Silver nanoparticles embedded over porous metal organic frameworks for carbon dioxide fixation via carboxylation of terminal alkynes at ambient pressure

Accepted Manuscript Silver nanoparticles embedded over porous MOF for CO2 fixation via carboxylation of terminal alkynes at ambient pressure Rostam Ali Molla, Kajari Ghosh, Biplab Banerjee, Md. Asif Iqubal, Sudipta K. Kundu, Sk. Manirul Islam, Asim Bhaumik PII: DOI: Reference:

S0021-9797(16)30322-8 http://dx.doi.org/10.1016/j.jcis.2016.05.037 YJCIS 21281

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

20 January 2016 22 April 2016 19 May 2016

Please cite this article as: R.A. Molla, K. Ghosh, B. Banerjee, Md. Asif Iqubal, S.K. Kundu, Sk. Manirul Islam, A. Bhaumik, Silver nanoparticles embedded over porous MOF for CO2 fixation via carboxylation of terminal alkynes at ambient pressure, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis. 2016.05.037

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Silver nanoparticles embedded over porous MOF for CO2 fixation via carboxylation of terminal alkynes at ambient pressure Rostam Ali Molla,a Kajari Ghosh,a Biplab Banerjeeb, Md. Asif Iqubal, c Sudipta K. Kundub, Sk. Manirul Islam,*,a Asim Bhaumik,*,b a

Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, W.B., India

b

Department of Material Science, Indian Association for the Cultivation of Science, Jadavpur,

Kolkata –700032, India c

Department of Chemistry, IIT Roorkee, Roorkee 247667, Uttarakhand, India

Abstract : Ag nanoparticles (NPs) has been supported over a porous Co(II)-salicylate metalorganic framework to yield a new nanocatalyst AgNPs/Co-MOF and it has been thoroughly characterized by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), energy dispersive X-ray spectrometry (EDX), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflection spectroscopy (DRS) and N2 adsorption/desorotion analysis. The AgNPs/Co-MOF material showed high catalytic activity in the carboxylation of terminal alkynes via CO2 fixation reaction to yield alkynyl carboxylic acids under very mild conditions. Due to the presence of highly reactive AgNPs bound at the porous MOF framework the reaction proceeded smoothly at 1 atm CO2 pressure. Moreover, the catalyst is very convenient to handle and it can be reused for several reaction cycles without appreciable loss of catalytic activity in this CO2 fixation reaction, which suggested a promising future of AgNPs/Co-MOF nanocatalyst. *

Address for correspondences.

E-mail: [email protected] (Sk. Manirul Islam); E-mail: [email protected] (Asim Bhaumik)

1

Keywords: CO2 fixation, metal organic frameworks; Ag nanoparticles; carboxylation of terminal alkynes; expansion of carbon chain.

Introduction: Successful utilization of carbon dioxide to an available feedstock for the production of organic fine chemicals is one of the major research interests today, especially in the context of sustainable global environment [1-7]. Taking into account the reduction of fossil fuels with time and the greenhouse effect on the environment, development of highly efficient methods for chemical fixation of carbon dioxide (CO2) in reactive organic molecule is gaining huge momentum over the years. The increasing interest of carbon dioxide as C1 building block in both pharmaceutical laboratories and academic interests due to its low-cost, high abundance, non toxic and massive potential as a renewable carbon source and a carbonyl reagent alternative to highly toxic CO [8-12]. Up to now, various methodologies have been developed to transform CO2 into useful chemicals such as esters formic acid, ureas, methane, methanol, formamidine derivatives, benzimidazoles, etc [13-22]. In recent time carbon dioxide capture and sequestration has been a highly fascinating research topic [23-26]. This idea has been called as ‘carbon dioxide capture and utilization’ (CCU) in the literature [27-30]. However, synthetic approaches using carbon dioxide as feedstock under atmospheric CO2 pressure is still in fancy. The alkynyl carboxylic acids are found highly important compounds in medicinal chemistry and the huge utility as a synthon in organic synthesis, making them particularly interesting targets for drugs and fine chemicals synthesis [31]. Unsaturated carboxylic acids are attractive starting molecules in organic synthesis and are favorite skeletons in a large number of natural products, agrochemicals, or pharmaceutically relevant compounds such as vancomycin, blopress, prandin, lipitor, and so on [32]. 2

