(NH4)2[UO2Cl4]·2H2O, a new uranyl tetrachloride with ammonium charge-balancing cations

While several actinyl tetrahalides have been synthesized and their structures reported, (NH4)2(UO2Cl4)·2H2O represents a new uranyl tetrachloride salt synthesized in a slow evaporation from a 2 M hydrochloric acid solution. Its optical properties were measured by diffuse reflectance and luminescence spectroscopies, while powder X-ray diffraction confirmed an ammonium chloride impurity phase.

A new uranyl tetrachloride salt with chemical formula, (NH 4 ) 2 [UO 2 Cl 4 ]Á2H 2 O, namely, diammonium uranyl tetrachloride dihydrate, 1, was prepared and crystallized via slow evaporation from a solution of 2 M hydrochloric acid. As confirmed by powder X-ray diffraction, the title compound crystallizes with an ammonium chloride impurity that formed as a result of the breakdown of a triazine precursor. The (UO 2 Cl 4 ) 2À dianion is charge balanced by ammonium cations, while an extensive hydrogen-bond network donated from structural water molecules stabilize the overall assembly. Compound 1 adds to the extensive collection of actinyl tetrachloride salts, but it represents the first without an alkali cation for purely inorganic compounds. Diffuse reflectance and luminescence spectra show typical absorption and emission behavior, respectively, of uranyl materials.

Chemical context
Hexavalent actinides such as uranium, neptunium, and plutonium exist in aqueous solution as the linear triatomic actinyl cation, with formula (AnO 2 ) 2+ . The actinyl ion coordinates a variety of poly-and mono-atomic anions such that four to six atoms bond in the equatorial plane (Burns, 2005;Lussier et al., 2016). In part due to their ease of synthesis, structural simplicity, and high symmetry, the actinyl tetrahalide family of compounds has remained a relevant subclass of materials over several decades and has led to a deeper understanding of actinide electronic structure, bonding, and optical properties, among many others. The actinyl tetrahalides have general formula (AnO 2 X 4 ) 2À (where An = U VI , Np VI , and Pu VI and X = Cl À and Br À ) and have been studied to investigate periodic trends in f-element chemistry. Of the numerous compounds that include this anionic complex, the Cs + salt with formula Cs 2 (AnO 2 Cl 4 ) has been one of the most extensively characterized actinyl compounds. The uranyl structure was first reported in 1966 (Hall et al., 1966) with an improved model reported in 1991 (Watkin et al., 1991). In that time, it was used to quantitatively assign infrared (Ohwada, 1975) and Raman (Ohwada, 1980) active bands of the uranyl ion, which were found to be at 916 cm À1 and 831 cm À1 , respectively. Improvements in analytical (i.e. X-ray absorption spectroscopies) and computational techniques (i.e. density functional theory calculations) over time have advanced our understanding in the electronic and molecular orbital energies of the uranyl ion in Cs 2 (UO 2 Cl 4 ), providing strong evidence that actinide atoms can bind with significant covalent character (Denning, 2007;Vitova et al., 2015). Luminescence spectroscopy, Raman spectroscopy, and computational works have also been used to study bond-length changes of the uranyl ion with respect to different pressures in Cs 2 (UO 2 Cl 4 ) (Osman et al., 2016;Warzecha et al., 2019). Beyond the Cs salt, systematic studies into actinyl bond strength changes as a function of metal center (i.e. An = U VI , Np VI and Pu VI ) have been reported for organic-based counter-cations (Schnaars & Wilson, 2013;Surbella III et al., 2017;Schnaars & Wilson, 2018). Quite recently, focus has been placed on the cationic influence on supramolecular assembly as well as actinyl bondstrength changes (Schnaars & Wilson, 2013;Surbella III et al., 2016;Carter et al., 2018;Pyrch et al., 2020;Augustine et al., 2023). Despite these numerous studies with actinyl tetrahalide species, we report a new inorganic uranyl tetrachloride not charge-balanced by an alkali cation, with formula (NH 4 ) 2 (UO 2 Cl 4 )Á2H 2 O (compound 1).

