Synthesis of a Keggin-type polyoxoselenidotungstate via site-selective oxygen-to-selenium substitution

Abstract

Selenium, a group 16 (chalcogen) element, can endow metal oxides with unique properties when replacing oxygen atoms from specific sites. Polyoxometalates (POMs), a class of anionic metal oxide clusters, exhibit structure-dependent properties and applications. Despite the potential of chalcogen substitution, the replacement of oxygen atoms in POMs with chalcogens has been rarely explored. In a recent study, we demonstrated site-selective oxygen-to-sulfur substitution in the Keggin-type POM [SiW₁₂O₄₀]⁴⁻. Building on this, we now report the first synthesis of a polyoxoselenidotungstate, featuring terminal selenido ligands (W=Se bonds), using a site-selective oxygen-to-selenium substitution reaction. By reacting [SiW₁₂O₄₀]⁴⁻ with Woollins' reagent (2,4-diphenyl-1,3,2,4-diselenadiphosphetane 2,4-diselenide) in organic solvents, all twelve terminal oxido ligands (W=O) were selectively converted to selenido ligands (W=Se). The resulting compound [SiW₁₂O₂₈Se₁₂]⁴⁻ retains the Keggin-type framework and exhibits distinct optical and electronic properties owing to the incorporated selenium atoms. These findings pave the way for the systematic modification of oxygen sites in POMs with the heavier chalcogens sulfur and selenium, opening new avenues for tailoring their properties and expanding their utility across diverse fields of materials science.

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Introduction

Metal chalcogenides (i.e. oxides, sulfides, and selenides) represent a versatile class of semiconductive materials with diverse properties and resulting applications spanning catalysis, optics, electrochemistry, energy storage, sensing, and medicine.^1,2 Their structures and constituent chalcogen elements play a critical role in determining their properties and applications.² In particular, the unoccupied 3d orbital of a selenium atom can contribute to bonding with metal atoms, which often imparts electron storage/transportation and catalytic properties.³ However, among the wide range of metal chalcogenide clusters,^3c–f selenides are less commonly explored compared to oxides and sulfides, underscoring the need for efficient and precise synthetic approaches for selenide-containing cluster compounds.

Polyoxometalates (POMs) are anionic molecular metal oxides of the early transition metals (W^VI, Mo^VI, V^V, Nb^V, and Ta^V). They display well-defined structures with tunable properties such as acidity/basicity, redox behavior, and photochemical activity, enabling their broad applications in catalysis, medicine, energy storage and conversion, sensing, electronics, and battery technologies.⁴ Their properties can be tailored by altering their structures, atomic composition, or electronic states. To date, various metal atoms, metal oxide clusters, and metal nanoclusters have been incorporated into POM frameworks by substitution of their metal sites or by derivatization of lacunary POMs containing vacant metal sites.^5–7 While organic ligands have been widely substituted for oxygen sites in POMs,^8,9 anion-substituted POMs, i.e. where oxygen atoms are replaced by simple anions, such as halides or chalcogenides, have garnered relatively less attention.¹⁰ To date, several sulfido (S²⁻)-containing POMs have been synthesized via aggregation of small cationic metal sulfide species (e.g. [M₂O₂S₂]²⁺; M = Mo^V, W^V), where sulfur atoms serve as bridging sites (i.e. M–S–M; M = Mo^V, W^V),¹¹ or by replacing terminal oxido ligands (O²⁻) on Nb or Ta atoms in [(O=M)PW₁₁O₃₉]⁴⁻ and [(O=M)W₅O₁₈]³⁻(M = Nb^V, Ta^V) with sulfido ligands (S²⁻).¹² In contrast, reports on POMs possessing selenido ligands (Se²⁻) remain scarce, likely due to their low stability. Only two reports have been documented: Keplerate-type (M₇₂Mo^V₆₀Se₆₀; M = Mo^VI, W^VI) featuring bridging selenido ligands (i.e. Mo^V–Se²⁻–Mo^V),¹³ Keggin-type [(Se=M)PW₁₁O₃₉]⁴⁻ (M = Nb^V, Ta^V) and the Lindqvist anion [(Se=Nb)W₅O₁₈]³⁻ where terminal selenido ligands are coordinated to a niobium or tantalum site.¹⁴