From the aspect of atom and step-economical chemistry approaches for the preparation of carboxylic acid derivatives with CO2 as a carboxylative reagent under mild conditions are the direct and the shortest route for the synthesis of carboxylic acids [33-35]. In this context, the research groups of Goossen, Zhang, Lu and Kondo have reported several methodologies for the carboxylation of terminal alkynes [36-43]. Several metals have been used for these processes reported in literature, among which silver catalyzed approaches have proven to be the most effective for providing access to the desired compounds in good yields and high selectivities [4043]. But these methods experienced several drawbacks because most of these reactions are carried out in the presence of homogeneous catalysts making the separation or purification and recovery of the expensive metal catalysts very difficult, or even impossible. The green and costeffective concerns linked with the contamination issues of biologically active compounds are very crucial when dealing with large scale-synthesis and industrial processes. In order to overcome problems related to the homogeneous catalysts, we have developed several heterogeneous catalytic systems for studying organic reactions [44-49]. Although several homogeneous silver (I) salt catalyzed carboxylation of terminal alkynes are known in literature but there are only few reports on heterogeneous Ag-catalysts in this context [50]. Nanoparticles are more reactive than their metal salts because smaller size of the metal nanoparticles leads to high surface to volume ratio and consequently, large number of potential active sites would be available to the substrates. These advantageous features helped in the improvement of their catalytic activity [51-53]. However, metal nanoparticles are not stable as such due to elevated surface energy. During a reaction, metal nanoparticles are destabilized by highly active surface atoms. Hence immobilization of metal nanoparticles into an appropriate solid support has come into practice to boost the stability of the material [54]. In this context, 3

finely dispersed silver nanoparticles have been immobilized over the microporous metal-organic framework (MOFs) [55,56] to obtain highly reactive catalysts. Metal-organic frameworks are one of the most widely studied crystalline porous materials made from metal ions and polyfunctional organic ligands [57]. Considerable research interest has been paid over these materials in recent years, not only for their wide diversity of framework types but also for many useful applications in catalysis [58-63]. In this particular case for the carboxylation of terminal alkynes with CO2 we have chosen metal organic framework (MOF) as the solid support because MOFs has established their huge potential for carbon dioxide capture due to their extra-high porosity [64-68] and tunable nature of pores. For the carboxylation of terminal alkynes with CO2 catalyzed by heterogeneous catalysts only few materials are reported. Keeping the above drawbacks and environmental concerns in mind, herein we report a porous Co(II)-salicylate metal-organic framework [69] supported Ag nano catalyst that exhibits brilliant activity in catalyzing the terminal alkynes with CO2 for the synthesis of alkynyl carboxylic acids under ambient conditions. Experimental Chemicals Cobalt(II) chloride hexahydrate [Co(II)Cl2, 6H2O], sodium salicylate [C7H5O3Na], NaBH4 and AgNO3 were purchased from Sigma-Aldrich. For solvent extraction Chloroform (CHCl3) and methanol (MeOH) were used. All other chemicals were obtained from Merck and used without further purification. Using a standard procedure solvents were dried and distilled. Physical measurements The FT-IR spectra of the samples were recorded from 400 to 4000 cm-1 on a Perkins Elmer FT-IR 783 spectrophotometer using KBr pellets. Thermogravimetric analysis (TGA) was 4

carried out using a Mettler Toledo TGA/DTA 851e. Specific surface area of the sample was measured by adsorption of nitrogen gas at 77 K and applying the Brunauer-Emmett-Teller (BET) calculation. Prior to adsorption, the samples were degassed at 250 0C for 3h. UV-Vis spectra were taken using a Shimadzu UV-2401PC doubled beam spectrophotometer having an integrating sphere attachment for solid samples. Powder X-ray diffraction (XRD) patterns of different samples were analyzed with a Bruker D8 Advance X-ray diffractometer using Ni– filtered Cu Kα (λ=0.15406 nm) radiation. Transmission electron microscopy (HR-TEM) images of the mesoporous polymer were obtained using a JEOL JEM 2010 transmission electron microscope operating at 200 kV. The reaction time of the products were quantified (GC data) by Varian 3400 gas chromatograph equipped with a 30 m CP-SIL8CB capillary column and a flame ionization detector. NMR spectra were recorded on a Varian Mercury plus NMR spectrometer in pure deuterated solvents. Synthesis of Co-MOF: 1.6 g of sodium salicylate [C7H5O3Na] was added slowly in 20 mL water and mixture was stirred for few minutes. Similarly, 3.8 g of cobalt(II) chloride hexahydrate [CoCl2, 6H2O] was dissolved in 5 mL water and then it was added drop by drop to the above solution. At ambient temperature the solution was allowed to stir for 2h. Pink precipitation was appeared when NaOH solution was added dropwise into the solution and maximum precipitation was achieved at pH 10. At room temperature the whole solution was stirred for 4h. Then it was transferred into a Teflon lined airtight autoclave, aged at 348 K for 24 h. The solid product was filtered and washed several times with distilled water and dried at 333 K. For solvent exchange, 500 mg of solid product was placed in 50 mL anhydrous methanol for 72 h. Freshly distilled methanol has

