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There is an increasing interest to replace precious metal-based catalysts by earth-abundant nonprecious metals due to higher costs, toxicity, and declining availability of the former. Here, the synthesis of a well-defined supported nickel hydrogenation catalyst prepared by surface organometallic chemistry is reported. For this purpose, [LNi(μ-H)]2 (L = HC(CMeNC6H3(iPr)2)2) was grafted on partially dehydroxylated silica to give a homobimetallic H- and O(silica)-bridged Ni2 complex. The structure of the latter was confirmed by infrared spectroscopy, X-ray absorption near-edge structure, and extended X-ray absorption fine structure analyses as well as hydride titration studies. The immobilized catalyst was capable of hydrogenating alkenes and alkynes at low temperatures without prior activation. As an example, ethene can be hydrogenated with an initial turnover frequency of 25.5 min-1 at room temperature.

CH3 OH formation rates in CO2 hydrogenation on Cu-based catalysts sensitively depend on the nature of the support and the presence of promoters. In this context, Cu nanoparticles supported on tailored supports (highly dispersed M on SiO2 ; M=Ti, Zr, Hf, Nb, Ta) were prepared via surface organometallic chemistry, and their catalytic performance was systematically investigated for CO2 hydrogenation to CH3 OH. The presence of Lewis acid sites enhances CH3 OH formation rate, likely originating from stabilization of formate and methoxy surface intermediates at the periphery of Cu nanoparticles, as evidenced by metrics of Lewis acid strength and detection of surface intermediates. The stabilization of surface intermediates depends on the strength of Lewis acid M sites, described by pyridine adsorption enthalpies and 13 C chemical shifts of -OCH3 coordinated to M; these chemical shifts are demonstrated here to be a molecular descriptor for Lewis acid strength and reactivity in CO2 hydrogenation.

More than 1 million workers are exposed routinely to carcinogenic benzene, contained in various consumer products (e.g., gasoline, rubbers, and dyes) and released from combustion of organics (e.g., tobacco). Despite strict limits (e.g., 50 parts per billion (ppb) in the European Union), routine monitoring of benzene is rarely done since low-cost sensors lack accuracy. This work presents a compact, battery-driven device that detects benzene in gas mixtures with unprecedented selectivity (>200) over inorganics, ketones, aldehydes, alcohols, and even challenging toluene and xylene. This can be attributed to strong Lewis acid sites on a packed bed of catalytic WO3 nanoparticles that prescreen a chemoresistive Pd/SnO2 sensor. That way, benzene is detected down to 13 ppb with superior robustness to relative humidity (RH, 10-80%), fulfilling the strictest legal limits. As proof of concept, benzene is quantified in indoor air in good agreement (R-2 >= 0.94) with mass spectrometry. This device is readily applicable for personal exposure assessment and can assist the implementation of low-emission zones for sustainable environments., Advanced Science, 9 (4), ISSN:2198-3844

Radical-mediated gas-phase reactions play an important role in the conversion of methane under non-oxidative conditions into olefins and aromatics over iron-modified silica catalysts. Herein, we used operando photoelectron photoion coincidence spectroscopy to disentangle the elusive C2+ radical intermediates participating in the complex reaction network. Our experiments pinpointed different C2-C5 radical species that allow for a stepwise growth of the hydrocarbon chains. Herein, propargyl radicals (H2C-C≡C-H) are identified as essential precursors for the formation of aromatics, which contribute with the formation of heavier hydrocarbon products via hydrogen abstraction - acetylene addition routes (HACA mechanism). These results provide comprehensive mechanistic insights that are relevant for development of methane valorization processes., Angewandte Chemie. International Edition, 60 (45), ISSN:1433-7851, ISSN:1521-3773, ISSN:0570-0833

A silanol-free silica with high surface area was prepared by postfunctionalization of silica by means of sol–gel reaction with triethoxysilane followed by a high-temperature dehydroxylation treatme...

