Crystal structure of dioxidobis(pentane-2,4-dionato-κ2 O,O′)[1-phenyl-3-(pyridin-4-yl)propane-κN]uranium(VI)

[UO2(acac)2(ppp)] is constructed from one uranyl(VI) unit, two anionic acetylacetonate (acac) ligands and one 1-phenyl-3-(pyridin-4-yl)propane (ppp) ligand. The U atom exhibits a UNO6 pentagonal–bipyramidal coordination geometry; two uranyl(VI) O atoms are located at the axial positions, whereas four O atoms from two chelating bidentate acac ligands and one N atom of a ppp ligand form the equatorial plane.


Chemical context
The structural properties of uranyl(VI) complexes are interesting from the viewpoint of nuclear fuels reprocessing and actinide waste treatment. In most commercial reprocessing plants, spent nuclear fuels are treated by the Purex method, in which uranium and plutonium are extracted from a nitric acid solution of spent nuclear fuels using tributyl-phosphate/ n-dodecane. Uranium in the nitric acid solution exists as uranyl(VI) ([O U O] 2+ ) complexes. However, the Purex method has a few problems; for example, as the processing takes place on a relatively large scale, a large amount of extractant is necessary (Ikeda et al., 2004;Suzuki et al., 2012) Attempts to find other suitable coordinating ligands are therefore being undertaken. A number of structural studies of uranyl(VI) -diketonate complexes have been reported by ourselves and others (Alcock et al., 1984(Alcock et al., , 1987Huuskonen et al., 2007;Kannan et al., 2001;Kawasaki & Kitazawa, 2008;Kawasaki et al., 2010;Sidorenko et al., 2009;Tahir et al., 2006;Takao & Ikeda, 2008). In particular, acetylacetonate (acac), is the simplest -diketonate ligand and an important coordinating ligand for uranium.
We report herein the synthesis and crystal structure of a novel uranyl(VI) acetylacetonate (acac) complex with the pyridine-based ligand ppp [ppp = 1-phenyl-3-(pyridin-4yl)propane] (Seth, 2014) (Fig. 1). The uranium(VI) atom exhibits a pentagonal-bipyramidal coordination geometry: two uranyl(VI) oxygen atoms (O1 and O2) are located in the axial positions and four oxygen atoms (O3, O4, O5 and O6) from two chelating bidentate acac ions, together with one nitrogen atom (N1) of the ppp molecule, form the equatorial plane. The bond lengths around U1 (Table 1) decrease in the order U-N > U-O acac > U O. The dihedral angle between the pyridine ring of the ppp molecule and the equatorial plane around U1 is 49.43 (12) . The above structural properties are similar to those in the majority of previously characterised [UO 2 (acac) 2 L] (L = pyridine derivative ligand) complexes (Alcock et al., 1984;Kawasaki & Kitazawa, 2008;Kawasaki et al., 2010). The conformation of the ppp molecule is GG 0 (Fig. 2). The dihedral angle between the pyridine ring and the phenyl ring in the ppp molecule is 26.96 (13) .

Supramolecular features
A packing diagram of title complex is shown in Fig. 3. The molecules are stacked along the b axis, held together by van der Waals' interactions only. Significant intermolecularand C-HÁ Á Á interactions are not found.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed at calculated positions [C(CH)-H = 0.93, C(CH 2 )-H = 0.97 and C(CH 3 )-H = 0.96Å ] and allowed to ride on their parent atoms with U iso (H) = 1.2U eq (CH,CH 2 ) and U iso (H) = 1.5U eq (CH 3 ).

Figure 1
The molecular structure of [UO 2 (acac) 2 (ppp)]. Displacement ellipsoids are drawn at the 50% probability level and H atoms have been omitted for clarity.

Dioxidobis(pentane-2,4-dionato-κ 2 O,O′)[1-phenyl-3-(pyridin-4-yl)propane-κN]uranium(VI)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.003 Δρ max = 0.88 e Å −3 Δρ min = −0.64 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.