The (cloromethyl)dihalosilanes X2HSiCH2Cl (X = F, Cl, Br, I): Synthesis, multinuclear NMR spectroscopy and rotational isomerism examined by Raman spectroscopy

Article abstract of DOI:10.1002/ejic.200400305

(Chloromethyl)diphenylsilane Ph₂HSiCH₂Cl (1) was prepared from Ph₂SiHCl and LiCH₂Cl, in situ, and was then treated with CF₃SO₃H/LiCl and CF₃SO ₃H/LiBr to give the (chloromethyl)dihalosilanes Cl ₂HSiCH₂Cl (2) and Br₂HSiCH₂Cl (3). I₂HSiCH₂Cl (4) was formed quantitatively in a protodearylation reaction between liquid hydrogen iodide and 1. Fluorination of 4 with ZnF₂ gave F₂HSiCH₂Cl (5), albeit in low yields. Compounds 1-5 were characterised by multinuclear NMR spectroscopy ( ²⁹Si, ¹³C, ¹⁹F) and vibrational spectroscopy. The temperature-dependent vibrational Raman spectra of 2, 3 and 4 prove that they are mixtures of gauche and anti rotamers in the liquid state. Enthalpy differences, ΔH(H_anti - H _gauche), were determined for the liquids by applying the van't Hoff relation, with the gauche rotamer being lower in energy for 3 and 4, and the anti rotamer for 2. The vibrational spectra were assigned with the help of ab initio calculations of equilibrium geometries and harmonic fundamental frequencies. Harmonic symmetry force constants and potential energy distributions were calculated from the ab initio Hessian matrices. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400305

FULL PAPER
The (Chloromethyl)dihalosilanes X2HSiCH2Cl (X ؍ F, Cl, Br, I): Synthesis,
Multinuclear NMR Spectroscopy and Rotational Isomerism Examined by
Raman Spectroscopy
Karl Hassler,*[a] Reinhard Hummeltenberg,[a] and Günter Tekautz[a]
Keywords: Infrared spectroscopy / NMR spectroscopy / Raman spectroscopy / Silanes / Synthesis
(Chloromethyl)diphenylsilane Ph2HSiCH2Cl (1) was pre-
pared from Ph2SiHCl and LiCH2Cl, in situ, and was then
treated with CF3SO3H/LiCl and CF3SO3H/LiBr to give the
gauche and anti rotamers in the liquid state. Enthalpy differ-
ences, ∆H (Hanti − Hgauche), were determined for the liquids
by applying the van’t Hoff relation, with the gauche rotamer
being lower in energy for 3 and 4, and the anti rotamer for
2. The vibrational spectra were assigned with the help of ab
initio calculations of equilibrium geometries and harmonic
fundamental frequencies. Harmonic symmetry force con-
(chloromethyl)dihalosilanes
Cl2HSiCH2Cl
(2)
and
Br2HSiCH2Cl (3). I2HSiCH2Cl (4) was formed quantitatively
in a protodearylation reaction between liquid hydrogen iod-
ide and 1. Fluorination of 4 with ZnF2 gave F2HSiCH2Cl (5),
albeit in low yields. Compounds 1−5 were characterised by stants and potential energy distributions were calculated
multinuclear NMR spectroscopy (29Si, 13C, 19F) and vibra- from the ab initio Hessian matrices.
tional spectroscopy. The temperature-dependent vibrational
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Raman spectra of 2, 3 and 4 prove that they are mixtures of
Germany, 2004)
Introduction
bearing no other substituents but hydrogen and halogen
atoms. They can be considered as the link between halogen-
ated hydrocarbons and their heavier analogues the halogen-
ated disilanes and trisilanes. The chemical behaviour of
both the SiϪSi bond and the SiϪC bond is strongly
influenced by the substituents at the molecular skeleton.
