Efficient Lanthanide Catalyzed Debromination and Oligomeric Length

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Efficient Lanthanide Catalyzed Debromination and Oligomeric Length-Controlled Ullmann Coupling of Aryl Halides Borja Cirera, Jonas Björk, Roberto Otero, José M. Gallego, Rodolfo Miranda, and David Ecija J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02172 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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The Journal of Physical Chemistry

Efficient Lanthanide Catalyzed Debromination and Oligomeric Length-Controlled Ullmann Coupling of Aryl Halides B. Cirera,1 J. Björk,2,* R. Otero1,3, J.M. Gallego,4 R. Miranda,1,3 and D. Ecija1,* 1

IMDEA Nanoscience, Cantoblanco, Madrid, Spain

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Department of Physics, Chemistry and Biology, IFM, Linköping University, Sweden

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Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, Cantoblanco, Madrid, Spain

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Instituto de Ciencia de Materiales de Madrid, CSIC, 28049 Madrid, Spain

ACS Paragon Plus Environment

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ABSTRACT: Lanthanide elements play a vital role in a broad range of high-tech applications and there is an increasing interest in their catalytic activity, particularly in organo-metallics. However, their catalytic role on surfaces remains unexplored. Here, we present a scanning tunneling microscopy and density functional theory study of the debromination, contacting and coupling of dibromine terphenyl species with Dy (f-block element) and Ag (d-block element) adatoms, respectively. We show that Dy debrominates the targeted species more efficiently than Ag adatoms at room temperature, promoting the formation of unprecedented C-Dy-C organometallic supramolecules versus C-Ag-C parallel chains for the Ag case. DFT calculations corroborate our results showing an almost spontaneous debromination process with Dy compared to Ag. Upon annealing, for samples containing Dy the formation of C-Ag-C metalorganic bonds and concomitant C-C coupling is inhibited, giving rise to a self-assembly of debrominated monomers, showing only a minority number of covalent dimer species. For samples without Dy covalent chains of irregular length are promoted. Our studies open new avenues for using lanthanide elements as efficient dehalogenation catalysts. Furthermore, we illustrate their potential as inhibitors of uncontrolled C-C coupling reactions, of great relevance for fine tuning the length of polymeric compounds.


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INTRODUCTION Coordination chemistry on surfaces in ultra clean environments has emerged as a powerful tool to engineer metallosupramolecular architectures,1-20 with prospects in sensing,4,21-26 catalysis,27-30 molecular electronics and spintronics.15,21,25,31-36 On-surface synthetic studies have provided a single molecule view of the catalytic role of noble metals supports on the dehalogenation of aryl halides (C-I and C-Br bonds), subsequently used to steer the formation of C-C bonds and thus to provide a cornucopia of nanomaterials with great potential application.37-51 Importantly, this catalytic activity is enhanced by the addition of extrinsic atoms like Cu, Pd and Pt,52,53 in which dehalogenation gives rise to the formation of organo-metallic intermediates and, concomitantly, covalent species upon thermal annealing. However, despite the importance of lanthanide complexes as low toxic and efficient catalysts for dehalogenation in solution, the catalytic role of extrinsic lanthanide elements on surfaces remains unexplored.54 Here, we report a combined scanning tunneling microscopy (STM) and density functional theory (DFT) study of the debromination of 4,4''-dibromo-p-terphenyl (DBTP) species on Ag(111) (cf. Figure 1a) and their subsequent chemical transformations with temperature both in the absence and presence of Dy atoms. In the absence of Dy, we observe the formation of different supramolecular phases when DBTP species are deposited at RT, signaling initial steps of debromination. Further annealing to 330 K leads to full debromination and formation of CAg-C organo-metallic wires. Subsequent annealing to 520 K results in the formation of covalent wires (C-C bonds). In the presence of Dy this scenario is completely altered. The post-deposition of Dy atoms on previously grown submonolayer of DBTP on Ag(111) held at RT results in the formation of randomly distributed Dy centers of irregular sizes, which are linked by fully debrominated DBTP species, thus illustrating unprecedented organo-metallic55-57 species (C-Dy-


