New Type of 2,6-Bis(imidazo[1,2-a]pyridin-2-yl)pyridine-Based Ruthenium Complexes: Active Catalysts for Transfer Hydrogenation of Ketones

Article abstract of DOI:10.1021/om501119x

Neutral and cationic ruthenium(II) complexes bearing a symmetrical 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine were synthesized and structurally characterized by NMR analysis and X-ray crystallographic determinations. These complexes have exhibited good catalytic activity in the transfer hydrogenation of ketones. In refluxing isopropyl alcohol, the conversion of the substrates reached up to 99%, and a TOF value of 356400 h^-1 with 0.1 mol % catalyst was achieved. (Figure Presented).

Full text of DOI:10.1021/om501119x

Article
pubs.acs.org/Organometallics
New Type of 2,6-Bis(imidazo[1,2‑a]pyridin-2-yl)pyridine-Based
Ruthenium Complexes: Active Catalysts for Transfer Hydrogenation
of Ketones
Ke Li, Jun-Long Niu, Ming-Ze Yang, Zhen Li, Li-Yuan Wu, Xin-Qi Hao,* and Mao-Ping Song*
College of Chemistry and Molecular Engineering, Henan Key Laboratory of Chemical Biology and Organic Chemistry, Zhengzhou
University, No. 100 of Science Road, Zhengzhou 450001, People’s Republic of China
S
* Supporting Information
ABSTRACT: Neutral and cationic ruthenium(II) complexes
bearing a symmetrical 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine
were synthesized and structurally characterized by NMR analysis
and X-ray crystallographic determinations. These complexes have
exhibited good catalytic activity in the transfer hydrogenation of
ketones. In refluxing isopropyl alcohol, the conversion of the
substrates reached up to 99%, and a TOF value of 356 400 h−1
with 0.1 mol % catalyst was achieved.
coordinating functionality in a polydentate ligand for
homogeneous catalysis.
INTRODUCTION
■
Transfer hydrogenation (TH) of carbonyl compounds
catalyzed by transition metal complexes has been well explored
and applied as the most useful catalytic method of hydro-
genation of ketones for the production of alcohols due to the
easy manipulations, high reactivities, and wide substrate scope.1
Ruthenium(II) complexes have represented the most exten-
sively investigated catalysts.2 In this area, Ru(II) complexes
containing N-tosylethylenediamine or β-amino alcohol ligands
developed by Noyori and co-workers have exhibited efficient
catalytic activity in the transfer hydrogenation of ketones and
imines.3 Baratta et al. have documented that cyclometalated
Ru(II) complexes containing 2-(aminomethyl)pyridine ligands
can be used as powerful catalysts for the transfer hydrogenation
of ketones.4 Yu’s group has also reported versatile pyridyl-based
pseudo-N3 ligands and their active Ru(II) complexes for the
transfer hydrogenation of ketones.5 Although various types of
ligands and their transition metal complexes have been
developed for transfer hydrogenation, highly active catalytic
systems are still strongly desired to extend the substrate scope
and enhance the reaction efficiency. Recently non-phosphorus
ligands have attracted more and more attention for their
versatile application, and the most common phosphine-free
ligands are those with tridentate pyridyl-based N3 or pseudo-N3
ligands where the N donors are pyridines, amines, oxazoline,
imidazolines, benzimidazole, pyrazole, or other N-containing
heterocycles, which have been well documented and success-
fully employed in organic synthesis, homogeneous catalysis, and
fabrication of functional materials.6 Substituted imidazo[1,2-
a]pyridine compounds attracted great attention recently owing
to their applications for pharmaceutical7 and organic light-
emitting diodes (OLEDs).8 To the best of our knowledge,
imidazo[1,2-a]pyridine itself has seldom been used as a
We have recently worked on symmetrical 1,3-bis(2′-
imidazolinyl)benzene (Phebim) and 1,3-bis(benzimidazol-2′-
yl)benzene (Bzimb) and unsymmetrical 2-aryl-6-(oxazolinyl)-
pyridine, N-substituted-2-aminomethyl-6-phenylpyridine, and
oxazolinyl-pyrazolyl- or oxazolinyl-diethylamino benzene li-
gands to construct metal complex catalysts.9 The Pt-, Pd-, Ni-,
Rh-, Ir-, and Ru-ligand complexes were prepared successfully,
and some of them exhibited high efficiency and selectivity in
many catalytic reactions such as Friedel−Crafts alkylation,9i,l
alkynylation of trifluoropyruvates with terminal alkynes,9f
allylation of aldehydes,9j carbonyl-ene reaction of trifluoropyr-
uvates,9c,h enantioselective hydrophosphination of enones with
diphenylphosphine,9g and asymmetric Michael addition.9k The
Pt-Phebim9m and Pt-Bzimb9d,e complexes were found to be
luminescent in solution at room temperature. As a continuation
of our work, herein we report in detail our studies on the
synthesis of the neutral and cationic ruthenium(II) complexes
bearing a symmetrical 2,6-bis(imidazo[1,2-a]pyridin-2-yl)-
pyridine. Investigations on their application to the transfer
hydrogenation of ketones are also presented.
