Interactions between Divalent Metal Ions and an Octacoordinate Macrocyclic Ligand

A dinucleating hexaazadiphenol macrocyclic ligand, 15,31-dimethyl-3,11,19,27,33,35-hexaazapentacyclo[27.3.1.1.5,91.13,171.21,25]hexatriaconta-5,7,9(33),13,15,17(34),21,23,25(35),29,31,1(36)-dodecaene-34,36-diol (H2L),
forms a number of protonated, neutral, and/or hydroxo mononuclear, homodinuclear, and heterodinuclear complexes
with the divalent metal ions Cu2+, Cd2+, Mn2+, and Zn2+, controlled by the stoichiometry of the metal ion and
ligand as well as the pH values of the solution. Their stability constants and species distribution as a function of
p[H] are determined. The pH potentiometric studies show that the dinuclear complexes are formed via the
mononuclear chelates in which two kinds of coordination patterns are observed. One is that the metal ions are
complexed by exactly half of coordination sites of the dinucleating macrocycle (N3O-), and the other is that the
metal ions occupy salen-like sites of the macrocycle (N3O22-). In the 2:1 systems (2:1 molar ratio of metal ion
to ligand), the mononuclear species predominate in acidic solutions while the dinuclear species predominate in
basic solutions, except for the case of copper. The protonated mononuclear complex [H2LZn](NO3)2·5H2O forms
triclinic crystals, of space group P1, with a = 10.7797(12) Å, b = 10.9047(12) Å, c = 17.0176(15) Å, α =
106.857(9)°, β = 95.822(8)°, γ = 100.191(9)°, and Z = 2; the neutral heterodinuclear complex [LZnCdCl2]·6H2O forms monoclinic crystals, of space group C2/c, with a = 16.234(5) Å, b = 15.976(9) Å, c = 29.829(11)
Å, α = 90°, β = 90.28(2)°, γ = 90°, and Z = 8.