English

• Cis-trans isomerism has received great interest in many research areas because of its intriguing structural features associated with wide potential applications (for example, biology, catalysis, optics, magnetism, and medicine field) [1-5]. The isomerization of organic ligand-bridged d-transition metal complexes can result in alternations in intrinsic properties, including optical, redox, and magnetic properties, originating from the behavior of d-electrons. This phenomenon holds promise for the development of new functional molecular systems [6]. The reversible reaction of cis-trans isomerization can be effectively toggled by diverse external stimuli, thus achieving the molecular switching role [7]. For example, reversible switching can be achieved through light and temperature variations, thereby inducing changes in molecular shape, size, and dipole moment [8].

  Polyoxometalates (POMs), as inorganic multidentate coordination compounds, possess the capability of self-assembly and ability in constructing novel architectures through the incorporation of appropriate organic ligands and metal ions. Thus, POMs hold the promise for the realization of cis-trans isomerism [9]. Notably, the conversion from trans to cis-form has been observed in the field of POMs [10-12]. In 2003, Pope et al. demonstrated the initiation of syn-anti interconversion alongside the irreversible formation of minor amounts of other diastereomers through the dissociation and rearrangement of enantiomeric polytungstate ligands [10]. Cronin et al. reported the reversible cis-trans isomerism of a dumbbell-shaped POM-organic hybrid molecule, TBA₁₀H₂[{P₂V₃W₁₅O₅₉(OCH₂)₃NHCO}₂(C₅H₃N)₂], triggered by Zn²⁺ cation in DMSO solution [11]. In 2017, our group also noticed the rapid isomerization of trans-[Ce(α₂-P₂W₁₇O₆₁)₂]¹⁷⁻ species to its cis-isomer counterpart in solution [12]. Within this context, the construction of POMs exhibiting cis-trans isomerism with identical metal nuclearity assumes significance. These compounds not only diversify POM chemistry in terms of structure and properties but also offer a unique platform for comparative studies on structural assembly and structure-property relationships. However, due to synthetic complexities, known instances of POMs displaying cis-trans isomerism remain relatively scarce [10, 12-16].

  Intriguingly, the basicity and nucleophilicity of Nb/Ta-bound oxygen atoms in Nb/Ta-substituted POMs are enhanced, leading to the elimination of peroxo groups and subsequent polymerization of the polyanion [17]. Notably, cis-trans isomers are easily detectable during the polymerization process. In 2014, Yue et al. isolated mixed-addenda Nb/W POM with 'trans' peroxo groups, [{H₆P₂W₁₂Nb₄O₅₉(NbO₂)₂}₂]⁸⁻ [13]. One year later, a 'cis'-dimer, [{Nb₆(O₂)₄P₂W₁₂O₅₇}₂]²⁰⁻, was obtained by our group [14]. Inspired by these findings, we pursued the synthesis of Ta-substituted POMs. However, the synthesis of Ta-substituted POMs remains relatively unexplored compared with Nb-substituted POMs (Tables S1 and S2 in Supporting information). The first Ta-substituted POM was reported in 1994 [18]. To date, only two compounds containing Ta-substituted POMs with peroxo units, [SiW₉(TaO₂)₃O₃₇]⁷⁻ and [P₂W₁₅O₅₉(TaO₂)₃]⁹⁻, have been obtained [19]. Furthermore, only one crystalline salt, [SiW₉(TaO₂)₃O₃₇]⁷⁻, has been characterized. Part of the challenge is that peroxylated Ta-cluster exhibits a propensity to form amorphous solids rather than crystalline salts, different than its Nb analogues [19]. In this paper, the monomer of Ta-substituted POM K₁₁Li[P₂W₁₂(TaO₂)₆O₅₆]·19H₂O (1) and two cis-trans-isomers, K₁₃Li₆H-cis-[P₂W₁₂Ta₄(TaO₂)₂O₅₉]₂·61H₂O (2), and KNa₃Li₄H₁₂-trans-[P₂W₁₂Ta₄(TaO₂)₂O₅₉]₂·37H₂O (3), have been synthesized. These compounds incorporate various POM anions with differing charges, resulting in distinct proton sources and prompting investigation into their proton-conducting properties. Notably, the highest recorded proton conductivity values of 1−3 are 6.88 × 10⁻⁴, 1.13 × 10⁻², and 2.00 × 10⁻² S/cm, respectively. This study represents the first systematic exploration of the proton-conducting properties of polyperoxo POMs.

