ISSN 1066-3622, Radiochemistry, 2009, Vol. 51, No. 4, pp. 345–349. © Pleiades Publishing, Inc., 2009. Original Russian Text © G.V. Sidorenko, M.S. Grigor’ev, V.V. Gurzhiy, D.N. Suglobov, I.G. Tananaev, 2009, published in Radiokhimiya, 2009, Vol. 51, No. 4, pp. 303–307. 345 Crystal and Molecular Structure of Uranyl Acetylacetonate Dimer, [UO 2 (C 5 H 7 O 2 ) 2 ] 2 G. V. Sidorenko a , M. S. Grigor’ev b , V. V. Gurzhiy c , D. N. Suglobov a , and I. G. Tananaev b a Khlopin Radium Institute, Research and Production Association, Federal State Unitary Enterprise, St. Petersburg, Russia b Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia c Geological Faculty, St. Petersburg State University, St. Petersburg, Russia Received January 12, 2009 Abstract—The crystal and molecular structure of uranyl acetylacetonate dimer was determined by single crys- tal X-ray diffraction. The compound crystallizes in the tetragonal system, a = 7.9420(2), c = 40.1240(13) Å (at 100 К), Z = 4, space group P4 1 2 1 2. Dimeric uranyl acetylacetonate molecules in the crystal are formed by bridging bonding of one of O atoms of the acetylacetonate ligands with U atoms, so that the coordination poly- hedra of U atoms (distorted pentagonal bipyramids) share a common equatorial edge. The dimer has a nonpla- nar structure, being significantly bent along the conventional line connecting the bridging O atoms. ȕ-Diketonates, in particular, their simplest represen- tatives, acetylacetonates, are among the most important coordination compounds of metals, including acti- nides. Acetylacetonates of actinides(IV) (Th, U, Np, Pu) are well known [1]. The crystal and molecular structure of U(IV) acetylacetonate has been determined by Titze [2]. As for uranyl, the first report on synthesis of its acetylacetonate was published as far back as 1904 [3]. Later the compound was prepared in the non- solvated form (here and hereinafter, by “nonsolvated” is meant “containing no additional ligands”) [4], and Comyns, Gatehouse, and Wait [5] examined uranyl acetylacetonate in detail. In particular, they prepared nonsolvated uranyl acetylacetonate, its monohydrate, and complexes with ethanol and a neutral acetylace- tone molecule. They also made ebullioscopic measure- ments and concluded that the nonsolvated compound is dimeric in a benzene solution. Later we examined the structure of uranyl acetylacetonate and its complexes (adducts) with neutral ligands by IR spectroscopy us- ing isotope substitution of oxygen in the uranyl group [6]. In the IR spectrum of the nonsolvated chelate, we observed a complex pattern in the range of absorption of the uranyl group, suggesting vibration interaction of closely located uranyl groups. This pattern was qualita- tively interpreted assuming formation of a dimer by bridging coordination of oxygen atoms of the acety- lacetonate ligand. This coordination increases the coor- dination number of uranyl to 5, as in adducts of uranyl ȕ-diketonates [1]. The IR spectra also suggested non- parallel arrangement of the uranyl groups in the dimer. Peaks of binuclear ions were also detected in the mass spectrum of uranyl acetylacetonate [7]. However, up to now the crystal and molecular structure of nonsolvated uranyl acetylacetonate has not been determined by di- rect structural methods. Available structural data on uranyl acetylacetonate refer only to adducts with neu- tral ligands: enamino derivatives of acetylacetone [8– 11], neutral acetylacetone molecule [12], triphenyl- phosphine oxide [13], pyridine [14, 15], dimethylfor- mamide [16], tetrahydrofuran [17], water [18], and also to a mixed-ligand complex of the composition [2,4,6-Me 3 PyH] + [UO 2 (acac)(NO 3 ) 2 ] – [18]. In this study we examined the crystal and molecular structure of nonsolvated uranyl acetylacetonate UO 2 (C 5 H 7 O 2 ) 2 . PACS numbers: 61.10.Nz, 61.66.Fn DOI: 10.1134/S106636220904002X EXPERIMENTAL By vacuum sublimation at 220–250°C of uranyl acetylacetonate monohydrate synthesized according to [5], we obtained orange crystals of the nonsolvated complex, one of which was selected for X-ray diffrac- tion analysis. Despite coordination unsaturation of the complex, short manipulations with the crystal in air to