J. Phys. Chem. C 2009, 113, 13877–13885
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Enantioselective Chemisorption on Model Chirally Modified Surfaces: 2-Butanol on r-(1-Naphthyl)ethylamine/Pd(111) Luke Burkholder,† Darı´o Stacchiola,‡ Jorge. A. Boscoboinik,† and Wilfred. T. Tysoe*,† Department of Chemistry and Biochemistry and Laboratory for Surface Studies, UniVersity of WisconsinsMilwaukee, Milwaukee, Wisconsin 53211, and Department of Chemistry, Michigan Technological UniVersity, Houghton, Michigan 49931 ReceiVed: April 24, 2009; ReVised Manuscript ReceiVed: June 10, 2009
The enantioselective adsorption of propylene oxide and 2-butanol is explored on R-(1-naphthyl)-ethylamine (NEA) covered Pd(111) using temperature-programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM). The saturation coverage of NEA is ∼0.1 monolayers (ML), and it thermally decomposes to desorb hydrogen and HCN. 2-Butanol adsorbs enantioselectively on NEA-modified Pd(111), while propylene oxide does not, emphasizing the importance of hydrogen-bonding interactions. An enantioselectivity ratio, ER, of ∼2 (corresponding to an ee of ∼33%) is found at an NEA coverage of ∼0.055 ML, where 2-butanol adsorbs on the Pd(111) substrate. A second regime is found in which 2-butanol adsorbs on an NEA-covered surface with a maximum ER ∼ 1.8 (corresponding to an ee of ∼29%). This interaction appears to cause the NH2 group to reorient to facilitate hydrogen bonding interactions between 2-butanol and the amine group, and the heat of adsorption of ∼35 kJ/mol is typical of -OH · · · NH2 hydrogen bond strengths. 1. Introduction In principle, nonchiral surfaces can be rendered enantioselective in two ways. First, chiral sites can be obtained on surfaces of achiral materials by generating faces that have unequal Miller indices,1-3 and it may be possible to generate such metal surfaces using chiral adsorbates that modify the substrate structure to “imprint” the surface to render it chiral.4,5 Alternatively, enantioselective surfaces can be created by adsorbing a chiral modifier. One of the first successful reported examples of enantioselectivity in the heterogeneously catalyzed conversion of organic compounds was that of the hydrogenation of β-keto esters over supported nickel catalysts modified with tartaric acid.6-9 More recently, R-keto esters have been hydrogenated enantioselectively using supported platinum catalysts modified with cinchona alkaloids.10-15 Such chiral modifiers have been proposed to act in a collective fashion so that several modifiers create a chiral pocket,16 to form a chiral “template” and indeed the formation of a wide range of ordered superstructures have been identified following the adsorption of small chiral adsorbates on metal surfaces.17-19 Alternatively, the chiral modifiers could be capable of forming individual chiral complexes with the reactant to direct its adsorption geometry, in a one-to-one interaction between the modifier and reactant. Chiral modifiers, which possess a similar molecular architecture to cinchonidine have been identified, in particular R-(1-naphthyl)-ethylamine (NEA), which contains a similar bicyclic ring to anchor the molecule to the surface and chiral side group,20-22 and it has been shown to act as a chiral modifier.23-26 In this case, under catalytic conditions, NEA reacts with the pyruvate to form a secondary amine, which provides the actual chiral site.25 However, NEA itself is sufficiently stable to allow it to be dosed onto surfaces in ultrahigh vacuum and is therefore an attractive * Corresponding author. E-mail:
[email protected]. † University of WisconsinsMilwaukee. ‡ Michigan Technological University.
option for fundamental study. In this case, the lack of ordered structures in scanning tunneling microscopy (STM) experiments implies that this does not act in a concerted fashion to form a template, but acts as a one-to-one modifier.22 It has also been proposed, based on correlating the structures of prochiral reactants with their enantioselectivity, that the one-to-one chiral interaction in NEA occurs through hydrogen-bonding interactions both with the amine and with hydrogens on the naphthyl ring.27,28 Such interactions have been observed directly by STM.29 It has been demonstrated that enantioselective chemisorption can be measured on chirally modified surfaces in ultrahigh vacuum thereby, in principle, allowing the relationship between the surface structures and the resulting (chemisorptive) enantioselectivity to be explored in detail.30-34 It has been found, for example, that enantioselective chemisorption of propylene oxide occurs on surfaces modified by 2-butanol, where hydrogenbonding interactions between 2-butanol and the propylene oxide were found to be central to inducing enantioselectivity.35 Similar enantioselective chemisorption and reactions were found on surfaces modified by amino acids.34 Here, this approach is extended in this work to examining enantioselective chemisorption of propylene oxide and 2-butanol on NEA-modified Pd(111) to experimentally identify whether hydrogen-bonding interactions control enantioselectivity. While platinum appears to yield the highest values of ee for the hydrogenation of alkyl pyruvates to alkyl lactates,10-15,36-43 enantioselective hydrogenation has been found using palladium.44-47 In this case, 2-butanol is used as a probe for enantioselectivity rather than as a modifier. No chemisorptive enantioselectivity is found for propylene oxide on NEA-modified surfaces, while the adsorption of 2-butanol is affected, suggesting that hydrogenbonding interactions are central to imparting enantioselectivity in NEA. However, two enantioselective regimes are found: one over a narrow coverage range at submonolayer coverages of NEA and another regime at higher coverages.
