Three-Dimensional Supermolecules Assembled from a Tris(Zn²⁺−cyclen) Complex and Di- and Trianionic Cyanuric Acid in Aqueous Solution (Cyclen = 1,4,7,10-Tetraazacyclododecane)

Two new three-dimensional supermolecules having closed hollow frameworks were isolated by self-assembly of a tris(Zn2+−cyclen) (Zn3L3) (L3 = 1,3,5-tris(1,4,7,10-tetraazacyclododecan-1-ylmethyl)benzene) with di- (CA2-) or trideprotonated cyanuric acid (CA3-) in aqueous solution (cyclen = 1,4,7,10-tetraazacyclododecane). One was a very stable 2:3 complex of Zn3L3 and CA2- formed above pH 6, which was isolated from a 2:3 mixture of Zn3L3 and cyanuric acid (CA) in aqueous solution and characterized by 1H NMR and potentiometric pH titration. The X-ray crystal structure showed that each CA2- is sandwiched between two Zn2+−cyclen moieties extended from two tris(Zn2+−cyclen). The crystal data: formula C75H147N36O36Cl3Zn6 ((Zn3L3)2−(CA2-)3·3NO3·3ClO4·6H2O), Mr = 2627.84, monoclinic, space group P21/n (No. 14), a = 27.327(3) Å, b = 29.127(2) Å, c = 16.025(2) Å, β = 90.56(1)°, V = 12754(1) Å3, Z = 4, R = 0.091, and Rw = 0.110. The second one was an unexpected supramolecular complex by 4:4 self-assembly of Zn3L3 and CA3-, which was isolated by allowing a 1:1 mixture of Zn3L3 and CA in aqueous solution to stand at pH 11.5. Its X-ray crystal analysis showed a highly symmetric 4:4 assembling complex having an inner hollow, which was schematically represented as a truncated tetrahedron made by binding four equilateral triangles and four scalene hexagons to each other through CA3-−Zn2+ bonds. Crystal data of the 4:4 supramolecular complex: formula C144H344N72O88Zn12 ((Zn3L3)4−(CA3-)4·12NO3·40H2O), Mr = 5277.29, cubic, space group I4̄3m (No. 217), a = 23.430(4) Å, V = 12862(3) Å3, Z = 2, R = 0.068, and Rw = 0.086. The 2:3 supercomplex is stable in H2O, while the 4:4 complex is stable only in solid or in DMSO solution and tends to go back to the 2:3 complex in the presence of H2O. The 2:3 and 4:4 complexes were equivalent in terms of acid−base equilibrium, and direction of the disproportionation to the 4:4 complex was favored due to its insolubility.