Syntheses, characterization and catalytic activities of half-sandwich ruthenium complexes with naphthalene-based Schiff base ligands

Article abstract of DOI:10.1080/00958972.2017.1284322

Four ruthenium(II) p-cymene complexes with naphthalene-based Schiff base ligands [Ru(p-cymene)LCl] (2a–2d) have been synthesized and characterized. The half-sandwich ruthenium complexes were characterized by ¹H and ¹³C NMR spectra, elemental analyses, and infrared spectrometry. The molecular structures of 2a, 2b, and 2c were confirmed by single-crystal X-ray diffraction. Furthermore, these half-sandwich ruthenium complexes are highly active catalysts for the hydrogenation of nitroarenes to anilines using NaBH₄ as the reducing agent in ethanol at room temperature.

Full text of DOI:10.1080/00958972.2017.1284322

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Syntheses, characterization and catalytic activities
of half-sandwich ruthenium complexes with
naphthalene-based Schiff base ligands
Wei-Guo Jia, Zhi-Bao Wang, Xue-Ting Zhi, Jia-Qin Han & Ying Sun
To cite this article: Wei-Guo Jia, Zhi-Bao Wang, Xue-Ting Zhi, Jia-Qin Han & Ying Sun (2017):
Syntheses, characterization and catalytic activities of half-sandwich ruthenium complexes
with naphthalene-based Schiff base ligands, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2017.1284322
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1284322
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Date: 20 January 2017, At: 10:37

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2017.1284322
Syntheses, characterization and catalytic activities of half-sandwich ruthenium
complexes with naphthalene-based Schiff base ligands
WEI-GUO JIA∗, ZHI-BAO WANG, XUE-TING ZHI, JIA-QIN HAN and YING SUN
College of Chemistry and Materials Science, Center for Nano Science and Technology, The Key Laboratory of
Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials,
Anhui Normal University, Wuhu, 241000, China
Four ruthenium(II) p-cymene complexes with naphthalene-based Schiff base ligands
[
Ru(p-cymene)LCl] (2a-2d) have been synthesized and characterized. The half-sandwich
1
13
ruthenium complexes were characterized by H and C NMR spectra, elemental analyses and
infrared spectrometry. The molecular structures of 2a, 2b and 2c were confirmed by
single-crystal X-ray diffraction. Furthermore, these half-sandwich ruthenium complexes are
highly active catalysts for the hydrogenation of nitroarenes to anilines using NaBH as the
4
reducing agent in ethanol at room temperature.
Keywords: Half-sandwich; Ruthenium; Structure; Nitroarene; Reduction
∗
Corresponding author. Email: wgjiasy@mail.ahnu.edu.cn
1

1
. Introduction
Ruthenium(II)-based catalysis provides an efficient and sustainable route for organic syntheses,
including transfer hydrogenation reactions of unsaturated substrates (ketone, imine and alkene)
[1-4] and C-H bond activation [5, 6], which are important in synthetic organic chemistry. Many
ruthenium complexes, especially half-sandwich ruthenium(II) complexes, have been widely used
for these transformations [7-15]. Thus, preparation of half-sandwich ruthenium complexes and
studying their catalytic properties is interesting. The half-sandwich ruthenium units with
three-legged, piano-stool fragments are easy to prepare and are air-stable, where the piano-stool
legs can be variably connected with N-, O-, S-, or P-donor ligands [16-25]. The majority of
half-sandwich ruthenium complexes have been prepared directly via reaction between various
organic donors and half-sandwich ruthenium dimer precursors. Schiff base compounds are
important N-based ligands that are readily modifiable to allow fine tuning of steric and electronic
properties of the metal center [26]. In previous studies, we have designed a series of
half-sandwich ruthenium(II) complexes containing pyridyl-salicylimine; thiazol-salicylimine;
benzothiazol-salicylimine and phenyl-salicylimine ligands to catalyze nitroarene reduction
[27, 28]. The promising efficiency of these catalysts may be partially attributed to the strong
electron-donating ability of the nitrogen and oxygen donors from Schiff base ligands.
Continuing our research in nitroarene reduction reaction catalyzed by half-sandwich
ruthenium complexes, we prepare new half-sandwich ruthenium complexes with
naphthalene-based Schiff base ligands and hope to exploit their chemistry in catalytic
applications. In comparison with previously reported half-sandwich ruthenium systems,
ruthenium catalysts containing bulky naphthalene groups may exhibit higher catalytic activity for
nitroarene reduction. Preliminary results indicate that these complexes can catalyze nitroarene
reduction in the presence of NaBH in ethanol. The electronic effects on their catalytic behavior
4
of nitroarene reduction are also discussed.
2

