Trapping Aluminum Hydroxide Clusters with Trisilanols during Speciation in Aluminum(III)–Water Systems: Reproducible, Large Scale Access to Molecular Aluminate Models

Article abstract of DOI:10.1002/anie.201604305

To gain molecular level insights into the properties of certain functions and units of extended oxides/hydroxides, suitable molecular model compounds are needed. As an attractive route to access such compounds the trapping of early intermediates during the hydrolysis of suitable precursor compounds with the aid of stabilizing ligands is conceivable, which was tested for the aluminum(III)/water system. Indeed, trisilanols proved suitable trapping reagents: their presence during the hydrolysis of AlⁱBu₂H in dependence on the amount of water used allowed for the isolation of tri- and octanuclear aluminum hydroxide cluster complexes [Al₃(μ₂-OH)₃(THF)₃(PhSi(OSiPh₂O)₃)₂] (1) and [Al₈(μ₃-OH)₂(μ₂-OH)₁₀(THF)₃(p-anisylSi(OSiPh₂O)₃)₄] (2). 1 can be regarded as the Al(OH)₃cyclic trimer, where six protons have been replaced by silyl residues. While 2 features a unique [Al₈(μ₃-OH)₂(μ₂-OH)₁₀]¹²⁺core. In contrast to most other known aggregates of this type, 1 and 2 can be readily prepared at reasonable scales, dissolve in common solvents, and retain an intact framework even in the presence of excessive amounts of water. This finding paves the way to future research addressing the reactivity of the individual functional groups.

Full text of DOI:10.1002/anie.201604305

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International Edition: DOI: 10.1002/anie.201604305
German Edition: DOI: 10.1002/ange.201604305
Aluminate Models
Hot Paper
Trapping Aluminum Hydroxide Clusters with Trisilanols during
Speciation in Aluminum(III)–Water Systems: Reproducible, Large
Scale Access to Molecular Aluminate Models
Kapil Shyam Lokare, Nicolas Frank, Beatrice Braun-Cula, Itziar Goikoetxea, Joachim Sauer,
and Christian Limberg*
Dedicated to Professor Hans-Joachim Freund on the occasion of his 65th birthday
Abstract: To gain molecular level insights into the properties of
certain functions and units of extended oxides/hydroxides,
suitable molecular model compounds are needed. As an
attractive route to access such compounds the trapping of
early intermediates during the hydrolysis of suitable precursor
compounds with the aid of stabilizing ligands is conceivable,
which was tested for the aluminum(III)/water system. Indeed,
trisilanols proved suitable trapping reagents: their presence
during the hydrolysis of AliBu2H in dependence on the amount
of water used allowed for the isolation of tri- and octanuclear
aluminum hydroxide cluster complexes [Al3(m2-OH)3(THF)3-
aluminates and aluminosilicates,[5] which account for a big
part of the reactivity of soil.
We were thus interested in trapping early intermediates
formed during the hydrolysis of suitable aluminum precursor
compounds to access well-defined species that promise to
provide valuable information. In the long term, the latter
could help to set the ground even for control of speciation. We
hoped to achieve trapping by 1) carefully dosing water into
anhydrous solvents, 2) adding suitable stabilizing ligands that
would saturate some valencies around the aluminum centers
with ligands/residues confining aggregation and reactivity at
the remaining sites. The resulting compounds could then be
regarded as molecular models for extended oxides/hydrox-
ides, giving way to studies seeking insights on the properties of
certain functions and units on the molecular level.
(PhSi(OSiPh2O)3)2]
(1)
and
[Al8(m3-OH)2(m2-OH)10-
(THF)3(p-anisylSi(OSiPh2O)3)4] (2). 1 can be regarded as
the Al(OH)3 cyclic trimer, where six protons have been
replaced by silyl residues. While
2 features a unique
[Al8(m3-OH)2(m2-OH)10]12+ core. In contrast to most other
known aggregates of this type, 1 and 2 can be readily prepared
at reasonable scales, dissolve in common solvents, and retain
an intact framework even in the presence of excessive amounts
of water. This finding paves the way to future research
addressing the reactivity of the individual functional groups.
