An uncommon multicentered ZnI-ZnIbond-based MOF for CO2fixation with aziridines/epoxides

Article abstract of DOI:10.1039/d1cc01865e

A novel cluster-based MOF with uncommon multicentered Zn^I-Zn^Ibonds {[K_1.2Na_2.8Zn^I₈(HL)₁₂]·4H₂O}_n(HL = tetrazole monoanion) (1) was synthesized, which showed higher stability than the reported Zn^I-Zn^Ibonded compounds. Moreover,1can effectively and circularly catalyze the cyclization of CO₂and aziridines or epoxides with five substituent groups. Importantly, this is the first time that the catalytic properties of MOFs with multicentered metal-metal bonded clusters as the catalyst have been studied.

Full text of DOI:10.1039/d1cc01865e

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An uncommon multicentered ZnI–ZnI bond-based
MOF for CO2 fixation with aziridines/epoxides†
Cite this: Chem. Commun., 2021,
57, 7537
Chun-Shuai Cao,ab Ying Shi,a Hang Xua and Bin Zhao
*
a
Received 26th April 2021,
Accepted 25th June 2021
DOI: 10.1039/d1cc01865e
rsc.li/chemcomm
A novel cluster-based MOF with uncommon multicentered ZnI–ZnI endow them with unexpected functions, which traditional
bonds {[K1.2Na2.8ZnI8(HL)12]Á4H2O}n (HL = tetrazole monoanion) (1) materials do not possess. To our knowledge, only a few
was synthesized, which showed higher stability than the reported cluster-based MOFs featuring uncommon LOS metal–metal
ZnI–ZnI bonded compounds. Moreover, 1 can effectively and cir- bonds have been reported in previous studies.10
cularly catalyze the cyclization of CO2 and aziridines or epoxides
On the other hand, carbon dioxide (CO2), as the primary
with five substituent groups. Importantly, this is the first time that greenhouse gas in the atmosphere, has caused various environ-
the catalytic properties of MOFs with multicentered metal–metal mental problems, such as global warming and ocean acidifica-
bonded clusters as the catalyst have been studied.
tion, etc.11 Therefore, it is an urgent task to develop novel
materials to effectively catalyze the chemical fixation of CO2.
Cluster-based metal–organic frameworks (MOFs) have been The chemical fixation or conversion of CO2 has been focused
attracting explosive research interest not only due to their on to make significant progress in the sustainable development
outstanding structural designability, but also extensive applica- of the environment and overcome these issues,12 which not
tion in storage/separation,1 magnetism,2 optical properties,3 only reduces CO2 in the atmosphere but also generates high-
especially catalysis,4 etc. Actually, polynuclear metal clusters value chemical products, for instance, cyclic carbonates,13
bridging organic ligands to assemble MOFs could greatly alkynyl carboxylic acids,14 oxazolidinones15 and others.
promote the catalytic efficiency directly due to more catalytic Among these compounds, oxazolidinones as important inter-
sites.5 To date, many cluster-based MOFs have been documen- mediates in organic synthesis, have attracted many researchers’
ted and explored; for example, eight Ti6O6 clusters and twenty- attention.16 To date, several catalytic systems have shown
four Cu2I2 dimers assemble a truncated octahedral sodalite admirable performance in the cyclization of CO2 and
cage,6 Zr6/Zr8 clusters assembled in NPF-2007 and a series of aziridines,17 whereas the recyclable performance still restricts
stable heterometallic Fe2M cluster-based MOFs (NNU-31-M, their further applications. Additionally, cyclic carbonates also
M = Co, Ni, Zn).8 In these explorations, it should be noted that show a wide range of applications in batteries, dyeing, textiles
polynuclear metal clusters in MOFs usually exhibit a common and drug intermediates.18 Though various homogeneous cata-
oxidation state rather than an uncommon low oxidation-state lysts (ionic liquids, metal–salen complexes, etc.) have been
(LOS). In fact, the electronic, luminescence, magnetic and applied in the synthesis of cyclic carbonates during the actual
catalytic properties of MOFs are intrinsically associated with industrial process,19 the reaction requires harsh conditions.
