12626 | Chem. Commun., 2018, 54, 12626--12629 This journal is©The Royal Society of Chemistry 2018

Cite this:Chem. Commun., 2018,

54, 12626

On-surface reactions of aryl chloride andporphyrin macrocycles via merging two reactivesites into a single precursor†

Chen-Hui Shu,a Yu-Li Xie,a An Wang,a Ke-Ji Shi,a Wei-Feng Zhang,*b

Deng-Yuan Li*a and Pei-Nian Liu *a

The reaction of aryl chloride and porphyrin macrocycles, which are

merged into a single precursor, has been achieved on Cu(111).

Scanning tunneling microscopy analysis of the oligomer products

showed that the adjacent porphyrin moieties linked mainly by the

phenyl group with the porphyrin macrocycle.

Reactions that form C–C bonds are powerful tools in syntheticchemistry and are widely used in the synthesis of naturalproducts, pharmaceuticals, and organic molecular materials.1

Starting with the work in 2007,2 on-surface reactions to formthe C–C bond have been extensively studied.3,4 Most of thesereactions use a single precursor featuring a single reactive site,such as aryl–aryl coupling (Ullmann-type reaction),4a,b alkynecoupling (Glaser-type reaction),4c,d alkene coupling,4e alkanecoupling,4f and decarboxylative polymerization.4g Such reactionshave been used extensively to construct diverse macromolecularsystems,5 including polymeric chains,6 hyper-branched oligomers,7

graphene ribbons,8 porous molecular networks,9 2D covalentorganic frameworks,10 and other stuctures.11

For the construction of robust low-dimensional architectureswith structural diversity, on-surface reactions involving twodifferent precursors have been developed.12–14 Such reactions, toform C–C bonds, usually involve the cross-coupling of porphyrinbromides with aryl bromides12 and the cross-coupling of terminalalkynes13 or alkenes14 with aryl halides. On-surface reactionsbetween two precursors pose two challenges over surface reactionsinvolving one precursor: (1) different precursors may assemble onthe surface in separate phases, limiting their collisions with eachother; (2) the self-reaction within aggregated precursors on thesurface is easier than the cross-reaction with another precursor.

Avoiding self-aggregation of the same precursor and promotingthe cross-reaction of different precursors remain substantialchallenges in on-surface synthesis.

Porphyrins play decisive roles in many important biologicalprocesses,15 and they have been extensively investigated for theconstruction of conjugated molecular wires, tapes, rings, anddiscrete oligomeric arrays on surfaces.16 Reactions of porphyrinmacrocycles on surfaces have been developed, including intra-molecular cyclodehydrogenation,17 and the reaction betweenporphyrin macrocycles.18 However, few on-surface reactionsbetween a porphyrin macrocycle and other reactive sites havebeen reported.12,19

In this work, we developed a strategy of merging two reactivesites into a single precursor to avoid self-aggregation of twoprecursors during the on-surface reaction. Using this approach,we achieved the cross-coupling/cyclodehydrogenation reactionof aryl chloride with a porphyrin macrocycle on Cu(111). Thecombination of scanning tunneling microscopy (STM) analysis anddensity functional theory (DFT) calculations confirmed the productsas arising from the reaction between the two reactive sites.

In the first stage of our investigation, 5,15-bis(4-chlorophenyl)-10,20-diphenylporphyrin (Cl2TPP) molecules were depositedonto a clean, single-crystal Cu(111) surface held at B200 K.The STM image of the as-deposited molecules shows saddle-shape characteristics. Besides the isolated molecules, there are twotypes of interactions (‘‘V-type’’ and ‘‘T-type’’) in the molecules toform chain-like structures (Fig. 1a and b). The high-resolution STMimage shows that the ‘‘V-type’’ interaction might originate from thehalogen bonds of phenyl-chlorines,20 and the ‘‘T-type’’ interactionmight originate from the hydrogen bonds of phenyl-chlorines andthe macrocycle.17c

