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. 2023 Feb 10;8(7):6638–6649. doi: 10.1021/acsomega.2c07137

Amide Functionalized Mesoporous MOF LOCOM-1 as a Stable Highly Active Basic Catalyst for Knoevenagel Condensation Reaction

Sheereen Afaq †, Muhammad Usman Akram †, Wasif Mahmood Ahmed Malik †,§, Muhammad Ismail †, Abdul Ghafoor †, Muhammad Ibrahim ∥, Mehr un Nisa ⊥, Muhammad Naeem Ashiq †, Francis Verpoort ‡,*, Adeel Hussain Chughtai †,*
PMCID: PMC9948166  PMID: 36844569

Abstract

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Acyl-amide is extensively used as functional group and is a superior contender for the design of MOFs with the guest accessible functional organic sites. A novel acyl-amide-containing tetracarboxylate ligand, bis(3,5-dicarboxy-pheny1)terephthalamide, has been successfully synthesized. The H4L linker has some fascinating attributes as follows: (i) four carboxylate moieties as the coordination sites confirm affluent coordination approaches to figure a diversity of structure; (ii) two acyl-amide groups as the guest interaction sites can engender guest molecules integrated into the MOF networks through H-bonding interfaces and have a possibility to act as functional organic sites for the condensation reaction. A mesoporous MOF ([Cu2(L)(H2O)3]·4DMF·6H2O) has been prepared in order to produce the amide FOS within the MOF, which will work as guest accessible sites. The prepared MOF was characterized by CHN analysis, PXRD, FTIR spectroscopy, and SEM analysis. The MOF showed superior catalytic activity for Knoevenagel condensation. The catalytic system endures a broad variety of the functional groups and presents high to modest yields of aldehydes containing electron withdrawing groups (4-chloro, 4-fluoro, 4-nitro), offering a yield > 98 in less reaction time as compared to aldehydes with electron donationg groups (4-methyl). The amide decorated MOF (LOCOM-1-) as a heterogeneous catalyst can be simply recovered by centrifugation and recycled again without a flagrant loss of its catalytic efficiency.

Introduction

The chemical industry has appeared as a vivacious part of the world economy.1 Conversely, the manufacture of fine chemicals results in an enormous scale of environmentally injurious waste material. Heterogeneous catalysis is taking on a progressively imperious role in chemical production frequently with an outcome of a crucial drop in waste.2 For environmental and economic motives, there is an enormous enticement to substitute the homogeneous catalytic systems with effectual and green heterogeneous systems.3 Heterogeneous catalysis is better than homogeneous catalysis for minimized waste, reusability, clean and green products, and easier separation.4

Being a progressive class of the porous materials, metal organic frameworks have elite features of eternal porosity,5 extreme crystallinity,6 unrivaled flexibility,7 unvarying pore sizes,8 constant and manageable morphologies,9 exceptional thermal stability,10 remarkably larger surface areas,11 and additional edges of lesser density,12 i.e., 0.13 g cm–3, along with exclusive features of pore size tunability,13 adaptable host–guest interactivity,14 enhanced pore dimensionality (98 Å),15 and functionality.16

Over the past few years, considerable efforts have been enamored for designing and building transition-metal-atom-based metal–organic frameworks having multifunctional ligand bridging moieties that tend to impart structure multiplicity as well as fascinating features, for instance, molecular recognition,17 therapeutics and diagnostics in biomedicine,18−20 magnetism,21−23 molecular separation,24 thin film systems,25 drug delivery,26 solar cells,27 electric conductance,28 carriers for nanomaterials,29 oxygen evolution reaction,30 capture of CO2,31 sensing,32 luminescence,33 adsorption,34 renewable energy,35,36 gas storage,37,38 separation,39 proton conduction,40 heterogeneous catalysis,41−45 asymmetric heterogeneous catalysis,46 electrolysis,47 and drug delivery.48 Particularly, MOFs have attribute features that include (i) dynamic behaviors in response to the guest molecules, (ii) well-ordered porous structures, and (iii) flexible, designable channel surface functionalities. Various kinds of ligands in copious synthetic approaches have been employed in order to produce some inimitable frameworks. Among those, polypyridine,49 hybrid ligands,50 polycyanate,51 or polycarboxylate,52 in which a range of spacer groups attach either cyanate or pyridine or carboxylate or a combination of all, are common choices for the building of metal organic networks.

One of utmost divergent fields of MOF investigation is their use as a heterogeneous catalyst. Actually, catalysis using a MOF as a catalyst depends on active catalytic sites within the MOF framework; both metal centers and organic ligands within the framework of MOF constrain to proficient catalytic activities.53 In past two decades, utilization of MOFs as a heterogeneous catalyst has offered a connotation surge as MOFs have been deliberated upon as an environmentally friendly alternative for heterogeneous catalysis. The separation of reaction target products, fewer leaching problems, and reusability of the catalyst make MOFs dynamic heterogeneous catalysts. The high porosity, multifunctionality, and pore tunability are engaging MOFs at the frontier between enzymes and zeolites. A wide diversity of MOF frameworks have been constructed with diverse linkers as well as numerous transition metals and have been evaluated in the heterogeneous catalysis for a variety of organic reactions, but still numbers of MOF materials are unexplored for catalysis. Hence, practice of the MOFs in catalysis is enormously wide and growing incessantly.