Recently, we reported site-selective oxygen-to-sulfur substitution reactions in a series of Keggin-type POMs, [XW₁₂O₄₀]ⁿ⁻ (X = Al^III, Si^IV, Ge^IV, and P^V), successfully yielding polyoxosulfidotungstates (also known as polyoxothiometalates), [XW₁₂O₂₈S₁₂]ⁿ⁻.^15,16 In these transformations, sulfurizing agents selectively replaced all twelve terminal oxido ligands of [XW₁₂O₄₀]ⁿ⁻ with sulfido ligands while retaining the characteristic Keggin-type structure. The resulting [XW₁₂O₂₈S₁₂]ⁿ⁻ compounds exhibited distinctive visible-light absorption characteristics, electronic structures, and electrochemical properties. Notably, the [XW₁₂O₂₈S₁₂]ⁿ⁻ compounds could not be synthesized through the conventional dehydrative condensation of oxo(thio)anions (e.g. [WS_xO_4−x]²⁻), a common route in POM synthesis. This indicates that [XW₁₂O₂₈S₁₂]ⁿ⁻ can only be obtained via site-selective oxygen-to-sulfur substitution. These findings underscore the potential of site-selective anion substitution as a powerful strategy for introducing unique structures, properties, and functions into POMs—although its application has thus far been limited to sulfur substitution. Given that selenium, like oxygen and sulfur, is a group 16 element, it should be capable of incorporation into POM frameworks via site-selective substitution, potentially enabling the discovery of previously unreported selenium-containing POMs, specifically polyoxoselenidotungstates.

Herein, we report the synthesis of the Keggin-type polyoxoselenidotungstate [SiW₁₂O₂₈Se₁₂]⁴⁻ through a site-selective oxygen-to-selenium substitution of [SiW₁₂O₄₀]⁴⁻ (Fig. 1). All twelve terminal oxygen atoms in [SiW₁₂O₄₀]⁴⁻ were replaced by selenium atoms. This study demonstrates the first successful synthesis of a polyoxoselenidotungstate featuring terminal W=Se bonds. Selenium incorporation results in notable changes in the optical properties and electronic structure of [SiW₁₂O₂₈Se₁₂]⁴⁻ compared to those of both [SiW₁₂O₄₀]⁴⁻ and known polyoxosulfidotungstates [XW₁₂O₂₈S₁₂]ⁿ⁻. These findings indicate that site-selective oxygen-to-chalcogen substitution offers a precise route for the post-synthetic modification of POM structures and properties. This approach promotes the high-throughput design of POM-based, selenium-containing materials, enhancing their potential and broadening their application range.

Fig. 1 Schematic representation of syntheses of Keggin-type polyoxosulfidotungstate ([SiW₁₂O₂₈S₁₂]⁴⁻; previous work)¹⁵ and polyoxoselenidotungstate ([SiW₁₂O₂₈Se₁₂]⁴⁻; this work) through the site-selective oxygen-to-chalcogen substitution of Keggin-type polyoxotungstate [SiW₁₂O₄₀]⁴⁻, and the photographs of 1 mM acetonitrile solutions of tetra-n-butylammonium (TBA) salts of each polyanion (i.e. (TBA)₄[SiW₁₂O₂₈E₁₂] (E = O, S, and Se)).

