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A functionalized polyoxometalate by hexanuclear copper–amino acid coordination complexes
Inorganic Chemistry Communications 10 (2007) 299–302 www.elsevier.com/locate/inoche
A functionalized polyoxometalate by hexanuclear copper–amino acid coordination complexes Haiyan An, Enbo Wang *, Yangguang Li, Zhiming Zhang, Lin Xu Key Laboratory of Polyoxometalate Science of Ministry of Education, Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University, Changchun 130024, PR China Received 5 October 2006; accepted 13 November 2006 Available online 21 November 2006
Abstract A new functionalized polyoxometalate by transition metal–amino acid coordination complexes, H4[Na(H2O)2][Cu6Na(gly)8 (H2O)2][BW12O40]2 Æ 13H2O (1) (gly = glycine) has been synthesized and characterized by elemental analysis, IR spectroscopy, TG analysis, and single crystal X-ray diffraction. Compound 1 possesses a 3D framework, which is built up of [BW12O40]5 building blocks, hexanuclear [Cu6Na(gly)8] coordination clusters and Na+ ions. Furthermore, magnetic measurements show ferromagnetic interactions for compound 1. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Functionalization; Polyoxometalates; Amino acids; Copper; Magnetic properties
Polyoxometalates (POMs), as anionic early transitionmetal oxide clusters, bear many properties that make them attractive for applications in catalysis, biology, magnetism, optics and medicine [1,2]. In this area, there has been an increasing interest in constructing functionalized POMbased materials by incorporation of organic or metalorganic moieties, originating from their importance in various fields such as medicine and magnetochemistry [3,4]. One potentially designed route to gain functionalized POMs is based on pre-synthesized interesting POMs as building blocks, and then link them up with organic molecules or secondary metal coordination complexes as bridging subunits by standard synthetic technique, which can control the overall cluster architectures and properties [5,6]. However, the approach is still in its infancy, thus representing a fantastic challenge in current synthetic chemistry and material science [7,8]. Recently, the remarkable amino acid molecules seized our attention, which were considered as the appropriate *
Corresponding author. Tel./fax: +86 431 5098787. E-mail addresses: [email protected], [email protected] (E. Wang). 1387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2006.11.007
functional groups because of their flexible coordination modes and the physiological importance. A few interesting functionalized POMs by directly connecting amino acids with polyoxomolybdate clusters have been successfully reported by Pope, Yamase and Kortz et al., showing discrete or 1D structures [9–11]. A recent study by Wu et al. demonstrated that amino acids are prone to binding with transition metal cations to form multinuclear metal clusters with magnetic characteristics [12]. To date the structures containing metal–amino acid subunits and POMs, especially high-dimensional architectures, remained largely unexplored [13], although, they may possess potential physiological and magnetic applications. Therefore, our current research interest is focused on acquiring 3D functionalized POMs by transition metal–amino acid coordination clusters as linkers. The exploitation of such species not only increases the structural diversity of functionalized POM materials, but also provides new insights into the relationships between the structure and function of these materials. On the other hand, we choose Keggin-type POMs [BW12O40]5 as the basic building blocks because of its high charge density and its potential values [14]. Herein, we
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successfully obtained a new 3D compound, H4[Na(H2O)2][Cu6Na(gly)8(H2O)2][BW12O40]2 Æ 13H2O (1) (gly = glycine), which is constructed from [BW12O40]5 anionic clusters, hexanuclear [Cu6Na(gly)8] cages and Na+ ions. To our best knowledge, it is the first time to connect Keggintype POM motifs with hexanuclear metal–amino acid clusters to construct a 3D covalent framework. Furthermore, magnetic properties of compound 1 have been studied in the temperature range 2–300 K, displaying ferromagnetic interactions. Compound 1 has been obtained in by adding CuCl2 Æ 2H2O (0.2 mmol), NaClO4 (0.4 mmol), and gly (0.4 mmol) to the 40 mL water solution of K5 [BW12O40] Æ 15H2O [15] (0.2 mmol). The mixture was heated for 2 h at 80 °C, and then filtered. The