English

• 0.   Introduction

  In recent decades, coordination polymers (CPs) have attracted considerable research interest as an important class of materials, primarily due to their diverse structural architectures^[1-5] and outstanding functional properties. These materials show promising applications in areas such as chemical sensing^[6-7], gas storage and separation^[8-9], and catalytic processes^[10-13]. This growing attention is further driven by the favorable attributes of metal ions and their complexes, including low cost, widespread availability, biological activity, photochemical responsiveness, and rich coordination behavior that allows compatibility with numerous ligand varieties^[14-16]. Of the many organic linkers available, aromatic carboxylic acids stand out as the most commonly employed building blocks in assembling coordination polymers and related frameworks^{[5, 12-13]}. Present research efforts are increasingly focused on designing novel carboxylate-based ligands featuring multiple binding sites, which are then systematically studied for their potential in constructing advanced metal-organic frameworks.

  As a continuation of our previous studies on functional CPs derived from commercially available carboxylic acid linkers^{[12-13, 17-18]}, we have now investigated the flexible dicarboxylate ligand bis(4-carboxyphenyl)urea (H₂cada) as an underexplored linker to construct coordination polymers^[19-22]. H₂cada was chosen owing to its four potential coordination sites, which enable diverse coordination modes. In this study, we report the hydrothermal synthesis and characterization of three new CPs assembled from metal(Ⅱ) salts, H₂cada, and two auxiliary ligands acting as mediators of crystallization. These products exhibit diverse 1D chain structures, formulated as [Zn(μ-cada)(bipy)(H₂O)]_n (1), [Zn(μ₃-cada)(phen)·H₂O]_n (2) and [Cd(μ₃-cada)(phen)]_n (3), where bipy=2, 2′-bipyridine and phen=1, 10-phenanthroline. Structural and topological features, as well as catalytic performance of the obtained products in the condensation reaction between benzaldehyde and malononitrile, are described below. This work expands the family of CPs constructed from flexible carboxylate linkers and underscores the promising catalytic potential of these materials in heterogeneous catalysis.

  1.   Experimental

  1.1   Reagents and physical measurements

  All reagents and solvents utilized were of analytical grade and were used as received, without any additional purification. Carbon, hydrogen, and nitrogen were determined using an Elementar Vario EL elemental analyzer. The 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 apparatus functioned at a voltage of 40 kV and a current of 40 mA, with data acquired over a 2θ range from 5° to 45°. Additionally, ¹H NMR spectra were captured using a JNM ECS 400M spectrometer.

  1.2   Synthesis of complex 1

  In a reaction setup, 0.027 g (0.20 mmol) of ZnCl₂ was combined with 0.060 g (0.20 mmol) of H₂cada, bipy (0.031 g, 0.20 mmol), along with 0.016 g (0.40 mmol) of NaOH and 10 mL of water. The mixture was stirred for 15 min at room temperature and then transferred to a 25 mL Teflon-lined stainless steel container. This vessel was heated to 160 ℃ for 3 d, after which it was slowly cooled to room temperature at a rate of 10 ℃·h^-1. The resulting colourless block-shaped crystals of 1 were extracted by hand and rinsed with distilled water, yielding 52% based on H₂cada. Anal. Calcd. for C₂₅H₂₀ZnN₄O₆(%): C 55.83, H 3.75, N 10.42; Found(%): C 56.07, H 3.73, N 10.72. IR (KBr, cm^-1): 3 333m, 3 072w, 1 648m, 1 594w, 1 567m, 1 525s, 1 433m, 1 305w, 1 236w, 1 175w, 1 113w, 1 013w, 975w, 871m, 851w, 778m, 725w, 640w, 621w.

