Half-sandwich ruthenium complexes with Schiff base ligands bearing a hydroxyl group: Preparation, characterization and catalytic activities

Article abstract of DOI:10.1002/aoc.5289

Three half-sandwich ruthenium(II) complexes with hydroxyl group functionalized Schiff-base ligands [Ru(p-cymene)LCl] (2a-2c) have been synthesized and characterized. All ruthenium complexes were fully characterized by ¹H and ¹³C NMR spectra, mass spectrometry and infrared spectrometry. The molecular structure of ruthenium complex 2c was confirmed by single-crystal X-ray diffraction methods. Furthermore, these half-sandwich ruthenium complexes were found to exhibit high catalytic activity for nitro compounds reduction using NaBH₄ reducing agent in the presence of cetyltrimethylammonium bromide (CTAB) in water at room temperature.

Full text of DOI:10.1002/aoc.5289

Received: 14 August 2019
DOI: 10.1002/aoc.5289
Revised: 9 September 2019
Accepted: 14 September 2019
F U L L P A P E R
Half‐sandwich ruthenium complexes with Schiff base
ligands bearing a hydroxyl group: Preparation,
characterization and catalytic activities
Wei‐Guo Jia
| Zhi‐Bao Wang | Xue‐Ting Zhi
The Key Laboratory of Functional
Molecular Solids, Ministry of Education,
Anhui Laboratory of Molecular‐Based
Materials (State Key Laboratory
Three half‐sandwich ruthenium(II) complexes with hydroxyl group functional-
ized Schiff‐base ligands [Ru(p‐cymene)LCl] (2a‐2c) have been synthesized and
1
13
characterized. All ruthenium complexes were fully characterized by H and
C
Cultivation Base), College of Chemistry
and Materials Science, Anhui Normal
University, Wuhu 241002, China
NMR spectra, mass spectrometry and infrared spectrometry. The molecular
structure of ruthenium complex 2c was confirmed by single‐crystal X‐ray dif-
fraction methods. Furthermore, these half‐sandwich ruthenium complexes
were found to exhibit high catalytic activity for nitro compounds reduction
Correspondence
Wei‐Guo Jia, College of Chemistry and
Materials Science Anhui Normal
University, No.189, south road jiuhua,
Wuhu city 241002, Anhui Province P.R.
China
using NaBH reducing agent in the presence of cetyltrimethylammonium bro-
4
mide (CTAB) in water at room temperature.
Email: wgjiasy@ahnu.edu.cn
KEYWORDS
nitro compound, reduction, ruthenium, Schiff‐base, structure
Funding information
Natural Science Foundation of Anhui
Province, Grant/Award Number:
1708085MB44; Natural Science Founda-
tion of the Anhui Higher Education Insti-
tutions of China, Grant/Award Number:
KJ2016A845; National Nature Science
Foundation of China, Grant/Award
Number: 21102004
1
| INTRODUCTION
coordinated by the hydroxyl group functionalized
bipyridine ligands show unique catalytic properties, such
Metal and ligand synergistic effect is an important cata-
lysts design concept, many efficient catalysts were applied
as transfer hydrogenation reaction, CO hydrogenation
2
[
18–20]
and formic acid decomposition.
The hydroxyl group
[1–8]
in a wide variety of chemical transformations.
The
functionalized bipyridine reversibly generates hydroxyl,
keto, and oxyanion structure, which will significantly
improve water‐soluble and the hydrogenation ability of
complexes containing functional ligands with two or
more distinct states can adjust the electronic configura-
tion of the metal center, accept or donate proton, and
induce reversible structural changes in the processes of
substrate activation and product formation. The com-
plexes based on “synergistic” ligands have exhibited
[21]
half‐sandwich metal complex under basic conditions.
Chen and co‐worker found that the ruthenium com-
plexes with unsymmetrical NNN ligands with 2‐
hydroxypyridyl fragment showed high catalytic activity
for β‐alkylation of secondary alcohols with primary alco-
[9,10]
remarkable catalytic activity.
During the past
[
22]
decades, half‐sandwich metal (M = Rh, Ir and Ru) com-
hols.
Himeda and co‐worker discovered an efficient
plexes were widely used as powerful catalysts in organic
proton‐responsive and water‐soluble half‐sandwich irid-
ium complex bearing one or more hydroxyl groups func-
tionalized bipyridine ligands for highly CO2
[11–17]
transformation and anticancer reagent.
