Stability, structure and dynamics of cationic lanthanide(III) complexes of N,N′-bis(propylamide)ethylenediamine-N, N′-diacetic acid

The cationic [Ln(EDTA-PA(2))](+) complexes (EDTA-PA(2) = EDTA-bispropylamide) have been characterised by a multinuclear NMR study. Y-89 and C-13 NMR data indicate the formation of 1 : 1 and 1 : 2 (Ln : ligand) complexes in aqueous solution. The stability constants of these complexes, as determined by potentiometric measurements, are log K-GdL = 10.3 and log K-GdL2 = 14.3. C-13 Relaxation times of the Nd3+ complex show hexadentate binding of the organic ligand via the two amines, the two carboxylates and the two amide oxygen atoms. The complexes are present in solution as a mixture of three isomers: two trans forms and a cis one. Luminescence measurements demonstrate that both Eu3+ and Tb3+ complexes are nona-coordinated at low concentrations (similar to10(-3) M). Three water molecules then complete the coordination sphere. At higher concentrations, the complexes exist in solution as a mixture of nona- and octa-coordinated species, the relative concentration of the latter increases with increasing concentration as a consequence of intermolecular interactions operating in aqueous solutions. Data sets obtained from variable-temperature O-17 NMR at 7.05 T and variable-temperature H-1 nuclear magnetic relaxation dispersion (NMRD) on the Gd3+ complex were fitted simultaneously to give insight into the parameters governing the water H-1 relaxivity. Fast rotation limits the relaxivity at 10-40 MHz.