English

• 0.   Introduction

  The field of coordination polymers has attracted tremendous attention due to their structural and topological diversity as well as their potential applications as functional materials^[1-11]. In the last ten years, organic carboxylate ligands have been widely used in synthesizing coordination polymers due to the strong coordination ability of the carboxyl group and the rich coordination modes^{[5-6, 12-15]}. Among them, ether-bridged carboxylic acids have been extensively applied as versatile building blocks toward the assembly of metal - organic architectures^[16-17].

  The 2, 3, 3', 4'-diphenyl ether tetracarboxylic acid (H₄deta) is a good bridging ligand for constructing coordination polymers^[18], under considering structural semirigidity, which has multiple coordinate sites involving four carboxylate oxygen atoms and one O - ether donor. Knoevenagel condensation is one of the imperative and essential condensation processes in synthetic organic chemistry, in which α, β - unsaturated products formed via carbon - carbon double bond involve a nucleophilic addition reaction between active methylene and carbonyl compounds followed by a dehydration reaction^[19-23]. Products obtained are extensively used as specialty chemicals and intermediates in the synthesis of fine chemicals such as carbocyclic, substituted alkenes, biologically active compounds, therapeutic drugs, calcium antagonists, natural products, functional polymers, coumarin derivatives, flavors, and perfumes. Transition metal -catalyzed Knoevenagel condensation reactions have recently received much attention^[24-26], mainly due to the low price and moderate toxicity of the catalysts in combination with their high activity.

  Herein, we report the synthesis, crystal structures, and catalytic activity of two Co(Ⅱ) coordination polymers with H₄deta, 2, 2'-biimidazole (H₂biim) and 1, 10- phenanthroline (phen) ligands.

  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 dif - fraction patterns (PXRD) 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(μ₃-deta)(H₂biim)₃(H₂O)₂]_n (1)

  A mixture of CoCl₂·6H₂O (0.048 g, 0.2 mmol), H₄deta (0.035 g, 0.1 mmol), H₂biim (0.027 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. Orange blockshaped crystals were isolated manually, and washed with distilled water. Yield: 35% (based on H₄deta). Anal. Calcd. for C₃₄H₂₈Co₂N₁₂O₁₁(%): C 45.45, H 3.14, N 18.71; Found(%): C 45.62, H 3.12, N 18.59. IR (KBr, cm^-1): 3 384m, 2 972w, 1 628w, 1 549s, 1 483m, 1 430m, 1 398s, 1 377s, 1 341m, 1 271w, 1 231w, 1 182w, 1 120w, 1 084w, 1 049w, 991w, 907w, 867w, 828w, 756w, 694m, 624w.

  1.3   Synthesis of {[Co₂(μ₆-deta)(phen)₂]·H₂O}_n (2)

  Synthesis of 2 was similar to 1 except using phen (0.040 g, 0.2 mmol) instead of H₂biim. Purple block - shaped crystals of 2 were isolated manually, and washed with distilled water. Yield: 60% (based on H₄deta). Anal. Calcd. for C₄₀H₂₄Co₂N₄O₁₀(%): C 57.30, H 2.88, N 6.68; Found(%): C 57.12, H 2.86, N 6.70. IR (KBr, cm^-1): 3 660w, 3 428w, 3 066w, 1 626s, 1 581s, 1 514m, 1 492w, 1 425m, 1 391s, 1 377s, 1 302w, 1 262w, 1 235w, 1 151w, 1 089w, 969w, 924w, 894w, 840m, 818w, 770m, 725m, 685w, 641w.

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

  1.4   Structure determination

  Two single crystals with dimensions of 0.25 mm× 0.20 mm×0.18 mm (1) and 0.23 mm×0.22 mm×0.20 mm (2) were collected at 293(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^[27]. All non - hydrogen atoms were refined anisotropically. All the hydrogen atoms were positioned geometrically and refined using a riding model. A summary of the crystallography data and structure refinements for 1 and 2 is given in Table 1. The selected bond lengths and angles for compounds 1 and 2 are listed in Table 2. Hydrogen bond parameters of compounds 1 and 2 are given in Table 3 and 4.

