Three helical chain-based 3D coordination polymers: Solvent-induced syntheses, tunable structures and catalytic properties for the Strecker reaction

Article abstract of DOI:10.1039/c9ce00972h

Three three-dimensional (3D) helical chain-based coordination polymers [Sc(aip)(OH)·(H₂O)]_n (1), [Sc₂(aip)₃]_n (2) and [Sc₂(nip)₃]_n (3) (H₂aip = 5-aminoisoph

Full text of DOI:10.1039/c9ce00972h

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DOI: 10.1039/C9CE00972H
ARTICLE
Three Helical chain-based 3D Coordination polymers: Solvent-
Induced Syntheses, Tunable Structures and Catalytic property for
Strecker reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Juan Chai, Ping Zhang, Xiangxiang Shi, Jing Sun, Li Wang * and Yong Fan*
DOI: 10.1039/x0xx00000x
Three three-dimensional (3D) helical chain-based coordination polymers [Sc(aip)(OH)∙(H2O)]n (1), [Sc2(aip)3]n (2) and
[Sc2(nip)3]n (3) (H2aip=5-aminoisophthalic acid, H2nip=5-nitroisophthalic acid) have been synthesized through solvent
induction without any chiral auxiliary systematically. We first synthesized CP 1, a 3D network，which contains intertwined
left- and right-hand helical chains {-ScO6-OH-}n. By changing the polarity of solvents from CH3CN/H2O to H2O, genuine single
helix of 2 was constructed, in which adjacent Sc(COO)6 polyhedra are connected alternately by aip2- ligands to form a 3D
framework with intertwined parallel left- hand helical chains {-Sc1(COO)-Sc2(COO)-}n. Then by changing the substituent of
ligand, CP 3 was obtained with right-handed helical chains. As for 3, the aip2- ligands bridge Sc(III) ions alternately into an
infinite right-handed helical chains {-Sc(COO)-}n, which serve as secondary building units (SBU) to generate a 3D framework.
Solid-state circular dichroism spectrum confirms that 3 is optically active. We also studied catalytic properties of 1-3 in the
Strecker reaction. Importantly, 1 shows excellent catalytic activity under mild conditions and can be reused for at least four
times without significant reduction in catalytic ability.
In addition, helical/nonhelical transitions and even
inversion of helicity are influenced by all intermolecular
interactions and various external factors have the potential to
Introduction
As a foundation of genetic codes in biological systems, helicity
has attracted great interest due to its intriguing structures1-4
change the sign and degree of helicity26. In 2003, Inoue et al.
reported a supramolecular Zn2H∙L, in which the higher degree
of supramolecular chirality induction was observed in CH2Cl2
than in CCl427. In 2004, Moore et al. prepared a helicene-
containing foldamer in which the choice of solvent drastically
changed the transfer of chiral information28. Although helical
CPs have been designed and synthesized, the control of helicity
during the constructions still remains difficult.
.
Both left- and right-handed helical chains are ubiquitous in
nature5. Inspired by sophisticated biological helices, scientists
have developed some helical structures to explore the origin of
asymmetry in living systems6, and their potential applications in
advanced materials such as the separation of enantiomers,
asymmetric catalysis and nonlinear optical devices7-9
.
Coordination polymers (CPs), as a flourishing subclass of
molecular engineering materials, built from the ordered
assembly of organic ligands and metal ions or clusters have offer
a unique platform for the construction of helical structure with
specific network topologies10-16. Generally, helical CPs are
constructed by coordination interactions, hydrogen bonding
interactions, π-π stacking and argentophilic interactions17-19, of
which ligand to metal coordination interaction is the most
efficient supramolecular synthon for the constructions of helical
CPs20-22. Up to date, for the design and constructions of helical
CPs, to use enantiopure or racemic ligands as building blocks
may be an available process23,24. Besides enantiopure and
racemic building blocks, helical chains can also be constructed
by achiral building blocks25.
