Communication pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
A Chromium-Substituted Polyoxoniobate with High Ionic Conductivity Zheng-Wei Guo,† Yi Chen,† Dan Zhao,‡ Yan-Lan Wu,† Li-Dan Lin,† and Shou-Tian Zheng*,† †
College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China Fuqing Branch of Fujian Normal University, Fuqing, Fujian 350300, China
‡
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S Supporting Information *
the study of the assembly of POMs with toxic Cr ions remains largely unexplored. In PONb chemistry, only a few Crcontaining PONbs, including {Cr 2Nb10}, {CrNb9}, and {CrNb6},7,12 have been isolated to date. Here, we report a new high-nuclearity Cr-substituted PONb, [Na 2 (H 2 O) 9 ] 2 {[Na 4 K 6 (H 2 O) 10 ][Cr 2.5 Nb 27.5 O 66 (OH) 20 (H2O)2]2}·nH2O (1; n ≈ 100), which has the following features: (1) Compound 1 is not only a rare Cr-containing PONb but also the largest molecular Cr-containing PONb known to date. (2) The molecular PONb polyoxoanion [Cr2.5Nb27.5O66(OH)20(H2O)2]7− (1a) in 1 has a brand-new structure. (3) Solid 1 exhibits high ionic conductivity. Compound 1 was obtained via a joint process of hydrothermal reaction and solvent evaporation (section 1 in the Supporting Information). First, a mixture of K7HNb6O19·13H2O, CrCl3· 6H2O and Na2CO3 was vigorously stirred in 10 mL of a 0.02 M Na2CO3/NaHCO3 buffer solution (pH 9) for ca. 1 h and then heated at 140 °C for 3 days. After cooling to room temperature, the filtrate was kept at room temperature for 2 weeks to generate light-green crystals. Compound 1 is insoluble in water, and the purity of bulk sample 1 was confirmed by powder X-ray diffraction (PXRD; Figure S1). The molecular structure of 1 consists of two unknown Crsubstituted 30-nuclearity hetero-PONbs 1a (Figure 1a), each of which was built from two kinds of subunits: four 6-nuclearity clusters {Cr 0. 5 Nb 5. 5 O 19 } and one 6-nuclearity ring {Cr0.5Nb5.5O30}. As shown in Figure 1b, each 6-nuclearity subunit {Cr0.5Nb5.5O19} can be described as a Lindqvist-type cluster, with its one metal site simultaneously occupied by both Cr and Nb. The four disordered metal sites from the four {Cr0.5Nb5.5O19} subunits are refined freely to give Cr/Nb ratios of 0.521/0.479, 0.533/0.467, 0.505/0.495, and 0.519/0.481, thus all of which are fixed to 0.5/0.5. By sharing the terminal O atoms from the disordered Cr0.5Nb0.5O6 octahedra, every two {Cr0.5Nb5.5O19} subunits are connected together to form a dimer of {CrNb11O37} (Figure S2). Different from the {Cr0.5Nb5.5O19} composed of six edge-sharing MO6 octahedra (M = Nb or Cr), the 6-nuclearity {Cr0.5Nb5.5O30} subunit contains six corner-sharing MO6 octahedra to give a circular structure (Figure 1c), in which two of the six metal sites are found to be simultaneously occupied by both Cr and Nb. These disordered sites are refined freely to give Cr/Nb ratios of 0.240/ 0.760 and 0.242/0.758. Thus, both of them are fixed to 0.25/ 0.75. Besides single-crystal X-ray analysis, the amount of Cr ions
ABSTRACT: A rare and novel Cr-substituted polyoxoniobate (PONb), [Cr2.5Nb27.5O66(OH)20(H2O)2]7−, has been synthesized by a combination of hydrothermal and conventional solution methods. The PONb shows an unknown tetrameric structure, which is the largest Crcontaining PONb to date. Interestingly, every two PONb tetramers can be joined together by alkali-metal cations to form a discrete cubelike ionic cluster. The obtained PONb not only enriches the very limited members of the Crsubstituted PONb family but also exhibits high ionic conductivity.
