三个二羧酸配体配位聚合物的合成、结构表征和荧光性质

崔培培 孙 悦 查 奕 刘圣楠张梦欣 曹际云 王 琦 王晓晴

(1德州学院生命科学学院，德州 253023)

(2中北大学化学与化工学院，太原 210023)

0 Introduction

Coordination polymers (CPs) are metal-ligand compounds based on inorganic metal ions/metal clusters and organic ligands through coordination bonds to form 1D, 2D, and 3D extended structures[1-3]. As a new type of organic-inorganic hybrid material, it has potential applications in photoluminescence properties,magnetism, catalysis, humidity, and pH response[4-6].Because the structures and properties of CPs mainly depend on the nature of building blocks, the choice of ligands is usually the first step in the design and synthesis of CPs. Among them, the ligand can be divided into rigid and flexible ones. Generally speaking, rigid ligands are more likely to promote the directional synthesis of the structure and function of CPs. Compared with rigid ligands, flexible ones with diverse configurations can exhibit different conformations during selfassembly, which makes it more difficult to predict the final structures[7-8]. Besides some structures that cannot be obtained through rigid ligands can be synthesized by flexible ones[9-10]. Due to the difficulty in predicting the structure and properties, the construction of complexes based on flexible ligands has attracted many researchers to study. Up to now, many CPs based on flexible ligands have been reported.Cao reported adiatopology CP with ferroelectric and second-order nonlinear optical properties based on the flexible tetracarboxylate organic linker tetrakis[4-(carboxyphenyl)oxamethyl]methane acid and Zn （Ⅱ）ions[11]. Based on flexible carboxylate ligands with acylamide groupsN,N′,N″-tris(isophthalyl)-1,3,5-benzenetricarboxamide, (5-(3,5-dicarboxybenzamido)isophthalic acid, 5,5′-(oxalyl bis(azanediyl))diisophthalic acid and 5-(5-((3,5-dicarboxyphenyl)carbamoyl)pyridin-2-yl)isophthalic acid, several frameworks with high-uptake of CO2were reported by Bai′s group[12-14].

Except for the nature of building blocks, intermolecular non-covalent interactions are an important part of influencing the CPs′ structures and properties.Much more elaborate studies have been required because inter-molecular non-covalent interactions determine the fashion of molecular structure and packing in the solid state[15-17]. Inter-molecular non-covalent interactions mainly include van der Waals forces,hydrogen bonding,π…πstacking interaction, and so on. Among them, hydrogen bonds are an important research content and design method in the crystal engineering of CPs and appear to be very useful interactions as they typically have sufficient strength and directionality[18-20]. For example, hydrogen-bonding interactions between the guest ions in the channels and the —OH groups of the organic ligand are thought to reduce the vibrational movements of the channels and affect the quenching effect in luminescence[21]. As another example, hydrogen bonds can be achieved by bridging two ligands that are coordinated to a metal center to effectively lead to supramolecular bidentate ligands, leading to catalysts with superior properties in a variety of metal-catalyzed transformations[22].

In the past few years, our group has designed and synthesized a series of CPs based on flexible carboxylate ligands and hydrogen-bonding interactions are also frequently discussed[23-26]. Based on our previous works,in this study,we choose flexible dicarboxylic acids 2,2′-(1,4-phenylenebis(methylene))bis(sulfanediyl)dibenzoic acid(H2L1)and 2,2′-(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)bis(sulfanediyl) dibenzoic acid(H2L2)as ligands(Scheme 1).Three CPs have been synthesized and the formulas are {[Ni(L1)(H2O)4]·2H2O}n(1),[Zn(L1)(DMA)2]n(2),and[Co(L2)(DMF)2]n(3)(DMA=N,N- dimethylacetamide, DMF=N,N- dimethylformamide). The single-crystal X-ray diffraction analysis shows that the structures of 1-3 are 1D chains, which can form 3D frameworks by hydrogen bonding interaction. In 1-3, the conformation of the ligands are all in anti-conformation. In addition, the thermal stability and fluorescence properties of the complexes were investigated.