Numerous experimental investigations show a weakening of
the SiϪC bond of carbosilanes (a term coined by Fritz[6] for
compounds in which the molecular framework is exclusively
built up by SiϪC bonds) upon chlorination at the carbon
atom, whereas chlorination at the silicon atom has the op-
posite effect, causing a bond strengthening. Ab initio stud-
ies of chlorinated silaethanes confirm these effects.[7] For
instance, the bond-dissociation energies of SiCl3CH3,
SiH3CH3, and SiH3CCl3 were calculated as 392, 369 and
314 kJ/mol, respectively. With disilanes, the bond strengths
decrease in the order SiCl3SiCl3 Ͼ SiH3SiCl3, Ͼ SiH3SiH3,
which is supported by experimental and ab initio SiϪSi
force constants and 29Si,29Si coupling constants, for in-
stance.[8] The effect of Si chlorination can be rationalised
by Bent’s rule,[9] which states that bonds to electronegative
substituents possess more p-character, thereby increasing
the s-character of the bonds to electropositive atoms; more
s-character usually means a stronger bond. Upon C-chlori-
nation, Bent’s rule should also be at work, but the effect is
overridden by the repulsive force between the positive
charges at the Si and C atoms due to the short SiϪC dis-
tance. Moreover, the steric repulsion between the chlorine
atoms at carbon — the Cl···Cl distance is in fact consider-
ably shorter than the van der Waals distance — is reduced
α-(Halomethyl)silanes are versatile starting materials
for the synthesis of organosilicon compounds such as
silylmethyllithium derivatives or main-group derivatives
M-CHnX2ϪnSiR3, where X is a halogen.[1] However, only a
handful of methods, each one with its own limitations, can
be used for their preparation. These comprise the direct
chlorination of methylsilanes, MeSiR3 with Cl2
[2]
or
Cl2SO2,[3] a method which does not tolerate silyl, vinyl or
aryl groups as the R substituent. Another synthetic ap-
proach uses silyllithiums, LiSiR3 as precursors, treating
them with, for instance, CH2Cl2 to form CH2ClSiR3.[4]
However, this method is not widely applicable due to a lack
of suitable silyllithiums. A third method with a wider range
of applications is the alkylation of chlorosilanes by a (halo-
methyl)lithium XnH3ϪnCLi (n ϭ 1, 2, 3; X ϭ halogen),
which is usually generated in situ. Pannell and co-workers
have prepared a considerable number of (chloromethyl)sil-
anes with this method that contain functionalities such as
SiH, SiCl, SiSi, Si-vinyl, Si-allyl and Si-phenyl.[5] The intro-
duction of these functional groups was a major step for-
ward, as they can be replaced by other substituents without
affecting the halomethyl functionality.
We are interested in the synthesis and spectroscopic
properties of silaethanes, silapropanes and disilapropanes
[a]
Institute of Inorganic Chemistry, University of Technology,
Stremayrgasse 16, 8010 Graz, Austria
Fax: ϩ43-316-873-8701
E-Mail: hassler@anorg.tu-graz.ac.at
Eur. J. Inorg. Chem. 2004, 4259Ϫ4265
DOI: 10.1002/ejic.200400305
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K. Hassler, R. Hummeltenberg, G. Tekautz
Si-aryl ϩ HX Ǟ SiX ϩ arylH (method 1)
FULL PAPER
in the free CCl3 radical upon dissociation. This reduction of
intramolecular repulsion energy certainly provides a further
contribution to the weakening of the bond.
Si-aryl ϩ CF3SO3H Ǟ SiOSO2CF3 ϩ arylH
SiOSO2CF3 ϩ LiX Ǟ SiX ϩ Liϩ[OSO2CF3]Ϫ (method 2)
If chlorine is replaced by other halogens, the relative con-
tributions of these two effects will certainly vary. Fluorine
substitution is expected to increase the positive charge on
the carbon atom, thereby also increasing the SiϪC repul-
sion. Conversely, the nonbonding halogenϪhalogen repul-
sion will decrease. Substitution with bromine should de-
crease the positive charge and increase the nonbonding in-
teractions, an effect which will be even more pronounced
with iodine. For instance, no (triiodomethyl)silanes have
been reported in the literature so far. C2I6 is also unstable
and has not been prepared yet.
