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A novel POM-based inorganic porous framework: Synthesis, structure and properties
Inorganic Chemistry Communications 36 (2013) 166–169
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A novel POM-based inorganic porous framework: Synthesis, structure and properties Yingying Sun, Jing Lu ⁎, Dacheng Li, Jianmin Dou Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252059, P.R. China
a r t i c l e
i n f o
Article history: Received 10 July 2013 Accepted 29 August 2013 Available online 4 September 2013 Keywords: Multifunctional porous material Adamantane network Helical chain
a b s t r a c t A novel POM-based inorganic porous framework, (H2en)3[Mn3P4Mo4O28] 1 (en = ethylenediamine),has been hydrothermally synthesized and characterized by elemental analyses, IR spectroscopy, TG analyses, and single crystal X-ray diffraction. In compound 1, PO4 groups link Mn atoms to form three-fold interpenetrated adamantane networks; the Mo-O helical chains connect with the adamantane networks through μ2-bridging oxygen atoms and PO4 groups. The PO4, MnO4, MnO6 and MoO6 polyhedrons share corners and connect with each other to form a 3-D complicated framework with 1-D channels. And the channels are filled with the diprotonated ethylenediamine cations. The magnetic behavior of 1 is antiferromagnetic coupling. Futhermore, compound 1 exhibits good thermal stability and active electrochemical behavior. © 2013 Elsevier B.V. All rights reserved.
In the past decade, metal-organic frameworks (MOFs), as a new class of crystalline porous materials, have become one of the most prolific areas of research in chemistry and materials [1,2]. Through the combination of diverse metal-based building units and numerous organic linkers, MOFs possess rich structures and adjustable pore size, which are quite different from traditional porous zeolites [3]. Compared with zeolite materials, MOFs have one major disadvantage, which is their weak stability. Therefore, it is exigent and challenging to obtain new types of porous materials that combine the sophistication and versatility of MOFs with the thermal stability of zeolites. Besides versatility and stability, multifunctionality is another demand for new types of porous materials. Polyoxometalates (POMs), as a unique class of metal-oxide clusters with controllable shape and size, highly negative charges, and oxo-enriched surface, are often connected by various organic linkers or metal–organic bridges to form multifunctional porous frameworks with potential applications in magnetic, medicine assays, biological analysis, as well as gas storage, ion exchange and catalysis [4–11]. Among the numerous POM-based porous frameworks, most of them are constructed from classical POM or transition-metal-substituted POMs clusters. But those POM-based frameworks with organic compositions usually possess low stability. So, a promising way to obtain new types of multifunctional porous materials with both versatility and stability is to link diverse POM units through inorganic linkers to construct pure inorganic porous frameworks. Although POM-based inorganic porous materials possess good stability and multifunctionality, to the best of our knowledge, studies on their designed synthesis and properties are quite scarce [12]. Herein, ⁎ Corresponding author. Fax: +86 635 8239001. E-mail addresses: [email protected], [email protected] (J. Lu). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.08.027
