Three-Dimensional Co(II)-MetalâOrganic Frameworks with Varying

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3D Co(II)-MOFs with Varying Porosity and Open Metal Sites toward Multipurpose Heterogeneous Catalysis under Mild Conditions Santanu Chand, Shyam Chand Pal, Manas Mondal, Subrata Hota, ARUN PAL, Rupam Sahoo, and Madhab C. Das Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00823 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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Crystal Growth & Design

Revised Manuscript ID: cg-2019-00823x

3D Co(II)-MOFs with Varying Porosity and Open Metal Sites toward Multipurpose Heterogeneous Catalysis under Mild Conditions Santanu Chand‖, Shyam Chand Pal‖, Manas Mondal, Subrata Hota, Arun Pal, Rupam Sahoo and Madhab C. Das* Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, WB, India. E-mail: [email protected] Abstract: In recent years heterogeneous catalysis is becoming one of the most active domain in the research of Metal-Organic Frameworks (MOFs). Here, two 3D Co(II)-MOFs with the open metal sites and exposed azo functionality on the MOF backbone have been constructed via mixed ligand assembly. Both the MOFs, 1 ({[Co2(1,4-NDC)2(L)(H2O)2(µ2H2O)]·(DMF)2(H2O)}n) and 2 {[Co(fma)(L)(H2O)2]·S}n [1,4-NDC = 1,4-naphthalene dicarboxylic acid, fma = fumaric acid, L = 3,3' azobis pyridine and S = disordered solvents] exhibit 3D frameworks with metal bound aqua ligands. These metal bound aqua ligands, as well as the lattice solvent molecules, could simply be removed upon activation affording the desolvated frameworks 1a and 2a respectively maintaining original crystallinity with varying number of open metal sites. Although, crystallographic analysis revealed porous structure for both the MOFs (34.4% and 14.3% void volume for 1 and 2 respectively), 1 showed permanently microporous nature with BET surface area of 197 m2g-1 and moderate CO2 uptake capacity as established through gas sorption study. Both the MOFs exhibit efficient catalytic activity for the chemical fixation of CO2 to cyclic carbonate in presence of a cocatalyst, cyanosilylation reaction, Knoevenagel condensation under solvent free and mild conditions and thus demonstrating their multipurpose heterogeneous catalytic nature. The limited pore space decorated with the exposed metal sites and the functional azo groups were efficient for size selective heterogeneous catalysis with varying catalytic efficiencies. A systematic comparison in their catalytic performances could be made with the establishment of structure-function relationship. Besides, both the MOFs can easily be separated out from the reaction mixtures and reused for at least four cycles without any loss of catalytic activity and structural integrity.

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Introduction Metal-organic framework (MOFs) have received extensive attention as porous hybrid material owing to their unique structural features and useful application ranging from gas storage, separation, catalysis, chemical sensing, drug delivery, magnetism, proton conduction, and so on.1-33 Usually, MOFs are constructed from linear or bent polycarboxylic acid, which acts as a linker between metal ions or their cluster to give the final three dimensional (3D) porous structure.34,35 In general, the structural arrangement of the building unit and the subsequent overall framework topology plays a key factor for the bulk property of the particular system.36-38 In recent years, MOFs having well-defined crystal structures, modifiable pore topology, excellent tailorability, high surface areas have presented huge potential, especially in the field of heterogeneous catalysis.4-10, 39-54 The catalytic property of MOFs can be readily controlled by the proper selection of metal ion and organic ligands. Based on the literature reports, there are different types of the active sites in the MOF which can catalyse the heterogeneous catalysis reactions (structural defect site, Lewis acid centre, open metal site and MOFs with a functional linker).9,10 There are two fundamental aspects of CO2 related research: sequestration and its utilization. While sequestration deals with CO2 gas separation and storage with porous materials, utilization of CO2 is focused on its chemical reactions as C1 feedstock. The increasing level of CO2 in the atmosphere has encouraged the researcher to find out ways to selectively capture CO2 from various sources of gas mixtures, in particular, separation from landfill or biogas (50% CO2 and 50% CH4) and flue gas (15% CO2 and 85% N2).55-57 On the other hand, CO2 is an abundant C1 resource and its chemical conversion to a value-added product such as dimethyl carbonate, N, Nꞌ disubstituted urea, cyclic carbonate, urethanes, formic acid and so on are both environmentally and practically attractive.58 Therefore, the formation of cyclic carbonates via metal-catalysed cycloaddition of CO2 and epoxide attracts immense attention among the catalysis community.59 Cyclic carbonates are relatively valuable as pharmaceutical/fine chemicals intermediate in addition to their several biomedical applications.58 The 100% atom economic ring extension reaction of epoxide with CO2 generating cyclic carbonates is a conventional and well-known procedure catalyzed by a collection of numerous homo and heterogeneous catalysis like Schiff base, simple metal oxide, salen complex and supported ionic liquids catalyst.60,61 Although there are several MOFs reported catalysing such reaction which required high pressure (>2M Pa) and high Page no:2 ACS Paragon Plus Environment

