Chapter 6 Catalysis with Chirally Modified Metal Surfaces: Scope and Mechanisms Angelo Vargas, Angelo Vargas Department of Chemical and Applied Biosciences, ETH Zurich, Switzerland, Wolfgang-Pauli-Str. 10, ETH Hönggerberg, HCI E 133, CH-8093 Zürich, SwitzerlandSearch for more papers by this authorCecilia Mondelli, Cecilia Mondelli Department of Chemical and Applied Biosciences, ETH Zurich, Switzerland, Wolfgang-Pauli-Str. 10, ETH Hönggerberg, HCI E 133, CH-8093 Zürich, SwitzerlandSearch for more papers by this authorAlfons Baiker, Alfons Baiker Department of Chemical and Applied Biosciences, ETH Zurich, Switzerland, Wolfgang-Pauli-Str. 10, ETH Hönggerberg, HCI E 133, CH-8093 Zürich, SwitzerlandSearch for more papers by this author Angelo Vargas, Angelo Vargas Department of Chemical and Applied Biosciences, ETH Zurich, Switzerland, Wolfgang-Pauli-Str. 10, ETH Hönggerberg, HCI E 133, CH-8093 Zürich, SwitzerlandSearch for more papers by this authorCecilia Mondelli, Cecilia Mondelli Department of Chemical and Applied Biosciences, ETH Zurich, Switzerland, Wolfgang-Pauli-Str. 10, ETH Hönggerberg, HCI E 133, CH-8093 Zürich, SwitzerlandSearch for more papers by this authorAlfons Baiker, Alfons Baiker Department of Chemical and Applied Biosciences, ETH Zurich, Switzerland, Wolfgang-Pauli-Str. 10, ETH Hönggerberg, HCI E 133, CH-8093 Zürich, SwitzerlandSearch for more papers by this author Book Editor(s):Michelangelo Gruttadauria, Michelangelo Gruttadauria Department of Molecular and Biomolecular Sciences (STEMBIO), Section of Organic Chemistry, University of Palermo, Viale delle Scienze, Ed. 17, 90128 Palermo, ItalySearch for more papers by this authorFrancesco Giacalone, Francesco Giacalone Department of Molecular and Biomolecular Sciences (STEMBIO), Section of Organic Chemistry, University of Palermo, Viale delle Scienze, Ed. 17, 90128 Palermo, ItalySearch for more papers by this author First published: 07 July 2011 https://doi.org/10.1002/9781118087992.ch6Citations: 1 AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onFacebookTwitterLinked InRedditWechat Summary This chapter contains sections titled: Introduction to chirally modified metal surfaces Asymmetric reactions at chirally modified metal surfaces The development of a model for the Cinchona alkaloid modified platinum asymmetric hydrogenation Conclusions References Citing Literature Catalytic Methods in Asymmetric Synthesis: Advanced Materials, Techniques, and Applications RelatedInformation

The interfacial coupling of a nanoscale oxide material to a metal surface creates a hybrid system with novel and often unprecedented physical and chemical properties that are not shared by the individual components of the system. The dimensionality of the oxide phase adds a further parameter to a oxide-metal hybrid system, which may be used as a tunable model for the study of emergent phenomena of low-dimensional (low-D) materials. Low-D oxide nanostructures on well-defined metal surfaces can be fabricated with atomic-scale design control using bottom-up directed self-assembly and a surface science methodology. Here we discuss the elastic and electronic effects introduced by the oxide-metal interface and how they determine the novel structure concepts that are encountered in low-D oxide nanophases on metal surfaces. The systems of interest are the oxides of Mn, Co, and Ni, which in monoxide form share a common rock-salt structure in the bulk and antiferromagnetic ordering behaviour. Their properties in low-D hybrid systems on noble metal surfaces are investigated here. The phase diagrams of 2-D oxide nanolayers reflect the flexibility of the oxidation states of the respective metals cations: this is illustrated by Mn oxides on Pd surfaces, which display a very complex phase behaviour, whereas the Ni oxide phases on Pd or Ag substrates are much simpler. A complex nanoscale morphology as a result of the interplay of elastic and electronic effects has been found for Mn oxide nanostripes on stepped Pd surfaces and for 2-D Ni oxide nanostructures embedded in a Ag(100) substrate. The magnetic response of 2-D Mn oxides show emergent behaviour and the high chemical reactivity of 1-D Mn and Ni oxide nanowires is emphasized. Finally, some preliminary results on the growth of low-D ceria nanostructures on Au will be briefly presented.