  The crystal data of 1 was collected using synchrotron X-ray radiation at 150 K (Table S3 in Supporting information) and further characterized by NMR (Fig. S1 in Supporting information), IR (Figs. S2 and S3 in Supporting information), ICP (Table S4 in Supporting information), and mass spectrometry (Figs. S6 and S7, Table S5 in Supporting information). Compound 1 exhibits a excellent thermal stability that could retain at a wide pH range, very different from [P₂W₁₂(NbO₂)₆O₅₆]¹²⁻ (Figs. S2−S8 in Supporting information) [20]. This implies that the peroxo group on Ta is more stable than that on Nb, as consistent with previous reports [21]. The X-ray structure of 1a (Fig. 1) is similar to [P₂W₁₂(NbO₂)₆O₅₆]¹²⁻, a contiguous longitudinal strip of six {NbO₂} groups (one on each cap position and two on each belt position) replaced by six {TaO₂} groups. Two Ta atoms in cap sites are ligated by μ₄—O atoms, and four Ta atoms in belt sites are connected by μ₃—O atoms (Fig. S12 in Supporting information). And the average Op-Ta-Op angle is 43.95° (Table S7 in Supporting information). The Ta peroxo linkages span 1.39(1)–1.52(1) Å with a mean of 1.48 Å, in agreement with the Op-Op distance of uncoordinated O₂²⁻ (1.49 Å) and [P₂W₁₂(NbO₂)₆O₅₆]¹²⁻ (1.43 A). The average Ta-Op distance of 1.97 Å is longer than Ta-Op distance in the [P₂W₁₂(NbO₂)₆O₅₆]¹²⁻. The remaining chemical bonds within 1a share similar bond lengths with [P₂W₁₂(NbO₂)₆O₅₆]¹²⁻ (Tables S8 and S9 in Supporting information). Worth mentioning that the configuration of {P₂W₁₂} remains unchanged in 1a, with the 12 W atoms maintaining a + 6 oxidation state and exhibiting the characteristic {WO₆} octahedral geometry (Table S10 and Fig. S13 in Supporting information). To elaborate the stacking mode of 1a more clearly, Li⁺, K⁺ ions, and lattice water molecules are omitted, with the three-dimensional (3D) supramolecular packing architecture and simplified packing of 1a along the a, b, and c-axis (Figs. S14 and S15 in Supporting information). It is evident that 1a aligns in the "−A − B − A − B−'' mode. Additionally, visualizing each 1a unit as a rectangle, the simplified three-dimensional packing confirms the "−A − B − A − B−" arrangement of 1a. Moreover, it becomes evident that the packing modes of adjacent layer A and layer B are not identical (Figs. S14 and S15).

  Figure 1

  

  Figure 1.  (a) Polyhedral and (b) ball-and-stick representation of 1a. Top view of (c) the polyhedral and (d) ball-and-stick representation of 1a. W, yellow; O, red; P, lime; Ta, aqua.