T
he incorporation of transition metals (TMs) into polyoxometalates (POMs) has been attracting attention because of their intriguing structural diversity and composite properties derived from the combination of TMs and POMs.1−3 Up to now, although a large number of TM-substituted POMs with various shapes, sizes, and compositions had been isolated, the main achievement was obtained in polyoxotungstate (POT) chemistry. The synthesis of new polyoxoniobates (PONbs) is of great interest because of their promising applications in basecatalyzed reactions, photocatalysis, virology, and nuclear-waste treatment.4−12 However, because of the relatively low reaction activity of niobate species, the development of PONbs lags far behind that of lacunary POTs. In recent years, the introduction of TMs into PONbs represents a feasible way to diversify the structures of the very limited PONb family. Some representative examples include Ti-containing PONbs {Ti12Nb6}, {TiNb9}, and {Ti2Nb8},6 Mn-containing PONb {MnNb9},7 Co-containing PONbs {Co14Nb56} and {Co8Nb24},8 Cu-containing PONbs {Cu24Nb56} and {Cu25.5Nb56},9 and the largest heterometal-containing PONbs {Ln12W12Nb72} (Ln = Y, La, Sm, Eu, Yb).10 Recently, Niu, Wang, and co-workers also reported a new Fe-containing PONb, {Fe3Nb25}, and a new Lncontaining PONb, {Eu3Nb48}.11 Nevertheless, the integration of TMs and PONbs is challenging, and known TM-containing PONbs are still limited because of the strong basic reaction conditions (usually >10) required for the dissolution of starting niobate species, under which most TM ions easily precipitate as hydroxides, especially for relatively highly charged (>2+) metal ions. Additionally, during the past 2 decades, chemists have made great efforts to study the assembly of POMs with nontoxic or low-toxicity bivalent metal ions, such as Co2+, Ni2+, and Cu2+. By contrast, © XXXX American Chemical Society
Received: December 19, 2018
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DOI: 10.1021/acs.inorgchem.8b03529 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 2. Comparison of the known trimeric PONb {Nb24O72} and tetrameric PONb {Nb32O96} with tetrameric PONb 1a.
[Cr2.5Nb27.5O66(OH)20(H2O)2]2} (1b), composed of 2 1a and 10 alkali-metal ions. It is worth noting that the cubelike structure of the octamer-like polyanion 1b is reminiscent of that seen in {Cu24Nb56}.9 Bond valence sum (BVS) calculations13 indicate that 20 of 88 O atoms from 1a are OH groups with BVS values in the range of 1.1−1.5, and the two terminal O atoms of the two disordered Cr/Nb sites from the circular {Cr0.5Nb5.5O30} subunit are H2O with a BVS value of 0.18 (Figure S5). Thus, the charges of polyoxoanion 1a and its dimeric ionic cluster 1b are 7− and 4−, respectively. Each huge ionic cluster 1b is exactly chargebalanced by two discrete dimeric Na−H 2 O clusters [Na2(H2O)9]2+. In 1, 1b clusters are stacked with a parallel packing pattern along the a-, b-, and c-axis directions to form the overall 3D structure, where the [Na2(H2O)9]2+ dimers fill the gaps between 1b clusters (Figure 3). As an inorganic ionic compound of 1, it is valuable to study its ionic conduction properties. First of all, the dependence on the relative humidity (RH) of compound 1 was tested over a range of 55−98% at 30 °C. At 55% RH, the conductivity (σ) was
Figure 1. Views of the structures of (a) polyoxoanion 1a, (b) subunit {Cr0.5Nb5.5O19}, (c) subunit {Cr0.5Nb5.5O30}, and (d) an alkali-metalbridging ionic cluster in 1.