  Table 1

  

  Table 1.  Crystal data for compounds 1 and 2

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  Parameter	1	2
  Chemical formula	C34H28Co2N12O11	C40H24Co2N4O10
  Molecular weight	898.54	838.49
  Crystal system	Triclinic	Monoclinic
  Space group	P1	P21/c
  a / nm	0.740 11(6)	1.107 14(10)
  b / nm	1.022 55(14)	1.586 84(10)
  c / nm	2.421 49(13)	1.997 32(15)
  α/(°)	100.288(8)
  β/(°)	91.398(6)	105.398(9)
  γ/(°)	100.304(9)
  V / nm3	1.771(3)	3.383(5)
  Z	2	4
  F(000)	916	1 704
  θ range for data collection / (°)	3.326-25.048	3.327-25.049
  Limiting indices	-8 ≤ h ≤ 8, -11 ≤ k ≤ 12, -28 ≤ l ≤ 21	-13 ≤ h ≤ 8, -18 ≤ k ≤ 18, -21 ≤ l ≤ 23
  Reflection collected, unique (Rint)	6 257, 3 869 (0.062 9)	5 979, 3 420 (0.105 7)
  Dc / (g·cm-3)	1.685	1.646
  μ / mm-1	1.019	1.052
  Data, restraint, parameter	6 257, 1, 556	5 979, 1, 505
  Goodness-of-fit on F2	1.041	1.023
  Final R indices [I≥2σ(I)] R1, wR2	0.064 9, 0.129 2	0.059 5, 0.092 6
  R indices (all data) R1, wR2	0.113 2, 0.169 2	0.118 6, 0.117 4
  Largest diff. peak and hole / (e·nm-3)	583 and -846	577 and -537

  Table 2

  

  Table 2.  Selected bond distances (nm) and bond angles (°) for compounds 1 and 2

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  1
  Co1—O1	0.218 4(4)	Co1—O4A	0.234 5(5)	Co1—N1	0.214 9(4)
  Co1—N4	0.207 5(4)	Co1—N5	0.215 9(4)	Co1—N8	0.210 2(4)
  Co2—O6	0.214 7(4)	Co2—O7	0.219 7(4)	Co2—O10	0.201 9(4)
  Co2—O11	0.218 0(4)	Co2—N9	0.208 2(5)	Co2—N12	0.218 1(5)
  N4—Co1—N8	178.6(2)	N1—Co1—N4	79.30(16)	N8—Co1—N1	101.12(16)
  N4—Co1—N5	100.74(17)	N5—Co1—N8	78.77(17)	N1—Co1—N5	177.4(2)
  O1—Co1—N4	94.65(17)	N8—Co1—O1	86.67(17)	N1—Co1—O1	90.06(17)
  N5—Co1—O1	92.56(17)	N4—Co1—O4A	87.12(17)	N8—Co1—O4A	91.55(17)
  N1—Co1—O4A	89.88(17)	N5—Co1—O4A	87.50(17)	O1—Co1—O4A	178.18(14)
  O10—Co2—N9	104.93(19)	O10—Co2—O6	154.36(18)	O6—Co2—N9	100.53(17)
  O11—Co2—O10	87.77(15)	O11—Co2—N9	88.52(18)	O11—Co2—O6	89.95(15)
  O10—Co2—N12	98.07(16)	N9—Co2—N12	79.5(2)	O6—Co2—N12	89.37(16)
  O11—Co2—N12	167.7(2)	O10—Co2—O7	93.96(16)	O7—Co2—N9	160.77(16)
  O6—Co2—O7	60.89(15)	O11—Co2—O7	95.92(17)	O7—Co2—N12	94.47(18)
  2
  Co1—O1	0.209 0(4)	Co1—O4A	0.209 0(3)	Co1—O7B	0.209 4(4)
  Co1—O8B	0.210 6(3)	Co1—N1	0.213 3(4)	Co1—N2	0.215 8(4)
  Co2—O2	0.202 4(3)	Co2—O3A	0.202 7(3)	Co2—O8B	0.206 3(4)
  Co2—O9B	0.239 0(4)	Co2—N3	0.208 8(5)	Co2—N4	0.214 8(4)
  O1—Co1—O4A	90.88(15)	O4A—Co1—O7B	100.25(14)	O1—Co1—O7B	166.12(14)
  O8B—Co1—O4A	88.86(13)	O1—Co1—O8B	90.97(13)	O7B—Co1—O8B	81.08(13)
  O4A—Co1—N1	90.22(16)	O1—Co1—N1	105.00(15)	O7B—Co1—N1	83.39(16)
  O8B—Co1—N1	164.02(15)	O4A—Co1—N2	164.18(15)	O1—Co1—N2	83.49(16)
  O7B—Co1—N2	87.76(15)	O8B—Co1—N2	105.96(14)	N1—Co1—N2	77.07(16)
  O2—Co2—O3A	96.20(14)	O2—Co2—O8B	90.71(14)	O8B—Co2—O3A	100.16(15)
  O2—Co2—N3	95.43(16)	N3—Co2—O3A	100.80(16)	N3—Co2—O8B	157.36(15)
  N4—Co2—O2	170.27(15)	N4—Co2—O3A	92.39(15)	N4—Co2—O8B	92.28(16)
  N3—Co2—N4	78.41(18)	O2—Co2—O9B	89.72(13)	O3A—Co2—O9B	158.05(15)
  O9B—Co2—O8B	58.52(12)	N3—Co2—O9B	99.65(15)	N4—Co2—O9B	83.96(14)
  Symmetry codes: A: x, y-1, z for 1; A: -x+1, y+1/2, -z+1/2; B: x+1, y, z for 2.

