Temperature-induced Sn(II) supramolecular isomeric frameworks as promising heterogeneous catalysts for cyanosilylation of aldehydes

Article abstract of DOI:10.1007/s11426-019-9621-x

Two novel Sn(II) supramolecular isomeric frameworks, with the identical formula of {(NH₂Me₂)₂[Sn(BDC)(SO₄)]}_n, Sn-CP-1-α (1) and Sn-CP-1-β (2) (H₂BDC=terephthalic acid) were synthesized und

Full text of DOI:10.1007/s11426-019-9621-x

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https://doi.org/10.1007/s11426-019-9621-x
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Temperature-induced Sn(II) supramolecular isomeric frameworks
as promising heterogeneous catalysts for cyanosilylation of
aldehydes
1
,2†
3†
1
1*
4
Kai Sheng , Li-Ming Fan , Xue-Fei Tian , Rakesh Kumar Gupta , Linna Gao ,
1
1*
Chen-Ho Tung & Di Sun
1
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering,
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China;
2
School of Aeronautics, Shandong Jiaotong University, Jinan 250037, China;
3
Department of Chemistry, College of Science, North University of China, Taiyuan 030051, China;
4
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China
Received August 27, 2019; accepted September 20, 2019; published online December 11, 2019
Two novel Sn(II) supramolecular isomeric frameworks, with the identical formula of {(NH Me ) [Sn(BDC)(SO )]} , Sn-CP-1-α
2
2 2
4
n
(
1) and Sn-CP-1-β (2) (H BDC=terephthalic acid) were synthesized under solvothermal condition and fully characterized by
2
single crystal X-ray diffraction (SCXRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-
Vis), elemental analyses, and thermogravimetric analysis (TGA). Interestingly, the structures of 1 and 2 are governed by the
temperature of the reaction, suggesting a temperature-induced supramolecular isomerism. The supramolecular isomers are
2–
primarily caused by the different bridging alignments of SO4 . Compounds 1 and 2 display 2D layer and 3D framework with
4
different topologies, non-interpenetrated 4 -sql and two-fold interpenetrated 4-connected dia topology, respectively. Due to
Lewis acid properties of coordinatively unsaturated Sn(II) sites in CPs, they have been utilized as heterogeneous catalyst for the
cyanosilylation of aldehydes with an excellent conversion yield over 99% under solvent-free conditions.
supramolecular isomerism, Sn(II) coordination polymers, catalysis, cyanosilylation
Citation:
Sheng K, Fan LM, Tian XF, Gupta RK, Gao L, Tung CH, Sun D. Temperature-induced Sn(II) supramolecular isomeric frameworks as promising
heterogeneous catalysts for cyanosilylation of aldehydes. Sci China Chem, 2019, 62, https://doi.org/10.1007/s11426-019-9621-x
The current research interests in coordination polymers
CPs) are due to their fascinating architectures and mis-
refers to the existence of more than one crystal structure that
has the same chemical composition and building blocks
[7,8]. Polymorphism in CPs are governed by many key
factors such as reaction medium, temperature, counter ions,
pH values, and solvent [9–12]. Among them, the effect of
temperature is one of the most significant variables [9,13]. In
the recent past, majority of the supramolecular isomeric CPs
are available either on transition metals or lanthanides [7–
12]. It should be noted that polymorphism in main group
metal complexes, particularly tin complexes are barely ex-
plored. To the best of our knowledge, only a few examples of
polymorphism in Sn(IV) complexes are available, but the
(
cellaneous applications, including gas storage, lumines-
cence, catalysts, sensors, and drug delivery [1,2]. The real
applications of CPs are directly associated with their crys-
talline properties of materials [3,4]. Therefore, the design
and synthesis of desired CPs with predictable metal-organic
connectivity is still challenging due to supramolecular iso-
merism [5,6]. Supramolecular isomerism or polymorphism
†
*
These authors contributed equally to this work.
Corresponding authors (email: rkg@sdu.edu.cn; dsun@sdu.edu.cn)
©
Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019
chem.scichina.com link.springer.com

