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J. Phys. Chem. C 2010, 114, 17836–17844
BINAP Adsorption on Palladium: A Combined Infrared Spectroscopy and Theoretical Study Sven Reimann,† Atsushi Urakawa,†,‡ and Alfons Baiker*,† Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Ho¨nggerberg, HCI, CH-8093 Zurich, Switzerland and Institute of Chemical Research of Catalonia (ICIQ), 43007 Tarragona, Spain ReceiVed: July 29, 2010; ReVised Manuscript ReceiVed: September 13, 2010
The adsorption of (rac)-BINAP on palladium nanoparticles as well as on a Pd film has been investigated with the aim to unravel the adsorption geometry of BINAP applied for the chiral modification of Pd metal surfaces in catalysis. The Pd nanoparticles with a narrow size distribution were characterized by attenuated total reflection (ATR)-IR spectroscopy. The adsorption geometry of BINAP was determined from the analysis of the experimental IR spectra and their comparison with the spectrum calculated by DFT using a model where BINAP is adsorbed on the (111) surface of a Pd cluster. The studies revealed that BINAP adsorbs with its C2 symmetry axis perpendicular to the Pd surface, and due to the steric hindrance by the phenyl rings connected to the two P atoms, the main interaction with the surface occurs via the two aromatic rings that are oriented parallel to the surface and not directly via the two P atoms. The remaining two phenyl rings are orientated in a tilted position toward the surface. To examine the generality of the proposed adsorption geometry, we also studied the adsorption of (rac)-BINAP from an organic liquid phase on a Pd model film by in situ ATR-IR spectroscopy combined with modulation excitation spectroscopy (MES). Two methods for enhancing the transient sorption of BINAP were applied: (a) temperature increase, and (b) introduction of coadsorbates which could compete with BINAP for adsorption sites on the Pd surface. The latter method proved to be powerful for adsorption studies at moderate temperature when the substrate strongly interacts with the surface and signals from surface adsorbates are very weak. 1. Introduction Chiral surfaces have attracted considerable interest in the past years because of their broad variety of possible applications such as enantioselective heterogeneous catalysis,1 chiral sensing,2,3 as well as chiral separation.4 Among the various methods used for the creation of chiral surfaces,1,5 the adsorption of chiral molecules on a metal surface proved to be most powerful.1 Several techniques like TEM,6-8 XPS and LEED,9,10 NEXAFS,11 IR,12-14 and SERS15 have been applied to investigate the properties and surface adsorption as well as reaction processes at chiral surfaces. However, apart from the thoroughly investigated metal surfaces modified with cinchona alkaloids1 and tartaric acid,16 relatively little is known about surfaces modified by other compounds used in asymmetric catalysis. In the past years, some highly enantioselective catalytic processes based on BINAP-stabilized nanoparticles17-19 or supported metal catalysts modified with BINAP20,21 have been reported. The first studies of tertiary phosphines adsorbed on different metals22-26 confirmed the considerably strong interaction between phosphines and metal surfaces. Enantiopure (R)or (S)-BINAP adsorbed on Au and Pd nanoparticles has also been studied. The optical activity of such particles was demonstrated by circular dichroism spectroscopy,19,27 but only little information emerges from these studies about the interaction of BINAP with the surface and about its adsorption * To whom correspondence should be addressed. E-mail: baiker@ chem.ethz.ch. † Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich. ‡ Institute of Chemical Research of Catalonia (ICIQ).
geometry. This information is obviously of central importance to comprehend the origin of enantioselection and further optimize the catalytic processes. Here we aim to uncover the adsorption configuration of BINAP on two kinds of Pd surfacessPd nanoparticles and a Pd model thin filmsby means of attenuated total reflection infrared (ATR-IR) spectroscopy. ATR-IR spectroscopy has proven to be a powerful tool to investigate surface processes, as only a small volume is probed by the evanescent electric field, which is formed at the interface between the internal reflection element (IRE) and the adjacent medium.12,28 Because of the small probed volume, samples can be measured very sensitively when they reside close to the IRE. For the elucidation of BINAP adsorption on the model Pd film, we studied the transient BINAP sorption behavior by ATR-IR spectroscopy in combination with modulation excitation spectroscopy (MES)29 to sensitively and selectively detect surface BINAP adsorbed on Pd. Spectral interpretations of surface-adsorbed BINAP were supported and verified by density functional theory (DFT) electronic structure calculations, explicitly treating the Pd surface by a cluster model. Such combined theoretical and spectroscopic studies have proven their value for the firm interpretation of spectral features in various cases.30,31 2. Experimental Section 2.1. Chemicals, TEM, and NMR. Ethylbenzene (99%, Sigma-Aldrich) and dichloromethane (99.5%, J.T.Baker) were dried and stored in sealed flasks over molecular sieve (4A, Zeochem). Pd(OAc)2 (99%, Fluka), (rac)-BINAP (g98%, Fluka), N2 (5.0, PanGas), 10% CO in He (4.6, PanGas) and 10% H2 in He (4.6, Messer) were used as received. TEM
