An Ultrastable, Easily Scalable and Regenerable Macrocycle-Based Hydrogen-Bonded Organic Framework

Introduction

In the preceding decades, crystalline porous materials, encompassing zeolites,¹ metal–organic frameworks,² and covalent organic frameworks,³ have revealed a multitude of potential applications in adsorption,⁴ separation,⁵ and catalysis.⁶ To evolve into a practical material, a crystalline porous substance typically needs to fulfill an array of additional prerequisites: cost-effectiveness, synthesis scalability, stability, processability, ease of manipulation, and application specificity.^7–9 In recent years, hydrogen-bonded organic frameworks (HOFs), assembled via hydrogen bonds (H-bonds), have emerged as a pivotal subset within the realm of crystalline porous materials.^10–12 They are distinguished by their inherent solution processability and regenerability.^11,13 However, due to the considerably weaker nature of H-bonds in comparison to covalent and coordination bonds, the majority of HOFs tend to collapse upon the removal of solvent molecules or become damaged when subjected to harsh conditions.¹⁴ By opting for covalent organic macrocycles (COMs) as the building blocks for constructing macrocycle-based HOFs (mHOFs), wherein multiple urea,¹⁵ amide,¹⁶ α-hydroxyketone,¹⁷ carboxyl,¹⁸ and amine¹⁹ motifs interconnect ( Supporting Information Figure S1), these mHOFs showcase notable stability due to their intricate net of multiple H-bonding interactions. Notwithstanding, the synthesis of these COMs typically requires multiple steps and yields limited quantities, making them difficult to scale up for industrial applications. Therefore, there is a pressing need for a scalable one-pot synthesis of mHOFs using commercially available building blocks to accelerate the development of these materials.

Acylhydrazone bond is a type of N-substituted imine that possesses error-checking and self-correcting abilities, exhibiting a high degree of stability.^20–23 Acylhydrazones can be conveniently prepared in high yield through the condensation of hydrazides and carbonyl compounds (aldehydes or ketones) under mild conditions in the presence of Brønsted acids²³ or metal templates (Figure 1a).^24,25 The majority of reported acylhydrazone-linked COMs have been constructed from hydrazides and aldehydes, demonstrating exceptional stability, even in water.²⁶ In contrast, the formation and breakage of acylhydrazone bonds between hydrazides and ketones require higher energy due to the larger steric hindrance and higher electron density at the carbonyl carbon atom of ketones.²³ Moreover, acylhydrazones, possessing donors (D) and acceptors (A), have a propensity to undergo spontaneous self-assembly through intermolecular H-bonds (N–H⋯O) between the amide units.^27,28 Apart from the Z and E configurations in the C=N bond,²⁹ the C–N bond in the amide unit has a partial double-bond character, affording two potential conformational isomers, namely anti and syn modes, in the acylhydrazone bond.³⁰ In the anti mode, they typically connect through a single H-bond (Figure 1b), whereas in the syn mode, they can assemble via double H-bonds (Figure 1c). Despite being uncommon in the literature, the syn mode undeniably plays a pivotal role in crafting a stable semirigid macrocycle that incorporates acylhydrazone bonds.³¹ We hypothesize that enhancing the rigidity of the macrocycle can effectively constrain the acylhydrazone bond into a specific conformation, thereby promoting spontaneous aggregation through hydrogen bonding interaction. Additionally, the presence of methyl groups in acylhydrazones derived from ketones may serve as a protective shield against nucleophilic attacks, thus augmenting their stability. Accordingly, the synthesis of acylhydrazone-linked macrocycles with increased rigidity holds significant promise in the construction of stable mHOFs, given the facile availability of ketones and hydrazides.

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Herein, we report the discovery of a facile, one-pot, catalyst-free, high-yielding, gram-scale synthesis of a crystalline porous mHOF-SYSU101 through the condensation of hydrazides and ketones. Notably, the resulting mHOF-SYSU101 exhibits remarkable stability in solid state, where discrete acylhydrazone-linked macrocycles are meticulously assembled via dense H-bond networks. Remarkably, even upon dissolution of mHOF-SYSU101 in trifluoroacetic acid (TFA), the [2 + 2] macrocycle molecule retains its integrity for an unprecedented duration of one week. Furthermore, an intriguing phenomenon is observed whereby injection of a TFA solution of mHOF-SYSU101 into dimethyl sulfoxide (DMSO) or methanol instantaneously regenerates the crystalline porous framework. We expect that this work could provide a promising method for facile preparation and processing of crystalline porous materials, opening new avenues for their potential applications.

