Synthesis, single crystal structure and efficient catalysis for alcohol oxidation of a novel Ru(II) complex with both a N,N,N-tridentate ligand and a pyridinedicarboxylate

Article abstract of DOI:10.1016/j.poly.2015.08.047

A novel N,N,N-tridentate ligand known as 2-(2-pyridylmethylamino)ethylbenzimidazole (pymaeb) was designed and synthesized. This ligand in combination with disodium pyridine-2,6-dicarboxylate (pydic) reacted with RuCl₃ to afford a novel complex Ru[2-(2-pyridymethylimino)ethylbenzimidazole]pyridinedicarboxylate [Ru(pymieb)(pydic)] which was characterized by NMR, IR, HR-MS and single crystal X-ray diffraction. Crystal structure analysis revealed that the complex has a distorted octahedral geometry. The complex showed excellent activity for the oxidation of various alcohols with TBHP as oxidation under mild and solvent-free conditions.

Full text of DOI:10.1016/j.poly.2015.08.047

Accepted Manuscript
Synthesis, single crystal structure and efficient catalysis for alcohol oxidation
of a novel Ru(II) complex with both a N,N,N-tridentate ligand and a pyridine-
dicarboxylate
Yuecheng Zhang, Liu Liu, Xiaohui Cao, Jiquan Zhao
PII:
S0277-5387(15)00727-5
DOI:
http://dx.doi.org/10.1016/j.poly.2015.08.047
POLY 11686
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
30 June 2015
28 August 2015
Please cite this article as: Y. Zhang, L. Liu, X. Cao, J. Zhao, Synthesis, single crystal structure and efficient catalysis
for alcohol oxidation of a novel Ru(II) complex with both a N,N,N-tridentate ligand and a pyridinedicarboxylate,
Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.08.047
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Synthesis, single crystal structure and efficient catalysis for
alcohol oxidation of a novel Ru(II) complex with both a N,N,N-
tridentate ligand and a pyridinedicarboxylate
Yuecheng Zhang, Liu Liu, Xiaohui Cao, Jiquan Zhao*
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin
300130, PRC. E-mail: zhaojq@hebut.edu.cn; yczhang@hebut.edu.cn; Fax: +86 22 60202926
Tel: +86 22 60202926; +862260204279
Abstract:
A
novel
N,
N,
N-tridentate
ligand
known
as
2-(2-
pyridylmethylamino)ethylbenzimidazole (pymaeb) was designed and synthesized. This ligand in
combination with disodium pyridine-2, 6-dicarboxylate (pydic) reacted with RuCl3 to afford a
novel
complex
Ru[2-(2-pyridymethylimino)ethylbenzimidazole]pyridinedicarboxylate
[Ru(pymieb)(pydic)] which was characterized by NMR, IR, HR-MS and single crystal X-ray
diffraction. Crystal structure analysis revealed that the complex has a distorted octahedral
geometry. The complex showed excellent activity for the oxidation of various alcohols with TBHP
as oxidation under mild and solvent-free conditions.
Keywords: N, N, N-tridentate ligand; ruthenium(II) complex; tert-butyl hydroperoxide; oxidation;
alcohols
1. Introduction
The oxidation of alcohols to the corresponding carbonyl compounds plays an important role in
both laboratory and synthetic industrial applications [1-6]. In traditional oxidation processes, large
amounts of metal oxidants and toxic and volatile organic solvents were extensively used, which
would generate large amounts of byproducts and cause pollution to the environment [7-11]. It
would be of great economical and ecological importance to develop benign and clean processes by
using molecular oxygen, hydrogen peroxide and tert-butyl hydrogen peroxide (TBHP) as oxidants.
With this view, a variety of transition metal based catalysts [12-26], especially ruthenium
compounds have been intensively studied due to their sufficiently selective and tolerant to many
other functional groups [17-26]. For example, Ishii and his co-workers employed a phosphine
ruthenium Ru(PPh3)3Cl2 in combination with hydroquinone as a catalyst system in the aerobic
oxidation of primary alcohols to their corresponding aldehydes in PhCF3 [17]. They also applied
Ru(PPh3)3Cl2 supported on active carbon to the oxidative cleavage of vic-diols by dioxygen to the
corresponding aldehydes [18]. Some ruthenium-nitrogen ligand complexes were also developed in
the oxidation of alcohols. Katsuki successfully achieved the oxidation of primary alcohols to
aldehydes in the presence of secondary alcohols under aerobic conditions by using a
(nitrosyl)Ru(salen) complex as catalyst [19-21]. Ji developed a protocol for the selective aerobic
oxidation of various alcohols to carbonyl compounds catalyzed by ruthenium(III) meso-
tetraphenylporphyrin chloride in the presence of isobutyraldehyde [22]. In this case the dosage of
1

