Diruthenium(II)-NNN pincer complex catalysts for transfer hydrogenation of ketones

Article abstract of DOI:10.1039/c6dt03620a

Dinuclear ruthenium(ii)-NNN pincer complexes bearing a π linker-supported bis(pyrazolyl-imidazolyl-pyridine) ligand were synthesized and structurally characterized, and they exhibited excellent catalytic activity for the transfer hydrogenation of ketones in refluxing isopropanol, reaching TOF values up to 1.3 × 10⁶ h^-1. Compared with the corresponding mononuclear Ru(ii)-NNN pincer complexes, the bimetallic complexes could be applied at concentrations as low as 0.03 mol% Ru and demonstrated remarkably enhanced catalytic activity in the transfer hydrogenation reactions of ketones. The high catalytic activity of the diruthenium(ii) complexes is attributed to the excellent stability and possible cooperativity of the two coordinated Ru(ii) metal centers through the π linker. The present synthetic methodology has established an applicable strategy to construct highly active bimetallic NNN pincer complex catalysts.

Full text of DOI:10.1039/c6dt03620a

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DOI: 10.1039/C6DT03620A
PAPER
Diruthenium(II)‐NNN Pincer Complex Catalysts for Transfer
Hydrogenation of Ketones†
Huining Chai,a Qingfu Wang,a Tingting Liu,a and Zhengkun Yu*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
Dinuclear ruthenium(II)‐NNN pincer complexes bearing a π linker‐supported bis(pyrazolyl‐imidazolyl‐pyridine) ligand were
synthesized, structurally characterized, and exhibited excellent catalytic activity for the transfer hydrogenation of ketones
in refluxing isopropanol, reaching TOF values up to 1.3×106 h‐1. As compared with the corresponding mononuclear Ru(II)‐
NNN pincer complexes, the bimetallic complexes could be applied at concentrations as low as 0.03 mol % Ru and were
demonstrated remarkably enhanced catalytic activity in the transfer hydrogenation reactions of ketones. The high catalytic
activity of the diruthenium(II) complexes is attributed to the excellent stability and possible cooperativity of the two
coordinated Ru(II) metal centers through the π linker. The present synthetic methodology has established an applicable
strategy to construct highly active bimetallic NNN pincer complex catalysts.
DOI: 10.1039/x0xx00000x
www.rsc.org/
efficient bimetallic transition metal complex catalysts have been
reported in this area to date.10 We recently established a strategy for
constructing highly active mononuclear ruthenium(II)-NNN pincer
Introduction
Construction of highly active metal complex catalysts has been a
challenging task in homogeneous catalysis and organic synthesis.1
Organometallic complexes that incorporate more than one metal
centers may exhibit unusual reactivity and/or catalytic activity due to
the cooperative electronic and steric effects from the ligand and
metal centers.2 Particularly, bimetallic complexes have been paid
much attention for their potential to effect efficient catalysis.3 In this
context, an array of bimetallic complex catalysts have recently been
documented. Diruthenium complexes for selective intramolecular
allylic C-H amination4a and 1,3-insertion of carbenes into N-H and
O-H bonds,4b were reported. Dinuclear Au(II) complexes acted as the
key intermediates for gold-catalyzed heteroarylation.4c Bimetallic
boryl-cobalt (or nickel) complexes catalyzed olefin hydrogenation.4d
Both dinuclear cobalt and ruthenium N-heterocyclic complexes
promoted the oxidation of water.4e,4f Oxo-bridged bimetallic
titanium(salen) complexes exhibited high catalytic activity in the
enantioselective cyanation of aldehy-des,4g and bimetallic
oxovanadium complexes effected the enantioselective oxidative
coupling of 2-naphthols.4h The cooperativity effects of zirconium-
based bimetallic complexes played a key role in the olefin
polymerization catalysis.4i Heterobimetallic Ru-Cu complexes
facilitated E-selective semi-hydrogenation of alkynes.4j
complex catalysts for transfer hydrogenation of ketones.11,12 In this
regard, unsymmetrical tridentate pyrazolyl-imidazolyl-pyridines and
pyrazolyl-NH-pyrazolyl-pyridine were used as the ligands to
synthesize the corresponding Ru(II)-NNN pincer complex
catalysts.11 At the outset of our investigations, we envisioned that the
same strategy might be applied for the construction of bimetallic
Ru(II)-NNN pincer complex catalysts by linking two such NNN
coordinating functionalities with a π-electronic platform. Herein, we
disclose the synthesis and catalytic activity of diruthenium(II)
bis(pyrazolyl-imidazolyl-pyridine) complexes for the transfer
hydrogenation of ketones (Scheme 1).
