MetalâOrganic Frameworks as Efficient Heterogeneous Catalysts for

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Adenine-Based Zn(II)/Cd(II) Metal−Organic Frameworks as Efficient Heterogeneous Catalysts for Facile CO2 Fixation into Cyclic Carbonates: A DFT-Supported Study of the Reaction Mechanism Yadagiri Rachuri,† Jintu Francis Kurisingal,† Ramesh Kumar Chitumalla,‡ Srimai Vuppala,‡ Yunjang Gu,† Joonkyung Jang,‡ Youngson Choe,† Eringathodi Suresh,*,§ and Dae-Won Park*,†

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†

Division of Chemical and Biomolecular Engineering and ‡Department of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of Korea § Analytical and Environmental Science Division and Centralized Instrument Facility, Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar 364 002, India S Supporting Information *

ABSTRACT: We synthesized two new adenine-based Zn(II)/Cd(II) metal−organic frameworks (MOFs), namely, [Zn2(H2O)(stdb)2(5H-Ade)(9H-Ade)2]n (PNU-21) and [Cd2(Hstdb)(stdb)(8H-Ade)(Ade)]n (PNU-22), containing auxiliary dicarboxylate ligand (stdb = 4,4′-stilbenedicarboxylate). Both MOFs were characterized by multiple analytical techniques such as single-crystal X-ray diffraction (SXRD), powder X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, scanning electron microscopy, as well as temperature program desorption and Brunauer−Emmett−Teller measurements. Both MOFs were structurally robust and possessed unsaturated Lewis acidic metal centers [Zn(II) and Cd(II)] and free basic N atoms of adenine molecules. They were used as heterogeneous catalysts for the fixation of CO2 into five-membered cyclic carbonates. Significant conversion of epichlorohydrin (ECH) was attained at a low CO2 pressure (0.4 MPa) and moderate catalyst (0.6 mol %)/cocatalyst (0.3 mol %) amounts, with over 99% selectivity toward the ECH carbonate. They showed comparable or even higher catalytic activity than other previously reported MOFs. Because of high thermal stability and robust architecture of PNU-21/PNU-22, both catalysts could be reused with simple separation up to five successive cycles without any considerable loss of their catalytic activity. Densely populated acidic and basic sites in both Zn(II)/Cd(II) MOFs facilitated the conversion of ECH to ECH carbonate in high yields. The reaction mechanism of the cycloaddition reaction between ECH and CO2 is described by possible intermediates, transition states, and pathways, from the density functional theory calculation in correlation with the SXRD structure of PNU-21.

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INTRODUCTION Carbon dioxide (CO2) is a greenhouse gas that substantially accounts for climate change.1 The sequestration of atmospheric CO2 discharged as a byproduct from petrochemical industries/power plants and the consumption of fossil resources strongly contributes to the rising CO2 concentration in the atmosphere, which exceeded 410 ppm in 2018.2 To complement the use of environmentally friendly energy sources instead of fossil fuels is another way to tackle this major global challenge: changing the raw material base for the chemical industry, from petrochemicals to more sustainable and renewable alternatives.3−7 The reduction of anthropogenic emissions can be influenced by the storage and chemical utilization of CO2 into value-added products.8−13 CO2 has attracted considerable attention not only as a greenhouse gas but also as an abundant C1 feedstock for organic transformations, due to its free availability, nontoxicity, nonflammability, and simplicity in handling. Therefore, researchers worldwide have been working intensively to exploit this carbon © XXXX American Chemical Society

source for several organic transformations, including cyclic carbonates. Cyclic carbonates can be used as alternative, nonflammable aprotic polar solvents, as monomers for polycarbonate synthesis, and as chemical intermediates in organic synthesis.14−25 Various homogeneous catalysts have been employed for the syntheses of cyclic carbonates by the cycloaddition reaction of CO2 and epoxides, such as transitionmetal complexes, ionic liquids, organocatalysts, and quaternary ammonium or phosphonium salts.26−37 However, these systems suffer from separation, purification, and recycling problems, which severely limit their industrial applications. Conversely, recyclable multifunctional heterogeneous catalysts with good chemical and thermal stability and robust frameworks, which have high surface area/porosity and good adsorption capacity, have been developed for efficient heterogeneous catalytic reactions.38−41 In this regard, several Received: March 21, 2019

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DOI: 10.1021/acs.inorgchem.9b00814 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

%)], due to its high number of acidic and basic sites in both catalysts.

types of heterogeneous catalysts such as metal oxides, zeolites, functional polymers, and silica-supported salts have been explored for the syntheses of cyclic carbonates by the utilization of CO2.38−41 Metal−organic frameworks (MOFs)/coordination polymers (CPs), also referred to as multidimensional porous coordination polymers, are constructed from metal ions/clusters and organic linkers42,43 and have been identified not only for their aesthetic structural topologies but also for their diverse applications in the areas of gas adsorption and separation,44,45 sensing,46,47 drug delivery,48 and catalysis.49−53 Particularly, in the area of heterogeneous catalysis, MOFs are well-celebrated as porous solid catalysts due to their well-defined and uniformly distributed active catalytic sites that are provided by the open metal centers, their large external and internal surface areas through proper pores or channels for the selectivity of the substrates, their recovery, and recyclability.54−58 Moreover, their Lewis acidic metal centers and basic functional groups make them efficient heterogeneous catalysts in various catalytic reactions.59,60 MOFs were also largely explored for their ability to adsorb CO2,61,62 and very few of them were exploited as heterogeneous catalysts for the conversion of CO2 into value-added chemicals.63−66 Biomolecules are alternatives to toxic organic building blocks for the syntheses of Bio-MOFs applied in biomedicine, sensors, and even for sustainable catalysis.67−69 Besides using amino acids and other biocompatible organic ligands for synthesizing MOFs,70 nucleobases are another attractive family of ligands that can be incorporated within the MOFs structure.70 In addition to their rigid structure and multiple metal binding sites, noncovalent interactions including hydrogen bonds and π···π stacking of these nucleobases are exploited as rigid functionalized building blocks in the construction of stable MOFs.71,72 Among all nucleobases, adenine (Hade) has been widely used in the syntheses of discrete metal complexes and CPs, or MOFs with transition metals.73−76 Because of its rigidity and five potential coordination sites, (i.e., two imidazolate and two pyrimidine N atoms and an −NH2 functional group) adenine is a potential nucleobase for the syntheses of CPs/MOFs. Over the years, many porous/ nonporous Ade-based MOFs with diverse structures and topologies have been reported.71,77 Many of them have been applied in the field of drug delivery, sensors, gas adsorption, and selective separation of CO2/CH4; they also exhibit lowdensity micro and mesoporosity, high surface areas, and large pore volumes.71−73,78 Recently, we and others developed sustainable MOFs for CO2 utilization as heterogeneous catalysts in the syntheses of cyclic carbonates.79−83 In furtherance of our quest to establish new functionalized MOF catalysts, we herein report adenine-based Zn(II)/Cd(II) metal−organic frameworks: [Zn2(H2O)(stdb)2(5H-Ade)(9HAde)2]n (PNU-21) and [Cd2(Hstdb)(stdb)(8H-Ade)(Ade)]n (PNU-22) (stdb = 4,4′-stilbenedicarboxylate), synthesized by solvothermal method; their structures are characterized by single-crystal X-ray diffraction (SXRD) and other analytical techniques. Taking advantage of unsaturated metal centers and basic N atoms, the application of both MOFs as potential heterogeneous catalysts for the fixation of CO2 into cyclic carbonates was investigated. The catalysts showed excellent catalytic activity in the syntheses of cyclic carbonates from the cycloaddition of epoxide and CO2, at a low pressure and using moderate amounts of the catalysts [0.4 MPa, 70 mg (0.6 mol

