Unusual transformations of difluorosilanone F2Si=O

Article abstract of DOI:10.1134/S107036320902008X

The structure and properties of fluorosilicon polymer (fluorosil) formed by the reaction of phenyltrifluorosilane with aliphatic alcohols have been studied by the methods of IR, ¹⁹F, ²⁹Si NMR spectroscopy, high temperature mass-spect

Full text of DOI:10.1134/S107036320902008X

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 2, pp. 215–220. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © M.G. Voronkov, S.V. Basenko, E.V. Boyarkina, E.F. Kalistratova, S.I. Lopatin, I.A. Gebel, L.E. Zelenkov, 2009, published in
Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 2, pp. 227–232.
Unusual Transformations of Difluorosilanone F2Si=O
M. G. Voronkov, S. V. Basenko, E. V. Boyarkina, E. F. Kalistratova,
S. I. Lopatin, I. A. Gebel, and L. E. Zelenkov
Favorskii Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: voronkov@irioch.irk.ru
Received July 31, 2008
Abstract―The structure and properties of fluorosilicon polymer (fluorosil) formed by the reaction of
phenyltrifluorosilane with aliphatic alcohols have been studied by the methods of IR, 19F, 29Si NMR
spectroscopy, high temperature mass-spectrometry, derivatography and atom emission analysis. Due to its high
reactivity, this polymer readily reacts with glass of the reaction vessel extracting the ions of all metals entering
into its composition. Fluorosil formed in a quartz, teflon or polypropylene reactors is characterized by low
stability and is slowly decomposed to SiF4 and SiO2. Apparently, fluorosil is the product of incorporation of
SiF4 into the SiO2 matrix.
DOI: 10.1134/S107036320902008X
In recent years, our scientific school is engaged in
studies of the reactions of the C–Si bond cleavage in
phenyltrifluorosilane (PTS) aiming at their use in
organoelement and organic synthesis [1–11].
1115°C leads only to formation of F3SiOSiF3 [16]. It
was incidentally reported that upon interaction of
difluorosilylene F2Si with oxygen perfluoropolysil-
oxanes are formed [17]. It was also reported that
interaction of SiF4 with metal oxides, salts of oxygen-
containing acids, fiberglass or sodium silicate at 400–
900°C results in (F3Si)2O, (F3SiO)2SiF2 and a viscous
mixture of, apparently, cyclic and linear perfluoropoly-
siloxanes (F2SiO)n [18]. All these compounds turned
out to be unstable with respect to H2O (atmosphere
moisture) and HF and readily disproportionate to SiO2
and SiF4. Upon evacuation of this mixture the poly-
mers eliminate HF and are solidified into a cracked
glassy mass which upon further heating to 300°C in
vacuum is transformed into very solid but voluminous,
roentgen-amorphous SiO2 [19].
We have made up our mind to elaborated on the
basis of this reaction the method of generation of
difluorosilanone F2Si=O in liquid phase and under
mild conditions.
Earlier it was reported that the molecule of F2Si=O
is generated only under unique conditions, for
example, upon the reaction of silicon monooxide with
fluorine on argon matrix at UV irradiation [12]. It was
also reported that difluorosilanone is formed in the gas
phase by successive reactions of tert-butoxide anion
Me3CO– with tetrafluorosilane under the conditions of
ion-cyclotron resonance by the following scheme [13]:
Difluorosilanones were investigated by the methods
of computer chemistry. Calculations of the reaction of
hydrogenation of the F2Si=O molecule allowed to
obtain the information on geometric and electronic
characteristics of the reagents, transition states and the
products of their interaction [20]. The calculated F–Si
and Si=O bond lengths in this molecule are equal to
1.556 and 1.482 Å, and the FSiF and FSiO bond angles
to 104.8° and 127.6°, respectively. The Si=O bond in
the F2Si=O molecule is more strong than in H2Si=O.
The activation barrier for hydrogenation of F2Si=O is
lower and the thermal effect is higher than those for
t-C4H9O– + SiF4 → F3SiO– + t-C4H9F,
F3SiO– + SiF4 → SiF5– + F2SiO.
The method of preparation of the mixture of F2SiO,
F3SiOSiF3, F3SiOSiF2OSiF3, F3SiSiF2SiF3 in the gas
phase by heating of the mixture of SiO2 and CaF2 to
2000–2500°C has been patented [14].