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temperature (>80°C) and thus causes high energy cost. However, under milder condition, only a handful of MOFs such as MMFC-2, MMPF-9 and HKUST-1 effectively catalyse such reaction. Therefore, the development of MOFs as a heterogeneous catalyst to produce cyclic carbonates under milder condition are highly desired.58,59 Besides CO2 cycloaddition, cyanosilylation is another important organic reaction to produce cyanohydrins from aldehydes, which are major organic compounds for both biological processes and synthetic chemistry as well.62 They can readily be altered into essential species such as β-amino alcohols or α-hydroxycarboxylic acids.63 On the other hand, Knoevenagel condensation (condensation between carbonyl compounds and activated methylene compounds) is one of the fundamental reactions in order to form carbon-carbon bonds.64 This kind of reaction is extensively used for the production of vital drug intermediates.64 The homogeneous catalysts such as metal oxides or amines are frequently used to catalyse such reactions, however, they do have their inherent limitations. The problem related to homogeneous catalyst essentially includes recycling of the catalyst and the difficulty in separation of product.60,61 Hence, the development of efficient heterogeneous MOF-catalysts featuring reusability and easier separation, together with non-leaching property towards multipurpose reactions such as CO2 cycloaddition, cyanosilylation and Knoevenagel condensation of aldehydes are of great interest that deserves to be explored.4-10, 39-54

On the other hand, it would be preferable to carry out the catalysis reactions without the presence of solvents, simply by using liquid substrate and reagents. Solvent-free environments are favourable not only from avoiding releases of organic vapours to the atmosphere but also the solid catalyst become easily reusable without any byproduct.65 Considering all those points, we have designed and assembled two 3D Co(II)-based MOFs bearing open metal sites, from mixed ligand assembly of two acid ligands and 3,3' azobis pyridine spacer bearing azo functionality, for their potential usage in multipurpose heterogeneous catalysis. Both the MOFs contain unsaturated Co(II) centre (s) to act as Lewis acidic sites and the accessible azo groups of the spacer to serve as a Lewis base centre. Interestingly, as ideal Lewis acid-base bifunctional catalyst, both the MOFs offer great performance in various scope of substrates with satisfactory recyclability in three different types of catalysis: CO2 to cyclic carbonate, cyanosilylation reaction and Knoevenagel condensation in solvent-free condition. Varying the number of open metal sites, accessible pore volumes, permanent porosity and CO2 sorption capability enabled us to establish a structure-catalytic function relationship. Although a variety of MOFs has been explored for Page no:3 ACS Paragon Plus Environment


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their various highly efficient catalytic performances, most of them are reported to catalyze single or at most two reactions simultaneously, however, to the best of our knowledge, reports on MOFs toward multipurpose heterogeneous catalysis are yet to be reported. Experimental section Materials and reagents The spacer L was prepared according to a previously reported method.66,67 All the chemicals used in this work were commercially available, and of reagent grade quality and used as received without additional purifications. Synthesis of {[Co2(1,4-NDC)2(L)(H2O)2(µ2-H2O)]·(DMF)2(H2O)}n, 1. A mixture of Co(NO3)2·6H2O (0.1 mmol, 0.029 g), 1,4-NDC (0.1 mmol, 0.0216 g) and linker L (0.05 mmol, 0.009 g) was taken in a mixed solvent (DMF : MeOH : Water 1:2:1 ) was heated to 120 °C under autogenous pressure in a Teflon line stainless steel