These properties of the SiϪC bond should also influence
the barriers to internal rotation and conformer stabilities.
Our attention, therefore, is mainly focussed on silaethanes
and silapropanes that can display rotational isomerism. The
existence of at least two different conformers in silaethanes
and silapropanes demands the presence of EH2X or EHX2
groups (X ϭ halogen, E ϭ C or Si) in the molecule. A
literature search revealed that only a handful of these com-
pounds have been prepared so far, such as ClH2CSiCl2H[10]
and ClH2CSiH2X (X ϭ F, Cl, Br);[11] no such compounds
with iodine have been described. Moreover, the literature
search revealed a complete absence of general procedures
for the preparation of (halomethyl)silanes with SiH2X or
SiHX2 groups. We have therefore set out to develop general
synthetic methods suitable for the preparation of a wide
range of (halomethyl)dihalosilanes.
Method 1 usually gives better overall yields than 2.
From our experience with these two methods we expect
that they can also be used successfully with (bromomethyl)-,
(iodomethyl)- and even (fluoromethyl)silanes, though we
have not yet tested them with these substrates.
The choice of which of the two methods to apply for a
specific problem is dictated by practical considerations. If
the desired carbosilane has a boiling point which is near
that of the hydrocarbon arylH formed (which is the case for
benzene and CClH2SiCl2H), purification becomes difficult.
If one wishes to apply method 1 a larger aryl group can be
introduced; for instance, phenyl groups can be replaced by
tolyl or mesityl groups. Method 2 offers the advantage that
phenyl groups still can be used as substituents, because ben-
zene is easily separated from the trifluoromethanesulfony-
loxysilane by evaporation in vacuo. Thus, commercially
available phenylsilanes can be used as starting materials, re-
ducing the time needed for the synthesis. As shown in
Scheme 1, chlorodiphenylsilane was treated with CH2ClLi,
generated in situ from bromochloromethane and lithium at
low temperatures,[13] forming (chloromethyl)diphenylsilane
1. When a solution of 1 in toluene was treated with two
equivalents of trifluoromethanesulfonic acid, the two phe-
nyl groups were cleaved off, giving benzene and (chlorome-
thyl)bis(trifluoromethanesulfonyloxy)silane (1a) without
noticeable side reactions. Toluene and benzene were then
removed by evaporation in vacuo, giving the pure (chloro-
methyl)sulfonyloxysilane as a colourless oily liquid. Upon
addition of LiCl or LiBr, the (halomethyl)silanes 2 and 3
were formed in yields not exceeding 30%, presumably due
to SiϪC cleavage.
Results and Discussion
Syntheses
Tetrachlorosilaethane, CClH2SiCl3, is readily obtained by
direct chlorination of trichloromethylsilane with Cl2.[12]
However, it is difficult to transform it into CClH2SiCl2H or
CClH2SiClH2, as it is not easy to achieve a selective chlori-
nation of the SiH3 group. Recently, a two-step reaction se-
quence was reported[10] involving a reduction with LiAlH4
and subsequent chlorination with SnCl4.
(ClH2C)SiCl3 ϩ LiAlH4 Ǟ (ClH2C)SiH3
(ClH2C)SiH3 ϩ SnCl4 Ǟ (ClH2C)SiCl2H
Dichloro(chloromethyl)silane was prepared for vi-
brational spectroscopic purposes. No NMR shifts and
physical data were reported, and the synthesis seems inap-
propriate for the preparation of larger quantities due to the
volatility of (ClH2C)SiH3.
The introduction of aryl substituents into (chloromethyl)-
silanes offers the possibility to replace them with Cl, Br or
I by using either liquefied hydrogen halides HX or trifluoro-
Scheme 1. Synthesis of (chloromethyl)dihalosilanes
Compound 4 was prepared by method 1, treating 1 with
methanesulfonic acid in combination with the lithium hal- liquid HI. Dry gaseous hydrogen iodide was easily obtained
ides LiX. Both methods tolerate chloromethyl groups as from tetrahydronaphthalene and iodine as described in the
well as SiϪH and SiϪSi functionalities, give good to excel- literature,[14] and purified by trap-to-trap condensation.
lent yields, and are widely applicable.