the catalyst was reduced dramatically. Recently, Ji and his co-workers synthesized a ruthenium-
nitrogen ligand complex, ruthenium-bis(benzimidazole)pyridinedicarboxylate [27]. By applying
this complex as catalyst, primary and secondary alcohols were oxidized to aldehydes and ketones
in good yield and excellent selectivity under mild conditions with aqueous hydrogen peroxide as
oxidant. The tridentate nitrogen liand bis(benzimidazole)pyridine is of very importance in
determining the catalysis of the complex. This work indicates that it is possible to obtain
complexes with various catalytic performances by designing and synthesizing novel ligands in
homogeneous catalysis.
Our interest has been aroused by ruthenium-catalyzed oxidation reactions with its wide range of
applicability and broad variation of ligand types in homogeneous catalysis, especially in the
oxidation of alcohols. In this regard, we focus on designing and synthesizing a novel N,N,N-
tridentate ligand with both benzimidazole and
pyridylmethylamino)ethylbenzimidazole (pymaeb). This ligand in combination with disodium
pyridine-2,6-dicarboxylate (pydic) reacted with RuCl3 to afford complex Ru[2-(2-
pyridylmethylimino)ethylbenzimidazole]pyridinedicarboxylate [Ru(pymieb)(pydic)]. The
pyridine moieties, known as 2-(2-
a
complex showed excellent performances in the oxidation of alcohols with TBHP as oxidant under
mild and solvent-free conditions without any additives. Herein, we report the synthesis,
characterization and catalysis of the novel complex.
2. Experimental
2.1 Reagent and apparatus
2, 6-Pyridinedicarboxylic acid, ꢀ-alanine, 1,2-diaminobenzene, 2-pyridinecarbaldehyde and di-
tert-butyl dicarbonate were purchased from Tianjin Keruisi Fine Chemical Co., LTD. Alcohols,
ruthenium chloride and t-butyl hydroperoxide (TBHP) (70%) were obtained from Alfa Aesar
China (Tianjin) Co., Ltd. Disodium pyridine-2, 6-dicarboxylate (pydic) was prepared by treatment
of 2,6-pyridinedicarboxylic acid with sodium hydroxide. Other reagents were commercially
available and used without further purification.
1H NMR and 13C NMR spectra were recorded with TMS as internal standard on a Brucker AC-
P 400 type nuclear magnetic resonance spectrometer. Infrared spectra were obtained from a
Bruker Vector 22 type instrument in the range 4000~400 cm-1 using a KBr pellet. Mass spectra
were recorded on a VG ZAB-HS high-resolution spectrometer.
2.2 Synthesis of 2-(2-pyridylmethylamino)ethylbenzimidazole (pymaeb)
(1) Synthesis of 3-(t-butyloxycarbonyl)aminopropionic acid (1)
To ꢀ-alanine (4.45g, 0.050mol) in a 250 ml flask was added slowly 1N aqueous solution of
NaOH (50 ml) at 0~5℃. Then a solution of di-tert-butyl dicarbonate (11.78g, 0.055mol) in 20ml
of dioxane was added. The resulting mixture was slowly warmed to room temperature and
2

maintained overnight under stirring. After reaction the mixture was evaporated under vacuum and
the residue was treated with 6N HCl to adjust the pH to 2~3. The mixture was extracted with ethyl
acetate, washed with water, dried over Na2SO4 and concentrated under vacuum successively. The
residue was recrystallized from ethyl acetate/n-hexane to give a white solid (8.35g, yield 88%).
M.p. 76.1~77.6℃. 1H NMR (400 MHz, CDCl3, TMS), ꢁ(ppm): 1.45 (s, 9H, -(CH3)3), 2.59 (s, 2H,
CH2), 3.40 (s, 2H,CH2),5.06 (s,1H,NH); IR(KBr, cm-1) ꢂ: 3440, 2981, 2701, 2632, 2574, 1705,
1515.
(2) Synthesis of N-(2-amino)phenyl-3-(t-butyloxycarbonyl)amino propionamide (2)
To a solution of 1 (7.56g, 0.04mol) in anhydrous dichloromethane (80ml) were added 1-(3-
dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC·HCl) (11.50g, 0.06mol) and 1,2-
diaminobenzene (6.48g, 0.06mol) at 0~5℃. The reaction mixture was stirred at room temperature
for 7.5 h. Then the solution was quenched with saturated sodium carbonate solution, and extracted
with ethyl acetate. The combined extracts were dried over Na2SO4 and evaporated under vacuum.
The residue was recrystallized from ethyl acetate to afford a white solid (5.76g, yield 52%). M.p.
1
139.4~140.1℃. H NMR(400 MHz, CDCl3, TMS), ꢁ(ppm): 1.43 (s, 9H, - (CH3)3), 2.61 (t,
J=5.6Hz, 2H, CH2), 3.46~3.50 (m, 2H, CH2), 4.13 (s, 2H, NH2), 5.25 (s, 1H, Ph-NH),
6.75~6.83(m, 2H, PhH), 7.04~7.08 (m, 1H, PhH), 7.21~7.26 (m, 1H, PhH), 7.74 (s, 1H, BocNH);
IR (KBr, cm-1) ꢂ: 3458, 3372, 3328, 2978, 1689, 1648, 1533, 875, 745.
(3) Synthesis of 2-(2-(t-butyloxycarbonyl)amino)ethyl-1H-benzimidazole (3)
Intermediate 2 (8.38g, 0.03mol) was dissolved in glacial AcOH (35ml). The resulting solution
was heated at 65℃ for 2.5 h. After reaction the mixture was evaporated under vacuum, and the
residue was treated with saturated sodium carbonate solution. The resulting mixture was extracted
with ethyl acetate and the combined extracts were dried over Na2SO4. The filtrate was evaporated
to give a white solid (7.56g, yield 96%). M.p. 176.3~177.3℃. 1H NMR (400 MHz, CDCl3, TMS),
ꢁ(ppm): 1.41 (s, 9H, -(CH3)3), 3.18 (t, J=6.0Hz, 2H, CH2), 3.68~3.72(m, 2H, CH2), 5.37 (s, 1H,
Ph-NH), 7.21~7.27 (m, 2H, PhH), 7.55~7.57 (m, 2H, PhH); IR(KBr, cm-1) ꢂ:3427, 2972, 1719,
1502, 1417, 869, 752.
(4) Preparation of 2-aminoethylbenzimidazole·2TFA(4)
Intermediate 3 (7.84g,0.03mol) was dissolved in TFA (40 ml), and the resulting solution was
stirred at room temperature for 0.5 h. Ether-petroleum mixture (1:1, v/v) was added to the solution
until a white solid product precipitated. The mixture was filtered to give the product (11.56g, yield
1
99%). M.p. 157.6~158.5. H NMR (400 MHz, D2O, TMS), ꢁ(ppm): 3.52~3.57 (m, 4H, (CH2)2),
7.50~7.53 (m, 2H, PhH), 7.68~7.71 (m, 2H, PhH); IR (KBr, cm-1) ꢂ: 1680, 1468, 1428, 841, 755.
(5) Synthesis of ligand pymaeb
To a solution of 4 (11.29g, 0.03mol) in anhydrous methanol (50ml) was added triethylamine
(16ml, 0.11mol) under nitrogen atmosphere. 2-Pyridinecarbaldehyde (2.80ml, 0.03mol) was added
dropwise. The resulting solution was stirred at 10 for 15min, then NaBH4 (2.18g, 0.06mol) was
3

