Atomic-Scale Visualization of the Stepwise Metal-Mediated Dehalogenative Cycloaddition Reaction Pathways: Competition between Radicals and Organometallic Intermediates.

Dehalogenative cycloaddition reaction is a powerful strategy to generate new ring scaffolds with π-conjugated features on a surface, and thus holds great promise toward atomically precise electronic devices or nanomaterials. The ortho-dihalo substitution provides a good strategy to realize cycloaddition. However, the limited understanding of intermediate states involved hinders mechanistic exploration for further precise design and optimization of reaction products. Now, the evolutions of competing surface-stabilized radicals and organometallic intermediates in real space were visualized toward the formation of dominant conjugated four-membered ring connections. From the interplay of scanning tunneling microscopy and density functional theory calculations, the stepwise metal-mediated dehalogenative cycloaddition pathway is elucidated both experimentally and theoretically. The results provide fundamental insights into the intermediate states involved in on-surface synthesis.