Solution 1H NMR and X-ray crystal structures of Ca 2+, Zn 2+ and Cd 2+ complexes with 12− and 13-membered macrocycles, dioxotetraazacycloalkanediacetic acids

Abstract

Listen

Translate article

Structures in aqueous solutions of the complexes of Ca 2+, Zn 2+ and Cd 2+ with 2,9-dioxo-1,4,7,I0-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 1H NMR spectroscopy at different pD. The formation constants of the Cd 2+ complexes were determined by potentiometric titrations and compared with those of the Ca 2+ and Zn 2+ complexes. X-Ray crystal analyses were carried out on Ca 2+ and Cd 2+ complexes with (13edtapn) 2− and the free ligand (12edtaen)H 2, The ligand (12edtaen)H 2, (C 12H 20N 4O 6) crystallized in the orthorhombic space group Pbcnwith a = 14.478(2) A ̊ , b = 9.605(1) A ̊ , c = 9.75I(1) A ̊ , 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(C 13H 20N 4O 6) 2(H 2O) 4]·15H 2O, crystallized in the triclinic space group P 1 with a = I0.969(2) A ̊ , b = 11.399(2) A ̊ , c = 10.931(2) A ̊ , α = 94.278(2)°, β = 94.009(2)°, γ = 111.971(2)° and Z= 1 . The coordination geometry around a Ca 2+ 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 Ca 2+ ions, leading to the formation of a centrosymmetric binuclear structure. The Cd(13edtapn) complex, [Cd(C 13H 2N 4O 6)(H 2O)] 2·9H 2O, crystallized in the orthorhombic space group Pccn with a = 16.260(2) A ̊ , b = 17.627(2) A ̊ , c = 15.251(2) A ̊ , and Z = 4 . A, distorted trigonal prism is formed around a Cd 2+ 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 Å. The 1H 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.