Ruthenium(II) complex catalysts bearing a pyridyl-supported pyrazolyl-imine ligand for transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.jorganchem.2009.05.028

A new class of hemilabile unsymmetrical 2-(1-arylimino)-6-(pyzazol-1-yl)pyridine ligands and their ruthenium(II) and nickel(II) NNN complexes were synthesized. The Ru(II) complex catalysts have been fully characterized and exhibited good to excellent catalytic activity in the transfer hydrogenation (TH) of ketones in refluxing 2-propanol. These results have demonstrated rare examples of active ruthenium(II) NNN complex catalysts that do not feature a N-H functionality for TH of ketones.

Full text of DOI:10.1016/j.jorganchem.2009.05.028

Journal of Organometallic Chemistry 694 (2009) 3068–3075
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
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Ruthenium(II) complex catalysts bearing a pyridyl-supported pyrazolyl-imine
ligand for transfer hydrogenation of ketones
c
a
Miao Zhao a,b, Zhengkun Yu b, , Shenggang Yan , Yang Li
*
a Department of Polymer Materials, College of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116012, PR China
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, PR China
c Electromechanics and Materials Engineering College, Dalian Maritime University, Dalian, Liaoning 116026, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of hemilabile unsymmetrical 2-(1-arylimino)-6-(pyzazol-1-yl)pyridine ligands and their
ruthenium(II) and nickel(II) NNN complexes were synthesized. The Ru(II) complex catalysts have been
fully characterized and exhibited good to excellent catalytic activity in the transfer hydrogenation (TH)
of ketones in refluxing 2-propanol. These results have demonstrated rare examples of active ruthe-
nium(II) NNN complex catalysts that do not feature a N–H functionality for TH of ketones.
Ó 2009 Elsevier B.V. All rights reserved.
Received 20 March 2009
Received in revised form 18 May 2009
Accepted 21 May 2009
Available online 27 May 2009
Keywords:
Arylimino
Pyrazolylpyridine
Ruthenium
Transfer hydrogenation
Ketones
1. Introduction
paid much attention due to their potential applications in homoge-
neous catalysis, organic synthesis, materials and physical chemis-
Transfer hydrogenation (TH) with 2-propanol as the hydrogen
source is the most promising alternative method to replace hydro-
genation for reduction of ketones to alcohols [1]. Ruthenium(II)
complexes have proven to be the efficient catalysts for this pur-
pose. Among the most efficient catalyst systems are the Ru(II) com-
plexes bearing a monotosylated 1,2-diamine or aminoalcohol
ligand discovered by Noyori and co-workers, which can offer high
catalytic activity and selectivity due to a N–H effect [2]. Following
their pioneering work, a lot of related ligands and transition metal
complexes have been reported in this area [3,4]. Baratta et al. re-
cently reported highly active ruthenium(II) 2-(aminomethyl)pyri-
dine (ampy) phosphane complex catalysts for TH of ketones
which have shown an acceleration effect by the N–H functionality
in the ampy ligand [5–8]. Several Ru(II) complex catalysts featuring
no N–H functionality have also been documented for TH of ketones
[9–12]. In order to construct highly active transition metal com-
plex catalysts, development of versatile ligands has been strongly
desired. Nitrogen-containing heterocyclic ligands have been paid
more and more attention in the fields of coordination chemistry,
homogeneous catalysis and organic synthesis because organome-
tallic complexes bearing nitrogen donor ligands usually exhibit
high reactivities. Planar tridentate NNN ligands have recently been
try [13–15]. However, examples of unsymmetrical planar
tridentate NNN ligands and their transition metal complexes have
only been scattered [16–20]. Recently, we found that phosphine-
free unsymmetrical pyridyl-supported 2,6-(mixed N-heterocycles)
ligands can demonstrate a dynamic ‘‘on and off” chelating effect for
the metal center during catalysis, and their transition metal com-
plexes usually exhibit enhanced catalytic activity and selectivity
[21–26]. In our complex catalysts, although pyrazolyls themselves
are poor ligands to late transition metals, they can exhibit both
suitable electron-donating ability (coordination ability) to stabilize
the metal center and hemilability to enhance the catalytic activity
of the complex catalysts bearing a planar pyrazoly-containing NNN
ligand. Highly active Ru(II) complex catalysts such as 1–3 have
been developed from our laboratories (Chart 1). During the course
to extend the generality of our protocol to construct highly active
Ru(II) NNN complex catalysts, imino was applied to act as a coor-
dinating arm in the unsymmetrical planar pyridyl-supported pyr-
azolyl-N-moiety ligands. Bis(imino)pyridines have been well
known to construct complex catalysts for olefin polymerization
and organic synthesis [27]. In other cases, an imino functionality
is usually used to construct a bidentate ligand [28], although rare
examples of planar NNX ligands containing an imino functionality
have also been documented [29]. Herein, we report synthesis of
ruthenium(II) complexes bearing a unsymmetrical 2-(1-arylimi-
no)-6-(pyzazol-1-yl)pyridine ligand and their catalytic activity in
transfer hydrogenation of ketones.