Two kinds of strategies that are presently utilized to functionalize the MOF channels are immobilization of coordinatevely unsaturated or open metal sites (OMSs) and exordium of the functional groups to deliver guest-accessible functional organic sites (FOSs).54,55 There is emergent curiosity in making use of OMSs for precise gas adsorption and Lewis acid catalysis, but lesser attention has been concerned with the study of the functional organic sites, regardless of their significance. A scarcity of reports on the FOS is due to the intricacy of constructing guest-accessible functional organic sites on the pore exterior: these functionalities tend to coordinate the metal ions, coming out in the frameworks where FOSs are entirely blocked.56 In contrast to utilizing OMSs to build functionalized MOFs, a study on the addition of the FOSs in the MOFs is lesser reported, which is probably due to the complexity endured in making guest accessible functional organic sites on pore surfaces of MOF because the organic functionalities tend to interrelate with the metal ions that come out in blocking functional organic sites within the MOF frameworks.

Even though MOFs have provided an advanced platform for heterogeneous catalysis and multifold publications regarding MOF catalysis have been recounted, still there is a challenge to establishing their durability, specificity, and catalytic activity under all provided reaction conditions accordingly. Including all categories of heterogeneous catalysts together with MOFs diminishes when used for a prolonged time of activity. Plenty of research is available that primarily addresses the reliability of MOFs, though a minor number of publications emphasize the turnover numbers as well as turnover frequencies and consequently compare them with those of other relevant catalysts. By means of spectroscopic and textural techniques, the catalyst deactivation can be easily tracked.57 Comparatively lower thermal stability is another determinant that hinders MOF applications for reactions taking place over higher temperatures. Equally, leaching of inorganic metal entities or that of organic linker entities in a reaction medium is a loathsome practice for the heterogeneous catalysis route. Hence, it must be settled throughout the reaction and can be easily spotted either by means of the hot filtration test or by means of chemical analysis based on the reaction filtrate, consequently. Ultimately, there is a prerequisite of analytical review-based research for justifying the catalytic performances of MOFs; additionally their chemical as well as thermal consistency in catalysis must be screened out thoroughly.

By incorporating the fundamental functional groups like pyridyl,58 amide,59 and amino,60 extremely dynamic solid basic heterogeneous MOF catalysts can be achieved. While incorporated basic organic functional groups are an essential part of organic linkers in MOFs, leaching of metal does not happen, resulting in a steady system.

One more predominant functional moiety is acylamide, which possesses both an electron acceptor site (−NH moiety) as well as an electron donor site (−C=O moiety) simultaneously and hence has been ascertained as a strong entrant for the synthesis of MOF edifices enriched with FOSs that are easily accessible by incoming guest molecules.61 A number of groups have synthesized the functionalized MOF edifices by simply engaging the acylamide moiety as a FOS with the aim to achieve efficient catalysis as well as selective adsorption within the frameworks exclusively. Numerous research groups have productively constructed the functionalized metal organic frameworks by utilizing the amide groups as functional organic sites for the catalysis and/or selective adsorption in MOF networks.61 The Knoevenagel condensation of aromatic and aliphatic ketones or aldehydes with active methylenes (containing two electron-attracting groups) was reported by Knoevenagel.62 Knoevenagel’s first papers in this field were concerned with condensation of diethyl malonate with ethyl benzoylacetate and formaldehyde.62

Knoevenagel condensation is a productive reaction for the formation of C=C bonds having various applications in the synthesis of pharmaceuticals (metallothionein-3 (MT3) ligand, 2-pyrone derivatives) and fine chemicals.63−65 Knoevenagel condensation is a strategic step in the industrial scale manufacture of antimalarial drug lumifantrine (a constituent of coartem).66,67 The reaction has a wide range of applications regarding coumarin and its derivatives’ synthesis that is a significant moiety of organic compounds and is also employed as additives in both cosmetics and the food industry.68 Knoevenagel condensation has been employed in the total synthesis of a multitargeted kinase inhibitor.69 It has various relevances in the production of fine chemicals and heterocyclic as well as carbocyclic compounds of the biological corollary.63 There is bountiful connotation in the Knoevenagel products as a virtue of their biological activity; for example, the tryphostins (a class of tyrosine phosphorylation inhibitor), such as α-cyano-thiocinnamide, are recognized to inhibit the autophosphorylation of EGFR (epidermal growth factor receptor), in addition to possessing antiproliferative effects on the keratinocytes.70

