Synthesis and crystal structure of triethylammonium hexabromidouranate(IV) dichloromethane monosolvate

The synthesis and crystal structure determination of (Et3NH)2[UBr6]·CH2Cl2 is reported.

We, however, observe reduction of UBr 5 to uranium(IV) as [UBr 6 ] 2and the protonation of NEt 3 . It is plausible that ethylene glycol serves as the proton source for the formation of the HNEt 3 + cations; however, we do not know where the glycolate anions end up. We also do not know what the reducing agent for the reduction of U V to U IV ) is, or if UBr 5 is simply unstable under these conditions and is converted to UBr 4 and 0.5 Br 2 . We also do not know how UBr 5 is dissolved, that is, whether U 2 Br 10 molecules or other mono-or polynuclear complexes, such as of gylcolates, are present in solution. Elemental bromine may be present within the brown solution and act as an oxidizing agent under the formation of the Br À anions required to constitute the [UBr 6 ] 2anions. For the reactions to be stoichiometric, some leftover U species should have been formed that we did not observe. In summary, the detailed formation of the title compound (Et 3 NH) 2 [UBr 6 ]ÁCH 2 Cl 2 remains unclear.

Supramolecular features
Sections of the crystal structure, illustrating the hydrogenbonding situation, are shown in Fig. 2. The hydrogen bonds were inspected visually and those with angles less than 134 were removed from the analysis. The Br3 and Br5 atoms of the [UBr 6 ] 2anion act as acceptors for the bifurcated N-HÁ Á ÁBr hydrogen bond. The other HNEt 3 + cation (with N2) also forms a N-HÁ Á ÁBr hydrogen bond, however, not bifurcated. Hydrogen-bond lengths and angles are given in Table 2. Furthermore, C-HÁ Á ÁHal hydrogen-bond-like interactions between the HNEt 3 + cations and the Br atoms of the [UBr 6 ] 2anion as well as to the Cl atoms of the dichloromethane molecules are also present. Overall, a three-dimensional hydrogen-bonded network results. An overview of the hydrogen-bond lengths between the cations, anion and solvent molecule in the compound reported here is given in Table 2. The C-HÁ Á ÁBr hydrogen bonds in (Ph 3 EtP) 2 [UBr 6 ] (Caira et al., 1978) range from 2.782 (1) to 3.504 (2) Å . An example for N-HÁ Á ÁBr hydrogen bonds is (C 6 H 8 NS 3 ) 2 [UBr 6 ] (Conradi et al., 1986), with lengths of 2.81 (9) Å for the interactions. These bond lengths are comparable with the presented data.

Figure 1
Section of the crystal structure of (Et 3 NH) 2 [UBr 6 ]ÁCH 2 Cl 2 , illustrating the asymmetric unit. Displacement ellipsoids are shown at the 70% probability level at 100 K and H atoms are drawn with an arbitrary radius.

Synthesis and crystallization
50 mg of UBr 5 (0.08 mmol, 1.00 eq) were dissolved in 2 mL of predried DCM and 0.06 mL of NEt 3 (40 mg, 0.39 mmol, 5.00 eq.) were added. Then, after stirring briefly, 0.01 mL of ethylene glycol (10 mg, 0.20 mmol, 2.50 eq.) were added dropwise. After two h, the reaction mixture was filtered and the obtained brown filtrate was cooled to 241 K. The product was obtained in crystalline form after three days as brown plates. A selected crystal was investigated by X-ray diffraction.
As only a few crystals precipitated from the cold filtrate, the yield could not be determined, but it can be assumed that it was rather low. No further analysis was carried out on the few minute crystals or the filtrate. UBr 5 was synthesized according to the literature (Deubner et al., 2019).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were positioned geometrically (N-H = 1.00Å , C-H = 0.98-0.99Å ) refined using a riding model with U iso (H) = 1.2U eq (N,C) or 1.5U eq (C methyl ). The maximum and minimum residual electron densities are located close to the U atom at distances of 0.77 and 1.19 Å , respectively.

Figure 2
The hydrogen bonds and hydrogen-bond-like interactions (dashed lines) present in the structure of the title compound. (a) and (b) show the interactions of the HNEt + cation with N1, (c) of the HNEt + cation with N2, and (d) shows the interactions of DCM. Displacement ellipsoids within each subfigure are shown at the 70% probability level at 100 K and H atoms are drawn with an arbitrary radius. See Table 2 for symmetry operators.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.