Solvent-dependent variations of both structure and catalytic performance in three manganese coordination polymers

Article abstract of DOI:10.1039/c8dt01103f

Three new manganese 4′-(3,5-dicarboxyphenyl)-2,2′:6′,2′′′-terpyridine (H₂DATP) metal-organic framework materials have been generated through regulating the ratios of a binary solvent mixture (DMA/H₂O) under solvothermal conditions. Compound 1 {[Mn₂(DATP)(HDATP)(H₂O)₄](OH)·10H₂O}_n displaying a one-dimensional (1D) chainlike structure was crystallized from the DMA/H₂O mixture with a molar ratio of 1:1, while the two-dimensional (2D) layer species, {[Mn(DATP)(H₂O)]·2H₂O}_n (2) was produced by increasing the ratio of DMA/H₂O to 5:1. Interestingly, the crystallization in pure DMA yields a three-dimensional (3D) interpenetrating network {[Mn(DATP)]·4H₂O}_n (3), featuring higher solvent stability and pH stability than compounds 1 and 2. It is proved that solvent not only influences the structural transformation process of crystals but also has a significant effect on their properties. These three compounds present different catalytic performances in the CO₂ cycloaddition to epoxides with various substituent groups into corresponding cyclic carbonates, and only 3 can serve as an efficient and recyclable catalyst at mild temperature.

Full text of DOI:10.1039/c8dt01103f

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Solvent-dependent variations of both structure
and catalytic performance in three manganese
coordination polymers†
Cite this: DOI: 10.1039/c8dt01103f
Xiao-Min Kang, Wen-Min Wang, Lin-Hong Yao, Hong-Xia Ren* and Bin Zhao
*
Three new manganese 4’-(3,5-dicarboxyphenyl)-2,2’:6’,2’’’-terpyridine (H2DATP) metal–organic frame-
work materials have been generated through regulating the ratios of a binary solvent mixture (DMA/H2O)
under solvothermal conditions. Compound 1 {[Mn2(DATP)(HDATP)(H2O)4](OH)·10H2O}n displaying a one-
dimensional (1D) chainlike structure was crystallized from the DMA/H2O mixture with a molar ratio of 1 : 1,
while the two-dimensional (2D) layer species, {[Mn(DATP)(H2O)]·2H2O}n (2) was produced by increasing
the ratio of DMA/H2O to 5 : 1. Interestingly, the crystallization in pure DMA yields a three-dimensional (3D)
interpenetrating network {[Mn(DATP)]·4H2O}n (3), featuring higher solvent stability and pH stability than
compounds 1 and 2. It is proved that solvent not only influences the structural transformation process of
crystals but also has a significant effect on their properties. These three compounds present different
catalytic performances in the CO2 cycloaddition to epoxides with various substituent groups into corres-
ponding cyclic carbonates, and only 3 can serve as an efficient and recyclable catalyst at mild
temperature.
Received 22nd March 2018,
Accepted 26th April 2018
DOI: 10.1039/c8dt01103f
rsc.li/dalton
have been reported so far and solvents play different roles in
Introduction
the coordination chemistry:13 (1) solvent as
a ligand;
In the past two decades, metal–organic frameworks (MOFs), as (2) solvent as a guest molecule; (3) solvent as both ligand and
an important class of porous materials, have stimulated tre- guest molecule; and (4) solvent as a template-directed agent.
mendous interest on account of their diversified architectures1 Hence, rationally controlling the ratio of solvents may yield
as well as a wide range of potential applications, including various coordination polymers with different properties.