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EXPERIMENTAL SECTION

Materials and Methods. 6-Aminopurine (HAde), 4,4′-stilbenedicarboxylic acid (H2stdb), Zn(NO3)2·6H2O, and Cd(NO3)2·4H2O were purchased from Sigma-Aldrich. Double-distilled water and dimethylformamide (DMF) were used as solvents in the syntheses of MOFs. Solvents were used as received without further purification. CHN analyses were performed using a Vario Micro cube system, Germany. The Fourier transform infrared (FT-IR) spectra were recorded using the KBr pellet method on a PerkinElmer, G-FTIR spectrometer. Thermogravimetric (TG) analyses were performed using Mettler Toledo and Netzsch. X-ray powder diffraction (PXRD) data were collected using a PANalytical Empyrean (PIXcel 3D detector) system with Cu Kα radiation. Single-crystal structures were determined using a Bruker Smart Apex (charge-coupled device (CCD)) diffractometer. UV−Vis spectra were recorded using a Shimadzu UV-3101PC spectrometer, and the photoluminescence spectra were recorded at room temperature using a Fluorolog Horiba Jobin Yvon spectrophotometer. The X-ray photoelectron spectroscopy (XPS) analyses of the catalysts were performed using a θ probe AR-XPS system [Thermo Fisher Scientific (U.K.)]. The instrument used a monochromated Al Kα X-ray source (hv = 1486.6 eV), which has an energy of 150 W. CO2 and NH3 Temperature program desorption (TPD) profiles were acquired with a chemisorption analyzer (BEL-CAT) as follows. Before the measurements, 0.05 g of the sample was activated in He (30 mL min−1), at 280 °C, for 1 h. The sample was subsequently exposed to the pulse of CO2 or NH3 at 40 °C, for 1 h. The sample was then flushed with He (30 mL min−1), for 1 h. TPD measurements were performed by raising the temperature from 40 to 600 °C at a rate of 5 °C min−1. N2 and CO2 adsorption isotherms and BET surface area were measured using Micromeritics ASAP 2020 instrument. Field emission scanning electron microscopy (FE-SEM, Zeiss supra 40 VP) images of PNU21 and PNU-22were captured using an S-4200 field emission scanning electron microscope, at 5.00 kV. The cyclic carbonate yields were assessed by a gas chromatograph (GC) (Agilent Technologies, HP 6890 A) rigged with a 30 m × 0.25 μm capillary column (HP-5) using a flame-ionized detector. The metal concentrations in the leaching test established by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis using an Activa, JY Horiba (1.5 kW, 40.68 MHz, 130−800 mm) fitted with a monochromatic high dynamic range detector (HDD) and a polychromatic photomultiplier tube (PMT) detector. X-ray Crystallography. A summary of the crystallographic data and details of data collection for PNU-21 and PNU-22 are given in Table S1. Single crystals with suitable dimensions were chosen under an optical microscope after immersion in paratone oil, after which they were mounted for data collection. The intensity data for both crystals were collected using Mo Kα (λ = 0.710 73 Å) radiation, on a Bruker SMART APEX diffractometer equipped with a CCD area detector, at 150 K. The data integration and reduction were processed with SAINT software.84 Empirical absorption correction was applied to the collected reflections with SADABS.85 The structures were solved by direct methods using SHELXTL and were refined on F2 by the full-matrix least-squares technique using the program SHELXL97.86,87 All non-hydrogen atoms were refined anisotropically, until convergence was attained. The hydrogen atoms attached to the organic moieties present in both MOFs are either located from the difference in the Fourier map or fixed stereochemically. Details of the crystal data and refinement parameters and selected bond lengths and angles are provided in Tables S1 and S2. Synthesis of [Zn2(H2O)(stdb)2(5H-Ade)(9H-Ade)2]n (PNU-21). Ade (52 mg, 0.38 mmol), H2stdb (46 mg, 0.17 mmol), and Zn(NO3)2·6H2O (120 mg, 0.40 mmol) were dispersed in 7 mL of DMF−H2O (1:2.5); the reaction mixture was stirred for ∼15 min in air and sealed in a 14 mL Teflon-lined autoclave, which was heated at 413 K for 96 h. Slow cooling of the reaction mixture to room B


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Inorganic Chemistry

Figure 1. Coordination environment around Zn and Cd metal centers in PNU-21 (a) and PNU-22 (b).

