Surface Adatom Mediated Structural Transformation in Bromoarene

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Surface Adatom Mediated Structural Transformation in Bromoarene Monolayers: Precursor Phases in Surface Ullmann Reaction Qitang Fan, Liming Liu, Jingya Dai, Tao Wang, Huanxin Ju, Jin Zhao, Julian Kuttner, Gerhard Hilt, J. Michael Gottfried, and Junfa Zhu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06787 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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Surface Adatom Mediated Structural Transformation in Bromoarene Monolayers: Precursor Phases in Surface Ullmann Reaction Qitang Fan1, 3†, Liming Liu2†,‡, Jingya Dai1, Tao Wang1, Huanxin Ju1, Jin Zhao2*, Julian Kuttner3, Gerhard Hilt3, J. Michael Gottfried3*, Junfa Zhu1* 1

National Synchrotron Radiation Laboratory and Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230029, P.R. China, [email protected] 2 Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China, [email protected] 3 Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Str., 35032 Marburg, Germany, [email protected]

ABSTRACT Structural transformations of supramolecular systems triggered by external stimuli maintain great potential for application in the fabrication of molecular storage devices. Using combined ultrahigh vacuum scanning tunneling microscopy (UHV-STM), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculation, we observed the surface adatom mediated structural transformation from 4,4’’-dibromo-m-terphenyl (DMTP)-based halogen-bonded networks to DMTP-Cu(Ag) coordination networks on Cu(111) and Ag(111) at low temperatures. The halogen-bonded networks, which were formed on Cu(111) at 97 K and on Ag(111) at 93 K, consist of intact DMTP molecules stabilized by triple Br…Br bonds. The DMTP-Cu(Ag) coordination networks form on Cu(111) at 113 K and on Ag(111) at 103 K. They contain alternatingly arranged intact DMTP molecules and Cu(Ag) adatoms stabilized by weak C-Br…Cu(Ag) coordination bonds. Annealing the DMTP-Ag structure to 333 K leads to the initiation of C-Br bonds scission. This observation suggests that the DMTP-Ag coordination network represents the intermediate phase ready for dehalogenation, which is the first step of the surface Ullmann reaction. Keywords: structural transformation, supramolecular structure, bromoarene, Ullmann coupling, surface adatom, scanning tunneling microscopy, X-ray photoemission spectroscopy

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Structural transformations of supramolecular systems triggered by external stimuli have attracted much attention for their potential application in the fabrication of molecular-scale electronics and devices.1-6 Generally, these stimuli include both physical and chemical factors such as temperature,6-10 electrical current,11 concentration,12-16 precursor coverage,17 and introduction of metal atoms.18, 19 Metal-atom induced structural transformations are typically accompanied by the formation of strong coordination or organometallic bonds between the metal atoms and precursor ligands. For instance, Ni atoms added to cytosine on Au(111) were reported to induce the structural transformation from hydrogen-bonded chains to coordination-bonded clusters.18 Similarly, for dicarbonitrile-pentaphenyl on Cu(111), the intrinsic surface Cu adatoms induced the structural transition from an N … H hydrogen-bonded kagome structure to N-Cu coordination-bonded honeycomb networks.19 Haloarenes on coinage metal surfaces have emerged as another interesting class of systems in which metal-atom induced structural transformations occur. In typical cases of such systems, the intact haloarene precursors form halogen- or hydrogen-boned nanomeshes as long as the temperature is low enough to prevent dissociation of the carbon-halogen bonds. Raising the temperature leads to dehalogenation and, as a result, the initial molecular nanomeshes transform to organometallic networks through the formation of carbon-metal (C-M) bonds between metal atoms and dehalogenated precursor molecules.20-27 The metal atoms incorporated in the C-M bonds are intrinsic surface adatoms, which facilitate the dehalogenation of precursors.28 Thus, this process can be considered as a structural transformation triggered by surface adatoms. These transformations are essentially driven by the formation of rather strong coordination or organometallic bonds, which involve a substantial charge transfer between the organic species and the metal atoms.28-30 However, structural transformations triggered by weak interactions between metals and intact organic molecules have scarcely been reported.31-34 Here, using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and X-ray photoemission spectroscopy (XPS) combined with density functional theory (DFT) calculations, we demonstrate the surface adatom mediated structural transformation on both Cu(111) and Ag(111) from 4,4’’-dibromo-meta-terphenyl (DMTP)-based halogen-bonded networks to a DMTP-Cu(Ag) coordination network. This DMTP-Cu(Ag) coordination network is stabilized by weak Coulomb attraction between Cu(Ag) adatoms and DMTP molecules (C-Br…Cu(Ag) coordination bond) as revealed by the DFT calculated differential potential distribution. Within the DMTP-Cu(Ag) network, the Cu(Ag) adatoms may be able to keep their intrinsic properties and may maintain activity for single-atom catalysis due to their high coordinative unsaturation.35 In addition, our results suggest that the DMTP-Cu(Ag) coordination network is the direct intermediate phase that transforms into the organometallic phase. This observation sheds light on the initial stage of the mechanism of the surface Ullmann reaction of haloarenes on metal surfaces. RESULTS AND DISCUSSION Structural transformation on Cu(111). Deposition of 0.4 ML DMTP onto Cu(111) held at 90 K and subsequent annealing to 97 K lead to the formation of a large-area two-dimensional (2D) ordered molecular network with tri-lobed cavities, as revealed by the STM image shown in Figure 1a. Figure 1b shows the magnified view of the white framed region in Figure 1a. In this structure, each three adjacent corner motifs are joined by one bright vortex node. DMTP 2


