Polyoxometalate condensation and transformation mediated by adaptive coordination-assembled molecular flasks

Abstract

Here we report polyoxometalate (POM) condensation or transformation reactions mediated by adaptive coordination-assembled molecular flasks. Addition of Na₂SiO₃ to the (Mo₆O₁₉)₂⊂1·(NO₃)₈ complex containing Lindquist-type clusters as guests leads to the formation of a new (SiMo₁₂O₄₀)⊂2·(NO₃)₈ host–guest complex, where the in situ generated Keggin-type cluster served as a trigger for the host transformation from cage 1 to isomeric bowl 2. Conversion from 1 to 2 driven by the in situ condensation was found to be 27.5-fold faster than the direct templation with independently prepared SiMo₁₂O₄₀⁴⁻. As a comparison, cage 1 was noticed to bind only one W₆O₁₉²⁻ cluster in its cavity, and the formation of (W₁₀O₃₂)⊂2·(NO₃)₈ as the main product and (SiW₁₂O₄₀)⊂2·(NO₃)₈ as the minor host–guest complex was observed when it was used for the above condensation reaction, highlighting the crucial role of encapsulation in cavity-confined POM transformations. The reaction processes and the final structure of all the new host–guest complexes have been investigated by NMR, ESI-TOF-MS and SCXRD. Our findings not only showcase a unique example of inorganic-reaction-driven responsive supramolecular system, but also provide a new approach for the preparation of functional POMs⊂cage composite materials.

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Introduction

Enzymes are biological catalysts with substrate-binding cavities that exhibit conformational dynamics and rich active sites, essential for efficient catalysis.^1–3 Inspired by these natural systems, scientists have developed coordination molecular containers based on supramolecular interactions to mimic enzyme functionality. The confined spaces of coordination-assembled cages, often referred to as ‘molecular flasks’, provide unique local microenvironments that allow for the encapsulation of guest molecules and endow them with reactivity distinctly different from the bulk solvent.^4–12 The tailored pockets of molecular flasks play a vital role in cage-promoted reactions through three major factors: preorganization and proximity of substrates to yield an unusual regioselectivity,^13,14 local concentration enrichment to accelerate reactions,^15,16 and reaction intermediate stabilization to lower the energy and enthalpy barrier.^17–19 Despite extensive research on constructing stimuli-responsive supramolecular hosts with adaptive confined cavities,^20–34 examples of using such adaptive hosts as molecular flasks for inorganic chemical transformations remain scarce.

Polyoxometalates (POMs) are a well-known class of inorganic compounds that have shown wide-spread applications in diverse fields.^35–39 In nature, polyoxomolybdates can be stored in a cage-like Mo-storage protein (MoSto) from Azotobacter vinelandii.^40–42 Biological protein pockets are able to engineer condensation processes and stabilize the resulting fragile POM species by shielding against hydrolysis.⁴³ In laboratory, POMs are usually generated using bottom-up synthetic approaches, through acidic condensation of tetraoxometalates MO₄²⁻ (M = Mo, W etc.), the outcome of which is very sensitive to synthetic variables, such as the pH, ionic strength, concentration etc.^44,45 POM transformation can happen by adjusting the solution conditions or by modifying the structural “vacancy” sites of lacunary POMs.⁴⁶ Although an artificial encapsulation approach has been demonstrated by installing POM anions into host systems via non-covalent and specific interactions,^47–59 the condensation or transformation of POM guests inside discrete adaptive coordination cages has been seldom targeted so far.

