Synthesis and solid-state molecular structure of bi(silacyclohexyls) C5H10SiX-XSiC5H10 with X = H, PH, F, CL, BR, and i

Article abstract of DOI:10.1080/10426507.2012.723655

The synthesis and purification of novel bi(silacyclohexyls) C ₅H₁₀SiX-XSiC₅H₁₀ with X = H (1), Ph (2), F (3), Cl (4), Br (5), and I (6), which are composed of two silacyclohexyl rings connected by a silicon-sili

Full text of DOI:10.1080/10426507.2012.723655

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Phosphorus, Sulfur, and Silicon and the
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Synthesis and Solid-State Molecular
Structure of Bi(silacyclohexyls)
C5H10SiX-XSiC5H10 with X = H, PH, F,
CL, BR, and I
Thomas Kern a , Roland Fischer a , Michaela Flock a & Karl Hassler a
a Institute of Inorganic Chemistry , University of Technology , Graz ,
Austria
Accepted author version posted online: 27 Aug 2012.Published
online: 05 Jul 2013.
To cite this article: Thomas Kern , Roland Fischer , Michaela Flock & Karl Hassler (2013) Synthesis
and Solid-State Molecular Structure of Bi(silacyclohexyls) C5H10SiX-XSiC5H10 with X = H, PH, F,
CL, BR, and I, Phosphorus, Sulfur, and Silicon and the Related Elements, 188:7, 931-940, DOI:
10.1080/10426507.2012.723655
To link to this article: http://dx.doi.org/10.1080/10426507.2012.723655
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Phosphorus, Sulfur, and Silicon, 188:931–940, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.723655
SYNTHESIS AND SOLID-STATE MOLECULAR
STRUCTURE OF BI(SILACYCLOHEXYLS)
C5H10SIX-XSIC5H10 WITH X = H, PH, F, CL, BR, AND I
Thomas Kern, Roland Fischer, Michaela Flock,
and Karl Hassler
Institute of Inorganic Chemistry, University of Technology, Graz, Austria
The synthesis and purification of novel bi(silacyclohexyls) C5H10SiX-XSiC5H10 with X = H
(1), Ph (2), F (3), Cl (4), Br (5), and I (6), which are composed of two silacyclohexyl rings
connected by a silicon–silicon single bond, is described. Molecular structures for 2, 4, 5, and 6
were determined employing X-ray crystallography. Surprisingly, the SiSi bond lengths increase
in the order I < Br < Cl, contrary to what is observed for methylated disilanes Me2XSiSiXMe2.
The bond lengthening can be traced back to 1, 3 nonbonded interactions between the halogen
atoms and the axial hydrogen atoms in the SiC5 ring, which also cause a decrease of the SiSiX
bond angles with increasing size of the halogen atom. Both rings substitute each other in the
equatorial position for 4, 5, and 6, but in the axial position for 2.
Keywords Silacyclohexanes; synthesis; x-ray diffraction
INTRODUCTION
Cyclohexane C6H12 has been and still is the archetype and cornerstone for discussing
and comparing the conformational equilibria of saturated heterocyclic six-membered rings
composed of main group elements such as carbon, silicon, germanium, oxygen, sulfur,
nitrogen, or phosphorus. Not surprisingly, a huge number of studies on the conformational
properties of cyclohexanes composed of second row elements C, O, and N have been
published in the literature.1 Much less effort has been directed toward heterocycles whose
backbones incorporate third or fourth row elements. Most intensely studied rings are
silacyclohexanes with up to three silicon atoms and thiacyclohexanes with up to five sulfur
atoms.1
Our group focuses on conformational equilibria of cyclohexanes composed of just
third or fourth row elements such as cyclohexasilanes Si6R12, cyclohexagermanes Ge6R12
or silicon–sulfur and germanium–sulfur rings such as Si3S3R6 and Ge3S3R6.2 Just as
in cyclohexane, these rings are characterized by the presence of chair, boat, and twist
conformations, albeit with smaller energy differences between conformers and smaller
Received 9 July 2012; accepted 17 August 2012.
The authors thank the “Fonds zu Fo¨rderung der wissenschaftlichen Forschung, FWF”, Vienna, for financial
support of project P21272-N19.
Address correspondence to Karl Hassler, Institute of Inorganic Chemistry, University of Technology,
Stremayrgasse 9, A-8010 Graz, Austria. E-mail: karl.hassler@tugraz.at
931