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stays intact on Cu(111) at temperatures lower than 170 K,36 as also evidenced by the Br 3p XP spectra in Figure S1 in the supporting information (SI). Therefore, the 120° corner motifs are attributed to intact DMTP molecules. The bright node then consists of three DMTP molecular terminals (C-Br groups) arranged in a vortex manner. The node is stabilized by the formation of triple Br…Br halogen bonds, as supported by the DFT-calculated structural model of the network in Figure 1c. The theoretical unit cell (26.6 × 26.6 Å2) agrees very well with the experimental one (26.6 × 26.7 Å2) in Figure 1b. Figure 1c also presents the DFT-calculated differential potential of the network (with yellow clouds representing the attractive potential with isosurface of 3.8 × 10-5 eV). The attractive potential (see the inset in Figure 1c for details) in the center of the nodes accounts for the Coulomb attraction between the three C-Br groups, i.e., the formation of Br···Br halogen bonds. The stabilization of the network with halogen bonds observed here is qualitatively very similar to that reported for the Sierpinski-type fractal network formed by the same molecule on Ag(111)37 and the honeycomb network formed by a similar molecule, 4,4’’-dibromo-para-terphenyl (DBPTP), on Cu(111).20 Annealing the sample in Figure 1a to 113 K leads to the transformation of the halogen-bonded network into islands of a densely packed structure, as shown by Figure 1d. To reveal more details of this structure, the magnified STM image (Figure 1e) of the white framed region in Figure 1d was recorded. The protrusions (marked with red circles) at the two terminals of a corner motif are attributed to C-Br groups,38 taking into account the XPS evidence that the DMTP molecules are intact. Moreover, weak protrusions with hexagonal arrangement (white rhomboid) are observed between the DMTP molecules. One of these protrusions is marked by the blue arrow in Figure 1e. Since XPS proves that the DMTP molecules stay intact on Cu(111) at 113 K, it can be ruled out that these weak protrusions are Br adatoms. Furthermore, it is a common case for organic monolayers to capture metal adatoms on surfaces, according to previous work.31, 32, 39-42 Therefore, the weak protrusions are attributed to Cu adatoms, which are intrinsically present on the substrate. The DMTP molecules and Cu adatoms are arranged alternatingly along the [0 1 -1] direction with a unit cell of 10.1 × 30.9 Å2. Note that the structure in Figure 1e lacks long range order along the [-1 1 0] direction due to the encounter of two different domains with relative lateral dislocation. To further understand the interactions between DMTP and Cu adatoms, DFT calculations were performed. Figure 1f shows the DFT-calculated adsorption model and differential potential for this structure. The lattice constant (unit cell) is optimized to 10.2 × 30.7 Å2, in good agreement with the experimental value. The corresponding attractive potential (yellow clouds in Figure 1f with isosurface of 3.8 × 10-5 eV, see the inset for details) is increased in the regions around the Cu adatoms and therefore indicates attractive Coulomb interactions between the Cu adatoms and the adjacent DMTP molecules. Therefore, this structure is attributed to a DMTP-Cu coordination network stabilized by Coulomb attraction. Additional evidence for this structural transformation from a DMTP-based halogen-bonded network to a DMTP-Cu coordination network is the in-situ observed collapse of the halogen-bonded network in Figure 1a. As shown by Figure S2a in the SI, the STM image of the same region as Figure 1a recorded 7 min later shows the collapse of the halogen-bonded network and the formation of DMTP-Cu coordination network. Additionally, the coexistence of these two phases has also been observed in other regions of the sample, 3