10.1021/jp903793n CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
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2. Experimental Section Temperature-programmed desorption (TPD) data were collected in an ultrahigh vacuum chamber operating at a base pressure of 8 × 10-11 Torr that has been described in detail elsewhere48 where desorbing species were detected using a Dycor quadrupole mass spectrometer placed in line of sight of the sample. The temperature ramp and data collection were controlled using LabView software. All TPD spectra were recorded using a heating rate of 2.7 K/s in this study. This chamber was also equipped with a double-pass, cylindrical mirror analyzer for Auger spectroscopy measurements and an ion-sputtering source for sample cleaning. Reflection absorption infrared spectroscopy (RAIRS) data were collected using a Bruker Equinox infrared spectrometer equipped with a liquid-nitrogen-cooled, mercury-cadmium telluride detector. The complete light path was enclosed and purged with dry, CO2-free air. Data were typically collected for 1000 scans at a 4 cm-1 resolution. X-ray photoelectron spectra (XPS) were collected in another chamber operating at a base pressure of 2 × 10-10 Torr, which was equipped with a Specs X-ray source and a double-pass cylindrical mirror analyzer. Spectra were typically collected with an Mg KR X-ray power of 250 W and a pass energy of 50 eV. Scanning tunneling microscopy (STM) images were collected in an ultrahigh vacuum system containing an experimental and sample-preparation chamber. The commercial experimental chamber (RHK) contained an eddy-current damped sample mount, and the sample could be heated by electron beam heating. The STM scan head was based on the “beetle” design and incorporated an additional piezo tube for more precise scanning. The tungsten wire used for STM experiments was annealed in vacuo by hanging a weight onto the wire and heating resistively until the wire broke to cause rapid cooling and the formation of tungsten crystallites.49 The tip was formed electrochemically using a procedure that stopped etching immediately after the tip had formed. Finally, the oxide layer was removed by electron beam heating in ultrahigh vacuum (Vtip ) 500 V, Itip ) 200 µA, for 200 s).50 The end of the tip was imaged by transmission electron microscopy (TEM), and only well-formed tips were used. The Pd(111) substrate (1 cm diameter, 0.5 mm thick) was cleaned using a standard procedure, which consisted of cycles of argon ion bombardment (2 kV, 1 µA/cm2) and annealing in 4 × 10-8 Torr of O2 at 1000 K. The cleanliness of the sample was judged using Auger spectroscopy and oxygen titration where O2 instead of CO desorbs following O2 adsorption when the sample is carbon free. Following each TPD experiment, the surface was briefly annealed once again in O2 to regain cleanliness. NEA was adsorbed on the Pd(111) surface using a homebuilt source, which was differentially pumped using a turbomolecular pump. (r)/(s)-NEA (Acros, 99% purity) were stored in glass vials. The sample vial containing NEA was cooled by an ethylene glycol-dry ice mixture (T ) 265 K) to provide reasonable NEA deposition rates. NEA was dosed onto Pd(111) via a glass tube that was placed ∼1 cm from the sample to avoid contaminating other parts of the chamber. (r)/(s)-2-Butanol (Aldrich, 99%) and (r)/(s)-propylene oxide (Acros, p.a.) enantiomers were further purified by cycles of freeze-pump-thaw before use. 3. Results 3.1. NEA Adsorption on Pd(111). In order to gauge the coverage of NEA adsorbed directly onto the surface and to
Figure 1. Plot of the relative desorption yield of 13CO measured from the integrated temperature-desorption profiles for the exposure of 3 L of 13CO to a surface covered by NEA, plotted as a function of NEA exposure time. Shown also in this figure is a depiction of NEA.
calibrate the dosing source, a series of experiments was carried out by dosing the surface with NEA and then saturating the vacant metal sites by exposing the surface to carbon monoxide, where the carbon monoxide coverage was measured using temperature-programmed desorption. This experiment was carried out using 13CO to avoid artifacts due to background CO adsorption. The relative yield of carbon monoxide, and therefore the proportion of vacant metal sites, is plotted versus NEA exposure (in seconds) in Figure 1.51 Shown as an inset to this figure is a depiction of NEA. The relative CO coverage shows an exponential decrease as a function of NEA exposure that reaches a plateau at a relative CO coverage of 0.11 ( 0.03 monolayers (ML) after an NEA dosing time of ∼700 s. However, complete suppression of CO adsorption only occurs after a total NEA exposure from the dosing source of ∼1400 s. Similar results are obtained using RAIRS experiments, except that the shape of the curve is slightly different since dipole-dipole interactions between carbon monoxide molecules means that the integrated absorbance of CO does not scale linearly with the coverage. A comparison of the integrated area of the C 1s XPS signal of NEA with a known coverage of carbon monoxide52 yields an NEA coverage of 0.10 ( 0.02 ML (where, in this case, the coverage is referenced to the number of palladium atoms on the (111) face). An STM image of a surface covered just by first-layer NEA is shown in Figure 2. This reveals that there is no long-range order, although there are small regions in which triangular motifs of adsorbed NEA can be seen, some of which are indicated on the image. The separation between adsorbates in this region is ∼8 Å, in agreement with the expected size of NEA and also with the diameter of NEA measured on a Pt(111) surface (at lower coverages) by STM.22 The absolute NEA coverage measured from these STM data yield a value of 0.099 ( 0.005 ML, in good agreement with the value measured by XPS. In the following, the monolayer saturation is taken to occur at a dose of ∼700 s and the relative coverages are calculated from the value of FS, where F is the NEA flux and S is the sticking probability, as described previously.51 Thus, a relative
2-Butanol on R-(1-Naphthyl)ethylamine/Pd(111)
Figure 2. STM image (30.37 nm × 30.37 nm) of NEA dosed onto a Pd(111) surface at ∼150 K and heated to 300 K to desorb the second layer obtained with a tunneling potential of 98.4 mV at a current of 296 pA. Also indicated on this figure are some of the triangular motifs formed on the surface.
Figure 3. Hydrogen (2 amu) temperature-programmed desorption data for various coverages of (s)-(1-naphthyl) ethylamine ((s)-NEA) adsorbed on Pd(111) at 80 K collected using a heating rate of 2.7 K/s, where the NEA coverages are marked adjacent to the corresponding spectra.
coverage of 1.0 corresponds to an absolute coverage (referenced to the number of palladium atoms on the surface) of ∼0.1 ML. 3.2. Surface Chemistry of NEA on Pd(111). Shown in Figure 3 are a series of 2 amu (hydrogen) TPD profiles collected following the adsorption of (s)-NEA on Pd(111) at 80 K. The peak at ∼350 K is due to a small amount of hydrogen adsorbing from the background,53 and the majority of the hydrogen resulting from NEA decomposition desorbs in peaks at ∼500, 625, and ∼700 K. A substantial amount of HCN (27 amu) was found to desorb from the surface predominantly in a broad feature centered at ∼575 K (Figure 4), with a tail extending to ∼850 K due to reaction products of the amine group. No cyanogen was found to desorb from the surface. The variation in integrated desorption yields of these products (hydrogen at 2 amu, HCN at 27 amu) is plotted in Figure 5 along with the
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Figure 4. HCN (27 amu) temperature-programmed desorption data for various coverages of (s)-(1-naphthyl) ethylamine ((s)-NEA) adsorbed on Pd(111) at 80 K collected using a heating rate of 2.7 K/s, where the NEA coverages are marked adjacent to the corresponding spectra.
