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Chalcogen Bonding and Hydrophobic Effects Force Molecules into Small Spaces.

Supramolecular capsules are desirable containers for the study of molecular behavior in small spaces, and offer applications in transport, catalysis and material science. We report here the use of chalcogen bonding to form container assemblies stable in water. Cavitands 1-3 functionalized with 2,1,3-benzoselenadiazole walls were synthesized in good yields from resor-cin[4]arenes. The solid state single crystal X-ray structure of 3 showed a dimeric assembly cemented together through multi-ple Se⋅⋅⋅N chalcogen bonds. Binding of hydrophobic and amphiphilic guests in D2O was investigated by 1H NMR methods and revealed host:guest assemblies of 1:1, 2:1 and 2:2 stoichiometries. Small guests such as n-hexane or cyclohexane as-sembled as 2:2 capsular complexes, larger guests like cyclohexane carboxylic acid or cyclodecane formed 1:1 cavitand complexes, and longer linear guests like n-dodecane, cyclohexane carboxylic acid anhydride and amides created 2:1 capsu-lar complexes. The 2:1 complex of the capsule with cyclohexane carboxylic acid anhydride was stable over two weeks, showing that the seam of chalcogen bonds is "waterproof". Selective uptake of cyclohexane over benzene and methyl cy-clohexane over toluene was observed in aqueous solution with the capsule. Hydrophobic forces and hydrogen bonding at-tractions between guest molecules such as 3-methylbutanoic acid stabilized the assemblies in the presence of the competing effects of water. The high polarizability and modest electronegativity of Se provide a capsule lining complementary to guest C-H bonds. The 2,1,3-benzoselenadiazole walls impart an unusually high magnetic anisotropy to the capsule environment, which is supported by DFT calculations.