autoclave for 48 h,

followed by slow cooling to room temperature at 5 o/min to give 1 as pink colour block shape crystals in 70% yield based on the metal. Elemental analysis for ‘C40H42CoN6O14’, calcd: C, 53.99%; H, 4.76%; N, 9.45%. Found: C, 54.01%; H, 4.75%; N, 9.43%. FT-IR (KBr pellet, cm−1): 3350.1(br), 3070.2(w), 2920.6(w), 1613.2(s), 1453.7(m), 1425.9(s), 1355.2(s), 1251.4(m), 1083.7(m), 1014.3(m), 783.9(m), 686.1(m), 637.8(w), 588.4(m), 445.5(m). Synthesis of {[Co(fma)(L)(H2O)2]·S}n, 2. A mixture of Co(NO3)2·6H2O (0.1 mmol, 0.029 g), fumaric Acid (fma) (0.1 mmol, 0.011 g) and linker L (0.05 mmol, 0.009 g) was taken in 3 ml DMF solution and stirred for 1 h at room temperature. The solution was filtered and kept for slow evaporation at ambient temperature. Red crystals were obtained after fifteen days in 60% yield based on the metal. FT-IR (KBr pellet, cm−1): 3105.8(br), 1542.4(s), 1471.2(w), 1428.1(w), 1384.9(s), 1194.3(m), 1098.2(w), 1028.9(w), 976.7(m), 811.1(s), 687.4(s), 645.4(w), 519.6(m). Results and Discussion Single crystal X-ray analysis shows that 1 crystallizes in monoclinic crystal system with space group P21/n constructing a 3D polymeric framework formulated as {[Co2(1,4NDC)2(L)(H2O)2(µ2-H2O)]·(DMF)2(H2O)}n. The primary building unit consists of two cobalt (II) ions (Co (1) and Co (2)), two 1,4-NDC anions, one spacer (L), one bridging water molecule (µ2-H2O), two coordinated water molecules, two lattice DMF molecules and one water molecules. As illustrated in Figure 1a Co(1) is six-coordinated by four carboxyl oxygen Page no:4 ACS Paragon Plus Environment

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atom from 1,4 NDC, one O atom from µ2-H2O and one pyridyl N atom from spacer L to complete a distorted octahedral geometry. The bond distances and bond angles around the metal centres in 1 are in the normal range and are listed in Table S2, SI. Co(2) is also sixcoordinated by two chelating carboxylate groups, two coordinated water molecules (Ow1 and Ow2), one pyridyl N atom and one O atom from µ2-H2O. Two Co(II) ions are bridged by two bis (monodentate) carboxyl groups and one aqua ligand with µ2 bridging mode forming a dimeric {Co2(COO)4} unit with a 2D square shape layered network (Figure 1b). The 2D layered network is further connected by the spacer (L) to generate an extended 3D network (Figure 1c). The separation distance between the two Co(II) metal centres along Co(1)···L···Co(1) and Co(2)···L···Co(2) are 10.832 and 12.007Å, respectively. The framework contains a total 34.4% solvent-accessible volume (1484.0 Å3 of the unit cell volume of 4308.9 Å3) as estimated through PLATON 68 calculation with the formation of a channel (5.0 x 8.2 Å2 considering van der Waals radii) along the crystallographic ‘a’ axis (Figure 1c). The Topological analysis of 1 using TOPOS revels that the framework of 1 can be simplified to a 3D uninodal 6-connected rob topology with the point symbol {48.66.8} as depicted in Figure 1d. Several H-bonding interactions among solvent molecules and the constituent ligands further stabilize the framework (Table S3, SI).

