Lanthanide Complexes Based on a 1,7-Diaza-12-crown-4 Platform Containing Picolinate Pendants: A New Structural Entry for the Design of Magnetic Resonance Imaging Contrast Agents

We have synthesized a new macrocyclic ligand, N,N′-Bis[(6-carboxy-2-pyridyl)methyl]-1,7-diaza-12-crown-4 (H2bp12c4), designed for complexation of lanthanide ions in aqueous solution. The X-ray crystal structure of the GdIII complex shows that the metal ion is directly bound to the eight donor atoms of the bp12c4 ligand, the ninth coordination site being occupied by an oxygen atom of a carboxylate group of a neighboring [Gd(bp12c4)]+ unit, while the structure of the LuIII analogue shows the metal ion being only eight-coordinate. The hydration numbers obtained from luminescence lifetime measurements in aqueous solution of the EuIII and TbIII complexes suggest an equilibrium in aqueous solution between a dihydrated (q = 2), ten-coordinate and a monohydrated (q = 1), nine-coordinate species. This has been confirmed by a variable temperature UV−vis spectrophotometric study on the EuIII complex. 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 DFT (B3LYP) level. The results indicate that the change in hydration number occurring around the middle of the lanthanide series is accompanied by a change in the conformation adopted by the complexes in solution [Δ(λλλλ) for q = 2 and Λ(δλδλ) for q = 1]. The structure calculated for the YbIII complex (Λ(δλδλ)) is in good agreement with the experimental structure in solution, as demonstrated by the analysis of the YbIII-induced paramagnetic 1H shifts.