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QUIMICA INORGANICA

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  1.25 Noble-Gas Chemistry DS Brock and GJ Schrobilgen , McMaster University, Hamilton, ON, Canada B Z ˇ emva , Jozˇef Stefan Institute, Ljubljana, Slovenia ã  2013 Elsevier Ltd. All rights reserved. 1.25.1 Introduction  7561.25.1.1 Discovery of Noble-Gas Reactivity 7561.25.1.2 Noble-Gas Chemistry Reviews (2000–11) 756 1.25.2 Compounds in which the Formal Oxidation Number of Xenon Is Less Than One  7571.25.2.1 Polynuclear Xenon Cations 757 1.25.2.1.1 The Xe 2 þ cation  757 1.25.2.1.2 The Xe 4 þ cation  7571.25.2.2 Xenon–Gold Cations 7571.25.2.3 Prospects for Xenon–M(Pt,Pd,Ag) Bonds and the HgXe þ Cation 7591.25.2.4 Computational Treatment of Gold Complexes 760 1.25.3 Xe(II) Compounds  7601.25.3.1 Cationic Species 760 1.25.3.1.1 Xenon(II)–carbon bonded cations  760 1.25.3.1.2 Xenon derivatives of the OTeF 5  and OSeF 5  groups  765 1.25.3.1.3 Xenon–nitrogen bonded cations  766 1.25.3.1.4 Xenon halides and oxide fluorides  7741.25.3.2 Neutral Xenon(II) Species 777 1.25.3.2.1 Xenon(II)  carbon bonded species  777 1.25.3.2.2 FXeONO 2  and XeF 2  HNO 3  782 1.25.3.2.3 XeF 2  as a ligand  7831.25.3.3 The XeF 3  Anion 792 1.25.4 Xe(IV) Compounds  7941.25.4.1 Neutral Xe(IV) Species 794 1.25.4.1.1 [Mg(XeF 2 )(XeF 4 )][AsF 6 ] 2  794 1.25.4.1.2 XeOF 2 , F 2 OXeN  CCH 3 , XeOF 2   n HF, and XeO 2  7951.25.4.2 Xe(IV) Cations 7971.25.4.3 The XeOF 3  Anion 798 1.25.5 Xe(VI) Compounds  7991.25.5.1 X-ray Crystal Structures of XeF 6  7991.25.5.2 Cationic Species 799 1.25.5.2.1 [XeF 5 ][ m -F(OsO 3 F 2 ) 2 ], [XeF 5 ][OsO 3 F 3 ], and [Xe 2 F 11 ][OsO 3 F 3 ]  799 1.25.5.2.2 (OsO 3 F 2 ) 2  2XeOF 4  and [XeF 5 ][SbF 6 ]  XeOF 4  800 1.25.5.2.3 [XeF 5 ] 3 [Ti 4 F 19 ]  801 1.25.5.2.4 XeO 2 F þ and FO 2 XeFXeO 2 F þ 801 1.25.6 Xe(VIII) Compounds  8041.25.6.1 NMR Studies of XeO 4  and [Na 4 ][XeO 6 ] 804 1.25.6.1.1 XeO 4  804 1.25.6.1.2 [Na 4 ][XeO 6 ]   x  H 2 O (  x  ¼ 0, 2)  805 1.25.7 Kr(II) Compounds  8051.25.7.1 KrF 2  as a Ligand 8051.25.7.2  a -KrF 2  8061.25.7.3 Fluoride Ion Donor Properties of KrF 2 , and KrF þ and Kr 2 F 3 þ Salt Formation 806 1.25.7.3.1 [KrF][MF 6 ] (M ¼ As, Sb, Bi, Au)  807 1.25.7.3.2 [Kr 2 F 3 ][SbF 6 ]  KrF 2 , ([Kr 2 F 3 ][SbF 6 ]) 2  KrF 2 , [Kr 2 F 3 ][AsF 6 ]  [KrF][AsF 6 ], and [Kr 2 F 3 ][PF 6 ]  nKrF 2  8081.25.7.4 KrF 2  and the Synthesis of TcOF 5  810 1.25.8 Thermochemistries of Known and Unknown Ionic Noble-Gas Compounds  810 1.25.9 Noble-Gas Molecules Characterized by Mass Spectrometry and Matrix Isolation  8101.25.9.1 Matrix-Isolated Noble Gases Bonded to Non-metals 811 1.25.9.1.1 The  XeF 3  and HXeO  Radicals  8131.25.9.2 Coordination of Noble Gases to Transition Metal Oxides 8131.25.9.3 Noble-Gas Species Observed by Mass Spectrometry 814 1.25.10 Synthetic Applications of XeF 2  8141.25.10.1 XeF 2  as an Oxidizing and Fluorinating Agent 8141.25.10.2 Inorganic Syntheses 814 Comprehensive Inorganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-097774-4.00128-5  755  1.25.10.2.1 XeF 2  as a fluorinating