New topological Co2(BDC)2(DABCO) as a highly active heterogeneous catalyst for the amination of oxazoles via oxidative C-H/N-H couplings

Article abstract of DOI:10.1039/c5cy01145k

Herein, we report the synthesis, structural characterization, and catalytic properties of a new cobalt-based metal-organic framework, Co₂(BDC)₂(DABCO) (where BDC = 1,4-benzenedicarboxylate and DABCO = 1,4-diazabicyclo[2.2.2]octane), namely VNU-10 (where VNU = Vietnam National University). Single-crystal X-ray diffraction analysis revealed that VNU-10 crystallizes in the hexagonal space group P6/mmm. The structure of this material is comprised of two-dimensional kgm layers which are interconnected by DABCO pillars to afford an overall three-dimensional porous architecture. VNU-10 exhibited exceptional catalytic activity toward the direct amination of oxazoles via C-H/N-H couplings while a previously reported topological isomer, Co₂(BDC)₂(DABCO), with the sql topology, displayed poor activity. Leaching tests indicated that homogeneous catalysis via leached active cobalt species is unlikely. Furthermore, the VNU-10 catalyst was facilely isolated from the reaction mixture and reused several times without degradation of the catalytic reactivity.

Full text of DOI:10.1039/c5cy01145k

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New topological Co2(BDC)2(DABCO) as highly active heterogeneous catalyst for
amination of oxazoles via oxidative C-H/N-H couplings
Thach N. Tu,a, b,† Khoa D. Nguyen,a,† Truong N. Nguyen,a Thanh Truong,a,* and Nam T.
S. Phana,*
aFaculty of Chemical Engineering, HCMC University of Technology, VNUꢀHCM, 268 Ly Thuong Kiet,
District 10, Ho Chi Minh City, Vietnam.
bCenter for Molecular and NanoArchitecture (MANAR), VNUꢀHCM, Ho Chi Minh City 721337, Vietnam.
†These authors contributed equally.
*To whom correspondence should be addressed: tvthanh@hcmut.edu.vn and ptsnam@hcmut.edu.vn
Abstract
Herein, we report the synthesis, structural characterization, and catalytic properties of a
new cobaltꢀbased metalꢀorganic framework, Co2(BDC)2(DABCO) (where BDC = 1,4ꢀ
benzenedicarboxylate and DABCO = 1,4ꢀdiazabicyclo[2.2.2]octane), termed VNUꢀ10
(where VNU = Vietnam National University). Single crystal Xꢀray diffraction analysis
revealed VNUꢀ10 crystallizes in the hexagonal space group P6/mmm. The structure of
this material is comprised of twoꢀdimensional kgm layers, which are interconnected by
DABCO pillars to afford an overall threeꢀdimensional porous architecture. VNUꢀ10
exhibited exceptional catalytic activity toward the direct amination of oxazoles via Cꢀ
H/NꢀH couplings while a previously reported topological isomer, Co2(BDC)2(DABCO),
with the sql topology, displayed poor activity. Leaching tests indicated that homogeneous
catalysis via leached active cobalt species is unlikely. Furthermore, the VNUꢀ10 catalyst
was facilely isolated from the reaction mixture and reused several times without
degradation of the catalytic reactivity.
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Keywords: metalꢀorganic frameworks; porous materials; amination; CꢀH
functionalization; heterogeneous catalyst.
1. Introduction
Molecules containing arylꢀ and heteroarylamine moieties are frequently found in a
variety of biologically active natural products, polymers, as well as functional materials
[1,2]. Traditional routes to access these molecules most often employ nitrenoid chemistry
[3,4]. Since the pioneering work by Buchwald and Hartwig [5,6], metalꢀmediated CꢀN
bond formation directly from CꢀH bonds have attracted an increasing amount of interest
[7]. To perform such transformations, nitrogen moieties have been commonly installed
by using protected amines or hydroxylamine derivatives as starting materials [8,9].
Simple amine coupling partners employed in direct CꢀN bond formation of
unfunctionalized arenes or heterocyclic compounds have rarely been reported. Typically,
palladium and rhodium were exploited as catalysts for these transformations, mostly in
the intramolecular fashion [10ꢀ13]. The first method for copperꢀcatalyzed directed
amination of arene CꢀH bonds was developed by the Yu group [14]. Subsequent reports
have demonstrated ortho amination of 2ꢀphenylpyridine derivatives through the use of
copper salts [15ꢀ17]. Furthermore, Miura, Chang, and Schreiber independently described
procedures for deprotonative thiazole and oxazole amination under copper catalysis
[8,18,19]. The first example of silverꢀmediated benzoxazole amination by formamides or
parent amines has also been disclosed [20]. Furthermore, it was recently reported that
cobalt and manganese salts are catalytically active for the transformation [21]. By
performing reactions under acidic conditions, Chang and coꢀworkers were able to direct
oxidative amination of a variety of azoles in the presence of a peroxide oxidant [21].
Despite the significant progress that has been made toward developing highly active
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homogeneous catalysts, the development of more economically and environmentally
efficient protocols, most notably by using heterogeneous catalytic systems, for this
transformation has yet to be reported.