Finally, the possibility of generation of F2SiO in the
reaction of SiO2 with PbF2 at 700–900°C has been
discussed [15]. The reaction of SiO2 with SiF4 at 450–
215

216
VORONKOV et al.
H2Si=O. It also follows from the calculations that the
highest occupied molecular orbital of F2Si=O contains
the electron pair of the silicon–oxygen π-bond while in
the H2Si=O molecule it is occupied only by the lone
pair of the oxygen atom [21]. According to modern
high-level ab initio calculations with expanded basis
set of the F2Si=O molecule, the interatomic distances
F–Si, Si=O and the angle FSiF are equal to 1.555(1)
and 1.498(1) Å, and 104.7(3)°, respectively [22].
Recently, high-level [B3LYP/6-311G(d,p)] calculations
of geometrical and spectroscopic parameters of the
F2Si=O molecule in comparison with other dihalo-
silanones X2Si=O [X = Cl, Br, I] were performed. The
bond lengths F–Si, Si=O and the angle FSiO are equal
to 1.586°, 1.510 Å, and 127.9°, respectively [23].
hexachlorocyclotrisiloxane (Cl2SiO)3 (the product of
cyclooligomerization of [Cl2Si=O], benzyloxypenta-
chlorodisiloxane and benzyloxyheptachlorotrisiloxane:
the products of insertion of one or two molecules of
[Cl2Si=O] into the Si–O bond of the starting
PhCH2OSiCl3) [27]. The yield of these compounds is
4, 11 and 1.5%, respectively.
From the GC and MS data, the thermal reaction
(600–620°C) of dichlorosilanone generated from
PhCH2OSiCl3 with permethylcyclosiloxanes (Me2SiO)n
(n = 3–4) leads to insertion of the dichlorosilanone
molecule into the Si–O–Si group in these heterocycles
in up to 37% yield [28].
Based on the developed by us theory of formation
and destruction of siloxane structures [29] as well as
on the published data [27, 28] we decided to use for
generation of difluorosilanone the reaction of geminal
decomposition of alkoxyfluorosilanes (RO)4–nSiFn (n =
2–3). It was found that the reaction of PTS with
aliphatic alcohols at 65–110°C proceeds with rupture
of the C–Si and C–F bonds to give rise to benzene, alk-
oxyfluoro- and tetraalkoxysilanes and a small amount
of phenylalkoxyfluorosilanes [3]:
B3LYP density functional calculations predicted
the spectroscopic parameters of the F2Si=O molecule
and its geometry, angle FSiO ~126°. The energy of
bonding of the fluorine atom with the SiO group is
80.1 kcal mol–1. In other X2Si=O molecules the energy
of the X–Si bond (X = Cl, Br) is substantially lower:
–40.9 and –29.3 kcal mol–1, respectively. Harmonic
vibrations and isotope shifts in the vibrational
spectrum of F2Si=O were calculated. Bonding
characteristics were studied in different approxima-
tions (natural bond orbital approach, topological
analysis of the charge density and functions of the
localized electron) [24].
C6H5Si(OR)nF3–n
C6H5SiF3 + n ROH
C6H6 + (RO)nSiF4–n
R = CH3, C2H5, C4H9, C6H13, C8H17, CH2C6H5; n = 1–3.
Recently, the IR frequencies and intensities in the
spectrum of F2Si=O were calculated and compared
with the experimental data [25]. Semiempirical method
MNDO–PM3 gave heat of formation for the F2Si=O
molecule of 229.4 kcal mol–1 which is lower than the
earlier found value of 267.9 kcal mol–1 [26]. These
values are in good agreement with ab initio
calculations using the 6-31G(d) basis set.
Surprisingly, it turned out that these reactions are
followed by the formation of a white powder-like
precipitate which is insoluble in regular solvents and
from the data of elemental analysis corresponds to
formula F2SiO. The yield of this inorganic polymer
(which we called “fluorosil”) increases with increase
the length of the alkyl radical R from CH3 to C8H17
reaching for R = C6H13 ~20% to the starting PTS
whereas for R = CH3, C2H5 its yield does not exceed 5%.
We believe that fluorosil is apparently the product of
disproportionation of the intermediately formed
alkoxytrifluoro- or dialkoxydifluorosilanes.
