Conformational properties of 1-halogenated-1-silacyclohexanes, C 5H10SiHX (X = Cl, Br, I): Gas electron diffraction, low-temperature NMR, temperature-dependent Raman spectroscopy, and quantum-chemical calculations

Article abstract of DOI:10.1021/om4005725

The molecular structures of axial and equatorial conformers of cyclo-C ₅H₁₀SiHX (X = Cl, Br, I) as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction, dynamic nuclear magnetic resonance, temperature-dependent Raman spectroscopy, and quantum-chemical calculations applying CCSD(T), MP2, and DFT methods. According to the experimental and calculated results, all three compounds exist as a mixture of two chair conformers of the six-membered ring. The two chair forms of C_s symmetry differ in the axial or equatorial position of the X atom. In all cases, the axial conformer is preferred over the equatorial one. When the experimental uncertainties are taken into account, all of the experimental and theoretical results for the conformational energy (E_axial - E_equatorial) fit into a remarkably narrow range of -0.50 ± 0.15 kcal mol^-1. It was found by NBO analysis that the axial conformers are unfavorable in terms of steric energy and conjugation effects and that they are stabilized mainly by electrostatic interactions. The conformational energies for C₆H₁₁X and cyclo-C ₅H₁₀SiHX (X = F, Cl, Br, I, At) were compared using CCSD(T) calculations. In both series, fluorine is predicted to have a lower conformational preference (cyclohexane equatorial, silacyclohexane axial) than Cl, Br, and I. It is predicted that astatine would behave very similarly to Cl, Br, and I within each series.

Full text of DOI:10.1021/om4005725

Article
pubs.acs.org/Organometallics
Conformational Properties of 1‑Halogenated-1-Silacyclohexanes,
C H SiHX (X = Cl, Br, I): Gas Electron Diffraction, Low-Temperature
5
10
NMR, Temperature-Dependent Raman Spectroscopy, and Quantum-
†
Chemical Calculations
‡
,⊥
‡,#
∥,∇
‡
‡
,‡
́
́
́
Sunna O. Wallevik, Ragnar Bjornsson, Agust Kvaran, Sigridur Jonsdottir, Ingvar Arnason,*
Alexander V. Belyakov, Thomas Kern, and Karl Hassler
§
∥
‡
Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland
§
Saint-Petersburg State Technological Institute, Saint-Petersburg 190030, Russia
∥
̈
Technische Universitat Graz, Stremayergasse 16, A-8010 Graz, Austria
*
S Supporting Information
ABSTRACT: The molecular structures of axial and equatorial
conformers of cyclo-C H SiHX (X = Cl, Br, I) as well as the
5
10
thermodynamic equilibrium between these species was
investigated by means of gas electron diffraction, dynamic
nuclear magnetic resonance, temperature-dependent Raman
spectroscopy, and quantum-chemical calculations applying
CCSD(T), MP2, and DFT methods. According to the
experimental and calculated results, all three compounds
exist as a mixture of two chair conformers of the six-membered
ring. The two chair forms of C symmetry differ in the axial or
s
equatorial position of the X atom. In all cases, the axial
conformer is preferred over the equatorial one. When the
experimental uncertainties are taken into account, all of the
experimental and theoretical results for the conformational energy (E − E
) fit into a remarkably narrow range of −0.50
axial
equatorial
−1
±
0.15 kcal mol . It was found by NBO analysis that the axial conformers are unfavorable in terms of steric energy and
conjugation effects and that they are stabilized mainly by electrostatic interactions. The conformational energies for C H X and
6
11
cyclo-C H SiHX (X = F, Cl, Br, I, At) were compared using CCSD(T) calculations. In both series, fluorine is predicted to have a
5
10
lower conformational preference (cyclohexane equatorial, silacyclohexane axial) than Cl, Br, and I. It is predicted that astatine
would behave very similarly to Cl, Br, and I within each series.
INTRODUCTION
Scheme 1
■
The stereochemistry of cyclohexane (1) is among the best-
explored areas in organic stereochemistry. The chair-to-chair
inversion in cyclohexane is well-understood, and the Gibbs free
2
,3
⧧
energy of activation for the step chair → half-chair → twist is
−1
generally accepted to be 10.1−10.5 kcal mol . Far fewer
investigations have been reported on silicon-containing six-
membered rings. In silacyclohexane (6), the activation energy is
4,5
about one-half of the value for cyclohexane. The conforma-
tional equilibria of a large number of monosubstituted
cyclohexanes have been studied. Winstein and Holness defined
the A value as the thermodynamic preference for the equatorial
conformation over the axial one (see Scheme 1 for the
6
bonded to the cyclohexane ring. When the substituent becomes
bulkier its equatorial preference generally increases. The
simplest alkyl groups (methyl, ethyl, isopropyl, and tert-butyl)
definition of A). A positive A value corresponds to a
preference for the equatorial conformer, and ΔG = G − G
0. All energy differences herein will be presented as (axial −
ax
eq
>
equatorial). As a rule, in monosubstituted cyclohexanes, the
substituent prefers the equatorial position of the chair
conformation. Rare exceptions are substituents having mercury
Received: June 18, 2013
Published: November 15, 2013
©
2013 American Chemical Society
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Article
have been used as examples. This tendency has in a classical
way been ascribed to 1,3-syn-axial interactions between the
substituent in the axial position and axial hydrogens on the ring
equatorial (eq). Each form possesses C symmetry with a chair
conformation of the six-membered ring. The radial distribution
functions for 8 and 9 are shown in Figures 1 and 2, respectively.
s
7
carbon atoms at positions 3 and 5. Toward the end of the last
century, the accepted A values were 1.74, 1.79, 2.21, and 4.9
kcal mol− for Me, Et, i-Pr, and t-Bu, respectively.
