Expanding the Family of Pyclen-Based Ligands Bearing Pendant Picolinate Arms for Lanthanide Complexation.

We report a detailed characterization of lanthanide complexes with two azaligands based on the pyclen macrocycle containing two picolinate and one acetate pendant arms. The two picolinate arms are attached to either opposite (L3) or adjacent (L4) amine nitrogen atoms of the macrocyclic platform. The X-ray structures of the Yb complexes show nine coordination of the ligand to the metal ion, a situation that is also observed for EuL4 in the solid state. The EuL3 complex forms centrosymmetric dimers in the solid state joined by μ-η:η carboxylate groups, which results in 10-coordinate Eu ions. The emission spectra of EuL3 measured in HO and DO solution reveal the presence of a hydration equilibrium involving a nine-coordinate species lacking inner-sphere water molecules and a monohydrated 10-coordinate species. The Eu complexes present modest emission quantum yields in buffered aqueous solution (Φ = 16 and 22% for EuL3 and EuL4, 0.1 M tris buffer, pH 7.4), while the corresponding Tb complexes present very high emission quantum yields under the same conditions (∼90%). H NMR studies show that the complexes of L3 present a fluxional behavior in DO solution, while those of L4 are more rigid. The analysis of the Yb-induced NMR shifts of YbL4 indicates that the complex presents a structure in solution similar to that observed in the solid state. The Gd complexes present very high thermodynamic stability constants (log K = 23.56(2) and 23.44(2) for GdL3 and GdL4, respectively). The corresponding pGd values (pGd = -log[Gd] with c = 1 × 10 M and c = 1 × 10 at pH 7.4) of 20.69 (GdL3) and 21.83 (GdL4) are higher than that of Gd(dota) (pGd = 19.21). The Gd complexes also show remarkable inertness with respect to their proton-assisted dissociation, with dissociation half-life times of 50 min (GdL3) and 20 h (GdL4) in 1 M HCl.