1 Experimental

1.1 Materials and methods

The ligands H2L1and H2L2were synthesized according to the procedure reported[27-28]. Other chemicals and solvents are commercially available of reagent grade and were used as received without further purification. Elemental analysis for C, H, N, and S was performed on a Perkin-Elmer 240C Elemental Analyzer.FTIR spectra were recorded in a range of 400-4 000 cm-1on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets. At room temperature, powder X-ray diffraction (PXRD) measurements were performed on a Bruker D8 Advance X-ray diffractometer using CuKαradiation (λ=0.154 18 nm), in which the X-ray tube was operated at 40 kV and 40 mA (2θ=5°-50°). Thermogravimetric analysis (TGA) was performed on a simultaneous SDT 2960 thermal analyzer under N2with a heating rate of 10 ℃·min-1from 30 to 650 ℃.UV-Vis spectrum was obtained using the Shimadzu UV2700 spectrophotometer at room temperature.Solidstate fluorescence spectrum, the quantum yield, and fluorescence lifetime were obtained on Edinburgh FLS980 Series of fluorescence spectrometers.

1.2 Synthesis of complexes 1-3

1.2.1 Synthesis of complex 1

A mixture of H2L1(20.50 mg, 0.05 mmol), orotic acid (7.80 mg, 0.05 mmol), KOH (11.20 mg, 0.20 mmol), and 5.0 mL H2O was stirred for 0.5 h. Then Ni(NO3)2·6H2O (29.08 mg, 0.10 mmol) in 5.0 mL H2O was added. The resultant solution was sealed in a 20 mL bottle and heated at 90 ℃for three days.After cooling to room temperature, a large amount of precipitation and a small amount of green block-shaped crystals of 1 were obtained. The yield of the crystals was 15%.Anal. Calcd. for C22H28O10S2Ni(%)： C, 45.93; H, 4.91;N, 0; S, 11.15. Found(%)： C, 46.03; H, 4.76; N, 0.12;S, 11.45. IR (KBr, cm-1)： 1 660 (m), 1 599 (s), 1 530(m),1 360(w),1 320(m),1 063(m),1 018(w),859(w),828(w),776(m),655(m),537(w).

1.2.2 Synthesis of complex 2

A mixture of ZnSO4·H2O (6.00 mg, 0.33 mmol)and H2L1(9.00 mg, 0.022 mmol) was added in DMA/EtOH (1∶1,V/V, 10.0 mL). The resultant solution was sealed in a 20 mL bottle and heated at 90 ℃for three days. After cooling to room temperature, colourless block-shaped crystals of 2 were obtained in a 45% yield.Anal.Calcd.for C30H34O6N2S2Zn(%)：C,55.59;H,5.29;N,4.32;S,9.89.Found(%)：C,55.32;H,5.71;N,4.78;S,10.12.IR(KBr,cm-1)：1 619(s),1 584(m),1 466(w), 1 426 (m), 1 376 (m), 1 160 (m), 1 100 (m), 1 024(w),859(m),763(w),638(w),610(w),535(w).

1.2.3 Synthesis of complex 3

A mixture of Co(NO3)2·6H2O (29.10 mg, 0.10 mmol) and H2L2(23.30 mg, 0.05 mmol) was added in DMF/EtOH/H2O (5∶2∶1,V/V, 10.0 mL). The resultant solution was sealed in a 20 mL bottle and heated at 90 ℃for three days.After cooling to room temperature,red block-shaped crystals of 3 were obtained in 55% yield.Anal.Calcd.for C32H38O6N2S2Co(%)：C,57.39;H,5.72;N,4.18;S,9.58.Found(%)：C,57.01;H,5.95;N,4.31;S,10.06.IR(KBr,cm-1)：1 620(s),1 588(m),1 569(m),1 528(s),1 416(s),1 377(w),1 105(w),1 058(w),866(m),810(w),753(m),716(m),677(w),660(w).

