Construction of Orientationally Ordered Organometallic Chains by Directional Noncovalent Interactions

Achieving orientational control during on-surface synthesis remains a major challenge because conventional approaches mainly rely on steric effects or substrate templating, which often lack chemical versatility. Here, we demonstrate that directional noncovalent interactions can regulate hierarchical reaction pathways through molecular recognition, enabling the formation of orientationally ordered surface nanostructures. Using 4,7-dibromobenzo[c][1,2,5]thiadiazole (2Br-BTD) on Ag(111) as a model system, we combine scanning tunneling microscopy, bond-resolved noncontact atomic force microscopy, and density functional theory calculations to reveal a sequential reaction pathway involving self-assembled monomers, cis-organometallic dimers, trans-covalent dimers, and ultimately alternating organometallic chains. Cooperative S···N chalcogen bonding is found to critically and selectively stabilize the cis-organometallic nodes throughout this process, driving both initial dimer formation and subsequent ordered chain synthesis. Chemically matched control experiments with 4,7-dibromobenzo[c][1,2,5]selenadiazole (2Br-BSe) and 5,8-dibromoquinoxaline (2Br-Qx) further demonstrate that analogous orientational selectivity can be achieved through strengthened Se···N chalcogen bonding and C-H···N hydrogen bonding, respectively. By contrast, the heteroatom-free analogue 1,4-dibromonaphthalene (2Br-Nap), which lacks such directional interaction motifs, exhibits no such selectivity. These results establish directional noncovalent interactions as an effective means of steering surface reaction pathways, offering new insights into the rational design of orientationally ordered low-dimensional nanostructures.