we report a novel POM-based inorganic porous framework (H2en)3 [Mn3P4Mo4O28] 1. It exhibits a novel three-dimensional framework constructed from Mn-Mo-P-O cluster with diprotonated ethylenediamine as charge-compensating cations in the channels. And the magnetic and electrochemistry properties of compound 1 are also investigated. Compound 1 was synthesized by the hydrothermal technique [13]. The experimental XRD pattern agrees well with the simulated one generated on the basis of the single-crystal analyses for 1 (Fig. S1), indicating the phase purity of the products. Thermogravimetric analysis experimental results (Fig. S2) show that compound 1 is stable until 253 °C and then loses 14.30% of its weight between 253-379 °C, corresponding to the release of three diprotonated ethylenediamine molecules (Calcd. 14.23%). No weight loss and no heat effect signal are observed from 380 °C to 760 °C, showing that the host framework could retain structural integrity. Single-crystal X-ray structural analysis reveals that compound 1 crystallizes in tetragonal crystal system, I41/a space group [14] and there are two independent Mn (Mn1, 1/4 occupied; Mn2, halfoccupied) atoms, one PO4 group, one Mo atom, three μ2-O atoms and three-quarter protonated ethylenediamine molecules in the asymmetric unit. As shown in Fig. 1, these two crystallographically independent Mn atoms exhibit different coordination environments where the Mn1 atom adopts a tetrahedral geometry, which is coordinated by four oxygen atoms (O3, O3D, O3E and O3G) from four adjacent PO4 groups, and the Mn2 atom exhibits a distorted octahedral geometry, coordinated by two oxygen atoms (O2 and O2B) from two PO4 groups and four oxygen atoms (O6C, O6F, O7 and O7B). The Mo1 atom also exhibits a distorted octahedral geometry, coordinated by two oxygen atoms (O1 and O4A) from PO4 groups, two oxygen atoms (O6 and O7) from two adjacent MnO6 groups, and two oxygen atoms (O5 and O5A) from two adjacent MoO6 groups. Thus,
Y. Sun et al. / Inorganic Chemistry Communications 36 (2013) 166–169
Fig. 1. The coordination environments of atoms in 1 (all H atoms are omitted for clarity). Symmetry code: A. 1.25−y, −0.25 + x, −0.25 + z; B. 1.5−x, 0.5−y, 0.5−z; C. 1.25−y, −0.75 + x, 0.25−z; D. 0.25 + y, 0.75−x, −0.25−z; E. 0.75−y, −0.25 + x, −0.25−z; F. 0.75 + y, 1.25−x, 0.25−z; G. 0.25 + y, 1.25−x, 0.25 + z.
the PO4 group is coordinated with two Mn atoms (Mn1 and Mn2) and two Mo atoms via its four oxygen atoms. The MnO4 tetrahedron is surrounded by four PO4 tetrahedrons, the MnO6 octahedron is surrounded by two PO4 tetrahedrons and four MoO6 octahedrons, and the MoO6 octahedron is surrounded by two PO4 tetrahedrons, two MnO6 octahedrons and two other MoO6 octahedrons. Bondvalence calculations for compound 1 suggest that the Mo, Mn, P and O atoms adopt +6, +2, +5 and -2 oxidation states, respectively, thus the [Mn3P4Mo4O28] cluster exhibits a -6 oxidation state. So the ethylenediamine moieties are diprotonated for charge balance. For the crystal structure of 1, the connections between the atoms are interesting and complicated. Firstly, in order to analysize the crystal structure expediently, only the Mn atoms and PO4 groups are considered. As shown in Fig. 2a, the twenty-four PO4 groups link ten Mn1 and twelve Mn2 atoms to form an adamantane cage with the Mn1∙∙∙Mn2 distance of 5.92 Å and Mn1∙∙∙Mn1 distance of 11.84 Å. In this cage, each Mn2 atom lies in the middle of the edge and links with two Mn1 atoms; while each Mn1 atom is sited on the corner of the adamantane, and links four Mn2 atoms from the cage itself and adjacent ones. So, the adamantane cage can be extended into three-dimensional adamantane networks (Fig. 2b). Secondly, if only Mo atoms and the μ2-O atoms are considered, it can be found that the μ2-bridging oxygen atom O5 and its symmetrical ones link the Mo atoms to form left- and right-handed helical chains (Fig. 2c) along c axis with the helix pitch 10.83 Ǻ. As shown in Fig. 3a, each Mo atom connects with two Mn1 atoms through the PO4 groups and two Mn2 atoms through μ2- O6 and O7 atoms. That is to say, the Mo-O helical chains are connected with the
