Hierarchical Dehydrogenation Reactions on a Copper Surface

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Hierarchical Dehydrogenation Reactions on a Copper Surface Qing Li, Biao Yang, Jonas Björk, Qigang Zhong, Huanxin Ju, Junjie Zhang, Nan Cao, Ziliang Shi, Haiming Zhang, Daniel Ebeling, Andre Schirmeisen, Junfa Zhu, and Lifeng Chi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12278 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Journal of the American Chemical Society

Hierarchical Dehydrogenation Reactions on a Copper Surface Qing Li1‡, Biao Yang1‡, Jonas Björk2, Qigang Zhong1,3, Huanxin Ju4, Junjie Zhang1, Nan Cao1, Ziliang Shi5, Haiming Zhang1, Daniel Ebeling3, Andre Schirmeisen3, Junfa Zhu4, Lifeng Chi1* 1

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials &devices Soochow University, Suzhou 215123, China; 2 Department of Physics, Chemistry and Biology, IFM, Linköping University, 58183 Linköping, Sweden; 3 Institute of Applied Physics (IAP), Justus Liebig University Giessen, Heinrich-Buff-Ring, 16, 35392 Giessen, Germany; 4 National Synchrotron Radiation Laboratory and Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei, Anhui 230029, China; 5 The Center for Soft Condensed Matter Physics & Interdisciplinary Research, Department of Physics, Soochow University, Suzhou 215006, China. ABSTRACT: Hierarchical control of chemical reactions is being considered as one of the most ambitious and challenging topics in modern organic chemistry. In this study, we have realized the one-by-one scission of the X-H bonds (X=N and C) of aromatic amines in a controlled fashion on the Cu(111) surface. Each dehydrogenation reaction leads to certain metal-organic supramolecular structures, which were monitored in single-bond resolution via scanning tunneling microscopy and non-contact atomic force microscopy. Moreover, the reaction pathways were elucidated from X-ray photoelectron spectroscopy measurements and density functional theory calculations. Our insights pave the way for connecting molecules into complex structures in a more reliable and predictable manner, utilizing carefully tuned step-wise on-surface synthesis protocols.

Removal of hydrogen atoms from reactants are mostly thermodynamically unfavorable processes1-3. The dehydrogenation reactions therefore usually require oxidants or sacrificial hydrogen acceptors4-7, which lead to wasteful byproduct generations. In this regard, acceptorless dehydrogenation (AD) reactions, in which the only by-product is hydrogen gas, have acquired much attention recently8-10. Since X-H (X=N, C, O…) bonds are ubiquitous among the hydrogen rich organic reactants, selective hierarchical AD reactions become particularly challenging. The difficulty not only arises from the selectivity and overcoming the reaction barriers, but also from monitoring hierarchical AD reactions with single-bond resolution. The development of “on-surface synthesis”11-28, which combines scanning tunneling microscopy (STM) and the ultrahigh vacuum (UHV) techniques, provides alternative means to study the AD reactions. STM observation enables imaging the target reactants and products at single-molecule scale. The entire reaction process takes place under UHV condition, which establishes an ideal clean model system for mechanism studies29-35. With this technique, the scission of hydrogen atoms on metal surfaces has been successfully visualized, e.g. C-H activations36-45, N-H activations46,47, and O-H activations29,48-51. Importantly, the detached hydrogen atoms mostly desorb from the metal surface directly, without forming byproducts except for hydrogen gas44. These surface assisted reactions therefore can be ascribed to the AD reactions, though it remains challenging to separate the

reaction products from the metal substrates. Moreover, several groups have reported the controllable hierarchical reactions on surfaces52,53. However, most of the attempts involve the Ullmann coupling. A precise control of the hierarchical scission of hydrogen atoms is still lacking.

Scheme 1. Scheme of the hierarchical dehydrogenation reactions of the aniline. Each step of the dehydrogenation reaction results in certain metal-organic connected supramolecular structure. The challenge can be overcome by designing appropriate reactants resulting in reaction products with preserved structural integrity during the hierarchical dehydrogenation reactions. As a consequence, the difficulty of steric hindrance can be naturally overcome. Herein, by applying aromatic amine 4,4''-Diamino-p-terphenyl (DATP), we successfully realized the scission of the X-H bonds (X=N and C) of the reactants one by one with good controllability on a Cu(111) surface (Scheme 1). Each step of the dehydrogenation reaction was monitored and evidenced by the formation of corresponding metal-organic supramolecular structures.

