Anchorage of a ruthenium complex into modified MOF: Synergistic effects for selective oxidation of aromatic and heteroaromatic compounds

Article abstract of DOI:10.1039/c5ra18179h

The structure of IRMOF-3 was modified with pyridine-2-aldehyde and then the Schiff base moieties were used to anchor ruthenium complex to this metal-organic framework. The prepared catalyst was characterized by X-ray powder diffraction (XRD), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and FTIR methods. The results show that the prepared catalyst acquires a high potential for selective oxidation of aromatic and heteroaromatic compounds under mild conditions and easy workups.

Full text of DOI:10.1039/c5ra18179h

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Anchorage of a ruthenium complex into modified
MOF: synergistic effects for selective oxidation of
aromatic and heteroaromatic compounds†
Cite this: RSC Adv., 2015, 5, 101013
Khalil Tabatabaeian,* Mohammad Ali Zanjanchi, Nosrat. O. Mahmoodi
and Tooraj Eftekhari
The structure of IRMOF-3 was modified with pyridine-2-aldehyde and then the Schiff base moieties were
used to anchor ruthenium complex to this metal–organic framework. The prepared catalyst was
characterized by X-ray powder diffraction (XRD), diffuse reflectance spectroscopy (DRS), scanning
electron microscopy (SEM), transmission electron microscopy (TEM) and FTIR methods. The results show
Received 6th September 2015
Accepted 13th November 2015
DOI: 10.1039/c5ra18179h
that the prepared catalyst acquires
a high potential for selective oxidation of aromatic and
www.rsc.org/advances
heteroaromatic compounds under mild conditions and easy workups.
phase oxidations since they have advantages such as facile
recovery and recycling, and amenability to continuous opera-
1
. Introduction
7
MOFs possess unique properties among the various classes of tion as compared to their homogeneous counterparts. The
microporous and mesoporous materials generating interests MOF encapsulation approach invites comparison to earlier
for many applications. MOFs are remarkable for applications in studies of oxidative catalysis by zeolite-encapsulated Fe(por-
1
2
3,4
9,10
11,12
gas adsorption, separations, and catalysis, mostly because of phyrin)
as well as Mn(porphyrin) systems.
The rational
their chemical tunability, high porosities, and adequate design of perfect nanoporous catalysts that have a resemblance
5
thermal stability. In fact MOFs bridge the gap between zeolites to redox enzymes could be a hope in the eld of oxidation
and mesoporous materials. MOF pore systems range from catalysis. Direct oxidation of arenes is an easy and effective
ultramicropore to mesopore. Unique features of MOFs also method, and much research have been carried out to develop
include exceptionally high porosities, well-dened crystalline practical methods which would be applicable to various are-
13,14
structures, designable channel surface functionalities, frame- nes.
The common oxidation of arenes requires a large excess
work exibility, and exible dynamic behavior in response to of metal oxidant such as chromium(VI) oxide. Since the metal
guest molecules. The broad choice of structure that facilitates residues are environmentally undesirable and oen cause
pore size tunability would be a great opportunity for the design problems during reaction and work-up, improvement of selec-
of MOFs with pore openings suitable for generating size and tive arene oxidations requiring only a catalytic amount of metal
shape selectivity. However, MOFs still have several issues to be reagent in combination with an appropriate stoichiometric
solved. Thermal, hydrothermal and chemical stability are oxidant is a great challenge. Direct oxidation to diones has been
among the issues. Also, high material costs due to the use of reported in low yields by the oxidation with highly toxic osmium
15
organic linkers that are not commercially available from tetroxide. From an economical point of view, sodium period-
industry are concerned. This principle was also used in the ate (NaIO ) is inexpensive, readily available and generally in this
covalent functionalization of various MOFs by carrying out type of oxidation preferred to other. Although NaIO is not an
4
4
organic reactions directly on MOF crystals. Catalytic oxidation is atom efficient and environmentally benign oxidant because it
widely used in the manufacture of bulk and ne chemicals from gives stoichiometric amounts of inorganic waste, however it use
6
–8
organic hydrocarbons in liquid phase as well as gas phase.
Liquid phase oxidations generally employ soluble metal salts or unavoidable. In the previously reported methods ruthenium
for oxidation of stable cancerous polyaromatic arenes is
16–21
complexes in combination with oxidants such as O (or air), oxide species play a key role for this type of oxidation.
To
2
H O , or organic hydroperoxides (TBHP). But, heterogeneous overcome the prolonged reported reaction times we oxidized
2
2
solid catalysts have also been given some attention in liquid various arenes and heteroaromatic compounds by using ultra-
sonic irradiation. The results were surprising, since previous
works with Ru-catalyzed oxidations of arenes had resulted in
Department of Chemistry, Faculty of Sciences, University of Guilan, P.O. Box:
22
either dicarboxylic acids, tetracarboxylic acids or dioles. The
advantage of ultrasound is forming acoustic cavitations in
liquids, resulting in the initiation or enhancement of the
4
†
1
1335-1914, Iran. E-mail: Taba@guilan.ac.ir; Fax: +981333333262
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra18179h
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chemical activity in the solution. Moreover, MOFs appear to be increase the yield of products and reduction of wastes that
one of the excellent candidates as oxidative catalysis due to their formed during the preparation of IRMOF-3, in addition to the
frameworks which contain a larger content of reducible metal stirring and rising temperature of the reaction, the time was
23–27
ions than zeolite frameworks.
