O Cage with Selective

Aug 15, 2017 - Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical En...
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Assembly of TeO32− Ions Embedded in an Nb/O Cage with Selective Decolorization of Organic Dye Zhijie Liang, Junjun Sun, Dongdi Zhang, Pengtao Ma, Chao Zhang, Jingyang Niu,* and Jingping Wang* Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004 P. R. China S Supporting Information *

ABSTRACT: A novel 24-niobic-2-tellurite, [H2Te2Nb24O72]14−, was isolated by incorporating tellurite anions into a polyoxoniobate cage. The synthesized cluster represents the first example of a sandwich-type polyoxoniobate with the largest telluroniobate aggregate. Furthermore, the hybrid material acts as an efficient catalyst for the decolorization of basic fuchsin.

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eteropolyoxometalates (HPOMs) are well-known because of their wide range of tunable electronic and physical properties, which result in their extensive use in catalysis, materials science, and even in medicine.1−4 Its important to select heteroanion templates for the synthesis and structural isolation of HPOMs, which might play an important role in obtaining novel molecules. In polyoxotungstates, several excellent researches explored whether a redox-active heteroanion (TeO32−) was effective in templating diverse structures in the self-assembly process. 5−10 However, heteropolyniobate (HPON) chemistry, one set of HPOMs, remains relatively unexplored compared to their close periodic neighbors W, Mo, and V. In fact, after Nyman’s initial discovery in 2002,11 some more compounds based upon “conventional” heteroanion templates (e.g., SiO44−, PO43−, GeO44−, and AsO43−) have been reported, mostly with a niobate Keggin framework (Figure S1). In contrast, the heteroanion (TeO32−) with a lone pair of electrons embedded in HPON has not yet been reported. Thus, it is possible to generate fascinating HPON architectures whereby the structures are directed by the tellurite anions. Besides this, disposal of organic dyes is a matter of serious concern because of their poor biodegradation, toxicity, and unsightliness.12 In recent years, polyoxometalates have emerged as potential catalysts in the degradation of organic dyes.13,14 It is thus meaningful to explore a heterogeneous system with good recyclability. Herein, we report a novel 24-Nb−O cage encapsulating two pyramidal [TeIVO3]2− templates: [H2Te2Nb24O72]14− (1a) isolated as K4[{Cu(en)2(H2O)}4{Cu(en)2}(H2Te2Nb24O72)]· 8H2O (1), which represents the first example of a telluirumcontaining sandwich-type HPON with an unprecedented hexaniobium ring in the sandwiched sites (Figure 1a). It is worth noting that 1 catalyzed the decolorization of basic fuchsin (BF) and could be isolated for reuse. Compound 1 was synthesized from an alkaline mixture of K7HNb6O19·13H2O with TeO2 to which copper amine cations were subsequently added.15 The phase purity of 1 was © 2017 American Chemical Society

Figure 1. (a) Ball-and-stick representation of 1a. (b) Polyhedral and ball-and-stick representations of two [TeNb9O33]17− fragments. (c) Side view of [Nb6O24]18−. (d) Top view of the large cavity.

characterized by the powder X-ray diffraction (PXRD) pattern (Figure S6). The unprecedented 24-niobic-2-tellurite polyanion presents four interesting remarkable features: (1) it is the largest telluroniobate aggregate isolated so far among the existing HPON family; (2) the {TeNb9} building block is observed for the first time; (3) six Nb atoms occupy the “belt” positions, displaying a chair-conformation-like cyclohexane; (4) it represents the first example of a hexaniobium sandwich-type polyoxometalate.16,17 A detailed crystallographic analysis reveals that 1a consists of two B-α-[TeNb9O33]17− (TeNb9) subunits and a central distorted ring shape [Nb6O24]18−. The core TeNb9 is derived from the self-assembly of TeO2 and [Nb6O19]8−, where a Te atom links to three O atoms from the edge-shared triad of NbO6 Received: April 4, 2017 Published: August 15, 2017 10119