All the aforementioned experimental data and
quantum chemical calculations have nothing to do with
the possibility of transformation of difluorosilanone
into the cyclic or linear oligomeric or polymeric per-
fluorosiloxane structures.
Earlier it was reported that dichlorosilanone
Cl2Si=O is the intermediate product in thermolysis of
benzyloxytrichlorosilane occurring at 500–650°C [27–
28] by the scheme:
2 ROSiF3 → (RO)2SiF2 + SiF4,
(RO)2SiF2 → ROК + [O=SiF2].
The necessity to use higher alcohols for splitting
the C–Si bond in PTS by alkanols is due to the fact
that elongation of the alkyl radical prevents more deep
disproportionation of ROSiF3 and (RO)2SiF2 to (RO)4Si.
PhCH2OSiCl3 → PhCH2Cl + [O=SiCl2].
Intermediate formation of Cl2Si=O is witnessed by
the presence in the condensate obtained at 580°C of
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217
UNUSUAL TRANSFORMATIONS OF DIFLUOROSILANONE F2Si=O
The obtained polymer, being washed with chloro-
form and dried in vacuum, contains 1.20% C and
0.67% H. This is apparently caused by the presence of
residual phenyl and alkoxy groups in the sample that
was corroborated by the data of thermography, mass
spectrometry and NMR.
pairs in the trans position in the SiF4 plane. Strict C4v
symmetry of the octahedral surrounding of the silicon
atom is caused by the alternative prohibition rule for
symmetric and asymmetric vibrations. In the IR
spectrum of fluorosil only the asymmetric vibration for
which (∂M/∂Q ≠ 0) is active.
The ratio of the fluorine atoms and hydroxyl groups
determines the intensity of the ν(Si–F)ax and ν(Si–O)
bands which in the starting fluorosil are of comparable
intensity. The axial location of the two oxygen atoms
in the octahedron O2SiF4 is confirmed by a similar
structure of complexes 2H2O·SiF4 [30].
On the derivatogram of the polymer prepared by
destruction of PTS with hexanol (see figure), its
gradual heating to 400°C is followed by almost 20%
loss of the mass. During further heating, two
exothermic maxima appear on the derivatogram at 430
and 570°C. Each of them is followed by the loss of
approximately 10% of the mass of the starting
polymer. From the data of elemental analysis the F/Si
atom ratio in the polymer undergone thermal treatment
to 460°C (α-fluorosil) is 3:2. Further heating of α-
fluorosil to 610°C gives rise to β-fluorosil with the
same F/Si atom ratio of 3:2. Finally, heating of β-
fluorosil to 800°C gives affords γ-fluorosil in which
the F/Si atom ratio remains unchanged, 3:2. From the
data of elemental analysis, after heating of the starting
polymer to 460°C the content of fluorine in it drops
from 46.7% to 32.0% (in α-fluorosil). On the contrary,
after heating of α-fluorosil to 610°C the content of
fluorine in the product of thermolysis (β-fluorosil)
increases to 39.4%. After further heating of the formed
β-fluorosil to 800°C the fluorine content in γ-fluorosil
becomes 26.6%. In the IR spectrum of the starting
fluorosil (without thermal treatment) the most intensive
is the Si–F absorption band at ν(Si–F) ~730 cm–1. This
is indicative of the octahedral arrangement of the
silicon atoms and of polyamorphous structure of the
polymer; νs(Si–O) vibration is represented by fre-
quency ~1040 cm–1. The complex wide band νas(Si–O)
with the central maximum at 1088 cm–1 is splitted into
three components.
In the IR spectrum of α-fluorosil the intensity of the
band at 730 cm–1 is small. Simultaneously, a new wide
band with maximum at 580 cm–1 appeared. In the IR
spectrum of β-fluorosil the band at 730 cm–1 disap-
peared almost completely and the intensity of the band
at 580 cm–1 substantially increased. In the IR spectrum
of γ-fluorosil the intensity of the wide band at 1080
cm–1 unexpectedly drastically dropped. All other bands
are identical to those in the spectra of α- and β-
fluorosil.
In the 29Si solid state NMR spectrum of the starting
polymer (without thermal treatment) three signals with
chemical shifts –78, –110, and –184 ppm are observed.