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C) on surfaces. Gentle annealing to 330 K reveals polymorphism, showing the presence of majority debrominated DBTP species and a minority of debrominated DBTP dimers, coexiting with three-dimensional irregular metallic clusters containing Dy and presumably Ag. Final annealing to 460 K signals the only survival of the metallic clusters. Our study illustrates the extreme efficiency of lanthanide elements to debrominate aryl halide species on surfaces, inhibiting the Ullmann coupling beyond dimers and the formation of subsequent byproducts, and thus open novel avenues to de-halogenation protocols on surfaces, as well as promising pathways for fine-tuning the length of in-situ synthesized polymers on surfaces.

METHODS The experiments were performed in a custom designed ultra-high vacuum system that hosts a low-temperature Omicron scanning tunneling microscope, where the base pressure was below 5×10-10 mbar. All STM images were taken in constant-current mode with electrochemically etched tungsten tips, applying a bias (Vb) to the sample and at a temperature of ~77 K. The Ag(111) substrate was prepared by standard cycles of Ar+ sputtering (800 eV) and subsequent annealing to 723 K for 10 minutes. DBTP species are commercial from Sigma Aldrich and were deposited by organic molecular-beam epitaxy (OMBE) from a quartz crucible held at 380 K onto a clean Ag(111) at room temperature. Dy atoms were evaporated by means of electron beam evaporation from an outgassed Dy rod, with the sample held at a separation of 12.5 cm, and with typical deposition times of 45 seconds at 5.5 mA of emission to achieve partial debromination, and of 60 seconds at 7.1 mA of emission for full debromination, respectively. If necessary, in a subsequent step, the samples were annealed with a thermal gradient of 1º K/s and kept at the


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desired temperature for 30 minutes. Next, they were cooled down to room temperature with a thermal gradient of -1º K/s and finally transferred to the STM stage held at 77 K. Periodic density functional theory (DFT) calculations were performed with the VASP code,58 using the projector-augmented wave method.59 To treat the problem of DFT to handle felectrons (due to self-interaction errors), a PAW potential was used in which the f-electrons are partly frozen in the core. More precisely, 9 f-electrons were kept frozen in the core, treating the Dy with a valency of 3. Plane waves were expanded up to a kinetic energy cutoff of 400 eV. Exchange-correlation effects were described by the version of the van der Waals density functional (vdWDF)60 introduced by Hamada61 and denoted as rev-vdWDF2, which has shown to accurately describe adsorption of different polycyclic aromatic hydrocarbons on Ag(111).62 The Ag(111) surface was approximated by a four layered slab separated by a vacuum region of 15 Å. In all calculations we used ‫݌‬ሺ5 × 5ሻ super cells together with 5 × 5 k-point sampling, except for the debromination on the flat Ag(111), where a 5 × 5 k-point sampling was used. The employed k-point samplings ensure that reported energies are converged within 10 meV. Structural optimizations were performed until the forces on all atoms were smaller than 0.1 eV/Å, except for the bottom two layers of the Ag slab which were kept frozen. Transition states were calculated using a combination of the climbing image nudged elastic band (CI-NEB)63 and Dimer methods.64 For the CI-NEB calculations the number of images was adjusted specifically for each transition-state calculation such that the tangent along the path was well described, using typically 15−20 images. The CI-NEB method was used to find a rough estimate of the transition state, which was used as input for the Dimer method. The structural optimizations of transition states were performed until the forces acting on the atoms on the central images, in the Dimer method, were smaller than 0.02 eV/Å.


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RESULTS AND DISCUSSION Chemical Transformations of DBTP on Ag(111) upon Temperature Annealing.

