Crystal structure of a trigonal polymorph of aqua­dioxidobis(pentane-2,4-dionato-κ²O,O′)uranium(VI)

The title compound, [UO₂(acac)₂(H₂O)] consists of a uran­yl(VI) unit ([O=U=O]²⁺) coordinated to two monoanionic acetyl­acetonate (acac, C₅H₇O₂) ligands and one water mol­ecule. The asymmetric unit includes a one-half of a uranium atom, one oxido ion, one-half of a water mol­ecule and one acac ligand. The coordination about the uranium atom is distorted penta­gonal–bipyramidal. The acac ligands and O_w atom comprise the equatorial plane, while the uranyl O atoms occupy the axial positions. Inter­molecular hydrogen bonding between complexes results in the formation of two-dimensional hexa­gonal void channels along the c-axis direction with a diameter of 6.7 Å. The monoclinic (P2₁/c space group) polymorph was reported by Alcock & Flanders [(1987). Acta Cryst. C43, 1480–1483].

1. Chemical context

Nuclear forensics applications often require the development of source materials for isotope-dilution mass-spectrometry measurements. One method for preparing actinide source materials includes the preparation of volatile compounds which can be deposited onto a conductive surface from the vapor phase. An alternative method involves electrochemical reduction to the zero-valent metal with concurrent deposition onto the electrode surface. This requires an organo-soluble actinide precursor. Hexavalent actinide complexes with β-diketonates are possibilities for either of these methods. They are neutrally charged, and with appropriate substituents on the β-diketonate may be volatile (Johnson et al., 2017). β-Diketonates also provide a platform to prepare the organosoluble precursors required for electrochemical reduction.

2. Structural commentary

The mol­ecular structure of the title compound, [UO₂(acac)₂(H₂O)] 1, was determined by single crystal X-ray diffraction. An ORTEP plot of the mol­ecular structure is shown in Fig. 1 and selected geometric parameters are listed in Table 1. The complex crystallizes in the trigonal Pc1 space group with one-half mol­ecule per asymmetric unit, while the other half is generated by a twofold axis running through the U and O_w atoms. In the mol­ecular structure, the U^VI center resides in a distorted penta­gonal–bipyramidal coordination environment, with the equatorial positions occupied by the four O atoms of two chelating monoanionic acac ligands, and one water mol­ecule, while the two oxido ions reside at the axial positions trans to each other. The equatorial plane composed of O1, O2 (and the two other symmetry-equivalent atoms) and O4 deviates noticeably from planarity [with a mean deviation of 0.172 (3) Å]. The two six-membered chelate rings composed of U1, O1, C1, C2, C3 and the symmetry-equivalent atoms deviate significantly from planarity [mean deviation, 0.211 (3) Å]. The dihedral angle between the two chelate best-fit planes is 26.02 (13)°.

Table 1
Selected geometric parameters (Å, °)

U1—O4	2.392 (4)	U1—O2	2.361 (3)
U1—O1	2.381 (2)	U1—O3	1.769 (2)
O1—U1—O4	75.17 (6)	O3—U1—O1^i	86.27 (10)
O1^i—U1—O1	150.33 (12)	O3—U1—O1	92.50 (11)
O2—U1—O4	144.24 (7)	O3—U1—O2	86.10 (11)
O2^i—U1—O1	139.25 (9)	O3^i—U1—O2	97.81 (12)
O2—U1—O1	70.00 (9)	O3—U1—O3^i	175.21 (17)
O3—U1—O4	87.61 (9)
Symmetry code: (i) .

Figure 1 Mol­ecular structure of aqua­dioxidobis(pentane-2,4-dionato-κ^2O,O′)uranium(VI). Displacement ellipsoids are drawn at the 50% probability level.

3. Supra­molecular features

Examination of the extended structure revealed a prominent inter­molecular hydrogen bonding inter­action (O4—H4⋯O1) involving one of the O atoms of the acac ligand and the O_w atom (Fig. 2, Table 2). This inter­action results in pairing of two mononuclear units, eventually consolidating the extended structure. The packing pattern along the c-axis direction reveals an extended pattern with considerably large hexa­gonal void channels, each one surrounded by six other smaller void channels. The void volume was determined using contact surface maps (which offer an estimate of the volume that could be filed by guest mol­ecules) to be 325.41 ų, representing 13.4% of the unit-cell volume (Barbour, 2006). This inter­esting packing pattern is shown in Figs. 3 and 4.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O4—H4⋯O1^ii	0.72 (6)	2.01 (5)	2.704 (3)	164 (7)
Symmetry code: (ii) .

