Self-assembly of high-nuclear core–shell polyoxovanadates with Lindqvsit templates

Abstract

Only rarely have polyoxometalates been recorded to form core–shell clusters, which presumably relates to the scarcity of appropriate building blocks. Herein, two high-nuclear core–shell polyoxophosphovanadates [M₆O₁₉]²⁻⊂[Na₈V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]²⁺ (M = W 1, Mo 2) are synthesized utilizing Lindqvist polyoxometalates as templates. Both clusters exhibit a nested three-shell architecture characterized by an octahedron@cube@octahedron configuration. The outer shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻ consists of two simple {VO₅} and {PhPO₃} building blocks connected alternately and exhibits the same octahedral geometry as the inner core [M₆O₁₉]²⁻ due to anionic template effects. To the best of our knowledge, compounds 1 and 2 represent the first core–shell polyoxovanadates templated by Lindqvist polyoxometalates. Furthermore, compound 1 demonstrates efficacy and stability as a catalyst for the oxidation of sulfide. This work provides a new perspective on the construction of novel core–shell polyoxometalates.

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Introduction

Polyoxometalates (POMs), which are discrete, large-sized clusters of polyoxoanions composed of W, Mo, V, Nb, and Ta, have been the focus of research due to their structural diversity, modifiability, and multi-functionality.^1–6 As an important branch of POMs, polyoxovanadates (POVs) have unique electronic and geometrical properties and can condense to form highly symmetric and elegant structures, which are widely used in the fields of energy, catalysis, and biomedicine.^7–13 The vanadium center exhibits versatile coordination geometries during the condensation, including the {VO₄} tetrahedron, {VO₅} pyramid, and {VO₆} octahedron. Notably, {VO₅} pyramids can assemble into various inorganic POV cages that encapsulate guests through self-condensation or coordination with other heterogeneous metals, such as {V₁₅O₃₆},¹⁴ {V₁₆O₄₂},¹⁵ {V₁₈O₄₂},¹⁶ {V₂₂O₅₄},¹⁷ {V₃₄O₈₂},¹⁸ {V₃₀Sb₈O₇₈}¹⁹ and the largest fullerene-like {V₃₀Nb₁₂}. Furthermore, the modification and functionalization of polyoxovanadates with organophosphates have led to the development of a series of functional hybrid POV cages, including {V₁₂P₈},²⁰ {V₁₄P₈},²¹ and {V₂₀W₂P₂₀}²² (Fig. 1a). These elegant and complex structures are usually synthesized by one-pot reactions, from the condensation of {VO₅} polyhedra and phosphate groups into {V_xP_y} building blocks of different topological configurations, including {V₂P₄}, {V₅P₄} and {WV₅P₅}, and then further self-assembled (Fig. 1b). However, it is important to note that these POV cages tend to have relatively small nuclear and sizes, which may be influenced by the size of the anionic guest (template) that facilitates the assembly of the POVs.

Fig. 1 (a) Representative high-nuclear polyoxophosphovanadates; (b) the {V_xP_y} building blocks. Color code: red, O; pink, P; green, V. H atoms are omitted for clarity.