The

deposition of DBTP on Ag(111) held approximately at room temperature affords a majority twodimensional porous network65 (to be termed phase α, cf. Figure 1a,b), stabilized by Br···Br bonds (average projected distance of ~ 2.8 ± 0.5 Å) and Br···H bonds (with projected distances ranging from 1.8 ± 0.5 Å to 3.2 ± 0.5 Å). Importantly, some organo-metallic chains based on debrominated DBTP species linked by C···Ag···C bonds are detected, with a C···Ag projected distance of 2.0 ± 0.5 Å, thus signaling initial steps of debromination and connection to Ag adatoms (cf. Figure 1c-d, to be termed phase β).

Figure 1. Evolution of DBTP Species on Ag(111) by Increasing Substrate Temperature. ah) STM images and atomistic models. a-d) Substrate held at room temperature. a-b) Phase α based on the supramolecular assembly of intact species. c-d) Minority phase β illustrating organo-metallic C-Ag-C chains of fully debrominated species. a-d) Carbon, hydrogen, bromine


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and silver atoms are depicted by green, white, yellow, and grey solid circles, respectively e) Annealing at 340 K leads to debromination and the subsequent formation of C-Ag-C organometallic wires, which are stabilized laterally by bromine atoms. Inset: High resolution STM image and atomistic model of the organo-metallic wires. f-g) Further annealing at 440 K affords the creation of covalent oligomers of distinct length. f) Large scale STM topograph. g) High resolution image and atomistic model, displaying the conversion of former organometallic wires into perfectly aligned 1D oligomers joint laterally by bromine atoms. h) Final annealing to 540 gives rise to covalent polimers. e-h) Carbon and hydrogen atoms are depicted by green and white, respectively. To emphasize visibility bromine and silver atoms are illustrated by bigger light green and grey circles. Image details: a) 19 nm x 19 nm, Vb= -1.0 V, I= 10 pA; c) 7.9 nm x 7.9 nm, Vb= 1.0 V, I= 50 pA; e) 9.3 nm x 9.3 nm (Inset: 2 nm x 2nm), Vb = 1 V, I = 100 pA; f) 21.2 nm x 21.2 nm, Vb = -2 V, I = 0.95 nA; g) 12.5 nm x 12,5 nm, Vb = -0.5 V, I = 0.2 nA; h) 25 nm x 25 nm (Inset: 1.8 nm x 1.8 nm), Vb = -1 V, I = 200 pA. Further annealing to 340 K leads to full debromination and the subsequent formation of one-dimensional organo-metallic supramolecular wires based on C···Ag···C bonds linked laterally by Br···H interactions (phase γ, cf. Figure 1e).66 An increase of the annealing temperature to 440 K gives rise to the formation of oligomers of distinct length (cf. Figure 1f-g), in agreement to results on Cu(111),67

thus

signaling the onset of the Ullmann coupling reaction. Subsequent heating to 520 K allows the creation of two-dimensional supramolecular islands formed by covalent nanowires (cf. Figure 1h), to be termed phase δ. As previously encountered on other coinage surfaces,49,68 these onedimensional nanowires are aligned parallel to each other and joint together thanks to the presence of interstitial bromine atoms via C-H···Br···H-C interactions.


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Role of Dysprosium as a Dehalogenation Catalyst. Next, we have inspected the influence of deposition of Dy on a submonalayer of DBTP grown on Ag(111), addressing the onsurface catalytic activity of lanthanide elements towards aryl halides upon thermal annealing. After deposition of minute amounts of Dy holding the sample at room temperature, as shown in Figure 2a, the majority of Dy forms irregular clusters (bright protrusions) coexisting with supramolecules. High resolution STM images of selected areas allow us to discern intramolecular details. Figure 2a-c shows the formation of tetrameric and heptameric supramolecules based on intact DBTP species and stabilized by Br···Br and Br···H bonds. Importantly, we also detect the formation of: i) pure organo-metallic supramolecules based on links between debrominated species and Dy centers through on-surface C···Dy···C interactions in a three-fold fashion, with a projected C···Dy bond distance of 2.4 Å ± 0.5 Å (cf. Figure 2d,e), and ii) hybrid pentameric supramolecules, showing the simultaneous expression of Br···Br / Br···H bonds and C···Dy···C interactions (cf. Figure 2b,c). Taking into account the average distribution of the distinct nanostructures, it can be concluded that despite sufficient molecular coverage, Dy adatoms tend to form small clusters, being peripherically surrounded by debrominated species. By increasing the amount of Dy (cf. Figure 2f), a scenario of fully debrominated molecular species emerges, in which the linkers are surrounding the Dy clusters or forming organometallic C-Dy-C oligomers with projected C···Dy distances of ~ 2.4 Å ± 0.5 Å (cf. Figure 2g-h and Figure S1). Importantly, no signature of phase α is detected, corroborating the efficient debromination of the molecular species upon adequate Dy dosage.