DOI: 10.1021/acs.inorgchem.7b00860 Inorg. Chem. 2017, 56, 10119−10122

Communication

Inorganic Chemistry octahedra, respectively. The [TeO3]2− ions, with a lone pair of electrons acting as heteroanion templates, bonded to three O atoms in a trigonal configuration, effectively giving rise to “open” lacunary units instead of “closed” Keggin-type clusters. Besides, because of the large radius of Te4+, the two TeNb9 building blocks are hindered upon joining together. The TeNb9 subunit is observed in a polyoxoniobate (PON) for the first time, although it is common in polyoxotungstate.18−20 It is noted that two TeNb9 subunits are linked by Nb−O bridges in a face-to-face manner, with a “half-unit” rotating by about 60° (Figure 1b). The slight difference lowers the symmetry of 1a from D3h to C3v. Also, the heteroatoms are located on a crystallographic 3-fold axis with a Te···Te (virtual) distance of 5.1273(10) Å. The metallaringtype moiety, [Nb6O24]18−, is a hinge chelating to two TeNb9 ligands (Figure 1c), in which six Nb atoms are connected in an orderly model, giving rise to an intriguing chair configuration if all of the O atoms are omitted (Figure S2). Alternatively, six NbO6 moieties match extremely well with the structure of the crown ether 18-crown-6 (Figure S3). In this way, compound 1 represents the first member of sandwich-type telluriumcontaining PON, interestingly differing from the traditional sandwich type in the sandwiched sites.21,22 Given the six equatorial Nb atoms, 1a can also be seen as a niobium cage embedding in two pyramidal tellurite anions [TeO3]2− with an average Te−O distance of 1.86 Å. Furthermore, the linkages among Nb, O, and Te atoms create an oval cavity (Figure 1d), as reflected by a deviation of ca. 1.5 Å between the “length” of 6.6 Å (radius of the O center in the plane of three Nb centers and one Nb atom) and the “width” of 5.1 Å (diameter of opposite Te centers perpendicular to the previous plane). Therefore, the compound is also the second but largest telluroniobate aggregate in the literature to date. In addition, copper−organic fragments have been successfully grafted onto the {Te2Nb24O72} cluster, forming a ribbon-like geometry (Figure 2a); K cations sew together the strands to form

a two-dimensional plane (Figure 2b). The two protons should be added for charge-balance considerations and bond-valence-sum calculations.23 The protons maybe delocalize over the entire architecture (Table S3 and Figure S5). The formula is verified further based on different analytical methods, namely, elemental analysis (C, H, and N), IR, thermogravimetric analysis (TGA), and UV−vis (Figures S7−S10). The ability of 1 to catalyze the decolorization of BF was studied. In a blank experiment, the decolorization rate of a BF solution was only 4.59% for 1 h. The decolorization rate can achieve 92.35% by the addition of 1.6 × 10−5 mol of 1 (Figure 3a,b). Subsequently, if the addition of 1 was increased to 2.4 ×

Figure 3. (a) Absorption spectra of a BF aqueous solution in the presence of compound 1 (1.63 × 10−5 mol). (b) Conversion of BF (K) with the reaction time (t) of 1. The conversion of BF (K) can be expressed as K = (C0 − Ct)/C0, where C0 represents the UV intensity of BF at the initial time (t = 0) and the UV absorption intensity at a given time (t). (c) Plot of Ct/C0 of BF versus time with different usage amounts of 1. (d) Five recycles of the decolorization of BF.

10−5 mol, (C0 − Ct)/C0 was a little lower than that of 1.6 × 10−5 mol, indicating that the optimal dosage was 1.6 × 10−5 mol (Figures 3c and S12). In contrast, when the above reaction was carried out in the presence of either [Cu(en)2]2+ or TeO2, Ct/C0 was found to be around 1, thereby clearly confirming the active catalyst for the decolorization of our compound, while the decolorization rate of a BF solution using Nb2O5 was 28.38% for 1 h (Figure S13). Meanwhile, comparative experiments were further performed by using K7HNb6O19·13H2O, which was completely soluble in our reaction medium, although it has a catalytic effect on BF. The insolubility nature of 1 is of great importance because it allows the catalyst to be recovered and reused for several consecutive reaction runs. The recyclability of the catalyst system was also tested, and the results show that it could be recycled at least five times with a small gradual decline in yield to 92.35%, 81.54%, 73.68%, 75.38%, and 68.46% (Figure 3d). The slight decline corresponded to the loss of catalyst. Interestingly, the PXRD patterns and IR spectra are nearly identical before and after five cycles of catalysis (Figures S14 and S15). Thus, it is concluded that 1 has good repeatability, with remarkable stability in the present experimental conditions. Further catalytic tests of 1 for different organic dyes concluded the exclusive selectively of 1 toward the decolorization of BF (Table S5).

