The on-surface Ullmann coupling of aromatic molecules has emerged as the most successful approach to synthesize atomically precise carbon nanostructures with unique electronic properties including molecular wires, graphene nanoribbons and two-dimensional conjugated polymers. Despite substantial progress in determining the mechanism of this reaction, the most fundamental question of whether the coupling is catalyzed directly by surface atoms or adatoms remains unanswered. In this work, the feasibility of the adatom creation and adatom-catalyzed Ullmann coupling of iodo-, bromo- and chlorobenzene on Cu(111), Ag(111) and Au(111) surfaces is examined using density functional theory modeling. Analysis of competing pathways reveals that two phenyl intermediates extract a silver atom from Ag(111) surface faster (energy barrier 0.43 eV) than they form the carbon-carbon bond (0.62 eV). However, on Cu(111) and Au(111), the extraction process is slower (0.71 eV Cu, 0.36 eV Au) than the C-C formation (0.49 eV Cu, 0.14 eV Au). The adatom creation is greatly facilitated by the strengthening of phenyl-metal bonds upon the extraction. However, if these bonds are too strong they create an insurmountable barrier for the subsequent adatom-catalyzed C-C coupling, as on Cu(111) and Ag(111) (1.78 eV Cu, 1.52 eV Ag). In contrast, Au adatoms that do not bind phenyl groups strongly can catalyze the C-C bond formation almost as efficiently (0.20 eV) as surface atoms (0.14 eV). Our results explain why adatoms are difficult to observe during surface-confined reactions and how their presence can lead to defects in the assembled nanostructures. The revealed trends can facilitate design of efficient on-surface reactions.
Adatoms in the surface-confined Ullmann coupling of phenyl groups
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