Reactivity of Cp*Al towards Silanols: Formation and Hydrolysis of Alumosiloxanes

Article abstract of DOI:10.1002/ejic.201800564

Treatment of [Cp*Al]₄ (1) (Cp* = pentamethylcyclopentadienyl) with various silanols gave access to the compounds [Al{OSi(OtBu)₃}₃(DMAP)] (3) (DMAP = 4-dimethylaminopyridine), [HNEt₃][Al(OSiPh₃)_4<

Full text of DOI:10.1002/ejic.201800564

A Journal of
Accepted Article
Title: Reactivity of Cp*Al Towards Silanols: Formation and Hydrolysis
of Alumosiloxanes
Authors: Philipp Wittwer, Adrian Stelzer, and Thomas Braun
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800564
Link to VoR: http://dx.doi.org/10.1002/ejic.201800564

10.1002/ejic.201800564
European Journal of Inorganic Chemistry
FULL PAPER
Reactivity of Cp*Al Towards Silanols: Formation and
Hydrolysis of Alumosiloxanes
Philipp Wittwer,[a] Adrian Stelzer,[a] and Thomas Braun*[a]
Abstract:
Treatment
of
[Cp*Al]4
(1)
(Cp*
=
which was treated with HOSi(OtBu)3. Note that subvalent
aluminum compounds, such as [Cp*Al]4 (1), can be valuable
tools to access unusual binding motifs and complexes of
aluminum.[6] Thus, H. W. Roesky et al. showed that 1 inserts into
the Si-F bond of SiF2Ph2 to give [{(Cp*AlF)2SiPh2}2].[7] Later, the
synthesis of the molecular alumoxane [{(NacNac)DippAl}2O] by
pentamethylcyclopentadienyl) with various silanols gave access to
the compounds [Al{OSi(OtBu)3}3(DMAP)] (3) (DMAP
dimethylaminopyridine), [HNEt3][Al(OSiPh3)4] (4),
=
4-
and
[Al7(OH)9{(OSiiPr2)2O}6] (6). ESI mass spectrometry revealed that 6
exchanges its outer hydroxyl groups in the presence of H218O water.
Hydrolysis of 4 led to the formation of [HNEt3][Al(OH)(OSiPh3)3] (5),
but only in the presence of additional NEt3. The structures in the
solid state of 6·toluene, 6·(THF)3, 6·(H2O·2THF)3, and 5 were
determined by X-ray crystallography.
oxygenation
of
[(NacNac)DippAl]
(NacNacDipp
=
CH{(CMe)(2,6-iPr2C6H3N)}2) with O2 was reported.[8] Additionally,
[(NacNac)DippAl] undergoes an oxidative addition reaction when
treated with a broad scope of E-H compounds, thus as in the
reaction with HOiPr giving [(NacNac)DippAl(H)OiPr].[9] Although
AlOSi linkages are accessible from Al(III) compounds and
silanols,[10] Al(I) complexes might lead to different structural
motifs.
Introduction
To the best of our knowledge, no reports on the reactivity of
[Cp*Al]4 (1) towards compounds containing hydroxyl groups
have been published, except for the slow decomposition in the
presence of water.[11] In this contribution we present studies on
the reactivity of [Cp*Al]4 (1) towards various silanols to give
alumosiloxanes. The compounds were studied with regard to
their reactivity towards water.
Alumosilicates play a crucial part as catalysts in fundamental
industrial processes,[1] e.g. crude oil cracking.[1h] A mechanistic
understanding of their formation is needed in order to design
catalysts without laborious screening processes. One step
towards this goal comprises the synthesis of molecular
alumosiloxanes, as they can be regarded as model structures
for early steps in zeolite formation.
Considerable contributions were reported by Veith and
coworkers.[2]
For
example,
the
alumosiloxane
[Al4(OH)4(OSiPh2OSiPh2O)4] consists of a ring of four [AlO4]
tetrahedra which are linked by OH groups, and four siloxanediol
ligands arranged around this core.[2c] Deprotonation of the
bridging aluminum hydroxyl groups is possible on using amine
bases. More recently, Limberg et al. reported the synthesis of
[Al3(µ2-OH)3(THF)3{PhSi(OSiPh2O)3}] which is characterized by
three OH-linked tetrahedral coordinated aluminum atoms in its
core.[3] The hydrolysis of both compounds led to larger clusters
with a core featuring octahedral coordinated aluminum atoms,
which are linked by bridging hydroxide ligands. Barron et al.
reported on the synthesis of the cluster [Al10(OH)16(OSiEt3)14]
that shows a motif which resembles moieties found in boehmite
Results and Discussion
Treatment of HOSi(OtBu)3 with [Cp*Al]4 (1) at 80°C led to gas
evolution and after ten minutes to the formation of a colorless
gel (Scheme 1). 1H NMR measurements of the gel showed
signals corresponding to iso-butene, Cp*H and H2. Similar
decomposition reactions were reported in the past for Al
compounds containing -OSi(OtBu)3 moieties yielding an
alumosilicate gel of an unknown structure, although in the
present case, the gelation occurs faster.[6g] When the reaction
solution was heated for only five minutes and then immediately
cooled down to room temperature, or when the reaction was
1
or diaspore.[4]
A
comparable structural motif was also
followed in situ at 80 °C via H NMR spectroscopy, additional
established in two different clusters obtained by hydrolysis of
[{(tBuO)3SiO}AlO]4.[5] The latter compound has a heterocubane
structure and was prepared by the reaction of [Cp*Al]4 (1) (Cp* =
pentamethylcyclopentadienyl) with O2 or N2O to yield [Cp*AlO]4,
signals were detected. The 1H NMR spectrum of the reaction
mixture after being stirred at 80°C for five minutes shows signals
at room temperature for [Cp*Al]4 (1), HOSi(OtBu)3, Cp*H, H2 and
for an additional Cp* species at  = 1.92 ppm, as well as
resonances for tBu groups in the region of  = 1.621.45 ppm.
