Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Design and Construction of a Chiral Cd(II)-MOF from Achiral Precursors: Synthesis, Crystal Structure and Catalytic Activity toward C−C and C−N Bond Forming Reactions Vijay Gupta and Sanjay K. Mandal* Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, Manauli PO, S.A.S. Nagar, Mohali, Punjab 140306, India
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S Supporting Information *
ABSTRACT: Using achiral components, a V-shaped dicarboxylic acid (H2L) and a conformationally flexible bidentate linker (bpp), a thermally stable chiral metal organic framework {[Cd(bpp)(L)(H2O)]· DMF}n (1), where H2L = 4,4′-(dimethylsilanediyl)bis-benzoic acid, bpp = 1,3-bis(4-pyridyl)propane and DMF = N,N-dimethylformamide, has been solvothermally synthesized and crystallographically characterized. It consists of 1D helical chains linked at the cadmium centers resulting in an overall 2D framework. Its microporous nature was confirmed by gas-sorption measurements. Upon thermal activation of 1, where both guest DMF molecules present in the 1D open channels and the coordinated H2O molecules are removed, its active metal site shows Lewis acid character to be an excellent heterogeneous catalyst for the C−C (Knoevenagel condensation reaction) and C−N (Strecker) bond forming reactions.
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INTRODUCTION For some time special efforts have been directed to engineering metal−organic frameworks (MOFs) not only because of their potential applications as functional materials1−4 but also in view of their assortment of captivating architectures and topologies.5 In particular, there is currently a substantial demand for sensible synthetic strategies to permit the construction of chiral MOFs due to their intriguing potential applications in enantioselective catalysis,6 chiral separation,7 nonlinear optical materials,8 and magnetic materials.9 Basically, there are two methods to generate chiral MOFs with building blocks used for the construction of coordination frameworks. One is the likelihood of incorporating chiral centers in either metal complexes or ligands, while the second involves achiral ligands with no chiral auxiliaries for acquiring either a chiral framework or enantiomers by spontaneous resolution.10 Until now, most of the chiral MOFs and coordination polymers are synthesized by the first method using enantiopure ligands, which induces the homochirality in the resulting framework. On the other hand, the second method is more difficult,11 but attractive12 for the chiral MOFs using achiral components, which can be easily prepared compared to chiral components. The chirality can be conveyed from achiral precursors through crystallization even without the use of enantiopure building blocks.13 In the literature, very few examples of self-assembled chiral MOFs from achiral precursors have been reported.14 The chirality in these frameworks arises because of the spatial organization (e.g., helix) of achiral precursors, but the interpenetration of © XXXX American Chemical Society
opposite-handed helices often yields nonporous and achiral structures.15 The other possibility involves individual homochiral crystals that are formed via a process known as spontaneous resolution.13 However, the bulk sample tends to be an equal mixture of crystals with opposite handedness. So far, our understanding of the bulk homochiral crystallization from achiral precursors is an unpredictable16 but exceptionally attractive process. Therefore, the design and development of synthesis of chiral MOFs from achiral components is essential to acquire the comprehensive expertise of inducing chirality into the MOF architectures. Owing to the chirality in coordination architectures, in most cases chirality arises by the helical secondary structures.12b,17 The V-shaped exobidentate ligands are well-known to impart helicity owing to their natural ability to endorse a twisted conformation in the resulting framework.18 In addition, the limited literature reports11b,19 reveal the uniqueness of conformationally flexible ligands for the synthesis of chiral frameworks adopting special geometries to induce chirality into the overall framework. In view of these facts, we chose to use a V-shaped dicarboxylic acid 4,4′-(dimethylsilanediyl)bis-benzoic acid (H2L) along with a conformationally flexible 1,3bis(4-pyridyl)propane linker (bpp) (Scheme 1) for their introduction into MOFs with a motive to engineer chirality into the overall architecture. We envisioned that the natural ability of V-shaped ligands to endorse a twisted conformation Received: November 28, 2018
A
DOI: 10.1021/acs.inorgchem.8b03307 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
