Ruthenium(II) complex catalysts bearing a pyridyl-based benzimidazolyl-benzotriazolyl ligand for transfer hydrogenation of ketones

Article abstract of DOI:10.1021/om400298c

Air- and moisture-stable ruthenium(II) complexes bearing a unsymmetrical 2-(benzimidazol-2-yl)-6-(benzotriazol-1-yl)pyridine ligand were synthesized and structurally characterized by NMR analysis and X-ray crystallographic determinations. These complexes have exhibited excellent catalytic activity in the transfer hydrogenation of ketones in refluxing 2-propanol, reaching final TOFs up to 176400 h^-1. The corresponding RuH complex was isolated and is proposed as the catalytically active species by controlled experiments. The high catalytic activity of the Ru(II) the complex catalysts is attributed to the hemilabile unsymmetrical coordinating environment around the central metal atom in the complexes and presence of a convertible benzimidazolyl NH functionality in the ligand.

Full text of DOI:10.1021/om400298c

Article
pubs.acs.org/Organometallics
Ruthenium(II) Complex Catalysts Bearing a Pyridyl-Based
Benzimidazolyl−Benzotriazolyl Ligand for Transfer Hydrogenation of
Ketones
Wangming Du,† Ping Wu,† Qingfu Wang,† and Zhengkun Yu*,†,‡
†Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, People’s
Republic of China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354
Fenglin Road, Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: Air- and moisture-stable ruthenium(II) complexes bearing a
unsymmetrical 2-(benzimidazol-2-yl)-6-(benzotriazol-1-yl)pyridine ligand
were synthesized and structurally characterized by NMR analysis and X-
ray crystallographic determinations. These complexes have exhibited
excellent catalytic activity in the transfer hydrogenation of ketones in
refluxing 2-propanol, reaching final TOFs up to 176400 h−1. The
corresponding RuH complex was isolated and is proposed as the catalytically
active species by controlled experiments. The high catalytic activity of the
Ru(II) the complex catalysts is attributed to the hemilabile unsymmetrical
coordinating environment around the central metal atom in the complexes
and presence of a convertible benzimidazolyl NH functionality in the ligand.
metrical coordinating arms usually exhibit much higher catalytic
activity due to the hemilabile electronic and steric properties of
the ligand. Unfortunately, such highly active complex catalysts
have less been investigated.12 Benzotriazole has been widely
used as a synthetic auxiliary in organic synthesis, and some of
its derivatives can be utilized as important UV photostabilizers.
Due to the three adjacent nitrogen donors, benzotriazole can
also act as a versatile ligand in coordination chemistry for the
preparation of supramolecular complexes and functional
materials.13 However, benzotriazole itself has seldom been
used as a coordinating functionality in a polydentate ligand for
homogeneous catalysis.
During our ongoing exploration of highly active transition-
metal complex catalysts for homogeneous catalysis,14 we have
disclosed that both benzimidazolyl and pyrazolyl NH
functionalities in a Ru(II) complex catalyst can show a
remarkable acceleration effect on the TH or ATH reactions
of ketones.15 Such an acceleration effect is attributed to easy
deprotonation of the unprotected NH functionalities of the
ligands in the complex catalysts under basic conditions by
releasing one HCl molecule and simultaneously forming
catalytically active species for TH or ATH reactions of ketones
(Scheme 1).14−16 With a successful strategy to construct highly
active RuII−NNN and −NNC complex catalysts in hand, we
envisioned that benzotriazolyl might be utilized as a
coordinating moiety to establish pyridyl-based NNN ligands.
INTRODUCTION
■
Transfer hydrogenation (TH) of unsaturated CO and CN
bonds has been well explored1 as a promising reduction
strategy and is considered as good complement to hydro-
genation reactions.2 In this area, Noyori et al. have documented
that N-tosylethylenediamine Ru(II) complexes3 and their later
versions, i.e., β-amino alcohol Ru(II) complexes,4 can be used
as powerful catalysts for asymmetric transfer hydrogenation
(ATH) of ketones and imines, and cyclometalated Ru(II) and
Os(II) complexes containing 2-(aminomethyl)pyridine ligands
developed by Baratta and co-workers5 have also exhibited
efficient catalytic activity in the (asymmetric) transfer hydro-
genation of ketones. Tetradentate chiral NNPP ligands and
their iron(II) complexes usually demonstrate moderate catalytic
activity for the same reactions under mild conditions.6
Although various types of ligands and their transition-metal
complexes have been developed for TH or ATH reactions,7
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 applications,8 and polydentate
ligands such as 2,2′:6′,2″-terpyridines (terpy),9 2,6-bis(imino)-
pyridines,10 and 2,6-bis(oxazolinyl)pyridines (Pybox)11 have
been well documented and successfully employed in organic
synthesis, homogeneous catalysis, and fabrication of functional
materials. Among the aforementioned cases, most of the
reported complex catalysts bear ligands containing two
symmetrical coordinating arms, while the complex catalysts
supported by a polydentate ligand containing two unsym-
Received: April 8, 2013
© XXXX American Chemical Society
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a
Scheme 1. NNN and NNC Ligands Developed in Our
Laboratories
Scheme 3. Synthesis of Complexes 6−8
Herein, we report the synthesis of Ru(II) complexes bearing a
pyridyl-based benzimidazolyl−benzotriazolyl NNN ligand and
their catalytic behaviors in the TH of ketones.
