Solution structure and dynamics of lanthanide(III) complexes of diethylenetriaminepentaacetate: a two-dimensional NMR analysis

Abstract

The solution structure and dynamics of lanthanide(III) complexes of diethylenetriaminepentaacetate (DTPA) have been investigated by {sup 1}H NMR. Two-dimensional (2D) exchange spectroscopy (EXSY) enables the determination of solution dynamics. At low temperatures (0-25{degree}C) the complexes Pr(DTPA){sup 2{minus}}, Eu(DTPA){sup 2{minus}}, and Yb(DTPA){sup 2{minus}} are in slow exchange on the NMR time scale, and all 18 proton chemical shifts are resolved. Raising the temperature causes the number of resonances to decrease from 18 to 9, consistent with exchange between two enantiomers. The rates of exchange of the complexes follow the order Pr(III) < Eu(III) < Yb(III). Utilization of 2D correlation spectroscopy (COSY) along with EXSY data as constraints enables assignments of the chemical shifts via calculations of the dipolar shifts. These calculations afford many reasonable solutions with low R factors, and only through the use of the 2D COSY and EXSY data can certain solutions be rejected. The calculations also confirm that the previously published crystal structure of Nd(DTPA){sup 2{minus}} can quite satisfactorily explain the solution structure. In addition , these data indicate that the central acetate moiety is coordinated to the metal in solution.