10.1021/jp107089n 2010 American Chemical Society Published on Web 09/29/2010
BINAP Adsorption on Palladium investigations were performed using a Philips CM30ST microscope (FEI; LaB6 cathode operated at 300 kV, point resolution ≈ 2 Å). The scanning transmission electron microscopic (STEM) investigation was performed on a field emission transmission electron microscope Tecnai 30F (FEI; SuperTwin lens with Cs ) 1.2 mm), operated at 300 kV. For both TEM and STEM measurements, some droplets of the reaction mixture of the freshly prepared nanoparticles or of a solution of washed and dried nanoparticles dissolved in dichloromethane were deposited on a holey carbon foil supported on a copper grid. Solution 31P NMR spectra were recorded on a Bruker Avance 200 spectrometer at a resonance frequency of 81 MHz for 31P. For the NMR measurements the washed and dried Pd nanoparticles were dispersed in THF-d8 under N2. 2.2. Synthesis of the Pd Nanoparticles. The Pd nanoparticles were synthesized according to a known procedure with a slight modification.32 Briefly, (rac)-BINAP (0.051 mmol, 31.9 mg) and Pd(OAc)2 (0.064 mmol, 14.3 mg) were dissolved in dichloromethane (200 mL) under N2. After stirring the yellow solution for 30 min, the N2 atmosphere was replaced by 5 bar H2. After 24 h, the dark brown reaction mixture was washed once with a saturated NaHCO3 aqueous solution and twice with water. After removing dichloromethane under reduced pressure the remaining powder was washed with pentane until no trace of free (rac)-BINAP was detected by 31P NMR. The clean Pd nanoparticles were then dried for three days under vacuum (10-6 mbar) at room temperature. 2.3. ATR-IR Spectroscopy. IR spectra of the solid samples were measured at 4 cm-1 resolution on a Bruker VERTEX 70 spectrometer equipped with an ATR-IR attachment (Harrick, MVP) and a liquid nitrogen cooled MCT detector. The BINAP adsorption studies at the solid-liquid interfaces were also conducted using ATR-IR technique to minimize IR light absorption by the solvent. A home-built stainless steel flowthrough cell33 mounted on an ATR-IR attachment (OPTISPEC) within the sample compartment of a Bruker IFS-66/S spectrometer equipped with a liquid nitrogen cooled MCT detector was used. Spectra were recorded at 4 cm-1 resolution. A ZnSe crystal was used for the internal reflection element (IRE, bevel of 45°, 52 × 20 × 2 mm3, Crystran Ltd.). The cell temperature was regulated by means of a thermostat. The Pd film was prepared by physical vapor deposition using a Balzers BAE-370 vacuum coating system, as described in detail elsewhere.34 The coating materials (Al2O3, Balzers, 99.3% and Pd, Balzers, 99.99%) were heated in a graphite crucible by means of an electron beam. First, a 50 nm Al2O3 film was deposited on the IRE at a rate of 1.0 Å/s followed by the deposition of 2 nm Pd at a rate of 0.5 Å/s. The properties of the film characterized by XPS and STEM are reported elsewhere.35 Pure ethylbenzene and a 1 mM solution of (rac)-BINAP in ethylbenzene were stored in two separated glass bubble tanks, which allowed saturating the solutions with N2 or CO. The tanks were connected with the flow-through cell by Teflon tubing. Solutions were admitted into the cell by means of a peristaltic pump (ISMATEC, Reglo 100) located after the cell. The flow rate was set to 0.5 mL/min. Two interconnected pneumatically activated Teflon valves (PARKER PV-1-2324), synchronized with the acquisition of spectra, were used to control the liquid flow direction and selection from the two tanks. After purging the sample compartment of the spectrometer with dry air for 1 h, the Pd film was reduced in gas phase by purging 10% H2 in He through the cell for 1 h at 50 °C. Instead of a common liquid phase reduction of PdO and removal of
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17837 carbonates using a H2-saturated solvent, we have employed gas phase reduction because in liquid phase reduction water accumulation on the film could not be avoided. After the reduction, neat ethylbenzene saturated with N2 was admitted to the cell for 30 min. When experiments were carried out with CO, CO-saturated ethylbenzene was additionally admitted for 30 min over the film. After the stabilization of the signals, the transient adsorption behavior of BINAP was studied by means of MES30 by periodically switching between the admitted solution of (rac)-BINAP and neat solvent, in some cases saturated with CO. One period consisted of admission of (rac)BINAP containing solution for 600 s and subsequent admission of neat (N2 or CO-saturated) solution for 600 s. Fifty spectra were recorded per period. The initial three periods were used to stabilize the response to reach a quasi-stationary state. Afterward, six periods were measured and averaged into one period to enhance the signal-to-noise ratio. 