Thermally Induced Single-Crystal-to-Single-Crystal Transformation

Article pubs.acs.org/crystal

Thermally Induced Single-Crystal-to-Single-Crystal Transformation and Heterogeneous Catalysts for Epoxidation Reaction of Co(II) Based Metal−Organic Frameworks Containing 1,4-Phenylenediacetic Acid Jintana Othong, Jaursup Boonmak,* Jintae Ha, Somying Leelasubcharoen, and Sujittra Youngme Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand S Supporting Information *

ABSTRACT: Four new metal−organic frameworks (MOFs) based on 1,4-phenylenediacetic acid (1,4-H 2 phda), [Co(1,4-phda)(4,4′-bpa)] (1), [Co(1,4-phda)(4,4′-bpp)] (2), [Co(1,4-phda)(4,4′-bpa)(H2O)2] (3), and [Co(1,4-phda)(4,4′-bpa)] (3a) (4,4′-bpa = 1,2-bis(4-pyridyl)ethane and 4,4′-bpp = 1,3-bis(4-pyridyl)propane) were successfully synthesized. The various synthetic methods play an important role in the formation of diverse structural frameworks. Compound 1 shows a 3D framework, while 2, 3, and 3a exhibit 2D layered MOFs with different architectures. The irreversible thermally induced single-crystal-to-single-crystal transformation with chromotropism from 3 to 3a was observed, which was established by the breakage and reformation of coordination bonds around Co(II) centers. The orientation of coordinated 4,4′-bpa ligand and conformation of 1,4-phda ligand play a key role on pore opening in 3 and pore closing in 3a. Furthermore, the functional properties as heterogeneous catalysts of these MOFs including epoxidation of alkenes and photocatalytic performance for MB degradation have been investigated. The heterogeneous catalytic properties of 1, 2, and 3a exhibit high catalytic activity with good catalyst stability.

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INTRODUCTION Metal−organic frameworks (MOFs) are well-known to have diverse intriguing topologies with tunable porous characteristics. They also are utilized as molecular material that can exhibit flexibility induced by physical and chemical stimuli.1,2 Especially, such flexibility involves ligand conformational changes and, most drastically, cleavage and formation of the coordination bond. In this work, single-crystal-to-single-crystal (SC-SC) transformations in MOFs with replacement of coordination water molecules with other organic linkers under thermal condition may change the structural architecture and the physical and chemical properties.3−5 Additionally, MOFs are of great interest as they have potential applications in gas storage,6−8 heterogeneous catalysts,9−11 magnetism,12−14 luminescence,15−17 and photocatalysis.18−20 Nowadays, the usage of MOFs as catalysts has becomes a hot topic.21 Comparing the traditional homogeneous catalyst, MOFs as heterogeneous catalysts are superior because of easy recovery and separation and disposal of spent catalyst.22 Among the recent applications of MOFs as catalysts, the epoxidation of alkenes is the key chemical process in synthetic organic chemistry and the chemical industry for generating products such as plasticizer, surfactants, paints, cosmetics, epoxy resins, and pharmaceuticals.23−25 Especially, styrene oxide acts as a raw material leading to the formation of phenethyl alcohol used in perfumes and as a chemical intermediate for the treatment of fibers and textiles.26 In a traditional catalytic procedure, oxidizing agents (MnO2 or CrO3) used for epoxidation of alkenes are pollutants. Thus, in recent years, the © XXXX American Chemical Society

development of greener processes using organic peroxide (R-OOH; R = alkyl chain), such as tert-butyl hydroperoxide (t-BuOOH) and hydrogen peroxide (H2O2), are most popular due to environmental friendliness. Moreover, it is well-known that the transition metal (Co, Ti, Mn, V, and Mo) ions act as active species for high selectivity of the epoxidation reaction.27−31 In this work we focus on 1,4-phenylenediacetic acid (H2phda) as a dicarboxylate ligand based on following considerations: (i) H2phda ligand may show a variety of coordination modes and conformations. (ii) Strong coordination of carboxylates leads to good thermal stability. (iii) The flexibility of H2phda ligands create changeable frameworks via SC-SC transformation. In addition, 4,4′-bpa and 4,4′-bpp are used as coligands for dimensional extension. Phenyl and pyridyl groups of H2phda and N-donor linkers can stabilize the chargetransferred state. Herein, we report four novel MOFs, namely, [Co(1,4-phda)(4,4′-bpa)] (1), [Co(1,4-phda)(4,4′-bpp)] (2), [Co(1,4-phda)(4,4′-bpa)(H2O)2] (3), and [Co(1,4-phda)(4,4′-bpa)] (3a). X-ray structural determination shows that 1 exhibits a 3D coordination framework with pcu α-Po topology while the others display 2D coordination frameworks with different structural architectures (Scheme 1). The flexibility of linker, preparative method, and temperature for the synthesis play an important role Received: December 8, 2016 Revised: February 15, 2017

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DOI: 10.1021/acs.cgd.6b01788 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Scheme 1. Synthetic Details of All Compounds Reported in This Paper