The ratio of 1 : unknown species : Cp*H is approximately
4 : 12 : 84. The 1H,29Si HMBC NMR spectrum shows signals that
can be assigned to HOSi(OtBu)3 and [Al(OSi(OtBu3)3].[6g]
However, in the 27Al NMR spectrum of the reaction solution a
broad singlet at  = -58 ppm (1/2 ≈ 340 Hz) for an unknown
aluminum species was observed in addition to the signal for
unreacted [Cp*Al]4 (1) at  = -80 ppm. A resonance which could
[a]
P. Wittwer, Dr. A. Stelzer, Prof. Dr. T. Braun
Department of Chemistry, Humboldt-Universität zu Berlin
Brook-Taylor-Straße 2, 12489 Berlin (Germany)
E-mail: thomas.braun@cms.hu-berlin.de
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800564
European Journal of Inorganic Chemistry
FULL PAPER
be assigned to [Al(OSi(OtBu3)3] was not observed. It appears
plausible, that the resonances at  = 1.92 ppm in the H NMR
refinement. ESI-MS studies of 3 in THF showed a signal at
m/z = 961.56 for the cation [NaAl{OSi(OtBu)3}3(DMAP)]+.
[Al{OSi(OtBu)3}3(DMAP)] (3) was also characterized by IR-ATR
spectroscopy. The spectrum shows an absorption band at
1630 cm1. A ring breathing mode of pyridine adsorbed at Lewis
acidic centers are reported in this region.[14] Bands at
comparable energies are also reported for other pyridine-
derivatives coordinated at an aluminum centre,[15] such as for
[(Cl)2Al(2-O2CPh)(4-Me-C5NH4)2] (1627 cm1)[15a] or [Et2Al(η-
1
spectrum and the resonance at  = -80 ppm in the 27Al NMR
spectrum are caused by the same unidentified compound, the
identity of which remains unclear. Comparable 27Al shifts to the
unknown aluminum species were reported for the aluminum
halides [Cp*AlX2]2 (X = Br, Cl) at  = -45.5 ppm and -53 ppm,
respectively.[12] Thus the unknown species likely posseses a Cp*
ligand and might be a dimeric Cp*Al(III) compound. When
adventitious water is present in the reaction mixture, after two
O2CC6H4-2-NH)AlEt2(4-Me-C5NH4)]
(1616 cm1).[15d]
The
days
crystals
of
the
literature
known
cluster
coordinated Lewis base strongly enhances the thermal stability
of compound 3, as it shows no sign of decomposition at 90°C.
The stability towards hydrolysis is also increased, as there is no
indication of decomposition upon direct exposure to H2O at room
temperature.
[Al4(OH)6(H2O)2(OSi(OtBu)3)6][5] (2) were identified by X-ray
crystallography.
In order to gain more information about the aforementioned
unidentified aluminum species, DMAP was added after five
minutes to a reaction mixture of HOSi(OtBu)3 and [Cp*Al]4 (1)
which was heated at 80°C. Comparison of the NMR spectra at
room temperature before and after the DMAP addition showed
that the signals corresponding to the unknown compound were
still present after the addition of the Lewis base. The NMR data
for 3 were also observed. Hence, DMAP did not react with the
unknown aluminum compound, whereas the formation of 3 is
likely due to a reaction of [Al(OSi(OtBu3)3] with DMAP.
Treatment of [Cp*Al]4 (1) with HOSiPh3 at 80°C in the presence
of NEt3 led to the precipitation of [HNEt3][Al(OSiPh3)4] (4) and
release of H2 from the reaction solution. The conversion is
complete after 10 minutes (Scheme 2). A stoichiometric amount
of the amine with regard to the silanol is needed to avoid side
reactions. Hydrolysis of 4 with water in THF occurred only in the
presence of NEt3 after heating to 60 °C for 15 hours yielding
[HNEt3][Al(OH)(OSiPh3)3] (5). This observation is in contrast to
the reactivity reported for [Li(THF)4][Al(OSiPh3)4],[16] which
hydrolyses immediately upon treatment with H2O to furnish
[Al(OSiPh3)3(H2O)]. In principle, the anion in 5 can also be
regarded as the deprotonated form of the known compound
[Al(OSiPh3)3(H2O)].[17] Indeed, 5 can also be synthesized by
addition of NEt3 to [Al(OSiPh3)3(H2O)] (Scheme 2).
Scheme 1. Reactivity of [Cp*Al] (1) towards HOSi(OtBu)3.
Treatment of [Cp*Al]4 (1) with HOSi(OtBu)3 at 80°C in the
presence of 4-dimethylaminopyridine (DMAP) gave the
mononuclear compound [Al{OSi(OtBu)3}3(DMAP)] (3), Cp*H and
H2 (Scheme 1). The 1H NMR spectrum shows two resonances in
the aliphatic region, one for the butoxy groups at  = 1.59 ppm
and one for the amine-bound methyl groups at  = 1.88 ppm,
whereas the two resonances at  = 6.20 ppm and  = 9.00 ppm
can be attributed to the meta and ortho bound hydrogen atoms
at the DMAP ring, respectively. The 1H,29Si HMBC NMR
spectrum shows a correlation between the signals caused by the
butoxy groups and a resonance in the 29Si domain at  = -97
ppm. No signal could be detected in the 27Al NMR spectrum.
