Ruthenium complexes bearing an unsymmetrical pincer ligand with a 2-hydroxypyridylmethylene fragment: Active catalysts for transfer hydrogenation of ketones

Article abstract of DOI:10.1039/c6dt00034g

Five ruthenium(ii) complexes were synthesized, including (HO-C₅H₃N-CH₂-C₅H₃N-C₅H₄N)Ru(PPh₃)Cl₂ (3), [(HO-C₅H₃N-CH₂-C₅H₃N-C₅H₄N)Ru(PPh₃)₂Cl][PF₆] (4) and [(HO-C₅H₃N-CH₂-C₅H₃N-C₅H₄N)Ru(PPh₃)₂OH][PF₆] (5) bearing an unsymmetrical pincer NNN ligand with a 2-hydroxypyridylmethylene fragment, and [(CH₃O-C₅H₃N-CH₂-C₅H₃N-C₅H₄N)₂Ru][Cl]₂ (6) and [(CH₃O-C₅H₃N-CH₂-C₅H₃N-C₅H₄N)₂Ru][PF₆]₂ (7) containing 2-methoxypyridylmethylene moieties. 4 reacts with H₂O at room temperature to give 5 whose crystal structure reveals the existence of intramolecular hydrogen-bonding between its two -OH groups. 3 exhibits high catalytic activity for transfer hydrogenation of ketones.

Full text of DOI:10.1039/c6dt00034g

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DOI: 10.1039/C6DT00034G
Journal Name
ARTICLE
Ruthenium complexes bearing an unsymmetrical pincer ligand
with a 2‐hydroxypyridylmethylene fragment: active catalysts for
transfer hydrogenation of ketones
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
c
ab
Jing Shi, Bowen Hu, Dawei Gong, Shu Shang, Guangfeng Hou and Dafa Chen*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Five ruthenium (II) complexes were synthesized, including (HO‐C
N‐C N)Ru(PPh Cl][PF ] (4) and [(HO‐C N‐CH ‐C N‐C
pincer NNN ligand with a 2‐hydroxypyridylmethylene fragment, and [(CH
(CH O‐C N‐CH ‐C N‐C N) Ru][PF (7) containing 2‐methoxypyridylmethylene moieties. 4 reacts with H
temperature to give 5 whose crystal structure reveals the existence of intramolecular hydrogen‐bonding between its two ‐
OH groups. exhibits high catalytic activity for transfer hydrogenation of ketones.
5
H
3
N‐CH
2
‐C
5
H
3
N‐C
OH][PF
N‐CH
5
H
4
N)Ru(PPh
] (5) bearing an unsymmetrical
‐C N‐C N) Ru][Cl] (6) and
O at room
3 2 5 3 2
)Cl (3), [(HO‐C H N‐CH ‐
C
5
H
3
5
H
4
3
)
2
6
5
H
3
2
5
H
3
5
H
4
N)Ru(PPh
)
3 2
6
3
O‐C
5
H
3
2
5
H
3
5
H
4
2
2
[
3
5
H
3
2
5
H
3
5
H
4
2
6
]
2
2
3
Introduction
Metal‐ligand cooperative complexes have important applications in
1
bond activation and catalysis. For example, Milstein’s pyridine‐
based PNP‐Ru and PNN‐Ru complexes exhibit an interesting metal‐
ligand cooperation based on aromatization‐dearomatization
processes, and have been exploited for a series of hydrogenation
Fig. 1 The active site of [Fe]‐hydrogenase.
1
,2
reactions.
Transition‐metal complexes with 2‐hydroxypyridyl (2‐HOPy)
derivatives are another group of metal‐ligand bifunctional catalysts,
and have also been applied to many catalytic reactions, such as
oxidation of alcohols, hydrogenation of CO2 and transfer
A number of bidentate structural models of [Fe]‐hydrogenase
9
,13‐18
have been synthesized,
but none of them show any catalytic
1
3
acitvity, except a semisynthetic iron complex and a ruthenium
1
4
3
‐8
complex. Inspired by [Fe]‐hydrogenase, Szymczak et al. recently
developed pincer‐type ruthenium complex
(dhtp)Ru(PPh ) Cl][PF ) with 6,6′‐dihydroxy terpyridine (dhtp) as a
hydrogenation of ketones, and so on. In the catalytic cycles,
a
aromatization‐dearomatization might also be involved, via the
3
‐8
[
tautomerization of 2‐HOPy and 2‐pyridone.
Besides these
3 2
6
ligand, and demonstrated its capability of catalyzing transfer
synthetic examples, 2‐HOPy is also employed by nature to activate
H2, e.g. [Fe]‐hydrogenase. In the active site of [Fe]‐hydrogenase,
there is a unique bidentate 2‐hydroxy‐6‐acylmethylenepyridyl (2‐
1
9
hydrogenation of ketones in 2‐propanol. Since a secondary
4
coordination sphere could influence the catalytic activities, in this
9
,10
present work we choose another ligand 6‐([2, 2′‐bipyridin]‐6‐
ylmethyl) pyridin‐2‐ol (2) for the Ru complexes. Compared to dhtp,
HO‐6‐CH CO‐Py) derivative coordinating with Fe (Fig. 1).
