English

• 0.   Introduction

  Nowadays, the design and synthesis of transition metal coordination polymers (CPs) showing adjustable characteristics and adaptable architectures have become a focal point in contemporary coordination chemistry, attributed to their wide-ranging attributes and promising applications^[1-5]. Such materials are capable of multifunctional applications and hold considerable promise across numerous domains, such as gas adsorption^[6-11], catalyst^[12-19], lighting detection^[20-23], including applications in the biomedical field^[24]. Their straightforward synthesis, adjustable structure, and notable resilience in chemical reactions have positioned them as a crucial focus within the field of materials research.

  The study of CPs has attracted considerable attention for their remarkable capabilities in the field of catalysis, including adjustable pore structures, high stability, choice, catalytic activity, and easy recovery, rendering them highly effective for catalysis in heterogeneous systems^{[9,12-19,25-27]}. The Knoevenagel condensation, a prominent reaction in the realm of organic chemistry, highlights the significant role of CPs, as demonstrated in studies^[25-27], which serves as a key and commonly employed process for forming carbon-carbon (C—C) connections, crucial components in the synthesis of a variety of organic substances, medications, and advanced materials^[28-29]. In addition, the process of Knoevenagel condensation usually takes place with the assistance of catalytic agents, which may include either Lewis acids or bases, which enhance reactant activation and promote more effective bond formation^[30-31]. So catalytic polymers have gained recognition as effective agents for this process, unlocking new opportunities owing to their varied structural designs and reactive sites, which provide superior benefits in terms of their ability to be reused compared to traditional catalysts, reusable, and under gentle operational settings.

  In the context of our continuing research on integrating commercially obtainable carboxylic acids as connectors for the purpose of crafting functional CPs^{[13,25-26,32-33]}, this study explores the viability of utilizing H₄dbda (4,4′-dihydroxy-[1,1′-biphenyl]-3,3′-dicarboxylic acid). The use of the dicarboxylic acid as a biphenyl-dicarboxylate bridge for the formation of innovative CPs is being evaluated. With this consideration, a total of three complexes were synthesized and evaluated, employing various metal(Ⅱ) salts, H₄dbda, and various auxiliary ligands. These materials display a variety of structural characteristics, a combination of 0D dimer (1) and 2D metal-organic networks (2 and 3). The obtained complexes include [Cu₂(μ-H₂dbda)₂(phen)₂]·2H₂O (1), [Ni(μ-H₂dbda)(μ-bpb)(H₂O)₂]_n (2), and [Cd(μ-H₂dbda)(μ-bpa)]_n (3), where phen=1,10-phenanthroline, bpb=1,4-bis(pyrid-4-yl)benzene, and bpa=bis(4-pyridyl)amine. In addition, the catalytic capabilities of these complexes were examined in relation to the Knoevenagel reaction, showing their capability to act as proficient catalysts in organic transformations.

  1.   Experimental

  1.1   Reagents and physical measurements

  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 spectra were 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 complex 1

  A mixture of CuCl₂·2H₂O (0.034 g, 0.20 mmol), H₄dbda (0.055 g, 0.20 mmol), phen (0.040 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. Green needle-shaped crystals of 1 were isolated manually and washed with distilled water. Yield: 47% (based on H₄dbda). Anal. Calcd. for C₂₆H₁₈CuN₂O₇(%): C 58.48, H 3.40, N 5.25; Found(%): C 58.26, H 3.42, N 5.28. IR (KBr, cm^-1): 3 429m, 3 060w, 1 613s, 1 559m, 1 517w, 1 459s, 1 428w, 1 406w, 1 344w, 1 309w, 1 240w, 1 144w, 1 106w, 1 048w, 932w, 886w, 847w, 774w, 721m, 698w, 636w, 594w.

  1.3   Synthesis of complex 2

  A mixture of NiCl₂·6H₂O (0.048 g, 0.20 mmol), H₄dbda (0.055 g, 0.20 mmol), bpb (0.046 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 160 ℃ for 3 d, followed by cooling to room temperature at a rate of 10 ℃·h^-1. Green block-shaped crystals of 2 were isolated manually, washed with distilled water, and dried. Yield: 43% (based on H₄dbda). Anal. Calcd. for C₃₀H₂₄NiN₂O₈(%): C, 60.13; H, 4.04; N, 4.68. Found(%): C, 60.43; H, 4.02; N, 4.71. IR (KBr, cm^-1): 3 433w, 3 105w, 1 620m, 1 562s, 1 474m, 1 400s, 1 355w, 1 284w, 1 246w, 1 155w, 1 068w, 948w, 877m, 823m, 728w, 690w, 644w, 558w.

