A new Mn(II)-based metal-organic framework: synthesis, structure and magnetic property

LI Fenfang, LIU Xiuping, ZHANG Baozhu

(College of Chemistry and Chemical Engineering, Jinzhong University, Jinzhong 030600, Shanxi, China)

Abstract: A novel Mn(II)-organic framework [Mn(Hcbic)]n(H3cbic =1-(4-carboxybenzyl)-1H-benzoimidazole-5, 6-dicarboxylic acid) is assembled and characterized by X-ray single crystal analysis. The Mn-MOF features one-dimensional left and right-handed double helical chains with screw-pitch of about 0.390 2 nm and the 4-methyl benzoic acid groups of Hcbic2- ligands in MOF-1 play many ribbons distributing in the two sides of the 2D networks. Magnetic susceptibility measurements indicate the presence of antiferromagnetic exchange interaction in MOF-1.

Keywords: Mn(II) complex; crystal structure;1-(4-carboxybenzyl)-1H-benzoimidazole-5, 6-dicarboxylic acid;magnetic property

Metal-organic frameworks (MOFs) are a class of materials constructed from the joining of organic linkers with metal ions or clusters. In the last decades, depending on their structure, chemical composition, particle size, MOFs have great potential for multiple applications such as hydrogen, methane storage capture[1], separation of CO2[2], water adsorption[3], solvent sponge behavior[4],controlled drug entrapment and release[5], heterogeneous catalysis[6], luminescence[7], etc. While many of these applications are based on the framework porosity, yet MOF materials also have interesting magnetic properties because the magnetic metal ions and their coupling can be tailored[8]in the MOF structure through the incorporation of magnetic moment carriers such as paramagnetic metals, open-shell organic ligands, or both.

As magnetism is a cooperative phenomenon, a connection between moment carriers at distances within interacting range is necessary; carboxylic-based and nitrogen-based ligands have proved to have good superexchange pathways for magnetic couplings. In addition, first-row transition metals magnetic complexes, due to their intrinsic paramagnetic and multi-valence properties[9-11], have the characteristics of small volume, low density, easy processing and various structures, and exhibit potential applicationsin the fields of spintronic devices and high-density information storage[12], as well as for understanding deeply the magneto-structural relationships. For example, the manganese atom with 3d54s2has many oxidation states, high spin electron configuration with many single electrons. Therefore, the manganese complexes have become a kind of important molecular-based magnetic ones due to their particular electron configuration.

Among the different types of organic linkers including O, N, P or S-containing molecules, the N-donor ligands such as azole (imidazole,triazole and tetrazole) and its derivative susually contribute to the assembly of intriguing MOFs[13-16]. These heterocycles can be easily deprotonated to form corresponding azolate anions, leading to the strong coordination ability for each individual nitrogen atom, thus giving rise to a great deal of robust networks.

With this in mind, an undeveloped ligand 1-(4-carboxybenz-yl)-1H-benzoimidazole-5,6-dicarboxylic acid (H3cbic) has been selected to build Mn(II)-MOF. H3cbic contains three carboxylate and one imidazole groups, therefore a lot of bridging modes could be adopted in the formation of MOFs. In addition, H3cbic is a flexible ligand, compared with the rigid ligands, flexible ones have more advantages in that their flexibility and conformational freedom allow them to conform to the coordination environment of the transition-metal ions.Given these considerations, we successfully synthesized a new double-layer 2DMOF[Mn(Hcbic)]n. The synthesized samples were characterized by X-ray single-crystal and powder diffractions, thermal gravimetric analysis and infrared spectra. Their magnetic properties were also studied.

1 Experimental

1.1 Materials and general methods

All chemicals with the analytical grade and solvents were purchased and used as received. Fourier transform (FT) IR spectra were taken on a BRUKER TENSOR27 spectrometer in the 4 000-400 cm-1region with KBr pellets. Elemental analyses of C, H and N were recorded on a CHNO-Rapid instrument. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance X-ray diffractometer with CuKαradiation (λ=0.154 18 nm). The calculated PXRD patterns were generated from the single-crystal X-ray diffraction data using PLATON software. The fluorescence spectrum was measured on a Varian Cary Eclipse Fluorescence spectrophotometer. Thermogravimetric analyses (TGA) were carried out with a Dupont thermal analyzer in the temperature range 25-800 ℃ under a N2flow with a heating rate of 5 ℃·min-1.