the electronic configurations of metal ions, namely the oxida- Therefore, it is necessary to seek novel catalysts to accelerate
tion state of central metal clusters.9 Therefore, uncommon LOS the cycloaddition of CO2 and aziridines or epoxides under mild
metal–metal bonds or metal centers in cluster-based MOFs will conditions. Recently, MOFs with high porosity and active sites
have been considered as ideal candidates for catalyzing the
chemical fixation of CO2, such as CO2 conversion with terminal
alkynes,20 aziridines,21 epoxides22 or propargylic amines.23
Nevertheless, to the best of our knowledge, MOF-based cata-
lysts containing uncommon LOS metal–metal bonds have never
been reported hitherto.
In this contribution, a 3D metal–organic framework {[K1.2Na2.8
ZnI8(HL)12]Á4H2O}n (HL = tetrazole monoanion) (1, † and ‡) containing
uncommon multicentered ZnI–ZnI bonds was prepared through an
a Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry,
MOE and Collaborative Innovation Center of Chemical Science and Engineering
(Tianjin), Nankai University, Tianjin 300071, China.
E-mail: zhaobin@nankai.edu.cn
b College of Environmental Science and Engineering, Nankai University,
Tianjin 300350, China
† Electronic supplementary information (ESI) available: Synthesis, TG, PXRD and
ICP-AES results. CCDC 1822099. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/d1cc01865e
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in situ [2+3] cycloaddition reaction. Interestingly, compound 1 can representative ZnI-containing Zn2R2 dimers,26 revealing
effectively catalyze the cycloaddition of CO2 and aziridines or epoxides, the formation of ZnI–ZnI bonds. In order to compare seve-
and the catalyst can be recycled at least five times still keeping high ral MOFs Na2.6K1.4{[Na(DMF)]8[ZnI8(HL)4(L)8]}Á8H2OÁ2H2LÁDMF
1
catalytic efficiency. Furthermore, H NMR spectroscopy proves that 1 (Zn8-1),10a Na3K2.33{K4[Na(DMF)3]1.33[ZnIIBr]1.33[ZnI8(HL)4(L)8]}Á
can enhance the nucleophilicity and activate the aziridines or epoxides, 2H2OÁ2H2LÁ4DMF(Zn8-2),10a {[ZnI ZnII3(H2O)6(HL)12](OH)2Á
8
which plays an important role in ring-opening of the substrates. 13H2O}n (Zn8-3),10c and {[ZnI ZnII3(H2O)2(HL)12](OH)2Á
8
Importantly, for the first time, we report a unique and multi- 13H2O}n (Zn8-4)10c (HL = tetrazole monoanion), the detailed
functional ZnI–ZnI bonded MOF for catalyzing the cycloaddition of structural information of five cluster-based MOFs contain-
CO2 and aziridines or epoxides, which enlarges the extensive applica- ing uncommon multicentered ZnI–ZnI bonds is summarized
tion of LOS metal–metal bonded MOFs.
