Assembly of Si-substituted heteropolyoxotantalate architecture

Abstract

The heteropolyoxotantalates (hetero-POTas) represent a fascinating class of compounds characterized by diverse structural variations and outstanding physicochemical properties. However, the development of hetero-POTas remains an intriguing challenge due to factors such as the indigent solubility, the terribly sluggish reaction kinetics, and the high alkaline reaction atmosphere of the POTa category. In this study, the first hetero-POTa containing Si element, Li₇KNa₁₁H₆[Si₂Ta₂₄(O₂)₂₀O₅₂(OH)₉]·85H₂O (1), was created through self-assembly in a neutral solution. The polyoxoanions embrace 5 tetrahedral {[Ta(O₂)O₅]₄}, 4 {TaO₆} and 1 {Si₂O₇} unit. The compound is the largest hetero-POTa at present. Additionally, 1 has a decent proton conductivity performance with proton conductivity of 3.63 × 10⁻³ S cm⁻¹.

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Introduction

Polyoxometalates (POMs) are an extensive category of polynuclear anionic metal oxo bunches, which have received substantial attention in the fields of magnetism, materials science, catalysis, biomedicine, photoluminescence, and proton conductors.^1–10 Due to the strong negative charge and hydrophilia of POM, POM-based proton-conductive materials have been extensively studied in recent years.^11–16 For example, in 2018, Zang et al. used {MnV₁₃} as the building block to synthesize two POM-based MOF compounds with excellent proton conductivity.¹³ To explore the effect of proton concentration and carrier on the proton conduction, Lan et al. identified four POMs with organic molecules containing varying proton contents in 2019. This finding offers valuable theoretical insights for investigating the properties of these compounds at the molecular level.¹⁴ A Preyssler-type POM functionalized with a nine-coordinate Eu³⁺ ion was found by Sadakane et al. in 2021, and the compound was mixed with poly(allylamine), with the amine groups serving as the protonation sites. This led to a notable enhancement in the proton conductivity of POMs.¹⁵ Moreover, Li et al. used POM as a multifunctional enhancer to manufacture a non-volatile flexible hybrid polymer electrolyte with enhanced conductive, tensile and adhesive features.¹⁶

POMs are classified as isopolyoxoanions or heteropolyoxoanions depending on whether they contain heteroanions (such as SO₄²⁻ and PO₄³⁻).¹⁷ To date, many isopolyoxometalates with a maximum metal nuclearity number of 368 in a separate group have been detected.^18–22 Lacunary heteropolyoxoanions are preferred for reactions with almost any metal moiety. This preference has resulted in a significant increase in the number of heteropolyoxometalates over the last few decades.^23–27 Lacunary heteropolyoxoanions present a viable and efficient maneuver that incorporates heteroatoms into the POM framework to enhance the structural diversity of POMs and modify their physicochemical properties.^28–30 Some important subfamilies of POM include polyoxomolybdate/polyoxotungstate/polyoxovanadate, which have received significant attention from researchers for numerous years.^31–38 Since 2002, Nyman et al. have prepared successfully two heteropolyoxoniobates, K₁₂[Ti₂O₂][SiNb₁₂O₄₀]·16H₂O and Na₁₄[H₂Si₄Nb₁₆O₅₆]·45.5H₂O;³⁹ the heteropolyoxoniobates have undergone significant advancements in the past decade (Table S1†).

