A New Molybdenum-Oxide-Based Organic−Inorganic Hybrid

Jun 11, 2010 - The magnetic investigations reveal that the complex exhibits interesting spin-canting antiferromagnetism, and the structure studies sho...
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DOI: 10.1021/cg100349d

A New Molybdenum-Oxide-Based Organic-Inorganic Hybrid Compound Templated by 5-(2-Pyridyl)tetrazole with New Topology and Canted Antiferromagnetism

2010, Vol. 10 3218–3221

Ping Dong,†,‡ Qi-Kai Zhang,† Fei Wang,†,‡ Shan-Ci Chen,†,‡ Xiao-Yuan Wu,† Zhen-Guo Zhao,†,‡ and Can-Zhong Lu*,† †

State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and ‡Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. China Received March 18, 2010; Revised Manuscript Received May 15, 2010

ABSTRACT: A new three-dimensional Mo/O/MnII/tetrazole compound [Mn(ptz)Mo2O7] (1) (ptz = 5-(2-pyridyl)tetrazole) was synthesized by a hydrothermal method. It represents the first example of a molybdenum-oxide-based organic-inorganic hybrid compound templated by the ligand which connects both molybdenum and secondary metal simultaneously. The magnetic investigations reveal that the complex exhibits interesting spin-canting antiferromagnetism, and the structure studies show that compound 1 is a (4,6)-connected framework with novel (33,42,5)(34,42,62,74,82,9)(34,44,52,64,7)2 topology that has never been reported to date.

Introduction In recent years, more and more attention has been paid to the rational design of new metal oxide based organic-inorganic hybrid materials because of the versatility of structure and exhibition of useful electronic, magnetic, optical, and mechanical properties for functional materials construction.1 One effective approach for the design of novel hybrid oxide materials exploits the organic molecules to modify inorganic microstructures. Zubieta and co-workers successfully apply this strategy to study the structural chemistries of the molybdenum oxide/ligand/metal.2-5 For convenience of classification, this system can be divided into three subclasses based on the role of the ligand: namely, the organic subunit incorporated as organoammonium cation;6 the ligand coordinated to the Mo oxide skeleton;7 and the ligand coordinated to heterometals.8 The development of new ligand systems is very important for the complexity of the structure of the Mo/O/ M0 /ligand family. Tetrazole, with four nitrogen electrondonating atoms, allows it to serve as either a multidentate or a bridging building block in supramolecular assemblies with interesting properties.9 On the other hand, the bulk magnetic properties of hybrid organic-inorganic materials are of special interest.10 A general strategy to prepare molecule-based magnetic materials is connecting paramagnetic transition metal ions with appropriate bridging ligands through a bottom-up approach. On this basis, we introduce the tetrazole and the secondary metal manganese into the molybdenum oxide complexes in order to obtain compounds with novel structure and interesting magnetic properties. In this paper we report a result of our attempts, a three-dimensional Mo/O/MnII/tetrazole compound [Mn (ptz) Mo2O7] (1) (ptz = 5-(2-pyridyl)tetrazole) by hydrothermal reaction. To the best of our knowledge, compound 1 represents the first example of a molybdenumoxide-based organic-inorganic hybrid compound templated *Corresponding author. Fax: (þ86)- 591-83714946. Telephone: (þ86)591-83705794. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 06/11/2010

by the ligand which connects both molybdenum and secondary metal simultaneously. The magnetic investigations reveal that the compound exhibits interesting spin-canting antiferromagnetism, and the structure studies show that it features a (4,6)connected framework with novel (33,42,5)(34,42,62,74,82,9)(34,44,52,64,7)2 topology that has never been reported before. Experimental Section Materials and Physical Measurements. The ligand 5-(2-pyridyl)tetrazole was prepared according to the literature.11 Other reagents and solvents employed were commercially available and used as received without further purification. Elemental analyses of C, H, and N were carried out with an Elementar Vario MICTO. The Fourier transform IR spectrum was obtained on a Perkin-Elmer Spectrum-one instrument using KBr pellets in the range 4000400 cm-1. The powder X-ray diffraction (PXRD) pattern was measured on a Rigaku DMAX2500 powder diffractometer at 40 kV and 100 mA using Cu KR radiation (λ = 1.54056 A˚). Thermal stability studies were carried out on a NETSCHZ STA 449C thermoanalyzer under N2 (30-1200 °C range) at a heating rate of 10 K/min. The polycrystalline magnetic study was performed on a Quantum Design PPMS model 6000 magnetometer in the temperature range from 2 to 300 K. Synthesis of 1. A mixture of Mn(CH3COO)2 3 4H2O (0.070 g, 0.28 mmol), ptz (0.10 g, 0.68 mmol), (NH4)6Mo7O24 (0.150 g, 0.12 mmol), and H2O (10 mL) was heated at 160 °C for 3 days in a sealed 23 mL Teflon-lined stainless steel vessel under autogenously pressure after adjustment of the pH to 3.0 by the addition of concentrated HNO3 solution. Then the autoclave was cooled to room temperature within 1 day. Orange block crystals were isolated, washed with water, and air-dried. Yield: 0.06 g (42% based on Mn). Anal. Calcd (%) for C6H5MnMo2N5O7 (505.97): H, 0.79; C, 14.05; N, 13.6. Found (%): H, 1; C, 14.24; N, 13.84. IR data (KBr, cm-1): 582.59w, 751.3m, 1121.8w, 1384.5m, 1632.9w, 3445.7s. X-ray Crystallography. The single crystal of compound 1 in the present work was mounted on a glass fiber for the X-ray diffraction analysis. Data sets were collected on a Rigaku AFC7R equipped with graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) from a rotating anode generator at 293 K. Intensities were corrected for LP factors and empirical absorption using the ψ scan technique. The structure was solved by direct methods and refined on F2 with full-matrix least-squares techniques using the Siemens SHELXTL r 2010 American Chemical Society