Figure 2 Representation of the O—H⋯O inter­molecular hydrogen-bonding inter­actions in aqua­dioxidobis(pentane-2,4-dionato-κ^2O,O′)uranium(VI)

Figure 3 Formation of a ring structure extracted from the packing pattern of aqua­dioxidobis(pentane-2,4-dionato-κ^2O,O′)uranium(VI).

Figure 4 Packing pattern of aqua­dioxidobis(pentane-2,4-dionato-κ^2O,O′)uranium(VI) along the c axis.

4. Database survey

A scrutiny of the CSD (Conquest version 2.0, 2021; Groom et al., 2016) revealed two other crystal structures of [UO₂(acac)₂(H₂O)], 2 and 3, available for comparison [there are few others for which coordinates are not available, see: Dornberger-Schiff & Titze (1969) and Comyns et al. (1958)]. Structure 2 is a polymorph of 1, while structure 3 is a pyrazine solvate of [UO₂(acac)₂(H₂O)]. Selected metric parameters of 1–3 are listed in Table 3. Structures 1 and 2 differ in their crystal packing arrangement. Whereas 1 crystallizes in the trigonal Pc1 space group with half a mol­ecule per asymmetric unit, 2 crystallizes in the monoclinic P2₁/c space group with the asymmetric unit consisting of the complete mol­ecule (Alcock & Flanders, 1987). The differences in metric parameters between structures 1 and 2 are subtle (albeit with a slightly longer U—O_w distance in 2), the differences being attributed to the different crystal packing. In case of 1, classical hydrogen bonding dictates the packing pattern, while in 2, the inter­molecular inter­actions are chemically inconsequential. Structure 3 is also quite similar to 1 and 2. However, 3 crystallizes in the triclinic P space group with one inter­stitial pyrazine mol­ecule (the asymmetric unit contains the full [UO₂(acac)₂(H₂O)] mol­ecule and two half-mol­ecules of pyrazine). The pyrazine mol­ecules within the structure are engaged in hydrogen bonding with the coordinated water mol­ecule [O_w—H—-N(pyrazine), H⋯N = 1.95 (2) Å and O_w⋯N = 2.765 (5) Å; Kawasaki & Kitazawa 2008], thus preventing the supra­molecular organization of the uranium complex (seen in 1). Inter­estingly, the lower density of 1 compared to those of 2 and 3 (1.99, 2.27 and 2.03 g cm⁻³ for 1, 2, and 3 respectively) is attributed to the large voids within the hexa­gonal channels.

Table 3
Comparison of selected metric parameters (Å)

	1^a	2^b	3^c
U—O(H2O)	2.392 (4)	2.489 (8)	2.409 (4)
U=(oxo)	1.769 (2)	1.743 (6)	1.776 (3)
U—O(acac)	2.371 (3)	2.339 (6)	2.354 (4)
Notes: (a) this work; (b) Alcock & Flanders (1987); (c) Kawasaki & Kitazawa (2008).

5. Synthesis and crystallization

To a vial containing 377 mg (3.8 mmol) of 2,4-penta­nedione (acac) in 7 mL of THF was added 20 mL of an aqueous solution containing 0.94 mmol of UO₂(OAc)₂(H₂O)₂. The reaction mixture rapidly changed color from colorless to yellow. A 10 M aqueous solution of KOH was added dropwise to the reaction mixture until the pH was approximately 9 (500 µL). The color of the solution intensified to a dark yellow concurrent with the formation of a suspension. The suspension was extracted with 50 mL of toluene, and the resultant yellow organic solution was dried over Na₂SO₄. After reducing the volume by 50% at reduced pressure, the remaining solution was allowed to evaporate at room temperature. Over the course of 1 week, yellow crystals were formed (150 mg, 34% yield).

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 4. The water O atom was freely refined. C-bound H atoms were positioned geometrically (C—H = 0.03–0.96) and refined as riding with U_iso(H) = 1.2–1.5U_eq(C).

