Trinuclear ruthenium carbonyl complexes with salicylaldimine ligands as efficient catalysts for the oxidation of secondary alcohols

Article abstract of DOI:10.1016/j.jorganchem.2020.121647

A series of novel trinuclear ruthenium carbonyl complexes [μ-?²-2-OC₆H₄-CH=N-Ar)]₂Ru₃(CO)₈ [Ar = Ph (8), C₆H₄-4-Me (9), C₆H₄-4-CF₃ (10), C₆H₄-4-Cl (11), C₆H₃-2,6-Me₂ (12), C₆H₃-2,6-Et₂ (13)] and [μ-?²-2-OC₆H₄-CH=N-C₆H₃-2,6-ⁱPr₂]Ru₃(CO)₉ (14) were designed and synthesized. All the seven novel complexes were fully characterized by elemental analysis, IR and NMR spectroscopy. Molecular structures of 8, 11, 13 and 14 were further confirmed by single-crystal X-ray diffraction. The catalytic performance of these complexes in the oxidation of secondary alcohols was explored and it was found the combination of such complexes and N-methylmorpholine-N-oxide (NMO) exhibits high catalytic activities for the oxidation of secondary alcohols, giving the corresponding carbonyl compounds in excellent yields.

Full text of DOI:10.1016/j.jorganchem.2020.121647

Journal of Organometallic Chemistry 932 (2021) 121647
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Trinuclear ruthenium carbonyl complexes with salicylaldimine ligands
as efficient catalysts for the oxidation of secondary alcohols
Zhiqiang Haoa,1, Ying Lia,1, Zhihong Mab,∗, Zhangang Hana, Jin Lina,∗, Guo-Liang Luc
a National Demonstration Center for Experimental Chemistry Education, Hebei Key Laboratory of Organic Functional Molecules, College of Chemistry &
Material Science, Hebei Normal University, Shijiazhuang 050024, China
b School of Pharmacy, Hebei Medical University, Shijiazhuang 050017, China
c Auckland Cancer Society Research Centre, Faculty of Medical and Health Sciences, The University of Auckland, Private Bag 92019, Auckland 1142, New
Zealand
a r t i c l e i n f o
a b s t r a c t
A series of novel trinuclear ruthenium carbonyl complexes [μ-ƞ2-2-OC6H4-CH=N-Ar)]2Ru3(CO)8 [Ar = Ph
(8), C6H4-4-Me (9), C6H4-4-CF3 (10), C6H4-4-Cl (11), C6H3-2,6-Me2 (12), C6H3-2,6-Et2 (13)] and [μ-ƞ2-2-
OC6H4-CH=N-C6H3-2,6-iPr2]Ru3(CO)9 (14) were designed and synthesized. All the seven novel complexes
were fully characterized by elemental analysis, IR and NMR spectroscopy. Molecular structures of 8, 11,
13 and 14 were further confirmed by single-crystal X-ray diffraction. The catalytic performance of these
complexes in the oxidation of secondary alcohols was explored and it was found the combination of such
complexes and N-methylmorpholine-N-oxide (NMO) exhibits high catalytic activities for the oxidation of
secondary alcohols, giving the corresponding carbonyl compounds in excellent yields.
Article history:
Received 6 November 2020
Revised 1 December 2020
Accepted 1 December 2020
Available online 3 December 2020
Keywords:
Salicylaldimine
Ruthenium complexes
Alcohol oxidation
NMO
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
which could oxidize a wide variety of primary and secondary al-
cohols to the respective aldehydes and ketones with moderate to
Transformation of alcohols to the corresponding carbonyl
derivatives is one of the fundamental chemical reactions both
in organic synthesis research and fine chemical industries [1–4].