catalysis,2 luminescent sensors,3 gas adsorption/separation,4
On the other hand, carbon dioxide (CO2), one of the major
magnetism,5 etc. Generally, the practical applications of greenhouse gases in the atmosphere, and industrial and
materials are closely related to their architectural character- human activities, should be responsible for climate change
istics. However, it still remains a great challenge to design and and global warming.14 At the same time, as a nontoxic, cheap,
synthesize preferable crystalline materials with desired struc- abundant, renewable and attractive C1 building block, CO2 can
tures and properties due to lots of factors in the determination be used potentially in synthetic chemistry.15 Recently, the CO2
of building block packing and growth, such as the reaction sol- transformation, as an effective method for CO2 capture and
vents,6 pH value,7 ratio of the reactants and reaction tempera- sequestration, has gained considerable attention due to the
ture.8 Subtle changes may play decisive roles in the formation generation of high-value chemicals, including cyclic carbon-
of the final topology and geometry structure.9 Importantly, a ates, urethanes, formic acid, dimethyl carbonate and others.16
reaction solvent is a crucial component in the domain of both In these investigations, one of the most promising catalysis
kinetic and thermodynamic aspects in the coordination transformations is the CO2 cycloaddition with epoxides to
process, which can directly influence the crystal growth,10 form five-membered cyclic carbonates by a 100% atom-econ-
structural interpenetration,11 crystalline dimensionality and omical reaction. In particular, the desired target cyclic carbon-
morphology.12 Several examples of solvent-dependent MOFs ates could be utilized as an important chemical intermediate
as well as a large number of fine chemicals, making them
useful in wide application.17 As far as we know, although
College of Chemistry and Key Laboratory of Advanced Energy Material Chemistry,
various catalysts have been applied in the CO2 coupling reac-
Nankai University, Tianjin, 300071, China. E-mail: zhaobin@nankai.edu.cn
tion, including zeolites, metal oxides, functional polymers and
†Electronic supplementary information (ESI) available. CCDC 1821375, 1822245
silica-supported salts, most of them own high catalytic reac-
and 1822246. For ESI and crystallographic data in CIF or other electronic format
tion temperature and pressure.18 Based on the catalytic mecha-
see DOI: 10.1039/c8dt01103f
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nism proposed in corresponding reports, Lewis-acid active M = 1170.68), calcd: C 47.15, H 4.78, N 7.17. Found: C 47.86,
sites play a key role in CO2 transformation. In comparison, H 4.79, N 7.11. IR (KBr, cm−1) (Fig. S3a†): 3201(s), 1606(vs),
MOFs possessing some Lewis-acid active sites can be treated 1553(vs), 1441(s), 1358(vs), 1241(s), 1159(m), 1088(m), 1005(s),
as efficient catalysts for CO2 cycloaddition into cyclic carbon- 770(s), 723(s), 659(vs), 571(w).
ates at mild temperature and 0.1 MPa.19 Importantly, the CO2
Synthesis of {[Mn(DATP)(H2O)]·2H2O}n (2). The synthesis of
transformation without byproducts is in accordance with compound 2 is similar to that of {[Mn2(DATP)(HDATP)(H2O)4]
green chemistry and atom economy.20
(OH)·10H2O}n (1), excepting that 3 mL H2O and 3 mL DMA are
In this contribution, three coordination polymers have replaced by 1 mL H2O and 5 mL DMA. The mixture was sealed
been obtained by varying the ratio of a binary solvent mixture in a 7 mL capped vial and was heated at 100 °C for 3 days
(DMA/H2O): a 1D chain species ({[Mn2(DATP)(HDATP)(H2O)4] under autogenous pressure and then cooled gradually down to
(OH)·10H2O}n (1)),
a
2D sheet structure ({[Mn(DATP) room temperature at the rate of 2 °C h−1. Bright yellow block
(H2O)]·2H2O}n (2)), and a 3D net framework ({[Mn(DATP)]·4H2O}n single crystals were collected (yield: 95%). Elemental analysis
(3) (4′-(3,5-dicarboxyphenyl)-2,2′:6′,2′′′-terpyridine) (H2DATP)) (see (%) for compound 2 (C23H19MnN3O7, M = 504.34), calcd:
in Scheme S1†). As the ratio of DMA/H2O changes, the number C 54.72, H 3.77, N 8.33. Found: C 54.78, H 3.78, N 8.48. IR
of H2O molecules as ancillary ligands coordinated to the manga- (KBr, cm−1) (Fig. S3b†): 3401(m), 3071(w), 1606(s), 1547(vs),
nese ion decreases, and all the water molecules are fully replaced 1436(s), 1353(vs), 1247(m), 1159(w), 1088(m), 1012(s), 870(m),
by H2DATP gradually. Interestingly, the variation of solvents leads 788(s), 730(vs), 652(s), 588(w).
