Synthesis, Crystal Structure, Thermal Stability, Luminescence and Magnetic Property of a New MnII Coordination Polymer①

YANG Hong-Li CHEN Fang HE Xiong LI Yan ZHANG Xiu-Qing



Synthesis, Crystal Structure, Thermal Stability, Luminescence and Magnetic Property of a New MnIICoordination Polymer①

YANG Hong-Li CHEN Fang HE Xiong LI Yan ZHANG Xiu-Qing②

(541004)

One new coordination polymer, [Mn(Hbtc)(bpy)(H2O)2]1, was synthesized by the reaction of manganese(II) salt under pH-controlled hydrothermal conditions with unsymmetrical polycarboxylic acid of 1,2,4-benzenetricarboxylic acid (H3btc) and 2,2΄-bipyridine (bpy). The structure of complex 1 was characterized by single-crystal X-ray diffraction, IR spectra, powder X-ray analysis, thermal gravimetric analysis (TGA), UV-Vis spectrometry, photoluminescence and magnetic susceptibility. Complex 1 is a one-dimensional (1) polymeric single-chain structure. Photoluminescence property is quite similar to the bpy ligand. Magnetic susceptibility measure- ment indicates that 1 shows a weak ferromagnetic coupling between the MnIIions.

MnIIcoordination polymer, crystal structure, 1,2,4-benzenetricarboxylic acid, photoluminescence, magnetic property;

1 INTRODUCTION

Coordination polymers (CPs) have increasingly attracted considerable attention for their charming structures and potential applications, such as in gas absorption and separation, luminescence, catalysis, molecular magnetism, sensors,[1-12]. However, polycarboxylic ligands which have diversified coordination modes and flexible conformation should be excellent bridging ligands for the con- struction of CPs, so the self-assembly process driven by metal-carboxylate coordination has been exploited as a useful tool for the crystal-engineering design of CPs with extended network structures. The desired CPs can be designed and synthesized by the appropriate selection of metal-salt and bridging ligands to extend them into polymeric networks. It is because of the complexity and diversity of their structures that they have a wide range of application properties. S. Shibu Prasad reported the construction of two novel 1CPs of cobalt and copper with 4,4¢-oxybis(benzoic acid) and N-donor ligand-1H-imidazoleand studied on their photoluminescence[13]. In addition, conforma- tional flexibility of the ligand also plays a decisive role in determining both the dimensionality and topology of the final structure.

In order to construct the complexity of CPs struc- tures, polycarboxylic acid ligands[14, 15]have attracted more and more attention in the research field of coordination chemistry, since they can bridge two or more metal centers in diverse fashions through its multidentate O-atom donors. Therefore, organic aromatic multicarboxylate ligands have been favored by a wide range of scholars, such as terephthalate, trimesic acid,. However, these symmetrical carboxylic acid ligands have been extensively studied. The research of asymmetric carboxylic acid ligands such as H3btc is not much reported. H3btc is a rigid, high asym- metric ligand with point groupC[16]. This feature may be exploited to form compounds with non- centrosymmetric unit cell due to asymmetric carboxylic acid distribution which can interlink in bridge and/or chelate coordination modes. Thus, they show different conformations and packing modes of carboxyl groups, giving rise to a wide kind of dihedron angles between the plane of carbonyl system and the benzene ring. This angular geometry can be combined with the metal coor- dination site to build self-assembled CPs.

Before that, one MnIICP based on H3btc and bpy ligands has been early described in detail[17]. Based on this fact, we present here another MnIICP with H3btc and bpy ligands by volatilization at room temperature. In this study, we describe the synthesis, single-crystal X-ray diffraction analysis, IR spectra, powder X-ray analysis, TGA, UV-Vis spectrometry, photoluminescence and magnetic susceptibility of complex 1.

2 EXPERIMENTAL

2.1 Materials and instruments

All the materials were purchased (Alarich, Aladdin, or Alfa Aesar) without further purification. The crystal structure was determined on an Agilent G8910A CCD diffractometer. FT-IR spectra were recorded as KBr pellets from 4000 to 400 cm-1on a Bio-Rad FTS-7 spectrophotometer. Power X-ray diffraction (XRD) data were collected on a Bruker D8 Advance X-ray diffractometer with Curadiation (= 1.5418 Å). The thermal behavior was carried out by a SDTQ 600 apparatus. UV-Vis absorption spectra were recorded on a Cary100 UV-Visible Spectrophotometer. The luminescence spectrum for the solid sample was recorded at room temperature on a RF-4600 fluorescence spectro- photometer under the same conditions. Magnetic measurement was carried out with a Quantum Design PPMS model 600 magnetometer to 5T for complex 1.

2.2 Synthesis of [Mn(Hbtc)(bpy)(H2O)2]n (1)

A mixture of MnCl2·4H2O (0.0396 g, 0.2 mmol), H3btc (0.0420 g, 0.2 mmol), bpy (0.0312 g, 0.2 mmol), and H2O (15 mL) was sealed and two drops of triethylamine were added in a 25 mL Teflon- lined autoclave under autogenous pressure at 393 K for 3 days. After cooling to room temperature and filtering into the beaker for two weeks or so, color- less needle crystals were obtained in a yield of 41% based on MnIIion. IR (KBr, cm-1): 3438(b), 1674(m), 1569(s), 1488(w), 1402(s), 1367(w), 1287(m), 1246(m), 772(s), 603(w).

