Engineering geometric metamorphosis in {P₆M₂Mo₁₆O₇₃}-based high-nuclearity metal superclusters: from tetrahedral to square assemblies via topological transformations

Abstract

The synthesis of superclusters is vital for understanding their self-assembly mechanisms, requiring precise control over composition, orientation, and connectivity. The tenets of reticular chemistry afford a systematic approach to the architectural design and regulation of molecular entities, thereby enabling the fulfillment of preconceived structural objectives. Herein, the strategic introduction of transition metal ions, Co^II and Ni^II, has enabled the controlled formation of two novel closely related anionic superclusters based on the {Sr⊂P₆M₂Mo₁₆O₇₃} clusters: Co₁₂Sr₄Mo₈₀P₃₆ (1) and Na₄Ni₂₃Sr₄Mo₈₈P₅₂ (2). The former manifests a tetrahedral structure, while the latter, derived from topological transformations of cluster 1, exhibits a unique square geometry. Structural elucidation revealed that the integration of Mo^V ions in a reduced state, in concert with distinct transition metal ions (Co^II and Ni^II), plays a pivotal role in determining the geometric profiles of these superclusters. Furthermore, clusters 1 and 2 demonstrate efficacy as heterogeneous catalysts, significantly promoting the transformation of methyl phenyl sulfide (MPS) to methyl phenyl sulfoxide (MPSO).

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Introduction

Molecular self-assembly, especially for high-nuclearity metal superclusters, relies on the intrinsic topological and chemical properties of molecules, as well as the surrounding environment, leading to the spontaneous formation of stable structures without external intervention.^1–3 This process is critically important in the design and synthesis of nanomaterials with architectural precision.⁴ By mimicking the intricate strategies of biological systems, chemists have successfully created a wide range of molecular materials with unique structures using self-assembly techniques.^5–7 However, the limitations inherent in synthesis methods and the complexity of assembly dynamics pose significant challenges to achieving desired molecular configurations.^8–10 As synthetic technology continues to evolve, a principle-based approach to controllable molecular design has become essential for the realization of targeted molecular structures.^11–15 The concept of ‘reticular chemistry’^16–18 enables the deductive progression from simple to complex molecular structures by precisely controlling the connectivity between ‘nodes’ and ‘linkers’ within geometrically predefined secondary building units (SBUs).^19,20 This systematic approach reveals the relationships between molecular structures, paving the way for the orchestration of complex structural assemblies with unprecedented accuracy.²¹

Polyoxometalates (POMs) have attracted increasing attention due to their unparalleled structural characteristics.^22,23 The polynuclear essence of POMs positions them as fundamental units for constructing rigid and directionally stable frameworks, which profoundly influences the design of crystalline materials with preordained structures and properties.²⁴ Consequently, POMs are regarded as promising SBUs in the realm of self-assembly, facilitating the creation of innovative superclusters. In recent years, Yang,^25–27 Lan,^28,29 Zheng,^30,31 Wang^32,33 and other research groups have meticulously used POMs with well-defined architectures as SBUs to construct metal superclusters exhibiting diverse geometries, such as {Na₁₉Ni₁₂W₃₁},²⁵ fullerene-like {V₃₀Nb₁₂} cage,²⁸ keplerate-type {Mo₁₃₂},²⁹ core–shell-type Ag₈@Nb₁₆₂ ³¹ and triangular prism-shaped {Mo₄₂Ti₁₂}.³³ Notably, POMs adorned with Mo exhibit a more variability of geometric configurations compared to their V, Nb, Ta, and W counterparts, unlocking the vast potential for the synthesis of molecular clusters with exceptional structural attributes.^34–36

