Article pubs.acs.org/IC
Solid-State Structures of XeO3 James T. Goettel and Gary J. Schrobilgen* Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4M1, Canada S Supporting Information *
ABSTRACT: The solid-state structure of xenon trioxide, XeO3, was reinvestigated by low-temperature single-crystal X-ray diffraction and shown to exhibit polymorphism that is dependent on the crystallization conditions. The previously reported α-phase (orthorhombic, P212121) only forms upon evaporation of aqueous HF solutions of XeO3. In contrast, two new phases, β-XeO 3 (rhombohedral, R3) and γ-XeO 3 (rhombohedral, R3c), have been obtained by slow evaporation of aqueous solutions of XeO3. The extended structures of all three phases result from XeO---Xe bridge interactions among XeO3 molecules that arise from the amphoteric donor−acceptor nature of XeO3. The Xe atom of the trigonal-pyramidal XeO3 unit has three Xe---O secondary bonding interactions. The orthorhombic α-XeO3 displays the greatest degree of variation among the contact distances and has a significantly higher density than the rhombohedral phases. The ambient-temperature Raman spectra of solid α- and γ-XeO3 have also been obtained and assigned for the first time.
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without detonation when slowly warmed from 40 to 120 °C.7 The activation energy for decomposition of XeO3 to the elements (131 kJ mol−1) was determined by thermogravimetric and chromatographic analyses, which gave an extrapolated halflife of 4.4 years at room temperature.8 No phase transitions were reported between 50 and 106 °C in the latter study. The Raman spectrum of a 2.0 M aqueous solution of XeO39 and the IR spectrum of XeO3 recorded in a AgCl pellet have also been reported, but the Raman spectrum of solid XeO3 was unknown.10 The isolation and low-temperature (−173 °C) single-crystal X-ray structures of two new phases of XeO3 and a higherprecision X-ray structure of the original orthorhombic phase are reported herein. Solid XeO3 was also characterized for the first time by room-temperature Raman spectroscopy.
INTRODUCTION Xenon trioxide (XeO3) is the first xenon oxide to have been synthesized and structurally characterized. The colorless solid is very deliquescent and highly shock- and temperature-sensitive, releasing 402 ± 8 kJ mol−1 1 of energy at the slightest provocation. Pitzer2 predicted that xenon oxides would be unstable and possibly explosive. Bartlett and Rao3 were the first to experimentally verify this prediction when a white crystalline powder that had been isolated as the product resulting from the hydrolysis of XeF4 detonated. The event unfortunately resulted in the hospitalization of both researchers. At the time, the identity of the redox disproportionation product, XeO3, was unknown. The product was initially formulated by Bartlett and Rao3 as either Xe(OH)4 or XeO2·2H2O. Two independent research groups subsequently synthesized the compound and showed it to be XeO3 by vibrational spectroscopy4 and singlecrystal X-ray diffraction. 5 The X-ray crystal structure unambiguously established the trigonal-pyramidal geometry of XeO3 (C3v),5 which had been previously predicted by the valence-shell electron-pair repulsion model of molecular geometry.6 The crystals used for the X-ray structure determination of XeO3 were grown by the slow evaporation of an aqueous solution of XeO3 that had been prepared by hydrolysis of XeF4. The crystal morphology was described as rod-shaped.5 A second study described additional crystal morphologies of XeO3 that were obtained by hydrolysis of XeF6 in the atmosphere at different relative humidities; however, powder X-ray diffraction studies and single-crystal X-ray structure determinations of these phases were not pursued.4 Xenon trioxide was shown to undergo decomposition to Xe and O2 © XXXX American Chemical Society
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RESULTS AND DISCUSSION Synthesis of XeO3 and Crystal Growth. Xenon trioxide was synthesized by the hydrolyses of XeF6 and XeF4 according to eqs 1 and 2, respectively. The hydrolysis of XeF4, which proceeds through the bright-yellow intermediate, XeO2,11 yields a larger molar excess of HF with respect to XeO3. XeF6 + 3H 2O → XeO3 + 6HF