5

been exchanged in every 24 h interval. The trapped pore blocking agents present in the asprepared sample was entirely substituted during volatile solvent exchanging. Synthesis of Colloidal Ag Nanoparticles: For the synthesis of colloidal Ag nanoparticles, 100 mg AgNO 3 was added to 10 ml of acetonitrile containing 0.5 mmol TRIS and was stirred for 2 min. Then, 4.5 ml solution of NaBH4 (0.7 mmol/mL) in ethanol was added drop-wise under stirring. The stirring was continued for another 5h, and the resulting nanocolloid was stored at 4°C. Synthesis of AgNPs/Co-MOF catalyst: At room temperature, 500 mg of Co-MOF was dispersed in 10 ml of TRIS-stabilized AgNPs and stirred for 4h. The color of the colloidal nanoparticles gradually disappeared while stirring. After 1h of stirring the supernatant solution was colourless, but the colour of Co-MOF changed to black, indicating the loading of Ag-NPs over the surface of Co-MOF. AgNPs/CoMOF was separated using centrifugation (Scheme 1). The AgNPs/Co-MOF material was washed with distilled water and dried at room temperature. The loading of Ag-NPs in Co-MOF was further confirmed from the spectral measurements. The observed Ag-loading in Co-MOF was 4.40 wt% by AAS. pH=10

CoCl2. 4H2O + Na-Salicylate

Hydrothermal treatment, 24h at 343k

Co-MOF

AgNO3 Tris

+ H2O

NaBH 4

Stir

Co-MOF RT

RT

= Ag nano

6

AgNPs / Co-MOF

Scheme 1: Schematic illustration of the formation of AgNPs/Co-MOF

General procedure of carboxylation reaction: A 50 mL high-pressure reactor was charged with DMF (5.0 mL), 1.0 mmol of terminal alkynes, AgNPs/Co-MOF (50 mg), and Cs2CO3 (1.5 mmol). After that CO2 (1 atm) was introduced into the reaction mixture under stirring at 80 0C for 14 h. After completion of the reaction, the reaction mixture was cooled to room temperature and transferred to the 2(N) potassium carbonate solution (5 mL) under stirring for 30 min. The mixture was washed with CH2Cl2 and the aqueous layer was acidified with concentrated HCl to pH=1 at low temperature, then extracted with ethyl acetate. The combined organic layers were washed with saturated NaCl solution, dried over Na2SO4 and filtered. The solvent was removed in vacuum to afford the isolated acid product. The products were identified by their retention times in comparison with authentic samples.

7

Results and discussion: The crystalline nature of Co-MOF and AgNPs/Co-MOF are quite evident from their respective powder XRD patterns as shown in Figure 1. A distinctive sharp diffraction peak of Co-MOF is observed at 2θ= 6.87 degree (Fig. 1a) has been shifted to higher 2θ for the AgNPs/Co-MOF material. Further, for AgNPs/Co-MOF the overall reflection intensities are much reduced with respect to that of Co-MOF. This could be attributed to the immobilization of the guest silver nanoparticles at the surface of the porous framework. Wide angle powder XRD pattern of the AgNPs/Co-MOF (Fig. 1c) showed the extra prominent diffraction peaks at 2θ = 38.06, 44.24, 64.66 and 77.43 degrees, representing (111), (200), (220) and (311) crystal planes of zero valent Ag FCC crystal. Small angle XRD pattern (Fig. 1b) of Ag/Co-MOF suggests that the crystalline nature of Co-MOF slightly reduced due to deposition of silver nanoparticles. However, still the AgNPs/Co-MOF material existed in the crystalline state [70].