However, a signal for 3 in the 27Al MAS NMR spectrum was
detected at iso = 55.0 ppm, compatible with a tetracoordinated
13]
Al center.[12a,
This signal shows a nearly axial electric field
gradient (Q = 0.05) and strong quadrupolar coupling
(Q = 1.669 MHz), which is consistent with the suggested
structure for 3 in Scheme 1. The geometry is also supported by
X-ray single crystallography, however, strong disorder of a
majority of the butoxy groups impair the quality factors of the
Scheme 2. Synthesis of [HNEt3][Al(OSiPh3)4] (4) and formation of
[HNEt3][Al(OH)(OSiPh3)3] (5).
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10.1002/ejic.201800564
European Journal of Inorganic Chemistry
FULL PAPER
Scheme 3. Formation of [Al7(OH)9{(OSiiPr2)2O}6] (6).
in [Li(THF)4][Al(OSiPh3)4] (4’ = 0.98), and the structure is close
to an ideal tetrahedral one (4’ = 1).
ESI-MS studies of the alumosiloxanes 4 and 5 revealed signals
for the cation (m/z = 102.13) and the anions [Al(OSiPh3)4]
(m/z = 1127.33) and [Al(OH)(OSiPh3)3] (m/z = 869.26). No
signal in the 1H NMR spectrum could be detected for the
ammonium protons or OH group in 4 or 5. The ATR-IR spectrum
of 5 shows weak broadened absorption bands at 3140 cm-1 and
3673 cm-1, which can be attributed to the NH and OH units of
separated ions, respectively,[18] and furthermore two broad
absorption bands in the regions of 2000-2550 cm-1 and
2550-2760 cm-1 for the hydrogen bonding.[19] Since no such
interaction occurs in 4, its IR-ATR spectrum shows a more
intense and less broadened absorption band corresponding to
the NH vibration at 3144 cm-1 and none of the other discussed
features. The 27Al NMR spectra of 4 ( = 54.8 ppm) and 5
( = 59.8 ppm) each show a signal in the typical range for an Al
center with tetrahedral coordination of oxygen atoms.[12a] Both
signals differ in shape, and the spectrum of 4 shows a sharp
signal with a line width of 1/2 ≈ 10 Hz, while in contrast a broad
singlet is detected for 5 (1/2 ≈ 270 Hz).[13] For comparison the
27Al NMR spectrum of [Li(THF)4][Al(OSiPh3)4] was also
measured and it exhibits a signal at  = 54.9 ppm with a
comparable line width to the resonance for 4.[16] The structure of
[HNEt3][Al(OH)(OSiPh3)3] (5) was determined by X-ray
crystallography (Figure 1), whereas measurements of single
crystals of [HNEt3][Al(OSiPh3)4] (4) revealed a severe disorder of
the cations. The cation in 5 is hydrogen bonded to the oxygen
atom of the hydroxyl group at the anion. The AlOH bond
(Al1O4 1.772(1) Å) is elongated when compared to the
distances of the three AlOSi bonds (averaged 1.728(1) Å),
which are close to the corresponding bond lengths in
Figure 1. Structure of [HNEt3][Al(OH)(OSiPh3)3] (5). Thermal ellipsoids are
drawn at 50% probability level, the hydrogen bond is depicted as a dashed
line, and hydrogen atoms except for the oxygen and nitrogen bound ones
were omitted for clarity. Selected distances [Å] and angles [°]: Al1O1
1.7250(9); Al1O4 1.7719(10); Si1O1 1.6002(9); N1…O4 2.661(2);
O1Al1O4 108.34(5).
We also investigated the reaction of [Cp*Al]4 (1) with the
disiloxanediol iPr2Si(OH)(µ-O)Si(OH)iPr2 in the presence of
water, which yielded the cluster [Al7(OH)9{(OSiiPr2)2O}6] (6) after
stirring at 95°C for two days (Scheme 3). The formation of H2
and Cp*H was also detected via NMR spectroscopy. Crystals of
6·toluene suitable for X-ray diffraction studies were obtained by
slow cooling of a hot saturated toluene solution (Figure 2a). A
representation of the oxygen coordinating polyhedra of the Al7
core is shown in Figures 2b and 2c (for an animated rotating
representation of the inner core, see the GIF file in the SI). The
structure consists of an inner octahedral Al(OH)6 core which is
linked to three pairs of corner-shared AlO4 tetrahedra via its
hydroxyl groups. Additional OH groups link the six AlO4 units to
form the three pairs. The polyhedral view in Figure 2b reveals
the C3 symmetry of the aluminum core in 6. The AlO4 tetrahedra
pairs can be transformed into each other by C3 rotation.
[Li(THF)4][Al(OSiPh3)4] (averaged 1.729(1) Å).[16] Such
difference in distances between AlOSi and terminal AlOH
bonds at AlO4 tetrahedra was also found for
a
[NMe4]4[Al4O12Si4(OH)8] (1.78(3) Å vs averaged 1.70(2) Å), but
the latter compound does not exhibit an interaction between the
cations and the hydroxyl groups.[20] Literature reveals only four
published compounds characterized by X-ray crystallography
which exhibit a terminal O3AlOH moiety.[21] The geometry
index[22] derived from the bond angles for the aluminum atom of
5 (4’ = 0.96) is comparable to the one in the [Al(OSiPh3)4] anion
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10.1002/ejic.201800564
European Journal of Inorganic Chemistry
FULL PAPER
Furthermore, the AlO4 pairs are connected via siloxanediolate
ligands in such a way that three AlO4 and three siloxanediolates
form one moiety bound to a face of the inner Al(OH)6 core. The
six inner hydroxyl groups form hydrogen bonds to the oxygen
atoms of six of the twelve AlOSi units in the cluster. The
structure of 6 does formally not obey Loewensteins rule, which
states that AlOAl linkages in zeolitic frameworks (i.e. between
tetrahedral AlO4 units) are avoided.[23] To the best of our
knowledge there is only one comparable compound to 6
reported in the literature.[2b, 2d] [Al7(OH)9{(OSiPh2)2O}6] was
characterized by X-ray crystallography and exhibits a similar
AlOSi framework and three diethyl ether molecules hydrogen
bonded to the outer hydroxyl groups, but no other analytical data
were presented.
