Enol Tautomerization

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Gold/Isophorone Interaction Driven by Keto/Enol Tautomerization Christian Stiehler, Niklas Nilius, Janne Nevalaita, Karoliina Honkala, and Hannu Häkkinen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06254 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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The Journal of Physical Chemistry

Gold/Isophorone Interaction Driven by Keto/Enol Tautomerization

Christian Stiehler,1,* Niklas Nilius,1,† Janne Nevalaita,2 Karoliina Honkala,2 Hannu Häkkinen 2 1

Institute of Physics, Carl von Ossietzky University, D-26111 Oldenburg, Germany

2

Departments of Physics and Chemistry, Nanoscience Center, University of Jyväskylä, P.O. Box 35, FI-40014 Finland

ABSTRACT The binding behavior of isophorone (C9H14O) to Au adatoms and clusters deposited on MgO/Ag(001) thin films is investigated by scanning tunneling microscopy (STM) and density functional theory (DFT). The STM data reveal the formation of various metal/organic complexes, ranging from Au1/isophorone pairs to larger Au aggregates with molecules bound along their perimeter. DFT calculations find the energetically preferred keto-isophorone to be unreactive towards gold, while the enoltautomer readily binds to Au monomers and clusters. The interaction is governed by electrostatic forces between the hydroxyl group of the enol and negative excess charges residing on the ad-gold. The activation barrier between keto- and enol-isophorone is calculated to be 0.76 eV, rendering tautomerization feasible at room-temperature. Our study provides evidence for an enol-driven chemical reaction of an organic molecule whose thermodynamic equilibrium lies far on the keto-side.

* Present address: Siemens AG, Rohrdamm 88, D-13629 Berlin, Germany † Corresponding author: [email protected], phone +49-441-798-3152

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INTRODUCTION

The complex adsorption and reaction behavior of organic molecules is directly related to their structural flexibility, i.e. their ability to form isomers. Typical isomerization processes include tilting, rotation and position switches of functional groups and enable the molecule to adapt to a given binding environment. Tautomerization is a specific form of structural isomerization, in which spatially adjacent single and double bonds change place and a hydrogen atom or proton migrates to a new position within the molecule. The classical example is the keto/enol tautomerization, in which the C=O group of a ketone transforms to the –OH group of an alcohol with a C=C bond in an adjacent position.1 The room-temperature equilibrium for the reaction typically lies far on the keto-side, for instance at 99.9997% for keto-acetone. However, a more balanced equilibrium is reported for specific examples, e.g. for ethenol-ethanal mixtures.2 The chemical reactivity of organic molecules might still be governed by the enol-tautomer that is often more reactive than the keto-form.3 Enolization occurs spontaneously if the anti-bonding orbital of the C=O group overlaps with a nearby H-atom (α-hydrogen) that, as a consequence, loses electron density and becomes acidic.1 The acidic hydrogen then migrates towards the carbonyl oxygen, forming the enol. Enolization is often catalyzed by acidic environments, in which the carbonyl oxygen temporarily binds a proton from the environment. The subsequent charge deficiency in the molecule is accounted for by changing a single into a double bond, followed by the release of the attached hydrogen. Apart from thermally-driven processes, tautomerization can be induced by electrons and light interacting with the molecule.4,5 A typical example is stilbene isomerization via UV-light, a process that is of interest for various photochromic applications. From a surface-science perspective, tautomerization has been explored in molecular ensembles as well as on the single-molecule level.6,7 For the latter, the scanning tunneling microscope (STM) turned out to be the ideal tool to image molecules before and after a tautomeric reaction induced by laser or electron irradiation.8,9 In addition, tautomerization could be stimulated with the tunneling electrons from an STM tip directly, providing fascinating insights into the quantum efficiency and threshold energy for such processes.10,11 In this paper, we address the adsorption characteristic of isophorone (C9H14O) on an MgO surface covered with small amounts of gold. Isophorone, an unsaturated cyclic ketone, occurs almost exclusively in the keto-form, as tautomerization to the enol is hindered by a substantial energy barrier in the gas phase. The keto-tautomer exhibits a strong molecular dipole that makes it relevant for probing electrostatic forces to the negatively-charged Au species occurring on thin MgO films.12 Indeed a variety of gold/isophorone complexes are detected with STM, although DFT calculations do not find any significant binding between both entities. This discrepancy may be resolved with the formation of enol-tautomers that are able to interact with the charged Au adatoms and clusters via electrostatic coupling. Possible tautomerization mechanisms are discussed at the end of the paper.

