Lanthanide complexes of iminocarboxylate ligands derived from 1,4,7-triazacyclononane: Structural characterisation and relaxivity of the Gd^III and luminescence of the Eu^III complexes

A new asymmetric derivative of 1,4,7-triazacyclononane with two iminocarboxylic pendant arms attached to the macrocyclic ring has been designed and prepared. Lanthanide complexes of L¹ and of its previously reported symmetric analogue L, containing three pendant arms, are described. The complexes [Ln(L)] (Ln = Gd, Eu, Dy) and [Ln(L¹)X] (Ln = Y, Gd, Eu, Dy) (X = Cl^- or CH₃COO^-) have been synthesised by Schiff-base condensation of 1,4,7-tris(2-aminoethyl)-1,4,7-triazacyclononane (1) or 1,4-bis(2-aminoethyl)-1,4,7-triazacyclononane (2), respectively, with sodium pyruvate on a lanthanide ion template. The complexes [Gd(L)] and [Gd(L¹)(CH₃CO₂)] have been structurally characterised by X-ray crystallography. ¹H NMR spectroscopic investigation of diamagnetic [La(L)] and [Y(L¹)] in D₂O reveals that these complexes show a different pH dependence for hydrolysis of the C=N bonds in L and L¹. Hydrolysis occurs within several hours for [Y(L¹)] at neutral pH, while acidic media are required to achieve the same rate of hydrolysis of [La(L)]. A similar value for the number of water molecules (q) directly bound to the metal centre for lanthanide complexes of L (q = 0) and L¹ (q = 1) has been estimated by three different approaches: NMR relaxivity studies for Gd^III complexes, luminescence spectroscopy for Eu^III complexes and Dy^III-induced ¹⁷O NMR water shift experiments for Dy^III complexes.