that correlates with ligand electron donor ability. Complex 4c achieves over 7300 turnovers of H2 in 30 h, which is among the highest reported for a molecular noble metal-free system. The initial photochemical step is reductive quenching of Fl* by TEA because of the latter's greater concentration. When system concentrations are modified so that oxidative quenching of Fl* by catalyst becomes more dominant, system durability increases, with a system lifetime of over 60 h. System variations and cyclic voltammetry experiments are consistent with a CECE mechanism that is common to electrocatalytic and photocatalytic hydrogen production. This mechanism involves initial protonation of the catalyst followed by reduction and then additional protonation and reduction steps to give a key Ni-H(-)/N-H(+) intermediate that forms the H-H bond in the turnover-limiting step of the catalytic cycle. A key to the activity of these catalysts is the reversible dechelation and protonation of the pyridine N atoms, which enable an internal heterocoupling of a metal hydride and an N-bound proton to produce H2.