Markus Hölscher
RWTH Aachen University
Network
Latest external collaboration on country level. Dive into details by clicking on the dots.
Publication
Featured researches published by Markus Hölscher.
Journal of the American Chemical Society | 2011
Frank M. A. Geilen; Barthel Engendahl; Markus Hölscher; Jürgen Klankermayer; Walter Leitner
Selective hydrogenation of biogenic carboxylic acids is an important transformation for biorefinery concepts based on platform chemicals. We herein report a mechanistic study on the homogeneously ruthenium/phosphine catalyzed transformations of levulinic acid (LA) and itaconic acid (IA) to the corresponding lactones, diols, and cyclic ethers. A density functional theory (DFT) study was performed and corroborated with experimental data from catalytic processes and NMR investigations. For [Ru(TriPhos)H](+) as the catalytically active unit, a common mechanistic pathway for the reduction of the C═O functionality in aldehydes, ketones, lactones, and even free carboxylic acids could be identified. Hydride transfer from the Ru-H group to the carbonyl or carboxyl carbon is followed by protonation of the resulting Ru-O unit via σ-bond metathesis from a coordinated dihydrogen molecule. The energetic spans for the reduction of the different functional groups increase in the order aldehyde < ketone < lactone ≈ carboxylic acid. This reactivity pattern as well as the absolute values are in full agreement with experimentally observed activities and selectivities, forming a rational basis for further catalyst development.
Journal of the American Chemical Society | 2012
Chidambaram Gunanathan; Markus Hölscher; Fangfang Pan; Walter Leitner
The nonclassical ruthenium hydride pincer complex [Ru(PNP)(H)(2)(H(2))] 1 (PNP = 1,3-bis(di-tert-butyl-phosphinomethyl)pyridine) catalyzes the anti-Markovnikov addition of pinacolborane to terminal alkynes yielding Z-vinylboronates at mild conditions. The complex [Ru(PNP)(H)(2)(HBpin)] 2 (HBpin = pinacolborane), which was identified at the end of the reaction and prepared independently, is proposed as the direct precursor to the catalytic cycle involving rearrangement of coordinated alkyne to Z-vinylidene as a key step for the apparent trans-hydroboration.
Chemistry: A European Journal | 2012
Andreas Uhe; Markus Hölscher; Walter Leitner
A prototypical catalytic cycle for the direct carboxylation of unactivated arene C-H bonds with CO(2) based on ruthenium(II) pincer complexes as catalysts is proposed and investigated by density functional theory (DFT) methods. The energetic span model is used to predict the turnover frequency (TOF) of various potential catalysts, evaluating their efficiency for this reaction. In addition to modifications of the catalyst structure, we also investigated the effect of the substrate, the solvent, and the influence of a base on the thermodynamics and kinetics of the reaction. Turnover frequencies in the range of 10(5)-10(7) h(-1) are predicted for the best systems. Alternative reaction pathways that might prevent the reaction are also investigated. In all cases, either the respective intermediates are found to be unstable or activation barriers are found to be very high, thereby indicating that these alternative pathways will not interfere with the proposed catalytic cycle. As a result, several ruthenium pincer complexes are suggested as very promising candidates for experimental investigation as catalysts for the carboxylation of arene C-H bonds with CO(2).
Chemistry: A European Journal | 2010
Thomas Pullmann; Barthel Engendahl; Ziyun Zhang; Markus Hölscher; Antonio Zanotti-Gerosa; Alan Dyke; Giancarlo Franciò; Walter Leitner
New derivatives of the Quinaphos ligands and the related Dihydro-Quinaphos ligands based on the more flexible 1,2,3,4-tetrahydroquinoline backbone have been prepared and fully characterised. A general and straightforward separation protocol was devised, which allowed for the gram-scale isolation of the R(a),S(c) and S(a),R(c) diastereomers. These new phosphine-phosphoramidite ligands have been applied in the Rh-catalysed asymmetric hydrogenation of functionalised olefins with the achievement of excellent enantioselectivities (> or = 99%) in most cases and turnover frequency (TOF) values of up to > or = 20,000 h(-1). These results substantiate the practical utility of readily accessible Quinaphos-type ligands, which belong to the most active and selective category of ligands for Rh-catalysed hydrogenation known to date.
