Half-sandwich ruthenium-based versatile catalyst for both alcohol oxidation and catalytic hydrogenation of carbonyl compounds in aqueous media

Article abstract of DOI:10.1002/aoc.4875

A series of half-sandwich ruthenium-based catalysts for both alcohol oxidation and carbonyl compounds hydrogenation have been synthesized through metal-induced C–H bond activation based on benzothiazole ligands. The neutral ruthenium complexes 1–4 were fully characterized by UV–vis, NMR, IR, and elemental analysis. Molecular structures of complexes 1 and 3 were further confirmed by X-ray diffraction analysis. All complexes exhibited high activity for the catalytic oxidation of a variety of alcohols with ^tBuOOH as oxidants to give carbonyl compounds with high yields in water. Moreover, these half-sandwich complexes also showed high efficiency for the catalytic hydrogenation of carbonyl compounds in a methanol–water mixture. The catalyst could be reused for at least five cycles without any loss of activity. The catalytic system also worked well for various kinds of substrates with either electron-donating or electron-withdrawing groups.

Full text of DOI:10.1002/aoc.4875

Received: 30 October 2018
DOI: 10.1002/aoc.4875
Revised: 29 November 2018
Accepted: 2 December 2018
F U L L P A P E R
Half‐sandwich ruthenium‐based versatile catalyst for both
alcohol oxidation and catalytic hydrogenation of carbonyl
compounds in aqueous media
Xin‐Chao Qiao1 | Dan‐Chen Qiu1 | Nan Lin1 | Jing‐Wei Zhu1 | Wei Deng1 | Zi‐Jian Yao1,2,3
1 School of Chemical and Environmental
A series of half‐sandwich ruthenium‐based catalysts for both alcohol oxidation
Engineering, Shanghai Institute of
Technology, Shanghai 201418, China
2 Key Laboratory of Synthetic and Self‐
and carbonyl compounds hydrogenation have been synthesized through metal‐
induced C–H bond activation based on benzothiazole ligands. The neutral
Assembly Chemistry for Organic
ruthenium complexes 1–4 were fully characterized by UV–vis, NMR, IR, and
Functional Molecules, Shanghai Institute
of Organic Chemistry, Chinese Academy
of Sciences, 345 Lingling Road, Shanghai
200032, China
3 State Key Laboratory of Coordination
Chemistry, Nanjing University, Nanjing
210023, China
elemental analysis. Molecular structures of complexes 1 and 3 were further
confirmed by X‐ray diffraction analysis. All complexes exhibited high activity
t
for the catalytic oxidation of a variety of alcohols with BuOOH as oxidants
to give carbonyl compounds with high yields in water. Moreover, these half‐
sandwich complexes also showed high efficiency for the catalytic hydrogena-
tion of carbonyl compounds in a methanol–water mixture. The catalyst could
be reused for at least five cycles without any loss of activity. The catalytic sys-
tem also worked well for various kinds of substrates with either electron‐
donating or electron‐withdrawing groups.
Correspondence
Zi‐Jian Yao, School of Chemical and
Environmental Engineering, Shanghai
Institute of Technology, Shanghai, 201418,
China.
Email: zjyao@sit.edu.cn
KEYWORDS
Funding information
catalysis, coordination, half‐sandwich complex, X‐ray analysis
Science and Technology Commission of
Shanghai Municipality, Grant/Award
Numbers: 1435700 and 16DZ2270100;
Shanghai Municipal Education Commis-
sion, Grant/Award Number: 16CG64;
Shanghai Young Teacher Training Pro-
gram, Grant/Award Number:
ZZZZyyx16005; Shanghai Science and
Technology Committee, Grant/Award
Number: 16DZ2270100; Open project of
Key Laboratory of Synthetic and Self‐
Assembly Chemistry for Organic Func-
tional Molecules, Grant/Award Number:
K2018‐6; Natural Science Foundation of
Shanghai, Grant/Award Number:
16ZR1435700; Chenguang Scholar of
Shanghai Municipal Education Commis-
sion, Grant/Award Number: 16CG64;
National Natural Science Foundation of
China, Grant/Award Number: 21601125
Appl Organometal Chem. 2019;e4875.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 13
https://doi.org/10.1002/aoc.4875