In comparison, the development of heteropolyoxotantalates (hetero-POTas) poses a significant challenge due to factors such as the indigent solubility, the sluggish reaction kinetics, and the high alkaline reaction atmosphere of the POTa category.^40–42 Automatically, the development of hetero-POTa is currently on a nascent platform (Table S2†). Two hetero-POTas with P as the heteroatom [P₄(TaO₂)₆O₂₅]¹²⁻ (cis-P₄Ta₆) and [P₄(TaO₂)₆O₂₄]¹⁰⁻ (trans-P₄Ta₆) were found by our group in 2017.⁴³ In the subsequent years, hetero-POTas salts containing As and Se atoms, in addition to their rare earth derivatives, [Ln(H₂O)₆{H₄(TaO₂)₆As₄O₂₄}]³⁻ (Ln = Sm, Eu, Tb, Dy, Er, Tm, Yb, Lu), [(TaO₂)₆Se₄(OH)₄O₁₇]⁴⁻ (Se₄Ta₆), [Ln(H₂O)₆(TaO₂)₆Se₄(OH)₃O₁₈]²⁻ (Ln = Tb, Dy, Ho, Er, Tm, Yb), were discovered successfully.^44,45 In 2022, three transition-metal-based hetero-POTas species, [Ni₂(H₂O)₄{P₄Ta₆(O₂)₆O₂₄}]⁶⁻, [Zn/Cd(H₂O)₄{P₄Ta₆(O₂)₆O₂₄}]⁸⁻, were acquired.⁴⁶ Our group obtained the above compounds under acidic conditions. In 2023, Zheng et al. obtained three hetero-POTas with [P₂O₇Ta₅O₁₄]⁷⁻ (P₂Ta₅) cluster, [Cu(en)₂(H₂O)₂]₂[Cu(en)₂][P₂O₇Ta₅O₁₄]⁻, [Cu(en)₂(H₂O)₂][Cu(en)₂(H₂O)]{[Cu-(en)₂]_1.5[(P₂O₇)Ta₅O₁₄]}, [Cu(en)₂]{[Cu(en)₂]₂[(P₂O₇)Ta₅O₁₄]}, under alkaline conditions.⁴⁷ Presently, only hetero-POTas containing the heteroatom of Se, As, and P have been obtained (Fig. 1), and their maximum nuclearity is only 6. In pursuit of advancing the rapid evolution of hetero-POTas, our research has delved deeply into this area, aiming to introduce additional heteroatoms into POTas to design clusters with diverse structures. Fortunately, we obtained the Si substituted hetero-POTas Li₇KNa₁₁H₆[Si₂Ta₂₄(O₂)₂₀O₅₂(OH)₉]·85H₂O under weakly acidic conditions. The proton conductivity of the compound has been systematically investigated, revealing a notable impact of lithium ions on proton conductivity.

Fig. 1 (a) Polyhedral structures and publication year of representative high nuclearity hetero-POTa: cis-P₄Ta₆, trans-P₄Ta₆, As₄Ta₆, Se₄Ta₆, P₂Ta₅. (b) The formation of [Si₂Ta₂₄(O₂)₂₀O₅₂(OH)₉]²⁵⁻ polyoxoanions by introducing Si to POTas. (P, As, Se, Si, yellow; Ta, sea blue; O, red and lavender).