Angewandte Chemie | 2016
Kai Christian Rohmann; Jens Kothe; Matthias W. Haenel; Ulli Englert; Markus Hölscher; Walter Leitner
Abstract The novel [Ru(Acriphos)(PPh3)(Cl)(PhCO2)] [1; Acriphos=4,5‐bis(diphenylphosphino)acridine] is an excellent precatalyst for the hydrogenation of CO2 to give formic acid in dimethyl sulfoxide (DMSO) and DMSO/H2O without the need for amine bases as co‐reagents. Turnover numbers (TONs) of up to 4200 and turnover frequencies (TOFs) of up to 260 h−1 were achieved, thus rendering 1 one of the most active catalysts for CO2 hydrogenations under additive‐free conditions reported to date. The thermodynamic stabilization of the reaction product by the reaction medium, through hydrogen bonds between formic acid and clusters of solvent or water, were rationalized by DFT calculations. The relatively low final concentration of formic acid obtained experimentally under catalytic conditions (0.33 mol L−1) was shown to be limited by product‐dependent catalyst inhibition rather than thermodynamic limits, and could be overcome by addition of small amounts of acetate buffer, thus leading to a maximum concentration of free formic acid of 1.27 mol L−1, which corresponds to optimized values of TON=16×103 and TOFavg≈103 h−1.
Chemistry: A European Journal | 2010
Andreas Uhe; Markus Hölscher; Walter Leitner
The catalytic hydroamination of ethylene with ammonia was investigated by means of density functional theory (DFT) calculations. An initial computational screening of key reaction steps (C-N bond formation, N-H bond cleavage), which are assumed to be part of a catalytic cycle, was carried out for complexes with the [M(L)]-complex fragment (M=Rh, Ir; L=NCN, PCP; NCN=2,5-bis(dimethylaminomethyl)benzene, PCP=2,5-bis- (dimethylphosphanylmethyl)benzene). Based on the evaluation of activation barriers, this screening showed the rhodium compound with the NCN ligand to be the most promising catalyst system. A detailed investigation was carried out starting with the hypothetical catalyst precursor [Rh(NCN)(H)(2)(H(2))] (1). A variety of activation pathways to yield the catalytically active species [Rh(NCN)(H)(NH(2))] (5), as well as [Rh(NCN)(C(2)H(5))(NH(2))] (17), were identified. With 5 and 17 several closed catalytic cycles could be calculated. One of the calculated cycles is favoured kinetically and bond-forming events have activation barriers low enough to be put into practice. The calculations also show that for experimental realisation the synthesis of 1 is not necessary, as the synthesis of 17 would establish an active catalyst directly without the need for activation. Oligomerisation of ethylene would be possible in principle and would be expected as a competitive side reaction. Accordingly not only ethylamine would be observed in an experimental system, as amines with longer carbon chains also can be formed.
Journal of the American Chemical Society | 2013
Thomas Georg Ostapowicz; Carina Merkens; Markus Hölscher; Jürgen Klankermayer; Walter Leitner
The synthesis of a novel class of bifunctional ruthenium hydride complexes incorporating Lewis acidic BR(2) moieties is reported. Determination of the molecular structures in the solid state and in solution provided evidence for tunable interaction between the two functionalities. Cooperative effects on the reactivity of the complexes were demonstrated including the activation of small Lewis basic molecules by reversible anchoring at the boron center.
Zeitschrift für Naturforschung B | 2012
Markus Hölscher; Christoph Gürtler; Wilhelm Keim; Thomas Müller; Martina Peters; Walter Leitner
With the growing perception of industrialized societies that fossil raw materials are limited resources, academic chemical research and chemical industry have started to introduce novel catalytic technologies which aim at the development of economically competitive processes relying much more strongly on the use of alternative carbon feedstocks. Great interest is given world-wide to carbon dioxide (CO2) as it is part of the global carbon cycle, nontoxic, easily available in sufficient quantities anywhere in the industrialized world, and can be managed technically with ease, and at low cost. In principle carbon dioxide can be used to generate a large variety of synthetic products ranging from bulk chemicals like methanol and formic acid, through polymeric materials, to fine chemicals like aromatic acids useful in the pharmaceutical industry. Owing to the high thermodynamic stability of CO2, the energy constraints of chemical reactions have to be carefully analyzed to select promising processes. Furthermore, the high kinetic barriers for incorporation of CO2 into C-H or C-C bond forming reactions require that any novel transformation of CO2 must inevitably be associated with a novel catalytic technology. This short review comprises a selection of the most recent academic and industrial research developments mainly with regard to innovations in CO2 chemistry in the field of homogeneous catalysis and processes. Graphical Abstract Carbon Dioxide as a Carbon Resource – Recent Trends and Perspectives
Chemistry: A European Journal | 2010
Markus Hölscher; Walter Leitner
