English

• Crystalline porous materials are of fundamental importance in the areas of science and technology owing to the wide applications in adsorption, separation, catalysis, energy storage and conversion [1, 2], etc. In generally, covalent bond and coordination bond appear as the most favorable choices to construct such materials regarding the network stability and diversity [3-6]. The representatives including well-known zeolites [4], metal-organic frameworks (MOFs) [5] and covalent-organic frameworks (COFs) [6]. Recently, hydrogen bond has been recognized as a new type of directional force to build highly stable and porous hydrogen-organic frameworks (HOFs) [7]. In this context, ionic bond with sufficient strength has also been recently developed as effective driving force to fabricate crystalline porous ionic salts (CPISs) [8, 9]. The unique ionic bond builds strong electrostatic filed within the highly polar channels/pores, which can favor the interaction, accommodation and stabilization of polar guests, thus leading to facile applications in proton conducting [10, 11], heterocatalysis [12, 13], selective absorption [14, 15] and molecular devices [16]. Considering the nondirectionality of ionic bond, several strategies have been proposed to reinforce ionic bond and rigidity of such porous ionic salts by using directional hydrogen bond as supplementary driving force or adopting intrinsically porous and rigid species as basic building blocks [8, 9]. Despite the latest development, construction of CPISs exhibiting permanent porosity and functionality is still challenging.

  Polyoxometalates (POMs) are a class of anionic metal-oxo clusters built from transition metals and oxygen that demonstrate tremendous structural diversities and wide-ranging applications including catalysis, energy storage and conversion, electronics, and medicine [17-25]. Due to the inherent anionic nature and rigid skeleton lined with oxygen atoms as potential hydrogen donors, POMs are promising candidates as building blocks to synthesize CPISs [9]. As such, a series of CPISs have been reported by Mizuno and Uchida groups using either flexible cation tetrabutyl ammonium [26] or rigid cationic clusters [Cr₃O(OOCCH₂CN)₆(H₂O)₃]⁺ [27] and [ε-Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺ [28]. These porous frameworks not only exhibit large pores and open channels, but also can make use of the highly polar surfaces achieving high-performance catalysis for allylation of an aldehyde and pinacol rearrangement.

  On the other hand, metal-organic cages (MOCs) are regarded as discrete metal-organic assemblies with intrinsic porosities, tunable structures and functionalities [29, 30]. Judicious selection of proper building blocks/functionalities or by postmodification, the MOCs can become charged [31-35]. Due to the well-defined polyhedral framework and rigidity, these charged species have been successfully adopted by Bloch and coworkers to build porous salts via self-assembly of MOCs of opposite charge [36], which exhibited enhanced gas uptake as compared with single component. In view of the unique advantages of POMs and MOCs as charged building blocks, Ohba and Ouay have very recently combined a series of Keggin-type POMs with cationic zirconium-based MOCs to afford a novel kind of CPISs [37], which exhibited high porosity and faster reduction by hydrazine than the pristine POMs. This indicate that combination of MOCs and POMs could be a very efficient strategy to fabricate porous solids with tailored functionalities and better performance than the single component.

  Herein we report the synthesis of a series of CPISs [W₁₀O₃₂]{[Cp₃Zr₃(μ₃-O)(μ₂-OH)₃]₂(BPDC)₃}₂·12DMA·2CCl₄ (1, Cp = cyclopentadiene, H₂BPDC = biphenyl-4,4′-dicarboxylic acid), [W₁₀O₃₂]{[Cp₃Zr₃(μ₃-O)(μ₂-OH)₃]₂(BPYDC)₃}₂·8DMA·10H₂O (2, H₂BPYDC = 2,2′-bipyridine-5,5'-dicarboxylic acid) and [SiW₁₂O₄₀]{[Cp₃Zr₃(μ₃-O)(μ₂-OH)₃]₂(BPDC)₃}₂·10DMA·2 CCl₄·6H₂O (3) assembled from POMs {W₁₀}/{SiW₁₂} and Zr-based cationic capsules. All the compounds exhibit permanent porosity due to the ordered arrangement of POMs and capsule building blocks that is guided by both electrostatic interaction and hydrogen bonds, in which compound 3 showed the highest accessible specific surface area of 33 m²/g. Based on the synergistic effect from {W₁₀} and {[Zr₃(Cp)₃(BPDC)₂(μ₃-O)(μ₂-OH)₃]}²⁺ capsule, the porous salt 1 showed much improved photocatalytic activity and selectivity towards aerobic oxidation of aromatic alcohols to aldehydes than {W₁₀} under the same condition, indicating the advantages of CPISs in catalysis.

