Pyclen-Based Ligands Bearing Pendant Picolinate Arms for Gadolinium Complexation

We report the synthesis of two pyclen-based regioisomer ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca1(15),11,13-triene) functionalized with picolinic acid pendant arms either at positions 3,9-pc2pa (L5) or 3,6-pc2pa (L6) of the macrocyclic fragment. The ligands were prepared by regiospecific protection of one of the amine nitrogen atom of the macrocycle using Boc and Alloc protecting groups, respectively. The X-ray structure of the Gd(III) complex of L5 contains trinuclear [(GdL5)3(H2O)3] 3+ entities in which the monomeric units are joined by 2- 1 : 1 carboxylate groups. However, the 1H and 89Y NMR spectra of its Y(III) analogue support the formation of monomeric complexes in solution. The Tb(III) complexes are highly luminescent, with emission quantum yields of up to 50% for [TbL5] + . The luminescence lifetimes recorded in H2O and D2O solutions indicate the presence of a water molecule coordinated to the metal ion, as also evidenced by the 1H relaxivities measured for the Gd(III) analogues. The Gd(III) complexes present very different exchange rates of the coordinated water molecule (kex 298 = 87.1 and 1.06 106 s -1 for [GdL5] + and [GdL6] + , respectively). The very high water exchange rate of [GdL5] + is associated to the steric hindrance originated by the coordination of the ligand around the water binding site, which favors a dissociatively activated water exchange process. The Gd(III) complexes present rather high thermodynamic stabilities (logKGdL = 20.47 and 19.77 for [GdL5] + and [GdL6] + , respectively). Furthermore, these complexes are remarkably inert with respect to their acid-assisted dissociation, in particular the complex of L5.