In our previous reports,^34,60,61 a mitosis-like host transformation from Pd₄L₂ cage 1 to a unique conjoined Pd₆L₃ twin-cage 3 has been observed (Scheme 1A), driven by an organic self-coupling dimerization reaction of ortho-quinone methide precursors. Here we present the cavity-confined syntheses of Keggin-type POM clusters (SiM₁₂O₄₀⁴⁻, M = Mo) from Lindquist-type Mo₆O₁₉²⁻ precursors and SiO₃²⁻, which exert the induced-fit power to force the structural conversion from cage 1 to isomeric bowl 2 (Scheme 1B). The same conversion also proceeds, but at a much slower rate, via the direct templating of independently made SiMo₁₂O₄₀⁴⁻. As a comparison, the reaction of the other Lindqvist-type W₆O₁₉²⁻ cluster with SiO₃²⁻ mediated by cage 1 led to not only Keggin-type SiW₁₂O₄₀⁴⁻ but also W₁₀O₃₂⁴⁻, both also accompanied by the induced-fit cage-to-bowl transformation. These POM synthesis reactions mediated by adaptive cages have been thoroughly investigated by IR, UV-vis, NMR, ESI-TOF-MS, and SCXRD.

Scheme 1 (A) Organic self-coupling dimerization reaction triggered cage to twin-cage transformation. (B) Inorganic POM condensation reaction induced cage to bowl transformations.

Results and discussion

Cage 1·(NO₃)₁₂ and (Mo₆O₁₉)₂⊂1·(NO₃)₈ were synthesized according to our previous work (Fig. S1–S4†).^60,61 The optimized structure of (Mo₆O₁₉)₂⊂1·(NO₃)₈ was simulated using molecular mechanical modeling (Fig. S5†). Surprisingly, when 1 equiv. of Na₂SiO₃ was added into the aqueous solution of (Mo₆O₁₉)₂⊂1·(NO₃)₈, a dramatic ¹H NMR change was observed after heating at 70 °C for 12 h, accompanied by the darkening of the pale-yellow solution (Fig. 1, S6 and S7†). Two sets of protons signals consisting of 12 aromatic signals were observed after the reaction. The resonances of H_e in the CH₂ groups of the ligand (L) split into two doublets, along with a new set of splitting signals of H_d and H_c in the pyridinium group, all of which experienced two different magnetic environments. The DOSY spectrum shows that all the new signals have the same diffusion coefficient (D = 1.75 × 10⁻¹⁰ m² s⁻¹, Fig. 1D and S8†), different from that of (Mo₆O₁₉)₂⊂1·(NO₃)₈ (D = 2.43 × 10⁻¹⁰ m² s⁻¹, Fig. S3†). The host–guest complex was characterized by high-resolution ESI-TOF-MS (Fig. 1E, S9 and S10†). Highly resolved +8 peaks observed at m/z = 520.9201 could be assigned to [Pd₄L₂(SiMo₁₂O₄₀)]⁸⁺. It is noted that the ¹H NMR of the inclusion complex (Mo₆O₁₉)₂⊂1·(NO₃)₈ had almost no change even after heating at 70 °C for three days (Fig. S11†). Based on the above results, we inferred that a new POM species, SiMo₁₂O₄₀⁴⁻, was formed in the system by the condensation of Mo₆O₁₉²⁻ and SiO₃²⁻, accompanied by the structural transformation of cage 1.

Fig. 1 Synthesis of the Keggin-type SiMo₁₂O₄₀⁴⁻ anion within the adaptive cage. ¹H NMR spectra (400 M, D₂O, 298 K) of (A) 1·(NO₃)₁₂, (B) (Mo₆O₁₉)₂⊂1·(NO₃)₈, and (C) (SiMo₁₂O₄₀)⊂2·(NO₃)₈ after heating the solution of (Mo₆O₁₉)₂⊂1·(NO₃)₈ at 70 °C with Na₂SiO₃ for 12 h. (D) ¹H DOSY spectrum of (SiMo₁₂O₄₀)⊂2·(NO₃)₈, and (E) ESI-TOF-MS of (SiMo₁₂O₄₀)⊂2·(NO₃)₈, showing the observed and simulated isotopic patterns of the +8 peak; (F) the chemical structure of (SiMo₁₂O₄₀)⊂2·(NO₃)₈.

