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

Kohlgruber, T.A.; Surbella III, R.G.

doi:10.1107/S2056989023005753

research communications

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Volume 79| Part 8| August 2023| Pages 702-706

https://doi.org/10.1107/S2056989023005753

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

Tsuyoshi A. Kohlgruber ^a and Robert G. Surbella III ^a ^*

^aPacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA, 99354, USA
^*Correspondence e-mail: robert.surbella@pnnl.gov

Edited by S. Parkin, University of Kentucky, USA (Received 25 May 2023; accepted 29 June 2023; online 7 July 2023)

A new uranyl tetrachloride salt with chemical formula, (NH₄)₂[UO₂Cl₄]·2H₂O, 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 (UO₂Cl₄)²⁻ 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.

Keywords: ammonium; optical properties; uranyl tetrachloride; X-ray diffraction; crystal structure.

CCDC reference: 2278035

1. Chemical context

Hexavalent actinides such as uranium, neptunium, and plutonium exist in aqueous solution as the linear triatomic actinyl cation, with formula (AnO₂)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 (AnO₂X₄)²⁻ (where An = U^VI, Np^VI, and Pu^VI and X = Cl⁻ and Br⁻) 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 Cs₂(AnO₂Cl₄) 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⁻¹ and 831 cm⁻¹, 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 Cs₂(UO₂Cl₄), 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 Cs₂(UO₂Cl₄) (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 bond-strength 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 (NH₄)₂(UO₂Cl₄)·2H₂O (compound 1).

2. Structural commentary

Compound 1 crystallizes in the space group P $[\overline{1}]$ . The uranyl tetrachloride dianion (UO₂Cl₄)²⁻ 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 (UO₂Cl₄)²⁻ 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 (UO₂)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.

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 2
The crystal packing observed in compound 1 as viewed along the a-axis.

3. 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 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 hydrogen-bond network in the extended structure.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1w—H1A⋯Cl1ⁱ	0.95 (6)	2.36 (7)	3.283 (2)	165 (7)
O1w—H1B⋯Cl2ⁱⁱ	0.95 (5)	2.35 (5)	3.268 (2)	163 (5)
N1—H2A⋯O1w	0.87 (3)	2.02 (3)	2.843 (2)	157 (3)
N1—H2B⋯Cl1ⁱⁱⁱ	0.87 (4)	2.68 (4)	3.441 (2)	148 (3)
N1—H2C⋯O1	0.87 (2)	2.32 (4)	3.014 (3)	137 (4)
N1—H2C⋯Cl1^iv	0.87 (2)	2.77 (3)	3.4060 (17)	131 (4)
N1—H2D⋯O1w^v	0.87 (3)	2.03 (3)	2.887 (3)	169 (3)
Symmetry codes: (i) ; (ii) x, y, z+1; (iii) x, y+1, z; (iv) ; (v) .

Figure 3
The seven unique hydrogen-bond interactions shown with the hydrogen-bond distances from donor hydrogen atom to acceptor atom.

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

4. Database survey

Compound 1 is the first inorganic uranyl tetrachloride charge-balanced 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 (AnO₂Cl₄)²⁻ 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 (UO₂Cl₄)²⁻-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 ).

With respect to the CSD, there are numerous reports with ammonium-based charge-balancing species (Di Sipio et al., 1974a ,b ; Bois et al., 1976a ,b ; Rogers et al., 1987 ; Gatto et al., 2004 ; Schnaars & Wilson, 2013; Biswas et al., 2017 ; Serezkhina et al., 2021 ). Compound 1 has ammonium with a water molecule, while one report has ammonium with crown ethers (Rogers et al., 1987). The other ammonium-based cations include organic-functional groups (Di Sipio et al., 1974a,b; Bois et al., 1976a,b; Gatto et al., 2004; Schnaars & Wilson, 2013; Biswas et al., 2017; Serezkhina et al., 2021). Other types of cations that charge-balance (UO₂Cl₄)²⁻ in the CSD include pyridinium-based (Graziani et al., 1975 ; Bombieri et al., 1978 ; Marsh, 1988 ; Pospieszna et al., 2008 ; Deifel & Cahill, 2009 ; Baker et al., 2010 ; Andrews & Cahill, 2012 ; Lhoste et al., 2013 ; Hashem et al., 2013 ; Surbella III et al., 2016, 2017; Carter et al., 2018; Mishra et al., 2019 ; Pyrch et al., 2020), phenanthrolinium-based (Di Sipio et al., 1981 ), imidazolium-based (Zalkin et al., 1983 ; Qu et al., 2014 ; Kohlgruber, 2022 ), and phosphonium-based (Brown et al., 1996 ; Schnaars & Wilson, 2014 ) species. Other (UO₂Cl₄)²⁻ complexes have crystallized in the presence of separate metal complexes (Moody & Ryan, 1979 ; Rogers et al., 1987, 1990 ; Pons y Moll et al., 2001 ; Hashem et al., 2014 ; Falaise et al., 2015 ; Zhang et al., 2017 ; Schöne et al., 2018 ), crown ethers (Wang et al., 1986 ; Rogers et al., 1987, 1991 ; Rogers & Benning, 1991 ; Evans et al., 2002 ) and porphyrins (Mishra et al., 2019). In total, there are over 60 known uranyl tetrachloride crystal structures in the CSD. Reference codes for these compounds can be found in the supporting information.