Basket-type phosphomolybdate ([P₆Mo₁₈O₇₃]ⁿ⁻, denoted as {P₆Mo₁₈}) represents a distinguished subclass within non-classical Mo-based POMs.³⁷ Its composition, integrating four-vacant γ-Dawson {P₂Mo₁₄} clusters with a handle-like {P₄Mo₄} segment,³⁸ is exquisitely tailored for the role of ‘vertices’ in the edification of supramolecular architectures.³⁹ Intriguingly, the handle-like {P₄Mo₄} segment nestled within the {P₆Mo₁₈} cluster presents a unique feature where some Mo atoms reside in a lower oxidation state.⁴⁰ This not only elevates the surface charge density, thereby activating the surface oxygen atoms, but also endows the structure with distinctive redox properties and the capability for reversible multi-electron transfer.⁴¹ Such characteristics mark the handle structure as an auspicious target for modification, with the potential to enhance the formation of supercluster assemblies by incorporating more adaptable coordination sites. In 2022, our group achieved a pivotal advancement by successfully substituting Mo^VI on the lateral aspects of the handle-like {P₄Mo₄} with Co ions, resulting in the {P₄Co₂Mo₂} variant, and subsequently engineered a tetrahedral supercluster based on the substituted basket type {Pb⊂P₆Co₂Mo₁₆}.⁴² In the current research, leveraging the flexibility in mathematical geometric transformations, our investigation has meticulously explored the feasible topological alterations of the tetrahedral supercluster, thereby uncovering an enchanting geometric metamorphosis. As shown in Scheme 1, the act of slicing through two opposed edges within the tetrahedral framework results in a transformation where the structure tends to culminate in a square arrangement. This implies that, while maintaining the integrity of the ‘vertices’, it is indeed feasible to derive a quadrilateral supercluster merely by manipulating the angles at which the vertex clusters and their interconnecting elements converge.

Scheme 1 The transformation from a tetrahedral structure to a square structure.

Herein, we achieved the synthesis of two clusters featuring closely related topological architectures through the meticulous manipulation of molecular structures. Initially, we obtained a tetrahedral supercluster, (HL)₁₅[(H₂P₂Mo^V₂O₉)₄ (H₆P₂Co₂Mo^V₄O₁₆)₂(H₄Sr⊂P₆Co₂Mo^V₂Mo^VI₁₄O₇₃)₄]·ca.142H₂O (Co₁₂Sr₄Mo₈₀P₃₆, 1), where L denotes the benzimidazole ligand. This supercluster is isomorphic to our previous work, with its vertex units constructed from {Sr⊂P₆Co₂Mo₁₆}. Subsequently, by finely tuning the metallic composition,¹⁴ we adeptly altered the vertex angles and elongated the edges, resulting in the formation of a square supercluster, (HL)₁₈[P₂Mo^V₂O₉]₂[P₁₀Ni_4.5Mo^V₈O₅₁]₂[NaPNiMo^VL_0.5O₉]₄[Sr⊂P₆Ni₂Mo^VMo^VI₁₅O₇₃]₂[Sr⊂P₆Ni₃Mo^VMo^VI₁₅O₇₅]₂·ca.90H₂O·2C₂H₅OH(Na₄Ni₂₃Sr₄Mo₈₈P₅₂, 2), whose vertices consist of {Sr⊂P₆Ni₂Mo₁₆} SBUs (Scheme S1 and Fig. S1†). Notably, the {P₄M₂Mo₂} (M = Co^II or Ni^II) subunits are situated within the ‘handle’ region of the basket-like SBUs and are instrumental in the assembly of the clusters through their interactions with various connecting units (Fig. S2†). To ascertain the structure of the synthesized clusters, we employed single-crystal X-ray diffraction (SCXRD) for comprehensive analysis. Concurrently, a series of other characterization techniques including powder X-ray diffraction (PXRD), infrared spectroscopy (IR), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were performed to demonstrate the phase purity, chemical composition, morphology, distribution of constituent elements, and oxidation state of the clusters 1–2 (Fig. S3–S7†). Moreover, clusters 1–2 can act as heterogeneous catalysts that efficiently oxidize a variety of thioethers into their corresponding sulfoxides.