filtrate was kept for two months at ambient conditions, and blue block crystals of 1 were isolated in about 25% yield (based on Cu) [16]. In the IR spectrum of compound 1, the characteristic peaks at 958, 902, 819, 625, 526 and 423 cm 1 demonstrate that [BW12O40]5 is a Keggin structure [17]. Single crystal X-ray structural analysis [18,19] revealed that compound 1 exhibits a unique 3D frame, which is built up from 2D layers by the self-assembly of [BW12O40]5 polyoxoanions and hexanuclear [Cu6Na(gly)8] coordination complex cages, further united by Na+ cations. The [BW12O40]5 heteropolyanion in compound 1 retains the well-known Keggin structure with a Td point symmetry (see Fig. 1). The central B atom, in the form of {BO4} tetrahedron, resides in the polyoxoanion of compound 1. The
˚ to 1.67(3) A ˚ are conB–O distances varied from 1.42(4) A sistent with the results of previous study [17]. The W–O bond lengths fall into four classes: W–Ot 1.59(3)– ˚ , W–Ob 1.76(4)– ˚ , W–Ot1 1.67(3)–1.74(3) A 1.75(4) A ˚ ˚ , according to the 2.10(5) A, and W–Oc 2.24(5)–2.51(5) A literature [13]. These results indicate that the {BO4} tetrahedron and {WO6} octahedra are in the distortion. The hexanuclear [Cu6Na(gly)8] coordination complex cage is similar to the reported structure Na[Cu6Na(gly)8(H2O)2](ClO4)6 Æ 2H2O [20]. They all have three unique Cu(II) atoms in the asymmetric unit. Four Cu2+ ions are located at the equatorial vertices of a regular non-bonding octahedron, and the other two Cu2+ ions are situated at the axial vertices to form a hexanuclear cationtic [Cu6(gly)8]4+ cage. Each gly group acts as a tridentate ligand to bridge two neighboring copper atoms. One unique Na+ ion in the center of the cage has electrostatic interactions with the surrounding eight oxygen atoms of the amino acids ˚ ]. The structural differences between the [Na–O 2.63 A [Cu6Na(gly)8] cage and the reported one are centered on the coordination modes in two of three unique Cu atoms. In the Na[Cu6Na(gly)8(H2O)2](ClO4)6 Æ 2H2O, two Cu atoms adopt square planar geometry, while in compound 1 both Cu(1) and Cu(3) link to two carboxyl oxygen atoms and two nitrogen atoms from two different gly molecules, and one terminal oxygen atom from the Keggin polyoxo˚ , and Cu(3)–O 2.452(6) A ˚ ] to anion [Cu(1)–O 2.538(4) A complete a distorted square pyramid geometry. The Cu(2) centers a square pyramid geometry, which is defined
Fig. 1. ORTEP drawing of compound 1 with thermal ellipsoids at 50% probability. Partial water molecules are omitted for clarity.
H. An et al. / Inorganic Chemistry Communications 10 (2007) 299–302
by four carboxyl oxygen atoms from four different gly mol˚ ], ecules, and one water molecule [Cu(2)–OW 2.280(5) A ˚ which is shorter than 2.353 A in the reported structure. Also, the other crystallographically independent Na(1) exists outside the cage. In 1 Na(1) has a distorted octahedral coordination environment, coordinated by two terminal oxygen atoms from two polyoxoanions, and four water ˚. molecules [21]. The average Na(1)–O distance is 2.20 A Such [Cu6Na(gly)8] cages are covalently bonded to terminal oxo-groups of the Keggin clusters via the Cu(1) and Cu(3) sites of each hexa-copper moiety. Each Keggin unit acts as a bidentate ligand coordinating to two adjacent cages through the terminal oxygen atoms in the structure. This kind of connection modes results in the formation of a 2D window-like sheet (see Fig. 2). These 2D layers can be coupled with each other through Na+ ions as linkers to form a 3D architecture (shown in Fig. S2). Notably, such hexanuclear cage structure is rare in coordination polymers, and extended structure constructed from POM building blocks and such hexanuclear metal cages have never been found, perhaps owing to the increasing steric hindrance to limit their combination. Extensive hydrogen-bonding interactions exist among water molecules filled in the frameworks, the polyoxoanions and ligands, and little cavities can be observed. The bond valence sum calculations [22] indicate that all W sites are in the +6 oxidation state, and all Cu sites are in the +2 oxidation state in 1. The temperature dependence of the vMT product for 1 is shown in Fig. 3. At 300 K, vMT is 2.47 cm3 K mol 1 (4.45 lB), which is slightly higher than the expected value 2.25 cm3 K mol 1 (4.24 lB, considering g = 2) of six iso-
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Fig. 3. The temperature dependence of reciprocal magnetic susceptibility vM1 (solid square) and the product vMT (open square) for compound 1.