[M]
N
N
N
N
N
N
M
unsymmetrical
NNN ligand
mononuclear complex
π linker
π linker
[M]
M
N
N
N
N
N
N
N
N
N
N
N
N
M
Transition metal-catalyzed transfer hydrogenation has been well
explored as a potentially useful method for the reduction of ketones
to alcohols.5 Mononuclear transition metal complex catalysts have
usually been employed for this purpose,6-9 whereas only a few less
π linker-supported
bis(NNN) ligand
bimetallic complex
Scheme 1. The design strategy of highly active bimetallic NNN pincer
complex catalysts.
aDalian Institute of Chemical Physics, Chinese Academy of Sciences, 457
Zhongshan Road, Dalian 116023, P. R. China. E-mail: zkyu@dicp.ac.cn; Fax
+86-411-84379227; Tel: +86-411-84379227
Results and discussion
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road,
Shanghai 200032, P. R. China
Bis(pyrazolyl-imidazolyl-pyridine) ligand 5 was synthesized by a
multistep synthetic procedure as shown in Scheme 2. Condensation
of 6-(3,5-dimethyl-1H-pyrazol-1-yl)picolinimidate (1)12d with N,N′-
(4,5-diamino-1,2-phenylene)bis(4-methylbezenesulfonamide)(2)13,14
†Electronic supplementary information (ESI) available: Experimental details
and NMR spectra. See DOI: 10.1039/
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by a modified literature method15 afforded tosyl-protected NNN
The NMR analyses of new complexes 6, 7, and 9 are consi-
View Article Online
DOI: 10.1039/C6DT03620A
compound 3 in 86% yield. Depro-tection of 3 with concentrated stent with their compositions, and 6 and 7 showed very similar
sulfuric acid16 gave diamino-substituted NNN compound 4 (39%). resonance signals in their 1H NMR spectra. The NH resonance signal
Further condensation of 4 with 1 in glacial acetic acid and followed in 6 was absent because of the H-D exchange in CD3OD. The 31P{1H}
by neutralization with aqueous ammonia yielded the target π linker- NMR signals of 6 and 7 appeared at 21.4 and 26.6 ppm as singlets,
supported bis(NNN) ligand 5 (96%) (see the Supporting Information respectively, suggesting different electronic impact from the metal
for details).
centers and equivalence of the PPh3 ligands in the two PPh3 pairs
which are positioned trans to each other in the bimetallic complexes.
Due to the poor solubility of 6 and 7 their 13C{1H} NMR spectra
were not successfully collected. Complex 9 features a Ru-H bond
which was signaled as a triplet at -6.7 ppm in the 1H NMR spectrum,
and in its 31P{1H} NMR spectrum a doublet splitted by the Ru-H
hydrogen appeared at 48.5 ppm for the two trans-PPh3 ligands.
The molecular structures of complexes 7 and 9 were further
confirmed by the X-ray single crystal structural determinations (see
the Supporting Information for details). In the solid state, bimetallic
complex 7 exhibits a centrosymmetrical pattern and the two metal
centers are positioned in a distorted octahedral environment
established by the tridentate NNN coordinating functionality, two
trans-PPh3 ligands, and one chloride, respectively (Figure 1). The P-
Ru-P angle in 7 is 177.4°, which is close to that number of 178.8° in
the mononuclear complex 8.11b The average Ru-P bond length in
complex 7 is 2.426 Å, longer than that (2.391 Å) in 8,11b revealing
that the metal centers in 7 are in a much more loose environment
than the mononuclear metal center in 8. These data suggests that the
bimetallic complex 7 may act as a more catalytically active catalyst
than the mononuclear complex 8. The mononuclear RuH complex 9
presents a coordination pattern similar to that in complexes 6, 7, and
8,11b and its P-Ru-P angle and Ru-H bond are 169.6° and 1.52 Å
(Figure 2), respectively. It is noteworthy that in the X-ray
crystallographic molecular structures of complexes 7 and 9, the unit
cells include a region of disordered solvent molecules, which could
not be modeled as discrete atomic sites. PLATON/SQUEEZE was
used to calculate the diffraction contribution of the solvent
molecules and, thereby, to produce a set of solvent-free diffraction
intensities. The SQUEEZE calculations showed the residual electron
density amounted to 30 electrons per unit cell of complex 7 and 52
electrons per unit cell of complex 9, corresponding to three
molecules of water in complex 7 and one molecule of toluene in
complex 9.
Me
Me
Me
H
H2N
H2N
NHTs
NHTs
(i)
OMe
(ii)
N
N
N
N
N
N
N
+
NH
N
NHTs
Me
Me
112d
213,14
3
NHTs
Me
Me
H
N
H
N
1
N
N
N
Me
Me
N
N
N
N
(iii)
N
N
N
N
NH2
N
Me
N
H
NH2
4
5
Conditions: (i) AcOH, 118 oC, 4 h, 86%. (ii) conc. H2SO4, 100 oC, 4h,
then Na2CO3, 39%. (iii) AcOH, 118 oC, 4 h, 96%.
