Synthesis, Structure, and Magnetic Properties of a Novel

COMMUNICATION pubs.acs.org/crystal

Synthesis, Structure, and Magnetic Properties of a Novel Polyoxometalate
Organic Framework with Unusual (5,6)-Connected Topology Yi-Ming Xie,†,‡ Rong-Ming Yu,† Xiao-Yuan Wu,† Fei Wang,† Jian Zhang,† and Can-Zhong Lu*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 35002, China ‡ College of Material Science and Engineering, Huaqiao University, the Key Laboratory for Functional Materials of Fujian Higher Education, Quanzhou, Fujian 362021, China

bS Supporting Information ABSTRACT: A novel three-dimensional (3D) polyoxometalate
organic framework [Cu2(4,40 -bpz)4(Cu2Mo6O22)]n (1) with unprecedented transition metal substituted octametalates has been hydrothermally synthesized and structurally characterized. Two [Cu2Mo6O22]4
polyoxometalate units are linked by two Cu2+ ions into a polymeric chain running along the b-axis, and the resulting chains are further interconnected by 4,40 -bpz (4,40 bpz = 3,30 ,5,50 -tetramethyl-4,40 -bipyrazole) ligands into a 3D framework with interesting (5,6)-connected topology. The variable temperature magnetic study of 1 indicates the presence of four independent Cu2+ centers and a weak ferromagnetic coupling behavior between them.

P

olyoxometalates (POMs) usually are composed of VB and VIB transition metals (VV, NbV, TaV, MoVI,V, and WVI,V) in their high oxidation states bounded together by oxygen atoms to form a large 3D metal
oxygen cage.1 In the last decades, more and more attention has been paid to the rational design of new POM-based organic
inorganic hybrid materials due to their structural versatility and abundant potential applications.2 The structures of some POMs are derived from a lacunary POMs structure by inserting one or more hetero transition metal atoms or from a parent POMs structure by replacing one or more addenda atoms by hetero transition metal atom(s), yielding a structure called a substituted structure.3 The chemistry of substituted POMs has been investigated enthusiastically recently for both synthetic interest and their extensive potential applications.4 Most of the substituted POMs are derivatives of Keggin type and Wells
Dawson type clusters.5 To the best of our knowledge, no example of a substituted octametalate POM has been reported so far. Furthermore, despite the practical and fundamental importance of metal
organic framework materials, the designed synthesis of such materials with modular inorganic polyoxometalate frameworks remains an elusive goal.6 The attractive and structurally simple 3,30 ,5,50 -tetramethyl-4,40 -bipyrazole (4,40 -bpz) ligand was widely used for the construction of metal organic frameworks.7 It has an angle of rotation around 50
90° and, very often, will result in noncollinear orientation of two N
M vectors. However, it is never involved in the self-assembly of polyoxometalate-based materials. Herein we report the synthesis, characterization, and magnetic properties of a new polyoxometalate
organic framework, r 2011 American Chemical Society

[Cu2(4,40 -bpz)4(Cu2Mo6O22)]n (1), which contains an unprecedented dicopper substituted octamolybdate unit (TMSP), [Cu2Mo6O22]4
, and exhibits an interesting (5,6)-connected topology. Green crystals of 1 were synthesized by the self-assembly of CuSO4 3 5H2O, MoO3 3 H2O, 4,40 -bpz, and H3PO4 under hydrothermal conditions at 160 °C for 4 days.8 They were formulated on the basis of X-ray structural analysis,9 elemental analysis, and thermogravimetric analysis. This compound is very stable in air at ambient temperature and is almost insoluble in most common solvents, such as water, acetonitrile, chloroform, acetone, and toluene. The purity of the compound is confirmed by powder X-ray diffraction study (Figure S1 of the Supporting Information). Great efforts have been made to synthesize the isostructural crystals using other metal salts (such as Fe, Co, Ni, Cd, etc.) instead of copper; however, no rewarding results have been obtained so far. The title compound crystallizes in the monoclinic space group P21/c. The asymmetric unit contains halves of four 4,40 -H2bpz ligands, a copper(II) ion and half of a dicopper substituted [Cu2Mo6O22]4
polyoxometalate unit. The dicopper substituted [Cu2Mo6O22]4
unit is unprecedented. Even though monosubstituted or multiply transition metal substituted polyoxometalates have been investigated extensively recently, most Received: June 23, 2011 Revised: September 19, 2011 Published: October 12, 2011 4739

dx.doi.org/10.1021/cg200791k | Cryst. Growth Des. 2011, 11, 4739–4741

Crystal Growth & Design

COMMUNICATION

Figure 3. The (5,6)-connected topology observed in 1. Figure 1. (a) Dicopper substituted octamolybdate [Cu2Mo6O22]4
cluster anion; (b) 1D bimetallic oxide chains [Cu2(Cu2Mo6O22)]n along the b-axis.