Scheme 1 summarises the syntheses of 1Ϫ5. Fluorination
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(Chloromethyl)dihalosilanes
FULL PAPER
of 4 with various fluorinating agents such as ZnF2 turned
out to be difficult due to extensive SiϪC bond cleavage. The
yields achieved were so small that only the NMR spectra
of 5 could be obtained, albeit in good quality. We will there-
fore report on the fluorination of 5 in conjunction with the
synthesis of (bromomethyl)halosilanes and (iodomethyl)-
halosilanes.
NMR Spectroscopy
Compounds 2Ϫ5 were characterised by multinuclear
NMR spectroscopy (13C, 19F, 29Si), and 1 by 29Si NMR
only. Table 1 summarises the chemical shifts and coupling
constants that were obtained. In the proton-coupled 29Si
NMR spectra of 2, 3 and 4, the signal of the Si nucleus is
split into a doublet of triplets due to one- and two-bond
SiϪH coupling. The 13C NMR signal splits into a triplet of
doublets. The 29Si and 13C NMR spectra of 5 are presented
in Figure 1. The unusual appearance of the 29Si NMR spec-
trum, which consists of two triplets and two quadruplets,
results from the near equality of 1JSi,H and 1JSi,F, the differ-
ence being almost equal to 2JSi,H. Each quadruplet is there-
fore an overlap of two triplets. The 13C NMR spectrum is
composed of six triplets, as expected for a first-order spec-
trum.
Table 1. Chemical shifts[a] (ppm) and coupling constants (Hz) for
the (chloromethyl)dihalosilanes 1؊5
δ(29Si) δ(13C) δ(19F) JSi,H 2JSi,H JC,H JC,H JC,F JSi,F
1
1
2
2
1
1
Ϫ17.6
1a Ϫ30.3
204.9 4.3
335.6
300.0 4.3
299.0 4.3
292.0 4.3
Figure 1. Splitting of the 29Si (top) and the 13C NMR signal (bot-
tom) of CH2ClSiF2H due to indirect spinϪspin coupling
2
3
4
5
ϩ0.9 29.9
Ϫ13.4 29.6
Ϫ85.2 28.1
144.0 16.0
146.0 15.0
146.0 11.0
141.0 23.5 18.3 305.0
Ϫ29.1 23.8 Ϫ141.5 301.0 4.8
Raman Spectroscopy and Rotational Isomerism
[a]
δ(29Si) and δ(13C) are referenced against TMS; δ(19F) is refer-
At ambient temperatures, the (chloromethyl)dihalosil-
anes 2Ϫ5 exist as mixtures of gauche [C1 symmetry,
ω(ClCSiH) ഠ 52°] and anti [Cs, ω(ClCSiH) ϭ 180°] con-
formers, as illustrated in Figure 2 with the help of New-
man projections.
enced against CCl3F.
The 29Si NMR chemical shifts of 1Ϫ5 do not follow a
linear dependence on the electronegativities of the directly
bonded halogens, which is not surprising, as it is known
that 29Si chemical shifts depend on the sum of the electro-
negativities of the substituents in a nonlinear manner.[15]
The shift maximum for 2 can easily be explained on that
basis.