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  The structure of 2 is analogous to that of 3, both displaying idealized C2v symmetry (Fig. 2). The polyanion 2 possesses peroxo groups 'cis' to each other, formed by two [P₂W₁₂Ta₄(TaO₂)₂O₅₉]¹⁰⁻ fragments (Figs. 2a and b). These fragments are fused together through the two parallel, equatorial Ta–O–Ta bridges, with a mirror plane relationship between them (Figs. 2c and d). Each of the two peroxo-Ta atoms, with a peroxo-bond length of 1.45 Å, resides in belt sites and is coordinated by four µ₂—O, one µ₃—O, and one terminal η₂-coordinated peroxo ligand, resulting in a distorted pentagonal-bipyramidal coordination sphere, whereas four oxo-Ta atoms exhibit octahedral coordination (Fig. S16 and Table S11 in Supporting information). The structure of 2a shows that peroxo groups in the cap sites of {(TaO₂)₆P₂W₁₂} preferentially reduce to the O atom of −2 (Table S12 in Supporting information), while the peroxy group in belt sites is more inclined to polymerization. This cis-dimeric configuration offers insights into possible Ta–O–Ta linkage growth mechanisms, facilitating further aggregation of subunits with Ta atoms from equatorial sites. While 3a shows a different double Ta–O–Ta bridged Wells-Dawson-dimer with 'trans' {TaO₂} groups, reassembling two [P₂W₁₂Ta₄(TaO₂)₂O₅₉]¹⁰⁻ entities suspended by double Ta–O–Ta bridges (Figs. 2e−h). In the {TaO₂} group, the distance between two oxygen atoms (1.14–1.23 Å) is notably shorter than that of peroxide bonds (Tables S13 and S14 in Supporting information). Notably, the structure of 2a differs from that of the {(NbO₂)₆P₂W₁₂} dimer previously reported by our group [14], where peroxo groups reside in the belt sites and cap sites are preserved. While 3a is isomorphic with Na₄[H₆P₂W₁₂Nb₄O₅₉(NbO₂)₂]₂·48H₂O reported by Yue et al. in 2014 [13]. The 2D stacking arrangements of 2a and 3a exhibit similar –AAA– arrangement pattern in the bc, ac and ab planes (Figs. S17 and S18 in Supporting information). The 3D stacking arrangements and simplified views of anions of 2a and 3a are depicted in Figs. S19 and S20 (Supporting information).

  Figure 2

  

  Figure 2.  (a) Polyhedral and (b) ball-and-stick representation of 2a. (e) Polyhedral and (f) ball-and-stick representation of 3a. Top view of the polyhedral representation of (c) 2a and (g) 3a. Ball-and-stick representation of {(TaO₂)₆} in (d) 2a and (h) 3a. W, yellow; O, red; P, lime; Ta, aqua.

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  The intrinsic properties of POMs, including their hydrophilicity, swift charge transfer ability and chemical stability, provide a solid foundation for their application in proton-conductive materials [22-26]. Compounds 1–3 possess abundant H₂O molecules, H⁺ cations, terminal oxygen atoms, and high stability under hydrous conditions. Within this context, proton conductivity of 1–3 was monitored by alternating current impedance measurements using compacted pellets under variable relative humidity (RH) at room temperature (Fig. S23 in Supporting information). At 25 ℃, the conductivity value (σ) of 1–3 was recorded as 4.62 × 10⁻⁶, 1.66 × 10⁻⁵, and 7.03 × 10⁻⁵ S/cm at 55% RH. As RH increased, σ gradually reached maximal values of 5.01 × 10⁻⁵, 2.12 × 10⁻³, and 2.88 × 10⁻³ S/cm at 90% RH, separately (Table S15 in Supporting information). The pronounced dependence of σ on RH in 2 suggests that water molecules act as proton carriers or major components in hydrogen-bond networks during proton transfer processes (Fig. 3a) [27, 28]. Consistently, water vapor absorption carried out at 25 ℃ (Fig. 3b and Fig. S24 in Supporting information) revealed enhanced water vapor adsorption of 1–3 along with increasing humidity, indicating their humidity-dependent proton conduction behavior. Under high humidity, the maximum water vapor uptake of 1–3 reached 162.82, 743.74, and 326.41 cm³/g, respectively, highlighting their high hydrophilicity. Noted that the better water absorption performance was observed with increased Li⁺ concentration in the structures, because Li⁺ cation possess a higher ionic potential than K⁺, Na⁺ cation (r/z, where r and z represent ionic radius and charge, respectively) [29]. Impressively, compound 2 exhibits superior water uptake capacity across the entire RH range measured compared to 1 and 3 and notably surpasses that of several porous POM and MOF materials as well (Table S16 in Supporting information).

