Synthesis of a polymer–ruthenium complex Ru(pbbp)(pydic) and its catalysis in the oxidation of secondary alcohols with TBHP as oxidant

Article abstract of DOI:10.1007/s11243-016-0112-x

A polymer–ruthenium complex Ru(pbbp)(pydic) was synthesized from the reaction of poly-2,6-bis(benzimidazolyl)pyridine (pbbp) with RuCl₃ and disodium pyridine-2,6-dicarboxylate (pydic). The Ru(pbbp)(pydic) was characterized thoroughly by spectroscopic methods. ICP analysis revealed that the percentage of complexation of 2,6-bis(benzimidazolyl)pyridine unit in pbbp was about 83%. The complex was tested as a heterogeneous catalyst for the oxidation of secondary alcohols to their corresponding carbonyl compounds in solvent-free conditions using aqueous tert-butyl hydroperoxide as oxidant. The developed catalytic system exhibited high activity and broad functional group compatibility, allowing a variety of secondary alcohols, including substituted secondary benzylic alcohols and secondary aliphatic ones, to be oxidized to the corresponding ketones in high yields. This Ru(pbbp)(pydic) could be recycled for several times, but it dissolved in part in the reaction mixture during the catalytic run leading to gradual deactivation of the catalyst with repeated runs.

Full text of DOI:10.1007/s11243-016-0112-x

Transit Met Chem
DOI 10.1007/s11243-016-0112-x
Synthesis of a polymer–ruthenium complex Ru(pbbp)(pydic)
and its catalysis in the oxidation of secondary alcohols with TBHP
as oxidant
Yuecheng Zhang1 Ruosi Chu1 Hongyu Zhang1 Jiquan Zhao1
•
•
•
Received: 1 November 2016 / Accepted: 7 December 2016
Ó Springer International Publishing Switzerland 2016
Abstract A polymer–ruthenium complex Ru(pbbp)(pydic)
was synthesized from the reaction of poly-2,6-bis(benzimi-
dazolyl)pyridine (pbbp) with RuCl3 and disodium pyridine-
2,6-dicarboxylate (pydic). The Ru(pbbp)(pydic) was char-
acterized thoroughly by spectroscopic methods. ICP analysis
revealed that the percentage of complexation of 2,6-
bis(benzimidazolyl)pyridine unit in pbbp was about 83%.
The complex was tested as a heterogeneous catalyst for the
oxidation of secondary alcohols to their corresponding car-
bonyl compounds in solvent-free conditions using aqueous
tert-butyl hydroperoxide as oxidant. The developed catalytic
system exhibited high activity and broad functional group
compatibility, allowing a variety of secondary alcohols,
including substituted secondary benzylic alcohols and sec-
ondary aliphatic ones, to be oxidized to the corresponding
ketones in high yields. This Ru(pbbp)(pydic) could be
recycled for several times, but it dissolved in part in the
reaction mixture during the catalytic run leading to gradual
deactivation of the catalyst with repeated runs.
involve the use of stoichiometric or even over-stoichio-
metric chromium(VI) oxide [4–7], MnO2 or hypervalent
iodine compounds [8–13] as oxidants, which can generate
large amounts of waste and cause environmental problems.
Therefore, it is crucial to develop cleaner catalytic oxidation
systems to realize the oxidation processes. From the view-
points of both economy and environmental protection, the
use of molecular oxygen as the terminal oxidant is most
promising, because molecular oxygen is widely available
and only water as byproduct is formed in the reaction in
principle. For this purpose, many catalytic systems have
been developed, and the ones based on stable nitroxyl
radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)
with various transition metal salts as co-catalysts are the
most reported [14–17]. However, the catalytic systems
developed using molecular oxygen as terminal oxidant only
showed high efficiency in the oxidation of primary alcohols,
and unfavorable results were generally received in the cases
of secondary alcohols as substrates. Therefore, it is essential
to contrive protocols to convert secondary alcohols to the
corresponding ketones.
