Synthesis and structure of three new alkaline earth metal-organic

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Synthesis and structure of three new alkaline earth metal-organic frameworks with high thermal stability as catalyst for Knoevenagel condensation Leila Asgharnejad, Alireza Abbasi, Mahnaz Najafi, and Jan Janczak Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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Crystal Growth & Design

1

Synthesis and structure of three new alkaline earth metal-organic

2

frameworks with high thermal stability as catalyst for

3

Knoevenagel condensation

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Leila Asgharnejad1, Alireza Abbasi2*, Mahnaz Najafi2, Jan Janczak3

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6

1

7

8

2

School of Chemistry, Alborz Campus, University of Tehran, Tehran, Iran.

School of Chemistry, College of Science, University of Tehran, P.O. Box 14155-6455, Tehran, Iran.

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10

3

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, PO Box 1410, 50-950 Wrocław, Poland.

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12 13

*Corresponding

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Prof. Alireza Abbasi

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School of Chemistry, College of Science,

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University of Tehran, Tehran, Iran.

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Tell: +98-21-61113644

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Fax: +98-21-66495291

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Email: [email protected]; [email protected]

author:

1 ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Three new alkaline earth metal-organic frameworks (MOFs), [Mg2(BDC-OH)2(DMF)3] (Mg-

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HBDC), [Ca(BDC-OH)(DMF)2] (Ca-HBDC) and [Sr(BDC-OH)(DMF)] (Sr-HBDC) (BDC-OH

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= 2-hydroxyterephthalate(2-) anion and DMF = N,N-dimethylformamide), were synthesized by

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the assembly of BDC-OH and nitrate salts of the metal ions under solvothermal conditions.

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Single-crystal structure analysis revealed that Ca-HBDC and Sr-HBDC are isostructural to their

26

corresponding

27

terephthalate(2-) anion (BDC), respectively. The MOFs were also characterized by FT-IR,

28

thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD). The obtained MOFs

29

were applied as heterogeneous basic catalysts for Knoevenagel condensation reaction at room

30

temperature. The catalysts showed good catalytic activity and structural stability in the

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condensation reaction and could be reused without observable loss of activity.

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Keywords: Alkaline earth MOFs, Crystal structure, Heterogeneous catalysts, Knoevenagel

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condensation reaction

MOFs

containing

2,5-dihydroxyterephthalalate(2-)

anion

(H2dhtp)

and

34 35

36

37 38 39 40 41


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1. INTRODUCTION

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Metal-organic frameworks (MOFs) are inorganic-organic hybrid materials constructed from

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organic linkers and metal ions (or clusters) possessing the advantages of both inorganic and

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organic components with unique properties.1-3 To synthesize MOFs, metal cations such as

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transition,4 rare-earth,5 lanthanide,6 main group7,

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organic linkers. Alkaline earth MOFs based on Mg, Ca, Sr and Ba have been synthesized,

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however have not been extensively studied compared to lanthanide and transition metal-based

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MOFs.10, 11

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Unique features of alkaline earth MOFs such as reduced toxicity, low density, stability in air and

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fairly low cost make them interesting candidate to be studied.12 These MOFs have found

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applications in gas adsorption,13,

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alkaline earth MOFs were applied as solid bases for “based-catalyzed reactions” including

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Knoevenagel condensation, aldol condensation, and Michael addition.18

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Knoevenagel condensation between a C=O group and an activated methylene group is important

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in drug industry and can be catalyzed by zeolites, Lewis acids, organometallic catalysts and

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amino-functionalized supports. Some of these materials involve using hazardous solvents and

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suffer from some limitations such as lack of reusability and high catalyst loading.19,

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research groups have been working on the development of MOFs as basic catalysts with

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advantages over conventional inorganic counterparts. In this connection condensation reactions

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was carried out to assess the basic properties of some MOFs such as M2(BTC)(NO3)(DMF) (M =

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Ba/Sr, H3BTC = 1,3,5-benzenetricarboxylic acid)21, Ba(pdc)H2O22 and Mg3(pdc)(OH)3(H2O)2

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(H3pdc = 3,5-pyrazoledicarboxylic acid).16

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catalysis15,

16

8

and actinide metals9 can assemble with

and luminescent.17 In the field of catalysis,

20

Many



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Alkaline earth MOFs exhibited acidic, basic or acidic-basic sites which can interact with