Results and discussion

Crystal structure of Co₁₂Sr₄Mo₈₀P₃₆ and Na₄Ni₂₃Sr₄Mo₈₈P₅₂

In the synthesis of the two clusters, Na₂MoO₄·2H₂O served as the molybdenum source, while H₃PO₄ was utilized as the phosphorus source. To facilitate the formation of basket-like structural units, SrCl₂·6H₂O was incorporated into the reaction mixture. To realize the structural regulation of two superclusters, distinct metal salts, CoCl₂·6H₂O and NiSO₄·6H₂O were introduced separately. In addition, the introduction of ligand L reinforces the structural stability through hydrogen bonding interaction, significantly enhancing thermal and solvent resistance. Simultaneously, it enables dynamic charge compensation via proton transfer mechanisms, leading to the electrostatic stabilization of the anionic metal-oxo core. This process effectively balances the overall charge of the system through both coordination and non-covalent interactions with the metal centers.⁴³ SCXRD analysis reveals that cluster 1 crystallizes within the C2/c space group of the monoclinic system. The structure comprises a heterometallic anion cluster, Co₁₂Sr₄Mo₈₀P₃₆, with dimensions of approximately 29.2 × 29.2 × 22.7 Å (Fig. S8a†), 15 protonated ligands (HL), and 142 lattice water molecules. The tetrahedral Co₁₂Sr₄Mo₈₀P₃₆ cluster can be visualized as having four basket-like [Sr⊂P₆Co₂Mo^V₂Mo^VI₁₄O₇₃]¹⁶⁻ (abbr. {Sr⊂P₆Co₂Mo^V₂Mo^VI₁₄}) cluster-based SBUs serving as ‘vertices’, with two [P₂Co₂Mo^V₄O₁₆]⁶⁺ (abbr. {P₂Co₂Mo^V₄}) and four [P₂Mo^V₂O₉]²⁺ (abbr. {P₂Mo^V₂}) acting as ‘edges’ (Fig. 1). Specifically, the basket-shaped {Sr⊂P₆Co₂Mo₁₆} SBU is bifurcated into a basket portion, centered on a Sr^II ion and linked to multiple Mo^VI through oxygen atoms and {PO₄} groups, and a handle portion, which connects to the {PO₄} group of the basket via two Mo^V and two Co^II ions (Fig. S9†).

Fig. 1 Structural analysis of Co₁₂Sr₄Mo₈₀P₃₆ (1): (a) the basket-like {Sr⊂P₆Co₂Mo^V₂Mo^VI₁₄} SBU as node A; (b) the trans-{P₂Mo^V₂} SBU; (c) the {P₂Co₂Mo^V₄} SBU; (d) the {P₂Mo^V₂} SBU as linker A connects to two nodes A; (e) the {P₂Co₂Mo^V₄} SBU as linker B connects to two nodes A; (f) the tetrahedral structure and its topology of Co₁₂Sr₄Mo₈₀P₃₆ (1).

Within this tetrahedral Co₁₂Sr₄Mo₈₀P₃₆ cluster architecture, the two types of SBUs, {P₂Mo^V₂} and {P₂Co₂Mo^V₄}, exhibit distinct structural attributes. The {P₂Mo^V₂} SBU is constituted by the bridging of two Mo^V ions via oxygen atoms, with the coordination sites on either side engaging with two {PO₄} groups to form a trans-configuration (Fig. 1b). Notably, the presence of the {PO₄} groups allows the binuclear molybdenum cluster to expose specific sites that perfectly align with the Co^II ions and {PO₄} on one side of the handles of two adjacent basket-like {Sr⊂P₆Co₂Mo^V₂Mo^VI₁₄} SBUs, thereby facilitating the connection with Co^II ions on the handles of neighboring basket-shaped SBUs (Fig. S10a and S10b†). Conversely, the {P₂Co₂Mo^V₄} structure is formed by the coordination of four Mo^V ions around a binuclear cobalt cluster core, with each terminal oxygen atom of the Mo^V ions connecting to the Mo^V ion on the rims of neighboring basket-like SBUs to form {Mo^V₂} units (Fig. 1c and S10c†). Moreover, if each {Sr⊂P₆Co₂Mo^V₂Mo^VI₁₄} SBU at the vertex is regarded as node A, and {P₂Mo^V₂} and {P₂Co₂Mo^V₄} at the junctions are respectively denoted as linker A (Fig. 1d) and linker B (Fig. 1e), the cluster Co₁₂Sr₄Mo₈₀P₃₆ can be simplified as a tetrahedral geometric structure (Fig. 1f). Within the tetrahedral geometry, it is evident that each node A is linked to two linkers A, and one linker B. The angle between the two linker A is measured at 67.2°, and a similar angle of 56.6° is observed between linker A and linker B (Fig. S11a and S11b†). Additionally, any set of three coplanar nodes A gives rise to two distinct dihedral angles: one resulting from the intersection of two planes through linker A, which is 65.5°, and the other formed by the intersection of two planes through linker B, which is 83.2° (Fig. S11c and S11d†). Moreover, the angle between linker A and the plane defined by the three nodes A is 57.2°, whereas the angle associated with linker B is 48.4° (Fig. S11e and S11f†). Despite these deviations from the angles of an ideal tetrahedron, the intricate molecular assembly process allows these structures to be considered highly compatible with the tetrahedral molecular geometry.