Metalꢀorganic frameworks (MOFs) are a new class of porous materials with distinct
structural properties, which has led to intensive examination of their properties for
potential applications in several areas, including gas storage, sensors, thin films, optics,
drug carriers, and catalysis [22ꢀ26]. MOFs have also been pursued as heterogeneous
catalysts by virtue of inherent catalytically active sites within the frameworks or as
supporting materials for other catalysts [27ꢀ31]. There have been many reports of MOFs
having been investigated as heterogeneous catalysts or catalyst supports for a variety of
organic transformations including cross coupling, oxidations, and multicomponent
reactions [32ꢀ39]. Among the most popular MOF catalysts, zincꢀ [28ꢀ31], copperꢀ [40ꢀ
44], and nickelꢀbased [45] structures have received the most interest. The use of Coꢀ
based MOFs as catalysts has mainly focused on oxidation or olefin epoxidation reactions
[46ꢀ48]. Reported procedures using heterogeneous Co(II)ꢀMOFs for cross coupling
reactions are rare due to the fact that mostly lowꢀvalent cobalt species, Co(0), are found
to be active under organometallic viewpoints [49]. It has been recently demonstrated that
the catalytic activities of MOFs containing transition metals were greatly affected by their
structural topologies as well as the organic linkers [50]. In this work, by modifying the
synthetic conditions, we report the synthesis and discovery of a new topological isomer
of Co2(BDC)2(DABCO), termed VNUꢀ10. Full structural analysis was performed in
conjunction with single crystal Xꢀray diffraction analysis to support the new structure.
Remarkably, VNUꢀ10 exhibited exceptional catalytic activity toward direct amination of
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azoles by NꢀH bonds at room temperature while no activity was observed when the
previously reported Co2(BDC)2(DABCO) topological isomer was employed.
2. Experimental section
2.1. Materials and instrumentation
All reagents and starting materials were obtained commercially from SigmaꢀAldrich and
Merck, and were used as received without any further purification unless otherwise
noted. The single crystal Xꢀray diffraction data for VNUꢀ10 was collected on a Bruker
D8 Venture diffractometer equipped with a PHOTONꢀ100 CMOS detector.
Monochromatic microfocus Cu Kα radiation (λ = 1.54178 Å) operated at 50 kW and 1.0
mA was used. Prior to data collection, a single crystal of VNUꢀ10 was cooled down to
100 K by chilled nitrogen flow controlled by a Kryoflex II system. Unit cell
determination was performed in the Bruker SMART APEX II software suite. The data
sets were reduced and a multiꢀscan spherical absorption correction was implemented in
the SCALE interface [51]. The structures were solved with direct methods and further
refined by the fullꢀmatrix leastꢀsquares method in the SHELXꢀ97 program package [52].
After locating framework atoms in the difference Fourier maps, the SQUEEZE routine in
PLATON was run to remove scattering from the disordered guest molecules inside the
pores [53]. Crystallographic data for VNUꢀ10 has been deposited in the Cambridge
Crystallographic Data Centre (deposition no. 1063411). This data can be obtained, free of
charge, via the web (www.ccdc.cam.ac.uk/data_request/cif). Powder Xꢀray diffraction
patterns (PXRD) were collected using a Bruker D8 Advance equipped with a Ni filtered
Cu Kα radiation (λ = 1.54178 Å) source. The diffractometer was also equipped with an
antiꢀscattering shield that prevented incident diffuse radiation from hitting the detector.
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Nitrogen physisorption measurements were conducted using a Micromeritics 2020
volumetric adsorption analyzer system. Samples were pretreated by heating under
vacuum at 150 °C for 3 h. A Netzsch Thermoanalyzer STA 409 was used for
thermogravimetric analysis (TGA) with a heating rate of 10 °C/min under a nitrogen
atmosphere. Scanning electron microscopy studies were conducted on a JSM 740
scanning electron microscope (SEM). Transmission electron microscopy studies were
performed using a JEOL JEM 1400 Transmission Electron Microscope (TEM) at 100 kV.
In a typical experiment, VNUꢀ10 sample was dispersed on holey carbon grids prior to
TEM observation. Elemental analysis with atomic absorption spectroscopy (AAS) was
performed on an AAꢀ6800 Shimadzu. Fourierꢀtransform infrared (FTꢀIR) spectra were
obtained on a Nicolet 6700 instrument, with samples being dispersed on potassium
bromide pallets. The chemisorption experiments were studied in a Micromeritics 2020
analyzer. For hydrogen temperature programmed reduction (H2ꢀTPR), the sample was
outgassed at 100 °C for 30 min with helium, then cooled down to room temperature, and
exposed to 50 mL/min of 10% H2/Ar as the temperature ramped at 2.5 °C/min to 600 °C.
The amount of hydrogen consumption was determined from TCD signal intensities,
which were calibrated using an Ag2O reference sample.
Gas chromatographic (GC) analyses were performed using a Shimadzu GC 2010ꢀPlus
equipped with a flame ionization detector (FID) and an SPBꢀ5 column (length = 30 m,
inner diameter = 0.25 mm, and film thickness = 0.25 ꢁm). The temperature program for
GC analysis held samples at 120 °C for 1 min; heated them from 120 to 180 °C at 50
°C/min; held them at 180 °C for 1 min; heated them from 180 to 280 °C at 50 °C/min and
held them at 280 °C for 3 min. Inlet and detector temperatures were set constant at 280
°C. Diphenyl ether was used as an internal standard to calculate reaction conversions.
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GCꢀMS analyses were performed using a Hewlett Packard GCꢀMS 5972 with a RTXꢀ
5MS column (length = 30 m, inner diameter = 0.25 mm, and film thickness = 0.5 ꢁm).
