Preparation and reactivity of compounds containing a carbon-xenon bond

Article abstract of DOI:10.1021/om010490j

The chemistry of carbon-xenon compounds is one of the newest in the field of organo-element chemistry. The specific character of carbon-xenon chemistry can be ascribed to the fact that in all Xe^II and Xe^IV compounds hypervalent bonds-symmetrical or asymmetrical-with with a high partial positive charge on Xe are present. Different from the class of high-oxidation-state organometallic derivatives of d metals, C-Xe compounds have low coordination numbers and, additionally, contain three or two lone electron pairs. It is the intent of this review to present the still limited number of synthetic approaches to C-Xe compounds and to organize their presently known reactivities into categories. It is hoped that C-Xe compounds can be introduced in the future as precursors in synthetic organic and organoelement chemistry.

Full text of DOI:10.1021/om010490j

4750
Organometallics 2001, 20, 4750-4762
Reviews
P r ep a r a tion a n d Rea ctivity of Com p ou n d s Con ta in in g a
Ca r bon -Xen on Bon d
Hermann-J osef Frohn*
Fachgebiet Anorganische Chemie, Gerhard-Mercator-Universita¨t Duisburg,
Lotharstrasse 1, D-47048 Duisburg, Germany
Vadim V. Bardin
N. N. Vorozhtsov Institute of Organic Chemistry, 9 Academician Lavrentjev Avenue,
630090 Novosibirsk, Russia
Received J une 8, 2001
The chemistry of carbon-xenon compounds is one of the newest in the field of organo-
element chemistry. The specific character of carbon-xenon chemistry can be ascribed to
the fact that in all XeII and XeIV compounds hypervalent bondsssymmetrical or asymmetricals
with a high partial positive charge on Xe are present. Different from the class of
high-oxidation-state organometallic derivatives of d metals, C-Xe compounds have low
coordination numbers and, additionally, contain three or two lone electron pairs. It is the
intent of this review to present the still limited number of synthetic approaches to C-Xe
compounds and to organize their presently known reactivities into categories. It is hoped
that C-Xe compounds can be introduced in the future as precursors in synthetic organic
and organoelement chemistry.
I. In tr od u ction
noxenon(II) derivatives containing a relatively wide
range of organic substituents have been prepared. In
2000 the first C-XeIV compound was reported.10
An early review in 199311 dealt only with the begin-
nings of C-Xe chemistry, and the 1993 review of
Zhdankin12 focused comprehensively on the numerous
attempts to realize carbon-xenon bonds. Thus, it is
useful to review the new developments in this active
area at the present time.
P r op er ties of XeF 2 a n d XeF 4 a n d th e Rela ted
Ca tion s [XeF ]+ a n d [XeF 3]+ w ith Releva n ce for th e
F or m a tion of C-Xe Bon d s. The bond between xenon
and the fluorine atoms in XeF2 and XeF4 is best
described in terms of MO theory:13 e.g., for XeF2 as a
3c-4e bond. This triad usually is described as a hyper-
valent bond.14 Typical characteristics of such hyper-
valent bond triads are the strongly polar Xe-F bond
with a weak covalent component (formal bond order 0.5)
and a distinctly electrostatic component. Overall, XeF2
This review covers the preparative chemistry of the
organic compounds of xenon(II) and -(IV). Consequently,
C-Xe chemistry in the gas phase1 and in the matrix2
is excluded, as well as the formation of C-Xe species
by radiochemical methods3 (â- decay of C-131I com-
pounds).
When in 1962 the paradigm of the inertness of noble
gases had been overcome by the discovery of the first
Xe(II) compound by Bartlett,4 an intensive research in
noble-gas chemistry was initiated. A result of this
activity was the early discovery of the binary xenon
fluorides XeF2, XeF4, and XeF6, which are directly
accessible by reactions of the elements. On the basis of
reactions of these fluorides, bonds of xenon to oxygen,
nitrogen and xenon were unambiguously realized.5-7
The first compounds containing a C-XeII bond were
reported in 1989.8,9 Since then a wide variety of orga-
* To whom correspondence should be addressed. E-mail: frohn@
uni-duisburg.de.
(1) (a) Holtz, D.; Beauchamp, J . L. Science 1971, 173, 1237-1238.
(b) Hovey, J . K.; McMahon; T. B. J . Am. Chem. Soc. 1986, 108, 528-
529.
(2) Pettersson, M.; Lundell, J .; Ra¨sa¨nen, M. Eur. J . Inorg. Chem.
1999, 729-737.
(5) Sladky, F. Noble Gases. In Main Group Elements Group VII and
Noble Gases; Gutmann, V., Ed.; Butterworth: London, 1973; pp 1-52.
(6) Zˇemva, B. Noble Gases: Inorganic Chemistry. In Encyclopedia
of Inorganic Chemistry; King, R. B., Ed.; Wiley: New York, 1994; Vol.
5, pp 2660-2680.
(7) Holloway, J . H.; Hope, E. G. Adv. Inorg. Chem. 1999, 46, 51-
100.
(3) (a) Carlson, T. A.; White, R. M. J . Chem. Phys. 1962, 36, 2883-
2887. (b) Nefedov, V. F.; Toropova, M. A.; Levchenko, A. V. Ra-
diokhimiya 1967, 9, 138-139; Radiochemistry (Moscow) 1967, 9, 166-
167. (c) Toropova, M. A.; Nefedov, V. F.; Levchenko, A. V.; Saikov, Y.
P. Radiokhimiya 1968, 10, 611-613; Radiochemistry (Moscow) 1968,
10, 601-603. (d) Toropova, M. A.; Nefedov, V. F.; Levchenko, A. V.;
Matveev, O. G. Radiokhimiya 1968, 10, 613; Radiochemistry (Moscow)
1968, 10, 604. (e) Toropova, M. A.; Nefedov, V. F.; Levchenko, A. V.;
Saikov, Y. P. Radiokhimiya 1968, 10, 616-619; Radiochemistry
(Moscow) 1968, 10, 608-611.
(8) Naumann, D.; Tyrra, W. J . Chem. Soc., Chem. Commun. 1989,
47-50.
(9) Frohn, H.-J .; J akobs, S. J . Chem. Soc., Chem. Commun. 1989,
625-627.
(10) Frohn, H.-J .; LeBlond, N.; Lutar, K.; Zˇemva, B. Angew. Chem.,
Int. Ed. 2000, 39, 391-393.
(11) Frohn, H.-J . Nachr. Chem. Tech. Lab. 1993, 41, 956-964.
(12) Zhdankin, V. V. Izv. Akad. Nauk Ser. Khim. 1993, 1849-1857;
Russ. Chem. Bull. 1993, 42, 1763-1771.
(4) Bartlett, N. Proc. Chem. Soc. 1962, 218.
(13) Rundle, R. E. J . Am. Chem. Soc. 1963, 85, 112-113.
10.1021/om010490j CCC: $20.00 © 2001 American Chemical Society
Publication on Web 10/12/2001

