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urea eutectic mixture
Inorganic Chemistry Communications 23 (2012) 14–16
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A high nuclear lanthanide-containing polyoxometalate aggregate synthesized in choline chloride/urea eutectic mixture Lin Liu, Shi-Ming Wang, Wei-Lin Chen ⁎, Ying Lu, Yang-Guang Li, En-Bo Wang ⁎ Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Renmin Street No. 5268, Changchun, Jilin, 130024, PR China
a r t i c l e
i n f o
Article history: Received 31 March 2012 Accepted 15 May 2012 Available online 24 May 2012 Keywords: Polyoxometalate Ionic liquid Eutectic mixture Lanthanide
a b s t r a c t A high nuclear lanthanide-containing polyoxometalate aggregate {[(CH3)3N(CH2)2OH]2(NH4)12}[Ce4(Mo4) (H2O)16(Mo7O24)4]·8H2O has been successfully synthesized in choline chloride/urea eutectic mixture. This method in synthesizing this anti-HIV activity aggregate has the advantages of convenience, facility, saving time and absence of additional noxious organic reagents. It has been characterized by XPS, IR, UV–vis spectra, TG analyses, power X-ray diffraction and single crystal X-ray diffraction. The CV property of the compound has also been tested. © 2012 Elsevier B.V. All rights reserved.
Ionic liquids have become one of the alternative solvents in the polyoxometalate (POM) synthesis area, because they possess the following advantages: nonvolatility, high fluidity, low melting temperature, high boiling temperature and nonflammability [1–6]. Many new POMs compounds including isopolyacid and heteropolyacid, sandwich structures, high-nuclear transition metal substituted POMs and POM-based metal–organic frameworks are prepared with this kind of solvent [7–11]. And also a deep eutectic mixture is a nice solvent for the synthesis of new coordination polymers [12,13]. A eutectic mixture is a kind of ionic liquid, which has the general properties of ionic liquids. However, it has its own special properties compared with the imidazole ionic liquids. The pure state of eutectic mixture can be obtained only through simply mixing the two components together mechanically [14], a eutectic mixture is the proper solvent for many different types of inorganic precursors. Even some ‘insoluble’ metal oxides were found to have significant solubility in a eutectic mixture. In addition, it does not react with water and it is biodegradable [15]. The choline chloride/urea eutectic mixture we used in this paper possesses very low toxicity compared with the imidazole ionic liquids [14,16]. Choline chloride is vitamin B4, an additive in chicken feed, and urea is a kind of common fertilizer. Many zeolites and coordination polymers were isolated from choline chloride/urea eutectic mixture under high temperature [17–19]. And some POM-based hybrids were also obtained from it under room temperature. On the basis of previous researches, polyoxomolybdate precursors perform good solubility in choline chloride/urea eutectic mixture, and the terminal oxygen atoms of the polyoxomolybdates ⁎ Corresponding authors. Tel./fax: + 86 431 85098787. E-mail addresses: [email protected] (W-L. Chen), [email protected] (E-B. Wang). 1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2012.05.025
exhibit high activities to coordinate with the metal atoms in this kind of eutectic mixture [20–22]. With the anti-HIV activity of the polyoxometalate PM-104 reported by Yamase et al., there are several investigations of the structure and the luminescence properties of this kind of polyoxometalate [23–25], but the reported synthesis method is complicated, so it is crucial to find more convenient ways of synthesis to promote expanded production. On the basis of the aforementioned considerations, we successfully isolated a heptamolybdate-based cluster with the formula {[(CH3)3 N(CH2)2OH]2(NH4)12}[Ce4(Mo4)(H2O)16 (Mo7O24)4]·8H2O (1) from a eutectic mixture for the first time. The method avoids lower yields, strict lower temperature condition (5 °C) and additional noxious organic reagent CH3OH, thus proving to be convenient and environmentally friendly. Compound 1 was synthesized at room temperature [26]. The nonvolatility of the eutectic mixture plays a crucial role in the synthesis process. Compared with the reported method, it simplifies the synthesis conditions. It does not need a low temperature (5 °C) and closed system in using the choline chloride/urea eutectic mixture as solvent compared with the reported method [25], under room temperature and open system would be appropriate. The reaction precursors (NH4)6Mo7O24·4H2O and Ce(NO3)3·6H2O show perfect solubility in this solvent and the product of 1 performs worse solubility in the solvent, so the product can be obtained in a relatively short time. We avoided using the noxious organic reagent CH3OH, thus making the process more environmentally friendly. What is more, there is no other additional reagent added, which follows the concept ‘atom economy’ and green chemistry. Through trial and error, the proper amount of reactants was found. The pH value was adjusted with 5 M HNO3, the choline chloride/urea eutectic mixture would not decompose with the addition of a small amount of dilute nitric acid.
