Coadsorption of Cinchona Alkaloids and Oxygen on Pt(111): A

Article pubs.acs.org/JPCC

Coadsorption of Cinchona Alkaloids and Oxygen on Pt(111): A Theoretical Study Konstanze R. Hahn† and Alfons Baiker*,‡ †

Department of Physics, University of Cagliari, Cittadella Universitaria, I-09042 Monserrato, Italy Department of Chemistry and Applied Biosciences, ETH Zurich, Hönggerberg, HCI, CH-8093 Zurich, Switzerland

‡

S Supporting Information *

ABSTRACT: The adsorption (binding energy, configuration, and spatial orientation) of the cinchona alkaloids cinchonidine and cinchonine, which are used to bestow different chirality to catalytically active noble metal surfaces, has been studied in the presence of coadsorbed oxygen on a model Pt(111) surface using density functional theory. The influence of oxygen was analyzed at 1/36, 0.5, and 1 monolayer coverage, mimicking the adsorption behavior of the cinchona alkaloids on partially and fully oxidized surfaces. Both cinchona alkaloids are destabilized with increasing oxygen coverage; however, adsorption of these molecules on a fully oxidized Pt surface leads to a drastic increase of their stability. The influence of coadsorption with oxygen on the adsorption of the cinchona alkaloids is compared to that of the earlier reported coadsorption with hydrogen. In the absence of coadsorbed oxygen or hydrogen, the most stable adsorption mode is where the quinoline moiety of the cinchona alkaloids lays nearly parallel to the surface. With increasing coverage of oxygen or hydrogen, the coadsorbed cinchona alkaloids become tilted, resulting in a different spatial orientation of the cinchona alkaloids. Possible implications of the coadsorption behavior for the practical use of cinchona alkaloids as chiral modifiers of Pt are discussed.

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In previous spectroscopic8 and theoretical studies,9 it has been shown that the orientation of the adsorbed cinchona alkaloid is strongly influenced by coadsorbed hydrogen, a situation that is encountered during asymmetric hydrogenation. Depending on the surface coverage of coadsorbed hydrogen, the tilting angle and thus the geometry of the chiral site formed by the adsorbed alkaloid change, which in turn can affect the enantiodifferentiating power of the chiral modifier. While coadsorption of the cinchona modifiers with hydrogen on Pt surfaces has been investigated experimentally and theoretically, to our knowledge, no such studies exist for their coadsorption with oxygen in spite of its possible relevance for asymmetric induction in noble-metal-catalyzed oxidation reactions such as e.g. the oxidative kinetic resolution of racemic secondary alcohols.10 Despite the fact that this reaction has so far not been established on cinchona modified Pt, it seems feasible because Pt is a well-known catalyst for alcohol oxidation.11 However, a basic requirement is that the cinchona alkaloids are structurally stable in the presence of coadsorbed oxygen and are suitably adsorbed on the noble metal surface for enantiodifferentiation.

INTRODUCTION

There are various strategies for developing chiral solid catalysts that facilitate enantiodifferentiation in chemical reactions.1 Among them, the chiral modification of platinum group metals by suitable chiral organic molecules has proven to be an interesting strategy for developing catalysts for asymmetric hydrogenation of activated CO and CC bonds.2−4 Various chiral molecules have been applied for the chiral modification among which cinchona alkaloids proved to perform best. The most frequently applied cinchona alkaloids are cinchonidine (CD) and cinchonine (CN), which are diastereomers differing in the absolute configuration at the C9 and C8 atoms (Scheme 1). Their application as chiral modifiers generally leads to opposite product enantiomers in the asymmetric hydrogenation of activated ketones.2 The efficiency of the adsorbed cinchona alkaloid in enantiodifferentiation is affected by the binding mode, conformation, and spatial orientation the modifier assumes upon adsorption on the noble metal surface. The binding mode and orientation of the adsorbed modifier are crucial for the geometry of the chiral site where the interaction between adsorbed modifier and substrate occurs during chiral induction. Understanding of this chirality transfer is a prerequisite for the rational design of chiral catalysts. Different experimental and theoretical strategies have been pursued to shed some light on these chirality transfer processes.5−7 © XXXX American Chemical Society

Received: July 15, 2016 Revised: August 18, 2016

A

DOI: 10.1021/acs.jpcc.6b06991 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Chemical Structure of the Diastereomers Cinchonidine and Cinchonine

prediction of exact values of the adsorption energy. Therefore, we decided to avoid the addition of empirical potentials to describe dispersion interactions which can potentially add another source of errors. The Pt surface, exposing the (111) crystallographic plane, has been simulated with a slab of four atomic layers and a 6 × 6 repetition of the unit cell in the surface plane resulting in a total number of 144 Pt atoms in the slab. The top two layers of the slab have been allowed to relax. A vacuum of 25ds, where ds is the slab thickness, has been added to avoid interactions with the periodic images of the slabs. A schematic of the studied cinchona alkaloids is shown in Scheme 1. The binding energy EB of cinchonidine and cinchonine has been calculated according to eq 1, where EPt+mol is the total energy of the (oxidized/hydrogenated) Pt(111) slab with the CD or CN molecule adsorbed, EPt refers to the total energy of the clean, oxidized, and reduced Pt(111) surface, respectively, without the adsorbed CD or CN molecule, and Emol,gas is the energy of the corresponding molecule in the gas phase.

With this in mind, we have studied theoretically the coadsorption of the two diastereomers, CD and CN, with oxygen on a Pt(111) surface. The oxygen coverage has been increased from 1/36 of a monolayer to a complete monolayer and the change of the adsorption mode and spatial orientation of the chiral modifier has been elucidated. The theoretical study of oxygen coadsorption with the cinchona alkaloids provides fundamental insight into the stability and orientation of cinchona alkaloids coadsorbed with oxygen and thus provides an important criterion for the feasibility of the application of cinchona alkaloids as enantiodifferentiating modifiers on platinum in the presence of oxygen.