Yellow block crystals suitable for SCXRD were obtained by slow evaporation of an aqueous solution of the host–guest complex (Fig. S46†). The inclusion complex crystallized in a hexagonal crystal system with the P6₃ space group. Remarkably, the crystal structure reveals that the Pd₄L₂-type cage 1 transformed into a structural isomer known as bowl 2, benefited by the semirigid feature of the ligand with flexible p-xylene linkers. A single Keggin-type α-SiMo₁₂O₄₀⁴⁻ anion is tightly accommodated in the cavity of bowl-shaped host structure 2 (Fig. 2A). Multiple hydrogen bonding interactions between the p-xylene group and SiMo₁₂O₄₀⁴⁻ (C–H⋯O=Mo), with distances ranging from 2.3778(1) to 2.9933(1) Å, were observed in the structure (Fig. S50†). Independent gradient model (IGM) analysis provides a visual depiction of the noncovalent bonding interactions (Fig. 2B and S48†), from which we inferred that the SiMo₁₂O₄₀⁴⁻ anion is stabilized in the cavity by electrostatic and anion–π interactions as well as multiple intermolecular hydrogen bonding interactions. Additionally, in the crystal packing, three host–guest complexes aggregate into a trimer via π–π interactions between p-xylene and TPT panels (center-to-center distances: 3.6822(3)–3.8283(2) Å) (Fig. S50†).

Fig. 2 (A) Crystal structure of (SiMo₁₂O₄₀)⊂2·(NO₃)₈. (B) Visualized intermolecular binding iso-surface between 2 and SiMo₁₂O₄₀⁴⁻ (δg_inter = 0.003). (C) Cavity volumes of cage 1 and bowl 2 calculated using the MoloVol program based on the crystal structures.

The bowl 2 skeleton adopts the C_2v molecular symmetry (Fig. 2C), in contrast to the D_2d-symmetry of cage 1, which aligns well with the NMR analysis. The cavity volume of bowl 2 was calculated to be ca. 963 ų using MoloVol calculations based on the crystal structure (Fig. 2C and S52†),⁶² which is larger than that of cage 1 (ca. 914 ų). This difference explains why the large-sized SiMo₁₂O₄₀⁴⁻ anion is encapsulated by bowl 2 rather than cage 1. According to Rebek's “55% rule”,⁶³ optimal binding between the host and guest can be expected when the occupancy factor falls within the range 0.55 ± 0.09. The SiMo₁₂O₄₀⁴⁻ anion has a maximum size of approximately 10.4 Å and a molecular volume of ca. 658 ų (Fig. S53†). The occupancy factor of the bowl-cavity space by SiMo₁₂O₄₀⁴⁻ is ca. 68%, smaller than that (72%) in the cage-cavity, which makes it more suitable for the optimal binding than 1. Therefore, shape complementarity between Keggin-type SiMo₁₂O₄₀⁴⁻ and the bowl complex 2 drives the supramolecular structural transformation.

To shed light on the POM condensation induced-fit cage-to-bowl transformation mechanism, independently prepared (TBA)₄SiMo₁₂O₄₀ (TBA = [(n-C₄H₉)₄N]⁺) was treated with 1·(NO₃)₁₂ in D₂O solution. As observed in the ¹H NMR spectra, cage 1 cannot encapsulate the Keggin-type SiMo₁₂O₄₀⁴⁻ in the cavity. Time-dependent ¹H NMR spectra with heating were collected to monitor the solution of (Mo₆O₁₉)₂⊂1·(NO₃)₈ after adding 1 equiv. of Na₂SiO₃, as well as the solution containing SiMo₁₂O₄₀⁴⁻ and cage 1 (Fig. 3 and S12–S15†). Upon heating at 70 °C, signals assignable to the host–guest complex (SiMo₁₂O₄₀)⊂2·(NO₃)₈ gradually evolved, implying the simultaneous POM condensation and induced-fit cage transformation. For the in situ POM condensation driven cage-to-bowl conversion, over 92% transformation from (Mo₆O₁₉)₂⊂1·(NO₃)₈ to (SiMo₁₂O₄₀)⊂2·(NO₃)₈ was observed within 12 h. In sharp contrast, only 61% conversion for SiMo₁₂O₄₀⁴⁻-induced structural transformation was observed even after heating at 70 °C for 8 d. Based on pseudo-first order reaction kinetics, a 27.5-fold higher rate constant of in situ-POM-condensation driven cage-to-bowl transformation was estimated, compared to that driven by SiMo₁₂O₄₀⁴⁻ (Fig. 3D).

