A Simple and Convenient Route to Arylxenon(II) Tetrafluoroborates

Article abstract of DOI:10.1515/znb-1999-1203

An improved synthesis of arylxenon(II) salts is reported. The series of fluoro-containing arylxenon(II) tetrafluoroborates (aryl = C₆F₅, 2,3,4,5-C₆HF₄, 3,4,5-C₆H₂F₃ and 3,5-C₆H₃F₂) are prepared by the reaction of xenon difluoride with the corresponding aryldifluoroboranes. The salts [C₆F₅Xe] [BF₄] and [2,3,4,5-C₆HF₄Xe] [BF₄] are long-term stable in anhydrous HF (aHF) solution at rt, while [3,4,5-C₆H₂F₃Xe] [BF₄] and [3,5-C₆H₃F₂Xe] [BF₄] are converted into 1,2,3,5-tetrafluorobenzene and 1,3,5-trifluorobenzene, respectively, within a few hours.

Full text of DOI:10.1515/znb-1999-1203

A Simple and Convenient Route to Arylxenon(II) Tetrafluoroborates
H.-J. Frohn3, H. Franke3, and V. V. Bardinb
aFachgebiet Anorganische Chemie, Gerhard-Mercator-Universität Duisburg,
Lotharstr. 1, D-47048 Duisburg, Germany
bInstitute of Organic Chemistry, 630090 Novosibirsk, Russia
Reprint requests to Prof. Dr. H.-J. Frohn.
Fax: (+49) 203 379 2231. E-mail: frohn@uni-duisburg.de
Z. Naturforsch. 54 b, 1495-1498 (1999); received October 4, 1999
Xenon Difluoride, Fluorinated Aryldifluoroboranes, Fluorinated Arylxenon(II) Salts, Fluorine-
Aryl Substitution, Thermal Decomposition
An improved synthesis of arylxenon(II) salts is reported. The series of fluoro-containing
arylxenon(II) tetrafluoroborates (aryl = C6F5, 2,3,4,5-C6HF4, 3,4,5-C6H2F3 and 3,5-CöH3F2)
are prepared by the reaction of xenon difluoride with the corresponding aryldifluoroboranes.
The salts [CöFsXe] [BF4] and [2,3,4,5-C6HF4Xe] [BF4] are long-term stable in anhydrous HF
(aHF) solution at rt, while [3,4,5-C6H2F3Xe] [BF4] and [3,5-C6H3F2Xe] [BF4] are converted
into 1,2,3,5-tetrafluorobenzene and 1,3,5-trifluorobenzene, respectively, within a few hours.
Introduction
- CF3 S(0)20H - CCI3F system at -40 °C resulted
in low yields of arylxenon(II) compounds and, in
many cases, in the non-regiospecific replacement
of hydrogen by xenon(II) [7].
In 1989 the first preparation of arylxenon(II)
salts, pentafluorophenylxenon(II)
pentafluoro-
phenyl(fluoro)borates, by reaction of tris(penta-
fluorophenyl)borane and xenon difluoride was re-
ported [1 , 2 ].
We have elaborated a new reaction sequence
for the preparation of organoxenon(II) tetrafluo-
roborates based on the interaction of xenon diflu-
oride with organyldifluoroboranes generated in situ
from the corresponding potassium aryltrifluorobo-
rates and boron trifluoride in an inert solvent [8 ]. We
present now the fruitful application of aryldifluoro-
boranes for the synthesis of some fluoro-containing
arylxenon(II) tetrafluoroborates. Most of the aryl
groups under investigation have the presence of hy-
drogen atom(s) in orr/2o-position(s) in common.
(C6F5)3B + XeF2 CH-C^ o roceCN >
[C6F5Xe] [(C6F5)„BF4_n]
(1)
Later this procedure was extended to some other
arylxenon(II) salts [3 -6 ]. However, the prepara-
tion and purification of boranes with electron-
withdrawing substituents in the aryl group is a
delicate process owing to the high tendency for
tetraarylborate formation and the moisture-sensiti-
vity of the boranes. A disadvantage of the tradi-
tional synthetic approach is the presence of nu-
cleophilic arylfluoroborate anions in combination
with the electrophilic arylxenon(II) cation. This fact
makes it neccessary to convert arylxenon(II) arylflu-
oroborates into salts with less nucleophilic anions
like [AsF6]_ in order to increase the thermal stabil-
ity. Furthermore, it should be mentioned, that only
one third of the aryl groups Ar is transferred to Xe(II)
(eq. (2 )).