As a suitable aluminum precursor we chose AliBu2H.
Polysilanols were considered appropriate trapping ligands
because, from the vast amount of aluminosilicates known
from nature, it is obvious that siloxides provide favorable
environments for aluminum cations, and this also translates
into synthetic molecular chemistry.[6]
Recently, we reported a new tripodal trisilanol ligand
precursor PhSi(OSiPh2OH)3 (LH3), which seemed suitable
for the purposes outlined above.[7] Its synthesis involved
dioxirane as an oxidant for Si-H functional groups in the last
step, though, which is a difficult reagent to prepare and handle
and limits the amount of product that can be obtained. Hence,
we have developed an alternative procedure for this step and
have also utilized the resulting route to access derivatives,
such as a representative with an p-anisyl residue in place of
the phenyl residue at the bridgehead silicon atom, which was
desirable for various reasons: 1) while LH3 does not show any
T
he world around us is determined by metal oxides, which
are ubiquitous in nature and in our engineered everyday life.
A plethora of different applications depend on metal oxides,
which is not surprising as a great variety is at our disposal.
Typically metal oxides are produced from aqueous solution,
both in materials science and geochemistry,[1] and some of
their properties can already be determined at rather early
stages[2] during speciation and nucleation.[3] Hence, these
processes receive a lot of attention. Within recent years,
application of modern analytical methods have led to
a significant amount of progress in clarifying the behavior
of molecular silicates in water as they assemble into solid
silica.[4] Far less is known in this context concerning
1
signal in the aliphatic region of the H NMR spectrum (the
aromatic region of which is rather complex), p-anisylSi-
(OSiPh2OH)3 (L’H3) exhibits an additional singlet at higher
field that can serve as a fingerprint for its reaction products, as
well as to monitor reactivity, and 2) introduction of the
methoxy group alters the crystallization properties. The
synthesis of both ligands was accomplished by following the
protocol delineated in Scheme 1. In the decisive last steps the
trisilanes were simply converted into the corresponding
trisilanols using 10% Pd/C in aqueous degassed THF. Large
reaction scales and relatively inexpensive starting materials
make LH3, and L’H3 (see the Supporting Information for the
[*] Dr. K. S. Lokare, N. Frank, Dr. B. Braun-Cula, Dr. I. Goikoetxea,
Prof. Dr. J. Sauer, Prof. Dr. C. Limberg
Humboldt-Universitꢀt zu Berlin, Institut fꢁr Chemie
Brook-Taylor-Straße 2, 12489 Berlin (Germany)
E-mail: christian.limberg@chemie.hu-berlin.de
Homepage: http://www2.hu-berlin.de/chemie/aglimberg/
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201604305.
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Scheme 1. Synthesis of LH3 and L’H3.
crystal structure of L’H3) some of the more readily prepared
tridentate trisilanol ligand precursors available.[8]
Figure 1. a) Molecular structure of 1·(THF) and b) its core motif.
Hydrogen atoms and an additional disordered solvent molecule are
omitted for clarity. Atoms: silicon (gray), oxygen (red), aluminum
LH3 and L’H3 were then tested with regard to their
potential to trap aluminum hydroxide clusters formed upon
hydrolysis of AliBu2H, on the way to solid Al(OH)3 or
AlOOH. Treating a solution of LH3 with 1.5 equiv of AliBu2H
in THF/hexane (1:1) in the presence of equimolar amounts of
water (relative to aluminum), afforded a colorless product
ꢀ
(yellow). Selected average bond lengths (ꢂ): Al O(H), 1.790(2); ca.
