Surface-assisted Ullmann coupling

Abstract

Surface-assisted Ullmann coupling is the workhorse of on-surfaces synthesis. Among the various couplings that were successfully transferred from solution to solid surfaces, Ullmann coupling is arguably the most reliable, controllable, and widespread coupling reaction. The basic reaction scheme is straightforward: halogenated precursors are deposited onto solid surfaces, normally of coinage metals. In the adsorbed state the halogen substitutents are split off by virtue of the surface's reactivity, thereby generating acitvated species that subsequently recombine by forming C-C bonds. Ullmann coupling is well suited for reticular synthesis of novel organic nanostructures: ideally, the halogen substitution pattern of the precursor - which becomes the monomer upon dehalogenation - predetermines dimensionality and topology of the covalent nanostructures. Also in many relevant systems, side-reactions do not occur. However, in reality topological defects, competing C-H activation on more reactive surfaces, and reaction intermediates render this seemingly simple coupling reaction not only more complex, but also more interesting for fundamental research. This feature article aims to provide an account of the vast amount of already published work and tries to destill important findings and currents trends in surface-assisted Ullmann coupling.

Molecular Nanowire Bonding to Epitaxial Single‐Layer MoS2 by an On‐Surface Ullmann Coupling Reaction

“…This organosulfur molecule was chosen since it can probe if the thiophenic sulfur in 2,8‐DBDBT, with a known affinity to the MoS 2 edges, can act as a linker to MoS 2 ,30,31 or if the interaction favors C to MoS 2 bonding via elimination of the terminal bromine. Once a single DBDBT molecule is fixed to the MoS 2 , addition of more will then be able to propagate the growth of poly‐DBT nanowires by regular on‐surface Ullmann coupling reactions 17,19. The mechanism of the Ullmann coupling involves sequential debromination of the DBDBT molecule followed by interlinking by CC coupling, as shown in Scheme 1.…”

“…Once a single DBDBT molecule is fixed to the MoS 2 , addition of more will then be able to propagate the growth of poly-DBT nanowires by regular on-surface Ullmann coupling reactions. [17,19] The mechanism of the Ullmann coupling involves sequential debromination of the DBDBT molecule followed by interlinking by CC coupling, as shown in Scheme 1. Figure 2a shows a typical STM image of isolated poly-DBT nanowires, obtained after deposition of 2,8-DBDBT onto a clean Au(111) surface held at 150 K and imaged at 110 K. [32] Ullmann coupling reactions have been triggered even at this low temperature on Au(111), resulting in DBT molecules selfassembled into a characteristic chain network ( Figure 2b).…”

“…This approach has also produced 2D particles of TMDCs with an alternating sequence of p‐doped and n‐doped sections that are seamlessly connected to form an in‐plane 2D architecture reminiscent of a traditional MOSFET device 16. In parallel, surface‐assisted coupling reactions have been developed to generate a very wide array of molecular nanostructures based on organic systems 17–19. Ullmann coupling reactions have attracted great interest due to the advantages in 1D‐2D nanostructure fabrication of, e.g., graphene nanoribbons, polymers and molecular nanowires 20–22.…”

“…[16] In parallel, surface-assisted coupling reactions have been developed to generate a very wide array of molecular nanostructures based on organic systems. [17][18][19] Ullmann coupling reactions have attracted great interest due to the advantages in 1D-2D nanostructure fabrication of, e.g., graphene nanoribbons, polymers and molecular nanowires. [20][21][22] In light of the interest in hybrid 2D devices, it would be highly interesting to develop versatile methods for well-defined bonding of 2D materials, such as TMDCs, with such molecular nanostructures.…”

Gold‐Catalyzed Cross‐Coupling Reactions: An Overview of Design Strategies, Mechanistic Studies, and Applications

“…[357][358][359][360] In 2011, Karimi and Esfahanir eportedU llmann coupling catalyzed by Au nanoparticles supported on periodic mesoporouso rganosilica (Au/PMO). [357][358][359][360] In 2011, Karimi and Esfahanir eportedU llmann coupling catalyzed by Au nanoparticles supported on periodic mesoporouso rganosilica (Au/PMO).…”