2.3 X-ray crystal structure determination

Single-crystal X-ray diffraction data for 1 were collected on an Agilent G8910A CCD diffracto- meter with Moradiation (= 0.71073 Å) at 293(2) K. The collected data were restored with SAINT and the SADABS was used for multiple absorbance correction. Multiple absorption is used for calibration. Using Olex2[18], the structure of 1 was solved with the SHELXS solution program by direct methods and refined with the SHELXL refinement program using Least-Squares minimiza- tion[19]. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were positioned geometrically and refined as riding. Selected bond lengths and bond angles are listed in Table 1.

3 RESULTS AND DISCUSSION

3.1 Structure analysis

X-ray crystallographic analysis of complex 1 indicates that it crystallizes in the monoclinic space group. As shown in Fig. 1, the unit of 1 contains one MnIIcation, one Hbtc2-anion, one bpy ligand, and two coordination water molecules. MnII(1) is coordinated by two carboxylate oxygen atoms (O(1), O(4B)) from two different Hbtc2-anions and two nitrogen atoms (N(1), N(2)) from one bpy molecule and two oxygen atoms (O(7), O(8)) from two water molecules. The six coordination atoms form distorted octahedral geometry around the MnII(1) cation. The equatorial plane is made by N(1), N(2), O(7) and O(8) with the mean deviation of 0.1425 Å. The axis position is occupied by O(1) and O(4B). As shown in Fig. 2, the MnIIcenters are connected through monodentate coordination of carboxyl groups from the Hbtc2-anions in a2-1:0:1:0:0:0fashion to form a 1chain structure. The adjacent Mn···Mn separation is 7.5165 Å. In the similar MnIIcomplex previously reported, the H3btc ligands are completely deprotonated so that the btc3-anions only adopt one kind of bridging modes through-bidentate bridging and bidentate chelation of carboxyl groups in a4-1:1:1:1:1:1fashion to form a 2layer structure. But the bpy ligands are all coordinated by bidentate chelating for two MnIIcomplexes.

Fig. 1. ORTEP view of the coordination environment of the MnIIatom in 1 with thermal ellipsoids at the 30% probability level (Hydrogen atoms were omitted for clarity)

Fig. 2. View of the 1polymeric single-chain structure and the-stacking interaction of complex 1

Table 1. Selected Bond Lengths (Å) and Bond Angles (º)

Symmetry code: (i)+1,,

The weak-stacking interaction exists in this chain structure. And the center distance between the nearly parallel benzene and pyridine rings is 3.7130 Å. In addition, as shown in Fig. 3, the chains are stabilized by the O–H···O hydrogen bonds between carboxyl groups and water molecules, with the hydrogen bond lengths and angles listed in Table 2.

Fig. 3. Packing drawing of complex 1

Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°)

Symmetry codes: (i),, 1+z; (ii)1+,,; (iii),, –1+

3.2 IR spectrum

IR spectra of carboxylate complexes can provide information to distinguish the coordination modes of carboxyl groups[20, 21]. As shown in Fig. 4, the broad band centered at 3438 reveals the O–H characteristic stretching vibration which indicates the existence of coordinated H2O molecules for 1. The characteristic vibration band at 1674 cm-1is attributed to the(C=O) stretching vibration. The characteristic vibration bands at 1488 and 1402 cm-1are attributed to the benzene vibration. The characteristic vibration bands at 1287 and 1246 cm-1are attributed to the(C–O) stretching vibration. Obvious bands observed at 1569 and 1367 cm-1are associated with asymmetric(OCO)asand symmetric(OCO)sstretching vibrations of the carboxyl groups. The Δ((OCO)as–(OCO)s) value is 202 cm-1, showing the presence of monodentate mode of carboxyl groups in the Hbtc2-ligand.

3.3 Powder X-ray diffraction analysis

As shown in Fig. 5, the phase purity of synthesized sample of 1 has been characterized by PXRD. The experimental PXRD pattern corre- sponds well with the pattern simulated form the single-crystal data, indicating the pure phases. However, the difference in reflection intensities between the simulated and experimental patterns may be attributed to variation in preferred orien- tation of the powder sample during the collection of experimental PXRD data.

3.4 Thermal gravimetric analysis

In order to examine the thermal stability of 1, thermal gravimetry (TG) was carried out between 293 and 1173 K under nitrogen flow. As shown in Fig. 6, complex 1 is stable up to 387 K, and then begins to decompose. After 387 K, not only the loss of coordinated water molecules, but also the skeleton of structure began to collapse. From 387 to 1173 K, a mass loss is 83.26% (calcd.: 84.41%, presumably the residue is MnO). And the weight drops sharply between 633 and 838 K with a mass loss of 34.04% which could be attributed to the loss of bpy ligands (calcd.: 34.30%).

