Selective oxidation of alcohols with hydrogen peroxide catalyzed by hexadentate binding 8-quinolinolato manganese(III) complexes

Article abstract of DOI:10.1016/j.jcat.2008.11.011

A series of hexadentate 8-quinolinolato manganese(III) complexes were synthesized and proven to own a distorted octahedral geometry via elemental analysis, solid UV-vis spectroscopy and Hartree-Fock/3-21G+ calculation. These Mn(III) complexes were found to be more efficient than their corresponding tetradentate 8-quinolinolato manganese(II) and salen-Mn^IIIOAc for the oxidation of alcohols in acetone medium, being due to their special hexadentate binding structures that could open an axial Mn{single bond}O bond to form the more active pentadentate structures in the presence of aqueous hydrogen peroxide, as supported by UV-vis spectra. The halogen substituents in ligand's aryl ring could significantly enhance the catalytic activities and 5-chloro-7-iodo-8-quinolinolato manganese(III) gave the highest turnover number (TON). A reasonable mechanism for the present catalytic system was proposed.

Full text of DOI:10.1016/j.jcat.2008.11.011

Journal of Catalysis 261 (2009) 110–115
Contents lists available at ScienceDirect
Journal of Catalysis
www.elsevier.com/locate/jcat
Selective oxidation of alcohols with hydrogen peroxide catalyzed by hexadentate
binding 8-quinolinolato manganese(III) complexes
∗
Zhengpei Ye, Zaihui Fu , Sheng Zhong, Fang Xie, Xiaoping Zhou, Fenglan Liu, Dulin Yin
Key Laboratory of Resource Fine-Processing and Advanced Materials of Hunan Province and Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research
(Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of hexadentate 8-quinolinolato manganese(III) complexes were synthesized and proven to own
a distorted octahedral geometry via elemental analysis, solid UV–vis spectroscopy and Hartree–Fock/
3-21G+ calculation. These Mn(III) complexes were found to be more efficient than their corresponding
tetradentate 8-quinolinolato manganese(II) and salen-MnIIIOAc for the oxidation of alcohols in acetone
medium, being due to their special hexadentate binding structures that could open an axial Mn–O
bond to form the more active pentadentate structures in the presence of aqueous hydrogen peroxide,
as supported by UV–vis spectra. The halogen substituents in ligand’s aryl ring could significantly enhance
the catalytic activities and 5-chloro-7-iodo-8-quinolinolato manganese(III) gave the highest turnover
number (TON). A reasonable mechanism for the present catalytic system was proposed.
Received 17 September 2008
Revised 12 November 2008
Accepted 12 November 2008
Available online 2 December 2008
Keywords:
Alcohols
Catalysis
Hydrogen peroxide
Hexadentate structure
8-Quinolinolato Mn(III) complexes
Selective oxidation
© 2008 Elsevier Inc. All rights reserved.
1. Introduction
In our previous works [19,20], we have reported that 8-quin-
olinolato manganese(III) complexes (Q3MnIII) could efficiently cat-
The selective liquid-phase oxidation of alcohols to the corre-
sponding carbonyl compounds is of significant importance in the
fine chemicals and pharmaceutical industries. And these products
represent important intermediates for the preparation of various
other compounds [1,2]. Some strong oxidants such as KMnO4,
CrO3, and HNO3 [3,4] were applied traditionally to alcohol oxi-
dation, which might result in much serious pollution and some
potential risks in the process of operation. In terms of atom effi-
ciency and environment friendly, after oxygen, aqueous hydrogen
peroxide (H2O2) is a very attractive oxidant for industrial applica-
tions since water is the only by-product, and it is easy to be dealt
with after reactions. The latest studies have therefore focused on
the catalytic alcohol oxidations with H2O2 as the terminal oxidant
[5–10]. Moreover, transition metal-catalyzed oxidation of alcohols
is of current interest, and the various effective catalysts have been
reported, such as molybdenum and tungsten [11–13], ruthenium
[14], cobalt [15], manganese [16], iron [17] and rhenium [18]-based
catalysts. However, based on the fact that most of the metal-
containing catalysts are expensive and some of them may lead to
the environmental pollution, some poisonous reagents are required
as solvents [12,13] with a time-consuming process [11] and so on,
it becomes more and more urgent to focus on finding out a more
efficient and environmental friendly catalytic system.
alyze the epoxidation of olefins with H2O2 in water–acetone media
containing ammonium acetate and acetic acid. More recently, we
found that the Q3MnIII complexes could catalyze the selective ox-
idation of some primary and secondary alcohols with high TON
and efficiency of H2O2 utility in acetone medium without any ad-
ditives. Herein, we report the initial results we have obtained.
