Control of Polymorphisms and Functions in All-Inorganic Ionic Crystals Based on Polyaluminum Hydroxide and Polyoxometalates

Article abstract of DOI:10.1021/acs.cgd.6b00555

Two kinds of polymorphs of all-inorganic ionic crystals were obtained by the combination of oppositely charged molecular ions (macroions) with T_d symmetry, Keggin-type polyaluminum hydroxide [?-Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺ (Al₁₃) and polyoxometalates (POMs) [α-CoW₁₂O₄₀]^6- (POM6), [α-1,2,3-SiV^V₃W₉O₄₀]^7- (POM7), or [α-1,2,3-SiV^IVV^V₂W₉O₄₀]^8- (POM8) in aqueous solutions. The needle-type crystals were grown by dissolution-recrystallization of the precipitates, which formed immediately by combining Al₁₃ and POMs. On the other hand, the plate-type crystals were grown by adding inorganic salts (e.g., NaCl) into the aqueous solution containing the needle-type crystals: Dissolution of the needle-type crystals occurred followed by precipitation of the plate-type crystals, which is the more insoluble and stable polymorph with larger numbers of hydrogen bonds between the constituent ions. The plate-type crystals possess one-dimensional channels with large apertures (ca. 12 ? × 6 ?) and showed high activities as heterogeneous solid acid catalysts in pinacol rearrangement.

Full text of DOI:10.1021/acs.cgd.6b00555

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Article
Control of Polymorphisms and Functions in All-Inorganic Ionic
Crystals based on Polyaluminum Hydroxide and Polyoxometalates
Kosuke Mizuno, Takuto Mura, and Sayaka Uchida
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00555 • Publication Date (Web): 09 Aug 2016
Downloaded from http://pubs.acs.org on August 15, 2016
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Control of Polymorphisms and Functions in All-
Inorganic Ionic Crystals based on Polyaluminum
Hydroxide and Polyoxometalates
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†
,‡
†,‡
†,§,*
Kosuke Mizuno, Takuto Mura, and Sayaka Uchida
†Department of Basic Sciences, School of Arts and Sciences, The University of Tokyo, 3-8-1
Komaba, Meguro-ku, Tokyo 153-8902, Japan.
§JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
Abstract
Two kinds of polymorphs of all-inorganic ionic crystals were obtained by the combination of
oppositely charged molecular ions (macroions) with T
d
symmetry, Keggin-type polyaluminum
hydroxide [-Al13
O
4
(OH)24(H
2
O)12]7+ (Al13) and polyoxometalates (POMs) [-CoW12 40]6
O
V
7
IV
V
W O
40]8 (POM8), in aqueous
(POM6), [-1,2,3-SiV
3
W
9
O
40] (POM7), or [-1,2,3-SiV V
2
9
solutions. The needle-type crystals were grown by dissolution-recrystallization of the precipitates,
which formed immediately by combining Al13 and POMs. On the other hand, the plate-type
crystals were grown by adding inorganic salts (e.g., NaCl) into the aqueous solution containing
the needle-type crystals: Dissolution of the needle-type crystals occurred followed by precipitation
of the plate-type crystals, which is the more insoluble and stable polymorph with larger numbers
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of hydrogen-bonds between the constituent ions. The plate-type crystals possess one-dimensional
channels with large apertures (ca. 12 Å × 6 Å), and showed high activities as heterogeneous solid
acid catalysts in pinacol rearrangement.
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Introduction
Polymorphism is the ability of a solid to exist in more than one crystal structure. Polymorphs
can exhibit different physicochemical properties, and one of the long-standing challenges in
science is prediction and control of polymorphism.1-5 In biomineralization, polymorph selection is
a key issue for survival: Mollusks selectively deposit a specific polymorph of CaCO
3
(calcite or
2
,6
aragonite) under the control of biopolymers. Polymorphism of CaCO
3
is one of the most studied
subjects in crystal engineering because of its abundant existence in biosphere and lithosphere, and
2,6
its wide industrial applications as fillers and construction materials. Polymorphism of TiO has
2
also been a popular topic due to its high photocatalytic activity, high stability, low cost, and low
7
toxicity. Various reaction conditions, such as concentrations of reactants and additives,
temperature, solvent have significant influence on the crystallization of these inorganic oxides. In
pharmacy, polymorphism of drug substances has been investigated intensively because drug
efficacy is strongly related to the solubility and stability of the polymorphs.4
Crystal engineering using molecular building blocks is an important line of research to
develop general strategies to form molecular crystals with various internal structures (i.e.,
polymorphs) and functions for applications in gas adsorption, heterogeneous catalysis, etc.3,5
Various kinds of interactions between the molecular building blocks, such as coordination bonds,
hydrogen bonds, ionic interaction, and van der Waals interaction have been explored. Toward this
end, molecular ions (macroions) with a Keggin-type structure have been a popular motif because
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of its easy synthesis and high molecular symmetry (T ), which may lead to formation of
d
polymorphs with different functions.8-12 Especially, Keggin-type polyoxometalate anions (POMs),
which are nano-sized anionic metal-oxygen clusters of early transition metals, have been
stimulating research in broad fields of sciences.13-16 Kwon et al., have studied the structures and
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water adsorption properties of porous ionic crystals composed of POMs and Keggin-type
7+ 17
polyaluminum hydroxide ([-Al13
O
4
(OH)24(H
2
O)12] , Al13).8-10 In one of their studies, two kinds

