Synthesis, Structure and Magnetic Property of a New MOF with Triazolate and Azido Heterobridges①

WANG Ye ZHAO Hong LIU Zhong-Yi YANG En-Cui ZHAO Xiao-Jun

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Synthesis, Structure and Magnetic Property of a New MOF with Triazolate and Azido Heterobridges①

WANG Ye ZHAO Hong LIU Zhong-Yi YANG En-Cui②ZHAO Xiao-Jun

(300387)

A new magnetic MOF with cyclic triazolate and linear azido mediators, [Co2(trz)3(N3)]1 (trz-= 1,2,4-triazolate), was hydrothermally synthesized and structurally and magnetically characterized. 1 crystallizes intherhombohedral63/space group with== 10.0716(17),= 7.5860(14)?,= 666.4(2)?3, D= 1.814 g/cm3,M= 364.09,= 2,(000) = 360,= 2.499 mm-1, the final= 0.0676 and= 0.1952 for 229 observed reflections with> 2(). Complex 1 consists of linear {Co(trz)3}chains passing through a6rotation axis, which are interconnected with tetrahedral CoIIion by3-trz-ligands into a hexagonal three-dimensional antiferromagnetic framework.

triazolate, azide, magnetism, crystal structure

1 INTRODUCTION

Magnetic metal-organic frameworks (MOFs) constructed from paramagnetic transition metal ions and organic bridging mediators[1-2]have recently received great interest due to their potential applica- tions in information storage and novel photomag- netic devices[3-4]as well as the important under- standing for the fundamentally magnetostructural correlations[5-6]. As compared to the diverse transi- tion metal ions with different magnetic nature and-electron counts, the rational design and successful preparation of new organic magnetic mediator or the combinations of different short ligands have become more and more important for the applicable magnets. In this regard, the mixed heterobridges constructed from cyclic azolates (imdazolate, triazolate, terazolate, and their diverse derivatives) and chain-like short mediators (such as azide[7-11], SCN-[12], carboxylate group[13-16], and so on) have already fabricated lots of promising MOFs with beautiful structures and intriguing magnetic beha- viors. For example, a scarce non-interpenetratedframework with isotropic CuIIion and mixed triazolate-azido ligands has been previously repor- ted, showing spin-canted antiferromagnetic pheno- menon at low temperature[7]. By varying the magnetic nature of the spin carriers, Gao and his coworkers have recently obtained a cobalt(II) chain with mixed azide and tetrazolate bridges, which exhibits the coexistence of AF ordering, SCM- derived multi-relaxation dynamics, field-induced metamagnetism and field-modified spin-glass-like dynamics[17]. In spite of these interesting and valuable results, to the best of our knowledge, the blend of the azide and unsubstituted triazolate mediators has been limited investigated by far[7, 17]and is still a challenging topic. As a continuous investigation along this line, herein, cyclic 1,2,4- triazole (Htrz) and linear azido ligands are selected as mixed heterobridges to perform the self-assembly reaction with anisotropic CoIIsource under con- trollable conditions. As a result, a novel hexagonal three-dimensional (3D) framework with linear {Co(trz)3}chains and tetrahedral CoIIions inter- connected by3-trz-connectors was hydrother- mally generated and structurally characterized. Strong antiferromagnetic interactions without the spontaneous magnetization were observed between the nearest neighbors of the octahedral and tetra- hedral CoIIions that were mediated by the3-trz-ligand.

2 EXPERIMENTAL

2.1 Reagents and instruments

All the starting materials employed herein were commercially purchased from Acros and used as received without further purification. Doubly deionized water was used for the conventional synthesis. Elemental analyses for C, H and N were carried out with a CE-440 (Leeman-Labs) analyzer. FT-IR spectrum (KBr pellet) was taken on an Avatar-370 spectrometer (Nicolet) in the range of 4000～400 cm–1. Powder X-ray diffraction (PXRD) pattern was obtained from a Rigaku D/max-2500 diffractometer at 60 kV and 300 mA for Curadiation (= 1.5406 ?), with a scan speed of 2 deg/min and a step size of 0.02o in 2. The simu- lated PXRD patterns were calculated using single- crystal X-ray diffraction data and processed by the free Mercury v1.4 program provided by the Cam- bridge Crystallographic Data Center. Magnetic susceptibility was acquired on a Quantum Design (SQUID) magnetometer MPMS-XL-7 with phase- pure crystalline samples. Diamagnetic corrections were calculated using Pascal's constants, and an experimental correction for the sample holder was applied.