Introduction
Generally, hydrogen peroxide (H2O2) and TBHP are also
considered to beenvironmentally benignoxidants, though they
are slightly inferior compared to molecular oxygen. Various
transition metals or their complexes were found to be efficient
in the oxidation of secondary alcohols to the corresponding
ketones with H2O2 or TBHP as oxidant [18–23]. Among the
transition metal complexes, several Ru complexes showed
excellent performances in the reaction [24–27]. For instance,
Ru(terpyridine)(2,6-pyridinedicarboxylate) [hereafter abbre-
viated as Ru(terpy)(pydic)] performed very well in the oxi-
dation of secondary alcohols to the ketones with H2O2 as
oxidant in the absence of any co-catalysts or organic solvents
[24]. Another ruthenium complex termed ruthenium-bis(ben-
zimidazole)(pyridinedicarboxylate) also showed high
Oxidation of alcohols to the corresponding carbonyl com-
pounds is of great importance both in academia and in
industry [1–3]. Traditional methods to perform this reaction
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-016-0112-x) contains supplementary
material, which is available to authorized users.
& Jiquan Zhao
zhaojq@hebut.edu.cn
1
School of Chemical Engineering and Technology, Hebei
University of Technology, Tianjin 300130, People’s Republic
of China
123

Transit Met Chem
efficiency in oxidation reactions with H2O2 as the oxidant [25].
Very recently, an N,N,N-tridentate ligand Ru complex—
Ru(pymieb)(pydic) designed and synthesized by us exhibited
excellent activity in the oxidation of various secondary alco-
hols to the ketones with TBHP as oxidant under mild and
solvent-free conditions [26]. In spite of the excellent perfor-
mances of these ruthenium complexes in the reaction, it is very
difficult to separate the expensive complexes from the reaction
mixture, making them impossible to recycle. Heterogenization
of the ruthenium complexes was used to overcome this diffi-
culty. For instance, a silica gel immobilized [(Me3-
were obtained from Tianjin Guangfu Fine Chemical
Research Institute. The ligand pydic was prepared by
treatment of pyridine-2,6-dicarboxylic acid with sodium
hydroxide. The alcohols were obtained from Alfa Aesar
China (Tianjin) Co., Ltd.
1H NMR spectra were measured in DMSO-d6 and
CDCl3 on a Bruker AC-P 400 spectrometer with tetram-
ethylsilane as an internal standard. The metal content of the
catalyst samples was analyzed using a PerkinElmer Opfima
7300V type ICP-AES instrument. Before analysis, the
accurately weighed sample was carefully added into a
flask, and then nitric acid and perchloric acid were added.
The suspension was heated to dissolve the sample, and the
obtained solution was subjected to analysis. FTIR spectra
were recorded on a Bruker Vector 22 instrument in the
wave number range of 400–4000 cm-1 with the samples in
the form of KBr pellets. UV–Vis spectra were collected in
the range of 200–800 nm on a Varian Cary 300 UV–visible
spectrophotometer. Thermogravimetric analysis of the
pbbp and Ru(pbbp)(pydic) was performed on an SDT/
Q600 thermogravimetric analyzer at a temperature range of
20–1200 °C with a temperature ramp of 10 °C/min under
an air environment. Scanning electron microscopy (SEM)
was carried out with a Nova Nano SEM450 instrument and
an Octane Pro Det. Oxidation reaction samples were ana-
lyzed by a Shandong Lunan Ruihong Gas Chromatograph
(SP-7800A) equipped with an FID detector and an SE 54
column (30 m 9 0.5 lm).
tacn)RuIII(CF3COO)2(H2O)]CF3CO2
complex
pre-
(Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane)
pared by Che et al. showed high activity and good recyclability
in the oxidation of secondary alcohols to the ketones with
TBHP as oxidant [27]. Recently, we successfully immobilized
Ru(terpy)(pydic) onto MCM-41 using a multi-step grafting
method [28]. This immobilized ruthenium complex exhibited
excellent performance in the oxidation of various secondary
alcohols to the corresponding ketones with TBHP as oxidant
and had the advantages of easy recovery and good reusability,
too. Though the heterogenization of the complexes received
promising results, the immobilization processes were gener-
ally complicated. It was reported that a polymer containing the
tridentate 2,6-bis(benzimidazol-2-yl)pyridine moiety was
easily synthesized by the condensation reaction between pyr-
idine-2,6-dicarboxylic acid and 3,30-diaminobenzidine
tetrahydrochloride dihydrate in the presence of polyphospho-
ric acid, and this polymer can coordinate with ruthenium to
give polymer–metal complexes exhibiting good thermal sta-
bilities [29]. Inspired by this report, we have used this polymer
in combination with pydic coordinating with RuCl3 to form a
polymer–metal complex with a Ru[bis(benzimidazol-2-
yl)pyridine](2,6-pyridinedicarboxylate) moiety. The resulting
polymer–ruthenium complex Ru(pbbp)(pydic) was applied to
the oxidationof secondary alcohols with TBHP as oxidant. The
catalytic results showed thatthe complex has good activity and
could be recycled several times in the reaction.
