Triple Molecular Recognition as a Directing Element in the Formation of Host−Guest Complexes with p-Sulfonatocalix[4]arene and β-Cyclodextrin

We have investigated a mixture consisting of p-sulfonatocalix[4]arene (CX4), β-cyclodextrin (β-CD), and 2,3-diazabicyclo[2.2.2]oct-2-ene (1) and its bridgehead-substituted derivative (2) in the absence and presence of Zn²⁺. In the absence of Zn²⁺, four equally populated host−guest complexes exist in solution, as projected from their comparable binding constants (ca. 1000 M⁻¹). However, upon the addition of Zn²⁺, the formation of a ternary complex, CX4·1·Zn²⁺, is induced by a synergy of three supramolecular interactions (Coulombic, hydrophobic, and weak metal−ligand bonding). Concomitantly, the CX4·2 complex is destabilized by competitive binding, which drives the system toward a state where only two complexes predominate: namely, CX4·1·Zn²⁺ and β-CD·2. Known binding constants for the multiple equilibria were used to model the complex system, and the results were consistent with experimental data obtained from 1D and 2D NMR as well as induced circular dichroism (ICD) spectroscopy. The combined results demonstrate how a subtle interplay between cooperative and competitive binding can be exploited to design a complex multicomponent sorting system.