TIAN Yu-Bin WANG Meng, ZHANG Wen, SONG Nn-Nn, SONG Xin-Jin, HU Wei-Bing FENG Fu,

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Syntheses, Structures, and Properties of Two Cobalt(II) Coordination Complexes Based on (Fluorene-9,9-diyl) dipropanoic Acid and 1,3-Bis(imidazol-1-yl)butane Ligands①

TIAN Yu-BinaWANG Menga, bZHANG Wena, bSONG Nan-Nana, bSONG Xin-Jiana, bHU Wei-Bingb②FENG Fua, b②

a(445000)b(445000)

Two new cobalt(II) coordination complexes, namely [Co(HL)2(biim)(H2O)2]n·nH2O (1) and [Co(EtL)2(biim)]n·3n(H2O)·2n(EtHL) (2) (H2L = (fluorene-9,9-diyl)dipropanoic acid, biim = 1,3-bis(imidazol-1-yl)butane), have been synthesized using the same starting reactants but different solvent medium. The two complexes exhibit distinctly different structures. Compound 1 exhibits a one-dimensional linear chain structure. However, complex 2 reveals a one-dimensional zigzag chain structure. Thermogravimetric analyses (TGA) and luminescent properties of these two complexes have been discussed.

coordination complexes, fluorene, luminescent properties;

1 INTRODUCTION

In the recent decade, the construction of novel metal-organic frameworks (MOFs) continues to attract great interest due to not only their potential applications in magnetochemistry, chemical sensors, gas adsorbents and catalysis[1-5], but also their intriguing variety of topologies and architectures[6-8]. To date, a lot of multitopic polycarboxylate rigid ligands have been successfully used to construct various extended structures with metal ions. Compared with the rigid ligands, the aliphatic and semi-rigid carboxylic ligands, serving as a type of flexible bridge-linker favorable for constructing novel structures, have been widely used in building MOFs[9].

Recently, we have designed a multi-carboxylate ligand (fluorene-9,9-diyl)dipropanoic acid (H2L, Scheme 1a) to construct novel MOFs. The ligand H2L has been chosen mainly based on the two special characteristics: 1) fluorescence (fluorene and its derivatives have good fluorescent characteristics. The products may exhibit some interesting lumine- scent properties). 2) flexibility and multifunctional carboxyl coordination sites.

In order to synthesize novel MOFs, we selected two different types of ligands H2L and N-donor ligand biim (Scheme 1b) as the organiclinkers. The reaction of the organic linkers (H2L and biim) with cobalt ion was performed in a same reactant but different solvent medium, and two Co(II) complexes, namely, [Co(HL)2(biim)(H2O)2]n·nH2O (1) and [Co(EtL)2(biim)]n·3n(H2O)·2n(EtHL) (2), were obtained. These two complexes were characterized by X-ray diffraction, and the results indicate that they form different MOF morphologies under dif- ferent solvent medium in which a 1D linear chain (1) and a 1D zigzag chain (2) were able to be respec- tively obtained.

Scheme 1. Schematic illustration of ligand (fluorene-9,9-diyl)dipropanoic acid (H2L, a) and ligand 1,3-bis(imidazol-1-yl)butane (biim, b)

2 EXPERIMENTAL

2. 1 Reagents and measurements

IR spectra were recorded as KBr pellets on a Perkin Elmer spectrometer. C, H and N elemental analyses were performed on an Elementar Vario MICRO E III analyzer. TGA was performed on a NETZSCH STA 449C thermo-gravimetric analyzer in flowing N2with a heating rate of 10 °C·min-1. Powder X-ray diffraction patterns (PXRD) were acquired on a Rikagu Smartlab X-ray diffractometer operating at 40 kV and 30 mA with Curadiation (= 1.5406 nm). X-ray data were collected using a BRUKER SMART APEX-CCD diffractometer with Cu-radiation (= 0.71073 ?). All chemicals purchased were of reagent grade and used without further purification.The H2L was prepared according to the literature method[10].