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3-D adamantane network and filled in the channels along c direction (Fig. 3b and c). As shown in Fig. 3c, each left-handed Mo-O helical chain is surrounded by four right-handed ones, and each right-handed helical chain is enclosed by four left-handed ones. The number of the two handed chains is identical, so, compound 1 is mesomer. It must be mentioned that in the 3-D adamantane network, the nearest Mn∙∙∙Mn distance along c direction is 32.49 Ǻ, which is three times of the helix pitch. Thus it can be supposed that the Mo-O helical chain may connect with three adamantane networks at the same time. In fact, according to the structure analysis results, the 3-D adamantane networks are threefold interpenetration (Fig. 3c and d). The distance of adjacent networks is 10.83 Ǻ, which is equal to the helix pitch. In a word, The PO4, MnO4, MnO6 and MoO6 polyhedrons share corners with each other to form a 3-D complicated network with the 1-D channels along [101] direction (shown in Fig. 4). The channels are filled with diprotonated ethylenediamine cations with N-H…O hydrogen bonds interactions (N…O distance in the range of 2.8-3.2 Å) between the host and guest. PLATON calculations indicate that diprotonated ethylenediamine cations occupy a volume of 1206.4 Å3 per unit cell (37.4% of the crystal lattice) for 1. The magnetic susceptibility of compound 1 was performed on polycrystalline samples (m = 25.19 mg) in the temperature range of 2-300 K in a 1500 Oe magnetic field. The temperature dependence of 1/χM and χMT is shown in Fig. 5. Since Mo(VI) (4d0, s = 0) ions do not possess an effective magnetic moment, they make no contribution to magnetism. So the magnetism may be attributed solely to the presence of Mn(II) (3d5, S = 5/2) ions. At room temperature, the χMT product is 13.66 cm3 K mol- 1, which is slightly larger than the expected value 13.12 cm3 · K · mol-1 for three uncoupled Mn(II) ions taking into account the g value of 2.0. On lowering the temperature, the χMT value of 1 slowly decreases from 13.66 cm3 K mol-1 at 300 K to 3.54 cm3 · K · mol-1 at 2.0 K. Such a behavior of χMT curve indicates a mainly antiferromagnetic interaction in 1. The χ-1 M vs T plot is well fitted by the Curie–Weiss law from 300 K to 2 K with the Curie constant of 13.99 cm3 · K · mol-1 and Weiss temperature of -8.20 K. The negative Weiss constant further indicates that the antiferromagnetic interactions exist in compound 1. The electrochemical characterization of compound 1 in 1-CPE was performed in 1 M H2SO4 aqueous solutions at room temperature. Fig. 6 shows the cyclic voltammogram behaviors at a potential range from -1.35 to 1.0 V for 1-CPE at different scan rates. The cathodic peak potentials are shifted slightly towards the negative direction, and the corresponding anodic peak potentials are shifted slightly towards the positive direction with increasing scan rates from 50 to 250 mV · s-1, suggesting that the redox processes are surface-controlled [15,16]. It can be seen five irreversible redox peaks with mean peak potentials -1020 mV(I), -26 mV(II), 79 mV (III), 326 mV (IV) and 726 mV (V) (scan rate: 250 mV), respectively. Redox peaks II–II′, III–III′, IV–IV′
Fig. 2. (a) PO4 groups link Mn atoms to form a diamondoid cage; (b) View of the 3-D diamondoid framework in 1; (c) Mo atoms are connected to form left- and right-handed helical chains extending along c direction through μ2-bridging oxygen atoms.
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Y. Sun et al. / Inorganic Chemistry Communications 36 (2013) 166–169
a) c)
b)
d)
Fig. 3. (a) The ball-stick representation of the connections between Mo–O helical chains and Mn atoms; (b) Topological representation of a single adamantane cage interpenetrated by Mo– O helical chains; (c) The connections between adamantane networks and Mo–O helical chains; (d) The three-fold interpenetrated adamantane networks viewed along different directions.