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METHODS Density functional theory (DFT) calculations of this study have been performed using the Quickstep module12 of the CP2K program package,13 a suite of programs developed to efficiently perform electronic structure and molecular dynamics calculations based on the Gaussian and plane waves formalism.14 Interactions with the frozen atomic core of atoms are described with norm-conserving pseudopotentials while valence electrons are treated explicitly.15 For O, N, and C atoms, respectively six, five, and four electrons have been considered in the valence shell, and a triple-ζ valence plus double polarization basis set has been adopted for their valence electrons. The same basis set has been used for H. Pt atoms have been simulated with 18 valence electrons and a triple-ζ basis set. The energy cutoff of the electron density has been set to 600 Ry and the relative cutoff to 50 Ry. Preliminary calculations showed the default value of 40 Ry for the relative cutoff to be insufficient for reliable results. Exchangecorrelation potentials are modeled within the general gradient approximation (GGA) using the Perdew−Burke−Ernzerhof (PBE) functional.16 Spin-polarization has been considered in all calculations. With these configurations, the lattice parameter of Pt has been optimized to be 3.965 Å. In the present calculations, dispersion interactions have not been treated explicitly. Several methods have been developed recently to account for van der Waals interactions within the DFT formalism, and it has been shown to be crucial for the correct description of bond lengths and adsorption energies.17 However, computational feasible methods for systems including large amounts of heavy atoms, such as the addition of empirical potentials,18 are based on fitting parameters including new sources of inaccuracies. In this study we are rather interested in the qualitative trends of coadsorbed cinchona alkaloids with oxygen on Pt and in particular on the comparison to the coadsorption with hydrogen than in the reproduction and

E B = E Pt + mol − (E Pt + Emol,gas)

(1)

The binding energy of oxygen has been calculated similarly, but here the reference state in gas phase is the O2 molecule and EB,O2 is normalized to the number n of O atoms (eq 2). E B,O2 =

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1

(

E Pt + nO − E Pt + n 2 EO2 n

) (2)

RESULTS Adsorption of Oxygen. Oxygen adsorption on Pt(111) has been simulated for isolated O atoms (θO = 1/36 ML) and at higher coverages of 0.25, 0.5, and 1 ML on high-symmetry adsorption sites on top of Pt atoms, on fcc (face-centered cubic), and on hcp (hexagonal closed packed) sites. The binding energy EB,O2 has been calculated with respect to molecular O2 (eq 2). For isolated O, adsorption at a top site is not stable. The O atom initially placed on a Pt top site diffuses to an hcp where a binding energy of −0.36 eV is calculated (Table 1). The most stable configuration is found at an fcc site with a binding energy of −0.81 eV, in agreement with experiments showing O atoms to adsorb on Pt on 3-fold hollow sites.19 Similarly, previous theoretical studies showed the fcc site to be most favored for O adsorption on Pt at a coverage of 0.25 ML.20−22 For molecular oxygen, we have calculated a formation energy of −3.52 eV. Considering this, the binding energy on an fcc site corresponds to −4.33 eV with respect to atomic oxygen in the gas phase. This B


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The Journal of Physical Chemistry C Table 1. Oxygen Adsorption: Binding Energy EB,O2 on Pt(111) with Respect to Gaseous, Molecular O2a

coadsorption of quinoline with oxygen. The stability of quinoline on clean Pt(111) has been investigated for several configurations adopted from previous calculations.27 The binding energy on clean Pt(111) has been calculated to be −1.88 eV. Coadsorption with O destabilizes the system. Already at a low oxygen coverage of 1/36 ML, the quinoline molecule is by 0.1 eV less stable resulting in a binding energy of −1.78 eV (see Table 2). This indicates repulsive interactions between the quinoline molecule and the oxygen atom and is also confirmed by the lower (less negative) binding energy of oxygen (EB,O2 = −0.74 eV) compared to isolated O adsorption (−0.81 eV). On the reduced Pt surface at θH = 1/36 ML, destabilization of quinoline is less pronounced (ΔE = 0.04 eV). Oxygen transfer to the N atom destabilizes the system even more. A binding energy of −0.61 eV is calculated when O binds to the N atom of the quinoline molecule (Figure S1). The oxidation of N is endothermic by 1.17 eV (Table 2) and slightly higher than the standard enthalpy of formation of NO (0.94 eV28). Transfer of H to N, on the other hand, stabilizes the system by 0.30 eV. Destabilization of the system is as well observed when O transfers to one of the C atoms, however, less pronounced (ΔE = 0.5 eV). In the case of H coadsorption, transfer to C results in a destabilization by 0.07 eV. At a higher coverage (θ = 0.5 ML), various distributions of O and H have been simulated. The most stable configurations are found when the atomic species are distributed homogeneously on the Pt surface, leading to a destabilization of 0.75 and 0.74 eV in the case of oxidized and reduced Pt surface, respectively. The effect is almost the same for O and H. Thus, repulsive interactions at θ = 0.5 ML leading to destabilization can be mainly attributed to steric hindrance rather than chemical effects. When the atomic species are distributed homogeneously, several factors have to be considered that affect the stability of the system. These include the repulsive interactions between the preadsorbed atomic species and the molecule, reaction of the atomic species with the molecule, and poisoning of active sites by the atomic species. For a homogeneous distribution of O, adsorption of quinoline becomes endothermic with a positive binding energy of 0.67 eV corresponding to a destabilization by 1.80 eV with respect to the segregated O distribution. In this configuration the quinoline molecule forms an angle of 33° with the Pt surface (Figure S2a). The molecule becomes more stable (by 0.32 eV) when one of the O atoms transfers to a C atom of the quinoline ring (Figure S2c). In this configuration, the inclination angle of quinoline is 25°. No stable configuration has been found for O transfer to N.