CoC22H20N2O4: C, 60.69; H, 4.60; N, 6.44%. Found: C, 60.75; H, 4.63; N, 6.58%. FT-IR (KBr pellet cm−1): νas(OCO) = 1611 and 1567 cm−1, νs(OCO) = 1426 and 1399 cm−1, δ(CN)pyridine ring = 830 cm−1. Preparation of [Co(1,4-phda)(4,4′-bpp)] (2). The same solvothermal condition of 1 was used for synthesis of 2. 1,3-Bis(4pyridyl)propane (4,4′-bpp) (0.5 mmol, 99 mg) was used instead of 4,4′-bpa. The mole ratio of metal salt:4,4′-bpp:1,4-H2phda:KOH of 3:5:3:6 was used. Purple block crystals of 2 were obtained in 32% yield (43 mg) based on metal salt. Anal. Calcd for CoC23H22N2O4: C, 61.47; H, 4.90; N, 6.24%. Found: C, 61.58; H, 4.96; N, 6.33%. FT-IR (KBr pellet cm−1): νas(OCO) = 1613 and 1566 cm−1, νs(OCO) = 1425 and 1406 cm−1, δ(CN)pyridine ring = 814 cm−1. Preparation of [Co(1,4-phda)(4,4′-bpa)(H2O)2] (3). The solvent mixture of water and N,N-dimethylformamide (2 mL, 1:1 v/v) was layered over an aqueous solution (2 mL) of Co(NO3)2·4H2O (0.1 mmol, 31 mg) in a glass vial (8 mL). 1,4-Phenylenediacetic acid (1,4-H2phda) (0.1 mmol, 19 mg), 1,2-bis(4-pyridyl)ethane (4,4′-bpa) (0.1 mmol, 19 mg), and KOH (0.2 mmol, 13 mg) were dissolved in a solution of water and methanol (3 mL, 1:2 v/v) and stirred well for 10 min. Then, this solution was slowly layered on the previous solution containing the metal salt. The vial was sealed at room temperature. After 1 week, pink block crystals were obtained in 47% yield (22 mg) based on metal salt. Anal. Calcd for CoC22H24N2O6: C, 56.01; H, 5.09; N, 5.94%. Found: C, 55.75; H, 5.03; N, 5.87%. FT-IR (KBr pellet cm−1): νas(OH) = 3351 cm−1, νas(OCO) = 1612 and 1571 cm−1, νs(OCO) = 1419 and 1377 cm−1, δ(CN)pyridine ring = 840 cm−1. Preparation of [Co(1,4-phda)(4,4′-bpa)] (3a). Red−violet block crystals of 3a were synthesized by heating compound 3 at 80 °C for 20 min. Anal. Calcd for CoC22H20N2O4: C, 60.69; H, 4.60; N, 6.44%. Found: C, 60.71; H, 4.65; N, 6.53%. FT-IR (KBr pellet cm−1): νas(OCO) = 1603 and 1571 cm−1, νs(OCO) = 1428 and 1394 cm−1, δ(CN)pyridine ring = 831 cm−1. X-ray Crystallography. The crystallographic data were measured on Bruker D8 Quest PHOTON100 CMOS detector with graphitemonochromated Mo Kα radiation using the APEX2 program.32 The diffraction data were integrated using SAINT,33 which was also applied for corrections to Lorentz and polarization effects while the absorption correction was performed with the SADABS program.34 The structure solutions were achieved by direct methods. All non-hydrogen atoms were anisotropically refined by full-matrix least-squares on F2 using the

in the structural variation. In addition, the single-crystal to singlecrystal transformation from 3 to 3a indicates that the breakage and formation of coordinate bonds contribute to pore opening and closing for 3 and 3a, respectively. Moreover, compounds 1, 2, and 3a exhibit highly efficient catalytic properties for the epoxidation of alkenes. In addition, compound 1 displays excellent photodegradation of methylene blue (MB) under UV irradiation.

