Holli

Efficient and recyclable ruthenium catalysts were synthesized from readily available polystyrene- or silica-supported phosphine ligands. Catalysts bound to the polymer support through an ether linkage showed good to excellent activity towards the N-alkylation of primary and secondary amines to afford the alkylated products in 62–99 % yield at 120–140 °C. The supported phosphine ligand/ruthenium ratio was found to be crucial for higher catalytic activity and lower ruthenium leaching. The continuous flow N-alkylation of amines was demonstrated by using the supported catalyst in a column reactor. By adopting the hydrogen-borrowing strategy, the synthesis of the anti-Parkinson agent Piribedil was established in 98 % yield at 140 °C.

Support group steals the show: An efficient Ru-based heterogeneous catalyst from readily available supported phosphine ligands is developed. The nature of the linkage and the extent of ruthenium incorporation are crucial in determining the catalytic activity. The catalyst can be recycled and used under continuous flow in a packed-bed reactor. The alkylation of cyclic amines is achieved in excellent yield at moderate temperatures in the absence of any external base.

Following recent progress towards understanding the structure of the particulate methane monooxygenase in methanotrophic bacteria, it is now possible to attempt the development of laboratory catalysts for the conversion of methane into MeOH under ambient conditions. To this end, a class of tricopper complexes that are capable of efficiently oxidizing small hydrocarbon substrates at room temperature has recently been developed. In this Minireview, we describe the development of a tricopper cluster to accomplish the catalytic conversion of methane into MeOH, as well as a number of small n-alkanes into their corresponding alcohols and ketones, with high efficiencies. The properties of this robust catalytic system are discussed.

Three and easy: A class of tricopper complexes has been developed as mimics of the catalytic site of particulate methane monooxygenase. These Cu^ICu^ICu^I clusters are capable of the efficient oxidation of methane into MeOH upon activation by O₂ at room temperature. The conversion is catalytic if H₂O₂ is used to drive the turnover.

Recent reports on homogeneous catalytic transformation of carbon dioxide by iridium complexes have prompted us to review the area. Progress on new iridium catalysts for carbon dioxide transformations should take into account the interaction of carbon dioxide with the iridium center, which seems to be governed by the oxidation state of iridium and the nature of the carbon dioxide molecule. Most examples of iridium catalyzed carbon dioxide reductions are based on Ir^III centers. These reactions take place through outer-sphere mechanisms, by means of nucleophilic attack on the carbon atom. In all the reported systems, the nucleophile is always a hydrido ligand coordinated to a Ir^III center. Future challenges on iridium catalyzed functionalization of carbon dioxide include the development of efficient electrophiles, compatible with the inclusion of appropriate nucleophiles, which would allow the preparation of value-added organic molecules using CO₂ as C1 feedstock.

CO₂ activation: The interaction of iridium complexes with carbon dioxide and subsequent homogeneous catalytic processes reported so far are reviewed. In general, outer-sphere mechanisms seem to prevail in the catalytic functionalization of carbon dioxide by iridium complexes.

The environmentally friendly catalyst iron titanate (FeTiO_x) was reported to be very active for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR), with high N₂ selectivity and H₂O/SO₂ durability in the medium temperature range, and the specific microstructure of iron titanate crystallites as the active phase was determined. In consideration of the probable existence of a redox cycle between Fe³⁺ and Fe²⁺ species in the NH₃-SCR reaction, the deoxidation behavior of the FeTiO_x catalyst in an H₂ temperature-programmed reduction process was studied extensively by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) methods. Owing to the presence of an electronic inductive effect between Fe and Ti species in the unique edge-shared Fe³⁺(O)₂Ti⁴⁺ structure, the reducibility of Fe³⁺ species in the FeTiO_x catalyst was greatly enhanced compared with that in pristine Fe₂O₃, leading to the higher oxidation ability of Fe species in FeTiO_x. In the H₂ temperature-programmed reduction process, the well-dispersed Fe³⁺ species in iron titanate crystallites could be totally converted into Fe²⁺ in the form of ilmenite FeTiO₃ below 500 °C, whereas pristine Fe₂O₃ could only be reduced to Fe₃O₄ up to this temperature point. The typical NH₃-SCR reaction is usually conducted below 500 °C, and the enhanced oxidation ability of Fe³⁺ species in FeTiO_x catalyst is responsible for its excellent catalytic NO_x reduction performance at low temperatures. Based on XANES linear fitting and EXAFS curve-fitting results, the specific deoxidation process of the FeTiO_x catalyst was proposed, which can provide useful information for the characterization of the microstructure and redox ability of active sites simultaneously in mixed oxide catalysts for certain catalytic reactions.