We have chosen isophthalic acid derivatives 5-
aminoisophthalic acid (H2aip), 5-nitroisophthalic acid (H2nip),
and scandium (III) as original materials to generate potential
helical structures. Carboxylates are well known to possess
versatile coordination modes, strong coordination ability, and
high structural stability. More importantly, these ligands
potentially generated helical structures in previous reports. As
expected, we first synthesized CP 1, a 3D network，which
contains intertwined left- and right-hand helical chains {-ScO6-
OH-}n. By changing the polarity of solvents, genuine single helix
of
CP
and
2
3
was constructed. Then by changing the substituent of ligand,
was obtained with right-handed helical chains (Fig. 1). In
, adjacent Sc(COO)6 polyhedra are connected alternately
2
3
by ligands to form 3D frameworks, which contain intertwined
parallel left- hand helical chains {-Sc1(COO)-Sc2(COO)-}n and
right-handed helical chains {-Sc(COO)-}n, respectively.
Furthermore,
1 presented high catalytic activity under mild
College of Chemistry, Jilin University, Changchun 130012, Jilin, P. R. China.
E-mail: mrfy@jlu.edu.cn; lwang99@jlu.edu.cn.
conditions for the Strecker reaction, and can be easily recycled
and reused for four successive cycles without significant loss of
catalytic activity.
† Electronic Supplementary Information (ESI) available: CCDC 1585707 for
1585711 for and 1585712 for . For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/x0xx00000x.
1,
2
3
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ARTICLE
Journal Name
2): A mixture of H2aip (0.0181 g, 0.1 mmol),
Sc(NO3)∙6H2O (0.2 mmol∙mL , 0.5 mL) was added to H2O (3 mL)
in a 23 mL Teflon-lined autoclave and then heated under
autogenous pressure at 100 °C for one day, then cooled to room
temperature under ambient conditions. The pink block crystals
were obtained by filtration and washed with distilled water, and
[Sc2(aip)3]n (
View Article Online
DOI: 10.1039/C9CE00972H
-1
dried in air. Yield: 80% for
2 (based on Sc(NO3)∙6H2O). Elemental
analysis (%) for : Anal. Calc.: C, 45.93; H, 2.39; N, 6.70; Found:
2
C, 45.82; H, 2.55; N, 7.87. IR (KBr pellet, cm-1): 3646 (s), 3476 (s),
3381 (s), 3217 (m), 1535 (s), 1461 (s), 1381 (s), 1317 (w), 1137
(w), 997 (s), 893 (m), 783 (s), 629 (w).
[Sc2(nip)3]n (3): A mixture of H2nip (0.0211 g, 0.1 mmol),
Sc(NO3)3∙6H2O (0.2 mmol∙mL-1, 0.5 mL) was added to CH3CN (3
mL) in a 23 mL Teflon-lined autoclave and then heated under
autogenous pressure at 100 °C for two days, then cooled to
room temperature under ambient conditions. The colorless
block crystals were obtained by filtration and washed with
Fig. 1 Rational modulation from left- and right- handed double-helical
chains to single helical chains (1-3).
distilled water, and dried in air. Yield: 72% for
3 (Sc(NO3)3∙6H2O).
Elemental analysis (%) for : Anal. Calc.: C, 40.17; H, 1.26; N,
3
5.86; Found: C, 40.25; H, 1.35; N, 5.50. IR (KBr pellet, cm-1): 3089
(s), 2950 (w), 2881 (w), 1682 (m), 1625 (s), 1547 (s), 1461 (s),
1397 (m), 1346 (w), 1087 (m), 914 (m), 722 (s), 583 (w).
Experimental section
Materials and methods.
Single-Crystal X-ray Diffraction.
All reagents and solvents were commercially available and used
without further purification. Fourier-transform infrared (FT-IR)
spectra were recorded using the KBr pellets on a Nicolet Impact
410 spectrometer between 400 and 4000 cm-1. Powder X-ray
diffraction (PXRD) data were recorded on an SHIMADAZU XRD-
6100 diffractometer equipped with Cu Kα radiation (λ = 1.5418
Å). Elemental analyses (C, H and N) were collected on an
Elementar Vario EL cube CHNOS Elemental Analyzer.