2. 2 Syntheses of the complexes

2. 2. 1 Synthesis of [Co(HL)2(biim)(H2O)2]n·nH2O (1)

A mixture of H2L (0.062 g, 0.2 mmol), biim (0.038 g, 0.2 mmol), CoCl2(0.065 g, 0.5 mmol), NaOH (0.016 g, 0.4 mmol)and10 mL of water-ethanol (1:1 (v/v)) solution was placed in a Teflon reactor (15 mL) and heated at 150oC for 3 days. After the mixture had been cooled to room temperature at a rate of 10oC·h-1, purple crystals of complex 1 were obtained in 35% yield (based on biim).Anal. Calcd. for C48H54CoN4O11: C, 62.47; H, 5.86; N, 6.07%. Found: C, 62.61; H, 5.72; N, 6.16%. IR (KBr, cm-1): 3411(m, -OH), 3062(m, Ar–H), 1721(s, -C=O), 1609(m, Ar–H), 1459(s, Ar–H), 1382(m, -CH2), 1260(m, O–C–O), 1062(m, C–O), 719(m, Ar-H).

2. 2. 2 Synthesis of [Co(EtL)2(biim)]n·3n(H2O)·2n(EtHL) (2)

The synthesis of complex 2 followed the almost uniform procedure as for complex 1 except that ethanol (10 mL) was used instead of a mixed solution water-ethanol (1:1 (v/v)). Purple crystals of complex 2 were obtained in 27% yield (based on biim). Anal. Calcd. for C94H106CoN4O19: C, 68.16; H, 6.40; N, 3.38%. Found: C, 68.37; H, 6.31; N, 3.47%. IR (KBr, cm-1): 3421(s, -OH), 3035(m, Ar–H), 2985(m, -CH2) 1711(s, -C=O), 1611(s, Ar–H ), 1521(m, Ar–H), 1271(s, C–O–C), 1071(w, C–O), 781(m, Ar–H).

2. 3 Crystal structure determination

Single-crystal X-ray data for complexes 1 and 2 were collected on a Bruker Apex (II) Duo diffrac- tometer using graphite-monochromated Mo(= 0.71073 ?) radiation at room temperature. Empirical absorption correction was applied. The structures were solved by direct methods and refined by full-matrix least-squares methods on2using the SHELX-97 software[11]. All non-hydrogen atoms were refined anisotropically. All of the hydrogen atoms were placed in the calculated positions. For complexes 1 and 2, the crystal data and structure refinements are summarized in Table 1, and the selected bond lengths, bond angles and hydrogen bonds parameters are shown in Tables 2, 3 and 4, respectively.

Table 1. Crystal and Structure Refinement Data for Complexes 1 and 2

Table 3. Selected Bond Angles (°) in Complexes 1 and 2

Table 4. Hydrogen Bond Parameters in Complexes 1 and 2 (?, °)

Symmetry code: b = –, 1–, 1–

3 RESULTS AND DISCUSSION

3. 1 Crystal structures

3. 1. 1 Crystal structure of [Co(HL)2(biim)(H2O)2]n·nH2O (1)

Complex 1, obtained in a mixed solution of water and ethanol, crystallizes in the monoclinic space group21/and exhibits aone-dimensional linear chain polymer. In 1, the Co(II) ion is six-coordinated in a distorted octahedral coordination environment and lies on a common twofold axis in this system with four pendent arms from two individual HL-anions and two biim ligands penetrating into different direction. The Co–N bond length is 2.118(2) ? and the Co–O bond lengths are in the range of 2.0892(19)～2.133(2) ?[12]. It's asymmetric unit contains one Co(II) ion, two mono-deprotonated HL-anions, two coordinated biim ligand molecules, two coordinated water molecules and one free water molecule, as illustrated in Fig. 1. In the crystal packing (Fig. 2), the Co(II) unit and free water molecules are linked into a 3D net structure by hydrogen bonds (Table 4).