  1.3   Synthesis of complex 2

  ZnCl₂ (0.027 g, 0.20 mmol), H₂cada (0.060 g, 0.20 mmol), phen (0.040 g, 0.20 mmol), NaOH (0.016 g, 0.40 mmol), and 10 mL of H₂O were gently mixed at ambient temperature for 15 min. The mixture was then enclosed in a stainless steel container lined with Teflon, totaling 25 mL in capacity, and heated at 160 ℃ for 3 d. Subsequently, the container was allowed to reach ambient temperature at a controlled pace of 10 ℃·h^-1. Manually, the distinct colourless block-like crystals of complex 2 were obtained, rinsed thoroughly with purified water, and then allowed to dry. Yield: 46% (based on H₂cada). Anal. Calcd. for C₂₇H₂₀ZnN₄O₆(%): C 57.72, H 3.59, N 9.97; Found(%): C 57.46, H 3.56, N 10.14. IR (KBr, cm^-1): 3 519w, 3 280w, 3 058w, 1 630m, 1 605s, 1 542w, 1 482w, 1 412w, 1 382w, 1 332s, 1 235s, 1 176m, 1 142w, 1 105w, 1 016w, 869w, 845w, 798m, 765w, 727m, 643w, 613w.

  1.4   Synthesis of complex 3

  CdCl₂·H₂O (0.040 g, 0.20 mmol), H₂cada (0.060 g, 0.20 mmol), phen (0.040 g, 0.20 mmol), NaOH (0.016 g, 0.40 mmol), and H₂O (10 mL) were agitated at ambient temperature for 15 min, and subsequently the mixture was placed into a Teflon-lined stainless steel container with a volume of 25 mL, and heated at 160 ℃ for 3 d. Then the temperature was decreased to ambient conditions at a controlled rate of 10 ℃·h^-1. Manually isolated, colourless block-shaped crystals of 3 were obtained, and subsequently rinsed with distilled water. The yield was recorded at 45% (based on H₂cada). Anal. Calcd. for C₂₇H₁₈CdN₄O₅(%): C 54.88, H 3.07, N 9.48; Found(%): C 55.14, H 3.09, N 9.44. IR (KBr, cm^-1): 1 663w, 1 609m, 1 537s, 1 406s, 1 297w, 1 229w, 1 171w, 1 144w, 1 101w, 1 009w, 951w, 894w, 856m, 779m, 728m, 698w, 636w, 617w. These complexes are insoluble in water and common organic solvents, such as methanol, ethanol, acetone, and DMF.

  1.5   Structure determinations

  A total of three individual crystals with sizes of 0.05 mm×0.03 mm×0.03 mm (1), 0.06 mm×0.05 mm×0.04 mm (2), and 0.06 mm×0.04 mm×0.03 mm (3) were selected, and the data were collected at 303(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-squares 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 and included in structure factor calculations in the final stage of full-matrix least-squares refinement. The hydrogen atoms of water molecules in 1 and 2 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 complexes 1-3 are listed in Table S2. Hydrogen bond parameters of complexes 1-3 are given in Table S3.

  1.6   Catalytic the Knoevenagel condensation reaction of aldehydes

  In a typical protocol, benzaldehyde (0.50 mmol), malononitrile (1.0 mmol), and catalyst (typically 2.0% of molar fraction) were combined in CH₃OH (1.0 mL), and the obtained suspension was stirred for 10-60 min at 25 ℃. The catalyst was then isolated by centrifugation, and the filtrate was evaporated under reduced pressure, resulting in a crude solid. This was dissolved in CDCl₃ and analyzed by ¹H NMR spectroscopy (JNM ECS 400M spectrometer) for product quantification (Fig.S1). In catalyst recycling experiments, the catalyst was removed by centrifugation, washed with CH₃OH, dried at 25 ℃, and reused in subsequent catalytic tests. Effects of catalyst and solvent, as well as substrate scope with other aldehydes, were investigated following the above-described procedure.

  2.   Results and discussion

  2.1   Description of the structure

  2.1.1   Structure of complex 1

  In the asymmetric unit of 1, there are one Zn(Ⅱ) center, a μ-cada^2- block, a terminal bipy, and a H₂O ligand. The Zn1 ion reveals a distorted trigonal bipyramidal {ZnN₂O₃} geometry, filled by two carboxylate oxygen atoms from two μ-cada^2- units one O atom from the H₂O ligand as well as a pair of nitrogen atoms contributed by one bipy ligand (Fig.1). The cada^2- linkers display a μ-coordination (mode Ⅰ, Scheme 1) and assemble the Zn1 ions into a 1D chain (Fig.2) with a 2C1 topology (Fig.3). Furthermore, the adjacent chains assemble into a 2D layer through hydrogen bonding (Fig.4 and Table S3).