of polymorphs with Al13 and [-H
2
W
12
O
40]6 as building blocks crystallized from the same
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aqueous solution. Mellot-Dranznieks et al., have predicted the crystal structures of polymorphs
7

composed of [-PMo12
O
36(OH) ] , benzenedicarboxylates, and Zn(II) ions by computational
4
simulations, and some of the polymorphs were actually synthesized and applied as an
electrocatalyst for reduction of bromate.11,12 However, most of the studies involving polymorphism
have been carried out with the aid of organic molecules or polymers, which are designed to assist
the self-assembly process of building blocks.
According to these considerations, we attempted to control not only the polymorphism but
also the functions of all-inorganic porous ionic crystals. Two kinds of polymorphs (needle and
plate-type crystals) were obtained by the combination of oppositely charged macroions with T
d
6

V
W O
40]7 (POM7), or [-
symmetry (Al13 and POMs: [-CoW12
O
40] (POM6), [-1,2,3-SiV
3
9
IV
W O
40]8 (POM8)) in aqueous solutions. The needle-type crystals were grown by
1
,2,3-SiV VV
2
9
dissolution-recrystallization of the precipitates, which formed immediately by combining Al13 and
POMs. The plate-type crystals were obtained by adding inorganic salts (e.g., NaCl) into the
aqueous solution containing the needle-type crystals. It is well known in protein crystals that
addition of salts can tune the interactions among solute, solvent, and precipitate, and induces
1
8
polymorphism. Dissolution of the needle-type crystals occurred followed by precipitation of the
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plate-type crystals, which is the more insoluble and stable polymorph with larger numbers of
hydrogen-bonds between the constituent ions. The plate-type crystals possessed one-dimensional
channels with large apertures (ca. 12 Å × 6 Å) and showed high activities as heterogeneous solid
acid catalysts in pinacol rearrangement.
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Experimental Section
Synthesis
7+
The solution containing [-Al13
Into 200 mL of water, 12 g of AlCl ∙6H
solution A). Into 450 mL of water, 4.5 g of NaOH was added (0.25 mol L NaOHaq), and this
O
4
(OH)24(H O)12] (Al13) was prepared according to ref.19:
2

1
3
2
O was added (0.25 mol L AlCl aq) and stirred at 353 K
3

1
(
solution was slowly added to solution A over 15 min under continuous stirring. K [-
6
V
CoW12
O
40]∙nH
2
O (POM6) and K
6
H[-1,2,3-SiV
3
W
9
O
40]∙nH
2
O (POM7) were synthesized
O (POM8) was obtained by
according to previous reports.20,21
K
6
H
2
[-1,2,3-SiV V
IV
V
W
O40]∙nH
2
9
2
V
reducing K
6
H[-1,2,3-SiV
3
W
9
O
40]∙nH O with aqueous ascorbic acid.
2

1
The ionic crystals (Al13-POMs) were synthesized as follows: 20 mL of 6.0 mmol L aqueous
solution of the potassium salts of POM were added to 20 mL of Al13 solution, followed by
immediate and spontaneous formation of precipitates (solution B). Solution B was left under
ambient conditions, and needle crystals of POM6-N, POM7-N, and POM8-N were obtained
through dissolution-recrystallization within 3 days (yields were 70−90% based on POM).

1
Alternatively, 20 mL of 6.0 mmol L aqueous solution of the potassium salts of POM were added
to 20 mL of Al13 solution containing NaCl (1.2 g (20 mmol) for POM6 and 2.4 g (40 mmol) for
POM7 and POM8) (solution C). Solution C was left under ambient conditions, and plate crystals
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of POM6-P, POM7-P, and POM8-P were obtained within 3 days (yields were ca. 70−90% based
on POM). Al13-POMs were characterized by IR spectroscopy (Figure S1), diffuse-reflectance UV-
vis spectroscopy (Figure S2), solid state 27Al-MASNMR spectroscopy (Figure S3),
thermogravimetry (Figure S4), X-ray photoelectron spectroscopy (XPS) (Figure S5), elemental
analysis, and single crystal X-ray diffraction (XRD). The powder XRD patterns of Al13-POMs
were totally different from the raw materials (Figure S6). Plate-type crystals could be obtained by
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using additives other than NaCl (Figure S7: LiCl, KCl, RbCl, AlCl
3
) or by adding NaCl to solution
B.
Characterization
Inductively coupled plasma optical emission spectrometry (ICP-OES) (Agilent Technologies,
ICP-OES720) was used for the quantitative analysis of inorganic elements. Atomic absorption
spectroscopy (AAS) analysis (Hitachi, ZA3000) was used for the quantitative analysis of alkali
metals. See Figure S8 for the calibration curves of Na ( = 589 nm) and K ( = 766.5 nm). Prior
to the ICP-OES and AAS measurements, conc. HNO (1 mL) was added to ca. 10 mg of Al13-
3
POMs to dissolve the solid completely into water (100 mL). In order to check the absence of
chlorine, XPS was conducted with a JPS-90SX spectrometer (JEOL) equipped with a magnesium
anode (Mg KeV) operating at 8 kV-10 mA. IR spectra were measured by KBr pellet
method with a JASCO FT/IR 4100 spectrometer (JASCO) equipped with a TGS detector. Diffuse-

