Ruthenium complex catalysts supported by a bis(trifluoromethyl)pyrazolyl- pyridyl-based NNN ligand for transfer hydrogenation of ketones

Article abstract of DOI:10.1021/om401144u

Ru(III) and Ru(II) complexes bearing a tridentate 2-(3′,5′- dimethylpyrazol-1′-yl)-6-(3″,5″-bis(trifluoromethyl) pyrazol-1″-yl)pyridine or 2-(benzimidazol-2′-yl)-6-(3″, 5″-bis(trifluoromethyl)pyrazol-1″-yl)pyridine ligand were synthesized and applied to the transfer hydrogenation of ketones. The Ru(II) complex was structurally confirmed by the X-ray crystallographic analysis and achieved up to 2150 turnover numbers and final TOFs up to 29700 h^-1 in the transfer hydrogenation of ketones. The benzimidazolyl moiety with an unprotected NH functionality in the ligand exhibited an enhancement effect on the catalytic activity of its RuCl₃ complex in the ketone reduction reactions, reaching a final TOF value up to 35640 h^-1. The controlled experiments have revealed that the compatibility of the trifluoromethylated pyrazolyl and unprotected benzimidazolyl is crucial for the establishment of the highly active catalytic system.

Full text of DOI:10.1021/om401144u

Article
pubs.acs.org/Organometallics
Ruthenium Complex Catalysts Supported by a
Bis(trifluoromethyl)pyrazolyl−Pyridyl-Based NNN Ligand for Transfer
Hydrogenation of Ketones
Wangming Du,† Qingfu Wang,† Liandi 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: Ru(III) and Ru(II) complexes bearing a tridentate 2-(3′,5′-
dimethylpyrazol-1′-yl)-6-(3″,5″-bis(trifluoromethyl)pyrazol-1″-yl)pyridine
or 2-(benzimidazol-2′-yl)-6-(3″,5″-bis(trifluoromethyl)pyrazol-1″-yl)-
pyridine ligand were synthesized and applied to the transfer hydrogenation
of ketones. The Ru(II) complex was structurally confirmed by the X-ray
crystallographic analysis and achieved up to 2150 turnover numbers and final
TOFs up to 29700 h−1 in the transfer hydrogenation of ketones. The
benzimidazolyl moiety with an unprotected NH functionality in the ligand
exhibited an enhancement effect on the catalytic activity of its RuCl3
complex in the ketone reduction reactions, reaching a final TOF value up
to 35640 h−1. The controlled experiments have revealed that the
compatibility of the trifluoromethylated pyrazolyl and unprotected benzimidazolyl is crucial for the establishment of the highly
active catalytic system.
ransition-metal-catalyzed transfer hydrogenation (TH) of
unsaturated substrates has attracted much attention in
methane. Dias and others have revealed that fluoro substituents
on the pyrazolyl rings can exert a significant effect on the metal
centers of their complexes.12 The relevant metal complexes
usually exhibited unique properties such as great stabilities
toward air and thermal conditions, and some structurally
unique and unstable complexes could be stabilized by these
ligands. Although 3,5-bis(trifluoromethyl)pyrazolyl has been
employed as a unique functionality, it has seldom been used for
fabricating transition-metal complex catalysts.13
We have recently worked on unsymmetrical pyrazolyl−
pyridyl-based NNN ligands to construct highly active
ruthenium(II) complex catalysts for the transfer hydrogenation
of ketones (Scheme 1).14 Highly active Ru(II) complex
catalysts bearing a pyrazolyl−(1H-imidazolyl) pyridine (B and
C)14d,f or pyrazolyl−(1H-pyrazolyl) pyridine (D)14j NNN
ligand were synthesized. With these results in hand, we
reasonably envisioned that combination of the distinctive 3,5-
bis(trifluoromethyl)pyrazolyl functionality with unprotected
benzimidazolyl or electron-rich pyrazolyl may lead to tridentate
NNN ligands with unique properties suitable for constructing
transition-metal complex catalysts. Herein, we report the
synthesis of two bis(trifluoromethyl)pyrazolyl−pyridyl-based
NNN ligands and their application in ruthenium-catalyzed TH
reactions of ketones.
T
recent years,1 not only because it provides reliable reduction
methods2 under mild conditions with simple procedures but
also because it is considered as a promising alternative for direct
hydrogenation reactions. N-Tosyldiphenylethylene and the
Ru(II) complexes developed by Noyori et al. have been
demonstrated as the most powerful catalysts in the asymmetric
transfer hydrogenation of ketones and imines.3 2-
(Aminomethyl)pyridine-based cyclometalated complexes re-
ported by Baratta and co-workers have been proven as another
class of efficient catalysts for the transfer hydrogenation of
ketones.4 Other types of ligands and their transition-metal
complex catalysts have also been established to achieve broader
substrate scopes and improve reaction efficiency.5 Among the
above established catalytic systems, Ru(II) complexes have
represented the most extensively investigated catalysts, while
their Ru(III) analogues have been seldom explored in the
transfer hydrogenation of ketones.6,7 Trifluoromethylated
molecules have been demonstrated to have versatile applica-
tions in various areas8 due to the inherent properties of CF3,
such as high electronic negativity, lipophilicity, H-bonding
formation, etc. The electronic properties of a pyrazolyl moiety
could be substantially modified by incorporating a CF3 group
into its backbone, which thus dramatically alters both the
physical and chemical properties of its metal complexes.9−11
3,5-Bis(trifluoromethyl)pyrazolyl was used to build scorpionate
ligands and bidentate (3,5-bis(trifluoromethyl)pyrazolyl)-
Received: November 25, 2013
© XXXX American Chemical Society
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refluxing ethanol formed Ru(III) complex 5 (85%). The
reaction of 5 with an equivalent amount of PPh3 and
triethylamine in ethanol gave Ru(II) complex 6 (55%).