Figure 4. Plot of χmT vs T for 1.

Figure 2. The 3D microporous framework of 1.

of the reported compounds are the derivatives of Keggin or Dawson type polyoxometalates, and no transition metal substituted octaoxometaltes have been reported so far. In the [Cu2Mo6O22]4
unit, only three molybdenum atoms and one copper atoms are crystallographically independent, of which two of the molybdenum atoms are coordinated by six oxygens to form a distorted octahedral geometry; one molybdenum is coordinated to five oxygen atoms with a distorted triangular bipyramidal coordination geometry. The coordination geometry around the copper atom is a distorted square pyramid with four oxygen atoms and one nitrogen atom from a 4,40 H2bpz ligand. The molybdenum polyhedra and copper polyhedra in [Cu2Mo6O22]4
are connected together through edge-sharing to form a building block which can be described as a derivative of γ-[Mo8O26]4
in which two of the octahedral MoO6 fragments are replaced by the distorted square pyramids of CuO4N. The Mo
O distances are in the normal range. The Cu
O bond in the axial position of CuO4N (2.355(6) Å) is much longer than the Cu
O (avg 1.95(1) Å) and Cu
N (1.905(8) Å) distances in equatorial positions due to the Jahn
Teller effect. POM-based metal
organic polymeric compounds have been investigated extensively over the past two decades. However, such compounds with partially metal-substituted POM as building

blocks are rarely reported. In the structure of the present compound, two [Cu2Mo6O22]4
units are bridged by two Cu(II) cations to form infinite [Cu2(Cu2Mo6O22)]n chains running along the b direction (Figure 1). Each chain is further linked to six neighborhood chains by the 4,40 -H2bpz bridging ligands, generating a 3D framework (Figure 2). The coordination geometry of the bridging Cu(II) is also a distorted square pyramid with two oxygen atoms from two [Cu2Mo6O22]4
units and three nitrogen atoms from three 4,40 -H2bpz ligands. To the best of our knowledge, no example of analogous metal substituted octamolybdate units and bimetallic oxide chains has ever been documented so far in the field of polyoxometalates chemistry. From a topological viewpoint, it is interesting that the 3D framework of 1 can be described as a novel (5,6)-connected topology (Figure 3). Topological analysis was performed by using Topos10 and Systre11 programs. The analysis reveals that the title compound features a 2-nodal topological structure through assigning the Cu1 atom and the dicopper substituted octamolybdate unit [Cu2Mo6O22]4
as two kinds of nodes, and the μ2-4,40 -H2bpz ligand as the linkers. According to the determined crystal structure of 1, each Cu1 atom can be defined as a 5-connected node. Likewise, the [Cu2Mo6O22]4
unit connecting six Cu1 can be defined as a 6-connected node. Thus, the net of the 3D framework represents a (5,6)-connected topology. The vertex symbols for the Cu1 node and the [Cu2Mo6O22]4
unit node are {46 3 64} and {48 3 66 3 8}, respectively, and the molar ratio 4740

