On-Surface Heck Reaction of Aryl Bromides with Alkene on Au(111)with Palladium as CatalystKe-Ji Shi,† Chen-Hui Shu,† Cheng-Xin Wang, Xin-Yan Wu, He Tian, and Pei-Nian Liu*

Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials, State Key Laboratory ofChemical Engineering and School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130Meilong Road, Shanghai 200237, China

*S Supporting Information

ABSTRACT: The on-surface Heck reaction of aryl bromides with terminal alkene has been achieved for the first time. Withpalladium as the catalyst, cross-coupling of porphyrin-derived aryl bromides with terminal alkene proceeds with high selectivityon an Au(111) surface, as determined by scanning tunneling microscopy at the single molecular level. Density functional theorycalculations suggest that the on-surface Heck reaction proceeds via debromination of aryl bromide, addition to the CC bond,and elimination of hydrogen, ultimately affording the cross-coupling product.

On-surface reactions, defined as the processes by whichorganic reactions take place on surfaces, have attracted

considerable attention and contribute new perspectives toorganic chemistry.1 Ever since the first pioneering work in2007,2 numerous conventional organic reactions in solution havebeen achieved on surfaces, including Ullmann coupling;2,3

homocoupling of alkanes,4 alkenes,5 and alkynes;6 imineformation;7 condensation of boron acids;8 Bergman cyclization;9

acylation;10 cycloaddition of acetyls;11 cyclodehydrogenation;12

and azide−alkyne reactions.13 On-surface homocoupling reac-tions, especially Ullmann coupling of aryl halides, have been usedextensively to construct diverse macromolecular systems,including polymeric chains,14 hyperbranched oligomers,15

graphene ribbons,16 porous molecular networks,17 2D covalentorganic frameworks,18 and other stuctures.19

Cross-coupling reactions in solution are extremely powerfultools for creating C−C single bonds and generating moleculardiversity. They allow highly selective joining of organic moietiesA and B into A−B20 while avoiding formation of A−A or B−B.Because most of the organic molecules contain consecutive C−Cbonds, these reactions are tremendously useful in the synthesis oforganic compounds, including pharmaceuticals and conjugatedorganic materials. For example, the Heck reaction, whichinvolves cross-coupling of aryl halides with terminal alkenes,allows substitution at CC bonds, affording a diversity of usefulalkene molecules (Scheme 1).21

Realizing cross-coupling reactions such as the Heck reactionon surfaces may allow the construction of a variety ofnanostructures that cannot be generated via homocoupling, forexample, organic functional materials with a D−π−A structure.However, developing on-surface cross-coupling reactions is

Received: March 22, 2017Published: May 16, 2017

Scheme 1. Heck Reaction on Au(111) with Pd as Catalyst

Letter

pubs.acs.org/OrgLett

© 2017 American Chemical Society 2801 DOI: 10.1021/acs.orglett.7b00855Org. Lett. 2017, 19, 2801−2804

challenging because it requires inhibiting homocoupling ofidentical precursor molecules while efficiently promoting cross-coupling of different precursor molecules. On-surface cross-coupling reactions have so far been limited to cross-coupling ofporphyrin bromide with aryl bromide22 and Sonogashirareaction of alkynes with aryl halides,23 but the selectivity ofboth reactions is not satisfactory. In the on-surface Sonogashirareactions of phenylacetylene with phenyl halides, homocoupledspecies are the major products on Au(111),23a Au(100),23b

Ag(100)23c surfaces. This is because the two reactants stronglyprefer to form homomolecular islands on the surface instead ofintermixing; as a result, cross-coupling occurs only at islandboundaries.23a

Here, we report the first on-surface Heck reaction of arylbromides with alkene on Au(111) with Pd as catalyst. Usingscanning tunneling microscopy (STM) at single-moleculeresolution, we investigated the reactions of 5,15-bis(4-bromophenyl)-10,20-diphenylporphyrin (A1) and 5,10,15,20-tetrakis(4-bromophenyl)porphyrin (A2) with 4-vinyl-1,1′-bi-phenyl (B). Both reactions formed the cross-coupling productshighly selectively, with no obvious formation of homocouplingproducts. Density functional theory (DFT) calculations suggestthat the reaction proceeds via debromination of aryl bromide andaddition to the CC bond and elimination of hydrogen,ultimately affording the cross-coupling product.In our approach, A1 was deposited onto a pristine surface of

Au(111) held at room temperature inside a commercial ultrahighvacuum (UHV) system (base pressure: ∼ 2 × 10−10 mbar)equipped with a variable-temperature STM (SPECS, Aarhus150). The STM image revealed a close-packed monolayerstructure in which A1 molecules adopted the same orientation(Figure 1a). Each A1 molecule had four neighbors, and thedistance between two porphyrins was 1.87± 0.05 nm, suggestingthat van der Waals forces drive the alignment.24

