Exchange of Hydrogen between a Platinum Surface and a Tertiary

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Exchange of Hydrogen between a Platinum Surface and a Tertiary Amine: An ab Initio Molecular Dynamics Investigation Angelo Vargas,*,† Gianluca Santarossa,‡ and Alfons Baiker* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, H€onggerberg, HCI, 8093 Zurich, Switzerland ABSTRACT: The presence of a tertiary amine plays a role in accelerating the catalysis of ketones hydrogenation on a platinum surface covered by hydrogen. Ab initio molecular dynamics has been applied to achieve an atomistic understanding of this base effect. The interaction of trimethylamine with hydrogen adsorbed on a platinum nanoparticle has been simulated for 22 ps revealing that hydrogen increases its oscillatory modes upon interaction with the base therefore its activation. Nonetheless in the case of trimethylamine, competing interactions appear in the form of skeletal hydrogen (from the methyl group) interacting with the metal particle. Such interactions distract the action of the base and predominate at the end of the simulation. The interaction of the trimethylamine molecule with hydrogen adsorbed on platinum is compared to the corresponding interaction of the quinuclidine moiety of cinchonidine, which is the most widely applied chiral modifier for the platinum catalyzed enantioselective hydrogenations. The behavior of the quinuclidine moiety of cinchonidine on a Pt(111) surface is shown to be similar to the one of trimethylamine, since it also interacts with surface hydrogen causing an increase of the oscillatory amplitudes of the Pt-H bonds. But the skeleton of the alkaloid and its fixed adsorption mode via the quinoline moiety keeps the quinuclidine base in position for interacting with surface hydrogen, resulting in a greater stability of interaction between the base and the surface. In fact the quinuclidine base cannot give secondary skeletal interactions due to its optimally constrained position with respect to the metal. The investigation sheds some light on the mechanism of hydrogen activation on metal surfaces by means of tertiary amines and underlines the originality of metal surface chemistry with respect to homogeneous chemistry.

1. INTRODUCTION The study at the molecular level of the reactivity of hydrogen adsorbed on platinum surfaces is of fundamental importance for understanding heterogeneous hydrogenation catalysis based on this metal. In the following, we focus our attention on the hydrogen exchange between an amine and a platinum surface (Ptsurf.-H-N exchange), both as a general topic of interest in the investigation of the reactivity of surfaces and in view of the implications on the mechanism of the asymmetric hydrogenation of ketones on cinchona-modified platinum.1-4 Ptsurf.-H-N exchange resembles a Brønsted acid-base reaction in the homogeneous phase, where an acid and a conjugated base, or vice versa, exchange a proton according to the relative acid constant. In the case of heterogeneous reactions with metals, the acid is a metal surface (either the surface of a particle or a low Miller index surface) with chemisorbed hydrogen. Electron energy loss spectroscopy (EELS) experiments have shown that during coadsorption of pyridine and hydrogen on Pt(110) the pyridyl cation is formed.5,6 From a phenomenological point of view, experiments in catalysis have evidenced that tertiary amines can further accelerate the reaction of catalytic hydrogenation of ketones on a supported platinum catalyst.7 The role played by tertiary amines r 2011 American Chemical Society

in further accelerating the catalytic hydrogenation of ketones seems to indicate that they participate in the activated complex, thus resulting in a further lowering of the transition-state energy of the catalytic hydrogenation process. A similar but more pronounced rate acceleration in ketones hydrogenation occurs also in the presence of a cinchona alkaloid (Figure 1). Cinchona alkaloids are used as chiral surface modifiers for the asymmetric hydrogenation of ketones or CdC, and within the context of this reaction protocol they are viewed as chemical agents able to locally modify a metal surface generating chiral sites on originally achiral metal sites.1-4 Such alkaloids include in their structure a quinuclidine moiety, therefore a tertiary amine, that has been shown to be critical for the process of asymmetric hydrogenation2,8since its N-methylation inhibits its catalytic activity.7 Furthermore in situ experiments have shown that under reaction conditions a hydrogen bond is formed between the N-atom of the quinuclidine ring of the alkaloid and the ketocarbonil group of the substrate.9 Using a static model of surface and adsorbed cinchonidine (CD) surface modifier, we have Received: August 27, 2010 Revised: November 24, 2010 Published: January 19, 2011 1969

