Metal organic frameworks á heterogeneous catalysts for the synthesis ò quinazolinones and pyridines

19 1.4 T HE QUINAZOLINONES SYNTHESIS OF 2- ARYLINDOLES WITH AMINES UTILIZING C U -MOF-74 AS AN EFFICIENT HETEROGENEOUS CATALYST .... 31: One-pot oxidative synthesis of quinazolinones

NGUYEN THI NGOC TRAN

METAL-ORGANIC FRAMEWORKS AS HETEROGENEOUS CATALYSTS

FOR THE SYNTHESIS OF

QUINAZOLINONES AND PYRIDINES

M ENG THESIS

Major: Chemical engineering Major ID: 60 52 03 01

CÔNG TRÌNH ĐƯỢC HOÀN THÀNH TẠI TRƯỜNG ĐẠI HỌC BÁCH KHOA – ĐHQG – HCM

Cán bộ hướng dẫn khoa học 1 : GS.TS Phan Thanh Sơn Nam

(Ghi rõ họ, tên, học hàm, học vị và chữ ký) Cán bộ hướng dẫn khoa học 2 :

(Ghi rõ họ, tên, học hàm, học vị và chữ ký) Cán bộ chấm nhận xét 1 : PGS.TS Nguyễn Thị Phương Phong

(Ghi rõ họ, tên, học hàm, học vị và chữ ký) Cán bộ chấm nhận xét 2 : TS Lê Vũ Hà

(Ghi rõ họ, tên, học hàm, học vị và chữ ký)

Luận văn thạc sĩ được bảo vệ tại Trường Đại học Bách Khoa, ĐHQG TP HCM ngày 12 tháng 01 năm 2019

Thành phần Hội đồng đánh giá luận văn thạc sĩ gồm:

(Ghi rõ họ, tên, học hàm, học vị của Hội đồng chấm bảo vệ luận văn thạc sĩ)

ĐẠI HỌC QUỐC GIA TP.HCM

TRƯỜNG ĐẠI HỌC BÁCH KHOA

CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM

Độc lập - Tự do - Hạnh phúc

NHIỆM VỤ LUẬN VĂN THẠC SĨ

Họ tên học viên: NGUYỄN THỊ NGỌC TRÂN MSHV:1770010

Ngày, tháng, năm sinh: 19/11/1994 Nơi sinh: Long An Chuyên ngành: Kỹ thuật hóa học Mã số : 60520301

I TÊN ĐỀ TÀI:

Metal-organic frameworks as heterogeneous catalysts for the synthesis of

quinazolinones and pyridines

II NHIỆM VỤ VÀ NỘI DUNG:

- Sử dụng xúc tác dị thể Cu-MOF-74 cho phản ứng tổng hợp quinazolinones

- Sử dụng xúc tác dị thể MOF VNU-20 cho phản ứng tổng hợp pyridines

III NGÀY GIAO NHIỆM VỤ : 13/08/2018

IV NGÀY HOÀN THÀNH NHIỆM VỤ: 12/01/2019

ACKNOWLEDGEMENTS

First and foremost, I would like to thank Prof Phan Thanh Son Nam for the financial support for this project and also gave me guidance on this thesis with his comprehensive knowledge Working with them is an honor and a valuable experience for me

Especially, my profound gratitude is expanded to all the teaching staffs of the Organic Chemistry Department, for the valuable information provided by them in their respective fields Their unconditional love and support have always accompanied with every achievement in my life

In addition, I would like to thank my talented and loyal friends: Mr Phuc H Pham and Mr Vu H H Nguyen, Miss Tram T Van, Miss Que T D Nguyen for their encouragement and support during my hardest time Their advices made me always have the positive attitude and helped me complete this thesis

Finally, I would like to express my sincere gratitude to my parents Their love, encouragement and continuous support have always been with me in every achievement I get in my life

Ho Chi Minh City, December, 2018

Nguyen Thi Ngoc Tran

ABSTRACT

Generally, this thesis focuses on applying metal-organic frameworks as efficient solid catalysts for diverse transformations According to that, two MOFs were successfully synthesized by solvothermal method A crystalline porous copper-based metal-organic framework named Cu-MOF-74 was generated from Cu(NO3)2.3H2O and 2.5-dihydroxyterephthalic acid while a mixed-linker iron-based MOF named VNU-20 [Fe3(BTC)(NDC)2·6.65H2O] was prepared from 1,3,5-benzenetricarboxylic acid, 2,6-naphthalenedicarboxylic acid and FeCl2 Physical characterizations of the solid catalysts were obtained by several analysis techniques including powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), and atomic absorption spectroscopy (AAS) The results indicated that the desired structures of the MOFs were obtained

For the first time, the Cu-MOF-74 was used as a heterogeneous catalyst for the reaction between 2-phenylindole and phenethylamine to afford the 3-phenethyl-2-phenylquinazolin-4(3H)-one in excellent conversion Indeed, the reaction offered many advantages as compared to previous works including low catalyst loading, and milder conditions VNU-20 was found to be more active for the cyclization of ketoxime carboxylates and dibenzyl ether than several conventional molecular and MOF-based heterogeneous catalysts, which has not mentioned in previous reports yet These MOFs not only exhibited high catalytic possibilities but also could be reused for several times without any considerable decline in efficiency Due to the benefits of quinazolinone and pyridine derivatives in pharmaceutical and chemical industry, the scope of the reactions was expanded by varying many substrates to obtain a broad range of desired products

CONTENTS

ACKNOWLEDGEMENTS iv

ABSTRACT v

CONTENTS vi

LIST OF FIGURES ix

LIST OF SCHEME xi

LIST OF TABLES xiv

LIST OF ABBREVIATION xv

CHAPTER 1: LITERATURE REVIEW 1

1.1 M ETAL - ORGANIC FRAMEWORKS (MOF S ) 1

1.1.1 General introduction 1

1.1.2 General methods for the synthesis of MOFs 3

1.1.3 Application of MOFs 4

1.2 I NTRODUCTION TO C U -MOF-74 AS AN EFFICIENT HETEROGENEOUS CATALYST 9

1.3 I NTRODUCTION TO IRON - BASED METAL - ORGANIC FRAMEWORKS AND IRON -BASED MOF VNU-20 [F E 3 (BTC)(NDC) 2 6.65H 2 O] AS A HETEROGENEOUS CATALYST 19

1.4 T HE QUINAZOLINONES SYNTHESIS OF 2- ARYLINDOLES WITH AMINES UTILIZING C U -MOF-74 AS AN EFFICIENT HETEROGENEOUS CATALYST 25