Here we show that a well- also increased from 24 h to 60 h. However increasing a time of
known MOF material, can be subjected to a sequence of reaction was only improveds the better crystal growth e.g. for
chemical reactions suitable for complexation of ruthenium. IRMOF-3 aer 36, 48 and 60 h, no improve in the yield of
IRMOF-3 represents a good model for a post-synthetic covalent product was observed. Elemental analysis was performed on
ꢀ
modication study. Current research includes heterogenization samples outgassed under vacuum (120 C, 12 h). Zn
4
O(ATA)
3
:
of homogeneous catalysts, through covalent attachment to (% calcd/found), C 35.4/35.5, H 1.86/1.9, N 5.16/5.21, FT-IR:
inorganic–organic matrices such as IRMOF-3 with mutual 3369 (br), 1620 (sh), 1568 (s), 1419 (s), 1385 (s), 1258 (m), 1155
advantages of both homogenous and heterogeneous catalysis. (w), 1064 (w), 948 (w), 898 (w), 836 (w), 768 (m), 582 (w), 517 (w).
Our synthesized catalyst exhibits catalytic activity as a reusable
2.1.4. Synthesis of IRMOF-3-PI, [Zn O(ATA)
(PITA)x]
4
3ꢁx
and green catalyst in oxidation of arenes and several hetero- (PITA ¼ 2-pyridyl-imine terephthalate). The isoreticular func-
aromatic compounds. It also could be used for desulfurization tionalized IRMOF-3-PI was synthesized according to the reported
29
of gas oil reactions containing heteroaromatic impurities. With procedure. (0.428 g) of pyridine-2-aldehyde was rst dissolved in
further exploration of this system, we developed a new Ru- 15 mL of dry CH CN. To this solution, freshly prepared des-
containing catalyst, IRMOF-3-PI-Ru, exploiting post-synthesis olvated IRMOF-3 (1 g) was immersed, and it was then allowed to
modication strategy of MOF. stand for 5 days under an inert (nitrogen) atmosphere. Aer
days, the resulting pale yellow colored solid was collected by
3
5
centrifugation, washed with dry CH CN, and dried in vacuo at
3
2
. Experimental
ꢀ
1
20 C. Elemental analysis was performed on samples outgassed
2.1. Materials and apparatus
ꢀ
under vacuum (120 C, 12 h). Anal. calcd for Zn
4
O(ATA)2.79
2
.1.1. Instrumentation. Ultrasonication was performed in (PITA)0.21: (% calc/found), C 37.20/37.10, H 1.93/1.92, N 5.50/5.45.
a TECNO-GAZ Tecna 3 ultrasonic cleaner with a frequency of 50/ 2.1.5. Synthesis of IRMOF-3-PI-Ru, [Zn O(ATA)3ꢁx(PITA-
0 Hz and a normal power of 250 W. The reaction ask was Ru(CO) ]. To obtain isoreticular metalation, a mixture of
4
6
4 x
)
located in the water bath of the ultrasonic cleaner, and the Ru
temperature of the water bath was controlled by a current of water drous CH
(CO)12 (0.3 mmol, 0.053 g) and 1 g of IRMOF-3-PI in anhy-
3
ꢀ
Cl
was stirred under reux condition at 40 C for the
2
2
at room temperature. IR spectra were recorded on a Shimadzu time specied. Where upon, the yellow crystalline material
FTIR-8400S spectrometer. XRD measurements were performed on became brown within several minutes. Aer 12 h, the brownish
˚
a Philips PW1840 diffractometer with Cu-Ka radiation (1.5418 A), yellow-colored material was washed three times with 10 mL
ꢀ
ꢁ1
ꢀ
scan rate 0.1 2q s and within a range of 2q of 4–60 . Scanning portions of CH
2 2
Cl . The crystals were then immersed in dry
electronmicrographs were taken on a LEO 1430 VP instrument. CH Cl , and the solvent was refreshed every 24 h for 3 days to
2
2
The metal contents were analyzed using inductively coupled yield IRMOF-3-PI-Ru. The solid phase was washed repeatedly
1
plasma (ICP; Labtam 8440 plasma lab). H NMR spectra were through Soxhlet extraction with ethanol and acetone and dried
13
ꢀ
obtained on a BrukerDRX-500 Avance spectrometer and C NMR at 60 C overnight, leading to ruthenium anchored IRMOF-3-PI
spectra were obtained on a BrukerDRX-125 Avance spectrometer. as a brownish powder. The amount of ruthenium metal in
1
13
Chemical shis of H and C NMR spectra were expressed in catalyst was found to be 2.62 w% based on ICP analysis.
