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

Abstract

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 β0, Δr H 0 m, Δr S 0 m, Δr G 0 m) 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 β1 0 and log β2 0 of about 6 and 9, respectively. The respective complexation enthalpies Δr H 0 m,1 and Δr H 0 m,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.

Study of Aqueous Am(III)-Aliphatic Dicarboxylate Complexes: Coordination Mode-Dependent Optical Property and Stability Changes

“…In Section , it was shown that the highest complexation order is two for Mala and Mal and one for the other ligands. These results indicate that Mala binds to Am­(III) via a tridentate binding mode, as also suggested by Taube et al based on IR and EXAFS results . A bidentate binding mode is suggested for the other dicarboxylate and acetate ligands.…”

“…These spectroscopic data consistently indicate side-on bidentate binding of a Mal ligand; each of the two carboxylate groups in a Mal ligand binds to an Am­(III) ion via a monodentate binding mode, resulting in a 6-membered ring structure formation, as depicted in Figure (b). The same changes were observed in the IR absorption spectra of the Mala ligand (Figure (b)), which were reported to form a tridentate chelating structure with two carboxylates and a hydroxyl group bound to a Nd­(III) ion . This similarity between the Mal and Mala complexes further supports a side-on binding of the Mal.…”

“…The formation constants of the Am(III)-Mala complexes (log β 0 1,1 = 6.36 ± 0.12, log β 0 1,2 = 9.66 ± 0.03) determined in this study are comparable to those of Nd(III)-Mala (log β 0 1,1 = 6.0 ± 0.4, log β 0 1,2 = 9.1 ± 0.7). 20 Comparisons between Mala and other ligands provide further insights into the binding structures depending on the properties of the functional groups involved. The Mala and Suc ligands have the same alkyl chain length between the carboxylate groups; however, Mala contains an additional alpha-hydroxyl group (Figure 1(a)).…”

“…The effect of alkyl chain length on the physicochemical behaviors of aqueous Am­(III) ion was examined in terms of the optical properties, binding stability, and coordination modes. Malate (Mala) and acetate (Ac) were also studied for comparison, although structural investigations on their Am­(III) complexes have already been reported. , Their speciation and formation constants were determined using a UV–vis spectrophotometry coupled with an LWCC. TRLFS and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy were used to examine the change in coordination modes depending on the ligand type, especially the alkyl chain length.…”

Towards Self-Organized Anodization of Aluminum in Malic Acid Solutions—New Aspects of Anodization in the Organic Acid

“…The black oxide was related with incorporation of carbon-bearing elements, such as carbonates, carboxylates, carbonyls of aluminum, or amorphous carbon into the AAO walls [21,41,42]. The violet-blue-green colouration of AAO fabricated in malic acid may indicate the incorporation of Al(III)-malic acid complexes into the AAO framework, since some metal-malate complexes demonstrate the absorption in the visible part of the spectrum [43,44]. 6.…”