Switching from Separated to Contact Ion-Pair Binding Modes with Diastereomeric Calix[4]pyrrole Bis-phosphonate Receptors

We describe the design, synthesis and conformational assignment of three diasteromeric bis-phosphonate cavitands based on an aryl extended calix[4]pyrrole tetraol scaffold. The diastereoisomers differ in the relative spatial orientation of the P=O groups installed in their upper rims. We demonstrate that these compounds act as heteroditopic receptors for ion pairs forming ion-paired 1:1 complexes with alkylammonium (quaternary and primary) chloride salts in dichloromethane (DCM) solution and in the solid-state. 1H NMR titrations indicate that the complexes are highly stable thermodynamically and kinetically. In the case of tetraalkyl-phosphonium/ammonium chloride guests, the host featuring the two P=O groups directed outwardly with respect to the aromatic cavity, 4oo, produces the more thermodynamically stable complexes. Conversely, for the primary alkyl ammonium chloride the most effective receptor is the diastereoisomer 4ii with the two P=O groups converging on top of the aromatic cavity. In the non-polar DCM solvent, the size of the quaternary cation has a strong impact in the thermodynamic stability of the complexes and their binding geometry. We use 2D-ROESY experiments to map out the binding geometries of the 1:1 complexes formed in solution. We report that the 1:1 complexes of the 4oo host with the chloride salts have a separated arrangement of the bound ion-pair. In contrast, those of the 4ii host display a close-contact arrangement. We also investigate the same complexation processes in acetonitrile (ACN) solution. Both the salt and the initially formed anionic complex are fully dissociated in this more polar solvent. The receptors show an analogous trend in their binding affinities for quaternary phosphonium/ammonium chloride salts to the one seen in DCM solution. However, in ACN solution the magnitudes of the binding affinities are reduced significantly and the size of the cation does not have an important effect on them. In addition, the inversion in the trend of relative binding affinities of the complexes, which was revealed in DCM solution, is eradicated in ACN when changing the cation substitution from quaternary to primary.