should be ascribed to three consecutive two-electron redox processes of Mo [17,18], respectively. Redox peaks I–I′ and V–V′ may correspond to the redox processes of the Mn2+ ions [19,20]. Thus the electrochemical results indicate that compound 1 is a potential electrocatalyst and modified electrode material. In conclusion, a novel POM-based inorganic porous framework has been synthesized by hydrothermal techniques, in which PO4 groups link Mn atoms to form three-fold interpenetrated adamantane networks. The Mo-O helical chains connect with the adamantane networks to form a 3-D complicated network with the 1-D channels filled by diprotonated ethylenediamine cations. This POM-based inorganic
Fig. 4. The MnO4 (cyan), PO4 (purple), the MnO6 (cyan) and MoO6 (dark yellow) polyhedrons share corners with each other to form 3-D complicated networks with 1-D channels (the channels are filled by diprotonated ethylenediamine cations, which are omitted for clarity).
porous framework shows remarkable stability and excellent electrocatalytic activity. Acknowledgement This work was supported by the Nation Nature Science Foundation of China (No. 21101086 and No. 21041002). Appendix A. Supplementary material X-ray crystallographic files for compound 1 and additional figures. CCDC number: 932067. The data can be obtained free of charge via http://www.ccdc.cam.ac.uk/deposit (or from the
Fig. 5. Temperature dependence of χMT and 1/χM for compound 1.
Y. Sun et al. / Inorganic Chemistry Communications 36 (2013) 166–169
Fig. 6. Cyclic voltammograms of 1-CPE in 1 M H2SO4 aqueous solution at different scan rates (from inner to outer: 50, 100, 150, 200, 250 mV · s-1).
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK). Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.inoche. 2013.08.027. References [1] Z.Y. Li, G.S. Zhu, G.Q. Lu, S.L. Qiu, X.D. Yao, J. Am. Chem. Soc. 132 (2010) 1490–1491. [2] J.J. Jiang, L. Li, M.H. Lan, M. Pan, A. Eichhofer, D. Fenske, C.Y. Su, Chem. Eur. J. 16 (2010) 1841–1848. [3] X.F. Liu, M. Oh, M.S. Lah, Inorg. Chem. 50 (2011) 5044–5053. [4] L.X. Shi, X.W. Zhang, C.D. Wu, Dalton Trans. 40 (2011) 779–781. [5] B. Botar, Y.V. Geletili, P. Kögerler, D.G. Musaew, C.L. Hill, J. Am. Chem. Soc. 128 (2006) 11268–11277.
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[6] F.J. Ma, S.X. Liu, C.Y. Sun, D.D. Liang, G.J. Ren, F. Wei, Y.G. Chen, Z.M. Su, J. Am. Chem. Soc. 133 (2011) 4178–4181. [7] M. Inoue, T. Suzuki, Y. Fujita, M. Oda, N. Matsumoto, T. Yamase, J. Inorg. Biochem. 100 (2006) 1225–1233. [8] H.D. Zeng, G.R. Newkome, C.T. Hill, Angew. Chem. Int. Ed. 39 (2000) 1771–1774. [9] A. Dolbecq, P. Mialane, F. Sécheresse, B. Keitab, L. Nadjo, Chem. Commun. 48 (2012) 8299–8316. [10] E. Coronado, C.J. Gómez-García, Chem. Rev. 98 (1998) 273–296. [11] H. Deng, C.J. Doonan, H. Furukawa, R.B. Ferreira, J. Towne, C.B. Knobler, B. Wang, O.M. Yaghi, Science 327 (2010) 846–850. [12] S.G. Mitchell, T. Boyd, H.N. Miras, D.L. Long, L. Cronin, Inorg. Chem. 50 (2011) 136–143. [13] The compound 1 was hydrothermally synthesized under autogenous pressure. A mixture of Mn(OAc)2 · 4H2O (0.1 g), Dy(OAc)3 · 10H2O (0.1 g), (NH4)6Mo7O24 · 4H2O (0.2 g), ethylenediamine (0. 