English

• 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 as syn-syn, syn-anti, and anti-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^-1 on a Bruker TENSOR27 Spectrometer. Powder X-ray diffraction (PXRD) data were collected on a Rikagu Smartlab X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å) in the 2θ range 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 N₂ while 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 1000 Oe.

  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 MoKα radiation (λ = 0.71073 Å) at 298(2) K. Cell parameter was determined using SMART software. Absorption correction was made via SADABS program^[19]. The structure was solved by direct methods employed in the SHELXS-2014 program and refined by full-matrix least-squares methods against F² with 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

  

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

  DownLoad: CSV

  Bond	Dist		Bond	Dist		Bond	Dist
  Mn(1)-O(5)	2.157(1)		Mn(1)-O(5i)	2.157(1)		Mn(1)-O(3i)	2.182(1)
  Mn(1)-O(3)	2.182(1)		Mn(1)-N(3ii)	2.264(1)		Mn(1)-N(3iii)	2.264(1)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(5)-Mn(1)-O(5i)	180.0		O(5)-Mn(1)-O(3i)	94.61(4)		O(5i)-Mn(1)-O(3i)	85.40(4)
  O(5)-Mn(1)-O(3)	85.40(4)		O(5i)-Mn(1)-O(3)	94.60(4)		O(3i)-Mn(1)-O(3)	180.0
  O(5)-Mn(1)-N(3ii)	90.10(5)		O(5i)-Mn(1)-N(3ii)	89.90(5)		O(3i)-Mn(1)-N(3ii)	85.16(4)
  O(3)-Mn(1)-N(3ii)	94.84(4)		O(5)-Mn(1)-N(3iii)	89.90(5)		O(5i)-Mn(1)-N(3iii)	90.10(5)
  O(3i)-Mn(1)-N(3iii)	94.84(4)		O(3)-Mn(1)-N(3iii)	85.16(4)		N(3ii)-Mn(1)-N(3iii)	180.1(0)
  Symmetry codes 1: i −x, −y + 1, −z + 1; ii −x, −y, −z + 1; iii x, y + 1, z; iv x, y − 1, z

  Table 2

  

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

  DownLoad: CSV

  D-H···A	D-H	H···A	D···A	∠D-H···A
  O(6)-H(6A)···O(3v)	0.82	1.91	2.732(1)	179
  O(6)-H(6B)···N(2)	0.82	2.06	2.876(1)	176
  O(5)-H(5B)···O(4vi)	0.81	2.02	2.834(1)	174
  O(5)-H(5A)···O(2vii)	0.84	1.90	2.730(1)	166
  O(1)-H(1)···O(6viii)	0.82	1.76	2.582(1)	174
  C(10)-H(10)···O(4ix)	0.93	2.56	3.465(1)	164
  C(7)-H(7)···O(1iv)	0.93	2.53	3.412(1)	158
  Symmetry codes: 1: iv x, y − 1, z; v x + 1, y, z; vi x − 1, y, z; vii −x, −y + 1, −z; viii −x + 1, −y + 1, −z; ix −x + 1, −y, −z + 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 2D fingerprint plots were displayed by using the standard 0.5~2.5 Å view with the d_e and d_i distance 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 of P$ \overline 1 $. 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)₂ 18-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 1D chain along the b direction. 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 1D double chains to form a 3D structure. Interestingly, the distance between pairs of benzene planes of two Htia⁻ groups in the 3D supramolecular 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.

  Figure 1

  

  Figure 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)

  DownLoad: Full-Size Img PowerPoint

  Figure 2

  

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

  DownLoad: Full-Size Img PowerPoint

  Compared with complex 2 that has been reported^[18], similar 1D chain 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 or π···π interactions.

  3.2 Molecular Hirshfeld surface (HS) study

  The HS maps of the structures are illustrated in Fig. 3, showing the surfaces are mapped over a d_norm^[25]. The spots especially shaped large deep red depressions on the d_norm surfaces, 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, d_e > d_i represents that the molecule could be the donor, otherwise an acceptor should be d_e > d_i.

  Figure 3

  

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

  DownLoad: Full-Size Img PowerPoint

  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.

  Figure 4

  

  Figure 4.  PXRD patterns for samples 1 and 2

  DownLoad: Full-Size Img PowerPoint

  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.