A number of catalysts have been noted for catalyzing this C=C bond forming reaction (Knovengel condensation); for instance, zeolites,71 organo-catalysts,72 surfactants,73 ionic liquids immobilized in the mesoporous silica,74 or surface alteration in the presence of the alkali metal ions.75 But from an application viewpoint, a number of these catalysts experience drawbacks such as using carcinogenic and hazardous materials. Furthermore, the Knoevenagel condensation reaction is commonly catalyzed by acids or bases and necessitates extreme reaction conditions such as high reaction temperatures76 and microwave irradiation.77 An acylamide adorned Cu-MOF was reported by Cheng et al.78 For Knoevenagel condensation, the mentioned MOF presented exceptional catalytic performance. A number of functionalized MOFs having acylamide enriched organic linkers were reported by Kitagawa et al. These functionalized MOFs presented fascinating selective adsorption as well as appealing heterogeneous catalytic performances.79 Similarly, a Cu(II)-organic framework was reported by Kostakis et al. that possessed a flexible terepthaloyl-bis-glycinate linker that displays a dexterous heterogeneous catalysis for 3,5-di-tert-butylcatechol (DTBC) oxidation.80 The Bai81,82 and the Sun83 research groups have produced a sequence of MOFs having acyl-amide-containing carboxylate linkers; all of the prepared MOFs reveal remarkabe CO2 capture capacity. The research about the acyl-amide adorned MOFs is relatively inadequate due to the intricacy in fabricating the guest accessible acyl-amide entities as guest accessible sites with cavities, as these functional moieties are labile to interrelate with the metal ions. Adorning MOF edifices containing various functional organic sites will enrich functionalized metal organic framework chemistry and will definitely expedite the achievement of enhanced performances of respective MOFs.

We successfully synthesized a new nanosized acylamide decorated tetracarboxylate linker, namely, bis(3,5-dicarboxy-pheny1)terephthalamide (H4L), presented in Scheme 1. The prepared H4L ligand possesses numerous fascinating attributes as follows: (i) a multiplicity of structures because of rich coordination patterns developed by four carboxylate moieties that serve equally as coordination sites and (ii) two acyl-amide assemblies as the guest accessible sites that can encourage guest molecules assimilated into the MOF framework through H-bonding interactions and contain the ability to function as a guest interaction FOS. Herein, we report the synthesis and characterization of [Cu2(L)(H2O)3]Sx·n (S, solvent molecule: water or DMF molecule, and x is the number of S). Moreover, catalytic properties of prepared MOFs in the Knoevenagel condensation of diverse aldehydes (benzaldehyde, p-substituted benzaldehydes) with the active methylene compound (malononitrile) were explored. The catalysis results illustrated that the MOF exhibits good activity in the base-catalyzed Knoevenagel condensation of aromatic aldehydes exclusive of a considerable loss of activity.

Scheme 1. Structure of Linker (Bis(3,5-dicarboxy-pheny1)terephthalamide) Showing Their Potential Connectivity through Carboxylate (Coordination Sites) and Amide Linkage As the Guest Interaction Sites.

Scheme 1

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Result and Discussion

Numerous metal organic frameworks have been built with the amide adoring manifold carboxylate linkers. In the recent past, Zou et al. have reported an iso-structural MOF with cobalt, synthesized from bis(3,5-dicarboxy-pheny1)terephthalamide with lon topology 4-linked CO2 groups and the nanosized tetra-carboxylate linker.84 We also designed and synthesized Cu-MOF (LOCOM-1) from an amide-containing tetra-carboxylate ligand (bis(3,5-dicarboxyphenyl)-tere-phthalamide) containing paddle wheel SBUs [Cu2(COO)4] (secondary building units) using a mixed solvent DMF/H2O (ratio 3:1) solvothermal process.85

The acylamide groups in the linker increases the arms of the linker, which can possibly produce a large internal cavity in the Cu-MOF and moreover increases the flexibility of the linker, which would facilitate making both boat and chair conformations in the lon network. The blue block crystals of [Cu2(L)(H2O)3]·4DMF·6H2O were obtained solvothermally by reacting H4L and Cu(NO3)23H2O in the presence of a binary solvent (having composition 60% dimethylformamide (DMF) along with 20% water) at 85 °C for 3 days.

The PXRD analysis of LOCOM-1 is shown in Figure 1a. The PXRD results demonstrate that the experimental PXRD patterns have intense peaks, evidencing a good, pure phase of the crystal. The TGA curve disclosed a 25% weight loss in a temperature range of 20–280 °C, which corresponds to four DMF and nine water guest molecules eliminated (overall calculated weight loss is 24.78%). The TGA record for Cu-MOF is emblematic of the two steps of weight loss for guest molecules: four DMF molecules and nine H2O molecules per formula unit. Hence, TGA analysis of the prepared MOF pointed out that prepared material was thermally stable up to a temperature range of 300 °C; beyond this range, prepared Cu-MOF gradually decomposes and ends at above 510 °C (Figure 1b).

Figure 1.

Figure 1

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PXRD (a) and TGA (b) of Cu-MOF.