L. Liu et al. / Inorganic Chemistry Communications 23 (2012) 14–16
15
Scheme 1. The coordination mode of the polyoxoanion of 1.
Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the monoclinic space group P2(1)/c [27]. There is a tetrahedral coordinated molybdenum atom in the center of the polyanion. The bond angles of O\Mo\O vary from 110.6(3)° to 108.4(3)°, distorting slightly from the perfect tetrahedral. Each oxygen atom of the central {MoO4} is bonded to a cerium atom. Each of the four cerium atoms performs nine-fold coordinated to nine oxygen atoms, one of them is from the central {MoO4}, four of them are from three different terminal oxygen atoms of {Mo7O24} unit and the last four are from aqua ligands. The coordination details of the structure are illustrated in Scheme 1. The structure of the unit cell of the cluster is shown in Figs. S1 and S2. The bond lengths of the Ce\O can be reduced to three types. Firstly, the cerium atoms coordinated with the oxygen atoms of the {MoO4} unit, the bond lengths are in the range of 2.421(6)–2.69(2) Å; secondly, the cerium atoms coordinated with the oxygen atoms of the water ligands, the bond lengths are in the range
Fig. 1. XPS spectrum of Ce3d (a) and Mo3d (b) for 1.
of 2.509(8)–2.692(8) Å; thirdly, the cerium atoms coordinated with the terminal oxygen of the heptamolybdate, the bond length scale is 2.475(6)–2.546(6) Å. There are two types of Mo\O bonds in the cluster: the Mo\O bond lengths in the center {MoO4} are in the range of 1.772(7)–1.746(5) Å; the bond lengths of Mo\O in the heptamolybdate are in the range of 1.680(9)–2.503(9)Å. The details of the selected bond length and bond angle are listed in Tables S1 and S2. The oxidation state of the molybdenum center and cerium center were identified by X-ray photoelectron spectrometer (XPS) (see Fig. 1). XPS for cerium center (see Fig. 1a) exhibits two peaks at ca. 885.8 and ca. 881.8 eV in the energy region of Ce3d5/2, two other peaks at ca. 904.5 and ca. 902.1 eV in the energy region of Ce3d3/2, consistent with the Ce III oxidation state [28–31]. The XPS for molybdenum center (Fig. 1b) shows a peak at 235.4 eV in the energy region of Mo3d3/2 and a peak 232.4 eV in the energy region of Mo3d5/2, which is consistent with the Mo VI oxidation state [28,32,33]. The IR spectrum of compound 1 is shown in Fig. S3, the characteristic peaks at 574, 619, 889 and 935 cm− 1 for 1 are attributed to ν(Mo\Ot), ν(Mo\Ob) and ν(Mo\Oc) vibrations of the polyanion. In addition, the other peaks at 1654, 1619, 1473, 1411 and 1083 cm- 1 for 1 are assigned to the vibrations of the choline cations and the urea. The UV–vis spectrum of compound 1 is recorded in aqueous solution at a concentration of 1.0 × 10 − 3 mol L − 1. The resulting spectrum is shown in Fig. S4. A main peak at 209 nm is originated from the O → Mo LMCT band. The TG curve of compound 1 is shown in Fig. S5, there are two weight loss steps. The first weight loss of 9.1% in the temperature range of 80–205 °C corresponds to the loss of water molecules, choline cations, lattice water of the polyanion. The second weight loss of 6.8% corresponds to the loss of water molecules coordinated to the cerium atoms. The whole weight loss of 15.2% is in agreement with the calculated value 14.7%. The electrochemical property of the compound is shown in Fig. 2. The cycle voltammetry (CV) of compound 1 was recorded in the pH= 4.0 (0.2 M H2SO4 + Na2SO4) buffer solution at the scan rate of 100 mV s− 1. Two reversible redox peaks appear in the potential range from +0.05 to −0.4 V versus Ag/AgCl as shown in Fig. 2. The mean potentials E1/2 = (Epa + Epc)/2 are −0.213 and −0.047 V, respectively. The two redox peaks could be ascribed to MoVI/MoV. In order to check the phase purity of 1, the powder X-ray diffraction (PXRD) patterns of 1 were recorded at room temperature. As shown in Fig. S6, the peak positions of experimental (Fig. S6a) are all almost consistent with the simulated patterns (Fig. S6b), which indicates the good phase purity of 1. The differences in intensity may be due to the preferred orientations of the crystalline powder samples. The result indicates that the compound formed in the eutectic mixture also has good phase purity. In summary, the cerium-containing POM aggregate {[(CH3)3 N(CH2)2OH]2(NH4)12}[Ce4(Mo4)(H2O)16(Mo7O24)4]·8H2O with anti-HIV activity has been successfully synthesized in choline chloride/urea eutectic mixture. This method avoids the disadvantages of poor solubility, lower yields, additional noxious organic reagents, thus proving to be convenient and environmentally friendly. We have successfully introduced this green and facile chemical way to synthesize high-quality POM aggregates.
