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Xenon Trioxide Adducts of O - Donor Ligands; [(CH 3 ) 2 CO] 3 XeO 3 ,
[(CH 3 ) 2 SO] 3 (XeO 3 ) 2 , (C 5 H 5 NO) 3 (XeO 3 ) 2 , and [(C 6 H 5 ) 3 PO] 2 XeO 3
Katherine M. Marczenko,[a,b]^ James T. Goettel,[a,b]^ and Gary J. Schrobilgen*[b]
Dedication ((optional)) Abstract: Oxygen coordination to the Xe(VI) atom of XeO 3 was observed in its adducts with triphenylphosphine oxide, dimethylsulfoxide, pyridine-N-oxide, and acetone. The crystalline adducts were characterized by low-temperature, single-crystal X-ray diffraction and Raman spectroscopy. Unlike solid XeO 3 , which detonates when mechanically or thermally shocked, the solid [(C 6 H 5 ) 3 PO] 2 XeO 3 , [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 , and (C 5 H 5 NO) 3 (XeO 3 ) 2 adducts are insensitive to mechanical shock, but undergo rapid deflagration when ignited by a flame. Both [(C 6 H 5 ) 3 PO] 2 XeO 3 and (C 5 H 5 NO) 3 (XeO 3 ) 2 are air-stable whereas [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 slowly decomposes over several days and [(CH 3 ) 2 CO] 3 XeO 3 undergoes adduct dissociation at room temperature. The xenon coordination sphere of [(C 6 H 5 ) 3 PO] 2 XeO 3 is a distorted square pyramid which provides the first example of a five-coordinate XeO 3 adduct. The xenon coordination spheres of the remaining adducts are distorted octahedra comprised of three Xe---O secondary contacts that are approximately trans to the primary Xe–O bonds of XeO 3. Quantum- chemical calculations were used to assess the Xe---O adduct bonds, which are predominantly electrostatic σ-hole bonds between the nucleophilic oxygen atoms of the bases and the σ-holes of the xenon atoms.
Introduction
X enon trioxide is a strong oxidant that rapidly oxidizes primary
and secondary alcohols to CO 2 and H 2 O.[1,^2 ]^ Solid XeO 3 readily detonates when subjected to mild mechanical or thermal shock, decomposing to Xe and O 2 gases with the liberation of 402 ±8 kJ mol–^1 of energy.[^3 ]^ In its solid-state structures, XeO 3 has three short Xe---O contacts between the oxygen atoms of neighboring XeO 3 molecules and the electrophilic XeVI^ atom.[^4 ] The electrostatic potential of the xenon atom at and in the vicinity of the C 3 v - axis of XeO 3 is positive and only slightly lower in energy than the three regions of maximum electrostatic potential, the σ-holes, which occur opposite to its polar-covalent Xe–O double bonds.[^5 −^7 ]^ The xenon–ligand bonds of XeO 3 adducts are best described as predominantly electrostatic, (weakly covalent) interactions between the highly electrophilic σ- holes of the Xe atom and the nucleophilic region of the electronegative ligand atom.[^5 ,^7 ,^8 ]^ Secondary bonding interactions occur approximately trans to the three primary Xe–O bonds, as observed for the N - coordinated pyridine and pyridinium salt adducts[^7 ]^ and nitrile adducts[^8 ]^ of XeO 3. In the case of (CH 2 CH 2 O) 5 XeO 3 , the σ-bonding interactions that occur between the electrophilic xenon atom and the five nucleophilic oxygen atoms of the crown ether ligand result in a xenon coordination number of 8, the highest coordination number observed thus far for XeVI^ in an XeO 3 adduct.[^5 ]^ Prior to the present study, there were no examples in which the number of secondary bonds with the xenon atom of XeO 3 was less than three. The 15-crown-5 adduct of XeO 3 , (CH 2 CH 2 O) 5 XeO 3 ,[^5 ]^ is thus far the only structurally documented example of XeO 3 coordinated to an organic oxygen base. Although early studies showed that primary and secondary alcohols quantitatively oxidized,[1]^ XeO 3 does not oxidize pure t - butanol, but dissolves to form solutions which are stable for up to six weeks.[2] Titrations of these solutions with M[ t - BuO] (M = K or Rb) yielded shock-insensitive precipitates having short-term stabilities whose formula weights suggested the formation of M[ t - BuO–XeO 3 ] t - BuOH. Structural characterizations of the species isolated and described in the latter study have not been forthcoming. In the present study, the syntheses and structural characterizations of XeO 3 coordination complexes of several E– O bonded ligands (E = C, S, N, P) provide further insights into the Lewis acid behavior of XeO 3. The complexes have been characterized in the solid-state by low-temperature, single- crystal X-ray diffraction, and Raman spectroscopy, and by gas- phase quantum-chemical calculations, which have been used to aid in the assignment of vibrational frequencies and to assess the relative strengths of the Xe---O adduct bonds.