2
2
. Experimental
.1. Materials and measurements
Commercial reagents were analytical grade and used as received from Aladdin and Energy
chemical. All operations were carried out under a pure nitrogen atmosphere using standard
Schlenk techniques. All solvents were purified and degassed by standard procedures. The Schiff
base compounds (1a-1d) and [Ru(p-cymene)(μ-Cl)Cl]
2
were synthesized according to
1
13
procedures described [29-31]. H and C NMR spectra were recorded on a 300 MHz or 500
MHz NMR spectrometer at room temperature. Chemical shifts (δ) are given in ppm relative to
1
13
internal TMS and are internally referenced to residual H and C solvent resonances. IR spectra
were recorded on a Nicolet AVATAR-360IR spectrometer. Elemental analyses were performed on
a PerkinElmer 2400 CHN analyzer.
2
2
1
.2. Synthesis
.2.1. Synthesis of the Schiff-bases 1a-1d
a: 2-Hydroxy-1-naphthaldehyde (516.6 mg, 3.0 mmol) and aniline (0.273 ml, 3.0 mmol) were
3
heated to 75 °C for 3 h in the presence of a catalytic amount of CH COOH (0.028 ml, 1.5 mmol)
in 12 ml MeOH. The solvent was removed under reduced pressure, and the obtained solid was
1
washed with 2 ml EtOH three times. Yellow solid; yield: (555.8 mg, 75%). H NMR (300 MHz,
CDCl ): δ 15.50 (s, 1H, OH), 9.30 (s, 1H, CHN), 8.08 (d, J = 9.0 Hz, 1H, Ar-H), 7.79 (d, J =
3
9
1
1
1
.0 Hz, 1H, Ar-H), 7.70 (d, J = 6.0 Hz, 1H, Ar-H), 7.64-7.22 (m, 7H, Ar-H), 7.08 (d, J = 9.0 Hz,
-1
H, Ar-H); IR (KBr, cm ): 3432 (m), 2368 (m), 1622 (vs), 1594 (s), 1543 (s), 1520 (s), 1491 (s),
346 (s), 1318 (s), 1142 (m), 838 (s), 759 (vs), 687 (s).
b: 2-Hydroxy-1-naphthaldehyde (516.6 mg, 3.0 mmol) and p-toluidine (0.330 ml, 3.0 mmol)
were heated to 75 °C for 3 h in the presence of a catalytic amount of CH COOH (0.028 ml,
3
1
.5 mmol) in 12 ml MeOH. The solvent was removed under reduced pressure, and the obtained
1
solid was washed with 2 ml EtOH three times. Yellow solid; yield: (736.0 mg, 94%). H NMR
(
300 MHz, CDCl
3
): δ 15.62 (s, 1H, OH), 9.29 (s, 1H, CHN), 8.07 (d, J = 6.0 Hz, 1H, Ar-H), 7.78
3