2 of 13
QIAO ET AL.
9 complexes have been prepared which exhibit good cat-
alytic activity for olefin polymerization and imine reduc-
tion in our previous work.[22] As an ongoing project,
four kinds of mononuclear half‐sandwich ruthenium
complexes was prepared based on benzothiazole ligands.
Experimental results show that these complexes can serve
as the catalysts for both oxidation of alcohols and hydro-
genation of ketones under proper conditions in aqueous
solution. The C, N‐chelate ruthenium complexes were
fully characterized by various spectroscopic techniques.
The steric and electronic effects on the catalytic activity
of the complexes were also investigated.
1 | INTRODUCTION
Half‐sandwich late transition‐metal complexes have been
widely used in many catalytic reactions because of their
good stability and high catalytic activity in the reac-
tion.[1,2] Therefore [(η6‐arene)MCl2]2 and [Cp#MCl2]2
(arene: η6‐C6H6, p‐cymene; Cp#: η5‐C5H5, η5‐C5Me5; M:
Co, Ir, Rh, Ru) are often utilized as the metal precursors
to prepare diverse half‐sandwich transition‐metal com-
plexes due to the following reasons: (i) the transition
metal precursors [Cp#MCl2]2 (M = Co, Ir, Rh, Ru) are
easily synthesized with high yields by reactions of metal
chlorides with conjugated ligands; (ii) The hemisphere
of the metal center is perfectly shielded by the Cp#
ligands, minimizing the possibility of undesired side‐
reactions; (iii) The redox property and solubility of these
transition metal complexes is enhanced by introducing
various types of substituents to cyclopentadienyl ring.[3]
Among different types of reported half‐sandwich late
transition metal complexes, ruthenium complexes often
showed good catalytic activity in transfer hydrogenation
reactions,[4,5] C–H bond functionalization (alkylation,[6–
2 | RESULTS AND DISCUSSION
2.1 | Synthesis of half‐sandwich
ruthenium complexes 1–4
Benzothiazole ligands L1–L4 were prepared in high
yields through cyclocondensation reaction from cheap
and available aldehydes and thioanilines under air.[23]
With the ligands in hand, the reactivity of these nitroge-
nous donor ligands with 0.5 equivalent ruthenium pre-
cursor [(p‐cymene)RuCl2]2 was investigated in the
presence of NaOAc. As shown in Scheme 1, half‐
sandwich ruthenium complexes 1–4 were furnished in
methanol through metal‐induced C–H bond activation
arylation,[9] sulfonylation,[10] and allylation[11]) and
8]
water oxidation reactions.[12–14] Additionally, the good
stability against water of half‐sandwich ruthenium com-
plexes makes them ideal candidates to perform catalytic
organic transformations in aqueous solution.[15]
Aldehydes and ketones, often obtained from catalytic
oxidation of alcohols, are important reaction intermedi-
ates in pharmaceuticals and fine chemical industry.[16–
18]
The key point of this process is the activity and selec-
tivity of the catalyst, as well as the utilization of environ-
mental friendly solvents. Owing to the good catalytic
activity and stability against water, ruthenium derivatives
play a particular role in the potential application for the
production of carbonyl compounds through alcohol oxi-
dation in aqueous solution.[19] Thus the catalytic activity
of half‐sandwich ruthenium complexes in these reactions
is worthy investigating.[20]
In search for late transition metal complexes with high
catalytic activity and novel structures, we are interested
in the construction of half‐sandwich complexes based
on the ligands with N‐donor because the electronic and
steric effects around the metal center can be easily
altered.[21] Several cyclometallated half‐sandwich group
FIGURE 1 UV–vis spectra of complexes 1–4 (2.0 × 10–5 M in
CH3CN)
SCHEME 1 Synthesis of Half‐sandwich Ruthenium Complexes 1–4

QIAO ET AL.
3 of 13
FIGURE 2 Molecular structures of 1 (a) and 3 (b) with thermal ellipsoids drawn at the 30% level. All hydrogen For Peer Review atoms are
omitted for clarity
TABLE 2 Crystallographic Data and Structure Refinement
at elevated temperature in moderate isolated yields. They
form air‐ and moisture‐stable, orange to red crystalline
solids which are soluble in alcohol, acetone and CH2Cl2,
but are insoluble in non‐polar solvents such as diethyl
ether and hexane. Sodium acetate is an essential reactant
in the reaction because this compound served as both
base and ligand in the C‐H activation process. No product
was observed without the addition of NaOAc. The
benzothiazole ligands act as the C, N‐coordination mode
ligands in the products which are supported by the IR
and NMR spectroscopies and elemental analysis. No C,
S‐chelate mode complexes were found in the reaction,
indicating the stronger coordination ability of N atom in
the ligands. No decomposition was observed after
refluxing the ruthenium complexes in toluene for several
hours suggested the thermal stability of complexes 1–4.
The TGA curves confirm that these complexes are stable
at high temperature up to 300 °C (see supporting infor-
mation). Complexes 1–4 were fully characterized by vari-
ous spectroscopic techniques and elemental analysis.
Parameters for 1 and 3a
1
3
Chemical Formula
C23H22ClNRuS
480.99
C24H21ClF3NRuS
549.00
FW
T/[K]
λ/Å
296(2)
173(2)
0.71073
Orthorhombic
P212121
7.7543(4)
10.5919(4)
24.3280(12)
90
0.71073
Monoclinic
P2(1)/c
17.4712(10)
7.7869(4)
16.0352(9)
90
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
90
93.540(2)
90
γ/°
90
V/Å3
1998.13(16)
4
2177.4(2)
4
Z
ρ/Mg m−3
μ/mm−1
F(000)
θ range/°
1.599
1.675
TABLE 1 Selected Bond Lengths (Å) and Angles (°) for Com-
plexes 1 and 3
5.798
5.506
976
1104
Bond lengths (Å) and angles (°)
1
3
4.816–57.118
4.808–56.993
19243
Reflections collected 12927
Ru(1)–C(1)
2.055(5)
2.111(4)
2.058(3)
2.100(2)
Data/restraints/
param.
4065/0/263
4433/18/299
Ru(1)–N(1)
Ru(1)–Cl(1)
C(1)–C(6)
2.4097(11)
1.416(7)
1.443(7)
1.321(6)
1.385(6)
112.4(4)
118.2(4)
77.67(18)
115.1(3)
2.4044(7)
1.403(4)
1.446(4)
1.315(3)
1.393(3)
112.8(2)
117.5(2)
77.44(9)
115.85(16)
Goodness‐of‐fit on
1.044
1.099
F 2
C(6)–C(7)
Final R indices
[I > 2σ(I)a]
R1 = 0.0329,
wR2 = 0.0827
R1 = 0.0291,
wR2 = 0.0694
C(7)–N(1)
C(8)–N(1)
Rint
0.0483
0.0465
C(1)–C(6)–C(7)
C(6)–C(7)–N(1)
C(1)–Ru(1)–N(1)
Ru(1)–N(1)–C(7)
Largest diff.peak/
hole (e Å−3
1.631/ –1.445
0.839/ –0.537
)
2
aR1
=
Σ|| F 0|‐| F c||/Σ| F 0|(based on reflections with F 0
>
2σF2).
(0.095P)2];
2
wR2
=
[Σ[w(F02‐Fc2)2]/Σ[w(F02)2]]1/2
;
w
=
1/[σ2( F 0
)
+
P = [max(F02, 0) + 2Fc2]/3(also with F0 > 2σF2).
2

