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Solvothermal synthesis, structures and properties of two new octamolybdate-based compounds with tetrazole- and pyridyl-containing asymmetric amide ligands
Inorganic Chemistry Communications 71 (2016) 9–14
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Solvothermal synthesis, structures and properties of two new octamolybdate-based compounds with tetrazole- and pyridyl-containing asymmetric amide ligands Xing Rong, Hongyan Lin ⁎, Danna Liu, Xiang Wang, Guocheng Liu, Xiuli Wang ⁎ Department of Chemistry, Bohai University, Jinzhou 121000, P.R. China
a r t i c l e
i n f o
Article history: Received 6 June 2016 Received in revised form 23 June 2016 Accepted 30 June 2016 Available online 01 July 2016 Keywords: Octamolybdate Mo\ \N bond Zn(II) compound Electrocatalysis Photocatalysis Luminescent properties
a b s t r a c t The solvothermal reactions of tetrazole- and pyridyl-containing asymmetric amide ligands [N-(pyridin-3yl)isonicotinamide (PIA) and 3-(1H-tetrazole-1-acetic acid amido)pyridine (TAAP)] with Zn(NO3)2 in the presence of [Mo7O24]6− anions lead to the formation of two new compounds, namely H4[(Mo8O26)(PIA)2]·H2O (1) and H4[Zn2(TAAP)4(H2O)4](Mo8O26)2·16H2O (2), which have been characterized by single-crystal X-ray diffraction, IR spectra, elemental analysis, TGA and PXRD analysis. Compound 1 is a 3D supramolecular structure derived from the 0D [(Mo8O26)(PIA)] cluster containing Mo\\N bond. Compound 2 features a two-fold interpenetrating 3D network with [Mo8O26]4− anions as templates and possess 4-connected lvt net. Furthermore, the electrochemical, photocatalysis and luminescent properties of compound 2 have been investigated. © 2016 Published by Elsevier B.V.
Polyoxometalates (POMs), as a unique class of inorganic metal oxide clusters, have been regarded as excellent candidates to obtain metal-organic frameworks (MOFs) due to their versatile architectures, various properties such as catalytic activity, magnetism, electrochemistry and photochemistry [1–5]. Nowadays, rational design and synthesis of POM-based MOFs with the expected structure and properties is still a long-standing challenge since organic ligands, metallic ions, solvents, reaction temperature, the pH value of the solution and counterions, etc., may easily affect the structure of POM-based MOFs [6–10]. Among these affecting factors, the organic linkers usually play more roles in the formation of various topological structures [11–15]. Wherein the pyridyl-containing amide ligands as one of the outstanding N/Odonor organic linkers have been effectively selected to generate POM-based MOFs because they can connect metal centers through their pyridyl groups or the amide groups [16,17]. Compared with other N-containing rigid ligands, the advantages of the pyridyl-amide ligands have been shown in the following aspects: (i) the amide groups with both the N\\H hydrogen donor and C_O hydrogen acceptor may conduce to the formation of hydrogen bonds and supramolecular structures; (ii) the flexible nature allows the ligands to bend freely to satisfy the coordination requirement of metal centers. Although, by
⁎ Corresponding authors. E-mail addresses: [email protected] (H. Lin), [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.inoche.2016.06.029 1387-7003/© 2016 Published by Elsevier B.V.
using the pyridyl-amide ligands, some POM-based MOFs with diverse topological networks and interesting functional properties have been prepared [18,19], as far as we know, only a series of symmetric pyridyl-amide ligands such as flexible or semi-rigid bis-pyridyl-bis-amide have been introduced into the POMs-based compounds [20,21]. However, up to now, the researches on POM-based MOFs derived from the asymmetric pyridyl-containing amide ligands are still quite rare [22]. Whether the asymmetric pyridyl-amide ligands could be chosen to construct the POM-based MOFs with different structures and interesting properties? Which inspires us to do this study. In this work, we chose two asymmetric pyridyl-amide ligands combined with tetrazole or/and pyridyl groups, namely, N(pyridin-3-yl)isonicotinamide (PIA) and 3-(1H-tetrazole-1-acetic acid amido)pyridine (TAAP) to assemble with typical Zn(II) ions in the presence of [Mo7O24]6 − under solvothermal conditions. Fortunately, two compounds H 4 [(Mo 8 O 26 )(PIA) 2 ]·H2 O (1) and H4 [Zn2 (TAAP) 4(H2O) 4](Mo 8O26) 2·16H 2 O (2) have been obtained. To the best of our knowledge (according to CCDC search), the POM-based MOFs constructed by introducing PIA/TAAP ligands into POMs systems are still very limited (Scheme 1). On the other hand, contrast to the metal Cu/Ag ions, Zn(II) ion as the metal center in the POMbased MOFs is rare, and the POM-based compounds constructed by Zn(II) ions and pyridyl-amide ligands have not been reported so far [23,24]. It's worth noting that these two title POM-based compounds with different structures are synthesized under the same solvothermal conditions only by using two different asymmetric
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Scheme 1. The tetrazole- and pyridyl-containing asymmetric amide ligands used in this work.
pyridyl-amide ligands. Furthermore, the luminescent properties, electrochemical and photocatalysis properties of the title compounds were investigated.