  1.4   Synthesis of complex 3

  A mixture of CdCl₂·H₂O (0.040 g, 0.2 mmol), H₄dbda (0.055 g g, 0.20 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 160 ℃ for 3 d, followed by cooling to room temperature at a rate of 10 ℃·h^-1. Colourless block-shaped crystals of 3 were isolated manually and washed with distilled water. Yield: 45% (based on H₄dbda). Anal. Calcd. for C₂₄H₁₇CdN₃O₆(%): C 51.86, H 3.08, N 7.56; Found(%): C 52.17, H 3.10, N 7.53. IR (KBr, cm^-1): 1 628m, 1 594s, 1 556m, 1 513m, 1 471m, 1 421s, 1 333w, 1 286w, 1 248w, 1 210m, 1 159w, 1 098w, 1 059w, 1 017m, 957w, 902w, 871w, 816m, 728w, 690m, 644w, 590w, 559m.

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

  1.5   Structure determinations

  The data of three single crystals with dimensions of 0.04 mm×0.03 mm×0.02 mm (1), 0.05 mm×0.03 mm×0.02 mm (2), and 0.07 mm×0.04 mm×0.03 mm (3) were collected at 150(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 in 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. 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.

  2.   Results and discussion

  2.1   Description of the structure

  2.1.1   Crystal structure of complex 1

  The structure of complex 1 reveals a dimeric complex that contains one Cu(Ⅱ) ion, one μ-H₂dbda^2- linker, one phen ligand, and one lattice water molecule in the asymmetric unit (Fig.1). The five-coordinated Cu1 center displays a distorted {CuN₂O₃} quadrangular pyramidal environment that is constructed from three oxygen atoms of two μ-H₂dbda^2- linkers and a pair of N atoms of the second ligand phen. The distances of Cu—N [0.200 2(18)-0.200 9(2) nm] and Cu—O [0.188 5(2)-0.244 4(2) nm] bonds agree with typical literature data^[26,32]. Two H₂dbda^2- ligands act as μ-H₂dbda^2- linkers via monodentate carboxylate groups (Scheme 1, mode Ⅰ), which assemble two Cu1 centers to give a cyclic Cu₂ complex with a Cu…Cu separation of 1.229 6(4) nm (Fig.2). Such Cu₂ molecular units are involved in an intermolecular H-bonding forming a 2D network (Fig.3).

  Figure 1

  

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

  Symmetry code: A: -x+1, -y+1, -z+1.

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  Scheme 1

  

  Scheme 1.  Coordination modes of H₂dbda^2- ligands in complexes 1-3

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  Figure 2

  

  Figure 2.  Cu₂ dimer structure

  Symmetry code: A: -x+1, -y+1, -z+1.

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  Figure 3

  

  Figure 3.  Two-dimensional H-bonded layer viewed along the a-axis

  Phen ligands are omitted for clarity.

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  2.1.2   Crystal structure of complex 2

  The asymmetric unit of 2 possesses one crystallo-graphically independent Cu(Ⅱ) ion with 50% occupancy, half a μ-H₂dbda^2- block, half a μ-bpb second ligand, and a H₂O ligand (Fig.4). As shown in Fig.4, the Ni1 ion is six-coordinated and adopts a distorted octahedral {NiN₂O₄} geometry formed by two carboxylate O atoms from two different μ-H₂dbda^2- blocks, two O atoms from two H₂O ligands and two N atoms from two distinct μ-bpb second ligands. The Ni—O distances are 0.206 5(2)-0.207 6(2) nm, whereas the Ni—N distances are 0.209 2(2)-0.210 0(2) nm; these bonding parameters are agree with those observed in other Ni(Ⅱ) complexes^[25,33]. In 2, the H₂dbda^2- block acts as a μ-spacer (Scheme 1, mode Ⅱ), in which the carboxylate groups exhibit the bidentate modes. The bpb auxiliary ligand adopts a bridging coordination mode. The neighboring Ni(Ⅱ) centers are linked by μ-H₂dbda^2- and μ-bpb blocks, giving rise to a 2D framework (Fig.5). This complex discloses a 2D structure composed of the 4-linked Ni1 nodes, 2-linked H₂dbda^2- nodes, and 2-connected bpb linkers with a sql topology and a vertex symbol of (4⁴.6²) (Fig.6).