1
8−10
Evidence
questioning the model of 1,3-syn-axial interactions is starting to
11
appear. Wiberg et al. revised the A values for the three lightest
alkyl groups and reported values of 1.80, 1.75, and 1.96 kcal
−1
mol for Me, Et, and i-Pr, respectively (all values reported with
−1
the error limit of ±0.02 kcal mol ), and the authors concluded
that there was no evidence of 1,3-syn-axial interactions with the
axial hyrogens of C3 and C5. Taddei and Kleinpeter examined
1
2,13
the role of hyperconjugation in substituted cyclohexanes.
Using atoms in molecules (AIM) analysis, Cuevas and co-
workers concluded that the t-Bu group is more stable when it
adopts the axial position in cyclohexane but produces
14
destabilization of the cyclohexyl ring. Clearly the conforma-
tional preferences in monosubstituted cyclohexanes are not
fully understood. In recent years, A values for some
monosubstituted derivatives of 6 have been reported.
Figure 1. (top) Experimental (open circles) and calculated (solid
lines) radial distribution curves of the C H SiHCl molecule. (bottom)
5
10
Difference curve for the optimized mixture of the conformers.
1
5−18
19
Methyl
and phenyl substituents were found to have
positive A values, albeit much lower in magnitude than for the
2
0−22
corresponding cyclohexane analogues.
such as CF3
Other substituents
2
3,24
25
and SiH3 were found to prefer the axial
position, contrary to their cyclohexane analogues.
Theoretical studies have confirmed the increased preference
for the axial position when monosubstituted silacyclohexanes
2
6,27
are compared with monosubstituted cyclohexanes.
A
unified model that can explain the conformational properties
of both ring systems has not yet been presented. Therefore,
more information would be valuable. The A values of the
halocyclohexanes 2−5 have been reported a number of times
using different methods (for a summary of the data, see refs 9
and 20). The experimental and theoretical A values for 2 vary
Figure 2. (top) Experimental (open circles) and calculated (solid
−1
from 0.1 to 0.4 kcal mol depending on the method, but most
lines) radial distribution curves of C H10SiH−Br molecule. (bottom)
5
−1
Difference curve for the optimized mixture of the conformers.
of the experimental values lie between 0.2 and 0.3 kcal mol .
−1
Most of the values for 3, 4, and 5 are close to 0.5 kcal mol .
The fluoro compound 7 has been studied extensively by our
group, applying not only experimental methods such as gas
electron diffraction (GED), dynamic nuclear magnetic
resonance (DNMR), microwave spectroscopy (MW), and
temperature-dependent Raman spectroscopy but also quantum-
Important structural parameters along with the equilibrium
compositions in the gas phase are given in Table 1 for 8, 9, and
1
0. In all cases there is a clear preference for the axial form. A
structural model of the axial form along with atomic numbering
is shown in Figure 3.
2
8,29
chemical (QC) calculations.
A separate infrared and Raman
As can be seen from Table 1 and ref 28, the Si−C bond is
30
study has recently appeared. An axial preference for the F
enlarged in the series C H SiHX with X = F, Cl, Br, and I,
5
10
−1
substituent (A values from −0.1 to −0.5 kcal mol ) was
manifested by all of the experimental methods and calculations.
We then embarked upon a comprehensive study of the
conformational properties of the chloro-, bromo-, and iodo-
substituted silacyclohexanes (8−10, respectively). In this paper,
we present results from GED, DNMR, and Raman experiments
as well as QC calculations. The GED experiment for compound
starting from 1.854(2) Å for X = F through 1.859(2), 1.860(2)
to 1.868(5) Å, respectively. This may be rationalized with the
35
use of Bent’s rule, according to which a central atom tends to
direct hybrids of higher p character toward the more
electronegative substituents. This leads to higher s character
of the hybrids directed toward the less electronegative
substituents attached to the same central atom. Because the
size of hybrids of higher p character is larger than those of
higher s character, the bonds adjacent to the more electro-
negative substituent have to become shorter. Hence, the Si−C
bond has to become shorter as the electronegativity of the
halogen atom increases in going from I to F.
3
1
10
has been reported separately, and short notices of the
32 33
GED results for 8 and 9 have been given. Recently, a
separate study by Klaeboe and co-workers on the vibrational
spectra (infrared and Raman) of 1-chlorosilacyclohexane (8)
34
has been published.
NMR Spectroscopy. Above about 140 K, the 13C NMR
spectra show rapid inversion of all of the compounds 8−10.