  Table 3

  

  Table 3.  Hydrogen bond parameters of compound 1

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  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  N2—H1…O2A	0.094 7	0.178 4	0.272 3	170.51
  N3—H2…O1A	0.081 9	0.199 5	0.279 1	163.95
  N6—H3…O3B	0.091 4	0.177 7	0.268 1	169.88
  N7—H5…O4B	0.087 3	0.188 8	0.275 0	169.01
  N10—H11…O9C	0.072 5	0.204 4	0.271 4	154.08
  N11—H12…O9C	0.085 1	0.191 5	0.274 3	163.89
  O10—H1W…O8D	0.082 0	0.179 0	0.261 0	177.84
  O10—H2W…O7E	0.085 0	0.190 6	0.275 6	179.75
  O11—H3W…O8D	0.085 5	0.193 2	0.278 5	175.56
  Symmetry codes: A: x-1, y, z; B: x+1, y-1, z; C: x-1, y-1, z; D: -x+2, -y+1, -z+1; E: -x+1, -y+1, -z+1.

  Table 4

  

  Table 4.  Hydrogen bond parameters of compound 2

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  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  O10—H1W…O6	0.086 3	0.206 1	0.280 7	144.38
  O10—H2W…N2A	0.085 0	0.264 9	0.349 9	179.65
  Symmetry code: A: -x+1, y+1/2, -z+1/2.

  CCDC: 2069400, 1; 2069401, 2.

  1.5   Catalytic activity for Knoevenagel condensation reaction of aldehydes

  Prior to the catalytic activity study, compound 1 was activated in a vacuum oven at 210 ℃ for 10 h. In a typical test, a suspension of an aromatic aldehyde (0.50 mmol, benzaldehyde as a model substrate), malononitrile (1.0 mmol), and catalyst (Molar fraction: 2%) in methanol (1.0 mL) was stirred at room temperature. 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 solid was dissolved in CDCl₃ and analyzed by ¹H NMR spectroscopy for quantification of products (Fig. S1, Supporting information). To perform the recycling experiment, the catalyst was isolated by centrifugation, washed with dichloromethane, 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

  X - ray crystallography analysis reveals that compound 1 crystallizes in the triclinic system space group P1. As shown in Fig. 1, the asymmetric unit of 1 bears two crystallographically unique Co(Ⅱ) ions (Co1 and Co2), one μ₃ - deta^4- block, three H₂biim moieties, and two H₂O ligands. The six-coordinated Co1 ion exhibits a distorted octahedral {CoN₄O₂} environment, which is occupied by two carboxylate O donors from two different μ₃ -deta^4- blocks and four N atoms from two H₂biim moieties. The Co2 center is also six - coordinated and forms a distorted octahedral {CoN₂O₄} geometry. It is completed by two carboxylate O atoms from one μ₃ - deta^4- block, two O donors from two H₂O ligands, and two N atoms from the H₂biim moiety. The Co—O and Co—N bond distances are 0.218 4(4) - 0.234 5(5) nm and 0.207 5(4) - 0.218 1(5) nm, respectively; these are within the normal ranges observed in related Co(Ⅱ) compounds^{[6, 28-29]}. In compound 1, the ligand deta^4- adopts a coordination mode Ⅰ (Scheme 1) with four COO^- groups being uncoordinated, monodentate or bidentate. In deta^4- ligand, a dihedral angle (between two aromatic rings) and a C—O_ether—C angle are 78.88° and 118. 26°, respectively. The μ₃ -deta^4- blocks connect Co ions to give a 1D chain (Fig. 2). Adjacent chains are assembled into a 2D supramolecular sheet through O— H…O and N—H…O hydrogen bonds (Table 3 and Fig. 3).