1.5 T HE CYCLIZATION REACTIONS OF KETOXIME ACETATES AND DIBENZYL ETHER TO PRODUCE PYRIDINES UTILIZING MOF VNU-20 AS A HETEROGENEOUS CATALYST 35

1.6 A IMS AND OBJECTIVES 49

CHAPTER 2: EXPERIMENTAL SECTION 51

2.1 M ATERIALS AND I NSTRUMENTATION 51

2.2 S YNTHESIS OF THE METAL - ORGANIC FRAMWORKS (MOF S ) 53

2.3 C ATALYTIC TESTS 54

2.3.1 Catalytic studies in the expansion reaction to produce 2-arylquinazolinones 54 2.3.2 Catalytic studies in the cyclization reaction of ketoxime acetates and dibenzyl ether to synthesize 2,4,6-triphenyl pyridine 55

CHAPTER 3 RESULT AND DISCUSSION 58

3.1 T HE C U -MO F -74- CATALYZED B AEYER -V ILLIGER OXIDATION EXPANSION REACTION TO SYNTHESIZE 2- ARYLQUINAZOLINONES 58

3.1.1 Synthesis and characterization of Cu-MOF-74 58

3.1.2 Catalytic studies in the synthesis of 2-arylquinazolinones 63

3.1.2.1 Effect of temperature on the reaction 64

3.1.2.2 Effect of solvent on the reaction 66

3.1.2.3 Effect of reactant molar ratio on the reaction yield 67

3.1.2.4 Effect of catalyst quantity on the reaction yield 68

3.1.2.5 Effect of different catalysts on the reaction yield 69

3.1.2.6 Leaching test 71

3.1.2.7 Catalyst reusability 72

3.1.2.8 Effect of different substituents on the reaction 75

3.1.2.9 Conclusion 77

3.2 T HE MIXED - LINKER MOF VNU-20- CATALYZED CYCLIZATION REACTIONS OF KETOXIME ACETATES AND DIBENZYL ETHER TO PRODUCE SYMMETRICAL PYRIDINES 77

3.2.1 Synthesis and characterization of VNU-20 77

3.2.2 Catalytic studies in the synthesis of symmetrical pyridines 78

3.2.2.1 Effect of temperature on the reaction 79

3.2.2.2 Effect of solvent on the reaction 80

3.2.2.3 Effect of ratio reactants on the reaction 82

3.2.2.4 Effect of catalyst amount on the reaction 83

3.2.2.5 Effect of time on the reaction 84

3.2.2.6 Effect of oxidant on the reaction 85

3.2.2.7 Effect of oxidant amount on the reaction 86

3.2.2.8 Effect of antioxidant on the reaction 87

3.2.2.10 Pyridine test 90

3.2.2.11 Catalyst reusability 91

3.2.2.12 Effect of different catalysts on the reaction 93

3.2.2.13 Effect of different atmospheres on the reaction 96

3.2.2.14 Effect of different substituents on the reaction 97

3.2.2.15 Plausible mechanism 101

3.2.2.16 Conclusion 105

CHAPTER 4 CONCLUSION 107

REFERENCES 109

APPENDIX A: CALIBRATION CURVE 118

APPENDIX B: GC YIELD 121

APPENDIX C: CHARACTERIZATION DATA 129

LIST OF FIGURES

Figure 1 1: Progress in the synthesis of ultrahigh porosity MOFs The values in

parentheses represent the pore volume (m3/ g) of these materials [4] 1

Figure 1 2: Growth of the Cambridge Structural Database (CSD) and MOF entries

since 1972 [5] The inset shows the MOF self-assembly process from building blocks: metals (red spheres) and organic ligands (blue struts) 2

Figure 1 3: Overview of synthesis methods, possible reaction temperatures, and final

reaction products in MOFs synthesis [10] 3

Figure 1 4: Interaction of a substrate molecule, S, with a metal site, M, through (a)

expansion of the coordination sphere around the metal ion; or (b) (reversible)

displacement of one of the ligands [22] 6

Figure 1 5: Color changes during the dehydration of Cu3(BTC)2(H2O)3.xH2O to give

Cu3(BTC)2, and subsequent readsorption of the aldehyde to give

Cu3(BTC)2(C6H5CHO)x [24] 7

Figure 1 6: General structure and selected examples of ligands containing

coordinative and reactive functional groups [22] 8

Figure 1 7: Crystal structure of a MOF-74 (left) and metal oxide chains connected by

organic linkers (right) O, red; C, black, H, white; metal, blue [33] 10

Figure 1 8: Solvothermal synthesis of MOF structures [35] 11

Figure 1 9: CO2 adsorption–desorption isotherms at different temperatures of

Cu2(dhtp) [34] 13

Figure 1 10: Total yields of the products that result from the oxidation of

cyclohexene in the presence of M–MOF-74 and without catalyst (blank) with TBHP 14

Figure 1 11: Comparison of different types of acid catalysts for the acylation of

vertically by BTC3− and NDC2−, respectively (a, e and f) to form the orange-red

crystals (d) with structure highlighted with a rectangular window of 6.0 × 8.7 Å2 (c) Atom colors: Fe, blue and orange polyhedra; C, black; O, red All H atoms are omitted for clarity [67] 20

Figure 1 15: Pyridine core and several pyridine derivatives [87, 88] 36 Figure 3 1: PXRD patterns of the simulated (a) and synthesized (b) Cu-MOF-74 58 Figure 3 2: FT-IR spectra of terephthalic acid and the Cu-MOF-74 60 Figure 3 3: TGA curve of the Cu-MOF-74 61

Figure 3 6: Pore size distribution of Cu-MOF-74 62

Figure 3 7: Isotherm linear plot of Cu-MOF-74 63

Figure 3 8: Effect of temperature on the reaction yield 65

Figure 3 9: Effect of solvent on the reaction yield 66

Figure 3 10: Effect of reactant molar ratio on the reaction yield 68

Figure 3 11: Effect of catalyst quantity on the reaction yield 69

Figure 3 12: Effect of homogeneous catalysts on the reaction yield 70

Figure 3 13: Effect of heterogeneous catalysts on the reaction yield 71

Figure 3 14: Leaching test indicated no contribution from homogeneous catalysis of active species leaching into reaction solution 71

Figure 3 15: Catalyst recycling studies 72

Figure 3 16: PXRD patterns of the simulated (a) and synthesized (b) Cu-MOF-74 74

Figure 3 17: FT-IR spectra of the Cu-MOF-74 74

Figure 3 18: Effect of different temperatures on the reaction yield 79

Figure 3 19: Effect of solvent to the reaction 80

Figure 3 20: Effect of molar ratio of dibenzyl ether /(E)-acetophenone O-acetyl oxime acetate on the reaction yield 82