a
Legend: (i) RuCl2(PPh3)3, 2-propanol, 82 °C, 3 h, 83% (6a), 86%
(6b); (ii) K2CO3, CH2Cl2, 39 °C, 5 h, 91%; (iii) K2CO3, 2-propanol,
82 °C, 3 h, 72%; (iv) K2CO3, 2-propanol, 82 °C, 3 h, 75%.
RESULTS AND DISCUSSION
■
revealing the presence of two identical PPh3 ligands in each
complex. Complex 7 showed a 0.5 ppm shift in the 31P{1H}
NMR spectrum as compared with that of 6a, which is in
accordance with our previous observation,15 suggesting that the
coordination pattern of the coordinating benzimidazolyl
nitrogen atom in 7 was changed by means of extrusion of
Synthesis of Ligands and Their Ru(II) Complexes.
Reaction of 2,6-dibromopyridine (1) with benzotriazole under
solvent-free conditions afforded 2 in 72% yield. Aldehyde 3 was
obtained from the reaction of 2 with nBuLi followed by
treatment with DMF. Condensation of 3 with 1,2-phenyl-
enediamine gave ligand 4 in nitrobenzene upon heating at 150
°C. The N-methyl derivative of 4, i.e., ligand 5, was readily
prepared from methylation of 4 by MeI in the presence of NaH
(Scheme 2).
1
HCl from 6a. Complex 8 features a Ru−H bond. Its H and
31P{1H} NMR spectra showed a triplet at −6.17 ppm for Ru−
H and a doublet at 46.5 ppm for the two trans-PPh3 ligands,
respectively. The single-crystal structures of complexes 6a and
8 were further confirmed by X-ray crystallographic determi-
nations (Figures 1 and 2).
a
Scheme 2. Synthesis of Ligands 4 and 5
a
Legend: (i) benzotriazole, 180 °C, 3 h, 72%; (ii) nBuLi, −78 °C,
DMF, 62%; (iii) 1,2-phenylenediamine, 150 °C, 6 h, 65%; (iv) MeI,
NaH, THF, 65 °C, 3 h, 92%.
Ru(II) complexes 6a,b were obtained in 83% and 86% yields,
respectively, by reacting equimolar amount of ligand 4 or 5
with RuCl2(PPh3)3 in refluxing 2-propanol (Scheme 3). The
ionic complex 6a was further converted to neutral complex 7 by
K2CO3 base in refluxing CH2Cl2 through releasing 1 equiv of
HCl. RuH complex 8 was isolated in 72% yield from the
reaction of 6a in basic 2-propanol, and it could also be prepared
by reacting complex 7 with K2CO3 in 2-propanol, which reveals
that β-H elimination was involved in the transformations.
Characterization of Ru(II) Complexes 6−8. The NMR
analyses of the complexes are in agreement with their
Figure 1. Molecular structure of complex 6a with hydrogen atoms and
a chloride anion omitted for clarity.
In the solid state, the cationic metal center of complex 6a is
surrounded by the tridentate NNN ligand 4, two PPh3 ligands,
and a chloride anion in a distorted-bipyramidal environment,
with another dissociated chloride anion in the vicinity (Figure
1). The P−Ru−P angle in 6a is 178.37(7)°, suggesting that the
two PPh3 ligands are almost linearly positioned (Table 1).