Reviews
Organometallics, Vol. 20, No. 23, 2001 4751
and XeF4 have no permanent dipole moment. Despite
the high opposite partial charges on Xe and F, there is
no intermolecular interaction in crystalline XeF2
product with a half-life of ∼30 min at room temperature
and ∼2 weeks at -192 °C and a “vapor pressure
significantly larger (?) than that of XeF2” was not
unambiguously characterized or reproduced success-
fully. Schmeisser et al. tried to add CF3 radicals to Xe0
in analogy to the direct synthesis of XeF2 by addition
of fluorine atoms to Xe0, but without success.26 The
successful introduction of O and N groups into the XeII
moiety by metathesis stimulated different groups to
examine metathetical methods for carbon-xenon(II)
bond formation. By direct analogy with the introduction
of oxygen- and nitrogen-containing groups Seppelt and
coworkers27 reacted the CH acid [FSO2]3CH with XeF2
but observed only the formation of [FS(O)2]3CF. A
similar result was obtained in the reactions of K[CH3-
(NO2)2C] with XeF2 in CH2Cl2.28
Or ga n oxen on iu m (II) Sa lts by Su bstitu tion of
Bor on in Or ga n obor a n es (Xen od ebor yla tion ). In
the case of the nucleophilic introduction of organic
groups into XeF2 it is necessary to overcome the low
electrophilicity of the Xe center in XeF2. Additionally,
the lack of a permanent dipole moment in XeF2 explains
its low reactivity. One solution to overcome these
problems is the use of organic group transfer reagents
with a Lewis acidic center, which polarizes the hyper-
valent F-Xe-F bond and increases the electrophilicity
of the xenon atom. However, the degree of polarization
should be not too high, because too strong a polarization
favors the oxidation ability of [FXe]+. The first exami-
nation of this concept was carried out using an ap-
propriate triarylborane as the source of the organic
group. Tris(pentafluorophenyl)borane was found to react
with xenon difluoride with formation of arylxenonium
fluoroborates8,9,29 (eq 1).
15
(shortest intermolecular distance Xe‚‚‚F ) 3.42 Å) and
16
there are only very weak ones in crystalline XeF4
(>3.22 Å; the sum of van der Waals radii of Xe and F is
3.47 Å).17
Xenon difluoride shows no tendency to form stable
adducts with nucleophiles, even with the fluoride anion.
The absence of 18[F]-19[F] fluorine exchange under
neutral or basic conditions in the system (Xe19F2
+
18[F]-)18 is in agreement with the property of XeF2 of
forming no adducts.
In contrast to XeF2, the molecules XeF4 and XeF6,
with a higher partial charge on Xe, form fluoride ion
adducts.19,20 XeF2 interacts readily with Lewis acids as
a consequence of the relatively high negative partial
charge of the fluorine atoms. Depending on the strength
of the Lewis acid, the solid-state structures of such
adducts show that the Xe-F distance involving the
interacting F atom becomes longer (mainly an electro-
static Xe‚‚‚F interaction) but without complete separa-
tion, whereas the second Xe-F bond becomes shorter
and more covalent (2c-2e bond). The [FXe]+ cation as
the extreme case is a strong oxidizing agent (EA )
10.6,21 10.9 eV22) and is not compatible with organic
nucleophiles. The electronegativity of XeII in XeF2 is 2.4
and increases to 2.5 in [FXe]+ (calculated by the cor-
relation of Allred and Rochow). The increase in elec-
tronegativity is accompanied by the change of Mulliken
charges on F from -0.666 to -0.284 and on Xe from
1.331 to 1.284 for the transition from XeF2 to [FXe]+
(ab initio calculations: RHF, LANL2DZ).23 A similar
tendency was calculated for the change from XeF4 (Xe,
2.494; F, -0.623) to [XeF3]+ (Xe, 2.283; 2 Fax, -0.500;
CH2Cl2
3
F
eq, -0.286).24 The comparison of the fluoride donor
(C6F5)3B + XeF2
or CH CN8
ability shows that XeF2 is a better fluoride donor than
XeF4. The higher effective charge on XeIV makes the
fluoride abstraction more difficult.
[C6F5Xe][(C6F5)nBF4-n] (1)
n ) 3, CH3CN;8 n ) 1, CH2Cl2;9 n ) 2, CH2Cl2
29
II. Cu r r en t P r ep a r a tive Meth od s of C-XeII Bon d
The attempted use of pentafluorophenyl-containing
Lewis acids having central atoms other than boron with
a tetrahedral or trigonal-bipyramidal configuration was
not successful.
F or m a tion
Attem p ted F or m a tion of Ca r bon -Xen on Com -
p ou n d s. In 1979 Lagow25 claimed to have synthesized
Xe(CF3)2 by reacting XeF2 with CF3 radicals. The waxy
Sch em e 1
(14) (a) Musher, J . I. Angew. Chem. 1969, 81, 68-83. (b) Lattman,
M. Hypervalent Compounds. In Encyclopedia of Inorganic Chemistry;
King, R. B., Ed.; Wiley: New York, 1994; Vol. 3, pp 1496-1511. (c)
Brel, V. K.; Zefirov, N. S. Hypervalent Compounds of Xenon. In
Chemistry of Hypervalent Compounds; Akiba, K., Ed.; Wiley-VCH:
New York, 1999; pp 389-408.
(15) Levy, H. A.; Agron, P. A. J . Am. Chem. Soc. 1963, 85, 241-
242.
(16) Templeton, D. H.; Zalkin, A.; Forrester, J . D.; Williamson, S.
M. J . Am. Chem. Soc. 1963, 85, 242.
(17) Bondi, A. J . Phys. Chem. 1964, 68, 441-451.
(18) (a) Schrobilgen, G. J .; Firnau, G.; Chirakal, R.; Garnett, E. S.
J . Chem. Soc., Chem. Commun. 1981, 198-199. (b) Patrick, T. B.;
J ohry, K. K.; White, D. H.; Bertand, W. S.; Mokhtar, R.; Kilbourn, M.
R.; Welck, M. J . Can. J . Chem. 1986, 64, 138-141.
(19) Christe, K. O.; Curtis, E. C.; Dixon, D. A.; Mercier, H. P.;
Sanders, J . C. P.; Schrobilgen, G. J . J . Am. Chem. Soc. 1991, 113,
3351-3361.
(20) Peterson, S. W.; Holloway, J . H.; Coyle, B. A.; Williams, J . M.
Science 1971, 173, 1238-1239.
(21) Sladky, F.; Bulliner, P.; Bartlett, N. J . Chem. Soc. A 1969,
2179-2188.
(22) Schrobilgen, G. J . Lewis Acid Properties of Fluorinated Nobel
Gas Cations. In Synthetic Fluorine Chemistry; Olah, G. A., Chambers,
R. D., Prakash, G. K. S., Eds.; Wiley: New York, 1992; pp 1-30.
(23) Frohn, H.-J .; Klose, A.; Schroer, T.; Henkel, G.; Buss, V.; Opitz,
D.; Vahrenhorst, R. Inorg. Chem. 1998, 37, 4884-4890.
Scheme 1 illustrates the course of the xenodeboryla-
tion reaction: the interaction of boron with one basic
fluorine atom of XeF2 polarizes the xenon-fluorine
(24) Frohn, H. J . Abstracts of Papers; 219th National Meeting of
the American Chemical Society, San Franscisco, CA, March 26-30,
2000; American Chemical Society: Washington, DC, 2000; FLUO 0048.
(25) Turbini, L.; Aikman, R.; Lagow, R. J . Am. Chem. Soc. 1979,
101, 5833-5834.
(26) Schmeisser, M.; Walter, R.; Naumann, D. Z. Anorg. Allg. Chem.
1980, 464, 233-239.
(27) Klo¨ter, G.; Pritzkow, H.; Seppelt, K. Angew. Chem. 1980, 92,
954-955.
(28) Tselinskii, I. V.; Mel’nikov, A. A.; Varyagina, L. G.; Trubitsin,
A. E. Zh. Org. Khim. 1985, 21, 2490-2493.
(29) Frohn, H.-J .; J akobs, S.; Henkel, G. Angew. Chem., Int. Ed.
Engl. 1989, 28, 1506-1507.

4752 Organometallics, Vol. 20, No. 23, 2001
Reviews
bond, and the hypervalent bond becomes asymmetric.
Consequently, the electrophilicity of Xe increases as well
as the oxidation potential. On the other side, the
interaction of boron with a fluorine bonded to xenon
leads to tetracoordination at boron. Simultaneously, the
nucleophilicity of the C6F5 group at boron increases.
Boranes seem to offer the best geometry for a successful
transition state in the fluorine-organyl substitution on
XeF2.
In dichloromethane suspensions the salts [XC6H4-
Xe][(XC6H4)nBF4-n] are thermally unstable and decom-
pose already above ca. -40 °C. More stable tetrafluoro-
borate salts are obtained when the triarylboranes were
reacted in the presence of BF3‚OMe2 (eq 6).
CH2Cl2
Ar3B + 3XeF2 + 2BF3·OMe2
8
3[ArXe][BF4] + 2OMe2 (6)
The oxidation property of XeF2 itself and of its
polarized form in the interaction with boranes limits the
choice of appropriate organic groups. The demands of
high nucleophilicity and stability to oxidation are best
realized by fluorinated aryl groups. The electron-
withdrawing fluorine substituents make the C-C σ
system electron-poor, whereas the π system becomes
enriched by p-p π back-bonding. In organoxenonium
cations in which C(1) is part of a π system, Xe increases
the π-electron density on C(1) by polarization and this
strengthens the C-Xe bond electrostatically.23 However,
in alkylxenonium cations with fluorine atoms bonded
at C(1), the C-Xe bond is weakened by repulsion of the
highly positive partial charges on C(1) and Xe.
An unusual variant of xenodeborylation is the nu-
cleophilic transfer of the C6F5 group in anhydrous HF
(aHF).30 It should be mentioned that XeF2 is very
soluble in aHF (9.88 mol/kg, 30 °C),31 whereas (C6F5)3B
is practically insoluble even at room temperature and
undergoes solvolysis only very slowly (72 h), yielding
C6F5H and [BF4]-. Xenon difluoride (3 equiv) is con-
sumed within 4 h when reacted with (C6F5)3B in aHF
at ca. -30 °C. The main products are [C6F5Xe]+ and
[BF4]-. Minor byproducts are [(C6F5)2BF2]-, [cyclo-C6F7-
BF3]-, and C6F6 (eq 2).
Ar ) 2-C6H4F, 4-C6H4F, 2,6-C6H3F2,34
2,4,6-C6H2F3,35 4-C6H4Cl, 2,6-C6H3Cl2,
2,4,6-C6H2Cl3
35,36
The majority of the salts obtained by this method
were not isolated but were characterized unambiguously
1
in solution by their H, 19F, and 129Xe NMR spectra.
All these routes to arylxenonium(II) salts are based
on Ar3B and are limited by the availability of the
corresponding triarylboranes. The synthesis and puri-
fication of arylboranes with electron-poor aryl groups
is a delicate procedure which in many cases results in
low yields.37 Recently a convenient access to the salts
[ArXe][BF4] was developed on the basis of the reaction
of XeF2 with in situ generated aryldifluoroboranes38
(eq 7).
BF3/CH2Cl2
XeF2/CH2Cl2
K[ArBF3](s)
8 ArBF2
8 [ArXe][BF4]
(7)
-40 °C
-K[BF4](s)
Ar ) C6HnF5-n (n ) 1-5)
Evidently, this approach is general and can be used
also for the introduction of other organic groups. This
was demonstrated by the preparation of trifluorovinyl-
xenonium tetrafluoroborate39 (eq 8) and the first organic
compound of xenon(IV), trans-difluoro(pentafluoro-
phenyl)xenonium(IV) tetrafluoroborate,10 which will be
discussed later.
aHF
3XeF2 + (C6F5)3B -30 °C8
3[C6F5Xe]+ + [BF4]- + 2[F(HF)n]- (2)
In aHF all C6F5 groups of the borane are transferred
to xenon, in contrast to the reaction in CH2Cl2. Indeed,
the fluoroborate anions [(C6F5)nBF4-n]- cannot transfer
C6F5 groups directly to xenon difluoride, but in aHF they
undergo abstraction of fluoride by HF (eq 3).
BF3/CH2Cl2
XeF2/CH2Cl2
K[CF2dCFBF3](s)
8 CF2dCFBF2
8
-40 °C
-K[BF4](s)
[CF2dCFXe][BF4]
(8)
[(C6F5)nBF4-n]- + mHF u
85%
(C6F5)nBF3-n + [F(HF)m]- (3)
A related procedure was used for the preparation of
some alkynylxenonium(II) tetrafluoroborates40 (eq 9).
The abstraction of fluoride occurs faster than the
oxidation of the aryl group in the fluoroborate anion by
the polarized XeF2 molecule (eq 4).
BF3/CH2Cl2
XeF2/CH2Cl2
t-BuCtCLi
8 Li [t-BuCtCBF3]
8
-100 °C
-40 °C
F-Xe-F···HF + [C6F5BF3]- aHF8
[cyclo-C6F7BF3]- + Xe0 (4)
[t-BuCtCXe][BF4] (9)
(30) Frohn, H.-J .; Schroer, T. J . Fluorine Chem., accepted for
publication.
(31) Nikolaev, A. V.; Opalovskii, A. A.; Nazarov, A. S.; Tret’yakov,
G. V. Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. 1972, 21-27.
(32) Frohn, H.-J .; Rossbach, C. Z. Anorg. Allg. Chem. 1993, 619,
1672-1678.
In this interesting modification of xenodeborylation
the resulting arylxenonium cations are accompanied by
a mixture of [BF4]- and [F(HF)n]- anions.
The method of xenodeborylation could be extended to
hydrogen-containing arylxenonium(II) (aryl)fluoro-
borates as long as the aryl group bears at least one
electron-withdrawing substituent X (eq 5).
(33) Frohn, H.-J .; J akobs, S.; Rossbach, C. Eur. J . Solid State Inorg.
Chem. 1992, 29, 729-738.
(34) Naumann, D.; Butler, H.; Gnann, R.; Tyrra, W. Inorg. Chem.
1993, 32, 861-863.
(35) Butler, H.; Naumann, D.; Tyrra, W. Eur. J . Solid State Inorg.
Chem. 1992, 29, 739-758.
(36) Naumann, D.; Tyrra, W.; Pfolk, D. Z. Anorg. Allg. Chem. 1994,
620, 987-992.
(XC6H4)3B + XeF2 f [XC6H4Xe][(XC6H4)nBF4-n] (5)
(37) Naumann, D.; Butler, H.; Gnann, R. Z. Anorg. Allg. Chem. 1992,
618, 74-76.
32
X ) 3-F,32 4-F,32,33 3-CF3,32,33 4-CF3