Next, we dosed Bmolecules onto the sample at approximately−20 °C. The STM image showed that B molecules tended to

assemble side by side to form a close-packed array (Figure 1b).Most A1 molecules (green ellipse) were scattered among arraysof B molecules (white ellipse), while some porphyrin moleculesaggregated into small patches. We tried to couple A1 and B byannealing the sample to 150 °C for 20 min. However, STMresults showed no Heck reaction product P1; instead, alkene Bdesorbed from the Au(111) surface and porphyrin A1 self-assembled on the surface (see Figure S1). These results showthat the Au(111) surface by itself does not effectively promotethe cross-coupling reaction of aryl bromide and alkene.To enhance the reactivity of the reactants on Au(111), we

dosed Pd onto Au(111) (0.09 ML) to act as catalyst in the Heckreaction of aryl bromide A1 with alkene B. After loading Pd ontothe sample containing molecules A1 and B at room temperatureand then annealing to 150 °C for 20 min, porphyrin moleculesP1 with two tails were generated and packed together closely toform islands (Figure 1c). Porphyrins lined up one by one at aseparation of 1.38± 0.05 nm (Figure 1d), suggesting that van derWaals forces drive the alignment.24 The porphyrins werearranged with their biphenyl groups on alternating sides. Thelength of each biphenyl side chain was 1.04 ± 0.05 nm, and thedistance between two side chains was 0.62 ± 0.05 nm. Theseresults suggest that the biphenyl chains did not interact strongly.The full length of the molecule along the two tails was 4.08 ±0.05 nm, consistent with the DFT-calculated length of 4.04 nmfor molecule P1. The length of the molecule along the twophenyl substitutions was 1.78 ± 0.05 nm, consistent with theDFT-calculated length of 1.81 nm. Note that the image of P1simulated by DFT calculation was shown in the corner of Figure1d, which is identical with the STM image of P1. P1 self-assembled into a periodic structure with the unit cell dimensionsof 2.73 ± 0.05 nm along the long side, 1.38 ± 0.05 nm along theshort side, and an angle of 107°. The simulated packing of fourmolecules of P1 was shown in Figure 1d, which fit well with theexperimental image. These results demonstrate that under ourexperimental conditions the aryl bromide group of A1underwent Heck reaction with the CC group of alkene B toform product P1. This on-surface cross-coupling reactionshowed excellent selectivity, and no obvious homocouplingproducts of A1 or dimerization product of B were observed. Thereaction of the full monolayer of monomer Bwas also carried outunder the same reaction conditions, but no dimerization productwas observed, similar to the reported result on Au(111).5 Theseresults show that Pd as catalyst on Au(111) allows efficient Heckreaction of aryl bromide with alkene, while the Ullmann reactionof aryl bromide and dehydrogenative dimerization of alkene areprohibited. Note that the metalation of Pd into porphyrin ring ofP1might occur, although it could not be determined by the STMimage.25

To verify the high reactivity and selectivity of this on-surface,Pd-catalyzed Heck reaction, we attempted the reaction withporphyrin molecule A2 containing four phenyl bromide groups.After deposition on Au(111) at room temperature, A2moleculesformed a close-packed monolayer structure (Figure 2a). Each A2molecule had four neighbors, and the distance between adjacentporphyrins was 1.58 ± 0.05 nm, suggesting that van der Waalsforces drive the alignment.24 Then Bmolecules were dosed ontothe surface at approximately −20 °C. The two differentmolecules tended to assemble side-by-side into a close-packedstructure (Figure 2b). Finally, Pd was loaded onto the surfacecontaining moleculesA2 (blue ellipse) andB (white ellipse) heldat room temperature. The sample was annealed to 150 °C for 20min, generating porphyrin molecules P2 with four tails, which

Figure 1. (a) Self-assembly of A1 molecules on Au(111) at roomtemperature (12 × 12 nm2, −2.0 V, −0.06 nA). (b) Self-assembly of A1and B molecules on Au(111) at −20 °C (20 × 20 nm2, −2.5 V, −0.08nA). (c) Heck reaction of A1 and B on Au(111) after dosing Pd andannealing at 150 °C (26 × 26 nm2, −2.0 V, −0.08 nA). (d) Zoomed-inimage of the Heck reaction products (8 × 8 nm2, −2.0 V, −0.11 nA).