As previously outlined, the tetrahedral architecture can be transformed into a square arrangement through the truncation of its two opposing edges. Careful manipulation of the experimental parameters, including the substitution of CoCl₂·6H₂O with NiSO₄·6H₂O and fine-tuning the pH, has led to the successful synthesis of the targeted cluster 2, which resembles the structure of a square cluster. Cluster 2 crystallizes in the triclinic system with the P1̄ space group, and its structure encompasses a complex metal anion cluster, Na₄Ni₂₃Sr₄Mo₈₈P₅₂, with dimensions of approximately 27.4 × 27.4 × 25.9 Å (Fig. S8b†), along with 18 protonated ligands (HL), 90 water molecules of crystallization and 2 ethanol molecules. A notable feature of cluster 2 is the presence of not only fully protonated ligands but also four ligands with an occupancy of 1/2 to coordinate with Ni^II ions in the Na₄Ni₂₃Sr₄Mo₈₈P₅₂ cluster. The structural affinity between 2 and 1 is most evident at the vertices, which are composed of four basket-like POMs-based clusters. These POMs-based clusters fall into two distinct types: the [Sr⊂P₆Ni₂Mo^VMo^VI₁₅O₇₃]¹⁵⁻ (abbr. {Sr⊂P₆Ni₂Mo^VMo^VI₁₅}) SBU and the [Sr⊂P₆Ni₃Mo^VMo^VI₁₅O₇₅]¹⁶⁻ (abbr. {Sr⊂P₆Ni₃Mo^VMo^VI₁₅}) SBU. The latter features an additional Ni^III ion at the base of the basket, which differs significantly from the Ni^II ion at the handle position. This discovery represents a significant breakthrough in the field of basket-shaped POMs structures. Furthermore, a significant distinction from cluster 1 is the varied oxidation states of Mo atoms within the basket-like POMs at the vertices, exhibiting both +V and +VI states (Fig. S12†). Critically, the asymmetric occurrence of Mo^V in the basket-like POMs structure leads to a marked change in the coordination environment at the handle of the basket. In cluster 1, the {Sr⊂P₆Co₂Mo^V₂Mo^VI₁₄} SBUs at each vertex are interconnected by two types of SBUs: {P₂Mo^V₂} or {P₂Co₂Mo^V₄}; however, in cluster 2, each basket-like POM, {Sr⊂P₆Ni₂Mo^VMo^VI₁₅} and {Sr⊂P₆Ni₃Mo^VMo^VI₁₅} at each vertex, is linked through three distinct types of SBUs: [P₂Mo^V₂O₉]²⁺ ({P₂Mo^V₂}), [P₁₀Ni_4.5Mo^V₈O₅₁]^2.5− ({P₁₀Ni_4.5Mo^V₈}), and [NaPNiMo^VL_0.5O₉]⁵⁻ ({NaPNiMo^VL_0.5}) (Fig. 2b–d and S13†).

Fig. 2 Structural analysis of Na₄Ni₂₃Sr₄Mo₈₈P₅₂ (2): (a) the basket-like {Sr⊂P₆Ni₂Mo^VMo^VI₁₅} SBU as node B and {Sr⊂P₆Ni₃Mo^VMo^VI₁₅} SBU as node C; (b) the cis-{P₂Mo^V₂} SBU; (c) the {P₁₀Ni_4.5Mo^V₈} SBU; (d) the hammer-like {NaPNiMo^VL_0.5} SBU; (e) the {P₂Mo^V₂} and {P₁₀Ni_4.5Mo^V₈} SBUs connecting to nodes B and C to form linker C; (f) two {NaPNiMo^VL_0.5} SBUs connect to nodes B and C to form linker D; (g) the square structure and its topology structure of Na₄Ni₂₃Sr₄Mo₈₈P₅₂ (2).