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Coordination between molecular ligands and metal adatoms leads to stable, well-ordered supramolecules. The triangle contour of the products is intact during the hierarchical dehydrogenation reactions. Combining STM, non-contact atomic force microscopy (nc-AFM) imaging, X-ray photoelectron spectroscopy (XPS) measurements with density functional theory (DFT) calculations, we explored the reaction pathways.

images (Figure 1e and Figure S3) resemble the ones acquired experimentally, providing an explanation why a smaller CuCu distance is observed in STM experiments as the Cu adatoms are positioned off-center with respect to their corresponding lobes. Furthermore, the maxima associated with the Cu adatoms in the simulated STM image in Figure 1d are separated by 1.51-1.53 nm, coinciding with the observed distance. This way, our hypothesized structural model of complex 1 (Figure 1d) could be confirmed. We could only observe isolated complex 1 product at the present stage. This is because only one hydrogen atom of the amino group (1st H) is removed, such that each resulting imino group can only coordinate with one Cu adatom, preventing the interlinking of the triangles. To further prove this, control experiments were performed by preparing a sample with higher coverage. As shown in Figure 1f, no connected triangles are observed though the triangles occupy almost the entire Cu(111) surface.

Figure 1. Formation of complex 1 by detaching one hydrogen atom from each amino group. (a) STM topographic image of the DATP adsorbed Cu(111) surface. The inset gives the structural model of the DATP. (b) The STM topographic image after annealing the surface shown in (a) at 340 K for 10 mins. (c)-(e) Highly resolved STM image (c), optimized structural model (d) and simulated STM image (e) of complex 1. (f) The STM topographic image of complex 1 with high coverage. Tunneling parameters are It=20 pA and Vb=-1 V for (a), (b) and (f); It=100 pA and Vb=-100 mV for (c). Figure 2. Formation of linked triangle structures by detaching two hydrogen atoms from the amino group. (a) STM topographic image after annealing the DATP adsorbed Cu(111) surface at 360 K for 10 mins. (b) Zoomed STM topographic image of the marked region in (a). (c) Structural model of (b). (d) STM topographic image after annealing the DATP adsorbed Cu(111) surface at 360 K for 30 mins with a higher coverage. (e) Zoomed STM topographic image of the marked region in (d). (f), Structural model of (e). Tunneling parameters are It=20 pA and Vb=-500 mV for all the STM images.

Figure 1a shows a representative STM image (taken at 77 K) after deposition of 0.05 ML DATP on a Cu(111) surface held at room temperature (RT). The fuzzy pattern indicates the fast movement of the adsorbed monomers, suggesting that both the molecule-substrate and molecule-molecule interactions are rather weak at 77 K. By taking the STM image at lower temperature (4.2 K), at which the mobility of the molecules is suppressed, individual monomers become visible (Figure S1), thus confirming the presence of DATP on the Cu(111) surface. The fuzzy pattern transforms dramatically to triangle-shaped objects after annealing the sample at 340 K for 10 mins, as seen in Figure 1b, which we refer to as complex 1. A high-resolution image (Figure 1c) reveals that complex 1 is constructed by three DATP monomers, jointed via visible “darker protrusion” at each vertex. According to previous reports46,47, amino groups can lose one of their hydrogen atoms (1st H) and each resulting imino group is able to coordinate with one copper adatom. Based on this, we performed density functional calculations, and the optimized model is depicted in Figure 1d. This model allows for two kinds of orientations of complex 1 on the copper surface, which is in nice agreement with the STM observations, as shown in Figure 1b. The calculated Cu-Cu distance in complex 1 is 1.62 nm, slightly larger than that observed experimentally (1.53±0.05 nm) (Figure 1c, the statistical analysis of the size of complex 1 can be found in Figure S2). Importantly, the simulated STM

The triangles connect with each other when increasing the annealing temperature to 360 K, as highlighted by the white ellipses in Figure 2a. Figure 2b gives the high resolution STM image of the newly formed rhombus-shaped supramolecular product. The transverse molecule coordinates with four Cu atoms, as indicated by the arrows in Figure 2b. Based on this, a structural model is given in Figure 2c. The reason that leads to the structural evolution is rather simple: at elevated temperatures a second hydrogen atom of the amino group (2nd H) of some reactants is detached. As a result, the imino groups further transform, enabling such N atoms to coordinate with two Cu adatoms. By further increasing the coverage (0.3 ML) and the annealing time (30 mins), more H atoms of the imino groups were split-off, leading to heavier coordination of DATP (Figure 2d). In particular, larger annular coordination


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Journal of the American Chemical Society complexes can be formed if all the H atoms of the amino groups were dissociated, as shown in Figure 2e and 2f. The kinetic effect arising from the relatively strong metal organic interaction prevents the formation of larger scale defect free 2D networks.

protrusions within complex 2 (Figure 3b and 3d). Calculations also show that a maximum LDOS appears close to the center of the two Cu adatoms. The distance between the maxima of the protrusions in the simulated images is around 1.31 nm, agreeing well with our STM observations (Figure 3b, see Figure S5 for more information).