Scheme 2. Synthesis of π linker‐supported bis(NNN) ligand 5.
Treatment of ligand 5 with RuCl2(PPh3)3 in a 1:2 molar ratio in
refluxing isopropanol formed the cationic diruthenium(II)-NNN
pincer complex 6 (92%) (Scheme 3). Alternatively, reacting ligand 5
with 2 equiv of RuCl3·xH2O in refluxing ethanol afforded a
paramagnetic ruthenium complex in 85% yield, which was then
reduced to give the neutral diruthenium(II)-NNN pincer complex 7
(80%) in the presence of PPh3 ligand and Et3N base. Complex 6 was
treated with excessive amount of K2CO3 base by extrusion of two
molecules of HCl also afforded complex 7 (80%). In order to
compare the catalytic properties of the bimetallic complexes with
their corresponding mononuclear NNN pincer complexes complex 8
was prepared by our previuosly reported procedure,11b,11c which was
reduced to its corresponding hydride complex, that is, RuH-NNN
pincer complex 9, in refluxing isopropanol under basic conditions
(Scheme 4).
2+
Me
H
N
Cl
N
N
NPPh
Me
Me
PPh3
N
(i) Ru(PPh3)3Cl2
3N
2Cl-
Ru
P N
N
Ru
Cl
Ph3
Me
N
Ph3
P
N
H
6
5
(iv)
Me
Me
(ii) RuCl3.xH2
O
N
N
Cl
Ru
PN
N
N
NPPh
Me
Me
PPh3
N
3N
(iii) PPh3, Et3
N
Ru
Cl
Ph3
N
Ph3
P
N
7
Conditions: (i) RuCl2(PPh3)3, iPrOH, reflux, 0.1 MPa N2, 6 h, 92%. (ii)
RuCl3 ⋅xH2O, EtOH, reflux, 0.1 MPa N2, 5 h, 85%. (iii) PPh3, Et3N,
EtOH, reflux, 0.1 MPa N2, 6 h, 80%. (iv) K2CO3, CH2Cl2, reflux, 0.1
MPa N2, 5 h, 80%.
Scheme 3. Synthesis of diruthenium(II) complexes 6 and 7.
Me
Me
Me
Me
H
N
N
(i)
(ii)
N
N
N
N
PPh3
N
N
N
Ru
Cl
N
N
N
PPh3
N
N
N
Ru
H
Me
Me
Ph P
Ph
P
3
3
[11b]
8
9
Figure 1. Molecular structure of diruthenium(II)‐NNN pincer complex 7.
Conditions: (i) Ru(PPh3)3Cl2, Et3N, toluene, 110 oC, 0.1 MPa N2, 2 h,
85%; (ii) K2CO3, iPrOH, 82 oC, 0.1 MPa N2, 6 h, 61%.
Next, the catalytic activities of complexes 6-9 were
Scheme 4. Synthesis of Ru(II) complexes 8 and 9.
comparatively investigated. Under the typical conditions for transfer
2 | Dalton Trans 2016, 00, 1‐6
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hydrogenation of ketones,11-13 these four Ru(II) NNN pincer
complexes were tested as the catalysts by using the same loading of
the metal content, that is, 0.1 mol % Ru, for the transfer
hydrogenation reactions of acetophenone, 3′-methyl-acetophenone,
2′-chloroacetophenone, 2-acetylnaphthalene, and 2-octone in
refluxing isopropanol, respectively. In all the cases the reactions
reached 95-99% yields, exclusively forming the corresponding
alcohol products (see Table S1 in the Supporting Information for
details). It was found that the bimetallic complex pair 6 and 7, and
the mononuclear complex pair 8 and 9, exhibited identical catalytic
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3
97
7
8
2
3
4
DOI: 10.1039/C6DT03620A
60
98d
0.5 (15)f
120
99 (99)f
98d
7
8
1 (40)c
5 (60)c
97 (80)c
98d (80)c
7
8
0.5
10
98
7
8
5
6
7
8
98d
1
98
95
7
8
20
3
97
95
7
8
10
3
5
97
95
7
8
3
96
96
7
8
9
Figure 2. Molecular structure of RuH NNN pincer complex 9.