Results and Discussion
The general procedure of the improved synthe-
sis consists of the sequential defluoridation of K
[ArBF3] by BF3 in CH2C12 at -40 to -50 °C, the sep-
aration of the dichloromethane solution of ArBF2
from solid K [BF4] followed by the reaction of the
aryldifluoroborane with XeF2 which results in the
precipitation of the desired arylxenon(II) tetrafluo-
roborate.
Ar3B + XeF2 CH-C'2 >[ArXe][Ar2BF2]
(2)
KTArRF-,1
+BF3,-K [B F 4]
E3J -4 0 to -5 0 °C ,C H 2Cl2
K |A
The alternative pathway to [ArXe] [0 S(0 )2CF3]
by reaction of ArH with XeF2 in the CF3C(0 )0 H
ArBF2
[ArXe][BF4]
(3)
0932-0776/99/1200-1495 $ 06.00 © 1999 Verlag der Zeitschrift für Naturforschung, Tübingen •www.znaturforsch.com
K
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H.-J. Frohn et al. ■A Simple and Convenient Route to Arylxenon(Il) Tetrafluoroborates
Table I. Preparation of [ArXe] [BF4].
Pentafluorophenylxenon(II) (1), 2,3,4,5-tetra-
fluorophenylxenon(II) (2), 3,4,5-trifluorophenyl-
xenon(II) (3), and 3,5-difluorophenylxenon(II) (4)
tetrafluoroborates were prepared in high yields. It
should be mentioned that aryldichloroboranes can
also be used as starting materials. For instance,
salt 1 was obtained by the reaction of an excess
of xenon difluoride with a mixture of CöFsBC^ and
c 6f 5b f 2.
CH.Cb,
ml
ArBF2,
XeF2, [ArXe] [BF4],
mmol yield in mmol (%)
mmol
3
2
1.50
C6F5BF2,
1.40
C6F5BF2 + C6F5BCI2
(21 : 79), 0.50
2,3,4,5-C6F4HBF1,
1.23
1.26 (90)
0.93
0.29 (58)
2.5
4
1.26
1.97
1.38
1.03(84)
1.58(81)
0.57 (41)
3,4,5-C6F3H.BF7,
1.95
1 (C6F5BC12 + C6F5BF2) + 1.9 XeF? CH-C1- >
10
[C6F5Xe] [BF4] + Cl2 + Xe
(4) 3,5-C6F2H3BF2,
1.53
However, the use of chloroboranes has no advan-
tage over the substrates Ar3B or C6F5BF2 because
50 % of xenon difluoride is lost in a preceeding
redox reaction.
be stored for four years at rt without decomposition.
In contrast to the stability of arylxenon(II) salts with
fluorine in ortho-position, 3 was converted in aHF
quantitatively into tetrafluorobenzene 5 within 12 h.
It should be emphasized that the total decomposition
of 3 in HOTf solution at rt resulted in the formation
of a mixture of 5 and 3,4,5-trifluorophenyl triflate
(7) (18: 82).
1 and the new hydrogen-containing arylxenon
compounds 2, 3 and 4 are white solids. The latter
show reduced stability with diminishing number of
fluorine atoms in the aromatic moiety. Indeed, 1 and
2 did not decompose at rt in a dry argon atmosphere
over one year. Compound 3 remained unchanged
at least for 4 d whereas 4 underwent total decom-
position within a few hours under the same condi-
tions. The suspensions of the arylxenon tetraflu-
oroborates 3 and 4 (no orr/io-fluorine atoms) in
dichloromethane showed the lowest thermal stabil-
[3,4,5-C6H2F3Xe] [BF4] ^ F-orCH~C1-. >
3
1.2.3.5-C6H2F4 + BF3 + Xe
(5)
5
ity. Both salts are converted completely into 1,2,3,5- [3,4,5-C6H2F3Xe] [BF4]
>
tetrafluorobenzene (5)or 1,3,5-trifluorobenzene (6),
3
respectively, within a few hours.
