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 (N3O2 2-). 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 P 1, 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.