Evolution rom [Zn₉] to a record-high [Zn₅₄] subblock and engineering a hierarchical supramolecular framework for enhanced iodine uptake

Abstract

Hierarchical supramolecular frameworks are being designed and constructed for various applications, yet the controlled assembly and process understanding incorporating giant building blocks remains a great challenge. Here, we report a strategy of “rivet” substitution and “hinge” linkage for the controlled assembly of the hierarchical supramolecular framework. The replacement of two “rivet” ethylene glycol (EG) molecules for triangular prism [Zn₉] (a small block in 1) with a 1,3-propanediol (PDO) provides space for a “hinge” linkage from adjacent ligands, thus providing a hierarchical (from micro- to mesopores, from the internal cavity to external surface) supramolecular framework (2) based on a coordinative subblock with the record number of zinc ions ([Zn₅₄]). Time-dependent powder X-ray diffraction and ESI-MS technology were used to assess the in situ evolution process: logically progressing from [Zn₉] to [Zn₁₈], then to [Zn₂₇], and finally to [Zn₅₄]. The sequential transformation entails two types of half-opening cavities and two types of internal microcages. Further aggregation of [Zn₅₄] in dia topology engenders the formation of a one-dimensional channel (10 Å), and an additional mesocage with a volume of 16 × 16 × 55 ų. The diverse pore system exhibits an impressive uptake capability (3.19 g g⁻¹) for iodine vapor at 75 °C and effective ethylene purification. Our investigations represent a valuable avenue for assembling a giant subblock and hierarchical supramolecular framework, facilitating multi-functional molecular accommodation.

The design of a carboxylate–pyridine–triazole integrated ligand, coupled with a strategy of “rivet” substitution and “hinge” linkage, facilitates the construction of the largest [Zn₅₄] cluster in a hierarchical supramolecular framework.

Introduction

The interest in supramolecular chemistry has progressively increased with the synthesis of intricate structures.^1–5 This is because the use of “programmed” building blocks allows for the assembly of cluster- or spacer-based supramolecules with unique physicochemical properties compared to their precursors.^6–9 Interestingly, spacer-based supramolecules (e.g. coordination cages) exhibit promising cavities, which show emerging functions, such as synergistic catalysis, ionic conductivity, enantioselective recognition, and gas separation.^10–12

Analogous to metal–organic frameworks,¹³ coordination-driven assemblies, together with non-interfering interactions, have emerged as an effective approach for the construction of spacer-based supramolecules.^14–19 For example, the coordination between palladium(ii) and the ligand (with a bending angle of 149°) selectively assembles into a [Pd₂₄L₁₄₈] complex.²⁰ However, when the bending angle of the ligand is increased to 152°, a supramolecule of [Pd₃₀L₂₆₀] can be assembled, with 144 components and a type of nearly spherical cage (∼6.8 nm).²¹ Similarly, a cuboctahedral nanocage, containing 13 cavities, but two types only, has been prepared from a C_3v-symmetric ligand, where the pregnant molecular nanoball can be converted into a single nanoball by ligand (C_2v) exchange.²² Therefore, the changes in geometric complementarity, the steric constraints, and also the external stimuli can influence the self-assembly of the subunits. In other words, the preparation of desired architectures by molecular self-assembly with a few or dozens of components, as well as the accurate understanding and regulation of the interplays involved, is still inherently challenging, and particularly huge when it comes to the one with a hierarchical (from micro- to mesopores and from the internal cavity to external surface) pore system established from a large number of components (>100) comparable to that seen in nature, and has been rarely reported.^21,23

During our exploration of coordinative clusters,^24–29 a series of M₄ cubane and M₇ brucite disk clusters, a cubane-based triangular [Co₁₂] single-molecule magnet, and the largest chiral cobalt cluster of [Co₁₆] have been synthesized.²⁷ According to the structural correlation information of the solid/solution phase using mass spectrometry technology, the transformation mechanisms of step-by-step assembly, competitive assembly, assembly/disassembly/in situ reaction/reassembly were proposed. This crucial and fundamental information not only helps to gain insights into the evolution of molecular fragments but also contributes to developing effective strategies for controllable assembly. Given the observation on the assembly process of the [Co₁₆] supercluster, a new structure with separate Co₄O₄ cubes has been prepared after step-by-step elimination and substitution of inner bridging ligands. However, the tuned assembly makes both the size and components of the sub-block smaller, restricted by the flexible configuration and the limited coordination sites.