RESULTS AND DISCUSSION
■
Synthesis of Ligand and the Ru(II) Complexes.
Reaction of pyridine-2,6-dicarboxylic acid with ethanol in
toluene, containing a few drops of H2SO4 as catalyst, afforded
pyridine-2,6-dicarboxylate (1) in 88% yield. The 2,6-diacetyl-
pyridine (2) was obtained from the reaction of 1 with ethyl
acetate and metallic sodium followed by treatment with 20%
aqueous HCl. Bromination of 2 with N-bromosuccinimide
Received: November 6, 2014
© XXXX American Chemical Society
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Scheme 1. Synthesis of Ligand 3
Scheme 2. Synthesis of Ru(II) Complexes 4 and 5
(NBS) as brominating agent in the presence of p-
toluenesulfonic acid monohydrate afforded the intermediate
α,α′-dibromo-2,6-diacetylpyridine, which was refluxed with 2-
aminopyridine and sodium hydrogen carbonate in acetonitrile
for 7 h to produce the desired ligand 3 (Scheme 1).
Neutral and cationic ruthenium(II) complexes 4 and 5 were
obtained in 86% and 64% yields, respectively, by reacting an
equimolar amount of ligand 3 with RuCl2(PPh3)3 in refluxing
toluene or isopropyl alcohol and NEt3 (Scheme 2). Both
complexes were stable in air.
Characterization of Ru(II) Complexes 4 and 5. The
NMR analyses of the complexes are in agreement with their
compositions. The 1H NMR signals of the imidazo[1,2-
a]pyridin-CH in ligand 3 appear at 8.64, 8.61, 7.64, 7.31, and
6.96 ppm, and those of the Ru(II) complex 4 are shifted
downfield to 8.87, 8.66, 8.14, 7.62, and 7.14 ppm, revealing that
the imidazo[1,2-a]pyridine is coordinated to the metal center.
The 31P NMR signal of complex 4 in DMSO-d6 appeared at
34.0 ppm, suggesting no obvious alteration of the environment
around the PPh3 ligand, and that of complex 5 in CD2Cl2
appeared at 24.5 ppm, revealing the presence of two identical
PPh3 ligands. The single-crystal structures of complexes 4 and 5
were further confirmed by X-ray crystallographic determination
(Figures 1 and 2).
In the solid state, the six-coordinated Ru(II) center is in a
distorted pseudo-octahedral coordination environment in each
case. In the molecular structure of 4 (Figure 1), the Ru−N
distances are significantly shorter for the central N(3)
(1.996(4) Å) than for the terminal N(1) and N(4) atoms
(2.101(4) and 2.106(4) Å, respectively). The bulky PPh3 ligand
has its plane oriented normal to the central chelate ligand plane.