  Figure 1

  

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

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

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  Scheme 1

  

  Scheme 1.  Coordination modes of deta^4- ligand in compounds 1 and 2

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  Figure 2

  

  Figure 2.  Perspective of 1D metal-organic chain along a axis

  H₂biim and H₂O ligands are omitted for clarity; Symmetry codes: A: x, y-1, z; B: x, y+1, z

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  Figure 3

  

  Figure 3.  Perspective of 2D H-bonded network along a axis

  Dashed lines present the hydrogen bonds

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  2.1.2   Structure of compound 2

  The asymmetric unit of compound 2 contains two crystallographically unique Co(Ⅱ) ions (Co1 and Co2), one μ₆ -deta^4- block, two phen moieties, and one lattice water molecule. As depicted in Fig. 4, both Co centers are six - coordinated and display a distorted octahedral {CoN₂ O₄} geometry. It is taken by four carboxylate O atoms from three individual μ₆ -deta^4- blocks and two N donors from the ligand phen. The bond lengths of Co—O are in a range of 0.202 4(3)-0.239 0(4) nm, while the Co—N bonds are 0.208 8(5) - 0.215 8(4) nm, being comparable to those found in some reported Co compounds^[28-30]. In 2, the deta^4- block acts as a μ₆ -linker (mode Ⅱ, Scheme 1), in which four carboxylate groups adopt monodentate, μ - bridging bidentate or tridentate modes. Besides, the μ₆ - deta^4- ligand is considerably bent showing a dihedral angle of 67.68° (between two aromatic rings) and the angle of C—O_ether—C being 121.04°. Two adjacent Co1 and Co2 ions are joined via three carboxylate groups from three independent μ₆ - deta^4- ligands to form a Co₂ subunit with a Co1…Co2 distance of 0.352 7(2) nm (Fig. 4). These di - cobalt subunits are further linked together through the remaining carboxylate groups of μ₆ -deta^4- blocks to form a 2D metal - organic network (Fig. 5). Compounds 1 and 2 were assembled under similar conditions except for the type of auxiliary ligand used (H₂biim for 1 and phen for 2), but they show different structures.

  Figure 4

  

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

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

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  Figure 5

  

  Figure 5.  View of 2D metal-organic network parallel to ab plane

  Phen ligands are omitted for clarity; Symmetry codes: A: x-1, y, z; B: -x+1, y-1/2, -z+1/2; C: -x+2, y-1/2, -z+1/2; D: x+1, y, z; E: -x+2, y+1/2, -z+1/2; F: -x+1, y+1/2, -z+1/2

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  2.2   TGA for compounds 1 and 2

  To determine the thermal stability of compounds 1 and 2, their thermal behaviors were investigated under nitrogen atmosphere by TGA. As shown in Fig. 6, compound 1 lost its two coordinated water molecules in a range of 96 - 203 ℃ (Obsd. 4.2%, Calcd. 4.0%). Decomposition of the sample occurred above 305 ℃, corresponding to the removal of deta^4- and H₂ biim ligands attached to the Co ions. For 2, one weight loss (Obsd. 2.2%, Calcd. 2.1%) in a range of 84 - 243 ℃ corresponds to the removal of one lattice water molecule; the remaining sample started to decompose above 342 ℃, corresponding to the removal of deta^4- and phen ligands attached to the Co ions.