Figure 2. One-dimensional (a) and two-dimensional (b) frameworks of 1. 10120

DOI: 10.1021/acs.inorgchem.7b00860 Inorg. Chem. 2017, 56, 10119−10122

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Inorganic Chemistry

(2) Long, D.-L.; Burkholder, E.; Cronin, L. Polyoxometalate clusters, nanostructures and materials: From self-assembly to designer materials and devices. Chem. Soc. Rev. 2007, 36, 105−121. (3) Wassermann, K.; Dickman, M. H.; Pope, M. T. Self-Assembly of Supramolecular Polyoxometalates: The Compact, Water-Soluble Heteropolytungstate Anion [As 12 III Ce 16 III (H 2 O) 36 W 148 O 524 ] 76− . Angew. Chem., Int. Ed. Engl. 1997, 36, 1445−1448. (4) Hill, C. L. Progress and challenges in polyoxometalate-based catalysis and catalytic materials chemistry. J. Mol. Catal. A: Chem. 2007, 262, 2−6. (5) Chen, W.-C.; Li, H.-L.; Wang, X.-L.; Shao, K.-Z.; Su, Z.-M.; Wang, E.-B. Assembly of Cerium(III)-Stabilized Polyoxotungstate Nanoclusters with SeO32−/TeO32− Templates: From Single Polyoxoanions to Inorganic Hollow Spheres in Dilute Solut ion. Chem. - Eur. J. 2013, 19, 11007−11015. (6) Gao, J.; Yan, J.; Mitchell, S. G.; Miras, H. N.; Boulay, A. G.; Long, D.-L.; Cronin, L. Self-assembly of a family of macrocyclic polyoxotungstates with emergent material properties. Chem. Sci. 2011, 2, 1502− 1508. (7) Gao, J.; Yan, J.; Beeg, S.; Long, D.-L.; Cronin, L. Assembly of Molecular “Layered” Heteropolyoxometalate Architectures. Angew. Chem., Int. Ed. 2012, 51, 3373−3376. (8) Yan, J.; Long, D.-L.; Wilson, E. F.; Cronin, L. Discovery of Heteroatom-“Embedded” Te ⊂{W18O54} Nanofunctional Polyoxometalates by Use of Cryospray Mass Spectrometr. Angew. Chem., Int. Ed. 2009, 48, 4376−4380. (9) Ismail, A. H.; Nsouli, N. H.; Dickman, M. H.; Knez, J.; Kortz, U. The 20-Tungsto-4-Tellurate(IV) [H2Te4W20O80]22− and the 15Tungstotellurate(IV)) [NaTeW15O54]13−. J. Cluster Sci. 2009, 20, 453−465. (10) Zhan, C.; Cameron, J. M.; Gao, J.; Purcell, J. W.; Long, D.-L.; Cronin, L. Time-Resolved Assembly of Cluster-in-Cluster {Ag12}-in{W76} Polyoxometalates under Supramolecular Control. Angew. Chem., Int. Ed. 2014, 53, 10362−10366. (11) Nyman, M.; Bonhomme, F.; Alam, T. M.; Rodriguez, M. A.; Cherry, B. R.; Krumhansl, J. L.; Nenoff, T. M.; Sattler, A. M. A General Synthetic Procedure for Heteropolyniobates. Science 2002, 297, 996− 998. (12) Nelson, C. R.; Hites, R. A. Aromatic Amines in and near the Buffalo River. Environ. Sci. Technol. 1980, 14, 1147−1149. (13) Yan, G.; Wang, X.; Ma, Y.; Cheng, X.; Wang, Y.; Li, Y. A new paratungstate-A-based organiceinorganic hybrid compound: Synthesis, structure and photocatalytic property of [Co(en)3]2[H2W7O24]·8H2O. Solid State Sci. 2013, 17, 146−150. (14) Lü, J.; Lin, J.-X.; Zhao, X.-L.; Cao, R. Photochromic hybrid materials of cucurbituril and polyoxometalates as photocatalysts under visible light. Chem. Commun. 2012, 48, 669−671. (15) Synthesis of 1: The procedure involves the formation of two solutions. Solution A comprises K7H[Nb6O19]·13H2O (0.33 g, 0.24 mmol) and TeO2 (0.04 g, 0.25 mmol) in distilled water (8.33 mL). Meanwhile, in a separate beaker, ethanediamine (0.21 mL) was added to a stirred clear solution of CuCl2·2H2O (0.17 g, 1.00 mmol) in water (0.83 mL), yielding solution B. Solution B was added directly into solution A. The pH was adjusted to 11.90 using KOH (2 mol·L−1). The resulting mixture was stirred for 15 min, sealed in a Teflon-lined steel autoclave, kept at 160 °C for 4 days, and then cooled to room temperature. Black block crystals were obtained by filtering, washed with distilled water, and dried in air. Yield: 20% based on K7H[Nb6O19]· 13H2O. Elem anal. Calcd for C20H106Cu5K4N20Te2Nb24O84: H, 2.17; C, 4.87; N, 5.68. Found: H, 2.28; C, 4.72; N, 5.57. IR (KBr): 3265, 1585, 1051, 893, 857, 771, 736, 669, 593, 537, 465 cm−1. (16) Chang, S.; Qi, Y.-F.; Wang, E.-B.; Zhang, Z. Two novel sandwiched-type polyoxotungstates containing Zn6 transition-metal cluster: Syntheses, structures and luminescent property. Inorg. Chim. Acta 2009, 362, 453−457. (17) Zhao, Z.; Zhou, B.; Zheng, S.; Su, Z.; Wang, C. Hydrothermal synthesis, crystal structure and magnetic characterization of three hexaM substituted tungstoarsenates (M = Ni, Zn and Mn). Inorg. Chim. Acta 2009, 362, 5038−5042.

