Inclusion of an Ion Triplet Complex within a Noncovalently Assembled Supramolecular Dimeric Capsular Assembly

Stabilization of an ion triplet complex within a noncovalently assembled supramolecular capsular assembly remains an unmet challenge in both self-assembly and calix[4] pyrrole chemistry. Herein, we describe the synthesis and ion binding properties of a meso-aryl two-wall calix[4] pyrrole 1 bearing two 4-(methylthio)phenyl groups at diametrically opposed meso-positions. A combination of 1H NMR spectral titrations and isothermal titration calorimetry (ITC) studies revealed that receptor 1 forms 1:1 complexes with halide anions in an acetonitrile solution. In the presence of TMAF, receptor 1 dimerizes as an interlocking pair of horseshoes encapsulating an unprecedented ion triplet (F–·TMA+·F–) (TMA = tetramethylammonium) complex in the solid state. On the other hand, the TBACl (TBA = tetrabutylammonium), TEACl (TEA = tetraethylammonium), and TMACl complexes of 1 are stabilized as noncapsular 1D columnar assemblies of 1/1 stoichiometric complexes. Importantly, a rare discrete tetrameric tetrahedral water cluster is found trapped within the hydrophobic exterior cavity of the 1·TMACl crystal lattice. Density functional theory (DFT) calculations were employed to elucidate the differing solid-state behavior of the calix[4] pyrrole receptor with TMACl and TMAF salts. To the best of our knowledge, a tetrahedral water cluster stabilized within an organic supramolecular cavity has not been reported in the literature.