lated spin-only Cu2+ ions. On cooling, vMT continuously increases, and then exhibits an abrupt increase to reach a maximum value of 4.33 cm3 K mol 1 at 2.0 K. The shape of this curve is typical of an overall ferromagnetic coupling. Fits of the curve for 1/vM versus T plots of 1 to the Curie– Weiss law give good results in the temperature range of 2– 300 K with C = 2.48 cm3 K mol 1 and h = 1.91 K for 1. These values confirm the overall ferromagnetic character of the magnetic interaction in compound 1. The magnetic properties of 1 are similar to those for the hexanuclear copper clusters [23], indicating that the magnetic behavior of 1 is that of the cation. In addition, ESR studies have been undertaken to elucidate the electronic properties of the Cu2+ in compound 1 (shown in Fig. S4). The effective g value is 2.12, which compares well with that reported in the literatures [24]. In summary, a novel architecture constructed from the pre-synthesized Keggin clusters as precursors and copper–amino acid coordination complexes as functional groups was successfully prepared using a rational strategy by standard synthetic techniques. Two main points deserve to be emphasized: (i) from the structural point of view, compound 1 represents the first example of 3D covalent frameworks containing Keggin-type POMs and hexanuclear [Cu6Na(gly)8] cages and (ii) from the magnetic point of view, in compound 1, owing to the existence of the hexanuclear [Cu6Na(gly)8] cluster the whole compound exhibits ferromagnetic interactions, which is one of the very few cases of POM-based materials. Acknowledgement The authors thank the National Natural Science Foundation of China (20371011) for financial support.
Fig. 2. Polyhedral and ball-stick view of the 2D assembly layer based on Keggin-type POMs and hexanuclear [Cu6Na(gly)8] cages along the a axis in 1 (color code: B, yellow; W, green; Cu, blue; O, red; N, oxford blue; C, gray). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Appendix A. Supplementary materials CCDC 614807 contains the supplementary crystallographic data for 1. These data can be obtained free of
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charge via http://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; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche. 2006.11.007. References [1] (a) M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer, Berlin, 1983; (b) V. Soghomonian, Q. Chen, R.C. Haushalter, J. Zubieta, C.J. O’Connor, Science 259 (1993) 1596. [2] (a) A. Mu¨ller, S.Q.N. Shah, H. Bo¨gge, M. Schmidtmann, Nature 397 (1999) 48; (b) K. Fukaya, T. Yamase, Angew. Chem., Int. Ed. 42 (2003) 654; (c) X.K. Fang, T.M. Anderson, C.L. Hill, Angew. Chem., Int. Ed. 44 (2005) 3540. [3] C.D. Wu, C.Z. Lu, H.H. Zhuang, J.S. Huang, J. Am. Chem. Soc. 124 (2002) 3836. [4] (a) J.R. Gala´n-Mascaro´s, C. Gime´nez-Saiz, S. Triki, C.J. Go´mezGarcı´a, E. Coronado, L. Ouahab, Angew. Chem., Int. Ed. 34 (1995) 1460; (b) H.Y. An, Y. Lan, Y.G. Li, E.B. Wang, N. Hao, D.R. Xiao, L.Y. Duan, L. Xu, Inorg. Chem. Commun. 7 (2004) 356; (c) L.C. Li, D.Z. Liao, Z.H. Jiang, S.P. Yan, Inorg. Chem. 41 (2002) 1019. [5] (a) B.B. Xu, Z.H. Peng, Y.G. Wei, D.R. Powell, Chem. Commun. (2003) 2562; (b) M.I. Khan, E. Yohannes, D. Powell, Chem. Commun. (1999) 23; (c) H.Y. An, Y.G. Li, E.B. Wang, D.R. Xiao, C.Y. Sun, L. Xu, Inorg. Chem. 44 (2005) 6062. [6] (a) F. Bonhomme, J.P. Larentzos, T.M. Alam, E.J. Maginn, M. Nyman, Inorg. Chem. 44 (2005) 1774; (b) H.Y. An, D.R. Xiao, E.B. Wang, Y.G. Li, L. Xu, New J. Chem. 29 (2005) 667; (c) H.Y. An, E.B. Wang, D.R. Xiao, Y.G. Li, L. Xu, Inorg. Chem. Commun. 