SOLUTION H-1-NMR AND X-RAY CRYSTAL-STRUCTURES OF CA2-MEMBERED AND 13-MEMBERED MACROCYCLES, DIOXOTETRAAZACYCLOALKANEDIACETIC ACIDS(, ZN2+ AND CD2+ COMPLEXES WITH 12)

Structures in aqueous solutions of the complexes of Ca2+, Zn2+ and Cd2 + with -dioxo-1,4,7,10-tetraaza-4,7-cyclododecanediacetic acid, abbrev iated as (12edtaen)H-2, and dioxo-1,4,7,10-tetraaza-4,7-cyclotridecane diacetic acid, (13edtapn)H-2, were studied by H-1 NMR spectroscopy at different pD. The formation constants of the Cd2+ complexes were deter mined by potentiometric titrations and compared with those of the Ca2 and Zn2+ complexes. X-Ray crystal analyses were carried out on Ca2+ a nd Cd2+ complexes with (13edtapn)(2-) and the free ligand (12edtaen)Hz . The ligand (12edtaen)H-2 (C12H20N4O6) crystallized in the orthorhomb ic 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 paralle l 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 rigi dity 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, g amma 111.971(2)degrees and Z = 1. The coordination geometry around a C a2+ ion is a distorted square antiprism formed by two carboxylate oxyg en atoms, two amine nitrogen atoms and one amide oxygen atom from a li gand molecule, a carboxylate oxygen atom from a neighboring chelate mo lecule, and two oxygen atoms from water molecules. One of the carboxyl ate oxygen atoms bridges two Ca2+ ions, leading to the formation of a centrosymmetric binuclear structure. The Cd(13edtapn) complex, [Cd(C13 H20N4O6)(H2O)](2) . 9H(2)O, crystallized in the orthorhombic space gro up Pccn with a = 16.260(2) Angstrom, b = 17.627(2) Angstrom, c = 15.25 1(2) Angstrom and Z = 4. A distorted trigonal prism is formed around a Cd2+ ion by two carboxylate oxygen atoms, two amine nitrogen atoms, o ne amide oxygen atom and a water oxygen atom. The other amide oxygen a tom is weakly coordinated to the metal ion with a Cd-O distance of 2.8 57 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 f ormation of a seven-coordination geometry. The NMR spectra of the (12e dtaen)2- complexes indicate that two carbonyl oxygen atoms in the amid e groups are alternately coordinated to a central metal ion in the com plexes in solution. The alternation rate in [Cd(12edtaen)](0) is highe r than the NMR frequency, but in [Zn(12edtaen)]0 the alternation rate is close to the NMR frequency. The intramolecular exchange of the coor dination sites decreases the lifetimes of the M-N bonds in these (12ed taen)(2-) complexes. The stabilities of the [ML](0) complexes with (12 edtaen)H-2 are almost identical with those of the corresponding metal complexes with (13edtapn)H-2. The dynamic properties of the metal chel ates are, however, different, due to the rigidity of the macrocyclic r ing framework.