Half-Sandwich Ru(II) Complexes with N,O-Chelate Ligands: Diverse Catalytic Activity for Amine Synthesis in Water

Article abstract of DOI:10.1021/acs.organomet.0c00554

Several types of β-ketoamino based N,O-coordinate half-sandwich ruthenium complexes have been synthesized in moderate to good yields. The stable ruthenium complexes displayed good and diverse catalytic efficiency in reductive amination between aldehydes and amines in aqueous solution. The method gave a facile route for one-pot synthesis of diverse complicated amines with a low catalyst loading by using cheap and less-toxic HCOOH or clean H2 as hydrogen source. Catalyst Ru1 showed the highest catalytic activity of 190 h-1 TOF value in the reductive amination reaction of benzaldehyde with aniline. The corresponding amine products were furnished in excellent yields under the standard catalysis system. The efficient and diverse catalytic activity, broad substance scope, mild conditions, and environmentally benign solvent made this system potentially applicable in industrial production. Ruthenium complexes were characterized using NMR, elemental analysis, and IR techniques to confirm their structure.

Full text of DOI:10.1021/acs.organomet.0c00554

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Article
Half-Sandwich Ru(II) Complexes with N,O-Chelate Ligands: Diverse
Catalytic Activity for Amine Synthesis in Water
Xue-Jing Yun, Chun Ling, Wei Deng, Zhen-Jiang Liu,* and Zi-Jian Yao*
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ABSTRACT: Several types of β-ketoamino based N,O-coordinate half-
sandwich ruthenium complexes have been synthesized in moderate to
good yields. The stable ruthenium complexes displayed good and diverse
catalytic efficiency in reductive amination between aldehydes and amines
in aqueous solution. The method gave a facile route for one-pot synthesis
of diverse complicated amines with a low catalyst loading by using cheap
and less-toxic HCOOH or clean H2 as hydrogen source. Catalyst Ru1
showed the highest catalytic activity of 190 h−1 TOF value in the
reductive amination reaction of benzaldehyde with aniline. The corresponding amine products were furnished in excellent yields
under the standard catalysis system. The efficient and diverse catalytic activity, broad substance scope, mild conditions, and
environmentally benign solvent made this system potentially applicable in industrial production. Ruthenium complexes were
characterized using NMR, elemental analysis, and IR techniques to confirm their structure.
catalysts are attractive because they could be employed in
different reaction conditions.
INTRODUCTION
■
Amine compounds play an important role in pharmaceutical
chemistry, fine chemical production, and biologically active
molecules synthesis, because the amino group is frequently
found in many natural and agricultural products.1 Reduction of
imines is a convenient and direct method to prepare amino
compounds. Methods for reduction of imines include: (a)
reduction of imines with equivalent metal hydride reducing
reagent (lots of waste);2 (b) transfer hydrogenation of imines
in the presence of a hydrogen source such as isopropanol or
HCOOH;3−6 (c) catalytic hydrogenation of imines using
transition metal catalysts showed another preparation method
of amines.7,8 The disadvantage of both methodologies
mentioned above is two steps required: imines synthesis and
their reduction.
Reductive amination reactions between carbonyl com-
pounds and amines in the presence of a reducing agent
represent a highly efficient and facile route to amine synthesis
because of the readily available starting materials, convenient
one-pot synthetic procedure, and broad substance scope.2c,9
Thus, various types of catalysts (including heterogeneous and
homogeneous) have been prepared and employed in the
reductive amination reaction due to the prominent efficiency in
carbon−nitrogen bond formation.10−14 For heterogeneous
catalysts, one of the most notable drawbacks is their poor
chemoselectivity in RA reactions.9f Development of efficient
homogeneous catalysts of RA reactions (including transfer
hydrogenation and catalytic hydrogenation) has attracted
considerable attention in recent years.15,16 However, the
catalysts with good efficiency in both hydrogenative and
transfer hydrogenative RA reactions are rarely reported.17 Such
The half-sandwich moiety has emerged as a important
building block in organometallic complexes synthesis and
catalysis due to their high stability, good solubility, easy
modification, and excellent catalytic activity.18−21 The detailed
advantages of the half-sandwich moiety are as follows: (i) the
dichloro-bridged metal precursor can be easily prepared in
good yields through inorganic metal salts; (ii) the metal center
was protected by the half-sandwich structure which avoided
the side reactions; (iii) the electron and steric effects can be
tuned conveniently by choosing different substituents. Among
these types of complexes, half-sandwich Ru(II) complexes
were frequently employed as the building blocks in the
preparation of diverse compounds which showed particular
activity in catalysis, coordination-driven self-assembly, and
biochemistry.22−25 Recent study indicated that the half-
sandwich Ru(II) complexes with nitrogen-containing ligands
showed good catalytic efficiency in diverse organic reac-
tions.26,27 However, there are limited studies of RA reactions
̈
using half-sandwich Ru(II) complexes. For example, Ozpozan
and co-workers reported transfer hydrogenative RA reactions
using a neutral N,N-chelate half-sandwich ruthenium complex
Received: August 19, 2020
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a
Scheme 1. Synthesis of the Ligands and Half-Sandwich Ru(II) Complexes
a
t
Reaction conditions: (i) 1-dihydroindanone (1.0 equiv), HCOOEt (2.0 equiv), BuOK (1.5 equiv), 0 °C, 30 min; rt, overnight; (ii) aromatic
amines (1.1 equiv), HCOOH (3−5 drops), MeOH, rt, 24 h; (iii) [(p-cymene)RuCl2]2 (0.5 equiv), NaOAc (2.0 equiv), CH3OH, 50 °C, 8 h.