ppm downeld from tetramethylsilane. Melting points were Elemental analysis was performed on samples outgassed under
ꢀ
measured on a Bȕchi Melting Point B-540 instrument and are vacuum (120 C, 12 h). Anal. calcd for Zn
4
O(ATA)2.79 (PITA-
uncorrected. Elemental analyses were made by a Carlo-Erba Ru(CO)
EA1110 CNNO-S analyzer and agreed with the calculated values. 5.2. The amount of Ru in the nal solid was determined by
.1.2. Materials. All materials were purchased from Merck the atomic absorption spectrometer: calcd: Ru 2.56%; found,
4
)0.21 (% calcd/found), C 35.5/35.4, H 1.86/1.90, N 5.28/
2
ꢁ
2
and Sigma-Aldrich and used without further purication. Ru 2.6% (2.4 ꢂ 10 mol%).
Ru (CO)12 (99%) was purchased from Acros.
2.1.6. Synthesis of MOF-5. MOF-5 were prepared following
.1.3. Preparation of IRMOF-3. IRMOF-3, Zn
was synthesized according to the literature with slight modi- two sharp peaks were obtained at 1580 and 1373 cm
3
3
0–32
2
4
O(C
8
H
5
NO
4
)
3
the procedure reported previously.
FTIR spectra of MOF-5,
ꢁ1
.
,
28
ꢁ1
cations. 0.9683 g zinc nitrate tetrahydrate and 0.2293 g of 2- Several small peaks were seen in the range of 1225 to 950 cm
amino-1,4-benzenedicarbocyclic acid were added to 30 mL of 900 to 670 and 500 cm . A broad peak at 3395 cm (ESI S1†).
ꢁ
1
ꢁ1
0
ꢀ
ꢀ
solvent N ,N-dimethyl formamide (DMF). The resulting Diffraction peaks at around 2q ¼ 6.9 and 9.87 are readily
suspension was stirred for 3 h at room temperature then heated recognized from the XRD pattern (ESI S2†). The observed
ꢀ
30
under autogeneous pressure at 105 C for 24 h in a Teon-lined diffraction peaks agree with the MOF-5.
autoclave. Aer decanting the hot mother liquor and rinsing
2.1.7. Synthesis of MIL-101. MIL-101 was synthesized
with DMF, the product was immersed in CHCl
during which the activation solvent was decanted and freshly several times, drying at 100 C. FTIR spectra and the XRD
replenished three times. The solvent was removed under patterns of MIL-101 are plotted in (ESI S3 and S4†). The main
vacuum at room temperature, yielding pale yellow cube-shaped diffraction peaks were at 2q ¼ 3.25, 5.10, 8.41, 9.02, 16.49, 17.40,
crystals (yield over 90% based on 2-aminoterephthalic acid). To
33
for 3 days, according to reported procedure. The sample was washing
3
ꢀ
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and relative diffraction intensities of the prepared sample was
19,34
found to be the same as the standard data for MIL-101.
2.2. General procedure for oxidation of aromatic and
heteroaromatic compounds under ultrasonic irradiation
conditions
2
.2.1. Ultrasonic assisted IRMOF-3-PI-Ru catalyzed oxida-
tion of aromatic (arenes) and heteroaromatic (benzothiophene)
compounds. A 20 mL ask was charged with CH Cl (4 mL), H
CN (4 mL), pyrene (202.25 mg, 1 mmol) and NaIO
2
2
2
O
(
5 mL), CH
3
4
(
1 g, 4.68 mmol, ꢃ4 eq.). IRMOF-3-PI-Ru (10 mg) was added into
the ask and the dark brown suspension was irradiated at room
temperature for 50 min. At rst, the mixture of dichloromethane
and water was found inactive when tested as a catalyst for the
Scheme 1 Procedure for obtaining MOFs containing the ruthenium
Schiff base complex.
ꢁ
oxidative cleavage of pyrene in the CH Cl /H O/IO system.
2
2
2
4
However, when CH CN was added to this inactive system, it
3
rapidly lead to pyrene-4,5-dione formation. Addition of water, on
the other hand is unavoidable for it is the only solvent that
dissolves sodium metaperiodate. It is also well known that
The free NH
cation via a simple condensation reaction, producing a 2-
pyridylimine (R–N]C–C N) moiety, useful as a bidentate
Pyridylimine formation activated the framework
2
groups were thus modied by covalent modi-

5
H
4
29,35–37
3
addition of CH CN to reaction mixtures in which catalysis had
ligand.
ceased (due to precipitation of the ruthenium catalyst in the toward metal sequestering, demonstrated by anchoring Ru to
course of an oxidation) results in dissolution of the precipitate the wall of IRMOF-3. The preparative procedure is summarized
and restoration of full catalytic activity. It should be noted that
in Scheme 1.
CH CN and water alone are not effective; the third solvent
3
The FTIR spectra of IRMOF-3, IRMOF-3-PI, and IRMOF-3-PI-
Ru were measured in the KBr pellet (Fig. 1).