4 of 13
QIAO ET AL.
The generation of half‐sandwich ruthenium products
1–4 was supported by their H NMR spectra. In compar-
elemental analysis and spectroscopic data of these com-
plexes demonstrate that their structures are similar to
each other.
1
ison of the free ligand, the disappearance of one proton
signal in 1–4 from downfield range of δ 9.00–7.00 ppm
suggested the C–H activation in phenyl ring. The chem-
ical shifts of p‐cymene ring protons of these complexes
shifted downfield on coordination interaction, when
compared to the [(p‐cymene)RuCl2]2 precursor. Addi-
tional confirmation of the C, N‐chelation of the ligands
is given by the chemical and magnetic inequivalence of
the p‐cymene ring protons, which appear as four distinct
In order to investigate the effects of the metal center
and coordination mode on the electronic structure of
the half‐sandwich ruthenium complexes, the UV–vis
absorption spectra of the complexes 1–4 were measured
in acetonitrile (Figure 1). All spectra exhibit strong
absorption bands in the region of 200–250 nm, which
can be assigned to the π–π* transitions of the molecular
orbitals localized on the ligands. The lower intensity
absorption bands in the range of 300–360 nm were
assigned to metal‐to‐ligand charge transfer (MLCT) from
the filled 4d orbital to the empty π* orbital.[26]
1
doublets in the H NMR spectra as a consequence of the
stereogenic nature of the ruthenium center. This obser-
vation was in a good agreement with related compounds
in previous report.[24] All the ruthenium complexes show
a characteristic absorption band in the range 1410–
1450 cm−1 in their IR spectra, which can be assigned to
the vibration of the C=N bond of the thiazole ring. These
C=N absorption band all shifted to lower frequencies
when compared to those of the free benzothiazole
ligands (1440–1490 cm−1). The red shift reflected the
coordination interaction between the ligand and [(p‐
cymene)Ru] moiety. This is consistent with the increase
in the electron density on the ruthenium (II) center
caused by the coordination of the C=N group, which
resulted in increasing the back bonding to the nitrogen
and hence a lower C=N stretching vibration.[25] The
2.2 | Crystal structures of half‐sandwich
ruthenium complexes
To clearly elucidate the structures of these half‐sandwich
ruthenium complexes, an X‐ray analysis is desired. Single
crystals suitable for X‐ray diffraction analysis were
obtained by the slow diffusion of hexane into a saturated
solution of the ruthenium complexes in dichloromethane.
Complexes 1 and 3 crystallized in the orthorhombic space
group P212121 and the monoclinic space group P21/c,
respectively (Figure 2). Selected geometric data are given
TABLE 3 Optimization for the Catalytic Oxidation of 1‐Phenylethanola
Entry
Cat.
Oxidant
T/oC
Con./%
Yield/%b
TON
TOF/h−1
1
1
1
1
1
–
1
1
1
2
3
4
1
H2O2
rt
rt
40
90/92
68
33
83/85c
59
330
830
590
–
165
415
295
–
2
tBuOOH
3
NaIO4
rt
4
O2
rt
trace
trace
71
trace
trace
63d
92
5
tBuOOH
tBuOOH
tBuOOH
tBuOOH
tBuOOH
tBuOOH
tBuOOH
tBuOOH
rt
–
–
6
rt
6300
920
930
900
960
920
620
3150
460
465
450
480
460
310
7
50
80
50
50
50
50
>99
>99
98
8
93
9
90
10
11
12
>99
98
96
92
70
62e
aReaction conditions: 1‐phenylethanol (1.0 mmol), catalyst (0.1 mol%), oxidant (3.0 mmol), reaction time: 2 hr.
bYield was determined by GC analysis, n‐tridecane was used as internal standard.
creaction time: 5 hr.
dCatalyst loading: 0.01 mol%.
eReaction media: pH = 3.

QIAO ET AL.
5 of 13
in Table 1, and their crystallographic data are summa-
rized in Table 2. Both complexes show a typical three‐
legged piano‐stool geometry with the metal center being
coordinated by the η6‐p‐cymene and the C and N atoms,
as well as a chloride ligand. The metal center has a
distorted‐octahedral environment, assuming that the η6‐
p‐cymene group occupies three fac coordination sides.[26]
The air and thermal stability of these ruthenium com-
plexes is presumably caused by the formation of the
cyclometallated structure. The distances of Ru–N bonds
in the two complexes (2.111(4) Å (1) and 2.100(2) Å (3))
are within the range of known values for the bond in sim-
ilar ruthenium complexes.20e, f The Ru–C bond lengths in
complexes 1 and 3 are 2.055(5) Å and 2.058(3) Å, respec-
tively, which are also consistent with previous reports.[27]
The shorter Ru–C bond length compared with the Ru–N
distance suggests that the stronger interaction between
metal center and carbon atom. The five‐membered ring
Ru(1)–C(1)–C(6)–C(7)–N(1) of complex 3 is almost planar
with the dihedral angle of 1.9o between planes of N(1)–
Ru(1)–C(1) and C(1)–C(6)–C(7)–N(1), whereas the
cyclometallated ring in complex 1 is folded with the dihe-
dral angle of 3.9o. The sum of the inner angles of the five‐
membered ring of 539.91o further confirmed the planarity
of complex 3. Non‐covalent inter‐molecular interactions
of Ru–Cl···H (aromatic) in the crystal packing structure
of the two complexes are observed.
2.3 | Catalytic oxidation of alcohols
Alcohol oxidation catalyzed by group 8 and 9 metal com-
plexes represents an efficient route to generate carbonyl
substrates.[28] In view of the previous results, we decided
to investigate the catalytic activity of these ruthenium
complexes for alcohols oxidation in aqueous solution.
Optimal conditions were obtained after screening differ-
ent factors such as oxidant, reaction temperature and cat-
alyst loading. The oxidation of 1‐phenylethanol was
studied as a model reaction at a catalyst loading of
t
0.1 mol%. BuOOH was proved to be the best oxidant
t
among different candidates (such as H2O2, BuOOH,
NaIO4) (Table 3, entries 1–3). Bubbling oxygen gas
through the reaction mixture did not induce any oxida-
tion over 6 hr (Table 3, entry 4). Prolonged reaction time
showed no obvious influence on the conversion (Table 3,
entry 2). No products were found in the absence of the
FIGURE 3 Plausible Mechanism for the Ru‐Catalyzed Oxidation
of Alcohols based on previous report20d
TABLE 4 Scope of the Oxidation of Alcohols under the Catalysis of Complex 3a
aReaction conditions: substrate (1.0 mmol), catalyst (0.1 mol%), tBuOOH (3.0 mmol), reaction time: 2 hr. Yield was
determined by GC analysis, n‐tridecane was used as internal standard.
bisolated yields.