2
Although the exact mechanism of [Fe]‐hydrogenase is still needed
2
is not only structurally closer to the active site of [Fe]‐
to be explored, the 2‐HO‐6‐CH CO‐Py ligand must play a critical
2
1
1,12
hydrogenase, but also more flexible, which might result in both
meridional and facial Ru products, showing some interesting
role.
‐hydroxy group acts as the proton acceptor. Recently, the first
experimental evidence supported this mechanism, and indicated
Theoretical studies have suggested that the deprotonated
1
2
2
2
0
reactivity. We expect the catalytic activity of the ruthenium
complexes with the new ligand to be improved significantly. Herein
we report the synthesis, reactivity and catalytic transfer
hydrogenation activity of several ruthenium complexes supported
by 2.
1
3
the 2‐hydroxy group was essential for H activation.
2
a.
School of Chemical Engineering & Technology, Harbin Institute of Technology,
Harbin 150001, P.R. China. E‐mail: dafachen@hit.edu.cn.
State Key Laboratory of Elemento‐organic Chemistry, Nankai University, Tianjin
b.
c.
300071, P.R. China.
Key Laboratory of Functional Inorganic Material Chemistry (MOE); School of
Chemistry and Materials Science, Heilongjiang University, Harbin 150080, P. R.
China.
Results and discussion
Synthesis and characterization of the ligands and their Ru(II)
complexes
Electronic Supplementary Information (ESI) available: Crystallographic data for
complex and NMR spectra of the new compounds. For ESI and crystallographic
5
data in CIF or other electronic format, see DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C6DT00034G
Scheme 1 Synthesis of ligands 1 and 2.
As shown in Scheme 1, 1 was given by the reaction of NH
2
NH
2
with
Fig. 2 Solid‐state structure of
5
. The thermal ellipsoids are displayed at a
3
0% probability. Hydrogen atoms not involved in hydrogen bonds, phenyl
(6‐methoxypyridin‐2‐yl)(6‐(pyridin‐2‐yl)pyridin‐2‐yl)methanone,
‐
rings on PPh ligands and PF anion have been omitted for clarity. Selected
3
6
o
which was synthesized by the treatment of the known complex
methyl [2,2'‐bipyridin]‐6‐carboxylate with one equivalent of the
lithium salt of 2‐bromo‐6‐methoxy‐pyridine. Hydrolysis of the
methoxyl group of 1 with HBr (40% in water) afforded 2, which is an
bond distances (Å) and angle ( ): Ru(1)‐N(1), 2.081(7); Ru(1)‐N(2), 2.031(6);
Ru(1)‐N(3), 2.151(6); Ru(1)‐P(1), 2.402(3); Ru(1)‐P(2), 2.437(3); Ru(1)‐
O(1W), 2.135(4); O(1)‐O(1W), 2.423(8); C(16)‐O(1), 1.341(10); N(2)‐Ru(1)‐
O(1W), 172.4(2); N(1)‐Ru(1)‐N(3), 171.2(2); P(1)‐Ru(1)‐P(2), 173.09(7).
unsymmetrical
hydroxypyridylmethylene fragment.
When 2 was treated with RuCl (PPh ) in refluxing methanol for
pincer
NNN
ligand
with
a
2‐
of water left in solvents, although they had been dried before use.
When the mixture of 4 and 5 was added with excess of water, 4
transformed to 5 completely. Interestingly, 3 could be re‐formed
when the mixture of 4 and 5 (or pure 5) was treated with excess of
2
3 3
24
hours,
a
pincer‐type complex (HO‐C H N‐CH ‐C H N‐
5 3 2 5 3
1
C H N)Ru(PPh )Cl (3) was formed with 59% yield (Scheme 2). 3 is a
5
4
3
2
HCl (Scheme 3). Each of the H NMR spectra of 4 and 5 shows one
1
red color complex. Its H NMR spectrum shows two doublets at 3.94
and 3.40 ppm for the ‐CH ‐ group, indicating that the two Cl atoms
singlet for the ‐CH ‐ group (3.55 and 3.84 ppm, respectively),
2
3
1
2
indicating the trans configuration of the PPh groups. The P NMR
3
3
1
are in cis positions. The P NMR shows one singlet at 58.1 ppm,
which is comparable to that of the NNN‐pincer ruthenium complex
for PPh groups in 4 and 5 are quite similar (22.5 ppm for 4, and
3
2
6.7 ppm for 5), and are both comparable to that of
1
9
L Ru(PPh )Cl (L = 2‐(1H‐pyrazol‐3‐yl)‐6‐(3,5‐dimethylpyrazol‐1‐yl)‐
1
3
2
1
[(dhtp)Ru(PPh ) Cl][PF ).
3
2
6
2
1a
pyridine) synthesized by Yu′s group.
Complex 3 further reacted with NH PF and PPh in methanol,
The molecular structure of 5 was further confirmed by X‐ray
crystallography (Fig. 2). The hydrogen atom of the Ru‐OH could not
be located exactly. The Ru(1)‐O(1W) distance (2.135(4) Å) is in the
4
6
3
but unexpectedly, a mixture of two ionic complexes [(HO‐C H N‐
5
3
2
2b,23
CH ‐C H N‐C H N)Ru(PPh ) Cl][PF ] (4) and [(HO‐C H N‐CH ‐C H N‐
2
5
3
5
4
3 2
6
5
3
2
5
3
range of Ru‐OH bonds.