Results and Discussion
Syntheses Solid XeO 3 detonates on contact with liquid DMSO and acetylacetone but readily dissolves in acetone without incident. Xenon trioxide is very soluble and stable in acetone for up to several months at room temperature and its solutions have proven useful in synthetic applications.[^5 ]^ When acetone solutions of XeO 3 were allowed to evaporate at room temperature, solid, unsolvated XeO 3 was recovered. Slow cooling of these solutions from 20 to −78 °C resulted in the formation of clear, colorless, block-shaped crystals of [(CH 3 ) 2 CO] 3 XeO 3 ( 1 ) (eq 1) which were stored at −78 °C to prevent acetone loss due to adduct dissociation. The adduct readily dissociated when slowly warmed to room temperature under dynamic vacuum to give solid XeO 3. However, when warmed to room temperature under ca. 1 atm of dry N 2 , the adduct dissociated to give a solution of XeO 3 in acetone. 3(CH 3 ) 2 CO + XeO 3 [(CH 3 ) 2 CO] 3 XeO 3 (1) [a] Both authors contributed equally. [b] K. M. Marczenko, Dr. J. T. Goettel, Prof. G. J. Schrobilgen Department of Chemistry, McMaster University Hamilton, ON L8S 4M1 (Canada) Email: schrobil@mcmaster.ca Supporting information and the ORCID identification number(s) for the authors of this article can be found under: −78 °C acetone
An aqueous solution containing a 1:6 molar ratio of XeO 3 :HF was mixed with DMSO at room temperature. Slow evaporation of the mixture yielded large, plate-shaped crystals of [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 ( 2 ) (eq 2). The crystalline product was significantly less shock-sensitive than solid XeO 3 , but slowly decomposed at room temperature to (CH 3 ) 2 SO 2 , Xe, and O 2 (eq
- over a period of several days. 3(CH 3 ) 2 SO + 2XeO 3 [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 (2) [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 3(CH 3 ) 2 SO 2 + 2Xe + 3 ⁄ 2 O 2 (3) Slow addition of an acetone solution of C 5 H 5 NO to solid XeO 3 resulted in detonation, however stable solutions of C 5 H 5 NO and XeO 3 in a 3:2 molar ratio ( vide supra ) were obtained by dissolution of the ligand in aqueous XeO 3. Evaporation of the solution yielded a large, rod-shaped crystal of (C 5 H 5 NO) 3 (XeO 3 ) 2 ( 3 ) (eq 3) which was insensitive to mechanical shock and was cut, without incident, into smaller fragments for an X-ray crystal structure determination. The crystalline adduct underwent rapid deflagration when brought into contact with a flame. 3(C 5 H 5 )NO + 2XeO 3 (C 5 H 5 NO) 3 (XeO 3 ) 2 (4) The reaction between XeO 3 and (C 6 H 5 ) 3 P in CH 3 CN afforded a fine, white precipitate of [(C 6 H 5 ) 3 PO] n XeO 3 ∙(CH 3 CN) x. Attempts to grow crystals of the solvate that were suitable for an X-ray crystal structure determination were unsuccessful, however layering of a CH 3 CN solution of P(C 6 H 5 ) 3 onto an aqueous solution of XeO 3 at room temperature resulted in the formation of clear, colorless, block-shaped crystals of [(C 6 H 5 ) 3 PO] 2 XeO 3 ( 4 ) that were stable at room temperature (eqs 5 and 6). The reaction between XeO 3 and P(C 6 H 5 ) 3 in acetone at
- 78 oC also yielded ( 4 ) as large, block-shaped crystals. (C 6 H 5 ) 3 P + XeO 3 (C 6 H 5 ) 3 PO + Xe + O 2 (5) 2 (C 6 H 5 ) 3 PO + XeO 3 [(C 6 H 5 ) 3 PO] 2 XeO 3 ( 6 ) The reaction of XeO 3 with 1,2-bis(diphenylphosphino)- ethane in CH 3 CN or acetone resulted in mixtures of decomposition products. Raman spectroscopy confirmed that the products did not contain XeO 3. However, when the reaction was carried out at 0 °C by initially layering a CH 3 CN solution of 1,2-bis(diphenylphosphino)ethane onto an aqueous solution of XeO 3 , a crystalline precipitate formed (eq 7 ) which was stable at room temperature and consisted of small, block-shaped crystals of [(C 6 H 5 ) 2 POCH 2 ] 2 XeO 3 ∙CH 3 CN (Figure S1). [(C 6 H 5 ) 2 PCH 2 ] 2 + 2XeO 3 [(C 6 H 5 ) 2 POCH 2 ] 2 XeO 3 ∙CH 3 CN