as catalyst. But the reaction conversions are not very high and
only aldehydes are tolerant in such condition.28
heating Ru1−Ru4 in toluene for 8 h. This result may indicate
good thermal stability of these complexes. The UV-vis spectra
of complexes Ru1−Ru4 in CH3CN solution were charac-
terized in order to investigate the electronic structures and the
coordination mode of the metal center. As shown in Figure 1,
Due to the diverse catalytic efficiency and good activity of
the half-sandwich Ru(II) complexes, the investigation of the
synthesis and application of this type of complex with a novel
structure is interesting and valuable. Herein, we reported four
types of N,O-chelate half-sandwich Ru(II) complexes contain-
ing a β-ketoamino motif. Experimental results confirmed their
good catalytic activity in the one-pot synthesis of amines
through transfer hydrogenation reductive amination reactions
between diverse amine derivatives and aldehydes in the
presence of HCOONa/HCOOH. Moreover, the same catalyst
also successfully catalyzes hydrogenative reductive amination
of carbonyl substrates with H2. The corresponding amine
products formed through C−N bond formation were furnished
smoothly in good to excellent yields in aqueous media under
mild conditions. All ruthenium complexes were fully
characterized by IR, NMR, and other techniques.
RESULTS AND DISCUSSION
■
Figure 1. UV−vis spectra of complexes Ru1−Ru4 (2.5 × 10−5 mol
Synthesis of β-Ketoamino Ligands and Half-Sand-
wich Ru(II) Complexes. β-Ketoamino ligands L1−L4 have
been prepared by the modified procedures based on the
previous report.29 The intermediate (β-diketones) were
produced by the interaction of ethyl formate and 1-
dihydroindanone using potassium t-butoxide as base in
anhydrous Et2O. The ligands L1−L4 were given in 72−88%
isolated yields by the condensation between substituted
aromatic amines and a β-diketone intermediate in EtOH
promoted by a catalytic amount of formic acid as the catalyst.
The broad and strong hydroxyl group stretch band at
approximately 3390 cm−1 in the IR spectra of L1−L4 indicated
the ligands existed in the enol form. In the 1H NMR spectra of
the ligands, a broad signal was observed in the range of 11.0−
11.5 ppm, which confirmed the enol form of the ligands. Four
types of new half-sandwich Ru(II) complexes Ru1−Ru4 with
the ligands were afforded in moderate yields through the
reaction between β-ketoamino ligands and [(p-cymene)-
RuCl2]2 using sodium acetate as base in methanol at 50 °C
for 8 h (Scheme 1). Ruthenium complexes Ru1−Ru4 were
obtained as a brown powder and exhibited good moisture and
air stability. Decomposition of these complexes has not been
observed after several weeks in air. The Ru(II) complexes were
soluble in DMSO, dichloromethane, toluene, acetonitrile,
DMF, and THF, giving brown yellow solutions, but showed
poor solubility in nonpolar solvents such as petroleum ether
and cyclohexane.
