Co(II)-MOF: A Highly Efficient Organic Oxidation Catalyst with Open

Oct 26, 2015 - A 3D Co(II)-MOF with open Co(II) sites can be a highly active recyclable heterogeneous catalyst for organic oxidation. .... Lanthanide(...
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Co(II)-MOF: A Highly Efficient Organic Oxidation Catalyst with Open Metal Sites Jian-Cheng Wang, Feng-Wen Ding, Jian-Ping Ma, Qi-Kui Liu, Jun-Yan Cheng, and Yu-Bin Dong* College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, P. R. China S Supporting Information *

ABSTRACT: A porous Co(II)-MOF (1) was synthesized by the combination of a bent imidazole-bridged ligand and p-phthalic acid (PTA) with Co(OAc)2 under solvothermal conditions. This Co(II)MOF (1) is able to undergo a reversible MeOH substitution reaction on the Co(II) center via a single-crystal-to-single-crystal process. The desolvated Co(II)-MOF (2) with the open Co(II) sites is very stable (up to 350 °C). Furthermore, 2 is a highly active heterogeneous catalyst for various organic substrates oxidation in the presence of tertbutyl hydroperoxide (TBHP) under milder conditions. The importance of open Co(II) sites in 2 for the organic substrates oxidation is directly evidenced by the single-crystal X-ray diffraction.

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INTRODUCTION Organic oxidation, especially the inert C−H bond of hydrocarbons, constitutes one of the most desirable but challenging issues.1 For example, the oxidation of cyclohexane is the main industrial way to a mixture of cyclohexanone (K) and cyclohexanol (A), the important intermediates in the production of Nylon-6 and Nylon-66, and also raw materials for the production of adipic acid.2a,b The present commercial process for cyclohexane oxidation is carried out at ca. 150 °C and 1−2 MPa pressure, affording 5−7% conversion to so-called KA-oil.2c,d In addition to that, the oxidation of olefins and alcohols is also important, not only for fundamental study but also for fine chemical production.3 Therefore, the development of more efficient catalysts, especially efficient versatile catalysts, for organic oxidation is highly demanded and important. On the other hand, traditional oxidation catalysts based on transition metal molecular complexes such as Co, Cu, Fe, and Mn are usually homogeneous ones, which makes their cyclic utilization almost impossible. For environmental and natural resource issues, the construction of new types of heterogeneous catalysts is highly valued.4 Fortunately, metal−organic frameworks (MOFs), as an emerging class of functional porous materials, provide a new tunable platform to fabricate heterogeneous catalysts.5 As we know, MOFs themselves are inherent heterogeneous catalysts due to their inorganic− organic hybrid composition and polymeric nature. Furthermore, the construction of MOF-type heterogeneous catalysts is convenient, and they can be easily prepared via assembly or a postsynthetic approach. However, catalysis based on MOFs is far less explored as compared to the huge amount of known MOF structures. Among various MOFs, the MOFs with open © XXXX American Chemical Society

metal sites are more appreciated. They are widely accepted to be the important candidates for applications such as catalysis,6 adsorption/separation,7 storage,8 and sensing.9 In this contribution, we report the synthesis of a Co(II)MOF (1) that can undergo a reversible MeOH binding at the Co(II) atom in a single-crystal-to-single-crystal (SC-SC) fashion. The desolvated Co(II)-MOF (2), which contains the coordinatively unsaturated Co(II) sites, is porous and robust. Furthermore, it is a high-efficiency recyclable versatile heterogeneous catalyst for cyclohexane, olefins, and alcohols oxidations under milder conditions.

