A highly stable Zn₉-pyrazolate metal–organic framework with metallosalen ligands as a carbon dioxide cycloaddition catalyst

Abstract

A three-dimensional (3D) metal–organic framework constructed from unprecedented Zn₉O₂(OH)₂(pyz)₁₂ (pyz = pyrazolate) clusters and Ni(salen)-derived linkers was reported. The MOF exhibits high catalytic activity for CO₂ cycloaddition reactions with excellent stability. The MOF catalyst can be recycled at least 15 times without loss of activity and the turnover number (TON) per Ni site can reach as high as 2887.

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As emerging materials, porous metal–organic frameworks (MOFs) have attracted much attention for applications in gas storage and separation,¹ chemical sensing,^2,3 catalysis,⁴ and many other fields over the past two decades.⁵ MOFs are constructed from organic linkers, usually polycarboxylates^6,7 or polyazolates,^8,9 and metal/metal cluster nodes, and thus are of rich structural and functional diversity that allows their fine-tuning and modification to meet different requirements of the mentioned applications. However, challenges remain. Chemical stability is one of the major concerns for further applications of MOFs.^10–12 Several strategies have been developed to design more stable MOFs: (i) increasing the metal–ligand interaction strength with high pK_a ligands (e.g. azoles) and/or high-valent metal ions (e.g. Zr⁴⁺,Cr³⁺);^13,14 (ii) reducing the contact between water or aqueous attacking reagents and the coordination bonds of host frameworks;¹⁵ (iii) increasing the rigidity of ligands to enhance the stability.¹⁶ MOFs based on azolate donors usually show better stability than those of carboxylate donors because the sp² N atoms tend to adopt saturated coordination and thus become locally hydrophobic and unavailable for interaction with attacking reagents.^17,18 Meanwhile, interpenetrated motifs also significantly enhance the framework stability.¹⁹ It is not only because interpenetrated networks could reduce the pore size and increase the wall thickness, but also due to the higher strength of non-covalent interactions in interpenetrated structures which lock the linkers in place and prevent their displacement.¹⁵

Metallosalen complexes are one of the earliest developed and most widely explored categories of asymmetric transition metal catalysts.^20–22 Since the early 1990s, metallosalen complexes have found catalytic applications for olefin epoxidation,²³ aldehyde cyanation,²⁴ epoxide ring-opening²⁵ and CO₂ cycloaddition reactions.^26,27 Heterogenized metallosalen catalysts have been constructed as surface-grafted silica or resins,²⁸ MOFs,^29–31 covalent organic frameworks (COFs),^32,33 and porous organic polymers (POPs)³⁴ for better catalyst separation and recyclability. However, it is still challenging to balance the catalytic activity/selectivity with material stability. MOF catalysts based on metallosalen-derived polycarboxylate linkers have shown exceptional catalytic performance compared to other heterogeneous metallosalen catalysts,^35–38 but the stability of those materials is not yet satisfactory.¹⁷

Herein, we report the synthesis and catalytic applications of a zinc-pyrazolate MOF constructed from Ni(salen) bispyrazolate ligands and unprecedented Zn₉ clusters. The zinc-pyrazolate connection allows the MOF to withstand acid and base treatments as well as boiling water. The MOF also exhibits high catalytic activity and exceptional recyclability for CO₂-epoxide cycloaddition reactions.

