N-heterocyclic carbene adsorption states on Pt(111) and Ru(0001).

N-heterocyclic carbene ligands (NHCs) are increasingly used to tune the properties of metal surfaces. The generally greater chemical and thermal robustness of NHCs on gold, as compared to thiolate surface ligands, underscores their potential for a range of applications. While much is now known about the adsorption geometry, overlayer structure, dynamics, and stability of NHCs on coinage elements, especially gold and copper, much less is known about their interaction with the surfaces of Pt-group metals, despite the importance of such metals in catalysis and electrochemistry. In this study, reflection absorption infrared spectroscopy (RAIRS) is used to probe the structure of benzimidazolylidene NHC ligands on Pt(111) and Ru(0001). The experiments exploit the intense absorption peaks of a CF3 substituent on the phenyl ring of the NHC backbone to provide unprecedented insight into adsorption geometry and chemical stability. The results also permit comparison with literature data for NHC ligands on Au(111) and to DFT predictions for NHCs on Pt(111) and Ru(0001), thereby greatly extending the known surface chemistry of NHCs and providing much needed molecular information for the design of metal-organic hybrid materials involving strongly reactive metals.

Substrate Tumbling in a Chemisorbed Diastereomeric α-Ketoester/1-(1-Naphthyl)ethylamine Complex.

Scanning tunneling microscopy (STM) data for α-ketoester/1-(1-naphthyl)ethylamine complexes on Pt(111) reveal a tumbling motion that couples two neighboring binding states. The interconversion, resulting in prochiral inversion of the α-ketoester, occurs in single complexes without breaking them apart. This is a surprising observation because the overall motion requires rotation of the α-ketoester away from the surface without branching exclusively into diffusion away from the complex or desorption. The multi-step interconversion is rationalized in terms of sequences of bound states that combine transient H-bond interactions with the chiral molecule and weakened adsorption interactions with the metal. The observation of tumbling in single long-lived complexes is of relevance to self-assembly and directed molecular motion on surfaces, to ligand-controlled surface reactions, and most directly to stereocontrol in asymmetric heterogeneous catalysis.

Generation and conversion of an N-heterocyclic carbene on Pt(111).

The formation of N-heterocyclic carbenes on a platinum surface is demonstrated. Surface vibrational spectroscopy measurements isolate steps in the transformation from 1,3-diisopropylbenzimidazolium hydrogen carbonate to surface carbene. As the temperature is further increased, the reactive metal transforms the isopropyl groups leading to the effective formation of a new surface carbene.

Monitoring interconversion between stereochemical states in single chirality-transfer complexes on a platinum surface.

Elementary steps in enantioselective heterogeneous catalysis take place on the catalyst surface and the targeted synthesis of a desired enantiomer requires the implantation of chiral information at the surface, which can be achieved-for example-by adsorbing chiral molecules. Studies of the structures of complexes formed between adsorbed prochiral reagents and chiral molecules yield information on the forces exerting stereocontrol, but further insight could be gained by studying the dynamics of their interactions. Here, using time-lapsed scanning tunnelling microscopy and density functional theory, we observe coupling between multiple stereochemical states within individual non-covalently bonded chirality-transfer complexes on a metal surface. We identify two modes of transformation between stereochemical states and find that the prochiral reagent can sample several complexation geometries during the lifetime of a complex, switching between states of opposing prochirality in the process. These results provide insight on the contribution of individual stereochemical states to the overall enantioselectivity of reactions occurring on catalyst surfaces.

Structure and Dynamics of Individual Diastereomeric Complexes on Platinum: Surface Studies Related to Heterogeneous Enantioselective Catalysis.