2
Sheng et al. Sci China Chem
examples of temperature-induced polymorphism in Sn(II)
CPs are not described so far [14–16].
On the other hand, cyanosilylation of carbonyl compounds
with trimethylsilyl cyanide (TMSCN) is an effective method
to synthesize cyanohydrins, which are important precursors
for the synthesis of natural and pharmaceutical compounds,
in presence of Lewis acid catalyst [17,18]. In this context,
numerous metal complexes and coordination polymers have
been studied as homogeneous catalysts; however, hetero-
geneous catalysts are limited [19–23]. Therefore, the design
and development of heterogeneous catalyst, which is in-
expensive, solvent free and easy to handle, are highly de-
sirable. The large size and stereo chemical effect of the lone
pair of tin endow them Lewis acid characteristics as a con-
sequence superior catalytic properties [15,24,25]. To our best
knowledge, the application of a Sn(II) complex as a het-
erogeneous catalyst for cyanosilylation of aldehydes under
solvent-free conditions has not been testified.
Scheme 1 Schematic illustration of synthesis of isomers 1 and 2, and the
crystal images under a microscope (color online).
+
two NH Me cations (Figure 1(a, b)). Each Sn(II) ion was
2
2
2–
surrounded by four O atoms from two different BDC li-
2
–
gands and two SO4 anions, forming the distorted {SnO4}
tetrahedrons which can be better described as a seesaw
structure with τ =0.88 for 1 and τ =0.84 for 2 [29]. The linear
Hence, terephthalic acid ligand has been selected to de-
velop some supramolecular Sn(II) isomeric CPs with ex-
cellent heterogeneous catalytic activity. Remarkably,
Jacobson et al. [26], have synthesized some tin ter-
ephthalates metal complexes A Sn (BDC) (H O) , A=Li,
4
4
O5–Sn1–O6 forms the plank and O1–Sn1–O3 forms the
pivot in 1. The O5–Sn1–O6 and O1–Sn1–O3 planes are
nearly perpendicular to each other with a dihedral angle
(DA) of around 88.3°. Similarly, in compound 2, O5–Sn1–
O8 and O2–Sn1–O3 forms the plank and pivot, respectively.
The DA of around 89.3° was observed with the nearly per-
pendicular O5–Sn1–O6 and O1–Sn1–O3 planes. The Sn–O
bond lengths span from 2.165(3) Å (Sn1–O1) to 2.502(5) Å
(Sn1–O6A) for 1 (Figure 1(c)), and 2.163(4) Å (Sn1–O3) to
2.660(5) Å (Sn1–O8C) for 2 (Figure 1(d)), respectively,
which are comparable with the earlier reports [26,27,30]
(Tables S3 and S4) (symmetry codes: A: 1+x, y, z; C: 0.5−x,
−0.5+y, z).
2
2
3
2
x
Na, K, Rb, Cs, under hydrothermal condition at 200 °C for
d in the presence of alkali metal hydroxides. Further, a 3D
6
framework structure of Sn O(BDC) was reported by the
3
2
same group at 180 °C for 24 h [27].
All above discussions clearly indicated that the Sn(II) CPs
are still quite rare in CPs field, compared to other metal
congeners. Keeping all these considerations and also inspired
by our previous work [28], we have successfully isolated two
supramolecular isomeric Sn(II) CPs, with exactly the same
chemical composition of {(NH Me ) [Sn(BDC)(SO )]} by
2
2 2
4
n
adjusting the reaction temperature. Compounds 1 and 2 show
D layer and 3D framework with different topologies, non-
Although, the asymmetric unit is similar, a minor differ-
ence leads to huge structural differences. In the construction
of 1 and 2, carboxyl groups of BDC ligands all adopt
2
4
2–
interpenetrated 4 -sql and two-fold interpenetrated 4-con-
nected dia network, respectively. Furthermore, hetero-
geneous catalytic activities have been established towards
the cyanosilylation of aldehydes with TMSCN in detail.
Compounds 1 and 2 were synthesized under solvothermal
condition by the reaction of H BDC and SnSO at different
monodentate coordination modes and act as bridging linkers
to connect the Sn(II) ions, giving 1D [Sn(BDC)] chains with
n
the separated Sn···Sn distances being 11.39 and 11.44 Å for
1, and 11.34 and 11.42 Å for 2, respectively. The DA ori-
ginated by the two BDC rings is 10.9° for 1, and 13.2° for 2.
2
4
temperatures (180 °C for 1; 150 °C for 2) using similar
concentration of reactants (Scheme 1). 1 and 2 have the
identical composition, but different crystal structures sig-
nifying temperature-dependent supramolecular isomerism,
which has been supported by single-crystal X-ray diffraction
(SCXRD) analysis. The morphology of the crystals appear-
ance under a microscope is depicted in Scheme 1.
Single crystal X-ray diffraction analyses show that 1 and 2
crystallize in the monoclinic P2 /n and orthorhombic Pbca
1
space groups, respectively (Tables S1 and S2, Supporting
Information online). Asymmetric units are similar, consist of
Figure 1 The asymmetric units of 1 (a) and 2 (b); the coordination en-
vironments of Sn(II) centres of 1 (c) and 2 (d). Colour code: Sn, brownish
green; S, yellow; O, red; C, grey (color online).
2–
2–
one Sn(II) ion, two half BDC ligand, one SO anion, and
4

