Oxidation of ethylbenzene (derivatives) catalyzed by a functionalized ionic liquid combined with cationic (tetrakis(N-methyl-4-pyridinium)porphyrinato) manganese(III) and anionic phosphotungstate

Article abstract of DOI:10.1007/s00706-010-0336-y

A functionalized ionic liquid (IL) combined with cationic (tetrakis(N-methyl-4-pyridinium)porphyrinato)manganese(III) and anionic phosphotungstate was prepared and applied as the catalyst for ethylbenzene (derivative) oxidations without involvement of the auxiliary axial ligands. A synergetic catalytic effect between the cations and the counteranions in this functionalized IL was observed in terms of activity and stability. Although anionic phosphotungstate ([PW₁₂O₄₀]^3-) showed negligible contribution to the activation of ethylbenzene, it stabilized the manganese porphyrin against oxidative degradation. The in situ UV-Vis analysis of the oxidation of the functionalized IL by PhIO indicated that with the incorporation of [PW₁₂O₄₀]^3- as the counteranion the formation of the μ-oxo Mn^IV porphyrin dimer was completely suppressed, resulting in improved catalytic performance for the corresponding manganese porphyrin. Springer-Verlag 2010.

Full text of DOI:10.1007/s00706-010-0336-y

Monatsh Chem (2010) 141:859–865
DOI 10.1007/s00706-010-0336-y
ORIGINAL PAPER
Oxidation of ethylbenzene (derivatives) catalyzed
by a functionalized ionic liquid combined with cationic
(tetrakis(N-methyl-4-pyridinium)porphyrinato)manganese(III)
and anionic phosphotungstate
•
Wei Liu Qing-Xia Wan Ye Liu
•
Received: 12 January 2010 / Accepted: 20 May 2010 / Published online: 18 June 2010
Ó Springer-Verlag 2010
Abstract A functionalized ionic liquid (IL) combined
with cationic (tetrakis(N-methyl-4-pyridinium)porphyrina-
to)manganese(III) and anionic phosphotungstate was
prepared and applied as the catalyst for ethylbenzene
(derivative) oxidations without involvement of the auxil-
iary axial ligands. A synergetic catalytic effect between the
cations and the counteranions in this functionalized IL was
observed in terms of activity and stability. Although
anionic phosphotungstate ([PW12O40]3-) showed negligi-
ble contribution to the activation of ethylbenzene, it
stabilized the manganese porphyrin against oxidative deg-
radation. The in situ UV-Vis analysis of the oxidation of
the functionalized IL by PhIO indicated that with the
incorporation of [PW12O40]3- as the counteranion the
formation of the l-oxo MnIV porphyrin dimer was com-
pletely suppressed, resulting in improved catalytic
performance for the corresponding manganese porphyrin.
some of their properties, like good chemical stability,
volatility, etc., have been challenged [1–5]. By varying the
cations and anions that constitute the IL’s structure, it is
possible to design ILs with the required functionalities [6].
This strategy may provide ways to reinforce the catalytic
behavior exerted by the cations and anions synergistically,
which is regarded as particularly important not only for the
design of liquid-phase catalysis experiments, but also for
the development of hybrid organic/inorganic functional
materials with diverse properties [7, 8].
Metalloporphyrins are well known for their ability to
catalyze oxidation of hydrocarbons by using single oxygen
atom donors, such as iodosylbenzene (PhIO), hypochlo-
rites, and hydroperoxides [9, 10]. All metalloporphyrins
suffer from deactivation, oxidative degradation, intermo-
lecular self-aggregation through p–p interaction, and
susceptibility to the formation of an unreactive l-oxo
porphyrin dimer ((por)M–O–M(por)) [10–12]. Many
strategies have been developed to tune the reactivity of
metalloporphyrins, including the introduction of bulky and/
or electron-withdrawing substituents at meso positions, and
the use of nitrogenous bases as axial ligands [13–15].
