5 Figure 2.1: Standard line of coupling reaction between benzothiazole and iodobenzene.. 19 Figure 3.5: Effect of temperature on reaction yields.. 24 Figure 3.8: Effect of catalyst amoun
LITERATURE REVIEW
Metal-Organic Frameworks (MOFs)
Metal-organic frameworks (MOFs) are an emerging class of porous materials created from metal-containing nodes (also known as secondary building units or SBUs) and organic linkers The flexibility of constituents including geometry, size, and functionality makes MOFs have ultrahigh porosity (extending beyond 50% of the MOF crystal volume) and enormous internal surface areas (range from 1000 to 10,000 m 2 /g) [1] Due to their odd structures, MOFs have become one of the fastest growing fields in chemistry demonstrated through the escalating number of structures, publications, citation as well as research scope
Figure 1.1: Some of inorganic secondary building units and organic linkers [1]
Almost all MOF researches can be ascribed to the following five developments: cluster chemistry, synthetic modification, structure determination, interdisciplinary growth of MOF research and especially the ever-expanding potential in applications [2] Although challenges in commercialization still exist, new ideas for MOF application have been made strides in exploration In this themed issue, many publications contribute an overview of the latest progress in energy transfer, light harvesting, photocatalytic proton and CO2 reduction, as well as water oxidation using MOFs [3] However, the most noteworthy highlight in recent years is the application of MOFs in heterogeneous supramolecular catalysis, notably for liquid phase reactions [4]
It is because many outstanding advantages of heterogeneously catalytic systems based on MOFs can replace the incident weakness of homogeneous catalysis such as supplying high transition metal content as well as reactive centers, separating products from reaction system especially metal catalysts and limiting utilization of collateral ligands Moreover, recoverable and reusable ability is also a strong point besides reaction space controlled through pore size of MOFs
Regarding choosing MOFs-based catalysts, some factors are carefully considered like framework structure, cluster coordination, ligand essentiality, metal centers, etc Via transition metals, there are many publications finding replacement of organopalladium complexes in reaction Notably among them, a series of investigations about nickel catalysis for coupling reaction has received significant attention as less expensive and less toxic alternative to “standard” palladium catalysis [6] Several key properties of nickel, such as facile oxidation addition and ready to access to multiple oxidation states, have allowed the development of a broad range of innovative reactions Therefore, Nickel-MOF catalysts are also the worthy objects for studies about heterogeneous catalysis.
Nickel-Metal-Organic Frameworks in Catalysts Study
As mentioned above, Nickel-MOFs are expected to be an auspicious alternative that not only have the similar catalytic activity of nickel-based homogeneous catalysis with many advantages in chemical industry but also overcome the defects of palladium catalyst [7, 8] in particular and homogeneous catalysis in general In fact, there is the ever-expanding number of researches employing Nickel-MOFs to catalyze organic reactions
1.2.1 Recent applications of Ni-MOF in catalysis
In 2012, Nam T S Phan and his group announced the successful use of Ni(HBTC)BPY in catalysis for arylation between aldehydes and arylboronic acids to produce diarylmethanols (Scheme 1.1) They proved that the catalyst could be reused without a remarkable reduction in catalytic activity [9]
Scheme 1.1: Aryl coupling reaction with Ni(HBTC)BPY as catalyst [9]
In 2013, they achieved the goal of applying Ni2(BDC)2(DABCO) as an efficient heterogeneous catalyst for C-H direct activation of benzoxazole with aryl boronic acids In particular, this Ni-MOF could be easily removed from the result mixture and reused with good maintenance in performance (Scheme 1.2) [10]
Scheme 1.2: Direct heteroarene C-H arylation reactions between azoles and arylboronic acids catalyzed by Ni2(BDC)2(DABCO) [10]
At the same time, Huanfeng Jiang and co-workers reported the synthesis of cyclic carbonates by cycloaddition of CO2 to epoxides using Ni(salphen)-based MOF under relatively mild conditions The MOF catalyst with a high local density of cooperative layer Ni(salphen) motifs, showed the better performance compared to the monomeric homogeneous catalyst (Scheme 1.3) [11]
Scheme 1.3: The synthesis of cyclic carbonates catalyzed by Ni(salphen)-based