Figure 3. Formation of complex 2 by the ortho C-H activations. (a) STM topographic image after annealing the DATP adsorbed Cu(111) surface at 400 K for 10 mins. (b)-(d) Highly resolved STM image (b), optimized structural model (c) and simulated STM image (d) of complex 2. Tunneling parameters are It=20 pA and Vb=-500 mV for (a) and It=500 pA, Vb=-100 mV for (b).

Figure 4. N1s line scan of the X-ray photoelectron spectra on a DATP adsorbed Cu(111) surface. The annealing temperatures are indicated beside the spectra. To further confirm the presence of N-Cu coordination motifs in both complex 1 and 2, XPS experiments were carried out. As shown in Figure 4, two peaks are visible in the N1s spectrum after depositing DATP on a Cu(111) surface at RT. The main peak centered at a binding energy (BE) of 400.2 eV is ascribed to the pristine amino groups54. The small shoulder centered at 398.7 eV represents the N-Cu coordination via the N-H activations. The spectra show that partial dehydrogenations of amino groups take place even at RT on Cu surfaces. In fact, we do observe a small amount of complex 1 after leaving the prepared sample at RT in the UHV chamber overnight. Upon annealing, the intensity of the main peak decreases significantly and almost vanishes after 390 K annealing. On the other hand, the peak centered at 398.7 eV dominates, suggesting that most amino groups lost their H atoms and coordinate with Cu adatoms. The peak centered at 398.7 eV remains unchanged up to 470 K annealing, which agrees with the structural model shown in Figure 2 and Figure 3 that N-Cu coordination retains during the hierarchical dehydrogenation reactions. The observed sequential evolution of individual reactants to (i) isolated complex 1, (ii) extended supromolecules (Figure 2), and (iii) complex 2 points to controllable thermally triggered chemical reactions on the copper surface. The process involves the hierarchical N-H and C-H activations, accompanied by metal-organic coordination. Furthermore, the triangle contour of the products remains unaltered during the successive structural evolution, providing templates for the study of structurally persistent and space confined surface catalytic reactions.

Surprisingly, further structural evolutions took place after annealing the sample at 400 K for 10 mins, and we refer to the new product as complex 2. Although the contour of the supramolecules remains unaltered, as seen in Figure 3a and 3b, brighter protrusions were discernible at the vertex of complex 2, which are different from the darker ones observed in complex 1 (Figure 1c). In addition, the side of the triangle transform from a rectangular shape in complex 1 (Figure 1c) to an isosceles triangle in complex 2 (Figure 3b). The difference cannot be ascribed to tip effects, because both the darker and brighter spots at the vertex of the triangle can be found simultaneously within some intermediate-stated supramolecules (Figure S4). The distance between the brighter protrusions at the triangle vertex in complex 2 is 1.29±0.03 nm (Figure 3b and Figure S5), smaller than that of the complex 1 (Figure 1c). We assign the structural evolution from complex 1 to complex 2 and to the formation of the C-Cu-C bonds via the ortho C-H activation of anilines derivatives. With this hypothesis, we obtained the relaxed structure of complex 2, in which both the N and ortho phenyl C atoms have hydrogen atoms detached and are bonded with Cu adatoms, as shown in Figure 3c. The corresponding simulated STM images (Figure 3d, and Figure S6) agree nicely with that observed experimentally. Furthermore, the calculations reveal that the local density of states (LDOS) of the C-Cu-C bonds is stronger than that of the N-Cu-N bonds. This explains why we observe the “darker protrusions” and “brighter protrusions” in Figure lc and 3b, respectively. The coexistence of the C-Cu-C bonds and the N-Cu-N bonds leads to the elongated shape of the



Figure 5. STM topographic and nc-AFM frequency shift images of the formed supramolecules at each stage. (a)-(d) STM topographic image of complex 1 (a), the diamond-shaped product (b), the “intermediate state”, partially converting from complex 1 to complex 2 (c) and complex 2 (d). (e)-(h) corresponding nc-AFM frequency shift images. The scanning parameters are Vbias=100 mV, It=10 pA for STM and ∆Z =-120 pm to -170 pm for nc-AFM.