The half‐
sandwich metal (M = Rh, Ir and Ru) complexes
Appl Organometal Chem. 2019;e5289.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
2 of 9
https://doi.org/10.1002/aoc.5289

3
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JIA ET AL.
hydrogenation and formic acid decomposition under mild
are given in ppm relative to internal TMS and are inter-
[23,24]
1
13
conditions.
They found that the catalysts bearing
nally referenced to residual H and C solvent resonances.
IR spectra were recorded on a Niclolet AVATAR‐360IR
spectrometer. Elemental analyses were performed on a
PerkinElmer 2400 CHN analyzer. Mass spectra were
obtained with MicroTof (Bruker Daltonics, Bremen, Ger-
many) spectrometers.
hydroxyl group act as cooperating ligands would revers-
–
ibly deprotonated to generate an oxyanion (O ) to
[25]
enhance the catalytic efficiency.
Zhang and co‐worker
reported Cp*Ir complexes with ortho‐hydroxyl group
functionalized bipyridine ligands exhibit remarkably high
catalytic
activity
for
hydrogenation
of
5‐
[26]
hydroxymethylfurfural in acidic water.
The catalytic
2.2 | Synthesis
efficiencies of Cp*Ir complex with four hydroxyl groups
functionalized pyrimidine ligands were promoting fur-
ther for hydrogenation of CO2.
2.2.1 | Synthesis of the Schiff‐Base ligand
1a‐1c
[27]
Water is readily available, inexpensive, nontoxic and
environmentally benign solvent in nature, and has gained
2‐hydroxy‐1‐naphthaldehyde (516.6 mg, 3.0 mmol) and
corresponding 2‐amino‐4‐substitute phenol (3.0 mmol)
were stirred in MeOH (15 ml) and were heated to 75 °C
for three hours in the presence of catalytic amount of
acetic acid. The obtained yellow solids were filtered off,
washed with EtOH for three times (1 ml) and dried under
the vacuum in an oven (80 °C) overnight.
[
28–
immense attention in transfer hydrogenation reaction.
31]
Thus, efforts towards hydrogenations of unsaturated
compounds with new water‐soluble transition metal com-
plexes with functional synergistic ligands are required. We
previously reported the synthesis and catalytic activity of a
series of half‐sandwich ruthenium complexes containing
[
32–
1
[
N,O] ligand such as benzo oxazoline and Schiff‐base.
1a: Yield: (760.6 mg, 96%). H NMR (300 MHz, DMSO‐
3
6]
These complexes catalyze nitro compounds reduction
d6): δ 15.70 (d, J = 9.0Hz, 1H), 10.31 (s, 1H), 9.48
(d, J = 9.0Hz, 1H), 8.37 (d, J = 9.0Hz, 1H), 7.92
(d, J = 9.0Hz, 1H), 7.78 (d, J = 9.0Hz, 1H), 7.65
(d, J = 6.0Hz, 1H), 7.46 (t, J = 6.0Hz, 1H), 7.24
(t, J = 6.0Hz, 1H), 7.09 (t, J = 6.0Hz, 1H), 7.00‐6.90
using NaBH reducing agent under basic conditions. More-
4
over, we demonstrated that the active species during catal-
ysis is Ru‐H intermediate. With respect to the development
of our target catalyst, the necessary feature is water solubil-
ity with no additives.
13
(m, 2H), 6.77 (d, J = 9.0Hz, 1H). C NMR (75 MHz,
Encouraged by these results and in our continuing
interest in the nitro compounds reduction, herein we
preparation a general and effective half‐sandwich ruthe-
nium complexes with hydroxyl group incorporate in
Schiff bass ligands that efficiently catalyzes the nitro com-
DMSO‐d ): δ 178.1, 149.9, 148.9, 138.4, 134.4, 129.1,
6
129.0, 128.6, 127.2, 126.3, 125.6, 123.5, 120.3, 120.2,
–1
118.0, 116.4, 108.2. IR (KBr cm ): 3483 (s), 3033 (m),
1632 (s), 1585 (m), 1549 (s), 1514 (s), 1459 (s), 1360 (s),
1140 (s), 745 (s).
1
pounds reduction in water using NaBH as the reductant.