Ni-MOF-74 is an isostructural microporous material with a three-dimensional honeycomb-like network structure formed by coordinating nickel ion with organic linkers (Figure 1.3) Helical chains of cis-edge connect nickel oxygen octahedral at the intersections of honeycomb Nearest helices are of opposite handedness The diameter of channels in the honeycomb is about 1.1 nm The carboxylic acid groups as well as the hydroxyl groups of the linker are deprotonated during the synthesis and all of the oxygen anions are coordinated to nickel atoms In detail, five of total oxygen atoms are coordinated to each nickel atom, while the sixth is connected with a water molecule which points towards the cavity [12] On heating, Ni-MOF-74 immediately starts to lose its solvent water giving unsaturated (open) metal sites- strong adsorption sites which may enhance the interaction with the adsorbed species as well as the catalytic metal sites (Figure 1.4) [13]
Figure 1.3: Scheme of Ni-MOF-74 crystal structure Ball-and-stick representation of SBU (a); SBU with Ni shown as polyhedral (b); view of crystalline framework with inorganic SBUs linked together via the benzene ring of 2,5-dihydroxyterephthalic acid (c) [12]
Figure 1.4: Crystal structures of Ni-MOF-74 in the hydrated (a) and dehydrated form (b) [13]
In addition, Ni-MOF-74 is found to be stable up to over 350 o C under nitrogen atmosphere, while in air the structure is rapidly destroyed at the temperature over 250 o C The volume of empty channels accounts for 58% of the total volume with pore volume of 0.41 cm 3 /g and Langmuir surface area of 1083 m 2 /g [13]
The different feature of Ni-MOF-74 compared to other Ni-MOFs is the open site of cluster with five coordinate bonds instead of six as normally and honeycomb network structure cause the ever largest porous volume These can bring to Ni- MOF-74 favorable advantages when using as heterogeneous catalyst, particularly for reaction with unwieldy reagents However, the fact that there is only one investigation focus on catalytic ability of this material (scheme 1.4) [14] while recent tendency in MOF application essentially research the aspect of absorption
Scheme 1.4: Products generated from the oxidation of cyclohexene through either the radical (top) or epoxidation (bottom) routes [14].
Arylation of Acidic C-H Heterocycles with Aryl Halide under Nickel
Heterobiaryl species are ubiquitous in natural products and pharmaceuticals [15-18] Due to critical role of heterobiaryls, developments to connect heteroarenes and aromatic nuclei have become an attractive topic in chemical synthesis [19]
Generally, the C-H bond arylation of heteroarenes holds an important role in the overall synthetic methods [20-24] More than that, catalysis is also a concerned aspect with the transition metal, particularly nickel, was proposed to be the most appropriate catalyst
In 2009, Kenicchiro and his group described nickel-based catalysis for the coupling of azoles and aryl halides [25] Accordingly, Ni(OAc)2/bipy was utilized to catalyze the coupling between benzothiazole and iodobenzene in the presence of LiOtBu in dioxane at 85 o C, giving 80% yield of 2-phenylbenzothiazole (Scheme 1.5)
Scheme 1.5: Arylation of benzothiazole with iodobenzene catalyzed by
In the same year, Miura has reported similar conditions for direct coupling between benzothiazole and aryl bromides The reaction was catalyzed by NiBr2/1,10- phenanthroline in the presence of LiOtBu as the base in diglyme at 150 o C, giving moderate yield of product (Scheme 1.6) [26]
Scheme 1.6: Miura’s protocol for nickel-catalyzed heterobiaryl synthesis [26]
On the other hand, Febuxostat, a drug for treatment of gout and hyperuricemia, was synthesized by Teijin Pharma with Ni(OAc)2/bipy catalytic system Thiazole was cross-coupled with iodoarene in 1,4-dioxane to form the corresponding product
Subsequent treatment with CF3COOH afforded 62-67% overall yield of the product (Scheme 1.7) [27]
Scheme 1.7: Synthesis of drug for treatment of gout and hyperuricemia catalyzed by
Approach
The undeniable needs of pharmaceutical industry to synthesize substituted azoles as well as the previous research in coupling arylation commonly conducted on nickel homogeneous catalysis show the critical role of nickel to generate valuable compounds containing oxazole motifs Besides, under the progressive outlook of green chemistry, scientists and engineers are encouraged to find the effective alternatives, especially in reusing catalyst and separating products after reaction process On the other hand, the Ni-MOF-74 with structural characteristics and potential applications as mentioned above seems to be completely suitable candidate in replacing nickel homogeneous catalysis for heteroaryl synthesis
Therefore, herein, an effort to apply this material in reaction between aryl halides and C-H heterocycles has been conducted (Scheme 1.8)
Scheme 1.8: Direct arylation reaction of benzothiazole and iodobenzene catalyzed by the Ni-MOF-74
A range of factors will be examined to find out the optimal condition for this reaction such as temperature, time, and base, ratio of reagents, ratio of catalysts, solvent and other relevant factors In addition, to confirm the heterogeneity of the Ni-MOF-74, leaching and reusability are also tested.