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Figure 2. Debromination of DBTP Species and Subsequent Formation of Organo-Metallic Irregular Architectures by the Addition of Dy at Room Temperature. a) Long-range STM image revealing the formation of Dy clusters surrounded by molecular debrominated linkers and supramolecular nanostructures, for insufficient amounts of Dy. b) High resolution STM image of intact supramolecules (right) coexisting with hybrid supramolecules (left) c) Ball-and-stick atomistic model of b). d) High resolution STM image of a trimeric organometallic supramolecule


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based on C-Dy-C interactions. e) Ball-and-stick atomistic model of d). f) Long-range STM image revealing the formation of Dy clusters surrounded by molecular debrominated linkers and organometallic C-Dy-C nanostructures, for sufficient amount of Dy to achieve full debromination. g) High resolution STM topograph of an organo-metallic pentamer imaged with a molecular tip. h) Ball-and-stick atomistic model of g). Carbon, hydrogen, bromine and Dy atoms are depicted in green, white, yellow and orange, respectively. Image details: a) 55 nm x 55 nm, Vb = -1.25 V and I = 100 pA; b) 11.0 nm x 6.8 nm, Vb = -1 V and I = 20 pA; d) 5.2 nm x 5.2 nm, Vb = -1.25 V and I = 100 pA; f) 100 nm x 100 nm, Vb= -1V, I = 15 pA.

Dyprosium as an Inhibitor of Ullmann Coupling Reaction. Further annealing of the sample to 330 K gives rise to a profound change in the self-assembly. As shown in Figure 3, the former irregular Dy clusters, laterally decorated by debrominated linkers, have been dissolved giving rise to three-dimensional irregular clusters (cf. Figure 3a) and supramolecular architectures affording distinct assemblies (cf. Figures 3b-f). Importantly, all molecular species are debrominated, and the vast majority of them are monomers (over 99%) and only ~1% of species are covalent dimers, i.e. the C-C bond formation between debrominated monomers is hindered (see discussion below), in contrast to the catalytic role of d-block transition metals for the homo-coupling of aryl bromides on Au(111).52 Bromine atoms are still present within the distinct supramolecular architectures and visualized as dim protrusions, with an identical appearance as those encountered during the annealing of DBTP species on Ag(111) (see above) and in recently published results.40,47,69,70 Thus, Dy atoms are highly efficient in debrominating aryl halides, while in majority preserving the length integrity of the monomers, precluding the subsequent Ullmann coupling reaction beyond dimers.


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Interestingly, regarding self-assembly, a cornucopia of phases is found including pure monomeric or dimeric phases, and monomeric-dimeric mixtures. The dominant phase is shown in Figure 3a-d, to be termed phase ζ, and consists of a close-packed assembly of equally oriented monomers forming small domains with a rhombic unit cell defined by unit cell vectors a=15.3 ± 0.5 Å, b=9.9 ± 0.5 Å, spanning an angle of 50°. Figure 3e reflects that monomers can also form star-like assemblies. In addition, the very minority of covalent dimers can give rise to ordered (cf. Figure 3) or disordered phases (cf. Figure S2). We can distinguish up to three ordered phases incorporating dimeric species: i) racemic supramolecules made up of monomers and dimers (cf. Figure 3f), ii) pure close-packed assemblies (cf. Figure 3g), and iii) close-packed racemic mixtures (cf. Figure 3h). All these phases are stabilized thanks to CH···Br interactions between bromine adatoms and adjacent molecular species. The effect of the surface is reflected by the alignment of both monomers and dimers along one of the high symmetry directions.