Figure 3 21: Effect of catalyst amount on the reaction yield 83

Figure 3 22: Effect of time on the reaction 84

Figure 3 23: Effect of oxidant on the reaction 85

Figure 3 24: Effect of oxidant amount on the reaction 86

Figure 3 25: Effect of antioxidant on the reaction 88

Figure 3 26: Leaching test indicated no contribution from homogeneous catalysis of active species leaching into reaction solution 89

Figure 3 27: Pyridine test 90

Figure 3 28: Catalyst reusing studies 91

Figure 3 29: FT-IR analyses of the new (a) and recovered (b) catalyst 92

Figure 3 30: PXRD determination of the new (a) and recovered (b) catalyst 93

Figure 3 31: Effect of different homogeneous catalyst on the reaction 93

Figure 3 32: Effect of different heterogeneous catalyst on the reaction 95

Figure 3 33: Effect of different atmospheres on reaction yield 96

Scheme 1 4: The catalytic activity of Cu-MOF-74 in some typically base-catalyzed

reactions, a) Knoevengel condensation rection b) Micheal reaction [31] 16

Scheme 1 5: The coupling reaction of amines and -carbonyl aldehydes [39] 17

Scheme 1 6: The reaction between dibenzyl ether and 2-acetyl phenol utilizing

Cu-MOF-74 catalyst [40] 17

Scheme 1 7: The three-component coupling reaction of 2-pyridincarboxaldehyde,

piperidine, and phenylacetylene using Cu-MOF-74 catalyst [41] 18

Scheme 1 8: The synthesis of imidazo[1,5-a]pyridines via oxidative amination of the

C(sp3)–H bond using Cu-MOF-74 [42] 18

Scheme 1 9: The hydroacylation of 1-alkynes with glyoxal derivatives using the

Cu-MOF-74 catalyst [43] 19

Scheme 1 25: 1,5-benzodiazepine synthesis via cyclocondensation of 1,2-diamines

with ketones using MOF-235 as an efficient heterogeneous catalyst [48] 21

Scheme 1 26: Direct C-C coupling of indoles with alkylamides via oxidative C−H

functionalization using Fe3O(BDC)3 as a productive heterogeneous catalyst [49] 21

Scheme 1 27: Direct arylation of benzoazoles with aldehydes utilizing metal–organic

framework Fe3O(BDC)3 as a recyclable heterogeneous catalyst [50] 21

Scheme 1 28: Synthesis of alkenylazaarenes using the direct alkenylation of

2-substituted azaarenes with carbonyls via C−H bond activation [51] 22

Scheme 1 29: Oxidant-promoted formation of coumarins using Fe3O(BPDC)3 as an efficient heterogeneous catalyst [52] 22

Scheme 1 30: Direct C–N coupling of azoles with ethers via oxidative C–H activation

under metal–organic framework catalysis [53] 23

Scheme 1 31: One-pot oxidative synthesis of quinazolinones using Fe(BTC) as

efficient heterogeneous catalysts by Oveisi and co-workers [54] 23

Scheme 1 32: Cross-coupling of coumarin and N,N-dimethylaniline utilizing

VNU-20 as a heterogeneous catalyst [55] 24

Scheme 1 33: The cross-dehydrogenative coupling of 6-methylcoumarin with

mesitylene using the VNU-20 catalyst [56] 24

Scheme 1 10: The reductive N-heterocyclization of

N-(2-nitrobenzoyl)azacy-cloheptane to prepare the corresponding azacycloheptano[2,1-b]-4(3H)-quinazolinone

[61] 26

Scheme 1 11: The Palladium-Catalyzed Reaction of o-Iodoanilines with

Carbodiimides and Carbon Monoxidea (a) The Palladium-Catalyzed

Scheme 1 12: One-pot condensation of anthranilic acid, ortho esters (or formic acid)

and amines (a) [63] Synthesis of 4(3H)-quinazolinones using La(NO3)3.6H2O and

PTSA under solvent-free conditions (b) [64] 28

Scheme 1 13: The synthesis of 3-aminoalkyl-2-arylaminoquinazolin-4(3H)-one and 3,3’-disubstituted bis-2-arylaminoquinazolin-4(3H)-ones [65] 28

Scheme 1 14: Cu-catalyzed synthesis of quinazolinone derivatives (a) [66]; Microwave-assisted synthesis of quinazolinone derivatives via rapid iron-catalyzed cyclization (b) [67] 29

Scheme 1 15: Niementowski synthesis of modified quinazolinones [68] 29

Scheme 1 16: Synthesis of 2,3-disubstituted quinazolinones from N-(o-halophenyl)imidoyl chlorides or imidates [70] 30

Scheme 1 17: Synthesis of 3-substituted and 2,3-disubstituted quinazolinones via Cu-catalyzed aryl amidation [71] 31

Scheme 1 18: Fe-catalyzed method for the synthesis of 2,3-diarylquinazolinones [72]. 31

Scheme 1 19: Oxidative radical skeletal rearrangement of 5-aryl-4,5-dihydro-1,2,4-oxadiazoles into quinazolinones [73] 32

Scheme 1 20: Palladium-catalyzed carbonylative synthesis of quinazolinones with 2-aminobenzamide (a) [74] with 2-aminobenzonitriles (b) [75] 32

Scheme 1 21: Oxidative synthesis of quinazolinones [76] 33

Scheme 1 22: The reaction between 2-aminobenzamide and benzyl alcohol using Fe3O(BPDC)3 catalyst [77] 33

Scheme 1 23: Synthesis of pyrido-fused quinazolinone derivatives via copper catalyzed domino reaction [78] 34

Scheme 1 24: The reaction of 2-arylindoles with amines or ammoniums via Baeyer-Villiger oxidation expansion [59] 34

Scheme 1 34: Conventional method for construction of pyridine [89-92] 37

Scheme 1 35: The Chichibabin reaction [87] 37

Scheme 1 36: The aza-Diels–Alder approach to pyridine derivatives [93] 38

Scheme 1 37: Synthesized 1-substituted 2-[(2S)-2-pyrrolidinyl] pyridine from L-proline [94] 39

Scheme 1 38: Microwave-assisted organic synthesis of substituted pyridines from 1,3-dicarbonyl compounds and aldehydes [95] 40