Sheng et al. Sci China Chem
3
2
–
The Sn···Sn···Sn angles along the linear axis of BDC li-
gand was found to be 101.42° for 1 and 102.62° for 2, re-
2–
spectively. Meanwhile, the SO4 occupied the pole position
to bridge the Sn(II) ions to form 1D [Sn(SO )] chain with
4
n
Sn···Sn distances being 6.83 Å for 1, and 6.58 Å for 2, re-
spectively. Sharing the Sn(II) ions, the wavy 2D layer of 1
was constructed (Figure 2(a, c)), which can be simplified into
4
a 4 -sql sheet (Figure 3(a)). While, 3D two-fold inter-
penetrated 4-connected uninodal networks was constructed
for 2 (Figure 2(b, d)), which can be simplified as a dia fra-
6
mework with Schläfli symbol {6 } and extended point
Figure 3 (a) The 4-connected sql sheet for 1 and (b) the two-fold inter-
penetrated 4-connected dia net for 2 (color online).
symbol [6 .6 .6 .6 .6 .6 .]. Based on interpenetration classi-
2
2
2
2
2
2
fication and nomenclature, the type of interpenetration in 2
belongs to class IIa, Z=2, which suggests two identical in-
terpenetrated nets are generated by means of space group
interpenetration symmetry elements, here are 2-fold axes
along crystallographic b and c directions, and no inter-
penetrating translations are allowed in this kind of inter-
penetration (Figure 3(b)) [31].
O8–Sn1, 2. The plane generated through Sn1–O5–S1–O6 is
parallel to each other in 1, whereas in 2, torsion angle is
17.33°. The Sn···Sn···Sn angles of 180° in 1 cause more
regular geometry and a 2D layer sheet is formed finally,
2
–
along SO4 axis. Whereas, in 2, Sn···Sn···Sn angle of
169.61° causes more distortion from the regular geometry
and a 3D structure is generated finally. These geometric
differences together result into the formation of 2D layer
non-interpenetrated sheet in 1, and two-fold interpenetrated
3D framework in 2. Thus the bridging orientations of the
The 2D layered non-interpenetrated sheet of 1 and two-
fold interpenetrated 3D framework of 2 are true supramo-
lecular isomers because they have different network struc-
tures but exactly the same chemical compositions. The
differences in their networks are associated with the altera-
tion in their coordination environment. The key difference
between the structures of these isomers is the bridging or-
2
–
SO4 cause different connectivity between the metal centers
and thus lead to the formation of true supramolecular isomers
1 and 2.
2–
ientation of the SO around the Sn(II) centers. In 1, the
4
2–
2–
SO4 ions show a uniform orientation, wherein each SO4
The UV-Vis absorption spectra of 1 and 2 were measured
in the solid state (Figure S1, Supporting Information online)
using diffusion-reflection mode. The absorption maxima of 1
and 2 exhibited at 308 and 312 nm respectively in the ul-
traviolet region. These absorption bands are assigned to s-p
transitions of Sn(II) systems [32,33].
Thermogravimetric analysis (TGA) was carried out to
check the thermal stabilities of both Sn(II) CPs. The features
of thermal stability are illustrated in Figure S2. Both com-
pounds are stable up to ca. 180 °C, and exhibit similar weight
loss pattern. For 1, the first major weight loss (35.2%) oc-
curred in the range of 180–320 °C, attributed to the removal
of organic ligand BDC (34.8%). Similarly, 2 also showed a
weight loss of 34.5% consistent to the loss of organic ligand
binds with another metal center with the bond angles of Sn1–
O5–S1, 124.51°, Sn1–O6–S1, 120.45° and O5–S1–O6,
2
–
1
05.40° for 1 (Figure 2(a)). However, in 2, SO4 ions adopt
alternate arrangement on binding two further metal centers
Figure 2(b)) with the bond angles of Sn1–O5–S1, 120.45°;
Sn1–O5–S1, 115.56°; and O5–S1–O6, 111.67°. The DAs of
6.51° and 30.21° were observed with the plane defined by
S1–O5–Sn1 and S1–O6–Sn1, 1; and S1–O5–Sn1 and S1–
(
4
(34.8%) in the temperature range 180–350 °C. Then, the
weight loss above aforesaid temperature leads to the loss of
NH Me , SO , and thermally stable final product corre-
2
2
4
sponding SnO. The TGA analyses show that all compounds
are thermally stable and can meet the conditions for catalytic
reactions as discussed below.
Based on the single-crystal structures of two Sn(II) CPs,
II
both compounds show coordinatively unsaturated Sn cen-
ters in frameworks, which encourages us to evaluate their
activity as heterogeneous catalysts towards Lewis acid pro-
moted reactions. Here, we choose a typical reaction for
testing Lewis acidity catalysis: cyanosilylation of aldehyde,
2
–
Figure 2 The alignment of SO4 (a) 2D layered non-interpenetrated
2–
structure of 1 (c); alternate arrangement of SO4 connecting to another
metal centers (b) two-fold interpenetrated 3D structure of 2 (d). Colour
code: Sn, brownish green; S, yellow; O, red; C, grey (color online).