Experimental Methods

Chemicals and materials

2,6-Diacetylpyridine ( DAP, 98%) was purchased from Jiangsu Aikon (Jiangsu, China). Terephthalic dihydrazide ( THA, 98% and 70%) and solvents were purchased from Energy Chemicals (Shanghai, China). Manganese perchlorate hexahydrate (Mn(ClO₄)₂·6H₂O, 99%) was purchased from Sigma-Aldrich Company (Shanghai, China). All the reagents and solvents were used as received without further purification unless otherwise noted. Seawater was obtained from South China Sea nearing the Zhuhai campus of Sun Yat-sen University (Zhuhai, China). Before use, the seawater was filtered to remove the solids.

Method

Nuclear magnetic resonance (NMR) spectra were recorded on Bruker spectrometers (Avance III 600 or Avance III 500) (Bruker, Karlsruhe, German). High resolution time-of-flight mass spectrometry measurements were performed on a Bruker ultraflex MALDI TOF mass spectrometer (Bruker, Karlsruhe, German). The Fourier-transform infrared spectroscopy (FT-IR) spectra were determined using a Bruker EQUINOX 55 spectrometer (Bruker, Karlsruhe, German). UV–vis-NIR absorption spectra were recorded on a Shimadzu UV-3600 spectrophotometer (SHIMADZU, Tokyo, Japan). Water contact angles were measured using a KRUSS DSA100 contact angle meter (KRÜSS, Hamburg, German). Raman spectra were recorded on a Renishaw inVia laser micro-Raman spectrometer (Renishaw, London, England). The conductivity of seawater was obtained by a METTLER TOLEDO DDSJ-319L electric conductometer (Mettler-Toledo International Inc., Greifensee, Switzerland). Digital photographs were obtained by a Nikon Y-TV55 camera (Nikon Corporation, Tokyo, Japan). Thermogravimetric analysis (TGA) was carried out on a NETZSCH TG209 system (NETZSCH, Shanghai, China). Inductively coupled plasma mass spectrometry (ICP-MS) analysis was performed on a Thermo Fisher Scientific iCAP Q instrument (ThermoFisher Scientific, Shanghai, China). The single-crystal X-ray diffraction (SCXRD) data were collected at Beamline 17B of Advanced Light Source at Shanghai Synchrotron Radiation Facility (Shanghai, China) or using a Bruker D8 VENTURE PHOTON II MetalJet (Bruker, Karlsruhe, German). Powder X-ray diffraction (PXRD) measurements were recorded on a Bruker D8 powder X-ray diffractometer (Bruker, Karlsruhe, German) with Cu-Kα radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) images were recorded using a SU8010 ultrahigh revolution field emission scanning electron microscope (Hitachi, Tokyo, Japan). Elemental distribution was examined by thermal field emission environmental energy dispersive spectroscopy (Quanta 400F; ThermoFisher Scientific, Shanghai, China). All cryo-electron microscopy (cryo-EM) experiments were performed on a ThermoFisher Scientific Titan Krios G3i electron microscope (ThermoFisher Scientific, Shanghai, China) operated at 300 kV. N₂ adsorption measurements were carried out using an ASAP 2460 system (Micromeritics, Shanghai, China). Water vapor adsorption measurements were investigated on a Quantachrome instrument (Quantachrome, Florida, USA).

Synthesis of mHOF-SYSU101

mHOF-SYSU101 can be prepared by direct condensation reaction ( Method 1) and dynamic transformation reaction ( Method 2), respectively. Single crystals of mHOF-SYSU101 for X-ray diffraction were obtained from Method 2. More experimental details and characterizations are available in the Supporting Information.

Density functional theory calculations

All density functional theory (DFT) calculations were performed by using the Vienna Ab-initio Simulation Package.³² The detailed DFT methods are provided in the Supporting Information.