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as illustrated in Figure S2b. The DMTP-Cu coordination network were observed on Cu(111) up to 150 K, at which temperature, the structural pattern of the network changes fast, as shown by Figure S3a and S3b, mainly due to a higher thermal agitation. The dissociation of the C-Br bond is expected to proceed at 170 K, according to the previous work.35

Figure 1. Overview STM images taken after deposition of 0.4 ML DMTP molecules onto Cu(111) surface held at 90 K followed by annealing to (a) 97 K and (d) 113 K. (b),(e) show the zoom-in STM image of the white framed region in panel (a) and (d), respectively. (c), (f) show the DFT-calculated adsorption model and differential potential for the structures in panels (b) and (e). The rhomboids in panel (b), (c), (e) upper region, (e) lower region and (f) shows the experimental and theoretical unit cells of the supramolecular structures. The corresponding angles of the unit cells are 117°, 120°, 118°, 122°, and 120° respectively. Yellow clouds in (c) and (f) represent the positive differential potential. Dark gray spheres represent carbon atoms; dark red, Br; light pink, H; light blue, substrate copper. The insets in (c) and (f) shows the magnified views of the differential potential with distances (in unit of Å) between the related atoms labelled. STM images in panel (a) and (d) are recorded at 97 K and 113 K, respectively. Tunneling parameters: (a) U = 1.9 V, I = 0.09 nA; (b) U = 1.3 V, I = 0.06 nA; (d) U = 1.9 V, I = 0.17 nA; (e) U = 1.2 V, I = 0.1 nA.

In view of this, the process of the structural transformation can be interpreted as follows. The initial formation of the halogen-bonded network is partly a kinetically driven process, i.e., this network forms faster than the more stable densely packed phase. In contrast, the subsequent slow transformation of the halogen-bonded network into the densely packed phase is a thermodynamically driven process resulting in the formation of a more stable phase. These findings represent a typical manifestation of Ostwald's step rule,43, 44 which states that normally the least stable polymorph crystallizes first. After this, a slow transition into a thermodynamically more stable structure occurs. In our case, the halogen-bonded network is kinetically favored, because it can form with a very low barrier. In contrast, the 4


