Giant Ln 30 -Cluster-Embedded Polyoxotungstate Nanoclusters with Exceptional Proton-Conducting and Luminescent Properties

Abstract

Open AccessCCS ChemistryCOMMUNICATION5 Sep 2022Giant Ln30-Cluster-Embedded Polyoxotungstate Nanoclusters with Exceptional Proton-Conducting and Luminescent Properties Zhong Li, Zhi-Hao Lv, Hao Yu, Yan-Qiong Sun, Xin-Xiong Li and Shou-Tian Zheng Zhong Li State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108 Google Scholar More articles by this author , Zhi-Hao Lv State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108 Google Scholar More articles by this author , Hao Yu State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108 Google Scholar More articles by this author , Yan-Qiong Sun State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108 Google Scholar More articles by this author , Xin-Xiong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108 Google Scholar More articles by this author and Shou-Tian Zheng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101573 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Although the synthesis of giant lanthanide (Ln)-containing polyoxotungstates (POTs) has long been actively pursued, it remains challenging to encapsulate high-nuclearity Ln–O clusters in POT shells because of the highly complicated assembly process. This work demonstrates a simple synthetic strategy of using lacunary POT precursors to assemble with in situ generated Ln–O clusters in the presence of basic N-containing ligands, which exert a slow-releasing effect for Ln3+ ions in alkaline conditions. With this strategy, two giant Ln30-cluster-embedded polyoxometalates (POMs) H38Na10K14(TMEDA)8[Ln30Ge12W107O420(OH)2(H2O)14]·solvents ( 1Ln) (Ln = Sm, Eu) have been successfully synthesized, which not only represent two rare high-nuclearity POM macromolecules with over 100 W atoms but also a unique Ge2W7⋐Ln30⋐W100 multishell configuration. The incorporated Ln–O clusters in [Ln30Ge12W107O420(OH)2(H2O)14]62− ( Ln30W107) with a nuclearity of 30 are larger than any known Ln-clusters observed in POM chemistry. Additionally, they contain the largest number of Sm and Eu centers for any POMs reported to date. Interestingly, the unique structural features and the high-nuclearity Ln–O clusters endow Ln30W107 with excellent proton conductivities over a wide temperature range and unusual luminescence properties. Download figure Download PowerPoint Introduction Diverse magnetic and electronic properties of 4f lanthanide (Ln) ions have rendered the Ln-containing polyoxotungstates (POTs) a prime target in new polyoxometalates (POMs) material synthesis over the past 20 years.1–18 Of particular interest is the creation of POTs with a high number of Ln centers or high-nuclearity Ln–O clusters because of their extraordinary structural features and attractive electronic, optical, and magnetic properties (e.g., large uniaxial anisotropy parameter, multiple configurations, and high-spin ground state) relevant to appealing applications in many fields such as single-molecule magnets, photoelectric molecular materials, and high-density information storage.12–14 However, the discovery of such POTs proves challenging and is usually by trial and error because their syntheses involve remarkably complex assembly processes and are hard to predict or design. To date, the known POTs with more than 20 Ln centers are restricted to several examples, such as {Ce20W100},15 {Ce24 W120},16 {Ln27W106},17 and {Dy30Co8W108}.18 A survey of reported giant Ln-containing POTs from other groups indicates two characteristic phenomena. One is that nearly all POTs were obtained in acidic solutions due to the difficulty in synthesizing Ln-containing POTs under basic reaction conditions, which often leads to fast precipitation of highly oxophilic Ln ions ( Supporting Information Table S1). The other is that the Ln ions incorporated in almost all those giant POTs are spatially separated from each other by POT polyoxoanions instead of being joined together