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substrates.18, 23 These MOFs with high and uniform dispersion of metal-oxygen bonds are well-

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known basic species.18 Acidic sites in these catalysts can be due to the existence of unsaturated

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metal centres24, 25 or potential Lewis acid sites which will be created upon MOF activation.23, 26

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Moreover, polyhedral symmetry distortions in alkaline earth MOFs can facilitate the formation

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of vacant position in the coordination sphere and generate Lewis acid sites.27

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Here, we have synthesized three MOFs (i.e. M-HBDC (M = Mg, Ca and Sr)) based on alkaline

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earth metal ions and BDC-OH ligand and additionally their catalytic activity for Knoevenagel

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condensation reaction at room temperature has been investigated.

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2. EXPERIMENTAL

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2.1 Materials and characterization

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Powder X-ray diffraction (PXRD) data were collected on a PANalytical X'Pert PRO instrument

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using Cu Kα radiation (λ = 1.5406 Å). FT-IR spectra were recorded using a Bruker Equinox 55

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spectrometer equipped with a single reflection diamond ATR system. TGA experiments were

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conducted in N2 atmosphere by means of Dupont 951 Thermogravimetric Analyzer. The results

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of the catalytic tests were analysed by a gas chromatograph (Agilent 7890A) equipped with a

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capillary column (HP-1) and a flame ionization detector (FID). Gas chromatography-mass

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spectrometry (GC-MS) was performed on an Agilent 7890A GC system equipped with an

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Agilent Technologies 5975C VL MSD with Triple Axis Detector Mass Spectrometer with a Rtx-

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5MS column.

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2.2 Synthesis of Mg-HBDC


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The BDC-OH ligand was synthesized according to previous method reported in the literature.28

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To synthesize Mg-HBDC, Mg(NO3)2.6H2O (0.026 g, 0.101 mmol) and BDC-OH (0.018 g, 0.099

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mmol) were added into the mixture of DMF (2.5 mL) and ethanol (1 mL) in a glass vessel. The

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vessel was sealed and the mixture was stirred for 30 min followed by keeping at 90 ºC for 20 h.

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After slow cooling, needle-like crystals of Mg-HBDC were isolated and washed with DMF for

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several times. Elemental analysis found/calcd.: C, 48.03/47.88; H, 4.39/4.50; N, 6.82/6.70.

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2.3 Synthesis of Ca-HBDC

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Ca(NO3)2.4H2O (0.025 gr, 0.106 mmol) and BDC-OH (0.018 g, 0.099 mmol) were added to

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DMF (2.5 mL) and ethanol (1 mL) in a vessel. The vessel was sealed and the mixture was stirred

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for 30 min at room temperature and then kept at 80 ºC for 24 h. After slow cooling, plate-like

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crystals of Ca-HBDC were collected and washed with DMF for several times. Elemental

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analysis found/calcd.: C, 45.63/46.02; H, 4.83/4.69; N, 7.42/7.67.

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2.4 Synthesis of Sr-HBDC

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Needle-like crystals of Sr-BDC were synthesized identical to the procedure mentioned for Mg-

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HBDC, except that Sr(NO3)2 (0.026 g, 0.123 mmol) was used instead of Mg(NO3)2.6H2O, and

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the reaction temperature was 115 ºC. Elemental analysis found/calcd.: C, 38.80/38.76; H,

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3.43/3.25; N, 4.20/4.11.

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2.5 Catalytic activity

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To perform the Knoevenagel condensation, 0.06 mmol (based on metal) of the desired MOF was

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dispersed in solvent (3mL). Benzaldehyde (1.1 mmol) and ethyl cyanoacetate (0.9 mmol) were

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added to the suspension. The condensation reaction was performed at room temperature for 1 h

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while stirring. The solid product was dissolved by the addition of DMF (6 mL) and then

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analyzed by GC and GC-MS.



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2.6 X-ray single crystal data collection and refinement

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Single-crystal X-ray diffraction data for the MOFs were collected on a four-circle  geometry

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KUMA KM-4 diffractometer equipped with a two-dimensional area CCD detector using

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graphite-monochromated Mo Kα radiation. The structures were solved by the direct method

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using SHELXT and refined using SHELXL-2014/7 program.29 All non-hydrogen atoms were

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refined anisotropically. H atoms were treated as riding atoms in geometrically idealized positions

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with Uiso=1.2Ueq of C of aromatic ring or 1.5Ueq of C for DMF. The CCDC numbers are

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1571177 (Mg-HBDC), 1860716 (Ca-HBDC) and 1571180 (Sr-HBDC). The crystallographic

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data are summarized in Table 1.