Tremendous efforts have been devoted to the progress of catalysts
for alcohol oxidation in the past decade [5–13]. It is a pivotal sub-
ject for scientists to search for environment-friendly oxidants in-
stead of the toxic and hazardous stoichiometric traditional oxidants
and inexpensive, efficient and recyclable catalysts. Transition metal
complexes supported by the ligands including N- and N,O-donors
have been accepted as catalysts for the oxidation of alcohols [14–
18]. Among the transition metal catalysts, ruthenium complexes
are intensively probed due to their diverse valence states and
rich structures. Besides, stable nitroxyl radicals, such as (2,2,6,6-
tetramethyl-1-piperidinyloxyl (TEMPO), 9-azabicyclo[3.3.1]nonane
N-oxyl (ABNO), particularly N-methylmorpholine-N-oxide (NMO),
have been used as oxidant for the moderate and selective oxida-
tion of primary, secondary, cyclic, allylic, aliphatic, and benzylic
alcohols to their corresponding carbonyl compounds [19–22]. For
instance, Mondal and co-workers reported two ruthenium com-
plexes cis-(CO)-trans-(X)-[Ru(CO)2(HL)X2] (where X = Cl and I),
high conversions in presence of NMO [23].
Schiff bases are well-known ligands and Schiff bases-
coordinated metal complexes not only have
a wide range of
applications in many fields including analytical, pharmaceutical
and biological, but also play an important role in organic syn-
thesis and catalysis [24–32]. In the past few years, our team
has reported some ruthenium complexes supported by pyridine-
imine/pyridylalkanol ligands and their catalytic behavior in the
oxidation of alcohols [33–36]. The chelation effects of Schiff base
ligands inspired us to explore the structural modification of the
triruthenium unit by varying the side chain of the Schiff base lig-
ands. In this paper, we present the synthesis and characterization
of several new ruthenium carbonyl complexes with salicylaldimine
ligands. Furthermore the catalytic activity of these complexes for
the oxidation of secondary alcohols in the presence of NMO as
oxidant was also studied.
2. Results and discussion
2.1. Synthesis of trinuclear ruthenium carbonyl complexes 8-14
The precursor, salicylaldimine ligands [2-HOC6H4-CH=N-Ar]
∗
Corresponding authors.
[Ar
=
C6H5 (1); C6H4-4-Me (2); C6H4-4-CF3 (3); C6H4-4-Cl
E-mail addresses: mazhihong@hebmu.edu.cn (Z. Ma), linjin@hebtu.edu.cn (J. Lin).
These authors contributed equally to this work.
(4); C6H3-2,6-Me2 (5); C6H3-2,6-Et2 (6); C6H3-2,6-iPr2 (7)] were
1
https://doi.org/10.1016/j.jorganchem.2020.121647
0022-328X/© 2020 Elsevier B.V. All rights reserved.

Z. Hao, Y. Li, Z. Ma et al.
Journal of Organometallic Chemistry 932 (2021) 121647
Scheme 1. Synthesis of complexes 8-14.
Fig. 2. Molecular structure of complex 14, showing 30% probability ellip-
soids (H atoms are omitted for clarity). Selected bond parameters: Ru(1)-O(1)
2.102(11), Ru(2)-O(1) 2.143(11), Ru(1)-N(1) 2.162(15), Ru(1)-Ru(2) 2.7943(17), Ru(1)-
Ru(3) 2.7745(19), Ru(2)-Ru(3) 2.793(2), C(20)-Ru(1)-O(1) 99.7(7), C(20)-Ru(1)-
N(1) 101.9(7), O(1)-Ru(1)-N(1) 86.2(5), O(1)-Ru(1)-Ru(3) 83.2(3), N(1)-Ru(1)-Ru(3)
163.5(4), Ru(1)-Ru(3)-Ru(2) 60.25(5).
Fig. 1. Molecular structure of complex 8, showing 30% probability ellipsoids (H
atoms are omitted for clarity). Selected bond parameters: Ru(1)-O(1) 2.110(3),
Ru(2)-O(2) 2.106(3), Ru(1)-N(1) 2.195(4), Ru(2)-N(2) 2.181(4), Ru(1)-Ru(2) 3.0586(7),
Ru(1)-Ru(3) 2.7896(7), Ru(2)-Ru(3) 2.7867(6), C(27)-Ru(1)-O(1) 97.57(19), O(1)-
Ru(1)-O(2) 75.56(12), C(27)-Ru(1)-N(1) 97.0(2), O(1)-Ru(1)-N(1) 84.50(14), O(1)-
Ru(1)-Ru(3) 83.43(9), Ru(2)-Ru(3)-Ru(1) 66.528(17).
