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Elucidating the binding modes of N-heterocyclic carbenes on a gold surface Anne Bakker, Alexander Timmer, Elena Kolodzeiski, Matthias Freitag, Hong Ying Gao, Harry Moenig, Saeed Amirjalayer, Frank Glorius, and Harald Fuchs J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06180 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018
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Journal of the American Chemical Society
Anne Bakker1,2, Alexander Timmer1,2, Elena Kolodzeiski1,2,4, Matthias Freitag3, Hong Ying Gao1,2, Harry Mönig1,2, Saeed Amirjalayer*1,2,4, Frank Glorius*3, Harald Fuchs*1,2 1
Physikalisches Institut, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany. 2Center for Nanotechnology, Heisenbergstraße 11, 48149 Münster, Germany. 3Organisch-Chemisches Institut, Westfälische Wilhelms-Universität, Corrensstraße 40, 48149 Münster, Germany. 4Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität, Corrensstraße 40, 48149 Münster, Germany. Supporting Information Placeholder ABSTRACT: Tuning the binding mode of N-heterocyclic carbenes (NHCs) on metal surfaces is crucial for the development of new functional materials. To understand the impact of alkyl sidegroups on the formation of NHCs species at the Au(111) surface, we combined scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. We reveal two significantly different binding modes depending on the alkyl length. In case of the short alkyl-substituent an up-standing configuration with one Au adatom is preferred, whereas the longer alkyl groups result exclusively in NHC-AuNHC complexes lying flat on the surface. Our study highlights how well-defined structural modifications of NHCs allow to control the local binding motif on surfaces, which is important to design designated catalytic sites at interfaces.
NHCs have developed into a powerful tool with a broad range of applications as catalysts and as ligands for p-block elements and transition metals.1,2 In particular, to transfer the unique electronic, optical and catalytic properties of gold into functional materials,3 NHCs allow electronic and steric stabilization of gold centers by their strong σ-donating properties and their tunable side-groups.4,5 Therefore, not only NHC-gold complexes,6,7 but also NHCfunctionalized nanoparticles8-12 and surfaces13,14 have been realized. In all these cases the carbene carbon is directly connected to a gold center. In general, for the anchoring at surfaces, four different, stable binding modes can be defined for a single NHC. However, hypothetically a continuum of orientations is possible. Considering the orientation of the central NHC ring with respect to the surface, the molecule can stand up or lie down, either bonded directly to the surface or via an adatom (Figure 1.A). Since these different orientations will potentially influence the reactivity and/or selectivity for catalytic applications, a better understanding of this atomic binding is crucial. In recent years, STM-investigations have shown that NHCs can form ordered monolayers on Au(111), Ag(111) and Cu(111) surfaces.15-18 It is known that the electronic and steric properties of the nitrogen-substituents can strongly influence the reactivity of the metal.10 Therefore, it is important to investigate the effect of these side-groups on the binding and orientation of NHCs on surfaces. Previously, we could show that including a bulkier side group can induce the transition between the Upsurf and Upad binding mode (Figure 1.A).16 However, flat-lying NHCs
Downsurf or Downad), which would allow direct access to the reactive gold center, have not been observed yet. Interestingly, our calculations of the binding mode of 1,3-dibutylimidazolium (IBu) on Au(111) reveal two stable configurations: Upsurf and Downsurf (Figure 1.C). In contrast to IMe, IBu energetically prefers the Down surf binding mode by Δ(IBu)= 5.6 kJ/mol (Δ(IMe)= -3.2 kJ/mol). This indicates that due to the increased van der Waals (vdW) interactions with the surface a down configuration is possible. Thus, we investigated the effect of elongation of the nitrogen-substituents (methyl vs. butyl, Figure 1.B) on the binding mode on Au(111).
Figure 1. A) Schematic representation of the different binding modes of NHCs on surfaces. In the Upsurf and Upad binding modes the molecule stands perpendicular to the surface, without and with an adatom respectively. The Downsurf and Downad binding modes represent flat lying NHCs, respectively without and with an adatom. B) Chemical structures of the investigated molecules. C) Balland-stick model of the DFT optimized stable configurations of IBu on Au(111) (top and side view).
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bright dot. A shoulder is visible on each side of the bright protrusion, which is attributed to the imidazolium ring. Furthermore, the distance measured here (0.90±0.04 nm) fits well to the expected dimension from the optimized IBu-Au-IBu complex (0.84 nm). Additionally, a hexagonal unit cell with sides of 1.34±0.04 nm and 1.40±0.03 nm and angles of 68.7±0.5° and 111.3±0.5° can be determined.