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Figure 3. Supramolecular architectures after codepositing DBTP species and Dy on Ag(111) held at room temperature followed by annealing to 330 K. The gentle annealing affords the formation of fully debrominated monomers and the minority on-surface synthesis of covalent species, mainly dimers, giving rise to distinct supramolecular architectures (see text for details). a-c, e-h) STM topographs. Image details: a) 60 nm x 60 nm, Vb = -1 V and I = 25 pA; b) 24.6 nm x 24.6 nm, Vb = -0.5 V and I = 200 pA; c) 5 nm x 5 nm, Vb = -0.5 V and I = 200 pA; etop panel): 7.3 nm x 4.1 nm, Vb = -1.25 V and I = 50 pA; e-bottom panel): 11 nm x 6 nm, Vb = 0.75 V and I = 50 pA; f-top panel): 10.4 nm x 6.0 nm, Vb = -1 V and I = 10 pA; f-bottom panel): 24.8 nm x 11.7 nm, Vb = -1 V and I = 10 pA; g) 8 nm x 8 nm, Vb = -0.5 V and I = 20 pA; and h) 9.3 nm x 9.3 nm, Vb = -0.75 V and I = 50 pA. d) Ball-and-stick model of the self-assembly of phase ζ. The black cross shows the high symmetry directions of Ag(111). Carbon and hydrogen atoms are depicted by green and white, respectively. To emphasize visibility bromine atoms are illustrated by bigger light green circles. Next, we track the evolution of the sample with thermal annealing. Surprisingly, despite annealing up to 420 K, almost no sign of C-Ag-C formation, nor increase of C-C bonds, is detected. To investigate this overall intriguing phenomenon, we performed a series of controlled experiments. First, we varied the dosage of Dy deposited on a submonolayer coverage of DBTP. i) For sufficient Dy to debrominate all DBTP species, annealing to 330 K expresses solely the monomer phase and the minority cornucopia of phases reported above, but no hint of C-Ag-C bonds. Further annealing signals the survival of the monomer phase up to 420 K (cf. Figure S3) and full desorption at 460 K (cf. Figure S4). ii) However, for insufficient Dy and upon annealing to 330 K, we see the presence of the monomer phase (ζ) surrounded by C-Ag-C bonds metal-


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organic chains (cf. Figure S5). In both experiments, the dosage of Dy was sufficient to decorate the steps, thus limiting the supply of Ag adatoms from steps. Second, we invert the order of deposition of the adsorbates, evaporating in the first place Dy to cover the steps as a facile source of Ag adatoms (cf. Figure S6). Mimicking the Dy dosages of the above-mentioned experiments and holding the sample at RT gives rise to steps fully decorated by Dy (cf. Figure S6a,b). Subsequent evaporation of submonolayer coverage of DBTP affords the expression of phase α, i.e., the supramolecular self-assembly of intact species (cf. Figure S6c,d). Since Dy atoms are at the steps, they do not participate in the debromination of the DBTP species. Annealing to 330 K gives rise solely to small C-Ag-C metal-organic chains, much less frequent than without the presence of Dy and remarkably no C-Dy-C bonds (cf. Figure S6e,f). Thus, the covering of the steps by Dy limits the main source of Ag adatoms, though at 330 K, they are present and active to form a few C-Ag-C coordinative links. Importantly, this experiment provides evidence of the importance of Ag adatoms in the reaction pathways for the Ullmann coupling on the clean Ag(111) surface. Based on these controlled experiments we conclude that Dy alters the reaction pathway of DBTP on Ag(111), which is rationalized as follows: 1) DBTP is absorbed intact on Ag(111) at RT. 2) Subsequent deposition of sufficient Dy gives rise to the debromination of DBTP species and formation of C-Dy-C organo-metallic supramolecules. 3) Annealing to 330 K breaks the CDy-C bonds, creating debrominated monomers that diffuse and form a close-packed assembly, in which bromine atoms play a stabilizing role. Importantly, these monomer species do not engage into C-Ag-C bonds. Only less than 1% of monomers are engaged into the formation of covalent dimers. 4) Further annealing to 420 K signals the survival of the monomer phase with almost no expression of C-Ag-C bonds, nor an increase of C-C links.


