Hierarchical Activation of C-Br and Cyano ortho C-H to Form Chiral Organometallics on Ag(111)

The combination of dehalogenative coupling and dehydrogenative coupling represents a promising avenue toward the on-surface synthesis of sophisticated nanostructures. Previous works have largely focused on the sequential activations of C-Br and C-H on pristine aromatic rings. The activation of ortho C-H on functionalized aromatic rings as the second step is rarely reported. In this work, we employ a precursor molecule equipped with bromine and cyano substituents at both ends for dehalogenative and dehydrogenative couplings, respectively. With stepwise annealing from 250 to 365 K, C-Br activation proceeds progressively, accompanied by an increase in organometallic (OM) dimers and the formation of delicate self-assemblies on Ag(111). Surprisingly, upon further annealing, the C-C coupling for OM dimers fails to occur. Instead, OM dimer dissociation and cyano ortho C-H activation are initiated, giving birth to chiral OM multimers. Plausibly, the cyano modification significantly reduces the ortho C-H activation barrier and increases the C-C coupling barrier. In general, this work serves as a paradigm of the hindered second step of Ullmann coupling not due to steric reason. The principle may be extended to the surface reactions of other functionalized aryl halides.