Dehydrofluorination as a residue-free selective route to C-C bond formation at metal surfaces.

Directing covalent on-surface polymerization of molecular precursors through rational synthetic design has been impaired by the lack of control over reaction sites and by limited chemical routes to covalent bond formation between precursors, resulting in limited control of the outcomes and parasitic surface reaction by-products. Here, using scanning tunnel microscopy, temperature programmed desorption and electron spectroscopies, we investigate the mechanism of a recently reported chemical route to C-C bond formation: dehydrofluorination on metal surfaces. We demonstrate that this chemical route, involving specifically C-H/F-C pairs, favors the elimination of HF in the gas phase as a single step therefore eliminating reaction by-products at the surface of metals. Unlike dehalogenation, we find that the remarkable selectivity of the dehydrofluorination reaction renders this C-C bond formation strategy chemo-selective as well as potentially regio-selective, if employed with a properly designed molecular precursor. Additionally, we demonstrate that the catalytic role of the metal substrate can be used to steer reaction pathways and select between dehydrofluorination and dehydrogenation reaction. For these reasons, the dehydrofluorination reaction, largely unexplored on metal surfaces, could become a valuable tool for on-surface synthesis.