On the other hand, heteropolyanion salts (or polyoxo-
metalates, POMs) have been widely investigated as ‘green’
catalysts in the oxidation of hydrocarbons [16–18]. The
associations of heteropolyanions with organic groups, like
benzotriazole [19], 2,20-biquinoline [20], 8-hydroxyquino-
line [21, 22], urea [23], and glycine [24], have been
described with potential optical, magnetic, or electric
properties rather than as catalysts. Associations of hetero-
polyanions with metal complexes like metalloporphyrins
have been scarcely reported [25, 26].
Keywords Functionalized ionic liquid ꢀ
Manganese porphyrin ꢀ Metalloporphyrin ꢀ
Polyoxometallate ꢀ Catalytic oxidation
Introduction
Ionic liquids (ILs) have been widely used as reaction
media, which can offer better reaction selectivity, higher
yields, and recyclability with ‘green’ attributes, although
W. Liu ꢀ Q.-X. Wan ꢀ Y. Liu (&)
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, Chemistry Department,
East China Normal University,
In order to exploit the individual catalytic properties of
metalloporphyrin and heteropolyanions, in this study, the
ionic compound combined with cationic (tetrakis(N-
3663 North Zhongshan Road, Shanghai, China
e-mail: yliu@chem.ecnu.edu.cn
123

860
W. Liu et al.
methyl-4-pyridinium)porphyrinato)manganese(III) and anio-
nic phosphotungstate (2, Scheme 1) was prepared and
applied as a catalyst for ethylbenzene oxidation without
additional involvement of the axial ligands. In regard to the
matched compatibility (miscibility), the room temperature
IL N-butylpyridinium tetrafluoroborate ([Bpy]BF4) was
used as the corresponding solvent. For comparison, (tet-
rakis(N-methyl-4-pyridinium)porphyrinato)manganese(III)
hexafluorophosphate ([MnIIITMPyP][PF6]5, 1) [27] and
N-butylpyridinium phosphotungstate ([Bpy]3[PW12O40], 3)
[28] were prepared and investigated in parallel.
according to 12.6% weight loss over the range 350–
600 °C.
FT-IR spectra in Fig. 2 showed that for 2 the charac-
teristic bands of the typical a-Keggin-type [PW12O40]3-
were observed in the region of 1,100–600 cm-1 without
significant changes in comparison to those of H3[PW12O40]
(1,077 (P–Oa), 978 (terminal W–Od), 896 (inter-octahedral
W–Ob–W), 800 (intra-octahedral W–Oc–W) cm-1) [29].
Besides these bands, the peaks at 3,100–3,000 cm-1
ascribed to C–H vibrations, and 1,638 cm-1 ascribed to
C=N vibration in the porphyrin ring were observed,
implying the desired association of the cationic
[MnIIITMPyP]5? with [PW12O40]3- anions. The UV-Vis
spectra of 2 in DMF (Fig. 2) showed the typical absorbance
bands corresponding to the cationic [MnIIITMPyP]5?
(Soret band, 462 nm; Q bands, 564 and 600 nm) and
[PW12O40]3- (CT band, 268 nm), respectively. The spec-
troscopic characterization of 2 by FT-IR and UV-Vis
indicated that the individual properties of the cations and
the anions in 2 were not altered dramatically, suggesting a
simple charge interaction between them [26].
Results and discussion
Synthesis and characterization of 1 and 2
Since (tetrakis(N-methyl-4-pyridinium)porphyrinato)man-
ganese(III) iodide salt and a-Keggin-type phosphotungstic
acid (H3[PMo12O40]ꢀxH2O) are highly soluble in water, the
association of [PMo12O40]3- anions with large cations of
[MnIIITMPyP]5? leads to the precipitation of insoluble
compounds. This behavior guaranteed the desired associ-
ation of the cationic Mn porphyrins with [PW12O40]3-
anions at the stoichiometric ratio.