2.4. Computational Method. Geometry optimization of BINAP and BINAP on Pd (111) surface and their vibrational (IR intensity and normal mode) analysis were performed with the unrestricted B3PW91 hybrid functional36,37 using Gaussian 03.38 The Pd surface was modeled by a cluster consisting of 64 Pd atoms. The Pd-Pd distance of the cluster was kept constant to the bulk parameters (2.75065 Å) while all degrees of freedom of BINAP were set free. The spin state with multiplicity of 15 was found to be the ground state of the Pd cluster, and therefore the same multiplicity was considered when BINAP is adsorbed on the Pd surface. A 6-311G(d,p) basis set was applied for all the atoms except Pd. For Pd, the LanL2DZ effective core potential basis set was used.39 All calculations were performed as an isolated system or complex without solvent effects. Calculated vibrational frequencies were scaled by the factor of 0.98. 3. Results and Discussion 3.1. Synthesis of (rac)-BINAP Modified Pd Nanoparticles. The (rac)-BINAP-modified Pd nanoparticles were synthesized by reducing palladium acetate with hydrogen in the presence of (rac)-BINAP in a substoichiometric amount, typically 0.8 equivalent relative to Pd. Varying the amount of (rac)-BINAP did not lead to changes in the particle size. However, at a lower (rac)-BINAP/Pd ratio of 0.6, some agglomeration of the particles was observed, as evidenced by TEM investigations (not shown). This might be due to lower stability of the Pd nanoparticle surface by the reduced amount of (rac)-BINAP. A similar effect was observed for Pd nanoparticles stabilized with various amines.40 In general, the amount of (rac)-BINAP must be carefully controlled to ensure that only the diphosphine is adsorbed on the Pd surface. Oxidized derivatives of (rac)BINAP, such as (rac)-BINAPO, also adsorb on Pd surfaces. However, their interaction with the metal is much weaker, so that (rac)-BINAP is able to replace these derivatives given that it is present in a comparably high concentration.41 Special care must be taken in the washing and subsequent drying procedures of the nanoparticles, since remaining water disturbs the IR measurement. TEM analysis of the nanoparticles before and after washing and drying did not show any changes in their size. As seen from the TEM and STEM pictures (Figure 1), the formed particles are spherical and of similar size with a diameter around 2 nm. A statistical analysis of the STEM images indeed revealed a narrow size distribution of the particles (Figure 2). The 31P NMR spectrum of the washed nanoparticles in THF shows one single signal at 26.7 ppm (Figure S1, Supporting Information). The observed chemical shift is typical for BINAP coordinated to Pd0.23 The appearance of one single 31P NMR signal indicates
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Figure 1. TEM and STEM images of freshly prepared (rac)-BINAP stabilized Pd nanoparticles.
Figure 2. Size distribution of (rac)-BINAP stabilized Pd nanoparticles. 1100 nanoparticles were analyzed for this purpose.
that there is only one kind of (rac)-BINAP containing Pd species present in the solution, e.g., only Pd nanoparticles. The absence of the signal of uncoordinated BINAP around -16 ppm shows that the Pd nanoparticles were purified efficiently, which is of great importance for the subsequent spectroscopy investigation of the Pd nanoparticles. 3.2. IR Study of (rac)-BINAP-Modified Pd Nanoparticles. IR spectroscopy combined with DFT calculations was employed to verify the presence of (rac)-BINAP on the surface of the Pd nanoparticles, assign the bands, and determine the adsorption geometry of the diphosphine. First, the IR bands of nonadsorbed, powdered (rac)-BINAP (Figure 3a) were assigned with the help of the calculated spectrum of isolated BINAP in vacuum (Figure 3c). A good agreement between the two spectra was found, although the calculated spectrum neglects the intermolecular BINAP-BINAP interactions, which are presumably present in the experimental (rac)-BINAP powder spectrum. The heavy P atoms of BINAP act as insulators and disconnect the vibrations of the different molecule parts from each other. Hence distinct vibrations from the phenyl rings as well as from the binaphthyl group were clearly observed. Several bands that are important for the identification of surface-adorbed (rac)-BINAP are described. The three bands at 1070, 1087, and 1113 cm-1 in the calculated spectrum for (rac)-BINAP powder were not well resolved in the calculated spectrum. The corresponding calculated band at 1089 cm-1 consists of contributions from both asymmetric and symmetric in-plane C-C-H deformation vibrations of the phenyl rings. In addition to the assignment by
Figure 3. Experimental ATR-IR spectra of (a) (rac)-BINAP powder and (b) (rac)-BINAP adsorbed on Pd nanoparticles as well as the calculated spectra of (c) (rac)-BINAP and (rac)-BINAP adsorbed on Pd nanoparticles (d) without and (e) with the surface selection rule consideration. The band that changes considerably upon BINAP adsorption on Pd are marked with asterisks.