Figure 1. a) Coordination environment around Co(II) metal centre in 1. Color code: Co, pink; O, red; N, blue; C, grey; H, cyan. b) Four coordinated NDC2− with a Co(II) unit generating a 2D {Co2(COO)4} layered network. c) Representation of the packing diagram showing 1D Page no:5 ACS Paragon Plus Environment


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channels along ‘a’ axis. d) A schematic illustration of the network presenting the ‘rob’ topology. The single-crystal X-ray determination of the structure for 2 also reveals a 3D coordination framework constituted by fma2- and a N, N'-donor spacer L. 2 crystallizes in monoclinic crystal system with C2/c space group formulated as {[Co(fma)(L)(H2O)2]·S}n. The asymmetric unit of 2 consists of one half of Co(II) metal ion, half deprotonated fumaric acid, half of the L spacer and one coordinated water molecule. Each Co(II) atom is sixcoordinated with two N-atoms ligated from two different L spacer in axial position and four O atoms, among them two from symmetry-related water molecules and two different fma2units and thus giving rise to a distorted octahedral configuration {CoO4N2} (Figure 2a). The Co–O bond distances range from 2.067 to 2.075 Å whereas the Co–N bond distance is 2.173 Å which are in the normal range and are listed in Table S4, SI. The Co(II) ions are coordinated by fumarate dianion and N, N'-donor spacer L in an alternating fashion to form a 2D [Co(fma)(L)] network (Figure 2b). These 2D layers are further pillared by spacer L, generating a 3D pillar-layered framework that houses 1D channels with an approximate diameter of 3.6 Å which is much smaller in size than found in 1 (Figure 2c). The PLATON68 calculation after removing disordered solvent molecule indicates that 2 affords a porosity of 14.3% of unit cell volume (254.6 Å3 per unit cell volume of 1776.5 Å3) which is again substantially lower than the 34.4% porosity found in 1. The framework of 2 can be simplified to a 3D uninodal, 4-connected ‘cds’ topology with the point symbol {65.8} as depicted in Figure 2d. One H-bonding and one non-covalent intermolecular interactions among the ligands further stabilized the framework (Table S5, SI).

Figure 2. a) Coordination environment around Co(II) metal centre in 2. Color code: Co, pink; O, red; N, blue; C, grey; H, cyan. b) Representation of a 2D view of [Co(fma)(L)] unit. c) Page no:6 ACS Paragon Plus Environment

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Representation showing channels along the crystallographic ‘c’ axis. d) A schematic representation of the network showing the cds topology. To investigate the purity of the crystals in the bulk phase, PXRD experiment was carried out for both the samples. The PXRD pattern of bulk samples of both the cases are in good agreement with the as-synthesized ones, suggesting the phase purity in bulk synthesis (Figure S1-S2, SI). It is well known that chemical stability is essential for the intended practical applications. To illustrate the chemical stability of both the MOFs, the PXRD patterns were recorded after immersing the MOFs in various solvents such as MeOH, THF, acetone, acetonitrile, and water for 24 h. 1 shows high stability in common organic solvents, however, when treated in water, change in peak positions in PXRD patterns is observed suggesting formation of a different phase (Figure S3, SI) probably due to the partial hydrolysis of metal-ligand bonds. On the other hand, for 2, the PXRD pattern of the dried sample after soaking in different solvents including water reveals exact similarity with assynthesized pattern indicating its high chemical stability (Figure S4, Supporting Information). TGA profile of 1 shows a gradual weight loss up to 175 °C due to the removal of DMF and H2O molecules (calc. 22.5 % for two lattice DMF and one water; three coordinated water molecules). The network is stable until 270 °C and after that, a steady decomposition could be observed. On the other hand, the framework of 2 shows a gradual weight loss up to 150 °C due to the elimination of disordered lattice solvent as well as metal-bound H2O molecules followed by steady weight loss corresponding to the degradation of the framework (Figure S5, Supporting Information). The permanent porosity of the MOFs was investigated by gas sorption studies. Before gas sorption measurement, as synthesized MOFs were exchanged with dry chloroform for 48 h (10 times) and then subsequently activated at 373K under high vacuum for 14 h. The activated frameworks retained the original crystallinity as confirmed through PXRD analysis (Figure S1-S2, SI). It was found that the activated samples of 1 and 2 (1a and 2a respectively) do not adsorb any N2 at 77K, indicating the energy of adsorbate and the thermal energy of the framework are not high enough to permit the diffusion of the adsorbate molecule through the pores.26 It is also well known that at this temperature the diffusion of nitrogen through the micropores of the framework is very sluggish and diffusional limitation at this temperature may influence the adsorption in ultramicropores.26 However, 1a adsorbed 84 cm3g-1 of CO2 at 195K/1 bar which corresponds to a BET surface area of 197 m2g-1 (Figure 3a) and thus establishing its permanent microporous nature. On the other hand, 2a did Page no:7 ACS Paragon Plus Environment


