Triruthenium carbonyl complexes containing bidentate pyridine–alkoxide ligands for highly efficient oxidation of primary and secondary alcohols

Article abstract of DOI:10.1002/aoc.5292

Reactions of substituted pyridylalkanol 6-CH₃PyCH₂CH(OH)R (R?=?Ph (L¹H), R?=?4-CH₃C₆H₄ (L²H), R?=?4-OCH₃C₆H₄ (L³H), R?=?4-ClC₆H₄ (L⁴H), R?=?4-BrC₆H₄ (L⁵H), R?=?4-CF₃C₆H₄ (L⁶H)) with Ru₃(CO)₁₂ in refluxing tetrahydrofuran afforded the corresponding ruthenium carbonyl complexes [6-CH₃PyCH₂CHRO]₂Ru₃(CO)₈ (R?=?Ph (1a), R?=?4-CH₃C₆H₄ (1b), R?=?4-OCH₃C₆H₄ (1c), R?=?4-ClC₆H₄ (1d), R?=?4-BrC₆H₄ (1e), R?=?4-CF₃C₆H₄ (1f)) in good yields. These ruthenium complexes were well characterized using elemental analysis and Fourier transform infrared and NMR spectroscopies. Furthermore, their crystal structures were determined using single-crystal X-ray diffraction analysis. Complexes 1a–1f were found to be highly active toward oxidation of a wide range of primary and secondary alcohols to corresponding aldehydes and ketones within 5?minutes in the presence of N-methylmorpholine-N-oxide as oxidant.

Full text of DOI:10.1002/aoc.5292

Received: 19 July 2019
DOI: 10.1002/aoc.5292
Revised: 14 September 2019
Accepted: 19 September 2019
F U L L P A P E R
Triruthenium carbonyl complexes containing bidentate
pyridine–alkoxide ligands for highly efficient oxidation of
primary and secondary alcohols
Xiaohui Yue† | Xinlong Yan† | Shuaicong Huo | Qing Dong | Junhua Zhang |
Zhiqiang Hao
| Zhangang Han | Jin Lin
National Demonstration Center for
Experimental Chemistry Education, Hebei
Key Laboratory of Organic Functional
Molecules, College of Chemistry and
Material Science, Hebei Normal
Reactions of substituted pyridylalkanol 6‐CH3PyCH2CH(OH)R (R = Ph (L1H),
R = 4‐CH3C6H4 (L2H), R = 4‐OCH3C6H4 (L3H), R = 4‐ClC6H4 (L4H), R = 4‐
BrC6H4 (L5H), R = 4‐CF3C6H4 (L6H)) with Ru3(CO)12 in refluxing tetrahydro-
furan afforded the corresponding ruthenium carbonyl complexes [6‐
CH3PyCH2CHRO]2Ru3(CO)8 (R = Ph (1a), R = 4‐CH3C6H4 (1b), R = 4‐
OCH3C6H4 (1c), R = 4‐ClC6H4 (1d), R = 4‐BrC6H4 (1e), R = 4‐CF3C6H4 (1f))
in good yields. These ruthenium complexes were well characterized using
elemental analysis and Fourier transform infrared and NMR spectroscopies.
Furthermore, their crystal structures were determined using single‐crystal
X‐ray diffraction analysis. Complexes 1a–1f were found to be highly active
toward oxidation of a wide range of primary and secondary alcohols to
corresponding aldehydes and ketones within 5 minutes in the presence of N‐
methylmorpholine‐N‐oxide as oxidant.
University, Shijiazhuang 050024, China
Correspondence
Jin Lin, College of Chemistry and Material
Science, Hebei Normal University,
Shijiazhuang 050024, China.
Email: linjin64@126.com
Zhiqiang Hao
Email: haozhiqiang1001@163.com
Funding information
Education Department Foundation of
Hebei Province, Grant/Award Numbers:
QN2019036 and ZD2018005; Hebei Natu-
ral Science Foundation of China, Grant/
Award Numbers: B2017205006 and
B2019205087; Science Foundation of
Hebei Normal University, Grant/Award
Numbers: L2017Z02 and L2018B08
KEYWORDS
alcohol oxidation, NMO, ruthenium complexes
1 | INTRODUCTION
view of environmental concerns, green chemistry,
energy‐effectiveness and especially atom efficiency, sub-
Transition metal‐catalyzed methodologies have been uti-
lized for a variety of organic synthesis processes to
improve the catalytic activity and selectivity of cata-
lysts.[1] The selective oxidation of alcohols to carbonyl
compounds is one type of reaction that has attracted
much attention. Some sustainable methods have been
developed.[2] Traditionally, this type of reaction is per-
formed with high‐valent transition metal salts, notably
chromium(VI)‐based[3] and manganese (VII)‐based
reagents,[4] or hypervalent iodine compounds[5] as oxi-
dants, which involve serious environmental issues. In
stantial numbers of new catalytic systems are continu-
ingly being investigated and developed. In particular,
the combination of transition metal catalysts and envi-
ronmentally benign oxidants or bases represents one of
the best alternatives in this field. Many heterogeneous[6]
and homogeneous[7] catalysts have been developed using
various metals, such as Co,[8] Ce,[9] Ni,[10] Fe,[11] Mo,[12]
Pd,[13] Rh,[14] Re,[15] Pt,[16] Ru[17] and Ir.[18] For instance,
Malakooti and Feghhi[19] investigated the aerobic oxida-
tion of benzyl alcohols using MoOx–pyridine wires as a
catalyst and hydrogen peroxide or molecular oxygen as
green oxidant. Aromatic alcohols were efficiently oxi-
dized to the corresponding ketones or aldehydes in the
†Xiaohui Yue and Xinlong Yan contributed equally to this work.