EXPERIMENTAL SECTION
Materials and Instruments
All reagents and starting materials were obtained commercially from Sigma- Aldrich, Acros and Merck, and were used as received without any further purification unless otherwise noted List of chemicals is presented through table 2.1
The suitably analytic methods were used to examine MOF characteristics and confirm reaction results The instrumental details are listed as follows:
- Thin layer chromatography was performed using Merck Silica Gel 60 F254 and visualized by UV irradiation at 254 nm and 365 nm Columns packed with Himedia silica gel (230-400 mesh) was used to operate column chromatography, with the solvents bought from Chemsol
- X-ray powder diffraction (XRD) patterns were recorded on a D8 Advance Bruker powder diffractometer with a Cu Kα radiation source
- Scanning electron microscopy studies were conducted on a S-4800 Scanning Electron Microscope (SEM)
- Transmission electron microscopy studies were performed using a JEOL JEM 1010 Transmission Electron Microscope (TEM) at 80 kV The Ni-MOF-74 sample was dispersed on holey carbon grids for TEM observation
- Elemental analysis with atomic absorption spectrophotometry (AAS) was performed on an AA-6800 Shimadzu
- Fourier transform infrared (FT-IR) spectra were obtained on a Nicolet 6700 instrument, with samples being dispersed on potassium bromide pellets
- Gas chromatography (GC) analysis was performed using a Shimadzu GC 2010-Plus equipped with a flame ionization detector (FID) and an SPB-5 column (length = 30 m, inner diameter = 0.25 mm), and film thickness = 0.25 àm) The temperature program for main reactions was holding samples at 100 o C for 1 minute, then heating them from 100 o C to 280 o C at 40 o C/min and holding for 4.50 minutes Inlet and detector temperatures were set constant at 280 o C Diphenyl ether was used as an internal standard
- GC-MS analysis was performed using a Hewlett Packard GC-MS 5972 with a RTX-5MS column (length = 30 m, inner diameter = 0.25 mm, and film thickness
= 0.25 àm) The temperature program for GC-MS analysis hold samples at 60 o C for 2 minutes, then heated them to 280 o C at 10 o C min -1 and held them at 280 o C for
5 minutes The inlet temperature was set to a constant value of 250 o C MS spectra were compared with the spectra in the NIST library
- The 1 H and 13 C NMR spectra were recorded on a Bruker spectrometer operating at 500 MHz for 1 H NMR and a Bruker AV 500 MHz spectrometer operating at 500 MHz for 1 H and 125 MHz for 13 C with tetramethylsilane as internal standard The chemical shifts (δ) are expressed as values in parts per million (ppm) and the coupling constant (J) is given in hertz (Hz) Spin multiplicities are described as s (singlet), br (broad singlet), d (doublet), t (triplet), q (quartet), and m (multiplet).