The observed peaks for IRMOF-3 are conrmed by
21
component, CH
mixture was poured into 50 mL of water and the organic phase
was separated. The aqueous phase was extracted with CH Cl
Cl extracts were combined with the
2 2
Cl , plays an important role. The reaction
28
2
2
comparing with FTIR vibration bands in literatures. In the
FTIR spectrum of IRMOF-3-PI appearance of a new band at
(
3 ꢂ 10 mL). The CH
2
2
ꢁ1
organic phase, washed with brine (3 ꢂ 10 mL) and dried over 1655 cm (nC]N), that were absent in the case of IRMOF-3,
anhydrous sodium sulfate to give a dark orange solution. The clearly indicates that the IRMOF-3 framework has been organ-
solution was concentrated under reduced pressure and the
ically modied as shown in Fig. 1.
mixture was puried by preparative TLC (n-hexane : EtOAc,
In IRMOF-3-PI-Ru, the vibration bands for both the azome-
thine group and the C]N moiety of the pyridine ring shied to
the lower frequency range and are not well-resolved due to
overlapping with the band that appeared for the bending
10 : 3), pyrene-4,5-dione as main product was obtained at room
temperature with a 74% yield. A similar procedure was appli-
cable for the other reported products listed in Table 3.
2.2.2. General procedure for oxidation of solid suldes to
2
vibration mode of the –NH group. Upon complexation of Ru
their corresponding sulfoxides and sulfones under ultrasonic with the ligand, the C]N stretching frequency is shied to
ꢁ1
irradiation conditions. To a mixture of sulde (1 mmol) and lower frequency (nC]N ¼ 1650 cm ) indicating formation of Ru
ꢁ1
3
0% aq. H O (1 mmol) in EtOH (4 mL), catalyst (10 mg) was
2 2
complex (Fig. 1). In addition, a broad band at 1968 cm (nC^O)
added and the mixture was irradiated in the presence of ultra-
sound at room temperature for the time specied. Completion
of the reaction was indicated by TLC (n-hexane/EtOAc, 10 : 4).
ꢀ
Aer completion, the reaction mixture was cooled to 5 C. The
catalyst precipitated as solid at the bottom layer and was easily
separated by ltration. The product was at the upper liquid layer.
The organic layer decanted and the solution was concentrated
under reduced pressure to afford the essentially crud products.
Further purication was achieved by short column chromatog-
raphy on silica gel with n-hexane : EtOAc as eluent.
3. Results and discussion
3.1. Catalyst synthesis and characterization
28
Following the procedure reported previously and were acti-
3
0
vated by guest exchange with CHCl
3
,
followed by vacuum
ꢀ
drying at 120 C prior to further treatment.
Fig. 1 FTIR spectra of IRMOF-3, IRMOF-3-PI and IRMOF-3-PI-Ru.
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ꢀ
ꢀ
showed the characteristic peak at 2q ¼ 6.8 (200), 9.6 (220) and
ꢀ
ꢀ
1
1
3.7 (400). However, another peak appeared at 15.35 (420),
ꢀ
ꢀ
9.39 and 24.62 . The corresponding hkl indices of the most
important diffraction peaks are indicated. The XRD pattern of
IRMOF-3 corresponds well with the literature data, indicating
38–40
that the desired material is obtained.
Comparison of the X-
ray powder diffraction patterns of IRMOF-3, IRMOF-3-PI, and
IRMOF-3-PI-Ru showed that peak patterns of IRMOF-3 were not
altered. This clearly shows that the framework structure of
IRMOF-3 remained intact upon organic functionalization as
well as metal complex formation in IRMOF-3 (Fig. 2).
Diffuse Reectance Spectroscopy (DRS) of the catalyst
IRMOF-3-PI-Ru was performed on the solid catalyst (Fig. 3). The
DRS spectrum featured a strong band at 362 nm, which can be
ascribed to the charge transfer transition arising from p-elec-
tron interactions between the metal and ligand. A comparison
between spectrums of IRMOF-3-PI and IRMOF-3-PI-Ru, shows
that the molecule remains unchanged during the process of
anchoring (Fig. 3).
The SEM images (Fig. 4a and b), shows that the IRMOF-3 and
IRMOF-3-PI-Ru have a highly crystalline structure, and Ru is
located within the network (Fig. 4). The SEM image of IRMOF-3
indicates that the particles have cubic topological ordered with
a crystal size of ꢃ50–100 nm (Fig. 4a). The SEM image of the
prepared catalyst is shown in Fig. 4b. The particle size according
to Scherrer equation calculated from the XRD data was ꢃ19 nm.
Fig. 2 X-ray diffraction patterns of IRMOF-3, IRMOF-3-PI and IRMOF-
3
-PI-Ru.
which represent the stretching vibrations of carbon monoxide
groups (CO). This is related to the decomposition of this metal
cluster with conventional heating to afford Ru(CO)
4
and
subsequently coordination to the imine nitrogen (Scheme 1).