Figure 2. A) STM-overview of the open, hexagonal self-assembled structure of IBu on Au(111) (2.1V, 15pA). B) Close-up image of the initial self-assembled structure of IBu consisting of hexagons (dotted line) that touch at the sides. Each hexagon contains 6 equal features indicated by crosses. All features include a bright dot (Auatom) and four arms (butyl N-substituents) which points to the formation of IBu-Au-IBu complexes (1.8V, 11pA). C) STM-overview of the same sample after annealing (100°C, 20 minutes) where a second, closer-packed self-assembled structure appears (2.0V, 12pA). D) Further annealing completely converts the open, hexagonal networks into the close-packed structure (1.0V, 38pA). STM imaging on an almost full monolayer of IBu on Au(111) (See SI for experimental details) reveals an open hexagonal self-assembled structure which conceals the Au(111) herringbone reconstruction (Figure 2.A). Figure 2.B shows a close-up image, allowing to identify the hexagonal arrangement (dotted line). Each hexagon in the network consist of 6 similar features, containing a bright center dot and four arms forming a cross-shape. The arms interlock in a chiral fashion inside one hexagon. Due to a high mobility, the molecules can only be observed in large islands and not at low coverage. The self-assembly is also dynamic inside the layer. Subsequent images illustrate how the empty center of some of the hexagons gets filled and emptied again (SI, Figure S.2). Annealing of a full monolayer of the open hexagonal structure at 100°C for 20 minutes resulted in a new self-assembled structure where the molecules are linearly ordered rather than in hexagons. Initially, this new structure appeared next to the original self-assembled structure (Figure 2.C). Further annealing completely converts the hexagonal structure into the linear self-assembled structure (Figure 2.D). Here, the herringbone reconstruction of the Au(111) appears again. Although the molecular layer now is more close-packed (0.50 molecules/nm2 to 0,61 molecules/nm2), it still consists of the same features as can be seen from the high-resolution image in Figure 3.A. The presence of four arms at one bright dot suggest two molecules lying flat on the surface, connected by an Au adatom (as depicted as overlay in Figure 3.A), rather than the DFT suggested Downsurf binding mode in Figure 1.C. The length of each arm of these IBu-Au-IBu complexes is measured to be 0.62±0.04 nm, which is in good agreement with the calculated values of 0.64 nm (SI, Figure S.3). This orientation and the NHC-Au-NHC complex formation are further supported by the height-profile in Fig. 3.C which is measured across a
To elucidate the impact of the alkyl groups on the binding mode, we next investigated the adsorption behavior of IMe using the same conditions. In line with our previously reported work16 an ordered, close-packed layer was obtained (Figure 3.B and S.4). The molecular protrusions appear symmetric in all directions, which is confirmed by the height-profile in Figure 3.D that shows no shoulder. Furthermore, the dimension (0.64±0.04 nm) fits to the expected width (0.50 nm) for a single IMe. The methyl side-groups of IMe, however, are too small for an unambiguous prove of the binding mode of IMe by STM. From the close-up image in Figure 3.B a hexagonal unit cell of 0.83±0.02 nm by 0.91±0.03 nm (68.3±1.2° and 111.7±1.2°) was determined for IMe on Au(111), which is significantly smaller compared to the unit cell of IBu. Furthermore, when compared to clean Au(111) a small broadening of the herringbone ridges from ~6.3 nm to ~6.7 nm was found for IMe. In contrast, no broadening was observed for the self-assembly of IBu, indicating that less atoms are removed from the upper atomic layer of the surface compared to IMe.19 This is in line with our proposed IBu-Au-IBu complexes, in which two IBu molecules share a single Au-atom, whereas in the reported Upad-configuration for IMe16 one molecule is bound to one Au atom.
Figure 3. A) Close-up STM image of the close-packed self-assembling structure in Figure 2.D including the chemical structure and proposed orientation of the complexes. The hexagonal unit cell and the position of the height-profile in Figure 2.C are indicated (2.0V, 120pA). B) Close-up STM-image of IMe on Au(111) showing the regular self-assembly. The hexagonal unit cell and the position of the height-profile in Figure 3.D are indicated (-1.0V, 30pA). C) Height-profile measured as indicated in Figure 3.A. The bright protrusion contains two shoulders which are attributed to the IBu rings. D) Height-profile measured as indicated in Figure 3.B showing the protrusion is symmetric and smaller than IBu.