Results and Discussion

Synthesis and characterization of mHOF-SYSU101

DAP and THA are commercially available at low cost. Herein, THA with purities of 98% and 70% are denoted as THA-98 and THA-70, respectively. In recent studies, we have successfully utilized metal ions as templates to facilitate the multicomponent self-assembly of DAP and hydrazine derivatives of THA-98, leading to the synthesis of a series of covalent metallacycles.²⁵ For instance, a one-pot reaction of DAP, THA-98, and Mn(ClO₄)₂·6H₂O in methanol resulted in the formation of 2-Mn with high yield (Figure 2a). In this study, considering the stability of acylhydrazone bonds, we were motivated to explore the possibility of directly condensing DAP and THA-98 in the absence of metal ion templates (Figure 2a, Method 1). Given the configurations and rigid skeletons of the reactants, we hypothesized that the direct assembly could yield polymeric structures, smaller COMs, or their mixtures. Due to the poor solubility of THA-98 in other solvents, we selected DMSO as the reaction solvent. We systematically screened various reaction conditions, including temperatures, concentrations, and reaction time, for the solvothermal condensation of DAP with THA-98 in a 1∶1 ratio. All screening experiments resulted in the formation of light-yellow precipitates with varying weights ( Supporting Information Table S1). Subsequently, the crystallinity of the isolated solids was evaluated using PXRD. Remarkably, with the exception of solid obtained from the reaction at 80 °C, which exhibited low crystallinity, all other solids displayed similar, strong, and sharp PXRD peaks, indicative of the presence of the same crystalline phase ( Supporting Information Figure S2, denoted as mHOF-SYSU101). Further investigation of the morphology of mHOF-SYSU101 using SEM revealed that it was highly dependent on the reaction temperature ( Supporting Information Figure S3). The transformation from an amorphous state at 80 °C to rod-shaped microcrystals at 140 °C clearly suggests that the formation of mHOF-SYSU101 is governed by thermodynamic control, wherein a self-correction phenomenon occurs. Subsequent optimization of the reaction conditions at 140 °C led to the formation of rod-shaped microcrystals in all experiments, although the crystals were too small for SCXRD analysis, possibly due to the rapid nucleation and hindered crystal growth during the crystallization process. Furthermore, mHOF-SYSU101 was found to be insoluble in most common solvents except for TFA, suggesting that its formation may involve the assembly of discrete molecules via strong noncovalent interactions ( Supporting Information Figure S4).³³ Accordingly, a mixed solution of TFA and dichloromethane was used to dissolve mHOF-SYSU101 for matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS). In the mass spectrum (Figure 2b), three peaks at m/z = 643.254, m/z = 665.233, and m/z = 681.200 were identified to a [2 + 2] macrocycle, corresponding to [M + H⁺]⁺, [M + Na⁺]⁺, and [M + K⁺]⁺, respectively. The measured isotope pattern matched that of the simulated one. The ¹H NMR spectrum of mHOF-SYSU101 showed two distinct signals for methyl groups at 2.64 and 2.76 ppm, indicating that the [2 + 2] macrocycle adopts a C₂ symmetry (Figure 2c). The assignment of the remaining protons was achieved through a judicious combination of two-dimensional (2D) NMR spectroscopic techniques ( Supporting Information Figures S5 and S6). Time-dependent NMR measurements further demonstrated the exceptional stability of the [2 + 2] macrocycle, as they retained their cyclic structure in TFA for a period of one week ( Supporting Information Figure S7). The subsequent recrystallization of mHOF-SYSU101 in a TFA/acetic acid solution yielded large single crystals, denoted as H₂-SYSU101. SCXRD analysis of H₂-SYSU101 confirmed the presence of protonated [2 + 2] macrocycles, thereby providing definitive evidence for the existence of [2 + 2] macrocycles in mHOF-SYSU101 ( Supporting Information Figure S14 and Table S3). Despite exhaustive efforts, attempts to obtain large crystals of mHOF-SYSU101 for SCXRD through direct condensation reactions were unsuccessful.

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Considering the phenomenon of self-healing observed in the SEM results, as well as the reported stimuliresponsive dynamic covalent bonds implicated in the conversion between discrete organic cages and polymers,^34–36 our research efforts were directed towards manipulating the crystallization process and impeding the reaction kinetics to obtain suitable crystals for SCXRD. Specifically, we aimed to retard the reaction rate and modify the crystallization process by disassembling the original macrocycles, an endeavor that entails cleaving the pre-existing bonds and thus decelerating the reaction. The framework of 2-Mn comprises coordination and acylhydrazone bonds, both of which are stimuliresponsive dynamic bonds, reversible and sensitive to temperature. Hence, we conjectured whether 2-Mn could be transformed into mHOF-SYSU101 at high temperature by breaking these reversible bonds. Intriguingly, heating the solution of 2-Mn in DMSO to 120 °C resulted in the formation of single crystals ranging from 10–20 μm in size (Figure 2d and Supporting Information Figure S8), being suitable for synchrotron radiation X-ray diffraction. The PXRD patterns of these crystals matched that of the powder solid obtained from Method 1 ( Supporting Information Figure S9), thereby confirming the successful transformation from 2-Mn to mHOF-SYSU101. This transformation necessitates cleavage of the existing coordination and acylhydrazone bonds, which curtails the rate of cyclization and nucleation compared to the direct method. Furthermore, we infer that crystalline mHOF-SYSU101 represents a thermodynamically favored product under these conditions, promoting dynamic transformations.