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L. Liu et al. / Inorganic Chemistry Communications 23 (2012) 14–16
Fig. 2. Cyclic voltammograms of 2 × 10− 3 M of 1 at pH = 4 (0.2 M H2SO4 + Na2SO4) buffer solution at the scan rate of 100 mV s− 1.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21001022 and 21131001), Science and Technology Development Project Foundation of Jilin Province (No. 20100169 and 201201072), the Fundamental Research Funds for the Central Universities (No. 11QNJJ014 and 11SSXT141) and the Analysis and Testing Foundation of Northeast Normal University. Appendix A. Supplementary materials Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC reference number 862608 for compound 1. The data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223/336-033; E-mail: [email protected]). Supplementary data containing chemicals and measurement, TG, UV and more structural figures and data of compound 1 associated with this article can be found. Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2012.05.025. References [1] E.R. Parnham, R.E. Morris, Ionothermal synthesis of zeolites, metal–organic frameworks, and inorganic–organic hybrids, Acc. Chem. Res. 40 (2007) 1005–1013. [2] P. Wasserscheid, W. Keim, Ionic liquids—new “solutions” for transition metal catalysis, Angew. Chem. Int. Ed. 39 (2000) 3772–3789. [3] R.E. Morris, Ionothermal synthesis—ionic liquids as functional solvents in the preparation of crystalline materials, Chem. Commun. (2009) 2990–2998. [4] E. Ahmed, M. Ruck, Ionothermal synthesis of polyoxometalates, Angew. Chem. Int. Ed. 51 (2012) 308–309. [5] E. Ahmed, M. Ruck, Homo- and heteroatomic polycations of groups 15 and 16. Recent advances in synthesis and isolation using room temperature ionic liquids, Coord. Chem. Rev. 255 (2011) 2892–2903. [6] E. Ahmed, M. Ruck, Chemistry of polynuclear transition-metal complexes in ionic liquids, Dalton Trans. 40 (2011) 9347–9357. [7] N. Zou, W.L. Chen, Y.G. Li, W.L. Liu, E.B. Wang, Two new polyoxometalates-based hybrids firstly synthesized in the ionic liquids, Inorg. Chem. Commun. 11 (2008) 1367–1370. [8] W.L. Chen, B.W. Chen, H.Q. Tan, Y.G. Li, Y.H. Wang, E.B. Wang, Ionothermal syntheses of three transition-metal-containing polyoxotungstate hybrids exhibiting the photocatalytic and electrocatalytic properties, J. Solid State Chem. 183 (2009) 310–321. [9] S.W. Lin, W.L. Liu, Y.G. Li, Q. Wu, E.B. Wang, Z.M. Zhang, Preparation of polyoxometalates in ionic liquids by ionothermal synthesis, Dalton Trans. 39 (2010) 1740–1743.