1b: Yield: (793.0 mg, 95%). H NMR (300 MHz, DMSO‐
4
Such Schiff‐base ligands bearing OH unit are consider as
d6): δ 15.67 (d, J = 9.0Hz, 1H), 10.05 (s, 1H), 9.45
(d, J = 9.0 Hz, 1H), 8.37 (d, J = 9.0 Hz, 1H), 7.76
(d, J = 6.0 Hz, 2H), 7.65 (d, J = 9.0 Hz, 1H), 7.46
(t, J = 6.0 Hz, 1H), 7.24 (t, J = 6.0 Hz, 1H), 6.87 (s, 2H),
“
proton‐responsive ligands”. This property makes them
pH‐switchable and enables modification of the polarity
and electron‐donating ability of the ligand, thus tuning
the catalytic activity and water‐solubility of the com-
13
6.76 (d, J = 12.0 Hz, 1H), 2.29 (s, 3H). C NMR (75
[23–25]
plexes.
MHz, DMSO‐d ): δ 128.3, 149.4, 146.6, 138.4, 134.4,
6
1
29.4, 129.3, 128.5, 127.6, 126.3, 125.7, 123.4, 120.1,
–1
2
2
| EXPERIMENTAL SECTION
.1 | Materials and measurements
118.1, 118.1, 116.3, 108.1, 20.8. IR (KBr cm ) : 3445 (m),
022 (m), 2923 (m), 1621 (vs), 1593 (s), 1544 (s), 1516 (s),
1459 (m), 1352 (vs), 1140 (s), 887 (m), 813 (s), 750 (s).
3
1
1
c: Yield: (860.6 mg, 96%). H NMR (300 MHz, DMSO‐
Commercial reagents were analytical grade and used as
received. All the operations were carried out under a pure
nitrogen atmosphere using standard Schlenk techniques.
All solvents were purified and degassed by standard proce-
dures. The Schiff base compounds (1a–1c) and [Ru(p‐
d6): δ 15.63 (d, J = 9.0Hz, 1H), 10.54 (s, 1H), 9.51
(d, J = 9.0Hz, 1H), 8.45 (d, J = 9.0 Hz, 1H), 8.10 (s, 1H),
7.81 (d, J = 9.0Hz, 1H), 7.67 (d, J = 9.0 Hz, 1H), 7.47
(t, J = 9.0Hz, 1H), 7.26 (t, J = 9.0 Hz, 1H), 7.10
(d, J = 9.0 Hz, 1H), 6.96 (d, J = 6.0Hz, 1H), 6.79
13
cymene)(μ‐Cl)Cl] were synthesized according to proce-
(d, J=9.0Hz, 1H). C NMR (75 MHz, DMSO‐d ): δ
2
6
[37–41] 1
dures described in the previous literature.
H and
C NMR were recorded on a 300 MHz or 500 MHz NMR
spectrometer at room temperature. Chemical shifts (δ)
177.5, 150.7, 147.9, 138.6, 134.2, 130.7, 129.4, 128.6,
13
126.5, 126.5, 125.1, 124.1, 123.8, 120.7, 117.7, 117.5,
–1
108.6. IR (KBr cm ) : 3439 (w), 3060 (m), 1632 (vs),

JIA ET AL.
4 of 9
1
1
8
582 (m), 1546 (s), 1505 (m), 1489 (m), 1349 (m), 1146 (s),
82 (m), 833 (s), 800 (m), 745 (s).