8

Figure 1. Powder XRD pattern of Co-MOF (a) and AgNPs/Co-MOF (b and c) materials

HR-TEM images of Ag-NPs/Co-MOF is shown in Figure 2 (a-c). These HR-TEM images indicated the presence of high electron density dark spots all over the sample, which are mostly spherical in nature. These spherical particles are assigned to AgNPs of Ag-NPs/Co-MOF. The size of the particles ranges from 4.5±0.3nm. Particles are well distributed over the rod like surface and having homogeneous size distribution over the surface. TEM image (Figure 2 d) of reused catalyst reveals that nanoparticles are well stabilized at the MOF surfaces even after six reaction cycles. TEM-EDX (Fig. S3) investigation suggested the existence of carbon, oxygen, cobalt and silver in Ag-NPs/Co-MOF.

9

Figure 2. HR-TEM images (a-c) of fresh Ag-NPs/Co-MOF and (d) reused catalyst (after 6th run)

In Figure 3, solid state EPR spectrum of Ag-NPs/Co-MOF is shown. This EPR signal provides useful information about the nature of the Ag atoms in AgNPs/Co-MOF. This figure suggested a symmetric signal with calculated g value of 1.99 corresponding to this EPR spectrum (Fig. 3b). The symmetrical signal could be responsible to the existence of Agnanoparticles [71] encapsulated over the porous Co-MOF. Further, this EPR spectrum suggested another clear signal with g value 2.16 (Fig. 3a) lacking of any hyperfine structure, which could be attributed to the presence of Co+2 ions [72] in Ag-NPs/Co-MOF.

10

Figure 3 The X-band EPR spectrum of Ag-NPs/Co-MOF

The N2 adsorption-desorption isotherms of the Ag-NPs/Co-MOF composite is shown in Fig. 4. The isotherms exhibited a steady increase in N2 uptake with the increase in the high relative pressure P/P0 of N2. In addition, a hysteresis loop in the desorption isotherm is observed, which could be attributed to the interparticle voids. Microporosity could be attributed to fabrication of the metal-organic framework, whereas mesoporosity is mainly attributed to interparticle void spaces together with the assembly of surface AgNPs. The BET surface area and pore volume of the Co-MOF composite are 250.0 m2g-1 and 0.22 cm3g-1, respectively [69]. The BET surface area of AgNPs/Co-MOF composite is 106.7 m2g-1. This decrease in BET surface area could be attributed to the deposition of silver nanoparticles at outer surface of CoMOF. Significant reduction in the surface area to 106.7 m2g-1 in Ag-NPs/Co-MOF suggested that heavier silver nanoparticles are grafted at the surface of the Co-MOF.

11

Volume Adsorbed (ccg-1 at STP)

60

40

20

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Figure 4. N2 adsorption/desorption isotherms of Ag-NPs/Co-MOF. Adsorption points are marked with filled circles whereas the desorption ones are marked with empty circles. Catalytic activity Recently, we have reported several efficient methodologies for the carbonylation [73-75] of aryl halides using carbon monoxide as a C1 source. However, CO is a toxic gas and thus here we have explored the catalytic activity of the AgNPs/Co-MOF for the carboxylation of terminal alkynes using CO2 as the C1 source. Initial studies focused on tentative feasibility of the 3phenylpropiolic acid synthesis from ethynylbenzene and CO 2 by optimizing reaction conditions that could be useful to a variety of substituted ethynylbenzene. Chemoselectivity of the reaction is very much dependent on the reaction conditions, i.e. solvent, base, nature of the nucleophile, the catalyst amount and most importantly the pressure of CO2.