Appl Organometal Chem. 2019;e5292.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5292

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YUE ET AL.
catalytic system. Yamaguchi and co‐workers[20] reported
a new catalytic system for the dehydrogenative oxidation
of alcohols utilizing a Cp*Ir complex supported by a func-
tional C,N‐chelate ligand. Mashima and co‐workers[21]
synthesized a series of cerium(III) complexes bearing
Schiff base ligands which were used as catalysts in N‐oxyl
radical‐free aerobic alcohol oxidation.
(NMO) as an oxidant under mild reaction conditions.
Most of the reactions could be completed within
5 minutes.
2 | RESULTS AND DISCUSSION
2.1 | Synthesis and characterization of 6‐
methylpyridine–alkoxide ligands
The selective oxidation of alcohols using ruthenium‐
based catalytic system has been studied widely, because
ruthenium complexes are found to have excellent redox
capacity compared with other transition metal complexes.
Several research groups have reported important progress
in this area. Bäckvall and co‐workers[22] reported aerobic
oxidation of secondary alcohols with a Shvo catalyst.
Most alcohols were oxidized to ketones in high yields
with high turnover frequency. However, a stoichiometric
amount of 2,6‐dimethoxy‐1,4‐benzoquinone and cobalt–
salen complex were necessary to complete this catalytic
cycle. Ruthenium nanoparticles immobilized on thiol‐
based dendrimer‐functionalized nano‐silica were synthe-
sized by Mohammadpoor‐Baltork and co‐workers.[23]
The catalytic activity of the ruthenium nanocomposite
as a heterogeneous catalyst was investigated in the oxida-
tion of alcohols with t‐BuOOH and the corresponding
products were obtained in moderate to high yields.
Recently, Goswami and co‐workers[24] studied the aerobic
oxidation reaction of alcohols using a ruthenium‐
coordinated azoaromatic pincer as catalyst with two
equivalents of t‐BuOK under positive pressure of oxygen.
Mild reaction conditions and highly efficient catalytic sys-
tems without the need for any external base are highly
desired in this direction.
Ligands L1H–L6H were synthesized according to a litera-
ture procedure and identified using NMR spectros-
copy.[25] The synthetic route for all ligands is shown in
scheme 1. ((6‐Methylpyridin‐2‐yl)methyl)lithium (from
2,6‐dimethylpyridine and n‐BuLi) reacted with aldehyde
derivatives, which were then hydrolyzed with ammo-
nium chloride aqueous solution to form the correspond-
ing 6‐methylpyridine ethanol ligands in moderate yields.
Ligands L5H and L6H have not been reported previously.
1
The H NMR spectra of L5H and L6H exhibited one sin-
glet at 6.52 and 6.71 ppm, respectively, for the OH proton,
in which the singlets were more downfield compared to
their counterparts L1H–L4H (6.05–6.51 ppm), owing to
the strong withdrawing groups Br and CF3 at the 4‐
position of phenyl ring. The doublets at 6.89, 7.05 and
7.30 (for L5H), 6.90 and 7.06 ppm (for L6H) and multiplets
centered at 7.46 ppm (for L5H) and 7.56 ppm (for L6H)
clearly represented the resonance of pyridine and phenyl
ring protons. In addition, the infrared (IR) spectra of
ligands L5H and L6H showed strong absorption bands at
3202 and 3184 cm−1, respectively, due to stretching fre-
quency of the OH group.
In the work reported herein, we synthesized a series of
ruthenium‐based carbonyl complexes bearing N,O‐
bidentate pyridine–alkoxide compounds as organic
ligands. It was found that the ruthenium carbonyl com-
plexes exhibit high efficiency for oxidation of alcohols to
aldehydes or ketones with N‐methylmorpholine‐N‐oxide
2.2 | Synthesis and characterization of
ruthenium carbonyl complexes
Thermal reaction of ligands L1H–L6H and Ru3(CO)12 in
refluxing tetrahydrofuran (THF) afforded bis‐chelate
ruthenium carbonyl complexes [6‐CH3PyCH2CHRO]
2Ru3(CO)8 (R = Ph (1a), R = 4‐CH3C6H4 (1b), R = 4‐
OCH3C6H4 (1c), R = 4‐ClC6H4 (1d), R = 4‐BrC6H4 (1e),
R = 4‐CF3C6H4 (1f), respectively) (Scheme 2). Recrystalli-
zation of complexes 1a–1f from dichloromethane–n‐hex-
ane gave air‐ and moisture‐stable orange crystals. All
SCHEME 1 Synthesis of 6‐methylpyridine–alkoxide ligands
SCHEME 2 Synthesis of ruthenium
carbonyl complexes

YUE ET AL.
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the ruthenium complexes were characterized using IR
and NMR spectroscopies, elemental analysis and single‐
crystal X‐ray diffraction.
The IR spectra of all ruthenium complexes 1a–1f
displayed three strong absorption peaks at 1905–
2075 cm−1 due to the presence of several types of terminal
C≡O bonds. In addition, the characteristic ν(OH) bands
at around 3178–3399 cm−1 of free ligands were absent
in the spectra of the Ru complexes, suggesting that the
deprotonated hydroxyl groups were coordinated to the
1
metal center. In the H NMR spectra of the ruthenium
complexes, the ―OH signals at around 6.05–6.71 ppm
disappeared, and the ―CH― group signals were shifted
downfield to 3.14–3.17 ppm compared to those of free
ligands, respectively, indicating the coordination of
pyridine–alkoxide ligands to the ruthenium atom via the
deprotonation of the hydroxyl groups. The signals of
―CH2― group in the complexes were split into two
doublet peaks at 3.77–3.89 and 2.68–2.74 ppm with
respect to the ligands, respectively, which suggested that
the ―CH2― groups were in a different chemical environ-
ment. Meanwhile, the 13C NMR spectrum of the ruthe-
nium complexes showed four signals in the range
190.7–205.5 ppm for the terminal metal carbonyl.