  Single crystals of 1-3 were synthesized by a solvothermal method from a mixture of zirconocene dichloride (Cp₂ZrCl₂), related ligands and POMs (Scheme 1, see Supporting information for detailed synthesis). Single-crystal X-ray structural analysis reveals that 1 crystallizes in the monoclinic space group P2₁/c, while 2 and 3 crystallize in the triclinic space group P-1. The asymmetric units of 1, 2 and 3 are composed of one molecular capsule and half POM cluster (Fig. S1 in Supporting information). Because 1-3 are structurally similar, we chose 1 to exemplify the structure features. In 1, [Cp₃Zr₃(μ₃-O)(μ₂-OH)₃] SBUs serve as vertices and the three associated ligands act as edges forming a cationic Zr-BPDC capsule, which is surrounded by two anionic [W₁₀O₃₂]⁴⁻ clusters via electrostatic interaction and H bonds between μ₂-OH and surface O atoms on {W₁₀} in the range of 2.6644(1) Å to 2.8202(1) Å (Fig. 1a). Meanwhile, one [W₁₀O₃₂]⁴⁻ interacts with four adjacent Zr-BPDC capsules by H bonds (Fig. 1b), thus giving rise to a 3D porous framework containing three different channels (A-C) along the c-axis, with the opening of largest channel C being ~5.8 Å × 2.7 Å (Fig. 1c). The solvent-accessible volume of 1 is ~4471.5 ų (39.6%) as calculated by PLATON [38]. Similar to 1, 2 and 3 are CPISs constructed by POM clusters and cationic Zr-capsules through electrostatic interaction and H bonding. However, the packing mode of 2 and 3 is different from that of 1, and no obvious open channels can be identified in 2, while 3 exhibits opening channel with the size of ~3.5 Å × 3.5 Å along the a-axis (Fig. S2 in Supporting information). According to PLATON analysis [31], the solvent-accessible volumes of 2 and 3 are ~2358.9 ų (40.9%) and ~2128.9 ų (37.2%), respectively.

  Scheme 1

  

  Scheme 1.  Self-assembly of 1-3 from Cp₂ZrCl₂, related ligands (L: H₂BPDC, X = C; H₂BPYDC, X = N), and POMs: [W₁₀O₃₂]⁴⁻ or [SiW₁₂O₄₀]⁴⁻.

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  Figure 1

  

  Figure 1.  (a, b) O-H···O hydrogen bonds formed between cationic Zr-BPDC capsule and anionic [W₁₀O₃₂]⁴⁻ clusters. (c) Representation of the 3D framework of 1 with three different channels along the c axis, hydrogen atoms are omitted for clarity.

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  The phase purity of 1-3 was verified by the good consistence between their experimental and simulated PXRD patterns (Fig. S3 in Supporting information). The FT-IR spectra of 1-3 further confirm the presence of both POMs and Zr-capsules in the samples (Fig. S4 in Supporting information). Thermogravimetric analysis showed that the overall framework of 1-3 could be stable up to 275-300 ℃ (Fig. S5 in Supporting information). The microporous character of 1-3 was confirmed by N₂ adsorption at 77 K (Fig. S6 in Supporting information), as all samples presented N₂ adsorption isotherms with the following specific surface areas (S_BET): 1: 31 m²/g; 2: 24 m²/g; 3: 33 m²/g. The UV–vis absorption spectra of 1 and 2 well inherit the feature of {W₁₀} with the representative broad peak centered at 365 nm, and 3 exhibits slight blue-shift of λ_max to 335 nm compared with free {SiW₁₂} (Fig. S7 in Supporting information). It is well known that {W₁₀} and Keggin-type POMs are excellent photocatalysts [39], coupled with porous capsules, 1-3 can therefore have great potential in photocatalysis.