heated to 100 °C for 48 h. Colorless rod-shaped crystals of 1 were filtered out without cooling the mixture. Upon repeated washings with DMF followed by water, these crystals were air-dried. Yield: 145 mg (69%), based on the metal salt. Anal. Calcd for C32H37N3O6SiCd (MW 700.13): Calc. %C, 54.89; %H, 5.33; %N, 6.00 and Found: %C, 55.05; %H, 6.10; %N, 5.88. Selected FTIR peaks (KBr, cm−1): 1670 (s), 1579 (s), 1530 (s), 1400 (s), 1307 (w), 1250 (m), 1225 (w), 1100 (s), 1017 (m), 812 (s), 767 (s), 724 (m), 659 (w), 509 (m). Typical Experimental Procedure for the Knoevenagel Reaction. In a typical reaction, a mixture of an aromatic aldehyde (0.10 mmol), an active methylene compound (0.12 mmol), the activated 1 (2 mg, 3 mol %) and 1 mL methanol was taken in a capped glass vial. The reaction mixture was allowed to stir at room temperature (25−30 °C) for 60 min. After that, the solid catalyst was separated by centrifugation and washed with chloroform to recover the catalyst. The supernatant was evaporated by using a rotary evaporator and the crude product thus obtained was redissolved in CDCl3 and analyzed by 1H NMR spectroscopy. Typical Experimental Procedure for the Strecker Reaction. In a typical reaction, a mixture of aldehyde/ketone (0.10 mmol), aniline (0.10 mmol), and TMSCN (0.12 mmol) was taken in a Schlenk tube along with 2 mg (3 mol %) of the activated 1. The reaction mixture was allowed to stir at room temperature (25−30 °C) for 6 h under nitrogen atmosphere in a solvent free condition. After that, 1 mL chloroform was added and the solid catalyst was separated by centrifugation. The supernatant was evaporated by using rotary evaporator and the crude product thus obtained was redissolved in CDCl3 and analyzed by 1H NMR spectroscopy. Recycling of 1. After the catalytic reaction, the solid catalyst was collected by centrifugation, washed with chloroform, and dried for 5 h under vacuum for its reuse in the next experiment.
Scheme 1. Chemical Structure and Schematic Representation of the Bent Dicarboxylic Acid H2L and Conformationally Flexible Ligand bpp
and the rotation freedom of Py−CH2−CH2−CH2−Py bonds of the bis(4-pyridyl)propane may act cooperatively to lock the overall framework in a chiral conformation. Herein, we report the synthesis of a Cd(II)-based helically chiral MOF, {[Cd(bpp)(L)(H2O)]·DMF}n (1) (where, DMF = N,N-dimethylformamide), using achiral organic ligands and without the need of any chiral auxiliaries such as enantiopure solvents, additives, catalysts, or a template. The polymer chain in 1 consists of Cd(II) ion and L2− arranged along the twofold screw axis in the form of a helix. These 1D helical chains are further linked at the cadmium centers forming a 2D framework. Further, the existence of exposed Lewis acidic metal sites in 1 has been demonstrated by its heterogeneous catalytic activity toward the carbon−carbon (Knoevenagel condensation reaction) and carbon−nitrogen (Strecker) bond forming reactions.
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RESULTS AND DISCUSSION Synthesis and Structural Characterization. Compound 1 was synthesized under solvothermal conditions by heating a
EXPERIMENTAL SECTION
Materials and Physical Measurements. All reactions carried out with n-BuLi required oven-dried glasswares and an atmosphere of nitrogen. Both diethyl ether and tetrahydrofuran (THF) were dried and purified by standard protocols (predried with KOH and distilled over benzophenone-Na) prior to use. All chemicals and other solvents used for synthesis were obtained from commercial sources and were used as received, without further purification. H2L was synthesized by following the literature procedure20a and characterized by 1H and 13C NMR spectra recorded at room temperature on a 400 MHz Bruker ARX400 NMR spectrometer. FTIR spectra of samples as KBr pellets were recorded in the 4000− 400 cm−1 range on a PerkinElmer Spectrum I spectrometer. TG (thermogravimetric) analysis was carried out in the temperature range of 25−500 °C (at a heating rate of 10 °C/min) under a dinitrogen atmosphere using an aluminum pan as a sample holder on a Shimadzu DTG-60H instrument. Elemental analysis (C, H, and N) was carried out using Leco TruSpec Micro CHNS analyzer. For the single crystal X-ray diffraction, see details in the Supporting Information. Power Xray diffraction (PXRD) measurements were carried out on a Rigaku Ultima IV diffractometer having settings and attachments as reported earlier20b over a 2θ range of 5−50° with a scanning speed of 2° per minute using a 0.02° step. Solid state CD spectrum was recorded on Chirascan CD instrument. Gas sorption experiments were performed using a BELSORP MAX (BEL JAPAN) volumetric adsorption analyzer at different temperatures. Ultrapure (99.995%) N2, He, and CO2 gases were used for the adsorption measurements. Each sample was pretreated prior to data collection by grounding it to a powder and heating at 373 K under very high vacuum for 24 h followed by purging with nitrogen gas on cooling. For sorption isotherms at 273 and 298 K, the set temperature was maintained with a circulating water bath connected to a chiller. Synthesis of {[Cd(bpp)(L)(H2O)]·DMF}n (1). A mixture of H2L (90 mg, 0.3 mmol), bpp (60 mg, 0.3 mmol), and cadmium nitrate tetrahydrate (92.5 mg, 0.3 mmol) was dissolved in 10 mL DMF. To this solution, 5 mL water was added to obtain a clear solution, which was then transferred to a glass vial. The glass vial was sealed and