The catalytic performance of 2 in ethylbenzene
oxidation
Thermogravimetry/differential thermal gravimetry
(TG/DTG) analyses (Fig. 1) indicated that the thermal
stabilization of 2 was dramatically improved in compar-
ison to that of 1, and that the [PW12O40]3- anion (in 2
and 3) was thermally stable up to 800 °C. Compound 1
was destructed at ca. 310 °C due to the dissociation of
The oxidation of ethylbenzene was selected as a model
reaction to evaluate the catalytic performance of 2. The
reaction conditions were optimized in terms of solvents
([Bpy]BF4 and DMF), oxidants (PhIO, H2O2, PhI(OAc)2,
NaClO, and m-chloroperoxybenzoic acid), catalyst con-
centration, molar ratio of oxidant to substrate, reaction
time, and temperature. Under the optimal conditions
(0.4 mol% catalyst, [Bpy]BF4 as a solvent, 1.5 equiv. of
PhIO, 1 h, 30 °C), 2 exhibited better activities than 1 (entry
2 vs. 1, Table 1), suggesting that the incorporation of
[PW12O40]3- gave rise to the activation of ethylbenzene.
The formation of ketone was due to the further oxidation of
alcohol by excess PhIO. In order to further explain the role
of the [PW12O40]3- anion in ethylbenzene oxidation, the
-
unstable PF6 anions. Compound 2 decomposed at ca.
520 °C, corresponding to the collapse of the porphyrin
framework. TG/DTG analysis also revealed that the
stoichiometric ratio of [MnIIITMPyP]5? to [PW12O40]3-
in 2 was 0.6, which was consistent with 13.5% weight
loss over the range 400–750 °C due to the CHN loss for
the porphyrin ring system. The stoichiometric ratio of
[Bpy]? to [PW12O40]3- in 3 was determined to be 3,
N
N
N
N
N
N
N
Mn(III)
N
N
Mn(III)
N
Bu
N
3
3
N
N
[PW12O40
]
[PW12O40]
N
[PF6]
N
0.2
0.6
3
N
N
[MnIIITMPyP][PF6]5 (1)
[MnIIITMPyP]0.6[PW12O40] (2)
[Bpy]3[PW12O40] (3)
Scheme 1
123

Oxidation of ethylbenzene (derivatives) catalyzed by a functionalized ionic liquid
861
6
5
4
3
2
1
0
Fig. 1 TG/DTG analyses of
1–3 in air flow
1
0.000
2
0.00
6
5
4
3
-0.001
-0.002
-0.003
-0.004
-0.005
-0.006
-0.01
-0.02
-0.03
-0.04
-0.05
-0.06
100 200 300 400 500 600 700 800
Temperature/oC
100 200 300 400 500 600 700 800
Temperature/oC
14
3
0.005
13
12
11
10
9
0.000
-0.005
-0.010
-0.015
-0.020
-0.025
-0.030
8
7
6
100 200 300 400 500 600 700 800
Temperature/oC
activity of 3 in [Bpy]BF4 was examined in entry 3, which
indicated that the contribution of [PW12O40]3- to the
activation of ethylbenzene was negligible.
the improved catalytic behavior of 2 in terms of activity
and stability was observed undoubtedly. In order to eluci-
date the specific role of [PW12O40]3- in 2, the evolving
oxidation of 2 by PhIO was monitored by UV-Vis
spectroscopy.
Repeated use of 2 in [Bpy]BF4 (Table 2) showed that 2
could be recycled for 4 runs without much activity loss. The
UV-Vis spectra showed that after 4 runs, the typical Soret
band of 2 (470 nm) was still clearly observed. In the absence
of [PW12O40]3-, the activity of 1 was lost rapidly with
accompanying rapid decay of its Soret band. These results
implied that the stability of 2 against oxidative degradation
was greatly improved in comparison to that of 1.
The oxidation of 1 and 2 in [Bpy]BF4 by PhIO was
analyzed at ambient temperature. Because the behavior of
the Mn porphyrin units was considered solely, the spectra
in the region 380–650 nm was recorded. The UV-Vis
spectra (Fig. 3a) showed that the typical Soret band
(470 nm) and Q bands (574 and 610 nm) of 1 were
observed in the first 30 min, but were accompanied by a
slight decay due to the oxidative degradation. After ca.