Thermogravimetric analyses (TGA) were conducted on a Perkin-
Elmer TGA 7 thermogravimetric analyzer in atmospheric
The crystallographic data sets for
Rigaku R-AXIS RAPID IP diffractometer with graphite-
monochromated Mo Kα (0.71073 Å) radiation, while was
1 and 3 were collected on a
2
collected on a Bruker SMART APEX-II CCD diffractometer by
using graphite-monochromated Mo Kα radiation (k = 0.71073 Å)
at room temperature. The structures were solved by using
direct methods and refined on F2 with full-matrix least-squares
techniques with the SHELX-97 crystallographic software
package29. All nonhydrogen atoms were refined anistropically.
All the hydrogen atoms were placed in fixed, calculated
positions using a riding model. Solvent molecules were removed
by SQUEEZE subroutine in PLATON30. The final molecular
formulas of 1-3 were derived from crystallographic data
combined with elemental and thermogravimetric analyses data.
The crystal data, the data-collection details for 1-3 are
summarized in Table S1, selected bond lengths and angles are
given in Tables S3-S5. CCDC-1585707, 1585711 and 1585712
contain the supplementary crystallographic data for this paper.
1
environment at a heating rate of 10 °C·min-1. H NMR spectra
were measured with a Bruker Avance 400 console at a
frequency of 400 MHz. The circular dichroism (CD) spectra were
measured on a MOS-450 spectrophotometer (KBr pellets).
Ultraviolet-visible (UV-Vis) adsorption spectra were recorded
on
a
Rigaku-UV-3100
spectrophotometer.
N2
adsorption/desorption experiments were performed at 77 K
using a Micromeritics ASAP 2010N analyzer.
Synthesis and Characterization of 1-3.
Sample activation.
[Sc(aip)(OH)∙(H2O)]n (1): A mixture of H2aip (0.0181 g, 0.1
Prior to the catalytic test, the as-prepared 1-3 samples were
soaked in CH3OH for 3 days. Further, the samples were filtrated
and dried at 150 °C for 12 h under vacuum to remove the guest
mmol), Sc(NO3)3∙6H2O (0.2 mmol∙mL-1, 0.5 mL) was added to
CH3CN (5 mL) in a 23 mL Teflon-lined autoclave and then heated
under autogenous pressure at 100 °C for two days, then cooled
to room temperature under ambient conditions. The brown
block crystals were obtained by filtration and washed with
molecules, generating the activated 1-3
.
Catalytic experiment.
distilled water, and dried in air. Yield: 85% for
1 (based on
For the catalytic experiments, activated catalysts, aldimine
(0.14 mmol) and trimethylsilyl cyanide (TMSCN) (47 μL) in CDCl3
(2.4 mL) were sequentially added to a standard 20 mL vial. The
reaction mixtures were stirred at room temperature. The
Sc(NO3)3∙6H2O). Elemental analysis (%) for : Anal. Calc.: C,
1
37.07; H, 3.10; N, 5.41; Found: C, 37.35; H, 3.05; N, 5.50. IR (KBr
pellet, cm-1): 3441 (s), 1625 (m), 1573 (m), 1546 (s), 1393 (s),
1035 (m), 988 (m), 780 (s), 709 (s), 633 (w), 574 (w), 527 (w).
1
reactions were monitored by H NMR spectroscopy and the
conversion yield was determined from the ratio of the integral
2 | J. Name., 2012, 00, 1-3
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ARTICLE
of the product signal in relation to the sum of integrals of all resulting in a novel 2D layer along b axis (Fig. 2b).VTiehweAsrteiclelaOynelinres
2-
DOI: 10.1039/C9CE00972H
signals (aldehyde, aldimine and product).
are further connected by bridging aip ligands to construct a 3D
framework with 1D channels with the interior cross-section
dimensions of ca. 7.5×7.5 Å2, along c axis (Fig. 2c). The adjacent
opposite handedness helical {-ScO6-OH-}n chains were parallel
Results and discussion
to each other, resulting in the entire crystal structure of 1 being
mesomeric. From the topological point of view, each O1 atom
is connected by two Sc3+ ions and two aip2− ligands, which can
be simplified as a four-connected node; each aip2- ligand is
simplified by a two-way linker linking O1 atom, which is in the
center of two Sc3+ centers. Accordingly, the whole network can
be extended to a 3D 2,2,4-connected network with the
{102∙12∙162∙18}{10}3 topological symbol (Fig. 2d).