Figure 2. (a) 2D network in PNU-21 down b-axis, (b) ribbon motif generated by parallel alignment of [Zn2(HAde)2]n, linked by a carboxylate oxygen of stdb by the μ2-η1η1 mode of coordination, (c) 6-connected uninodal-type hexagonal 2D network topology of PNU-21. by gas chromatography; the product purities were examined by 1H NMR spectroscopy.

temperature resulted in a pale yellow crystalline material in 74% yield. The elemental analysis results of C42H32N10O9Zn2 (%) are as follows. Calcd: C = 53.02; H = 3.39; N = 14.72; found: C = 52.51; H = 3.69; N = 14.24. FT-IR (KBr): 3606 (s), 3445 (br), 3118 (w), 3079 (w), 1931 (w), 1695 (s), 1630 (m), 1591 (m), 1547 (m), 1473 (m), 1360 (s), 1189 (s), 1146 (m), 954 (s), 954 (s), 853 (s), 788 (s), 735 (s), 631 (s), 574 (m), 523 (s), 456 (w). Synthesis of [Cd2(Hstdb)(stdb)(8H-Ade)(Ade)]n (PNU-22). HAde (52 mg, 0.38 mmol), H2stdb (42 mg, 0.15 mmol), and Cd(NO3)2·4H2O (120 mg, 0.38 mmol) were dispersed in 7 mL of DMF−H2O (1:2.5), and the reaction mixture was stirred for ∼15 min and sealed in a 14 mL Teflon-lined autoclave, which was heated at 413 K for 96 h. After slow cooling of reaction mixture to the room temperature, a pale yellow crystalline material was obtained in 74% yield. The elemental analysis results for C42H30N10O8Cd2 (%) are as follows. Calcd: C = 49.09, H = 2.94, N = 13.63; found: C = 48.74, H = 3.23, N = 5.13. FT-IR (KBr): 3393 (w), 3319 (w), 3101 (m), 1931 (w), 1691 (w), 1665 (w), 1608 (m), 1543 (s), 1470 (w), 1368 (s), 1329 (m), 1268 (m), 1202 (s), 1168 (m), 954 (w), 858 (s), 788 (s), 705 (s), 626 (s), 570 (m), 509 (s), 461 (w). Procedure for CO2−Epoxide Cycloaddition Reaction. The solventless synthesis of the cyclic carbonates by the cycloaddition reaction of CO2 and a corresponding epoxide was performed with the requisite amount of activated MOF−catalyst (activated at 80 °C for 8 h) and cocatalyst, in a 25 mL stainless steel autoclave reactor equipped with a magnetic stirrer, at 400 rpm. The reaction vessel was pressurized with the predecided amount of CO2 to achieve the different temperature/time for each reaction. After the reaction, the reactor was cooled to less than 5 °C using an ice bath, and the excess CO2 was ventilated. The internal standard, that is, DCM/toluene, was added to the product, and the cyclic carbonate yields were analyzed

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RESULTS AND DISCUSSION Characterization of PNU-21 and PNU-22 Catalysts. Solvothermal reactions of M(NO3)2·xH2O (where M = Zn, Cd), the respective metal salt, adenine, and H2stdb in DMF and water, at 413 K, for 96 h, produced pale yellow crystalline materials: PNU-21 and PNU-22. The molar ratios of the reactants were carefully adjusted and optimized to obtain the pure bulk form of the materials. The pure catalytic material was characterized by various analytical techniques, such as SXRD, PXRD, thermogravimetric analysis (TGA), FT-IR, XPS, and SEM. Crystal and Molecular Structure of [Zn 2 (H 2 O)(stdb)2(5H-Ade)(9H-Ade)]n (PNU-21). Single-crystal X-ray diffraction unveiled that PNU-21 crystallizes in the triclinic crystal system with the P1 space group and that it features a two-dimensional (2D) coordination framework. The asymmetric unit of PNU-21 is composed of two crystallographically unique Zn(II) (Zn1 and Zn2), two molecules each of adenine in a neutral zwitterionic form, and completely deprotonated stdb along with a single coordinated water molecule. The coordination geometries around the respective metal centers of Zn1 and Zn2 were found to be distorted tetrahedral (Td) and trigonal bipyramidal (TBP), respectively (Figure 1a & Figure S1). The tetracoordination around Zn1 is provided by imidazole nitrogen atoms (N1 and N6) from different Hade moiety, and the O3 and O2 of the symmetrically disposed stdb C


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Inorganic Chemistry Scheme 1. Coordination Modes and Pronated Forms of stdb and Ade in PNU-21 and PNU-22

Figure 3. Intermolecular hydrogen-bonding interactions (O−H···O and N−H···O) between coordinated water molecule/adenine and the carboxylate of stdb ligands in PNU-21.

6.089 and 6.132 Å, respectively. Interestingly, another [Zn2(Hade)2]n zigzag chain aligned almost in a parallel manner with the first chain, in which a 5H-Ade heterocyclic ring with opposite orientation creates strong π···π interactions, allowing close proximity between the chains with the mean plane distance between the heterocyclic rings (3.34 Å). In addition to the mentioned π···π stacking, the parallel chains are further interlocked by coordination through the carboxylate oxygen atoms (O5 and O6) from one of the stdb ligands in a syn−anti fashion: the Zn2 metal centers generating a ribbon motif along the a-axis, with a dangling stdb moiety on either side. The Zn2···Zn2 distance between the parallel chain, linked via carboxylate oxygen atoms, is 4.472 Å, and the closest Zn1··· Zn1 distance is 8.609 Å. Parallelly aligned [Zn2(Hade)2]n zigzag chains, which are refurbished into ribbon motifs with unsaturated coordination on Zn1, are tethered from either end with the adjacent ribbon motif through the completely deprotonated second stdb ligand and are shaped into a 2D framework oriented along the ab-plane (Figure 2a). Thus, only one carboxylate group from the first stdb ligand is involved in the μ2-η1η1:η0η0 mode of coordination, in interlocking the parallel [Zn2(Hade)2]n strands, and the second stdb creates μ2η1η0:η1η0 (Scheme 1), in linking the inter ribbon motif, which generates the two-dimensional network. Topological analysis with the program TOPOS revealed that the two-dimensional sheets composed of Zn(II), adenine, and stdb ligands in PNU-

ligand, while the pentacoordination, with respect to Zn2, is provided by carboxylate oxygen atoms (O5 and O6) and imidazole nitrogen (N2 and N7), from different stdb/Hade ligands, respectively, and an oxygen atom (O9) from water. Five potential coordination sites, namely, two pyrimidine, two imidazolate nitrogen atoms, and the −NH2 group, are available in the case of the adenine moiety and can adopt neutral tautomeric88 and protonated forms89,90 upon coordination with metals. To engender a diverse network topology in the MOFs, adenine has a good tendency to shape supramolecular architectures alone or in combination with auxiliary ligands.88 Adenine molecules can acquire protonation during the self-assembly of the coordination networks in the combination of metal centers, including dicarboxylic acids.69 This is exemplified well in our previous report, where adenine undergoes protonation during self-assembly to generate a 2D network.73 The SXRD data of PNU-21 revealed that both the protonated adenine molecules (5H-Ade and 9H-Ade) exist as neutral entities with formal opposite charges on N2, N5 and N7, N9, respectively. The migration of protons from N5, N9 accelerates the coordination of the adenine molecules with alternate Zn1 and Zn2 metal centers, through imidazole nitrogen atoms (N1, N2 and N6, N7) to form a onedimensional (1D) infinite [Zn2Hade2]n zigzag chain along aaxis (Figure 2b). The Zn···Zn separations within the [Zn2(Ade)2]n motif, through a μ2-bridged adenine moiety for Zn1···5H-Ade···Zn2 and Zn1···9H-Ade···Zn2, are found to be D