[10] S.W. Lin, W.L. Chen, Z.M. Zhang, W.L. Liu, E.B. Wang, Tetrakis(1-ethyl-3methylimidazolium) β-hexacosaoxidooctamolybdate, Acta Crystallogr. E 64 (2008) m954. [11] H. Fu, Y.G. Li, Y. Lu, W.L. Chen, Q. Wu, J.X. Meng, X. Wang, Z.M. Zhang, E.B. Wang, Polyoxometalate-based metal−organic frameworks assembled under the ionothermal conditions, Cryst. Growth Des. 11 (2011) 458–465. [12] C.H. Zhan, F. Wang, Y. Kang, J. Zhang, Lanthanide-thiophene-2,5-dicarboxylate frameworks: ionothermal synthesis, helical structures, photoluminescent properties, and single-crystal-to-single-crystal guest exchange, Inorg. Chem. 51 (2012) 523–530. [13] J. Zhang, T. Wu, S. Chen, P. Feng, X. Bu, Versatile structure-directing roles of deep-eutectic solvents and their implication in the generation of porosity and open metal sites for gas storage, Angew. Chem. Int. Ed. 48 (2009) 3486–3490. [14] A.P. Abbott, D. Boothby, G. Capper, D.L. Davies, R.K. Rasheed, Deep eutectic solvents formed between choline chloride and carboxylic acids: versatile alternatives to ionic liquids, J. Am. Chem. Soc. 126 (2004) 9142–9147. [15] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Novel solvent properties of choline chloride/urea mixtures, Chem. Commun. (2003) 70–71. [16] R.P. Swatloski, J.D. Holbrey, S.B. Memon, G.A. Caldwell, K.A. Caldwell, R.D. Rogers, Using Caenorhabditis elegans to probe toxicity of 1-alkyl-3-methylimidazolium chloride based ionic liquids, Chem. Commun. (2004) 668–669. [17] E.R. Cooper, C.D. Andrews, P.S. Wheatley, P.B. Webb, P. Wormald, R.E. Morris, Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues, Nature 430 (2004) 1012–1016. [18] J.H. Liao, P.C. Wu, Y.H. Bai, Eutectic mixture of choline chloride/urea as a green solvent in synthesis of a coordination polymer: [Zn(O3PCH2CO2)]·NH4, Inorg. Chem. Commun. 8 (2005) 390–392. [19] J. Zhang, T. Wu, S.M. Chen, P.Y. Feng, X.H. Bu, Versatile structure-directing roles of deep-eutectic solvents and their implication in the generation of porosity and open metal sites for gas storage, Angew. Chem. Int. Ed. 48 (2009) 3486–3490. [20] S.M. Wang, Y.W. Li, X.J. Feng, Y.G. Li, E.B. Wang, New synthetic route of polyoxometalate-based hybrids in choline chloride/urea eutectic media, Inorg. Chim. Acta 363 (2010) 1556–1560. [21] S.M. Wang, W.L. Chen, E.B. Wang, Two chain like B-type-Anderson-based hybrids synthesized in choline chloride/urea eutectic mixture, J. Clust. Sci. 21 (2010) 133–145. [22] S.M. Wang, W.L. Chen, E.B. Wang, Y.G. Li, F.X. Jia, L. Liu, Three new polyoxometalate-based hybrids prepared from choline chloride/urea deep eutectic mixture at room temperature, Inorg. Chem. Commun. 13 (2010) 972–975. [23] T. Yamase, Photo- and electrochromism of polyoxometalates and related materials, Chem. Rev. 98 (1998) 307–325. [24] J.T. Rhule, C.L. Hill, D.A. Judd, R.F. Schinazi, Polyoxometalates in medicine, Chem. Rev. 98 (1998) 327–357. [25] K. Burgemeister, D. Drewes, E.M. Limanski, I. Küper, B. Krebs, Formation of large clusters in the reaction of lanthanide cations with heptamolybdate, Eur. J. Inorg. Chem. 13 (2004) 2690–2694. [26] Compound 1 was synthesized using the following procedure: (NH4)6Mo7O24·4H2O (1.24 g, 1.00 mmol) was added to 10 mL choline chloride/urea eutectic mixture in a 25 mL beaker, Ce(NO3)3·6H2O (0.1 g, 0.23 mmol) was added