Surface-Assisted Wurtz Coupling for the Stereoselective Synthesis of Covalent Organic Frameworks.

Surface-confined precision synthesis of 2D covalent organic frameworks (COFs) represents a crucial bridge connecting molecular design with macroscopic material properties. In this work, we achieve the first surface-confined synthesis of monolayer COF via Wurtz coupling, demonstrating prominent stereoselectivity through temperature modulation. At reduced temperature (78 K), the reaction preferentially follows a trans-coupling pathway, enabling the successful synthesis of metastable COF that maintains structural integrity during thermal treatment due to surface confinement. Conversely, deposition at elevated temperature (298 K) promotes thermodynamically stable cis-coupling porous nanoribbons. Combined ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and density functional theory (DFT) calculations elucidate the fundamental thermodynamic-kinetic competition mechanism governing the C-C coupling reactions at the molecular level: while the trans-coupling pathway benefits from a lower activation barrier, the cis-coupling configuration represents the thermodynamic minimum. Notably, this synthetic approach demonstrates pronounced generality, as confirmed by its successful implementation with another triphenylbenzyl bromide precursor to yield monolayer COF. This work deepens the understanding of irreversible bond formation in on-surface synthesis and establishes kinetic trapping as a powerful strategy for achieving precise stereoselective control in Wurtz coupling.