Lanthanide complexes of a new nonadentate ligand derived from 1,4,7-triazacyclononane: synthesis, structural characterisation and NMR spectroscopic studies

A new route to the synthesis of 1,4,7-tris(2-aminoethyl)-1,4,7-triazacyclononane has been introduced. This polyamino derivative of 1,4,7-triazacyclononane has been used to synthesise a new ligand (L) by Schiff-base condensation with sodium pyruvate in the presence of lanthanide(III) (Ln) ions as templating agents to form the complexes [Ln(L)] (Ln = Y, La, Sm, Yb). The ligand L has nine donor atoms comprising three amine and three imine N-donors and three carboxylate O-donors, and forms thermodynamically and kinetically stable Ln(III) complexes in which the three pendant arms of the ligand wrap around the nine-co-ordinate Ln(III) centres. Complexes with La(III), Sm(III) and Y(III) have been structurally characterised. All the complexes are isostructural and the co-ordination polyhedron about the lanthanide centre is in each case slightly distorted tricapped trigonal prismatic, with the two triangular faces of the prism formed by the macrocyclic N-donors and the carboxylate O-donors. NMR spectroscopic studies on the diamagnetic Y(III) and La(III) complexes and on the paramagnetic Yb(III) and Sm(III) complexes indicate that L imposes a very rigid co-ordination cage around the metal centre.