English

• 0.   Introduction

  Coordination polymers (CPs) represent a highly diverse and popular class of compounds known for their multifarious structures and significant applications^[1-5], including but not limited to sensing^[6-7], gas sorption^[8-9], and catalysis^[10-12]. This spectrum of applications is intricately dependent on the structural characteristics of CPs and the type of metal ions and ligands they are constructed from^[13-15]. Among the plethora of organic ligands used to assemble CPs, aromatic multicarboxylic acids constitute the most common and widely explored type of linkers on account of their high diversity, good chemical and thermal stability, facile functionalization, and coordination versatility^{[11-12, 16-18]}.

  Within our general research objective aimed at exploring some commercially available but still little-studied multi-carboxylic acids as potential linkers for the design of functional CPs^{[11-12, 19-21]}, we have focused on the present work on an aminocarboxylate ligand, namely 5, 5′-azanediyldiisophthalic acid (H₄adip). The selection of the ligand has relied on the following considerations. (1) This tetracarboxylic acid possesses eight potential coordination sites that can realize a high diversity of coordination patterns. (2) The presence of the NH group might allow performing based-catalyzed reactions, such as the Henry and Knoevenagel condensation reactions, the classic and versatile C—C bond-forming reactions in synthetic chemistry^[22-25]. (3) H₄adip has not yet been widely applied in CPs^[26-27].

  Motivated by all these facts, we report herein the hydrothermal synthesis, isolation, characterization, and elucidation of crystal structures and topological features, as well as the evaluation of the catalytic performance of the three CPs. These compounds, comprising Zn(Ⅱ) and Co(Ⅱ), were assembled utilizing H₄adip as a linker in conjunction with different supporting ligands, specifically N-donor mediators of crystallization. The resulting products have been formulated as {[Zn₂(μ₆-adip)(phen)₂]·4H₂O}_n (1), {[Co₂(μ₆-adip)(bipy)₂]·4H₂O}_n (2), and [Co₂(μ₄-adip)(μ-bpa)₂]_n (3), where phen=1, 10-phenanthroline, bipy=2, 2′-bipyridine, bpa=bis(4-pyridyl)amine. In addition to extending the application of CPs driven by H₄adip-derived linkers, three synthesized compounds act as remarkable heterogeneous catalysts in the Henry reaction. Of significant interest is the system based on CP 3, which could lead to a higher conversion of 4-nitrobenzaldehyde into the corresponding product. The study further explored catalyst reusability, the scope of substrates, and the impact of various reaction parameters.

  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, and 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 compound 1

  A mixture of ZnCl₂ (0.027 g, 0.20 mmol), H₄adip (0.034 g, 0.10 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. Colourless block-shaped crystals of 1 were isolated manually, and washed with distilled water. Yield: 47% (based on H₄adip). Anal. Calcd. for C₂₀H_15.5ZnN_2.5O₆(%): C 53.11, H 3.45, N 7.74; Found(%): C 53.32, H 3.42, N 7.78. IR (KBr, cm^-1): 3 488m, 3 252w, 3 073w, 1 615w, 1 578s, 1 537s, 1 428 m, 1 407s, 1 379s, 1 317w, 1 220w, 1 147w, 1 106w, 1 025 w, 926w, 894w, 834w, 845m, 817w, 788m, 723m, 646w.

  1.3   Synthesis of compound 2

  The synthesis was the same as that of compound 1, except that ZnCl₂ and phen were replaced by CoCl₂·6H₂O (0.048 g, 0.20 mmol) and bipy (0.031 g, 0.20 mmol). Pink needle-shaped crystals of compound 2 were isolated manually, washed with distilled water, and dried. Yield: 45% (based on H₄adip). Anal. Calcd. for C₁₈H_15.5CoN_2.5O₆(%): C, 51.26; H, 3.70; N, 8.30. Found(%): C, 51.53; H, 3.72; N, 8.25. IR (KBr, cm^-1): 3 484w, 3 281w, 3 089w, 1 605w, 1 570s, 1 537s, 1 448m, 1 403s, 1 391s, 1 317w, 1 289w, 1 248w, 1 163w, 1 147 w, 1 065w, 1 025w, 935w, 894w, 812w, 792m, 763m, 731m, 654w.

