Ruthenium carbonyl complexes supported by pyridine-alkoxide ligands: Synthesis, structure and catalytic oxidation of secondary alcohols

Article abstract of DOI:10.1039/c9nj03222c

Five novel trinuclear ruthenium complexes [[PyCHC(RC₆H₄)O]₂Ru₃(CO)₈] [R = 4-OMe (6), 4-Br (7), 4-CF₃ (8)] and [[PyCH₂CH(RC₆H₄)O]₂Ru₃(CO)₈] [R = 2-Br (9), 2-CF₃ (10)], were synthesized by treating Ru₃(CO)₁₂ with two equivalents of the corresponding pyridine-alcohols PyCH₂CH(RC₆H₄)OH [1-5, R = 4-OMe, 4-Br, 4-CF₃, 2-Br and 2-CF₃] in refluxing toluene. The structures of 6-10 were fully characterized by IR and NMR spectroscopy, elemental analysis and single-crystal X-ray diffraction. They were found to be efficient catalysts for the oxidation of secondary alcohols by NMO, giving the corresponding ketones in good to excellent yields within 15 min, of which [PyCHC(4-OCH₃C₆H₄)O]₂Ru₃(CO)₈ (6) is the best.

Full text of DOI:10.1039/c9nj03222c

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Ruthenium carbonyl complexes supported by
pyridine-alkoxide ligands: synthesis, structure and
Cite this:DOI: 10.1039/c9nj03222c
catalytic oxidation of secondary alcohols†
Siqi Zong,‡a Kang Liu,‡a Xiaohui Yue,a Zhiqiang Hao,*a Zhihong Ma,*b
a
Zhangang Han,a Guo-Liang Luc and Jin Lin
*
Five novel trinuclear ruthenium complexes [[PyCHQC(RC6H4)O]2Ru3(CO)8] [R = 4-OMe (6), 4-Br (7),
4-CF3 (8)] and [[PyCH2CH(RC6H4)O]2Ru3(CO)8] [R = 2-Br (9), 2-CF3 (10)], were synthesized by treating
Ru3(CO)12 with two equivalents of the corresponding pyridine-alcohols PyCH2CH(RC6H4)OH [1–5,
R = 4-OMe, 4-Br, 4-CF3, 2-Br and 2-CF3] in refluxing toluene. The structures of 6–10 were fully char-
acterized by IR and NMR spectroscopy, elemental analysis and single-crystal X-ray diffraction. They were found
to be efficient catalysts for the oxidation of secondary alcohols by NMO, giving the corresponding ketones in
good to excellent yields within 15 min, of which [PyCHQC(4-OCH3C6H4)O]2Ru3(CO)8 (6) is the best.
Received 21st June 2019,
Accepted 11th August 2019
DOI: 10.1039/c9nj03222c
rsc.li/njc
environmentally benign and clean oxidant.10,11 However, due
to the limited oxidation capacity of H2O2, the catalytic system
Introduction
The oxidation of alcohols to their corresponding aldehydes/ has to be assisted by carboxylic acid or H2SO4 as an additive to
ketones is of significant importance in organic synthesis.1–3 achieve high efficiency. Hence, it is in urgent demand for a
Alcohol oxidation reaction is widely investigated in both laboratories mild, efficient and less toxic oxidation reaction system.
and industrial applications due to the importance of its valuable
Considerable progress has been made to the dehydrogenizing
products such as aldehydes and ketones, which are widely used in oxidation of alcohols catalyzed by transition metal complexes. These
dyestuff, polymer precursors, pharmaceutical and agrochemical transition metal catalysts including ruthenium, palladium, copper,
industries.4 Traditionally, such transformations have been per- and cobalt have been invented for the oxidation of alcohols.12–16
formed with stoichiometric inorganic oxidants, such as chromium Many examples of alcohol oxidations involve ruthenium com-
reagents5 or permanganate,6 that resulted in the generation of large pounds as catalysts. Ruthenium compounds are intensively inves-
amounts of waste. To reduce serious environmental pollution, many tigated because of their rich structural and various valence states.
efforts have been devoted to developing more atom efficient and For instance, Zhang et al. reported several 2-(biphenylazo)phenolate
greener methods. Molecular oxygen was one of the green oxidants ligands supported ruthenium complexes that successfully achieved
and water was the only by-product, which is convenient and eco- high catalytic activity in the presence of NMO (N-methylmorpholine-
friendly.7 However, in most catalytic systems using O2 as oxidant, N-oxide) without diminishing chemoselectivity.17 Recently, our
the additives e.g. TEMPO and base are also needed, which makes group reported ruthenium carbonyl complexes based on N,O-
the reaction system more complicated.8,9 Compared to oxygen, bidentate pyridine-alkoxide ligands and their catalytic oxidation
hydrogen peroxide is a good alternative and can act as an behavior towards alcohols.18 Inspired by these studies and based
on our continuing efforts in creating novel ruthenium organome-
tallic complexes, herein we report the synthesis, characterization of
several ruthenium carbonyl complexes bearing pyridine-alkoxide
ligands and their catalytic activity in the oxidation reaction of
secondary alcohols in combination with NMO.