1.3 Crystallographic data collection and refinement

Diffraction data for 1-3 were collected on a Bruker Apex ⅡCCD with graphite monochromated MoKαradiation source (λ=0.071 073 nm) inφ-ωscan mode.The SAINT program was used for the integration of the diffraction data and the intensity correction for the Lorentz and polarization effects. The structures of 1-3 were solved by direct methods and all non-hydrogen atoms were refined anisotropically onF2by the fullmatrix least-squares technique using the SHELXL crystallographic software package. The riding model was used to refine the hydrogen atom isotropic in the computational position. Multi-scan absorption corrections were applied by using the SADABS program. For 1-3,the details of the crystal parameters, data collection,and refinements are summarized in Table 1. Selected bond lengths and angles for 1-3 are listed in Table S1(Supporting information). The parameters of hydrogen bonds for 1-3 are listed in Table S2.

Table 1 Crystal data and structure refinements for complexes 1-3

CCDC：2270091,1;2270092,2;2270094,3.

2 Results and discussion

2.1 Crystal structures of complexes 1-3

2.1.1 Crystal structure of complex 1

The crystal structure of complex 1 was determined by single-crystal X-ray diffraction analysis. Crystallographic analysis indicates that 1 is a triclinic complex crystallizing in theP1 space group.There are two Ni（Ⅱ）ions in the asymmetric unit and the coordination environment around Ni （Ⅱ）is the same. The coordination number of Ni（Ⅱ）is six and the coordination geometry of Ni（Ⅱ）is a distorted octahedron structure. Among them,four oxygen atoms from water molecules form a quadrilateral plane. In the trans axial positions, there are two oxygen atoms from (L1)2-ligands (Fig.1a). Each Ni （Ⅱ）ion links two (L1)2-ligands, and each (L1)2-ligand links two Ni（Ⅱ）ions. Thus, the adjacent (L1)2-ligands by the coordination of carboxylate groups link together with Ni （Ⅱ）to form zigzag chains (Fig.1b). In 1, there are many hydrogen bonding interactions. When discussing hydrogen bonding interactions, free water molecules have been omitted for clarity. Among them, the adjacent chains based on Ni1 can form 2D bilayer by intermolecular hydrogen bonding interactions, such as O10—H10A…O11,O10—H10B…O1,O11—H11A…O12, and O11—H11B…O4 (Fig.S1a). At the same time, the adjacent chains based on Ni2 also can form 2D bilayer by intermolecular hydrogen bonding interactions, mainly including O13—H13A…O7, O14—H14A…O6, O16—H16A…O6, O14—H14A…O16,and O14—H14B…O7 (Fig.S1b). The bilayers based on Ni1 and the bilayers based on Ni2 can form 3D framework by intermolecular hydrogen bonding interactions C35—H35A…O2 and C37—H37…O2(Fig.1c).

Fig.1 (a)Coordination environment around the Ni（Ⅱ）center in complex 1 with 50% thermal ellipsoid probability,where hydrogen atoms and free water molecules are omitted for clarity;(b)Zigzag chains in 1,where the light green chain based on Ni1 and the red chain based on Ni2;(c)3D framework structure of 1 based on bilayers with short contact indicated by dashed lines(lime：C35—H35A…O2;black：C37—H37…O2)

2.1.2 Crystal structure of complex 2

The structural analysis shows that complex 2 crystallizes in the monoclinicC2/cspace group. The asymmetric unit of 2 consists of half Zn （Ⅱ）ion, half (L1)2-ligand, and one coordinated DMA molecule. The coordination number of Zn （Ⅱ）is four and the coordination geometry of Zn （Ⅱ）is a distorted tetrahedron occupied by O1, O1i, O3, and O3i(Fig.2a). Among them, O1 comes from the carboxylate group of (L1)2-ligand and O3 comes from coordinated DMA molecule. Two carboxylate groups of one (L1)2-ligand link two adjacent Zn （Ⅱ）ions to form a chain (Fig.2b). Intermolecular hydrogen bonding interaction also exists in 2. As shown in Fig.2c and 2d,the adjacent chains can form a 3D framework by the H-bonding C18—H18A…O1.