added in portions carefully. After 3 h the reaction was quenched with water, and the mixture was
evaporated under vacuum to give a residue. The resulting residue was dissolved in ethyl acetate,
washed with water, dried over Na2SO4. The filtrate was evaporated under vacuum to remove the
solvent. The residue was recrystallized from ethyl acetate to give a white solid product pymaeb
1
(4.72g, yield 65%). M.p. 142.1~143 . H NMR (400 MHz, CDCl3, TMS), ꢁ(ppm): 3.15 (s, 4H,
CH2CH2), 4.08 (s, 2H, CH2), 7.19~7.28 (m, 4H, PhH), 7.56~7.58 (m, 2H, PyH), 7.67 (t, J= 4.0Hz,
1H, PyH), 8.68 (d, J=8Hz, 1H, PyH); 13C NMR (100.6 MHz, CDCl3) ꢁ(ppm): 28.8, 46.4 (NH-
CH2-CH2), 54.0 (NH-CH2-Py), 114.7 (benzene ring), 121.8 (pyridine ring), 122.4 (benzene ring),
122.6 (pyridine ring), 136.8 (pyridine ring), 138.4 (benzene ring), 149.2 (pyridine ring), 154.7
(N=C-NH), 158.8 (pyridine ring); IR (KBr, cm-1) ꢂ: 3272, 3052, 2839, 1590, 1542, 1432, 1272,
750. HRMS(ESI+): m/z calcd. for C15H16N4 + H+: 253.1453; found: 253.1450.
2.3 Synthesis of complex
To a solution of pymaeb (0.76g, 3.0mmol) and RuCl3 (0.62g, 3.0mmol) in methanol (23ml),
pydic (0.63g, 3.0mmol) in MeOH/H2O (1:1, 30 ml) was added in air. The resulting reaction
mixture was heated at 65 for 3h. After reaction the reaction mixture was cooled to room
temperature. The dark violet precipitate was collected by filtration to give one part of product
(0.56g). The filtrate was evaporated, washed with hot MeOH/H2O (1:1, 20 ml), cooled to room
temperature. Another part of the dark violet precipitate was formed, and collected by filtration
(0.15g). Finally the product (0.71g, yield 46%) was received. 1H NMR (400 MHz, CD3OD, TMS),
ꢁ(ppm): 3.58 (t, J=8.0Hz, 2H, CH2), 5.10 (t, J=8Hz, 2H, CH2), 6.34 (d, J=8.4Hz, 1H, PhH),
6.89~6.93 (m, 1H, PhH), 7.16~7.19 (m, 2H, PhH), 7.28~7.31 (m, 1H, PyH), 7.47 (d, J=8.0Hz, 1H,
PyH), 7.71~7.74 (m, 1H, PyH), 8.18 (d, J=8.0Hz, 1H, PyH), 8.32~8.41 (m, 3H, PyH), 9.56 (s, 1H,
N=CH); IR (KBr, cm-1) ꢂ: 3419, 2910, 1615, 1561, 1522, 1514, 1462, 752, 690; HRMS(ESI +):
m/z calcd. for C22H17O4N5Ru + H+: 518.0402; found: 518.0405. UV/Vis (MeOH) ꢃmax=378 nm,
526nm.
2.4 Catalytic oxidation of alcohols
In a typical process, into a 5 ml two-necked, round-bottom flask equipped with a magnetic
stirrer and a thermometer were added the ruthenium complex [Ru(pymieb)(pydic)] (0.002mmol)
and the substrate alcohol (2mmol) successively. The mixture was heated to 40 ℃ under stirring.
Then t-butyl hydroperoxide (TBHP) was added dropwise to the mixture and the temperature was
kept at 40 ℃until completion of the reaction. The reaction samples were analyzed on a Shandong
Lunan Ruihong Gas Chromatograph (SP-6800A) equipped with a FID detector and a SE 30
column (30 m×0.5 mm). The conditions used in gas chromatography were temperature of the
detector 280 , column temperature 130–220 (varying with alcohols), pressure of the carrier gas
0.05–0.07 MPa (varying with alcohols).
4

2.5 X-ray structure determination
Crystal suitable for X-ray crystallography was obtained by slow diffusion of THF to a solution
of the complex in DMF at room temperature. The single crystal X-ray diffraction data collection
for the complex was performed with a Rigaku Saturn70 CCD diffractometer, equipped with
graphite monochromatized Mo radiation with a radiation wavelength of 0.071073 nm. All the
calculation was accomplished by using program of SHELXS-97 and SHELXL-97.
3. Results and discussion
3.1 Synthesis of complex
The synthesis route of the ruthenium complex is shown in Scheme 1. First, the ligand pymaeb
was synthesized in five steps. In the first step the Boc protected ꢀ-alanine, 3-(t-
butyloxycarbonyl)aminopropionic acid (1), was smoothly synthesized from ꢀ-alanine and di-tert-
butyl dicarbonate. In the next step the carboxy group of 1 condensed with one amino group of 1,2-
diaminobenzene in the presence 1-(3-dimethyl aminopropyl)-3-ethylcarbodiimide hydrochloride
(EDC·HCl) as condensing agent to afford the amide (2) in moderate yield. This amide underwent
a ring closure upon intra-molecular condensation of the carbonyl group with the ortho amino
group to form the benzimidazole ring readily to give intermediate 3. The protecting group BOC
was easily removed by resolving 3 in TFA, and the released amino group reacted with TFA in situ
to form a salt. The obtained salt proceeded reductive amination with 2-pyridinealdehyde using
NaBH4 as reducing agent and triethylamine as acid scavenger to give the ligand pymaeb. The
reaction of RuCl3 with pymaeb and pydic in 1:1:1 molar ratio in refluxing MeOH/H2O (1:1)
afforded the complex as shown in Scheme 1. The formation of the complex involves the
dehydrogenation of pmaeb to 2-(2-pyridymethylimine)ethylbenzimidazole (pymieb), while the
Ru(III) is reduced to Ru(II).
Scheme 1. Synthesis route for the ruthenium complex
3.2 Spectral characterization
The structure of pymaeb and the complex were characterized by 1H NMR, FT-IR and HR-MS
1
thoroughly. In the H NMR spectrum of pmaeb (Fig. S1) resonance at 3.98 ppm (single peak) is
5