The modification of heterogeneous catalysts through the chemisorption of chiral molecules is a method to create catalytic sites for enantioselective surface reactions. The chiral molecule is called a chiral modifier by analogy to the terms chiral auxiliary or chiral ligand used in homogeneous asymmetric catalysis. While there has been progress in understanding how chirality transfer occurs, the intrinsic difficulties in determining enantioselective reaction mechanisms are compounded by the multisite nature of heterogeneous catalysts and by the challenges facing stereospecific surface analysis. However, molecular descriptions have now emerged that are sufficiently detailed to herald rapid advances in the area. The driving force for the development of heterogeneous enantioselective catalysts stems, at the minimum, from the practical advantages they might offer over their homogeneous counterparts in terms of process scalability and catalyst reusability. The broader rewards from their study lie in the insights gained on factors controlling selectivity in heterogeneous catalysis. Reactions on surfaces to produce a desired enantiomer in high excess are particularly challenging since at room temperature, barrier differences as low as ∼2 kcal/mol between pathways to R and S products are sufficient to yield an enantiomeric ratio (er) of 90:10. Such small energy differences are comparable to weak interadsorbate interaction energies and are much smaller than chemisorption or even most physisorption energies. In this Account, we describe combined experimental and theoretical surface studies of individual diastereomeric complexes formed between chiral modifiers and prochiral reactants on the Pt(111) surface. Our work is inspired by the catalysis literature on the enantioselective hydrogenation of activated ketones on cinchona-modified Pt catalysts. Using scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations, we probe the structures and relative abundances of non-covalently bonded complexes formed between three representative prochiral molecules and (R)-(+)-1-(1-naphthyl)ethylamine ((R)-NEA). All three prochiral molecules, 2,2,2-trifluoroacetophenone (TFAP), ketopantolactone (KPL), and methyl 3,3,3-trifluoropyruvate (MTFP), are found to form multiple complexation configurations around the ethylamine group of chemisorbed (R)-NEA. The principal intermolecular interaction is NH···O H-bonding. In each case, submolecularly resolved STM images permit the determination of the prochiral ratio (pr), pro-R to pro-S, proper to specific locations around the ethylamine group. The overall pr observed in experiments on large ensembles of KPL-(R)-NEA complexes is close to the er reported in the literature for the hydrogenation of KPL to pantolactone on (R)-NEA-modified Pt catalysts at 1 bar H2. The results of independent DFT and STM studies are merged to determine the geometries of the most abundant complexation configurations. The structures reveal the hierarchy of chemisorption and sometimes multiple H-bonding interactions operating in complexes. In particular, privileged complexes formed by KPL and MTFP reveal the participation of secondary CH···O interactions in stereocontrol. State-specific STM measurements on individual TFAP-(R)-NEA complexes show that complexation states interconvert through processes including prochiral inversion. The state-specific information on structure, prochirality, dynamics, and energy barriers delivered by the combination of DFT and STM provides insight on how to design better chiral modifiers.

Stereodirection of an α-ketoester at sub-molecular sites on chirally modified Pt(111): heterogeneous asymmetric catalysis.

Chirally modified Pt catalysts are used in the heterogeneous asymmetric hydrogenation of α-ketoesters. Stereoinduction is believed to occur through the formation of chemisorbed modifier-substrate complexes. In this study, the formation of diastereomeric complexes by coadsorbed methyl 3,3,3-trifluoropyruvate, MTFP, and (R)-(+)-1-(1-naphthyl)ethylamine, (R)-NEA, on Pt(111) was studied using scanning tunneling microscopy and density functional theory methods. Individual complexes were imaged with sub-molecular resolution at 260 K and at room temperature. The calculations find that the most stable complex isolated in room-temperature experiments is formed by the minority rotamer of (R)-NEA and pro-S MTFP. The stereodirecting forces in this complex are identified as a combination of site-specific chemisorption of MTFP and multiple non-covalent attractive interactions between the carbonyl groups of MTFP and the amine and aromatic groups of (R)-NEA.

Scanning Tunneling Microscopy Measurements of the Full Cycle of a Heterogeneous Asymmetric Hydrogenation Reaction on Chirally Modified Pt(111).

The hydrogenation of a prochiral substrate, 2,2,2-trifluoroacetophenone (TFAP), on Pt(111) was studied using room-temperature scanning tunneling microscopy (STM) measurements. The experiments were carried out both on a clean surface and on a chirally modified surface, using chemisorbed (R)-(+)-1-(1-naphthyl)ethylamine, ((R)-NEA), as the modifier. On the nonmodified surface, introduction of H2 at a background pressure of ∼1 × 10(-6) mbar leads to the rapid break-up of TFAP dimer structures followed by the gradual removal of all TFAP-related images. During the latter step, some monomers display an extra protrusion compared to TFAP in dimer structures. They are attributed to a half-hydrogenated intermediate. The introduction of H2 to a mixture of (R)-NEA and TFAP on Pt(111) leads to the removal of TFAP without any change in the population of the modifier, as required for an efficient chirally modified catalyst.

Direct observation of molecular preorganization for chirality transfer on a catalyst surface.

The chemisorption of specific optically active compounds on metal surfaces can create catalytically active chirality transfer sites. However, the mechanism through which these sites bias the stereoselectivity of reactions (typically hydrogenations) is generally assumed to be so complex that continued progress in the area is uncertain. We show that the investigation of heterogeneous asymmetric induction with single-site resolution sufficient to distinguish stereochemical conformations at the submolecular level is finally accessible. A combination of scanning tunneling microscopy and density functional theory calculations reveals the stereodirecting forces governing preorganization into precise chiral modifier-substrate bimolecular surface complexes. The study shows that the chiral modifier induces prochiral switching on the surface and that different prochiral ratios prevail at different submolecular binding sites on the modifier at the reaction temperature.

Activation in prochiral reaction assemblies on Pt(111).

Trifluoroacetophenone (TFAP) forms C O...H-C bonded dimers and trimers at room temperature on Pt(111). It is proposed that these systems mimic the prochiral carbonyl-chiral modifier interaction in the enantioselective hydrogenation of TFAP on cinchona-modified Pt catalysts. That is, the activation of TFAP in homomolecular assemblies at racemic sites is expected to be roughly the same as in the diastereomeric complex formed at chiral sites. This interpretation suggests a reason why alpha-phenyl ketones do not display a strong measured rate enhancement effect in the Orito reaction.