Ni-MOF-74 Synthesis
Flowchart 2.1: Ni-MOF-74 synthetic process
- Stir in 15 min - Capped tightly in a 15ml pressure tube - Place in a 120 o C oven
- Wash with THF (3 x10ml) for 3 days - Evaporate 140 o C, 6h 2,5-dihydroxylterephtalic acid THF Ni(OAc)2.4H2O
In a 15 mL pressure tube capped tightly, a solution of Ni(OAc)2.4H2O (0.252 g, 1 mmol) in 7.5 mL of water and a solution of DHTA (0.102 g, 0.5 mmol) in 7.5 mL of THF were mixed under sonication for 15min and then placed in a 120 o C oven for 24h After cooling to room temperature, the mother liquor was removed and the solid product was washed three times with THF (10 mL) and immersed in 10 mL of THF THF solvent (10 mL) was exchanged at room temperature one per day over the next three days The product was then heated to dryness and evacuated under vacuum to 140 o C After 6 hours, the sample was cooled to room temperature, yielding 0.1633 g (94%) of the Ni-MOF-74 in the form of yellow fined-crystalline solid.
Catalytic Research
- Stir, 15 min, RT - Capped tightly in a 8ml vial - Heat
The Ni–MOF–74 was used as a catalyst for the nickel-catalyzed coupling of iodobenzene derivatives and benzothiazole to firm phenylbenzothiazole derivatives
In a typical experiment, a predetermined amount of Ni–MOF–74 was added to the 8 ml vial a mixture of iodobenzene (0.1030 g, 0.5 mmol), benzothiazole (0.1379 g, 1 mmol), base Li2CO3 (0.0746 g, 1 mmol), and diphenyl ether (0.085g, 0.5 mmol) as standard substaince in 1–Methoxy–2–(2–methoxyethoxy)ethane (Diglyme) (1 ml) under condition at argon atmosphere, 160 o C for 20 hours The catalyst concentration was based on the molar ratio of nickel/iodobenzene The reaction yield was monitored by withdrawing aliquots from the reaction mixture, diluting with ethylacetate (2 ml), quenching with an aqueous KOH solution (1%, 1 ml), and then drying over anhydrous Na2SO4 before analyzing by GC with reference to diphenyl ether, and further confirming product identity by GC-MS, 1 H NMR, and
For the leaching test, the catalytic reaction was stopped after 12h, analyzed by GC, and filtered to remove the solid catalyst The reaction solution was then stirred for an additional 4h The reaction progress if any was monitored by GC as previously described
To investigate the recoverability of the Ni-MOF-74, the catalyst was filtered from the reaction mixture after the experiment, washed with copious amounts of mixed solvent and then dried at 120 o C under vacuum for 6 h, and reused for the next run under identical conditions The mixed solvent includes 1,4-dioxane, THF, ethyl acetate, hexane, methanol and distilled water in proportion as follows 40:30:5:1:10:1 Regarding to characterizing the reused Ni-MOF-74 catalyst, the after-reaction Ni-MOF-74 was also examined by XRD and FT-IR
The sensitivity of GC instrument for different substances is not completely equal
So that to convenience as well as limit errors in determining reaction performance through on GC analysis, a standard line was established based on molar ratios between diphenyl ether and product 2-phenylbenzothiazole
Table 2.2: Molar ratios and peak area ratios in GC between diphenylether and 2- phenylbenzothiazole n2−phenylbenzothiazole ndiphenyl ether 0.02 0.2 0.4 0.6 0.8 1
Figure 2.1: Standard line of coupling reaction between benzothiazole and iodobenzene y = 1.3373x - 0.0179 R² = 0.9998
S2 − pheny lb enzothiazole Sdi pheny l et her n2−phenylbenzothiazole ndiphenyl ether
GC yield of 2-phenylbenzothiazole is calculated by the following formula:
S2-phenylbenzothiazole, Sdiphenylether: peak areas of 2-phenylbenzothiazole and diphenyl ether on GC diagram
ndiphenyl ether, niodobenzene: molar amount of diphenyl ether and iodobenzene in real.