To reverse this trend and also to enrich the family of supramolecular chemistry, we report here a strategy of “rivet” substitution and “hinge” linkage for the controlled assembly of a hierarchical supramolecular framework with a giant subunit from that with a small cluster (Scheme 1). To achieve this goal, a judicious choice of appropriate molecular building blocks is very important. The ligand (6,6′-(1H,1H′-[3,3′-bi(1,2,4-triazole)]-5,5′-diyl)dipicolinic acid: H₄btdpa) was selected for the following reasons: (a) three types of coordination groups of carboxylate, pyridine, and triazole were integrated into one linear-dentate shape ligand with twelve sites to bind metal ions, (b) the inner pyridine-triazole skeleton supports bidirectional coordination capacity while the two terminal carboxylate groups provide significant potential to extend the superstructure, (c) the multiple ring rotation around the C–C bond between adjacent structural motifs (four aromatic rings and two carboxylates) facilitates linker flexibility for self-adjusted coordination; (d) the large dimensions of the ligand may meet the demand for stabilizing nanoclusters/nanocages.³² Additionally, of the 3d metal ions, zinc is particularly attractive in terms of its relative flexibility in adopting different coordination environments, including the coordination number and geometry. Based on these considerations, a supramolecule 1 was prepared at first, in which the ethylene glycol (EG) strengthens the small triangular prism [Zn₉] subcluster. After replacing these short “rivet” molecules with relatively longer 1,3-propanediol (PDO), extra coordination sites on terminal Zn ions on [Zn₉] were linked together through the “hinge” carboxylates in btdpa, forming a giant [Zn₅₄] nanocage (114 components) with four kinds of micropores. Further aggregation of this nanocage in dia topology offers a new superstructure with significant mesopores (55 Å), allowing impressive capabilities in iodine adsorption and ethylene purification.

Scheme 1. Illustration of the strategy of “rivet” substitution and “hinge” linkage for controlled assembly of a giant coordinative cluster ([Zn₅₄]) from a small one ([Zn₉]) in the supramolecular framework, in comparison with previous benchmark Zn(ii) clusters.^30,31.

Results and discussion

Rhombic crystals of 1 ([Zn₉(btdpa)₃(EG)₆]·5.5H₂O) were obtained through the solvothermal reaction of H₄btdpa, Zn(NO₃)₂·6H₂O and EG. 1 crystallizes in the R3̄c space group (Tables S1 and S2†). In 1, three linearly packed Zn ions are connected by two ligands at an angle of about 60°, forming a triangular prism nanotube. Additionally, each edge of this cluster is strengthened by two “rivet” EGs (Fig. 1a and b), where the two O atoms adopt the μ₂-η¹,η² mode. The nanotubes are connected by hydrogen bonds (O_COO–H–O_EG) and form a supramolecular framework with small channels of about 4.4 Å (Fig. 1c and S2†). In this regard, if the two “rivet” molecules on the edge of 1 can be replaced by a molecule with two O atoms in the μ₃-η²,η² mode, there may be extra space on the two terminal Zn atoms, allowing for coordination expansion as subunits.

Fig. 1. Schematic representation of the synthesis and structure of supramolecule 1: involving a ligand and metal center (a), structure view of 1 (b) and its packing mode (c), inset c is the photo of 1. H atoms were omitted for clarity.

To ensure the μ₃-η²,η² mode, the distance between the two O atoms on the “rivet” should fit the configuration of the trinuclear edges in 1. Compared with EG, PDO with three carbons in the center may be a candidate. To validate this idea, 1 was soaked in a PDO/N,N′-dimethylformamide (DMF) solution for the synthesis of the supramolecule with an expanded subunit. As expected, octahedral crystals [Zn₅₄(btdpa)₁₈(PDO)₁₈(HN(CH₃)₂)_21.6]·16PDO·30DMF (2) were found (Fig. 2), which has a coordinative cluster with the largest number of Zn(ii) ions, compared to the benchmark clusters with 25 and 32 Zn(ii) ions (Fig. S3†).^30,31 The crystal has a cell volume of up to 198 173 ų in the Fd3̄ space group. Firstly, it is found that the regular triangular prism of 1 changed to become a highly distorted one, because two of the three [Zn–Zn–Zn] edges were not straight anymore. The two μ₂-η¹,η² EGs on each edge of 1 have been replaced by one μ₃-η²,η² PDO, that is, there are just three PDO molecules for each Zn₉ species in 2 (Fig. S1–S4†). There is an obvious coordination number lowering from single μ₆-fashion through ten coordination atoms in 1 to mixed μ₅-coordination through nine atoms and μ₄-coordination through eight atoms in 2 for the primary ligand of btdpa.