Inspection of the structure of
1 suggests that it is possible
to separate the quasi-double-helical chain, forming single
helical chains by changing reaction conditions. When the
polarity of solvent was increased by introducing H2O instead of
CH3CN, although the H2aip ligand was still deprotonated, the 3D
Synthesis.
As is well known, many factors in the synthetic process should
be considered, such as reaction temperature, solvents, ligands
and concentration of staring materials, in which polarity of
mixed solvents and substituent groups of ligands maybe more
likely to affect the reaction results. As depicted in Scheme 1, all
the CPs were prepared in the presence of isophthalic acid
derivatives ligands by solvent induced methods.
structure of
2 with different single helical chains was isolated
without chiral additives.
Distinguishingly,
1 was obtained by the reaction of
Scheme 1 The synthetic route to 1-3.
Sc(NO3)3·6H2O, H2aip, and CH3CN/H2O mixed solution. Then, by
changing the solvent (replacing CH3CN/H2O with H2O), as the
polarity of the solvent increased, the double-helical interactions
were eliminated and single helix of
2 was acquired. Furthermore,
3
was also self-assembled into helical chains structure under
CH3CN/H2O mixed solution system by the H2nip ligand and
Sc(NO3)3·6H2O. In addition, the structural diversities among the
three CPs also rely heavily on the substituent groups of ligands.
1
and
2 were obtained by the ligand H2aip (NH2) and
Fig. 2 (a) View of the left- and right-handed helical chains in 1. (b) 2D layer
structure of 1 along b axis. (c) 3D open framework of 1 (hydrogen atoms,
lattice water molecules are omitted for clarity). Color scheme: carbon, gray;
nitrogen, blue; oxygen, red; metal, green. (d) Simple topology of 1.
Sc(NO3)3·6H2O. Based on these, by changing the substituent
group of ligand (NO2 instead of NH2), CP
successfully.
3 was obtained
2
crystallizes in chiral cubic space group P213. The
Crystal structures
asymmetric unit of 2
contains two Sc3+ ions, one aip2- ligand. Sc1
Single-crystal X-ray analysis of CP 1 reveals that it crystallizes in
ion is surrounded by six carboxylic acid oxygen atoms from six
different aip2- ligands, displaying a octahedral coordination
geometry Sc1(COO)6. With a coordination geometry similar to
that of Sc1, Sc2 ion is coordinated to six oxygen atoms
(Sc2(COO)6) from six individual aip2- ligands. Sc-O bond lengths
fall in the region of 2.047(4)-2.098(4) Å and O-Sc-O bond angles
the tetragonal space group I41/amd. The asymmetric unit of 1
contains one central Sc3+ ion, a half aip2- ligand, one OH- group
and one lattice water molecule. The Sc3+ ion is six coordinated
environment and exhibits octahedral geometry, in which six O
atoms are from four individual aip2- ligands and two O atoms
are from two OH- groups. Sc-O bond lengths fall in the region of
2.0554(19)-2.148(3) Å and O-Sc-O bond angles varies from
varies from 85.49(15) to 177.95(14)° in
to those reported in the related Sc-CPs32.
In
, the aip2- ligand shows the same coordination mode to
link four Sc3+ ions (Scheme S1). Two adjacent Sc(COO)6
polyhedra in
are connected alternately by aip2- ligands to form
a 3D framework (Fig 3a). Unlike , the 3D structure of appears
2, all of which are similar
85.32(18) to 176.9(2)° in
those of similar Sc-CPs31.