2c: Yield (145.4 mg, 64%). H NMR (300 MHz, CDCl ):
3
δ 8.48 (s, 1H), 7.65 (d, J = 9.0 Hz, 2H), 7.55 (d, J = 6.0 Hz,
1H), 7.36‐7.23 (m, 2H), 7.19‐7.11(m, 2H), 7.04 (d, J = 6.0 Hz,
2H), 5.47 (d, J = 3.0 Hz, 2H), 5.00 (d, J = 6.0 Hz, 1H), 4.24
(
d, J = 6.0 Hz, 1H), 2.76‐2.67 (m, 1H), 2.18 (s, 3H), 1.25
13
2
.2.2 | Synthesis of half‐Sandwich ruthe-
(d, J = 6.0 Hz, 3H), 1.16 (d, J = 9.0 Hz, 3H). C NMR
(125 MHz, CDCl ): δ 167.7, 159.6, 150.4, 146.4, 137.5,
135.0, 129.3, 128.6, 128.2, 127.1, 125.2, 124.3, 124.2,
23.0, 119.4, 119.1, 108.9, 101.6, 99.2, 88.5, 85.2, 83.8,
nium complexes with naphthalene‐based
Schiff Base ligands 2a‐2c
3
1
A solution of [Ru(p‐cymene)(μ‐Cl)Cl] (122.4 mg, 0.20
80.9, 30.9, 23.4, 21.8, 18.8. Anal. Calcd. for
C H NO RuCl : C 57.15, H 4.44, N 2.47 Found: C
2
mmol), schiff base (0.50 mmol), and K CO (69.0 mg,
2
3
27 25
2
2
0
.50 mmol) in MeOH (15 ml) was purged with N and
57.24, H 4.47, N 2.53. MS (MALDI‐TOF): calcd. for
2
+
+
then stirred for 4 hr at room temperature. The reaction
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 petro-
leum ether (2 : 1) to give dark red half‐sandwich ruthe-
C H NO RuCl [M‐Cl] 532.0617, found 532.0598. IR
27
25
2
1
–
(KBr cm ) : 3447 (m), 3059 (m), 2926 (m), 2926 (m),
2849 (m), 1617 (s), 1599 (s), 1576 (s), 1532 (vs), 1466
(vs), 1328 (s), 1282 (s), 1190 (s), 904 (w), 827 (s), 746 (s),
672 (s), 555(m)
nium complexes.
1
2
a: Yield (145.2 mg, 68%). H NMR (300 MHz, CDCl ):
3
2
.2.3 | General procedure for the reduc-
δ 8.53 (s,1H), 8.35 (s,1H), 7.64 (d, J = 9.0 Hz, 2H), 7.55
tion of nitro compounds with ruthenium
catalysts
(
(
(
(
(
d, J = 6.0 Hz, 1H), 7.33‐7.26 (m, 2H), 7.17‐6.98
m, 5H), 5.45 (s, 2H), 4.95 (d, J = 6.0 Hz, 1H), 4.19
d, J = 3.0 Hz, 1H), 2.76‐2.67 (m, 1H), 2.18 (s, 3H), 1.24
Ruthenium complex (0.003 mmol, 0.01 equiv) was dis-
solved in solvent (2.0 ml), then appropriate nitro com-
pounds (0.3 mmol, 1.0 equiv) CTAB (0.2 mmol) and
13
d, J = 6.0 Hz, 3H), 1.14 (d, J = 6.0 Hz, 3H). C NMR
125 MHz, CDCl ): δ 167.2, 159.6, 151.4, 146.1, 137.1,
3
1
1
8
35.1, 129.3, 128.83, 128.0, 127.0, 125.2, 124.5, 122.7,
19.8, 119.4, 118.0, 109.0, 101.2, 99.2, 88.5, 85.2, 83.9,
1.2, 30.9, 23.4, 21.7, 18.8. Anal. Calcd. for
NaBH (45.4 mg, 1.2 mmol, 4.0 equiv) was added. Subse-
4
quently, the resulting mixture was stirred at room tem-
perature in closed vessel. After completion of the
reaction (monitored by TLC), the crude reaction mixture
was extraction with ether (3×2 ml). After solvents were
removed in vacao from combined organic extracts, the
crude products loaded directly onto a column of silica
gel and purified by column chromatography petrol ether
and ethyl acetate (1 : 3) to get the corresponding
C H NO RuCl: C 60.84, H 4.92, N 2.63 Found: C
27
26
2
6
0.88, H 4.87, N 2.73. MS (MALDI‐TOF): calcd. for
+
+
C H NO Ru [M‐Cl] 498.1007, found 498.0990. IR
27
26
2
1
–
(KBr cm ) : 3458 (m), 3059 (m), 2961 (s), 2926 (s), 2853
(m), 1614 (s), 1599 (s), 1576 (s), 1535 (vs), 1465(vs), 1423
(s), 1381(s), 1354 (s), 1328(s), 1282(s), 1190(s), 1160 (s),
1
093 (m), 985(w), 820 (s), 746 (vs), 671(s), 557 (w).
[
34,42]
products.