The C(16)‐O(1) distance (1.341(10)) is
C H N)Ru(PPh ) OH][PF ] (5) was constantly produced (Scheme 3).
5
4
3 2
6
consistent with a C‐O single bond, indicating a neutral NNN ligand
‐
This was because the ‐Cl in 4 could be easily substituted by the ‐OH
scaffold. Remarkably, similar to complex [(dhtp)Ru(PPh ) Cl](PF ),
3
2
6
group in the presence of water at room temperature, giving the
the O(1)‐O(1W) distance (2.423(8) Å) indicates strong
intramolecular hydrogen‐bonding interaction between the two
2
2
product 5, accompanied by the loss of HCl (Scheme 3).
A
1
9
completely pure 4 was not obtained due to the presence of a trace
hydroxyl groups.
To compare the reactivity and catalytic efficiency of Ru(II)
complexes with different secondary coordination spheres, ligand 1
was also treated with RuCl2 (PPh ) in refluxing methanol.
3 3
Interestingly, the only identified product was [(CH O‐C H N‐CH ‐
3
5
3
2
C H N‐C H N) Ru][Cl] (6), no matter if the ratio of 1 to RuCl (PPh )
3 3
5
3
5
4
2
2
2
was 1:1 or 2:1. 6 could also react with NH PF to give [(CH O‐C H N‐
4
6
3
5 3
2
4
CH ‐C H N‐C H N) Ru][PF ] (7) (Scheme 4). 6 and 7, different
2
5
3
5
4
2
6 2
Scheme 2 Synthesis of 3.
from 3‐5, have two NNN ligands coordinating with Ru(II). The
difference might be because the coordination ability of 1 is higher
Scheme 3 Synthesis of
4
and
5
.
Scheme 4 Synthesis of 6 and 7.
2
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a
Table 2. Transfer hydrogenation of ketone catalyzed by complex 2.View Article Online
Table 1. Optimization of reaction conditions for the transfer hydrogenation of
acetophenone catalyzed by Ru (II) complexes 3 and 5‐7.
a
DOI: 10.1039/C6DT00034G
entry
ketone
Time
GC yield (Isolated
yield) (%)
final TOF
(h )
‐1 b
(min)
O
3
3
3
3
1
2
3
4
Me
10
97 (93)
99 (94)
98 (91)
92 (87)
1.1610
1.1910
1.1810
1.1010
O
b
entry
cat.
5
base
PrOK
PrOK
PrOK
PrOK
time(min)
Yield(%)
80%
95%
0%
Me
10
10
10
i
i
i
i
1
2
3
4
5
6
7
8
8
8
F
O
3
Me
6
8
Cl
O
7
8
0%
Me
Br
3
NaOH
KOH
15
15
15
15
15
15
15
120
73%
90%
90%
98%
62%
12%
0%
3
O
3
t
F
Cl
Br
5
10
95 (90)
1.1410
3
BuOK
Me
i
3
PrOK
O
c
i
3
3
3
3
9
3
PrOK
6
7
8
9
Me
10
10
10
10
15
90
25
93 (89)
92 (87)
91 (88)
96 (92)
95 (90)
94 (90)
96 (91)
1.1210
1.1010
1.0910
1.1510
760
d
i
10
3
PrOK
O
O
i
11
PrOK
Me
Me
12
3
49%
a
i
General conditions: ketone, 2.0 mmol (2M in 50ml PrOH);
o
b
c
F
ketone/base/cat.
=
200/10/1;
N
2
(0.1 MPa); 82 C.
GC yield.
d
O
Ketone/base/cat. = 1000/50/1. Reaction was carried out under an air
atmosphere.
Me
Cl
O
than 2, which exists as a pyridone form in solution. The results
indicate that when ‐PyOH is changed to ‐PyOMe group, the
coordination mode of transition‐metal complexes, as well as their
1
0
Me
O
4
reactivity, might be changed accordingly.
11
125
Me
O
O
Catalysis
O
O
12
Me
Me
461
Complexes 3, 5‐7 were then tested as the catalysts for the transfer
hydrogenation of acetonphenone in refluxing isopropyl alcohol
i
13
150
86 (80)
69
(Table 1). Firstly, PrOK was chosen as the base, and the reactions
were performed with
a
molar ratio of 200/10/1 for
O
ketone/base/catalyst under a nitrogen atmosphere. It is shown in
Table 1 (entries 1‐2) that complex 3 gave 15% higher yield than 5.
Compared to 5, 3 might be easier to be deprotonated by an
14
15
16
17
120
60
92 (85)
96 (92)
97
92
O
21a
Me
192
external base, to give an intermediate with an open site. 6 and 7
i
did not show any reactivity, because they could not react with PrOK
O
to give an open site (entries 3‐4). Other bases such as NaOH, KOH
3
8
1.4610
t
i
and BuOK were also tested, but PrOK gave the best performance, a
O
98% yield within 15 min (entries 5‐8). If the catalyst was reduced to
40
99
297
0.1%, the yield decreased to 62% (entry 9). When the reactions
4
were carried out under air or in absence of a base or catalyst, the
yields were negligible or much lower (entries 10‐12).
On the basis of the results above, the conditions for entries 2 and
O
c,d
18
120
120
53
n.d.