The microscope image of LOCOM-1 shows bright bluish shaped rhombohedron shaped crystals (Figure S5). Figure 2 illustrated the SEM images of the Cu-MOF obtained from the ligand (H4L) along with metal salt, namely, Cu(NO3)2·3H2O inside a binary mixed solvent, i.e., DMF/H2O, having single drop of 2 M HCl as well. The SEM micrograph revealed the attainment of a precise block of bluish crystalline material as was anticipated. Some rhombohedra with distinct facets can be recognized (Figure 2a,b).

Figure 2.

Figure 2

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(a) SEM image of Cu-MOF (10 μm) representing crystals of Cu-MOF and (b) single rhomohedral crystal of Cu-MOF.

FTIR spectra of Cu-MOF and H4L are shown in Figure 3. The FTIR pattern of the mesoporous Cu-MOF divulged the existence of the two significant bands at 1587 and 1370 cm–1 that designate vibrations of the carboxylate symmetric stretching (υ-COO– 1400–1310 cm–1) and antisymmetric stretching (υ-COO– 1630–1500 cm–1), which were lower than the stretching vibration of carbonyl of the carboxylic acid group (υ C=O, 1760–1690 cm–1) professed for the free carboxylic groups in H4L (Figure 3). The two mentioned bands were associated with the carboxylate ion (−COO–) vibrations that confirm the existence of the carboxylate ions in the Cu-MOF framework. The resilient absorption band of the carbonyl group in H4L at 1711 cm–1 disappeared in Cu-MOF, indicating that full deprotonation of the carboxylic acid moieties in the Cu-MOF proves the change of the carboxyl moieties into the carboxylate ones and moreover acclaims that the H4L ligand becomes coordinated with Cu(II) when the reaction is accomplished. The coordination of the Cu ions with the provided ligand, namely, H4L, ensues in the efficacious synthesis of Cu-MOF. An apparent band for N–H was perceived at 3554 cm–1 that was indicative of the presence of amide N–H coordinated with water in the MOF. The amide I band appearing at 1668 cm–1 in the ligand shifted down to 1626 cm–1 in MOF. The bands lower than 900 cm–1 fit in for different C–H bond bending vibrations that usually tend to lie in the 900–800 cm–1 range, whereas C=C–H aromatic ring deformation bands usually lie in the 780–670 cm–1 range. For H4L, a peak at 1333 cm–1 was observed that points out the presence of C–N stretching vibrations. Later, this mentioned peak value in prepared MOF spectra was dropped to 1276 cm–1.

Figure 3.

Figure 3

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FTIR spectrum of H4L and Cu-MOF.

Permanent porosity of mesoporous [Cu2(L)(H2O)3]·4DMF·6H2O is checked by their N2 sorption isotherm at 77 K. First, the Cu-MOF was soaked in CH3OH for 3 days and then in CH2Cl2 for 3 days also and finally evacuated at room temperature after about 2 h to get the actuated Cu-MOF prior to BET analysis. The N2 adsorption–desorption isotherm of Cu-MOF demonstrates the typical type-IV curve, that is, illustrating pore compression with distinct sorption hysteresis (Figure 4a). The N2 sorption isotherm results describe the existence of mesopores with 352 cm3/g of N2 uptake for Cu-MOF (64.725 m2/g of BET surface area and 71.15 m2/g of Langmuir surface area). The pore diameter (width) and pore volume observed were 26.65 nm and 0.43 cm3/g, respectively. The CO2 and CH4 sorptions were also performed at STP to probe the uptake of CO2 and CH4 in MOFs (Figure 4c and b). The sorption study of both gases proves that MOFs possess a maximum CO2 and CH4 of 13.87 and 4.54 cm3/g, respectively. The higher adsorption value of CO2 is primarily because of the small molecular size of CO2 in contrast to CH4.

Figure 4.

Figure 4

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(a) Nitrogen sorption isotherm, (b) CH4 sorption isotherm, and (c) CO2 sorption isotherm.

Because the prepared MOF is not only of interest for gases, in order to further explore the permanent porosity of prepared MOFs, organic dye adsorptions in water were investigated. Two different kinds of dyes, methyl orange (MO) and methyl blue (MB), were chosen as models for the experiments. A total of 10 mg of MOF was poured into 10 mL of an aqueous organic dye solution of 100 mg·L–1 at room temperature under stirring. The magnetic stirring of prepared dye solutions having an adsorbent was carried out and was upheld for a fixed time frame (12 h) at 25 °C and kept in the dark to ensure the establishment of an adsorption–desorption equilibrium. After centrifugation, with the help of UV–vis spectroscopy, the plasma was evaluated, or after dilution (if necessary). For the [Cu2(L)(H2O)3]·4DMF·6H2O (Cu-MOF), the dye adsorption percentages studied were 88.59% for MB and 21.19% for MO (Figure 5). The difference in adsorption rates could be attributed to the different molecular sizes of used dyes. However, the percentage adsorption for MB dye was comparatively higher than that of MO percentage adsorption. A possible reason for the higher percentage of MB could be related to its smaller molecular size, while the size of methyl orange was comparatively larger so molecules of MB dye can easily penetrate inside pores of the prepared MOF and hence can comparatively elevate its adsorption rate accordingly.

Figure 5.