(
d, J = 9.0 Hz, 1H, Ar-H), 7.70 (d, J = 9.0 Hz, 1H, Ar-H), 7.40-7.18 (m, 5H, Ar-H), 7.07 (d, J =
-1
9
1
1
3
.0 Hz, 1H, Ar-H), 2.39 (s, 3H, p-Me); IR (KBr, cm ): 3447 (m), 3023 (w), 2915 (w), 2854 (w),
617 (vs), 1581 (s), 1540 (m), 1512 (s), 1326 (s), 968 (m), 853 (m), 815 (s), 738 (s).
c: 2-Hydroxy-1-naphthaldehyde (516.6 mg, 3.0 mmol) and 4-chloroaniline (382.7 mg,
.0 mmol) were heated to 75 °C for 3 h in the presence of a catalytic amount of CH COOH
3
(0.028 ml, 1.5 mmol) in 12 ml MeOH. The solvent was removed under reduced pressure, and the
obtained solid was washed with 2 ml EtOH three times. Yellow solid; yield: (792.4 mg, 94%).
1
H NMR (300 MHz, CDCl ): δ 15.29 (s, 1H, OH), 9.33 (s, 1H,CHN), 8.10 (d, J = 6.0 Hz, 1H,
3
Ar-H), 7.82 (d, J = 9.0 Hz, 1H, Ar-H), 7.73 (d, J = 9.0 Hz, 1H, Ar-H), 7.53 (d, J = 9.0 Hz, 1H,
-1
Ar-H), 7.43-7.27 (m, 5H, Ar-H), 7.11 (d, J = 9.0 Hz, 1H, Ar-H); IR (KBr, cm ): 3447 (m),
3
8
1
064 (w), 3033 (w), 1617 (vs), 1578 (m), 1563 (s), 1540 (m), 1486 (s), 1326 (s), 1165 (m),
58 (w), 820 (s), 746 (s), 723 (w).
d: 2-Hydroxy-1-naphthaldehyde (516.6 mg, 3.0 mmol) and 4-nitroanline (414.4 mg, 3.0 mmol)
were heated to 75 °C for 3 h in the presence of a catalytic amount of CH COOH (0.028 ml,
3
1
.5 mmol) in 12 ml MeOH. The solvent was removed under reduced pressure, and the obtained
1
solid was washed with 2 ml EtOH three times. Red solid; yield: (727.1 mg, 83%). H NMR
(
300 MHz, CDCl
3
): δ 14.87 (s, 1H, OH), 9.38 (s, 1H, CHN), 8.34 (d, J = 9.0 Hz, 1H, Ar-H), 8.12
(
d, J = 6.0 Hz, 1H, Ar-H), 7.87 (d, J = 9.0 Hz, 1H, Ar-H), 7.75 (d, J = 9.0 Hz, 1H, Ar-H), 7.56 (t,
-1
J = 9.0 Hz, 1H, Ar-H), 7.46-7.37 (m, 3H, Ar-H), 7.12 (d, J = 9.0 Hz, 1H, Ar-H); IR (KBr, cm ):
3
444 (m), 3070 (w), 1621 (s), 1581 (s), 1547 (s), 1504 (s), 1336 (vs), 1287 (s), 1107 (s), 860 (s),
8
28 (s), 760 (s).
2
.2.2. Synthesis of half-sandwich ruthenium complexes with naphthalene-based Schiff base
ligands 2a-2d
A solution of [Ru(p-cymene)(μ-Cl)Cl] (122.4 mg, 0.20 mmol), the Schiff base (0.50 mmol, 1a
2
2 3
(123.5 mg), 1b (130.6 mg), 1c (140.5 mg), 1d (146.0 mg)) and K CO (69.0 mg, 0.50 mmol) in
MeOH (15 mL) was purged with N and then stirred for 4 h at room temperature. The reaction
2
4