dx.doi.org/10.1021/cg200791k |Cryst. Growth Des. 2011, 11, 4739–4741

Crystal Growth & Design of the Cu1 nodes and the [Cu2Mo6O22]4
unit nodes is 2:1; thus, the point symbol for this (5,6)-connected net is (46 3 64)2(48 3 66 3 8). Temperature dependence measurements of molar magnetic susceptibility for 1 have been investigated under an applied field of 1000 G in the temperature range 2
300 K, illustrating in the form of χmT versus T (Figure 4). The curve shows a typical paramagnetic interaction.12 The χmT value of 1.37 emu K mol
1 for 1 at 300 K is consistent with the Curie value (1.50 emu K mol
1) for four isolated Cu2+ (S = 1/2). As the temperature decreases, the χmT value undergoes a gradual increase. This magnetic behavior indicates the existence of ferromagnetic interactions between magnetic centers. In the temperature range 100
300 K, the inverse magnetic susceptibility data are fitted with the Curie
Weiss law, affording parameters of C = 1.50 emu K mol
1 and θ = 3.06 K. The positive Weiss constants (θ) suggest that the title compound exhibits a weak ferromagnetic coupling. This coupling is induced by the spatial geometries such as the angle of Cu
Mo
Cu. The thermal stability of the compound was studied using thermogravimetric analysis (Figure S3 of the Supporting Information). To further test the thermal stability of compounds, heating
cooling experiments were carried out according to the TGA result and monitored by XRD. Compared to the XRD data for the original crystal of 1, as shown in Figure S4 of the Supporting Information, the XRD patterns for 1 at 320 °C in air remained unchanged, indicating that the structural integrity of the framework remains. When the samples were heated to 340 °C, the XRD pattern of 1 changed completely, revealing the collapse of the framework. It is clear that the title compound is stable up to ca. 320 °C. The high thermal stability and insolubility in common solvent make it an excellent potential magnetic material. In conclusion, a 3D polyoxometalate-containing metal
organic hybrid compound with an unprecedented transition metal substituted polyoxometalates (TMSPs) [Cu2Mo6O22]4
unit and a [Cu2(Cu2Mo6O22)]n bimetallic oxide chain was synthesized. It displays a novel (5,6)-connected topology. The title compound may be a potential candidate as magnetic material, owing to its magnetism, thermal stability, and insolubility in common polar and nonpolar solvents. Considering that a large variety of TMSPs can be used in this synthetic strategy, further work is underway in our laboratory.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1 and S2, and experimental procedures and crystallographic data for compound 1. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: (+86)-591-83714946. Telephone: (+86)-591-83705794. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Chinese Academy of Sciences (KJCX2-EW-H01, KJCX2-YW-319), the 973 key program of the MOST (2010CB933501, 2012CB821705), the National Natural Science Foundation of China (20873150, 20821061, 20973173, 50772113, and 91022008), and the Natural Science Foundation of Fujian Province (2009HZ0004-1, 2009HZ0006-1, 2006L2005).