12

R

Ag NPs/Co-MOF OH HCl Base H + CO2 R C Solvent O 80 0C Scheme 2 AgNPs/Co-MOF catalyzed carboxylation of terminal alkynes

To test the catalytic activity of Ag-NPs/Co-MOF catalyst, the carboxylation of terminal alkynes into propiolic acids has been carried out as illustrated in Scheme 2. In this reaction 1ethynylbenzene is converted to 3-phenylpropiolic acid under 1 atm CO2 pressure. In our initial studies the carboxylation reaction of 1-ethynylbenzene is carried out by using K2CO3 as the base in DMF solvent medium. The reaction is conducted in a 50-mL autoclave at 50 0C under CO2 atmosphere for 12 h and we got a reasonable yield of phenylpropiolic (32%). Ag NPs /Co-MOF Cs2CO3 + CO2 DMF 80 0C

OH

HCl C

O

Scheme 3 AgNPs/Co-MOF catalyzed carboxylation of 1-ethynylbenzene

To obtain the most suitable reaction conditions, a variety of bases and solvents are screened (Table 1). In order to recognize the efficient reaction conditions for promoting the carboxylation reaction, the reaction is conceded under various temperature and reaction time (Table 2). In addition, the presence of a base could strongly affect the carboxylation reaction. A variety of inorganic and organic bases were employed to investigate the influenced of base on the catalytic activity. A good number of the bases, such as Na2CO3, K2CO3, CsOAc, DBU and DBN were less efficient for this reaction. But tBuOK and Cs2CO3 played a vital role in the carboxylation reaction. For tBuOK the yield is reasonable (76%), whereas for Cs2CO3 maximum 13

catalytic activity is observed. We found that by using Cs2CO3 as base in DMF at 80 0C AgNPs/Co-MOF showed 98% yield of the product for the carboxylation of 1-ethynylbenzene (Table 1).

Table 1 Effect of base and solvent on carboxylation of 1-ethynylbenzene Entry Base

Solvent Yielda (%) DMF 22

1

K2CO3

2

t

BuOK

DMF

76

3

CsOAc

DMF

59

4

Cs2CO3

DMF

98

5

DBU

DMF

36

6

DBN

DMF

31

7

Na2CO3

DMF

25

8

Cs2CO3

dioxane

55

9

Cs2CO3

Toluene

-

10

Cs2CO3

DMSO

51

11

Cs2CO3

THF

23

Reaction conditions:1-ethynylbenzene (1.0 mmol), AgNPs/Co-MOF (50 mg), base (1.5 mmol), CO2 (1.0 atm), reaction temp. 80 0C, solvent (5 mL), 14 h, aGC yield.

Considering the carboxylation of 1-ethynylbenzene as the representative reaction, various solvents are screened to confirm the solvent effect. The reaction is carried out in various polar solvents like, DMF, DMSO, THF and dioxane. Among them DMF is found to be most effective (Table 1). The use of toluene as solvent did not work for this reaction (Table 1, entry 9).

14

Therefore, DMF is chosen as the medium for this carbonxlation reaction. This is because DMF is not only a good solvent for Cs2CO3, but also a good solvent for CO2 (DMF is a weak base) [76]. The carboxylation reaction is established to be responsive to the reaction temperature. At lower temperatures (40-60 0C) only small to good yield is obtained (Table 2, entries 1-3). A reaction temperature of 80 0C was found to be most favourable for the model reaction (Table 2, entry 5). In this method we got an exciting inspection. The yield was decreased at reaction temperature higher than optimum temperature (90 0C). This may be due to fact that the silver propynoate intermediate is not stable at high temperatures. The intermediate may decompose through decarboxylation at high temperature. Also the reaction was carried out for different time ranging from 8 h to 14 h and it was found that at 14 h the conversion was 98% at given conditions.

Table 2 Effect of temperature and reaction time on carboxylation of 1-ethynylbenzene Entry Temperature Time Conversion(%)a (0C) (h) 1

40

14

28

2

50

14

51

3

60

14

69

4

70

14

73

5

80

14

98

6

90

14

81

7

80

8

56

8

80

10

68

9

80

12

81

10

80

16

98

15

11b

80

14

23

12c

80

14

46

Reaction conditions: 1-ethynylbenzene (1.0 mmol), AgNPs/Co-MOF (50 mg), Cs2CO3 (1.5 mmol), CO2 (1.0 atm), DMF (5 mL).a GC yields, bCo-MOF (50 mg) used instead of AgNPs/CoMOF, CAgNPs used instead of AgNPs/Co-MOF.

Variation of yield with respect to catalyst amount has also been investigated (Figure 5). An increase in the catalyst amount from 10 mg to 50 mg proceeds in an increase in the yield up to 98%. Additional increase in catalyst amount had no thoughtful effect on the yield of the required product.