3 ml), and water (10 ml) was neutralized to pH 5 with H3PO4 under continuous stirring and sealed in a 15 ml Teflon-lined autoclave, which was heated to 180 °C for five days. After slow cooling to room temperature, red block crystals of 1 were separated as a major phase (75% yield based on Mo) together with a small amount of an unidentified white powder. Anal. Calc. For 1: C, 5.51%; N, 6.43%; H, 2.31%; P, 9.48; Mn, 12.61%; Mo, 29.37%, Found: C, 5.62%; N, 6.55%; H, 2.42%; P, 9.41%; Mn, 12.49%; Mo, 29.46%. IR (KBr, cm- 1): 3422.4 m, 1611.8 m, 1506.9w, 1455.3w, 1384.8w, 1347.0w, 1320.7w, 1094.3 s, 990.9 s, 953.5 s, 827.3 s, 739.4 s. [14] Crystal data for 1: C6H30Mn3Mo4N6O28P4, M = 1306.82, tetragonal, a = 17.2470(19) Å, b = 17.2470(19) Å, c = 10.8317(11) Å, α = 90.00° β = 90.00° γ = 90.00° V = 3222.0(6) Å3, T = 293(2) K, space group I 41/a, Z = 4, 2712 reflections measured, 1415 independent reflections (Rint = 0.0734). The final R1 values were 0.0687 (I N 2σ(I)). The final wR(F2) values were 0.1669 (all data). [15] G.F. Hou, L.H. Bi, B. Wang, L.X. Wu, CrystEngComm 13 (2011) 3526–3535. [16] L.Zh. Wu, H.Y. Ma, Zh.G. Han, Ch.X. Li, Solid State Sci. 11 (2009) 43–48. [17] A.X. Tian, J. Ying, J. Peng, J.Q. Sha, H.J. Pang, P.P. Zhang, Y. Chen, M. Zhu, Z.M. Su, Cryst. Growth Des. 8 (2008) 3717–3724. [18] Z.G. Han, Y.L. Zhao, J. Peng, Q. Liu, E.B. Wang, Electrochim. Acta 51 (2005) 218–224. [19] P.-E. Car, B. Spingler, S. Weyeneth, J. Patscheider, G.R. Patzke, Polyhedron 52 (2013) 151–158. [20] Y.W. Li, Y.G. Li, Y.H. Wang, X.J. Feng, Y. Lu, E.B. Wang, Inorg. Chem. 48 (2009) 6452–6458.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A novel POM-based inorganic porous framework: Synthesis, structure and properties Yingying Sun, Jing Lu ⁎, Dacheng Li, Jianmin Dou Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252059, P.R. China
a r t i c l e
i n f o
Article history: Received 10 July 2013 Accepted 29 August 2013 Available online 4 September 2013 Keywords: Multifunctional porous material Adamantane network Helical chain
a b s t r a c t A novel POM-based inorganic porous framework, (H2en)3[Mn3P4Mo4O28] 1 (en = ethylenediamine),has been hydrothermally synthesized and characterized by elemental analyses, IR spectroscopy, TG analyses, and single crystal X-ray diffraction. In compound 1, PO4 groups link Mn atoms to form three-fold interpenetrated adamantane networks; the Mo-O helical chains connect with the adamantane networks through μ2-bridging oxygen atoms and PO4 groups. The PO4, MnO4, MnO6 and MoO6 polyhedrons share corners and connect with each other to form a 3-D complicated framework with 1-D channels. And the channels are filled with the diprotonated ethylenediamine cations. The magnetic behavior of 1 is antiferromagnetic coupling. Futhermore, compound 1 exhibits good thermal stability and active electrochemical behavior. © 2013 Elsevier B.V. All rights reserved.