One-bond SiϪH coupling constants usually increase with
increasing electronegativities (sum of the electronegativities)
of the substituents, which can be interpreted in terms of
isovalent hybridisation. Bonds to electronegative elements
possess more p-character, which is balanced by more s-
character in the SiϪH bond. As the Fermi contact term
1
Figure 2. Newman projections of anti (left) and gauche (chlorome-
thyl)dihalosilanes
dominates JSi,H, the coupling constant increases. Interest-
1
ingly, the values of JSi,H for 2Ϫ5 show only a slight in-
crease by no more than 9 Hz with increasing electronegativ-
ity of the halogens. For the silanes H2SiI2 and H2SiF2, the
difference is 7.4 Hz.[15]
As long as the barriers separating the two conformers are
well above 2.5 kJ/mol (ϭ RT at room temperature), they
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K. Hassler, R. Hummeltenberg, G. Tekautz
FULL PAPER
behave as distinct vibrating entities and can be identified by normal modes as group vibrations by use of the potential-
their IR and Raman vibrational spectra. For SiϪC and energy distributions (PEDs). The program ASYM40[17] was
SiϪSi bonds this condition is usually fulfilled. For assigning used for these calculations. A factor of 0.92 was used to
bands to individual rotamers, a normal coordinate analysis scale the calculated frequencies, and (0.92)2 to scale the
based upon ab initio calculations of vibrational frequencies force constants. Table 3 and 4 summarise the wavenumbers
and force constants is normally carried out for both con- in the Raman and IR spectra of 3 and 4, the scaled and
formers.
unscaled ab initio wavenumbers, and the descriptions of the
The rotational isomerism of dichloro(chloromethyl)silane modes as group vibrations based on calculated PEDs. The
has been investigated recently by FT-IR spectroscopy on spectra of 2 are not repeated here as they have been re-
solutions in liquid xenon, with the more stable form being ported previously.[10]
the gauche conformer (∆Hexp. ϭ 4.34 Ϯ 0.48 kJ/mol).[10]
Table 2. Selected structural constants (pm, deg.) and relative ener-
gies (kJ/mol) for the conformers of 2؊5 as predicted by the ab
initio calculations
The result is consistent with ab initio calculations at the
MP2/6-31G(d) level of theory, but is also reproduced cor-
rectly at the HartreeϪFock level. Values for ∆H obtained
on solutions in noble gases are supposed to differ only
slightly from values in the gas phase.
CClH2SiF2H CClH2SiCl2H CClH2SiBr2H CClH2SiI2H
anti gauche anti gauche anti gauche anti gauche
For the present study, we have recorded the Raman spec-
tra for the liquids of 2, 3 and 4. Figure 3 presents the Ra-
man spectrum of 3 at two temperatures, illustrating the
variations that occur upon cooling. In all cases we only ob-
tained glassy solids at low temperatures that contained both
CSi
SiH
SiX
187.8 187.8 188.9 188.8 189.3 189.1 189.9 189.7
147.2 146.7 147.4 146.9 147.6 147.1 147.7 147.3
158.1 158.6 205.2 206.2 222.2 223.2 245.4 246.5
158.1
205.3
222.2
254.4
CSiH 111.5 114.2 109.3 112.4 108.8 112.3 107.9 111.5
conformers at the temperature of liquid nitrogen. No crys- CSiX 109.9 107.3 110.4 106.9 110.5 106.7 110.7 106.6
ClCSi 112.5 110.3 114.1 110.9 114.5 111.1 114.9 111.3
tallisation was observed.
ClCSiH 180.0 52.2 180.0 52.2 180.0 52.0
180.0 52.3
6.3 0.0
Erel
4.8
0.0
5.7 0.0
6.2
0.0
The value of the enthalpy difference can be determined
from temperature-dependent vibrational spectra by apply-
ing the van’t Hoff relationship, which is derived under the
assumption that ∆H is independent of temperature
ln(Ia/Ig) ϭ Ϫ∆H/RT ϩ const.
where Ia and Ig are the intensities of the Raman or IR
bands for rotamer a and g, respectively. Plotting ln(Ia/Ig)
versus 1/T thus results in a straight line whose slope is equal
to Ϫ∆H/R. ∆H values were obtained by a least-squares fit
from three line pairs for 2 and 3, and from two pairs for 4.
As an example, two van’t Hoff plots are presented in Fig-
Figure 3. Raman spectrum of dibromo(chloromethyl)silane in the
range 50Ϫ900 cmϪ1 at 22 °C and Ϫ100 °C.