100

Yield (%)

80 60 40 20 0 0

10

20

30

40

50

60

Catalyst amount (mg)

Figure 5. Effect of catalyst amount on carboxylation of 1-ethynylbenzene Reaction conditions: 1-ethynylbenzene (1.0 mmol), AgNPs/Co-MOF (50 mg), Cs2CO3 (1.5 mmol), CO2 (1.0 atm), 80 0C, DMF (5 mL), 14 h. Up to now, the studies on the influence of the size of silver particle on the catalytic activity have been mostly restricted to particles that are bigger than 10 nm in diameter. Small sized (<10 nm) 16

nanoparticles are most effective than larger sized nanoparticles due to the quantum size effects. It is still limited due to the complexity in the synthesis of small sized silver nanoparticles. In recent times, silver nanoclusters with controlled core size have been effectively synthesized [77-79] which make it feasible to study the catalytic activity of Ag nano clusters towards carboxylation of terminal alkynes. Here, we have synthesized 4.5±0.3 nm silver nanoparticles and used CoMOF as the support, which is highly porous. So, very tiny silver nano particles can be finely immobilized at its surface. Immobilized silver nanoparticles are found to be highly active than their homogeneous form. In literature Co-MOFs are often employed as an active material for CO2 gas capture. This property of Co-MOF increased the CO2 concentration around the COMOF and facilitated the reaction. On the other hand Co-MOF can also catalyse the reaction by activating the terminal hydrogen of alkynes but yield was low thus we have loaded silver nanoparticles over the Co-MOF and AgNPs/Co-MOF was found to be more active species due to presence of small sized active silver nanoparticles. So Co-MOF plays two important crucial roles; one is as a support of silver nanoparticles and other one is as supporting active centre for the reaction. Thus, AgNPs/Co-MOF catalyst could be highly reusable and effective for the carboxylation reaction. Porous nature of Co-MOF material increased the catalyst uniqueness. To inspect the extent of this carboxylation reaction, a range of terminal alkynes were tied with CO2 in DMF in the presence of AgNPs/Co-MOF catalyst. The investigational results are summarized in Table 3. With a distinct set of rules in hand, the substrate possibility with a range of terminal alkynes was explored. As exposed in Scheme 2, the procedure well tolerated various electronic and steric substituents of the terminal alkynes, all ensuing in good to outstanding yields. The substitutions of electron withdrawing and electron donating groups on the phenyl ring of 1-ethynylbenzene had considerable power on the result of the reaction. 1-ethynylbenzene 17

bearing either electron-donating or electron-withdrawing substituents, afforded the analogous caboxylative products in good to excellent yields. CH3- and -OMe as the electron-donating groups or NO2 as electron-withdrawing group on 1-ethynylbenzene resulted in slightly less yields than 1-ethynylbenzene (Table 3, entries 1-4). Nearly comparable yields were obtained with 1-ethynyl-4-methoxylbenzene and 1-ethynyl-4-nitrobenzene (Table 3, entries 3 and 4). Overall, terminal aromatic alkynes with an electron withdrawing group are deactivated and often inert to many transformations. With the electron withdrawing group on the phenyl ring, the nucleophilicity of the C1 carbon of alkynes was dropped. Therefore, the carboxylative yield of 4nitro-1-ethynylbenzene (Table 3, entry 4) was low compared to the 1-ethynylbenzene. Overall reaction depends on three major steps; (i) formation of silver acetylide intermediate from terminal alkynes in the presence of base and this step is more facile when strong electron withdrawing group is present at aryl ring. (ii) CO2 insertion into the silver acetylide intermediate is the 2nd step. (iii) The 3rd step involved the nucleophilic attack at silver propynoate intermediate to form the desired acid product (after acidification) and another silver acetylide intermediate to complete the cycle (scheme 4). Now 3rd step is slowed down when strong electron withdrawing group is present at aryl ring, on the other hand, step 1 is slowed down when strong electron donating group is present at aryl ring. Thus in our case moderately donating group (methyl) gave the higher product compared to strong donating or withdrawing group. For substituted aryl alkynes this transformations proceeded smoothly without any side product formation. Interestingly, this method was applicable to the 1,4-diethynylbenzene (Table 3, entry 7). Even heteroaromatic alkyes proved effective for this carboxylative coupling as depicted with the thiophene and pyridine ring system leading to the products (Table 3, entries 5 and 6).