5. 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 single-crystal X-ray diffraction. Powder-diffraction data was collected using a Rigaku Ultima IV Diffractometer with Cu Kα radiation and a linear position-sensitive detector. The analysis revealed an ammonium chloride, NH₄Cl, 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.

6. Refinement

Crystal data, data collection, and structure refinement details are summarized in Table 2. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were found in Fourier difference maps, and their positions refined with positional restraints.

Table 2
Experimental details

Crystal data
Chemical formula	(NH₄)₂[UO₂Cl₄]·2H₂O
M_r	483.95
Crystal system, space group	Triclinic, P $[\overline{1}]$
Temperature (K)	100
a, b, c (Å)	6.6574 (4), 6.6954 (4), 7.4018 (4)
α, β, γ (°)	99.827 (2), 93.879 (2), 117.354 (1)
V (Å³)	284.69 (3)
Z	1
Radiation type	Mo Kα
μ (mm⁻¹)	15.17
Crystal size (mm)	0.10 × 0.03 × 0.03

Data collection
Diffractometer	Bruker D8 Venture
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.460, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	24049, 2803, 2803
R_int	0.042
(sin θ/λ)_max (Å⁻¹)	0.842

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.017, 0.041, 1.09
No. of reflections	2803
No. of parameters	76
No. of restraints	12
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	2.94, −1.97
Computer programs: APEX4 and SAINT (Bruker, 2014 ), SHELXT (Sheldrick, 2015a ), SHELXL (Sheldrick, 2015b ), CrystalMaker (CrystalMaker, 2014 ), and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 2278035

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989023005753/pk2687sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989023005753/pk2687Isup2.hkl

Thermal Ellipsoidal plots and data, (e.g., Powder X-ray diffraction, diffuse reflectance spectroscopy, and photo luminescence spectroscopy). DOI: https://doi.org/10.1107/S2056989023005753/pk2687sup3.docx

checkCIF report

Computing details top

Data collection: APEX4 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: CrystalMaker (CrystalMaker, 2014); software used to prepare material for publication: publCIF (Westrip, 2010).

Diammonium uranyl tetrachloride dihydrate top

Crystal data top

(NH₄)₂[UO₂Cl₄]·2H₂O	Z = 1
M_r = 483.95	F(000) = 218
Triclinic, P1	D_x = 2.823 Mg m⁻³
a = 6.6574 (4) Å	Mo Kα radiation, λ = 0.71073 Å
b = 6.6954 (4) Å	Cell parameters from 9764 reflections
c = 7.4018 (4) Å	θ = 3.5–36.7°
α = 99.827 (2)°	µ = 15.17 mm⁻¹
β = 93.879 (2)°	T = 100 K
γ = 117.354 (1)°	Block, yellow
V = 284.69 (3) Å³	0.10 × 0.03 × 0.03 mm

Data collection top

Bruker D8 Venture diffractometer	2803 reflections with I > 2σ(I)
Radiation source: microsource Diamond II	R_int = 0.042
φ and ω scans	θ_max = 36.8°, θ_min = 3.5°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −11→11
T_min = 0.460, T_max = 0.747	k = −11→11
24049 measured reflections	l = −12→12
2803 independent reflections

Refinement top

Refinement on F²	Primary atom site location: dual
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.017	Hydrogen site location: difference Fourier map
wR(F²) = 0.041	All H-atom parameters refined
S = 1.09	w = 1/[σ²(F_o²) + (0.0306P)²] where P = (F_o² + 2F_c²)/3
2803 reflections	(Δ/σ)_max = 0.001
76 parameters	Δρ_max = 2.94 e Å⁻³
12 restraints	Δρ_min = −1.97 e Å⁻³