After annealing the sample to 423 K, Cl2TPP formed chain-likestructures (Fig. 1c). The high-resolution STM image analysisshows the periodic chain-like structures, with 20 � 0.2 Å distancebetween two molecular centers (Fig. 1d). This separation distancematches the value of 19.8 Å predicted by DFT calculations. Thelinear periodic structure appears to correspond to the C–Cu–Corganometallic intermediate, in which porphyrin units are linked

a Shanghai Key Laboratory of Functional Materials Chemistry,

State Key Laboratory of Chemical Engineering, School of Chemistry & Molecular

Engineering, East China University of Science and Technology, 130 Meilong Road,

Shanghai, 200237, China. E-mail: liupn@ecust.edu.cn, dengyuanli@ecust.edu.cnb Key Laboratory of Photovoltaic Materials of Henan Province, Henan University,

Kaifeng 475004, China. E-mail: wfzhang6@163.com

† Electronic supplementary information (ESI) available: Synthesis of precursors,supplemental STM images and calculation results. See DOI: 10.1039/c8cc07652a

Received 22nd September 2018,Accepted 10th October 2018

DOI: 10.1039/c8cc07652a

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by a copper atom. This result is similar to the reaction ofCl–(C6H4)3–Cl on Cu(111).10a

After further annealing this sample to 523 K, the linearorganometallic chain vanished (Fig. 1e). Most molecules remainedas disconnected entities which assembled to form short chainsand aggregated structures via the intermolecular interactions (seethe top-right inset in Fig. 1e). Surprisingly, only a few molecules(about 6%) underwent the homo-coupling reaction to generate thecovalent bond with the center to center distance of 1.75 nm12 (seethe lower left inset, Fig. 1e). Our previous work demonstrated thatthe aryl chloride Cl–(C6H4)3–Cl, which consisted of three phenylenes,could readily undergo homo-coupling on Cu(111).10a Our results inthe present study suggested that the homo-coupling of Cl2TPP mightbe inhibited by its strong adsorption to Cu(111) to prevent lateralmovement and thereby a lack of approach between the porphyrins.It is noteworthy that about 8% of porphyrin molecules decomposedunder the current conditions.

Annealing the sample at 523 K also induced intramolecularcyclodehydrogenation and the formation of a C–C bond

between the phenyl group and Cb of the porphyrin macrocycle,which afforded planar porphyrin derivatives 1–4 as described inprevious studies (Fig. 1f).17a,c The experimental results alsomatched the simulated STM images of 1–4 based on DFTcalculations (see Fig. S1 in the ESI†). The proportions of species1–4 in all porphyrin molecules were estimated at about 37%,25%, 14% and 16%, respectively.

These studies with Cl2TPP give insights into the reactivity ofporphyrin-derived aryl chloride on Cu(111). One is that strongadsorption between Cl2TPP and the surface of Cu(111) inhibitson-surface homo-coupling of Cl2TPP. Another one is that theCb–H of the porphyrin macrocycle can be activated in cyclode-hydrogenation on Cu(111). Therefore, the cross-coupling of arylchloride with a porphyrin macrocycle might be implementedon Cu(111).

Next we explored the coupling of 5,15-bis(4-chlorophenyl)-porphyrin (Cl2DPP) on Cu(111). Cl2DPP molecules were depositedonto the surface of Cu(111) held at B200 K. The molecules self-assembled into ladder-like lines tend to align along [11%2] and theequivalent directions of the substrate (Fig. 2a). Fig. 2c shows ahigh-resolution STM image of the molecules which reveals astructure of oval shape with a dim center. Based on relatedstudies with other surfaces,12,17a,20 the porphyrin cores probablylie closer to the substrate and the phenyl moieties rotate out ofthe molecular plane at a large dihedral angle, generating thebright protrusions along the major axis. The length of eachmolecule (19.5 Å) in the ladder-like structure matches the lengthof DFT-relaxed Cl2DPP (19.3 Å). The molecules tend to alignside-by-side with their long axes along [10%1] or the equivalentdirections of the substrate (Fig. 2c).