30 min, a new peak at ca. 430 nm, which was assigned to
the l-oxo MnIV porphyrin dimer [10–12], gradually grew
to reach a maximum intensity accompanied by the con-
current decay of absorbance at 470 nm. Once the l-oxo
In situ oxidation of 1 and 2 by PhIO monitored
by UV-Vis spectroscopy
The results in Table 1 showed that [PW12O40]3- exhibited
no contribution to the activation of ethylbenzene. However,
123

862
W. Liu et al.
4
3
2
1
0
Fig. 2 FT-IR (KBr) and UV-Vis
(DMF) spectra of 1–3
1
3
H3[PW12O40]
2
3
1
2
H3PW12O40
3200 3000 28001800 1600 1400 1200 1000 800 600
300
400
500
600
700
Wavenumber/cm-1
Wavelength/nm
Table 1 Comparison of
catalysts 1–3 for ethylbenzene
oxidation
Entry
Catalyst
Solvent
Conversion (%)
Selectivity (%)
1-Phenylethanol
1-Phenylethanone
1
2
3
4
5
6
1
2
3
1
2
3
[Bpy]BF4
[Bpy]BF4
[Bpy]BF4
DMF
39
53
71
56
–
29
44
–
\3
17
Catalyst 0.4 mol% (2 lmol),
ethylbenzene 500 lmol, PhIO
750 lmol, solvent 2 cm3,
reaction temperature 30 °C,
reaction time 1 h
10
8
90
92
–
DMF
23
DMF
\3
–
MnIV porphyrin dimer was formed, it underwent more
severe oxidative destruction than the corresponding MnIII
porphyrin complex due to the increased electron density in
the porphyrin ring, which led to the large drop of the
absorbance intensities for lines 6–9.
Comparatively, with DMF as the solvent, the oxidation
of 2 or 1 by PhIO exhibited very similar spectral patterns
like in Fig. 3c. In ca. 30 min, not only was the l-oxo
MnIV porphyrin dimer (435 nm) formed rapidly, but also
the severe degradation of the manganese porphyrins
occurred simultaneously. During 30 min monitoring,
most of the Mn(III, IV) porphyrin complexes had been
oxidatively destructed into non-conjugated fragments
(line 6).
In contrast, when the oxidation of 2 by PhIO was moni-
tored under the same conditions, the obtained UV-Vis
spectra in Fig. 3b always exhibited the typical Soret band of
2 at ca. 470 nm without shifting during 60 min, revealing
that the presence of the [PW12O40]3- counteranion in 2 as a
p-electron-rich reservoir could suppress the formation of the
less reactive l-oxo MnIV porphyrin dimer, which could
account for the improved activity and stability of 2 in
[Bpy]BF4 toward ethylbenzene oxidation. Hence, it was
believed that the p–p interaction and the electrostatic force
between [MnIIITMPyP]5? cations and [PW12O40]3- anions
in 2 could counteract the p–p stacking of porphyrin itself.
Generality of 2-[Bpy]BF4 for oxidation of ethylbenzene
derivatives
To examine the generality of 2-[Bpy]BF4 catalysis, sev-
eral ethylbenzene derivatives were selected and
investigated and results are shown in Table 3. For the
substrates without conjugated character, like n-hexane,
123

Oxidation of ethylbenzene (derivatives) catalyzed by a functionalized ionic liquid
863
substituents (Me, OMe) at the para position of ethylben-
zene afforded better conversions due to the increased
electron density at the a carbon of ethylbenzene.