In addition, the intricate array of SBUs within cluster 2, coupled with their diverse connection modes, complicates the construction of the ‘edge’ architecture for cubic clusters. A detailed analysis reveals that the components forming the edges comprise three distinct SBUs: {P₂Mo^V₂}, {P₁₀Ni_4.5Mo^V₈}, and {NaPNiMo^VL_0.5}. Notably, the {P₂Mo^V₂} SBU is also present in cluster 1, but in cluster 2, it assumes a cis-configuration, with the {PO₄} group situated on the same side of the {Mo^V₂} cluster (Fig. 2b). In Fig. 2c, the {P₁₀Ni_4.5Mo^V₈} SBU emerges as a novel structural element in 2. Excluding the consideration of the upper six-coordinate Ni^III ions, this unit exhibits a high degree of symmetry, consisting of four Ni^II ions and eight Mo^V ions. The intriguing aspect of this structure is that it can be regarded as the doubling of the {P₂Co₂Mo^V₄} SBU from cluster 1 in the arrangement of metal ions. The increase in the number of metal ions leads to the enlargement of the {P₁₀Ni_4.5Mo^V₈} SBU, thereby expanding the range for modulating its coordination angles relative to adjacent units. Consequently, the integration of two SBUs, {PMo^V₂} and {P₁₀Ni_4.5Mo^V₈}, allows for the interconnection of {Sr⊂P₆Ni₂Mo^VMo^VI₁₅} and {Sr⊂P₆Ni₃Mo^VMo^VI₁₅}, thereby constructing one type of edge of the square cluster 2 (Fig. 2e). The {NaPNiMo^VL_0.5} SBUs differ from the first two symmetric SBUs in cluster 2, it can be imagined as a hammer-like structure. In detail, each SBU contains a hexa-coordinate Ni^II ion, a four-coordinate Na^I ion, a {PO₄} group, a hexa-coordinate Mo^V ion, and a ligand L. The binuclear cluster formed by the bridging of Ni^II and Na^I ions serves as the hammerhead, which is connected to the mononuclear {MoO₆} cluster via {PO₄} groups to constitute the hammer handle. At the hammerhead portion, Ni^II and Na^I ions are coordinated to Mo^VI and Ni^II ions located at the handle of the basket-like {Sr⊂P₆Ni₂Mo^VMo^VI₁₅} SBUs, while the Mo^V ion within the {MoO₆} cluster of the hammer handle links with the Mo^V ion at the handle of the basket-like {Sr⊂P₆Ni₃Mo^VMo^VI₁₅} SBUs. Ultimately, a pair of {NaPNiMo^VL_0.5} SBUs together link {Sr⊂P₆Ni₂Mo^VMo^VI₁₅} and {Sr⊂P₆Ni₃Mo^VMo^VI₁₅}, creating another type of edge for the square cluster 2 (Fig. 2f). To further gain a clearer understanding of structural connectivity, these two types of edges can be further conceptualized as linkers C and D, with {Sr⊂P₆Ni₂Mo^VMo^VI₁₅} and {Sr⊂P₆Ni₃Mo^VMo^VI₁₅} considered as nodes B and C, respectively, thereby cluster 2 can be abstracted to a square configuration (Fig. 2g). The simplified structure finds that the angles between adjacent edges are 88.5° and 91.5°, respectively (Fig. S14†). Despite these angles deviating slightly from the ideal 90° of a square, the intricate molecular assembly process permits them to be regarded as highly compatible with cubic molecular geometry. The crystallographic data of clusters 1 and 2 are summarized in Table S1,† while the bond lengths (Å) are detailed in Tables S2 and S3.† In our comparative analysis of the two cluster structures, we have elucidated the pivotal roles of Mo, Ni, and Co ions in the assembly of 1 and 2. Through bond valence sum analysis (BVS) and subsequent XPS analysis, we have ascertained the specific oxidation states of the Mo ions (Tables S4, S5† and Fig. 4a, b). The structural composition examination reveals that Mo^VI ions predominantly occupy the ‘vertex’ positions within these clusters, constituting the basket-like main body of the {Sr⊂P₆M_nMo₁₆} (M = Co or Ni, n = 2 or 3) clusters, with variations only observed in the ‘handle’ regions. Conversely, Mo^V ions are primarily situated at the connection points, or ‘edges’ of each cluster. This discovery has inspired us to further explore the important role of Mo^V ions in the structure.

Our analysis has uncovered that the segments of Mo^V ions in both clusters adopt a {Mo^V₂} cluster configuration (Fig. 3a). This {Mo^V₂} unit has previously been identified in polynuclear clusters assembled from polyoxomolybdates, such as {Mo₇₄},⁴⁴ {Co₁₆Mo₁₆P₂₄}⁴⁵ and {Mo₆₄Ni₈Ln₆},⁹ and its diverse coordination modes provide a reliable basis for the aggregation of additional metal ions. Concurrently, the incorporation of transition metals Ni and Co, and their interaction with the {Mo^V₂} clusters, are instrumental in the formation of multinuclear assemblies. The structures of the two clusters were found to harbor similar [P₄M₂Mo^V₂O₂₇]¹⁰⁻ (abbr. {P₄M₂Mo^V₂}, M = Co or Ni) units, each interconnected by {P₂Mo^V₂} structural units through the bridging action of two Ni and Co ions, respectively. A further examination of the electron density distribution within these structural units reveals that in Co₁₂Sr₄Mo₈₀P₃₆ (1), the {Mo^V₂} unit, in conjunction with two Co ions and four {PO₄} groups, forms the {P₄Co₂Mo^V₂} structural unit with a symmetrical electron density distribution (Fig. 3b). This symmetrical electron density distribution facilitates an antisymmetric arrangement of the connecting basket-like {Sr⊂P₆Co₂Mo₁₆} SBUs on either side of the structural unit (Fig. 3c), which is crucial for the formation of the tetrahedral cluster in 1. In contrast, the electron density within the {P₄Ni₂Mo^V₂} structural unit observed in Na₄Ni₂₃Sr₄Mo₈₈P₅₂ (2) exhibits an asymmetric distribution (Fig. 3d), which results in its linkage with {Sr⊂P₆Ni₂Mo₁₆} and {Sr⊂P₆Ni₃Mo₁₆} SBUs, the latter featuring an additional Ni ion at the base, facilitating the arrangement of the basket-type {Sr⊂P₆Ni_nMo₁₆} clusters in mirror symmetry (Fig. 3e).