Scheme 1 39: Synthesis of pyridines via palladium-catalyzed iminoannulation of internal acetylenes [96] 40

Scheme 1 40: Ring-closing metathesis strategy for pyridine synthesis using acrylamide entry to synthesize pyridines [98] 41

Scheme 1 41: Rhodium-catalyzed cycloaddition reaction for the formation of pyridine derivatives [99, 100] 42

Scheme 1 42: Transition metal -catalyzed cross-coupling of activated pyridines 42

Scheme 1 44: Synthesis of functionalized pyridines via Cu-catalyzed

three-component cascade annulation reaction [81] 44

Scheme 1 45: The cyclization between (E)-acetophenone O-acetyl oxime acetate and

N,N-dimethylaniline utilizing iron-organic framework catalyst [104] 44

Scheme 1 46: Metal-free assembly of polysubstituted pyridines from oximes [105,

106] 45

Scheme 1 47: Methodology synthesized 2,4,6, tri-substituted pyridines [110-113] 46 Scheme 1 48: Coupling reaction of oximes acetates with toluene derivatives via

Csp3–H bond activation [80] 47

Scheme 1 49: Some recent methodology synthesized 2,4,6- trisubstituted pyridines

via oxime derivatives [80, 114-116] 48

Scheme 1 50: The reaction between 2-phenylindole and 2-phenylethanamine utilizing

Cu2(dhtp) catalyst 50

Scheme 1 51: The cyclization of ketoxime acetates and dibenzyl ether using VNU-20

as a heterogeneous catalyst 50

Scheme 2 1: Synthetic reaction of Cu2(dhtp) or Cu-MOF-74 [34] 53

Scheme 2 2: Self-assembling synthesis of the reddish-yellow crystal (VNU-20) [44].

54

Scheme 3 1: The reaction between 2-phenylindole and 2-phenylethanamine utilizing

Cu-MOF-74 catalyst 64

Scheme 3 2: The cyclization of (E)-acetophenone O-acetyl oxime acetate and

dibenzyl ether utilizing VNU-20 as a heterogeneous catalyst 78

Scheme 3 3: Control experiments 103 Scheme 3 4: Plausible reaction pathway 105

LIST OF ABBREVIATION

MOFs Metal-organic frameworks

SBUs Secondary building units

VNU Vietnam national university

DABCO 1, 4-diazabicyclo [2.2 2] octane

TEMPO 2, 2, 6, 6-tetramethylpiperidine-1-oxyl radical

FT-IR Fourier transform infrared

MS Mass Spectrometry

NMR Nuclear Magnetic Resonance

AAS Atomic absorption spectroscopy

SEM Scanning Electron Microscope

SBU Secondary building unit

TBHP tert-butyl hydroperoxide

DTBP Di-tert-Butyl peroxide

TGA Thermal Gravimetric Analyzer

TEM Transmission electron microscopy

H2(dhtp) 2,5-dihydroxyterephthalic acid

H2OBA 4,4′-oxybis(benzoic) acid

Dobdc 2, 5-dioxido-1, 4-benzenedicarboxylate

Dpa Di (2-pyridyl) amine

NIST The National Institute of Standards and Technology

CHAPTER 1: LITERATURE REVIEW

1.1 Metal-organic frameworks (MOFs)

1.1.1 General introduction

Metal-organic frameworks (MOFs), also known as porous coordination polymers , are compounds consisting of metal ions clusters linked together by organic bridging ligands with wide range of well – defined topology to form one-, two-, three-dimensional structures [1] MOFs have unusually large surface areas and tailorable pore sizes The most striking properties of MOFs are their large pore volumes that have been unsurpassed by any other porous material to date In comparison with other solid matters such as zeolites, carbons and oxides, the porosity of MOFs have come up

to 90% free volume and the enormous internal surface areas have extended beyond

7000 m2/g (Figure 1 1) [2] For instance, MOF–200 and MOF–210 (Zn4O(BBC)2 and (Zn4O)3(BTE)4(BPDC)3, respectively) act as an extensive class of crystalline materials with the ultrahigh surface areas (4530 m2/g and 6240 m2/g, respectively) and porosities (90% and 89% volume) [3]

Figure 1 1: Progress in the synthesis of ultrahigh porosity MOFs The values in

parentheses represent the pore volume (m3/ g) of these materials [4]

An almost exponential growth of the structures of MOFs has been seen in the Cambridge Structural Database (CSD) during the last decade The combination of so far unreached porosity, surface area, pore size and wide chemical inorganic–organic composition recently has created a strikingly increasing trend each year for all

structure types (Figure 1 2)

Figure 1 2: Growth of the Cambridge Structural Database (CSD) and MOF entries since 1972 [5] The inset shows the MOF self-assembly process from building blocks:

metals (red spheres) and organic ligands (blue struts)

Transition metal ions such as copper, zinc, nickel, iron are frequently used as the inorganic components of MOFs For the organic linker, there are an extensive variety

of choices Ligands with rigid backbones are often preferred, because the rigidity helps

to sustain the open-pore structure and easily predicts the network geometry for expected application The linkers can be neutral, anionic, or cationic The neutral and anionic organic linkers are most commonly use such as pyrazine and 4,4’-bipyridine (bpy) [6] and carboxylates because they have the ability to aggregate metal ions into clusters and therefore the frameworks are more stable [7] Cationic organic ligands are relatively little used, because of their low affinities for cationic metal ions [8]

1.1.2 General methods for the synthesis of MOFs

During the last two decades, the synthesis of MOFs has attracted significant attentions The main goal in MOF synthesis was to establish the synthesis conditions that lead to defined inorganic building blocks without decomposition of the organic linker [9] MOFs are often synthesized by means of solvothermal in which the reactions are carried out in an organic solvent at high-temperature in closed vessels This method is relatively simple and can produce large-scale MOFs; However, it typically takes long reaction times, from several hours up to several months, depending upon the MOF of interest and the reaction solvents, reaction temperatures, reagent concentrations, and other factors [8] Besides that conventional electric (CE), electrochemistry (EC), mechanochemistry (MC), microwave-assisted and ultrasonic

(US) methods have been employed (Figure 1 3)

Figure 1 3: Overview of synthesis methods, possible reaction temperatures, and final

reaction products in MOFs synthesis [10].

