Cis/Trans Stereoselectivity of 6,6”-Dibromo-2,2’:6’,2”-Terpyridine Steered by Alkali Metal Salts on Metal Surfaces

Abstract

Controlling stereoselectivity in competing on-surface reactions remains a fundamental challenge for the bottom-up synthesis of covalent nanostructures. Herein, we employ scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations to investigate on-surface self-assembly and reaction of 6,6”-dibromo-2,2’:6’,2”-terpyridine (DT) on Au(111) and Cu(111). After thermal annealing, DT molecules yield trans-chain structures and the nitrogen-doped porous nanocarbon networks (n-NPCNs). The resulting 3-NPCNs exhibit an electronic bandgap of ∼1.35 eV. Importantly, we successfully demonstrate that the alkali-metal salts act as effective external additives for steering the stereoselectivity of DT reaction. Co-deposition of NaCl or KCl significantly enhances cis-ring formation. A maximum cis-ring selectivity of 91.9 ± 3.6% is achieved with NaCl on Cu(111), while KCl also substantially promotes cis-ring formation. DFT calculations suggest that the enhanced cis-ring selectivity is associated with electrostatic stabilization between alkali metal ions and the N-rich cavity of the cis-ring structure. These findings provide mechanistic insight into the stereoselective control of on-surface reactions and offer an effective approach for controlling the construction of low-dimensional covalent nanoarchitectures.