Reactivity of the Sterically Demanding Siloxanediol Mes2Si(OH)(μ-O)Si(OH)Mes2 Towards Water and Ether Molecules

Article abstract of DOI:10.1002/chem.201702393

A series of isotopologues of the siloxanediol Mes₂Si(OH)(μ-O)Si(OH)Mes₂ (3 a) (Mes=2,4,6-trimethylphenyl) were synthesized by reactions of the corresponding disiloxane precursors Mes₂Si(μ-O)₂SiMes₂ (2

Full text of DOI:10.1002/chem.201702393

A Journal of
Accepted Article
Title: Reactivity of the Sterically Demanding Siloxanediol Mes2Si(OH)
(µ-O)Si(OH)Mes2 Towards Water and Ether Molecules
Authors: Thomas Braun, Philipp Roesch, Ulrike Warzok, Martin Enke,
Robert Müller, Caspar Schattenberg, Christoph Schalley,
Martin Kaupp, and Philipp Wittwer
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To be cited as: Chem. Eur. J. 10.1002/chem.201702393
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Reactivity of the Sterically Demanding Siloxanediol
Mes
2
Si(OH)(µ-O)Si(OH)Mes Towards Water and Ether Molecules
2
[
a]
[b]
[c]
[c]
[c]
Philipp Roesch, Ulrike Warzok, Martin Enke, Robert Müller, Caspar Schattenberg, Christoph A.
[
b]
[c]
[a]
[a]
Schalley,* Martin Kaupp,* Thomas Braun,* and Philipp Wittwer
Abstract:
Mes Si(OH)(µ-O)Si(OH)Mes
were synthesized by reactions of the corresponding disiloxane
A
series of isotopologues of the siloxanediol
bonding driven self-association and anion binding of various 1,3-
disiloxanediols.[3] Organosilanols and siloxanediols in particular,
can be regarded as models for surface-bound silanol groups,[
which are abundant structural motifs in silicates and silica
materials. Hydroxo groups at adjacent silicon centers in a Si-O-
Si moiety can be generated from strained surface-located oxo
bridges by hydrolysis,[ and the latter have been identified as
possible defect sites on a silica surface.[6] Although several
2
2
(3a) (Mes 2,4,6-trimethylphenyl)
=
4]
17
precursors Mes
2
Si(µ-O)
2
SiMes
2
(2a), Mes
2
Si(µ- O)
2
SiMes
2
(2b) or
18
17
18
Mes
2
Si(µ- O)
2
SiMes (2c) with an excess of H O, H O or H O.
2
2 2 2
NMR and IR signal assignments for the siloxanediols in benzene are
supported by quantum-chemical calculations, which indicate small
energy differences between trans and cis conformers, the latter of
5]
1
which exhibits an intramolecular hydrogen bond. H NMR as well as
2 2
molecular siloxanes containing a four-membered Si O ring unit
IR data suggest the presence of a mixture of both conformers in
were described earlier, their hydrolysis and behavior towards
water is less explored.[ West and co-workers reported the
7]
6 6 2 2
C D . Hydrogen-bonded adducts of Mes Si(OH)(µ-O)Si(OH)Mes
with ethers such as diethylether, dimethoxyethane or dioxane were
observed in the solid state, where they form polymeric chain-like
hydrolysis of Mes
hydrolysis product Mes
investigated further. In this article, we present studies on the
2
Si(µ-O)
2
SiMes
Si(OH)(µ-O)Si(OH)Mes
2
, but the behavior of the
was not
2
2
17
1
[8]
structures. The latter appear to be stable only in the crystal. O{ H}
NMR and IR data in THF solution suggest an interaction of 3a with at
least one THF molecule, whereas diethylether appears not to
interact. Water adducts form neither in solution nor in the solid state
water
reactivity
of
the
tetramesityl-1,3-disiloxanediol
Mes Si(OH)(µ-O)Si(OH)Mes
2
2
and its interactions with several
ether molecules in solution, in the gas phase and in the solid
state. The experiments are complemented by density functional
theory (DFT) calculations.
1
7
1
as indicated by NMR and ATR IR data. O{ H} NMR and ESI-MS
experiments illustrate the remarkably high stability of the
siloxanediols towards water and show no evidence for intra- or
intermolecular oxygen-exchange reactions. In marked contrast, a
stepwise exchange of all three oxygen atoms – including the one in
Results and Discussion
the Si-O-Si bridge
–
occurred in the gas phase, when
18
18
18
−
[
Mes
2
Si( OH)(µ- O)Si( O)Mes
2
]
2
was treated with H O in the
Synthesis and Properties in Solution
hexapole of an ESI FT-ICR mass spectrometer. The scrambling
between the bridging and the other oxygen atoms likely proceeds
As originally reported by West et al.,[9] the oxygenation of the
2 2
through cyclic Si O intermediates.
disilene Mes
corresponding disiloxane Mes
hydrolyzed to provide the siloxanediol Mes
2
Si=SiMes
2
(1, Scheme 1) with O
Si(µ-O) SiMes (2a), which is then
Si(OH)(µ-
(3a).[10] Analogously, a reaction of the disilene 1
2
2
yields the
2
2
2
2
O)Si(OH)Mes
Introduction
with 17O
or 18O
(2b) and Mes
2
2
2
generates the isotopologues Mes Si(µ-
1
7O)
18
2
SiMes
2
2
Si(µ- O)
2
2
Mes (2c), respectively.
