Base-free non-noble-metal-catalyzed hydrogen generation from formic acid: scope and mechanistic insights.

The iron-catalyzed dehydrogenation of formic acid has been studied both experimentally and mechanistically. The most active catalysts were generated in situ from cationic Fe(II) /Fe(III) precursors and tris[2-(diphenylphosphino)ethyl]phosphine (1, PP3 ). In contrast to most known noble-metal catalysts used for this transformation, no additional base was necessary. The activity of the iron catalyst depended highly on the solvent used, the presence of halide ions, the water content, and the ligand-to-metal ratio. The optimal catalytic performance was achieved by using [FeH(PP3 )]BF4 /PP3 in propylene carbonate in the presence of traces of water. With the exception of fluoride, the presence of halide ions in solution inhibited the catalytic activity. IR, Raman, UV/Vis, and EXAFS/XANES analyses gave detailed insights into the mechanism of hydrogen generation from formic acid at low temperature, supported by DFT calculations. In situ transmission FTIR measurements revealed the formation of an active iron formate species by the band observed at 1543 cm(-1) , which could be correlated with the evolution of gas. This active species was deactivated in the presence of chloride ions due to the formation of a chloro species (UV/Vis, Raman, IR, and XAS). In addition, XAS measurements demonstrated the importance of the solvent for the coordination of the PP3 ligand.

Synthesis and characterization of new iridium photosensitizers for catalytic hydrogen generation from water.

Novel phenylazole ligands were applied successfully in the synthesis of cyclometalated iridium(III) complexes of the general formula [Ir(phenylazole)(2)(bpy)]PF(6) (bpy=2,2'-bipyridine). All complexes were fully characterized by NMR, IR, and MS spectroscopic studies as well as by cyclic voltammetry. Three crystal structures obtained by X-ray analysis complemented the spectroscopic investigations. The excited-state lifetimes of the iridium complexes were determined and showed to be in the range of several hundred ns to multiple µs. All obtained iridium complexes were active as photosensitizers in catalytic hydrogen evolution from water in the presence of triethylamine as a sacrificial reducing agent. Applying an in situ formed iron-based water reduction catalyst derived from [HNEt(3)](+) [HFe(3)(CO)(11)](-) and tris[3,5-tris-(trifluoromethyl)-phenyl]phosphine as the ligand, [Ir(2-phenylbenz-oxazole)(2)-(bpy)]PF(6) proved to be the most efficient complex giving a quantum yield of 16% at 440 nm light irradiation.

Hydrogen evolution from water/alcohol mixtures: effective in situ generation of an active Au/TiO2 catalyst.

Gold standard: Au/TiO(2) catalysts, easily prepared in situ from different Au precursors and TiO(2), generate hydrogen from water/alcohol mixtures. Different alcohols, and even glucose, can serve as sacrificial reductants. The best system produces hydrogen on a liter scale, and is stable for more than two days. Deuteration studies show that proton reduction is likely the rate-limiting step in this reaction.

Efficient dehydrogenation of formic acid using an iron catalyst.

Hydrogen is one of the essential reactants in the chemical industry, though its generation from renewable sources and storage in a safe and reversible manner remain challenging. Formic acid (HCO(2)H or FA) is a promising source and storage material in this respect. Here, we present a highly active iron catalyst system for the liberation of H(2) from FA. Applying 0.005 mole percent of Fe(BF(4))(2)·6H(2)O and tris[(2-diphenylphosphino)ethyl]phosphine [P(CH(2)CH(2)PPh(2))(3), PP(3)] to a solution of FA in environmentally benign propylene carbonate, with no further additives or base, affords turnover frequencies up to 9425 per hour and a turnover number of more than 92,000 at 80°C. We used in situ nuclear magnetic resonance spectroscopy, kinetic studies, and density functional theory calculations to explain possible reaction mechanisms.

General and selective iron-catalyzed transfer hydrogenation of nitroarenes without base.

The first well-defined iron-based catalyst system for the reduction of nitroarenes to anilines has been developed applying formic acid as reducing agent. A broad range of substrates including other reducible functional groups were converted to the corresponding anilines in good to excellent yields at mild conditions. Notably, the process constitutes a rare example of base-free transfer hydrogenations.

Hydrogen storage in formic acid amine adducts.

Formic acid, containing 4.4 wt% of hydrogen, is a non-toxic liquid at ambient temperature and therefore an ideal candidate as potential hydrogen storage material. Formic acid can be generated via catalytic hydrogenation of CO2 or bicarbonate in the presence of an amine with suitable ruthenium catalysts. In addition selective dehydrogenation of formic acid amine adducts can be carried out at ambient temperatures with either ruthenium phosphine catalyst systems as well as iron-based catalysts. In detail we obtained with the [RuCl2(benzene)]2/dppe catalyst system a remarkable TON of 260,000 at room temperature. Moreover applying Fe3(CO)12 together with tribenzylphosphine and 2,2':6',2"-terpyridine under visible light irradiation a TON of 1266 was obtained, which is the highest activity known to date for selective dehydrogenation of formic acid applying non-precious metal catalysts.