Scheme 1. Strategy for Constructing Ruthenium Complex
Catalysts
In a fashion similar to the preparation of 3, compound 9 was
obtained from the reaction of 2,6-dibromopyridine with
hydrazine hydrate followed by cyclization with 1,1,1,5,5,5-
hexafluoropentane-2,4-dione (Scheme 3). Dehydration of 9
a
Scheme 3. Synthesis of Ligands 12 and 13
Initially, we tried to introduce a 3,5-bis(trifluoromethyl)-
pyrazolyl functionality to a pyridyl backbone by the palladium-
or copper-catalyzed cross-coupling reaction of 2-bromopyridine
with 3,5-bis(trifluoromethyl)pyrazole in a fashion similar to that
for the preparation of 2,6-bis(3,5-dimethylpyrazol-1-yl)-
pyridine.14b Unfortunately, the reaction did not occur
regardless of using palladium and copper catalysts or changing
to harsh conditions. This consequence may be attributed to the
highly electron-deficient properties of 3,5-bis(trifluoromethyl)-
pyrazole. An indirect procedure was then developed to
synthesize the target bis(trifluoromethyl)pyrazolyl−pyridyl-
based NNN ligand 4 (Scheme 2). Starting from the
a
Legend: (i) 85% N2H4 hydrate, nBuOH, 120 °C, 12 h, 67%; (ii)
1,1,1,5,5,5-hexafluoropentane-2,4-dione, 66 °C, THF, 12 h, 97%; (iii)
concentrated H2SO4, HOAc, 120 °C, 5 h, 98%; (iv) nBuLi, DMF, −78
°C, 60%; (v) 1,2-phenylenediamine, PhNO2, 150 °C, 10 h, 55%; (vi)
MeI, NaH, THF, 66 °C, 6 h, 93%.
a
Scheme 2. Synthesis of Ligand 4 and Complexes 5 and 6
under the acidic conditions gave 2-bromo-6-(3′,5′-bis-
(trifluoromethyl)pyrazolyl)pyridine (10). Formylation of 10
with DMF in THF produced 11 (60%), which was condensed
with 1,2-phenylenediamine to produce the desired ligand 12
(55%). Methylation of 12 by methyl iodide formed its
derivative: that is, ligand 13. In a manner similar to the
synthesis of complexes 5 and 6, we failed to obtain the desired
Ru(III) complex product from the reaction of ligand 12 or 13
with RuCl3·xH2O, and in the presence of PPh3 and Et3N the
corresponding Ru(II) complex product was not successfully
collected either. In all our attempts to isolate the desired
Ru(III) and Ru(II) complexes bearing ligands 12 and 13, only a
mixture of complexes and no pure complex products 14−17
could be obtained. This result is presumably attributed to the
highly electron-deficient properties of the 3,5-bis-
(trifluoromethyl)pyrazolyl functionality, which may not be
very compatible with the benzimidazolyl moiety in the ligand as
compared with that in ligand 4, resulting in variable Ru(III)
species under the reaction conditions.
a
Legend: (i) 85% N2H4 hydrate, nBuOH, 120 °C, 20 h, 65%; (ii)
1,1,1,5,5,5-hexafluoropentane-2,4-dione, 66 °C, THF, 12 h, 95%; (iii)
concentrated H2SO4, HOAc, 120 °C, 5 h, 90%.; (iv) RuCl3·xH2O,
ethanol, 78 °C, 5 h, 85%; (v) PPh3, triethylamine, ethanol, 60 °C, 3 h,
55%.
nucleophilic substitution reaction of 2-bromo-6-(3′,5′-
dimethylpyrazolyl)pyridine (1) with hydrazine hydrate in
butanol on heating, pyridyl hydrazine 2 was obtained in 65%
yield. Cyclization of 2 with 1,1,1,5,5,5-hexafluoropentane-2,4-
dione in refluxing THF afforded the intermediate product 3
(95%), which was efficiently dehydrated to give the desired
ligand 4 (90%) in the presence of concentrated H2SO4.
Treatment of 4 with an equivalent amount of RuCl3·xH2O in
All the intermediate compounds, ligands, and complex 6
1
were characterized by H, 13C{1H}, 31P{1H}, and 19F{1H} (if
any) NMR and HRMS techniques or elemental analysis.
Ru(III) complex 5 is paramagnetic, and its NMR spectra could
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not be successfully collected. The HRMS (ESI-TOF) analysis
of complex 5 revealed two peaks featuring values of 546.9343
([M − Cl]+) and 604.8874 ([M + Na]+), which are consistent
with the calculated values: i.e., 546.9339 and 604.8925,
respectively. The elemental analysis suggests its composition
as expected. The molecular structure of complex 6 was further
confirmed by the X-ray crystallographic determination (Figure
1). In the solid state, the six-coordinated metal center is
highly efficient example of Ru(III)-catalyzed transfer hydro-
genation of ketones.6,7 Various substituents were tolerated, and
electron-withdrawing groups such as chloro, bromo, and fluoro
usually facilitated the reaction, whereas an o-trifluoromethyl
group deteriorated the reaction efficiency (Table 1, entry 13).