1
, these values are consistent with
2
In the structure of
1, all carboxylate groups of H2aip ligands
2
are completely deprotonated (Scheme S1). The ScO6 units are
linked to each other through OH- groups, which generate
intertwined left- and right-hand helical chains {-ScO6-OH-}n with
a helical pitch of 11.964 Å, as shown in Fig. 2a. These two types
of opposite chiral chains are alternately linked by aip2- ligands,
1
2
Fig. 3 (a) 3D framework of 2 along c axis. (b) 3D [Sc(COO)6] framework of 2 showing the
left-handed helical chains. (c) The left-handed helical chain in 2. (hydrogen atoms are
omitted for clarity). Color scheme: carbon, gray; nitrogen, blue; oxygen, black; metal,
red.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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ARTICLE
Journal Name
question in molecular design level. With further sVtiuewdyArtiinclteoOntlhinee
DOI: 10.1039/C9CE00972H
1 and 2, it can be found that the solvents used in
structures of
experiments have obvious effects on the formation of products
with different helices, left- and right-hand helical chains {-ScO6-
OH-}n for
1 and left- hand helical chains {-Sc1(COO)-Sc2(COO)-}n
for . When CH3CN/H2O is used as solvent, Sc(III) ions are
2
coordinated by aip2-, forming left- and right-hand double helices
{-ScO6-OH-}n. The double helices are further connected by aip2-
ligands, resulting in the formation of a 3D meso framework with
1D channels. Upon changing solvent polarity from CH3CN/H2O
to contain intertwined left- hand helical chains {-Sc1(COO)-
Sc2(COO)-}n running along the screw axis, which are parallel to
each other with the same handedness and a screw pitch of
13.99 Å, as shown in Figs. 3b and 3c.
3
crystallizes in the chiral cubic space group I213.
structural characteristics very similar to . Nevertheless, there
are still some differences between the two compounds.
Asymmetric unit of
consists of one Sc(III) ion and a half nip2-
3 shows
2
3
ligand. Sc(III) ion is coordinated by six oxygen atoms, all of which
are from six nip2- ligands, overall describing a ScO6 six-
coordinated environment whose geometry resembles
a
Fig. 4 (a) The right-handed helical chain in 3. (b) 3D [Sc(COO)6] framework of 3 showing
the right-handed helical chains. (c) 3D framework of 3 along a axis (hydrogen atoms are
omitted for clarity). Color scheme: carbon, gray; nitrogen, blue; oxygen, red; metal,
green.
significantly octahedron. Sc-O bond lengths fall in the region of
2.0564(16)-2.0950(16) Å and O-Sc-O bond angles varies from
87.04(7) to 175.25(7)° in 3, all of which are similar to those
reported in the related Sc-CPs33. As depicted in Fig. 4a, the
carboxylate groups bridge Sc(III) ions alternately into an infinite
right-handed helical chain {-Sc(COO)-}n with a pitch of 14.3757
Å. The helix is generated around the crystallographic 21 screw
axis, which is consistent with the compound crystallizing in the
I213 space group. Each helical chain serves as a secondary
building unit (SBU) and is further linked to its four adjacent
neighbors with the same handedness along a axis through nip2-
linkers, generating a homochiral 3D framework, as shown in Fig.
to H2O, although the ligands are still deprotonated, ligand
spatial conformations have changed (Scheme S1, a and b). The
Sc(III) ions are connected by ligands alternately, only giving rise
to one kind of single helix {-Sc1(COO)-Sc2(COO)-}n for
However, is racemic and no optical activity is observed.
Nevertheless, it can be seen from the structure of that two
2.
2
1
carboxylic acid groups of H2aip ligand are completely
symmetrical, which lead to generate extended mesomeric
networks. Furthermore, obviously, the twisted spatial
conformations of aip2- ligands result in the helicity of the
4c. The chirality in structure
3 is a result of the 21 axis in the
symmetry. Therefore, the chirality in the present compound is
derived from the presence of helices.
molecular units for
solvent H2O into CH3CN/H2O and using H2nip (NO2) instead of
H2aip (NH2), the chiral with right- handed helical chains {-
Sc(COO)-}n was constructed, which is similar to in the structure.