Results and discussion

Synthesis of Keggin-type [SiW₁₂O₂₈Se₁₂]⁴⁻

We first explored the site-selective oxygen-to-selenium substitution of [SiW₁₂O₄₀]⁴⁻ by reacting its tetra-n-butylammonium (TBA) salt with selenium-containing reagents. Previously, we reported that 1,3,2,4-dithiadiphosphetane-2,4-dithione derivatives (R–P₂S₄–R),¹⁷ such as Lawesson's reagent (i.e., R = –C₆H₄OMe)¹⁵ act as effective sulfurizing agents for the site-selective oxygen-to-sulfur substitution of [SiW₁₂O₄₀]⁴⁻ (Fig. 1). In contrast, other sulfur-based reagents, including bis(trimethylsilyl)sulfide, tetraphosphorous decasulfide (P₄S₁₀), triphenylphosphine sulfide, and dimethyl trisulfide, exhibit little to no reactivity toward [SiW₁₂O₄₀]⁴⁻. Building upon these findings, we used Woollins' reagent (2,4-diphenyl-1,3,2,4-diselenadiphosphetane 2,4-diselenide)¹⁸ that possesses an analogous four-membered ring structure with terminal selenido ligands (Fig. 1). Taking into account the solubilities of both (TBA)₄[SiW₁₂O₄₀] and Woollins' reagent we selected a mixed solvent of acetonitrile/1,2-dichloroethane for the reaction. At room temperature (∼25 °C), the reaction of (TBA)₄[SiW₁₂O₄₀] and Woollins' reagent (seven equivalents with respect to (TBA)₄[SiW₁₂O₄₀]) proceeded minimally, as indicated by electrospray ionization mass (ESI-mass) spectrometry and the negligible color change of the reaction solution. However, heating the mixture to 60 °C for 20 h caused the colorless solution of (TBA)₄[SiW₁₂O₄₀] to turn dark red (see the ESI† for details). The final red crystalline product was obtained by washing with dichloromethane and recrystallizing from a mixed solvent of acetonitrile and diethyl ether (47% yield).

The ESI-mass spectrum of the product dissolved in acetonitrile displayed a signal at m/z = 2542.458 (z = 2), corresponding to [(TBA)₆SiW₁₂O₂₈Se₁₂]²⁺ (theoretical m/z = 2542.483) (Fig. 2a). This result indicates the formation of the [SiW₁₂O₂₈Se₁₂]⁴⁻ anion, wherein all 12 oxygen atoms of [SiW₁₂O₄₀]⁴⁻ are replaced by selenium atoms. Elemental analysis further confirmed the molecular formula of the product to be (TBA)₄[SiW₁₂O₂₈Se₁₂]. In addition, the ESI-mass spectrum of (TBA)₄[SiW₁₂O₂₈Se₁₂] showed no significant change over one week in an acetonitrile solution containing 1 vol% water (ca. 10 000 equivalents with respect to (TBA)₄[SiW₁₂O₂₈Se₁₂]), indicating that (TBA)₄[SiW₁₂O₂₈Se₁₂] exhibits high stability under these conditions (Fig. S1†).

Fig. 2 (a) ESI-mass spectrum of (TBA)₄[SiW₁₂O₂₈Se₁₂] in acetonitrile (positive ionization mode). (b) ¹⁸³W NMR spectra of (TBA)₄[SiW₁₂O₄₀] (black), (TBA)₄[SiW₁₂O₂₈S₁₂] (blue), and (TBA)₄[SiW₁₂O₂₈Se₁₂] (red) in dimethylsulfoxide-d₆.