  Aromatic aldehydes are versatile building blocks in the agricultural, pharmaceutical and perfume industries [40, 41]. The traditional industrial manufacture generally involves stoichiometric oxidation or chlorization-hydrolysis method, leading to serious environment pollution [42, 43]. Photocatalytic oxidation have emerged as a green methodology for the synthesis of useful synthons, and {W₁₀} has been well-established as privileged homogeneous photocatalyst for a variety of organic transformations [44]. Therefore, we sought to explore the utility of 1-3 for the selective photocatalytic aerobic oxidation of aromatic alcohols to arylaldehydes. Initially, benzyl alcohol was taken as a model compound to investigate the photoactivities of 1-3. Firstly, we used 1 as catalyst, screening the reaction solvents, light sources, reaction time and the amount of cat.1 (Tables S2 and S3 in Supporting information). Benzaldehyde can be obtained in high conversion and selectivity when 1 mol% cat.1 was employed to perform photooxidation in benzotrifluoride (BTF) under irradiation of 45 W 385 nm LEDs with 1 atm O₂ balloon for 22 h. It is worth noting that the conversion rate can still reach 97% when reacting directly under air (Table S4 in Supporting information, entry 1), therefore the air was used as oxygen source in the following reaction exploration. Under the same conditions, with 2 or 3 as the catalyst, the yield was significantly lower than 1 (Table S4, entries 2 and 3), which confirmed that 1 had the best performance. This could be ascribed to the intrinsically higher photocatalytic activity {W₁₀} than {SiW₁₂} (Table S4, entries 7 and 8) owing to that {W₁₀} can form an active excited state through LMCT under UV light irradiation more efficiently than {SiW₁₂} [39], and the presence of open channels accessible for better substrate diffusion in 1. Control experiments in the absence of a catalyst or light and in the N₂ atmosphere led to no obvious conversion of the benzyl alcohol, demonstrating that the catalyst, light irradiation and oxygen are essential for the reaction. (Table S4, entries 4-6). Notably, using {W₁₀}, Zr-BPDC capsules or the mixture of the two components as catalysts, both the conversion and selectivity were lower than 1 (Table S4, entries 7, 9 and 10), suggesting a synergetic effect of Zr-BPDC and {W₁₀} in a porous framework.

  With optimized reaction conditions, the scope of oxidation reaction of aromatic alcohols was then investigated. To our delight, 1 exhibited high photocatalytic activity for the selective oxidation of a range of aromatic alcohols to their corresponding aldehydes (Table 1). The benzyl alcohols bearing electron-donating substituents of p-CH₃ and p-OCH₃ gave a high conversion (99% and 95%) and selectivity (> 99%). Meanwhile, benzyl alcohol possessing electron-withdrawing p-Cl could also be selectively oxidized to aldehyde with high conversion in a shorter time of 12 h (99%, entry 4), while the p-NO₂ derived aldehyde was obtained in mild yield of 51% owing to the strong electron-withdrawing effect (Table 1, entry 5). Besides, heterocyclic aromatic alcohols such as 2-pyridinemethanol, 4-hydroxymethylpyridine and 3-thienylmethanol, which are difficult to arrest catalysis, also provided moderate to high yields for the desired aldehydes, respectively (Table 1, entries 6-8). Larger 2-naphthalenemethanol can be transformed to 2-naphtanalenealdehyde smoothly with slightly lower yield of 75% owing to the steric hindrance (Table 1, entry 9). Overall, 1 exhibited the broad applicability with high selectivity (> 99%). ICP analysis of reaction mixture revealed almost no leaching of Zr and W. In view of the heterogeneous nature of 1 during catalysis, we studied the recyclability of 1. After run 1, the conversion of benzyl alcohol dropped to 66%, and after the run 2 the conversion further dropped to 42%. PXRD analysis of the recovered sample indicated the porous framework of 1 gradually collapsed after each cycle (Fig. S8 in Supporting information).

