English

• 0.   Introduction

  As one of the most promising crystalline materials, metal-organic architectures have attracted tremendous attention in the past few decades because of their diversities^[1-3], and display extensive potential applications in gas adsorption/separation^[4-5], sensing^[6-7], heterogeneous catalysis^[8-12], bioactivity^[13-14], and so on^[15-16].

  The synthesis of coordination polymers (CPs) can be adjusted for generating specific structures and topologies with property-driven applications^[17-19]. Despite considerable progress made in this area, it is often challenging to predict the structures of CPs during the self-assembly generation, since a variety of factors can affect their crystallization. Such factors may include structural features of organic linkers and supporting ligands^[20-22], coordination behavior of metal ions^[23-24], type of solvent^[25-26], reaction temperature^[27-28], and pH value^[29-30].

  Aromatic multi-carboxylic acids represent the most important class of organic linkers for CPs, owing to their versatile coordination chemistry, good thermal stability, adjustable level of deprotonation, and ability to participate in different noncovalent interactions^{[9, 11-12, 24]}. Recently, our groups have been focused on designing functional CPs based on multi-carboxylate biphenyl or triphenyl blocks, which feature a diversity of coordination modes toward different metal(Ⅱ) centers^{[9, 31-32]}. As a further continuation of this research line, we have selected in the present study a little explored ether-bridged tetracarboxylate ligand, 5-(2, 5-dicarboxyphenoxy)isophthalic acid (H₄dia), and tested it as an adaptable linker for constructing new metal-organic architectures by hydrothermal synthesis.

  Herein, we report the synthesis, crystal structures, and catalysis of three Co(Ⅱ), Cu(Ⅱ), and Cd(Ⅱ) CPs with H₄dia and phen (1, 10-phenanthroline)/bipy (2, 2′-bipyridine)/bpa (bis(4-pyridyl)amine).

  1.   Experimental

  1.1   Reagents and physical measurements

  All chemicals and solvents were obtained from commercial suppliers. H₄dia was acquired from Jinan Henghua Sci. & Tec. Co., Ltd. Carbon, hydrogen, and nitrogen of the compounds 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.

  1.2   Synthesis of [Co₂(μ₄-dia)(phen)₂(H₂O)₂]_n (1)

  A mixture of CoCl₂·6H₂O (0.048 g, 0.2 mmol), H₄dia (0.035 g, 0.1 mmol), phen (0.040 g, 0.2 mmol), NaOH (0.016 g, 0.4 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 120 ℃ for 3 d, followed by cooling to room temperature at a rate of 10 ℃·h^-1. Pink block-shaped crystals were isolated manually, and washed with distilled water. Yield: 45% (based on H₄dia). Anal. Calcd. for C₄₀H₂₆Co₂N₄O₁₁(%): C 56.09, H 3.06, N 6.54; Found (%): C 55.83, H 3.08, N 6.51. IR (KBr, cm^-1): 3 622w, 3 297w, 3 067w, 1 615m, 1 588m, 1 543m, 1 519w, 1 458 w, 1 426s, 1 382s, 1 342s, 1 310w, 1 225w, 1 201w, 1 128 w, 1 080w, 967w, 919w, 871w, 850m, 803w, 778w, 727 m, 698w, 642w.

  1.3   Synthesis of [Cu₂(μ₆-dia)(bipy)₂]_n (2)

  A mixture of CuCl₂·2H₂O (0.034 g, 0.2 mmol), H₄dia (0.035 g, 0.10 mmol), bipy (0.031 g, 0.20 mmol), NaOH (0.016 g, 0.40 mmol), and H₂O (10 mL) was stirred at room temperature for 15 min, then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 120 ℃ for 3 d, followed by cooling to room temperature at a rate of 10 ℃·h^-1. Blue block-shaped crystals were isolated manually, washed with distilled water, and dried. Yield: 43% (based on H₄dia). Anal. Calcd. for C₃₆H₂₂Cu₂N₄O₉(%): C 55.32, H 2.84, N 7.17; Found(%): C 55.54, H 2.82, N 7.21. IR (KBr, cm^-1): 1 638s, 1 615s, 1 543s, 1 579m, 1 475w, 1 442w, 1 357s, 1 285w, 1 249 w, 1 209w, 1 161w, 1 125w, 1 080w, 1 060w, 1 027w, 967w, 923w, 899w, 859w, 771m, 730w, 678w, 649w.

