Binding of Phosphate with a Simple Hexaaza Polyammonium Macrocycle

Abstract

A mixture of dihydrogen phosphate and phosphoric acid has been crystallized with a hexaprotonated 26-membered polyammonium macrocycle, 1,4,7,14,17,20-hexaazacyclohexacosane, as the counterion. The complex crystallizes in the monoclinic space group P2₁/c with unit cell parameters of a = 10.006(2) Å, b = 12.525(1) Å, c = 19.210(2) Å, β = 102.91(1)°, and V = 2346.6(5) ų. The hexaprotonated macrocycle is located on a crystallographic center of inversion and is surrounded by eight phosphate anions. Six of the phosphates are dihydrogen phosphates (H₂PO₄^-), and the other two are neutral phosphoric acid molecules. Intricate hydrogen-bonding networks, involving the anionic and neutral phosphates and the protonated macrocycle, dominate the crystal lattice. Potentiometric studies using NaCl as the supporting electrolyte indicate high formation constants for the triprotonated macrocycle, H₃L³⁺, with PO₄^3- at pH ∼9.5 (log K = 4.55(4)), for the tetraprotonated macrocycle, H₄L⁴⁺, with monohydrogen phosphate, HPO₄^2-, at pH ∼8.0 (log K = 6.01(3)), and for ditopic complexes with H₅L⁵⁺ and H₆L⁶⁺ and dihydrogen phosphate, H₂PO₄^-, at pH ∼4.0 (log K = 6.16(6)) and pH ∼2.5 (log K = 6.44(5)), respectively. The ditopic behavior in the simple polyazamacrocycle receptor is a somewhat unusual occurrence, as is the finding of phosphoric acid species in the crystal structure.