Both associative and dissociative H(2)O adsorption on SnO(2)(110), TiO(2)(110), and Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces have been investigated at low ((1)/(12) monolayer (ML)) and high coverage (1 ML) by density functional theory calculations using the Gaussian and plane waves formalism. The use of a large supercell allowed the simulation at low symmetry levels. On SnO(2)(110), dissociative adsorption was favored at all coverages and was accompanied by stable associative H(2)O configurations. Increasing the coverage from (1)/(12) to 1 ML stabilized the (associatively or dissociatively) adsorbed H(2)O on SnO(2)(110) because of the formation of intermolecular H bonds. In contrast, on TiO(2)(110), the adsorption of isolated H(2)O groups ((1)/(12) ML) was more stable than at high coverage, and the favored adsorption changed from dissociative to associative with increasing coverage. For dissociative H(2)O adsorption on Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces with Ti atoms preferably located on 6-fold-coordinated surface sites, the analysis of the Wannier centers showed a polarization of electrons surrounding bridging O atoms that were bound simultaneously to 6-fold-coordinated Sn and Ti surface atoms. This polarization suggested the formation of an additional bond between the 6-fold-coordinated Ti(6c) and bridging O atoms that had to be broken upon H(2)O adsorption. As a result, the H(2)O adsorption energy initially decreased, with increasing surface Ti content reaching a minimum at 25% Ti for (1)/(12) ML. This behavior was even more accentuated at high H(2)O coverage (1 ML) with the adsorption energy decreasing rapidly from 145.2 to 101.6 kJ/mol with the surface Ti content increasing from 0 to 33%. A global minimum of binding energies at both low and high coverage was found between 25 and 33% surface Ti content, which may explain the minimal cross-sensitivity to humidity previously reported for Sn(1-x)Ti(x)O(2) gas sensors. Above 12.5% surface Ti content, the binding energy decreased with increasing coverage, suggesting that the partial desorption of H(2)O is facilitated at a high fractional coverage.

By means of ab initio molecular dynamics, we have investigated the molecular adsorption of acetaldehyde on Pt 13 nanoparticles in the presence of coadsorbed hydrogen on the surface of the metal particle. The acetaldehyde molecules exclusively interact with low-coordinated metal atoms of the nanoparticle while they remain inert toward direct interaction with adsorbed hydrogen, thus confirming the key role of the metal as intermediate binding site for hydrogenation. At room temperature within a time scale of picoseconds aldehyde-metal bonds are formed. Coadsorbed hydrogen decreases the reactivity of the aldehyde molecules toward the metal. Kinetically, the first adsorption modes to occur are of the eta(1) type, either via the oxygen or via the carbon atom. A tendency for double adsorption on the same metal site is observed. Upon addition of ammonia molecules to the simulation box, the interesting phenomenon of the conversion of eta(1) to eta(2) carbonyl bonding appears, mediated by the adsorption of the ammonia nitrogen to a platinum atom. This investigation highlights the richness of the interaction modes of a carbonyl group with a platinum nanoparticle, reached in the very brief time scale of a few picoseconds. In particular the adsorption modes of the aldehyde are modified by the presence of a second electron-donor molecule, such as another aldehyde molecule or an ammonia molecule, in the latter case even changing the adsorption mode of the carbonyl moiety from eta(1) to eta(2).

The efficiency of propylamino functionalization of magnetic silica-coated FeOx nanoparticles prepared by different methods, including coprecipitation and flame aerosol synthesis, has been evaluated by attenuated total reflection infrared spectroscopy (ATR-IR) combined with a specific surface reaction, thus revealing the availability of the grafted functional groups. Large differences in the population of reactive groups were observed for the investigated materials, underlining the tight relation between the structure of nanoparticles and their suitability for organic functionalization. The materials possessed different core structure, surface area, and porosity, as evidenced by transmission electron microscopy and nitrogen adsorption-desorption isotherms. Grafting of aminopropyl groups using a standard procedure based on reaction with (3-aminopropyl) trimethoxysilane as source of the propylamino groups was performed, followed by classical dry analysis methods to determine the specific concentration of the organic functional groups (in mmol g(-1) of material). ATR-IR spectroscopy in a specially constructed reactor cell was applied as wet methodology to determine the chemically available amount of such functional groups, showing that the materials possess largely different loading capacity, with a variability of up to 70% in the chemical availability of the organic functional group. The amount of (3-aminopropyl) trimethoxysilane used for functionalization was optimized, thus reaching a saturation limit characteristic of the material. (C) 2010 Elsevier B. V. All rights reserved.