FIGURE 2 Perspective view of 1b (CCDC: 1907697) with
thermal ellipsoids drawn at 30% probability level. Hydrogens are
omitted for clarity. Selected bond lengths (Å) and angles (deg.): Ru
(1)–Ru(3) 2.7949(6), Ru(1)–Ru(2) 3.0521(5), Ru(2)–Ru(3) 2.7931(6),
Ru(1)–N(1) 2.289(4), Ru(1)–O(1) 2.134(3), Ru(2)–O(1) 2.137(3); Ru
(2)–Ru(3)–Ru(1) 66.213(13), Ru(1)–O(1)–Ru(2) 91.25(10), O(1)–Ru
(1)–Ru(2) 44.41(7), O(1)–Ru(2)–Ru(1) 44.34(7), N(1)–Ru(1)–Ru(3)
173.97(9), N(1)–Ru(1)–Ru(2) 117.11(9)
The molecular structures of ruthenium complexes 1a–
1f were further determined using X‐ray crystallographic
studies and are shown in Figures 1–6 together with
selected bond distances and angles. The crystallographic
data for all complexes are presented in Tables S2 and S3
in the supporting information. Complex 1a crystallized
FIGURE 3 Perspective view of 1c (CCDC: 1907698) with thermal
ellipsoids drawn at 30% probability level. Hydrogens are omitted for
clarity. Selected bond lengths (Å) and angles (deg.): Ru(1)–Ru(3)
2.7838(7), Ru(1)–Ru(2) 3.0645(7), Ru(2)–Ru(3) 2.7871(8), Ru(1)–N
(1) 2.312(5), Ru(1)–O(1) 2.133(4), Ru(2)–O(1) 2.132(4); Ru(1)–Ru
(3)–Ru(2) 66.745(18), Ru(2)–O(1)–Ru(1) 91.86(14), O(1)–Ru(1)–Ru
(2) 44.05(10), O(1)–Ru(2)–Ru(1) 44.09(10), N(1)–Ru(1)–Ru(3) 174.54
(12), N(1)–Ru(1)–Ru(2) 118.11(12)
FIGURE 1 Perspective view of 1a (CCDC: 1905995) with thermal
ellipsoids drawn at 30% probability level. Hydrogens are omitted for
clarity. Selected bond lengths (Å) and angles (deg.): Ru(1)–Ru(3)
2.7804(5), Ru(1)–Ru(2) 3.0612(5), Ru(2)–Ru(3) 2.7736(5), Ru(1)–N
(1) 2.299(3), Ru(1)–O(1) 2.126(3), Ru(2)–O(1) 2.127(3); Ru(2)–Ru
(3)–Ru(1) 66.895(12), Ru(1)–O(1)–Ru(2) 92.07(11), O(1)–Ru(1)–Ru
(2) 43.98(7), O(1)–Ru(2)–Ru(1) 43.95(8), N(1)–Ru(1)–Ru(3) 174.81
(9), N(1)–Ru(1)–Ru(2) 118.80(9)
in the monoclinic space group P21/c. The unit cell of 1a
contains three crystallographically independent mole-
cules which possess similar connectivity and only one
molecule is depicted in Figure 1 for clarity. Complexes
1b–1f crystallized in the monoclinic space group P21/n.

4 of 11
YUE ET AL.
FIGURE 6 Perspective view of 1f (CCDC: 1907701) with thermal
ellipsoids drawn at 30% probability level. Hydrogens are omitted for
clarity. Selected bond lengths (Å) and angles (deg.): Ru(1)–Ru(3)
2.7823(9), Ru(1)–Ru(2) 3.0765(9), Ru(2)–Ru(3) 2.7849(9), Ru(1)–N
(1) 2.303(6), Ru(1)–O(1) 2.135(5), Ru(2)–O(1) 2.142(5); Ru(1)–Ru
(3)–Ru(2) 67.09(2), Ru(1)–O(1)–Ru(2) 92.01(18), O(1)–Ru(1)–Ru(2)
44.09(12), O(1)–Ru(2)–Ru(1) 43.91(12), N(1)–Ru(1)–Ru(3) 173.94
(16), N(1)–Ru(1)–Ru(2) 118.03(16)
FIGURE 4 Perspective view of 1d (CCDC: 1907699) with
thermal ellipsoids drawn at 30% probability level. Hydrogens are
omitted for clarity. Selected bond lengths (Å) and angles (deg.):
Ru(1)–Ru(3) 2.780(2), Ru(1)–Ru(2) 3.064(2), Ru(2)–Ru(3) 2.805(3),
Ru(1)–N(1) 2.296(18), Ru(1)–O(1) 2.155(14), Ru(2)–O(1) 2.101(13);
Ru(1)–Ru(3)–Ru(2) 66.53(5), Ru(2)–O(1)–Ru(1) 92.1(5), O(1)–Ru
(1)–Ru(2) 43.3(4), O(1)–Ru(2)–Ru(1) 44.7(4), N(1)–Ru(1)–Ru(3)
173.5(5), N(1)–Ru(1)–Ru(2) 116.4(5)
atom center is surrounded by four CO groups. Ru (1)
and Ru (2) coordinate via two pyridine nitrogen atoms
and are bridged by two μ‐O atoms acting as a three‐
electron donor. Two metal–metal bonds should exist
according to the 18‐electron rule.[26] The Ru(1)–Ru(2) dis-
tances in the range 3.0476(11)–3.0765(19) Å are longer
than those in Ru3(CO)12 (Ru–Ru = 2.8512(4)–2.8595
(4) Å),[27] which indicates that the Ru–Ru bond is non‐
existent between Ru(1) and Ru(2) atoms. The Ru(1)–Ru
(3) and Ru(2)–Ru(3) bond lengths are in the range
2.7736(5)–2.7949(6) Å, which are similar to the Ru–Ru
distance found in the [(Cp*Ru)3‐(μ‐η2:η2‐PhCNMe)(μ‐H)
2] complex (Ru–Ru = 2.7814(5) Å).[28] Three ruthenium
atoms are bound to form an isosceles triangle. The two
Ru–N bond lengths in the range 2.289(4)–2.341(4) Å are
almost equal and are in agreement with those of 2.257
(5) and 2.186(5) Å observed in {μ‐η2‐CH[(2‐C5H4N)(N‐
C6H5)]}2Ru2(CO)4(μ‐CO).[29] The bond angles of Ru(1)–
O(1/2)–Ru(2) formed from ruthenium atoms and
deprotonated oxygen atoms are 91.33(10)–92.7(11)° and
close to those in Ru3(CO)8(C12H10NO2)2 and Ru3(CO)8
(C7H8NO2)2 (89.57°, 88.84° and 88.43°, 88.41°, respec-
tively).[30] Two deprotonated oxygen atoms, Ru(1) and
Ru(2) form a four‐membered ruthenacycle (Ru(1)–O(1)–
Ru(2)–O(2)). The dihedral angles of the four‐membered
ruthenacycle in all complexes vary from 55.47° to
61.09°, suggesting the four atoms (Ru(1), O(1), Ru(2)
and O(2)) are nonplanar and slightly distorted.
