Charge-Transfer-Regulated Site-Selective Activation: Divergent On-Surface Coupling Pathways of Multi-Reactive-Site Amide Precursors on Au(111) and Ag(111)

Abstract

Controlling the reaction selectivity of multi-reactive-site precursors and understanding its underlying mechanism are crucial for on-surface synthesis. However, directing the activation of different functional groups toward specific reaction pathways remains a fundamental challenge. Herein, we report a comprehensive investigation of the substrate-dependent reactivity of 2,8-dibromo-5H-dibenzo[b,e]azepin-5-one (2,8-DBDA), a multi-reactive-site precursor containing both C–Br bonds and amide groups, on Au(111) and Ag(111) surfaces using scanning tunneling microscopy and density functional theory (DFT) calculations. On Au(111), selective activation of C–Br bonds triggers Ullmann-type C–C coupling, yielding zigzag chains, cyclic trimers, and S-shaped oligomers, while the amide groups remain intact to mediate hydrogen bonding. In contrast, on Ag(111), C–Br cleavage together with N–H deprotonation leads to the formation of N–Ag–N and N–Ag–C intermediates, which further evolve into N–C and C–C cross-coupled heterocycles. The strong electronic interactions and interfacial charge redistribution on Ag(111) substantially lower the energy barriers for both C–Br and N–H bond activation, enabling multisite reactivity, as confirmed by DFT calculations. This work demonstrates that the choice of metal substrate is a powerful strategy to steer the reactivity of multi-reactive-site precursors, providing fundamental insights into the rational design of on-surface synthetic pathways for complex functional nanostructures.