the calculation, comparison of the experimental spectrum with that of free triphenylphosphine from the literature42 allowed to assign the bands as follows: 1070 cm-1 (Ph, C-C-H asym. def.), 1087 and 1113 cm-1 (Ph, C-C-H sym. def.). A broad band was observed at 1259 cm-1, consisting of asymmetric inplane C-C-H deformation vibrations from the naphthyl moiety. The signal at 1309 cm-1 can be attributed to in-plane C-C-H asymmetric deformation vibrations from the phenyl groups and is in agreement with the data reported for triphenylphosphine,42 although our calculation suggested some contributions from the naphthyl unit. Furthermore, three bands were found between 1400 and 1500 cm-1, at 1432, 1481, and 1500 cm-1. The first two bands are attributed to asymmetric (1432 cm-1) and symmetric (1481 cm-1) C-C stretching vibrations from the phenyl groups, whereas the broad band at 1500 cm-1 can be assigned to symmetric C-C stretching vibrations from the naphthyl moiety. Finally, the signal at 1583 cm-1 was assigned to symmetric C-C stretching vibrations from all four phenyl rings. Again, the vibrational bands of the phenyl groups are in good agreement with the literature.42
BINAP Adsorption on Palladium Figure 3b shows the IR spectrum of the (rac)-BINAPmodified Pd nanoparticles presented in the previous section. Obviously, the bands originating from (rac)-BINAP have significantly changed in their band intensity and position compared to those of the (rac)-BINAP powder (Figure 3a): the band at 1087 cm-1 was enhanced and blue-shifted by 10 cm-1 to 1097 cm-1. The broad band at 1259 cm-1 increased remarkably in intensity and a small blue-shift to 1263 cm-1 was observed. In contrast, the signal at 1309 cm-1 lost some intensity and shifted slightly to 1311 cm-1. Note that the intensity increase or decrease has been judged based on the relative band comparison in the whole range; it is not possible to compare the band absorbance of the (rac)-BINAP powder and the (rac)BINAP-modified Pd nanoparticles because of the completely different form of the samples. Between 1400 and 1500 cm-1, the band at 1482 cm-1 remarkably lost intensity and red-shifted slightly by 2 cm-1. The band at 1432 cm-1 was slightly blueshifted to 1434 cm-1. Alternatively, the band at 1583 cm-1 lost almost all of its intensity for the (rac)-BINAP-modified Pd nanoparticles. The observed band shifts are generally small and are a good indication of (rac)-BINAP coordination to the Pd surface. Similar band shifts of other tertiary phosphines upon adsorption on a metallic surface have been reported.42 In the case of (rac)BINAP, the shifts are not prominent because, similar to triphenylphosphine, the aromatic substituents and the increased steric hindrance weaken the interaction of the P atoms with the metallic surface. The only band that shows a considerable shift is at 1097 cm-1. In this case, a strong interaction of the corresponding phenyl group with the surface can be expected. 3.3. Theoretical Study of (rac)-BINAP Adsorbed on Pd(111) Surface. To gain deeper insights into the adsorption geometry of BINAP on Pd, the adsorption geometry of BINAP on a Pd (111) surface, taking a cluster consisting of 64 Pd atoms as a model, was investigated and the corresponding normal modes were determined by DFT calculations. As a starting point for the geometry optimization, BINAP was placed above the Pd surface in a way that the two P atoms point toward the surface and the C2 axis of BINAP is perpendicular to the surface since one would expectsin analogy to homogeneous complexessthe strongest interaction with the surface there. The optimized adsorption geometry is shown in Figure 4. One of the two phenyl rings connected to one P atom is oriented in a tilted position toward the surface (phenyl A), whereas the second phenyl ring is oriented parallel to the surface (phenyl B). Interestingly, the two tilted phenyl rings (A1 and A2) at the different P atoms are not oriented in the same way. Consequently, the adsorbed BINAP found in the optimization lost its C2 symmetry upon adsorption. Furthermore, the parallel adsorption of the remaining two phenyl rings on the surface led to a decrease of the dihedral angle between the two naphthyl rings compared to the nonadsorbed BINAP. Comparison with typical dihedral angels in PdBINAP complexes as revealed by a search in the Cambridge Structural Database (CSD), confirmed that the coordination of BINAP to Pd generally leads to a decrease of the dihedral angle of BINAP (indicated in Figure 5a) from typically 92° in noncoordinating BINAP to 60-80° in coordinating forms (Figure 5a). The dihedral angle of 77° in the adsorbed BINAP obtained by our calculation seems therefore reasonable. The distance between the C atoms of the phenyl rings adsorbed parallel to the Pd surface is around 2 Å, which is a reasonable length for a Pd-C bond as the CSD statistical analysis revealed (Figure 5b). According to this analysis, the length for a
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Figure 4. Calculated optimized structure of BINAP adsorbed on a Pd (111) surface. The naphtyl moiety is orientated with its C2 axis perpendicular to the surface, whereas one of the two phenyl rings connected to each P atom adsorbs parallel to the surface (B) and the other one is orientated in a tilted manner to the surface (A). Colors: Pd: green, C: gray; H: with; P: orange.
coordinative Pd-P bond typically lies between 2.2 and 2.4 Å (Figure 5c). The distance between the P atoms and the surface is around 2.8 Å. Therefore, the interaction between the Pd surface and the P atoms of the adsorbed BINAP is not as strong as in homogeneous Pd complexes, most likely because of the geometrical constraints given by the adsorption of the two phenyl rings, which prevents strong coordination of the P atoms on the Pd surface. On the basis of this optimized adsorption geometry of BINAP on a Pd surface we calculated the corresponding IR spectrum (Figure 3d). The spectral features differed remarkably from the isolated BINAP (Figure 3c), showing poor agreement with the spectral features of (rac)-BINAP adsorbed on Pd nanoparticles (Figure 3b). We therefore took the surface selection rule43 into account: the calculated IR intensity of every band shown in Figure 3d was scaled by a factor obtained from the projection of dynamic dipole moment of respective normal modes onto the normal axis relative to the Pd surface. The resulting IR spectrum is greatly simplified and shows better agreement with the spectrum of BINAP interacting with the Pd nanoparticles (Figure 3e). Generally, all bands have been red-shifted using the same scaling factor (0.98). This may be due to the increased number of basis functions to describe the adsorbed BINAP electronic structure by the presence of the Pd atoms, which possibly led to more accurate description of the vibrational frequency (i.e., increased scaling factor closer to 1). For the sake of consistency, we show all spectra with the same scaling factor of 0.98. As seen in Figure 3, parts a and b, there were several notable differences in the spectrum of (rac)-BINAP adsorbed on the surface of the Pd nanoparticles (bands marked with asterisks in Figure 3b). The first remarkable change occurs in the signal of the phenyl ring vibrations (Ph, C-C-H def.) around 1100 cm-1. According to our theoretical study, one of the two phenyl rings denoted as B (Figure 4) and connected to a P atom undergoes surface adsorption. Consequently, the spectral contributions from
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Figure 5. Statistical analysis of CSD searches for (a) dihedral angels between the two naphthyl moieties in BINAP and derivatives of it in Pd complexes, (b) Pd-C bond lengths, and (c) Pd-P bond lengths.
