Selective on-surface covalent coupling based on metal-organic coordination template

Abstract

Control over on-surface reaction pathways is crucial but challenging for the precise construction of conjugated nanostructures at the atomic level. Herein we demonstrate a selective on-surface covalent coupling reaction that is templated by metal-organic coordinative bonding, and achieve a porous nitrogen-doped carbon nanoribbon structure. In contrast to the inhomogeneous polymorphic structures resulting from the debrominated aryl-aryl coupling reaction on Au(111), the incorporation of an Fe-terpyridine (tpy) coordination motif into the on-surface reaction controls the molecular conformation, guides the reaction pathway, and finally yields pure organic sexipyridine-p-phenylene nanoribbons. Emergent molecular conformers and reaction products in the reaction pathways are revealed by scanning tunneling microscopy, density functional theory calculations and X-ray photoelectron spectroscopy, demonstrating the template effect of Fe-tpy coordination on the on-surface covalent coupling. Our approach opens an avenue for the rational design and synthesis of functional conjugated nanomaterials with atomic precision.

Fig. 1

Reaction pathways of p-DBTB on Au(111). The precursor 1,4-bis(6,6”-dibromo-[2,2’:6’,2”-terpyridin]-4’-yl)benzene (p-DBTB) is flexible as the bipyridine (bpy) fragments can adopt cis- or trans-conformation. a Uncontrolled C–C coupling. The p-DBTB forms inhomogeneous polymorphic structures after annealing at 430–661 K, where diverse bonding modes coexist. The as-formed C–C bonds are highlighted in red. b Fe-tpy coordination templated macrocyclization. The template effect of the Fe-tpy coordination leads to a distinct reaction pathway, generating C-Au-C organometallic intermediates after annealing at 397–566 K and conjugated SPy-p-Ph nanoribbons at 578–705 K

Fig. 2

Molecular conformers and assembled structures of intact p-DBTB. a STM overview of the Kagome networks monitored at T = 113 K. A unit cell is highlighted by the rhombus (a = b = 3.35 nm, γ = 60°). b Most of the molecules appear in the trans-D_2h shape. A “defect”, C_2v-conformer, is indicated by an ellipse. Tentative structural model of a trimeric cluster is overlaid. Carbon, gray; nitrogen, blue; bromine, red; hydrogen, white. c Br···Br halogen bonds of a trimeric cluster. d–f Magnified topographs of the trans-D_2h, C_2v and cis-D_2h conformers, overlaid by the corresponding molecular models. g A close-packed island composed of cis-D_2h conformers observed at 293 K. A “defect”, C_2v conformer, is marked by an ellipse. h Tentative structural model of the close-packed structure. i Spontaneous conformational transition of the molecules monitored at 293 K. One C_2v conformers (marked by an arrow) was transformed into a cis-D_2h conformer during a continuous scanning in 3 min. Scale bars: (a) 5 nm; (b) 2 nm; (g) 3 nm. Data acquisition conditions (U, I, T): (a, b, d and e) −1.5 V, 0.2 nA, 113 K; (f) −1.2 V, 0.1 nA, 293 K; (g, i) −1.2 V, 0.1 nA, 293 K

Fig. 3

Uncontrolled debrominated aryl-aryl coupling reaction. a STM overview of the sample after annealing at 661 K. b–g Three typical conjugated sub-structures and the corresponding structural models. The conformations of bpy fragments are highlighted by ovals in different colors (trans, blue; cis, green). Different as-formed bpy-bpy connections are observed, namely trans-trans, trans-cis and cis-cis. h Statistic analysis of the molecular conformers (based on the conformations of tpy terminals). i Statistic analysis of the three typical C-C coupling modes of the bpy-bpy connections. Scale bars: (a) 5 nm; (b, d, and f) 2 nm. Data acquisition conditions (T = 293 K): (a) −1.0 V, 0.4 nA; (b) −0.5 V, 0.2 nA; (d) −0.6 V, 0.15 nA; (f) −0.5 V, 0.08 nA

Fig. 4

Molecular conformers and assembled structures in the presence of Fe. a STM overview of the chain structures observed when the sample held at 100 K. b Close-up inspection of the chains. Most molecules are cis-D_2h conformers. c Tentative structural model. Fe coordination centers are represented by purple spheres. Scale bars: (a) 10 nm; (b) 3 nm. Data acquisition conditions (T = 100 K): (a, b and c) −1.8 V, 0.2 nA

Fig. 5

C-Au organometallic intermediates after annealing at 397–566 K. a Overview of the organometallic chains after annealing at 487 K. b Close inspection of the chain structure. Black arrows denote the missing of Au atoms. The white arrow marks a debrominated tpy end. Three types of spots are marked by different symbols (plus sign + , cross sign × and circle ○). c High-resolution STM image (U = −0.05 V) of the chains (4.1 × 1.6 nm²). Additional Fe atoms, as marked by white “ × ”, are resolved. d–f DFT optimized structure of the organometallic chain: top view (d), side view (e) and simulated STM topograph (U = -0.05 V) (f). Fe, purple; Au adatom, yellow; C, gray; H, white. Scale bars: (a) 10 nm; (b) 2 nm. Data acquisition conditions (T = 293 K): (a) −1.2 V, 0.14 nA; (b) 0.3 V, 0.05 nA; (c) −0.05 V, 0.1 nA

Fig. 6

SPy-p-Ph nanoribbons formed after annealing at 578-705 K. a STM overview of a sample annealed at 705 K. b Close-up inspection of the nanoribbons. The measured periodicity of the chain is 1.75 ± 0.05 nm. Labels I, II and III denote the three modes for the SPy macrocycles. c High-resolution STM image (left), DFT-optimized structure (middle) and the simulated STM topograph (right, U = −0.5 V) of the I-mode SPy. d, e High-resolution STM images of the II and III-mode. f The dependence of the abundance of I, II and III-mode SPy units on the annealing temperatures (650 K, 673 K and 705 K). Scale bars: (a) 10 nm; (b) 3 nm. Data acquisition conditions: (a) −0.8 V, 0.2 nA, 105 K; (b, c and e) −0.5 V, 0.2 nA, 133 K; (d) −1.5 V, 0.3 nA, 293 K

Fig. 7

Fe-tpy templated debrominated C–C coupling of m-DBTB. a An STM overview with the sample annealed at 457 K. b High-resolution STM of the organometallic chains with structural models overlaid. The Fe-coordination is represented by purple spheres, while the central sphere is bold to highlight the specific state of the central Fe atom. c High-resolution image of SPy units (III-mode). d STM overview of the SPy-m-Ph ribbons after annealing at 547 K. e, f Segments of the ribbon and the structural model. g–j The C-Au-C organometallic and C-C coupled hyper-ring units with their corresponding structural models. Scale bars: (a, d) 10 nm; (b, c, e, g and i) 2 nm. Data acquisition conditions (T = 293 K): (a) −1.0 V, 0.5 nA; (b) −0.5 V, 2.0 nA; (c) −1.2 V, 0.5 nA; (d) −1.0 V, 0.2 nA. (e) −1.0 V, 1.2 nA; (g) −1.0 V, 1.0 nA; (i) −1.0 V, 0.2 nA