  Figure 5

  

  Figure 5.  TG plot of complex 1

  DownLoad: Full-Size Img PowerPoint

  3.4 Magnetic properties

  Mn(Ⅱ) ions endowed with d⁵ electronic configuration could induce paramagnetic behavior in a magnetic field. As expected, the magnetic behaviors of 1 and 2 were successfully recorded and shown as χ_M^-1 and χ_MT versus T plots with extremely similar trends in Fig. 6.

  Figure 6

  

  Figure 6.  Plots of χ_MT and χ_M vs. T for 1 (a) and 2 (b) (Inset: plot of χ_M⁻¹ vs. T)

  DownLoad: Full-Size Img PowerPoint

  The experimental χ_MT values 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⁻¹ of one uncorrelated theoretical spin value (Mn(Ⅱ): S = 5/2, g = 2.0). Upon cooling, the χ_MT values steadily increase up to 4.4 cm³·K·mol⁻¹ for 1 and 4.5 cm³·K·mol⁻¹ for 2 from room temperature to 10.0 K, and then the values rapidly decrease down to 4.25 cm³·K·mol⁻¹ for 1 and 4.3 cm³·K·mol⁻¹ for 2 at 2.0 K. Gradually monotonically increasing trends reflect the ferromagnetic behavior between 300 and 10 K. While, the rapid decrease of χ_MT to 4.25 cm³·K·mol⁻¹ for 1 and 4.3 cm³·K·mol⁻¹ for 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 law χ = C/(T − θ) with C = 2.0 (1) and 1.0 (2) cm³·K·mol⁻¹ and θ = 4.0 (1) and 2.0 (2) K. The positive θ values 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 1D chain 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.

  
  
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• 

  Figure 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)

  下载: 全尺寸图片 幻灯片
  

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

  下载: 全尺寸图片 幻灯片
  

  Figure 3  HS of INDP mapped with d_norm (left) and Fingerprint plots (right) (the semitransparent surface shows the enclosed molecule)

  下载: 全尺寸图片 幻灯片
  

  Figure 4  PXRD patterns for samples 1 and 2

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  Figure 5  TG plot of complex 1

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  Figure 6  Plots of χ_MT and χ_M vs. T for 1 (a) and 2 (b) (Inset: plot of χ_M⁻¹ vs. T)

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

  Bond	Dist		Bond	Dist		Bond	Dist
  Mn(1)-O(5)	2.157(1)		Mn(1)-O(5i)	2.157(1)		Mn(1)-O(3i)	2.182(1)
  Mn(1)-O(3)	2.182(1)		Mn(1)-N(3ii)	2.264(1)		Mn(1)-N(3iii)	2.264(1)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(5)-Mn(1)-O(5i)	180.0		O(5)-Mn(1)-O(3i)	94.61(4)		O(5i)-Mn(1)-O(3i)	85.40(4)
  O(5)-Mn(1)-O(3)	85.40(4)		O(5i)-Mn(1)-O(3)	94.60(4)		O(3i)-Mn(1)-O(3)	180.0
  O(5)-Mn(1)-N(3ii)	90.10(5)		O(5i)-Mn(1)-N(3ii)	89.90(5)		O(3i)-Mn(1)-N(3ii)	85.16(4)
  O(3)-Mn(1)-N(3ii)	94.84(4)		O(5)-Mn(1)-N(3iii)	89.90(5)		O(5i)-Mn(1)-N(3iii)	90.10(5)
  O(3i)-Mn(1)-N(3iii)	94.84(4)		O(3)-Mn(1)-N(3iii)	85.16(4)		N(3ii)-Mn(1)-N(3iii)	180.1(0)
  Symmetry codes 1: i −x, −y + 1, −z + 1; ii −x, −y, −z + 1; iii x, y + 1, z; iv x, y − 1, z

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

  D-H···A	D-H	H···A	D···A	∠D-H···A
  O(6)-H(6A)···O(3v)	0.82	1.91	2.732(1)	179
  O(6)-H(6B)···N(2)	0.82	2.06	2.876(1)	176
  O(5)-H(5B)···O(4vi)	0.81	2.02	2.834(1)	174
  O(5)-H(5A)···O(2vii)	0.84	1.90	2.730(1)	166
  O(1)-H(1)···O(6viii)	0.82	1.76	2.582(1)	174
  C(10)-H(10)···O(4ix)	0.93	2.56	3.465(1)	164
  C(7)-H(7)···O(1iv)	0.93	2.53	3.412(1)	158
  Symmetry codes: 1: iv x, y − 1, z; v x + 1, y, z; vi x − 1, y, z; vii −x, −y + 1, −z; viii −x + 1, −y + 1, −z; ix −x + 1, −y, −z + 1

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