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Table 1 Crystal structure and data refinement for M-HBDC (M = Ca, Mg, Sr) Compound Mg-HBDC Ca-HBDC Sr-HBDC Empirical formula C25H28Mg2N3O13 C14H17CaN2O7 C11H11NO6Sr Molecular weight 627.12 365.38 340.83 Temperature (K) 100(2) 100(2) 150(2) Wavelength, Mo Kα (Å) 0.71073 0.71073 0.71073 Space group Pbca C2/c P32 a (Å) 18.0520(8) 18.583(2) 10.6058(4) b(Å) 17.7654(7) 9.3784(11) 10.6058(4) c (Å) 18.5665(9) 9.6955(12) 9.8468(6) α (°) 90 90 90 β (°) 90 93.044(9) 90 γ (°) 90 90 120 Cell volume (Å3) 5954.3(5) 1687.3(4) 959.21(9) Z 8 4 3 ρ (g cm-3) 1.399 1.438 1.770 μ (mm-1) 0.150 0.410 4.237 Total reflections 66441 9309 11637 Unique reflections 7431 2109 3129 Observed reflections [F2> 2σ(F2)] 3996 1793 2796 Rint 0.0628 0.0401 0.0524 Data/restraints/parameters 7431/0/407 2109/0/117 3129/ 1/175 Flack parameter -0.014(7) Goodness-of-fit (GOF) on F2 1.000 1.002 1.004 R [F2> 2σ(F2)] (R1, wR2) *) 0.0813, 0.1432 0.0489, 0.1152 0.0476, 0.1031 R (all data) (R1, wR2)*) 0.2000, 0.1834 0.0595, 0.1226 0.0576, 0.1092 Δρmax, Δρmin (e Å–3) 0.975, -0.340 0.759, -0.354 1.142, -.0521 *)wR={Σ [w(F 2–F 2)2]/ΣwF 4}½; w–1=σ2(F 2) + (aP)2 + bP where P = (F 2 + 2F 2)/3. The a and o c o o o c b parameters are 0.0683 and 1.7255 for Mg-HBDC, 0.0523 and 4.6123 for Ca-HBDC and 0.0592 and 1.1135 for Sr-HBDC.


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3. RESULTS AND DISCUSSION

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Ying Yang et al. previously reported Mg3(BDC-NH2)3(DMF)4 and Sr(BDC-NH2)(DMF) based

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on BDC-NH2 ligand (BDC-NH2 = 2-aminoterephthalate).14 Inspired by the synthetic process of

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these MOFs, we made an attempt to isolate the M-HBDC (M = Mg, Ca and Sr) crystals by

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utilizing BDC-OH instead of BDC-NH2. Firstly, we achieved the isolation of Sr-HBDC at 115

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ºC by altering the synthesis conditions including ratios of starting materials and solvents as well

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as reaction time and temperature of the solvothermal treatment. Then, almost the same ratios of

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starting materials and solvents were applied for the isolation of Ca-HBDC and Mg-HBDC. We

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should note that decreasing reaction temperature made it possible to obtain Ca-HBDC and Mg-

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HBDC at 80 ºC and 90 ºC, respectively.

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3.1 Structure description of Mg-HBDC

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The Mg-HBDC structure crystallizes in orthrhombic with Pbca space group and contains two

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crystallographically unique magnesium ions. The asymmetric unit of this compound, shown in

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Fig. 1a, includes two Mg2+, three DMF molecules and two BDC-OH ligands. In one of the BDC-

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OH ligands, the hydroxyl group appeared in two positions with the occupation of 0.591(6) and

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0.409(6) for O152 and O151 atoms (Fig. 1a), so it has two orientations.

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Figure 1 a) Asymmetric unit and b) 3-D structure of Mg-HBDC, showing MgO6 octahedra linked by

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BDC-OH ligands.