- Xe + ½O 2 ( 7 ) The extreme shock sensitivity of solid XeO 3 is largely attributable to extended ---O 3 Xe---O(XeO 3 ) networks that occur in all three solid phases of XeO 3 [5]^ which provide an efficient means to propagate the detonation shock wave throughout the crystal lattice. Although the shock-sensitivity of ( 1 ) could not be explicitly tested due to adduct dissociation and loss of acetone upon warming to room temperature, at no point during the synthesis, Raman spectral acquisition, crystal isolation, and X- ray structure determination did the crystalline adduct detonate. Crystalline samples of ( 2 ), ( 3 ), and ( 4 ) were shock-insensitive but underwent rapid deflagration when ignited. As in the cases of the stable XeO 3 N - base adducts, (C 6 H 5 N) 3 XeO 3 ,[^7 ]^ (4- (CH 3 ) 2 NC 5 H 4 N)XeO 3 ∙H 2 O,[^7 ]^ and the O - base adduct, (CH 2 CH 2 O) 5 XeO 3 ,[^5 ]^ the crystal structures of ( 1 ), ( 2 ), ( 3 ), and ( 4 ) lack extended ---O 3 Xe---O(XeO3) networks ( vide infra ). The structural units of these adducts are well isolated which also significantly diminishes their shock sensitivities. Rapid oxidation of soft Lewis bases was observed at room temperature when the Lewis base center is phosphorus, whereas slower oxidation occurred (ca. 12 h) at low temperatures (– 78 oC) or over several days at room temperature when the Lewis base center is sulfur. X-ray Crystallography A summary of X-ray crystal data and refinement results is provided in Table 1. The single-crystal X-ray structures of [(CH 3 ) 2 CO] 3 XeO 3 ( 1 ), [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 ( 2 ), (C 5 H 5 NO) 2 (XeO 3 ) 2 ( 3 ), and [(C 6 H 5 ) 3 PO] 2 XeO 3 ( 4 ) are shown in Figures 1 – 4, respectively. The crystal structure of [(C 6 H 5 ) 2 POCH 2 ] 2 XeO 3 ∙CH 3 CN ( 5 ) is provided in Figure S1. Important bond lengths and angles are listed in Table 2, and more extensive lists of geometric parameters are given in Tables S1–S4 of the Supporting Information. The primary Xe–O bond lengths (1.762(1)–1.779(1) Å) and O–Xe–O bond angles (100.20(5)–103.2(2)°) lie within the bond length and bond angle ranges of the three solid phases of XeO 3 ((1.7558(11)–1.7801(11) Å and 100.51(5)−105.09(6)°).[^4 ]^ The trans- O−Xe---O contact angles of the adducts described in this study lie within the range 152.46(5)−172.80(4)° and are comparable to those of σ-hole bonded Group 14–17 adducts (155−180°).[^6 ]^ The xenon coordination spheres of ( 1 )–( 3 ) are distorted octahedra, whereas that of compound ( 4 ) is a distorted square pyramid that may be described as being derived from a distorted octahedron by removal of one secondary Xe---O bonding interaction from the octahedral coordination sphere of xenon. The empirical bond valence method of I. D. Brown[^9 ]^ was used to determine the contributions of individual Xe---O contacts to the total bond valence of xenon in compounds ( 1 ), ( 2 ), ( 3 ), and ( 4 ) (Table S5). Inclusion of all Xe---O contacts provides bond valence values that are very close to the ideal value of 6 for XeVI. The Xe---O bond valences, which span 0.089–0. v.u., are consistent with very weak covalent bonding between O and Xe, in accordance with their experimental bond lengths (Table 2). [(CH 3 ) 2 CO] 3 XeO 3 (1). The structural unit of ( 1 ) consists of two crystallographically unique [(CH 3 ) 2 CO] 3 XeO 3 units (Figure 1.1) in which the XeO 3 molecules are each coordinated to three acetone molecules through Xe---O bonds (2.738(2)–2.778(2) Å). The C–O bond lengths (1.210(3)–1.223(4) Å) show little variation and are slightly elongated with respect to those of solid acetone (1.208(3), 1.209(3) Å).[1^0 ]^ The ability of acetone to form a 3: adduct with Xe contrasts with RCN (R = CH 3 , CH 2 CH 3 ), which form RCNXeO 3 and (RCN) 2 XeO 3 but not (RCN) 3 XeO 3 .[^8 ]^ These RT RT RT H 2 O HF(aq) RT CH 3 CN/H 2 O