2. Experimental
2.1. Materials and apparatus
Benzyl alcohol, cyclohexanol, octanol-1, octanol-2, 1,2-propylene
glycol, trans-2-hexen-1-ol, tetrahydrofuran (THF), ethanol, acetone,
50 wt% aqueous H2O2, 70 wt% aqueous tert-butyl-hydroperoxide
(TBHP) and solvents were at least A.R. grade. C, H and N element
analyses were performed on a Vario EL III CHN elemental analyzer
and UV–vis spectrum were recorded from 200 to 700 nm on UV-
3310 spectrophotometer.
2.2. Preparation of Q3MnIII complexes
The title complexes were prepared based on the published
procedures with a little modification [19]. The general produc-
ing procedure is as follows: 5 ml of Mn(OAc)2 aqueous solution
(1 mol L−1) was added dropwise to 50 ml of stirred THF con-
taining 15 mmol of 8-hydroxyquinoline (8-HQ) or its derivative,
Corresponding author. Fax: +86 731 8872531.
*
E-mail address: fzhhnu@tom.com (Z. Fu).
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.11.011

Z. Ye et al. / Journal of Catalysis 261 (2009) 110–115
111
Scheme 1. The structures of Q3MnIII complexes.
followed by adding 30 wt% aqueous H2O2 (0.57 g, 5 mmol) into
Table 1
Mn and CHN elemental Contents (mass percentage) of Q3MnIII complexes.a
the solution. Then, the reaction mixture was refluxed for 2 h, and
the resulting precipitate was filtrated and washed with ethanol
for at least three times. After air-drying, a deep-yellow solid (80–
92% yield) was obtained and denoted as 1a to 1e (see Scheme 1).
The specific surface area of these Q3MnIII samples, which was
measured by nitrogen volumetric adsorption-desorption method
at 77 K on a Micromeritics TriStar 3000 apparatus, is very low
(Sg, 10–35 m2 g−1) and attributed mostly to the external poros-
ity (>95%). For the sake of comparison, a tetradentate binding
5-chloro-7-iodo-8-quinolinolato MnII (Q2MnII) was prepared via
using two equivalents of ligand in the absence of aqueous H2O2.
And a traditional salen-MnIIIOAc complex was also prepared from
N,N-bis(salicylidene) ethylethylenediamine and Mn(OAc)2 follow-
ing reported procedures [21].
Samples
Mn
1a
1b
9.60
1c
8.16
1d
5.64
1e
Found
Calcd.
11.36
11.29
5.66
5.68
9.32
7.93
5.72
C
Found
Calcd.
66.44
66.50
54.82
54.90
46.02
46.68
33.72
33.71
34.00
33.45
H
Found
Calcd.
3.65
3.70
2.60
2.54
1.76
1.73
1.25
1.25
1.47
1.24
N
Found
Calcd.
8.59
8.60
7.13
7.12
6.04
6.05
4.51
4.37
4.32
4.34
a
CHN elemental analyses were conducted on a Vario EL III Elemental Analyzer
(Germany). Found and Calcd. list the actually measured values and the theoretically
calculated ones based on a hexadentate mode, respectively. See Scheme 1 in the
paper for definitions of 1a–e.
2.3. General procedure of catalytic oxidation experiments
The general procedure for alcohol oxidation is described as fol-
lows. A solution of 50 wt% H2O2 (3 mmol) in acetone (2 ml)
was added to a stirred mixture of alcohol (9 mmol) and catalyst
◦
Q3MnIII (0.002 mmol) at room temperature (25 C). Oxidation pro-
cess for such heterogeneous mixture was carried out on a magnetic
−1
stirring apparatus with a higher stirring speed than 2500 r min
,
and such stirring speed, as proven by our experiments (see en-
tries 6 and 7 in Table 2), could efficiently eliminate the influence
of mass transfer resistance on the reaction. After H2O2 was con-
sumed completely via an inspection with potassium iodide-starch
test paper, the catalyst was separated from the reaction mixture by
filtration, and the filtrate was analyzed on an Agilent 6890N gas
chromatography (GC) with a HP-5 quartz capillary column (30 m
× 0.32 mm × 0.25 μm) and flame ionization detector (FID) us-
ing ultrapure nitrogen as carrier gas (rate 1.0 ml min−1). Both the
◦
injector and detector temperature were 250 C, and the column
◦
temperature was between 100 and 180 C. The products were con-
firmed by GC-MS with those of the authentic samples.