Reviews
Stang and Zhdankin assumed that this reaction
Organometallics, Vol. 20, No. 23, 2001 4753
isolated in 10-15% yield, whereas the other compounds
were only detected in the reaction mixture by means of
their 13C, 19F, and 129Xe NMR spectra. No arylxenonium
compounds were obtained with benzene, toluene, aniline,
phenol or biphenyl or with 1,3,5-(CF3)3C6H3. The ori-
entation of the xenodeprotonation suggests the electro-
philic route of this process. However, the low yields and
the nonregiospecificity make the xenodeprotonation
inferior as a alternative preparative method to the
xenodeborylation approach, despite its scientific inter-
est.
proceeds via the alkynyltrifluoroborate.40 However, it
should be remembered that all other preparations of
organoxenonium salts with electrophilic cations proceed
under acidic conditions. Therefore, it seems more prob-
able that in the presence of the excess of BF3 the real
substrate might be the corresponding alkynyldifluo-
roborane. The nature of the key reactive alkynyl ele-
ment intermediate in the reaction of RCtCSiMe3 (R )
Et, Pr, t-Bu, Me3Si) with XeF2 in the presence of
BF3‚OEt2 (CH2Cl2/-70 to -50 °C)40 is not clear (eq 10).
Dia r ylxen on , XeAr 2, a n d Ar ylxen on iu m F lu o-
r id e, Ar XeF , Der iva tives by th e Ba se-Ca ta lyzed
Nu cleop h ilic In tr od u ction of th e Ar yl Gr ou p C6F 5
in to th e XeF 2 Molecu le. In addition to the previously
discussed methods of xenodeborylation and xenodepro-
tonation in acidic media Naumann et al. reported
recently an new method of introducing the C6F5 group
into XeF2 using basic catalysis. XeF2 reacts with 2 equiv
of C6F5SiMe3 after addition of a catalytic amount of
[NMe4] F, forming Xe(C6F5)2 (eq 13). With 1 equiv of
the silane C6F5SiMe3, C6F5XeF was formed with an
CH2Cl2
RCtCSiMe3 + XeF2 + BF3·OEt2 -70 to -50 °C8
[RCtCXe][BF4] (10)
R ) Et, Pr, t-Bu, Me3Si
The formation of the alkynylxenonium tetrafluoro-
borates was proven by their 129Xe and 13C NMR spectra.
No 129Xe resonance was observed when R was Ph-
CtC.40 None of the alkynylxenonium salts were iso-
lated.
45
admixture of Xe(C6F5)2 (eq 14). In neither case was a
Ar ylxen on iu m (II) Sa lts by Su bstitu tion of Hy-
d r ogen in Ben zen e Der iva tives (Xen od ep r oton a -
tion ). Numerous efforts to replace hydrogen in aromatic
compounds by an electrophile generated from XeF2 and
a Lewis acid were unsuccessful. Usually they led to the
addition of fluorine to the C-C double bond or resulted
in fluorodeprotonation.41,42 Naumann et al. offered the
unique electrophilic XeII system generated by the reac-
tion of XeF2 in sequence with CF3CO2H and CF3SO3H
in CCl3F at -40 °C. The nature of the resulting reagent
was not elucidated. The asymmetric compound CF3CO2-
XeO3SCF3 or its protonated form was assumed to be the
reactive key intermediate in the substitution of hydro-
gen in benzene derivatives by the XeO3SCF3 group43,44
(eqs 11 and 12).
yield reported.
[[NMe4]F]
XeF2 + 2C6F5SiMe3 -60 to -40 °C8
Xe(C6F5)2 + 2Me3SiF (13)
[[NMe4] F]
XeF2 + C6F5SiMe3 -60 to -40 °C8
C6F5XeF + Xe(C6F5)2 + Me3SiF (14)
solvents ) C2H5CN, CH3CN, CH2Cl2
It should be mentioned that under similar conditions
the silane CF3SiMe3 gave only C2F6 and no trifluoro-
methylxenon(II) compounds were detected.45
The intermediate generation of arylxenon(II) com-
pounds in the reactions of 4-RC6H4SiMe3 with XeF2
2. CF3SO3H
XeF2 1. CF CO H 8 Xe(O2CCF3)2 CCl F/-40 °C8
3
2
CCl3F/-40 °C
41,46,47
3
as well as in the fluoride-catalyzed reactions of
42,46,48
CF3CO2XeO3SCF3 (11)
CF3CO2XeO3SCF3 + Ar-H CCl F8
RC6F4SiX3 (X ) Me, F)
was proposed. Later
Ramsden showed that 4-R-C6H4SiMe3 forms 4-R-C6H4F
on reaction with XeF2 only in the presence of an acidic
Pyrex surface but not in FEP (copolymer of tetra-
fluoroethylene and hexafluoropropylene) vessels.49 The
Lewis acid catalyzed interaction of XeF2 with silyl- and
germylpentafluorobenzenes C6F5MX3 (M ) Si, Ge; X )
Me, Et, F) resulted in ring-fluorination products.50
Cova len t Com p ou n d s Ar XeY fr om Ar ylxen on -
iu m (II) Sa lts. Despite the weak nucleophilicity of the
[AsF6]- anion, there is a significant cation-anion
contact with a distance Xe‚‚‚F of 2.672(5) Å and a nearly
linear C(1)-Xe‚‚‚F triad (174.2(3)°) in solid [C6F5Xe]-
[AsF6].23 This contact is 23% shorter than the sum of
3
[ArXe][O3SCF3] + CF3CO2H (12)
Ar ) 2,4,6-C6H2Cl3, 2,4,6-C6H2F3, 2-F-5-NO2C6H3,
2-F-5-CF3C6H3, 3,5-(CF3)2C6H3, 2,4-C6H3F2,
3-NO2C6H4, 3,5-(NO2)2C6H3, C6H4Cl and C6H4F
(isomer mixtures)
The compounds with Ar ) 2,4,6-C6H2F3, 2-F-5-
NO2C6H3, 2-F-5-CF3C6H3, and 3,5-(CF3)2C6H3 were
(38) Frohn, H.-J .; Franke, H.; Bardin, V. V. Z. Naturforsch. 1999,
54b, 1495-1498.
(39) Frohn, H.-J .; Bardin, V. V. Chem. Commun. 1999, 919-920.
(40) Zhdankin, V. V.; Stang, P. J .; Zefirov, N. S. J . Chem. Soc., Chem.
Commun. 1992, 578-579.
(45) Maggiarosa, N.; Naumann, D.; Tyrra, W. Angew. Chem., Int.
Ed. 2000, 39, 4588-4591.
(46) Bardin, V. V.; Stennikova, I. V.; Furin, G. G.; Leshina, T. V.;
Yakobson, G. G. Zh. Obshch. Khim. 1988, 58, 2580-2588; J . Gen.
Chem. USSR 1988, 58, 2297-2303.
(41) Bardin, V. V.; Yagupol’skii, Y. L. Xenon Difluoride. In New
Fluorinating Agents for Organic Synthesis; German, L. S., Zemskov,
S. V., Eds.; Springer: Berlin, 1989; pp 1-34.
(47) Lothian, A. P.; Ramsden, C. A. Synlett 1993, 753-755.
(48) Frohn, H.-J .; Bardin, V. V. J . Organomet. Chem. 1995, 501,
155-159.
(42) Bardin, V. V.; Shchegoleva, L. N.; Frohn, H.-J . J . Fluorine
Chem. 1996, 77, 153-159.
(43) Naumann, D.; Tyrra, W.; Gnann, R.; Pfolk, D. J . Chem. Soc.,
Chem. Commun. 1994, 2651-2653.
(49) Ramsden, C. A.; Smith, R. G. J . Am. Chem. Soc. 1998, 120,
6842-6843.
(44) Naumann, D.; Tyrra, W.; Gnann, R.; Pfolk, D.; Gilles, T.; Tebbe,
K.-F. Z. Anorg. Allg. Chem. 1997, 623, 1821-1834.
(50) Bardin, V. V.; Frohn, H.-J . J . Fluorine Chem. 1993, 60, 141-
151.