(1)
6XeF4 + 12H 2O → 2XeO3 + 3O2 + 4Xe + 24HF
(2)
As a consequence of redox disproportionation, only one-third of the xenon is converted to XeO3 in eq 1. Disproportionation Received: October 5, 2016
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DOI: 10.1021/acs.inorgchem.6b02371 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. Summary of Crystal Data and Refinement Results for α-, β-, and γ-XeO3 space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z (molecules/unit cell) MW (g mol−1) calcd density (g cm−3) T (°C) μ (mm−1) reflns collected BASF R1a wR2b a
α-XeO3
β-XeO3
P212121 5.19230(10) 6.10570(10) 8.0059(2) 90 90 90 253.808(9) 4 197.30 4.692 −173 13.297 17188 0.04028 0.0145 0.0319
R3 4.15170(12) 4.15170(12) 4.15170(12) 80.1860(13) 80.1860(13) 80.1860(13) 68.741(6) 1 197.30 4.331 −173 12.274 4581 0.50508 0.0254 0.0615
γ-XeO3 R3c 11.04930(10) 11.04930(10) 15.4569(2) 90 90 120 1634.27(4) 24 197.30 4.372 −173 12.391 22727 0.31105 0.0163 0.0340
R3c 11.1998(5) 11.1998(5) 15.5598(8) 90 90 120 1690.27(17) 24 197.30 4.228 17 11.980 10557 0.37634 0.0185 0.0365
1
R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)] /2.
proceeds through the formation and decomposition of XeIV and XeII intermediates.12−16 Hydrogen fluoride was initially removed by either co-evaporation with H2O or by neutralization with MgO followed by centrifugation of precipitated MgF2. Because small amounts of residual magnesium salts contaminated the solutions and interfered with crystallization, coevaporation of HF and H2O without neutralization was preferred. In the course of a study of adduct formation between XeO3 and alkyl nitriles,17 a solution of XeO3 in CH3CH2CN was allowed to evaporate from an FEP fluoroplastic reaction vessel in a fume hood. This resulted in block-shaped crystals of γXeO3 having a large hexagonal unit cell volume [1690.3(2) Å3, 17 °C]. Upon cooling of the single crystal used for structure determination from 17 to −173 °C, the unit cell contracted to 1634.27(4) Å3 with no evidence for a phase transition. Crystals grown by the evaporation of an aqueous XeO3 solution in a fume hood had a morphology and unit cell dimensions identical to those of crystals grown by the evaporation of a CH3CH2CN solution of XeO3. Although a large proportion of the crystalline samples grown in fume hood environments spontaneously detonated, presumably because of air turbulence, it was possible to show that these crystals also had the same unit cell parameters as γ-XeO3. Slower, more controlled crystal growth was achieved by placing aqueous XeO3 solutions inside partially sealed plastic containers. This allowed very slow water evaporation to take place and resulted in well-defined, block-shaped crystals of βXeO3. The crystals were found to have a small unit cell volume [68.741(6) Å3] when compared with that previously reported for orthorhombic α-XeO3 (261.77 Å3).5 In the present work, initial attempts to crystallize α-XeO3 from aqueous XeO3 solutions failed. In order to replicate the crystallization conditions used in the original structure report,5 XeF4 was used to generate XeO3 (eq 2) in aqueous HF. However, difficulties controlling the humidity levels frequently caused crystalline samples to absorb atmospheric moisture and redissolve. In addition, the ratio of XeF4 to H2O used for crystal growth was not provided in the original paper which contributed to difficulties encountered in the growth of αXeO3 crystals. Crystalline α-XeO3 was reliably obtained by
dissolution of solid XeO3 in 48% HF followed by slow evaporation in polystyrene Petri dishes inside a fume hood. Long, needle-shaped crystals were obtained that had the previously reported unit cell.5 No phase transition was observed upon cooling a single crystal of α-XeO3 from room temperature to −173 °C. X-ray Crystal Structures of α-, β-, and γ-XeO3. Data collection details and other crystallographic information pertaining to the three phases of XeO3 are provided in Tables 1 and S1. α-XeO3. The structure of the P212121 phase obtained in this study at −173 °C (Figure 1) is in agreement with the room-
Figure 1. Low-temperature single-crystal X-ray structure of α-XeO3 (P212121). Thermal ellipsoids are shown at the 50% probability level.