  1.4   Synthesis of compound 3

  The synthesis was the same as that of compound 2, except that phen was replaced by bpa (0.034 g, 0.2 mmol). Pink block-shaped crystals of compound 3 were isolated manually, and washed with distilled water. Yield: 45% (based on H₄adip). Anal. Calcd. for C₃₆H₂₅Co₂N₇O₈(%): C 53.95, H 3.14, N 12.23; Found(%): C 53.71, H 3.12, N 12.28. IR (KBr, cm^-1): 3 259w, 1 625w, 1 597s, 1 577s, 1 516m, 1 441m, 1 345s, 1 317 m, 1 213w, 1 057w, 1 021m, 950w, 905w, 857w, 826m, 781w, 726w, 658w, 602w. These compounds are insoluble in water and common organic solvents, such as methanol, ethanol, acetone, and DMF.

  1.5   Structure determination

  Three suitable single crystals with dimensions of 0.08 mm×0.04 mm×0.03 mm (1), 0.10 mm×0.03 mm×0.02 mm (2), and 0.07 mm×0.03 mm×0.03 mm (3) were collected at 301(2) K on a Bruker SMART APEX Ⅱ CCD diffractometer with Mo Kα (λ=0.071 073 nm). Semi-empirical absorption corrections were applied using the SADABS program. The structures were solved by direct method and refined by full-matrix least-square on F² using the SHELXTL-2014 program. All non-hydrogen atoms were refined anisotropically. All the hydrogen atoms 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. Some highly disordered solvent molecules in 1 and 2 were removed using the SQUEEZE routine in PLATON^[28]. The final amount of solvent molecules was estimated from the data of elemental and thermogravimetric analyses. A summary of the crystallography data and structure refinements for compounds 1-3 is given in Table S1 (Supporting information). The selected bond lengths and angles for 1-3 are listed in Table S2. Hydrogen bond parameters of 3 are given in Table S3.

  2.   Results and discussion

  2.1   Description of the structure

  2.1.1   Compounds 1 and 2

  Single-crystal X-ray diffraction analyses reveal that the structures of compounds 1 and 2 are similar and have 3D framework structures. The structure of 1 is described below as a selected example. The asymmetric unit of 1 contains one crystallographically unique Zn(Ⅱ) ion, a half of μ₆-adip^4- ligand, one phen moiety, and two lattice water molecules. As shown in Fig. 1, the Zn1 ion is six-coordinated by four carboxylate O atoms from three individual μ₆-adip^4- blocks and two N atoms from the phen ligand, constructing a distorted octahedral {ZnN₂O₄} geometry. The Zn—O bond lengths range from 0.206 0(2) to 0.237 3(2) nm, whereas the Zn—N bonds vary from 0.211 8(2) to 0.215 1(2) nm; these bonding parameters are comparable to those found in other reported Zn(Ⅱ) compounds^{[12, 19, 21]}. In 1, the adip^4- ligand exhibits the coordination mode Ⅰ (Scheme 1), in which four deprotonated carboxylate groups show bidentate or bridging bidentate modes. Two Zn1 ions are connected by two carboxylate groups from two distinct μ₆-adip^4- linkers, generating a Zn₂ subunit (Fig. 2). These subunits are connected by the remaining carboxylate groups of μ₆-adip^4- linkers to form a 3D framework (Fig. 3). From a topological viewpoint, it is composed of the 3-linked Zn1 and 6-linked μ₆-adip^4- nodes that are arranged into a binodal 3, 6-connected framework with a new topology and a point symbol of (4².6.)₂(4⁴. 6².8⁸.10) (Fig. 4).

  Figure 1

  

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

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

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  Scheme 1

  

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

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  Figure 2

  

  Figure 2.  Structure of Zn₂ subunit

  Symmetry code: A: -x+3/2, -y+1, z.

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  Figure 3

  

  Figure 3.  Three-dimensional metal-organic frameworks of compound 1 viewed along the c-axis

  The phen ligands are omitted for clarity; Symmetry codes: A: -x+3/2, -y+1, z+1; B: x-1/2, y, -z+2; C: -x+1, -y+1, -z+2; D: -x+1, y+1/2, z+3/2; E: -x+3/2, y+1/2, -z+5/2; F: x-1/2, -y+1/2, z+3/2; G: x, -y+1/2, -z+5/2; H: -x+2, -y+1, -z+2.

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  Figure 4

  

  Figure 4.  Topological representation of a 3, 6-connected framework with a new topology viewed along the c-axis

  Cyan: 3-linked Zn1 nodes; Gray: centroids of 6-connected μ₆-adip^4- nodes.