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The coordination environment of Mg 1 is filled with one O atom of a DMF molecule (Mg1-O21,

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2.116(3) Å) and five O atoms from BDC-OH ligand with the Mg1-O bond lengths from 2.005(3)

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to 2.229(3) Å. The BDC-OH in the structure of Mg-HBDC displayed two different coordination

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modes. Two BDC-OH ligands, in which both carboxylate groups have the same coordination

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fashion, exhibit the η1:η1:µ2-COO- coordination mode and bond to Mg1 through O11 and O14

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atoms (Scheme 1a). Two of the ligands around Mg1 adopt the coordination mode and in which

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the carboxylate groups in each ligand exhibit both η1:η1:µ2-COO- and η1:η2:µ2-COO-

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coordination modes and the hydroxyl group is adjacent to the carboxylate with η1:η1:µ2-COO-

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fashion (Scheme 1b). One of these ligands is coordinated to Mg1 through O4 and O3 (with

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η1:η2:µ2-COO- fashion) while the other is connected to the metal via its O1 atom (with η1:η1:µ2-

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COO- mode). The Mg2 centre is six-coordinated and binds to two O atoms of two DMF

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molecules (Mg2-O41, 2.069(3) Å and Mg2-O31, 2.117(3) Å) as well as four O atoms belonging

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to BDC-OH ligands with Mg2-O bond distances value from 2.041(3) to 2.096(3) Å. Three O

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atoms around Mg2 (O3, O12 and O13) come from the ligand with η1:η1:µ2-COO- coordination 8 ACS Paragon Plus Environment

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mode. The selected bond lengths and angles are given in Table S1 and are in agreement with

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literature.26 The octahedrons of MgO6 around Mg1 and Mg2 are joined in the corner via O3 and

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are further interconnected by BDC-OH ligands to generate the 3-D MOF of Mg-HBDC seen in

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Fig. 1b. To the best of our knowledge, there is only one report, [(CH3)2NH2][Mg3(OH)(BDC-

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OH)(TPT)] (TPT = 2,4,6-tri(4-pyridinyl)-1,3,5-triazine), for magnesium MOF based on BDC-

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OH,

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[(CH3)2NH2][Mg3(OH)(H2dhtp)3(H2O)3],

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[Mg2(H2dhtp)1.5(DMAc)4]Cl·DMAc were previously reported (H4dhtp = 2,5-dihydroxy-

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terepthalic

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[Mg2(H2dhtp)1.5(DMAc)4]Cl·DMAc displayed similar coordination modes to those observed for

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BDC-OH ligand in Mg-HBDC. Unlike this structure, the hydroxyl groups in Mg-HBDC are not

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coordinated to the metal centres and just involve in hydrogen bonding interactions.

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Topological representation of an underlying two-nodal 3,5-connected framework is characterized

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by {63}{69.8} point symbol (gra topological type), wherein {63} notation represents node

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centered in the ligand (blue), and {69.8} notation represents node centered on Mg ion (magenta)

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(Figure 2a).

however

acid

magnesium

and

DMAc

frameworks

=

based

on

H4dhtp

ligand

such

[(CH3)2NH2][Mg3(OH)(H2dhtp)3(TPT)]30

N,N-dimethylacetamide).31

The

H4dhtp

ligand

as and

in

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Scheme 1 Coordination modes of BDC-OH ligand observed in the prepared MOFs.

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176 177

Figure 2 Topological representation of a) Mg-HBDC along c axis, b) Ca-HBDC along b axis and c) Sr-

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HBDC along c axis.

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3.2 Structure description of Ca-HBDC

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Structural analysis of Ca-HBDC exhibits a 3-D extended structure that crystallizes in the

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monoclinic space group C2/c. The molecular structure of Ca-HBDC is displayed in Fig. 3a and

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contains one Ca2+, half of BDC-OH and one DMF in the asymmetric unit. The 2-

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hydroxyterephthalate(2-) anion ligand lies in the inversion centre, so the hydroxyl group


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statistically occupied two positions (see Fig. 3a). The metal centre is six-coordinated and is

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connected to four O atoms belonging to four carboxylate groups of BDC-OH ligands with Ca-O

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bonds value from 2.2938(15) to 2.3287(15) Å and two O atoms from DMF molecules (Ca-O,

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2.3737(17) Å). The bond lengths for this structure, given in Table S2, lie within the range of

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those reported previously.31, 32 Each BDC-OH ligand in this MOF exhibits η1:η1:µ2-COO- mode

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for the carboxylate groups (Scheme 1a). The adjacent Ca2+ ions are linked by two bridging BDC-