Catalyst preparation
Synthesis of pbbp
3,30-Diaminobenzidine
tetrahydrochloride
dihydrate
(1.98 g, 5 mmol) was gradually added to freshly prepared
PPA (50 g, 85% P2O5) under nitrogen atmosphere at about
80 °C. After eliminating the hydrochloride, pyridine-2,6-
dicarboxylic acid (0.83 g, 5 mmol) was added into the
solution. The resulting slurry was maintained at 150 °C for
about 24 h and then at 200 °C for 30 h, respectively. The
viscous polymer solution was poured into hot water, and the
mixture was stirred vigorously for 1 h in order to eliminate
any residual of monomer and PPA. The solid was filtered off
and washed with aqueous sodium hydroxide solution (5%).
Then it was washed with water till the pH of the filtrate
reached 7. The solid was then purified by washing with water
in a Soxhlet extractor and finally dried under vacuum for
Experimental
Reagents and apparatus
All the reagents and solvents were purchased from com-
mercial sources and used as received without further
purification. 3,30-Diaminobenzidine tetrahydrochloride
dehydrate (98%) was purchased from Source Leaf Bio-
logical Technology Co., Ltd. RuCl3Á3H2O was supplied by
Xi’an Kaili Chemical Company. Pyridine-2,6-dicarboxylic
acid (98%) and TBHP (70%) were purchased from Energy
Chemical, China. H2O2 (30%) and all solvents, AR grade,
1
24 h (1.21 g, yield 78%). H NMR (400 MHz, DMSO-d6,
TMS) d(ppm): 7.70–7.77(m, 2H), 7.79–7.94(m, 2H),
8.07–8.25(m, 3H), 8.38–8.44(m, 2H), 13.11–13.21(br, 2H).
IR(KBr, cm-1) m: 3407, 1626, 1596, 1562, 1437, 1304, 992,
803. UV/Vis (DMF) kmax = 266, 357 nm.
123

Transit Met Chem
Synthesis of Ru(pbbp)(pydic)
Results and discussion
To a solution of RuCl3Á3H2O (131 mg, 0.5 mmol) in
MeOH (8 ml) was added a solution of pydic (156 mg,
0.5 mmol) in MeOH–H2O (2:1, 12 ml) under nitrogen
atmosphere. The mixture was heated and refluxed for 1 h
and then cooled to room temperature. The solvent was
evaporated under vacuum. The residue was dissolved in
DMF (200 ml), and pbbp (150 mg, 0.5 mmol, calculated
from the molecular weight of the repeating unit) was
added to the above DMF solution under stirring. The
mixture was heated at 100 °C for 24 h under nitrogen
atmosphere. The mixture became homogeneous during the
course of the reaction. The reaction mixture was cooled to
room temperature and poured into diethyl ether (150 ml).
A dark green precipitate was formed and collected by
suction filtration. The product was washed thoroughly
with methanol and was collected as a dark green solid
(0.23 g, yield 82%). The ruthenium content of the catalyst
was analyzed using ICP and was found to be 14.5%. IR
(KBr, cm-1) m: 3400, 1620, 1564, 1450, 1386, 1308,
1150, 1020, 803, 472. UV/Vis (DMF) kmax = 264, 360,
586 nm.
Synthesis of Ru(pbbp)(pydic)
The synthesis route of the Ru(pbbp)(pydic) is shown in
Scheme 1. The polymer pbbp was prepared as described in
the literature [29]. The condensation reaction between 3,30-
diaminobenzidine tetrahydrochloride dihydrate and pyr-
idine-2,6-dicarboxylic acid smoothly afforded pbbp in high
yield in the presence of PPA as the reaction medium. The
physical properties of the solid product including solubility
in organic acids such as methanesulfonic acid, formic acid,
trifluoroacetic acid and 1H NMR are identical with the
literature report [29].