to the architectural transformation of these three compounds
Synthesis of {[Mn(DATP)]·4H2O}n (3). The synthesis of com-
from a 1D chain to a 3D framework, and from nonpenetration to pound 3 is similar to that of 1, excepting that the mixed
interpenetration. Furthermore, there are differences in the stabi- solvent is substituted by 6 mL pure DMA. The reactants were
lities of 1–3, and compound 3 is more stable than 1 and 2 in sealed in a 7 mL capped glass container and were heated at
various organic solvents and acid/base solutions with the pH 100 °C for 72 hours under autogenous pressure. After cooling
range from 2 to 13. Diverse catalytic activities of 1–3 were observed slowly to room temperature, bright yellow block-shaped crys-
in the CO2 cycloaddition with epoxides into five-membered tals of 3 were obtained (yield: 94%). Elemental analysis (%) for
cyclic carbonates, but only compound 3 can be reused six compound 3 (C23H21MnN3O8, M = 522.30), calcd: C 52.84,
times at mild temperature.
H 4.02, N 8.04. Found: C 52.91, H 3.97, N 8.10. IR (KBr, cm−1
)
(Fig. S3c†): 3389(m), 3059(w), 612(s), 1553(vs), 1436(s),
1353(vs), 1247(m), 1159(w), 1088(w), 1012(s), 870(m), 782(s),
730(vs), 652(vs), 594(w).
Experimental section
Materials and general methods
Crystallographic determination
All chemicals and solvents used for the syntheses are reagent
grade without further purification. The new ligand (H2DATP)
was characterized by IR and 1H NMR analyses (Fig. S1 and
S2†). Powder X-ray diffraction (PXRD) data were recorded on a
Rigaku D/Max-2500 diffractometer using Cu Kα radiation (λ =
1.54056 Å), with a scan speed of 5° min−1 in the 2θ = 3–60°
region. The Fourier transform infrared (FT-IR) spectra were
recorded on a Bruker Tensor 27 spectrophotometer using KBr
pellets in the range of 400–4000 cm−1. Elemental analyses (EA)
for C, H, and N were performed by a PerkinElmer elemental
analyzer. Thermogravimetric analyses (TGA) were carried out
on a Netzsch TG 209 TG-DTA analyzer with heating the crystal-
line samples from room temperature to 700 °C at a rate of
10 °C min−1 under a nitrogen atmosphere.
Suitable single crystals of 1–3 were selected for single-crystal
X-ray diffraction analysis under a nitrogen atmosphere. The
crystal data were recorded on an Oxford diffractometer
SuperNovaTM equipped with a graphite-monochromatic Mo Kα
radiation (λ = 0.71073 Å) using a ω–ϕ scan technique and cor-
rected for Lorentz-polarization effects. The single-crystal struc-
tures of 1–3 were solved by directed methods and refined by
full-matrix least-squares techniques based on F2 using
SHELXS-97 and SHELXL-97 programs.21,22 All the non-hydro-
gen atoms were refined with anisotropic thermal parameters
while H atoms attached to C atoms were placed in calculated
positions and refined using the riding model. The water H
atoms were located from the difference maps. The SQUEEZE
routine within the PLATON software package was employed to
remove the solvent contribution in compounds 1–323 and the
guest molecules could be confirmed by elemental analysis and
Syntheses of compounds 1–3
Synthesis of {[Mn2(DATP)(HDATP)(H2O)4](OH)·10H2O}n (1). thermogravimetric analysis. All crystallographic data and
A mixture of 0.03 mmol Mn(Ac)2·2H2O (0.0073 g), 0.025 mmol refinement details for 1–3 are listed in Table 1 and selected
H2DATP (0.0099 g), 3 mL H2O and 3 mL DMA (N,N’-dimethyl- bond lengths and angles are shown in Table S1.† All the
acetamide), together with 100 µL HNO3, was sealed in a 7 mL crystal data of 1–3 have been deposited with the Cambridge
capped vial. The vial was heated at 100 °C for 72 h under auto- Crystallographic Data Centre (1821375, 1822245, 1822246†).