4754 Organometallics, Vol. 20, No. 23, 2001
Reviews
the van der Waals radii of Xe and F of 3.47 Å.17 The
contact to the weakly nucleophilic [AsF6]- anion em-
phasizes the electrophilic character of the arylxenonium
cation. The linear triad C(1)-Xe‚‚‚F is best described
as an asymmetric hypervalent bond with a nearly 2c-
2e C(1)-Xe bond and a mainly electrostatic Xe‚‚‚F
interaction. A similar structural pattern was found for
[2,6-C6H3F2Xe][BF4].51
(Pentafluorophenyl)xenonium acylates were obtained
in aqueous solution by the reaction of [C6F5Xe][AsF6]
with cesium acylates. The pentafluorobenzoate,57 tet-
rafluoroterephthalate, and trifluoromethanesulfonate58
derivatives were prepared in this manner (eq 19).
D2O
[C6F5Xe][AsF6] + Cs[O2CC6F5] -Cs[AsF ]8
6
The description of this structural motif as an asym-
metric, angled fluoride bridge between the arylxenon-
ium cation and the Lewis acid AsF5 may be an alter-
native one. By emphasizing in this way the Lewis acidic
nature of the [C6F5Xe]+ cation, it is understandable that
the mononuclear C6F5XeF molecule can exist as well as
the fluoride-bridged dinuclear cation [(C6F5Xe)2F]+. The
heterogeneous reaction of [C6F5Xe][AsF6] with an equi-
molar amount of [NMe4]F in CH2Cl2 leads to
C6F5XeF.52-54 The latter is not available under acidic
conditions from the reaction of a [FXe]+ source and a
[C6F5]- substrate. C6F5XeF is a covalent compound,
which is soluble in CH2Cl2 (eq 15).
C6F5XeO2CC6F5V
(19)
80%
Alternative routes to arylxenonium triflates involve
the reaction of [ArXe][BF4] salts with Me3SiOTf
(eq 20) or the xenodeprotonation reaction of xenon
trifluoroacetate triflate with substituted benzene de-
rivatives (see above).43,44
59
CH3CN, CH2Cl2
[2,6-C6H3F2Xe][BF4] + TfOSiMe3 -20 °C, 4 days 8
[2,6-C6H3F2Xe][OTf] + Me3SiF + BF3 (20)
In [2,6-C6H3F2Xe][OTf] there is an asymmetric
C-Xe‚‚‚O triad with a strong Xe‚‚‚O contact of 2.687(9)
Å.44
[C6F5Xe][AsF6](s) + [NMe4]F CH Cl /-78 °C8 C6F5XeF
2
2
[NMe4][AsF6](s)
(15)
The C-Xe‚‚‚O triad in (pentafluorophenyl)xenon(II)
pentafluorobenzoate becomes significantly more sym-
metrical and the bond covalent. Consequently, the
compound is soluble in CH2Cl2. The crystal structure
shows the xenon atom linearly (178.1(1)°) bonded to
carbon and oxygen with a Xe-O distance of 2.367(3) Å,
which is 32% shorter than the sum of the van der Waals
radii of Xe and O (3.52 Å17).
The nucleophilic xenon-bonded fluorine atom of C6F5-
XeF can react with a further [C6F5Xe]+ cation to yield
[(C6F5Xe)2F][AsF6]. This salt is insoluble in CH2Cl2 but
soluble in polar CH3CN52-54 (eq 16).
-78 °C
C6F5XeF + [C6F5Xe][AsF6]
8
CH2Cl2
C6F5XeF is a useful starting material for introducing
a second organic group into the C6F5Xe moiety. The first
approach consists of the homogeneous reaction with an
organic transfer reagent of weak Lewis acidity such as
organocadmium compounds or organotrimethylsilanes.
Alternatively, a second aryl group can be introduced
under basic conditions (fluoride catalysis;45 see above).
[(C6F5Xe)2F][AsF6]V (16)
In a similar way, two C6F5-Xe-Cl compounds had
been isolated earlier. The first was the covalent com-
pound C6F5XeCl, which is soluble in CH2Cl2. The second
is a salt containing the dinuclear [(C6F5Xe)2Cl]+ cat-
ion.55,56 The latter features a nearly symmetrical, bent
Xe-Cl-Xe bridge, with Xe(1)-Cl-Xe(2) ) 176.0(3)°,
Xe(1)-Cl ) 2.847(2) Å, and Xe(2)-Cl ) 116.96(8) Å (eqs
17 and 18).
C6F5XeF reacts rapidly with Cd(C6F5)2 at low tem-
54,60
perature with formation of Xe(C6F5)2
(eq 21).
CH2Cl2
C6F5XeF + 0.5Cd(C6F5)2
8 Xe(C6F5)2 + 0.5CdF2
-78 °C
CH2Cl2/-78 °C
(21)
[C6F5Xe][AsF6](s) + 4-ClC5H4N·HCl
8
-[4-ClC5H4NH][AsF6]V
C6F5XeCN is formed in the reaction of C6F5XeF with
Me3SiCN54,60 (eq 22).
C6F5XeCl
85%
(17)
CH2Cl2
CH2Cl2/-78 °C
C6F5XeF + Me3SiCN
8 C6F5XeCN + Me3SiF
2[C6F5Xe][AsF6](s) + 6Me3SiCl -6Me SiF, -AsCl , -Cl 8
-78 °C
3
3
2
(22)
[(C6F5Xe)2Cl][AsF6]V
(18)
91%
Similar to the synthesis of bis(pentafluorophenyl)-
xenon, Xe(2,4,6-C6H2F3)2 and the asymmetric diaryl-
xenon(II) compound C6F5Xe(2,4,6-C6H2F3) were ob-
tained by reaction of 2,4,6-C6H2F3XeF with Cd(2,4,6-
(51) Gilles, T.; Gnann, R.; Naumann, D.; Tebbe, K.-F. Acta Crys-
tallogr. 1994, C50, 411-413.
(52) Frohn, H.-J .; Schroer, T.; Theissen, M. New Molecules with
Xenon-Carbon Bond. Presented at the 12th European Symposium on
Fluorine Chemistry, Berlin, Germany, August-September 1998; Paper
B37.
(53) Theissen, M.; Frohn, H.-J . Pentafluorophenylxenon Fluoride.
Presented at the 8th Deutscher Fluortag, Schmitten, Germany,
September 1998; Paper 24.
(54) Frohn, H.-J .; Theissen, M. Angew. Chem., Int. Ed. 2000, 39,
4591-4593.
(56) Frohn, H.-J .; Schroer, T.; Henkel, G. Angew. Chem., Int. Ed.
1999, 38, 2554-2556.
(57) Frohn, H.-J .; Klose, A.; Henkel, G.; Angew. Chem., Int. Ed. Engl.
1993, 32, 99-100.
(58) Frohn, H.-J .; Klose, A. Unpublished results.
(59) Naumann, D.; Gnann, R.; Padelidakis, V.; Tyrra, W. J . Fluorine
Chem. 1995, 72, 79-82.
(55) Frohn, H.-J .; Schroer, T. Synthesis, Molecular Structure and
Reactivity of the First Compounds Containing Xenon(II)-Chlorine
Bonds. Presented at the 15th International Symposium on Fluorine
Chemistry, Vancouver, BC, Canada, August 1997; Paper Ba(2) C-4.
(60) Frohn, H.-J .; Theissen, M. C6F5XeF: An Important Key Sub-
strate for the Synthesis of New Xenon(II) Compounds. Presented at
the 16th International Symposium on Fluorine Chemistry, Durham,
U.K., J uly 2000; Poster 2P-20.