The first two signals belong to tetrahedral silicon, the
signal at –110 ppm corresponding to the silicon atom
surrounded by four groups OSi<, that is, Q4 type, or
(–O)4Si fragment. The signal at –78 ppm belongs to
the silicon atom of the type Q2 with the two other
substituents being fluorine atoms, thus corresponding
to the –OSiF2 fragment. The signal at –184 ppm
belongs to hexacoordinate silicon atom in the anion
[SiF6]2-. For more detailed assignment of the signals of
1
silicon the measurements with H–29Si cross polariza-
tion were performed and the 29Si MAS spectra of Si(O–)4
after adsorption of water were measured. The 29Si MAS
Comparison of the IR spectra of the starting
fluorosil in KBr and in vaseline oil suggests that in the
range of stretching vibrations of hydroxyl groups the
band at 3440 cm–1 corresponding to ν(O–H) in water
(always presenting in KBr) has a low-frequency shoulder
at ~3370 cm–1. IR spectrum in vaseline oil which is free
of water contains a single symmetric band at 3370 cm–1.
The position and the shape of this band (with half-
width of ~300 cm–1) are characteristic for ionic band
O–Hδ+. The band of δ(O–Hδ+) deformational vibrations
has the maximum at 1650 cm–1.
100
90
TG
80
70
60
50
40
30
20
10
0
570°
430°
DTA
0
100 200 300 400 500 600 700 800 T, °C
The singlet shape of the ν(Si–F)(λ6 Si–F) suggests
that four fluorine atoms are identical and are located in
Thermogram of fluorosil.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 2 2009

218
VORONKOV et al.
Mass spectra of the volatile components of fluorosil
chemical shift being somewhat changed also upon
further heating of the sample (downfield shift).
According to calculations, this effect may be indicative
of an increase of the number of oxygen atoms in the
surrounding of the silicon atom.
Temperature, °С
Comp.
Ion
M/Z
no.
250°C
2.0
6.3
4.3
3.0
2.6
3.3
5.3
3.3
5.0
2.6
8.9
36.4
1.0
2.3
7.6
6.3
2.3
3.3
100
5.2
3.3
2.3
350°C
1.1
450°C
1
F+
19
20
27
29
33
39
41
42
43
47
48
49
50
51
55
56
57
68
85
86
87
104
I
HF
3.0
2.0
19F MAS NMR spectra are rather complex. In the
spectrum of the original fluorosil (without thermal
treatment) the most intensive are the signals at –128,
–137, and –145 ppm. After thermal treatment at 310°C
the signal at –128 ppm disappears together with the
29Si signal at –78 ppm belonging to the tetracoordinate
silicon atom. The signals at –137 and –145 ppm, ap-
parently, relate to hexacoordinate silicon atom bound
by the –OSiO– link. In the 19F MAS NMR spectrum of
β-fluorosil these signals are lacking, the spectrum
consists of a large number of wide lines. The presence
of the Si…F…Si fluorine links in the structure of
fluorosil is, apparently, witnessed by the signal at –145
ppm. The terminal fluorine atoms are shifted
downfield (–8, –52 ppm).
II
C2H+3
C2H5+
CH2F+
C3H+3
C3H5+
III
IV
1.1
1.6
V
VI
1.5
1.1
VII
VIII
IX
+
C3H6
C3H+7
1.5
CFO+
CHFO+
CH2FO+
C4H+2
2.3
2.8
10.5
44
X
16.6
72.7
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
XX
XXI
XXII
The polymer synthesized by protolysis of PTS with
hexanol was investigated by the method of high
temperature mass spectrometry at 250, 350 and 450°C;
the masses observed are presented in the table.
C4H+3
C4H+7
1.9
1.9
The data of the table demonstrate that upon ther-
molysis of the starting polymer at the temperatures
given above F+., HF, F3Si+ (the product of dissociative
ionization of SiF4) and SiF4 are formed I, II, XIX–
XXII, respectively. Hydrocarbon ions III, IV, VI-IX,
XIII–XVII result from destruction of groups C6H13O
or dicyclohexyl ether C6H13OC6H13 remained in the
polymer. Finally, the formation of ions V, X–XII,
XVIII is apparently due to decomposition of geminal
groups –Si(OC6H13)F. From the isotopic composition,
ions XII and XVIII do not contain silicon.
C4H+8
C4H+9
CH2F2O+
28SiF3+
29SiF3+
30SiF3+
SiF4
4.5
100
5.3
2.8
100
5.2
3.4
3.2
2.7
2.0
At temperatures higher than 350°C the C6H13O2FSi+
ion with m/z 164 appears in the mass spectrum corro-
borating the destruction of the geminal group >Si(OR)F.