  1.4   Synthesis of {[Cd₂(μ₅-dia) (μ-bpa)₂(H₂O)]·H₂O}_n (3)

  A mixture of CdCl₂·H₂O (0.040 g, 0.2 mmol), H₄dia (0.035 g, 0.1 mmol), bpa (0.034 g, 0.2 mmol), NaOH (0.016 g, 0.4 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 120 ℃ for 3 d, followed by cooling to room temperature at a rate of 10 ℃·h^-1. Colourless block-shaped crystals were isolated manually and washed with distilled water. Yield: 47% (based on H₄dia). Anal. Calcd. for C₃₆H₂₈Cd₂N₆O₁₁(%): C 45.73, H 2.98, N 8.89; Found(%): C 45.85, H 3.00, N 8.86. IR (KBr, cm^-1): 3 631w, 3 259, 3 164w, 3 064w, 1 613s, 1 593s, 1 540s, 1 521s, 1 480w, 1 445w, 1 425w, 1 373s, 1 341w, 1 249w, 1 209m, 1 141w, 1 081w, 1 014m, 977w, 921w, 902w, 861w, 845w, 814w, 786w, 734w, 693w, 641w.

  The compounds are insoluble in water and common organic solvents, such as methanol, ethanol, acetone, and DMF.

  1.5   Structure determination

  Three single crystals with dimensions of 0.23 mm× 0.20 mm×0.18 mm (1), 0.21 mm×0.20 mm×0.18 mm (2), and 0.20 mm×0.19 mm×0.15 mm (3) were collected at 293(2) K on a Bruker SMART APEX Ⅱ CCD diffractometer with Cu Kα (λ =0.154 178 nm). The structures were solved by direct methods and refined by fullmatrix least-square on F² using the SHELXTL-2014 program. 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 included in structure factor calculations in the final stage of full-matrix least-squares refinement. The hydrogen atoms of water molecules were located by different maps and constrained to ride on their parent O atoms. For 1, the O_ether atom of the dia^2- ligand was split over two sites with refined occupancies of 0.55 and 0.45. Two benzene rings of the ligand were also split over two sites with refined occupancies of 0.55 and 0.45, 050 and 0.50, respectively. 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 compounds 1 and 3 are given in Table S3 and S4.

  1.6   Catalysis of the Knoevenagel condensation reaction of aldehydes

  In a typical test, a suspension of aromatic aldehyde (0.50 mmol, benzaldehyde as a model substrate), ethyl cyanoacetate (1.0 mmol), and catalyst (typically x=2%) in methanol (1.0 mL) was stirred at 50 ℃. After the desired reaction time, the catalyst was removed by centrifugation, followed by 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 the 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.

  2.   Results and discussion

  2.1   Description of the structure

  2.1.1   Structure of compound 1

  This compound is a 1D CP and its asymmetric unit encloses two Co(Ⅱ) centers (Co1 and Co2), one μ₄-dia^4- block, two phen moieties, and two H₂O molecules (Fig. 1). Both Co centers are six-coordinated and assume a distorted octahedral {CoN₂O₄} geometry, which is occupied by three carboxylate oxygen atoms from two μ₄-dia^4- ions, one H₂O molecule as well as two N atoms from the phen moiety. The Co—O (0.198 1(5)-0.224 7(4) nm) and Co—N (0.209 0(6)-0.214 1(6) mn) bonds show typical distances^{[12, 24, 31]}. The dia^4- block functions as a μ₄-bridging ligand with four COO^- groups being monodentate or bidentate (mode Ⅰ, Scheme 1). In the μ₄-dia^4-, two aromatic rings reveal a dihedral angle of 75.94° and a C—O_ether—C angle of 114.79°. These carboxylate functionalities unite the Co(Ⅱ) centers to form a 1D double chain (Fig. 2).

  Figure 1

  

  Figure 1.  Drawing of the asymmetric unit of compound 1 with a 30% probability of thermal ellipsoids

  H atoms are omitted for clarity; Symmetry code: A: x-1, y, z

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  Scheme 1

  

  Scheme 1.  Coordination modes of the dia^4- ligands in compounds 1-3

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  Figure 2

  

  Figure 2.  A perspective of 1D double chain along the b-axis

  phen and H₂O ligands are omitted for clarity

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  2.1.2   Structure of compound 2