To facilitate the identification of vibrational bands be- ure 4.
longing to the various conformers, ab initio calculations of
Table 5 summarises the ∆H values together with the rel-
equilibrium geometries, harmonic frequencies and force evant band pairs and the statistical error. In these experi-
constants (Hessian matrices) were performed at the ments, severe interference from overlapping with overtones
HartreeϪFock level of theory for 2Ϫ5. Inner filled electron or combination bands occur quite often. Fermi resonances
shells were replaced by the electron core potentials (ECPs) between overtones and fundamentals are also encountered
reported by Stevens, Krauss and Basch.[16] A 6-31G** quite frequently. It is also a prerequisite of the method that
double-zeta basis set was used for the valence electrons. By the bands of the two conformers both have reasonably high
use of the ECPs, relativistic corrections for the heavier hal- intensity and are sitting on a flat baseline. These difficulties
ogens are taken into account. Table 2 summarises the most have been reviewed recently by Klaboe.[18]
important geometric parameters for both conformers of
The discrepancies between the experimental ∆H values
2Ϫ5 — anti and gauche — as well as the relative confor- obtained from the different band pairs can easily be ex-
mational energies predicted by the ab initio calculations. plained on that basis. Average ∆H values of Ϫ0.57 Ϯ 0.37,
The Hessian matrices, which represent the force constants 0.21 Ϯ 0.07 and 0.56 Ϯ 0.50 were obtained for 2, 3 and 4,
defined in Cartesian coordinates, were transformed into respectively, by neglecting the statistical uncertainties re-
force fields defined with symmetry coordinates which are sulting from the least-squares fits. Due to the uncertainties
linear combinations of bond distortions and angle distor- mentioned above, the standard deviations are quite large.
tions. With the symmetry force fields, normal coordinate
Opposite to the gaseous state, the anti rotamer of
analyses were carried out which allow the description of the CH2ClSiHCl2 is slightly more stable in the liquid. This can
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(Chloromethyl)dihalosilanes
FULL PAPER
Table 3. Comparison of observed and calculated wavenumbers (cmϪ1) for the fundamental vibrations of anti and gauche dibromo(chloro-
methyl)silane
Fundamental
Obsd.
anti
gauche
Ra(l)
Ir(l)
ab initio
scaled
ab initio
scaled
CH2 antisymmetric stretch
CH2 symmetric stretch
SiH stretch
CH2 deformation
CH2 wag
2974 ms
2930 vs
2216 vs
1377 w
1171 vw
2980 m
2940 vs
2220 s
3302.4
3232.1
2382.1
1532.4
1322.5
3038
2974
2192
1410
1217
3320.9
3246.6
2408.9
1534.0
1320.2
3055
2987
2216
1411
1215
1386 s
1177 ms
1125 w
1100 mw
823 vs
795 vs
763 vs,b
667 vs
CH2 twist
SiH bend
SiH bend
CCl stretch
CH2 rock
SiC stretch
SiC stretch
1091 vw
826 w
1217.0
878.6
875.2
863.3
759.8
1120
808
805
794
699
1214.3
904.8
837.8
851.4
727.8
705.9
1117
832
771
783
670
649
773 w,b
666 m
649 sh
606 w
587 w
466 w
435 w
397 vs
385 sh
309 w
234 w
216 w
197 vs
162 w
130 sh
119 s
641.5
590
SiBr2 antisymmetric stretch
SiBr2 antisymmetric stretch
SiBr2 symmetric stretch
SiBr2 symmetric stretch
SiBr2 wag
463 vs
489.4
419.4
450
386
483.9
445
395 vs
380 sh
307 mw
403.3
336.9
371
310
ClSiC bend
222.7
205.6
205
189
SiBr2 wag
SiBr2 twist
SiBr2 deformation
SiBr2 deformation
SiBr2 twist
174.9
132.5
161
122
120.8
106.2
60.2
111
98
55
96.3
52.4
89
48
torsion
Table 4. Comparison of observed and calculated wavenumbers (cmϪ1) for the fundamental vibrations of anti and gauche (chloromethyl)di-
iodosilane
Fundamental
Obsd.