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not adsorb CO2 under similar condition confirming its non-microporosity and less affinity towards CO2. 1a took up moderate CO2 uptake amount of 1.6 and 1.2 mmol at 273 and 295 K respectively under 1 bar (Figure 3b) whereas 2a showed negligible uptake under such conditions (Figure S6, SI).

Figure 3. (a) CO2 adsorption of 1a at 195K at 1 bar. (b) CO2 sorption isotherms of 1a at 273 K and at 295 K. (Solid and open symbols represent adsorption and desorption, respectively). Cycloaddition of CO2 to various epoxides Given the open metal sites in both the MOFs and capacity to adsorb CO2 at ambient conditions for 1, we decided to estimate its performances as Lewis acid based catalyst in the wide range of heterogeneous catalysis such as, cycloaddition of CO2 and epoxides to form cyclic carbonates, cyanosilylation reaction and Knoevenagel condensation under mild conditions at 1 bar pressure. The presence of open metal site in 1 is very important for CO2 fixation as it is already established that CO2 molecules generally occupy positions near the unsaturated metal centers at low pressure.55-57 Moderate CO2 uptake capacity on 1a, the exposed metal sites as well as Lewis basic sites on both the frameworks of 1 and 2 make these MOFs potential heterogeneous catalysts for cycloaddition reaction of CO2 with a various epoxides. The reaction was performed with styrene oxide, activated catalyst 1a and tetrabutyl ammonium bromide as co-catalyst in a two-neck round-bottom flask at room temperature with a pressure of 1 bar CO2 for 24 h. However, the conversion of the corresponding carbonate was found to be only ~ 20%, therefore, the temperature of the reaction mixture was increased stepwise and finally 80 °C was selected as the optimum temperature. Keeping the temperature fixed at 80 °C, the reaction time was incresed successively from 1 h and finally optimised to 12 h (Figure S7, SI). The conversion of the product from CO2 under this condition was found to be 93%. Page no:8 ACS Paragon Plus Environment

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The catalytic reaction was also investigated in the presence of both spacer and acid ligand; however, no desired product could be obtained. Co(NO3)2·6H2O was also tested as a catalyst; only 20% yield was obtained. Scheme 1. Cycloaddition reaction of epoxide with CO2 catalysed by 1a and 2a O O

Catalyst 1-2 (0.5 mol%)

CO2

+

TBAB o 80 C, 1 atm CO2

R

Entry

Catalyst(0.5 mol %)

O

O

R

Epoxidation Reaction Conversion (%)# 55

Cyanosilylation Recation Conversion (%) $ 52

Knovenagel Reaction Conversion (%)&

1

MOF1 (Un activated)

60

2

MOF2 (Un activated)

50

47

52

3

Spacer (L)

No

No

No

4

No

No

No

5

1,4 Napthlene Dicarboxylic Acid Succinic Acid

No

No

No

6

Co(NO3)2·6H2O

20

28

30

7

TBAB (10 mol%)