  CP 2 shows a 3D framework structure. The asymmetric unit contains two Cu centers (Cu1 and Cu2), a μ₆-dia^4- linker, and two bipy moieties. As shown in Fig. 3, both Cu1 and Cu2 centers are five-coordinate and feature a distorted square pyramidal {CuN₂O₃} environment with the τ parameter of 0.113 or 0.078 (τ=0 or 1 for a regular square pyramidal or trigonal-bipyramidal geometry, respectively)^[33]. The Cu1 and Cu2 centers are surrounded by three carboxylate O atoms from three μ₆-dia^4- spacers and two N atoms from the bipy moiety. The Cu—O (0.193 2(5)-0.235 3(4) nm) and Cu—N (0.199 7(5)-0.203 5(5) nm) bonds are within standard values^{[11, 31]}. The dia^4- ligand takes a μ₆ -coordination fashion with COO^- groups being monodentate or bidentate (mode Ⅱ, Scheme 1). The μ₆-dia^4- moiety displays the dihedral angles between the aromatic rings of 81.06° and the C—O_ether—C angle of 115.54°. Two crystallographically Cu1 and Cu2 ions are bridged through two carboxylate groups or two carboxylate O atoms from two different dia^4- blocks, respectively, giving rise to two Cu₂ subunits (Fig. 4). These Cu₂ subunits are multiply interlinked by the remaining carbox-ylate groups of the dia^4- blocks to form a 3D metal-organic framework (Fig. 5)

  Figure 3

  

  Figure 3.  Drawing of the asymmetric unit of compound 2 with a 30% probability of thermal ellipsoids

  H atoms are omitted for clarity; Symmetry codes: A: x+1, y, z; B: -x+1, -y, -z+1; C: -x+1, -y+1, -z; D: x, y+1, z

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  Figure 4

  

  Figure 4.  Two Cu₂ units of compound 2

  Symmetry codes: E: -x+2, -y, -z+1; F: -x+1, -y+1, -z

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  Figure 5

  

  Figure 5.  Three-dimensional metal-organic framework viewed along the b-axis

  Symmetry codes: E: -x+2, -y, -z+1; F: -x+1, -y+1, -z

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  2.1.3   Structure of compound 3

  This compound also has a 3D metal-organic framework. The asymmetric unit contains two Cd(Ⅱ) centers (Cd1 and Cd2), a μ₅-dia^4- block, two μ-bpa moieties, an H₂O ligand, and a lattice water molecule. The Cd1 center is seven-coordinate and assumes a distorted pentagonal bipyramidal {CdN₂O ₅} environment, which is populated by four carboxylate O atoms from two μ₅-dia^4- blocks, one H₂O ligand, and two N donors of two different μ-bpa moieties (Fig. 6). The Cd2 center is six-coordinate and features a distorted octahedral {CdN₂O₄} geometry, which is occupied by four carboxylate oxygen atoms from three μ₅-dia^4- ligands as well as two N atoms from two different μ-bpa moieties. The Cd—O (0.226 4(3)-0.249 1(4) nm) and Cd—N (0.228 1(4)-0.238 8(4) nm) bonds show typical distances^{[24, 31-32]}. The dia^4- block acts as a μ₅-spacer (mode Ⅲ, Scheme 1), in which four COO^- groups are μ-bridging bidentate or bidentate. In the μ₅-deta^4-, two aromatic rings reveal a dihedral angle of 75.00° and a C—O_ether—C angle of 116.11°. The adjacent Cd(Ⅱ) atoms are connected via μ₅-dia^4- blocks and μ-bpa moieties to form a 3D metal-organic framework (Fig. 7).

  Figure 6

  

  Figure 6.  Drawing of the asymmetric unit of compound 3 with a 30% probability of thermal ellipsoids

  H atoms are omitted for clarity except for NH groups; Symmetry codes: A: x, y-1, z; B: -x, y-1/2, -z+1/2; C: -x+1, -y+1, -z+1; D: -x+1, y-1/2, -z+1/2; E: x, -y+1/2, z+1/2

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  Figure 7

  

  Figure 7.  Three-dimensional metal-organic framework viewed along the a-axis and c-axis

  Water molecules are omitted for clarity

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  2.2   TGA analysis

  To determine the thermal stability of CPs 1-3, their thermal behaviors were investigated under a nitrogen atmosphere by TGA. As shown in Fig. 8, compound 1 lost its two coordinated H₂O molecules in a range of 168-214 ℃ (Obsd. 4.4%, Calcd. 4.2%), followed by the decomposition at 314 ℃. Compound 2 does not contain H₂O and solvent molecules and its framework maintained stability up to 271 ℃. For 3D CP 3, the TGA curve displayed a loss of one lattice and one coordinated water molecule between 54 and 162 ℃ (Obsd. 3.6%, Calcd. 3.8%), whereas a dehydrated solid was then stable up to 346 ℃.