  Figure 4

  

  Figure 4.  Drawing of the asymmetric unit of complex 2 with 30% probability thermal ellipsoids

  Symmetry codes: A: -x+1/2, y, -z+3/4; B: x, y+1, -z; C: -x+1, -y+2, z; D: -x+1/2, y, -z+3/4.

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  Figure 5

  

  Figure 5.  Two-dimensional metal-organic network viewed along the c-axis

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  Figure 6

  

  Figure 6.  Topological representation of a 4,2,2-connected network with a sql topology viewed along the c-axis

  Green: 4-connected Ni1 nodes, gray: centroids of 2-connected μ-H₂dbda^2- nodes, dark blue: centroids of 2-connected μ-bpb linkers.

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  2.1.3   Crystal structure of complex 3

  The structure of 2D CP 3 bears one cadmium(Ⅱ) ion, one μ-H₂dbda^2- block, and a bpa auxiliary ligand within the asymmetric unit. Each Cd(Ⅱ) ion is six-coordinated in a deformed {CdN₂O₄} octahedral geometry, which is taken by four O atoms from two μ-H₂dbda^2- blocks and two N atoms from two distinct bpa auxiliary ligands (Fig.7). The H₂dbda^2- block acts as a μ-linker (Scheme 1, mode Ⅲ) through its bidentate carboxylate groups. A 2D sheet is formed by bridging cadmium centers through the μ-H₂dbda^2- and μ-bpa linkers (Fig.8). This 2D network is composed of the 4-linked Cd1 nodes, 2-linked μ-H₂dbda^2- nodes, and 2-connected μ-bpa linkers (Fig.9). It is a sql topology with a vertex symbol of (4⁴.6²) (Fig.9).

  Figure 7

  

  Figure 7.  Drawing of the asymmetric unit of complex 3 with 30% probability thermal ellipsoids

  Symmetry codes: A: -x+1/2, -y+2, z-1/2; B: x, y+1, z.

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  Figure 8

  

  Figure 8.  Two-dimensional metal-organic network viewed along the a-axis

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  Figure 9

  

  Figure 9.  Topological representation of a 4,2,2-connected network with a sql topology viewed along the a-axis

  Turquoise: 4-connected Cd1 nodes, gray: centroids of 2-connected μ-H₂dbda^2- nodes, dark blue: centroids of 2-connected μ-bpa linkers

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  2.2   Thermal stability of the complexes

  To determine the thermal stability of complexes 1-3, their thermal behaviors were investigated under a nitrogen atmosphere by TGA. As shown in Fig.10, complex 1 lost its two lattice water molecules in a range of 79-107 ℃ (Obsd. 3.2%, Calcd. 3.4%), followed by the decomposition at 208 ℃. For complex 2, there was a release of two H₂O ligands between 69 and 108 ℃ (Obsd. 5.9%, Calcd. 6.0%), whereas a dehydrated sample remained stable up to 351 ℃. Complex 3 does not contain any coordinated or crystallization water, and its framework shows stability on heating until 324 ℃.

  Figure 10

  

  Figure 10.  TGA curves of complexes 1-3

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  2.3   Catalytic Knoevenagel reaction

  Earlier investigations have highlighted the effective use of CPs as catalysts in facilitating Knoevenagel reaction processes^{[13,18,27,33]}. In our current research, we examined the potential of complexes 1-3 to act as heterogeneous catalysts in reactions involving a range of aldehydes and propanedinitrile, carried out in methanol at room temperature, employing benzaldehyde as a reference substrate. The synthesis of 2-benzylidenemalononitrile was conducted (Scheme 2). In addition, the influence of various reaction conditions was examined, including factors like the amount of catalyst used and its ability to be reused, reaction time, solvent based, and substrate scope.

  Scheme 2

  