FIGURE 5 Perspective view of 1e (CCDC: 1907700) with thermal
ellipsoids drawn at 30% probability level. Hydrogens are omitted for
clarity. Selected bond lengths (Å) and angles (deg.): Ru(1)–Ru(3)
2.7871(12), Ru(1)–Ru(2) 3.0476(11), Ru(2)–Ru(3) 2.7876(11), Ru(1)–
N(2) 2.302(8), Ru(1)–O(1) 2.141(6), Ru(2)–O(1) 2.144(6); Ru(1)–Ru
(3)–Ru(2) 66.28(3), Ru(1)–O(1)–Ru(2) 90.7(2), O(1)–Ru(1)–Ru(2)
44.70(17), O(1)–Ru(2)–Ru(3) 83.84(16), N(2)–Ru(1)–Ru(3) 173.9(2),
N(2)–Ru(1)–Ru(2) 117.1(2)
In the solid state, all the Ru compounds feature a
trinuclear unit in the crystal lattice. In all structures,
two ruthenium atom (Ru (1) and Ru (2)) centers are
surrounded by two 6‐methylpyridine–alkoxide ligands
and two CO groups, respectively. The third ruthenium

YUE ET AL.
5 of 11
CH3CN was found to be a suitable solvent. The effect of
catalyst loading on the reaction was also studied. When
the loading of complex 1a increased from 1.0 to 1.5 mol
%, the yield of product increased slightly (Table 1, entry
6 versus entry 5). Interestingly, the catalyst loading
reduced to 0.5 mol% did not significantly decrease the
yield (Table 1, entry 7 versus entry 5). On further reduc-
ing the catalyst loading to 0.25 mol%, the yield of
acetophenone dropped considerably (Table 1, entry 8).
When the reaction was carried out in absence of 1a, the
target product was obtained in only 23% yield. Therefore,
0.5 mol% of 1a was chosen to be the best catalyst loading.
In addition, a control experiment showed that no oxi-
2.3 | Catalytic oxidation of alcohols
Next, the catalytic behaviors of ruthenium complexes 1a–
1f were investigated toward oxidation of alcohols using
NMO as oxidant. In order to obtain the most optimal con-
ditions, we set out to evaluate various factors affecting the
reaction using 1‐phenylethanol as a model substrate and
ruthenium complex 1a as the Ru precursor. Since the oxi-
dants play a crucial role during the oxidation process, var-
t
ious oxidants, such as O2, TEMPO/O2, NMO, BuOOH
and H2O2, were studied under the same reaction condi-
tions (Table S1). Among the various oxidants, NMO
showed better oxidation capacity for oxidation of 1‐
phenylethanol to give the product in 86% yields (Table
S1, entry 5). Then, optimized conditions using NMO as
an oxidant were further investigated and the results are
summarized in Table 1.
In the presence of NMO (2 mmol) in refluxing toluene,
complex 1a (1 mol%) afforded acetophenone in 51% yield
(Table 1, entry 1). Varying the solvent from toluene
to 1,4‐dioxane, CH2Cl2, THF and acetonitrile, the
acetophenone product was afforded in 60%, 86%, 53%
and 91% yields, respectively (Table 1, entries 2–5). Thus,
TABLE 1 Optimization for oxidation of 1‐phenylethanol cata-
lyzed by complex 1aa
NMO
Solvent
FIGURE 7 Profile of the percentage yield of acetophenone as a
function time. Reaction conditions: 1‐phenylethanol (1.0 mmol),
complex 1a (0.5 mol%), NMO (2.5 mmol), CH3CN (3.0 ml)
Entry Catalyst (mol%) (mmol) (ml)
Yield (%)b
1
1
2
Toluene
51
2
1
2
1,4‐Dioxane 60
3
1
2
CH2Cl2
THF
86
53
91
92
91
74
23
0
TABLE 2 Comparison for oxidation of 1‐phenylethanol catalyzed
by complexes 1a–1fa
4
1
2
5
1
2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
6
1.5
0.5
0.25
—
0.5
0.5
0.5
0.5
0.5
2
7
2
8
2
Entry
Catalyst
NMO (mmol)
Solvent
Yield (%)b
9
2
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
2.5
2.5
2.5
2.5
2.5
2.5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
93
95
96
89
85
80
10
11
12
13
14c
—
1.5
2.5
3
82
93
94
81
2.5
aReaction conditions: 1‐phenylethanol (1.0 mmol), catalyst 1a (0.5 mol%),
aReaction conditions: alcohol (1.0 mmol), catalyst (0.5 mol%), CH3CN
(3.0 ml), 5 minutes.