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.
E-mail: haozhiqiang1001@163.com, linjin64@126.com; Fax: +86-311-80787431
b School of Pharmacy, Hebei Medical University, Shijiazhuang 050017, China.
E-mail: mazhihong-1973@163.com
c Auckland Cancer Society Research Centre, Faculty of Medical and Health Sciences,
Experimental
General considerations
The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
† Electronic supplementary information (ESI) available. CCDC 1485420, 1487566,
1505318, 1543437 and 1543441. For ESI and crystallographic data in CIF or other
All manipulations were carried out under an inert argon atmo-
sphere using a Schlenk line. Solvents were distilled from
electronic format see DOI: 10.1039/c9nj03222c
‡ These authors contributed equally to this work.
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appropriate drying agents under N2 before use. All reagents 160.5, 167.4, 203.7, 204.0, 204.6, 204.8. IR (uCO, KBr, cmÀ1):
were purchased from commercial sources and used without 2068(m), 2010(m), 1977(m), 1928(m).
purification. Neural alumina for column chromatography. 1H
Synthesis of [[PyCH2CH(2-BrC6H4)O]2Ru3(CO)8] (9). A similar
and 13C NMR spectra were recorded on Bruker AV III-500 procedure as described above was followed, PyCH2CH(2-BrC6H4)-
instrument in DMSO, while IR spectra were recorded as KBr OH (4) (0.26 g, 0.94 mmol) was reacted with Ru3(CO)12 (0.30 g,
disks on Thermo Fisher is50 spectrometer. X-ray measurements 0.47 mmol) in refluxing toluene for 12 h. [[PyCH2CH(2-BrC6H4)O]2-
were made on a Bruker AXS SMART 1000 CCD diffractometer Ru3(CO)8] (9) was obtained (0.24 g, 44% yield) as yellow crystals.
with graphite monochromated Mo Ka (l = 0.71073 Å) radiation. M.p. 189.6–190.4 1C. Anal. calcd for C35H24Br2Cl2N2O10Ru3: C,
Elemental analyses were performed with a VarioEL III analyzer. 36.04; H, 2.07; N, 2.40. Found (%): C, 36.32; H, 2.24; N, 2.66.
Synthesis of [[PyCH C(4-OCH3C6H4)O]2Ru3(CO)8] (6). A 1H NMR (500 MHz, d6-DMSO): d: 8.77 (d, 2H, J = 8.5 Hz, Py-H), 8.34
Q
solution of PyCH2CH(4-OMeC6H4)OH (1) (0.22 g, 0.94 mmol) (d, 2H, J = 7.0 Hz, Py-H), 8.19 (d, 4H, J = 8.0 Hz, Ar-H), 8.06 (t, 2H,
and Ru3(CO)12 (0.30 g, 0.47 mmol) in 30 mL of toluene was J = 9.5 Hz, Py-H), 7.96 (d, 2H, J = 10.0 Hz, Py-H), 7.83 (d, 4H,
heated at reflux for 6 h. The mixture was allowed to cool to room J = 5.0 Hz, Ar-H), 4.37 (d, 2H, J = 9.1 Hz, CH2), 3.04 (d, 2H, J = 9.2
temperature before solvent was evaporated under vacuum. The Hz, CH2), 2.88–2.82 (m, 2H, CH). 13C NMR (125 MHz, d6-DMSO):
residue was chromatographed on an alumina column with a d: 50.2, 96.8, 123.0, 125.6, 126,8, 127.9, 128.8, 132.0, 138.1, 145.9,
mixture of petroleum ether and ethyl acetate (v/v = 2 : 1) as eluent. 153.3, 161.2, 194.2, 203.7, 204.0, 204.1. IR (uCO, KBr, cmÀ1): 2072(s),
The crude product was recrystallized from dichloromethane and 1983(s), 1925(s), 1909(s).