Electron-donating substituents, that is, methyl and methoxy,
made the ketone substrates more electron rich and thus reacted
less efficiently than their electron withdrawing group(s) bearing
analogues. Increasing the steric hindrance of the ketones
required longer time and/or a higher catalyst loading for the
reaction to reach satisfactory yields (Table 1, entries 19−21). It
was observed that acetophenones bearing an ortho substituent
usually reacted more quickly than those bearing meta or para
substituent(s) (Table 1, entries 3 vs 4 and 5, 7 vs 8 and 9, 10 vs
11 and 12, and 16 vs 17 and 18), except for the case of using
the ketone bearing a sterically hindered o-CF3 substituent
(Table 1, entry 13). Aliphatic ketones were also reduced to the
corresponding alcohols, and the cyclic substrates cyclo-
pentanone and cyclohexanone exhibited much higher reactivity
than their acyclic analogue: i.e., 2-heptanone (Table 1, entries
22−24). On the basis of the observations we reasonably
propose that during the TH reaction Ru(III) complex 5 is
initially reduced to a Ru(II) species which subsequently
interacts with the alkoxide (e.g., iPrOK) to form a catalytically
active RuH species to initiate the TH reaction.14k
The reduction of ketones was also performed by using 0.2
mol % of complex 6 as catalyst. As shown in Table 1, complex 6
exhibited a much higher catalytic activity than its Ru(III)
analogue 5, reaching 99% yield and a final TOF value of up to
29700 h−1 within 1 min (Table 1, entry 4). Similar electronic
and steric effects of the substituents from the ketone substrates
were observed (Table 1). In particular, complex 6 was
catalytically superior to our previously reported Ru(II) complex
A bearing a 2,6-bis(3′,5′-dimethylpyrazol-1′-yl)pyridine ligan-
d.14a,b Under the same conditions with 0.2 mol % A as catalyst,
the TH reaction of ketones only formed the corresponding
alcohols in up to 99% yield with the highest final TOF (5940
h−1) over a period of 5 min.14a These results suggest that the
hemilability of unsymmetrical ligands is crucial for their
transition-metal complexes to exhibit high catalytic activity.
The TON values ranged between 450 and 495 in the cases of
using 0.2 mol % 6 as catalysts, which are higher than those
achieved by using 1 mol % complex 5. However, on a 10 mmol
scale of the ketone in the presence of 0.04 mol % 6 as catalyst,
the TON reached 2150 (Table 1, entry 4). The modest TON
values are presumably attributed to the fact that 3,5-
bis(trifluoromethyl)pyrazolyl is a relatively weak coordinating
group, and complexes 5 and 6 bearing a ligand containing such
a coordinating functionality could not be stabilized enough and
were gradually decomposed to deactivation during the reaction.
The NH functionality of benzimidazolyl has been proven as a
great accelerating element in the ruthenium(II) complex
catalysts developed by our group,14 and such complexes can
be efficiently applied in the TH reactions of ketones. Because
pure Ru(III) and Ru(II) complexes 14−17 could not be
successfully prepared, a combination of RuCl3·xH2O and ligand
12 or 13 in a 1/1 molar ratio was used as the catalyst for the
TH reaction of ketones under the standard conditions. By
means of a loading of 0.5 mol % RuCl3·xH2O and 0.5 mol % of
12 in refluxing 2-propanol, the TH reaction of ketones was
efficiently realized (Table 2). In most cases, >97% yields were
obtained within 10 min, and the reaction of 2′-chloroaceto-
phenone reached 99% yield with a final TOF value of 35640
Figure 1. Molecular structure of complex 6. One molecule of
crystallized CHCl3 and half a molecule of crystallized cyclohexane are
omitted for clarity.
surrounded by ligand 4, two chlorides, and one PPh3 ligand in a
distorted-bipyramidal environment, which is similar to that of
our previously reported Ru(II) complex bearing a 2,6-bis(3′,5′-
dimethylpyrazol-1′-yl)pyridine ligand.14a The Cl(1)−Ru−
Cl(2) angle is 85.72(3)°, suggesting that the two Cl atoms
are cis positioned. The P−Ru−N(1), P−Ru−N(3), and P−
Ru−N(5) angles are 92.74(8), 93.03(8), and 92.83(8)°,
respectively, indicating that the PPh3 ligand is nearly vertically
situated above the planar ligand. The N(3)−Ru−Cl(2) angle is
170.39(8)°, revealing that Cl(2) is positioned trans to the
pyridyl nitrogen atom. The Ru−P bond length is 2.3237(9) Å,
which is longer than that (2.2927(14) Å) in the aforemen-
tioned complex,14a suggesting that the ruthenium metal center
in 5 is situated in a more loose environment as compared with
that in the Ru(II) complex bearing a 2,6-bis(3′,5′-dimethylpyr-
azol-1′-yl)pyridine ligand. Such a structural element usually
enhances the catalytic activity of a metal complex catalyst.