Upon cooling below 140 K, the spectra show significant line
broadening and gradual splitting of the signals into two
components, indicating a mixture of two conformers. This
effect is shown in Figure 4 for carbons C3 and C5 in compound
RESULTS AND DISCUSSION
GED Analysis. Structure refinements of compounds 8 and 9
■
were carried out with least-squares analyses of the experimental
molecular intensity curve sM(s). According to QC calculations,
two stable conformers of C H SiHX exist: axial (ax) and
5
10
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Table 1. Structural Parameters of 1-Halo-1-silacyclohexane
a
Molecules by Gas-Phase Electron Diffraction
X = Cl (8)
X = Br (9)
X = I (10)
−
ΔG°298 (kcal mol−1)b
−0.43(18)
67(5)
−0.82(21)
80(5)
−0.59(22)
73(7)
b
χax (%)
Bond Lengths (Å)
Si−C
1.859(2)
2.073(2)
2.063(2)
1.534(3)
1.529(3)
1.116(4)
1.860(2)
1.868(5)
2.458(6)
2.447(6)
1.535(9)
1.528(9)
1.090(8)
Si−Xax
Si−Xeq
C2−C3
C3−C4
2.232(2)
2.221(2)
1.528(3)
1.523(3)
(C−H)av
1.103(3)
c
Bond Angles (deg)
C2−Si−C6
C3−C4−C5
107.7(6)
106.2(4)
105.5(10)
112.2(26)
116.8(15)
116.7(7)
d
d
d
(H−C−H)av
106.4
106.8
106.8
C2−C3−C4
Si−C2−C3
X−Si−C2
C2−Si−C6−C5
Si−C6−C5−C4
C6−C5−C4−C3
114.2(7)
110.3(4)
110.3(5)
−42.2(15)
51.6(12)
−62.7(14)
115.5(9)
111.0(3)
109.7(3)
−42.6(8)
51.3(7)
114.1(13)
110.2(7)
109.3(10)
−42.6(27)
55.8(18)
−59.6(16)
−67.4(26)
a
Atom numbering is shown in Figure 3. The R factors are 5.9, 5.4 and
9
.5% for 8, 9, and 10, respectively. Values in parentheses are 3σ.
b
c
Relative standard free energy (−ΔG°) and mole fraction (χ). H −
C2−C3 = H −C3−C2 = H −C3−C4 = H −C3−C2. Fixed.
Figure 4. Simulation of the 13C NMR signals for C3 and C5 of
ax
d
eq
eq
ax
compound 8 in a mixture of CD Cl , CHFCl , and CHF Cl in a ratio
2
2
2
2
of 1:1:3 at low temperatures. Experimental spectra are on the left, and
calculated spectra are on the right.
the C3(5) pair of carbon atoms. Chemical shifts derived from
the NMR spectra recorded at the lowest temperatures were
assumed to represent conditions of negligible ring inversion.
⧧
Average values for ΔG are listed in Table 2. The equilibrium
e→a
Table 2. Parameters Relevant to Conformational Equilibria
and Rates of Exchange Derived from Dynamic NMR
1
3
Simulations of C NMR spectra (C3 and C5 Atoms)
parameter
8
9
10
coalescence point (K)
127(5)
5.3(1)
132(5)
5.4(2)
132(5)
5.4(2)
Figure 3. Molecular model and atom numbering.
ΔG (kcal mol−1
⧧
)
a
e→a
b
Ke→a
4.88(4)
−0.35(3)
5.99(4)
−0.40(3)
5.99(4)
−0.40(3)
c
8
. The low-temperature spectra of 9 and 10 follow the same
ΔGe→a (kcal mol−1
)
pattern. The higher-field (lower δ) component signal is much
a
Average values over the temperature ranges 107−150 K (8), 110−
larger in all cases. In previous work on related compounds, we
b
1
50 K (9), and 113−132 K (10). K is the equilibrium constant for
e→a
1
3
have shown that the C chemical shifts of the ring carbon
the equatorial to axial inversion derived from ax:eq signal intensity
c
atoms C3 and C5 have lower δ values when the substituent is in
ratios over the temperature ranges specified in footnote a. ΔG
=
e→a
1
5,25
the axial position compared with that in the equatorial one.
The same results have been derived by computational studies
−RT ln(K ) at T = 128.5 K.
e→a
1
3
for 8, 9 and 10. On the basis of the C NMR signal weights
hence relative populations) and QC chemical shift calcu-
constants (Ke→a), and hence the free energy changes (ΔGe→a),
for the equatorial to axial transformations at temperatures close
to the coalescence points were determined from the relative
signal intensities (Table 2).
(
lations, we conclude that the same holds for the C2(6) and
C3(5) ring carbon atoms. No clear splitting of the C4 signals is
observed, whereas the QC chemical shift calculations predict
the ring C4 atoms to have slightly higher δ values when the
substituent is in the axial position compared with the equatorial
one. For more information, see the Experimental Section and
the Supporting Information.
Raman Spectroscopy. In previous publications, we used
temperature-dependent Raman spectroscopy to analyze the
ratio of axial and equatorial conformers in Si-substituted
1
7,25,28,37
silacyclohexanes.
The application of the method to this
problem has been described in detail in one of the
25
Dynamic NMR simulations of the spectra obtained using the
publications, and therefore, only a brief description will be
given here. Temperature-dependent Raman spectra of com-
pounds are typically analyzed using the van’t Hoff relation,
ln(A /A ) = −ΔH/RT + constant, where A and A are the
36
software WinDNMR, as shown in Figure 4, allowed the
determination of the rate constants (ke→ ⧧a ) and the correspond-
ing free energies of activation (ΔGe→a) as functions of
temperature. The most reliable results were obtained from
1
2
1
2
intensities of the vibrational bands belonging to two different
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conformers of the molecule. Either the heights or areas of the
bands can be used for the A /A ratio. The relation is correct
1
2
under the assumption that ΔH and the Raman scattering
coefficients are temperature-independent.
In our previous reports on related silacyclohexanes, we used
pairs of Raman bands belonging to the axial and equatorial
conformers. As a rule, the symmetric Si−C2(6) stretching
mode and/or the Si−X stretching mode (X = substituent) have
been found to be well or moderately well separated in the
sample spectrum, and the intensity ratio has shown variation
with temperature. Assignment to the axial or equatorial
conformer is made by comparison with calculated vibrations.
In this contribution, we used the Si−X (X = Cl, Br, I)
stretching vibrations for the axial and equatorial conformers for
all of the compounds 8−10. The experimental and calculated
frequencies are summarized in Table 3.
