Complex formation of tetravalent actindies with small carboxylate ligands


Complex formation of tetravalent actindies with small carboxylate ligands

Hennig, C.; Ikeda-Ohno, A.; Takao, S.; Takao, K.; Kraus, W.; Scheinost, A. C.

Tetravalent actinides form strong complexes with the carboxylic acids. Although there is a number of thermodynamic data reported [1-2] there is no information available on the complex structure in aqueous solution.
We used EXAFS spectroscopy to estimate the complex structure of Th4+, U4+, Np4+ and Ce4+ with different carboxylates (RCOO–; R = H, CH3, C2H2NH2) in aqueous solution [3-6]. The structural information from EXAFS data is limited to radial pair distribution functions. To overcome this limitation we forced single crystals to precipitate in different stability ranges of the solution species. Whether or not a species is preserved in a crystal structure was subsequently investigated by EXAFS spectroscopy. The crystal structure, revealed
by single-crystal diffraction, and combined with EXAFS spectroscopy on liquid and solid samples, provides precise information of coordination and structure of the related solution species. These studies show that in all of the investigated aqueous systems hexanuclear complexes [M6O4(OH)4(RCOO)12] appear, which become predominant with increasing ligand concentration as well as increasing pH, and dominate finally the species distribution.
An example is shown in Figure 1. EXAFS spectra were recorded on samples in aqueous solution with 0.05 M Th(IV), 1 M glycine and pH values ranging from 0.5 to 3.2. The spectrum at pH 0.5 shows a single peak representing mononuclear complexes of Th(OH)n (4n)+ with n  2. With increasing pH, the first peak in the Fourier transform becomes more asymmetric and finally splits. A second peak appears at R+ = 3.75 Å, and its intensity rises with increasing pH. This latter feature represents a Th-Th scattering interaction. The data fit indicate the appearance of a hexanuclear complex. This complex is stable until pH 3.2. At higher pH values precipitates a crystalline material. The EXAFS spectrum of this precipitate is identical with that of the solution at pH 3.2, indicating that the structure of the solution species remains preserved in the crystal structure.
Crystals from the precipitate were used for structure analysis by X-ray diffraction. A drawing of the crystal structure is shown in Figure 2, which is in turn representative for the coordination of the predominating solution species at pH 3.2.
The appearance of hexanuclear complexes in aqueous solution corresponds with both, the onset of the metal hydrolysis at the one hand, and the deprotonation of the carboxylic function at the other hand. This results in a competing reaction between hydrolysis and ligation. The hydrolysis results in a polymerization via oxo and hydroxo bonds, whereas the carboxylic function of the ligand results in the formation of 12 terminating chelate rings providing charge neutrality of the hexanuclear core and preventing further polymerization. Most of the thermodynamic data of actinide(IV) carboxylates are estimated assuming mononuclear solution species.
Our studies indicate that future work on tetravalent actinide carboxylates in aqueous solution need consideration of hexanuclear species.
[1] G.M. Sergeev, Radiokhimija 22, 536 (1980).
[2] A. Bismondo, L. Rizzo, G. Tomat, Inorg. Chim. Acta 74, 21 (1983).
[3] S. Takao, K. Takao, W. Kraus, F. Emmerling, A.C. Scheinost, G. Bernhard, C. Hennig, Eur. J. Inorg. Chem. 4771 (2009).
[4] K. Takao, S. Takao, A.C. Scheinost, G. Bernhard, C. Hennig, Inorg. Chem. 51, 1336 (2012).
[5] C. Hennig, S. Takao, K. Takao, S. Weiss, W. Kraus, F. Emmerling, A.C. Scheinost, Dalton Trans. 41, 12818 (2012).
[6] C. Hennig, A. Ikeda-Ohno, W. Kraus, S. Weiss, P. Pattison, H. Emerich, P.M. Abdala, A.C. Scheinost, Inorg. Chem. 52, 11734 (2013).

Keywords: actinides; xafs; thorium; uranium; neptunium; cerium

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