Organosilanols show interesting properties,[1] as their ability for
hydrogen bonding can be associated, for example, with
condensation reactions and applications in catalysis or drug
design.[2] Recently, Franz and co-workers reported the hydrogen
18
Their hydrolysis with H
2
O, H 1
2
7O or H
2
O thus provides the
complete series of differently O-labeled siloxanediols (3a – 3g).
Interestingly, the hydrolysis of 2a – 2c does neither occur when
reacted with excess water in C
one equivalent of degassed water in [D
refluxing the mixture for seven days. Only when the disiloxanes
6
D
6
, nor when treated with only
8
]THF even after
[
[
[
a]
b]
c]
P. Roesch, P. Wittwer, Prof. Dr. T. Braun
2
a – 2c were reacted in [D
8
]THF with an excess of water at
Department of Chemistry, Humboldt Universität zu Berlin
Brook-Taylor-Straße 2, 12489 Berlin (Germany)
E-mail: thomas.braun@cms.hu-berlin.de
U. Warzok, Prof. Dr. C. A. Schalley
Institut für Chemie und Biochemie, Freie Universität Berlin,
Takustraße 3, 14195 Berlin (Germany)
60 °C for three days, clear solutions were obtained, from which
the siloxanediols were separated after work-up as colorless
powders that can be handled under ambient conditions.
Remarkably, all attempts to synthesize 3a by hydrolysis of the
dichlorosilane Mes
react with water to yield Ph
contrast, the formation of 3a was observed within two hours
when 2a was reacted in C with an equimolar amount of
dihydroxodiphenylsilane Ph Si(OH) in presence of NEt . The
siloxanediol 3d was also prepared using this approach. The
condensation of Ph Si(OH)2 to give the
2
SiCl
2
failed, although Ph
2
SiCl
2
is known to
E-Mail: schalley@zedat.fu-berlin.de
.[11] In
M. Enke, C. Schattenberg, Dr. R. Müller, Prof. Dr. M. Kaupp
Institut für Chemie, Technische Universität Berlin
Theoretische Chemie/Quantenchemie, Sekr. C7
Straße des 17. Juni 135, 10623 Berlin (Germany)
E-mail: martin.kaupp@tu-berlin.de
2
Si(OH)(µ-O)Si(OH)Ph
2
6 6
D
2
2
3
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.2017xxxxx.
2
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the 1H, Si HMBC NMR spectra of the compounds 3a, 3b, 3d
29
2 3
cyclohexaphenyltrisiloxane (Ph SiO) generates stoichiometric
amounts of water, which react in situ under basic conditions with
a or 2b (Scheme 1). For comparison, 2a was also heated with
an excess of water in deuterated benzene in the presence of
NEt . This leads only to an incomplete conversion to the
disiloxanediol after 24 hours. Apparently, in situ-generated water
reacts differently when compared to bulk water. An
intramolecular condensation of the siloxanediols to give a cyclic
disiloxane was not observed, even when they were reacted with
NaOH in a water/THF mixture at elevated temperature.
and 3f at δ = -37.6 ppm in [D ]THF show a significant upfield
8
2
shift (∆δ = 30 ppm) in comparison to the data for the precursors,
due to the reduced strain after ring opening. The value is also in
good agreement with the calculated NMR chemical shifts (Table
1, see below for further details on calculations) and comparable
literature data.[3, 14] The 17O{ H} NMR spectrum of 3b shows a
broad resonance at δ = 47 ppm for a silanol group, whereas for
3d and 3f the 17O{ H} NMR spectrum reveals two signals at δ =
84/86 and δ = 48/49 ppm for the bridging oxygen atom and the
silanol group, respectively. The 17O{ H} NMR chemical shifts are
again in very good agreement with the computed values and
furthermore match well with data for comparable
3
1
1
1
H2O or H217
or H218
O
O
L
L
O
L
L
L
Si
E
O2
O
O
[D8]THF (1:50)
Si
O
H
Si
Si
L
L
L
60 °C, 3 d
organosiloxanes and silanols such as for phenyl-substituted
polysiloxanes and triphenyl or trimethyl silanol.[12, 15]
H
2a
a E = 16
b E = 17O
c E = 18O
3
3
3
O
Table 1: Calculated average (PBE0/IGLO-II//PBE0-D3(BJ)/def2-TZVP) and
H2O
D8]THF (1:50)
L
L
experimental O{ H} and H NMR chemical shifts (δ) in ppm.
17
1
1
E
L
L
L
L
17O2
17O
17O
L
L
L
L
Si
E
Si
O
H
[
Si Si
Si
Si
L
L
C6D6
60 °C, 3 d
17
1
1H NMR (OH)
O{ H} NMR
H
1
2b
3
d E = 17O
comp
exp.
calcd.
exp.
calcd.