prepared following the previously reported procedure [37]. Sub-
sequently, the corresponding trinuclear ruthenium carbonyl com-
plexes [μ-ƞ2-2-OC6H4-CH=N-Ar]2Ru3(CO)8 [Ar = Ph (8), C6H4-4-
Me (9); C6H4-4-CF3 (10); C6H4-4-Cl (11); C6H3-2,6-Me2 (12); C6H3-
2,6-Et2 (13)] and [μ-ƞ2-2-OC6H4-CH=N-C6H3-2,6-iPr2]Ru3(CO)9
(14) were obtained in 33-83% yields by treating Ru3(CO)12 with the
ligands 1-7 respectively (Scheme 1). All the new complexes were
purified by column chromatography, and well characterized by el-
emental analysis, IR and NMR spectroscopy. Their IR spectra reveal
strong absorption bands at 2088-1930 cm−1 for the stretching fre-
quencies of the terminal carbonyls, ν(CO). In the proton NMR spec-
tra, the phenyl protons show peaks in the range of 6.34~7.49 ppm
and the imine (CH=N) protons shows a singlet at about 7.80 ppm.
In the 13C NMR spectra, the carbonyls show resonances from 192.0
to 205.0 ppm.
long to monoclinic crystal system with P2(1)/c space group, while
complex 13 crystallizes in triclinic crystal system with space P-1
group. X-ray diffraction analyses that these complexes present sim-
ilar molecular configurations, in which two salicylaldimines ligands
are coordinated to the metal Ru and each salicylaldimine ligand is
coordinated to a ruthenium atom forms stable six-membered ring.
A μ2-O atom acts as a three-electron donor that bridges Ru(1) and
Ru(2) in these complexes. The distance between the two neigh-
boring unbonded Ru(1) and Ru(2) atoms in complexes 8, 11 and
˚
13 is in the range of 3.058-3.087 A, which is larger than the nor-
˚
mal bonded Ru-Ru distances (2.70-2.95 A). The bond distances of
˚
˚
˚
˚
Ru-N [2.195(4) A, 2.181(4) A for 8; 2.168(3) A, 2.216(3) A for 11;
˚
˚
˚
2.198(5) A, 2.189(6) A for 13] are close to that of 2.221(3) A and
˚
2.210(3) A in complex [PyCH2CH(Ph)O]2Ru3(CO)8 [38]. The bond
angles of Ru(1)-O-Ru(2) [91.90(13)°, 91.95(13)° for 8; 92.77(10)°,
92.46(10)° for 11; 92.78(17)°, 91.89 (17)° for 13] are slightly bigger
than that those in complexes [PyCH=C(Ph)O]2Ru3(CO)8 [39] and
Ru3(CO)8(μ-OC6H4OMe-2)2 [40] [90.78°, 90.78° and 89.9°, 90.7°
respectively]. The Ru-C bond distances in the carbonyl ligands are
in normal range and similar to the values from Ru3(CO)12 [41].
Complex 14 has only one molecular of salicylaldimine ligand
coordinated to the Ru3(CO)9 unit owing to the steric hindrance
of the two iso-propyl groups. Three ruthenium atoms formed an
approximately isosceles triangle with two longer Ru-Ru distances
2.2. Single crystal X-ray diffraction
Complexes 8, 11, 13 and 14 were further characterized by X-
ray single-crystal analysis. Perspective views of each structure are
shown in Figs. 1 and 2 and Figs. S1-S2, with selected bonds angles
and distances, respectively. The experimental details and crystallo-
graphic data are summarized in Table S1 (see Supporting Informa-
tion).
˚ ˚
[Ru(1)-Ru(2) 2.7943(17) A; Ru(2)-Ru(3) 2.793(2) A] and one short
As shown in Figs. 1 and 2 and Figs. S1-S2, complexes 8, 11 and
13 are similar trinuclear ruthenium carbonyl complexes. Crystal-
lographic data indicate that crystals of complexes 8 and 11 be-
˚
Ru-Ru distance [Ru(1)-Ru(3) 2.7745(19) A]. The Ru-O and Ru-N
˚
bond lengths are 2.102(11), 2.143 (11) and 2.162 (15) A respectively,
2

Z. Hao, Y. Li, Z. Ma et al.
Journal of Organometallic Chemistry 932 (2021) 121647
Table 1
Table 3
Catalytic oxidation of different alcohols using complex 14a.