RESULTS AND DISCUSSION
Analytical Results of The synthesized Ni-MOF-74
of the Ni-MOF-74 synthesized was analyzed by the X-ray diffraction powder (XRD) (Figure 3.1) Sharp peaks in XRD patterns detected at 2θ = 6.98 o and 11.98 o confirmed that the structure is similar to the previous simulated patterns [28, 29]
Figure 3.1: XRD pattern of the synthesized the Ni-MOF-74 (blue) compared with the simulated pattern (black)
FT-IR spectra of the Ni-MOF-74 and 2,5-dihydroxyterephthalic acid were demonstrated in Figure 3.2 The stretching vibration of the carbonyl group of the carboxylic acid was found at nearly 1650 cm -1 in the FT-IR spectrum of 2,5- dihydroxyterephthalic acid The presence of the carboxylate ions formed by the deprotonation of –COOH groups in 2,5-dihydroxyterephthalic acid upon the reaction with nickel(II) ions resulted in the lower value for C=O stretching vibration in the case of the Ni-MOF-74 with the strong peak at 1556.49 cm -1 in the corresponding spectrum The results were in good agreement with reported data [12]
Figure 3.2: FT-IR spectra of the synthesized MOF and DHTA
The information about morphology and surface properties, the scanning electron microscopy of the Ni-MOF-74 was measured (Figure 3.3).The SEM image indicated that the Ni-MOF-74 consisted of crystallite agglomerates and the average particle size is less than 4 àm in diameter, which was consistent with the previous reports [28, 30] Furthermore, the TEM micrograph revealed a complex porous structure as expected (Figure 3.4)
Figure 3.3: SEM images of the synthesized Ni-MOF-74
Figure 3.4: TEM images of the synthesized Ni-MOF-74
Nickel content in the synthesized Ni-MOF-74 was analyzed by Atomic absorption spectroscopy (AAS) demonstrated a nickel amount of 33.3%, which was lined with the theoretical value of 37.3%
In conclusion, the Ni-MOF-74 was successfully synthesized from Ni(OAc)2.4H2O and 2,5-dihydroxyterephthalic acid by a solvothermal method in THF and water at 120 o C providing yield of 94% The properties of the Ni-MOF-74 characterized by variety different techniques were appropriate to those of previous studies With high porosity, thermal stability, and high content of unsaturated nickel sites in the structure, this MOF becomes a promising candidate for heterogeneous catalysis for organic transformations Next, the catalytic performance of the synthesized Ni-MOF-74 on the arylation of benzothiazole and iodobenzene was studied.
Results of Catalytic Research
The Ni-MOF-74 was employed for direct C-H coupling reaction of benzothiazole with iodobenzene to produce 2-phenylbenzothiazole (Scheme 3.1) with several studied factors as mentioned above Experimental data and processes are also conducted according to the reaction process as outlined
Scheme 3.1: C-H arylation reaction between benzothiazole and iodobenzene
Until now, mechanisms of nickel-catalyzed arylations of azoles with aryl halides still stayed unclear However, several previous experiments were consistent with the proposed Ni 0 /Ni II catalytic cycle including: 1) production of Ar-Ni-X through oxidative addition of Ar-X to Ni 0 ; 2) azole nickelation to produce Ar-Ni-Het; and 3) reductive elimination, generating heterobiaryl product Het-Ar and Ni 0 It is assumed that base play an important role in the azole nickelation step and are probably involved both in the product-generating catalytic cycle and in the generation of catalytically active Ni 0 species from Ni salts [31] Moreover, Kenichiro Itami and his group assumed that the nickelation could be either (a) deprotonation of heteroarene with LiOtBu, followed by transmetalation with Ar-Ni-X or (b) concerted metalation-deprotonation of heteroarene with Ar-Ni-OtBu, which could possibly be generated from Ar-Ni-X and LiOtBu [25] The Ni-MOF-74-catalyzed direct arylation studied here could be assumed to follow this mechanism of homogeneous catalysis
Scheme 3.2: Proposed mechanism for nickel-catalyzed heterobiaryl synthesis [31]
