Tuning water-exchange rates on (carboxymethyl)iminobis(ethylenenitrilo)tetraacetate (dtpa)-type gadolinium(III) complexes

Abstract

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.