基于芳香四羧酸构筑的两种配位聚合物的荧光及磁性质

翟丽军 张 婕 高玲玲 高 婷 贾焦焦 牛宇岚 胡拖平＊,

(1 中北大学理学院化学系，太原 030051)

(2 太原工业学院化学与化工系，太原 030008)

Coordination polymers (CPs) have caused extensive attention from researchers as a new kind of functional hybrid materials because they have fascinating structures and extensive application in gas storage and separation[1-4], optics properties[5-10], magnetism[11-12], catalysis[13-14]and drug sustained release[15],etc. At present, there have also been a lot of reports on the research of CPs in luminescence probe.Compared with traditional instrument methods,luminescence sensing is considered as a promising method due to its advantages of quick response, high sensitivity, low cost, simple operation, and so on[16-18].

In the past few years, with the rapid development of economy and the explosive growth of the population, the problems such as environmental pollution and public health are arousing more and more concerns[19]. Fe3+is not only necessary for metabolism,but also widely used in industry[20]. However, excessive amounts of Fe3+in the human body are harmful, and Fe3+can contaminate the environment by being carelessly discarded[21]. While Cr6+is widely used in diverse industrial applications, improper disposal of Cr6+will pollute the living conditions of people[22]. So,it is very urgent to synthesize materials that can selectively and sensitively identify these ions. In recent years, researchers have agreed that CPs are one of the promising alternatives for luminescent probe. One of the biggest challenges is the design and construction of CPs with stable structure and ideal function. In the process of self-assembly, the construction of the desired CPs depends on organic ligands and metal ions[23]. In terms of ligands, studies have shown that aromatic polycarboxylic acids are extensively used to construct CPs due to the following advantages: firstly, their multiple binding nodes；secondly, their carboxylate groups can be partially or completely deprotonated to form multiple structures；furthermore, aromatic conjugated systems can coordinate with d10center ions forming CPs materials with excellent luminescence properties. At the same time,the introduction of the second nitrogen-containing ligand is helpful to construct CPs with novel structures.

So, based on the ligands of p-terphenyl-3,3″,5,5″-tetracarb oxylic acid (H4tptc), 1,4-bis(imidazol-1-ylmethyl) benzene (1,4-bimb) or 1,2-bis(imidazol-1-ylmethyl) benzene (1,2-bimb) (Scheme 1), two novel CPs, namely {[Zn2(tptc)(1,4-bimb)2]·H2O}n(1) and {[Ni(tptc)0.5(1,2-bimb)(H2O)]·H2O}n(2), have been synthesized under solvothermal method and characterized by luminescence properties (1) and magnetic properties (2).

Scheme 1 Structures of the ligands

1 Experimental

1.1 Materials and physical measurements

All chemicals were purchased commercially and used without further purification. IR (KBr pellet)spectra were recorded under a FTIR-8400S spectrometer in a range of 4 000 ～400 cm-1. Thermogravimetric analyses (TGA) were collected on a METTLER TGA analyzer at a heating rate of 10 ℃·min-1under N2atmosphere from room temperature to 750 ℃.Elemental analyses (C, H, and N) were performed by using a PerkinElmer 2400C elemental analyzer (EA).Powder X-ray diffraction (PXRD) was performed using a Rigaku D/Max-2500 PC diffractometer (Mo Kα radiation, λ=0.154 06 nm) at 50 kV, 30 mA with the 2θ range of 5°～50°. Luminescence spectra were performed on Hitachi F4600 spectrophotometer.Magnetic properties were measured by Quantum Design MPMS-XL-7 SQUID magnetometer.

1.2 Synthesis of the complexes

1.2.1 Synthesis of {[Zn2(tptc)(1,4-bimb)2]·H2O}n(1)

Zn(NO3)2·6H2O (0.015 mmol, 4.5 mg), H4tptc(0.005 mmol,2.1 mg),1,4-bimb (0.005 mmol,1.2 mg),0.15 mL NaOH aqueous solution (0.5 mol·L-1) and 1 mL H2O/DMF (1∶1, V/V) were mixed in a stainless steel vessel (25 mL), kept at 130 ℃for 72 h and then naturally cooled to ambient temperature to obtain colorless crystals.Yield:41%(based on Zn).Elemental analysis Calcd. for C50H40N8O9Zn2(%): C, 58.48； H,3.91；N,10.85.Found(%):C,58.54；H,3.95；N,10.89.IR(KBr,cm-1):3 455(m),1 630(s),1 524(vs),1 393(s),742 (s), 733 (s), 683 (m), 678 (m) (Supporting information, Fig.S1).

