High-pressure NMR kinetics .80. Tuning water-exchange rates on (carboxymethyl)iminobis(ethylenenitrilo)tetraacetate (dtpa)-type gadolinium(III) complexes

A variable-temperature and -pressure, multiple-field O-17 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 [GdL2(H2O)](-) and [GdL3(H2O)](-) complexes [k(ex)(298) = (1.9 +/- 0.1) x 10(6) and (1.3 +/- 0.1) x 10(6) S-1] are smaller than observed for [Gd(dtpa)(H2O)](2-); that of the ethoxybenzyl derivative [GdL1(H2O)](-) is k(ex)(298) = (3.6 +/- 0.1) x 10(6) S-1. High positive activation volumes have been obtained for all three complexes studied (Delta V-double dagger = 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.