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  2.1.2   Compound 3

  The asymmetric unit of compound 3 possesses one crystallographically independent Co(Ⅱ) ion, a half of μ₄-adip^4- block, and one μ-bpa ligand. As shown in Fig. 5, the Co1 ion is tetra-coordinated and adopts a distorted tetrahedral {CoN₂O₂} geometry formed by two carboxylate O atoms from two different μ₄-adip^4- blocks and two N atoms from two distinct μ-bpa ligands. The Co—O distance is 0.193 7(4) nm, whereas the Co—N distances are 0.204 3(5)-0.205 0(6) nm; these bonding parameters agree with those observed in other Co(Ⅱ) compounds^{[11-12, 21]}. In 3, the adip^4- block acts as a μ₄-spacer (mode Ⅱ, Scheme 1), in which the carboxylate groups exhibit the monodentate modes. The bpa auxiliary ligand adopts a bridging coordination mode. The neighboring Co(Ⅱ) centers are linked by μ₆-adip^4- and μ-bpa blocks, giving rise to a 3D framework (Fig. 6). This 3D structure is composed of the 4-linked Co1 nodes, 4-linked μ₄-adip^4- nodes, and 2-connected μ-bpa linkers (Fig. 7). It is a qtz (quartz, 4/6/h1) topology and a point symbol of (6⁴.8²) (Fig. 7).

  Figure 5

  

  Figure 5.  Drawing of the asymmetric unit of compound 3 with a 30% probability of thermal ellipsoids

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

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  Figure 6

  

  Figure 6.  Three-dimensional metal-organic framework viewed along the c-axis

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

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  Figure 7

  

  Figure 7.  Topological representation of a 2, 4-connected framework with a qtz (quartz, 4/6/h1) topology viewed along the c-axis

  Pink: 4-connected Co1 nodes; Gray: centroids of 4-connected μ₄-adip^4- nodes; Dark blue: centroids of 2-connected μ-bpa linkers.

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  2.2   Thermal stability 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 four lattice water molecules in a range of 36-138 ℃ (Calcd. 8.0%, Obsd. 8.2%, ), followed by the decomposition at 332 ℃. For compound 2, four lattice water molecules were released between 42 and 156 ℃ (Calcd. 8.5%, Obsd. 8.6%), whereas a dehydrated sample remained stable up to 309 ℃. Compound 3 does not contain any coordinated or crystallization water, and its framework shows stability on heating until 434 ℃.

  Figure 8

  

  Figure 8.  TGA curves of compounds 1-3

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  2.3   Henry reaction catalyzed by the CPs

  A few amide-based CPs have been reported to act as catalysts for the Henry reaction^[21-22]. The compounds prepared in this study, particularly Co(Ⅱ) CP 3, have both Lewis acid (Co²⁺ with unsaturated coordination) and basic center (amide group). Therefore, it presents a promising feature to act as a bifunctional catalyst. In addition, on account of their insolubility in common solvents, their use as heterogeneous catalysts should be particularly promising. So, we probed CPs 1-3 as heterogeneous catalysts in this reaction using assorted aldehydes with nitroethane. As a model substrate, 4-nitrobenzaldehyde was treated with nitroethane at 70 ℃ in a methanol medium to form a β-nitro alcohol product (Scheme 2, Table 1). Various parameters encompassing solvent selection, reaction temperature and duration, catalyst loading, recyclability, and substrate diversity were assessed.

  Scheme 2

  

  Scheme 2.  Henry reaction of 4-nitrobenzaldehyde with nitroethane (model substrate) catalyzed by CPs 1-3

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  Table 1

  

  Table 1.  Optimization of conditions for Henry reaction of 4-nitrobenzaldehyde with nitroethane catalyzed by CPs 1-3

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  Entry	Catalyst	Time / h	Temperature / ℃	Catalyst loadinga / %	Solvent	Yieldb / %	Selectivity (nsyn/nanti)
  1	3	1	70	4.0	CH3OH	34	54∶46
  2	3	2	70	4.0	CH3OH	55	55∶45
  3	3	4	70	4.0	CH3OH	65	55∶45
  4	3	6	70	4.0	CH3OH	74	55∶45
  5	3	8	70	4.0	CH3OH	83	56∶44
  6	3	10	70	4.0	CH3OH	88	55∶45
  7	3	12	70	4.0	CH3OH	91	56∶44
  8	3	16	70	4.0	CH3OH	91	55∶45
  9	3	12	25	4.0	CH3OH	17	55∶45
  10	3	12	60	4.0	CH3OH	75	56∶44
  11	3	12	80	4.0	CH3OH	89	55∶45
  12	3	12	70	3.0	CH3OH	82	55∶45
  13	3	12	70	5.0	CH3OH	91	56∶44
  14	3	12	70	4.0	H2O	78	55∶45
  15	3	12	70	4.0	CH3CN	12	45∶55
  16	3	12	70	4.0	THF	65	46∶54
  17	3	12	70	4.0	C2H5OH	53	55∶45
  18	1	12	70	4.0	CH3OH	68	54∶46
  19	2	12	70	4.0	CH3OH	64	55∶45
  20	Blank	12	70		CH3OH	1	45∶55
  21	CoCl2	12	70	4.0	CH3OH	10	44∶56
  22	H4adip	12	70	4.0	CH3OH	2	45∶55
  a The loading amount of the catalyst is expressed in mole fractions; b Calculated by 1H NMR spectroscopy: Yield=nproduct/naldehyde×100%.