Optimization of oxidation of 1-phenylethanol using complex 8a.
Entry
Substrate
Product
Yieldb [%]
Entry Cat.8 [mol%] Oxidant (mmol) Temp [°C] Solvent Time [h] Yieldb [%]
1
1.0
1.0
1.0
1.0
1.0
1.0
–
H2O2 ( 3.0)
NMO (1.0)
NMO (2.0)
NMO (3.0)
NMO (4.0)
–
80
80
80
80
80
80
80
80
80
80
80
80
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
CH3CN
toluene
acetone
CH3CN
6
6
6
6
6
6
6
6
6
1
33
38
49
66
55
–
2
1
2
83
96
3
4
5
6
7
NMO (3.0)
NMO (3.0)
NMO (3.0)
NMO (3.0)
NMO (3.0)
NMO (3.0)
7
8
1.0
1.0
1.0
1.0
1.0
26
73
84
84
70
9
10
11
12
CH3CN 0.5
CH3CN 0.3
3
93
a
Reaction conditions: 1.0 mmol 1-phenylethanol, 5 mL solvent.
Isolated yield was determined by column chromatography.
b
4
5
90
92
Table 2
Catalytic oxidation of 1-phenylethanol using complexes 8-14a.
6
89
Entry
Cat.
Yieldb [%]
1
2
3
4
5
6
7
8
84
85
62
65
78
79
83
9
7
8
80
86
10
11
12
13
14
a
Reaction conditions:1.0 mmol substrate, 1.0 mol% catalyst, 3.0
mmol NMO, 5 mL acetonitrile, 80°C, 30 min.
b
9
>99
Yield was determined by column chromatography.
10
95
which are consistent with the reported values in the similar Ru-
Schiff base complexes [42].
11c
12c
95
2.3. Catalytic application
In order to develop an optimal catalytic system, we set out to
evaluate various experimental conditions using 1-phenylethanol as
the model substrate and complex 8 as the catalyst. The results are
summarized in Table 1. Using H2O2 as oxidant, the reaction gave
the acetophenone in only 33% yield (Table 1, entry 1). The yield in-
creased to 38% by replacing H2O2 with NMO and further improved
to 66% using 3 equiv. of NMO (Table 1, entries, 2-4). But the yield
of acetophenone decreased significantly when 4 equiv. of NMO was
employed (Table 1, entry 5). In the absence of either NMO or the
Ru catalyst, the formation of acetophenone was not observed or
its yield was very poor (Table 1, entries 6, 7). Then, the solvent
effect on the reaction was screened. It was found that CH3CN was
the best solvent. Eventually, a high yield of 84% was achieved using
CH3CN as the solvent and NMO (3.0 mmol) as the oxidant (Table 1,
entry 10). To find out the best reaction duration, two more entries
were carried (Table 1, entries 11-12) and 30 min was found to be
a suitable reaction time. In summary, the pre-optimized condition
is as follows. A solution of 1-phenylethanol (1.0 mmol), complex 8
(1.0% equiv.) and NMO (3 equiv.) in acetonitrile (5 mL) was stirred
at 80°C for 30 min under 1 atmosphere of argon.
92
94
13c
a
Reaction conditions:1.0 mmol alcohol, 1.0 mol% catalyst, 3.0 mmol
NMO, 5 mL acetonitrile, 80°C, 30 min.
b
Yield was determined by column chromatography.
c
Yield was analyzed by GC, 3h.
Subsequently, in order to select the best catalyst, the other six
Ru complexes 9-14 were then tested under the pre-optimized re-
action condition. The results in Table 2 show each of the com-
plexes 8-14 appears highly active in catalyzing oxidation of 1-
phenylethanol. 8, 9 and 14 are more active than the others, and
the catalytic activities of these three complexes are very similar
(Table 2, entries 1, 2 and 7). According to the earlier reports [43–
45], the possible mechanism of such catalytic oxidation involves
RuIV = O species. Generally, complexes with only one ligand have
been found to be more efficient than the analogous complexes
with two ligands, possibly because the space of Ru metal centers
3