It was reported that substituted terpyridine or 2,6-
bis(benzimidazolyl)pyridine in combination with pydic can
coordinate with RuCl3 to afford the corresponding
Ru(L)(pydic) complex (L = BINOL-terpyridine or 2,6-
bis(N-octadecylbenzimidazol-2-yl)pyridine) [30, 31].
Therefore, RuCl3 was employed as the ruthenium source to
prepare the polymer–ruthenium complex Ru(pbbp)(pydic).
Pre-coordination of pydic with RuCl3 was required to
facilitate the dissolution of pydic in DMF. The
Ru(pbbp)(pydic) was synthesized by heating a mixture of
pbbp with RuCl3–pydic in DMF at 100 °C in high yield.
Ru(pbbp)(pydic) has much higher solubility in DMF than
pbbp. The enhancement in solubility in coordination of pbbp
with ruthenium in polar aprotic solvent was also observed in
other ruthenium–polymer complexes [29]. The ruthenium
content determined by ICP was 14.5%, from which it can be
estimated that the percentage of complexation was about
83%, or the ratio of x to y in Scheme 1 was 83:17.
Catalytic oxidation of alcohols
In a typical process, into a 5-ml two-necked round-bottom
flask equipped with a magnetic stirrer were added
Ru(pbbp)(pydic) (0.002 mmol) and alcohol (2 mmol)
successively at room temperature. The mixture was
heated to 60 °C under stirring, and then TBHP (70%
aqueous solution) was slowly dropped in 0.5 h. The
reaction was monitored by GC equipped with a SE 54
column (30 m 9 0.5 lm). After reaction, the product was
purified by column chromatography over silica gel (elu-
ent: n-hexane/ethyl acetate) and characterized by 1H
NMR.
Characterization
The Ru(pbbp)(pydic) was firstly characterized by FTIR,
and the spectra of the complex and pbbp are shown in
Scheme 1 Synthesis route for Ru(pbbp)(pydic)
123

Transit Met Chem
Fig. 1. The absorption band at 1626 cm-1 in the spectrum
of pbbp (a) is assigned to the C=N stretching in the imi-
dazole, and the bands at 1596 and 1562 cm-1 are assigned
to the C=N stretching vibration of the pyridine ring [29].
Compared with that of pbbp, the spectrum of
Ru(pbbp)(pydic) (b) exhibits an additional band at
1620 cm-1, attributed to the C=O stretching in the pydic
[25], and a weak band at 472 cm-1 attributed to Ru–N
vibration [32]. Besides, all the characteristic peaks in the
spectrum of ruthenium-bis(benzimidazole)pyridinedicar-
boxylate complex [Ru(bbp)(pydic)] [25] can be found in
that of Ru(pbbp)(pydic). These results indicate the forma-
tion of the Ru(pbbp)(pydic).
Fig. 2. In the spectrum of pbbp (a), the bands maxima at
266 and 357 nm are, respectively, derived from the p–p*
transitions of phenyl and C=N groups [29, 33]. After the
formation of Ru(pbbp)(pydic) (b), an additional weak
absorption band at 586 nm is observed, it is assigned to the
metal–ligand charge transfer [MLCT, d(Ru) ? p*(pyr-
idine)] transition, which is a characteristic transition band
in ruthenium polypyridine complexes [29, 34].
The thermal stability of pbbp and Ru(pbbp)(pydic) was
investigated by thermogravimetric analysis (TGA). Fig-
ures 3 and 4 show the TG and DSC curves for pbbp and
Ru(pbbp)(pydic), respectively. As shown in Figs. 3 and 4,
both the samples of pbbp and Ru(pbbp)(pydic) show an
initial endothermic stage occurring in the range of
50–150 °C, with a weight loss of 10%, being ascribed to
the desorption of physisorbed water. In Fig. 3, the second
weight loss beginning at about 530 °C is due to the
exothermic oxidative decomposition of pbbp [29]. In
Fig. 4, the second stage between 300 and 450 °C is cor-
responding to the exothermic oxidative degradation of the
complex. A weight loss of 65% takes place in this stage.