to form polynuclear Ln–O clusters. Most likely the acidic reaction conditions do not favor the formation of the O2− or OH− intermediates required for the condensation formation of Ln–O clusters, and thus oxophilic Ln ions prefer to coordinate with POT polyoxoanions and act as linkers between POTs rather than form clusters ligated by POTs. In contrast, alkaline conditions might be more beneficial for forming large Ln–O clusters if we could effectively control Ln hydrolysis and avoid the formation of precipitates. Based on the above, we have been motivated in the past few years to create giant Ln–O clusters embedded POTs under alkaline conditions through a simple synthetic strategy of using lacunary POT precursors to assemble with in situ generated Ln–O clusters in the presence of basic N-containing ligands (e.g., ethylenediamine, imidazole, piperazine, and so on).17,19–21 The use of basic N-containing ligands is crucial for the following reasons: (1) they offer moderate alkaline reaction conditions for the clustering of Ln3+ ions without the introduction of hydroxides; (2) they exert a slow-releasing effect for Ln3+ ions (i.e., provide a low concentration of Ln3+ ions) via coordinating N-donors to avoid the rapid precipitation of Ln3+ ions in alkaline solutions, and (3) they form relatively weak coordination interactions with oxophilic Ln ions and avoid the formation of stable Ln-organic complexes. With this strategy, giant 27-Ln-106-W-containing POTs {Ln27Ge10W106} (Ln = La and Ce) were obtained in 2017.17 The success of this strategy offers a tantalizing hope that it might be possible to realize a greater level of Ln–O cluster aggregation. Herein, we report two giant Ln30-cluster-embedded POTs H38Na10K14 (TMEDA)8[Ln30Ge12W107O420(OH)2 (H2O)14]·solvents ( 1Ln) (Ln = Sm, Eu), which are obtained from moderate alkaline reaction conditions by using weakly coordinating and highly sterically hindered N,N,N′,N′-tetramethylethylenediamine (TMEDA) as a basic ligand for the regulation of reaction alkalinity and slow release of Ln3+ ions. Besides being a rare giant POMs with more than 100 W centers, the polyoxoanions [Ln30Ge12W107O420(OH)2(H2O)14]62− ( Ln30W107), encapsulating a kind of basket-like 30-nuclearity Ln30 cluster, represent the POMs with the highest-nuclearity Ln–O clusters reported thus far. What is more, the nuclearity of Ln30 is also larger than those of all previously known high-nuclearity transition metal clusters (e.g., Mn19,22 Co16,23 Ni25,24 Cu20,25 Ag18,26 Fe28,27 and so on28–31) that POMs can stabilize. Furthermore, compounds 1Ln show high proton conductivity from −40 to 85 °C and irregular temperature-dependence luminescence. Results and Discussion Polyoxoanion Ln30W107 was obtained from the reaction of K8Na2[A-a-GeW9O34]·25H2O32,33 with Ln(NO3)3·6H2O and TMEDA in water at 140 °C and pH 8.2 for 3 days. The formulations were determined by single-crystal X-ray diffractions and inductively coupled plasma analyses ( Supporting Information Table S2). To balance charges, 38 protons must be added to 1Ln, but they could not be located and were assumed to be delocalized on the overall cluster structures, which is common in POMs.15–31 Given Ln30W107 are analogs, the detailed structural description is provided only for Eu30W107. One uncommon feature of Eu30W107 is that the outmost shell consists of as many as 10 dilacunary POTs. To date, intercluster aggregation of more than 10 lacunary POTs is quite rare in POMs chemistry.12–17,34–36 Of these limited nanoclusters, only three examples contain more than three kinds of lacunary POTs, including {Ce24W120},16 {Ln27W106},17 and {Mn40W224}.36 Eu30W107 represents the fourth example, where 10 lacunary POTs are divided into three types derived from different saturated Keggin POTs, including four {α(1,5)-GeW10O38} (α(1,5)), four {α(1,8)-GeW10O38} (α(1,8)), and two {β(4,11)-GeW10O38} (β(4,11)) (Figure 1a).15–17 Although these lacunary POTs are known, they have been found for the first time to exist simultaneously within a molecular