Z. Hao, Y. Li, Z. Ma et al.
Journal of Organometallic Chemistry 932 (2021) 121647
Taken previous reports into account [20,23,35,36,46], a possible
mechanism for the oxidation of secondary alcohols catalyzed by Ru
complex 14 is depicted in Scheme 2. Initially, Ru complex 14 is ox-
idized by one molecule of NMO to form a oxo-ruthenium species
A, which then reacts with substrate alcohol to give a Ru-alcoholate
intermediate B. Subsequently, B undergoes β-hydride elimination
to produce the ketone, along with one molecule of H2O and a
coordinatively-unsaturated coordination unsaturated species D. Fi-
nally, D is oxidized by another NMO to regenerate the species A to
finish the catalytic cycle.
3. Conclusions
The reactions of seven salicylaldimine ligands [2-HOC6H4-
CH=N-Ar] [Ar = Ph (1); C6H4-4-Me (2); C6H4-4-CF3 (3); C6H4-4-
Cl (4); C6H3-2,6-Me2 (5); C6H3-2,6-Et2 (6); C6H3-2,6-iPr2 (7)] with
Ru3(CO)12 were investigated. All the novel Ru complexes were fully
characterized by spectra analysis. The X-ray diffraction analysis of
complexes 8, 11, 13 and 14 confirmed the trinuclear structure. The
sterically hindered salicylaldimine ligands (such as 7) prefer to
form single-ligand-coordinated complexes (such as 14). In addition,
the catalytic performance of these ruthenium carbonyl complexes
has been investigated preliminarily. The combination of [μ-η2-2-
OC6H4-CH=N-C6H3-2,6-iPr2]Ru3(CO)9 (14) and NMO showed high
efficiency and broad functional group compatibility toward the ox-
idation of secondary alcohols to form the ketone derivatives in ex-
cellent yields. Efforts to discover more unusual ruthenium com-
plexes and their applications in catalysis are currently in progress.
4. Experimental
4.1. General procedure
All manipulations were carried out under an inert argon atmo-
sphere using standard Schlenk lines. Solvents were distilled from
appropriate drying agents under nitrogen before use. All the other
chemical reagents and starting materials were purchased from
commercial sources and used as received unless otherwise indi-
cated. 1H and 13C NMR spectra were measured on a Bruker AV III-
500 instrument in CDCl3. Infrared spectra were recorded as KBr
disks using a Thermo Fisher iS50 spectrometer. Elemental analy-
ses were performed on a VarioEL Ⅲ analyzer. X-ray intensity data
were measured on a Bruker AXS SMART 1000 CCD diffractometer
Scheme 2. Proposed mechanism for the oxidation of secondary alcohols.
is more open, and it is easier for substrates to coordinate. Thus
complex 14 was chosen as the optimized catalyst.
Lastly, the substrate scope was examined using complex
˚
with graphite monochromated Mo Kα (λ = 0.71073 A) radiation.
14 as the catalyst. As shown in Table 3,
a variety of sec-
ondary alcohols can be oxidized efficiently to furnish the cor-
responding ketones in high to excellent yields. The present
condition worked well for the 1-phenylethanols with electron-
donating or electron-withdrawing substitutes at different posi-
tions (Table 3, entries 2-8). Among them, those with electron-
donating groups such as methoxy and methyl (Table 3, entries 2-
3) gave higher yield than those with electron-withdrawing groups
(Table 3, entries 4-8). Excellent yields were also obtained for
oxidation of diphenylmethanol and 1-(naphthalen-2-yl)ethan-1-ol
(Table 3, entries 9-10), which are equally efficient compared with
[RuCl(L)(CO)(PPh3)2]{L = N-[di(alkyl/aryl)carbamothioyl benzamide
derivatives]}/NMO system [20]. To our delight, three less-reactive
second alkyl alcohols, i.e. cyclopentanol, cyclohexanol and 2-
hexanol proceeded smoothly to give the corresponding ketones in
> 90 yields by simply extending the reaction time to 3 h (Table 3,
entries 11-13). The present catalytic system is highly efficient
for these substrates and yields of the products are higher than
those obtained from [Ru(L)(PPh3)Cl2]]{L = 2-(phenylthio/seleno)-
N-(pyridin-2-ylmethyl)ethan-amine}/NMO [46] and our previously
reported [PyCH=C(RC6H4)O]2Ru3(CO)8] (R = 4-OMe, 4-Br and 4-
CF)/NMO catalytic systems [36].