* Corresponding author. Tel./fax: +86 411 8437 9227.
E-mail address: zkyu@dicp.ac.cn (Z. Yu).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.05.028

M. Zhao et al. / Journal of Organometallic Chemistry 694 (2009) 3068–3075
3069
signal of 8a in CD3OD appeared at 50.3 ppm. For complex 8b, it
showed a much better solubility in CDCl3 than 8a due to introduc-
tion of a methyl group to the imino moiety and its 31P{1H} NMR
signal was shown at 42.7 ppm. Although pure ligand 7 was ob-
tained, its pure Ru(II) complex 9 could not be successfully isolated
because it was easily oxidized to 8a by air in solution. Thus, com-
plex 9 was always obtained as a mixture containing 8a.
Me
Me
Me
Me
Me
Me
Me
Me
H
N
Cl
N
N
N
N
N
N
N
N
N
N
N
N
L
Cl
N
N
Ru
Ru
Cl
Ru
Cl
Ph3P
Ph3P
Ph3P
Cl
L = DMSO
1 [23]
2 [24]
3 [26]
Chart 1.
2.2. Synthesis of ligand 6c and complex 11
In a fashion similar to the synthesis of ligands 6a and 6b, rela-
tively bulky ligand 6c was prepared in 72% yield by condensation
of 5b with 2,6-diisopropylaniline (Scheme 2). Unexpectedly, under
the same conditions employed for synthesis of complexes 8 and 9,
ligand 6c did not react with RuCl2(PPh3)3 to form the desired com-
plex 10, which is attributed to the steric hindrance from the methyl
group on the imino group and the two isopropyls on the aryl moi-
ety. However, treatment of 6c with NiCl2ꢀ6H2O in THF afforded
Ni(II) complex 11 in 90% yield. The NMR spectra of 11 in solution
could not be obtained due to its paramagnetic property. The IR
stretching vibrations of the C@N bonds in the imino, pyrazolyl
and pyridyl moieties and the aromatic C@C bonds in complex 11
varied from 12 to 100 cmꢁ1 as compared with those of the free
ligand 6c. The red shift phenomena of the C@N bonds suggest coor-
dination of the three N-donor atoms to the metal center. An
analogous complex of 11 was also obtained from the 1:1 molar ra-
tio reaction of 6a and NiCl2ꢀ6H2O in THF, but its X-ray single crystal
structure was not successfully determined. The stretching vibra-
2. Results and discussion
2.1. Synthesis of ligands 6 and 7, complexes 8 and 9
Aldehyde 5a and ketone 5b were synthesized by reaction of 2-
bromo-6-(3,5-dimethylpyrazol-1-yl)pyridine [26] with n-BuLi in
THF followed by treatment with N,N-dimethylformamide (DMF)
or N,N-dimethylacetamide (DMA). Condensation of 5 with 4-tolui-
dine in the presence of p-TsOH and 4A molecular sieves in refluxing
toluene afforded 2-(1-tolylimino)-6-(3,5-dimethylpyrazol-1-yl)pyri-
dines 6 in 83–90% yields. Reduction of 6a with NaBH4 in methanol
produced 2-(aminomethyl)-6-(3,5-dimethylpyrazol-1-yl)pyridine
7 (Scheme 1). Reactions of ligands 6 and 7 with RuCl2(PPh3)3 in
refluxing toluene gave complexes 8 and 9, respectively. Complex
8b was only isolated in 20% yield presumably due to the introduc-
tion of a methyl group to the imino moiety which affects the
coplanarity of the NNN ligand 6b in the complex. Complexes 8
are purple and 9 is yellow in the solid state and solution. Exposure
of a yellow solution of 9 in CH2Cl2 to air with stirring at room tem-
perature for 2 h led to a purple solution containing 8a. During the
purification of 9 by flash silica gel column chromatography in air,
transformation of yellow 9 to purple 8a was always accompanied
that pure complex 9 could not be obtained. It has been known that
cationic and pincer-type Ru(II) amine complexes can be oxidized to
Ru(II) imino complexes by oxygen [30,31]. Complex 8a is bestowed
with a poor solubility in organic solvents and its satisfied 13C{1H}
NMR spectrum was not successfully collected. The 31P{1H} NMR
tion of the C@N bond in this complex appeared at 1578 cmꢁ1
red-shifted by 15 cmꢁ1 as compared with that of 11.