1
reflectance UV-vis spectra were measured in the range of 1250040000 cm (800250 nm) with
a V-550 UV-vis spectrometer (JASCO). The samples were prepared by grinding and diluting the
2
7
1
compounds (ca. 10 mg) with ca. 1 g of NaCl. Solid state Al and H-MASNMR spectra were
measured with a Bruker AVANCE III 400 WB spectrometer (Bruker) equipped with a 4 mm
standard probe operating at 400.18 MHz and a MAS rate of 10 kHz. Thermogravimetry was
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conducted with a Thermo Plus 2 thermogravimetric analyzer (Rigaku) with α-Al
2
O as a reference
3
−
1
under a dry N
2
flow (100 mL min ) in the temperature range of 303−773 K. Powder XRD patterns
were measured with a New advance D8 X-ray diffractometer (Bruker) by using Cu K radiation
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(
 = 1.54056 Å, 40 kV-40 mA) at 1.8 deg min . Scanning electron microscope (SEM) images
were obtained with JSM-6700F (JEOL) operating at an accelerating voltage of 5 kV. N (77 K)
2
adsorption isotherms were measured using a Belsorp-max volumetric adsorption apparatus (BEL
Japan). Prior to the measurement, about 0.1 g of crystals were ground and treated in vacuo at 298
K for 3 h.
Catalytic reaction
Reactions were carried out in a glass reactor equipped with a magnetic stirrer. In a typical run,
a mixture of pinacol (2.0 mmol), naphthalene (0.8 mmol, internal standard), and Al13-POM (0.2
mmol) in toluene (3 mL) were stirred under air at 373 K. The reaction progress was followed by
gas chromatography using a GC-2014 (Shimadzu) fitted with an InertCap 5 capillary column (GL
Sciences) and a flame ionization detector. As a reference, Na[-Al13
O
4
(OH)24(H
2
O)12](SO
4
)
4
(Al13-Sulfate) was synthesized and used as a solid catalyst under the same reaction condition. Al13
-

1
Sulfate was synthesized according to ref. 19: 250 mL of 0.1 mol L Na
2
SO was added to 340
4
mL of Al13 solution, and 2.0 g of colorless crystals of Al13-Sulfate were obtained after 1 day (see
Figure S9 for the crystal structure).
Single-crystal XRD and structure analysis
XRD data were collected at 93 K with a CCD 2D detector by using a Rigaku Mercury
diffractometer with graphite-monochromated MoK radiation ( = 0.71069 Å). The structures
were solved by direct methods (SHELX97), expanded by Fourier techniques, and refined by full-
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matrix least-squares against F with the SHELXL-2014 package. PLATON/ SQUEEZE was used
in the refinements. CCDC-1456905, 1456906, and 1456907 contains the supplementary
crystallographic data for POM6-N, POM7-N, and POM8-N, respectively. CCDC-1465341
contains the supplementary crystallographic data for POM6-P. Void analyses were carried out
with a structure visualization program Mercury (CCDC). Single crystals suitable for X-ray
structure analysis could not be obtained for POM7-P and POM8-P. The powder XRD patterns of
POM7-P and POM8-P were analogous to that of POM6-P, showing that these compounds
possess the same porous structure (see results and discussion for the details). Tables S1 and S2
show the selected bond distances and angles for POM6-N and POM6-P, respectively. Table S3
summarizes the possible hydrogen bonds (O-O < 3.0 Å) and their distances in POM6-N and
POM6-P.
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Elemental analysis
According to the results of thermogravimetry (Figure S4) and quantitative analysis of
inorganic elements, chemical formulae of Al13-POMs were determined as follows. POM6-N: [-
Al13
O
4
(OH)25(H
O, POM7-N: [-Al13
(OH)24(H O)12][-1,2,3-SiV
,2,3-HSiV 40]∙35H O, POM8-P: [-Al13
2
O)11][-CoW12
O
40]∙34H
(OH)24(H
40] ∙ 43H
(OH)24(H
2
O, POM6-P: [-Al13
O)12][-1,2,3-SiV
O, POM8-N: [-Al13
O)12][-1,2,3-HSiV
O
4
(OH)25(H
2
O)11][-CoW12
O ]
40
∙
42H
2
O
4
2
3
W
9
O
40] ∙ 32H
2
O, POM7-P: [-
(OH)24(H O)12][-
40]∙45H O.
Al13
O
4
2
3
W
9
O
2
O
4
2
1
3
W
9
O
2
O
4
2
3
W
9
O
2
AAS (K, Na), XPS (Cl), and ion-exchange chromatography (Cl) showed that Al13-POMs do not
contain Na, K, or Cl (see Figures S5 and S8 for the details). Elemental analysis for POM6-N
(
calc): Al 8.0 (7.7), Co 1.32 (1.29), W 50.6 (48.4). POM6-P: Al 8.0 (7.5), Co 1.30 (1.25), W 48.9
46.9). POM7-N: Al 8.8 (8.6), Si 0.64 (0.69), V 3.8 (3.7), W 40.5 (40.4). POM7-P: Al 8.6 (8.2),
(
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Si 0.75 (0.65), V 3.9 (3.6), W 39.6 (38.6). POM8-N: Al 8.2 (8.5), Si 0.68 (0.68), V 3.7 (3.7), W
3
9.8 (39.9). POM8-P: Al 7.8 (8.1), Si 0.65 (0.68), V 3.7 (3.5), W 38.2 (38.2).
Results and Discussion
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Blue needle-type crystals of POM6-N (Figure 1) were grown via dissolution-recrystallization
of the precipitate, which formed immediately by mixing the aqueous solution of Al13 with POM
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
27
([-CoW12O40] ). IR (Figure S1) and solid state Al-MASNMR (Figure S3) spectra confirmed
that the molecular structures of Al13 and POM were essentially retained in POM6-N.20-22 The
powder XRD pattern of the precipitate was analogous to that of POM6-N (Figure S10), showing
that the structures were basically the same. The crystal structure of POM6-N (Figure 1 and Table
6