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Non-oxidative CH4 coupling is promoted by silica with incorporated iron sites, but the role of these sites and their speciation under reaction conditions are poorly understood. Here, silica-supported iron(II) single sites, prepared via surface organometallic chemistry and stable at 1020 °C in vacuum, are shown to rapidly initiate CH4 coupling at 1000 °C, leading to 15-22 % hydrocarbons selectivity at 3-4 % conversion. During this process, iron reduces and forms carburized iron(0) nanoparticles. This reactivity contrasts with what is observed for (iron-free) partially dehydroxylated silica, that readily converts methane, albeit with low hydrocarbon selectivity and after an induction period. This study supports that iron sites facilitate faster initiation of radical reactions and tame the surface reactivity.

Cu supported on SiO2 can be used to directly convert methane to methanol in a stepwise process with no intrinsic need for a zeolite support. Effects of parameters such as the O-2 activation temperature, activation time, CH4 reaction temperature, CH4 partial pressure (pCH(4)), and Cu wt % on methanol yield were investigated. Increasing the O-2 activation temperature in the 200-800 degrees C range significantly improved the methanol yield, and when carried out at 800 degrees C, a methanol yield of 11.5 mu mol/g(ctalyst) was obtained after reaction with methane at 200 degrees C for the sample with 2 wt % Cu. Yield per mole of Cu increased exponentially from 1.0 to 59.1 mmol with decreased Cu wt % from 30 to 1, respectively. The increase in the O-2 activation time also strongly influenced the yield which corresponded to the increase in yield by a factor of >2 between 1 and 8 h. Increasing PCH4 from 0.05 to 1 atm resulted in a 5-fold increase in yield after activation at 450 degrees C; however, it resulted in at least 20% lower yields after activation at 800 degrees C showing that active sites of different nature were formed at different activation temperatures. The increase in yield with ramped O-2 activation temperature correlated with the dehydration of the samples as evidenced by X-ray absorption near-edge spectroscopy (XANES) and via mass spectroscopy (MS) traces of H2O during the O-2 activation step., Turkish Scientific and Technological Research Council (Scientific and Technological Research Council of Turkey (TÜBİTAK))

A method for enantioselective hydrogenation of cyclic imines with gaseous hydrogen has been developed. Easily accessible Noyori-Ikariya Ru(II) and Rh(III) complexes can be used directly without an inert atmosphere. Substrate activation has been achieved by trifluoroacetic acid. A new hydroxyl-functionalized complex is reported, showing high activity in transfer hydrogenation.

Only four types of dimeric precursors [RuCl2(η6-arene)]2 for the synthesis of Noyori’s half sandwich diamine catalysts [RuCl(TsDPEN)(η6-arene)] are commercially available, yet so far no study has tried to systematically evaluate how these systems perform during the asymmetric transfer hydrogenation of various 3,4-dihydroisoquinolines (i.e., the typical substrates for Noyori asymmetric transfer hydrogenation benchmarking). Experiments combined with molecular modeling allowed us to assess their properties and formulate a hypothesis clarifying the difference in enantioselectivity of these systems.

A new [Ru(II)( η 6 - p -cymene)(1 R ,2 R )- N -((1 S ,2 S )-borneol-10-sulfonyl)-1,2-diphenylethylenediamine] catalyst for the asymmetric transfer hydrogenation of both 1-alkyl and 1-aryl dihydroisoquinolines has been isolated. For the first time in this type of reaction, the catalyst employs an N-alkylsulfonyl group instead of N-arylsulfonyl.

The asymmetric transfer hydrogenation of five dihydroisoquinolines (DHIQs) was studied by NMR spectroscopy. The DHIQs differed by substitution with methoxy groups, which had a significant effect upon the reaction performance in terms of reaction rate and enantioselectivity. The differences are most probably related to the basicity of DHIQs.

We present, to the best of our knowledge, the first parametric study of the asymmetric transfer hydrogenation of imines catalyzed by a Noyori-type catalytic complex based on ruthenium. A model imine for this study was 1-methyl-3,4-dihydroisoquinoline, and a well-known complex RuCl(η 6 - p -cymene)((1 S ,2 S )- N - p -toluenesulfonyl-1,2-diphenylethylenediamine) was chosen as the model catalyst. The reactions were performed in the presence of a formic acid–triethylamine mixture as the source of hydrogen. The parameters examined include general parameters, for example, concentration, temperature, and substrate-to-catalyst molar ratio, as well as parameters specific to this particular reaction, such as the amount of the hydrogenation mixture used, the ratio of its components, or the inhibitive effect of carbon dioxide. During this study, several unexpected parameters worth further investigation have emerged.

