Highly Selective Intermolecular Cross-Coupling Reactions via a Persistent Radical Effect on Ag(111).

Achieving highly selective cross-coupling between two radicals remains a long-standing challenge due to competing homocoupling pathways. Here, we report a high-yield radical-based cross-coupling reaction on Ag(111), enabled by the persistent radical effect. Using bond-resolving scanning tunneling microscopy (BR-STM), we directly visualize reaction precursors, intermediates, and products at the single bond level. In our model system, we fulfill the stringent requirement of the persistent radical effect: coexisting high concentrations of persistent radicals with low concentrations of transient radicals. As a result, we achieve an unexpectedly high cross-coupling selectivity with yields exceeding 99%. Combining the experimental finding is combined with density functional theory (DFT) calculations, the reaction mechanism is clarified. Moreover, as radical cross-coupling is diffusion-controlled, we elucidate the kinetics of the persistent radical effect through systematic numerical modeling, Monte Carlo simulations, and control experiments on Cu(111).