III) MetalâOrganic Framework for Selective

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Functional Inorganic Materials and Devices

A Redox-Active Cobalt(II/III) Metal-Organic Framework for Selective Oxidation of Cyclohexene Tao Zhang, Yue-Qiao Hu, Tian Han, Yuan-Qi Zhai, and Yan-Zhen Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19323 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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ACS Applied Materials & Interfaces

A Redox-Active Cobalt(II/III) Metal-Organic Framework for Selective Oxidation of Cyclohexene Tao Zhang,a Yue-Qiao Hu,*,a,b Tian Han,a Yuan-Qi Zhaia and Yan-Zhen Zheng*,a a

Frontier Institute of Science and Technology (FIST), State Key Laboratory for Mechanical

Behavior of Materials, MOE Key Laboratory for Nonequilibrium Synthesis of Condensed Matter and School of Science, Xi’an Jiaotong University, 99 Yanxiang Road, Xi’an, Shaanxi 710054, P. R. China. b

Key Laboratory of Advanced Molecular Engineering Materials, College of Chemistry and

Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013, China.

KEYWORDS. metal-organic framework • cobalt • catalysis • oxidation • cyclohexene

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Abstract. We report herein a new cobalt(II/III) mixed-valance metal-organic framework (MOF) fomulated as [CoIICo2III(µ3-O)(bdc)3(tpt)]·Guest 1, where bdc = benzene-1,4-dicarboxylate and tpt = 2,4,6-tri(4-pyridinyl)-1,3,5-triazine, can be used as a redox-active heterogeneous catalyst for selective oxidation of cyclohexene on the allylic position without destroying the adjacent double bond. Two oxidants were chosen to demonstrate this result. For using tert-butyl hydroperoxide, the conversion rate is 63 % and only allylic oxidation products (tert-butyl-2cyclohexenyl-1-peroxide, 86 %; 2-cyclohexen-1-one, 14 %) are found. While if using O2 as oxidant, a total conversion of 38% is achieved and also, only the allylic oxidation products (cyclohexenyl hydroperoxide, 72 %, 2-cyclohexen-1-one, 20 % and cyclohex-2-en-1-ol, 8%) are found. The absence of any adductive on the double-bond may be due to the unique radical chain mechanism triggered by the mixed-valent [CoIICo2III(µ3-O)] centers.


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INTRODUCTION The catalytic property of metal–organic frameworks (MOFs) draw a growing attention in recent years owing to the significant porosity, diversified ligand decoration and variable coordination chemistry of metal ions.1,2 Peroxidation of cyclohexene, as one of the model reactions for selectively introducing peroxy bond into the allylic position (without breaking the adjacent and reductive double bond), was initially developed by Kharasch et al in 1950s.3 After years of investigation, this reaction has been known to involve transition metal centers such as copper, manganese and cobalt, for higher efficiency of catalytic activities.4,5 However, these reactions are homogeneous. To achieve larger scale synthesis, the development of heterogeneous catalyst is more desired. MOFs constructed by metal-containing secondary building units (SBUs) and organic linkers, are designable for this purpose.6 The pioneering work using a cobalt(II) MOF towards catalyzing cyclohexene oxidation was demonstrated by Volkmer et al, the best conversion is only 27.5 % (for MFU-1).7,8 Moreover, due to the reductive nature of the double bond many other oxidative products were also detected (Scheme 1), indicating low selectivity. Later on, Van Der Voort et al found MIL-47 which contains one-dimensional O-VIV-O chain exhibiting a much higher conversion (up to 60 %),9 but the metal leaches out during the reaction and diverts the reaction pathways. Thus, the selectivity is also low.

Scheme 1. Typical oxidation products of cyclohexene. Blue: allylic oxidation products; orange: double-bond oxidation products.


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Enlightened by these studies we assume that MOFs with redox active metal centers are important to catalyze the oxidation reaction. In this context, the trigonal planar trimer [M3(µ3O)(RCOO)6], in which the metal ions exhibit flexible oxidation states, drew our attention, especially for M = Co.10 In contrast to its Cr, Fe, Ni analogues, triangular [Co3(µ3-O/OH)] SBUs have only been found in a few MOFs, including [Co3(OH)(pdc)3]·H2O (pdc = pyridine-3,5dicarboxylate),11 [CoII3(µ3-O)(OH)(cpt)3(H2O)2]·guest (Hcpt = 4-(4′-carboxyphenyl)-1,2,4triazole)

