Direct Observation of Radical-Arene Contact Complexes Provides Structural Insights into Selectivity in Radical...

Radical-arene contact complexes have long been proposed as key intermediates for selectivity control in radical reactions, but their solution-phase structures and dynamics underlying this role have remained experimentally inaccessible. Here, we use time-resolved X-ray solution scattering to directly observe transient I•-arene contact complexes in benzene and mesitylene. Our results show that I•-benzene adopts an η1/η2-like off-center geometry rather than an η6 π-complex. In contrast, I•-mesitylene exhibits a localized η1 geometry directed toward a CH site. DFT calculations support the relative energetic preferences of these contact motifs, and AIMD simulations reveal rapid dynamic averaging among closely related off-center configurations in benzene but a much narrower orientational range in mesitylene. These findings resolve the detailed structures, interaction motifs, and dynamics of I•-arene contact complexes and show that aromatic substitution substantially reshapes each of these features. Taken together, these results provide a structural and dynamic framework for understanding selectivity in radical reactions.