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densely packed phase contains Cu atoms, the formation of which has a considerable barrier (0.74 eV).45 Annealing at 113 K accelerates the formation of the densely packed phase, because the necessary Cu atoms are produced at a higher rate (a factor of approximate 13). Structural transformation on Ag(111). A similar structural transformation from a halogen-bonded to a coordination network was observed for DMTP on the Ag(111) surface. Figure 2a shows the STM image (taken at 93 K) of a 2D network formed after deposition of 0.5 ML DMTP onto Ag(111) held at 88 K. As illustrated by the magnified view in Figure 2b, this network is very similar to that formed on Cu(111) at 97 K, apart from the broadened lobes of the cavities. The Br 3p XP spectra in Figure S4 indicate that the network consists of intact DMTP molecules. An adsorption model of the network on the basis of DFT-calculations is shown in Figure 2c. The optimized unit cell (31.8 × 31.8 Å2) agrees well with the experimental unit cell (31.3 × 31.5 Å) in Figure 2b. The larger unit-cell of the halogen-bonded network formed on Ag(111) than on Cu(111) indicates considerable DMTP-substrate interaction. Otherwise, identical networks would form irrespective of the substrate material. This is also supported by the formed different nanostructures on different crystallographic planes of the Cu crystal, i.e., Cu(111) and Cu(110) (see Figure S5 for the structure formed by deposition of DMTP on Cu(110) at 90 K). Additionally, the formation of triple Br…Br halogen bonds is confirmed by the attractive potential (with isosurface of 3.8 × 10-5 eV) in the region between three C-Br groups in Figure 2c (see the inset for details). Again, the network is stabilized by Coulomb attraction between neighboring Br atoms, very similar to that on Cu(111). Close inspection of Figure 2a reveals between the network domains some fuzzy areas, which are ascribed to diffusing DMTP molecules. This indicates that the halogen-bonded network is not stable and starts to deconstruct on the Ag(111) surface at 93 K. After annealing the sample in Figure 2a to 103 K, the islands of the network shrink and the fuzzy regions expand, as shown in Figure 2d. Furthermore, small islands of a DMTP-Ag coordination network occur in Figure 2d, as marked with white circles. The detailed structure of this DMTP-Ag coordination network is confirmed by the following observations: First, the zoom-in STM image displayed in Figure 2e contains alternatingly arranged corner motifs (attributed to DMTP molecules) and protrusions (attributed to Ag atoms, one example marked with a blue arrow). This is very similar to the DMTP-Cu coordination network in Figure 1e. Note that the asymmetry appearance of the two legs in the corner motifs derives from the slight thermo-drift along the vertical direction during scanning. Additional image of the DMTP-Ag coordination network (on the same sample as Figure 2e) with less thermos-drift has been given in Figure S6, however, with lower resolution. Second, as proven by the Br 3d XP spectra in Figure 4, the DMTP molecules stay intact on Ag(111) at temperatures lower than 283 K, which excludes that the bright protrusions are Br adatoms. The possibility that the bright protrusions are reconstruction of the Ag(111) surface can be eliminated by the measured apparent height (0.81 Å) of them as shown in Figure S7. The attribution of the protrusion to multiple Ag atom cluster could also be excluded due to the short distances (3.5 Å shown in inset of Figure 2f) between the Ag adatom and the adjacent H atoms in the DMTP molecules nearby. Figure 2f shows the DFT-calculated adsorption model and differential potential of the DMTP-Ag coordination network. Note that the favorite adsorption sites of Ag adatoms on the Ag(111) surface are the atop sites (Figure 2f), which is in contrast to the adsorption 5


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sites of Cu adatoms on Cu(111) (hollow sites, Figure 1f). This has been verified by the DFT calculated adsorption energies for single Ag adatom located at the atop (-2.35 eV) and hollow (-2.28 eV) sites of Ag(111) surface (see Figure S8 for details). The optimized unit cell (11.6 × 34.7 Å2) agrees well with the experimental one (11.3 × 34.0 Å2) in Figure 2e. The attractive potential (with isosurface of 3.8 × 10-5 eV) in Figure 2f (see the inset for details) reveals also Coulomb attraction between DMTP and Ag adatoms. Similar to the case on Cu(111), the formation of the halogen-bonded network on Ag(111) is also kinetically preferred. Subsequently, this metastable phase undergoes slow transformation into the more stable DMTP-Ag coordination network by capturing Ag adatoms. Annealing the substrate to higher temperature (103 K) accelerates this structural transformation process resulting in the observation of larger islands of the coordination network. It should be mentioned that the transformation of halogen-bonded network to the coordination network on Ag(111) is irreversible. This is evidenced by the DMTP-Ag coordination network (Figure S9, recorded at 90 K) observed after deposition of DMTP on Ag(111) at 300 K and subsequent cooling to 90 K, in contrast to the halogen-bonded network formed after deposition of DMTP on Ag(111) at 90 K shown in Figure 2a. Moreover, it further confirms that the DMTP-Ag coordination network is thermodynamically more stable.