The Zn₉-pyrazolate MOF Zn₉O₂(OH)₂(L)₆ was obtained by a solvothermal reaction between Zn(OAc)₂·6H₂O and the Ni(salen)-derived ligand H₂L (Fig. 1b and Scheme S1†) in a DMF/H₂O/trifluoroacetic acid mixture solution. As shown in the scanning electron microscopy (SEM) image, the crystalline MOF particles are of oblique dodecahedron shape with a uniform size of 20 μm (Fig. 1d). The exact formula and structure of the MOF were determined by single-crystal X-ray diffraction (SCXRD) studies. Preliminary reflection data reduction indicates that the MOF crystallizes in the cubic Fd3̄m space group. Further structure refinement revealed that the L²⁻ linker exhibits symmetry-related, two-fold disorder. To better describe the structure, the apparent symmetry of the framework was arbitrarily lowered to the chiral F4₁32 space group with a racemic twinning parameter accounting for the disorder. The MOF has a formula of Zn₉O₂(OH)₂(L)₆ and contains an unprecedented [Zn₉O₂(OH)₂(pyz)₁₂] cluster secondary building unit (SBU) (Fig. 1a). In the SBU, eight Zn atoms are located at the eight vertexes of a cuboid with an additional Zn atom at the center. The central Zn(II) atom adopts a distorted tetrahedral ZnO₄ coordination sphere, while the four O atoms connect to eight vertical Zn(II) atoms in a μ₃-O fashion (Fig. 1a). The twelve pyrazolate groups coordinate to the Zn₉ cuboid cluster from the directions of twelve edges; therefore, each vertical Zn(II) ion is tetracoordinated in a ZnN₃O fashion. The Zn–Zn distances between vertical and central Zn(II) ions are 3.06 Å and 3.14 Å, respectively, and the distances between vertical Zn(II) ions are 3.58 Å, showing no intermetallic bonding. Previous studies on zinc-pyrazolate coordination chemistry have found [Zn₄O(pyz)₆] and [Zn₃O(pyz)₃] clusters, and ZnN₄ tetrahedron SBUs;^18,39–41 yet to the best of our knowledge, the [Zn₉O₂(OH)₂(pyz)₁₂] cluster has never been reported. The Zn₉O₂(OH)₂ cluster SBU is connected to twelve adjacent SBUs through Ni(salen)-based bis(pyrazolate) linkers to form a 3D network with face-centered cubic unit (fcu) topology, the same as that constructed from Ni₈ clusters and linear bis(pyrazolate) linkers.⁴² The MOF adopts a two-fold interpenetration structure, which is unprecedented in previous Ni₈(pyrazolate) MOFs.^42–45 The Zn₉ cluster of one network is located in the tetrahedral cavity formed by another network with 50% occupancy (Fig.S4†). The two interpenetrating networks are symmetrically related by the 4₁ screw axis and pseudo d glide plane. Because of the interpenetration, the pore size of the Zn₉O₂(OH)₂(L)₆MOF is only 10 Å as measured from the single crystal structure, a relatively small value compared to the ligand length. The powder X-ray diffraction (PXRD) pattern of the bulk MOF sample matches well with that from single-crystal structure simulation, confirming the phase purity (Fig. 1e). X-ray photoelectron spectroscopy (XPS) shows that both Zn and Ni are in the +2 oxidation state (Fig.S5†).

Fig. 1 Construction of the Zn₉O₂(OH)₂(L)₆ MOF (Zn: turquoise; Ni: lime; N: blue; O: red.): (a) the Zn₉O₂(OH)₂(pyrazolate)₁₂ node; (b) the Ni(salen)-derived bis(pyrazolate) ligand; (c) two-fold interpenetration structure of the Zn₉O₂(OH)₂(L)₆ MOF; (d) SEM image of Zn₉O₂(OH)₂(L)₆; (e) simulated and experimental PXRD patterns of Zn₉O₂(OH)₂(L)₆; (f) N₂ sorption isotherms of the Zn₉O₂(OH)₂(L)₆ MOF (inset: pore size distribution by the NLDFT method).

The N₂ sorption isotherms (Fig. 1f) of Zn₉O₂(OH)₂(L)₆ revealed a typical type I behavior with a Brunauer–Emmett–Teller (BET) surface area of 934 m²g⁻¹. Non-local density functional theory (NLDFT) pore size distribution analysis gave a total pore volume of 0.215 cm³g⁻¹ with a pore size of 12.7 Å (Fig. 1f), which is in good agreement with the two-fold interpenetration structure. The CO₂ sorption isotherm (Fig. S7a†) gave a maximum CO₂ uptake of 168 cm³g⁻¹. The Zn₉O₂(OH)₂(L)₆ MOF retained its crystallinity even after sorption tests as shown by the unchanged diffraction peaks (Fig.S7b†).