  Figure 6

  

  Figure 6.  TGA curves of compounds 1 and 2

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  2.3   Catalytic activity for Knoevenagel condensation reaction

  Given the potential of cobalt(Ⅱ) coordination compounds to catalyze organic reactions^{[6, 30-31]}, we explored the application of 1 and 2 as heterogeneous catalysts in the Knoevenagel condensation reaction of benzaldehyde as a model substrate to give 2-(phenylmethylene)- propanedinitrile. Typical tests were carried out by reacting a mixture of benzaldehyde, malononitrile, and a Co(Ⅱ) complex catalyst in methanol at room temperature (Scheme 2, Table 5). Such effects as reaction time, catalyst loading, solvent composition, catalyst recycling, and finally substrate scope were investigated.

  Scheme 2

  

  Scheme 2.  Knoevenagel condensation reaction of benzaldehyde (model substrate) catalyzed by Co coordination compounds

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  Table 5

  

  Table 5.  Knoevenagel condensation reaction of benzaldehyde with malononitrile catalyzed by Co(Ⅱ) coordination compounds

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  Entry	Catalyst	Time / min	Catalyst loadinga / %	Solvent	Yieldb / %
  1	1	10	2.0	CH3OH	41
  2	1	20	2.0	CH3OH	56
  3	1	30	2.0	CH3OH	64
  4	1	40	2.0	CH3OH	80
  5	1	50	2.0	CH3OH	93
  6	1	60	2.0	CH3OH	100
  7	1	60	2.0	H2O	90
  8	1	60	2.0	C2H5OH	98
  9	1	60	2.0	CH3CN	85
  10	1	60	2.0	CHCl3	67
  11	1	60	1.0	CH3OH	92
  12	1	60	1.5	CH3OH	98
  13	1	55	2.5	CH3OH	100
  14	2	60	2.0	CH3OH	89
  15	Blank	60	—	CH3OH	23
  16	CoCl2·6H2O	60	2.0	CH3OH	33
  17	H4deta	60	2.0	CH3OH	30
  aMolar fraction; b Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

  Upon using compound 1 as the catalyst (Molar fraction: 2%), a high conversion rate of 100% of benzaldehyde into 2-(phenyl methylene)- propanedinitrile was reached after 60 min in methanol at room temperature (Table 5, Entry 6). The product was accumulated with a yield increase from 41% to 100% on prolonging the reaction from 10 to 60 min (Table 5, Entry 1 - 6). The influence of catalyst amount was also investigated, revealing a product yield growth from 92% to 100% on increasing the loading of catalyst from 1% to 2.5% (Table 5, Entry 6 and 11 - 13). In addition to water, other solvents were tested, in particular, the reaction showed a comparable efficiency in H₂O and ethanol (90% and 98% product yields, respectively). Acetonitrile and chloroform were less suitable (85% and 67% product yields, respectively). It should be highlighted that under similar reaction conditions, the Knoevenagel condensation of benzaldehyde was significantly less efficient in the absence of catalyst (only 23% product yield) or when using H₄deta (30% yield) or CoCl₂· 6H₂O (33% yield) as catalysts (Table 5, Entry 15-17).

  The results show that compound 1 is more active than compound 2. Although a relationship between structure and catalytic activity in the present study can not be clearly established, the highest conversation shown by compound 1 may eventually be associated with its 1D structure for easily accessible metal centers, together with the presence of the open metal sites^{[30, 32]}.

  We also compared the activities of catalyst 1 in the reactions of other substituted aromatic aldehydes with malononitrile, and the corresponding yields were in a range of 54% to 100% (Table 6). Aryl aldehydes bearing strong electron - withdrawing substituents (e. g., nitro and chloro) exhibited high activities (Table 6, Entry 2-5), which may be related to an increase in the electrophilicity of the substrate. Aldehydes containing electron - donating groups (e. g., methyl) showed lower reaction yields (Table 6, Entry 6-8), as expected.

  Table 6

  

  Table 6.  Knoevenagel condensation reaction of various aldehydes with malononitrile catalyzed by compound 1^a

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  Entry	Substituted benzaldehyde substrate (R—C6H4CHO)	Product yieldb / %
  1	R=H	100
  2	R=2-NO2	100
  3	R=3-NO2	100
  4	R=4-NO2	100
  5	R=4-Cl	100
  6	R=4-OH	54
  7	R=4-CH3	98
  8	R=4-OCH3	76
  a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), catalyst 1 (Molar fraction: 2.0%), and CH3OH (1.0 mL) at 25 ℃; b Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

  To examine the stability of 1 in the Knoevenagel condensation reaction, we tested the recyclability of this heterogeneous catalyst. For this purpose, upon completion of a reaction cycle, we separated the catalyst by centrifugation, washed it with CH₂Cl₂, and dried it at room temperature before its further use. We found that for catalyst 1, the catalytic system maintained the high activity over at least five consecutive cycles with the yields being 100%, 100%, 99%, and 98% for the second to the fifth run, respectively. According to the PXRD data (Fig.S2), the structure of 1 was essentially preserved after five catalytic cycles.