Therefore, we can conclude that Ru(pbbp)(pydic) is
stable when it is employed as catalyst below 300 °C.
The samples of pbbp (a), fresh Ru(pbbp)(pydic) (b),
Ru(pbbp)(pydic) after the first run (c) and the third run
(d) were analyzed using SEM, and the results are presented
in Fig. 5. The SEM images show that the sample of pbbp
consists of irregular mass particles, and the samples of
fresh and used Ru(pbbp)(pydic) retain nearly the same
morphology to that of the pbbp. Part fragmentation of
Ru(pbbp)(pydic) was found in the catalytic runs by com-
paring the images of the used samples with that of the fresh
one.
The electronic spectra for dilute solutions of pbbp and
Ru(pbbp)(pydic) in DMF (2.5 9 10-5 M) are shown in
Fig. 1 FTIR spectra of pbbp (a) and Ru(pbbp)(pydic) (b)
Fig. 2 UV–Vis spectra of pbbp (a) and Ru(pbbp)(pydic) (b)
Fig. 3 TG and DSC curves of the thermal degradation of pbbp
123

Transit Met Chem
conversion of 1-phenylethanol reached up to 99% with a
selectivity of 99% toward acetophenone in 4 h at 40 °C
(Table 1, entry 7). The catalytic reaction was also carried
out in different solvents. However, the reaction proceeded
slowly, and only moderate conversions of 1-phenylethanol
were obtained in more than 5 h in the solvents tested
(Table 1, entries 3–6).
The parameters influencing the reaction were optimized
under solvent-free conditions. First, the effect of reaction
temperature on the reaction was examined, and the results
are listed in Table 2. From the table, we can find that it
took 15 h to finish the reaction when it was conducted at
25 °C (Table 2, entry 1). Increasing temperature can
accelerate the reaction (Table 2, entries 2–4), and the
reaction time to completion was only 2 h at 60 °C
(Table 2, entry 3). Further increasing temperature can still
increase the reaction rate, but it may cause safety problems
in large-scale application of TBHP. Therefore, 60 °C was
selected as the reaction temperature in the subsequent
experiments.
Fig. 4 TG and DSC curves of the thermal degradation of
Ru(pbbp)(pydic)
Catalytic activity
The Ru(pbbp)(pydic) was employed as a catalyst in the
oxidation of secondary alcohols to the corresponding
ketones. First, the catalytic performance of Ru(pbbp)(py-
dic) in the reaction was evaluated using 1-phenylethanol as
a model substrate. Initially, various oxidants including O2,
H2O2, TBHP were screened in the reaction under solvent-
free conditions. As shown in Table 1, almost no reaction
took place in the case of oxygen as oxidant (Table 1, entry
1). Unexpectedly, low conversion was obtained in the case
of 30% aqueous H2O2 as oxidant (Table 1, entry 2), which
is very different from that catalyzed by the corresponding
complex Ru(terpy)(pydic) [24]. To our delight, TBHP
showed good performance in the reaction and the
The catalyst loading was optimized at 60 °C keeping the
other parameters constant. As shown in Table 3, the use of
Ru(pbbp)(pydic) is essential. Only 47.3% conversion of
1-phenylethanol was achieved in 10 h in the absence of the
catalyst (Table 3, entry 1). Increasing the amount of cata-
lyst from 0.05 to 0.2 mol%, the reaction time required to
finish the reaction decreased from 5 h to 1.5 h (Table 3,
entries 2–4). However, when the catalyst loading was
increased from 0.1 to 0.2 mol%, only 0.5 h was saved to
finish the reaction. Therefore, the optimal loading of
Ru(pbbp)(pydic) was chosen as 0.1 mol%.