1.1.3 Application of MOFs

MOFs with permanent porosity and their variety and multiplicity than any other class of porous materials have made MOFs ideal candidates for storage of fuels (hydrogen and methane), capture of carbon dioxide, (gas adsorption) and catalysis applications [4]

In 1998, MOF-2 [Zn(BDC)] is the first carbon dioxide adsorption material [11] and

to date, MOF-200 with 2437 mg/g at 50 bar and 298 K have the best excess carbon dioxide uptake [3] The development in the chemistry of MOFs came in 1999, MOF-

5, the first robust and highly porous material , have gas adsorption measurements, which revealed 61% porosity and a Brunauer- Emmett-Teller (BET) surface area of

2320 m2/g (2900m2/g Langmuir) These values are substantially higher than those commonly found for zeolites and activated carbon [12] MOFs have also been use to separate toxic molecules, hydrocarbon and water from complex compounds For instance, Cu2(PZDC)2(Pyz) (PZDC = pyrazine-2,3-dicarboxylate; Pyz = pyrazine) selectively takes up acetylene over carbon dioxide through hydrogen bonding between acetylene and oxygen atoms on the MOF internal surface [13] Besides that, the melamine-MOFs were also used as an absorbent for the removal of heavy metal Pb(II) from waste water [14] One of the earliest examples of a dynamic separation was performed using a gas chromatographic column filled with MOF-508 [Zn2(BDC)2(BPy)] to separate alkanes such as n-pentane, n-hexane, 2,2-

dimethylbutane, and 2-methylpentane [15] In biomedical chemistry, iron-containing BioMIL-1 MOF, which was built up from non-toxic iron and the therapeutically active linker nicotinic acid, showed higher loading for nicotinic acid (up to 75%) as compared to the native MOF structures and exhibited controlled drug delivery [16] The most attractive application of MOFs is as heterogeneous catalysts for organic chemical reactions due to the fact that they can be easily separated and recycled from the reaction systems [17] MOFs are composed entirely of strong bonds (e.g., C-C, C-

H, C-O, and M-O), they show high thermal stability ranging from 250° to 500°C[18]

a challenge to make chemically stable MOFs because of their susceptibility to displacement reactions when treated with solvents over extended periods of time (days) [4] Moreover, high open metal sites as well as abundant metal content in the structure of MOFs give a higher catalytic activity than zeolites, in comparison [20] So MOFs have several unquestionable advantages included the variety of structures and the ability for large-scale production [21]

Catalysis at the metal sites

Metal ions or clusters in the structure of numerous MOFs can directly coordinate to the substrate to catalyze a chemical transformation, as well as, as-synthesized active MOFs Coordination of the substrate to the metal requires either an expansion of the

coordination sphere of the metal ion, or a displacement of one of the ligands (Figure

1 4) However, the crystalline integrity do not collapse as a consequence of the local

distortions produced upon substrate coordination [22] For example, employing copper imidazolate, [Cu(im)2] and copper pyrimidinolate, [Cu(2-pymo)2] for aerobic liquid phase oxidation of activated paraffins was investigation In this study, the different reactivities of [Cu(2-pymo)2] and [Cu(im)2] was described by principle DFT calculations on MOF model clusters According to that, [Cu(im)2] has a more adaptable crystalline framework than [Cu(2-pymo)2], which allows that the copper sites expand their coordination sphere from 4 to 5 upon interaction with ·OH radical species On the contrary, binding of the same radical to [Cu(2-pymo)2] produces the displacement of one of the 2-pymo ligands from the coordination sphere around the central Cu site A hypothesis that a higher energy would be required in the case of [Cu(2-pymo)2] to break a Cu-pyrimidine bond than in the case of [Cu(im)2] in which only a rearrangement of the ligands is required to accommodate the ·OH radical leads

to the better performance of [Cu(im)2] than [Cu(2-pymo)2] in higher alkane conversion, higher selectivity and low accumulation of alkylhydroperoxides in the reaction medium [23]

Figure 1 4: Interaction of a substrate molecule, S, with a metal site, M, through (a)

expansion of the coordination sphere around the metal ion; or (b) (reversible)

displacement of one of the ligands [22]

Besides, MOFs with coordinatively unsaturated sites are MOFs in which one of the coordination positions of the metal centers is occupied by a labile ligand, which can be removed without causing the collapse of the crystalline structure In most cases, the labile ligands are solvent molecules that, when thermally removed, leave a free coordination position in the metal, which become available for adsorbed substrates The correspoding metal center which will be prone to accept electron density from any donor will behave as a Lewis acid center When suitable oxidizing agents, such as O2,

H2O2 or hydroperoxydes, are present in the reaction medium, the resulting MOF can contribute as redox catalyst [22] For instance, the liquid phase cyanosilylation of benzaldehyde using Cu3(BTC)2 was reported by Kclaus and co-workers In this situation, the Lewis acid copper(II) sites of the Cu2-paddle-wheel become accessible for the coordination of the aldehyde It was observed that physically and chemically bound water molecules are easily removed from the host material by heating the

sites accessible for other molecules [24] Other examples in previous literature were Mukaiyama-aldol condensation [25], Friedel–Crafts benzilation [26], and the oxidation

of alcohols [27] sulfides, olefins, paraffins [28]

Figure 1 5: Color changes during the dehydration of Cu3(BTC)2(H2O)3.xH2O to give

Cu3(BTC)2, and subsequent readsorption of the aldehyde to give

Cu3(BTC)2(C6H5CHO)x [24]

Catalysis at the organic linkers

MOFs catalyze a chemical reaction not only at the metal sites but also at the organic linkers in some cases in which MOFs contain functional groups at the organic linker Therefore, the catalytic function is located at the organic linker and not at the metal site It is obvious that the linkers that form this type of MOFs need to contain two different types of organic functional groups: coordinative functional groups, G1, that coordinate to the metal sites to hold the crystalline framework; and reactive functional groups, G2, which are not coordinated to the metals and will be responsible for the

catalytic properties of the material (Figure 1 6) However, it is complicated to

generate MOFs with reactive functional groups free and accessible to catalytic

substrates To meet the demand, the ligand used must be soluble and the functional group must be resistant under the synthesis conditions [22] A prototypic example of MOFs having two types of functional groups is the Friedel−Crafts reaction between pyrroles and nitroalkenes utilizing the material Nu-601 which contains 2D layers of Zn paddlewheel dimmers connected to the urea ligand depicted and pillared with 4,40-bipyridine Nu-601 was found to be an active hydrogen-bond-donor that showed a significantly higher rate of substrate consumption versus the control reaction [29]

Figure 1 6: General structure and selected examples of ligands containing

coordinative and reactive functional groups [22]