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and MCF-18 series.13 In all of these MOFs, the [Co3(µ3-O/OH)] SBUs are in-situ

formed during the solvothermal reactions in the presence of carboxylates and N-heterocyclic ligands. Hence, we reason that the presence of both N-heterocyclic ligands and carboxylates in basic condition is critical to achieve the [CoIICo2III(µ3-O)] SBUs. To ensure the large porosity, the triangular N-heterocyclic ligand 2,4,6-tri(4-pyridinyl)-1,3,5triazine (tpt) and a long dicarboxylate ligand benzene-1,4-dicarboxylate (bdc) were chosen. After solvothermal treatment, we isolated a mixed-valence cobalt(II/III) based MOF, [CoIICo2III(µ3O)(bdc)3(tpt)]·Guest 1 (Guest = 4DMF and 3H2O), with large porosity. Two oxidants were chosen to demonstrate the catalytic ability of 1 for oxidizing cyclohexene. By using tert-butyl hydroperoxide (TBHP) as an oxidant the total conversion rate is 63 %; while if using O2 as an oxidant the total conversion rate is 38 %. These conversion rates are better than many other reported MOFs to date (see below). More importantly, only the allylic oxidation products are detectable for both reactions, and the selectivity towards peroxide product is up to 86 % and 72 % for TBHP and O2, respectively. We postulate such a high selectivity is due to the unique radical mechanism promoted by the mixed-valent Co3 unit.

RESULTS AND DISCUSSION


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Compound 1 crystalizes in hexagonal space group P63/mmc. The planar triangular [CoIICo2III(µ3-O)] SBU is extended by six dicarboxylates and three tpt ligands. (Figure 1a). The Co atom shows a six-coordinate octahedral geometry consisting of four O atoms from bdc ligands (Co-O = 2.093 Å), one µ3-O2– (Co-O = 2.025 Å) and one N atom from tpt ligand (Co-N = 2.170 Å). Topologically, the tpt ligand is treated as three connected node, while the [CoIICo2III(µ3-O)] SBU can be considered as a nine connected node. Thus, the 3D framework of 1 exhibits a non-interpenetrating (3,9)-connected network with the point symbol {421.615}{43}.14 The framework consists of two kinds of microporous cages, namely cage A and cage B, with dimensionality of 12.9 × 12.9 × 7.6 Å3 and 10.9 × 10.9 × 15.3 Å3, respectively (Figure 1b). For cage A, it is a cavity inside a dicapped trigonal antiprism formed by six [CoIICo2III(µ3-O)] SBUs, two facing tpt ligands and six bdc ligands. Cages A are running along the c axis and are separated by the tpt ligands. For cage B, it is the cavity inside a trigonal bipyramid formed by five [CoIICo2III(µ3-O)] SBUs and six bdc ligands. Each cage B is surrounded by six cages A and interconnected through the void space between ligands (Figure 1c and d). The porous structure affords total void space of 58.8 % crystallographic volume. X-ray photoelectron spectroscopy (XPS) was used to reveal the valence state of the cobalt ions. As shown in Figure 2a, the peaks with binding energies of 779.6 eV and 795.6 eV are attributed to 2p3/2 and 2p1/2 electrons of the CoIII ions. Meanwhile, the peaks at 781.2 eV and 796.9 eV belong to the 2p3/2 and 2p1/2 electrons of the CoII ions. Moreover, the peaks at 784.1 eV and 800.4 eV are attributed to the CoII satellites and the peaks at 803.0 eV and 786.8 eV are attributed to the CoIII satellites. All these observations correspond to the presence of mixedvalance [CoIICoIII2(µ3-O)] SBUs, in good agreement with the crystallographic result.


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Figure 1. The building blocks (a) , structures of Cage A and Cage B (b), the 3D crystal packing (c) and the surface view of the 3D porous structure (d) of 1. Color: Co, violet; C, gray; N, blue, O,red, H atoms are omited for clarity. To further confirm the present of µ3-O2− rather than µ3-OH−, we perform the Infrared (IR) measurement on the activated 1 (Figure S7). As we all known, the hydroxyl group has typical band at 3670-3580 cm−1 with strong intensity, which belongs to the O-H stretching vibrations. However, the band was not observed in our case. In addition, experientially, µ3-O2− is usually planar coordinated with the metal ions, but the µ3-OH− is often coordinated out of the plane. Thus, we can safely conclude that µ3-O2− is the bridging group in 1 rather than µ3-OH−. Also, the temperature-dependent direct-current (dc) susceptibility was measured to reveal the spin state of 1. As shown in Figure 2b, the χMT product at room temperature is 9.66 cm3 K mol−1, higher than the spin-only value of one CoII ion and two high-spin CoIII ions (7.87 cm3 K mol−1 for g = 2.0),15 owing to the orbital contribution of CoII ions. Upon cooling, χMT decreases fast and finally reaches to 1.94 cm3 K mol−1 at 2 K, which is close to the value of a magnetically


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isolated CoII ion (1.73 cm3 K mol−1 for Seff = 1/2 with g = 4.3).16 In addition, the magnetization at 2.0 K increases rapidly in low field but slowly in high field, finally reaches 2.98 µB at 7 T. Fitting the data above 150 K with Curie–Weiss law gives C = 12.22 cm3 K mol−1 and θ = −74.96 K. The large negative value of Weiss constant results from strong antiferromagnetic interactions as well as the spin-orbit coupling of CoII ions. To quantitatively analysize the magnetic interaction, the dc susceptibility data above 30 K are modelled by the following Hamiltonian using PHI program:17