Complex 8 features a molecular structure similar to that of 6a,
but with a discrete Ru−H bond (1.55(4) Å). Such a Ru−H
bond length is in the region of those reported (∼1.53 Å).16b,17
The presence of two Ru−P bonds (2.340(14) and 2.3197(14)
1
compositions. In the H NMR spectra, the NH protons in
ligand 4 and its Ru(II) complex 6a exhibited singlets at 12.94
and 15.65 ppm, respectively, and their resonance signals
disappeared in complexes 7 and 8, suggesting the formation of
Ru−N bonds in the complexes under basic conditions. In the
31P{1H} NMR spectra of complexes 6a,b and 7, the resonance
signals appeared at 22.2, 22.0, and 22.7 ppm, respectively,
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that electron-withdrawing substituents such as chloro, bromo,
fluoro, and trifluoromethyl usually facilitated the TH reactions
(entries 3−12 and 20, Table 2), while acetophenones bearing
an electron-donating 3′-methyl or 3′- or 4′-methoxy substituent
required a longer reaction time to reach high conversions
(entries 14−16, Table 2). Unexpectedly, 2′-methylacetophe-
none reacted quickly to achieve 97% conversion over a period
of 10 min (entry 13, Table 2).
Under the same conditions, complex 6b was also employed
as the catalyst for TH of ketones. Selected examples are
presented in Table 3. In comparison with 6a, complex 6b had a
much lower catalytic activity for the TH of ketones. All the
tested ketone substrates reached 95−98% conversions within 3
h, with 980 h−1 as the highest final TOF value (entry 3, Table
3). However, in the case of 4′-chloroacetophenone reduction,
complex 6a promoted the reaction to completion in 97% yield
with a final TOF value of 29100 h−1 within 2 min (entry 5,
Table 2). These results suggest that the NH functionality in a
coordinating benzimidazolyl moiety plays a crucial role in
enhancing the TH reaction rate of ketones.
TH Reaction Mechanism. The catalytic behaviors of
complexes 6a, 7, and 8 were comparatively explored in the
TH reactions of acetophenone, propiophenone, 2′-fluoroace-
tophenone, and cyclohexanone, respectively (Table 4). All
three complexes almost exhibited the same catalytic activity
under the stated conditions, suggesting that complex 8 may be
the catalytically active species for the TH reactions. Under the
controlled conditions, complex 8 was further tested to catalyze
the TH reaction of acetophenone in the absence of a base,
resulting in the desired product in 96% yield over a period of 20
h (eq 3); neither 6a nor 7 could catalyze the same reaction
Figure 2. Molecular structure of complex 8.
Å), three Ru−N bonds (2.030(4), 2.048(4), and 2.105(4) Å),
P−Ru−N angles (88.13(11)−98.16(11)°), and P(1)−Ru−
P(2) and N(3)−Ru−H angles (163.19(5) and 177.3(14)°)
reveals a six-coordinated metal center of complex 8 in a
distorted-bipyrimidal environment with the hydride trans to the
N(3) atom of the pyridyl backbone. Unlike our previously
prepared RuH complex supported by a tridentate NNC
ligand,14a which easily decomposes upon exposure to air,
complex 8 can be kept in air at ambient temperature for 5
months without any decomposition. Such excellent stability is
partially attributed to the stabilized coordinating environment
from the pyridyl−benzimidazolyl−benzotriazolyl NNN ligand.
Transfer Hydrogenation of Ketones. In a fashion similar
to our previously reported procedures for TH or ATH
reactions of ketones,14−16 complex 6a was applied as a catalyst
for the transfer hydrogenation of ketones (Table 2). Alcohols
were produced as the sole products by means of 0.1 mol %
catalyst in refluxing 2-propanol under a nitrogen atmosphere.
The reaction of acetophenone was nearly complete within 5
min, reaching 97% yield for the alcohol product with a final
TOF value of 11640 h−1 (entry 1, Table 2). In comparison with
our previously reported RuII−NNN or −NNC catalyst systems,
complex 6a exhibited an excellent catalytic activity for the TH
of ketones (eq 2).14 Thus, the reduction of a variety of
acetophenones, aryl alkyl ketones, and aliphatic cyclic and
acyclic ketones was efficiently performed. In most of the cases
shown in Table 2, the TH reactions reached ≥97% yields
within 2−10 min, achieving up to 99% conversions for the
ketone substrates and final TOF values up to 176400 h−1 (entry
20, Table 2). The electronic and steric effects were obvious, in
under base-free conditions. These results suggest that both 6a
and 7 act as the precatalysts for the TH reactions of ketones,
because they could be readily converted to RuH complex 8
under basic conditions (Scheme 2), and 8 behaved as the
catalytically active species. This transformation can be initiated
Table 1. Selected Bond Distances (Å) and Angles (deg) for 6a and 8
Complex 6a
Ru−N(1)
Ru−P(1)
N(1)−Ru−N(5)
P(2)−Ru−Cl(1)
2.086(5)
2.3930(19)
157.7(2)
91.88(6)
Ru−N(3)
Ru−P(2)
P(1)−Ru−P(2)
N(3)−Ru−P(1)
2.086(5)
Ru−N(5)
Ru−Cl(1)
N(3)−Ru−Cl(1)
N(5)−Ru−P(1)
2.019(5)
2.4298(18)
178.37(7)
91.19(14)
2.4405(17)
177.68(15)
88.52(16)
Complex 8
Ru(1)−N(3)
Ru(1)−P(1)
N(1)−Ru−P(2)
N(3)−Ru−P(1)
2.030(4)
Ru(1)−N(5)
Ru(1)−P(2)
N(5)−Ru−P(2)
N(1)−Ru−P(1)
2.048(4)
Ru(1)−N(1)
Ru(1)−H(1)
N(3)−Ru−H
P(2)−Ru−Cl(1)
2.105(4)
1.55(4)
2.3404(14)
93.39(11)
98.16(11)
2.3197(14)
89.52(10)
95.67(11)
177.3(14)
163.19(5)
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Table 2. Transfer Hydrogenation of Ketones Catalyzed by
6a
Table 3. Transfer Hydrogenation of Ketones Catalyzed by
6b
b
b
a
b
Determined by GC analysis. Conditions: ketone, 2.0 mmol (0.1 M
in 20 mL iPrOH); complex 6b, 0.1 mol %; ketone/iPrOK/cat. =
1000:20:1; 0.1 MPa; 82 °C.