1
2
b: Yield (142.5 mg, 65%). H NMR (300 MHz,
CDCl ): δ 8.52 (s,1H), 8.16 (s,1H), 7.64 (t, J = 9.0 Hz,
3
2
7
4
2
H), 7.55 (d, J = 9.0 Hz, 1H), 7.31 (t, J = 6.0 Hz, 1H),
.15‐6.97 (m, 4H), 6.84 (s,1H), 5.45 (d, J = 3.0 Hz, 2H),
.96 (d, J = 6.0 Hz, 1H), 4.22 (d, J = 6.0 Hz, 1H), 2.76‐
.67 (m, 1H), 2.38 (s, 3H), 2.18 (s, 3H), 1.25 (d,
2.3 | X‐ray structure determination
Diffraction data of 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
13
J = 9.0 Hz, 3H), 1.15 (d, J = 9.0 Hz, 3H). C NMR
125 MHz, CDCl ): δ 167.1, 159.5, 149.0, 145.8, 136.9,
(
3
1
1
8
35.2, 129.3, 129.2, 129.2, 128.0, 127.0, 125.3, 124.8,
22.7, 119.5, 117.7, 109.0, 101.2, 99.1, 88.5, 85.3, 84.1,
1.2, 30.9, 23.4, 21.7, 21.0, 19.8. Anal. Calcd. for
solved by direct methods and subsequently refined on
2
F
by using full‐matrix least‐squares techniques
[
43]
(SHELXL),
SADABS absorption corrections were
[
44]
C H NO RuCl: C 61.48, H 5.16, N 2.56 Found: C
applied to the data,
all non‐hydrogen atoms were
28
28
2
6
1.56, H 5.22, N 2.45. MS (MALDI‐TOF): calcd. for
refined anisotropically, and hydrogen atoms were located
at calculated positions. All calculation was performed
using the Bruker Smart program. A summary of the crys-
tallographic data and selected experimental information
are given in Table S1, selected bond angles and distances
are given in Table S2.
+
+
C H NO Ru [M‐Cl] 512.1164, found 512.1151. IR
28
28
2
1
–
(KBr cm ): 3456 (s), 3060 (w), 2967 (w), 2873 (w), 1615
(s), 1599 (s), 1579 (m), 1535(s), 1503 (m), 1464 (vs),
1
7
428(m), 1371 (m), 1286(m), 1190 (w), 983(w), 830 (s),
50 (s), 671 (m), 580 (w).

5
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JIA ET AL.
3
3
| RESULTS AND DISCUSSION
.1 | Synthesis of Schiff Base compounds
confirmed by positive‐mode electrospray ionization mass
spectrometry (ESI‐MS) (Figures S14‐S16, supporting
information).
and half‐Sandwich ruthenium complexes
X‐ray diffraction of single crystals for complex 2c was
obtained. The crystal was grown by slow diffusion of
diethyl ether into a concentrated solution of the complex
in methanol solution. The molecular structure of 2c is
shown in Figure 1. The crystal was solved with and
The naphthalene‐based Schiff base compounds 1a‐1c
were prepared by condensation of 2‐hydroxy‐1‐
naphthaldehyde with different 2‐amino‐4‐substitute phe-
nol in high yields in methanol solvent according to the
monoclinic P2 /c space groups. As shown in Figure 1,
1
[37–40]
published method.
A few drops of CH COOH were
each ruthenium atom is coordinated by p‐cymene, one
nitrogen atom and one oxygen atom of Schiff base ligand
and one chlorine atom adopted typical piano‐stool config-
uration. The Ru‐O distances (2.0673(10)) and Ru‐N dis-
tances (2.0909(13)) are consistent with Ru‐N/Ru‐O bond
length values reported for those of half‐sandwich ruthe-
3
added to the mixture to accelerate the condensation reac-
tion. The dark red half‐sandwich complexes [(p‐cymene)
LRuCl] (2a‐2c) were obtained by treatment of 2 equiv of
the schiff base (1a‐1c) with [(p‐cymene)Ru(μ‐Cl)Cl] in
2
the presence of K CO in MeOH at room temperature
2
3
[
45–48]
(Scheme 1). The pure half‐sandwich ruthenium com-
nium complexes.
The dihedral angle between naph-
plexes (2a‐2c) were achieved by column chromatography
on silica gel using ethyl acetate/petroleum ether (2:1, v:v).
All complexes have been characterized by a combination
of NMR spectroscopy, mass spectrometry, and X‐ray crys-
tallography. The half‐sandwich ruthenium complexes are
air and moisture stable, soluble in chlorohydrocarbon,
alcohol, acetonitrile solvents but slightly soluble in water.
