Substituent effects control the self-association of molecular clips in the crystalline state

We report the X-ray crystal structure of 11 molecular clips and analyze the influence of substituents ( e. g., OMe, Me, and NO2) and their location on the observed crystal packing. Molecular clips 3a and 3b form tapelike structures in the crystal due to pi-pi interactions between the aromatic walls. Compounds 3d, 3eC, and 3fC form dimers driven by critical C-H center dot center dot center dot O interactions and then form tapes driven by pi-pi interactions in the crystal. These two building motifs, pi-pi and C-H center dot center dot center dot O interactions, can be used to rationalize the enantio- and diastereoselectivity observed in the X-ray crystal structures of the remaining five molecular clips. For example, the C-H center dot center dot center dot O interactions are found to dictate the formation of homochiral dimers in the structures of (+/-)-3eT and (+/-)-3fT and to control the diastereoselective formation of 6a(2)-6c(2) dimeric motifs with internal p-dimethoxy-o-xylylene walls. Overall, the results suggest that substituent effects that induce even weak intermolecular interactions ( e. g., C-H center dot center dot center dot O) can be used to reliably control crystal packing within glycoluril-based systems.