cluster. As shown in Figure 1b, the arrangement of the 10 POTs are vesicle-like distorted elongated square bipyramids with two β-Keggin divacant β(4,11) POTs being at the two opposite end vertices surrounded by four triangular faces and eight α-Keggin divacant POTs (four α(1,5) and four α(1,8)) being at the eight side vertices surrounded by two square faces and two triangular faces, leading to an approximate C2-symmetry arrangement with the C2 axis passing through the centers of a pair of opposite square faces. Figure 1 | (a) View of two kinds of saturated Keggin POTs and three lacunary derivations found in Eu30W107. (b) View of the arrangement of the 10 POTs in the vesicle-like Eu30W107. GeO4, yellow; WO6, red, green, or purple. Download figure Download PowerPoint Another uncommon feature of Eu30W107 is that the vesicle-like arrangement of the 10 lacunary POTs encloses an irregular 30-nuclear Eu cluster {Eu30O160(OH)2(H2O)14} (Eu30) formed by five Eu–O subunits (Figure 2a), including two 5-nuclear {Eu5O25 (H2O)3} (Eu5), one 6-nuclear {Eu6O34(OH)2(H2O)4} (Eu6), and two 7-nuclear {Eu7O40(H2O)2} (Eu7) (Figure 2b). The Eu5 comprises a triangular Eu3O17 trimer of three face-sharing EuO8 bicapped trigonal prisms with two EuO8 polyhedra further face-sharing with one EuO6(H2O)2 bicapped trigonal prism and one EuO8(H2O) tricapped trigonal prism ( Supporting Information Figure S1). The Eu6 displays a C2-symmetric bar-shaped hexamer made up of two {Eu2O11(OH)(H2O)2} dimers connected by one {Eu2O12(OH)2} dimer via two Eu–OH–Eu bridges, in which the dimer at both ends comprises one bicapped trigonal prism EuO7(OH) and one tricapped trigonal prism EuO7(H2O)2 combined via face-sharing. The middle dimer is formed by two edge-sharing EuO7(OH) bicapped trigonal prisms ( Supporting Information Figure S1). The Eu7 can be regarded as two edge-sharing triangular trimers of {Eu3O19(H2O)} and {Eu3O18(H2O)} attached by an additional EuO9 tricapped trigonal prism. In Eu7, the {Eu3O19(H2O)} is composed of three face-sharing tricapped trigonal prisms (two EuO9 and one EuO8(H2O)), while the {Eu3O18(H2O)} consists of three face-sharing Eu–O polyhedra of two EuO9 tricapped trigonal prisms and one EuO7(H2O) bicapped trigonal prism that shares a face with the additional EuO9 polyhedron ( Supporting Information Figure S1). All the OH and H2O groups in Eu30 are also identified by bond valence sum calculations ( Supporting Information Table S4). Figure 2 | Structures of Eu30W107(a) and Eu30 (b), respectively. α(1,5), green; α(1,8), purple; β(4,11), red; GeO4, yellow; WO6, red, green, or purple; Eu5, turquoise; Eu6, bright green; Eu7, blue. Download figure Download PowerPoint The whole Eu30 presents a unique basket-like structure with an approximate C2-symmetry. The “basket body” is formed by a pair of Eu7 units oriented 180° for each other ( Supporting Information Figure S2), which are joined together in a parallel double-stranded fashion by sharing two vertices. Then, two Eu5 units, acting as a “basket handle”, are opposite each other and attached to the two sides of the “basket body” through vertex-sharing with Eu7 units. Finally, the Eu6 unit is in the basket via edge-sharing with the two Eu7 units, giving the overall Eu30 with the Eu-O/OH/H2O bond lengths ranging from 2.241(1) to 2.978(1) Å. Interestingly, the large inner cavity of basket-like Eu30 is partitioned into multiple C2-related small voids by the bar-shaped Eu6 across the cavity, allowing the intercalation of small structural units in pairs. As shown in Supporting Information Figure S3, two W2O10 dimers of two edge-sharing WO6 octahedra are trapped in the side voids of the basket via edge-sharing with a Eu5, a Eu6, and a Eu7 units, while two WO6 octahedra fill in the bottom voids of the basket via edge-sharing with a Eu7 unit and corner-sharing with a Eu6 and a Eu7 unit. Except for W–O polyhedra, two GeO4 tetrahedra also reside in the top voids of the basket via edge-sharing with a Eu5 unit and corner-sharing with a Eu5 and a Eu6 