Fig.2 (a)Coordination environment around the Zn（Ⅱ）center in complex 2 with 50% thermal ellipsoid probability,where hydrogen atoms are omitted for clarity;(b)Chain structure in 2;(c)Hydrogen bonding interaction(C18—H18A…O1ii)in 2 indicated by pink dashed lines;(d)3D structure of 2 with intermolecular hydrogen bonding interaction indicated by dashed lines

2.1.3 Crystal structure of complex 3

Crystallographic analysis indicates that complex 3 crystallizes in the triclinicP1 space group.In the structure of 3,the asymmetric unit consists of half Co（Ⅱ）ion,half (L2)2-ligand, and one coordinated DMF molecule,which is similar to 2. Different from 2, the coordination number of Co（Ⅱ）is six in 3. The coordination geometry of Co（Ⅱ）is a distorted octahedron structure occupied by two carboxylate groups and two DMF molecules(Fig.3a). Four oxygen atoms of two carboxylate groups form a plane and two coordinated DMF molecules in the axial direction. Each Co （Ⅱ）ion links two (L2)2-ligands and each(L2)2-ligand link two Co（Ⅱ）ions.Thus,the adjacent (L2)2-ligands by the coordination of carboxylate groups link together with Co（Ⅱ）to form chains(Fig.S2). Like 1 and 2, intermolecular hydrogen bonding interactions also exist in 3. By intermolecular Hbonding interactions C4—H4…O1 and C3—H3…O2,the adjacent chains along thea- andc-axes can form 2D sheets (Fig.3b). The adjacent sheets further form 3D framework by C15—H15C…O2(Fig.3c and 3d).

2.2 Synthesis and comparison of the complexes

Complexes 1-3 were synthesized and characterized based on the flexible dicarboxylate ligands H2L1and H2L2. Although the synthesis conditions are different, each metal ion links two ligands and each ligand links two metal ions to form 1D chains in 1-3. Under the reaction conditions of 1, ZnSO4·H2O or Co(NO3)2·6H2O replaced Ni(NO3)2·6H2O, other things being equal, no crystals produced. In similar situations,Ni(NO3)2·6H2O or Co(NO3)2·6H2O replaced ZnSO4·H2O and Ni(NO3)2·6H2O or ZnSO4·H2O replaced Co(NO3)2·6H2O, there was still no crystal formation.When H2L1and H2L2replaced each other, no crystals were produced. In 1-3, the carboxylate groups of the corresponding ligands all lose hydrogen ions to coordinate with metal ions.The deprotonation of the carboxylic acids was confirmed by crystal structural analyses as described above as well as by IR spectra. In IR spectra, no obvious bands between 1 700 and 1 800 cm-1were found(Fig.S3).

2.3 TGA and PXRD for the complexes

The thermal stability of CPs is important for their application research, so TGA curves of complexes 1-3 were measured in an N2atmosphere from 30 to 650 ℃(Fig.4).The result analysis indicated that before 170 ℃the weight loss of 1 was 17.9%, which is in accordance with the loss of free water molecules and coordinated water molecules (Calcd. 18.8%). Further weight loss was observed at about 320 ℃owing to the decomposition of 1. For 2, there was little weight loss before 175 ℃and it showed a weight loss of 26.0% before 230 ℃,which corresponds to the release of DMA molecules (Calcd. 26.5%). Over 285 ℃, 2 began to decompose. For 3, it had little weight loss before 200 ℃.Then there was a weightless platform until 270 ℃and the weight loss was 21.4%, which is the same as the loss of coordinated DMF molecules (Calcd. 21.5%).Over 300 ℃,3 began to decompose.