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EXPERIMENTAL SECTION

Materials and Measurements. All the chemicals were obtained from commercial sources and used without further purification. Intermediate A (Scheme 1) for ligand L was prepared according to the literature method.10 Infrared (IR) spectra were obtained in the 400− 4000 cm−1 range using a Bruker ALPHA FT-IR Spectrometer. Elemental analyses were performed on a PerkinElmer model 2400 analyzer. 1H NMR data were collected on an AM-300 and Varian Advance 600 spectrometer. Chemical shifts are reported in δ relative to TMS. All crystal data were obtained by Agilent SuperNova X-ray single crystal diffractometer. GC-MS analysis data were performed on a J&K S011525−300 gas chromatographic (Agilent 6890GC5973MS). The separation data were obtained by Agilent 1260 Infinity HPLC system equipped with an Agilent C18 reverse phase column (150 × 4.6 mm, 5 μm). Thermogravimetric analyses were carried out on a TA Instrument Q5 simultaneous TGA under flowing nitrogen at a heating rate of 10 °C/min. XRPD patterns was obtained on D8 Received: August 23, 2015

A

DOI: 10.1021/acs.inorgchem.5b01938 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of L

Scheme 2. Synthesis of 1 and 2

Synthesis of 2 ([Co3L(PTA)2.5(OAc)]·solvent). 1 was allowed to stand at ambient temperature for ca. 15 min to generate 2 quantitatively. IR (KBr pellet cm−1): 3425(s), 1613(s), 1506 (m), 1384 (vs), 1149(w), 1017(vw), 1016(w), 918(w), 823(m), 750(v), 544(m). Elemental analysis(%): calcd for C49H35Co3N4O12: C 56.12, H 3.36, N 5.34; found: C 56.52, H 3.46, N 5.33. Synthesis of 1′ ([Co3L(CH3OH) (PTA)2.5(OAc)]·solvent). 2 was soaked in MeOH at ambient temperature for 24 h to afford 1′ quantitatively. IR (KBr pellet cm−1): 3425(s), 1614(s), 1569(s), 1505 (m), 1384 (vs), 1129(w), 1072(vw), 1017(w), 824(m), 750(v), 527(m). Typical Catalytic Procedure for Cyclohexane. A mixture of cyclohexane (10 mL), TBHP (0.31 mmol, 30.8 μL) and C5H6Cl (0.10 mmol, 10.16 μL, as internal reference) was stirred at ambient temperature for 5 min, and then 2 (2%) was added inside. The mixture was stirred at 60 °C for 22 h (monitored by GC) to afford the corresponding KA oil. The conversions and products were determined by GC-MS. 2 was recovered by centrifugation and filtration, and then directly reused in the next run under the same reaction conditions. Typical Catalytic Procedure for Styrene and Its Derivatives. 2 (2%) was added to a MeCN solution (10 mL) of the substrate (1.0 mmol) and TBHP (2.0 mmol), and the mixture was stirred at 60 °C (monitored by GC) to afford the corresponding products. The conversions and products were determined by GC-MS. 2 was recovered by centrifugation and filtration, and then directly reused in the next run under the same reaction conditions. Typical Catalytic Procedure for Benzyl Alcohol and Its Derivatives. 2 (2%) was added to a CH3CN (10 mL) solution of alcohol substrates (1 mmol) and TBHP (2 mmol), the mixture was stirred under aerobic conditions. The conversions and products were determined by GC. X-ray Structure Determination.11 All single-crystal X-ray intensity data were measured at 100 K on a Agilent SuperNova CCD-based diffractometer (Cu Kα radiation, λ = 1.54184 Å). After determination of crystal quality and initial tetragonal unit cell parameters, a hemi sphere of frame data was collected. The raw data frames were integrated with CrysAlisPro, Agilent Technologies, Version 1.171.36.32 (release 02−08−2013 CrysAlis171. NET) (compiled Aug 2 2013, 16:46:58). Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK