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Journal of the American Chemical Society Our STM results suggest two different binding modes for IMe and IBu on Au(111), respectively. In the case of IMe, theoretical investigations could show that, after adsorption at the surface (Upsurf), the molecule fully extracts an Au atom from the surface resulting in the Upad binding mode.16 Although NHC-Au-NHC complexes have been reported,16-18 corresponding mechanistic studies are missing. Focusing on the formation mechanism, we therefore performed DFT geometry optimizations using either the Upsurf- or Downsurf configuration of two adjacent IMe molecules as starting situation. Starting from the Upsurf configuration, no complex formation was observed. In contrast, a geometry optimization starting from two flat-lying IMe molecules on the surface (Figure 4.A), both molecules subsequently bind to the same surface atom (Figure 4.B-C) and finally pull out an Au atom from the surface resulting in an NHC-Au-NHC complex (Figure 4.D). Therefore, our calculations reveal a reasonable pathway for complex formation on Au(111), where the Downsurf binding mode likely acts as a precursor state. The calculations show no indication of charge accumulation for the Upad binding mode and the NHC-Au-NHC complex, so that, in agreement with the XPS data (see SI) both species can be considered as neutral at the surface
of -0.6 eV was calculated for the complex. This is in good agreement with the experimental results and supports our proposed NHC-Au-NHC and Upad-binding modes for IBu and IMe respectively. The C 1s signal was measured and calculated as well for both compounds and showed the same trend as the N 1s data (SI, S6).
Figure 5. A) Experimental XPS results for the N 1s spectral region of IBu (top) and IMe (bottom) on Au(111). B) Top- and side-view of the IMe-Au-IMe complex on Au(111), which is used to simulate the theoretical core-level shifts in XPS. C) Side-view of the Upad configuration on Au(111) which is used to simulate the theoretical core-level shifts in XPS. Figure 4. Stills from subsequent structural DFT optimizations showing: A) the starting situation with two IMe molecules flat on the surface with the reactive carbon atoms pointing towards each other. B) Both molecules bind to the same Au-atom on the surface, similar to the Downsurf binding mode. C) The atom is pulled out of the plane, similar to the Downad binding mode. D) In the final situation two carbenes share the Au-atom in an IMe-Au-IMe complex on the surface, similar to the observed structure in the STM images. To further confirm our assignment and to obtain an insight in the electronic coupling between NHC and the metal, we investigated both compounds by XPS. Especially the N 1s binding energy can be used as a marker for the NHC-metal binding.20 The N 1s data of IBu and IMe are plotted in Figure 5.A. The N 1s binding energy for IMe is found to be 401.0 eV, which is in good agreement with other reports of IMe on Au(111)17, NHC-coated Au-nanoparticles21 and NHC-coated nanocrystals22. In case of IBu, the N 1s peak is considerably shifted by -0.5 eV to 400.5 eV when compared to IMe. This relatively large shift cannot be solely explained by the elongation of the alkyl chain but rather reflects an electronic change of the inner core of the NHCs by bonding to different gold species and a different distance to the surface. To verify this and explain the corresponding XPS shift, we performed additional DFT calculations. Focusing on the impact of the different configurations of the NHC ring, we used IMe as reference system in the Upadd configuration and as NHC-Au-NHC complex (Figure 5.B-C). From these optimized structures the NHC-Au bond length is measured to be 2.04 Å in both configurations, similar to previously reported NHC-Au bond lengths.10,15,23,24 The distance between the nitrogen atom and the surface was calculated to be 5.20 Å in the Upad-configuration and 3.31 Å for the flat-lying complex. Comparing the IMe-Au-IMe complex with the upstanding IMe system, an N 1s core level shift
In summary, our theoretical and experimental results reveal a consistent picture of the binding modes of the NHCs on Au(111). Based on this, we can conclude that IBu forms, after adsorption in the Downsurf configuration, flat-lying IBu-Au-IBu complexes, due to the abundance and mobility of IBu. The corresponding carbene with a shorter alkyl chain (IMe) binds in an Upad-configuration. Consequently, the nature of the N-substituents has a strong influence on the binding of NHCs to the surface. The elongation from methyl to butyl increases the vdW interaction with the surface and, therefore, changes the binding mode from up to down. To study the intermediate regime with respect to the interaction with the surface, 1,3-di-isopropylimidazolium (IiPr) was investigated (Figure S.7). Since the isopropyl side groups are bigger than methyl (IMe) but provide less surface interaction than IBu due to the bulky shape, an intermediate vdW interaction is expected. Our combined DFT, STM and XPS results show an Upad-configuration for IiPr, which further confirms our conclusion that the NHC structure defines the binding mode at the surface. This knowledge is highly important for the controlled design of NHCs for future on-surface applications like heterogeneous catalysts or electronic devices.
The Supporting Information is available free of charge on the ACS Publications website. It contains additional STM images, geometrical analysis of the self-assembly, C1s XPS data, chemical synthesis and experimental and computational details.
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[email protected] The authors declare no competing financial interests.
This work was generously supported by the Deutsche Forschungsgemeinschaft through collaborative research center SFB 858 (project B03), and through projects AM 460/2-1, MO 2345/41 and FU 299/19. Furthermore we thank NanoAnalytics GmbH and Damla Yesilpinar for technical support.
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