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.