the phenyl rings B are largely lost since the dynamic dipole moments of the corresponding in-plane C-C-H deformation vibrations are canceled out according to the surface selection rule because of their parallel adsorption on the Pd surface. In the case of the nonadsorbed phenyl rings A1 and A2, the deformation vibration of the two phenyl groups appear differently because of different tilt angles with respect to the Pd surface. The phenyl ring A1 is oriented less tilted to the Pd surface (∼30°) than A2 (∼50°). According to the calculation, the prominent band at 1097 cm-1 can be assigned to asymmetric C-C-H vibrations of the phenyl ring A2, whose dynamic dipole moment has a large component in the direction perpendicular to the surface, hence explaining the experimentally observed enhancement (Figure S2, Supporting Information). The band at 1120 cm-1 in the spectrum of the powdered (rac)-BINAP sample was assigned to symmetric C-C-H vibrations from both phenyl rings A1 and A2. For these vibrational modes, the dynamic dipole moments are oriented more parallel toward the surface than those of the asymmetric in-plane C-C-H vibrations. Therefore, there was no remarkable enhancement in the IR intensity.
Reimann et al. The intensity of the broad band at ∼1263 cm-1, assigned to in-plane C-C-H deformation vibrations of the naphthyl groups, increased upon (rac)-BINAP adsorption on the Pd surface. It is therefore most likely that the decrease of the dihedral angle between the two naphthyl groups led to a more perpendicular orientation of the dynamic dipole moment of the asymmetric in-plane C-C-H vibrations to the surface. The band at 1309 cm-1 shows a considerable intensity upon adsorption of (rac)BINAP. According to our calculations, the signal originates from asymmetric in-plane C-C-H vibrations from all four phenyl groups. The contributions from the phenyl rings B are lost because of their parallel adsorption on the surface, and the enhancement probably originates from asymmetric vibrations of the phenyl rings A1 and A2. The intensity enhancement of the two bands was correctly reproduced by the calculation taking into account the surface selection rule (Figure 3e) at 1250 and 1290 cm-1. Between 1400 and 1500 cm-1, the most significant change was the intensity decrease of the band at 1482 cm-1. The calculated spectrum (Figure 3e) showed greatly decreased IR intensities compared to the free (rac)-BINAP, too (Figure 3c). This band originates from C-C stretching vibrations of both types of phenyl rings, A and B. Upon adsorption, the contributions from the two phenyl rings B are lost, and the remaining symmetric C-C stretching vibrations of the phenyl rings A are, as in the case of the band intensity at ∼1100 cm-1, not enhanced. Interestingly, the signal at 1500 cm-1 did not show any notable change in neither experiment nor calculation upon surface adsorption. This is reasonable because that signal arises from symmetric C-C stretching vibrations of the binaphthyl moiety, which has a large vibrational component in the surfaceperpendicular direction. For the signal at 1434 cm-1, a slight decrease in intensity was observed. According to the calculations for the adsorbed BINAP the band consists of symmetric C-C stretching vibrations from the phenyl rings B and asymmetric C-C stretching vibrations from the binaphthyl group. Again, the contributions from the phenyl rings B are lost due to their parallel adsorption on the Pd surface. As in the case for the broad signal at 1267 cm-1, the vibrations from the binaphthyl group are enhanced and partly compensate the loss in intensity caused by the disappearance of the contributions from the phenyl rings. It is remarkable that the signal at 1583 cm-1 nearly vanished for the (rac)-BINAP adsorbed on Pd nanoparticles. The vibrational mode consists of symmetric C-C stretching vibrations from all four phenyl rings and loses its intensity almost completely; the contributions from the phenyl rings B disappear upon adsorption, and the vibrations from the phenyl rings A are not enhanced because of close to parallel direction of the dynamic dipole moment change. 3.4. Transient (rac)-BINAP Adsorption on a Pd Film from the Liquid Phase. Reactions with BINAP adsorbed on nanoparticles are preferably carried out in the liquid phase.18,19,32,44 We therefore investigated the transient adsorption/desorption behavior of (rac)-BINAP on Pd in the liquid phase using in situ ATR-IR MES. A conventional adsorption study did not yield useful results because of the very weak absorbance of (rac)-BINAP compared to the solvent molecule (ethylbenzene). Hence, it was necessary to study (rac)-BINAP adsorption using the transient technique to better compensate solvent signals with the aid of MES. Prior to the transient adsorption study, a reference spectrum of (rac)-BINAP dissolved in ethylbenzene was recorded using ATR-IR MES at 50 °C. (Figure 6). During the first half of the
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Figure 6. (bottom) Time-resolved IR spectra of (rac)-BINAP acquired in the ATR-IR MES experiment by modulating a 1 mM solution of (rac)-BINAP in ethylbenzene (first half of the whole period) against N2-saturated ethylbenzene (second half) on a clean IRE at 50 °C. The last spectrum of the period at 1200 s was used as background. (top) Near in-phase spectrum (φPSD ) 0°)29 of the time-resolved spectra. The bands that change considerably upon BINAP adsorption on Pd are marked with asterisks.