Results and discussion

Structure description

This structure is intriguing for four distinct reasons: first, as a member of the heteropolyoxoanions, the compound exhibits a ring-shaped polyoxoanion structure, which remains relatively uncommon in scientific literature. It reminds us of the same of cyclic heteropolyanion including {P₈W₄₈},^48,49 {P₅W₃₀},⁵⁰ {Se₆W₃₉},⁵¹ and {Se₈W₄₈};⁵² seconds, it represents the first Si-containing hetero-POTa and is the largest molecular cluster within the category of hetero-POTas; third, the structure comprises both {TaO₆} and {TaO₅(O₂)} building units, which were initially identified; fourth, it was observed for the first time that four {TaO₅(O₂)} units were connected by a μ₄-O bond (Fig. 1). The single-crystal X-ray structural analysis shows that 1 belongs to the triclinic space group P1̄ (Table S3†). 1 comprises [Si₂Ta₂₄(O₂)₂₀O₅₂(OH)₉]²⁵⁻ polyoxoanions (1a), 6 H⁺, K⁺, 11 Na⁺, and 7 Li⁺ counter cation, and 85 lattice water molecules. The 1a with pentagon configuration is composed of 5 tetrahedral conventional tetrahedral coordination polyhedrals (Fig. S1 and S2†): three corners of {[Ta(O₂)O₅]₄} (denoted as {Ta₄}), four Ta atoms (denoted as {TaO₆}) and one {Si₂O₇} unit (Fig. 2a and b). All the Si atoms display {SiO₄} tetrahedra, bonded within the cluster by three Si–O–Ta, and the one Si–O bond connects with another Si atom (Fig. 2c). The four Ta atoms with octahedral geometry act as a bridge connecting two neighboring {Ta₄} units (Fig. 2d). The {Ta₄} unit with regular tetrahedral configuration can be visualized as four Ta atoms connected by μ₄-O (Fig. 2e–g). This is different from the previously reported hetero-POTas, which are connected by μ₃-O (Fig. S3†).^43–45 The presence of μ₄-O indicates the potential formation of extensive clusters by hetero-POTas. Furthermore, the {Ta₄} unit can be considered as a peroxo-{Ta₆} moiety that releases two {Ta(O₂)} groups (one is located in an equatorial position while the other is in an axial position) (Fig. 3 and S4–S6†). Each of the four Ta atoms in the {Ta₄} with a twisted-pentagonal-bipyramidal is surrounded by a single μ₄-O atom, terminal η₂-coordinated peroxy group, and four μ₂-O atoms. Notably, the distances of 20 peroxy bands in the polyanion range from 1.496(17) to 1.546(18) Å (Table S4†), are longer than those for noncoordinated O₂²⁻ (1.49 Å), peroxotantalum-substituted polyoxotungstates (1.50 Å).^42,53 It is important to note that the 1 possesses the highest number of peroxo groups at present (Table S5†). The Ta–O distances in 1 is in the span of 1.886(12)–2.204(11) Å (Table S5†). The average Ta–Op distance of 1.99 Å is 0.3 Å longer than that in the anion {P₄Ta₆}, whereas the average Op–Ta–Op angle of 44.8° is close to that in {P₄Ta₆} (45.3°), and {Se₄Ta₆} (44.6°) (Table S6†).^43,54 The space packing of 1a is intriguing. The 2-D stacking of 1 demonstrates a regular arrangement in the form of –AAA–, and each of the layers is parallel to the surrounding ones (Fig. S7†). However, the 3 × 3 × 3 supramolecular 3-D stacking framework and the simplified representations illustrate that the polyoxoanions of 1 are regularly arranged in the pattern –ABAB– along the a, b, and c axes. This stacking arrangement of polyoxoanions mitigates steric hindrance (Fig. S8†).⁵⁵

Fig. 2 (a) The polyhedral representation of 1a. (b) The ball-and-stick representation of 1a. The polyhedral, simplified diagram, and ball-and-stick representation of {Si₂O₇} (c), {TaO₆} (d), {Ta₄} (e and g). Color code: sea blue and green spheres: Ta; lavender spheres: μ₄-O spheres; red spheres: O; yellow spheres: Si.

Fig. 3 Ball-and-stick representations of [Ta₆O₁₉]⁸⁻ ({Ta₆}) (a), supposed peroxo-{Ta₆} (b), and {Ta₄} (c). Color code: sea blue spheres: Ta; red spheres: O.

ESI-MS spectrometry

Electrospray ionization mass spectrometry (ESI-MS) analysis is depicted in Fig. S9.† The complete anionic structure of 1a can be readily distinguished as three broad peaks spanning from m/z 1000 to 1625. The identification of isotopic patterns, in conjunction with the m/z spacing observed between the clusters of peaks, shows that the analyzed anionic fragments possess similar compositions with charge states of 6− (m/z 1037.07), 5− (m/z 1256.17) and 4− (m/z 1579.08) (Fig. S10†). Three signals correspond to species of [K₄NaLiH₂₂Si₂Ta₂₄(O₂)₂₀O₅₂(OH)₉]⁶⁻ (m/z 1037.04), [K₅NaLi₄H₁₉Si₂Ta₂₄(O₂)₂₀O₅₂(OH)₉]⁵⁻ (m/z 1256.24) and [K₅Na₂Li₆H₁₇Si₂Ta₂₄(O₂)₂₀O₅₂(OH)₉]⁴⁻ (m/z 1579.06) (Table S7†).