  Figure 1

  

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

  H atoms are omitted for clarity; Symmetry code: A: x+1/2, -y+3/2, z+3/2.

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  Scheme 1

  

  Scheme 1.  Coordination modes of cada^2- ligands

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  Figure 2

  

  Figure 2.  One-dimensional metal-organic chain of 1 viewed along the a-axis

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  Figure 3

  

  Figure 3.  Topological representation of a chain of 1 with 2C1 topology viewed along the a-axis

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  Figure 4

  

  Figure 4.  Two-dimensional supramolecular network of 1 viewed along the c-axis

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  2.1.2   Structure of complex 2

  In the asymmetric unit of this 1D coordination polymer, there is one Zn(Ⅱ) center, a μ₃-cada^2- block, a terminal phen ligand, and one lattice water molecule (Fig.5). The Zn1 center is five-coordinated and adopt distorted trigonal bipyramidal {ZnN₂O₃} geometry, which is populated by two N_phen atoms and three carboxylate O donors from two μ₃-cada^2- linkers (Fig.5). The cada^2- blocks function as μ₃-linkers (mode Ⅱ, Scheme 1) and assemble the Zn1 centers into a 1D chain (Fig.6) with a (4, 4)(0, 2) topology (Fig.7). Furthermore, the adjacent chains assemble into a 2D layer through hydrogen bonding (Fig.8 and Table S3).

  Figure 5

  

  Figure 5.  Drawing of the asymmetric unit of complex 2 with 30% probability thermal ellipsoids

  H atoms and lattice water molecules are omitted for clarity; Symmetry codes: A: -x+3/2, -y+3/2, -z+3/2; B: x, -y+3/2, z-1/2.

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  Figure 6

  

  Figure 6.  One-dimensional metal-organic chain of 2 viewed along the b-axis

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  Figure 7

  

  Figure 7.  Topological representation of a chain of 2 with (4, 4)(0, 2) topology viewed along the b-axis

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  Figure 8

  

  Figure 8.  Two-dimensional supramolecular network of 2 viewed along the b-axis

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  2.1.3   Structure of complex 3

  The asymmetric unit of this 1D coordination polymer is composed of one Cd(Ⅱ) ion, a μ₃-cada^2- linker, and a terminal phen ligand (Fig.9). The Zn1 ion adopts a distorted octahedral {CdN₂O₄} geometry filled by four carboxylate O atoms from three μ₃-cada^2- ligands and two N donors from the terminal phen ligand. The cada^2- linkers display a μ₃-coordination (mode Ⅲ, Scheme 1) and assemble the Cd1 ions into a 1D chain (Fig.10) with a fes topology and a point symbol of (4.8²) (Fig.11). Furthermore, the adjacent chains assemble into a 2D layer through hydrogen bonding (Fig.12 and Table S3).

  Figure 9

  

  Figure 9.  Drawing of the asymmetric unit of complex 3 with 30% probability thermal ellipsoids

  H atoms are omitted for clarity; Symmetry codes: A:-x+2, y, -z+3/2; B:-x+1, -y+1, -z+1.

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  Figure 10

  

  Figure 10.  One-dimensional metal-organic chain of 3 viewed along the c-axis

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  Figure 11

  

  Figure 11.  Topological representation of a chain of 3 with a fes topology viewed along the c-axis

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  Figure 12

  

  Figure 12.  Two-dimensional supramolecular network of 3 viewed along the a-axis

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  2.2   TGA of the complexes

  TGA of complexes 1-3 was conducted under an inert nitrogen atmosphere over the temperature range of 25-800 ℃ (Fig.13). Complex 1 had a weight loss between 272 and 331 ℃ due to the release of one coordinated water molecule (Obsd. 3.1%, Calcd. 3.3%). The decomposition of the dehydrated sample started at 383 ℃. Similarly, a release of one lattice water molecule (Obsd. 3.4%; Calcd. 3.2%) was observed in the TGA curve of complex 2 between 90 and 206 ℃, followed by the decomposition at 267 ℃. Complex 3 has no coordinated or crystallization water, and its metal-organic network remained stable on heating up to 270 ℃.