4.2. [μ-η2-2-OC6H4-CH=N-Ph]2Ru3(CO)8 (8)
In a round-bottom flask were added [2-HOC6H4-CH=N-C6H5]
(1) (0.472 g, 2.4 mmol) and Ru3(CO)12 (0.30 g, 0.47 mmol)
and 20 mL toluene. The reaction mixture was stirred at reflux
for 9 hours. After the mixture was cooled to room tempera-
ture, the solvent was evaporated under reduced pressure. The
residue was chromatographed on an Al2O3 column with petroleum
ether/ethyl acetate (v/v = 8:1) as the eluent. Pure crystals suit-
able for X-ray crystallography were obtained by recrystallization
from dichloromethane and n-hexane. 8: yellow crystals (0.146 g,
33% yield). M.p. 169.0~169.5°C. Anal. Calcd. for C34H20N2O10Ru3:
C, 44.40; H, 2.19; N, 3.05. Found (%): C, 44.19; H, 2.08; N, 2.98;
1H NMR (CDCl3, 500 MHz): δ 6.49 (t, 4H, J = 8.0 Hz, Ar-H), 6.70
(d, 2H, J = 8.5 Hz, Ar-H), 6.84 (t, 2H, J = 7.5 Hz, Ar-H), 7.11 (q,
4H, J = 7.5 Hz, Ar-H), 7.22 (d, 6H, J = 6.5 Hz, Ar-H), 7.82 (s, 2H,
N=CH); 13C NMR (CDCl3, 125 MHz): δ 118.9, 122.2, 122.4, 123.1,
126.6, 128.6, 135.9, 136.2, 155.8, 162.7, 165.4, 192.6, 202.6, 203.9,
205.2; IR (υCO, KBr, cm−1): 2047 (m), 2027 (m), 1996 (s), 1985 (s),
1935 (m).
4

Z. Hao, Y. Li, Z. Ma et al.
Journal of Organometallic Chemistry 932 (2021) 121647
4.3. [μ-η2-2-OC6H4-CH=N-C6H4-4-Me]2Ru3(CO)8 (9)
CH3), 1.22 (d, 3H, J = 6.0 Hz, CH3), 1.44 (d, 6H, J = 6.0 Hz, CH3),
3.28-3.40 (m, 2H, CH), 7.03 (t, 1H, J = 7.5 Hz, Ar-H), 7.21 (d, 1H,
J = 7.5 Hz, Ar-H), 7.28-7.37 (m, 4H, Ar-H), 7.56 (t, 1H, J = 7.5 Hz,
Ar-H), 7.97 (s, 1H, N=CH); 13C NMR (CDCl3, 125 MHz): δ 23.2, 24.9,
25.8, 27.9, 28.0, 120.2, 121.3, 123.5, 124.2, 124.4, 127.4, 135.8, 136.2,
138.8, 151.5, 164.5, 167.1, 180.5, 190.9, 194.4, 197.6, 203.4, 203.6,
204.9; IR (υCO, KBr, cm−1): 2088 (s), 2052 (s), 2001 (s), 1969 (m),
1951 (m).
Yield: 0.239 g, 53%. M.p. 176.4~177.6°C. Anal. Calcd. for
C36H24N2O10Ru3: C, 45.62; H, 2.55; N, 2.96. Found (%): C, 45.24;
H, 2.31; N, 2.59; 1H NMR (CDCl3, 500 MHz): δ 2.36 (s, 6H, CH3),
6.37 (d, 4H, J = 8.0 Hz, Ar-H), 6.70 (d, 2H, J = 7.5 Hz, Ar-H), 6.83
(t, 2H, J = 7.0 Hz, Ar-H), 7.01 (d, 4H, J = 8.0 Hz, Ar-H), 7.10 (t, 4H,
J = 7.5 Hz, Ar-H), 7.78 (s, 2H, N=CH); 13C NMR (CDCl3, 125 MHz):
δ 21.0, 118.9, 122.2, 122.3, 123.2, 129.1, 135.8, 136.1, 136.5, 153.7,
162.6, 165.2, 192.8, 202.7, 204.0, 205.3; IR (υCO, KBr, cm−1): 2010
(m), 2001 (s), 1989 (s), 1937 (m).