in Table S2, ESI.† Eight ZnI ions locate at the vertices of the
The molecular structures of compound 1 were determined distorted cube to form a ZnI8 core with D4h symmetry in Zn8-1
by X-ray single-crystal diffraction, manifesting that 1 crystal- and D2d symmetry in Zn8-2. Their ZnI–ZnI bond lengths are
lized in a cubic crystal system with space group Im3 (Table S1, 2.292/2.4810 and 2.394/2.766/2.2713 Å, respectively.10a
%
ESI†). In addition, K+ and Na+ ions in 1 were characterized by Meanwhile, eight Zn1 ions as vertices construct a perfect
inductively coupled plasma-atomic emission spectrometry cubic ZnI -cluster with Oh symmetry in Zn8-3, possessing
8
(ICP-AES), and the number of isolated H2O molecules was identical Zn–Zn bond lengths of 2.443 Å.10c In Zn8-4, eight
determined by TG analysis. X-ray photoelectron spectra (XPS) Zn1 ions construct a slightly distorted cube with different
indicate that the binding energy of Zn-2p3/2 is 1020.8 eV ZnI–ZnI bond lengths of 2.412 and 2.394 Å, forming a [ZnI ]
8
(Fig. S1, ESI†), which is lower than the corresponding 2p3/2 cube with D4h symmetry.10c Due to the ZnI cubic cluster
8
binding energy value of 1021.8 eV in ZnII.24 This can match well being supported by twelve tetrazole monoanions to con-
with the values of K-edge energy of Zn with a +1 oxidation state struct a [ZnI (HL)12 4À cluster, compound 1 possesses twelve
]
8
in our previous studies,10 suggesting that the oxidation state of equal ZnI–ZnI bond lengths of 2.357 Å with D4h symmetry
4À
Zn ions in Zn8-clusters is +1. ZnI ions may originate from the (Fig. 1b and e). The unique [ZnI (HL)12
]
cluster is linked
8
4À
reduction of ZnII ions caused by the cleavage of C–C bonds. to the eight nearest [ZnI (HL)12
]
clusters by eight bridged
8
Each asymmetric unit consists of one crystallographically inde- K+/Na+ ions to construct the novel 3D structure with a 1D
pendent ZnI ion, one K+/Na+ ion and one tetrazole monoanion. channel (Fig. 1c and d). The ZnI cubic core connecting
8
The Zn1 ion coordinates to three other Zn1 (Zn1A, Zn1B and with twelve tetrazole monoanions can be considered as a
Zn1C) ions and three N1 (N1, N1D and N1F) atoms from three 12-connected node, and each tetrazole monoanion can be
different tetrazole monoanions. Eight Zn (Zn1, Zn1A, Zn1B, regarded as a 3-connected node, while a K+/Na+ ion can be
Zn1C, Zn1D, Zn1E, Zn1F and Zn1G) ions assemble the unique simplified as a 6-connected node. Based on the above
Zn8 cubic cluster with ZnI–ZnI distance of 2.357 Å (Fig. 1a and e). simplification style, the resulting network in 1 can be
The ZnI–ZnI distance is obviously shorter than the sum of described as a (3,6,12)-connected framework with new topology,
Pauling’s single-bond metallic radii of 2.50 Å,25 and falls in and the Schlafli symbol for the net is {42Á6}12{424Á636Á86}
the ZnI–ZnI bond length range (2.295–2.418 Å) of the reported {46Á86Á103}4 as shown in Fig. S2, ESI.†
The thermogravimetric analysis (TGA) shows that 1 remains
as an intact framework until about 232 1C (Fig. S3, ESI†). The
powder X-ray diffraction (PXRD) of 1 and activated sample
indicate that the prepared samples are in pure phase and still
remain stable (Fig. S4, ESI†). After immersing in six common
solvents for 24 h, the PXRD patterns of 1 consistent with the
simulated one showed that 1 exhibited high solvent-stability
(Fig. S5, ESI†). As a result, compound 1 shows higher stability
than reported ZnI–ZnI bonded compounds, which easily
decompose spontaneously into Zn0 and ZnII compounds in
air/solvent due to the nature of covalent MI–MI bonded inter-
actions, namely, two-center two-electron bonding.27 Based on
our previous theoretical analysis performed to investigate the
electronic structure of the MI8 (M = Zn or Mn) cubic clusters,10
the special chemical stability and the nature of chemical
bonding in compound 1 can be illuminated. ZnI is character-
ized with 3d104s1 valence electron structure. Thus, the effective
Fig. 1 The4Àframework structure of 1. (a) ZnI8 cube core; (b) the
overlap of Zn 4s orbitals as well Zn 3d and 4p orbitals
[Zn8(HL)12
[Zn8(HL)12
]
]
cluster in 1; (c) the connection relation among the
cluster and K+/Na+ ions; (d) the 3D framework of 1; (e)
constructs the molecular orbitals of the [ZnI8] cubic cluster.