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EXPERIMENTAL SECTION

Materials. All chemicals were purchased commercially for synthesis and employed without further purification. The metal salts, 1,4-phenylenediacetic acid, 1,2-bis(4-pyridyl)ethane, and 1,3-bis(4pyridyl)propane were obtained from Sigma-Aldrich and Alfa Aesar. Physical Measurements. FT-IR spectra (KBr pellets) were recorded on a PerkinElmer Spectrum One FT-IR spectrophotometer in the region 4000−450 cm−1. CHN analyses were carried out on a PerkinElmer PE 2400CHNS analyzer. UV−vis diffuse reflectance spectra were measured on a PerkinElmer Lambda2S spectrophotometer, in the range of 400−1100 nm in the solid state. The UV−vis spectra for the dye solution were recorded on a Shimadzu UV 2450 spectrometer. Thermogravimetric analysis (TGA) measurements were determined on TG-DTA 2010S MAC apparatus under N2 atmosphere in the temperature range 40−800 °C with a heating rate of 10 °C min−1. Powder X-ray diffraction (PXRD) patterns were collected on a Bruker D8 ADVANCE diffractometer with monochromatic Cu Kα radiation, and the 2θ range of 5−50°. Photocatalytic experiments: 400 W xenon lamp (λ = 365 nm) was used for UV light source. The epoxide products from catalytic reaction were detected on an HP Agilent 6890 gas chromatograph with HP-5 capillary column (phenylmethylsiloxane 30 m × 320 μm × 0.25 μm) and a FID detector. The leaching of metal was carried out on ICP-AES PerkinElmer Optima 3300 DV. Preparation of [Co(1,4-phda)(4,4′-bpa)] (1). A mixture of Co(NO3)2·6H2O (0.1 mmol, 31 mg), 1,2-bis(4-pyridyl)ethane (4,4′-bpa) (0.1 mmol, 18 mg), 1,4-H2phda (0.1 mmol, 20 mg), and KOH (0.2 mmol, 12 mg) in the solutions of water, methanol, and N,N-dimethylformamide (8 mL, 4:3:1 v/v) was sealed in a Teflon lined stainless steel container (15 mL) and heated at 125 °C for 1 day. After cooling to room temperature, blue-violet crystals of 1 were obtained in 63% yield (29 mg) based on metal salt. Anal. Calcd for B



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Table 1. Crystallographic Data for Compounds 1−3a

a

compound

1

2

3

3a

formula molecular weight T (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) μ (Mo Kα) (mm−1) data collected unique data (Rint) R1a/wR2b [I > 2σ(I)] R1a/wR2b [all data] GOF max/min electron density (e Å−3)

Co C22 H20 N2 O4 435.33 293(2) monoclinic P21/n 10.1553(3) 14.3751(5) 13.5043(5) 90 103.102(1) 90 1920.09(11) 4 1.506 0.926 4778 3707(0.0338) 0.0376/0.0801 0.0590/0.0874 1.030 0.383/−0.264

Co C23 H22 N2 O4 449.36 293(2) monoclinic P21/c 10.0616(3) 22.0955(8) 10.4369(4) 90 117.599(1) 90 2056.27(13) 4 1.452 0.867 5096 4047(0.0224) 0.0359/0.0830 0.0521/0.0905 1.036 0.352/−0.208

Co C22 H24 N2 O6 471.36 293(2) triclinic P1̅ 5.6342(3) 8.9463(4) 10.8036(5) 92.714(3) 97.603(3) 90.490(3) 539.11(5) 1 1.452 0.837 2665 1965(0.0732) 0.0550/0.1126 0.0910/0.1255 1.061 0.640/−0.527

Co C22 H20 N2 O4 435.33 293(2) triclinic P1̅ 10.1127(8) 10.3263(7) 11.0903(8) 62.716(2) 70.066(2) 80.067(2) 967.43(12) 2 1.495 0.919 3959 2998(0.0458) 0.0406/0.0740 0.0688/0.0821 1.041 0.422/−0.316

R = ∑∥oF|−|cF∥/∑|oF|. bRw = {∑[w(|oF|−|cF|)/2/[∑[w|oF|2]}1/2.

SHELXTL software.35 The H atoms of all coordinated water molecules in 3, which were found via difference Fourier maps, were then restrained at fixed positions and isotropically refined while the other H atoms were placed in calculated positions and isotropically refined. The crystallographic data and selected bond lengths and angles for all compounds are summarized in Tables 1 and S1. Catalytic Epoxidation. The epoxidation reactions were performed in a 50 mL two-necked flask that was connected with a reflux condenser. The catalysts (10 mg), acetonitrile (12 mL) as solvent, and alkenes (10 mmol) as substrates were mixed and stirred at 300 rpm. The mixture was heated to 70 °C and then tert-butyl hydroperoxide (14.4 mmol) was added to the reaction which was carried out in air. The products were quantitatively identified by gas chromatography. After the reaction, the filtered MOF catalysts were reused by rinsing with acetone and drying in vacuum. The next cycles were performed in the same reaction condition. The heterogeneity of the epoxidation reaction was examined by filtration of catalyst after a reaction time of 5 h. Photocatalytic Experiments. The photodegradation was carried out in a typical process. 40 mg of 1 was added to 80 mL of MB (40 ppm) solution and continuously stirred for about 30 min in the dark. The 400 W xenon lamp was used as the UV source for photodegradation in the mixture. To separate the catalyst, the reaction solution was centrifuged and a sample solution (3 mL) was detected at the different times (0, 15, 30, 45, 60, 75, 90 min, respectively). After filtration, the concentration of MB was detected by the UV−vis spectrophotometry (λmax 665 nm).