Advance X-ray powder diffractometer with Cu Kα radiation (λ = 1.5405 Å). Synthesis of L. Synthesis of B. A mixture of A (10.58 mmol, 4.00 g) and NaH (11.64 mmol, 0.28 g) in THF (50 mL) was stirred at 80 °C, and then CH3CH2I (11.64 mmol, 2.63 g) was added in the reaction system. The mixture was further stirred at 80 °C to afford crude product. B was obtained as a yellow crystalline solids (2.89 g, 68%) after purification by chromatography on silica gel using CH3COOEt as the eluent. IR(KBr pellet cm−1): 3442 (m), 2981(m), 1916(vw), 1637(m), 1500(vs), 1471(s), 1387(s), 1067(s), 1009(vs), 948(s), 838(vs), 818(s), 660(m), 561(m). 1H NMR (400 MHz, CDCl3, 25 °C, TMS, ppm): δ = 8.31 (s, 1H, -C3HN2), 7.63− 7.65(d, J = 8.0 Hz, 2H, -C6H4), 7.32−7.39(m, 4H, -C6H4), 7.21− 7.23(d, J = 8.0 Hz, 2H, -C6H4), 3.93−3.98 (q, J = 10.0 Hz, 2H, −CH2−), 1.31−1.34 (t, J = 9.0 Hz, 3H, −CH3). Elemental Analysis(%): calcd for C17H14N2Br2: C 50.28, H 3.47, N 6.90, Br 39.35; found: C 50.45, H 3.49, N 6.92, Br 39.54. Synthesis of L. A mixture of B (5.00 mmol, 2.03g), pyridine-4boronic acid (12.00 mmol, 1.48g), K2CO3 (40.00 mmol, 5.53 g), tetrakis(triphenylphosphine)palladium (0.50 mmol, 0.57 g) in a mixed solvent system (EtOH: H2O: toluene =3:2:3) was stirred at reflux for 48 h. Ligand L was obtained as yellow crystalline solids (1.70 g, 84.2%) after chromatography purification. IR (KBr pellet cm−1): 3423(ms), 3032(vw), 2969(vw), 2360(vw), 1595 (vs), 1496 (m), 1397(m), 1189(w), 1109(w), 992(w), 950(w), 857(w), 815(vs), 766(vw), 736(vw), 658(w), 543(w), 515(w). 1H NMR (300 MHz, CDCl3, 25 °C, TMS, ppm): δ = 8.75−8.77 (d, J = 6.0 Hz, 2H, -C6H4N), 8.63− 8.75 (d, J = 6.0 Hz, 2H, -C6H4N), 7.81−7.83 (d, J = 6.0 Hz, 2H, -C6H4), 7.75 (s, 1H, -C3HN2), 7.62−7.66 (m, 4H, -C6H4N), 7.51− 7.58 (m, 6H, -C6H4), 3.92−3.99 (q, 2H, −CH2−), 1.29−1.40 (t, 3H, −CH3). Elemental Analysis(%): calcd for C27H22N4: C 80.57, H 5.51, N 13.92; found: C 80.64, H 5.49, N 13.67. Synthesis of 1 ([Co3L(CH3OH)0.5(H2O)0.5(PTA)2.5(OAc)]·solvent). A mixture of L (4.06 mg, 0.01 mmol), p-phthalic acid (PTA) (1.66 mg, 0.01 mmol), Co(OAc)2 (0.01 mmol, 2.13 mg) and MeOH (2 mL) was sealed in a glass tube and then heated at 120 °C for 72 h. After the mixture was allowed to cool to room temperature (50 h), red-purple crystals of 1 were isolated. Yield, 78%. IR (KBr pellet cm−1): 3424(s), 2360(w), 1613(s), 1568(s), 1504 (m), 1384 (vs), 1072(vw), 1016(vw), 824(m), 750(v), 524(m). B

DOI: 10.1021/acs.inorgchem.5b01938 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structural Refinement Parameters for 1, 2, and 1′a formula fw λ/Å crystal system space group T/K a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcak/g·cm−3 μ/mm−1 F (000) GOF data/restraints/parameters R1 [I > 2σ(I)] wR2 [all data] a

1

2

1′

C49.50H38Co3N4O13 1073.63 1.54184 Triclinic P1̅ 100.00(10) 11.5791(4) 15.2623(6) 19.2483(9) 70.394(4) 76.587(3) 82.732(3) 3112.6(2) 2 1.146 6.639 1096 1.046 11720/7/636 0.0552 0.1453