n-hexane to give [[PyCHQC(4-OCH3C6H4)O]2Ru3(CO)8] (6) as a
Synthesis of [[PyCH2CH(2-CF3C6H4)O]2Ru3(CO)8] (10). A
yellow powder (0.20 g, 43% yield). Single crystals suitable for similar procedure as described above was followed, PyCH2CH(2-
X-ray crystallographic determination were grown by slow evaporation CF3C6H4)OH (5) (0.25 g, 0.94 mmol) was reacted with Ru3(CO)12
of a solution in CH2Cl2 and n-hexane at ambient temperature. M.p. (0.30 g, 0.47 mmol) in refluxing toluene for 12 h. [[PyCH2CH(2-
243.8–244.5 1C. Anal. calcd for C36H24N2O12Ru3: C, 44.13; H, 2.47; N, CF3C6H4)O]2Ru3(CO)8] (10) was obtained (0.25 g, 50% yield) as
1
2.86. Found (%): C, 44.15; H, 2.66; N, 2.55. H NMR (500 MHz, yellow crystals. M.p. 179.6–180.2 1C. Anal. calcd for C36H22F6N2-
d6-DMSO): d 8.54 (d, 2H, J = 5.0 Hz, Py-H), 7.62 (t, 2H, J = 7.5 Hz, O10Ru3: C, 40.80; H, 2.09; N, 2.64. Found (%): C, 40.47; H, 2.38;
Py-H), 7.34 (t, 4H, J = 8.5 Hz, Ar-H), 7.27 (t, 2H, J = 6.5 Hz, Py-H), N, 2.88. 1H NMR (500 MHz, d6-DMSO): d: 8.57 (d, 2H, J = 10.0 Hz,
7.13 (d, 2H, J = 8.0 Hz, Py-H), 6.91 (d, 4H, J = 8.5 Hz, Ar-H), 5.50 Py-H), 7.96 (t, 2H, J = 9.5 Hz, Py-H), 7.83 (d, 2H, J = 9.5 Hz, Py-H),
(s, 2H, CHQCH), 3.83 (s, 6H, CH3). 13C NMR (125 MHz, 7.72 (d, 4H, J = 9.0 Hz, Ar-H), 7.67 (d, 4H, J = 10.0 Hz, Ar-H), 7.60
d6-DMSO): d: 46.0, 114.0, 121.9, 124.0, 127.3, 136.6, 137.1, (d, 2H, J = 8.5 Hz, Py-H), 4.59 (d, 2H, J = 9.8 Hz, CH2), 3.12 (d, 2H,
148.8, 159.1, 160.1, 202.7, 203.1, 204.2, 205.4. IR (uCO, KBr, cmÀ1): J = 9.8 Hz, CH2), 3.02–2.96 (m, 2H, CH). 13C NMR (125 MHz,
2068(m), 2030(m), 1982(s), 1916(s).
d6-DMSO): d: 29.7, 30.6, 74.2, 122.5, 125.9, 127.4, 128.1, 132.8,
Synthesis of [[PyCH C(4-BrC6H4)O]2Ru3(CO)8] (7). A similar 137.9, 148.4, 153.4, 194.1, 203.3, 203.9, 204.0. IR (uCO, KBr, cmÀ1):
procedure as described above was followed, PyCH2CH(4-BrC6H4)OH 2070(m), 1999(m),1984(m), 1920(m).
(2) (0.26 g, 0.94 mmol) was reacted with Ru3(CO)12 (0.30 g,
Q
0.47 mmol) in refluxing toluene for 6 h. [[PyCHQC(4-BrC6H4)-
O]2Ru3(CO)8] (7) was obtained (0.18 g, 36% yield) as yellow
crystals. M.p. 277.7–280.3 1C. Anal. calcd for C34H18Br2N2O10Ru3:
C, 37.90; H, 1.68; N, 2.60. Found: (%): C, 38.20; H, 1.86; N, 2.74.
1H NMR (500 MHz, d6-DMSO): d: 7.83 (t, 2H, J = 10.0 Hz, Py-H),
7.54 (d, 4H, J = 10.0 Hz, Ar-H), 7.49 (d, 2H, J = 6.5 Hz, Py-H), 7.44
(d, 4H, J = 10.0 Hz, Ar-H), 7.37 (d, 2H, J = 9.5 Hz Py-H), 7.30 (d, 2H,
J = 10.0 Hz, Py-H), 5.90 (s, 2H, CHQCH). 13C NMR (125 MHz,
d6-DMSO): d: 119.7, 122.9, 127.2, 128.9, 130.8, 131.3, 139.0, 145.4,
152.8, 203.0, 203.5, 204.1, 204.5. IR (uCO, KBr, cmÀ1): 2067(w),
1994(m), 1970(m), 1924(w).