= −2 ∙ + ∙ − 2 ∙ + 1

where J1 and J2 are the exchange coupling between spins, g is Landé factor. Using S1 = S3 = SCo(III) = 2, S2 = SCo(II) = 3/2 ion, and fixed g1 = g3 = 2.30 we obtained the best fit of J1 = −6.68 cm−1, J2 = −2.61 cm−1 and g2 = 2.43 (Figure 2b, inset). The simulation further confirms the presence of mixed-valance [CoIICoIII2(µ3-O)] SBUs, in which CoIII ions are high spin and antiferromagnetic coupled. The cyclic voltammetry (CV) measurement was performed to reveal the redox activity of the cobalt ions (Figure 2d). The redox couple [CoIICoIII2(µ3-O)]6+/[CoIII3(µ3-O)]7+ was observed at E1/2 = −825 mV vs Ag/Ag+ and the separation between the oxidation and the reduction peaks is 140 mV. Such a separation comes from slow electron transfer kinetics/low conductivity of the MOF materials.18 The quasi-reversibility of the redox process was confirmed by the calculated ratio of | / | ≈ 1, and the shapes of CV curves maintain after three cycles, indicating 1 is stable during the redox process. In addition, detailed electrochemical measurements give the molar ratio of CoII:CoIII = 1:1.7 (see Supporting Information for detail), which in agree with the presence of mixed-valance CoIICoIII SBUs.


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(b)

3+

10 9

3

χMT / cm K mol

−1

Co 2+ Co

8 7

3.0

6

2.5

M / Νβ

(a)

5 4

805

800

795

790

785

780

0.0

(c)

50

i/A

−1

dV/dW (cm g nm )

1.6

50

3

3

4

5

6

7

150

200

250

300

-0.6

-0.4

First cycle Second cycle Third cycle

0.0

1.2

−1

3

100

2

T/K

20.0µ

200 150

100

(d)

Adsorption Desorption

1

H/T

-1

250

0

0

775

Binding Energy / eV

2.00 K 3.00 K 4.00 K 5.00 K

1.5

0.5

2 810

2.0

1.0

3

Amount Absorbed (cm g , STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-20.0µ 0.8 0.4

-40.0µ 0.0 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Pore Width / nm

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 P / P0

-60.0µ

-1.4

-1.2

-1.0

-0.8

E / V vs Ag/AgCl

Figure 2. (a) Co 2p XPS spectra of 1. (b) χMT versus T plot for 1 under 1000 Oe dc field, Solid line is the best fit. Inset: top, the potential magnetic structure in 1, down, field dependent magnetization of 1, Solid lines are a guide for the eye. (c) The N2 adsorption isothremal at 77 K. Inset, the pore size distribution. (d) Cyclic voltammogram of 1 in 0.1 M NBu4PF6 in degassed CH3CN with scan rate 50 mV/s. The N2 adsorption isothermal of 1 at 77 K shows a type-I microporous feature (Figure 2c). The gas uptake increases sharply in low pressure region and up to 206 cm3 g−1 (STP) at 0.22 P/P0, attributing to monolayer adsorption. Then the N2 uptake increases slowly and finally reaches 256 cm3 g−1 at 1 bar. For the degasing process, no hysteresis was observed. The Langmuir surface area is calculated to be 1008 m2g−1. Furthermore, The pore size distribution was calculated by


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dV/dW plot (Figure 2c, inset), and two maximum values appear at 0.58 nm and 0.70 nm, corresponding to cage A and cage B, respectively. Since compound 1 displays significant porosity as well as redox activity and stability, it may be a good heterogeneous catalysis for selectively oxidizing alkene. Thus, we chose cyclohexene as a model substrate to test this hypothesis. Two oxidants are chosen as representatives. The first one is the tert-butylhydroperoxide (TBHP). After activation, equimolar reactants of cyclohexene and TBHP were mixed in the present of 1 at 70 oC. The conversion rates were monitored in situ by gas chromatography-mass spectrocopy (GC-MS) using 1,2,4-trichlorobenzene as internal standard. During the first two hours (Figure 3a), a 40 % conversion was observed, which corresponds to a turnover frequency of 144 h−1 (based on Co3 SBU). In the following 22 hours a total 63 % conversion was observed, corresponding to a turnover number 360.

(a)

70

(b)

Toatal conversion b a1

70

Toatal conversion a1 b

60

Conversion / %

60

Conversion / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 40 30 20 10

50 40 30 20 10

0 0

5

10

15

Time / h

20

25

0

1

2

3

Run Number

4

Figure 3. (a) The conversion rate of the oxidation of cyclohexene using TBHP catalyzing by 1. (b) The recyclability test for 1.

Table 1. Selected MOFs with catalytic activity towards oxidation of cyclohexene using TBHP as an oxidant.


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Complex Conv% a% b% Other% Ref [CuII (p-L)2][a] 82 56 19 25 19 63 86 14 0 Here 1 [CoII (BPB)][b] 62 83

III) MetalâOrganic Framework for Selective

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