Fig.4 TGA curves of complexes 1-3

The phase purities of the bulk samples of 1-3 were investigated by PXRD at room temperature (Fig.S4). The measured PXRD patterns of the samples matched well with the simulated patterns, which indicates that the samples of complexes 1-3 are pure phase.

2.4 Fluorescence property

Due to the possible application of CPs based ond10transition metal centers in luminescent sensors,photochemistry, and electroluminescent displays, the fluorescence property of CPs constructed from Zn （Ⅱ）and organic ligands has attracted much attention[29].Thus, the solid-state fluorescence properties of H2L1ligand and 2 were investigated at room temperature.The excitation and emission spectra of H2L1and 2 were all measured. In the excitation spectrum, the strongest excitation peak was around 242 nm (Fig.S5). When the excitation wavelength was 242 nm, there were two emission peaks for H2L1and 2. For H2L1, the smaller emission band was atca. 276 nm, and the maximum emission band was atca. 390 nm. For 2, the smaller emission band was atca. 281 nm, and the maximum emission band was atca. 395 nm (Fig.5). Thus, the emission spectrum of 2 was similar to the free H2L1ligand.So the emission of 2 can be assigned to an intraligandπ→π* electronic transition. Compared to the free ligand H2L1, the emission maxima of 2 was a small amount of red-shift. The reason may be attributed to the deprotonation of H2L1and the coordination of (L1)2-to Zn2+.

Fig.5 Solid-state emission spectra of H2L1 and 2 at room temperature

Furthermore, the solid-state UV-Vis spectra of H2L1and 2 were investigated at room temperature. The results show that the absorption wavelength range of H2L1and 2 was similar and they had absorption peaks between 200 and 420 nm, which indicates that H2L1and 2 have good light absorption ability in the UV-Vis light region (Fig.6). For H2L1, the maximum absorption band was atca.332 nm,which should be considered asπ→π* electronic transition of the ligand. Compared with H2L1, the intensity of the maximum absorption peak of 2 was weaker, which may be caused by the deprotonation of H2L1and the coordination between(L1)2-and Zn2+.For 2,at around 270 nm,the absorption peak weakened, which can be attributed to the coordination of (L1)2-to Zn2+and the formation of Zn—O bonds[30-31].

Fig.6 Solid-state UV-Vis absorption spectra of H2L1 and 2 at room temperature

Fluorescence lifetime was also measured at room temperature in the solid state (Fig.S6). The attenuation curves of H2L1and 2 can be well fitted into the twoexponential function：R(t)=B1exp(-t/τ1)+B2exp(-t/τ2)[32-36].Two time components were included, a fast decay (τ1)and a slower component (τ2). According to the mean lifetime equation：τm=B1τ1+B2τ2[32-33], the fluorescence lifetimes of H2L1and 2 were calculated to be 1.18 and 3.65 ns, respectively, which indicates that the fluorescence lifetimes are very short.

3 Conclusions

Based on flexible dicarboxylate ligands 2,2′-(1,4-phenylenebis(methylene))bis(sulfanediyl)dibenzoic acid(H2L1)and 2,2′-(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)bis(sulfanediyl) dibenzoic acid (H2L2),{[Ni(L1)(H2O)4] ·2H2O}n(1), [Zn(L1)(DMA)2]n(2) and[Co(L2)(DMF)2]n(3) were prepared under solvothermal conditions. The molecular structures of complexes 1-3 were established by single-crystal X-ray diffraction analyses. The ligands all display anti-conformation and 1-3 all have zigzag chain structures,which further form 3D frameworks by hydrogen bonding interaction.Solidstate photoluminescent measurements reveal that complex 2 manifests the characteristic emission bands of the H2L1ligand.

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