Chemical reactions may take place in a pure phase of gas or liquid or at the interface of two phases (gas–solid or liquid–solid). Recently, the emerging field of “surface-confined coupling reactions” ...has attracted intensive attention. In this process, reactants, intermediates, and products of a coupling reaction are adsorbed on a solid–vacuum or a solid–liquid interface. The solid surface restricts all reaction steps on the interface, in other words, the reaction takes place within a lower-dimensional, for example, two-dimensional, space. Surface atoms that are fixed in the surface and adatoms that move on the surface often activate the surface-confined coupling reactions. The synergy of surface morphology and activity allow some reactions that are inefficient or prohibited in the gas or liquid phase to proceed efficiently when the reactions are confined on a surface. Over the past decade, dozens of well-known “textbook” coupling reactions have been shown to proceed as surface-confined coupling reactions. In most cases, the surface-confined coupling reactions were discovered by trial and error, and the reaction pathways are largely unknown. It is thus highly desirable to unravel the mechanisms, mechanisms of surface activation in particular, of the surface-confined coupling reactions. Because the reactions take place on surfaces, advanced surface science techniques can be applied to study the surface-confined coupling reactions. Among them, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are the two most extensively used experimental tools. The former resolves submolecular structures of individual reactants, intermediates, and products in real space, while the latter monitors the chemical states during the reactions in real time. Combination of the two methods provides unprecedented spatial and temporal information on the reaction pathways. The experimental findings are complemented by theoretical modeling. In particular, density-functional theory (DFT) transition-state calculations have been used to shed light on reaction mechanisms and to unravel the trends of different surface materials. In this Account, we discuss recent progress made in two widely studied surface-confined coupling reactions, aryl–aryl (Ullmann-type) coupling and alkyne–alkyne (Glaser-type) coupling, and focus on surface activation effects. Combined experimental and theoretical studies on the same reactions taking place on different metal surfaces have clearly demonstrated that different surfaces not only reduce the reaction barrier differently and render different reaction pathways but also control the morphology of the reaction products and, to some degree, select the reaction products. We end the Account with a list of questions to be addressed in the future. Satisfactorily answering these questions may lead to using the surface-confined coupling reactions to synthesize predefined products with high yield.
By using biphenyl‐2‐ylphosphines functionalized with a remote tertiary amino group as a ligand, readily available acetylenic amides are directly converted into 2‐aminofurans devoid of any ...electron‐withdrawing and hence deactivating/stabilizing substituents. These highly electron‐rich furans have rarely been prepared, let alone applied in synthesis, because of their high reactivities and low stabilities associated with the electron‐rich nature of the furan ring. In this work, these reactive furans smoothly undergo either in situ intermolecular Diels–Alder reactions to deliver highly functionalized/substituted aniline products or intramolecular ones to furnish carbazole‐4‐carboxylates in mostly good to excellent yields. This work offers general and expedient access to this class of little studies electron‐rich furans and should lead to exciting opportunities for their applications.
What a ligand! With bifunctional ligands specifically designed for gold catalysis, acetylenic amides are efficiently transformed into 2‐aminofurans in a single step. The highly electron‐rich nature of these furans makes them difficult to access by other means, but also endows them with exceptional synthetic value. D‐A=Diels–Alder, EWG=electron‐withdrawing group.
Precisely introducing topological defects is an important strategy in nanographene crystal engineering because defects can tune π‐electronic structures and control molecular assemblies. The ...synergistic control of the synthesis and assembly of nanographenes by embedding the topological defects to afford two‐dimensional (2D) crystals on surfaces is still a great challenge. By in‐situ embedding ladder bipyrazinylene (LBPy) into acene, the narrowest nanographene with zigzag edges, we have achieved the precise preparation of 2D nonbenzenoid heteroacene crystals on Au(111). Through intramolecular electrocyclization of o‐diisocyanides and Au adatom‐directed 2+2 cycloaddition, the nonbenzenoid heteroacene products are produced with high chemoselectivity, and lead to the molecular 2D assembly via LBPy‐derived interlocking hydrogen bonds. Using bond‐resolved scanning tunneling microscopy, we determined the atomic structures of the nonbenzenoid heteroacene product and diverse organometallic intermediates. The tunneling spectroscopy measurements revealed the electronic structure of the nonbenzenoid heteroacene, which is supported by density functional theory (DFT) calculations. The observed distinct organometallic intermediates during progression annealing combined with DFT calculations demonstrated that LBPy formation proceeds via electrocyclization of o‐diisocyanides, trapping of heteroarynes by Au adatoms, and stepwise elimination of Au adatoms.
Two‐dimensional (2D) nonbenzenoid heteroacene crystals are synthesized via the in‐situ embedding of ladder bipyrazinylenes (LBPys) on Au(111). Through the intramolecular electrocyclization of o‐diisocyanides and Au adatom‐directed 2+2 cycloaddition, the nonbenzenoid heteroacene products are produced with high chemoselectivity, and give rise to the molecular 2D assembly via LBPy‐derived interlocking hydrogen bonds.