932
T. KERN ET AL.
barriers that separate them. Interconversions of conformers on the time-scale of NMR-
spectroscopy therefore are fast, and very low temperatures are needed to slow them down
sufficiently. For instance, a temperature of about −160 ◦C had to be used for investigating
the chair to chair interconversion of Si6Me12 with 13C-NMR spectroscopy, with all the
problems caused by the low solubility at that temperature.3 A spectroscopic method with a
shorter timescale such as vibrational Raman spectroscopy coupled with quantum chemical
calculations therefore is a much more convenient method for investigating these rings, as we
have shown with Si6Me12,4 Si6Me11X (X = F, Cl, Br, and I)5 and Ge6Me12.6 It is therefore
quite natural to apply the method to larger systems, such as bi(cyclohexasilanyls) Si12H22
and Si12Me22 as shown in Scheme 1, which we are investigating currently. Si12Me22 is
a formidable conformational problem, because each Si6 ring can adopt chair, twist, and
boat conformation. Moreover, rotational isomerism around the pivot SiSi bond is present
with the further complication that methylated oligosilanes display SiSiSiSi dihedral angle
minima at ≈90 ◦, termed ortho conformations, besides the usual ≈180 ◦ and ≈ 60 ◦ anti
and gauche minima.7
Scheme 1 Molecular structures of bicyclohexyl, bi(silacyclohexyl) and bi(cyclohexasilanyl). H-atoms have been
omitted for clarity.
We therefore decided to include 1,1ꢁ-bi(silacyclohexyl) derivatives C5H10SiX-
XSiC5H10 with X = hydrogen or halogen (Scheme 1) in the study, as these represent
simpler systems due to the fact that twisted conformers of the SiC5 rings have large relative
energies of about 16 kJmol−1.8 At ambient temperatures, these states are populated to
an almost negligible extent, thereby simplifying the conformational problem. However,
rotational isomerism of the pivot SiSi bond is maintained.
Much to our surprise, no such compounds have been described in the literature
yet. We therefore have set out to develop a synthetic method for their preparation and to
compare their structures and conformations with bicyclohexyl C6H11-C6H11, which is also
shown in Scheme 1. So far, bicyclohexyl has been investigated with gas phase electron
diffraction,9 and the solid state molecular structure of Si12Me22 has been elucidated with
X-ray crystallography.10
RESULTS AND DISCUSSION
Synthesis
Compounds 1–6 were synthesized following two different routes as shown in
Scheme 2. The first one starts from 1,5-dichloropentane, which with magnesium gives
dichloro-μ-1,5-pentanediyldimagnesium. Its reaction with hexachlorodisilane affords

SYNTHESIS AND SOLID-STATE MOLECULAR STRUCTURE
933
Scheme 2 Synthesis of C5H10SiPh-PhSiC5H10 (2) and C5H10SiCl-ClSiC5H10 (4).
a mixture of C5H10SiCl(SiCl3) and 4, from which 4 can be separated by fractional
crystallization. The reaction of Si2Cl6 with dibromo-μ-1,5-pentanediyldimagnesium has
been described previously.11 Due to bromine/chlorine exchange a mixture of partially
brominated chlorosilanes was obtained from which no pure compounds could be
separated.
The second route starts from known 1-chloro-1-phenylsilacyclohexane C5H10SiPhCl,
which was reduced to 2 by the use of sodium.
As shown in Scheme 3, 2 and 4 were transformed into 1, 3, and 5 employing standard
methods used for the synthesis of alkyldisilanes such as reduction with LiAlH4 or removal
of the phenyl groups with trifluoromethanesulfonic acid and subsequent transformation of
the trifluoromethanesulfonyloxysilane into a halosilane with an alkali metal halide.12 6 was
synthesized from 1 by hydrogen/halogen exchange with CHI3.
Scheme 3 Synthesis of 1, 3, 5 and 6.

