Synthesis and studies of chemical properties of arylthallium(III) bis(tetrafluoroborates)

Article abstract of DOI:10.1134/S107036321003014X

Synthesis of arylthallium(III)bis(tetrafluoroborates) was carried out and their pyrolysis was studied. Synthetic procedure for preparation of fluoroaromatic compounds is developed.

Full text of DOI:10.1134/S107036321003014X

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 3, pp. 455–457. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © I.F. Gun’kin, 2010, published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 3, pp. 432–434.
Synthesis and Studies of Chemical Properties
of Arylthallium(III)bis(tetrafluoroborates)
I. F. Gun’kin
Vavilov Saratov State Agricultural University, ul. Sokolovaya 335, Saratov, 410005 Russia
e-mail: rector@ssau.saratov.ru
Received June 10, 2009
Abstract―Synthesis of arylthallium(III)bis(tetrafluoroborates) was carried out and their pyrolysis was studied.
Synthetic procedure for preparation of fluoroaromatic compounds is developed.
DOI: 10.1134/S107036321003014X
Arylthallium(III) salts are used in organometallic
and organic synthesis. Carbon-thallium bond in the
arylthallium(III) salts is fairly labile (E = 125 kJ mol–1)
suggesting an easy substitution of thallium fragment
leading to the formation of various substituted arenes.
Arylthallium(III) derivatives attract the attention first
of all because they are inert with respect to various
functional groups, and in the course of functionnaliza-
tion the substituting group selectively occupies the
position of thallium atom.
It was found that arylthallium bis(tetra-
fluoroborates) I–III while heating in argon without a
solvent undergo the oxidative decomposition to give
fluoroaromatic compounds IV–VI, thallium(I) tetra-
fluoroborate, and BF3 [reaction (2)].
Ar
ArTl(BF4)2
ArF + TlBF4 + BF3,
(2)
t, °C
IV−VI
ArF = C6H5F (IV), 4-CH3C6H4F (V), 3,4(CH3)2C6H3F(VI).
Yields of fluoroaromatic compounds are 55–70%.
The developed procedure for the preparation of mono-
fluoroaromatic compounds is interesting because the
starting substances I–III can be synthesized con-
siderably easy. Note that monofluoroaromatic com-
pounds are usually prepared according to the Balz–
Schiemann reaction [1–3]. Synthesis of mono-
fluoroarenes by the reaction of arylthallium difluoride
and boron trifluoride in boiling cyclohexane was
reported [4]. 9-Fluorocarboranes with the B–F bond
were synthesized by the reaction of carboranylthal-
lium(III) bis(trifluoroacetates) with the boron trifluo-
ride etherates [5]. Hence, the developed synthesis of
fluoroaromatic compounds supplements the Balz–
Schiemann method.
The aim of this work is the synthesis of aryl-
thallium bis(tetrafluoroborates) and the investigation
of their chemical properties.
We synthesized previously unknown arylthallium
(III) bis(tetrafluoroborates) I–III by the reaction of
arylthallium(III) dihydroxide with HBF4 in ether at
room temperature [reaction {1)].
ether
ArTl(BF4)2 + 2H2O,
(1)
ArTl(OH)2 + 2HBF4
I−III
Ar = C6H5 (I); 4-CH3C6H4 (II); 3,4(CH3)2C6H3 (III).
Compounds I–III are insoluble in ether and
precipitate as hygroscopic oils, and only after thorough
drying over P2O5 in a vacuum at room temperature
they gradually crystallize. All the attempts to purify
compounds I–III by recrystallization failed because
they gradually decompose on heating. That is why
these substances were purified at room temperature by
reprecipitation (see Experimental).
In the course of the reaction (2) thallium converts
from the trivalent state to univalent, and gaseous boron
trifluoride is evolved. For improving the yields of
fluoroarenes IV–VI the starting tetrafluoroborates I–
III should be thoroughly dried. Hence, pyrolysis of
arylthallium(III) bis(tetrafluoroborates) can be
successfully used for the selective introduction of
fluorine in the aromatic ring.
455