assigned to the 2 protons of the methylene attached to the pyridyl ring; the resonances at
3.57~3.60 and 5.09~5.11 ppm are assigned to the 4 protons of the ethylene group. The 13C NMR
1
of the pmaeb as shown in Fig. S2 is in agreement with that of the expected structure. In the H
NMR spectrum of the complex (Fig. S3) the resonance corresponding to the 2 protons of the
methylene attached to the pyridyl ring in pymaeb is disappeared; instead, a resonance at 9.56 ppm
(s, 1H) which can be assigned to that of the 1 proton of imino group formed from the
dehydrogenation of the aminomethyl moiety attached to the pyridyl ring in pymaeb is observed.
In the FT-IR spectrum of pymaeb two bands respectively exhibited at 1590 and 1542 cm-1 are
assigned to the C=N stretching vibrations of pyridine and benzimidazole moieties [28]. These
bands shift to 1561 and 1514 cm-1 in the ruthenium complex, indicating the coordination of
pyridine and benzimidazole nitrogens to the ruthenium (Fig. S29). In addition the band around
3272 cm-1 due to the N-H vibration of the secondary amino group in pymaeb disappears in the
ruthenium complex, indicating the secondary amine is converted to other group in the complex
[29].
The UV–Vis absorption spectrum of the complex was investigated in the 200–800 nm range in
MeOH as shown in Fig. S30. The spectrum of the complex shows bands in the region 230-530nm.
The ruthenium complex is diamagnetic, indicating the presence of ruthenium in the +2 oxidation
6
state. The ground state of ruthenium(II) in an octahedral environment is 1A1g, arising from the t2g
1
3
3
1
configuration, and the excited states corresponding to the t2g5eg configuration are T1g, T2g, T1g
1
1
3
1
3
1
and T2g. Hence, four bands corresponding to the transitions A1g ꢄ T1g, A1g ꢄ T2g, A1g
ꢄ
1
1T1g and 1A1g ꢄ T2g are possible in order of the increasing energy [30]. The bands appearing in
the region 230-280 nm are assigned to intraligand transitions [24, 31]. These bands are designated
as ꢅ-ꢅ* and n-ꢅ* transitions for the electrons localized on the aromatic system. The band due to
charge transfer transitions arising from the metal t2g level to the unfilled ꢅ* molecular orbital of
the ligand appears around 378nm. The electronic spectral band at 526nm is assigned to d–d
transition. The pattern of the electronic spectrum indicates the presence of an octahedral geometry
around ruthenium(II) ion [32,33].
3.3 X-ray crystallography of ruthenium complex
The crystal structure of the complex was determined by means of X-ray structure
determination. The molecular structure of the complex is illustrated in Fig. 1. Details of the X-
ray experiment, crystal parameters, data collections and refinements are summarized in Table 1,
and the selected bond lengths and angles are listed in Table 2.
Crystal structure analysis revealed that the complex belongs to a monoclinic system with space
group P2(1)/c. As shown in Fig.1, the crystal structure of the complex contains one water
molecule. The central ruthenium atom is coordinated by pymieb from the dehydrogenation of
pymaeb in the formation of the complex, as well as pybic. The pymieb coordinates to ruthenium
ion via the pyridine nitrogen (N4), benzimidazole nitrogen (N1) and imine nitrogen (N3). The
6

Fig.1. Molecular structure of ruthenium complex
pydic binds the metal center at the pyridine nitrogen (N5) and carboxylate oxygen (O1, O3). The
pymieb coordinates to Ru on one plane as a N,N,N-tridentate ligand and the pydic acts as a
dianionic O,N,O-tridentate ligand on another plane, in which they are nearly perpendicular to each
other. The geometry around Ru1 can be described as a distorted octahedral geometry, in which the
four nitrogen atoms occupy the four equatorial positions, while the two oxygen atoms are in the
axial with a trans-position. The selected bond lengths and angles for the complex are shown in
Table 2. The distances of Ru(1)-N(1), Ru(1)-N(3) and Ru(1)-N(4) are 2.101(4), 2.018(4)
and2.056(4) Å, respectively, which indicates that imino group coordinates to Ru(II) more strongly
than benzimidazolyl and pyridyl groups in pymieb. The distance of Ru(1)-N(5) is 1.967(4) Å
shorter than the ones of Ru(1)-N(1), Ru(1)-N(3) and Ru(1)-N(4). This difference arises from
higher steric hindrance of pymieb than pybic.
Table 1 Crystallographic data for the complex
Empirical
Size / mm
0.22 × 0.18× 0.12
C22H17 N5O4Ru・H2O
formula
Formula weight
534.49
F(000)
1080
Temperature /K
Wavelength/ Å
Crystal system
113(2)
Absorption coefficient/ mm-1
ꢆ range for data collection / (°)
Limiting indices
0.674
0.71073
Monoclinic
2.17 to 27.95
-12 ≤h≤11, -32≤k≤32,
-14 ≤l≤14
space group
a / Å
P2(1)/c
Reflections collected
unique
21979
9.7261(19)
5830 [Rint = 0.0435]
7