agent in the preparation of fluorofullerenes  814 1.25.10.2.2 The role of XeF 2  in the synthesis of Ni 2 F 5  and its oxidation by KrF 2  814 1.25.10.2.3 Reactions of tri(9-anthryl) derivatives of phosphorus and bismuth  815 1.25.10.2.4 Syntheses of organotellurium(IV) diazides and triazides  815 1.25.10.2.5 Syntheses of Pd(II), Pd(IV), Pt(II), and Pt(IV) fluoride complexes  815 1.25.10.2.6 Oxidative carbonylation of Fe(CO) 5  in HF/SbF 5  and HF/BF 3  816 1.25.10.2.7 Syntheses of polyfluoroorganoiodine(V) tetrafluorides  816 1.25.10.2.8 Syntheses of Ir(III) fluoride complexes  816 1.25.10.2.9 Fluorination of dibenzoselenophene and dibenzo(1,2)diselenine  8161.25.10.3 Organic Syntheses 8161.25.10.4 Applications of [ 18 F]XeF 2  to the Syntheses of  18 F-Labeled Radiopharmaceuticals for Positron EmissionTomography 817 1.25.11 Conclusion  817 Acknowledgments  817 References  817 1.25.1 Introduction 1.25.1.1 Discovery of Noble-Gas Reactivity  The joint discovery of argon by Lord Rayleigh (born John William Strutt) and Sir William Ramsay has been describedin an essay by John Meuring Thomas. 1  The discovery of argon(lazy) and helium (its presence in the sun had been spectro-scopically detected before its discovery on Earth, hence it wasnamed after the Greek god of the Sun, Helios) led Ramsay toconjecturethatanewfamilyofchemicalelementsmustexistinthe periodic table, which are chemically inert. Ramsey,together with his student, Morris W. Travers, isolated threenew elementary gases – neon (new), krypton (hidden), and xenon (strange). With the discovery of radon (named after itssrcinal source, radium) some years later, the noble-gas family  was complete.Over the years immediately following their isolation, thechemical reactivities of the noble gases were investigated.Unfortunately, the majority of experiments were performed with argon and krypton because these gases were considerably more accessible than xenon. Even today, the chemistry of argon is limited to matrix-isolation studies, while the kryptoncompounds that have been isolated in macroscopic quantitiesare thermodynamically unstable and are therefore challenging to synthesize and to handle. In 1933, Professor Don Yost andhis graduate student, Albert Kaye, passed electrical dischargesthrough gaseous mixtures of xenon and fluorine. Up until that time, they came closest to the isolation of a xenon fluoride. 2  At that time, their failure to induce chemical reactivity was takenas proof that the noble gases were indeed inert. This negativefinding was in accordance with the current electronic theory of chemical bonding and the octet rule which held that eight electrons in the valence orbital are the most stable elec-tronic configuration. This became a dogma which wasespoused in practically all chemistry textbooks of the timeand up until 1962.It was not until 23 March 1962 that this dogma was aban-doned. Professor Neil Bartlett, then at the University of BritishColumbia, reacted xenon, with its complete octet of valenceelectrons, with the potent oxidant, PtF 6 , to give a yellow prod-uct which was initially formulated as ‘[Xe][PtF 6 ]’, the first truechemical compound of a noble gas. 3  The discovery, however, was not so straightforward. It began with the attempted puri-fication of PtF 4  which entailed heating PtF 4  in a stream of dilutedfluorineinaPyrexglassapparatus. 