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...

Artificial photosynthesis (AP) is a promising method of converting solar energy into fuel (H 2 ). Harnessing solar energy to generate H 2 from H + is a crucial process in systems for artificial photosynthesis. Widespread application of a device for AP would rely on the use of platinum-free catalysts due to the scarcity of noble metals. Here we report a series of cobalt dithiolene complexes that are exceptionally active for the catalytic reduction of protons in aqueous solvent mixtures. All catalysts perform visible-light-driven reduction of protons from water when paired with as the photosensitizer and ascorbic acid as the sacrificial donor. Photocatalysts with electron withdrawing groups exhibit the highest activity with turnovers up to 9,000 with respect to catalyst. The same complexes are also active electrocatalysts in 1∶1 acetonitrile/water. The electrocatalytic mechanism is proposed to be ECEC, where the Co dithiolene catalysts undergo rapid protonation once they are reduced to . Subsequent reduction and reaction with H + lead to H 2 formation. Cobalt dithiolene complexes thus represent a new group of active catalysts for the reduction of protons.

A novel class of derivatized acetylacetonate (acac) linkers for robust functionalization of TiO2 nanoparticles (NPs) under aqueous and oxidative conditions is reported. The resulting surface adsorbate anchors are particularly relevant to engineering photocatalytic and photovoltaic devices since they can be applied to attach a broad range of photosensitizers and photocatalytic complexes and are not affected by humidity. Acac is easily modified by CuI-mediated coupling reactions to provide a variety of scaffolds, including substituted terpy complexes (terpy = 2,2':6,2''-terpyridine), assembled with ligands coordinated to transition-metal ions. Since Mn-terpy complexes are known to be effective catalysts for oxidation chemistry, functionalization with Mn(II) is examined. This permits visible-light sensitization of TiO2 nanoparticles due to interfacial electron transfer, as evidenced by UV-vis spectroscopy of colloidal thin films and aqueous suspensions. The underlying ultrafast interfacial electron injection is complete on a subpicosecond time scale, as monitored by optical pump-terahertz probe transient measurements and computer simulations. Time-resolved measurements of the Mn(II) EPR signal at 6 K show that interfacial electron injection induces Mn(II) --> Mn(III) photooxidation, with a half-time for regeneration of the Mn(II) complex of ca. 23 s.

The generation of H2 from protons and electrons by complexes of cobalt has an extensive history. During the past decade, interest in this subject has increased as a result of developments in hydrogen generation that are driven electrochemically or photochemically. This article reviews the subject of hydrogen generation using Co complexes as catalysts and discusses the mechanistic implications of the systems studied for making H2. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

No abstract

No abstract

The new ligand bitriazole-2-ylidene (bitz) has been successfully coordinated to Ru(II), Pd(II), Rh(I), Rh(II), and Rh(III), showing its wide chemical versatility. Reaction of the bitz ligand precursor salt, 1,1′-methyl-4,4′-bi-1,2,4-triazolium diiodide, with [RhCl(cod)]2 under mild conditions afforded chelate and bis-chelate Rh(III) complexes, as well as an unexpected metal−metal bounded di-Rh(II) species. The reaction of the precursor salt of bitz with [(η6-p-cymene)RuCl2]2 and NaOAc afforded a mixture of chelate and dimetallic Ru(II)-arene complexes. The reaction of the salt with Pd(OAc)2 led to a chelate complex. In an unexpected, purely organic reaction, the precursor salt rearranges to a new C−C bound bitriazole on reaction with strong base in the absence of the metal. The electron-donor power of the bitz ligand, estimated by DFT computations, proved to be very low for an NHC and comparable with those of dmpe and dipy. Some of the new complexes proved to be catalytically active in hydrogen transfer from iPrOH.