The composition and thermodynamic stability of the (110) surface of Sn1-xTixO2 rutile solid solutions was investigated as a function of Ti-distribution and content up to the formation of a full TiO2 surface monolayer. The bulk and (110) surface properties of Sn1-xTixO2 were compared to that of the pure SnO2 and TiO2 crystal. A large supercell of 720 atoms and a localized basis set based on the Gaussian and plane wave scheme allowed the investigation of very low Ti-content and symmetry. For the bulk, optimization of the crystal structure confirmed that up to a Ti-content of 3.3at.%, the lattice parameters (a, c) of SnO2 do not change. Increasing further the Ti-content decreased both lattice parameters down to those of TiO2. The surface energy of these solid solutions did not change for Ti-substitution in the bulk of up to 20at.%. In contrast, substitution in the surface layer rapidly decreased the surface energy from 0.99 to 0.74J/m2 with increasing Ti-content from 0 to 20at.%. As a result, systems with Ti atoms distributed in the surface (surface enrichment) had always lower energies and thus were thermodynamically more favorable than those with Ti homogeneously distributed in the bulk. This was attributed to the lower energy necessary to break the TiO bonds than SnO bonds in the surface layer. In fact, distributing the Ti atoms homogeneously or segregated in the (110) surface led to the same surface energy indicating that restructuring of the surface bond lengths has minimal impact on thermodynamic stability of these rutile systems. As a result, a first theoretical prediction of the composition of Sn1-xTixO2 solid solutions is proposed.

The presence of a tertiary amine plays a role in accelerating the catalysis of ketones hydrogenation on a platinum surface covered by hydrogen. Ab initio molecular dynamics has been applied to achieve an atomistic understanding of this base effect. The interaction of trimethylamine with hydrogen adsorbed on a platinum nanoparticle has been simulated for 22 ps revealing that hydrogen increases its oscillatory modes upon interaction with the base therefore its activation. Nonetheless in the case of trimethylamine, competing interactions appear in the form of skeletal hydrogen (from the methyl group) interacting with the metal particle. Such interactions distract the action of the base and predominate at the end of the simulation. The interaction of the trimethylamine molecule with hydrogen adsorbed on platinum is compared to the corresponding interaction of the quinuclidine moiety of cinchonidine, which is the most widely applied chiral modifier for the platinum catalyzed enantioselective hydrogenations. Th...

The description of the conformational space generated by metal nanoparticles is a fundamental issue for the study of their physicochemical properties. In this investigation, an exhaustive exploration and a unified view of the conformational space of a gold nanocluster is provided using a Au 12 cluster as an example. Such system is characterized by coexisting planar/quasiplanar and tridimensional conformations separated by high-energy barriers. The conformational space of Au 12 has been explored by means of Born-Oppenheimer ab initio metadynamics, i.e., a molecular dynamics simulation coupled with a history dependent potential to accelerate events that might occur on a long time scale compared to the time step used in the simulations (rare events). The sampled conformations have complex, in general not intuitive topologies that we have classified as planar/quasiplanar or tridimensional, belonging to different regions of the free energy surface. Three conformational free energy basins were identified, one for the planar/quasiplanar and two for the tridimensional structures. At thermodynamic equilibrium, the planar/quasi-planar and tridimensional conformations were found to coexist, to be fluxional and to be separated by high-free-energy barriers. The comparison between the free energy and the potential energy revealed the relevance of the entropic contribution in the equilibrium distribution of the conformations of the cluster.