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Table 1. Crystal Data and Structure Refinement Results for Compound 1 empirical formula C6H5MnMo2N5O7 formula weight 505.97 cryst syst orthorhombic space group Pna2 (1) a (A˚) 7.487(4) b (A˚) 11.314(6) c (A˚) 13.166(7) R (deg) 90 β (deg) 90 γ (deg) 90 1115.2(10) V (A˚3) Z 4 3 3.014 Fcalcd (g/cm ) 3.378 μ (mm-1) GOF 1.059 a 0.0380 R1 (I > 2σ(I)) b 0.1126 wR2 (all data) P P P P a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = { [w(Fo2 - Fc2)2/ [w(Fo2)2]]1/2. Table 2. Selected Bond Lengths (A˚) and Angles (deg) for Compound 1a Mo(1)-O(6)#5 Mo(2)-O(5)#5 Mo(2)-O(7)#1 O(2)-Mo(1)-O(4) O(2)-Mo(1)-O(6) O(4)-Mo(1)-O(6) O(2)-Mo(1)-O(1) O(4)-Mo(1)-O(1) O(6)-Mo(1)-O(1) O(2)-Mo(1)-O(6)#5 O(4)-Mo(1)-O(6)#5 O(6)-Mo(1)-O(6)#5 O(1)-Mo(1)-O(6)#5 O(2)-Mo(1)-N(2) O(4)-Mo(1)-N(2) O(6)-Mo(1)-N(2) O(6)#5-Mo(1)-N(2) O(3)-Mo(2)-O(5)#5 O(3)-Mo(2)-O(1) O(5)#5-Mo(2)-O(1) O(3)-Mo(2)-O(7)#1 O(5)#5-Mo(2)-O(7)# O(1)-Mo(2)-O(7)#1 O(3)-Mo(2)-O(5) O(5)#5-Mo(2)-O(5)

2.181(7) 1.763(7) 1.962(7) 104.9(3) 103.9(3) 99.3(3) 99.3(3) 149.7(3) 92.3(3) 87.1(3) 85.1(3) 166.5(4) 78.1(3) 166.7(3) 78.6(3) 88.0(3) 80.3(3) 102.3(3) 103.7(3) 98.9(4) 101.1(3) 95.3(3) 148.0(3) 88.8(3) 168.5(2)

Mn(1)-O(2)#3 Mn(1)-O(3)#6 Mn(1)-N(4)#2 O(1)-Mo(2)-O(5) O(7)#1-Mo(2)-O(5) O(3)-Mo(2)-N(1) O(5)#5-Mo(2)-N(1) O(1)-Mo(2)-N(1) O(7)#1-Mo(2)-N(1) O(5)-Mo(2)-N(1) O(7)-Mn(1)-O(4) O(7)-Mn(1)-O(2)#3 O(4)-Mn(1)-O(2)#3 O(7)-Mn(1)-O(3)#6 O(4)-Mn(1)-O(3)#6 O(2)#3-Mn(1)-O(3)#6 O(7)-Mn(1)-N(4)#2 O(4)-Mn(1)-N(4)#2 O(2)#3-Mn(1)-N(4)#2 O(3)#6-Mn(1)-N(4)#2 O(7)-Mn(1)-N(5) O(4)-Mn(1)-N(5) O(2)#3-Mn(1)-N(5) O(3)#6-Mn(1)-N(5) N(4)#2-Mn(1)-N(5)

2.143(7) 2.249(7) 2.270(9) 80.9(3) 79.6(3) 168.5(3) 89.1(3) 75.2(3) 76.5(3) 79.7(3) 173.5(3) 98.1(3) 87.6(3) 82.1(3) 92.3(3) 179.0(3) 85.9(3) 97.1(3) 92.9(3) 86.2(3) 94.7(3) 82.2(3) 89.3(3) 91.7(3) 177.7(3)

Figure 1. View of the coordination environment of metal atoms (all hydrogen atoms are omitted for clarity).