L−1 in CH3CN).
the absorption bands around 300−350 nm were assigned to
the metal-to-ligand charge transfer (MLCT) n→π* and π→π*
transition, and the MLCT dπ→π* transition bands were
shown in the range of 400−450 nm.28 Similar UV-vis spectra
of the four ruthenium complexes indicated their structures
were analogous with each other. No peaks were found at
approximately 3400 cm−1 in IR spectra of the half-sandwich
ruthenium complexes, which may suggest the Ru−O bond
formation through the deprotonation of the −OH group. The
imine CN group stretch bands at approximately 1660−1680
cm−1 are shifted to a lower frequency of 1620−1640 cm−1,
probably caused by the coordination interaction between the
ruthenium center and the N, O donors.30 The disappearance of
the broad peak at approximately 11.40−11.30 ppm which was
1
assigned to the −OH proton in the H NMR spectra of the
ligands further confirmed the Ru−O bond formation. The
signals at approximately 7.85 ppm were ascribed to the proton
of the imine group. All the proton signals of the aromatic
1
protons in the ligands of H NMR spectra of the ruthenium
complexes were observed in the region of 7.81−7.20 ppm, and
four groups of signals shown in the region of 5.50−4.20 ppm
were ascribed to the aromatic protons in the [(p-cymene)Ru]
motif. The mass spectra and elemental analysis both confirmed
the expected structure of Ru1−Ru4. To elucidate the exact
structure of the ruthenium complex, single crystal X-ray
determination of half-sandwich complex Ru4 was performed.
The target Ru(II) products were further characterized by 1H
1
NMR and IR spectroscopies. The change of the H NMR
spectra of ruthenium complexes has not been observed after
B
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The ruthenium complex crystallized in the monoclinic space
group P21c (Figure 2). Geometric data are listed in the
half-sandwich ruthenium complexes Ru1−Ru4 was inves-
tigated.
The reaction was initially studied in water by choosing
benzaldehyde with aniline as the substrates under the catalysis
of Ru1 using HCOOH as hydrogen source. It is noteworthy
that the pH value is crucial for this reaction, because the
hydrogenation of the carbonyl compound is a compatible
pathway during the RA reaction. The RA reaction was carried
out at 80 °C for 2 h at a molar ratio of the substrate to the
catalyst Ru1 (S/C) of 200:1 using HCOOH/HCOONa to
adjust the pH value of this catalytic system. As shown in Figure
3a, the pH value of 4.8 is the best point based on experimental
results, because the imine is protonated, whereas the carbonyl
compound is in its free form so that its hydrogenation is
inhibited. The alcohol production was negligible under such
pH value. Low conversions were observed when the pH value
< 3 due to the generation of imine in the process was inhibited
in a strong acidic environment. Both imine reduction and
carbonyl hydrogenation were inhibited when the pH value > 6.
These results were also consistent with that of the previous
reports.31 Additionally, time dependent amine yields and
conversions were also investigated; the study displayed that the
RA reaction with catalyst Ru1 was fast and both conversions
and amine yields were obtained up to 95% after 5 h under the
conditions (Figure 3b).
With the best pH value in hand, we carried out other control
experiments to optimize the RA reaction condition. 1a was
obtained in 96% yield at 80 °C for 5 h with 0.5 mol % catalyst
loading of Ru1. Similar results were observed when the
reaction was carried out at 60 °C, whereas lower yields were
furnished when the reaction temperature decreased to 50 °C
(Table 1, entries 1−3). The amount of the catalyst loading was
investigated to obtain the highest TOF value. The yields of 1a
were nearly the same when the catalyst loading was in the
range of 0.5−0.1 mol %. The catalyst loading is much lower
than that of related ruthenium complexes (0.5−1.0 mol %)
reported previously.32,33 However, 1a was furnished with low
yield when 0.05 mol % catalyst loading was used in the
Figure 2. Crystal structure of complex Ru4 with thermal ellipsoids
drawn at the 30% level. Selected bond lengths (Å) and angles (deg):
Ru(1)−N(1), 2.1118(14); Ru(1)−O(1), 2.0758(12); Ru(1)−Cl(1),
2.4288(5); C(11)−N(1), 1.425(2); C(1)−O(1), 1.276(2); O(1)−
Ru(1)−N(1), 89.82(5); O(1)−Ru(1)−Cl(1), 86.57(4); Ru(1)−
O(1)−C(1), 124.26(11); Ru(1)−N(1)−C(11), 118.99(11).
Supporting Information (Table S1), and the characteristic
bond lengths and angles which are consistent with the reported
values19 are summarized in the footnote of Figure 2. The
ruthenium center of this complex displayed a typical three-
legged piano-stool conformation.