Reviews
Organometallics, Vol. 20, No. 23, 2001 4755
C6H2F3)2 and of C6F5XeF with Cd(2,4,6-C6H2F3)2,
respectively.61
those in aryl groups increase the relative stability of the
C-Xe bond. Aryl groups with σ-electron-withdrawing
character prevent the transfer of positive charge from
Xe to the aryl group. Reduction of the electron acceptor
ability of the aryl group increases the electrophilicity
of C(1), which will be accompanied by a weakening of
the C-Xe bond. Anions of low nucleophilicity are
favored for organoxenonium salts. In the case of kineti-
cally labile anions such as fluoroelementate anions
[(C6F5)mEFn+1]-, we have to consider additionally the
contribution of their solvolysis in basic solvents, which
can thereby produce the strongly nucleophilic fluoride
anion. The presence of either nucleophilic anions or
basic solvents destabilizes the corresponding organo-
xenonium salt, because the interaction of a nucleophile
at Xe(II) weakens the 2c-2e C-Xe bond.
C6F5XeF does not react with Me3SiC6F5 alone in CH2-
Cl2 (weakly acidic conditions). The composition of the
reaction mixture obtained under F- catalysis in CH2-
Cl2 strongly depends on the concentration. In dilute
solution the aryl nucleophile generated from Me3SiC6F5
attacks the solvent (protodesilylation) but in highly
concentrated solutions the desired product, Xe(C6F5)2,
was obtained.62
III. Th e F ir st Syn th esis of a n Ar ylxen on iu m (IV)
Sa lt by Su bstitu tion of Bor on in Or ga n obor a n es
(Xen od ebor yla tion )
Recently, C-Xe bond formation could be extended to
XeIV. Xenon tetrafluoride is a weaker fluoride donor
than XeF2 and possesses a higher oxidation potential
than XeF2. An additional disadvantage of XeF4 is its low
solubility in aHF,31 a solvent which is stable toward
oxidation. In the reaction of XeF4 with B(C6F5)3 in aHF
only [C6F5Xe]+ salts were obtained instead of Xe(IV)
salts because of the spontaneous reaction of the elec-
Suitable solvents for ionic [RXe][Y] compounds are
polar basic ones such as CH3CN, CH3NO2, H2O, and
CF3CH2OH, which predominantly solvate the cation, or
strongly acidic ones such as aHF, CF3SO3H or SO2,
which preferably solvate the anion and result thereby
in a comparatively naked cation.
trophilic [C6F5XeF2]+ cation with the initially formed
V. Th er m a l Sta bility of Or ga n oxen on iu m Sa lts
a n d Cova len t Com p ou n d s Ar XeY (Ar ) Ar yl)
-
nucleophilic [(C6F5)nBF4-n
]
anions. With C6F5BF2 in
CH2Cl2 the reaction was successful10 (eq 23), whereas
In many cases the data for the thermal stability of
organoxenon compounds are only qualitative, especially
for thermally very labile species, because the decompo-
sition points depend strongly on the purity of the
compounds, on the rate of heating, and on other
experimental details. The most reliable information
exists for the salt [C6F5Xe][AsF6]. The pure salt melts
at 102 °C and decomposes slowly above 125 °C (eq 24).
CH2Cl2
XeF4 + C6F5BF2 -55 to -40 °C8 [C6F5XeF2][BF4] (23)
in CH3CN no substitution of a fluorine atom by an aryl
group occurred. Probably, in the basic solvent the acidity
of the aryldifluoroborane is not high enough to polarize
XeF4 effectively.
The bright yellow compound [C6F5XeF2][BF4] is ther-
mally less stable than the analogous XeII compound
[C6F5Xe][BF4] and decomposes above -20 °C.
In addition, recently the formation of the cations
[C6F5XeF2]+ and [C6F5Xe]+ was detected spectroscopi-
cally by Schrobilgen in the reaction of XeF6 with
B(C6F5)3 in SO2ClF. Also formed were oxidized fluoro-
aromatic compounds.63
[C6F5Xe][AsF6](s) g125 °C8 C6F6 + AsF5 + Xe0 (24)
In acetonitrile solution the decomposition already pro-
ceeds very slowly even at room temperature and is
accelerated on heating to 80 °C, forming C6F5H and
C12F10 (6.7:1 at 20 °C and 12:1 at 80 °C) (eq 25). In
CH3CN
[C6F5Xe][AsF6]
8 C6F5H + C12F10 + Xe0 (25)
IV. Rea ctivity of Ca r bon -Xen on Com p ou n d s
80 °C, 4 h
At the beginning of this section some general infor-
mation about the reactivity of organoxenon compounds
shall be given. The reactivity of organoxenonium(II) and
-(IV) salts is determined by the electrophilic character
of the cations. In covalent C-Xe compounds with
symmetric or asymmetric 3c-4e bonds the high partial
positive charge on Xe has a strong influence on the
reactivity. Most information about reactivity deals with
reactions of organoxenonium(II) salts with nucleophiles.
The reactivity, as well as the stability, of organo-
xenonium salts [RXe][Y] depends on the nature of the
organic group R, the counteranion [Y]-, and, if in
solution, on the nature of the solvent.
aqueous solution the stability of the salt [C6F5Xe][AsF6]
is significantly lower than in CH3CN.23
Presumably, if we neglect the interaction of the
electrophilic cation with the fluoride ion, possibly present
as a solvolysis product of [AsF6], the decomposition of
the donor-acceptor complex [C6F5Xe‚‚‚NCMe][AsF6]
takes place. Related complexes with substituted py-
ridines in acetonitrile solution show a remarkable
dependence of their thermal stability on the basicity of
the nitrogen atom and are less thermally stable than
the parent salt [C6F5Xe][AsF6]64 (eqs 26 and 27).
total dec
All known organoxenonium salts have in common
that Xe(II) or Xe(IV) is bonded to a carbon atom which
is part of a π system. More extended π systems such as
[C6F5Xe·NC5H3(CH3)2-2,6][AsF6]
8
-40 °C
100% (C6F5H + C12F10) + Xe0 (26)
(61) Frohn, H. J . Synthetic Approaches to Organo Xenon Com-
pounds. Presented at the International Chemical Congress of Pacific
Basin Societies, Honolulu, HI, December 2000; Paper 1176.
(62) Frohn, H. J .; Theissen, M. Unpublished results.
[C6F5Xe·NC5H3F2-2,6][AsF6] total dec8
20 °C
35% (C6F5H + C12F10) + Xe0 +
(63) Schrobilgen, G. J .; Becher, M. Personal communication.
65% C-arylated pyridines (27)

4756 Organometallics, Vol. 20, No. 23, 2001
Reviews
AsF5·CH3CN
-40 to 20 °C
In the thermolysis of the solid adducts [C6F5Xe‚
N-base][AsF6] (N-bases are substituted pyridines) N-
and C-pentafluorophenylation of the corresponding
pyridines is preferred over formation of C6F5H and
[C6F5Xe][(C6F5)2BF2]
8
[C F Xe][AsF ]
+ (C6F5)2BF·CH3CN (34)
6
5
6
78%
64
C12F10 (eq 28).
VII. Rea ction s of Or ga n oxen on iu m (II) Sa lts
w ith Nu cleop h iles
[C6F5Xe·NC5H5][AsF6](s) 30 °C8
[C6F5-NC5H5][AsF6] (58%) + C6F5C5H4N (24%) +
(C6F5H + C12F10) (12%) + Xe0 (28)
Rea ction s w ith Ha lid e An ion s. According to NMR
spectral data, organoxenonium salts exist in highly
acidic media such as superacids as separated ions with
solvated anions and nearly naked organoxenonium
cations. The latter possess a high electrophilicity and
high reactivity toward nucleophiles. On the other hand,
the reactivity of nucleophiles is diminished in super-
acidic media. Thus, (pentafluorophenyl)xenonium hexa-
fluoroarsenate does not react with KF or [Me4N]Cl in
aHF solution at room temperature over days. The same
inertness toward fluoride and chloride anions in aHF
is found also for (nonafluorocyclohexen-1-yl)xenonium
hexafluoroarsenate.66
The covalent XeII compounds C6F5XeO2CC6F5,57 C6F5-
XeCl,56 and C6F5XeF54 in the solid state are thermally
less stable than [C6F5Xe][AsF6] and evolve xenon al-
ready at ca. 35 °C (fast heating), whereas in solution
the decomposition takes place at significantly lower
temperatures (eqs 29-31).
C6F5XeOC(O)C6F5 ∼35 °C8 C6F5OC(O)C6F5 + Xe0
(29)
Alternatively, reactions with bromide and iodide
anions occur readily in aHF and lead to the correspond-
ing bromo and iodo derivatives with reductive elimina-
tion of Xe66-68 (eqs 35-37).
C6F5XeCl ∼36 °C8 C6F5Cl + Xe0
(30)
(31)
C6F5XeF ∼20 °C8 C6F6 + Xe0
4 h
It is interesting that solid (C6F5Xe)2Cl][AsF6] shows
a thermal stability similar to that of [C6F5Xe][AsF6] and
decomposes at 100 °C, yielding Xe and C6F5Cl, in
addition to C6F6, AsF5, and [C6F5Xe][AsF6].56
VI. An ion Meta th esis Rea ction s of
Ar ylxen on iu m F lu or obor a tes
The thermal stability of organoxenonium salts [RXe]-
[Y] is strongly influenced by the nucleophilicity of the
anion (comparing salts with the same [RXe] cation). This
influence can be demonstrated with [ArXe][ArnBF4-n
]
salts. As one goes from n ) 3 to n ) 0, the thermal
stability under the same conditions increases signifi-
cantly. It should be mentioned that the anions [Arn-
-
BF4-n
]
have two sites available for electrophilic at-
tack: the carbon atom C(1) of the aryl group and the
fluorine atom bonded to boron.
One route to more thermally stable [ArXe][Y] salts
uses the conversion of [ArXe][ArnBF4-n] salts, which
were formed initially in the reaction of Ar3B with XeF2
into salts with more weakly nucleophilic anions. This
can be achieved, for example, by treatment with fluoride
23
anion acceptors such as AsF5‚CH3CN23,29,65 or AsF5(g)
.
By this procedure pure arylxenononium(II) hexafluoro-
arsenates of high thermal stability were obtained (eqs
32-34).
In basic solvents such as CH3CN and C2H5CN,
fluorine-containing aryl-, cycloalkenyl-, and vinyl-
xenonium salts show similar reactivity patterns toward
“soft” anionic nucleophiles such as bromide and iodide
(eqs 38 and 39).
CH2Cl2
(C6F5)3B + XeF2
8 [C6F5Xe][(C6F5)2BF2] (32)
-50 °C
AsF5(g)/CH2Cl2
[ArXe][Y] + MX CH CN8 ArX + Xe0 + MY (38)
[C6F5Xe][(C6F5)2BF2]
8
3
-40 to 20 °C
[C F Xe][AsF ]
+ (C6F5)2BF (33)
6
5
6
X ) I, Y ) ArBF3, Ar ) C6F5,9 3-CF3C6H4,
4-CF3C6H4, 4-C6H4F;33,32 X ) I, Y ) AsF6,
Ar ) C6F5;69 X ) Br, Ar ) C6F5, Y ) ArBF3,9 AsF6
76%
(64) Frohn, H.-J .; Schroer, T.; Henkel, G. Z. Naturforsch. 1995, 50b,
1799-1810.
69