Figure 5. Plot of the integrated desorption yield at 2 (9), 27 (O), and 51 (1) amu as a function of (s)-NEA coverage, where the coverage is measured using CO titrations (Figure 1).
51 amu signal intensity, due to the most intense fragment of NEA in our mass spectrometer. Lines are plotted onto these data as a guide to the eye. Molecular NEA desorbs in a sharp state centered at ∼265 K, and a broader profile with a peak at ∼325 K that extends to ∼400 K (see inset of Figure 7), and no NEA desorbs below a coverage of ∼0.7 ML. The yields of the reaction products increase with increasing NEA coverage and saturate at ∼1 ML as expected. The yield of molecular NEA (51 amu signal) increases for relative NEA coverages greater than ∼0.9. This NEA ultimately completely blocks the adsorption of 13CO (Figure 1). The corresponding RAIRS data for (r)-NEA adsorbed on Pd(111) at ∼80 K are displayed in Figure 6 as a function of coverage. The top trace in Figure 6 displays the infrared spectrum of ∼10 ML of (r)-NEA on Pd(111), and the frequencies and their assignments are summarized in Table 1 on the basis of assignments for naphthalene54 and ethylamine55,56
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Figure 6. Reflection absorption infrared spectra of (r)-NEA adsorbed on clean Pd(111) as a function of coverage. The (r)-NEA coverages are marked adjacent to the corresponding spectra. The (r)-NEA coverages were determined using the data in Figure 1 (see text).
TABLE 1: Vibrational Assignments for (s)-NEA Based on the Vibrational Spectra for Naphthalene42 and Ethylamine43,44a (s)-NEA vibrational frequency/cm-1 10 ML/Pd(111)
assignment
782, 802 873 914 1122 1168 1259 1365 1396 1447 1512 1598
naphthyl, B3u, NH2 wag CC symmetric stretch naphthyl, B3u CH3 rock, naphthyl, B1u naphthyl, B2u naphthyl, B1u CH3 symmetric deformation, naphthyl, B2u CH2 wag, naphthyl, B1u CH3 asymmetric deformation naphthyl, B2u naphthyl, B1u
a The symmetries for the naphthyl vibrational modes are for the D2h point group of naphthalene.
assuming that the NEA spectrum is a superposition of the features of the individual moieties. The infrared spectrum of multilayers of naphthalene deposited on the Pd(111) single crystal is in good agreement with published data. It is not clear which of the most intense features at 782 and 802 cm-1 can be assigned to the B3u ring mode and the NH2 wagging mode since they are relatively close in frequency. The multilayer of naphthalene alone has its most intense feature at 777 cm-1 due to the B3u out-of-plane ring mode, while the NH2 wagging mode is at 773 cm-1 in the gauche conformation of ethylamine and at 789 cm-1 in the trans conformation. The most intense features in the multilayer spectrum are due to an out-of-plane bending mode of the naphthyl ring and an NH2 wagging mode (782 and 802 cm-1) with all of the other features being substantially weaker. When the NEA coverage is lower than a monolayer, the only clearly discernible feature is at ∼1365 cm-1, which may contain contributions from the B1u ring mode and the symmetric methyl bending mode, while the 782 and 802 cm-1 modes are essentially absent. However, the B1u mode is very weak in naphthalene, suggesting that the ∼1365 cm-1 peak is due to the symmetric methyl bending mode,
Burkholder et al. implying, based on the surface infrared selection rules,57 that the dynamic dipole moment of this mode is oriented close to perpendicular to the surface. Since this is assigned to a symmetric methyl mode, this suggests that the C-C bond of the methyl group is oriented close to perpendicular to the surface. The corresponding stretching modes are only weakly evident since the infrared detector used to collect these spectra is not very sensitive in this region. Increasing the NEA coverage to 1.25 ML results in additional intense features appearing at ∼782 and 802 cm-1. The absence of in-plane ring modes of B1u and B2u symmetry at this coverage (1.25 ML) suggests that the plane of the naphthyl ring still lies close to parallel to the surface. However, the appearance of the features at ∼782 and 802 cm-1 at higher coverages may suggest that it adopts a more tilted or random adsorption mode at higher coverages. The origin of the absence of the ∼782 and 802 cm-1 modes at submonolayer coverages is not clear. One possible, although rather unphysical, explanation is that the naphthyl ring is oriented perpendicular to the surface so that these modes are therefore symmetry forbidden, although this geometry would result in the appearance of modes with B1u or B2u symmetry that are not seen. Naphthalene adsorbs onto a Ag(111) surface with the molecular plane parallel to the surface, although it tilts in the second layer and the B3u naphthalene modes are clearly detected on this surface in accord with this geometry.58,59 However, NEXAFS experiments for NEA adsorbed on Pt(111) suggest that the presence of the ethylamine group causes the molecule to tilt with respect to the surface.22 One possible explanation for the absence of the NH2 and naphthyl ring modes is that their frequencies are lowered due to bonding of the amine group with the surface and thus not detectable within the range that can be accessed by our spectrometer. It is not unreasonable to assume that the lone pair of the amine group can interact with the surface and significant red shifts of the NH2 wagging mode by 30-50 cm-1 have been observed for amines on transition-metal surfaces,60-62 and the B3u mode of naphthalene itself is found to shift to slightly lower frequencies on Pd(111) in the monolayer. In addition, a geometry in which the amine group is bonded to the surface would lead to the C-CH3 bond being oriented close to perpendicular to the surface. Thus, the appearance of the peaks at ∼782 and 802 cm-1 will be taken, in the following, to indicate that the amine group in NEA is no longer bonded directly to the surface. The effect of annealing a surface covered by 1.8 ML of (s)NEA to various temperatures is displayed in Figure 7. These experiments were performed by heating to the indicated temperature for a period of 10 s and allowing the sample to cool once again to 80 K, following which the infrared spectrum was recorded. Shown for comparison is the 51 amu (NEA) TPD profile also collected at a relative coverage of 1.8 ML. The peak at ∼1371 cm-1 is almost unaffected by heating to ∼400 K and only disappears after the sample has been heated to ∼450 K, due to the onset of the thermal decomposition of NEA (Figure 3). However, heating to ∼300 K results in a significant diminution in intensity of 782 and 802 cm-1 peaks, with the 802 cm-1 feature decreasing relatively more rapidly in intensity. By ∼350 K, the ∼802 cm-1 feature has almost disappeared, both features have attenuated substantially by 400 K, due to NEA desorption (see inset), and both disappear almost completely on heating to ∼450 K. 3.3. Enantioselective Adsorption on NEA-Modified Pd(111). Initial experiments were carried out using (r)- and (s)-propylene oxide as a probe for enantioselectivity. In this case, an enantioselective ratio, defined as {Θ((s)-PO)/(s)-NEA)}/{Θ((r)-
2-Butanol on R-(1-Naphthyl)ethylamine/Pd(111)
Figure 7. Reflection absorption infrared spectra of 1.8 ML of (s)NEA adsorbed on clean Pd(111) as a function of annealing temperature. The annealing temperatures are marked adjacent to the corresponding spectra. The (s)-NEA coverage was determined using the data in Figure 1 (see text). Shown as an inset is a 51 amu (NEA) TPD profile collected at the same coverage (1.8 ML) as that used to collect the infrared data.