Single crystals of 6·(3 THF·H2O)1.5 suitable for X-ray
crystallography were formed by dissolving compound 6 in wet
THF and subsequent slow evaporation of the solvent. The
obtained structure is comparable to the one obtained from a
toluene solution, but additionally the three outer hydroxyl groups
form hydrogen bonds either to THF or alternatively to water
molecules, which are additionally bound to two THF molecules
via hydrogen bridges (Figure 2d and 2e). Refinement set the
ratio of the independent moieties 6·(THF)3 and 6·(H2O·2THF)3 to
55 : 45. Table 1 summarizes selected bond lengths of
6·(3 THF·H2O)1.5 and 6·toluene. While the central Al(OH)6 part of
both structures is virtually identical, certain bonds of the outer
framework differ. A small but significant difference is observed
for the AlOH and SiOAl bonds at the outer hydroxyl groups
(Figure 2a: Al2O7/Si6O18; Figure2d/e: Al2O2/Si1O3, Table
1, entries 1-3). These bond lengths are shorter when the
hydroxyl groups are involved in an intermolecular hydrogen
bond in 6·(3 THF·H2O)1.5 compared to 6·toluene.
Table 1. Selected bond lengths of 6·toluene[a] and 6·(3 THF·H2O)1.5
.
6·toluene
1.823(1)
1.620(1)
1.634(1)
1.893(1)
1.771(1)
1.698(1)
1.717(1)
6·(3 THF·H2O)1.5
1.796(2)
O3AlO(H)AlO3 [Å]
SiOAl [Å]
1.605(3)
[b]SiOAl [Å]
1.616(3)
Al1O(H)AlO3 [Å]
O3AlO(H)Al1 [Å]
AlOSi [Å]
1.896(2)
Figure 2. a) Structure of 6·toluene, b) & c) polyhedral view of the AlO4 (blue)
and AlO6 (green) units of 6, d) Structures of 6·(THF)3 and e) 6·(H2O·2THF)3.
Thermal ellipsoids are set at 50% probability level, hydrogen bonds are
depicted as lightblue dashed lines, the iPr groups and hydrogen atoms except
for those bound to an O atom were omitted for clarity. Selected bond lengths
[Å] and angles [°]: 6·toluene: Al1-O1 1.8912(18); Al2-O1 1.7748(18);
1.769(2)
1.699(3)
[b]AlOSi [Å]
1.722(3)
Al2-O7 1.8249(19);
O1-Al1-O6179.61(10);
Al2-O10
O1-Al2-O7 98.50(9);
1.7002(18);
Si1-O10
O1-Al2-O10
1.6229(19);
112.26(9);
[a] averaged bond length of chemically equivalent bonds, [b] with
intramolecular hydrogen bond at the oxygen atom
6·(THF/H2O)3: Al1-O1 1.896(2); Al2-O1 1.769(2); Al2-O2 1.796(2); Al2-O3
1.699 (3); Si1-O3 1.605(3); O1-Al1-O1’ 179.12(14); O1-Al2-O2 101.31(13);
O1-Al2-O3 111.52(14).
The ATR-IR spectrum of compound 6 shows a broad absorption
band between 3560 to 2850 cm-1, which is assigned to OH
bonds involved in hydrogen bonding.[18] The 29Si NMR spectrum
of [Al7(OH)9{(OSiiPr2)2O}6] (6) displays resonances for two
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10.1002/ejic.201800564
European Journal of Inorganic Chemistry
FULL PAPER
chemically inequivalent silicon atoms at  = -23.1 and -24.8 ppm.
This is consistent with the presence of intramolecular hydrogen
bonding of the inner hydroxyl groups, which only affects half of
the silicon atoms. Likewise, the 1H and 13C NMR data for
compound 6 show more than only one set of signals for the iPr
groups. The methyl groups at the iPr moieties show eight signals
at  = 2018 ppm, because of prochirality and the lower
symmetry due to the hydrogen bonds. Similarly, the CH groups
display four signals at  = 1715 ppm. The 1H NMR spectrum
reveals two signals at  = 7.87 and 5.55 ppm with an integral
ratio of 3:6 corresponding to the outer and inner hydroxyl
groups, respectively. In the 27Al NMR spectrum a signal at
 = 5.2 ppm of 6 was detected, which is a typical chemical shift
for octahedral coordinated AlO6 nuclei.[12a] No signals for the six
outer AlO4 units were identified in solution. However, in the 27Al
MAS NMR spectrum a good agreement of measured and fitted
Figure 3. ESI-MS data of reaction solutions for the synthesis of 6 after
different reaction times (see Figure S4 in ESI for bigger picture).
data for cluster
iso = 5.32 ppm with a Gaussian line (linewidth ~500 Hz, i.e.