6 of 13
QIAO ET AL.
catalyst (Table 3, entry 5). When the catalyst amount was
lowered to 0.01 mol %, the yield decreased obviously in
6 hr (Table 3, entry 6). The yield of the product was pro-
moted (up to 92%) at elevated temperature of 50 °C
(Table 3, entry 7), however, further elevating reaction
temperature showed little influence on the yield of the
product (Table 3, entry 8). All half‐sandwich complexes
1–4 were employed in the reaction and there was no
remarkable effect on changing the ligand substitution
(Table 3, entries 9–11). Slightly higher catalytic activity
exhibited by complex 3 (Table 3, entry 10) could be attrib-
uted to the strong electron withdrawing nature of the CF3
group on the ligand which may stabilize the intermediate
oxo species based on the reported plausible mechanism of
this reaction.20d The product obtained with low yield
under acidic conditions (Table 3, entry 12) support the
reported plausible mechanism (Figure 3), because the
aquation reaction, representing the likely catalyst activa-
tion step, is probably hindered at lower pH.20d, f In addi-
tion, another possible mechanism of this reaction is the
involvement of the Ru‐OOBut hydroxo species, and the
feature of this mechanism is the proton formation pro-
t
duced by mixing the Ru catalyst and BuOOH.20d This
mechanism was ruled out based on the experimental
results, because there is little change in the pH value after
t
mixing Ru catalyst and BuOOH in 1: 1 molar ratio at
50 °C for 2 hr.
The study was extended to other alcohols with this
catalytic system in order to establishing its scope after
obtaining the optimum conditions for the oxidation.
Complex 3 was employed in the reaction because the
highest TON value was afforded when this complex
was used as catalyst. A series of substrates were used in
the reaction and the results showed that high
conversions for aromatic alcohols with both electron
withdrawing group and electron donating groups were
observed (Table 4). The corresponding aromatic alde-
hydes and ketones were given in good yields (5a–k). It
is noteworthy that the reaction rates of the substrates
with ortho‐substituted groups (5f) are comparable to
those of para‐substituted groups, and this result is
different from the catalytic activity showed by the
TABLE 5 Hydrogenation of ketones under Catalysis of Half‐sandwich Ruthenium Complexesa
entry
Cat.
Substrate
Time
T/oC
P/atm
Solvent
Con./%
Yield/%b
TON
TOF/h−1
1
1
2
3
4
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
Acetophenone
3
3
3
3
3
3
3
3
6
2
3
3
3
3
3
3
3
3
3
80
80
80
80
100
50
80
80
80
80
80
80
80
80
80
80
80
80
80
5
5
5
5
5
5
8
2
5
5
5
5
5
5
5
5
5
5
5
MeOH
>99
99
93
90
186
180
182
186
188
156
186
102
186
150
114
36
62
2
MeOH
60
3
MeOH
98
91
60.7
62
4
MeOH
>99
>99
86
93
5
MeOH
94
62.7
52
6
MeOH
78
7
MeOH
>99
62
93
62
8
MeOH
56
34
9
MeOH
>99
84
93
31
10
11
12
13
14
15
16
17
18
19
MeOH
75
75
MeOH
67
57c
38
CH2Cl2
25
18
12
Toluene
THF
26
23
46
15.3
39.3
24.7
61.3
60.7
50.7
–
65
59
118
74
H2O
42
37
MeOH/H2O (1/1)
MeOH/H2O (1/3)
MeOH/H2O (1/5)
MeOH
>99
>99
83
92
184
182
152
–
91
76
trace
tracec
aReaction conditions: acetophenone (1.0 mmol).
bYield was determined by GC analysis, n‐tridecane was used as internal standard.
cPMe3 (1.0 mol%) was used as additive.

QIAO ET AL.
7 of 13
cationic N,N‐chelate group 8 and 9 half‐sandwich transi-
tion metal complexes.[29] However, 3, 5‐Me group‐
substituted 1‐phenylethanol (5 l) showed moderate con-
version under the catalysis of complex 3 probably
because of the steric effects. Alkyl substrates gave lower
conversion (5 m–o) in comparison with aromatic sub-
strates, which is similar to the results in a previous
report.20d Complex 3 showed similar catalytic activity
but broader substrates scope compared with those of
half‐sandwich ruthenium analogous,[29] However, for
the oxidation of alkyl alcohols, the catalytic efficiency
of these neutral complexes is lower than that of ruthe-
nium analogous with the tridentate type ligands.20b
On the other hand, reduction of aldehydes and
ketones is one of the important research areas in medi-
cine, organic synthesis and flavor industry.[30] Catalytic
hydrogenation with late transition metal complexes by
using clean H2 as starting material represents the sus-
tainable and green chemical process, when compared
with the traditional reduction methods using metal
hydrides as reduction reagents.[30] Thus the catalytic
hydrogenation activity of carbonyl substrates by using
complexes 1–4 as catalysts was investigated. Preliminary
results showed that hydrogenation of acetophenone
gave the highest yield when the reaction was performed
at the temperature of 80 °C and pressure of 5 atm
(Table 5, entries 1–10). All the half‐sandwich ruthenium
complexes were efficient in the reaction and no remark-
able differences of activity were found (Table 5, entries
1–4). Decreasing the catalyst loading (from 0.5 mol%
to 0.1 mol%) led to a lower yield of product (Table 5,
entry 11). Solvent played an important role in the reac-
tion and MeOH was the best choice (Table 5, entries
12–14). The hydrogenation process did not work well
in water (Table 5, entry 15), however, 1‐phenylethanol
was obtained in good yield when mixed solvent of
MeOH/H2O (v/v = 1/3) was used (Table 5, entries 16–
18). Having established the optimum conditions for the
reaction, a variety of substrates were employed in the
reaction. Corresponding alcohols were afforded in high
TABLE 6 Scope of the Catalytic Hydrogenation of Carbonyl Compoundsd
aReaction conditions: substrates (1.0 mmol), catalyst loading: 0.5 mol%, 80 °C, 3 hr, 5 atm, solvent: 2 ml,
MeOH/H2O (1/3). Yield was determined by GC analysis, n‐tridecane was used as internal standard.
bisolated yields.
FIGURE 4 1H NMR investigation of the formation of ruthenium hydride species.

