Syntheses, crystal structures, and catalytic properties of three zinc(Ⅱ), cobalt(Ⅱ) and nickel(Ⅱ) coordination polymers constructed from 5-(4-carboxyphenoxy)nicotinic acid
Weizhong LING , Xiangyun CHEN , Wenjing LIU , Yingkai HUANG , Yu LI
The research field of coordination polymers (CPs) has witnessed a remarkable expansion since its discovery in the mid-twentieth century. This is due to these compounds′ unique and versatile properties, which can be tailored by manipulating the organic ligand and metallic ion components in their structures[1-4]. The structural diversity results in a lot of applications, such as sensing[5-6], gas sorption[7-8], and catalysis[9-11], making CPs a subject of continued interest in the field of materials chemistry.
In the catalysis field, the significance of CPs lies in their high selectivity, activity, and facile recovery, particularly for heterogeneous catalysis due to their limited solubility in a range of organic solvents. Knoevenagel condensation reaction, the reaction between carbonyl compounds, such as aldehydes/ketones, with active methylene compounds, such as malononitrile, represents an effective route for the formation of C—C bonds and has many applications in the production of important molecules, such as drugs[12-13] and fragrances[14-15].
In pursuit of our general research line on probing various commercially available carboxylic acids as linkers for designing functional CPs[10-11, 16-17], in the current work we have chosen a trifunctional nicotinic acid derivative as a main building block, namely 5-(4-carboxyphenoxy)nicotinic acid (H2cpna). This compound was tested as a principal building block for the hydrothermal synthesis of CPs. The following reasons have governed the selection of H2cpna. (1) This trifunctional ligand contains one pyridine and one phenyl ring interconnected by a rotatable O-ether group that can provide a subtle conformational adaptation. (2) H2cpna contains three different types of functionalities (i.e., —COOH, N-pyridyl, and O-ether). It has six potential coordination sites, which can result in diverse coordination patterns and high dimensionalities, especially when acting as a multiply bridging spacer. (3) Although some coordination compounds bearing the ligand were reported[18-21] and their luminescence and magnetism were studied, their potential catalytic activities were ignored[18-21]. So the present work provides us with a good chance to research this field.
Therefore, based on the above reasons, we designed and synthesized three (Zn(Ⅱ), Co(Ⅱ), and Ni(Ⅱ)) CPs based on ligand H2cpna and dpea (1, 2-di(4-pyridyl)ethane)/dpey (1, 2-di(4-pyridyl)ethylene). In this article, we report the syntheses, crystal structures, and catalytic properties of these CPs.
All chemicals and solvents were of AR grade and used without further purification. Carbon, hydrogen, and nitrogen were determined using an Elementar Vario EL elemental analyzer. IR spectrum was recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer. Thermogravimetric analysis (TGA) data were collected on a LINSEIS STA PT1600 thermal analyzer with a heating rate of 10 ℃·min-1. Powder X-ray diffraction (PXRD) patterns were measured on a Rigaku-Dmax 2400 diffractometer using Cu Kα radiation (λ=0.154 06 nm); the X-ray tube was operated at 40 kV and 40 mA; the data collection range was between 5° and 45°. Solution 1H NMR spectra were recorded on a JNM ECS 400M spectrometer.
A mixture of ZnCl2 (0.027 g, 0.20 mmol), H2cpna (0.052 g, 0.20 mmol), dpea (0.037 g, 0.20 mmol), NaOH (0.016 g, 0.40 mmol), and H2O (10 mL) was stirred at room temperature for 15 min, and then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 160 ℃ for 3 d, followed by cooling to room temperature at a rate of 10 ℃·h-1. Colorless block-shaped crystals of 1 were isolated manually, and washed with distilled water. Yield: 52% (based on H2cpna). Anal. Calcd. for C19H13ZnN2O5(%): C 55.03, H 3.16, N 6.75; Found(%): C 54.82, H 3.14, N 6.80. IR (KBr, cm-1): 1 633s, 1 603s, 1 554s, 1 505w, 1 408s, 1 351m, 1 298w, 1 245s, 1 214w, 1 166w, 1 095w, 1 074w, 1 038w, 963w, 915w, 862w, 840w, 782m, 698w, 664w.