Scheme 2. Synthesis of 1
mixture of Cd(NO3)2·4H2O, bpp, and H2L along with DMF/ H2O (2/1) mixture at 100 °C for 48 h (Scheme 2). Colorless rod-shaped crystals of 1 suitable for single crystal X-ray diffraction analysis were isolated by filtration in 69% yield. Based on the single crystal X-ray diffraction analysis (Table S1, Supporting Information), 1 crystallizes in the chiral space group P21212 (No. 18) and has a 2D structure derived from Cd−bpp−L2− helical chains [Flack parameter = 0.001(7)]. As shown in Figure 1a, the asymmetric unit consists of one Cd(II) ion, one bpp, one L2−, and one coordinated H2O molecule along with one lattice DMF molecule. An ORTEP view of the asymmetric unit with an atom numbering scheme is also shown in Figure S4 in SI. The Cd center has a distorted pentagonal bipyramidal geometry with an N2O5 coordination environment due to binding of four oxygen atoms (O1, O2, O3, O4) from carboxylates of two different L2− in a chelated fashion and one oxygen (O5) from coordinating water at the equatorial positions (Table S2, Supporting Information). The two apical positions are occupied by two nitrogen atoms (N1 and N2) from two different bis-pyridyl ligands. The chelated mode of carboxylate binding with the metal center is evident by the difference (Δν = 179 cm−1) in the asymmetric (ν = 1579 cm−1) and symmetric (ν = 1400 cm−1) carbonyl stretching of carboxylates in the FTIR spectrum of 1 (Figure S3, Supporting Information). A strong peak at 1670 cm−1 B
DOI: 10.1021/acs.inorgchem.8b03307 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. X-ray crystallographic analysis of 1: (a) A view of the asymmetric unit and the coordination environment around the Cd (II) center. (b) Perspective view of the 2D framework; lattice DMF molecules present in the 1D channels are shown as their space-fill model. (c, d) Representation of the 21 screw axis along b-axis and the twofold rotation axis along c-axis. (e) Space fill view of the framework where different 1D helical chains fused at the Cd(II) centers are shown in different colors.
Figure 2. Simplified node and linker representation for 1, indicating 4-connected uninodal net, sql topology.
the framework shown in Figure 1e depicts the fusion of different 1D helical chains at the Cd(II) centers. Clearly, 1 is a noninterpenetrating MOF, and its microporous nature established by gas adsorption studies supports this further (vide infra). To simplify the rather intricate structure of 1, the Cd(II) center can be considered as a 4-connected node and the ligands L2− and bpp as bent linkers, respectively. Further examination of the node and linker representation reveals that the framework exhibits a 4-connected uninodal net, sql topology, with Schlafli point symbol {44.62} (Figure 2) as determined by the TOPOS23 program (Table S3, Supporting Information). Further structural analysis of 1 reveals that the metal center is not stereogenic and can not induce chirality; thus, the Vshaped dicarboxylate and the conformationally flexible bpp
corresponds to the carbonyl stretching frequency of the lattice DMF molecule. The Cd(II) centers act as a link between the helical chains giving rise to an overall 2D framework with 1D open channels with dimensions of 10.9 × 12.1 Å2 (7.1 × 9.0 Å2 without van der Waals radii),21 which are filled with lattice DMF molecules (Figure 1b). The total potential solvent-accessible void volume for 1 was estimated to be 17.8% (577 Å3 out of the unit cell volume 3224 Å3) using the PLATON software.22 The dicarboxylate ligand (L2−) adopting a helical chain conformation, based on its connectivity with the Cd(II) centers in 1, can be observed by considering the 21 screw axis along the baxis (Figure 1c). On the other hand, the twofold rotation axis along the c-axis provides the overall connectivity of all components (Figure 1d). Furthermore, a space fill view of C
DOI: 10.1021/acs.inorgchem.8b03307 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Substrate Scope for the Knoevenagel Condensation Reaction Catalyzed by 1a
Table 2. Substrate Scope for the Strecker Reaction Catalyzed by 1a
a
Reaction conditions: Aldehyde (0.1 mmol), active methylene compound (0.15 mmol), and reaction time: 60 min. bAverage percent yield for a set of triplicate runs, calculated by 1H NMR of the crude products. cNumber of moles of product per mole of catalyst. dIsolated yield. eBlank reaction.