8
in Table 1 were selected as the optimal conditions (except the
O
c
19
88
n.d.
reaction times), and they were later applied in the reduction
experiments of a variety of ketones (Table 2). For all the substrates,
the reaction times were optimized, to make sure the yields reached
to 90% or above (except entry 13 due to a relative slow reaction
a
i
General conditions: ketone, 2.0 mmol (2M in 50 mL of PrOH);
o
b
ketone/base/cat. = 200/10/1; N
yield. Yield determined by GC/MS; yield of carbonyl hydrogenation
product. Note other products are observed, see text. 1 mol% PPh added.
3
2
(0.1 MPa); 82 C. TOF calculated with GC
c
d
speed). The transfer hydrogenation reactions for aryl ketones gave
3
86−99% yields within 8−150 min, reaching final TOFs up to 1.1910
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−
1
6,21a
h
(Table 2, entries 1‐15). Acetophenone was reduced to 1‐ Yamaguchi et al.,
we propose a possible mechanism in Scheme
View Article Online
DOI: 10.1039/C6DT00034G
i
phenylethanol in approximately quantitative (97%) yield within 10 5. In the presence of PrOK, 3 firstly transforms to an intermediate A
min (Table 2, entry 1). No obvious reactivity changed was found with an open site via dearomatization, by extrusion of one molecule
when electron withdrawing groups such as fluoro, chloro and of HCl. This intermediate then reacts with PrOH to form a Ru(II)
i
bromo were introduced into the phenyl group (91%‐99%, Table 2, alkoxide species B. The Ru‐H intermediate C, regarded as the
entries 2‐9), while adding electron‐donating groups decreased the catalytically active species, is then formed through ‐H elimination,
reactivity (86−96% yield, Table 2, entries 10, 11 and 13). by releasing one molecule of acetone. Addition of a ketone to C
Benzophenone and 2‐acetonaphthone reacted smoothly to give the produces another Ru(II) alkoxide intermediate D. After the
desired product in 92−96% yields within 120 min (Table 2, entries elimination of one molecule of alcohol, A is re‐formed and the cycle
14−15). Aliphaꢀc ketone showed moderate reacꢀvity (Table 2, entry is finished. Attempts to isolate the intermediates were unsuccessful
1
7), while cyclohexanone exhibited highest reactivity, with a TOF probably due to their unstability. For example, we have tried the
3
−1
i
value of 1.4610
h
(Table 2, entry 16). To the best of our reactions of 3 with PrOK (1 eq. and 2 eq.) and K CO (1 eq. and 10
2
3
i
knowledge, the highest reported TOF value in transfer eq.) in PrOH, and 3 with KN(SiMe ) (1 eq.) in THF, however, no
hydrogenation of ketones was 2.510 h , which was reported by identified product was isolated. The reaction of 3 with NaHBEt (1
Baratta’group. Stradiotto’ group also reported a Ru(II) catalyst eq.) in THF also did not give any characterized Ru‐H product. Since
showing high catalytic activity (up to 2.210 h ). Besides, Yu et no intermediate has been isolated, the detailed mechanistic
al. developed a series of NNN‐pincer Ru(II) complexes showing analyses are needed in further experiments.
3
2
6
‐1
3
2
5a
5
‐1 25b
2
1
excellent catalytic activity. Our system is still needed to be
improved.
To further understand the reaction mechanism, unsaturated Conclusions
aliphatic ketones were also tested. 5‐hexen‐2‐one was converted to
In summary, a ruthenium (II) complex 3 bearing a NNN ligand
a mixture of hydrogenation products with high conversion (93%)
over 120 min, and the ratio of 1: 0.6: 0.45 obtained for 5‐hexen‐2‐ol,
with a 2‐hydroxypyridylmethylene fragment was synthesized by the
reaction of RuCl (PPh ) with 2. 3 could further react with NH PF
2
3 3
4
6
2‐heptanone and 2‐hexanol, respectively, with a 53% yield of 5‐
and PPh3 to afford 4. 4 shows a reactivity with H O at room
2
hexen‐2‐ol (Table 2, entry 18). Different from the result of
temperature in the absence of a base to give 5, accompanied by the
loss of HCl. The crystal structure of 5 shows strong intramolecular
hydrogen‐bonding interaction between the two hydroxyl groups.
Surprisingly, when 1 was treated with RuCl (PPh ) , the complex 6
‐
[
(dhtp)Ru(PPh ) Cl](PF ), addition of
1
mol% PPh3 did not
3
2
6
significantly change the selectivity, which might be because the
1
9
dissociation of PPh was not involved in the catalytic cycle. For 6‐
3
2
3 3
methyl‐5‐hepten‐2‐one, a mixture of 6‐methyl‐5‐hepten‐2‐ol, 6‐
methylheptan‐2‐one and 6‐methylheptan‐2‐ol in a 1: 0.06: 0.02
ratio was obtained (95% conversion) (Table 2, entry 19), which
means that the chemoselectivity goes much higher when the steric
environment around the olefin is increased.
with two NNN ligands was formed. The structural difference
between 3 and 6 indicates that a small change in the ligands (‐PyOH
to ‐PyOMe) might result in a significant change in the coordination
mode and reactivity of transition‐metal complexes. 3 exhibits
higher catalytic activity in transfer hydrogenation of ketones
The low selectivity for 5‐hexen‐2‐one suggests the reactions may
‐
compared against [(dhtp)Ru(PPh ) Cl](PF ) with a symmetrical
1
9,21,26
3 2
6
follow an inner‐sphere hydrogenation pathway.