Consequently, the PPh3 ligand adopts a position cis to the
nitrogen atoms from the N3-pincer ligand and leads to a cis
orientation of the chlorido ligand Cl(1). The P(1)−Ru−Cl(2)
Figure 1. Molecular structure of complex 4 with hydrogen atoms
omitted for clarity. Selected bond lengths (Å) and angles (deg):
Ru(1)−N(1), 2.101(4); Ru(1)−N(3), 1.996(4); Ru(1)−N(4),
2.106(4); Ru(1)−Cl(1), 2.4675(12); Ru(1)−Cl(2), 2.5135(11);
Ru(1)−P(1), 2.2805(11); N(3)−Ru(1)−N(1), 79.38(15); P(1)−
Ru(1)−Cl(1), 94.13(4); P(1)−Ru(1)−Cl(2), 176.68(5); Cl(1)−
Ru(1)−Cl(2), 88.64(4); N(3)−Ru(1)−P(1), 91.68(10); N(3)−
Ru(1)−Cl(1), 174.00(10).
angle is 176.68(5)°, revealing that Cl(2) is positioned trans to
the P atom. The PPh3 ligand exerts a notable trans effect, which
results in a relatively longer Ru−Cl(2) bond distance of
2.5135(11) Å versus that of Ru−Cl(1) (2.4675(12) Å).
Complex 5 features a molecular structure similar to that of 4
(Figure 2). The remaining chloride ligand resides at a trans
coordination site relative to the central pyridine nitrogen atom,
forming a parallelogram skeleton with good planarity. The two
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Table 1. Optimization of Reaction Conditions for the
Transfer Hydrogenation of Acetophenone Catalyzed by
a
Ru(II) Complexes 4 and 5
b
entry
cat.
base
time (min)
yield (%)
1
2
3
4
5
6
7
8
4
5
4
4
4
4
4
4
4
4
iPrOK
iPrOK
iPrOK
tBuOK
tBuONa
KOH
10
10
1
94
70
79
68
76
96
96
89
96
48
NR
NR
1
1
1
NaOH
NaOH
NaOH
NaOH
NaOH
1
1/6
1/6
10
10
10
c
9
d
10
11
12
4
a
Reaction conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH);
b
ketone/base/cat. = 2000/20/1; Ar (0.1 MPa); 82 °C. GC yield.
c
d
Ketone/base/cat. = 1000/10/1. Reaction was carried out under an
air atmosphere.
Figure 2. Molecular structure of complex 5 with hydrogen atoms and a
chloride anion omitted for clarity. Selected bond lengths (Å) and
angles (deg): Ru(1)−N(3), 1.993(4); Ru(1)−N(2), 2.095(3);
Ru(1)−Cl(1), 2.4334(10); Ru(1)−P(1), 2.4152(8); N(3)−Ru(1)−
N(2), 78.98(9); N(3)−Ru(1)−P(1), 92.90(2); N(3)−Ru(1)−Cl(1),
180; N(2)−Ru(1)−Cl(1), 101.02(9); P(1)−Ru(1)−Cl(1), 87.10(2);
N(2)−Ru(1)−N(2A), 157.95(17); P(1)−Ru(1)−P(1A), 174.21(4).
On the basis of the above results, the molar ratio of 1000/
10/1 for ketone/base/catalyst, 0.1 mol % of complex 4 in
refluxing isopropyl alcohol, and NaOH as the base under an
argon atmosphere were considered as the optimized reaction
conditions. The reaction was extended to other ketones in the
transfer hydrogenation under the optimized conditions to
evaluate the scope of the system (Table 2). Thus, the reduction
of a variety of acetophenones, aryl alkyl ketones, and aliphatic
cyclic ketones was efficiently performed. In most of the cases
shown in Table 2, the transfer hydrogenation reactions
obtained 93−99% yields within 10−120 s, reaching final
TOFs of up to 356 400 h−1 (Table 2, entries 1−3, 5, 6, 11−16,
and 23). The reduction yields of 4′-tert-butylacetophenone, 3′-
methoxyacetophenone, and 4′-methoxyacetophenone furnished
90−99% yields over a period of 30 min (Table 2, entries 7, 18,
and 19). By using 0.5 mol % of catalyst 4, the reaction of 2′-
methoxyacetophenone, 3′-methoxyacetophenone, and 4′-me-
thoxyacetophenone gave the product in 99% yield within 1 min
(Table 2, entries 17−19). The transfer hydrogenations of
fluoro-substituted acetophenones were also accomplished, but
only achieved moderate yields within 10−120 s (85−90%
yields, Table 2, entries 8−10). Benzophenone, 1-acetonaph-
thone, and 2-acetonaphthone reacted smoothly to give the
desired product in 97−99% yields within 10 min (Table 2,
entries 20−22). Aliphatic ketones showed moderate reactivity
(Table 2, entries 24 and 25), but cyclohexanone exhibited high
reactivity (Table 2, entry 23).