Powder X-ray diffraction patterns of IRMOF-3, IRMOF-3-PI,
and catalyst IRMOF-3-PI-Ru were recorded (Fig. 2). The result-
ing XRD pattern and related indexes (hkl) of the crystal IRMOF-3
Fig. 3 Diffuse reflectance spectroscopy (DRS) of IRMOF-3-PI and. Fig. 5 Nitrogen adsorption–desorption isotherms of IRMOF-3 on the
IRMOF-3-PI-Ru. top and IRMOF-3-PI-Ru on the bottom.
Fig. 4 SEM images of (a) IRMOF-3, (b) IRMOF-3-PI-Ru and TEM image of IRMOF-3-PI-Ru (c).
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a
Table 1 Optimization of reaction conditions for oxidation of arenes and heteroaromatic compounds
b
ꢀ
Entry
Catalyst (mg)
Ru content (mg)
NaIO
4
/arene
Time (min)
T ( C)
Yield (%)
1
2
3
4
5
6
Without catalyst (0)
IRMOF-3-PI-Ru (5)
IRMOF-3-PI-Ru (10)
IRMOF-3-PI-Ru (15)
IRMOF-3-PI-Ru (25)
IRMOF-3-PI-Ru (35)
0.00
0.12
0.24
0.36
0.60
0.84
12/1
4/1
4/1
4/1
4/1
4/1
360
60
50
50
50
50
50
r.t
r.t
r.t
r.t
r.t
5
c
65
74
74
75
76
a
b
c
ꢀ
Isolated yield. Measured by ICP. Preliminary activation of the as-prepared IRMOF-3-PI-Ru under vacuum at 120 C.
incorporation process and so offers an excellent matrix to
support highly dispersed PI-Ru species. The places with darker
contrast could be assigned to the presence of PI-Ru particles
and located in the IRMOF-3 cavities (Fig. 4c).
The nitrogen adsorption–desorption isotherms of IRMOF-3
and catalyst IRMOF-3-PI-Ru are shown in Fig. 5. The nitrogen
sorption study revealed that the BET surface area of IRMOF-3
2
ꢁ1
3
ꢁ1
was 2450 m g (pore volume ¼ 1.01 cm g ). IRMOF-3-PI-
2
ꢁ1
Ru shows the BET surface area of a value of 1374 m g with
3
ꢁ1
pore volume of 0.77 cm g . Change in surface area as well as
pore volume IRMOF-3-PI-Ru was smaller than IRMOF-3. It seems
space occupied by the Schiff-base moiety has not changed
substantially aer complex formation with ruthenium (ESI S5†).
3.2. Catalytic activity studies
This catalyst is an excellent heterogeneous system for rapid
Scheme 2 Proposed mechanism for the oxidation of pyrene.
oxidation of arenes and functionalized suldes to sulfoxides
and sulfones with NaIO under mild conditions. It is remark-
4
able that the reaction tolerates oxidatively sensitive functional
groups and the sulfur atom is selectively oxidized. The inuence
of different reaction parameters on the efficiency of the oxida-
tion process was evaluated in the model reaction depicted in
Table 1. Typical results for the ultrasonic-assisted IRMOF-3-PI-
Ru-catalyzed oxidation of arenes: a mixture of arene (1 mmol)
However, the particle dimension obtained by SEM image indi-
cated to be ꢃ50–100 nm. This difference can be attributed to the
aggregation of the particles (Fig. 4a and b).
TEM analyses indicate that the continuous 3D pore structure
of the IRMOF-3 is robust enough to survive the PI-Ru
Table 2 The catalytic activity tests of IRMOF-3-PI-Ru in a comparison with several homogeneous and heterogeneous prepared catalysts in the
presence of ultrasonic
ꢀ
a
Entry
Catalyst (mg)
NaIO
4
(1a(1b)(2a))/arene
Time (min) 1a(1b)(2a)
T ( C) 1a(1b)(2a)
Yield (%) 1a(1b)(2a)
b
1
2
3
4
5
6
IRMOF-3-PI-Ru (10)
4(8)(4)/1
4(8)(4)/1
4(8)(4)/1
4(8)(4)/1
4(8)(4)/1
4(8)(4)/1
50(75)(65)
r.t(40)(r.t)
r.t(40)(r.t)
r.t(50)(r.t)
r.t(50)(r.t)
r.t(40)(r.t)
r.t(40)(r.t)
74(55)(65)
40(30)(60)
55(40)(59)
50(40)(55)
45(30)(55)
40(30)(50)
IRMOF-3 (10)
RuCl
90(120)(110)
120(150)(120)
120(150)(120)
110(130) (120)
120(150)(180)
c
3 2
$nH O (10)
Ru (CO)12 (10)
3
d
MOF-5 (10)
MIL-101 (10)
e
a
b
ꢀ
c
22
d
ꢀ
Activated at 120 C
Isolated yield. Preliminary activation of the as-prepared IRMOF-3-PI-Ru under vacuum at 120 C. Reported previously.
ꢀ
e
ꢀ
ꢀ
and 180 C, respectively. Activated at 120 C and 180 C, respectively.
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and NaIO
4
(1 g, 4.68 mmol, 4 eq.) in a system consisting of H
2
O, mechanistic pathway for the oxidation of pyrene is presented in
CH Cl , CH CN (5, 4, 4 mL) with different catalytic amounts and Scheme 2.