H217O/[D8]THF (1:50)
H O/[D ]THF (1:50)
3e E = 18
O
L =
2b*
3b*
108
47
109.0
47.1a
42.7b
‒
‒
2
8
H218
5.90
6.49a
7.05b
O
L
L
E
18O2
Si
E
18O
L
L
Si
E
H
L
L
[D8]THF (1:50)
L
L
Si
Si
18O
60 °C, 3 d
H
a
a
b
6.49a
7.05b
2c
E = 17
f O
g E = 18O
3d*
3d†
3f*
86, 49
84, 48
86, 49
91.6 , 47.1
5.91
2.42
5.90
3
3
b
9
1.1 , 42.7
1
.0 equiv. NEt3
1.0 equiv. Ph2Si(OH)2
c
c
3.22c
1.92d
L
L
L
L
L
L
L
82.9, 39.6
E
E
Si
Si
Si
E
Si
E
C6D6, 2 h, 60 °C
1/3 (Ph2SiO)3
L
d
d
O
H
90.6, 38.5
-
H
a
a
6.49a
7.05b
‒
a E = 16O
b E = 17O
3a E = 16
3d E = 17
91.6 , 47.1
2
O
O
2
b
b
9
1.1 , 42.7
Scheme 1: Oxygenation of 1 and subsequent ring opening with an excess of
water in [D ]THF and alternative synthetic route to access 3a and 3d via in situ
H
2
O†
0.2, −13.6
‒
‒
8
6 6.
formed equimolar amounts of water in C D
†
*
in [D
8
]THF; calcd. with two THF molecules without PCM solvent model; in
a
cis
b
C
6
D
6
; calcd. with PCM for benzene.
Cis conformer 3a -2THF. Trans
conformer 3a -2THF. Cis conformer 3acis
trans
c
.
d
Trans conformer 3atrans.
The 17O{ H} NMR spectrum of 2b shows a broad resonance at δ
1
=
108 ppm. This value is in good agreement with the calculated
NMR shift (Table 1, see also below). Due to the high strain of
the siloxane ring, the signal appears at lower field when
compared to the resonances of non-strained siloxane species.[
In the H NMR spectra of 3a and the 17O isotopologues 3b, 3d
and 3f in [D ]THF, a resonance at δ = 5.91 ppm for the hydroxo
8
groups was detected. This is in good agreement with signals for
terminal silanol groups of the structurally similar 1,3,5,7-
1
In contrast to the spectra in [D
compound 3d in C
8
]THF, the H NMR spectrum of
12]
6
D
6
shows a broad signal for the hydroxo
1
groups at δ = 2.42 ppm, which is in accordance with a rapid
exchange of OH protons. Most likely, a fast conformational
equilibrium as well as intramolecular dynamics associated with
the hydrogen bridge in a cis conformer are responsible for
proton exchange. However, intermolecular hydrogen bonding of
tetrahydroxy-1,3,5,7-tetraphenylcyclotetrasiloxanes
]THF.[13] The resonances appear at rather low field,
presumably because of hydrogen bonding to [D ]THF molecules.
The resonance shifted to δ = 5.50 ppm when an NMR sample of
a was heated to 333 K, possibly indicating an entropy-driven
decrease in hydrogen bonding. The signals in the 29Si domain of
in
[D
8
3
d
is rather unlikely, since the self-association of the
8
comparable tetraphenyl-1,3-disiloxanediol was only observed at
high concentrations or in the solid state.[
spectrum of 3d in C changes negligibly when compared to
the spectra in [D ]THF. After addition of an excess of THF to a
3, 16]
The O{ H} NMR
17
1
3
6 6
D
8
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C
6
D
6
solution of 3d, the 1H NMR signal of the silanol groups
appear at slightly lower field (∆δ = 0.97 ppm) which agrees with
the suggested interaction of 3d with [D ]THF. Addition of excess
Et O to a solution of 3d in C did not lead to any change of the
signals for the hydroxo groups in the H NMR or O{ H} spectra.
Thus, Et O does not seem to interact with 3d in C solution.
17O NMR shifts of the cis arrangements seem to fit the
experimental values better. However, the opposite is true when
comparing measured and calculated 29Si chemical shifts. Hence,
a prediction of which conformer is predominantly (or exclusively)
present in solution is not feasible based only on the calculated
NMR data alone. On the other hand, the results also do not rule
out fast dynamic interconversions of conformers and rapid OH
proton exchanges in solution, as suggested above for the
8
2
6 6
D
1
17
1
2
6 6
D
To gain closer insight into possible solvent effects on the
chemical shifts in the 1H NMR spectra from DFT calculations,
different models of 3a have been employed. In particular, we
focused on six distinct structures from optimizations in the gas
phase: 3acis, which exhibits a cis arrangement of the silanol
interpretation of experimental 1H NMR shifts of 3d in C
D .
6 6
Computationally, the cis and trans arrangements are
energetically closer than the predictive accuracy, given the
magnitude of dependencies on computational aspects and
particularly on environmental effects (cf. Table 2 and Table S3).
A rough conformational scan suggests an upper limit of 36 kJ
…
groups (O-Si Si-O = 25.5°) and shows intramolecular bridged
OH moieties; 3atrans, which exhibits a corresponding trans
…
cis
…
conformation (O-Si Si-O = 161.0°); 3a -2THF (O-Si Si-O =
5.3°) which has been modeled with explicit coordination of two
4
mol-1 for the barrier between 3acis and 3atrans
.
