Lanthanide complexes of iminocarboxylate ligands derived from 1,4,7-triazacyclononane: structural characterisation and relaxivity of the GdIII and luminescence of the EuIII 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 Schiffbase condensation of 1,4,7-tris(2-aminoethyl)-1,4,7-triazacyclononane (1) or 1,4-bis(2-aminoethyl)-1,4,7triazacyclononane (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 CN 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 GdIII complexes, luminescence spectroscopy for EuIII complexes and DyIII-induced 17O NMR water shift experiments for DyIII complexes.