Fig. 1. Diffuse reflectance UV–vis spectra of the various solid Q3MnIII complexes
(1a–1e).
3. Results and discussion
3.1. Characterizations of Q3MnIII complexes
due to the increase of electron-withdrawing effect of halogen sub-
stituents on the ligand’s aryl ring. In addition, the two very weak
bands in the low energy region (530–583 nm) could also be ob-
served upon the Q3MnIII complexes, which may be attributed to
the spin-allowed d–d transitions of the centered Mn ions, sup-
porting that the Q3MnIII complexes own a distorted octahedral
geometry [23].
The three dimensional structure mode of a typical sample 1e
was established by Hartree–Fock /3–21G+ with Gaussian-03 soft-
ware and their Mn–O and Mn–N bond distances calculated from
this mode are given in Fig. 2. The theoretical calculation results
indicate that three Mn–O bonds in 1e complex are not equal to
each other, and the Mn1–O51 bond is the longest among the Mn–O
bonds. A similar situation could also be noticed upon three Mn–
N bonds. This should be correlated with the Jahn–Teller effect of
Table 1 lists the Mn and CHN elemental contents of all the
Q3MnIII complexes, wherein the found values for 1a–e are in good
agreement with those calculated from the corresponding formulae
of the hexadentate structures.
The diffuse reflectance UV–vis spectra of all the solid Q3MnIII
complexes are very similar to each other (Fig. 1), consisting of
several higher energy bands in the 230–346 nm region that are
probably due to the π → π∗ transitions of ligands. The complexes
also exhibit a strong band at ca. 394–437 nm, which should be as-
signed to the metal to ligand charge transfer (MLCT) absorption,
as reported for other Mn(III) complexes [22]. Notably, the halo-
genated Q3MnIII samples commonly result in the red shift of the
MLCT band, and such a red shift effect gradually strengthens with
increasing the number of halogen substituents, being most likely

112
Z. Ye et al. / Journal of Catalysis 261 (2009) 110–115
Table 2
The oxidation of alcohol with aqueous H2O2 catalyzed by Q3MnIII complexes.a
Entry Catalyst Substrate
Products
Time TON Sel.%g
(h)
1
2
3
4
1a
1b
1c
1d
1e
1e
1e
1e
5
5
5
5
5
5
5
5
5
5
630 >99
887 >99
988 >99
1076 >99
1320 >99
855 >99
1322 >99
130 >99
882 >99
216 >99
5
6b
7c
8d
9e
10f
Q2MnII
Salen-
MnIIIOAc
11
1e
3
1205 82
18
12
1e
8
610 59
Fig. 2. The optimum three dimensional structures of Q3MnIII complex 1e, selected
bond distances (Å): Mn1–O15, 1.889; Mn1–O51, 1.919; Mn1–O52, 1.887; Mn1–N16,
2.046; Mn1–N41, 2.016; Mn1–N42, 2.061.
41
Mn3+ ions with d4 [24], which can induce the distortion of the
octahedral Q3MnIII complexes.
13
14
1e
1e
8
8
915 >99
3.2. Catalytic tests
Cyclohexanol oxidation with aqueous H2O2 was used to check
the catalytic performances of Q3MnIII, Q2MnII and salen-MnIIIOAc
complexes, and the results are listed in Table 2. To our delight,
the hexadentate Q3MnIII complexes exhibited modest to high TON
and excellent selectivity for cyclohexanone in acetone medium, in
which the TON followed an increasing sequence of 1e > 1d > 1c
> 1b > 1a, indicating that the 5- and 7-halogenated Q3MnIII cat-
alysts (1b–1e) significantly improved the catalytic efficiency (en-
tries 2–5). This is most likely due to the increase of electron-
withdrawing effect of halogen substituents on the ligand’s aryl
ring, and the high resistance of halogenated 8-HQ ligands against
oxidative degradation of the Q3MnIII, as reported for chlorinated
porphyrin-MnIII [25]. Notably, using TBHP instead of H2O2 as an
oxidant results in a poor TON and the rapid deactivation of cata-
lyst 1e (entry 8). Using a tetradentate Q2MnII gave a relative low
TON compared to the corresponding hexadentate 1e (entry 9), and
a much lower TON was obtained using a traditional salen-MnIIIOAc
as catalyst (entry 10).
