8 Halogens and noble gases

By E. G. HOPE Department of Chemistry, University of Leicester, Leicester LE 7 7RH, UK

1 Introduction

This chapter reviews the 1996 literature for the elemental halogens and noble gases and compounds containing these elements in their positive oxidation states onIy. Publica- tions which include reference to halide, polyhalide or oxohalide anions as counter ions are not described.

2 Halogens

Two reports have detailed recent developments in the producrion of elemental fluor- ine; the first outlined fundamental modifications to the established fluorine cell tech- nology' while the second offered an alternative, solid-state, electrochernicaI source of pure A uorine gas based on the electrochemical decomposition of a conducting solid- fluoride electr~lyte .~ In the latter paper, the authors outlined the use of a solid solution of LaF, containing 3-10% BaF, as the electrolyte and suggested that this system offers significant practical advantages over the established methodology which is derived directly from Moissan's 1886 extraction of fluorine.

The most commun reactions of the halogens are oxidations and the expansion in the scope and apphcation of elemental fluorine in this context is continuing. Reports during 1994 have included the fluorination of cyclic l ,?-diket~nes,~ lY3-dithiolanes,4 purines (where regiospecific derivatisation at the 8-position has been reported for the first and [60]-fullerene and some higher f~lIerenes,~ and the generation of some new N-fluoro nitrogen systems7 The value of fluorine gas as a reagent in a wide range of organic chemistry, even for the synthesis of products which do not contain fluorine, has been revieweda and elegantly i l l ~ s t r a t e d . ~ Consecutive papers describe the surpris- ing room-temperature fluorination of some of the platinum metals by elemental fluorine in anhydrous HF in the presence of alkali-metal fluorides or ammonium fluoride. An apparatus designed for the preparation and characterisation, using ma trix-isolation infrared spectroscopic techniques, of molecular species formed when fluorine reacts with high-temperature metal surfaces, has been reported.' Oxidation reactions involving the heavier halogens are rnure widespread: papers of note include an important hazard warning associated with the generation of explosive nitro- gen-chlorine derivatives during the reaction of chlorine with ammonium comp1exes,l2

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106 E. G. Hope

the synthesis of the [SbX,][Sb(OTeF,),] (X = C1 or Br) salts13 and the crucial function of bromine in the synthesis of a Iuminescent conjugated polymer for LED app1i~ations.l~ Although equilibrium constants for the hydrolysis of aqueous Br, have been extensively investigated for 85 years, considerable disagreement exists over the values of K, [equation (l)] ( K , = kl/,k1). The reinvestigation of this reaction as a function of ionic strength at 25 "C and as a function of temperature at constant ionic strength (p E 0 M) offers definitive kinetic data.15

& I

k - 1 Br2(aq) + H 2 0 HOBr + Br- + H + (1)

Donor-acceptor complexes, particularIy involving the halogens, have been exten- sively studied since the interactions in such species are one of the most fundamental processes in chemistry and may piay a key role in many chemical and biological processes. The structure of { 1,2,4,5-(EtS),C6H, - . . Br,Im nicely illustrates this type of interaction where the Br-Br bond length is increased from 228 (gas phase) to 241 pm (adduct).16 Two intercalation compounds, CsF-Br, and 2CsF*Br,, were isolated from the reaction of CsF with bromine." In the 1: 1 compound, a single-crystal X-ray investigation revealed eclipsed Cs . . - F layers (Cs' positioned above Cs ') whereas, from X-ray- and neutron-powder diffraction data, the layers in the 2: 1 compound are staggered (Fig. 1). In both compounds, the Br-Br distance is larger than that for free gaseous bromine suggesting some charge transfer from fluoride to bromine. The interaction between iodine and porous materials, investigated spectrochemicalfy, has been proposed as a molecular probe for the quantitative evaluation of zeolite donor strengthla and may be controlIed by topotactical insertion in, for example, poro~i l s . '~

A feature article on the formation and characterisation, in the gas phase by ground- state rotational spectroscopy, of pre-reactive intermediates of halogens (and interhaIo- gens, see beIow) with a series of Lewis bases, offered a general introduction, including experimental details, and a summary of most of the results in this area.20 New linear HCN ~ - - F, and CH,CN - - - F, intermediates have been described.2' A theoretical paper on H,O - -. F,, H,O - - - C1, and H,O . . . ClF adducts has suggested that a lone pair on water interacts with chlorine and chlorine monofluoride; however, in the fluorine adduct, the fluorine nucIei are more effectively screened such that the water lone pairs have little invoivement in complex information and the interaction, this adduct is, therefore, more like a classical van der Wads interaction.22