4 Inthisway,Bartlett obtained the salt [O 2 ][PtF 6 ]. 5 Because his finding was not universally accepted, he searched for the right candidate toprove the extraordinary one-electron affinity of PtF 6  which,according to his calculations, should exceed 7 eV. 6 His subse-quent reasoning and the experimental plan were brilliant. Herealized that the first ionization potential of Xe (12.129 eV) ismarginally higher than the first ionization potential of O 2 (12.075 eV). Because O 2  could be directly oxidized to [O 2 ][PtF 6 ] with PtF 6 , 5 he proceeded to oxidize xenon gas withPtF 6  and obtained the first compound of a noble-gas element. This experiment has been proclaimed to be among the tenmost beautiful chemical experiments performed in the history of chemistry. 7  The product, ‘[Xe][PtF 6 ]’, was amorphous andeven today its structure is not fully understood. Details con-cerning the nature of ‘[Xe][PtF 6 ]’ are discussed in Section 1.25.3.1.4.2  and by Graham et al. 8 1.25.1.2 Noble-Gas Chemistry Reviews (2000–11) During the period 2000–11 (inclusive), several reviews haveappeared that deal with a broad spectrum of topics in noble-gas chemistry: general noble-gas chemistry; 9 129  Xe nuclear magnetic resonance (NMR) in noble-gas chemistry; 10 synthe-ses, properties, and chemistry of Xe(II) fluoride; 11 compoundsthat contain Xe–Cbonds; 12  Xe–N-bonded compounds derivedfrom NSF 3 ; 13 inert matrices for low-temperature isolationand spectroscopy of noble-gas species; 14 krypton chemistry; 15 goldandmercurycationsofxenon; 16 calculationstodeterminethe structures of complex es formed by a noble gas and a coinage metal monohalide; 17  XeF 2  as a ligand; 18  xenon NMR spectroscopy, theory and applications; 19 a critical review of experimental and theoretical advances in noble-gas chemistry during the prior 20 years; 20 NMR studies of structure andbonding in xenon and krypton compounds; 21 and a review summarizing the structures of neutral and cationic kryptonand xenon-fluoro species. 22 Chapter 9.04 , which relates tohypervalent bonding, should also be consulted. 756  Noble-Gas Chemistry  1.25.2 Compounds in which the Formal OxidationNumber of Xenon Is Less Than One 1.25.2.1 Polynuclear Xenon Cations 1.25.2.1.1 The Xe  2  þ cation  The Xe 2 þ cation is readily formed in thegas phase by collisionsinvolving excited xenon atoms (eqn [1]). Xe * þ  Xe !  Xe 2 þ þ e  [1] The Xe 2 þ cation was established in the gas phase by massspectrometry, 23 photoionization, 24–26 and by elastic scattering studies. 27,28  TheXe 2 þ cationwasfirstdescribedinthecondensedphase 29  when Xe 2 þ  was formed by reaction of [O 2 ][SbF 6 ] with xenon gas (500 Torr). It is a bright green intermediate product in the reaction of Xe and [O 2 ][SbF 6 ] which gave yellow [XeF][Sb 2 F 11 ] as the final product (eqn [2]). 