1.2.2 Synthesis of {[Ni(tptc)0.5(1,2-bimb)(H2O)]·H2O}n(2)

Ni(NO3)2·6H2O (0.03 mmol, 8.8 mg), H4tptc (0.01 mmol, 4.2 mg), 1,2-bimb (0.02 mmol, 4.8 mg) and 8 mL CH3CN/H2O (1∶1, V/V) were placed in a 25 mL autoclave and heated to 130 ℃for 72 h. After slowly being cooled to ambient temperature, green crystals were gained. Yield: 46% (based on Ni). Elemental Analysis Calcd. for C25H23N4NiO6(%): C, 56.16； H,4.30；N,10.48.Found(%):C,56.24；H,4.25； N,10.68.IR(KBr,cm-1):3 402(vs),1 613(vs),1 546 (vs),1 526(s),1 402(vs),1 371(s),986(m),977(m). 870 (s), 782(s), 773 (s), 685 (m), 653 (m) (Fig.S1).

1.3 X-ray crystallographic study

All crystallographic data were collected on a Bruker APEX ⅡCCD diffractionmeter using Mo Kα radiation (λ=0.071 073 nm) at 25 ℃. The structures were determined by direct methods and refined by the full-matrix least-squares method based on F2using SHELXL program and OLEX 2[24-25]. All nonhydrogen atoms were refined with anisotropic displacement parameters and hydrogen atoms were placed geometrically and refined using a riding model. Crystal structural parameters, some selected bond lengths and angles are listed in Table 1 and Table S1,respectively. The topology of CPs was analyzed by using TOPOs 4.0 program package[26].

CCDC: 1911100, 1； 1911101, 2.

Table 1 Crystal structure data and refinement parameters of 1 and 2

2 Results and discussion

2.1 IR spectra

The absorption spectrum at 3 440 ～3 460 cm-1corresponds to the characteristic peak of the stretching vibration of the O-H group in water molecules. The peaks at 1 393 cm-1(1) or 1 376 cm-1(2) and 1 630 cm-1(1) or 1 546 cm-1(2) are attributed to the symmetric and asymmetric stretching vibration of the carboxylate group, respectively. For 1～2, the lack of strong peak in a range of 1 690～1 730 cm-1demonstrates that the H4tptc ligand is completely deprotonated(Fig.S1).

2.2 Descriptions of crystal structures

2.2.1 Crystal structure of {[Zn2(tptc)(1,4-bimb)2]·H2O}n(1)

Complex 1 crystallizes in the monoclinic system with the P21space group. Its asymmetric unit contains two Zn2+ions, one tptc4-linker, two 1,4-bimb linkers and one lattice water molecule.Both Zn2+ions are fourcoordinated and exhibit distorted tetrahedron geometries (Fig.1a). Each Zn2+ion is bound to two oxygen atoms of two distinct tptc4-linkers and two nitrogen atoms provided by two 1,4-bimb ligands. The bond angles around Zn2+range from 97.7° to 129.1°,and the bond lengths of Zn-O and Zn-N vary from 0.193 4 to 0.197 1 nm and 0.200 4 to 0.202 1 nm,respectively.

The H4tptc ligands in 1 are completely deprotonated. All carboxylate groups adopt monodentate bridging coordination modes to link Zn2+ions forming a 2D network (Fig.1b), which are further expanded by 1,4-bimb linkers to construct 3D structures (Fig.1c).Topologically, complex 1 reveals a 2-nodal (4,4)-c network with the topology of (86) by denoting Zn2+ions and H4tptc ligands to be 4-c nodes, respectively.

2.2.2 Crystal structure of {[Ni(tptc)0.5(1,2-bimb)(H2O)]·H2O}n(2)

Complex 2 crystallizes in the triclinic system P1 space group. There are one Ni2+ion, half of tptc4-linkers, one 1,2-bimb, and one coordinated water molecule in the asymmetric unit of 2. As exhibited in Fig.2a, each Ni2+ion is coordinated by three carboxylate O atoms (Ni1-O1 0.214 26 nm, Ni1-O2 0.214 44 nm, Ni1-O3i0.202 80 nm), a lattice water O atom(Ni1-O5W 0.207 47 nm), and two N atoms of two 1,4-bimb (Ni1-N1 0.205 47 nm and Ni-N4ii0.206 35 nm),presenting a pseudo-octahedral geometry. The bond angles around Ni1 range from 61.46° to 179.23°.