1,2,3,5-C6H2F4 + 3,4,5-C6H2F30 S (0 )2CF3 (6)
The fluoro-containing arylxenon(II) tetrafluorob-
orates 1 - 4 are soluble in basic MeCN as well
as in strong anhydrous acids. The solution of 1 in
MeCN slowly decomposes at rt similarly to the cor-
responding hexafluoroarsenate [6] to give mainly
C6HF5, (C6F5)2 and traces of CöFö after 23 d. The
5
7
The aHF solution of 4 was the least stable in this
series and conversion of 4 into trifluorobenzene 6
proceeded already at -30 °C.
decomposition of the trifluorophenylxenon salt 3 [3,5-Q,H,F2Xe] [BF4] aHF0” co”-c'- >
was complete within 18 h under the same condi-
tions yielding a complex mixture which contains
1,2,3,5-tetrafluorobenzene. 4 remained unchanged
in an CD3CN - EtCN solution at -60 °C for 7 h,
but was converted into 1,3,5-trifluorobenzene (6) at
-30°C.
4
1.3.5-C6H3F3 + BF3 +X e
(7)
6
These experimental results show the clear inter-
dependence between the presence of carbon-bonded
Solutions of the tetra- and pentafluorophenyl- fluorine atoms in the [BF4]~ salts of arylxenon
xenon(II) salts 2and 1in acidic media were signifi-
cations and the thermal stability and reactivity to-
cantly more stable than in basic media. No changes wards strong acids with weakly nucleophilic anions.
of these salts were detected in the anhydrous HF The influence of the electronic structure in partially
(aHF) solutions at rt during 13 - 16 d. The solution fluorinated arylxenon(II) cations on their reactivity
will be discussed elsewhere.
of the related [C6F5Xe] [AsF6] salt in HOTf could
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H.-J. Frohn et al. ■A Simple and Convenient Route to Arylxenon(II) Tetrafluoroborates
Table II. NMR spectra of [ArXe] [BF4].
Ar
Solvent r[°C] <5(H)
6(F)a
F-4
<5(Xe)
F-2
F-3
F-5
F-6
HF
-123.26 -151.53 -137.85 -151.53 -123.26 -3940.6
J [Hz]: (2,3) 11, (2,4) 6, (2,6) 6C, (3,4) 19, (3,5) 5C, (F-2,Xe) 58.6;
59.3b, (F-3,Xe) 18
C6F5
-10
MeCN
HF
-30
-10
-125.48 -155.04 -142.26 -155.04 -125.48 -3785.8
J [Hz]: (2,4) 4, (3,4) 20, (F-2,Xe) 68; 67b
c 6f 5
2,3,4,5-C6HF4
8.70
-120.20 -143.25 -140.60 -127.57
-3840.6
J [Hz]: (2,3) 15, (2,4)7, (2,6)4, (3,4) 18, (3,5) 5, (3,6)3, (4,5) 18,
(4,6) 7, (F-2,Xe) 54.8; 54.7 b, (H-6,Xe) 14.3b
2,3,4,5-C6HF4 d
2,3,4,5-C6HF4
HF
8.65
-119.94 -143.35 -140.90 -127.82
-3836.3
-3712.2
-10
-30
J [Hz]: (3,4) 16, (4,5) 16, (F-2,Xe) 55.5 b, (H-6,Xe) 16b
MeCN
8.11 -122.54 -147.29 -144.35 -132.12
J [Hz]: (2,3) 18, (2,4) 8, (2,5) 5, (2,6) 5, (3,4) 19, (3,5) 9, (4,5) 20,
(4,6) 7, (5,6) 9, (F-2,Xe) 63.4; 63. lb
HF
3,4,5-C6H2F3
3,4,5-C6H2F3
3,5-C6H3F2
3,5-C6H3F2
-10
-30
-60
-60
8.54
-120.87 -147.15 -120.87
-3704.4
-3606.6
-3708.0
-3605.8
b
J [Hz]: (2,3) 5, (2,4) 5, (3,4) 18, (H-2, Xe) 17.6
MeCN
8.05
-125.89 -150.37 -125.89
J [Hz]: (2,3) 6, (3,4) 19
HF,
8.46 (H-2,6)
8.06 (H-4),
-96.59
-96.59
J [Hz]: (Xe, H-2,6) 19.4
e
7.94 (H-2,6),
7.59 (H-4)
-101.42
-101.42
J [Hz]: unresolved signal
*
a 19F resonances of the anion [BF4 ] were located at -148.71 (1), -148.03 (2), -147.56 (3) in acetonitrile solution, at
-149.43 e (4) ppm, and at -148.09 (1), -148.50 (2), -148.16 (3) and -148.31 (4) ppm in aHF solution;b From 129Xe
NMR spectrum;c Apparent constant;d Counteranion [Asfö]~; e In CD3CN - EtCN (1 : 3).