Table 3. Experimental (Room Temperature) and Calculated
Si−X Stretching Frequencies for Compounds 8, 9, and 10
−
1
(
cm )
a
experimental
equatorial
calculated
axial
axial
equatorial
8
9
474
389
324
538
432
402
472
391
313
537
436
394
1
0
a
B3LYP38,39/6-31+G(d,p)40−45 for 8 and 9, B3LYP/aug-cc-pVDZ46,47
for 10.
Low-temperature spectra were recorded for pure 8 and 9 at
temperatures varying from 300 to 195 K at 15 K intervals.
Spectra were also recorded for the two compounds in THF and
heptane solution. Low-temperature spectra were recorded for
pure 10 and for 10 in toluene solution at temperatures varying
from 290 to 190 K at 10 K intervals.
Signal overlap makes the Raman spectra of 8 and 10
somewhat more complicated in the Si−X stretching region than
the spectrum of 9. Shown in the bottom panel of Figure 5 is the
room-temperature Raman spectrum of neat 10 over the
−1
wavenumber range 200−800 cm ; the middle panel shows
−1
the expanded spectrum over the range 300−420 cm , and
−1
calculated spectra over the 280−420 cm range are shown in
the upper panel. More information on band deconvolution and
the temperature dependence of the spectra of the three
compounds is given in the Supporting Information. Figure 6
−1
shows the van’t Hoff plots for the 324/402 cm band pair of
pure 10 using both peak heights (top panel) and peak areas
Figure 5. Raman spectrum of C H SiHI over the range 200−800
−1
5
10
(
bottom panel).
−1
cm (bottom), expanded spectrum over the range 300−420 cm
1
The resulting ΔH values are listed for compounds 8−10 in
(
middle), and calculated spectra over the range 280−420 cm− (top).
Tables 4, 5, and 6, respectively. The listed entries are averaged
values from measured peak heights and peak areas. We estimate
that ±0.15 kcal mol is a fair limit for the error involved in the
ΔH results. We note that the polarity of the solvent does not
significantly influence ΔH. We also note that our results for 8
are in good agreement with the average value reported by
silacyclohexanes, the inversion path starting from the axial
conformer consists of a half-chair/sofa-like transition state from
which the molecule can move into a twist form of relatively
high energy. The molecule then goes through a boat form into
a more stable twist form at the midpoint of the path. The
molecule then proceeds further through a boat transition state,
a twist minimum, and a half-chair/sofa transition state before it
ends up in the equatorial form.
High-level ab initio calculations were carried out in order to
get accurate potential energy differences between the axial and
equatorial conformers of all three compounds. Shown in Tables
4−6 are calculated relative energies with thermodynamic
−1
−1
Klaeboe and co-workers (−0.67 kcal mol ) using other band
pairs.
3
4
COMPUTATIONAL STUDIES
■
The minimum-energy pathways for the chair-to-chair inversion
of compounds 8, 9, and 10 (and 7 for comparison) are shown
in Figure 7 and are overall very similar. Similarly to previous
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differences are being compared (i.e., no entropy effects). There
is worse agreement between the theoretical and GED free
energy differences for the bromine compound, where a larger
population of the axial conformer is found in the GED
experiment; however, this is not seen in the low-temperature
NMR experiment.
A systematic 0.15−0.19 kcal/mol difference in the low-
temperature free energy values from theory and the NMR
experiments is evident, although the trend in the values is the
same.
We note that we did not attempt to take into account solvent
effects in the computations, as such effects are very hard take
into account accurately.
A closer look at Tables 4−6 reveals that when the
experimental uncertainties are taken into account, all of the
experimental and theoretical results for the conformational
energy (axial − equatorial) for compounds 8, 9, and 10 fit into
−1
a remarkably narrow range of −0.50 ± 0.15 kcal mol .
In order to better understand the reasons for the conforma-
tional preference of 1-halo-1-silacyclohexane molecules we
4
8−51
carried out a natural bond order (NBO) analysis.
With the
use of NBO analysis, we performed a decomposition of the
(L)
total electronic energy into a Lewis component, E , and a
(
NL)
non-Lewis component, E
(see Table 7). The Lewis
component corresponds to the localized structure that has
populations of each occupied orbital equal to two electrons.
Thus, the Lewis energy nearly exactly incorporates steric and
electrostatic interactions and the non-Lewis component
5
0,51
corresponds to all types of conjugations.
It can be seen
from the data in Table 7 that on the basis of the non-Lewis
components, the effects of conjugations predict the equatorial
conformer to be more preferable. Next, NBO analysis was used
52−54
Figure 6. Van’t Hoff plots for the 324/402 cm−1 band pair of neat
to calculate the total steric energy.
It follows from the data
C H SiHI using (bottom) band areas and (top) band heights.
shown in Table 7 that the steric energy is considerably higher in
the axial conformer than in the equatorial conformer for all
compounds. Thus, the axial conformers of 1-X-1-silacyclohex-
ane molecules (X = Cl, Br, I) are examples of stabilization of
the form that is unfavorable from the point of view of steric
energy and conjugation effects and is determined mainly by
electrostatic interactions.
5
10
corrections at experimental temperatures compared to the
experimental (GED, Raman, NMR) energy differences. The 0
K potential energy differences (ZPE-exclusive) evaluated at the
CCSD(T)/CBS level of theory for the three compounds as well
as the fluoro compound (ΔE = −0.15 kcal/mol) show a trend
of increasing stability of the axial conformer in going from the
lightest halogen derivative to the heaviest. This trend can also
be seen in the values obtained from the various experiments.