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Inorganic Chemistry

Figure 4. (a) 3D network in PNU-22, (b) [Cd2(Ade)(8H-Ade)]n zigzag loop motif present in PNU-22, (c) O−H···N and N−H···O interactions between adenine and the carboxylic group of Hstdb ligands in PNU-22.

octahedral N3O3 coordination for Cd2 is governed by carboxylate oxygen atoms (O5, O6, and O7) from two independent dicarboxylates, and imidazole nitrogen atoms (N2, N7 and N10) from two different heterocycles. Cadmium metal centers are linked by an anionic form of adenine, through the deprotonation of an imidazole nitrogen (Ade), and the neutral form (8H-Ade) by migration of hydrogen with formal opposite charges on N7 and N8. As depicted in Figure S2, Ade is involved in a μ2-type coordination through the imidazole nitrogen atoms (N1 and N6) linking the adjacent Cd1 and Cd2, while 8H-Ade is making a μ3-type coordination, which involves both imidazole nitrogen atoms (N2 and N7) and additional pyrimidine nitrogen (N10), generating a [Cd2(Ade)(8H-Ade)]n zigzag loop motif. In the [Cd2(Ade)(8H-Ade)]n motif, the Cd1···Cd2 separation distance by the μ2-bridged Ade is 6.381, and the μ3-bridged 8H-Ade is 6.381 and 6.417 Å, respectively; the closest Cd1···Cd2 distance within the bilayer is 4.21 Å (Figure 4b). The heterocyclic rings of the 8H-Ade moiety create a very strong π···π stacking interaction, which is reflected in the mean plane distance between them (3.19 Å), in the formation of the bilayer. Two different stilbene dicarboxylic acid moieties (one with monoprotonated (Hstdb) and the other completely deprotonated (stdb)forms) are involved in bridging the [Cd2(Ade)(8H-Ade)]n zigzag loops, in a bid to generate the twodimensional network in PNU-22. Thus, [Cd2(Ade) 8H-Ade]n loops oriented along b-axis are pillared by a completely deprotonated stdb along c-axis, from either side, involving the μ3-η2η1:η1η0 mode of coordination between them, and Hstdb, as a hanging moiety, is involved in the μ2-η1η1:η0η0 mode of coordination between Cd1 within the loop holding free terminal carboxylic acid. The overall network topology in PNU-22 can be described as a 3,4,5-connected three-nodal network with point symbols {4.62.83}{42.6}2{43.64.83} (Figure 5). The hanging Hstdb moiety from the adjacent loops overlaps and is involved in the effective π···π stacking of the aromatic phenyl rings (3.67 A), and the free carboxylic acid (−COOH) creates O−H···N and N−H···O hydrogen-bonding interactions with 8H-Ade [O4−H4···N9: H4···N9 = 1.93; N8−H8A···O3: H8A···O3 = 2.15] (Figure 4c). In addition to these interactions H8B and amino hydrogen H5A of the adenine moiety create a bifurcated N−H···O interaction with the carboxylate oxygen, O7, of stdb, to stabilize the hydrogen-

21 formed a neutral 6-connected uninodal-type hexagonal 2D network topology with a point symbol {36.46.53} (Figure 2c). The packing of PNU-21 revealed that the 2D nets are oriented almost diagonally to the bc-plane (down a-axis), and they are involved in strong inter- and intramolecular hydrogenbonding interactions. The coordinated water molecule creates an intermolecular O−H···O interaction with the uncoordinated carboxylate oxygen, which links the adjacent 2D nets, thereby generating a hydrogen-bonded three-dimensional (3D) framework. Thus, H9c from the coordinated water atom as a donor interacts with the uncoordinated carboxylate oxygen (O8) of the dangling stdb, and the same oxygen (O8) is further involved as a bifurcated donor in the N−H···O interaction with H9A of 9H-Ade (Figure 3). The amino hydrogen atoms (H10A and H10B) of 9H-Ade create interand intramolecular N−H···O interactions with uncoordinated/ coordinated carboxylate oxygen atoms O7 and O3, respectively, of different stdb ligands. The migrated amino hydrogen, H5C, of 5H-Ade also creates a strong N−H···O interaction with the free carboxylate oxygen (O2) of the stdb linker. In addition to all the mentioned interactions, inter- and intramolecular C−H···O and C−H···N supramolecular interactions, facilitated between the adenine and stdb ligands, stabilize the 2D network in the crystal lattice. Details of all the pertinent hydrogen-bonding interactions with symmetry codes present in PNU-21 are given in Table S3. Crystal and Molecular Structure of [Cd2(Hstdb)(stdb)(8H-Ade)(Ade)]n (PNU-22). PNU-22 is composed of Cd(II) metal nodes linked by mixed ligands, and it represents a 3D framework (Figure 4a), which crystallizes in the triclinic space group P1. The asymmetric unit of PNU-22 consists of one molecule each of mono and completely deprotonated H2stdb (Hstdb and stdb), one molecule each of adeninate (Ade) in anionic and zwitterionic (8H-Ade) forms, along with two crystallographically independent Cd1 and Cd2 metal ions. In the 3D network, both metal ions adopted different coordination geometries: pentacoordinated Cd1 holds a distorted trigonal bipyramidal, and hexacoordinated Cd2 adopted a distorted octahedral geometry (Figure 1b & Figure S2). The distorted trigonal bipyramidal coordination of Cd1 is provided by carboxylate oxygen atoms (O1, O2, and O5) from three different dicarboxylates and imidazole nitrogen atoms (N1 and N6) from two heterocycles, while the distorted E