Combined
1
9
ligand, but its catalytic efficiency is still needed to be improved
the mechanisms of L Ru(PPh )Cl and the transition‐metal
1
3
2
compared to other NNN‐pincer ruthenium complexes developed by
complexes with 2‐HOPy derivatives developed by Fujita and
2
1
Yu’s group. The catalytic pathway might follow an inner‐sphere
hydrogenation mechanism, and might involve a metal‐ligand
cooperation, which is based on the aromatization/dearomatization
via protonation/deprotonation of the 2‐hydroxypyridyl group.
Detailed mechanistic studies are still ongoing in our lab.
Experimental Section
General Considerations. All manipulations were carried out
under an inert nitrogen atmosphere using a Schlenk line. Solvents
were distilled from appropriate drying agents under N before use.
2
All reagents were purchased from commercial sources. Liquid
compounds were degassed by standard freeze‐pump‐thaw
2
7
procedures prior to use. RuCl (PPh ) and methyl [2,2'‐bipyridin]‐
2
3 3
2
8
6
‐carboxylate were prepared as previously described, respectively.
The H, P and C NMR spectra were recorded on a Bruker Avance
00 spectrometer. The H NMR chemical shifts were referenced to
1
31
13
1
4
residual solvent as determined relative to Me Si (δ = 0 ppm). The
4
Scheme 5 Proposed mechanism.
3
1
1
P{ H} chemical shifts were reported in ppm relative to external
1
3
1
8
5% H PO . The C{ H} chemical shifts were reported in ppm
3 4
4
| J. Name., 2012, 00, 1‐3
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relative to the carbon resonance of CDCl
analyses were performed on a Perkin‐Elmer 240C analyzer. High‐ 1H), 7.84‐7.74 (m, 2H), 7.37‐7.27 (m, 3H), 6.43 (d, J = 9 Hz, 1H), 6.13
resolution mass spectrum (HR‐MS) was recorded on a Varian 7.0T (d, J = 7 Hz, 1H), 4.09 (s, 2H). C NMR (100 Hz, DMSO, ppm): 163.0,
FTICR‐MS by ESI technique. X‐ray diffraction studies were carried 156.8, 155.0, 154.8, 149.2, 147.0, 140.9, 138.0, 137.1, 124.1, 123.4,
out in an Xcalibur E X‐ray single crystal diffractometer. Data 120.4, 118.4, 117.0, 104.6, 40.6.
3
(77.0 ppm). Elemental ppm): 8.66 (d, J = 4 Hz, 1H), 8.44 (d, J = 8 Hz, 1H), 8.32 (d, J = 8 Hz,
View Article Online
DOI: 10.1039/C6DT00034G
1
3
collections were performed using four‐circle kappa diffractometers
Synthesis of 3:
equipped with CCD detectors. Data were reduced and then
Method a: A solution of 2 (0.17 g, 0.63 mmol) and RuCl (PPh )
2
3 3
2
9
corrected for absorption. Solution, refinement and geometrical (0.60 g, 0.63 mmol) was refluxed in dried methanol (75 mL) with
calculations for all crystal structures were performed by SHELXTL‐97, stirring for 24 h. The mixture was allowed to cool to room
and PLATON SQUEEZE was used to remove the undetermined temperature and the organic phase was evaporated under vacuum.
3
0
disordered solvent.
The crude product was recrystallized with CH Cl /ether to give 3 as
2 2
Synthesis of (6‐methoxypyridin‐2‐yl)(6‐(pyridin‐2‐yl)pyridin‐2‐ a red powder (0.51 g, 59%). Anal. Calcd for C H Cl N OPRu: C,
3
4
28
2 3
1
yl)methanone:
58.54; H, 4.05; N, 6.02. Found: C, 58.33; H, 4.03; N, 6.08. H NMR
To a solution of n‐BuLi (1.44 mL, 1.60 M, 2.30 mmol) in 10 mL dry (400 MHz, CD OD, ppm): 8.54 (t, J = 8 Hz, 2H), 8.34 (d, J = 6 Hz, 1H),
3
o
tetrahydrofuran at ‐78 C was added 2‐bromo‐6‐methoxy‐pyridine 8.00 (m, 2H), 7.74 (t, J = 8 Hz, 1H), 7.41‐7.33 (m, 4H), 7.04‐7.19 (m,
(
0.44 g, 2.30 mmol) in 25 mL tetrahydrofuran. After 30 min of 13H), 7.00 (d, J = 8 Hz, 1H), 6.85 (d, J = 8 Hz, 1H), 3.94 (d, J = 14 Hz,
o
31
stirring at ‐78 C, methyl [2,2'‐bipyridin]‐6‐carboxylate (0.50 g, 2.30 1H), 3.40 (d, J = 14 Hz, 1H). P NMR (162 MHz, CD OD, ppm): 58.1.
mmol) was added. The reaction mixture was stirred at ‐20 C for 3 h
3
o
Method b：2 mL of HCl was added to a solution of 5 (0.52 g, 0.50
and left at room temperature overnight. After addition of 15 mL of mmol) in MeOH (50 mL) drop by drop at room temperature, after
water the reaction mixture was extracted with dilute HCl. The water 24 h, the red precipitate was collected, washed with copious
phase was neutralized by NaOH in water and extracted with CH Cl . amounts of ether and dried under vacuum to provide 3 as a red
2
2
The crude product was purified by column chromatography on silica powder (0.21 g, 60%).
