基于衣康酸配体构筑的两个锌配合物的合成、结构和荧光性质

尹卫东　李桂连　李晓玲　辛凌云　马录芳　刘广臻

(洛阳师范学院化学化工学院，洛阳　471022)



基于衣康酸配体构筑的两个锌配合物的合成、结构和荧光性质

尹卫东李桂连李晓玲辛凌云马录芳刘广臻＊

(洛阳师范学院化学化工学院，洛阳471022)

摘要：通过水热法合成了两种锌配位聚合物{[Zn(ic)(bip)]·2H2O}n(1)和[Zn(ic)(bpe)]n(2)(H2ic=衣康酸，bip=3，5-二(1-咪唑基)吡啶，bpe=1，2-二(4-吡啶基)乙烯)，并通过X射线单晶衍射和元素分析对其结构进行了表征。配合物1和2均为含有一维金属-羧酸链的二维(4，4)格子层结构。此外，对它们的热重、粉末X射线衍射和固体荧光性能进行了考察。与配体bip相比，1的发射光谱发生了明显的蓝移(～78 nm)，可能归因于配体到金属的电荷转移；2显示与游离的bpe配体相似的荧光性质，轻微的红移可能是因为与金属离子之间的配位作用导致的。

关键词：衣康酸；配合物；水热合成；荧光性质

国家自然科学基金(No.21571092)、河南省高校科技创新人才(No.14HASTIT017)、河南省高校科技创新团队(No.14IRTSTHN008)和河南省科技计划项目(No.152102310348)资助。

＊通信联系人。E-mail：gzliuly@126.com

In recent years, the design and synthesis of metalorganic frameworks (MOFs) has become an active area in the field of crystal engineering and supramolecular chemistry, not only for their structural characteristics, such as manifold coordination modes, intriguing architecture, and porosity, but also for their potential applications as functional materials, such as gas storage, small molecule separation, ion exchange, catalysis, luminescent sensing[1-8]. Multidentate O,N-donor organic molecules with excellent coordination sites are goodcandidates for the assembly process to construct one-, two-, three-dimensional coordination polymers[9-11]. Aromatic dicarboxylate and tricarboxylate ligands have been represented as the most common choices in the O-donor ligands to date, because their conformational rigidity provides the scaffolding both for inducing crystallization and promoting the structural stability necessary for reversible solid-state transformations[12-13]. In contrast, the flexible aliphatic acids are a new family of flexible multicarboxylates with long-spanning carbon chain groups and have rarely been studied[14-17], affording some interesting structures including helices and interpenetrating networks; this may originate from the flexibility of the carbon chain groups because the flexible groups can easily adjust their conformations to meet various coordination requirements of the metal ion by their “breathing”ability in the solid state.

Furthermore, it is proven that N-donor ancillary ligands can mediate the structures of coordination polymers through the cooperative coordination together with the carboxylate group to meet the requirement of coordination geometries of metal ions in the assembly process[18]. In this regard, some flexible N-donor ancillary ligands such as 1,2-bi(4-pyridyl)ethane (bpa), 1,2-bi(4-pyridyl)ethylene (bpe), and 1,3-bi(4-pyridyl)propane (bpp) are the most common auxiliary ligands to combine with polycarboxylates main ligands, and have obtained a series of MOFs with different structures and properties[19-22]. However, there are only a few reports about the 3,5-bis (1-imidazoly)pyridine as a rigid N-donor ligand with five potential N coordination sites to synthesize the coordination polymers[23-24]. On the basis of the aforementioned points, we obtained two zinc coordination polymers under similar hydrothermal reaction conditions with the flexible itaconic acid (H2ic) and bpe, bip as N-donor ancillary ligands, namely, {[Zn(ic)(bip)]·2H2O}n(1) and [Zn(ic)(bpe)]n(2). Both compounds are characterized by elemental analysis, single crystal X-ray crystallography, fluorescence properties, thermal stabilities and powder X-ray diffraction.

1 Experimental

1.1 Materials and methods

All reagents and solvents were commercially available and used as received. Elemental analyses for C, H and N were performed on a Flash 2000 organic elemental analyzer. Thermogravimetric analyses were carried out on a SII EXStar 6000 TG/DTA6300 analyzer heated from 25 to 870℃with a heating rate of 10℃· min-1in N2atmosphere. Powder X-ray diffraction (PXRD) patterns were taken on a Bruker D8-ADVANCE X-ray diffractometer with Cu Kα radiation (λ=0.154 18 nm). Solid fluorescence spectra were performed on an Aminco Bowman Series 2 luminescence spectrometer at room temperature.