Figure 7. (bottom) Time-resolved IR spectra of (rac)-BINAP acquired in the ATR-IR MES experiment by modulating a 1 mM solution of (rac)-BINAP in ethylbenzene (first half of the whole period) against N2-saturated ethylbenzene (second half) on a Pd-coated IRE at 50 °C. The last spectrum of the period at 1200 s was used as background. (top) Near in-phase spectrum (φPSD ) 0°)29 of the time-resolved spectra. The bands that change considerably upon BINAP adsorption on Pd are marked with asterisks.
measuring period, a 1 mM solution of (rac)-BINAP was admitted to the ATR-IR cell, and the solution was altered to neat solvent in the second half. Upon switching from the (rac)-BINAP solution to neat ethylbenzene, all BINAP bands disappeared immediately, indicating no or only negligible interaction of (rac)-BINAP with the clean ZnSe surface. The near in-phase spectrum of this MES experiment (which can be conveniently approximated as a difference spectrum between the two solutions with high sensitivity) shows identical features to the spectrum of powder (rac)-BINAP (Figure 3a). This implies that there is no significant difference in the intermolecular interactions between powdered (rac)-BINAP and (rac)-BINAP in the liquid phase, respectively. MES was in this case useful to enhance the signal-to-noise ratio and to unambiguously identify the bands originating from (rac)-BINAP in solution, at maximum 900 microabsorbance, by perfectly compensating the solvent bands. When the IRE was covered with a Pd film, the spectral features as well as their temporal behavior changed to a great extent. The observed differences from the reference spectrum (1 mM (rac)-BINAP, Figure 7) resemble those already observed for the (rac)-BINAP powder and the (rac)-BINAP on the Pd nanoparticles (Figure 3a, 3b). The enhancement of the band at 1091 cm-1 indicates (rac)-BINAP adsorption on the Pd surface. Furthermore, the deposition of Pd on the IRE induced the dynamic adsorption and desorption of (rac)-BINAP; the absorbance of (rac)-BINAP bands gradually increased over the period of the (rac)-BINAP flow and decreased over the period of the neat solvent flow. This clearly shows the surface (rac)-BINAP concentration increase and decrease, respectively, as it is illustrated best by the signal at 1434 cm-1. Consequently, the band at 1259 cm-1, although very weak, became visible over time in the first half of the measurement cycle. Generally, the observed bandssexcept for those at 1091 cm-1 and 1434 cm-1sare weak because of the differential nature of the spectra; the 2D time-resolved spectra are shown taking the last spectrum of the period (i.e., the last spectrum under the neat solvent flow) and the near in-phase spectrum shown in Figure 7 shows by nature only the difference in the signals under the two solution flows because of the phase-sensitive detection used in MES.29 Since the interaction of (rac)-BINAP with a Pd surface is
considerably strong under the given conditions, the fraction of (rac)-BINAP that adsorbs and desorbs over time is small. The raw spectrum clearly showed that BINAP is strongly bound to the surface and only a small part of it adsorbs and desorbs reversibly at 50 °C (not shown). In addition, negligible interaction of (rac)-BINAP with alumina was confirmed by the same technique (Figure S3, Supporting Information). It should be noted that the MES experiment with (rac)-BINAP in the presence of the Pd film led to a large, periodic movement of the baseline, which was not observed for the reference measurements with the clean IRE and the alumina film deposited on the IRE. A careful baseline correction was therefore necessary. The observed periodic baseline shift during periodic changes of the (rac)-BINAP concentration gives valuable information because such shifts originate from the changes in the electronic properties and therefore the refractive index of Pd. This gives evidence that the Pd surface was covered and reduced by BINAP.41 Similar effects have already been observed in the case of oxidized and reduced Pd.45,46 Since the applied MES technique can only detect dynamically adsorbing and desorbing BINAP during one MES cycle (i.e., stably adsorbed BINAP under the BINAP concentration change cannot be observed in MES), we looked for possibilities to boost the amount of such dynamically adsorbing BINAP in order to enhance the absorbance of the detectable BINAP fraction. This challenge was tackled by the following two approaches: (a) increasing the temperature, and (b) inducing competitive surface adsorption. 3.4.1. (a) Increasing the Temperature. First, the system temperature was increased from 50 °C to 80 °C to accelerate the desorption processes. All other parameters were kept identical to the previous experiment (cf. Figure 7); the result of the MES experiment is shown in Figure 8. In general, the spectral features were considerably simplified and highlight the surface-adsorbed (rac)-BINAP (bands marked by asterisks in Figure 3b). This in turn confirms that the spectral features of (rac)-BINAP adsorbed on the Pd nanoparticles indeed originated from the surface-adsorbed (rac)-BINAP and that the features are similar when (rac)-BINAP adsorbs on Pd at the solid-liquid
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Figure 8. (bottom) Time-resolved IR spectra of (rac)-BINAP acquired in the ATR-IR MES experiment by modulating a 1 mM solution of (rac)-BINAP in ethylbenzene (first half of the whole period) against N2-saturated ethylbenzene (second half) on a Pd-coated IRE at 80 °C. The last spectrum of the period at 1200 s was used as background. (top) Near in-phase spectrum (φPSD ) 0°)29 of the time-resolved spectra. The bands that change considerably upon BINAP adsorption on Pd are marked with asterisks.
