J. Phys. Chem. C 2011, 115, 1355–1360
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Tuning Aryl-CH · · · O Intermolecular Interactions on Pt(111)† Vincent Demers-Carpentier, Marc-Andre´ Laliberte´,‡ Yunxiang Pan,§ Gautier Mahieu,‡ Ste´phane Lavoie,‡ Guillaume Goubert,‡ Bjørk Hammer,§ and Peter H. McBreen*,‡ De´partement de Chimie, UniVersite´ LaVal, Que´bec, QC, Canada G1 V OA6, and Department of Physics and Astronomy, UniVersity of Aarhus, 8000 Århus C, Denmark ReceiVed: August 23, 2010; ReVised Manuscript ReceiVed: December 2, 2010
Scanning tunneling microscopy (STM) data are reported for the room-temperature adsorption of 2,2,2trifluoroacetophenone (TFAP), 2,2,2-trifluorovinylbenzene (TFVB), octafluoroacetophenone (OFAP), and methyl benzoate (MB) on Pt(111). The objective of the study is to establish the role of aryl-CH · · · O bonding in forming self-assembled low-nuclearity structures at room temperature and to compare aryl-CH · · · O bonding by ester and ketone carbonyl functions. The STM images clearly evidence the formation of homochiral dimers and trimers of TFAP, and density functional theory (DFT) calculations reveal aryl-CH · · · O bonding as the driving force for dimer formation. In contrast to TFAP, chemisorbed TFVB and OFAP do not form such self-assembled structures as they lack carbonyl and aryl-CH groups, respectively. The self-assembly of MB on Pt(111) differs from that of TFAP, in that it can form structures stabilized by one, as distinct from two, aryl-CH · · · O bonds. The results are discussed with respect to the enantioselective hydrogenation of R-ketoesters on cinchona modified Pt catalysts. Introduction The asymmetric catalytic site created by the adsorption of a chiral modifier on a reactive metal surface is an example of a functional nanoscopic system. For asymmetric synthesis to take place, the prochiral substrate must undergo enantiotopic recognition in the chiral pocket created by the chemisorbed modifier. As yet, only a handful of reactions have been able to achieve satisfying results, mostly because of the lack of a comprehensive mechanistic understanding as well as the complex set of conditions necessary to optimize ee, reaction rate, and catalyst longevity.1 One of these systems is the Orito reaction, the asymmetric hydrogenation of activated ketones on chirally modified platinum.2 Among the most widely studied examples of the reaction is the enantioselective hydrogenation of methyl pyruvate to (R)-methyl lactate (ee g 95%) on platinum bearing a low surface coverage of cinchonidine. It is typically carried out at approximately room temperature and involves the formation of a 1:1 cinchonidine/methyl pyruvate complex.1,3 With the objective of rationalizing a range of experimental observations on the reaction, Lavoie et al.4 proposed a mechanistic model in which H-bonding at two points in the 1:1 complex yields the precise geometry necessary for the asymmetric process. In addition to a conventional N+H · · · O interaction between the modifier and the ester carbonyl of methyl pyruvate, the model specifies an aryl-CH · · · O bond between the prochiral ketone and the chemisorption activated aromatic group of the chiral modifier. A number of examples of the participation of aryl-CH · · · O bonding in molecular self-assembly on surfaces are to be found in the literature. These include the formation of porous networks of anthraquinone on Cu(111),5 2,2,2-trifluoroacetophenone (TFAP) dimers and trimers on Pt(111),6 monolayers of 1,5-bis(2-dioxaalkyl)-anthracenes on HOPG,7 4-trans-2-(pyrid-4-yl†
Part of the “Alfons Baiker Festschrift”. * Corresponding author. E-mail:
[email protected]. ‡ Universite´ Laval. § University of Aarhus.