  Figure 8

  

  Figure 8.  TGA curves of compounds 1-3

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  2.3   Catalysis of the Knoevenagel condensation reaction

  Considering the potential of different metal (Ⅱ) coordination compounds to act as catalysts in the Knoevenagel condensation^{[9, 34-36]}, we probed compounds 1-3 as heterogeneous catalysts in this reaction using assorted aldehydes with ethyl cyanoacetate. As a model substrate, benzaldehyde was treated with ethyl cyanoacetate at 50 ℃ in a methanol medium to form a 2-cyano-3-phenyl-2-propenoic acid ethyl ester product (Scheme 2, Table 1). The influence of different reaction parameters (i.e., reaction time, temperature, solvent type, catalyst loading and recycling, and substrate scope) was studied.

  Scheme 2

  

  Scheme 2.  Cu-catalyzed the Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate

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  Table 1

  

  Table 1.  Cu-catalyzed Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate

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  Entry	Catalyst	Time/h	Temperature/℃	Catalyst loading/%	Solvent	Yield*/%
  1	2	1	50	2.0	CH3OH	53
  2	2	2	50	2.0	CH3OH	77
  3	2	3	50	2.0	CH3OH	90
  4	2	4	50	2.0	CH3OH	100
  5	2	4	50	1.0	CH3OH	96
  6	2	4	40	2.0	CH3OH	93
  7	2	4	50	2.0	H2O	57
  8	2	4	50	2.0	C2H5OH	55
  9	2	4	50	2.0	CH3CN	37
  10	2	4	50	2.0	CHCl3	32
  11	1	4	50	2.0	CH3OH	81
  12	3	4	50	2.0	CH3OH	74
  13	Blank	4	50	—	CH3OH	2
  14	CuCl2	4	50	2.0	CH3OH	13
  15	H4dia	4	50	2.0	CH3OH	15
  * Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

  Compound 2 revealed the highest activity, resulting in a quantitative conversion of benzaldehyde to 2-cyano-3-phenyl-2-propenoic acid ethyl ester (Table 1 and Fig. S1). Compound 2 was used to research the influence of different reaction parameters. The yield was accumulated with a yield increase from 53% to 100% on prolonging the reaction from 1 to 4 h (Table 1, Entry 1-4). The influence of catalyst amount was also investigated, revealing a product yield growth from 96% to 100% on increasing the loading of the catalyst (molar fraction) from 1% to 2% (Entry 4 and 5). Entry 6 showed that 2% compound 2 catalyzed the reaction and produced the product in 93% at 40 ℃; however, a mildly elevated temperature of 50 ℃ led to nearly quantitative. In addition to methanol, other solvents were tested. Water, ethanol, acetonitrile, and chloro-form were less suitable (Yield: 32%-57%).

  In comparison with 2, compounds 1 and 3 were less active, resulting in the maximum product yields in a range of 74%-81% (Entry 11 and 12, Table 1). It should be highlighted that under similar reaction conditions, the Knoevenagel condensation of benzaldehyde was significantly less efficient in the absence of the catalyst (Yield: 2%) or when using H₄dia (Yield: 15%) or CuCl₂ (Yield: 13%) as catalysts (Entry 13-15, Table 1). Although a connection between catalytic activity and the structural characteristic of the catalyst may not be fully established in the present work, the superior activity of compound 2 might be attributed to the existence of more easily accessible Lewis acid metal sites^{[35, 37]}.

  Different substituted benzaldehyde substrates were used to study the substrate scope in the Knoevenagel condensation with ethyl cyanoacetate. These tests were run under optimized conditions (2.0% of 2, CH₃OH, 50 ℃, 4 h). The corresponding products were obtained in the yields varying from 36% to 100% (Table 2). Benzaldehydes containing a strong electron-withdrawing group (e.g., nitro, and chloro substituent in the ring) revealed the best efficiency (Entry 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 (Entry 7 and 8, Table 2).

  Table 2

  

  Table 2.  Knoevenagel condensation reaction of various aldehydes with ethyl cyanoacetate catalyzed by compound 2^a

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  Entry	Substituted benzaldehyde substrate (R—C6H4CHO)	Yieldb/%
  1	R=H	100
  2	R=2-NO2	100
  3	R=3-NO2	100
  4	R=4-NO2	100
  5	R=4-Cl	83
  6	R=4-OH	40
  7	R=4-CH3	65
  8	R=4-OCH3	36
  a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), catalyst 2 (2.0%), and CH3OH (1.0 mL) at 50 ℃; b Calculated by 1H NMR spectroscopy.