interface. Compared to the measurement at 50 °C (Figure 7), the intensity of the broad band at 1259 cm-1 markedly increased. Strong interaction of (rac)-BINAP with the Pd surface was also confirmed by the gradual increase and decrease of BINAP bands within one period. Compared to the measurement at 50 °C, (rac)-BINAP desorption under the neat solvent flow was faster as pronounced in the evolution of the band at 1434 cm-1. 3.4.2. (b) Co-adsorption of CO. In cases where the related target reaction proceeds at room temperature, it is not desirable to increase the system temperature. Therefore different approaches have been sought for, and the addition of another, strongly adsorbing reagent was considered. The idea was to enhance the desorption of (rac)-BINAP by addition of a molecule that adsorbs competitively on the Pd surface, thereby accelerating the BINAP desorption. Because of its simple applicability and high affinity toward the Pd surface, CO was selected. Besides, CO is a good test molecule for probing the electronic properties of a given metal surface because its stretching vibrational frequency changes depending on the electronic state of the metal surface.47 The neat solvent in the second half of the period, previously saturated with N2, was saturated with CO and an identical MES experiment ((rac)-BINAP vs CO) was performed at 50 °C (Figure 9). Compared to the experiment where the neat solvent without CO was used (Figure 7), the bands around 1090, 1263, and 1309 cm-1, indicative of surface-adsorbed (rac)-BINAP, were clearly enhanced. Interestingly, these bands decayed faster during the second half period in the presence of CO as evidenced from the temporal evolution of the band at 1435 cm-1 (Figure 9) in comparison to the evolution of the same band in Figure 7. This is reasonable considering that the surface Pd sites are competitively accessed by both (rac)-BINAP and CO. Apparently, the presence of CO induces faster desorption of (rac)BINAP from the surface than in the case without CO. In addition, a striking effect of this approach was observed for the signal at 1435 cm-1. This band underwent a red-shift by ∼6 cm-1 during BINAP adsorption and underwent a blue-shift under CO-saturated neat solvent flow (Figure 10). This indicates that the electronic environment for the adsorbed BINAP on the
Reimann et al.
Figure 9. (bottom) Time-resolved IR spectra of (rac)-BINAP acquired in the ATR-IR MES experiment by modulating a 1 mM solution of (rac)-BINAP in ethylbenzene (first half of the whole period) against CO-saturated ethylbenzene (second half) on a Pd-coated IRE at 50 °C. The last spectrum of the period at 1200 s was used as background. (top) Near in-phase spectrum (φPSD ) 0°)29 of the time-resolved spectra. The bands that changes considerably upon BINAP-adsorption on Pd are marked with asterisks.
Figure 10. Selected region (1420-1445 cm-1) of the spectra shown in Figure 9.
surface changes over timesinduced by the surface-adsorbed COssince such a large band shift was not observed in the previous MES experiments (Figures 7 and 8). In other words: the surface electronic states were modified by the adsorbates, particularly by CO. The state of the adsorbed CO was investigated by analyzing the CO stretching region of the spectra (Figures S4 and S5, Supporting Information) from the MES experiments where two solutions were alternately admitted over the Pd surface: (1) N2and CO-saturated solvent and (2) (rac)-BINAP solution and COsaturated solvent. In the absence of (rac)-BINAP (case (1)), CO could ad- and desorb slowly during the experiment. The spectral bands indicate prominent, broad bridged-adsorbed CO at 1945 cm-1 as well as small band of atop (linear) CO at 2069 cm-1.35,48-50 The picture changes when (rac)-BINAP competitively accesses the surface. The relative ratio of bridged to atop adsorbed CO decreased; implying that some surface sites, i.e., bridged ones, were preferentially occupied by BINAP. Apparently, the adsorption of CO in the second half period was quicker when (rac)-BINAP was present on the surface. However, this
BINAP Adsorption on Palladium
Figure 11. (bottom) Time-resolved IR spectra of (rac)-BINAP acquired in the ATR-IR MES experiment by modulating a 1 mM solution of (rac)-BINAP in CO-saturated ethylbenzene (first half of the whole period) against CO-saturated ethylbenzene (second half) on a Pd-coated IRE at 50 °C. The last spectrum of the period at 1200 s was used as background. (top) Near in-phase spectrum (φPSD ) 0°)29 of the timeresolved spectra. The bands that change considerably upon BINAP adsorption on Pd are marked with asterisks.