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

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  The activity observed with complex 1 was the most pronounced, facilitating the conversion of benzaldehyde to 2-benzylidenemalononitrile with > 99% conversion (Table 1 and Fig. S1). We conducted experiments using complex 1 to assess how varying the reaction parameters impacted the outcomes. By increasing the reaction duration from 10 to 60 min, the yield had a remarkable rise from 51% to more than 99% (Table 1, entries 1-6, Fig. S2). In addition, the quantity of catalyst was assessed for its impact, revealing that increasing catalyst loading (molar fraction) from 1% to 2% enhanced the product yield from 96% to > 99% (Table 1, entries 6 and 11). Additionally, various solvents were evaluated in addition to methanol, including water, ethanol, acetonitrile, and chloroform were found to be less effective, ranging from 65% to 98% in terms of product yields (Table 1, entries 7-10). Nevertheless, unlike complex 1, the activities of complexes 2 and 3 are comparatively diminished, yielding maximum product percentages between 81% and 84% (Table 1, entries 12 and 13). This could be linked to the inclusion of Cu²⁺ within complex 1, which possesses unsaturated coordination sites^[13,28]. The efficiency of the Knoevenagel reaction involving benzaldehyde was significantly diminished when using H₄dbda as the catalyst, resulting in a yield of only 28%, while CuCl₂ provided a slightly better yield of 32% or in the absence of any catalyst, resulting in a mere 21% yield of the product (Table 1, entries 14-16).

  Table 1

  

  Table 1.  Catalytic Knoevenagel condensation of benzaldehyde with malononitrile

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  Entry	Catalyst	t / min	T / ℃	xcat / %	Solvent	Yield* / %
  1	1	10	25	2.0	CH3OH	51
  2	1	20	25	2.0	CH3OH	69
  3	1	30	25	2.0	CH3OH	81
  4	1	40	25	2.0	CH3OH	91
  5	1	50	25	2.0	CH3OH	96
  6	1	60	25	2.0	CH3OH	> 99
  7	1	60	25	2.0	H2O	98
  8	1	60	25	2.0	C2H5OH	95
  9	1	60	25	2.0	CH3CN	87
  10	1	60	25	2.0	CHCl3	65
  11	1	60	25	1.0	CH3OH	96
  12	2	60	25	2.0	CH3OH	84
  13	3	60	25	2.0	CH3OH	81
  14	Blank	60	25		CH3OH	21
  15	CuCl2	60	25	2.0	CH3OH	32
  16	H4dbda	60	25	2.0	CH3OH	28
  * The yield (Y) was calculated by 1H NMR spectroscopy: Y=nproduct/naldehyde×100%.

  In order to assess the compatibility of various substrates in the Knoevenagel condensation involving malononitrile, a range of benzaldehyde derivatives underwent testing under refined conditions utilizing complex 1 as a catalyst (2.0% in methanol at 25 ℃ for 1 h). The observed yields for the products showed considerable variation, spanning from 37% to more than 99% (Table 2). Those benzaldehydes featuring prominent electron-withdrawing groups, including substituents like nitro or halogens in the aromatic ring, demonstrated the greatest reactivity due to their enhanced electrophilic nature (Table 2, entries 2-7). Conversely, the presence of electron-donating substitutions such as methyl or methoxy led to lower product yields (Table 2, entries 10 and 11). The performance of complex 1 was assessed by conducting several cycles of reactions. Following each cycle, the catalyst was isolated via centrifugation, washed with methanol, and subsequently dried at room temperature in the air before reuse. The findings indicated that complex 1 provided remarkable catalytic efficacy for a minimum of five applications, delivering high yields of > 99% in the second cycle and sustaining high efficiency with 99%, 98%, and 96% in the third, fourth, and fifth cycles (Fig. S3). Additionally, the analysis of PXRD demonstrated that the structural composition of complex 1 remained intact (Fig. S4), while the emergence of some additional diffraction peaks and a slight increase in peak width were noted; these observations are typical following several catalytic cycles and could stem from the presence of impurities or slight alterations in the crystal structure. Despite these changes, complex 1 demonstrated catalytic activity comparable to, or even better than, other similar metal-carboxylate-based heterogeneous catalysts (Table S4)^[30,34-37].

  Table 2

  

  Table 2.  Knoevenagel condensation of various aldehydes with malononitrile catalyzed by complex 1^a

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  Entry	Substituted benzaldehyde substrate	Yieldb / %
  1	Benzaldehyde	> 99
  2	2-Nitrobenzaldehyde	> 99
  3	3-Nitrobenzaldehyde	> 99
  4	4-Nitrobenzaldehyde	> 99
  5	4-Chlorobenzaldehyde	> 99
  6	4-Bromobenzaldehyde	> 99
  7	4-Fluorobenzaldehyde	> 99
  8	Pyridine-3-aldehyde	> 99
  9	Pyridine-4-aldehyde	> 99
  10	4-Methylbenzaldehyde	96
  11	4-Methoxybenzaldehyde	65
  12	4-Hydroxybenzaldehyde	37
  a Reaction conditions: aldehyde (0.5 mmol), propanedinitrile (1.0 mmol), catalyst 1 (2.0%), and CH3OH (1.0 mL) at 25 ℃; b Calculated by 1H NMR spectroscopy.