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Figure 1. Alkaloids of the cinchona family. When used as modifiers for the catalytic asymmetric hydrogenation of ketones, the quinoline base anchors the alkaloid to the surface and the stereogenic carbon atoms C8 and C9 impart the critical stereochemistry to the environment while the quinuclidine (tertiary amine) is able to form hydrogen bonds with the incoming substrate.

recently proposed that the quinuclidine moiety of CD rapidly exchanges hydrogen with the platinum surface, apparently without an appreciable activation barrier.10 In the present contribution we use ab initio molecular dynamics (MD) to investigate in some detail the process of Ptsurf.-H-N exchange. First we investigate the interaction between trimethylamine (TMA) and a nanoparticle of platinum bearing coadsorbed hydrogen. The MD simulation shows the exchange of H that nonetheless remains prevalently attached to the metal. Then we analyze the interaction of CD adsorbed on a Pt(111) surface with coadsorbed hydrogen. Also in this case during the MD simulation we observed a rapid hydrogen exchange between metal and the quinuclidine moiety, albeit with different typical times. In both cases coadsorbed hydrogen easily overcomes the activation barrier between the metal to the amine at 300 K. In the context of the simulations with CD we will discuss the special behavior of cinchona alkaloids as surface modifiers, in relation to the activity of the quinuclidine moiety during asymmetric hydrogenation of ketones and analyze the previously proposed reaction mechanism of this reaction in the light of the present investigation.

2. COMPUTATIONAL METHODS Gaussian and plane wave (GPW) formalism11,12 as implemented in the CP2K code13 has been used for the Born-Oppenheimer molecular dynamics (BOMD) simulations. We use the latest implementation of the GPW formalism in the Quickstep module of the CP2K program package, a suite of programs aimed at performing efficient electronic structure calculations and molecular dynamics at different levels of theory.14 The Kohn-Sham orbitals are expanded in terms of contracted Gaussian type orbitals (GTO), X CR i jR ðrÞ ψi ðrÞ ¼ ð1Þ R

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where ψi is the molecular orbital corresponding to the ith Kohn-Sham state, {jR} are the basis set functions, and {CR i} are the expansion coefficients. The auxiliary PW basis set is, instead, used only to expand the electronic charge density for the calculation of the Hartree potential. To limit the number of PW basis functions, the interaction of the valence electrons with frozen atomic cores is described using norm-conserving, dual-space type pseudopotentials (PP).15 For Pt atoms we used a PP including all of the electrons up to the 5s levels in the core, thus treating the 18 electrons, corresponding to the 5p and 5d, explicitly in the valence. The GTO basis set adopted is optimized for the specific pseudopotentials on the basis of the Goedecker-Teter-Hutter (GTH) method. We used the triple-ζ valence (TZV) basis set for the metal and TZV2P for the lighter elements. For the auxiliary PW expansion of the charge density, the energy cutoff has been set at 300 Ry. The exchange and correlation term was modeled using the PerdewBurke-Ernzerhof (PBE) functional.16 For the solution of the selfconsistent field (SCF) equations, we used an optimizer based on orbital transformations, which scales linearly in the number of basis functions .17 It has been already demonstrated that this optimization algorithm, in combination with the GPW linear scaling calculation of the Kohn-Sham matrix, can be used for applications with several thousands of basis functions.18 A time step of 0.5 fs and a wave function convergence of 10-5 guarantee energy conservation during the dynamics. The sampling of the canonical ensemble at 300 K is obtained by coupling to a thermostat using the canonical sampling through a velocity rescaling (CSVR) scheme19 with a time constant of 100 fs.