1
) was analogous to that of the ionic crystal composed of Al13 and [-

W O40] reported by
12
Kwon et al.10,23 In the crystal structure of POM6-N, Al13 and POMwere arranged alternately in a
tetragonal cell by electrostatic interactions and hydrogen bonds. Straight channels existed along
the [001], [110], and [1-10] directions, which were interconnected to each other. The minimum
aperture of the channel was ca. 6 Å × 6 Å. Elemental analysis and thermogravimetry (Figure S4)

1
suggested that POM6-N contained about 34 mol mol of water of crystallization. Single crystal
X-ray structure analysis could locate 25 mol mol1 of water molecules, which existed in the
channels and were hydrogen-bonded to other water molecules and/or to the constituent ions.
When NaCl was added to the aqueous mixture of Al13 and POM, the precipitate dissolved
completely and blue plate-type crystals of POM6-P were formed (Figure 1 and Table 1).
Elemental analysis and single crystal X-ray structure analysis showed that POM6-P and POM6-
N are polymorphs. POM6-P crystallized in an orthorhombic cell, Al13 and POM each formed
columns along the a-axis, and these columns lined up along the c-axis by electrostatic interactions
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and hydrogen bonds. Two-dimensional layers were formed in the ac-plane, which stacked along
the b-axis, and disordered POMs existed in the interlayer space. As a result, one-dimensional
channels existed along the a-axis with a minimum aperture of ca. 6 Å × 12 Å. Elemental analysis
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
1
and thermogravimetry (Figure S4) suggested that POM6-P contained 42 mol mol of water of

1
crystallization, while only 14.5 out of 42 mol mol could be located with single crystal X-ray
structure analysis. The water of crystallization existed in the channels and were hydrogen-bonded
to other water molecules and/or to the constituent ions. Notably, POM6-P could be obtained with
additives other than NaCl (see Figure S7 for the powder XRD patterns of the products). The effects
of the additives will be discussed later.
Synthesis of porous ionic crystals was also carried out with other POMs having negative
V
7
IV
V
W O
40]8), which resulted in the
charges of 7 ([-1,2,3-SiV
3
W
9
O
40] ) or 8 ([-1,2,3-SiV V
2
9
formation of orange and black crystals of POM7-N and POM8-N, respectively. The powder XRD
patterns of POM6-N, POM7-N, and POM8-N (Figure 2a-d) were analogous. The crystal
structures of POM7-N and POM8-N were solved by single crystal X-ray analysis (Table 1).
Synthesis of polymorphs was also attempted by adding NaCl to the synthetic solutions of POM7-
N and POM8-N, which resulted in the formation of POM7-P and POM8-P, respectively. The
powder XRD patterns of POM6-P, POM7-P, and POM8-P (Figure 2e-h) were analogous. These
results show that anion charge has little effect toward the crystal structure in the range of 6 to
8.24,25