3 Interhalogen compounds and polyhalide anions

The generation of an extensive range of pre-reactive intermediates of ClF with Lewis bases23-26 have been described; the H,O adduct lZ4 compares well with an earlier theoretical prediction,22 whilst the data for the ammonia25 and trimethylamine,, pre-reactive intermediates indicate increasing ionic contributions to the valence-bond descriptions and the formation of Mulliken inner-complexes, e.g. [(CH,),NCl JF. Bromine triffuoride reacts with M(C,F,), (M = Zn, Cd or Hg) to give (C,F,)BrF,, whilst in the presence of additional Lewis acids bromonium cations, [(C,F,),Br] +, are formed.,' The molecular structures of [(C,F,),Br]BF, and [(C,F,),Br]AsF, reveal distorted square-planar co-ordination at bromine, with asymmetric hypervalent

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Halogens and noble gases 107

c

8 e

~ E m m W . 5 “ ‘ :: F 2; i % L 0 %

+ f u p + e z c % S ” , r r n ~ m u m ? c

( i i )

Fig. 1 (Reproduced by permission from Chem. Eur. .I., 1996,2, 1303.)

ORTEP plots of the (i) CsF-Br, and (ii) 2CsF*Br, structures

bonds, in which the anions and cations form infinite -3r-F-€%/As-F- zigzag chains.27 New cations containing the unusual Se-Se bridge have been synthesized by the reaction of selones with IBr or ICl’’ and the binding energy (8.06 kcal mol- ’) in the donor-acceptor ICI-diethyl ether complex has been determined by iodine PES.”

Pol yiodides continue to provide a rich area for investigation offering various degrees of catenation and a wide variety of geometrical arrangements. In contrast to the intercalation compounds formed in the reaction of CsF with bromine, CsF and iodine react to give Cs,I, and, probably, CsIF6.17 The conformations of the triiodide and tribrornide anions in a series of crystal structures have been classified3’ and phase transitions in triiodide salts of binuclear ferrocene derivatives with long alkyl chains have been investigated by 12’1 Mossbauer spec t ro~copy .~~ Slow cooling of the product

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108 E. G. Hope

2

3

obtained by the reaction of NH,I-I,-Au at 500°C afforded crystals of [NH& [(Au14)Au12(p-14)] where the near-linear bridging 1,' ligands link to form a chain network.32 The nature of the cation can have a pronounced effect on the structure of the anion; [N-propylurotropiniumlI, and [IV-propylurotropiniumlI, crystallize with 'normal' anions (I5-, V-shaped; IT-, pyramidal)33 whilst the 15- anion in [Pr",N]I, consists of almost-squared nets of I- ions connected by four Iz molecules in which the cations are enclosed by a mesh made up of twelve iodine atoms. Similarly, the I, - anion in [Pr",N]I, adopts a centrosymmetric Z-shape in which linear, symmetrical, 1,- units and two iodine molecules form twisted rope Four types of polyiodide anions (I3- , 1133- , I , 2 2 - and II6*-) have been structurally characterised as their dimethyldiphenylammonium salts. 35 The I 33 - anion consists of zigzag chains of iodide ions and iodine molecules in which the iodide ions are co-ordinated to 15- groups. The 1122- anion 2 consists of two Is- groups bridged by an iodine molecule and the 11,2 - anion 3 consists of two I7 - groups also bridged by an iodine molecule. In a further development of the co-ordination chemistry of thioether crown ligands with main-group centres, Schroder and c o - ~ o r k e r s ~ ~ have described the synthesis of the binuctear [([16]aneS4)MIM([16]aneS4)]IL from the reaction of EM([ 16]aneS4)] [PF6I2 (M = Pd or Pt) with [Bu",N]I,. The anion, which can be described either as two 1,- groups linked uia an iodide ion or two 1,- groups and two iodine molecules linked via an iodide ion, forms a 14-membered polyhalide ring (9.657 x 12.640A) around the complex cation with the metal-bridgingiodide at the centre of the ring(Fig. 2); these rings also link into an infinite two-dimensional sheet. The authors proposed

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Halogens and noble gases

Fig. 2 Structure of [([14JaneS,)PdIPd([l4]aneS,)jCI,1,1 (Reproduced by permission from Chem. Comrnun., 1996,2207.)

that this represents the first synthesis of a cydic polyhalide array in which the complex cation acts as a template.36 Raman spectroscopy and X-ray crystal structure analysis of [(Bu'NH)Ph,P]ICI, and [(Bu'NH)Ph,P]IBr, showed linear non-centrosymrnetric and linear centrosymrnetric anions re~pectively.~~ In contrast, the anion in [K(crypt-2- 2-2)][1(ICN)J is bent at an angle of 89" at the central iodine