The single crystal structure analysis reveals that compound 1 crystallizes in the monoclinic system with space group C2c [25] and is a 3D supramolecular structure constructed by the 0D [(Mo8O26)(PIA)] clusters containing Mo\\N bonds. As shown in Fig. 1a, the [(Mo8O26)(PIA)] cluster in compound 1 contains one [Mo8O26]4 − anion and one PIA. It should be noticed that the Zn(II) anion is absent in the system. Notably, only one N atom from the 4-pyridyl group of each PIA ligand is coordinated to the Mo1 ion of the [Mo8O26]4 − anion with the Mo1\\N1 bond distance of 2.145 Å in 1, and the remaining N atom of 3-pyridyl group is non-coordinated, namely H4[(Mo8O26)(PIA)2] unit. Such 0D [(Mo8O26)(PIA)] clusters are further interlinked by hydrogen bonding interaction (C2\\H2A···O11, 2.322 Å; C8–H8…O13, 2.335 Å; N2–H2…O12, 2.322 Å; Table S2)
Fig. 1. (a) The structure of [(Mo8O26)(PIA)] clusters in 1. (b) The 1D supramolecular chain in 1. (c) The detail hydrogen bonding interactions. (d) The 2D supramolecular layer constructed by the H-Bond interactions view the ab plane and (e) view the ac plane. (f) The detail hydrogen bonding interactions. (g) The 3D supramolecular framework formed by the hydrogen bonding interactions.
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between PIA and [Mo8O26]4 − anions of the adjacent butterfly-like H4[(Mo8O26)(PIA)2] units to yield 1D supramolecular chain structure (Fig. 1b). Further, the adjacent chains are extended by another hydrogen bonding interaction (C10\\H10…O3, 2.569 Å; C11\\H11…O10, 2.543 Å; Fig. 1c) between the 3-pyridyl groups and [Mo8O26]4− anions into a 2D supramolecular layer (Fig. 1d and Fig. 1e). Finally, the neighboring 2D layers are further linked by hydrogen bonding interaction (C9\\H9…O2, 2.400 Å; Fig. 1f) to generate a 3D supramolecular network (Fig. 1g). These weak supramolecular interactions consolidate the structure of 1 in cooperation with the coordination interaction.
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According to the structure of 1, we have found the central metal ions Zn(II) was absent in the structure and the 3-pyridyl groups of PIA was also uncoordinated with ions, which may be due to the pH value of the system. Could we construct the diverse structures by choosing the asymmetric pyridyl-amide ligand with the more coordination sites such as the tetrazole-group amide that might coordinate with metal ions? Inspired by this idea, when the TAAP instead of PIA was used in the same experimental conditions as those for the synthesis of 1 [26], blockshaped light blue crystals of 2 was obtained. Single crystal X-ray study
Fig. 2. (a) The coordination environments around Zn(II) ions in 2. (b) The 3D single framework constructed by the TAAP and Zn(II). (c) The hydrogen bond interaction among the adjacent 3D frameworks. (d) View of the two-fold interpenetrating 3D network. (e) The hydrogen bond interaction between the TAAP and the [Mo8O26]4− anions. (f) The 3D network of 2 in the presence of [Mo8O26]4− anions. (g) The topology of the two-fold interpenetrating 3D network of 2 without [Mo8O26]4− anions and (h) with [Mo8O26]4− anions.
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reveals that compound 2 crystallizes in the orthorhombic space group Cmca [27], which has similar structure to Cu(II)/Co(II)-POM-based MOFs just reported by our group [22]. The asymmetric unit of 2 consists of two crystallographically independent Zn(II) ions (Zn1 and Zn2 occupancy rate is 0.5 and 0.5), two half of [Mo8O26]4 − anions, two TAAP ligands, two coordinated water molecules and sixteen lattice water molecules. In order to balance the charges, four protons were added to the molecular formula. As shown in Fig. 2a, both the six-coordinate Zn1 and Zn2 atoms possess a distorted octahedral {ZnN4O2} environment. The Zn1 and Zn2 centers are surrounded by four N atoms from four different TAAP and further coordinated by two water molecules. The Zn\\O bonds range from 2.133(15) to 2.205(13) Å, whereas the Zn\\N distances vary from 2.119(12) to 2.165(11) Å (Table S1), which are comparable to those found in other reported Zn(II) compounds [28]. Each Zn2+ ion is bonded with four different TAAP ligands to form a 3D framework as in 2 (Fig. 2b), which is further perpendicularly interpenetrating with the other adjacent 3D framework to afford a two-fold interpenetrating 3D network (Fig. 2c). The two-fold interpenetrating 3D framework is further stabilized by the hydrogen bonding interactions between\\CH of the methylene moiety and the tetrazole N atom of TAAP ligand from the adjacent 3D frameworks (Fig. 2d and Fig. S1), the hydrogen bonding interactions (Fig. 2e) between TAAP and the O atoms from the [Mo8O26]4− anions (Fig. 2f and Table S2). If the connection of [Mo8O26]4− anions is ignored, a computation of the voids using PLATON [29] suggests a value of 12,857.1 Å3, corresponding to 64.8% of the unit cell volume. However, the voids are occupied by the temple [Mo8O26]4− anions, resulting in the structure of 2 is nonporous. To get a better insight into the structure of the complicated 3D network of 2, by denoting Zn(II) and TAAP as 4-connected nodes and 2connected nodes (Fig. S2), respectively, the whole structure of 2 can be simplified as an uninodal 4-connected 3D network with the lvt topology, its Schläfli symbol is (42.84), as depicted in Fig. 2g. Based on the above structural analyses of 1 and 2, we can see that these two compounds are quite different structures obtained under the same conditions by using different organic amide-based ligands, which clearly indicates the coordination sites of the N-donor ligands play the key role in constructing the final structures for the aiming compounds. The PXRD experiments for 1 and 2 were carried out to confirm whether the crystal structures are truly representative of the bulk materials. As shown in Figs. S3–S4, the main peak positions of PXRD experiments are fairly good agreement with the computer-simulated compound patterns, which clearly confirms the phase purity of the asprepared products. The