456
GUN’KIN
As known, arylthallium bis(trifluoroacetates)
3,4-Dimethylphenylthallium bis(tetrafluoroborate)
(III). Yield 74.4%, decomposition temperature 67–74°C.
IR spectrum, ν, cm–1: 3062, 3013 (C–Har); 2958 (CH3),
1628, 1488 (C=Car); 1052 (B–F); 850, 804, 776, 749
[δ(C–Har)]. Found, %: C 19.82, H 1.83, Tl 42.32.
C8H9B2F8Tl. Calculated, %: C 19.89, H 1.88, Tl 42.30.
undergo disproportionation in acetone at 56°C to give
diarylthallium(III) trifluoroacetate [6]. We have shown
that arylthallium(III) bis(tetrafluoroborates) synthesized
behave analogously [reaction (3)].
acetone
PhTl(BF4)2
Ph2TlBF4 + TlBF4.
(3)
56°C
General procedure for preparing fluoroaromatic
compounds IV–VI. The installation for distillation
was connected with a vessel for washing of gases filled
with aqueous NaF solution and flushed with dry argon.
After that 0.05 mol of tetrafluoroborate I–III was
placed into the flask and the system was additionally
flushed with argon. The flask was heated at 70–90°C
when the liberation of gaseous products began.
Fluoroaromatic compounds were condensed in the
condenser and collected in the receiver. Boron
trifluoride was absorbed with sodium fluoride solution.
At the end of the process reaction temperature was
increased to 115–155°C and maintained for additional
30 min. After that the installation was flushed with
argon and gradually cooled. Fluoroaromatic com-
pounds were additionally purified by distillation The
residue after pyrolysis containing thallium(I)
tetrafluoroborate and the tarry products was treated
with water, filtered, and thallium(I) salt was isolated.
VII
EXPERIMENTAL
IR spectra of compounds I–III were registered on a
Specord-80 spectrometer in mineral oil and hexa-
chlorobutadiene.
Arylthallium dihydroxides were synthesized
according to [7].
General procedure for preparation of aryl-
thallium bis(tetrafluoroborates) (I–III). To
a
solution of 0.05 mol of the corresponding arylthallium
hydroxide in 50 ml of ether 0.12 mol of aqueous HBF4
was added under intense stirring. After dissolution of
the precipitate and the formation of the oily product
the reaction mixture was stirred for 10 min. After that
the oils obtained were separated from ether, placed in
the broad beaker, and evacuated. After removing the
admixtures of ether compounds I–III were dried over
P2O5 in a vacuum for a week at room temperature in
the darkness. In the course of handling they gradually
crystallized. Tetrafluoroborates I–III are well soluble
in water, ethanol, acetonitrile, and DMF, and poorly
soluble in ether. They are insoluble in the nonpolar
solvents. Compounds I–III were purified by
reprecipitation. They were dissolved in acetonitrile,
and the solutions obtained were added to ether or to
3:1 ether–pentane. The precipitates formed were dried
at room temperature in a vacuum for 2 days over P2O5.
Fluorobenzene (IV). Yield 55%, bp 85°C, nD20
1.4652, published data [4]: bp 84.5°C, nD20 1.4650.
4-Fluorotoluene (V). Yield 65%, bp 117°C, nD20
1.4678, published data [4]: bp 116°C, nD20 1.4670.
3,4-Dimethylfluorobenzene (VI). Yield 70%, bp
151°C, nD20 1.4857, published data [4]: bp 151.5°C, nD20
1.4864.
Diphenylthallium tetrafluoroborate (VII). A
solution of 1.656 g of compound I in 20 ml of acetone
was refluxed for 0.5 h. After that the volatile sub-
stances were distilled off, and the residue was three
times treated with hot 9:1 benzene–ethanol. Solvent
was distilled off, the precipitate was filtered off,
washed with benzene, and dried in a vacuum at 50°C
to give 0.350 g (44%) of compound VII, mp 304–
306°C. IR spectrum, ν, cm–1: 3050, 2964, 2952 (C–Har);
1581, 1479, 1433 (C=Car); 765, 725, 686 [δ(C–Har)],
1035 (B–F). Diphenylthallium tetrafluoroborate VII
prepared by us using the exchange reaction between
Ph2TlCl and TlBF4 in aqueous pyridine; after
crystallization from 9:1 benzene–ethanol it melted at
306–308°C, and its IR spectrum was identical to that
obtained in the above-described experiment.
Phenylthallium bis(tetrafluoroborate) (I). Yield
81.2%, decomposition temperature 113–115°C. IR
spectrum, ν, cm–1: 3060, 2960, 2930 (C–Har); 1548,
1476, 1440 (C=Car); 1040 (B–F), 768, 756, 720, 704,
676 [δ(C–Har)]. Found, %: C 15.75, H 1.01, Tl 44.87.
C6H5B2F8Tl. Calculated, %: C 15.84, H 1.11, Tl 44.91.
p-Tolylthallium bis(tetrafluoroborate) (II). Yield
70%, decomposition temperature 105–109°C. IR
spectrum, ν, cm–1: 3068, 3064, 3020 (C–Har); 2962,
2862 (CH3); 1628, 1488, 1464 (C=Car); 1032 (B–F);
790, 756, 726, 724 [δ(C–Har)]. Found, %: C 17.81, H
1.43, Tl 43.62. C7H7B2F8Tl. Calculated, %: C 17.92; H
1.50, Tl 43.57.
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SYNTHESIS AND STUDIES OF CHEMICAL PROPERTIES
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4. Taylor, E.C., Bigham, E., Johnson, D.K., and McKillop, A.,
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