4 Halogen oxides and organoiodine oxygen compounds

The focus of attention in halogen-oxygen chemistry is the action of these compounds in the atmosphere. The transcripts of the 1995 Nobel lectures include discussions on the role of halogen radicals in ozone d e s t r u ~ t i o n ~ ~ whilst the influence of chloro- fluorocarbons and their replacements are reviewed4' and dem~nstrated.~' Tirne- resolved flash photolysis of iodine and ozone gave, in addition to evidence of 10,

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110 E. G. Hope

4 h {R=HorN02)

5

electronic bands attributed to the 010 radical42 whilst ab initio calculations on HBrO, indicated that it may play a role in night-time stratospheric bromine chemis- try.43 Rotational spectra for Br,O and OBrO have been recorded and inter~reted.~, The C10, radical 4 is generated by thermaI decomposition of c1206 or Cl,O, and characterised by IR and UV/vIS spectroscopies under matrix-isolation conditions. The structural parameters were calculated from normal co-ordinate analysis and indicate a dynamic Jahn-Teller The kinetics of the reaction of ClO, with lignin, as a model cornpound for the pulp bleaching process, have indicated that chlorination does not occur via direct reaction but via released HOCl or C12.46 The harmonic force field for CIO,F has been refined during a study of the microwave FT spectrum of an '*O-enriched mooolabelled sample.47

The rate of Friedel-Crafts acylation using lanthanum triflate as a catalyst has been shown to be significantly enhanced by the addition of lithium perchlorate; the com- bined system reputedly offers significant advantages over the established pro- c e d u r e ~ . ~ * During the past 15 years, 200 articles have been published on the chlorite-iodide reaction; Lengyel et ~ 1 . ~ ' have recently obtained, for the first time, a self-consistent set of rate constants from kinetic measurements and modeIling studies of the four components within this system. The X-ray crystal structures of MIO, (M = K, Rb or Cs), first structurally characterised between 1926 and 1937 and now redetermined using modern fa~ilities,~' have revealed isolated 10, tetrahedra. The structure of Be(H4IO,),*4H,O was first reported, incorrectly, in 1941; a redetermina- tion of the structure has indicated that this complex is the first beryllium periodate in which slightly deformed Be(OH,), tetrahedra and cis-IO,(OH), octahedra are held together by eight hydrogen bonds.51 The same workers have also determined the structures of the related Mg(103),-10H,052 and [Ni(OH,)6][H,10,]53 complexes. Further work on the tetrabutylarnmonium periodate oxidation of alcohols54 and the syn-dihydroxylation of alkenes using the RuCl,-NaIO, catalyst has been r e p ~ r t e d . ~ '

The last decade has witnessed a substantial renaissance in hypervalent iodine chemistry and a number of new reactions have been discovered exploiting the low toxicity, ready availability and ease of handling of many of these reagents; the litera- ture in this area (19m-1!295) has been comprehensively reviewed.56 Reactions using iodosylbenzene have been preeminent in this area inchding a recent report of high yield 8-functionaIization in the treatment of triisopropykilylenol ethers with PhIO-trimethylsiIy1 a ~ i d e . ~ ' The related iminoiodine ArINTs compound, (Ar = C,H, or 2-MeC6H,) have found application in the asymmetric catalytic syn- thesis of s u l f i m i d e ~ ~ ~ and exhibit unusual polymeric ass~ciation.~' The determination of two crystal structures of (o-tolyi) INTs compounds have revealed quasi-two-co- ordination at iodine, but while one contains centrosymmetric dimers held together via I - ~. N interactions which link uia I - - . 0 associations into ladder polymers, the second contains no short I - - . N distances and polymerises exclusively via 1. . - 0 interactions

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Halogens and noble gases 111

Fig. 3 ORTEP Plots of two structures of (o-toly1)JNTs compounds (Reproduced by permission from Inorg. Chern., 1996,35,275.)