It was observed that at 160 o C the highest GC yield of 90% was afforded after 20 hours Meanwhile, decreased temperature led to significant drop in the reaction rate, with GC yield of 36%, 10%, and under 2% being detected at the temperature of 150 o C, 140 o C, and lower 130 o C, respectively Therefore, 160 o C was the chosen temperature for the optimization stage In comparison with reported data (the highest temperature recorded at 150 o C) [26], this temperature was slightly higher due to using weaker base in previous works More than that, heterogeneous catalyst reaction possessed more difficulty than homogeneous system
Figure 3.5: Effect of temperature on reaction yields
Reaction condition: benzothiazole/iodobenzene/Li2CO3 = 2:1:2 (in epuivelent mol) under argon atmosphere for 20 hours 7 mol% Ni-MOF-74 as catalyst was calculated according to 0.5 mmol iodobenzene in 1 mL diglyme
The next important factor studied was reaction time Direct arylation reactions between benzothiazole and iodobenzene were examined the performance after the different periods of time such as 8, 12, 16, 20, and 24 hours Typically, the increase in the reaction time resulted in increasing the reaction yields with 7%, 47%, and 76% GC yield being observed after 8, 12, and 16 hours When the reaction time was increased to 20 hours, the highest GC yield of 90% was achieved However, increasing the reaction time longer than 20 hours was found to be unnecessary since the reaction yield seemed to be remain stable (Figure 3.6) This reaction time is an improvement compared with previous researches up to 36 hours [25]
Figure 3.6: Effect of time on reaction yields
Reaction condition: benzothiazole/iodobenzene/Li2CO3 = 1:0.5:1 (in mmol) in 1 mL diglyme under the presence of 7 mol% Ni-MOF-74 catalyst and argon atmosphere The study was carried out at the temperature 160 o C
Various different solvents including toluene, diglyme, DMPU, NMP, DMF, DMSO, and DMAc were investigated Among these solvents, diglyme was the most effective with 90% GC yield The other solvents such as toluene, DMPU, NMP, DMF, and DMSO were found to be not suitable with less than 2% GC yield (Figure 3.6) The first research of the heterogeneous direct arylation of benzothiazole with iodobenzene in diglyme using CuO nanoparticles as the catalyst at reflux temperature (160 o C) by Wang and co-workers; however, only trace amount of 2- phenylbenzothiazole was detected [32] On the other hand, similar reactions were conducted in dioxane solvent, which was not appropriate with this method due to high reaction temperature 160 o C [27]
Figure 3.7: Effect of solvent on reaction yields
Reaction condition: benzothiazole/iodobenzene/Li2CO3 = 1:0.5:1 (in mmol) under the presence of 7 mol% Ni-MOF-74 catalyst and argon atmosphere at 160 o C for 20 hours The researched solvents was used with amount of 1 mL
Different percentages of catalyst in the range of 0-10 mol% were tested to determine the optimal amount of catalyst Ni-MOF-74 for the arylation reaction It was found that optimal yield of 2-phenylbenzothiazole was achieved at 7 mol% Ni- MOF-74 Decreasing the amount of catalyst led to significant drop of reaction rate with 75% and 58% GC yield for the reaction using 5 mol% and 3 mol% of catalyst, respectively However, using more than 7 mol% catalyst led to a slight reduction in the reaction rate It should be noted that no reaction occurred in the absence of the Ni-MOF-74 (Figure 3.8)
Figure 3.8: Effect of catalyst amount on reaction yields
Reaction condition: benzothiazole/iodobenzene/Li2CO3 = 2:1:2 (in equivalent mol) in 1 mL diglyme under argon atmosphere at 160 o C for 20 hours The amount of Ni- MOF-74 was calculated according to 0.5 mmol iodobenzene
Several different bidentate ligands such as 2,2’-bipyridine, 1,10-phenathroline, EDA, TMEDA and 4-methyl-o-phenylenediamine were employed to examine the role of Ni-MOF-74 ligand and whether the needs of further support ligands The result showed that unlike the homogeneous catalytic system with combination between ligands and trasition metals to increase the reaction yield, using bidentate ligands in this case was unnecessary or even caused a serious decrease in performance To be specific, there was no surveyed ligand exceeded 5% GC yield (Figure 3.9)
Figure 3.9: Effect of different ligands on reaction yields