The temperature program for GCꢀMS analysis heated samples from 60 to 280 °C at 10
°C/min and held them at 280 °C for 2 min. Inlet temperature was set constant at 280 °C.
1
MS spectra were compared with the spectra gathered in the NIST library. H and 13C
NMR spectra were recorded in CDCl3 using TMS as an internal standard on a Bruker
NMR spectrometer at 500 MHz and 125 MHz, respectively. Elemental microanalyses
(EA) were performed in the Microanalytical Laboratory of the College of Chemistry at
UC Berkeley, using a Perkin Elmer 2400 Series II CHNS elemental analyzer.
2.2. Synthesis of the metal-organic framework Co2(BDC)2(DABCO), VNU-10
In a typical synthesis procedure, a mixture of 1,4ꢀbenzenedicarboxylic acid (H2BDC) (0.1
g, 0.60 mmol), 1,4ꢀdiazabicyclo[2.2.2]octane (DABCO) (0.075 g, 0.67 mmol), and
Co(NO3)2·6H2O (0.1 g, 0.34 mmol) was dissolved in a solvent mixture of N,Nꢀ
dimethylformamide (DMF) (20 mL), CH3COOH (2 mL, 0.01 mmol), and HCl (20 ꢁl,
0.24 ꢁmol). The resulting solution was then dispensed equally to ten different 10 mL
vials. The vials were heated at 120 °C in an isothermal oven for 12 h. After cooling the
vials to room temperature, the solid product was removed by decanting the mother liquor
and then washed with DMF (3 × 10 mL) to remove any unreacted species. The DMF
solvent was exchanged with dichloromethane (DCM) (3 × 10 mL) at room temperature.
The product was then dried at 120 °C for 4 h under vacuum, yielding green needleꢀ
shaped crystals of VNUꢀ10 (76% based on Co(NO3)2·6H2O).
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2.3. Catalytic studies
A mixture of benzoxazole (0.101 mL, 1 mmol), acetic acid (0.114 mL, 2.0 mmol), and
diphenyl ether (0.15 mL, 0.95 mmol), as an internal standard, in acetonitrile (5 mL) was
added together into a 25 mL round bottom flask containing VNUꢀ10 catalyst (0.0143 g, 5
mol%). The catalyst amount was calculated with respect to the cobalt/benzoxazole molar
ratio. The reaction mixture was magnetically stirred for 3 min to disperse the catalyst
throughout the liquid phase. Piperidine and tertꢀbutyl hydroperoxide (TBHP) were then
added. The resulting mixture was continuously stirred at room temperature for 1 h.
Reaction conversion was monitored by withdrawing aliquots from the reaction mixture at
different time intervals, followed by quenching with aqueous KOH solution (5% (w/w), 1
mL). The organic components were then extracted into ethyl acetate (2 mL), dried over
anhydrous Na2SO4, and then analyzed by GC with reference to diphenyl ether. The
product identity was further confirmed by GCꢀMS, 1H NMR, and 13C NMR. In recycling
studies, the catalyst was separated from the reaction mixture by centrifugation, washed,
and heated with copious amounts of DMF at 100 °C for 2 h. The recovered VNUꢀ10 was
then activated under vacuum at room temperature for 4 h, and reused for the next run
under identical conditions. For the leaching test, the catalytic reaction was stopped after 5
min, analyzed by GC, and centrifuged to remove the solid catalyst. The reaction solution
was then stirred for an additional 55 min. Reaction progress, if any, was monitored by
GC as previously described.
3. Results and discussion
3.1. Catalyst synthesis and characterization
Kaskel and coꢀworkers previously reported the formation of a Co2(BDC)2(DABCO)
framework with a pillared sql topology, composed of squareꢀgrid layers and DABCO
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pillars (Fig. 1) [54]. Isoreticular (having the same topology) structures have also been
reported for Zn and Ni [55]. In the presence of CH3COOH and HCl additives to the
synthetic conditions, a different topology for Co2(BDC)2(DABCO), which we term
VNUꢀ10, was obtained.
To clarify the effect of added acids on the formation of new topology, experiments using
different amount of acids were conducted (See Supporting Information). Specifically, the
color of DMF solution containing Co(NO3)2•6H2O shifted from purple to deep blue when
2ꢀ
HCl was added. This suggests the formation of (CoCl4)2ꢀ from Co(H2O)6 . Interestingly,
the results also indicated that pure phase of new topology was selectively formed under
restricted conditions. Thus, using less or more amount of acids than the reported numbers
led to the formation of previous sql topology. Though the effect of CH3COOH and HCl
to the formation of VNUꢀ10 is abundantly unclear, changing of pH and anionic
environment (AcOꢀ, Clꢀ) are likely the key factors.
Single crystal Xꢀray diffraction analysis of this topological isomer revealed that the
structure consisted of cobalt paddlewheel units that are connected, in a slightly bent
fashion, by four BDC linkers (Fig. 1, Table S1). This cluster formation coupled with the
bent nature of the linkage allowed the formation of twoꢀdimensional kagome (kgm)
sheets (Fig. 1). Furthermore, DABCO ligands on the apical sites of the paddlewheel
cobalt clusters link each kgm sheet to one another. Consequently, a threeꢀdimensional
framework is formed with 4.5 Å triangular and 15 Å hexagonal channels running along
the c axis and a ~3.5 Å rectangular channel along the a and b axes (Fig. 1). It is noted that
the pillared kgm topology has been observed for Zn2(BDC)2(DABCO) and
Ni2(BDC)2(DABCO) isomers [55ꢀ58], however, this is the first report of the Coꢀbased
version.