18

Table 3 AgNPs/Co-MOF catalyzed carboxylation of terminal alkynes with CO2a. alkynes

Entry

Product

Conv. (%)b (c) O

98 (96)

C OH

1

O C OH

2

94 (92)

O C OH

3

91 (88)

MeO MeO

O C OH

4

92 (90)

O 2N

O2 N

O C OH

5

S S

19

92 (89)

6

O OH

84 (82)

N

N

O C

d

7

OH

86 (83)

O

C OH

a

Reaction conditions: alkyne (1.0 mmol), AgNPs/Co-MOF (50 mg), Cs2CO3 (1.5 mmol), CO2

(1.0 atm), 80 0C, DMF (5 mL), 14 h, bGC yields, c Isolated yield, dalkyne (0.5 mmol).

Proposed mechanism The complete mechanistic pathway for this CO2 fixation reaction has not been determined yet. However, based on previous reports, a probable reaction mechanism is revealed in Scheme 4 [80-81]. For silver-catalyzed C-H activation of terminal alkynes, silver acetylide is the key intermediate. It is known that the Ag-carbon bond is active for carbon dioxide insertion reaction. The silver acetylide intermediate (A) is produced through the reaction of the terminal alkyne and AgNPs/Co-MOF in the presence of a base. Consequent CO2 introduction into the polar Ag-C bond will form propynoate intermediate (B), in which it will go through metathesis with the terminal alkyne under basic conditions. This step would discharge propiolic acid and restore intermediate (A). Though, it must be noted that the silver propynoate intermediate B is unstable at high temperatures. In this reaction, intermediate B may decompose over heat to restructuring (A) during a decarboxylation. 20

H R

Ag Cs2CO3

O

R

Ag

(A)

C OH R

H3 O+ CO2

O

R

C OAg

H R

(B)

Scheme 4 Proposed mechanism of AgNPs/Co-MOF catalyzed carboxylation of alkyne. Table 4: Comparison with other reported system Entry 1

Catalyst

Reaction conditions

Ag@MIL-101 1-ethynylbenzene (1.0 mmol), catalyst

Yield (%)

Ref.

96.5

50

94

83

98

82

94

80

(70 mg), Cs2CO3 (1.5 mmol), CO2(1.0 atm), 50 0C, DMF (5 mL), 15 h. 2

3

bridged 1-ethynylbenzene (1.0 mmol), Cs2CO3 bis(amidate) (2.0 mmol), Catalyst (0.04 mmol), CO2 rare-earth metal amides (1 atm), DMSO (5 mL), 24h. P-NHC-Ag catalyst

1-ethynylbenzene (1.0 mmol), Cs2CO3 (1.2 mmol), DMF (5 mL), P-NHC-Ag catalyst (5.0 mg), CO2 (1.0 atm), room temperature, 20h.

4

AgI

1-ethynylbenzene (2 mmol), 20 mL DMF, Cs2CO3 (1.5 mmol), Catalyst AgI 21

(1 mol%), CO2 (0.2MPa), 12h, 50 0C. 5

Copper-NHC 1-ethynylbenzene (1.0 mmol), Cs2CO3

95

37

98

This work

(1.2 mmol), CO2 (2.5 atm), DMF (5 mL), 120 0C, 14 h. 6

Ag/Co-MOF alkyne (1.0 mmol), AgNPs/Co-MOF (50 mg), Cs2CO3 (1.5 mmol), CO2 (1.0 atm), 80 0C, DMF (5 mL), 14 h.

Catalyst reusability For the heterogeneous catalysis catalyst life is one of the major issues. We examined the reusability of AgNPs/Co-MOF catalyst in the carboxylation reaction of 1-ethynyl-4methylbenzene (Fig. 6). After the completion of the reaction catalyst has been removed through simple filtration and washed with methanol later acetone then dried in low pressure at 50 0C. Under optimum reaction conditions the next run was tested using recovered catalyst. The catalyst displayed nearly similar activity up to six reaction cycles. Stability of the supported catalyst has been confirmed from the observation of no catalyst deterioration under the reaction conditions. The Ag concentration in the filtrate is about 1.5 ppb as observed by inductively coupled plasmamass spectrometric (ICP-MS) analysis, which recommended that silver escape from the catalyst (0.0021%) is insignificant. These studies undoubtedly established that metal does intact to a significant level with the support (Co-MOF), and only insignificant amount of Ag escape during the course of carboxylation reaction.

