A Multinuclear NMR Study on the Structure and Dynamics of Lanthanide(III) Complexes of the Poly(amino carboxylate) EGTA4- in Aqueous Solution

The structures and intramolecular dynamics of [Ln(EGTA)(H2O)]- (Ln = La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+ and EGTA4- = 3,12-bis(carboxymethyl)-6,9-dioxa-3,12-diazatetradecanedioate(4-)) in aqueous solution have been investigated by variable temperature 1H and 13C NMR. The quantitative analysis of the proton hyperfine shifts and considerations about the stereochemical nonrigidity indicate the occurrence of a structural change along the lanthanide series with the crossover between Sm and Eu. Changes of coordination number from 10 to 9 to 8 are proposed to occur across the series. The experimental data from 17O NMR, EPR and NMRD studies for the Gd3+ complex are treated using a self-consistent theoretical model in a simultaneous multiple parameter least-squares fitting procedure (Powell, D. H.; Ni Dhubhghaill, O. M.; Pubanz, D.; Helm, L.; Lebedev, Y. S.; Schlaepfer, W.; Merbach, A. E. J. Am. Chem. Soc. 1996, 118, 9333). An intermolecular dipole-dipole electronic relaxation mechanism that has very recently been described for Gd3+ complexes (Powell et al., op. cit.) is included in the data treatment. The high water exchange rate of [Gd(EGTA)(H2O)]- of kex298= (3.1 ± 0.2) × 107 s-1 is explained in terms of a limiting dissociative exchange mechanism (DV* = +10.5 ± 1.0 cm3mol-1), favored by the steric constraints on the water binding site that are revealed by the study on the solution structure.