Structural commentary
Compound 1 crystallizes in the space group P1. The uranyl tetrachloride dianion (UO 2 Cl 4 ) 2À is composed of a U VI metal center that is coordinated to two terminal, axial oxygen atoms and four equatorial chlorine atoms as shown in Fig. 1. The (UO 2 Cl 4 ) 2À dianion adopts a square-bipyramidal coordination geometry with D 4h point group symmetry. The U VI atom sits on a center of inversion symmetry, resulting in a linear uranyl (UO 2 ) 2+ cation with a U1-O1 bond distance of 1.7745 (14) Å and O1-U1-O1 angle of 180 . The U VI atom is also coordinated to two crystallographically unique chlorine atoms with U1-Cl1 and U1-Cl2 bond distances of 2.6752 (5) Å and 2.6623 (4) Å , respectively. The two Cl1-U1-Cl2 bond angles measure 88.855 (15) and 91.145 (15) , and O1-U1-Cl1, bond angles also slightly deviate from 90 . The U-O (Lussier et al., 2016) and U-Cl (Surbella III et al., 2016) bond lengths are typical for these compounds. The structure contains one crystallographically unique structural water molecule (O1w) with two O-H covalent bonds with restrained bond lengths near 0.95 Å , and one crystallographically unique ammonium cation (N1) is present to provide charge balance to the overall structure. There are four N-H covalent bonds with restrained bond lengths that are approximately 0.87 Å . The extended crystal structure is shown in Fig. 2.

Supramolecular features
A hydrogen-bond network consisting of seven unique interactions exists between ammonium cations, water molecules, and uranyl tetrachloride units as depicted in Fig. 3 and as The crystal packing observed in compound 1 as viewed along the a-axis.

Figure 1
The uranyl tetrachloride anionic unit along with a crystallographically unique water molecule and ammonium cation. Displacement ellipsoids for non-hydrogen atoms are shown at 50% probability.

Figure 3
The seven unique hydrogen-bond interactions shown with the hydrogenbond distances from donor hydrogen atom to acceptor atom. tabulated in Table 1. Each water molecule donates two hydrogen bonds via H1A and H1B donor atoms to two separate uranyl tetrachloride complexes. On the other hand, each ammonium cation donates hydrogen bonds in three dimensions to three separate uranyl tetrachloride units and two separate water molecules, stabilizing the overall crystal structure into a complex network. Fig. 4 shows the hydrogenbond network in the extended structure.

Database survey
Compound 1 is the first inorganic uranyl tetrachloride chargebalanced with a non-alkali metal in the Inorganic Crystal Structure Database (Zagorac et al., 2019). With respect to structures in the ICSD, Cs salts of the (AnO 2 Cl 4 ) 2À species have been reported for U (Hall et al., 1966;Watkin et al., 1991;Tutov et al., 1991;Schnaars & Wilson, 2013), Np (Wilkerson et al., 2007), and Pu (Wilkerson & Scott, 2008;Schnaars & Wilson, 2013). Other charge-balancing cations reported in the ICSD for U VI and Pu VI include Rb (Anson et al., 1996;Schnaars & Wilson, 2013) and tetramethylammonium (Schnaars & Wilson, 2013), while that of Np includes (UO 2 Cl 4 ) 2À -doped Np VI (Wilkerson & Berg, 2009) and a mixed Np V/VI oxidation state Cs salt (Alcock et al., 1986). Although there is a tetramethylammonium salt in the ICSD, we consider it as a better member of the Crystal Structure Database (CSD) given the presence of organic-based (i.e. C-H bonds) components in the structure (Groom et al., 2016).

Synthesis and crystallization
Concentrated hydrochloric acid, HCl, (Sigma-Aldrich, 37%) was diluted to 2 M. Then, 0.0366 g (0.44 mmol) of 1,3,5-triazine (Sigma-Aldrich, 97.0%) was dissolved into 1 mL of 2 M HCl in a 1-dram borosilicate glass reaction vial. Uranyl acetate dihydrate (0.10216 g; 2.4 mmol) was added to this solution and allowed to dissolve completely. The vial was placed uncapped in a 20 mL centrifuge tube on a bed of desiccant. The centrifuge tube was capped, and the reaction solution was allowed to evaporate for 3 weeks until large yellow crystals formed. It was noticed that compound 1 partially dissolves in ethanol, affecting the preparation for characterization beyond singlecrystal X-ray diffraction. Powder-diffraction data was collected using a Rigaku Ultima IV Diffractometer with Cu K radiation and a linear position-sensitive detector. The analysis revealed an ammonium chloride, NH 4 Cl, impurity phase along with compound 1. Diffuse reflectance and luminescence spectra were also collected for the mixed-phase material and can be found in the supporting information along with the powder-diffraction data.  (3) 169 (3) Symmetry codes: (i) Àx; Ày þ 1; Àz þ 2; (ii) x; y; z þ 1; (iii) x; y þ 1; z; (iv) Àx; Ày þ 1; Àz þ 1; (v) Àx þ 1; Ày þ 2; Àz þ 2.

Figure 4
The crystal packing along with the hydrogen-bond network observed in compound 1 as viewed slightly offset along the a-axis.

Special details
Geometry. All estimated standard deviations (esds), except those pertaining to the dihedral angle between two least squares (ls) planes, are estimated using the full covariance matrix. 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.