Therefore, the solvent polarity and spatial conformations of
ligands together lead to the structural differences among
and . Such an accurate strategy to obtain ordered helical
2. Along these lines, further changing the
The asymmetric units of
crystallographically independent Sc(III) ions, respectively. For
H2aip ligands link the Sc(III) ions one by one, then forming a 1D
single helical chain. Whereas, for , the coordination mode of
1 and 2 contain one and two
2
,
3
2
1
two carboxylic acid groups in H2aip ligand are mirror
symmetrical (Scheme S1). Every carboxyl connects two
crystallographically independent Sc(III) ions respectively to
form intertwined left- and right-hand helical chains. Then,
1, 2
3
coordination polymers, which utilizes the rigid bridging H2aip
and H2nip ligands by solvent effect, is actually effective.
Therefore, the helicity induction based on solvent polarity
is very significant for the study of chiral transformation. The
polarity of solvent dramatically affects dispersion forces
between CPs and solvent molecules to afford some
perturbation to intramolecular interactions35.
under the help of aip2- ligands interactions,
structure. Perhaps symmetrical coordination patterns
contribute to the formation of double-stranded chains for . In
, the asymmetric carboxyl coordination mode maybe the
1 shows a 3D
1
2
asymmetrical origin of the entire single crystal structure
(Scheme S1). If the NO2 replaces NH2 in ligands, the
coordination mode of carboxyl groups may be changed
Characterization
compared to
1 and 2. In previously report, we have found that
Powder X-ray diffraction (PXRD) patterns for the bulk samples
the H2nip (NO2) ligand could be deprotonated by NaOCH3 in
H2O/CH3CH2OH, and the auxiliary ligand bimb favors the
formation of helix chain34. Therefore, using H2nip instead of
H2aip and changing the solvent into H2O/CH3CN compared with
were measured at room temperature and the diffraction peaks
perfectly match well with the simulated patterns, confirming
the phase purity of the obtained crystal samples (Figs. S1, S4
and S6). The intensity differences may be due to the preferred
orientation of the crystalline samples. Thermogravimetric
analyses (TGA) experiments were performed on pure single
1
and 2, CP 3 consisting of single-helical chains were prepared
successfully by spontaneous resolution.
In the absence of any chiral source, to induce chirality into
crystal structures from achiral building blocks is a crucial
crystal samples of 1, 2 and 3 under air atmosphere with a
4 | J. Name., 2012, 00, 1-3
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heating rate of 10 °C·min-1 in the range of 25-800 °C.
ARTICLE
displays Copper(I)/copper (II)-salen CP reported by Li et al. could be used
1
View Article Online
a continuous weight loss (13.8 wt.%) in the temperature range as an efficient heterogeneous multifunctiDoOnaI:l1c0a.1t0a3l9y/sCt9fCoEr0t0h9r7e2eH-
of 25-220 °C, which corresponds to the loss of the coordinated component Strecker reactions with 99% conversion rate50.
OH- groups, lattice water molecules in the framework (calc. 13.5
To evaluate the effectiveness of 1-3 as catalysts, compare
wt.%). The sharp weight loss above 400 °C corresponds to the the ability of each of the catalytic sites in Strecker reactions of
decomposition of the framework (Fig. S11). The TGA plots benzylidene-phenyl-amine, catalytic studies were divided into
indicate that
2 and 3 display continuous weight loss (75.5 wt.% two categories. About activated catalyst 1, 1D channels with
and 80.9 wt.%) in the temperature range of 330-550 °C and 420- dimensions of approximately 7.5×7.5 Å2 decorated with
520 °C, respectively, which correspond to the loss of the ligands pendent -NH2 groups, acts as lewis acid and base cooperative
in the frameworks (calc. 78.0 wt.% and 80.8 wt.%). (Figs. S12 catalyst in Strecker reaction. However, for activated catalysts
and S13). The IR spectra of CPs show the characteristic and , Sc(III) ions as lewis acid catalytic centers during the
bands of the carboxylic groups in the usual region. The peaks reaction. At first, Strecker reaction of benzylidene-phenyl-
observed at 1546, 1535, 1547 cm-1 for
are assigned to the amine, TMSCN, and different catalysts at ambient temperature
stretching bands of νas(COO-), respectively, while the peaks was performed, and the results are listed in Table 1. Since
1-3
2
1
-
3
3
1-3
observed at 1393, 1381, 1397 cm-1 can be assigned to the are insoluble and stable in most organic solvents (Figs. S2, S5
stretching bands of νs(COO-) (Figs. S8-S10). In addition, the and S7), meanwhile, a great deal of literatures prove that
bands at 3420 and 3390 cm-1 correspond to the symmetric and chloroform performed well as a reaction medium for Strecker
asymmetric stretching of the primary amine36,37 in
respectively.