Complex 5 was then applied as the catalyst for transfer
hydrogenation of ketones (Table 1). Under the standard
conditions, a wide range of acetophenones, aliphatic cyclic and
acyclic ketones were reduced to their corresponding alcohols
using 1 mol % 5 as the precatalyst in refluxing 2-propanol under
a nitrogen atmosphere. Most of the substrates reached ≥97%
conversions and TON values of ca. 100 within 30 min. In
particular, both 2′-chloroacetophenone and 2′,4′-dichloroace-
tophenone were reduced to the corresponding alcohols in 99%
yields within 1 min, achieving the highest final TOF value of
5940 h−1 (Table1, entries 3 and 6), demonstrating a rare and
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e
Table 1. Transfer Hydrogenation of Ketones Catalyzed by 5 or 6
a
b
c
Determined by GC analysis. Data given in parentheses for the cases of using 6 as catalyst. The reaction was conducted on a scale of 10 mmol of
d
e
ketone with 0.04 mol % of 6 in 100 mL of iPrOH. Using 2 mol % of 5. Conditions: ketone, 2.0 mmol (0.1 M in 20 mL of iPrOH); complex 5 (1
mol %), ketone/iPrOK/cat. 5 = 100/15/1, or complex 6 (0.2 mol %), ketone/iPrOK/cat. 6 = 500/15/1; 0.1 MPa of N2,; 82 °C.
h−1 over a period of 20 s (Table 2, entry 3), demonstrating one
of the best results in Ru(III)-catalyzed TH reactions of ketones
under phosphine-free conditions to date.6 The TON values
were around 200 in the cases of using 0.5 mol % RuCl3·xH2O/
12 as catalyst, which could be increased to 720 on a 10 mmol
scale of the ketone in the presence of 0.1 mol % RuCl3·xH2O/
12 (Table 2, entry 3). The modest TON values are also
presumably attributed to the fact that 3,5-bis(trifluoromethyl)-
pyrazolyl is a relatively weakly coordinating functionality which
could not effectively stabilize the central metal of the complex
catalyst during the reaction.
The interaction of RuCl3·xH2O with ligand 12 possibly
generated variable Ru(III) complex 14, which was readily
transformed to 14′ by extrusion of one molecule of HCl in the
presence of a base and then reduced to Ru(II) species E under
the reaction conditions. During the reaction species E may be
stabilized by the solvent, ketone substrate, and/or alcohol
product, and it may also coexist with its dimer. The precatalyst
E reacted with iPrOK base to initiate the catalytic cycle, as we
D
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d
Table 2. Transfer Hydrogenation of Ketones Catalyzed by RuCl3·xH2O/12
a
b
Determined by GC analysis. The reaction was conducted on a 10 mmol scale of ketone with 0.1 mol % RuCl3·xH2O/ligand 12 as catalyst in 100
c
d
mL of iPrOH. Data given in parentheses for the cases of adding 0.5 mol % PPh3. Conditions: ketone, 2.0 mmol (0.1 M in 20 mL of iPrOH);
RuCl3·xH2O, 0.5 mol %; ligand 12, 0.5 mol %; iPrOK, 7.5 mol %; 0.1 MPa of N2; 82 °C.
previously reported.14f,h,i,k Although most of the substrates
could be efficiently reduced to their corresponding alcohols, the
reaction of some electron-rich and sterically hindered ketones
required a longer time to achieve satisfactory yields. To our
delight, the reaction efficiency of these relatively stubborn
ketones was obviously improved by adding 0.5 mol % of PPh3
to the catalytic system (Table 2, entries 19−21 and 23). Such
an acceleration effect is ascribed to the coordination of PPh3
ligand to the ruthenium center, which stabilizes the catalytically
active species during the reaction. After all the volatiles were
evaporated under reduced pressure, the reaction residue was
analyzed by a 31P{1H} NMR determination to reveal a singlet at
29.4 ppm for a PPh3 ligand coordinated to the ruthenium
atom.14
In order to make comparison with ligand 12, ligand 13 with
an N-methyl benzimidazolyl moiety was applied in the catalytic
system for the TH reaction of ketones. It was found that the
combination of RuCl3·xH2O and 13 could only exhibit a
modest catalytic activity under the same conditions. For the five
tested ketone substrates, a much longer time (30−120 min)
was required to get acceptable yields (Table 3) in comparison
with those cases (3−20 min) using the RuCl3·xH2O/12 catalyst
system (Table 2). For example, 4′-bromoacetophenone was
converted to the corresponding alcohol in 97% yield within 3
min by using the RuCl3·xH2O/12 catalyst system, reaching a
final TOF value of 3880 h−1 (Table 2, entry 9), whereas the
same reaction only achieved 84% yield within 1 h by means of
RuCl3·xH2O/13 as catalyst, exhibiting a final TOF value of 168
E
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PPh3.18 In the absence of a phosphine ligand, the Ru(II) species
generated from the in situ reduction of the Ru(III) precatalyst
may not be effectively stabilized by the ligand, so that
performance of the Ru(III) complex catalyst was inferior to
that of its corresponding Ru(II) analogue bearing a PPh3 ligand.
Thus, we reasonably proposed an in situ formed Ru(II) hydride
as the catalytically active species to promote the TH reaction
via an inner-sphere pathway.
Scheme 4. In Situ Generation of Complex 14
The mechanism is depicted by means of complex 5 as the
precatalyst (Scheme 5). Ru(III) complex 5 is initially reduced
Scheme 5. Proposed Mechanism
Table 3. Transfer Hydrogenation of Ketones Catalyzed by
b
RuCl3·xH2O/13
to Ru(II) species E in situ. A subsequent ligand substitution
with iPrOK forms Ru(II)−alkoxide F, and the subsequent β-H
elimination from F generates Ru(II)−H intermediate G with
release of acetone. Coordination to the metal center and
insertion to the Ru−H bond in G by the carbonyl of a ketone
substrate yields Ru(II)−alkoxide I, which undergoes alcohol
metathesis to furnish the desired product and complete the
catalytic cycle.