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OH with the distance of 4.980 Å and form an eight-membered ring Ca2C2O4 seen in Fig 3b. The

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3-D extended structure shows channels running along c-axis and the coordinated DMF molecules

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are arrayed towards the channels (Fig. 3c). The Ca-HBDC is the first example of MOF based on

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Ca2+ and BDC-OH ligand. This structure is isostructural to previously reported

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[Ca(H2dhtp)(DMF)2] (H2dhtp = 2,5- dihydroxyterephthalate anion) containing functionalized

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BDC ligand.32

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Topological analysis of this structure exhibits a two-nodal 4,4-connected framework

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characterized by {42.84} point symbol (pts topological type), wherein node centered in the ligand

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is drawn in magenta, and node centered on Ca ion is drawn in blue (Figure 2b).

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201 202



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Figure 3 a) Molecular structure of Ca-HBDC with thermal displacement ellipsoids at the 50 %

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probability level, showing (non-hydrogen) atom-labelling scheme of the asymmetric unit and b) the

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structure viewed along [010] axis, exhibiting the CaO6 octahedra and c) 3-D extended network of Ca-

206

HBDC.

207 208

3.3 Structure description of Sr-HBDC

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Compound Sr-HBDC crystallizes in trigonal P32 space group. As seen in Fig. 4a, the

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asymmetric unit of Sr-HBDC contains one Sr2+, one DMF molecule and one BDC-OH ligand.

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Each metal centre is coordinated to eight O atoms including one O from DMF molecule (O6) and

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seven O atoms of five BDC-OH anionic ligands to form bicapped octahedrons around Sr2+ ion.

213

Two carboxylate groups in each BDC-OH displayed different coordination modes (Scheme 1c).

214

The Sr2+ is bound to two O atoms (O3 and O4) from BDC-OH ligands in which the carboxylate

215

groups adopt the η2:η2:µ3-COO- coordination mode. The other two BDC-OH ligands around Sr2+

216

exhibit carboxylate groups with η1:η2:µ2-COO- fashion. One of these molecules is coordinated

217

through O1 and the other is connected to Sr2+ via O1 and O2 atom. The coordination site of the

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metal ion is completed with two O atoms (O3 and O4) from one BDC-OH ligand containing a

219

carboxylate group with η2:η2:µ3-COO- coordination mode. In the bicapped octahedron, the Sr-O

220

bond lengths ranges from 2.489(6)-2.742(5) Å and the shortest Sr-O6 length is from DMF

221

molecule. The bond lengths and angles presented in Table S3, are in agreement with the those

222

reported in the literature.33 The cis and trans O-Sr-O angles deviate from the corresponding ideal

223

angles of 90º and 180º in a bicapped octahedron.

224


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225 226 227

Figure 4 a) Asymmetric unit of Sr-HBDC, showing thermal displacement ellipsoids at the 50 %

228

probability level and b) 3-D extended network displaying SrO8 bicapped octahedral linked by BDC-OH.

229 230

The adjacent bicapped octahedral are joined together through their faces to form chains along c

231

axis which are linked via bridging BDC-OH molecules and generate a 3-D network containing

232

triangular channels. Two types of channels exist in this structure; the smaller ones are empty

233

while the larger ones are occupied with DMF molecules. To the best of our knowledge, Sr-

234

HBDC is the first structure of strontium MOF which is constructed by BDC-OH ligand. The

235

MOF is isostructural to [Sr(BDC)(DMF)] (BDC = terephthalate(2-) ligand) that is formed based

236

on non-functionalized BDC ligand.34

237

Topological representation of an underlying two-nodal 5,5-connected framework is characterized

238

by {43.64.83}{46.64} point symbol (unknown topological type), wherein {43.64.83} notation

239

represents node centered in the ligand (blue), and {46.64} notation represents node centered on Sr

240

ion (magenta) (Figure 2c).

241



Page 14 of 38

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3.4 PXRD of M-HBDC (M = Mg, Ca, Sr)

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The PXRD patterns of the MOFs are illustrated in Fig. 5. The purity of the prepared MOFs can

244

be confirmed by comparing the experimental PXRD patterns to their corresponding simulated

245

patterns. The simulated PXRD of Ca-HBDC and Sr-HBDC are similar to the patterns of

246

[Ca(H2dhtp)(DMF)2] and [Sr(BDC)(DMF)] reported by Liang et al.