29–31  The cation has only exhibited stability in HF/SbF 5  solutions. A very pale greensolution was observed in AsF 5  at low temperature, but there was no indication of Xe 2 þ formation in HSO 3 F, BrF 5 , or IF 5 solutions. 31 2Xe þ 2 O 2 ½  SbF 6 ½ !  Xe 2 ½  SbF 6 ½ þ  O 2 ½  SbF 6 ½ þ O 2 !  XeF ½  Sb 2 F 11 ½ þ  Xe þ 2O 2  [2] The brightgreen,paramagnetic Xe 2 þ ion has been characterizedby Raman (123 cm  1 ), ultraviolet (UV)–visible (335 and710 nm), and electron spin resonance (ESR) spectroscopy. 29,31 Solutions of Xe 2 þ are stable indefinitely at room tempera-ture under a pressure of xenon gas and can also be prepared by the irreversible reaction of a limited amount of water or other reducing agents (e.g., Pb or Hg) with SbF 5  solutions of [XeF][Sb 2 F 11 ] (eqns [3] and [4]). 29,31  The reaction between [XeF][Sb 2 F 11 ]andXegasinSbF 5 solventisreversible,withthee xtent of reaction being influenced by the Xe pressure (eqn [5]). 31,32 4 XeF ½  Sb 2 F 11 ½ þ 3Pb 6Hg  ð Þ! 2 Xe 2 ½  SbF 6 ½ þ 3Pb SbF 6 ½  2  3 Hg  2 ½  SbF 6 ½  2    [3]8 XeF ½  Sb 2 F 11 ½ þ 18H 2 O ! 4 Xe 2 ½  SbF 6 ½ þ 12 H 3 O ½  SbF 6 ½ þ 3O 2 [4]3Xe þ  XeF ½  Sb 2 F 11 ½ þ 2SbF 5 P 2 Xe 2 ½  Sb 2 F 11 ½   [5]Reaction[5] wassubsequentlyrepeatedanditwasshownthatthepresence of HF and the resulting superacid, HF/SbF 5 , is essentialfor Xe 2 þ formation. 33  A dark green solution of [Xe 2 ][Sb 2 F 11 ],SbF 5 , and HF yielded crystalline [Xe 2 ][Sb 4 F 21 ] at    30   C. The X-ray crystal structure showed that the Xe 2 þ cations and Sb 4 F 21  anions were well separated, with Xe  F contacts ( > 3.22 A ˚) that areonly slightly shorterthan or approach the sumof the vander  Waals radii of xenon and fluorine (3.63 A ˚). 34  The Xe–Xe bond(3.087(1)A ˚) has been cited as the longest bond between main-group elements. 33  The open-chain tetrameric fluoroantimonate(V) anion, which was observed for the first time, has the highest fluorideionaffinity (FIA) among theknownSb n F n  1  ( n ¼ 1–4)anions,whichisapparentlynecessarytoovercomethehighFIAof theXe 2 þ cation. 33 1.25.2.1.2 The Xe  4 þ cation Green solutions of the Xe 2 þ ion in SbF 5  reversibly convert intodark blue solutions at high pressures of Xe gas (30–50 bar). 35 Under these conditions, the solvent is a homogeneous mixtureof SbF 5  and liquid Xe. The reversible color change, from blue togreen, was achieved by varying the temperature which, in turn,altered the amount of dissolved xenon. The Xe 32 þ , Xe 3 þ , and Xe 4 þ cations were considered as possible causes for the bluecolor. It appears unlikely that single crystals of the blue speciescan be grown because of the high viscosity of SbF 5 , whichhas been the only appropriate solvent for the generation of this cation, and the high Xe pressures that are required tostabilize the proposed Xe 4 þ cation. The characterization of the blue species was therefore based on spectroscopic data(UV–visible absorption, Raman, infrared (IR), and electronparamagnetic resonance (EPR) spectroscopy) and comparisons withtheoretical calculations. Itwas concluded thatthe Xe 4 þ ionis likely the srcin of the blue color. A linear ( D 1 h ) structure with Xe–Xe bond lengths of 3.529 (terminal bonds) and 3.190(central bond) A ˚ was calculated as the energy-minimizedstructure of Xe 4 þ . 