b / Å
c / Å
25.145(5)
10.903(2)
Completeness to theta = 25.02
Absorption correction
98.2 %
Semi-empirical from
equivalents
ꢇ/ (°)
90
Max. and min. transmission
Data / restraints / parameters
Goodness-of-fit on F2
0.9235 and 0.8658
ꢀ/ (°)
112.07(3)
90
5830 / 0 / 306
ꢈ/ (°)
1.029
V / Å3
Z
2471.2(9)
4
Final R indices [I>2σ(I)]
R indices (all data)
R1 = 0.0740, wR2 = 0.1876
R1 = 0.0802, wR2 = 0.1932
5.959 and -1.518
Dc / (mg·m-3)
1.437
Largest diff. peak and hole/(e·Å-3)
Table 2 Selected bond lengths (Å) and angles (°) for the complex
Ru(1)-N(1)
Ru(1)-N(3)
2.101(4)
Ru(1)-N(4)
2.056(4)
Ru(1)-O(1)
2.109(3)
2.121(4)
Bond
2.018(4)
Ru(1)-N(5)
1.967(4)
Ru(1)-O(3)
N(1)-Ru(1)-N(3)
N(1)-Ru(1)-N(4)
N(1)-Ru(1)-N(5)
N(1)-Ru(1)-O(1)
N(1)-Ru(1)-O(3)
91.08(17)
N(3)-Ru(1)-N(4)
78.74(17)
173.44(17)
99.94(14)
103.14(15)
94.94(17)
N(4)-Ru(1)-O(1)
N(4)-Ru(1)-O(3)
N(5)-Ru(1)-O(1)
N(5)-Ru(1)-O(3)
O(1)-Ru(1)-O(3)
91.63(16)
91.91(18)
78.35(14)
78.61(14)
156.90(14)
168.98(16) N(3)-Ru(1)-N(5)
95.35(16)
94.31(14)
86.26(16)
N(3)-Ru(1)-O(1)
N(3)-Ru(1)-O(3)
N(4)-Ru(1)-N(5)
Angle
3.4 Catalytic oxidation of alcohols
Table 3 The oxidation of 1-phenylethanol catalyzed by the ruthenium complex with different oxidants
Entry
Oxidant a
Solvent
CH3CN
CH3CN
CH3CN
Time（h）
Conversion (%)b
Selectivity (%)b
1
2
3
O2
1.0
1.0
1.0
1.5
>99
>99
>99
30% H2O2
70% TBHP
11.2
48.6
8

4
5
6
70% TBHP
70% TBHP
70% TBHP
CH3CN
CH3OH
__
10.0
5.0
76.9
56.1
99.9
>99
>99
>99
2.5
Reaction condition: 1-phenylethanol (2 mmol), catalyst (0.1 mol%), oxidant (10 mmol), 40℃, solvent (2 ml).
a O2 1 atm; b Determined by GC.
The complex was employed as a catalyst for the oxidation of various alcohols to the
corresponding carbonyl compounds with TBHP as oxidant. Initially, different oxidants including
O2, H2O2 and TBHP were examined in the oxidation of 1-phenylethanol as a model substrate with
acetonitrile as solvent. As shown in Table 3 almost no reaction took place in the case of
atmospheric oxygen as oxidant (Table 3, entry 1). Unexpectedly, very low conversion was
received when 30% aqueous H2O2 was used as oxidant, which is very different from the same
reaction catalyzed by a similar complex Ru(bbp)(pybic) (Table 3, entry 2)[27]. TBHP showed
good performance in the reaction. The conversion of 1-phenylethanol reached up to 48.6% in 1 h,
and increased with reaction time (Table 3, entries 3 and 4).
The reaction was also run in methanol and solvent-free conditions, respectively. It can be seen
that the reaction proceeded slowly in methanol, and only moderate conversions of 1-phenylethanol
were obtained in more than 5 h (Table 3, entries 4 and 5). However, 1-phenylethanol was
quantitatively converted to acetophenone in 2.5 under solvent-free conditions. It is advantageous
to perform the reaction under solvent-free conditions due to no requirement of solvent separation.
Table 4 Necessity of the ruthenium complex in the oxidation of 1-phenylethanol with TBHP as oxidant
Entry
Catalyst
Ru(pymieb)(pydic)
Ru(pymieb)(pydic)
RuCl3
Amount (mol%)
Time (h)
Conversion (%)a
Selectivity (%)a
1
2
0
0.10
3
2.5
3
28.6
99.8
43.0
64.4
71.6
>99
>99
>99
>99
>99
3
0.10
4
RuCl3/pymaeb
0.10/0.10
0.10
8
5b
[Et3NH]2[Ru(dipic)Cl3]
8
Reaction conditions: 1-phenylethanol 2 mmol, TBHP 6 mmol, catalyst 0.002mmol (0.1 mol%), reaction
temperature 40℃. a Conversions and selectivity were determined by GC (area normalization method); b Complex
[Et3NH]2[Ru(dipic)Cl3] is obtained from direct coordination of RuCl3 with 2,6-pyridinedicarboxylic acid in the
presence of triethylamine [34] .
Control experiments were performed with 1-phenylethanol as substrate to evaluate the necessity
of the complex in the oxidation of alcohols with TBHP as oxidant. As shown in Table 4, the use of
complex Ru(pymieb)(pydic) is essential. Only 28.6 % of 1-phenylethanol was converted to
9