Table 2 Recycling of 1 and 2 in [Bpy]BF4 for ethylbenzene
oxidation
Run
1
2
Conversion Selectivity
(%)
Conversion Selectivity
(%)
(%)
(%)
Experimental
X-ola X-onea
X-ola X-onea
Reagents and analyses
1
2
3
4
39
30
24
15
71
30
26
25
29
70
74
75
53
50
51
51
56
30
27
27
44
70
73
73
All chemical reagents were purchased and used as
received. FT-IR spectra were recorded on a Nicolet
NEXUS 670 spectrometer. The 31P NMR spectra (with
85% H3PO4 sealed in a capillary tube as internal standard)
were recorded on a Bruker Avance 500 spectrometer. The
elemental analyses for Mn, P, and W were performed by an
inductively coupled plasma (ICP) emission spectrometer
(RIS Intrepid II XSP spectrometer, Thermo Electron Cor-
poration). TG/DTG was acquired by using a Mettler TGA/
SDTA 851e instrument and STARe thermal analysis data
processing system. TG/DTG measurements were run in air
flow with a temperature ramp of 10 °C min-1 between 50
and 800 °C. GC analyses were performed on a SHIMA-
DZU-14B chromatograph equipped with Rtx-Wax
capillary column (30 m 9 0.25 mm 9 0.25 lm). GC–MS
analyses were performed on an Agilent 6890 instrument
equipped with Agilent 5973 mass-selective detector. UV-
Vis spectra were recorded on a SHIMADZU-UV 2550
spectrophotometer with resolution of ca. 1 nm. The oxi-
dation of 2 (or 1) by PhIO in [Bpy]BF4 or DMF was
monitored in situ by means of their UV-Vis spectra. The
mixture upon adding the catalyst (1 or 2), [Bpy]BF4
(2 cm3), and PhIO (20 mg, in large excess) into a UV
cuvette (3 cm3) was stirred until the appointed time.
Catalyst 0.4 mol%, ethylbenzene 500 lmol, PhIO 500 lmol,
[Bpy]BF4 2 cm3, reaction temperature 30 °C, reaction time 1 h
a
X = 1-phenylethan
n-octane, n-chlorobutane, etc., the conversion was not
detectable mainly due to the immiscibility of the sub-
strate in the 2-[Bpy]BF4 system and the resultant mass
transfer limitation. As shown in Table 3, with substitu-
ents possessing electron-donating nature (Me, OMe) at
the para position of ethylbenzene, the increased electron
density at the a carbon of ethylbenzene led to the fast
formation of alkyl radicals and then the fast formation of
the corresponding products (entries 1, 2). With electron-
withdrawing groups (–Cl, –Br, –COOH) at para position,
the conversions of the substrates were relatively low due
to the decreased nucleophilicity at a carbon of the ethyl
side chain (entries 3–5).
Conclusion
Without involvement of the auxiliary axial ligands, the
ionic compound 2 combined with the cationic Mn
Synthesis
porphyrin ([MnIIITMPyP]5?
) and the a-Keggin-type
phosphotungstate ([PW12O40]3-) is an efficient catalyst in
the room temperature IL [Bpy]BF4 toward ethylbenzene
oxidation in terms of activity and stability, in comparison
to 1 which only possesses the catalytic site of [MnIIITM-
PyP]5? alone. The contribution of [PW12O40]3- anions to
the activation of ethylbenzene proved to be negligible, but
to the stability of [MnIIITMPyP]5? against the oxidative
degradation was of significance. The in situ UV-Vis anal-
ysis for the oxidation of 2 by PhIO indicated that with the
incorporation of [PW12O40]3- as the counteranions in 2 the
formation of the l-oxo MnIV porphyrin dimer with disad-
vantages of low reactivity and susceptibility to oxidative
degradation was completely suppressed, which greatly
favored the catalytic behavior of the corresponding
[MnIIITMPyP]5?. In a general study only ethylbenzene
derivatives with (partial) miscibility with [Bpy]BF4 could
be investigated. The substrates with electron-donating
Tetrakis(N-methyl-4-pyridinium)porphyrin
iodide
([H2TMPyP][I]4) and (tetrakis(N-methyl-4-pyridinium)-
porphyrinato)manganese(III) hexafluorophosphate (1) were
prepared according to Ref. [27].
(Tetrakis(N-methyl-4-pyridinium)porphyrinato)-
manganese(III) phosphotungstate ([MnIIITMPyP]0.6
[PW12O40]) (2, [C44H36N8Mn]0.6PW12O40)
(Tetrakis(N-methyl-4-pyridinium)porphyrinato)manganese
(III) iodide [27] (207 mg) dissolved in 100 cm3 deionized
water was treated dropwise with 30 cm3 of an aqueous
solution of 1.0 g phosphotungstic acid H3[PW12O40]ꢀxH2O.