mixture was separated from insoluble salts by filtration and dried in vacuo. The crude material
was subjected to silica gel chromatography with ethyl acetate and petroleum ether (1:1) to give
the dark red half-sandwich ruthenium complexes.
Chloro(p-cymene)(1-((phenylimino)methyl)naphthalen-2-ol-κ-O,N)ruthenium(II) 2a:
1
Yield: (140.6 mg, 68%). H NMR (300 MHz, CDCl ): δ 8.58 (s, 1H, CHN), 7.69-7.56 (m, 5H,
3
Ar-H), 7.53-7.44 (m, 2H, Ar-H), 7.36-7.25 (m, 2H, Ar-H), 7.19-7.10 (m, 2H, Ar-H), 5.37 (d, J =
6
6
1
1
9
6
2
1
.0 Hz, 1H, p-Cy), 5.30 (d, J = 6.0 Hz, 1H, p-Cy), 4.97 (d, J = 6.0 Hz, 1H, p-Cy), 4.19 (d, J =
.0 Hz, 1H, p-Cy), 2.62 (m, 1H, CHMe ), 2.13 (s, 3H, ArMe), 1.16 (d, J = 6.0 Hz, 3H, CHMe ),
2
2
1
3
.09 (d, J = 6.0 Hz, 3H, CHMe ). C NMR (125 MHz, CDCl ): δ 165.99, 159.62, 157.74,
2
3
35.83, 134.85, 128.86, 127.29, 126.70, 126.61, 125.40, 124.21, 121.88, 118.65, 108.16, 101.27,
7.88, 86.62, 84.35, 83.78, 80.43, 30.40, 22.82, 21.56, 18.57. Anal. Calcd. for C27 26NORuCl: C,
H
-1
2.72; H, 5.07; N, 2.71. Found: C, 62.75; H, 5.10; N, 2.78. IR (KBr, cm ): 3032 (w), 2968 (w),
870 (w), 1617 (vs), 1604 (vs), 1576 (s), 1536 (vs), 1453 (m), 1364 (s), 1179 (s), 1160 (m),
081 (m), 835 (m), 820 (s), 745 (s), 590 (s).
Chloro(p-cymene)(1-((p-tolylimino)methyl)naphthalen-2-ol-κ-O,N)ruthenium(II) 2b:
1
Yield: (137.8 mg, 65%). H NMR (500 MHz, CDCl
3
): δ 8.57 (s, 1H, CHN), 7.67 (d, J = 10.0 Hz,
1
H, Ar-H), 7.59 (d, J = 10.0 Hz, 1H, Ar-H), 7.55 (d, J = 5.0 Hz, 2H, Ar-H), 7.31-7.24 (m, 4H,
Ar-H), 7.16 (d, J = 10.0 Hz, 1H, Ar-H), 7.12 (d, J = 10.0 Hz, 1H, Ar-H), 5.38 (d, J = 5.0 Hz, 1H,
p-Cy), 5.30 (d, J = 5.0 Hz, 1H, p-Cy), 4.97 (d, J = 5.0 Hz, 1H, p-Cy), 4.22 (d, J = 5.0 Hz, 1H,
p-Cy), 2.64 (m, 1H, CHMe ), 2.45 (s, 3H, ArMe), 2.16 (s, 3H, ArMe), 1.18 (d, J = 5.0 Hz, 3H,
2
1
3
CHMe
2
), 1.19 (d, J = 10.0 Hz, 3H, CHMe
2
). C NMR (125 MHz, CDCl
3
): δ 166.29, 158.04,
1
1
1
2
1
57.86, 136.77, 136.07, 135.26, 129.70, 129.20, 127.61, 127.00, 125.78, 124.36, 122.19, 119.04,
08.57, 101.53, 101.49, 98.36, 87.12, 87.08, 84.74, 84.12, 80.99, 30.82, 23.21, 22.01, 21.47,
9.02. Anal. Calcd. for C H NORuCl: C, 63.33; H, 5.31; N, 2.64. Found: C, 63.39; H, 5.27; N,
2
8
28
-1
.70. IR (KBr, cm ): 3040 (w), 2964 (w), 2921 (w), 2870 (w), 1617 (vs), 1602 (vs), 1574 (s),
506 (w), 1451 (m), 1362 (s), 1179 (s), 1160 (m), 1088 (m), 832 (m), 820 (m), 743 (m), 588 (m).
5

