YAN Jun-Zhi ZHAO Dn LU Li-Ping

a (Taiyuan University, Taiyuan 030032, China)

b (Institute of Molecular Science, Shanxi University, Taiyuan 030006, China)

ABSTRACT One new Mn(II) coordination polymer, [Mn(Htia)2(H2O)2]n·2nH2O (1, H2tia = 4-(1,2,4-triazol-1-yl)isophthalic acid) has been synthesized in mixed solvents under solvothermal conditions. Further characterizations including single-crystal XRD, elemental analysis, IR spectroscopy, thermogravimetric analysis and powder XRD were performed to verify the structure. Complex 1 displays a one-dimensional (1D) chain and is similar to a reported Mn(II) complex 2 {[Mn(Htta)2(H2O)2]·2H2O}n. It crystallizes in triclinic P1 space group, with a = 7.5557(3), b = 7.5974(3), c = 11.8448(4) Å, α = 90.088(1)°, β = 95.863(1)°, γ = 113.668(1)°, V = 618.81(4) Å3,Z = 1, Mr = 591.36 g/mol, Dc = 1.587 mg/m3, μ = 0.609 mm-1, F(000) = 303, GOOF = 1.062, the final R = 0.0359 and wR = 0.0818 for 4272 observed reflections with I > 2σ(I). Based on two similar structures, Hirshfeld surface analysis confirmed both structures are mainly stabilized by O···H/H···O and C···H/H···C hydrogen bonds. Further,weak ferromagnetic behaviors between adjacent Mn(II) ions in 1D chain are obtained.

Keywords: manganese(II) complexes, (triazol-1-yl)iso/terephthalic acid, crystal structure, Hirshfeld surface,magnetic properties; DOI: 10.14102/j.cnki.0254-5861.2011-3186

1 INTRODUCTION

The inorganic-organic complexes with peculiar structures and potential applications have attracted much attention and seen great progress in recent years[1-3]. Especially in the field of magnetism, many manganese coordination polymers have been designed and synthesized[4,5]. The key to designing such material is to select a bridging ligand that can effectively construct novel structures and mediate the magnetic coupling[6,7]. Carboxylate groups are among the most extensively investigated bridges due to their various bridging modes such assyn-syn,syn-anti, andanti-anti[8,9], which can easily form extended coordination networks with diverse topologies, and efficiently mediate either ferromagnetic (FM)or antiferromagnetic (AFM) coupling[10]. The polycarboxylate ligands that contain multiple bridging moieties and multidentate chelating modes are one of the most widely used linkers in the design of polynuclear complexes with interesting magnetic properties. It is well-known that substituted 1,2,4-triazoles are widely used to construct novel MOFs due to their versatile coordination modes[11-13]. Specific structure could regulate magnetism behavior to realize the functionalization of the structure. In our previous work,several 1,2,4-triazole complexes have been reported[14-17],where manganese complexes[15]also showed different crystal structures and magnetic properties.

As continuation of our investigation of asymmetrically substituted 1,2,4-triazoles, herein, synthesis and characterization of one new manganese complex based on a bifunctional ligand 4-(1,2,4-triazol-1-yl)isophthalic acid (H2tia) are presented. We also compared a similar structural complex 2{[Mn(Htta)2(H2O)2]·2H2O}n[18]reported in previous literature based on 2-(1,2,4-triazol-1-yl)terephthalic acid (H2tta) with the title complex 1. Ligands H2tia and H2tta used are just different in the positions (para-/meta-) of one carboxyl group bonded with benzene ring. For structural investigation and comparison, Hirshfeld surface analysis may better find similarities and minor differences. Magnetic properties of two manganese complexes are described according to structural features in 300～2.0 K.