Co···Co distance of 4.579(5) Å. Each SBU-1 is connected via μ3-η1:η1:η1:η1 bridging mode of cis-1,4-phda generating a 2D sheet (Figure 1b). Moreover, these layers are gathered by μ2-4,4′-bpa ligands with Co···Co distance of 13.504(6) Å producing a 3D coordination framework. The network is simplified to a 6-connected uninodal pcu α-Po primitive cubic with point symbol {412.63} (Figure 1c and d). The two pyridine rings of 4,4′-bpa are inclined to each other with the dihedral angle of 13.70° and −CCHCHC− torsion angle of 172.96°. Crystal Structure of [Co(1,4-phda)(4,4′-bpp)] (2). Compound 2 crystallizes in the monoclinic, P21/c. The asymmetric unit consists of a Co(II) ion, a 1,4-phda, and a 4,4′-bpp (Figure S1). Each Co(II) ion is six-coordinate, surrounded by four oxygen atoms from three different 1,4-phda [Co−O of 2.004(2)−2.235(2) Å] and two nitrogen atoms from two different 4,4′-bpp with Co−N of 2.161(2) and 2.153(2) Å, forming a CoO4N2 distorted octahedral geometry. The SBU-2 in 2 is formed in a similar manner as the SBU-1 in 1 with the Co···Co distance of 4.159(5) Å (Type I, Scheme 2). In contrast, each SBU-2 is double-bridged by cis-1,4-phda in μ3-η1:η1:η1:η1 bridging mode to form 1D ladder-chains along the c axis (Figure 2a). Moreover, each chain is connected by μ2-4,4′-bpp ligands generating a two-dimensional (2D) network with 6-connected hxl topology and point symbol {36.46.53} (Figure 2b,c). The nearest interlayer intermetallic distance is 10.062(5) Å. The dihedral angle between the two planar rings of pyridines is 77.74°, which is larger than that of 1 as a result of the greater flexibility of the 4,4′-bpp than 4,4′-bpa. Moreover, C10H··· πcentroid (benzene ring) is perceived between the hydrogen atom of 4,4′-bpa and the aromatic ring of 1,4-phda with separation of 2.803 Å assisting the construction of the overall supramolecular framework of 2 (Figure S2 and Table S2). Crystal Structure of [Co(1,4-phda)(4,4′-bpa)(H2O)2] (3). Compound 3 crystallizes in the triclinic, P1̅. The asymmetric unit of compound 3 contains a Co(II) center located on an inversion center, a half of 4,4′-bpa, a half of dianionic 1,4-phda, and one coordinated water (Figure S3). Each Co(II) center

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RESULTS AND DISCUSSION Crystal Structure of [Co(1,4-phda)(4,4′-bpa)] (1). Compound 1 crystallizes in the monoclinic, P21/n. The asymmetric unit of 1 comprises a Co(II) ion, a 1,4-phda dianionic ligand, and one 4,4′-bpa (Figure 1a). Each Co(II) is a distorted octahedral CoO4N2 chromophore coordinated with four oxygen atoms from different 1,4-phda ligands [Co−O lengths of 2.015(2)−2.215(2) Å] and two nitrogen atoms from different 4,4′-bpa ligands [Co−N lengths of 2.151(2) and 2.144(2) Å]. In 1 the dinuclear unit (SBU-1) is formed by μ2-η1:η1-syn-anti bridging mode of cis-1,4-phda connecting between Co(II) centers (Type I, Scheme 2) with the C



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Figure 1. (a) Asymmetric unit of 1 with ellipsoids drawn at the 50% probability level. Symmetry code: (i) x, y, −1+z. (b) 2D framework of 1 formed by 1,4-phda ligands. (c) 3D framework of 1 formed by μ2-4,4′-bpa along the a axis. The CoO4N2 chromophore is shown as a purple polygon. (d) View of the α-Po topology.

structure is stabilized by intramolecular H-bonding between H atom of water molecule and unbound carboxyl oxygen atom of 1,4-phda (Figure S4 and Table S3). In addition, the 3D supramolecular framework of 3 is stabilized by interlayer H-bonding between the H atom of 4,4′-bpa and the coordinated carboxylic oxygen of 1,4-phda and the H-bond between H atom of coordinated water and the coordinated carboxylic oxygen of 1,4-phda (Figure S5 and Table S3). Syntheses and Structural Discussion. Four MOFs using flexible ligands as 1,4-H2phda, 4,4′-bpa, and 4,4′-bpp were successfully synthesized and structurally characterized (Scheme 1). The multicarboxylate ligands are known to tune the construction of MOFs with specific structural architecture. Although flexibility and coordinating groups of linkers have an influence on the structure of the compound, the synthetic method, and the temperature for the synthesis are also an important factor for assembling the desired MOFs. The remarkable divergence between the frameworks of 1, 3, and 3a denotes the major influence of the synthetic method and temperature. With the same reaction condition, changing the diffusion technique to a solvothermal technique at 125 °C causes the variation of dimensionality from the 2D layer in 3 to a 3D framework in 1, and coordination modes of 1,4-phda ligand are changed from type II in 3 to type I in 1. Hence the temperature effect and synthetic method play an important role in different structures in 3 and 1. However, with the same solvothermal technique at 125 °C, using the more flexible 4,4′-bpp linker instead of 4,4′-bpa leads to different structure and topology of compound 2 as compared to 1. 3a was prepared by heating the single crystals