Reviews
Organometallics, Vol. 20, No. 23, 2001 4757
Sch em e 2
[RXe][Y] + MX CH3CN or C2H5CN8 RX + Xe0 + MY (39)
X ) Br, I, R ) cyclo-1,4-C6F7, cyclo-C6F9,
cyclo-2-HC6F8, Y ) AsF6;70,68 X ) I,
However, in reactions with [Me4N][HF2] or anhydrous
[Me4N]F in CH3CN (the latter is not really stable at
room temperature) hexafluorobenzene, pentafluoroben-
zene, and decafluorobiphenyl are obtained in addition
to HF69 (Scheme 2).
It is likely that the reaction of [C6F5Xe][AsF6] with
CsF in acetonitrile proceeds via the [(C6F5Xe)2F]+ cation
(see section II). Its decomposition at -20 °C is ac-
companied by the chemically induced dynamic nuclear
polarization (CIDNP) of the 19F NMR signals of C6F5H
(C6F5D) and C12F10, indicating the intermediate genera-
tion of pentafluorophenyl radicals. The generation of the
latter species is supported by the formation of a mixture
of C6F5C6H4F isomers when the reaction was performed
in the presence of fluorobenzene (see later).69
A peculiar reaction route was found in the case of
(perfluorocycloalken-1-yl)xenonium hexafluoroarsenates
with the fluoride anion in CH3CN.71,66 Treatment of the
salt [cyclo-1,4-C6F7Xe][AsF6] with sodium fluoride re-
sults in a contraction of the ring, and perfluoro-3-
methylenecyclopentene is formed. This conversion can
be rationalized in terms of a preferential attack of the
“hard” anionic nucleophile fluoride at the “hard” elec-
trophilic C(2) site, rather than at the “softer” electro-
philic xenon(II) site71 (eq 41).
39
R ) CF2dCF, Y ) BF4
The reaction of the cycloalkenylxenonium salt [cyclo-
C6F9Xe][AsF6] with Cl- leads to 1-H-nonafluorocyclo-
hexene as the major product in addition to 1-chloronona-
fluorocyclohexene (ca. 4:3).66 Under the same condi-
tions, chloropentafluorobenzene and pentafluorobenzene
(traces) are obtained from the salt [C6F5Xe][AsF6],9
whereas the slow reaction of [2,4,6-C6H2F3Xe][BF4] with
Cl- results in 1,3,5-trifluorobenzene and 2,4,6-trifluo-
rochlorobenzene (4:1).35 The competitive reaction of
[C6F5Xe][AsF6] and [cyclo-C6F9Xe][AsF6] with chloride
anions in CH3CN results in the faster conversion of the
unsaturated derivative in comparison to the aromatic
salt.66 (eq 40).
The interaction of the salt [C6F5Xe][AsF6] with the
“hardest” nucleophile fluoride in acetonitrile proceeds
in a more complex way. The treatment of [C6F5Xe][AsF6]
with an excess of CsF at room temperature leads to the
evolution of xenon and the formation of C6F5H and
C6F5C6F5 in a 5.6:1 molar ratio (in CH3CN) or of C6F5D
and C6F5C6F5 (1.7:1) (in CD3CN).69 No C6F6 was de-
tected in either case, which is in contrast to the reported
reaction of [C6F5Xe][(C6F5)3BF] with KF in CH3CN.8
The different behavior of the “hard” chloride anion
and the “hardest” fluoride anion in aHF compared with
CH3CN toward electrophilic organoxenonium com-
pounds arises from the higher degree of protonation of
F- in aHF, which diminishes sufficiently the nucleo-
philicity of F-, and, on the other hand, from the lower
reducing potential of HCl compared to Cl- (Scheme 3).
(65) Frohn, H.-J .; Klose, A.; Henkel, G. GIT Fachz. Lab. 1993, 37,
752-755.
Sch em e 3
(66) Frohn, H.-J .; Bardin, V. V. Z. Anorg. Allg. Chem. 1996, 622,
2031-2034.
(67) Frohn, H.-J .; Bardin, V. V. Z. Naturforsch. 1996, 51b, 1011-
1014.
(68) Frohn, H.-J .; Bardin, V. V. Z. Naturforsch. 1998, 53b, 562-
564.
(69) Frohn, H.-J .; Klose, A.; Bardin, V. V.; Kruppa, A. J .; Leshina,
T. V. J . Fluorine Chem. 1995, 70, 147-154.
(70) Frohn, H.-J .; Bardin, V. V. J . Chem. Soc., Chem. Commun.
1993, 1072-1074.
The reaction of (nonafluorocyclohexen-1-yl)xenonium
hexafluoroarsenate as well as of the arylxenonium salt

4758 Organometallics, Vol. 20, No. 23, 2001
Reviews
Sch em e 4
ysis of the [(C6F5)nBF4-n] anions cannot be excluded. It
is known that different products result in the reaction
of [C6F5Xe][AsF6] with F- or [HF2]- anions in CH3CN.
The consumption of [C6F5Xe][AsF6] in CD3CN at 80
°C is accelerated and is complete in 1 h in the presence
of H2O (1 equiv), forming C6F5D (main product) and
(C6F5)2. A similar acceleration was found in the presence
of CH3OH (1 equiv) at room temperature, but instead
of C6F5D, pentafluorobenzene (C6F5H) was the main
product.58 In the presence of C6F5OH in CD3CN the salt
[C6F5Xe][AsF6] decomposes rapidly at g -20 °C and
more than 10 pentafluorophenyl derivatives are formed,
including C6F5D, (C6F5)2, and C6F5OC6F5. The last ether
becomes the main product in neat H2O at 0 °C.58
[C6F5Xe][AsF6] reacts spontaneously with C6F5SH as
well as with [NMe4][C6F5S] in CD3CN even at -40 °C
and forms (C6F5)2S, (C6F5S)2, and C6F5H. A CIDNP
effect is observed in the produced C6F5H when the
reaction is monitored by 19F NMR spectroscopy. The
nucleophilicity of the sulfur atom in the disulfide
(C6F5S)2 is lower than that in C6F5SH or in [C6F5S]-.
Thus, the reaction of [C6F5Xe][AsF6] with (C6F5S)2
proceeds in CH3CN only at 80 °C and is complete within
1 h, yielding (C6F5)2S (main product) and C6F5H (minor
product). The more nucleophilic disulfide (NCS)2 reacts
already at -40 °C, spontaneously forming C6F5SCN,
C6F5H, and (C6F5)2S. The last species is not a product
due to C6F5SCN attack on [C6F5Xe][AsF6] because
neither C6F5SCN nor (C6F5)2S react with the [C6F5Xe]+
cation in CH3CN at 80 °C. The arylxenonium salt is
converted only to C6F5H and (C6F5)2.58
[C6F5Xe][AsF6] with chloride, bromide, and iodide an-
ions in the basic CH3CN proceeds via the displacement
of coordinated CH3CN in [R-Xe‚‚‚NCCH3]+ by the
anionic nucleophile.
As a result of such displacement, the intermediate
[RXe-Hal] (R ) C6F5, cyclo-C6F9) is formed. The further
fate of [RXe-Hal] depends on the nature of Hal. For
Hal ) Br, I a single electron transfer (Scheme 4, path
1) with elimination of Xe0 from RXe• was proposed,
followed by the in-cage recombination of R• and Hal• to
RHal. When the halide anion is more nucleophilic and
more stable toward oxidation (Hal ) Cl, F), the carbon-
xenon bond in the intermediate RXe-Hal (asymmetric
hypervalent bond) will be elongated and cleaved prefer-
ably (Scheme 4, path 2). The escaping radical R•
abstracts hydrogen from solvent molecules. In the case
of the chloride anion both paths seem to compete. This
consideration is in agreement with data obtained from
the interaction of (pentafluorophenyl)xenonium hexa-
fluoroarsenate with pyridine derivatives of different
nucleophilicity64 and with the generation of the FXe•
radical in CH3CN.72
Rea ction s w ith P n ictid e Nu cleop h iles. Adduct
formation between the [C6F5Xe]+ cation and neutral
N-bases was investigated in detail. The molecular
structure of the salt [C6F5Xe‚‚‚NCCH3][(C6F5)2BF2]
shows a nearly linear asymmetric hypervalent triad
C-Xe‚‚‚N with a C-Xe‚‚‚N angle of 174.5(3)° and
distances C-Xe ) 2.092(8) Å and Xe‚‚‚N ) 2.681(8) Å.
These can be regarded in a borderline description as a
weak 2c-2e C-Xe bond and a strong ion-dipole
Xe‚‚‚N interaction.29
Rea ction s w ith Ch a lcogen Nu cleop h iles. The salt
[C6F5Xe][AsF6] is soluble in water and decomposes
slowly in aqueous solution at room temperature over
12 h.23 (Polyfluorophenyl)xenonium salts react with
aqueous acetonitrile formally by replacement of xenon
by hydrogen (eq 42).
Pyridines react with solutions of [C6F5Xe][AsF6] in
CH3CN and H2O or with its dichloromethane suspen-
sion to form the corresponding adducts64 (eq 43).
[C6F5Xe][AsF6] + NC5H5-nRn f
[C6F5Xe···NC5H5-nRn][AsF6] (43)
[ArXe]Y + H2O CH3CN8 ArH + Xe0 + Y- + ... (42)
Rn ) H, 2-F, 3-F, 2,6-F2, 4-CN, 4-CH3,
2,6-(CH3)2, 2,6-(t-Bu)2
Y ) BF4, Ar ) 2-C6H4F, 4-C6H4F, 2,6-C6H3F2,34
2,4,6-C6H2F3;35 Y ) OTf, Ar ) 2,6-C6H3F2;59
Y ) (C6F5)3BF8 or C6F5BF3,9 Ar ) C6F5
The X-ray structural analysis of the [C6F5Xe‚‚‚2,6-
C5H3F2N][AsF6] salt shows a linear C-Xe‚‚‚N arrange-
ment and distances C-Xe ) 2.087(5) Å and Xe‚‚‚N )
1
2.694(5) Å. The H, 19F, and 129Xe NMR spectra of the
These reactions may not all proceed by the same
route, because the list of examples contains both ther-
mally unstable as well as thermally stable salts. Ad-
ditionally, the formation of F- or [HF2]- in the hydrol-
adduct give evidence for a significant charge transfer
from nitrogen to xenon, and there is a good correlation
between the chemical shifts δ(F-4, C6F5) and the pKa
values of the pyridine bases.64
(71) Frohn, H.-J .; Bardin, V. V. Mendeleev Commun. 1995, 114-
A triphenyl(alkynyl)phosphonium tetrafluoroborate
was obtained by the electrophilic alkynylation of tri-
phenylphosphine40 (eq 44).
115.
(72) Bucher, G.; Scaiono, J . C. J . Am. Chem. Soc. 1994, 116, 10076-
10079.