Table 4
Experimental details

Crystal data
Chemical formula	[UO2(C5H7O2)2(H2O)]
Mr	486.26
Crystal system, space group	Trigonal, Pc1
Temperature (K)	298
a, c (Å)	19.5774 (9), 7.3264 (5)
V (Å^3)	2431.8 (3)
Z	6
Radiation type	Mo Kα
μ (mm^−1)	10.03
Crystal size (mm)	0.30 × 0.15 × 0.05
Data collection
Diffractometer	Bruker D8 Quest PHOTON II
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.457, 0.745
No. of measured, independent and observed [I > 2σ(I)] reflections	26571, 1493, 1426
Rint	0.027
(sin θ/λ)max (Å^−1)	0.603
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.016, 0.042, 1.13
No. of reflections	1493
No. of parameters	89
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å^−3)	0.51, −1.06
Computer programs: APEX3 and SAINT (Bruker, 2016), SHELXT2018/2 (Sheldrick, 2015a), SHELXL2018/3 (Sheldrick, 2015b), and OLEX2 (Dolomanov et al., 2009).

Supporting information

Computing details

Data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Aquadioxidobis(pentane-2,4-dionato-κ²O,O')uranium(VI)

Crystal data

[UO2(C5H7O2)2(H2O)]	Dx = 1.992 Mg m^−3
Mr = 486.26	Mo Kα radiation, λ = 0.71073 Å
Trigonal, P3c1	Cell parameters from 9890 reflections
a = 19.5774 (9) Å	θ = 3.2–25.4°
c = 7.3264 (5) Å	µ = 10.03 mm^−1
V = 2431.8 (3) Å^3	T = 298 K
Z = 6	Needle, yellow
F(000) = 1344	0.30 × 0.15 × 0.05 mm

Data collection

Bruker D8 Quest PHOTON II diffractometer	1426 reflections with I > 2σ(I)
ω–scan	Rint = 0.027
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θmax = 25.4°, θmin = 3.5°
Tmin = 0.457, Tmax = 0.745	h = −23→23
26571 measured reflections	k = −23→23
1493 independent reflections	l = −8→8

Refinement

Refinement on F^2	Primary atom site location: dual
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.016	Hydrogen site location: mixed
wR(F^2) = 0.042	H atoms treated by a mixture of independent and constrained refinement
S = 1.13	w = 1/[σ^2(Fo^2) + (0.0203P)^2 + 3.5627P] where P = (Fo^2 + 2Fc^2)/3
1493 reflections	(Δ/σ)max < 0.001
89 parameters	Δρmax = 0.51 e Å^−3
0 restraints	Δρmin = −1.06 e Å^−3

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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > 2σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

A suitable crystal of [UO2(acac)2(H2O)] was selected and mounted on a Bruker D8 Quest diffractometer. The crystal was kept at 298.0?K during data collection. Using Olex2 (Dolomanov, 2009) the structure was solved with the SHELXT (Sheldrick 2015) structure solution program using Intrinsic Phasing and refined with the SHELXL (Sheldrick 2015) refinement package using Least Squares minimisation.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
U1	0.38611 (2)	0.000000	0.750000	0.03158 (8)
O4	0.5083 (2)	0.000000	0.750000	0.0415 (8)
O1	0.45776 (16)	0.08101 (15)	0.4979 (3)	0.0442 (6)
O2	0.31937 (16)	0.06227 (16)	0.6287 (4)	0.0525 (7)
O3	0.42991 (16)	0.08004 (14)	0.9047 (3)	0.0453 (6)
C1	0.4697 (2)	0.1489 (2)	0.4498 (5)	0.0437 (9)
C2	0.4199 (3)	0.1769 (2)	0.4902 (6)	0.0524 (10)
H2	0.436802	0.229520	0.464721	0.063*
C3	0.3450 (3)	0.1311 (3)	0.5675 (5)	0.0503 (9)
C4	0.5432 (3)	0.1979 (3)	0.3397 (7)	0.0674 (13)
H4A	0.588238	0.204246	0.406406	0.101*
H4B	0.548844	0.248675	0.316338	0.101*
H4C	0.539460	0.171870	0.225884	0.101*
C5	0.2896 (3)	0.1632 (4)	0.5750 (7)	0.0742 (15)
H5A	0.249096	0.134214	0.663397	0.111*
H5B	0.266109	0.158127	0.457143	0.111*
H5C	0.318349	0.217860	0.609141	0.111*
H4	0.525 (3)	−0.018 (3)	0.694 (7)	0.073 (17)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
U1	0.03345 (9)	0.02596 (10)	0.03283 (11)	0.01298 (5)	0.00110 (3)	0.00219 (6)
O4	0.0423 (15)	0.044 (2)	0.0383 (19)	0.0222 (10)	−0.0026 (8)	−0.0052 (16)
O1	0.0552 (16)	0.0402 (14)	0.0435 (13)	0.0286 (12)	0.0131 (12)	0.0121 (11)
O2	0.0516 (16)	0.0580 (17)	0.0576 (18)	0.0345 (15)	0.0090 (13)	0.0207 (13)
O3	0.0560 (16)	0.0355 (13)	0.0447 (14)	0.0231 (12)	−0.0011 (12)	−0.0068 (11)
C1	0.054 (2)	0.0369 (19)	0.0394 (19)	0.0223 (18)	0.0035 (16)	0.0077 (15)
C2	0.068 (3)	0.041 (2)	0.054 (2)	0.032 (2)	0.011 (2)	0.0157 (18)
C3	0.071 (3)	0.061 (3)	0.0364 (19)	0.046 (2)	0.0031 (18)	0.0109 (18)
C4	0.062 (3)	0.054 (3)	0.082 (3)	0.025 (2)	0.022 (2)	0.027 (2)
C5	0.097 (4)	0.101 (4)	0.065 (3)	0.080 (3)	0.024 (3)	0.031 (3)