247

respectively (Fig. S1). It is observed that the structure and pore shapes of Ca-HBDC is similar to

248

[Ca(H2dhtp)(DMF)2] and the Sr-HBDC is isostructural to [Sr(BDC)(DMF)].

32

and Pan et. al.34,

249

250 251

Figure 5 PXRD patterns of M-HBDC (M = Mg, Ca, Sr). (The intensity differences in the experimental

252

and simulated patterns in b and c are due to the preferred orientation effects of the samples.)

253 254

3.5 Thermal stability of M-HBDC (M = Mg, Ca, Sr)

255

Thermogravimetric analyses of the MOFs were conducted to study the thermal stability of the

256

prepared compounds (Fig. 6). TGA analyses revealed that all three MOFs possess high thermally

257

stable. In the TG curve of Mg-HBDC, the initial weight loss below 140 ºC could be ascribed to

258

the removal of surface-adsorbed solvents. The MOF exhibits the first weight loss up to 200 ºC


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259

with the weight loss of 14 %, which could be due to the removal of one coordinated DMF

260

molecule. The removal of the other two DMF ligands occurs at 200-320 ºC with the weight loss

261

of 25 % (calculated 23.3 %) and then the framework collapse begins at 320 ºC. The Ca-HBDC

262

and Sr-HBDC show no weight loss up to 150 ºC and then the solvent removal and the

263

framework decomposition occur up to 900 ºC. The Ca-HBDC and Sr-HBDC lost 83.9 % and

264

76.6 % of their weight at 900 ºC, respectively.

265

266

Figure 6 TGA curves of M-HBDC (M = Mg, Ca, Sr).

267 268 269

3.6 Catalytic tests

270

To inspect the catalytic activity of the MOFs, they were employed as heterogeneous catalysts for

271

Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate. The condensation reaction

272

was performed at room temperature for 1 h and the results are summarized in Table 2. The

273

formation of the product (ethyl trans-α-cyanocinnamate) was confirmed by GC-MS (Fig. S2). It

274

can be seen that all three MOFs showed good catalytic activity in the reaction at room

275

temperature in all solvents. The MOFs displayed more catalytic activity in ethanol rather than



Page 16 of 38

276

dichloromethane and toluene. This can be explained with increasing polarity of the solvents as

277

ethanol> dichloromethane>toluene. Similar results were reported for the production of 4H-pyran

278

derivatives

279

naphthalenedicarboxylic acid), as catalyst and ethanol as polar protic solvent.26

280

The condensation reaction was carried out in ethanol without addition of the MOFs under similar

281

conditions to prove the importance of the catalysts in the process (Table 2, entry 13). No product

282

formation was observed in these conditions after 1 h, indicating the catalytic role of the MOFs in

283

the reaction.

using

Mg-based

MOF,

[Mg3(NDC)3(DMF)4].H2O

(NDC

=

2,6-

284 285

Table 2a Catalytic activity of the obtained MOFs for the Knoevenagel condensation of benzaldehyde and

286

ethyl cyanoacetate

Entry Catalyst Solvent Conversion (%)b 1 Mg-HBDC Toluene 80 2 Ca-HBDC Toluene 62 3 Sr-HBDC Toluene 58 4 Mg-HBDC Dichloromethane 85 5 Ca-HBDC Dichloromethane 64 6 Sr-HBDC Dichloromethane 63 7 Mg-HBDC Ethanol 95 8 Ca-HBDC Ethanol 89 9 Sr-HBDC Ethanol 92 10c Mg-HBDC Ethanol 82 11c Ca-HBDC Ethanol 78 12c Sr-HBDC Ethanol 80 13 None Ethanol 0 aReaction conditions: benzaldehyde (1.1 mmol), ethyl cyanoacetate (0.9 mmol), catalyst (0.06 mmol), solvent (3 ml), room temperature, 1 h . bGC yield is based on ethyl cyanoacetate. cCondensation reactions were carried out using the recovered catalysts. 287


Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


288 289

The reusability of the MOFs was also examined in the catalyzed condensation reaction and the

290

results are given in Table 2 (entry 10-12). To inspect the reusability, MOFs were applied for the

291

condensation of benzaldehyde with ethyl cyanoacetate using ethanol as solvent. After 1 h, the

292

white solid product was dissolved by the addition of DMF, the catalysts were separated by

293

filtration, washed with DMF and then ethanol for several times. The recycled catalysts were

294

reused for the reaction under similar conditions mentioned for the fresh MOFs. The results reveal

295

that the MOF catalysts can be reused for the reaction without noticeable decrease in their

296

catalytic activity.

