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Wang Haiying, Xu Han, Chen Chaolong, Zhu Yingjie, Zhang Yikang, Zhang Dongdi, Niu Jingyang. Parallelogram 3d-4f-5d heterometallic clusters based on trilacunary tungstoantimonates with excellent proton conductivity[J]. Chin. Chem. Lett., 2024, 35(3): 108486  

Haiying Wang^a, Han Xu^b, Chaolong Chen^c, Yingjie Zhu^a, Yikang Zhang^a, Dongdi Zhang^a,*, Jingyang Niu^a,*     

Polyoxometalates (POMs) are a class of anion metal oxygen clusters composed of early transition-metal, and have brought remarkable advances in catalysis, magnetism, biomedicine, materials science, and nanotechnology [1-12]. Generally, saturated POM clusters can be transformed into lacunary building blocks by removal of one or more {XO₆} octahedra under given conditions. The terminal oxygen at the lacunary site has strong nucleophilic nature because of its improved charge density. Therefore, lacunary POM derivatives are ideal candidates to coordinate with electrophilic transition (3d) or rare-earth (4f) metals [13]. However, their utilization is still limited isometallic cluster-incorporation POM derivatives, such as the representative high-nuclear 3d or 4f cluster-incorporation species {Mn₄₀P₃₂W₂₂₄} [14], {Fe₂₈P₈W₄₈} [15], {Co₁₆(PW₉)₄} [16], {Ni₂₅(SiW₉)₆}, [17] {Cu₂₀(P₈W₄₈)} [18], {Ce₂₀Ge₁₀W₁₀₀} [19] and {Ln₂₇Ge₁₀W₁₀₆} (Ln: La, Ce) [20].

The 3d-4f or 4d-4f heterometallic aggregates occupy a special place among POM-based molecular materials not only because of their inherent contributions of electrons, but also owing to the increasing exchange phenomenon between the 3d/4d and 4f centers [21-26]. Notably, most heterometallic aggregates-containing POM complexes (HMPOMs), especially for discrete molecular clusters, were restricted to binary mixed systems. From the viewpoint of POM structure, although dimeric or trimeric HMPOMs have been widely communicated, there are only four tetrameric HMPOMs (Fig. S1 in Supporting information) [27-30]. To our knowledge, no examples of POM-based trinary heterometallic aggregates have been reported so far. Therefore, it is a challenge to explore suitable synthetic conditions to incorporate 3d-4f-5d or 4d-4f-5d heterometallic aggregates into POM ligands.

The Lewis basicity of oxygen atom can form cation-cationic interactions with Lewis acidity of 3d or 4f centers, and thus act as hydrogen bond receptors to result in the excellent proton conductivity [31]. As a continuous interest in 3d-4f-POM systems [23, 24, 29], we herein present the synthesis and characterization of two unprecedented 3d-4f-5d HMPOMs Na₂₂[(SbW₉O₃₃)₄{La₃W₆MO₁₈(H₂O)₈(CH₃COO)₄}₂]·nH₂O(abbreviated as La₆Co₂, n = 60; La₆Mn₂, n = 75), which have remarkable features: (1) Comprise a distinct 3d-4f-5d aggregate {La₆W₁₂M₂} (M = Co/Mn), showing a structure novel for the polyanion chemistry of heterometallic clusters; (2) Display an unusual parallelogram tetrameric structure with dimension of ca. 2.3 × 1.8 × 2.6 nm³ (Fig. S3 in Supporting information); (3) Represent the first Mn-4f heterometallic cluster in tungstoantimonate chemistry (Fig. S2 and Table S1 in Supporting information) [29, 30, 32-37] (4) Show excellent proton conductivity at 65 ℃ and 90% RH.

Trilacunary tungstoantimonate ([SbW₉O₃₃]⁹⁻) was chosen as the POM building block because it offers a well-known affinity towards 3d or 4f electrophiles. More importantly, the presence of lone-pair electrons can prevent the formation of a fully closed saturated structure, which is in favor of forming high polymeric compounds [38]. As shown in Scheme 1, La₆Co₂ and La₆Mn₂ were synthesized by the subsequent addition of LaCl₃·7H₂O and CoCl₂·6H₂O or MnCl₂·4H₂O into the in-situ formed [SbW₉O₃₃]⁹⁻ solution. Notably, the same reaction leads to the formation of complex {Sb₂W₂₀Co₂} (Scheme 1 and Fig. S4 in Supporting information) without addition of La³⁺ [39]. Furthermore, a small amount of {La₃Co₂} (Scheme 1 and Fig. S4) [30] and {Sb₂W₂₀Co₂} [39] can be also isolated after the crystallization of La₆Co₂. We also found that the more La³⁺ is added, the less {La₃Co₂} is formed. Luckily, the three compounds can be easily distinguished by their different colors (Scheme 1 and Fig. S4). However, La₆Mn₂ is exclusively formed in the La/Mn system, indicating that the metal ion may play an essential role in regulating composition of the structure, which in turn guides the formation of HMPOMs. To some extent, the successful synthesis of La₆Co₂ and La₆Mn₂ may "open" a synthetic route to a new class of heterometallic clusters, giving access to novel building blocks that can be universally used to assemble clusters with a remarkable range of structures and properties.