95% conversion for styrene oxide with comparatively higher pressure (2

MPa,

100°C,

4h

for

Co-MOF-74

and

0.8

MPa,

80°C,

12h

for

[{Co(muco)(bpa)(2H2O)}·2H2O]n.80,81 In order to gain insight the scope of the catalysis reactions for MOF 1 and 2, a systematic approach has been taken by varying the size of the substrates. As shown in Table 2, the substrates with electron withdrawing group generally showed better conversions. For example, the cycloaddition of CO2 with epichlorohydrin and allylglycidyl ether were effectively catalyzed in high conversions (96% and 87%; 80% and 78%) (Figure S8-S9, SI). When propylene oxide, styrene oxide and 1,2-epoxy cyclohexane were used the desired product were generated with 97% and 90%; 93% and 86%; and 42% and 30% conversions for 1a and 2a respectively (Figure S10-S12, SI). Interestingly, the smaller size substrate like propylene oxide (4.35 × 3.41Å2), styrene epoxide (4.28 × 7.26 Å2) resulted the higher catalytic conversion. On the other side, aliphatic epoxide with long alkyl chain (Table 2, entries 7 and 8) were converted to the cyclic carbonate with low conversion due to larger size (8.19 × 3.39 and 13.32 × 3.39 Å2), respectively, compared to the pore size (∼5.0 × 8.2 Å2) of 1.69-71,82 The results are in accordance with the fact that small substrates can easily diffuse through the 1D channels which contain large numbers of open metal sites for 1a59,69-71,80,81,83,84 suggesting the usefulness of 1a as a promising heterogeneous catalyst for the cycloaddition of CO2. In addition, 2a exhibits systematically lower catalytic efficiency than 1a, due to its non-affinity towards CO2 as evident from CO2 sorption studies (Figure S8S20, SI). Again, for the case of 2, the 1D channels with approximate diameter of 3.6 Å is much smaller than the molecular sizes of most of the substrates used in the catalytic reaction and therefore, it is assumed that the catalytic reaction for the case of 2a might proceed mainly on the surface of the MOF. Although, 2a is nonporous to CO2, it showed surface adsorption behavior (Figure S6, SI). In addition, presence of open metal sites also makes 2a a potential catalyst for cycloaddition of CO2 to various epoxides with obvious lower efficiency than 1a. The characterization of the desired carbonates was performed by the 1H spectra. In order to confirm the role of open metal sites onto the frameworks towards catalysis, the catalysis reactions have been investigated without activating the MOFs i.e. 1 and 2. As expected the yield of the catalytic conversion is very low for both of the MOFs (55% for 1; 50% for 2). Therefore, this phenomenon confirms the fact that the presence of open metal sites is ultimately necessary to facilitate the catalytic reaction.69-71


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Table 2. Cycloaddition reaction of various substituted epoxides catalyzed by 1a and 2a. Entry

Substrate

TON

1.

Conversion (%) by 1a 97

TON

194


2.

96

192

87

174

3.

93

186

86

172

4.

42

84

30

60

5.

68

134

61

122

6.

80

160

78

156

7.

52

104

35

70

8.

49

98

30

60

180

Reaction conditions: epoxides (20 mmol, 1 equiv), catalyst (0.5 mol% based on Co(II)) TBAB (10 mol%). No solvent was added. The conversion was evaluated from the 1H NMR spectra. A tentative mechanism has been proposed for the formation of cyclic carbonates catalysed by the strong Lewis acid based catalyst of 1a and 2a on the account of some earlier reports, 85-87 as demonstrated in Scheme S1 (Supporting Information). Moreover, on the basis of hot filtration test, the catalysts were found to be entirely heterogeneous revealing no leaching of Co(II) ions (Figure S21 for 1a, S22 for 2a, SI). Cyanosilylation Reaction Taking into account the Lewis acidity and open metal sites, both the MOFs were also explored as the heterogeneous catalysts in the cyanosilylation reaction under solvent free condition. The results are summarized in the Table 3. Using 1a as catalyst the conversion reached up to 96% while for 2a it was 81% for benzaldehyde (Figure S23-S24, SI). To investigate the scope and limitation of this heterogeneous catalysis reaction, we studied the Page no:11 ACS Paragon Plus Environment