40
activities, respectively, and complexes 6 and 7 were much more
catalytically active than complexes 8 and 9, demonstrating a
catalytic activity order 6 = 7 >> 8 = 9. Thus, the comparative
investigation was extended to more ketone substrates by merely
employing complexes 7 and 8 as the catalysts (Table 1). It should be
noted that complexes 6 and 7 could be stored at room temperature
over half a year and did not lose their catalytic activity. Using the
bimetallic complex catalyst 7 all the substituted acetophenones, 2-
acetylnaphtha-lene, and 2-octone underwent the reactions to
exclusively yield the corresponding alcohol products over a period
of 0.5-10 minutes, while the mononuclear complex catalyst 8
exhibited a much lower catalytic activity, rendering the same
reactions close to completion within 5-240 minutes (Table 1). For
example, complex 7 catalyzed the reactions of acetophenone, 2′-
chloroacetophenone, 2′-fluoroacetophenone, and 4-(trifluoro-
methyl)acetophe-none to completion at 1, 0.5, 3, and 0.5 minutes,
while the same reactions were finished at 15, 120, 40, and 60
minutes by using complex 8 as the catalyst, respectively (Table 1,
entries 1, 3, 9, and 14). Complex 7 exhibited a remarkable
enhancement of catalytic activity, which is presuma-bly attributed to
its high stability and the possible cooperativity of the two coexisting
metal centers in the complex.2,3 However, the reactions underwent
slowly at 40 oC (Table 1, entries 1 and 4).
7
8
1
99
95
10
11
12
13
14
15
30
3
98
96
7
8
30
3
98
96d
7
8
120
0.5
100
94
7
8
120
3
97
96
7
8
10
0.5
60
100
37
7
8
10
60
99
95d
7
8
16
17
18
5
98
95
7
8
180
2
96
95
7
8
Table 1 Comparison of the catalytic activity of complexes 7 and 8.a
15
O
OH
O
0.1 mol % Ru
OH
Me
+
+
10
15
99
99d
7
8
iPrOK
R1
R2
R1
R2
Me
Me
Me
19
time
[min]
yieldb
[%]
entry
1
cat.
ketone
O
7
96
96d
7
8
20
21
Me
O
1 (90)c
15 (120)c
98 (82)c
97d (80)c
10
7
8
Me
10
99
94d,e
7
8
240
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aConditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); catalyst, 0.1 mol % the phenyl ring of the substituted acetophenonesVieewxhAirbticitleedOnlianne
Ru (ketone/iPrOK/Ru = 1000:20:1); 0.1 MPa N2, 82 ºC. bAverage value from acceleration effect on the reactions (Table 1, entries 3-8).
DOI: 10.1039/C6DT03620A
three parallel experiments by GC analysis. cResult from the reaction at 40 oC Surprisingly, in the cases of chloro-substituted acetophenones the
f
given in parentheses. dCited from reference 11b. eUsing 0.2 mol % Ru.
applicable catalyst loading was as low as 0.015 mol % 7 (0.03
Scale-up reaction conditions: ketone, 20.0 mmol (1.0 M in 20 mL iPrOH); mol % Ru), and the reactions were complete within 2-3 minutes,
catalyst 7 (0.005 mol %), 0.01 mol % Ru (ketone/iPrOK/Ru = 10000:20:1); reaching a maximum TOF value of 1.3×106 h-1 (Table 2, entries 3-5).
0.1 MPa N2, 82 ºC.
The substituent effect from electron-withdrawing fluoro, and
trifluoromethyl, and electron-donating methyl and methoxy varied,
rendering the reactions to be complete over a period of 0.5-30
minutes, achieving TOF values in the region of 3.2×104 − 1.1×106 h-1
(Table 2, entries 9-16). It should be noted that o-trifluoromethyl
substituent exhibited a remarkable acceleration effect: the reaction
was finished within 0.5 minute, achieving 100% conversion and a
TOF value of 1.1×106 h-1 (Table 2, entry 12). Unexpectedly,
sterically hindered benzophenone and 2-acetylnaphthalene smoothly
underwent the reaction to produce the corresponding alcohol
products in 96-98% yields (Table 2, entries 18 and 19). Using a
higher catalyst loading (0.04 mol %) 2-octone also efficiently reac-
The kinetics profiles of acetophenone, 4′-fluoroacetophenone,
3-methylacetophenone catalyzed by complexes 7 and 8 also revealed
a remarkable enhancement of catalytic activity of the bimetallic
complex (Figure 3). Additionally, complex 8 could not promote the
reactions to completion or just made the reactions cease at an
incomplete conversion of the ketone substrates when a catalyst
loading of < 0.1 mol % Ru was applied (see Table S2 in the
Supporting Information). These results have revealed that the
complex catalysts have a concentration limit applicable for the
transfer hydrogenation reactions, suggesting that bimetallic complex
7 is more stable than mononuclear complex 8 under the stated
reaction conditions, and can be applied at a lower concentration than
complex 8. Moreover, based on the kinetics profiles obtained from
the reaction of 3-methylacetophenone (Table 1, entry 17, and Figure
3), cooperativity of the two coordinated Ru(II) metal centers through
the π linker is possibly present to enhance the catalytic activity of the
diruthenium(II)-NNN pincer complex. Furthermore, the reaction was
scaled up to 10000:1 of ketone/Ru by using 20 mmol of 2′-
chloroacetophenone and 0.005 mol % complex 7, and the correspon-
ding alcohol product was obtained in 99% yield within 15 minutes
(Table 1, entry 3).