Amine Synthesis through Reductive Amination
under Ru Catalysis in Water. Reductive amination (RA)
reaction represents one of the simplest and efficient routes to
produce amines, because imine was generated in situ in the
reaction and the isolation of this intermediate was alleviated.2c
Recently, reductive amination catalyzed by transition metal
complexes from carbonyl compounds through transfer hydro-
genation is a green, direct, and versatile method for the
synthesis of amines.15,16 So such reaction under catalysis of
Figure 3. (a) Influence of the pH value on the RA reaction. Conditions: benzaldehyde (1.0 mmol), aniline (1.2 mmol), Ru1 (0.005 mmol),
HCOOH/HCOONa (2 mL), 80 °C, 2 h. Conversion and yield were monitored by 1H NMR. (b) Conditions: benzaldehyde (1.0 mmol), aniline
1
(1.2 mmol), Ru1 (0.005 mmol), HCOOH/HCOONa (2 mL, pH = 4.8), 80 °C. Conversion and yield were monitored by H NMR.
C
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a
Table 1. RA Reactions under Catalysis of Ruthenium Complexes
b
entry
cat./mol %
Ru1 (0.5)
T/°C
solvent
yield/%
TON
TOF/h−1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
80
60
50
60
60
60
60
60
60
60
60
60
60
60
60
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
96
95
87
95
95
79
96
82
192
190
174
475
950
1580
960
820
850
230
38.4
38
34.8
95
Ru1 (0.5)
Ru1 (0.5)
Ru1 (0.2)
Ru1 (0.1)
Ru1 (0.05)
Ru2 (0.1)
Ru3 (0.1)
Ru4 (0.1)
Ru1 (0.1)
Ru1 (0.1)
Ru1 (0.1)
Ru1 (0.1)
190
316
192
164
170
46
H2O
85
23
DMF
CH2Cl2
toluene
MeOH
H2O
NR
NR
81
NR
trace
810
162
[(p-cymene)RuCl2]2
H2O
a
Reaction conditions: benzaldehyde (1.0 mmol), aniline (1.2 mmol), solvent (2 mL), HCOOH/HCOONa (2 mL, pH = 4.8,
b
S:HCOOH:HCOONa = 1:4.7:11.8), 80 °C, 5 h. Yield was determined by GC analysis.
a b
,
Table 2. Substrate Scope of RA Reactions of Carbonyls Compounds with Amines
a
Reaction conditions: carbonyl substrates (1.0 mmol), amines (1.2 mmol), Ru1 (0.1 mol %), water (2 mL), HCOOH/HCOONa (2 mL, pH =
b
4.8, S:HCOOH:HCOONa = 1:4.7:11.8), 60 °C, 5 h. Yield was determined by GC analysis; isolated yields are provided in brackets.
reaction (Table 1, entries 4−6). The catalytic efficiency of Ru2
is comparable with that of Ru1, whereas Ru3 and R4 exhibit
lower catalytic activity in the RA reaction (Table 1, entries 7−
9). The influence of the solvent on the RA reaction was
monitored, and the results indicated that the property of the
solvent affected the product yields dramatically. Among the
different solvents employed in the reaction, MeOH showed
lower activities compared to water (Table 1, entries 10−13),
while others showed very low or no activities. Therefore, the
optimal condition was obtained after screening different effects
of the reaction. Target product 1a was given in 95% yield
under standard conditions. No reaction was observed in the
absence of the catalyst, and only a trace amount of product
were found when [(p-cymene)RuCl2]2 was used in the
reaction as catalyst under standard conditions (Table 1, entries
14 and 15).
D
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Figure 4. Gram-scale reductive amination reaction study
We next investigated the substrate scope of this reaction
under optimal conditions. We first explored the RA reaction of
aromatic aldehydes and aromatic amines. The results suggested
that all the expected amine products were furnished in good
yields (91−97%) at 50 °C in 5 h by using 0.1 mol % of Ru1
(Table 2, 1a−f). Both electron-donating and electron-with-
drawing groups substituted on benzaldehydes and aromatic
amines were tolerant in this reaction. Similar results were
observed for using ketones as substrates which were not used
as substrates in other ruthenium catalytic systems.33 Never-
theless, the related ruthenium catalyst reported by Zhu showed
a shorter reaction time (2 h) compared with that of our
system.32 The corresponding amines were generated in 87−
92% yields for the RA reactions of acetophenone derivatives
and aromatic amines (Table 2, 1g−l). RA reactions of aromatic
amines and aliphatic ketones were carried out, and these
carbonyl compounds showed comparable reactivity with that
of aromatic carbonyl substrates (Table 2, 1m−o). Reactions of
aliphatic amines with benzaldehyde also gave positive results
(Table 2, 1p and 1q), whereas low yields of the products were
obtained by using other ruthenium catalysts such as [RuCl(p-
cymene)(TsDPEN)] or N,N-chelate bipyridine-based ruthe-
nium complexes.32,33 Additionally, alcohols were always found
as the byproducts in most cases for the bipyridine-Ru catalytic
system.33 Secondary amines exhibited good reactivity with
benzaldehyde, whereas they showed low activity with ketones
in the reaction (Table 2, 1r and 1s). Inspired by the catalytic
recycling experiment results of the bipyridine-ruthenium
complex,33 we also tried the recyclability of our N,O-chelate
ruthenium complexes in the reductive amination reaction;
however, no positive results were observed.
metric reagents. Thus, the catalytic activity of Ru1 in the
reduction amination with molecular hydrogen was explored.