The direct reduction of N2 with H2 to give NH3 is one of the most challenging chemical transformations, as the N N triple bond of dinitrogen is very stable and unreactive. The heterogeneously catalyzed ammonia synthesis from N2 and H2 (Haber–Bosch process) was developed more than a century ago and has been investigated in detail ever since. In contrast, there is currently no molecular catalyst that is able to transform N2 to NH3 using H2 as the sole reductant. In living systems, the fixation of N2 and its transformation to NH3 occurs at the FeMoCo-cofactor of nitrogenases, where metal-bonded N2 reacts stepwise with protons (H ) and reduction equivalents such as ferredoxin. This biological reaction was complemented by an organometallic counterpart by Schrock et al. recently and the catalytic cycle as well as the influences of a variety of ligand structures were investigated theoretically in detail. Stimulated by these findings we became interested in investigating the direct reduction of N2 with H2 in the coordination sphere of transition metals computationally and recently identified complexes bearing pincer ligands as promising lead structures for this fundamental challenge. Albeit the ruthenium–pincer fragments allowed the construction of complete reaction pathways leading to NH3 from N2 and H2 several severe limitations were identified that need to be overcome before these transformations could lead to an active catalytic cycle. The two most important are: 1) catalytic cycles can be accessed from ruthenium–pincer complexes only when the N2 molecule is coordinated side-on to the metal and 2) the coordination of N2 must occur trans to the pincer backbone. Thermodynamically, ruthenium–pincer complexes with end-on coordinated N2 are much more stable than the side-on coordinated counterparts. Consequently one arrives at the question how a sideon bonded N2 molecule could be stabilized in late transition metal pincer complexes in such a way that side-on coordination is energetically competitive or even superior to end-on coordination. Here we report on a novel approach to resolve this issue, which is based on a bifunctional activation of the N2 molecule by a metal and a nonmetal center within a well-defined coordination sphere. The principle of stabilizing a metal-bonded N2 molecule by introduction of a second metal atom is well established. Three possible modes of how N2 can be placed between or near two metal atoms are possible: side-on, side-on; end-on, end-on; and side-on, end-on. N2 complexes relevant to N2 fixation and activation with all three binding modes are known and in the past decade the groups of Fryzuk and Chirik have contributed a wealth of work in this field. With side-on, side-on complexes such as [hC5Me4H)2Zr]2(m , h,h-N2) it was possible to transform N2 to NH3, by a stochiometric reduction. [10] These dinuclear zirconium compounds were studied by theoretical methods by the groups of Morokuma and Hirao, and their mode of action was elucidated. Starting with bis-end-on bonded N2 complexes no transformation of N2 to NH3 has been reported to the best of our knowledge. Instead of using two transition metal centers for the fixation/activation of N2 we reasoned that a Lewis acidic group within the ligand could play a stabilizing role. This leads to a new approach in which the center, at which the reduction is supposed to take place (i.e. the reactive center) is the transition-metal center of an organometallic complex (i.e. ruthenium), while the second (i.e. assisting center) is a Lewis acidic group (e.g. boron), which is part of one of the substituents of the pincer ligand. With this concept, one arrives at the general metal hydride structure A (Scheme 1). The design of the architecture of structure A has to fulfil several criteria: First, the interaction between the coordinated nitrogen and the Lewis-acidic site must be strong enough [a] Dr. M. Hçlscher, Prof. Dr. W. Leitner Institute of Technical and Macromolecular Chemistry RWTH Aachen University Worringerweg 1, 52074 Aachen (Germany) Fax: (+49) 241-8022177 E-mail : hoelscher@itmc.rwth-aachen.de leitner@itmc.rwth-aachen.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002010.
Chemistry: A European Journal | 2013
Andreas Uhe; Markus Hölscher; Walter Leitner
Very few cases of oxidative addition of NH(3) to transition-metal complexes forming terminal amide hydrides have been experimentally observed. Here, two examples with the iridium pincer complexes [Ir(PCP)(NH(3))] A1 with PCP = [κ(3)-(tBu(2)P-C(2)H(4))(2)CH](-) and [Ir(PSiP)(NH(3))] B1 with PSiP = [κ(3)-(2-Cy(2)P-C(6)H(4))(2)SiMe](-) were investigated by DFT calculations applying the M06L density functional to successfully reproduce the trend of the experimentally observed thermochemical stabilities. According to the calculations, the corresponding hydrido-amido complexes A2 and B2 are more stable than the corresponding ammine complexes by ΔG = -2.8 and -2.6 kcal mol(-1), respectively. Complexes such as A2 and B2 are ideally suited entry points to catalytic cycles for the hydroamination of ethylene with ammonia. Therefore, the relevant stationary points of the potentially available cycles were studied computationally to verify if these complexes can catalyze the hydroamination. As a result, complex A2 will clearly not catalyze the hydroamination as all energy spans calculated range close to 40 kcal mol(-1) or higher. The energy spans obtained with B2 are significantly lower in some cases and range around 35 kcal mol(-1), further indicating that no turnover can be expected. By systematically varying the structure of B2, the energy span could be reduced to 28.8 kcal mol(-1) corresponding to a TOF of 17 h(-1) at a reaction temperature of 140 °C. A reoptimization of relevant structures under the inclusion of cyclohexane as a typical solvent reduces the calculated TOF to 6.0 h(-1).