almost negligible quadrupolar distortion) and one signal with a
chemical shift of iso = 61.7 ppm, a quadrupolar coupling of Q
6 is achieved by fitting one signal at

To investigate how cluster 6 interacts with water, 6 was
dissolved in THF and an excess of 18O enriched water was
added to the solution. The ESI-MS data measured after one
week at room temperature show peaks at m/z = 1999.84,
=
1.175 MHz and a non-axial electric field gradient (Q = 1.00).
The former signal is again caused by the octahedral coordinated
aluminum atom, while the latter one corresponds to the AlO4
units and its isotropic chemical shift appears in the expected
range for fourfold oxygen coordinated aluminum atoms.[12a]
ESI-MS measurements of 6 reveal peaks at m/z = 1999.85 for
the [M-H] ion with the expected isotopic pattern (Figure 4
bottom). The formation of 6 was also monitored by ESI-MS and
data were collected after solutions being stirred at 95°C for 12,
24, 36 and 48 hours (Figure 3). Negatively charged clusters with
differing intensities at given reaction times were detected. The
data after 12 hours revealed signals of smaller clusters such as
[Al(L)2] or [Al(LH)2(L)] (L = iPr2Si(O)(µ-O)Si(O)iPr2), whereas
after 24 and 36 hours the intensity of signals with larger m/z
([Al2(LH)2(L)2(OH)], [Al3(L)4(OH)2], [Al6(OH)6(L)6-H]) increased.
After a reaction time of 48 hours, the most intense signal
corresponds to [Al7(OH)9{(OSiiPr2)2O}6H] (6H+). Of special
interest is the signal at m/z = 1921.84. The mass and isotopic
pattern fits to [Al6(OH)5(O){(OSi(iPr)2)2O}6] and differs to
[Al7(OH)9{(OSiiPr2)2O}6] (6) by the mass of one Al(OH)3 unit and
one proton. The signal is already observed after 12 hours
reaction time and increases in intensity over time, until its
corresponding anion presumably gets consumed, forming 6.
When the reaction solution was analyzed by 1H NMR
spectroscopy after 24 hours, several singlets in the region of
 = 104 ppm were detected. These signals are likely caused by
aluminum hydroxyl groups of various intermediate clusters, but
they could not be assigned further.
2001.84,
2003.84
and
2005.84
for
[Al7(OH)9-x(18OH)x{(OSiiPr2)2O}6] (x = 0-3) (Figure 4), which
revealed the incorporation of up to three 18O atoms, presumably
by exchange of hydroxyl groups. The reaction solution was then
exposed to air. Subsequent ESI-MS measurements after roughly
one and three weeks showed a decrease of peak intensity for
clusters with labeled oxygen atoms (Table 2). Obviously, the
ambient conditions led to exchange reaction with unlabeled air
1
moisture. When D2O was added to a [D8]THF solution of 6, H
NMR measurements showed a decrease in intensity of the
signal in the 1H NMR spectrum for the outer hydroxyl groups
(see SI). The experimental data therefore suggest, that only the
outer hydroxyl groups are exchanged in the presence of water.
In a separate experiment, 6 was exposed to an excess of water
1
in [D8]THF and the reaction solution was monitored by H NMR
spectroscopy, revealing partial decomposition over the course of
ten days.
Figure 4. ESI-MS spectra of 6 before (bottom, green) and one week after
treatment with H218O (top, blue).
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10.1002/ejic.201800564
European Journal of Inorganic Chemistry
FULL PAPER
silanols. This led to the generation of [Al{OSi(OtBu)3}3(DMAP)]
(3), [HNEt3][Al(OSiPh3)4] (4) and to [Al7(OH)9{(OSiiPr2)2O}6] (6),
when for the latter a siloxanediol was used. The reaction of 1
towards silanols gave, therefore, products which are accessible
from Al(III) staring compounds. Use of less than three
equivalents of silanols per aluminum atom did not change the
type of product, only its yield. This is in contrast to the known
Table 2. Percentage of incorporated 18O atoms of a THF solution of 6 after
different times.
Nr. of 18
O
1 week, inert
26 %
5 days later, air
21 days later, air
0
1
2
3
87 %
13 %

93 %
7 %

reactivity of [(NacNac)DippAl].[10] Hydrolysis of
4
yielded
45 %
[HNEt3][Al(OH)(OSiPh3)3] (5), which exhibits a rare example of a
terminal hydroxyl group at a molecular aluminum compound.
The compound is stabilized by a hydrogen bond. The cluster
[Al7(OH)9{(OSiiPr2)2O}6] (6) features formally a Loewenstein-
forbidden framework. NMR and MS data reveal an exchange of
its outer hydroxyl groups in the presence of water, which is
remarkable and unprecedented.
24 %
5%


Calculations on the exchange mechanism of OH bridges
between two AlO6 moieties with solvent water were performed
for the Al13--Keggin-ion by Rustad and Casey.[24] According to
those calculations, the first step involves a cleavage of the AlO
bonds of both aluminum atoms to the bulk structure, which
results in five-fold coordinated aluminum atoms. This is followed
by the coordination of solvent water at both Al atoms. Since
compound 6 shows tetrahedral geometry at the six outer
aluminum atoms, such a mechanism is unlikely, because it
would lead to at least one intermediate AlO3 moiety. Therefore,
an associative-type mechanism appears more plausible instead.
Addition of 20 Eq. NEt3 to a solution of cluster 6 in [D8]THF led
exclusively to the deprotonation of the outer hydroxyl groups, as
revealed by the disappearance of their corresponding peak in
the 1H NMR spectrum. The [HNEt3]+ cation is likely hydrogen
bound to the now deprotonated hydroxyl-bridge, in a similar
manner as in compound 5. An increase of the temperature to
60 °C for three days did not lead to any deprotonation of the
inner hydroxyl groups nor to decomposition. Removal of all
volatiles in vacuo restored the cluster 6 by reprotonation.