4.9. Catalytic oxidation of secondary alcohols
In general, the catalytic reaction was carried out under the
following conditions: 1.0 mmol of substrate alcohol, 1.0 mol% of
ruthenium complex and 3.0 mmol of NMO were mixed in 5.0 ml
of CH3CN. The solution was stirred at 80°C for 30 min under 1 at-
mosphere of argon. Solvent was removed under reduced pressure,
pure products were isolated by Al2O3 column and further identi-
fied by comparison with the authentic sample through 1H NMR
spectra.
4.4. [μ-η2-2-OC6H4-CH=N-C6H4-4-CF3]2Ru3(CO)8 (10)
Yield: 0.213 g, 42%. M.p. 210.2~210.6°C. Anal. Calcd. for
C36H18 F6N2O10Ru3: C, 39.25; H, 1.65; N, 2.54. Found (%): C, 39.01;
H, 1.41; N, 2.35; 1H NMR (CDCl3, 500 MHz): δ 6.58 (d, 4H, J = 8.0
Hz, Ar-H), 6.69 (d, 2H, J = 8.4 Hz, Ar-H), 6.89 (t, 2H, J = 7.2 Hz,
Ar-H), 7.11-7.17 (m, 4H, Ar-H), 7.49 (d, 4H, J = 8.4 Hz, Ar-H), 7.84
(s, 2H, N=CH); 13C NMR (CDCl3, 125 MHz): δ 29.2, 119.5, 122.2,
123.0, 123.2, 126.1, 136.6, 136.7, 158.0, 163.3, 166.1, 180.3, 191.9,
202.3, 203.5, 205.1; IR (υCO, KBr, cm−1): 2050 (s), 2010 (m), 1981
(s), 1938(m).
4.10. Procedure for the synthesis of acetophenone
1-phenylethanol (1.0 mmol, 0.122 g), NMO (3.0 mmol, 0.117 g)
and complex 14 (1.0 mol%, 0.0084 g) were mixed in 5.0 ml of
CH3CN. The solution was stirred at 80°C for 30 min under 1 atmo-
sphere of argon. The reaction mixture was cooled down to room
temperature and the solvent was directly concentrated under vac-
uum. The resulting residue was subjected to Al2O3 column chro-
matography (petroleum ether: ethyl acetate 10 : 1, v/v) to afford
the desired product acetophenone as a light yellow oil (0.1201 g,
83%), which was identified by NMR analysis.
4.5. [μ-η2-2-OC6H4-CH=N-C6H4-4-Cl]2Ru3(CO)8 (11)
Yield: 0.307 g, 64%. M.p. 185.6~186.2°C. Anal. Calcd. for
C34H18 Cl2N2O10Ru3: C, 40.31; H, 1.84; N, 2.83. Found (%): C, 39.98;
H, 1.65; N, 2.63; 1H NMR (CDCl3, 500 MHz): δ 6.39 (d, 4H, J = 8.5
Hz, Ar-H), 6.72 (d, 2H, J = 8.5 Hz, Ar-H), 6.86 (t, 2H, J = 7.5 Hz, Ar-
H), 7.12-7.29 (m, 8H, Ar-H), 7.78 (s, 2H, N=CH); 13C NMR (CDCl3,
125 MHz): δ 119.2, 122.1, 123.1, 123.7, 128.8, 132.3, 136.3, 136.3,
154.1, 162.9, 165.6, 192.1, 202.4, 203.6, 205.6; IR (υCO, KBr, cm−1):
2076 (m), 2015 (s), 2002 (s), 1988 (m), 1936 (m).