gel (petroleum ether/ethyl acetate, v:v = 10:1) to give (6‐
methoxypyridin‐2‐yl)(6‐(pyridin‐2‐yl)pyridin‐2‐yl)methanone (0.61 g,
Reaction of 3 with NH PF /PPh :
4 6 3
A solution of 3 (0.50 g, 0.72 mmol) and PPh (0.40 g, 14.4 mmol)
3
8
2
8
7
9%). HR‐MS (ESI): Calcd for C H N O +H, 292.1086; Found, was dissolved in dried methanol (75 mL), and NH PF (1.7 g, 10.43
1
7
1
3
3
2
4
6
1
92.1083. H NMR (400 MHz, CDCl , ppm): 8.68 (d, J = 4 Hz, 1H), mmol) was added with stirring. After 20 min, the precipitate was
3
.63 (d, J = 8 Hz, 1H), 8.37 (d, J = 8 Hz, 1H), 8.07 (d, J = 8 Hz, 1H), collected, washed with copious amounts of diethyl ether and dried
.99 (t, J = 8 Hz, 1H), 7.80‐7.73 (m, 3H), 7.31 (t, J = 6 Hz, 1H), 6.97 (t, under vacuum to provide a mixture of 4 and 5 as an orange‐red
1
3
1
J = 4 Hz, 1H), 3.87 (s, 3H). C NMR (100 MHz, CDCl , ppm): 192.3, powder (0.54 g). 4: H NMR (400 MHz, CDCl , ppm): 11.93 (s, 1H),
3
3
1
1
63.2, 155.3, 155.2, 153.8, 151.4, 149.0, 138.6, 137.2, 136.9, 124.8, 8.74 (d, J = 5 Hz, 1H), 7.88‐6.69 (m, 38H), 6.07 (d, J = 8 Hz, 1H), 3.55
3
1
1
24.0, 123.1, 121.2, 118.9, 114.6, 53.4.
(s, 2H). P NMR (162 MHz, CDCl , ppm): 22.5, ‐141.0. 5: H NMR
3
Synthesis of 1:
(400 MHz, CDCl , ppm): [14.54(s), 14.38(s) (total 1H)], 8.81 (d, J = 6
3
To a solution of (6‐methoxypyridin‐2‐yl)(6‐(pyridin‐2‐yl)pyridin‐2‐ Hz, 1H), 7.60 (m, 2H), 7.43 (d, J = 8 Hz, 1H), 7.37 (d, J = 8 Hz, 1H),
yl)methanone (1.00 g, 3.40 mmol) in 20 mL ethylene glycol was 7.88‐6.69 (m, 33H), 6.72 (d, J = 7 Hz, 1H), 5.83 (d, J = 8 Hz, 1H), 3.84
3
1
added NaOH (1.50 g) and NH NH (20.0 mL, 80% in water). The (s, 2H). P NMR (162 MHz, CDCl , ppm): 26.7, ‐141.0.
mixture was heated at 100 C for 5 h. Water (20 mL) was added and
the solution was neutralized by dilute HCl (1.0 M) and extracted
2
2
3
o
Synthesis of 5:
Method a: A solution of 2 (0.17 g, 0.63 mmol) and RuCl (PPh )
2
3 3
with ethyl acetate. The organic phase was dried over Na SO and (0.60 g, 0.63 mmol) was refluxed in dried methanol (75 mL) with
2
4
evaporated under vacuum. The crude product was purified by stirring for 24 h. The mixture was allowed to cool to room
column chromatography on silica gel (petroleum ether/ethyl temperature and diluted with methanol (95% in water, 75 mL) and
acetate, v:v = 4:1) to give 1 (0.77 g, 81%). HR‐MS (ESI): Calcd for then was filtered through a pad of Celite (~4 cm tall x 5 cm diam) in
1
C H N O+H, 278.1293; Found, 278.1300. H NMR (400 MHz, CDCl , the open air. The Celite pad was rinsed with methanol until the
1
7
15
3
3
ppm): 8.67 (br s, 1H), 8.45 (d, J = 8 Hz, 1H), 8.24 (d, J = 8 Hz, 1H), eluent became colorless. Solid NH PF (1.70 g, 10.43 mmol) was
4
6
7
.80 (t, J = 8 Hz, 1H), 7.73 (t, J = 8 Hz, 1H), 7.48 (t, J = 8 Hz, 1H), 7.28 added to the combined filtrates and the solution was placed in a ‐
(
m, 2H), 6.84 (d, J = 8 Hz, 1H), 6.58 (d, J = 8 Hz, 1H), 4.32 (s, 2H), 25 °C freezer overnight, during which time a red precipitate
1
3
3
1
1
.91 (s, 3H). C NMR (100 MHz, CDCl , ppm): 163.6, 159.0, 157.1, emerged. The precipitate was collected, washed with copious
3
56.3, 155.4, 148.9, 138.7, 137.0, 136.6, 123.4, 123.4, 121.1, 118.5, amounts of diethyl ether and dried under vacuum to provide the
15.9, 107.9, 53.1, 46.9.