Experimental Section
General Procedures
The SI contains the reports for the tested reactions with 6, as well as
selected spectra (1H, 27Al NMR, IR, ESI-MS) and tables (XRD, ESI-
MS).
All manipulations were carried out under an atmosphere of argon
using standard Schlenk techniques or in an argon-filled glovebox
(MBRAUN). THF, toluene, pentane and benzene were dried over
Solvona® and then distilled, degassed three times by freeze-pump-
thaw procedure and stored over activated 3 Å molecular sieve.
[26]
[27]
[Cp*Al]4
(1) and iPr2Si(OH)(µ-O)Si(OH)iPr2
were prepared
according to literature procedure. Deionized water was degassed
three times by freeze-pump-thaw procedure. The NMR spectra were
recorded at a Bruker AVANCE II 400, AVANCE II 500 NMR or a
Bruker DPX 300 spectrometer, all at 25 °C, except if otherwise
noted. The 1H NMR chemical shifts were referenced to residual
[D7]THF at  = 1.72 ppm or C6D5H at  = 7.16 ppm and the 13C NMR
chemical shifts were referenced to [D8]THF at  = 25.31 ppm or C6D6
at  = 128.06 ppm. The 29Si NMR spectra were referenced externally
to TMS at  = 0.0 ppm. 27Al{1H} NMR spectra were referenced
externally to AlCl3 in D2O at  = 0.00 ppm. 27Al MAS NMR spectra
have been recorded on a Bruker AVANCE400 spectrometer using a
2.5 mm MAS probe. The resonance frequency has been 104.6MHz
for 27Al. The MAS NMR spectra have been fitted by the program
DMFit.[28] Microanalyses were performed with a HEKAtech Euro EA
Elemental Analyzer. Infrared spectra were recorded at a Bruker
Vertex 70 spectrometer that was equipped with an ATR unit
(diamond).
When 1,8-bis(dimethylamino)naphthalene (‘proton-sponge’) was
used, no deprotonation was detected in [D8]THF after one day at
room temperature. However, when deuterated benzene was
used as solvent, deprotonation of the outer hydroxyl groups was
detected over the course of one day at room temperature. When
compound 6 was exposed to KOH, LiOH·H2O, or acetic acid,
decomposition takes place, and the 1H NMR spectra revealed
the formation of iPr2Si(OH)(µ-O)Si(OH)iPr2. As mentioned
above, the cluster
6 showed partial decomposition upon
exposure to an excess of water in [D8]THF over the course of
ten days. This decomposition was considerably faster when six
equivalents of NEt3 (relating to 6) were added and in that case
the decomposition was nearly completed after ten days (see SI).
Decomposition reactions outside of a certain pH range are not
surprising, considering also the amphoteric nature of Al(OH)3.
Accordingly, other aluminum hydroxide clusters are also only
stable in certain pH regions.[25]
Crystal Structure Determination
Diffraction data were collected on a Bruker D8 Venture spectrometer
at -173 °C using Mo-K ( = 0.71073 Å) radiation. Multi-scan
absorption corrections using SADABS[29] were applied on the data.
The structures were solved by intrinsic phasing (SHELXT-2013)[30]
and refined with the full matrix least squares method on F²
(SHELXL-2014, via ShelXle[31]). The hydrogen atoms were placed at
calculated positions and refined by using a riding model, except for
the protons on the hydroxyl groups in 5 and 6 as well as the amine
bound proton in 5. These were found in the electron density
difference fourier map and refined freely for 5 or refined with DFIX
Conclusions
The reaction of [Cp*Al]4 (1) towards various silanols revealed
that the subvalent Al atom is able to react with the OH moiety of
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800564
European Journal of Inorganic Chemistry
FULL PAPER
restraints for 6·toluene and 6·(3 THF·H2O)1.5. Due to strong disorder
of the iso-propyl groups in the solid structure of 6, the data resolution
quality was limited and in 6·(3 THF·H2O)1.5 one carbon atom could
not be refined anisotropically, while in 6·toluene one carbon atom
has an elongated ADP shape, but could not be better modelled.
Similar behavior was also observed in the outer residues of the only
published comparable structure [Al7(OH)9{(OSiPh2)2O}6] (CCDC
971744). As these problems occur in the outer sphere of the
molecule, it is only a minor influence on the central AlOSi
framework. The crystallographic data are summarized in Table S1 in
the SI. Supplementary crystallographic data for this paper have been
uploaded to the CCDC with the access numbers 1825941 for
6·(3 THF·H2O)1.5, 1825942 for 6·toluene and 1825943 for 5.
1630 (m), 2871 (w), 2906 (w), 2929 (w), 2971 (m) cm-1; ESI-MS: m/z
(%) = 961.56 (100) [M+Na+], 839.47 (47) [M-(C7H10N2)+Na+].
Synthesis of [HNEt3][Al(OSiPh3)4] (4)
NEt3 (0.25 mL, 1.8 mmol) and 8 mL toluene were added to a mixture
of [Cp*Al]4 (65 mg, 0.10 mmol) and HOSiPh3 (446 mg, 1.61 mmol). A
bubbler was connected and the solution stirred at 80 °C for 10
minutes. After filtration, the residue was washed two times with 2 mL
toluene. After dissolving the residue in 5 mL THF and filtration, 7 mL
of pentane were added to the solution, whereby a colorless solid
precipitated. The solvent was then removed by filtration and the
precipitate dried in vacuo, yielding [HNEt3][Al(OSiPh3)4] (4) as a
colorless solid (354 mg, 0.284 mmol, 71% yield).