8 (2005) 267. [7] (a) S. Bareyt, S. Piligkos, B. Hasenknopf, P. Gouzerh, E. Lacoˆte, S. Thorimbert, M.J. Malacria, J. Am. Chem. Soc. 127 (2005) 6788; (b) H.Y. An, Y.Q. Guo, Y.G. Li, E.B. Wang, J. Lu¨, L. Xu, C.W. Hu, Inorg. Chem. Commun. 7 (2004) 521. [8] (a) J. Tao, Y. Zhang, M.L. Tong, X.M. Chen, T. Yuen, C.L. Lin, X.Y. Huang, J. Li, Chem. Commun. (2002) 1342; (b) B.Z. Lin, S.X. Liu, Chem. Commun. (2002) 2126.
[9] F.B. Xin, M.T. Pope, J. Am. Chem. Soc. 118 (1996) 7731. [10] (a) M. Inoue, T. Yamase, Bull. Chem. Soc. Jpn. 68 (1995) 3055; (b) Z.B. Han, E.B. Wang, G.Y. Luan, Y.G. Li, H. Zhang, Y.B. Duan, C.W. Hu, N.H. Hu, J. Mater. Chem. 12 (2002) 1169. [11] U. Kortz, M.G. Savelieff, F.Y.A. Ghali, L.M. Khalil, S.A. Maalouf, D.I. Sinno, Angew. Chem., Int. Ed. 41 (2002) 4070. [12] (a) J.J. Zhang, T.L. Sheng, S.Q. Xia, G. Leibeling, F. Meyer, S.M. Hu, R.B. Fu, S.C. Xiang, X.T. Wu, Inorg. Chem. 43 (2004) 5472; (b) B.Q. Ma, D.S. Zhang, S. Gao, T.Z. Jin, C.H. Yan, G.X. Xu, Angew. Chem., Int. Ed. 39 (2000) 3644. [13] H.Y. An, E.B. Wang, D.R. Xiao, Y.G. Li, Z.M. Su, L. Xu, Angew. Chem., Int. Ed. 45 (2006) 904. [14] J.T. Rhule, C.L. Hill, D.A. Judd, R.F. Schinazi, Chem. Rev. 98 (1998) 327. [15] C.R. Deitcheff, M. Fournier, R. Franck, R. Thouvenot, Inorg. Chem. 22 (1983) 207. [16] Elem. anal. Found: W, 62.82; Cu, 5.22; C, 2.43; N, 1.74 (%). Calcd: W, 62.64; Cu, 5.41; C, 2.72; N, 1.59 (%). FTIR data (cm 1): 3450(s), 3292(s), 1640(s), 1599(s), 1454(m), 1410(m), 1337(m), 1248(m), 1116(m), 1048(w), 1000(w), 958(vs), 902(vs), 819(vs), 625(m), 526(m) and 423(m). [17] (a) S. Uchida, R. Kawamoto, N. Mizuno, Inorg. Chem. 45 (2006) 5136; (b) J.Y. Niu, Z.W. Zhao, J.P. Wang, P.T. Ma, J. Mol. Struct. 699 (2004) 85. [18] Crystal data for 1: H4[Na(H2O)2][Cu6Na(gly)8(H2O) 2][BW12O40]2 Æ ˚ , b = 15.302(3) A ˚, 13H2O, Triclinic, space group P1, a = 12.518(3) A ˚ , a = 74.07(3)°, b = 72.54(3)°, c = 77.95(3)°, V = c = 17.578(4) A ˚ 3, Z = 1, Dc = 3.812 mg/m3, F(0 0 0) = 3108, k(Mo Ka) = 3059.3(1) A ˚ 0.71073 A, T = 293 K, Rint = 0.0829. Structure solution and refinement based on 10,548 independent reflections with I > 2r (I) and 947 parameters gave R1 (wR2) = 0.1000 (0.1975) and S = 1.087. [19] (a) G.M. Sheldrick, SHELXL 97, Program for Crystal Structure Refinement, University of G} ottingen, Germany, 1997; (b) G.M. Sheldrick, SHELXL 97, Program for Crystal Structure Solution, University of G} ottingen, Germany, 1997. [20] S.M. Hu, W.X. Du, J.C. Dai, L.M. Wu, C.P. Cui, Z.Y. Fu, X.T. Wu, J. Chem. Soc., Dalton Trans. (2001) 2963. [21] N. Honma, K. Kusaka, T. Ozeki, Chem. Commun. (2002) 2896. [22] I.D. Brown, D. Altermatt, Acta. Crystallogr. Sect. B. 41 (1985) 244. [23] (a) L.Y. Wang, S. Igarashi, Y. Yukawa, T. Hashimoto, K. Shimizu, Y. Hoshino, A. Harrison, G. Aromi, R. Winpenny, Chem. Lett. 32 (2003) 202; (b) T. Yamase, K. Fukaya, H. Nojiri, Y. Ohshima, Inorg. Chem. 45 (2006) 7698. [24] D.M. Martino, M.C.G. Passeggi, R. Calvo, O.R. Nascimento, Phys. B: Condens. Matter 225 (1996) 63.
