Exploring the Coordination Chemistry and Potential Applications of PC3PA-Based Lanthanide Complexes: Synthesis, Solution Structure, Luminescence, and Relaxation Properties

The novel ligand H₃PC3PA was synthesized from pyclen and methyl-6-(bromomethyl)picolinate with 82% overall yield, and its rare-earth complexes (La, Eu, Gd, Tb, Lu, Y) were isolated and characterized by HR-MS and analytical HPLC. Solution ¹H and ¹³C NMR studies of the diamagnetic [Lu(PC3PA)] and [Y(PC3PA)] chelates evidenced non-coordination of the picolinate moiety at the N₆ position to the metal. The ¹H NMR spectrum of [Lu(PC3PA)] showed diastereotopic CH₂ signals corresponding to a single, rigid isomer in solution, while [Y(PC3PA)] displayed sharp signals at higher temperatures, and diffusion-ordered NMR spectroscopy (DOSY) confirmed a single monomer species. [Eu(PC3PA)] and [Tb(PC3PA)] in water showed broad absorption bands at 269 nm due to the picolinate chromophore. [Eu(PC3PA)] displays red emission with a splitting of the ⁷F_J manifold characteristic of a low-symmetry coordination environment, while [Tb(PC3PA)] shows typical ⁵D₄–⁷F_J transitions of Tb³⁺. Emission lifetimes confirmed monohydration of both complexes, in accordance with a non-coordinating picolinate pendant. The relaxivity of [Gd(PC3PA)], r_1p = 3.97 mM^–1 s^–1 (20 MHz, 298 K), is comparable to that of commercial MRI contrast agents. The dissociation half-life of [Gd(PC3PA)] (22 min at pH 1, 25 °C) is short in comparison to that of analogous complexes, evidencing that the non-coordinating picolinate accelerates proton-assisted dissociation.