4À
Based on the Hu¨ckel MO (HMO) analysis, the closed-shell
thermal ellipsoids are set at the 30% probability level. The Zn, K/Na, N, C
and H atoms were represented as blue, red, purple, gray and white balls,
respectively.
6
electron configuration of the [ZnI ] cube is (a1g)2(t1u
)
8
(t2g)0(a2u)0. The EPR spectrum of compound 1 showed no
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Table 1 Synthesis of various oxazolidinones from CO2 and aziridines with
catalyst 1a
signal, further proving the closed-shell electron configuration
and cubic aromaticity of the [ZnI8(HL)12]4À cluster (Fig. S6,
ESI†). Temperature dependence of magnetic susceptibility
measurement proved the diamagnetism of 1 (Fig. S7, ESI†).
The bonding pattern for eight MOs obeys the 6n + 2 rule,
representing cubic aromaticity; therefore, it can well explain
the intrinsic stability of [ZnI8] cluster-based MOFs.
Entry
1
Substrates
Conv.b
%
Yieldb
%
Regio-selc
98 : 2
The CO2 adsorption/desorption of the activated sample was
tested at 298 K (Fig. S8, ESI†), and exhibited hysteresis with CO2
uptake of 28.38 cm3 gÀ1 at ambient pressure, indicating that
the sample possessed potential ability to adsorb CO2. The N2
adsorption–desorption isotherms of 1 are characteristic of
type IV with a hysteresis loop in the range of 0.50–1.00 P/P0
(Fig. S9, ESI†), and the SBET of 1 is 31.06 m2 gÀ1. Considering
the abundant Lewis acid active sites of Zn, compound 1 was
chosen as the catalyst for a cycloaddition reaction of CO2 with
aziridines. And according to the reported methods,28 the
corresponding aziridines were synthesized as shown in Scheme
S1, ESI.† The optimized reaction conditions were investigated
by the model reaction of CO2 and 1-ethyl-2-phenylaziridine (1a),
as listed in Table S3, ESI.† Firstly, the reaction temperature was
explored (entries 1–4, Table S3, ESI†), and 70 1C was chosen as
the optimal temperature. Then the yield of 2a decreased
obviously with the reduction of CO2 pressure or additive
amount of TBAB (entries 5 and 6, Table S3, ESI†), indicative
of the fact that 2 MPa CO2 and 5% co-catalyst tetrabutylammo-
nium bromide (TBAB) were prerequisite. As a result, the opti-
mized reaction conditions should be 20 mg of catalyst 1 at 70 1C
and under 2 MPa CO2.
499
499
2
499
499
98
87
98 : 2
98 : 2
3
4
499
499
90
92
92 : 8
92 : 8
5
a
Reaction conditions: aziridines (2.0 mmol 1x), solvent-free, 20 mg of
catalyst 1 (based on metal center, about 5.3 mol%), TBAB (32.4 mg,
0.1 mmol), CO2 (2.0 MPa), 70 1C, 12 h. Determined by 1H NMR using
1,3,5-trimethoxybenzene as an internal standard. Molar ratio of
b
c
2x to 3x.
Several typical aziridines with different substituent groups
(ethyl, n-propyl, n-butyl, –Cl or –CH3) at N atoms or phenyl were
employed as substrates under the optimized reaction condi-
tions to further prove the catalytic potential of catalyst 1
(Table 1). Three N-substituted aziridines were tested in the
reaction (entries 1–3, Table 1). Both aziridines with ethyl and
n-propyl groups achieved better yields and selectivity than that
with an n-butyl group, which is possibly attributed to the higher
hindrance of the n-butyl group. In addition, aziridines bearing
different groups (–CH3 and –Cl) on the aromatic rings were also
investigated (entries 4 and 5, Table 1), and the yields of the
corresponding oxazolidinones ethyl-5-(4-chlorophenyl)-oxazolidin-
2-one (2d) and ethyl-5-p-tolyloxazolidin-2-one (2e) were high. 2d
showed low activity, which may be attributed to the electron-
withdrawing effect of the chloride group on the phenyl ring.28 In
conclusion, catalyst 1 can highly effectively catalyze the cycloaddi-
tion reaction of CO2 with an extensive range of aziridines.