  Figure 13

  

  Figure 13.  TGA curves of complexes 1-3

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  2.3   Catalytic Knoevenagel condensation reaction

  Coordination polymers are known as catalysts for various condensation reactions^{[10, 12-13, 17-18]}. In this study, we explored the obtained complexes 1-3 as heterogeneous catalysts in the condensation reaction between an aldehyde and a dinitrile. As model substrates, benzaldehyde and malononitrile were used. The reaction was carried out at 25 ℃ in methanol as a typical solvent, leading to the selective generation of 2-benzylidenemalononitrile (Scheme 2, Table 1). Additionally, the effects of different parameters, including catalyst loading (x_cat) and recyclability, reaction time, solvent choice, and substrate scope, were investigated.

  Scheme 2

  

  Scheme 2.  Catalytic Knoevenagel condensation reaction of benzaldehyde with malononitrile (model substrate)

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  Among the obtained complexes, complexes 1 and 2 revealed the highest catalytic activity, converting benzaldehyde to 2-benzylidenemalononitrile with the yields as high as 100% (Table 1 and Fig.S1). Complex 1 was selected to evaluate the influence of various reaction parameters. For example, when the reaction time was extended from 10 to 60 min (Table 1, entries 1-6), the product yield increased from 51% to 100%. The increase in the catalyst loading from 1% to 2% also boosted the product yield from 94% to 100% (Table 1, entries 6 and 11). We also screened alternative solvents, such as water, ethanol, acetonitrile, and chloroform, but these proved less effective than methanol, with the yields ranging from 67% to 98%. It should be mentioned that very high product yields (98% and 96%; Table 1, entries 7 and 8) could be obtained in water or ethanol that are considered green solvents. In contrast to complexes 1 and 2, complex 3 revealed lower activity, with maximum yields of 85% (Table 1, entry 13). This difference in activity can be attributed to the presence of Zn²⁺ centers with unsaturated coordination sites in complexes 1 and 2^{[13, 17-18]}. Blank tests showed that the condensation reaction was much less pronounced when using H₂cada (26% yield) or ZnCl₂ (31% yield) as catalysts (Table 1, entries 14-16). In the absence of a catalyst, the yield dropped to 20%.

  Table 1

  

  Table 1.  Catalytic Knoevenagel condensation reaction of benzaldehyde with malononitrile

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  Entry	Catalyst	Time/min	xcat/%	Solvent	Yield*/%
  1	1	10	2.0	CH3OH	51
  2	1	20	2.0	CH3OH	71
  3	1	30	2.0	CH3OH	82
  4	1	40	2.0	CH3OH	91
  5	1	50	2.0	CH3OH	96
  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	85
  10	1	60	2.0	CHCl3	67
  11	1	60	1.0	CH3OH	94
  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	H2cada	60	2.0	CH3OH	26
  * The yield (Y) was calculated by 1H NMR spectroscopy: Y=nproduct/naldehyde×100%.

  Several substituted benzaldehyde substrates were tested to explore the substrate scope in the condensation reaction with malononitrile. These studies were performed under optimized conditions (1 h of reaction time, 25 ℃, 2.0% of catalyst 1, methanol as solvent). The corresponding products were obtained in yields ranging from 45% to 100% (Table 2). Benzaldehyde substrates comprising strong electron-withdrawing groups, such as nitro, fluoro, chloro, and bromo, revealed the highest efficiency, likely due to the increased electrophilicity of the substrates (Table 2, entries 5-7). In contrast, benzaldehyde substrates with electron-donating groups, such as methyl or methoxy, resulted in lower yields (Table 2, entries 9 and 10).

  Table 2

  

  Table 2.  Knoevenagel condensation reaction of various aldehydes with malononitrile catalyzed by complex 1^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-F	100
  6	R=4-Cl	100
  7	R=4-Br	100
  8	R=4-OH	45
  9	R=4-CH3	95
  10	R=4-OCH3	71
  a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), complex 1 (2.0%), and CH3OH (1.0 mL) at 25 ℃ for 1 h; b Calculated by 1H NMR spectroscopy.