  Figure 3

  

  Figure 3.  (a) Humidity-dependent proton conductivity of 1–3 at 25 ℃. (b) Vapor sorption isotherms of 1–3 at 25 ℃. The Nyquist plots of (c) 1, (d) 2, and (e) 3 at 90% RH and various temperatures (25–95 ℃). (f) Arrhenius plots of 1–3 under 90% RH and different temperatures.

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  Considering temperature's significant influence on proton conductivity, the proton conductivities of 1–3 were further investigated at 25–95 ℃ under 90% RH (Figs. 3c–e). The σ values of 2 and 3 reached their optimum at elevated temperatures, achieving 1.13 × 10⁻² and 2.00 × 10⁻² S/cm at 95 ℃, while that of 1 increased significantly from 5.01 × 10⁻⁵ S/cm to 6.88 × 10⁻⁴ S/cm at 95 ℃, demonstrating typical temperature-dependent proton conductivity behavior (Table S17 in Supporting information). This enhancement may be attributed to the increased formation of hydronium ions from H₂O and H⁺ at elevated temperatures, consequently accelerating proton transport [30]. Notably, the proton conductivities of 1–3 surpass those of most POM-based proton-conducting membranes (Table S18 in Supporting information) [31]. To gain insight into their proton transfer mechanisms, the activation energies (E_a) of 1–3 at 90% RH and 25–95 ℃ were calculated according to the Arrhenius equation. The E_a values of 1–3 are calculated as 0.36, 0.24 and 0.27 eV, respectively, indicating the dominance of Grotthuss mechanism in the proton conduction process (establishing proton pathway via hydrogen-bonded networks, typically with E_a < 0.4 eV) (Fig. 3f) [32]. Additionally, the structural integrality of 1–3 remains intact after impedance testing, as confirmed by the PXRD and FT-IR studies (Figs. S25 and S26 in Supporting information).

  Interestingly, although they have the same composition and analogous structures, the proton conductivity of compounds 1–3 is significantly different. To elucidate these differences, we carefully analyzed the correlation between structure and proton-conducting properties. Firstly, we observed that the peroxy bond remains stable after proton conduction, yet it does not effectively contribute to proton conductivity. In addition, the high proton conductivities of 2 and 3 are derived from several factors: (1) The presence of absorbed or coordinated H₂O in the structures of 2 and 3 facilitate the construction of the hydrogen-bonded networks [33]. (2) The increased exposure of surface O atoms in 2 and 3 provide more sites for proton transfer, establishing an unhindered hydrogen bond network conductive to efficient proton transmission [28]. Compound 3 exhibits higher conductivity than 2 mainly for the presence of a large number of proton sources and cluster-bound protons, which play a more crucial role than crystal water in promoting proton conductivity to a certain extent. This phenomenon has been previously observed in reports [34, 35]. The proton transfer barrier reflects the fact that the proton suffers electron density depletion during the transfer from the proton donor to the proton acceptor [36]. Compound 2 has a lower energy barrier, indicating the presence of dense and extensive hydrogen-bonding network (Fig. S27 in Supporting information, the short distances of O···O vary from 2.7 Å to 2.9 Å) [37]. In addition, the state of water molecule in the 2 was further explored by in situ IR spectroscopy under water vapor condition. The characteristic absorption bands of water molecules around 3000–3700 and 1600 cm⁻¹ can be ascribed to OH stretching (ν(OH)) and HOH bending (δ(HOH)), respectively [38, 39]. It has been noted that the δ(HOH) absorption band of water molecules is less affected by the hydrogen bonding networks [40, 41]. On the contrary, the vibration location of ν(OH) bands of water molecules have shift to lower wave numbers with the increase of the number and strength of hydrogen bonds, due to the ν(OH) band of water molecules appears independently in the range of hydrogen bonds [42, 43]. Indeed, the characteristic absorption bands emerged tardily after the introduction of water vapor, and those bands could been deconvoluted into four Gaussian peaks, corresponding to distinct groups of water molecules in crystal structure of 2 and previous report on the state of water in Keggin-type and Preyssler-type POMs (Fig. 4a). The OH vibrations of water molecules in 2 can be categorized into four groups: (1) Those near a metal ion (K⁺) without hydrogen bonding, (2) those near a metal ion (K⁺) at a hydrogen-bonding distance with an O atom or oxide ion, (3) those at hydrogen-bonding distances with an O atom and/or oxide ion, and (4) those within hydrogen-bonding distance of two oxygen atoms of the constituent ion and/or another water molecule. Comparison with 1 reveals a distinct shift to lower wavenumbers of each band in 2, manifesting a strengthened hydrogen-bonding network present within in 2 (Fig. 4b and Fig. S28 in Supporting information).