monitored by 1H NMR spectroscopy (Fig. 2). After adding TBAB,
1 or 1/TBAB, the 1H signals of C–H (d E 2.45, 1.88, 1.60 ppm) in
1-ethyl-2-phenylaziridine become broad and decreased in
intensity, clearly demonstrating that 1 can enhance the nucleo-
philicity and activate the aziridines.29 According to the previous
studies, the reaction mechanism for the reaction of CO2 and
aziridines has been proposed. At first, the catalytic nature of 1
was proved, and different zinc sources [Zn(OAc)2, Zn(NO3)2,
Actually, as a heterogeneous catalyst, the recyclability of 1
was also necessary. Catalyst 1 can be recycled at least five times,
and still maintain high activity (Fig. S10, ESI†). The PXRD
patterns of 1 after recycling can keep well consistent with the
simulated one (Fig. S11, ESI†). Only o 2% leakage of Zn2+ in
the reaction filtrates was recorded by ICP-AES measurement
(Table S4, ESI†). The catalyst filtration test proved that 1 could
be used as a heterogeneous catalyst (Fig. S12, ESI†).
To explore the mechanism for this cycloaddition reaction
based on 1, the interactions between aziridines and 1 were
Fig. 2 Spectral changes on attempting activation of C–N bonds by
different systems (in CDCl3).
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ZnSO4 and ZnCl2] replacing 1 were employed to conduct the reaction Notes and references
(entries 7–10, Table S3, ESI†). The experimental results indicate that
‡ Crystallographic data for 1, {[K1.2Na2.8ZnI8(HL)12]Á4H2O}n (HL = tetrazole
monoanion), a = b = c = 14.5713(7) Å, a = b = g = 901, V = 3093.8(4) Å3,
the yields of zinc salts reacting with TBAB are less than 71%, which
mainly originates from the interaction between Zn2+ and BrÀ to
hinder the nucleophilicity of BrÀ.30 Thus, 1 plays a vital role in the
cycloaddition reaction, which may benefit from several factors: (1)
the 3D framework in 1 possessing 1D channels can enrich CO2 and
aziridines, and the reaction can be accelerated with the increased
concentration of reactants; (2) the steric hindrance between Zn2+
and the ligand restricts the coordination between Zn2+ and BrÀ,
leading to more nucleophilic attack of BrÀ on aziridines. A possible
mechanism is proposed based on the above analysis and litera-
ture:15,28 (a) firstly, the 3D structure of 1 enriches CO2 and substrate
molecules, and zinc sites coordinate with nitrogen atoms of the
aziridines; (b) secondly, BrÀ of TBAB attacks the aziridine molecule
at the C atoms by two pathways I and II resulting in opening rings
(Scheme S2, ESI†); (c) thirdly, the N anion of the ring-opened
intermediate attacks CO2 to form a carbamate salt; (d) OÀ attacks
the carbon atom to perform intramolecular ring-closure, and oxa-
zolidinones generate along with regeneration of 1 and TBAB. In
terms of selectivity, process I is the main one, which may originate
from ring-opening of the aziridine at the most easily substituted
carbon, and produce a more stable carbamate salt.
%
space group Im3, Z = 24, T = 123.60(14) K, Goodness-of-fit on
F2 = 1.569, the final R1 was 0.1208 (I 4 2s(I)) and wR1 was 0.1399 (all
data). CCDC†:1822099.
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This work was supported by the National Natural Science
Foundation of China (Grants 21625103, 21971125 and
21801183), Tianjin Natural Science Foundation (20JCQNJC01840,
20JCYBJC01200 and 20JCQNJC01940) and China Postdoctoral
Science Foundation (No. 2018M641637).
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Conflicts of interest
There are no conflicts to declare.
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