Fig. 2. Schematic representation of the synthesis, structure, and topology of 2: synthesis route and associated coordination change of the blocks before and after reaction (a); packing view of structure 2 (b), view of 2 and its four kinds of micropores (c–e); view of the mesopore generated by aggregation of [Zn₅₄] (f); simplified geometry of Zn₅₄ (g) and the hydrogen-bond connected network (h); dia topology (i), the green line indicates the macropore again.

Even so, the pyridine–triazole part in each independent btdpa still adopts the bilateral coordination with four Zn(ii) moieties. Coupled with μ³-chelated PDO linkage of another Zn(ii), the nano-nuclear building block can be maintained. The coordination changes in the inner pyridine-triazole strut also promote the slacking of the whole btdpa ligand, confirmed by the five independent Zn(ii) moieties with a more regular polyhedron geometry, and larger/multiple ring rotation of btdpa in 2, compared to that of 1 (Tables S3–S5†). As the concomitant benefit, one of the terminal carboxylates previously adopted μ₁-η¹ fashion transform to the μ₂-η¹,η¹ model using both of the two oxygen atoms, leading to the desired role of the carboxylate “hinge” to link two adjacent Zn₉-subunits. As a result, a giant nanocage of [Zn₅₄] with an octahedron configuration was constructed from six Zn₉-subunit blocks. The diameter of this cluster reaches 35.6 Å, comparable in size to some biological molecules (Fig. S5†). Thanks to the involved hydrogen bonds, the [Zn₅₄] aggregates together to form a supramolecular framework (Fig. 2b).

The structure of [Zn₅₄] is fascinating. The triangular prism nanotube in 1 distorts into an octahedral Zn₉-subunit (pore A) with an internal size of about 4.2 Å in 2 (Fig. S6†). In contrast to the usual pore expansion due to increased components, the coordination extension of the Zn₉-subunit in 2 leads to a collapsible arrangement with a central pore B measuring only 5.8 Å (Fig. 2c and S7†). This collapsible arrangement offers two types of bowl-shaped pores (C and D). The lower part of pore C is composed of three PDO molecules, while the upper part is made up of three Zn₉-subunits, resulting in an opening size of 11.0 Å and a depth of 12.6 Å (Fig. 2d and S8†). Next to pore C is another bowl-shaped pore D, which is wider (13.2 Å) but shallower (8.3 Å) (Fig. 2e and S9†). Additionally, six hydrogen bonds of O_COO⋯H–N_HN(CH3)2 with a short distance of 1.876 Å have been found between the two adjacent [Zn₅₄] clusters, yielding additional mesopore E (∼16 × 55 Ų) and a 1D channel (10 Å) along the [1 1 0] plane (Fig. 2f and S10†). Supramolecular chemistry has yielded fruitful results, whereas the creation of six different types of nanospaces, particularly ranging from micro-to mesopores, is unprecedented. To better understand structure 2, topology analysis was performed. Considering the four groups of hydrogen-bonds around [Zn₅₄], this subunit can be simplified as a 4-fold-connected node. After subsuming these doubly connected vertices into the linkage, a net with a dia topology was observed (Fig. 2g–i). Notably, the solvent-accessible volume calculated for the framework of 2 using PLATON is ca. 58.3%, which is almost five times larger than that of 1 (12.9%). The structures and phase purities of the as-synthesized 1 and 2 were confirmed by powder X-ray diffraction (PXRD) analyses (Fig. S11–S14†).