Catalysis with the advantages of the pore sites

The prominent role of MOFs in heterogenerous catalysis fascinated scientists to scrutinize this impact on the catalytic transformation By decreasing mass transport limitations, a high porosity of MOFs could facilitate the contact between the substrate

of the reaction and the catalytic sites Academically, the pore system of the solid can

be used as either a host matrix to introduce additional catalytic species, or as the nanometric reaction cavity where a chemical reaction takes place [22] As a host matrix, the regular system of channels and cavities of these solids can be used to encapsulate various kinds of species, including metal or metal oxide nanoparticles or molecular catalysts, this combine the properties of both the host and the guest Moreover, it can participate in the catalytic process contributing with additional functionalities not provided by the encapsulated moieties, such as acid or basic sites

cavity or the confinement space of the pore system can largely influence the product selectivity, and this is more likely to occur when a substrate or product of the reaction and the host matrix in which it is contained have similar dimensions One example is the reaction of o-methyl dibenzyl ketone inside the pores of the material [Co3(4,40-BPhDC)3(4,40-bpy)] In solution, the product distribution arises from the random coupling of the two benzyl radicals, giving a 25, 50, 25% ratio of the three possible A–

A, A–B and B–B diarylethanes (Scheme 1 1) However, when diffusion is restricted

due to confinement effects, the product distribution changes, favoring the asymmetric A–B diarylethane arising from recombination of the geminate radical pair [30]

Scheme 1 1:The photolysis of o-methyl dibenzyl ketone carried out inside the pores

to form a 3-D structure with honeycomb-like hexagonal that contains 1-D broad channels [31] The metal ions bond to oxygen atoms in square pyramidal geometry with coordination number of five After synthesis, the channels of MOF-74s are lined with guest molecules such as water or DMF molecules because the metal cations

coordinate oxygen atoms and guest molecules in octahedral geometry (Figure 1 7)

Upon desolvation, the metal coordination changes from octahedral to square pyramidal without compromising the framework integrity, leaving coordinatively unsaturated metal sites open to channels [32] The desolvated MOFs are called activated ones because they have active metal sites on the channels

Figure 1 7: Crystal structure of a MOF-74 (left) and metal oxide chains connected by

organic linkers (right) O, red; C, black, H, white; metal, blue [33]

Synthesis of Cu-MOF-74

The synthesis of Cu-MOF-74 is conducted by solvothermal method following the

procedure (Figure 1 8) [34] First, a mixture of 2,5-dihydroxyterephthalic acid and

Cu(NO3)2.3H2O was dissolved in a mixture of N,N’ -dimethylformamide (DMF) and isopropanol Then, the resultant solution was placed in an oven at 85oC for 18 h After cooling the sample to room temperature, the solid product was removed from mother liquor, washed in copious DMF and exchanged with methanol at room temperature The solid Cu-MOF-74 was then evacuated under vacuum at 150 °C for 5 h

Figure 1 8: Solvothermal synthesis of MOF structures [35].

Properties of Cu-MOF-74

Cu-MOF-74 contains a permanent microporosity with a BET specific surface area

of 1126 m2/ g, a pore volume of 0.57 cm3/ g and an average pore diameter 11oA The decomposition of Cu-MOF-74 is about 375 oC Thus, it shows a high thermal stability similar to that of other MOFs due to the strong bonding between copper ions and

oxygen atoms The Properties of Cu-MOF-74 is summarized below (Table 1 1)

Table 1 1:The Properties of Cu-MOF-74 [36]

For the outstanding properties, Cu-MOF-74 has become a promising candidate for heterogeneous catalysis and other application such as gas storage and separations

Empirical formula Cu2C8H2O6

Nomenclature Copper (II) 2,5-dioxidoterephthalate

Application of Cu-MOF-74

 Gas adsorption

In fact, there has been an intensive growth on the research of Cu-MOF-74 application, specifically as gas adsorption and heterogeneous catalyst In gas adsorption, Cu-MOF-74 is a promising material for CO2 separation/purification processes due to the extraordinary development of microporous structures with pore volumes and the ability of the framework components to enhance the affinity of the internal pore surface toward CO2 molecules The higher affinity between the material structure and the CO2 molecules is also related to the facts of a higher crystallinity and

a more effective activation of coordinately saturated copper sites by the solvent removal when 2-propanol is employed For instance, CO2 adsorption capacity at 1 bar and 45 °C increases from 3.0 wt% for the sample synthesized with DMF: water to 5.0 wt% for the sample synthesized with DMF : 2-propanol The effect of temperature on

CO2 adsorption has been also tested for the sample synthesized with DMF and

2-propanol Figure 1 9 illustrates that since the amount of CO2 adsorbed decreases as the temperature increases and when the pressure rises, the CO2 uptake will increase strongly [34]

Figure 1 9: CO2 adsorption–desorption isotherms at different temperatures of

Cu2(dhtp) [34]

 Cu-MOF-74 as a heterogeneous catalyst

Due to the conspicuous properties, Cu-MOF-74 as a heterogeneous catalyst has been employed for various organic reactions to create numerous products serving chemical and pharmaceutical industries In particular, Cu-MOF-74 exhibits as the redox catalyst or the catalytic activity in acid or base-catalyzed reaction

Scheme 1 2: The oxidation of cyclohexene [37].

As the redox catalyst, nanocrystalline Cu–MOF-74 material was prepared to use for the catalytic oxidation of cyclohexene using peroxides as oxidizing agents tert-

butylhydroperoxide (TBHP) at room temperature (scheme 1 2) This catalytic reaction

Due to the ultrahigh BET surface area of Cu-MOF-74, The nanocrystalline

Cu–MOF-74 materials were much more active than their micrometer-sized homologues (Figure

1 10) for the heterogeneous catalyst of the oxidation of cyclohexene [37]

Figure 1 10: Total yields of the products that result from the oxidation of

cyclohexene in the presence of M–MOF-74 and without catalyst (blank) with TBHP

As a reference acid catalyzed reaction, Cu-MOF-74 with open metal sites performs the catalysis of the reaction between anisole and acetyl chloride to form MAPs

(Scheme 1 3)

Scheme 1 3: Simplified reaction for the acylation of anisole with acetyl chloride [36].