1
and
2
,
reaction51,52. We used chloroform as reaction medium.
Table 1 Comparison of the performance of different catalysts in the Strecker reactiona.
The circular dichroism spectra (CD) analysis
The combination of single-crystal X-ray structure
determinations and circular dichroism spectra (CD) is the most
common way of gaining as much evidence as possible for the
homochirality or enantioenrichment of the bulk material. In
conversion yield(%)b
order to confirm the homochirality of
spectra were measured on crystals with a KBr pellet. The CD
spectrum of exhibits strong positive Cotton effects, indicating
that the crystals of are optically active (Fig. S15). However, the
CD spectrum of reveals the presence of racemic mixture in
2 and 3, the solid-state CD
time(h)
6.5
12
CP
58
91
99
1
CP
2
CP
3
none
trace
trace
trace
23
24
3
36
82
67
88
3
24
2
2.
a Reaction conditions unless specified otherwise: aldimine (0.14 mmol), TMSCN (47
μL), catalyst (0.02 mmol for 1, 0.01 mmol for 2 and 3), CDCl3 (2.4 mL) at room
temperature. b Yield calculated by 1H NMR with aldimines.
N2 adsorption properties
The permanent porosity of activated 1-3 were confirmed by N2
The results demonstrated that higher yields were obtained
at room temperature. It is necessary to mention that in the
absence of a catalyst, the reaction rate is slow, and even after
extending the reaction time, the reaction yields still remain low.
adsorption measurements. As shown in Fig. S20, N2-uptake (77
K) measurements gave typical I-type isotherms with an
adsorption of 35.5 cm3·g-1 for
1
. However, almost no sorptions
of N2 at 77 K were detected, particularly at low pressure,
implying non-porous structures of and . The surface areas of
Brunauer-Emmett-Teller (BET) from the N2 adsorption
In contrast with
2 and 3, 1 features the large open channels
2
3
(about 7.5 ×5.7 Å) and has accessibly functional NH2 groups
imparting basic character to the material. The subsequent
isotherms was 92.4 m2·g-1 for
1, and the surface areas of
Langmuir calculated was 123.4 m2·g-1.
experimental results show the catalytic activity of
better than the catalyst and (Table 1). Moreover, we
performed the FT-IR experiments of catalysts before and after
catalytic reactions (Figs. S8-S10). The FT-IR spectra of after
1 is obviously
2
3
Catalytic activity
1-3
Strecker reactions are one of the most classical ways for the
syntheses of aminonitriles, which are significant organic
intermediates in the synthesis α-amino acids, β-amino acids,
other nitrogen-containing heterocycles etc.38-40. Meanwhile,
Strecker reactions are well known as Lewis acid or base
catalyzed C-C bond formation. In the past decades, plenty of
homogeneous catalysts, such as metal salts, metal oxides,
metal-salen complexes, have been extensively utilized for
Strecker reactions41-45. Moreover, some CPs with catalytic sites
have been validated to be high catalytic activity for Strecker
catalytic reaction displayed a small peak at 2234 cm-1, which
resulted from the CN- vibration characteristics, implying the
presence of cyano-group in the catalysts53.
To further examine the general application of the catalyst 1, the
scope of the reaction was extended to the synthesis of various
aldimines under the above mentioned conditions (Table 2). The
results show that the catalytic activity decreases with an
increase in the size of aldimines (Table 2, entries 5, 6). The
reduced catalytic activity can be attributed to the steric effects,
which result in restricted diffusion of the bigger substrates into
the pore, and the observed catalytic activity could be mostly
reactions46-48
.
[InxGa1-x(O2C2H4)0.5(hfipbb)], prepared by
Gandara et al., showed how the activity of a heterogeneous
catalyst can be controlled in one-pot Strecker reaction49.