This ion disappears at temperatures above 500°C, and
the ion SiF3+ above 1024°C. Difluorosilanone was not
detected in the gas phase. Strange as it might seem, at
temperatures above 994°C an ion with m/z 157 was
detected in the mass spectrum of fluorosil. From its
mass and isotope distribution we identified this ion as
BaF+ the product of dissociative ionization of BaF2
molecule. Starting from 950 K in the mass spectrum
appeared ions AlF2+, BaF+ (at 1250K), and Ba+ and BaO+
(at ~1400 K).
cross polarization study suggests the lack of protons in
the second coordination sphere of the hexacoordinate
silicon atom. After the reaction of the fluorosil sample
with water the signal at –110 ppm disappears, ap-
parently, due to splitting of the Si(O–)4 fragments with
HF and water. The signal at –78 ppm does not change
after water treatment although protons appear in the
second coordination sphere that is indicative of the
formation of groups Si–OH.
In the 29Si MAS spectrum of the original sample of
α-fluorosil heated to 310°C only one line correspond-
ing to the hexacoordinate silicon remains, its width and
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219
UNUSUAL TRANSFORMATIONS OF DIFLUOROSILANONE F2Si=O
To prove these unexpected results we have studied
the elemental composition of fluorosil by the method
of atom emission analysis. The polymer turned out to
contain ions Na, K, Ba, B, Al and minor quantities of
Mg, Ca, Fe, Zr. Since there was no doubt in high
purity of all the reagents used by us, it became clear
that all these metals were extracted by the polymer
from laboratory glass which contains them [31].
voltage 70 eV. The temperature of the injector increased
stepwise and was 250, 350, 450, and >800°C.
IR spectra were recorded on a Specord IR-75 spec-
trometer in the range of 400–4000 cm–1 in KBr pellets.
Thermograms were obtained on a derivato-graph
MOM (Hungary). The rate of heating 5 deg min–1, in
air, reference compound (Al2O3), sensitivity 1/5.
Phenyltrifluorosilane was prepared by the reaction
of phenyltrichlorosilane with HF as described earlier
[32]. Absolute aliphatic alcohols were distilled before
use.
Therefore, a detailed study of the reaction of PTS
with aliphatic alcohols has shown that the material of
the reactor in which the reaction was carried out
(laboratory glassware) substantially affects the yield
and the composition of the products.
Reaction of PTS with methanol. Mixture of 8.1 g
(0.1 mol) of PTS and 1.6 g (0.1 mol) of methanol was
refluxed for 7–14 h. The precipitate formed was
filtered off, washed with CHCl3 and dried under
vacuum of 10 mm Hg at 20°C. Yield 0.15 g. Found,
%: F 46.79; Si 34.47. F2SiO. Calculated, %: F 46.29;
Si 34.22.
It prompted us to carry out the investigated reaction
of alcoholysis of PTS in a teflon reactor. It turned out,
that in this case the yield of fluorosil is minimal even
at reflux of the reaction mixture. Only negligible
amount of polymer is formed containing (to the time of
analysis) 42% Si and 15% F. With this, the content of
the later after 10 days decreased to 4%.
Reactions of PTS with other alcohols in the molar
ratios 1:1 and 1:2 as well as the reaction of PTS with
hexanol in CHCl3 were performed similarly. In all
cases the formation of insoluble inorganic polymer cor-
responding by composition to (F2SiO)n or SiO2·SiF4.
EXPERIMENTAL
13
19
29
1H, C, F and Si NMR spectra were registered
on a Avance-400 (Bruker) spectrometer under the
conditions of the magic angle spinning of the sample
using the Fourier transformation of the free induction
decay (FID) at frequencies 400, 100.6, 376.4, and
ACKNOWLEDGMENTS
This work was supported by the Council on grants of
the President of Russian Federation (garnt no. NSh-
4575.2006.3).
1
13
19
29
79.5 MHz for H, C, F and Si, respectively. The
rate of spinning was 2–20 KHz. FID was excitated by
a single pulse sequence, duration of the 90o pulse was
10 μs, delay for relaxation between the pulses was 10 s
for 13C, 19F, 29Si and 2 s for 1H.
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