The ¹⁸³W nuclear magnetic resonance (NMR) spectrum of (TBA)₄[SiW₁₂O₂₈Se₁₂] in dimethylsulfoxide-d₆ showed a single signal, indicating that all twelve W atoms are equivalent and (TBA)₄[SiW₁₂O₂₈Se₁₂] was obtained in high purity (Fig. 2b). Notably, the ¹⁸³W NMR signal of (TBA)₄[SiW₁₂O₂₈Se₁₂] (δ = 2015.8 ppm) appeared at a significantly lower field compared to those of (TBA)₄[SiW₁₂O₄₀] (δ = −91.8 ppm) and (TBA)₄[SiW₁₂O₂₈S₁₂] (δ = 1361.8 ppm). Density functional theory (DFT) was used to calculate ¹⁸³W NMR chemical shifts. The computed shifts for (TBA)₄[SiW₁₂O₂₈Se₁₂] (δ_cal = 1805 ppm) and (TBA)₄[SiW₁₂O₂₈S₁₂] (δ_cal = 1239 ppm) were in excellent agreement with experimental data, confirming the significant differences in the ¹⁸³W NMR chemical shift between these compounds. Furthermore, the calculated shift for (TBA)₄[SiW₁₂O₄₀] (δ_cal = −62 ppm) was consistent with its expected upfield position relative to (TBA)₄[SiW₁₂O₂₈Se₁₂] and (TBA)₄[SiW₁₂O₂₈S₁₂]. Despite the wide chemical shift range observed (≈2100 ppm), the ¹⁸³W NMR calculations strongly support the experimental results.

It is known that ¹⁸³W NMR chemical shifts are largely governed by the inverse of the energy gap between the unoccupied and occupied molecular orbitals. Considering the frontier orbital diagram (Fig. S2 and S3†), this involves the energy difference between occupied W^VI–ligand bonding orbitals with a predominant ligand component and unoccupied W^VI–ligand antibonding orbitals with a predominant metal component. As the ligand varies among O²⁻, S²⁻, and Se²⁻, the energies of the occupied orbitals increase relative to the predominantly metal based unoccupied orbitals. This leads to a reduction of the energy gap as the electronegativity of the ligand decreases (see the optical and electrochemical properties section), resulting in a downfield shift in the ¹⁸³W NMR spectrum.¹⁹ Therefore, the observed downfield shift of (TBA)₄[SiW₁₂O₂₈Se₁₂] relative to (TBA)₄[SiW₁₂O₂₈S₁₂] and (TBA)₄[SiW₁₂O₄₀] is consistent with the trend in chalcogen electronegativity: O (3.44) > S (2.58) > Se (2.55).

Structures of [SiW₁₂O₂₈E₁₂]⁴⁻ (E = O, S, and Se)

In the Raman spectrum of (TBA)₄[SiW₁₂O₄₀], two peaks appearing at 968 and 989 cm⁻¹ were assigned to the stretching vibrations of terminal W=O bonds.^15,20 In contrast, the Raman spectrum of (TBA)₄[SiW₁₂O₂₈Se₁₂] displayed no prominent peaks associated with W=O bonds but instead presented four intense peaks at 326, 342, 365, and 407 cm⁻¹, closely resembling the pattern observed for the [WSe₄]²⁻ anion (Fig. 3a and Table S1†).²¹ The observed wavenumbers were significantly lower than those reported for terminal W=S stretching vibrations (506 and 550 cm⁻¹) in (TBA)₄[SiW₁₂O₂₈S₁₂],¹⁵ supporting the incorporation of heavier selenium atoms in place of oxygen or sulfur atoms. Additionally, the infrared spectrum of (TBA)₄[SiW₁₂O₂₈Se₁₂] displayed two peaks at 349 and 327 cm⁻¹, likely attributable to stretching vibrations of W=Se bonds (Fig. S4†). These results confirm that all twelve terminal oxido ligands in (TBA)₄[SiW₁₂O₄₀] were selectively replaced by selenido ligands through the reaction with Woollins' reagent.

Fig. 3 (a) Raman spectra of (TBA)₄[SiW₁₂O₄₀] (black), (TBA)₄[SiW₁₂O₂₈S₁₂] (blue), and (TBA)₄[SiW₁₂O₂₈Se₁₂] (red). (b) Crystal structure of the anionic component of the tetraphenylphosphonium (TPP) salts of [SiW₁₂O₂₈Se₁₂]⁴⁻. Color code: dark blue, Si; black, W; red, O; light green, Se.