In summary, we have successfully synthesized pyridyl-
supported NNN ligands bearing a 3,5-bis(trifluoromethyl)-
pyrazolyl moiety and their Ru(III) and Ru(II) complexes.
These complexes and the combination of the NNN ligands
with RuCl3·xH2O exhibited very high catalytic activity in the
TH reaction of ketones, demonstrating one of the most
efficient Ru(III) catalysts for TH reactions of ketones.
a
b
Determined by GC analysis. Conditions: ketone, 2.0 mmol (0.1 M
in 20 mL iPrOH); RuCl3·xH2O, 0.5 mol %; ligand 13, 0.5 mol %;
iPrOK, 7.5 mol %; 0.1 MPa of N2; 82 °C.
h−1 (Table 3, entry 3). These results suggest that the
unprotected NH functionality in the benzimidazolyl moiety is
a key factor for constructing highly active transition-metal
complex catalysts for TH reactions of ketones.
The TH mechanism is unclear at present, although Ru(II)-
catalyzed TH reactions have been investigated15 with a Ru(II)−
H complex as the catalytically active species.16 In our cases of
using the Ru(III) precatalysts, reduction of Ru(III) to Ru(II)
may occur first.17 The 1H NMR analysis of the residue from the
1
in situ reduction of complex 5 reveals unambiguous H NMR
signals (see the Supporting Information), suggesting trans-
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
■
formation of the Ru(III) species, i.e., 5, to some Ru(II) species
1
because Ru(III) complex 5 is paramagnetic and its H and 13C
NMR signals could not be collected. In addition, the Ru(III)
precatalyst could be unambiguously reduced to the Ru(II)
species containing a Ru(II)−PPh3 bond in the presence of
1
dried and distilled prior to use by the literature methods. H and
13C{1H} NMR spectra were recorded on a Bruker DRX-400
F
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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 acetone-d6 (δ(1H), 2.05 ppm; δ(13C),
29.84 and 206.26 ppm). HRMS and elemental analysis were carried
out by the Analysis Center, Dalian University of Technology, and the
HRMS data given for compounds 8−10 refer to the 79Br isotope. All
of 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.
51−52 °C. 1H NMR (CD3COCD3, 400 MHz): δ 8.20 (m, 1 H, 4-H),
8.12 and 7.66 (d each, J = 8.2 Hz and J = 7.7 Hz, 1:1 H, 3-H and 5-H),
7.58 (s, 1 H, 4′-H), 6.06 (s, 1 H, 4″-H), 2.56 and 2.23 (s each, 3:3 H,
methyl of pyrazolyl). 13C{1H} NMR (CD3COCD3, 100 MHz): δ
152.6 and 150.4 (Cq each, C2 and C6), 148.5 and 141.7 (Cq each,
C3″ and C5″), 142.9 and 133.5 (q and Cq each, J = 39.3 Hz and J =
41.4 Hz, C3′ and C5′), 120.6 and 119.2 (q and Cq each, J = 266.9 Hz
and J = 267.2 Hz, CF3), 141.8, 117.1, and 116.0 (pyridyl CH), 109.7
(C4′), 108.6 (C4″), 13.12 and 13.11 (methyl of pyrazolyl). 19F{1H}
NMR (CD3COCD3, 400 MHz): δ −58.9 and −63.7 (s each, CF3).
HRMS: calcd for C15H11F6N5 375.0919, found 375.0926.
X-ray Crystallographic Studies. Red-brown single crystals of
complex 6 suitable for an X-ray structural determination were obtained
by recrystallization in CHCl3/cyclohexane (1/3 v/v) at −20 °C. The
X-ray single-crystal structure of compound 6 was determined 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
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 data and
refinement details for 6 are given in the Supporting Information.
Synthesis of Compound 2. Under a nitrogen atmosphere, a
mixture of 1 (4.02 g, 16.0 mmol) and 10 mL of hydrazine hydrate in
10 mL of nBuOH was stirred at 120 °C for 20 h. After the mixture was
cooled to ambient temperature, all of the volatiles were removed under
reduced pressure. Purification of the resultant residue by silica gel
column chromatography (eluent petroleum ether (60−90 °C)/ethyl
acetate, 5/1 v/v) afforded the desired product 2 as a brown solid (2.09
g, 65% yield). Mp: 160−162 °C. 1H NMR (CDCl3, 400 MHz): δ 7.48
(t, J = 8.2 Hz and J = 7.9 Hz, 1 H, 4-H), 7.06 and 6.43 (d each, J = 8.2
Hz and J = 7.7 Hz, 1:1 H, 3-H and 5-H), 6.24 (s, 1 H, 1′-H), 5.92 (s, 1
H, 4″-H), 3.90 (s, 2 H, 2′-H), 2.53 and 2.24 (s each, 3:3 H, methyl of
pyrazolyl). 13C{1H} NMR (CDCl3, 100 MHz): δ 159.9 and 151.8 (Cq
each, C2 and C6), 149.5 and 140.9 (Cq each, C3″ and C5″), 139.5,
108.6, and 105.6 (pyridyl CH), 103.7 (C4″), 14.5 and 13.6 (methyl of
pyrazolyl). HRMS: calcd for C10H13N5 203.1171, found 203.1174.