1.2 Synthesis of [Mn(Hcbic)]n

1-(4-Carboxybenzyl)-1H-benzoimidazole-5,6-dicarboxylic acid (18.33 mg, 0.05 mmol), MnCl2·4H2O (12.58 mg, 0.10 mmol) and a 7.0 mL of mixed solvents (acetonitrile/water, 3/4, V/V) were placed in a screw-capped autoclave and then heated at 160 ℃ for 72 h.The resulting yellow block crystals were obtained through filtration, washed with water and dried under the air, yield: 63%.

1.3 X-ray crystallography

Single-crystal X-ray diffraction data for compound MOF-1 were collected in the Beijing Synchrotron Radiation Facility (BSRF) beamline 3W1A, which were mounted on an MARCCD-165 detector (λ=0.072 00 nm) with the storage ring working at 2.5 GeV. In the process, the crystal was protected by liquid nitrogen at 100(2) K. Data was collected by the program MARCCD and processed using HKL 2000[17]. The structure was solved by direct methods employed in the program SHELXS-2014[17], and refined by full-matrix leastsquares methods againstF2with SHELXL-2016[18]. Atoms of benzoxy in the MOF-1 were found to be seriously disordered and they were refined with two parts of occupancies in the asymmetric unit. After all the non-H atoms were refined anisotropically, hydrogen atoms attached to C and O (hydroxyl) atoms were geometrically placed and refined using a riding model approximation, with C-H=0.093-0.096 nm and O-H=0.082 nm. The DISP instructions were used in the SHELXL2016 refinements in order to correct anomalous scattering values (F′ andF″) of elements for the synchrotron wavelength used. A summary of the crystallographic data, data collection and refinement parameters for all compounds is provided in Table 1.The selected bond lengths and bond angles for this compound are listed in Table 2.

Table 1 Crystal data and structure refinement for MOF-1

Table 2 The selected bond lengths(nm) and bond angles(°) for MOF-1

2 Results and discussion

2.1 Syntheses and characterization

A solvothermal reaction of H3cbic (1-(4-carboxybenzyl)-1H-benzoimi-dazole-5,6-dicarboxylic acid) and MnCl2·4H2O in mixed solvents of water and acetonitrile (volume ratio is 4∶3) gave the yellow block crystal MOF-1 in a high yield (63%). As depicted in Fig.1, The peaks of FT-IR pointed out that the strong broad absorption bands in the range of 3 474-3 653 cm-1in MOF-1 should be assigned to the characteristic vibrations of theνO-Hstretching frequencies of the benzoimidazole ring. The presence of strong bands around 1 702-1 690 cm-1in the FT-IR spectra indicates that -COOH group has not been completely deprotonated to generate -COO-anions, which is in agreement with their X-ray single structure, and the strong characteristic bands of the coordinated carboxylate groups appear at 1 589-1 535 cm-1for the asymmetric stretching and 1 367-1 315 cm-1for the symmetric one[19].

Fig.1 IR spectra of MOF-1

2.2 Structural description

Single-crystal X-ray structural data revealed that MOF-1 crystallizes in the monoclinic system with space groupP21/c, which exhibits 2Dlayer networks with many ribbons. The asymmetric unit consists of one crystallography independent Mn2+ion, and one Hcbic2-ligand (Fig.2). Each metal ion is hexa-coordinated with a distorted octahedral coordination geometry, MnNO5. One coordination site istaken up by a nitrogen atom on the benzoimidazole ring [Mn(1)-N(1), 0.222 4(5) nm] from a ligand. A symmetry generated O(1) and O(4) atoms from a different ligand [Mn(1)-O(1)ii, 0.222 7(4) nm, Mn(1)-O(4)ii, 0.213 4(5) nm] take up the other two coordination sites. Meanwhile the O1 atom also bridges the neighbouring metal ions (Fig.3a), so a symmetry generated O(1) atom from a different ligand also occupies one of the coordination sites [Mn(1)-O(1)i, 0.222 0(5) nm]. The remaining two coordination sites are taken up by two symmetry generated O(2) and O(3) atoms from two different ligands [Mn(1)-O(3)iv, 0.229 2(5) nm; Mn(1)-O(2)iii, 0.216 9(5) nm]. All Mn-O and Mn-N bond lengths are consistent with those reported in literatures[20]. So each Mn2+ion is linked to five symmetry-related ligands. The adjacent metal ions are alternately joined [Mn(1)-Mn(1), 0.390 2(2) nm] by aμ1,3-carboxyl bridge frombenzoimidazole ring to form a 1Dinorganic helix chains along the crystallographicbaxis (Fig.3b). Each of the double helixes consists of two interweaved right- and left-handed 21helixes by sharing manganese, carbon, and oxygen atoms, with screw-pitch of about 0.390 2 nm along thebaxis.These parallel inorganic chains are connected through N and O atoms of benzoimidazole-5,6-dicarboxylates from Hcbic2-ligands, forming a layer structure (Fig.3c) atbcplane. Interestingly, in the structure, neighboring Mn2+bridged by O(2) atom form Mn2O2units and the units are linked by Hcbic2-ligand to produce a 2Dnetwork having a rhomblike window with the dimensions of 0.799 8 nm×0.645 3 nm(Fig.3c). In detail, the structure is catenated with such identical rings while the 4-methylbenzoic acid groups of Hcbic2-ligands play many ribbons distributing in the two sides of the 2Dnetworks.