In the past decade, metal-organic frameworks (MOFs), as a new class of crystalline porous materials, have become one of the most prolific areas of research in chemistry and materials [1,2]. Through the combination of diverse metal-based building units and numerous organic linkers, MOFs possess rich structures and adjustable pore size, which are quite different from traditional porous zeolites [3]. Compared with zeolite materials, MOFs have one major disadvantage, which is their weak stability. Therefore, it is exigent and challenging to obtain new types of porous materials that combine the sophistication and versatility of MOFs with the thermal stability of zeolites. Besides versatility and stability, multifunctionality is another demand for new types of porous materials. Polyoxometalates (POMs), as a unique class of metal-oxide clusters with controllable shape and size, highly negative charges, and oxo-enriched surface, are often connected by various organic linkers or metal–organic bridges to form multifunctional porous frameworks with potential applications in magnetic, medicine assays, biological analysis, as well as gas storage, ion exchange and catalysis [4–11]. Among the numerous POM-based porous frameworks, most of them are constructed from classical POM or transition-metal-substituted POMs clusters. But those POM-based frameworks with organic compositions usually possess low stability. So, a promising way to obtain new types of multifunctional porous materials with both versatility and stability is to link diverse POM units through inorganic linkers to construct pure inorganic porous frameworks. Although POM-based inorganic porous materials possess good stability and multifunctionality, to the best of our knowledge, studies on their designed synthesis and properties are quite scarce [12]. Herein, ⁎ Corresponding author. Fax: +86 635 8239001. E-mail addresses: [email protected], [email protected] (J. Lu). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.08.027
we report a novel POM-based inorganic porous framework (H2en)3 [Mn3P4Mo4O28] 1. It exhibits a novel three-dimensional framework constructed from Mn-Mo-P-O cluster with diprotonated ethylenediamine as charge-compensating cations in the channels. And the magnetic and electrochemistry properties of compound 1 are also investigated. Compound 1 was synthesized by the hydrothermal technique [13]. The experimental XRD pattern agrees well with the simulated one generated on the basis of the single-crystal analyses for 1 (Fig. S1), indicating the phase purity of the products. Thermogravimetric analysis experimental results (Fig. S2) show that compound 1 is stable until 253 °C and then loses 14.30% of its weight between 253-379 °C, corresponding to the release of three diprotonated ethylenediamine molecules (Calcd. 14.23%). No weight loss and no heat effect signal are observed from 380 °C to 760 °C, showing that the host framework could retain structural integrity. Single-crystal X-ray structural analysis reveals that compound 1 crystallizes in tetragonal crystal system, I41/a space group [14] and there are two independent Mn (Mn1, 1/4 occupied; Mn2, halfoccupied) atoms, one PO4 group, one Mo atom, three μ2-O atoms and three-quarter protonated ethylenediamine molecules in the asymmetric unit. As shown in Fig. 1, these two crystallographically independent Mn atoms exhibit different coordination environments where the Mn1 atom adopts a tetrahedral geometry, which is coordinated by four oxygen atoms (O3, O3D, O3E and O3G) from four adjacent PO4 groups, and the Mn2 atom exhibits a distorted octahedral geometry, coordinated by two oxygen atoms (O2 and O2B) from two PO4 groups and four oxygen atoms (O6C, O6F, O7 and O7B). The Mo1 atom also exhibits a distorted octahedral geometry, coordinated by two oxygen atoms (O1 and O4A) from PO4 groups, two oxygen atoms (O6 and O7) from two adjacent MnO6 groups, and two oxygen atoms (O5 and O5A) from two adjacent MoO6 groups. Thus,
Y. Sun et al. / Inorganic Chemistry Communications 36 (2013) 166–169
Fig. 1. The coordination environments of atoms in 1 (all H atoms are omitted for clarity). Symmetry code: A. 1.25−y, −0.25 + x, −0.25 + z; B. 1.5−x, 0.5−y, 0.5−z; C. 1.25−y, −0.75 + x, 0.25−z; D. 0.25 + y, 0.75−x, −0.25−z; E. 0.75−y, −0.25 + x, −0.25−z; F. 0.75 + y, 1.25−x, 0.25−z; G. 0.25 + y, 1.25−x, 0.25 + z.