8 of 13
QIAO ET AL.
yields by using complex 1 as the catalyst (Table 6, 6a–j).
The reaction of heterocycle aldehyde also occurred
smoothly and gave the product in good yield (6 k).
Interestingly, for ethyl acetoacetate, the –COR group
was selectively reduced by the catalytic system and the
ester group remained (6 l). In comparison with water‐
soluble ruthenium analogous, the catalytic efficiency of
the complexes in this work is higher for the hydrogena-
tion of the carbonyl substrates,31a but lower than that of
the bipyridine‐based half‐sandwich rhthenium complex
(for the aliphatic substrates).31b In addition, the catalytic
activity of the reported half‐sandwich ruthenium
complexes is lower than that of the similar catalysts
employed in the transfer hydrogenation reactions.31c, d
To elucidate the possible mechanism of the hydroge-
nation process, the reactivity between the catalyst and
H2 was investigated. The NMR tube containing the solu-
tion of 1 (in dry d8‐toluene) was pressurized with H2
(5 atm). The solution was then kept at 80 °C and mon-
itored by 1H NMR. A singlet was observed at δ
−12.77 ppm (Figure 4), which indicating that the forma-
tion of metal hydride species.[32] It is supported, by
analogy with the mechanism of catalytic activation of
hydrogen by ruthenium chloride, that this step involves
the heterolytic splitting of H2 with formation of the
hydrido ruthenium active species.[33]
The conclusion was further confirmed by the experi-
mental result that little hydrogenation takes place when
PMe3 was used as additive in the hydrogenation process
under standard reaction conditions (Table 5, entry 19).
As mentioned above, the hydrogenation benefits from
higher H2 pressure. So the reaction was monitored under
different pressure. Experimental results also showed that
there is almost a linear correlation between the conver-
sions and H2 pressure, indicating that H2 is probably
involved in the rate‐determining step of the hydrogena-
tion (Figure 5).
Based on the above experimental results and previous
reports, the possible mechanism has been proposed.
Under high temperature and pressure of H2, half‐
sandwich ruthenium catalyst reacted with H2 molecule
to give the hydrido ruthenium intermediate as the cata-
lytic active species. The catalytic cycle is made up of
three steps: reversible coordination of H2, turnover‐
FIGURE 5 Effect of H2 pressure on the hydrogenation process.
Reaction conditions: 1.0 mmol of acetonphenone, 0.5 mol% of
catalyst, H2, 80 °C for 20 min. The conversion was determined by
1H NMR
FIGURE 6 Possible mechanism of the
hydrogenation process

QIAO ET AL.
9 of 13
FIGURE 7 Reuse of the catalyst in the
hydrogenation and oxidation process
controlling heterolytic cleavage of H2 into a hydride and
a proton to be taken up by the ketone, and subsequent
transfer of the hydride to the resulting oxonium cation.
Analogous ionic mechanism have been previously
reported (Figure 6).[34,35]
The recycle of the catalyst in the hydrogenation
process was investigated based on these results
(Table 6). The catalyst was separated from the reaction
mixture by removing all volatiles under vacuum.
Complex 1 could be reused for at least five cycles with-
out any loss of activity (Figure 7 (left), 93%, 94%, 92%,
92%, 90% for five runs, respectively). However, the
decrease of the yields was obviously observed for the
oxidation reactions when the catalyst was reused for
the third cycle (Figure 5 (right), 93%, 85%, 76% for three
runs, respectively).
4 | EXPERIMENTAL SECTION
4.1 | General data
All reactions were performed under an atmosphere of
nitrogen using standard Schlenk techniques. Chemicals
were purchased from commercial source without further
1
purification. H NMR (500 MHz) spectra were measured
with a Bruker DMX‐500 spectrometer. Elemental analysis
was performed on an Elementar vario EL III analyzer.
UV/vis absorption spectra were recorded using a UV 765
spectrophotometer with 1 cm path length quartz cuvettes.
IR (KBr) spectra were measured with the Nicolet FT‐IR
spectrophotometer. Ligands L1–L4 were prepared using
the modified method according to the literature.[23]
4.2 | Synthesis of benzothiazole ligands
L1–L4
3 | CONCLUSIONS
The mixture of 2‐aminothiophenol (1 mmol) and corre-
sponding aromatic aldehyde (1.2 mmol, 1.2 equiv.) in tol-
uene (5 ml) in a 20 ml Schlenk tube adapted with an air
balloon was heated at 100 °C for 24 hr. The reaction
was then monitored by TLC. Column chromatography
of the crude products (PE: EA = 10: 1) gave L1–L4 in
good yields.
In summary, we have described a series of stable half‐
sandwich ruthenium complexes with the C, N‐coordina-
tion mode based on benzothiazole ligands. Such com-
plexes feature the catalytic activity for both alcohol
oxidation and hydrogenation of aldehydes and ketones
in aqueous media. Oxidations reactions are occurred in
water and proceed cleanly and rapidly with high TON
values. On the other hand, the ruthenium complexes also
showed high efficiency for the hydrogenation of carbonyl
compounds by using H2 as the hydrogen source. A survey
of various kinds of substrates showed that both electron‐
poor and electron‐rich groups are tolerance to the oxida-
tion and hydrogenation reaction. Moreover, the catalyst
can be reused several times without the loss of obvious
catalytic activity. The good stability and prominent cata-
lytic efficiency of the half‐sandwich ruthenium com-
plexes proved the possibility of their application in
industrial production.
L1: white solid, 85% isolated yield. 1H NMR (500 MHz,
CDCl3): δ 8.11–8.08 (m, Ar–H, 3H), 7.92 (d, J = 8.0 Hz,
Ar–H, 1H), 7.51–7.49 (m, Ar–H, 4H), 7.41 (t, J = 8.0 Hz,
Ar–H, 1H). IR (KBr, disk): υ 1646, 1509, 1478, 1384,
1313, 1224, 962, 766 cm−1. Elemental analysis calcd (%)
for C13H9NS: C 73.90, H 4.29, N 6.63, found: C 73.93, H
4.27, N 6.68.
L2: white solid, 83% isolated yield. 1H NMR (500 MHz,
CDCl3): δ 8.05 (t, J = 9.0 Hz, Ar–H, 3H), 7.89 (d,
J = 8.0 Hz, Ar–H, 1H), 7.50 (t, J = 7.5 Hz, Ar–H, 1H),
7.37 (t, J = 8.0 Hz, Ar–H, 1H), 7.01 (d, J = 9.0 Hz, Ar–
H, 2H), 3.89 (s, OCH3, 3H). IR (KBr, disk): υ 1605, 1521,

