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 7FJ manifold characteristic of a low-symmetry coordination environment, while [Tb(PC3PA)] shows typical 5D4-7FJ transitions of Tb3+. Emission lifetimes confirmed monohydration of both complexes, in accordance with a non-coordinating picolinate pendant. The relaxivity of [Gd(PC3PA)], r1p = 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.