genous pressure and then cooled slowly down to room tem-
Catalysis
perature at 2 °C h−1. Bright yellow block single crystals were
obtained. The yield was 92% (based on Mn(Ac)2·2H2O). The general procedure for the CO2 coupling reaction with
Elemental analysis (%) for compound 1 (C46H56Mn2N6O23, epoxide to form cyclic carbonates is described as follows: first,
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Table 1 Crystal data and structure refinement parameters of compounds 1–3
1
2
3
Empirical formula
Formula weight
C46H56Mn2N6O23
1170.68
C23H19MnN3O7
504.34
C23H21MnN3O8
522.30
Temperature/K
Crystal system
Space group
293(2)
113(15)
Monoclinic
C2/c
293(2)
Tetragonal
P41212
Triclinic
ˉ
P1
a/Å
b/Å
c/Å
α/°
11.3671(5)
11.6113(3)
20.9861(7)
80.625(3)
74.837(3)
73.183(3)
2547.68(16)
2
27.489(5)
12.3570(16)
13.6835(17)
90
118.528(3)
90
4083.7(10)
8
17.3875(12)
17.3875(12)
18.3017(9)
90
90
90
5533.0(8)
8
β/°
γ/°
Volume/Å3
Z
ρcalc g cm−3
1.269
0.557
1.523
0.689
1.081
0.503
μ/mm−1
F(000)
1216
5.972 to 50.02
17 050
0.0207
1.028
R1 = 0.0421, wR2 = 0.1118
R1 = 0.0534, wR2 = 0.1198
0.617/−0.464
—
2072.0
6.04 to 50.018
25 670
0.0289
2152.0
6.464 to 50.012
11 972
0.0856
0.988
R1 = 0.0688, wR2 = 0.1338
R1 = 0.1025, wR2 = 0.1495
0.498/−0.299
0.08(3)
2θ range for data collection/°
Reflections collected
Rint
Goodness-of-fit on F2
Final R indices [I ≥ 2σ(I)]
Final R indices [all data]
1.061
R1 = 0.0359, wR2 = 0.1002
R1 = 0.0391, wR2 = 0.1022
0.772/−0.444
Δρmax/min (e Å−3
)
Flack parameter
—
2 mmol of the corresponding epoxide, 0.025 mmol tetrabutyl- while the reaction results in the generation of 2 in the DMA–
ammonium bromide (TBAB) (16.2 mg), and 3.05 mol% catalyst H2O ratio of 5 : 1. With the concentration of DMA increasing,
(based on Mn) were slowly added to a glass vial (10 mL). Then the number of water molecules coordinated to the central
CO2 (0.1 MPa) was introduced into the vial using a balloon metal ion decreases. When a pure DMA solvent was used, all
and the resulting reaction mixture was stirred at different the coordinated H2O molecules were fully replaced by H2DATP
temperatures. After 12 h, when the reaction was cooled down and 3 was isolated (see in Scheme 1). It is worth noting that
to room temperature, 0.025 mmol 1,3,5-trimethoxybenzene 0.1 mL HNO3 is applied to improve the yield and crystalline
(42 mg) and 3 mL CH2Cl2 were added to the reaction products. quality.