Overall the agreement with experiment is satisfactory, and we
suspect the differences to be mainly due to experimental
uncertainties, the imperfect estimations of entropic contribu-
tions in the calculations, and solvent effects. The agreement
between theory and the Raman experiments is very satisfactory,
which may be partly due to the fact that only enthalpic energy
CONCLUSIONS
■
With this contribution, all 1-halogen-substituted 1-silacyclohex-
anes have been studied by various experimental techniques and
high-level calculations. GED, DNMR, and temperature-depend-
ent Raman experiments all agree that for all of these derivatives
the axial conformer is preferred over the equatorial one. This
contradicts findings for the corresponding cyclohexane
derivatives. For a better comparison of the compounds, we
Table 4. Conformational Properties of C H SiHCl (8)
5
10
T = 0 K ΔE = E − Eeq
T = 300−195 K ΔH = H − Heq
T = 128.5 K A = G − Geq
T = 352 K A = G − Geq
ax
)
ax
ax
)
ax
)
(kcal mol−
1
(kcal mol
Calculations
−0.54
−1
)
(kcal mol
−1
(kcal mol
−1
method
CCSD(T)/CBS +
thermal corr.
−0.54
−0.50
−0.43
Experiments
GED
−0.43(18)
Raman neat
Raman in heptane
Raman in THF
DNMR
−0.58(15)
−0.40(15)
−0.60(15)
−0.35(6)
7
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Table 5. Conformational Properties of C H SiHBr (9)
5
10
T = 0 K ΔE = E − Eeq
T = 300−195 K ΔH = H − Heq
T = 128.5 K A = G − Geq
T = 352 K A = G − Geq
ax
)
ax
ax
)
ax
)
(kcal mol−
1
(kcal mol
Calculations
−0.62
−1
)
(kcal mol
−1
(kcal mol
−1
method
CCSD(T)/CBS +
thermal corr.
−0.62
−0.58
−0.49
Experiments
GED
−0.82(21)
Raman neat
Raman in heptane
Raman in THF
DNMR
−0.46(15)
−0.40(15)
−0.60(15)
−0.40(3)
Table 6. Conformational properties of C H SiHI (10)
5
10
T = 0 K ΔE = E − Eeq
T = 290−190 K ΔH = H − Heq
T = 128.5 K A = G − Geq
T = 352 K A = G − Geq
ax
)
ax
ax
)
ax
)
(kcal mol−
1
(kcal mol
−1
)
(kcal mol
−1
(kcal mol
−1
method
Calculations
−0.64
CCSD(T)/CBS +
thermal corr.
−0.63
−0.59
−0.48
Experiments
GED
−0.59(22)
Raman neat
Raman in toluene
DNMR
−0.67(15)
−0.72(15)
−0.40(3)
Figure 7. Minimum-energy path for chair-to-chair inversion of compounds 7−10.
have collected the calculated ΔE values at the CCSD(T)/CBS
EXPERIMENTAL SECTION
■
level for both series into one graph (Figure 8). Although the
Materials. The 1-chloro-1-silacyclohexane used as the starting
material for the synthesis listed below was prepared in slight variation
to the general preparation of silacyclohexanes described by West. It
should be pointed out that a mixture of chlorinated and brominated
substances is usually obtained by that method because of halogen
exchange between the di-Grignard reagent BrMg(CH ) MgBr (or the
synthesis and conformational experimental analysis on astatine
57
derivatives are not conceivable, we added At derivatives to the
calculations to explore further the periodic trend down the
halogen group. We may conclude four trends from Figure 8.
First, for both series the fluoro derivatives have rather similar
energies for the axial and equatorial conformers. Second, for Cl,
Br, and I, the cyclohexane derivatives have a distinct preference
for the equatorial conformer, whereas the silacyclohexanes have
a somewhat stronger preference for the axial conformer. Third,
the first two trends are in alignment with the experimental
results. Fourth, the calculations predict At to behave in a similar
way as Cl, Br, and I.
58
2
5
MgBrCl reaction salt) and SiHCl during the reaction. 1-Chloro-1-
3
silacyclohexane and 1-bromo-1-silacyclohexane may be obtained in
higher purity by reacting 1-phenyl-1-silacyclohexane with HCl and
HBr, respectively. The di-Grignard reagent was prepared in a
traditional way. All solvents were dried using appropriate drying
agents and distilled prior to use. Standard Schlenk techniques and an
inert atmosphere of dry nitrogen were used for all manipulations.
7
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Organometallics
Article
temperature. After the ampule was opened, all of the volatile
Table 7. Results of the NBO Analysis of 1-Halo-1-
components were condensed on a N (l)-cooled finger. The desired
silacyclohexane Molecules and Energy Decomposition of the
2
(
total)
(L)
product was collected (after the benzene byproduct had been
removed) by distillation under nitrogen at 138−140 °C (7.78 g,
72%) as a colorless liquid. The product was characterized by NMR and
Total Electronic Energy E
into Lewis E , Non-Lewis
(NL)
(ST)
(L−ST)
E
, Total Steric E , and Electrostatic E
Energies
a
(
See the Text)
1
MS. H NMR (400 MHz, CDCl ): δ 1.06−1.18 (m, 4H, CH ), 1.33−
3
2
1
.43 (m, 1H, CH
), 1.54−1.62 (m, 1H, CH
), 1.72−1.87
Cl
Br
I
2(ax/eq)
2(ax/eq)
13
1
(
m, 4H, CH ), 4.84−4.86 (m, 1H, SiH). C{ H} NMR (101 MHz,
2
ax
eq
ax
eq
ax
eq
29
CDCl ): δ 14.7, 23.3, 29.0. Si NMR (79 MHz, CDCl ): δ −21.9. MS
3
3
(
(
(
(
(
total)
L)
ΔE
ΔE
ΔE
ΔE
ΔE
0.0
0.89
2.73
0.0
0.0
0.83
2.53
0.0
0.0
0.0
0.95
2.86
(EI, 70 eV) m/z (%): 178 (79), 150 (100), 109 (54). HRMS m/z:
0.0
0.0
calcd for C H SiBr(79) 177.9813, found 177.9845.