Entries 11–15 further illustrate that the 1e was also very ef-
fective for the oxidations of other alcohols. Among the various
alcohols verified (Table 2), benzyl alcohol was found to be the
most reactive and gave high TON (ca. 1205 in entry 11). Entries 12
and 13 illustrate that secondary aliphatic alcohol was more reactive
than the corresponding primary one and gave a higher TON (915
vs 610). Notably, primary alcohols were inevitably oxidized to form
their secondary oxidation products carboxylic acids (entries 11, 12,
14); whereas secondary alcohols were oxidized to only form the
corresponding ketones without C–C chain cleavage (entries 5 and
13). Entry 14 shows that the primary and secondary hydroxyls in
1,2-propandiol could be simultaneously and equiprobably oxidized
to form pyruvic aldehyde (91%). Finally, entry 15 illustrates that
565 91
9
15
1e
8
958 56
44
a
Reaction condition: alcohol 9 mmol, catalyst 0.002 mmol, hydrogen peroxide
−1
◦
at 25 C.
3 mmol, acetone 2 ml, stirring speed 3000 r min
b
−1
Stirring speed 1000 r min
Stirring speed 2500 r min
TBHP as oxidant.
.
.
c
−1
d
e
Tetradentate binding 5-chloro-7-iodo-8-quinolinolato manganese(II) (Q2MnII) as
a catalyst.
f
Traditional Salen-MnIIIOAc as a catalyst.
The selectivity to alcohol oxide was based on alcohol.
g
the hydroxyl and C=C bond groups of trans-2-hexene-1-ol could
be nearly equiprobably oxidized to form the corresponding alde-
hyde and epoxide respectively, but the simultaneous oxidations of
the two groups did not occur.
In the following experiment, the effect of reaction time on
the catalytic efficiency and goal product selectivity was investi-
gated using 1e-catalyzed benzyl alcohol oxidation and the results
are illustrated in Fig. 3, Wherein the TON gradually increases be-

Z. Ye et al. / Journal of Catalysis 261 (2009) 110–115
113
Fig. 3. The time-dependent TON and benzaldehyde selectivity for 1e-catalyzed the
oxidation of benzyl alcohol with Hydrogen peroxide.
−5
Fig. 5. UV–vis spectra of (A) a solution of 1e in acetone (8.0 × 10
M, 5 ml) at
◦
10 C; (G) 6 h later of adding 50 wt% H2O2 (10 equiv relative to 1e) and cyclohex-
anol (200 equiv relative to 1e) to (A); (H) 12 h later; (I) 24 h later; (J) 36 h later;
(K) 48 h later. Inset was the amplified traces.
−5
Fig. 4. UV–vis spectra of (A) a solution of 1e in acetone (6.0 × 10
M, 5 ml) at
◦
10 C; (B) 12 h later of adding 50 wt% H2O2 (10 equiv relative to 1e) to (A); (C) 12 h
later of adding cyclohexanol (200 equiv relative to 1e) to (B); (D) 24 h later; (E) 36 h
later; (F) 48 h later. Inset was the amplified traces.
−5
Fig. 6. UV–vis spectra of (A) a solution of 1e in acetone (8.0 × 10
M, 5 ml) at
◦
10 C; (L) 6 h later of adding TBHP (10 equiv relative to 1e) to (A); (M) 12 h later;
(N) 1 h later of adding cyclohexanol (200 equiv relative to 1e) to (C); (O) 36 h later.
fore 3 h and reaches a plateau after 3 h; while benzaldehyde
selectivity slowly decreases in whole reaction process, in conse-
quence of benzaldehyde to be uninterruptedly oxidized to benzoic
acid.
diate. Finally, after 48 h, the trace in 350–390 nm fell back toward
the trace A (F vs A), being likely due to the disappearance of such a
new intermediate. We must note that: there is no clear absorbance
around 500 nm designed to the ligand to metal charge transfer of
the Q3MnV=O in the whole process, indicating that the Q3MnV=O
should not be responsible for the oxidation of alcohols using H2O2
as an oxidant. If cyclohexanol and H2O2 were added to the A solu-
tion simultaneously, the spectral change in the simulation of real
oxidation system (see Fig. 5), as expected, is very similar to that
of the above described, thus indicating that the spectral informa-
tion given in Fig. 4 can present the real situation of the present
oxidation system.