EB,O2 [eV] θO

top

1/36 0.25 0.5

1

0.41

fcc

hcp

−0.81 −0.36 −0.49 −0.51 −0.32 0.17 0.12

−0.36

distribution corrugated surface, O on different sites homogeneous [011̅] homogeneous [121̅] segregated corrugated surface, O on different sites

a

Adsorption of O has been simulated on several high-symmetric sites (top, hcp, and fcc) and at different coverages (θ = 1/36, 0.25, 0.5, and 1 ML). At 0.5 ML, different distributions of the O atoms have been considered.

is in good agreement with previous results where binding energies from −3.73 to −4.03 eV have been reported.20,21 In the latter studies, the difference in binding energy between hcp and fcc adsorption with respect to atomic oxygen is 0.36−0.47 eV,21,22 which is in excellent agreement to what we have calculated here (−0.45 eV). At 0.25 ML, O atoms initially placed at top sites diffuse partly to hcp and fcc sites, resulting in a configuration with O atoms adsorbed to equal parts at top, fcc, and hcp sites. The binding energy of this configuration is calculated to be −0.36 eV. When the coverage is increased to 0.5 ML, adsorption at fcc sites becomes less stable (EB = −0.51 eV) with respect to an isolated O atom, indicating repulsive interactions between adsorbates. Marginal differences are observed at 0.5 ML when O atoms are adsorbed along different crystallographic directions (Table 1). Orientation along [011̅] direction is slightly less stable (EB = −0.49 eV) than along [121̅] direction (EB = −0.51 eV). When O atoms are segregated in one corner of the slab, the binding energy is even more reduced to −0.32 eV, confirming repulsive interactions between adsorbates. Additional destabilization is observed when increasing the coverage to 1 ML, resulting in a positive binding energy of 0.41 and 0.17 eV for adsorption at top and fcc sites, respectively. Destabilization with increasing coverage is in agreement with previous DFT simulations.23 Adsorption of Quinoline. Earlier spectroscopic24,25 and theoretical studies26 indicated that the quinoline part plays a crucial role for stable anchoring of cinchona alkaloids to the Pt surface. With this in mind we have first examined the

Table 2. Binding Energy EB of Quinoline Adsorbed on Clean Pt (θ = 0 ML) and on Oxidized and Hydrogenated Pt(111) at Different O and H Coverages (Data for H Coadsorption from Ref 9) O/H uptake θ (ML)

on N

oxidized Pt

on C

0 1/36 1 1 0.5

reduced Pt

EB [eV]

αQ‑Pt [deg]

EB,O2 [eV]

EB [eV]

αQ‑Pt [deg]

EB,H2 [eV]

−1.88 −1.78 −0.61 −1.28 −1.13 0.67

0 0 0 0 0 33

−0.74 0.43 −0.24 −0.28 −0.35

−1.84 −2.14 −1.77 −1.14 −0.62 −0.93

0 8 0 0 46 22

−0.47 −0.77 −0.40 −0.41 −0.38 −0.40

0.35 −4.39

25 53

−0.37 0.05 −0.91

73

−0.35

1 1 1

1 3 2

C


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The Journal of Physical Chemistry C

(ΔEB = 0.31 eV). On the partially oxidized surface (θO = 1/2 ML) stabilization of the molecule is observed when O binds to one of the C atoms. Here, the energy gain with respect to the configuration without O transfer is 0.32 eV. At low coverage (θO = 1/36 ML), on the other hand, adsorption becomes less favorable when O binds to C. Adsorption of Cinchonidine. Effect of Oxygen Transfer. To investigate the effect of O transfer from the surface to the CD molecule, one O atom has been added to the adsorbed CD molecule at specific sites. The O atom has been placed close to the N atom of the quinoline group (N′), the N atom of the quinuclidine moiety, the C9′ atom of the quinoline group, and the C11 atom of the terminal vinyl group. In the reference state, the O atom has been placed on an fcc site furthest away possible from the molecule to minimize interactions between the two adsorbates. Despite the distance between CD and the O atom, the CD molecule is destabilized by 0.05 eV (EB = −2.26 eV) with respect to adsorption on the clean Pt surface (EB = −2.31 eV). Significant destabilization is observed when the O atom transfers to either of the N atoms (Figure 2a,c). In fact, the least

On the partially reduced Pt surface, the quinoline molecule is destabilized by only 0.52 eV with respect to a segregated H distribution and adsorption is still exothermic (EB = −0.62 eV, Figure S2b). The angle between quinoline and the Pt surface increases to 46°. In contrast to the oxidized surface, H transfer from the reduced Pt surface to the N site of the quinoline molecule (Figure S2d) leads to a configuration more stable by 0.31 eV than the one where no H transfer takes place. No stable configuration has been found for H transfer to C. On the fully oxidized Pt surface, a stable configuration is found when three O atoms react with the C atoms of the quinoline molecule (Figure S3a). The binding energy with respect to the fully oxidized Pt surface and the isolated quinoline molecule in gas phase is calculated to be −4.39 eV. This significant stabilization of the molecule with respect to lower O coverages possibly results from the protrusion of one of the Pt surface atoms from the surface. In this configuration, the quinoline molecule is inclined by 53° with respect to the surface plane. On the reduced Pt surface, on the other hand, the system follows the trend that higher H coverage leads to lower stability (EB = −0.91 eV, Figure S3b). The potential energy surface for quinoline adsorption on oxidized and reduced Pt as a function of the O and H coverage θ, respectively, is shown in Figure 1. In general, the quinoline

