Deciphering On-Surface C–C Coupling Reactions by Scanning Probe Imaging and Manipulation

On-surface synthesis has proven to be powerful for fabricating various low-dimensional covalent nanostructures with atomic precision that are challenging for conventional solution chemistry. Dehydrogenative C_aryl–C_aryl coupling is one of the most popular on-surface reactions, of which the mechanisms, however, have not been well understood due to the lack of microscopic insights into the intermediates that are fleetingly existing under the harsh reaction conditions.

Here, we bypass the most energy-demanding initiation step to generate and capture some of the intermediates at room temperature via cyclodehydrobromination of 1-bromo-8-phenylnaphthalene on Cu(111) [1]. Bond-level scanning probe imaging and manipulation in combination with DFT calculations allow the identification of chemisorbed radicals, cyclized intermediates and dehydrogenated products. These intermediates correspond to three main reaction steps, namely, debromination, cyclization, and H elimination. H elimination is the rate-determining step as evidenced by the predominant cyclized intermediates. Furthermore, we reveal a long-overlooked pathway of dehydrogenation, namely, atomic hydrogen-catalyzed H shift and elimination, based on the observation of intermediates for H shift and superhydrogenation and the proof of a self-amplifying effect of the reaction. This pathway is further corroborated by using deuterated precursors together with theoretical analysis on the reaction thermodynamics and kinetics.