Tuning water-exchange rates on (carboxymethyl)iminobis(ethylenenitrilo)tetraacetate (dtpa)-type gadolinium(III) complexes
Résumé
A variable-temperature and -pressure, multiple-field 17 O NMR study has been performed on the gadolinium(III) complexes of an ethoxybenzyl (L 1 ) and symmetric (L 2 ) and asymmetric (L 3 ) mono(methylamide) derivatives of (carboxymethyl)iminobis(ethylenenitrilo)tetraacetate (dtpa) in order to study water exchange and rotational dynamics. Electronic relaxation parameters were obtained from EPR measurements. The water-exchange rates on the [GdL 2 (H 2 O)] - and [GdL 3 (H 2 O)] - complexes [k ex 298 = (1.9 ± 0.1) × 10 6 and (1.3 ± 0.1) × 10 6 s -1 ] are smaller than that observed for [Gd(dtpa)(H 2 O)] 2- ; that of the ethoxybenzyl derivative [GdL 1 (H 2 O)] - is k ex 298 = (3.6 ± 0.1) × 10 6 s -1 . High positive activation volumes have been obtained for all three complexes studied (ΔV ‡ = 10.6 –12.7 cm 3 mol -1 ), indicating dissociatively activated water exchange. As a general rule, when amide groups substitute for carboxylates in gadolinium(III) polyaminopolycarboxylate complexes, the water-exchange rate is decreased by about a factor of 4 per substituted carboxylate, but the mechanism of the process is not affected. However, no influence on the water exchange is observed as a result of the introduction of large groups on the carbon backbone of the ligand, outside the first co-ordination sphere.