Figure 1. Potential energy surface for quinoline adsorption on oxidized Pt without O transfer to the molecule (red) and with O transfer to one of the C atoms (orange) and for adsorption on reduced Pt when no H transfer takes place (blue) and when H is transferred to the N atom (light blue). The numbers indicate the energy difference between different configurations and are given in eV. Data for H coadsorption from ref 9. Figure 2. Stable configurations of cinchonidine on Pt(111) in the presence of one O (a, c) and H (b, d) atom and transfer of the atomic species to the N atoms of the cinchonidine molecule. Configurations are shown for (a) O transfer and (b) H transfer to the N′ atom of the quinoline group and for (c) O transfer and (d) H transfer to the N atom of the quinuclidine moiety. Data for H coadsorption from ref 9.

molecule tends to destabilize at higher O and H coverage. This can be attributed to repulsive interactions between quinoline and the preadsorbed atomic species. One exception to this trend is the coadsorption with H at low coverage (θH = 1/36 ML) when the H atom binds to the N atom of the quinoline. This stabilizes the molecule by 0.26 eV with respect to adsorption of quinoline on clean Pt(111). More significant is stabilization when quinoline is coadsorbed with O at θO = 1 ML, which is by 5.06 eV more stable than quinoline on the partially (θ = 1/2 ML) oxidized surface (without O transfer to the molecule). On the reduced surface, quinoline is stabilized when H transfer to the N atom takes place. It has a similar effect at both a hydrogen coverage of θH = 1/36 ML (ΔEB = 0.30 eV) and of θH = 1/2 ML

stable configuration at an O coverage of θO = 1/36 ML is with the O transferred to the N atom of the quinuclidine moiety resulting in a binding energy of −1.30 eV (Table 3). Transfer of O to the C9′ is more favorable than transfer to the N atoms, however, still less stable by 0.32 eV than the reference state (Figure 3a). Oxidation of the vinyl group (CH2), on the other hand, leads to a stabilization of the system by 0.20 eV (EB = −2.47 eV, Figure 3c). D


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The Journal of Physical Chemistry C Table 3. Binding Energy EB of Cinchonidine Adsorbed on Pt(111) in the Presence of One O and H Atom (1/36 ML), Considering Various Possibilities of the Location of the Atomic Speciesa O O/H position

EB [eV]

on Pt on N′ on N on C9′ on CH2

−2.26 −1.52 −1.30 −1.93 −2.47

In the reference state, no O transfer takes place, and the O atoms are segregated on the surface to leave enough space for the molecule to adsorb. This is only possible up to a coverage of 0.5 ML. In these configurations, the cinchonidine molecule becomes destabilized with increasing O coverage, leading to an energy difference of ΔEB = 1.07 eV between adsorption on a clean Pt(111) surface and on the partially oxidized Pt surface with θO = 0.5 ML (Figure 4). Similar effects have been shown

H ΔEB [eV]

EB [eV]

ΔEB [eV]

0.74 0.96 0.32 −0.20

−2.27 −2.56 −2.45 −1.83 −1.94

−0.29 −0.18 0.44 0.33

a The effect of O (H) transfer to the cinchonidine molecule is represented in the relative energy ΔEB, referring to the energy when O (H) is adsorbed on an unaffected top Pt site. Data for H coadsorption from ref 9.

Figure 4. PES of CD adsorption on an oxidized and reduced Pt surface as a function of the O and H coverage without transfer of the preadsorbed atomic species O and H to the CD molecule. Configurations are shown for CD coadsorbed on Pt(111) with (a) O and (b) H at a coverage θ of 0.5 ML. Data for H coadsorption from ref 9.

previously on the reduced Pt surface: without H transfer the cinchonidine molecule becomes destabilized with increasing H coverage.9 When increasing the O and H coverage from 1/36 to 0.5 ML, the effect of destabilization is slightly more prominent on the oxidized surface (Figure 4). It is suggested that this is a result of the larger van der Waals radius of O leading to enhanced steric hindrance compared to H at the same coverage. Considering transfer of the surface atomic species O and H, the stability of the adsorbed CD molecule changes significantly. On the reduced surface, H transfer to the N′ atom of the quinoline group has been found to stabilize the adsorbed CD at low H coverage (1/36 ML) while further increase of the H coverage leads to a destabilization (Figure 5). In contrast, on the oxidized surface at θO = 1/36 ML, the CD molecule is less stable when the surface O atom binds to the N′ atom of the quinoline group, and only little variation in the stability is observed when increasing the coverage up to 0.5 ML (Figure 5). Instead, a significant stabilization is observed on the fully oxidized surface where the molecule has a binding energy of EB = −5.49 eV and is

Figure 3. Stable configurations of cinchonidine on Pt(111) in the presence of one O (a, c) and H (b, d) atom and transfer of the atomic species to the C atoms of the cinchonidine molecule. Configurations are shown for (a) O transfer and (b) H transfer to the C9 atom of the quinoline group and for (c) O transfer and (d) H transfer to the C11 atom of the vinyl group. Data for H coadsorption from ref 9.

This is in contrast to the behavior on the reduced surface. Equivalent simulations of H transfer have revealed a stabilization of the CD molecule when H is transferred to either of the N atoms by 0.29 and 0.18 eV (Figure 2b,d), while transfer to the C9′ atom of quinoline and the C11 atom of the vinyl group leads to a destabilization of the molecule by 0.44 and 0.33 eV (Table 3 and Figure 3b,d). Coverage Effect. The O coverage on Pt(111) has been changed to investigate its effect on the stability of the adsorbed cinchonidine molecule. In particular, a coverage θO of 1/36, 0.5, and 1 ML has been simulated, considering different configurations and O transfer processes at each coverage. E


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Figure 5. PES of CD adsorption on an oxidized and reduced Pt surface as a function of the O and H coverage when one of the surface O and H atoms transfer to the N′ atom of the quinoline group. Configurations are shown for CD coadsorbed on Pt(111) with (a1) O at θO = 0.5 ML and (a2) at θO = 1 ML and with (b1) H at θH = 0.5 ML and (b2) at θH = 1 ML. Pt atoms protruding from the surfaces are highlighted in red. Data for H coadsorption from ref 9.