3. RESULTS 3.1. Reaction between Trimethylamine and Hydrogen Adsorbed on a Pt13 Nanoparticle. To gain a first insight of

the interaction between a tertiary amine and hydrogen adsorbed on a Pt surface, we constructed a square simulation supercell with a side of 22 Å by placing at its center a Pt13 nanoparticle bearing six hydrogen atoms adsorbed on its surface. The platinumhydrogen system was obtained by running an MD simulation in the same supercell, including the bare metal particle, in its optimal configuration20 together with an excess of dihydrogen molecules, and by stopping the simulation when only six hydrogen atoms were adsorbed on the surface of the nanoparticle. For the purpose of our simulation it was in fact interesting to have a platinum nanoparticle where both metal surface sites and metal sites with chemisorbed hydrogen were exposed. Successively we included 15 molecules of TMA. The initial state is represented in Figure 2. From this initial configuration the simulation was run for ca. 22 ps at 300 K. After a short first period of equilibration, the nitrogen of a TMA molecule started interacting with one of the adsorbed hydrogen atoms (Figure 3a-c). The system remained in this state for ca. 14 ps during which time the hydrogen was rapidly exchanged between the metal and the amine. The distance between N and H and the distance between Pt and H oscillated. The oscillatory behavior of the N-H and Pt-H distances is shown in the diagrams in Figure 4. Figure 4a shows that after 14 ps the TMA disengages its interaction with the surface hydrogen. The effect of the detachment is best observed in the right part of Figure 4a, where the distribution of the TMA-H distances are depicted before (blue histogram) or after (red histogram) the detachment of the TMA molecule. While before the disengagement of the TMA the distribution of the N-H distances has a narrow, Gaussian-like shape, after the 1970

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Figure 2. Simulation supercell (22 Å  22 Å  22 Å) used for the investigation of the interactions between trimethylamine (TMA) and hydrogen adsorbed on a Pt13 nanoparticle.

interaction is broken the distribution has a broader shape, with several lower maxima. Figure 4b shows that the oscillatory behavior of the Pt-H distances has a higher amplitude during the interaction of H with TMA, while, as soon as the TMA disengages from the interaction with the hydrogen, the oscillatory amplitude diminishes. The distribution of the Pt-H bond distances, reported in the right part of Figure 4b, shows that the presence of TMA is affecting the interaction of the hydrogen with the metal particle. While the hydrogen atom interacts with the TMA, the distribution of distances is very broad, indicating a larger mobility of the hydrogen on the surface (blue histogram). On the contrary, the detachment of the TMA from the surface hydrogen has the effect of decreasing the flexibility of the Pt-H distance, thus restabilizing the H atom on the surface (red histogram). The larger distribution of distances found in the case of the interaction of the TMA molecule with the surface hydrogen is an indication of the ability of the TMA to activate the surface hydrogen. In other words, this behavior exemplifies the activation of hydrogen on the surface due to the interaction with the tertiary amine. During the simulation a second type of interaction occurs, between the methyl hydrogen atoms of the TMA and platinum atoms of the nanoparticle (Figure 3d). At the end of the simulation four such interactions are present, while the only interaction of the amine nitrogen with surface hydrogen has disappeared (Figure 3d). The surface of the metal particle is to a large extent occupied by such secondary interactions, while little space is left for the N-H bonds to occur. Interestingly during the simulation no direct binding between the tertiary amine and platinum atoms of the nanoparticle takes place. 3.2. Cinchonidine on Platinum(111). Figure 5 shows the simulation supercell used for the investigation of the interaction between two main conformers of CD (Surface Open4 (SO4) and Surface Open3 (SO3)) and hydrogen coadsorbed on a Pt(111) surface. This model is based on previous investigations on the adsorption of CD on a Pt(111) surface.10,21-23 The supercell had

Figure 3. Hydrogen exchange between platinum and TMA. In the proximity of the tertiary amine hydrogen is at first only attached to the metal (a), is then approached by the TMA nitrogen (b), and collected by the TMA (c). The exchange process is fast (units of picoseconds), but most of the simulation time H stays closer to the surface than to the TMA. After ca. 22 ps of simulation the TMA interacts with the metal particle only through the methyl hydrogen atoms (d).

dimensions of 16.8222 Å  14.5683 Å, and included a 6  4 platinum surface for a depth of four layers (Figure 5). In the z direction 25 Å of space guarantees that no interactions occur between the images of the supercell. The total number of Pt atoms explicitly considered was 96, and periodic boundary conditions were applied to reproduce the extension of the slab in the three dimensions of space. CD is adsorbed on a platinum slab via its quinoline moiety, and hydrogen is coadsorbed on the metal. CD was adsorbed in two different ways, namely, as SO(3) 1971

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Figure 4. Hydrogen exchange between platinum and TMA. In the graphs, the blue lines show the distances between the H atom and the metal nanoparticle or the TMA. The red dots indicate the presence of a bond between the H and the nanoparticle or the TMA. The horizontal dotted lines show the threshold values chosen for the determination of the presence of a bond. The vertical line shows the time at which the H atom detaches from the TMA. The distributions of bond distances are shown at the right of each graph. Graph a shows the variation of the N-H distances during the interaction of TMA with hydrogen: after 14 ps TMA discontinues its interaction with surface hydrogen, as evidenced by the increase of the N-H distance. Graph b shows the oscillatory behavior of the Pt-H distance: during the interaction with TMA the surface hydrogen experiences wider oscillation amplitudes, while, after the interaction with TMA ceases, the oscillation amplitudes decrease.