Transfer Hydrogenation Mechanism. The transfer
hydrogenation mechanism is unclear at present, although
Ru(II)-catalyzed transfer hydrogenation reactions have been
investigated with a Ru(II)-H complex as the catalytically active
species.1h,4g,10 Ligand dissociation is a common initiation step
for most Ru-based inner-sphere hydrogenation catalysts.10b,11
Thus, we examined whether exogenous PPh3 hindered catalytic
transfer hydrogenation mediated by 4. When an excess of PPh3
was added to the reaction mixture, the rate of transfer
hydrogenation catalyzed by the 4/NaOH system was
significantly affected (Figure 3). This result suggests that the
bulky PPh3 ligands are in a trans arrangement at the axial
positions, implying that the ruthenium metal center can be
effectively protected by the phenyl rings. The Ru−P bond
lengths in 5 (2.4152(8) and 2.4153(8) Å) are longer than that
in 4 (2.2805(11) Å), suggesting that the two PPh3 ligands in 5
have great trans influences on each other. The results suggest
that the ruthenium metal center in 4 is situated in a more loose
environment as compared with that in the Ru(II) complex 5,
bearing two PPh3 ligands. Such a structural element usually
enhances the catalytic activity of a metal complex catalyst.
Transfer Hydrogenation of Ketones by Ruthenium
Complexes. Complexes 4 and 5 were tested as the catalysts
for the transfer hydrogenation of acetophenone in refluxing
isopropyl alcohol (Table 1). The reaction was performed with a
molar ratio of 2000/20/1 for ketone/base/catalyst using 0.05
mol % of ruthenium complex as the catalyst in the presence of
iPrOK as base under an argon atmosphere. It can be seen from
the initial results in Table 1 (entries 1 and 2) that the
ruthenium complex 4 gave the best results, with a 94% yield
within 10 min. For comparison, iPrOK, tBuOK, tBuONa,
KOH, and NaOH were tested as the bases. Over a period of 1
min, the corresponding alcohol product from acetophenone
reached 79%, 68%, 76%, 96%, and 96% yields by GC analysis in
the reactions using iPrOK, tBuOK, tBuONa, KOH, and NaOH
as the base, respectively (Table 1, entries 3−7). NaOH was
selected as the reaction promoter for its fast dissolution and
high efficiency, although KOH also worked well as a base in the
reactions. On increasing the catalyst loading, the reaction was
accelerated. For example, with 0.1 mol % 4, acetophenone
reached a complete conversion to form the alcohol product
within 10 s (Table 1, entry 9).
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a
Table 2. Transfer Hydrogenation of Ketones Catalyzed by Complex 4
a
b
c
Reaction conditions: ketone, 2.0 mmol (0.1 M in 20 mL of iPrOH); ketone/base/cat. = 1000/10/1; Ar (0.1 MPa); 82 °C. Isolated yield. 0.5 mol
d
% 4 was used. GC yield.
present transfer hydrogenation reactions may follow an inner-
sphere mechanism.5i−k,10a
active catalysts for the transfer hydrogenation of ketones.10a,14
Coordination to the metal center and insertion to the Ru−H
bond in B by the carbonyl of a ketone substrate yields Ru(II)-
alkoxide D. Lastly, base-mediated alcohol metathesis with D
regenerates species A to furnish the desired product and
complete the catalytic cycle.