title compound as a red powder (0.45 g, 68%). A crystal suitable for
Synthesis of 2:
a single crystal X‐ray diffraction experiment was grown by vapor
A solution of 1 (4.30 g, 16.0 mmol) in 20 mL of HBr (40% in water) diffusion of diethyl ether into a dichloromethane solution of 5 at
was heated at reflux for 3 h. After cooling to room temperature, the room temperature. Anal. Calcd for C H F N O P Ru: C, 59.43; H,
52 44 6 3 2 3
yellow solution was neutralized by slow addition of a saturated 4.22; N, 4.00. Found: C, 59.04; H, 4.13; N, 3.84.
aqueous solution of NaOH (100 mL). The aqueous solution was Method b: In a J Young NMR tube, a solution of a mixture of 4
extracted with CH Cl (3 × 100 mL). The combined organic extracts and 5 (5 mg) in CDCl (0.5 mL) was added 2 drops of H O. The
2
2
3
2
1
were dried over Na SO , filtered, and concentrated to afford 2 as a solution was shaken for 30 min and the H NMR spectrum showed
milk white solid (3.00 g, 73%). Mp: 191 C. HR‐MS (ESI): Calcd for the mixture was almost completely transformed into 5.
C H N O+H, 264.1137; Found, 264.1140. H NMR (400 MHz, CDCl ,
2
4
o
1
Synthesis of 6:
1
6
13
3
3
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 5
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ARTICLE
Journal Name
Luca and R. H. Crabtree, Chem. Soc. Rev., 2013,V4ie2w,A1rt4ic4le0O;n(lihne)
2 3 3
A solution of 1 (0.35 g, 1.25 mmol) and RuCl (PPh ) (0.60 g, 0.63
C. Gunanathan and D. Milstein, Chem. Rev., 2014, 114, 12024;
DOI: 10.1039/C6DT00034G
mmol) was refluxed in dried methanol (200 mL) with stirring for 24
h. The mixture was allowed to cool to room temperature and the
organic phase was evaporated under vacuum. The crude product
was recrystallized with CH Cl /ether to give 6 as an orange powder
(
i) S. Kuwata and T. Ikariya, Chem. Commun., 2014, 50, 14290;
j) H. Li, B. Zheng and K.‐W. Huang, Coord. Chem. Rev., 2015,
(
2
93‐294, 116; (k) R. H. Morris, Acc. Chem. Res., 2015, 48,
2
2
1494; (l) J. R. Khusnutdinova and D. Milstein, Angew. Chem.
Int. Ed., 2015, 54, 12236.
For selected examples, see (a) J. Zhang, G. Leitus, Y. Ben‐
David and D. Milstein, Angew. Chem. Int. Ed., 2006, 45, 1113;
(
1
0.26 g, 54%). Anal. Calcd for C H Cl N O Ru: C, 56.20; H, 4.16; N,
34 30 2 6 2
1
2
1.57. Found: C, 56.25; H, 4.17; N, 11.45. H NMR (400 MHz, CD OD,
3
ppm): 8.54 (d, J = 8 Hz, 2H), 8.31‐8.23 (m, 4H), 8.07 (d, J = 8 Hz, 2H),
.85‐7.76 (m, 4H), 7.46 (d, J = 7 Hz, 2H), 7.08 (t, J = 6 Hz, 2H), 6.71 (d,
J = 7 Hz, 2H), 6.58 (d, J = 8 Hz, 2H), 4.81 (d, J = 14 Hz, 2H), 4.33(d, J =
4 Hz, 2H), 2.92 (s, 6H).
Synthesis of 7:
(
b) E. Balaraman, B. Gnanaprakasam, L. J. W. Shimon and D.
7
Milstein, J. Am. Chem. Soc., 2010, 132, 16756; (c) E.
Balaraman, C. Gunanathan, J. Zhang, L. J. W. Shimon and D.
Milstein, Nat. Chem., 2011,
3
, 609; (d) J. R. Khusnutdinova, J.
, 2416.
1
A. Garg and D. Milstein, ACS Catal., 2015,
5
3
4
5
6
A recent review: C. M. Moore, E. W. Dahl and N. K. Szymczak,
A solution of 6 (0.8 g, 1.1 mmol) was dissolved in dried methanol
Curr. Opin. Chem. Biol. 2015, 25, 9.
(
2
50 mL), and NH PF (1.7g, 11 mmol) was added with stirring. After
4 6
J. B. Geri and N. K. Szymczak, J. Am. Chem. Soc. 2015, 137,
0 min, the precipitate was collected, washed with copious
12808.
A. M. Royer, T. B. Rauchfuss and D. L. Gray, Organometallics,
2010, 29, 6763.
amounts of diethyl ether and dried under vacuum to provide 7 as
‐
an orange‐red powder (1.0 g, 77%). HR‐MS (ESI): Calcd for [M‐PF ],
6
(a) K.‐i. Fujita, N. Tanino and R. Yamaguchi, Org. Lett., 2007,
8
4
(
2
7
8
01.1116; Found, 801.1121. Anal. Calcd for C H F N O P Ru: C,
34 30 12 6 2 2
9, 109; (b) K.‐i. Fujita, T. Yoshida, Y. Imori and R. Yamaguchi,
1
3.18; H, 3.20; N, 8.89. Found: C, 43.39; H, 3.21; N, 8.73. H NMR
Org. Lett., 2011, 13, 2278; (c) R. Kawahara, K.‐i. Fujita and R.