The synthesis of compound 2 was the same as that of compound 1, except that ZnCl2 and dpea were replaced with CoCl2·6H2O (0.048 g, 0.20 mmol) and dpey (0.036 g, 0.20 mmol), respectively. Purple block-shaped crystals of 2 were isolated manually, washed with distilled water, and dried. Yield: 43% (based on H2cpna). Anal. Calcd. for C19H12CoN2O5(%): C, 56.04; H, 2.97; N, 6.88. Found(%): C, 56.55; H, 2.99; N, 6.84. IR (KBr, cm-1): 1 616s, 1 563m, 1 541s, 1 505w, 1 408s, 1 355m, 1 298w, 1 245s, 1 205w, 1 162w, 1 144 w, 1 100w, 1 064w, 1 034w, 962w, 910w, 862w, 782m, 699w, 664w.
The synthesis of compound 3 was the same as that of compound 2, except that ZnCl2 was replaced with NiCl2·6H2O (0.048 g, 0.2 mmol). Green block-shaped crystals of 3 were isolated manually, and washed with distilled water. Yield: 46% (based on H2cpna). Anal. Calcd. for C19H14NiN2O6(%): C 53.69, H 3.32, N 6.59; Found(%): C 53.86, H 3.30, N 6.71. IR (KBr, cm-1): 3 370w, 3 080w, 2 921m, 2 846m, 16 20m, 1 554m, 1 462 s, 1 399m, 1 373m, 1 294w, 1 254w, 1 205w, 1 162w, 1 095w, 1 074w, 1 025w, 972w, 906w, 849m, 835w, 778w, 720w, 690w, 641w.
These compounds are insoluble in water and common organic solvents, such as methanol, ethanol, acetone, and DMF.
Three single crystals with dimensions of 0.06 mm×0.05 mm×0.03 mm (1), 0.05 mm×0.04 mm×0.04 mm (2), and 0.06 mm×0.04 mm×0.03 mm (3) were collected at 304(2) K on a Bruker SMART APEX Ⅱ CCD diffractometer with Mo Kα (λ=0.071 073 nm). The structures were solved by direct methods and refined by full-matrix least-square on F2 using the SHELXTL-2014 program[22]. All non-hydrogen atoms were refined anisotropically. All the hydrogen atoms (except for the ones bound to water molecules) were placed in calculated positions with fixed isotropic thermal parameters in included in structure factor calculations in the final stage of full-matrix least-squares refinement. The h ydrogen atoms of water molecules in 3 were located by different maps and constrained to ride on their parent O atoms. A summary of the crystallography data and structure refinements for 1-3 is given in Table S1 (Supporting information). The selected bond lengths and angles for compounds 1-3 are listed in Table S2. Hydrogen bond parameters of compound 3 are given in Table S3.
In a typical test, aromatic aldehyde (0.50 mmol, benzaldehyde as a model substrate), malononitrile (1.0 mmol), and catalyst (typically molar fraction of 2.0%) were mixed in methanol (1.0 mL). The suspension was stirred at room temperature. After the desired reaction time, the catalyst was removed by centrifugation, followed by an evaporation of the solvent from the filtrate under reduced pressure to give a crude solid. This was dissolved in CDCl3 and analyzed by 1H NMR spectroscopy for quantification of products (Fig.S1). To perform the recycling experiment, the catalyst was isolated by centrifugation, washed with methanol, dried at room temperature, and reused. The subsequent steps were performed as described above.
Because single-crystal X-ray diffraction analysis reveals that compounds 1 and 2 have similar structures, only compound 1 is described herein. The asymmetric unit of 1 contains one crystallographically unique Zn(Ⅱ) ion, one μ3-cpna2- block, and a half of dpea moiety. As shown in Fig. 1, the Zn1 ion is four-coordinated by three carboxylate O atoms from three individual μ3-cpna2- blocks and one N atom from the auxiliary ligand (dpea), constructing a distorted tetrahedral {ZnNO3} geometry. The Zn—O bond lengths range from 0.193 6(2) to 0.197 8(2) nm, whereas the Zn—N bond is 0.201 8(2) nm. These bonding parameters are comparable to those found in other reported Zn(Ⅱ) compounds[4, 16-17]. In 1, the cpna2- ligand adopts the coordination mode Ⅰ (Scheme 1), in which two deprotonated carboxylate groups show a monodentate or bridging bidentate mode, and the nicotinate N atom remains uncoordinated. Within the μ3-cpna2- block, the dihedral angle between two aromatic rings is 75.42°, whereas the C—Oether—C angle is 117.26°. The dpea ligand adopts a bridging coordination fashion. Two adjacent Zn centers are held together through two carboxylate groups coming from two μ3-cpna2- spacers. As a result, the dizinc(Ⅱ) subunits with a Zn…Zn separation of 0.358 2(2) nm (Fig. 2) are formed and are further interlinked by the μ3-cpna2- and μ-dpea blocks to form a 2D sheet (Fig. 3). This 2D network discloses a trinodal 2,3,4-connected network with a new topology and a point symbol of (42.6.82.10)2(42.6)2(8) (Fig. 4). This network is composed of the 4-linked Zn1 nodes, 3-linked μ3-cpna2- nodes, and 2-connected μ-dpea linkers (Fig. 4). A significant feature of this structure concerns its two-fold parallel 2D+2D interpenetration (Fig. 5).