Chloro(p-cymene)(1-((4-chlorophenylimino)methyl)naphthalen-2-ol-κ-O,N)ruthenium(II)
1
2
c: Yield: (158.7 mg, 72%). H NMR (300 MHz, CDCl
3
): δ 8.53 (s, 1H, CHN), 7.68-7.54 (m, 4H,
Ar-H), 7.44 (t, J = 9.0 Hz, 2H, Ar-H), 7.32 (t, J = 6.0 Hz, 1H, Ar-H), 7.14 (t, J = 6.0 Hz, 2H,
Ar-H), 5.39 (d, J = 6.0 Hz, 1H, p-Cy), 5.32 (d, J = 6.0 Hz, 1H, p-Cy), 4.98(d, J = 6.0 Hz, 1H,
p-Cy), 4.26 (d, J = 6.0 Hz, 1H, p-Cy), 2.63 (m, 1H, CHMe ), 2.15 (s, 3H, ArMe), 1.18 (d, J =
2
1
3
6
.0 Hz, 3H, CHMe
58.43, 158.30, 136.57, 135.14, 132.66, 129.34, 129.30, 127.83, 127.03, 126.05, 125.75, 122.45,
18.91, 108.54, 101.77, 98.64, 87.10, 84.57, 83.75, 80.93, 30.87, 23.16, 22.04, 19.03. Anal.
Calcd. for C H NORuCl : C, 58.81; H, 4.57; N, 2.54. Found: C, 58.85; H, 4.60; N, 2.52. IR
2
), 1.10 (d, J = 6.0 Hz, 3H, CHMe
2
). C NMR (125 MHz, CDCl
3
): δ 166.72,
1
1
2
7
25
2
-1
(
KBr, cm ): 3036 (w), 2964 (w), 2925 (w), 2870 (w), 1615 (vs), 1602 (vs), 1581 (vs), 1504 (w),
1
451 (s), 1362 (vs), 1181 (s), 1139 (m), 822 (s), 745 (s), 652 (w).
Chloro(p-cymene)(1-((4-nitrophenylimino)methyl)naphthalen-2-ol-κ-O,N)ruthenium(II)
1
2
1
5
1
d: Yield: (157.4 mg, 70%). H NMR (500 MHz, CDCl ): δ 8.53 (s, 1H, CHN), 8.34 (d, J =
3
0.0 Hz, 2H, Ar-H), 7.87 (d, J = 10.0 Hz, 2H, Ar-H), 7.64 (t, J = 10.0 Hz, 2H, Ar-H), 7.56 (d, J =
.0 Hz, 1H, Ar-H), 7.34 (t, J = 10.0 Hz, 1H, Ar-H), 7.17-7.14 (m, 2H, Ar-H), 5.40 (d, J = 5.0 Hz,
H, p-Cy), 5.34 (d, J = 5.0 Hz, 1H, p-Cy), 4.98 (d, J = 5.0 Hz, 1H, p-Cy), 4.28 (d, J = 5.0 Hz, 1H,
p-Cy), 2.62 (m, 1H, CHMe
2
), 2.16 (s, 3H, ArMe), 1.18 (d, J = 10.0 Hz, 3H, CHMe
2
), 1.11 (d, J =
1
3
5
1
8
5
2
.0 Hz, 3H, CHMe ). C NMR (125 MHz, CDCl ): δ 167.59, 164.42, 158.26, 146.33, 137.38,
2
3
35.01, 129.46, 128.13, 127.12, 128.87, 125.75, 125.08, 122.85, 118.81, 108.76, 102.15, 99.10,
7.11, 84.35, 83.39, 80.91, 30.95, 23.10, 22.11, 19.05. Anal. Calcd. for C H N O RuCl: C,
2
7
25
2
3
-1
7.70; H, 4.48; N, 4.98. Found: C, 57.68; H, 4.47; N, 5.03. IR (KBr, cm ): 3044 (w), 2972 (w),
934 (w), 2874 (w), 1617 (s), 1602 (s), 1574 (s), 1530 (s), 1515 (s), 1455 (w), 1336 (vs),
1181 (s), 1111 (m), 820 (m), 743 (m), 588 (w).
2
.2.3. General procedure for the reduction of nitroarenes to anilines with half-sandwich
ruthenium catalysts
6

The half-sandwich ruthenium complex (0.003 mmol, 0.01 equiv) was dissolved in solvent
2.0 mL), and then the appropriate nitroarene (0.3 mmol, 1.0 equiv) and NaBH (45.4 mg,
.2 mmol, 4.0 equiv) were added. The resulting mixture was stirred at room temperature in a
(
4
1
closed vessel. After completion of the reation (monitored by TLC), the crude reaction mixture
was extracted with ether (3×2 mL). After the solvents were removed in vacuo from the combined
organic extracts, the crude products were loaded directly onto a column of silica gel and purified
by column chromatography using petroleum ether and ethyl acetate (1:3) to get the corresponding
products [27].
2
.3. X-ray structure determination
Diffraction data of 2a, 2b and 2c were collected on a Bruker AXS SMART APEX diffractometer
equipped with a CCD area detector using Mo Kα radiation (λ = 0.71073 Å). All the data were
collected at 298 K and the structures were solved by direct methods and subsequently refined on
2
F by using full-matrix least-squares techniques (SHELXL) [32]. SADABS absorption
corrections were applied to the data [33]. All non-hydrogen atoms were refined anisotropically,
and hydrogens were located at calculated positions. All calculations were performed using the
Bruker Smart program. A summary of the crystallographic data and selected experimental
information are given in table 1. Selected bond angles and distances are given in table 2.
3
3
. Results and discussion
.1. Synthesis of Schiff base compounds and half-sandwich ruthenium complexes
According to literature methods, the naphthalene-based Schiff base derivatives 1a-1d were
prepared by condensation of 2-hydroxy-1-naphthaldehyde with different p-substituted aryl
amines in a 1:1 mole ratio in methanol in the presence of a catalytic amount of CH
3
COOH
(scheme 1) [30, 31].
7