Single crystal analysis of mHOF-SYSU101

Using synchrotron radiation X-ray diffraction, the single-crystal structure of mHOF-SYSU101 was unambiguously solved and refined with a resolution of 0.94 Å. SCXRD results revealed that mHOF-SYSU101 crystallizes in the triclinic space group P-1, featuring a [2 + 2] covalent macrocycle in the unit cell ( Supporting Information Figure S15 and Table S4). As shown in Figure 3a, all amide units in the acylhydrazone bonds adopt the syn conformation and extend outward from the macrocycle. The two pyridyl diimine units display an antiparallel orientation to each other. Notably, in contrast to the structure of mHOF-SYSU102, which was synthesized under similar conditions but utilized 1,3-diacetylbenzene instead of DAP ( Supporting Information Figure S45), the pyridyl nitrogen atoms in mHOF-SYSU101 engage in intramolecular C–H⋯N interactions. These interactions exhibit H⋯N distances between 2.56 and 2.65 Å, and they substantially enhance the rigidity of the resultant macrocycle. In addition to the conventional syn-amide units, the electrostatic potential map of the macrocycle suggests that the electrostatically negative methyl groups may also participate in intermolecular H-bonding connections (Figure 3b). Each macrocycle connects with four adjacent macrocycles through four ADD-DDA hydrogen-bonded arrays along the bc planes to form a 2D layer, containing rectangular windows (Figure 3c,d). The ADD-DDA array comprises two C–H⋯O bonds with H⋯O distances of 2.29–2.47 Å and two short N–H⋯O bonds with H⋯O distances of 2.09–2.15 Å, respectively ( Supporting Information Table S5). To the best of our knowledge, such a hydrogen-bonded array has not been previously reported.^37,38 Furthermore, each 2D layer associates with neighboring layers via offset π–π stacking interaction with a distance of 3.95 Å, along the a axis to form extrinsically 1D rectangular channels of 7.4 × 8.7 Å (Figure 3e,f). PLATON analysis revealed that the total solvent-accessible volume of the framework is 16.8% of its unit cell. Importantly, the methyl groups on the pore surface can effectively protect the hydrogen-bonded arrays from the attack of polar solvent molecules.³⁹ We envisage that multiple cooperative noncovalent interactions in the framework could endow mHOF-SYSU101 with an outstanding stability.

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Phase purity, porosity, and stability of mHOF-SYSU101

The PXRD patterns of bulk powder mHOF-SYSU101 exhibited an excellent match with the simulated one, confirming the high phase purity of the product (Figure 4a). Variable temperature PXRD experiments revealed that mHOF-SYSU101 maintains its crystallinity up to 400 °C (Figure 4b). The TGA further substantiated its high thermal stability, with a decomposition temperature of up to 410 °C ( Supporting Information Figure S16). Besides, the porosity of mHOF-SYSU101 was investigated by N₂ sorption measurements at 77 K (Figure 4c), and the Brunauer–Emmett–Teller surface area was calculated to be 238 m² g⁻¹, proving its porous nature ( Supporting Information Figure S17). The pore size was calculated to be 7.3 Å, in good agreement with the crystal structure (Figure 4c, inset). Moreover, water vapor sorption isotherms measured at 298 K revealed that mHOF-SYSU101 adsorbs a negligible amount of water at low pressure, confirming the hydrophobic nature of its 1D channel ( Supporting Information Figure S18). Compacted solids of mHOF-SYSU101 showed a water contact angle of 25° ( Supporting Information Figure S19), indicative of the hydrophilic nature of the crystal surfaces. We inferred that the hydrophobic 1D channels result from the methyl groups on the pore surface, while the hydrophilic crystal surfaces are attributed to the abundant –NH– and –C=O groups exposed on the surface.