Reviews
Organometallics, Vol. 20, No. 23, 2001 4759
CH2Cl2
the electrophilic arylxenonium cation can abstract C6F5
groups from the fluoroborate anion (eqs 52 and 53). The
[t-BuCtCXe][BF4] + PPh3 -78 °C8
[t-BuCtCPPh3][BF4]
(44)
[C6F5Xe][(C6F5)3BF]
9
∆8 C6F5XeC6F5 + (C6F5)2BF
(52)
70-80%
The less electron-rich phosphine P(C6F5)3 is arylated
by an electrophilic route using [C6F5Xe][AsF6] in CH3-
CN solution or CH2Cl2 suspension (eqs 45 and 46).
C6F5XeC6F5 f 2[C6F5]• deuterated solvent8
C6F5D + (C6F5)2 (53)
CH3CN
[C6F5Xe][AsF6] + P(C6F5)3 80 °C/40 min8
fluoroborane (C6F5)2BF reacts like B(C6F5)3.
[P(C6F5)4][AsF6] + Xe0 + ... (45)
Aromatic compounds C6H5X also undergo pentafluo-
rophenylation in reactions with [C6F5Xe][Y] to give
2,3,4,5,6-pentafluorobiphenyls C6F5C6H4X.33,73 The reac-
tion rates diminish in the series X ) CH3 > F > CF3 ≈
CN > NO2, which does not contradict the electrophilic
nature of the process. However, detailed investigation
of the isomer distribution in the C6F5C6H4X products
shows unambiguously the radical character of the
pentafluorophenylation reaction73 (eq 54).
CH2Cl2
[C6F5Xe][AsF6](s) + P(C6F5)3 reflux/2.5 h8
[P(C6F5)4][AsF6](s) + Xe0 + ... (46)
Such quaternization of As(C6F5)3 by [C6F5Xe][AsF6]
to give [As(C6F5)4][AsF6] was not detected in refluxing
CH3CN or in CH2Cl2.58
Rea ction s w ith CH Acid s a n d Or ga n oelem en t
a n d Ar om a tic Com p ou n d s. The reactions of organo-
xenonium salts with negatively charged nucleophiles
and heteroatomic neutral n-electron donors as discussed
before result primarily in formation of donor adducts
at the xenon atom and/or the formal transfer of the
organic cation to the nucleophile. Formal similarity is
found in reactions of [C6F5Xe][(C6F5)2BF2] with some CH
acids in which the pentafluorophenyl group is added to
the deprotonated anionic species with elimination of
Xe0 33 (eqs 47 and 48).
[C6F5Xe][AsF6] + C6H5X CH CN8
3
C6F5C6H4X + Xe0 + [CH3CNH][AsF6] (54)
X ) CH3, F, CF3, CN, NO2
It is noteworthy that these reactions are accelerated
by the fluoride anion. For instance, the total conversion
of the salt [C6F5Xe][AsF6] to C6F5C6H4F is achieved
within 14 h at room temperature, while the CsF-
catalyzed process is complete within a few minutes.69
Pentafluorobenzenes C6F5X (X ) F, H, CN, SiMe3) as
well as C6F5I are not reactive under these conditions.9,73
The reported formation of the iodonium salt [(C6F5)2I][Y]-
in the reaction of [C6F5Xe][Y] (Y ) (C6F5)3BF) with
C6F5I8 may be explained by the presence of the anion
Y- which can act as a F- donor or via the formation of
C6F5IF2 from C6F5I and residual XeF2 followed by
interaction with (C6F5)3B.73 Moreover, the reaction of
the salt [C6F5Xe][AsF6] with C6H5I gives C6F5I (major
product) in addition to C6F5C6H4I, C6F5H, and
C6F5C6H5.73 A similar reaction mixture is obtained by
the radical pentafluorophenylation of iodobenzene using
C6F5NH2 and C5H11ONO. The predominant formation
of iodopentafluorobenzene here may be rationalized in
terms of attack of the electrophilic C6F5 radical on C6H5I
with subsequent decomposition of the unstable diaryli-
odonium radical [C6F5(C6H5)I]• with [C6H5] elimination
(cf. ref 74). C6F5I is favored over C6H5I by the stronger
bond of iodine to the C6F5 group because of the ad-
ditional high electrostatic bonding component.75
[C6F5Xe][(C6F5)2BF2] + PhCH2CN CH CN8
3
PhCH(C6F5)CN + PhC(C6F5)2CN + ... (47)
[C6F5Xe][(C6F5)2BF2] + Ph3CH f Ph3C(C6F5) + ...
(48)
The organoelement compounds Cd(C6F5)2 and B(C6F5)3
have in common a nucleophilic ipso-C atom, but they
react with [C6F5Xe][AsF6] by different routes, due to
their different Lewis acidities. In refluxing CD2Cl2
solution Cd(C6F5)2 reacts by two different paths. First,
nucleophilic attack of the C6F5 group on the electrophilic
[C6F5Xe]+ cation results in formation of C6F5D, (C6F5)2,
and Xe0. Second, the C6F5 group adds to the polarized
2C6F5Xe···FAsF5 + Cd(C6F5)2 f
2C6F5AsF4 + 2 C6F5XeF + CdF2V (49)
[AsF6]- anion in [C6F5Xe][AsF6] (eq 49), forming pen-
tavalent arsoranes, (C6F5)nAsF5-n (n ) 2, 3) (eq 50).
Perfluorocycloalkenylxenonium salts react with aro-
matic compounds such as arylxenonium salts70 (eq 55).
C6F5AsF4 + Cd(C6F5)2 f (C6F5)3AsF2 + CdF2V (50)
[C6F5Xe][AsF6] reacts with the Lewis acid B(C6F5)3
in CD2Cl2 at reflux to give C6F5D, C6F5BF2, (C6F5)2,
(C6F5)2BF, and AsF3 (reduction product of AsF5). The
result can be explained by the competition of the Lewis
acids AsF5 and B(C6F5)3 for the fluoride anion attached
to the [C6F5Xe]+ cation (eq 51). In a subsequent step,
C6F5Xe···FAsF5 + B(C6F5)3 7f
C6F5Xe···FB(C6F5)3 + AsF5v (51)
9
Rea ction s of th e Molten Sa lt [C6F 5Xe][AsF 6] w ith
Nu cleop h iles. The salt [C6F5Xe][AsF6] is one of the

4760 Organometallics, Vol. 20, No. 23, 2001
Reviews
most thermally stable organoxenon compounds. It melts
at 102 °C, forming a viscous liquid which decomposes
slowly above 125 °C with an electrophilic transfer of the
C6F5 group from xenon to one of the fluorine atoms of
the counteranion23 (eq 56).
only one reduction peak, which reflects the irreversible
electrochemical reduction of the arylxenonium cation to
the arylxenonium radical. The latter then undergoes
rapid elimination of xenon, generating the pentafluo-
rophenyl radical. As a result, pentafluorobenzene and
decafluorobiphenyl are the observed products77 (eq 61).
[C6F5Xe][AsF6]
8 C6F6 + AsF5 + Xe0 (56)
125-180 °C
[C6F5Xe]+ -8 [C6F5Xe]•
8 C6F5. CH CN8
+e
3
-Xe0
The thermal stability to 125 °C and the slow decom-
position of the melt until 180 °C can be used for
electrophilic pentafluorophenylation reactions of less
nucleophilic organoelement compounds. For instance,
the reactions of [C6F5Xe][AsF6] with (C6F5)3P, (C6F5)3-
As, and (C6F5)2S in the melt result in the formation of
the corresponding phosphonium, arsonium, and sulfo-
nium salts in good yield58,65,76 (eqs 57-59).
C6F5H + C12F10 (61)
The reduction potentials of both (pentafluorophenyl)-
xenonium tetrafluoroborate and hexafluoroarsenate are
equal to +0.45 V on a glassy-carbon electrode or +0.20
V on a platinum-disk electrode.
For the partially fluorinated salts [2,4,6-C6H2F3Xe]-
[BF4] and [2,6-C6H3F2Xe][BF4] substantially lower po-
tentials (Pt-disk electrode, -1.13 and -0.87 V, respec-
[C6F5Xe][AsF6] + (C6F5)3P 130 °C8
tively) were measured.78 However, the values of [C6H4-n
-
1.5 h
0
RnXe][Y] salts do not reflect the real redox properties
of these salts, as also is the case for the salts [2-C6H4-
FXe][BF4], [4-C6H4FXe][BF4], and [4-C6H4ClXe][BF4].78
The early deposition of a polyphenylene film on the
surface of the cathode as a result of a radical oligomer-
ization was assumed to be responsible for the measured
low potential values77 (eq 62).
[(C F ) P][AsF ]
+ Xe (57)
6
5 4
6
41%
[C6F5Xe][AsF6] + (C6F5)3As 150 °C8
1.5 h
[(C6F5)4As][AsF6] + [(C6F5)3AsF][AsF6] +
(C6F5)3AsF2 + AsF3 + Xe0 (58)
+
[C6H5-nHalnXe]
[C6H5-nHalnXe]+
- 8 [C6H5-nHaln]•
8
+e
[C6F5Xe][AsF6] + (C6F5)2S 140 °C8
-Xe0
1.5 h
[C6H5-nHaln-C6H5-nHalnXe]•+
8
0
[(C F ) S][AsF ]
+ Xe (59)
-Xe0, -H+
6
5 3
6
47%
[C6H5-nHaln-C6H4-nHaln]• f f f
In all cases C6F6 was found among the products.
However, heating [C6F5Xe][AsF6] with (C6F5)3Sb or
(C6F5)3Bi did not effect the electrophilic arylation of SbIII
or BiIII. The main products were (C6F5)nAsF5-n (n ) 2,
3), which probably arise from aryl-fluorine redistribu-
tion between [AsF6]- and tris(pentafluorophenyl)anti-
mony or -bismuth at elevated temperature (see for
comparison the reaction of [C6F5Xe][AsF6] with B(C6F5)3).
Bromo- and iodopentafluorobenzenes underwent elec-
trophilic pentafluorophenylation in the molten salt
[C6F5Xe][AsF6]76 (eq 60).
polyphenylenes -[C6H4-nHaln]m- (62)
VIII. Rea ction s of Or ga n oxen on iu m (II) Sa lts
w ith Electr op h iles
Polyfluoroaryl groups in arylxenonium cations are
resistant toward most electrophiles. The electron-
withdrawing effect of the fluorine substituents and
especially the influence of the positively charged xenon
atom are responsible for this behavior. No reactions
were detected between [C6F5Xe][AsF6] and the super-
acids aHF, FSO3H, CF3SO3H (at room temperature,
during the course of some years), (Cl2 + SbF5/HSO3F)
(-35 °C, 1 h), and [NO2][BF4] in FSO3H (90 °C, 36 h)
and between [2,3,4,5-C6HF4Xe][BF4] and Br2 (excess) in
CH3CN (room temperature, 22 h) or 100% HNO3-aHF
(room temperature, 12 h).79
[C6F5Xe][AsF6] + C6F5X 140 °C8
1 h
[(C6F5)2X][AsF6] + Xe0 (60)
X ) Br, I
However, treatment of the salt [C6F5Xe][AsF6] with
XeF2 in aHF results in xenon evolution and formation
of (perfluoro-1,4-cyclohexadien-1-yl)xenonium hexa-
fluoroarsenate. Further fluorination leads to (perfluo-
rocyclohexen-1-yl)xenonium hexafluoroarsenate in high
yield70 (eq 63).
Electr och em ica l Red u ction of Ar ylxen on iu m (II)
Sa lts. Although the mechanisms of reactions of aryl-
xenonium salts with π- or n-electron donors have not
been investigated in detail, many processes can be
described in terms of the reduction-oxidation termi-
nology. To determine the oxidation ability of arylx-
enonium salts, their electrochemical reduction in ac-
etonitrile solution was studied using cyclic voltammetry.
The cyclic voltammograms of [C6F5Xe][Y] (Y ) BF4,
AsF6) were measured in CH3CN on a glassy-carbon
electrode and on a platinum-disk electrode. They showed
Similar ring fluorination reactions take place in the
case of the [2,3,4,5-C6HF4Xe][Y] salts (Y ) AsF6, BF4),
(76) Frohn, H.-J .; Klose, A.; Henkel, G. Thermally Stable Aryl-
xenon(II) Compounds: The Salt [C6F5Xe][AsF6] and the Molecule
C6F5XeO2C6F5. Presented at the 11th Winter Fluorine Conference, St.
Petersburg, FL, J anuary 1993; Paper 58.
(77) Datsenko, S.; Ignat’ev, N.; Barthen, P.; Frohn, H.-J .; Scholten,
T.; Schroer, T.; Welting, D. Z. Anorg. Allg. Chem. 1998, 624, 1669-
1673.
(78) Naumann, D.; Gnann, R.; Ignat’ev, N.; Datsenko, S. Z. Anorg.
Allg. Chem. 1995, 621, 851-853.
(79) Frohn, H.-J .; Bardin, V. V. Unpublished results.
(73) Frohn, H.-J .; Klose, A.; Bardin, V. V. J . Fluorine Chem. 1993,
64, 201-215.
(74) Beringer, F. M.; Falk, R. A. J . Chem. Soc. 1964, 4442-4451.
(75) Bailly, F.; Barthen, P.; Frohn, H.-J .; Ko¨ckerling, M. Z. Anorg.
Allg. Chem. 2000, 626, 2419-2427.