Synthesis of Compound 3. Under a nitrogen atmosphere, a
mixture of 2 (2.17 g, 10.8 mmol), 1,1,1,5,5,5-hexafluoropentane-2,4-
dione (1.87 g, 9.0 mmol), and CF3COOH (75 μL, 5 mol %) in 50 mL
of THF was stirred under reflux for 12 h. After the mixture was cooled
to ambient temperature, all of the volatiles were removed under
reduced pressure. Purification of the resultant residue by silica gel
column chromatography (eluent petroleum ether (60−90 °C)/ethyl
acetate, 10/1 v/v) afforded the desired product 3 as a white solid (3.36
Synthesis of Complex 5. Under a nitrogen atmosphere, a mixture
of RuCl3·xH2O (341 mg, 1.3 mmol) and 4 (488 mg, 1.3 mmol) in
ethanol (20 mL) was refluxed for 5 h. After the mixture was cooled to
ambient temperature, the solid was filtered off, rinsed with diethyl
ether (3 × 10 mL), and dried under vacuum to afford 5 as a red-brown
powder (644 mg, 85% yield). Mp: >300 °C dec. Anal. Calcd for
C15H11Cl3F6N5Ru: C, 30.92; H, 1.90; N, 12.02. Found: C, 30.88; H,
1.85; N, 12.09. HRMS (ESI-TOF): calcd for C15H11Cl3F6N5Ru, [M −
Cl]+ 546.9339, found 546.9343; calcd for [M + Na]+ 604.8925, found
604.8874.
Synthesis of Complex 6. Under a nitrogen atmosphere, a mixture
of 5 (410 mg, 0.7 mmol), PPh3 (184 mg, 0.7 mmol), and triethylamine
(1.7 mL) in EtOH (15 mL) was stirred at 60 °C for 3 h. After the
mixture was cooled to ambient temperature, all of the volatiles were
removed under reduced pressure. Purification of the resultant residue
by silica gel column chromatography (eluent dichloromethane/
methanol, 40/1 v/v) afforded crude 6, which was further purified by
recrystallization in CHCl3/n-hexane (1/3 v/v) at −20 °C to give 6 as a
1
deep red-brown solid (312 mg, 55% yield). Mp: >300 °C dec. H
NMR (CD2Cl2, 400 MHz): δ 7.71 (m, 1 H, 4-H), 7.56 and 7.47 (d
each, J = 7.7 Hz and J = 7.3 Hz, 1:1 H, 3-H and 5-H), 7.32 (s, 1 H, 4′-
H), 7.30−7.03 (m, 15 H, PPh3), 6.12 (s, 1 H, 4″-H), 2.83 and 2.60 (s
each, 3:3 H, methyl of pyrazolyl). 13C{1H} NMR (CD2Cl2, 100 MHz):
δ 160.3 and 155.0 (Cq each, C2 and C6), 153.1 and 146.4 (Cq each,
C3″ and C5″), 148.5 and 136.0 (q and Cq, J = 43.0 Hz and J = 42.4
Hz, C3′ and C5′), 137.1, 116.6, and 115.8 (pyridyl CH), 135.0 (C4′),
134.9, 131.4, and 129.7 (CH of PPh3), 134.6 (Cq, PPh3), 120.5 and
120.0 (q and Cq, J = 270.8 Hz and J = 268.6 Hz, CF3), 110.4 (C4″),
16.9 (s) and 16.2 (s) (methyl of pyrazolyl). 19P{1H} NMR (CD2Cl2,
400 MHz): δ −58.40 and −58.42 (s each, CF3). 31P{1H} NMR
(CD2Cl2, 162 MHz): δ 38.9 (s, PPh3). Anal. Calcd for
C33H26Cl2F6N5PRu: C, 48.96; H, 3.24; N, 8.65. Found: C, 48.90; H,
3.22; N, 8.69.
Synthesis of Compound 8. Under a nitrogen atmosphere, a
mixture of 2,6-dibromopyridine (7; 23.70 g, 100.0 mmol) and 25 mL
of hydrazine hydrate in 25 mL of nBuOH was stirred at 120 °C for 12
h. After the mixture was cooled to ambient temperature, all of the
volatiles were removed under reduced pressure to give a crude product
which was subject to recrystallization in EtOAc/methanol (5/1 v/v) at
−4 °C to afford 8 as a brown solid (12.53 g, 67% yield). Mp: 109−111
°C. 1H NMR (CD3COCD3, 400 MHz): δ 8.63 (s, 1 H, 1′-H), 7.44 (t,
1 H, J = 8.3 Hz and J = 7.4 Hz, 4-H), 7.09 and 6.83 (d each, J = 8.3 Hz
and J = 7.4 Hz, 1:1 H, 3-H and 5-H), 4.05 (s, 2 H, 2′-H). 13C{1H}
NMR (CD3COCD3, 100 MHz): δ 158.2 and 139.1 (Cq each, C2 and
C6), 140.2 (C4), 117.4 and 105.3 (C3 and C5). HRMS: calcd for
C5H6BrN3 186.9745, found 186.9749.
1
g, 95% yield). Mp: 108−110 °C. H NMR (CD3COCD3, 400 MHz):
δ 7.95 (t, J = 8.1 Hz and J = 8.0 Hz, 1 H, 4-H), 7.50 and 7.32 (d each, J
= 8.0 Hz and J = 8.2 Hz, 1:1 H, 3-H and 5-H), 6.06 (s, 1 H, 4″-H),
3.91 and 3.61 (d each, J = 19.6 Hz, 1:1 H, 4′-H), 3.43 (s, 1 H, OH),
2.56 and 2.21 (s each, 3:3 H, methyl of pyrazolyl). 13C{1H} NMR
(CD3COCD3, 100 MHz): δ 153.1 and 150.8 (Cq each, C2 and C6),
149.7 and 140.6 (Cq each, C3″ and C5″), 139.9 and 94.4 (q and Cq
each, J = 39.0 and J = 33.4 Hz, C3′ and C5′), 123.4 and 120.1 (q, Cq, J
= 284.4 Hz and J = 267.7 Hz, CF3), 140.9, 111.1, and 109.5 (pyridyl
CH), 109.0 (C4″), 42.2 (C4′), 12.77 and 12.73 (methyl of pyrazolyl).