C49H35Co3N4O12 1048.60 1.54184 Triclinic P1̅ 100.00(10) 11.6664(6) 14.9859(8) 19.4221(11) 75.584(5) 75.343(5) 87.679(4) 3180.8(3) 2 1.095 0.823 1068 1.038 11323/0/618 0.0396 0.1049

C50H39Co3N4O13 1080.64 1.54184 Triclinic P1̅ 100.00(10) 11.5784(7) 15.3081(12) 19.2580(11) 70.388(6) 76.659(5) 83.048(6) 3125.0(4) 2 1.148 6.615 1104 0.945 11117/0/636 0.0706 0.1768

R1 = ∑∥F0| − |Fc∥/∑|F0|, wR2 = {∑[w(F02 − Fc2)2]/∑[w(F02)2]}1/2.

a large solvent-accessible volume of 1098.7 Å3 (35.3% of a total unit cell volume of 3112.6(2) Å3). The crystallographically unidentified MeOH molecules (confirmed by 1H NMR spectrum, Supporting Information) are located inside. Interestingly, compound 1 can undergo a reversible MeOH substitution at ambient temperature in which the single-crystal nature is maintained (Scheme 2). As shown in Figure 2, when 1 was allowed to stand at room temperature for a while (∼15 min), 2 was generated; meanwhile, the crystal changed color from pink to purple. Single-crystal X-ray diffraction revealed that 2 possesses the same structural motif as 1 (Table 1). Upon closer observation of the structure, the coordinated MeOH/ H2O molecule was detached from the Co(2) center. As indicated in Figure 2, the six-coordinated sphere of {Co(2)O5N} in 1 changed to a coordinatively unsaturated fivecoordinated {Co(2)O4N} in 2 while no coordination mode change for the other three types of Co(II) centers is observed during the transformation process. Compared to 1, the solvent accessible void volume in 2 effectively increased (1307.2 Å3, 41.1% of the total unit cell volume 3180.8(3) Å3) after removal of the coordinating MeOH molecules. In addition, 1 can be readily regenerated (1′) without losing its single crystallinity when 2 was soaked in MeOH for 24 h (Table 1). The simulated and measured XRPD patterns are well consistent with each other, indicating this reversible solvent substitution triggered SC-SC transformation is clean (Supporting Information). It is noteworthy that the encapsulated MeOH guest molecules in 2 can be completely removed at ca. 150 °C (monitored by thermogravimetric analysis (TGA), Supporting Information) and the empty framework with open Co(II) sites is stable up to ca. 350 °C based on TGA and XRPD measurements (Supporting Information). Such a highly thermally stable MOF with open metal sites could be used as the heterogeneous catalyst for the organic reactions carried out at higher temperature. The permanent porosity of compound 2 was determined by measuring nitrogen-gas (N2) adsorption at 77 K. It exhibits

scaling algorithm. Analysis of the data showed negligible crystal decay during data collection. The structure was solved by a combination of direct methods and difference Fourier syntheses, and refined by fullmatrix least-squares against F2, using the SHELXTL software package. The species in this region were too severely disordered to be modeled, and were treated with SQUEEZE/PLATON. These species include some CH3OH and H2O molecules. The contribution of the disordered species was removed from the structure factor calculations. The tabulated F(000), MW and density reflect known cell contents only. Eventually, all non-hydrogen atoms of the framework were refined with anisotropic displacement parameters. Hydrogen atoms were placed in geometrically idealized positions and included as standard riding atoms.

