Cationic two-dimensional inorganic networks of antimony oxide hydroxide for Lewis acid catalysis

Article abstract of DOI:10.1039/c8dt00188j

We have successfully synthesized a rare example of inorganic layered materials possessing a positive charge, which is well outside the isostructural set of layered double hydroxides. This layered architecture consists of two-dimensional corrugated [Sb₂O₂(OH)]⁺ layers with linear α,ω-alkanedisulfonate anions residing in the interlamellar space. This cationic material displays a chemical robustness under highly acidic aqueous conditions (pH = 1). Combining the robust nature and the high density of Sb^III sites on the exposed crystal facets, our cationic layered material is an efficient, recyclable catalyst for cyanosilylation of benzaldehyde derivatives with trimethylsilyl cyanide. In addition, the Lewis acidity of the Sb^III sites also catalyzes the ketalization of carbonyl groups under "green" solvent-free conditions.

Full text of DOI:10.1039/c8dt00188j

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Cationic two-dimensional inorganic networks of
antimony oxide hydroxide for Lewis acid catalysis†
Cite this: DOI: 10.1039/c8dt00188j
Received 16th January 2018,
Accepted 13th February 2018
Jinlin Yin and Honghan Fei
*
DOI: 10.1039/c8dt00188j
rsc.li/dalton
We have successfully synthesized a rare example of inorganic
Layered double hydroxides (LDHs) are an extensively
layered materials possessing a positive charge, which is well studied class of positively charged inorganic architectures,
3+
outside the isostructural set of layered double hydroxides. This with the general formula of [M2+
M
x(OH)2]An−x/n·mH2O,
1−x
layered architecture consists of two-dimensional corrugated where M2+ and M3+ are a range of metals (typically Mg2+ and
[Sb2O2(OH)]+ layers with linear α,ω-alkanedisulfonate anions resid- Al3+), x is the ratio of M3+/(M2+ + M3+), and An− represents the
ing in the interlamellar space. This cationic material displays a interlamellar anions (e.g. CO32−).13–15 This family of layered
chemical robustness under highly acidic aqueous conditions (pH = 1). materials have attracted considerable attention as hetero-
Combining the robust nature and the high density of SbIII sites on geneous and recyclable catalysts or catalyst supports for a
the exposed crystal facets, our cationic layered material is an variety of organic reactions, especially as solid base
efficient, recyclable catalyst for cyanosilylation of benzaldehyde catalysts.16–18 However, they are usually subject to decompo-
derivatives with trimethylsilyl cyanide. In addition, the Lewis sition in acidic medium, owing to the Brønsted basicity of
acidity of the SbIII sites also catalyzes the ketalization of carbonyl hydroxyl bridging ligands.19,20 Moreover, most LDHs require
groups under “green” solvent-free conditions.
an energy-intensive calcination activation pretreatment, and
suffer from difficulty in complete separation.21,22
Naturally occurring or synthetic crystalline inorganic materials,
such as zeolites and anionic clays, typically have a two-dimen-
sional (2-D) or three-dimensional (3-D) framework, in which
the overall charge is compensated by extra-framework cations
residing in pore cages/channels or interlamellar regions.1–3
Despite their significant importance in catalysis and cation
exchange, they have limited opportunities in anion-based
applications, including separation, recognition, trapping, cata-
lysis, etc.4,5 Based on the Inorganic Crystal Structure Database
(ICSD), fewer than 100 of >150 000 crystal structures have an
intrinsic cationic extended framework.6 Thus, it is highly
imperative to develop new inorganic 2-D or 3-D network frame-
works possessing an overall positive charge. In recent years,
several examples of 3-D cationic zeotype materials with extra-
framework mobile anions have been reported, showing poten-
tial applications to trap the pertechnetate (TcO4−) radionuclide
from nuclear waste streams.7–12
Meanwhile, only a handful of inorganic layered materials
bearing a positive charge have been constructed with lower
p-block metals.23–27 Among them, antimony (SbIII) possesses a
stereochemically active lone pair of electrons that allows for a
large variability in its coordination environment as well as a
high tendency to form trigonal pyramidal SbO3 species.