Desirable features for the computational investigation of catalytic surfaces and chemisorption phenomena are: i) large surface areas, in order to accommodate reactants, products, and possible surface functionalities, ii) the inclusion of finite temperature effects through the generation of molecular dynamics trajectories and, iii) basis sets constituted of localized orbitals. The application of the Gaussian and Plane Waves (GPW) formalism [1] in the description of the metallicity of Pt bulk, Pt(111) and Pt(100) surfaces is shown to yield excellent agreement with standard Plane Waves (PW) calculations in the evaluation of structural, electronic and dynamic properties for either bulk and surface systems [2]. The GPW formalism, with simulation cells of 400-800 atoms, can be safely used in the study of chemistry related problems involving transition metal surfaces. The methodology is applied to the study of chirally modified surfaces [3].

A magnetic Pt/SiO2/Fe3O4 catalyst consisting of chirally modified platinum supported on silica coated magnetite nanoparticles was prepared using an easy synthetic route and successfully applied for the enantioselective hydrogenation of various activated ketones. The magnetic catalyst modified with cinchonidine showed a catalytic performance (activity, enantioselectivity) in the asymmetric hydrogenation of various activated ketones in toluene comparable to the best known Pt/alumina catalyst used for these reactions. The novel catalyst can be easily separated from the reaction solution by applying an external magnetic field and recycled several times with almost complete retention of activity and enantioselectivity.

A series of peptides, all containing the natural amino acid tryptophan (Trp), have been used as chiral surface modifiers for asymmetric hydrogenation reactions on alumina supported platinum catalysts. The surface chiral sites have been investigated using density functional theory calculations in order to elucidate the structure of the asymmetric environment produced by the adsorption of the peptides on the metal surface. As a test reaction, ketopantolactone (KPL) has been asymmetrically hydrogenated using the modified catalyst. Catalytic results were tested against a computational model to shed light on the phenomenon of chiral induction, The choice of Trp as the first member of the peptidic chain is due to its structural resemblance to cinchona alkaloids, already successfully used as modifiers for asymmetric hydrogenation reactions, The amino acidic moiety of Trp was elongated by addition of other natural amino acids via a peptidic bond, and the resulting peptides were tested as chiral modifiers. The indole moiety of Trp anchors the peptide by adsorbing parallel to the metal surface, while the amino acidic chain forms the chiral environment that promotes the asymmetric reaction, It is shown that Trp-based peptides greatly extend the class of chiral Surface modifiers for heterogeneous enantioselective hydrogenations. Peptides are in fact in principle simple to prepare, give access to a huge variety of tridimensional structures able to form stable chiral surface sites, and allow introduction of functional groups through peptidic bond elongation. A preliminary screening carried out in this work using a relatively low number of peptides shows enantioselectivities up to 30% for a choice of standard test asymmetric reactions oil platinum catalyst. The main features of the chiral environment created by the peptidic modifiers on the metal surface are described by means of first principle computational modeling. The enantioselectivity is interpreted using a computational docking model of KPL within the chiral site.

The structure sensitivity of enantioselective hydrogenations on chirally modified metals was investigated using Pt nanoparticles of different shapes. All three samples had an average particle size of 10 nm, but the fraction of dominantly cubic, cubooctahedral, and octahedral particles varied with decreasing {100} and increasing {111} faces in the same order. In the absence of chiral modifier the hydrogenation of ethyl pyruvate was independent of the shape of the Pt nanoparticles; variation of the specific reaction rates did not exceed the experimental error on all self-prepared catalysts and on a commercial Pt/Al(2)O(3) used as reference. Addition of cinchonidine or quinine induced a significant rate enhancement by a factor of 4-15, and the rate was always higher with quinine. Also, 72-92% ees were achieved, and the reaction was shape selective: both the rate and the ee increased with increasing Pt{111}/Pt{100} ratio. A similar correlation in the hydrogenation of ketopantolactone confirmed that decarbonylation or aldol-type side reactions of ethyl pyruvate were not the reason for structure sensitivity. A combined catalytic and theoretical study revealed that the probable origin of the particle shape dependency of enantioselective hydrogenation is the adsorption behavior of the cinchona alkaloid. DFT studies of cinchonidine interacting with Pt(100) and Pt(111) terraces indicated a remarkably stronger interaction on the former crystallographic face by ca. 155 kJ/mol. The higher adsorption strength on Pt(100) was corroborated experimentally by the faster hydrogenation of the homoaromatic ring of the alkaloid, which fragment interacts the strongest with Pt during its adsorption. Thus, an ideal catalyst for the hydrogenation of activated ketones contains dominantly Pt{111} terraces, which crystallographic face is more active and affords higher enantioselectivity, combined with the higher stability of the modifier.