According to the related reported literature,5a−c,10a,12 we
were encouraged to propose the plausible mechanism in
Scheme 3, even though the Ru−H active specics of complex 4
was not isolated successfully. The transfer hydrogenation of a
ketone is initiated from complex 4. It interacts with the base
and isopropyl alcohol to form a Ru(II)-alkoxide A, which
undergoes β-H elimination to form a Ru−H species B and
releases of acetone. Intermediate B is presumably considered as
the catalytically active species, although it was not successfully
isolated by reacting complex 4 with NaOH or iPrOK in
refluxing isopropyl alcohol. The formation of Ru−H complexes
from Ru−Cl precursors has been documented,13 and such in
situ formed Ru−H species have been well-known to act as the
CONCLUSION
■
In summary, ruthenium(II) complexes bearing a symmetrical
2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine have been success-
fully synthesized and exhibited exceptionally high catalytic
activity in the transfer hydrogenation of ketones. In refluxing
isopropyl alcohol, a 99% conversion of the substrates and a
TOF value of 356 400 h−1 with 0.1 mol % catalyst loading can
be achieved.
D
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MS/MS System ESI spectrometer. Flash column chromatography was
performed on silica gel (200−300 mesh).
Synthesis of 2,6-Diacetylpyridine (2). To a solution of pyridine-
2,6-dicarboxylic acid (1.67 g, 0.01 mmol) in ethanol (14 mL) and
toluene (8 mL) was added three drops of concentrated sulfuric acid.
The mixture was heated to reflux and water removed by a Dean−Stark
trap for 12 h. The reaction mixture was cooled to room temperature,
and the solvent was evaporated under reduced pressure. The residue
was dissolved in water (20 mL) and neutralized by solid Na2CO3. The
aqueous solution was extracted with ether three times, and the organic
layer was washed with brine, dried over MgSO4, and evaporated to
dryness to give diethylpyridine-2,6-dicarboxylate (1) (1.98 g, 89%) as a
colorless solid. Mp: 41−42 °C (literature: 42−43 °C).15
Sodium (2.30 g, 0.1 mol) was added portionwise to a stirred
solution of 1 (4.46 g, 0.02 mol) in toluene (80 mL) and ethyl acetate
(50 mL) under argon, and the reaction mixture was refluxed at 120 °C
(oil bath) for 9 h. The mixture was cooled to room temperature and
stirred rapidly and continuously during the slow addition of the 20%
aqueous HCl (25 mL). The reaction mixture was then refluxed for 5 h.
After cooling to room temperature, it was neutralized by solid
Na2CO3. The aqueous solution was extracted with ether three times,
and the organic layer was washed with brine, dried over MgSO4, and
evaporated. The crude product was purified by column chromatog-
raphy on silica gel with CH2Cl2/petroleum ether (2/1) as eluent,
providing 2,6-diacetylpyridine (2) (2.10 g, 64%) as a white solid.. Mp:
81−83 °C (literature: 79−80 °C).16 1H NMR (400 MHz, CDCl3): δ
(ppm) 8.22 (d, J = 8.0 Hz, 2H), 8.00 (t, J = 8.0 Hz, 1H), 2.80 (s, 6H).
Synthesis of 2,6-Bis(imidazo[1,2-a]pyridin-2-yl)pyridine (3).
To a solution of 2,6-diacetylpyridine (163 mg, 1.0 mmol) and p-
toluenesulfonic acid monohydrate (571 mg, 3.0 mmol) in acetonitrile
(5 mL) was added NBS (445 g, 2.5 mmol). The reaction was refluxed
for 16 h. After the reaction mixture was cooled to room temperature,
the solvent was evaporated to afford a crude product, which was
filtered by a short silica gel chromatography column with CH2Cl2 as
eluent to give α,α′-dibromo-2,6-diacetylpyridine (white solid). Then
the obtained α,α′-dibromo-2,6-diacetylpyridine reacted with 2-amino-
pyridine (235 mg, 2.5 mmol) and sodium hydrogen carbonate (252
mg, 3.0 mmol) in acetonitrile (5 mL) at reflux for 7 h. After cooling
and filtration, the solvent was evaporated in vacuo, and purification by
column chromatography on silica gel with ethyl acetate as eluent gave
2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (3) (75 mg, 24%) as a
yellow solid. Mp: 220−223 °C. IR (KBr pellet): ν 3167, 3069, 3032,
1631, 1604, 1571, 1497, 1402, 1365, 1327, 1254, 1206, 1081, 931, 830,
Figure 3. Product distribution during the transfer hydrogenation of
acetophenone using 4 with PPh3. Conditions: ketone, 2.0 mmol (0.1
M in 20 mL of iPrOH); ketone/base/cat. = 1000/10/1; Ar (0.1 MPa);
82 °C.