2
2
3
of IRMOF-3-PI-Ru in a comparison with several homogeneous
and heterogeneous catalysts.
K-region oxidation of pyrene has been studied extensively
because of its suspected role in the carcinogenicity of fused
Although the precise mechanism of the reaction awaits further arenes. One of the oxidation products, pyrene-4,5-dione (1a), is
studies, by analogy with the previous works on ruthenium- also important because it serves as an intermediate in the
22
catalyzed oxidation of similar compounds
a proposed preparation of other fused-ring molecules, which are of interest
Table 3 Ultrasonic assisted IRMOF-3-PI-Ru catalyzed oxidation of arenes and heteroaromatic compounds
Irradiated
Non irradiated
Time (min)
a
ꢀ
b
b
Entry
1
Substrate
Products
T ( C)
Time (min)
50(75)
Yield (%)
Yield (%)
r.t(40)
r.t
74(55)
65
220(270)
240
60(50)
65
2
3
60
60
r.t
80
240
80
4
r.t
60
80
240
85
5
6
r.t
r.t
60
60
60
80
360
240
60
80
7
r.t
60
95
240
95
8
9
r.t
r.t
r.t
60
45
45
95
98
90
240
200
220
95
98
90
1
1
0
1
r.t
r.t
45
45
88
85
240
240
88
85
1
2
a
b
Products 1a–12a and 1b in ESI S5. Isolated yield.
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a
Table 4 Comparison of IRMOF-3-PI-Ru with other catalysts for the selective oxidation of sulfides in the presence of ultrasonic
d
d
Sulfoxide (b)
Sulfone (a)
b
c
e
e
Entry
Catalyst (mg)
H
2
O
2
(TBHP) : sulde (mol)
Time (min)
Yield (%)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
Without catalyst (—)
IRMOF-3-PI-Ru (5)
IRMOF-3-PI-Ru (10)
IRMOF-3-PI-Ru (15)
IRMOF-3 (10)
12 : 1
360
2(2)
2(2)
2(2)
120(120)
15(15)
22(25)
120(120)
150(150)
50(75)
96(96)
96(96)
96(96)
70(60)
95(95)
96(96)
96(96)
96(96)
—
—
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
55(55)
55(55)
55(55)
360(360)
80(80)
80(80)
360(360)
450(450)
82(80)
92(92)
92(92)
70(70)
90(90)
90(90)
90(90)
90(90)
3
RuCl (10)
Ru (CO)12 (10)
3
MOF-5 (10)
f
MIL-101 (10)
a
b
c
Reaction scale: 1-benzothiophene (1 mmol), oxidant and with catalysts (10 mg) in 3 mL solvent. Using 30% aq. H
2
O
2
.
tert-Butyl hydroperoxide
d
e
f
(TBHP) was indicated in parentheses in table. See ESI S5 and ref. 41–43. Isolated yield. The selective oxidation of suldes without ultrasonic
44
conditions.
from both a theoretical and a practical point of view. Further aldehyde 5a, benzoic acid 6a–7a, benzophenone 8a in good
oxidation of pyrene-4,5-dione gives pyrene-4,5,9,10-tetraone yields. The catalytic activity of the system toward heteroaromatic
(
1b), which has been used in metal complex formation and as compounds led to the interesting results. 1-Benzothiophene
21
a monomer in step growth polymerizations.
(BT), dibenzothiophene (DBT), 4,6-dimethyldi-benzothiophene
The model reaction was applied to the oxidation of pyrene 1. (4,6-DMDBT) and 4-methyldibenzothiophene (4-MDBT) gave
Pyrene-4,5-dione 1a was obtained at room temperature with corresponding sulfones 9a, 10a, 11a and 12a, respectively in
ꢀ
4
equiv. of NaIO
4
in 74% yield and irradiation at 40 C for 75 a high yield. The results are summarized in Table 3.
min using 8 equiv. of NaIO
4
afforded pyrene-4,5,9,10-tetraone
Although several studies on the oxidation of suldes by
1b in 55% yield. Treatment of chrysene 2 under the same various catalysts, especially industrial catalysts, but due to high-
condition to 1a gave 2a in good yield (Table 1).
performance MOFs are few studies in this area can be seen. So
The catalytic behavior of IRMOF-3-PI-Ru for the selective in continuation of our work on the catalytic properties of cata-
oxidation of pyrene 1 to pyrene-4,5-dione 1a, pyrene-4,5,9,10- lysts, herein, the catalytic behavior of IRMOF-3-PI-Ru and
tetraone 1b and chrysene 2 to chrysene-5,6-dione 2a in several prepared catalysts for the selective oxidation of suldes
a comparison with several homogeneous and heterogeneous to sulfoxides and sulfonesis reported in Table 4. The aromatic
prepared catalysts was shown in Table 2.
suldes selected for these studies include; BT, DBT, 4,6-DMDBT
In order to increase the yield of products various amount of and 4-MDBT. In order to examine the scope of this process, and
catalyst were employed. It was found that the best results were to demonstrate the diversity of the IRMOF-3-PI-Ru, the opti-
obtained in the presence of 10 mg of IRMOF-3-PI-Ru larger mized conditions were applied to 1-benzothiophene as shown
amount of catalyst, however, had no signicant effect on the in Table 4.
yield.