acetophenone in 3 h when no Ru(pymieb)(pydic) was added (Table 4, entry 1). However, the
conversion of 1-phenylethanol reached up to 99.8% in 2.5 h if only 0.10 % mole of
Ru(pymieb)(pydic) was introduced (Table 4, entry 2). When RuCl3 instead of Ru(pymieb)(pydic)
was used as catalyst, the conversion of 1-phenylethanol was 43.0 % in 3 h (Table 4, entry 3). If
both RuCl3 and pymaeb were introduced, the reaction was improved and the conversion of 1-
phenylethanol reached 64.4 % in 8 h (Table 4, entry 4). Complex [Et3NH]2[Ru(dipic)Cl3] pre-
prepared from direct coordination of RuCl3 with 2,6-pyridinedicarboxylic acid in the presence of
triethylamine also had poor performance compared to Ru(pymieb)(pydic). In the case of this
complex as catalyst the conversion of 1-phenylethanol was 71.6 % in 8 h (Table 4, entry 5).
The effect of reaction parameters was examined by running the reaction in solvent-free
conditions, and the results are listed in Table 5. As can be seen from Table 5, the reaction rate
increased with increasing the molar ratio of TBHP to 1-phenylethanol, and the reaction completed
in the same time if the molar ratio was higher than 3:1 (Table 5, entries 1-4). Increasing the
complex loading from 0.050 mol% to 0.20 mol% led to fast reaction, the reaction time required to
finish the reaction decreased from 5.0 h to 1.5 h (Table 5, entries 6 and 7). Meanwhile, increasing
the reaction temperature can accelerate the reaction (Table 5, entries 2, 8 and 9). From the results
we found the optimal amounts of TBHP and complex to be 3 equivalents and 0.1 mol%,
respectively, and the suitable reaction temperature to be 40℃.
Table 5 Optimization of reaction conditions under solvent free conditions a
Substrate：TBHPb
1：5
Complex（mol%）
Entry
Time (h)
2.5
Conv.c, d (%)
99.9
T (℃)
40
1
2
3
4
5
6
7
8
9
0.10
0.10
0.10
0.10
0.00
0.050
0.20
0.10
0.10
1：3
40
2.5
100
1：2
40
5.0
99.1
1：1.1
1：3
40
7.0
78.8
40
3.0
28.6
1：3
40
5.0
99.9
1：3
40
1.5
99.4
1：3
20
9.0
99.7
1：3
60
1.5
99.9
a 1-Phenylethanol (2 mmol); b Molar ratio; c Determined by GC; d Selectivity >99%
In order to evaluate the versatility of this novel catalytic system, the oxidation of various
alcohols to the corresponding carbonyl compounds was explored under the optimized reaction
conditions, and the results are shown in Table 6. As shown in Table 6, most of secondary benzylic
10

alcohols, including those bearing both electron-withdrawing and electron-donating groups para or
meta to hydroxyalkyl moiety, were selectively converted to their corresponding aromatic ketones
in excellent yields under the optimal reaction conditions (Table 6, entries 1-6, 10-14). These
alcohols are generally difficult to be oxidized by molecular oxygen catalyzed by copper/TEMPO
based catalysts [35-38]. However, the time required to finish the reaction was different especially
in the case of 4-fluoro-ꢇ-phenylethanol as a substrate (Table 6, entries 1-6). These results
demonstrated that electronic effects seem to have some effects on the reaction of substrate with a
substituent para to the -hydroxymethyl. The position of the substituent on the benzene ring has
certain effects on the reactivity of benzylic alcohols, too. The substrate with an o-substituent
showed poor reactivity compared to that with an m- or p-substituent due to the steric hindrance
(Table 6, entries 7, 8). For instance, only 49.5% conversion was obtained in 12.5 h in the case of
1-(2-chlorophenyl)ethanol as a substrate (Table 6, entry 8). Overall, the reaction rate was
determined by the combination of the electronic and steric effects. The complex showed moderate
activity in the oxidation of secondary aliphatic alcohols which are poor substrates in most of the
transition-metal catalyst systems (Table 6, entries 15, 16) [35-38]. 2-Isopropyl-5-
methylcyclohexanol gave very low conversion due to the big steric hindrance in the structure
(Table 6, entry 17).
The protocol was also applied to the oxidation of primary benzylic alcohols. Meanwhile, we
want to see whether it was possible to obtain selectivity towards either the aldehyde or the
carboxylic acid. Unfortunately, the selectivity was not controlled, the main products were the
carboxylic acids (Table 6, entries 18-22). However, the primary heterocyclic alcohols including 2-
thioyl methanol, 2-pyridyl methanol and 5-hydroxymethylfurfural were converted to the
corresponding aldehydes in moderate conversion, and no deep oxidation products were observed
(Table 6, entries 23-25). A drawback of this protocol is that it does not work well with aliphatic
alcohols similar to the results catalyzed by other Ru-based catalytic system [39].
3.5 Mechanism aspect
A ruthenium (III) oxo complex or ruthenium(III) species was observed in several similar
ruthenium complex-catalyzed oxidation systems [27,40,41]. In addition, a close-open equilibrium
of an axial Ru–O bond of Ru(pyboxazine)(pydic) in solution was also detected by Beller [41]. For
our complex Ru(pymieb)(pydic), the bond distance of Ru-O is longer than that of Ru-N (Table 2).
When DMSO and THF were diffused into the complex solution in DMF, a carboxyl group
dissociated from the central ruthenium ion by replacement of one DMSO molecule to form a
novel complex Ru(pymieb)(pydic)(DMSO). The detailed crystal structure of the formed complex
can be found in Supplementary information (Fig. S31). Based on these results in combination
with our experimental result that the oxidation reaction rate was related with the steric hindrance
of the substrate, and by reference of a mechanism proposed for a manganese(III) complex-
11

catalyzed oxidation of alcohols with TBHP as oxidant [42], we propose a possible mechanism for
the alcohol oxidation catalyzed by this ruthenium complex as shown in Scheme 2.
Table 6 Oxidation of various alcohols by TBHP in the presence of ruthenium complex
Conversiona
(%)
Selectivitya
(%)
Entry
1
Substrate
Product
Time (h)
2.5
100(95)
100
>99
>99
2
3
2.5
6.0
99.1
99.8(96)
100
>99
>99
>99
4
5
3.0
3.0
6
7
3.0
99.7(95)
88.9
>99
>99
12.0
8
9
12.5
4.0
49.5
99.9
>99
>99
12