Scheme 1

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Scheme 1. The synthetic route of La6M2, highlightling the different processes of La6Co2, {Sb2W20Co2} and {La3Co2}.

Single-crystal structural analyses reveal that the polyanions of La₆Co₂ and La₆Mn₂ are isostructural and crystallize in the same monoclinic space group C2/c (Table S2 in Supporting information). Therefore, only the structure of the polyanion of La₆Mn₂ is described in detail. As shown in Fig. 1, this cluster can be viewed as a tetrameric Keggin polyoxotungstate, in which a huge 3d-4f-5d heterometallic cluster {[La₃W₆MnO₁₈(H₂O)₈(CH₃COO)₄]₂} ({La₆W₁₂Mn₂}, Fig. 1c) are wrapped by four inorganic ligands (two {β-B-SbW₉O₃₃} (Fig. 1a) and two {α-A-SbW₉O₃₃} (Fig. 1b)). The four Keggin fragments are linked to each other in a parallelogram-like manner (Fig. 1d and Fig. S3), in which the two identical fragments are located at the opposite vertex position. Each {β-B-SbW₉O₃₃} fragment is linked to {La₆W₁₂Mn₂} core by four W-O-W bridges and two Mn-O-W bridges, while two additional La-O-W bridges are required between {α-A-SbW₉O₃₃} cluster and {La₆W₁₂Mn₂} core (Figs. S5 and S6 in Supporting information). The {La₆W₁₂Mn₂} core contains two MnO₆ octahedra (Fig. 1e), two {W₆O₂₈} clusters (Fig. 1f) and two {La₃O₂₄} species (Fig. 1g), resulting the first observation of 3d-4f-5d heterometallic cluster in POM chemistry. All the W and Mn atoms have octahedral coordination of oxygen atoms, while the La atoms exhibit three different coordination geometries, nine-coordination tricapped trigonal prism (Fig. S7a in Supporting information), ten-coordination double-capped tetragonal antiprism (Fig. S7b in Supporting information), and nine-coordination single-capped tetragonal antiprism (Fig. S7c in Supporting information), respectively. Notably, La₆Mn₂ represents the first example of Mn-4f mixed-cluster in tungstoantimonate chemistry.

Figure 1

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Fig. 1. The structure of (a) {β-B-SbW9O33}, (b) {α-A-SbW9O33}, (c) {La6W12Mn2}, polyanion in (d) La6Mn2, (e) {MnO6}, (f) {W6O28} and (g) {La3O24}. Code: {WO6}, blue/green octahedra; C, gray spheres; O, red spheres; Sb, gray-purple spheres; W, green spheres; Mn, rosy spheres; La, violet spheres.

The structure of the polyanion of La₆Mn₂ can be also described as comprising two dimeric half-units [(SbW₉O₃₃)₂{La₃W₆MnO₂₁(H₂O)₈(CH₃COO)₄}]¹⁷⁻ (Fig. S5), in which two trilacunary tungstoantimonate fragments are supported on the unprecedented heterometallic complex [La₃W₆MnO₂₁(H₂O)₈(CH₃COO)₄]⁺ through one corner-shared {MnO₆} octahedra, four W-O-W bridges and two La-O-W bridges. Two half-units are fused together via two La centers, and the apparent 180° rotation of the half-unit leads to a staggered tetragon.

Bond valence sum (BVS) calculations (Table S3 in Supporting information) [40] indicate that the Sb, W, La, Co and Mn atoms in compound La₆Co₂ and La₆Mn₂ are all in the +3, +6, +3, +2 and +2 oxidation state, respectively. Furthermore, the BVS calculations for all oxygen atoms in La₆Co₂ and La₆Mn₂ enable us to identify the presence of the eight terminal aqua ligands with values of 0.28–0.38. The resulting [(SbW₉O₃₃)₄{La₃W₆MO₁₈(H₂O)₈(CH₃COO)₄}₂]²²⁻ (M = Co/Mn) polyanions are perfectly charge-balanced with 22 sodium cations, in agreement with the results of elemental analysis.