NMO (2.5 mmol), solvent (3.0 ml), 5 minutes, 80°C.
bYield determined by GC.
cT = 60°C.
bDetermined by GC.

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YUE ET AL.
TABLE 3 Oxidation of secondary alcohols to ketones catalyzed by 1ca
Entry
Alcohol
Product
Yield (%)b
1
99 (95)
2
3
4
5
6
7
98 (93)
93 (87)
91 (86)
87 (84)
93 (90)
90 (86)
8
93 (90)
9
95 (91)
91 (87)
10
11
12
90 (87)
88 (85)
13
14
90 (86)
85
(Continues)

YUE ET AL.
7 of 11
TABLE 3 (Continued)
Entry
Alcohol
Product
Yield (%)b
15
87
16
84
aReaction conditions: alcohol (1.0 mmol), catalyst 1c (0.5 mol%), NMO (2.5 mmol), CH3CN (3.0 ml), 5 minutes.
bDetermined by GC; isolated yields are indicated in parentheses.
dized product formed when the reaction was carried out
in absence of oxidant (Table 1, entry 10). When the
amount of NMO was increased gradually from 1.5 to
2.5 mmol, the yield of product increased up to 93%
(Table 1, entries 7, 11 and 12). At a high amount of
NMO (3 mmol), the yield of target product charged only
slightly (Table 1, entry 13 versus entry 12). Then, the
effect of temperature on the reaction was investigated.
On lowering the temperature from 80 to 60°C, the yield
dropped from 93% to 81% (Table 1, entry12 versus entry
14). The appropriate reaction time was determined by
measuring yield as a function of reaction time. A plot of
percentage yield of acetophenone as a function of time
with complex 1a in the presence of NMO is shown in
Figure 7. It was obvious that a reaction time of 5 minutes
was suitable for the oxidation of alcohols, and the target
product was obtained with a high yield of up to 93%.
According to the data obtained (Table 1 and Figure 7),
the optimized reaction was carried out at the refluxing
temperature of acetonitrile in the presence of catalyst
(0.5 mol%) using NMO (2.5 mmol) as oxidant under nitro-
gen atmosphere in CH3CN.
Complexes 1b–1f were further screened in the oxida-
tion of 1‐phenylethanol under the same reaction
conditions in order to investigate the influence of several
α‐substituted ligands. As evident from Table 2, complexes
1b–1f displayed good to high catalytic activities, the yields
of acetophenone being in the range 80–96% (Table 2,
entries 2–6), which demonstrated that pyridine–alkoxide
ligands could adjust the catalytic activity of complexes
owing to their steric and electronic properties provided
by various substituents. Compared with complexes 1d–
1f, complexes 1a–1c showed a slightly higher catalytic
activity, and complex 1c showed the highest activity,
resulting in the target product in 96% yield (Table 2,
entry 3). The results indicated that the pyridine–alkoxide
ligands having an electron‐donating group could mark-
edly promote catalytic activity.
Then, the substrate scope was explored using complex
1c as precatalyst under the optimized conditions. The
results are summarized in Table 3. Aromatic secondary
alcohols bearing electron‐withdrawing or electron‐
donating groups at the ortho or para position of phenyl
ring were converted to corresponding ketones in high
yields (87–99%; Table 3, entries 1–7). 2‐Methyl‐ or
bromo‐substituted substrates displayed a slightly lower
activity probably owing to steric hindrance (Table 3,
entries 3 and 5). Several sterically hindered aromatic
secondary alcohols, such as diphenylmethanol,
1‐(naphthalen‐2‐yl)ethanol and 9H‐fluoren‐9‐ol, were
oxidized to target products with excellent yields (88–95%;
Table 3, entries 8–13). Aliphatic and cyclic secondary
alcohols were also oxidized to the corresponding ketones
in good yields (84–87%; Table 3, entries 14–16). The lower
catalytic activity toward aliphatic alcohols than benzylic
alcohols is owing to the differences of α‐C―H bond disso-
ciation energy between benzylic (82–85 kcal mol−1) and
aliphatic position (95–96 kcal mol−1).[31] The reactions
of various primary alcohols were further investigated
under identical conditions. It was found that substituted
benzyl alcohols with electron‐releasing or electron‐
withdrawing groups were oxidized to corresponding
aldehydes in excellent yields and selectivity (88–96%;
Table 4, entries 1–6). Representative heteroaromatic
alcohols, such as furan‐2‐ylmethanol and thiophen‐2‐
ylmethanol, were examined and showed slightly lower
yields compared with aromatic primary alcohols (80%
and 83%; Table 4, entries 7 and 8). It was assumed that
electron‐rich oxygen and sulfur atoms on the
heteroaromatic ring coordinated easily with metal atoms
and decreased the reactivity of the ruthenium catalyst.[32]
Cinnamyl alcohol and n‐butanol were also oxidized to
corresponding aldehydes with good yield (89% and 87%;
Table 4, entries 9 and 10). Cinnamyl alcohol was singly
converted to cinnamaldehyde, and the double bond
(C¼C group) was still present, which suggested that the

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YUE ET AL.
TABLE 4 Oxidation of primary alcohols to aldehydes catalyzed
by 1ca
Entry
Alcohol
Product
Yield (%)b
1
94 (88)
2
3
4
5
95 (90)
96 (91)
90 (86)
88 (84)
SCHEME 3 Oxidation of alcohols to benzaldehydes or ketones
on the gram scale catalyzed by complex 1c
was produced with a yield of 82% (Scheme 3). Furfuryl
alcohol underwent oxidation on 1.0 g scales to afford
the desired aldehyde in 70% yield (Scheme 3). These
results indicated the potential application of the ruthe-
nium catalytic system for the selective oxidation of pri-
mary and secondary alcohols.