Hydrogen atoms are omitted for clarity; Symmetry codes: A: -x+1, -y, -z; B: x+1, y+1, z; C: -x+1, -y+2, -z.
Cyan: 4-connected Zn1 nodes, gray: centroids of 3-connected µ3-cpna2− nodes, dark blue: centroids of 2-connected µ-dpea linkers.
The asymmetric unit of compound 3 possesses one crystallographically independent Ni(Ⅱ) ion, one μ3-cpna2- block, half of the auxiliary ligand (dpey), and one H2O ligand. As shown in Fig. 6, the Ni1 ion is six-coordinated and adopts a distorted octahedral {NiN2O4} geometry formed by three carboxylate O atoms from three distinct μ3-cpna2- blocks, one O donor from the H2O ligand, and two N atoms coming from one μ3-cpna2- and one dpey blocks. The Ni—O distances range from 0.205 2(2) to 0.219 7(2) nm, whereas the Ni—N distances are 0.206 5(2)-0.208 4(2) nm. These bonding parameters agree with those observed in other Ni(Ⅱ) compounds[16, 23]. In 3, the cpna2- ligand acts as a μ3-N, O3-spacer (mode Ⅱ, Scheme 1) with COO- groups showing a monodentate or bidentate mode. In μ3-cpna2-, the aromatic rings show a dihedral angle of 78.54°, while the C—Oether—C angle attains 118.14°. The dpey ligand adopts a bridging coordination mode. The μ3-cpna2- and μ-dpey spacers connect the neighboring Ni(Ⅱ) ions to furnish a 2D metal-organic network (Fig. 7). This structure reveals a 2D architecture assembled from the 4-linked Ni1 nodes, 3-linked μ3-cpna2- nodes, 2-connected μ-dpey linkers (Fig. 8), which is a new topology and a point symbol of (42.6.82.10)2(42.6)2(8).
Hydrogen atoms are omitted for clarity; Symmetry codes: A: -x+1, -y+1, -z+2; B: -x+2, -y+1, -z+2; C: -x+3, -y+2, -z.
Symmetry codes: A: -x+1, y, z; B: x+1, y, z; C: -x+3, -y+2, -z; D: -x+2, -y+1, -z+2.
Green: 4-connected Ni1 nodes, gray: centroids of 3-connected µ3-cpna2− nodes, dark blue: centroids of 2-connected µ-dpey linkers.
To determine the thermal stability of compounds 1-3, their thermal behaviors were investigated under a nitrogen atmosphere by TGA. As shown in Fig. 9, because compounds 1 and 2 do not contain H2O moieties, TGA curves reveal that the samples were stable up to 362 and 202 ℃, respectively, followed by a decomposition on further heating. The TGA curve of 3 showed a release of the H2O ligand between 192 and 238 ℃ (Obsd. 4.0%, Calcd. 4.2%), while the dehydrated solid remains stable up to 324 ℃.
Considering a recognized application of different metal(Ⅱ) CPs to act as catalysts in the Knoevenagel condensation reaction[11, 24-26], we probed compounds 1-3 as heterogeneous catalysts in this reaction using assorted aldehydes with malononitrile. As a model substrate, benzaldehyde was treated with malononitrile at 25 ℃ in a methanol medium to form a 2-benzylidenemalononitrile product (Scheme 2, Table 1). The influence of different reaction parameters (i.e., reaction time, solvent, catalyst loading and recycling, and substrate scope) was studied.