  CP 3 exhibited the highest activity, resulting in a 91% conversion of 4-nitrobenzaldehyde to two isomers of β-nitro alcohol products (Table 1 and Fig.S1). So, 3 was used to research the influence of different reaction parameters. The yield was accumulated with a yield increase from 34% to 91% on prolonging the reaction from 1 to 12 h (Table 1, entries 1-7). The influence of catalyst amount was also investigated, revealing a product yield growth from 82% to 91% on increasing the loading of the catalyst from 3.0% to 5.0% (entries 11-13). Entry 10 showed that 4.0% compound 3 was able to catalyze the reaction and produced the product in 75% at 60 ℃; however, a mild elevated temperature of 70 ℃ led to a yield of 91%. In addition to methanol, other solvents were tested. Water, ethanol, acetonitrile, and chloroform are less suitable (12%-78% product yields, respectively).

  In comparison with 3, CPs 1 and 2 are less active, resulting in the maximum product yields in the 64%-68% range (entries 18 and 19, Table 1). It should be highlighted that under similar reaction conditions, the Henry reaction of 4-nitrobenzaldehyde is significantly less efficient in the absence of the catalyst (only 1% product yield) or when using H₄adip (2% yield) or CoCl₂ (10% yield) as catalysts (entries 20-22, Table 1). Although there is no clear connection between the activity and structure of the catalyst, the superior performance of CP 3 might be related to the existence of open Co sites^{[21, 29-30]}.

  Different substituted benzaldehyde substrates were used to study the substrate scope in the Henry reaction with nitroethane. These tests were run under optimized conditions (4.0% 3, CH₃OH, 70 ℃, 12 h). The corresponding products were obtained in the yields varying from 23% to 91% (Table 2). Benzaldehydes containing a strong electron-withdrawing group (e.g., nitro, and chloro substituent in the ring) revealed the best efficiency (entries 2-5, Table 2), which can be explained by an increased electrophilicity of substrates. The benzaldehydes possessing an electron-donating functionality (e.g., methyl or methoxy group) led to lower product yields (entries 7 and 8, Table 2).

  Table 2

  

  Table 2.  Henry reaction of various aldehydes with nitroethane catalyzed by CP 3^a

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  Entry	Substituted benzaldehyde substrate (R-C6H4CHO)	Product yieldb / %	Selectivity (nsyn/nanti)
  1	R=H	81	52∶48
  2	R=2-NO2	83	57∶43
  3	R=3-NO2	87	55∶45
  4	R=4-NO2	91	55∶45
  5	R=4-Cl	82	51∶49
  6	R=4-OH	61	55∶45
  7	R=4-CH3	55	44∶56
  8	R=4-OCH3	23	45∶55
  a Reaction conditions: aldehyde (0.5 mmol), nitroethane (2.0 mmol), catalyst 3 (4.0%), and CH3OH (1.0 mL) at 70 ℃; b Calculated by 1H NMR spectroscopy.

  Finally, the recyclability of the catalyst 3 was tested. After each reaction cycle, the catalyst was separated via centrifugation, washed with CH₃OH, dried in air at about 25 ℃, and reused in the next cycle. The obtained results proved that CP 3 preserved the activity for at least five reaction cycles (the yields were 89%, 88%, 86%, and 85% for the second to fifth run, respectively). Besides, the PXRD patterns confirmed that the structure of 3 was maintained (Fig.S2), despite the appearance of several additional signals or widening of some peaks. These alterations might be expected after a few catalytic cycles and are explained by the presence of some impurities or a decrease in crystallinity. In a model reaction involving 4-nitrobenzaldehyde and nitroethane as substrates, the catalytic performance of 3 was comparable to other heterogeneous catalytic systems based on metal-carboxylate coordination compounds (Table S3) or superior if reaction time was taken into consideration^{[21, 29, 31-33]}.