2 EXPERIMENTAL

2. 1 Materials and methods

All reagents were purchased commercially and used without further purification. Elemental analyses (C, H, and N)were performed on an Elementar Vario EL III analyzer. FT-IR spectra were recorded from KBr pellets in the range of 4000～400 cm−1on a Bruker TENSOR27 Spectrometer.Powder X-ray diffraction (PXRD) data were collected on a Rikagu Smartlab X-ray diffractometer with Cu-Kαradiation(λ= 1.5406 Å) in the 2θrange of 5～50° at a rate of 5 °/min.Thermogravimetric (TG) study was carried out on a Dupont thermal analyzer under 20 mL/min flowing N2while ramping the temperature at a rate of 10 K/min from 313 to 1073 K.Variable temperature (2.0～300 K) magnetic susceptibilities of crystalline samples of the complexes were measured on a Quantum Design MPMS SQUID magnetometer with an applied field of 1000Oe.

2. 2 Synthesis of complex [Mn(Htia)(H2O)2]n·2nH2O (1)

A mixture of MnCl2·4H2O (0.0396 g, 0.2 mmol), H2tia(0.0233 g, 0.1 mmol), CH3CN (1.0 mL) and H2O (2.0 mL)was stirred at room temperature, then sealed in a 23.0 mL Teflon-lined stainless-steel vessel, heated at 393 K for 3 days,and cooled to room temperature, obtaining colourless crystals in 46.8% yield (based on Mn(II)). Anal. Calcd.C20H20MnN6O12(%): C, 40.62; H, 3.41; N, 14.21. Found (%):C, 39.78; H, 3.44; N, 14.52%. IR (cm−1): 3411 (s), 3137(m),1957 (w), 1691 (m), 1603 (s), 1510 (s), 1424 (m), 1379 (s),1300 (s), 1257 (m), 1129 (m), 1106 (m), 1041 (m), 977 (s),982 (w), 912 (m), 883 (m), 868 (w), 846 (w), 819 (w), 767 (s).

2. 3 Synthesis of reported complex[Mn(Htta)(H2O)2]n·2nH2O (2)

The synthesis of 2 was similar to that of 1 except H2tia was replaced by H2tta (0.0233 g, 0.1 mmol). Colourless crystals 2 were obtained in 57.6% yield (based on Mn(II)). The synthesis condition is quite different from the reported method[18]. Even with different temperature, solvents and metal/ligand ratios, the same structure is obtained, which may display the most stable state in kinetics and thermodynamics system.

2. 4 X-ray crystallography

Single-crystal X-ray diffraction data for complex were collected on a Bruker D8 venture diffractometer equipped with graphite-monochromatic MoKαradiation (λ= 0.71073 Å)at 298(2) K. Cell parameter was determined using SMART software. Absorption correction was madeviaSADABS program[19]. The structure was solved by direct methods employed in the SHELXS-2014 program and refined by full-matrix least-squares methods againstF2with SHELXL-2014[20]. After all non-H atoms were refined anisotropically, hydrogen atoms attached to carbon atoms were placed geometrically and refined using a riding model approximation. Selected bond lengths, angles and H-bonds are shown in Tables 1 and 2.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°) of Complex 1

Table 2. Hydrogen Bond Geometry (Å, °) for 1

2. 5 Hirshfeld surface (HS) calculations

Molecular Hirshfeld surface calculations were performed by using the CrystalExplorer software ver. 3.1[21]. Molecular Hirshfeld surfaces are shown as transparent to allow visualization of the molecular and connection environment in crystals. When the cif files of structures 1 and 2 were read into the CrystalExplorer program for analysis, all bond lengths to hydrogen were automatically modified to typical standard neutron values. The 2Dfingerprint plots were displayed by using the standard 0.5～2.5 Å view with thedeanddidistance scales displayed on the graph axes.

