Lanthanide(III) Complexes with Ligands Derived from a Cyclen Framework Containing Pyridinecarboxylate Pendants. The Effect of Steric Hindrance on the Hydration Number

Two new macrocyclic ligands, 6,6′-((1,4,7,10-tetraazacyclododecane-1,7-diyl)­bis­(methylene))­dipicolinic
acid (H2DODPA) and 6,6′-((4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)­bis­(methylene))­dipicolinic
acid (H2Me-DODPA), designed for complexation of lanthanide
ions in aqueous solution, have been synthesized and studied. The X-ray
crystal structure of [Yb­(DODPA)]­(PF6)·H2O shows that the metal ion is directly bound to the eight donor atoms
of the ligand, which results in a square-antiprismatic coordination
around the metal ion. The hydration numbers (q) obtained
from luminescence lifetime measurements in aqueous solution of the
EuIII and TbIII complexes indicate that the
DODPA complexes contain one inner-sphere water molecule, while those
of the methylated analogue H2Me-DODPA are q = 0. The structure of the complexes in solution has been investigated
by 1H and 13C NMR spectroscopy, as well as by
theoretical calculations performed at the density functional theory
(DFT; mPWB95) level. The minimum energy conformation calculated for
the YbIII complex [Λ­(λλλλ)]
is in good agreement with the experimental structure in solution,
as demonstrated by the analysis of the YbIII-induced paramagnetic 1H shifts. The nuclear magnetic relaxation dispersion (NMRD)
profiles recorded for [Gd­(Me-DODPA)]+ are typical of a
complex with q = 0, where the observed relaxivity
can be accounted for by the outer-sphere mechanism. However, [Gd­(DODPA)]+ shows NMRD profiles consistent with the presence of both
inner- and outer-sphere contributions to relaxivity. A simultaneous
fitting of the NMRD profiles and variable temperature 17O NMR chemical shifts and transversal relaxation rates provided the
parameters governing the relaxivity in [Gd­(DODPA)]+. The
results show that this system is endowed with a relatively fast water
exchange rate kex298 = 58 × 106 s–1.