Table 4. Transfer Hydrogenation of Ketones Catalyzed by
b
Ru(II) Complexes
a
GC yields of alcohols at 5 min by using the complex catalysts.
Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); catalyst, 0.1
b
mol %; ketone/iPrOK/catalyst = 1000:20:1; 0.1 MPa; 82 °C.
in situ by deprotonating the NH fuctionality of the
coordinating benzimidazolyl moiety in 6a to generate complex
7 and then forming 8 to trigger the TH reaction.14a,16b Thus, a
plausible inner-sphere pathway18 involving a RuH intermediate
as the catalytically active species19 is proposed (Scheme 4). The
reaction can directly start from complex 7 or from in situ
generated 7 by extrusion of HCl from the precursor complex 6a
with a base such as K2CO3 or iPrOK. The ruthenium(II)
hydride complex 8 is then formed by releasing one molecule of
acetone through β-H elimination from Ru(II) alkoxide F.
Coordination of a ketone substrate to 8 generates intermediate
G, followed by insertion of the ketone carbonyl into the Ru−H
bond to produce Ru(II) alkoxide H. The metathesis of H by 2-
propanol yields the desired alcohol product. Extrusion of an
a
b
Determined by GC analysis. Conditions: ketone, 2.0 mmol (0.1 M
in 20 mL iPrOH); complex 6a, 0.1 mol %; ketone/iPrOK/cat.=
1000:20:1; 0.1 MPa; 82 °C.
D
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refined anisotropically. All hydrogen atoms were placed in calculated
positions. Structure solution and refinement were performed by using
the SHELXL-97 package. The X-ray crystallographic files, in CIF
format, are available from the Cambridge Crystallographic Data
Centre on quoting the deposition numbers CCDC 927801 for 6a and
CCDC 927800 for 8. Copies of this information may be obtained free
of charge from the Director, CCDC, 12 Union Road, Cambridge CB2
IEZ, U.K. (fax, +44-1223-336033; e-mail, deposit@ccdc.cam.ac.uk;
web, http://www.ccdc.cam.ac.uk). X-ray crystallographic data and
refinement details for 6a and 8 are given in the Supporting
Information, and selected bond lengths and angles are given in
Table 1.
Scheme 4. Proposed Mechanism
Synthesis of Compound 2.
Under a nitrogen atmosphere, a mixture of 2,6-dibromopyridine (4.74
g, 20.0 mmol) and benzotrizole (3.57 g, 30.0 mmol) was stirred at 180
°C for 3 h. After the reaction mixture was cooled to ambient
temperature, 100 mL of dichloromethane was added and the solution
was filtered to separate the insoluble substances; all the volatiles were
removed under reduced pressure to give a crude product which was
subjected to purification by silica gel column chromatography (eluent
petroleum ether (60−90 °C)/ethyl acetate: 20/1, v/v) to afford the
desired product as a white solid (3.96 g, 72% yield). Mp: 80−82 °C.
1H NMR (CDCl3, 400 MHz): δ 8.51 (d, J = 8.3 and 8.0 Hz, 1 H, 5-H),
8.20 and 8.07 (d each, J = 8.0 and 8.2 Hz, 1:1 H, 5′-H and 8′-H), 7.73
and 7.58 (m, 2 H, 4-H and 3-H), 7.43 (m, 2 H, 6′-H and 7′-H).