ꢀ
ꢀ
O(H) O(THF), 2.579(5); Al O(Si), 1.707(2). Selected average bond
angles (8): (Si)O-Al-O(Si), 112.25(12); Al-O(H)-Al, 137.96(14);
O(H)-Al-O(H), 101.73(11).
that
was
identified
as
[Al3(m2-OH)3(THF)3(PhSi-
(OSiPh2O)3)2] (1; Scheme 2) by multinuclear NMR tech-
significant Brønsted acidity.[9] Such acidity should be expected
for a faithful model of bridging Al-O(H)-Al units in oxidic
surroundings as part of solids or surfaces, according to
Paulingꢀs electrostatic valence rule.[10] For terminal (-O)3Si-
OH and (-O)2Al-OH groups the ionic bond strengths of the
protons are described by Equation 1 and 2, respectively:
4
4
ð1Þ
ð2Þ
ꢀ2ðOÞ þ ðSiÞ ¼ ꢀ1
3
ꢀ2ðOÞ þ ðAlÞ ¼ ꢀ1
3
For a zeolitic Brønsted site, Si-O(H)-Al, the ionic bond
strength is described by Equation 3,
Scheme 2. Synthesis of the trinuclear aluminum cluster 1. Reagents:
H2O (1.5 equiv), AliBu2H (1.5 equiv).
4
4
3
4
ð3Þ
niques, infrared spectroscopy, and X-ray diffraction studies.
Block-shaped crystals of 1·(THF) were grown by slow
diffusion of hexane into a concentrated THF solution. The
molecular structure of 1 is depicted in Figure 1. Complex 1
can be regarded as the Al(OH)3 cyclic trimer, where six
protons have been replaced by silyl residues provided by two
clamping tripodal ligands. Well-characterized compounds
containing aluminum hydroxide moieties supported by
siloxide ligands are rare.[6b,e,f] Similarly, molecular aluminum
hydroxides with tetra-coordinated aluminum atoms in
general are scarce, as due to their Lewis and Brønsted acidity
Al-OH units tend to aggregate, forming AlO6 octahedra. The
core motif of 1 is remarkable. The protons belonging to the
Al-OH units have been found in the electron density map and
were freely refined; they interact with three THF molecules
(see Figure 1; a further THF molecule co-crystallizes without
a pronounced interaction). Notably, the Al-OH groups show
a single sharp resonance at 9.76 ppm in the 1H NMR [D8]THF
spectrum. In comparison to other molecular aluminum
hydroxide compounds, this chemical shift is quite high, even
if hydrogen bonding with the solvent THF is taken into
account (see the Supporting Information) and indicative of
ðSiÞꢀ2ðOÞ þ ðAlÞ ¼ ꢀ0:25
while for a bridging Al-O(H)-Al, as in the present case,
Equation 4 is used to determine the ionic bond strength:
3
4
3
4
ð4Þ
ðAlÞꢀ2ðOÞ þ ðAlÞ ¼ ꢀ0:5
Therefore, the acidity of Al-O(H)-Al entities are expected to
be lower than the Si-O(H)-Al units in zeolites but higher than
that of a terminal Si-OH or Al-OH group. The fact that the
1H NMR spectrum also shows signals corresponding to
unbound THF, indicates facile exchange of the THF
molecules observed to interact with each of the three
hydroxide groups in the solid state.
The hexagonal Al3(OH)3 unit is planar, with the average
intra-ring angle at oxygen (137.96(14)8) being significantly
larger than the one at aluminium (101.73(11)8) and also than
the Al–O–Al angles either found in the gas-phase electron
diffraction study on [Me2Al(m2-OMe)]3 (125.808),[11] or calcu-
lated for the model compound [H2Al(m2-OH)]3 (127.808).[12]
This is due to the constraints caused by the bulky trisiloxy
2
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ligand framework, which requires a slight increase in the
ꢀ
intra-ring Al–Al distance, while the Al O bond lengths
remain fixed.
To further analyze the nature of the Al3(OH)3 core, DFT
calculations were carried out. A structure optimization was
performed for the full unit cell of 1·(THF), as shown in
Figure 1. The obtained bond distances and angles are
provided in the Supporting Information (Table S2).