IR spectra of 1 and 2 are shown in Figs. S5–S6. The characteristic bands at 956, 892, 840, 668 cm−1 for 1 and at 940, 883, 840, 687 cm− 1 for 2 are attributed to the ν(Mo_O), ν(Mo\\O\\Mo) [30]. The thermal stability of 1 and 2 was examined by thermogravimetric analysis (TGA) and the results are shown in Fig. S7. The TGA curves for 1 displays initial weight losses of 2.21% from 180 °C, suggesting the loss of the free water molecules (calcd 1.62%). For 2, a total weight loss of 9.63% in the temperature range of 120–200 °C consists of the corresponding calculated values of 9.81% due to the loss of free and coordinated water molecules. The carbon paste electrodes (CPEs) [31] bulk-modified with POMMOFs have been widely applied in electrochemistry due to their advantages: high stability, low solubility in water and common organic solvents, easy to handle and prepare, and so on [32–35]. Based on this reason, the electrochemical activities of 2 was explored by using 2 bulk-modified CPE (2-CPE) in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution (because the electrochemical behaviors of compounds 1 and 2 are similar, so compound 2 has been taken as an example). The cyclic voltammograms of 2-CPE at different scan rates in the potential range of 700 to −180 mV are presented in Fig. 3a. The 2–CPE exhibits three pairs of redox peaks, which can be attributed to the redox of Mo8
Fig. 3. Cyclic voltammograms of 2-CPE in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution (a) at different scan rates (from inner to outer: 80, 120, 160, 200, 250, 300, 350, 400, 450, 500 mV s−1); (b) The dependence of anodic peak (II) and cathodic peak (II′) currents on scan rates for 2–CPE. (c) Cyclic voltammograms of 2-CPE in 0.1 M H2SO4 + 0.5 M Na2SO4 solution containing 0.0–12.0 mM KNO2.
polyanions [36]. The mean peak potentials E1/2 = (Epa + Epc)/2 are +352 (I–I′), +189 (II–II′) and − 50 mV (III–III′) (200 mV s−1) for 2– CPE. The peak potentials change gradually with the scan rates increasing: the cathodic peak potentials shift toward the negative direction and the corresponding anodic peak potentials to the positive direction. The peak currents are proportional to the scan rates up to 500 mV s−1, indicating that the redox processes of the 2–CPE is surface-controlled (Fig. 3b) [37].
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Fig. 4. Absorption spectra of the MB solution (a) and RhB solution (b) during the decomposition reaction under UV irradiation at the presence of compound 2; Photocatalytic decomposition rates of MB (c) and RhB (d) under UV irradiation (with and without catalyst).
As is known, some POMs can be used as electrocatalysts to catalyze the reduction of nitrite, bromate and hydrogen peroxide [38]. So, in this work, we investigated the electrocatalytic activities of 2–CPEs toward the reduction of nitrite. As can be seen from Fig. 3c, with the addition of nitrite, all of the reduction peak currents increase gradually and the corresponding oxidation peak currents decrease, suggesting that the reduction of nitrite is electrocatalyzed by the reduced species of Mo8 in compound 2 [37]. Organic dyes have been widely employed in textile, paper printing and plastics industries, thus the removal of organic dyes from industrial wastewater by photodegradation has attracted great attention in recent years. As is well known, many POMs show photocatalytic activities in the degradation of some organic substances under UV irradiation [39]. Herein, we selected organic dyes methylene blue (MB) and rhodamine B (RhB) as the model pollutants in aqueous media to investigate the photocatalytic performance of compounds 1 and 2 under UV irradiation from a 125 W high pressure Hg lamp through a typical process: 100 mg powder of the title compounds was mixed in a 90 mL MB/RhB (0.02 mmol) aqueous solution and magnetically stirred about 10 min. Then, every 20 min, 5.0 mL clear transparent sample was taken out for analysis by UV spectroscopy. As shown in Fig. 4a–b and Fig. S8a–S8b, the absorption peaks of MB/ RhB decreased obviously with increasing reaction time for compounds 1 and 2 under UV irradiation. Moreover, the concentrations of MB/RhB (C) versus reaction time (t) for the title compounds were plotted. The degradation ratio of MB/RhB reaches 85/59% for 1 and 97/49% for 2 during 100 min. (Fig. 4c–d and Fig. S8c–S8d), which illustrates that the title compounds possess good photocatalytic activities toward the decomposition of MB/RhB. Photoluminescent measurements of the compounds were also studied in the solid state at room temperature. As illustrated in Fig. S9, it can
be observed the main emission peaks of the compound 2 is at 385 nm and 462 nm (λex = 320 nm), whereas a strong emission peak (λem = 412 nm) upon excitation at 320 nm was observed for the free TAAP ligand. Compared with the free TAAP ligand, the emission bands of compound 2 exhibit red shifts and blue shifts, which indicate that the emission peaks of 2 may derive from the metal-to-ligand charge transfer (MLCT) and/or ligand-to-metal charge transfer (LMCT) [40–42]. In summary, under the solvothermal condition, we successfully synthesized two POM-based MOFs constructed by two asymmetric pyridyl-amide ligands in the presence of the [Mo7O24]6− anions. And they have quite different structures under the same conditions only by using different asymmetric organic amide-based ligands. The compound 1 features a 0D [(Mo8O26)](PIA)] cluster based on the Mo\\N bond, which is further interlinked by hydrogen bonding interactions to yield a 3D structure. The compound 2 represents a two-fold interpenetrating 3D network based on Zn(II) ions and to our knowledge, POMbased compounds constructed by Zn(II) ions were still limited. In addition, compound 2 exhibits good electrocatalytic activities toward the reduction of nitrite and the title compounds possess good photocatalytic properties for the degradation of MB/RhB under UV irradiation, which may be potential and valuable electrocatalysts or photocatalysts. Moreover, compound 2 has luminescent property.
