17558
J. Phys. Chem. C 2007, 111, 17558-17563
Influence of Modification Conditions on the Interaction of Methylacetoacetate with (R,R)-Tartaric Acid-Modified Ni{111} T. E. Jones and C. J. Baddeley* EaStCHEM School of Chemistry, UniVersity of St. Andrews, St. Andrews, Fife KY16 9ST, United Kingdom ReceiVed: June 8, 2007; In Final Form: September 10, 2007
Ni catalysts modified by the adsorption of the chiral molecule, (R,R)-tartaric acid, are known to be capable of catalyzing the enantioselective hydrogenation of β-ketoesters (e.g., methylacetoacetate).1 The adsorption of (R,R)-tartaric acid onto Ni{111} from solution was investigated with reflection absorption infrared spectroscopy as functions of modification temperature and pH. Under all conditions tested, RAIR spectra are consistent with the formation of several layers of nickel and/or sodium bitartrate. After washing, the majority of the overlayer is dissolved, leaving submonolayer quantities of adsorbate. At 350 K, the optimum modification temperature in terms of catalytic enantioselectivity, the washed surface exhibits a proportionately greater hydrogen tartrate content than at 300 K. When the modified surfaces are exposed to methylacetoacetate, the presence of the protonated tartrate species correlates with an enhancement in the diketone to enol tautomeric ratio of methylacetoacetate. Similarly, when the promoter NaBr is added to the modification solution, although little difference is observed in the RAIR spectra of adsorbed tartrate, there is a further increase in the diketone: enol ratio after exposure to methylacetoacetate. The implications for understanding the behavior of the enantioselective nickel catalyst are discussed.
Introduction One of the major goals of heterogeneous catalysis in the early 21st century is the development of economically viable enantioselective heterogeneous systems. For several decades, two closely related catalytic processes have received much attention in the academic literature. The hydrogenation of R-keto and β-keto esters has been demonstrated over Pt and Ni catalysts, respectively, with values of enantiomeric excess (ee) of >90%.1,2 In each case, the key step in the creation of an enantioselective catalyst is the adsorption, from solution, of chiral molecules. In the Pt-catalyzed system, typical modifiers include the cinchona alkaloids (e.g., cinchonidine/cinchonine). In Ni-based catalysis, the most successful modifiers are R-hydroxyacids (e.g., (R,R)-tartaric acid (H2TA), Figure 1) and R-aminoacids (e.g., (S)-alanine, (S)-glutamic acid). In recent years, surface science investigations have revealed considerable information on the surface chemistry underpinning such processes mainly via measurements on single-crystal surfaces under ultrahigh vacuum (UHV) conditions.3-9 In the Ni-catalyzed system, the enantioselectivity of the catalyst is dependent on a range of parameters including modification temperature, pH, and the presence of alkali halide promoters (particularly NaBr) in the modification solution. Among the key issues that are considered important are the molecular conformation of the chiral modifier, the extent to which the modifier is capable of etching the Ni surface, and the tautomeric form of the β-ketoester on the modified surface. Clearly, the influence of such parameters is more adequately understood if measurements are carried out in situ or on samples prepared from solution. We recently reported how pH and modification temperature influence the behavior of the simplest β-ketoester, methylacetoacetate (MAA), on glutamic acidmodified Ni{111}.10 In this Article, we use reflection absorption * To whom correspondence should be addressed. E-mail:
[email protected].
Figure 1. Schematic reaction scheme for the enantioselective hydrogenation of methylacetoacetate showing the diketone and enol tautomeric forms of methylacetoacetate and the chiral modifier, (R,R)-tartaric acid.
infrared spectroscopy (RAIRS) to study the adsorption of H2TA on Ni{111} from aqueous solution as functions of modification temperature, pH, and the addition of NaBr. In addition, we investigate the adsorption of MAA from tetrahydrofuran (THF) solution onto chirally modified surfaces. We discuss the implications of our findings for understanding the Ni-catalyzed system. Experimental Section All experiments were performed using a mechanically polished Ni{111} single crystal. The IR data were collected using a Digilab FTS 7000 spectrometer with a liquid nitrogen cooled MCT detector. Prior to each experiment, the Ni crystal was annealed at 1273 K in a 5% H2/Ar stream for 8 h and allowed to cool, also in hydrogen/argon, to room temperature.
10.1021/jp074450q CCC: $37.00 © 2007 American Chemical Society Published on Web 11/02/2007
Interaction of MAA with H2TA-Modified Ni{111}
J. Phys. Chem. C, Vol. 111, No. 47, 2007 17559
The “cleanliness” was analyzed using photoelastic modulation infrared reflection absorption spectroscopy (PEM-IRRAS) to detect the presence of observable surface molecular contaminants; clearly, we are unable to identify atomic contamination (e.g., C or S) by this approach. A single reflection (non-PEM) background spectrum (256 scans) was then acquired from the unmodified Ni{111} at 4 cm-1 resolution. The sample was immersed in either H2TA (Fluka g99.5%) solution or a solution consisting of a 1:1 molar ratio of H2TA and NaBr (Fluka g99.5%) for 900 s as functions of temperature and pH under constant agitation. Webb and Wells11 observed the enantioselectivity to pass through a maximum at a concentration of H2TA of ∼10 mM, while Keane12 reported the maxima at slightly higher modification solution concentrations of ∼30 mM. In this study, an H2TA concentration of 10 mM was employed. Similarly, a concentration of 10 mM NaBr was employed in relevant experiments. The solution pH was controlled by the addition of 1 M NaOH solution. After modification, the surface was dried using a flow of N2 and RAIRS data taken. The surface was then washed in Millipore water (18.2 MΩ) by submerging for 2 s and drying under N2 flow. A second background spectrum, corresponding to the washed H2TA covered Ni surface, was taken prior to immersion under constant agitation in a 50:50 mixture of methylacetoacetate (MAA; Fluka g99%) in tetrahydrofuran (THF) for 900 s. A further RAIR spectrum was subsequently acquired. THF was chosen as it is reported to be one of the more successful solvents used in the Ni-catalyzed enantioselective hydrogenation of MAA.1 In all cases, RAIRS data were acquired in air. Results Figure 2a shows the RAIR spectra following immersion of Ni{111} in 10 mM H2TA solution for 900 s at 300 K as a function of pH. The most intense bands are centered at 1622 and 1402 cm-1. These may be assigned as the asymmetric (νasym) and symmetric (νsym) stretching modes of the carboxylate groups of bitartrate (TA2-).7 The intensity of the bands at pH 2.5 is approximately 4 times larger than that after immersion at pH 9. UHV RAIRS experiments on Ni{111} following the adsorption of a monolayer of H2TA show that the intensity of absorption of the (νsym) mode at ∼1400 cm-1 is ∼0.2%.6 Hence, we conclude that several monolayers of tartrate are produced under these low pH conditions. In addition, because no sodium is added to the modification solution at low pH, we may also conclude that nickel tartrate is being produced under these conditions; that is, the surface is corroded by H2TA as has been widely reported.1 Figure 2b shows the RAIR spectra of Ni{111} immersed in 10 mM H2TA solution for 900 s at 300 K following a post modification wash. In each case, the intensity of the bands associated with adsorbed species attenuates by approximately a factor of 6 as compared to those shown in Figure 2a. The spectra remain dominated by the TA2- carboxylate bands, although now the intensity of the absorption bands is in the (sub)monolayer regime. With increasing pH, the intensity of these bands decreases to