Initial experiments were performed with aniline and
benzaldehyde as starting materials at 60 °C, with 0.1 mol %
catalyst in methanol. The pressure of H2 was crucial to reach
high conversion: from 1 to 3 atm, the conversion increased
from 42% to 96% over the 3 h reaction time (Table 3, entries
Table 3. RA Reactions under Catalysis of Ru1 Using H2 as
a
Hydrogen Source
b
entry cat./mol % T/°C H2/atm
solvent
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
CH2Cl2
toluene
DMF
THF
H2O
MeOH/H2O (1:1)
MeOH/H2O (1:1)
MeOH/H2O (1:1)
MeOH/H2O (1:1)
MeOH/H2O (1:1)
yield/%
1
2
Ru1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.01)
2 (0.1)
3 (0.1)
4 (0.1)
60
60
60
60
50
30
50
50
50
50
50
50
50
50
50
50
1
2
3
5
3
3
3
3
3
3
3
3
3
5
5
5
42
77
96
96
95
65
35
56
73
63
69
95
65
94
93
95
3
4
5
6
7
8
9
10
11
12
13
14
15
16
A gram-scale experiment of the reductive amination of
benzaldehyde or acetophenone with aniline under optimal
conditions was investigated with 0.1 mol % of Ru1 using a
HCOOH hydrogen source. Pure products 1a and 1g were
afforded in 90% and 87% isolated yields, respectively (Figure
4). A possible mechanism of the ruthenium catalyzed RA
reaction was proposed based on the experimental results and
related reports.15,16 As mentioned above, a lower pH value can
promote the protonation of imines and ketones, facilitating
their reductive process. So the RA reaction pathway may
contain three parts: ruthenium hydride formation, imine
protonation (imine was generated in situ from carbonyl
compounds and amines), and transfer hydrogenation proc-
ess.32,33
a
Reaction conditions: Under argon, an autoclave was charged with
the catalyst and solvent (2.0 mL), followed by benzaldehyde (1.0
mmol) and aniline (1.2 mmol), in that order. The autoclave was then
charged with H2 and heated in an oil bath. Yield was determined by
GC analysis.
b
1−3). However, further increase of the pressure did not show
any promotion of the reaction (Table 3, entry 4). The
temperature of 50 °C is proper for the reaction, and only 65%
yield of the desired product was observed when the reaction
was performed at a low temperature of 30 °C (Table 3, entries
5 and 6). Solvent played an important role in the reaction, and
MeOH was the best choice (Table 3, entries 7−10). The
reduction did not work well in water under such conditions
(Table 3, entry 11); however, 1a was obtained in good yield
when a mixed solvent of MeOH/H2O (v/v = 1/1) was used
(Table 3, entry 12). Decreasing the catalyst loading (from 0.1
mol % to 0.01 mol %) led to a lower yield of product; the yield
Catalytic hydrogenation with late transition metal complexes
using molecular hydrogen as reduction reagent represents the
sustainable and green chemical process, which is crucial for
large-scale industries. With respect to reducing agents,
molecular hydrogen is more beneficial compared to stochio-
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a b
,
Table 4. Substrate Scope of RA Reactions through Catalytic Hydrogenation Process
a
b
Reaction conditions: carbonyl substrates (1.0 mmol), amines (1.2 mmol), Ru1 (0.1 mol %), solvent (2 mL), 50 °C, 3 h, H2 (3 atm). Yield was
determined by GC analysis.
mixture was poured into a round-bottom flask; then the obtained
suspension was treated with HCOOH (EtOH solution) to pH < 7.
Corresponding aromatic amines (1.0 equiv) were then added to the
above mixture, and the dehydration condensation was performed
smoothly to afford the corresponding β-enaminoketonato ligands
L1−L4, respectively.
dropped from 94% to 65% even over the 8 h reaction time
(Table 3, entry 13). All the half-sandwich ruthenium
complexes were efficient in the reaction, and no remarkable
differences of activity were found (Table 3, entries 14−16).