Reaction condition: benzothiazole/iodobenzene/Li2CO3 = 1:0.5:1 (in mmol) in 1 mL diglyme under argon atmosphere at 160 o C for 20 hours with 7 mol% Ni-MOF- 74 as catalyst and the 14 mol% bidentate ligand were calculated according to amount of iodobenzene
The effect of concentration of reagents was next studied in direct arylation reaction
Obviously, when concentration of reagents was too low (0.25 mol of iodobenzene in 1 mL of diglyme), the contact ability between reagents and catalytic particles was dramatically poor, so that the reaction rate was reduced giving 6% of GC yield It was found that concentration of iodobenzene at 0.5 was the optimal condition affording 90% of yield after 20 hours Nevertheless, increasing concentration of reagents to over 0.5 M was not suitable It could be assumed that the more amount
LIGANDS of reactants, the more difficult for them to dissolve in the solution, leading to a reduction in reaction rate (Figure 3.10)
Figure 3.10: Effect of concentration of iodobenzene to GC yields
Reaction condition: benzothiazole/iodobenzene/Li2CO3 = 2:1:2 (in equivalent mol) in 1 mL diglyme under argon atmosphere at 160 o C for 20 hours The 7 mol% Ni- MOF-74 catalyst was counted according to amount of iodobenzene
3.2.7 Effect of reagent molar ratio
The impact of the reagent molar ratios were carried out through reactions with 1.0, 1.5, 2, and 2.5 equivalents of benzothiazole, respectively After 20 hours, the experimental results indicated that using less than 1.5 equivalents of benzothiazole resulted in a significant drop in the reaction rate, with 32% and 6% GC yields being detected for the case 1.5 and 1 equivalents of benzothiazole, respectively (Figure 3.13) In addition, when using molar ratio of 1:2, only trace of product was recorded This is reasonable with the proposed mechanism Moreover, using more than two equivalents of benzothiazole was found to be unnecessary as the GC yield did not increase Therefore, reactant molar ratio of 2:1 was chosen for further investigation
Figure 3.11: Effect of reagent molar ratio on reaction yields
Reaction condition: benzothiazole/Li2CO3 = 1:1 (in equivalent mol) in 1 mL diglyme under argon atmosphere at 160 o C for 20 hours The 7 mol% Ni-MOF-74 catalyst was counted according to amount of the least reagent
The effect of different bases on the reaction yield was examined (Table 3.1) The experimental results indicated the excellent performances obtained when using Li2CO3 (90%) and KCl (91%) There are no rest additives achieved the GC yields over 5% excepted Chloride salts including NaCl, KCl, MgCl2 and Chloride salt LiCl with GC yield at 81%, 91%, 37% and 66%, respectively This may cause by several reasons that need more investigations to make it clear
Table 3.1: Effect of various additives on reaction yields
GC yield (%) n benzothiazole : n I-ph RATIO OF REAGENTS
Reaction condition: benzothiazole/iodobenzene/additive 1:0.5:1 (in mmol) in 1 mL diglyme under argon atmosphere at 160 o C for 20 hours The 7 mol% Ni-MOF-74 catalyst was counted according to amount of iodobenzene
CONCLUSION
A crystalline porous metal-organic framework Ni-MOF-74 was successfully synthesized by solvothermal method using Ni(OAc)2.4H2O and 2,5- dihydroxyterephthalic acid with the yield of 94% The material was then characterized by a variety of different techniques including XRD, SEM, TEM, TGA, FT-IR, AAS, and BET
The synthesized Ni-MOF-74 catalyst exhibited as an efficient catalyst for the direct arylation reaction of benzothiazole with iodobenzene to get a good yield (89% isolated yield) The reaction conditions that favored to produce 2- phenylbenzothiazole were indicated like scheme 4.1
Scheme 4.1: The optimal condition of coupling reaction to form 2- phenylbenzothiazole under Ni-MOF-74 catalysis
In reusability and recoverability tests, the catalyst performed successful runs without loss of activity The leaching test showed that there was no contribution from the active sites of Ni-MOF-74 in the solution, confirming a successful heterogeneous catalysis system
In the future, the mechanism of the arylation of benzothiazole and iodobenzene catalyzed by the Ni-MOF-74 need to be investgated Moreover, the reaction scope will be expanded with other functional substituents especially azole counterparts
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