The mixture was stirred for 5 min and the supernatant was
obtained after centrifugation. The yield of the product was
Crystal structure description
determined by 1H NMR spectroscopy using 1,3,5-trimethoxy-
{[Mn2(DATP)(HDATP)(H2O)4](OH)·10H2O}n (1). Single-crystal
benzene as an internal standard and the relational expression
diffraction analysis reveals that compound 1 crystallizes in the
is shown in detail in Fig. S4.†
ˉ
triclinic system with the space group P1. Each asymmetric unit
is completed by two crystallographically independent Mn2+,
one independent DATP2− and HDATP−, four coordinated water
molecules and ten free water molecules. As shown in Fig. 1a,
Results and discussion
Syntheses
the seven-coordinated Mn2+ is connected by three N atoms
and two carbonyl oxygen atoms from one H2DATP, as well as
Three new Mn-based MOFs were prepared by the solvothermal two coordinated water molecules, presenting an irregular
method with good reproducibility (Fig. S5†) and characterized decahedron coordination geometry. The Mn–N distances fall
by single-crystal X-ray diffraction, PXRD, EA, TGA and IR into the range of 2.2755(19)–2.355(2) Å. The Mn–OCOO− average
analyses.
value is 2.3681(18) Å, while Mn–OH O bond lengths are slightly
2
The solvothermal syntheses of MOFs generally undergo a shorter [Mn1–O1 = 2.211(2) and Mn1–O2 = 2.242(2) Å], which
complicated process, which is influenced by lots of synthesis is similar to those in related Mn-based MOFs.24 A one-dimen-
parameters, including reaction temperature, concentration of sional chain structure is generated by end-on coordination
reactants, pH, counterion and types of solvents. As a primary modes of three N atoms and two carbonyl oxygen atoms from
component for the reaction, solvents play key roles in the H2DATP (Fig. 1b). The guest DATP2− and HDATP− anions form
growth of crystals and the structural transformation of MOFs. intermolecular charge-assisted hydrogen bonds between the
In our work, 1–3 were obtained via the hydrothermal reaction negatively charged and neutral carboxylate groups with a short
of Mn(Ac)2 and the H2DATP ligand in a binary solvent mixture O⋯O distance of 2.487(3) Å. Then, abundant OH O⋯OCOO− and
2
of DMA and H2O. In the presence of 0.1 mL HNO3 in DMA– O–H⋯O hydrogen bonds between the neighboring chains
H2O (6 mL, v/v = 1 : 1), compound 1 was produced at 100 °C, extend the chains into the 3D supramolecular framework
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Scheme 1 The assembly of compounds 1, 2 and 3 tuned by the solvent ratio.
Fig. 1 (a) The coordination environment of Mn2+ in 1. (b) The chainlike
structure of 1. All hydrogen atoms are omitted for clarity. Green: Mn; red: O;
blue: N; gray: C. (c) The 3D supramolecular framework of compound 1
through the H-bond interactions between the neighboring chains.
Fig. 2 (a) The coordination environment of Mn2+ in 2. (b) The 2D layer
structure of compound 2. (c) The 3-connected fes topological structure
for 2 with the Schläfli symbol of {4.8^2}. All hydrogen atoms are omitted
for clarity. Green: Mn; red: O; blue: N; gray: C.
(Fig. 1c). The distance of O6⋯H12 is 1.683 Å and the angle of
O6⋯H12–O12 is 166.447°.
{[Mn(DATP)(H2O)]·2H2O}n (2). When the ratio of DMA/H2O
was tuned to 5 : 1, the dimensionality of 2 changes correspond-
ingly. Single-crystal diffraction analysis demonstrates that com- from 56.10(4) to 170.90(5)°, which agrees well with those
pound 2 crystallizes in the monoclinic system with the space reported.25 In order to simplify the structure of 2, the 2D
group C2/c. As shown in Fig. 2, 2 presents a two-dimensional framework was constructed by the freely available TOPOS soft-
(2D) layer structure, and mononuclear Mn2+ bonds to one ware package. Then, each Mn ion acts as a 3-connected node,
crystallographically independent DATP2−, one oxygen atom and each H2DATP also serves as a 3-connected node linking
from the other DATP2− and one water molecule. Each DATP2− three Mn ions. As a result, such connectivity affords a 3-con-
possesses three N atoms and two carboxylate groups, leading nected fes topological framework with the Schläfli symbol of
to a double negative charge on the ligand. The Mn–OCOO− {4.8^2} (Fig. 2c).
average distance is 2.2795(46) Å and the Mn–OH O bond length
{[Mn(DATP)]·4H2O}n (3). Compound 3 was obtained in pure
2
is 2.1758(13) Å. Additionally, the O–Mn–O bond angles range DMA solvent, featuring a 3D framework structure (Fig. 3b).