Living on the iron edge: The deoxidation behavior of an environmentally friendly iron titanate catalyst for the selective catalytic reduction of NO_x with NH₃ is systematically studied by H₂ temperature-programmed reduction and X-ray absorption fine-structure methods. The significantly enhanced reducibility and redox ability of Fe species in the unique edge-shared Fe³⁺(O)₂Ti⁴⁺ structure improves the NO_x reduction efficiency at low temperatures.

Hydrogen evolution from water is a true challenge, yet it is possible with titanium complexes. In their Full Paper on page 13705 ff., D. Hollmann, T. Beweries, A. Brückner et al. unravel the mechanism of water activation and hydrogen formation on decamethyltitanocene triflate by using in situ spectroscopy supported by DFT calculations. The picture relates the flexibility of the titanium ligand sphere with that of a Titanflex® eyeglass frame.

The synthesis, processing, and performance of a low-cost monolithic battery electrode, produced entirely of natural and renewable resources, are reported. This anode material exhibits tunable electrochemical performance suitable for both high power and high energy applications. A synthesis method that directly results in electrically interconnected three-dimensional architectures is presented, where the carbon framework functions as current collector and lithium insertion material, eliminating the extra mass and expense of inactive materials in conventional designs. Fibrous carbon electrode materials are produced from solvent extracted lignin using scalable melt processing technology and thermal conversion methods. The resulting free-standing electrodes exhibit comparable electrochemical performance to commercial carbon-based anodes at a fraction of the materials and processing costs. Compositional and electrochemical characterization shows that carbonized lignin has a disordered nano-crystalline microstructure. The carbonized mats cycle reversibly in conventional aprotic organic electrolytes with Coulombic efficiencies over 99.9%. Moreover, lignin carbon fibers carbonized at 2000 °C can cycle reversibly in 1 m LiPF₆ in propylene carbonate.

A novel synthetic technique for lignin-based carbon fibers, where the fibers are fused to each other resulting in monolithic fibrous mats, is described. The unique fiber microstructures and mat morphologies provide favorable electrochemical performance for lithium ion battery anode applications. The fibers cycle reversibly versus Li metal in alkyl carbonate electrolytes.

The application of a ruthenium complex, bearing a sulfonated bis-N-heterocyclic carbene (NHC) ligand as catalyst precursor for the hydrogenation of aromatic compounds is reported. The reaction proceeds under mild conditions in aqueous phase. The treatment of the Ru complex with 40 bar H₂ at 60 °C in water (0.1 M KOH solution) leads to the formation of a catalytically active species, which can be stored and used in catalysis experiments. The catalyst is responsible for the hydrogenation of functionalized aromatic substrates exhibiting endo- and exo-CO bonds, which are relevant products in biomass conversion. Acetophenone is quantitatively reduced to 1-cyclohexyl ethanol with a selectivity of 100 %. Various other oxygen-functionalized aromatic substrates were also hydrogenated in moderate to quantitative conversions. To elucidate the nature of the catalytically active species, NMR, UV/vis, and ESI-MS experiments were undertaken, showing the presence of a mononuclear Ru hydride complex. TEM measurements of a sample of the catalyst solution did not indicate the presence of nanoparticles. Mechanistic investigations point towards a homogenous mechanism.

Takes like a duck to water: A Ru^II complex with sulfonated bis-N-heterocyclic carbene ligands is used as precatalyst for the hydrogenation of oxygen-functionalized aromatics in aqueous phase with H₂ at 60 °C. Optimization of the catalytic protocol using phenol and acetophenone as model substrates, a broadened substrate scope as well as characterization of the active Ru species before and after catalysis are presented.