To explore the chemical stability of Zn₉O₂(OH)₂(L)₆, the MOF samples were treated with HCl, NaOH and ammonia aqueous solutions at room temperature, and H₂O and HCl upon heating. In pH 2 HCl, 2.5% ammonia, and 2 M NaOH solutions, the MOF samples were stable for at least 210 days at room temperature as indicated by the PXRD patterns (Fig. 2). In pH 3 HCl and H₂O, the MOF samples were stable for at least 21 days at 100 °C (Fig. 2 and S8†). Therefore, Zn₉O₂(OH)₂(L)₆ exhibits better chemical stability compared with most pyrazolate-based MOFs (Table S3†).^46,47 Such good stability could be attributed to a combination of several effects. First, the Zn-pyrazole saturated coordination protects the metal centre from being attacked by a base.^10,48 Second, the two-fold interpenetrated structure reduces the MOF porosity and inhibits guest diffusion through MOF channels, making the MOF more stable.^15,49 Third, the pyrazolate ligand with hydrophobic t-butyl group modification results in a hydrophobic MOF with a water contact angle of 114° (Fig. S9†), further reducing the contact of the MOF with aqueous attacking reagents.⁴² All these effects taken together lead to the high chemical stability of Zn₉O₂(OH)₂(L)₆.

Fig. 2 PXRD patterns of the as-prepared Zn₉O₂(OH)₂(L)₆ and those after acid, base and heating treatments.

Great interest has been shown in converting CO₂ into value-added chemicals and reducing the greenhouse gas concentration in the atmosphere. The cycloaddition of CO₂ with epoxides (Scheme 1) has been widely proposed as an effective approach for that purpose due to its near-100% atomic efficiency.^50,51 Different types of catalysts, including organocatalysts,⁵² ionic liquids,⁵³ metallosalen complexes,²⁶ single atom catalysts⁵⁴ and other coordination compounds,^55,56 have been reported as effective catalysts for CO₂ cycloaddition reactions. POPs³⁴ and MOFs^38,57–60 have been reported as the heterogeneous version of those molecular catalysts to facilitate the separation and recycling processes. Given the high stability of the Ni(salen)-derived Zn₉O₂(OH)₂(L)₆ MOF, we expected that it could be a good candidate as a heterogeneous CO₂ cycloaddition catalyst. We first investigated its catalytic activity with styrene oxide as a model substrate. The reaction was almost complete in 24 hours at 80 °C in the presence of 1.5% MOF catalyst and 1.5% tetrabutylammonium bromide (TBAB) co-catalyst and under 7 bar CO₂ atmosphere. When the MOF and TBAB loading was decreased from 1.5% to 0.5%, the yield of the carbonate product only decreased from 99% to 95% (entries 1 and 2, Table 1), indicating the high catalytic activity of the MOF/TBAB catalyst. When either the MOF or TBAB was absent, the product yield was much lower under the same pressure and temperature conditions (Fig. 3a and entries 3 and 4, Table 1), showing that both the components are necessary for the catalysis. Using the Ni(salen)-derived ligand H₂L instead of the MOF gave a similar yield for styrene carbonate, showing that the ligand is the catalytically active component (entry 5, Table 1). Reducing the reaction temperature or pressure led to lower yields, yet such an effect can be compensated for by increasing the catalyst loading to 1% (entries 6–8, Table 1).

Scheme 1 CO₂ cycloaddition reaction catalyzed by the MOF and TBAB.

Fig. 3 (a) Styrene oxide conversions with different catalysts; (b) styrene oxide conversions at different runs in the reuse experiments for CO₂ cycloaddition. Reaction conditions: 80 °C, 7 atm CO₂, 0.5% MOF catalyst and 0.5% TBAB co-catalyst.