Geometric parameters (Å, º)

U1—O4	2.392 (4)	C1—C2	1.369 (6)
U1—O1	2.381 (2)	C1—C4	1.504 (5)
U1—O1^i	2.381 (2)	C2—H2	0.9300
U1—O2	2.361 (3)	C2—C3	1.400 (6)
U1—O2^i	2.361 (3)	C3—C5	1.502 (5)
U1—O3	1.769 (2)	C4—H4A	0.9600
U1—O3^i	1.769 (2)	C4—H4B	0.9600
O4—H4	0.71 (4)	C4—H4C	0.9600
O4—H4^i	0.71 (4)	C5—H5A	0.9600
O1—C1	1.279 (4)	C5—H5B	0.9600
O2—C3	1.261 (5)	C5—H5C	0.9600
O1^i—U1—O4	75.17 (6)	C1—O1—U1	130.3 (2)
O1—U1—O4	75.17 (6)	C3—O2—U1	131.1 (3)
O1^i—U1—O1	150.33 (12)	O1—C1—C2	124.0 (3)
O2—U1—O4	144.24 (7)	O1—C1—C4	115.4 (4)
O2^i—U1—O4	144.24 (6)	C2—C1—C4	120.5 (3)
O2^i—U1—O1	139.25 (9)	C1—C2—H2	118.0
O2—U1—O1^i	139.25 (9)	C1—C2—C3	124.0 (4)
O2^i—U1—O1^i	70.00 (9)	C3—C2—H2	118.0
O2—U1—O1	70.00 (9)	O2—C3—C2	124.0 (4)
O2^i—U1—O2	71.52 (13)	O2—C3—C5	116.7 (4)
O3—U1—O4	87.61 (9)	C2—C3—C5	119.3 (4)
O3^i—U1—O4	87.61 (9)	C1—C4—H4A	109.5
O3^i—U1—O1^i	92.50 (11)	C1—C4—H4B	109.5
O3—U1—O1^i	86.27 (10)	C1—C4—H4C	109.5
O3—U1—O1	92.50 (11)	H4A—C4—H4B	109.5
O3^i—U1—O1	86.27 (10)	H4A—C4—H4C	109.5
O3—U1—O2^i	97.81 (12)	H4B—C4—H4C	109.5
O3—U1—O2	86.10 (11)	C3—C5—H5A	109.5
O3^i—U1—O2	97.81 (12)	C3—C5—H5B	109.5
O3^i—U1—O2^i	86.10 (11)	C3—C5—H5C	109.5
O3—U1—O3^i	175.21 (17)	H5A—C5—H5B	109.5
U1—O4—H4^i	134 (4)	H5A—C5—H5C	109.5
U1—O4—H4	134 (4)	H5B—C5—H5C	109.5
H4—O4—H4^i	92 (8)
U1—O1—C1—C2	−26.9 (6)	O1—C1—C2—C3	−8.8 (7)
U1—O1—C1—C4	154.2 (3)	C1—C2—C3—O2	9.1 (7)
U1—O2—C3—C2	27.1 (6)	C1—C2—C3—C5	−169.0 (4)
U1—O2—C3—C5	−154.8 (3)	C4—C1—C2—C3	170.0 (4)

Symmetry code: (i) x−y, −y, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4···O1^ii	0.72 (6)	2.01 (5)	2.704 (3)	164 (7)

Symmetry code: (ii) −x+1, −y, −z+1.