  Figure 4

  

  Figure 4.  (a) In situ IR spectra of 2 under a water vapor pressure of 0.5, 1.5 and 2.5 kPa in the OH stretching region. (b) The ν(OH) region of water molecules obtained from in situ IR spectra of 1 and 2 under a water vapor pressure of 2.5 kPa (P/P₀ = 0.90). The observed bands were well reproduced by the sum (solid lines) of four Gaussian peaks (broken lines).

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  In summary, three stable Ta/W mixed-addendum POMs (1–3) based on hexalacunary {P₂W₁₂} building blocks were synthesized for the first time. Compounds 1–3 were comprehensively characterized using various physicochemical techniques, confirming their high solution stability and establishing them as novel additions to the POM family. In addition, compounds 1–3 were investigated as proton conductors for the first time, demonstrating excellent performance in proton conductivity. Their proton conduction mechanisms were explored, revealing that high concentrations of H⁺ and H₂O contribute to enhanced proton conductivity by facilitating the formation of continuous and extensive hydrogen-bond networks. These findings are significant for the rational design or optimization of POM structures to develop highly efficient proton conductors with enhanced mechanical and stability performances. And compounds 1–3 can serve as a structural motif to manufacture additional fascinating molecular clusters, promoting the advancement of POM chemistry.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Hanhan Chen: Writing – original draft, Formal analysis, Data curation, Conceptualization. Yahao Sun: Visualization, Validation, Investigation, Data curation. Mingyang Zhang: Resources, Investigation. Pengtao Ma: Formal analysis, Data curation. Jingping Wang: Supervision, Funding acquisition. Jingyang Niu: Writing – review & editing, Supervision, Funding acquisition.

  Acknowledgments

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

  Supplementary materials

  Experimental Procedures, BVS, PXRD, IR, TG, ICP, mass spectrometry, NMR, XPS and packing arrangements for the 1–3. CCDC numbers 2306434−2306436 contains the supplementary crystallographic data for this paper.

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110329.

  
  
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• 

  Figure 1  (a) Polyhedral and (b) ball-and-stick representation of 1a. Top view of (c) the polyhedral and (d) ball-and-stick representation of 1a. W, yellow; O, red; P, lime; Ta, aqua.

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  Figure 2  (a) Polyhedral and (b) ball-and-stick representation of 2a. (e) Polyhedral and (f) ball-and-stick representation of 3a. Top view of the polyhedral representation of (c) 2a and (g) 3a. Ball-and-stick representation of {(TaO₂)₆} in (d) 2a and (h) 3a. W, yellow; O, red; P, lime; Ta, aqua.

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  Figure 3  (a) Humidity-dependent proton conductivity of 1–3 at 25 ℃. (b) Vapor sorption isotherms of 1–3 at 25 ℃. The Nyquist plots of (c) 1, (d) 2, and (e) 3 at 90% RH and various temperatures (25–95 ℃). (f) Arrhenius plots of 1–3 under 90% RH and different temperatures.

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  Figure 4  (a) In situ IR spectra of 2 under a water vapor pressure of 0.5, 1.5 and 2.5 kPa in the OH stretching region. (b) The ν(OH) region of water molecules obtained from in situ IR spectra of 1 and 2 under a water vapor pressure of 2.5 kPa (P/P₀ = 0.90). The observed bands were well reproduced by the sum (solid lines) of four Gaussian peaks (broken lines).

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