THF molecules to the silanol groups, thus preventing
intramolecular hydrogen bridges; 3acis-1THF (O-Si Si-O
…
=
5
4.4°) with one THF molecule coordinated to both silanol
groups; the corresponding trans arrangement 3atrans-2THF (O-
…
Si Si-O = 163.0°) with two OH-coordinating THF molecules; and
3
atrans-1THF (O-Si Si-O = 162.0°) in which one silanol group is
…
uncoordinated (see Figure 1, Figures S1-S3 in the SI).
Models 3acis-2THF and 3atrans-2THF permit investigation of
possible effects of THF on the NMR spectra, as discussed
above. As shown in Table 1 (see also Tables S2 and S11 in the
17
29
SI), the obtained O and Si NMR chemical shifts for both
conformers are generally in good agreement with the
experimental values. Particularly with respect to the chemical
Figure 1: Molecular structures of conformers 3acis (left) and 3atrans (right) from
optimizations in the gas phase (PBE0-D3(BJ)/def2-TZVP; Si = yellow, O = red,
C = white, H = cyan).
shifts in the 1H NMR spectrum, the experimentally detected
1
8
H(OH) signal at δ = 5.91 ppm for 3a/b/d/f in [D ]THF is
reproduced well (δ = 6.49 (cis) / 7.05 (trans) ppm, average
1
values). Most importantly, the corresponding calculated
H
chemical shifts of the non-coordinated models are significantly
2 8
After heating of 3b or 3d in an H O/[D ]THF solution (1:50) to
upfield-shifted in comparison. We note that 3acis-1THF and
60 °C for several days, no generation of labeled water was
observed, and no change in the signal intensity in 17O{ H} NMR
spectra of 3b and 3d was found, which excludes any inter- or
intramolecular exchange in solution under these conditions. In
atrans-1THF give 17O and 29Si NMR chemical shifts close to the
1
3
corresponding doubly coordinated complexes. The same holds
true for the 1H (OH) shifts of the coordinated silanol groups,
although for 3acis-1THF the downfield shift is not as pronounced
as in the case of individually coordinated silanol groups
addition, a solution of 3d in C
6
D
6
was treated with a small
excess of H 1
7
O and, after three days of refluxing of the mixture,
2
no change of the 17O{ H} NMR spectrum for 3d occurred (Figure
2). We presume that no exchange of the silanol groups with
water takes place in solution. Micelle formation via the OH
groups was observed for tert-butylsilanetriol in aqueous media,
but can be excluded for the siloxanediol due to its steric bulk.[
However, the spectrum showed an additional signal at δ = -13.5
ppm, which can be assigned to separated water aggregates in
deuterated benzene. Note that H. Kusanagi assigned this
resonance to isolated water molecules stabilized by solvent
1
(averaged value δ = 4.78 ppm; cf. Table S2 in Supporting
Information). Overall, the computational results thus strongly
…
support the presence of OH THF hydrogen-bonding interactions
in 3a/b/d/f/[D
8
]THF solutions.
18]
For C as solvent, intermolecular solute-solvent hydrogen
6 6
D
bonding interactions are not expected. Nevertheless, in order to
account for polarization effects of the solvent, a simple self-
consistent reaction field (SCRF) polarizable continuum model
and integral equation formalism (IEF-PCM) has been employed
in the NMR chemical shift calculations for 3acis and 3atrans (see
SI for details). The thus obtained values for both conformers are
again in good agreement with the corresponding experimental
data (Table 2).
molecules, whereas emulsified water leads to
a broad
resonance at δ = 0.0 ppm.[
17, 19]
However, recent theoretical
investigations on the water-benzene system postulate larger
water clusters in benzene to be more stable in comparison to the
isolated water molecules coordinated by benzene.[20]
In general, a comparison of the calculated 17O, H, and 29Si NMR
data (Table 2 and Table S2, S11 in the SI) between the cis and
trans arrangement in each of the employed model pairs,
1
cis
trans
cis
trans
3
a
/3a
and 3a -2THF/3a
-2THF, shows that both
respective conformers exhibit comparable chemical shifts. The
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intensities for both conformers generally match fairly closely to
the experimental spectra (see Table S10 in the SI).
However, the computations show two distinct IR bands at 3748
cm-1 and 3630 cm for structure 3a with the cis-arrangement
-1
cis
of the OH groups, whereas 3atrans exhibits only a single band at
751 cm-1 (averaged value) for the O-H stretching modes. This
3
finding confirms the interpretation of the experimental spectra,
where the additional red-shifted band was attributed to the
intramolecular hydrogen-bridged siloxanediol moiety in 3acis
.
Figure 2: 17O{ H} NMR spectra of 3d (above), 3d + H
1
17O after removal of
2
1
7
excess water (middle), and H
2 6 6
O in C D (below) at 298 K. * emulsified
[
17]
water.
Negative-mode electrospray ionization mass spectrometry (ESI-
MS) generates singly deprotonated ions from siloxanediols 3a-g
when sprayed from 100 µM solutions in acetonitrile/THF (10:1).