Scheme 2. Coordination Modes of 1,4-phda Ligand in 1−3a: μ3-η1:η1:η1:η1 Bridging Mode (I); μ2-η1:η1 Bridging Mode (II)

adopts a distorted octahedral CoO4N2 chromophore (Figure 3a), with two carboxylate oxygens from different 1,4-phda and two coordinated water oxygens [Co−O distances of 2.105(2)− 2.116(2) Å] occupying the equatorial position, and two nitrogen atoms (N1, N1A) from different 4,4′-bpa occupying the axial sites [Co−N distances of 2.159(2) Å]. The Co(II) ions are bridged by trans-1,4-phda via carboxylate groups in μ2-η1:η1 bridging mode (Type II, Scheme 2), generating 1D-zigzag chains along the c axis. Each zigzag chain is further connected by μ2-4,4′-bpa ligands generating a two-dimensional (2D) network with the shortest interlayer intermetallic distance of 5.634(3) Å (Figure 3b). The network can be described as a four-connected uninodal sql topological type with point symbol 4 4 .6 2 (Figure 3c). Both pyridine rings of 4,4′-bpa are completely coplanar with torsion angle of 180° as the methanediyl group (−CH2−) in 4,4′-bpa lies on an inversion center. Moreover, the D



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Figure 2. (a) 1D chain of 2. (b) 2D layer formed by 1,4-phda and 4,4′-bpp. The CoO4N2 chromophore is shown as the purple polygon. (c) View of the hxl topology featuring a 6-connected uninodal net.

Figure 3. (a) Coordination environment of the Co(II) of 3 with ellipsoids drawn at the 50% probability level. Symmetry code: (i) 1-x,1-y,-z. (b) 2D network of 3. CoO4N2 chromophore is shown as the purple polygon. (c) View of the sql topology with a 4-connected uninodal net. E



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Figure 4. Water-induced irreversible SC-SC transformation from 3 → 3a showing the coordination environments around Co(II) ions for 3 and 3a (upper). The space filling models of the porous layer of 3 and nonporous layer of 3a with color change (lower).

dehydration process, single crystals of 3 were heated at 80 °C (20 min) and the color altered from pink to red−violet. The dehydrated single crystals of [Co(1,4-phda)(4,4′-bpa)] (3a) which are suitable for structural X-ray analysis were achieved (Figure 4). The dehydrated 3a crystallizes in the triclinic, P1̅ (Table 1). The significant change from 3 to 3a is the dimerization between Co(II) ions within a layer of 2D sheet of 3, generating the different 2D architectures. Each central Co(II) in 3a adopts a six-coordinate structure by four oxygens from three different 1,4-phda [Co−O distances of 2.015(2)− 2.215(2) Å] and two nitrogen atoms from different 4,4′-bpa [Co−N distances of 2.144(2) and 2.151(2) Å], generating octahedral CoO4N2 chromophore. The adjacent Co(II) ions are doubly bridged by carboxylate groups of different 1,4-phda with Co···Co distance of 4.579(5) Å (Figure 5a) forming a dimeric Co(II) unit (SBU-3a) that is in a similar manner to SBU-1. The dimeric units are further connected by cis-1,4-phda and μ2-4,4′-bpa leading to a 2D layer with the same sql topology. The nearest interlayer intermetallic separation is 11.090(10) Å (Figure 5b). The 2D networks are further connected by πcentroid···πcentroid between pyridine rings of 4,4′-bpa with distances of 3.972 Å and C2H··· πcentroid (pyridine ring) with distance of 2.861 Å (Figure 5b and Table S2). In the comparison with 3, the vacant sites of Co(II) ions caused by the loss of two coordination waters around octahedral Co(II) ions are filled by the unbound carboxylic oxygens from 1,4-phda in 3a (Figure 4). The irreversible thermally induced single-crystal-to-single crystal transformation involving broken and reformed bonds is seldom observed.36 The μ2-η1:η1 bridging mode (Type II, Scheme 2) in

of 3 at 80 °C and the color altered from pink to red−violet. During the thermally induced elimination of two coordinated water molecules in 3, the unbound oxygen atoms of the carboxylic group begin to interact with the cobalt(II) vacant sites to generate SBU-1 instead of SBU-2 in 3 (coordination modes: type II in 3 and type I in 3a) (Scheme 2). By comparison of the structure of 2 with 1, 3, and 3a, a given SBU unit is connected to four neighboring SBUs by the 4,4′-bpp bridges, whereas that of 1, 3, and 3a is extended by the 4,4′-bpa bridges to two adjacent SBUs, as a result of the greater flexibility of the 4,4′-bpp ligand than that of the 4,4′-bpa ligand. In other words, these effects play a pivotal role in the tunability of the structural architectures, topology, dimensionality, and SBU framework formation. Thermal Stabilities and Powder X-ray Diffraction. The thermal stability of 1−3 was investigated by TG analysis under N2 atmosphere in the range of temperature 40−800 °C (Figure S6). Compounds 1 and 2 are stable up to ca. 310 °C and ca. 250 °C, respectively, and then the structures start losing residues. Compound 3 exhibits the loss of two coordinated water molecules in the region of 40−80 °C (found 7.72%, calc.7.64%), and then the residues begin to decompose around 295 °C. The pure phase of the as-synthesized 1−3 is supported by powder X-ray diffraction (PXRD) (Figure S7). Irreversible Thermally Induced Single-Crystal-to-Single-Crystal Transformation with Chromotropism of [Co(1,4-phda)(4,4′-bpa)(H2O)2] (3) to [Co(1,4-phda)(4,4′-bpa)] (3a). From thermal analysis, compound 3 loses two coordinated water molecules in the range of 40−80 °C. To analyze whether single crystal is maintained during the F