934
T. KERN ET AL.
Scheme 4 1, 3-steric interaction between hydrogen atoms on C2 and C4 and halogen atom X.
X-Ray Crystallography
We were able to grow crystals of 2, 4, 5, and 6 from various solvents with a quality
suitable for X-ray analysis. ORTEP drawings of the molecular structures are presented in
Figure 1, and relevant structural parameters are summarized in Table 1. In all compounds,
the silacyclohexane rings possess the chair conformation. A substituent on the silicon atom
thus can adopt either the equatorial (e) or axial (a) position. The conformers therefore can
be classified as either aa (both rings adopt the axial position relative to each other), ae, or ee.
Figure 1 ORTEP plots (30% probabilities) of the solid state molecular structures of 2 (top left), 4 (top right),
5 (bottom left) and 6 (bottom right) (Color figure available online).

SYNTHESIS AND SOLID-STATE MOLECULAR STRUCTURE
935
Table 1 Selected bond lengths [pm] and bond angles [◦] for 2, 4, 5, and 6
2
4
5
6
Sil-SilA
Sil-Xl
Sil-Cl
235.90(5)
188.03(9)
188.18(9)
188.23(9)
109.47(3)
112.22(3)
111.21(3)
234.55(12)
209.53(9)
186.31(19)
186.2(2)
105.73(4)
116.52(7)
114.78(7)
234.09(12)
225.75(3)
186.27(9)
186.47(9)
104.95(15)
116.74(3)
115.35(3)
231.77(10)
248.48(5)
186.28(19)
186.28(19)
103.34(3)
114.26(7)
115.77(7)
Sil-C5
SilA-Sil-Xl
SilA-Sil-Cl
SilA-Sil-C5
As a silacyclohexyl substituent is quite bulky, the ee conformer is expected to constitute the
energetic global minimum for all compounds. Interestingly, the phenyl compound 2 shows
the SiC5 rings in the axial positions, which seems to be an effect of the crystal packing
forces.
Quite surprisingly, the SiSiX bond angles in the series Cl → Br → I decrease from
105.73(4) ◦ for Cl to 103.34(3) ◦ for I, despite an increase of the size of the halogen atom.
We attribute this to the repulsive 1,3-steric interaction of X with the two axial hydrogen
atoms on C-atoms 2 and 4, as shown in Scheme 4.
We do not attribute the decrease of the XSiSi bond angle to crystal packing forces
because it is reproduced quite nicely by preliminary quantum chemical DFT calculations,
which we will describe in a forthcoming article. They predict values of 105.4 ◦ (Cl), 104.7 ◦
(Br), and 104.3 ◦ (I). The calculated HX distances marked by arrows in Scheme 4 are 328.8
pm, 337.2 pm, and 351.3 pm for X = Cl, Br, and I, respectively. They compare well with the
sum of the van der Waals radii, listed in the literature (H: 120–145 pm, Cl: 170–190 pm, Br:
180–200 pm, I: 195–210 pm).13,14 It is worth mentioning that for substituted cyclohexanes
C6H11R with R = H, Cl, Me, and tert-Bu, the so called 1,3-syn-diaxial interaction has been
shown to be attractive rather than repulsive by quantum chemical calculations.15 The long
SiCl bond (as compared to CCl) can serve as an explanation for why an attractive interaction
turns into a repulsive one. Moreover, the van der Waals radius of a chlorine atom bonded
to silicon certainly is larger than in a CCl bond due to the differing electronegativities of C
and Si.
The SiSi bonds of 3, 4, and 5 shorten in the order Cl → Br → I, which is quite
unusual for disilanes. Normally, SiSi bond lengths of halogenated disilanes decrease with
increasing electronegativities of the halogen atoms, for instance in the hexahalodisilanes
X3SiSiX3.16a,16b The effect can be rationalized using Bent’s rule, which is an extension of
the VSEPR model, stating that bonds to electronegative substituents use more p-character
due to the shift of the electron cloud toward the electronegative atom.17 The bond to the most
electropositive substituent, which is the other silicon atom, thus gains more s-character, an
effect which is maximized for X = F. An increased s-character usually means a stronger
and consequently shortened bond.
The trend of bond shortening observed in the solid state, which contradicts simple
VSEPR theory, is also predicted by the preliminary quantum chemical calculations men-
tioned previously, suggesting that it originates from electronic causes. Detailed results of
the calculations including the influences of conformations on the bond lengths will be
published elsewhere.