A generalized two-point H-bonding model for catalytic stereoselective hydrogenation of activated ketones on chirally modified platinum.

The asymmetric hydrogenation of alpha-ketoesters on cinchona-modified supported platinum particles is a prototype reaction in heterogeneous chiral catalysis. The catalysis literature shows that the reaction is highly metal-specific, that it displays rate-enhancement with respect to the racemic reaction on the nonmodified surface, and that the observed stereoselectivity is a sensitive function of substrate and modifier structure. This set of observations has proven difficult to rationalize within the context of existing models for the mechanism of the Orito reaction. The most widely discussed mechanistic models are based on the formation of chemisorbed 1:1 complexes through H-bonding between the quinuclidine function of the cinchona modifier and the prochiral, keto-carbonyl, function of the substrate. Recent surface science studies, as well as advances in the area of C-H...O hydrogen bonding, suggest that chemisorption-induced polarization may lead to an aromatic-carbonyl H-bonding interaction between the aromatic anchor of the modifier and the coadsorbed substrate. By specifying that the aromatic C-H...O interaction is to the prochiral carbonyl and that it is accompanied by a H-bonding interaction between the ester carbonyl and the quinuclidine function, we show that it is possible to rationalize essentially all of the catalysis literature for the Orito reaction in terms of a single molecular mechanism. The generality of the proposed mechanistic model is demonstrated by addressing data from the literature for a representative range of substrates, modifiers, solvents, and metals. Results of catalytic tests on an asymmetric diketone substrate are presented in support of the model.

Creating, varying, and growing single-site molecular contacts.

The known range of chemisorption bonds forms the toolbox for the design of electrical contacts in molecular electronics devices. Double-bond contacts to technologically relevant materials would be attractive for a number of reasons. They are truly single-site, bonding to a single surface atom. They obviate the need for a thiol linkage, and they may be amenable to further modification through olefin-metathesis methodologies. We report olefin-metathesis methods for establishing, varying, and growing thermally stable double-bond contacts to molybdenum carbide, a conducting material.

Evidence for C-H...O=C bonding in coadsorbed aromatic-carbonyl systems on Pt(111).

The coadsorption of ethyl formate, acetone, and methyl pyruvate with benzene and naphthalene on Pt(111) was studied with reflection absorption infrared spectroscopy (RAIRS) and thermal desorption (TPD) measurements. Coadsorbed benzene or naphthalene are found to convert eta1- and eta2-states of ethyl formate and acetone into new states displaying slightly red-shifted carbonyl bands. Similarly, coadsorption converts the enediolate state of methyl pyruvate into a new adsorption geometry in which the carbonyl bands are silent. In each case, coadsorption of the aromatic leads to significantly modified carbonyl desorption spectra. The results suggest an attractive carbonyl-aromatic interaction that weakens or removes the direct interaction of the carbonyl function with the metal surface. The aromatic-carbonyl interaction is attributed to hydrogen bonding between C-H and C=O, enhanced by the chemisorption induced polarization of the aromatic.

Carbon-nitrogen place exchange on NO exposed beta-Mo2C.

Atomic nitrogen and oxygen were deposited on beta-Mo(2)C through dissociative adsorption of NO. Reflectance absorbance infrared spectroscopy (RAIRS), thermal desorption, and synchrotron X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the interplay between atomic nitrogen, carbon, and oxygen in the 400-1250 K region. The combination of the high resolution and high surface sensitivity offered by the synchrotron XPS technique was used to show that atomic nitrogen displaces interstitial carbon onto the carbide surface. Thermal desorption measurements show that the burnoff of the displaced carbon occurs at approximately 890 K. The incorporation of nitrogen into interstitial sites inhibits oxygen dissolution into the bulk. RAIRS spectroscopy was used to identify surface oxo, terminal oxygen, species formed from O(2) and NO on beta-Mo(2)C.

Dissociation of acetaldehyde on beta-Mo2C to yield ethylidene and oxo surface groups: a possible pathway for active site formation in heterogeneous olefin metathesis.

The dissociative adsorption of acetaldehyde on beta-Mo2C was studied using reflection absorption infrared spectroscopy. In contrast to what is observed for all metals previously studied, acetaldehyde undergoes selective carbonyl bond scission on the carbide surface. By comparison to calculated spectra, the surface product is identified as an oxo-ethylidene species. The study thus provides the first extended-range infrared spectrum of a propene metathesis initiator or propagator alkylidene. Aldehydes may be formed in the presence of olefins during the induction period of supported metal oxide olefin metathesis catalysts. Hence, the observed dissociative chemisorption of acetaldehyde suggests a possible answer to the question of how initiator sites are formed in heterogeneous olefin metathesis. This question has never been satisfactorily answered. In the proposed mechanism, aldehydes formed during the induction period subsequently react with the catalyst surface to generate alkylidene sites.