EXPERIMENT AND THEORY


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The experiments were performed in an ultrahigh-vacuum STM operated at liquid helium temperature. MgO films of 1-2 monolayer thickness, prepared via reactive Mg deposition onto an Ag(001) crystal in 1×10-6 mbar O2 at 550 K, were used as support.13 The films exhibited a sharp square-pattern in lowenergy electron diffraction, indicative of the (001) termination of rocksalt MgO. STM images revealed extended oxide islands coexisting with small, uncovered patches of silver. Isophorone, purified with several freeze-pump-thaw cycles, was dosed from a liquid flask at 300 K. Optimal coverage was obtained by exposing the sample at 1×10-8 mbar vapor pressure for 15s. In a subsequent step, 0.05-0.1 ML of gold was deposited from a thermal evaporator. Depending on the deposition temperature, either Au monolayer islands (300 K) or single adatoms (10 K) were found on the MgO surface.14 STM topographic images and differential conductance (dI/dV) maps, probing the local state-density, were recorded in the constant current mode and with a lock-in amplifier, respectively (25 mV Vmod, 957 Hz). The experiments were accompanied by spin-polarized DFT calculations, carried out with the GPAW code and the van-der-Waals-corrected BEEF functional, using real space grids of 0.2 Å periodicity.15,16,17 The system was modeled with a three-layer Ag(001) slab, freezing the bottom plane to bulk positions and covering the surface by an MgO monolayer whose O ions occupy Ag top sites. To mimic step sites, half of the MgO layer was removed. A 3×3 unit cell and 3×3×1 k-point sampling was used for systems containing an Au monomer, while 4×2 cells with 3×5×1 k-sampling were employed to model extended Au stripes. All structures were optimized until residual forces dropped below 0.05 eV/Å. The vacuum region between the non-periodic slabs was set to a minimum of 5 Å. Simulated STM images were obtained with the Tersoff-Hamann formalism.18

Fig.1: (a) Topographic STM image of an MgO/Ag(001) film after isophorone exposure (0.8 V, 50 pA, 30×30 nm2). Close-up images showing molecular assemblies on (b) the Ag(001) (12×7 nm2) and (c) the MgO surface (5×5 nm2). (d) Topography and (e) corresponding dI/dV map of MgO/Ag(001) after codosing single Au atoms and isophorone molecules (black and grey arrows) (18×18 nm2, 0.8 V). Dimer structures between Au1 and isophorone are marked with white arrows.



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RESULTS AND DISCUSSION

A topographic STM image of the MgO/Ag(001) surface covered with isophorone is shown in Fig. 1a. Individual molecules are clearly discernable as round protrusions with 2.8 Å height and 9 Å diameter. On the MgO film, they remain isolated or form small clusters containing up to 12 species, while aggregation into dense, hexagonal islands with 8.5Å periodicity is observed on pristine Ag patches (Fig. 1b). This observation suggests enhanced binding to silver as compared to wide-gap MgO, although calculated differences are small. The molecular clusters on the MgO exhibit characteristic sizes and shapes, pointing to the development of ‘magic’ islands (Fig. 1c). New features are detected after exposing the isophorone-covered surface to gold at low temperature. The Au monomers can be distinguished from the likewise spherical molecules by their dark circumference and a low contrast in dI/dV images taken at +0.5 V (Figs. 1d,e). The latter originates from the presence of excess electrons in the adatoms that have been transferred from the MgO/Ag support during adsorption.12 The oxide bands respond to the anionic gold with a local upward bending, producing the sombrero shapes observed in STM.14 While most of the pre-dosed isophorone remains isolated, a number of peculiar hybrid structures are identified on the surface after dosing. The smallest one appears to be an adatom/molecule pair with 7 Å center distance (Fig. 2). The different contrast clearly indicates the unequal nature of the two species, with the right one being assigned to an Au atom due to its sombrero shape and the left one to an isophorone molecule. The two entities are not only spatially adjacent but interact with each other, as deduced from the dI/dV maps shown in Fig. 2a. In empty-state images taken at +1.1 V, Au atom and molecule appear as dark and bright features, respectively, a contrast that reverses in filled-state maps taken at -0.8 V. More complex patterns, composed of bright and dark regions along the dimer axis, emerge at intermediate bias voltages. This delocalization effect points to an electronic coupling between the two partners. The bias range beyond ±1 V could not be probed here, as Au1/isophorone pairs easily dissociate at these conditions.