Asymmetric SbIII-oxo moieties can be interconnected and
extended to polymeric structures, which have lone pairs of
electrons capping the unsaturated coordination side of the
SbIII centers.28,29 More importantly, antimony-based homo-
geneous complexes are of great interest as catalysts, especially
in Lewis acid catalysis.30,31
Herein, we present the synthesis and structural characteriz-
ation of a highly unusual 2-D antimony oxide hydroxide with a
positively charged network that is well outside the extensively
studied LDHs. Unlike acid-sensitive LDHs, this cationic
material demonstrates a high chemical robustness under
acidic aqueous conditions (pH = 1) as well as a high thermal
stability up to ca. 250 °C. Combining the robust nature and
the high density of SbIII sites, the as-synthesized material is
used as an efficient, robust and recyclable Lewis acid catalyst
for both cyanosilylation and ketalization of carbonyl
substrates.
Shanghai Key Laboratory of Chemical Assessment and Sustainability, School of
Chemical Science and Engineering, Tongji University, 1239 Siping Rd.,
Shanghai 200092, P. R. China. E-mail: fei@tongji.edu.cn
†Electronic supplementary information (ESI) available: Experimental details and
additional characterization. CCDC 1546785. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c8dt00188j
Hydrothermal reactions of antimony acetate (Sb(OAc)3), di-
sodium 1,4-butanedisulfonate (BDSNa2), and perchloric acid
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metry with three bridging oxygens. The inert electron pair of
the SbIII center is pointing towards the interlamellar region
and capping the cationic layer (Fig. 2b). The other crystallogra-
phically independent SbIII center is positioned in the middle
of the layer, presenting a distorted tetragonal pyramid geome-
try with three bridging oxygens and a terminal hydroxide
ligand. To note, the inert pair effect offers all of the SbIII
centers with umbrella-type coordination environments. Two of
the three crystallographically distinct intralayer O atoms are
unprotonated and triply bridging to metal centers, while the
other O atom is protonated and singly bridging to Sb.
The cationic [Sb2O2(OH)]+ layer of TJU-2 is crystallographi-
cally distinct from the previously published SLUG-5 in several
aspects.32 The [Sb2O2(OH)]+ polyhedra in the cationic sheets of
TJU-2 are extended in three directions along the b–c plane,
forming a honeycomb-like array with 1-D channels (Fig. 2b).
However, the [Sb2O2(OH)]+ asymmetric unit in each layer of
SLUG-5 is connected in only one-direction along the c-axis,
and the adjacent chains are linked by isolated μ2-O into a
layer. In general, TJU-2 is a geometrically distinct and inher-
ently more stable layered corrugated structure with a Sb⋯OSO2
distance of 2.549(7) Å. This value is significantly longer than
the accepted value of the Sb–O covalent bond (1.9–2.2 Å, from
the Cambridge Structural Database), confirming the merely
electrostatic interactions between the cationic layers and
organosulfonate anions. In addition, the X-ray photoelectron
spectrum indicated a characteristic 3d binding energy for SbIII
(530.4 eV and 539.7 eV) in TJU-2 (Fig. S2†). Compared to
Sb2O3, the blue-shifted peak by 0.4 eV is probably due to the
weak interaction between the Sb centers and organosulfonate
anions. Based on these unprecedented structural features,
TJU-2 is a very unique cationic layered structure that is crystal-
lographically very distinct from LDHs as well as other cationic
materials.
Fig. 1 PXRD of TJU-2 before and after thermal or chemical treatment.
Simulated PXRD pattern from single-crystal data (black) is given for
comparison.
(HClO4) afforded colorless plate-like crystals of TJU-2 (TJU =
Tongji University), the structure of which is formulated as
[Sb2O2(OH)](BDS)0.5 (Fig. S1†). The structure can be syn-
thesized between and including temperatures of 150–175 °C
with a relatively high yield (>90%). The synthetic conditions
employing HClO4 were necessary, likely due to its performance
as a stabilizer as well as adjusting the synthetic aqueous solu-
tion to the highly acidic conditions (pH = 0.2). Moreover, the
high phase purity of TJU-2 was evidenced by experimental
powder X-ray diffraction (PXRD), matching well with the simu-
lated pattern from single-crystal data (Fig. 1). No apparent
other phase (e.g. Sb2O3) was observed in the as-synthesized
hydrothermal product. Elemental analysis of TJU-2 (C, 6.03%;
H, 1.07%; S, 8.09%) is in good agreement with the theoretical
values (C, 5.99%; H, 1.24%; S, 8.01%).
