English

• 0.   Introduction

  Metal-organic frameworks (MOFs) have attracted widespread attention due to their potential applications such as electrochemical biosensors^[1-3], anticounterfeiting luminescent material^[4-5], environmental protection^[6-8], and biological medicine^[9-10]. In 1999, Yaghi's group^[11] reported a particular example of porous material MOF-5, i. e., [ZnO₄(BDC)₃·(DMF)₈·(C₆H₅Cl)], which is constructed by organic linkers and ZnO₄ cluster units, and this specific area have developed dramatically during last two decades. Notably, the final structures of MOFs are affected by many factors such as solvent type, ligand structure, temperature, pH, and metal ions. It is imperative to achieve MOFs effectively by synthetic strategies such as "reticular chemistry"^[12] and "secondary building units"^[13]. Intriguingly, multi-topic bridging ligands could construct diverse coordination networks^[14] and in this particular area, mixed ligands consisting of bipyridyl and polycarboxyl compounds are reliable building blocks^[15]. Namely, bipyridyl ligands could coordinate with metal ions in the form of rod-like structures and meanwhile, polycarboxyl compounds could exist as anionic species, providing the diversity of coordination modes with metal ions. Specifically, during the assembly of coordination networks, mixed ligands containing acid and base species could unlock amazing potential for compensating charge balance, coordinating deficiency and so much more. Recently, we embarked on the study of MOF materials by mixed ligands strategy and those corresponding materials have demonstrated various structural characters and potential applications for dye degradation and catalytic usage. For example, seven novel MOFs have been fabricated by using the acid-base mixed-ligand synthetic strategy, i. e., a basic ligand (BPCH=2, 6-bis(4-pyridylmethylidene)cyclohexanone) and three acidic ligands (H₂TP=terephthalic acid, H₂IP=isophthalic acid, H₃TMA=1, 3, 5-benzenetricarboxylic acid) have been combined with transition metals, and those corresponding MOF materials have been explored to adsorb dye molecules^[16]. Recently, a particular example was that three MOFs were constructed by combining the rigid acidic ligand 2, 4, 6-tris (4-carboxyphenyl)-1, 3, 5-triazine (H₃TATB) and BPCH or 2, 6-bis(3-pyridylmethylidene)cyclohexanone (BPMC) with transition metals, those corresponding MOFs demonstrated high catalytic activity for Knoevenagel condensation under mild conditions^[17]. Interestingly, MOF materials could act as heterogeneous catalysts ^[18] and in particular, MOFs have been applied for organic transformations such as Knoevenagel reaction^[19-20], aldol condensation^[21], Friedel-Crafts alkylation^[22], oxidation^[23-25], and transesterification reaction^[26-27]. Specifically, the Knoevenagel condensation reaction of aldehydes is an important method for carbon-carbon bond formation in synthetic chemistry^[28] to achieve biologically significant target molecules^[29].

  In this contribution, we reported new cyclohexanone-based ligands 4-methyl-2, 6-bis-pyridin-3-ylmethylene-cyclohexanone (L₁) and 4-methyl-2, 6-bis (4-pyridin-4-yl-benzylidene)-cyclohexanone (L₂) and combined with H₂ IP to form four new MOFs, [Zn(IP) (L₁)]_n (1), {[Cd(IP) (L₁)]·H₂O}_n (2), {[Co(IP) (L₁)]·H₂O}_n (3), and [Zn(IP)(L₂)(H₂O)]_n (4) (Scheme 1). These compounds have been characterized by single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA). Regarding MOFs 1-3, their photocatalytic degradation properties for dye molecules have been explored. MOF 4 could act as a heterogeneous catalyst for the Knoevenagel reaction and its stability during the process has been determined. Moreover, a plausible mechanism has been proposed. This work not only demonstrated that the mixedligand synthetic strategy has the benefit of achieving various types of MOFs with aesthetic structural characters but also has the opportunity to achieve functional materials with meaningful properties such as dye degradation and heterogeneous catalytic activity.