The catalytic performance Cu-MOF-74 in comparison to other different acid catalysts in terms of relative anisole conversion and p-MAP yield under the same

acid catalyzed reaction such as H-ZSM-5 and BETA zeolitic materials and

Al-MCM-41 mesoporous materials In comparison with these inorganic catalysts (Figure 1 11),

the anisole conversion of Cu-MOF-74 is the highest Therefore, the acid capacity of copper atoms located into the hybrid MOF-74 phase as well as its remarkable surface area makes Cu-MOF-74 a promising material for acid catalyzed reactions, in particular for the acylation of anisole

Figure 1 11: Comparison of different types of acid catalysts for the acylation of

anisole [36]

Interestingly, Cu-MOF-74, which acts not only as a Lewis acid catalyst [38], but also a basic catalysts with well-defined active sites [31] For instance, Cu-MOF-74 was introduced by Valvekens as a catalyst in Knoevenagel condensation, Michael

conjugate addition reactions in previous report (Scheme 1 4) [31] According to this,

Cu-MOF-74 could be catalyzed the Knoevenagel condensation of benzaldehyde to malononitrile and the Michael addition reaction of ethyl cyanoacetate to methyl vinyl ketone, which afforded the moderate product yield The successful application of this MOF for these standard base-catalyzed reactions opens a new window for catalysis research using the intrinsic basicity of MOFs [31]

Scheme 1 4: The catalytic activity of Cu-MOF-74 in some typically base-catalyzed

reactions, a) Knoevengel condensation rection b) Micheal reaction [31]

In the framework of Cu-MOF-74, 2,5-dihydroxyterephthalic acid molecules are completely deprotonated, all of their oxygen atoms bond directly with the copper

centers (Figure 1 12) Therefore, these oxygen atoms, especially the phenolate ones,

exhibit Bronsted basicity, in other words, they have the ability to deprotonate the reactant molecules in the Knoevenagel condensation Furthermore, the coordinatively unsaturated copper ion adjacent to the phenolate oxygen atom can act as a docking site for the deprotonated reactant molecule The interplay of copper ions and phenolate oxygen atoms make up the active sites in Cu-MOF-74

Figure 1 12: Pores in the M2dobdc MOF (brown = carbon; orange = metal; red =

oxygen).[31]

Recently, there are many studies for utilizing Cu-MOF-74 as a heterogeneous catalyst for various reaction Typically, Cu-MOF-74 catalyst using for the coupling

2015 (Scheme 1 5) The outstanding result indicated that the reaction obtained 95%

yield in the presence of Cu-MOF-74, simultaneously, this heterogeneous catalyst could contribute to be highly recycled with 9 times, being superior to other homogeneous and heterogeneous for this coupling [39]

Scheme 1 5: The coupling reaction of amines and -carbonyl aldehydes [39]

At the first time, the direct esterification to produce O-acetyl substituted phenol esters utilizing dibenzyl ethers as acylating source assisted by Cu-MOF-74 as a

recyclable catalyst was explored by Lieu and co-workers (Scheme 1 6) This

transformation occurred in ease condition with the aid of DMSO as an effective solvent, t-BuOOH as an oxidant and Cu-MOF-74 as a recyclable catalyst, affording up

to 86% in product yield The feature that Cu-MOF-74 could reuse over the 6thcatalytic run without a noticeable deterioration in catalytic efficiency would be fascinated to the chemical industry [40]

Scheme 1 6:The reaction between dibenzyl ether and 2-acetyl phenol utilizing

Cu-MOF-74 catalyst [40]

In 2016, G.H.Dang et al announced the three-component coupling reaction of pyridincarboxaldehyde, piperidine, and phenylacetylene to form 3-phenyl-1-(piperidin-1-yl)indolizine using Cu-MOF-74 as a heterogeneous catalyst The coupling reaction was carried out in n-butanol under argon for 5 h at 100 oC (Scheme 1 7).The

2-yield of the product was 99% in in the presence of Cu-MOF-74 catalyst And the catalyst could be recovered and reused seven times without a significant degradation in catalytic activity [41]

Scheme 1 7: The three-component coupling reaction of 2-pyridincarboxaldehyde,

piperidine, and phenylacetylene using Cu-MOF-74 catalyst [41].

Synthesis of imidazo[1,5-a]pyridines via oxidative amination of the C(sp3)–H bond under air using metal–organic framework Cu-MOF-74 as an efficient heterogeneous

catalyst was also reported in 2016 by Nguyen and co-workers (Scheme 1 8) The C-N

coupling reaction between 2-benzoyl pyridine and benzylamine obtained the desired product with 71% yield after 8 hours Indeed, the reaction still afforded 67% yield of the product in the 8th run [42]

Scheme 1 8: The synthesis of imidazo[1,5-a]pyridines via oxidative amination of the

C(sp3)–H bond using Cu-MOF-74 [42]

And at that time,Cu-MOF-74 was also assessed for its catalytic activity in the hydroacylation of phenylacetylene with ethyl glyoxalate to form (E)-ethyl 2-oxo-4-

phenylbut-3-enoate in a yield of 93% after four hours (Scheme 1 9) Without a

significant degradation in catalytic activity of Cu-MOF-74 after reusing and recycling,

a 91% yield of product was still achieved in the eighth run [43]

Scheme 1 9: The hydroacylation of 1-alkynes with glyoxal derivatives using the

Cu-MOF-74 catalyst [43]

1.3 Introduction to iron-based metal-organic frameworks and iron-based

MOF VNU-20 [Fe 3 (BTC)(NDC) 2 6.65H 2 O] as a heterogeneous catalyst

Structure of MOF VNU-20 [Fe 3 (BTC)(NDC) 2 6.65H 2 O]

Up to now, a great number of reports using iron-based MOFs have been revealed; however, these works were mainly focused on oxo-centered trimers of octahedral Fe(III) secondary building units (SBUs) In addition to MOFs assembled from a single type of organic linker, a novel iron-based MOF VNU-20 (VNU = Vietnam National University) has been prepared and employed as an efficient heterogeneous catalyst for many coupling transformations MOF VNU-20, formulated as [Fe3(BTC)(NDC)2·6.65H2O] (BTC = 1,3,5-benzenetricarboxylate; NDC = 2,6-napthalenedicarboxylate), was constructed from mixed linkers of BTC3− and NDC2−with an infinite [Fe3(CO2)7]∞ rod SBU, which was rarely seen before (Figure 1 14)

[44]

Figure 1 13: The crystal structure of VNU-20 (b) are linked horizontally and

vertically by BTC3− and NDC2−, respectively (a, e and f) to form the orange-red crystals (d) with structure highlighted with a rectangular window of 6.0 × 8.7 Å2 (c) Atom colors: Fe, blue and orange polyhedra; C, black; O, red All H atoms are omitted

for clarity [44]

Application of iron-based MOFs and the mixed-linker iron-based MOF

VNU-20 [Fe 3 (BTC)(NDC) 2 6.65H 2 O] as heterogeneous catalyst

Over the past few years, the applications of iron-based metal-organic frameworks in catalysis have drawn an increased attention, especially in the field of C−H functionalization reactions [45-47] In 2015, Le and co-workers carried out the preparation of 1,5-Benzodiazepine through cyclocondensation of 1,2-diamines with

ketones using MOF-235 as an efficient heterogeneous catalyst (Scheme 1 25)

Excellent conversion to the desired product were achieved in the presence of 5 mol% MOF-235 catalyst and the molecule oxygen as the stoichiometric oxidant at 50 0C for

180 minutes In addition, the MOF could be reused ten times without degradation in

Scheme 1 10:1,5-benzodiazepine synthesis via cyclocondensation of 1,2-diamines with ketones using MOF-235 as an efficient heterogeneous catalyst [48].