To further investigate the structure, we conducted X-ray crystallographic analysis of [SiW₁₂O₂₈Se₁₂]⁴⁻ after replacing the TBA counterions with tetraphenylphosphonium (TPP). The crystallographic data confirmed that four TPP cations replaced the original four TBA cations. In the parent Keggin-type polyoxotungstate structure, the 40 oxygen atoms fall into three categories: four μ₄-oxygen atoms surrounding the central silicon atom, 24 μ₂-oxygen atoms bridging tungsten atoms (W–O–W), and 12 terminal oxygen atoms (W=O).²² The anionic structure of [SiW₁₂O₂₈Se₁₂]⁴⁻ (Fig. 3b and Table S2†) retains the α-Keggin-type framework, with all terminal W=O oxido ligands in [SiW₁₂O₄₀]⁴⁻ (Table S2 and S3†) replaced by terminal W=Se selenido ligands, while the other oxygen atoms, comprising μ₄-O and μ₂-O ligands, remain unaltered. These findings confirm that Woollins' reagent enables site-selective substitution of the terminal oxygen atoms in [SiW₁₂O₄₀]⁴⁻ with selenium, consistent with the results of ESI-mass and Raman spectral results. Bond valence sum (BVS) values for silicon (3.99, 4.04) and tungsten (6.22–6.42) in [SiW₁₂O₂₈Se₁₂]⁴⁻ indicate that their oxidation states remain at +4 and +6, respectively (Table S4†), closely matching those in [SiW₁₂O₄₀]⁴⁻ (Table S3†). BVS values of selenium atoms range from 1.62 to 2.00 (Table S5†) and confirm their identity as selenido (W=Se) rather than hydrogen selenido (W–SeH) ligands, consistent with the molecular formula (TPP)₄[SiW₁₂O₂₈Se₁₂].

A comparison of the crystal structures of [SiW₁₂O₄₀]⁴⁻, [SiW₁₂O₂₈S₁₂]⁴⁻, and [SiW₁₂O₂₈Se₁₂]⁴⁻ reveals a systematic increase in the average W=E bond length (E = O, S, and Se): 1.71 Å for W=O in [SiW₁₂O₄₀]⁴⁻, 2.15 Å for W=S in [SiW₁₂O₂₈S₁₂]⁴⁻,¹⁵ and 2.28 Å for W=Se in [SiW₁₂O₂₈Se₁₂]⁴⁻ (Table S6†). This trend correlates well with the increasing ionic radii of O²⁻, S²⁻, and Se²⁻.²³ Notably, the bond lengths of Si–μ₄-O, W–μ₄-O, and W–μ₂-O remain nearly unchanged across [SiW₁₂O₄₀]⁴⁻, [SiW₁₂O₂₈S₁₂]⁴⁻, and [SiW₁₂O₂₈Se₁₂]⁴⁻, indicating that oxygen-to-chalcogen substitution occurs selectively at the terminal sites without substantially affecting the internal framework.

To date, only two reports have described the synthesis of POMs featuring M=Se bonds i.e. [(Se=M)PW₁₁O₃₉]⁴⁻ (M = Nb^V, Ta^V) and [(Se=Nb)W₅O₁₈]³⁻.¹⁴ The Keggin-type [SiW₁₂O₂₈Se₁₂]⁴⁻ anion is the first example of a polyoxoselenidotungstate containing terminal selenium atoms bonded to W^VI. The W^VI=Se bond remains largely unexplored, with prior observations limited to tetrahedral [WSe₄]²⁻ and its derivatives²⁴ and a square-pyramidal {WSe₅} structure.²⁵ To our knowledge, polyoxoselenidotungstate [SiW₁₂O₂₈Se₁₂]⁴⁻ is the first molecular structure wherein terminal selenido ligands are coordinated to octahedral W^VI.