Figure 6. PES of CD adsorption on an oxidized and reduced Pt surface as a function of the O and H coverage when one of the surface O and H atoms transfers to the C11 atom of the terminal vinyl group. Configurations are shown for CD coadsorbed on Pt(111) with (a1) O at θO = 0.5 ML and (a2) at θO = 1 ML and with (b1) H at θH = 0.5 ML and (b2) at θH = 1 ML. Data for H coadsorption from ref 9.

more stable by 3.18 eV compared to adsorption on the clean surface. This effect can possibly be attributed to the protrusion of one Pt atom from the surface (Figure 5a2). A similar behavior is found when the O atom transfers to the vinyl terminal group of the quinuclidine moiety (Figure 6). Hardly any change in stability is observed up to a coverage of 0.5 ML. Increasing the coverage further to 1 ML, however, the molecule becomes significantly stabilized (EB = −4.37 eV). In this configuration, one additional O atom is transferred to the C9 atom of the quinoline group. This leaves some space on the surface, increasing the stability of the adsorbed O atoms (see coverage effect on O in the section above) and thus of the whole system. In this configuration, the stabilization cannot be

attributed to the surface Pt protrusion as it has been found when O transfers to the N′ atom. Instead, the stabilization results from the additional O transfer to the C atom leading to a reduction of the effective O coverage. Adsorption of Cinchonine. Effect of Oxygen Transfer. Oxygen transfer to cinchonine has been studied to compare with cinchonidine at a coverage of θO = 1/36 ML. The reference state with the O adsorbed on a remote fcc site is by 0.02 eV less stable than the adsorption on clean Pt. Cinchonine is destabilized when the O atom transfers to one of the N atoms (Figure 7) similar to the behavior of CD. However, in contrast to CD, the least stable configuration is with the O atom bound to the N′ atom of the quinoline group with a binding energy of −1.58 eV (Table 4). F


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of −2.04 eV (Figure 8). Similar to CD, the most stable configuration of CN in the presence of one O atom is found

Figure 7. Stable configurations of CN on Pt(111) in the presence of one O (a, c) and H (b, d) atom and transfer of the atomic species to the N atoms of the cinchonine molecule. Configurations are shown for (a) O transfer and (b) H transfer to the N′ atom of the quinoline group and (c) O transfer and (d) H transfer to the N atom of the quinuclidine moiety. Data for H coadsorption from ref 9.

Figure 8. Stable configurations of CN on Pt(111) in the presence of one O (a, c) and H (b, d) atom and transfer of the atomic species to the C atoms of the cinchonine molecule. Configurations are shown for (a) O transfer and (b) H transfer to the C9 atom of the quinoline group and for (c) O transfer and (d) H transfer to the C11 atom of the vinyl group. Data for H coadsorption from ref 9.

Table 4. Binding Energy EB of Cinchonine Adsorbed on Pt(111) in the Presence of One O and H Atom (1/36 ML), Considering Various Possibilities of the Location of the Atomic Speciesa O O/H position

EB [eV]

on Pt on N′ on N on C9′ on CH2

−2.35 −1.58 −1.66 −2.04 −2.44

when oxygen transfers to the vinyl group with a binding energy of −2.44 eV. It is slightly less stable compared to the corresponding adsorption of the CD molecule (EB = −2.47 eV). Similar to CD, the behavior of O transfer to the CN molecule from the oxidized Pt surface is in contrast to H transfer to CN from the reduced Pt surface. On the reduced surface, H transfer to the N atoms has been shown to stabilize the system while H transfer to the C atoms destabilizes the system (Table 4).9 In all configurations, independent of O and H transfer, the vinyl group of the CN molecule is pointing upward, indicating negligible interactions with the (oxidized or reduced) Pt surface (see Figures 7 and 8). The vinyl group of adsorbed CD, on the other hand, always points toward the Pt surface, suggesting attractive interactions (see Figures 2 and 3). Coverage Effect. The O coverage effect has been studied for different configurations of CN on the oxidized Pt surface with θO = 1/36, 0.5, and 1 ML in analogy to CD adsorption. When CN is inert to O bonding (no O transfer to the CN molecule), the behavior is comparable to adsorption of CD. On both oxidized and reduced surfaces CN is destabilized with increasing coverage (Figure 9). At high coverage (0.5 ML) the destabilization effect is more pronounced on oxidized Pt even though the molecule binds closer (Figure 9a) to the surface compared to the adsorption on reduced Pt (Figure 9a) indicating covalent bonds.

H ΔEB [eV]

EB [eV]

ΔEB [eV]

0.77 0.68 0.31 −0.09

−2.32 −2.57 −2.75 −1.90 −1.54

−0.25 −0.43 0.42 0.78

a

The effect of O (H) transfer to the cinchonidine molecule is represented in the relative energy ΔEB, referring to the energy when O (H) is adsorbed on an unaffected top Pt site. Data for H coadsorption from ref 9.

This is more stable by 0.06 eV than adsorption of CD with O transfer to N′ where a binding energy of −1.52 eV has been found (Table 3). In the case of O transfer to the N atom of the quinuclidine moiety, adsorption of CN is more favorable by 0.33 eV than CD adsorption. This difference results from the position of the N atom which is pointing toward the Pt surface in CN (Figure 7c) while it is pointing upward in CD (Figure 2c). When O binds to the C9′ atom of the quinoline group, the CN molecule is destabilized by 0.31 eV, resulting in a binding energy G


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Figure 9. PES of CN adsorption on an oxidized and reduced Pt surface as a function of the O and H coverage without transfer of the preadsorbed atomic species O and H to the CN molecule. Configurations are shown for CN coadsorbed on Pt(111) with (a) O and (b) H at a coverage θ of 0.5 ML. Data for H coadsorption from ref 9.