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Inorganic Chemistry

vibration modes, respectively. Furthermore, the XPS analytical technique was used to investigate the chemical state of both complexes. The inspection results of the XPS spectra of both catalysts are as follows (Figure 7 & Figure S6): four binding peaks around 284.6, 400, 532, and 1024.3/405.9 eV were assigned to C 1s, N 1s, O 1s, and Zn 2p/Cd 3d, respectively. The deconvoluted spectrum of C 1s spectra revealed three different peaks with similar binding energies 284.5/284.6, 286.2/285.6, and 289.4/287.6 eV, which were assigned to the CC, CO, and COO−/COOH functional groups in Zn(II)/Cd(II) catalysts, respectively.91 The N 1s spectrum fitted with three symmetrical peaks concentrated at 398.2, 397, 395.8 and 399.8, 399.7, 399.3, representing −NH3+, −NH2, and C−NC of the protonated Ade moieties in PNU-21 and PNU-22, respectively.92 The metal existences in both catalysts were confirmed by their Zn 2p1/2, Zn 2p3/2 and Cd 3d5/2, Cd 3d3/2 peaks, which appeared at 1043.5, 1020.4 and 412.8, 405.9 eV, respectively. The elemental compositions of both catalysts were assessed by CHN analysis, which is given in the synthesis section. The morphology and elemental distribution of the bulk catalysts were studied by FE-SEM and EDS, respectively. Both catalysts showed microcrystalline plates and agglomeration to spherical morphology, particularly in PNU22, and each element was distributed uniformly in both catalysts (Figure 8a−h & Figure S7). Moreover, the UV−vis absorbance spectra of both compounds were recorded in the solid state, at room temperature (Figure S8). The absorption spectra were observed in the range of 250−600 nm with absorption maxima at 386 and 381 nm for PNU-21 and PNU-22, respectively. The band gap energies of both complexes were calculated from the Kubelka−Munk (K-M) method, to be 2.91 and 2.93 eV for PNU-21 and PNU-22, respectively, which falls in the range of semiconductor materials (Figure S9). MOFs, which comprise d10 metals and conjugated organic linkers, can generate luminescence upon irradiation and are well-known in the field of luminescent materials for photonic applications.93,94 The fluorescence spectra of the free ligands and both complexes were also recorded at ambient temperature for comparative studies. The solid-state emission spectra of Ade and H2 stdb showed maximum emissions at 416 and 463 nm upon excitation at 295 and 330 nm, respectively. PNU-21 and PNU-22 contain d10 metal ions and mixed ligands (Ade and stdb); consequently, they exhibited a strong emission spectrum at 465 and 467 nm upon irradiation at 380 nm (Figure S10). The emission spectrum of both complexes can be attributed to the π*−π or π*−n transitions, which can be ascribed to a ligand-centered luminescence process based on the similarity

Figure 5. 3,4,5-Connected three-nodal network topology of PNU-22.

bonded supramolecular network. Details of all the H-bonding interactions with symmetry codes are provided in Table S3. PXRD, TGA, FT-IR, XPS, and SEM Analysis and Luminescence Properties. The bulk material of both catalytic materials was synthesized by doubling the reactant ratios, and the phase purities of the samples were confirmed by PXRD analysis. The experimental PXRD profiles of PNU-21 and PNU-22 are in good agreement with simulated patterns of SXRD, which clearly confirmed the phase purity of the bulk samples (Figure 6a & Figure S3). The thermal stabilities of both catalysts were established by TG analyses (Figure 6b & Figure S4). The coordinated water molecule of PNU-21 was expelled from the framework in the temperature range of 100− 250 °C, whereas the TGA profile of PNU-22 did not reveal any occluded solvent molecules, which may be due to its compact 3D structure. Being robust architectures, both catalysts showed high thermal stabilities up to 450 °C. The FT-IR analysis revealed the characteristic carbonyl asymmetric and symmetric stretching frequencies at 1695, 1591 cm−1 and at 1668, 1586 cm−1 for the carboxylate functional groups in PNU-21 and PNU-22, respectively (Figure S5). The distinct band at 1691 cm−1 is ascribed to the free carboxylic group (−COOH) in PNU-22. The stretching frequencies for the N−H group of adenine are in the range of 3393−3018 cm−1 in both catalysts. The broad band at 3445 cm−1 is attributed to the O−H stretching frequency of the coordinated water molecule in PNU-21. The medium and weak vibrational peaks in the fingerprint region at 523, 456 cm−1 for PNU-21 and at 509, 461 cm−1 for PNU-22 are assigned to the M−O and M−N

Figure 6. (a) Simulated and experimental PXRD patterns and (b) TGA profile of PNU-21. F


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Inorganic Chemistry

Figure 7. XPS spectra of PNU-21. (a) Full spectrum, (b) N 1s, (c) C 1s, and (d) Zn 2p spectrum.

Figure 8. (a) SEM images, EDS elemental mapping, and spectra of PNU-21 (a−d) and PNU-22 (e−h).

Table 1. Details of Acidic and Basic Sites in PNU-21 and PNU-22 acid/base sites concentration (mmol g−1 of catalyst)

acid/base strength temperature (°C) a

catalyst

temp at A

temp at B

PNU-21 PNU-22

228/231

403/406 418/430

b

temp at C 583/586 577/590

c

acidity/basicity at A

acidity/basicity at B

acidity/basicity at C

total acidity/basicity

2.039/0.695

11.446/10.498 16.109/14.743

9.484/8.538 6.174/5.817

22.97/19.7 22.3/20.56

A = Weak acidic/basic site (200−310 °C). bB = Medium acidic/basic site (310−500 °C). cC = Strong acidic/basic site (500−1000 °C).

