On-surface gold-catalyzed hydroamination/cyclization reaction of alkynes†

On-surface synthesis under ultra-high vacuum (UHV) conditions facilitates the fabrication of unique molecular compounds, replicating established in-solution protocols. However, the intramolecular hydroamination and cyclization (IHC) of alkynes on surfaces remain unexplored due to the challenges posed by the repulsion between the nitrogen lone pair and the alkyne π-system. Here we describe the first on-surface IHC of alkyne-functionalized molecular precursors in UHV environment on the Au(111) surface. Notably, the synthesis introduces two pyrrole groups into the quinoidal-based precursor, enabling the formation of two fused pyrrolo-benzoquinonediimine compounds not achievable in solution chemistry. To analyze the resulting reaction products, we utilized scanning tunneling microscopy and non-contact atomic force microscopy with single bond resolution, comparing these products to those obtained through traditional solution methods. In parallel to the experimental results, we provide a detailed computational description of the key role of single gold adatoms during the complete on-surface reaction.