Molecular Tectonics. Use of Br•••aryl Supramolecular Interactions for the construction of Organized Networks from 9,9’-spirobifluorene in the Crystalline State

Numerous studies, many based on statistical analyses of structural data from the Cambridge Structural Database (CSD, have highlighted the importance of halogen•••aryl interactions.Although the exact value of the binding energy is still a subject of debate, it is generally agreed that such interactions can play an important role in crystal packing.For example, previous work by Desiraju and collaborators has shown that Br•••aryl interactions generate characteristic supramolecular motifs, which may be useful for designing molecular solids [2].With this goal in mind, we set out to use 9,9'spirobifluorene (SBF) or tetrabrominated SBF derivatives as tectons [3] and to see if Br•••aryl interactions can be used to build predictable molecular networks.Several features made the SBF core an attractive candidate for this study: 1) SBF has D 2 d symmetry, is rigid and orients its aromatic rings and their substituents in distinct directions of space; 2) the synthesis of SBF derivatives has been described; and 3) derivatives of SBF tend to crystallize readily, frequently as inclusion complexes, which allows the study of interactions between the network and included guests.In the first part of our work, the two different tetrabromo-SBFs (substituted in the meta and para positions, respectively, relative to the spiro center) were synthesized either by a literature procedure or through a new route.Crystals were grown and the structures were solved.Both structures are close packed, and the molecules are linked together by Br•••Ar interactions and also by Br•••Br contacts.In the second part of our work, we co-crystallized un-substituted SBF with small brominated guests that orient 1, 2, 3 or 4 atoms of bromine in distinct directions.The structures of four co-crystals were solved, and analysis reveals the following points: 1) There are at most only a few direct interactions (CH•••) between the SBF units themselves; 2) by engaging in Br•••aryl interactions, the brominated guests play a role of connector between SBF units; iii) in this way, Br•••aryl interactions direct the packing of the crystal structures.