Activation energies of intermolecular dynamics in ethylenediaminetetraacetate complexes with lanthanides: An effect of the “gadolinium break”

Abstract

Earlier the monotonic increase of the free energy of chemical exchange processes along the series of lanthanide [Ln(H2O)n(EDTA4−)]− complexes was found [Babailov S.P., Magnet. Reson. Chem., 2012, 50, 12, 793] for the yttrium subgroup (Tb3+, Ho3+, Tm3+, Yb3+ and Lu3+). In the present paper 1H NMR measurements are reported for the aqueous solutions of [Ln(H2O)n(EDTA4−)]− complexes of the cerium subgroup {La3+ (I), Pr3+ (II) and Eu3+ (III); n=1, 2 and 3}. It is determined that the activation free energies (ΔG‡298) of the intermolecular EDTA4− ions exchange at [Ln(H2O)n(EDTA4−)]− complexes are 65±3, 65±3 and 64±3kJ/mol for I, II and III respectively (at pD=2.5). Comparison ΔG‡298 values for the light and the heavy lanthanides showed a “gadolinium break” of the dependence of the ΔG‡298 on the lanthanide cation numbers. The obtained results indicate that coordination compounds II and III may be considered as thermometric NMR sensor and lanthanide paramagnetic probes for in situ temperature control in solution.