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Figure 2. Overview STM images taken after deposition of 0.5 ML DMTP molecules onto a Ag(111) surface held at 88 K, followed by annealing to (a) 93 K and (d) 103 K. (b),(e) show the zoom-in STM images of the white framed region in panels (a) and (d), respectively. (c), (f) show the DFT-calculated adsorption models and differential potentials for the structures in panels (b) and (e). The rhomboids in panel (b), (c), (e) and (f) show the experimental and theoretical unit cells of the supramolecular structures. The corresponding angles of the unit cells are 122°, 120°, 119°, and 120° respectively. The yellow clouds in (c) and (f) represent the positive differential potential. Dark gray spheres represent carbon atoms; dark red, Br; light pink, H; gray, substrate silver. The insets in (c) and (f) shows the magnified views of the differential potentials with distances (in unit of Å) between the related atoms labelled. STM images in panel (a) and (d) are recorded at 93 K and 103 K, respectively. Tunneling parameters: (a), (b) U = 1.3 V, I = 0.14 nA; (c), (d) U = 1.4 V, I = 0.17 nA.

Initiation of the Ullmann reaction on Ag(111). Similar to the growth of the DMTP-Cu coordination network on Cu(111), the DMTP-Ag network grows faster at higher substrate temperature, due to the increasing rate of formation of Ag adatoms. This is evidenced by the formation of larger islands (see Figure 3a) of the DMTP-Ag network after annealing the sample in Figure 2d to 283 K. Figure 3b displays a high-resolution STM image of a section of the sample in Figure 3a. As outlined with white dotted lines (domain boundaries), several domains of the ordered DMTP-Ag network with different orientations are clearly seen. In addition, vacancy defects (marked by blue arrows) are typically observed at the domain boundaries, due to the disruption of the translational symmetry at these sites.

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Figure 3. Overview STM images taken after annealing the sample in Figure 2d to (a) 283 K and (c) 333 K. (b) High-resolution STM image of a section of the sample in panel (a) showing domain boundaries (white dotted curves). (d) Zoom-in STM image of the white framed region in panel (c) with overlaid molecular models. Dark gray spheres represent carbon atoms; dark red, Br; light pink, H; gray, silver. The black rhomboids in panels (b) and (d) show the experimental unit cells of the DMTP-Ag coordination network. The corresponding angles of the unit cells are 121° and 119° respectively. Tunneling parameters: (a) U = 1.1 V, I = 0.34 nA; (b) U = 0.21 V, I = 0.35 nA; (c), (d) U = 0.32 V, I = 0.33 nA.

Further annealing the sample in Figure 3a to 333 K leads to the emergence of “staple” shaped motifs inside the DMTP-Ag coordination network domain, as labeled by the blue arrows in Figure 3c. Figure 3d shows the zoom-in STM image of the blue framed region in Figure 3c. It reveals that the “staple” shaped motif contains two corners connected by a bright protrusion. On the basis of the discussions above, the corners and bright protrusions are attributed to meta-terphenyl (MTP) and Ag atoms, respectively. The corner-to-corner distance is measured to be 16.0 Å, in agreement with the reported distance of 15.9 Å between two adjacent p-terphenyl units in the organometallic chains formed from 8