Table 2 Transfer hydrogenation catalyzed by complex 7a,
O
O
OH
Me
O
0.025 mol % 7
iPrOK
R'
R'
+
+
Me
Me
Me
R
R
time
yieldb
[%]
TOFc
entry
R, R′
[min]
[h-1]
1
H, Me
H, Et
20
20
2
98
95
98
98
98
97
98
97
96
98
95
100
97
96
98
96
98
98
1.5×105
4.7×104
8.1×105
4.7×105
1.3×106
1.4×105
2.2×104
3.0×105
3.8×104
3.9×104
4.0×105
1.1×106
3.2×104
3.7×104
2.1×105
2.3×105
1.1×105
1.5×105
2
3d
4d
5 d
6
o-Cl, Me
m-Cl, Me
p-Cl, Me
o-Br, Me
m-Br, Me
p-Br, Me
o-F, Me
3
2
5
7
15
5
8
9
20
20
25
0.5
10
30
15
10
20
30
10
11
12
13
14e
15 e
16e
17f
18
m-F, Me
p-F, Me
o-CF3, Me
m-CF3, Me
o-Me, Me
m-Me, Me
p-Me, Me
m-OMe, Me
H, Ph
O
Me
TOF (h-1
at 50%
)
2.1×105 (using 7)
3.1×104 (using 8)
1.5×105 (using 7)
2.8×105 (using 8)
1.9×105 (using 7)
1.1×103 (using 8)
conversion
Figure 3. Representative kinetics profiles using 0.1 mol % Ru under the
conditions shown in Table 1.
Then, the catalytic activity of bimetallic complex 7 was further
explored at a lower catalyst loading (0.025 mol % 7), that is, 0.05
mol % Ru, for the transfer hydrogenation of ketones (Table 2). At
such a low catalyst concentration, the reaction of acetophenone
became relatively slow to be complete within 20 minutes, achieving
a TOF value of 1.5×105 h-1 at 50% conversation of the ketone
substrate (Table 2, entry 1). The chloro and bromo substituents on
19
20
15
96
98
4.2×105
1.2×104
20f
aConditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); catalyst 7, 0.025
b
mol % (ketone/iPrOK/Ru = 2000:20:1); 0.1 MPa N2, 82 ºC. Average value
c
from three parallel experiments by GC analysis. Turnover frequency (moles
4 | Dalton Trans 2016, 00, 1‐6
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of ketone converted per mol of Ru per hour) at 50% conversion of the ketone.
d0.015 mol % 7 (0.03 mol % Ru). e0.03 mol % 7. f0.04 mol % 7.
pressure gave the product as a green solid (130 mg, 39% yield). Mp:
DOI: 10.1039/C6DT03620A
198−201 °C. H NMR (DMSO-d6, 400 MHz): δ 11.74 (s, 1 H, NH),
View Article Online
1
8.04 and 7.66 (d each, J = 7.7 and 7.8 Hz, 1:1 H, 3-H and 5-H), 7.99
(t, J = 7.8 Hz, 1 H, 4-H), 6.83 and 6.74 (s each, 1:1 H, 8″-H and 5″-
H), 6.16 (s, 1 H, 4′-H), 4.68 and 4.37 (s each, 2:2 H, 5″-NH2 and 8″-
NH2), 2.71 (s, 3 H, C3′-CH3), 2.23 (s, 3 H, C5′-CH3). 13C{1H} NMR
(DMSO-d6, 100 MHz) δ 152.4 and 148.9 (s and Cq each, C2 and
C6), 147.7 and 141.1 (s and Cq each, C3′ and C5′), δ 145.9 (s and
Cq, C2″), 139.5 and 135.5 (s and Cq each, C4″ and C9″), 137.4 (s,
C4), 133.3 and 129.0 (s and Cq each, C7″ and C6″), 117.6 and 115.1
(s each, C8″ and C5″), 108.6 and 102.9 (s each, C3 and C5), 95.4 (s,
C4′), 13.9 (s, C3′-CH3), 13.4 (s, C5′-CH3). HRMS: calcd for
C17H17N7 319.1545, found 319.1556.