3
1H NMR (400.1 MHz, [D8]THF, 25 °C):  = 7.57 (d, J(H,H) = 8 Hz,
3
3
Reaction of [Cp*Al]4 (1) and HOSi(OtBu)3
24 H, o-Ar), 7.04 (t, J(H,H) = 8 Hz, 12 H, p-Ar), 6.80 (t, J(H,H) = 8
Hz, 24 H, m-Ar), 2.80 (q, 3J(H,H) = 7 Hz, 6 H, NCH2), 0.96 (t,
3J(H,H) = 7 Hz, 9 H, NCH2CH3) ppm; 13C NMR (100.6 MHz, [D8]THF,
25 °C):  = 142.4, 136.4, 128.0, 127.5, 47.6, 9.4 ppm; 27Al{1H} NMR
(104.3 MHz, [D8]THF, 25 °C):  = 54.8 ( ≈ 10 Hz) ppm; 1H, 29Si
HMBC (400.1/79.5 MHz, [D8]THF, 25 °C):  = 7.57/-30 ppm; IR
(neat, ATR/Diamond): ν̃ = 430 (m), 443 (m), 465 (m), 516 (s), 613
(w), 631 (w), 698 (s), 997 (w), 1024 (m), 1061 (m), 1108 (s), 1427
(w), 2996 (w), 3044 (w), 3063 (w), 3144 (w) cm-1; ESI-MS: m/z (%) =
[Cp*Al]4 (1) (5.2 mg, 0.0080 mmol) was dissolved in 0.4 mL C6D6
and in a second Schlenk tube HOSi(OtBu)3 (16.6 mg, 0.0628 mmol)
was dissolved in 0.6 mL C6D6. Both solutions were heated to 80 °C
and after stirring for 2 minutes, the hot HOSi(OtBu)3 solution was
quickly transferred into the solution of [Cp*Al]4 (1) via a cannula,
upon which a gas evolution occured. After stirring for 5 minutes, the
flask was immediately cooled down to room temperature with a cold
water bath and the solution was transferred into a NMR tube.
Signals are assigned to [Al(OSi(OtBu)3)3] (A), HOSi(OtBu)3 (S),
Cp*H (C) or [Cp*Al]4 (1) or belong to yet unidentified species (X).
1H NMR (500.1 MHz, C6D6, 25 °C):  = 4.47 (s, H2), 2.42 (q, 3J(H,H)
= 8 Hz, C), 1.92 (s, X), 1.90 (s, 1), 1.81 (s, C), 1.75 (s, C), 1.50 (s,
tBu, A), 1.39 (s, S), 1.00 (d, 3J(H,H) = 8 Hz, C); 13C NMR
(125.8 MHz, C6D6, 25 °C):  = 137.7 (C), 134.6 (C), 114.1 (X), 72.8
(A), 72.7 (X), 72.5 (X), 72.1 (X), 52.0 (C), 32.3 (X), 32.2 (X), 32.1 (X),
32.0 (A), 31.6 (S), 14.3 (C), 11.8 (C), 11.3 (C), 10.9 (Cp*, X) ppm;
27Al{1H} NMR (130.3 MHz, C6D6, 25 °C):  = -58.2 ( ≈ 340 Hz, X), -
79.4 ( ≈ 110 Hz, 1) ppm; 1H, 29Si HMBC (500.1/99.4 MHz, C6D6,
25 °C):  = 1.50/-95 (A), 1.39/-90 (S) ppm.

1127.33 (100) [Al(OSi(C6H5)3)4 ] (negative mode), 102.13 (100)
+
[HN(C2H5)3 ] (positive mode).
Synthesis of [HNEt3][Al(OH)(OSiPh3)3] (5)
a) H2O (1.2 µL, 0.067 mmol), NEt3 (0.02 mL, 0.1 mmol), and 2.5 mL
THF were added to [HNEt3][Al(OSiPh3)4] (4) (77 mg, 0.063 mmol) in
a Schlenk tube. The suspension was stirred at 60 °C for 15 hours.
After removal of the solvent in vacuo, the colorless sticky residue
was extracted with 15 mL diethyl ether. The solution was then
concentrated to about 2 mL and cooled down with an ice bath, which
lead to a colorless precipitate. The supernatant solution was filtered
off, the residue was washed two times with 0.5 mL cold diethyl ether
and dissolved in 1 mL THF. Storing the solution in the freezer
Synthesis of [Al{OSi(OtBu)3}3(DMAP)] (3)
at -26 °C for
3 days yielded [HNEt3][Al(OH)(OSiPh3)3] (5) as
[Cp*Al]4 (1) (100 mg, 0.154 mmol), HOSi(OtBu)3 (490 mg,
colorless crystals (81 mg, 0.040 mmol, 43% yield). Crystals suitable
1.85 mmol)
and 4-dimethylaminopyridine
(DMAP)
(75 mg,
for XRD measurements were obtained by evaporation of
concentrated THF solution exposed to air.
a
0.61 mmol) were dissolved in 10 mL toluene. The mixture was
heated to 80 °C and after stirring for 5 minutes, gas evolution
stopped and decolorization of the solution had occured. After stirring
for 10 minutes, heating was stopped and the solution was brought to
room temperature. All volatiles were removed in vacuo and the
resulting colorless solid was dissolved in 0.5 mL pentane. After
filtration, the solution was stored at  40 °C for 3 days to obtain
colorless crystals of (DMAP)Al(OSi(OtBu)3)3 (3) (249 mg, 265 mmol,
43%). The compound is highly soluble in apolar solvents like
benzene and pentane, poorly soluble in ethers like diethyl ether and
THF, but is insoluble in water.
b) After dissolving [Al(OSiPh3)3(H2O)] (176 mg, 0.202 mmol) in 4 mL
diethyl ether, NEt3 (0.04 mL, 0.3 mmol) was added, and the mixture
was afterwards stirred for two hours. The flask was cooled with an
ice bath and the reaction mixture after 20 minutes filtered. The
filtrate was dried in vacuo. [HNEt3][Al(OH)(OSiPh3)3] (5) (143 mg,
0.147 mmol, 73%) was obtained as a colorless powder.