to 2 mL choline chloride/urea eutectic mixture in another beaker. After they all dissolved in the solvent, the cerous nitrate solution was added to the heptamolybadte solution drop by drop. After stirring for 10 min, appropriate amount of dilute nitrate acid was added to above solution to adjust the pH value to 4–5, stirring for 3 h in air at room temperature. The dark yellow suspension was filtered and the filtrate was kept at room temperature. Yellow block crystals of 1 were isolated after 2 days. The products were collected by filtration, washed with absolute alcohol, and dried in vacuum desiccator at 80 °C for half an hour (yield: 82% based on Mo). Elemental Anal. Calcd (found) for compound 1 (wt.%) Mo 47.97 (48.51) Ce 9.66 (9.48), C 2.07 (1.89). IR (KBr disk, cm− 1): 3347 (br), 1627 (m), 1471 (w), 1409 (w), 1081 (m), 937 (m), 987 (s), 842 (m), 624 (s). [27] Crystal data for 1: Mo29Ce4O126C10H124N14, M = 5799.83, monoclinic, a = 22.021(4)Å, b = 31.341(6)Å, c = 29.608(6)Å, β = 101.24(3)° V = 20042(7) Å3, Z = 2, Dcalc = 1.817 mg m-3, T = 150(2) K, R1 = 0.0554. [28] T.L. Barr, C.G. Fries, F. Cariati, J.C.J. Bart, N. Giordano, A spectroscopic investigation of cerium molybdenum oxides, J. Chem. Soc., Dalton Trans. (1983) 1825–1829. [29] G. Praline, B.E. Koel, R.L. Hance, H.I. Lee, J.M. White, X-ray photoelectron study of the reaction of oxygen with cerium, J. Electron. Spectrosc. Relat. Phenom. 21 (1980) 17–30. [30] J.M. Pemba-Mabiala, M. Lenzi, J. Lenzi, A. Lebugle, XPS study of mixed cerium– terbium orthophosphate catalysts, Surf. Interface Anal. 15 (1990) 663–667. [31] G.M. Ingo, E. Paparazzo, O. Bagnarelli, N. Zacchetti, XPS studies on cerium, zirconium and yttrium valence states in plasma-sprayed coatings, Surf. Interface Anal. 16 (1990) 515–519. [32] M. Shimoda, T. Hirata, K. Yagisawa, M. Okochi, A. Yoshikawa, Deconvolution of Mo3d X-ray photoemission spectra γ-Mo4O11: agreement with prediction from bond length–bond strength relationships, J. Mater. Sci. Lett. 8 (1989) 1089–1091. [33] M. Anwar, C.A. Hogarth, R. Bulpett, An XPS study of amorphous MoO3/SiO films deposited by co-evaporation, J. Mater. Sci. 25 (1990) 1784–1788.
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A high nuclear lanthanide-containing polyoxometalate aggregate synthesized in choline chloride/urea eutectic mixture Lin Liu, Shi-Ming Wang, Wei-Lin Chen ⁎, Ying Lu, Yang-Guang Li, En-Bo Wang ⁎ Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Renmin Street No. 5268, Changchun, Jilin, 130024, PR China
a r t i c l e
i n f o
Article history: Received 31 March 2012 Accepted 15 May 2012 Available online 24 May 2012 Keywords: Polyoxometalate Ionic liquid Eutectic mixture Lanthanide
a b s t r a c t A high nuclear lanthanide-containing polyoxometalate aggregate {[(CH3)3N(CH2)2OH]2(NH4)12}[Ce4(Mo4) (H2O)16(Mo7O24)4]·8H2O has been successfully synthesized in choline chloride/urea eutectic mixture. This method in synthesizing this anti-HIV activity aggregate has the advantages of convenience, facility, saving time and absence of additional noxious organic reagents. It has been characterized by XPS, IR, UV–vis spectra, TG analyses, power X-ray diffraction and single crystal X-ray diffraction. The CV property of the compound has also been tested. © 2012 Elsevier B.V. All rights reserved.