5
11
NL)
ST)
1.84
3.76
0.0
1.70
1.91
5.41
0.0
0.0
1-Iodo-1-silacyclohexane (10). Iodotrimethylsilane (13.9 g, 69.4
mmol) was slowly added to a solution of 1-chloro-1-silacyclohexane
(8.5 g, 63.1 mol) in CH Cl (220 mL) under vigorous stirring. After
0.0
11.2
0.0
0.0
L−ST)
6.49
0.0
−1
13.8
8.27
2
2
a
complete addition, the reaction mixture was stirred for 3 days. The
desired product was collected (after CH Cl had been removed) by
Relative energies are in kcal mol . Calculations were performed at
the theoretical levels M06-2X /6-311++G**
and M06-2X/SDB -aug-cc-pVTZ for C H SiHBr and C H SiHI.
55
40−45
2
2
for C H SiHCl
5 10
56
distillation under nitrogen at 108−110 °C and 50 Torr (8.10 g, 35.8
5
10
5
10
1
mmol, 57%) as a colorless liquid. H NMR (400 MHz, CDCl ): δ
3
1
1
.16−1.24 (m, 2H, CH2(ax/eq)), 1.26−1.36 (m, 2H, CH2(ax/eq)), 1.37−
.45 (m, 1H, CH2
), 1.54−1.64 (m, 1H, CH
), 1.68−1.78
(ax/eq)
2(ax/eq)
1
(
1
m, 2H, CH ), 1.79−1.88 (m, 2H, CH ), 4.80 (s, J = 224.3 Hz,
2
2
H−Si
1
3
1
H, SiH). C{ H} NMR (101 MHz, CDCl ): δ 14.2, 23.7, 28.9.
3
29
1
Si{ H} NMR (79 MHz, CDCl ): δ −16.0.
3
GED Experiments. The electron diffraction patterns of 8 and 9
were recorded at M. V. Lomonosov Moscow State University on the
EMR-100 M electron diffraction apparatus. Nozzle temperatures were
4
6 and 54 °C for 8 and 51 and 54 °C for 9 for the long and short
nozzle-to-plate distances, respectively, and an accelerating voltage of
about 60 kV and a cubic sector were used. For the scanning on an
Epson Perfection 4870 photoscanner, three plates were selected for
the long nozzle-to-plate distance (362 mm) and three plates for the
short (194 mm) nozzle-to-plate distance. The wavelengths of the
electron beam were determined with the use of the scattering pattern
Figure 8. Conformational energies ΔE = Eax − Eeq for C H X and
6
11
from gaseous CCl and were equal to 0.049591 and 0.049402 Å for 8
4
C H SiHX.
5
10
and 0.049593 and 0.049459 Å for 9 for the long and short nozzle-to-
plate distances, respectively. Atomic scattering factors were taken from
5
9
the International Tables of Crystallography. Experimental back-
grounds were drawn as cubic spline functions to the difference
between the experimental and theoretical molecular intensity curves
with the use of a program developed by A.V.B. Observed intensity
1
-Phenyl-1-silacyclohexane. PhMgBr (14.9 g, 82.0 mmol) was
slowly added to 1-chloro-1-silacyclohexane (10.0 g, 74.0 mmol)
dissolved in Et O (90 mL) while stirring at 0 °C. The diethyl ether was
distilled off the reaction mixture and replaced by pentane. The reaction
mixture was then filtered under nitrogen and reduced pressure, and the
salt was discarded. Distillation of the reaction mixture under reduced
pressure (112−115 °C, 25 Torr) yielded 12.4 g (70.0 mmol, 95%) of
pure 1-phenyl-1-silacyclohexane.
2
−1
curves were recorded in the ranges s = 3.2−19.0 and 7.6−38.0 Å for
−
1
8
and s = 3.2−19.0 and 7.2−35.0 Å for 9 for the long and short
nozzle-to-plate distances, respectively. Both curves were digitized with
−1
increments of Δs = 0.2 Å [s = (4π/λ) sin θ/2, where λ is the electron
wavelength and θ is the scattering angle]. Least-squares structure
refinements were carried out with the use of the computer program
1-Chloro-1-silacyclohexane (8). Anhydrous HCl (2.4 g, 66.0
mmol) was condensed into a 100 mL ampule containing 1-phenyl-1-
silacyclohexane (10.5 g, 60.0 mmol), and the ampule was then sealed
off in vacuum. The ampule was inserted in a −78 °C cooling bath
6
0,61
KCED25M, which was developed by Norwegian researchers
and
modified by A.V.B. Weight matrices were diagonal. The short-distance
data were assigned 0.5 weight and the long-distance data 1.0 weight.
Estimated standard deviations calculated by the program were
multiplied by a factor of 3 to include added uncertainty due to data
(
methanol/dry ice bath). Dry ice was added to the cooling bath on a
regular basis for the next 2 days. The content of the ampule was then
allowed to stand in the cooling bath and slowly warm to room
temperature. After the ampule was opened, all of the volatile
components were condensed on a N (l)-cooled finger. The desired
product was collected (after the benzene byproduct had been
removed) by distillation under nitrogen at 120−122 °C (4.68 g,
8%) as a colorless liquid. The product was characterized by NMR and
MS. H NMR (400 MHz, CDCl ): δ 0.94−1.06 (m, 2H, CH ), 1.32−
.42 (m, 1H, CH2
m, 4H, CH ), 4.85−4.87 (m, 1H, SiH). C{ H} NMR (101 MHz,
CDCl ): δ 14.8, 23.1, 29.0. Si NMR (79 MHz, CDCl ): δ 7.9. MS
EI, 70 eV) m/z (%): 134 (90), 106 (100), 63 (76). HRMS m/z: calcd
for C H SiCl(35) 134.0319, found 134.0312.