The crystal structures of the polymorphs were studied in detail. Figure 3a shows the void
analysis of POM6-N with a probe diameter of 4 Å (grid spacing 0.5 Å). Water of crystallization
was removed from the structure and hydrogen atoms of Al13 were geometrically added to improve
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the accuracy of estimation. The void volume was 24.3% (2365 Å per unit cell, Z = 4) of the unit
cell volume. As described above, straight channels exist along the [001], [110], and [1-10]
directions, which were interconnected to each other, and the minimum aperture was ca. 6 Å × 6 Å.
Figures 3b and 3c show the local structures and possible hydrogen bonds between the molecular
ions of POM6-N. A Al13 cluster is surrounded by four POMs in a tetrahedral arrangement, and
each POM has three hydrogen bonds (O-O distance of < 3.0 Å) with the Al13 cluster (see Table S3
for the distances). Therefore, the total number of hydrogen bonds per Al13 in POM6-N is twelve.
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Void analysis of POM6-P was carried out by the same method (Figure 4a). The void volume
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was 21.6% (8311 Å per unit cell, Z = 16) of the unit cell, which is slightly smaller than that of
POM6-N (24.3%) because of the disordered POMs. In contrast to POM6-N with interconnected
three-dimensional channels, POM6-P possesses one-dimensional channels along the a-axis with
a minimum aperture of ca. 6 Å × 12 Å. Figures 4b and 4c show the local structures and possible
hydrogen bonds between the molecular ions of POM6-P. As shown in Figure 4b, a Al13 cluster
has two POMs and two Al13 in the nearest neighbors. There are three and two hydrogen bonds
between Al13-POM and Al13-Al13, respectively. In addition, the Al13 cluster has ten hydrogen
2
6
bonds with the disordered POMs (Figure 4c) (see Table S3 for the distances). Therefore, the total
number of hydrogen bonds per Al13 in POM6-N is twenty, which is larger than that of POM6-N
(twelve).
The powder XRD patterns of POM6-N and POM6-P broadened after evacuation at rt (Figure
S11), showing that water of crystallization are necessary to maintain the porous structure. The
crystal structures can be restored by exposure to water vapor. POM6-N and POM6-P showed low
porosity and BET surface area based on N adsorption at 77 K (Figure S12), in accordance with
2
1
6
27,28
the broad powder XRD patterns. However, we and other groups
have reported that such kind
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of host-guest systems also show interesting properties in guest-exchange, proton conduction, acid
catalysis, etc.
The polymorphs were utilized as heterogeneous solid acid catalysts in pinacol rearrangement
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(
Scheme 1), which is a typical acid reaction. Al13 exhibits acidity and has been used as a pillar
in clay-supported catalysts.30,31 Table 2 summarizes the results of pinacol rearrangement with Al13
POMs as heterogeneous solid catalysts at 373 K. The catalytic activities of the raw materials (Al13
-
-
sulfate, POM6, POM7, and POM8) were low (conv. 48%, yield of pinacolone 25%), which is
probably due to the non-porous structure so that reaction occurs only at the particle surface (Figure
32
S9). The catalytic activities of needle-type crystals (conv. 2131%, yield of pinacolone 818%)
were higher than that of Al13-sulfate, suggesting that pinacol molecules could enter the porous
structure. Acetone was formed by the oxidative dissociation of pinacol, which is probably due to
the reduction of vanadium moieties in POM7-N and POM8-N. Notably, the catalytic activities of
plate-type crystals (conv. 4576%, yield of pinacolone 2654%) were higher than those of the
needles, suggesting that channels with large apertures (ca. 6 Å × 12 Å and ca. 6 Å × 6 Å for the
3
3
plates and needles, respectively) is favorable. The powder XRD pattern of POM6-P after the
reaction (Figure 2i) was analogous to that before the reaction (Figure 2f), showing that crystal
structure of the catalyst is maintained during the reaction. SEM images of POM6-N and POM6-
P before and after the reaction (Figure S13) were similar, suggesting that particle size does not
affect the catalytic activities.
The high catalytic activities of plate-type crystals may be due to not only the large aperture of
1
channels but also high acid strength. In order to characterize acidity, solid state H-MASNMR
spectroscopy was utilized. It is well established that strengths of Brønsted acid sites increase with
increase in chemical shift of the solid state H-MASNMR signal.34 1H-MASNMR spectra of
1
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POM6-N and POM6-P with Al13-sulfate as a reference are shown in Figure 5. Both polymorphs
gave a single signal at 4.7 ppm, showing that the states of protons and thus the acidity are
essentially the same. The lower chemical shift compared with Al13-sulfate (5.4 ppm) is probably
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due to the leveling effect caused by the large amounts of water of crystallization in the channels.
These results suggest that the high catalytic activity of the plate-type crystals is due to the large
aperture of the channels, so that pinacol molecules may enter and diffuse through the porous
structure smoothly and Al13 exposed to the inner pore surface could participate in the reaction.
Finally, the effects of additives toward the formation of polymorphs were considered in the
case of POM7-N and POM7-P. When NaCl was added to the aqueous mixture containing POM7-
N, POM7-N dissolved followed by crystallization of POM7-P. Figure S14 shows the photo
images of POM7-N in aqueous solutions of sodium salts (NaCl, Na
2
SO
4
or Na
2
CO ). The solution
3
is colorless and transparent without any sodium salts showing that POM7-N is insoluble in water.
On the other hand, the solutions turned orange and turbid by the addition of sodium salts, showing
that POM7-N dissolved back into the solution. Focusing on the types of anion of the sodium salts,

2
turbidity of the solution or solubility of POM7-N increased in the order of Cl < SO
CO 2which is in line with the hardness of the anions as a Lewis base. Figure S15 shows the
photo images of POM7-N in aqueous solutions of chlorides (KCl, NaCl, or CaCl ). The solutions
4
<
3
2
turned orange and turbid by the addition of chlorides, showing that POM7-N dissolved back into
the solution. Focusing on the types of cation of the chlorides, turbidity of the solution or solubility