Figure 2. Inorganic Mn2þ-Mo2O72- framework of compound 1.

a Symmetry transformations used to generate equivalent atoms: (#1) -x, -y þ 2, z þ 1/2; (#2) -x, -y þ 2, z - 1/2; (#3) x - 1/2, -y þ 5/2, z; (#4) -x þ 1/2, y þ 1/2, z þ 1/2; (#5) x þ 1/2, -y þ 5/2, z; (#6) -x þ 1/2, y - 1/2, z - 1/2.

version 5 package of crystallographic software.12 All non-hydrogen atoms were refined anisotropically. The positions of the hydrogen atoms attached to carbon atoms were fixed at their ideal positions.

Results and Discussion Syntheses and General Characterization. Crystal Structure. Single-crystal X-ray diffraction of 1 reveals that it crystallizes in the noncentrosymmetric space group Pna21 (see Tables 1 and 2). The asymmetric unit of 1 contains one MnII cation, one Mo2O72- anion, and one ptz ligand. Each distorted octahedral coordination MnII center is coordinated by four μ2-bridging oxygen atoms (Mn1-O2C = 2.143(7), Mn1O3B = 2.248(7), Mn1-O4 = 2.122(6), Mn1-O7 = 2.106(6)), and two nitrogen atoms (Mn1-N4B = 2.270(8), Mn1-N5 = 2.278(8)) from two ptz ligands (Figure1). The Mo atoms are all six coordinated by five oxygen atoms and one nitrogen atom (Mo1-N2 = 2.339(7), Mo2-N1 = 2.399(8)) from a ptz ligand. The Mo1 and Mo2 atoms are connected through

Figure 3. The ptz serve as the template in the framework.

μ2-bridging O1 to form a Mo2O72- anion. The inorganic Mn2þ-Mo2O72- framework is an open three-dimensional network (Figure 2). It is suggested that the ptz ligands serve as templates during the assembly so that hybrid building blocks are aggregated around them (Figure 3). To the best

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Figure 4. 1D substructure of compound 1 (all hydrogen atoms are omitted for clarity).

Figure 6. Temperature dependence of χmT and χm-1 for two Mn2þ of 1 at H = 1 kOe from 2 to 300 K. The solid line is the best-fit according to Curie-Weiss law. Inset: Temperature dependence of χm for 1.

Figure 5. (4, 6)-connected topology of compound 1.

of our knowledge, one ligand as template connected by both molybdenum and the second metal atoms simultaneously is unprecedented in the chemistry of molybdenum-oxide-based organic-inorganic hybrid complexes. Every two neighboring MnII ions along the c axis are connected by the N4 and N5 donors and the N1 and N2 are used to connect the molybdenum atoms along the a and b directions (Figure 4). In this substructure, the intrachain Mn 3 3 3 Mn distance is 6.5882(3) and interchain ones are 6.8478(2) and 7.4865(4). This is significantly responsible for the magnetic properties, as discussed in detail later. For better insight into this intricate net, a topological analysis of compound 1 was performed. Topological analysis was performed with Topos13 programs. We can define the ptz ligand as a four-connected node and assume that the Mo1 and Mo2 are not distinguished. This means that there are three kinds of nodes in the structure of 1, namely a 4-connected ptz ligand, 6-connected Mo atoms, and 6-connected Mn1 atoms. Thus, the overall network topology of 1 can be represented as an unprecedent (4,6)-connected framework with the Sch€ alfli symbol of (33,42,5)(34,42,62,74,82,9)(34,44,52,64,7)2 (Figure 5). Magnetic Properties. The cryomagnetic properties were measured at 1000 Oe in the temperature range 2-300 K (Figure 6). The χmT value of 6.67 cm3 mol-1 K at 300 K is smaller than the spin-only one expected for two isolated Mn2þ ions (8.75 cm3 mol-1 K and S = 5/2). As the temperature is lowered, χmT first decreases gradually to a minimum of 2.5 cm3 mol-1 K at 8 K, then increases sharply to 4.04 cm3 mol-1 K at about 5.5 K, and finally drops rapidly upon further cooling. The best fit with the Curie-Weiss law gives C = 6.91 cm3 mol-1 K and θ = -10.9 K, indicating apparent intrachain antiferromagnetic interactions between Mn2þ ions. The low-temperature magnetic behaviors of 1 suggest that a mechanism of ferromagnetic-like correlations exists and develops into a weak ferromagnetic ordering. For an antiferromagnetic system, the ferromagnetic correlations can be attributed to spin canting. To fully characterize this