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Fig. 1. (a) Paddlewheel cobalt cluster unit and ballꢀandꢀstick model of 1,4ꢀbenzenedicarboxylate
used to construct VNUꢀ10. (b) Expansion node in the 4,4ꢀconnected sql grid. (c) Crystal structure
of DABCO pillared 4,4ꢀconnected sql framework (Co2(BDC)2(DABCO)) along c axis. (d)
Expansion node in the 3,4ꢀconnected kgm grid. (e) Crystal structure of DABCO pillared 3,4ꢀ
connected kgm framework (VNUꢀ10) along c axis. Atom colors: C, black; O, red; N, blue; and
Co, purple polyhedra.
Phase purity of the bulk sample was assessed by PXRD analysis. For this analysis, PXRD
patterns were calculated and generated based on structural models of
Co2(BDC)2(DABCO) for both the pillared kgm and sql topological isomers and
compared with the measured experimental pattern for VNUꢀ10. The results indicated that
the structure of the asꢀsynthesized VNUꢀ10 was consistent with the single crystal data for
the pillared kgm Co2(BDC)2(DABCO) isomer (Fig. 2a). Furthermore, elemental analysis
by AAS indicated a cobalt amount of 20.0%, which is line with the theoretical value of
20.9%. SEM observation displayed wellꢀshaped, high quality needle crystals (Fig. 2b).
Furthermore, the TEM micrograph revealed a complex porous structure as expected (Fig.
S3). Nitrogen isotherm measurements at 77 K confirmed the permanent porosity of
VNUꢀ10 with calculated BrunauerꢀEmmettꢀTeller and Langmuir surface areas of 2235
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and 2256 m2 gꢀ1, respectively (Fig. 2c). Thermal gravimetric analysis (TGA) of activated
VNUꢀ10 demonstrates the material’s high thermal stability and architectural robustness
(>340 °C) (Fig. 2d). In addition, FTꢀIR spectra of VNUꢀ10 indicated the presence of
DABCO ligand and bonded carboxylate organic linkers (Fig. S7). Finally, elemental
analysis results supported the molecular formula of Co2C22H26O11N2 (See Supporting
Information).
Fig. 2. (a) Simulated PXRD patterns of the pillared sql (green) and pillared kgm (blue)
Co2(BDC)2(DABCO) isomers as compared to the asꢀsynthesized VNUꢀ10 (red). It is noted that
the PXRD pattern of VNUꢀ10 satisfactorily corresponds with the simulated pattern for the
pillared kgm isomer. (b) Scanning electron microscopy image of VNUꢀ10 demonstrating the
material’s wellꢀdefined needleꢀshape. (c) N2 isotherm at 77 K. Filled and open circles represent
the adsorption and desorption branches, respectively. (d) Thermogravimetric analysis of
activated VNUꢀ10.
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3.2 Catalytic studies
In reaction screening, the catalytic activity of VNUꢀ10 was assessed in the direct
oxidative amination of benzoxazole with piperidine at room temperature using the
previously reported tertꢀbutyl hydroperoxide (TBHP) oxidant (Table 1). Kinetic studies
of all entries were placed in Supporting Information.
Table 1. Reaction condition development.a
Entry
Ratio
Amount of
Solvent
Acid
GC yield (%)
catalyst (%)
NꢀH/CꢀH
1
2
1
1.5
2
3
3
3
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CO2H
CH3CO2H
37
60
CH3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
CH3CO2H
none
3
81
4
2
2
2
2
2
2
2
2
2
2
2
5
1
5
5
5
5
5
5
5
5
5
98 (86)
58
5
6
CH2Cl2
Toluene
Dioxane
Ethanol
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
89
7
36
8
59
9
73
10
11
12
13
14b
8
C6H5CO2H
HCO2H
87
80
TFA
75
CH3CO2H
69
aVolume of solvent 2 mL, 1.0 mmol scale, 60 m. Conversion by GC analysis. bReaction using 1 equiv. of
acetic acid. Numbers in parentheses indicate the isolated yields. Kinetic studies of entries were placed in
Supporting Information.
Initial studies addressed the effect of the reagent molar ratio, which was
determined to have a significant effect on the reaction conversion. The amination reaction
with 3% catalyst could proceed to 81% conversion after 1 h when using two equivalents
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of piperidine, whereas a remarkable decrease in reaction efficiency was observed when
lower amounts of piperidine was employed (entries 1ꢀ3). Interestingly, close to
quantitative conversion was achieved for the reaction using 5% catalyst (entry 4). With
respect to solvents, reasonable reaction rates were also observed in dichloromethane and
ethanol (entries 6, 9). Other solvents, such as dioxane and toluene, were shown to be less
sufficient when <60% conversions were obtained (entries 7, 8). Previously reported
optimized conditions for homogeneous copperꢀcatalyzed amination of sp2 CꢀH bonds
involved the use of nonpolar solvents, such as toluene and xylene [18, 59, 60]. In
addition, when compared to acetic acid, benzoic and formic acid afforded slightly lower
activity while only 75% was obtained with trifluoroacetic acid (TFA) (entries 11ꢀ13).