a
Reaction conditions: Aldehyde/ketone (0.1 mmol), amine (0.1 mmol), TMSCN (0.12 mmol) and reaction time: 6 h. bAverage percent yield for a set of triplicate runs, calculated by 1H NMR of the crude products. cNumber of moles of product per mole of catalyst. d Isolated yield. eBlank reaction.
adopt a helical arrangement when coordinated to the metal ion, giving rise to a chiral network. The helical chirality of the framework was further established with the help of solid state circular dichroism (CD) measurement (Figure S6, Supporting Information). Powder X-ray diffraction data of the as-synthesized, solvent exchanged, and desolvated sample of 1 were recorded at room temperature to confirm whether the single crystal structure represents the bulk material or not (Figure S7, Supporting Information). It was found that the experimental powder patterns were in good agreement with the simulated powder pattern (obtained from the single crystal data) of 1, which confirmed the phase purity as well as retention of structural integrity upon thermal desolvation/activation. In order to test the thermal stability and structural variation as a function of temperature of 1, thermogravimetric analysis (TGA) was carried out for the single phase polycrystalline samples of assynthesized, solvent exchanged, as well as desolvated 1, under a dinitrogen atmosphere (Figure S8, Supporting Information). TGA data reveal that the compound is stable up to 200 °C, which has been further corroborated by temperature-depend-
ent powder X-ray diffraction analysis (Figure S9, Supporting Information). Coordinated water and the guest molecules in as-synthesized 1 can be removed by heating under vacuum to obtain its desolvated/activated form (Figure S10, Supporting Information). As indicated earlier, the activated compound with readily available open channels provides a surface with unsaturated Cd(II) sites with chiral pore environment. Porosity in the activated compound was examined by the gas-sorption measurements. Prior to the adsorption measurements, the methanol exchanged sample (∼100 mg) was activated by placing the sample under ultravacuum at 100 °C for 12 h. The low temperature gas sorption measurements reveal that 1 adsorbs an appreciable amount of CO2 (at 195 K) but much lower amount of N2 (at 77 K) (Figure S11, Supporting Information). The Brunauer−Emmett−Teller (BET) and the Langmuir surface areas were estimated to be 61 m2 g−1 and 109 m2 g−1, respectively, based on the CO2 adsorption measurement at 195 K (Figure S12 and S13, Supporting D
DOI: 10.1021/acs.inorgchem.8b03307 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. Plausible mechanisms for the Knoevenagel condensation and the Strecker reactions catalyzed by 1.
catalytic activity, the versatility of 1 was further tested by the reaction of different types of substituted aromatic aldehydes and active methylene compounds. It resulted in the corresponding α,β-unsaturated cyano/ester derivatives in good-to-excellent yields. A summary of the results is compiled in Table 1. There are reports where MOFs have been utilized for Strecker reaction, but the scope of d10 metal ion-based MOFs is very limited for this reaction.14a Therefore, benzaldehyde, trimethylsilyl cyanide (TMSCN) and aniline as the test substrates were added to 3 mol % of 1 placed in a Schlenk tube. Without adding a solvent the reaction was performed at room temperature under N2 atmosphere, while its progress was followed by TLC. The final conversion was calculated by 1H NMR spectroscopy (Figure S18, Supporting Information). As a control experiment, the reaction was repeated without any catalyst under the same conditions (Entry 14, Table 2; Figure S19, Supporting Information), where only 27% yield was obtained. The catalytic activity of 1 was further tested for different substrates, especially for the more reluctant ketone derivatives (Table 2). The results obtained for both the Knoevenagel and Strecker reactions reveal that the substrates with an electron with drawing group gave the maximum conversion while a drop in the product conversion was obtained for the substrates with electron donating substituents. This supports the fact that an electron withdrawing group increases the rate of a nucleophilic attack at the carbonyl or imine groups compared to an electron donating group. The stronger the electron-withdrawing ability of the substituent, the easier the activation of carbonyls or imines for nucleophilic attack is at the carbon center, and consequently faster the reaction is. In all cases, products were isolated and characterized by 1H and 13C NMR spectroscopy to confirm their identity and purity (Figures S20−S73, Supporting Information).