The structure and electronic properties of gold nanoparticles (Au 12, Au 13, Au 14, Au 15, Au 20, Au 34, and Au 55) have been investigated using Born-Oppenheimer ab initio molecular dynamic simulations of 50 to 80 ps in order to have an insight in the recently proposed fluxional character of nanosized gold. The dynamic changes in shape, symmetry, and atomic coordination of atoms within clusters, occurring in the time scale of picoseconds, which are characteristic of fluxionality, have been investigated for all the above systems at 300 K. Except for Au 20, all systems have been found to have fluxional properties. The extent and the type of fluxional behavior changed according to the number of atoms constituting the particle. At 300 K Au 12 and Au 13 rapidly generate several different topologies which cyclically interconvert. Au 14 shows a rotation of 8 external gold atoms around a core of six atoms. Au 15 is more rigid, but interestingly shows the interconversion between enantiomeric structures within the time scale of the simulation. Au 20 shows a high stability of the pyramidal topology and is the only one of the investigated systems not to show fluxionality within the assigned temperature and time scale. Au 34 and Au 55 show fluxionality of the outer shell and within the sampled time scale are able to change coordination of the outer shell atoms and thus open and close surface holes. For all the particles in study the structures forming the local minima were isolated and separately optimized, and the electronic properties of the thus obtained structures were analyzed.

Density functional computations of alkane reaction energies suffer from systematic errors whicha ccumulate with increasing system size. [1]An efficient way to correct the errors is to add an empirical atom pair wise interaction-correction, inspired by theLennard-Jones potential (R -6 dependence). [2] The presented results show that higher order correction terms( R -8 andR -10 dependent) together with theuniversal damping function of Tang and Toennis [3] reduce these errors more efficiently with even less empiricism.F or general applicability, the TT-damping function is augmented by as econd damping function in order to have negligible corrections at covalent distances.T he scope of this correction (dD10) is simultaneously expanded to intermolecular interactions.We test several combinations of first-principle functionalsa long with the new correction (PBE-dD10, PBEsol-dD10 and RGE2-dD10) and find that PBE-dD10 gives the most reliable results, closely followed by RGE2-dD10.The resultso utperform or match B2PLYP-D and M06-2X, two of the newer functionals with increased accuracy for weak interactions.0 10 20 SVWN5P BE TPSS B3LYPB 2PLYPM 06-2X PBE-dD10 MAD/ (kcal/mol) chains rings cages intramolecular dispersion

Characterization of organic functional groups grafted to magnetic nanoparticles often causes considerable difficulty because of lack of information concerning their accessibility and chemical reactivity. Classical solid-state techniques only allow gaining qualitative information and the determination of upper bounds of the total loading of organic moieties. To overcome this limitation we applied attenuated total reflection (ATR) infrared spectroscopy in a specially designed mechanically stirred in situ batch reactor-cell. Using the example of iron oxide magnetic nanoparticles embedded in a silica matrix that have been functionalized by grafting aminopropyl groups, we elucidated the accessibility and reactivity of amino groups. Reactivity tests using the transformation of the amine to benzaldimine revealed that less than 40% of the introduced amino groups reacted to the corresponding benzaldimine indicating that the theoretical loading provides only an upper bound of active functional groups. The reactivity of these groups has been followed by observing the transformation of the amine to the corresponding benzaldimine, revealing population density, accessibility, and reactivity of these groups. The present in situ technique facilitates the monitoring of the transformation of grafted functional groups and simultaneously provides quantitative and structural information when the material is suspended in a solvent system.