3.5 UV-Vis absorption spectra

UV-Vis absorption spectra of the ligands (H3btc and bpy) and the corresponding MnIIcomplex 1 in DMF solutions at 298 K are shown in Fig. 7. The spectral shape of 1 is similar to bpy, and different from the shape of H3btc. The UV-Vis spectrum of the free ligand (H3btc) exhibits two absorption peaks at. 267 and 288 nm. The former absorption peak could be assigned to the-* transition, and the latter could be attributed to the-*transition of the benzene rings[22]. Complex 1 has one obvious absorption peak at 278 nm, which is almost identical to that at 282 nm of ligand bpy. The absorption peaks of complex 1 are compared with ligands in different concentrations, and it is found that the absorption of complex 1 is due to bpy. The absorption peak at 278 nm in the complex results from-* transition from the bpy ligand. In addition, the decreased absorbance of complex 1 may be due to the metal-to-ligand charge transfer (MLCT).

Fig. 4. IR spectrum of complex 1

Fig. 5. XRD patterns of complex 1

Fig. 6. Thermal behavior of complex 1

Fig. 7. UV-Vis absorption spectra of H3btc, bpy and complex 1

3.6 Luminescent property

The MnIIcomplexes may have excellentfluore- scentproperties[23]. The luminescent properties of complex 1 and free ligands were examined in the solid state at room temperature (Fig. 8). Free H3btc, bpy and complex 1 exhibited emissions at= 330, 489 and 546, 490 and 548 nm with the corresponding excitation wavelengths of 278, 331 and 340 nm, respectively.

For 1, the emission band (= 330 nm) disap- pears with respect to H3btc which may be due to the center MnIImetal ion that can usually quench organic ligand luminescence[24]. Compared with bpy, complex 1 displays the slight red shift which may be assigned to intraligand transitions (*→or*→) luminescence emission when coordinated with the metal atoms[25]. But it has lower lumine- scent intensities than the bpy ligand. This is a consequence of the presence of MnIIions to cause the bpy ligand to change its internal charge distribution. Probably due to the transfer of metal to bty ligand (MLCT)[26]and strong conjugation effect of bty ligands in crystal packing, the emission band of complex 1 originates from bpy ligand-based luminescence.

3.7 Magnetic property

The temperature dependence of magnetic suscep- tibility for complex 1 was measured in the 2～300 K temperature range under an applied field of 1000 Oe. Theχ,χT versusplots are illustrated in Fig. 9. As can be seen from the plots of experi- mental data, the experimentalχTvalue at 300 K is 4.25 cm3·K·mol-1which is close to the spin-only MnIIcation (4.38 cm3·K·mol-1)[27]. A gradual increase inχTvalues of 1 is observed as the temperature is decreased until about 50 K and then it increases to a maximum value of 4.35 cm3·K·mol-1at 55 K. This suggests the operation of ferromagnetic interactions between MnIIions in complex 1[28]. The magnetic susceptibilities of 1 in the range of 50～300 K obey the Curie-Weiss law[29-31]with Weiss constant= 2.21 K and Curie constant= 4.21 cm3·K·mol-1. This fact leads us to believe that some degree of ferromagnetic exchange interactions does exist in 1. In order to understand quantitatively the magni- tudes of spin-exchange interaction, the magnetic analysis was carried out with the model of chains of equally spaced magnetic MnIIcenters. Good fits to the experimental data from room temperature to 50 K are obtained. The magnetic parameters thus determined are= 2.33 K and= 1.99. Below 50 K, theχTvalue decreases rapidly to 0.49 cm3·K·mol-1at 2 K. The data below 50 K can not satisfactorily fit this calculation. The interaction between MnIIions within the chain may be responsible for these magnetic behaviors below 50 K[32]. This behavior ofχTcurve and the positive values ofandindicatethe weak ferromagnetic coupling between the MnIIions in complex 1.

Fig. 8. Solid-state luminescent spectra of H3btc, bpy and 1 at room temperature

Fig. 9. Temperature dependence of magnetic susceptibilityχandχTfor complex 1

4 CONCLUSION

In this contribution we have presented onenew CP [Mn(Hbtc)(bpy)(H2O)2]1 based on the H3btc and bpy ligands under pH-controlled hydrothermal conditions. It is a 1polymeric single-chain struc- ture. The structure has been determined by X-ray diffraction analysis and spectroscopic charac- terization. The luminescence property was studied in the solid state at room temperature, and the emission band of complex 1 originates from bpy ligand-based luminescence.Magnetic susceptibility measurement indicates that 1 shows a weak ferro- magnetic coupling between the MnIIions.

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4 August 2018;

10 October 2018 (CCDC 1839225)

① Supported by the Natural Science Foundation of Guangxi Province (No. 2017GXNSFAA198268), the Foundation of Guangxi Key Laboratory of Electrochemical and Magneto-chemical Functional Materials (EMFM20161102) and the National Natural Science Foundation of China (No. 61765005)

. Zhang Xiu-Qing, born in 1979, associate professor. E-mail: glutchem@163.com

10.14102/j.cnki.0254-5861.2011-2145