Aniline–phenol recognition: from solution through supramolecular synthons to cocrystals

摘要

Aniline–phenol recognition is studied in the crystal engineering context in several 1:1 cocrystals that contain a closed cyclic hydrogen-bonded [⋯O—H⋯N—H⋯]2 tetramer supramolecular synthon (>II). Twelve cocrystals of 3,4,5- and 2,3,4-trichlorophenol with one of eight halogenated anilines have been characterized. Ten of these cocrystals contain an extended octamer synthon that is assembled with hydrogen bonding and π⋯π stacking that defines a Long-Range Synthon Aufbau Module (LSAM). The design strategy is, therefore, based on the construction and transferability of the LSAM, which is a dimer of tetramers. Using the LSAM concept, two short cell axes in the crystal structures can be predicted. Whilst one of them is dictated by synthon >II, the other one is dominated by π⋯π interactions. The third cell axis can also be predicted, in some cases, by systematic tuning of the halogen bonds. The design strategy is also verified in cocrystals of non-halogenated precursors. The observation of this large synthon in so many structures points to its stability and possible existence in solution. To this end, one-dimensional ¹H and ¹⁵N NMR studies, performed on the 3,4,5-trichlorophenol–3,5-dichloroaniline cocrystal in CDCl3, show characteristic downfield shifts that point to a π⋯π stacked structure and to the robustness of the hydrogen-bonded aggregates. Nuclear Overhauser effects point to hydrogen bonding between aniline and phenol molecules in the aggregates. Diffusion-ordered spectroscopy and T 1 inversion recovery experiments show that stacking is present in concentrated solution and lost at a certain dilution. A sequence of events is therefore established: molecules of the aniline and the phenol associate via hydrogen bonding to form tetramers, and tetramers subsequently stack to form octamers.