Based on these structures, it is evident that the strategy of “rivet” substitution and “hinge” linkage is effective for the controlled assembly of a supramolecular framework with a giant subunit from a much smaller one. Therefore, delving into the molecular-level replacement of the “rivet” may offer valuable insights into this strategy. During the reaction of 1 to 2, the uniformly dispersed crystals were evaluated by PXRD, while the corresponding solvent was detected by electrospray ionization mass spectrometry (ESI-MS).

As shown in Fig. 3a, the position of the typical [1 0 −2] peak of 1 did not change obviously after 2 hours of reaction, but became slightly broader and was associated with a very small shoulder peak. As the reaction progressed, the intensity of this shoulder peak gradually increased, causing the [1 0 −2] peak to split into two at 4 hours of reaction. This change indicates that 1 does not dissolve in the solvent and maintains its crystallinity initially. After 8 hours, the four peaks ([4 0 0], [3 3 1], [4 2 2], and [5 1 1]) belonging to 2 were observed, while the [1 0 −2] peak belonging to 1 disappeared. This indicates a gradual extension of the substitution reactions from the outside to the inside of the bulky crystal 1. This is because the presence of macrospores in 2 allows for convenient molecular exchange. Moreover, the relative intensity of the diffraction peaks of the collected crystals at 12 hours became much stronger than that at 8 hours, showing a completed “rivet” substitution and “hinge” linkage. This result has also been confirmed by time-dependent IR and Raman spectroscopy (Fig. S16 and S17†). In addition, the non-observable signal in the range of m/z = 1400 to 4000 of the reaction solvents confirms that the “rivet” substitution and “hinge” linkage occurs initially on the crystal surface and then extends into the crystal as the reaction progresses (Fig. S18†).

Fig. 3. PXRD patterns of the time-dependent reaction from 1 to 2 (a), ESI-MS of dissociated 2 and typical peaks (b and c), and predicted conversion process from 1 to 2 (d).

Due to the in vivo reaction, the gradual change process cannot be monitored at the molecular level by solvent ESI-MS. However, the dissociation of 2 provides valuable insight into the structural transformation (Fig. 3b and c, S19–S20†). The main species were found at 0 eV. These can be identified as the ion fragments of Zn₉⁻, Zn₁₈⁻ and Zn₂₇²⁻ by comparing the simulated and experimental isotope distributions. When the voltage was increased to 20 eV, the abundance of the Zn₂₇²⁻ ion peak decreased. This indicates further fragmentation of the Zn₂₇²⁻ species into smaller fragments. Therefore, the assembly of the [Zn₅₄] nanocluster may occur through the following process: EG is replaced by PDO on a [Zn₉] cluster, the carboxylate O of a ligand from another Zn₉ joins the coordination of the terminal Zn atoms, forming Zn₁₈ and then Zn₂₇ units. Further coordination from carboxylate O fuses two Zn₂₇ units together to form a [Zn₅₄] nanocluster (Fig. 3d).

The permanent porosities of 1 and 2 were evaluated using N₂ (77 K) adsorption isotherms (Fig. 4a and b). The rapid gas uptake at a lower pressure area can be attributed to the adsorption in micropores, while the uptake at the pressure of P/P₀ = 0.1–0.2 can be assigned as the adsorption in micro- and mesopores. In contrast, 1 exhibits negligible adsorption of N₂. The Brunauer–Emmett–Teller surface area of 2 was calculated to be 736 m² g⁻¹. Additionally, the evaluated pore size distribution varies in a wide range of ∼6 to 55 Å (Fig. 4b), agreeing well with the crystal structure.

Fig. 4. (a) Accessible inner surface highlighted as yellow in a unit cell of 2. (b) The 77 K N₂ adsorption isotherms of 1 and 2, and the measured pore size distribution of 2. (c) The time-dependent iodine vapor uptake performance of 2 at 75 °C under ambient pressure.