In La₆M₂, polyanions can be extended into a 2D metal framework if the peripheral Na⁺ ions are regarded as linkers (Fig. S9 in Supporting information). In addition, there are a large number of crystal water molecules in and between the 2D planes (Fig. S10 in Supporting information). Therefore, a hydrogen-bonded network would be formed by abundant water molecules, countercations and polyanions, used for proton transmission. Simultaneously, the unique "pseudoliquid pase" behavior of POMs [41] was also its unique advantage as a proton conductive material. Herein, the proton conductivity of La₆M₂ was investigated by AC impedance measurement using compacted pellets of the crystalline powder samples at relative humidity (RH) and temperature in a range of 45%−90% and 25–65 ℃, respectively. Figs. 2a and b show the real (Z′) and imaginary (-Z″) spectra of La₆M₂ in the RH range of 45%−90% at 25 ℃. Proton conductivity (σ) of La₆Co₂ increased from 2.3 × 10⁻⁴ S/cm to 3.2 × 10⁻³ S/cm when RH was increased from 45% to 90%. Under the same conditions, σ of La₆Mn₂ was increased from 3.8 × 10⁻⁴ S/cm to 2.6 × 10⁻³ S/cm, which was in the same order of magnitude as that of La₆Co₂. Similar conductivity was to be expected due to isomorphous crystals and similar void ratio of La₆Co₂ (28.1%) and La₆Mn₂ (27.8%). The water vapor adsorption tests of La₆M₂ showed positive interrelationships between water adsorption and RH (Fig. S11 in Supporting information). At 298 K and P/P₀ = 0.90, the water vapor adsorption capacity of La₆M₂ reached 74 cm³/g and 83 cm³/g, respectively. When maximum adsorption capacity was reached, proton conductivity also reached the maximum value, indicating that there was positive interrelationship between water adsorption and proton conduction. Furthermore, as shown in Figs. S12 and S13 (Supporting information), PXRD peaks of as-synthesized compound before and after proton conductivity measurement were basically in agreement with the corresponding simulated peaks, indicating that the structure of La₆M₂ remained intact during the measurement.

Figure 2

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Fig. 2. (a, b) Impedance spectra of La6M2 at 25 ℃ with 45%–90% RHs. (c, d) Impedance spectra at 90% RH with 25–65 ℃. (e, f) Arrhenius plots of proton conductivity for La6M2 under 90% RH.

Temperature-dependent conductivity measurements were carried out in temperatures ranging from 25 ℃ to 65 ℃ under 90% RH. Upon rising temperature to 65 ℃, conductivity increased to 1.3 × 10⁻² and 2.3 × 10⁻² S/cm for La₆Co₂ and La₆Mn₂, respectively (Figs. 2c and d). The elevated conductivities were attributed to the acceleration of water molecular motion at high temperatures [42]. Notably, at 65 ℃ and 90% RH, the conductivities of La₆M₂ show excellent conductivity among reported POM conductive materials [43, 44]. According to the Arrhenius equation (σT = σ₀exp(−E_a/k_bT)), activation energy (E_a) at 90% RH for La₆Co₂ and La₆Mn₂ were calculated to be 0.324 eV and 0.327 eV (< 0.4 eV) (Figs. 2e and f), respectively, suggesting that conduction proceeds were mainly dominated by Grotthuss mechanism [45, 46], and protons were hopping along conductivity pathway.

In summary, we report the construction of two unprecedented HMPOMs, in which four trilacunary tungstoantimonate building units are efficiently linked by the rare hybrid 3d-4f-5d cluster {La₆W₁₂M₂} (M = Co/Mn) to form a parallelogram tetrameric architecture. In our case, polyanion La₆Mn₂ represents the first heterometallic 3d-4f cluster in tungstoantimonate chemistry. More importantly, the new compounds are of interest in their excellent proton conductivity due to their 2D layer filled with water molecules, which is favorable to form an H-bonded network. The fact that 3d, 4f and 5d metals can be simultaneously incorporated into the lacunary POM structure extends the work to the systematic study of other 4f or 5f ions into this multicomponent system in order to achieve a better understanding of the interaction between lacuanry POM clusters and 3d/4f/5f centers.

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.

This work was supported by the National Natural Science Foundation of China (No. 22071045), the Excellent Youth Science Fund Project of Henan Province (No. 202300410042), the Natural Science Foundation of Henan Province (No. 232300420372), Henan University, and the State Key Laboratory of Physical Chemistry of the Solid Surface of Xiamen University.

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108486.