Figure 1. The structural units in the crystal structures of [(CH 3 )CO] 3 XeO 3 ( 1 ), [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 ( 2 ) viewed (a) perpendicular to and (b) along the Xe∙∙∙Xe axis, (C 5 H 5 NO) 3 (XeO 3 ) 2 ( 3 ), and [(C 6 H 5 ) 3 PO] 2 XeO 3 ( 4 ); thermal ellipsoids are shown at the 50% probability level. which belonged to the orthorhombic Cc space group ( Z = 4). Other than two Xe---O ligand bonds, there are no significant secondary contacts with the Xe atom of XeO 3 (Figure 1.4). Coordination of less than three ligands to XeO 3 has not been previously encountered. The low coordination number is a consequence of the steric bulk of the (C 6 H 5 ) 3 PO ligands which prevents intermolecular Xe---O bonding interactions with neighboring XeO 3 groups. The resulting xenon coordination sphere may be described as a distorted square pyramid. The lower coordination number results in Xe---O bonds (2.605(3) and 2.591(3) Å) that are significantly shorter than those of solid XeO 3 (2.678(2)–2.8387(12) Å). The shorter average primary Xe–O bond lengths of ( 4 ) result in a slightly higher XeVI^ bond valence contribution (5.601 v.u.) than the Xe–O bonds of (C 6 H 5 NO) 3 (XeO 3 ) 2 (5.536 v.u.). This compensatory shortening permits attainment of a total XeVI^ bond valence of 5.999 for ( 4 ) despite only two Xe---O bonding interactions. The P–O bonds (1.504(3), 1.508(3) Å) of the (C 6 H 5 ) 3 PO ligands are elongated relative to that of solid (C 6 H 5 ) 3 PO (1.484(1) Å) whereas the P–C bonds (1.804(6)–1.809(6) Å) do not differ significantly from those of solid (C 6 H 5 ) 3 PO (1.798(2)– 1.804(2) Å).[^19 ]^ The P–O bond of the azaborine adduct-cation of (C 6 H 5 ) 3 PO ([1,2-C 2 H 4 NB-OP[(C 6 H 5 ) 3 ]+, 1.5563(13) Å)[^18 ]^ is significantly longer than those of the XeO 3 adduct. Hirshfeld surfaces provide an unbiased means to identify close contacts in complex crystal structures.[2^0 ]^ The Hirshfeld surfaces of XeO 3 and (C 6 H 5 ) 3 PO in ( 4 ) were mapped with the d norm function[2^0 ]^ and are depicted in Figures 2 and S5, respectively. Contacts shorter than the sums of the xenon and oxygen van der Waals radii appear as circular red regions on a light blue surfaces. The two large, intense red areas on the Hirshfeld surfaces of ( 4 ) indicate that the adduct has only two Xe---O secondary bonds. Additional smaller, light red areas on these surfaces arise from O---C(H) hydrogen bonds between neighboring [(C 6 H 5 ) 3 PO] 2 XeO 3 molecules in the crystal lattice. Raman Spectroscopy Raman frequencies and assignments are provided for [(CH 3 ) 2 CO] 3 XeO 3 ( 1 ), [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 ( 2 ), (C 5 H 5 NO) 3 (XeO 3 ) 2 ( 3 ), (C 6 H 5 ) 3 PO) 2 XeO 3 ( 4 ), and [(C 6 H 5 ) 2 POCH 2 ] 2 XeO 3 ∙CH 3 CN ( 5 ) in Tables S6–S 9 , respectively, and the corresponding Raman spectra are depicted in Figures S 6 – S 10 of the Supporting Information. Vibrational assignments were aided by comparison with the published frequency assignments for XeO 3 [^4 ]^ and its nitrogen-base adducts,[^5 ,^7 ,^8 ]^ and with the calculated vibrational frequencies and intensities of the energy-minimized, gas-phase structures of ( 1 )–( 4 ) (Figures S11– 13 ; see Computational Results). Overall, the frequencies are better reproduced by the B3LYP method. The ligand modes were assigned by
Figure 2. The Hirshfeld surface of XeO 3 in [(C 6 H 5 ) 3 PO] 2 XeO 3. Two views of the surface are provided which highlight the five-coordinate xenon atom of XeO 3 and its secondary bonding interactions with the (C 6 H 5 ) 3 PO ligands. The Hirshfeld surfaces of the (C 6 H 5 ) 3 PO ligands are depicted in Figure S5. comparison with the published assignments for (CH 3 )CO,[2^1 ] (CH 3 ) 2 SO,[2^2 ]^ C 5 H 5 NO,[2^3 ]^ (C 6 H 5 ) 3 PO.[2^4 ]^ The Raman spectra of ( 2 )–( 5 ) were recorded in air at 20 oC, which also confirmed the room-temperature stabilities of the solid compounds and their insensitivities to atmospheric moisture. Although ( 2 ) slowly decomposed at room temperature by oxidation of the ligand to (CH 3 )SO 2 (eq 3), it was possible to obtain its Raman spectrum at room temperature without appreciable decomposition. The Raman spectrum of ( 1 ) was recorded at – 78 oC in a closed FEP sample tube under nitrogen to prevent adduct dissociation. The vibrational modes of the coordinated XeO 3 molecules of ( 1 )–( 4 ) summarized in Tables S6–S9 and were assigned by comparison with those obtained for 2.0 M[2^5 ]^ and 5.0 M[^4 ] aqueous XeO 3 [^25 ]^ and by comparison with the calculated frequencies and intensities of the