+
2+
of POM7-N increased in the order of K < Na < Ca which is also in line with the hardness of
the cations as a Lewis acid.
According to these observations, the formation of polymorphs can be explained qualitatively
as follows: Anions and cations of the additives interact with the needle-type crystals via
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electrostatic interaction, shield the attractive interaction between the constituent ions (Al13 and
POMs), and the constituent ions would peel off from the crystals causing dissolution. The plate-
type crystals crystallize subsequently, which is the more insoluble and probably stable form with
larger numbers of hydrogen bonds between the constituent ions.
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Conclusion
Two kinds of polymorphs of all-inorganic ionic crystals were obtained by the combination of
oppositely charged macroions with T symmetry (Al13 and POMs) in aqueous solutions. The
d
needle-type crystals were grown by dissolution-recrystallization of the precipitates, which formed
immediately by combining Al13 and POMs. On the other hand, the plate-type crystals were grown
by adding inorganic salts into the aqueous solution containing the needle-type crystals: Dissolution
of the needle-type crystals occurred followed by precipitation of the plate-type crystals, which is
the more insoluble and stable polymorph with larger numbers of hydrogen-bonds between the
constituent ions. The plate-type crystals possess one-dimensional channels with large apertures
(ca. 12 Å × 6 Å), and showed high activities as heterogeneous solid acid catalysts in pinacol
rearrangement. All these results show the control of polymorphism and functions in all-inorganic
porous ionic crystals.
Supporting Information
Table S1: Selected bond distances and angles for POM6-N. Table S2: Selected bond distances and
angles for POM6-P. Table S3: Possible hydrogen-bonds (O-O < 3.0 Å) and their distances in
POM6-N and POM6-P. Table S4: Pinacol to pinacolone rearrangement over various solid
catalysts and conditions. Figure S1: IR spectra. Figure S2: Diffuse-reflectance UV-vis spectra.
27
Figure S3: Solid state Al-MASNMR spectra. Figure S4: Thermogravimetry. Figure S5: Wide
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scan XPS spectra. Figure S6: Powder XRD patterns of Al13-POMs (POM6-P and POM6-N) and
the raw materials. Figure S7: Powder XRD patterns of POM6-P synthesized with additives other
than NaCl. Figure S8: Calibration curves of (a) Na ( = 589 nm) and (b) K ( = 766.5 nm) in AAS.
Figure S9: Crystal structure of Al13-sulfate. Figure S10: Powder XRD patterns of the precipitate
and the needle crystal (POM6-N). Figure S11: Powder XRD patterns of POM6-N and POM6-P
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before and after evacuation. Figure S12: N adsorption isotherms (77 K). Figure S13: SEM images.
2
Figure S14: Effects of anions of additives. Figure S15: Effects of cations of additives.
Corresponding Author
Sayaka Uchida, e-mail csayaka@mail.ecc.u-tokyo.ac.jp
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
‡
to the final version of the manuscript. These authors contributed equally.
Notes
The authors declare no competing financial interest.
Acknowledgements
This work was supported by JST-PRESTO and Grant-in-Aids for Scientific Research from the
Ministry of Education, Culture, Science, Sports, and Technology of Japan. Prof. Motoyuki Matsuo
and Dr. Katsumi Shozugawa (Univ. of Tokyo) are acknowledged for providing access to the ICP
instrument. Members of Prof. Noritaka Mizuno group (Univ. of Tokyo) are acknowledged for
providing access to the XPS and DLS instruments. Prof. Junko N. Kondo and Mr. Ryota Osuga
are acknowledged for the preliminary IR measurements to characterize the acid sites.
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References and Notes
1) Bernstein, J. Cryst. Growth and Des. 2011, 11, 632-650.
(
(2) Cartwright, J. H. E.; Checa, A. G.; Gale, J. D.; Gebauer, D.; Sainz-Diaz, C. I. Angew. Chem.
Int. Ed. 2012, 51, 11960-11970.
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45
46
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52
53
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55
56
57
58
59
60
(3) Desiraju, G. R. Angew. Chem. Int. Ed. 2007, 46, 8342-8356.
(4) Pudipeddi, M.; Serajuddin, A. T. M. J. Pharm. Sci. 2005, 94, 929-939.
(5) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629-1658.
(6) Xu, A. –W.; Dong, W. –F; Antonietti, M.; Cölfen, H. Adv. Funct. Mater. 2008, 18, 1307-
1
313.
(7) Hanaor, D. A. H.; Sorrell, C. C. J. Mater. Sci. 2011, 46, 855-874.
(8) Son, J. –H.; Choi, H.; Kwon, Y. –U. J. Am. Chem. Soc. 2000, 122, 7432-7433.
(
(
(
9) Son, J. –H.; Kwon, Y. –U. Inorg. Chem. 2004, 43, 1929-1932.
10) Son, J. –H.; Kwon, Y. –U. Inorg. Chim. Acta. 2005, 358, 310-314.
11) Rodriguez-Albelo, L. M.; Ruiz-Salvador, A. R.; Sampieri, A.; Lewis, D. W.; Gómez, A.;
Nohra, B.; Mialane, P.; Marrot, J.; Sécheresse, F.; Mellot-Draznieks, C.; Biboum, R. N.; Keita, B.;
Nadjo, L.; Dolbecq, A. J. Am. Chem. Soc. 2009, 131, 16078-16087.
(
12) Rodriguez-Albelo, L. M.; Ruiz-Salvador, A. R.; Lewis, D. W.; Gómez, A.; Mialane, P.;
Marrot, J.; Dolbecq, A.; Sampieri, A.; Mellot-Draznieks, C. Phys. Chem. Chem. Phys. 2010, 12,
632-8639.
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(
13) Hill, C. L. Ed. Thematic Issue on Polyoxometalates, Chem. Rev. 1998, 98, 1-390.
(14) Cronin, L.; Müller, A. Eds. Thematic Issue on Polyoxometalates, Chem. Soc. Rev. 2012, 41,
7325-7648.
0
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0
(15) Seino, S.; Kawahara, R.; Ogasawara, Y.; Mizuno, N.; Uchida, S. Angew. Chem. Int. Ed.
2
2
016, 55, 3987-3991.
(16) Kawahara, R.; Niinomi, K.; Kondo, J. N.; Hibino, M.; Mizuno, N.; Uchida, S. Dalton Trans.
016, 45, 2805-2809.
(17) Casey, W. H. Chem. Rev. 2006, 106, 1-16.
(18) Gillespie, C. M.; Asthagiri, D.; Lenhoff, A. M.; Cryst. Growth and Des. 2014, 14, 46-57.
(19) Wang, M.; Muhammed, M. Nanostr. Mat. 1999, 11, 1219-1229.
(20) Baker, L. C. J. Am. Chem. Soc. 1959, 81, 4744.
(
(
(
21) Domaille, P. J. J. Am. Chem. Soc. 1984, 106, 7677-7687.
22) Bradley, S. M.; Kydd, R. A.; Fyfe, C. A. Inorg. Chem. 1992, 31, 1181-1185.
6