Table 6 (continued)Oxidation of various alcohols by TBHP in the presence of ruthenium complex
Conversion a
(%)
Selectivity a
Entry
10
Substrate
Product
Time (h)
(%)
4.0
100(95)
97.2
>99
11
8.0
>99
12
13
14
2.0
1.0
1.5
100
99.7
>99
>99
>99
100(95)
15
16
17
4.5
13.0
5.0
55.1
83.9
27.0
98.9
>99
>99
18
19
3.5
7.5
99.6
99.5
88.5
95.6
13

Table 6(continued)Oxidation of various alcohols by TBHP in the presence of ruthenium complex
Conversiona
(%)
Selectivitya
Entry
20
Substrate
Product
Time (h)
(%)
11.5
99.7
99.9
99.8
92.7
21
22
6.0
95.6
88.3
10.0
23
24
25
2.0
3.5
7.0
44.5
78.4
60.3
>99
>99
87.9
Reaction conditions: alcohol 2 mmol, TBHP 6 mmol, catalyst 0.002mmol (0.1 mol%), reaction temperature 40℃.
a
Conversions and selectivity were determined by GC (area normalization method); data in parentheses were
isolated yields; all products were determined by 1H NMR.
In the oxidation process, the Ru(II) complex (I) readily interacts with TBHP to form a Ru(III)–
OOtBu species II through opening an axial Ru–O bond. Then species II forms the ruthenium (III)
oxo complex (III) through the heterolytic cleavage of O–O bond, and releases t-butyl alcohol. III
as a strong oxidant converts alcohol to the corresponding ketone or aldehyde through a transition
state (III), and simultaneously gives species V. The intermediate V, on the loss of the water
molecules through the re-coordination of the carboxy of pybic, eventually gives the regenerated
Ru(II) complex I.
14

Scheme 2 Possible catalytic cycle for the complex-catalyzed oxidation of alcohols with TBHP as oxidant
4. Conclusion
A N,N,N-tridentate ligand known as 2-(2-pyridymethylamino)ethylbenzimidazole (pymaeb)
was synthesized. This ligand in combination with disodium pyridine-2,6-dicarboxylate (pybic)
reacted with RuCl3 to afford a novel complex Ru(pymaeb)( pybic). During the reaction the liangd
pymaeb was dehydrogenated to 2-(2-pyridymethylimino)ethylbenzimidazole (pymaeb) and Ru(III)
was reduced to Ru(II). The Ru(II) complex was thoroughly characterized by NMR, IR, HR-MS
and single crystal X-ray diffraction. Crystal structure analysis revealed that the complex has a
distorted octahedral geometry. The ruthenium was employed as catalyst for the oxidation of
various alcohols with TBHP as oxidant. Under the catalysis of this complex secondary benzylic
alcohols and primary benzylic alcohols were quantitatively oxidized to their corresponding
ketones and acids, respectively, under mild and solvent-free conditions. This complex also showed
moderate activity in the oxidation of secondary aliphatic alcohols and primary heterocyclic
alcohols to the corresponding ketones and aldehydes under the same conditions.
Appendix A. Supplementary data
CCDC <1405850> contains the supplementary crystallographic data for Ru(pymaeb)( pybic).
CCDC <1417328> contains the supplementary crystallographic data for the complex
Ru(pymieb)(pydic)(DMSO). These data can be obtained free of charge via
15

http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgements
The authors are grateful for financial support from the National Natural Science Foundation of
China (no. 21276061); Natural Science Foundation of Hebei Province, China ( no. B2013202158 )
and Research Fund for the Doctoral Program of Higher Education of China (no. 20121317110010)
References
[1] R.A. Sheldon, J.K. Kochi, Metal-Catalyzed Oxidations of Organic Compounds, 1st ed.,
Academic Press, New York, 1981.
[2] V.R. Choudhary, A. Dhar, P. Jana, R. Jha, B.S. Uphade, Green Chem. 7 (2005) 768-770.
[3] M. Vazylyev, D. Sloboda-Rozner, A. Haimov, G. Maayan, R. Neumann, Top. Catal. 34
(2005) 93-99.
[4] M. Pagliaro, S. Campestrini, R. Ciriminna, Chem. Soc. Rev. 34 (2005) 837-845.
[5] A.K. Vannucci, J.F. Hull, Z. Chen, R.A. Binstead, J.J. Concepcion, T.J. Meyer, J. Am. Chem.
Soc. 134 (2012) 3972-3975.
[6] M.J. Schultz, R.S. Adler, W. Zierkiewicz, T. Privalov, M.S. Sigman, J. Am. Chem. Soc. 127
( 2005) 8499-8507.
[7] J. March, Advanced organic chemistry: Reactions, mechanisms, and structure, 4th ed., John
Wiley & Sons, New York, 1992.
[8] W.J. Mijs, D. Jong, C. Eds, Organic syntheses by oxidation with metal compounds, Plenum
Press, Inc. New York, 1986, pp. 423-443.
[9] D.B. Dess, J.C. Martin, J. Org. Chem. 48 (1983) 4155-4156.
[10] M. Frigerio, M. Santagostino, S. Sputore, G. Palmisano, J. Org. Chem. 60 (1995) 7272-7276.
[11] J.D. More, N.S. Finney, Org. Lett. 4 (2002) 3001-3003.
[12] (a) V.B. Sharma, S.L. Jain, B. Sain, J. Mol. Catal. A: Chem. 212 (2004) 55-59; (b) V.B.
Sharma, S.L. Jain, B. Sain, Tetrahedron Lett. 44 (2003) 383-386; (c) S.L. Jain, B. Sain, J.
Mol. Catal. A: Chem. 176 (2001) 101-104.
[13] (a) N. Jiang, A.J. Ragauskas, Tetrahedron Lett. 48 (2007) 273-276; (b) A.T. Radosevich, C.
Musich, F.D. Toste, J. Am. Chem. Soc. 127 (2005) 1090-1091; (c) P.R. Schreiner, A.
Wittkopp, Org. Lett. 4 (2004) 217-220.
[14] C.N. Kato, A. Shinohara, N. Moriya, K. Nomiya, Catal. Commun. 7 (2006) 413-416.
[15] D.R. Jensen, M.J. Schultz, J.A. Mueller, M.S. Sigman, Angew. Chem. Int. Ed. 115 (2003)
3940-3943.
16