gas-phase structures (Tables S6–S9, Figures S11–13). As previously shown for (CH 2 CH 2 O) 5 XeO 3 [5]^ and the N - base adducts of XeO 3 ,[^7 ,^8 ]^ coordination of XeO 3 to a Lewis base results in a shift of the most intense band, s(A 1 ), to lower frequency (778 ( 1 ); 769 ( 2 ); 761 ( 3 ); 776 ( 4 ) 772 ( 5 ) cm–^1 ) relative to that of 2.0 M[2^5 ]^ and 5.0 M[^4 ]^ aqueous XeO 3. The frequencies of the asymmetric XeO 3 stretching modes (as(E), 804, 830, 835 ( 1 ); 819, 826, 833 ( 2 ); 803, 810, 819 ( 3 ); 831 ( 4 ) 788, 826 ( 5 ) cm–^1 ) and bending modes (umb(A 1 ), (343 ( 1 ); 342, 350 ( 2 ); 342 ( 3 ) ; 345 ( 4 ) 303 ( 5 ) cm–^1 ), as(E), (314 ( 1 ); 307, 311 ( 2 ); 311 ( 3 ); 253, 289, 307 ( 4 ) 269 ( 5 ) cm–^1 )) are also shifted relative to those of XeO3(aq). Electronic structure calculations predict significant coupling of the asymmetric stretches of both XeO 3 molecules in ( 2 ), the symmetric and asymmetric stretches of both XeO 3 molecules in ( 3 ), and the umbrella and asymmetric bending modes of both XeO 3 molecules in ( 2 ) and ( 3 ). The split bands assigned to the asymmetric stretches result from in-phase and out-of-phase coupling. Although the calculations predict that both symmetric XeO 3 stretching bands and umbrella XeO 3 bending bands should appear in the Raman spectrum of ( 3 ), and ( 2 ) and ( 3 ), respectively, only a single band was observed and assigned to each of these modes. The complexation shifts obtained from the Raman spectra of the (PO) stretching mode of (C 6 H 5 ) 3 PO Lewis acid-base adducts were previously shown to be approximately proportional to the strengths of the Lewis acids.[^28 ]^ The bands assigned to (PO) of [(C 6 H 5 ) 3 PO] 2 XeO 3 (1155 cm−^1 ) are shifted 30 cm−^1 to lower frequency relative to free (C 6 H 5 ) 3 PO (1185 cm−^1 ), but are shifted by less than that for ((C 6 H 5 ) 3 PO) 2 SbCl 3 ( 1135 cm−^1 ),[^27 ] indicating that XeO 3 is a weaker Lewis acid than SbCl 3. The bands assigned to the (PO) of ((C 6 H 5 ) 3 PO) 3 BiCl 3 (1154, 1167 cm−^1 )[^27 ]^ and ((C 6 H 5 ) 3 PO) 2 BiBr 3 (1140, 1152 cm−^1 ) suggest the Lewis acid strength of XeO 3 is similar to that of BiCl 3 but less than that of BiBr 3. Computational Results The energy-minimized geometries (Figures S11–S13), with all frequencies real, were calculated for [(CH 3 ) 2 CO] 3 XeO 3 ( 1 ) (B3LYP, C 1 symmetry), [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 ( 2 ) (B3LYP/APFD, C 1 symmetry), (C 5 H 5 NO) 3 (XeO 3 ) 2 ( 3 ) (B3LYP, C 1 symmetry; APFD, C 2 symmetry), and (Ph 3 PO) 2 XeO 3 ( 4 ) (B3LYP, C 1 symmetry) using the Def2-TZVP basis set. Selected calculated bond lengths and bond angles for ( 1 )–( 4 ) are provided in Tables S1–S4. The calculated geometric parameters are in overall good agreement with values obtained from the crystal structures. As previously shown for (CH 2 CH 2 O) 5 XeO 3 ,[^5 ]^ the calculated Xe–O and Xe---O distances are in better agreement with the experimental values when calculated by the APFD method than when calculated by the B3LYP method, which overestimates these bond lengths. However, it was necessary to optimize the geometries of ( 1 ) and ( 4 ) by use of the B3LYP method owing to convergence failures when the APFD method was used. Consequently, the calculated Xe–O and Xe---O bond distances were slightly overestimated (Xe–O: ( 1 ) exptl, 1.762(1)–1.767(1), calcd, 1.784–1.786 Å; ( 4 ) exptl, 1.763(4)–1.772(4), calcd, 1.783– 1.785 Å; Xe---O: ( 1 ) exptl. 2.738(1)–2.780(1), calcd, 2.864– 2.866 Å; ( 4 ) exptl, 2.591(4)–2.605(3), calcd, 2.723–2.731 Å). The gas-phase structure of ( 2 ) reproduces the eclipsed conformation of the XeO 3 molecules in its crystal structure and the calculated Xe∙∙∙Xe distance (3.968 Å), which is slightly shorter than the experimental distance (3.997(1) Å). The calculated Xe–O (1.762–1.780 Å) and Xe---O (2.675–2.825 Å) bond lengths are in excellent agreement with their respective experimental values (Xe–O, 1.763(1)–1.772(1); Xe---O, 2.682(1)
electrostatic Xe---O bonding interactions between the nucleophilic oxygen atoms of the bases and the electrophilic σ- holes of the xenon atoms.
