SYNTHESIS AND NMR-STUDIES OF 3 PYRIDINE-CONTAINING TRIAZA-MACROCYCLICTRIACETATE LIGANDS AND THEIR COMPLEXES WITH LANTHANIDE IONS

The synthesis of three triazamacrocycles containing the pyridine moiet y and three acetate pendant arms (PCTA) is reported. The three systems differ due to the number of carbon atoms in the macrocyclic ring form ing ligands PCTA-[12], -[13], and -[14], endowed with different coordi nation capabilities toward lanthanide(III) ions. Microscopic protonati on sequences for the three ligands have been investigated by H-1 NMR s pectroscopy. Complexes of PCTA-[12], -[13], and -[14] with La(III), Gd (III), and Lu(III) have been prepared. Relaxometric measurements on th e aqueous solutions of the paramagnetic Gd(III) complexes in the prese nce of competitive ligands gave the following stability constants: log K-f = 20.8 for Gd-PCTA-[12], log K-f = 19.3 for GdPCTA-[13], and log K-f = 12.5 for GdPCTA-[14]. The measurement of water relaxation rates indicated a tendency to decrease the degree of hydration upon increasi ng the ring size. The VT H-1 and C-13-NMR spectra of the diamagnetic L a(III) and Lu(III) complexes exhibit a large variability of the soluti on structures dictated by the matching of the size of the lanthanide i on and the macrocyclic cavity. This results in noticeable differences in their stereochemical nonrigidity, hydration state, and thermodynami c stability. To some extent the changes observed in continuing from th e 12-14-membered ring macrocyclic complexes parallels the behaviors sh own by the octacoordinated lanthanide(III) complexes with DOTA and TET A. GdPCTA-[12] and -[13] feature promising properties in view of their possible use as contrast agents for magnetic resonance imaging.