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RESULTS AND DISCUSSION Crystal Structures and SC-SC Transformation. Compound 1 was simply synthesized as red-purple crystalline solids by the combination of L, p-phthalic acid (PTA) coligand, and Co(OAc)2 in MeOH under solvothermal conditions (120 °C) in good yield (Scheme 2). The single-crystal diffraction revealed that 1 crystallized in the triclinic space group P1̅ (Table 1). There are four crystallographically independent Co(II) centers in 1. They all adopt distorted octahedral coordination spheres, including {Co(1)O6}, {Co(2)O5N}, {Co(3)O4N2}, and {Co(4)O6}. As shown in Figure 1, six PTA coligands act as the bridges to link two Co(1) and one Co(2) atoms to generate the trinuclear {(Co(1)2Co(2)} core, while four PTA coligands and two coordinating OAc− anions to bind two Co(3) and one Co(4) atoms into the {Co(3)2Co(4)} core. Notably, a coordinating MeOH/H2O (1:1) solvent molecule is attached to the Co(2) center with a Co(2)− O(13) bond length of 2.130(3) Å. In the solid state, two sets of trinuclear Co3 clusters are further linked alternatively by L and PTA into a 3D framework. An inspection of Figure 1 reveals that the overall array of 1 contains a 3D intersecting channel system and wide open windows, with an approximate effective cross section of ca. 12 × 8 (down a), 10 × 8 (down b), 13 × 8 (down c), and 10 × 8 Å2 (down [−1 0 1]), which is reflected in C

DOI: 10.1021/acs.inorgchem.5b01938 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a−b) Two sets of trinuclear Co3 cluster cores and the coordination spheres of Co(1), Co(2), Co(3), and Co(4) found in 1. (c−f) Channels opening along the crystallographic a, b, c, and [−1 0 1] directions in 1.

Figure 2. Reversible MeOH substitution-triggered single-crystal-to-single-crystal transformation. The corresponding single-crystal pictures are given in the inset.

classic type I isotherms characterized by a sharp uptake under low relative pressures in the range P/P0 = 10−5−10−2 (Figure 3), a signature feature of microporous materials. The lack of hysteresis indicates that the adsorption and desorption mechanisms are similar and that the adsorption is reversible. The Brunauer−Emmett−Teller (BET) surface areas were found to be 595 m2 g−1 for 2 (Supporting Information). The

pore-size distributions of 2 were calculated by nonlocal density functional theory (NLDFT). Compound 2 shows a narrow pore width (∼5.5 Å, Supporting Information), which is slightly smaller than the theoretical values predicted from their crystal structures. Catalytic Properties of 2. Compound 2 with open Co(II) sites was used to test the catalytic activity for the solvent-free D

DOI: 10.1021/acs.inorgchem.5b01938 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 3. Summary of the Reported Cyclohexane Oxidation Reactions Using MOFs as the Heterogeneous Catalysts Catalyst PIZA-3 Cu3(μ−OH)(μ-pz)32+ Mn-TMPyP⊂rho-ZMOF NH4[Cu3(OH)(4carboxypyrazolato)3] ZnMn-RPM MIL-101 family MOF-253

Figure 3. N2 adsorption isotherms for compound 2 at 77 K. Red and black circles represent adsorption and desorption branches, respectively.

Au⊂MIL-53(Cr) and Au⊂MIL-101(Cr) Au−Pd⊂MIL-101

cyclohexane oxidation. Compound 2 was activated by heating at 70 °C (2 h) for removal of encapsulated MeOH guest before the oxidation reaction (Supporting Information). Table 2

CZJ-1 (MnIII-TCPP)] Zr-PCN-221(Fe)

Table 2. Solvent-Free Cyclohexane Oxidation Catalyzed by 2

NHPI/Fe(BTC) CoII-MOF (2), this work a

Solvent/Oxidant CH3CN/PhIO CH3CN/ HNO3/H2O2 cyclohexane/ TBHP cyclohexane/ TBHP CH2Cl2/ArINTs CH3CN/ TBHP-O2 cyclohexane/ 1.0 MPa O2 cyclohexane/ 1.2 MPa O2 cyclohexane/ 1.5 MPa O2 CH3CN/PhIO CH2Cl2/PhIO cyclohexane/ TBHP cyclohexane/ 6 bar O2 cyclohexane/ TBHP

Condition

a

Yieldref

r.t./2 h r.t./6 h

47.812 2.7−32.213

65 °C/24 h

91.514

70 °C/24 h

415

r.t./24 h 70 °C/8 h

2016 3817

160 °C/8 h

60.818

130 °C/8 h

31.319

160 °C/4 h

50.820

r.t./22 h r.t./6 h 65 °C/11 h

9421 20.622 92.323

160 °C/24 h

5.724

60 °C/22 h

88

Overall C−H conversion.