Proton conductivity

The presence of delocalized H⁺, abundant crystal water, and the high stability indicate the potential application of 1 as a proton conducting material.⁵⁶ The conductivity of powder sample of the compound was probed at 25 °C under varying humidity levels and the alteration in the conductivity with the rise in relative humidity (RH) was measured (Fig. 4a and S11†). With the increase in RH, the conductivity gradually increased from 1.23 × 10⁻⁷ S cm⁻¹ at 75% RH to 8.13 × 10⁻⁴ S cm⁻¹ at 95% RH (Table S8†). It is intuitively clear that proton conductivity of 1 is positively correlated with RH. This result shows that the proton conductivity in 1 is primarily facilitated by water mediation, and the rate of proton conductivity typically exhibits a significant increase by orders of magnitude as the RH rises.^14,57,58 Given that temperature is a crucial factor for promoting proton conductivity, we examined the conductivity with impedance as a function of temperature.^59,60 The conductivity at 95% RH rose rapidly to a maximum value of 3.63 × 10⁻³ S cm⁻¹ with rising temperature (Fig. 4b and Table S9†). The value is comparable to that of reported POTas (Table S10†) and a series of other POMs-based proton-conducting materials (Table S11†).^61–64 To investigate the effect of water and temperature on proton conductivity and the proton transport mechanism, Nyquist plots of 1 were analyzed at various temperatures and RH levels (Fig. 4c, S12–S17†). The reduction in the size of the semicircle is evident with rising temperature and RH, suggesting a gradual increase in proton conductivity (Tables S12–S15†). The increase in temperature may have led to enhanced carrier mobility and the development of more practicable proton transfer routes due to the presence of a higher number of water molecules at elevated RH levels.⁶⁵ To have preliminary insights into the proton-conducting mechanism, the activation energy (E_a) of 1a was calculated, and the values of E_a at various RH such as 75% RH, 80% RH, 85% RH, 90% RH, and 95% RH were found to be 1.04, 0.41, 0.37, 0.34, and 0.29 eV, respectively (Fig. 4c). These values illustrate that the proton transfer process in 1 follows efficacious Grotthuss mechanism at higher RH, whereas protons are transported through vehicles at lower RH. We can conclude that the conductivity and E_a of proton transport in 1 are significantly influenced by the RH. The XRD confirmed the preservation of the structure of 1 after the proton conductivity measurement (Fig. S18†). To enhance comprehension of the function of molecules of water in the formation of proton-conducting routes in 1, water vapor absorption and desorption isotherms were assessed at 25 °C (Fig. 4d). The adsorption of water vapor increases as humidity levels rise, a trend that aligns with the variations in proton conductivity. The highest water vapor uptake can reach 184.0 cm³ g⁻¹, indicating the high hydrophilicity of POM. The water adsorption property of 1 can be compared with other POMs.^14,61,66,67 The enhanced water absorption performance was observed in the presence of Li⁺ in the structure, attributed to the lower ionic potential of the Li⁺ cation compared to K⁺ and Na⁺ cations (z/r, where z and r are the ionic charge and radius).⁶⁸ More water molecules are introduced into the void in the crystals of 1 at higher RH (Fig. S19†). Denser hydrogen-bonding networks are naturally formed by adsorbed water molecules, facilitating the creation of effective proton hopping pathways and impeding the migration of cations.⁶⁹ This conclusion is supported by the hysteresis loop between the adsorption and desorption branches, while the delayed release of water molecules in the procedure for the desorption process is attributed to hydrogen bonding. Moreover, in situ infrared spectrum (IR) spectroscopy was used to investigate the status of water in the 1 under water vapor situation (Fig. 4e and Fig. S20†). The distinctive absorption bands observed in water molecules within the spectral range of 3000–3700 and 1600 cm⁻¹ can be ascribed to OH stretching (ν(OH)) and HOH bending (δ(HOH)), respectively.^70,71 Note that the δ(HOH) absorption band of water molecules is minimally influenced by the hydrogen bonding networks.^72,73 However, the vibrational position of the ν(OH) bands has migrated to lower wave numbers as the amount and intensity of hydrogen bonds increase. This shift occurs because the ν(OH) band appears independently within the range of hydrogen bonds.^74,75 As might have been expected, the characteristic absorption peaks emerged progressively following the introduction of water vapor and these bands could be effectively replicated by the combination of four Gaussian peaks. These peaks correspond to specific groups of water molecules within the crystal structure of compound 1, aligning with prior studies on the water molecule configurations in Keggin-type and Preyssler-type polyoxometalates (POMs) (Fig. 4e).^15,68 The OH vibrations of the molecules of water in 1 can be categorized into four distinct units: (a) those in close proximity to an alkali metal ion without engaging in hydrogen bonding, (b) those in close proximity to an alkali metal ion at a hydrogen-bonding distance with an O atom or oxide ion, (c) those at hydrogen-bonding distances with an O atom and/or oxide ion, and (d) those located within the hydrogen-bonding range of two oxygen atoms of the constituent ion and/or another water molecule. The structural analysis of 1 suggests the presence of H-bonds between water molecules and polyanions (Fig. 4f, S21, and Table S16†).