Figure 5

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UV–vis spectra of aqueous solutions of dyes with Cu-MOF: (a) MB, (b) MO.

Knoevenagel Condensation

Knoevenagel condensation is a broadly used reaction in the industry and research and is of great significance for numerous pharmaceutical products. Normally, organo-bases catalyze the mentioned renowned reaction, i.e., pyridine-based basic catalysts or piperidine-based basic catalysts. On the other hand, these homogeneous base catalysts seem to follow lengthy tedious reaction routes. Furthermore, they end up in undesirable side reactions. For instance, oligomerizations may result, requiring high temperatures for accomplishment, and recovering the used catalyst is also not an easy job. Numerous reports on the heterogeneous Knoevenagel catalysts, for example, magnetic base analogues or ionic liquids, modified zeolites, have been reported, leading to cleaner products while the complex neutralization processes can be skirted. Furthermore, these catalysts can be regenerated and recovered. Knoevenagel condensation of various aromatic, aliphatic, and heterocyclic aldehydes with malononitrile substrates using different catalysts first entails the heating of the reaction mixture and second requires comparably more time for reaction completion. Without exploiting any catalyst, the Knoevenagel condensation of various benzaldehydes involving active methylene substrates could not be accomplished.

All reactions were performed with different p-substituted benzaldehydes and malonitriles in DMSO-d6 using LOCOM-1 without using any organic or inorganic base. 1HNMR spectroscopy was used to observe the progress of the reaction and to conclude reaction yield. It took a time period of just 3–6.5 h for analyzing all reactions involving alkene derivative synthesis with an excellent productivity of 95–99% yield as well. The first step is the formation of a carbanion from malononitrile via basic Cu-MOF. The carbanion attacks the electron-deficient carbon (partially positive) of the benzaldehyde substrate, while the carbonyl oxygen (partially negative) present in benzaldehyde takes back the exact same proton that was taken out during enolate formation. The hydrogen from α-carbon was removed by a lone pair present on the newly formed hydroxyl group. Finally, the double bond (propanedinitrile-2-[(phenyl) methylene) formation is accomplished by involvement of a lone pair donated by α-carbon; meanwhile, elimination of a water molecule also occurs. As illustrated in Table 1, reaction of benzaldehyde gave the final product in 98% yield with 2451 TONs (Table 1, entry 1). High yields from 95 to 100% were attained for various benzaldehydes containing different functionalities at the p position, including electron donating and attracting groups (4-NO2, 4-Cl, 4-F, 4-Me). Impressively, 4- fluorobenzaldehyde and 4-chlorobenzaldehyde revealed reasonable reaction yields of 99 and 100%, respectively. It has been observed that reaction times were interrelated with the electronic effects on the benzaldehydes. Consequently, more reaction times are provided for electron-donating moieties in comparison to respective acceptor ones.

Table 1. Knoevenagel Condensation Reactions of Different Benzaldehydes with Malononitrile over Cu-MOFa.

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entry R time (minutes) yieldb (100%) TONC
1 H 390 98.05 2451
2 CH3 435 95.63 2390
3 F 180 99.92 2498
4 Cl 260 100 25000
5 NO2 20 99.4 2485
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a

Reaction conditions: All reactions were performed with 1 mmol of benzaldehyde and 1.1 mmol of malononitrile in 4 mL of DMSO-d6 using 0.005 g (0.04 mmol based on amide in MOF).

b

Determined by 1H NMR, based on starting materials.

C

Turnover number (TON): conversion of benzaldehyde (mol)/catalyst (mol) used.

In the case of benzaldehyde (1.0 mmol) and malononitrile (1.1 mmol) as substrates using 5 mg of catalyst, a 98% conversion was achieved. The conversion with time was monitored by 1H NMR. At different time intervals following the time frame sequence, samples were collected and then immediately were subjected to analysis for 1H NMR study. The conversions (yields) were computed by integration of the benzylic protons in each case (Table 2). The conversions of benzaldehye into propanedinitrile-2-[(phenyl)methylene] calculated were 2.91, 4.76, 5.66, 13.04, 37.88, 69.41, 85.05, and 98.05 at time intervals of 0, 5, 15, 30, 90, 210, 260, 330, and 390 min, respectively (Figure S7).

Table 2. 1H NMR Integration of the Characteristic Benzylic Protons (Starting, Product) and Conversions into Propanedinitrile-2-[(phenyl)methylene].

S. no time (minutes) integration starting integration product conversion (%)
1 0 1 0.00 0.00
2 5 1 0.03 2.96
3 15 1 0.05 4.76
4 30 1 0.06 5.66
5 90 1 0.15 13.04
6 210 1 0.61 37.88
7 260 1 2.27 69.41
8 330 1 6.07 85.85
9 390 1 64.66 98.05
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The Knoevenagel condensation of p-methylbenzaldehyde with malononitrile offered 85.6% yield in 435 min (Table S1). The high reaction time of p-methylbenzaldehyde is due to a strong + I effect (inductive effect) of the methyl group at the para position. The carbonyl group in benzaldehyde is more electrophilic than the carbonyl group in p-methylbenzaldehyde. The more electrophilic the carbonyl carbon of benzaldehyde, the more reactive it is toward the malononitrile. Table S1 shows the conversion of p-methylbenzaldehyde into the propanedinitrile-2-[(4-methyl phenyl)methylene] with different time intervals. The progress of the reaction monitored by 1H NMR and spectra of the condensation reaction are provided in the Supporting Information (Figure S8).