13C{1H} NMR (CDCl3, 100 MHz): δ 151.0 and 146.7 (Cq each, C2
and C6), 140.0 and 131.2 (Cq each, C4′ and C9′), 140.8, 125.3, and
119.9 (pyridyl CH), 129.3, 126.1, 114.7, and 112.5 (aromatic CH of
benzotriazolyl). HRMS: calcd for C11H7BrN4 273.9854, found
273.9865.
alkoxide anion by an alcohol in the final step can be facilitated
by a base, which rationalizes the inferior catalytic activity of 8
under base-free conditions.
CONCLUSION
■
Synthesis of Compound 3.
In summary, Ru(II) complexes bearing a unsymmetrical
pyridyl-based benzimidazolyl−benzotrizolyl NNN ligand have
exhibited excellent catalytic activity in the TH of ketones. The
structurally characterized RuH complex is proposed to be the
catalytically active species. The combination of readily
coordinating benzotriazolyl and NH-benzimidazolyl in a
unsymmetrical 2,6-pyridyl-supported ligand provides a route
to construct highly active transition-metal complex catalysts.
To a stirred mixture of 6.8 mL of nBuLi (1.6 M in hexanes, 10.8
mmol) and THF (40 mL) was added dropwise a solution of 2 (2.45 g,
9.0 mmol) in THF (40 mL) at −78 °C over a period of 30 min. After
the mixture was further stirred at −78 °C for another 30 min,
anhydrous DMF (1.4 mL, 18.0 mmol) was added. The mixture was
warmed to 0 °C within 1 h, and the reaction was then quenched with
methanol (10 mL). Saturated aqueous NH4Cl (150 mL) was added,
and the aqueous phase was extracted with CH2Cl2 (3 × 50 mL). The
combined organic phases were dried over anhydrous Na2SO4 and
filtered. All the volatiles were removed under reduced pressure, and
the resultant residue was subjected to purification by silica gel column
chromatography (eluent petroleum ether (60−90 °C)/diethyl ether:
8/1, v/v) to afford the desired product as a white solid (1.25 g, 62%
yield). Mp: 132−133 °C. 1H NMR (CDCl3, 400 MHz): δ 10.18 (s, 1
H, CHO), 8.74 and 8.56 (d each, J = 8.4 and 8.2 Hz, 1:1 H, 3-H and 5-
H), 8.13 (m, 2 H, 4-H and 5′-H), 7.96 (d, J = 7.5 Hz, 1 H, 8′-H), 7.65
and 7.49 (m, 2 H, 6′-H and 7′-H). 13C{1H} NMR (CDCl3, 100
MHz): δ 191.8 (CHO), 152.0 and 151.3 (Cq each, C2 and C6), 146.9
and 131.4 (Cq each, C4′ and C9′), 140.1, 120.0, and 118.7 (pyridyl
CH), 129.3, 125.4, 120.1, and 114.8 (aromatic CH of benzotriazolyl).
HRMS: calcd for C12H8N4O 224.0698, found 224.0698.
EXPERIMENTAL SECTION
■
General Considerations. All the manipulations of air- and/or
moisture-sensitive compounds were carried out under a nitrogen
atmosphere using the standard Schlenk techniques. The solvents were
dried and distilled prior to use by literature methods. 1H 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), CD2Cl2 (δ(1H), 5.32 ppm; δ(13C), 53.84
ppm), and DMSO-d6 (δ(1H), 2.50 ppm; δ(13C), 39.52 ppm).
Elemental and HRMS analyses were achieved by the Analysis Center,
Dalian University of Technology. 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 mesh). All chemical reagents were
purchased from commercial sources and used as received unless
otherwise indicated.
X-ray Crystallographic Studies. The X-ray single-crystal
structure studies of compounds 6a and 8 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 reflections. Intensities were
corrected for Lorentz and polarization effects and empirical
absorption. The structures were solved by direct methods and refined
by full-matrix least squares on F2. All non-hydrogen atoms were
Synthesis of Compound 4.
