Three N,N″-bis(amide) derivatives of
H
5
dtpa
(diethylenetriamine-N,N,N
![](https://www.rsc.org/images/entities/char_200a.gif)
′,N
![](https://www.rsc.org/images/entities/char_200a.gif)
″,N
![](https://www.rsc.org/images/entities/char_200a.gif)
″-pentaacetic acid),
H
3
L
1
= bis- (isopropylamide),
H
3
L
2
= bis(tert-butylamide)
and H
3
L
3
= bis(benzylamide), were
synthesized. Their protonation constants were determined by
potentiometric titration in 0.10 mol dm
-3
KCl and by
NMR pH titration at 25 ± 0.1 °C.
Stability and selectivity constants were measured to evaluate the
possibility of using the corresponding gadolinium(
III)
complexes as magnetic resonance imaging contrast agents. The formation
of the gadolinium(
III), copper(
II),
zinc(
II) and calcium(
II) complexes were
investigated quantitatively by potentiometry. The stability constant
determined for Gd
III
is larger than those for
Ca
II
, Zn
II
and Cu
II
for these
octadentate ligands. The selectivity constants and modified selectivity
constants of the amides for Gd
3+
over endogenously available
metal ions were calculated.