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packed closely into islands (Figure 2c). The measured length ofmolecule P2 was 4.02 ± 0.05 nm, similar to the length ofmolecule P1 (Figure 2d). The image of P2 simulated by DFTcalculations was also shown in the corner of Figure 2d, similar tothe STM image of P2. The porphyrins lined up one by one with aseparation of 2.70± 0.05 nm and interchain separation of 0.63±0.05 nm. These results indicate that the biphenyl chains did notinteract strongly. They also indicate that the aryl bromide groupofA2 underwent Heck reaction with the CC group of alkeneBto form product P2.To unravel the mechanism of the Heck reaction catalyzed by

Pd on Au(111), DFT calculations were performed usingsimplified benzene bromide and styrene as the reactants. Weanalyzed debromination and coupling steps using the climbingimage nudged elastic band (CI-NEB) technique implemented inthe plane wave-based Vienna ab initio simulation package(VASP).26 For the first step of debromination (Figure 3a), thecalculated reaction barrier energy Ebarrier (ETS1 − EIS1) is 0.83 eVon Au(111) with a Pd adatom as catalyst. In contrast, Ebarrier inthe absence of Pd adatom is 1.02 eV,27 consistent with ourobservations that the Pd adatom is an effective catalyst ofdebromination.28 Although the calculated reaction energy Ereact(EINT1 − EIS1) of debromination is 0.44 eV, the benzene radicalcan be further stabilized by another Pd adatom with anexothermal energy of −0.86 eV (see Figure S5).In the subsequent coupling step (Figure 3b), the benzene

radical stabilized by the Pd adatom and an isolate styrene were setas IS2, and the styrene approached the radical by coordinationwith Pd adatom to form the initial state IS3. Then the benzeneradical underwent addition to the head atom of the CC bondin the alkene to form intermediate INT2 via transition state TS2.The calculated reaction energy Ebarrier (ETS2 − EIS3) of this stepwas small (0.44 eV), and the resulting radical intermediate INT2was also stabilized by the Pd adatom. Subsequent elimination ofthe H atom (marked as red) in INT2 gave the final Heck productwith Ebarrier (ETS3 − EINT2) of 0.86 eV and Ereact (EFS − EIS3) of

−0.54 eV. These calculations show that the on-surface Heckreaction is exothermal. The calculation of similar Ebarrier forelimination and debromination suggests that once the aryl radicalforms on the surface, it can react smoothly with alkene togenerate the Heck reaction product through addition andelimination steps. Moreover, the debrominated radicals fromporphyrin-derived bromides A1 and A2 are less stable and havemuch higher adsorption energy in comparison with alkene B. Asa result, the radicals generated from A1 or A2 prefer to react withthe fast-moving alkene B instead of homocoupling with anotherradical. This is consistent with the experimental observation ofthe high selectivity of Heck reaction. It is noteworthy that theDFT-calculated barrier energy of the dimerization of alkene onmore active Cu(110) surface was reported as 1.35 eV,5

illustrating the dimerization of alkene might be more difficultthan Heck reaction.In summary, we describe here for the first time an on-surface

Heck reaction of aryl bromides with terminal alkene, which weanalyze using UHV-STM at single molecular level. Pd as catalystis essential on Au(111) to promote highly selective cross-coupling of porphyrin-derived aryl bromides A1 and A2 withalkene B to afford the products P1 and P2. DFT calculationssuggest that the reaction proceeds via debromination of arylbromide, addition to alkene and elimination of hydrogen. This

Figure 2. (a) Self-assembly of A2 molecules on Au(111) at roomtemperature (39× 39 nm2, inset 7× 7 nm2,−2.0 V,−0.08 nA). (b) Self-assembly of A2 and B molecules on Au(111) at −20 °C (13 × 13 nm2,−2.5 V, −0.06 nA). (c) Heck reaction of A2 and B on Au(111) afterdosing with Pd and annealing at 150 °C (34 × 34 nm2, −2.0 V, −0.08nA). (d) Zoomed-in image of Heck reaction products (10 × 10 nm2,−2.0 V, −0.07 nA).

Figure 3. DFT-calculated energy diagrams for (a) debromination and(b) coupling of Heck reaction on Au(111) with Pd as catalyst. Below theenergy diagrams are shown top and side views of the initial state (IS),transition state (TS), intermediate state (INT), and final state (FS) ofthe reactions. In panel (b), the red atom is the H atom eliminated fromthe intermediate.

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protocol may shed light to the development of more on-surfacecross-coupling reactions with high selectivity and may be appliedin the preparation of nano electronic materials and devices onultraclean solid surfaces in the future.

■ ASSOCIATED CONTENT*S Supporting Information

The Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.orglett.7b00855.

General procedures for the STM experiments and DFTcalculations; supplemental STM and calculation data(PDF)

■ AUTHOR INFORMATIONCorresponding Author

*E-mail: liupn@ecust.edu.cn.ORCID

He Tian: 0000-0003-3547-7485Pei-Nian Liu: 0000-0003-2014-2244Author Contributions†K.-J.S. and C.-H.S. contributed equally.Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Prof. Nian Lin (Hong Kong University of Science &Technology) for help with this work. This work was supportedby the National Natural Science Foundation of China (ProjectNos. 21672059, 21421004, 21561162003, and 21372072), theProgram for Eastern Scholar Distinguished Professor, theFundamental Research Funds for the Central Universities, andthe Programme of Introducing Talents of Discipline toUniversities (B16017).

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