Acknowledgements Financial supports of the National Natural Science Foundation of China (21171025, 21471021, 21501013 and 21401010) and Program for Distinguished Professor of Liaoning Province (No. 2015399) are gratefully acknowledged.
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[26] Synthesis of 1 and 2: A mixture of Zn(NO3)2·6H2O (0.15 g, 0.5 mmol), TAAP (0.020 g, 0.1 mmol), (NH4)6Mo7O24·4H2O (0.124 g, 0.1 mmol), H2O (5 mL) and CH3OH (5 mL) was added in a 12 mL glass vial and heated at 80 °C for 3 days. Compound 2 was prepared in the same way as that for 1 except for PIA (0.020 g, 0.1 mmol). After slow cooling to room temperature, colorless crystals of 1 and light blue crystals of 2 were filtered off, washed with distilled water, and dried in a desiccator at room temperature to give a yield of 20% and 23% based on Mo, respectively. Anal. Calcd for C22H26N6O30Mo8: C 16.29; H, 1.62; N, 5.18%. Found: C, 16.35; H, 1.68; N, 5.22%. Anal. Calcd for C32H68Zn2N25O75Mo16: C 10.48; H, 1.87; N, 9.54%. Found: C, 10.45; H, 1.90; N, 9.52%. IR for 1 (KBr pellet, cm−1): 3470 (s), 3080 (w), 2372 (m), 1691(w), 1655 (m), 1563 (m), 1052 (s), 956(s), 892(s), 840(s), 668(s). IR for 2: 3500(s), 3080(w), 2365(s), 1870(w), 1726(w), 1688(s), 1551(s), 1490(m), 1110(m), 940(s), 883(s), 840(s), 687(s). [27] Crystal data for compound 2: C32H68Zn2N25O75Mo16, F.W. = 3380.64, Orthorhombic, space group Cmca, a = 27.090(5) Å, b = 38.406(5) Å, c = 19.081(5) Å, α = 90.000°, β = 90.000°, γ = 90.000°, V = 19852(7) Å3, Z = 8, Dcalc = 2.260 mg/ m3, λ = 0.71069 Å, T = 293(2) K, R1 (wR2) = 0.0582 (0.1640), Bruker Smart Apex II CCD area detector, Mo Kα radiation. 16 water molecules in the compound 2 were omitted using the SQUEEZE option of the PLATON program. The structure was solved by direct methods and refined on F2 by full matrix least squares methods using SHELXTL 97. CCDC NO. 1481240. [28] Y.X. Hu, Y.T. Qian, W.W. Zhang, Y.Z. Li, J.F. Bai, Inorg. Chem. Commun. 47 (2014) 102–107. [29] A.J. Spek, Appl. Crystallogr. 36 (2003) 7–13. [30] Y.Q. Lan, S.L. Li, X.L. Wang, K.Z. Shao, D.Y. Du, H.Y. Zang, Z.M. Su, Inorg. Chem. 47 (2008) 8179–8187. [31] The compound 2 bulk modified CPE (2–CPE) was fabricated by mixing 0.10 g graphite powder and 0.030 g compound 2 in an agate mortar for approximately 30 min to achieve a uniform mixture; then 0.16 mL paraffin oil was added and stirred with a glass rod. The homogenized mixture was packed into a 3 mm inner diameter glass tube and the tube surface was wiped with weighing paper. The electrical contact was established with the copper wire through the back of electrode. [32] Z.G. Jiang, K. Shi, Y.M. Lin, Q.M. Wang, Chem. Commun. 50 (2014) 2353–2355. [33] Y. Zhang, J.Q. Shen, L.H. Zheng, Z.M. Zhang, Y.X. Li, E.B. Wang, Cryst. Growth Des. 14 (2014) 110–116. [34] J.J. Jiang, C. Yan, M. Pan, Z. Wang, H.Y. Deng, J.R. He, Q.Y. Yang, L. Fu, X.F. Xu, C.Y. Su, Eur. J. Inorg. Chem. 8 (2012) 1171–1179. [35] M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219–237. [36] X.D. Du, C.H. Li, Y. Zhang, S. Liu, Y. Ma, X.Z. You, CrystEngComm 13 (2011) 2350–2357. [37] X.L. Wang, C. Xu, H.Y. Lin, G.C. Liu, J. Luan, Z.H. Chang, RSC Adv. 3 (2013) 3592–3598. [38] J.Q. Sha, J. Peng, Y.Q. Lan, Z.M. Su, H.J. Pang, A.X. Tian, P.P. Zhang, M. Zhu, Inorg. Chem. 47 (2008) 5145–5153. [39] X.L. Wang, Z.H. Chang, H.Y. Lin, A.X. Tian, G.C. Liu, J.W. Zhang, Dalton Trans. 43 (2014) 12272–12279. [40] X.W. Wang, J.Z. Chen, J.H. Liu, Cryst. Growth Des. 7 (2007) 1227–1229. [41] H. Deng, Y.C. Qiu, Y.H. Li, Z.H. Liu, R.H. Zeng, M. Zeller, S.R. Batten, Chem. Commun. (2008) 2239–2241. [42] N. Chen, M.X. Li, P. Yang, X. He, M. Shao, S.R. Zhu, Cryst. Growth Des. 13 (2013) 2650–2660.