  Table 1

  

  Table 1.  Scope of the photocatalytic selective oxidation of alcohols using 1 as a photocatalyst.^a

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  To investigate the catalytic mechanism of the reaction as described above, the band gap of 1 was first estimated as 2.86 eV according to Tauc plot (Fig. S9 in Supporting information). Then the LUMO of 1 was estimated as −0.68 V versus the normal hydrogen electrode (NHE) according to the Mott-Schottky plots (Fig. S10 in Supporting information). The corresponding HOMO level at 2.18 V (vs. NHE) could be calculated by subtracting the LUMO level from the band gap. Because the LUMO position is more negative than E(O₂/O₂^•−) = −0.33 V, it is theoretically feasible to use 1 as a photocatalyst for the reduction of O₂ to O₂^•− [45]. Next, various radical scavengers have been used to explore the photoactive species in the oxidation of 4-methoxybenzylalcohol over 1. As shown in Fig. 2, no significant change of the conversion (90%) was observed by adding tert-butyl alcohol (5 equiv.) as the hydroxyl radical (^•OH) scavenger, and there was an abrupt decrease in conversion to 15% and 35% by the use of KI as the photo-generated holes (h⁺) and AgNO₃ as photo-generated electrons (e⁻) scavengers, indicating the photo-generated electrons and holes were involved in the catalytic process. In addition, adding p-benzoquinone (BQ) and 9,10-diphenylanthracene (DPA) as O₂^•− and ¹O₂ scavengers, a reduced conversion of 60% and 37% was observed, showing that O₂^•− and ¹O₂ were formed in the photocatalytic reaction and acted as oxidant species in the catalytic cycle. On the basis of these studies, a plausible mechanism is proposed (Fig. 3). Under 385 nm UV light irradiation, 1 will generate photogenerated carrier electrons, molecular O₂ is converted into active oxygen species (¹O₂ and O₂^•−) through energy or electron transfer. Meanwhile, the adsorbed aromatic alcohol in solution interacts with holes to form the corresponding radical cations, which further react with ¹O₂ or O₂^•−, leading to the formation of the corresponding aldehydes.

  Figure 2

  

  Figure 2.  Mechanism study using different scavengers with 4-methoxybenzylalcohol as model compound.

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  Figure 3

  

  Figure 3.  The possible photocatalytic oxidation mechanism of aromatic alcohols with 1 as photocatalyst.

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  In conclusion, three novel CPISs 1-3 are assembled from POM clusters and Zr-based cationic capsules through electrostatic interaction and hydrogen bonding. Owing to the presence of catalytic POM units, inherent porous Zr-capsule and polar channels that are accessible to substrates, 1-3 not only show permanent porosity but also can promote photocatalytic selective oxidation of aromatic alcohols to arylaldehydes. 1 presented best performance to afford a variety of aldehydes in moderate to high yield (36%-99%) and high selectivity (> 99%) because of the synergistic effect derived from {W₁₀} and Zr-capsule as well as open channels, which is inaccessible by {W₁₀} and thus results in better photocatalytic activity. This work not only greatly expands the structure library of CPISs, but also provides a viable approach to combine anionic clusters and cationic MOCs to build CPISs with pre-designed functionalities; this may further spur the development of more sophisticated CPISs bearing diverse structures as a new type of porous functional material.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by National Natural Science Foundation of China (Nos. 92161111, 21901037, 21901038), Shanghai Pujiang Program (No. 19PJ1400200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and Fundamental Research Funds for the Central Universities (No. 2232019G-07). We also thank the staff from BL17B1 beamline of National Facility for Protein Science in Shanghai (NFPS) at Shanghai Synchrotron Radiation Facility, for assistance during data collection.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.04.042.

  
  
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• 

  Scheme 1  Self-assembly of 1-3 from Cp₂ZrCl₂, related ligands (L: H₂BPDC, X = C; H₂BPYDC, X = N), and POMs: [W₁₀O₃₂]⁴⁻ or [SiW₁₂O₄₀]⁴⁻.

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  Figure 1  (a, b) O-H···O hydrogen bonds formed between cationic Zr-BPDC capsule and anionic [W₁₀O₃₂]⁴⁻ clusters. (c) Representation of the 3D framework of 1 with three different channels along the c axis, hydrogen atoms are omitted for clarity.

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  Figure 2  Mechanism study using different scavengers with 4-methoxybenzylalcohol as model compound.

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  Figure 3  The possible photocatalytic oxidation mechanism of aromatic alcohols with 1 as photocatalyst.

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  Table 1.  Scope of the photocatalytic selective oxidation of alcohols using 1 as a photocatalyst.^a

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