Optical and electrochemical properties

The UV-vis spectrum of (TBA)₄[SiW₁₂O₂₈Se₁₂] in acetonitrile displays two intense absorption bands at λ = 281 nm (ε = 1.9 × 10⁵ L mol⁻¹ cm⁻¹) and λ = 317 nm (ε = 2.0 × 10⁵ L mol⁻¹ cm⁻¹) (Fig. 4a). These bands are red-shifted compared to those of (TBA)₄[SiW₁₂O₄₀] (λ = 264 nm; ε = 4.6 × 10⁴ L mol⁻¹ cm⁻¹) and (TBA)₄[SiW₁₂O₂₈S₁₂] (λ = 271 nm; ε = 2.1 × 10⁵ L mol⁻¹ cm⁻¹).¹⁵ Additionally, the absorption tail of (TBA)₄[SiW₁₂O₂₈Se₁₂] extends to approximately 580 nm, significantly extended to longer wavelengths than those of (TBA)₄[SiW₁₂O₄₀] (λ = ca. 370 nm) and (TBA)₄[SiW₁₂O₂₈S₁₂]⁴⁻ (λ = ca. 470 nm), indicating a substantial alteration in the electronic structure. Consequently, the acetonitrile solution of (TBA)₄[SiW₁₂O₂₈Se₁₂] appears orange, whereas those of (TBA)₄[SiW₁₂O₂₈S₁₂] and (TBA)₄[SiW₁₂O₄₀] are pale yellow and colorless, respectively (Fig. 1).

Fig. 4 (a) UV-vis spectra of (TBA)₄[SiW₁₂O₄₀] (10 μM, black), (TBA)₄[SiW₁₂O₂₈S₁₂] (5 μmol L⁻¹, blue),¹⁵ and (TBA)₄[SiW₁₂O₂₈Se₁₂] (8 μmol L⁻¹, red) in acetonitrile. Inset: enlarged view. (b) Selected molecular orbitals of the [SiW₁₂O₂₈Se₁₂]⁴⁻ anion based on DFT calculations.

To further investigate the electronic structure of [SiW₁₂O₂₈Se₁₂]⁴⁻, we performed DFT calculations. In compound [SiW₁₂O₄₀]⁴⁻, the lowest unoccupied molecular orbital (LUMO) is primarily composed of W 5d orbitals, while the highest occupied molecular orbital (HOMO) is mainly derived from μ₂-O 2p orbitals.¹⁵ In contrast, the LUMO and LUMO+1 levels of [SiW₁₂O₂₈Se₁₂]⁴⁻ are primarily composed of W 5d orbitals, while the occupied orbitals include contributions from Se 4p orbitals (HOMO to HOMO−11), W–Se bonding π-orbitals (HOMO−12 to HOMO−23), and W–Se bonding σ-orbitals (HOMO−24 to HOMO−26). These orbitals are higher in energy than the μ₂-O 2p orbitals, such as HOMO−27 (Fig. 4b and S2†). Owing to the presence of these Se-derived occupied orbitals, the HOMO–LUMO gap of [SiW₁₂O₂₈Se₁₂]⁴⁻ (5.73 eV) is substantially smaller than that of [SiW₁₂O₄₀]⁴⁻ (6.85 eV) and slightly smaller than that of [SiW₁₂O₂₈S₁₂]⁴⁻ (5.86 eV).¹⁵ DFT calculations also reveal the formation of unoccupied W–Se antibonding π orbitals (e.g., LUMO+2 to LUMO+8). Based on the time-dependent DFT calculations, the absorption bands at λ = 281 and 317 nm in (TBA)₄[SiW₁₂O₂₈Se₁₂] were likely attributed to charge-transfer transitions from W–Se bonding π-orbitals (i.e. HOMO−12 to HOMO−23) and Se 4p orbitals (i.e. HOMO to HOMO−11) to W–Se antibonding π orbitals and W 5d orbitals (i.e. LUMO+9 to LUMO+11) (Fig. S5†). These results demonstrate the strong influence of selenium substitution on the electronic states and optical properties of POMs.