297

The PXRD patterns of the recovered MOFs are presented in Fig. 5. The diffraction peaks for the

298

recycled MOFs are similar to the peaks of the corresponding fresh MOFs, indicating that the

299

structure of the MOFs remained unchanged during Knoevenagel condensation.

300

The FT-IR spectra of the recycled MOFs (Fig. 7) correspond to the spectra of the pristine MOF

301

catalysts and displayed the characteristic peaks of the BDC-OH and DMF ligands. In the fresh

302

MOFs, the νas (C-O) vibrations of BDC-OH ligand appeared at 1650 and 1593 cm-1 (for Mg-

303

HBDC), 1652 and 1585 cm-1 (for Ca-HBDC) as well as 1658 and 1564 cm-1 (for Sr-HBDC).

304

The peak exists at 1398-1417 cm-1 in the spectra can be attributed to νs (C-O) vibrations of this

305

ligand.35-37 For all three MOFs, two peaks are seen around 670 and 960 cm-1 which can be

306

assigned to DMF ligands.26

307



Page 18 of 38

308

Figure 7 FT-IR of M-HBDC (M = Mg, Ca, Sr).

309 310

311

Knoevenagel condensation reactions are known to be catalyzed in the presence of acidic, basic

312

and acid-base bifunctional catalysts. Regarding MOFs containing carboxylate ligands, the Mn+-

313

O2- Lewis acid-base can catalyze Knoevenagel condensation.38 For instance, the catalytic activity

314

of

315

pyridylcarboxaldehydeisonicotinoylhydrazone)39 and [Zn2(oba)4(3-bpdh)2].4H2O (oba = 4,4'-

316

oxybis(benzoic acid) and 3-bpdh = N,N'-bis-(1 pyridine-3-yl-ethylidene)-hydrazine)40 have been

317

associated with the Mn+-O2- Lewis acid-base pair which can catalyze Knoevenagel condensation.

318

Concerning M-HBDC (M = Mg, Ca, Sr) MOFs reported here, the polarized Mn+-O2- bonds in the

319

structures endow the MOFs with cooperative Lewis acid-base sites which can synergistically

320

catalyze the reaction. Based on the reported mechanism for Knoevenagel condensation40 , the

321

alkaline earth ions impart the acidic sites and interact with the C=O group of benzaldehyde

322

which improves the electrophilicity of the carbon of C=O group. Also, the basic sites of the

323

MOFs which are associated with the O atoms of BDC-OH can interact with the methylene group

324

of ethyl cyanoacetate and facilitate its reaction with benzaldehyde.

[Zn(ADA)(L)].2H2O

(ADA

=

1,3-adamantanediacetic

acid

and

L

=

4-


Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


325

The catalytic activity of the prepared MOFs for Knoevenagel condensation of benzaldehyde with

326

ethyl cyanoacetate is compared to some reported catalysts (Table 3). The M-HBDC MOFs are

327

capable of catalyzing the condensation reaction at room temperature compared to some other

328

MOF catalysts (Entries 1 and 2). Also, our MOFs afforded high conversions (above 89 %) within

329

shorter reaction time (1 h) at room temperature using ethanol as green solvent, whereas other

330

alkaline earth MOFs in Table 3 (Entries 3 and 4) performed at higher temperatures in the

331

presence of toluene.

332

333

Table 3 Comparison of the catalytic activity of some MOFs in the Knoevenagel condensation of

334

benzaldehyde and ethyl cyanoacetate Entry

Catalyst

Substrates

1

UiO-66-NH2 (0.144 g)

Benzaldehyde (5 mmol), cyanoacetate (10 mmol) Benzaldehyde (8 mmol), cyanoacetate (7 mmol) Benzaldehyde (1 mmol), cyanoacetate (1 mmol) Benzaldehyde (1 mmol), cyanoacetate (1 mmol) Benzaldehyde (1.1 mmol), cyanoacetate (0.9 mmol) Benzaldehyde (1.1 mmol), cyanoacetate (0.9 mmol) Benzaldehyde (1.1 mmol), cyanoacetate (0.9 mmol)