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Solvothermal synthesis, structures and properties of two new octamolybdate-based compounds with tetrazole- and pyridyl-containing asymmetric amide ligands Xing Rong, Hongyan Lin ⁎, Danna Liu, Xiang Wang, Guocheng Liu, Xiuli Wang ⁎ Department of Chemistry, Bohai University, Jinzhou 121000, P.R. China
a r t i c l e
i n f o
Article history: Received 6 June 2016 Received in revised form 23 June 2016 Accepted 30 June 2016 Available online 01 July 2016 Keywords: Octamolybdate Mo\ \N bond Zn(II) compound Electrocatalysis Photocatalysis Luminescent properties
a b s t r a c t The solvothermal reactions of tetrazole- and pyridyl-containing asymmetric amide ligands [N-(pyridin-3yl)isonicotinamide (PIA) and 3-(1H-tetrazole-1-acetic acid amido)pyridine (TAAP)] with Zn(NO3)2 in the presence of [Mo7O24]6− anions lead to the formation of two new compounds, namely H4[(Mo8O26)(PIA)2]·H2O (1) and H4[Zn2(TAAP)4(H2O)4](Mo8O26)2·16H2O (2), which have been characterized by single-crystal X-ray diffraction, IR spectra, elemental analysis, TGA and PXRD analysis. Compound 1 is a 3D supramolecular structure derived from the 0D [(Mo8O26)(PIA)] cluster containing Mo\\N bond. Compound 2 features a two-fold interpenetrating 3D network with [Mo8O26]4− anions as templates and possess 4-connected lvt net. Furthermore, the electrochemical, photocatalysis and luminescent properties of compound 2 have been investigated. © 2016 Published by Elsevier B.V.
Polyoxometalates (POMs), as a unique class of inorganic metal oxide clusters, have been regarded as excellent candidates to obtain metal-organic frameworks (MOFs) due to their versatile architectures, various properties such as catalytic activity, magnetism, electrochemistry and photochemistry [1–5]. Nowadays, rational design and synthesis of POM-based MOFs with the expected structure and properties is still a long-standing challenge since organic ligands, metallic ions, solvents, reaction temperature, the pH value of the solution and counterions, etc., may easily affect the structure of POM-based MOFs [6–10]. Among these affecting factors, the organic linkers usually play more roles in the formation of various topological structures [11–15]. Wherein the pyridyl-containing amide ligands as one of the outstanding N/Odonor organic linkers have been effectively selected to generate POM-based MOFs because they can connect metal centers through their pyridyl groups or the amide groups [16,17]. Compared with other N-containing rigid ligands, the advantages of the pyridyl-amide ligands have been shown in the following aspects: (i) the amide groups with both the N\\H hydrogen donor and C_O hydrogen acceptor may conduce to the formation of hydrogen bonds and supramolecular structures; (ii) the flexible nature allows the ligands to bend freely to satisfy the coordination requirement of metal centers. Although, by
⁎ Corresponding authors. E-mail addresses: [email protected] (H. Lin), [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.inoche.2016.06.029 1387-7003/© 2016 Published by Elsevier B.V.
using the pyridyl-amide ligands, some POM-based MOFs with diverse topological networks and interesting functional properties have been prepared [18,19], as far as we know, only a series of symmetric pyridyl-amide ligands such as flexible or semi-rigid bis-pyridyl-bis-amide have been introduced into the POMs-based compounds [20,21]. However, up to now, the researches on POM-based MOFs derived from the asymmetric pyridyl-containing amide ligands are still quite rare [22]. Whether the asymmetric pyridyl-amide ligands could be chosen to construct the POM-based MOFs with different structures and interesting properties? Which inspires us to do this study. In this work, we chose two asymmetric pyridyl-amide ligands combined with tetrazole or/and pyridyl groups, namely, N(pyridin-3-yl)isonicotinamide (PIA) and 3-(1H-tetrazole-1-acetic acid amido)pyridine (TAAP) to assemble with typical Zn(II) ions in the presence of [Mo7O24]6 − under solvothermal conditions. Fortunately, two compounds H 4 [(Mo 8 O 26 )(PIA) 2 ]·H2 O (1) and H4 [Zn2 (TAAP) 4(H2O) 4](Mo 8O26) 2·16H 2 O (2) have been obtained. To the best of our knowledge (according to CCDC search), the POM-based MOFs constructed by introducing PIA/TAAP ligands into POMs systems are still very limited (Scheme 1). On the other hand, contrast to the metal Cu/Ag ions, Zn(II) ion as the metal center in the POMbased MOFs is rare, and the POM-based compounds constructed by Zn(II) ions and pyridyl-amide ligands have not been reported so far [23,24]. It's worth noting that these two title POM-based compounds with different structures are synthesized under the same solvothermal conditions only by using two different asymmetric
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Scheme 1. The tetrazole- and pyridyl-containing asymmetric amide ligands used in this work.
pyridyl-amide ligands. Furthermore, the luminescent properties, electrochemical and photocatalysis properties of the title compounds were investigated.