ted to form the target product (Table 2, entriy 20). These results have
illustrated
a rare efficient catalyst system for the transfer
hydrogenation of ketones at a low catalyst loading.8,11-13 It is
noteworthy that one of Baratta’s Ru(II)-NNC complexes could reach
a TOF value of 3.8×106 h-1 with 99% conversion within 0.5 minute
at 0.005 mol % catalyst loading in the transfer hydrogenation of 3′-
bromoacetophenone.8d
Based on the comparative investigation of the catalytic activity
of complexes 8 and 9 (see Table S1 in the Supporting Information)
we can reasonably conclude that Ru(II) hydride may act as the
catalytically active species for the transfer hydrogenation of
ketones.17 The Ru(II) hydride species is initially generated in situ
from the Ru(II)-Cl precatalyst by interaction with potassium
alkoxide under the stated reaction conditions. Then, the in situ
generated RuH species catalyzes the reduction of ketone via a
possible inner-sphere pathway18 (see Scheme S1 in the Supporting
Information).
Synthesis of compound 5: A mixture of methyl 6-(3,5-dimethyl-
1H-pyrazol- 1-yl)-picolinimidate (1) (80 mg, 0.3 mmol) and
compound 4 (115 mg, 0.3 mmol) in 10 mL glacial acetic acid was
stirred at reflux under nitrogen atmosphere for 4 h. The resultant
mixture was cooled to ambient temperature, and followed by
addition of 30 mL water and neutralization with aqueous ammonia
(25%, 5 mL). The precipitate was collected by filtration and dried
under reduced pressure. Further purification by silica gel column
chromatography (CH2Cl2/CH3OH, 20:1, v/v) afforded the target
product as a light yellow solid (150 mg, 96% yield). Mp: > 300 °C
Conclusions
In summary, diruthenium(II) bis(pyrazolyl-imidazolyl-pyridine)
pincer complexes have been successfully synthesized, and exhibited
very high catalytic activity for the transfer hydrogenation of ketones
at low catalyst loadings. The present protocol provides an applicable
route to highly active bimetallic NNN pincer complex catalysts for
homogeneous catalysis and organic synthesis.
1
dec. H NMR (TFA-d, 400 MHz): δ 8.20 (m, 4 H, 3-H and 5-H),
8.08 (t, J = 7.7 Hz, 2 H, 4-H), 7.67 and 7.65 (s each, 1:1 H, 6"-H),
6.21 (s, 2 H, 4′-H), 2.33 (s, 6 H, C3′-CH3), 2.20(s, 6 H, C5′-CH3).
13C{1H} NMR (TFA-d, 100 MHz) δ 149.8 (s and Cq, C6), 148.4 (s
and Cq, C3′), 147.1 (s and Cq, C2), 145.9 (s, C4), 142.8 (s and Cq,
C5′),139.2 and 130.3 (s and Cq each, C5″ and C4″), 124.2 and 120.1
(s each, C3 and C5), 111.0 (s, C4′), 100.5 (s C6″). IR (KBr pellets,
cm-1): 3402, 2926, 1634, 1596, 1580, 1483, 1464, 1440, 1408, 1384,
1364, 1289, 1246, 971, 815, 775, 724, 656. HRMS: calcd for
C28H24N10 500.2185, found 500.2187.
Experimental
General considerations
All the manipulations of air- and/or moisture-sensitive compounds
were carried out under nitrogen atmosphere using the standard
Schlenk techniques. The solvents were dried and distilled prior to
Synthesis of complex 6: Under nitrogen atmosphere, a mixture of
RuCl2(PPh3)3 (96 mg, 0.10 mmol) and ligand 5 (25 mg, 0.05 mmol)
in 2-propanol (10 mL) was stirred at reflux for 6 h. The resulting
mixture was cooled to ambient temperature and the resultant red
precipitate was filtered off, rinsed with diethyl ether (4×6 mL), and
dried under reduced pressure to afford the target complex product as
a red-brown crystalline solid (90 mg, 92% yield). Mp: >300 °C dec.
1H NMR (CDCl3/CD3OD, 400 MHz): δ 8.52 (s, 2 H, 6″-H), 7.59 (m,
2H, 4-H), 7.41 and 7.33 (d each, J = 8.6 Hz and J = 7.7 Hz, 2:2 H, 3-
H and 5-H), 7.17, 7.10 and 7.00 (m each, 24:12:24 H, 4×PPh3), 6.17
(s, 2 H, 4′-H), 2.77 and 2.18 (s each, 6:6 H, methyl of pyrazolyl).
Due to poor solubility, the 13C{1H} NMR is not avilable. 31P{1H}
NMR (CDCl3/CD3OD, 162 MHz): δ 21.6 (s, 4×PPh3). IR (KBr
pellets, cm-1): 3417, 3051, 1610, 1556, 1482, 1432, 1412, 1307,
1092, 741, 698, 521. Anal. Calcd for C101H87Cl4N10P4Ru2: C, 63.56;
H, 4.59; N, 7.34. Found: C, 63.15; H, 4.61; N, 7.32.