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Fig. 3 (a) Coordination mode of the Mn2+ ion in 3. (b) The 3D network of compound 3. (c) Representation of the two-fold interpenetrating structure
with a SrSi2 (srs) topology from two identical and independent 3D networks. All hydrogen atoms were omitted for clarity. Green: Mn; red: O; blue: N;
gray: C.
Single-crystal diffraction analysis displays that 3 crystallizes in molecules determines the coordination modes, giving rise to
the tetragonal system with the space group P41212. The asym- different structures.
metric unit consists of one isolated Mn2+, one independent
DATP2− as well as four free water molecules. As shown in
Influence of the solvents on the assembly of frameworks
Fig. 3a, each central Mn2+ adopting a six coordinated geometry H2DATP as a novel building linker was chosen for constructing
occupies one crystallographic site, which is coordinated by new MOFs on account of its multidentate characteristic to
three N atoms with the Mn–N distances from 2.248(6) to provide versatile coordination modes and the close relation-
2.349(6) Å and three carbonyl oxygen atoms. The Mn–O bond ship of coordination natures with the reaction parameters. It is
lengths fall in the range between 2.133(6) and 2.270(5) Å, and found that the ratio of solvents is of great importance in the
the O–Mn–O bond angles change from 57.72(18)° to 158.7(2)°, assembly of metal ions and ligands by the comparison of sum-
which matches well with those reported previously.26 In com- marizing synthetic conditions and architectures of 1–3. The
pound 3, a cavity with the pore of an independent network is 1D chain structure was generated under the condition of DMA/
12.142 × 12.142 Å2 (Fig. 1c). Two identical and independent 3D H2O = 1 : 1 while the 2D layer structure was produced in a rich-
frameworks interdigitate into each other, generating a two-fold DMA environment. When the pure DMA was used, the 3D
interpenetrating architecture. The 1D channels are observed 2-fold interpenetrating network was obtained. Due to the
along the [001] direction in 3, presenting the solvent-accessible strong competition between the H2DATP ligand and the water,
volume of 40.3%, calculated by the PLATON program.23 Better the number of coordinated water molecules acting as ancillary
insight into the 3D network can be attained by topology ana- ligands around the Mn(II) center (2 in 1, 1 in 2, 0 in 3) depends
lysis. In this network, each Mn ion can be regarded as a 3-con- on the water ratio in mixed solvents (see in Scheme S2†). The
nected node that coordinates to three H2DATP ligands, and high dimensional (3D) framework is inclined to form in pure
each H2DATP ligand belongs to a 3-connected spacer, which is DMA environment, which may be explained by more stronger
linked by three Mn ions. Thus, the two-fold interpenetrating metal–ligand coordination than metal–water interaction.
framework of 3 can be simplified into a 3-connected SrSi2 (srs) Therefore, the subtle change of solvents plays a crucial role in
topology with the point symbol of {10^3} (Fig. 3d). Comparing regulating the dimensionality and interpenetration of
compound 3 with 1 and 2, the absence of coordinated water structures.