  Scheme 1

  

  Scheme 1.  Structures of ligands L₁, L₂, and H₂IP

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  1.   Experimental

  1.1   Materials and measurements

  All the chemicals were received as reagent grade and used directly without further purification. IR spectrum was recorded on a Varian 640 FT/IR spectrometer with KBr powder in a range of 4 000-500 cm^-1. Elemental analysis (EA) was obtained from Elementar Vario MICRO. Powder X-ray diffraction (PXRD) pattern was collected on a DX-2600 spectrometer with Mo Kα radiation (λ =0.071 073 nm) at room temperature (U=40 kV, I=30 mA, 2 θ=5°-50°). Thermal gravimetric analyses (TGA) were carried out under N₂ flow on an SDT 2960 differential thermal analyzer at a heating rate of 10 ℃·min^-1. The UV-Vis measurements of the solution were carried out on a Thermo EV 201CP.

  1.2   Synthesis of ligands L₁ and L₂

  A mixture of 4-methylcyclohexanone (1.12 g, 10.0 mmol), 3-pyridinecarboxaldehyde (2.14 g, 20.0 mmol) was dissolved in 50 mL of C₂H₅OH, then 8 mL of 6 mol·L^-1 KOH solution was added dropwise. The mixture was stirred at room temperature for 10 h and the reaction solution was neutralized with dilute hydrochloric acid to pH 7, then the obtained yellow solid was filtered (Scheme S1, Supporting information). The final L₁ was purified by recrystallization with ethanol. FTIR (KBr, cm^-1): 3 032(w), 1 664(s), 1 606(s), 1 572(s), 997 (w), 707(m). ¹H NMR (400 MHz, CDCl₃): δ 8.71(s, 2H), 8.57(d, J=4.9 Hz, 2H), 7.75(d, J=7.6 Hz, 4H), 7.36(dd, J=8.2, 4.7 Hz, 2H), 3.03 (dd, J=15.9, 4.0 Hz, 2H), 2.54 (dd, J=15.7, 11.4 Hz, 2H), 1.95-1.87 (m, 1H), 1.08 (d, J=6.6 Hz, 3H).

  4-Methylcyclohexanone (1.12 g, 10 mmol) and 4-(4-pyridinyl)benzaldehyde (3.66 g, 20 mmol) was dissolved in 100 mL of C₂H₅OH, and 15 mL of 6 mol·L^-1 KOH solution was added by dropwise. L₂ was obtained under a similar reaction condition (Scheme S2). FTIR (KBr, cm^-1): 3 424(s), 3 032(w), 2 955(w), 1 665(w), 1 594(vs), 1 487(m), 1 404(m), 1 242(m), 1 148(s), 810 (vs), 703(m). ¹H NMR (400 MHz, CDCl₃): δ 8.71-8.65 (m, 2H), 7.84(s, 1H), 7.70(d, J=8.3 Hz, 2H), 7.59 (d, J= 7.9 Hz, 2H), 7.56-7.50 (m, 2H) 3.10(d, J=16.3 Hz, 1H), 2.58(t, J=13.6 Hz, 1H), 1.12(d, J=6.7 Hz, 2H).

  1.3   Synthesis of MOFs 1-3

  Zn(NO₃)₂·6H₂O (0.05 mmol, 15.0 mg), H₂IP (0.05 mmol, 8.3 mg), L₁ (0.05 mmol, 14.8 mg) were dissolved in 7 mL DMF/H₂O/CH₃CH₂OH (4∶2∶1, V/V) in a 10 mL glass vial, and then the resulting solution was stirred for about 30 min at room temperature, heated at 90 ℃ for 2 d. After the mixture was cooled to room temperature, yellow block crystals of MOF 1 were obtained. Yield: 83% (based on Zn). The preparation for MOF 2 or 3 was the same as that for 1 except that the transition metal salt was changed to Cd(NO₃)₂·4H₂O or Co(NO₃)₂·6H₂O, respectively. The yield of MOF 2 was 83% and that of MOF 3 was 74%. 1: FTIR (KBr, cm^-1): 3 442(w), 2 949(m), 1 673(m), 1 641(vs), 1 474(s), 1 333(vs), 1 202(s), 1 147(s), 1 000(w), 933(w), 745(s). 2: 3 440(w), 2 929(w), 1 672(w), 1 603(vs), 1 478(s), 1 386(vs), 1 247(w), 1 151(m), 1 030(w), 722(m). 3: 3 442(m), 2 926(w), 1 672(w), 1 616(vs), 1 534(m), 1 400(vs), 1 245(w), 1 142(m), 1 032(w), 750(m). Elemental analysis: C₂₇H₂₂N₂O₅Zn (MOF 1) Calcd. (%): C 62.38, H 4.27, N 5.39. Found(%): C 61.60, H 4.34, N 5.46. C₂₇H₂₂N₂O₅Cd (MOF 2, excluding H₂O) Calcd. (%): C 57.21, H 3.91, N 4.94; Found(%): C 55.21, H 4.27, N 4.35. C₂₇H₂₂N₂O₅Co (MOF 3, excluding H₂O) Calcd. (%): C 63.16, H 4.32, N 5.46; Found(%): C 61.39, H 4.67, N 5.89.