TGA after the catalytic experiment indicates that the total weight loss from room temperature to 1250 °C was 24.01%, which was in accordance with the theoretical value (23.39%) and the value of fresh sample (23.14%; Figure S16). Further, elemental analyses (C, H, and N) of compound 1 in different conditions were measured (Table S6). Calcd: H, 2.17; C, 4.87; N, 5.68. Found (crystal): H, 2.28; C, 4.72; N, 5.57. Found (catalyzed sample): H, 2.12; C, 4.99; N, 5.40. Therefore, compound 1 is decolorizing the dye, not adsorbing. In summary, an unprecedented hexaniobium sandwich-type HPON directed by a [TeO3]2− ion has been synthesized. Compound 1 represents the first member of a telluriumcontaining sandwich-type HPON and the largest telluroniobate aggregate. The isolation shows that the TeIV ions in 1 serve as templates to the key building units, thereby enabling the selfassembly of trivacant TeNb9. The activity of 1 indicates that it can be regarded as a potential catalyst in the decolorization of BF. In future work, we will explore more novel clusters with [TeO3]2− as the template as well as perform catalytic studies.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00860. Experimental section (materials and methods, crystal data, synthetic discussion, and structural discussion), supplementary structural figures, additional measurements (PXRD, IR, cyclic voltammetry, thermal gravimetric mass spectrometry, and solid-state UV−vis spectrum), and catalytic experiments for 1 (PDF) Accession Codes

CCDC 1405492 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 Authors

*E-mail: [email protected] (J.N.). *E-mail: [email protected] (J.W.). ORCID

Jingyang Niu: 0000-0001-6526-7767 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21371048 and 21671056), the Natural Science Foundation of Henan Province (Grant 134300510035), the Postdoctoral Foundation of Henan Province (Grant 2015031), and the 2015 Young Backbone Teachers Foundation from Henan Province (Grant 2015GGJS017).

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REFERENCES

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Inorganic Chemistry (18) Kortz, U.; Al-Kassem, N. K.; Savelieff, M. G.; Al-Kadi, N. A.; Sadakane, M. Synthesis and Characterization of Copper-, Zinc-, Manganese-, and Cobalt-Substituted Dimeric Heteropolyanions, [(αXW9O33)2M3(H2O)3]n− (n = 12, X = AsIII, SbIII, M = Cu2+, Zn2+; n = 10, X = SeIV, TeIV, M = Cu2+) and [(α-AsW9O33)2WO(H2O)M2(H2O)2]10− (M = Zn2+, Mn2+, Co2+). Inorg. Chem. 2001, 40, 4742−4749. (19) Kortz, U.; Savelieff, M. G.; Bassil, B. S.; Keita, B.; Nadjo, L. Synthesis and Characterization of Iron(III)-Substituted, Dimeric Polyoxotungstates, [Fe4(H2O)10(β-XW9O33)2]n− (n = 6, X = AsIII, SbIII ; n = 4, X = SeIV, TeIV). Inorg. Chem. 2002, 41, 783−789. (20) Limanski, E. M.; Drewes, D.; Droste, E.; Böhner, R.; Krebs, B. Syntheses and X-ray characterisation of novel tellurium-substituted lacunary polyoxotungstates containing VIV, CoII, NiII and ZnII as heteroatoms. J. Mol. Struct. 2003, 656, 17−25. (21) Kalinina, I. V.; Izarova, N. V.; Kortz, U. Bis[tetraruthenium(IV)]Containing Polyoxometalates: [{RuIV4O6(H2O)9}2Sb2W20O68(OH)2]4− and [{RuIV)4O6(H2O)9}2{Fe(H2O)2}2{β-TeW9O33}2H]−. Inorg. Chem. 2012, 51, 7442−7444. (22) Gaunt, A. J.; May, I.; Copping, R.; Bhatt, A. I.; Collison, D.; Fox, O. D.; Holman, K. T.; Pope, M. T. A new structural family of heteropolytungstate lacunary complexes with the uranyl, UO22+, cation. Dalton Trans. 2003, 3009−3014. (23) Brown, I. D.; Altermatt, D. Bond-valence parameters obtained from a systematic analysis of the Inorganic Crystal Structure Database. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244−247.

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DOI: 10.1021/acs.inorgchem.7b00860 Inorg. Chem. 2017, 56, 10119−10122