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

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aDepartment of Chemistry and Biochemistry, Florida International University, 11200 SW 8th St., Miami, FL 33199, USA
*Correspondence e-mail: cdares@fiu.edu

Edited by J. Ellena, Universidade de Sâo Paulo, Brazil (Received 24 September 2021; accepted 21 October 2021; online 1 January 2022)

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 Ow 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[Johnson, A. T., Parker, T. G., Dickens, S. M., Pfeiffer, J. K., Oliver, A. G., Wall, D., Wall, N. A., Finck, M. R. & Carney, K. P. (2017). Inorg. Chem. 56, 13553-13561.]). β-Diketonates also provide a platform to prepare the organosoluble precursors required for electrochemical reduction.

[Scheme 1]

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[link] and selected geometric parameters are listed in Table 1[link]. The complex crystallizes in the trigonal P[\overline{3}]c1 space group with one-half mol­ecule per asymmetric unit, while the other half is generated by a twofold axis running through the U and Ow atoms. In the mol­ecular structure, the UVI 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—O1i 86.27 (10)
O1i—U1—O1 150.33 (12) O3—U1—O1 92.50 (11)
O2—U1—O4 144.24 (7) O3—U1—O2 86.10 (11)
O2i—U1—O1 139.25 (9) O3i—U1—O2 97.81 (12)
O2—U1—O1 70.00 (9) O3—U1—O3i 175.21 (17)
O3—U1—O4 87.61 (9)    
Symmetry code: (i) [x-y, -y, -z+{\script{3\over 2}}].
[Figure 1]
Figure 1
Mol­ecular structure of aqua­dioxidobis(pentane-2,4-dionato-κ2O,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 Ow atom (Fig. 2[link], Table 2[link]). 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 Å3, representing 13.4% of the unit-cell volume (Barbour, 2006[Barbour, L. J. (2006). Chem. Commun. pp. 1163-1168.]). This inter­esting packing pattern is shown in Figs. 3[link] and 4[link].

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
O4—H4⋯O1ii 0.72 (6) 2.01 (5) 2.704 (3) 164 (7)
Symmetry code: (ii) [-x+1, -y, -z+1].
[Figure 2]
Figure 2
Representation of the O—H⋯O inter­molecular hydrogen-bonding inter­actions in aqua­dioxidobis(pentane-2,4-dionato-κ2O,O′)uranium(VI)
[Figure 3]
Figure 3
Formation of a ring structure extracted from the packing pattern of aqua­dioxidobis(pentane-2,4-dionato-κ2O,O′)uranium(VI).
[Figure 4]
Figure 4
Packing pattern of aqua­dioxidobis(pentane-2,4-dionato-κ2O,O′)uranium(VI) along the c axis.

4. Database survey

A scrutiny of the CSD (Conquest version 2.0, 2021; Groom et al., 2016[Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171-179.]) 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[Dornberger-Schiff, K. & Titze, H. (1969). Acta Chem. Scand. 23, 1685-1694.]) and Comyns et al. (1958[Comyns, A. E., Gatehouse, B. M. & Wait, E. (1958). J. Chem. Soc. 4655-4665.])]. Structure 2 is a polymorph of 1, while structure 3 is a pyrazine solvate of [UO2(acac)2(H2O)]. Selected metric parameters of 13 are listed in Table 3[link]. Structures 1 and 2 differ in their crystal packing arrangement. Whereas 1 crystallizes in the trigonal P[\overline{3}]c1 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[Alcock, N. W. & Flanders, D. J. (1987). Acta Cryst. C43, 1480-1483.]). The differences in metric parameters between structures 1 and 2 are subtle (albeit with a slightly longer U—Ow 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[\overline{1}] 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 [Ow—H—-N(pyrazine), H⋯N = 1.95 (2) Å and Ow⋯N = 2.765 (5) Å; Kawasaki & Kitazawa 2008[Kawasaki, T. & Kitazawa, T. (2008). Acta Cryst. E64, m673-m674.]], 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)

  1a 2b 3c
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[Alcock, N. W. & Flanders, D. J. (1987). Acta Cryst. C43, 1480-1483.]); (c) Kawasaki & Kitazawa (2008[Kawasaki, T. & Kitazawa, T. (2008). Acta Cryst. E64, m673-m674.]).