Reviews
Organometallics, Vol. 20, No. 23, 2001 4761
Sch em e 5
but this reaction is additionally accompanied by the
partial replacement of a hydrogen atom by fluorine68
(Scheme 5). It is noteworthy that under similar condi-
tions the tetrafluorobenzenes C6HF4X (X ) H, F, Br,
CF3, NO2) also undergo both reactions, ring fluorination
as well as fluorodeprotonation.42
atom and the pentafluorophenyl group for the formal
addition to the electron pair of I or P only the addition
of fluorine is observed.10
When the salt [C6F5Xe][AsF6] reacts with XeF2 in HF
which contains 1-2 equiv of H2O, new types of oxygen-
ation reactions are encountered. (Pentafluoro-1,4-cyclo-
hexadien-3-on-1-yl)xenonium hexafluoroarsenate is the
initially isolated product. In an excess of XeF2 and H2O
the latter salt undergoes epoxidation67 (eq 64).
X. Rea ction s of th e Cova len t Xen on (II)
Com p ou n d s R2Xe, RXeR′, a n d RXeHa l
These organoxenon derivatives were synthesized only
a short while ago. Consequently, only few examples of
their reactivity have been studied to date. Similar to
the fluorine atoms in XeF2, both organic groups in
(C6F5)2Xe carry a high negative charge which is con-
centrated on C(1). However, only one C-Xe bond is
cleaved by the action of aHF (eq 68) because in the
initially formed cation [C6F5Xe]+ the negative charge
on C(1) is strongly diminished.54
(C6F5)2Xe + nHF -40 °C8
C6F5H + [C6F5Xe][F(HF)n-1] (68)
As the reactive key intermediate, [HOXeF‚nHF] or a
related electrophilic species was assumed. Similar elec-
trophilic oxygenation reactions proceed with the di-
enylxenonium salt [cyclo-C6F7][AsF6]67 (eq 65) as well
The carbon-xenon bond in (C6F5)2Xe with bond order
0.5 is weaker than that in the cation [C6F5Xe]+ (bond
order 1). In CH3CN or CH2Cl2 solution total decomposi-
tion takes place within 24 or 9 h, respectively. In CH3-
CN (C6F5)2 is reported as the main product in addition
to C6F5H,45 whereas in CH2Cl2 a 10:1 ratio of C6F5H to
(C6F5)2 was found.54 Reactions of (C6F5)2Xe with mer-
cury45 and with iodine54 proceed formally with addition
of C6F5 radicals to Hg or I (eqs 69 and 70).
as with the polyfluoroarenes C6F5R (R ) F, H, CF3,
NMe3+, OCH2CF3, etc.)80 and demonstrate the general-
ity of this process. It is noteworthy that the CFdCXe+
moiety in cyclohexenyl derivatives in all cases is not
attacked.
(C6F5)2Xe + Hg CH CN8 (C6F5)2Hg + Xe (69)
3
-40 °C
CH2Cl2
(C6F5)2Xe + I2
8 2C6F5I + Xe
(70)
-78 °C
IX. [C6F 5XeF 2] [BF 4], a P oten tia l Electr op h ilic
F lu or in a tin g Agen t
The solution of C6F5XeCN in dichloromethane is less
stable than that of (C6F5)2Xe. At -40 °C total decom-
position proceeds within 2 h, forming C6F5CN and
C6F5H (4:1) as fluoroaromatic products. Reactions of
C6F5XeCN with aHF and I2 give the fluoroaromatic
products in the indicated ratio54 (eqs 71 and 72).
High fluorinating ability is characteristic of the salt
[C6F5XeF2][BF4]. The reactions with iodopentafluoro-
benzene or with P(C6F5)3 result in the oxidative fluori-
nation of the heteroatom with formation of the salt
[C6F5Xe][BF4] as the other product (eqs 66 and 67). This
C6F5XeCN + nHF f
CH3CN
[C6F5XeF2][BF4] + C6F5I
8
C F H C F CN
-40 °C
[C6F5Xe][F(HF)n-1
4
]
+
+
(71)
6
5
6
5
C6F5IF2 + [C6F5Xe][BF4] (66)
1
3
CH2Cl2
CH3CN
C6F5XeCN + I2 -78 °C8
[C6F5XeF2][BF4] + (C6F5)3P
8
-40 °C
C F I C F H C F CN
+
+
+ Xe (72)
6
5
6
5
6
5
(C6F5)3PF2 + [C6F5Xe][BF4] (67)
5
3
1
means that in the competition between the fluorine
A remarkable decomposition of C6F5XeF in CH2Cl2
proceeds above -30 °C. C6F5H and traces of C6F5Cl are
formed.54 The higher thermodynamic stability of [C6F5-
(80) Frohn, H.-J .; Bardin, V. V. Z. Naturforsch. 1996, 51b, 1015-
1021.

4762 Organometallics, Vol. 20, No. 23, 2001
Reviews
Xe][Y] salts compared to C6F5XeF becomes obvious in
the reaction of the covalent C6F5XeF with Lewis acids
preclude their existence and the preparation of these
two classes of xenon compounds is one of the “hot” points
in this field.
54
such as Me3SiOS(O)2CF3,45 C6F5BF2, and SiF4 (eqs
73-75).
Additionally, organoxenonium salts offer a promising
access to new xenon-element compounds because of
their lower oxidation potential in comparison to the
[FXe]+ cation which has been used traditionally as basis
for the preparation of new xenon-element bonds.
Further progress in carbon-xenon chemistry also will
deal with the application of organoxenon compounds in
synthesis: for instance, in the radical and, in particular,
the electrophilic introduction of organic groups R into
organic molecules and organoelement compounds. The
first examples of such applications have been given here.
They promise a good perspective for organoxenon com-
pounds as useful synthetic reagents.
C6F5XeF + Me3SiOS(O)2CF3 f [C6F5Xe][O3SCF3]
(73)
C6F5XeF + C6F5BF2 f [C6F5Xe][C6F5BF3] (74)
C6F5XeF + SiF4 f [C6F5Xe][SiF5]
(75)
XI. Con clu sion s a n d P r osp ects
At the present time the following types of organo-
xenon compounds are known: the mononuclear xenon-
ium(II) salts [RXe]+[Y]- (R ) aryl, polyfluoroalkenyl,
alkynyl), the binuclear xenonium(II) salts [(C6F5Xe)2Z]+-
[AsF6]- (Z ) F, Cl), the arylxenonium(IV) salt [C6F5-
XeF2]+[BF4]-, the covalent xenon(II) compounds C6F5-
XeZ (Z ) F, Cl, CN, OC(O)C6F5), the symmetric diorgano-
xenon(II) compounds R2Xe (R ) C6F5, 2,4,6-C6H2F3), and
the unsymmetrical diorganoxenon(II) compounds RXeR′
(R ) C6F5, R′ ) 2,4,6-C6H2F3).
Ack n ow led gm en t. We are grateful to the Deutsche
Forschungsgemeinschaft and the Russian Foundation
for Basic Research for support under grants FR687/11,
FR687/12, 436RUS113/588/0, and 00-03-04003 NNIO_a
and to all the co-workers who contributed to these
studies in the Duisburg group. Their names are cited
in the references.
To date there is no evidence for the existence of
alkylxenon derivatives (salts or covalent species). How-
ever, recent experimental and theoretical data do not
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