anti
gauche
Ra(l)
Ir(l)
ab initio
scaled
ab initio
scaled
CH2 antisymmetric stretch
CH2 symmetric stretch
SiH stretch
CH2 deformation
CH2 wag
CH2 twist
CCl stretch
2975 ms
2928 vs
2198 vs
1388 w
1175 vw
1090 vw
812 w
2983 m
2942 ms
2200 s
1390 s
1180 m
1180 m
820 vs
795 vs
3302.6
3231.3
2371.6
1532.0
1319.5
1216.4
870.2
3039
2973
2182
1409
1214
1119
801
3322.5
3246.6
2397.3
1533.9
1318.9
1216.7
845.9
3057
2987
2206
1412
1213
1119
778
SiH bend
789 w
853.7
785
896.4
825
SiH bend
SiH bend
766 w
724 w
851.1
783
810.1
745
CH2 rock
CH2 rock
SiC stretch
SiC stretch
SiI2 antisymmetric stretch
SiI2 symmetric stretch
SiI2 wag
687 vw
653 m
636 sh
596 w
403 mw
333 vs
280 m
753.5
693
660 vs
713.9
698.9
657
643
590 w
403 vs
333 ms
632.0
416.9
351.4
302.6
581
384
323
278
427.0
357.0
393
328
ClSiC bend
ClSiC bend
203 m
170 vs
210.0
193
167.5
154
SiI2 wag
SiI2 twist
SiI2 deformation
torsion
177.2
99.3
88.5
58.0
163
91
81
102.0
81.5
49.7
94
75
46
53
be explained by considering the dipole moments of the two moments for gauche and anti CH2ClSiHCl2 are 1.53 and
rotamers, as the conformer with the larger dipole moment is 3.22 D, respectively, and the latter is therefore stabilized in
normally stabilized in a polar solvent. The predicted dipole the liquid. anti CH2ClSiHBr2 and anti CH2ClSiHI2 are also
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K. Hassler, R. Hummeltenberg, G. Tekautz
FULL PAPER
phere of dry, oxygen-free nitrogen using Schlenk techniques. Sol-
vents were dried over sodium or potassium and distilled under N2
prior to use.
Raman spectra were recorded with a JobinϪYvon T64000 spec-
trometer equipped with a triple monochromator. A frequency-
doubled Nd:YAG laser (20 mW) operating at 532 nm served as
light source and a charge-coupled device (CCD) camera was used
as detector. The samples were condensed into 1 mm capillary glass
tubes that were sealed under nitrogen atmosphere. Variable-tem-
perature Raman spectra were obtained by mounting the capillary
on a copper block supplied with a heater and a thermocouple for
temperature monitoring. Liquid nitrogen was used for cooling.
Infrared spectra in the range from 3000Ϫ250 cmϪ1 were measured
with a PerkinϪElmer 883 spectrometer using a film of the pure
liquid between CsBr plates
NMR spectra of 2؊5 as solutions in CDCl3 were obtained with a
Bruker MSL 300 spectrometer.
(Chloromethyl)diphenylsilane (1): A solution of chlorodiphenylsil-
ane (55.17 g, 250 mmol) and bromochloromethane (32.63 g,
250 mmol) in 500 mL of THF was cooled to Ϫ70 °C, and 138 mL
of a 2  solution of BuLi in pentane was added dropwise over a
period of 2 h, whilst maintaining the temperature. After completion
of the addition the mixture was stirred at Ϫ70 °C for another 2
h. It was then allowed to warm to room temperature and stirred
overnight. After hydrolysis and extraction with n-hexane the or-
ganic layer was separated. Subsequently, the solvent is removed by
evaporation in vacuo and the liquid residue was fractionated, giving
20.7 g (88.9 mmol, 35%) of 1 as a colourless liquid. B.p.0.1 ϭ 125
°C. C13H13ClSi (232.77): calcd. C 67.07, H 5.63; found C 66.98,
H 5.6.