temperature structure previously reported by Templeton et al.5 except that the bond length and bond angle uncertainties are significantly improved and all atoms are now refined anisotropically. As expected, the calculated density of α-XeO3 is significantly greater at −173 °C (4.692 g cm−3) than that reported for the room-temperature structure (4.55 g cm−3).5 The asymmetric unit consists of a crystallographically unique XeO3 molecule. Each Xe atom (Table 2) has three Xe---O contacts with three adjacent XeO3 molecules that avoid the Xe valence electron lone pair (VELP). The variations among the three Xe---O contacts [2.6914(11), 2.8387(12), and 2.8403(11) Å] and their Xe---O---Xe contact angles B
DOI: 10.1021/acs.inorgchem.6b02371 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Table 2. Selected Bond Lengths (Å), Contact Distances (Å), Angles (deg), and Contact Angles (deg) of α-, β-, and γ-XeO3
From ref 5; X-ray data were obtained at room temperature. bFrom the present work; X-ray data were obtained at −173 °C. cValues obtained at 17 °C are given in Table S2.
a
γ-XeO3. The unit cell of γ-XeO3 belongs to the R3c space group and an asymmetric unit that contains two crystallographically unique Xe atoms (Figure 3). Each Xe atom has
[74.03(3), 101.24(3), and 107.25(2)°] are significantly greater than those of rhombohedral β-XeO3 and γ-XeO3 (vide infra). Variations among the primary Xe−O bond lengths [1.7558(11), 1.7669(10), and 1.7801(11) Å] and the primary O−Xe−O bond angles [100.51(5), 100.55(5), and 105.09(6)°] are also greater than those of the rhombohedral phases. The differences likely arise from the range of O-bridged coordination modalities that occur in α-XeO3. In both rhombohedral phases, all three O atoms have contacts with Xe atoms, whereas only two O atoms have contacts with Xe atoms [Xe---O = 2.6914(11), 2.8387(12), and 2.8403(11) Å] in the orthorhombic phase. An additional long Xe---O contact [3.272(1) Å], which is significantly shorter than the sum of the Xe and O van der Waals radii (3.68 Å), occurs along the C3-axis and between the three shorter Xe---O contacts where the Xe VELP is located. β-XeO3. The asymmetric unit of β-XeO3 contains only one crystallographically unique Xe−O bond (Figure 2) with a bond
Figure 3. Low-temperature single-crystal X-ray structure of γ-XeO3 (R3c). Thermal ellipsoids are shown at the 50% probability level.
three Xe---O contacts [2.716(2)−2.742(2) Å] with nearestneighbor XeO3 molecules. The Xe---O contacts avoid the Xe VELP, and the O−Xe−O angles [101.72(10)−102.52(11)°] are more open than the O---Xe---O angles [80.9(1) and 82.87(7)°], which is consistent with the spatial requirements that result from XeO double bond−double bond repulsions. The Xe(1) atom has contacts exclusively with three symmetryrelated O atoms [O(4)], whereas Xe(2) has contacts with three crystallographically unique O atoms [O(1), O(2A), and O(3A)]. The Xe−O primary bonds range from 1.763(2) to 1.766(2) Å and are equal to those of β-XeO3 [1.768(4) Å]. The densities calculated from the unit cell parameters (4.372 g cm−3 at −173 °C and 4.228 g cm−3 at 17 °C) are significantly less than those of α-XeO3 (4.692 g cm−3 at −173 °C; 4.55 g cm−3 at room temperature4) and β-XeO3 (4.331 g cm−3 at −173 °C). Raman Spectra of α- and γ-XeO3. The Raman spectra of solid α- and γ-XeO3 were recorded at 20 °C (Figures 4 and 5). The observed and calculated frequencies and mode descriptions are provided in Table 3. The assignments are based on the calculated gas-phase frequencies and aqueous solution Raman spectra of XeO3 and do not take into account stretching, deformation, and torsion modes arising from the XeO---Xe bridges of their extended structures. Attempts to record the Raman spectrum of β-XeO3 were made, but the spectrum was indistinguishable (see Figure S2) from that of γ-XeO3; it is therefore likely that β-XeO3 rapidly converts to γ-XeO3 under laser irradiation. Although recording of the Raman spectra at lower temperatures may impede the phase transition, the low-
Figure 2. Low-temperature single-crystal X-ray structure of β-XeO3 (R3). Thermal ellipsoids are shown at the 50% probability level.