  The achieved catalytic performance of compound 1 in the Knoevenagel condensation reaction of benzaldehyde with malononitrile is superior to those exhibited by the heterogeneous catalysts based on other metalcarboxylate coordination compounds (Table 7)^[33-37].

  Table 7

  

  Table 7.  Comparison among various catalysts for Knoevenagel condensation reaction between benzaldehyde and malononitrile

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  Catalyst	Catalyst loading / %	Solvent	t / h	T	Product yield / %	Ref.
  [Co2(μ3-deta)(H2biim)3(H2O)2]n	2	MeOH	1	RT	100	This work
  Zn3(OH)(ATTCA)2(H2O)]·C2H6NH2·4DMF·H2O	10	CH2Cl2	5	RT	94	[33]
  [Zn3(L)2(μ2-OH)2]n	4	H2O	8	90 ℃	78	[33]
  {[Ba3Zn4(TDP)2(HCO2)2(OH2)2]·7DMF·4H2O}n	3	Ethanol	1	60 ℃	99	[33]
  [Zn(L)(H2O)2]n·n(N-methylformamide)	3	MeOH	1.5	40 ℃	75	[33]
  [Pb(L)2]·2DMF·6H2O	3	CH3CN	24	RT	100	[33]

  3.   Conclusions

  In summary, we have synthesized two Co(Ⅱ) coordination polymers based on a tetracarboxylate ligand. Compound 1 discloses a 1D chain structure. Compound 2 features a 2D network. The catalytic properties of both compounds were investigated. Compound 1 revealed an excellent catalytic activity in the Knoevenagel condensation reaction of benzaldehyde at room temperature.

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

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

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

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  Scheme 1  Coordination modes of deta^4- ligand in compounds 1 and 2

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  Figure 2  Perspective of 1D metal-organic chain along a axis

  H₂biim and H₂O ligands are omitted for clarity; Symmetry codes: A: x, y-1, z; B: x, y+1, z

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  Figure 3  Perspective of 2D H-bonded network along a axis

  Dashed lines present the hydrogen bonds

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

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

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  Figure 5  View of 2D metal-organic network parallel to ab plane

  Phen ligands are omitted for clarity; Symmetry codes: A: x-1, y, z; B: -x+1, y-1/2, -z+1/2; C: -x+2, y-1/2, -z+1/2; D: x+1, y, z; E: -x+2, y+1/2, -z+1/2; F: -x+1, y+1/2, -z+1/2

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  Figure 6  TGA curves of compounds 1 and 2

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  Scheme 2  Knoevenagel condensation reaction of benzaldehyde (model substrate) catalyzed by Co coordination compounds

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  Table 1.  Crystal data for compounds 1 and 2

  Parameter	1	2
  Chemical formula	C34H28Co2N12O11	C40H24Co2N4O10
  Molecular weight	898.54	838.49
  Crystal system	Triclinic	Monoclinic
  Space group	P1	P21/c
  a / nm	0.740 11(6)	1.107 14(10)
  b / nm	1.022 55(14)	1.586 84(10)
  c / nm	2.421 49(13)	1.997 32(15)
  α/(°)	100.288(8)
  β/(°)	91.398(6)	105.398(9)
  γ/(°)	100.304(9)
  V / nm3	1.771(3)	3.383(5)
  Z	2	4
  F(000)	916	1 704
  θ range for data collection / (°)	3.326-25.048	3.327-25.049
  Limiting indices	-8 ≤ h ≤ 8, -11 ≤ k ≤ 12, -28 ≤ l ≤ 21	-13 ≤ h ≤ 8, -18 ≤ k ≤ 18, -21 ≤ l ≤ 23
  Reflection collected, unique (Rint)	6 257, 3 869 (0.062 9)	5 979, 3 420 (0.105 7)
  Dc / (g·cm-3)	1.685	1.646
  μ / mm-1	1.019	1.052
  Data, restraint, parameter	6 257, 1, 556	5 979, 1, 505
  Goodness-of-fit on F2	1.041	1.023
  Final R indices [I≥2σ(I)] R1, wR2	0.064 9, 0.129 2	0.059 5, 0.092 6
  R indices (all data) R1, wR2	0.113 2, 0.169 2	0.118 6, 0.117 4
  Largest diff. peak and hole / (e·nm-3)	583 and -846	577 and -537