Information). In order to determine the quantitative binding strength of CO2 molecules with the framework, the isosteric adsorption enthalpy (Qst) was calculated based on the Clausius−Clapeyron equation, using the adsorption isotherms collected at different temperatures (Figure S14 and S15, Supporting Information). The Qst value at zero loading was found to be ∼25 kJ mol−1, which indicates good binding affinity of CO2 with the framework. Catalysis Studies. Due to the microporous nature and coordinatively unsaturated metal site in 1 (Figure S10, Supporting Information), it was tested for its active role as a heterogeneous catalyst for Lewis acid promoted reactions. The Knoevenagel condensation and Strecker reactions are wellknown Lewis acid catalyzed C−C and C−N bond forming reactions used for the synthesis of benzylidene malononitriles and α-aminonitriles, respectively. As the Knoevenagel reaction deals with aldehydes and compounds having active methylene groups, it has wide applications for the preparation of several fine chemicals and pharmaceuticals. Additionally, the product benzylidene malononitrile is well-known for its biological activity.24 On the other hand, α-aminonitriles obtained from the three-component Strecker reaction are versatile building blocks for the synthesis of α-amino acids and their derivatives.25 To check the feasibility of 1 toward the Knoevenagel condensation reaction, the reaction between benzaldehyde and malononitrile was chosen as a test reaction. Both components were placed in a capped glass vial along with a catalytic amount (3 mol %) of 1. The reaction was performed at room temperature using methanol as reaction medium. The progress of the reaction was monitored by TLC and the final conversion was calculated by 1H NMR spectroscopy (Figure S16, Supporting Information). As a control experiment, the reaction was repeated without any catalyst under the same conditions (Entry 15, Table 1; Figure S17, Supporting Information). As expected, only 17% yield was achieved. Inspired by the good E
DOI: 10.1021/acs.inorgchem.8b03307 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
To determine the recyclability of the catalyst, it was separated via filtration, washed with chloroform and methanol, and dried under vacuum at 100 °C for 5 h to regenerate the active catalyst. In both cases, no loss of crystallinity or phase purity of the catalyst was observed even after three cycles of reactions, as evident from the PXRD patterns of the recovered catalyst (Figure S74, Supporting Information). In addition, the catalyst can be reused up to three cycles in both cases with a slight loss (∼8−10%) in its catalytic activity (Figure S75, Supporting Information). In order to find any catalyst leaching into the product stream, the catalyst was separated from the reaction mixture by centrifugation after a certain time and the supernatant was subjected to the same reaction conditions. No significant conversion was obtained (Figure S76, Supporting Information) indicating the heterogeneous nature of 1 and its active role as a catalyst in both the reactions. The efficiency of 1, when compared to the catalytic activity of various MOFs that have been used as heterogeneous catalysts in the Knovenagel and Strecker reactions, is found to be comparable with the reported examples.14a,26 Based on the studies described above, plausible mechanisms are presented in Figure 3. These are in accordance with reported proposals.26,27 The open Lewis-acidic metal center interacts with the electrophilic carbonyl or imine group to impart polarization of these groups, which leads to the enhancement of electrophilic character of the carbon atom of carbonyl or imine moiety. In addition, the Lewis basic O atom of the coordinated carboxylate group is known to impart basic character to the framework and can activate the corresponding nucleophile; thus, it acts as the driving force of the reaction.
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Corresponding Author
*E-mail:
[email protected]. ORCID
Sanjay K. Mandal: 0000-0002-5045-6343 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge IISER Mohali, India for financial support (to S.K.M.) and SRF from UGC New Delhi, India (to V.G.). The use of the X-ray facility at IISER Mohali is gratefully acknowledged.
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REFERENCES
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CONCLUSIONS In summary, we have reported the synthesis and structural characterization of a chiral Cd(II) MOF based on a relatively simple V-shaped dicarboxylic acid and conformationally flexible bis-pyridyl ligand, without the need of any chiral auxiliary. The chirality induction has occurred due to the helical arrangement of the polymeric chain around a twofold screw-axis. The overall 2D framework represents the 4connected uninodal net, sql topology. The microporous nature and the presence of open metal sites in the activated framework has been utilized for its catalytic activity toward Lewis acid catalyzed C−C and C−N bond forming Knoevenagel condensation and Strecker reactions, respectively. The structural integrity of the catalyst during the reaction was retained and thus provided its heterogeneous nature. Furthermore, this MOF can be easily separated and reused for three catalytic cycles without significant loss of its activity in both cases. Its utilization in many other organic transformations is the future plan in our laboratory.
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AUTHOR INFORMATION
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03307. Experimental details, characterization procedures, crystallographic data with refinement details, selected bond lengths and angles, TGA and PXRD patterns (PDF) Accession Codes
CCDC 1879636 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge F
DOI: 10.1021/acs.inorgchem.8b03307 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.8b03307 Inorg. Chem. XXXX, XXX, XXX−XXX