Figure 6. DFT study of the mechanism of the hierarchical dehydrogenation reactions. (a) Top and side views of local minima (S0a, S0b, S1a, S1b, S2, S3) and transition states (TS1, TS2, TS3) along the path. (b) Corresponding energy profiles. The activation energy for the N-H scission is the energy difference between TS1 and S0a, while the activation energy for the C-H scission is the energy difference between TS3 and S1b. To indisputably confirm the reaction process, the CO-tip ncAFM technique was utilized, allowing intramolecular singlebond indentifications55-60. Figure 5a-5d/Figure 5e-5h give the representative high-resolution STM and nc-AFM frequency shift images at each reaction stage. For isolated complex 1, the nc-AFM frequency shift image (Figure 5e) shows that each N atom bonds with one Cu adatom and forms N-Cu-N coordination. For diamond-shaped products, the nc-AFM frequency shift image (Figure 5f) elucidates that each N atom of the vertical molecule does bond with two Cu adatoms, agreeing nicely with the proposed structural model shown in Figure 2c. The AFM images clearly show that the formation and “linking” of the complex 1 are ascribed to the successive scission of the H atoms from the target amino group. Figure 5h depicts the nc-AFM frequency shift image of complex 2, while

Figure 5g shows an intermediate state, capturing the conversion from complex 1 to complex 2. Figure 5d implies that the brighter protrusion in complex 2 can be ascribed to the formation of both N-Cu-N and C-Cu-C bonds, confirming the proposed structural model shown in Figure 3c. The elongation of the protrusion in complex 2 is then a result of the superposition of the lower DOS of the nitrogen Cu and a relative stronger one of the carbon Cu. The Cu atoms are not visible in the frequency shift image, although the X-Cu-X (X=C, N) bonding is visible as a straight line. Similar phenomenon has been reported previously61. Careful analysis of the nc-AFM data shows that the distance between the nitrogen atoms and the carbon atoms of two connected DATP molecules is 3.7 Å and 3.6 Å, respectively (see Supplementary Figure S7 for details). This distance and the observed AFM


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Journal of the American Chemical Society and STM contrasts are in good agreement with our earlier measurements of C-Cu-C bonds between triphenylene molecules57. The distance is much longer than that for C-C (1.54 Å) or N-N (1.47 Å) covalent bonds, thus further confirming the formation of N-Cu-N and C-Cu-C bonds. Furthermore, the phenyl rings distort significantly in complex 2. DFT calculations reveal that the distortion arises from the different nature of N-Cu and C-Cu bonds (see Supplementary Figure S7 for details), which results in that the terphenyl components in complex 2 cannot maintain a planar formation, as shown in Figure S8. This also explains why complex 2 do not tend to link with each other. Extensive DFT calculations (Figure 6) were carried out to unravel the mechanisms of the N-H and C-H activations for a simple model system, namely aniline (C6H7N) on Cu(111). It was found that the initial removal of a hydrogen atom from the amino group has a significantly lower barrier than the subsequent dehydrogenation of the adjacent C atom (compare 1.04 eV to 1.67 eV). Both processes were considered to be assisted by Cu adatoms, as observed by our experiments, and pathways without adatoms resulted in larger barriers (see Figure S9 and Figure S10 for details). The N-H activation is accompanied by the formation of the N-Cu bond. On the other hand, for the C-H activation, the C-Cu bond is formed prior to the dehydrogenation step. Several additional pathways were considered. For example, the scenario in which the initial N-H activation is followed by the dehydrogenation of the imino group. The dehydrogenation of the imino group has barriers of 1.73 eV and 1.72 eV, without and with an additional Cu adatom, respectively (Figure S11 and Figure S12). These are slightly higher than the barrier for C-H activation (1.67 eV), which contradicts our STM observations that the dehydrogenation of the imino group is observed at lower temperatures than the C-H activation. However, in the experiments the C-H bond is protected by the triangular structure of complex 1, making it inaccessible for Cu adatoms, which may explain its higher activation temperature experimentally. Importantly, dehydrogenation of the ortho C-atom at the opposite side of the N atom (corresponding to a C-atom on the outer periphery of complex 1) has a barrier of 1.98 eV (Figure S13), explaining the selectivity towards dehydrogenating the ortho C-atom on the inside of the triangle. We also considered pathways where the initial N-H activation is followed by tautomerization of the C-H bond (Figure S14 and Figure S15), but which turned out to be significantly less favorable than other considered processes. The absolute values of the barriers depend on the chemical potential of the Cu adatoms. Here, we used as reference energy an isolated Cu adatom on the Cu(111) surface. Moreover, we used a simple aniline/Cu(111) model system, and enabled the phenol ring rotate freely during the calculation. However, the rotation of the phenol ring is suppressed after the formation of complex 1 in real situation. All these facts lead to that the calculated activation barriers can not accurately reflect the real reaction temperatures. The purpose of the calculations here was, however, to obtain a qualitative understanding of the reaction mechanism. Another study has reported aryl C-H activations by the copper catalysis at 500 K43, much higher than our case. We ascribed the low reaction barrier of the ortho C-H activation of DATP to the