The observed isotopic patterns are in good agreement with
those calculated for the different isotopologues. In excellent
agreement with the 17O NMR experiments, isotope scrambling[21]
did not occur, when for example 3g was reacted with an excess
16
of H
2
O in THF for three days at elevated temperature. No
incorporation of 16O was observed, an outcome which did not
even change, when an excess of aqueous NaOH solution (5
mM) was added (see Figure S27 in the SI). This result
emphasizes the remarkable inertness of the siloxanediols
against water in solution. Siloxanediol 3g undergoes, however, a
H/D exchange in methanol-OD at room temperature as easily
detected in the corresponding ESI mass spectra.
Figure 3: Part of the IR spectra of 3a in C D and in C D after addition of Et O,
6
6
6
6
2
In order to get more information on possible aggregates of the
siloxanediol in solution, IR spectra of 3a in the presence of ether
molecules or in the presence of water were acquired. Solutions
2 2
THF, H O and D O.
of 3a in C
respective donor molecules, and subsequently IR spectra were
measured (Figure 3). In C , 3a shows two broad absorption
6 6
D (0.01 M) were treated with five equivalents of the
Table 2: Calculated relative Gibbs free energies ∆ Grel and reaction free
-1
R
energies ∆ G (in kJ mol ; PBE0-D3(BJ)/def2-TZVP) for gas-phase optimized
cis
trans
cis
trans
cis
trans
conformers (3a , 3a
, 3a -1THF, 3a
-1THF, 3a -2THF, 3a
-2THF).
6 6
D
-
1
bands at 3676 and 3611 cm for intramolecular hydrogen-
bridged silanol moieties.[22] These absorption bands can be
attributed to the cis conformer. The band of the OH stretch
vibration of a trans conformer is presumably not visible due to
overlap with the more intense vibration of 3acis (see also
calculations below, Figure S7 and Table S10). In accordance
with the observations made by NMR spectroscopy, addition of
conformer
∆ Grel
R
∆ G
3
acis
12.5
0.0
4.5
0.0
0.0
1.4
-27.7a
atrans
-40.2a
3
3acis-1THF
-38.1b
Et
When a sample of 3a in C
band at 3339 cm-1 was detected, which further supports the
suggested hydrogen bonding to THF. In the presence of H O, 3a
shows absorption bands for the intramolecular bridging silanol
2
O did not lead to any change of the IR spectrum in this region.
3
atrans-1THFb
acis-2THF
-42.6b
6 6
D
was treated with THF, a broad
-45.6c
3
2
3atrans-2THFb
-44.2c
-
1
units at 3679 and 3609 cm , whereas the dissolved water in
a
b
c
-
1 [22]
2a + H
2
O → 3a; 2a + H
2
O + THF → 3a-1THF; 2a + H O + 2 THF → 3a-
2
C
D
6
D
6
results in a broadened band at 3300 – 3600 cm . With
O instead of H O the bands for OH groups appear at 3671
O was
2THF.
2
2
-
1
and 3613 cm . Note that H/D exchange in presence of D
2
extremely slow under these conditions. The experimental IR
data were compared with calculated IR bands based on gas-
phase DFT computations of 3acis and 3atrans. Similarly to the
Note that two of the O-H stretching frequencies are essentially
identical for both conformers. Computations with two explicit
THF molecules provide two peaks close in energy near 3300
results for the calculated NMR shifts, the computational data for
the computed IR band positions and corresponding relative
-
1
cm , while calculations with one coordinated THF give more
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Chemistry - A European Journal
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separated peaks at 3329/3429 cm-1 and 3752/3651 cm-1 for
cis/trans conformers (cf. Table S10 and Figures S6-S10 in
Supporting Information). As mentioned above, experimentally
only a weak, broad peak appears in this region, which is most
likely caused by the existence of various coordination modes of
THF in solution. This assumption is further supported by the
calculated, comparably small additional free energy gain from a
second THF coordination (Table 2).
H
O
L
H
L
L
L
O
Si
Si
O
Si
O
O Si
L
L
L
L
H
O
H
O
L
L
dme
O
Et2O
Si
O
Si
O
H
L
L
O
O
H
H
H
L
L
O
Si
O
L
3a
O
Si
L
Si
L
L =
O
L
O
L
Si
1
,4-dioxane
H
L
O
Solid State Structures
H
3
a•dme
3a•Et2O
L
Si
L
H
O
Crystallization of 3a from an ether solution led to the formation of
L
H O
O
O
Si
L
single crystals of 3a•Et
and 3a•dioxane were obtained (Scheme 2, Figures 4-6, Table
). The crystals were only stable in presence of the respective
2
O. In a similar way, crystals of 3a•dme
O
O
O
Si
L Si O
H
L
H
L
3
L
3a•dioxane
ethers or in an inert gas atmosphere, whereas after addition of
various polar and apolar solvents, 3a was liberated immediately.
The X-ray diffraction analysis revealed a chain of ether and
siloxanediol molecules connected by hydrogen bonding in each
case. Electron densities which were assigned to the hydrogen
atoms of the hydroxyl groups were found in the difference
Fourier maps and the hydrogen atoms were freely refined. All
O−H distances are identical at 0.9 Å (Table 3). The O-O
2
Scheme 2: Treatment of 3a to give 3a•Et O, 3a•dme and 3a•dioxane as
crystalline compounds.