3.3. Catalytic mechanism
The 1e oxidation system was further examined by UV–vis spec-
troscopy in order to investigate the catalytic mechanism for the
Q3MnIII-catalyzed alcohol oxidation, and the results are illustrated
in Fig. 4, Wherein A is the spectrum of 1e in acetone where an ab-
sorption band with λmax ∼ 412 nm is assigned to the MLCT [22].
When the A solution was treated with aqueous H2O2 for 12 h, the
MLCT absorbance decreased, whereas the trace in 365-382 nm ran
up (see B and its inlet in Fig. 4), which may be due to the cleav-
age of an axial Mn–O bond of 1e to form an intermediate. Then,
when adding cyclohexanol to the B solution further, the trace in
350–390 nm continuously ran up in 24 h (C), implying that cyclo-
hexanol interacted with this intermediate to form a new interme-
The above experiments were repeated using TBHP instead of
H2O2 and the obtained UV–vis spectra are illustrated in Fig. 6.
Trace L is the spectrum of adding TBHP to a solution of 1e in ace-
tone, in which a new absorption band centered around 500 nm

114
Z. Ye et al. / Journal of Catalysis 261 (2009) 110–115
Scheme 2. Possible catalytic cycle for the Q3MnIII-catalyzed cyclohexanol oxidation.
was developed and the maximum of which is obscured by the
tailing of strongly absorbing species at λmax > 500 nm due to the
formation of the Q3MnV=O, as observed similarly in the epoxida-
tion of olefins catalyzed by salen-MnIII with using PhIO and TBHP
as oxidants [26]. On the other hand, another absorption band cen-
tered at 360–370 nm increased continuously (traces L and M) even
after adding cyclohexanol to this solution (trace N), and was fi-
nally developed to a very strong absorbance at λmax ∼ 364 nm
correlated to the disappearance of the Q3MnV=O species (trace O).
Notably, the MLCT band of Q3MnIII at λmax ∼ 412 nm continuously
decreased and finally disappeared in the whole process. Correlat-
ing with the poor results obtained using TBHP (entry 8 of Table 2),
we assume that the absorption band at λmax ∼ 364 nm should be
assigned to the MLCT absorption of μ-oxomanganese(IV) dimer,
which is directly responsible for catalyst deactivation. In addition,
the formation of such deactived species (Eq. (1)) probably under-
other hand, the 6 may react with another Q3MnIII to form the
deactivated species 8, as supported by the above reaction and char-
acterization results.
4. Conclusions
In summary, for the first time we have developed the hex-
adentate Q3MnIII complexes as effective catalysts for the oxidation
of alcohols to the corresponding carbonyl compounds in acetone
medium. Besides, the present catalytic system has some distinct
characteristics as follows: (a) using only acetone as a solvent with-
out any additives; (b) employing very small amount of catalyst
(0.02 mol%); (c) giving high H2O2 efficiency; (d) facile operation.
Having found this kind of Q3MnIII complexes, we are interested
in exploiting the preparations of other 8-quinolinolato metal com-
plexes and their application in organic oxidations.
goes a pathway proposed by Srinivasan et al. for the salen-MnIII
catalyzed epoxidation of olefins [26].
-
Acknowledgments
Q3MnV=O + Q3MnIII → Q3MnIV–O–MnIVQ3.
(1)
We acknowledge the financial support for this work by the Na-
tional Natural Science Foundation of China (20873040, 20675029,
90713018, 20335020) and the Natural Science Foundation of Hunan
Province (05JJ40022).
Based on the above findings, a mechanism for the Q3MnIII
-
catalyzed alcohol oxidation with H2O2 (using cyclohexanol as an
example) is proposed as follows (see Scheme 2). Wherein the
Q3MnIII (1) can easily interact with H2O2 to form a pentaden-
tate Q3MnIII–OOH species 2 with pendant hydroxyl group through
opening an axial Mn–O bond, which corresponds to the decrease
and slight blue-shift of the MLCT band observed in the UV–vis
spectra. Then, the 2 reacts with cyclohexanol to give a corre-
sponding adduct 3, which may result in the obvious blue-shift and
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