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To investigate the chemical stability of mHOF-SYSU101, we immersed it in various solvents for 7 days, including pyridine, N-methylpyrrolidone (NMP), DMSO, dimethylformamide (DMF), N,N-diethylformamide, aqueous NaOH (1 M), aqueous HCl (1 M), water (25 °C), water (100 °C), and seawater. PXRD results indicated that mHOF-SYSU101 maintained its crystalline structure under all tested conditions (Figure 4d). We also analyzed the weight percentage of the residue obtained after each immersion, as shown in Figure 4e. In highly polar organic solvents, we hardly observed a weight loss of mHOF-SYSU101. While in other aqueous solutions at room temperature, the residue weight percentage could hold on to not less than 95%, probably due to the hydrolysis of a small amount of macrocycles on the crystal surface. Even in boiling water, the residue weight percentage was retained at 85%, which surpasses the reported imine-linked covalent organic frameworks.⁴⁰ These findings demonstrated that mHOF-SYSU101 exhibits ultrahigh thermal and chemical stabilities ( Supporting Information Table S6), which could be attributed to its robust acylhydrazone bonds, multiple H-bonding interactions, and hydrophobic micropores.

Analysis of ADD-DDA hydrogen-bonded array

C–H⋯O bond has garnered significant attention due to its pivotal role in biological structures,⁴¹ molecular recognition,⁴² and supramolecular chemistry,⁴³ and it also plays an important part in our hydrogen-bonded framework. Despite the well-known challenge of experimentally detecting this weak interaction,^44–46 we successfully identified the C–H⋯O interactions via variable temperature FT-IR. As shown in Figure 4f, two stretching vibration bands of the methyl groups were observed at 3176 and 3072 cm⁻¹ at 30 °C, respectively. Upon increasing the temperature from 30 to 200 °C, the peak at 3176 cm⁻¹ showed an obvious blueshift to 3186 cm⁻¹ and reduction in intensity, providing compelling evidence for the existence of C–H⋯O bonds.⁴⁷ In contrast, both of the N–H stretching band at 3280 cm⁻¹ and the C=O stretching band at 1668 cm⁻¹ exhibited a slight decrease in intensity at 200 °C ( Supporting Information Figure S20), indicating the presence of strong hydrogen-bonding interactions between the amide groups.⁴⁸

To further evaluate the C–H⋯O and N–H⋯O interactions in mHOF-SYSU101, we used Crystalexplorer17.5 software to analyze the Hirshfeld surface and 2D fingerprint spectra of the crystal structure.⁴⁹ In the Hirshfeld surface analysis, two distinct red dots were prominently visible on the surfaces of each ADD group, denoting strong intermolecular interactions between adjacent macrocycles within the 2D layer ( Supporting Information Figure S21). As shown in the 2D fingerprint plots, various noncovalent interactions, including H⋯O, H⋯N, H⋯C, C⋯C, and H⋯H, are found in the crystal structure of mHOF-SYSU101 ( Supporting Information Figure S22). Notably, a pair of striking spikes on the bottom left are identified to C–H⋯O and N–H⋯O interactions, which accounts for 18.4% of the total interactions ( Supporting Information Figure S23). Moreover, H⋯H interaction with a proportion of 47.2% disclosed an extremely dense packing characteristic of mHOF-SYSU101. These findings provide further evidence for the significant role of noncovalent interactions in the formation of mHOF-SYSU101.

Investigations on the formation mechanism of mHOF-SYSU101

To investigate the formation mechanism of mHOF-SYSU101, we performed in situ NMR experiment to monitor the condensation reaction of DAP with THA-98 in DMSO-d₆. The experiment consisted of two stages of heating up and constant temperature ( Supporting Information Figure S24). Upon heating, the signal of –NH₂ protons at 4.56 ppm gradually decreased and disappeared at 120 °C, indicating complete consumption of THA-98. A new singlet appeared at 14.87 ppm after 1 h at 120 °C, which is attributed to the large downfield shifts (up to 5 ppm) of the –NH– protons.⁵⁰ This phenomenon revealed that the –NH– moieties are involved in the formation of H-bonds. However, DMSO is a polar solvent that does not promote hydrogen bond formation.⁵¹ We therefore speculate that the self-assembly process involves the synergy of multiple noncovalent interactions. Notably, even after complete consumption of THA-98, the signal peaks of different methyl groups in the range of 2.20–2.80 ppm underwent dynamic transformation, indicating the presence of various intermediates in solution ( Supporting Information Figure S25). Furthermore, we observed that the low crystalline solid obtained at 80 °C could convert into crystalline mHOF-SYSU101 via self-correcting reaction in DMSO at 120 °C, which was confirmed by the changes in PXRD patterns and morphologies ( Supporting Information Figures S26 and S27).