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A mixture of 3 (1.12 g, 5.0 mmol) and 1,2-phenylenediamine (545 mg,
5.0 mmol) in nitrobenzene (30 mL) was stirred at 150 °C for 6 h. All
the volatiles were removed under reduced pressure at 80−100 °C, and
the resultant residue was subject to purification by silica gel column
chromatography (eluent petroleum ether (60−90 °C)/ethyl acetate/
triethylamine: 50/20/1, v/v/v) to afford the desired product as a white
solid (1.02 g, 65% yield). Mp: 258−259 °C. 1H NMR (DMSO-d6, 400
MHz): δ 12.94 (s, 1H, NH), 8.82 and 8.22 (d each, J = 8.4 and 8.3 Hz,
1:1 H, 3-H and 5-H), 8.41 and 8.31 (m each, 1:2 H, 5′-H, 6′-H and 7′-
H), 7.79 (m, 2 H, 5″-H and 8″-H), 7.69 (d, J = 7.8 Hz, 8′-H), 7.60 (t, J
= 8.4 and 8.2 Hz, 1 H, 4-H), 7.29 (m, 2 H, 6″-H and 7″-H); 13C{1H}
NMR (DMSO-d6, 100 MHz): δ 151.0 and 150.0 (Cq each, C2 and
C6), 148.3, 146.5, and 135.5 (Cq each, C2″, C4″, and C9″), 144.1 and
131.3 (Cq each, C4′ and C9′), 141.6, 121.5, and 119.9 (pyridyl CH),
130.0, 126.0, 119.9, and 115.3 (aromatic CH of benzotriazolyl), 124.2,
122.9, 116.2, and 113.0 (aromatic CH of benzimidazlyl). HRMS: calcd
for C18H12N6 312.1123, found 312.1129.
119.2, and 113.5 (aromatic CH of benzimidazolyl). 31P{1H} NMR
(CD2Cl2, 162 MHz): δ 22.2 (s, 2 × PPh3). Anal. Calcd for
C54H42Cl2N6P2Ru: C, 64.29; H, 4.20; N, 8.33. Found: C, 64.30; H,
4.22; N, 8.35.
Synthesis of Complex 6b.
Under a nitrogen atmosphere, a mixture of RuCl2(PPh3)3 (576 mg, 0.6
mmol) and 5 (196 mg, 0.6 mmol) in 2-propanol (10 mL) was refluxed
for 3 h, forming a red-brown microcrystalline solid. The mixture was
cooled to ambient temperature, and the solid was filtered off, washed
with diethyl ether (4 × 6 mL), and dried under vacuum to afford the
desired product as a red-brown crystalline solid (527 mg, 86% yield).
Synthesis of Compound 5.
1
Mp: >300 °C dec. H NMR (CDCl3, 400 MHz): δ 8.53 and 7.98 (d
each, J = 7.8 and 7.3 Hz, 1:1 H, 3-H and 5-H), 8.37 (br, 1 H, 4-H),
7.68, 7.54, and 7.44 (br each, 1:1:2 H, aromatic CH of benzotriazolyl),
7.31 and 7.23 (m each, 2:2 H, aromatic CH of benzimidazolyl), 7.10
and 6.90 (m each, 18:12 H, 2 × PPh3). 13C{1H} NMR (CDCl3, 100
MHz): δ 149.5 and 149.1 (Cq each, C2 and C6), 148.5, 146.2, and
136.0 (Cq each, C2″, C4″ and C9″), 141.2 and 131.4 (Cq each, C4′
and C9′), 133.2, 130.0, and 128.3 (CH of 2 × PPh3), 132.0, 122.6, and
119.0 (pyridyl CH), 129.5 (Cq of 2 × PPh3), 125.9, 125.8, 122.2, and
122.0 (aromatic CH of benzotriazolyl), 125.4, 123.8, 111.9, and 110.7
(aromatic CH of benzimidazolyl), 65.8 (N−CH3). 31P{1H} NMR
(CDCl3, 162 MHz): δ 22.0 (s, 2 × PPh3). Anal. Calcd for
C55H44Cl2N6P2Ru: C, 64.58; H, 4.34; N, 8.22. Found: C, 64.56; H,
4.37; N, 8.24.
A mixture of 4 (156 mg, 0.5 mmol) and NaH (160 mg, 60% in mineral
oil, 4.0 mmol) in 10 mL of THF was stirred at ambient temperature
for 30 min. Iodomethane (1.42 g, 1.0 mmol) was added, and the
reaction mixture was heated to reflux for 3 h. After the mixture was
cooled to ambient temperature, methanol (5 mL) was added to
quench the reaction. All the volatiles were evaporated under reduced
pressure, and the resultant residue was subjected to purification by
silica gel column chromatography (eluent petroleum ether (60−90
°C)/ethyl acetate: 4/1, v/v) to afford the desired product as a white
Synthesis of Complex 7.