But the ruthenium complexes are insoluble in non‐polar
solvents such as diethyl ether and hexane. Aqueous solu-
tions of the ruthenium complexes remain unchanged for
at least ten days with no sign of decomposition.
thalene group and phenyl moiety is 82.37°, which is
1
The H NMR spectra of complexes 2a‐2c display a dis-
tinct resonance shift of the Schiff base ligands protons in
comparison with the equivalent protons in the free com-
pounds. The proton signal at 15.6 ppm corresponds to
the OH group of the free ligand disappear in complexes
due to the coordination effect of the ligand with the
ruthenium metal. And proton singal appear at
δ = 8.5 ppm, which indicates the presence of free
hydroxyl group in the complexes. It was observed that
the broad and intense band at approximately 3450 cm–
due to the stretching vibration of hydroxyl group in infra-
red spectroscopy, which also confirmed the hydroxyl
group existence in the complexes. The main peaks owing
1
FIGURE 1 Molecular structure of 2c, some hydrogen atoms are
omitted for clarity 158 × 171 mm (96 × 96 DPI)
+
to the [M‐Cl] fragments of the complexes was also
SCHEME 1 Synthesis of half‐sandwich
ruthenium complexes (2a‐2c) 162 × 68mm
(
300 × 300 DPI)

JIA ET AL.
6 of 9
bigger than that of half‐sandwich ruthenium complexes
complexes was investigated in the presence of NaBH4
reducing agent.
And the optimized reaction conditions were screened
using 1‐chloro‐4‐nitrobenzene as the standard substrate,
the results were summerized in Table 1. The ruthenium
complexes (2a‐2c) catalysts were screened with 1 mol%
[49]
with naphthalene‐based Schiff base ligands.
cated that the hydroxyl group on phenyl ring has impor-
tant spatial effects intramolecular structure.
It is indi-
Furthermore, the dihedral angle between half‐sandwich
moiety and phenyl moiety is 14.83°. Non‐covalent inter‐
molecular interactions of C‐H···X in the crystal packing
structure of the complex are not observed.
and 4.0 equiv of NaBH in 2 ml ethanol at room temper-
4
ature for 0.5 hr. As shown in Table 1, the half‐sandwich
ruthenium complex 2c was the best catalyst toward the
nitro compounds reduction (Table 1, entries 1–3). When
the catalyst amount decreased, the high conversion was
needed with longer reaction time (Table 1, entries 4–6).
Then, the complex 2c was chosen as the optimal catalyst
to screen the various solvents (Table 1, entries 7–10). The
yield was inferior when MeOH, CH CN or CH Cl was
3
.2 | The nitro compounds reduction
catalyzed by ruthenium complexes
Aromatic anilines are quintessential blocks in the synthe-
sis of dyes, agrochemicals, pharmaceuticals, and pig-
3
2
2
used as the solvent. And 30% desired products were
detected in water (Table 1, entry 10). To our delight, the
yield was increased to 60% when 0.2 mmol CTAB was
added the reaction for 4 hr. The yield could be increased
to 95% by prolonging the reaction time to 7 hr (Table 1,
entries 11–14). The micellar formation with the help of
CTAB helps to overcome the phase‐transfer limitations
for water‐organic catalytic system and increase the
[50]
ments.
The typical method for the synthesis of
aromatic anilines is the reduction of nitro compounds
[51,52]
using NaBH4.
We and others have reported the
highly efficient nitro compounds reduction with half‐
[31,45]
sandwich ruthenium complexes catalysts.
With the
ruthenium complexes with hydroxyl group incorporate
in Schiff bass ligands in hand, the catalytic reactivity for
nitro compounds reduction with these ruthenium
[
53–58]
desired product yield in water.
A
control
a
TABLE 1 Optimization of the reaction conditions for 1‐chloro‐4‐nitrobenzene reduction using ruthenium complexes
−
1
b
Entry
Catalyst
Loading (mol.%)
Solvent
Additive
Time (h)
TON
TOF (h
)
Yield (%)
1
2
3
4
5
6
7
8
9
2a
2b
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
1
1
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
0.5
0.5
0.5
1
92
184
170
190
180
340
98
92
85
85
1
95
95
0.5
0.25
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
180
340
880
750
260
450
300
600
800
850
950
90
1
85
9
88
9
83
75
CH
3
CN
9
29
26
DCM
9
50
45
10
11
12
13
14
15
16
H
H
H
H
H
H
H
2
2
2
2
2
2
2
O
O
O
O
O
O
O
9
33
30
CTAB
CTAB
CTAB
CTAB
CTAB
4
150
160
142
136
60
5
80
85
6
7
95
24
24
No reaction
No reaction
a
4
Reaction conditions: 1‐chloro‐4‐nitrobenzene (0.3 mmol), NaBH (1.2 mmol), Ru catalysts, solvent (2 ml), Additive (0.2 mmol), room temperature.
b
Isolated yield.