1.2 Synthesis of {[Zn(ic)(bip)]·2H2O}n(1)

The starting mixture containing Zn(OAc)2·2H2O (0.065 g, 0.30 mmol), H2ic (0.026 g, 0.20 mmol), bip (0.021 g, 0.10 mmol), KOH (0.006 g, 0.10 mmol), EtOH (3 mL) and H2O (3 mL) was placed in a 23 mL Teflonlined autoclave at 120℃for 4 days, then cooled to room temperature. Colorless block crystals were obtained in ca. 75% yield. Elemental analysis Calcd. for C16H17N5O6Zn (%): C, 43.60; H, 3.88; N, 15.89. Found (%): C, 43.52; H, 3.95; N, 15.76.

1.3 Synthesis of [Zn(ic)(bpe)]n(2)

The mixture of Zn(OAc)2·2H2O (0.065 g, 0.30 mmol), H2ic (0.013 g, 0.10 mmol), bpe (0.036 g, 0.20 mmol), KOH (0.006 g, 0.10 mmol) and 6 mL H2O was placed in a 23 mL Teflon-lined autoclave. Colorless blocked crystals were obtained after 4 days at 120℃with 82% yield. Elemental analysis Calcd. for C17H12N2O4Zn (%): C, 54.64; H, 3.24; N, 7.50. Found (%): C, 54.58; H, 3.37; N, 7.46.

1.4 X-ray crystallography

The crystallographic data collections for complexes 1 and 2 were recorded on a Bruker SMART APEXⅡCCD diffractometer equipped with graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) by using φ-ω scan technique at room temperature. Absorption corrections were based on symmetry equivalent reflections using the SADABS program[25]. All structures were solved by direct methods with SHELXS-97 and refined on F2by full-matrix least-squares using the SHELXL-97 program package[26-27]. All hydrogen atoms were placed in calculated positions and refined isotropically with a riding model except for water H atoms, which were initially located in a difference Fourier map andincluded in the final refinement by use of geometrical restraints with d(O-H)=0.085 nm and Uiso=1.5Ueq(O). Some disordered guest water molecules in complex 2 are squeezed by PLATON/SQUEEZE program[28]. The details of the structure solutions and final refinements for two complexes are summarized in Table 1. Selected bond distances/angles and hydrogen bonds are listed in Table S1 and Table S2.

Table1　Crystal and structure refinement data for compounds 1 and 2

CCDC: 1428340, 1; 1428341, 2.

2 Results and discussion

2.1 Structure description of {[Zn(ic)(bip)]·2H2O}n(1)

The compound 1 crystallizes in monoclinic crystal system, space group P21/n and features a 2D layer composed of the metal-carboxylate chains and metal-bip helix chains with ic2-and bip as linkers, respectively. The asymmetric unit contains one crystallographic Zn2+cation, one ic2-dianion, one bip molecule, in addition to two guest water molecules. The Zn2 +cation adopts the distorted tetrahedral [ZnN2O2] coordination environment with two carboxylic oxygen atoms from two ic2-dianions and two nitrogen atoms from two bip coligands (Fig.1a). The Zn-O bond lengths are 0.193 82(19) and 0.19430(18) nm, and the Zn-N bond lengths are 0.199 4(2) and 0.202 9(2) nm, respectively.

The adjacent Zn2+ions are linked together to form a metal-carboxylate chain through ic2 -dianions adopting the monodentate coordination mode with the Zn…Zn distance of 0.703 72(4) nm (Fig.1b). In addition, the Zn2+ions are bridged by bip molecules to generate 1D helix chain along the b direction with the Zn…Zn separation of 1.154 94(6) nm, as shown in Fig.1c. A careful examination shows that the helix A is exclusively right-handed (R), whereas those in the neighboring chains B are lefthanded (L). These helix chains array in a parallel fashion and are further linked by the flexible ic2-anions to produce a 2D layer (Fig.1d). There exists very strong face-face stacking π…π interaction between the two completely parallel pyridine rings of two bip ligandsfrom two adjacent helix chains (Fig.1e). The centroids Cg1 (Coordinate: -0.694 68, 1.523 72, -0.462 17) and Cg2 (Coordinate: -0.305 32, 1.476 28, -0.537 83) are made up of atoms including C9, C10, N6, C11, C12 and C13 atoms. The centroid-centroid distance (Cg1-Cg2) and the dihedral angle between two pyridine rings is 0.358 39(1) nm and 0.0°, respectively.