Figure 7. Plots of temperature dependence of χm at different fields at 2-20 K. Inset: ZFC & FC curves at H = 50 Oe from 2 to 20 K.

phenomenon due to spin canting, field-cooled (FC) and zerofield-cooled (ZFC) susceptibilities were measured at 50 Oe below 30 K (Figure 7 inset). The divergence below 7 K indicates the irreversible transition of the weak ferromagnetism.14 To further characterize the weak ferromagnetism, field-cooled (FC) magnetizations were measured at different fields. As depicted in Figure 7, the susceptibility below 7.5 K is rather field-dependent and the magnetization shows no rise anymore above 500 Oe and tends to saturate at lower temperatures, indicating that the antiferromagnetic interaction is overcome by the external field. This kind of field-dependent magnetic behavior confirms the weak ferromagnetism due to spin canting. Further evidence for the spin canting in compound 1 comes from the magnetization vs filed plot at 2 K. The magnetization increases almost linearly and achieves 0.58 Nβ at 8 T (Figure 8 inset), far from the saturation (10 Nβ) for two spin-only MnII ions. The hysteresis loop measured at 2 K is consistent with a weak ferromagnetism (Figure 8). The remnant magnetization is 0.1 Nβ, and the coercive field is ca. 48.8 Oe. This is in agreement with the behavior of weak ferromagnetism due to spin canting. Thus, a conclusion may be given that the spontaneous magnetization observed is due to spin canting, with an estimated canting angle of 0.57°, calculated using the equation ψ = tan-1(Mr/Ms),15 where Mr is the remnant magnetization and Ms = gS is the expected saturation magnetization if all of the moments are aligned ferromagnetically. The temperature dependence of the ac magnetic susceptibilities shows that χm0 and χm00 signals have sharp maxima at about 6.5 K (TC), which verifies the magnetic phase transition and the absence of a frequency dependence (Figure 9). We can

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exhibits interesting spin-canting antiferromagnetism, and the structure studies show that it features a (4,6)-connected framework with a novel (33;42;5)(34;42;62;74;82;9)(34;44;52;64;7)2 topology that has never been reported before. This work may have contributed much to the research on the molybdenumoxide-based organic-inorganic hybrid complex with novel topologies and interesting magnetic properties.

Figure 8. The hysteresis loop measured at 2 K in the (3 T range. The inset denotes the field dependent isothermal magnetization for 1 at 2 K from 0 to 8 T.

Acknowledgment. This work was supported by the 973 key program of the MOST (2006CB932904, 2007CB815304, 2010CB933501), the National Natural Science Foundation of China (20873150, 20821061, 20973173, and 50772113), the Chinese Academy of Sciences (KJCX2-YW-M05, KJCX2YW-319), and the Fund of Fujian Key Laboratory of Nanomaterials (2006L2005). Supporting Information Available: X-ray crystallographic files in CIF format, simulated and experimental XRD powder patterns, and TGA profiles for compound 1. These materials are available free of charge via the Internet at http://pubs.acs.org.

References

Figure 9. The in-phase χm0 and out-of-phase χm00 components of the ac susceptibility of 1 measured at different frequencies (311, 511, 711, 1311 Hz) in an applied ac field of 3 Oe.

no doubt conclude that compound 1 is a spin canted antiferromagnet with weak ferromagnetism originating from spin canting below the Tc = 6.5 K. It is well-known that the occurrence of spin canting may arise from two mechanisms: (i) single-ion magnetic anisotropy or (ii) the so-called antisymmetric DzyaloshinskyMoriya (DM) exchange coupling.16 Because of the common isotropic character of the Mn2þ ion, the second factor should be responsible for the canting in 1 reported here. The 3D framework of 1 consists of linear chains of MnII, in which each pair is bridged by a μ-1, 2-ptz ligand. The chains are arrange parallel in the 3D framework and linked by a Mo2O7 anion. The main acentric-symmetry exchange pathways bridged by a ptz ligand may account for the antisymmetric interactions, whereas there are no inversion centers between the adjacent metal centers linked by a pair of μ-1,2-N-N bridges. Of course, the interchain also has a weaker contribution. The spin canting phenomenon of manganese complexes has been well studied,17-21 but compound 1 represents the first example of a manganese polymer that displays the spin canting antiferromagnetic behavior in metal oxide based organic-inorganic hybrid materials. Conclusions In summary, a new three-dimensional Mo/O/MnII/tetrozole compound 1 was prepared by a hydrothermal method. This compound represents the first example of a molybdenumoxide-based organic-inorganic hybrid complex templated by a ligand which connects both molybdenum and secondary metal. The magnetic investigations reveal that the complex

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