4
Sheng et al. Sci China Chem
since the cyanosilylation of aldehydes is an efficient route for
preparation of cyanohydrins, which are key intermediates in
the synthesis of various biologically active molecules
[17,18]. Firstly, we use solvent-free cyanosilylation of ben-
zaldehyde as a model reaction to explore the activity of 1 and
2
.
Figure 4 shows the kinetic reaction profile (conversion
versus time) of the reaction between benzaldehyde and
1
TMSCN. The values of conversion based on H NMR in-
1
tegrations and their corresponding H NMR spectra are
shown in Table S5 and Figures S4–S21, respectively. The
plots reveal that both 1 and 2 are excellent catalysts for
cyanosilylation reaction. Especially, 2 shows better catalytic
activity than 1 with the conversion up to more than 99%,
whereas that of 1 reaches only 96% under the same catalytic
reaction conditions (1 mol% loading and room temperature).
Such catalysis reaction kinetic behaviors were also supported
by time-dependent infrared spectra (Figure S22).
Figure 4 The kinetic reaction profile of the cyanosilylation catalysed by
1 mol% of 1 and 2 (based on H NMR integrations) (color online).
1
control experiment, without the catalyst, shows little con-
version of only 3% under the same conditions (entry 10).
In order to understand the heterogeneous catalytic nature
of the reaction, the catalyst is separated from the reaction
after 20 min by centrifugation and filtration. The filtrate is
allowed to stir under the same conditions for additional
100 min. No significant increase of conversion can be ob-
served (Figure S32), demonstrating the heterogeneous nature
of the catalysis. Moreover, the solid catalyst has been col-
lected after centrifugation, washed with chloroform and
methanol and dried at 65 °C for 5 h for recovery. The re-
cycling test shows that it can be reused at least for three
cycles with slight loss of activity (Figure S33).
Furthermore, aldehyde substrates with a larger conjugated
system (Table 1) are further examined by using 2 as the
1
catalyst. The corresponding H NMR spectra are shown in
Figures S23–S31. The complete conversion of o-Me-, m-Me-
and p-Me-substituted benzaldehydes can be achieved within
2
h at room temperature (entries 3–5). With the p-tBu-sub-
stituted benzaldehyde, the reaction is slightly slower and
lower conversion (entry 6). For bulkier naphthaldehyde, only
a 47% conversion is given in 2 h and it can also achieve full
conversion when the reaction time extends to 5 h (entry 7).
This steric hindrance effect indicates the bulkier substrates
are more difficult to access the catalytic sites [34,35]. The
fluorine substituted substrate can also be tolerated (entry 8).
For ketones, however, the reaction is much slower owing to
the lower activity of ketones than aldehydes (entry 9). The
In conclusion, two novel Sn(II) supramolecular isomeric
frameworks, Sn-CP-1-α and Sn-CP-1-β have been synthe-
sized and fully characterized. Interestingly, the structures of
1 and 2 are dependent on the temperature of the reaction at
a)
Table 1 Results of cyanosilylation of varied aromatic aldehydes/ketone in the presentence of Sn-CP-1-α orSn-CP-1-βat room temperature
b)
c)
–1
Entry
Catalyst
Ar
Ph
Ph
R
H
Conversion (%)
TOF (h )
1
2
3
4
5
6
7
8
9
1
96
48
2
H
>99
>99
>99
>99
95
49.5
49.5
49.5
49.5
47.5
23.5
49.5
2.5
2
o-MeC H
H
6
4
2
m-MeC H
H
6
4
2
p-MeC H
H
6
4
2
p-tBuC H
H
6
4
d)
2
1-Naphtyhyl
H
47 (99)
2
o-FC H
H
>99
5
6
4
2
Ph
Ph
Me
H
10
No catalyst
3
1.5
a) General reaction conditions: aldehyde/ketone (1 mmol), TMSCN (1.2 mmol), catalyst 1 or 2 (0.01 mmol), room temperature, 2 h. b) The conversions are
1
calculated based on H NMR integration of the product peaks in comparison to those of the aldehyde/ketone. c) TOFs are calculated as moles of product
formed per hour and per mole of Lewis acidic active site. d) Reaction time: 5 h.

Sheng et al. Sci China Chem
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