O
OH
NH2
N
R
OH
MeOH,CH COOH,reflux
3
+
R
1
1
1
1
a R = H
b R = CH3
c R = Cl
d R = NO2
Scheme 1. Synthesis of Schiff base compounds (1a-1d).
The dark red half-sandwich complexes [(p-cymene)LRuCl] (2a-2d) were obtained by
treatment of 2 equivalents of the Schiff base (1a-1d) with [(p-cymene)Ru(μ-Cl)Cl] in the
2
presence of K CO in MeOH at room temperature (scheme 2). The half-sandwich ruthenium
2
3
complexes (2a-2d) were isolated as pure complexes by chromatography on silica gel using ethyl
acetate and petroleum ether (1:1) as eluent in yields of 65-72%. All complexes have been
characterized by IR and NMR spectroscopy as well as elemental analysis. The half-sandwich
ruthenium complexes are air and moisture stable, soluble in chlorohydrocarbons, alcohol, and
acetonitrile but insoluble in water.
8

Cl
Cl
Ru Cl
OH
Ru
Ru
N
R
MeOH
O
Cl
2
2
N
R
+
K CO , RT
2
3
Cl
2
2
2
2
a R = H
b R = CH3
c R = Cl
d R = NO2
Scheme 2. Synthesis of half-sandwich ruthenium complexes (2a-2d).
1
The H NMR spectrum of 2d displays a distinct resonance shift of the Schiff base
ligand’s protons in comparison with the equivalent protons in the free compound. The proton
signals at 8.53, 8.34, 7.87, 7.64, 7.56, 7.34, 7.17, 5.40, 5.34, 4.98, 4.28, 2.62, 2.16, 1.18 and 1.11
1
ppm in the H NMR spectrum can be easily assigned to each of the corresponding hydrogen
atoms of 2d. The proton signal at 14.87 ppm corresponds to the OH group of the free ligand
which disappears in 2d due to coordination. Four doublets appear at δ = 5.40, 5.34, 4.98 and
4
.28 ppm with a coupling constant of J = 5.0 Hz, respectively, which indicates the presence of
the cymene ring [34, 35].
X-ray diffraction of single crystals for 2a, 2b and 2c were obtained. The crystals were
grown by slow diffusion of diethyl ether into a concentrated solution of the complexes in
methanol. The crystallographic data for 2a, 2b and 2c are summarized in table 1. The molecular
structures of 2a, 2b and 2c are shown in figure 1. All structures were solved with monoclinic
P2 /n space groups. As shown in figure 1, each ruthenium is coordinated by p-cymene, one
1
nitrogen and one oxygen of the Schiff base ligand, and one chloride adopting a typical distorted
piano-stool geometry. The Ru-O distances (2.0576(13) (2a), 2.0486(14) (2b) and 2.044(3) (2c))
and Ru-N distances (2.0857(15) (2a), 2.0772(16) (2b) and 2.078(3) (2c)) are consistent with
9

Ru-N/Ru-O bond length values reported for half-sandwich ruthenium complexes with [N,O]
anionic ligands [36-39], while the Ru-N bond lengths are shorter than those of half-sandwich
ruthenium complexes with diphenyl(2-pyridyl)phosphine ligands,
[(p-cymene)Ru(S=PPh2Py)Cl]PF6 (2.123(3)) and [(benzene)Ru(PPh2Py)Cl]PF6 (2.118(4)) [40].
The flexible dihedral angles (58.53° (2a), 65.95° (2b) and 65.71° (2c)) between the naphthalene
group and phenyl moiety of the Schiff base indicate the different special effects of different
substituent groups on the phenyl ring. Significant hydrogen bonding occurs in the crystal
structures of 2a-2c. Their chlorides are involved in intermolecular hydrogen bonds ((Cl and H of
cymene) (C(23)-C1(1) = 3.693(2) Å, C(23)-H(23)⋅⋅⋅C1(1) = 158° (2a); (C(24)-C1(1) =
3
.587(2) Å, C(24)-H(24)⋅⋅⋅C1(1) = 145° (2b) and (C(21)-C1(1) = 3.579(5) Å, C(21)-H(21)⋅⋅⋅C1(1)
145° (2c)). For 2a, adjacent units are connected to form a dimer through intermolecular
hydrogen bonds (figure 1(2)).
=
3
.2. The nitroarene reduction catalyzed by half-sandwich ruthenium complexes
The catalytic performance of half-sandwich ruthenium complexes 2a-2d has been evaluated for
reduction of nitroarenes using 1-chloro-4-nitrobenzene in the presence of NaBH (table 3). As
4
shown in table 3, 2d was very active towards reduction of nitroarenes. However, different
activities exhibited by 2a-2d suggest that electronic effects of the substituted groups influence
catalyst performance. For example, 2d with a -NO exhibited higher catalytic activity than the
2
other complexes (table 3, entries 1-4) because the electron-deficient metal center facilitated
formation of the hydride intermediate. Thus 2d was chosen as the optimal catalyst to screen
various solvents (table 3, entries 5-7). No desired products were detected in water due to the poor
solubility of the catalyst (table 3, entry 10). When the catalyst amount was decreased, the
conversion also decreased even with a longer reaction time (table 3, entry 11). No catalytic
activity was observed for nitroarene reduction without addition of a ruthenium complex (table 3,
entries 8 and 9). From these studies, the optimal conditions for the reduction of nitroarenes were
10