  1.4   Synthesis of MOF 4

  Zn(NO₃)₂·6H₂O (0.014 9 g, 0.05 mmol), H₂IP (0.008 3 g, 0.05 mmol), L₂ (0.022 1 g, 0.05 mmol), DMF (4 mL), H₂O (2 mL), C₂H₅OH (1 mL) was placed in a 10 mL glass vial and under ultrasonic for 30 min. The bottle was then placed in an oven at 80 ℃ for 48 h. Light yellow transparent crystals of MOF 4 were formed and isolated. Yield: 68% (based on Zn). FTIR (KBr, cm^-1): 2 950(m), 1 612(vs), 1 564(s), 1 491(w), 1 350(s), 1 227(m), 1 148(m), 1 000(w), 816(s), 750(s). Elemental analysis: C₃₉H₃₀N₂O₅Zn (MOF 4, excluding H₂O) Calcd.(%): C 69.70, H 4.50, N 4.17. Found(%): C 67.71, H 4.72, N 3.97.

  1.5   X-ray crystallography

  The suitable crystal of MOFs 1-4 was selected and characterized on an Oxford Diffraction Gemini R Ultra diffractometer with graphite-monochromated Mo Kα (λ =0.071 073 nm) at 296 K. The structure was solved by direct methods and refined on F² by fullmatrix least squares methods using the SHELXTL package^[30-31]. The hydrogen atoms attached to carbon atoms were placed at geometrically estimated positions. The SQUEEZE procedure has been used to deal with the disordered solvents. A summary of the crystal data and structure refinements of the compound is provided in Table S1. The selected bond lengths and angles of 1-4 are given in Table S2.

  1.6   Photocatalytic degradation of organic dyes by MOFs 1-3

  The MOF sample (20 mg) was added to an aqueous solution (50 mL) of rhodamine B (RhB, ρ =10 mg·L^-1) or pararosaniline hydrochloride (PH, ρ=10 mg·L^-1) and stirred for 30 min in the dark to make sure the adsorption-desorption equilibrium of the resulting solution. It was exposed to UV irradiation from an LP300WE lamp (λ=365 nm) and kept for stirring during the irradiation. At every 30-minute interval, 4 mL solution was taken out for the UV-Vis measurement.

  1.7   Catalytic Knoevenagel reaction

  The Knoevenagel reaction between benzaldehyde and malononitrile using the MOF 4 catalyst was carried out in a magnetically stirred round-bottom flask. Unless otherwise stated, a mixture of MOF 4 (40 mg) and benzaldehyde (0.11 mL, 1.0 mmol) was poured into a 25 mL flask containing 2 mL of MeOH. The catalyst concentration was calculated with respect to the zinc/benzaldehyde molar ratio. The reaction vessel was stirred (600 r·min^-1) for 10 min to disperse the MOF 4 in the liquid phase. A solution of malononitrile (0.07 g, 1.0 mmol) in MeOH (1 mL) was then added, and the resulting mixture was stirred at room temperature for 3 h. The crude product was purified by using silica gel column chromatography (eluent: 30% ethyl acetate in ether). The yield was calculated and analyzed using the ¹H NMR spectrum to determine the composition of the substance. The MOF 4 catalyst was recycled from the reaction mixture by filtration, washed with copious amounts of methanol, and dried under vacuum at 160 ℃ overnight. Then it was reused for the next run.