introduction of the amino directing group. Similarly, in our previous report, we found the introduction of hydroxyl directing group can reduce the barrier of the C-H bonds activation significantly on surfaces under UHV29. In addition, the C-H activation takes place with the contour of the supramolecules remaining unaltered. As a consequence, no additional energy is needed to adjust the phenyl orientations, which should lead to a reduction of the reaction barrier. To show the generality of the hierarchical dehydrogenation reactions, we designed another amine derivative reactant, the N,N-Diphenylbenzidine (DPB), as shown in Figure 7a. Similar to the DATP room temperature adsorption, the DPBs move fast on a Cu(111) surface at 77 K, such that the STM topographic image only shows fuzzy features. The fuzzy pattern transforms to a cross-linked structure after annealing the surface at 340 K for 10 mins, as shown in Figure 7b. The monomers are connected via N-Cu-N coordinations through N-H activations. Similar to that shown in Figure 1c, the DOS of the nitrogen coordinated Cu atoms are quite low, such that dark protrusions are visible at the Cu sites. Further annealing the sample at 420 K for 10 mins leads to additional transitions, which can be ascribed to the formation of N-Cu-C bonds via C-H activations. As explained above, the Cu adatoms bonded to carbon exhibits higher DOS, thus the protrusions change from dark (Figure 7b) to bright ones (Figure 7c). Note that the large scale STM image at each stage can be found in Figure S16. The structural transition of the DPB adsorbed Cu(111) surface provides evidence that the controllable hierarchical dehydrogenation reactions of amine derivatives are universal using on-surface synthesis techniques.

Figure 7. Structural transitions of the DPB adsorbed Cu(111) surfaces. (a) Structural model of DPB. (b) The STM topographic image after annealing the DPB adsorbed Cu(111) at 340 K for 10 mins. (c) The STM topographic image after annealing the DPB adsorbed Cu(111) at 420 K for 10 mins.



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Tunneling parameters are It=100 pA and Vb=-1 V for (b) and It=1 nA, Vb=100 mV for (c). In summary, we have obtained several levels of chemical transformations controllably on the Cu(111) surfaces. By thorough STM and high resolution nc-AFM studies, we found that the reaction products were dictated by hierarchical dehydrogenation reactions via N-H and C-H activations of aniline derivatives. Each step of the dehydrogenation reactions was monitored and evidenced by the formation of corresponding metal-organic coordinated supramolecular products structures. In combining with XPS measurements and DFT calculations, we have elucidated the respective reaction pathways. Our findings not only report alternative means for the construction of novel metal-organic coordinated structures, but also point out novel strategies for controlled hierarchical dehydrogenation reactions.

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ASSOCIATED CONTENT Supporting Information. Experiments and calculations methods; Evidence of the presence of DATP on Cu(111); The bias dependent STM topographic images and the corresponding simulated images of the supramolecule structures; Statistical analysis of the size of the supramolecule structures; Detail analysis of the AFM images; Other candidate reaction pathways. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the Collaborative Innovation Center of Suzhou Nano Science & Technology and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The National Supercomputer Centre in Sweden is acknowledged for allocating computational resources. This work was supported by National Science Foundation of China (21661132006, 21790053, 21622306, 91545127, 21473178, 21773222), National Major State Basic Research Development Program of China (2017YFA0205002, 2014CB932600) and Natural Science Foundation of Jiangsu Province (BK20140305 and BK20130285).

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Hierarchical Dehydrogenation Reactions on a Copper Surface

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