7
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JIA ET AL.
a
TABLE 2 Screening of substrates for nitro compounds reduction catalyzed by ruthenium complex 2c
a
4 2
Reaction conditions: nitro compounds (0.3 mmol), NaBH (1.2 mmol), Ru catalysts (0.1 mol %), H O (2 ml), CTAB (0.2 mmol), 7 hr, room temperature.
b
Isolated yield.
experiment without the addition of the catalyst or only
with CTAB, no desired product was observed for nitro
compounds reduction for 24 hr (Table 1, entry 15–16).
From these studies, it was confirmed that the optimal
conditions for the reduction of nitro compounds were
using ruthenium complex 2c catalyst (0.1 mol %) in the
also suitable for this catalytic system giving the desired
product in 80% and 88% yield, respectively.
To understand the mechanism of the ruthenium com-
plex catalyzed nitro compounds reduction further, half‐
sandwich ruthenium was used to react with NaBH4
under the standard conditions. The half‐sandwich ruthe-
nium hydride active species can be confirmed through
in stiu NMR studies (δ ‐10.17 ppm, Figure S13,
Supporting information). The possible mechanism of
nitro compound reduction is outer‐sphere mechanism
presence of four equivalent NaBH and 0.2 mmol CTAB
4
in water at room temperature. And the catalytic efficiency
has been improved greatly in comparison with previous
reported with half‐sandwich ruthenium catalyst for nitro
compounds reduction under greener reaction condi-
[33–35]
through Ru‐H intermediate as previous reported.
[33]
tion.
Compared with the previous literature using
half‐sandwich ruthenium complexes with naphthalene‐
based Schiff‐base ligand catalysts for nitro compounds
reduction, the catalytic efficiency have been improved
greatly and lower the catalyst loading from 3 mol% to
4
| CONCLUSION
In conclusion, we have synthesized and characterized
three half‐sandwich ruthenium complexes with the
hydroxyl group functionalized Schiff base ligands and
evaluated their catalytic activities for nitro compounds
[49]
0
.1 mol%, and the solvent changed to water.
The
hydroxy group introduced to the naphthalene‐based
Schiff‐base ligand have an important effect on the cata-
lytic results with half‐sandwich ruthenium complexes
for nitro compound reduction in the presence of NaBH4,
indeed. The water‐solubility of the complexes was also
increased when the hydroxy group in introduce to the
ligands.
reduction in the presence of NaBH in water with the
4
help of CTAB at room temperature. We have shown that
the efficiency of the catalytic reaction is improved by
introduction of one hydroxy group into Schiff base
ligands. The results illustrate that a simple ligand modifi-
cation imparts dramatic changes to catalysis.
We further evaluated the ruthenium complex cata-
lyzed nitro coumpounds reduction under optimal reac-
tion conditions, and the results are shown in Table 2.
As shown in Table 2, many functionalized nitro com-
pounds were investigated. We could see that the reactions
worked well, leading to the desired products in good to
excellent yield. Both electron‐withdrawing and electron‐
donating substrates were tolerate to the reaction. For 4‐
nitrobenzaldehyde, the ‐CHO group was also reduced to
Supplementary Material
CCDC 1946806 contains the supplementary crystallo-
graphic data for 2c. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223‐336‐033; or e‐mail: deposit@ccdc.cam.ac.uk.
‐
CH OH together with the nitro group (Table 2, 3l). Nota-
2
bly, the nitro group in N‐heterocyclic compound and was
NMR data and spectrum of ruthenium complexes

JIA ET AL.
8 of 9
ACKNOWLEDGMENTS
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Fujita, Chem. Rev. 2015, 115, 12936.
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 the
Natural Science Foundation of Anhui Province
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X‐T. Half‐sandwich ruthenium complexes with
Schiff base ligands bearing a hydroxyl group:
Preparation, characterization and catalytic
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