4.11. Crystallographic studies
4.6. [μ-η2-2-OC6H4-CH=N-C6H3-2,6-Me2]2Ru3(CO)8 (12)
Crystals of complexes 8, 11, 13 and 14 (CCDC 1479557, 1523388,
1538116, 1558723) of
a quality suitable for the X-ray deter-
minations were grown by slowly layering hexane onto their
dichloromethane solutions at room temperature. Data were col-
Yield: 0.206 g, 42%. M.p. 178.3~178.9°C. Anal. Calcd. for
C38H28N2O10Ru3: C, 46.77; H, 2.89; N, 2.87. Found (%): C, 46.98; H,
2.71; N, 2.65; 1H NMR (CDCl3, 500 MHz): δ 1.14 (s, 6H, CH3), 2.43
(s, 6H, CH3), 6.65 (d, 2H, J = 8.0 Hz, Ar-H), 6.69 (d, 2H, J = 7.5 Hz,
Ar-H), 6.82 (t, 2H, J = 7.0 Hz, Ar-H), 7.01 (t, 2H, J = 7.5 Hz, Ar-H),
7.13-7.21 (m, 6H, Ar-H), 7.77 (s, 2H, N=CH); 13C NMR (CDCl3, 125
MHz): δ 16.9, 19.1, 118.9, 121.0, 123.5, 125.9, 128.2, 128.5, 128.9,
129.1, 135.4, 136.2, 152.5, 163.7, 166.2, 192.4, 201.6, 203.4, 205.1; IR
(υCO, KBr, cm−1): 2071 (s), 2041 (m), 1991 (s), 1963 (m), 1930 (s).
lected on a Bruker AXS SMART 1000 CCD diffractometer with
˚
graphite-monochromated Mo Kα radiation (λ
=
0.71073 A).
Semiempirical absorption corrections were applied for all com-
plexes. All non-hydrogen atoms were refined anisotropically and
hydrogen atoms were introduced into calculated positions with
the displacement factors of the host carbon atoms. The structures
were refined on F2 by full-matrix least-squares methods using the
SHELXL-97 program package.
4.7. [μ-η2-2-OC6H4-CH=N-C6H3-2,6-Et2]2Ru3(CO)8 (13)
Declaration of Competing Interest
Yield: 0.412 g, 83%. M.p. 177.9~178.6°C. Anal. Calcd. for
C42H36N2O10Ru3: C, 48.88; H, 3.52; N, 2.71. Found (%): C, 48.96; H,
3.18; N, 2.34; 1H NMR (CDCl3, 500 MHz): δ 0.54 (t, J = 8.5 Hz, 6H,
CH3), 1.28 (t, J = 8.5 Hz, 6H, CH3), 1.19-1.25 (m, 2H, CH2), 1.77-1.85
(m, 2H, CH2), 2.79-2.94 (m, 4H, CH2), 6.63 (d, J = 8.5 Hz, 2H, Ar-H),
6.75 (d, J = 7.5 Hz, 2H, Ar-H), 6.82 (t, 2H, J = 7.5 Hz, Ar-H), 7.13-
7.17 (m, 4H, Ar-H), 7.20-7.24 (m, 4H, Ar-H), 7.84 (s, 2H, N=CH); 13C
NMR (CDCl3, 125 MHz): δ 15.0, 15.3, 23.4, 24.6, 118.9, 121.0, 123.6,
125.9, 126.4, 126.8, 134.2, 134.5, 135.2, 136.2, 151.5, 163.8, 165.9,
192.3, 201.4, 203.3, 204.8; IR (υCO, KBr, cm−1): 2071 (m), 1996 (s),
1965 (s), 1933 (m).
The authors declare no competing financial interest.
Acknowledgements
This work was supported by the Hebei Natural Science Foun-
dation of China (nos. B2017205006 and B2019205087), the Edu-
cation Department Foundation of Hebei Province (nos. ZD2018005
and QN2019036), and the Science Foundation of Hebei Normal Uni-
versity (nos. L2017Z02 and L2018B08).
4.8. [μ-η2-2-OC6H4-CH=N-C6H3-2,6-iPr2]Ru3(CO)9 (14)
Supplementary materials
Yield: 0.303 g, 78% yield. M.p. 190.2~190.7°C. Anal. Calcd. for
C28H22NO10Ru3: C, 40.24; H, 2.65; N, 1.68. Found (%): C,40.75; H,
2.41; N, 1.37; 1H NMR (CDCl3, 500 MHz): δ 1.13 (d, 3H, J = 6.0 Hz,
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2020.
121647.
5

Z. Hao, Y. Li, Z. Ma et al.
Journal of Organometallic Chemistry 932 (2021) 121647
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