35  The possible formation, under different experimental con-ditions, of higher xenon aggregates such as Xe 4   Xe n þ has alsobeen considered. 35 Calculations indicate that 12–18 very loosely bound xenon atoms are needed to stabilize a linear symmetric Xe 4 þ unit, with the outer xenon atoms at    4.40 A ˚from the Xe 4 þ core. 36–38  An earlier report suggested that larger  Xe n þ aggregates underwent photo-decomposition, leaving behind Xe 4 þ ion fragments. 39  Xenon cations of the type Xe n þ ( n  30) have been detectedin molecular beam experiments using mass spectrometry andtheir structures have been predicted by quantum-chemicalcalculations. 40  A prominent feature of these clusters is delocal-ization of their positive charges over their cores, which arecomprised of trimeric or tetrameric units held together by covalent bonds. The remaining xenon atoms are polarizedand bind to these positively charged cores. 40 1.25.2.2 Xenon–Gold Cations  There have been indications that metal–xenon bonds can beformed. The complexes (CO) 5 Mo   Xe, (CO) 5  M  Ng (M ¼ Cr, W; Ng  ¼ Kr, Xe), (CO) 5 Mn   Xe, and (CO) 5 Fe þ  Kr have beendetected in noble-gas matrices. 41–44 Quantum–chemical calcula-tions were reported for (CO) 5 M  Ng (M ¼ Cr, Mo, W;Ng  ¼  Ar, Kr, Xe). 45 Short-lived transients containing xenon– or krypton–metal bonds have been observed in supercritical Xeand Kr solutions. 46–51  The AuXe þ ion has been detected by massspectrometrywithacalculatedbondlengthof2.57 A ˚ anda bond energy of 125.60  12.56 kJ mol  1 , 52 and ArAuCl andKrAuCl have been observed by microwave spectroscopy which yielded Ar–Au and Kr–Au bond lengths of 2.47 and 2.52 A ˚,respectively. 53  The reaction between AuF 3  and Xe in HF/SbF 5  solution yielded [AuXe 4 ][Sb 2 F 11 ] 2 , which was obtained as a dark red,crystalline solid at   78   C. 54 Removal of Xe at   40   C yielded Au[SbF 6 ] 2 , a rare example of an Au(II) salt. The formation of  AuXe 4 þ from Au[SbF 6 ] 2  is reversible; thus, for the preparationof [AuXe 4 ][Sb 2 F 11 ] 2 , a moderately high xenon pressure(10 bar) was required. 54 Four xenon–gold cations have been isolated as their saltsfrom the Au 2 þ  /HF/SbF 5  /Xe system with the species depending primarily on the HF solvent acidity (concentration of SbF 5 ), Xepressure, and temperature. At a high SbF 5  concentration(0.5 molof SbF 5  in 1 mol ofHF), [AuXe 4 ][Sb 2 F 11 ] 2  crystallized Noble-Gas Chemistry  757  in two crystallographic phases, triclinic ( Figure 1 ) andtetragonal ( Figure 2 ), which only differ with respect to their cation–anion interactions. 55  At lower Xe pressures, which wereachieved by pumping on solutions of [AuXe 4 ][Sb 2 F 11 ] 2 , the[AuXe 2 ][Sb 2 F 11 ] 2  salt formed. The effect of acidity was appar-ent when equimolar amounts of finely divided gold and XeF 2  were reacted with 10–12 bar of Xe in superacid media com-prised of   2:5, 10:7, and 5:1 molar ratios of HF:SbF 5 , yielding  trans -[AuXe 2 ][SbF 6 ], [Au 2  Xe 2 F][SbF 6 ] 3 , and  trans -[AuXe 2 F][SbF 6 ][Sb 2 F 11 ], respectively. 