A brown solid immediately precipitated from the solution.
The resultant mixture was stirred vigorously at room
temperature for 3 h. The brown precipitates were collected
after centrifugation and then washed with deionized water
and ethanol. A dark-brown product of 2 (352 mg) was
123

864
W. Liu et al.
Fig. 3 UV-Vis spectra
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(b)
(a)
recorded for the continuously
evolving processes of a 1
oxidized by PhIO in [Bpy]BF4;
b 2 oxidized by PhIO in
[Bpy]BF4; c 2 oxidized by PhIO
in DMF [conditions: 1 (or 2)
ca. 3.0 9 10-5 M, solvent
2 cm3, PhIO 20 mg (in large
excess), room temperature]
1
1
1
2
3
4
5
6
7
8
2
1
2
3
4
5
6
7
8
1
2 + PhIO-3 min
2 + PhIO-10 min
2 + PhIO-20 min
2 + PhIO-30 min
2 + PhIO-40 min
2 + PhIO-50 min
2 + PhIO-60 min
1 + PhIO-3 min
1 + PhIO-10 min
1 + PhIO-20 min
1 + PhIO-30 min
1 + PhIO-40 min
1 + PhIO-50 min
1 + PhIO-60 min
8
6
8
7
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength/nm
Wavelength/nm
3.5
(c)
1
2
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1
2
3
4
5
6
2 + PhIO-3 min
2 + PhIO-10 min
2 + PhIO-15 min
2 + PhIO-20 min
2 + PhIO-30 min
2
3
4
6
5
400
450
500
550
600
650
Wavelength/nm
obtained after drying in vacuo. Due to the paramagnetism
of 2, its 1H NMR signals were unobservable. UV-Vis
(DMF): kmax = 268 (s, CT transition of [PW12O40]3-), 462
(s, Soret band), 564 and 600 (w, Q bands) nm; FT-IR
d6): d = -14.0 ppm; UV-Vis (DMF): kmax = 263 nm;
ꢀ
UV-Vis ([Bpy]BF4): kmax = 304 nm; FT-IR (KBr): m =
3,136 (w), 3,080(w), 2,965(w), 2,936(w), 2,871(w), 1,633
(s, C=N), 1,483 (s, C=C), 1,083 (m, P–Oa), 982 (s, terminal
W–Od), 896 (m, inter-octahedral W–Ob–W), 808 (s, intra-
octahedral W–Oc–W) cm-1
.
ꢀ
(KBr): m = 3,099 and 3,052 (w, C–H), 1,635 (m, C=N),
1,077 (P–Oa), 978 (terminal W–Od), 896 (inter-octahedral
W–Ob–W), 800 (intra-octahedral W–Oc–W) cm-1
.
General procedure for oxidation of ethylbenzene
(derivatives) catalyzed by 2 in [Bpy]BF4
N-Butylpyridinium phosphotungstate ([Bpy]3[PW12O40])
(3) [28]
Compound 3 was prepared as described in Ref. [28] and
then dried in an oven at 90 °C. 31P NMR (200 MHz, DMF-
To 2 cm3 [Bpy]BF4 and 500 lmol ethylbenzene (ana-
lytical grade reagent used as received) was added 7 mg
123

Oxidation of ethylbenzene (derivatives) catalyzed by a functionalized ionic liquid
865
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a
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The remaining ionic liquid phase was dried in vacuo and
used without further treatment for next the run. In each run,
due to the stoichiometric consumption of the oxidant,
110 mg PhIO (500 lmol) was added additionally besides
the substrate.
Acknowledgments The research was financially supported by the
National Natural Science Foundation of China (No. 20533010,
20673039), and the Science and Technology Commission of Shang-
hai Municipality (09JC1404800).
28. Wang S, Liu W, Wan Q, Liu Y (1009) Green Chem 11:1589
29. Weng Z, Wang J, Jian X (2008) Catal Commun 9:1688
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