Interplay of Halogen and Weak Hydrogen Bonds in the Formation of Magic Nanoclusters on Surfaces

Halogen bonding has recently been recognized as an interaction whose relevance is on par with hydrogen bonding. While observed frequently in solution chemistry, the significance of halogen bonds in forming extended supramolecular structures on surfaces is less explored. Herein, we report on the self-assembly of chlorobenzene molecules adsorbed on the Cu(111) surface into nanosized clusters at submonolayer coverages, where the molecular planes are close to parallel to the surface. A comprehensive study of the role of intermolecular interactions through both halogen and weak hydrogen bonds on nanocluster formation is presented, gained by combining the results of temperature-programmed desorption, reflection-absorption infrared spectroscopy, scanning tunneling microscopy, and density functional theory calculations. Based on an unprecedented precise determination of the molecules' orientation within the clusters, the binding motifs that lead to the formation and stability of nanoclusters with magic sizes are identified and explained. A complex and delicate interplay of halogen bonds with weak hydrogen bonds, van-der-Waals forces, and surface-adsorbate interactions leads to a preference for hexamers and tetramers with an observable propensity for halogen bonding over weak hydrogen bonding when adsorbed to the Cu(111) surface.