Experimental Section
Caution! Solid XeO 3 detonates when subjected to mild thermal or mechanical shock. Great care must be taken during the syntheses and handling of XeO 3 and its adducts. For these reasons, the quantity of XeO 3 used for each synthesis was limited to 5–15 mg. Reaction vessels used in this study were fabricated from FEP (hexafluoropropylene, tetrafluoroethylene copolymer) fluoroplastic, which is inert to attack by HF, strong oxidants, and fluorinating agents; and generally does not shatter to produce sharp shards when a detonation occurs at the reagent scales used in this study. Appropriate protective equipment (face shield, leather gloves, ear protection) must be used preparing and handling XeO 3 and its adducts. Apparatus and Materials. All volatile, air-sensitive, corrosive fluorides were handled on metal vacuum lines constructed of nickel, 316 stainless steel, and FEP fluoroplastic. Xenon hexafluoride was prepared by reaction of xenon (99.995%, Air Products and Chemicals, Inc.) and F 2 (98+%, Air Products and Chemicals, Inc.) similar to the method described by Malm and Chernick.[^28 ]^ Small amounts of XeF 4 impurity, identified by two bands of nearly equal intensity at 502 and 543 cm−^1 in the Raman spectrum, also formed XeO 3 when hydrolyzed[^29 ]^ and therefore did not interfere with the synthesis of XeO 3. An aqueous solution of XeO 3 was prepared by hydrolysis of XeF 6 as previously described.[^4 ]^ An aliquot (ca. 10 mg, 0.056 mmol XeO 3 ) of the resulting solution, XeO 3 ∙6HF(aq), was transferred into a 3 × 3 cm FEP tray by use of a pipette fabricated from FEP tubing. Water and HF were removed from XeO 3 by evaporation overnight in a fume hood to give solid XeO 3 which was transferred to a plastic (HDPE) desiccator and dried over 4 Å molecular sieves for 2−3 h to remove surface moisture. [(CH 3 ) 2 CO] 3 XeO 3 (1). In a typical synthesis (eq 1), acetone (ca. 0.2 mL) was cautiously pipetted onto dry XeO 3 (ca. 6 mg, 0.03 mmol) in an FEP tray. The solid XeO 3 rapidly dissolved to give a clear, colorless solution which was then pipetted into a ¼-in FEP reactor. Clear, colorless, block- shaped crystals of ( 1 ) grew upon slow cooling from 20 to −78 °C. The solvent was removed under dynamic vacuum at −78 to −60 °C and the reactor was backfilled at −78 oC with N 2 as soon as solvent removal was complete. The sample was stored at −78 °C to prevent adduct dissociation and acetone loss. [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 (2). In a typical synthesis (eq 2), a stoichiometric excess of (CH 3 ) 2 SO (ca. 0.2 mL, 2.8 mmol) was mixed with an aliquot of aqueous XeO 3 ∙6HF (ca. 0.2 mL, 0.056 mmol of XeO 3 ) which had been obtained by hydrolysis of XeF 6 at room temperature. Slow evaporation of the mixture at room temperature inside a fume hood yielded large, colorless, plate-shaped crystals of ( 2 ). The crystalline material had a waxy appearance and was shock insensitive, but slowly decomposed to (CH 3 ) 2 SO 2 , Xe, and O 2 (eq 3) at room temperature over a period of several days. (C 5 H 5 NO) 3 (XeO 3 ) 2 (3). In a typical synthesis (eq 4), water (ca. 0.2 mL) was pipetted onto dry XeO 3 (ca. 12 mg, 0.067 mmol) in an FEP tray. An aqueous solution of C 5 H 5 NO (ca. 7 mg, 0.07 mmol) was mixed with an aqueous solution of XeO 3. Evaporation of the resulting solution yielded a single, rod-shaped crystal of (C 5 H 5 NO) 3 (XeO 3 ) 2 (1.0 × 0.2 × 0.2 cm^3 ). The crystal was insensitive to mechanical shock and was easily cut into smaller pieces for crystal mounting. Crystalline (C 5 H 5 NO) 3 (XeO 3 ) 2 underwent rapid deflagration when brought into contact with a flame. [(C 5 H 5 ) 3 PO] 2 XeO 3 (4). In a typical synthesis (eqs 5 and 6), water (ca. 0. mL) was pipetted onto dry XeO 3 (ca. 10 mg, 0.056 mmol) in an FEP tray and transferred to a ¼-in FEP reaction vessel by use of an FEP pipette. A CH 3 CN solution of (C 6 H 5 ) 3 P (12 mg, 0.067 mmol) was layered onto an aqueous solution of XeO 3 at 0 °C. The reaction