1
use by the literature methods. H and 13C{1H} NMR spectra were
recorded on a Bruker DRX-400 spectrometer and all chemical shift
values refer to δTMS = 0.00 ppm, CDCl3 (δ(1H), 7.26 ppm; δ(13C),
77.16 ppm) and DMSO-d6 (δ(1H), 2.50 ppm; δ(13C), 39.52 ppm).
Elemental and HRMS analysis were achieved by the Analysis Center,
Dalian University of Technology and Dalian Institute of Chemical
Physics, Chinese Academy of Sciences. All the melting points were
uncorrected. TLC analysis was performed by using glass-backed
plates coated with 0.2 mm silica gel. Flash column chromatography
was performed on silica gel (200-300 meshes). All chemical reagents
were purchased from commercial sources and used as received
unless otherwise indicated.
Synthesis of ligands and complexes
Synthesis of complex 7: Under nitrogen atmosphere a mixture of
RuCl3·xH2O (26 mg, 0.10 mmol) and ligand 5 (25 mg, 0.05 mmol)
in ethanol (20 mL) was stirred at reflux for 5 h. After the mixture
was cooled to ambient temperature, the resultant precipitate was
filtered off, rinsed with diethyl ether (3×5 mL), and dried under
reduced pressure to afford a brown powder (38 mg, 0.04 mmol). The
brown powder was then reacted with PPh3 (43 mg, 0.16 mmol) and
triethylamine (1.0 mL) in EtOH (5 mL) at reflux for 6 h under
nitrogen atmosphere. After the mixture was cooled to ambient
Synthesis of compound 4: A mixture of compound 3 (628 mg, 1.0
mmol) and 5 mL concentrated sulfuric acid was heated at 100 °C for
4 h. The resulting dark violet mixture was carefully poured into 50
mL ice water. Treatment of the resultant mixture with saturated
aqueous Na2CO3 until the solution became basic (pH>8), forming a
clear green solution. The green solution was then extracted with
dichloromethane (3×60 mL). The combined organic phase was dried
over MgSO4, and filtered. Removal of all the volatiles under reduced
This journal is © The Royal Society of Chemistry 2016
Dalton Trans 2016, 00, 1‐6 | 5
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Green Chemistry
temperature, the resultant precipitate was filtered off, rinsed with refinement were performed by using the SHELXL-97VipewacAkrtaicglee.OTnlihnee
DOI: 10.1039/C6DT03620A
diethyl ether (3×5 mL), and dried under reduced pressure to give the X-ray crystallographic files, in CIF format, are available from the
complex product as a red powder (60 mg, 80% yield). Single crystals Cambridge Crystallographic Data Centre on quoting the deposition
suitable for X-ray crystallographic determination were grown from numbers CCDC 1053207 for 7 and CCDC 1002186 for 9. Copies of
the recrystalolization of 7 in CHCl3/CH3OH/n-hexane (20:1:60, this information may be obtained free of charge from the Director,
v/v/v) at 25 °C.
CCDC, 12 Union Road, Cambridge CB2 IEZ, UK (Fax: +44-1223-
Mp: >300 °C dec. 1H NMR (CDCl3/CD3OD, 400 MHz): δ 8.49 (s, 2 336033; e-mail: deposit@ccdc.cam.ac.uk or www: http://www.
H, 6″-H), 7.54 (t, J = 8.1, 2 H, 4-H), 7.34 and 7.30 (d each, J = 8.0 ccdc.cam.ac.uk).
Hz and J = 8.4 Hz, 2:2 H, 3-H and 5-H), 7.14, 7.07 and 6.97 (m each,
24:12:24 H, 4×PPh3), 6.11 (s, 2 H, 4′-H), 2.73 and 2.19 (s each, 6:6 A typical procedure for the transfer hydrogenation reactions of
H, methyl of pyrazolyl). Due to poor solubility, the 13C{1H} NMR is
ketones
not avilable. 31P{1H} NMR (CDCl3/CD3OD, 162 MHz): δ 26.6 (s,
4×PPh3). IR (KBr pellets, cm-1): 3420, 3035, 1608, 1553, 1519, 1482, A catalyst solution was prepared by dissolving complex 7 (11.3 mg,
1465, 1438, 1318, 1091, 744, 696, 516. Anal. Calcd for 0.006 mmol) in 2-propanol (60 mL). Under nitrogen atmosphere, a
C101H85Cl2N10P4Ru2: C, 66.08; H, 4.67; N, 7.63. Found: C, 66.15; H, mixture of ketone (2.0 mmol), 10 mL of the catalyst solution (0.001
4.61; N, 7.54.