[16] (a) N. Jiang, A.J. Ragauskas, J. Org. Chem. 71 (2006) 7087-7090; (b) P. Gamez, I.W. C.E.
Arends, R.A. Sheldon, J. Reedijk, Adv. Synth. Catal. 346 (2004) 805-811; (c) G. Ragagnin,
B. Betzemeier, S. Quici, P. Knochel, Tetrahedron 58 (2002) 3985-3991.
[17] A. Hanyu, E. Takezawa, S. Sakaguchi, Y. Ishii, Tetrahedron Lett. 39 (1998) 5557-5560.
[18] E. Takezawa, S. Sakaguchi, Y. Ishii, Org. Lett. 1 (1999) 713-715.
[19] A. Miyata, M. Murakami, R. Irie, T. Katsuki, Tetrahedron Lett. 42 (2001) 7067–7070.
[20] A. Miyata, M. Furukawa, R. Irie, T. Katsuki, Tetrahedron Lett. 43 (2002) 3481–3484.
[21] H. Egami, S. Onitsuka, T. Katsuki, Tetrahedron Lett. 46 (2005) 6049-6052.
[22] H.B. Ji, Q.L. Yuan, X.T. Zhou, L.X. Pei, L.F. Wang, Bioorg. Med. Chem. Lett. 17 (2007)
6364-6368.
[23] N. Gunasekaran, R. Karvembu, Inorg. Chem. Commun. 13 (2010) 952-955.
[24] M.M. Tamizh, K. Mereiter, K. Kirchner, R. Karvembu, J. Organomet. Chem. 700 (2012)
194-201.
[25] M.M. Subarkhan, R. Ramesh, Spectrochim. Acta Part A 138 (2015) 264–270.
[26] S.K. Sarkar, M.S. Jana, T.K. Mondal, C. Sinha, Appl. Organometal. Chem. 28 (2014) 641–
651.
[27] X.T. Zhou, H.B. Ji, S.G. Liu, Tetrahedron Letters 54 (2013) 3882–3885.
[28] M. Karnan, V. Balachandran, M. Murugan, M.K. Murali, Spectrochim. Acta Part A 130
(2014) 143–151.
[29] H.J. Guadalupe, J. Narayanan, T. Pandiyan, J. Mol. Struct. 989 (2011) 70–79.
[30] N. Sathya, G. Raja, N.P. Priya, C. Jayabalakrishnan, Appl. Organometal. Chem. 24 (2010)
366–373.
[31] A.A. Soliman, Spectrochim. Acta Part A 53 (1997) 509-515.
[32] J.L. Pratihar, S. Bhaduri, P. Pattanayak, D. Patra, S. Chattopadhyay, J. Organomet. Chem.
694 (2009) 3401-3408.
[33] R. Karvembu, S. Hemalatha, R. Prabhakaran, K. Natarajan, Inorg. Chem. Commun. 6 (2003)
486-490.
[34] Y. F. Xie, H. Zhu, H. T. Shi, A. Q. Jia, Q. F. Zhang, Inorg. Chem. Commun. 428 (2015) 147-
153.
[35] P. Gamez, I. W. C. E. Arends, J. Reedijk, R. A. Sheldon, Chem. Commun. (2003) 2414–2415.
[36] P. J. Figiel, M. Leskelä , T. Repo, Adv. Synth. Catal. 349 (2007) 1173–1179.
[37] Z. Lu, T. Ladrak, O. Roubeau, J. Toorn, S. J. Teat, C. Massera, P. Gamez, J. Reedijk, Dalton
Trans. (2009) 3559–3570.
[38] O. Das, T.K. Paine, Dalton Trans. 41 (2012) 11476–11481.
[39] O. Verho, M. D. V. Dilenstam, M. D. Kärkäs, E. V. Johnston, T. Äkermark, Jan-E. Bäckvall,
B. Åkermark, Chem. Eur. J. 18 (2012) 16947 – 16954.
[40] M.K. Tse, C. Döbler, S. Bhor, M. Klawonn, W. Mägerlein, H. Hugl, M. Beller, Angew.
Chem. Int. Ed. 43 (2004) 5255 –5260.
17

[41] M.K. Tse, S. Bhor, M. Klawonn, G. Anilkumar, H. Jiao, A. Spannenberg, C. Döbler, W.
Mägerlein, H. Hugl, M. Beller, Chem. Eur. J. 12 (2006) 1875 – 1888.
[42] Z. Ye, Z. Fu, S. Zhong, F. Xie, X. Zhou, F. Liu, D. Yin, J. Catal. 261 (2009) 110–115.
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Graphical abstract synopsis
A novel Ru(II) complex Ru(pymieb)(pydic) was synthesized and characterized by NMR,
FT-IR, HR-MS and single crystal X-ray diffraction. The complex showed excellent activity
for the oxidation of various alcohols with TBHP as oxidant under mild and solvent-free
conditions.
19

Graphical abstract pictogram
Ru(pymieb)(pydic)
20