A multinuclear NMR study on the structure and dynamics of lanthanide(III) complexes of the poly(amino carboxylate) EGTA(4-) 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 EGTA(4-) = 3,12-bis(carboxymethyl)-6,9-dioxa-3,12-diazatetradecanedioate(4-)) in aqueous solution have been investigated by variable temperature H-1 and C-13 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 O-17 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. Sec. 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 k(ex)(298) = (3.1 +/- 0.2) x 10(7) s(-1) is explained in terms of a limiting dissociative exchange mechanism (Delta V double dagger = +10.5 +/- 1.0 cm(3)mol(-1)), favored by the steric constraints on the water binding site that are revealed by the study on the solution structure.