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4,4’’-dibromo-p-terphenyl and Ag atoms.46 Therefore, we conclude that the "staple" motif consists of two DMTP fragments linked by a C-Ag-C bond. The two brighter terminals of the “staple” motif indicates the existence of two intact C-Br groups. The “staple” motifs are therefore assigned to cis-configured Br-(MTP)-Ag-(MTP)-Br, as illustrated by the molecular model overlaid in Figure 3d. The weaker circular protrusions located at the corners of the “staples” are attributed to chemisorbed Br adatoms (overlaid with big brown spheres). This is further supported by the stoichiometric ratio (2:1) of the number of weak circular protrusions and “staples”, because two Br adatoms are produced by forming one Br-(MTP)-Ag-(MTP)-Br unit. The partial dissociation of C-Br bonds at 333 K is further confirmed by the high-resolution C 1s and Br 3d XP spectra in Figure 4, taken after deposition of 1 ML DMTP onto Ag(111) held at 260 K and subsequent annealing to 283, 333 and 393 K. At 283 K, the Br 3d5/2 binding energy (BE) of 70.9 eV (dark green filled line) is indicative of intact C-Br bonds, in agreement with previous work.24, 47 The deconvolution of the corresponding C 1s XP spectrum obtained at 283 K supports the interpretation of DMTP remaining intact: the peak located at 286.0 eV (blue filled line) is assigned to carbon bound to bromine, in accord with previous work.47, 48 The other peak located at 285.0 eV (brown filled line) originates from the other sp2 carbons in DMTP.44 Furthermore, the area ratio of these two peaks is 0.64 : 4.81 = 0.133, in accordance with the stoichiometric ratio (2 : 16 = 0.125) of two carbons in C-Br versus the other 16 sp2 carbons. After annealing the monolayer to 333 K, another Br 3d peak (bright green filled line) with a Br 3d5/2 BE of 68.3 eV appears, accompanied by an intensity attenuation of the Br-C related peak (dark green filled line). According to previous work,24, 47, 36 the observed Br 3d core level shift of 2.3 eV toward a lower BE can be attributed to the chemisorbed Br on metal surfaces. This suggests that the C-Br bonds start to dissociate at 333 K. The formation of Br adatoms also partly explains the shifts of the Br-C related Br 3d5/2 peak from 70.9 to 70.6 eV and of the C 1s peak (brown) from 285.0 to 284.5 eV; these shift are partly related to the work-function increase caused by the chemisorbed Br atoms.36 In addition, the resulting terminal carbon atoms now bind to Ag adatoms forming C-Ag bonds. This is confirmed by the deconvolution of the corresponding C 1s spectrum at 333 K: besides the peaks related to Br-C (blue filled peak, 285.5 eV) and the other sp2 carbons (brown filled peak, 284.5 eV), another peak located at a BE of 283.4 eV (orange filled peak) appears. This peak is assigned to carbon with an organometallic bond to silver adatoms, in agreement with the expected lower binding energies for carbon linked to metal atoms.48 Further annealing the monolayer to 393 K leads to increased intensities of chemisorbed Br components (bright green filled peak) in the Br 3d and C-Ag components (orange filled peak) in the C 1s spectra, in line with the fact that higher substrate temperature prompts the dissociation of C-Br bonds and the formation C-Ag bonds.

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Figure 4. C 1s and Br 3d core level spectra of one monolayer (1 ML) DMTP on Ag(111) deposited at 260 K followed by annealing to different temperatures: 283 K, 333 K and 393 K. The 283 K XP spectra were collected at 283 K; the 333 K and 393 K spectra were collected at 300 K. Experimental data after Shirley background subtraction are shown as empty circles, and fitted Voigt functions are shown as solid filled lines. The C 1s and Br 3d spectra are collected using an incident photo energy of 350 eV and 150 eV, respectively. The peak shifts are caused by work-function changes as explained in the text.