6
2
correlation and an estimated scale uncertainty of 0.1%. The observed
and calculated molecular intensity curves are compared in Figures 9
and 10. The GED experiment for 1-iodo-1-silacyclohexane has been
2
3
1
published elsewhere.
Low-Temperature NMR Experiments. A 400 MHz NMR
spectrometer (Bruker Avance 400) was used for all of the NMR
experiments. A solvent mixture of CD Cl , CHFCl , and CHF Cl in a
5
1
3
2
1
(
), 1.52−1.60 (m, 1H, CH
), 1.72−1.85
2
2
2
2
(ax/eq)
2(ax/eq)
13
13
1
ratio of 1:1:3 was used for low-temperature C NMR measurements
on 8 and 9. Compound 10 could not be measured in the same freon
mixture because of limited solubility at low temperatures. A different
2
29
3
3
(
solvent mixture of CD Cl , CHF Cl, and CF Br in a ratio of 1:2:2
5
11
2 2 2 3
1
-Bromo-1-silacyclohexane (9). Anhydrous HBr (5.3 g, 66.0
allowed us to perform measurements on 10 successfully. The
temperature of the probe was calibrated by means of a type K
(chromel/alumel) thermocouple inserted into a dummy tube. The
readings are estimated to be accurate within ±2 K. The NMR spectra
were loaded into the data-handling program IGOR (WaveMetrics) for
analysis, manipulations, and graphic display. Line shape simulations of
mmol) was condensed into a 100 mL ampule containing 1-phenyl-1-
silacyclohexane (10.5 g, 60.0 mmol), and the ampule was then sealed
off in vacuum. The ampule was inserted in a −78 °C cooling bath
(
methanol/dry ice bath). Dry ice was added to the cooling bath on a
regular basis for the next 2 days. The content of the ampule was then
allowed to stand in the cooling bath and slowly warm to room
36
the NMR spectra were performed using the WinDNMR program.
7
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Organometallics
Article
The minimum-energy pathways for the chair-to-chair inversions of
compounds 7−10 were calculated in redundant internal coordinates
6
3
64
with the STQN(Path) method as implemented in Gaussian 09 at
4
0−45
the B3LYP/6-31G(d)
level of theory (def2-SVP basis set and
6
5
ECP used for iodine ).
High-level ab initio calculations were carried out on MP2/cc-
4
6,47
pVTZ
optimized geometries (using the cc-pVTZ-PP and ECP on I
6
6
and At ). The energy differences at the CCSD(T)/CBS (CBS =
complete basis set) level were then estimated by performing large-basis
MP2 calculations that were extrapolated to the basis set limit and then
applying a CCSD(T) correction to the MP2/CBS value:
CCSD(T)/CBS
MP2/CBS
CCSD(T)/small basis
ΔE
≈ ΔE
−
+ (ΔE
ΔEMP2/small basis)
The MP2 calculations were performed with correlation-consistent
basis sets up to the aug-cc-pV5Z level (using the aug-cc-pVXZ-PP
basis set and ECP for iodine and astatine). The energy difference
obtained at the MP2/aug-cc-pV5Z level was estimated as being
sufficiently close to the basis set limit, MP2/CBS. The
Figure 9. (top) Experimental (open circles) and calculated (solid line)
molecular intensity curves for C H SiHCl. (bottom) Difference curve
for the optimized mixture of the conformers.
5
10
CCSD(T)/small basis
MP2/small basis
(
ΔE
− ΔE
) term was calculated at the
CCSD(T)/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels. All of the
67
MP2 and CCSD(T) calculations performed with Molpro 2006.1.
Thermal corrections to enthalpies and free energies were calculated
6
8
65
from B97-1 /def2-TZVPP harmonic vibrational frequencies (ac-
companying def2 ECP on iodine). This CCSD(T)/CBS protocol has
1
7
been used in previous studies on silacyclohexanes.
All of the enthalpy and entropy corrections to the conformational
energies were calculated at the same level (B97-1/def2-TZVPP) for
consistency. The B97-1 functional is known to predict good harmonic
6
9
frequencies and enthalpy and entropy corrections. A problem with
harmonic frequency calculations for six-membered rings persists,
however. Six-membered rings include a number of low-frequency
vibrations that are known to be badly predicted by the harmonic
approximation. As they contribute significantly to the entropy, errors
in the entropy correction and hence the computed free energy
Figure 10. (top) Experimental (open circles) and calculated (solid
line) molecular intensity curves for C H SiHBr. (bottom) Difference
curve for the optimized mixture of the conformers.
13
5
10
differences can be expected. The absolute C NMR shielding
constants of the carbon nuclei were calculated with the GIAO
7
0,71
72−74
method
using the PBE1 functional
and a mixed basis set
7
5
consisting of aug-pcS-2 on carbon atoms and def2-TZVPP on all
other atoms (including the def2 ECP on iodine). Shielding constants
were converted to chemical shifts by reference to C shielding
Low-Temperature Raman Experiments. Raman spectra were
recorded with a Jobin Yvon T64000 spectrometer equipped with a
triple monochromator and a CCD camera. The samples were filled
into 1 mm capillary glass tubes and irradiated by the green 532 nm line
of a frequency-doubled Nd:YAG laser (Coherent, DPSS model 532-
13
constants in TMS. All of the geometries (including the TMS standard)
were optimized at the MP2/cc-pVTZ level. The magnitudes of the
relative shieldings are in reasonable agreement with experiment (slight
overestimation), and their signs confirmed the expected assignment in
2
0, 10 mW). Spectra were recorded from pure compound and in
which the ring carbon nuclei (apart from C4) are more shielded in the
heptane and THF solutions. A continuous-flow cryostat (Oxford
Instruments OptistatCF) using liquid nitrogen for cooling was
employed for the low-temperature measurements.