Heterogeneous catalytic hydrogenation of ketones is an important synthetic route to alcohols, but the detailed reaction pathway of this common reaction is still unknown. In particular, different opinions exist concerning the nature of the surface intermediates that eventually react with the activated surface hydrogen: eta(1)(0) and eta(2)(C,O) adsorbed intermediates have been postulated by different authors, leading to different surface reaction pathways. Here we studied the hydrogenation of aromatic ketones activated by an ester group in a-position. Ethyl benzoylformate (1) was hydrogenated on Pt/Al2O3 under mild conditions, but insertion of one or two o-substituents into the aromatic ring diminished or completely eliminated the reactivity of the ketone. The dramatic difference between the reactivities of 1 and ethyl mesithylglyoxylate (5) prompted us to investigate the adsorption geometries and energies of the two ketones by electronic structure calculations on a model platinum (111) surface. The calculations revealed that the presence or absence of o-substitution on the phenyl ring strongly affects the interaction mode of the C = O moiety with the metal surface. In particular, o-substitution suppresses adsorption modes where the keto-carbonyl group is bound to the metal in eta(2)(C,O) mode. Following such observations the reactivity of aromatic ketones is discussed, and a correlation between adsorption mode and reactivity of the ketone towards hydrogenation is proposed that could be critical for the further investigation of a complete reaction mechanism. (C) 2007 Elsevier B.V. All rights reserved.

We present a study on structural and electronic properties of bulk platinum and the two surfaces (111) and (100) comparing the Gaussian and plane wave method to standard plane wave schemes, normally employed for density functional theory calculations on metallic systems. The aim of this investigation is the assessment of methods based on the expansion of the Kohn-Sham orbitals into localized basis sets and on the supercell approach, in the description of the metallicity of Pt. Electronic structure calculations performed at Gamma-point only on supercells of different sizes, from 108 up to 864 atoms, are compared to the results obtained for the unit cell of four Pt atoms where the k-point expansion of the wave function over Monkhorst-Pack grids up to (10x10x10) has been employed. The evaluation of the two approaches with respect to bulk properties is done through the calculation of the equilibrium lattice constant, the bulk modulus, and the total and the d-projected density of states. For the Pt(111) and Pt(100) surfaces, we consider the relaxation of the first layers, the surface energies, the work function, the total density of states, as well as the center and filling of the d bands. Our results confirm that the accuracy of two approaches in the description of electronic and structural properties of Pt is equivalent, providing that consistent supercells and k-point meshes are used. Moreover, we estimate the supercell size that can be safely adopted in the Gaussian and plane wave method in order to obtain the same reliability of previous theoretical studies based on well converged plane wave calculations available in literature. The latter studies, in turn, set the level of agreement with experimental data. In particular, we obtain excellent agreement in the evaluation of the density of states for either bulk and surface systems, and our data are also in good agreement with previous works on Pt reported in literature. We conclude that Gaussian and plane wave calculations, with simulation cells of 400-800 atoms, can be safely used in the study of chemistry related problems involving transition metal surfaces.

Docking of molecules to surface chiral sites generated by adsorbed chiral organic modifiers is a fundamental step in heterogeneous metal-catalyzed asymmetric hydrogenation. The understanding of such docking events is nonetheless limited by the technical difficulties in obtaining experimental submolecular information for complex ads ' orbed structures. In addition an accurate theoretical analysis is computationally very demanding due to the need of including the metal surface. The present investigation explores by means of density functional theory the supramolecular docking structures of ketopantolactone within the chiral sites formed by cinchonidine adsorbed on a platinum surface, a crucial step in the enantioselective hydrogenation of ketopantolactone over cinchonidine-modified platinum. The study is performed using periodic slabs exposing Pt(111) surfaces of (6 x 6) and (8 x 8) atoms with a depth of four layers and including complete relaxation of the first three. Twenty six docking sites are investigated thus covering the most complete configurational space until now. The implications of the physi- and chernisorption of ketopantolactone in the docking have been also explored, thus revealing the role of weak and strong adsorption of the substrate in the formation of precursor states for hydrogen uptake within a docking site. The calculations on the (8 x 8) periodic slab were performed in order to decouple the energy related to the adsorption of the substrate from the energy involved in the docking interactions with the adsorbed alkaloid. Such a surface is in fact apt to accommodate both the chiral surface site and the noninteracting substrate. The study revealed that the substrate likely approaches the chiral site either from solution or from the physisorbed state. The resulting scenario gives a basic understanding of the elements involved in the docking of substrates within the chiral sites formed by cinchona alkaloids on platinum, and constitutes a fundamental stage for the comprehensive clarification of enantioselectivity at chirally modified surfaces.