Preparation o f arylxenon(II) tetrafluoroborates (general
procedure)
Experimental
NMR spectra were measured on Bruker spectrome-
ters WP 80 SY (’H at 80.13 MHz and 19F at 75.39
MHz) and AVANCE DRX 500 ('H at 500.13 MHz, 19F
at 470.59 MHz and 129Xe at 138.34 MHz) with TMS
('H), CCI3F (i9F), and XeOF4 (l29Xe) as references.
The IR spectra were measured on the Bruker IFS 66
spectrometer in polyethylene pellets. GC-MS analysis
was performed with the Hewlett Packard HP G 1800A
instrument.
Xenon difluoride was added portionwise to the stirred
solution of the aryldifluoroborane in dichloromethane
at -40 to -50 °C. After 1 - 2 h the suspension was
warmed slowly to rt. After centrifugation the mother
liquor was decanted and the arylxenon(II) tetrafluorobo-
rate was washed with several portions of CH2C12 and
finally dried in vacuum at rt (Table I). Salt 4 was isolated
at < -40 °C.
The reaction of a mixture (21 : 79) of C6F5BF2 and
C6F5BCF with XeF2 was performed in a similar manner.
NMR spectra are presented in Table II.
Pentafluorophenylxenon(II) tetrafluoroborate 1. IR:
441,433,381,349, 308, 278, 224, 203, 132, 116 cm "1.
Preparations of K [ArBF3] and ArBF2 (Ar = CöFs,
2,3,4,5-C6HF4, 3,4,5-C6H2F3 and 3,5-C6H3F2) were re-
cently described [8]. The yield of aryldifluoroboranes was
determined from the 19F NMR spectra of the solutions in
dichloromethane using 1,1,2-trichlorotrifluoroethane as
quantitative internal reference.
Anhydrous HF was electrolytically dried and trifluo-
romethanesulfonic acid was distilled before use. All ma-
nipulations with arylxenon(II) salts were carried out in a
dry argon atmosphere in FEP traps. For NMR measure-
ments FEP inliners were used.
Elemental analysis for CöBFgXe (385.15 g/mol)
Calcd C 18.7 F44.4%,
Found C 19.4 F44.3%.
2,3,4,5-Tetrafluorophenylxenon(II) tetrafluoroborate
2. IR: 434, 384, 359, 338, 321, 292, 282, 212, 194, 168,
148, 117 cm“ 1.
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H.-J. Frohn et al. • A Simple and Convenient Route to Arylxenon(II) Tetrafluoroborates
Elemental analysis forCöHBFsXe (367.91 g/mol)
Stability o f [ArXe][BF
or EtCN-MeCN
4
] solutions in MeCN
Calcd C 19.6 H 0.27 F41.3 %,
Found C 18.6 H 0.23 F42.0%.
A. An acetonitrile solution of 1 was kept at rt and
showed 60 % conversion after 13 d and total conversion
after 23 d ( l9 F NMR). Products were CöHFs, (CöF5)2
(10 : 1), CöFö (traces) besides an unidentified compound
C6F5X [(5(F): -143.38 (F-2,6), -153.10 (F-4) and -161.26
(F-3,5) ppm)]. The broad resonance at ca. -149 ppm was
assigned to fluorine bonded to boron, presumably [BF4]- .
