New Insight into the Photochemical Reaction Mechanism of Uranyl Citrate by NMR and DFT


New Insight into the Photochemical Reaction Mechanism of Uranyl Citrate by NMR and DFT

Kretzschmar, J.; Steudtner, R.; Tsushima, S.

A sound understanding of the major reaction mechanisms is crucial to handle uranium containing waste appropriately. This means both the synthesis of unique compounds and the treatment of uranium occurring in or released into the environment. In an environmental context, uranium occurs in two main redox states: mobile U(VI) and immobile U(IV).
Due to both its model character in U(VI) complexation by chelating polycarboxylates and the citrate being a ubiquitous occurring ligand, particularly being important in the citric acid cycle in vivo, the uranyl citrate system itself [1–4] and also its photoreaction [5, 6 and refs. cited therein] is already repeatedly investigated, but still not fully understood.
This investigation provides not only further insight into the U(VI)-citrate complexation, but also a better understanding of the (photo-)redox chemistry of uranium in general.
Here we want to present the reaction pathway of the U(VI) citrate complex photooxidation to its degradation products ketoglutaric acid, acetoacetic acid and acetone with concomitant CO2 formation by several decarboxylation steps and the formation of U(IV). The oxidation state of the latter is indicated by NMR showing 1H chemical shifts > 50 ppm and proven by UV-vis. Moreover, the yielded U(IV) appears as soluble complexes of citrate and its degradation products. The identity of the formed compounds was experimentally proven by one- and twodimensional NMR methods and confirmed by DFT calculations.
The photoreaction starts by irradiating the sample with light from a simple light source such as the sun or a commercial mercury lamp. Interestingly, the initial chemical alteration starts by a single electron transfer from a citrate molecule, being hydrogen bonded to the fifth remaining coordination site not occupied by U(VI)–coordinating citrate. Most likely the intermediate, i.e., not observable U(V) disproportionates fast to U(VI) and the aforementioned U(IV).

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[2] E. Ohyoshi, J. Oda, A. Ohyoshi, Bull. Chem. Soc. Jap. 1975, 48, 227–229.
[3] S. P. Pasilis and J. E. Pemberton, Inorg. Chem. 2003, 42, 6793–6800.
[4] A. Günther, R. Steudtner, K. Schmeide, G. Bernhard, Radiochim. Acta 2011, 99, 535–541.
[5] H. D. Burrows and T. J. Kemp, Chem. Soc. Rev. 1974, 3, 139–165.
[6] A. J. Francis and C. J. Dodge, DAE-BRNS Biennial Symposium on Emerging Trends in Separation Science and Technology (SESTEC) 2008 (BNL-80322-2008-CP).

Keywords: uranium; citrate; photoreaction; reaction mechanism; NMR spectroscopy; DFT calculation

  • Poster
    Advancing the Chemistry of the f-elements: Dalton Discussion 14, 28.-30.07.2014, Edinburgh, United Kingdom

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