Figure 8. 2-Butanol (45 amu) temperature-programmed desorption data for (a) (r)- and (s)-2-butanol adsorbed on 0.54 ML of (r)-1-(1naphthyl) ethylamine ((r)-NEA) and (b) (r)- and (s)-2-butanol adsorbed on 0.88 monolayers of (r)-NEA adsorbed on Pd(111) at 80 K collected using a heating rate of 2.7 K/s, where the NEA coverages are marked adjacent to the corresponding spectra and are measured using CO titrations (see text).
PO)/(s)-NEA)} where Θ((s)-PO) and Θ((r)-PO) refer to the coverages of (s)- and (r)-propylene oxide, respectively, measured by TPD, is unity, within experimental error (see the Supporting Information). This indicates that NEA does not provide an effective enantioselective modifier for propylene oxide. Experiments were also carried out using 2-butanol as a probe. Shown in Figure 8 are the 45 amu (the most intense fragment of 2-butanol in our mass spectrometer) TPD data obtained following adsorption on a surface covered by 0.54 ML of (r)NEA (Figure 8a) and by 0.88 ML of (r)-NEA (Figure 8b), where
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Figure 9. Plot of the enantioselectivity ratio for the adsorption of (s)and (r)-2-butanol on (r)-NEA modified Pd(111) as a function of NEA coverage for NEA adsorbed on the palladium surface (9) and on top of the (r)-NEA (O).
the NEA was dosed at 80 K, where these refer to relative coverages. It has been found previously that 2-butanol desorbs from Pd(111) between ∼285 and 235 K depending on coverage.63 The feature at ∼240 K is thus assigned to desorption from the clean Pd(111) surface, and the intensity of this feature decreases with increasing NEA coverage and is completely absent when the coverage exceeds ∼0.75 ML due to blocking by NEA; thus, 2-butanol is blocked more rapidly than 13CO (Figure 1). Clearly, there is enhanced adsorption of (r)compared to (s)-2-butanol on a surface modified by (r)-NEA. In addition, a small amount (a few percent) of 2-butanone is formed from surface 2-butoxide species that then undergo β-hydride elimination to form the ketone, but this is sufficiently small that it does not influence the enantioselectivity measurements. A feature is detected at ∼155 K (Figure 8a), and as the NEA coverage increases, the ∼240 K feature is suppressed and only a feature at ∼140 K is evident (Figure 8b). Note that there is no evidence for the formation of a ketone from 2-butanol giving rise to the low-temperature feature. The chemisorptive enantioselectivity is measured from the integrated desorption areas between ∼140 and 155 K and at ∼240 K (see Figure 8) following the adsorption of (s)- and (r)2-butanol on various coverages of (r)-NEA where the enantioselectivity ratio (ER) is calculated from {Θ((r)-2-butanol)/(r)NEA)}/{Θ((s)-2-butanol)/(r)-NEA)}, where Θ((s)-2-butanol) and Θ((r)-2-butanol) refer to the coverages of (s)- and (r)-2butanol respectively. This is related to the enantiomeric excess (ee) by the following: ee ) (ER - 1)/(ER + 1). The results are plotted in Figure 9 for the low- (9) and high-temperature (O) states. Below an (r)-NEA coverage of ∼0.4 ML, 2-butanol desorbs only at ∼240 K, while above (r)-NEA coverages of ∼0.75 ML it desorbs exclusively in the ∼140-155 K state. Two maxima are found for the enantioselective ratio. The first is for 2-butanol which desorbs at ∼240 K (9), which shows a peak over a narrow NEA coverage range centered at ∼0.55 ML, which has a maximum value of ER ) 2.0 ( 0.1 (ee ) 33 ( 2%). Similar data are plotted for the ∼140-155 K state for desorption from a surface with higher NEA coverages (Figure 9) showing that this surface also exhibits enantioselective chemisorption (O). This, however, occurs over a much wider coverage range with a maximum value of ER ) 1.8 ( 0.2 (ee ) 29 ( 4%) at a coverage of ∼0.85 ML.
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Burkholder et al. clearly identified by comparison with the spectrum in Figure 10a, and the additional features due to NEA are marked by asterisks. Most striking is the observation that the spectrum for (r)-2-butanol on a surface covered by (r)-NEA (Figure 10b) shows the distinct features at 782 and 802 cm-1 that are seen neither on the clean surface nor in the presence of (s)-2-butanol (Figure 10c).