4
using 2d as catalyst (1 mol %) in the presence of four equivalents of NaBH at room
temperature.
With the optimal reaction condition in hand, we started to expand the scope and
efficiency of this methodology. Many functionalized anilines were obtained in excellent yields
(
table 4). As shown in table 4, electron-withdrawing and electron-donating substituents on the
aromatic ring were converted to the desired products in excellent yields. In the case of
-nitrobenzaldehyde, the -CHO group was reduced together with the -NO group. Under the
4
2
same reaction conditions, -NHCOMe was unchanged (table 4, entries 11 and 12).
The half-sandwich ruthenium hydride active species can be confirmed through in situ
NMR studies (δ -10.18 ppm, Supporting Information). The proton signal at -10.18 ppm
corresponds to a hydride, which indicates the presence of a Ru-H intermediate [41, 42]. A
possible mechanism of nitroarene reduction is an outer-sphere mechanism through a
half-sandwich ruthenium hydride intermediate as previously reported (scheme 3) [28].
NO2
O
N
H
Ru
Cl
Ru Cl
Ru
H
O
Cl
O
O
O
H
N
NaBH4
N
HN
NaBH4
NaBH4
NO
outer-sphere mechanism
RuH
NH2
two more cycle
Cl
Cl
Scheme 3. Probable mechanism for the reduction of nitroarenes catalyzed by ruthenium
complexes.
11

4
. Conclusion
We have synthesized and characterized four half-sandwich ruthenium complexes with
naphthalene-based Schiff base ligands and evaluated their ability as catalysts for nitroarene
reduction in the presence of NaBH in ethanol at room temperature. The hydrogenation of
4
nitroarenes exhibits varied functional group compatibility and provides anilines in excellent
yields. Moreover, a combination of spectroscopic studies and X-ray crystallographic studies also
confirmed the molecular structure of the half-sandwich ruthenium complexes.
Appendix. Supplementary material
The crystallographic data (excluding structure factors) for the structures in this paper have been
deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK. Copies of the data can be obtained free of charge on quoting the depository
numbers CCDC-1496634 (2a), CCDC-1496633 (2b) and CCDC-1496635 (2c) (Fax:
+
44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Acknowledgement
We acknowledge financial support from the National Nature Science Foundation of China
(
21102004), and the Natural Science Foundation of the Anhui Higher Education Institutions of
China (KJ2016A845), and Special and Excellent Research Fund of Anhui Normal University,
and Training Programs of Innovation and Entrepreneurship for Undergraduates of Anhui Normal
University (201610370080).
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15

(1)
(2)
(3)
(4)
Figure 1. (1) Molecular structure of 2a with thermal ellipsoids drawn at the 50% ll evel. (2) The
dimer structure connected by intermolecular hydrogen bonds in 2a. (3) Molecula rr structure of 2b
with thermal ellipsoids drawn at the 50% level. (4) Molecular structure of 2c with thermal
ellipsoids drawn at the 50% level; all hydrogens are omitted for clarity.
1
6

