ISSN 1066-3622, Radiochemistry, 2009, Vol. 51, No. 6, pp. 567–575. © Pleiades Publishing, Inc., 2009. Original Russian Text © G.V. Sidorenko, M.S. Grigor’ev, V.V. Gurzhiy, S.V. Krivovichev, D.N. Suglobov, 2009, published in Radiokhimiya, 2009, Vol. 51, No. 6, pp. 495–502. 567 Crystal and Molecular Structure of Adducts of Uranyl Pivaloyltrifluoroacetonate with Hexamethylphosphoramide and of Uranyl Trifluoroacetylacetonate with Trimethyl Phosphate G. V. Sidorenko a , M. S. Grigor’ev b , V. V. Gurzhiy c , S. V. Krivovichev c , and D. N. Suglobov a a Khlopin Radium Institute, Research and Production Association, Federal State Unitary Enterprise, St. Petersburg, Russia; e-mail: gevasid@mail.ru 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 February 25, 2009 Abstract—Crystal and molecular structures of adducts of uranyl pivaloyltrifluoroacetonate with hexamethyl- phosphoramide [UO 2 (PTFA) 2 (HMPA)] (I) and of uranyl trifluoroacetylacetonate with trimethyl phosphate [UO 2 (TFA) 2 (TMP)] (II) were determined. Compound I crystallizes in the monoclinic system, space group P2 1 /n; a = 16.9384(3), b = 9.1090(2), c = 20.9844(4) Å, β = 101.5337(10)°, V = 3172.34(11) Å 3 (at 100 К); Z = 4. Compound II crystallizes in the rhombic system, space group Pbca; a = 17.8214(4), b = 7.7786(2), c = 30.9176(7) Å, V = 4285.97(18) Å 3 (at 100 К); Z = 8. In both cases, the cis isomer in which the neutral ligand is located between the trifluoromethyl groups is realized. Compound I differing from II by the stronger branching of ligand periphery is characterized by stronger structural deformations in the crystal. Key words: uranyl, β-diketonates, adducts, crystal and molecular structure, single crystal X-ray diffraction Among adducts of uranyl β-diketonates, the com- plex of uranyl pivaloyltrifluoroacetonate with hexa- methylphosphoramide [UO 2 {Bu t C(O)CHC(O)CF 3 }· {OP(NMe 2 ) 3 }] [UO 2 (PTFA) 2 (HMPA)] (I) is of par- ticular interest. Firstly, this compound exhibits very high thermal stability compared to related complexes: It can be heated an inert atmosphere to at least 270°С without noticeable decomposition. The saturated vapor pressure of the complex, attained in so doing, exceeds 10 3 Pa [1]. The complex can be distilled without de- composition not only in a vacuum, but also at atmos- pheric pressure in an inert gas flow. Secondly, in con- trast to virtually all the adducts of uranyl β-diketonates with aprotic ligands that we examined in our previous studies [2, 3], in which there are no prerequisites for cation–cation interactions, in the case of I the uranyl group is asymmetric, as indicated by splitting of the antisymmetric stretching vibration band of the 18 O=U= 16 O group into two components. Splitting was observed in the spectrum of not only crystalline but also matrix-isolated sample [4], suggesting in- tramolecular nature of the effect. This splitting was attributed to distortion of the symmetry of the complex due to steric hindrance produced by the branched ligands. Among adducts of uranyl β-diketonates with aprotic ligands, similar splitting was observed only in the adduct of uranyl dipivaloylmethanate with HMPA, having a still more branched structure [4]. Therefore, it is interesting to examine the structure of this complex in more detail with the aim to quantitatively evaluate the extent of its asymmetry and to reveal factors re- sponsible for unusually high thermal stability of the complex. Also, of certain interest is possible isomer- ism of complex I with an unsymmetrical β-diketone. For comparison we examined the structure of the ad- duct of uranyl trifluoroacetylacetonate with trimethyl phosphate [UO 2 (TFA) 2 (TMP)] (II) [1]. This complex differs from I by the smaller extent of branching of the ligand periphery and by the lower donor power of the neutral ligand [5]. PACS numbers: 61.10.Nz, 61.66.Fn DOI: 10.1134/S1066362209060034 EXPERIMENTAL Crystals of I were prepared by recrystallization from hexane (vacuum distillation led to melting of the sample, and crystals of the required quality did not