  2.   Results and discussion

  2.1   Structural analysis of MOF 1

  Single crystal X-ray diffraction analysis reveals that MOF 1 crystallizes in the monoclinic crystal system with space group P2₁/n, and the asymmetric unit consists of one Zn²⁺ ion, one L₁ ligand, and one IP^2- ligand (Fig. 1a). The Zn²⁺ ion is four-coordinated, and are composed of two oxygen atoms (O2, O4^ⅰ) from two IP^2- anions and two nitrogen atoms (N1^ⅱ, N2) from two L₁ ligands (Fig. 1b). The Zn—O and Zn—N bond distances are in the ranges of 0.194 1(2)-0.194 6(2) nm and 0.204 5(3)-0.208 5(3) nm, respectively (Table S1). A 2D layer structure could be observed (Fig. 1c) and the topological analysis indicates that 1 is a (2, 4)-connected net with Schläfli symbol (6⁴.8.10)(6) rationalized by the Topos program (Fig. 1d).

  Figure 1

  

  Figure 1.  Structure of MOF 1: (a) asymmetric unit; (b) coordination environment of Zn²⁺ ion; (c) 2D structure; (d) topological structure

  Symmetry codes: ^ⅰ 0.5+x, 1.5-y, -0.5+z; ^ⅱ 0.5-x, 0.5+y, 0.5-z.

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  2.2   Structural characterization of MOFs 2-3

  MOFs 2-3 crystallize in the triclinic crystal system with the P1 space group. They have similar structures and 2 is described as an example. It contains one Cd²⁺ ion, one L₁ ligand, and one IP^2- ligand (Fig. 2a). The Zn²⁺ ion is six-coordinated and links with four oxygen atoms (O1^ⅰ, O2^ⅰ, O3^ⅱ, O4) from three IP^2- anions and two nitrogen atoms (N1, N2^ⅲ) from two L₁ ligands (Fig. 2b). The Cd—O and Cd—N bond distances are in the ranges of 0.225 2(3)-0.245 7(3) nm and 0.231 9(3)-0.234 8(3) nm, respectively (Table S1). A 2D layer structure has been observed (Fig. 2c) and the topological analysis indicated that 2 is a 4-connected net with Schläfli symbol (4⁴.6²) rationalized by the Topos program (Fig. 2d).

  Figure 2

  

  Figure 2.  Structure of MOF 2: (a) asymmetric unit; (b) coordination environment of Cd²⁺ ion; (c) 2D structure; (d) topological structure

  Symmetry codes: ^ⅰ 1+x, y, z; ^ⅱ 1-x, -y, -z; ^ⅲ-1+x, -1+y, -1+z.

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  2.3   Structural analysis of MOF 4

  MOF 4 crystallizes in the triclinic crystal system with the P1 space group. The asymmetric unit contains one Zn ion, one L₂ ligand, one IP^2- ligand, and an H₂O molecule (Fig. 3a). The pyridyl ring is connected with Zn1 ion (Fig. 3b) and the IP^2- ligand containing two carboxyl groups link two Zn1 ions as well (Fig. 3c). Then a 1D chain structure along the a-axis could be observed (Fig. 3d). The Zn1 ion is four-coordinated with three oxygen atoms (O1, O5, O3^ⅱ) from two IP^2- anions, one H₂O molecule and nitrogen atom (N1) from L₂ ligand with the tetrahedron coordination mode (Fig. 3e and 3f). Aided by hydrogen bondings and π-π stacking interactions, supramolecular stackings are arranged as depicted in Fig. 4.

  Figure 3

  

  Figure 3.  Structure of MOF 4: (a) asymmetric unit; (b) coordination of L₂ ligand; (c) coordination of IP^2- ligand; (d) 1D chain; (e, f) coordination environment of Zn1

  Symmetry codes: ^ⅰ 1+x, y, z; ^ⅱ-1+x, y, z.

