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

The cationic [Ln(EDTA-PA2)]+ complexes (EDTA-PA2 
= EDTA-bispropylamide) have been characterised by a multinuclear NMR study. 89Y and 13C 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 KGdL 
= 10.3 and log KGdL2 
= 14.3. 13C 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 (∼10−3 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 17O NMR at 7.05 T and variable-temperature 1H nuclear magnetic relaxation dispersion (NMRD) on the Gd3+ complex were fitted simultaneously to give insight into the parameters governing the water 1H relaxivity. Fast rotation limits the relaxivity at 10–40 MHz.