In 2016, Doan and co-workers successfully utilized Fe3O(BDC)3 as recycled heterogeneous catalyst for the direct C-C coupling of indoles with alkylamides via

oxidative C-H functionalization (Scheme 1 26) The reaction could only progress in

the presence of this Fe-MOF to obtain 90% yield with high selectivity after just 60 minutes Furthermore, this strategy contributes to the green eligibility for the coupling regard to the simplicity of reusability without substantial deterioration in catalytic activity [49]

Scheme 1 11:Direct C-C coupling of indoles with alkylamides via oxidative C−H functionalization using Fe3O(BDC)3 as a productive heterogeneous catalyst [49]

In the meantime, a new method for the direct arylation of benzoazoles with aldehydes in the existence of Fe3O(BDC)3 as a productively heterogeneous catalyst

was developed by Doan and co-workers (Scheme 1 27) Instead of aryl halides in the

conventional methodology, the precursor benzaldehyde featured inexpensive and commercially available abilities In the catalysis of Fe3O(BDC)3, the corresponding product was obtained the best yield 93% for 3h when the temperature was raised to

100 0C [50]

Scheme 1 12:Direct arylation of benzoazoles with aldehydes utilizing metal–organic

framework Fe3O(BDC)3 as a recyclable heterogeneous catalyst [50]

Otherwise, many investigations on metal organic framework Fe3O(BPDC)3 have gained intensive concerns for the catalysis chemistry In 2016, Dang and co-worker successfully achieved 2-alkenylazaarenes with 88% yield via direct alkenylation of 2-substituted azaarenes with carbonyls using 10 mol% Fe3O(BPDC)3 (Scheme 1 28)

The Fe3O(BPDC)3 exhibited the better performance in catalytic activity than other heterogeneous and homogeneous catalyst in this transformation; in addition, the combination of Fe3O(BPDC)3 and co-catalyst accelerated the yield significantly [51]

Scheme 1 13:Synthesis of alkenylazaarenes using the direct alkenylation of

2-substituted azaarenes with carbonyls via C−H bond activation [51]

In the same years, the synthesis of coumarins from salicylaldehydes and activated

methylene compounds was conducted by Lieu and co-workers (Scheme 1 29) When

homogeneous catalysts were using, the reaction yield only gained 20%; In contrast, the heterogeneous catalyst Fe3O(BPDC)3 could promoted the transformation to reach the excellent yield 96% Indeed, this transformation could occur under mild temperature, heterogeneous and base-free conditions should be of an advantage [52]

Scheme 1 14:Oxidant-promoted formation of coumarins using Fe3O(BPDC)3 as an

efficient heterogeneous catalyst [52]

Interestingly, for the direct C-N coupling, Nguyen and co-workers proceeded the formation of azole derivatives from azoles with ethers via oxidative C-H activation by using Fe3O(BPDC)3 as recyclable solid catalyst (Scheme 1 30) It was noted that 90%

yield of the expected product was also recorded when only 5 mol% Fe-MOF was used

in the mild condition, confirming the significance of this chemical activation protocol

Scheme 1 15: Direct C–N coupling of azoles with ethers via oxidative C–H activation

under metal–organic framework catalysis [53]

Additionally, Oveisi and co-workers described the potential catalytic utility of Fe(BTC) that makes it quite attractive for sustainable industrial chemistry The porous iron-based MOF, Fe(BTC), showed high activity catalysis in the oxidative cyclization

of methylenebisnaphthols to the corresponding spirodienones (Scheme 1 31a)

Simultaneously, the modern tandem process between benzyl alcohols and aminobenzamide with the aid of Fe(BTC) and oxidant to produce quinazolin-4(3H)-ones Fe(BTC) was also explored by Oveisi These works consistently have the advantages such as availability of MOF, inexpensive catalyst, mild reaction

o-conditions, reasonable yields, and simple experimental procedures (Scheme 1 31b)

[54]

Scheme 1 16: One-pot oxidative synthesis of quinazolinones using Fe(BTC) as

efficient heterogeneous catalysts by Oveisi and co-workers [54]

As previously mentioned, although reports on catalytic activity of mixed-linker MOFs were reported widely in recent years, mixed-linker MOFs containing Fe(II)-

[Fe3(BTC)(NDC)2·6.65H2O] called VNU-20 has been recently explored and become a potential candidate in the field of organic reaction catalysis For the first time, in 2018, Pham and co-workers applied MOF VNU-20 into the transformation of coumarins

with N,N-dimethylaniline through the direct C–H bond activation (Scheme 1 32) It

was noteworthy that the excellent yield was still preserved in the 5th run utilizing the recovered catalyst Furthermore, VNU-20 exhibit the high catalytic performance than that of other MOFs in the coupling transformation, which confirms the practical possibility in catalytic synthesis [55]

Scheme 1 17: Cross-coupling of coumarin and N,N-dimethylaniline utilizing

VNU-20 as a heterogeneous catalyst [55]

Besides, To expand the catalytic applications of MOF VNU-20 to the dehydrogenative coupling of coumarins with alkylbenzenes, cycloalkanes, ethers, and formamides, Doan and co-workers conducted the reaction between 6-methylcoumarin

cross-and mesitylene using the VNU-20 catalyst (Scheme 1 33) The desired product with

89% yield was obtained with the combination of DTBP as the oxidant and DABCO as the additive led Heterogeneous catalysis was confirmed for the cross-dehydrogenative coupling transformation utilizing the VNU-20 catalyst, and the contribution of active iron species in liquid phase was insignificant [56]

Scheme 1 18: The cross-dehydrogenative coupling of 6-methylcoumarin with

mesitylene using the VNU-20 catalyst [56]

All in all, there have been a remarkable increase in the utilization of iron-based MOFs as efficiency and recyclable catalysts for the chemical synthesis of numerous