Based on the above results, we propose a plausible mechanism of the formation of mHOF-SYSU101 ( Supporting Information Figure S28). Initially, a condensation reaction between DAP and THA-98 leads to a mixture of soluble precyclic oligomers, [2 + 2] macrocycles, and small macrocycle aggregates. As the concentration of [2 + 2] macrocycles reaches the critical aggregation concentration (CAC), synergistic noncovalent interactions drive their self-assembly to form crystal nuclei, which subsequently grow into microcrystals. This aggregation-induced crystallization process further promotes the transformation of precyclic oligomers and small macrocycle aggregates to the [2 + 2] macrocycles.⁵² The almost quantitative yield obtained suggests that the CAC of the [2 + 2] macrocycles is exceptionally low in DMSO. Due to the strong intermolecular interactions, the aggregated [2 + 2] macrocycles are strongly retained in the microcrystals and are difficult to break away from it, leading to the insoluble and ultrastable properties of mHOF-SYSU101 in DMSO.

Scalable synthesis of mHOF-SYSU101

It is worth noting that the price of THA primarily hinges on its purity, as detailed in Supporting Information Table S2. To further curtail expenses, we endeavored to synthesize mHOF-SYSU101 employing THA-70. ¹H NMR analysis unveiled that the impurity within THA-70 is ethyl 4-(hydrazinecarbonyl)benzoate ( Supporting Information Figure S10). Intriguingly, this impurity can likewise engage in condensation with DAP, yielding low yields. Through rigorous experimentation, we ascertained that by orchestrating the reaction between THA-70 and DAP in a 3∶2 ratio, mHOF-SYSU101 could be produced with an exceptional yield of 92.0%. Notably, the solvent exhibited the capacity for recycling thrice ( Supporting Information Figure S11). Employing THA-70 and maintaining a reaction ratio of 3∶2 between THA-70 and DAP, we successfully executed the synthesis of mHOF-SYSU101 on a multigram scale. The yield of this reaction correlates with the volume of DMSO used. Notably, by reducing the volume of the reaction solvent to an optimal level, the yield can be remarkably enhanced to an impressive 96.0% (Figure 4g and Supporting Information Figures S12 and S13). The amalgamation of low-cost raw materials, solvent recycling, and multigram-scale synthesis bestows mHOF-SYSU101 with pronounced advantages poised to reverberate within practical applications.

Rapid regeneration of mHOF-SYSU101

Noncovalent interactions endow HOFs materials with solution processability and regenerability.^11,13,53 Furthermore, strategies employing reversible solid-solution-solid transformations enhance the castability and recyclability of these materials, thereby overcoming a substantial barrier impeding their practical application. Typically, most reported HOFs materials can be regenerated via a dissolution-recrystallization process in common solvents such as DMSO, NMP, and DMF. However, this approach is not viable for mHOF-SYSU101, since these polar solvents are insufficient to break its dense hydrogen-bonded networks. Nevertheless, rapid-regeneration of mHOF-SYSU101 was achieved immediately by a direct injection of a TFA solution of mHOF-SYSU101 into DMSO (Figure 5a and Supporting Information Video S1). The PXRD pattern of the recovered solids confirmed the successful regeneration of mHOF-SYSU101 ( Supporting Information Figure S29). SEM results indicated that the rapid crystallization process induced the formation of mHOF-SYSU101 nanocrystals (Figure 5b). DFT calculations shed further light on the regeneration process (Figure 5c and see Supporting Information Figures S31, S32 and Tables S7, S9). Introduction of protonated macrocycles and trifluoroacetate ions into DMSO (State 1) causes DMSO to act as a Lewis base. By abstracting protons from the protonated macrocycles, it generates the original macrocycles and protonated DMSO (State 2). This transition is inherently exothermic, as evidenced by a 2.0 eV decrease in the energy profile, facilitating its spontaneous progression. The robust ADD-DDA hydrogen-bonded connectivity, coupled with π–π stacking interactions, triggers the rapid nucleation of original macrocycles and simultaneous growth of crystal along the a, b, and c axes ( Supporting Information Table S8), thereby accomplishing the regeneration of mHOF-SYSU101.

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Low-electron-dose cryo-EM is a powerful tool for observing the atomic-scale structure of crystalline porous materials.⁵⁴ However, visualizing the microstructure of HOFs materials using this method remains challenging.⁵⁵ On the one hand, the crystal sizes of HOFs reported in the literature usually reach the micrometer scale, which make them difficult for electron beam to penetrate the crystals. On the other hand, noncovalently linked porous HOFs materials are sensitive to electron beam. Therefore, visualization of the microstructure of HOFs materials always encounters rapid amorphization of the target voxel.⁵⁶ Benefiting from its ultrastability and nanometer size, we successfully visualized the crystallographic structure of the regenerated mHOF-SYSU101 using low-electron-dose cryo-EM. As shown in Figure 5d,e, a series of lattice planes along the >0 1 0>, >0 1 1>, and >1 1 0> directions were observed. The ordered 1D tubular channels and layered structures were clearly visible. Importantly, the observed microstructure is in excellent agreement with that obtained by SCXRD analysis.