Ionic liquids have become one of the alternative solvents in the polyoxometalate (POM) synthesis area, because they possess the following advantages: nonvolatility, high fluidity, low melting temperature, high boiling temperature and nonflammability [1–6]. Many new POMs compounds including isopolyacid and heteropolyacid, sandwich structures, high-nuclear transition metal substituted POMs and POM-based metal–organic frameworks are prepared with this kind of solvent [7–11]. And also a deep eutectic mixture is a nice solvent for the synthesis of new coordination polymers [12,13]. A eutectic mixture is a kind of ionic liquid, which has the general properties of ionic liquids. However, it has its own special properties compared with the imidazole ionic liquids. The pure state of eutectic mixture can be obtained only through simply mixing the two components together mechanically [14], a eutectic mixture is the proper solvent for many different types of inorganic precursors. Even some ‘insoluble’ metal oxides were found to have significant solubility in a eutectic mixture. In addition, it does not react with water and it is biodegradable [15]. The choline chloride/urea eutectic mixture we used in this paper possesses very low toxicity compared with the imidazole ionic liquids [14,16]. Choline chloride is vitamin B4, an additive in chicken feed, and urea is a kind of common fertilizer. Many zeolites and coordination polymers were isolated from choline chloride/urea eutectic mixture under high temperature [17–19]. And some POM-based hybrids were also obtained from it under room temperature. On the basis of previous researches, polyoxomolybdate precursors perform good solubility in choline chloride/urea eutectic mixture, and the terminal oxygen atoms of the polyoxomolybdates ⁎ Corresponding authors. Tel./fax: + 86 431 85098787. E-mail addresses: [email protected] (W-L. Chen), [email protected] (E-B. Wang). 1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2012.05.025
exhibit high activities to coordinate with the metal atoms in this kind of eutectic mixture [20–22]. With the anti-HIV activity of the polyoxometalate PM-104 reported by Yamase et al., there are several investigations of the structure and the luminescence properties of this kind of polyoxometalate [23–25], but the reported synthesis method is complicated, so it is crucial to find more convenient ways of synthesis to promote expanded production. On the basis of the aforementioned considerations, we successfully isolated a heptamolybdate-based cluster with the formula {[(CH3)3 N(CH2)2OH]2(NH4)12}[Ce4(Mo4)(H2O)16 (Mo7O24)4]·8H2O (1) from a eutectic mixture for the first time. The method avoids lower yields, strict lower temperature condition (5 °C) and additional noxious organic reagent CH3OH, thus proving to be convenient and environmentally friendly. Compound 1 was synthesized at room temperature [26]. The nonvolatility of the eutectic mixture plays a crucial role in the synthesis process. Compared with the reported method, it simplifies the synthesis conditions. It does not need a low temperature (5 °C) and closed system in using the choline chloride/urea eutectic mixture as solvent compared with the reported method [25], under room temperature and open system would be appropriate. The reaction precursors (NH4)6Mo7O24·4H2O and Ce(NO3)3·6H2O show perfect solubility in this solvent and the product of 1 performs worse solubility in the solvent, so the product can be obtained in a relatively short time. We avoided using the noxious organic reagent CH3OH, thus making the process more environmentally friendly. What is more, there is no other additional reagent added, which follows the concept ‘atom economy’ and green chemistry. Through trial and error, the proper amount of reactants was found. The pH value was adjusted with 5 M HNO3, the choline chloride/urea eutectic mixture would not decompose with the addition of a small amount of dilute nitric acid.
L. Liu et al. / Inorganic Chemistry Communications 23 (2012) 14–16
15
Scheme 1. The coordination mode of the polyoxoanion of 1.
Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the monoclinic space group P2(1)/c [27]. There is a tetrahedral coordinated molybdenum atom in the center of the polyanion. The bond angles of O\Mo\O vary from 110.6(3)° to 108.4(3)°, distorting slightly from the perfect tetrahedral. Each oxygen atom of the central {MoO4} is bonded to a cerium atom. Each of the four cerium atoms performs nine-fold coordinated to nine oxygen atoms, one of them is from the central {MoO4}, four of them are from three different terminal oxygen atoms of {Mo7O24} unit and the last four are from aqua ligands. The coordination details of the structure are illustrated in Scheme 1. The structure of the unit cell of the cluster is shown in Figs. S1 and S2. The bond lengths of the Ce\O can be reduced to three types. Firstly, the cerium atoms coordinated with the oxygen atoms of the {MoO4} unit, the bond lengths are in the range of 2.421(6)–2.69(2) Å; secondly, the cerium atoms coordinated with the oxygen atoms of the water ligands, the bond lengths are in the range
Fig. 1. XPS spectrum of Ce3d (a) and Mo3d (b) for 1.