and SO(4). The two conformations differ by the solid angle τ1 between the quinoline moiety and the rest of the molecule (Figure 1).22 Investigations concerning the nature of such adsorption sites have been previously published;21-23 we here briefly mention that SO(4) and SO(3) are the most thoroughly investigated among the surface conformations of CD and that all other surface conformations can derive from these two by simple conformational rearrangements around the carbon atoms C8 and C9 (Figure 1). Importantly a switch between SO(4) and SO(3) implies a desorption-readsorption step. For this reason the set of conformations present on the surface is probably strongly biased by the first adsorption event. In the present study, for the sake of completeness we have investigated the dynamic behavior of both SO(4) and SO(3) adsorption conformations, during 5 ps of MD. It has been shown that when CD is adsorbed on a Pt surface, if no surface hydrogen is present, the most stable adsorption mode of the alkaloid corresponds to the chemisorption of both the quinoline and quinuclidine moieties to the platinum surface. Such adsorption conformation in fact can count on the additional binding energy of the quinuclidine nitrogen to the platinum.22 Nonetheless when the metal surface is covered with hydrogen, the metal sites are shielded. Therefore we have chosen to perform our simulations starting from CD conformations where the quinuclidine moiety is not interacting with the metal, thus observing the preferential interaction pattern of the tertiary

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Figure 5. Simulation supercell of CD adsorbed on Pt(111) with coadsorbed hydrogen. SO(3) and SO(4) differ by the values of the solid angle τ1 (Figure 1) between the quinoline moiety and the other part of the molecule.

amine with either chemisorbed hydrogen or metal sites. Within this kind of simulation the relative energies of the SO(4) and SO(3) conformations in the presence of coadsorbed H atoms are better assessed dynamically. The energy oscillations at 300 K of both systems are larger than the energy differences between the two adsorbed conformations. In other terms, supposing that statically one of the conformers was more stable than the other, at 300 K the difference in energy is lost in the noise of the oscillations of the total energy. It should be stressed that in any case the dynamic evolution of the energies at a given temperature is more representative of the system under investigation than the energies of single conformers at 0 K obtained from static calculations. From this point of view, at 300 K the stabilities of the two surface conformers are comparable. During both dynamics simulations (SO(4) and SO(3)) at 300 K we observe that quinuclidine nitrogen bends to the surface, interacts with surface hydrogen, and removes it from the metal (Figures 6 and 7 show snapshots taken from the MD trajectories). This process of exchange occurs in the picosecond time scale. This dynamic behavior confirms that the activation barrier of the exchange process is lower than kBT at 300 K, as we had 1972