Guided by these mechanistic insights, we tested replacing DMSO with low-cost methanol as the Lewis base responsible for extracting protons from the protonated macrocycles. Upon introducing a TFA solution of mHOF-SYSU101 to methanol, mHOF-SYSU101 was also immediately regenerated ( Supporting Information Figure S29), confirming the successful reversible solid-solution-solid transformation of mHOF-SYSU101. The obtained mHOF-SYSU101 nanocrystals could be well dispersed in methanol for several weeks ( Supporting Information Figure S30), demonstrating significant potential for porous membrane fabrication.⁵⁷ To our best knowledge, such a convenient methodology for regenerating crystalline porous materials remains unprecedented in the literature.

Iodine adsorption

¹³¹I₂ and ¹²⁹I₂ are common radioactive species in nuclear waste and accidents, and effectively removing these radioactive iodine compounds from nuclear waste is paramount for both human well-being and the preservation of the ecological environment. Until now, the endeavor to develop practical porous iodine adsorption materials for the nuclear power industry, which must concurrently possess the ability to seamlessly absorb iodine, provide high adsorption efficiency, maintain exceptional thermal stability, and enable reversible, reproducible regeneration, has remained a crucial and formidable undertaking.^58,59 mHOF-SYSU101 possesses 1D hydrophobic channels with suitable sizes, exhibiting great advantages in iodine capture.⁵⁹ Therefore, we performed iodine capture experiments using mHOF-SYSU101 as the adsorbent. Initially, the completely activated mHOF-SYSU101 (96.3 mg) was soaked in a saturated solution of I₂ in cyclohexane for 24 h at room temperature, and a brown solid (104.4 mg, denoted as I₂@mHOF-SYSU101) was obtained after filtration and drying ( Supporting Information Figure S33). To better understand the mechanism of iodine adsorption, we conducted PXRD, gas sorption, TGA, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDS) analysis. The PXRD pattern of I₂@mHOF-SYSU101 confirmed that mHOF-SYSU101 retained its crystal structure after absorbing I₂ ( Supporting Information Figure S34). The increased intensity of the high-angle peaks indicated that the 1D channels in mHOF-SYSU101 were occupied by iodine, which was consistent with the N₂ sorption result ( Supporting Information Figure S35). TGA revealed that the weight of iodine in I₂@mHOF-SYSU101 was calculated as 10% ( Supporting Information Figure S36). In comparison to the pristine mHOF-SYSU101, two new bands at 107 and 164 cm⁻¹ were observed in the Raman spectrum of I₂@mHOF-SYSU101, which were attributed to the symmetric stretching vibrations of I₃⁻ and I₅⁻, respectively (Figure 6a). The peaks at 620.53 and 618.09 eV in the XPS spectrum of I₂@mHOF-SYSU101 were assigned to I₂ and I₃⁻, respectively ( Supporting Information Figures S37 and S38). The presence of I₃⁻ and I₅⁻ polyiodides suggested that iodine interacts with the pyridyl groups surrounding the 1D channel to form a charge-transfer complex.⁶⁰ SEM imaging showed that the morphology of mHOF-SYSU101 was maintained after adsorption, and a uniform distribution of iodine in mHOF-SYSU101 was detected by EDS ( Supporting Information Figure S39).

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Furthermore, the preferred adsorption configuration of I₂ in I₂@mHOF-SYSU101 was investigated by DFT calculations. As shown in Supporting Information Figure S40, the I₂ molecule locates at the space between the diagonal methyl groups, interacting with the hydrogen atoms at a distance of 2.65 Å. The adsorption energy was estimated to be −0.23 eV. The charge density difference of I₂@mHOF-SYSU101 also showed the charge accumulation between the I₂ molecule and methyl groups ( Supporting Information Figure S41), further proving the interaction between the I₂ molecule and the framework.