a

between CO2 and free pyrimidine N atoms and −NH2 functional groups of MOF framework. Moreover, the acidic and basic sites in PNU-21 and PNU-22 were analyzed by NH3 and CO2 TPD analyses. The acid and base strength of these catalysts were described by three regions, namely, weak, medium, and strong acidic/basic sites, over the temperature ranges of 200−310, 310−500, and 500−1000 °C.95 As depicted in Figure S13 and Table 1, the weak to strong acidic and basic sites in both catalysts were found in the range of 180−600 °C. Different desorption peaks were observed for the binding capacity of NH3 for each catalyst. The weak to strong acidic sites observed are as follows: for PNU-21, at 228 °C (2.039 mmol g−1), 403 °C (11.446 mmol g−1), and 583 °C (9.484 mmol g−1). Medium and strong acidic sites were found at 418 °C (16.109 mmol g−1) and 577 °C (6.174 mmol g−1)

in the wavelength observed in the case of H2stdb (λem = 463 nm). Additionally, both complexes emitted a blue fluorescent color upon exposure to UV radiation at 250 and 365 nm (Figure S11). To study the surface areas of PNU-21 and PNU-22, gas sorption analysis was investigated for activated samples. Prior to gas analysis, as-synthesized samples were activated at 80 °C for 8 h. The BET analysis by N2 gas adsorption at 77 K revealed that both MOFs are type-III nonporous materials, and external surface areas were found to be 2.56 m2 g−1 for PNU21 and 1.5 m2 g−1 for PNU-22 (Figure S12a). CO2 adsorption of both MOFs reached the maximum uptakes up to 0.26 and 1.4 cm3 g−1 for PNU-21 and PNU-22 at 298 K, respectively (Figure S12b). Both MOFs showed good CO2 uptake compared to the N2 uptake probably due to the interaction G


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Inorganic Chemistry

in the ECH conversion, with an excellent selectivity of more than 99%, probably due to the combined role of the active metal centers in the MOF framework (Lewis acidic centers) and the nucleophilic bromide ion from TBAB (Table 2, entries 11 and 12). It has been observed, from our previous studies, that the bromide ion is the preferred nucleophile for creating a crucial synergistic effect with the metal ions in the coupling reaction of epoxide to CO2.96,97 We calculated turnover number (TON) and turnover frequency (TOF) of PNU-21 (160, 20 h−1) and PNU-22 (140, 17.5 h−1). The results are compared with other previously reported MOFs as shown in Table S4. They showed comparable or even higher catalytic activity than other previously reported MOFs. Figure 9 shows the effects of the reaction variables on the ECH carbonate synthesis, which employs both catalysts under semi-batch conditions, for understanding the optimal reaction conditions. As expected, the ECH conversion increased with an increase in the concentration of the PNU-21/PNU-22 catalysts (Figure 9a). We examined the catalyst concentration in the range of 0.2−1.0 mol % at 80 °C and 0.4 MPa CO2 pressure, for 8 h with a 0.3 mol % cocatalyst. Maximal ECH conversions of 96% and 85% were observed at a 0.6 mol % concentration of PNU21 and PNU-22 catalysts, respectively, and no further progress in the ECH conversion for the 0.8 and 1.0 mol % catalyst concentrations was observed. Temperature-dependent catalytic activity investigations of both the PNU-21 and PNU-22 catalysts are given in Figure 9b. Apparently, both catalysts facilitated low ECH conversions (especially PNU-22) at a low reaction temperature, while they provided good ECH conversions at 80 °C, for a reaction time of 8 h and 0.4 MPa CO2 pressure. A slight increment in the ECH conversion is observed at 100 °C for both catalysts. Thus, 0.6 mol % catalyst and 80 °C reaction temperature is identified as the optimal catalyst concentration and reaction temperature. To investigate the influence of CO2 pressure on the catalytic activity, reactions were performed in the range of 0.1−1.2 MPa CO2 pressure (Figure 9c). Both PNU-21 and PNU-22 catalysts are active at the atmospheric pressure of CO2 but with low ECH conversions under the reaction condition of 0.6 mol % catalyst concentration and at 80 °C for 8 h. The ECH conversion increased as the pressure increased from 0.1 to 0.4 MPa. A sudden rise in the ECH conversion is observed when the pressure reached 0.4 from 0.2 MPa. When the pressure was further raised to 1.2 MPa, the ECH conversion decreased slightly. A similar trend of pressure dependence was reported in our previous study.79 The decrease in the ECH conversion at elevated pressures is due to the dilution effect, which retards the interaction between the epoxide and catalyst. The effect of the reaction duration on the PNU-21/PNU-22-catalyzed reaction is depicted in Figure 9d. ECH conversion gradually increased within 4 to 8 h, and the maximal conversion was obtained at 8 h. It was found that a reaction duration of 8 h and 0.4 MPa CO2 pressure are needed to attain the maximum ECH conversion, at 80 °C, in the presence of 0.6 mol % PNU-21/ PNU-22 catalyst. Following the performances of these catalysts in the conversion of ECH to ECH carbonate, the efficiencies of both catalysts were further investigated over the other aliphatic cyclic/acyclic and aromatic epoxides. As shown in Table 3, both catalyst systems could convert all epoxides (propylene oxide, allyl glycidyl ether, and styrene oxide) to the corresponding cyclic carbonates effectively, under 0.4 MPa pressure, at 80 °C, and for 8 h, up to some extent; however,

for PNU-22. The stronger acidic sites were attributed to the coordinately unsaturated tetrahedral and trigonal bipyramidal Zn centers in PNU-21. In the same line, the basic sites were observed by the desorption peaks of CO2 at various temperatures for both catalysts. As usual, three desorption peaks are observed at 231 °C (0.695 mmol g−1), 406 °C (10.498 mmol g−1), and 586 °C (8.538 mmol g−1), which are indicative of weak to strong basic sites of PNU-21, and the medium and strong basic sites of PNU-22 were detected by the CO2 desorption peaks at 430 °C (14.743 mmol g−1) and 590 °C (5.817 mmol g−1), respectively. The supramolecular interaction between CO2, the free pyrimidine N atoms, and the −NH2 functional groups of adenine are responsible for binding the CO2 effectively. In addition to this, the free lone pairs on the O atoms of the carboxylate also enhanced the binding affinity toward CO2 in both catalysts. The Zn(II)/ Cd(II) metal centers in both catalysts can act as acidic sites to activate the reaction substrates; the nitrogen atoms of the adenine molecule can bind CO2 molecules to accelerate the formation of cyclic carbonates. Cycloaddition Reaction of Epoxide and CO2. The catalytic potential of PNU-21 and PNU-22 was evaluated in the coupling reaction of CO2, using epichlorohydrin (ECH) as the model epoxide under semibatch conditions. As is widely known, the formation of epichlorohydrin carbonate is not observed in the absence of a catalyst, under an employed reaction condition of 80 °C, and 0.4 MPa CO2 pressure for 8 h, with 25 mmol ECH (Table 2, entry 1). Under similar Table 2. Catalytic Activity Comparison of All Catalysts for the Cycloaddition Reaction of ECH and CO2a entry

catalyst

conversionb (%)

selectivityb (%)