19F{1H} NMR (CD3COCD3, 400 MHz): δ −68.0 and −81.0 (s each,
CF3). HRMS: calcd for C15H13F6N5O 393.1024, found 393.1029.
Synthesis of Compound 4. Under a nitrogen atmosphere, a
mixture of 3 (1.97 g, 5.0 mmol) and concentrated H2SO4 (ca. 1 mL)
in 20 mL of HOAc was stirred under reflux for 5 h. After the mixture
was cooled to ambient temperature, all of the volatiles were removed
under reduced pressure. The residue was neutralized by saturated
aqueous Na2CO3, washed with 20 mL of water, and then extracted by
dichloromethane (3 × 10 mL). All of the volatiles were removed under
reduced pressure. Purification of the resultant residue by silica gel
column chromatography (eluent petroleum ether (60−90 °C)/ethyl
acetate, 25/1 v/v) afforded 4 as a white solid (1.69 g, 90% yield). Mp:
Synthesis of Compound 9. Under a nitrogen atmosphere, a
mixture of 8 (8.98 g, 40.0 mmol), 1,1,1,5,5,5-hexafluoropentane-2,4-
dione (8.08 g, 40.0 mmol), and CF3COOH (0.3 mL, 5 mol %) in 100
mL of THF was stirred under reflux for 12 h. After the mixture was
cooled to ambient temperature, all of the volatiles were removed under
reduced pressure. Purification of the resultant residue by silica gel
column chromatography (eluent petroleum ether (60−90 °C)/diethyl
ether, 50/1 v/v) afforded 9 as a white solid (14.63 g, 97% yield). Mp:
1
33−34 °C. H NMR (CD3COCD3, 400 MHz): δ 7.74 (t, J = 9.6 Hz
and J = 8.0 Hz, 1 H, 4-H), 7.40 and 7.27 (d each, J = 9.6 Hz and J =
7.7 Hz, 1:1 H, 3-H and 5-H), 3.89 and 3.58 (d each, J = 19.6 Hz, 1:1
H, 4′-H), 3.12 (s, 1 H, OH). 13C{1H} NMR (CD3COCD3, 100
MHz): δ 154.5 and 137.8 (Cq each, C2 and C6), 141.2 and 94.3 (q
G
dx.doi.org/10.1021/om401144u | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
and Cq each, J = 39.5 Hz and J = 34.2 Hz, C3′ and C5′), 123.2 and
120.1 (q, J = 284.5 Hz and J = 226.7 Hz, CF3), 141.4, 122.1, and 110.3
(pyridyl CH), 42.0 (C4′). 19F{1H} NMR (CD3COCD3, 400 MHz): δ
−67.5 and −79.7 (s each, CF3). HRMS: calcd for C10H6BrF6N3O
376.9598, found 376.9596.
were evaporated under reduced pressure, and the resultant residue was
subject to purification by silica gel column chromatography (eluent
petroleum ether (60−90 °C)/ethyl acetate, 5/1 v/v) to afford the
desired product 13 as a white solid (382 mg, 93% yield). Mp: 129−
1
130 °C. H NMR (CD3COCD3, 400 MHz): δ 8.68 (d, J = 3.9 Hz, 1
H, 4″-H), 8.18 (t, J = 8.0 Hz, 1 H, 4-H), 8.02 and 7.85 (d each, J = 8.0
Hz and J = 8.1 Hz, 1:1 H, 3-H and 5-H), 7.65 and 7.39 (m each, 2:2 H,
aromatic CH of benzimidazolyl), 3.86 (N-Me). 13C{1H} NMR
(CD3COCD3, 100 MHz): δ 150.8 and 148.6 (Cq each, C2 and
C6), 148.6 (C4″), 143.4, 141.4, and 136.0 (Cq each, C2′, C4′ and
C9′), 142.3 and 133.3 (q and Cq each, J = 37.9 Hz and J = 40.1 Hz,
C3″ and C5″), 139.8, 125.5, and 123.4 (pyridyl CH), 120.4 and 119.1
(q and Cq, J = 268.5 Hz and J = 268.7 Hz, CF3), 122.5, 120.0, 118.6,
and 110.4 (aromatic CH of benzimidazolyl), 30.15 (N-Me). 19F{1H}
NMR (CD3COCD3, 400 MHz): δ −57.6 and −62.0 (s each, CF3).
HRMS: calcd for C18H11F6N5 411.0919, found 411.0921.
Typical Procedure for the Catalytic Transfer Hydrogenation
of Ketones. The catalyst solution was prepared by dissolving complex
6 (32.3 mg, 0.04 mmol) in 2-propanol (20.0 mL). Under a nitrogen
atmosphere, a mixture of ketone (2.0 mmol), 2.0 mL of the catalyst
solution (0.004 mmol), and 2-propanol (17.4 mL) was stirred at 82 °C
for 10 min. Then, 0.6 mL of 0.1 M iPrOK (0.06 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 concentrated 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.