Figure 10. Infrared spectrum of (a) 4.5 L 2-butanol adsorbed on Pd(111) at 80 K compared with the spectra of 4.5 L of (b) (r)-2-butanol and (c) (s)-2-butanol adsorbed on surfaces precovered with 1.0 ML of (r)-NEA, then warmed to 150 K to remove multilayer 2-butanol.
TABLE 2: Vibrational Assignments of RAIRS Signals for (r)-2-Butanol Adsorbed on Clean Pd(111) and for (r)- and (s)-2-Butanol Adsorbed on (r)-NEA-Covered Pd(111)
assignmenta
2-butanol/ Pd(111), multilayer
(r)-2-butanol/ (r)-NEA/ Pd(111)
(s)-2-butanol/ (r)-NEA/ Pd(111)
δa(CH3) δS(CH3) δip(OH), δ(CH) F(CH3) ν(CO) ν(CC), F(CH3) F(CH3) ν(C-C-C), F(CH3) F(CH2) νS(C-C-O)
1460 1377 1330 1158 1122 1034 992 966 914 820
1457 1400
1457 1400 1317 1158 1122 1034 992 966 914 814
1158 1122 1034 992 966 914 810
ν stretching; δ deformation; F rocking; s symmetric; a asymmetric; ip in-plane. a
In order to attempt to probe the interactions that are responsible for the enantioselectivity displayed in Figure 9, infrared data were collected for (r)- and (s)-2-butanol adsorbed on an (r)-NEA-covered surface at an (r)-NEA coverage of ∼1 ML (Figure 10). These data were collected for an NEA coverage of 1 ML, rather than at the coverage which exhibits the maximum enantioselectivity (0.85 ML) since it is easier to precisely calibrate the coverage of one monolayer of NEA in the infrared spectrometer as this occurs just before the appearance of the intense ∼782 and 802 cm-1 features thereby ensuring identical NEA coverages are used for each experiment. Shown for comparison is the spectrum for (s)-2-butanol alone adsorbed on clean Pd(111) at 80 K (Figure 10a). These features have been assigned previously and the assignments are summarized in Table 2 for (s)-2-butanol on clean and (r)-NEA-covered Pd(111),whichareslightlymodifiedfrompreviousassignments,30,31 with the in-plane OH bending mode being assigned to the peak at 1330 cm-164 rather than at 1158 cm-1.65 The spectrum obtained for (r)-2-butanol on an (r)-NEA-modified surface is shown in Figure 10b and that for (s)-2-butanol on the same surface shown in Figure 10c. The features due to 2-butanol are
4. Discussion 4.1. NEA Adsorption and Reaction on Pd(111). The adsorption of NEA has been studied previously on Pt(111) surfaces, and a tilt angle of 46 ( 5° of the naphthyl plane was measured using near-edge X-ray absorption fine structure (NEXAFS) and was found to bond via the naphthyl ring and the amine group.22 The coverage of NEA on Pd(111) surface was gauged using 13CO titrations (Figure 1) allowing the coverage to be presented for the subsequent results, where the absolute saturation coverage at completion of the first monolayer is 0.10 ( 0.02 monolayers from XPS and 0.099 ( 0.005 monolayers by STM (referenced to the number of palladium atoms on the (111) surface; Figure 2). This value is larger than the corresponding saturation coverage reported for Pt(111), which is ∼1 NEA molecule per 18 platinum atoms (yielding a coverage of ∼0.06). On the basis of the van der Waals’ areas of naphthalene (∼60 Å2)66,67 or the dimensions measured in STM (∼70 Å2)22 compared to the area occupied by a palladium atom on the (111) surface (∼6.6 Å2), close-packed coverages for flat-lying NEA on Pd(111) can be expected to be between 0.094 and 0.11 molecules per substrate palladium atom, in good agreement with the measured values. Note that this would allow the NEA to be accommodated in a configuration in which the naphthyl ring is close to parallel to the surface. It is notable in the blocking data of Figure 1 that ∼11% of the 13CO found on the clean surface can still adsorb onto the surface at an NEA coverage of 1 ML and is only completely suppressed at higher NEA coverages. This is also evident in the STM images (Figure 2), where there are small, relatively ordered areas where the intermolecular spacing is ∼8 Å with less-ordered regions in between that could accommodate (labeled) CO. One possible explanation for this effect might be that NEA adsorbs at some sites on the surface but is relatively immobile because of strong bonding to the surface. If such random adsorption results in gaps between the adsorbed NEA that are too small to easily accommodate additional NEA, these gaps, as shown in Figure 2, will be available to adsorb additional 13 CO. The data of Figure 5 show that additional NEA can be accommodated onto the surface as the exposure exceeds that needed to produce 1 ML, finally resulting in complete suppression of 13CO adsorption (Figure 1), and the additional NEA desorbs molecularly (Figure 7). It is likely that adsorbing the additional NEA will sterically prevent all of the NEA molecules from being bonded with the naphthyl ring parallel to the surface and may account for the tilt angle measured on Pt(111) by NEXAFS. Whether this occurs by the additional NEA being adsorbed in a highly tilted geometry in the interstices between the previously adsorbed NEA or whether this occurs by a concerted tilt of all NEA molecules is not known. NEA decomposes on Pt(111) predominantly by desorption of hydrogen as well as N2, HCN, and C2N2.22 Similar results are found for the chemistry on Pd(111). Close to saturation, hydrogen desorption from Pt(111) displays three features at 410, 550, and 650 K, corresponding to the features seen at ∼500, 625, and 700 K from Pd(111) (Figure 3), the higher temperatures reflecting the lower reactivity of palladium than platinum. No