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Figure 6. PXRD patterns for 3, 3a and thermally treated samples of 3 at different temperatures.

μ2-4,4′-bpa. Furthermore, the 2D structure of 3a is stabilized by various hydrogen bonds among the carboxylate oxygen of 1,4-phda ligand and pyridyl hydrogen of μ2-bpa (Table S3) and various πcentroid··· πcentroid, C−H··· πcentroid interactions (Figure 5b, Figure S10, and Table S2). These results agree with the TG curve of 3a which is stable up to about 300 °C (Figure S11). Catalytic Properties. Epoxidation of alkenes is one of the well-known reactions in the industry for synthesis organic products which is catalyzed by many metal salts under both homogeneous and heterogeneous catalysts.37−39 Herein, the heterogeneous catalytic activity of MOFs was carried out using tert-butyl hydroperoxide (t-BuOOH) as an oxidant in acetonitrile media at 70 °C for 20 h. Owing to the high thermal stabilities of compounds 1, 2, and 3a, they were used as catalysts for the alkenes epoxidation. To find the suitable conditions for epoxidation, the effects of solvent, oxidant, and temperature of the reaction were examined by using trans-stilbene as a model substrate. Generally, the solvent plays a key role in the oxidation of trans-stilbene.40,41 Among acetonitrile, acetone, and DMF, the weak donor solvent acetonitrile was chosen because the higher conversion and selectivity were observed (Table 2). The stronger coordinating solvent may compete for the coordination efficiency around the metal center leading to low conversion which is indicative of the Co(II) center acting as the active site for this reaction.

Figure 5. (a) 1D chain of 3a. (b) 2D layer of 3a. The intralayer C−H···πcentroid interactions and πcentroid···πcentroid interactions are represented in red and black dashed lines, respectively.

3 changes to a μ3-η1:η1:η1:η1 bridging mode in 3a (Type I, Scheme 2), and the Co···Co distance is significantly shortened from 10.613(5) Å in 3 to 4.579(5) Å in 3a, causing the great rearrangement of this structural motif in 3a. The packing diagram of the layer in 3 shows rectangular channels along the a axis whereas that of 3a is completely filled with 1,4-phda and 4,4′-bpa ligands, giving a nonporous layer of 3a (Figure 4). In addition, the change of coordination modes of 1,4-phda affects the geometrical isomerism of flexible 1,4-phda which is converted from trans to cis-isomerism in 3a. In addition, changes in orientation and conformation of 4,4′-bpa were observed. The dihedral angle between the two planar rings of pyridine in 4,4′-bpa twists from 0° in 3 to 87.83° in 3a resulting in πcentroid···πcentroid and C−H···πcentroid interactions within layers of 3a. The conformation of μ2-bpa is anti with the torsion angle of 172.81° in 3a while that of 3 is 180°. Furthermore, the UV−vis spectra obviously exhibit the corresponding bands observed in 3 and 3a denoting the distorted octahedral geometry of Co(II) (Figure S8). However, the anhydrous 3a exhibits a slightly red-shifted absorption broad band with λmax ∼ 530 nm, yielding the red−violet color indicating the different coordination Co(II) environments in 3 and 3a. The structural transformation behavior in 3 and 3a was also confirmed by IR spectra (Figure S9) that the ν(O−H) around 3351 cm−1 is not observed in 3a. The PXRD patterns (Figure 6) clearly show that 3 remains stable upon heating to 70 °C. Then, above 80 °C, compound 3 is entirely dehydrated forming anhydrous 3a. These results confirm the irreversible thermally induced single-crystal-to-single-crystal transformation from 3 → 3a. To investigate the rehydration process, compound 3a was immersed in water for 1 month. However, the reconversion of 3a to 3 was not observed. This result may be attributed to the more stable dimeric unit as SBUs in 3a which are further connected via covalent bond of

Table 2. Effect of Solvent on the Oxidation of trans-Stilbene with t-BuOOH Catalyzed by 3aa entry

solvent

conversionb (%)

selectivityc (%)

1 2 3

CH3COCH3 CH3CN DMF

76 82 78

28 57 42

a

Conditions: a mixture of trans-stilbene (10 mmol), MOFs (0.06 mol %), and t-BuOOH (20 mmol) in acetonitrile (12 mL) was stirred at 70 °C for 20 h. bDetermined by GC. cEpoxide yield/ % conversion.

