Sc(III) Complexes of 1,4,7-Triazacyclononane-1,4,7-triacetic Acid and Its Monoamides

Scandium(III) complexes with H₃nota and its N-ethyl (H₂L¹) and N,N-diethyl (H₂L²) monoamides were studied in the solid state and solution. Potentiometric measurements showed high stabilities of the binary Sc^III complexes with the ligands (log K_[Sc(L)] = 19.50, 16.64, and 17.94 for H₃nota, H₂L¹ and H₂L,² respectively) and weak coordination of the second ligand molecule under the ligand excess. The chemical model was confirmed by ⁴⁵Sc NMR. Multinuclear NMR spectroscopy was used to study the ternary Sc^III-ligand-oxalate systems. The results showed the presence of species with 2:1 and 1:1 [Sc(L)]-to-oxalate stoichiometry. Ternary complexes with the nota-monoamides form two isomers differing in the position of oxalate versus amide pendant arm. In the solid state, the [Sc(L)] complexes form oligomers interconnected through ligand carboxylate groups. Crystallization from solutions containing H₂O₂ or oxalate anions yielded ternary complexes. In all binary and ternary complexes, the Sc^III ion is octacoordinated with the N₃O₄O₁ coordination mode. All of the carboxylate groups of the ligands are coordinated in the O₄-plane. The apical position is occupied by an oxygen atom of water, peroxide, or carboxylate anion. The apical Sc-O bonds are usually longer compared to those of the oxygen atoms forming the O₄-plane.