Finally, to investigate the electrochemical properties of a series of (TBA)₄[SiW₁₂O₂₈E₁₂] (E = O, S, and Se), we carried out the cyclic voltammetry measurements in acetonitrile containing (TBA)ClO₄ (0.10 mol L⁻¹; Fig. 5). The cyclic voltammograms of (TBA)₄[SiW₁₂O₂₈E₁₂] showed reversible redox behavior. The first redox wave of (TBA)₄[SiW₁₂O₂₈Se₁₂] appeared at −1.08 V vs. Ag/Ag⁺, similar to those observed for (TBA)₄[SiW₁₂O₄₀] and (TBA)₄[SiW₁₂O₂₈S₁₂].^15,16 In contrast, the second reduction wave showed a distinct trend, with the reduction potential shifting positively in the order of E = O (−1.62 V), S (−1.48 V), and Se (−1.43 V).

Fig. 5 Cyclic voltammograms of (TBA)₄[SiW₁₂O₄₀] (black line), (TBA)₄[SiW₁₂O₂₈S₁₂] (blue line), and (TBA)₄[SiW₁₂O₂₈Se₁₂] (red line) in acetonitrile containing (TBA)ClO₄ (0.10 mol L⁻¹; 100 mV s⁻¹).

Conclusions

This study demonstrates the synthesis of a polyoxoselenidotungstate through the site-selective substitution of terminal oxido ligands (W=O) in the parent polyoxotungstate with selenido ligands (W=Se) using Woollins' reagent. The resulting selenium-containing polyoxoselenidotungstate, [SiW₁₂O₂₈Se₁₂]⁴⁻, retains the α-Keggin-type framework of its precursor, [SiW₁₂O₄₀]⁴⁻ and exhibits high stability in acetonitrile, while exhibiting a distinct electronic structure. Specifically, [SiW₁₂O₂₈Se₁₂]⁴⁻ features molecular orbitals derived from Se 4p and W–Se bonding/antibonding orbitals, leading to extended absorption into the visible-light region (up to ca. 580 nm), in contrast to [SiW₁₂O₄₀]⁴⁻ and the previously reported [SiW₁₂O₂₈S₁₂]⁴⁻. This study expands the range of chalcogen-substituted metal oxides, demonstrating precise oxygen-to-selenium substitution at the terminal sites. Such structural modifications open new avenues for tailoring the electronic properties of POM-based materials, with potential applications in (photo)catalysis, sensing, optics, energy conversion, and battery technologies.

Data availability

The data supporting this manuscript are available in the ESI and available on request. Crystallographic data for TPP salts of [SiW₁₂O₄₀]⁴⁻ and [SiW₁₂O₂₈Se₁₂]⁴⁻ have been deposited at the CCDC (deposition numbers 2440303 and 2440307).

Author contributions

K. Yonesato and K. S. designed the project and experiments. K. Yonesato performed the major part of experiments. Y. W. contributed to the ¹⁸³W NMR measurements and manuscript revision. M. P.-B. and R. J. E. contributed to the ¹⁸³W NMR measurements, calculations and manuscript revision. K. Yonesato, K. S., and K. Yamaguchi cowrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from JSPS KAKENHI (21K20552, 22H04971, 24H02210, and 25K01782), JST FOREST (JPMJFR213M), and the Mizuho Foundation for the Promotion of Sciences. Single-crystal X-ray diffraction measurements at SPring-8 were conducted with the approval of the Japan Synchrotron Radiation Research Institute (proposal numbers 2024A1880 and 2024B1868). A part of the computations was performed at the Research Center for Computational Science, Okazaki, Japan (Project: 24-IMS-C101 and 25-IMS-C101). The authors are grateful to EPSRC and JSPS for financial support through the INPOMS UK-Japan Core-to-Core Network Grant (EP/S031170/1). M. P.-B. thanks Prof. Josep M. Poblet and the Quantum Chemistry Group at Universitat Rovira i Virgili (URV), Tarragona, for computational resources.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, Tables S1–S4 and Fig. S1–S3. CCDC 2440303 and 2440307 for tetraphenylphosphonium salts of [SiW₁₂O₄₀]⁴⁻ and [SiW₁₂O₂₈Se₁₂]⁴⁻, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5sc03340c

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