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DFT Insights. To gain additional insights into this unprecedented reactivity of a metal towards an aryl halide species, we performed DFT calculations. First of all, we studied the debromination of bromobenzene on Ag(111), both in the presence and absence of Dy. The debromination barrier aided by an Ag adatom is slightly lower than on the atomically flat surface, showing that the Ag adatom indeed has some reducing effect on the surface. The reaction is initiated by the formation of an intermediate structure (IntS), in which the adatom interacts directly with the Br atom. This is followed by the debromination step with a potential energy barrier of 0.85 eV (comparing the energies of TS2 and IntS). On the flat surface, the corresponding barrier is 0.98 eV (slightly larger than previously reported71 due to higher numerical accuracy and slightly different density functional in the current study). For a comparison of the debromination on the atomically flat, and Ag-adatom aided process, see Figure S7.

Figure 4. Dehalogenation reaction of bromobenzene on Ag(111) in the absence/presence of Dy. For both reactions top and side views of initial state (IS), intermediate state (IntS), final state (FS) and transition states (TS1, TS2) are depicted, accompanied by energy profiles.


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Hydrogen, carbon, bromine and Dy atoms are shown in white, grey, red and blue solid circles, respectively. In the presence of Dy (cf. Figure 4, right panel), the dehalogenation can be seen as an almost spontaneous process. First, an intermediate is formed between the Dy atom and bromobenzene, which decreases the energy of the system compared to having Dy and bromobenzene separate, demonstrating a strong attraction between the two entities. However, the resulting intermediate state (IntS) is not an actual local minimum, but is rather located at a very flat region of the potential energy surface. This is concluded from the fact that no barrier separates the intermediate state from the final dehalogenated structure (FS), in which the resulting bromine atom and phenyl radical are chemisorbed to the Dy atom, similar to the case with an Ag adatom. Notably, the net reaction energy is -2.70 eV in the presence of a Dy atom. In other words, the Dy adatom favors the dehalogenation process both by making it, in practice, spontaneous, and significantly more exothermic compared to having the reaction catalyzed by an Ag adatom. From Figure 4 it is also inferred that both the interaction of a Br atom and the phenyl radical with Dy is stronger than with an Ag adatom. It costs 0.34 eV to remove the Br from the Ag, while the corresponding energy cost is 1.09 eV in the case of Dy (compare FS’ to FS for both scenarios). Furthermore, the final state in which the phenyl radical is chemisorbed to a Ag or Dy adatom, while the Br is adsorbed on the flat surface, is 1.05 eV more stable with Dy (compare the energies of FS’ for both scenarios). In addition, we inspected the capacity of Dy to reduce the amount of Ag adatoms diffusing on the surface. We performed DFT calculations to compare the clustering of Ag adatoms around (i) a single Ag adatom and (ii) around a Dy adatom on Ag(111). The results are summarized in Figure S8, and explained in the SI. In short, the clustering of Ag adatoms around