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Table 2 Cycloaddition reaction of CO2 with styrene oxide under
different conditionsa
Powder X-ray diffraction (PXRD) studies and thermal behavior
(TGA)
The PXRD patterns of 1–3 are shown in Fig. S6.† Good consist-
ency between the simulated and experimental peaks demon-
strates high phase purity of 1–3. In order to evaluate the
solvent and pH stabilities of 1–3, these crystal samples were
immersed in various common solvents for about 12 h but only
the PXRD patterns of 3 agree well with the simulated one
(Fig. S7†). Then 1–3 were treated with different acid/base solu-
tions for 12 h, respectively, but the comparison of simulated
results with experimental ones revealed that only 3 can remain
intact in the solutions with the pH range from 2.0 to 13.0
(Fig. S8†). Therefore, compound 3 possesses higher solvent
stability and pH stability than 1 and 2. Moreover, the thermal
stabilities of 1–3 were explored (Fig. S9†). For compound 1, the
TGA curve displays a weight loss of 21.78% (calculated
21.53%) in the range from 25 °C to 347 °C, which is attributed
to the removal of ten free and four coordinated water mole-
cules. 2 and 3 were observed to lose 10.65% (calculated
10.71%) and 13.42% (calculated 13.78%) weight, respectively,
before the collapse of their structures, corresponding to the
removal of free and coordinated water molecules. Then, these
frameworks begin to collapse at nearly 355 °C (346 °C for 1,
355 °C for 2, and 361 °C for 3), indicating the good thermal
stabilities of 1–3.
Entry
Catalyst 3 [mg]
TBAB [%]
T [°C]
Conv.b [%]
1
2
3
4
32
32
32
32
32
32
0
0
32
0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0
50
60
70
80
90
100
70
70
70
70
59
73
99
94
93
91
13
11
15
48
5
6
7c
8d
9
10
2.5
a Reaction conditions: styrene oxide (240.2 mg, 2.0 mmol), solvent-free,
catalyst 3 (32 mg, 0.061 mmol), TBAB (16.2 mg, 0.025 mmol), CO2 (0.1
MPa), 12 h, 32 mg catalyst 3 loading (based on Mn, about 3.05 mol%).
b Yield of the product determined by 1H NMR spectroscopy using
1,3,5-trimethoxybenzene as an internal standard. c MnCl2 (12.07 mg,
0.061 mmol). d Mn(Ac)2 (10.55 mg, 0.061 mmol).
yields in 3 (entries 2 and 3 in Table 3), respectively), revealing
that the electron-withdrawing groups contribute to the ring-
opening of substrates by nucleophilic attack. Inversely, when
Catalytic studies
Given the manganese Lewis-acid sites in 1–3, the catalytic per- 1,2-epoxyhexane and epoxy-2-methylpropane with the electron-
formances are evaluated for the cycloaddition of CO2 with donating group were used, a dramatic reduction in the yields
epoxides as a model reaction under 1 atm CO2 pressure at of desired products was detected, as indicated by 51% for-
different temperatures with 2.5 mmol% TBAB as a co-catalyst mation of 1,2-epoxyhexane carbonate (entry 4 in Table 3) and
for 12 h. Styrene oxide is selected as a probe substrate to inves- 42% formation of epoxy-2-methylpropane carbonate (entry 5 in
tigate the optimized conditions, and the corresponding results Table 3) with 3 as the catalyst (51% and 41% yields in 1
show that the yields of the desired products rise first and then (entries 4 and 5 in Table S3a†), and 59% and 39% yields in 2
fall from 50 to 100 °C using 3.05 mol% 1–3 as a catalyst (based (entries 4 and 5 in Table S3b†)). Additionally, under identical
on Mn), respectively (Tables S2a and S2b† and Table 2). Both conditions, the cyclohexene oxides were converted into the
lower and higher temperatures can reduce the catalytic activity. corresponding products with low yields (entry 6 in Table 3,
The optimum value (99%) of styrene carbonate can be observed Table S3a and S3b†) possibly due to the large steric hindrance
at a mild temperature of 70 °C for 1 and 3 (entry 3 in Table S2a† of the substrate. The productivities of cyclic carbonates cata-
and Table 2), while the highest yield (98%) of the desired lyzed by MnCl2 and Mn(Ac)2 are 13% and 11%, respectively
product was obtained at 90 °C for 2 (entry 3 in Table S2b†), (entries 7 and 8 in Table 2), showing that the inorganic salts
which may be attributed to the different structures of 1–3. with various anions can’t work effectively in this catalytic
However, only 48% conversion to 4-phenyl-1,3-dioxolan-2-one reaction.