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  Figure 4

  

  Figure 4.  Hydrogen bonding interactions (a) and π-π packing patterns (b) in MOF 4

  Symmetry codes: ^ⅰ 2+x, -2+y, -1+z; ^ⅱ 2-x, -1-y, -z.

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  2.4   PXRD and TGA

  As shown in Fig.S3, the simulated PXRD patterns and experimental results of MOFs 1-4 were consistent in the main locations, demonstrating the single-phase purity of those bulk samples.

  The thermal stability of MOFs 1-4 has been investigated (Fig.S4). The TGA curve of MOF 1 showed one step of weight loss. The skeleton began to collapse around 340 ℃ with a weight loss of 59.6%. The TGA curves of MOFs 2 and 3 had similar characters. Starting from 340 ℃, the significant steps of weight loss were 78.5% and 68.8%, respectively, indicating the organic skeleton began to collapse. Regarding MOF 4, around 330℃, a weight-loss step of 74.4% was observed

  2.5   Photocatalytic degradation of dyes

  MOFs have shown their potential as photocatalysts in the treatment of polluted water^[33]. Fig. 5 shows the photocatalytic effect of MOFs 1-3 on RhB and PH dyes under irradiation for 5 h, respectively. Fig. 6 shows the degradation rates of MOFs 1-3 on RhB dye were 21.2%, 20.4%, and 20.0% and those on PH were 28.4%, 26.4%, and 26.7%, respectively. Regarding MOF 4, there was no photodegradation affection (Supporting information).

  Figure 5

  

  Figure 5.  UV-Vis spectra varying with the irradiation time of RhB dye (a-c) and PH dye (d-f) in the presence of MOFs 1-3

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  Figure 6

  

  Figure 6.  Degradation rates of the RhB and PH solutions in the presence of MOFs 1-3

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  2.6   Catalytic activity of MOF 4

  To explore the catalytic activity of MOF 4, its performance in the Knoevenagel reaction has been evaluated^[34-35]. Several reaction factors such as reaction time, cycling usage, and solvent types have been tested to reveal the suitable parameters for this particular reaction (Scheme 2).

  Scheme 2

  

  Scheme 2.  Knoevenagel condensation reaction of benzaldehyde and malononitrile

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  Firstly, different solvents such as methanol, dichloromethane, ethyl acetate, tetrahydrofuran, and acetonitrile have been used. Table 1 shows that methanol is the most suitable solvent for the catalytic experiment. The reaction between benzaldehyde and malononitrile was also monitored in the blank, as well as using free ligand L₂ precursor, H₂IP, and the metal salt Zn(NO₃)₂·6H₂O, instead of MOF 4. In the absence of the catalyst, the reaction showed a trace yield at room temperature. Only 10%-35% of product yield could be observed by using Zn(NO₃)₂·6H₂O, H₂IP or L₂. The effect of reaction time in the Knoevenagel condensation showed that 3 h was the best reaction time and further increasing the reaction time had no significant effect on reaction yield (Fig. 7).

  Table 1

  

  Table 1.  Optimization of the parameters for Knoevenagel reaction between benzaldehyde and malononitrile with MOF 4 as the catalyst*

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  Catalyst	Solvent	Yield / %
  MOF 4	Methanol	71.4
  	Dichloromethane	27.7
  	Ethyl acetate	15.6
  	Tetrahydrofuran	28.9
  	Acetonitrile	18.0
  Zn(NO3)2	Methanol	32.9
  L2	Methanol	14.4
  H2IP	Methanol	10.2
  Blank	Methanol	8.9
  * The molar fraction of the catalyst was 3%, and the reaction time was 3 h.

  Figure 7

  

  Figure 7.  Yield of Knoevenagel reaction between benzaldehyde and malononitrile with MOF 4 as the catalyst over reaction time (in methanol)

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  The reusability of MOF 4 has been evaluated. After the reaction, it was separated by centrifugation, washed with copious amounts of methanol, and dried under vacuum at 160 ℃. The recovered MOF 4 was analyzed by PXRD and reused in further reaction under identical conditions. This procedure has been repeated three successive times. MOF 4 catalyst could be reused without a significant loss in activity and structure has not changed (Fig. 8).