Fig. 3 (a) The structure of {Mo^V₂} cluster. (b) The electron density distribution of the {P₄Co₂Mo^V₂} unit. (c) The anti-symmetric structure formed by the {P₂Mo^V₂} unit and two basket-like {Sr⊂P₆Co₂Mo₁₆} SBUs in 1. (d) The electron density distribution of the {P₄Ni₂Mo^V₂} unit. (e) The mirror-symmetrical structure formed by {P₂Mo^V₂} unit and the basket-like {Sr⊂P₆Ni_nMo₁₆} SBUs (n = 2 or 3) in 2.

Notably, the structural integrity of {P₄M₂Mo^V₂} units was found to be dependent on Mo^V–Co/Ni interactions. Substitution of Mo^V with Mo^VI ions resulted in divergent structural outcomes (Fig. S15†), confirming the essential role of {Mo^V₂} clusters and Co^II/Ni^II ions in the assembly of clusters 1 and 2. The systematic experimental investigation has demonstrated that beyond the characteristic clusters 1 and 2, the substitution of Sr^II ions with other metal salts of analogous radius, such as Pb^II ions, still permits the manifestation of significant structural orientation upon the incorporation of Co^II and Ni^II ions. The addition of Co^II exclusively yields tetrahedral cluster structures, whereas the introduction of Ni^II preferentially facilitates the formation of square clusters. Notably, the inclusion of other transition metal ions did not yield crystalline products. This observation conclusively affirms the critical role of Co^II and Ni^II in governing the structural assembly of {Mo^V₂} clusters.

Catalytic oxidation of MPS to MPSO

The polynuclear nature of metal superclusters holds a unique appeal in the domain of catalysis, leveraging the potent synergistic interactions between metal centers to significantly enhance the efficiency and vitality of chemical transformations. Sulfones and sulfoxides are critical intermediates in chemical synthesis, and their utility in the chemical sector is a testament to their invaluable role.^46–48 In particular, sulfoxides are frequently used as chiral auxiliaries and oxygen-transfer reagents.^49,50 The selective oxidation of sulfides is an effective route to producing sulfoxides. Thus, the oxidation of methyl phenyl sulfide (MPS) was selected as an important reaction model, and a comprehensive evaluation of the catalytic capabilities of clusters 1–2 was conducted. Cluster 2 has been chosen as the primary catalyst due to its associated superior yield. An in-depth investigation has been conducted into the intricacies of solvent choice, catalyst dosage, oxidant dosage, and temperature, which influence the performance of the catalytic system (Table S6†).

Our initial exploration involved an evaluation of several solvents, including acetonitrile (CH₃CN), methanol (MeOH), ethanol (EtOH), and 1,4-dioxane (Diox). The results indicated that the highest efficiency was obtained in EtOH, where the conversion rate reached an impressive 99.5%, with a selectivity of 98.9% (Table S6,† entries 1–4). The amount of catalyst was found to exert minimal influence on the conversion of MPS; however, it played a significant role in determining product selectivity. The highest selectivity for methyl phenyl sulfoxide (MPSO) was attained when the amount of catalyst was fixed at 0.25 μmol (Table S6,† entries 3 and 5–7). The amount of oxidant plays a critical role in the catalytic system. Elevating the dosage of tert-butyl hydroperoxide (TBHP) improves the conversion of MPS and the selectivity for MPSO. Nevertheless, when the TBHP is increased to 0.8 mmol, the selectivity for MPSO drops from 98.9% to 98.2% (Table S6,† entries 3 and 8–11), signifying that an excessive oxidant leads to over-oxidation. Consequently, 0.75 mmol is determined to be the optimal dosage of TBHP to avert the oxidation of MPS to sulfone. Additionally, temperature is a pivotal factor in the kinetics of the catalytic reaction. Investigations conducted at 40, 50, 60, and 70 °C revealed optimal performance at 60 °C (Table S6,† entries 3 and 12–14). The optimized reaction conditions were found to be: 0.5 mmol MPS, 0.25 μmol catalyst, 0.75 mmol TBHP as the oxidant, EtOH as the solvent, and a reaction time of 50 min at 60 °C. Under these optimal conditions, the conversion of MPS reached 99.5% with a selectivity for MPSO of 98.9% (Table 1, entry 2). The scalability of the catalytic reaction is crucial for its industrial application; hence, the potential for scale-up was examined. The results indicated that the reaction maintained excellent catalytic efficiency when scaled up by a factor of 10, achieving a conversion of 99.2% for MPS and a selectivity of 95.5% for MPSO (Fig. 4c and S16; Table S6,† entry 15).

