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

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 similar to 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.