22

Figure 6 Catalyst reusability test of the AgNPs/Co-MOF catalyst.

Conclusions In conclusion, we have reported the synthesis of the immobilized silver nanoparticles over a porous metal organic framework material to yield AgNPs/Co-MOF catalyst and its successful use for the carboxylation reaction of terminal alkynes under CO2 atmosphere. Both electron-donating and electron-deficient terminal alkynes are active for this CO2 fixation reaction and the nature of the solvent, base and temperature are proved to be crucial for this transformation. The AgNPs/Co-MOF catalyst is highly moisture and air stable, and the precursors for the synthesis of the catalyst are commercially available and inexpensive. Moreover, the catalyst is reused for six consecutive cycles without any considerable loss of catalytic activity. Further work is in progress to broaden the scope of this catalytic system for other organic transformations. This carbon dioxide fixation strategy reported herein can be used for the expansion of the carbon chain through activation of C-H bonds of the terminal alkynes and this could open new avenues in environmental research. Acknowledgements SMI acknowledges DST-SERB, UGC (Project Sanction F.No.-43-180/2014(SR) and DST-W.B.,

Project

No

811(Sanc)/ST/P/S&T/4G-8/2014 23

for

financial support.

RAM

acknowledges UGC, New Delhi, India for his Maulana Azad National Fellowship (F117.1/2012-13/MANF-2012-13-MUS-WES-9628/SA-III). SK acknowledges CSIR, New Delhi for his senior research fellowship. AB thanks DST, New Delhi for DST-SERB and DSTUKIERI project grants. We also acknowledge DST and UGC for providing funds to the University of Kalyani under FIST, PURSE and SAP programs. References [1] Louie, J. Org. Chem. 9 (2005) 605-623. [2] M. Mori, Eur. J. Org. Chem. (2007) 4981-4993. [3] T. Sakakura, J. C. Choi, H. Yasuda, Chem. Rev. 107 (2007) 2365-2387. [4] M. Aresta, A. Dibenedetto, Dalton Trans. (2007) 2975-2992. [5] A. Siewniak, K. Jasiak, S. Baj, Appl. Catal. A: Gen. 482 (2014) 266-274. [6] L. L. Yang, L. Yu, G. Q. Diao, M. Sun, G. Cheng, S. Y. Chen, J. Mol. Catal. A: Chem. 392 (2014) 278-283. [7] K. Huang, C. L. Sun, Z. J. Shi, Chem. Soc. Rev. 40 (2011) 2435-2452. [8] R. Zevenhoven, S. Eloneva, S. Teir, Catal. Today 115 (2006) 73-79. [9] M. Aresta, A. Dibenedetto, I. Tommasi, Energy Fuels 15 (2001) 269-273. [10] P. Tundo, M. Selva, Acc. Chem. Res. 35 (2002) 706-716. [11] G. A. Olah, Angew Chem. Int. Ed. 44 (2005) 2636-2639. [12] T. Aida, S. Inoue. Acc. Chem. Res. 29 (1996) 39-48. [13] M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann, F. E. Khn, Angew. Chem. Int. Ed. 50 (2011) 8510-8537. [14] C. Das, N. Gomes, B. O. Jacquet, C. Villiers, P. Thury, M. Ephritikhine, T. Cantat, Angew. Chem. Int. Ed. 51 (2012) 187-190. [15] G. A. Olah, G. K. S. Prakash, A. Goeppert, J. Am. Chem. Soc. 133 (2011) 12881-12898.

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Silver nanoparticles embedded over porous MOF for CO2 fixation via carboxylation of terminal alkynes at ambient pressure Rostam Ali Molla,a Kajari Ghosh,a Biplab Banerjee,b Md. Asif Iqubal, c Sudipta K. Kundu,b Sk. Manirul Islam,*,a Asim Bhaumik,*,b a

Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, W.B., India

b

Department of Material Science, Indian Association for the Cultivation of Science, Kolkata –

700032, c

Department of Chemistry, IIT Roorkee, Roorkee 247667, Uttarakhand, India

Ag nanoparticles has been supported over a porous Co(II)-salicylate metal-organic framework material and resulting AgNPs@CO-MOF showed excellent catalytic efficiency in the carboxylation of terminal alkynes via CO2 fixation reaction to yield alkynyl carboxylic acids under mild reaction conditions.

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