Inclusion of an Ion Triplet Complex within a NoncovalentlyAssembled 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 <b>1</b> 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 <b>1</b> forms 1:1 complexes with halide anions in an acetonitrile solution. In the presence of TMAF, receptor <b>1</b> 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 <b>1</b> 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 <b>1</b>·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.