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  Table 2.  Selected bond distances (nm) and bond angles (°) for compounds 1 and 2

  1
  Co1—O1	0.218 4(4)	Co1—O4A	0.234 5(5)	Co1—N1	0.214 9(4)
  Co1—N4	0.207 5(4)	Co1—N5	0.215 9(4)	Co1—N8	0.210 2(4)
  Co2—O6	0.214 7(4)	Co2—O7	0.219 7(4)	Co2—O10	0.201 9(4)
  Co2—O11	0.218 0(4)	Co2—N9	0.208 2(5)	Co2—N12	0.218 1(5)
  N4—Co1—N8	178.6(2)	N1—Co1—N4	79.30(16)	N8—Co1—N1	101.12(16)
  N4—Co1—N5	100.74(17)	N5—Co1—N8	78.77(17)	N1—Co1—N5	177.4(2)
  O1—Co1—N4	94.65(17)	N8—Co1—O1	86.67(17)	N1—Co1—O1	90.06(17)
  N5—Co1—O1	92.56(17)	N4—Co1—O4A	87.12(17)	N8—Co1—O4A	91.55(17)
  N1—Co1—O4A	89.88(17)	N5—Co1—O4A	87.50(17)	O1—Co1—O4A	178.18(14)
  O10—Co2—N9	104.93(19)	O10—Co2—O6	154.36(18)	O6—Co2—N9	100.53(17)
  O11—Co2—O10	87.77(15)	O11—Co2—N9	88.52(18)	O11—Co2—O6	89.95(15)
  O10—Co2—N12	98.07(16)	N9—Co2—N12	79.5(2)	O6—Co2—N12	89.37(16)
  O11—Co2—N12	167.7(2)	O10—Co2—O7	93.96(16)	O7—Co2—N9	160.77(16)
  O6—Co2—O7	60.89(15)	O11—Co2—O7	95.92(17)	O7—Co2—N12	94.47(18)
  2
  Co1—O1	0.209 0(4)	Co1—O4A	0.209 0(3)	Co1—O7B	0.209 4(4)
  Co1—O8B	0.210 6(3)	Co1—N1	0.213 3(4)	Co1—N2	0.215 8(4)
  Co2—O2	0.202 4(3)	Co2—O3A	0.202 7(3)	Co2—O8B	0.206 3(4)
  Co2—O9B	0.239 0(4)	Co2—N3	0.208 8(5)	Co2—N4	0.214 8(4)
  O1—Co1—O4A	90.88(15)	O4A—Co1—O7B	100.25(14)	O1—Co1—O7B	166.12(14)
  O8B—Co1—O4A	88.86(13)	O1—Co1—O8B	90.97(13)	O7B—Co1—O8B	81.08(13)
  O4A—Co1—N1	90.22(16)	O1—Co1—N1	105.00(15)	O7B—Co1—N1	83.39(16)
  O8B—Co1—N1	164.02(15)	O4A—Co1—N2	164.18(15)	O1—Co1—N2	83.49(16)
  O7B—Co1—N2	87.76(15)	O8B—Co1—N2	105.96(14)	N1—Co1—N2	77.07(16)
  O2—Co2—O3A	96.20(14)	O2—Co2—O8B	90.71(14)	O8B—Co2—O3A	100.16(15)
  O2—Co2—N3	95.43(16)	N3—Co2—O3A	100.80(16)	N3—Co2—O8B	157.36(15)
  N4—Co2—O2	170.27(15)	N4—Co2—O3A	92.39(15)	N4—Co2—O8B	92.28(16)
  N3—Co2—N4	78.41(18)	O2—Co2—O9B	89.72(13)	O3A—Co2—O9B	158.05(15)
  O9B—Co2—O8B	58.52(12)	N3—Co2—O9B	99.65(15)	N4—Co2—O9B	83.96(14)
  Symmetry codes: A: x, y-1, z for 1; A: -x+1, y+1/2, -z+1/2; B: x+1, y, z for 2.