Fig. 3 Time-dependent ¹H NMR (400 M, D₂O, 298 K) for (A) (Mo₆O₁₉)₂⊂1·(NO₃)₈ after adding 1 equiv. of Na₂SiO₃ and heating at 70 °C and (B) the SiMo₁₂O₄₀⁴⁻ induced structural transformation from 1 to 2. (C) Conversion and (D) pseudo-first-order kinetic plots for the above two cage transformation reactions.

We also examined a similar condensation process with another Lindqvist W₆O₁₉²⁻ cluster. Host–guest NMR titration experiments and ESI-TOF-MS revealed that only one W₆O₁₉²⁻ cluster could be trapped in the cavity of 1 (Fig. S16–S20†). Then we wondered whether the reaction of (TBA)₂W₆O₁₉ and Na₂SiO₃ in the presence of 1·(NO₃)₁₂ could also produce a similar Keggin-type product. The solution of 1 equiv. of Na₂SiO₃ and 2 equiv. of W₆O₁₉²⁻ with 1·(NO₃)₁₂ was heated for 15 h, resulting in a complicated ¹H NMR spectrum (Fig. S21†). The formation of the main product (W₁₀O₃₂)⊂2·(NO₃)₈, as well as the minor product (SiW₁₂O₄₀)⊂2·(NO₃)₈ was confirmed by ESI-TOF-MS (Fig. S22†). Observation of +4 and +3 charged species assigned to (W₁₀O₃₂)⊂2·(NO₃)₈ and (SiW₁₂O₄₀)⊂2·(NO₃)₈ provides the direct evidence. In addition, other signals attributable to [Pd₄L₂(NO₃)₃(W₆O₁₉)(HW₂O₇)(H₂O)₃]⁶⁺ and [Pd₄L₂(NO₃)₅(W₆O₁₉)(HW₂O₇)(H₂O)₃]⁴⁺ were observed in the mass spectrum. The cage-to-bowl transformation was then also checked with independently made SiW₁₂O₄₀⁴⁻. Time-dependent ¹H NMR spectra of 1·(NO₃)₁₂ and 1 equiv. of SiW₁₂O₄₀⁴⁻ with heating at 70 °C exhibit 27% yield of clean (SiW₁₂O₄₀)⊂2·(NO₃)₈ (Fig. S23†) without the formation of other POM species. ESI-TOF-MS analysis of (SiW₁₂O₄₀)⊂2·(BF₄)₈ revealed a clear series of multivalent signals assignable to [Pd₄L₂(BF₄)_nSiW₁₂O₄₀]⁸⁻ⁿ (n = 0–4) (Fig. S24†). These results indicate that the full-encapsulation of the small POM precursors inside the cage cavity is crucial for efficient condensation reactions.