Fig.1 (a) View of the asymmetric unit showing the local coordination environments of Zn2+ion in 1 with the ellipsoids drawn at the 50% probability level; (b) View of a section of the 1D chain by ic2-anions and Zn2+cations; (c) View of a section of helix chains by bip molecules and Zn2+ions; (d) Polyhedral view of a 2D wave layer extending in compound 1; (e) View of face-face stacking π…π interaction between the two pyridine rings of two bip ligands

As a result of the presence of two free water molecules in 1, there exist abundant intralayer H-bond interactions between the guest waters and carboxylate O atoms of the ic2-anions (O(5W)-H(2W)…O(1), O(6W)-H(3W)…O(2)i, O(6W)-H(3W)…O(4)ii, O(6W)-H(4W)…O(3)i), and between the guest water O atom and the N atom of the bip coligand (O (5W)-H (1W)…N(6)ii; Symmetry codes:ix+1, y, z;iix+3/2, -y+1/2, z+1/2;iiix-1, y, z-1. The adjacent layers stack together in a -ABAB-motif through Van der Waals interaction and interlayer face-face stacking π…π interactions between the pyridine rings and imidazole rings of two bip ligands from two different layers to accomplish its 3D supermolecular framework (Fig.S1). The centroids Cg3 (Coordinate: -0.805 32, 0.023 72, -0.037 83) and Cg5 (Coordinate: -0.194 68, -0.023 72, 0.037 83) are made up of atoms including C9, C10, N6, C11, C12 and C13 atoms, moreover, the centroids Cg4 (Coordinate: -0.453 92, 0.079 71, -0.208 66) and Cg6 (Coordinate: -0.546 08, -0.079 71, 0.208 66) are made up of atoms including N1, C6, C7, N2 and C8 atoms. The centroidcentroid distance (Cg3-Cg4 and Cg5-Cg6) and the dihedral angle between two pyridine rings is 0.393 20(1) nm and 6.462°respectively.

2.2 Structure description of [Zn(ic)(bpe)]n(2)

The compound 2 crystallizes in monoclinic crystal system, space group P2/c and features a 2D wave layer composed of the metal-carboxylate chains with ic2-and bpe as linkers. The asymmetric unit contains a half crystallographic Zn2+ion, a half ic2-dianion and a half bpe molecule. The Zn atom adopts the distorted tetrahedral [ZnN2O2] coordination environment, and is coordinated with two carboxylic oxygen atoms from two ic2 -dianions and two nitrogen atoms from two bpe coligands (Fig.2a). Two Zn-O bond lengths are 0.192 0(5) nm, and two Zn-N bond lengths are 0.205 2(5) nm, respectively.

The adjacent Zn2+ions are linked together to form a metal-carboxylate chain through ic2 -dianions adoptingthe monodentate coordination mode with the Zn…Zn distance of 0.865 26(12) nm (Fig.2b). The metal-carboxylate chains further extend through bpe coligands to form one 2D (4,4) grid layer with the side length of 1.346 14(16) or 0.865 26(12) nm (Fig.2b). The parallel layers stack together in a -AAA- sequence along b axis through Van der Waals interaction. Fortunately, such an arrangement makes each cavity open completely (Fig.2c), which provides the solvent accessible volume of about 26.7% in the unit cell after removal of solvent analyzed by PLATON/SQUEEZE program.