Table 1 Optimization of reaction conditions for the cycloaddition of CO₂ with styrene oxide

Entry	Catalyst (%)	TBAB (%)	P (atm)	T (°C)	Conversion (%)
a The ligand H2L was used instead of the MOF.
1	1.5	1.5	7	80	99
2	0.5	0.5	7	80	95
3	0.5	0	7	80	8
4	0	0.5	7	80	34
5^a	0.5	0.5	7	80	90
6	0.5	0.5	4	80	68
7	0.5	0.5	7	60	67
8	1	1	4	80	95

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The high chemical stability of the Zn₉O₂(OH)₂(L)₆ MOF allows easy recycling of the catalysts. After the reaction, the MOF catalyst was simply filtered off and washed to remove product residues in the pores, and then it was added to another batch of reactants. In this way, the MOF catalyst can be recycled at least 15 times without any loss in its catalytic activity (Fig. 3b). A total turnover number (TON) of more than 2887 was achieved after 15 cycles. SEM, PXRD and N₂ sorption experiments showed that the MOF maintains the original morphology, structure and porosity after catalysis (Fig.S11†).

The catalytic activity of the Zn₉O₂(OH)₂(L)₆ MOF for substrates with different sizes was also investigated. With the increasing molecular sizes of the substrates, the isolated yields for the corresponding carbonate products showed a descending trend of 89%, 30%, and 9% for epichlorohydrin, styrene oxide, and stilbene oxide, respectively (Table S4†). The size-dependent conversion, therefore, demonstrates that the catalysis takes place in the MOF channels and not only on the external surface.

We next explored the catalytic activity of the MOF/TBAB system with different epoxide substrates. As shown in Table 2, the MOF/TBAB catalyst showed good conversion for epichlorohydrin (99%), phenyl glycidyl ether (98%) and cyclopentene oxide (82%) substrates, and moderate conversion for cyclohexene oxide (24%). It is worth noting that cyclohexene oxide is one of the very inactive substrates owing to its steric hindrance originating from the bicyclic structure.⁶¹ However, the yield of cyclohexene carbonate can be optimized to 82% by increasing the catalyst loading to 1.5%. This yield outperformed many previously reported heterogeneous catalysts under similar loading, temperature, and pressure conditions (Table S5†). The results fully demonstrate that the MOF has excellent catalytic activity and provides a convincing experimental basis for industrial applications.

Table 2 Cycloaddition reactions of CO₂ with different epoxide substrates

Entry	Substrate	Catalyst (%)	TBAB (%)	Conversion (%)	Yield (%)
1		0.5	0.5	99	96
2		0.5	0.5	98	97
3		0.5	0.5	82	78
4		0.5	0.5	24	19
5		1.5	1.5	82	79

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In summary, we reported the synthesis of a Zn-pyrazolate MOF with an unprecedented Zn₉O₂(OH)₂(pyz)₁₂ cluster and a two-fold interpenetration network structure. This MOF exhibits excellent chemical and stability in pH 2–14 aqueous solutions as well as in boiling water or weak acids. The MOF efficiently catalyzes CO₂ cycloaddition reactions with various epoxide substrates, showing its high activity and exceptional recyclability. Consequently, this work will offer significant guidance in the design and synthesis of highly stable metal–pyrazolate MOFs, thereby promoting the discovery of their structures as well as potential applications.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work is financially supported by the National Key Research and Development Program of China (2017YFA0700103) and the National Natural Science Foundation of China (Grant No. 22005306). The authors greatly thank Prof. Li-Xin Wu (FJIRSM, CAS) for the help with contact angle measurements, Prof. Xin-Qiang Fang (FJIRSM, CAS) for the help with ee determination, and Bai-Tong Liu (FJIRSM, CAS), Jia-Wei Chen (XMU) and Shi-Rui Zhang (SNNU) for the help with crystallography and N₂ sorption analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available.CCDC 2128369. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1qi01555a

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