Fig. 4 (a and b) The XPS spectra of Mo ions in clusters 1 and 2. (c) The time-dependent relationship of the catalytic oxidation of MPS following a 10-fold scale-up. (d) The cyclic experiment of 2; (e and f) the Raman spectra and PXRD patterns of the 2 before and after the catalytic reaction.

Table 1 Catalytic oxidation of MPS with different catalysts^a

Entry	Catalyst	Conv.^b (%)	Sel.^c (%)
a Reaction conditions: 0.25 μmol of catalyst, 0.5 mmol of MPS, 2 mL EtOH, 0.75 mmol 70% TBHP, 60 °C, 50 min. b Conversion and selectivity were determined by GC using naphthalene as an internal standard. c Selectivity to MPSO, the byproduct was sulfone. d Without catalyst. e Without TBHP.
1	Co12Sr4Mo80P36	99.3	97.6
2	Na4Ni23Sr4Mo88P52	99.5	98.9
3^d	—	38.2	99.0
4^e	Co12Sr4Mo80P36	13.8	100.0
5^e	Na4Ni23Sr4Mo88P52	11.0	100.0
6	Na2MoO4·2H2O	56.2	98.4
7	CoCl2·6H2O	61.0	99.4
8	NiSO4·6H2O	57.9	98.3
9	SrCl2·6H2O	51.6	98.9
10	L	42.8	98.2
11	Na2MoO4·2H2O + CoCl2·6H2O + SrCl2·6H2O + L	91.6	99.1
12	Na2MoO4·2H2O + NiSO4·6H2O + SrCl2·6H2O + L	94.9	98.0

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In the optimized experimental conditions, the oxidation reaction of MPS with Co₁₂Sr₄Mo₈₀P₃₆ as a catalyst also achieved a high conversion rate of 99.3%, and the selectivity towards MPSO reached 97.6% (Table 1, entry 1). Totally, clusters 1–2 as catalysts exhibit exceptional performance in the MPS oxidation reaction, markedly outperforming the majority of POM-based catalysts reported in the literature, as shown in Table S7.† The reusability and stability of the catalyst are important indexes for evaluating its catalytic oxidation performance. After being reused for up to five cycles, catalysts 1–2 sustained their catalytic efficacy, with conversion efficiencies above 99% and selectivity exceeding 95%, which underscores the recyclability of catalysts (Fig. 4d and S17†). Furthermore, to evaluate the structural integrity of catalysts 1 and 2 following the catalytic reaction, PXRD patterns, and IR spectra were tested before and after the reaction (Fig. 4f and S18, S19†). These analyses affirm the integrity of catalyst structures, implying their robust stability during the catalytic process.

To delve into the factors contributing to its superior catalytic performance, a suite of comparative experiments was conducted. Initial experiments investigated the catalytic efficiency of reactions devoid of catalyst or TBHP, confirming the pivotal role of catalyst-oxidant interactions in driving the reaction forward (Table 1, entries 3–5). The results revealed that the omission of either the catalyst or the oxidant markedly suppresses the oxidation of MPS, accentuating the pivotal role of TBHP as the predominant provider of oxygen atoms within the oxidation mechanism. Subsequently, comparative experiments were conducted using Na₂MoO₄·2H₂O, CoCl₂·6H₂O, NiSO₄·6H₂O, SrCl₂·6H₂O, and ligand L as catalysts respectively to further confirm the catalytic role of each component in the oxidation reaction of MPS. When used individually, the conversion rates of MPS were 56.2%, 61.0%, 57.9%, 51.6%, and 42.8%, while the selectivity for MPSO was 98.4%, 99.4%, 98.3%, 98.9%, and 98.2%, respectively as shown in entries 6–10 of Table 1. This indicates that when a single component is used as a catalyst, although a high MPSO selectivity can be maintained, the conversion rate of MPS is only at a moderate level. In Table 1 entries 11 and 12, when physical mixtures are used as catalysts, the synergistic catalytic effect among the components significantly increases the conversion rate of MPS to 91.6% and 94.9%, while the selectivity reaches 99.1% and 98.0%, respectively. This result fully proves the efficient promoting role of multi-component catalysts in the oxidation process of MPS. The findings indicate that while the catalytic efficacy of the physically blended system may not attain the level of target clusters 1 and 2, it nonetheless distinctly demonstrates the substantial facilitation afforded by the multi-component catalyst within the MPS oxidation pathway, underscoring its significance in enhancing reaction efficiency.