Table 1. Crystallographic data and structure refinement parameters for ruthenium complexes.
2
a
2b
2c
Empirical formula
C H ClNORu
C H ClNORu
28 28
C H Cl NORu
2
7
26
27 25
2
Formula weight
517.01
531.03
551.45
Crystal system, Space group
Monoclinic, P2
10.5933(7)
16.1394(11)
14.3720(10)
108.4530(10)
2330.8(3), 8
1.473
1
/n
Monoclinic, P2
12.7524(7)
14.8292(8)
12.9306(7)
94.1600(10)
2438.8(2), 4
1.446
1
/n
1
Monoclinic, P2 /n
a (Å)
b (Å)
c (Å)
β (°)
12.724(7)
14.803(8)
12.935(7)
94.074(6)
2430(2), 4
1.507
3
Volume (Å ), Z
3
D
c
(mg / m )
-1
μ (Mo-Kα) (mm )
F(000)
0.806
0.772
0.885
1056
1088
1120
θ range (°)
1.955 ~ 27.573
2.329 ~ 27.453
2.094 ~ 27.682
Limiting indices
-13, 13; -21, 20; -18, 18
5319 / 4902 [0.0507]
27.573 (98.8 %)
5319 / 0 / 283
-16, 16; -16, 18; -16, 16
5454/4439 [0.0246]
27.453 (97.4%)
5454 / 0 / 293
-16, 16; -19, 19; -16, 16
5590/4861 [0.1056]
27.682 (98.3 %)
5590 / 0 / 286
Reflections / unique [R(int)]
Completeness to θ (°)
Data / restraints / parameters
2
Goodness-of-fit on F
1.062
1.028
1.088
a
R
1
, wR
2
[I > 2σ(I)]
R
1
= 0.0253, wR
2
= 0.0644
R
1
= 0.0278, wR
2
= 0.0665
R
1
2
= 0.0751, wR = 0.2340
R , wR (all data)
R = 0.0282, wR = 0.0672
R = 0.0389, wR = 0.0719
R = 0.0834, wR = 0.2442
1
2
1
2
1
2
1
2
a
2
2
2
2 2 1/2
R = Σ||F |-|F ||/Σ|F |; wR = [Σw(|F |-|F |) /Σw|F | ] .
1
o
c
o
2
o
c
o
17

Table 2. Selected bond lengths (Å) and angles (°) for ruthenium complexes.
2
a
2b
2c
Ru1-O1
Ru1-N1
Ru1-Cl1
Cycent-Ru
2.0576(13)
2.0857(15)
2.4368(5)
1.6639(1)
2.0486(14)
2.0772(16)
2.4235(7)
1.6675(3)
2.044(3)
2.078(3)
2.4204(17)
1.6661(7)
a
N1-Ru1-O1
N1-Ru1-Cl1
O1-Ru1-Cl1
Cycent-Ru-O1
Cycent-Ru-N1
Cycent-Ru-Cl1
86.99(5)
85.88(5)
85.94(5)
124.20(3)
131.33(4)
127.89(2)
87.05(6)
84.34(5)
84.61(5)
125.38(4)
131.01(5)
128.99(2)
87.02(12)
84.44(9)
84.57(10)
125.35(1)
131.07(1)
128.83(5)
a
Cycent is the computed cymene ring carbon centroid.
18

a
Table 3. Optimization of the reaction conditions for 1-chloro-4-nitrobenzene reduction using ruthenium complexes.
b
Entry Catalyst Solvent Time (h) Yield (%)
1
2
3
4
5
6
7
8
9
1
2a
2b
2c
EtOH
EtOH
EtOH
EtOH
MeOH
MeCN
1
1
1
1
9
9
4
2
5
2
1
86
88
90
2d
2d
2d
2d
2d
2d
2d
95
10
40
H O
2
No reaction
c
EtOH
EtOH
EtOH
EtOH
95
d
90
e
0
55
11
No reaction
a
Reaction conditions: 0.3 mmol 1-chloro-4-nitrobenzene, 1.2 mmol NaBH4,
b
c
Ru catalysts (3 mol %), solvent (2 mL), room temperature; Isolated yield;
d
e
2
d (1 mol %); 2d (0.5 mol %); 2d (1 mol %), 0.6 mmol NaBH .
4
19

a
Table 4. Screening of substrates for nitroarene reduction catalyzed by 2d.
b
Entry Substrate
NO2
Product
Time (h) Yield (%)
NH2
2
2
2
2
2
2
88
97
95
92
90
92
1
NO2
NO2
NO2
ΝΗ2
ΝΗ2
ΝΗ2
2
NC
NC
Cl
Br
I
3
Cl
4
Br
NO2
NH2
5
I
NO2
NH2
6
CH3
NO2
CH3
NH2
2
92
7
CH3
CH3
NO2
NH2
2
2
2
95
90
96
8
H C
H C
3
3
NO2
NH2
9
1
HOH C
HOH C
2
2
NO2
NO2
NH2
0
H N
2
H N
2
NH2
2
95
11
OHC
HOH C
2
20

NO2
ΝΗ
2
2
95
1
2
H NOC
2
H NOC
2
a
Reaction conditions: 0.3 mmol nitroarene, 1.2 mmol NaBH , 2d (0.003 mmol, 1 mol %),
4
b
EtOH (2 mL), room temperature. Isolated yield.
21

22