the PO4 group is coordinated with two Mn atoms (Mn1 and Mn2) and two Mo atoms via its four oxygen atoms. The MnO4 tetrahedron is surrounded by four PO4 tetrahedrons, the MnO6 octahedron is surrounded by two PO4 tetrahedrons and four MoO6 octahedrons, and the MoO6 octahedron is surrounded by two PO4 tetrahedrons, two MnO6 octahedrons and two other MoO6 octahedrons. Bondvalence calculations for compound 1 suggest that the Mo, Mn, P and O atoms adopt +6, +2, +5 and -2 oxidation states, respectively, thus the [Mn3P4Mo4O28] cluster exhibits a -6 oxidation state. So the ethylenediamine moieties are diprotonated for charge balance. For the crystal structure of 1, the connections between the atoms are interesting and complicated. Firstly, in order to analysize the crystal structure expediently, only the Mn atoms and PO4 groups are considered. As shown in Fig. 2a, the twenty-four PO4 groups link ten Mn1 and twelve Mn2 atoms to form an adamantane cage with the Mn1∙∙∙Mn2 distance of 5.92 Å and Mn1∙∙∙Mn1 distance of 11.84 Å. In this cage, each Mn2 atom lies in the middle of the edge and links with two Mn1 atoms; while each Mn1 atom is sited on the corner of the adamantane, and links four Mn2 atoms from the cage itself and adjacent ones. So, the adamantane cage can be extended into three-dimensional adamantane networks (Fig. 2b). Secondly, if only Mo atoms and the μ2-O atoms are considered, it can be found that the μ2-bridging oxygen atom O5 and its symmetrical ones link the Mo atoms to form left- and right-handed helical chains (Fig. 2c) along c axis with the helix pitch 10.83 Ǻ. As shown in Fig. 3a, each Mo atom connects with two Mn1 atoms through the PO4 groups and two Mn2 atoms through μ2- O6 and O7 atoms. That is to say, the Mo-O helical chains are connected with the
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3-D adamantane network and filled in the channels along c direction (Fig. 3b and c). As shown in Fig. 3c, each left-handed Mo-O helical chain is surrounded by four right-handed ones, and each right-handed helical chain is enclosed by four left-handed ones. The number of the two handed chains is identical, so, compound 1 is mesomer. It must be mentioned that in the 3-D adamantane network, the nearest Mn∙∙∙Mn distance along c direction is 32.49 Ǻ, which is three times of the helix pitch. Thus it can be supposed that the Mo-O helical chain may connect with three adamantane networks at the same time. In fact, according to the structure analysis results, the 3-D adamantane networks are threefold interpenetration (Fig. 3c and d). The distance of adjacent networks is 10.83 Ǻ, which is equal to the helix pitch. In a word, The PO4, MnO4, MnO6 and MoO6 polyhedrons share corners with each other to form a 3-D complicated network with the 1-D channels along [101] direction (shown in Fig. 4). The channels are filled with diprotonated ethylenediamine cations with N-H…O hydrogen bonds interactions (N…O distance in the range of 2.8-3.2 Å) between the host and guest. PLATON calculations indicate that diprotonated ethylenediamine cations occupy a volume of 1206.4 Å3 per unit cell (37.4% of the crystal lattice) for 1. The magnetic susceptibility of compound 1 was performed on polycrystalline samples (m = 25.19 mg) in the temperature range of 2-300 K in a 1500 Oe magnetic field. The temperature dependence of 1/χM and χMT is shown in Fig. 5. Since Mo(VI) (4d0, s = 0) ions do not possess an effective magnetic moment, they make no contribution to magnetism. So the magnetism may be attributed solely to the presence of Mn(II) (3d5, S = 5/2) ions. At room temperature, the χMT product is 13.66 cm3 K mol- 1, which is slightly larger than the expected value 13.12 cm3 · K · mol-1 for three uncoupled Mn(II) ions taking into account the g value of 2.0. On lowering the temperature, the χMT value of 1 slowly decreases from 13.66 cm3 K mol-1 at 300 K to 3.54 cm3 · K · mol-1 at 2.0 K. Such a behavior of χMT curve indicates a mainly antiferromagnetic interaction in 1. The χ-1 M vs T plot is well fitted by the Curie–Weiss law from 300 K to 2 K with the Curie constant of 13.99 cm3 · K · mol-1 and Weiss temperature of -8.20 K. The negative Weiss constant further indicates that the antiferromagnetic interactions exist in compound 1. The electrochemical characterization of compound 1 in 1-CPE was performed in 1 M H2SO4 aqueous solutions at room temperature. Fig. 6 shows the cyclic voltammogram behaviors at a potential range from -1.35 to 1.0 V for 1-CPE at different scan rates. The cathodic peak potentials are shifted slightly towards the negative direction, and the corresponding anodic peak potentials are shifted slightly towards the positive direction with increasing scan rates from 50 to 250 mV · s-1, suggesting that the redox processes are surface-controlled [15,16]. It can be seen five irreversible redox peaks with mean peak potentials -1020 mV(I), -26 mV(II), 79 mV (III), 326 mV (IV) and 726 mV (V) (scan rate: 250 mV), respectively. Redox peaks II–II′, III–III′, IV–IV′
Fig. 2. (a) PO4 groups link Mn atoms to form a diamondoid cage; (b) View of the 3-D diamondoid framework in 1; (c) Mo atoms are connected to form left- and right-handed helical chains extending along c direction through μ2-bridging oxygen atoms.