,
2.3. X-ray crystallographic studies
It was very difficult to obtain single crystals of complex 8a.
Eventually, its single crystals suitable for the X-ray single crystal
structure determination was grown by vapor diffusion of diethyl
ether into a saturated solution of 8a in CH3CN at ambient temper-
ature. The single crystals of complex 11 were collected by vapor
Me
Me
Me
(i)
(ii)
Me
R
N
N
N
Br
N
N
N
Me
N
N
N
O
O
4
R =: H (5a)
Me (5b)
Me
Me
5b
Me
Me
(i)
Me
(iii)
R = H
R
N
N
N
N
N
N
Me
iPr
N
HN
N
N
N
N
Me
Me
Me
Me
6c
Me
R =: H (6a), Me (6b)
7
iPr
(iv)
(v)
(ii)
.
RuCl2(PPh3)2
NiCl2
6H2O
N
X
(iii)
Me
Me
Me
Me
air
R
N
N
N
N
N
N
Me
Me
Me
R = H
Cl
Cl
Me
Me
iPr
N
NH
Ru
Cl
Ru
Cl
9
N
N
N
N
N
iPr
Cl
N
N
Ph3P
Ph3P
Ru
Ni
Cl
Me
Me
Me
Cl
Ph3P
R =: H (8a), Me (8b)
Cl
iPr
iPr
o
Conditions: (i) n-BuLi, THF -78 − 0 C, DMF for 5a (48%), DMA
for 5b (50%). (ii) 4-toluidine, p-TsOH, 4A MS, toluene, reflux, 48 h,
90% for 6a, 83% for 6b. (iii) NaBH4, MeOH, r.t. to reflux, 54%. (iv)
1.0 equiv RuCl2(PPh3)3, toluene, N2, 110 oC, 2 h, 80% for 8a; 24 h,
20% for 8b. (v) RuCl2(PPh3)3, toluene, N2, 110 oC, 2 h, 72%.
10
11
Conditions: (i) 2,6-diisopropylaniline, p-TsOH, 4A MS,
toluene, reflux, 48 h, 72%. (ii) RuCl2(PPh3)3, 1.0 equiv.;
o
.
toluene, N2, 110 C, 2 h. (iii) NiCl2 6H2O, 1.0 equiv.;
THF, r.t., 4 h, 90%.
Scheme 1. Synthesis of ligands 6a, b and 7, complexes 8 and 9.
Scheme 2. Synthesis of ligand 6c and complex 11.

3070
M. Zhao et al. / Journal of Organometallic Chemistry 694 (2009) 3068–3075
Table 1
Table 2
Crystallographic date and refinement details for 8a and 11.
Selected bond distances (Å) and angles (°) for 8a and 11.