13
axial conformer than in the equatorial one. All of the calculated
C
NMR shielding constants are available in the Supporting Information.
ASSOCIATED CONTENT
Supporting Information
COMPUTATIONAL DETAILS
■
■
*
S
All of the calculations for direct comparison and use with GED
experiments were carried out at the MP2(full)/SDB-aug-cc-pVTZ
level of theory for Br and I atoms and the MP2(full)/aug-cc-PVTZ
level of theory for all other atoms. The theoretical molecular force
fields were used to calculate mean vibrational amplitudes and
vibrational correction terms necessary for GED analysis. To reduce
the number of refined parameters, the following assumptions were
made on the basis of MP2 results. Only the geometric parameters of
the axial conformer were refined, and the parameters of the equatorial
form were tied to those of the axial conformer using the calculated
differences. For the axial conformer, the difference between the nearly
equal C2−C3 and C3−C4 bond lengths was constrained to the
calculated value. All of the C−H bonds, the H−C−H angles, and the
H −C2−C3, H −C3−C2, H −C3−C4, and H −C3−C2 angles
Simulated spectra and parameters derived from the DNMR
1
13
analysis, NMR spectra ( H and C) for the title compounds,
selected Raman spectra, figures showing deconvolution of
signals and van’t Hoff plots, calculated 13C NMR chemical
shifts for the title compounds, MP2-optimized geometries, and
a file of all computed Cartesian coordinates in a format for
convenient visualization. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: ingvara@hi.is.
■
*
ax
eq
eq
ax
were set equal. Angles that define the orientation of the C−H bonds
were set to calculated values. The optimized theoretical geometry
parameters were found to be in good agreement with experimental
ones.
Present Addresses
ICI Rheocenter, Reykjavik University, Innovation Center
Iceland, Keldnaholti, IS-112 Reykjavik, Iceland.
⊥
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Organometallics
Article
#Max-Planck Institute for Chemical Energy Conversion,
(23) Girichev, G. V.; Giricheva, N. I.; Bodi, A.; Gudnason, P. I.;
Jonsdottir, S.; Kvaran, A.; Arnason, I.; Oberhammer, H. Chem.Eur. J.
2007, 13, 1776.
Stiftstrasse 32-34, D-45470 Mu
Takeda Austria GmbH, St. Peter-Straße 25, A-4020 Linz,
̈
lheim an der Ruhr, Germany.
∇
(24) Girichev, G. V.; Giricheva, N. I.; Bodi, A.; Gudnason, P. I.;
Austria.
Jonsdottir, S.; Kvaran, A.; Arnason, I.; Oberhammer, H. Chem.Eur. J.
Notes
2009, 15, 8929.
́
́
(25) Wallevik, S. O.; Bjornsson, R.; Kvaran, A.; Jonsdottir, S.;
The authors declare no competing financial interest.
†
Arnason, I.; Belyakov, A. V.; Baskakov, A. A.; Hassler, K.;
Oberhammer, H. J. Phys. Chem. A 2010, 114, 2127.
Conformations of Silicon-Containing Rings. 11. For Part 10,
see ref 1.
(26) Bjornsson, R.; Arnason, I. Phys. Chem. Chem. Phys. 2009, 11,
8
689.
ACKNOWLEDGMENTS
(27) Bodi, A.; Bjornsson, R.; Arnason, I. J. Mol. Struct. 2010, 978, 14.
(
■
́
28) Bodi, A.; Kvaran, A.; Jonsdottir, S.; Antonsson, E.; Wallevik, S.
́
Financial support from RANNIS − The Icelandic Centre for
́
O.; Arnason, I.; Belyakov, A. V.; Baskakov, A. A.; Ho
Oberhammer, H. Organometallics 2007, 26, 6544.
29) Favero, L. B.; Velino, B.; Caminati, W.; Arnason, I.; Kvaran, A. J.
̈
lbling, M.;
Research (Grant 080038021) is gratefully acknowledged. Use
of the computing resources of the EaStCHEM Research
Computing Facility are acknowledged. A.V.B. is grateful to
RFBR (Project 12-03-91330-NNIOa) and the Government of
RF (Project 11.G34.31.0069) for financial support. T.K. and
K.H. thank the Austrian Science Foundation FWF (Fonds zur
(
Phys. Chem. A 2006, 110, 9995.
(30) Klaeboe, P.; Aleska, V.; Nielsen, C. J.; Horn, A.; Guirgis, G. A.;
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(
31) Belyakov, A. V.; Baskakov, A. A.; Berger, R. J. F.; Mitzel, N. W.;
́
Oberhammer, H.; Arnason, I.; Wallevik, S. O. J. Mol. Struct. 2012,
012, 126.
32) Belyakov, A. V.; Baskakov, A. A.; Naraev, V. N.; Rykov, A. N.;
̈
Forderung der wissenschaftlichen Forschung, Vienna) for
financial support of Project P 21272-N19.
1
(
Oberhammer, H.; Arnason, I.; Wallevik, S. O. Russ. J. Gen. Chem. 2011,
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