mixture was allowed to stand undisturbed overnight during which time clear, colorless, block- shaped crystals of ( 4 ) grew. The latter were separated from the mother liquor by removal of the supernatant with an FEP pipette. The dry crystals were stable at room temperature. [(C 6 H 5 ) 3 PO] n XeO 3 ∙ x CH 3 CN. Triphenylphosphine (8.5 mg, 0.032 mmol) was dissolved in a CH 3 CN solution of XeO 3 [^8 ]^ (5 mg, 0.03 mmol) at −10 °C. Slow cooling afforded a fine white precipitate which was shown by Raman spectroscopy to be consistent with the formation of [(C 6 H 5 ) 3 PO] n XeO 3 ∙ x CH 3 CN. Attempts to grow crystals suitable for an X- ray crystal structure determination were unsuccessful. [(C 6 H 5 ) 2 POCH 2 ] 2 XeO 3 ∙CH 3 CN (5). In a typical synthesis (eq 7), water (ca. 0.2 mL) was pipetted onto dry XeO 3 (ca. 3 mg, 0.02 mmol) in an FEP tray and transferred to a ¼-in FEP reaction vessel by use of an FEP pipette. A concentrated solution of DPPE (1,2-bis(diphenylphosphino)- ethane) in CH 3 CN (6 mg, 0.02 mmol) was layered onto an aqueous solution of XeO 3 at 0 °C. The reaction mixture was allowed to stand undisturbed for 4 h during which time small block-shaped crystals of ( 5 ) grew that proved to be stable at room temperature for up to several days. X-ray Crystallography. (a) Crystal Mounting. Single crystals of [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 , (C 5 H 5 NO] 3 (XeO 3 ) 2 and [(C 6 H 5 ) 3 PO] 2 XeO 3 were submerged in a perfluoropolyether (Fomblin oil) in the open atmosphere and were selected and mounted at room temperature at the tip of a dual- thickness polymer loop (MiTeGen, Ithaca, NY; MicroMount;™^100 – 500 μm) using Halocarbon 25-5S grease (Halocarbon Products Corporation, River Edge, NJ). Single crystals of [(CH 3 ) 2 CO] 3 XeO 3 and [(C 6 H 5 ) 2 POCH 2 ] 2 XeO 3 ∙CH 3 CN were mounted on an X-ray diffractometer at low temperatures using a previously described procedure.[3^0 ]^ Dry, crystalline samples of the latter compounds were carefully poured into a ¾-in. FEP cold trough, which was maintained at −8 5 ±5 ° by passage of a cold stream of dry N 2 gas. While in the cold trough, a crystal was affixed to a nylon cryoloop (MiTeGen MicroMounts TM) that had been dipped in Fomblin Z- 25 oil. The oil fully solidified when the cryoloop and attached crystal were transferred and mounted in the cold stream of the diffractometer, thereby firmly attaching the crystal to the cryoloop. (b) Collection and Reduction of X-ray Data. Crystals were centered on a Bruker SMART APEX II diffractometer equipped with an APEX II 4K CCD (charge-coupled device) area detector and a triple-axis goniometer that was controlled by the APEX II Graphical User Interface (GUI) software.[3^1 ] A Bruker Triumph curved crystal monochromator was used with a Mo Kα (λ = 0.71073 Å) radiation source for all compounds. Diffraction data collection at −173 °C consisted of ω- and -scans collected at 0.5o intervals. The crystal-to-detector distance was 4.960 cm (compounds ( 1 ) and ( 3 )) or 4.954 cm (compounds ( 2 ), ( 4 ), and ( 5 )) and data collection
was carried out in a 512 x 512 pixel mode using 2 x 2 pixel binning. The data were processed by use of the APEX III GUI software.[^32 ]^ The SADABS[3^3 ]^ program was used for scaling the diffraction data. Two domains (0.4824(6) and 0.5176(6)) were located in reciprocal space for compound ( 4). The data were integrated, merged, and the TWINABS[^34 ] program was used to scale and process the reflection data. (c) Solution and Refinement of the Structures. The XPREP program[3^5 ] was used to confirm unit cell dimensions and crystal lattices. All calculations were carried out using the SHELXTL-plus[3^5 ]^ and the Olex2[^36 ]^ software packages for structure determination, solution refinement, and molecular graphics. Space group choices were confirmed using Platon.