There is a very good agreement between experimental
and calculated bond distances: the differences are below
0.02 ꢁ for the three intra-ring Al–Al distances, and below
Scheme 3. Synthesis of the octanuclear aluminum cluster 2. Reagents:
ꢀ
0.01 ꢁ for the average Al O bond length. For the three intra-
H2O (3 equiv), AliBu2H (2 equiv).
ring angles at oxygen and at aluminum, the differences
between DFTand X-ray diffraction results are in the range of
0.4 to 2.38.
ꢀ
For the O(H) O(THF) contacts, the X-ray analysis shows
one shorter (O9···O18) and two slightly longer (0.04 ꢁ; O7···O16,
O8···O17) distances. The PBE + D results for these distances
are too short (0.04–0.07 ꢁ), which is a known feature of this
functional.[13]
The position of protons is typically difficult to derive from
X-ray diffraction data but is readily available from DFT
structure optimization. The calculations show that the bridg-
ing Al-O(H)-Al hydroxide groups form strong hydrogen
bonds with THF, but the protons are not transferred.
Evidence for the presence of strong hydrogen bonds is
also provided by the red-shifted OH stretching frequencies
associated with the bridging Al-O(H)-Al groups. Using the
w/rOH correlation of Nachtigall[14] and the OH bond distances,
wavenumbers of 2938, 2791, and 2728 cmꢀ1 were obtained for
O7H (1.028 ꢁ), O8H (1.043 ꢁ), and O9H (1.044 ꢁ), respec-
tively (unfortunately, these vibrational energies could not be
determined in the experimental spectrum because of severe
masking). These values can be compared with computed
wavenumbers for bridging hydroxide groups in different types
of aluminosilicates.
Figure 2. a) Molecular structure of 2 and b) its core motif depicted
with a ligand skeleton. Hydrogen atoms and disordered solvent
molecules are omitted for clarity. Atoms: silicon (gray), oxygen (red),
ꢀ
aluminum (yellow). Selected bond lengths (ꢂ): Al4 O19, 1.944(2);
ꢀ
ꢀ
ꢀ
Al3 O20, 1.897(2); Al1 O13, 1.726(2); Al2 O18, 1.794(2). Selected
bond angles (8):O14-Al-O13, 111.62(13); Al2-O16-Al3, 126.45(11).
in tetrahedral ligand environments. The first set (Al3 and
Al3’) corresponds to aluminum atoms bonded to neighboring
aluminum centers through four m2-OH units (average
ꢀ
ꢀ
The predicted wavenumbers for our bridging AlIV-O(H)-
AlIV group in 1 fit well with the experimentally obtained IR
spectra for the interaction of THF with the H-b zeolite.[15] The
band associated with the acidic Si-O(H)-AlIV site at 3614 cmꢀ1
underwent a red-shift to 2900(ꢁ 50) cmꢀ1 upon adsorption.
An obvious question was now: what sort of product is
obtained if the water content is increased in the system that
leads to 1? It was expected that more bonds would be
hydrolyzed and a larger cluster obtained. And indeed,
employing 1.5 equiv of water (with respect to aluminum)
led to the formation of an octanuclear cluster, which only
crystallized when L’H3 (instead of LH3) was employed (see
Scheme 3). Crystals of [Al8(m3-OH)2(m2-OH)10(THF)3(p-
anisylSi(OSiPh2O)3)4] (2) suitable for an X-ray diffraction
study were obtained by slow diffusion of hexane into
a concentrated THF/toluene (4:1) solution; Figure 2 shows
the derived molecular structure. The X-ray diffraction
analysis revealed the formation of an octanuclear
aluminum(III) complex containing an [Al8(m3-OH)2(m2-
OH)10]12+ core and the remaining twelve valencies around
these aluminum centers are saturated by four tripodal ligands,
thus making it an overall neutral species. Complex 2 contains
four unique aluminum centers; two are in octahedral and two
Al OH, 1.888(2) ꢁ) and one m3-OH ligand (Al OH,
1.928(2) ꢁ), and the octahedral coordination sphere is
ꢀ
completed by a THF donor (Al O(THF), 1.940(2) ꢁ). The
second set (Al4 and Al4’) also consists of six-coordinate
aluminum atoms bonded to three m2-OH ligands (average
ꢀ
ꢀ
Al OH, 1.870(2) ꢁ), two m3-OH ligands (average Al OH,
ꢀ
1.944(2) ꢁ) and a siloxide unit (Al O(Si), 1.7855(19) ꢁ). A
third pair (Al1 and Al1’) is only four-coordinate, with
aluminum atoms connected to two arms of the same ligand
backbone (average Al O(Si), 1.725(3) ꢁ), one arm of
a second ligand backbone (Al O(Si), 1.743(2) ꢁ), and one
m2-OH unit (Al OH, 1.810(2) ꢁ). Finally, a fourth set (Al2
and Al2’) is surrounded by two arms of the same ligand
backbone (average Al O(Si), 1.710(2) ꢁ), and two m2-OH
units (average Al OH, 1.788(2) ꢁ). All Al OH distances
concur with distances observed previously (see the Support-
ing Information).