Core–shell POMs demonstrate unique structural characteristics during their assembly, which are influenced by the size, shape, and charge of the anionic templates.^3,23,24 These anionic templates, whether pre-introduced or synthesized in situ, serve a crucial role in guiding the structural configuration of the clusters. Typically, most core–shell POVs are constructed with small-size anionic guests (e.g., ClO₄⁻, N₃⁻, halides, etc.) as templates, such as [Cl₄⊂V₁₈O₂₅(H₂O)₂(PhPO₃)₂₀]⁴⁻, [N₃⊂V₁₄O₂₂(OH)₄(PhPO₃)₈]⁷⁻,²⁵ and {V₁₆As₈}, {V₁₆As₁₀}, {V₂₀As₈}, and {V₂₄As₈} mediated by {X_x(H₂O)_6y} (X = Br, Cl; y = 2, 4, 6).²⁶ However, only rarely have polyoxometalates mediated by anionic templates based on large-size POMs been found to form core–shell nanoclusters. Only a few wheel-shaped core–shell polyoxomolybdates have been reported, such as {Mo₃₆⊂Mo₁₅₀},²⁷ {PW₁₂/PW₁₈/P₅W₃₀⊂Mo₂₄Fe₁₂}²⁸ and {P₅W₃₀⊂Mo₂₂Fe₈}.²⁹ Unfortunately core–shell POV cages utilizing polyanions as templates have not been reported to date. Consequently, we endeavored to employ POM-based anionic templates to modulate the {V_xP_y} building blocks, facilitating the synthesis of high-nuclear core–shell POVs. Herein, we successfully synthesized two core–shell POVs [M₆O₁₉]²⁻⊂[Na₈V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]²⁺ (M = W 1, Mo 2) with Lindqvist POM template, presenting a new building block {VO(PhPO₃)₄} (Fig. 1b). The outer shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻ can be viewed as self-assembled from six {V^VO(PhPO₃)₄} vertices and twelve {V^IVO₅} edges with the same octahedral geometry as the inner [M₆O₁₉]²⁻ template. To our knowledge, compounds 1 and 2 represent the first and largest high-nuclear core–shell POVs encapsulating a POM template. Compound 1 exhibits good thermal and chemical stability alongside good catalytic oxidative activity towards sulfur ethers, efficiently converting substrates such as phenyl methyl sulfide at room temperature.

Experimental

Synthesis of [W₆O₁₉]²⁻⊂[Na₈V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]²⁺

VOSO₄·xH₂O (45 mg, 0.28 mmol), Na₂WO₄·4H₂O (70 mg, 0.21 mmol), and phenylphosphonic acid (70 mg, 0.44 mmol) were dissolved in 1 mL of N,N-dimethylformamide and 3 mL of acetonitrile. The mixture was heated to 160 °C for 3 days in a 12 mL Teflon-lined stainless-steel vessel, and gradually cooled to room temperature. The light green crystal 1 was obtained. Yield: 35.8% based on VOSO₄·xH₂O. Elemental analysis (EA) % calcd: C: 31.26; H: 2.44; O: 29.73; P: 12.34. Found: C: 31.29; H: 2.41; O: 29.75; P: 12.36.

Synthesis of [Mo₆O₁₉]²⁻⊂[Na₈V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]²⁺

VOSO₄·xH₂O (15 mg, 0.10 mmol), Na₂MoO₄·4H₂O (10 mg, 0.04 mmol), and Phenylphosphonic acid (10 mg, 0.06 mmol) were placed in a 2 mL glass tube. Then 1 mL N,N-dimethylformamide, 150 μL ethylenediamine, and 3 mL acetonitrile were placed in a 12 mL Teflon-lined stainless-steel vessel. Glass tubes containing the reactants were heated in a 12 mL Teflon-lined stainless-steel vessel at 150 °C for 3 days, and gradually cooled to room temperature. The yellow crystal 2 was obtained. Yield: 21.2% based on VOSO₄·xH₂O. Elemental analysis (EA) % calcd: C: 29.84; H: 2.51; O: 26.84; P: 10.95. Found: C: 29.85; H: 2.49; O: 26.87; P: 10.98.