936

SYNTHESIS AND SOLID-STATE MOLECULAR STRUCTURE
937
CONCLUSIONS
In this article, we describe preparative procedures for the synthesis and purification
of novel bi(silacyclohexyls) C5H10SiX-XSiC5H10 with X = Ph, H, F, Cl, Br, and I, which
are composed of two silacyclohexyl rings connected with a silicon–silicon bond. The
compounds with X = Ph, Cl, Br, and I crystallize readily, and X-ray diffraction experiments
show that the two rings substitute each other in an equatorial position for X = Cl, Br, and I
(ee isomer), but in axial position for X = Ph (aa isomer) caused by crystal packing forces.
Moreover, the SiSi bond lengths increase in the order I → Br → Cl, just opposite to what
has been found for other halogenated disilanes, for instance MeX2SiSiX2Me.23 This is due
to 1,3-nonbonded interactions between the halogen atoms and the axial hydrogen atoms in
the ring, which can be relieved in methylated disilanes by internal rotation of the methyl
groups.
EXPERIMENTAL PART
General
All manipulations were carried out under an inert atmosphere of dry nitrogen using
standard Schlenk techniques. All solvents were deoxygenated and dried prior to use with
Al2O3, using a column purification system from Innovative Technology. 1H, 13C, and 29Si
NMR spectra were recorded in solutions of C6D6 employing a INOVA 300 spectrometer
1
(299.953 MHz for H, 59.592 MHz for 29Si, and 75.430 MHz for 13C). Standard INEPT
1
1
pulse sequences were used for recording the 29Si{ H} and 13C{ H} spectra. All shifts are
referenced against TMS. Due to ring inversion, r.t. proton signals generally are broad,
sometimes splitting into broad doublets. 13C and 29Si nuclei produce sharp signals due to
lower resonance frequencies.
Synthesis of 1,1ꢀ-Dichloro-bi(silacyclohexyl) (4)
A solution of of dichloro-μ-pentanediylmagnesium (35.05 g, 185 mmol) in di-
ethylether (100 mL), which was prepared from Cl(CH2)5Cl (154.05 g) and of Mg (8.99 g) ac-
cording to the literature,18 was added dropwise to a solution of hexachlorodisilane (24.85 g,
92 mmol) in diethylether (400 mL) at a temperature of 0 ◦C. After completion, the reaction
mixture was stirred for 2 h. Diethylether was then replaced by toluene, and the solution
was separated from the salts by filtration. Toluene was then removed by evaporation in
vacuo (i.v.) Crude 4 crystallized from the oily and colorless residue and was separated by
decantation. A further crystallization from toluene afforded pure 4 (5.05 g, 18.7 mmol,
20.3%) with a crystal quality sufficient for X-ray analysis.
Elemental analysis C10H20Si2Cl2 (267.34): calc. C 44.93, H 7.54%; found C 44.95,
1
H 7.48% H-NMR: δ = 1.75–1.56 ppm (m, 8H), δ = 1.39–1.27 ppm (m, 2H), δ =
1.18–1.05 ppm (m, 2H), δ = 0.98–0.86 ppm (m, 8H).
1
13C{ H}-NMR: δ = 29.1 ppm, δ = 23.3 ppm, δ = 15.3 ppm.
29Si{ H}-NMR: δ = 11.3 ppm.
1
1
1
1
The measured H-, 13C{ H}-, and 29Si{ H}-NMR spectra of 4 are presented in the
supplemental materials available online (Figures S1, S2, and S3) to illustrate their general
appearance, which is also typical for the other compounds described in this publication.