Yamaguchi, J. Am. Chem. Soc., 2012, 134, 3643; (d) R.
Kawahara, K.‐I. Fujita and R. Yamaguchi, Angew. Chem. Int.
Ed. 2012, 51, 12790; (e) K.‐I. Fujita, T. Uejima and R.
Yamaguchi, Chem. Lett. 2013, 42, 1496; (f) K.‐i. Fujita, W. Ito
400 MHz, CD COCD , ppm): 8.71 (d, J = 8 Hz, 2H), 8.45 (d, J = 8 Hz,
3 3
H), 8.36 (t, J = 8 Hz, 2H), 8.20 (d, J = 8 Hz, 2H), 7.91‐7.86 (m, 4H),
.55 (d, J = 8 Hz, 2H), 7.22 (m, 2H), 7.02 (d, J = 6 Hz, 2H), 6.72 (d, J =
Hz, 2H), 5.00 (d, J = 14 Hz, 2H), 4.52(d, J = 14 Hz, 2H), 3.04 (s, 6H).
General procedure for the catalytic transfer hydrogenation of
ketones: The catalyst solution was prepared by dissolving complex
and R. Yamaguchi, ChemCatChem 2014, 6, 109; (g) K.‐i. Fujita,
Y. Tanaka, M. Kobayashi and R. Yamaguchi, J. Am. Chem. Soc.
2014, 136, 4829.
7
(a) J. F. Hull, Y. Himeda, W.‐H. Wang, B. Hashiguchi, R.
Periana, D. J. Szalda, J. T. Muckerman and E. Fujita, Nat.
3
(69.7 mg, 0.10 mmol) in 2‐propanol (10.0 mL) and internal
standard solution was prepared by dissolving dodecane (340 mg,
.0 mmol) in 2‐propanol (10.0 mL). Under an N atmosphere, the
Chem., 2014,
Muckerman, E. Fujita and Y. Himeda, Energy Environ. Sci.,
012, , 7923; (c) W.‐H. Wang, J. T. Muckerman, E. Fujita and
Y. Himeda, ACS Catal. 2013, , 856; (d) N. Onishi, S. Xu, Y.
4, 383; (b) W.‐H. Wang, J. F. Hull, J. T.
2
2
mixture of a ketone (2.0 mmol), 1.0 mL of the catalyst solution (0.01
mmol), 1.0 mL of the internal standard solution (0.2 mmol), and 2‐
2
5
3
Manaka, Y. Suna, W.‐H. Wang, J. T. Muckerman, E. Fujita and
Y. Himeda, Inorg. Chem. 2015, 54, 5114.
I. Nieto, M. S. Livings, J. B. Sacci, L. E. Reuther, M. Zeller and
E. T. Papish, Organometallics, 2011, 30, 6339.
For selected reviews, see: (a) M. J. Corr and J. A. Murphy,
Chem. Soc. Rev., 2011, 40, 2279; (b) S. Dey, P. K. Das and A.
Dey, Coord. Chem. Rev., 2013, 257, 42; (c) T. R. Simmons, G.
Berggren, M. Bacchi, M. Fontecave and V. Artero, Coord.
Chem. Rev. 2014, 270‐271, 127; (d) W. Lubitz, H. Ogata, O.
Rüdiger and E. Reijerse, Chem. Rev. 2014, 114, 4081; (e) T. Xu,
D. Chen and X. Hu, Coord. Chem. Rev. 2015, 303, 32.
propanol (10.0 mL) was stirred at 82 °C for 10 min. Then 0.2 mL of a
i
0
.5 M PrOK (0.10 mmol) solution in 2‐propanol was introduced to
8
9
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 precooled to 0 °C for GC analysis. After the reaction was
completed, the reaction mixture was condensed under reduced
pressure and subjected to purification by flash silica gel column
chromatography to afford the corresponding alcohol product,
which was identified by comparison with the authentic sample
through NMR and GC analysis.
10 (a) S. Shima, O. Pilak, S. Vogt, M. Schick, M. S. Stagni, W.
Meyer‐Klaucke, E. Warkentin, R. K. Thauer and U. Ermler,
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Acknowledgements
1
1 T. Hiromoto, E. Warkentin, J. Moll, U. Ermler and S. Shima,
This work was supported by the National Natural Science
Foundation of China (Nos. 21302028 and 21201049) and
Harbin Science and Technology Bureau (2013RFLXJ002).
Angew. Chem. Int. Ed., 2009, 48, 6457.
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2 (a) X. Yang and M. B. Hall, J. Am. Chem. Soc., 2009, 131
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Notes and references
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1
For selected reviews, see: (a) T. Ikariya, K. Murata and R. 14 K. F. Kalz, A. Brinkmeier, S. Dechert, R. A. Mata and F. Meyer,
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,
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Angew. Chem. Int. Ed., 2013, 52, 4744; (e) C. Gunanathan
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6
| J. Name., 2012, 00, 1‐3
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Journal Name
ARTICLE
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DOI: 10.1039/C6DT00034G
The synthesis, reactivity and catalytic transfer hydrogenation activity of three metal-ligand cooperative
ruthenium (II) complexes (3-5) bearing an unsymmetrical pincer NNN ligand with a 2-hydroxypyridylmethylene
fragment were reported.