The STM image further reveals that, after annealing to393 K, the Cl2DPP molecules are linked together into extendedlinear organometallic chains basically along [11%2] or the equiva-lent directions of the substrate (Fig. 2b). Fig. 2d shows a one-dimensional organometallic chain with a periodic structurein which the porphyrin units alternate with copper atoms.Adjacent porphyrins are 20 � 0.2 Å apart, matching the valueof 19.8 Å predicted by DFT calculations. It is noteworthy thatthe metallization of the porphyrin molecule by the copper atomoccurred partially at 393 K and completed at 453 K (see Fig. S2in the ESI†). The intramolecular cyclodehydrogenation alsocompleted at 453 K, while only a small portion of the cross-coupling reaction between the phenyl group and the porphyrinmacrocycle occurred. These results suggested that the cross-coupling step followed the intramolecular cyclodehydrogena-tion step (see Fig. S2 in the ESI†).

When the sample was further annealed to 500 K, the linearorganometallic chains disappeared. Most adjacent porphyrinmolecules appeared to be connected at the porphyrin macro-cycle via the phenyl group, generating oligomer products(Fig. 2e). This result suggested that the cross-coupling reactionoccurred between the phenyl group and the porphyrin macro-cycle. The annealing process at 500 K also induced intra-molecular cyclodehydrogenation of the phenyl groups withthe porphyrin macrocycle (Fig. 2g). Compared with the resultsobtained at 453 K (C1 : C2 E 4 : 1 and C3 : C4 E 3 : 1), the

Fig. 1 STM images of Cl2TPP: (a) deposited onto Cu(111) held at B200 K;(b) zoom-in image after deposition at B200 K; (c) after annealing to 423 K;(d) zoom-in image of an organometallic chain; (e) after annealing at 523 K, thelower left inset indicates the homo-coupling product and the top-right insetindicates the chain-like aggregation of molecules; (f) zoom-in image afterannealing at 523 K. Scanning conditions: (a and b) I = 0.13 nA, U = �1.96 V;(c and d) I = 0.08 nA, U = �1.79 V; (e and f) I = 0.10 nA, U = �1.96 V.

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12628 | Chem. Commun., 2018, 54, 12626--12629 This journal is©The Royal Society of Chemistry 2018

sample annealed to 500 K gave the results of C1 : C2 E 2 : 1 andC3 : C4 E 1 : 1, suggesting that the cyclodehydrogenation stepmight follow the cross-coupling step.

Structural fitting of close-up STM images demonstrates acertain extent of aryl chloride homo-coupling in the oligomerstructures (Fig. 2f). Statistical analysis of the products indicatesthat the cross-coupling of the phenyl group with the porphyrinmacrocycle predominated over the homo-coupling of aryl chloride(86% vs. 14%, Fig. 2h). The selectivity might be explained bythe strong adsorption of Cl2DPP to Cu(111) and the difficultyof splitting C–Cu–C bonds (80% remained intact at 453 K; seeFig. S2 in the ESI†) to generate the active porphyrin–phenyl radical.Compared with the homo-coupling reaction of two active radicals,the cross-coupling of one radical with the abundant porphyrinmacrocycles is dynamically favorable.

To clarify the influence of different halogens on the cross-coupling, we explored the reactivity of 5,15-bis(4-bromophenyl)-porphyrin (Br2DPP) on Cu(111). Depositing Br2DPP onto Cu(111)held at B200 K led to one-dimensional self-assembled structuressimilar to Cl2DPP, although the current molecular coverage wasa little lower (see Fig. S3a and b in the ESI†). Annealing thesample to 393 K led to the formation of organometallic chains,surrounded by Br atoms (see Fig. S3c and d in the ESI†). Incontrast, chlorine atoms were not observed beside the organo-metallic chains of Cl2DPP, which might reflect the highermobility of Cl atoms on Cu(111).