However, in 3a•dioxane the backbone angle of 133.71(14)° is
smaller and can be classified at the lower end of the usual Si-O-
[1a]
Si bond angles of 135-180°. Pietschnig and coworkers found
separations in the hydrogen bridges of 3a•Et
2
O are O1-O4
.845(2) Å and O3-O4 2.764(2) Å. One of the O-O contacts in
a•dme is slightly shorter (2.816(3) and 2.755(3) Å).
an angle of 137.59° for (DMP)Si(OH)
intramolecular hydrogen bridging of the silanol moieties. For
DMP)Si(OH) (µ-O)Si(OH) (DMP)•(THF) and (DMP)Si(OH) (µ-
O)Si(OH) (DMP)•(DMF), which are also characterized by
2 2
(µ-O)Si(OH) (DMP) due to
2
3
3
2
Å
(
2
2
2
a•dioxane exhibits even shorter values of 2.776(3) Å and
.682(3) Å. In general, the data are in good agreement with O-O
2
intermolecular hydrogen bonds in the solid state, the Si-O-Si-
distances in (DMP)Si(OH)
.7800(12) Å and 2.692(4) Å (DMP = 2,6-dimesitylphenyl).[2n]
The Si-O bond lengths to the bridging oxygen atom in the
backbone of 3a•Et O are 1.6276(17) Å and 1.6231(17) Å. The
corresponding distances in 3a•dme exhibit identical values of
.6242(18) Å and 1.6234(18) Å), whereas in 3a•dioxane the Si-
O distances are slightly increased (1.642(2) Å and 1.639(2) Å).
a•dme and 3a•Et O exhibit identical Si-O-Si backbone angles
2 2
(µ-O)Si(OH) (DMP)•(THF, DMF) of
[2n]
angles are 136.22(5)° and 132.35(17)°, respectively.
For the
2
structures of Ph Si(OH)(µ-O)Si(OH)Ph three values were
2
2
reported (147.8(3)°, 157.0(3)° and 162.5(3)°) caused by packing
effects.[16]
2
1
3
2
of 152°. Both angles are in accordance to values described for
many other siloxanes.[1a]
2
Table 3: Bond angles and bond lengths of 3a•Et O, 3a•dme and 3a•dioxane [°, Å]
…
calcd
compound
Si-O-H･･･O
∢(Si-O-Si)
∢(O-Si Si-O)
d(O-H)
0.90(2)
0.90(2)
0.89(3)
0.88(3)
0.89(4)
0.89(4)
d(O-H･･･O)
1.97(2)
1.96(2)
1.89(4)
1.95(3)
1.84(4)
1.90(4)
d(O･･･O)
2.845(2)
2.764(2)
2.755(3)
2.816(3)
2.682(3)
2.776(3)
3
3
3
a•Et
2
O
Si1-O1-H1･･･O4
Si2-O3-H3･･･O4
Si1-O1-H1･･･O4
Si2-O3-H3A･･･O5
Si1-O1-H1･･･O4
Si2-O3-H3A･･･O5
152.67(11)
162.49
160.16
65.33
a•dme
152.48(12)
133.71(14)
a•dioxane
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Chemistry - A European Journal
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2
Figure 4: Part of the chain structure of 3a•Et O. C-centred hydrogen atoms
are omitted for clarity. The ellipsoids are drawn at the 30 % probability level.
As mentioned earlier, the potential energy hypersurface of the
silicon-oxygen linkage is very shallow, thus the Si-O-Si angle
can easily respond to any bending enforced by hydrogen
bridges.[ However, the smaller Si-O-Si angle in 3a•dioxane
might be explained by the less flexible nature of the dioxane
linker connecting the silanediols. The chain of molecules
connected via H-bonding runs parallel to the a cell axis and is
23]
Figure 6: Part of the chain structure of 3a•dioxane. C-centred hydrogen
atoms are omitted for clarity. The ellipsoids are drawn at the 30 % probability
level (top). Two asymmetric units of 3a•dioxane connected by a glide plane
operation. The organic ligands were omitted for clarity (below, Si = yellow, O =
red, C = grey, H = white).
associated with a glide plane (see also in Figure 6). 3a•Et
2
O and
…
3a•dme exhibit a dihedral O-Si Si-O angle of 160° and 162°,
suggesting an anti-like arrangement of the two SiOH moieties.
a•dioxane shows a much smaller angle of 65°. This angle is
3
consistent with a gauche-like conformation.