was obtained using 2.5 mol% TBAB alone in the absence of a
Recyclability is an essential and important factor for hetero-
catalyst at 70 °C (entry 10, Table 2), indicating that the nucleo- geneous catalysts in application, and the residual products of
philic attack of Br− to epoxide will be enhanced tremendously compounds 1–3 could be separated from the reaction mixture
with the help of manganese Lewis-acid sites.
through centrifugation and filtration, respectively. Then, the
Under the optimized conditions, the catalytic coupling of catalytic reactions of the residual products on CO2 conversion
CO2 with other epoxides substituted by different functional were carried out to explore the reusability of 1–3. As illustrated
groups has been conducted to examine the effectiveness of in Fig. 4, 3 can be recycled at least six times without significant
1–3. A relatively high catalytic activity has been observed for decrease in catalytic activity, whereas 1 and 2 can’t catalyze the
the cycloadditions of epoxy chloropropane and glycidyl phenyl CO2 cycloaddition repeatedly, which is explained by the limit-
ether with CO2 into corresponding cyclic carbonates (95% and ation of their chemical stabilities. The PXRD patterns of used
92% yields in 1 (entries 2 and 3 in Table S3a†), 90% and 91% 3 are in line with the simulated ones, confirming that 3 can
yields in 2 (entries 2 and 3 in Table S3b†), and 94% and 92% remain stable after six successive recycles (Fig. S10†). Then,
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Based on reported work,27 taking 3 as an example, the
plausible mechanism of the CO2 coupling reaction has been
studied carefully, as shown in Fig. S12.† First of all, the oxygen
atom from epoxide bonds with unsaturated Mn Lewis-acid
sites in 3 and the epoxy ring is activated. Compared with 1 and
2, more space is available for the activation of the epoxy ring
by the catalytic action of 3 due to the change in coordination
number from seven to six. Secondly, the ring-opening process is
started by the attack of nucleophilic Br− on the less-hindered C
of epoxides.28 Subsequently, CO2 reacts with the oxygen anion
of the opened epoxy ring to generate an alkylcarbonate anion
and the corresponding cyclic carbonate is obtained finally
through ring-closure. We deduce that the synergistic effect of 3
and TBAB has facilitated the chemical transformation of CO2
into cyclic carbonates. In truth, it is necessary to examine inter-
mediates during the CO2 coupling and the research in this
direction will be explored in the future.
Table 3 Yields of the cycloaddition reaction of CO2 with various epox-
ides under optimized conditionsa
Catalyst
3 [mg]
Yieldb
[%]
Entry
1
Subs
Prod
32
32
32
99
94
92
2
3
In conclusion, three new coordination polymers have been
prepared in the presence of mixed solvents with advisable
ratios and characterized structurally, featuring a 1D chain
species, 2D layer structure and 3D network with two-fold inter-
penetration for 1–3, respectively. Structural variation con-
trolled by solvents further makes a difference in the properties
of 1–3. The chemical stability analysis reveal that only com-
pound 3 can resist various organic solvents and acid/base solu-
tions with pH range from 2 to 13. Different catalytic activities
are detected for compounds 1–3 in the synthesis of cyclic car-
bonates from CO2 and epoxides. Importantly, only 3 can serve
as a high-efficiency heterogeneous catalyst in CO2 cyclo-
addition with excellent cyclic performance.
4
5
32
32
51
42
6
32
4
a Reaction conditions: epoxides (2.0 mmol), catalyst 3 (32 mg, about
3.05 mol%, based on Mn), TBAB (16.2 mg, 2.5 mmol%), solvent-free,
CO2 (0.1 MPa). b Determined by 1H NMR spectroscopy using 1,3,5-tri-
methoxybenzene as an internal standard.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the NSFC (21625103, 21571107,
and 21421001), and the 111 Project (B12015).
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