On reduced Pt, a continuous destabilization of the adsorbed CN molecule with increasing H coverage has been found when one H atom binds to the N′ atom of the quinoline group. On the oxidized surface, the trends are similar up to a coverage of θO = 0.5 ML. Increasing the O coverage up to a fully oxidized Pt surface, a significant stabilization of the system is observed (Figure 10). This behavior is identical to the trends for CD adsorption. Several processes are taking place that can explain this stabilization by 7.04 eV when increasing the coverage from 0.5 to 1 ML. In the configuration on the fully oxidized Pt surface, O atoms are transferred to five C atoms of the quinoline group in addition to the N′ atom. Considering the stability effects that have been reported in the previous section with O transfer to C atoms being favored over O transfer to N atoms (Table 4) at low coverage (1/36 ML), this can possibly lead to some extent to a stabilization of the CN molecule on the surface. However, it is suggested that the stabilization mainly results from the protrusion of Pt atoms from the surface, which has been found as well in the case of CD adsorption (Figure 5a2). In the stable configuration of CN, two Pt atoms protrude from the surface (Figure 10a2), which possibly also explains the higher binding energy of CN (EB = −8.42 eV) compared to CD (EB = −5.50 eV). When O is transferred to the C11 atom of the terminal vinyl group, the O coverage hardly affects the stability up to θO = 0.5 ML (ΔEB = 0.03 eV with respect to the clean surface). In contrast, on the fully oxidized surface, the system is remarkably stabilized by 4.78 eV (with respect to θO = 0.5 ML) in analogy to

Figure 10. PES of CN adsorption on an oxidized and reduced Pt surface as a function of the O and H coverage when one of the surface O and H atoms transfer to the N′ atom of the quinoline group. Configurations are shown for CN coadsorbed on Pt(111) with (a1) O at θO = 0.5 ML and (a2) at θO = 1 ML and with (b1) H at θH = 0.5 ML and (b2) at θH = 1 ML. Pt atoms protruding from the surfaces are highlighted in red. Data for H coadsorption from ref 9.

CD adsorption and to the situation where different O transfer has been simulated. Again, in the configuration at 1 ML, one Pt atom protrudes from the surface plane (Figure 11), possibly leading to the remarkable stabilization.

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DISCUSSION Adsorption of Quinoline. Quinoline adsorption on an oxidized Pt surface becomes less favorable with increasing O coverage, suggesting repulsive interactions between the oxygen atoms and the quinoline molecule. For homogeneous distribution of O atoms on the surface, when no transfer of O surface atoms to the molecule takes place, the binding energy becomes even positive indicating an endothermic adsorption process and H


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surface is thermodynamically less favorable compared to adsorption on clean Pt. However, on the fully oxidized Pt surface, a remarkable stabilization of the quinoline molecule is observed with a binding energy of −4.29 eV. In this configuration, several surface O atoms react with the quinoline molecule. The transfer of O atoms to the molecule indicate a decrease of the O coverage on the surface, thus leading to a stabilization of the system. Furthermore, it induces the protrusion of surface Pt atoms, which can additionally contribute to the stabilization of the system. This effect has not been observed on the reduced surface. Adsorption of Cinchona Alkaloids. Adsorption trends on the oxidized Pt surface are similar for both investigated cinchona alkaloids cinchonidine (CD) and cinchonine (CN). In contrast to the reduced surface, where H transfer to the N atoms of the molecules leads to more stable configurations,9 oxygen transfer destabilizes the system in particular when O atoms bind to the N atoms of the molecules. A stabilization is only observed when O binds to the C11 atom of the terminal vinyl group (Tables 3 and 4, Figures 2 and 7). On the reduced surface, configurations of CD and CN change significantly with increasing H coverage. This is in particular presented by an inclination of the quinoline group with respect to the surface plane. In the case of CN, the inclination is observed in two different directions, at high coverage mostly with the vinyl group and the N′ atom pointing toward the surface. In contrast, on the oxidized surface, the inclination of CD and CN is only marginal without any obvious trends with increasing O coverage. Another effect observed on the reduced Pt surface with higher H coverage is the breaking of the C2−C3 bond in the case of nonparallel adsorption of the CD molecule and transfer of a surface H atom to the C11 of the terminal vinyl group.9 This effect does not occur for CN. On the oxidized surface, CD and CN behave in a similar way when O transfer to the molecules takes place. At a fully oxidized surface, the adsorption of the cinchona alkaloids changes significantly compared to adsorption on the reduced surface. In general, at high O coverage, the surface O atoms tend to bind to the C atoms of the molecules leaving space on the surface and thus reducing the repulsive interactions between O surface atoms. The accumulation of O atoms close to the molecule further results in a protrusion of Pt atoms from the surface (Figures 5a2, 10a2, and 11a2). Both effects lead to a drastic stabilization (decrease of the binding energy) of the system (Figures 4−6 and 9−11). Finally, it should be stressed that our analysis of the adsorption mode of the cinchona alkaloids does account for a solvent-free system under UHV (ultrahigh vacuum) conditions. Therefore, a possible influence of an interacting solvent molecule under reaction conditions cannot be ruled out. Practical Implication of Oxygen Coadsorption. As mentioned in the Introduction, the adsorption of cinchona alkaloids in the presence of oxygen is particularly interesting in the light of a possible application of cinchona alkaloids for chiral modification of platinum in the presence of oxygen, which may be relevant e.g. for the oxidative kinetic resolution of alcohols (Scheme 2). Albeit this reaction has to our knowledge not yet been experimentally established, its feasibility is supported by our study, which shows that the cinchona alkaloids are stable when coadsorbed with oxygen keeping their chiral pockets. Other practical implications are connected with the role of the presence of small amounts of oxygen in the reaction mixture of the Pt-catalyzed asymmetric hydrogenation of activated ketones.