Fig. 5 SEM images of pbbp
(a), fresh Ru(pbbp)(pydic) (b),
Ru(pbbp)(pydic) after the first
run (c) and Ru(pbbp)(pydic)
after the third run (d)
123

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Table 1 Oxidation of 1-phenylethanol catalyzed by Ru(pbbp)(pydic)
with different oxidants
Table 3 Effect of the amount of catalyst on the oxidation of
1-phenylethanol
Entry Oxidanta
Solvent Time (h) Conv. (%)b Select. (%)b
Entry
Ru(pbbp)(pydic)
(mol%)
Time (h)
Conv. (%)a
Select. (%)a
1
2
3
4
5
6
7
O2
–
–
5.0
5.0
–
–
1
2
3
4
–
10.0
5.0
47.3
[99
[99
[99
[99
[99
[99
[99
30% H2O2
46.1
74.0
65.6
59.2
52.0
[99
[99
[99
[99
[99
[99
[99
0.05
0.1
0.2
70% TBHP CH3CN 5.0
70% TBHP MeOH 5.0
70% TBHP Toluene 5.0
2.0
1.5
70% TBHP AcOEt
70% TBHP
5.0
4.0
Reaction condition 1-phenylethanol (2 mmol), oxidant (6 mmol),
reaction temperature 60 °C
–
a
Determined by GC
Reaction condition 1-phenylethanol (2 mmol), catalyst (0.1 mol%),
oxidant (6 mmol), solvent (2 ml), reaction temperature 40 °C
a
O
1 atm
2
b
Determined by GC
H
O
O
Ru(pbbp)(pydic)
TBHP
Table 2 Effect of reaction temperature on the oxidation of
1-phenylethanol
Entry Temperature (°C) Time (h) Conv. (%)a Select. (%)a
1
2
3
4
25
40
60
80
15.0
4.0
2.0
1.0
[99
[99
[99
[99
[99
[99
[99
[99
Reaction condition 1-phenylethanol (2 mmol), catalyst (0.1 mol%),
oxidant (6 mmol)
a
Determined by GC
Fig. 6 Effect of the amount of TBHP on the oxidation of
1-phenylethanol
Generally, increasing the amount of TBHP can accel-
erate the reaction effectively. However, excessive TBHP
will cause work-up complications and generate additional
wastes. In order to avoid over-loading of TBHP, the oxi-
dation reaction was tracked under the molar ratio of oxi-
dant to substrate from 1.0:1 to 2.1:1, and the results are
shown in Fig. 6. As shown in Fig. 6, the conversion of
1-phenylethanol increased with reaction time under all
TBHP loadings. However, the reaction could not finish in
7.5 h if the molar ratio of TBHP to substrate was lower
than 1.7:1. When the molar ratio of TBHP to substrate was
equal to or higher than 1.7:1, the reaction could finish in
7.5 h. Therefore, the most suitable molar ratio of TBHP to
1-phenylethanol was 1.7:1. In this case 1-phenylethanol
was almost quantitatively converted to acetophenone in
7.5 h.
Based on the experimental results, we concluded that the
optimal conditions for the oxidation of 1-phenylethanol are
substrate 2 mmol, catalyst 0.1 mol%, molar ratio of TBHP
to substrate 1.7:1 and reaction temperature 60 °C. Under
the optimal conditions, 1-phenylethanol was almost com-
pletely oxidized to acetophenone in 7.5 h in the absence of
solvent.
123

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Having obtained the optimal conditions, the
Ru(pbbp)(pydic) was applied to the oxidation of various
secondary and several primary alcohols to screen the ver-
satility of the Ru(pbbp)(pydic)–TBHP. As shown in
Table 4, most of the secondary benzylic alcohols, including
the ones with both electron-donating and electron-with-
drawing groups, were selectively converted to the corre-
sponding ketones in high yields under the optimal reaction
conditions (Table 4, entries 1–10, 13–15). However, the
reaction rate changed with the substrate structures. Gener-
ally, the electron-donating substituents resulted in longer
reaction times (Table 4, entries 2–10), which indicated that
electronic properties of the substituents have some effects on
the reaction. The substituent position on the benzene ring has
obvious influence on the reactivity of the secondary benzylic
alcohols. The substrate with an o-substituent reacted slowly
compared to the one with an m- or p-substituent due to the
steric hindrance of the o-substituent (Table 4, entries 2, 3,
5–7). For instance, only 38.5% conversion was obtained in
the oxidation of 1-(2-chlorophenyl)ethanol in 12 h (Table 4,
entry 7). Higher TBHP loading and longer reaction time
were required to get high yields in the oxidation of 1-(2-
and dried thoroughly. The recovered catalyst was subjected
to the next run under the same experimental conditions.