Reactions with 1 equiv. of acid or no acid produced conversions of 69% and 8%,
respectively (entries 10, 14).
Table 2. Effect of various oxidants on reaction yields.
Entry Oxidant
GC yield (%)
1
2
3
4
5
6
H2O2
19
30
DTBP
K2S2O8
trace
< 5
92
None, only air
PhI(OAc)2
TBHP in Ar
97
aVolume of solvent 2 mL, 1.0 mmol scale, 60 m. Kinetic studies
of entries were placed in Supporting Information
The effect of various oxidants on reaction yields was also investigated (Table 2).
The direct amination reaction was carried out at room temperature in CH3CN at 5 mol%
VNUꢀ10 catalyst in the presence of two equivalents of tested oxidant and two equivalents
of acetic acid. In particular, reactions using hydrogen peroxide and diꢀtertꢀbutyl peroxide
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proceeded with difficulty, affording only 19% and 30% conversions, respectively (entries
1,2). The inorganic oxidant, K2S2O8, was found to be unsuitable with only a trace amount
of product being detected (entry 3). Similarly, low reaction conversion was observed
when only air was employed as the reaction oxidant (entry 4). In contrast, reactions that
used PhI(OAc)2 as the oxidant resulted in good conversion while role of air in reaction
atmosphere was showed to be negligible (entries 5, 6). The use of peroxide oxidant in the
pioneering work of Chang was proposed to form corresponding alkoxy and alkylperoxy
radicals, which further abstract the hydrogen from reaction intermediates [21].
To highlight the advantages of our catalyst, the activity of the VNUꢀ10 was
compared to other similarly related MOFs, including Co2(BDC)2(DABCO) with squareꢀ
grid layers (Co2(BDC)2(DABCO)sql), as well as other common Co(II) salts (Table 3).
Under the optimized conditions, only 12% and 21% conversions were detected for the
case of Ni2(BDC)2(DABCO)kgm and Cu2(BDC)2(DABCO)kgm, respectively (entries 2,3).
Similar results were obtained when DABCO ligand was removed from CoꢀMOFs
structure (entry 4). Unprecedentedly, low reaction conversion was obtained when
reported Co2(BDC)2(DABCO)sql was employed (entry 5). The remarkably high catalytic
activities of pillared kgm layered frameworks can be rationalized by their larger cavities.
CoꢀZIFꢀ67 is ineffective with an unappreciable amount of product being observed (entry
6). Other solid cobaltꢀbased catalysts such as magnetic ferrite CoFe2O4 and CoꢀZeolite X
also exhibited poor activity under tested conditions (entries 7, 8). Interestingly, common
Co(II) salts resulted in similar or slightly slower conversions when compared to the
VNUꢀ10 catalyst. However, it is worth mentioning that purification of contaminated
metals in final products of homogeneous catalysis often represents a problematic issue,
especially in the pharmaceutical industry [61].
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Table 3. Reactivity of related heterogeneous and homogeneous catalystsa
Entry Type
Catalyst
GC yield (%)
1
2
3
MOFs
VNUꢀ10 [Co2(BDC)2(DABCO)kgm
]
97
12
21
Ni2(BDC)2(DABCO)kgm
Cu2(BDC)2(DABCO)kgm
4
5
Co(BDC)(DMF) (CoꢀMOFꢀ71)
Co2(BDC)2(DABCO)sql
28
30
6
CoꢀZIFꢀ67
CoFe2O4
33
9
7
8
CoꢀZeoliteꢀX
18
90
79
93
9
Common salts CoCl2
Co(NO3)2
Co(OAc)2
10
11
aVolume of solvent 2 mL, 1.0 mmol scale. Kinetic studies of entries were placed in Supporting
Information.
In order to determine if active cobalt species that leaches out from the solid VNUꢀ
10 catalyst contribute to the total conversion of the direct benzoxazole amination
reaction, a control experiment was carried out (Fig. 3). In such an experiment, the direct
benzoxazole amination reaction was performed at room temperature in acetonitrile with 5
mol% VNUꢀ10 catalyst, two equivalents of piperidine, two equivalents of tertꢀbutyl
hydroperoxide, and two equivalents of acetic acid. The VNUꢀ10 catalyst was removed by
simple centrifugation from the reaction mixture 5 min after the reaction had been
initiated. The reaction solution was then transferred to a new reactor vessel, and stirred
for an additional 55 min at room temperature with aliquots being sampled at different
time intervals. It was observed that almost no further conversion was detected for the
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direct benzoxazole amination reaction after the VNUꢀ10 catalyst was separated from the
reaction mixture. Moreover, ICPꢀMS of reaction filtrate revealed that less than 20 ppm of
Co was presented. These observations confirm the heterogeneous nature of the developed
protocol and that catalysis by leached active species is unlikely.
Fig. 3. Leaching test with catalyst removal during reaction course. Conversion percentage
as a function of reaction time in the presence of the VNUꢀ10 catalyst (blue) and once
VNUꢀ10 was removed 5 min after the reaction started (red).
In addition, to further address the issue of where the catalysis takes place,
experiments using CoꢀMOF catalysts with different particle sizes were performed under
identical conditions (Fig. S21). It was found that similar reaction yields were obtained
(Scheme 1). About 10 % drop with CoꢀMOFs possessing large particle size might come
from the diffusion limitation. In addition, the size of the pore according to the CoꢀMOFs
structure is 15 Å running along the c axis while the kinetic diameters of aromatic
o
o
reactions and product are calculated to be 5 A and 8ꢀ10 A, respectively [62]. Though
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further studies are needed for better understanding, it is likely that reactions occur inside
the cavities.