With the optimized reaction conditions in hand, we explored
the generality of the reaction. The corresponding amines
through reductive amination were furnished in good to
excellent yields with both electron-donating and electron-
withdrawing substituents (Table 4). Interestingly, 1s was
furnished in higher yield compared with the result given in the
HCOOH/HCOONa catalytic system.
1
L1: Light yellow solid, 79% isolated yield. H NMR (500 MHz,
CDCl3): δ 11.36 (d, J = 12.8 Hz, 1H), 7.84 (d, J = 7.6 Hz, 1H), 7.52
(dt, J = 11.4, 9.5 Hz, 3H), 7.41 (t, J = 7.1 Hz, 1H), 7.34 (t, J = 7.9 Hz,
2H), 7.10 (d, J = 7.9 Hz, 2H), 7.05 (t, J = 7.3 Hz, 1H), 3.67 (d, J =
10.0 Hz, 2H) ppm. IR (KBr, disk): υ 3378, 1667, 1553, 1471, 1239,
1227, 958, 746 cm−1. Elemental analysis calcd (%) for C16H13NO: C
81.68, H 5.57, N 5.95, found: C 81.61, H 5.60, N 5.97.
L2: Yellow solid, 72% isolated yield. 1H NMR (500 MHz, CDCl3):
δ 11.35 (d, J = 13.1 Hz, 1H), 7.83 (d, J = 7.5 Hz, 1H), 7.51 (dd, J =
19.7, 9.6 Hz, 3H), 7.40 (t, J = 7.1 Hz, 1H), 7.14 (d, J = 8.1 Hz, 2H),
7.00 (d, J = 8.3 Hz, 2H), 3.66 (s, 2H), 2.32 (s, 3H). IR (KBr, disk): υ
3389, 1672, 1581, 1455, 1264, 1221, 952, 749 cm−1. Elemental
analysis calcd (%) for C17H15NO: C 81.90, H 6.06, N 5.62, found: C
81.85, H 6.08, N 5.66.
CONCLUSIONS
■
In summary, we have synthesized a series of N,O-chelate half-
sandwich ruthenium complexes which displayed prominent
catalytic efficiency in reductive amination for the direct
synthesis of amine derivatives in water. The RA reactions
involved a variety of carbonyl substrates with amine derivatives
and have been accomplished by using cheap and less toxic
HCOOH or clean H2 as hydrogen source. Various types of
amines were obtained in high yields under mild reaction
conditions. The broad substrate scope, high catalytic activity,
low catalyst loading, and environmentally friendly solvent
made the catalytic system attractive in industrial production.
1
L3: Pale yellow solid, 88% isolated yield. H NMR (500 MHz,
CDCl3): δ 11.34 (d, J = 11.8 Hz, 1H), 7.84 (d, J = 7.6 Hz, 1H), 7.57−
7.47 (m, 3H), 7.41 (t, J = 7.3 Hz, 1H), 7.30 (d, J = 8.8 Hz, 2H), 7.02
(d, J = 8.8 Hz, 2H), 3.68 (s, 2H). IR (KBr, disk): υ 3391, 1679, 1580,
1438, 1272, 1236, 946, 752 cm−1. Elemental analysis calcd (%) for
C16H12ClNO: C 71.25, H 4.48, N 5.19, found: C 71.31, H 4.45, N
5.21.
L4: Yellow solid, 80% isolated yield. 1H NMR (500 MHz, CDCl3):
δ 11.30 (d, J = 11.8 Hz, 1H), 7.84 (d, J = 7.7 Hz, 1H), 7.58−7.47 (m,
3H), 7.43 (dd, J = 17.8, 10.3 Hz, 2H), 7.21−7.14 (m, 2H), 6.99 (d, J
= 8.0 Hz, 1H), 3.68 (s, 2H). IR (KBr, disk): υ 3404, 1671, 1586,
1442, 1274, 1239, 960, 758 cm−1. Elemental analysis calcd (%) for
C16H12BrNO: C 61.17, H 3.85, N 4.46, found: C 61.21, H 3.88, N
4.50.
Synthesis of Half-Sandwich Ruthenium Complexes Ru1−
Ru4. β-Enaminoketonato ligands L1−L4 (0.5 mmol, 1.0 equiv), [(p-
cymene)RuCl2]2 (0.25 mmol, 0.5 equiv), and NaOAc (1.0 mmol, 2.0
equiv) were stirred at 50 °C in methanol (5 mL) for 8 h. After the
reaction, the mixture was filtered and evaporated to give the crude
products which were further purified by silica gel column
chromatography (CH2Cl2:EA = 5:1) to furnish the pure target half-
sandwich ruthenium complexes in moderate yields.