Meanwhile,O6-H…O4i(i:1-x,y-1/2, -z+1/2) hydrogen bonds between the ribbons and the inorganic chains spread over the whole structure to assemble a 3DH-bonding framework as shown in Fig.3d.Interatomic distance is 0.271 6(1) nmand angle is 165° indicated strong hydrogen bonds. The simplified structure of the MOF-1 can be classified as 5-connected topology with the topological notation (47.63) (Fig.3e) from TOPOS program analysis[21].

The experimental powder X-ray diffraction patterns of MOF-1 agree well with those simulated from thecrystal structure, demonstrating phase purity of MOF-1 (Fig.4a).In addition, the thermogravimetric analysis (TGA) curve of MOF-1 showed the 2Dnetwork structure to be stable in the range of room temperature to 400 ℃ (Fig.4b).

Fig.2 Structure of the asymmetric unit in MOF-1, showing the atom-numbering scheme with a MnNO5coordination enviroment. Displacement ellipsoids are drawn at the 30% probability level and the minor disorder components have been omitted for clarity.Symmetry code: (i) -x, -y+1, -z; (ii) x, -y+3/2, z+1/2; (iii) -x,-y+2, -z; (iv) x,-y+1/2, z+1/2

Fig.3 (a)The geometry of the coordination polyhedron around the Mn(II) cation in MOF-1. (b) The infinite 1D zigzagchain along the b axis by a μ1,3-carboxyl bridge from benzoimidazole ring. (c)The illustration of the 2D layer in the bc plane. (d) 3D supramolecular network constructed by hydrogen bonding O-H…O. (e) The topology net of MOF-1 with Schl?fli symbol (47.63)

Fig.4 (a) Comparison of the simulated from the X-ray single-crystal structure and experimental PXRD patterns of MOF-1. (b) Thermogravimetric analysis (TGA) curve of MOF-1

2.3 Magnetic properties

Variable-temperature magnetic susceptibilities of MOF-1 were measured in the temperature range of 2.0-300 K with an applied magnetic field of 1 000 Oe. As shown in Fig.5, theχMTvalue of MOF-1 is 5.18 cm3·K·mol-1at 300 K which is lower thanonespin-only value (g=2.0 andS=5/2) 4.35 cm3· K· mol-1. On lowering temperature,the χMTfirst decreases slowly until～50 K and then sharply to reach a value of 0.017 cm3·mol-1·K at 2 K. The shape of this curve is typical of a dominant antiferromagnetic coupling in the MOF-1.The Curies-Weiss fit in the range of 2.0 to 300 K affords a Curie constant ofC=6.37 cm3·K·mol-1and a Weiss constant ofθ= -72.68 K.

Fig.5 Plots of the variation between theM(left),MT(right)and the temperature in MOF-1The graph of χM-1 and the temperature T is shown in the middle graph

3 Conclusions

In summary, a unique 2DMOF-1 framework containingone-dimensional left and right-handed double helical chains was synthesized with high thermostability.In MOF-1,five coordinate atoms of seven sites take part in the two-dimensional layered structure. The structure is catenated with an identical rhomblike window while the 4-methylbenzoic acid groups of Hcbic2-ligands play many ribbons distributing in the two sides of the 2Dnetworks. In addition, magnetic susceptibility measurements indicate the presence of antiferromagnetic exchange interaction in MOF-1.