Scheme 3. Proposed Mechanism
1
739 cm−1. H NMR (400 MHz, DMSO-d6): δ 8.64 (d, J = 6.8 Hz,
2H), 8.61 (s, 2H), 8.04 (d, J = 7.2 Hz, 2H), 7.97 (t, J = 7.6 Hz, 1H),
7.64 (d, J = 9.2 Hz, 2H), 7.31 (t, J = 7.8 Hz, 2H), 6.96 (t, J = 6.6 Hz,
2H). 13C NMR (100 MHz, DMSO-d6): δ 152.4, 144.8, 144.5, 137.8,
127.3, 125.4, 118.8, 116.9, 112.7, 111.5. HRMS (positive ESI, m/z):
[M + H]+ calcd for C19H14N5 312.1249, found 312.1244.
Synthesis of Neutral Ruthenium Complex 4. A mixture of
RuCl2(PPh3)3 (96 mg, 0.1 mmol) and compound 3 (31 mg, 0.1
mmol) in toluene (10 mL) was refluxed for 24 h under an argon
atmosphere. The mixture was cooled to ambient temperature to
precipitate a red-brown microcrystalline solid. The solid was filtered
off, washed with diethyl ether (3 × 10 mL), and dried under reduced
pressure to afford complex 4 (64 mg, 86%) as a red-brown crystalline
solid. Mp: >300 °C. IR (KBr pellet): ν 3125, 1637, 1506, 1475, 1401,
1
1295, 1092, 751, 697 cm−1. H NMR (400 MHz, DMSO-d6): δ 8.87
EXPERIMENTAL SECTION
■
(s, 2H), 8.66 (d, J = 6.8 Hz, 2H), 8.14 (d, J = 9.2 Hz, 2H), 7.86−7.76
(m, 3H), 7.62 (t, J = 8.0 Hz, 2H), 7.20−7.08 (m, 11H), 7.01 (t, J = 6.6
Hz, 6H). 13C NMR (100 MHz, DMSO-d6): δ 153.7, 146.6, 145.5,
134.9, 132.7, 132.6, 131.2, 130.8, 129.4, 129.3, 128.7, 127.8, 127.7,
119.5, 116.2, 114.8, 114.3. 31P{1H} NMR (121 MHz, DMSO-d6): δ
34.0 (s, PPh3). HRMS (positive ESI, m/z): [M − Cl]+ calcd for
C37H28ClN5PRu 710.0814, found 710.0818.