length of 1.768(4) Å. Each Xe atom has three equivalent Xe---O contacts [2.754(4) Å], which are slightly longer than those of γ-XeO3 [2.716(2)−2.742(2) Å]. The O−Xe−O bond angle [102.9(2)°] is comparable to those of γ-XeO 3 [101.72(10), 101.93(11), and 102.52(11)°], where the O--Xe---O contact angle [72.3(2)°] is significantly less than those of α-XeO3 [74.03(3), 101.25(3), and 107.24(2)°] and γ-XeO3 [80.2(1)−81.7(1)°]. The XeO3 units of β-XeO3 are all orientated in the same direction (Figure 2), consistent with the polar space group R3 of this phase. The densities of the rhombohedral phases (β-XeO3, 4.331 g cm−3; γ-XeO3, 4.372 g cm−3) are very similar. C
DOI: 10.1021/acs.inorgchem.6b02371 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
fluoride was removed by passing XeO3 solutions through a Zr[HPO4]/ZrO column.8 For example, the νsym(PO4) mode of aqueous [Ca]3[PO4]2 occurs at 938 cm−1,18 which is very close to the band at 933 cm−1. These bands were not observed in 2.0 and 5.0 M solutions prepared in the course of the present study. The Raman spectrum of a 5.0 M aqueous solution of XeO3 (Figure 6) has frequencies that are intermediate with
Figure 4. Raman spectrum of α-XeO3 at 20 °C. The symbol † denotes an instrumental artifact.
Figure 6. Raman spectrum of a frozen concentrated aqueous solution of XeO3 at −140 °C. Symbols denote FEP sample tube lines (*) and an instrumental artifact (†).
respect to those of 2.0 M aqueous XeO3 and those of the solid, suggesting aggregate formation. The vibrational frequencies obtained for the 2.0 M aqueous solution are in good agreement with those calculated for gas-phase XeO3 (Table 3), suggesting that XeO3 does not undergo significant aggregation at lower concentrations. The number of bands observed in the Raman spectra of αXeO3 and γ-XeO3 exceeds the number of bands predicted for gas-phase XeO3. The very weak bands at 343(3), 777 (sh), 820(2), and 838(1) cm−1 in the Raman spectrum of γ-XeO3 occur at frequencies that are very similar to those obtained for 5.0 M aqueous XeO3 (Table 3) and may be attributed to small amounts of dissolved XeO3 that result from the absorption of atmospheric moisture. The remaining bands are attributed to vibrational coupling within the crystallographic unit cell (factorgroup splitting; vide infra). The possible formation of XeO3 hydrates through hydrogen-bonding or O---Xe donor−acceptor interactions between H2O and XeO3 was also considered as a possible source of the additional vibrational bands observed in the spectra of solid XeO3. However, attempts to crystallize such
Figure 5. Raman spectrum of γ-XeO3 at 20 °C. The symbol † denotes an instrumental artifact.
temperature Raman spectra of the solid phases of XeO3 were not recorded because of the likelihood of detonations occurring upon cooling. When suddenly cooled to −196 °C, solid samples consistently detonated because of thermal shock. Xenon trioxide in the gas phase possesses six vibrational modes under C3v symmetry which belong to the irreducible representations 2A1 + 2E, where the A1 and E modes are both Raman- and IR-active. The Raman spectrum of XeO3, which had only been previously reported for a 2.0 M aqueous solution, consisted of four bands at 833 [ν3(E), νasym], 780 [ν1(A1), νsym], 344 [ν2(A1), δumbrella], and 317 [ν4(E), δasym] cm−1.8 Three additional weak bands at 460, 524, and 933 cm−1 were attributed to minor unknown species. These bands are presumably due to impurities that were introduced when
Table 3. Selected Experimental and Calculated Raman Frequencies (cm−1) and Assignments of XeO3 α-XeO3a,b 857(8), 819(26), 809(63) 764(100), 754, sh 351(1), 323(3) 311(42), 297(11) 146(5) 111(10), 99(16)
γ-XeO3a,b 855(1), 838(1), 827(