EXPERIMENTAL SECTION
■
General Data. The half-sandwich ruthenium complexes Ru1−
Ru4 were prepared under an inert atmosphere using standard Schlenk
techniques. Chemicals were used as commercial products without
further purification. 1H NMR (500 MHz) spectra were measured with
a Bruker DMX-500 spectrometer. Elemental analysis was performed
on an Elementar vario EL III analyzer. IR (KBr) spectra were
measured with the Nicolet FT-IR spectrophotometer. UV/vis
absorption spectra were recorded using a UV 765 spectrophotometer.
Synthesis of Ligands L1−L4. 1-Dihydroindanone (1.32 g, 10.0
mmol), ethylformate (1.61 mL, 20.0 mmol), and tBuOK (1.68 g, 15.0
mmol) were added to anhydrous diethyl ether (20 mL) at 0 °C. The
mixture was kept stirring at this temperature for 30 min, then warmed
to room temperature and stirred overnight. After the reaction, the
F
https://dx.doi.org/10.1021/acs.organomet.0c00554
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Article
Ru1: Brown solid, 62% isolated yield. 1H NMR (500 MHz,
CDCl3): δ 7.87 (d, J = 5.5 Hz, 1H), 7.62 (d, J = 7.7 Hz, 2H), 7.41−
7.32 (m, 6H), 7.24 (d, J = 7.3 Hz, 1H), 5.39 (d, J = 6.1 Hz, 1H), 5.36
(d, J = 6.2 Hz, 1H), 4.96 (d, J = 5.5 Hz, 1H), 4.19 (d, J = 5.7 Hz, 1H),
3.53−3.42 (m, 2H), 2.64−2.58 (m, 1H), 2.17 (s, 3H), 1.17 (d, J = 7.0
Hz, 3H), 1.11 (d, J = 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ
173.67, 159.72, 154.72, 145.77, 140.67, 128.95, 128.62, 126.39,
125.64, 124.86, 124.46, 121.92, 106.12, 100.24, 98.28, 86.71, 83.05,
82.27, 81.14, 33.26, 30.50, 22.92, 21.58, 18.69. Elemental analysis
calcd (%) for C26H26ClNORu: C 61.84, H 5.19, N 2.77, found: C
61.92, H 5.20, N 2.71. HRMS calcd for M (C26H26ClNORu): [M −
Cl]+: m/z 470.1058; found: 470.1066.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00554.
■
sı
1H NMR and 13C NMR spectra of L1−L4 and
ruthenium complexes Ru1−Ru4, NMR data of 1a−1s,
and crystallographic data (PDF)
Accession Codes
CCDC 2034758 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Ru2: Brown solid, 66% isolated yield. 1H NMR (500 MHz,
CDCl3): δ 7.86 (d, J = 4.2 Hz, 1H), 7.50 (d, J = 8.0 Hz, 2H), 7.33 (d,
J = 15.9 Hz, 4H), 7.18 (s, 1H), 7.16 (s, 1H), 5.37 (s, 1H), 5.36 (s,
1H), 4.95 (d, J = 5.6 Hz, 1H), 4.20 (d, J = 5.5 Hz, 1H), 3.52−3.40
(m, 2H), 2.61−2.58 (m, 1H), 2.40 (s, 3H), 2.18 (s, 3H), 1.17 (d, J =
7.0 Hz, 3H), 1.11 (d, J = 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3):
δ 173.51, 157.30, 154.60, 145.47, 140.73, 129.26, 128.80, 126.36,
124.57, 124.36, 121.97, 105.99, 100.10, 98.29, 86.74, 83.02, 82.27,
81.24, 33.28, 30.51, 22.87, 21.64, 21.02, 18.73. Elemental analysis
calcd (%) for C27H28ClNORu: C 62.48, H 5.44, N 2.70, found: C
62.55, H 5.41, N 2.76. HRMS calcd for M (C27H28ClNORu): [M −
Cl]+: m/z 484.1214; found: 484.1223.