10 of 13
QIAO ET AL.
1485, 1310, 1302, 1225, 968, 833 cm−1. Elemental analysis
calcd (%) for C14H11NOS: C 69.68, H 4.59, N 5.80, found:
C 69.70, H 4.57, N 5.86.
C 57.43, H 4.61, N 2.91, found: C 57.45, H 4.63, N 2.90. MS
(ESI, m/z): 446 [M–Cl]+.
2: brown yellow solid, 53.0% yield. 1H NMR
(500 MHz, CDCl3) δ 8.31 (d, J = 8.0 Hz, Ar–H, 1H),
7.81–7.78 (m, Ar–H, 2H), 7.62–7.57 (m, Ar–H, 2H), 7.40
(t, J = 7.5 Hz, Ar–H, 1H), 6.63 (d, J = 6.0 Hz, Ar–H,
1H), 5.88 (d, J = 6.0 Hz, p‐cymene, 1H), 5.70 (d,
J = 6.0 Hz, p‐cymene, 1H), 5.39 (d, J = 6.0 Hz, p‐cymene,
1H), 5.12 (d, J = 6.0 Hz, p‐cymene, 1H), 3.94 (s, OCH3,
3H), 2.29–2.21 (m, CH (CH3)2, 1H), 2.12 (s, p‐cymene‐
CH3), 0.92 (d, J = 7.0 Hz, CH (CH3)2, 3H), 0.77 (d,
J = 7.0 Hz, CH (CH3)2, 3H). 13C NMR (100 MHz, CDCl3):
δ 185.6 (s, C=N), 173.8 (s, CAr(NS)–N=C), 160.2 (s, C–S),
151.1 (s, CAr), 132.8 (s, CAr), 131.7 (s, CAr), 127.1 (s,
CAr), 127.0 (s, CAr), 124.8 (s, CAr), 124.0 (s, CAr(NS)),
122.7 (s, CAr(NS)), 120.6 (s, CAr(NS)), 109.6 (s, CAr(NS)),
102.3 (s, Ccymene), 98.8 (s, Ccymene), 90.3 (s, Ccymene),
89.8 (s, Ccymene), 82.4 (s, Ccymene), 80.5 (s, Ccymene), 55.3
(s, OCH3), 30.9 (s, CH (CH3)2), 22.7 (s, CH3), 21.6 (s,
CH (CH3)2), 18.9 (s, CH (CH3)2). IR (KBr, disk): υ 1668,
1577, 1428, 1384, 1311, 1224, 1031, 985, 756 cm−1. Ele-
mental analysis calcd (%) for C24H24ClNORuS: C 56.41,
H 4.73, N 2.74, found: C 56.36, H 4.79, N 2.77. MS
(ESI, m/z): 476 [M–Cl]+.
L3: white solid, 80% isolated yield. 1H NMR (500 MHz,
CDCl3): δ 8.22 (d, J = 8.0 Hz, Ar–H, 2H), 8.12 (d,
J = 8.0 Hz, Ar–H, 1H), 7.94 (d, J = 7.5 Hz, Ar–H, 1H),
7.76 (d, J = 8.0 Hz, Ar–H, 2H), 7.55 (t, J = 8.0 Hz, Ar–
H, 1H), 7.45 (t, J = 7.5 Hz, Ar–H, 1H). IR (KBr, disk): υ
1618, 1593, 1484, 1323, 1253, 1173, 971, 760 cm−1. Ele-
mental analysis calcd (%) for C14H8F3NS: C 60.21, H
2.89, N 5.02, found: C 60.23, H 2.83, N 5.06.
L4: white solid, 75% isolated yield. 1H NMR (500 MHz,
CDCl3): δ 8.08 (d, J = 8.0 Hz, Ar–H, 1H), 8.04 (d,
J = 8.5 Hz, Ar–H, 2H), 7.92 (d, J = 7.5 Hz, Ar–H, 1H),
7.52–7.46 (m, Ar–H, 3H), 7.42 (t, J = 7.5 Hz, Ar–H, 1H).
IR (KBr, disk): υ 1643, 1589, 1474, 1399, 1251, 1090,
972, 756 cm−1
. Elemental analysis calcd (%) for
C13H8ClNS: C 63.54, H 3.28, N 5.70, found: C 63.55, H
3.30, N 5.60.
4.3 | Synthesis of ruthenium complexes 1–
4
1
3: orange red solid, 58.8% yield. H NMR (500 MHz,
A mixture of [(p‐cymene)RuCl2]2 (0.1 mmol), NaOAc
(0.4 mmol), and corresponding ligands L1–L4 (0.2 mmol)
was stirred at 50 °C in 10 ml of methanol for 6 hr. The
mixture was filtered and evaporated to give the crude
products which were further purified by silica gel column
chromatography (CH2Cl2: EA = 20: 1) to afford pure
cyclometalated half‐sandwich ruthenium complexes in
