Binding of Phosphate with a Simple Hexaaza Polyammonium Macrocycle

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) Å3. 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 PO43- at pH ∼9.5 (log K = 4.55(4)), for the tetraprotonated macrocycle, H4L4+, with monohydrogen
phosphate, HPO42-, 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.