Results and discussion

Compounds 1 and 2 (Fig. 2a and S1†) have similar structures and both of them crystallize in a trigonal system with R3̄ space group, with the only difference being the central polyanionic templates {W₆O₁₉}²⁻ and {Mo₆O₁₉}²⁻, respectively (Fig. S2†). The crystallographic data are listed in Table S1.† The core–shell structure of 1 is constructed from an anionic template {W₆O₁₉}²⁻ and cationic [Na₈V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]²⁺ shell with external dimensions of 2.3 × 2.3 × 2.3 nm³. The core–shell structure of 1 can be described as a nesting of three distinct Platonic polyhedra: a central {W₆O₁₉} octahedron, an intermediate {Na₈} cube, and an outer {V₁₈P₂₄} octahedron (Fig. 2b). The {W₆O₁₉}²⁻ octahedron (Fig. 2c) is composed of six {WO₆} octahedra interconnected through shared edges with O_h symmetry. The W–O_μ2 bond lengths ranging from 1.921(4) Å to 1.929(8) Å, while the W–O_μ6 bond lengths vary from 2.248(8) Å to 2.324(12) Å, and terminally oxygenated W–O_t bond lengths span from 1.691(10) Å to 1.694(8) Å (Table S2†). Each face of the {W₆O₁₉}²⁻ octahedron is constituted of equilateral triangles. Encapsulating the {W₆O₁₉}²⁻ guest is a {Na₈} cube (Fig. 2d), with a W atom situated at the center point of each {Na₄} square. Each Na⁺ in the {Na₈} cube is connected to the outer shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻ (Fig. 2e and S3†) via a μ₆:η²:η²:η²:η²:η²:η² coordination mode to form a concave hexagonal {NaV^IV₃P₃O₆} unit (Fig. 3a). The resultant hexagonal {V^IV₃P₃O₆} clusters (Fig. 3b) are located within the plane of eight equilateral triangles {V^V₃V^IV₃P₃O₆} (Fig. 3c) with side lengths of 9.73 Å in the octahedral shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻ (Fig. 3d). The triangular plane {V^V₃V^IV₃P₃O₆} is formed by a hexagonal {V^IV₃P₃O₆} cluster connected to the three {V^VO₅} vertices via P–O–V bonds, with the sides of each triangle comprising two {V^VO₅} units and one {V^IVO₅} unit. Eventually, eight equilateral triangles {V^V₃^IVV₃P₃O₆} connected by shared {V^V₂V^IV} edges combine to form an octahedral shell with O_h symmetry [V^V₆V^VI₁₂O₁₈(PhPO₃)₂₄]⁶⁻. It is noteworthy that the edges and faces constituting the octahedral shell and the inner core template {W₆O₁₉}²⁻ are parallel to one another. The assembly configuration of the inner core and outer shell is characteristic of a ‘Russian doll’. The distinction between the structures of 2 and 1 is that the {Mo₆O₁₉}²⁻octahedron is slightly smaller due to differing radii of the metal atoms, with Mo–Oμ₂ bond lengths ranging from 1.944(7) Å to 1.957(9) Å, Mo–Oμ₆ bond lengths of 2.337(10) Å, and the terminal oxygen Mo–O_t bond length at 1.62(11) Å (Table S3†).

Fig. 2 (a) and (b) Polyhedral and schematic diagrams of compounds 1 and 2, respectively, (c) {M₆O₁₉}²⁻ cluster, (d) {Na₈} cluster, (e) {V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄}⁶⁻ cluster. Color code: red, O; pink, P; yellow, Na; green, V^IV; light green, V^V; blue, W/Mo. H atoms are omitted for clarity.

Fig. 3 Three different self-assembly schemes for the octahedral shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻. (a) {NaV^IV₃P₃O₆} cluster, (b) {V^IV₃P₃O₆} cluster, (c) {V^V₃V^IV₃P₃O₆} cluster, (d, g, and j) octahedral shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻, (e) {V^VP₄} cluster, (f) {V^IVO₅} unit, (h) square pyramidal building blocks {V^VV^IV₄(PhPO₃)₄} and (i) square ring {V^V₄V^IV₄(PhPO₃)₁₆}. Color code: red, O; pink, P; yellow, Na; green, V^IV; light green, V^V. H atoms and phenyl are omitted for clarity.