Selectivity (%)

a

Entry

Conversion (%)a

K

A

1 2 3 4 5

85 87 88 87 83

34 30 28 29 33

66 70 72 71 67

TMPyP⊂rho-ZMOF,14 CZJ-1,21 and Zr-PCN-221(Fe)23 performed better than Co(II)-MOF (2). On the other hand, some reported results did not provide solid evidence for the catalyst reusability or the reused MOF-catalysts that display a stable catalytic activity. Therefore, 2 herein could be a beneficial complement for the MOFs to heterogeneous cyclohexane oxidation catalysts. In order to gain insight into the heterogeneous nature of 2, a leaching test was carried out. As indicated in Figure 4, no further reaction took place without 2 after initiation of the oxidation reaction at 14 h. This finding indicates that no leaching of the catalytically active sites occurs and that 2 exhibits a typical heterogeneous catalyst nature. As a heterogeneous catalyst, 2 can be reused. After each catalytic cycle, 2 could be easily recovered by centrifugation and filtration, and then directly reused in the next run under the same reaction conditions. As shown in Table 2, KA oil was still obtained in an ideal yield (83%) after five catalytic cycles. Actually, the catalytic activity of 2 only gradually dropped and the conversion of cyclohexane was still above 70% at run 10 within 22 h. As indicated in Figure 4, after five consecutive catalytic runs, 2 was demonstrated to be highly crystalline from XRPD patterns. In addition, the 1H NMR performed on the reaction filtrate, which was obtained by hot filtration during reaction, indicated that no proton resonances related to L were detected, which further confirmed 2 as a heterogeneous catalyst is stable under reaction conditions. Typical XPS data of 2 before and after the oxidation reaction were shown in Figure 4. The Co 2p core lines of the samples are split into Co 2p3/2 (781.1 eV) and Co 2p1/2 (796.8 eV) main peaks accompanied by satellite bands at 785.8 and 802.3 eV, respectively. Prominent Co 2p3/2 satellite bands are indicative of a highspin Co(II) state.26 No difference was observed before and after the catalytic reactions, further implying 2 is stable during the reaction processes. Furthermore, SEM images (Figure 5) revealed that the morphology of the Co(II)-framework is

Conversion is determined by GC-MS.

shows that 2 exhibits a high catalytic activity for cyclohexane oxidation. The treatment of cyclohexane with TBHP in the presence of 2 at 60 °C for 22 h afforded the KA oil in 83−88% yields (Table 2, entries 1−5 and Supporting Information). No difference in yield was observed after 22 h of monitoring by GC. Additionally, this 2-catalyzed oxidation is clean, and almost no other oxidation products were detected. This is different from the most reported MOFs-catalyzed cyclohexane oxidation results (Table 3):12−24 the cyclohexanol is the preferable product (∼70%), and a relatively smaller amount of cyclohexanone (∼30%) is produced (Table 2). High selectivity of cyclohexanol species, however, was observed on a series of metal-containing molecular sieves.25 It is worth pointing out that the oxidation reaction described herein needs no additional organic solvents except the reaction substrates. Thus, as this oxidation is easy to manipulate and avoids the use of toxic and volatile solvents, it could be considered as a clean catalytic organic synthesis approach. In addition, the coordinating solvent molecules play a key role in this cyclohexane oxidation. For example, 1 catalyzed the cyclohexane oxidation with MeOH/H2O to result in a very low conversion (24 h,