Fig. 4 (a) Humidity-dependent proton conductivity of 1 at 25 °C. (b) Nyquist plots for 1 at various temperatures and 95% RH conditions. (The solid lines are best fits.) (c) ln(σT) vs. 1000/T plots of 1 at different RH. (d) The water vapor adsorption and desorption isotherm of 1 at 25 °C (e) In situ IR spectra of 1 under a water vapor pressure of 0.5 kPa, 1.5 kPa, and 2.5 kPa in the OH stretching region. (f) The hydrogen bonding networks in 1. The yellow dashed lines represent hydrogen bonds. Color code: sea blue octahedron: Ta; red spheres: O; purple spheres: O of water molecules; green spheres: Na; grey spheres: K.

Conclusions

In summary, the compound Li₇KNa₁₁H₆[Si₂Ta₂₄(O₂)₂₀O₅₂(OH)₉]·85H₂O (1) was successfully synthesized through the self-assembly process of K₈[Ta₆O₁₉]·17H₂O in an aqueous solution of H₂O₂. 1a with a pentagon configuration is composed of 5 tetrahedral {Ta₄}, 4 {TaO₆}, and a {Si₂O₇} unit. At 95% RH, the conductivity of 1 reaches a maximum value of 3.63 × 10⁻³ S cm⁻¹ under 75 °C. The E_a showed that the primary proton conductivity mechanism was the vehicle mechanism at lower RH and Grotthuss mechanism at higher RH. In essence, this work elucidates the potential for discovering novel types of POTa and establishes a groundwork for utilizing POTa in proton conductivity applications.

Author contributions

Hanhan Chen designed the experiments and synthesized the title compound, investigation, visualization, methodology, formal analysis, writing and editing. Haojie Xu on the characterization of the title compounds and carried out alternating current impedance measurements. Xinyi Ma reviewed the manuscript. Pengtao Ma provided detailed refinements on the crystal structures. Jingping Wang and Jingyang Niu conceived the idea, supervised the project, funding acquisition and review. All authors discussed and co-wrote the manuscript.

Data availability

The data supporting this article have been incorporated into the ESI.†

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

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22071044, 21771054 and 22171071).

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Footnote

† Electronic supplementary information (ESI) available: Materials, measurements and syntheses, and the additional tables and figures containing the unit cell, XRD, IR, TGA, and Nyquist plots of 1. CCDC 2326212 of 1. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01540a

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