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a Dy adatom is more stable by 80 meV per added Ag adatom, compared to the clustering around an Ag adatom. Although, in this case, the effect of the Dy is quite subtle, it is clear that even a single Dy atom has a favorably effect on the nucleation of atoms. This phenomenon, in addition to the decoration of Ag steps by Dy, may explain a diminishing number of available Ag adatoms in the presence of Dy, resulting in fewer C-Ag complexes. Finally, we discuss the origins of the inhibition by Dy of the Ullmann coupling. As illustrated in Figure 1f-g, in the absence of Dy, intermediate annealing gives rise to the formation of dimers, trimers, tetramers and polymers of distinct length within the C···Ag···C metallosupramolecular chains. We never observed a single oligomer outside the supramolecular chains, which reveals the crucial role of the confinement within the chains in the coupling reaction. In this atomistic process, a Ag adatom is removed from the C···Ag···C bond, and then since the debrominated termini are very close, facing each other and vibrating, the formation of the C-C bond (the dimer and so on) is enhanced. On the contrary, in the presence of Dy, the molecules are debrominated and peripherically linked to the Dy clusters. In this geometry, the monomers are not facing each other, but in an open architecture. When slightly annealed the C···Dy bond is broken, and the monomers diffuse predominantly forming the ζ phase, in which the residual bromine atoms play a crucial role stabilizing the supramolecular architecture via Br···H bonds (cf. Figure 2a-c). Additionally, the decoration of the step edges further diminishes the catalytic sites for monomers to react. The overall result is that the Ullmann coupling is greatly inhibited and less than 1% of the species present on the surface are dimers. Thus, our results highlight the importance of metal-organic coordination in subsequent reaction pathways, which is of relevance for Ullmann coupling schemes.

CONCLUSIONS


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In summary, we have shown the distinct role of metal adatoms in dehalogenating, contacting and coupling molecular species equipped with terminal bromines. Intrinsic surface d-block Ag adatoms are able to debrominate the molecular compounds at sufficient temperature, affording C-Ag-C organometallic assemblies, which upon annealing gives rise to parallel covalent wires via Ullmann coupling reaction joint laterally by bromine atoms. On the contrary, deposition of Dy at room temperature catalyzes the full debromination of species (provided sufficient Dy coverage) and the formation of unprecedented C-Dy-C organo-metallic supramolecules. DFT calculations of bromobenzene justify these results revealing a potential energy barrier of 0.85 eV for the debromination reaction with a Ag adatom, whereas the reaction in the presence of a Dy adatom is almost spontaneous. In addition, Dy decorates silver steps and traps Ag adatoms, precluding the formation of C-Ag-C bonds, provided enough Dy dosage. Importantly, further annealing in the presence of Dy inhibits the Ullmann reaction, i.e., a very few of the debrominated species are covalently linked, producing dimers and trimers. Based on these evidences, we conjecture that Dy adatoms are very efficient in dehalogenating molecular species. However, they inhibit the Ullmann coupling reaction beyond dimers. This inhibition behavior is attributed to the open coordination geometry in Dydirected metal-organic clusters versus the confined metallosupramolecular architecture imposed by C-Ag-C bonds. Our studies will pave new avenues for catalyzing chemical reactions with metal adatoms addressing the importance of their electronic nature: d versus f valence electrons. Remarkably, Dy as member of the lanthanide family is shown to be efficient in dehalogenation reactions of


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relevance in green chemistry and in organic synthesis. Furthermore, it is illustrated to be relevant for inhibiting uncontrolled coupling reactions on surfaces, revealing a potential way for designing polymers on surfaces with controlled length at the nanoscale, an area in which further research is necessary to increase the production yield.

ASSOCIATED CONTENT Supporting Information. STM images of the formation of Dy irregular clusters surrounded by debrominated DBTP species. STM images of random supramolecular architectures based on debrominated DBTP precursors. STM image showing the preservation of phase ζ upon annealing to 420 K. STM images signaling the desorption of molecular species at 460 K. STM image illustrating the coexistance of C-Ag-C and C-C bonds for insufficient amounts of Dy. STM images showing the sequestering of Ag adatoms by Dy on Ag(111). Comparison of the dissociation of bromobenzene with and without an Ag adatom. Calculations of the clustering of Ag adatoms around a silver and a dysprosium adatom. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGEMENTS Work supported by the EC FP7-PEOPLE-2011-COFUND AMAROUT II program, the Spanish Ramón and Cajal Program (nº RYC-2012-11133), the Spanish Ministerio de Economía y Competitividad (projects FIS 2013-40667-P, FIS 2015-67287-P), the Comunidad de Madrid (projects MAD2D, NANOFRONTMAG (S2013/MT-2850)), and the IMDEA Foundation.

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Efficient Lanthanide Catalyzed Debromination and Oligomeric Length

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