Furthermore, aromatic aldehydes having electron withdrawing substitution (4-nitro, 4-chloro, 4-fluoro benzaldehyde) offer yields > 98 in less reaction time as compared to benzaldehaydes having an electron donationg group (Figures S9–S11). Electronic inductive and mesomeric effects can be used to explicate divergent reactivities among different benzaldehydes. Both fluorine and chlorine groups have an electron withdrawing effect on neighboring carbon, accomplishing it electron deficiently. This in sequence has a substantial inductive effect (I) on the aldehydic carbonyl carbon atom. As in the case of fluoro and chlorobenzaldehydes, 99% conversion is achieved within 180 and 260 min, respectively (Tables S2, S3). The conversion of p-fluorobenzaldehyde and p-chlorobenzaldehyde into propanedinitrile-2-[(4-fluorophenyl)methylene] and propanedinitrile-2-[(4chlorophenyl)methylene] with a time interval is given in Tables S2 and S3, respectively. The high yield of propanedinitrile-2-[(4-fluorophenyl)methylene] in contrast to propanedinitrile-2-[(4chloro phenyl)methylene] is due to the high electron withdrawing effect of fluoro as compared to chloro.

Knoevenagel condensation of 4-nitrobenzaldehyde with malononitrile gave 98% conversion within 5 min (Table S4). The high reactivity of nitrobenzaldehyde can be attributed to the strong negative resonance effect of the nitro group at the para position. The observed order of reactivity of electron withdrawing benzaldehyes was −NO2 > −F > −Cl. The nitro group, for instance, is characteristic of −I and −M effects, while halogen groups have a −I and +M effect. The −I effect of the halogen group is more dominant than its +M effect. The nitro group has improving −I and –M effects, both. That is why p-nitrobenzaldehyde gives a Knoevenagel condensation product in less reaction time as compared to p-fluoro and p-chlorobenzaldehye.

Though a lot of studies have been conducted that discuss MOF networks having acid properties,4,86−89 only a few reports of the PCPs having base properties are found in the literature.90−92 To typify the basic properties of Cu-MOF, the Knoevenagel condensations catalyzed by Cu-MOF done at two sites (−NH, −C=O) are simultaneously present inside amide moieties in Cu-MOF and differ from each other with respect to their functions. In Cu-MOF, the −NH (amino group) moiety acts as a hydrogen donor, whereas carbonyl carbon (−C=O) acts as an electron donor. A recent study described the MOFs with a basic carbonyl oxygen atom acting as catalytic interface sites on the channel wall.91 So, for our prepared Cu-MOF edifice, we also anticipated that the −C=O moiety (base sites) offers guest interaction sites accordingly. A plausible mechanism can be inferred for the Knoevenagel reaction over amide moieties acting as FOSs in our prepared amide functionalized Cu-MOF in Scheme 2.

Scheme 2. Proposed Mechanism for Knoevenagel Reaction of the Benzaldehyde with Malononitrile in Prepared Cu-MOF.

Scheme 2

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According to the proposed mechanism, activation of the methylene carbon of malononitrile ensues due to the basic site (−C=O moiety) of the amide group (step 1; Scheme 2). At the same time (step 2), removal of the acidic proton of the methylene carbon of the malononitrile and given to the carbonium anion happens at the basic site of prepared Cu-MOF (−C=O). Next is the proton shift (P.S) from the carbonyl oxygen atom of the amide to the carbonyl oxygen atom of the benzaldehyde to make the carbon atom of the benzyl group more electrophilic in nature in order to facilitate the attack for the carbonium ion and removal of water. After the proton shift, the carbonion attacks the electrophilic site of benzaldehyde, namely, the carbonyl moiety, which results in intermediate generation (step 3), which undergoes suitable rearrangement for making dehydration possible (step 4) and eventually leads to final product synthesis. According to the above mechanism, the strength of basicity of the oxygen ion is very imperative for activation of the reactants and subsequent rearrangement of the intermediate inside pores of MOF.

Finally, reusability and recovery of Cu-MOF were conducted. When the condensation reaction between aromatic aldehyde and malononitrile was accomplished, the prepared catalyst (Cu-MOF) was recovered from the reaction mixture by filtration; after that, the catalyst was thoroughly washed away by using ethanol (3 × 10 mL) for complete removal of any obnoxious residue that had adhered onto the catalyst. Subsequently, recovered Cu-MOF (LOCOM-1) catalyst was used again in the next catalytic cycle without any extra treatments.