1 2 3 4 5 6 7 8 9 10 11 12

none Zn(NO3)·6H2O Cd(NO3)2·4H2O H2stdb Ade Zn/H2stdb/Ade Cd/H2stdb/Ade TBAB PNU-21 PNU-22 PNU-21/TBAB PNU-22/TBAB

19 21 12 17 28 22 61 44 32 96 85

98 98 97 98 98 98 >99 >99 >99 >99 >99

a

Reaction conditions: epichlorohydrin (ECH) = 25 mmol, catalyst = 0.6 mol %, TBAB = 0.3 mol %, pressure = 0.4 MPa CO2, temperature = 80 °C, time = 8 h. bDetermined by GC.

conditions, neither metal precursors [Zn(NO3)2·6H2O and Cd(NO3)2·4H2O)] nor ligands (H2 stdb and Ade) separately yielded any noticeable conversion of ECH (Table 2, entries 2− 5). Meanwhile, the combination of the metal precursors and ligands afforded a slight increase in the ECH conversion but with less selectivity toward the ECH carbonate (Table 2, entries 6 and 7). An ECH carbonate yield of 61% (Table 2, entry 8) was provided by 0.3 mol % of the cocatalyst (tetrabutylammonium bromide (TBAB)). Both heterogeneous catalysts (PNU-21 and PNU-22) individually facilitated ECH conversions of 44% and 32%, respectively, along with more than 99% selectivity under the same reaction conditions (Table 2, entries 9 and 10). However, the PNU-21/TBAB and PNU22/TBAB binary catalyst systems manifested a tremendous rise H


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Inorganic Chemistry

Figure 9. Effect of reaction parameters for PNU-21/PNU-22 on the cycloaddition of ECH and CO2: (a) catalyst amount (80 °C, 8 h, 0.4 MPa PCO2), (b) reaction temperature (0.6 mol %, 8 h, 0.4 MPa PCO2), (c) pressure (0.6 mol %, 80 °C, 8 h), and (d) reaction time (0.6 mol %, 80 °C, 0.4 MPa PCO2).

toward the active sites of PNU-21 and PNU-22. The results clearly demonstrate that both amine-functionalized catalysts exhibit efficient heterogeneous catalytic performances in the cycloaddition of a variety of epoxides under moderate reaction conditions (small amount of catalyst [72 mg (0.6 mol %)] and 0.4 MPa CO2 pressure). In all the catalytic reactions, the cyclic carbonate yields were estimated by GC, and the purities were confirmed by 1H NMR analyses of the isolated products (Supporting Information). Moreover, the efficiencies of the catalysts were investigated by a recyclability test for the conversion of ECH to ECH carbonate. Both catalysts exhibited good conversions and selectivities up to five cycles without loss of their structural stabilities, which was confirmed by the PXRD analysis (Figure 10 & Figure S14a,b). Further, the heterogeneous nature of both catalysts was also confirmed by leaching test. The filtrates after five catalytic cycles showed a

Table 3. Syntheses of Cyclic Carbonates from Various Epoxides

a

Reaction conditions: epoxide = 25 mmol, catalyst = 0.6 mol %, TBAB = 0.3 mol %, pressure = 0.4 MPa CO2, temperature = 80 °C, time = 8 h. Yields are determined by GC with more than 99% of selectivity of corresponding cyclic carbonates.

cyclohexene oxide exhibited very low conversion with negligible yields (11 and 8%; Table 3, entry 6). The catalytic performance, which was clearly observed for epichlorohydrin, propylene oxide, and allyl glycidyl ether with good conversion rates in the cycloaddition reactions with CO2, may be due to the small size of epoxides, which can easily access the active sites (acidic and basic sites) on the surface of the catalysts, whereas low conversions were observed for styrene oxide and cyclohexene oxide, due to their bulky and steric hindrance

Figure 10. Catalyst reusability at 80 °C, for 8 h, with 0.4 MPa CO2 pressure, 0.6/0.3 mol % catalyst/cocatalyst, using 25 mmol of epichlorohydrin (>99% selectivity toward ECH carbonate). I


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Inorganic Chemistry Scheme 2. Proposed Mechanism for the Cycloaddition of ECH and CO2, Catalyzed by PNU-21

Figure 11. Relative free energy diagram of the PNU-21/TBAB-catalyzed cycloaddition of ECH and CO2. All the distances in the representative images of the intermediates and transition states are in angstroms. The hydrogen atoms are omitted here for clarity.

very small amount of Zn metal leaching of 0.065 mg mL−1 for PNU-21 and Cd leaching of 0.07 mg mL−1 for PNU-22, respectively. Thus, both the catalysts were proved to be suitable for cycloaddition reaction as heterogeneous catalysts. Cycloaddition Reaction Mechanism: DFT-Supported Study. The mechanism of the cycloaddition reaction between CO2 and ECH, catalyzed by PNU-21, is proposed based on previous reports, experimental shreds of evidence, and density functional theory (DFT) periodic calculations (Scheme 2). We

investigated theoretically the cycloaddition reaction mechanism by means of DFT. The mechanistic study of the cycloaddition of ECH to CO2 is performed in three pathways, namely, noncatalyzed, TBAB-catalyzed, and PNU-21/TBAB cocatalyzed. All the simulations were performed using the Gaussian 16 program98 with a meta-hybrid GGA functional M06.99 We used the mixed basis set 6-31G(d) for H, C, N, O, and Cl, whereas the heavier atoms (Zn and Br) were treated with a “double-ξ” quality basis set consisting of Hay and J