In view of this, the DMTP-Ag coordination network represents the direct intermediate phase ready for the debromination process. Particularly, the topology of the alternatively up-and-down arrangement of the DMTP molecules in DMTP-Ag coordination network is similar to the trans-linked MTP units in C-Ag-C bonded organometallic chains after debromination completed as illustrated by Figure S10. This is also the case for the DMTP-Cu coordination network, which undergoes Ullmann reaction at higher temperatures, as reported in our previous studies.45, 49 The discovery of this DMTP-Ag(Cu) coordination networks sheds light on the mechanism of the surface Ullmann reaction. Most importantly, it shows that the metal adatoms are very near the C-Br bonds already before their dissociation. It therefore very likely that the bond scission is catalyzed by these metal adatoms rather than by metal atoms in the terraces. Moreover, the Ag(Cu) adatoms in the DMTP-Ag(Cu) coordination network may be of high activity because only weak Coulomb attraction between Ag(Cu) and DMTP exists. Due to the coordinatively unsaturated character of these metal atoms, the here described networks may have the potential for application in single-atom catalysis.35 CONCLUSIONS In conclusion, the surface adatom mediated structural transformation from DMTP-based halogen-bonded networks to a DMTP-Cu(Ag) coordination network has been observed on Cu(111) and Ag(111). The halogen-bonded networks, which were observed on Cu(111) at 97 K and on Ag(111) at 93 K, contain intact DMTP molecules stabilized by triple Br…Br bonds. Due to the different lattice matching on the two substrates, the halogen-bonded network forms broader tri-lobed cavities on Ag(111) than on Cu(111). Thus, the former network has a 10


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larger unit cell (31.8 Å × 31.8 Å on Ag) than the latter one (25.6 Å × 25.6 Å on Cu). The DMTP-Cu(Ag) coordination networks form on Cu(111) at 113 K and on Ag(111) at 103 K, along with the deconstruction of the halogen-bonded networks. Both DMTP-Cu and DMTP-Ag networks consist of alternatingly arranged intact DMTP molecules and Cu or Ag adatoms. As a result, their topologies are qualitatively very similar. In contrast, owing to the larger atomic radius of Ag, the unit cell of the DMTP-Ag structure (11.6 Å × 34.7 Å) is slightly larger than that of the DMTP-Cu structure (10.2 Å × 30.7 Å). The DFT-calculated differential potential distributions reveal that the DMTP-Cu(Ag) coordination networks are stabilized by Coulomb attraction between DTMP and the Cu or Ag adatoms. Therefore, the Cu or Ag adatoms are proposed as indispensable components for the formation and stabilization of DMTP-Cu(Ag) coordination networks. With the increasing of the temperature of Ag(111) to 283 K, the islands of the DMTP-Ag networks expand due to the higher concentration (and faster rate of formation) of Ag adatoms at higher temperatures. The transformation from the initially formed, less stable halogen-bonded network to the more stable DMTP-Cu(Ag) coordination network is a thermodynamically favorable process and a typical manifestation of Ostwald's step rule. Annealing the DMTP-Ag coordination network to 333 K leads to the initiation of C-Br bonds scission and the formation of organometallic Br-(MTP)-Ag-(MTP)-Br dimers. This indicates that the DMTP-Ag network is the direct intermediate phase ready for surface Ullmann reaction and suggests that the metal adatoms play a decisive role in catalyzing the C-Br bond dissociation. Due to their coordinatively unsaturated character, the Cu(Ag) adatoms in the DMTP-Cu(Ag) networks may also show high activity in other reactions and thus may find applications in single-atom catalysis. EXPERIMENTAL AND COMPUTATIONAL DETAILS The experiments were performed in a two-chamber UHV system, which has been described previously,50 with a background pressure below 10-10 mbar. The scanning tunneling microscope is a SPECS STM 150 Aarhus with SPECS 260 electronics. All voltages refer to the sample and the images were recorded in constant current mode. Moderate filtering (Gaussian smooth, background subtraction) has been applied for noise reduction. The Cu(111) and Ag(111) single crystals with alignments of better than 0.1° relative to the nominal orientation were purchased from MaTecK, Germany. Preparation of a clean and structurally well controlled metal surfaces was achieved by cycles of bombardment with Ar+ ions and annealing at 800 K. As described elsewhere,49 4,4’’-dibromo-1,1’:3’,1’’-terphenyl (4,4''dibromo-meta-terphenyl, DMTP) was made from 4-bromophenylacetylene in a short reaction sequence utilizing a Grubbs-enyne metathesis reaction and a regioselective cobalt-catalyzed Diels-Alder reaction followed by mild oxidation. DMTP was vapor-deposited from a commercial Kentax evaporator with a Ta crucible held at 369 K. STM images were recorded at a sample temperature of 300 K or at the indicated temperatures. The DMTP coverage of 1 monolayer (ML) means a full layer of coordination network in Figure 2a. This equals 0.042 DMTP molecules per surface copper atom or 0.054 DMTP per surface silver atom. The deposition rate of DMTP was 0.11 ML per minute (ML/min) unless indicated otherwise. Coverages were derived from STM images. The XPS measurements were performed on the Catalysis and Surface Science Endstation located in National Synchrotron Radiation Laboratory (NSRL), Hefei. A detailed description of the endstation can be found elsewhere. 51 The XPS spectra were collected at an emission angle of 60° with respect to the 11