Fig.2 (a) View of the asymmetric unit showing the local coordination environments of Zn atom in 2 with the ellipsoids drawn at the 50% probability level; (b) Polyhedral view of a 2D (4,4) layer extending in compound 2; (c) View of 3D packing of three adjacent layers with obviously observed free pore volume

2.3 Thermogravimetric analyses and powder X-ray diffractions

The thermogravimetric analyses (TGA) of 1 and 2 performed by heating the polycrystalline samples display significantly different thermal degradations between room temperature and 870℃in N2atmosphere(Fig.3). The TGA curve for 1 suggests that the first weight loss of 8.14% from room temperature to 130℃corresponds to the loss of two lattice water molecules (Calcd 8.18%). The framework can keep stability up to about 250℃, and then begins a series of weight loss and does not stop until heating ends owing to the decomposition of organic ligands. The observed final mass remnant of 43.38% can not be specifically identified and may be the mixtures of metal oxide and carbonaceous material, because the theoretical remaining mass of 18.47% is calculated by assuming a final phase ZnO. The TGA curve of 2 displays the first weight loss of 5.53% appears from 55 to 115℃, indicating probably the existence of 1.25H2O per formula unit (Calcd. 5.68%) in light of some residual diffraction peaks squeezed by PLATON program. The observed final mass remnant of 20.74% up to 870℃likely representing deposition of ZnO is agreement with the calculated value of 20.78% . The powder X-ray diffractions of two compounds are in good agreement with the patterns simulated from the respective singlecrystal data, implying their good phase purity (Fig.S2 and Fig.S3).

Fig.3 TGA curves for complexes 1 and 2

2.4 Photoluminescent properties

The solid-state photoluminescent properties of 1 and 2 as well as the free powder ligands have been investigated in the solid state at room temperature, as illustrated in Fig.4. There is no emission observed for the free H2ic ligand in a rather wide excitation wavelength, which may be attributed to the absence of the conjugation effect. The solid free bpe and bip ligands exhibit fluorescent emission bands at 367 nm(λex=334 nm) and 420 nm (λex=290 nm), which can probably be assigned to the π-π* and/or n-π* transitions[29-31]. The emission maximum for complex 1 locates at 342 nm upon excitation at a wavelength of 290 nm and the emission maximum occurs at 385 nm upon excitation at 280 nm for 2. The emission spectrum for compound 1 displays obvious blue shift compared with the free bip ligand, which may be assigned to ligandto-metal charge-transfer (LMCT) according to the literature[32-35]. In addition, complex 2 displays strong emission spectrum feature similar to that of the powdered free bpe molecule, which should be assigned as the π-π* and/or n-π* transition of the bpe linker, but the slight red-shift may be attributed to the unique coordination of bpe to the Zn centre increasing the ligand conformational rigidity, thereby reducing the nonradiative decay of the intraligand (π-π*) excited state[9].

Fig.4 Solid-state emission spectra of compound 1 and 2 along with the powdered free bip and bpe ligands

3 Conclusions

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

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Syntheses, Structures and Fluorescent Properties of Two Zinc Coordination Polymers Based on Itaconic Acid

YIN Wei-Dong LI Gui-Lian LI Xiao-Ling XIN Ling-Yun MA Lu-Fang LIU Guang-Zhen＊
(College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, Henan 471022, China)

Abstract:Two zinc coordination polymers {[Zn(ic)(bip)]·2H2O}n(1) and [Zn(ic)(bpe)]n(2) (H2ic=itaconic acid, bip=3,5-bis(1-imidazoly)pyridine, bpe=1,2-bi(4-pyridyl)ethylene) were synthesized hydrothermally and characterized structurally by single-crystal X-ray diffractions, elemental analysis. Both complexes 1 and 2 display 2D (4,4) grid layers containing 1D metal-carboxylate chains. Thermogravimetric Analyses (TGA), powder X-ray diffractions (PXRD) and fluorescence properties for compounds 1 and 2 are also investigated. The solid state fluorescence spectra indicate that the emission spectrum of complex 1 shows obvious blue shift (～78 nm) compared to free bip coligand due to the ligand-to-metal charge-transfer (LMCT) effect. The compound 2 shows similar emission spectrum to the free bpe ligand, but the slight red-shift may be attributed to the complexation effect between the bpe molecule and Zn atoms. CCDC: 1428340, 1; 1428341, 2.

Keywords:itaconic acid; coordination polymers; hydrothermal synthesis; fluorescence property

收稿日期：2015-10-21。收修改稿日期：2016-01-18。

DOI：10.11862/CJIC.2016.063

中图分类号：O614.24+1

文献标识码：A

文章编号：1001-4861(2016)04-0662-07