3
1H NMR (300.1 MHz, [D8]THF, 25 °C):  = 7.59 (d, J(H,H) = 7 Hz,
3
3
18 H, o-Ar), 7.16 (t, J(H,H) = 7 Hz, 9 H, p-Ar), 7.00 (t, J(H,H) = 7
Hz, 18 H, m-Ar), 2.49 (q, 3J(H,H) = 7 Hz, 6 H, NCH2), 0.83 (t,
3J(H,H) = 7 Hz, 9 H, NCH2CH3) ppm; 13C NMR (75.5 MHz, [D8]THF,
25 °C):  = 141.3, 136.2, 128.9, 127.8, 45.7, 8.9 ppm; 27Al{1H} NMR
3
1H NMR (300.1 MHz, C6D6, 25 °C):  = 9.00 (d, J(H,H) = 7 Hz, 2 H,
o-DMAP), 6.20 (d, 3J(H,H) = 7 Hz, 2H, m-DMAP), 1.88 (s, 6H,
N(CH3)2), 1.59 (s, 81 H, tBu) ppm; 13C NMR (75.5 MHz, C6D6,
25 °C):  = 155.7 (DMAP), 148.6 (DMAP), 105.6 (DMAP), 71.7
(OC(CH3)3, 38.0 (N(CH3)2), 32.3 (OC(CH3)3) ppm; 27Al MAS NMR
(104.3 MHz, neat, 25 °C, rot = 20 kHz):  = 55.0 (Q = 1.669 MHz,
1
(104.3 MHz, [D8]THF, 25 °C):  = 59.2 ( ≈ 270 Hz) ppm; H, 29Si
HMBC (400.1/79.5 MHz, [D8]THF, 25 °C):  = 7.59/-27.6 ppm; IR
(neat, ATR/Diamond) ν̃ = 430 (m), 445 (m), 473 (m), 507 (s), 514 (s),
617 (w), 636 (w), 669 (m), 698 (s), 743 (m), 997 (w), 1023 (m), 1045
(m), 1107 (m), 1427 (m), 2000-2550 (br), 2550-2760 (br), 2997 (w),
3020 (w), 3045 (w), 3064 (w), 3140 (w), 3673 (w) cm-1; ESI-MS: m/z

Q = 0.05) ppm; 1H, 29Si NMR HMBC)(500.1/99.4 MHz, C6D6, 25 °C):
 = 1.59/-97 ppm; IR (neat, ATR/Diamond): ν̃ = 405 (w), 430 (w), 472
(m), 480 (w), 507 (w), 514 (w), 569 (w), 583 (w), 659 (w), 698 (m),
769 (w), 826 (m), 1021 (s), 1042 (s), 1072 (m), 1134 (w), 1193 (m),
1238 (m), 1361 (m), 1386 (w), 1459 (w), 1473 (w), 1551 (w),

(%) = 869.26 (100) [Al(OH)(OSi(C6H5)3)3 ] (negative mode), 102.13
+
(100) [HN(C2H5)3 ] (positive mode).
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800564
European Journal of Inorganic Chemistry
FULL PAPER
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Synthesis of [Al7(OH)9{(OSiiPr2)2O}6] (6)
10mL toluene and 0.1 mL THF were added to a mixture of [Cp*Al]4
(65 mg, 0.10 mmol) and iPr2Si(OH)(µ-O)Si(OH)iPr2 (96 mg,
0.34 mmol) in a Schlenk flask. Then H2O (10 µL, 0.56 mmol) was
added and the reaction solution was stirred at 95 °C for 48 hours.
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1H NMR (400.1 MHz, [D8]THF, 25 °C):  = 7.87 (s, 3 H, OH), 5.55 (s,
6 H, OH), 1.21-0.79 (m, iPr) ppm; 13C NMR (75.5 MHz, [D8]THF, 25
°C):  = 19.6, 19.4, 19.1, 19.0, 18.9, 18.6, 18.6, 18.4, 16.6, 16.4,
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50 °C):  = -23.0, -24.6 ppm; 27Al MAS NMR (104.3 MHz, neat, 25
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2865 (m), 1463 (m), 1385 (w), 1365 (w), 1245 (w), 1162 (w), 1085
(m), 1033 (s), 985 (s), 884 (s), 717 (m), 692 (s), 642 (m), 609 (m),
502 (m), 462 (m), 441 (m), 405 (m) cm-1; ESI-MS: m/z (%) = 1999.85
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43.22, H 8.92; found: C 43.88, H 8.95.
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10.1002/ejic.201800564
European Journal of Inorganic Chemistry
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Philipp Wittwer, Adrian Stelzer, Thomas
Braun*
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Reactivity of Cp*Al Towards Silanols:
Formation and Hydrolysis of
Alumosiloxanes
The reaction of [Cp*Al]4 with various silanols was studied. Treatment with a
siloxanediol gave [Al7(OH)9{(OSiiPr2)2O}6] (6), which exhibits OH exchange
reactions of the outer hydroxide groups in the presence of H218O.
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