A functionalized polyoxometalate by hexanuclear copper–amino acid coordination complexes Haiyan An, Enbo Wang *, Yangguang Li, Zhiming Zhang, Lin Xu Key Laboratory of Polyoxometalate Science of Ministry of Education, Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University, Changchun 130024, PR China Received 5 October 2006; accepted 13 November 2006 Available online 21 November 2006
Abstract A new functionalized polyoxometalate by transition metal–amino acid coordination complexes, H4[Na(H2O)2][Cu6Na(gly)8 (H2O)2][BW12O40]2 Æ 13H2O (1) (gly = glycine) has been synthesized and characterized by elemental analysis, IR spectroscopy, TG analysis, and single crystal X-ray diffraction. Compound 1 possesses a 3D framework, which is built up of [BW12O40]5 building blocks, hexanuclear [Cu6Na(gly)8] coordination clusters and Na+ ions. Furthermore, magnetic measurements show ferromagnetic interactions for compound 1. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Functionalization; Polyoxometalates; Amino acids; Copper; Magnetic properties
Polyoxometalates (POMs), as anionic early transitionmetal oxide clusters, bear many properties that make them attractive for applications in catalysis, biology, magnetism, optics and medicine [1,2]. In this area, there has been an increasing interest in constructing functionalized POMbased materials by incorporation of organic or metalorganic moieties, originating from their importance in various fields such as medicine and magnetochemistry [3,4]. One potentially designed route to gain functionalized POMs is based on pre-synthesized interesting POMs as building blocks, and then link them up with organic molecules or secondary metal coordination complexes as bridging subunits by standard synthetic technique, which can control the overall cluster architectures and properties [5,6]. However, the approach is still in its infancy, thus representing a fantastic challenge in current synthetic chemistry and material science [7,8]. Recently, the remarkable amino acid molecules seized our attention, which were considered as the appropriate *
Corresponding author. Tel./fax: +86 431 5098787. E-mail addresses: [email protected], [email protected] (E. Wang). 1387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2006.11.007
functional groups because of their flexible coordination modes and the physiological importance. A few interesting functionalized POMs by directly connecting amino acids with polyoxomolybdate clusters have been successfully reported by Pope, Yamase and Kortz et al., showing discrete or 1D structures [9–11]. A recent study by Wu et al. demonstrated that amino acids are prone to binding with transition metal cations to form multinuclear metal clusters with magnetic characteristics [12]. To date the structures containing metal–amino acid subunits and POMs, especially high-dimensional architectures, remained largely unexplored [13], although, they may possess potential physiological and magnetic applications. Therefore, our current research interest is focused on acquiring 3D functionalized POMs by transition metal–amino acid coordination clusters as linkers. The exploitation of such species not only increases the structural diversity of functionalized POM materials, but also provides new insights into the relationships between the structure and function of these materials. On the other hand, we choose Keggin-type POMs [BW12O40]5 as the basic building blocks because of its high charge density and its potential values [14]. Herein, we