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  Table 3.  Hydrogen bond parameters of compound 1

  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  N2—H1…O2A	0.094 7	0.178 4	0.272 3	170.51
  N3—H2…O1A	0.081 9	0.199 5	0.279 1	163.95
  N6—H3…O3B	0.091 4	0.177 7	0.268 1	169.88
  N7—H5…O4B	0.087 3	0.188 8	0.275 0	169.01
  N10—H11…O9C	0.072 5	0.204 4	0.271 4	154.08
  N11—H12…O9C	0.085 1	0.191 5	0.274 3	163.89
  O10—H1W…O8D	0.082 0	0.179 0	0.261 0	177.84
  O10—H2W…O7E	0.085 0	0.190 6	0.275 6	179.75
  O11—H3W…O8D	0.085 5	0.193 2	0.278 5	175.56
  Symmetry codes: A: x-1, y, z; B: x+1, y-1, z; C: x-1, y-1, z; D: -x+2, -y+1, -z+1; E: -x+1, -y+1, -z+1.

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  Table 4.  Hydrogen bond parameters of compound 2

  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  O10—H1W…O6	0.086 3	0.206 1	0.280 7	144.38
  O10—H2W…N2A	0.085 0	0.264 9	0.349 9	179.65
  Symmetry code: A: -x+1, y+1/2, -z+1/2.

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  Table 5.  Knoevenagel condensation reaction of benzaldehyde with malononitrile catalyzed by Co(Ⅱ) coordination compounds

  Entry	Catalyst	Time / min	Catalyst loadinga / %	Solvent	Yieldb / %
  1	1	10	2.0	CH3OH	41
  2	1	20	2.0	CH3OH	56
  3	1	30	2.0	CH3OH	64
  4	1	40	2.0	CH3OH	80
  5	1	50	2.0	CH3OH	93
  6	1	60	2.0	CH3OH	100
  7	1	60	2.0	H2O	90
  8	1	60	2.0	C2H5OH	98
  9	1	60	2.0	CH3CN	85
  10	1	60	2.0	CHCl3	67
  11	1	60	1.0	CH3OH	92
  12	1	60	1.5	CH3OH	98
  13	1	55	2.5	CH3OH	100
  14	2	60	2.0	CH3OH	89
  15	Blank	60	—	CH3OH	23
  16	CoCl2·6H2O	60	2.0	CH3OH	33
  17	H4deta	60	2.0	CH3OH	30
  aMolar fraction; b Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

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  Table 6.  Knoevenagel condensation reaction of various aldehydes with malononitrile catalyzed by compound 1^a

  Entry	Substituted benzaldehyde substrate (R—C6H4CHO)	Product yieldb / %
  1	R=H	100
  2	R=2-NO2	100
  3	R=3-NO2	100
  4	R=4-NO2	100
  5	R=4-Cl	100
  6	R=4-OH	54
  7	R=4-CH3	98
  8	R=4-OCH3	76
  a Reaction conditions: aldehyde (0.5 mmol), malononitrile (1.0 mmol), catalyst 1 (Molar fraction: 2.0%), and CH3OH (1.0 mL) at 25 ℃; b Calculated by 1H NMR spectroscopy: nproduct/naldehyde×100%.

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  Table 7.  Comparison among various catalysts for Knoevenagel condensation reaction between benzaldehyde and malononitrile

  Catalyst	Catalyst loading / %	Solvent	t / h	T	Product yield / %	Ref.
  [Co2(μ3-deta)(H2biim)3(H2O)2]n	2	MeOH	1	RT	100	This work
  Zn3(OH)(ATTCA)2(H2O)]·C2H6NH2·4DMF·H2O	10	CH2Cl2	5	RT	94	[33]
  [Zn3(L)2(μ2-OH)2]n	4	H2O	8	90 ℃	78	[33]
  {[Ba3Zn4(TDP)2(HCO2)2(OH2)2]·7DMF·4H2O}n	3	Ethanol	1	60 ℃	99	[33]
  [Zn(L)(H2O)2]n·n(N-methylformamide)	3	MeOH	1.5	40 ℃	75	[33]
  [Pb(L)2]·2DMF·6H2O	3	CH3CN	24	RT	100	[33]

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•