Synthesis, characterisation and application of iridium(III) photosensitisers for catalytic water reduction.

The synthesis of novel, monocationic iridium(III) photosensitisers (Ir-PSs) with the general formula [Ir(III)(C^N)(2)(N^N)](+) (C^N: cyclometallating phenylpyridine ligand, N^N: neutral bidentate ligand) is described. The structures obtained were examined by cyclic voltammetry, UV/Vis and photoluminescence spectroscopy and X-ray analysis. All iridium complexes were tested for their ability as photosensitisers to promote homogeneously catalysed hydrogen generation from water. In the presence of [HNEt(3)][HFe(3)(CO)(11)] as a water-reduction catalyst (WRC) and triethylamine as a sacrificial reductant (SR), seven of the new iridium complexes showed activity. [Ir(6-iPr-bpy)(ppy)(2)]PF(6) (bpy: 2,2'-bipyridine, ppy: 2-phenylpyridine) turned out to be the most efficient photosensitiser. This complex was also tested in combination with other WRCs based on rhodium, platinum, cobalt and manganese. In all cases, significant hydrogen evolution took place. Maximum turnover numbers of 4550 for this Ir-PS and 2770 for the Fe WRC generated in situ from [HNEt(3)][HFe(3)(CO)(11)] and tris[3,5-bis(trifluoromethyl)phenyl]phosphine was obtained. These are the highest overall efficiencies for any Ir/Fe water-reduction system reported to date. The incident photon to hydrogen yield reaches 16.4% with the best system.

Photocatalytic hydrogen generation from water with iron carbonyl phosphine complexes: improved water reduction catalysts and mechanistic insights.

An extended study of a novel visible-light-driven water reduction system containing an iridium photosensitizer, an in situ iron(0) phosphine water reduction catalyst (WRC), and triethylamine as sacrificial reductant is described. The influences of solvent composition, ligand, ligand-to-metal ratio, and pH were studied. The use of monodentate phosphine ligands led to improved activity of the WRC. By applying a WRC generated in situ from Fe(3) (CO)(12) and tris[3,5-bis(trifluoromethyl)phenyl]phosphine (P[C(6)H(3)(CF(3))(2)](3), Fe(3)(CO)(12)/PR(3)=1:1.5), a catalyst turnover number of more than 1500 was obtained, which constitutes the highest activity reported for any Fe WRC. The maximum incident photon to hydrogen efficiency obtained was 13.4% (440 nm). It is demonstrated that the evolved H(2) flow (0.23 mmol H(2) h(-1) mg(-1) Fe(3)(CO)(12)) is sufficient to be used in polymer electrolyte membrane fuel cells, which generate electricity directly from water with visible light. Mechanistic studies by NMR spectroscopy, in situ IR spectroscopy, and DFT calculations allow for an improved understanding of the mechanism. With respect to the Fe WRC, the complex [HNEt(3)](+)[HFe(3)(CO)(11)](-) was identified as the key intermediate during the catalytic cycle, which led to light-driven hydrogen generation from water.

Iron-catalyzed hydrogen production from formic acid.

Hydrogen represents a clean energy source, which can be efficiently used in fuel cells generating electricity with water as the only byproduct. However, hydrogen generation from renewables under mild conditions and efficient hydrogen storage in a safe and reversible manner constitute important challenges. In this respect formic acid (HCO(2)H) represents a convenient hydrogen storage material, because it is one of the major products from biomass and can undergo selective decomposition to hydrogen and carbon dioxide in the presence of suitable catalysts. Here, the first light-driven iron-based catalytic system for hydrogen generation from formic acid is reported. By application of a catalyst formed in situ from inexpensive Fe(3)(CO)(12), 2,2':6'2''-terpyridine or 1,10-phenanthroline, and triphenylphosphine, hydrogen generation is possible under visible light irradiation and ambient temperature. Depending on the kind of N-ligands significant catalyst turnover numbers (>100) and turnover frequencies (up to 200 h(-1)) are observed, which are the highest known to date for nonprecious metal catalyzed hydrogen generation from formic acid. NMR, IR studies, and DFT calculations of iron complexes, which are formed under reaction conditions, confirm that PPh(3) plays an active role in the catalytic cycle and that N-ligands enhance the stability of the system. It is shown that the reaction mechanism includes iron hydride species which are generated exclusively under irradiation with visible light.

Hydrogen generation: catalytic acceleration and control by light.

The ruthenium-catalyzed generation of hydrogen from formic acid is significantly accelerated by light.

Hydrogen generation at ambient conditions: application in fuel cells.

The efficient generation of hydrogen from formic acid/amine adducts at ambient temperature is demonstrated. The highest catalytic activity (TOF up to 3630 h(-1) after 20 min) was observed in the presence of in situ generated ruthenium phosphine catalysts. Compared to the previously known methods to generate hydrogen from liquid feedstocks, the systems presented here can be operated at room temperature without the need for any high-temperature reforming processes, and the hydrogen produced can then be directly used in fuel cells. A variety of Ru precursors and phosphine ligands were investigated for the decomposition of formic acid/amine adducts. These catalytic systems are particularly interesting for the generation of H2 for new applications in portable electric devices.