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| Entry | Catalyst | t / min | Catalyst loadinga / % | Solvent | Yieldb / % |
| 1 | 1 | 10 | 2.0 | CH3OH | 51 |
| 2 | 1 | 20 | 2.0 | CH3OH | 69 |
| 3 | 1 | 30 | 2.0 | CH3OH | 78 |
| 4 | 1 | 40 | 2.0 | CH3OH | 86 |
| 5 | 1 | 50 | 2.0 | CH3OH | 94 |
| 6 | 1 | 60 | 2.0 | CH3OH | 100 |
| 7 | 1 | 60 | 2.0 | H2O | 98 |
| 8 | 1 | 60 | 2.0 | C2H5OH | 96 |
| 9 | 1 | 60 | 2.0 | CH3CN | 87 |
| 10 | 1 | 60 | 2.0 | CHCl3 | 64 |
| 11 | 1 | 60 | 1.0 | CH3OH | 95 |
| 12 | 2 | 60 | 2.0 | CH3OH | 100 |
| 13 | 3 | 60 | 2.0 | CH3OH | 85 |
| 14 | Blank | 60 | — | CH3OH | 20 |
| 15 | ZnCl2 | 60 | 2.0 | CH3OH | 31 |
| 16 | H2cpna | 60 | 2.0 | CH3OH | 25 |
| a Expressed in molar fraction; The yield (Y) was calculated by 1H NMR spectroscopy: Y=nproduct/naldehyde×100%. | |||||
Compounds 1 and 2 revealed the highest activity, resulting in a 100% conversion of benzaldehyde to 2-benzylidenemalononitrile (Table 1 and Fig.S1). Because compound 1 had a higher yield when it was synthesized, this polymer was used to research the influence of different reaction parameters. The yield was accumulated with a yield increase from 51% to 100% on prolonging the reaction from 10 to 60 min (Table 1, entries 1-6). The influence of catalyst amount was also investigated, revealing a product yield growth from 95% to 100% on increasing the loading of catalyst from 1% to 2% (entries 6 and 11). In addition to methanol, other solvents were tested. Water, ethanol, acetonitrile, and chloroform were less suitable (product yields of 64%-98%, respectively).
In comparison with 1 and 2, compound 3 is less active, resulting in a maximum product yield of 85% (entry 13, Table 1). It should be highlighted that under similar reaction conditions, the Knoevenagel condensation reaction of benzaldehyde with malononitrile was significantly less efficient in the absence of a catalyst (only 20% of product yield) or when using H2cpna (25%) or ZnCl2 (31%) as catalysts (entries 14-16, Table 1). Although no correlation between activity and the structure of the catalyst can be drawn, the superior performance of compounds 1 and 2 can be attributed to the existence of the non-saturated coordination site in metal centers[27-28].
Different substituted benzaldehyde substrates were used to study the substrate scope in the Knoevenagel condensation of benzaldehyde with malononitrile. These tests were run under optimized conditions (x1=2.0%, CH3OH, 25 ℃, 60 min). The corresponding products were obtained in the yields varying from 43% to 100% (Table 2). Benzaldehydes containing a strong electron-withdrawing group (e.g., nitro, and chloro substituent in the ring) revealed the best efficiency (entries 2-5, Table 2), which can be explained by an increased electrophilicity of substrates. The benzaldehydes possessing an electron-donating functionality (e.g., methyl or methoxy group) led to lower product yields (entries 7 and 8, Table 2).
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| Entry | Substituted benzaldehyde substrate (R-C6H4CHO) | Product yieldb / % | Separation yield / % |
| 1 | R=H | 100 | 98 |
| 2 | R=2-NO2 | 100 | 98 |
| 3 | R=3-NO2 | 100 | 97 |
| 4 | R=4-NO2 | 100 | 96 |
| 5 | R=4-Cl | 100 | 97 |
| 6 | R=4-OH | 43 | 40 |
| 7 | R=4-CH3 | 96 | 92 |
| 8 | R=4-OCH3 | 72 | 68 |
| a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), catalyst 1 (2.0%), and CH3OH (1.0 mL) at 25 ℃; b Calculated by 1H NMR spectroscopy. | |||
Finally, the recyclability of catalyst 1 was tested. After each reaction cycle, the catalyst was separated via centrifugation, washed with CH3OH, dried in air at about 25 ℃, and reused in the next cycle. The obtained results prove that compound 1 preserved the activity for at least five reaction cycles (the yields were 100%, 100%, 98%, and 97% for the second to fifth run, respectively). Besides, the PXRD patterns confirm that the structure of 1 was maintained (Fig.S2), despite the appearance of several additional signals or widening of some peaks. These alterations might be expected after a few catalytic cycles and are explained by the presence of some impurities or a decrease in crystallinity.