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a) c)
b)
d)
Fig. 3. (a) The ball-stick representation of the connections between Mo–O helical chains and Mn atoms; (b) Topological representation of a single adamantane cage interpenetrated by Mo– O helical chains; (c) The connections between adamantane networks and Mo–O helical chains; (d) The three-fold interpenetrated adamantane networks viewed along different directions.
should be ascribed to three consecutive two-electron redox processes of Mo [17,18], respectively. Redox peaks I–I′ and V–V′ may correspond to the redox processes of the Mn2+ ions [19,20]. Thus the electrochemical results indicate that compound 1 is a potential electrocatalyst and modified electrode material. In conclusion, a novel POM-based inorganic porous framework has been synthesized by hydrothermal techniques, in which PO4 groups link Mn atoms to form three-fold interpenetrated adamantane networks. The Mo-O helical chains connect with the adamantane networks to form a 3-D complicated network with the 1-D channels filled by diprotonated ethylenediamine cations. This POM-based inorganic
Fig. 4. The MnO4 (cyan), PO4 (purple), the MnO6 (cyan) and MoO6 (dark yellow) polyhedrons share corners with each other to form 3-D complicated networks with 1-D channels (the channels are filled by diprotonated ethylenediamine cations, which are omitted for clarity).
porous framework shows remarkable stability and excellent electrocatalytic activity. Acknowledgement This work was supported by the Nation Nature Science Foundation of China (No. 21101086 and No. 21041002). Appendix A. Supplementary material X-ray crystallographic files for compound 1 and additional figures. CCDC number: 932067. The data can be obtained free of charge via http://www.ccdc.cam.ac.uk/deposit (or from the
Fig. 5. Temperature dependence of χMT and 1/χM for compound 1.
Y. Sun et al. / Inorganic Chemistry Communications 36 (2013) 166–169
Fig. 6. Cyclic voltammograms of 1-CPE in 1 M H2SO4 aqueous solution at different scan rates (from inner to outer: 50, 100, 150, 200, 250 mV · s-1).
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK). Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.inoche. 2013.08.027. References [1] Z.Y. Li, G.S. Zhu, G.Q. Lu, S.L. Qiu, X.D. Yao, J. Am. Chem. Soc. 132 (2010) 1490–1491. [2] J.J. Jiang, L. Li, M.H. Lan, M. Pan, A. Eichhofer, D. Fenske, C.Y. Su, Chem. Eur. J. 16 (2010) 1841–1848. [3] X.F. Liu, M. Oh, M.S. Lah, Inorg. Chem. 50 (2011) 5044–5053. [4] L.X. Shi, X.W. Zhang, C.D. Wu, Dalton Trans. 40 (2011) 779–781. [5] B. Botar, Y.V. Geletili, P. Kögerler, D.G. Musaew, C.L. Hill, J. Am. Chem. Soc. 128 (2006) 11268–11277.
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