unit. Notably, an additional WO6 unit inserts into the gap between the two Eu5 units and connects them by vertex-sharing with one Eu5 and face-sharing with another Eu5. The asymmetric arrangement of the additional WO6 between Eu5 units reduces the symmetry of Eu30W107 from C2-symmetry to E-symmetry. The proton conductivity of many POM materials can exceed 10−2 S cm−1 at moderate temperatures (50–100 °C) ( Supporting Information Table S3), but usually drops dramatically at subzero temperatures due to water solidification, which limits their widespread applications. The good stability and the presence of rich protons in 1Ln point to using them as proton conducting materials. Thus, proton conductivities of 1Ln were measured by alternating current impedance spectroscopy using a two-electrode configuration between 107 and 1 Hz. First, the humidity-dependent conductivities were measured at room temperature (RT) (25 °C). The results showed that the conductivities of 1Sm and 1Eu are 7.48 × 10−4 S cm−1 and 6.54 × 10−4 S cm−1 at 55% relative humidity (RH), respectively, which increase to 3.27 × 10−3 S cm−1 and 2.51 × 10−3 S cm−1 at 98% RH (Figure 3a and Supporting Information Figure S4). The proton conductivities of 1Ln at 55% RH and RT are among the highest reported for POMs at low RH and RT. Additionally, high proton-conducting POMs usually display at least one order of magnitude enhanced proton-conducting ability as the RH increases from low RH (e.g., 55%) to 98% at RT.37 Whereas, upon varying the RH from 55% to 98%, the conductivities of 1Ln increase about four times, indicating their relatively low humidity dependency. Second, the temperature-dependent proton conductivities were measured in the range of 25–85 °C at 98% RH, indicating that the conductivities of 1Sm and 1Eu reach 2.05 × 10−2 S cm−1 and 1.95 × 10−2 S cm−1 at 85 °C (Figure 3b and Supporting Information Figure S4), respectively, which are also among the highest reported for POMs ( Supporting Information Table S3). We attribute the excellent proton conductivity to the presence of rich proton carriers within the structure of 1Ln, including water molecules, terminal oxygen atoms, and organic amines, which help to form hydrogen-bonded proton “hopping” networks.38,39 Figure 3 | Proton conductivity of 1Sm. (a) RH-dependent Nyquist plots at 25 °C. (b and c) Temperature-dependent Nyquist plots at 98% RH. (d) Arrhenius plots and the linear least-squares fit at 98% RH. Download figure Download PowerPoint The relatively low humidity-dependent characteristic and high proton conductivity incited us to further explore the conducting performance of 1Ln at subzero temperature. Given that the proton-conducting properties of 1Sm are slightly superior to 1Eu, 1Sm was chosen as the representative for the subzero conducting test in the temperature range of −40 °C to 0 °C. Interestingly, 1Sm showed a high proton conductivity of 1.78 × 10−5 S cm−1 even at −40 °C (Figure 3c), which is much better than known crystalline POM proton conductor at −40 °C.11 The activation energy (Ea) of 1Sm and 1Eu were calculated by Arrhenius equation σT = σ0 exp(Ea/kbT) to be 0.30 and 0.32 eV (Figure 3d and Supporting Information Figure S4), respectively, suggesting that their proton transfer processes follow the Grotthus mechanism where typically the Ea is below 0.4 eV.37–39 Compared with the Ea (0.3 eV) above RT, the Ea of 1Sm under subzero temperature increases to 0.38 eV, still belonging to the Grotthus mechanism (Figure 3d and Supporting Information Figure S5). Powder X-ray diffraction after testing confirmed that structures of 1Ln remained intact throughout the whole measurement process ( Supporting Information Figure S6). Ln-containing POMs have drawn abundant attention owing to their intriguing luminescence sensitized by energy transfer via oxygen ligand to metal charge-transfer.40,41 However, due to the difficulty in synthesizing POMs containing high-nuclearity Ln–O clusters, particularly for those with intense luminescence, their luminous performance