of 2.509(8)–2.692(8) Å; thirdly, the cerium atoms coordinated with the terminal oxygen of the heptamolybdate, the bond length scale is 2.475(6)–2.546(6) Å. There are two types of Mo\O bonds in the cluster: the Mo\O bond lengths in the center {MoO4} are in the range of 1.772(7)–1.746(5) Å; the bond lengths of Mo\O in the heptamolybdate are in the range of 1.680(9)–2.503(9)Å. The details of the selected bond length and bond angle are listed in Tables S1 and S2. The oxidation state of the molybdenum center and cerium center were identified by X-ray photoelectron spectrometer (XPS) (see Fig. 1). XPS for cerium center (see Fig. 1a) exhibits two peaks at ca. 885.8 and ca. 881.8 eV in the energy region of Ce3d5/2, two other peaks at ca. 904.5 and ca. 902.1 eV in the energy region of Ce3d3/2, consistent with the Ce III oxidation state [28–31]. The XPS for molybdenum center (Fig. 1b) shows a peak at 235.4 eV in the energy region of Mo3d3/2 and a peak 232.4 eV in the energy region of Mo3d5/2, which is consistent with the Mo VI oxidation state [28,32,33]. The IR spectrum of compound 1 is shown in Fig. S3, the characteristic peaks at 574, 619, 889 and 935 cm− 1 for 1 are attributed to ν(Mo\Ot), ν(Mo\Ob) and ν(Mo\Oc) vibrations of the polyanion. In addition, the other peaks at 1654, 1619, 1473, 1411 and 1083 cm- 1 for 1 are assigned to the vibrations of the choline cations and the urea. The UV–vis spectrum of compound 1 is recorded in aqueous solution at a concentration of 1.0 × 10 − 3 mol L − 1. The resulting spectrum is shown in Fig. S4. A main peak at 209 nm is originated from the O → Mo LMCT band. The TG curve of compound 1 is shown in Fig. S5, there are two weight loss steps. The first weight loss of 9.1% in the temperature range of 80–205 °C corresponds to the loss of water molecules, choline cations, lattice water of the polyanion. The second weight loss of 6.8% corresponds to the loss of water molecules coordinated to the cerium atoms. The whole weight loss of 15.2% is in agreement with the calculated value 14.7%. The electrochemical property of the compound is shown in Fig. 2. The cycle voltammetry (CV) of compound 1 was recorded in the pH= 4.0 (0.2 M H2SO4 + Na2SO4) buffer solution at the scan rate of 100 mV s− 1. Two reversible redox peaks appear in the potential range from +0.05 to −0.4 V versus Ag/AgCl as shown in Fig. 2. The mean potentials E1/2 = (Epa + Epc)/2 are −0.213 and −0.047 V, respectively. The two redox peaks could be ascribed to MoVI/MoV. In order to check the phase purity of 1, the powder X-ray diffraction (PXRD) patterns of 1 were recorded at room temperature. As shown in Fig. S6, the peak positions of experimental (Fig. S6a) are all almost consistent with the simulated patterns (Fig. S6b), which indicates the good phase purity of 1. The differences in intensity may be due to the preferred orientations of the crystalline powder samples. The result indicates that the compound formed in the eutectic mixture also has good phase purity. In summary, the cerium-containing POM aggregate {[(CH3)3 N(CH2)2OH]2(NH4)12}[Ce4(Mo4)(H2O)16(Mo7O24)4]·8H2O with anti-HIV activity has been successfully synthesized in choline chloride/urea eutectic mixture. This method avoids the disadvantages of poor solubility, lower yields, additional noxious organic reagents, thus proving to be convenient and environmentally friendly. We have successfully introduced this green and facile chemical way to synthesize high-quality POM aggregates.