The single crystal structure analysis reveals that compound 1 crystallizes in the monoclinic system with space group C2c [25] and is a 3D supramolecular structure constructed by the 0D [(Mo8O26)(PIA)] clusters containing Mo\\N bonds. As shown in Fig. 1a, the [(Mo8O26)(PIA)] cluster in compound 1 contains one [Mo8O26]4 − anion and one PIA. It should be noticed that the Zn(II) anion is absent in the system. Notably, only one N atom from the 4-pyridyl group of each PIA ligand is coordinated to the Mo1 ion of the [Mo8O26]4 − anion with the Mo1\\N1 bond distance of 2.145 Å in 1, and the remaining N atom of 3-pyridyl group is non-coordinated, namely H4[(Mo8O26)(PIA)2] unit. Such 0D [(Mo8O26)(PIA)] clusters are further interlinked by hydrogen bonding interaction (C2\\H2A···O11, 2.322 Å; C8–H8…O13, 2.335 Å; N2–H2…O12, 2.322 Å; Table S2)
Fig. 1. (a) The structure of [(Mo8O26)(PIA)] clusters in 1. (b) The 1D supramolecular chain in 1. (c) The detail hydrogen bonding interactions. (d) The 2D supramolecular layer constructed by the H-Bond interactions view the ab plane and (e) view the ac plane. (f) The detail hydrogen bonding interactions. (g) The 3D supramolecular framework formed by the hydrogen bonding interactions.
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between PIA and [Mo8O26]4 − anions of the adjacent butterfly-like H4[(Mo8O26)(PIA)2] units to yield 1D supramolecular chain structure (Fig. 1b). Further, the adjacent chains are extended by another hydrogen bonding interaction (C10\\H10…O3, 2.569 Å; C11\\H11…O10, 2.543 Å; Fig. 1c) between the 3-pyridyl groups and [Mo8O26]4− anions into a 2D supramolecular layer (Fig. 1d and Fig. 1e). Finally, the neighboring 2D layers are further linked by hydrogen bonding interaction (C9\\H9…O2, 2.400 Å; Fig. 1f) to generate a 3D supramolecular network (Fig. 1g). These weak supramolecular interactions consolidate the structure of 1 in cooperation with the coordination interaction.
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According to the structure of 1, we have found the central metal ions Zn(II) was absent in the structure and the 3-pyridyl groups of PIA was also uncoordinated with ions, which may be due to the pH value of the system. Could we construct the diverse structures by choosing the asymmetric pyridyl-amide ligand with the more coordination sites such as the tetrazole-group amide that might coordinate with metal ions? Inspired by this idea, when the TAAP instead of PIA was used in the same experimental conditions as those for the synthesis of 1 [26], blockshaped light blue crystals of 2 was obtained. Single crystal X-ray study
Fig. 2. (a) The coordination environments around Zn(II) ions in 2. (b) The 3D single framework constructed by the TAAP and Zn(II). (c) The hydrogen bond interaction among the adjacent 3D frameworks. (d) View of the two-fold interpenetrating 3D network. (e) The hydrogen bond interaction between the TAAP and the [Mo8O26]4− anions. (f) The 3D network of 2 in the presence of [Mo8O26]4− anions. (g) The topology of the two-fold interpenetrating 3D network of 2 without [Mo8O26]4− anions and (h) with [Mo8O26]4− anions.
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reveals that compound 2 crystallizes in the orthorhombic space group Cmca [27], which has similar structure to Cu(II)/Co(II)-POM-based MOFs just reported by our group [22]. The asymmetric unit of 2 consists of two crystallographically independent Zn(II) ions (Zn1 and Zn2 occupancy rate is 0.5 and 0.5), two half of [Mo8O26]4 − anions, two TAAP ligands, two coordinated water molecules and sixteen lattice water molecules. In order to balance the charges, four protons were added to the molecular formula. As shown in Fig. 2a, both the six-coordinate Zn1 and Zn2 atoms possess a distorted octahedral {ZnN4O2} environment. The Zn1 and Zn2 centers are surrounded by four N atoms from four different TAAP and further coordinated by two water molecules. The Zn\\O bonds range from 2.133(15) to 2.205(13) Å, whereas the Zn\\N distances vary from 2.119(12) to 2.165(11) Å (Table S1), which are comparable to those found in other reported Zn(II) compounds [28]. Each Zn2+ ion is bonded with four different TAAP ligands to form a 3D framework as in 2 (Fig. 2b), which is further perpendicularly interpenetrating with the other adjacent 3D framework to afford a two-fold interpenetrating 3D network (Fig. 2c). The two-fold interpenetrating 3D framework is further stabilized by the hydrogen bonding interactions between\\CH of the methylene moiety and the tetrazole N atom of TAAP ligand from the adjacent 3D frameworks (Fig. 2d and Fig. S1), the hydrogen bonding interactions (Fig. 2e) between TAAP and the O atoms from the [Mo8O26]4− anions (Fig. 2f and Table S2). If the connection of [Mo8O26]4− anions is ignored, a computation of the voids using PLATON [29] suggests a value of 12,857.1 Å3, corresponding to 64.8% of the unit cell volume. However, the voids are occupied by the temple [Mo8O26]4− anions, resulting in the structure of 2 is nonporous. To get a better insight into the structure of the complicated 3D network of 2, by denoting Zn(II) and TAAP as 4-connected nodes and 2connected nodes (Fig. S2), respectively, the whole structure of 2 can be simplified as an uninodal 4-connected 3D network with the lvt topology, its Schläfli symbol is (42.84), as depicted in Fig. 2g. Based on the above structural analyses of 1 and 2, we can see that these two compounds are quite different structures obtained under the same conditions by using different organic amide-based ligands, which clearly indicates the coordination sites of the N-donor ligands play the key role in constructing the final structures for the aiming compounds. The PXRD experiments for 1 and 2 were carried out to confirm whether the crystal structures are truly representative of the bulk materials. As shown in Figs. S3–S4, the main peak positions of PXRD experiments are fairly good agreement with the computer-simulated compound patterns, which clearly confirms the phase purity of the asprepared products. The