2-Butanol on R-(1-Naphthyl)ethylamine/Pd(111) cyanogen or nitrogen was found to desorb from Pd(111) while HCN desorbed at ∼575 K with a tail extending to above 800 K on Pd(111) (Figure 4), while it desorbs at ∼550 K with a high-temperature tail from Pt(111).22 The effect on the infrared spectrum of heating a surface covered by ∼1.8 ML of NEA is shown in Figure 7. Shown as an inset is the 51 amu (NEA) desorption spectrum collected at the same coverage. The peaks at ∼782 and 802 cm-1 attenuate slightly on heating to ∼250 K, corresponding to the onset of the ∼260 K NEA desorption peak. Heating to ∼300 K results in a more drastic diminution in intensity of these features corresponding to the completion of the ∼260 K desorption state. However, in this case, the 802 cm-1 peak appears to attenuate more rapidly than the ∼782 cm-1 feature. Since the amine group is likely to be more reactive than the naphthyl ring, this may suggest some dehydrogenation of the amine group and implies that the 802 cm-1 mode is associated with the amine group. The methyl mode at ∼1365 cm-1 is still present indicating that the methyl group remains intact. Note that the ∼782 and 802 cm-1 peaks are only detected for NEA coverages that exceed one monolayer and thus provide no information regarding the NEA decomposition for the initially adsorbed NEA. Dehydrogenation of the amine group over this temperature range will only desorb a small proportion of the total hydrogen, which will not easily be detected in TPD (Figure 3). The ∼782 and 802 cm-1 features continue to decrease in intensity on heating to ∼400 K and are almost absent when the surface has been annealed to ∼450 K. This is in accord with the TPD data (see Figure 7, inset). The methyl mode at ∼1365 cm-1 persists even on heating to ∼400 K indicating that it is stable to this temperature and this mode only disappears on heating to ∼450 K. This is in accord with the TPD data where the onset of significant hydrogen desorption occurs at 450 K (Figure 3). However, hydrocarbon fragments remain on the surface that are invisible to infrared spectroscopy, and these fragments finally desorb as hydrogen (Figure 3) and HCN (Figure 4) at higher temperatures. 4.2. Enantioselective Adsorption on NEA-Modified Pd(111). Enantioselective chemisorption of propylene oxide and 2-butanol was explored on NEA-covered surfaces. Experiment with propylene oxide showed no enantioselectivity (see the Supporting Information). However, measuring the enantioselective adsorption of 2-butanol on NEA-covered Pd(111) (Figure 9) shows two distinct enantioselective regimes. In the first regime, a maximum enantioselective ratio of ∼2 is measured at an NEA coverage of ∼0.55 ML, and this occurs over a rather narrow coverage range. A second enantioselective regime is found at higher NEA coverages with a maximum enantioselectivity ratio of ∼1.8 at a coverage of ∼0.85 (Figure 9). The desorption temperature of 2-butanol in the first enantioselective regime (peaking at an NEA coverage of 0.55) is ∼240 K, close to that found for 2-butanol desorption from clean Pd(111). This indicates that 2-butanol, in this case, is similarly bonded to the surface, via its oxygen lone pairs.63 Hydrogen bonding interactions have previously been shown to be important in the enantioselective interaction between propylene oxide and 2-butanol (where 2-butanol was used as a chiral modifier in that case). The amine group will provide the major hydrogenbonding site in NEA, although it has also been suggested that the hydrogens on the naphthyl ring are also capable of forming hydrogen bonds on Pt(111).27-29 Precise hydrogen-bonding energies are not available for all of these surface interactions. However, empirical relationships between the strength of the hydrogen bond and the difference in the pKA of the hydrogen
J. Phys. Chem. C, Vol. 113, No. 31, 2009 13883 donor and the pKB of the acceptor (∆pKa) can be useful in gauging the relative strengths of possible interactions where smaller values of ∆pKa yield relatively stronger hydrogen bonds.68 The hydrogen-bond strength between an amine donor and a cyclic ether acceptor (∆pKa ∼ 43) is classified as weak and may account for the lack of enantioselectivity when using propylene oxide as a chiral probe. If an amine group bonded to the surface can only act as a hydrogen donor and the hydroxyl group as a hydrogen acceptor, then this would lead to a value of ∆pKa ∼ 40 and would also be a weak interaction. However, if the amine can be a hydrogen-bonding acceptor and the 2-butanol acts as a donor, then the value of ∆pKa is about 6 leading to a medium strong interaction. As argued above, the amine group is likely to be coordinated to the surface, thus weakening this interaction. For comparison, the ∆pKa for hydrogen bonding between two 2-butanol molecules is ∼20 leading to a medium strength interaction. If the 2-butanol hydrogen-bond strengths are larger than that between 2-butanol and NEA in the low-coverage regime, this would account for the lack of enantioselectivity found at low NEA coverages. In this case, the 2-butanol molecules would preferentially hydrogen bond to each other rather than interact with NEA. As the NEA coverage increases, it may be that the available space precludes the adsorption of two adjacent 2-butanol molecules so that 2-butanol then hydrogen bonds enantiospecifically to NEA. Note that 2-butanol adsorption is suppressed more rapidly by NEA than is carbon monoxide (Figure 1) and is completely blocked from direct contact with the Pd(111) surface at the NEA coverage of ∼0.75 ML. More detailed considerations of such effects will rely on information on the way in which NEA is distributed on the surface at lower coverages and STM experiments are underway to measure the NEA distribution at lower coverages. A second enantioselective regime appears at higher NEA coverages (above ∼0.6 ML) where 2-butanol adsorption onto Pd(111) via the oxygen lone pairs is now suppressed. In order to explore the nature of the enantioselective interactions in this coverage regime, Figure 10 compares the infrared spectrum of (s)-2-butanol adsorbed on clean Pd(111) (Figure 10a) with those of (r)- and (s)-2-butanol adsorbed on (r)-NEA (Figure 10b and c, respectively). The peak assignments of the 2-butanol are summarized in Table 2. Of particular interest is the δip(OH) mode, which appears at 1330 cm-1 for 2-butanol on Pd(111). This mode shifts in frequency depending on whether the O-H groups are hydrogen bonded, where the mode at 1330 cm-1 