Crystal structure determination
Single crystals of complexes 6–10 (CCDC 1485420, 1487566,
1505318, 1543437, 1543441†) suitable for X-ray diffraction were
obtained by crystallization from their dichloromethane-n-
hexane solutions. Data collection were performed on a Bruker
AXS SMART 1000 CCD diffractometer, using graphite mono-
chromated Mo Ka radiation (j/o scan, l = 0.71073 Å). Semi-
empirical absorption corrections were applied for all complexes.
The structures were solved by direct methods and refined by
full-matrix least-squares. All calculations were done using the
SHELXL-97 program system. All non-hydrogen atoms were
refined anisotropically and hydrogen atoms were introduced
into calculated positions with the displacement factors of the
host carbon atoms.
Q
Synthesis of [[PyCH C(4-CF3C6H4)O]2Ru3(CO)8] (8). A similar
procedure as described above was followed, PyCH2CH(4-CF3C6H4)-
OH (3) (0.25 g, 0.94 mmol) was reacted with Ru3(CO)12 (0.30 g,
0.47 mmol) in refluxing toluene for 6 h. [[PyCHQC(4-CF3C6H4)-
O]2Ru3(CO)8] (8) was obtained (0.23 g, 46% yield) as yellow crystals.
M.p. 186.8–187.3 1C. Anal. calcd for C36H18F6N2O10Ru3: C, 40.96;
H, 1.72; N, 2.65. Found (%): C, 40.76; H, 2.15; N, 2.83. 1H NMR
(500 MHz, d6-DMSO): d: 8.49 (d, 2H, J = 6.0 Hz, Py-H), 7.98
(d, 4H, J = 8.0 Hz, Ar-H), 7.81 (t, 2H, J = 7.5 Hz, Py-H), 7.70 (d, 4H,
J = 8.0 Hz, Ar-H), 7.43 (d, 2H, J = 8.5 Hz, Py-H), 7.06 (d, 2H,
J = 6.5 Hz, Py-H), 6.37 (s, 2H, CHQCH). 13C NMR (125 MHz,
d6-DMSO): d: 65.5, 76.1, 103.5, 128.3, 129.1, 132.2, 138.6, 153.3,
Scheme 1 Synthesis of complexes 6–10.
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Table 1 Crystal data and structure refinement parameters for 6–10
Complex
6
7
8
9
10
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (1)
b (1)
C36H24N2O12Ru3
979.78
298(2)
0.71073
Triclinic
%
P1
8.8906(7)
13.3485(11)
17.5144(15)
74.8410(10)
86.893(2)
83.248(2)
1991.7(3)
2
C34H18Br2N2O10Ru3 C36H18F6N2O10Ru3 C34H22Br2N2O10Ru3ÁCH2Cl2 C36H22F6N2O10Ru3
1077.53
298(2)
0.71073
Monoclinic
P2(1)/c
9.711(2)
19.058(4)
20.157(4)
90
1055.73
298(2)
1166.49
298(2)
1059.77
298(2)
0.71073
Monoclinic
P2(1)/c
9.1966(7)
17.7452(13)
23.7016(18)
90
100.465(2)
90
3803.7(5)
4
0.71073
Monoclinic
P2(1)/n
9.0954(9)
32.782(3)
13.3879(12)
90
91.7520(10)
90
3989.9(6)
4
0.71073
Triclinic
P1
%
12.4137(11)
12.4513(12)
13.0078(13)
94.7580(10)
97.7260(10)
107.663(2)
1882.0(3)
2
98.567(3)
90
3689.0(13)
4
g (1)
V (Å3)
Z
Dcalc (g cmÀ3
)
1.634
1.183
964
1.940
3.436
1.851
1.264
1.942
3.314
1.870
1.277
m (mmÀ1
F(000)
)
2072
2072
2256
1036
Crystal size (mm)
y range(1)
Reflections collected
R(int)
0.48 Â 0.40 Â 0.20 0.40 Â 0.38 Â 0.35 0.42 Â 0.42 Â 0.40 0.42 Â 0.15 Â 0.14
0.43 Â 0.40 Â 0.32
2.50–25.02
9486/6525
0.0294
2.31–25.02
10 113/6845
0.0608
2.31–25.02
18 110/6502
0.0593
2.46–25.02
18 793/6678
0.0333
2.41–25.02
19 975/7003
0.0745
Completeness to y
Absorption correction
97.5%
99.9%
99.5%
99.4%
98.2%
Semi-empirical from equivalents