moderate yields.
CDCl3) δ 8.49 (d, J = 8.0 Hz, Ar–H, 1H), 8.41 (d,
J = 8.0 Hz, Ar–H, 1H), 7.91 (d, J = 8.0 Hz, Ar–H,
1H), 7.74 (d, J = 7.5 Hz, Ar–H, 1H), 7.69 (t,
J = 7.5 Hz, Ar–H, 1H), 7.52 (t, J = 7.5 Hz, Ar–H, 1H),
7.32 (d, J = 8.0 Hz, Ar–H, 1H), 5.96 (d, J = 6.0 Hz, p‐
cymene, 1H), 5.75 (d, J = 6.0 Hz, p‐cymene, 1H), 5.45
(d, J = 6.0 Hz, p‐cymene, 1H), 5.21 (d, J = 6.0 Hz, p‐
cymene, 1H), 2.26–2.20 (m, CH (CH3)2, 1H), 2.15 (s, p‐
cymene‐CH3), 0.91 (d, J = 7.0 Hz, CH (CH3)2, 3H),
0.76 (d, J = 7.0 Hz, CH (CH3)2, 3H). 13C NMR
1: brown solid, 54.5% yield. 1H NMR (500 MHz, CDCl3):
δ 8.39 (d, J = 8.5 Hz, Ar–H, 1H), 8.27 (d, J = 7.5 Hz, Ar–H,
1H), 7.86 (d, J = 8.0 Hz, Ar–H, 1H), 7.68 (d, J = 7.5 Hz, Ar–
H, 1H), 7.64 (t, J = 7.5 Hz, Ar–H, 1H), 7.45 (t, J = 7.5 Hz,
Ar–H, 1H), 7.24 (t, J = 7.5 Hz, Ar–H, 1H), 7.09 (t,
J = 7.5 Hz, Ar–H, 1H), 5.92 (d, J = 6.0 Hz, p‐cymene,
1H), 5.73 (d, J = 6.0 Hz, p‐cymene, 1H), 5.38 (d,
J = 6.0 Hz, p‐cymene, 1H), 5.14 (d, J = 6.0 Hz, p‐cymene,
1H), 2.27–2.21 (m, CH (CH3)2, 1H), 2.12 (s, p‐cymene‐
CH3), 0.91 (d, J = 7.0 Hz, CH (CH3)2, 3H), 0.76 (d,
J = 7.0 Hz, CH (CH3)2, 3H). 13C NMR (100 MHz, CDCl3):
δ 183.3 (s, C=N), 174.5 (s, CAr(NS)–N=C), 151.0 (s, C–S),
139.9 (s, CAr), 139.5 (s, CAr), 132.1 (s, CAr), 130.1 (s, CAr),
127.2 (s, CAr), 125.8 (s, CAr), 125.4 (s, CAr(NS)), 123.0 (s,
CAr(NS)), 122.8 (s, CAr(NS)), 121.2 (s, CAr(NS)), 102.5 (s,
Ccymene), 99.1 (s, Ccymene), 90.6 (s, Ccymene), 90.0 (s, Ccymene),
82.2 (s, Ccymene), 80.4 (s, Ccymene), 30.9 (s, CH (CH3)2), 22.7
(s, CH3), 21.7 (s, CH (CH3)2), 19.0 (s, CH (CH3)2). IR (KBr,
disk): υ 1646, 1575, 1431, 1383, 1317, 1263, 1040, 986,
757 cm−1. Elemental analysis calcd (%) for C23H22ClNRuS:
(100 MHz, CDCl3): δ 183.3 (s, C=N), 173.3 (s, CAr(NS)
–
N=C), 150.9 (s, C–S), 142.6 (s, CAr), 135.8 (s, 2C, CAr),
132.3 (s, CAr), 130.5 (s, CAr), 130.3 (q, J = 280 Hz,
CF3) 126.0 (s, CAr), 123.0 (s, CAr(NS)), 121.5 (s, CAr(NS)),
120.0 (s, CAr(NS)), 103.3 (s, CAr(NS)), 99.8 (s, CAr(NS)),
90.7 (s, Ccymene), 90.1 (s, Ccymene), 82.8 (s, Ccymene), 80.5
(s, Ccymene), 30.9 (s, CH (CH3)2), 22.6 (s, CH3), 21.6 (s,
CH (CH3)2), 19.0 (s, CH (CH3)2). IR (KBr, disk): υ
1646, 1532, 1434, 1385, 1316, 1167, 1108, 992, 768 cm
−1. Elemental analysis calcd (%) for C24H21ClF3NRuS:
C 52.50, H 3.86, N 2.55, found: C 52.39, H 3.90, N
2.75. MS (ESI, m/z): 514 [M–Cl]+.
4: brown yellow solid, 63.9% yield. 1H NMR (500 MHz,
CDCl3) δ 8.36 (d, J = 8.0 Hz, Ar–H, 1H), 8.22 (d,
J = 8.0 Hz, Ar–H, 1H), 7.87 (d, J = 8.0 Hz, Ar–H, 1H),
7.66 (t, J = 7.5 Hz, Ar–H, 1H), 7.60 (d, J = 8.5 Hz, Ar–
H, 1H), 7.48 (t, J = 7.5 Hz, Ar–H, 1H), 7.07 (d,
J = 8.0 Hz, Ar–H, 1H), 5.93 (d, J = 6.0 Hz, p‐cymene,