information must be judged carefully since the total amount of surface-bound CO decreased based on the absorbance in case of a competitive BINAP adsorption. Finally, we carried out another competitive adsorption experiment using CO during the whole period ((rac)-BINAP + CO vs CO). Figure 11 shows the spectra of the MES experiment. Generally, we could again confirm the bands originating from surface-adsorbed (rac)-BINAP with slightly higher intensities than without CO present (Figure 7). However, the bands were less prominent than in the previously examined case ((rac)BINAP vs CO). Notable features were the absence of the band shifts at about 1434 cm-1 as observed for BINAP vs CO (Figure 10) and a much narrower bridged CO band during the experiment (Figure S6, Supporting Information). The former shows that the electronic structure of the Pd surface was not altered considerably by the continuous presence, hence by continuous interaction and adsorption of CO on Pd. The latter implies that the continuous presence of CO and also the presence of (rac)BINAP restrict the type of surface Pd sites occupied by CO and, equivalently, by (rac)-BINAP, replacing the same sites. The narrow CO bands imply that (rac)-BINAP reversibly adsorbed and desorbed preferably on more defined Pd surfaces, i.e., large terraces, and not corners and defects. The competitive adsorption-induced transient adsorption study introduced in this work appears to be an elegant way to study the adsorption geometry of surface species. Using CO, the desorption of the substrate BINAP has been enhanced efficiently at relatively low temperature. Under such mild conditions, the desorption rate is normally low. Also, by introducing CO in either one of the half periods or the whole period, we could gain information such as preferential adsorption sites of BINAP, which cannot be accessed otherwise. 4. Conclusions The ATR-IR investigation of Pd nanoparticles modified with (rac)-BINAP revealed clear differences between the spectra of powdered (rac)-BINAP, (rac)-BINAP dissolved in ethylbenzene, and (rac)-BINAP adsorbed on a Pd surface. Good agreement was found between the experimental and the
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17843 simulated IR spectra. The latter were derived from calculations of the optimized adsorption geometry of BINAP on a Pd cluster, taking the surface selection rule into account. BINAP adsorbs with its C2 symmetry axis perpendicular to the Pd surface, and due to steric hindrance by the phenyl rings connected to the two P atoms, the main interaction with the surface occurs via the two aromatic rings that are oriented parallel to the surface and not directly via the two P atoms. The remaining two phenyl rings are orientated in a tilted position toward the surface. Our in situ ATR-IR MES study showed that the adsorption geometry of BINAP at the solid-liquid interface on a model Pd film does not differ from that observed for the (rac)-BINAP adsorbed on Pd nanoparticles. Furthermore, two approaches to increase the fraction of reversibly adsorbed BINAP were successfully applied: (i) increase of the temperature, and (ii) introduction of a coadsorbing molecule such as CO. The latter method proved to be particularly useful to study the dynamic sorption behavior under mild conditions where the substrate is strongly bound. While the present study was performed with (rac)-BINAP, in asymmetric catalysis the corresponding pure enantiomers are applied for chiral modification of metal surfaces. We assume that the principal adsorption mode of the two enantiomers is similar but with different enantiofaces, as previously observed for the well-investigated modification of platinum with the two pseudoenantiomers cinchonidine and cinchonine.51,52 The study also revealed that the interaction of BINAP with the surface is weaker than intuitively expected. Especially the experiments at higher temperatures and in the presence of a strong coadsorbate demonstrated that the modifier is easily removed from the surface under certain reaction conditions. This could explain the usually rather high amount of modifiers needed in such surface-catalyzed reactions.20,32,53,54 Furthermore, as the stability of nanoparticles is directly linked to the sorption behavior of the used stabilizers,40 such transient adsorption studies may help to find reaction conditions under which the used nanoparticles are stable and their agglomeration can be prevented. Finally, the knowledge of the adsorption geometry of BINAP may be used in the future to explain the mechanism of enantioselection in reactions catalyzed by BINAP-stabilized nanoparticles. Acknowledgment. Financial support by the Swiss National Science Foundation is kindly acknowledged. We thank Dr. Frank Krumeich for the STEM and TEM measurements and the Electron Microscopy ETH Zurich, EMEZ, for the measuring time. Supporting Information Available: The 31P NMR spectrum of the (rac)-BINAP-modified nanoparticles, the data of the reference measurements, and the CO spectra of the MES experiments with CO are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mallat, T.; Orglmeister, E.; Baiker, A. Chem. ReV. 2007, 107, 4863– 4890. (2) Bodenho¨fer, K.; Hierlemann, A.; Seemann, J.; Gauglitz, G.; Koppenhoefer, B.; Go¨pel, W. Nature 1997, 387, 577–580. (3) McKendry, R.; Theoclitou, M. E.; Rayment, T.; Abell, C. Nature 1998, 391, 566–568. (4) Ahuja, S. Chiral Separations by Chromatography; Oxford University Press: Washington, DC, 2000. (5) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483–2487. (6) Zhao, X. Y. J. Am. Chem. Soc. 2000, 122, 12584–12585. (7) Schunack, M.; Petersen, L.; Ku¨hnle, A.; Lægsgaard, E.; Stensgaard, I.; Johannsen, I.; Besenbacher, F. Phys. ReV. Lett. 2001, 86, 456–459.
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