Fig.2: (a) Topography and dI/dV maps of an Au1 (right) / isophorone (left) pair on MgO (2.5×2.5 nm²). (b) Simulated maps of a corresponding pair formed between an Au monomer and enol isophorone. Comparable intensity patterns are observed in both cases.

In addition to dimer structures, larger compounds containing several gold and isophorone species were detected on the MgO surface. Figure 3a depicts a linear Au trimer with one attached molecule, while 4 ACS Paragon Plus Environment

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two Au oligomers plus two/three molecular ligands are identified in Fig. 3b. The complex, biasdependent hybridization patterns observed in the corresponding dI/dV maps point to an electronic coupling between the different entities also in this case. Note that linear Au clusters comprising up to seven atoms were revealed on MgO films even in absence of isophorone.19 They develop due to an energetic incentive to separate the excess charges of the involved atoms by localizing them on both sides of the 1D chain. Finally, large monolayer islands containing ten to several hundreds of atoms were found to interact with isophorone.20 The number of attached molecules is governed by the perimeter length, with each ligand requiring about 15 Å of space. The 60 Å-wide island in Fig. 3c, for example, is able to bind eleven molecules. These Aun/isophorone complexes exhibit an unexpected stability, as demonstrated with two subsequent STM images in Fig. 3d. The depicted Aun cluster (n∼10) underwent a spontaneous 25° rotation during scanning without losing its five molecular ligands. Although we cannot decide whether the molecules only slipped along the edge or the island participated in the motion, the integrity of the structure is clearly maintained. Fig.3: Topographic and dI/dV maps of Aun/isophorone complexes on MgO/Ag(001) films. The assemblies are tentatively composed of (a) Au3/one molecule (0.6 V, 2.5×2.5 nm2), (b) Au5/two molecules and Au3/ three molecules (5.5×5.5 nm2) and (d) Au10/five molecules (4.0×4.0 nm2). The latter complex experienced a 25° rotation during scanning, which does not involve the spatially adjacent molecule marked with an open circle. (c) Large Au island binding eleven isophorone molecules (0.6 V, 8.0× 8.0 nm2).

To obtain insight into the nature of gold/isophorone interactions on MgO/Ag(001), we have performed DFT calculations including van der Waals forces. While flat MgO binds the keto-isophorone only weakly (-0.69 eV), oxide step edges exert substantial electrostatic forces onto the molecular dipole (1.21 eV). No additional binding is revealed to Au adatoms on the MgO film, as reflected by a steadily increasing distance between both species during structural optimization (Fig. 4). The bond formation is hindered, because the anionic gold exerts a Coulomb repulsion to the negatively charge carbonyl oxygen, while the C=C bond on the ring, as the positive charge-center of keto-isophorone, is inaccessible to gold due to steric repulsion of the molecular side groups. The situation changes for the enoltautomer, being formed by shifting hydrogen from a C-atom in the ring to the terminal oxygen and removing the double bond at the keto-group (Fig.4).1,21 In the gas phase, the keto-tautomer is energetically preferred over the enol by 0.64 eV. This energy difference reduces to 0.18 eV for surface-bound species, as the enol is able to interact with both, bare MgO and ad-gold. Whereas the binding to MgO 5 ACS Paragon Plus Environment


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is mediated by Coulomb forces between the hydroxyl-oxygen and a surface Mg2+ ion, the hydroxylhydrogen gets attracted by the anionic Au species (Fig.4c). To enable comparison with experiment, the STM appearance of an Au1/isophorone pair is modeled in the Tersoff-Hamann scheme. We concentrate on the enol-tautomer, as the keto-form does not interact with gold. Despite an actual Au-hydroxyl distance of 2.27 Å, the corresponding maxima are separated by 7 Å in the simulation (Fig. 2b). Moreover, the isophorone appears brighter than the Au atom in the topographic channel, in agreement with the experimental data. Simulated conductance maps reproduce the bias-dependent contrast evolution, in which the bright side moves from the Au1 to the enolisophorone when going from empty to filled-state images. The enol-tautomer shows attractive coupling also to an Au stripe, used to mimic the Au islands in the experiment (Fig. 4b). The binding shows strong site selectivity and occurs preferentially along the island perimeter, where the enolhydroxyl is able to couple simultaneously to Au edge atoms and the MgO surface. Conversely, no binding is revealed on top of the Au stripe, as the stabilizing contribution of the MgO film vanishes. Keto-isophorone, on the other hand, experiences no net-attraction towards the Au islands and binding energies are similar on bare MgO and edge/top positions of the Au stripes. This implies that molecular adsorption is non-selective with respect to the Au stripe in this case. The observed binding preference of isophorone to the island perimeter thus provides additional evidence that the enol- and not the ketotautomer governs the interaction with gold.