Max. and min. transmission 0.7978/0.6006
0.3793/0.3403
0.6318/0.6188
0.6540/0.3366
0.6853/0.6096
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
R1, wR2[I 4 2s(I)]
Full-matrix least-squares on F2
6845/0/480
1.081
0.0687, 0.1768
0.0998, 0.1932
1.229
6502/0/462
1.047
0.0651, 0.1381
0.1253, 0.1607
0.950
6678/0/570
1.102
0.0445, 0.0771
0.0810, 0.0912
1.627
7003/0/515
1.027
0.0501, 0.0843
0.1121, 0.0967
0.682
6525/0/514
1.140
0.0466, 0.1256
0.0589, 0.1312
0.810
R1, wR2 (all data)
Max. peak/(e ÅÀ3
)
Mini. peak/(e ÅÀ3
CCDC
)
À0.612
À1.012
À0.662
À0.656
À0.836
1485420
1487566
1505318
1543437
1543441
Procedure for the oxidation of secondary alcohols
In the 1H NMR spectra of these complexes, the OH was not
observed, suggesting the deprotonation the hydroxyl groups of
the ligands. Complexes 6–8 showed four groups of peaks at
7.06–8.54 ppm for the pyridyl protons and two doublets at 6.91–
7.98 ppm for phenyl protons, plus a singlet for the CQC double
bond protons at 5.50–6.37 ppm. Complexes 9 and 10 showed
four groups of peaks at 7.60–8.77 ppm for the pyridyl protons,
A typical procedure using the synthesized complexes as catalyst
and NMO as oxidant as follows. A mixture of 1.0 mmol of
second alcohol, 0.04 mmol of the ruthenium complex and 3.0 mmol
of NMO in 8 mL of CH3CN was heated at 80 1C for 15 min. Solvent
was removed under reduced pressure and the resulting residue was
purified by alumina column chromatography. Eluting with a mix-
ture of petroleum ether and ethyl acetate (v/v = 30 : 1), gave the
desired product. The target product was identified by 1H NMR.
Results and discussion
Synthesis and characterization of ruthenium complexes
Pyridine containing alcohols ligands PyCH2CH(Ar)OH [(Ar = 4-
OMeC6H4 (1); 4-BrC6H4 (2); 4-CF3C6H4 (3); 2-BrC6H4 (4); 2-CF3C6H4
(5))] were synthesized according to the literature methods.19,20
PyCH2CH(4-RC6H4)OH [R = OMe (1); R = Br (2); R = CF3 (3)]
were treated with Ru3(CO)12 in refluxing toluene for 6 h. The
corresponding triruthenium clusters [PyCHQC(4-RC6H4)O]2-
Ru3(CO)8 [R = OMe (6); R = Br (7); R = CF3 (8)] were obtained
in 36–46% yields. After PyCH2CH(2-RC6H4)OH [R = Br (4);
R = CF3 (5)] were treated with Ru3(CO)12 in refluxing toluene
for 12 h, the corresponding triruthenium clusters [PyCH2CH(2-
RC6H4)O]2Ru3(CO)8 [R = Br (9); R = CF3 (10)] were isolated in
47–50% yields (Scheme 1).
Fig. 1 Perspective view of 6 with thermal ellipsoids are drawn at 30%
probability level. Hydrogen atoms have been omitted for clarity. The
selected bond lengths (Å) and angles (1): Ru(1)–N(1) 2.185(6), Ru(2)–N(2)
2.201(6), Ru(1)–O(1) 2.116(5), Ru(2)–O(3) 2.113(5), Ru(1)–Ru(3) 2.7759(9),
Ru(2)–Ru(3) 2.7845(9); C(31)–Ru(2)–N(2) 96.9(3), O(1)–Ru(1)–N(1) 89.7(2),
Ru(1)–Ru(3)–Ru(2) 56.96(2), C(30)–Ru(1)–N(1) 97.7(3), O(3)–Ru(2)–N(2)
89.1(2), O(1)–Ru(2)–Ru(1) 45.38(12), Ru(2)–O(1)–Ru(1) 90.58(18).
In the IR spectra, all the complexes displayed four strong
absorptions around 2072–1909 cmÀ1 due to the terminal carbonyls.