According to the same conditions to the oxidation of pyrene 1, aromatic compounds has been investigated. Six catalysts such
anthracene 3, phenanthrene 4, naphthalene 5, biphenyl 6, trans- as IRMOF-3-PI-Ru, IRMOF-3, RuCl , Ru (CO)12, MOF-5, MIL-101
The effect of kind of catalysts on the oxidation of hetero-
3
3
stilbene 7, 1,1-diphenyl-ethylene 8 in Table 3 gave corresponding were selected. Catalytic amounts of catalysts were irradiated in
anthracene-9,10-dione 3a, phenanthrene 9,10-dione 4a, phthal- the presence of ultrasound in an open vessel at room
a
Table 5 Study of the solvent and optimization of reaction conditions for oxidation of heterosulfides
Sulfoxide (b)
Time (min)
Sulfone (a)
Time (min)
ꢀ
ab
ab
Entry
Solvent
Temp ( C)
H
2
O
2
(TBHP) : sulde
Yield (%)
Yield (%)
1
2
3
4
5
6
7
8
EtOH
EtOH
EtOH
EtOH
MeOH
THF
r.t
r.t
r.t
40
r.t
r.t
r.t
r.t
4(10) : 1
3(5) : 1
<2(2)
<2(2)
2(2)
2(2)
2(2)
4(4)
2(2)
2(2)
96(95)
96(95)
96(96)
96(96)
96(96)
94(94)
96(96)
96(96)
55(55)
55(55)
55(55)
55(55)
55(55)
60(60)
55(55)
60(60)
95(95)
92(92)
92(92)
92(92)
92(92)
90(90)
92(92)
92(92)
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
2(3.4) : 1
Toluene
CH CN
3
a
b
Reaction scale: 1-benzothiophene (1 mmol), oxidant and catalysts (10 mg) in 3 mL solvent. Isolated yield.
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temperature. The disappearance of the starting materials and
appearance of target products were monitored by TLC. In order
to optimize the reaction conditions, oxidation reaction of 1-
benzothiophene as a model compound was examined using
3
(
0% aq. H
Table 4). One of the main factors in the oxidative system was
the amount of H (or TBHP) and catalyst. Thus, it was of great
interest to explore the inuence of the H (or TBHP) dosages
on removal of sulde. With equal molar amounts of H (or
2 2
O (or TBHP) under various reaction conditions
2 2
O
2 2
O
2 2
O
TBHP) and 1 benzothiophene, the corresponding oxidize
compounds 1-benzothiophene 1-oxide and 1-benzothiophene
1
,1-dioxide respectively were produced with 100% selectivity.
According to the results in Table 4, optimal reaction conditions
were achieved at H (or TBHP) dosages to sulde 2 : 1 (or
.4 : 1) respectively and 10 mg of IRMOF-3-PI-Ru at room Fig. 6 Reusability of IRMOF-3-PI-Ru as catalyst in oxidation of 1-
2 2
O
3
temperature was used. To compare the relative reactivity of six benzothiophene.
prepared different catalysts, the oxidation of 1-benzothiophene
was carried out under the same reaction conditions (Table 4,
reactivity decreased in the order of DBT > 4,6-DMDBT > 4-MDBT
BT. BT exhibited the lowest reactivity due to the different
electron density on the sulfur atom related to the other
substrate. In summary, presented catalytic ndings conrm
that the MOF structure has a signicant impact on the catalytic
properties.
entries 3, 5–9). The data indicate that IRMOF-3-PI-Ru is
considerably more efficient catalyst than others. At the end of
the reaction, the catalyst was recovered by cooling the reaction
>
ꢀ
mixture to 5 C, dried in vacuum and weighed.
Solid sulfur compounds are insoluble in aqueous media, it
was necessary to use a co-solvent that solubilizes these
compounds in water. In this study, EtOH was used as the co-
solvent since it is green and biologically less objectionable in
the environment compared to other solvents. Table 5 entries 6–
The merit of the present work in comparison with recently
44–49
reported protocols
and other industrial catalysts, this is the
amounts of the catalysts used, reaction times and yields of the
products, selective oxidation under mild conditions, easy
workups and reasonably nontoxic nature.
8
indicate that the metal organic framework moiety plays
a signicant role as Phase Transfer Catalyst (PTC). A compar-
ison between EtOH and several other solvents was shown in
Table 5.
Based on the above results, the catalytic prototype can be
seen in Scheme 3.
The recoverability and reusability of the catalyst (IRMOF-3-
PI-Ru) was examined in the oxidation reaction of 1-benzothio-
phene 9a. The catalyst was separated aer each run, washed
ꢀ
with MeOH and acetone, dried in an oven at 60 C, the recov-
ered catalyst was reused in subsequent reactions without
For dibenzothiophene the corresponding dibenzothiophene
a signicant decrease in activity even aer ve runs (Fig. 6).
