Lanthanide(III) Complexes with a Reinforced Cyclam Ligand Show Unprecedented Kinetic Inertness

Lanthanide­(III) complexes of a cross-bridged cyclam derivative containing two picolinate pendant arms are kinetically inert in very harsh conditions such as 2 M HCl, with no dissociation being observed for at least 5 months. Importantly, the [Ln­(dota)]− complexes, which are recognized to be extremely inert, dissociate under these conditions with lifetimes in the range ca. 1 min to 12 h depending upon the Ln3+ ion. X-ray diffraction studies reveal octadentate binding of the ligand to the metal ion in the [Eu­(cb-tedpa)]+ complex, while 1H and 13C NMR experiments in D2O point to the presence of a single diastereoisomer in solution with a very rigid structure. The structure of the complexes in the solid state is retained in solution, as demonstrated by the analysis of the Yb3+-induced paramagnetic shifts.