Furthermore, the octahedral shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻ can be described from the point of view of the building blocks. It can be viewed as a self-assembly of six {V^VO₅(PhPO₃)₄} square pyramids (Fig. 3e) serving as vertices and twelve {V^VIO₅} units (Fig. 3f) as edges. Each {V^IVO₅} edge is bordered by two neighboring {V^VO₅(PhPO₃)₄} vertices through four V–O–P bonds. All V atoms exhibit two different coordination modes. The V^V ion at the apex is connected to the four surrounding {PhPO₃} ligands via four μ₂-O, resulting in a square pyramid {V^VO₅(PhPO₃)₄} centered on {V^VO₅}, where the P–O–V bond angles vary from 149.27° to 150.23° (Table S4†). Conversely, the V^IV ion, which constitutes an edge, is also connected to the surrounding {PhPO₃} ligand via four μ₂-O, but it forms a nearly planar {V^IVO₅(PhPO₃)₄} square building block (Fig. S4†) centered on {V^IVO₅}, with all V–O–P bonds curved in-plane at an angle of 158.23° to 160.02° (Table S5†). Significantly, the square pyramidal {V^VO₅(PhPO₃)₄} and {V^IVO₅(PhPO₃)₄} building blocks are observed for the first time in polyoxophosphovanadates.

In addition, the octahedral shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻ can also be viewed as consisting of two larger square pyramidal building blocks {V^VV^IV₄(PhPO₃)₄} (Fig. 3h) encircling a square {V^V₄V^IV₄(PhPO₃)₁₆} ring (Fig. 3i) with fourfold symmetry. In particular, the square {V^V₄V^IV₄(PhPO₃)₁₆} ring cluster is interconnected by four {V^VO₅(PhPO₃)₄} square pyramids and four {V^IVO₅(PhPO₃)₄} square shared {PhPO₃} groups with all {PhPO₃} groups and V=O bonds are orientated outward. It is intriguing to note that the {V^VV^IV₄(PhPO₃)₄} square pyramid presented here is different from the reported {V₅P₄} pyramid (Fig. S5†).²⁵ At first glance, exchanging the positions of V^IV and PO³⁻ in the bottom squares of the pyramid {V^VV^IV₄(PhPO₃)₄} evolves into {V₅P₄}. The distinction between the two structures primarily arises from the sequential arrangement in which V^IV/PO³⁻ is connected to the center V^V cation. In the {V^VV^IV₄(PhPO₃)₄} unit, a V^V center coordinates with four {PhPO₃} groups through four μ₂-O bonds, forming a convex square unit that subsequently connects to the surrounding four V^IV ions via eight μ₂-O bonds to form a square pyramid. Conversely, in the {V₅P₄} unit, a V^V center is first linked to the four surrounding V^IV ions through four μ₃-O bonds, resulting in a {V₅} convex square unit that is further linked to the four {PhPO₃} groups via eight μ₂-O bonds, thereby forming a {V₅P₄} square pyramid. Additionally, the square {V^V₄V^IV₄(PhPO₃)₁₆} ring cluster is distinct from the {V₅P₅} ring,²² mainly in the valence state of vanadium and the coordination mode of vanadium with PO₃⁻ (Fig. S6†). The {V₅P₅} building block is an expanded ring with 5-fold symmetry consisting of five {V^IVO₅} units alternately linked with five {PhPO₃} ligands, where only two oxygen atoms on the diagonal of the chassis in the {V^IVO₅} pyramid are linked to neighboring {PhPO₃} ligands. In contrast, the square {V^V₄V^IV₄(PhPO₃)₁₆} ring cluster comprises four {V^VO₅ (PhPO₃)₄} pyramids alternately linked with four {V^IVO₅} units, with all V–O bonds connected to {PhPO₃} ligands. Notably, this planar square {V^V₄V^IV₄(PhPO₃)₁₆} ring, composed of vanadium centers with varying valences, is presented in POVs for the first time.