Fig.4: (a) Ball models for keto and enol-isophorone with two sticks marking the double bonds. Adsorption geometry of both tautomers to (b) Au stripes and (c) Au monomers on the MgO/Ag(001) film. The Ag, Mg and O atoms are depicted in light grey, green and brown, respectively, while the Au is yellow. The molecular C, the carbonyl O and H atoms are shown with grey, red and white color.

Our DFT calculations indicate that the observed gold/isophorone interaction on MgO/Ag(001) may be the result of a tautomerization, producing the reactive enol-species. In the gas phase, the keto/enol equilibrium lies far on the keto-side, as the latter is energetically preferred by ∼0.5 eV. Only 1 out of 108 molecules should thus be in the enol-form at 300 K, insufficient to explain the observed reactivity. 6 ACS Paragon Plus Environment

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Also, IRAS measurements performed on the isophorone/Ru(0001) system at saturation coverage find only the keto-form.20 This discrepancy calls for a tautomerization reaction to be possible on the oxide film. To explore the underlying mechanism, we have calculated the barrier for keto/enol tautomerization on MgO/Ag(001) in presence of an Au adatom. We find a barrier of 0.76 eV, a value that renders a conversion energetically feasible at room-temperature (Fig. 5). The transition state is reached by abstracting a α-hydrogen next to the carbonyl oxygen and forming a hydroxyl group on the MgO surface. In a second step, the oxide OH dissolves again and the hydrogen moves to the carbonyl oxygen. The process might be promoted by negatively-charged Au species on MgO/Ag(001), being able to exchange charges with the isophorone and thus to release a temporary electron deficiency in the molecule during tautomerization.22 In other words, the anionic gold accounts for the missing electron at the carbon cage before the new C=C bond is established in the enolate. Note that the MgO/Ag(001) film is known to stimulate charge-driven processes, such as the growth of 1D and 2D Au islands19,23 or the activation of molecular oxygen.24,25 The reaction pathway depicted in Fig. 5 suggests an endothermic process, implying that tautomerization is unfavorable at thermal equilibrium. Still, the enol-form is kinetically accessible according to the calculated barrier of 0.76 eV. In order to rule out alternative schemes, we have checked whether enolformation may be catalyzed by exposed silver patches or MgO/Ag boundaries on the surface. Our DFT calculations reveal no evidence for these scenarios. In fact, keto-isophorone binds ∼0.7 eV stronger to Ag(001) than its enol-counterpart, which renders spontaneous keto/enol tautomerization on Ag(001) unlikely. Moreover, test experiments in which the area of open Ag patches has been reduced to a minimum revealed similar results regarding the isophorone adsorption behavior.

Fig.5: Potential curve along the reaction coordinate of keto/enol tautomerization on MgO/Ag(001) films in presence of an Au adatom.

CONCLUSIONS

Isophorone was found to attach to gold nanostructures on MgO/Ag(001) thin films, producing different hybrid structures, e.g. Au1/isophorone pairs and islands surrounded by molecular species. DFT calculations could reproduce these results only by assuming enol- and not keto-isophorone to be the active species. Given a calculated transition barrier of 0.76 eV in presence of Au adatoms, formation of the enol-tautomer is indeed feasible at room temperature although the keto-form is thermodynami7 ACS Paragon Plus Environment


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cally preferred. Let’s note that theory only provides a plausible interpretation of the experiment at this stage, because direct proof for the tautomerization reaction is missing. Verification of the proposed mechanism requires additional experiments, for instance via vibrational spectroscopy, to demonstrate the presence of enol-tautomers on the MgO/Ag(001) surface.

ACKNOWLEDGEMENTS: The experiments have been performed at the Chemical Physics Department of the FritzHaber-Institute Berlin and we are grateful to his director H.J. Freund for his permission to use the data in this publication. N.N. is grateful for a DFG grant ‘Exploring photocatalytic reactions at the atomic scale’. C.S. thanks the Studienstiftung for a fellowship. J. N. acknowledges a PhD grant from the Eemil Aaltonen foundation. The computer resources were provided by CSC- The Finnish IT center for Science.

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Enol Tautomerization

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