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L. Liu et al. / Inorganic Chemistry Communications 23 (2012) 14–16
Fig. 2. Cyclic voltammograms of 2 × 10− 3 M of 1 at pH = 4 (0.2 M H2SO4 + Na2SO4) buffer solution at the scan rate of 100 mV s− 1.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21001022 and 21131001), Science and Technology Development Project Foundation of Jilin Province (No. 20100169 and 201201072), the Fundamental Research Funds for the Central Universities (No. 11QNJJ014 and 11SSXT141) and the Analysis and Testing Foundation of Northeast Normal University. Appendix A. Supplementary materials Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC reference number 862608 for compound 1. The data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223/336-033; E-mail: [email protected]). Supplementary data containing chemicals and measurement, TG, UV and more structural figures and data of compound 1 associated with this article can be found. Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2012.05.025. References [1] E.R. Parnham, R.E. Morris, Ionothermal synthesis of zeolites, metal–organic frameworks, and inorganic–organic hybrids, Acc. Chem. Res. 40 (2007) 1005–1013. [2] P. Wasserscheid, W. Keim, Ionic liquids—new “solutions” for transition metal catalysis, Angew. Chem. Int. Ed. 39 (2000) 3772–3789. [3] R.E. Morris, Ionothermal synthesis—ionic liquids as functional solvents in the preparation of crystalline materials, Chem. Commun. (2009) 2990–2998. [4] E. Ahmed, M. Ruck, Ionothermal synthesis of polyoxometalates, Angew. Chem. Int. Ed. 51 (2012) 308–309. [5] E. Ahmed, M. Ruck, Homo- and heteroatomic polycations of groups 15 and 16. Recent advances in synthesis and isolation using room temperature ionic liquids, Coord. Chem. Rev. 255 (2011) 2892–2903. [6] E. Ahmed, M. Ruck, Chemistry of polynuclear transition-metal complexes in ionic liquids, Dalton Trans. 40 (2011) 9347–9357. [7] N. Zou, W.L. Chen, Y.G. Li, W.L. Liu, E.B. Wang, Two new polyoxometalates-based hybrids firstly synthesized in the ionic liquids, Inorg. Chem. Commun. 11 (2008) 1367–1370. [8] W.L. Chen, B.W. Chen, H.Q. Tan, Y.G. Li, Y.H. Wang, E.B. Wang, Ionothermal syntheses of three transition-metal-containing polyoxotungstate hybrids exhibiting the photocatalytic and electrocatalytic properties, J. Solid State Chem. 183 (2009) 310–321. [9] S.W. Lin, W.L. Liu, Y.G. Li, Q. Wu, E.B. Wang, Z.M. Zhang, Preparation of polyoxometalates in ionic liquids by ionothermal synthesis, Dalton Trans. 39 (2010) 1740–1743.
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The products were collected by filtration, washed with absolute alcohol, and dried in vacuum desiccator at 80 °C for half an hour (yield: 82% based on Mo). Elemental Anal. Calcd (found) for compound 1 (wt.%) Mo 47.97 (48.51) Ce 9.66 (9.48), C 2.07 (1.89). IR (KBr disk, cm− 1): 3347 (br), 1627 (m), 1471 (w), 1409 (w), 1081 (m), 937 (m), 987 (s), 842 (m), 624 (s). [27] Crystal data for 1: Mo29Ce4O126C10H124N14, M = 5799.83, monoclinic, a = 22.021(4)Å, b = 31.341(6)Å, c = 29.608(6)Å, β = 101.24(3)° V = 20042(7) Å3, Z = 2, Dcalc = 1.817 mg m-3, T = 150(2) K, R1 = 0.0554. [28] T.L. Barr, C.G. Fries, F. Cariati, J.C.J. Bart, N. Giordano, A spectroscopic investigation of cerium molybdenum oxides, J. Chem. Soc., Dalton Trans. (1983) 1825–1829. [29] G. Praline, B.E. Koel, R.L. Hance, H.I. Lee, J.M. White, X-ray photoelectron study of the reaction of oxygen with cerium, J. Electron. Spectrosc. Relat. Phenom. 21 (1980) 17–30. [30] J.M. Pemba-Mabiala, M. Lenzi, J. Lenzi, A. Lebugle, XPS study of mixed cerium– terbium orthophosphate catalysts, Surf. Interface Anal. 15 (1990) 663–667. [31] G.M. Ingo, E. Paparazzo, O. Bagnarelli, N. Zacchetti, XPS studies on cerium, zirconium and yttrium valence states in plasma-sprayed coatings, Surf. Interface Anal. 16 (1990) 515–519. [32] M. Shimoda, T. Hirata, K. Yagisawa, M. Okochi, A. Yoshikawa, Deconvolution of Mo3d X-ray photoemission spectra γ-Mo4O11: agreement with prediction from bond length–bond strength relationships, J. Mater. Sci. Lett. 8 (1989) 1089–1091. [33] M. Anwar, C.A. Hogarth, R. Bulpett, An XPS study of amorphous MoO3/SiO films deposited by co-evaporation, J. Mater. Sci. 25 (1990) 1784–1788.