  3.   Conclusions

  In summary, we have synthesized three Zn(Ⅱ) and Co(Ⅱ) CPs based on an aminocarboxylate ligand. Compounds 1-3 disclose 3D frameworks. The catalytic performances of these CPs were investigated. CP 3 revealed an effective catalytic activity in the Henry reaction at 70 ℃.

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

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

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  Figure 2  Structure of Zn₂ subunit

  Symmetry code: A: -x+3/2, -y+1, z.

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  Figure 3  Three-dimensional metal-organic frameworks of compound 1 viewed along the c-axis

  The phen ligands are omitted for clarity; Symmetry codes: A: -x+3/2, -y+1, z+1; B: x-1/2, y, -z+2; C: -x+1, -y+1, -z+2; D: -x+1, y+1/2, z+3/2; E: -x+3/2, y+1/2, -z+5/2; F: x-1/2, -y+1/2, z+3/2; G: x, -y+1/2, -z+5/2; H: -x+2, -y+1, -z+2.

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  Figure 4  Topological representation of a 3, 6-connected framework with a new topology viewed along the c-axis

  Cyan: 3-linked Zn1 nodes; Gray: centroids of 6-connected μ₆-adip^4- nodes.

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  Figure 5  Drawing of the asymmetric unit of compound 3 with a 30% probability of thermal ellipsoids

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

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  Figure 6  Three-dimensional metal-organic framework viewed along the c-axis

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

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  Figure 7  Topological representation of a 2, 4-connected framework with a qtz (quartz, 4/6/h1) topology viewed along the c-axis

  Pink: 4-connected Co1 nodes; Gray: centroids of 4-connected μ₄-adip^4- nodes; Dark blue: centroids of 2-connected μ-bpa linkers.

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  Figure 8  TGA curves of compounds 1-3

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  Scheme 2  Henry reaction of 4-nitrobenzaldehyde with nitroethane (model substrate) catalyzed by CPs 1-3

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  Table 1.  Optimization of conditions for Henry reaction of 4-nitrobenzaldehyde with nitroethane catalyzed by CPs 1-3

  Entry	Catalyst	Time / h	Temperature / ℃	Catalyst loadinga / %	Solvent	Yieldb / %	Selectivity (nsyn/nanti)
  1	3	1	70	4.0	CH3OH	34	54∶46
  2	3	2	70	4.0	CH3OH	55	55∶45
  3	3	4	70	4.0	CH3OH	65	55∶45
  4	3	6	70	4.0	CH3OH	74	55∶45
  5	3	8	70	4.0	CH3OH	83	56∶44
  6	3	10	70	4.0	CH3OH	88	55∶45
  7	3	12	70	4.0	CH3OH	91	56∶44
  8	3	16	70	4.0	CH3OH	91	55∶45
  9	3	12	25	4.0	CH3OH	17	55∶45
  10	3	12	60	4.0	CH3OH	75	56∶44
  11	3	12	80	4.0	CH3OH	89	55∶45
  12	3	12	70	3.0	CH3OH	82	55∶45
  13	3	12	70	5.0	CH3OH	91	56∶44
  14	3	12	70	4.0	H2O	78	55∶45
  15	3	12	70	4.0	CH3CN	12	45∶55
  16	3	12	70	4.0	THF	65	46∶54
  17	3	12	70	4.0	C2H5OH	53	55∶45
  18	1	12	70	4.0	CH3OH	68	54∶46
  19	2	12	70	4.0	CH3OH	64	55∶45
  20	Blank	12	70		CH3OH	1	45∶55
  21	CoCl2	12	70	4.0	CH3OH	10	44∶56
  22	H4adip	12	70	4.0	CH3OH	2	45∶55
  a The loading amount of the catalyst is expressed in mole fractions; b Calculated by 1H NMR spectroscopy: Yield=nproduct/naldehyde×100%.

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  Table 2.  Henry reaction of various aldehydes with nitroethane catalyzed by CP 3^a

  Entry	Substituted benzaldehyde substrate (R-C6H4CHO)	Product yieldb / %	Selectivity (nsyn/nanti)
  1	R=H	81	52∶48
  2	R=2-NO2	83	57∶43
  3	R=3-NO2	87	55∶45
  4	R=4-NO2	91	55∶45
  5	R=4-Cl	82	51∶49
  6	R=4-OH	61	55∶45
  7	R=4-CH3	55	44∶56
  8	R=4-OCH3	23	45∶55
  a Reaction conditions: aldehyde (0.5 mmol), nitroethane (2.0 mmol), catalyst 3 (4.0%), and CH3OH (1.0 mL) at 70 ℃; b Calculated by 1H NMR spectroscopy.

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