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The Journal of Physical Chemistry C previously proposed .10 Interestingly also in this case interactions between the quinuclidine nitrogen and the platinum surface do not occur within our simulation time. Hydrogen exchange seems therefore to be the fastest chemical reaction occurring when CD is adsorbed on platinum in the presence of coadsorbed hydrogen. From this point of view there is no difference between the behavior of SO(4) and SO(3) adsorbed on the surface and in the presence of coadsorbed hydrogen (we will show the differences in the next lines). Interestingly the quinuclidine nitrogen does not interact with only one hydrogen atom. The conformational flexibility of adsorbed CD allows a broader field of action of the tertiary amine. In fact in the case of the SO(4) conformation a hydrogen picked up from a metal site is delivered back to another metal site (Figure 6). In the case of the SO(3) conformation of CD the H atom is removed and replaced on the same metal site, but the hydrogen rapidly changes its position between two adjacent metal atoms, so that the sites of interest for the interaction of H with CD are two also in this case. This is shown in Figure 8: the scope of the attack for the quinuclidine nitrogen is constituted of two metal sites, different for each conformation. While the SO(4) conformation interacts with hydrogen atoms on sites B1 and B2 (Figure 8a), the SO(3) conformation interacts with H atoms from sites A1 and A2 (Figure 8b). It is worth noting that the kinetic energy of the system allows the surface H atoms to jump over the different metal sites and to adsorb with both top and bridge adsorption modes, while interacting with the CD molecule. The obvious implication for enantioselectivity is that different hydrogen uptake (or the interaction of protonated CD with surface platinum) poses different geometrical constraints to the active conformation of CD toward its further reactivity with a substrate. Therefore SO(4) and SO(3) conformations have in principle different enantioselective potentials which are also due to the different interaction with surface hydrogen. Figures 9 and 10 show the analysis of the variation of the N-H bonds (Figures 9a and 10a), of the Pt-H bonds (Figures 9b,c and 10b,c) and the variation of L€owdin charges on the quinuclidine nitrogen (Figures 9d and 10d) during the simulation, for both SO(4) (Figure 9) and SO(3) (Figure 10) surface conformations of CD. The N-H distance shows a very neat oscillating behavior for both conformations (Figures 9a and 10a). During most of the simulation time the H atom is bound to the metal, while it binds to nitrogen for ca. 0.5 ps with a periodicity of little more than 2 ps. The distance between platinum and hydrogen is shown in two graphics, the first (Figures 9b and 10b) being a measure of the distance of H from the plane of the metal, while the second measures the distance of H from the sites B1 and B2 (Figures 9c) and A1 and A2 (Figures 10c). Distances of H from the plane of the metal are less smooth compared to the N-H distances (Figures 9b and 10b), but follow closely the same periodicity. It must be noted that the loss of smoothness in the graphics of the distances of H from Pt is due to the interaction of hydrogen with more than one metal site. The oscillation is perturbed by the potential felt by a neighboring metal site. This is seen well in the graphics in Figures 9c and 10c, where the distances of hydrogen from the metal sites B1 and B2 and A1 and A2 are shown, respectively. Even when hydrogen is not interacting with the quinuclidine nitrogen, the distances between H and the metal sites B1, B2 and A1, A2 increase, since the H migrates to other metal sites. Only when returning to B1, B2 and A1, A2 metal sites CD is able to form N-H bonds again. Analysis of the charges (Figures 9d and 10d) shows that in correspondence to the transfer of hydrogen to the quinuclidine nitrogen, the latter

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Figure 6. Molecular view of the hydrogen exchange between CD and platinum for the SO(4) surface conformation. Panels a-d represent different phases of the process during which the quinuclidine nitrogen interacts with a surface hydrogen, removes it, and redeposits it (in this case on a different metal site).

decrease its electronic charge (becomes more positive) hinting to a process that resembles proton transfer. Table 1 reports the average L€owdin charges of the quinuclidinic N, the interacting H 1973

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Figure 8. Metal sites of attack of the quinuclidine base. Hydrogen is picked up from sites B1 and B2 by CD in the SO(4) conformation, while they are picked up from sites A1 and A2 in the SO(3) conformation.

could be observed by comparing the SO(3) with the SO(4) conformations of the CD. When it is adsorbed on the surface, the interacting H atom has a L€owdin charge of 0.05, while when it is taken by the CD the L€owdin charge becomes more positive (0.13). The modification of the charge of the H atom when it adsorbs to the CD is an indication of the increased acidity of the atom when attached to the alkaloid. Accordingly, the N atom of the quinuclidine partially loses its negative charge (-0.10) after the interaction with the H atom (0.11). The hydrogen uptake also affects the charges on the Pt surface. While the Pt surface is almost neutral before the hydrogen uptake (-0.05), it becomes negatively charged after the event (-0.56). The negative charge is localized on the first Pt layer, in the vicinity of the pick-up sites (A1, A2 for SO(3) or B1, B2 for SO(4) conformations), where the enantiodifferentiation is assumed to occur.

Figure 7. Molecular view of the hydrogen exchange between CD and platinum for the SO(3) surface conformation. Panels a-d represent different phases of the process during which the quinuclidine nitrogen interacts with a surface hydrogen, removes it, and redeposits it on the metal.

and the Pt surface for the SO(4) and SO(3) adsorption modes of CD, when the H is adsorbed on the surface and when it is interacting with the CD molecule. No remarkable differences

4. DISCUSSION The previous simulations show a pattern of interaction between tertiary amines and platinum in the presence of adsorbed hydrogen. TMA preferentially interacts as a base with surface adsorbed hydrogen rather than with the metal sites. During ca. 22 ps. of dynamics no interaction event has been observed between the nitrogen atom of TMA and the metal. The acid-base type reaction is nonetheless hindered by the occurrence of interactions between the hydrocarbon part of the TMA with the metal. Hydrogen atoms of the methyl groups are favored in such interaction because they are more numerous than N atoms (ratio 1:9), more exposed, and have no tendency to bind with surface hydrogen atoms. It has been experimentally shown that tertiary amines accelerate the catalytic hydrogenation of 1974