Recovered catalyst efficiency was assessed again using the same model reaction. Notably, over 97% conversion of the benzaldehyde was retained even after five recycles. As shown in Table 3, a reaction yield of 97% was achieved after five cycles, suggesting that there is no significant loss of activity of the catalyst in process. The results divulged that Cu-MOF maintains easy recovery without losing evident activity. A consistency for PXRD patterns for both fifth-recycled Cu-MOF and a fresh run catalyst was observed that signified an exceptional chemical constancy possessed by the prepared catalyst (Figure S6). The PXRD patterns of the Cu-MOF corroborate that the skeleton of the catalyst remained intact after five recycles.

Table 3. Investigation into the Reuse of Cu-MOFa.

entrya runb time (minutes) yieldc (%)
1. 1 390 98
2. 2 390 98
3. 3 390 98
4. 4 390 97
5. 5 390 97
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a

After each run, the catalyst (Cu-MOF) was isolated by filtration, washed with the ethanol, and dried for the next cycle.

b

Reaction conditions: All reactions were performed with 1 mmol of benzaldehyde and 1.1 mmol of malononitrile in 4 mL of DMSO-d6 using 0.005 g (0.04 mmol based on amide in MOF) of the as-synthesized Cu-MOF for 390 min.

c

Determined by 1HNMR, based on starting materials.

The effectiveness of Cu-MOF in comparison with numerous catalysts for Knoevenagel condensation within our study is given in Table 4. The obtained TON is better than that of the reported ones. Moreover, in the present work, we did the reaction at room temperature to achieve 98% conversion with 2462 TONs with benzaldehyde and malononitrile. Table 4 represents the Knoevenagel reaction studied with a higher temperature, longer reaction time, and large catalyst loading (amount). In evaluation with other reported catalysts, our Cu-MOF has benefits of being highly active, easy to prepare, and rather cheap and is easily recyclable without a perceptible loss of catalytic activity even after five recycles.

Table 4. A Divergence of Catalytic Activity of Numerous MOFs in the Knoevenagel Reaction of Malononitrile and Benzaldehyde.

entry benzaldehyde (mmol) malononitrile (mmol) catalyst quantity temp time yield (%) TON ref
1 10 20 JNU-402-NH2 c 0.06 mmol 80 °C 1 h 99 1665 (93)
2. 2.1 2 Cd-MOF 0.08 mmol r.t 12 h 98 1225 (94)
3 2 2 fiber catalyst PANEPF 0.04 mmol 78 °C 4 h 98 2450 (95)
4 10 10 ZnCl2 1.0 mmol 100 °C 10 min 90.9 90.9 (96)
5 10 10 sodium benzoate 0.1 mmol   10 min 98.7 987 (97)
6 10 10 LaCl3·7H2O 1.0 mmol 85 °C 1 h 95 95 (98)
7 0.5 0.5 ZnO-3 2.0 mol % r.t 15 min 92 46 (99)
8 1.0 1.1 [Cd(DDIH)2H2O]n 3.0 mol % r.t 15 min 96 32 (100)
9 1.0 1.1 (Zn(ADA)(L)]·2H2O)n 1.0 mol % r.t 1 h 99 99 (101)
10 10 20 NUC-54a 0.30 mol % 60 °C 5 h 98 327 (102)
11 1 1 Cu-MOF (LOCOM-1) 0.04 mmol r.t 6 h 98 2462 this work
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Conclusions

In summary, we have successfully synthesized a mesoporous Cu-MOF by using an acylamide-containing tetracarboxylate ligand in order to generate the amide FOS within the MOF, which will work as a guest accessible site. The typical illustrious base-catalyzed reaction, namely, the Knoevenagel condensation reaction, was selectively endorsed with excellent yields. The electronic effects of different groups have an impact on the reaction times, with electron donating groups increasing this time obdurately compared to the electron attracting groups. Cu-MOF displays a unique recoverable framework; the mesoporous Cu-MOF can be easily recovered and revealed similar catalytic activity to the initial one even after five cycles. The presented study is predominantly applicable regarding the perspective of the porous solid-state chemistry for the creation of innovative materials having remarkable characteristics resulting from flexible ligand involvement. This result recommended that coordination of transition metal ions and the designed amide functionalized organic linker can give a functionalized MOF with functional organic sites elucidating the base catalytic performance. MOF functionalized with the FOS heralds the next advance in the field of porous materials that can be utilized as a novel enthralling class of heterogeneous catalysts and adsorbents.

Experimental Section

Materials

All consumed chemicals were procured from Sigma-Aldrich and well-reputed international and local suppliers. All chemicals were used mainly as such, but when required they were purified via typical techniques, i.e., distillation and recrystallization.

