Structures in aqueous solutions of the complexes of Ca2+, Zn2+ and Cd2+ with 2,9-dioxo-1,4,7,10-tetraaza-4,7-cyclododecanediacetic acid, abbreviated as (12edtaen)H-2, and 2,9-dioxo-1,4,7,10-tetraaza-4,7-cyclotridecanediacetic acid, (13edtapn)H-2, were studied by H-1 NMR spectroscopy at different pD. The formation constants of the Cd2+ complexes were determined by potentiometric titrations and compared with those of the Ca2+ and Zn2+ complexes. X-Ray crystal analyses were carried out on Ca2+ and Cd2+ complexes with (13edtapn)(2-) and the free ligand (12edtaen)Hz. The ligand (12edtaen)H-2 (C12H20N4O6) crystallized in the orthorhombic space group Pbcn with a = 14.478(2) Angstrom, b = 9.605(1) Angstrom, c = 9.751(1) Angstrom and Z = 4. The molecule has a C-2 axis parallel to the molecular plane of the macrocyclic ring. The C-N bond of the amide group has a partial double bond character and increases the rigidity of the ring framework. The Ca(13edtapn) complex, [Ca-2(C13H20N4O6)(2)(H2O)(4)]. 15H(2)O, crystallized in the triclinic space group P (1) over bar with a = 10.969(2) Angstrom, b = 11.399(2) Angstrom, c = 10.931(2) Angstrom, alpha = 94.278(2)degrees, beta = 94.009(2)degrees, gamma 111.971(2)degrees and Z = 1. The coordination geometry around a Ca2+ ion is a distorted square antiprism formed by two carboxylate oxygen atoms, two amine nitrogen atoms and one amide oxygen atom from a ligand molecule, a carboxylate oxygen atom from a neighboring chelate molecule, and two oxygen atoms from water molecules. One of the carboxylate oxygen atoms bridges two Ca2+ ions, leading to the formation of a centrosymmetric binuclear structure. The Cd(13edtapn) complex, [Cd(C13H20N4O6)(H2O)](2) . 9H(2)O, crystallized in the orthorhombic space group Pccn with a = 16.260(2) Angstrom, b = 17.627(2) Angstrom, c = 15.251(2) Angstrom and Z = 4. A distorted trigonal prism is formed around a Cd2+ ion by two carboxylate oxygen atoms, two amine nitrogen atoms, one amide oxygen atom and a water oxygen atom. The other amide oxygen atom is weakly coordinated to the metal ion with a Cd-O distance of 2.857 Angstrom. The H-1 NMR spectra of [Cd(13edtapn)](0) and [Zn(13edtapn)](0) in aqueous solutions show that two carbonyl oxygen atoms in the amide groups are coordinated to a central metal ion resulting in the formation of a seven-coordination geometry. The NMR spectra of the (12edtaen)2- complexes indicate that two carbonyl oxygen atoms in the amide groups are alternately coordinated to a central metal ion in the complexes in solution. The alternation rate in [Cd(12edtaen)](0) is higher than the NMR frequency, but in [Zn(12edtaen)]0 the alternation rate is close to the NMR frequency. The intramolecular exchange of the coordination sites decreases the lifetimes of the M-N bonds in these (12edtaen)(2-) complexes. The stabilities of the [ML](0) complexes with (12edtaen)H-2 are almost identical with those of the corresponding metal complexes with (13edtapn)H-2. The dynamic properties of the metal chelates are, however, different, due to the rigidity of the macrocyclic ring framework.