938
T. KERN ET AL.
Synthesis of 1, 1ꢀ–diphenyl-bi(silacyclohexyl) (2)
A solution of 1-chloro-1-phenylsilacyclohexane (18.65 g, 88.5 mmol) prepared by
following the literature procedure was dissolved in of toluene (100 mL).19 An excess of
sodium (5.6 g, 243.5 mmol) was added, and the reaction mixture was refluxed for 3 days
(72 h). The resulting greenish solution was separated from the salts by decantation, and
the solvent was removed by evaporation i.v. The resulting oily residue was fractionated
by a Kugelrohr distillation, giving 2 (6.1 g, 17.4 mmol, 20%) as a colorless oil, which
crystallized slowly at r.t. The quality of the crystals turned out to be sufficient for an X-ray
analysis.
Elemental analysis C22H30Si2 (350.65): calc. C 75.36, H 8.62%; found C 75.60 H
8.83%.
1H-NMR: δ = 7.39–7.35 ppm (m, aromatic 4H), δ = 7.23–7.13 ppm (m, aromatic
6H), δ = 7.39–7.35 ppm (m, 4H), δ = 1.74–1.55 ppm (m, 8H), δ = 1.41–1.23 ppm (m,
4H), δ = 1.06–0.90 ppm (m, 8H).
1
13C{ H}-NMR: δ = 134.6 ppm (aromatic C), δ = 133.2 ppm (aromatic C), δ =
130.3 ppm (aromatic C), δ = 128.0 ppm (aromatic C), δ = 30.1 ppm, δ = 24.6 ppm, δ =
10.9 ppm.
1
29Si{ H}-NMR: δ = −25.6 ppm.
Synthesis of bi(silacyclohexyl) (1)
To a solution of 4 (2.17 g, 8.1 mmol) in diethyl ether (50 mL) was added a solution
◦
of LiAlH4 (0.30 g, 8 mmol) in diethyl ether (30 mL) at a temperature of 0 C. After
completion, the reaction mixture was stirred overnight and then added dropwise to an ice-
cooled aqueous solution of H2SO4 (2N). The aqueous phase was separated with the help of a
separatory funnel, and washed twice with diethyl ether. The combined organic phases were
then dried with Na2SO4. The oily residue that was obtained after the removal of the diethyl
ether was fractionated with a Kugelrohr distillation, giving pure 1 (1.57 g, 7.94 mmol, 98%)
as a colorless oil (Kp0.1 ≈ 65 ◦C). Attempts to obtain crystals at low temperatures failed.
Therefore, the purity of the compound was checked with NMR spectroscopy.
1H-NMR: δ = 4.0 ppm (s, 2H), δ = 1.96–1.87 ppm (m, 8H), δ = 1.66–1.57 ppm (m,
4H), δ = 1.07–0.92 ppm (m, 8H).
1
13C{ H}-NMR: δ = 30.4 ppm, δ = 26.6 ppm, δ = 9.3 ppm.
1
29Si{ H}-NMR: δ = -39.7 ppm (s), 1J(SiH) = 195 5 Hz.
Synthesis of 1, 1ꢀ-Difluorobi(silacyclohexyl) (3) and 1,
1’-dibromo-bi(silacyclohexyl) (5)
2 (1.32 g, (3.76 mmol) was dissolved in toluene (60 mL). The solution was cooled
◦
to −30 C, and CF3SO3H (0.68 mL, 7.71 mmol) was added with a syringe. After about
2 h, C5H10Si(OTf)-(TfO)SiC5H10 had formed quantitatively as checked with 29Si-NMR
spectroscopy (δ(29Si) = +18.7 ppm). Toluene and the excess of CF3SO3H were then
removed by evaporation i.v. The oily residue was then dissolved in diethyl ether (30 mL),
and an ample excess of an alkali metal halide was added (for 3: LiF; for 5: LiBr). The
resulting suspension was stirred at r.t. for 3 days. Then diethyl ether was removed by
evaporation i.v., and the residue was suspended in dry toluene. The solution that was