3 RESULTS AND DISCUSSION

3. 1 Structural description

X-ray crystallographic analysis reveals complex 1 crystallizes in triclinic system with space group ofP1. As shown in Fig. 1, the Mn(II) coordination consists of four O atoms (O(3), O(3i), O(5), O(5i)) in the equatorial plane and two triazolyl axial N (N(3ii), N(3iii)) atoms. The connectivity of 1 is comparable to 2 {[Mn(Htta)2(H2O)2]·2H2O}n[18], which is based on a meta-substituted triazolyl carboxylic acid H2tta similar to H2tia ligand. H2tia still keeps one protonated carboxyl group because no base was introduced, which makes complex 1 the same coordination mode with literature.Central Mn(II) falling in symmetrical center makes each Mn(II) ion joined by double ligands to result in the Mn2(Htia)218-membered cycle with a diameter approximately 7.60 and 8.30 Å in 1 (7.15 and 8.83 Å in 2), and afford a 1Dchain along thebdirection. As shown in Fig. 2, two strands of ligands are held together by Mn(II) atoms and wrapped around each other to form a double chain with Mn···Mn distances of 7.5974(3) Å in 1 (7.1565(2) Å in 2).Even though the Mn···Mn distance is long, it still leads to weak magnetic interactions, as analyzed in the magnetic part of this context. In addition, there are two intramolecular hydrogen bonds (C-H···O) (Table 2) to stabilize the structure,and the other ones (O-H···O) from coordinated and free water molecules connect primarily the adjacent 1Ddouble chains to form a 3Dstructure. Interestingly, the distance between pairs of benzene planes of two Htia−groups in the 3Dsupramolecular network of 1 is 4.266 Å (Fig. 2), which is longer than 3.267 Å in complex 2, which may be attributed to the different dihedral angle between the triazole and benzene rings due to the steric hindrance of para-/metacarboxyl group in the ligand.

Fig. 1. View of the structure of 1 with 30% thermal ellipsoids(Symmetry codes: i −x, −y + 1, −z + 1; ii −x, −y, −z + 1; iii x, y + 1, z)

Fig. 2. View of the 3D structures of 1(a) and 2(b) through weak interactions

Compared with complex 2 that has been reported[18],similar 1Dchain structures are observed as expected due to the same coordination sites of two similar partially deprotonated ligands. But different bond distances and bond angles around the centre metal promote two different octahedra with varying degrees of distortion. For example, the Mn-O bond lengths on the equatorial plane is 2.157(1) and 2.182(1) Å (2.1605(16) and 2.224(2) Å reported in 2), and the axial Mn-N of 2.264(1) Å in 1 is comparable with 2.260(2) Å in 2. But all the parameters including bond distances and bond angles are within a reasonable range and match the values reported for similar coordination Mn(II) complexes[15,22-24]. Unlike 2 {[Mn(Htta)2(H2O)2]·2H2O}n,uncoordinated para-substituted carboxylate group will generate smaller steric hindrance in Htia, displaying a smaller dihedral angle between the triazole ring and benzene ring,which will be reflected on H bonding orπ···πinteractions.

3. 2 Molecular Hirshfeld surface (HS) study

The HS maps of the structures are illustrated in Fig. 3,showing the surfaces are mapped over adnorm[25]. The spots especially shaped large deep red depressions on thednormsurfaces, reflecting the effective and strong hydrogen bonding contacts. Light red spots may be indicative of comparable weak C···H contacts. The leading interactions are also visible in the fingerprint plots, which can be decomposed into diverse interaction types to highlight particular atom-pair close contacts. In the fingerprint plots,de>direpresents that the molecule could be the donor, otherwise an acceptor should bede>di.

Fig. 3. HS of INDP mapped with dnorm (left) and Fingerprint plots (right) (the semitransparent surface shows the enclosed molecule)

In complex 1, the proportions of H···O/O···H contacts are 16.3 and 18.0% derived from solvent water molecules and carboxyl groups. The proportions of C···H/H···C interactions comprise 2.1 and 6.2% of entire HS surfaces for each molecule. Finally, supramolecular self-assembly and stabilization are derived from those H bonding contacts.Similarly, the contributions C···H/H···C interactions in 2 are 3.2 and 4.9% and the H···O/O···H contacts comprise 13.6 and 17.8%. The HS analysis shows both structures are stabilized mainly by O···H/H···O and C···H/H···C hydrogen bonds.Furthermore, compared with 2, the O···H/H···O contacts of 1 are slightly increased (～2%) due to the uncoordinated para-/metacarboxyl group. Minor structural changes generate slightly different weak interactions. The fact is indeed different O-H···O bonding between H2O and carboxylate oxygen. We can see free water molecules are located quite differently. Thus, the Hirshfeld surface analysis could analyze and get insight into weak interactions in the self-assembly system.

