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

Abstract 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.