SYNTHESIS AND SOLID-STATE MOLECULAR STRUCTURE
939
decanted from the residual salts contained the desired compounds, which were purified
further by either fractional distillation (3) or crystallization (5).
C10H20Si2F2, 3: (234.44): After removal of toluene by evaporation i.v., 3 was purified
◦
by a Kugelrohr distillation (Kp0.1 ≈ 70 C). 3 (0.75g, 3.19 mmol, 84.9%) was obtained
as a colorless liquid, whose purity was checked with multinuclear NMR-spectroscopy. All
attempts to grow crystals failed.
1H-NMR: δ = 1.78–1.56 ppm (m, 8H), δ = 1.56–1.53 ppm (m, 4H), δ =
1.03–0.93 ppm (m, 8H).
1
13C{ H}-NMR: δ = 29.6 ppm (s), δ = 23.5 ppm (s), δ = 14.8 ppm (dd), 2J(CF) =
7.0Hz, 3J(CF) = 2.2 Hz.
1
29Si{ H}-NMR: δ = 21.1 ppm (dd), 1J(SiF) = 318.5 Hz, 2J(SiF) = 32.8 Hz.
C10H20Si2Br2, 5:
Elemental analysis C10H20Si2Br2 (356.25): calc. C 33.72, H 5.66%; found C 34.23,
H 5.65%.
1H-NMR: δ = 1.69–1.59 ppm (m, 8H), δ = 1.34–1.29 ppm (m, 4H), δ =
1.11–0.97 ppm (m, 8H).
13C{ H}-NMR: δ = 28.7 ppm, δ = 23.1 ppm, δ = 14.6 ppm.
1
29Si{ H}-NMR: δ = 7.9 ppm.
1
Synthesis of 1, 1ꢀ-Diiodo-bi(silacyclohexyl) (6)
1 (2.83 g, 14.3 mmol) was dissolved in heptane (200 mL). CHI3 (5.0 g, 12.6 mmol)
◦
was then added under vigorous stirring, and the reaction mixture was kept at 90 C for a
period of 20 h. The mixture was then allowed to come to r.t. Subsequently, the solution
was decanted from the excess of CHI3. The compound was then purified by fractional
crystallization from toluene giving 6 (about 1 g, 15%) as colorless crystals.
Elemental analysis C10H20Si2I2 (450.25): calc. H 4.48%; found 4.48%
1H-NMR: δ = 1.66–1.41 ppm (m, 8H), δ = 1.25–1.19 ppm (m, 4H), δ =
1.10–0.55 ppm (m, 8H).
1
13C{ H}-NMR: δ = 26.2 ppm, δ = 21.1 ppm, δ = 11.4 ppm.
1
29Si{ H}-NMR: δ = −10.4 ppm.
X-Ray Structure Determination
Crystals suitable for X-ray structural analyses were immersed in inert oil under a
nitrogen atmosphere, selected and mounted on the tip of a glass fibre. Diffraction data were
collected at 100 K on a Bruker D8 Kappa diffractometer equipped with a SMART APEX
II CCD detector with Mo Kα (λ = 0.71073Å) radiation.
Data were integrated with SAINT20 and empirical methods as implemented in SAD-
ABS21 were used to correct for absorption effects. Structures were solved with direct
methods using SHELXS-97. SHELXL-97 was used for refinement against all data by full-
matrix least-squares methods on F2.22 All nonhydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were refined isotropically on calculated posi-
tions using the riding model implemented in SHELXL-97. All diagrams were draw with
30% probability thermal ellipsoids and all hydrogen atoms were omitted for clarity.
Crystallographic data (excluding structure factors) for the structures of compounds 2,
4, 5, and 6 reported in this article are summarized in Table 2 and have also been deposited

940
T. KERN ET AL.
with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-
868302 (2), CCDC-868300 (4), CCDC-868299 (5), and CCDC- 868301 (6). These data
can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; Fax
+44 1223 336033; E-mail deposit@ ccdc.cam.ac.uk).
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