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Inorganic Chemistry Wadt’s effective core potentials (LanL2DZ ECP).100,101 The method and level of theory used in this study were successfully applied to other similar thermochemical investigations.102 To validate the method and basis set, we investigated the mechanism of the noncatalyzed cycloaddition of ECH to CO2. The energy barrier for the rate-determining step (RDS), that is, epoxide ring opening, is 61.96 kcal/mol (Figure S15), which is in very good agreement with the earlier reports (55− 63 kcal/mol).103,104 The geometric optimization was followed by vibrational frequency analysis, and all the reactants, intermediates (Int), and products were confirmed with no imaginary frequencies, whereas the transition states (TS) were confirmed with only one imaginary frequency. The potential energy profile of the TBAB-catalyzed cycloaddition of ECH to CO2 is depicted in Figure S16. The relative energy of ECH is considered as 0.0 kcal/mol and is lowered to −5.74 kcal/mol upon the addition of TBAB. In the next step, the bromide ion of TBAB attacks the β-carbon atom of ECH, resulting in the formation of a bromo-substituted intermediate (Int-2) through epoxide ring opening. The activation energy for this ring-opening step, that is, the RDS, is 39.60 kcal/mol. The energy barrier for the CO2 addition (TS2) is found to be very low (0.40 kcal/mol), and thereafter, an intermediate (Int-4) is formed with a relative energy of −9.40 kcal/mol. With ring closure (Int-5), the energy reduces to −20.91 kcal/mol, and the final product is formed with a relative energy of −14.63 kcal/mol. In the PNU-21/TBAB cocatalyzed cycloaddition, we modeled a simplified PNU-21 MOF structure with 73 atoms (Figure S17) used in the DFT simulations. For computational simplicity, we used a simple Br− ion to represent the TBAB, which was involved in the ECH ring opening. Figure 11 shows the potential energy surface profile along the reaction coordinate with the representative structures. The energy of ECH is reduced by 6.35 kcal/mol with the addition of PNU-21 (Int-1), and it further drops to −34.96 kcal/mol, due to the close proximity of Br− ion from TBAB (Int-2). The Br− ion interacts with the β-carbon of the ECH at a distance of 3.42 Å in Int-2. The ring opening of ECH by the nucleophilic attack of Br− ion is believed to be a crucial step (RDS) in this cycloaddition process, which is associated with an energy barrier of 7.58 kcal/mol (TS1). The ring-opening step resulted in a bromo-substituted ECH (Int-3) with a relative energy of −35.34 kcal/mol. Another transition state (TS2) is formed with the addition of CO2, with an activation energy of 4.12 kcal/mol. The intermediate (Int-5) is formed after the attack of CO2 on the negatively charged oxygen atom of the bromosubstituted ECH, and it is located at −48.31 kcal/mol. After the formation of Int-5, the ring-closure step (Int-6) takes place at a relative energy of −45.77 kcal/mol. Finally, the ECH carbonate is formed at the relative energy of −14.68 kcal/mol, as the final product on the potential energy surface. The mechanistic study of the cycloaddition of ECH to CO2 reveals that the activation energy for the RDS in the PNU-21/TBAB cocatalyzed process (7.58 kcal/mol) is lower than that of the TBAB-catalyzed (39.60 kcal/mol) and noncatalyzed processes (61.96 kcal/mol). On the basis of the single-crystal structure and possible intermediates, and transition states and pathways from DFT calculations, we proposed a detailed catalytic reaction mechanism for the cycloaddition reaction of ECH and CO2, to form ECH carbonate, catalyzed by PNU-21 and bromide anions of TBAB (Scheme 2). As evidenced by SXRD, PNU-21

possesses two types of unsaturated Zn metal centers: one is Zn1 (Td) and the other is Zn2 (TBP). According to the TPD results, PNU-21 contains high volumes of acid−base sites, owing to their unsaturated metal centers, −NH2 functionalization, and other basic N atoms. In the reaction mechanism, the epoxide (ECH) first binds with an unsaturated Lewis acidic Zn site through the oxygen atom of epoxide. The active Br− anion of TBAB attacks the less-hindered carbon of the epoxide to open three-membered epoxide rings. The CO2 polarized by one of the −NH2 groups of Ade molecules interacts with an oxygen anion of the opened epoxy ring to form an alkyl carbonate anion, and a consequent ring-closure step results in the corresponding cyclic carbonate. The catalyst regenerates as the product ECH carbonate detaches from the tetrahedral Zn metal center by the addition of another new ECH molecule. It could be assumed that the synergetic effect of the unsaturated Zn metal centers and basic N atoms in PNU-21 provided the active sites for the reactive substrates to facilitate the cycloaddition of epoxides and CO2 for the formation of cyclic carbonates.

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CONCLUSION Two adenine-based Zn(II)/Cd(II)MOFs were successfully synthesized and characterized by SXRD and other analytical techniques. SXRD revealed that the PNU-21 and PNU-22 structures have 2D and 3D rigid and robust architectures, respectively, with coordinately unsaturated metal environments. Both MOFs showed high thermal stabilities up to 450 °C. The coordinatively unsaturated Zn and Cd metal centers and basic N atoms in both MOFs showed potential acid−base heterogeneous binary catalytic efficiencies for the synthesis of cyclic carbonates by the cycloaddition of epoxide and CO2. Both MOF materials exhibited good catalytic performances at a low pressure and moderate catalyst amount, that is, PNU-21 achieved maximum conversion with 96% of ECH conversion, whereas PNU-22 achieved a lower conversion (85%). The reaction parameters such as the amount of catalyst, temperature, pressure, and time were systematically investigated and optimized to obtain good conversion and yield of the corresponding cyclic carbonates from CO2 and epoxide in high selectivity. Both catalysts showed good recyclability up to five cycles while maintaining the structural integrity of their frameworks. Because of their binary catalytic efficiency, and by the synergistic effect of unsaturated metal centers activating the epoxides and basic nitrogen sites on the adenine moiety, the binding of CO 2 in both MOFs is supported, which demonstrates the efficient catalytic conversion by cycloaddition of CO2 and epoxides to cyclic carbonates in good yields and reusability.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00814. Crystal structure diagrams of PNU-21/PNU-22, PXRD pattern, FT-IR spectra, XPS spectra, EDS mapping, UV, bandgap energy profiles, PL spectra, digital images under UV light, TPD spectra, PXRD profiles of recycled catalyst, N2/CO2 gas adsorption isotherms and crystallographic information tables of both MOFs, and NMR spectra of the cyclic carbonates (PDF) K


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CCDC 1895565−1895566 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge 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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (D.-W.P.) *E-mail: [email protected]. (E.S.) ORCID

Yadagiri Rachuri: 0000-0003-2979-2876 Ramesh Kumar Chitumalla: 0000-0002-9523-7056 Joonkyung Jang: 0000-0001-9028-0605 Youngson Choe: 0000-0001-9536-113X Eringathodi Suresh: 0000-0002-1934-6832 Dae-Won Park: 0000-0003-4668-4906 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Research Foundation of Korea (NRF) through Basic Research Program (2019-057644), Brain Korea 21 Plus Program, and also the Korea Research Fellowship Program through the NRF, funded by the Ministry of Science and ICT (2016H1D3A1936765). We gratefully acknowledge the analytical support for the SXRD technique by AESD&CIF of CSIR-CSMCRI.

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