is further confirmed by inductively coupled plasma analysis (Table S1). Additionally, the oxidation states of all Cr ions in 1 have been determined as 3+ by X-ray photoelectron spectroscopy measurement (Figure S3). Further, two {CrNb 11 O 37 } dimers and one 6-ring {Cr0.5Nb5.5O30} fragment are joined together via a corner- and edge-sharing mode to give rise to a new 30-nuclearity Crsubstituted PONb, 1a. Because of the disordered distribution, the average Nb/Cr−Oμ−2 bond length [1.823(5) Å] and the average Nb/Cr−Oμ−3 [2.178(3) Å] are shorter than the average Nb−Oμ−2 bond length [1.987(3) Å] and the average Nb−Oμ−3 bond length [2.398(5) Å], respectively. Notably, polyoxoanion 1a not only enriches the very limited family of Cr-substituted PONbs but also represents so far the largest single molecule of Cr-substituted PONb. Additionally, it has been known that trimeric PONb {Nb24O72} is composed of three Lindqvist-type {Nb6O19} clusters and one circular 6-nuclearity ring {Nb6O30} and known tetrameric PONb {Nb32O96} is comprised of four Lindqvist-type {Nb6O19} clusters and one circular 8-nuclearity ring {Nb8O42}.4c It is interesting to point out that polyoxoanion 1a, consisting of four Lindqvist-type {Cr0.5Nb5.5O19} clusters and one 6-nuclearity ring, seems to be an interphase between {Nb24O72} and {Nb32O96} (Figure 2). The linkage between the {Cr0.5Nb5.5O19} clusters and the 6ring {Cr0.5Nb5.5O30} generates a hole in the center of structure 1a, which can trap two K+ ions to give a bi-K-decorating PONb, {K2(H2O)4Cr2.5Nb27.5O66(OH)20(H2O)2}. Further, every two {K2(H2O)4Cr2.5Nb27.5O66(OH)20(H2O)2} can be held together by two additional K+ and four Na+ ions via ionic bonds to form a large cubelike discrete nanocluster, {[Na 4 K 6 (H 2 O) 10 ][Cr2.5Nb27.5-O66(OH)20(H2O)2]2}, with a size of 1.6 × 1.9 × 2.0 nm3, which cannot be further linked by alkali-metal cations into an infinite extended structure (Figure S4). Therefore, compound 1 also can be described as a rare 70-nuclearity ionic molecular cluster, {[Na4K6(H2O)10]-
Figure 3. 3D stacking structure of 1b and {Na2(H2O)9} in 1. B
DOI: 10.1021/acs.inorgchem.8b03529 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry evaluated to be 5.9 × 10−5 S cm−1 and increased slowly to 6.6 × 10−4 S cm−1 when the humidity reached 98% (Figure 4a).
channels might attribute to the high conductivity. Some similar POM solids with high conductivity and the vehicle mechanism have also been reported.16 Finally, PXRD patterns confirmed that the structure of 1 remains intact before and after ionic conduction experiments (Figure S6). In conclusion, we succeeded in synthesizing a rare Crsubstituted high-nuclearity PONb, 1a, with a brand-new structure. The synthesis of new Cr-substituted PONbs has long been a challenge and has been limited to a few relatively low-nuclearity structures. PONb 1a reported here enriches the very limited members of the Cr-substituted PONb family. Especially, 1a is composed of 30 metal atoms, which is the largest single-molecular Cr-substituted PONb. Additionally, negatively charged 1a can interact with alkali-metal cations to give a larger ionic molecular cluster 1b, which is cocrystallized with binuclear sodium−water clusters. Besides the novel structure, the ionic conduction experiments reveal that solid 1 may be an excellent ionic conductive material.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03529. Experimental details and additional tables, figures, and characterizations (PDF) Accession Codes
CCDC 1885821 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shou-Tian Zheng: 0000-0002-3365-9747 Notes
The authors declare no competing financial interest.
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Figure 4. (a) Nyquist plots for 1 at different RHs and T = 30 °C. (b) Nyquist plots for 1 at different temperatures and 98% RH. (c) Arrhenius plots of the conductivity of 1.
ACKNOWLEDGMENTS This work was financially supported by the NSF of China (Grants 21773029 and 21371033).
Meanwhile, the temperature dependence was measured over the range 30−70 °C with the RH at 98%. As the temperature changed from 30 to 70 °C, the Nyquist plots revealed that the conductivity value received a continuous promotion and reached a maximum of 6.4 × 10−2 S cm−1 (Figure 4b), which is among the best values reported thus far for POM materials (Table S2).14 Further, on the basis of the Arrhenius formula (σT = σ0 exp(−Ea/kbT), the activation energy of 0.96 eV was obtained by the linear regression analysis, which indicated that conduction was mainly carried out by the “vehicle” mechanism (Figure 4c).15 Generally, alkali-metal ions are likely to transmit through direct diffusion, which is the main transmission model of the vehicle mechanism. Additionally, the vehicle mechanism does not require an infinite hydrogen-bonding network, which is consistent with the structural feature of ionic solid 1. So, in 1, lots of discrete alkali-metal ions, water molecules, and large
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DOI: 10.1021/acs.inorgchem.8b03529 Inorg. Chem. XXXX, XXX, XXX−XXX