2.2 Synthesis of [Co2(trz)3(N3)]n (1)

A mixture of Htrz (48.3 mg, 0.7 mmol), NaN3(32.5 mg, 0.5 mmol), Co(NO3)2×6H2O (145.5 mg, 0.5 mmol), and doubly deionized water (10.0 mL) was sealed in a 23.0 mL Teflon-lined stainless-steel autoclave, which was heated at 200℃ for 72 h under autogenous pressure. After the mixture was cooled to room temperature at a rate of 1.9℃×h?1, blue block-shaped crystals suitable for X-ray analysis were obtained directly, washed with water, and dried in air (Yield: 40% based on Htrz). Anal. Calcd. for C3H3CoN6: C, 19.79; H, 1.66; N, 46.17%. Found: C, 19.80; H, 1.67; N 46.16%. FT-IR (KBr, cm–1): 3420(w), 3111(w), 2064(w), 1505(s), 1458(w), 1420(m), 1384(m), 1305(m), 1269(m), 1206(w), 1157(m), 1077(m), 1032(w), 1008(m), 883(m), 664(m).

2.3 Structure determination

A suitable single crystal of 1 with dimensions of 0.10mm × 0.08mm × 0.07mm was carefully selected and glued on a thin glass fiber. Diffraction intensities of the complex were collected on a Bruker APEX-II CCD diffractometer equipped with graphite-monochromatic Moradiation with a radiation wavelength of 0.71073 ? by using the-scan mode at 296(2) K. A total of 3220 reflections with 250 unique ones (int= 0.0582) were measured in the range of 2.34≤≤24.81, of which 229 were observed with> 2().

There was no evidence of crystal decay during data collection. Semi-empirical absorption correc- tions were applied, and the program SAINT was used for the integration of diffraction profiles[18]. The structures were solved by direct methods and refined with full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 pro- grams[19, 20]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The positions of hydrogen atoms bound to carbon were generated geometrically and allowed to ride on their parent carbons before the final cycle of refinement. The final= 0.0676,= 0.1952 (= 1/[2(2) + (0.1277)2+ 4.0721], where= (F2+ 2F2)/3) for 229 observed reflections with> 2(),= 1.070, (D)max= 1.582, (D)min=-0.570, and (D/)max= 0.000. Selected bond distances and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Symmetry transformation: A = –, –, 2 –; B=–,, 2 –; C=–, –,; D=,–, 2 –; E= –,–,; F = 1 –+, 1 –, 5/2 –; G1 –,–,

3 RESULTS AND DISCUSSION

3.1 Description of the crystal structure

Complex 1 presents a hexagonal 3D coordination framework with linear {Co(trz)3}chains intercon- nected with three-connected CoIItetrahedra by3-trz-ligands, in which the linear azido anions just act as terminal spacers to complete the metal coordination spheres. The fundamentally structural unit of 1 contains two crystallographically inde- pendent CoIIions respectively located at different special positions, half of an anionic3-trz-ligand with a mirror plane perpendicular to the molecular plane, and one strictly linear N3-anion with a 2-fold rotation axis passing through the central N atom. As shown in Fig. 1a, located at an inversion center and a6symmetry axis, the Co(1) is coordinated to six N donors from six symmetry-related trz-anions, adopting a perfect octahedral coordination geometry with the Co-N separation of 2.125(5) ? (Table 1). By contrast, the unique Co(2) atom that locates at a3axis is tetra-coordinated in a CoN4tetrahedral coordination sphere completed by three trz-and one azido anions, in which the Co-Nazideis considerably longer by 0.13 ? than those of Co-Ntriazolyl(Table 1). Each crystallographically unique trz-ligand in 1 adopts a3-N1,N2,N4-bridging mode to coordinate with two mirror-symmetric Co(1) and Co(1A) ions and one Co(2) site. The linear azido anion passing through the3axis of Co(2) site just adopts a terminally monodentate mode to complete the Co(2) coordination sphere.