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two doublets at 7.67–8.19 ppm for phenyl protons and two P2(1)/n space group for 9. Their molecular structures are depicted
doublets at 3.04–4.59 ppm for the methylene protons, plus a in Fig. 1–5 together with selected bond distances and angles,
multiplet for the methylidyne protons at 2.82–3.02 ppm.
respectively. X-ray diffraction analyses confirm that these com-
plexes are similar triruthenium carbonyl clusters, in which two
Crystal structures of complexes 6–10
Crystals of complexes 6–10 suitable for X-ray diffraction were
obtained by slow evaporation of their dichloromethane–hexane
solutions. Crystallographic data and experimental details are
given in Table 1. The crystallographic data of these complexes
in CIF are provided in ESI.† Crystallographic data indicate that
crystals of complexes 6 and 10 belong to triclinic crystal system
%
with P1 space group, while complexes 7–9 crystallize in mono-
clinic crystal system with P2(1)/c space group for 7 and 8, and
Fig. 4 Perspective view of 9 with thermal ellipsoids are drawn at 30%
probability level. The thermal ellipsoids are displayed at 30% probability.
Hydrogen atoms have been omitted for clarity. The selected bond lengths
(Å) and angles (1): Ru(1)–N(2) 2.206(6), Ru(2)–N(1) 2.206(5), Ru(1)–O(1)
2.151(4), Ru(2)–O(2) 2.138(4), Ru(1)–Ru(3) 2.7840(9), Ru(2)–Ru(3) 2.7795(8);
C(28)–Ru(1)–N(2) 94.9(3), O(1)–Ru(1)–N(2) 92.36(18), Ru(3)–Ru(1)–Ru(2)
56.763(19), C(29)–Ru(2)–N(1) 95.8(3), O(2)–Ru(2)–N(1) 92.82 (18), O(1)–
Ru(1)–Ru(3) 83.43(11), Ru(2)–O(1)–Ru(1) 90.59(16).
Fig. 2 Perspective view of 7 with thermal ellipsoids are drawn at 30%
probability level. The thermal ellipsoids are displayed at 30% probability.
Hydrogen atoms have been omitted for clarity. The selected bond lengths (Å)
and angles (1): Ru(1)–N(2) 2.223(7), Ru(2)–N(1) 2.207(8), Ru(1)–O(1) 2.176(6),
Ru(2)–O(2) 2.168(6), Ru(1)–Ru(3) 2.8043(12), Ru(2)–Ru(3) 2.7898(12); C(27)–
Ru(1)–N(2) 97.1(4), O(1)–Ru(1)–N(2) 90.8(3), Ru(1)–Ru(3)–Ru(2) 66.40(3),
C(29)–Ru(2)–N(1) 93.2(4), O(2)–Ru(2)–N(1) 91.1(3), O(2)–Ru(1)–Ru(3)
84.22(16), Ru(2)–O(1)–Ru(1) 91.1(2).
Fig. 3 Perspective view of 8 with thermal ellipsoids are drawn at 30%
Fig. 5 Perspective view of 10 with thermal ellipsoids are drawn at 30%
probability level. The thermal ellipsoids are displayed at 30% probability.
probability level. The thermal ellipsoids are displayed at 30% probability.
Hydrogen atoms have been omitted for clarity. The selected bond lengths (Å) Hydrogen atoms have been omitted for clarity. The selected bond lengths (Å)
and angles (1): Ru(1)–N(2) 2.180(5), Ru(2)–N(2) 2.188(5), Ru(1)–O(2) 2.183(4),
and angles (1): Ru(1)–N(1) 2.186(6), Ru(2)–N(2) 2.238(6), Ru(1)–O(1) 2.131(4),
Ru(2)–O(1) 2.1694(4), Ru(1)–Ru(3) 2.7899(7), Ru(2)–Ru(3) 2.8069(8); C(31)–
Ru(2)–O(2) 2.134(4), Ru(1)–Ru(3) 2.7891(8), Ru(2)–Ru(3) 2.7965(8); C(31)–
Ru(2)–N(2) 96.4(3), O(1)–Ru(2)–N(2) 89.35(17), Ru(1)–Ru(3)–Ru(2) 65.765(19), Ru(2)–N(2) 98.5(3), O(1)–Ru(2)–N(2) 95.74(18), Ru(3)–Ru(1)–Ru(2) 57.102(18),
C(29)–Ru(1)–N(1) 96.6(2), O(1)–Ru(1)–N(1) 90.66 (16), O(1)–Ru(2)–Ru(3) C(29)–Ru(1)–N(1) 91.6(3), O(2)–Ru(1)–N(1) 95.56(19), O(2)–Ru(1)–Ru(3)
82.40(11), Ru(2)–O(1)–Ru(1) 90.40(4).
82.87(11), Ru(1)–O(1)–Ru(2) 90.46(16).