23) Because of the charge-mismatch between Al13 (+7) and [-CoW12
(OH)24(H O)12][-CoW12 40](OH)∙26H O with a hydroxide ion
(OH)25(H O)11][-CoW12 40]∙26H O. Synthetic solution of POM6-N
O40] (6), POM6-N
can be formulated as [-Al13
as in ref. 10 or as [-Al13
O
4
2
O
2
O
4
2
O
2
is acidic (pH = 4), so we decided that the inclusion of a hydroxide ion in the formula is
inappropriate in our case.
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IV
V
W O
40]8 (8) in
(
24) Because of the charge-mismatch between Al13 (+7) and [-1,2,3-SiV V
2
9
+
POM8-N and POM8-P, a proton (H ) was added to the formula.
40], [-
25) Ionic crystals could not be obtained with Keggin-type POMs of [-BW12O
(
10
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


SiW12O40] , or [-PW12O40] .
(26) According to Figure 4a, there are nine hydrogen-bonds between Al13 and the disordered
POM. There is another disordered POM beneath the Al13, so there are eighteen hydrogen-bonds
between Al13 and the disordered POM. Out of the nine oxygen atoms of POM, which are in
hydrogen-bonding distances with the Al13, one and eight oxygen atoms have occupancies of 1 and
0
.5, respectively. Therefore, there are 1 × 2 + 0.5 × 16 = 10 hydrogen-bonds between Al13 and the
disordered POMs.
(
27) Biradha, K.; Dennis, D.; MacKinnon, N. K. R.; Sharma V. K.; Zaworotko, M. J. J. Am.
Chem. Soc. 1998, 120, 11894-11903.
(
28) Wang, H.; Xu, X.; Johnson, N. M.; Dandala N. K. R.; Ji, H. F. Angew. Chem. Int. Ed. 2011,
50, 12538-12541.
(
29) Pinacol to pinacolone rearrangement over various solid catalysts and conditions are
summarized in Table S4. Note that there are many superior catalysts for this reaction. In this work,
catalytic reaction was utilized as a characterization.
(
30) Pinnavaia, T. J.; Tzou, M. –S.; Landau, S. D.; Raythatha, R. H. J. Mol. Catal. 1984, 27,
1
95-212.
(31) Kooli, F. Micropor. Mesopor. Mat. 2014, 184, 184-192.
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(
32) The reaction also proceeded with the POMs as catalysts because partial dissolution of the
POMs was observed and acidic protons were contained in POM7 and POM8.
33) Note that the carbon balances (total yield / conv.) of the plate-type crystals were about 90%,
(
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probably because relatively large amounts of reactant and products were absorbed in the catalysts.
(34) Freude, D.; Hunger, M.; Pfeifer, H. Chem. Phys. Lett. 1986, 128, 62-66.
(35) Atkins, P. W. Shriver and Atkins' Inorganic Chemistry, Fifth Edition, Oxford University
Press, 2010, Chap. 4.
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Table 1. Crystal data
POM6-N
POM7-N
POM8-N
Tetragonal
P4 /ncm
POM6-P
Orthorhombic
Ibca
Crystal system
Space group
Tetragonal
Tetragonal
P4
2
/ncm
P4
2
/ncm
2
a = 18.583(3)
a = 18.559(12)
c = 27.896(16)
a = 18.567(5)
c = 27.942(8)
a = 26.659(5)
b = 31.288(6)
c = 46.176(9)
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Unit cell dimensions c = 28.121(5)
Volume
9711(3)
4
9608(10)
4
9633(4)
4
38515(13)
16
Z