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ARTICLE
Journal Name
due to surface catalysis by Sc(III) ions exposed to the surface of synthesis of α-aminonitriles was also investigated by three-
View Article Online
DOI: 10.1039/C9CE00972H
component Strecker reactions of a variety of aldehydes and
the CPs.
amines with TMSCN using activated
conditions.
The results of the reaction are summarized in Table S2. To
our delight, benzaldehyde showed good conversion rate (99%)
which further inspired us to check the more reluctant substrate,
different aldehydes and amines, for the reaction. The results of
1 under identical reaction
Table 2 Strecker reaction of various aldimines catalyzed by activated 1a.
the catalytic activity of 1 reveal that both aldehydes of electron-
entry
1c
aldimine
reaction time conversion
(h)
24
withdrawing as well as amines of electron-donating groups give
good yields.
yield(%)b
trace
PXRD pattern was further applied to verify the framework
robustness of the recovered
1 sample. As depicted in Fig. S3,
similar diffraction peaks can be observed before and after the
reaction, indicating the integrity of the framework structure. It
is noticed that the catalyst
reaction system and could be reused at least four times with
only a slight decrease in the reactivity (Fig. S16).
1 can be simply filtrated from the
2
3
12
24
90
85
Conclusions
In summary, we successfully designed and prepared three 3D
Sc-CPs with different helical chains from isophthalic acid
derivatives by solvent induced methods.
1 displays a 3D
4
5
6
24
24
24
5
network with ScO6 units connected by OH- groups, exhibiting
left- and right- handed double helical chains. Interestingly, by
changing the polarity of solvents, the 3D structures can be
successfully induced from
1 with double helical chains to 2 with
99
51
single helix. In , adjacent Sc(COO)6 polyhedra are connected
2
alternately by aip2- ligands to form a 3D framework, which
contains intertwined parallel left- hand helical chains {-
Sc1(COO)-Sc2(COO)-}n. Then by changing the substituent group
of ligand (NO2 replaces NH2) and solvents, the right- handed
single helical structure of
3 was constructed. As for 3
, the nip2-
ligands bridge Sc(III) ions alternately into infinite right-handed
helical chains {-Sc(COO)-}n. Each helical chain serves as a
secondary building unit (SBU) connected by ligands to generate
a 3D framework. Furthermore, 1 exhibits good catalytic activity
a Reaction conditions unless specified otherwise: aldimine (0.14 mmol), TMSCN (47
μL), catalyst (0.02 mmol), CDCl3 (2.4 mL) at room temperature. b Yield calculated
by 1H NMR with aldimines. c The same reaction conditions without catalyst 1.
for the Strecker reactions. Solvent-induced switching of helicity
may open up a promising approach to rationally design and
control helical structures.
Besides the fact that small-scale substrates have better
efficiencies than the bulk-sized samples for the Strecker
reaction, the results show that the presence of the electron-
donating group on the aldimines leads to greater desirability for
the reaction. 4-Substituted aldimines with an electron-donating
group (-OCH3) gave higher conversions than those with
electron-withdrawing group (-CF3) (Table 2, entries 2-4). As an
electronic-donating group, -OCH3 group can result in the carbon
atom of aldimine being polarized and attacked by TMSCN
affording the corresponding α-amino nitriles. Thus, the high
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the financial support
through the National Natural Science Foundation of China
(Grant Nos. 21171065, 21201077 and 21401005).
efficiency observed for aldimines suggest the utility of
promising heterogeneous catalyst for Strecker reaction.
1 as a
References
Compared to the binary reaction, three-component Strecker
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6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9CE00972H
Graphical abstract
Three three-dimensional (3D) helical chain-based coordination polymers have
been synthesized through solvent induction without any chiral auxiliary
systematically. We first synthesized CP 1, which contains intertwined left- and
right-hand helical chains {-ScO6-OH-}n. By changing the polarity of solvents from
CH3CN/H2O to H2O, genuine single helix of 2 was constructed. Then by changing the
substituent of ligand, CP 3 was obtained with right-handed helical chains.