  Finally, the recyclability of catalyst 2 was tested. After each reaction cycle, the catalyst was separated via centrifugation, washed with CH₃OH, dried in air at ca. 25 ℃, and reused in the next cycle. The yields were 100%, 99%, 97%, and 96% for the second to the fifth run, respectively. The obtained results proved that compound 2 preserved the activity for at least five reaction cycles. Besides, the PXRD patterns (Fig. 9) confirm that the structure of 2 is maintained, despite the appearance of several additional signals or the 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 terms of the observed activity, compound 2 generally appears to be superior in the Knoevenagel condensation of aldehydes if compared to several reported CPs (Table S5).

  Figure 9

  

  Figure 9.  PXRD patterns for 2

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  3.   Conclusions

  In summary, we have synthesized three Co(Ⅱ), Cu(Ⅱ), and Cd(Ⅱ) CPs (1-3) based on a tetracarboxylate ligand. Compounds 1-3 disclose a 1D double chain and 3D framework, respectively. The catalytic properties of these compounds were investigated. Compound 2 revealed excellent catalytic activity in the Knoevenagel condensation reaction at 50 ℃.

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

  
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  Figure 1  Drawing of the asymmetric unit of compound 1 with a 30% probability of thermal ellipsoids

  H atoms are omitted for clarity; Symmetry code: A: x-1, y, z

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  Scheme 1  Coordination modes of the dia^4- ligands in compounds 1-3

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  Figure 2  A perspective of 1D double chain along the b-axis

  phen and H₂O ligands are omitted for clarity

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  Figure 3  Drawing of the asymmetric unit of compound 2 with a 30% probability of thermal ellipsoids

  H atoms are omitted for clarity; Symmetry codes: A: x+1, y, z; B: -x+1, -y, -z+1; C: -x+1, -y+1, -z; D: x, y+1, z

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  Figure 4  Two Cu₂ units of compound 2

  Symmetry codes: E: -x+2, -y, -z+1; F: -x+1, -y+1, -z

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  Figure 5  Three-dimensional metal-organic framework viewed along the b-axis

  Symmetry codes: E: -x+2, -y, -z+1; F: -x+1, -y+1, -z

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  Figure 6  Drawing of the asymmetric unit of compound 3 with a 30% probability of thermal ellipsoids

  H atoms are omitted for clarity except for NH groups; Symmetry codes: A: x, y-1, z; B: -x, y-1/2, -z+1/2; C: -x+1, -y+1, -z+1; D: -x+1, y-1/2, -z+1/2; E: x, -y+1/2, z+1/2

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  Figure 7  Three-dimensional metal-organic framework viewed along the a-axis and c-axis

  Water molecules are omitted for clarity

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  Figure 8  TGA curves of compounds 1-3

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  Scheme 2  Cu-catalyzed the Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate

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  Figure 9  PXRD patterns for 2

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  Table 1.  Cu-catalyzed Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate

  Entry	Catalyst	Time/h	Temperature/℃	Catalyst loading/%	Solvent	Yield*/%
  1	2	1	50	2.0	CH3OH	53
  2	2	2	50	2.0	CH3OH	77
  3	2	3	50	2.0	CH3OH	90
  4	2	4	50	2.0	CH3OH	100
  5	2	4	50	1.0	CH3OH	96
  6	2	4	40	2.0	CH3OH	93
  7	2	4	50	2.0	H2O	57
  8	2	4	50	2.0	C2H5OH	55
  9	2	4	50	2.0	CH3CN	37
  10	2	4	50	2.0	CHCl3	32
  11	1	4	50	2.0	CH3OH	81
  12	3	4	50	2.0	CH3OH	74
  13	Blank	4	50	—	CH3OH	2
  14	CuCl2	4	50	2.0	CH3OH	13
  15	H4dia	4	50	2.0	CH3OH	15
  * Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

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  Table 2.  Knoevenagel condensation reaction of various aldehydes with ethyl cyanoacetate catalyzed by compound 2^a

  Entry	Substituted benzaldehyde substrate (R—C6H4CHO)	Yieldb/%
  1	R=H	100
  2	R=2-NO2	100
  3	R=3-NO2	100
  4	R=4-NO2	100
  5	R=4-Cl	83
  6	R=4-OH	40
  7	R=4-CH3	65
  8	R=4-OCH3	36
  a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), catalyst 2 (2.0%), and CH3OH (1.0 mL) at 50 ℃; b Calculated by 1H NMR spectroscopy.

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