Speaker
Dr
Christoph Hennig
(The Rossendorf Beamline at ESRF, BP 220, F-38043 Grenoble, France)
Description
Tetravalent actinides and lanthanides form strong complexes with carboxyl containing ligands. Such complexes play an important role in technological processes as well as in biological and environmental systems. So far, most of the thermodynamic data of actinide(IV) and lanthanide(IV) carboxylates are estimated by assuming mononuclear solution species [1]. We applied comprehensive studies with EXAFS, UV-Vis-NIR spectroscopy and X-ray diffraction on Th4+, U4+, Np4+ and Ce4+ carboxylates (RCOO–; R = H, CH3, CHR’NH2; R’ = H, CH3, CH2SH) in aqueous solution and solid state [2-5]. Our studies reveal that in all of the investigated systems hexanuclear complexes appear, which become predominant with increasing metal and ligand concentration and increasing pH, and dominate finally the species distribution. We present here their structure, stability constants, and the mechanisms of complex formation. The appearance of hexanuclear complexes in aqueous solution coincides with the onset of the An(IV) hydrolysis on the one hand, and the deprotonation of the carboxylic function on the other hand. This results in a competing reaction between hydrolysis and ligation. The hydrolysis induces a polymerization of the metal ions via oxo and hydroxo bonds, whereas 12 carboxylic ligands provide charge neutrality of the hexanuclear core and prevent further polymerization. Our studies indicate that future work on tetravalent actinide carboxylates in aqueous solution requires consideration of these hexanuclear species.
REFERENCES
[1] Casellato, U.; Vigato, P. A.; Vidali, M. Coord. Chem. Rev. 1978, 26, 85-159.
[2] Takao, S.; Takao, K.; Kraus, W.; Emmerling, F.; Scheinost, A.C.; Bernhard, G.; Hennig, C. Eur. J. Inorg. Chem. 2009, 4771-4775.
[3] Takao, K.; Takao, S.; Scheinost, A.C.; Bernhard, G.; Hennig, C. Inorg. Chem. 2012, 51, 1336-1344.
[4] Hennig, C.; Takao, S.; Takao, K.; Weiss, S.; Kraus, W.; Emmerling, F.; Scheinost, A.C. Dalton Trans. 2012, 41, 12818-12823.
[5] Hennig, C.; Ikeda-Ohno, A.; Kraus, W.; Weiss, S.; Pattison, P.; Emerich, H.; Abdala, P.M.; Scheinost, A.C. Inorg. Chem. 2013, 52, 11734-11743.
Primary author
Dr
Christoph Hennig
(The Rossendorf Beamline at ESRF, BP 220, F-38043 Grenoble, France)
Co-authors
Dr
Andreas, C. Scheinost
(The Rossendorf Beamline at ESRF, BP 220, F-38043 Grenoble, France)
Dr
Atsushi Ikeda-Ohno
(Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, D-01314 Dresden, Germany)
Dr
Koichiro Takao
(Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, 152-8550 Tokyo, Japan)
Dr
Shinobu Takao
(Innovation Research Center for Fuel Cells, The University of Electro-Communications, 1-5-1, Chofugaoka, Chofu-shi, 182-8585 Tokyo, Japan)
Mr
Stephan Weiss
(Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, D-01314 Dresden, Germany)
Werner Kraus
(BAM Federal Institute for Materials Research and Testing, Richard-Willstätter-Str. 11, D-12489 Berlin, Germany)
