Self-assembled hemicapsules with inherent functionalities: modeling of a supramolecular electrostatic self-assembly

An approach to functional self-assembled hemicapsules is described consisting of the use of multivalent (valency ≥ 4) and divalent components, the functional groups of which have a relatively weak binding affinity. Electrostatic self-assembly of tetrakis(pyridiniummethyl)cavitand hemispheres (H) and doubly charged anions (A) in polar media gives rise to an equilibrium mixture that consists, as detected with ESI-MS, of hemicapsule H2A3, capsule H2A4, and other ion-pair associates. Fitting 1H NMR data with a model that includes (hemi)capsules and ion-pair associates gave an effective molarity (EM) for the intramolecular assembly of the host (H) with sulfate (A) of 0.19 ± 0.02 M and binding constants of the functional [2 + 3] hemicapsules H2A3 and the [2 + 4] capsules H2A4 in methanol of 3.25 × 1012 M-4 and 3.45 × 1015 M-5, respectively. A substantial amount of the functional [2 + 3] hemicapsules H2A3 with respect to [2 + 4] capsules H2A4 is present in solution, with ratios of H2A3 to H2A4 of 5.67−0.43 in the studied concentration range (0.1−25 mM of [H]tot). The [2 + 3] hemicapsules H2A3 built with sulfate linkers incorporate guests between the closely positioned pyridinium planes.