Electrocatalytic Benzylic C-H Activation Enables Direct Au-C Single-Molecule Junctions

Achieving selective C-H bond activation under mild, sustainable conditions remains a major challenge. An externally applied bias provides a reagent-free driving force for promoting bond transformations, yet ensemble-averaged measurements often obscure transient intermediates and mask the intrinsic reactivity of individual chemical bonds. Here, we employ in situ scanning tunnelling microscopy break-junction (STM-BJ) measurements to directly interrogate electrocatalytic benzylic C-H activation at the single-molecule level, revealing the formation of highly conductive Au-C covalent junctions. Complementary theoretical calculations, electrochemical analyses, and radical trapping experiments indicate that the Au-C bond formation proceeds via a single-electron transfer (SET) mediated cleavage of the benzylic C-H bond, ultimately resulting in covalent coupling to the gold electrode. This strategy enables the direct transformation of nominally inert C-H bonds into robust Au-C junctions under mild conditions, while simultaneously capturing the associated bond-forming dynamics with single-bond resolution. These findings establish a mechanistic framework for bias-driven benzylic C-H activation and advance C-H functionalization toward atomically precise electrochemical manipulation of molecular transformations.