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[link]. 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 Uiso(H) = 1.2–1.5Ueq(C).

Table 4
Experimental details

Crystal data
Chemical formula [UO2(C5H7O2)2(H2O)]
Mr 486.26
Crystal system, space group Trigonal, P[\overline{3}]c1
Temperature (K) 298
a, c (Å) 19.5774 (9), 7.3264 (5)
V3) 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[Krause, L., Herbst-Irmer, R., Sheldrick, G. M. & Stalke, D. (2015). J. Appl. Cryst. 48, 3-10.])
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−1) 0.603
 
Refinement
R[F2 > 2σ(F2)], wR(F2), 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[Bruker (2016). APEX3 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]), SHELXT2018/2 (Sheldrick, 2015a[Sheldrick, G. M. (2015a). Acta Cryst. A71, 3-8.]), SHELXL2018/3 (Sheldrick, 2015b[Sheldrick, G. M. (2015b). Acta Cryst. C71, 3-8.]), and OLEX2 (Dolomanov et al., 2009[Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2009). J. Appl. Cryst. 42, 339-341.]).

Supporting information


Computing details top

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) top
Crystal data top
[UO2(C5H7O2)2(H2O)]Dx = 1.992 Mg m3
Mr = 486.26Mo Kα radiation, λ = 0.71073 Å
Trigonal, P3c1Cell parameters from 9890 reflections
a = 19.5774 (9) Åθ = 3.2–25.4°
c = 7.3264 (5) ŵ = 10.03 mm1
V = 2431.8 (3) Å3T = 298 K
Z = 6Needle, yellow
F(000) = 13440.30 × 0.15 × 0.05 mm
Data collection top
Bruker D8 Quest PHOTON II
diffractometer
1426 reflections with I > 2σ(I)
ω–scanRint = 0.027
Absorption correction: multi-scan
(SADABS; Krause et al., 2015)
θmax = 25.4°, θmin = 3.5°
Tmin = 0.457, Tmax = 0.745h = 2323
26571 measured reflectionsk = 2323
1493 independent reflectionsl = 88
Refinement top
Refinement on F2Primary atom site location: dual
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.016Hydrogen site location: mixed
wR(F2) = 0.042H atoms treated by a mixture of independent and constrained refinement
S = 1.13 w = 1/[σ2(Fo2) + (0.0203P)2 + 3.5627P]
where P = (Fo2 + 2Fc2)/3
1493 reflections(Δ/σ)max < 0.001
89 parametersΔρmax = 0.51 e Å3
0 restraintsΔρmin = 1.06 e Å3
Special details top