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successfully obtained a new 3D compound, H4[Na(H2O)2][Cu6Na(gly)8(H2O)2][BW12O40]2 Æ 13H2O (1) (gly = glycine), which is constructed from [BW12O40]5 anionic clusters, hexanuclear [Cu6Na(gly)8] cages and Na+ ions. To our best knowledge, it is the first time to connect Keggintype POM motifs with hexanuclear metal–amino acid clusters to construct a 3D covalent framework. Furthermore, magnetic properties of compound 1 have been studied in the temperature range 2–300 K, displaying ferromagnetic interactions. Compound 1 has been obtained in by adding CuCl2 Æ 2H2O (0.2 mmol), NaClO4 (0.4 mmol), and gly (0.4 mmol) to the 40 mL water solution of K5 [BW12O40] Æ 15H2O [15] (0.2 mmol). The mixture was heated for 2 h at 80 °C, and then filtered. The filtrate was kept for two months at ambient conditions, and blue block crystals of 1 were isolated in about 25% yield (based on Cu) [16]. In the IR spectrum of compound 1, the characteristic peaks at 958, 902, 819, 625, 526 and 423 cm 1 demonstrate that [BW12O40]5 is a Keggin structure [17]. Single crystal X-ray structural analysis [18,19] revealed that compound 1 exhibits a unique 3D frame, which is built up from 2D layers by the self-assembly of [BW12O40]5 polyoxoanions and hexanuclear [Cu6Na(gly)8] coordination complex cages, further united by Na+ cations. The [BW12O40]5 heteropolyanion in compound 1 retains the well-known Keggin structure with a Td point symmetry (see Fig. 1). The central B atom, in the form of {BO4} tetrahedron, resides in the polyoxoanion of compound 1. The
˚ to 1.67(3) A ˚ are conB–O distances varied from 1.42(4) A sistent with the results of previous study [17]. The W–O bond lengths fall into four classes: W–Ot 1.59(3)– ˚ , W–Ob 1.76(4)– ˚ , W–Ot1 1.67(3)–1.74(3) A 1.75(4) A ˚ ˚ , according to the 2.10(5) A, and W–Oc 2.24(5)–2.51(5) A literature [13]. These results indicate that the {BO4} tetrahedron and {WO6} octahedra are in the distortion. The hexanuclear [Cu6Na(gly)8] coordination complex cage is similar to the reported structure Na[Cu6Na(gly)8(H2O)2](ClO4)6 Æ 2H2O [20]. They all have three unique Cu(II) atoms in the asymmetric unit. Four Cu2+ ions are located at the equatorial vertices of a regular non-bonding octahedron, and the other two Cu2+ ions are situated at the axial vertices to form a hexanuclear cationtic [Cu6(gly)8]4+ cage. Each gly group acts as a tridentate ligand to bridge two neighboring copper atoms. One unique Na+ ion in the center of the cage has electrostatic interactions with the surrounding eight oxygen atoms of the amino acids ˚ ]. The structural differences between the [Na–O 2.63 A [Cu6Na(gly)8] cage and the reported one are centered on the coordination modes in two of three unique Cu atoms. In the Na[Cu6Na(gly)8(H2O)2](ClO4)6 Æ 2H2O, two Cu atoms adopt square planar geometry, while in compound 1 both Cu(1) and Cu(3) link to two carboxyl oxygen atoms and two nitrogen atoms from two different gly molecules, and one terminal oxygen atom from the Keggin polyoxo˚ , and Cu(3)–O 2.452(6) A ˚ ] to anion [Cu(1)–O 2.538(4) A complete a distorted square pyramid geometry. The Cu(2) centers a square pyramid geometry, which is defined
Fig. 1. ORTEP drawing of compound 1 with thermal ellipsoids at 50% probability. Partial water molecules are omitted for clarity.