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influence of various substituents at the phenyl ring and found that conversion are very sensitive to the electronic properties of the substituents attached to the phenyl ring (Table 3). Aldehydes with electron withdrawing group attached to the phenyl ring are much more reactive than electron donating group at the phenyl ring (Figure S25-S28, SI). On the other hand, shape and size of the substrates influence the % conversion of the products to some extent, for example, the longer hexanal showed significantly lower conversion (57% and 44%) than the other aromatic substrates. As given in Table 3, 1a exhibited higher cyanosilylation catalytic activity than 2a. This can be attributed to the microporous nature of 1a as evident from BET surface area analysis as opposed to non-porous nature of 2a. Although, the conversions are systematically lower for 2a than found in 1a as expected, but still could be considered quite high for aromatic aldehydes. This might be due to porous nature of 2 as determined through crystallographic analysis, although less compared to 1 as estimated from void volume analysis with PLATON68 (34.4% vs 14.3%). Moreover, the higher catalytic efficiency of 1a could also be correlated to the higher number of active metal sites within the framework (three open metals sites in 1 vs two in 2 per unit formula). To the best of our knowledge, only a few MOFs have been reported to be capable of efficiently catalyzing cyanosilylation reaction with such efficiency under solvent free conditions.88-90 Furthermore, Table S7 (Supporting Information) provides the comparison of the catalytic performances of MOF 1a and 2a with those for various other MOF-based catalytic systems reported in the literature in terms of time, temperature and conversion. A proposed mechanism for cyanosilylation reaction of aldehydes catalyzed by 1a is presented in Scheme S2, SI. 90 Hot filtration test was performed to check the heterogeneity of the catalyst, showing the catalysis reaction is entirely heterogeneous (Figure S29 for 1a, S30 for 2a, SI). Scheme 2. Cyanosilylation reaction of aldehydes with TMSCN catalysed by 1a and 2a. O H

H + TMSCN

CN OTMS

Catalyst 1-2 (0.5 mol%)

R

60 oC, 6h

R

Table 3.cyanosilylation reaction of various substituted aldehydes catalyzed by 1a and 2a. Entry

Substrate

Conversion (%) by 1a

TON

Conversion (%) by 2a

TON


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96

192

81

162

2.

99

198

72

144

3.

96

192

83

166

4.

99

198

78

156

5.

90

180

68

136

6.

98

196

89

178

7.

57

114

44

88

1.

Reaction conditions: aldehyde (2 mmol, 1 equiv), catalyst (0.5 mol% based on Co(II)) trimethylsilyl cyanide (6 mmol, 3equiv). No solvent was added. The conversion was evaluated from the 1H NMR spectra. Knoevenagel Condensation Reaction Presence of moderate porosity as revealed through single crystal structure determination, abundant Lewis basic and open metal sites for both the MOFs inspired us to explore the catalytic reaction for Knoevenagel condensation of an aldehyde with active methylene compound under solvent free condition. As shown in Table 4, the conversion reached up to above 90% for 2-benzylidenemalonitrile catalyzed by both the MOFs (Figure S31-S32, SI). To understand whether the catalysis reaction proceeds inside the pore or surface of the MOFs, various substituted aldehyde groups as substrate were taken and extended the scope of the study under similar condition. As shown in Table 4, in the case of slightly elongated aldehyde, conversion for hexanal was decreased (33% and 31% for 1a and 2a respectively) significantly compared to smaller aldehydes for both the MOFs. Furthermore, in



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Scheme 3. Knoevenagel condensation reaction of substituted aldehyde with malononitrile catalyzed by 1a and 2a. O H + NC CN R

Catalyst 1-2(0.5 mol%)

CN CN

H

60 oC, 6h R

Table 4. Knoevenagel condensation of various substituted aldehyde catalyzed by 1a and 2a. Entry

Substrate

TON

1.


TON

190

Conversion ( %) by 2a 93

2.

94

188

92

184

3.

97

194

95

190

4.

92

184

78

156

5.

90

180

80

160

6.

99

198

96

192

7.

33

66

31

62

186

Reaction conditions: aldehyde (2 mmol, 1 equiv), catalyst (0.5 mol% based on Co(II)) malanonitrile (10 mol, 5 equiv) under neat condition. The conversion was evaluated from the 1H NMR spectra. the presence of electron withdrawing group i.e p-chloro benzaldehyde and p-nitro benzaldehyde, excellent conversions are achived (97% and 95%; 99% and 96%) (Figure S33S34, SI), however, for electron donating groups i.e. p-anisaldehyde and p-methyl benzaldehyde the conversions were systematically lower (92% and 78% ; 94% and 92%) (Figure S35-S36, SI), which suggests that the electrophilicity of carbonyl group increases in Page no:14 ACS Paragon Plus Environment