Table 3. Effect of Oxidants on the Oxidation of transStilbene with t-BuOOH Catalyzed by 3aa entry

oxidant

conversionb (%)

selectivityc (%)

1 2

H2O2 t-BuOOH

20 82

13 57

a

Conditions: a mixture of trans-stilbene (10 mmol), MOFs (0.06 mol %), and oxidant (20 mmol) in acetonitrile (12 mL) was stirred at 70 °C for 20 h. bDetermined by GC. cEpoxide yield/ % conversion.

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Table 4. Selective Epoxidation of Alkenes Catalyzed by Compounds 1, 2, and 3aa

Conditions: a mixture of alkenes (10 mmol), mofs (0.06 mol %) and t-BuOOH (20 mmol) in acetonitrile (12 mL) was stirred at 70 °C for 20 h. Determined by GC. cEpoxide yield/ % conversion. dWithout MOFs. eWithout t-BuOOH. f The fifth cycle.

a

b

Figure 7. Recycle experiment for epoxidation of trans-stilbene for 1 (a), 2 (b), and 3a (c).

provided significantly higher conversion and selectivity. The temperature effect was evaluated in the range 55−70 °C. The results showed that the conversion (82%) of trans-stilbene

The influence of different oxidants on the catalysis was studied as shown in Table 3. When t-BuOOH and hydrogen peroxide were used as the oxidant in CH3CN media, the t-BuOOH H



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heterogeneous catalysts for epoxidation. The higher conversion of the substrate reaches 82% after 20 h (Figure 8). A possible mechanism of this epoxidation reaction may give rise to a change of oxidation state of the cobalt center which involves the formation of t-BuOO• radical species to interact with styrene, generating styrene oxide.11,43 The catalytic activity of 3a was compared with the previously reported Co-based MOFs as heterogeneous catalysts (Table 5). It is known that many types of active species have been employed for epoxidation catalysis, including unsaturated metal nodes, oxidizing agent, solvent, and porosity to enhance the catalytic activity. However, octahedral Co(II) based MOF performing heterogeneous catalyst is rare (entries 1, 2, and 6 Table 5). It is obvious that the solvent, oxidants, and reaction temperature play a major role in the conversion of epoxides. Photocatalytic Properties. The solid-state UV−vis spectra show the absorption characteristics of 1−3a (Figure S8). The intense UV absorption bands at 291, 291, 288, and 291 nm for 1, 2, 3, and 3a are attributed to ligand-to-metal charge transfer (LMCT). Moreover, absorption peaks of all compounds in the visible region were found at 482(518), 479(524), 506, and 482(532) nm, respectively, which derived from the d−d transition within Co(II) (d7) ions. The band gaps of all compounds were determined by a solid-state UV−vis diffuse reflectance spectra at room temperature. From the equation αhν2 = K(hν − Eg)1/2 (where K is a constant, Eg is the band gap energy, hν is the discrete photo energy, and α is the absorption coefficient),46 the extrapolated values (the straight lines to the x axis) of hν at α = 0 give absorption edge energy corresponding to Eg = 2.80, 3.00, 3.20, and 3.25 eV for 1, 2, 3, and 3a, respectively. Thus, compound 1 was examined for photocatalytic reaction due to the lowest energy band gap. Herein, methylene blue (MB) as a model cationic dye was selected to evaluate the photocatalytic effeciency in the purification of wastewater and the characteristic absorption of MB about 665 nm was monitored. A mixed solution of 1 (40 mg) and 40 ppm MB (80 mL) solution was stirred in the dark for 30 min to eliminate the effect of absorption on the surfaces. Without catalyst, the MB solution was slightly decomposed (Figure 9). Interestingly, with catalyst (1), MB decomposed by 98% after 90 min under UV irradiation (Figure 9). The results show a high photocatalytic efficiency for 1. In addition, after photocatalysis, the powder of 1 was obtained by filtration and the PXRD pattern is similar to that of the original crystalline phase, implying that MOFs are stable after photocatalysis (Figures S13).

Figure 8. Epoxidation of trans-stilbene in acetonitrile at 70 °C catalyzed by 3a.