1
solid (150 mg, 92% yield). Mp: 197−198 °C. H NMR (CDCl3, 400
MHz): δ 8.47 and 7.86 (d each, J = 8.3 and 7.5 Hz, 1:1 H, 3-H and 5-
H), 8.32 and 8.12 (m each, 2:2 H, aromatic CH of benzotriazolyl),
7.58(t, J = 8.3 and 7.5 Hz, 1 H, 4-H), 7.46 and 7.36 (m each, 2:2 H,
aromatic CH of benzimidazolyl), 4.22 (s, 3 H, M-Me). 13C{1H} NMR
(CDCl3, 100 MHz): δ 150.3 and 149.4 (Cq each, C2 and C6), 149.0,
146.7, and 137.0 (Cq each, C2″, C4″, and C9″), 142.3 and 131.2 (Cq
each, C4′ and C9′), 139.9, 124.9, and 122.9 (pyridyl CH), 128.8,
120.1, 120.0, and 113.5 (aromatic CH of benzotriazolyl), 123.7, 123.6,
115.2, and 109.9 (aromatic CH of benzimidazolyl), 32.4 (N−CH3).
HRMS: calcd for C19H14N6 326.1280, found 326.1284.
Under a nitrogen atmosphere, a mixture of complex 6a (504 mg, 0.5
mmol) and K2CO3 (690 mg, 5.0 mmol) in 10 mL of dichloromethane
was refluxed for 5 h. After it was cooled to ambient temperature, the
reaction mixture was passed through a short pad of Celite which was
rinsed with 5 mL of dichloromethane. The filtrate was collected, and
all the volatiles were removed under reduced pressure to afford the
desired product (443 mg, 91% yield) as a red solid. Mp: >300 °C dec.
1H NMR (CD2Cl2, 400 MHz): δ 8.39 (d, J = 6.6, 5′-H), 8.21 and 8.06
(d each, J = 7.3 and 7.4 Hz, 1:1 H, 3-H and 5-H), 7.50 (m, 4 H, 6′-H,
7′-H, 8′-H, and 4-H), 7.38 and 6.89 (d each, J = 7.8 and 8.2 Hz, 1:1 H,
5″-H and 8″-H), 7.30 and 6.98 (m each, 6″-H and 7″-H), 7.19, 7.10,
and 6.97 (m each, 12:6:12 H, 2 × PPh3). 13C{1H} NMR (CD2Cl2, 100
MHz): δ 152.7 and 148.8 (Cq each, C2 and C6), 146.4, 132.7, and
132.6 (Cq each, C2″, C4″, and C9″), 143.8 and 131.3 (Cq each, C4′
and C9′), 134.1, 122.0, and 120.8 (pyridyl CH), 133.4, 129.3, and
127.7 (CH of 2 × PPh3), 130.2 (Cq, 2 × PPh3), 129.0, 125.7, 108.0,
and 107.0 (aromatic CH of benzotriazolyl), 122.8, 119.0, 118.8, and
115.7 (aromatic CH of benzimidazolyl). 31P{1H} NMR (CD2Cl2, 162
MHz): δ 22.7 (s, 2 × PPh3). Anal. Calcd for C54H41ClN6P2Ru: C,
66.70; H, 4.25; N, 8.64. Found: C, 66.76; H, 4.22; N, 8.61.
Synthesis of Complex 6a.
Under a nitrogen atmosphere, a mixture of RuCl2(PPh3)3 (480 mg, 0.5
mmol) and 4 (156 mg, 0.5 mmol) in 2-propanol (10 mL) was refluxed
for 3 h, forming a red-brown microcrystalline solid. The mixture was
cooled to ambient temperature, and the solid was filtered off, washed
with diethyl ether (3 × 10 mL), and dried under vacuum to afford the
desired product as a red-brown crystalline solid (418 mg, 83% yield).
Single crystals suitable for X-ray crystallographic determination were
grown from the recrystallization of 6a in CHCl3/n-hexane (1/3, v/v)
at 25 °C. Mp: >300 °C dec. 1H NMR (CD2Cl2, 400 MHz): δ 15.65 (s,
1 H, NH), 8.33 and 8.08 (d each, J = 8.2 and 7.8 Hz, 1:1 H, 3-H and 5-
H), 7.65 and 7.26 (m each, 2:2 H, aromatic CH of benzimidazolyl),
7.53 (m, 4 H, aromatic CH of benzotriazolyl), 7.39 (br, 1 H, 4-H),
7.18, 7.11, and 6.92 (m each, 12:6:12 H, 2 × PPh3). 13C{1H} NMR
(CD2Cl2, 100 MHz): δ 150.0 and 149.2 (Cq each, C2 and C6), 148.9,
146.3, and 134.9 (Cq each, C2″, C4″, and C9″), 141.9 and 131.3 (Cq
each, C4′ and C9′), 134.7, 123.3, and 121.6 (pyridyl CH), 133.2,
129.6, and 127.9 (CH of 2 × PPh3), 129.8 (Cq, 2 × PPh3), 130.0,
129.5, 108.7, and 108.3 (aromatic CH of benzotriazolyl), 126.0, 125.2,
Synthesis of Complex 8.