The results are summarized in Table 5. Comparing the
results in the second run with those in the first run, the
conversion of 1-phenylethanal decreased by about 6.1%. In
the third run, the conversion of 1-phenylethanal to ace-
tophenone was only 70.8% in 7.5 h. To elucidate the
deactivation reasons, the ruthenium content of the filtrate
after each run was analyzed using ICP, and the results are
presented in Table 5. The ICP analysis confirmed the
presence of ruthenium in the filtrate, which was the deac-
tivation reason of the Ru(pbbp)(pydic) catalyst. In order to
investigate whether the leakage of ruthenium into filtrate is
caused by dissolving of the polymer ruthenium complex in
the reaction mixture or breaking down from the polymer
ligand, an additional catalytic experiment was carried out.
To the filtrate of the first run was added 5 mmol of
1-phenylethanol, then 8.5 mmol of TBHP was dropped
slowly, the 1-phenylethanol was almost quantitatively
converted to acetophenone with extending reaction time.
This result indicated that the deactivation of the polymer–
ruthenium is mainly due to the dissolution of the ruthenium
complex into the reaction mixture.
chlorophenyl)ethanol
and
1-(2-methylphenyl)ethanol
(Table 4, entries 4, 8). The length of the aliphatic chain of
the substrate has obvious effect on the reaction. Only 59.6%
conversion was obtained in 14 h under the optimized con-
ditions in the oxidation of 1-phenyl-1-propanol (Table 4,
entry 11). Increasing TBHP loading and extending reaction
time improved the reaction. When the molar ratio of TBHP
to substrate was increased to 2.2:1, 1-phenyl-1-propanol was
quantitatively converted to propiophenone in 16 h (Table 4,
entry 12). Overall, the reaction of the secondary alcohols
was controlled by the combined results of electronic and
steric effects. The Ru(pbbp)(pydic) showed moderate
activity in the oxidation of secondary aliphatic alcohols
(Table 4, entries 16–21), which are poor substrates in the
oxidation with other transition metal catalyst systems
[14, 35]. Due to the big steric hindrance in the structure 2-
isopropyl-5-methylcyclohexanol gave very low conversion
(Table 4, entries 22, 23). The catalytic oxidation system was
also used in the oxidation of several primary benzylic
alcohols to screen whether it is feasible to get either the
aldehyde or the carboxylic acid. The results indicated that it
is difficult to control the selectivity, and the main products
were the corresponding carboxylic acids in all the cases
(Table 4, entries 24–28).
In view of the above results, the recycling procedure for
the catalyst was changed to the following manner. After
each catalytic run, the product acetophenone was distilled
off in vacuum and the residue was used directly in the
subsequent run. The results listed in Table 6 show that the
conversion of 1-phenylethanol decreased slightly in the
second run. The reusability of the Ru(pbbp)(pydic) catalyst
was improved in this manner compared to recycling the
catalyst by filtration. However, the catalyst still deactivated
gradually with further recycling. In the fourth run, the
conversion of 1-phenylethanal to acetophenone was 84.8%
in 7.5 h. In this case the deactivation of the Ru(pbbp)(py-
dic) catalyst could be ascribed to the decomposition of the
complex in the catalytic run.
A hot filtration test was carried out to confirm the
heterogeneity of the reaction. In the experiment 0.1% mol
of Ru(pbbp)(pydic) was suspended in the mixture of sol-
vent and 1-phenylethanol or neat 1-phenylethanol under
stirring for 30 min at 60 °C, and then the catalyst was
removed by hot filtration. The filtrate was subjected to
oxidation by TBHP. The reaction processes were moni-
tored by GC, and the results are listed in Table 7. For
comparison, the oxidation of 1-phenylethanol both in the
absence and in the presence of Ru(pbbp)(pydic) was also
performed, and the results are listed in Table 7, too. From
the table, it can be seen that low conversion of 1-pheny-
lethanol was obtained in the oxidation of the filtrates in 5 h,
no matter whether the extracts are acetonitrile, toluene and
1-phenylethanol itself (Table 7, entries 1–3). Slightly low
conversion of 1-phenylethanol was received in the absence
Recycling of catalyst
Finally, a recycling test was carried out to evaluate the
stability as well as the reusability of Ru(pbbp)(pydic). For
each cycle, the catalyst was separated from the reaction
mixture by filtration, washed extensively with methanol
123

Transit Met Chem
Table 4 Oxidation of various alcohols with TBHP catalyzed by Ru(pbbp)(pydic)
Time
(h)
Conv.