Previous reports have shown that the rapid transformation of W₆O₁₉²⁻ to W₁₀O₃₂⁴⁻ occurs when water is added to a methanolic solution containing W₆O₁₉²⁻.^64,65 Thus the transformation of W₆O₁₉²⁻ without SiO₃²⁻ was also examined in our system (Fig. 4A). When 2 equiv. of the W₆O₁₉²⁻ anion was added to the H₂O solution of 1·(NO₃)₁₂ and heated at 70 °C, ¹H NMR spectra show the disappearance of the starting material along with the coinstantaneous evolvement of two sets of new species (Fig. 4B and C). ESI-TOF-MS reveals the formation of new host–guest complexes assignable to (W₁₀O₃₂)⊂Pd₄L₂ (Fig. 4D, S27 and S28†). ¹H NMR spectra of 1·(NO₃)₁₂ with the addition of independently prepared (TBA)₂W₁₀O₃₂ showed almost identical NMR spectra (Fig. S34†). Based on the NMR of 1·(NO₃)₁₂ and (TBA)₄W₁₀O₃₂ (Fig. S29–S32†), along with the symmetry of the cage and bowl, one set of signals is assigned to (W₁₀O₃₂)⊂1·(NO₃)₈ and the other to (W₁₀O₃₂)⊂2·(NO₃)₈. This is further supported by the ESI-TOF-MS analyses (Fig. S36†).

Fig. 4 (A) Structural transformation from cage 1 to bowl 2 induced by the W₆O₁₉²⁻ conversion reaction and (B) time-dependent ¹H NMR (400 M, D₂O, 298 K) for (W₆O₁₉)₂⊂1·(NO₃)₈ upon heating at 70 °C. (C) The corresponding conversion yield and (D) ESI-TOF-MS spectrum showing the observed and simulated isotopic patterns of the +8 peak of the resulting inclusion complex.

SCXRD provided direct proof of the structural transformation. After heating W₆O₁₉²⁻ in the aqueous solution of 1·(NO₃)₁₂ at 70 °C for 2 days, single crystals of (W₁₀O₃₂)⊂2·(NO₃)₈ were obtained by slow vapor diffusion of THF into the system. X-ray structural analysis reveals a 1 : 1 host–guest complex, where a W₁₀O₃₂⁴⁻ anion was encapsulated into the cavity of bowl 2 (Fig. 5A). The average diagonal Pd–Pd distances in the structure of (W₁₀O₃₂)⊂2·(NO₃)₈ are 17.3563(34) Å and 19.3029(38) Å. IGM analysis revealed that electrostatic, anion–π interactions and multiple hydrogen bonds work together to stabilize this host–guest complex (Fig. 5B and S49†). As the maximum size and the volume of W₁₀O₃₂⁴⁻ are ca. 12.0 Å and 558 ų, respectively, the occupancy factor of W₁₀O₃₂⁴⁻ in the bowl-cavity of 2 is ca. 58%, compared to 61% in the cage cavity of 1 (Table S1†). W₁₀O₃₂⁴⁻ also can be shape-matched well with the bowl-shape space of 2 and multiple non-covalent interactions drive the guest-reaction induced-fit structural transformation.

Fig. 5 The top view for the X-ray structure of inclusion complexes (A) (W₁₀O₃₂)⊂2·(NO₃)₈, and (C) (SiW₁₂O₄₀)⊂2·(NO₃)₈, showing the diagonal Pd–Pd distances. IGM analysis for (B) (W₁₀O₃₂)⊂2·(NO₃)₈ and (D) (SiW₁₂O₄₀)⊂2·(NO₃)₈ (δg_inter = 0.003). The color scale shows a range of interaction strengths: strong attraction (blue), weak contacts (green), and nonbonding repulsion (red).

Crystallization of the (SiW₁₂O₄₀)⊂2·(NO₃)₈ complex was also successful. In the crystal structure, the complex crystallized in the monoclinic crystal system with the P2₁/n space group. One SiW₁₂O₄₀⁴⁻ anion sits inside the bowl-shaped cage 2, similar to SiMo₁₂O₄₀⁴⁻ (Fig. 5C and D). IGM analysis confirmed similar non-covalent interactions in stabilizing this iso-structural host–guest complex as observed with (SiMo₁₂O₄₀)⊂2·(NO₃)₈.