Powder X-ray diffraction analysis (Fig. S7†) revealed that the experimental peaks of 1 and 2 closely matched the simulated curves, confirming its good phase purity. The FTIR spectrum (Fig. S8†) of 1 exhibited characteristic absorption peaks at 1076 cm⁻¹, 725 cm⁻¹, and 450 cm⁻¹, which are attributed to the telescopic vibration of the terminal oxygen W=O, the asymmetric telescopic vibration and the bending vibration of the W–O–W, respectively. The V=O bond stretches occurred at 1025 cm⁻¹ and 992 cm⁻¹, while the V–O bond stretches were observed at 696 cm⁻¹ and 564 cm⁻¹. The V–O–P bond stretches at 1148 cm⁻¹ and 1121 cm⁻¹, and the monosubstituted absorption peak of the phenyl groups at 756 cm⁻¹; C=C bond stretches occurred at 1430 cm⁻¹, 1484 cm⁻¹, and 1669 cm⁻¹; and the absorption band at 3054 cm⁻¹ is caused by the C–H stretching vibration. The X-ray photoelectron spectra (XPS) (Fig. S9 and S10†) of W in compound 1 showed four peaks fitted at 38.8 eV, 37.5 eV, 35.1 eV, and 33.6 eV corresponding to the binding energies of the W(VI) 4f_5/2 satellite peak, the W(VI) 4f_7/2 satellite peak, and the binding energies of W(VI) 4f_5/2 and W(VI) 4f_7/2, thus proving that the W atoms of compound 1 have the valence state +VI. The XPS spectrum of V 2p shows two peaks at 513.2 eV and 520.6 eV corresponding to the binding energies of V^IV2p_3/2 and V^IV2p_1/2, respectively, and peaks at 514.8 eV and 521.9 eV corresponding to the binding energies of V^V2p_3/2 and V^V2p_1/2, respectively, which confirms that the atomic valence states of V are +IV and +V.³⁰ In addition, charge balance analysis and bond valence calculations (BVS) indicate that the oxidation state of the W atoms in compound 1 is +VI. The oxidation states of the V atoms are +IV and +V, which is consistent with the X-ray photoelectron spectra (Tables S6 and S7†).

The stability of POMs is crucial for both scientific research and industrial applications. Therefore, we further investigated the thermal and solvent stability of compound 1. Thermogravimetric analysis (Fig. S11†) of compound 1 showed a stable plateau between 240 °C and 350 °C, indicating that compound 1 maintains its structural integrity at this temperature and has good thermal stability. Furthermore, after immersing 1 mg of compound 1 in 2 ml of solvents with different polarities (e.g., MeCN, MeOH, EtOH, H₂O, DMF, CH₂Cl₂) for 72 h, 1 can retain its structural integrity, which was consistent with the morphology before immersion, indicating that compound 1 exhibits high solvent stability (Fig. S12†).

In the presence of a catalyst, sulfides can be converted to sulfone and sulfoxide, which are essential intermediates in organic synthesis, thereby reducing the emission of hazardous substances sulfur oxides (SOx). Therefore, the exploration of catalysts with good catalytic activity for sulfur ether compounds is of great significance for industrial applications and environmental protection.³¹ Polyoxovanadates exhibit advantageous electron transfer capabilities and redox activity, making them suitable as environmentally friendly molecular catalysts.^32–34 It is essential to investigate the oxidative desulfurization catalytic performance of compound 1 to further elucidate the relationship between molecular structure and its functional characteristics. Firstly, the oxidative desulfurization was carried out in different solvents at room temperature with methyl phenyl sulfide (MBT) (0.2 mmol) as a substrate, compound 1 (0.00125 mmol) as oxidative desulfurization catalyst, 30% hydrogen peroxide (2 mmol) as oxidant, and biphenyl as counterpart. The experimental outcomes were monitored using liquid chromatography (LC). The results showed that optimal catalytic performance was achieved when the reaction solvent was MeOH (2 mL) with 99% conversion of MBT within 10 min (Table S8†). Under this solvent condition, the oxidizing agent was replaced with an equivalent amount of tert-butyl hydroperoxide, which showed a slightly poorer oxidation effect and 78.63% MBT conversion at the same time. Furthermore, in the absence of compound 1, the conversion of MBT was only 19.25%, indicating that compound 1 is the key substance facilitating the catalytic oxidation process.