According to the literature, the possible mechanism of MPS catalytic oxidation was speculated due to the interaction between POMs and TBHP to produce active peroxide species.^51–54 This hypothesis was substantiated through Raman spectroscopic analysis. As shown in Fig. S20† and Fig. 4e, the significant enhancement of the peak at 670 cm⁻¹ after the catalytic reaction is attributed to the O–O stretching vibration. A mechanism for the catalytic reaction initiated by the Mo center was proposed in Fig. S21.† In the catalytic oxidation process, the Mo centers within the catalyst generate an electrophilic peroxide intermediate B under the influence of TBHP. Subsequently, the nucleophilic S atom of the substrate attacks this intermediate, leading to the formation of the precarious intermediate C. An intramolecular electron migration ensues, converting intermediate C into intermediate D, which ultimately succumbs to rapid decomposition, liberating the corresponding sulfoxide. Given the catalyst exhibited excellent catalytic activities in the oxidation of MPS, 2 was used as the catalyst to explore more thioether analogues to prove its versatility. As presented in Table S8,† cluster 2 exhibits high efficiency in catalyzing the oxidation of various sulfides, achieving outstanding conversion rates and high selectivity for sulfoxides.

Conclusions

In conclusion, we have achieved the synthesis of two rare superclusters exhibiting closely related topological architectures through meticulous manipulation of the molecular framework. Initially, a tetrahedral supercluster Co₁₂Sr₄Mo₈₀P₃₆ (1) was procured wherein the vertex entities are constituted by {Sr⊂P₆Co₂Mo^V₂Mo^VI₁₄} moieties and the edges are constructed by two different SBUs. Subsequently, a purposeful refinement of the metallic constituents was executed to adeptly modify the vertex orientations and elongate the interstitial edges, culminating in the generation of a square-shaped supercluster Na₄Ni₂₃Sr₄Mo₈₈P₅₂ (2) with vertices formed by {Sr⊂P₆Ni₂Mo^VMo^VI₁₅} units and {Sr⊂P₆Ni₃Mo^VMo^VI₁₅} units and edges constructed from three distinct SBUs. Of particular significance, the {P₄M₂Mo₂} subunits (M = Co^II or Ni^II) are positioned within the ‘handle’ region of the basket-like SBUs, where they critically facilitate the assembly process through their interactive engagement with a variety of SBUs. In addition, clusters 1–2 demonstrated exceptional heterogeneous catalytic activity, capably facilitating the oxidation of MPS to MPSO with great selectivity and a conversion yield approaching 99%. Concurrently, clusters 1–2 efficiently catalyzed the oxidation of other diverse thioethers to their corresponding sulfinyl derivatives. Throughout the catalytic process, the superclusters exhibit remarkable stability. This work validates the critical role of the predictive capability of structural manipulation in the assembly of superclusters within the domain of reticular chemistry, and underscores the promising potential of POM-based materials for applications in the field of catalysis.

Author contributions

Xiaoyan Zhang: writing – original draft, visualization, validation, formal analysis, investigation. Hui Li: validation, formal analysis, data curation. Na Xu: writing – review & editing, methodology, validation, funding acquisition. Xiaodong Liu: investigation, formal analysis. Xiu-Li Wang: writing – review & editing, validation, methodology, funding acquisition.

Data availability

All the data that support the findings of this study are available in this paper and its ESI.† Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under the CCDC deposition numbers 2323884 (for Co₁₂Sr₄Mo₈₀P₃₆) and 2323886 (for Na₄Ni₂₃Sr₄Mo₈₈P₅₂).†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 22271021, 21971024, 22201021, 22101030).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2323884 and 2323886. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi00460h

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