Synthesis of Compound 10. Under a nitrogen atmosphere, a
mixture of 9 (7.34 g, 20.0 mmol) and concentrated H2SO4 (ca. 2 mL)
in 40 mL of HOAc was stirred at 120 °C for 5 h. After the mixture was
cooled to ambient temperature, all of the volatiles were removed under
reduced pressure, and the resultant residue was neutralized by
saturated aqueous Na2CO3, washed with 20 mL of water, and
extracted with dichloromethane (3 × 15 mL). All of the volatiles were
removed under reduced pressure, and the resulting residue was subject
to purification by silica gel column chromatography (eluent petroleum
ether (60−90 °C)/diethyl ether, 100/1 v/v) to afford 10 as a white
solid (7.04 g, 98% yield). Mp: 45−46 °C. 1H NMR (CD3COCD3, 400
MHz): δ 8.06 (t, J = 7.9 Hz and J = 7.7 Hz, 1 H, 4-H), 7.98 and 7.79
(d each, J = 8.0 Hz and J = 7.7 Hz, 1:1 H, 3-H and 5-H), 7.55 (s, 1 H,
4′-H). 13C{1H} NMR (CD3COCD3, 100 MHz): δ 149.9 and 139.0
(Cq each, C2 and C6), 143.1 and 134.0 (q and Cq each, J = 39.8 Hz
and J = 42.2 Hz, C3′ and C5′), 142.2, 128.8, and 115.5 (pyridyl CH),
120.4 and 119.1 (q and Cq, J = 267.0 Hz and J = 267.0 Hz, CF3),
109.9 (C4′). 19F{1H} NMR (CD3COCD3, 400 MHz) δ −59.4 and
−64.0 (s each, CF3). HRMS: calcd for C10H4BrF6N3 358.9493, found
358.9495.
Synthesis of Compound 11. To a stirred mixture of 10 (2.87 g,
8.0 mmol) in 100 mL of THF was added 4.0 mL of nBuLi solution
(2.4 M in hexanes, 9.6 mmol) at −78 °C over a period of 30 min. After
the mixture was stirred at −78 °C for another 30 min, anhydrous DMF
(1.3 mL, 16.7 mmol) was added. The mixture was warmed to 0 °C
within 2 h and quenched with methanol (10 mL). Saturated aqueous
NH4Cl (100 mL) was added, and the aqueous phase was extracted
with CH2Cl2 (3 × 50 mL). The combined organic phase was dried
over anhydrous Na2SO4 and filtered. All of the volatiles were removed
under reduced pressure, and the resultant residue was subject to
purification by silica gel column chromatography (eluent petroleum
ether (60−90 °C)/diethyl ether, 10/1 v/v) to afford 11 as a white
solid (1.48 g, 60% yield). Mp: 57−58 °C. 1H NMR (CD3COCD3, 400
MHz): δ 10.26 (s, 1H, CHO), 8.66 (m, 1 H, 4′-H), 8.20 (m, 1 H, 4-
H), 7.92 and 7.69 (d each, J = 7.4 Hz and J = 8.1 Hz, 1:1 H, 3-H and
5-H). 13C{1H} NMR (CD3COCD3, 100 MHz): δ 181.5 (CHO),
150.4 and 148.7 (Cq each, C2 and C6), 148.7 (C4′), 141.6 and 135.1
(q and Cq each, J = 40.0 Hz and J = 42.3 Hz, C3′ and C5′), 139.8,
125.8, and 119.4 (pyridyl CH), 120.1 and 119.0 (q and Cq, J = 268.0
Hz and J = 269.5 Hz, CF3). 19F{1H} NMR (CD3COCD3, 400 MHz) δ
−55.8 and −63.0 (s each, CF3). HRMS: calcd for C11H5F6N3O
309.0337, found 309.0337.
Synthesis of Compound 12. A mixture of 11 (1.30 g, 4.2 mmol)
and 1,2-phenylenediamine (454 mg, 4.2 mmol) in nitrobenzene (20
mL) was stirred at 150 °C for 10 h. All of 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, 4/1 v/v) to afford 12 as a
white solid (917 mg, 55% yield). Mp: 223−224 °C. 1H NMR
(CD3COCD3, 400 MHz): δ 12.91 (s, 1 H, N−H), 8.62 (d, J = 3.8 Hz,
4″-H), 8.15 (m, 1 H, 4-H), 7.94 and 7.61 (d and m, J = 8.1 Hz, 1:1 H,
3-H and 5-H), 7.74 and 7.34 (m each, 2:2 H, aromatic CH of
benzimidazolyl). 13C{1H} NMR (CD3COCD3, 100 MHz): δ 150.7
and 140.0 (Cq each, C2 and C6), 148.6 (C4″), 142.0 and 132.9 (q and
Cq each, J = 37.9 Hz and J = 40.2 Hz, C3″ and C5″), 139.8, 125.4, and
118.6 (pyridyl CH), 120.4 and 119.1 (q and Cq, J = 268.6 Hz and J =
269.1 Hz, CF3), 123.09 (broad, benzimidazlyl). 19F{1H} NMR
(CD3COCD3, 23 °C, 400 MHz): δ −56.8 and −61.5 (s each, CF3).
HRMS: calcd for C17H9F6N5 397.0762, found 397.0772.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and a CIF file giving experimental
procedures, analytical data, and NMR spectra of the new
compounds and X-ray crystallographic data for 6. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for Z.K.Y.: zkyu@dicp.ac.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the National Natural Science Foundation of
China (21272232).
■
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dx.doi.org/10.1021/om401144u | Organometallics XXXX, XXX, XXX−XXX