Of course, a few aluminum hydroxide clusters have been
reported in the literature already,[16,6f] mostly unligated
polyoxoaluminates, and one of them published by Casey
and co-workers also contained eight Al atoms:
[Al8(m3-OH)2(m2-OH)12(H2O)18]10+(aq.) was prepared from
aluminum metal, aqueous H2SO4, and catalytic amounts of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
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seven years.[16k] This example illustrates that compounds of
this kind were usually obtained after long reaction times,
often in low yields, some only through crystal picking, and
solution studies could not be performed: upon re-dissolution
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study, and upon dissolution it stays intact, as evidenced by its
NMR spectra. The structure depicted in Figure 2 contains six
unique OH ligands, and indeed the 1H NMR spectrum of 2—
besides a complex resonance pattern in the aromatic region—
is characterized by six individual -OH signals observed at
5.37, 4.82, 3.23, 3.09, 2.80, and 1.99 ppm (see the Supporting
Information); their lower chemical shifts, compared to those
found for 1, can be rationalized by the fact that the
surrounding aluminum ions are primarily in octahedral
environments and thus possess lower acidity according to
Paulingꢀs rules. Only two 29Si NMR resonances at ꢀ75.5 and
ꢀ74.5 ppm were observed for the bridgehead Si atoms of the
four ligands around the Al8 core of complex 2 indicating that
also in solution they are pairwise equivalent. Furthermore,
with NMR spectroscopy it was shown that upon treatment of
such solutions with excessive water the framework of the
complex remained intact. That is, no irreversible cleavage
reactions take place, thereby allowing the reversible water
reactivity of such clusters to be studied; the same is true for 1.
In conclusion, our new synthetic approach offers access to
an exclusive class of compounds that can be readily prepared
at reasonable scales, and which may serve as kinetic models
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water interfaces in common organic solvents. Also investiga-
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Acknowledgements
We are grateful to The Collaborative Research Centre—CRC
1109 “Understanding of Metal Oxide/Water Systems at the
Molecular Scale: Structural Evolution, Interfaces, and
Dissolution”, and the Humboldt-Universitꢂt zu Berlin for
financial support. We wish to thank Dr. Andrea Zehl for
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Keywords: aluminum · aluminosilicates · clusters · hydrolysis ·
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Received: May 3, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Aluminate Models
K. S. Lokare, N. Frank, B. Braun-Cula,
I. Goikoetxea, J. Sauer,
C. Limberg*
&&&& — &&&&
Trapping Aluminum Hydroxide Clusters
with Trisilanols during Speciation in
Aluminum(III)–Water Systems:
Reproducible, Large Scale Access to
Molecular Aluminate Models
Picking out early aggregates: Tripodal
trisilanols stabilize aluminum hydroxide
aggregates formed during the hydrolysis
of AliBu2H with different sizes dependent
on the amount of water added.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!