Figure 11. PES of CN adsorption on an oxidized and reduced Pt surface as a function of the O and H coverage when one of the surface O and H atoms transfers to the C11 atom of the terminal vinyl group. Configurations are shown for CN coadsorbed on Pt(111) with (a1) O at θO = 0.5 ML and (a2) at θO = 1 ML and with (b1) H at θH = 0.5 ML and (b2) at θH = 1 ML. Pt atoms protruding from the surfaces are highlighted in red. Data for H coadsorption from ref 9.

is less stable by 2.55 eV than quinoline adsorption on clean Pt(111). Destabilization on the oxidized surface is more pronounced compared to quinoline adsorption on a reduced Pt surface when increasing the hydrogen coverage (Figure 1). In contrast to the reduced surface, where H transfer to the N atom of quinoline stabilizes the system,9 O transfer to the molecule always destabilizes the system. This is most prominent when O binds to N. Oxygen transfer to a C atom destabilizes the system by 0.5 and 1.48 eV for an O coverage of 1/36 and 0.5 ML, respectively, while O transfer to N leads to a destabilization by 1.17 and 2.55 eV, respectively. It follows that quinoline is unlikely to react with surface oxygen and adsorption on an oxidized Pt I


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indicate the feasibility of the application of cinchona alkaloids for chiral modification of Pt in the presence of oxygen, which is a necessary prerequisite for extending their scope to asymmetric oxidations.

Scheme 2. Oxidative Kinetic Resolution of Racemic Secondary Alcohol

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06991. Additional configurations of quinoline adsorbed on Pt(111) (Figures S1−S3) (PDF)

Previous studies on the effect of small amounts of oxygen in the reaction mixture of the asymmetric hydrogenation of ethyl pyruvate indicated a beneficial effect on activity and enantioselectivity in the presence of small amounts of oxygen/ air.29,30 Toukoniitty and Murzin29 ascribed it to the cleaning of the platinum surface from decomposition products (e.g., CO) of the reactant by oxygen, while Guan et al.30 attributed it to the presence of more electrophilic platinum species in the oxidized state promoting the interaction between the chiral modifier and substrate. A possible influence of the presence of oxygen on the adsorption mode of the modifier was not considered. Our theoretical studies indicate that an additional effect may have to be considered: oxygen coadsorption affects the adsorption mode and spatial orientation of the adsorbed cinchona alkaloids thereby changing the chiral space where the enantiodifferentiating interaction between chiral modifier and substrate occurs.31 This aspect should also be taken into account when explaining the effect of oxygen addition to the reaction mixture of asymmetric hydrogenation of ketones over cinchona alkaloid modified noble metals.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone +41 44 632 3153 (A.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.B. thanks the foundation Claude & Giuliana for supporting this work.

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REFERENCES

(1) Gellman, A. J.; Tysoe, W. T.; Zaera, F. Surface Chemistry for Enantioselective Catalysis. Catal. Lett. 2015, 145, 220−232. (2) Mallat, T.; Orglmeister, E.; Baiker, A. Asymmetric Catalysis at Chiral Metal Surfaces. Chem. Rev. 2007, 107, 4863−4890. (3) Blaser, H.-U.; Studer, M. Cinchona-modified Platinum Catalysts: From Ligand Acceleration to Technical Processes. Acc. Chem. Res. 2007, 40, 1348−1356. (4) Kyriakou, G.; Beaumont, S. K.; Lambert, R. M. Aspects of Heterogeneous Enantioselective Catalysis by Metals. Langmuir 2011, 27, 9687−9695. (5) Demers-Carpentier, V.; Goubert, G.; Masini, F.; Lafleur-Lambert, R.; Dong, Y.; Lavoie, S.; Mahieu, G.; Boukouvalas, J.; Gao, H.; Rasmussen, A. M. H.; et al. Direct Observation of Molecular Preorganization for Chirality Transfer on a Catalyst Surface. Science 2011, 334, 776−780. (6) Meemken, F.; Hungerbühler, K.; Baiker, A. Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy. Angew. Chem., Int. Ed. 2014, 53, 8640−4. (7) Lemay, J.-C.; Dong, Y.; Groves, M. N.; Demers-Carpentier, V.; Goubert, G.; Lafleur-Lambert, R.; Boukouvalas, J.; Hammer, B.; McBreen, P. H. A Comparative Study of Diastereomeric Complexes Formed by a Prochiral Substrate and Three Structurally Analogous Chiral Molecules on Pt(111). Surf. Sci. 2016, 646, 13−18. (8) Motobayashi, K.; Tomioka, R.; Uchida, T.; Osawa, M. Effect of Hydrogen on the Orientation of Cinchonidine Adsorbed on Platinum: An ATR-SEIRAS Study. Chem. Lett. 2015, 44, 770−772. (9) Hahn, K. R.; Seitsonen, A. P.; Baiker, A. Chiral Modification of Platinum: Ab Initio Study of the Effect of Hydrogen Coadsorption on Stability and Geometry of Adsorbed Cinchona Alkaloids. Phys. Chem. Chem. Phys. 2015, 17, 27615−27629. (10) Pellissier, H. Catalytic Non-Enzymatic Kinetic Resolution. Adv. Synth. Catal. 2011, 353, 1613−1666. (11) Mallat, T.; Baiker, A. Oxidation of Alcohols with Molecular Oxygen on Solid Catalysts. Chem. Rev. 2004, 104, 3037−3058. (12) VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. J. QUICKSTEP: Fast and Accurate Density Functional Calculations Using a Mixed Gaussian and Plane Waves Approach. Comput. Phys. Commun. 2005, 167, 103−128. (13) CP2K Developers, version 2.3.2012; www.cp2k.org.

