First-principles exploration of the versatile configurations at an alkynyl-protected coinage metal(111) interface

Abstract

Alkynyl groups (R–CC–) have attracted intense interest recently as alternative ligands to thiolates to protect atomically precise coinage metal nanoclusters, and more than two dozen compositions have been structurally resolved. However, structure determinations indicated that the interface shows strong metal sensitivity, where a staple motif is the common structural feature at the interface of Au–alkynyl nanoclusters, while the bridging motif dominates at the RCC–/Ag and RCC–/Cu interface. To understand their interfacial differences, we employed density functional theory (DFT) calculations to examine the versatile interfacial structures between CH₃CC– and the coinage metal surface; both the (111) surface as well as a surface with a metal adatom are investigated. We find that the alkynyl/gold(111) interface does prefer to form the staple motifs, and a linear flat-lying staple motif is preferred. The adatom occupies the bridge position and two CH₃CC– ligands lie diagonally at the fcc hollow sites with the CC bond interacting with the surface Au by both σ- and π-coordination modes. In contrast, the bridging motif is energetically more favored on Ag(111) and Cu(111). The alkynyl carbons form strong σ, π- or σ-only bonds with the surface Ag/Cu, forming μ₃-bridge coordination over the fcc hollow site. The binding strengths have the order of Cu(111) > Ag(111) > Au(111). The difference in structural preference is attributed to the intrinsic metal attributes with the different gaps of energetic penalty for surface energy, adatom creation and vacancy formation. The other two reasons are the differences of alkynyl–metal bond character and vdW interaction. We further show that this structure preference is also the preferred bonding mode of CH₃CC– on M₅₅ clusters and the CH₃S–/M(111) interface. Our insights greatly facilitate the structural elucidation and provide useful guidelines for future structure predictions in alkynyl-protected metal nanosystems.