(Fig. 3).59 In spite of the extensive interest in organoiodine oxygen compounds, little is known about the related peroxy systems, probably because of their tendency to decompose. One such type of compound, 5, has been found to offer a new route to the room-temperature oxidation of benzyl and ally1 ethers.60

5 Cationic iodine and other organoiodine compounds

4-Methyl(difluoroiodo)benzene has been structurally characterised6' and used for the selective fluorination (at the &-position) of fi-ketoesters.62 Phenyliodine(111) bis(tri- fluoroacetate) has been used in a novel and efficient synthesis of sulfur-containing he te ro~ycfes ,~~ in the intramolecular oxidative phenol coupling of phenol-ether de- r i v a t i v e ~ , ~ ~ and in the asymmetric synthesis of isostegane derivative^.^^ Penta- fluoroiodobenzene has been oxidised regiospecifically to iodoheptafluoro- 1,4-cy-

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112 E.G. Hope

6

clohexadiene using IF,-BF3. Subsequent cautious oxidation using diluted elemental fluorine in dichloromethane at - 78 "C has afforded the first perfluorovinyliodine(Il1) compound 6 [equation (2)].66 Treatment of CFJ with [Me,N]X (X = C1 or Br) gave [Me,N] [CF,IX]; the related CF,IF- anion was obtained via thermal degradation of the intermediate resulting from the treatment of the heavier halide species with CF, 0C1.67

OTf OTf 7

The ligand-free palladium acetate-catalysed coupling of various aryl-atkenyl- and -alkynyLiodonium salts (including [PhJ JBF,, [(PhC=C)IPh]BF,, PhI(OW)(OTs) and Zefirov's reagent, 7) with terminal alkynes has been reported.68 Stang and co- w o r k e r ~ , ~ ~ in a continuation of their work on cationic tetranuclear macrocyclic squares, have described the seif-assembly of optically active nanoscale-size assemblies linked via iodoniurn units.

6 Noblegases

The fundamental high-resolution molecular spectroscopy of small van der Wads clusters of SF, in liquid helium droplets (ca. 4000 He atoms) has been described.7* The observation of rotationaI structure indicates that the embedded species rotate nearly freely, even at 0.37 K, suggesting that these dropfets are probably superfluid and provide a uniquely cold yet gentle matrix for high-resohion spectroscopy. Xenon has been used to cataIyse the isomerisation of CH,NO, to CH,ONO within the reaction region of a selected-ion flow tube (SIFT) mass s p e ~ t r o m e t e r . ~ ~ The authors proposed a mechanism involving direct 'interaction' of xenon with nitromethane promoting the methyl-cation shuttle. The heavy-atom effect of xenon has been reported to be en- hanced by adsorption to zeolites72 and further work on the structures of zeolites by lz9Xe NMR has been outlined.73 Early work on the incorporation of noble gases into fullerenes indicated a very low gas uptake; concentration to 30% enrichment by column chromatography has now been reported.74 Fundamental vibrational spectro- scopic studies of unstable adducts, such as BF,-CO and CH,CHF-HCl, in liquid argon have been carried out under equilibrium conditions. 75 The first time-resolved IR study of the photolysis of [M(CO)6] (M = Cr, Mo ur W) in supercritical argon, krypton or xenon has permitted the observation of the established [M(CO),(noble gas)] adducts. Further competitive kinetic studies gave data on the strength of the metal-noble-gas i n t e r a ~ t i o n . ~ ~

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Halogens and nobk gases 113

7 NobIe-gas compounds

Noble-gas compounds have continued to captivate theoretical scientist^."-^^ The 29Xe NMR chemical shifts of XeF,,, XeF,,- + and XeO,F,- ,, (n = 1-3) have been

studied theoretically by ab initio finite perturbation theory. The calculated values agree with experimental data and indicate that the dominant term in the xenon chemical shift is the paramagnetic term.” Following its spectroscopic identification in 1995, XeH, has been identified as a transient species obtained by annealing xenon-hydrocarbon systems irradiated with fast electrons at 15 K;*l following reac- tion with trans-but-2-ene, XeH, has been described as either a highly reactive hydro- genation agent or an ‘active form’ of rnolecuIar hydrogen. A new thermal catalytic synthesis of XeF, and KrF, has been proposed.82 The principal application of noble-gas compounds outside noble-gas chemistry is oxidation with or without fluor- ination. Xenon difluoride has been used for the oxidation of Te(C6HJ4 in the synthesis of Te(C,H,),, the first neutral compound comprising a hexaarylated The reaction of tetrafluorobenzenes with XeF, in anhydrous HF has been shown to occur either by addition of fluorine to the ring or substitution of hydrogen by f l~or ine . ’~ In water, in the presence of HF, XeF, acts as an electrophilic oxygenation agent in the production of cyclohexa- 174-dienones from pentaf l~oroarenes .~~ Krypton difluoride has been used in the synthesis of [NF,]J3F,82 and [TcOF,].~~

