Thermodynamic and Structural Studies on the Ln(III)/An(III) Malate Complexation.

The complexation of the trivalent lanthanides Nd(III) and Eu(III) and of the actinide Am(III) with malate was studied using a multi-method approach. The combination of structural and thermodynamic studies was required for the interpretation of the stoichiometry and thermodynamic data (log ß0, Δr H0m, Δr S0m, Δr G0m) of the lanthanide/actinide malate complexes leading to a profound molecular understanding of the system. The structure-sensitive methods vibrational spectroscopy and extended X-ray absorption fine structure spectroscopy complemented with quantum-mechanical ab initio molecular dynamics calculations revealed a tridentate ring structure of the respective metal complexes. The metal is coordinated by two carboxylate groups and a hydroxyl group. UV-vis, laser fluorescence, and calorimetric studies consistently yielded two complex species having a 1:1 and a 1:2 (metal/malate) stoichiometry. Parallel factor analysis and iterative transformation factor analysis were applied to decompose experimental spectra into their single components and to determine stability constants. The 1:1 and 1:2 Nd(III) malate complexation constants determined by isothermal titration calorimetry were extrapolated to zero ionic strength using the specific ion interaction theory, yielding log ß10 and log ß20 of about 6 and 9, respectively. The respective complexation enthalpies Δr H0m,1 and Δr H0m,2 showed average values of 5 kJ·mol-1 which are typical for small organic molecules. The comparison of Nd(III) and Am(III) malate complexes showed that the malate binding motif, the speciation, and the thermodynamics can be transferred from lanthanides(III) to actinides(III) supporting the 4f/5f element homology.