6
94 (90)
3 | CONCLUSIONS
7
8
9
80 (76)
83 (80)
89 (86)
A series of novel trinuclear ruthenium complexes sup-
ported by bidentate pyridine–alkoxide ligands have been
synthesized and well characterized. The structures of the
ruthenium complexes were further confirmed using
single‐crystal X‐ray diffraction. Ruthenium complexes
1a–1f showed efficient catalytic activities and good
functional‐group compatibility for oxidation of primary
and secondary alcohols, leading to the formation of a vari-
ety of aldehyde and ketone derivatives in good to excellent
yields within only 5 minutes. To the best of our knowledge,
the present catalytic system is the first example of alcohol
oxidation within 5 minutes in the presence of NMO as oxi-
dant.[29,33] Exploration of the potential applications of
these ruthenium carbonyl complexes is in progress.
10
87 (81)
aReaction conditions: alcohol (1.0 mmol), catalyst 1c (0.5 mol%), NMO
(2.5 mmol), CH3CN (3.0 ml), 5 minutes.
bDetermined by GC; isolated yields are indicated in parentheses.
present ruthenium catalytic system showed good selectiv-
ity and tolerance. All primary alcohols were oxidized
selectively to corresponding aldehydes and trace amounts
of over‐oxidation products (carboxylic acids) were
obtained.
4 | EXPERIMENTAL
4.1 | General considerations
All manipulations were carried out under a nitrogen
atmosphere using standard Schlenk and vacuum‐line
techniques. Chemicals obtained from commercial sup-
pliers were of reagent grade and used without further
purification. Toluene, THF, dioxane, acetonitrile, dichlo-
romethane and n‐hexane were dried using the general
methods and degassed before use. Melting points were
2.4 | Scale‐up experiment
Exploring the catalytic system for a scaling up of synthe-
ses is also very meaningful. When 1‐phenylethanol and
benzyl alcohol were scaled up to 1.0 g in the ruthenium
catalytic system, acetophenone was synthesized with
excellent yields (93%; Scheme 3). Cyclohexanone also

YUE ET AL.
9 of 11
determined using an SGW X‐4A digital melting point
apparatus. IR spectra were recorded as potassium bro-
mide pellets using a Thermo Fisher iS 50 spectrometer
in the range 4000 to 400 cm−1 and elemental analyses
were performed using a Vario EL III analyzer. NMR spec-
tra were recorded using a Zhongke‐Niujin 400 MHz NMR
spectrometer at room temperature. 1H NMR chemical
shifts in ppm indicate downfield shift relative to
tetramethylsilane (0 ppm), and were referenced relative
to the signal of the solvent (CDCl3). Ligands L1H–L4H
have been reported in the literature[25a] and the new
ligands L5H and L6H were synthesized according to a lit-
erature procedure.[25b]
CH), 3.05 (d, 2H, J = 8.3 Hz, CH2), 2.56 (s, 3H, CH3). 19
F
NMR (376 MHz, CDCl3, δ, ppm): −62.29. 13C NMR
(101 MHz, CDCl3, δ, ppm): 158.6, 157.5, 148.2, 137.4,
126.7, 126.2, 125.3, 125.3, 121.6, 120.7, 72.9, 44.9, 24.4.
4.4 | Synthesis of complex 1a
In a 50 ml Schlenk flask were placed L1H (0.190 g,
0.938 mmol), Ru3(CO)12 (0.300 g, 0.469 mmol) and THF
(20 ml), and the mixture was heated to refluxing temper-
ature for 12 hours under a nitrogen atmosphere. After the
mixture was cooled to room temperature, the solvent was
removed in vacuo. The residue was chromatographed on
an Al2O3 column (eluent CH2Cl2–petroleum ether, 1/3),
and subsequent recrystallization from CH2Cl2–hexane
gave 1a (0.228 g, 51%) as orange crystals; m.p. 160–161°
C. 1H NMR (400 MHz, CDCl3, δ, ppm): 7.52 (t, 2H,
J = 7.6 Hz, Py‐H), 7.37–7.30 (m, 6H, Ar‐H), 7.26 (d, 4H,
J = 6.6 Hz, Ar‐H), 6.92 (d, 2H, J = 7.6 Hz, Py‐H), 6.84
(d, 2H, J = 7.5 Hz, Py‐H), 3.84 (d, 2H, J = 9.5 Hz, CH2),
3.20–3.14 (m, 2H, CH), 2.74 (d, 2H, J = 14.9 Hz, CH2),
1.78 (s, 6H, CH3). 13C NMR (101 MHz, CDCl3, δ, ppm):
205.5, 203.4, 202.8, 192.2, 163.9, 162.0, 148.8, 137.7,
4.2 | Synthesis of L5H
2,6‐Dimethylpyridine (3.22 g, 30 mmol) was dissolved in
dry THF (15 ml) and the resulting solution was cooled to
−78°C. Then n‐BuLi (1.6 M in hexanes, 19 ml, 30 mmol)
diluted in 15 ml of THF was added slowly with vigorous
stirring. The solution was stirred at −78°C for 1 hour,
followed by the addition of p‐bromobenzaldehyde (5.55 g,
30 mmol). The mixture was stirred at −78°C for another
3 hours, then warmed to ambient temperature, and contin-
uously stirred at ambient temperature overnight to afford a
pale yellow‐orange solution. The solution was then hydro-
lyzed with saturated NH4Cl solution. The orange layer was
separated, washed with water and dried over anhydrous
MgSO4 for several hours. The solvent was removed under
reduced pressure and the residue was placed in an Al2O3
column. Elution with ethyl acetate–petroleum ether(1/4
v/v) afforded yellow solid L5H (4.80 g, 54.7%); m.p. 94–
128.5, 127.4, 125.9, 124.9, 123.1, 80.2, 55.5, 28.5. IR (υCO
,
KBr, cm−1): 2073 (s), 1992 (s), 1909 (s). Anal. Calcd for
C36H28N2O10Ru3 (%): C, 45.43; H, 2.96; N, 2.94. Found
(%): C, 45.51; H, 3.03; N, 2.80.