[^37 ]^ The final refinements were obtained by introducing anisotropic thermal parameters and the recommended weightings for all atoms except the hydrogen atoms, which were placed at locations derived from a difference map and refined as riding contributions with isotropic displacement parameters that were 1.2 times those of the attached carbon atoms. The maximum electron density in the final difference Fourier map was located near the xenon atom. Raman Spectroscopy. The Raman spectra were recorded on a Bruker RFS 100 FT-Raman spectrometer using 1064-nm excitation, 300 mW laser power, and ±0.5 cm−^1 resolution as previously described.[3^8 ] Computational Details. Density-functional theory (DFT) was employed to study the electronic structures of XeO 3 , [(CH 3 ) 2 CO] 3 XeO 3 , [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 , (C 5 H 5 NO] 3 (XeO 3 ) 2 , [(C 6 H 5 ) 3 PO] 2 XeO 3 , and the free ligands. All basis sets were obtained online from the EMSL Basis Set Exchange.[^39 ]^ Quantum-chemical calculations were carried out by use of the program Gaussian 09[4^0 ]^ for geometry optimizations and to create wavefunction files. The GaussView[4^1 ]^ program was used to visualize the vibrational displacements that form the basis for the vibrational mode descriptions given in Table S 6 – S9. The CrystalExplorer 3.1[4^2 ]^ program was used for Hirshfeld surface analyses. Natural bond orbital analyses were pcarried out by use of APFD densities in conjunction with the NBO program.[4^3 ]
Acknowledgements
We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for support in the form of a Discovery Grant (G.J.S.) and an Alexander Graham Bell CGS- award (J.T.G). We thank Dr. Hélène P. A. Mercier for her valued comments relating to the manuscript and for her assistance with the vibrational assignments, and Dr. James Britten for his assistance with the structure solution of [(C 6 H 5 ) 3 PO] 2 XeO 3. We are also grateful for the computational resources provided by SHARCNet (Shared Hierarchical Academic Research Computing Network, www.sharcnet.ca) and compute Canada. Keywords: oxygen-base adducts • xenon trioxide • noncovalent bonding • single-crystal X-ray diffraction • quantum-chemical calculations [1] B. Jaselskis, J. P. Warriner, Anal. Chem. 1966 , 38 , 563–564. [2] B. Jaselskis, J. P. Warriner, J. Amer. Chem. Soc. 1969 , 91 , 211– 212. [ 3 ] S. R. Gunn in Noble-Gas Compounds (Ed.: H. H. Hyman), University of Chicago Press: Chicago, IL, 1963 , pp. 149–151. [ 4 ] J. T. Goettel, G. J. Schrobilgen, Inorg. Chem. 2016 , 55 , 12975–12981. [ 5 ] K. M. Marczenko, H. P. A. Mercier, G. J. Schrobilgen, Angew. Chem. Int. Ed. 2018 , 57 , 12448–12452. [ 6 ] P. Scilabra, G. Terraneo, G. Resnati, J. Fluorine Chem. 2017 , 203 , 62, and references therein. [ 7 ] J. T. Goettel, H. P. A. Mercier, G. J. Schrobilgen, J. Fluorine Chem. 2018 , 211 , 60–69. [ 8 ] J. T. Goettel, K. Matsumoto, H. P. A. Mercier, G. J. Schrobilgen, Angew. Chem. Int. Ed. 2016 , 55 , 13780–13783. [ 9 ] I. D. Brown, The Chemical Bond in Inorganic Chemistry , IUCr Monographs in Crystallography, 12, Oxford University Press: Oxford, UK, 2002. [1 0 ] D. R. Allan, S. J. Clark, R. M. Ibberson, S. Parson, C. R. Pulham, L. Sawyer, Chem Commun. 1999 , 8¸ 751. [1 1 ] C. Laurence, J. F. Gal in Lewis Basicity and Affinity Scales: Data and Measurement. John Wiley & Sons: West Sussex, UK, 2009 , pp. 85–109. [1 2 ] S. Hoyer, T. Emmler, K. Seppelt, J. Fluorine Chem. 2006 , 127 , 1415–
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Entry for the Table of Contents (Please choose one layout) Layout 1: FULL PAPER Xenon trioxide displays a distorted square-pyramidal geometry with the unusual Xe coordination number of five in its adduct with triphenylphosphine oxide. Acetone, dimethylsulfoxide, and pyridine N - oxide also coordinate to XeO 3 to form complexes that, unlike solid XeO 3 , are remarkably insensitive to mechanical shock. K. M. Marczenko, J. T. Goettel, G. J. Schrobilgen* Page No. – Page No. Xenon Trioxide Adducts of O - Donor Ligands; [(CH 3 ) 2 CO]XeO 3 , [(CH 3 ) 2 SO] 3 (XeO 3 ) 2 , (C 5 H 5 NO) 3 (XeO 3 ) 2 , and [(C 6 H 5 ) 3 PO] 2 XeO 3
