Synthesis and NMR Studies of Three Pyridine-Containing Triaza Macrocyclic Triacetate Ligands and Their Complexes with Lanthanide Ions

The synthesis of three triazamacrocycles containing the pyridine moiety and three acetate pendant arms (PCTA) is reported. The three systems differ due to the number of carbon atoms in the macrocyclic ring forming ligands PCTA-[12], -[13], and -[14], endowed with different coordination capabilities toward lanthanide(III) ions. Microscopic protonation sequences for the three ligands have been investigated by 1H NMR spectroscopy. Complexes of PCTA-[12], -[13], and -[14] with La(III), Gd(III), and Lu(III) have been prepared. Relaxometric measurements on the aqueous solutions of the paramagnetic Gd(III) complexes in the presence of competitive ligands gave the following stability constants:  log Kf = 20.8 for Gd-PCTA-[12], log Kf = 19.3 for GdPCTA-[13], and log Kf = 12.5 for GdPCTA-[14]. The measurement of water relaxation rates indicated a tendency to decrease the degree of hydration upon increasing the ring size. The VT 1H and 13C-NMR spectra of the diamagnetic La(III) and Lu(III) complexes exhibit a large ...