Size of VNUꢀ10 (main
8 µm
98
30 µm
96
70 µm
84
distribution)
Yield
Scheme 1. Reactions with different Co2(BDC)2(DABCO) particle sizes. See SI for
more details.
For the development of more environmentally benign procedures, the ability to
recover and reuse the VNUꢀ10 catalyst was investigated. The reaction was then carried
out and upon completion, the catalyst was separated from the reaction mixture by
centrifugation. The recovered VNUꢀ10 was washed with acetonitrile and DMF, activated
under vacuum at room temperature for 4 h, and then reused in a following reaction under
identical conditions as the previous run. It was found that the VNUꢀ10 catalyst could be
recovered and reused several times without any significant degradation in catalytic
activity. Indeed, a conversion of 80% was achieved in the 10th run (Fig. 4). PXRD
analysis of the reused catalyst confirmed that the crystallinity of VNUꢀ10 was retained
over the course of the recycling studies (Fig. 5). Furthermore, the FTꢀIR spectra exhibited
a similar absorption as compared to that of the fresh VNUꢀ10. These results demonstrated
the potential of VNUꢀ10 as a heterogeneous catalyst for modern organic transformations.
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100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Run
Fig. 4. Catalyst recycling studies.
Fig. 5. PXRD (a) and FTꢀIR spectra (b) of the fresh (black) and reused (red) VNUꢀ10.
Reactions under optimized conditions using different amines were described
(Table 4). Functionality with secondary amines occurs at the weakest CꢀH bond in
benzoxazole. Piperidines with substituents at C3 and C4 are active and aminated products
were obtained in excellent yields (entries 2, 3). Amination by other cyclic secondary
amines such as morpholine and 4ꢀmethylpiperazine afforded reasonable yields with 53%
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and 57%, respectively (entries 4, 5). The optimized conditions were also applicable for
cross coupling reactions of Nꢀmethylbenzylamine (entry 6). Significant improvement in
reaction efficiency of aliphatic acyclic amine was achieved when reactions were
conducted at 60 °C (entry 7). Interestingly, a heterocyclic amine, which has not been
reported under homogeneous conditions, was crossꢀcoupled in moderate yield (entry 8).
Table 4. Reaction scope with respect to different amine substrates.
Entry
1
Amine
Product
Yields (%)
86
89
2
3
4
82
53
5
6
57
54
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40
7
8
95b
30b
Conditions: solvent (2 mL), benzoxazole (0.5 mmol).b reaction at 60
°C .
4. Conclusions
In summary, a new topological isomer of Co2(BDC)2(DABCO), VNUꢀ10, was
synthesized by a solvothermal method. Single crystal Xꢀray diffraction analysis of VNUꢀ
10 revealed the structure to be built from DABCOꢀpillared kagome layers that produced
triangular and hexagonal channels of sufficient size to carry out important organic
transformations. VNUꢀ10 was also characterized by a host of techniques, including
PXRD, SEM, TEM, FTꢀIR, TGA, AAS, and N2 physisorption measurements.
Furthermore, this material was used as an efficient heterogeneous catalyst for direct
amination reactions of oxazoles. Excellent conversions with a variety of amines were
obtained. Most remarkably, VNUꢀ10 offered significantly higher activity than that of
Co2(BDC)2(DABCO) with the pillared sql structure as well as other Coꢀbased
heterogeneous catalysts. VNUꢀ10 was confirmed to be recyclable without a significant
degradation in catalytic activity. Finally, leaching tests showed no contribution from
homogeneous leached active cobalt species.
Acknowledgements
The Vietnam National University – Ho Chi Minh City (VNUꢀHCM) is
acknowledged for financial support through project B2015ꢀ20ꢀ03 (the catalyst synthesis
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and characterization and the catalytic studies). The work at MANAR (single crystal
preparation and structural determination of VNUꢀ10) was supported by Vietnam National
University – Ho Chi Minh City (project B2011ꢀ50ꢀ01TĐ). We thank Mr. J.ꢀJ. Yang (UC
Berkeley) for his assistance with elemental microanalysis measurements; Mr. Kyle E.
Cordova, Mr. Ha L. Nguyen, and Ms. Ashley M. Osborn at MANAR for their valuable
discussions.
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References
1.
R. Hili, A.K. Yudin, Nature Chem. Biol. 2 (2006) 284ꢀ287.
2.
A. Armstrong, J.C. Collins, Angew. Chem. Int. Ed. 49 (2010) 2282ꢀ2285.
3.
K.ꢀH. Ng, A.S.C. Chan, W.ꢀY. Yu, J. Am. Chem. Soc. 132 (2010) 12862ꢀ12864.
A.N. Vedernikov, K.G. Caulton, Chem. Commun. (2004) 162ꢀ163.
E. Paul, J. Patt, J.F. Hartwig, J. Am. Chem. Soc. 116 (1994) 5969ꢀ5970.
A.S. Guram, S.L. Buchwald, J. Am. Chem. Soc. 116 (1994) 7901ꢀ7902.
