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 that are stable in water. Cavitands 1–3 functionalized with 2,1,3-benzoselenadiazole
walls were synthesized in good yield from resorcin[4]­arenes. The solid-state
single-crystal X-ray structure of 3 showed a dimeric
assembly cemented together through multiple 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 assembled
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 capsular complexes. The 2:1 complex
of the capsule with cyclohexane carboxylic acid anhydride was stable
over 2 weeks, showing that the seam of chalcogen bonds is “waterproof”.
Selective uptake of cyclohexane over benzene and methyl cyclohexane
over toluene was observed in aqueous solution with the capsule. Hydrophobic
forces and hydrogen-bonding attractions 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 density functional theory calculations.