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Figure 9. Oscillatory behavior of the N-H and Pt-H distances during the evolution of the trajectory obtained by ab initio molecular dynamics for CD SO(4) (a-c) and the L€owdin charges on the quinuclidine nitrogen (d) for CD SO(4). In the graphs, the vertical lines show the time at which the H atom detaches from the surface. The N-H bond has a smooth oscillatory behavior (a); the distance of H from the plane of the platinum surface follows the same periodicity as the N-H bond (b); the distances between the platinum B1 and B2 sites and exchanged H show a more complex behavior, since the hydrogen atom can migrate to platinum atoms other than B1 and B2 (c); the L€owdin charges on the quinuclidine nitrogen become more positive when the N-H bond is formed.

Figure 10. Oscillatory behavior of the N-H and Pt-H distances during the evolution of the trajectory obtained by ab initio molecular dynamics (a-c) and the L€owdin charges on the quinuclidine nitrogen (d) for CD SO(3). In the graphs, the vertical lines show the time at which the H atom detaches from the surface. The N-H bond has a smooth oscillatory behavior (a); the distance of H from the plane of the platinum surface follows the same periodicity as the N-H bond (b); the distances between the platinum B1 and B2 sites and exchanged H shows a more complex behavior, since the hydrogen atom can migrate to platinum atoms other than A1 and A2 (c); the L€owdin charges on the quinuclidine nitrogen become more positive when the N-H bond is formed.

ketones on platinum catalysts, with an efficiency that is proportional to the basicity of the amine. This further catalysis (the primary given by the metal itself) could be attributed to the activation of surface hydrogen by interaction with an amine. The increased amplitude of the oscillations of adsorbed hydrogen can decrease the energy barrier to hydrogen transfer. The increase of the catalytic effect with basicity can be interpreted as being due to the possibility of a stronger base to increase the oscillatory

amplitude to a larger extent. Experimentally the acceleration of the hydrogenation rate of ketones in the presence of a tertiary amine (base effect) is only moderate, but always present. The moderate proportion to which the base effect occurs can be interpreted as being due to the competing adsorption of the amine on the metal in positions that do not favor the interaction of the amino nitrogen with surface hydrogen. This competing adsorption can hinder the accessibility of the acid (hydrogenated 1975

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Table 1. L€ owdin Charges of the Quinuclidinic N, the Interacting H, and the Pt Surface for the SO(4) and SO(3) Adsorption Modes of CD, during the Interaction of H with the Surface or with the CD Molecule SO(4) N surface H CD-H

H

SO(3) Pt

N

H

Pt

-0.10

0.05

-0.05

-0.09

0.04

0.02

0.11

0.13

-0.56

0.13

0.11

-0.48

surface) from the tertiary amine, thus limiting its catalytic activity. A different situation appears when the tertiary amine is conformationally constrained as in the case of the quinuclidine moiety of cinchona alkaloids. It has been noted that the effect of rate acceleration in the hydrogenation of ketones caused by cinchona alkaloids is much more pronounced than the base effect of quinuclidine alone.7 The simulations of the behavior of adsorbed CD in the presence of adsorbed hydrogen show that the quinuclidine moiety can activate surface hydrogen very effectively without interference from the hydrocarbon skeleton, because the tertiary amine is kept in position by the anchoring of the alkaloid to the surface, guided by the quinoline moiety. In such arrangement the base (quinuclidine moiety of CD) preferentially directs the N atom toward the metal surface and can interact with hydrogen without interference. The picked-up hydrogen possess stronger acidic characteristics compared to the hydrogen on the metal surface. This may as well contribute to the rate acceleration noted in the presence of CD. We have already noted in the previous paragraph that the sites of preferential attack of the base are also constrained by the conformation, being different according to the adsorption conformation of CD. Note that the enantiodifferentiation originates from the interaction of the adsorbed chiral modifier (cinchona alkaloid) with the substrate via hydrogen bonding between the N-atom of the quinuclidine moiety and the oxygen of the ketocarbonyl group of the substrate. The adsorbed diastereomeric transition complex determines the chiral space responsible for the enantiodifferentiation as exemplified and explained in previous studies.2 The hydrogen in this bonding stems either from the protonation of the N-atom of the quinuclidine ring (in protic solvents) or hydrogen exchange with the platinum surface. The hydrogen exchange investigated in the present study is thus crucial for the enantioselective hydrogenation. One further source of rate increase operating for chirally modified surfaces is the chiral recognition itself, that provides a favorable entropic effect by increasing the local concentrations of the reactant in the proximity of the modifier.23 This rationalization of the ligand acceleration effect on the basis of entropic contributions has been proposed by Page and Jencks for the case of biological catalysts.24 Interestingly, it has been recently shown that the presence of the ketone substrate in the reaction medium also accelerates the catalytic hydrogenation of the alkaloid itself (at the level of the quinoline anchoring group), probably by perturbation of its optimal adsorption mode.25 The question of which surface conformation of the alkaloid is responsible for enantioselectivity has been addressed elsewhere.23,25 Another important aspect of the phenomenon of rapid hydrogen exchange between CD and platinum surface is the blocking of the nucleophilic activity of the quinuclidine base. Our simulations show that at 300 K the activation energy necessary for hydrogen exchange is easily overcome by the kinetic energy of