AUTHOR INFORMATION
Corresponding Authors
■
Zi-Jian Yao − School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai
201418, China; Key Lab of Synthetic Chemistry of Natural
Substances, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, Shanghai 200032, China; orcid.org/
0000-0003-1833-1142; Phone: +86-21-60877231;
Email: zjyao@sit.edu.cn; Fax: +86-21-60873335
Zhen-Jiang Liu − School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai
201418, China; orcid.org/0000-0001-6850-9485;
Email: zjliu@sit.edu.cn
Ru3: Brown solid, 70% isolated yield. 1H NMR (500 MHz,
CDCl3): δ 7.86 (d, J = 6.6 Hz, 1H), 7.59 (d, J = 8.5 Hz, 2H), 7.35 (d,
J = 8.9 Hz, 4H), 7.27 (s, 1H), 7.26 (s, 1H), 5.38 (s, 2H), 4.95 (d, J =
4.7 Hz, 1H), 4.26 (d, J = 5.0 Hz, 1H), 3.53−3.41 (m, 2H), 2.64−2.58
(m, 1H), 2.18 (s, 3H), 1.18 (d, J = 7.0 Hz, 3H), 1.12 (d, J = 7.0 Hz,
3H). 13C NMR (125 MHz, CDCl3): δ 174.34, 158.22, 154.71, 145.81,
140.41, 131.00, 129.24, 128.67, 126.52, 126.24, 124.48, 122.13,
106.11, 100.26, 98.47, 82.18, 81.34, 33.18, 31.44, 30.56, 29.70, 18.76.
Elemental analysis calcd (%) for C26H25Cl2NORu: C 57.89, H 4.67, N
2.60, found: C 57.94, H 4.62, N 2.55. HRMS calcd for M
(C26H25Cl2NORu): [M − Cl]+: m/z 504.0668; found: 504.0660.
Ru4: Brown solid, 65% isolated yield. 1H NMR (500 MHz,
CDCl3): δ 7.86 (d, J = 7.1 Hz, 1H), 7.81 (s, 1H), 7.61 (d, J = 8.4 Hz,
1H), 7.37 (t, J = 11.7 Hz, 4H), 7.28 (s, 1H), 7.24 (d, J = 7.9 Hz, 1H),
5.41 (d, J = 6.1 Hz, 1H), 5.36 (d, J = 6.1 Hz, 1H), 5.01 (d, J = 5.6 Hz,
1H), 4.26 (d, J = 5.7 Hz, 1H), 3.53−3.43 (m, 2H), 2.65−2.59 (m,
1H), 2.19 (s, 3H), 1.19 (d, J = 7.0 Hz, 3H), 1.12 (d, J = 7.0 Hz, 3H).
13C NMR (125 MHz, CDCl3): δ 174.75, 160.99, 154.60, 145.97,
140.38, 129.93, 129.33, 128.61, 128.06, 126.52, 124.51, 123.81,
122.18, 122.01, 106.44, 100.88, 98.13, 86.55, 83.24, 82.56, 80.61,
33.16, 30.55, 22.92, 21.51, 18.67. Elemental analysis calcd (%) for
C26H25BrClNORu: C 53.48, H 4.32, N 2.40, found: C 53.51, H 4.36,
N 2.33. HRMS calcd for M (C26H25BrClNORu): [M − Cl]+: m/z
548.0163; found: 548.0170.
General Procedure for RA Reactions under HCOOH/
HCOONa System. Carbonyl compounds (1.0 mmol), amines (1.2
mmol), and complex Ru1 (0.001 mmol) were added in water (2 mL),
then a water solution of HCOOH/HCOONa (2.0 mL, pH = 4.8).
The mixture was stirred at 60 °C for 5 h. After reaction, the solution
was extracted with EtOAc and dried over Na2SO4, filtered, and
evaporated to give the crude products which were further purified by
silica gel column chromatography (CH2Cl2/EA = 6:1) to give the
corresponding products.
Authors
Xue-Jing Yun − School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai
201418, China
Chun Ling − School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai
201418, China
Wei Deng − School of Chemical and Environmental Engineering,
Shanghai Institute of Technology, Shanghai 201418, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00554
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (No. 21601125), the Chenguang Scholar
of Shanghai Municipal Education Commission (No. 16CG64),
the Shanghai Gaofeng & Gaoyuan Project for University
Academic Program Development, and the research funding of
Shanghai Institute of Technology (ZQ2020-10 and XTCX-
2020-24).
General Procedure for RA Reactions under H2 Atomsphere.
Carbonyl compounds (1.0 mmol), amines (1.2 mmol), and Ru1
(0.001 mmol) and MeOH/H2O (2 mL, v/v = 1/1) were charged in a
5 mL vial with a magnetic bar. The vial was then transferred to an
autoclave. The autoclave was purged with H2 (3 atm) via three cycles
of pressurization/venting, then pressurized with H2 (3 atm) and
disconnected from the H2 source. The autoclave was heated to 50 °C.
After stirring for 3 h, the autoclave was cooled and the pressure was
slowly released. The resultant mixture was extracted with diethyl ether
(2 × 5 mL) and dried over anhydrous Na2SO4. Solvent was
evaporated under vacuum. The residue was dissolved in hexane and
analyzed by GC-MS.
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