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CONCLUSIONS The density functional theory calculations indicate a general destabilization of cinchona alkaloids (CD and CN) on the Pt(111) surface with increasing oxygen coverage. This effect is more prominent compared to that of the coadsorption of the cinchona alkaloids and hydrogen on Pt(111). No significant differences in configuration and stability exist in the adsorption of the two cinchona alkaloids on the oxidized surface in contrast to the adsorption on the hydrogen covered surface where different bond breaking and bond formation processes are observed for CD and CN. Cinchona alkaloids are destabilized when coadsorbed oxygen atoms transfer to the N atoms of the chiral modifier whereas in the case of a hydrogenated Pt surface the H transfer to N atoms results in a stabilization of the system. Adsorption of the molecules on a fully oxidized Pt surface leads to a drastic increase of stability. This behavior is attributed to the transfer of several O atoms to the cinchona alkaloid leaving space on the surface and thus reducing repulsive interactions between the adsorbates. Furthermore, at such a coverage, Pt atoms tend to protrude from the surface, which might further benefit the adsorption of the cinchona alkaloids. Both oxygen and hydrogen coadsorption affect the spatial orientation of the coadsorbed cinchona alkaloid, which in turn changes the chiral site where in the asymmetric reaction the enantiodifferentiating interaction between the chiral modifier and the substrate occurs. The calculations indicate stable adsorption of the cinchona alkaloids on the oxidized as well as reduced Pt(111) surface, which is a necessary requirement for bestowing chirality to Pt surfaces for oxidative and reductive asymmetric catalysis. While the potential of cinchona alkaloids as chiral modifier for asymmetric hydrogenation of ketones has been demonstrated in numerous studies, their potential for asymmetric reactions in the presence of oxygen such as the oxidative kinetic resolution of secondary alcohols still needs to be established. The present findings J


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The Journal of Physical Chemistry C (14) Lippert, G.; Hutter, J.; Parrinello, M. A Hybrid Gaussian and Plane Wave Density Functional Scheme. Mol. Phys. 1997, 92, 477−487. (15) Goedecker, S.; Teter, M.; Hutter, J. Separable Dual-Space Gaussian Pseudopotentials. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 1703−1710. (16) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (17) Tkatchenko, A.; Romaner, L.; Hofmann, O. T.; Zojer, E.; Ambrosch-Draxl, C.; Scheffler, M. Van der Waals Interactions Between Organic Adsorbates and at Organic/Inorganic Interfaces. MRS Bull. 2010, 35, 435−442. (18) Grimme, S. Semiempirical GGA-type Density Functional Constructed with a Long-range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (19) Steininger, H.; Lehwald, S.; Ibach, H. Adsorption of Oxygen on Pt(111). Surf. Sci. 1982, 123, 1−17. (20) Michaelides, A.; Hu, P. Catalytic Water Formation on Platinum: A First-Principles Study. J. Am. Chem. Soc. 2001, 123, 4235−4242. (21) Grabow, L. C.; Gokhale, A. A.; Evans, S. T.; Dumesic, J. A.; Mavrikakis, M. Mechanism of the Water Gas Shift Reaction on Pt: First Principles, Experiments, and Microkinetic Modeling. J. Phys. Chem. C 2008, 112, 4608−4617. (22) Lynch, M.; Hu, P. A Density Functional Theory Study of CO and Atomic Oxygen Chemisorption on Pt(111). Surf. Sci. 2000, 458, 1−14. (23) Miller, S. D.; Kitchin, J. R. Relating the Coverage Dependence of Oxygen Adsorption on Au and Pt fcc(111) Surfaces Through Adsorbate-induced Surface Electronic Structure Effects. Surf. Sci. 2009, 603, 794−801. (24) Ferri, D.; Bürgi, T. An in Situ Attenuated Total Reflection Infrared Study of a Chiral Catalytic Solid-Liquid Interface: Cinchonidine Adsorption on Pt. J. Am. Chem. Soc. 2001, 123, 12074−12084. (25) Kubota, J.; Zaera, F. Adsorption Geometry of Modifiers as Key in Imparting Chirality to Platinum Catalysts. J. Am. Chem. Soc. 2001, 123, 11115−11116. (26) Vargas, A.; Baiker, A. First Principles Study of the Conformations of Cinchonidine on a Pt(111) Surface. J. Catal. 2006, 239, 220−226. (27) Santarossa, G.; Iannuzzi, M.; Vargas, A.; Baiker, A. Adsorption of Naphthalene and Quinoline on Pt, Pd and Rh: a DFT Study. ChemPhysChem 2008, 9, 401−413. (28) Chase, M. W. NIST-JANAF Thermochemical Tables, 4th ed.; J. Phys. Chem. Ref. Data 1998, 11951. (29) Toukoniitty, E.; Murzin, D. Y. Effect of Cinchonidine and Dissolved Oxygen in Continuous Enantioselective Hydrogenation of Ethyl Pyruvate. Catal. Lett. 2004, 93, 171−176. (30) Guan, Z.; Lu, S.; Li, C. Highly Oxidized Pt Species Stabilized Inside Carbon Nanotubes for Asymmetric Hydrogenation. Chin. J. Catal. 2015, 36, 1535−1542. (31) Vargas, A.; Ferri, D.; Bonalumi, N.; Mallat, T.; Baiker, A. Controlling the Sense of Enantioselection on Surfaces by Conformational Changes of Adsorbed Modifiers. Angew. Chem., Int. Ed. 2007, 46, 3905−3908.

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Coadsorption of Cinchona Alkaloids and Oxygen on Pt(111): A

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