A method for the analysis of the optical purity of a series of chiral substituted tetrahydroisoquinolines (THIQs) was developed. The method is based on pre-column derivatization of the analytes with the derivatization reagent (–)-(1R)-menthyl chloroformate. The derivatization reaction selectively gives diastereomeric carbamates that are resolvable on an achiral non-polar GC column. The developed technique covers variously substituted THIQs, which differ significantly in volatility, steric and electronic properties. In all cases, the resolution factors (R) exceeded the value of 1.5. The method represents a robust way of analysis of mixtures of THIQs, which are often present in various matrixes such as body fluids, tissues and reaction mixtures.

A chiral precursor of Mivacurium chloride, (R)-5′-methoxylaudanosine, was prepared using two different methods. The chiral resolution of racemic 5′-methoxylaudanosine, typically used in industry, was carried out in parallel with a procedure consisting of asymmetric transfer hydrogenation (ATH) and reductive methylation. A novel one-pot synthetic step was developed for the synthesis of racemic 5′-methoxylaudanosine. In both routes, the enantioselectivity was high but further purification was necessary to reach the level of a pharmaceutical standard. The individual synthetic steps reported herein can also be used for the synthesis of analogous bistetrahydroisoquinoline-based skeletal muscle relaxants.

Asymmetric transfer hydrogenation of the acyclic imine acetophenone N-benzylimine was studied by means of computational chemistry. Calculated transition states offer an explanation of why this proc...

The asymmetric transfer hydrogenation (ATH) of imines catalyzed by the Noyori-Ikariya [RuCl(η6-arene)(N-arylsulfonyl-DPEN)] (DPEN=1,2-diphenylethylene-1,2-diamine) half-sandwich complexes is a research topic that is still being intensively developed. This article focuses on selected aspects of this catalytic system. First, a great deal of attention is devoted to the N-arylsulfonyl moiety of the catalysts in terms of its interaction with protonated imines (substrates) and amines (components of the hydrogen-donor mixture). The second part is oriented toward the role of the η6-coordinated arene. The final part concerns the imine substrate structural modifications and their importance in connection with ATH. Throughout the text, the summary of known findings is complemented with newly-presented ones, which have been approached both experimentally and computationally.

This review is oriented toward the asymmetric transfer hydrogenation (ATH) of imines regarding mostly fundamental, yet important topics from the practical point of view. Development of analytical methods for the monitoring of ATH (i.e., kinetics and stereoselectivity) belongs to those topics, as well as studies on the influence of reaction conditions and structural variations on the reaction performance. The second part is devoted to the reaction mechanism with the emphasis on imine ATH and catalyst behaviour under acidic conditions. The review also addresses the asymmetric hydrogenation (AH) of ketones and imines using molecular hydrogen and the application of ATH in pharmaceutical projects. The contributions of our group to each area are included.

Asymmetric transfer hydrogenation (ATH) of cyclic imines using [RuCl(η(6)-p-cymene)TsDPEN] (TsDPEN = N-tosyl-1,2-diphenylethylenediamine) was tested with various aliphatic (secondary, tertiary) and aromatic amines employed in the HCOOH-base hydrogen donor mixture. Significant differences in reaction rates and stereoselectivity were observed, which pointed to the fact that the role of the base in the overall mechanism could be more significant than generally accepted. The hydrogenation mixture was studied by nuclear magnetic resonance (NMR), Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and vibrational circular dichroism (VCD) with infrared spectroscopy. The results suggested that the protonated base formed an associate with the active ruthenium-hydride species, most probably via a hydrogen bond with the sulfonyl group of the complex. It is assumed that the steric and electronic differences among the bases were responsible for the results of the initial ATH experiments.