G. Evano, N. Blanchard, M. Toumi, Chem. Rev. 108 (2008) 3054ꢀ3131.
T. Kawano, K. Hirano, T. Satoh, M. Miura, J. Am. Chem. Soc. 132 (2010) 6900ꢀ6901.
N. Matsuda, K. Hirano, T. Satoh, M. Miura., Org. Lett. 13 (2011) 2860ꢀ2863.
Y. Tan, J.F. Hartwig, J. Am. Chem. Soc. 132 (2010) 3676ꢀ3677.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
7655.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
E.T. Nadres, O. Daugulis, J. Am. Chem. Soc. 134 (2012) 7ꢀ10.
M. Wasa, J.ꢀQ. Yu, J. Am. Chem. Soc. 130 (2008) 14058ꢀ14059.
E.J. Yoo, S. Ma, T.ꢀS. Mei, K.S.L. Chan, J.ꢀQ. Yu, J. Am. Chem. Soc. 133 (2011) 7652ꢀ
X. Chen, X.ꢀS. Hao, C.E. Goodhue, J.ꢀQ. Yu, J. Am. Chem. Soc. 128 (2006) 6790ꢀ6791.
T. Uemura, S. Imoto, N. Chatani, Chem. Lett. 35 (2006) 482ꢀ483.
A. John, K.M. Nicholas, J. Org. Chem. 76 (2011) 4158ꢀ4162.
Q. Shuai, G. Deng, Z. Chua, D.S. Bohle, C.ꢀJ. Li, Adv. Synth. Catal. 352 (2010) 632ꢀ636.
Q. Wang, S.L. Schreiber, Org. Lett. 11 (2009) 5178ꢀ5180.
S.H. Cho, J. Yoon, S. Chang, J. Am. Chem. Soc. 133 (2011) 5996ꢀ6005.
S.H. Cho, J.Y. Kim, S.Y. Lee, S. Chang, Angew. Chem. Int. Ed. 48 (2009) 9127ꢀ9130.
J.Y. Kim, S.H. Cho, J. Joseph, S. Chang, Angew. Chem. Int. Ed. 49 (2010) 9899ꢀ9903.
H. Furukawa, K. E. Cordova, M. O'Keeffe, O. M. Yaghi, Science 341 (2013) 1230444.
Y.ꢀB. Zhang, H. Furukawa, N. Ko, W. Nie, H. J. Park, S. Okajima, K. E. Cordova, H.
Deng, J. Kim, O. M. Yaghi, J. Am. Chem. Soc. 137 (2015) 2641ꢀ2650.
24. D.T. Genna, A.G. WongꢀFoy, A.J. Matzger, M.S. Sanford, J. Am. Chem. Soc. 135 (2013)
10586ꢀ10589.
25.
N. T. T. Nguyen, H. Furukawa, F. Gándara, H. T. Nguyen, K. E. Cordova, O. M.
Yaghi, Angew. Chem. Int. Ed. 53 (2014) 10645ꢀ10648.
26.
27.
28.
29.
30.
M.ꢀL. Ma, C. Ji, S.ꢀQ.Zang, Dalton Trans. 42 (2013) 10579ꢀ10586.
Z.ꢀY. Gu, J. Park, A. Raiff, Z. Wei, H.ꢀC. Zhou, ChemCatChem, 6 (2013) 67ꢀ75.
P. Valvekens, F. Vermoortelea, D. D. Vos, Catal. Sci. Technol. 3 (2013) 1435ꢀ1445.
A. Corma, H. García, F.X. Llabrés i Xamena, Chem. Rev. 110 (2010) 4606ꢀ4655.
A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 41 (2012) 5262ꢀ5284.
21

Catalysis Science & Technology
Page 22 of 23
View Article Online
DOI: 10.1039/C5CY01145K
31.
32.
M. Yoon, R. Srirambalaji, K. Kim, Chem. Rev. 112 (2012) 1196ꢀ1231.
P. Wu, C. He, J. Wang, X. Peng, X. Li, Y. An, C. Duan, J. Am. Chem. Soc. 134 (2012)
14991ꢀ14999.
33.
A.S. Roy, J. Mondal, B. Banerjee, P. Mondal, A. Bhaumik, S.M. Islam, Applied
Catalysis A: General 469 (2014) 320ꢀ327.
34.
(a) N.T.S. Phan, T.T. Nguyen, P. Ho, K.D. Nguyen, ChemCatChem 5 (2013) 1822ꢀ
1831.(b) G. H. Dang, D. T. Le, T. Truong, N. T. S. Phan, J. Mol. Catal. A: Chem. 400 (2015)
162ꢀ169.
35.
Farrusseng, Green Chem. 11 (2009) 1729ꢀ1732.
36. E. PérezꢀMayoral, Z. Musilová, B. Gil, B. Marszalek, M. Položij, P. Nachtigall, J. Čejka,
Dalton Trans. 41 (2012) 4036ꢀ4044.
M. Savonnet, S. Aguado, U. Ravon, D. BazerꢀBachi, V. Lecocq, N. Bats, C. Pinel, D.
37.
38.
39.
M. Opanasenko, M. Shamzhy, J. Čejka, ChemCatChem 5 (2013) 1024ꢀ1031.
H. T. N. Le, T. V. Tran, N. T. S. Phan, T. Truong, Catal. Sci. Technol. 5 (2015) 851ꢀ859.
O.V. Zalomaeva, A.M. Chibiryaev, K.A. Kovalenko, O.A. Kholdeeva, B.S.