After annealing the sample to 500 K, the organometallicchains vanished and the cross-coupling/cyclodehydrogenationoccurred between the phenyl group and the porphyrin macro-cycle, similar to what was observed with Cl2DPP (see Fig. S3eand f in the ESI†). Statistical analysis indicated a lower cross-coupling selectivity for Br2DPP than that of Cl2DPP (72% vs.86%, Fig. 2h). This illustrates that Cl2DPP possesses a greaterpreference for the cross-coupling of the phenyl group with theporphyrin macrocycle than Br2DPP. Presumably, it might beattributed to the assembly of Br atoms around the porphyrinmacrocycles (see Fig. S3d and f in the ESI†), which increases thereaction hindrance of the porphyrin macrocycle, as indicatedby a previous report.21

Because the C–Cl bond is much weaker than the C–H bondsin benzene,10a,22 the cross-coupling only occurred at the para-position of the phenyl group. To elucidate which site of theporphyrin macrocycle (Cmeso–H and Cb–H) prefers to couplewith the phenyl group on Cu(111), DFT calculations for the C–Hactivation barrier in both cases were performed on this surface(Fig. 3). The activation barrier for Cb–H was calculated tobe 1.35 eV (TS1), which is 0.23 eV lower than that of Cmeso–H(1.58 eV, TS2). In addition, the ratio of Cb–H to Cmeso–H on theporphyrin macrocycle is 2 : 1. The statistical analysis of theproducts annealed to 500 K also indicated that the ratio of Cb–Hactivation (C1 and C2, Fig. 2g) to Cmeso–H activation (C3 and C4,Fig. 2g) is about 3 : 1. All these results demonstrate the favor-ability of the Cb–H activation in the porphyrin macrocycle inthe cross-coupling reaction.

In summary, we have reported the on-surface cross-coupling/cyclodehydrogenation reaction of aryl chloride withporphyrin macrocycles on Cu(111) via merging of the tworeactive sites into a single precursor. Based on the STM analysisin single molecular resolution and the DFT calculations, thereaction is proved to proceed via dehalogenation of the arylchloride to form C–Cu–C organometallic chains, then thephenyl group preferentially cross-couples with the Cb–H ofthe porphyrin macrocycle. Meanwhile, cyclodehydrogenationoccurs between the phenyl groups and the porphyrinmacrocycle. In addition, the similar porphyrin bromide under-goes the same cross-coupling/cyclodehydrogenation reaction,although the selectivity is a little lower. Our protocol provides anovel strategy to explore on-surface cross-coupling reactionswhile avoiding self-aggregation of precursors. It would shedlight on the fabrication of robust low-dimensional architectureswith structural diversity.

Fig. 2 (a) Self-assembled structure of Cl2DPP on Cu(111) at B200 K;(b) organometallic chains after annealing Cl2DPP to 393 K; (c) a zoom-inSTM image of the self-assembled structure; (d) a zoom-in STM image ofthe organometallic chain; (e) oligomer structure after annealing Cl2DPP at500 K. White ovals indicate the homo-coupling products; (f) a zoom-inSTM image of oligomer structure; (g) different types of coupling products;(h) statistical analysis of cross-coupling and homo-coupling products.Scanning conditions: (a–d) I = 0.05 nA, U = �1.6 V; (e and f) I = 0.07 nA,U = �1.7 V.

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This work was supported by the National Natural ScienceFoundation of China (Project No. 21672059, 21561162003 and21602059), the Program of the Shanghai Committee of Scienceand Technology (Project No. 18520760700) and the EasternScholar Distinguished Professor Program.

Conflicts of interest

The authors declare no competing financial interests.

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Fig. 3 DFT calculations for Cmeso–H and Cb–H activation in a porphyrinmacrocycle on Cu(111).

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