  Figure 8

  

  Figure 8.  Reusability of MOF 4 catalyst in Knoevenagel reaction between benzaldehyde and malononitrile

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  To extend the general applicability of MOF 4, its catalytic activity with several aromatic aldehydes with malononitrile has been explored. The formation of the desired product in each case was confirmed by ¹H NMR spectroscopy (Fig.S6-S12). As shown in Table 2, MOF 4 could convert benzaldehyde derivatives to those corresponding condensation products in mild yield.

  Table 2

  

  Table 2.  Knoevenagel condensation reaction of various aldehydes with malononitrile with catalyst MOF 4

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  Entry	R	Yield / %
  1	H	71.4
  2	4-Methyl	66.0
  3	2-Methyl	67.7
  4	4-Ethyl	63.2
  5	4-Methoxy	49.0
  6	4-Ethoxy	42.1

  The activity of MOF 4 for the Knoevenagel condensation reaction was compared with other catalysts (Table 3). Among those catalysts, JNU-401, UiO-66-NH₂, and Zn-6TEA have 3D structures with large holes and exhibit similar catalytic properties with MOF 4.

  Table 3

  

  Table 3.  Comparison of catalytic activity of various MOFs in the Knoevenagel condensation reaction of benzaldehyde and malononitrile

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  Entry	Catalyst	Amount of catalyst*	Experimental condition	Yield / %	Ref.
  1	JNU-401	6 mg	80 ℃, t=1 h	67	35
  2	MOF-PIL-AM	20 mg	Ultrasonic power of 150 W	100	36
  3	Pd@HPW@HP-UiO-66-NH2	20 mg	80 ℃, t=24 h	88	37
  4	UiO-66-NH2	50 mg	80 ℃, t=0.5 h	73	38
  5	Zn-CBS	2%	t=3 h	82	39
  6	10Zn@ZIF-67	2.5%	t=1.5 h	50	40
  7	Zn-6TEA	3%	40 ℃, t=1 h	74	41
  8	Zn2(ptaH)2·11H2O	5%	t=3 h	80	42
  9	[Ni4(μ6-MTB)2(μ2-H2O)4(H2O)4]·10DMF·11H2O	50 mg	130 ℃, t=6 h	78	43
  10	CSMCRI-15	1.5%	60 ℃, t=4 h	80	44
  11	MOF 4	40 mg	RT, t=3 h	71	This work
  * x% represents the molar fraction of the catalyst.

  A mechanism for the MOF 4 catalyzed Knoevenagel condensation reaction of aromatic aldehyde and malononitrile is proposed. The reaction initiates from the activation of aromatic aldehyde with the Lewis acid site Zn ion. The uncoordinated pyridine ring in MOF 4 serves as a base to promote the deprotonation of the active methylene group for the generation of carbanion intermediate Ⅱ. Next, the newly formed carbanion Ⅱ reacts with the activated carbonyl group Ⅰ on the aromatic aldehyde via nucleophilic attack. Lastly, the addition intermediate Ⅲ can be quickly converted to the 2-benzylidenemalononitrile product after obtaining one proton and losing water (Fig. 9).

  Figure 9

  

  Figure 9.  Proposed catalytic mechanism for Knoevenagel condensation reaction catalyzed by MOF 4

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  3.   Conclusions

  MOFs 1-4 were achieved by using a base/acid mixed ligand strategy, i.e., 4-methyl-2, 6-bis-pyridin-3-ylmethylene-cyclohexanone and 4-methyl-2, 6-bis(4-pyridin-4-yl-benzylidene)-cyclohexanone combined with isophthalic acid. Photocatalytic experiments with rhodamine B and pararosaniline hydrochloride have shown MOFs 1-3 have certain activity for dye degradation. MOF 4 could act as a heterogeneous catalyst for the Knoevenagel condensation under mild reaction conditions. It could be reused at least three times while maintaining its catalytic activity. By mixed ligand strategy, it is anticipated that diverse MOF materials could be obtained. Intriguingly, those functional materials can excel in dye degradation and catalytic application.