In a practical assessment, a seawater solution of iodine (ca. 90 ppm, see Supporting Information Figures S43) was used to test the ability of mHOF-SYSU101 to adsorb iodine in real environment. Time-dependent UV/vis experiments demonstrated that the characteristic absorption peaks of I₂ were rapidly reduced upon addition of mHOF-SYSU101, and the removal efficiency reached 99% in 2 min (Figure 6b,c). This fast adsorption rate is superior to those of most previously reported materials.⁵⁸ The remarkable performance of mHOF-SYSU101 is attributed to its 1D hydrophobic channels and the high dispersibility of sorbent in water ( Supporting Information Figure S18). Moreover, when mHOF-SYSU101 was employed as a filler, the corresponding filled column can directly and effectively adsorb I₂ from flowing seawater ( Supporting Information Video S2). Remarkably, the adsorption performance of mHOF-SYSU101 remained unchanged even after five cycles (Figure 6d). Upon desorption, both the crystalline nature and all Bragg peaks of mHOF-SYSU101 remained unaltered ( Supporting Information Figure S42). This confirmed the complete reversibility of the I₂ dynamic adsorption-desorption performance of mHOF-SYSU101, as well as its high stability throughout the process. Actual nuclear wastewater contains a diverse array of iodine species, including iodide, hypoiodite, and iodate ions, in addition to molecular iodine. To achieve comprehensive capture of these disparate forms from aqueous solutions, a cascade of sequential oxidation-reduction reactions is required to convert them to elemental iodine. ICP-MS analysis ascertained that mHOF-SYSU101, as a solid adsorbent, can proficiently capture 99% of the varied iodine forms within these complex wastewater treatment processes. Moreover, in the process of recovering the solid adsorbent, mHOF-SYSU101 could be reused up to three times with minimal loss in mass. This finding underscores its remarkable iodine adsorption proficiency and concurrently confirms its sufficient stability to withstand challenging redox conditions (Figure 4e). These exemplary performances highlight its considerable promise as an efficient adsorbent for I₂ molecules in real-world scenarios.

Conclusion

In summary, we have synthesized a macrocycle-based hydrogen-bonded organic framework mHOF-SYSU101, via a facile one-pot condensation reaction utilizing commercially available reactants. This method enabled us to obtain multigram-scale product of crystalline mHOF-SYSU1 with almost quantitative yield, even from a raw material with a purity of just 70%. The dynamic transformation from 2-Mn to mHOF-SYSU101 facilitates the growth of single crystals of mHOF-SYSU101 through controlled thermal induction of coordination and acylhydrazone bond cleavage and reformation. Assembled from discrete [2 + 2] macrocycles through distinctive ADD-DDA hydrogen-bonded arrays, mHOF-SYSU101 showcases exceptional thermal and chemical stabilities in its porous crystalline form, which are attributed to the combined effects of multiple noncovalent interactions and methyl-induced hydrophobicity of the pore surface. Notably, the permanent hydrophobic 1D channels within mHOF-SYSU101 facilitate rapid iodine adsorption, enabling it as a reusable solid adsorbent for the sequestration of various iodine species in complex and demanding environments. Additionally, when a TFA solution of mHOF-SYSU101 is injected into DMSO or methanol, the crystalline porous framework can be instantaneous regenerated. Such attributes accentuate its potential as a valuable iodine adsorbent, especially for nuclear power industry applications. Ongoing research in our laboratory is further delving into the potential of mHOF-SYSU101, particularly its promising capabilities in hydrocarbon separation and as an aqueous zinc-iodide battery separator material. Given the inherent versatility in crafting diverse mHOFs through modulating various combinations of acylhydrazides and ketones (see Supporting Information Figures S44–S51 and Table S10), we envisage mHOF-SYSU101 and its analogues will make significant strides in fields spanning from adsorption and separation to clean energy.

Supporting Information

Supporting Information is available and includes materials, methods, experimental procedures for synthesis and characterization, X-ray crystallographic data, details on the DFT calculations, and Iodine adsorption experiment. The Cambridge Crystallographic Data Center (CCDC) deposition number (CCDC 2302841-2302843, 2314406) contains the supplementary crystallographic data for this paper.

Conflict of Interest

The authors disclose competing financial interests as follows: A patent application in China (Application No. 2023114292004) related to the manuscript’s content is currently under review. The research presented herein may have implications for the commercial exploitation of this patent. Furthermore, the approval of this patent could yield potential economic benefits to the authors.

Funding Information

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant nos. 21272285, 22090061, and 3100041031094) and Natural Science Foundation of Guangdong Province (grant nos. 2021A1515010132 and 2022A1515010051). Single-crystal structure characterization was supported by the Shanghai Synchrotron Radiation Facility. Computational resources were provided by the National Supercomputer Center in Guangzhou.