In order to gather more information of 3a and its behavior
towards small donor molecules in the solid state, ATR IR spectra
of selected compounds were recorded. Figure 7 shows parts of
the ATR IR spectra of 3a, 3a•Et
comparison. The ATR IR spectrum of single crystals of 3a•Et
shows a broad band for the hydrogen bonded silanol groups to
2 2 2
O, 3a+H O and 3a+D O in
2
O
-
1
the ether molecule with a maximum at 3414 cm . This value is
in good agreement with data for silsesquioxane-diethylether
adducts.[
4b]
However, the spectrum of 3a exhibits a broad
absorption band between 3150 and 3600 cm-1 with a maximum
at 3366 cm-1 suggesting the presence of hydrogen bonding of
2 2
the siloxanediol molecules. Ph Si(OH)(µ-O)Si(OH)Ph exhibits a
comparable absorption band at 3425 cm-1 which has been
assigned to intermolecular hydrogen bridges.[11,
24]
This
observation is in contrast to the spectrum of 3a in C
6
D
6
, which
reveals intramolecular hydrogen bonding for the cis isomer.
After drying a wet solution of 3a in C , an ATR IR spectrum of
the remaining solid was measured. The spectrum for 3a+H
6 6
D
2
O
shows two absorption bands at 3590 and 3672 cm-1 for
intramolecular hydrogen bridged silanol groups, which is in good
accordance with the observations in solution. A broad band
Figure 5: Part of the chain structure of 3a•dme. C-centred hydrogen atoms
are omitted for clarity. The ellipsoids are drawn at the 30 % probability level.
between 3300 and 3500 cm-1 is due to bulk water, which
seemingly does not interact with 3a.[ When D
22]
2
O was used
2
instead of H O, absorption bands for the SiOH moieties (3595
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Chemistry - A European Journal
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-
1
and 3666 cm ) were found, whereas bands at 2500 and 2660
fragmentation series indicating the high stability of this moiety.
Furthermore, a new disiloxane Si O ring is formed during the
2 2
-
1
[22]
cm can be assigned to bulk D
2
O.
To the best of our
knowledge, only few molecular compounds exhibiting silanol
water contacts by hydrogen bonding in the solid state were
process as supported by our DFT calculations.
reported.[25] In case of the [Si(OH)
(bipy)
bipyridyl), the SiOH absorption stretch appears at 3500-3100
2
2 2 2
]I •2H O cation (bipy =
-
1 [25a]
cm .
2 2 2
Figure 7: Part of the ATR IR spectra of 3a, 3a•Et O, 3d+H O and 3d+D O.
Reactivity and Oxygen Atom Exchange in the Gas Phase
Tandem MS experiments were performed on siloxanediol-
derived ions aiming for a confirmation of the positions of the
different isotopically labeled oxygen atoms in the molecule by
investigation of the occurring gas phase fragmentations.
Furthermore, they might thereby provide reliable information on
possible oxygen-atom scrambling between the bridge and the
OH-positions of the siloxanediol in the gas phase. As a
representative example, the 3g derived ion was mass-selected
and subjected to collision-induced dissociation (CID) (Figure
Figure 8: a) Mass selection and b) collision-induced dissociation (CID)
experiment with [Mes ( OH)Si((µ- O)Si( O)Mes ] anions derived from 3g. c)
2 2
Fragmentation pathway including structures A-D as proposed by DFT
calculations (PBE0-D3(BJ)/def2-TZVPD).
1
8
18
18
-
An 18O-to- O-exchange experiment was conducted in the gas
16
18
18
18

2 2
phase on the triply labeled ion [Mes ( OH)Si(µ- O)Si( O)Mes ]
18
18
18

8
a,b). [Mes
2
( OH)Si(µ- O)Si( O)Mes
2
]
anions exhibit two
Si(µ-
Si( O)Mes-H] fragment
ions at m/z 467 and 347. The first fragmentation can be
rationalized by formal intramolecular substitution of
16O as the exchange reagent in the
ion derived from 3g using H
2
sequential losses of neutral mesitylene leading to [Mes
2
accumulation hexapole of a Fourier-transform ion-cyclotron
resonance (FT-ICR) mass spectrometer (for experimental details
and data analysis, see SI).[26] Figure 9 shows the development
18
18
-
18
18

2 2
O) Si( O)Mes] and [MesSi(µ- O)
a
a
of the isotope pattern over reaction time. Isotope pattern
analysis provides the relative intensities of the different
protonated mesityl ligand by the siloxide group of A yielding the
anionic siloxanediol oxide species B which exhibits a newly
16
-, 16
18O-, 16O O
18
- and 18
isotopologues E, F, G and H ( O
3
O
2
2
3
O -
2 2
formed four-membered Si O ring (Figure 8c). The proposed
labeled, respectively). In marked contrast to the experiments in
structures A and B (as well as C and D further below) were
obtained from DFT optimizations, while the corresponding
electronic structures were analyzed by natural resonance theory
18
16
solution reported above, the O/ O-exchange reaction occurs
and involves all three oxygen atoms, thus including also the Si-
O-Si bridge. A dissociation-reassociation mechanism, in which
cleavage of the bridge leads to two independent fragments
which then undergo oxygen-exchange reactions and finally
(NRT) on truncated model systems (see Schemes S1-S4). The
second gas-phase fragmentation product possesses the
structures C or D, which both feature a bridging mesityl ligand
and which are similar in energy according to our quantum-
chemical calculations (see Scheme S1). It is important to note
that the SiO-backbone stays intact throughout the entire
-5
reconnect, can be safely ruled out in the high vacuum of ca. 10
mbar inside the hexapole collision cell. Consequently, the
exchange of the bridging oxygen atom is most likely only
feasible when oxygen atom scrambling occurs through
a
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Chemistry - A European Journal
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strained, four-membered cyclic disiloxane intermediate (Scheme
[
27]
3
). The surprising finding that this exchange is observed in the
gas phase, but not in solution, may be rationalized by several
marked differences in the environments: the species
investigated in the gas phase is not the neutral siloxanediol, but
the corresponding anion. Ring formation – and thus oxygen
atom scrambling within the anion – is more favorable since the
-
Si-O moiety incorporated in the ion is certainly more nucleophilic
than a Si-OH group. This intramolecular reaction further benefits
from the absence of any solvation shell in the gas phase which

would reduce the nucleophilicity of the Si-O group. The same
argument holds true for the water addition. Finally, the binding
energy of the water molecule to the siloxanediol anion cannot be
liberated easily, as collisions with the surrounding medium occur
very infrequently under the conditions of the experiment.