4.5 | Synthesis of complex 1b
Using a procedure similar to that described above,
complex 1b was synthesized with the reaction of L2H
(0.211 g, 0.938 mmol) and Ru3(CO)12 (0.300 g,
0.469 mmol) in 20 ml of THF. Complex 1b was obtained
as orange crystals (0.276 g, 60%); m.p. 167–169°C. 1H
NMR (400 MHz, CDCl3, δ, ppm): 7.51 (t, 2H,
J = 7.6 Hz, Py‐H), 7.17–7.12 (m, 8H, Ar‐H), 6.92 (d, 2H,
J = 7.6 Hz, Py‐H), 6.82 (d, 2H, J = 7.4 Hz, Py‐H), 3.80
(d, 2H, J = 9.4 Hz, CH2), 3.19–3.13 (m, 2H, CH), 2.71
(d, 2H, J = 14.9 Hz, CH2), 2.40 (s, 6H, CH3), 1.81 (s, 6H,
CH3). 13C NMR (101 MHz, CDCl3, δ, ppm): 205.5, 203.4,
202.7, 192.2, 163.9, 162.1, 145.8, 137.6, 136.9, 129.1,
125.8, 124.7, 123.1, 79.9, 55.6, 28.6, 21.2. IR (υCO, KBr,
cm−1): 2070 (m), 1992 (m), 1905 (m). Anal. Calcd for
C38H32N2O10Ru3 (%): C, 46.58; H, 3.29; N, 2.86. Found
(%): C, 46.40; H, 3.34; N, 2.94.
1
96°C. H NMR (400 MHz, CDCl3, δ, ppm): 7.51 (t, 1H,
J = 7.7 Hz, Py‐H), 7.45–7.48 (m, 2H, Ar‐H), 7.30 (d, 2H,
J = 8.3 Hz, Ar‐H), 7.05 (d, 1H, J = 7.7 Hz, Py‐H), 6.89 (d,
1H, J = 7.6 Hz, Py‐H), 6.52 (s, 1H, OH), 5.09 (t, 1H,
J = 6.0 Hz, CH), 3.03 (d, 2H, J = 5.9 Hz, CH2), 2.56 (s,
3H, CH3). 13C NMR (101 MHz, CDCl3, δ, ppm): 158.7,
157.4, 143.3, 137.4, 131.3, 127.6, 121.4, 120.9, 120.6, 72.7,
44.9, 24.3.
4.3 | Synthesis of L6H
The synthesis of L6H followed analogous methods as
described above for the synthesis of L5H using 2, 6‐
dimethylpyridine (3.22 g, 30 mmol), n‐BuLi (1.6 M in hex-
anes, 19 ml, 30 mmol) and 4‐(trifluoromethyl)benzalde-
hyde (5.22 g, 30 mmol). L6H was obtained as yellow solid
(4.60 g, 54.5%); m.p. 80–81°C. 1H NMR (400 MHz, CDCl3,
δ, ppm): 7.59 (d, 2H, J = 8.0 Hz, Ar‐H),7.55–7.50 (m, 3H,
Py‐H, Ar‐H), 7.06 (d, 1H, J = 7.7 Hz, Py‐H), 6.90 (d, 1H,
J = 7.6 Hz, Py‐H), 6.71 (s, 1H, OH), 5.18–5.20 (m, 1H,
4.6 | Synthesis of complex 1c
Using a procedure similar to that described above, com-
plex 1c was synthesized with the reaction of L3H

10 of 11
YUE ET AL.
(0.230 g, 0.938 mmol) and Ru3(CO)12 (0.300 g, 0.469 mmol)
4.9 | Synthesis of complex 1f
in 20 ml of THF. Complex 1c was obtained as orange crys-
1
Using a procedure similar to that described above, complex
1f was synthesized with the reaction of L6H (0.267 g,
0.938 mmol) and Ru3(CO)12 (0.300 g, 0.469 mmol) in
20 ml of THF Complex 1f was obtained as orange crystals
(0.231 g, 45%); m.p. 170–172°C. 1H NMR (400 MHz, CDCl3,
δ, ppm): 7.64 (d, 4H, J = 8.4 Hz, Ar‐H), 7.57 (t, 2H,
J = 7.6 Hz, Py‐H), 7.40 (d, 4H, J = 7.7 Hz, Ar‐H), 6.96 (d,
2H, J = 7.6 Hz, Py‐H), 6.87 (d, 2H, J = 7.4 Hz, Py‐H), 3.89
(d, 2H, J = 9.4 Hz, CH2), 3.20–3.14 (m, 2H, CH), 2.72 (d,
2H, J = 15.0 Hz, CH2), 1.80 (s, 6H, CH3). 19F NMR
(376 MHz, CDCl3, δ, ppm): −62.14. 13C NMR (101 MHz,
CDCl3, δ, ppm): 205.1, 202.9, 202.8, 191.5, 163.5, 161.5
152.7, 137.9, 130.7 (q, JC–F = 32.0 Hz), 126.2, 125.6(q, JC–
tals (0.308 g, 65%); m.p. 158–160°C. H NMR (400 MHz,
CDCl3, δ, ppm): 7.51 (t, 2H, J = 7.6 Hz, Py‐H), 7.17 (d,
4H, J = 8.0 Hz, Ar‐H), 6.93 (d, 2H, J = 7.6 Hz, Py‐H),
6.90 (d, 4H, J = 8.7 Hz, Ar‐H), 6.82 (d, 2H, J = 7.4 Hz,
Py‐H), 3.87 (s, 6H, CH3), 3.78 (d, 2H, J = 9.5 Hz, CH2),
3.18–3.12 (m, 2H, CH), 2.70 (d, 2H, J = 14.8 Hz, CH2),
1.87 (s, 6H, CH3). 13C NMR (101 MHz, CDCl3, δ, ppm):
204.4, 202.3, 201.8, 191.2, 162.8, 161.0, 157.9, 139.9, 136.6,
125.9, 123.7, 122.1, 112.7, 78.5, 54.4, 28.7, 27.6. IR (υCO
,
KBr, cm−1): 2070 (s), 1994 (vs), 1909 (s). Anal. Calcd for
C38H32N2O12Ru3 (%): C, 45.11; H, 3.18; N, 2.77. Found
(%): C, 45.19; H, 3.25; N, 2.72.