Under a nitrogen atmosphere, a mixture of complex 6a (605 mg, 0.60
mmol) or complex 7 (583 mg, 0.60 mmol) and K2CO3 (828 mg, 6.0
mmol) in 15 mL of 2-propanol was refluxed for 3 h. After the mixture
F
dx.doi.org/10.1021/om400298c | Organometallics XXXX, XXX, XXX−XXX

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Article
was cooled to ambient temperature, all the volatiles were removed
under reduced pressure. An 8 mL portion of toluene was then added
to dissolve the crude product, followed by filtration to separate the
inorganic salts. The filtrate was condensed under reduced pressure and
then recrystallized in toluene/n-hexane (1/2, v/v) at −20 °C to give
the desired product as red-brown crystals (405 mg, 72% yield from 6a;
422 mg, 75% yield from 7). Mp: >300 °C dec. 1H NMR (CDCl3, 400
MHz): δ 8.01, 7.65, and 7.46 (m each, 1:3:3 H, pyridyl CH and
aromatic CH of benzotriazolyl), 7.33, 7.24, and 6.64 (m each, 1:2:1 H,
aromatic CH of benzimidazolyl), 7.13, 6.99, and 6.86 (m each, 12:6:12
H, 2 × PPh3), −6.17 (t, J = 23.9, Ru−H). 13C{1H} NMR (CDCl3, 100
MHz): δ 159.1 and 152.1 (Cq each, C2 and C6), 147.0, 145.9, 132.1
(Cq each, C2″, C4″, and C9″), 144.6 and 131.2 (Cq each, C4′ and
C9′), 133.7 (Cq, 2 × PPh3), 133.0, 128.4, and 127.3 (CH of 2 × PPh3),
132.7, 120.0, and 119.4 (pyridyl CH), 126.5, 124.3, 107.3, and 103.9
(aromatic CH of benzotriazolyl), 118.1, 118.0, 117.7, 115.9 (aromatic
CH of benzimidazolyl). 31P{1H} NMR (CDCl3, 162 MHz): δ 46.5 (d,
J(P,H) = 22.7 Hz, 2 × PPh3). Anal. Calcd for C54H42N6P2Ru: C,
69.15; H, 4.51; N, 8.69. Found: C, 69.19; H, 4.57; N, 8.64.
Typical Procedure for the Catalytic Transfer Hydrogenation
of Ketones. The catalyst solution was prepared by dissolving complex
6a (20.2 mg, 0.02 mmol) in 2-propanol (20.0 mL). Under a nitrogen
atmosphere, a mixture of the ketone (2.0 mmol), 2.0 mL of the
catalyst solution (0.002 mmol), and 2-propanol (17.6 mL) was stirred
at 82 °C for 10 min. Then, 0.4 mL of an 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 precooled to 0 °C for
GC analysis. After the reaction was complete, 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.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving NMR spectra of the new
compounds and X-ray crystallographic data for 6a and 8. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(6) (a) Mikhailine, A. A.; Maishan, M. I.; Lough, A. J.; Morris, R. H. J.
Am. Chem. Soc. 2012, 134, 12266−12280. (b) Sonnenberg, J. F.;
Coombs, N.; Dube, P. A.; Morris, R. H. J. Am. Chem. Soc. 2012, 134,
5893−5899. (c) Lagaditis, P. O. A.; Lough, J.; Morris, R. H. J. Am.
Chem. Soc. 2011, 133, 9662−9665. (d) Mikhailine, A. A.; Morris, R. H.
Inorg. Chem. 2010, 49, 11039−11044. (e) Lagaditis, P. O.; Lough, A.
J.; Morris, R. H. Inorg. Chem. 2010, 49, 10057−10066. (f) Mikhailine,
A.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2009, 131, 1394−
1395.
AUTHOR INFORMATION
Corresponding Author
*E-mail for Z.Y.: zkyu@dicp.ac.cn.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(7) For selected recent examples, see: (a) Johnson, T. C.; Totty, W.
We are grateful to the National Basic Research Program of
China (2009CB825300), and the National Natural Science
Foundation of China (21272232).
G.; Wills, M. Org. Lett. 2012, 14, 5230−5233. (b) Soni, R.; Collinson,
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J.; Clarkson, G. C.; Wills, M. Org. Lett. 2011, 13, 4304−4307. (c) Dıaz-
Valenzuela, M. B.; Phillips, S. D.; France, M. B.; Gunn, M. E.; Clarke,
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