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 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, 0. 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 forGd³⁺ complexes (Powell et al., op. cit.) is included in the data treatment. The high water exchange rale of [Gd(EGTA)(H₂O)]^- of k_eX²⁹⁸ = (3.1 ± 0.2) × 10⁷ s^-1 is explained in terms of a limiting dissociative exchange mechanism (ΔV‡ = +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.