the system. In the past years mechanistic proposals for the asymmetric hydrogenation of ketones on cinchona modified platinum have been put forward based on the assumption that the quinuclidine nitrogen could act as a nucleophile toward the prochiral ketone.26 The relevance of the nucleophilic activity of the quinuclidine moiety of cinchona alkaloids is questioned by our investigation, since in the presence of chemisorbed hydrogen the amino nitrogen results immediately blocked by a hydrogenation process that resembles the homogeneous reaction of protonation. Evidently this interaction blocks the nucleophilic activity of the quinuclidine nitrogen while it favors the H exchange between modifier and substrate. It should be noted that the above discussion requires the contribution of the metal surface, acting as a proton donor, and the effect of the constraints posed by surface adsorption to the adsorbed alkaloid. This behavior generates reaction mechanisms that are specific to the metal surface and that therefore deviate from those acting when the same molecules are homogeneously dissolved in a solvent system.

5. CONCLUSIONS Ab initio molecular dynamics simulations have been performed to analyze the interaction between tertiary amines and hydrogen adsorbed on a platinum surface. Trimethylamine interacts with the hydrogen adsorbed on the surface of a platinum nanoparticle mainly in two ways: through the amino nitrogen, thus resembling a Brønsted acid-base reaction, and through the methyl hydrogen atoms, thus hindering the acid-base type interaction. Within the time of the performed simulations (ca. 22 ps) the amine nitrogen never interacted directly with metal sites. During the acid-base type interaction the surface hydrogen bound to the amine increased its oscillation amplitude, thus becoming highly activated for further hydrogen transfer. The greater number of methyl hydrogens nonetheless favor interactions of the trimethylamine with the metal via the hydrocarbon skeleton, thus reducing the proportion of feasible acid-base type of interactions. The interaction of the quinuclidine moiety of CD with platinum surface hydrogen has also been investigated. The geometrical constraints given to the tertiary amine by the skeleton of the alkaloid and by its adsorption mode favor acid-base type of interactions, since the quinuclidine is set in the right position for attacking surface hydrogen through the amino nitrogen rather than with its hydrocarbon skeleton. The results of the simulations here presented are able to consistently interpret experimental results, show a mechanism of platinum-adsorbed hydrogen activation by means of a tertiary amine, and exclude to a large extent the possibility that cinchona alkaloids can perform catalysis via the nucleophilic activity of the amino nitrogen, since the activation barrier to its hydrogenation is easily overcome at room temperature and therefore to its deactivation as a nucleophile. The ab initio MD simulations provide a further level of rationalization in the complex mechanism of platinum catalyzed surface hydrogenation of ketones, and give an atomistic level of interpretation of metal surface acid-base reactions. From a broader point of view, the understanding achieved shows the originality of the reaction mechanisms occurring at the surface of a metal, as compared to those taking place in the homogeneous phase. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (A.B.); [email protected] (A.V.). Tel.: þ41 44 6323153. Fax: þ41 44 6321163. 1976

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Present Addresses †

BASF Construction Chemicals Europe Ltd., 8048 Zurich, Switzerland. ‡ Institute of Process Engineering, Department of Mechanical and Process Engineering, ETH Zurich, CH-8092 Zurich, Switzerland.

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dx.doi.org/10.1021/jp1081577 |J. Phys. Chem. C 2011, 115, 1969–1977