IR spectra of 1 and 2 are shown in Figs. S5–S6. The characteristic bands at 956, 892, 840, 668 cm−1 for 1 and at 940, 883, 840, 687 cm− 1 for 2 are attributed to the ν(Mo_O), ν(Mo\\O\\Mo) [30]. The thermal stability of 1 and 2 was examined by thermogravimetric analysis (TGA) and the results are shown in Fig. S7. The TGA curves for 1 displays initial weight losses of 2.21% from 180 °C, suggesting the loss of the free water molecules (calcd 1.62%). For 2, a total weight loss of 9.63% in the temperature range of 120–200 °C consists of the corresponding calculated values of 9.81% due to the loss of free and coordinated water molecules. The carbon paste electrodes (CPEs) [31] bulk-modified with POMMOFs have been widely applied in electrochemistry due to their advantages: high stability, low solubility in water and common organic solvents, easy to handle and prepare, and so on [32–35]. Based on this reason, the electrochemical activities of 2 was explored by using 2 bulk-modified CPE (2-CPE) in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution (because the electrochemical behaviors of compounds 1 and 2 are similar, so compound 2 has been taken as an example). The cyclic voltammograms of 2-CPE at different scan rates in the potential range of 700 to −180 mV are presented in Fig. 3a. The 2–CPE exhibits three pairs of redox peaks, which can be attributed to the redox of Mo8
Fig. 3. Cyclic voltammograms of 2-CPE in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution (a) at different scan rates (from inner to outer: 80, 120, 160, 200, 250, 300, 350, 400, 450, 500 mV s−1); (b) The dependence of anodic peak (II) and cathodic peak (II′) currents on scan rates for 2–CPE. (c) Cyclic voltammograms of 2-CPE in 0.1 M H2SO4 + 0.5 M Na2SO4 solution containing 0.0–12.0 mM KNO2.
polyanions [36]. The mean peak potentials E1/2 = (Epa + Epc)/2 are +352 (I–I′), +189 (II–II′) and − 50 mV (III–III′) (200 mV s−1) for 2– CPE. The peak potentials change gradually with the scan rates increasing: the cathodic peak potentials shift toward the negative direction and the corresponding anodic peak potentials to the positive direction. The peak currents are proportional to the scan rates up to 500 mV s−1, indicating that the redox processes of the 2–CPE is surface-controlled (Fig. 3b) [37].
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Fig. 4. Absorption spectra of the MB solution (a) and RhB solution (b) during the decomposition reaction under UV irradiation at the presence of compound 2; Photocatalytic decomposition rates of MB (c) and RhB (d) under UV irradiation (with and without catalyst).
As is known, some POMs can be used as electrocatalysts to catalyze the reduction of nitrite, bromate and hydrogen peroxide [38]. So, in this work, we investigated the electrocatalytic activities of 2–CPEs toward the reduction of nitrite. As can be seen from Fig. 3c, with the addition of nitrite, all of the reduction peak currents increase gradually and the corresponding oxidation peak currents decrease, suggesting that the reduction of nitrite is electrocatalyzed by the reduced species of Mo8 in compound 2 [37]. Organic dyes have been widely employed in textile, paper printing and plastics industries, thus the removal of organic dyes from industrial wastewater by photodegradation has attracted great attention in recent years. As is well known, many POMs show photocatalytic activities in the degradation of some organic substances under UV irradiation [39]. Herein, we selected organic dyes methylene blue (MB) and rhodamine B (RhB) as the model pollutants in aqueous media to investigate the photocatalytic performance of compounds 1 and 2 under UV irradiation from a 125 W high pressure Hg lamp through a typical process: 100 mg powder of the title compounds was mixed in a 90 mL MB/RhB (0.02 mmol) aqueous solution and magnetically stirred about 10 min. Then, every 20 min, 5.0 mL clear transparent sample was taken out for analysis by UV spectroscopy. As shown in Fig. 4a–b and Fig. S8a–S8b, the absorption peaks of MB/ RhB decreased obviously with increasing reaction time for compounds 1 and 2 under UV irradiation. Moreover, the concentrations of MB/RhB (C) versus reaction time (t) for the title compounds were plotted. The degradation ratio of MB/RhB reaches 85/59% for 1 and 97/49% for 2 during 100 min. (Fig. 4c–d and Fig. S8c–S8d), which illustrates that the title compounds possess good photocatalytic activities toward the decomposition of MB/RhB. Photoluminescent measurements of the compounds were also studied in the solid state at room temperature. As illustrated in Fig. S9, it can
be observed the main emission peaks of the compound 2 is at 385 nm and 462 nm (λex = 320 nm), whereas a strong emission peak (λem = 412 nm) upon excitation at 320 nm was observed for the free TAAP ligand. Compared with the free TAAP ligand, the emission bands of compound 2 exhibit red shifts and blue shifts, which indicate that the emission peaks of 2 may derive from the metal-to-ligand charge transfer (MLCT) and/or ligand-to-metal charge transfer (LMCT) [40–42]. In summary, under the solvothermal condition, we successfully synthesized two POM-based MOFs constructed by two asymmetric pyridyl-amide ligands in the presence of the [Mo7O24]6− anions. And they have quite different structures under the same conditions only by using different asymmetric organic amide-based ligands. The compound 1 features a 0D [(Mo8O26)](PIA)] cluster based on the Mo\\N bond, which is further interlinked by hydrogen bonding interactions to yield a 3D structure. The compound 2 represents a two-fold interpenetrating 3D network based on Zn(II) ions and to our knowledge, POMbased compounds constructed by Zn(II) ions were still limited. In addition, compound 2 exhibits good electrocatalytic activities toward the reduction of nitrite and the title compounds possess good photocatalytic properties for the degradation of MB/RhB under UV irradiation, which may be potential and valuable electrocatalysts or photocatalysts. Moreover, compound 2 has luminescent property.
