Material- and Orientation-Dependent Reactivity for Heterogeneously Catalyzed Carbon−Bromine Bond Homolysis

Abstract

Adsorption of the brominated aromatic molecule 1,3,5-tris(4-bromophenyl)benzene on different metallic substrates, namely Cu(111), Ag(111), and Ag(110), has been studied by variable-temperature scanning tunneling microscopy (STM). Depending on substrate temperature, material, and crystallographic orientation, a surface-catalyzed dehalogenation reaction is observed. Deposition onto the catalytically more active substrates Cu(111) and Ag(110) held at room temperature leads to cleavage of carbon−bromine bonds and subsequent formation of protopolymers, i.e., radical metal coordination complexes and networks. However, upon deposition on Ag(111) no such reaction has been observed. Instead, various self-assembled ordered structures emerged, all based on intact molecules. Also sublimation onto either substrate held at ∼80 K did not result in any dehalogenation, thereby exemplifying the necessity of thermal activation. The observed differences in catalytic activity are explained by a combination of electronic and geometric effects. A mechanism is proposed, where initial charge transfer from substrate to adsorbate, followed by subsequent intramolecular charge transfer, facilitates C−Br bond homolysis.

Temperature-controlled metal/ligand stoichiometric ratio in Ag-TCNE coordination networks

“…4 The inclusion of surface atoms in coordination networks with organic ligands has been thoroughly investigated on copper surfaces, where the high reactivity of the spontaneously formed surface adatoms facilitates the formation of metal-organic networks. Similar coordination networks with substrate adatoms have also been observed on other noble metal surfaces, such as Au(111), [5][6][7] Ag(110), 8 and Ag(111). 9,10 For silver and gold surfaces, the inclusion of the relatively unreactive surface adatoms into coordination networks is achieved by using very reactive molecules such as surface radicals arising from on-surface dehalogenation of halogenated compounds, [8][9][10] or very strong organic acceptors such as fluorinated tetracyanoquinodimethane (F4-TCNQ).…”

“…Similar coordination networks with substrate adatoms have also been observed on other noble metal surfaces, such as Au(111), [5][6][7] Ag(110), 8 and Ag(111). 9,10 For silver and gold surfaces, the inclusion of the relatively unreactive surface adatoms into coordination networks is achieved by using very reactive molecules such as surface radicals arising from on-surface dehalogenation of halogenated compounds, [8][9][10] or very strong organic acceptors such as fluorinated tetracyanoquinodimethane (F4-TCNQ). 7 While the participation of adatoms arising from the metal surface facilitates the preparation of coordination networks, it also limits our ability to tune their physico-chemical properties.…”

“…Of course, using surface atoms as metal centers in coordination networks determines the chemical nature of the metal centers, but moreover the coordination networks with substrate adatoms reported so far in previous studies show a unique value for the metal/ligand stoichiometric ratio. [1][2][3][4][5][6][7][8][9][10] Actually, raising the temperature should enhance the availability of surface adatoms, thereby enabling the formation of coordination networks with different metal/ligand stoichiometries. Different metal/ligand stoichiometries have readily been observed by codepositing Fe atoms and carboxylic acid molecules in different relative amounts.…”

Substrate, Molecular Structure, and Solvent Effects in 2D Self-Assembly via Hydrogen and Halogen Bonding

“…42 We ascribe the bright protrusion located at the center of the TBPT hexamers (dotted white circle in Figure 5a) to an electronic effect in the HOPG substrate caused by adsorbed molecules. 72 The alignment of the molecular pattern along the ⟨12̅ 10⟩ zigzag directions of the substrate further corroborates the hypothesis of coassembly of solvent molecules.…”

Probing halogen–halogen interactions in solution

“…A particularly interesting class of XB interaction is the halogen-halogen (XX) interaction, which occurs in particular between two halogen atoms that are bonded to (different) carbon atoms. Although weak, these XX interactions have been shown to be essential to the formation of numerous selfassembled monolayers [8][9][10][11][12][13][14] and crystalline structures. [15][16][17][18][19][20] For instance, the introduction of halogen atoms at key positions allows to organize molecules in the solid state to favour topochemical polymerization, 21 or inclusion complex formation.…”