Molecular Recognition with Ditopic Cavitand Re-Complexes

The synthesis of a series of cavitand receptors bearing increasing numbers of phosphonate groups at their upper rims and of the corresponding Re-based ditopic complexes is reported. The molecular recognition properties of these phosphonate cavitands towards both monotopic and ditopic N-methylpyridinium salts were investigated. The numbers and spatial dispositions of P=O units are pivotal for the complexation of guest ions. The removal of a single P=O bridge from the parent tetraphosphonate cavitand was sufficient to reduce the association constant by almost three orders of magnitude. Elimination of a second P=O unit completely abolished complexation in the AC distal isomer, both in monotopic and in ditopic hosts. The preorganised structure obtained by interconnection of two triphosphonate cavitands through a Re system showed moderate positive self-assembly cooperativity in the inclusion of ditopic N-methylpyridinium salts in both polar and nonpolar solvents. The speciation profile in ethanol indicates that the clamshell 1: 1 complex is dominant at low concentrations (< 10(-4) M) whereas the 1: 2 complex is preferred at higher concentrations of ditopic guest.