Single-crystal X-ray crystallography reveals that TJU-2 crystal-
lized in the P2(1)/c space group of the monoclinic crystal
system. The cationic architecture is defined via the
[Sb2O2(OH)]+ asymmetric units moving into 2-D cationic corru-
gated layers, while charge-balancing BDS anions reside in the
interlamellar regions (Fig. 2a). One of the two crystallographi-
cally independent SbIII centers is located at the outer surface
of the corrugated layer, and occupies a trigonal pyramid geo-
The triply bridging hydroxide ligands in LDHs result in the
basic nature of LDHs as well as chemical sensitivity under
acidic conditions, which limit their applications in environ-
mental and/or chemical processes. In contrast, our antimony
oxide based cationic topology is a hydrothermal product under
very acidic conditions (pH = 0.2). The chemical resistance of
Fig. 2 (a) Crystallographic view of TJU-2 along the c-projection. (b) Crystallographic view of one single layer of TJU-2 along the a-projection (anti-
mony: cyan, oxygen: red, white: hydrogen); the dashed lines show long intralayer contacts between antimony and oxygen.
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TJU-2 was examined by treating the samples with boiling
water, diluted HClO4 solution (pH = 1), and common organic
solutions (e.g. hexane) for 24 h. The high crystallinity of the
cationic layered architecture was well retained, as evidenced by
the PXRD patterns (Fig. 1), confirming that our cationic
materials overcome the acid-sensitive problem of LDHs. The
thermal behavior of TJU-2 was characterized by ex situ powder
X-ray thermodiffraction in air (Fig. 1) and thermogravimetric
analysis under N2 (Fig. S2†). The cationic topology was
thermally stable up to 250 °C, followed by the loss of BDS and
decomposition to Sb2O3.
Enantiomerically enriched cyanohydrins are presented as
fascinating chiral building blocks for both organic synthesis
and biomimetic catalysis.33 Lewis acid-catalyzed cyanosilyla-
tion has been a favorable synthetic strategy34,35 and numerous
examples of homogeneous catalysts have been studied.36,37
Recently, metal–organic frameworks (MOFs) with open metal
sites have been employed as heterogeneous catalysts for cyano-
silylation of carbonyl compounds, despite such materials
usually being moisture-sensitive and in need of activation
prior to catalysis.38–40
Fig. 3 IR transmittance spectra recorded after pyridine adsorption on
TJU-2 at 373 K (a) and 473 K (b). IR transmittance spectra recorded after
pyridine adsorption on Sb2O3 at 373 K (c) and 473 K (d).
entry 4). By comparison, control reactions of both no catalyst
and Sb2O3 gave no reaction product (Table 1, entries 1 and 2).
The significantly enhanced Lewis acid catalytic performance of
TJU-2 over Sb2O3 is likely due to the high density of SbIII cata-
lytic sites on the exposed facet of plate-like crystals. It was
further studied using the infrared spectroscopy of pyridine
adsorbed over TJU-2 and Sb2O3 (Fig. 3). Introduction of the
first dose of pyridine into TJU-2 leads to the appearance of the
bands at 1608, 1575, 1490, and 1449 cm−1, which are assigned
to the Lewis acid sites (Fig. 3).41,42 The rather weak band at
1545 cm−1 indicates the Brønsted acid site, probably owing to
the hydroxyl groups on the [Sb2O2(OH)]+ layers. The intensity
of the Lewis acid sites exhibited a merely slight decrease with
the activation temperature increasing from 373 K to 473 K,
thereby confirming the strong Lewis acid nature of TJU-2. In
contrast, Sb2O3 does not have strong affinity towards pyridine
at elevated temperature (473 K) (Fig. 3 and Table 2).
Given the highly robust nature and highly dense SbIII cata-
lytic sites of our cationic material, we sought to investigate the
catalytic performance of the as-synthesized TJU-2 in cyanosily-
lation of benzaldehyde derivatives. A time-dependent catalytic
performance was studied for the reaction between benz-
aldehyde and trimethylsilyl cyanide using a catalytic amount
of TJU-2 (Fig. S3†). After 16 h, TJU-2 exhibited nearly a quanti-
tative yield of 2-phenyl-2-trimethylsilyloxyacetonitrile (Table 1,
Table 1 SbIII-Catalyzed cyanosilylation of benzaldehyde derivativesa
The high crystallinity of the cationic layered architecture
was maintained throughout the catalysis, as confirmed by
PXRD (Fig. S4†). Between each run, the TJU-2 catalyst could be
completely recovered by filtration, washed with dichloro-
methane, and directly used for an additional six cycles of reac-
tions (Table S1†). Moreover, the catalytic performance of the
recovered TJU-2 was maintained, achieving a quantitative con-
version of benzaldehyde to the corresponding cyanosilylation
product (Table 1). Importantly, the heterogeneous nature of
the catalyst was confirmed by ICP, and merely <0.7% of total
SbIII in TJU-2 was detected in the supernatant of the solution.