Fig.1 (a) Coordination environment of Zn2+ions in 1； (b) 2D network of 1 based on Zn2+ion and tptc4-observed along a-axis； (c) 3D structure of 1 viewed along a-axis； (d) Overall topological network for complex 1

As shown in Fig.2b, four carboxylate groups of the H4tcpb ligand adopt two different coordination patterns (bridging mode and chelating mode). The H4tcpb ligands link the Ni2+ions to get an interesting 1D rectangle chain (Fig.3a), while 1,2-bimb ligands link with the Ni2+ions to form a 1D [Ni(1,2-bimb)]nlinear chain (Fig.3b). These two chains intertwine each other to form a 2D sheet by sharing metal centers (Fig.3c). Finally, 3D supramolecular structure is formed through O5W-H5WA …O2 and O5WH5WB…O4 hydrogen bonds interactions between the adjacent sheets (Fig.4). Topological analysis indicates that the framework of 2 can be simplified to a new(4,4)-c network with the point symbol of {4.64.8}2{42.64}, where H4tptc ligands and Ni2+ions are taken as 4-connected nodes, respectively.

Fig.2 (a) Coordination environment of Ni2+ions in 2； (b) Coordination mode of the H4tcpb ligand in complex 2

Fig.3 (a) One dimensional rectangle chain based on H4tcpb and Ni2+ions； (b) 1D [Ni(1,2-bimb)]n linear chain constructed by 1,2-bimb ligands and Ni2+ions； (c) View of the 2D network of 2

Fig.4 Hydrogen bonds between adjacent 2D layers of 2

Fig.5 Schematic diagram of the 4,4-connected net of 2 with the point symbol of{4.64.8}2{42.64}

2.3 Powder X-ray diffraction analyses and TGA analyses

To evaluate the phase purity of CPs, the PXRD patterns of the as-synthesized samples were analyzed at ambient temperature (Fig.S2). The key peaks of the experimental PXRD patterns were almost consistent with the simulated ones, indicating that the phase purity of CPs is good. The difference in strength may be caused by the preferred orientation of crystal powder samples.

As shown in Fig.S3, complex 1 exhibits a weightlessness of 1.76% (Calcd. 1.75%) below 159 ℃,which is attributed to the loss of a lattice water molecule. After that, its framework is stable below 400 ℃. As for 2, the first weight loss of 6.88% (Calcd.6.74%) corresponds to the release of a lattice water and a coordinated water molecule under 196 ℃. The framework of 2 started to break down after 396 ℃.

2.4 Luminescence properties

The luminescence properties of ligands (H4tptc and 1,4-bimb) and complex 1 were measured under room temperature (Fig.6). The emission spectra of H4tptc and 1,4-bimb ligands were observed at 408 and 457 nm (λex=280 nm), respectively, which can be attributed to the π-π*and π*-n transition[27-31].Furthermore, compared with the emission bands of H4tptc and 1,4-bimb, complex 1 was blue-shifted and exhibited an obvious emission maximum at 359 nm (λex=280 nm). This is due to structural changes in ligands because of coordination with metal ions, which greatly enhances the rigidity of CP and decreases the energy loss through radiation less decay[32].

Fig.6 Solid-state luminescence emissions of H4tptc,1,4-bimb and complex 1

In addition, from the perspective of practical application, the luminescence sensing properties of 1 in common solvents are also investigated. The ground samples were dispersed in different solvents (2 mL),including H2O, DMA, DMF, methanol, acetonitrile,ethanol, acetone, DMSO, etc., by ultrasonic treatment for 30 min to obtain uniform 1@solvent suspensions.As exhibited in Fig.7, the luminescence intensity of 1 depends on the types of the solvent. It is worth noting that 1 exhibited the strongest emission peak in H2O and the weakest emission in acetone, which may be the result of interaction between the network of CPs and solvent molecules with disparate polarities[33].

Fig.7 Luminescence intensities of complex 1 scattered in different organic solvents

Furthermore, because of water stability of complex 1, luminescence sensing properties of 1 towards metal ions in aqueous solution were carried out. The finely ground sample of 1 was dispersed in M(NO3)xaqueous solutions (0.01 mol·L-1, M=Na+,Cd2+, K+, Cu2+, Pb2+, Ag+, Fe3+, Mn2+, Cr3+, Zn2+, Co2+,Ni2+, Ba2+, Al3+, Hg2+, Ni2+) to form 1@M suspensions treated by the ultrasonic for 30 min. As presented in Fig.8, the luminescence intensity of 1 presents iondependent changes. Al3+and Na+ions enhanced its luminescence intensity, while the other cations reduced its luminescence intensities. Particularly, the luminescence intensity of 1 was almost completely quenched by Fe3+ion, implying that 1 can be one of the luminescent probes sensing Fe3+ion.