mmol), and 2-propanol (9.6 mL) was stirred at 82 °C for 5-10
minutes. Then, 0.4 mL of 0.1 M iPrOK (0.04 mmol) solution in 2-
propanol was introduced to initiate the reaction. At the stated time,
0.1 mL of the reaction mixture was sampled and immediately diluted
with 0.5 mL of 2-propanol pre-cooled at 0 °C, and filtered through a
short pad of celite to remove the complex catalyst to quench the
reaction. The resultant filtrate was used for GC analysis. After the
reaction was complete, the reaction mixture was quickly cooled to
ambient temperature, filtered through celite, condensed under
reduced pressure, and then subjected to purification by silica gel
column chromatography to afford the corresponding alcohol product
which was identified by comparison with the authentic sample
through NMR and GC analysis.
Synthesis of complex
7 from complex 6: Under nitrogen
atmosphere, a mixture of complex 6 (98 mg, 0.05 mmol) and K2CO3
(69 mg, 0.50 mmol) in dichloromethane (6 mL) was stirred at reflux
for 5 h. The resultant mixture was filtered through a short pad of
celite, and rinsed the celite with 5 mL dichloromethane. The
combined filtrate was evaporated all the volatiles under reduced
pressure to afford complex 7 (75 mg, 80% yield).
Synthesis of complex 9: Complex 8 (403 mg, 0.42 mmol) was
reacted with K2CO3 (138 mg, 1.0 mmol) in 10 mL refluxing iPrOH
for 6 h under nitrogen atmosphere. After cooled to ambient
temperature all the volatiles were removed under reduced pressure,
and 5 mL toluene was introduced to dissolve the crude product,
followed by filtration to separate the inorganic salts. The filtrate was
condensed to 1/3 volume under reduced pressure and then
recrystallized in toluene/n-hexane (1:2, v/v) at ambient temperature
to give the target complex product as red-brown crystals (238 mg,
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (21672209) for financial support of this research.
1
61% yield). Mp: >300 °C dec. H NMR (CDCl3, 400 MHz): δ 7.87
(d, J = 7.7 Hz 1 H, 3-H), 7.41 (m, 2 H, 4-H and 5-H), 7.18 (s, 1 H,
phenyl CH), 6.99, 6.91 and 6.85 (m each, 6:12:12 H, 2×PPh3), 6.74
and 6.49 (t each, J = 7.2 Hz and J = 7.3 Hz, 6"-H and 7"-H), 6.65 (d,
J = 8.0 Hz, 1 H, phenyl CH), 5.60 (s, 1 H, 4'-H), 2.52 (s, 3 H, C3'-
CH3), 1.17 (s, 3 H, C5'-CH3), -6.71 (t, J = 25.6 Hz, Ru-H); 13C{1H}
NMR (CDCl3, 100 MHz): δ 160.0 and 153.1 (Cq each, C2 and C6),
152.8 and 145.8 (Cq each, C3' and C5' ), 148.9, 147.3, 132.0 (Cq
each, C2", C4" and C9"), 140.9 (s, C4), 132.0 (Cq, 2×PPh3), 133.1,
128.2 and 127.3 (CH of 2×PPh3), 119.4, 119.2, 116.0, 110.8 (s each,
phenyl CH), 118.2 and 117.6 (s each, C3 and C5), 105.3 (s, C4'),
16.1 (s, C3'-CH3), 15.3(s, C5'-CH3); 31P{1H} NMR (CDCl3, 162
MHz): δ 48.5 (d, J(P,H) = 25.0 Hz, 2×PPh3). IR (KBr pellets, cm-1):
3438, 3042, 1840, 1600, 1553, 1497, 1478, 1456, 1434, 1408, 1391,
1351, 1321, 1088, 743, 698, 518. Anal. Calcd. for C53H45N5P2Ru: C,
69.57; H, 4.96; N, 7.65. Found: C, 69.51; H, 4.90; N, 7.64.
Notes and references
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Single crystal X-ray diffraction studies for complexes 7 and 9 were
carried out on a SMART APEX diffractometer with graphite-
monochromated Mo Kα radiation (λ= 0.71073 Å). Cell parameters
were obtained by global refinement of the positions of all collected
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direct methods and refined by full-matrix least squares on F2. All
non-hydrogen atoms were refined anisotropically. All hydrogen
atoms were placed in calculated positions. Structure solution and
6 | Dalton Trans 2016, 00, 1‐6
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DOI: 10.1039/C6DT03620A
Table of Contents
Diruthenium(II)-NNN Pincer Complex Catalysts for Transfer
Hydrogenation of Ketones
Huining Chai, Qingfu Wang, Tingting Liu, and Zhengkun Yu*
A strategy to construct highly efficient diruthenium(II)-NNN pincer complex catalysts
was established for transfer hydrogenation of ketones.