remains largely unexplored. Therefore, the luminescence properties of 1Ln were studied and all exhibit their characteristic emissions (Figure 4 and Supporting Information Figure S7). Due to attractive optical properties of Eu3+, 1Eu was chosen for detailed description of its luminescence behaviors. When monitoring Eu3+ emission at 615 nm, the excitation spectrum of 1Eu showed a band maximum around 395 nm (Figure 4a), corresponding to the 7F0→5L6 transition of Eu3+ ion. Under 395 nm excitation, 1Eu exhibited the characteristic 5D0–7FJ (J = 0–4) transitions of Eu3+ at 579, 594, 615, 652, and 701 nm (Figure 4b). The quantum yield (Ф) and luminescence lifetime (τ) for the supreme 5D0→7F2 emission peak were monitored. The Ф value of 1Eu is 39.09% (Figure 4c), and its decay curve can be well fitted by a single exponential function as I = A exp (−t/τ) with τ = 625 μs (Figure 4d). Figure 4 | Solid-state luminescence of 1Eu. (a) Excitation spectrum. (b) Emission spectrum. (c) Quantum yield. (d) Lifetime decay curves. (e and f) Temperature-dependence luminescence spectra. Download figure Download PowerPoint Generally, luminescence intensity of the compound relates largely to its ambient temperature due to non-radiative relaxation, energy transfer, or crossover processes. Interestingly, the luminescence of 1Eu is little affected by temperatures below RT but quite sensitive to temperatures above RT. As shown in Figures 4e and 4f, the temperature-dependent luminescence spectra of 1Eu were collected in the ranges of 77–300 K and 298–493 K, which revealed that its fluorescence intensity only slightly decreased by 0.02% per K upon increasing the temperature from 77 K to RT but decreased drastically by 4.77% per K from RT to 493 K. Generally, luminescence intensity of reported Ln-containing POMs decreases significantly with increasing temperature42–44; however, the luminescence intensity of 1Eu was almost undisturbed in the temperature ranges of 77–300 K. We attribute this to its high structural stability and crystallinity that can effectively suppress the non-radiative energy loss when the temperature varies in the low temperature range43–45 but also exhibits different temperature-dependence luminescence performances below RT and above RT ( Supporting Information Figures S8–S13). The temperature-persistent luminescence might endow 1Eu with the intriguing potential of modulating its response toward various gas sensitive monitoring or gas phase analyte at low temperature environment. Conclusion Giant Ln30-cluster embedded POTs Ln30W107 have been successfully obtained via the combination of lacunary POTs and Ln cations with basic N-containing organic ligands, illustrating the vast potential of such a combination in assembling giant Ln-containing POTs. Ln30W107 present new record high-nuclearity Ln–O aggregates in POMs chemistry. Compounds also have many interesting features including unique molecular architectures, excellent proton conductivity over a wide temperature range and charming temperature-dependence luminescence. This work demonstrates that the key to isolating giant Ln-containing POTs lies in using basic N-containing organic ligands to exert a slow-releasing effect for Ln3+ ions and offer moderate basic reaction conditions that favor the formation of the Ln–O clusters and the accompanying self-assembly reaction to give giant Ln-containing POTs, which are promising molecular materials for potential applications in the fields of luminescence, electricity, and magnetism. Supporting Information Supporting Information is available and includes additional experimental details, X-ray crystallographic data, extra structural figures for Eu30W107 ( Supporting Information Figures S1–S3), and additional figures for basic characterization and properties ( Supporting Information Figures S4–S13). Conflict of Interest The authors declare no conflict of interest. Funding Information We gratefully acknowledge the financial support from the NSFs of China (grant nos. 21773029, 21971039, and 22171045).