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On the basis of high values of group electronegativity, for
example, Xe?C6F5)2 and Xe?CN)2 should be favored for a new
class of symmetric C-Xe-C molecules, since their C?1) atom is
part of a polarizable p system and the strongly electron-
withdrawing components make the C-ligand electron-poor.
Herein we present a new concept for the synthesis of
covalent C6F5Xe-C and C6F5Xe-Y compounds, specific exam-
ples being Xe?C6F5)2 and C6F5XeCN[8]. This concept is based
on the new molecule C6F5XeF ?1)[9] which is the decisive key
substrate. The asymmetric hypervalent molecule 1 is formed
in 70% yield as the soluble product of the heterogeneous low-
temperature reaction of [C6F5Xe] salts withªnakedº fluoride
[NMe4]F in CH2Cl2 [Eq. ?1)]. The alternative approach, the
electrophilic substitution of E in C6F5-E with[FXe] does not
lead to 1, because the oxidation potential of [FXe] is too high
even for fluoroaromatics.
CH2 Cl2; À78 o
C
À!
[C6F5Xe][Y]?s) [NMe4]F
Y AsF6, BF4
C6F5XeF [NMe4][Y] #
slow
1
?1)
If the soluble source of fluoride in Equation ?1) is used in a
smaller than stoichiometric amount, multinuclear fluoro-
bridged xenonium species result [Eq. ?2)]:
CH2Cl2 ; À78 o
C
À!
2[C6F5Xe][AsF6]?s) [NMe4]F
slow
?2)
[?C6F5Xe)2F][AsF6] # [NMe4][AsF6] #
Ab initio calculations ?comparison in the gas phase) show
À
that the C Xe distance in 1 is longer than in the [C6F5Xe] ion
À
and the Xe F distance is greater than in XeF2. The latter
feature makes the FÀ ion a good leaving group. The
permanent dipole moment in 1 makes the successful attack
of nucleophiles on the electrophilic Xe center easier. With
Cd?C6F5)2 as an aryl transfer reagent it is possible to introduce
a second aryl group into 1 [Eq. ?3)]:
C6F5XeF, A Key Substrate in Xenon ± Carbon
Chemistry: Synthesis of Symmetric and
Asymmetric Pentafluorophenylxenon"ii)
Derivatives**
CH2Cl2 ; À78 o
C
À!
2C6F5XeF Cd?C6F5)2
2Xe?C6F5)2 CdF2 #
?3)
fast
1
2
The direct introduction of the C6F5 group into XeF2 within
the thermal-existence range of 2 is not successful with
Cd?C6F5)2, as the nucleophilicity of the aryl group ?no
permanent dipole moment) does not suffice for the substitu-
tion on XeF2. The nucleophilicity of the aryl group in
Me3SiC6F5 in CH2Cl2 is not high enough for the successful
F-C6F5 substitution in 1 [Eq. ?4)] ?see also ref. [10]):
Hermann-Josef Frohn* and Michael Theiûen
À
Salt-like compounds witha C Xe bond in the cationic part
[RXe] have been known since 1989, where R represents an
aryl,[1] alkenyl,[2] or alkynyl group[3]
C6F5XeO2CC6F5,[4] C6F5XeCl and [?C6F5Xe)2Cl]
weaker covalent C Xe bonds ?3c-4e bonds[6], asymmetric,
hypervalent bonds with different distinct heteropolar compo-
nents). The existence of the symmetric, hypervalent, molec-
.
In contrast
[5]
contain
À
À70!À40 o
C
=
À!
2 Me3SiF
C6F5XeF Me3SiC6F5
?4)
CH2Cl2 ; <10 h
[7]
ular compound Xe?CF3)2 is extremely doubtful, and un-
ambiguous proof of its constitution has not yet been provided.
However, withMe SiCN the CN group can be introduced
3
successfully into 1 [Eq. ?5)]:
[*] Prof. Dr. H.-J. Frohn, Dipl.-Chem. M. Theiûen
Fachgebiet Anorganische Chemie
CH2Cl2 ; À78 o
C
À!
C6F5XeF Me3SiCN
C6F5XeCN Me3SiF
?5)
spontaneous
Gerhard-Mercator-Universität Duisburg
Lotharstrasse 1, 47048 Duisburg ?Germany)
Fax : ?49)203-379-2231
1
3
We attribute the different reactivities of Me3SiC6F5 and
Me3SiCN more to differences in Lewis acidity than to steric
effects.
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemische Industrie.
Angew. Chem. Int. Ed. 2000, 39, No. 24
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