1,4,5,8,9,12‐Hexaazatriphenylene (HAT) is one of the smallest polyheterocyclic aromatic building blocks for forming conjugated metal–organic frameworks (cMOFs). However, the strong inter‐molecular ...steric hindrance impedes the growth of HAT‐based cMOFs. Here we employ on‐surface synthesis to grow single‐layer two‐dimensional cMOFs of M3(HAT)2 (M=Ni, Fe, Co). Using scanning tunnelling microscopy and density‐functional theory (DFT) analysis, we resolve that the frameworks comprise a hexagonal lattice of HAT molecules and a Kagome lattice of metal atoms. The DFT analysis indicates that Ni, Co and Fe carry a magnetic moment of 1.1, 2.5, and 3.7 μB, respectively. We anticipate that the small π‐conjugated core of HAT and strong bidentate chelating coordination give rise to appealing electronic and magnetic properties.
1,4,5,8,9,12‐Hexaazatriphenylene (HAT)‐based two‐dimensional conjugated metal–organic frameworks of M3(HAT)2 (M=Fe, Ni, Co) were synthesized by means of an on‐surface self‐assembly protocol which effectively overcomes the strong inter‐molecular steric hindrance.
3Radialenes 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 ...aza3radialenes 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 aza3radialenes. 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.
The first 1+1+1 cycloaddition reaction has been achieved on the Ag(111) surface to afford the aza3radialenes. The chlorine substituents in the isocyanides ensured the selectivity and orientational order of the products via the steric hindrance‐directed molecular assembly.
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.
The first on‐surface post‐functionalization of two supramolecular polymers (SPs) derived from the same porphyrin precursor has been achieved via cross‐coupling. Metal‐coordinated SPs were easily decorated at both edges with 4‐vinyl‐1,1′‐biphenyl via Heck reaction, while self‐assembled chiral SPs underwent enantioselective cross‐coupling with 4‐isocyano‐1,1′‐biphenyl to acquire homochiral functional units.
We have characterized the local electronic structure of a porphyrin-containing single-layer covalent organic framework (COF) exhibiting a square lattice. The COF monolayer was obtained by the ...deposition of 2,5-dimethoxybenzene-1,4-dicarboxaldehyde (DMA) and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin (TAPP) onto a Au(111) surface in ultrahigh vacuum followed by annealing to facilitate Schiff-base condensations between monomers. Scanning tunneling spectroscopy (STS) experiments conducted on isolated TAPP precursor molecules and the covalently linked COF networks yield similar transport (HOMO–LUMO) gaps of 1.85 ± 0.05 eV and 1.98 ± 0.04 eV, respectively. The COF orbital energy alignment, however, undergoes a significant downward shift compared to isolated TAPP molecules due to the electron-withdrawing nature of the imine bond formed during COF synthesis. Direct imaging of the COF local density of states (LDOS) via dI/dV mapping reveals that the COF HOMO and LUMO states are localized mainly on the porphyrin cores and that the HOMO displays reduced symmetry. DFT calculations reproduce the imine-induced negative shift in orbital energies and reveal that the origin of the reduced COF wave function symmetry is a saddle-like structure adopted by the porphyrin macrocycle due to its interactions with the Au(111) substrate.
Up to now, the fabrication of well-intergrown Co-based zeolitic imidazolate framework (ZIF) membranes on porous tubular supports is still a major challenge. We report here a heteroepitaxial growth ...for preparing well-intergrown Co-based ZIFs (ZIF-67 and ZIF-9) tubular membranes with high performance and excellent thermal stability by employing a thin layer of ZnO nanorods acting as both nucleation centers and anchor sites for the growth of metal-organic framework membranes. The results show that well-intergrown Co-ZIF-67 and Co-ZIF-9 membranes are successfully achieved on the ZnO nanorod-modified porous ceramic tubes. This highly active heteroepitaxial growth may be attributed to the fact that the (Zn,Co) hydroxy double salt intermediate produced in situ from ZnO nanorods acts as heteroseeds and enables the uniform growth of Co-based membranes. The H
/CO
selectivity of the as-prepared Co-ZIF-9 tubular membrane could reach about 23.8 and the H
/CH
selectivity of Co-ZIF-67 tubular membrane is as high as 45.4. Moreover, the membranes demonstrate excellent stability because of the ZnO nanorods as linkers between the membrane and substrate.
Dendrimers are homostructural and highly branched macromolecules with unique dendritic effects and extensive use in multidisciplinary fields. Although thousands of dendrimers have been synthesized in ...solution, the on-surface synthetic protocol for planar dendrimers has never been explored, limiting the elucidation of the mechanism of dendritic effects at the single-molecule level. Herein, we describe an on-surface synthetic approach to planar dendrimers, in which exogenous palladium is used as a catalyst to address the divergent cross-coupling of aryl bromides with isocyanides. This reaction enables one aryl bromide to react with two isocyanides in sequential steps to generate the divergently grown product composed of a core and two branches with high selectivity and reactivity. Then, a dendron with four branches and dendrimers with eight or twelve branches in the outermost shell are synthesized on Au(111). This work opens the door for the on-surface synthesis of various planar dendrimers and relevant macromolecular systems.
By using designed biphenyl-2-ylphosphines functionalized with a remote basic groups as ligands, N-alkynyl-o-nosylamides are directly converted to (1E,3E)-1-amido-1,3-dienes with excellent ...diastereoselectivities under gold catalysis. With allenamides as substrates, the gold-catalyzed isomerizations are high yielding and applicable to a broad substrate scope including various nitrogen protecting groups and exhibit unprecedented (3E)-selectivities for the distal C–C double bond and good regioselectivities. Combining this gold catalysis with one-pot Diels–Alder reactions leads to rapid assembly of valuable bicyclic compounds.
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