Balzhinimaev, V.P. Fedin, J. Catal. 298 (2013) 179ꢀ185.
40. (a) A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Adv. Synth. Catal. 351 (2009) 2271ꢀ
2276. (b) H. T.N. Le, T. T. Nguyen, P. H.L. Vu, T. Truong, N.T.S. Phan, J. Mol. Catal. A: Chem.
391 (2014) 74 ꢀ 82
41.
(a) A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Adv. Synth. Catal. 352 (2010) 711ꢀ717.
(b) N. T. T. Tran, Q. H. Tran, T. Truong, J. Catal. 320 (2014) 9ꢀ15.
42.
43.
A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Adv. Synth. Catal. 352 (2010) 3022ꢀ3030.
(a) L. Alaerts, E. Séguin, H. Poelman, F. ThibaultꢀStarzyk, P.A. Jacobs, D.E. De Vos,
Chem. Eur. J. 12 (2006) 7353ꢀ7363. (b) G. H. Dang, T. D. Nguyen, D. T. Le, T. Truong, N.T. S.
Phan, ChemPlusChem 79 (2014) 1129ꢀ1137.
44.
F.X. Llabrés i Xamena, A. Corma, J. Catal. 276 (2010) 134ꢀ140.
45. (a) N.T.S. Phan, C.K. Nguyen, T.T. Nguyen, T. Truong, Catal. Sci. Technol. 4 (2014)
(a) I. Luz, F.X. Llabrés i Xamena, A. Corma, J. Catal. 285 (2012) 285ꢀ291. (b) I. Luz,
369ꢀ377. (b) T. Truong, C. K. Nguyen, T. V. Tran, T. T. Nguyen, N. T. S. Phan, Catal. Sci.
Technol. 4 (2014) 1276ꢀ1285.
46.
Langstein, D. Volkmer, Angew. Chem. Int. Ed. 48 (2009) 7546ꢀ7550.
47. J. Zakzeski, A. Debczak, P.C.A. Bruijnincx, B.M. Weckhuysen, Applied Catalysis A:
General 394 (2011) 79ꢀ85.
M. Tonigold, Y. Lu, B. Bredenkotter, B. Rieger, Stefan Bahnmuller, J. Hitzbleck, G.
22

Page 23 of 23
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C5CY01145K
48.
Grunwaldt, A. Baiker, Chem. Eur. J. 18 (2012) 887ꢀ898.
49. (a) C. P. Lenges,M. Brookhart, B. E. Grant, J. Organomet. Chem. 528 (1997) 199; b) C.
M.J. Beier, W. Kleist, M.T. Wharmby, R. Kissner, B. Kimmerle, P.A. Wright, J.ꢀD.
P. Lenges, P. S.White, M. Brookhart, J. Am. Chem. Soc. 120 (1998) 6965; c) A. Wang, H. Sun,
X. Li, Organometallics 27 (2008) 5434.
50.
D. J. Tranchemontagne, J. L. MendozaꢀCortés, M. O’Keeffe, O. M. YaghiChem. Soc.
Rev., 38 (2009) 1257ꢀ1283.
51.
52.
C. H. Görbitz, ActaCryst. B55 (1999) 1090ꢀ1098.
(a) G. M. Sheldrick, SHELXL97 1997, Program for Crystal Structure Refinement,
University of Göttingen, Germany, 1997; (b) G. M. Sheldrick, SHELXL97, Program from Crystal
Structure Solution, University of Göttingen, Germany, 1997.
53.
54.
A. L. J. Spek, J. Appl. Crystallogr. 36 (2003) 7ꢀ13.
H. Wang, J. Getzschmann, I. Senkovska, S. Kaskel, Micropor. Mesopor. Mater. 116
(2008) 653ꢀ657.
55.
56.
57.
P. Maniam, N. Stock, Inorg. Chem. 50 (2011) 5085ꢀ5097.
H. Chun, J. Moon, Inorg. Chem. 46 (2007) 4371ꢀ4373.
T. Frisˇcˇic´, D.G. Reid, I. Halasz, R.S. Stein, R.E. Dinnebier, M.J. Duer, Angew. Chem.
Int. Ed. 122 (2010) 724ꢀ727.
58.
M. Kondo, Y. Takashima, J. Seo, S.K.S. Furukawa, CrystEngComm 12 (2010) 2350ꢀ
2353.
59.
D. Monguchi, T. Fujiwara, H. Furukawa, A. Mori, Org. Lett. 11 (2009) 1607ꢀ1610.
S. Guo, B. Qian, Y. Xie, C. Xia, H. Huang, Org. Lett. 13 (2011) 522ꢀ525.
60.
61.
(a) C. E. Garrett, K. Prasad, Adv. Synth. Catal. 346 (2004) 889. (b) C. J. Welch, J. A.ꢀ
Walker, W. R. Leonard, M. Biba, J. DaSilva, D. Henderson, B. Laing, D. J. Mathre, S. Spencer,
X. Bu, T. Wang, Org. Process Res. Dev. 9 (2005) 198.
62.
(a) H. Wang, M. Frenklach, Combust. Flame 96 (1994) 163. (b) J. Jae, G. A. Tompsett,
A. J. Foster, K. D. Hammond, S. M. Auerbach, R. F. Lobo, G. W. Huber, J. Catal. 279 (2011)
257–268
23