Seppelt and c o - ~ o r k e r s ~ ’ * ~ ~ have reported X-ray structural analyses of complexes containing the XeF7-, Xe,F,,- and XeF,*- anions (Fig. 4). In CsXeF,, obtained from CsF and XeF, in BrF, at 4”C, the anion is a capped octahedron) a geometry which is strictly obeyed due to the symmetry constraints of the cubic lattice system; this should be compared to most other main-group seven-co-ordinate compounds which are pentagonal bipyramidal. The Xe-F,,, bond is unusually long (210pm, c$ average Xe-FOh = 195pm) which may arise by interaction with the non-bonding eIectron pair or from the interaction with three caesium cations (the other fluorine atoms only show short contacts to two cations). In [NO,]Xe,F,,, the anion can be described as an adduct of XeF, - with XeF,. As such, this would represent the first structural determi- nation of a discrete XeF, molecule; in the solid state XeF, is a complicated mixture of fluorine-bridged tetramers and hexamers. In this description of the anion structure, the XeF,- part adopts a capped trigonal-prismatic arrangement in which the Xe-F,,, distance is the shortest bond and the Ione pair appears to point between the two Xe-F bonds trans to Xe-F,,,. The XeF, part has C,, symmetry with two short, two intermediate and two long Xe-F bonds and exhibits significant deviation from a regular octahedral structure, as seen previously for IF,- and SeF,’-. In the solvated structure of CS,[XeF8]*4BrF,, the anion adopts a near-regular square-antiprismatic structure, as seen 25 years ago for the NO+ salt, which may be compared to that for IF,-. The Xe-F bonds are longer than the I-F bonds which can be rationalised in terms of shielding by the centrosymmetric non-bonding electron pair o r by increased polarity of the A-F bonds due to the increased negative charge.

The systematic investigations of the xenon-carbon bond by Frohn and co- w o r k e r ~ ~ ~ * ~ * have been continued. The reactions of (nonafluorocyclohexen-l- yl)xenon(II) hexafluoroarsenate with halide ions have been shown to be solvent de- pendent. With iodide and bromide, in acetonitrile and anhydrous HF, xenon(r1) is

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114 E.G. Hope

V

( i i )

Fig. 4 Structures of units of (i) [NOJ[Xe,F,,] and ( i i ) Cs,[XeF,]*4BrF5 (Reproduced by permission from Angew. Chem., inr. Ed. Engl., 1995, 35, 1123 and Chem. Eur. J. , 1996,2, 398.)

displaced by the halide giving RX (X = Br or I). Chloride and fluoride do not react with the xenon salt in anhydrous H F but, in acetonitrile, the fluoride ions initiate the formation of alkenyl radicals which abstract hydrogen from the solvent affording RH. Chloride shows intermediate behaviour in acetonitrile giving both RH and RCI.89 As outlined above, the XeF,-H,O-HF system acts as a strong electrophilic oxygenation agent. Treatment of [C,F,Xe]AsF, and fC,F,Xe]AsF, 8 with this system resulted in only the second report of the transformation of an organic moiety bonded to xenon without Xe-C bond cleavage, equations (3) and (4)."

References

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4 R. D. Chambers, G. Sandford, M. E. Sparrowhawk and M. J. Atherton, J. Chem. Soc., Perkin Trans. I , 1996, Trans. 1, 1994,227 1.

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Halogens and noble gases 115

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1 1 S. 3. Osin, D. 1. Davaliatshin and J. S. Odgen, J. Fluorine Chem., 1996,76, 187. 12 M. Knothe and W. Hasenpusch, Inorg. Chem., 1996,35,4529. 13 W. 1. Casteel, jun., P. Kolb, N. LeBlond, H. P. A. Mercier and G. J. Schrobilgen, Inorg. Chem., f996,35,929. 14 Q. Pei and Y. Yang, J. Am. Chem. Soc., 1996,118,7416. 15 R.C. Beckwith, T. X. Wang and D. W. Margerum, Inorg. Chem., 1996,35,995. 15 H. Bock, 2. HavIas, A. Rauschenbach, C. Nather and M. Kleine, Chem. Commun., 1996,1529. 17 T. Drews, R. Marx and K. Seppelt, Chem. Eur. J., 1996,2,1303. 18 S.Y. Choi, Y.S. Park, S. B. Hong, and K. B. Yoon, J . Am. Chem. Suc., 1996,118,9377. 19 G. Wirnsberger, H. P. Fritzer, A. Popitsch, G. van de Goor and P. Behrens, Angew. Chem., Int. Ed. Engl.,

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22 T. Dahl and I. Rwggen, 1. Am. Chem. Soc., 1996,118,4152. 23 See for example, H. I . Bloemink, J. H. Holloway and A.C. Legon, Chern. Phys. Len., 1996, 250, 557; K.

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