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 (<b>L3</b>) or adjacent (<b>L4</b>) amine nitrogen atoms of the macrocyclic platform. The X-ray structures of the Yb³⁺ complexes show nine coordination of the ligand to the metal ion, a situation that is also observed for <b>EuL4</b> in the solid state. The <b>EuL3</b> complex forms centrosymmetric dimers in the solid state joined by μ₂-η¹:η¹ carboxylate groups, which results in 10-coordinate Eu³⁺ ions. The emission spectra of <b>EuL3</b> measured in H₂O and D₂O 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 Eu³⁺ complexes present modest emission quantum yields in buffered aqueous solution (Φ = 16 and 22% for <b>EuL3</b> and <b>EuL4</b>, 0.1 M tris buffer, pH 7.4), while the corresponding Tb³⁺ complexes present very high emission quantum yields under the same conditions (∼90%). ¹H NMR studies show that the complexes of <b>L3</b> present a fluxional behavior in D₂O solution, while those of <b>L4</b> are more rigid. The analysis of the Yb³⁺-induced NMR shifts of <b>YbL4</b> indicates that the complex presents a structure in solution similar to that observed in the solid state. The Gd³⁺ complexes present very high thermodynamic stability constants (log <i>K</i>_GdL = 23.56(2) and 23.44(2) for <b>GdL3</b> and <b>GdL4</b>, respectively). The corresponding pGd values (pGd = −log­[Gd³⁺]_free with <i>c</i>_L = 1 × 10^–5 M and <i>c</i>_Gd = 1 × 10^–6 at pH 7.4) of 20.69 (<b>GdL3</b>) and 21.83 (<b>GdL4</b>) are higher than that of Gd­(dota)⁻ (pGd = 19.21). The Gd³⁺ complexes also show remarkable inertness with respect to their proton-assisted dissociation, with dissociation half-life times of 50 min (<b>GdL3</b>) and 20 h (<b>GdL4</b>) in 1 M HCl