Fig 1. (a) Local coordination environments of CoIIions in 1 (H atoms were omitted for clarity. Symmetry codes: A =, –, 2 –; B =–,, 2 –; C =–, –,; D =,–, 2 –; E = –,–,; F = 1 –+, 1 –, 5/2 –; G = 1 –,–,). (b) 1D linear chain of 1 running along theaxis. (c) Hexagonal 3D framework and the topological representation of 1 (Terminal azido spacers were omitted for clarity)

As shown in Fig. 1b, the crystallographically equivalent Co(1) octahedra are periodically aggre- gated by the3-N1, N2, N4-trz-anions through a N(1), N(2) bridging mode, generating an infinitely linear {Co(trz)3}chain running along theaxis with the intrachain CoII×××CoIIdistance of 3.7930(7) ?. Crystallographically, all CoIIions along the {Co(trz)3}chain pass through a6axis, which could potentially produce a high-symmetry struc- ture. Furthermore, each linear chain of 1 is connected with Co(2) tetrahedra by the N3 position of trz-anion bound to the metal ion, leading to a hexagonal 3D coordination framework with the nearest Co(1)×××Co(2) separation of 5.9711(1) ? (Fig. 1c). Topologically, each3-trz-ligand in 1 aggregates three CoIIions and can be considered as a three-connected node. The octahedral Co(1) and tetrahedral Co(2) are respectively surrounded by six and three3-trz-ligands and can be treated respectively as six- and three-connected nodes. Thus, a (3,6)-connected network of 1 is obtained with a Schl?fli notation of (482)3(83)(4686103) generated by OLEX[21](Fig. 1c). Previously, a similar 3D framework to 1 with terminal chloride anion [Co2(trz)3Cl][22]was reported, which crystallizes from the orthorhombicspace group.

3.2 FT-IR spectrum and PXRD pattern

The characteristic bands at 3420 and 3111 cm–1should be assigned to the C–H stretching of the triazolate ring. The weak absorption at 2064 and 1458 cm–1could be respectively attributed to the asymmetric and symmetric stretching vibrations of azido group[7].Additionally, other multiple bands in the range of 800～1500 cm–1were associated with skeleton vibrations of the trz-ligand. In addition, the experimental and computer-simulated PXRD patterns of the bulk sample are in good agreement with each other, indicating the phase purity of the as-synthesized product. Notably, the thermal stability of 1 was not investigated due to the presence of the explosive azido anions.

3.3 Magnetic properties

Variable-temperature (2～300 K) magnetic suscep- tibility of 1 was measured under an applied field of 1 kOe. As shown in Fig. 2, theMproduct per CoII2subunit of 1 is 5.15 cm3·mol-1·K at 300 K, which is comparable with the theoretical value for two magnetically isolated CoIIions with octahedral and tetrahedral geometries[22].Upon cooling, theMvalue of 1 decreases monotonously and is down to 0.43 cm3·mol-1·K at 2.0 K. The temperature dependence of the reciprocal susceptibility (M-1) obeys the Curie-Weiss law between 35 and 300 K and gives= 5.75 cm3·mol-1·K and= –60.7 K, suggesting strong antiferromagnetic interactions between the neighboring spin carriers of 1. The isothermal magnetization of 1 measured at 2.0 K increases linearly with the enhanced magnetic field (Fig. 2 insert). The magnetization of 1 is 1.39at 50 kOe, which is far from the saturation value expected for two CoIIions (4～6) and confirms the antiferromagnetic interactions between the spin carriers of 1. Thus, complex 1 exhibits typically antiferromagnetic interactions without the spon- taneous magnetization, which is much different from its analogue [Co2(trz)3Cl] that behaves as a weak ferrimagnet[22]. From a viewpoint of magneto- structural relationships, the significant difference on the magnetic behaviors of the two samples is ascribed to the different microscopic symmetry of the 3D frameworks. Especially, the presence of6axis in 1 induces highly symmetric magnetic pathways, in which the magnetic moments from the octahedral and tetrahedral CoIIions are fully cancelled out.

Fig. 2. Temperature dependence of χMandM–1for 1 (Inset: field dependence of magnetization measured at 2.0 K)

4 CONCLUSION

In summary, a novel paramagnetic entity with linear azido and cyclic triazolyl heterobridges was hydrothermally generated and exhibits a hexagonal 3D framework constructed from linear {Co(trz)3}chains and tetrahedral CoIIions. Significantly, resulting from the full compensation of magnetic moments from the octahedral and tetrahedral cobalt(II) ions, only strong antiferromagnetic inter- actions without the spontaneous magnetizations were transferred by the cyclic triazolate ligand.

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7 January 2014;

25 March 2014 (CCDC 979027)

NNSFC (No. 21171129 and 21173157), the Program for Innovative Research Team in University of Tianjin TD52-5038), and Tianjin Educational Committee (2012ZD01)

. Yang En-Cui, born in 1974, professor, majoring in supramolecular chemistry. E-mail: encui_yang@163.com