  3.   Conclusions

  In summary, we have synthesized three Cu(Ⅱ)/Ni(Ⅱ)/Cd(Ⅱ) complexes based on a biphenyl-dicarboxylate ligand. The structural types of complexes 1-3 range from a molecular dimer (1) to 2D metal-organic networks (2 and 3). The catalytic performances of these complexes were investigated. Complex 1 revealed an effective catalytic activity in the Knoevenagel condensation reaction at room temperature.

  
  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

  Symmetry code: A: -x+1, -y+1, -z+1.

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  Scheme 1  Coordination modes of H₂dbda^2- ligands in complexes 1-3

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  Figure 2  Cu₂ dimer structure

  Symmetry code: A: -x+1, -y+1, -z+1.

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  Figure 3  Two-dimensional H-bonded layer viewed along the a-axis

  Phen ligands are omitted for clarity.

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  Figure 4  Drawing of the asymmetric unit of complex 2 with 30% probability thermal ellipsoids

  Symmetry codes: A: -x+1/2, y, -z+3/4; B: x, y+1, -z; C: -x+1, -y+2, z; D: -x+1/2, y, -z+3/4.

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  Figure 5  Two-dimensional metal-organic network viewed along the c-axis

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  Figure 6  Topological representation of a 4,2,2-connected network with a sql topology viewed along the c-axis

  Green: 4-connected Ni1 nodes, gray: centroids of 2-connected μ-H₂dbda^2- nodes, dark blue: centroids of 2-connected μ-bpb linkers.

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  Figure 7  Drawing of the asymmetric unit of complex 3 with 30% probability thermal ellipsoids

  Symmetry codes: A: -x+1/2, -y+2, z-1/2; B: x, y+1, z.

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  Figure 8  Two-dimensional metal-organic network viewed along the a-axis

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  Figure 9  Topological representation of a 4,2,2-connected network with a sql topology viewed along the a-axis

  Turquoise: 4-connected Cd1 nodes, gray: centroids of 2-connected μ-H₂dbda^2- nodes, dark blue: centroids of 2-connected μ-bpa linkers

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  Figure 10  TGA curves of complexes 1-3

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

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  Table 1.  Catalytic Knoevenagel condensation of benzaldehyde with malononitrile

  Entry	Catalyst	t / min	T / ℃	xcat / %	Solvent	Yield* / %
  1	1	10	25	2.0	CH3OH	51
  2	1	20	25	2.0	CH3OH	69
  3	1	30	25	2.0	CH3OH	81
  4	1	40	25	2.0	CH3OH	91
  5	1	50	25	2.0	CH3OH	96
  6	1	60	25	2.0	CH3OH	> 99
  7	1	60	25	2.0	H2O	98
  8	1	60	25	2.0	C2H5OH	95
  9	1	60	25	2.0	CH3CN	87
  10	1	60	25	2.0	CHCl3	65
  11	1	60	25	1.0	CH3OH	96
  12	2	60	25	2.0	CH3OH	84
  13	3	60	25	2.0	CH3OH	81
  14	Blank	60	25		CH3OH	21
  15	CuCl2	60	25	2.0	CH3OH	32
  16	H4dbda	60	25	2.0	CH3OH	28
  * The yield (Y) was calculated by 1H NMR spectroscopy: Y=nproduct/naldehyde×100%.

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  Table 2.  Knoevenagel condensation of various aldehydes with malononitrile catalyzed by complex 1^a

  Entry	Substituted benzaldehyde substrate	Yieldb / %
  1	Benzaldehyde	> 99
  2	2-Nitrobenzaldehyde	> 99
  3	3-Nitrobenzaldehyde	> 99
  4	4-Nitrobenzaldehyde	> 99
  5	4-Chlorobenzaldehyde	> 99
  6	4-Bromobenzaldehyde	> 99
  7	4-Fluorobenzaldehyde	> 99
  8	Pyridine-3-aldehyde	> 99
  9	Pyridine-4-aldehyde	> 99
  10	4-Methylbenzaldehyde	96
  11	4-Methoxybenzaldehyde	65
  12	4-Hydroxybenzaldehyde	37
  a Reaction conditions: aldehyde (0.5 mmol), propanedinitrile (1.0 mmol), catalyst 1 (2.0%), and CH3OH (1.0 mL) at 25 ℃; b Calculated by 1H NMR spectroscopy.

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