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 l,4,7-tris(2-aminoethyl)-l,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(ni) (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(in) complexes in which the three pendant arms of the ligand wrap around the nine-co-ordinate Ln(in) centres. Complexes with La(in), Sm(ni) and Y(m) have been structurally characterised. All the complexes are isostructural and the coordination 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(ni) and La(ni) complexes and on the paramagnetic Yb(in) and Sm(ni) complexes indicate that L imposes a very rigid co-ordination cage around the metal centre.