Hydrogen goes with the flow: Conversion of amides into amines is usually achieved with stoichiometric amounts of metal hydrides, which generate large amounts of waste. Catalytic hydrogenation represents an environmentally benign alternative for this conversion, whereas flow catalysis allows catalyst separation and high throughput. Here, we combine amide hydrogenation and flow catalysis for the first time.

Catalyst–substrate hydrogen bonds in artificial catalysts usually occur in aprotic solvents, but not in protic solvents, in contrast to enzymatic catalysis. We report a case in which ligand–substrate hydrogen-bonding interactions cooperate with a transition-metal center in alcoholic solvents for enantioselective catalysis. Copper(I) complexes with prolinol-based hydroxy amino phosphane chiral ligands catalytically promoted the direct alkynylation of aldehydes with terminal alkynes in alcoholic solvents to afford nonracemic secondary propargylic alcohols with high enantioselectivities. Quantum-mechanical calculations of enantiodiscriminating transition states show the occurrence of a nonclassical sp³-CH⋅⋅⋅O hydrogen bond as a secondary interaction between the ligand and substrate, which results in highly directional catalyst–substrate two-point hydrogen bonding.

Efficient enantioselective direct carbonyl addition of terminal alkynes is achieved through ligand–substrate two-point hydrogen bonds consisting of OH⋅⋅⋅O and sp³-CH⋅⋅⋅O interactions that cooperate with copper in alcoholic solvents (see picture).

The effective catalytic N-methylation of anilines using CO₂ as C₁ source and molecular hydrogen as reducing agent was demonstrated using the well-defined [Ru(triphos)(tmm)] catalyst. Secondary and primary (shown) aromatic amines were mono- or dialkylated, respectively, in high yields. N-methylation of amides coupled with the amide hydrogenation offers an efficient approach to unsymmetrical tertiary methyl/alkyl/aromatic amines.

The development of new energy materials that can be utilized to make renewable and clean fuels from abundant and easily accessible resources is among the most challenging and demanding tasks in science today. Solar-powered catalytic water-splitting processes can be exploited as a source of electrons and protons to make clean renewable fuels, such as hydrogen, and in the sequestration of CO₂ and its conversion into low-carbon energy carriers. Recently, there have been tremendous efforts to build up a stand-alone solar-to-fuel conversion device, the “artificial leaf”, using light and water as raw materials. An overview of the recent progress in electrochemical and photo-electrocatalytic water splitting devices is presented, using both molecular water oxidation complexes (WOCs) and nano-structured assemblies to develop an artificial photosynthetic system.

Turning a new leaf: Electrochemical and light-driven electrocatalytic water oxidation assemblies have been targeted to develop artificial photosynthetic system. Such “Artificial Leaves” are used to make H₂ and O₂ using water as a raw material. The design and performance of the water oxidation systems and standalone solar-to-fuel conversion devices are presented. Progress in the field and future perspectives of water splitting are also discussed.

Taking alcohol from the nanobar The cover picture shows the use of nickel and cerium nano-oxyhydride catalysts in hydrogen production from ethanol in the presence of water and oxygen. In their Full Paper on p. 2207 ff., L. Jalowiecki-Duhamel et al. describe the characterization and catalytic properties of the nano-compounds. Hydrogen production (45 mol %) and total conversion of ethanol remain almost constant over at least 75 h time-on-stream with only a small energy input (reaction in oven at 60 °C), thanks to the presence of hydride species stored in the solid.

Discotics studied by EPR: The first application of EPR spectroscopy to columnar discotic liquid crystal using a novel rigid-core nitroxide spin probe (see picture) is reported. EPR spectra measured at different temperatures across three phases of hexakis(n-hexyloxy)triphenylene show a strong sensitivity to the phase composition, molecular rotational dynamics, and columnar order.

Dehydrogenation of HCO₂H: The reaction mechanism for the dehydrogenation of formic acid catalyzed by a highly active and selective iron complex (see figure) has been studied by DFT. The most favorable pathway shows the hydride in Fe–H complexes acting as a spectator ligand throughout the catalytic cycle. This result opens up the Fe complex for modification in order to achieve more efficient and selective catalysts.