indicates that the O-H groups are not hydrogen bonded, and shifts to ∼1400 cm-1 when alcohols are hydrogen bonded.69 A shoulder is evident at ∼1400 cm-1 in the spectrum of Figure 10a, which grows with 2-butanol exposure due to this effect. The vibrational modes for (r)- and (s)-2-butanol adsorbed on (r)-NEA-covered Pd(111) can be assigned by direct comparison with the spectrum on clean Pd(111) and are summarized in Table 2, and the remaining features due to NEA itself are marked by asterisks in Figure 10b. The majority of the features for 2-butanol on NEA/Pd(111) have identical vibrational frequencies to those on the clean surface, although there are slight shifts in the δS(CH3) and νS(C-C-O) modes. However, the nonhydrogen bonded O-H bending mode is still discernible for (s)-2-butanol on (r)-NEA, albeit relatively weaker than on clean Pd(111), while there is no detectable feature at ∼1330 cm-1 for (r)-2-butanol on (r)-NEA. This implies that this mode is shifted due to hydrogen bonding. Hydrogen bonding to another hydroxyl group would increase the frequency to ∼1400 cm-1,69 while the weaker hydrogen bond to an amine group would be
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expected to result in a somewhat smaller shift and the resulting, relatively weak peak may be obscured by the relatively intense methyl deformation mode at ∼1377 cm-1 (Table 2). Nevertheless, these results suggest that hydrogen-bonding interactions are also important in imparting enantioselectivity to 2-butanol adsorption of NEA-covered Pd(111) at higher NEA coverages. The enantioselectivity in the high-NEA coverage regimes attains its maximum value at an NEA coverage of ∼0.85 ML (Figure 9), where the TPD data indicate that, at this NEA coverage, 2-butanol is no longer bonded to the Pd(111) surface from the absence of the ∼240 K desorption state (Figure 8). Note, however, that there are still vacant sites on the surface since CO can still adsorb on the Pd(111) surface at this coverage (Figure 1) and additional NEA can also adsorb (Figure 5). If the O-H group is not adsorbed on the Pd(111) surface, it will be sterically hindered from interacting with a surface-bonded NH2 group. Some hint as to how this interaction might occur comes from the features assigned to (r)-NEA (marked by asterisks in Figure 10b). Note that, at the NEA coverage used to collect the data shown in Figure 10, the infrared spectrum exhibits only very weak signals at 782 and 802 cm-1 (Figure 6). However, when (r)-2-butanol is adsorbed onto (r)-NEAcovered Pd(111), these features become more intense, while they are essentially absent when NEA is coadsorbed with (s)-2butanol. It was suggested above that the absence of these modes for NEA-coverages less than one monolayer adsorbed directly onto the Pd(111) surface (Figure 6) was due to the peaks being shifted out of the range that is accessible by our spectrometer due to the bonding of the NH2 group to the palladium surface. If this is indeed the case, the appearance of the 802 and 782 cm-1 features implies that these are no longer bonded to the surface and are shifted away from the surface to allow hydrogen bonding with (r)-2-butanol on (r)-NEA, but not with (s)-2butanol. Care must be taken with such a simple interpretation since hydrogen bonding to the NH2 group would be expected to increase it vibrational frequency.70 Nevertheless, the difference in the spectral region between 780 and 810 cm-1 does suggest difference conformations of (r)-NEA in the presence of (s)-or (r)-2-butanol. As further indication of a hydrogen-bonding interaction between the O-H group and the amine, the strength of this hydrogen-bonding interaction has been measured calorimetrically71 and from the variation in vapor pressures of mixtures,72 which leads to an estimate of the hydrogen-bonding interaction energy of ∼35 kJ/mol. The activation energy for 2-butanol desorption at high NEA coverages (with a peak temperature of ∼140-155 K; Figure 8) using the Redhead equation73 leads to a desorption activation energy of ∼33-35 kJ/mol. This is in reasonable agreement with the energy of the OH · · · NH2 hydrogen bond and provides additional evidence that this might be the interaction responsible for 2-butanol adsorption at high NEA coverages. Thus, two distinct chemisorptive enantioselective regimes are found for 2-butanol adsorption on NEA-modified Pd(111). The first yields a maximum enantioselectivity ratio of ∼2.0 at a relative NEA coverage of ∼0.55 ML. This is due to 2-butanol enantiospecifically interacting with NEA adsorbed on the surface via the hydroxyl group suggesting that this regime occurs due to a hydrogen-bonding interaction between the O-H of 2-butanol adsorbed onto the surface and the amine group of NEA also bonded to the surface. It has been suggested, based on the observation that NEA does not form ordered structures on Pt(111), that it acts as a one-to-one modifier and not in concert to form chiral adsorption sites (i.e., as a template).21 However,
Burkholder et al. the observation that the enantioselectivity peaks over a relatively narrow coverage range centered at an NEA coverage of ∼0.55 ML (Figure 9) implies that there is a templating component to the interaction. An analogous coverage dependence has been observed for the chemisorptive enantioselectivity of propylene oxide on NEA-modified Pt(111).74 One possible origin of the lack of enantioselectivity for NEA coverage below ∼0.5 ML may be that the 2-butanols interact more strongly with each other than they do with NEA and that chemisorptive enantioselectivity only occurs when the openings between the NEA molecules are sufficiently small to prevent more than one 2-butanol molecule from adsorbing in the resulting vacant site. In this case, 2-butanol would still interact directly with one NEA molecule, formally a one-to-one interaction, and the role of other NEA molecules would be to restrict the adsorption of more than one 2-butanol at a particular