Synthesis of Cationic Ruthenium Complex 5. Under an argon
atmosphere, a mixture of RuCl2(PPh3)3 (96 mg, 0.1 mmol),
compound 3 (31 mg, 0.1 mmol), 2-propanol (10 mL), and NEt3
(70 μL, 0.5 mmol) was refluxed with stirring for 10 h. After cooling to
ambient temperature, the solvent was evaporated in vacuo, and
General Considerations. Solvents were dried with standard
methods and freshly distilled prior to use if needed. Unless otherwise
noted, all the starting materials were commercially available and used
without further purification. Reactions for the preparation of Ru(II)
complexes as well as all the catalytic reactions were carried out under
an argon atmosphere. Melting points were measured on a melting
point apparatus and are uncorrected. Infrared spectra were obtained by
a spectrophotometer with KBr pellets. H and 13C NMR spectra were
1
all recorded using TMS as an internal standard. Data are reported as
follows: chemical shift (δ ppm), multiplicity (s = single, d = doublet, t
= triplet, q = quartet, m = multiplet), integration, and coupling
constants in hertz (Hz). HRMS were determined on a Q-Tof Micro
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purification by column chromatography on Al2O3 with CH2Cl2/
MeOH (10/1) as eluent gave complex 5 (64 mg, 64%) as red-brown
crystals. Mp: disintegrated at 230 °C. IR (KBr pellet): ν 3334, 1636,
2010, 110, 1663−1705. (e) Morris, R. H. Chem. Soc. Rev. 2009, 38,
2282−2291. (f) Wang, C.; Wu, X.; Xiao, J. Chem. Asian. J. 2008, 3,
1750−1770. (g) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40,
1
1612, 1574, 1504, 1477, 1432, 1400, 1296, 1089, 747, 697 cm−1. H
1300−1308. (h) Samec, J. S. M.; Backvall, J.-E.; Andersson, P. G.;
̈
NMR (400 MHz, CD2Cl2): δ 8.57 (s, 2H), 8.39 (d, J = 6.8 Hz, 2H),
8.18 (d, J = 9.2 Hz, 2H), 7.35 (s, 3H), 7.21 (dd, J = 6.8, 8.4 Hz, 2H),
7.14−7.09 (m, 18H), 6.93 (dd, J = 7.2, 8.4 Hz, 12H), 6.86 (dt, J = 0.8,
6.8 Hz, 2H). 13C NMR (100 MHz, CD2Cl2): δ 154.6, 147.8, 146.5,
133.83, 133.78, 133.73, 132.8, 132.7, 132.5, 132.4, 129.6, 128.6, 128.2,
128.14, 128.09, 126.9, 120.0, 119.1, 114.5, 114.0. 31P{1H} NMR (121
MHz, CD2Cl2): δ 24.5 (s, PPh3). HRMS (positive ESI, m/z): [M − Cl
− PPh3]+ calcd for C37H28ClN5PRu 710.0814, found 710.0816.
General Procedure for the Catalytic Transfer Hygrogenation
of Ketones. The catalyst solution was prepared by dissolving complex
4 (14.9 mg, 0.02 mmol) in 2-propanol (10.0 mL). Under an argon
atmosphere, the mixture of a ketone (2.0 mmol), 1.0 mL of the
catalyst solution (0.002 mmol), and 2-propanol (18.0 mL) was stirred
at 82 °C for 10 min. Then 1.0 mL of a 0.02 M NaOH (0.02 mmol)
solution in 2-propanol was introduced to initiate the reaction. The
reaction was monitored by GC analysis. After the reaction was
complete, the reaction mixture was evaporated and the residue was
purified by TLC on silica gel plates to afford the alcohol product. The
purified products were identified by 1H NMR spectra, and their
analytical data are given in the Supporting Information.
X-ray Diffraction Studies. Crystals of 4 and 5 (CCDC file
numbers 1027736 and 1027735) were obtained by recrystallization
from CH2Cl2/MeOH at ambient temperature. The data were collected
on an Oxford Diffraction Gemini E diffractometer with graphite-
monochromated Cu Kα radiation (λ = 1.541 84 Å for complex 4) and
Mo Kα radiation (λ = 0.7107 Å for complex 5) at ambient
temperature. The structures were solved by direct methods using
the SHELXS-97 program, and all non-hydrogen atoms were refined
anisotropically on F2 by the full-matrix least-squares technique, which
used the SHELXL-97 crystallographic software package.17 The
hydrogen atoms were included but not refined. Details of the crystal
structure determination are summarized in Table S1 in the Supporting
Information.
Brandt, P. Chem. Soc. Rev. 2006, 35, 235−248.
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ASSOCIATED CONTENT
* Supporting Information
■
S
A table giving crystallographic details for the Ru(II) complexes
4 and 5, figures giving NMR spectra of the new compounds 3−
5 and NMR spectra of the catalysis products, text giving
characterization data of the known catalysis products, and CIF
files giving crystallographic data for complexes 4 and 5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xqhao@zzu.edu.cn (X.-Q. Hao).
*Tel/fax: (+86)-371-6776-3869. E-mail: mpsong@zzu.edu.cn
(M.-P. Song).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21102135 and 21272217) for financial support of this
work.
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