Fig. 1. Molecular structure of complex 1

Fig. 2. 3-D net structure of complex 1

3. 1. 2 Crystal structure of [Co(EtL)2(biim)]n·3n(H2O)·2n(EtHL) (2)

Complex 2, obtained in ethanol solution, crystal- lizes in the orthorhombic space group21, and reveals a one-dimensional zigzag chain (Fig. 3). In crystallization process, H2L formed a new ligand EtHL due to the esterification of a -COOH of ligand H2L with ethanol. In 2, the Co(II) ion is six-coor- dinated by two nitrogen atoms from two biim ligands and four oxygen atoms from two EtL-anions in a distorted octahedral coordination environment. The Co–N bond lengths range from 1.992(7) to 2.040(7) ? and the Co–O bond distances fall in the 2.045(7)～2.046(7) ? range. The asymmetric unit of 2 contains one Co(II) ion, two deprotonated EtL-anions and two coordinated biim ligand molecules. In addition, this structure also contains intermole- cular hydrogen-bonding involving three free water molecules and two free EtHLmolecules, see Table 4 for detailed hydrogen-bonding geometry.

Fig. 3. Molecular structure (a) and 1-D zigzag structure (b) of complex 2

The experimental and simulated XRPD patterns of complexes 1 and 2 are shown in Fig. 4. Their peak positions are in good agreement with each other, indicating the phase purity of the product. The difference intensity may be due to the preferred orientation of the power sample[13].

Fig. 4. PXRD patterns of complexes 1 and 2

3. 2 Thermal stabilities

Thermal analyses for complexes 1 and 2 were per- formed from room temperature to 1000oC under a N2atmosphere, as shown in Fig. 5. For complex1, the first weight loss of 2.08% between 50 and 85oC is attributed to the loss of one guest water per formula unit (calcd. 1.99%). The second weight loss of 4.11% was observed in 90～125 °C, correspon- ding to the release of coordinated water (calcd. 3.98%). The third weight loss of 86.11% in the temperature range of 165～550 °C corresponds to the elimination of coordinated biim and HL?ligands (calcd. 87.64%). For 2, it underwent a two-step degradation process with the first weight loss of 3.31% between 50 and 90 °C, which is attributed to the loss of free water (calcd. 3.26%). The coordinated biim and EtL?ligands andfree EtHLmolecules were gradually lost at temperature from 170 till 600oC. The observed weight loss of 93.02% is in agreement with the expected value of 93.17%.

Fig. 5. TGA curves for complexes 1 and 2

3. 3 Photoluminescent properties

It is known that fluorene-containing complexes have held the attention of a lot of research groups worldwide because of their good optical properties and high luminescent efficiencies[14-16]. Thus the photoluminescent properties of complexes 1 and 2 were detected in solid states, as shown in Fig. 6. It is shown that complex 1 has an emission at 437 nm (ex= 318 nm), while 2 displays a stronger emission at 442 nm (ex= 323 nm). The free H2L ligand has stronger emission at 429 nm (ex= 311 nm). In contrast to the free ligand H2L, the photolumine- scence at longer wavelengths in complexes 1 and 2 should be ascribed to the coordination of metal cobalt(II) andthe* electron transfer in the H2L ligand, resulting in red-shift of values by 8 and 13 nm, respectively.The biim ligands seem to be no obvious contribution to their luminescence pro- perties.

Fig. 6. Solid-state excitation and emission spectra for complexes 1, 2 and the free H2L ligand powder

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24 November 2017;

19 March 2018

the Key projects of Hubei Provincial Education Department (D20171902)

Hu Wei-Bing (1959-). Tel: 15671883828, E-mail: chemistryhu@126.comFeng Fu (1978-). Tel: 15027224903, E-mail: fengfu2010@163.com

10.14102/j.cnki.0254-5861.2011-1900