Fig.8 Luminescence intensity of complex 1 in different cationic water solutions

To test the sensitivity of 1 for sensing Fe3+ion,the titration experiments were carried out. 2 mg crystal samples were dispersed in 3 mL aqueous solution to form 1@H2O suspensions, and then 30 μL Fe(NO3)3solution (0.01 mol·L-1) was gradually added into the above suspensions at a time. As can be seen in Fig.9, the luminescence intensity of 1@H2O suspension decreases gradually with the addition of Fe3+ion. When the concentration of Fe3+ion was 0.7 mmol·L-1, the luminescence intensity of 1 was almost completely quenched. The relationship between I0/I and the concentration of Fe3+ion can be expressed by the equation of I0/I=0.74exp(cM/0.26)+0.44, where cMrepresents the concentration of Fe3+, I0and I stand for luminescence intensities of 1@H2O and 1@Fe3+suspension, respectively. When the concentration of Fe3+ion is low, the Stern-Volmer (S-V) curve can be expressed as a linear equation of I0/I=1+KsvcM(Fig.9)[34]. The Ksvvalue of Fe3+was 5.29×103L·mol-1.The detection limit was calculated by 3σ/Ksvto be as low as 4.63×10-4mol·L-1(σ is the standard deviations by measuring the blank solution for 5 times at room temperature).

Fig.9 Influence of the addition of Fe3+ions on the emission spectra of 1 dispersed in water solution

The luminescence experimental procedure of anions is similar to that of cations except substituting M(NO3)xwith KnX (0.01 mol·L-1, X=Cr2O72-, HPO42-,SCN-,CO32-,H2PO4-,I-,HCO3-,PO43-,Br-,SCN-,SO42-,H2PO4-and Cl-). As exhibited in Fig.10, compared to other anions, Cr2O72-ion has the remarkable quenching effect to the luminescence of 1. Similarly, the titration experiment of Cr2O72-anion is shown in Fig.11, and the S-V curve is linear at low concentration and gradually deviated from linearity with the increasing of Cr2O72-concentration. The quenching constant (KSV)was calculated to be 6.15×103L·mol-1, which was higher than that of the reported MOFs for sensing Cr2O72-ions (Table S2)[35-36].

Fig.10 Luminescence intensity of complex 1 in aqueous solution containing different anions

Fig.11 Influence of the addition of Cr2O72-ions on the emission spectra of 1 dispersed in water solution

2.5 Quenching mechanism

In order to explore the mechanism of luminescence quenching, PXRD patterns of samples were measured before and after luminescence experiment,and the results showed that PXRD patterns of samples after luminescence experiment were almost identical with those of the original ones, which indicates that the structural collapse of complex is not the cause of luminescence quenching (Fig.S4). Furthermore, the UV-Vis absorption spectra (Fig.S5) showed that there were the partial overlap between the excitation band of 1 and the absorption band of Fe3+/Cr2O72-ions,which shows the competitive absorption of energy between the frameworks and that Fe3+/Cr2O72-ions is responsible for luminescence quenching[37].

2.6 Magnetic properties

The direct-current (dc) magneti c susceptibility of 2 was measured in a temperature range of 2～300 K under a 1 000 Oe applied field. As exhibited in Fig.12, the χMT value is 1.08 cm3·K·mol-1at room temperature, which is close to the theoretical value of 1.0 cm3·K·mol-1for one isolated Ni2+ion (S=1,g=2.0).The χMT value decreased slowly to 0.49 cm3·K·mol-1at 30 K, and then decreased sharply, which may be attributed to the antiferromagnetic interaction between Ni2+ions. Furthermore, the curve of the reciprocal susceptibilities( χM-1)vs T was well fitted by the Curie-W moeli-s1s alnadw :θχ=M-T2=.C68/( TK- θi)n,wahtiecmh pgearvaeturCe=r1a.n0g7e comf3·5K0·～300 K, confirming the antiferromagnetic interaction between Ni2+ions.

Fig.12 Temperature properties of χMT vs T and χM-1 vs T for 2

3 Conclusions

In short, two new CPs have been successfully synthesized by utilizing the mixed ligands strategy.Complex 1 shows a 3D network with the point symbol of (86), and complex 2 is a 3D supramolecular architecture formed by the H-bond interaction between the adjacent 2D layers. Furthermore, the luminescence properties show that 1 has good sensing selectivity for Fe3+/Cr2O72-ions in aqueous solution, which indicates 1 has potential application in the detection of Fe3+and Cr2O72-ions.Moreover,magnetic measurements indicate that there is the antiferromagnetic interaction between Ni2+ions in 2.

Acknowle dgements:The authors sincerely thank the National Natural Science Foundation of China (Grant No.21676258), the international Scientific and Technological Cooperation Projects of Shanxi Province (Grant No.201803D421080).Meanwhile, the authors honestly acknowledge the support of innovative investigated group of inorganic-organic mixed functional materials in North University of China.

Supporting information is available at http://www.wjhxxb.cn