0
S-oxide 98% yield and dibenzothiophene S,S -dioxide 96% yield
50
According to the methodology suggested by Sheldon et al.,
could be obtained in excellent yield in the presence of 10 mg of
IRMOF-3-PI-Ru and 2 : 1, H O (or 3.4 : 1 TBHP) : sulde ratio
2 2
with trace over-oxidation product. In case of 4,6-dimethyldi-
benzothiophene under the same conditions gave corresponding
we performed fast catalyst ltration tests for all the elaborated
catalytic systems. We ltered the catalyst, e.g., IRMOF-3-PI-Ru,
aer 22 min (ca. 50% 1-benzothiophene 9a conversion) and
allowed the ltrate to react further. No further substrate
conversion in the ltrate was observed aer removal of the
4
,6-dimethyldibenzothiophene S-oxide 96% yield and 4,6-
0
dimethyldibenzothiophene S,S -dioxide 95% yield respectively.
Also, 4-methyldibenzo thiophene gave corresponding 4-meth-
yldibenzothiophene S-oxide 95% yield and 4-methyl dibenzo-
0
thiophene S,S -dioxide 94% yield respectively. The oxidation
Scheme 3 The oxidation catalytic prototype of sulfides to sulfoxides
and sulfones.
Fig. 7 Catalyst filtration test for the oxidation of 1-benzothiophene.
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catalyst in the oxidations over IRMOF-3-PI-Ru (an example is 13 W. J. Mijs and C. R. H. I. de Jonge, Organic Syntheses by
shown in Fig. 7), indicating that the observed catalysis is truly
heterogeneous. A trace amount of ruthenium (less than 1 ppm)
Oxidation with Metal Compounds, Plenum Press, New York,
1986.
was determined in the ltrate by ICP-AES aer oxidation with 14 M. Hudlicky, Oxidations in Organic Chemistry ACS
26,51
H
2
O
2
.
Monograph, American Chemical Society, Washington DC,
The recovered catalyst was then subjected to X-ray powder
vol. 186, 1990.
diffraction and FTIR spectral analysis study. Comparison of 15 F. G. Oberender and J. A. Dixon, J. Org. Chem., 1959, 24, 1226.
X-ray diffraction patterns (see Fig. S6†) and FTIR spectra (see 16 F. X. Webster, J. Rivas-Enterrios and R. M. Silverstein, J. Org.
Fig. S7†), of the pristine compound and recovered catalyst
Chem., 1987, 52689.
convincingly demonstrates the structural integrity of the 17 H. R. Khavasi, S. S. Hosseiny Davarani and N. Safari, J. Mol.
compound retained aer the reactions.
Catal. A: Chem., 2002, 188, 115.
1
1
2
2
2
2
8 T. K. M. Shing, V. W.-F. Tai and E. K. W. Tam, Angew. Chem.,
Int. Ed. Engl., 1994, 33, 2312.
9 G. Balavoine, C. Eskenazi, F. Meunier and H. Riviere,
Tetrahedron Lett., 1984, 25, 3187.
0 A. J. Bailey, W. P. Griffith, A. J. P. White and D. J. Williams, J.
Chem. Soc., Chem. Commun., 1994, 1833.
4
. Conclusions
In summary, we have succeeded in designing a new heteroge-
neous catalyst for oxidation reactions by anchoring a Ru Schiff
base moiety into microporous IRMOF-3 by a post-synthetic
method. The catalyst shows high activity toward selective
oxidation of arenes and heteroaromatic compounds and for
oxidation of suldes to sulfoxides and sulfones. The possibility
of easy recycling of the catalyst and mild reaction conditions
make the catalyst cheap and highly desirable to address the
environmental concerns.
1 F. W. Harris, D. Zhang and J. Hu, J. Org. Chem., 2005, 70,
707–708.
2 K. Tabatabaeian, M. Mamaghani, N. O. Mahmoodi and
A. Khorshidi, Catal. Commun., 2008, 9, 416–420.
3 Y. K. Hwang, G. Ferey, U.-H. Lee and J.-S. Chang, in Liquid
Phase Oxidation via Heterogeneous Catalysis: Organic
Synthesis and Industrial Applications, ed. M. G. Clerici and
O. A. Kholdeeva, John Wiley & Sons, Inc, 2013, pp. 371–409.
Acknowledgements
24 A. Dhakshinamoorthy and H. Garcia, Chem. Soc. Rev., 2014,
750.
5
The authors thank the University of Guilan Research Council
for the partial support of this work.
2
2
2
5 K. Weissermel and H.-J. Harpe, Industrial Organic Chemistry,
Wiley-VCH, Weinheim, 4th edn, 2003.
6 N. V. Maksimchuk, K. A. Kovalenko, V. P. Fedin and
O. A. Kholdeeva, Chem. Commun., 2012, 6812.
7 A. Dhakshinamoorthy, M. Alvaro and H. Garcia, Catal. Sci.
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