vinyl) benzoic acid (PVBA) and 4-[(pyrid-4-yl-ethynyl)] benzoic acid (PEBA) paired rows on Ag(111),8 and 1-nitronaphthalene clusters on Au(111).9 The interaction is also observed in coadsorbed systems, as reported in previous work from our group10,11 and in work by Wei et al. on the coadsorption of pentacene and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(111).12 In order to form isolated structures such as 1:1 complexes or low molecularity clusters at ambient temperatures, in the absence of other stronger interactions, aryl-CH · · · O bonding needs to be strengthened through intramolecular or chemisorption activation. In the context of a study of the Orito reaction, we have reported examples of isolated aryl-CH · · · O bonded structures formed by the coadsorption of aromatics and carbonyl compounds on Pt(111).10,11 The stability of these clusters was attributed to the activation of the aromatic toward strong aryl-CH · · · O bonding by the charge redistribution accompanying chemisorption on the reactive surface. We have also reported aryl-CH · · · O hydrogen bonding as the driving force for the assembly of TFAP dimers and trimers on Pt(111).6 For TFAP, intramolecular activation by the electron withdrawing group (CF3) might contribute in addition to chemisorption activation. In this study, a set of related molecules are used to explore aryl-CH · · · O bonding. These molecules differ by the substitution of functional groups or atoms at key positions relative to TFAP, as summarized in Scheme 1. Octafluoroacetophenone (OFAP) is used to investigate a case where there are no aryl-CH bonds. 2,2,2-Trifluorovinylbenzene (TFVB) is used as an example of a molecule lacking a carbonyl function. Methyl benzoate (MB) is used to explore aryl-CH · · · OC bonding involving an ester function. TFAP, rather than acetophenone, is used as a reference molecule so as to avoid keto-enol tautomerization.13,14 Acetophenone undergoes enolization at room temperature leading to self-assembly involving primarily OH · · · OH interactions instead of aryl-CH · · · O bonding.14 In combination with the results of previous work on the coadsorption of aromatics and carbonyl compounds on Pt(111),10,11
10.1021/jp107972u 2011 American Chemical Society Published on Web 12/13/2010
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J. Phys. Chem. C, Vol. 115, No. 4, 2011
SCHEME 1: Substitution at the Functional Groups Involved in the Formation of Intermolecular Aryl-CH · · · O Bonds in 2,2,2-Trifluoroacetophenone (TFAP) Dimers Gives Octafluoroacetophenone (OFAP), 2,2,2-Trifluorovinylbenzene (TFVB), and Methyl Benzoate (MB)a
a In this study, these four molecules are used to investigate the role of aryl-CH · · · O bonding in forming self-assembled low-nuclearity structures at room temperature and to compare aryl-CH · · · O bonding by ester and ketone carbonyl functions.
the study provides insight on how aryl-CH · · · O bonding may be fine-tuned for applications in asymmetric synthesis on chirally modified metallic surfaces. Methods Section The scanning tunnelling microscopy experiments were carried out independently using two UHV systems. All four molecules were studied using an Omicron variable temperature STM. 2,2,2Trifluoroacetophenone and methyl benzoate were also studied using a SPECS Aarhus STM-150 system. The STM measurements were performed using -1.0 V sample bias and 1 nA tunneling current. The platinum surface was cleaned by cycles of Ar+ ion bombardment at 600 K and oxygen (2 × 10-7 Torr) treatment at 900 K. The purity of the products used was as follows: 2,2,2-trifluoroacetophenone, 99% Aldrich; octafluoroacetophenone, 97% Aldrich; 2,2,2-trifluorovinylbenzene; methyl benzoate, 99% Aldrich. The products were further purified by repeated freeze-thaw cycles in the gas-handling line. The density functional theory (DFT) calculations were performed with the SIESTA code that uses a numerical linear combination of an atomic-like orbital basis set for the oneelectron wave functions.15,16 Specifically, a double-ζ basis set plus polarization orbitals was used which provided a good computational convergence with respect to the basis set. For exchange-correlation effects we employed the revised Perdew-Burke-Ernzerhof (RPBE) functional which is developed within the generalized gradient approximation (GGA).17,18 Standard norm-conserving Troullier-Martins pseudopotentials were used to describe the interaction between ionic cores and valence electrons.19 All geometries were relaxed until forces on unconstrained atoms were