To verify the active sites, a moderate conversion yield (∼90%)
was achieved with Sb(OAc)3 as a homogeneous catalyst after
16 h (Table 1, entry 3), confirming the catalytic activity of Sb
sites. A series of benzaldehyde derivatives were chosen for the
Entry
Substrate
Catalyst
Time (h)
Yieldb (%)
1
2
3
4
5
Blank
Sb2O3
Sb(OAc)3
TJU-2
24
24
16
16
12
<1
<1
90 (1)
>99
>99
TJU-2
6
7
8
9
TJU-2
TJU-2
TJU-2
TJU-2
12
12
12
16
>99
>99
>99
>99
Table 2 Relative amount of Lewis sites determined from the infrared
spectra of adsorbed pyridine at 373 K and 473 K
Lewis sites (μmol g−1
Sample (373 K)
)
Lewis sites (μmol g−1
)
(473 K)
a Reaction conditions: 1.25 mmol benzaldehyde derivatives, 2.5 mmol
trimethylsilyl cyanide, 6 mol% catalyst (based on Sb), 5 mL CH2Cl2,
room temperature. b Yield was determined by 1H NMR, an average of TJU-2
377
123
148
0
two independent trials.
Sb2O3
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Table 3 SbIII-Catalyzed ketalization of 2-butanonea
thesized catalyst has many outstanding properties, including
high stability, high crystallinity and easy separation, and can
be directly used for catalysis without activation. This new type
of cationic extended metalate provides a synthetic route to
cationic inorganic materials with higher stability than LDHs
Entry
Catalyst
Sb (mol%)
Yieldb (%) for anion-related applications.
1
2
3
4
5
Sb(OAc)3
TJU-2
TJU-2 (2nd cycle)
TJU-2 (3rd cycle)
TJU-2 (4th cycle)
1.25
1.25
1.25
1.25
1.25
30 (3)
55 (3)
58 (4)
53 (3)
53 (3)
Conflicts of interest
The authors declare no competing financial interest.
a Reaction conditions: 40 mmol 2-butanone, 80 mmol ethylene glycol,
1.25 mol% (based on Sb) catalyst, solvent-free, 90 °C. b Yield was deter-
mined by 1H NMR, an average of two independent trials.
Acknowledgements
This work was supported by grants from the National Natural
Science Foundation of China (21501136 and 51772217), the
Recruitment of Global Youth Experts by China, the
Fundamental Research Funds for the Central Universities, and
substrate scope in the reaction, and nearly quantitative yield of
the corresponding cyanosilylation products was achieved using
the as-synthesized TJU-2. Overall, our cationic SbIII-based
material appears to be an efficient and recyclable catalyst for
cyanosilylation of benzaldehyde derivatives over a range of
substrates.
the Science
&
Technology Commission of Shanghai
Municipality (14DZ2261100).
Encouraged by the high catalytic activity observed for the
cyanosilylation reactions, we further studied the Lewis acid cat-
alysis of TJU-2 toward the ketalization of a ketone that usually
requires a stronger Lewis acid. A time-dependent Lewis acid-
catalyzed ketalization of 2-butanone with ethylene glycol was
investigated using TJU-2 (Fig. S5†). A mere 1.25 mol% of TJU-2
(based on Sb) afforded a 55% yield of the corresponding ketali-
zation product 2-ethyl-2-methyl-[1,3]-dioxolane (Table 3, entry 2).
In contrast, the control reaction of homogeneous Sb(OAc)3
gave a yield of merely 30% (Table 3, entry 1). These results
confirm that the high density of Sb catalytic sites on
TJU-2 made our cationic material an efficient Lewis acid cata-
lyst. Moreover, the heterogeneity of our cationic materials was
confirmed by ICP, which again showed that only 40 ppm Sb
(0.35% of total Sb in TJU-2) was present in the reaction solu-
tion. An additional three cycles were carried out to verify the
stability and recyclability of our cationic SbIII-based material,
and the yields were maintained at 53–58% (Table 3, entries
3–5). Moreover, a negligible loss of catalyst crystallinity was
observed throughout the four cycles, as evidenced by PXRD
(Fig. S6†).
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