clearly increased with increasing temperature. After optimizing conditions, the catalytic reaction was extended to the oxidation of other substrates as summarized in Table 4. Without MOFs as catalyst or without t-BuOOH as the oxidant (entries 1 and 2, Table 4), negligible conversions (10% and 5%, respectively) were observed. Using Co(NO3)2·6H2O as homogeneous catalyst, a higher conversion (87%) was obtained than that for 3a (82%), while the lower selectivity of epoxide in Co(NO3)2· 6H2O (36%) was obtained when compared with that of 3a (57%) (entries 4 and 5, Table 4). Comparison of 1, 2, and 3a, showed no significant changes in conversion, because all compounds contain similar SBUs (entries 5, 6, and 7, Table 4). All conversions of the styrene oxidation are higher than those of the trans-stilbene reaction. It may be owing to the styrene containing terminal alkene which is the high activity position for epoxidation reaction. However, selectivity of styrene oxide products is much lower than those of trans-stilbene oxide, because the styrene oxide could be oxidized easily (entries 8, 9, and 10, Table 4) generating to species such as aldehydes or ketones.42 The conversion and selectivity of cyclohexene oxide is lower than that of other oxide products (entries 11, 12, and 13). The reason may be attributed to cyclohexene being weak nucleophile and less active for epoxidation than strong nucleophiles such as trans-stilbene and styrene. The catalytic results for the fifth run demonstrated that the conversion of trans-stilbene decreases from 82% to 75% epoxide conversion (Figure 7). In order to confirm the stability of 3a after the fifth run recycle experiment, PXRD measurement was investigated (Figure S12). The results show closely identical diffraction peaks of catalysts, denoting that the crystal structure of 3a remained unchanged after catalysis. Lastly, the heterogeneity of the catalyst was confirmed (Figure 8). After catalyst (3a) removal from the reaction, no further substrate conversion occurred. In addition, the amount of cobalt species leached in the solution after the catalytic process was measured by ICP-AES. The ICP-AES analysis revealed negligible cobalt species leaching (0.4 ppm) into the reaction solution. All results demonstrate that all MOFs have good stability and act as good

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CONCLUSIONS In summary, four novel 2D and 3D Co(II)-MOFs have been successfully synthesized under various synthetic methods. The diverse structures of these compounds are contributed from the flexibility of linker, synthetic method, and temperature. Moreover, the various coordination modes and conformations

Table 5. Comparison of 3a with Previously Reported trans-Stilbene Oxidation Reaction Catalyzed by Co-MOFs entry

catalysts

conversion (%)

solvent/oxidant

condition

ref

1 2 3 4 5 6

[Co(H2-DHBDA) (bpe)]n Co6-Cp [Co(Hoba)2] NTU-Z30 Co(II)@Cr-MIL-101-P2I 3a

63 98.6 86 79.6 91 82

CH3CN/H2O2 DMF/O2 TBHP DMF/O2 Isobutylaldehyde/CH3CN CH3CN/TBHP

40 °C/24 h 120 °C/6 h 75 °C/6 h 100 °C/6 h 35 °C/5 h 70 °C/20 h

40 43 11 44 45 This work

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Figure 9. (a) Photograph of photocatalytic degradation of MB using 1 as catalyst for 0−90 min. (b) Photocatalytic decomposition of MB solution with 1, and without any catalyst. (c) Absorption spectra of the MB solution during the decomposition reaction.

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of both carboxylate groups in 1,4-phda also have an important influence on the structural architectures. The thermally induced irreversible single-crystal-to-single crystal transformation with chromotropism from 3 to 3a upon dehydration demonstrates pore opening and closing owing to the orientation of coordinated 4,4′-bpa ligand in both compounds. However, the rehydration process of 3a was unsuccessful on account of the more stable dimeric unit as SBUs in 3a which are further stabilized by various hydrogen bonds, πcentroid···πcentroid and C−H···πcentroid interactions. Furthermore, compounds 1, 2, and 3a have been investigated as heterogeneous catalysts for epoxidation of alkenes with t-BuOOH as oxidant in acetonitrile. The results show high conversion and high stability of the frameworks after reuse for five times. Although compound 3 is unstable for catalytic activity, the structural transformation from 3 to 3a significantly enhanced the framework stability which resulted in a stable enough product for catalytic activity in the epoxidation reaction. In addition, the photocatalytic performance of 1 showed that 1 is an effective and stable photocatalyst for the photodegradation of MB under UV light.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; fax: +6643-202-373. ORCID

Jaursup Boonmak: 0000-0002-6108-9569 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work is provided by The Thailand Research Fund: Grant BRG5980014, the Higher Education Research Promotion and National Research University Project of Thailand, through the Advanced Functional Materials Cluster of Khon Kaen University, and the center of Excellence for Innovation in Chemistry (PERCH−CIC), Office of the Higher Education Commission, Ministry of Education, Thailand.

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ABBREVIATIONS 1,4-phda, 1,4-phenylenediacetic acid; 4,4′-bpa, 1,2-bis(4pyridyl)ethane; 4,4′-bpp, 1,3-bis(4-pyridyl)propane; MB, methylene blue; SC-SC, single-crystal-to-single-crystal; t-BuOOH, tert-butylhydroperoxide; H2O2, hydrogenperoxide; SBU, secondary building unit; MOFs, metal−organic frameworks; LMCT, ligand-to-metal charge transfer; Eg, energy bandgap; hν, photo energy; α, absorption coefficient; K, constant; ICPAES, inductively coupled plasma-atomic emission spectrometry

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01788. Figures for diffuse-reflectance absorption spectra, XRPD patterns, packing structures, TGA plots, FTIR spectra, and tables for X-ray data collection (PDF)

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Accession Codes

CCDC 1450961−1450964 contains 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. J



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