The intriguing pore system encourages us to explore the function of 2. Given that radioactive I¹²⁹ with an extremely long half-life of up to 15.7 million years is a heavy waste in nuclear power stations, a new type of adsorbent with strong I₂ affinity and high uptake is quite useful for clean nuclear energy production. After exposure of activated 2 to iodine vapor at 75 °C, the maximum iodine absorption capacity reaches 3.19 g g⁻¹ (Fig. 4c). To the best of our knowledge, such iodine uptake data not only represents a new record in the metal–organic cage (previous record of 2.35 g per g AlOC-162-C)³³ or cluster-based adsorbents (2.246 g g⁻¹ for Ti₁₂Pb₅ based porous layers),³⁴ but also approaches those of best-performing pure MOFs like MOF-808 (2.18 g g⁻¹),³⁵ [Zr₆O₄(OH)₄(L⁷)₆] (2.79 g g⁻¹),³⁶ UPC-158-HCl (2.92 g g⁻¹),³⁷ and PCN-333-Al (4.42 g g⁻¹)³⁸ (Table S6†).³⁹ In addition, iodine enrichment can also be performed in solution.⁴⁰ It is notable that the iodine-loaded single crystal structure disclosed that the disordered iodine species are mainly concentrated in pore C, which might be ascribed to the abundance of triazole and pyridine nitrogen atoms on the surface of this pore (Fig. S21†). Meanwhile, these adsorbed iodine species can be released by immersing the I₂@2 in ethanol at room temperature (Fig. S22†).

To extend the functionality of 2, single-component adsorption isotherms of C₂H₂, CO₂, and C₂H₄ were collected. 2 shows a relatively higher C₂H₂ uptake (40.6 cm³ g⁻¹) compared to CO₂ (24.2 cm³ g⁻¹) and C₂H₄ (17.8 cm³ g⁻¹) at 298 K and 1 bar, showing potential for C₂H₄ purification (Fig. S23–S33†). To confirm this, dynamic breakthrough experiments were performed with the binary mixtures (C₂H₂/C₂H₄, 1/99, v/v, 2 mL min⁻¹) on 2. Pure C₂H₄ (6.6 min) eluted first from the column, while C₂H₂ broke through the sample at 18.6 min (Fig. S34†). The pure C₂H₂ (>99.5%) was also obtained via the subsequent desorption process. After changing the feed gas to a ternary mixture of C₂H₂/CO₂/C₂H₄ (1/9/90, v/v/v), C₂H₄ and CO₂ were eluted at 9.8 min and 15.7 min, respectively, while C₂H₂ broke through at 23.3 min (Fig. S35†). The obvious intervals confirmed that 2 retains the ability to purify C₂H₄ from mixtures containing C₂H₂ or CO₂/C₂H₂, confirmed by the calculated selectivity and binding energy (S28–S33†). Furthermore, no loss of capacity was observed during the cycling experiments, confirming the good framework stability of this hierarchical supramolecular framework (Fig. S36†).

Conclusions

We report a strategy of “rivet” substitution and “hinge” linkage for the controlled assembly of a hierarchical supramolecular framework. The rational design of a particular primary ligand, in combination with the auxiliary ligand and the Zn(ii), provides a mother supramolecular 1 with a small Zn₉ sub-cluster. The replacement of the “rivet” auxiliary ligands on Zn₉ by a flexible and longer one provides space for “hinge” linkage from adjacent ligands, yielding a complex and giant cluster ([Zn₅₄] with 114 components) that has the largest number of Zn(ii) ions and four types of micropores. Aggregation of the [Zn₅₄] cluster in dia topology creates a hierarchical supramolecular mesostructured 2 (∼55 Å), facilitating a record iodine vapor uptake (3.19 g g⁻¹) among cage or cluster type sorbents, as well as effective C₂H₂/C₂H₄ and C₂H₂/CO₂ separation. More importantly, the in situ substitution and linkage processes have been finely revealed via spectra and diffraction analysis. Such synergistic incorporation not only facilitates the construction of an unprecedented structure but also illustrates the importance of multidentate and self-adjusting ligands in oriented assembly that can have profound implications in coordination-driven supramolecular chemistry. Moving forward, the molecular insights of this strategy can be extended as an innovative tool for the advancement and enrichment of supramolecular families.

Data availability

All data can be found in the main text and ESI.†

Author contributions

M.-H. Z., J. D., and F.-P. H. conceived the idea of this work. Y. T., Q. D., and J. W. carried out the experiments and analysed the results. J. D., Q. D. and M.-H. Z. wrote the paper. All authors participated in discussions and gave valuable suggestions for the final draft.

Conflicts of interest

The authors declare no conflict of interest.