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surface normal. The C 1s and Br 3d spectra are taken with incident photon energies of 350 eV and 150 eV, respectively. For theoretical simulations, the density functional theory (DFT) based Vienna ab initio simulation package (VASP)52, 53 was adopted to optimize geometry and to calculate electronic properties. Besides ion-electron and electron-electron interaction, Perdew-Burke-Ernzerhof (PBE)54, 55 functional was chosen to describe the exchange correlation interaction between electrons. The slab models of DMTP on Cu(111) and Ag(111) were separated by a vacuum layer of 15 Å. All substrates were configured by 3 layers of atoms with the bottom layer fixed during geometry optimization processes. For elements involved in this study, the valence electron configurations are: C-2s22p2, H-1s1, Br-4s24p5, Cu-3d104s1 and Ag-4d105s1; for core electrons and ions, the projected-augmented wave method (PAW)56, 57 was used to simplify the ion presentations. Dispersive interaction was taken into consideration by adding the vdW-D2 correction.58 For ionic geometry optimization, the energy cutoff for plane wave basis is 400 eV and the criterion for the halt of relaxation iterations is that the force on each atom should be less than 0.02 eV/Å. For electronic structure calculations, the self-consistent field theory was used and the convergence criterion is 10-5 eV in total energy difference. The differential Potential is the electrostatic potential difference between the whole adsorption system and its individual parts, which is defined as: Pdiff = Ptotal – (Psubstrate + mPmolecule + nPinterstitial) m, n are the numbers of DMTP molecule and inserted metal adatoms in the halogen-bonded or coordination networks correspondingly. Ptotal, Psubstrate, Pmolecule, and Pinterstitial are electrostatic potentials for the total adsorbate system, substrate, single molecule, and single metal atom in the unit cells, respectively. The charge transfer analysis was accomplished by calculating the difference of charge partition from Bader charge analysis.59 The isosurfaces of differential potential were set uniformly to 3.8 × 10-5 eV. Except for the differential electrostatic potential, the separated electrostatic potentials for the clean Cu(111), Ag(111), halogen-bonded networks of DMTP in vacuum, Cu and Ag atoms in vacuum have been given in Figure S11, S12, and S13. These plots helps to clearly identify where the electrostatic potential changes. In the related figures (for electrostatic potential), the yellow marked region represents the increase of the electrostatic potential due to the binding of DMTP molecules on metal substrates. The increase of the electrostatic potential implies the increase of the electron density and thus gives intuitive comprehension of the binding information in real space. ASSOCIATED CONTENT Supporting Information X-ray photoelectron spectra; these materials are available free of charge via the Internet. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] *[email protected] † These authors contributed equally. 12


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‡

Current address: Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, Sichuan, 621999. ACKNOWLEDGEMENTS J.F.Z. acknowledges the financial support from the National Natural Science Foundation of China (21473178, 21773222, U1732272), the National Key R&D Program of China (2017YFA0403402), the Key Program of Research and Development of Hefei Science Center of CAS (2017HSC-KPRD001) and China Scholarship Counsil (201606345003). J.M.G. thanks the Deutsche Forschungsgemeinschaft for support through grant GO1812/2-1, SFB 1083, and the Chinese Academy of Sciences for a Visiting Professorship for Senior International Scientists (Grant No. 2011T2J33). Q.T.F thanks the Alexander von Humboldt-Foundation for Research Fellowship for Postdoctoral Researchers.

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Surface Adatom Mediated Structural Transformation in Bromoarene

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