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pyridylalkoxo ligands are simultaneously coordinated to two [Ru(1)–N(2)] and 2.207(8) Å [Ru(2)–N(1)], respectively. The Ru–C
ruthenium atoms with a m2-O atom acting as a three-electron and C–O distances in the carbonyl ligands are not significantly
donor. Each pyridyl group is coordinated to one ruthenium different from Ru3(CO)12. The Ru(1)–O–Ru(2) bond angle are
atom forms two six-membered rings. For complex 6 (Fig. 1), 91.1(2)1, and 91.0(3)1, respectively, slightly bigger than the
there is no significant metal–metal interaction between Ru(1) Ru–O–Ru bond angles in [PyCHQC(Ph)O]2Ru3(CO)8 [90.781].
and Ru(2) [Ru(1)–Ru(2) 3.0430(8) Å] in a formal sense. The In complex 8 (Fig. 3), the metal framework consists of a triangle
lengths of Ru(1)–Ru(3) and Ru(2)–Ru(3) are 2.7759(9) Å and
2.7845(9) Å, respectively, which are slightly longer than those in
Table 4 Oxidation of various substrates catalyzed by complex 6a
Ru3(CO)8(m-OC6H4OMe-2)2 [2.73 Å and 2.74 Å].21 The Ru–N
distances [Ru(1)–N(1) 2.185(6) Å; Ru(2)–N(2) 2.201(6) Å] are close
to those of 2.221(3) Å and 2.210(3) Å in [PyCH2CH(Ph)O]2-
Ru3(CO)8.22 The Ru(1)–O–Ru(2) bond angles [90.581, 91.611]
are comparable to those in Ru3(CO)8(m-OC6H4OMe-2)2 and
[PyCHQC(Ph)O]2Ru3(CO)8 [89.91, 90.71 and 90.781, 90.781,
respectively]. For complex 7 (Fig. 2), the distances between
the pyridyl nitrogen and the ruthenium atom are 2.223(7) Å
Entry
1
Substrate
Product
Yield (%)
90b
23
2
3
4
97b
95b
91b
Table 2 Optimization of the oxidation of 1-phenylethanol catalyzed by
complex 6a
Cat. 6
Oxidant
Temp.
(1C)
Time
Entry (mol%) (mmol)
Solvent
(min) Yieldb (%)
1
2
3
4
5
6
7
8
2.0
2.0
3.0
Blank
4.0
5.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
H2O2 (3.0) 80
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
15
15
15
15
15
15
15
15
15
35
72
83
20c
90
90
74
80
89
80
83
86
66
47
90
74
80
NMO (3.0) 80
NMO (3.0) 80
NMO (3.0) 80
NMO (3.0) 80
NMO (3.0) 80
NMO (1.0) 80
NMO (2.0) 80
NMO (5.0) 80
NMO (3.0) 80
NMO (3.0) 80
NMO (3.0) 80
NMO (3.0) 80
NMO (3.0) 80
NMO (3.0) 80
NMO (3.0) 50
NMO (3.0) 60
5
6
93b
90b
9
10
11
12
13
14
15
16
17
Toluene 15
CH2Cl2 15
Acetone 15
7
87b
89b
DMF
15
10
30
15
15
CH3CN
CH3CN
CH3CN
CH3CN
8
a
Reaction conditions: 1-phenylethanol (1.0 mmol), solvent (8 mL).
Isolated yield was determined by column chromatography. Catalytic
b
c
9
76b
93b
70c
reaction without any catalyst under identical conditions.
Table 3 Oxidation of 1-phenylethanol catalyzed by complexes 7–10a
10
11
12
Entry
Catalyst
Yieldb (%)
81c
77c
1
2
3
4
7
8
9
84
80
75
73
13
10
a
a
Reaction conditions: 1-phenylethanol (1.0 mmol), catalyst (4.0 mol%),
Reaction conditions: alcohol (1.0 mmol), catalyst (4.0 mol%), NMO
b
b
NMO (3.0 mmol), acetonitrile (8 mL), 80 1C, 15 min. Yield was (3.0 mmol), acetonitrile (8 mL), 80 1C, 15 min. Yield was determined
determined by column chromatography.
c
by column chromatography. Yield was determined by GC analysis, 3 h.