= 3.5 Hz),125.0, 123.3, 122.9 (q, JC–F = 270.1 Hz), 79.9,
F
55.0, 28.5. IR (υCO, KBr, cm−1): 2075 (s), 1993 (s), 1914
(s). Anal. Calcd for C38H26F6N2O10Ru3 (%): C, 41.96; H,
2.41; N, 2.58. Found (%): C, 41.88; H, 2.33; N, 2.72.
4.7 | Synthesis of complex 1d
Using a procedure similar to that described above, complex
1d was synthesized with the reaction of L4H (0.236 g,
0.938 mmol) and Ru3(CO)12 (0.300 g, 0.469 mmol) in
20 ml of THF. Complex 1d was obtained as orange crystals
(0.249 g, 52%); m.p. 165–167°C. 1H NMR (400 MHz, CDCl3,
δ, ppm): 7.54 (t, 2H, J = 7.6 Hz, Py‐H), 7.34 (d, 4H,
J = 8.5 Hz, Ar‐H), 7.20 (d, 4H, J = 7.9 Hz, Ar‐H), 6.96 (d,
2H, J = 7.5 Hz, Py‐H), 6.84 (d, 2H, J = 7.4 Hz, Py‐H), 3.79
(d, 2H, J = 9.4 Hz, CH2), 3.17–3.11 (m, 2H, CH), 2.68 (d,
2H, J = 14.9 Hz, CH2), 1.88 (s, 6H, CH3). 13C NMR
(101 MHz, CDCl3, δ, ppm): 204.2, 202.1, 201.8, 190.7,
162.6, 160.6, 146.1, 136.8, 131.9, 127.6, 126.2, 123.9, 122.2,
78.6, 54.2, 27.7. IR (υCO, KBr, cm−1): 2071 (s), 1990 (s),
1912 (s). Anal. Calcd for C36H26Cl2N2O10Ru3 (%): C,
42.36; H, 2.57; N, 2.74. Found (%): C, 42.29; H, 2.64; N, 2.80.
4.10 | Catalytic oxidation of alcohols with
NMO
Oxidation of alcohols was carried out using the com-
plexes in the presence of NMO as oxidant. A mixture of
an alcohol substrate (1 mmol), complex (0.005 mmol)
and NMO (2.5 mmol) in acetonitrile (3 ml) was refluxed
with stirring for 5 minutes under a nitrogen atmosphere.
The reaction was monitored by GC. After the reaction
was completed, the mixture was cooled to room tempera-
ture and condensed under reduced pressure. The residue
was subject to purification by Al2O3 column chromatog-
raphy using ethyl acetate–petroleum ether to afford the
corresponding aldehydes or ketones, which were identi-
fied using NMR analyses.
4.8 | Synthesis of complex 1e
Using a procedure similar to that described above, complex
1e was synthesized with the reaction of L5H (0.273 g,
0.938 mmol) and Ru3(CO)12 (0.300 g, 0.469 mmol) in
20 ml of THF. Complex 1e was obtained as orange crystals
(0.239 g, 46%); m.p. 178–179°C. 1H NMR (400 MHz, CDCl3,
δ, ppm): 7.54 (t, 2H, J = 7.6 Hz, Py‐H), 7.49 (d, 4H,
J = 8.4 Hz, Ar‐H), 7.14 (d, 4H, J = 7.8 Hz, Ar‐H), 6.97 (d,
2H, J = 7.3 Hz, Py‐H), 6.84 (d, 2H, J = 7.2 Hz, Py‐H), 3.77
(d, 2H,J = 9.4 Hz, CH2), 3.17–3.11 (m, 2H, CH), 2.68 (d,
2H, J = 14.9 Hz, CH2), 1.88 (s, 6H, CH3). 13C NMR
(101 MHz, CDCl3, δ, ppm): 205.2, 203.14, 202.8, 191.7,
163.7, 161.6, 147.7, 137.8, 131.6, 127.6, 124.9, 123.2, 120.9,
79.6, 29.7, 28.7. IR (υCO, KBr, cm−1): 2069 (s), 1993 (vs),
1911 (s). Anal. Calcd for C36H26Br2N2O10Ru3 (%): C,
38.97; H, 2.36; N, 2.52. Found (%): C, 38.82; H, 2.39; N, 2.40.
4.11 | Crystal structure determination
Good‐quality crystals of trinuclear ruthenium complexes
1a–1f suitable for single‐crystal X‐ray diffraction analysis
were obtained from the slow evaporation of a hexane–
CH2Cl2 solution. X‐ray crystallographic data were col-
lected with a Bruker AXS SMART 1000 CCD diffractome-
ter, using graphite monochromated Mo Kα radiation (φ/ω
scan, λ = 0.71073 Å). The structures were solved by direct
methods and refined by full‐matrix least‐squares on F 2.
All hydrogen atoms were placed in calculated positions.
Structure solution and refinement were performed using
the SHELXL‐97 package. The crystal data and summary
of X‐ray data collection are given in Tables S2 and S3.

YUE ET AL.
11 of 11
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support
from the Hebei Natural Science Foundation of China
(nos. B2017205006 and B2019205087), the Education
Department Foundation of Hebei Province (nos.
ZD2018005 and QN2019036) and the Science Foundation
of Hebei Normal University (nos. L2017Z02 and
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