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

Abstract

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.