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with Ru–Ru distances of 2.7899(7), 2.8069(8) and 3.0386(7) Å, the reaction shows very poor conversion (Table 2, entry 4).
respectively. The bond distances of the C(6)–C(7) [1.364(12) Å Variations of amount of catalyst, equivalent of NMO and solvent
(6), 1.354(15) Å (7), and 1.386(9) Å (8)] are close to standard CQC have been carried out to optimize the reaction condition. The
[1.337 Å]. Obviously, C(6)–C(7) is a double bond in complexes best yield of 90% was obtained by using CH3CN as the solvent,
6–8. For complex 9 (Fig. 4), the distances between the pyridyl NMO (3.0 equivalents) as the oxidant and 4 mol% complex 6 as
nitrogen and the ruthenium atom are 2.206(6) Å [Ru(1)–N(2)] catalyst at 80 1C for 15 min (entry 5). It is worth mentioning that
and 2.206(5) Å [Ru(2)–N(1)], respectively. The bond distances of extending time did not increase the yield (entry 15).
the C(6)–C(7) [1.509(8) Å (9), 1.529(10) Å (10)] are close to
The other four complexes were also tested under the optimized
standard C–C [1.54 Å]. Thus, C(6)–C(7) is a single bond in condition mentioned above. The results are shown in Table 3. All
complexes 9 and 10. Similar to complexes 6–9, there is a m2-O the complexes can catalyze the oxidation of 1-phenylethanol, of
atom that bridges Ru(1) and Ru(2) in complex 10 (Fig. 5).
which 6 is the best.
To establish the scope and limitations of this process,
oxidation of a variety of secondary alcohols was investigated
under the optimized reaction condition. As shown in Table 4,
Catalytic property of the complexes in the oxidation of
secondary alcohols
Initially, we selected 1-phenylethanol as a test substrate and the all 1-phenylethanols bearing various substitute groups can be
reaction results were summarized in Table 2. Firstly, complex 6 oxidized to give the corresponding ketones in moderate to good
was used as a precatalyst under the condition that 1.0 mmol of yields, in which those with electrondonating group such as
1-phenylethanol was treated with 3.0 mmol of H2O2 in 8 mL of methoxy and methyl (entries 2 and 3) achieved higher yield than
CH3CN at 80 1C under argon. As shown in Table 2, the reaction those with electronwithdrawing group (entries 4–8). This might
only gave acetophenone in 35% yield (entry 1). Then, NMO as also explain the low yield of the oxidation of 1-(naphthalen-2-
oxidant was examined. The desired product was obtained in yl)ethan-1-ol (entry 9). Surprisingly, the yield of the oxidation of
72% yield (entry 2). Blank reaction with the substrate was diphenylmethanol was excellent too (entry 10). In addition to
carried out under the same experimental conditions but without the 1-phenylethanols, alkyl second alcohols including two cyclic
any catalyst. It is to be noted that in the case of a blank reaction, alcohols and 2-hexanol were applied in the oxidation. They all gave
Scheme 2 Proposed mechanism for oxidation of secondary alcohols catalyzed by Ru/NMO system.
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´
5 G. Tojo and M. Fernandez, Oxidation of Primary Alcohols to
Carboxylic Acids: A Guide to Current Common Practice,
Springer, New York, 2010.
the corresponding ketones in good yields under same condition,
but much slower (3 h, entries 11–13). From the observations above,
it can be concluded that complex 6 is an highly active catalyst for the
oxidation of secondary alcohols by NMO.
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On the basis of the literatures24 and the above observations,
we might propose a mechanism to account for the ruthenium-
catalytic oxidation of secondary alcohols by NMO, as shown in
Scheme 2. First, the ruthenium complex A is oxidized by two
molecules of NMO to form the Ru3+QO species B. Then, the
alcohol attacks the Ru oxide species to produce a Ru-alcoholate
species C. Subsequently, the intermediate C undergoes b-hydride
elimination to produce the carbonyl compound, along with the
H2O and a coordination unsaturated species D, which is oxidized by
another two NMO to regenerate the species B for next catalytic cycle.
¨
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Conclusions
In summary, five new ruthenium complexes supported by
pyridine-alkoxide ligands were synthesized and fully characterized.
Their catalytic activity in the oxidation of second alcohols was
extensively investigated. Complex 6 was found to be a highly active
catalyst for the oxidation of secondary alcohols in the presence of
NMO, which provides an alternative method for the syntheses of
ketones particularly acetophenones with the advantages of short
reaction time, mild reaction conditions, and high yield. Further
studies are in progress to improve the catalytic activity by modify-
ing the ligand and expand the synthetic utility of these catalysts
including region- and stereo-selective reactions.
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Conflicts of interest
The authors declare no conflict of interest.
18 (a) Z. Hao, N. Li, X. Yan, Y. Li, S. Zong, H. Liu, Z. Han and
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Acknowledgements
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (No. 21372061,
21871076), the Hebei Natural Science Foundation of China (No.
B2017205006, B2019205087), the Education Department Foundation
of Hebei Province (No. ZD2018005, QN2019036) and the Natural
Science Foundation of Hebei Normal University (No. L2017Z02,
L2018B08).
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