2

3

4

5

6

7

MIL-101(Cr) (encapsulated with 20 wt % POM) (0.5 g) Mg2dobdc [dobdc4= 2,5dioxidoterephthalate] (0.073 g) Ba2(BTC)(NO3) [BTC3= 1,3,5benzenetricarboxylate] (0.05 g) Mg-HBDC (0.019 g)

Ca-HBDC (0.022 g)

Sr-HBDC (0.020 g)

Solvent

Temperature (ºC) 80

Time (h) 2

Conversion (%) 94

References

Ethyl

Ethanol (5 mL)

41

Toluene (5 mL)

40

1.25

~ 87

42

Ethyl

Toluene (2 mL)

70

24

14

43

Ethyl

Toluene (2 mL)

110

24

85

21

Ethyl

Ethanol (3 mL)

r.t.

1

95

This work

Ethyl

Ethanol (3 mL)

r.t.

1

89

This work

Ethyl

Ethanol (3 mL)

r.t.

1

92

This work

Ethyl



Page 20 of 38

335

CONCLUSIONS

336

We have synthesized three new MOFs (M-HBDC (M = Mg, Ca, Sr)) through solvothermal

337

assembly of alkaline earth metal salts and BDC-OH ligand. Ca-HBDC and Sr-HBDC structures

338

are the first MOFs reported based on Ca2+ and Sr2+ using BDC-OH ligand. Ca-HBDC is

339

isostructural to the known [Ca(H2dhtp)(DMF)2] constructed by H2dhtp2- ligand. Sr-HBDC

340

displayed a new topological type and is isostructural to the known [Sr(BDC)(DMF)] synthesized

341

using BDC ligand. The MOFs showed good stability and represented good catalytic activity for

342

Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate at room

343

temperature under mild conditions. The reported MOFs are able to catalyze Knoevenagel

344

condensation reaction within a short reaction time in ethanol as green solvent. Considering the

345

metal ions and their coordination environment, the prepared catalysts could be considered as

346

acid-base catalysts which catalyze the reaction synergistically. The alkaline earth MOFs with

347

excellent dispersion of active catalytic sites and high thermal stability will be attractive in the

348

field of heterogeneous catalysis in terms of sustainability.

349 350

ASSOCIATED CONTENT

351

Supporting Information

352

The following files are available free of charge at http://pubs.acs.org.

353

The selected bond distances and angles in the crystal structures of Mg-HBDC, Ca-HBDC and Sr-

354

HBDC (Table S1-S3), comparisons of simulated patterns of Ca-HBDC and Sr-HBDC with their

355

corresponding isostructural MOFs (Figure S1), GC-Ms for catalytic reaction with Mg-HBDC

356

(Figure S2).


Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


357

Cif files for Mg-HBDC, Ca-HBDC and Sr-HBDC were deposited in the Cambridge Structural

358

Database with CCDC deposition numbers 1571177, 1860716 and 1571180 respectively.

359 360

ACKNOWLEDGEMENTS

361

We gratefully acknowledge University of Tehran for the financial support.

362 363

REFERENCES

364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395

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497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515


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516


For Table of Contents Use Only

517

518

Synthesis and structure of three new alkaline earth metal-organic

519

frameworks with high thermal stability as catalyst for

520

Knoevenagel condensation

521

522

Leila Asgharnejad, Alireza Abbasi, Mahnaz Najafi, Jan Janczak

523

524 525



Page 26 of 38

526

Metal-organic frameworks (MOFs) were solvothermally synthesized using alkaline earth metal

527

ions and 2-hydroxyterephthalate(2-) anion (BDC-OH). The MOFs were employed as

528

heterogeneous catalysts for Knoevenagel condensation reaction at room temperature.

529


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graphical abstract 60x45mm (300 x 300 DPI)

ACS Paragon Plus Environment


Fig. 1 80x34mm (300 x 300 DPI)


Page 28 of 38

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Figure 2 99x24mm (300 x 300 DPI)



Figure 3 119x45mm (300 x 300 DPI)


Page 30 of 38

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Page 34 of 38

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Scheme 1 43x37mm (300 x 300 DPI)



Figure for Table 2 77x19mm (300 x 300 DPI)


Page 36 of 38

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Fig. S1 82x24mm (300 x 300 DPI)



Fig. S2 39x31mm (300 x 300 DPI)


Page 38 of 38

Synthesis and structure of three new alkaline earth metal-organic

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