adsorption site. Testing the veracity of such a model will rely on adsorbed NEA being relatively randomly adsorbed on the surface at lower coverages, and this effect is currently being explored using STM. The second regime yields a maximum enantioselective ratio of ∼1.8 at a relative NEA coverage of ∼0.85 ML (Figure 9). In this regime, the O-H group of 2-butanol can no longer interact with the Pd(111) surface. However, infrared spectroscopy (Figure 10) suggests the presence of enantiospecific hydrogen bonding of 2-butanol and perhaps some reorientation of the amine group to allow this interaction to occur to form a docking complex.6,75 The desorption activation energy of this 2-butanol is in accord with a hydrogen-bonding interaction between an O-H and amine and suggests that the chiral interaction can be quite adaptive. Again, the enantioselectivity in this regime varies with coverage, symptomatic of templating behavior. Presumably, the increase in enantioselectivity at higher coverages reflects the growth of regions of the surface completely covered by NEA, where the 2-butanol cannot adsorb directly onto the surface. It should also be mentioned that two enantioselective regions have been observed for cinchonidine-modified platinum catalysts,76 where the behavior is strikingly similar to that shown in Figure 9. In that case, the high-coverage regime was correlated with a tilt of the cinchonidine with respect to the surface, analogous to the behavior observed here, suggesting that there is a correspondence between the behavior measured in ultrahigh vacuum and the chiral catalyst. 5. Conclusions NEA adsorbs on Pd(111) in a state in which the amine group interacts with the Pd(111) surface, with a saturation coverage of ∼0.1 monolayers, referenced to the number of palladium atoms on the (111) surface, where some carbon monoxide can still be accommodated onto the surface due to the presence of vacancies in the NEA overlayer. However, additional NEA can still adsorb onto this surface, and the amine group appears not to be bonded to the surface in this case. Propylene oxide does not adsorb enantioselectively on an NEA-modified surface, while enantioselective chemisorption of 2-butanol is found in two NEA-coverage regimes, suggesting that hydrogen-bonding interactions play an important role in defining enantioselectivity. The first regime occurs at a relative NEA coverage of ∼0.55 monolayers, where 2-butanol adsorbs on the Pd(111) substrate, with a maximum enantioselectivity ratio of ∼2 (corresponding to an ee of ∼33%). A second regime is found in which 2-butanol adsorbs on top of an NEA-covered surface with a maximum enantioselectivity ratio of ∼1.8 (corresponding to an ee of ∼29%). This interaction may cause
2-Butanol on R-(1-Naphthyl)ethylamine/Pd(111) the NH2 group to reorient to facilitate hydrogen bonding interactions between 2-butanol and the amine group, and the heat of adsorption, in this case, is typical of -OH · · · NH2 hydrogen bond strengths of ∼35 kJ/mol. Acknowledgment. We gratefully acknowledge support of this work by the US Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under grant number DE-FG02-03ER15474 and the National Science Foundation under grant number CHE 0521328. Supporting Information Available: Plot of the enantioselectivity ratio for the adsorption of (s)- and (r)-propylene oxide on (s)-NEA modified Pd(111) as a function of NEA coverage. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Horvath, J. D.; Koritnik, A.; Kamakoti, P.; Sholl, D. S.; Gellman, A. J. J. Am. Chem. Soc. 2004, 126, 14988. (2) Power, T. D.; Sholl, D. S. J. Vac. Sci. Technol. A 1999, 17, 1700. (3) Gellman, A. J.; Horvath, J. D.; Buelow, M. T. J. Mol. Catal. A 2001, 167. (4) Zhao, X. Y.; Zhao, R. G.; Yang, W. S. Langmuir 2000, 16, 9812. (5) Zhao, X.; Perry, S. S.; Horvath, J. D.; Gellman, A. J. Surf. Sci. 2004, 563, 217. (6) Hoek, A.; Sachtler, W. M. H. J. Catal. 1979, 58, 276. (7) Izumi, Y. AdV. Catal. 1983, 32, 215. (8) Osawa, T.; Harada, T.; Tai, A. Catal. Today 1997, 37, 465. (9) Keane, M. A. Langmuir 1997, 13, 41. (10) Orito, Y.; Imai, S.; Niwa, S.; Gia, H. N. Yuki Gosei Kagaku Kyokaishi 1979, 37, 173. (11) Orito, Y.; Imai, S.; Niwa, S. J. Chem. Soc. Jpn. 1979, 1118. (12) Blaser, H.-U.; Jalett, H.-P.; Mu¨ller, M.; Studer, M. Catal. Today 1997, 37, 441. (13) Baiker, A. J. Mol. Catal. A 1997, 115, 473. (14) Wells, P. B.; Wilkinson, A. G. Top. Catal. 1998, 5, 39. (15) LeBlond, C.; Wang, J.; Liu, J.; Andrews, A. T.; Sun, Y.-K. J. Am. Chem. Soc. 1999, 121, 4920. (16) Webb, G.; Wells, P. B. Catal. Today 1992, 12, 319. (17) Raval, R. CATTECH 2001, 5, 12. (18) Humblot, V.; Barlow, S. M.; Raval, R. Prog. Surf. Sci. 2004, 76, 1. (19) Ortega Lorenzo, M.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376. (20) Stephenson, M. J.; Lambert, R. M. J. Phys. Chem. B 2001, 105, 12832. (21) Bonello, J. M.; Sykes, E. C. H.; Lindsay, R.; Williams, F. J.; Santra, A. K.; Lambert, R. M. Surf. Sci. 2001, 482-485, 485–207. (22) Bonello, J. M.; Williams, F. J.; Lambert, R. M. J. Am. Chem. Soc. 2003, 125, 2723. (23) Diezi, S.; Hess, M.; Orglmeister, E.; Mallat, T.; Baiker, A. 2005, 239, 49. (24) Maris, M.; Mallat, T.; Orglmeister, E.; Baiker, A. J. Mol. Catal. A: Chem. 2004, 219, 371. (25) Minder, B.; Schurch, M.; Mallat, T.; Baiker, A.; Heinz, T.; Pfaltz, A. J. Catal. 1996, 160, 261–268. (26) Heinz, T.; Wang, G.; Pfaltz, A.; Minder, A.; Schu¨rch, M.; Mallat, T.; Baiker, A. J. Chem. Soc., Chem. Commun. 1995, 1421. (27) Lavoie, S.; Laliberte´, M.-A.; McBreen, P. H. J. Am. Chem. Soc. 2003, 125, 15756. (28) Lavoie, S.; Laliberte´, M.-A.; Temprano, I.; McBreen, P. H. J. Am. Chem. Soc. 2006, 128, 7588. (29) Lavoie, S.; Mahieu, G.; McBreen, P. H. Angew. Chem. 2006, 45, 7404. (30) Stacchiola, D.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2002, 124, 8984. (31) Stacchiola, D.; Burkholder, L.; Tysoe, W. T. J. Mol. Catal. A: Chem. 2004, 216, 215.
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