Special details top

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
U1	0.500000	0.500000	0.500000	0.00924 (3)
Cl1	0.11039 (7)	0.32334 (9)	0.63968 (6)	0.01683 (7)
Cl2	0.27372 (8)	0.27569 (9)	0.15652 (6)	0.01779 (7)
O1	0.4614 (3)	0.7395 (3)	0.4746 (2)	0.0157 (2)
O1W	0.2602 (3)	0.7346 (3)	1.0758 (2)	0.0193 (2)
H1A	0.173 (10)	0.747 (13)	1.169 (7)	0.060 (19)*
H1B	0.253 (11)	0.589 (5)	1.071 (9)	0.048 (16)*
N1	0.2678 (3)	0.9378 (3)	0.7673 (2)	0.0165 (2)
H2A	0.227 (6)	0.850 (5)	0.846 (4)	0.028 (11)*
H2B	0.186 (5)	1.008 (6)	0.765 (5)	0.042 (14)*
H2C	0.247 (7)	0.855 (6)	0.657 (2)	0.042 (14)*
H2D	0.412 (2)	1.040 (5)	0.800 (5)	0.049 (16)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
U1	0.01069 (4)	0.01103 (4)	0.00762 (4)	0.00584 (3)	0.00291 (2)	0.00367 (2)
Cl1	0.01357 (14)	0.02264 (19)	0.01467 (15)	0.00771 (14)	0.00622 (12)	0.00655 (14)
Cl2	0.01902 (16)	0.02084 (18)	0.01029 (14)	0.00814 (14)	0.00083 (12)	0.00047 (13)
O1	0.0210 (5)	0.0150 (5)	0.0159 (5)	0.0113 (5)	0.0057 (4)	0.0067 (4)
O1W	0.0189 (6)	0.0234 (7)	0.0175 (6)	0.0096 (5)	0.0063 (5)	0.0093 (5)
N1	0.0180 (6)	0.0189 (6)	0.0126 (5)	0.0085 (5)	0.0027 (5)	0.0045 (5)

Geometric parameters (Å, º) top

U1—O1	1.7745 (14)	O1W—H1A	0.948 (10)
U1—O1ⁱ	1.7745 (14)	O1W—H1B	0.947 (10)
U1—Cl2	2.6623 (4)	N1—H2A	0.868 (9)
U1—Cl2ⁱ	2.6623 (4)	N1—H2B	0.872 (9)
U1—Cl1	2.6752 (5)	N1—H2C	0.870 (9)
U1—Cl1ⁱ	2.6752 (5)	N1—H2D	0.871 (9)

O1—U1—O1ⁱ	180.0	O1ⁱ—U1—Cl1ⁱ	89.87 (5)
O1—U1—Cl2	90.32 (5)	Cl2—U1—Cl1ⁱ	88.855 (15)
O1ⁱ—U1—Cl2	89.68 (5)	Cl2ⁱ—U1—Cl1ⁱ	91.145 (15)
O1—U1—Cl2ⁱ	89.68 (5)	Cl1—U1—Cl1ⁱ	180.000 (17)
O1ⁱ—U1—Cl2ⁱ	90.32 (5)	H1A—O1W—H1B	104 (6)
Cl2—U1—Cl2ⁱ	180.000 (11)	H2A—N1—H2B	109.7 (14)
O1—U1—Cl1	89.86 (5)	H2A—N1—H2C	109.8 (14)
O1ⁱ—U1—Cl1	90.13 (5)	H2B—N1—H2C	109.1 (14)
Cl2—U1—Cl1	91.145 (15)	H2A—N1—H2D	109.7 (14)
Cl2ⁱ—U1—Cl1	88.855 (15)	H2B—N1—H2D	109.1 (14)
O1—U1—Cl1ⁱ	90.14 (5)	H2C—N1—H2D	109.5 (14)

Symmetry code: (i) −x+1, −y+1, −z+1.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1w—H1A···Cl1ⁱⁱ	0.95 (6)	2.36 (7)	3.283 (2)	165 (7)
O1w—H1B···Cl2ⁱⁱⁱ	0.95 (5)	2.35 (5)	3.268 (2)	163 (5)
N1—H2A···O1w	0.87 (3)	2.02 (3)	2.843 (2)	157 (3)
N1—H2B···Cl1^iv	0.87 (4)	2.68 (4)	3.441 (2)	148 (3)
N1—H2C···O1	0.87 (2)	2.32 (4)	3.014 (3)	137 (4)
N1—H2C···Cl1^v	0.87 (2)	2.77 (3)	3.4060 (17)	131 (4)
N1—H2D···O1w^vi	0.87 (3)	2.03 (3)	2.887 (3)	169 (3)

Symmetry codes: (ii) −x, −y+1, −z+2; (iii) x, y, z+1; (iv) x, y+1, z; (v) −x, −y+1, −z+1; (vi) −x+1, −y+2, −z+2.

Acknowledgements

The authors thank Dr Aaron D. Nicholas for his feedback in preparing this manuscript.

Funding information

Funding for this research was provided by: U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Heavy Element Chemistry Program, FWP 73200 .

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