Figure 4. Van’t Hoff plots for the line pairs 687/653 cmϪ1 of
ClCH2SiHI2 (left) and 435/466 cmϪ1 of ClCH2SiHBr2 (right)
Dichloro(chloromethyl)silane (2): Triflic acid (6.46 g, 43.3 mmol)
was added dropwise at a temperature of Ϫ30 °C to a solution of 1
(5.01 g, 21.5 mmol) in 20 mL of toluene. After completion the mix-
ture was allowed to warm to room temperature and stirred for 5 h.
The benzene formed in the reaction and the toluene were com-
pletely removed in vacuo, this process was controlled by IR spec-
troscopy. Diethyl ether and 3 g of LiCl (100% excess) were added
and the mixture was then stirred at room temperature for 2 d. After
this time all volatile substances were condensed off. The diethyl
ether was removed very carefully from the mixture by distillation.
The liquid residue was fractionated under slightly reduced pressure
Table 5. Line pairs (cmϪ1) and experimental ∆H values (kJ/mol;
∆H ϭ Hanti Ϫ Hgauche) obtained from least-square fits for 2, 3 and 4
CH2ClSiHCl2
line pair
CH2ClSiHBr2
line pair
CH2ClSiHI2
line pair ∆H
∆H
∆H
508/495 Ϫ0.18Ϯ0.003 435/466 0.28Ϯ0.007 687/653 0.20Ϯ0.003
671/624 Ϫ0.92Ϯ0.006 387/396 0.19Ϯ0.005 596/636 0.91Ϯ0.008
680/624 Ϫ0.61Ϯ0.003 606/649 0.15Ϯ0.003
giving 0.9 g (6 mmol) of 2 (28.1%) as colourless liquid. B.p.200
42 °C. CH3Cl3Si (149.48).
ϭ
stabilized, but due to the smaller dipole moments and the
larger gauche-anti energy differences (Table 2) the stabilizing
effect is not able to reverse the sign of ∆H.
It is also of some note that the SiC valence force con-
stants calculated from the Hessian matrices by use of sym-
metry coordinates increase with increasing electronegativity
of the halogen substituents. The calculated values for gau-
che 5, 2, 3 and 4 [N/cm, scaled by (0.92)2] are: 2.87, 2.77,
2.73 and 2.67, respectively; the force constants for the anti
conformers are smaller by 0.03Ϫ0.04 N/cm.
Dibromo(chloromethyl)silane (3): Compound 3 was prepared using
the same procedure as for 2, except that LiBr was used instead of
LiCl. The following amounts of starting materials were used: 5.67 g
of 1 (24.36 mmol); 7.31 g of triflic acid and 5 g of LiBr. Fraction-
ation in vacuo yielded3 (1.4 g, 5.8 mmol; 24.1%) as a colourless
liquid. B.p.20 ϭ 43Ϫ45 °C. CH3Br2ClSi (238.38): calcd. Br 64.04;
found Br 64.18.
(Chloromethyl)diiodosilane (4): Compound 1 (4.37 g) and an excess
of HI were placed in a sealed glass tube and stored for one week at
room temperature. After removal of the excess HI and the benzene
formed during the reaction, the liquid residue was fractionated in
vacuo to give 3.7 g (mmol) of 4 (59.3%) as a colourless liquid.
B.p.0.1 ϭ 33Ϫ35 °C. CH3ClI2Si (332.39): calcd. I 76.36; found I
76.15.
Experimental Section
General Remarks: All compounds except (chloromethyl)diphenylsi-
lane are sensitive towards moisture; some of them, such as (chloro-
methyl)diiodosilane, are also sensitive towards oxygen. All synth-
eses and manipulations were therefore carried out under an atmos-
(Chloromethyl)difluorosilane (5): A solution of 4 (1.5 g, 4.5 mmol)
in 15 mL of hexachlorobutadiene was added at a pressure of
4264
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Eur. J. Inorg. Chem. 2004, 4259Ϫ4265

(Chloromethyl)dihalosilanes
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Early View Article
Published Online September 13, 2004
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