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 (Å2) top
xyzUiso*/Ueq
U10.38611 (2)0.0000000.7500000.03158 (8)
O40.5083 (2)0.0000000.7500000.0415 (8)
O10.45776 (16)0.08101 (15)0.4979 (3)0.0442 (6)
O20.31937 (16)0.06227 (16)0.6287 (4)0.0525 (7)
O30.42991 (16)0.08004 (14)0.9047 (3)0.0453 (6)
C10.4697 (2)0.1489 (2)0.4498 (5)0.0437 (9)
C20.4199 (3)0.1769 (2)0.4902 (6)0.0524 (10)
H20.4368020.2295200.4647210.063*
C30.3450 (3)0.1311 (3)0.5675 (5)0.0503 (9)
C40.5432 (3)0.1979 (3)0.3397 (7)0.0674 (13)
H4A0.5882380.2042460.4064060.101*
H4B0.5488440.2486750.3163380.101*
H4C0.5394600.1718700.2258840.101*
C50.2896 (3)0.1632 (4)0.5750 (7)0.0742 (15)
H5A0.2490960.1342140.6633970.111*
H5B0.2661090.1581270.4571430.111*
H5C0.3183490.2178600.6091410.111*
H40.525 (3)0.018 (3)0.694 (7)0.073 (17)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
U10.03345 (9)0.02596 (10)0.03283 (11)0.01298 (5)0.00110 (3)0.00219 (6)
O40.0423 (15)0.044 (2)0.0383 (19)0.0222 (10)0.0026 (8)0.0052 (16)
O10.0552 (16)0.0402 (14)0.0435 (13)0.0286 (12)0.0131 (12)0.0121 (11)
O20.0516 (16)0.0580 (17)0.0576 (18)0.0345 (15)0.0090 (13)0.0207 (13)
O30.0560 (16)0.0355 (13)0.0447 (14)0.0231 (12)0.0011 (12)0.0068 (11)
C10.054 (2)0.0369 (19)0.0394 (19)0.0223 (18)0.0035 (16)0.0077 (15)
C20.068 (3)0.041 (2)0.054 (2)0.032 (2)0.011 (2)0.0157 (18)
C30.071 (3)0.061 (3)0.0364 (19)0.046 (2)0.0031 (18)0.0109 (18)
C40.062 (3)0.054 (3)0.082 (3)0.025 (2)0.022 (2)0.027 (2)
C50.097 (4)0.101 (4)0.065 (3)0.080 (3)0.024 (3)0.031 (3)
Geometric parameters (Å, º) top
U1—O42.392 (4)C1—C21.369 (6)
U1—O12.381 (2)C1—C41.504 (5)
U1—O1i2.381 (2)C2—H20.9300
U1—O22.361 (3)C2—C31.400 (6)
U1—O2i2.361 (3)C3—C51.502 (5)
U1—O31.769 (2)C4—H4A0.9600
U1—O3i1.769 (2)C4—H4B0.9600
O4—H40.71 (4)C4—H4C0.9600
O4—H4i0.71 (4)C5—H5A0.9600
O1—C11.279 (4)C5—H5B0.9600
O2—C31.261 (5)C5—H5C0.9600
O1i—U1—O475.17 (6)C1—O1—U1130.3 (2)
O1—U1—O475.17 (6)C3—O2—U1131.1 (3)
O1i—U1—O1150.33 (12)O1—C1—C2124.0 (3)
O2—U1—O4144.24 (7)O1—C1—C4115.4 (4)
O2i—U1—O4144.24 (6)C2—C1—C4120.5 (3)
O2i—U1—O1139.25 (9)C1—C2—H2118.0
O2—U1—O1i139.25 (9)C1—C2—C3124.0 (4)
O2i—U1—O1i70.00 (9)C3—C2—H2118.0
O2—U1—O170.00 (9)O2—C3—C2124.0 (4)
O2i—U1—O271.52 (13)O2—C3—C5116.7 (4)
O3—U1—O487.61 (9)C2—C3—C5119.3 (4)
O3i—U1—O487.61 (9)C1—C4—H4A109.5
O3i—U1—O1i92.50 (11)C1—C4—H4B109.5
O3—U1—O1i86.27 (10)C1—C4—H4C109.5
O3—U1—O192.50 (11)H4A—C4—H4B109.5
O3i—U1—O186.27 (10)H4A—C4—H4C109.5
O3—U1—O2i97.81 (12)H4B—C4—H4C109.5
O3—U1—O286.10 (11)C3—C5—H5A109.5
O3i—U1—O297.81 (12)C3—C5—H5B109.5
O3i—U1—O2i86.10 (11)C3—C5—H5C109.5
O3—U1—O3i175.21 (17)H5A—C5—H5B109.5
U1—O4—H4i134 (4)H5A—C5—H5C109.5
U1—O4—H4134 (4)H5B—C5—H5C109.5
H4—O4—H4i92 (8)
U1—O1—C1—C226.9 (6)O1—C1—C2—C38.8 (7)
U1—O1—C1—C4154.2 (3)C1—C2—C3—O29.1 (7)
U1—O2—C3—C227.1 (6)C1—C2—C3—C5169.0 (4)
U1—O2—C3—C5154.8 (3)C4—C1—C2—C3170.0 (4)
Symmetry code: (i) xy, y, z+3/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O4—H4···O1ii0.72 (6)2.01 (5)2.704 (3)164 (7)
Symmetry code: (ii) x+1, y, z+1.
Comparison of selected metric parameters (Å) top
1a2b3c
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).
 

Funding information

Funding for this research was provided by: Idaho National Laboratory (contract No. 18A12-107); U.S. Nuclear Regulatory Commission (grant No. 31310018M0012).

References

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