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the former case and suppresses in the latter case. As ensembled in Table 4, 1a exhibited higher catalytic activity than 2a, due to more porous nature of 1a compared to 2a as determined through crystallographic and BET surface area analysis in conjunction with more accessible open metal sites in 1a compared to 2a. A systematic comparison on catalytic performances between 1a and 2a along with various other literature reports are given in the Table S8 (Supporting Information). Interestingly, both the MOFs offer superior catalytic activity with minimal catalytic loadings under mild and solvent free condition and thus making them as potential heterogeneous catalysts for Knoevenagel condensation reaction. A proposed mechanism for 1a-catalyzed knoevenagel condensation reaction is shown in Scheme S3, Suporting Information.91-93 It is assumed that the interaction between Lewis acidic sites (coordinatively unsaturated cobalt center) and the carbonyl group of the aldehyde improves the electrophilic character of the carbonylic carbon atom and thus assisting the attack by malononitrile.91-93 Furthermore, hot filtration data supported the heterogeneous nature of the catalyst for Knoevenagel reaction (Figure S37 for 1a, S38 for 2a SI). The recyclability is a very important issue for heterogeneous catalytic systems. The catalysts could be completely recovered and reused without showing any significant loss of activity at least for four catalytic cycles by simple filtration followed by washing with DMF, ethyl acetate, DCM and subsequently drying in oven as shown in Figure S39-S41 (Supporting Information). The integrity of frameworks is also retained during multiple cycles as confirm by PXRD analysis (Figure S42-S43, SI) and thus demonstrating their potential to act as multipurpose heterogeneous catalysts under milder conditions. Conclusion In conclusion, two new 3D Co(II)-MOFs with Lewis acid-Lewis base bifunctionalities were successfully developed based on organic dicarboxylic acids and azo linked N, N donor spacer and structurally characterized with varying porosities. The limited pore spaces decorated with exposed metal sites and N-rich functional azo groups were efficient for size selective chemical transformation of CO2 with epoxide into cyclic carbonate under solvent free and mild condition. The MOFs also showed excellent heterogeneous catalytic behaviour toward the cyanosilylation reactions and Knoevenagel condensation. A systematic comparison in their catalytic performances could be made based on their available open metal site concentrations, porosities and CO2 sorption capabilities. Apart from these, the catalysts can be easily be separated and reused for at least four cycles without any loss of Page no:15 ACS Paragon Plus Environment


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activity and structural integrity. Given the enormous library of organic polycarboxylic acids and N,N' donor spacer and less numbers of reported Co(II)-based MOFs with permanent porosity and open metal sites, this study may contribute to broaden the scope of Co(II)-based microporous MOFs for their potential applications in heterogeneous catalysis under mild conditions.

ASSOCIATED CONTENT Supporting Information (SI): Crystallographic data table, PXRD patterns, TGA profile, calculation details, selectivity comparison table, related catalytic reactions plots (PDF). CCDC 1896926 and 1896927 contains the supplementary crystallographic data for this paper. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID iD Madhab C. Das: 0000-0002-6571-8705 Notes ‖These authors contributed equally to this work

Acknowledgements S.C. and S.C.P. thanks IIT KGP for fellowship. M.C.D. gratefully acknowledges the financial support

received

from

SERB,

New

Delhi

as

Early

Carrier

Research

Award

(ECR/2015/000041).

Conflicts of Interest The authors declare no competing financial interest.

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Table of Content 3D Co(II)-MOFs with Varying Porosity and Open Metal Sites toward Multipurpose Heterogeneous Catalysis under Mild Conditions Santanu Chand‖, Shyam Chand Pal‖, Manas Mondal, Subrata Hota, Arun Pal, Rupam Sahoo and Madhab C. Das* Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, WB, India. E-mail: [email protected] ___________________________________________________________________________

Two new 3D Co(II)-MOFs with varying porosities and open metal sites have been constructed for three different types of heterogeneous catalysis such as CO2 to cyclic carbonate, cyanosilylation reaction and Knoevenagel condensation under solvent free and mild conditions.


Three-Dimensional Co(II)-MetalâOrganic Frameworks with Varying

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