(%)a
Select.
(%)a
Yield
(%)b
Entry
Substrate
Product
1
7.5
>99
>99
95.0
2
3
11
15
20
8
>99
62.8
>99
>99
>99
>99
>99
>99
>99
>99
94.1
49.7
88.2
95.2
93.3
4c
5
18
6
7
12
24
38.5
>99
>99
>99
>99
>99
28.5
90.2
90.1
8c
9
7.5
123

Transit Met Chem
Table 4 continued
10
11
9
>99
>99
>99
96.6
48.9
14
59.6
12d
13
14
15
16
17c
18
16
4
>99
>99
>99
>99
67.3
>99
57.9
>99
>99
>99
>99
>99
>99
>99
93.7
97.6
89.0
90.1
55.6
83.4
41.0
10
5
17.5
17
21
19e
20
17
33
>99
>99
>99
80.2
42.0
56.4
123

Transit Met Chem
Table 4 continued
21c
25
9
>99
17.2
39.2
99.7
97.8
95.9
98.0
95.1
>99
>99
>99
97.4
66.6
62.8
75.6
92.8
84.6
22
23c
24d
25d
26c
27d
28d
17
10
16
12
16
15
89.7
51.9
49.0
61.3
78.3
Reaction conditions alcohol (2 mmol), oxidant (3.4 mmol), catalyst (0.1 mol%), reaction temperature 60 °C
a
Determined by GC
b
Isolated yields, the product was purified by column chromatography over silica gel (eluent: n-hexane/ethyl acetate)
c
Oxidant: 6 mmol
d
Oxidant: 4.4 mmol
e
Oxidant: 4 mmol
1
All products were determined by H NMR
of catalyst compared to that in the oxidation of 1-pheny-
lethanol filtrate (Table 7, entry 4). However, the conver-
sion of 1-phenylethanol reached up to 95.6% in the
presence of Ru(pbbp)(pydic) (Table 7, entry 5). The results
indicated that the solubility of Ru(pbbp)(pydic) in the
reaction mixture was very low during the reaction, and
most of the reaction was catalyzed by the Ru(pbbp)(pydic)
in solid state.
123

Transit Met Chem
Table 5 Recycling of Ru(pbbp)(pydic) in the oxidation of 1-pheny-
lethanol by filtration
Table 7 Oxidation of 1-phenylethanol in hot filtrates with TBHP
OH
O
TBHP, 60 oC
Entry
Filtrate
Conv. (%)a
Select. (%)a
Entry
Run
Conv. (%)a
Select. (%)a
Ru (%)b
1
1-Phenylethanol/CH3CN
27.7
19.4
25.4
22.8
95.6
[99
[99
[99
[99
[99
1
2
3
1
2
3
[99
93.9
70.8
[99
[99
[99
10.6
11.5
18.4
2
1-Phenylethanol/Toluene
3
1-Phenylethanol
4b
5c
_
_
Reaction condition 1-phenylethanol (50 mmol), catalyst (0.05 mmol),
oxidant (85 mmol), reaction temperature 60 °C, reaction time 7.5 h
a
Reaction condition 1-phenylethanol (2 mmol), oxidant (3.4 mmol),
solvent (2 ml), reaction temperature 60 °C, reaction time 5 h
Determined by GC
b
The percentage of ruthenium in filtrate with the catalyst loading in
the first run
a
Determined by GC
b
The oxidation of 1-phenylethanol with TBHP in the absence of
catalyst
c
The oxidation of 1-phenylethanol with TBHP in the presence of
0.1% mol of catalyst
Table 6 Recycling of Ru(pbbp)(pydic) in the oxidation of
1-phenylethanol
which will decrease the solubility of the polymer–ruthe-
nium complex in solvents and improve its recyclability in
catalysis.
Entry
Run
Conv. (%)a
Select. (%)a
Acknowledgements The authors are grateful for financial supports
from the National Natural Science Foundation of China (No.
21276061) and Natural Science Foundation of Hebei Province, China
(No. B2013202158).
1
2
3
4
5
1
2
3
4
5
[99
97.4
92.3
84.8
77.2
[99
[99
[99
[99
[99
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