Halogen-bonded solvates of tetrahaloethynyl cavitands†

Halogen bonding (XB) has been receiving considerably increasing attention as it provides a complementary interaction to hydrogen bonding (HB). XB has been proven to play crucial roles in supramolecular chemistry, nanoscience, biosystems, and functional materials. Its directionality and tunability make XB an important noncovalent interaction, geometrically and thermodynamically, similar to HB; however, its allowance for hydrophobicity makes it a complementary force to hydrophilic HB. To exploit this interaction, many macrocyclic host molecules, functionalized with classical XB donor/acceptor pairs, have been designed. Resorcinarenes, a subclass of the calixarene family, result from the condensation reaction of resorcinol and an aldehyde, with a π-basic cavity that has been shown to bind organic cations. Easily available resorcinarene derivatives have been extensively studied due to their tuneable cavity size, solubility profile, and ligand specificity. Recently, we reported a series of pre-organized resorcinarene cavitands functionalized with four haloethynyl moieties. As part of our program to construct halogen-bonded capsules, we first investigated their XB donor ability using standard XB acceptors: the oxygen in 1,4-dioxane, the sp nitrogen in pyridine, and the bromide anion of tetrapropyl ammonium bromide. The results provided some insight into the specific features of these resorcinarene cavitand XB receptors, not only their XB related behaviour, but also their structural features in the solid-state. To rationally design and selectively assemble a halogenbonded capsule, it is necessary to understand the binding preferences of these macrocyclic XB donors with different solvents. As each solvent has unique polarity, acidity, hydrophobicity and nucleophilicity properties, the halogenbonding interactions between the XB donor and the solvent molecules have been found to be very sensitive to the nature of the solvent. Studying different resorcinarene cavitand XB donors in various solvents can shed light on the efficiency of halogen bonding. This allows us to extract, from the resulting crystal structures, an understanding of the basic factors that influence the formation and properties of different solvates. The role of the solvent in halogen bonding is often unpredictable and one cannot foresee if the solvent acts as an XB acceptor or just a passive solvent. In the current contribution, we present a detailed study of the crystallization of three tetrahaloethynyl cavitands (the rigid methylene-bridged bromoand iodoethynyl cavitands 1 and 2, and the more flexible ethylene-bridged iodoethynyl cavitand, 3, Fig. 1) from several different solvents.