3. 3 PXRD and thermogravimetric analysis

The experimental and simulated PXRD patterns of complexes 1 and 2 are shown in Fig. 4. The experimental PXRD pattern of the bulk product is in good agreement with the calculated XRD pattern from single-crystal X-ray diffraction results.

As illustrated in Fig. 5, complex 1 has similar thermogravimetric plot with that of 2 {[Mn(Htta)2(H2O)2]·2H2O}n[18],which matches well with their structural composition. The first weight-loss stage occurred about 361～450 K without any loss below 361 K, corresponding to the loss of four water molecules, including coordinated and lattice ones (expt.12.7%, calcd. 12.2%). The dehydrated complex then decomposes roughly at 548 K as indicated by the huge weight loss in the TG curve.

3. 4 Magnetic properties

Mn(II) ions endowed withd5electronic configuration could induce paramagnetic behavior in a magnetic field. As expected, the magnetic behaviors of 1 and 2 were successfully recorded and shown asχM−1andχMTversusTplots with extremely similar trends in Fig. 6.

Fig. 4. PXRD patterns for samples 1 and 2

Fig. 5. TG plot of complex 1

Fig. 6. Plots of χMT and χM vs. T for 1 (a) and 2 (b) (Inset: plot of χM−1 vs. T)

The experimentalχMTvalues of 1 and 2 at room temperature are 4.28 and 4.25 cm3·K·mol−1, which are slightly lower than 4.75 cm3· K·mol−1of one uncorrelated theoretical spin value (Mn(II):S= 5/2,g= 2.0). Upon cooling, theχMTvalues steadily increase up to 4.4 cm3·K·mol−1for 1 and 4.5 cm3·K·mol−1for 2 from room temperature to 10.0 K, and then the values rapidly decrease down to 4.25 cm3·K·mol−1for 1 and 4.3 cm3·K·mol−1for 2 at 2.0 K. Gradually monotonically increasing trends reflect the ferromagnetic behavior between 300 and 10 K. While, the rapid decrease ofχMTto 4.25 cm3·K·mol−1for 1 and 4.3 cm3·K·mol−1for 2 between 10 and 2.0 K is probably caused by weak intermolecular antiferromagnetic interactions and zero-Feld splitting. The data in the temperature range of 2.0～300 K obey the Curie-Weiss lawχ=C/(T−θ) withC= 2.0 (1) and 1.0 (2) cm3·K·mol−1andθ=4.0 (1) and 2.0 (2) K. The positiveθvalues are indicative of similar weak ferromagnetic interactions between adjacent Mn(II) ions in the network structure of complexes. By magnetic structure analysis, the observed weak magnetic behavior can be interpreted in the context of the structures:large organic linker may be the dominant pathway for magnetic exchange, the longer distance of Mn···Mn will contribute to weaker magnetic interactions. Above extremely similar magnetism indicate Mn···Mn distance and deliver linker are structurally similar.

4 CONCLUSION

In summary, two similar Mn(II) complexes have been synthesized under solvothermal conditions. Their structures and magnetic performances are comparable due to similar organic linkers (H2tia and H2tta). Both complexes have a 1Dchain arrangement with a mononuclear Mn(II) geometry. The Hirshfeld surfaces analysis shows both structures are stabilized mainly by O···H/H···O and C···H/H···C hydrogen bonds. Both complexes display weak ferromagnetic exchange,which is also comparable with their structures. Thus similar coordination and connection could rarely affect the performance.