55 Both [AuXe 4 ][Sb 2 F 11 ] 2  phases were stable up to   40   C. Warming above this temperatureresulted in melting accompanied by Xe loss, a color changefrom dark red to light orange, and formation of Au[SbF 6 ] 2 . 54  The crystal structure of triclinic [AuXe 4 ][Sb 2 F 11 ] 2  ( Figure 1 )consists of discrete AuXe 42 þ cations and Sb 2 F 11  anions. The Au atom is situated in the middle of a square plane of Xeatoms with the four Au   Xe distances ranging from 2.728(1)to 2.750(1)A ˚. The three closest Au  F cation–anion contactsare 2.671(5), 2.949(5), and 3.153(5)A ˚ and the Sb 2 F 11  anionsarebentattheirSb  F  Sbbridges.ThetriclinicphasehasoneparticularlyshortAu  Fcontact(Au  F12,2.671(5)A ˚)which,together with the square-planar AuXe 42 þ unit, results in asquare pyramidal arrangement. The contacts are w eaker (Au  F10, 2.927(9)A ˚) in the tetragonal phase  Figure 2 . 55  The Raman spectrum of triclinic [AuXe 4 ][Sb 2 F 11 ] 2  showed,in addition to bands attributable to the Sb 2 F 11  anion, a very strong band at 129 cm  1 . This band was assigned to the totally symmetric stretching vibration of AuXe 42 þ based on thefrequency predicted by ab initio and density functional theory (DFT) calculations. 54  ThemeanthermochemicalAu–XebondenergyforAuXe 42 þ in eqn [6] is estimated to be 210 kJ mol  1 , which is AuXe  2 þ 4  !  Au 2 þ þ 4Xe [6]in accordance with the thermal stability of [AuXe 4 ][Sb 2 F 11 ] 2 .Because gold is the most electronegative transition metal, alarge charge transfer from xenon to gold occurs in AuXe 42 þ , which is reflected in the calculated charge distribution of thecation, where most of the positive charge is located on the Xeatoms. 54 Reduction of Au 3 þ to Au 2 þ and its complexation with xenon only occur in the superacid medium, HF/SbF 5 . Theoverall reaction emphasizes, yet again, the role of ‘protons’(H 2 F þ ) and HF elimination in Au   Xe bond formation(eqn [7]). 54 Crystals of green [Xe 2 ][Sb 4 F 21 ] 33  were also identi-fied in the solid reaction mixture at   60   C. AuF 3 þ 6Xe þ 3H þ !  AuXe  2 þ 4  þ  Xe þ 2  þ 3HF [7] Thermally unstable, violet-black   cis -[AuXe 2 ][Sb 2 F 11 ] 2  andthermally very unstable, ochre-colored  trans -[AuXe 2 ][SbF 6 ] 2 ( Figure 3 ) crystals were isolated. 55 In the case of the XeXe1   Au1F1   F1Sb2Sb2  Figure 3  The structural unit in the X-ray crystal structure of  trans -[AuXe 2 ][SbF 6 ] 2 . Reproduced with permission from Drews, T.; Seidel, S.;Seppelt, K.  Angew. Chem. Int. Ed  .  2002 ,  41 , 454–456. Xe2Xe2  Xe3Xe1 AuF10F10  Figure 2  The structural unit in the X-ray crystal structure of thetetragonal phase of [AuXe 4 ][Sb 2 F 11 ] 2 . Data from Ref. 55 were used todraw this figure. Xe4Xe2Xe3Xe1 Au1F26F25F12Sb3Sb4Sb1Sb2 Figure 1  The structural unit in the X-ray crystal structure of the triclinic phase of [AuXe 4 ][Sb 2 F 11 ] 2 . Data from Ref. 55 were used to draw this figure. 758  Noble-Gas Chemistry
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