B. A solution of [3,4,5-C6H2F3Xe] [BF4] (0.08 mmol)
in MeCN (0.25 ml) was kept at rt for 18 h. The l9F
NMR spectrum confirmed the total decomposition of 3
and the formation of 1,2,3,5-tetrafluorobenzene (yield 14
%) besides a number of unidentified products.
C. Compound [3,5-C6H3F2Xe] [BF4] (0.14 mmol) was
dissolved in EtCN - CD3CN (3:1) (0.5 ml) at -60 °C. No
changes were observed within 7 h at this temperature, but
warming to -30 °C caused the quantitative decomposition
of 4 and the formation of 1,3,5-trifluorobenzene as the
main aromatic product (l9F NMR).
Stability o f [ArXe] jBF\ ] solutions in acids
A. [2,3,4,5-C6HF4Xe] [BF4] (0.10 mmol) was dis-
solved in aHF (0.2 ml) and kept at rt for 16 d. No changes
were detected (l9F NMR).
B. A solution of [3,4,5-C6H2F3Xe] [BF4] (0.13 mmol)
in aHF (0.3 ml) was kept at rt for 12 h. Quantitative con-
version of 3 into 1,2,3,5-tetrafluorobenzene was observed
(l9FNMR).
C. [3,4,5-C6H2F3Xe] [BF4] (0.07 mmol) was dissolved
in HOTf (0.1 ml) at -60 °C and kept at rt. The con-
version of 3 was ca. 50 % after 1.5 h and complete
after 16 h. The formation of 1,2,3,5-tetrafluorobenzene
and 3,4,5-C6H2F30S(0 )2CF3 as the only aromatic com-
pounds was confirmed by 19F NMR. The reaction mix-
ture was poured onto ice, the products were extracted
with dichloromethane and the extracts were dried with
MgSCU. The l9F NMR spectrum showed the presence of
1,2,3,5-tetrafluorobenzene and 3A 5-C6H2F30S(0 bCF3
(18 : 82) (quantitative total yield). The latter compound
was characterized by the l9F resonances at -130.6 (F-
3,5), -159.6 (F-4) and -73.0 (OSO2CF3) ppm [J, Hz: (2,3)
7.2, (2,4) 5.5 and (3,4) 20.3] and the 'H resonance at
7.07 ppm. The mass spectrum of the mixture contained
the molecular ions m/z 150 [1,2,3,5-C6H2F4]+ and 280
[3,4,5-C6H2F30S(0)2CF3]+.
Stability o f [ArXe][BF\] in CH
1
CI2 suspensions
A. A suspension of [3,4,5-C6H2F3Xe] [BF4] (0.08
mmol) in CH2CI2 (0.25 ml) was agitated at rt for 5 d.
The l9F NMR spectrum showed the total decomposition
of salt 3 and the formation of 1,2,3,5-tetrafluorobenzene
besides traces of unidentified products.
B. Compound [3,5-C6H3F2Xe] [BF4] (0.14 mmol)
was suspended in CH2CI2 (0.5 ml) at -60 °C and agi-
tated at rt. The quantitative conversion of 4 into 1,3,5-
trifluorobenzene was observed within 2 h (19F NMR).
D. A solution of [3,5-C6H3F2Xe] [BF4] (0.14 mmol)
in aHF (1 ml) was kept at -30 °C for 13 h. It
showed the formation of trifluorobenzene 6 (64 % con-
version of 4). At 0 °C the decomposition of 4 into
6, Xe and BF3 was complete within a few hours
(19F NMR).
Acknowledgements
We gratefully acknowledge financial support by Volks-
wagen Stiftung and Fonds der Chemischen Industrie.
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les, K.-F. Tebbe, Z. Anorg. Allg. Chem. 623, 1821
(1997).
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[3] H.-J. Frohn, St. Jakobs, Chr. Rossbach, Eur. J. Solid
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