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 P2(1)/c with unit cell parameters of a = 10.006(2) A ngstrom, b = 12.525(1) Angstrom, c = 19.210-(2) Angstrom, beta = 102.91(1)d egrees, and V = 2346.6(5) Angstrom(3). The hexaprotonated macrocycle is loc ated on a crystallographic center of inversion and is surrounded by eight p hosphate 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 pro tonated macrocycle, dominate the crystal lattice. Potentiometric studies us ing NaCl as the supporting electrolyte indicate high formation constants fo r the triprotonated macrocycle, H3L3+, With PO43- at pH similar to 9.5 (log K = 4.55(4)), for the tetraprotonated macrocycle, H4L4+, with monohydrogen phosphate, HPO42-, at pH similar to 8.0 (log K = 6.01(3)), and for ditopic complexes with H5L5+ and H6L6+ and dihydrogen phosphate, H2PO4-, at PH sim ilar to 4.0 (log K = 6.16(6)) and pH similar to 2.5 (log K = 6.44(5)), resp ectively. 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.