Next, we performed desulfurization experiments on thioether compounds with different electron-absorbing (electron-donating) effect substituents under suitable catalytic conditions to investigate the catalytic activity of compound 1 (Table 1). The experimental results showed that under the optimal conditions, ethyl phenyl sulfide and 4-methoxy thioanisole were almost completely converted in 10 min, 4-chlorothioanisole was completely converted in 20 min, diphenyl sulfide was completely converted in 1 h. In the case of dibenzothiophene, the reaction was almost non-responsive at room temperature, and the conversion rate could reach 52.64% after warming up to 50 °C and reacting for 3 h. These results indicate that compound 1 can catalyze compounds with different electron-withdrawing (electron-donating) effect substituents. Furthermore, substituents with electron-withdrawing effects and high space resistance are detrimental to the reaction. Meanwhile, increasing the reaction temperature can accelerate the catalytic oxidation.

Table 1 Results of different sulfur-containing aromatic substrates oxidation reactions catalyzed by 1 at different conditions^a

Entry	Substrate	Temperature (°C)	Time (min)	Conv.^b (%)
a Reaction conditions: substrate (0.2 mmol), compound 1 (0.00125 mmol), H2O2 (2 mmol), and MeOH (2 mL). b Isolated conversions were calculated by LC.
1		25	10	>99
2		25	10	>99
3		25	10	>99
4		25	10	89.64
			20	>99
5		25	10	45.38
			60	>99
6		25	10	Trace
		50	180	52.64

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The catalytic stability and recoverability of compound 1 were further investigated using MBT as a substrate. After one round of catalytic experiments, the reaction mixture was centrifuged, the precipitate was washed with methanol several times and observed under a microscope, revealing the complete morphology of crystal 1. Furthermore, the crystals were centrifuged and filtered after a period of catalytic reaction, and the remaining liquid continued to be reacted. The results showed that the system without crystals had no catalytic activity for MBT, which further confirmed that compound 1 was a stable non-homogeneous catalyst (Fig. S13†). Subsequently, the precipitate was dried and reintroduced into a new reaction for further catalysis. After five rounds of cyclic experiments, the conversion of MBT was still as high as 98.55% (Fig. S14 and Table S9†). The PXRD patterns before and after catalysis are in high agreement, indicating that compound 1 has good structural stability and recoverability in the catalytic process (Fig. S15†). Furthermore, a potential mechanism for the catalytic oxidation of sulfides by Compound 1 is detailed in the ESI (Fig. S16†).

Conclusions

In summary, two unique high-nuclear POVs featuring a three-layer core–shell structure based on Lindquist POM [M₆O₁₉]²⁻ templates are formed under solvothermal conditions, which are the first reported core–shell clusters mediated by a POM template in a POV system. The outer shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻ has the same octahedral geometry as the inner core [M₆O₁₉]²⁻ due to the templating effect. From the perspective of building units, the assembly of the octahedral shell [V^V₆V^IV₁₂O₁₈(PhPO₃)₂₄]⁶⁻ can originate from its vertices, edges, and faces, leading to the formation of three distinct assembly modes that derive three new building units, namely, the square pyramids {V^VO₅(PhPO₃)₄}, the {V^VV^IV₄(PhPO₃)₄}, and the equilateral triangles {V^V₃V^IV₃P₃O₆} building units. Furthermore, compound 1 exhibited good catalytic activity in catalyzing the oxidation of sulfides. This study provides a new insight into the synthesis of high-nuclear POMs with nested structures based on large-size POM templates to tune the ordered arrangement of building blocks.

Data availability

All relevant data are within the manuscript and its additional files.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the NSFC of China (no. 22271023 and 22371032).

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Footnote

† Electronic supplementary information (ESI) available: Additional characterization data, including PXRD patterns, IR and XPS spectra, TG curve and single-crystal X-ray diffraction data. CCDC 2401866 and 2401867. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02884h

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