William R. McNamara

A cobalt-dithiolene complex for the photocatalytic and electrocatalytic reduction of protons.

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.

Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions

Artificial photosynthesis (AP) is a promising method of converting solar energy into fuel (H2). Harnessing solar energy to generate H2 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 H2 formation. Cobalt dithiolene complexes thus represent a new group of active catalysts for the reduction of protons.

Cobalt complexes as artificial hydrogenases for the reductive side of water splitting

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.

Hydroxamate anchors for water-stable attachment to TiO2 nanoparticles

Surface functionalization of nanoparticles is of broad interest, such as for dye attachment in dye-sensitized solar cells (DSSCs) and photocatalysis. Visible-light photoexcitation of the dye gives interfacial electron transfer (IET) into the conduction band of a semiconductor host. In a Gr€atzel cell, TiO2 is functionalized with Ru polypyridyl complexes that attach via carboxylate substituents that permit ultrafast IET but are unstable in aqueous conditions. We now report on hydroxamate anchors for robust TiO2 functionalization even in aqueous conditions. Hydroxamate ligands bind tightly to transition metals, even in water. For example, bacterial siderophores that contain hydroxamates can dissolve Fe(III) from the oxide. Recent studies have reported binding of hydroxamic acids to TiO2. 8 Here, we investigate their potential as robust anchors for functionalization of TiO2 thinfilms commonly used in solar energy conversion and photocatalysis. We synthesize and deposit a hydroxamate-functionalized terpyridine and demonstrate visible-light sensitization of TiO2 and activation of Mn adsorbates by ultrafast IET by using spectroscopy and molecular modeling. The synthesis (Scheme 1) builds on prior methods and proceeds in two steps in good yield. The methyl ester (1) reacts with O-Bn hydroxylamine (BnONH2) in the presence of LiHMDS to give the corresponding ester. The ester is then deprotected with H2 and Pd/C to give the product 2. Degussa P25 TiO2 nanoparticles (NPs) were sensitized with a solution of 2 in dry EtOH using known techniques. The resulting sensitized nanoparticles were characterized using UV-visible and FTIR spectroscopy (see Fig. S1 and S2†). The spectroscopic data are consistent with 2 anchoring to TiO2 via the hydroxamate. Upon binding, the disappearance of a C]O stretch at 1635 cm 1 present in the IR of unbound 2 is consistent with a O–CR]N–O unit

Water-stable, hydroxamate anchors for functionalization of TiO2 surfaces with ultrafast interfacial electron transfer

A novel class of derivatized hydroxamic acid linkages for robust sensitization of TiO2 nanoparticles (NPs) under various aqueous conditions is described. The stability of linkages bound to metal oxides under various conditions is important in developing photocatalytic cells which incorporate transition metal complexes for solar energy conversion. In order to compare the standard carboxylate anchor to hydroxamates, two organic dyes differing only in anchoring groups were synthesized and attached to TiO2 NPs. At acidic, basic, and close to neutral pH, hydroxamic acid linkages resist detachment compared to the labile carboxylic acids. THz spectroscopy was used to compare ultrafast interfacial electron transfer (IET) into the conduction band of TiO2 for both linkages and found similar IET characteristics. Observable electron injection and stronger binding suggest that hydroxamates are a suitable class of anchors for designing water stable molecules for functionalizing TiO2.

Hydrogen Evolution Catalyzed by an Iron Polypyridyl Complex in Aqueous Solutions

Iron complexes containing tetradentate monophenolate ligands have been found to be highly active for the electrocatalytic reduction of protons to hydrogen gas. Catalysis occurs at -1.17 V vs SCE in CH3CN with a turnover frequency of up to 1000 s(-1) and a 660 mV overpotential. Interestingly, the catalyst activity is enhanced in the presence of water, achieving turnover frequencies of 3000 s(-1) with an overpotential of 800 mV, making it one of the most active iron electrocatalysts currently reported. The catalyst is also capable of generating hydrogen from purely aqueous buffer solutions of pH 3-5 with Faradaic efficiencies of 98%.

Iron Polypyridyl Complexes for Photocatalytic Hydrogen Generation

A series of Fe(III) complexes were recently reported that are stable and active electrocatalysts for reducing protons into hydrogen gas. Herein, we report the incorporation of these electrocatalysts into a photocatalytic system for hydrogen production. Hydrogen evolution is observed when these catalysts are paired with fluorescein (chromophore) and triethylamine (sacrificial electron source) in a 1:1 ethanol:water mixture. The photocatalytic system is highly active and stable, achieving TONs > 2100 (with respect to catalyst) after 24 h. Catalysis proceeds through a reductive quenching pathway with a quantum yield of over 3%.

Sulfinato Iron(III) Complex for Electrocatalytic Proton Reduction

We report the first example of a sulfinato Fe(III) complex acting as a highly active electrocatalyst for proton reduction. The sulfinate binds to the metal through oxygen, resulting in a seven-membered chelate ring that is likely hemilabile during catalysis. Proton reduction occurs at -1.57 V versus Fc/Fc(+) in CH3CN with an ic/ip = 13 in CH3CN (kobs = 3300 s(-1)) and an overpotential of 800 mV. The catalysis is first order with respect to [catalyst] and second order with respect to [trifluoracetic acid]. An 11% increase in catalytic activity is observed in the presence of water, suggesting that sulfinate moieties are viable functional groups for aqueous proton reduction catalysts.