In summary, we have synthesized three Zn(Ⅱ), Co(Ⅱ), and Ni(Ⅱ) CPs based on a trifunctional N, O-building block. All compounds disclose 2D networks. The catalytic properties of these compounds were investigated. Compounds 1 and 2 revealed effective catalytic activities in the Knoevenagel condensation reaction at room temperature.
Supporting information is available at
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Figure 1 Drawing of the asymmetric unit of compound 1 with a 30% probability of thermal ellipsoids
Hydrogen atoms are omitted for clarity; Symmetry codes: A: -x+1, -y, -z; B: x+1, y+1, z; C: -x+1, -y+2, -z.
Figure 4 Topological representation of a 2,3,4-connected network with a new topology viewed along the c-axis
Cyan: 4-connected Zn1 nodes, gray: centroids of 3-connected µ3-cpna2− nodes, dark blue: centroids of 2-connected µ-dpea linkers.
Figure 6 Drawing of the asymmetric unit of compound 3 with a 30% probability of thermal ellipsoids
Hydrogen atoms are omitted for clarity; Symmetry codes: A: -x+1, -y+1, -z+2; B: -x+2, -y+1, -z+2; C: -x+3, -y+2, -z.
Figure 7 Two-dimensional metal-organic network viewed along the b-axis
Symmetry codes: A: -x+1, y, z; B: x+1, y, z; C: -x+3, -y+2, -z; D: -x+2, -y+1, -z+2.
Figure 8 Topological representation of a 2,3,4-connected network with a new topology
Green: 4-connected Ni1 nodes, gray: centroids of 3-connected µ3-cpna2− nodes, dark blue: centroids of 2-connected µ-dpey linkers.
Scheme 2 Knoevenagel condensation reaction of benzaldehyde with malononitrile (model substrate) catalyzed by the CPs
Table 1. Knoevenagel condensation reaction of benzaldehyde with malononitrile catalyzed by compounds 1-3, ZnCl2, and H2cpna
| Entry | Catalyst | t / min | Catalyst loadinga / % | Solvent | Yieldb / % |
| 1 | 1 | 10 | 2.0 | CH3OH | 51 |
| 2 | 1 | 20 | 2.0 | CH3OH | 69 |
| 3 | 1 | 30 | 2.0 | CH3OH | 78 |
| 4 | 1 | 40 | 2.0 | CH3OH | 86 |
| 5 | 1 | 50 | 2.0 | CH3OH | 94 |
| 6 | 1 | 60 | 2.0 | CH3OH | 100 |
| 7 | 1 | 60 | 2.0 | H2O | 98 |
| 8 | 1 | 60 | 2.0 | C2H5OH | 96 |
| 9 | 1 | 60 | 2.0 | CH3CN | 87 |
| 10 | 1 | 60 | 2.0 | CHCl3 | 64 |
| 11 | 1 | 60 | 1.0 | CH3OH | 95 |
| 12 | 2 | 60 | 2.0 | CH3OH | 100 |
| 13 | 3 | 60 | 2.0 | CH3OH | 85 |
| 14 | Blank | 60 | — | CH3OH | 20 |
| 15 | ZnCl2 | 60 | 2.0 | CH3OH | 31 |
| 16 | H2cpna | 60 | 2.0 | CH3OH | 25 |
| a Expressed in molar fraction; The yield (Y) was calculated by 1H NMR spectroscopy: Y=nproduct/naldehyde×100%. | |||||
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Table 2. Knoevenagel condensation reaction of various aldehydes with malononitrile catalyzed by compound 1a
| Entry | Substituted benzaldehyde substrate (R-C6H4CHO) | Product yieldb / % | Separation yield / % |
| 1 | R=H | 100 | 98 |
| 2 | R=2-NO2 | 100 | 98 |
| 3 | R=3-NO2 | 100 | 97 |
| 4 | R=4-NO2 | 100 | 96 |
| 5 | R=4-Cl | 100 | 97 |
| 6 | R=4-OH | 43 | 40 |
| 7 | R=4-CH3 | 96 | 92 |
| 8 | R=4-OCH3 | 72 | 68 |
| a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), catalyst 1 (2.0%), and CH3OH (1.0 mL) at 25 ℃; b Calculated by 1H NMR spectroscopy. | |||
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