QIAO ET AL.
11 of 13
1H), 5.72 (d, J = 6.0 Hz, p‐cymene, 1H), 5.44 (d,
J = 6.0 Hz, p‐cymene, 1H), 5.17 (d, J = 6.0 Hz, p‐cymene,
1H), 2.27–2.21 (m, CH (CH3)2, 1H), 2.14 (s, p‐cymene‐
CH3), 0.92 (d, J = 7.0 Hz, CH (CH3)2, 3H), 0.77 (d,
J = 7.0 Hz, CH (CH3)2, 3H). 13C NMR (100 MHz, CDCl3):
δ 184.8 (s, C=N), 173.5 (s, CAr(NS)–N=C), 150.9 (s, C–S),
138.9 (s, CAr), 138.0 (s, CAr), 135.7 (s, CAr), 132.0 (s,
CAr), 127.3 (s, CAr), 126.5 (s, CAr), 125.6 (s, CAr(NS)),
123.4 (s, CAr(NS)), 122.9 (s, CAr(NS)), 121.1 (s, CAr(NS)),
103.0 (s, Ccymene), 99.4 (s, Ccymene), 90.6 (s, Ccymene), 89.9
(s, Ccymene), 83.0 (s, Ccymene), 80.5 (s, Ccymene), 30.9 (s,
CH (CH3)2), 22.6 (s, CH3), 21.7 (s, CH (CH3)2), 19.0 (s,
CH (CH3)2). IR (KBr, disk): υ 1646, 1540, 1425, 1384,
1319, 1130, 1079, 988, 763 cm−1. Elemental analysis calcd
(%) for C23H21Cl2NRuS: C 53.59, H 4.11, N 2.72, found: C
53.52, H 4.10, N 2.81. MS (ESI, m/z): 480 [M–Cl]+.
stirring for 3 hr, the autoclave was cooled and the pres-
sure was slowly released. The resultant mixture
extracted with diethyl ether (2 × 5 ml) and dried over
anhydrous Na2SO4. Solvent was evaporated under vac-
uum. The residue was dissolved in hexane and analyzed
by GC–MS. For isolated products: The reaction mixture
was extracted with diethyl ether (2 × 5 ml) and dried
over anhydrous Na2SO4. The extracts were evaporated
and the crude product was isolated by flash column
chromatography over silica using a hexane/EtOAc mix-
ture to give pure alcohol compounds following evapora-
tion. (For reuse of the catalyst: after GC analysis of the
reaction solution, all the reaction mixture was evapo-
rated under vacuum at high temperature (100 °C) for
1 hr (all volatiles were removed), then the residue solid
was used as the catalyst under standard reaction
conditions for the second cycle).
4.4 | General procedure for the oxidation
of alcohols
4.6 | X‐ray crystallography
In a typical run, a Schlenk tube was charged with alco-
hol (1.0 mmol), tBuOOH (3.0 mmol) and catalyst
(0.1 mol%) in water (2 ml). The resulting mixture was
stirred at 50 °C for 2 hr. Then, it was extracted with
diethyl ether (2 × 5 ml) and dried over anhydrous
Na2SO4. Solvent was evaporated under vacuum. The
residue was dissolved in hexane and analyzed by GC–
MS. For isolated products: The reaction mixture was
extracted with diethyl ether (2 × 5 ml) and dried over
anhydrous Na2SO4. The extracts were evaporated and
the crude product was isolated by flash column chroma-
tography over silica using a hexane/CH2Cl2 mixture to
give pure carbonyl compounds following evaporation.
(For reuse of the catalyst: after GC analysis of the
reaction solution, all the reaction mixture was evapo-
rated under vacuum at high temperature (100 °C) for
1 hr (all volatiles were removed), then the reaction of
the residue solid (using as catalyst), 1‐phenylethanol
Diffraction data of 1 and 3 were collected on a Bruker
Smart APEX CCD diffractometer with graphite‐
monochromated MoKa radiation (λ = 0.71073 Å). All
the data were collected at room temperature, and the
structures were solved by direct methods and subse-
quently refined on F 2 by using full‐matrix least‐squares
techniques (SHELXL).[36] SADABS[37] absorption correc-
tions were applied to the data, all non–hydrogen atoms
were refined anisotropically, and hydrogen atoms were
located at calculated positions. All calculations were
performed using the Bruker program Smart.
ACKNOWLEDGEMENTS
This work was supported by the National Natural
Science Foundation of China (No. 21601125), the
Chenguang Scholar of Shanghai Municipal Education
Commission (No. 16CG64), Natural Science Foundation
of Shanghai (No. 16ZR1435700), Open project of Key
Laboratory of Synthetic and Self‐Assembly Chemistry
for Organic Functional Molecules (K2018‐6), Shanghai
Science and Technology Committee (16DZ2270100),
Shanghai Young Teacher Training Program (No.
ZZZZyyx16005), Science and Technology Commission
of Shanghai Municipality (No. 1435700, 16DZ2270100),
and Shanghai Municipal Education Commission, (No.
16CG64).
t
(1.0 mmol), BuOOH (3.0 mmol) in water (2 ml) was
performed).
4.5 | General procedure for the
hydrogenation of ketones
In a typical run, the substrate (1.0 mmol), catalyst
(0.5 mol%) and MeOH/H2O (2 ml, v/v = 1/3) were
charged in a 5 ml vial with a magnetic bar. The vial
was then transferred to an autoclave. The autoclave
was purged with H2 (5 atm) via three cycles of
pressurization/venting, then pressurized with H2
(5 atm) and disconnected from the H2 source. The auto-
clave was heated to the desired temperature. After
CONFLICT OF INTEREST
There are no conflicts of interest to declare.

12 of 13
QIAO ET AL.
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ORCID
Zi‐Jian Yao https://orcid.org/0000-0003-1833-1142
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SUPPORTING INFORMATION
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in the Supporting Information section at the end of the
article.
How to cite this article: Qiao X‐C, Qiu D‐C, Lin
N, Zhu J‐W, Deng W, Yao Z‐J. Half‐sandwich
ruthenium‐based versatile catalyst for both alcohol
oxidation and catalytic hydrogenation of carbonyl
compounds in aqueous media. Appl Organometal
Chem. 2019;e4875. https://doi.org/10.1002/aoc.4875
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