3
Dcalc (g cm )
3.116
8444
2.828
7704
2.859
7828
3.131
F(000)
h, k, l range
33766


21/22, 19/22, 23/24, 23/24, 23/20, 23/23, 27/32, 37/37,
33/33
34/36
11.294
0.1509
0.2933
1.203
34/34
11.270
0.1453
0.3030
1.181
51/55
14.719
0.1479
0.3591
1.121

1

(Mo K) (mm )
(I > 2(I))
(all data)
GOF on F2
14.597
0.1243
0.2982
1.145
R
1
wR
2
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Table 2. Pinacol rearrangement reactiona
Yield [%]b
Catalyst
Conv. [%]
2,3-dimethyl
1,3-butadiene
pinacolone
acetone
-
POM6-N
POM7-N
POM8-N
POM6-P
POM7-P
POM8-P
POM6
31
22
21
76
72
45
6
18
8
13
4
0
9
0
1
2
3
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8
9
0
1
2
3
4
5
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8
9
0
1
2
3
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9
0
1
2
3
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8
9
0
1
2
3
4
5
6
7
8
9
0
12
54
26
32
5
5
2
15
14
5
0
24
3
1
0
POM7
8
2
1
3
POM8
8
2
2
2
Al13-sulfate
4
2
1
0
a
Reaction conditions: pinacol (2.0 mmol), naphthalene (0.8 mmol, internal standard), and
b
catalyst (0.2 mmol) in toluene (3 mL) at 373 K, 40 h. Only pinacolone, 2,3-dimethyl-
,3-butadiene, and acetone were detected as products.
1
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Scheme 1 Pinacol rearrangement. Pinacolone is the main product. 2,3-dimethyl-1,3-butadiene
(formed via dehydration) and acetone (formed via dissociative oxidation) are by-products.
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+
O
O
OH OH
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POM
Al13
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a
c
b
b
1
mm
1 mm
Figure 1. Molecular structures of Al13 and POM. Crystal structures and photo images of POM6-
N (left) and POM6-P (right). Green, purple, light blue, and dark blue polyhedra show the [WO ],
[CoO ], [AlO ], and [AlO ] units, respectively. The disordered POMs in POM6-P are depicted
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4
6
4
with a ball-and-stick model, where red, green, and purple spheres show the oxygen, tungsten, and
cobalt atoms, respectively.
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Figure 2. Powder XRD patterns of (a) POM6-N (calc), (b) POM6-N (exp), (c) POM7-N (exp),
(
d) POM8-N (exp), (e) POM6-P (calc), (f) POM6-P (exp), (g) POM7-P (exp), (h) POM8-P
exp), and (i) POM6-P (exp, after catalytic reaction) .
(
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Figure 3. (a) Void analysis of POM6-N (probe diameter 4 Å, grid spacing 0.5 Å). Voids are shown
in brown. Water of crystallization was removed from the crystal structure and hydrogen atoms of
Al13 were geometrically added to the molecular structure. Minimum aperture of the void is ca. 6
Å × 6 Å. Possible hydrogen-bonds (OO < 3.0 Å, shown by the gray lines) among the building
blocks in POM6-N in the (b) (11 0) and (c) (1 1 0) planes. Note that (c) is the view along the
green arrow in (b).
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(a)
c
b
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(
b)
c
a
(c)
c
b
Figure 4. (a) Void analysis (probe diameter 4 Å, grid spacing 0.5 Å) of POM6-P. Voids are shown
in brown. Water of crystallization was removed from the crystal structure and hydrogen atoms of
Al13 were geometrically added to the molecular structure. The minimum aperture of the void is ca.
6 Å × 12 Å. Possible hydrogen-bonds (O-O < 3.0 Å, shown by the gray lines) among the building
blocks in POM6-P in the (b) (1 0 1) and (c) (0 1 1) planes. Note that (c) is the view along the green
arrow in (b). In (c), another disordered POM exists beneath Al13, which is omitted for the clarity.
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10
20
30
Chemical shift [ppm]
1
Figure 5. Solid state H-MASNMR spectra (MAS = 10 kHz) of (a) Al13-sulfate, (b) POM6-N,
and (c) POM6-P.
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Control of Polymorphisms and Functions in All-Inorganic Ionic Crystals based on
Polyaluminum Hydroxide and Polyoxometalates
Kosuke Mizuno,†,‡ Takuto Mura,†,‡ and Sayaka Uchida†,§,*
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Two kinds of polymorphs were obtained by the combination of oppositely charged molecular ions
with T symmetry in aqueous solutions. The plate-type crystal was obtained by using additives,
d
and is the stable polymorph with larger numbers of hydrogen-bonds between the molecular ions.
The plate-type crystal possesses one-dimensional channels with large openings and showed high
activity as a solid catalyst.
controlled synthesis
＋
of polymorphs
+
NaCl
high catalytic activity
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