UV-vis absorption spectra also supported the POM synthesis driven supramolecular transformation processes (Fig. S37 and S38†). After heating the solution of the inclusion complex (Mo₆O₁₉)₂⊂1·(NO₃)₈ at 70 °C with NaSiO₃ in H₂O, the structural conversion to (SiMo₁₂O₄₀)⊂2·(NO₃)₈ was indicated by the appearance of a new absorption band tailing to the region until ca. 400 nm. In the IR spectra (Fig. S39†), the characteristic peaks of ν(Mo=O) and ν(O–Mo–O) for (Mo₆O₁₉)₂⊂1·(NO₃)₈ were observed at 956 and 800 cm⁻¹, respectively, which disappeared when forming the new host–guest complex of (SiMo₁₂O₄₀)⊂2·(NO₃)₈, along with new peaks at 945, 901, 795 cm⁻¹ assignable to SiMo₁₂O₄₀⁴⁻. As for the conversion from W₆O₁₉²⁻ to W₁₀O₃₂⁴⁻, new peaks in the region of 961–434 cm⁻¹ corresponding to W₁₀O₃₂⁴⁻ were observed after heating the sample of the mixture of W₆O₁₉²⁻ (2 equiv.) and 1·(NO₃)₁₂ (Fig. S40†). The peaks at 961, 893 and 804 cm⁻¹ correspond to ν(W–O_t), ν(W–O_b–W), and ν(W–O_c–W) of the [W₁₀O₃₂]⁴⁻ cluster, respectively.^48,66,67

Plausible mechanisms were proposed for the POM condensation/transformation induced cage-to-bowl conversion. In the case of Mo₆O₁₉²⁻, two precursors are encapsulated by the cavity of cage 1 and condensation happens after the addition of SiO₃²⁻. The resulting SiMo₁₂O₄₀⁴⁻ anion instigates the induced-fit transformation from cage 1 to bowl 2. In fact, when (TBA)₂Mo₆O₁₉ was treated with Na₂SiO₃ and heated at 70 °C for 4 days as a control, no SiMo₁₂O₄₀⁴⁻ anion was observed (Fig. S41†), suggesting the vital role of cage 1 in the synthesis of SiMo₁₂O₄₀⁴⁻. Similarly, no W₁₀O₃₂⁴⁻ or SiW₁₂O₄₀⁴⁻ was observable by heating (TBA)₂W₆O₁₉ and Na₂SiO₃ at 70 °C in H₂O/CH₃CN (v/v, 4/1) even for 4 days (Fig. S42†). Moreover, a remarkable stability of SiMo₁₂O₄₀⁴⁻ hydrolytically protected within the bowl host was confirmed in water, as ¹H NMR witnessed negligible change for the (SiMo₁₂O₄₀)⊂2·(NO₃)₈ complex even after standing at room temperature for one year (Fig. S44 and S45†).

Conclusions

In summary, we have discovered an induced-fit supramolecular transformation driven by in situ POM condensation reactions. Our work represents the first report on inorganic condensation reaction driven supramolecular structure transformation, which may provide new design principles for chemically fueled molecular machines. We also envision that efficient embedding of POMs into adaptive supramolecular hosts could be very promising for the development of functional hybrid materials.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data have been deposited at the CCDC under numbers 2360627, 2380035 and 2380036, and can be obtained from https://www.ccdc.cam.ac.uk/structures/.

Author contributions

Q.-F. S. and L.-X. C. conceived and designed this project. L.-X. C. carried out the synthesis, characterization, conducted the experiments and analyzed all the results. Y.-H. H. assisted with the synthesis. L.-P. Z. and Y.-T. C. performed the mass spectroscopy measurement. P.-M. C and X.-Q. G. contributed to the figure production. L.-X. C. and Q.-F. S. wrote the manuscript with input from all the authors.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (Grants 2022YFA1503300 and 2021YFA1500400), the National Natural Science Foundation of China (Grants 22171262, and 22171264), the National Science and Technology Council (NSTC) of Taiwan (113-2628-M-002-004), the Self-Deployment Project Research Program of Haixi Institutes, Chinese Academy of Sciences (CXZX-2022-GH02), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB1170000).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2360627, 2380035 and 2380036. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc08729a

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