Synthesis

Synthesis of H4L

Synthesis of the amide-containing tertra-carboxylate linker (H4L, bis(3,5-dicarboxy-pheny1)terephthalamide) was done using a modified version of a previously reported procedure.103 5-Aminoisophthalic acid (7.5 mmol, 1.358 g) was added to 10 mL of anhydrous tetrahydrofuran under N2. Terephthaloyl chloride (3.75 mmol, 0.6713 g) was dissolved with 10 mL of anhydrous THF and was added to the above solution dropwise at a 0–5 °C temperature. After complete addition, the temperature of the reaction was increased to room temperature. Continuous stirring of the mixture was done for a time frame of 24 h, and finally white solid was produced. The solid was later filtered as well as washed using THF. Finally, the product was added to a 100 mL beaker, and 30 mL of MeOH was added. The mixture was heated to dissolve excess 5-aminoisophthalic acid. The white solid was subjected to filtration and was thoroughly washed with 10 mL of EtOH (hot) and finally dried in a vacuum. The 1H NMR, 13C NMR, and FTIR spectras of the amide ligand are provided in the Supporting Information (Figures S1–S3). Yield = 3.20 g, 86%. Data for H4L, selected IR (Solid, cm–1): 3466, 3376, 3094, 1711, 1679, 1666, 1612, 1543, 1510, 1407, 1333, 1281, 1253, 1196, 1122, 9258, 860, 756, 663, 611, 593, 506. 13C NMR (DMSO-d6, δ ppm): 166.98 (COOH), 166.55 (CONH), 140.13 (C-NH2), 137.61 (C–C=O), 132.25 (C–COOH), 128.38 (C–H aromatic), 125.26. 1H NMR (DMSO-d6, δ ppm): 8.17 (4H, ArH), 8.25 (2H, ArH), 8.70 (4H, ArH), 10.75 (2H, CONH), 13.32 (4H, COOH).

Synthesis of [Cu2(L)(H2O)3]·4DMF·6H2O or Cu-MOF (LOCOM-1)

A 10 mL Teflon vial was acquired in which 4 mL of solvent mixture (DMF/H2O 3:1 in v/v) was added, and then the prepared ligand (H4L; 0.05 mmol, 24 mg) was added with a drop of 2 M HCl. To this solution, metal salt Cu(NO3)2·3H2O (0.15 mmol, 36 mg) was added in a Teflon vial, and the whole mixture was stirred to homogenize the prepared mixture. Then, the Teflon vial was closed and transferred to the inner steel jacket of an autoclave and heated for 72 h at 85 °C. The targeted Cu-MOF was achieved in the form of blue crystals (Figure S5). DMF washing of the prepared crystals was carried out and crystals were finally dried in the open air. Yield: 42 mg, 64%. [Cu2(L)(H2O)3]·4DMF·6H2O (Cu2C36H58N6O36N6) Anal. Calcd: C = 40.38%; H = 5.42%; N = 7.85%. Found: C = 41.85%; H = 4.56%; N = 7.23%.

Knoevenagel Condensation

In an model batch experiment, 5 mg of Cu-MOF (the basic catalyst) corresponding to 0.04 mmol of amide groups (based on the total amount of amide groups in Cu-MOF) was added into a solution of 1.1 mmol malononitrile in 4 mL of DMSO-d6 (solvent) in an Erlenmeyer flask. Finally, the addition of a simple benzaldehyde or p-substituted benzaldehyde (1 mmol) into the reaction mixture was done with continuous stirring at room temperature (Table 1). The 1HNMR analysis tool was employed to monitor the reaction progress periodically.

Physical Measurements

NMR (nuclear magnetic resonance) data (13C and 1H) were assembled on a Bruker DPX 500 MHz apparatus in DMSO-d6 solution, and the chemical shifts were accounted to the internal standard TMS (0 ppm). FTIR (Fourier transform infrared spectroscopy) spectra were confirmed on an Agilent Technologies 41630 apparatus. The elemental analysis was obtained on a Vario EL cube. PXRD (powder X-ray powder diffraction) peaks were verified using a Cu Kα radiation source on a D8 advanced Bruker-Powder-Diffractometer. The SEM (scanning electron microscopy) measurements were performed on a JSM740 Scanning Electron Microscope. A NETZSCH STA 409 PC/PG instrument was employed for recording TGA (thermogravimetric analyses) under a N2 environment with a heating rate of 10 °C/min. Pore size and BET surface measurements were done at 77 K with N2 sorption isotherms on the Micromeritics ASAP 2020 apparatus. Prior to the analysis, the prepared Cu-MOF effectively degassed overnight at a temperature of 150 °C as well.

Acknowledgments

The authors are very grateful to the Bahauddin Zakariya University, Multan 60800, Pakistan for providing the Research Facilities at the Institute of Chemical Sciences. The authors approvingly acknowledge State Key Laboratory of the Advanced Technology for Materials Synthesis and Processing; Center for Chemical and Material Engineering, The Wuhan University of Technology, Wuhan 430070, P.R. China, for characterization of samples.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07137.

  • 1H NMR, C NMR, and FTIR of H4L; PXRD and FTIR results and microscope images of Cu-MOF and PXRD before and after five cycles; 1H NMR spectroscopic determination of the products; and 1H NMR integration of the characteristic benzylic protons (starting, product) and conversions into products (PDF)

Author Contributions

The manuscript was written through equal contributions of the all authors and coauthors. All authors have approved the final form of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao2c07137_si_001.pdf (1.3MB, pdf)

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