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by four carboxyl oxygen atoms from four different gly mol˚ ], ecules, and one water molecule [Cu(2)–OW 2.280(5) A ˚ which is shorter than 2.353 A in the reported structure. Also, the other crystallographically independent Na(1) exists outside the cage. In 1 Na(1) has a distorted octahedral coordination environment, coordinated by two terminal oxygen atoms from two polyoxoanions, and four water ˚. molecules [21]. The average Na(1)–O distance is 2.20 A Such [Cu6Na(gly)8] cages are covalently bonded to terminal oxo-groups of the Keggin clusters via the Cu(1) and Cu(3) sites of each hexa-copper moiety. Each Keggin unit acts as a bidentate ligand coordinating to two adjacent cages through the terminal oxygen atoms in the structure. This kind of connection modes results in the formation of a 2D window-like sheet (see Fig. 2). These 2D layers can be coupled with each other through Na+ ions as linkers to form a 3D architecture (shown in Fig. S2). Notably, such hexanuclear cage structure is rare in coordination polymers, and extended structure constructed from POM building blocks and such hexanuclear metal cages have never been found, perhaps owing to the increasing steric hindrance to limit their combination. Extensive hydrogen-bonding interactions exist among water molecules filled in the frameworks, the polyoxoanions and ligands, and little cavities can be observed. The bond valence sum calculations [22] indicate that all W sites are in the +6 oxidation state, and all Cu sites are in the +2 oxidation state in 1. The temperature dependence of the vMT product for 1 is shown in Fig. 3. At 300 K, vMT is 2.47 cm3 K mol 1 (4.45 lB), which is slightly higher than the expected value 2.25 cm3 K mol 1 (4.24 lB, considering g = 2) of six iso-
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Fig. 3. The temperature dependence of reciprocal magnetic susceptibility vM1 (solid square) and the product vMT (open square) for compound 1.
lated spin-only Cu2+ ions. On cooling, vMT continuously increases, and then exhibits an abrupt increase to reach a maximum value of 4.33 cm3 K mol 1 at 2.0 K. The shape of this curve is typical of an overall ferromagnetic coupling. Fits of the curve for 1/vM versus T plots of 1 to the Curie– Weiss law give good results in the temperature range of 2– 300 K with C = 2.48 cm3 K mol 1 and h = 1.91 K for 1. These values confirm the overall ferromagnetic character of the magnetic interaction in compound 1. The magnetic properties of 1 are similar to those for the hexanuclear copper clusters [23], indicating that the magnetic behavior of 1 is that of the cation. In addition, ESR studies have been undertaken to elucidate the electronic properties of the Cu2+ in compound 1 (shown in Fig. S4). The effective g value is 2.12, which compares well with that reported in the literatures [24]. In summary, a novel architecture constructed from the pre-synthesized Keggin clusters as precursors and copper–amino acid coordination complexes as functional groups was successfully prepared using a rational strategy by standard synthetic techniques. Two main points deserve to be emphasized: (i) from the structural point of view, compound 1 represents the first example of 3D covalent frameworks containing Keggin-type POMs and hexanuclear [Cu6Na(gly)8] cages and (ii) from the magnetic point of view, in compound 1, owing to the existence of the hexanuclear [Cu6Na(gly)8] cluster the whole compound exhibits ferromagnetic interactions, which is one of the very few cases of POM-based materials. Acknowledgement The authors thank the National Natural Science Foundation of China (20371011) for financial support.
Fig. 2. Polyhedral and ball-stick view of the 2D assembly layer based on Keggin-type POMs and hexanuclear [Cu6Na(gly)8] cages along the a axis in 1 (color code: B, yellow; W, green; Cu, blue; O, red; N, oxford blue; C, gray). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Appendix A. Supplementary materials CCDC 614807 contains the supplementary crystallographic data for 1. These data can be obtained free of
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