  In addition, the recyclability of complex 1 was investigated by performing several reaction cycles with the same sample of the catalyst. After each cycle of the condensation reaction between benzaldehyde and malononitrile, the catalyst was recovered by centrifugation, washed with methanol, air-dried at 25 ℃, and then used again. The obtained results showed that complex 1 maintained its original activity for at least five cycles, leading to product yields of 100%, 99%, 97%, and 95% in the second through fifth runs, respectively. Furthermore, PXRD patterns of parent complex 1 and reused catalyst confirmed that its structure is preserved (Fig.S2), although some new signals were observed along with peak broadening. These changes, which are expected in the catalyst recycling experiments, likely result from impurities or a decrease in crystallinity. The catalytic performance of complexes 1 and 2 in this type of model condensation reactions is comparable to or even superior to those of other heterogeneous catalysts based on metal-carboxylate CPs (Table S4)^[23-27].

  3.   Conclusions

  In the present research study, we applied a hydrothermal method to assemble three new coordination polymers, using a still little explored bis(4-carboxyphenyl)urea (H₂cada) as the primary linker. The obtained complexes were fully characterized, and their crystal structures were determined by single-crystal X-ray diffraction. The products reveal different types of 1D chain networks with distinct topologies. The diversity in coordination modes of the cada^2- linkers is likely influenced by the nature of the metal(Ⅱ) centers and the N-donor auxiliary ligands. In addition, all the obtained products were screened as heterogeneous catalysts in the model condensation reaction between benzaldehyde and malononitrile. Remarkably, the Zn-based complexes 1 and 2 showed high catalytic activity and reusability, leading to almost quantitative product yield under optimized conditions.

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

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

  H atoms are omitted for clarity; Symmetry code: A: x+1/2, -y+3/2, z+3/2.

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  Scheme 1  Coordination modes of cada^2- ligands

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  Figure 2  One-dimensional metal-organic chain of 1 viewed along the a-axis

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  Figure 3  Topological representation of a chain of 1 with 2C1 topology viewed along the a-axis

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  Figure 4  Two-dimensional supramolecular network of 1 viewed along the c-axis

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  Figure 5  Drawing of the asymmetric unit of complex 2 with 30% probability thermal ellipsoids

  H atoms and lattice water molecules are omitted for clarity; Symmetry codes: A: -x+3/2, -y+3/2, -z+3/2; B: x, -y+3/2, z-1/2.

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  Figure 6  One-dimensional metal-organic chain of 2 viewed along the b-axis

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  Figure 7  Topological representation of a chain of 2 with (4, 4)(0, 2) topology viewed along the b-axis

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  Figure 8  Two-dimensional supramolecular network of 2 viewed along the b-axis

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  Figure 9  Drawing of the asymmetric unit of complex 3 with 30% probability thermal ellipsoids

  H atoms are omitted for clarity; Symmetry codes: A:-x+2, y, -z+3/2; B:-x+1, -y+1, -z+1.

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  Figure 10  One-dimensional metal-organic chain of 3 viewed along the c-axis

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  Figure 11  Topological representation of a chain of 3 with a fes topology viewed along the c-axis

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  Figure 12  Two-dimensional supramolecular network of 3 viewed along the a-axis

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  Figure 13  TGA curves of complexes 1-3

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  Scheme 2  Catalytic Knoevenagel condensation reaction of benzaldehyde with malononitrile (model substrate)

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  Table 1.  Catalytic Knoevenagel condensation reaction of benzaldehyde with malononitrile

  Entry	Catalyst	Time/min	xcat/%	Solvent	Yield*/%
  1	1	10	2.0	CH3OH	51
  2	1	20	2.0	CH3OH	71
  3	1	30	2.0	CH3OH	82
  4	1	40	2.0	CH3OH	91
  5	1	50	2.0	CH3OH	96
  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	85
  10	1	60	2.0	CHCl3	67
  11	1	60	1.0	CH3OH	94
  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	H2cada	60	2.0	CH3OH	26
  * 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 complex 1^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-F	100
  6	R=4-Cl	100
  7	R=4-Br	100
  8	R=4-OH	45
  9	R=4-CH3	95
  10	R=4-OCH3	71
  a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), complex 1 (2.0%), and CH3OH (1.0 mL) at 25 ℃ for 1 h; b Calculated by 1H NMR spectroscopy.

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