Therefore, the binding energy remains as vibrational excitation
within the intermediates and is available to overcome even
somewhat higher barriers for rearrangements which would be
prohibitive in solution. Consequently, the particular conditions in
Scheme 3: Proposed mechanism of 18O/16O-exchange reaction at siloxanediol
2 2
3g derived ion [Mes ( ] .
OH)Si(µ-18O)Si(18O)Mes
1
8
-
the high vacuum of the mass spectrometer provide mechanistic Conclusions
insight, which could not be obtained from solution experiments
and clearly point to the ability of the anion to undergo ring
Several isotopologues of Mes Si(OH)(µ-O)Si(OH)Mes2 (3a-g)
2
closure to four-membered Si
O
2 2
ring intermediates.
were prepared by reaction of Mes Si(μ-O) SiMes (2a),
2
18
2
2
17
Mes
2
Si(μ- O)
2
SiMes
2
(2b) or Mes
2
Si(μ- O)
2
SiMes
2
(2c) with
water isotopologues. In solution, an intra- or intermolecular
exchange of oxygen atoms in Mes Si(OH)(µ-O)Si(OH)Mes
appears not to occur. Tandem ESI-MS studies revealed that the
2
2
2 2
Si( OH)(µ- O)Si( O)Mes
]− fragments in the gas
1
8
18
18
anion [Mes
phase via loss of two neutral mesitylene, yielding a product with
a newly formed Si ring as supported by quantum-chemical
calculations. In marked contrast to the solution reactivity of 3a,
2 2
O
2 2 2
Mes Si( OH)(µ- O)Si( O)Mes O in the gas
]− reacts with H
18
18
18
[
phase in the hexapole of an ESI FT-ICR mass spectrometer and
an exchange of all three oxygen atoms was observed. The
incorporation at the Si-O-Si bridge occurs most likely through a
strained, four-membered cyclic disiloxane intermediate. IR
spectra of Mes Si(OH)(µ-O)Si(OH)Mes (3a) in C D solution
2 2 6 6
agree with the presence of both cis and trans conformers,
consistent with small energy differences revealed by DFT
2
calculations. Neither the presence of Et O nor of water led to
formation of hydrogen bonding in solution. In contrast, up to two
THF molecules seem to be able to accept intermolecular
hydrogen bonds from Mes
2
Si(OH)(µ-O)Si(OH)Mes
2
(3a). This
1
assumption was further substantiated by H NMR chemical shift
calculations using a model of 3a with explicit THF coordination
to the silanol groups. In the solid-state, interactions via hydrogen
bonding were observed with ether molecules, and chain
structures are formed. As in solution, water does not form any
2 2
hydrogen bridges to Mes Si(OH)(µ-O)Si(OH)Mes (3a) in the
solid state. This suggests that the generation of water
aggregates accompanied by an intramolecular hydrogen
bridging in 3a is preferred over an interaction of 3a to water.
Figure 9: Gas-phase 8O/16O-exchange experiment performed on 3g derived
1
2 2 2
ion [Mes ( ] with gaseous H O in the hexapole
OH)Si((µ- O)Si(18O)Mes
1
8
18
-
16
cell of the FTICR mass spectrometer after different reaction intervals (left) and
calculated isotopic patterns of the corresponding isotopologue ratios (right).
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Chemistry - A European Journal
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Chemistry - A European Journal
10.1002/chem.201702393
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Philipp Roesch, Ulrike Warzok, Martin
Enke, Robert Müller, Caspar
Schattenberg, Christoph A. Schalley,*
Martin Kaupp,* Thomas Braun* and
Philipp Wittwer
Page No. – Page No.
Properties of Tetramesitylsiloxanediol towards Water and Ethers: The
reactivity of the sterically demanding siloxanediol towards water and ether
molecules differs drastically when reactions in solution, the solid state and the gas-
Reactivity of the Sterically
Demanding Siloxanediol
phase were investigated. Isotopic labelling of oxygen atoms ( O, 18O) revealed no
1
7
2 2
Mes Si(OH)(µ-O)Si(OH)Mes Towards
18
Water and Ether Molecules
2
exchange or interaction with water in solution, but [Mes Si( OH)(µ-
18
O)Si( O)Mes
2
]− reacted in the gas phase.
18
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