Binding of Inorganic Oxoanions to Macrocyclic Ligands:  Effect of the Degree of Protonation on Supramolecular Assemblies Formed by Phosphate and [18]aneN₆

Five macrocycle-oxoanion adducts have been isolated from aqueous solutions containing 1,4,7,10,13,16-hexaazacyclooctadecane ([18]aneN₆, L) and phosphoric acid whose pH had been adjusted to selected values in the 1−8 range. Four products, (H₆L)(H₂PO₄)₆·2H₃PO₄ (1), (H₆L)(H₂PO₄)₆ (2), (H₄L)(H₂PO₄)₄·2H₂O (4), and (H₄L)(HPO₄)₂·7H₂O (5) crystallized from aqueous solutions at pH 1, 3, 6, and 8, respectively, while (H₄L)(H₂PO₄)₄ (3) crystallized on diffusion of EtOH into an aqueous reaction mixture at pH 6. Single-crystal X-ray structure determinations enabled an examination of supramolecular interactions between protonated forms of [18]aneN₆, phosphoric acid and its conjugate bases, and water of solvation. The macrocycle adopts a variety of conformations in order to accommodate the supramolecular constructs formed by the oxoanions and solvent molecules as the relative proportions of interacting species are altered. At pH 1 and 3, the fully protonated macrocycle, [LH₆]⁶⁺, is found with six H₂PO₄^- anions. At pH 6 and 8, the tetraprotonated macrocycle, [LH₄]⁴⁺, crystallizes with four H₂PO₄^- and two HPO₄^2-, respectively. Variations in the solute of crystallization are evident, with phosphoric acid being present at the lowest pH and water at pH 6 and 8. In 5, the seven unique water molecules form a string-of-pearls motif within which a new heptameric isomer, consisting of a water pentamer that uses a single water to interact with the other two unique water molecules, is found. Structures 1, 2, 4, and 5 exhibit η-3 H-bonding of ammonium protons to a single oxygen of the guest phosphates located above and below the macrocyclic ring. In 3, two phosphate oxygens of the cavity anion interact with the macrocycle, one of which participates in η-2 H-bonding with ammonium groups.