COMMUNICATION

’ REFERENCES (1) Pope, M. T.; M€uller, A. Polyoxometalate Chemistry: From Topology via Self-Assembly to Applications; Kluwer Academic: Dordrecht, 2001. (2) (a) Hill, C. L. Chem. Rev. 1998, 98, 1. (b) Dai, L. M.; You, W. S.; Li, Y. G.; Wang, E. B.; Huang, C. Y. Chem. Commum. 2009, 2721. (c) Judd, D. A.; Nettles, J. H.; Nevins, N.; Snyder, J. P.; Liotta, D. C.; Tang, J.; Ermolieff, J.; Schinazi, R. F.; Hill, C. L. J. Am. Chem. Soc. 2001, 123, 886. (d) Ma, B. Q.; Zhang, D. S.; Gao, S.; Jin, T. Z.; Yan, C. H.; Xu, G. X. Angew. Chem., Int. Ed. 2000, 39, 3644. (e) Geletii, Y. V.; Botar, B.; K€ogerler, P.; Hillesheim, D. A.; Musaev, D. G.; Hill, C. L. Angew. Chem., Int. Ed. 2008, 47, 3896. (f) Ritchie, C.; Moore, E. G.; Speldrich, M.; K€ogerler, P.; Boskovic, C. Angew. Chem., Int. Ed. 2010, 49, 7702. (g) Zheng, S. T.; Zhang, J.; Yang, G. Y. Angew. Chem., Int. Ed. 2008, 47, 3909. (3) (a) Zhao, J.-W.; Zhang, J.; Zheng, S.-T.; Yang, G.-Y. Chem. Commun. 2008, 570. (b) Fang, X.; K€ogerler, P. Chem. Commun. 2008, 3396. (c) Dutta, D.; Jana, A. D.; Debnath, M.; Mostafa, G.; Clerac, R.; Tojal, J. G.; Ali, M. Eur. J. Inorg. Chem. 2010, 5517. (4) (a) Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2010, 49, 6096. (b) Murad, A. A.; Juan, M. C. J.; Eugenio, C.; Carlos, M. G.; Alejandro, G. A. J. Am. Chem. Soc. 2008, 130, 8874. (c) Zheng, S.-T.; Zhang, J.; Li, X.-X.; Fang, W.-H.; Yang, G.-Y. J. Am. Chem. Soc. 2010, 132, 15102. (5) (a) Kortz, U.; Isber, S.; Dickman, M. H.; Ravot, D. Inorg. Chem. 2000, 39, 2915. (b) Maxym, V. V.; Ronny, N. J. Am. Chem. Soc. 2004, 126, 884. (c) Wang, J.-P.; Ma, P.; Shen, Y.; Niu, J.-Y. Cryst. Growth Des. 2007, 7, 603. (d) Ritchie, C.; Boyd, T.; Long, D.-L.; Ditzel, E.; Cronin, L. Dalton Trans. 2009, 1587. (e) M€uller, A.; Krickemeyer, E.; B€ogge, H.; Schmidtmann, M.; Peters, F.; Menke, C.; Meyer, J. Angew. Chem., Int. Ed. 1997, 36, 484. (6) (a) McGlone, T.; Streb, C.; Long, D.-L.; Cronin, L. Adv. Mater. 2010, 22, 4275. (b) Thiel, J.; Ritchie, C.; Miras, H. N.; Streb, C.; Mitchell, S. G.; Boyd, T.; Corella Ochoa, M. N.; Rosnes, M. H.; Mclver, J.; Long, D.-L.; Cronin, L. Angew. Chem., Int. Ed. 2010, 49, 6984. (c) Mitchell, S. G.; Streb, C.; Miras, H. N.; Boyd, T.; Long, D.-L.; Cronin, L. Nature Chem. 2010, 2, 308. (d) Long, D. L.; Burkholder, E.; Cronin, L. Chem. Rev. 2007, 36, 105. (7) (a) Rusanov, E. B.; Ponomarova, V. V.; Komarchuk, V. V.; StoeckliEvans, H.; Fernandez-Ibanez, E.; Stoeckli, F.; Sieler, J.; Domasevitch, K. V. Angew. Chem., Int. Ed. 2003, 42, 2499. (b) Ponomarova, V. V.; Komarchuk, V. V.; Boldog, I.; Chernega, A. N.; Sieler, J.; Domasevitch, K. V. Chem. Commun. 2002, 436. (c) Ganesan, P. V.; Kepert, C. J. Chem. Commun. 2004, 2168. (d) Zhang, J. P.; Horike, S.; Kitagawa, S. J. Am. Chem. Soc. 2008, 130, 907. (e) He, J.; Yin, Y. G.; Wu, T.; Li, D.; Huang, X. C. Chem. Commun. 2006, 2845. (8) Preparation of compound 1. MoO3 3 H2O (0.1 g, 0.62 mmol), CuSO4 3 5H2O (0.10 g, 0.40 mmol), 4,40 -H2bpz (0.05 g, 0.26 mmol), and H2O (13 mL) were well-mixed, and adjustment of the pH to 3.9 was made by the addition of 0.6 mL of a 0.1 M H3PO4 solution. The resulting blue mixture was heated in a 23 mL Teflon-lined reaction vessel at 160 °C for 4 days. Then the autoclave was cooled at 5 °C h
1 to room temperature. The green block crystals of compounds were isolated, washed with water and ethanol, and dried at room temperature. Yield: 0.13 g (39.6% based on Mo). Anal. Calcd (%) for C40H56Cu4Mo6N16O22: H 2.91, C 24.73, N 11.54. Found: H 3.07, C 23.56, N 10.77. IR spectrum (KBr pellets, ν/cm
1): 3853 (w), 3736 (w), 3444 (s), 3198 (s), 3081 (w), 2921 (w), 1627 (m), 1551 (m), 1454 (w), 1422 (m), 1288 (m), 1250 (w), 1192 (w), 1168 (m), 1095 (m), 945 (w), 925 (m), 880 (m), 858 (w), 769 (s), 749 (m), 659 (m), 630 (w). (9) Crystal data for 1: C40H56Cu4Mo6N16O22, Mr = 1942.81, monoclinic, space group P21/c, a = 8.392(4) Å, b = 12.451(6) Å, c = 29.364(14) Å, β = 93.075(9)°, V = 3064(2) Å3, Z = 2, T = 293(2) K, R1 (wR2) = 0.0758 (0.1885), and S = 1.040 for 397 parameters and 5560 reflections with I > 2σ(I). CCDC reference number 805910. (10) (a) Blatov, V. A.; Shevchenko, A. P.; Serezhkin, V. N. Acta Crystallogr. 1995, A51, 909. (b) Blatov, V. A.; Carlucci, L.; Ciani, G.; Proserpio, D. M. CrystEngComm 2004, 6, 377. (11) Delgado-Friedrichs, O.; O’Keeffe, M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2003, 59, 351–360. (12) Fujita, W.; Awaga, K.; Kondo, R.; Kagoshima, S. J. Am. Chem. Soc. 2006, 128, 6016. 4741

dx.doi.org/10.1021/cg200791k |Cryst. Growth Des. 2011, 11, 4739–4741