The structures and thermodynamics of complexes between water-soluble calix[4]arenes and dipyridinium ions

Abstract

Three crystalline complexes were prepared by the inclusion

complexation of p-sulfonatothiacalix[4]arene (TCAS) and psulfonatocalix[

4]arene (CAS) with 2,2 -dipyridinium (2-DPD;

(complexes 1 and 2, respectively), and TCAS with 4,4'-dipyridinium

(4-DPD; complex 3). The crystal structures show

that the calixarenes in 1 and 2 maintain their original cone

conformation, with shallow inclusion of 2-DPD, and assemble

themselves into bi-layer arrangements. However, the cone

shape of TCAS in 3 is disrupted by 4-DPD to assume the socalled

1,2-alternate conformation in the solid state, which is

similar to the 1,3-alternate case of CAS stabilized by 4-DPD

(4). The thermodynamics of this inclusion complexation were

further investigated by microcalorimetric titration in aqueous

solution. The obtained results indicated that the molecular

binding ability and selectivity of TCAS/CAS with DPDs is

entirely controlled by enthalpy gains accompanied by a

smaller negative entropic change; these are discussed from the viewpoint of electrostatic, hydrogen-bonding, π-stacking,

and van der Waals interactions, size/shape-fit, and a desolvation

effect between host and guest. The molecular selectivity

for the inclusion complexation of 2-DPD with CAS was found

to be nine times greater than that of 4-DPD with CAS. Combining

the present crystal structures and thermodynamic

parameters revealed that the position of the nitrogen atoms

in the DPDs is the crucial factor for controlling the binding

modes, molecular selectivity, conformational features, and

assembly behavior of the host upon complexation with

TCAS/CAS. This will help us to design large molecular

assemblies possessing highly supramolecular architectures

based on calixarenes by controlling exactly the guest

molecules.