Construction of Two-Dimensional Organometallic Coordination Networks with Both Organic Kagome and Semiregular Metal Lattices on Au(111).

Two-dimensional metal-organic networks (2D MONs) having heterogeneous coordination nodes (HCNs) could exhibit excellent performance in catalysis and optoelectronics because of the unbalanced electron distribution of the coordinating metals. Therefore, the design and construction of 2D MONs with HCNs are highly desirable but remain challenging. Here, we report the construction of 2D organometallic coordination networks with an organic Kagome lattice and a semiregular metal lattice on Au(111) via the in situ formation of HCNs. Using a bifunctional precursor 1,4-dibromo-2,5-diisocyanobenzene, the coordination of isocyano with Au adatom on a room-temperature Au(111) yielded metal-organic coordination chains with isocyano-Au-isocyano nodes. In contrast, on a high-temperature Au(111), a selective debromination/coordination cascade reaction occurred, affording 2D organometallic coordination networks with phenyl-Au-isocyano nodes. By combining scanning tunneling microscopy and density functional theory calculations, we determined the structures of coordination products and the nature of coordination nodes, demonstrating a thermodynamically favorable pathway for forming the phenyl-Au-isocyano nodes.

Two-Dimensional Crystal Transition from Radialene to Cumulene on Ag(111) via Retro-[2 + 1] Cycloaddition.

Two-dimensional (2D) crystal-to-crystal transition is an important method in crystal engineering because of its ability to directly create diverse crystal materials from one crystal. However, steering a 2D single-layer crystal-to-crystal transition on surfaces with high chemo- and stereoselectivity under ultra-high vacuum conditions is a great challenge because the transition is a complex dynamic process. Here, we report a highly chemoselective 2D crystal transition from radialene to cumulene with retention of stereoselectivity on Ag(111) via retro-[2 + 1] cycloaddition of three-membered carbon rings and directly visualize the transition process involving a stepwise epitaxial growth mechanism by the combination of scanning tunneling microscopy and non-contact atomic force microscopy. Using progression annealing, we found that isocyanides on Ag(111) at a low annealing temperature underwent sequential [1 + 1 + 1] cycloaddition and enantioselective molecular recognition based on C-H···Cl hydrogen bonding interactions to form 2D triaza[3]radialene crystals. In contrast, a higher annealing temperature induced the transformation of triaza[3]radialenes to generate trans-diaza[3]cumulenes, which were further assembled into 2D cumulene-based crystals through twofold N-Ag-N coordination and C-H···Cl hydrogen bonding interactions. By combining the observed distinct transient intermediates and density functional theory calculations, we demonstrate that the retro-[2 + 1] cycloaddition reaction proceeds via the ring opening of a three-membered carbon ring, sequential dechlorination/hydrogen passivation, and deisocyanation. Our findings provide new insights into the growth mechanism and dynamics of 2D crystals and have implications for controllable crystal engineering.

On-Surface Cross-Coupling Reactions.

On-surface synthesis, as a bottom-up synthetic method, has been proven to be a powerful tool for atomically precise fabrication of low-dimensional carbon nanomaterials over the past 15 years. This method relies on covalent coupling reactions that occur on solid substrates such as metal or metal oxide surfaces under ultra-high-vacuum conditions, and the achievements with this method have greatly enriched fundamental science and technology. However, due to the complicated reactivity of organic groups, distinct diffusion of reactants and intermediates, and irreversibility of covalent bonds, achieving the high selectivity of covalent coupling reactions on surfaces remains a great challenge. As a result, only a few on-surface covalent coupling reactions, mainly involving dehalogenation and dehydrogenation homocoupling, are frequently used in the synthesis of low-dimensional carbon nanosystems. In this Perspective, we focus on the development and synthetic applications of on-surface cross-coupling reactions, mainly Ullmann, Sonogashira, Heck, and divergent cross-coupling reactions.

Synthesis of Dendronized Polymers on the Au(111) Surface.

Dendronized polymers (DPs) consist of a linear polymeric backbone with dendritic side chains. Fine-tuning of the functional groups in the side chains enriches the structural versatility of the DPs and imparts a variety of novel physical properties. Herein, the first on-surface synthesis of DPs is achieved via the postfunctionalization of polymers on Au(111), in which the surface-confinement-induced planar conformation and chiral configurations were unambiguously characterized. While the dendronized monomer was synthesized in situ on Au(111), the subsequent polymerization afforded only short, cross-linked DP chains owing to multiple side reactions. The postfunctionalization approach selectively produced brominated polyphenylene backbone moieties by the deiodination polymerization of 4-bromo-4″-iodo-5'-(4-iodophenyl)-1,1':3',1″-terphenyl on Au(111), which smoothly underwent divergent cross-coupling reactions with two different isocyanides to form two types of DPs as individual long chains.

On-Surface Synthesis of [3]Radialenes via [1+1+1] Cycloaddition.

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross-conjugated π-electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non-contact atomic force microscopy, and time-of-flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction.

Ladder Phenylenes Synthesized on Au(111) Surface via Selective [2+2] Cycloaddition.

Ladder phenylenes (LPs) composed of alternating fused benzene and cyclobutadiene rings have been synthesized in solution with a maximum length no longer than five units. Longer polymeric LPs have not been obtained so far because of their poor stability and insolubility. Here, we report the synthesis of linear LP chains on the Au(111) surface via dehalogenative [2+2] cycloaddition, in which the steric hindrance of the methyl groups in the 1,2,4,5-tetrabromo-3,6-dimethylbenzene precursor improves the chemoselectivity as well as the orientation orderliness. By combining scanning tunneling microscopy and noncontact atomic force microscopy, we determined the atomic structure and the electronic properties of the LP chains on the metallic substrate and NaCl/Au(111). The tunneling spectroscopy measurements revealed the charged state of chains on the NaCl layer, and this finding is supported by density functional theory calculations, which predict an indirect bandgap and antiferromagnetism in the polymeric LP chains.

Post-Functionalization of Supramolecular Polymers on Surface and the Chiral Assembly-Induced Enantioselective Reaction.

Although post-functionalization is extensively used to introduce diverse functional groups into supramolecular polymers (SPs) in solution, post-functionalization of SPs on surfaces still remains unexplored. Here we achieved the on-surface post-functionalization of two SPs derived from 5,10,15-tri-(4-pyridyl)-20-bromophenyl porphyrin (Br-TPyP) via cross-coupling reactions on Au(111). The ladder-shaped, Cu-coordinated SPs preformed from Br-TPyP were functionalized through Heck reaction with 4-vinyl-1,1'-biphenyl. In the absence of Cu, Br-TPyP formed chiral SPs as two enantiomers via self-assembly, which were functionalized via divergent cross-coupling reaction with 4-isocyano-1,1'-biphenyl (ICBP). Surprisingly, this reaction was discovered as an enantioselective on-surface reaction induced by the chirality of SPs. Mechanistic analysis and DFT calculations indicated that after debromination of Br-TPyP and the first addition of ICBP, only one attack direction of ICBP to the chiral SP intermediate is permissive in the second addition step due to the steric hindrance, which guaranteed the high enantioselectivity of the reaction.