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

  
  
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  Scheme 1  Structures of ligands L₁, L₂, and H₂IP

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  Figure 1  Structure of MOF 1: (a) asymmetric unit; (b) coordination environment of Zn²⁺ ion; (c) 2D structure; (d) topological structure

  Symmetry codes: ^ⅰ 0.5+x, 1.5-y, -0.5+z; ^ⅱ 0.5-x, 0.5+y, 0.5-z.

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  Figure 2  Structure of MOF 2: (a) asymmetric unit; (b) coordination environment of Cd²⁺ ion; (c) 2D structure; (d) topological structure

  Symmetry codes: ^ⅰ 1+x, y, z; ^ⅱ 1-x, -y, -z; ^ⅲ-1+x, -1+y, -1+z.

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  Figure 3  Structure of MOF 4: (a) asymmetric unit; (b) coordination of L₂ ligand; (c) coordination of IP^2- ligand; (d) 1D chain; (e, f) coordination environment of Zn1

  Symmetry codes: ^ⅰ 1+x, y, z; ^ⅱ-1+x, y, z.

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  Figure 4  Hydrogen bonding interactions (a) and π-π packing patterns (b) in MOF 4

  Symmetry codes: ^ⅰ 2+x, -2+y, -1+z; ^ⅱ 2-x, -1-y, -z.

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  Figure 5  UV-Vis spectra varying with the irradiation time of RhB dye (a-c) and PH dye (d-f) in the presence of MOFs 1-3

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  Figure 6  Degradation rates of the RhB and PH solutions in the presence of MOFs 1-3

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  Scheme 2  Knoevenagel condensation reaction of benzaldehyde and malononitrile

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  Figure 7  Yield of Knoevenagel reaction between benzaldehyde and malononitrile with MOF 4 as the catalyst over reaction time (in methanol)

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  Figure 8  Reusability of MOF 4 catalyst in Knoevenagel reaction between benzaldehyde and malononitrile

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  Figure 9  Proposed catalytic mechanism for Knoevenagel condensation reaction catalyzed by MOF 4

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  Table 1.  Optimization of the parameters for Knoevenagel reaction between benzaldehyde and malononitrile with MOF 4 as the catalyst*

  Catalyst	Solvent	Yield / %
  MOF 4	Methanol	71.4
  	Dichloromethane	27.7
  	Ethyl acetate	15.6
  	Tetrahydrofuran	28.9
  	Acetonitrile	18.0
  Zn(NO3)2	Methanol	32.9
  L2	Methanol	14.4
  H2IP	Methanol	10.2
  Blank	Methanol	8.9
  * The molar fraction of the catalyst was 3%, and the reaction time was 3 h.

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  Table 2.  Knoevenagel condensation reaction of various aldehydes with malononitrile with catalyst MOF 4

  Entry	R	Yield / %
  1	H	71.4
  2	4-Methyl	66.0
  3	2-Methyl	67.7
  4	4-Ethyl	63.2
  5	4-Methoxy	49.0
  6	4-Ethoxy	42.1

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  Table 3.  Comparison of catalytic activity of various MOFs in the Knoevenagel condensation reaction of benzaldehyde and malononitrile

  Entry	Catalyst	Amount of catalyst*	Experimental condition	Yield / %	Ref.
  1	JNU-401	6 mg	80 ℃, t=1 h	67	35
  2	MOF-PIL-AM	20 mg	Ultrasonic power of 150 W	100	36
  3	Pd@HPW@HP-UiO-66-NH2	20 mg	80 ℃, t=24 h	88	37
  4	UiO-66-NH2	50 mg	80 ℃, t=0.5 h	73	38
  5	Zn-CBS	2%	t=3 h	82	39
  6	10Zn@ZIF-67	2.5%	t=1.5 h	50	40
  7	Zn-6TEA	3%	40 ℃, t=1 h	74	41
  8	Zn2(ptaH)2·11H2O	5%	t=3 h	80	42
  9	[Ni4(μ6-MTB)2(μ2-H2O)4(H2O)4]·10DMF·11H2O	50 mg	130 ℃, t=6 h	78	43
  10	CSMCRI-15	1.5%	60 ℃, t=4 h	80	44
  11	MOF 4	40 mg	RT, t=3 h	71	This work
  * x% represents the molar fraction of the catalyst.

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