Acknowledgements Financial supports of the National Natural Science Foundation of China (21171025, 21471021, 21501013 and 21401010) and Program for Distinguished Professor of Liaoning Province (No. 2015399) are gratefully acknowledged.
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[26] Synthesis of 1 and 2: A mixture of Zn(NO3)2·6H2O (0.15 g, 0.5 mmol), TAAP (0.020 g, 0.1 mmol), (NH4)6Mo7O24·4H2O (0.124 g, 0.1 mmol), H2O (5 mL) and CH3OH (5 mL) was added in a 12 mL glass vial and heated at 80 °C for 3 days. Compound 2 was prepared in the same way as that for 1 except for PIA (0.020 g, 0.1 mmol). After slow cooling to room temperature, colorless crystals of 1 and light blue crystals of 2 were filtered off, washed with distilled water, and dried in a desiccator at room temperature to give a yield of 20% and 23% based on Mo, respectively. Anal. Calcd for C22H26N6O30Mo8: C 16.29; H, 1.62; N, 5.18%. Found: C, 16.35; H, 1.68; N, 5.22%. Anal. Calcd for C32H68Zn2N25O75Mo16: C 10.48; H, 1.87; N, 9.54%. Found: C, 10.45; H, 1.90; N, 9.52%. IR for 1 (KBr pellet, cm−1): 3470 (s), 3080 (w), 2372 (m), 1691(w), 1655 (m), 1563 (m), 1052 (s), 956(s), 892(s), 840(s), 668(s). IR for 2: 3500(s), 3080(w), 2365(s), 1870(w), 1726(w), 1688(s), 1551(s), 1490(m), 1110(m), 940(s), 883(s), 840(s), 687(s). [27] Crystal data for compound 2: C32H68Zn2N25O75Mo16, F.W. = 3380.64, Orthorhombic, space group Cmca, a = 27.090(5) Å, b = 38.406(5) Å, c = 19.081(5) Å, α = 90.000°, β = 90.000°, γ = 90.000°, V = 19852(7) Å3, Z = 8, Dcalc = 2.260 mg/ m3, λ = 0.71069 Å, T = 293(2) K, R1 (wR2) = 0.0582 (0.1640), Bruker Smart Apex II CCD area detector, Mo Kα radiation. 16 water molecules in the compound 2 were omitted using the SQUEEZE option of the PLATON program. The structure was solved by direct methods and refined on F2 by full matrix least squares methods using SHELXTL 97. CCDC NO. 1481240. [28] Y.X. Hu, Y.T. Qian, W.W. Zhang, Y.Z. Li, J.F. Bai, Inorg. Chem. Commun. 47 (2014) 102–107. [29] A.J. Spek, Appl. Crystallogr. 36 (2003) 7–13. [30] Y.Q. Lan, S.L. Li, X.L. Wang, K.Z. Shao, D.Y. Du, H.Y. Zang, Z.M. Su, Inorg. Chem. 47 (2008) 8179–8187. [31] The compound 2 bulk modified CPE (2–CPE) was fabricated by mixing 0.10 g graphite powder and 0.030 g compound 2 in an agate mortar for approximately 30 min to achieve a uniform mixture; then 0.16 mL paraffin oil was added and stirred with a glass rod. The homogenized mixture was packed into a 3 mm inner diameter glass tube and the tube surface was wiped with weighing paper. The electrical contact was established with the copper wire through the back of electrode. [32] Z.G. Jiang, K. Shi, Y.M. Lin, Q.M. Wang, Chem. Commun. 50 (2014) 2353–2355. [33] Y. Zhang, J.Q. Shen, L.H. Zheng, Z.M. Zhang, Y.X. Li, E.B. Wang, Cryst. Growth Des. 14 (2014) 110–116. [34] J.J. Jiang, C. Yan, M. Pan, Z. Wang, H.Y. Deng, J.R. He, Q.Y. Yang, L. Fu, X.F. Xu, C.Y. Su, Eur. J. Inorg. Chem. 8 (2012) 1171–1179. [35] M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219–237. [36] X.D. Du, C.H. Li, Y. Zhang, S. Liu, Y. Ma, X.Z. You, CrystEngComm 13 (2011) 2350–2357. [37] X.L. Wang, C. Xu, H.Y. Lin, G.C. Liu, J. Luan, Z.H. Chang, RSC Adv. 3 (2013) 3592–3598. [38] J.Q. Sha, J. Peng, Y.Q. Lan, Z.M. Su, H.J. Pang, A.X. Tian, P.P. Zhang, M. Zhu, Inorg. Chem. 47 (2008) 5145–5153. [39] X.L. Wang, Z.H. Chang, H.Y. Lin, A.X. Tian, G.C. Liu, J.W. Zhang, Dalton Trans. 43 (2014) 12272–12279. [40] X.W. Wang, J.Z. Chen, J.H. Liu, Cryst. Growth Des. 7 (2007) 1227–1229. [41] H. Deng, Y.C. Qiu, Y.H. Li, Z.H. Liu, R.H. Zeng, M. Zeller, S.R. Batten, Chem. Commun. (2008) 2239–2241. [42] N. Chen, M.X. Li, P. Yang, X. He, M. Shao, S.R. Zhu, Cryst. Growth Des. 13 (2013) 2650–2660.