Bis(pyridinium) trans-tetrachloridodioxidouranate(VI) dioxane solvate

In the crystal structure of the title compound, (C5H6N)2[UCl4O2]·C4H8O2, the pyridinium cations occupy general positions and the anions and the solvent dioxane molecule are located on centres of inversion. The dioxane molecules are connected to two symmetry-related pyridinium cations via O—H⋯O hydrogen bonding. There are additional intermolecular C—H⋯Cl contacts, which are indicative of weak C—H⋯Cl interactions.

In the crystal structure of the title compound, (C 5 H 6 N) 2 [UCl 4 O 2 ]ÁC 4 H 8 O 2 , the pyridinium cations occupy general positions and the anions and the solvent dioxane molecule are located on centres of inversion. The dioxane molecules are connected to two symmetry-related pyridinium cations via O-HÁ Á ÁO hydrogen bonding. There are additional intermolecular C-HÁ Á ÁCl contacts, which are indicative of weak C-HÁ Á ÁCl interactions.
Data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis RED (Oxford Diffraction, 2006); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP (Siemens, 1989); software used to prepare material for publication: SHELXL97. The use of uranium as a source of energy has caused increasing attention which is focused on the problem of fuel reprocessing and waste storage. Much effort has been devoted, in recent years, to the preparation and characterization of specific complexing agents for the uranyl ion (UO 2 2+ ), so-called "uranophiles" with the objective of a possible application for the separation of uranium species in waste liquids from the nuclear fuel cycle and for the recovery and utilization of uranium from the sea water (Sessler et al. 2006). The title compound was isolated during our study on the synthesis and

S2. Experimental
The title compound was isolated during the slow diffusion of dioxane into pyridine hydrochloride solution of the uranyl(VI) Schiff base complex prepared through one-step template reaction of 2,6-diacetylpyridine with spermidine in the presence of uranyl(VI) acetateunder following conditions: to a mixture of uranyl acetate (42.5 mg, 0.1 mmol) in methanol (10 cm 3 ) and 2,6-diacetylpyridine (16,3 mg, 0.1 mmol) in methanol (10 cm 3 ), spermidine (0.016 cm 3 , 0.1 mmol) in methanol (10 cm 3 ) was added dropwise with stirring; the reaction wascarried out for 4 h, the solution volume was then reduced to 10 cm 3 by roto-evaporation and a yellow precipitate formed on addition of a small amount of diethyl ether was filtered off, washed with ether, and dried in vacuo.

S3. Refinement
The H atoms were positioned with idealized geometry and were refined isotropic using a riding model with U iso (H) = 1.2.
U eq (C,N) of the parent atom. Weak restraints (ISOR) were applied to the displacement parameters of C, N and O atoms.

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 > σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq U1