Complex 8a
8aꢀ0.5OEt2
11ꢀH2O
Ru(1)–N(1)
1.937(7)
2.295(3)
179.0(2)
Ru(1)–N(2)
2.126(7)
2.438(3)
155.3(3)
88.86(10)
92.7(2)
Ru(1)–
N(3)
2.063(8)
Empirical formula
Formula weight
T (K)
C38H38Cl2N4O0.50PRu C24H32Cl2N4NiO
761.66
293(2)
Ru(1)–P(1)
Ru(1)–Cl(2)
N(2)–
N(6)
1.335(12)
522.15
293(2)
N(1)–Ru(1)–
Cl(2)
N(2)–Ru(1)–
N(3)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P2(1)/c
15.5402(11)
31.771(2)
14.6564(10)
90
Monoclinic
P2(1)/c
9.5307(17)
13.585(3)
10.2132(19)
90
P(1)–Ru(1)–
Cl(1)
176.10(10) Cl(1)–Ru(1)–
Cl(2)
N(1)–Ru(1)–
Cl(1)
90.2(2)
P(1)–Ru(1)–
N(1)
a
(°)
b (°)
105.545(2)
90
6971.6(9)
8
1.451
0.684
103.712(3)
90
1284.7(4)
2
1.350
0.986
Complex 11
Ni–N(1)
Ni–Cl(2)
N(1)–Ni–N(2) 152.50(13) N(2)–Ni–N(3)
N(1)–Ni–
Cl(1)
c
(°)
1.992(4)
2.248(2)
Ni–N(2)
C(2)–N(6)
2.174(4)
1.280(7)
151.60(17)
112.51(7)
Ni–N(3)
2.107(5)
V (Å3)
Z
Dc (g cmꢁ3
)
94.98(13)
Cl(1)–Ni–
Cl(2)
l
(mmꢁ1
)
F (0 0 0)
3128
548
Crystal size (mm3)
0.48 ꢂ 0.28 ꢂ 0.06
1.81–25.50
36 583
12 945
0.1433
0.51 ꢂ 0.42 ꢂ 0.06
2.05–27.00
7373
4893
0.0847
3927
303
0.968
0.0685/0.0572
0.1433/0.1378
0.591 (ꢁ0.514)
h Limits (°)
Number of data collected
Number of unique data
Rint
2.13, and 2.06 Å, respectively, indicating that these N-donor atoms
are not equally coordinated to the metal (Table 2). The pyridyl
nitrogen, Ru(1) and Cl(2) atoms are nearly linearly positioned
(N(1)–Ru(1)–Cl(2), 179.0°) and the N(2)–Ru(1)–N(3) angle is
155.3°. The nickel atom in complex 11 is in a distorted pyramidal
Number of data observed with I > 2
Number of refined parameters
Goodness-of-fit (GOF) on F2
R (all data/observed data)
wR2 (ll data/observed data)
r
(I) 5481
822
0.850
0.1575/0.0968
0.2544/0.2156
1.802 (ꢁ1.632)
environment and also coordinated by the three
r-donor nitrogen
Residual qmax (e Åꢁ3
)
atoms (Fig. 2). The three Ni–N bonds are longer than those corre-
sponding Ru–N bonds in 8a (Table 2), suggesting that structure
of 11 is more loose than that of 8a.
diffusion of diethyl ether into a saturated solution of 11 in metha-
nol at ambient temperature. A non-coordinate Et2O molecule was
incorporated in the single crystals of 8a that its highly satisfied
crystallographic date could not be collected (Table 1). In the unit
cell, two differently oriented molecules of 8a shared a diethyl ether
molecule (Fig. 1). 6a acts as a planar tridentate NNN ligand in com-
plex 8a, and the ruthenium atom is situated in a distorted octahe-
dral environment with the PPh3 ligand and one chloride trans to
each other on the two sides of the ligand plane, which is crucial
to provide the complex with high catalytic activity for TH of
ketones in our cases [8b-f]. The unsymmetrical ‘‘pincer”-type
NNN ligand occupies the three meridional sites with the two
N-heterocycles and the imino moiety in a quasi-planar disposition,
and the second chloride is arranged trans to the pyridyl nitrogen
atom. The distances between the metal center and the pyridyl
nitrogen, pyrazolyl nitrogen, and imino nitrogen in 8a are 1.94,
2.4. Catalytic transfer hydrogenation (TH) of ketones
TH of acetophenone to 1-phenylethanol by 2-propanol was
tested with complexes 8 and 9 as the catalysts (Table 3). In a
0.1 M solution in refluxing 2-propanol, reduction of acetophenone
smoothly proceeded to form 1-phenylethanol as the only product
by means of 0.2 mol% complex catalyst in the presence of iPrOK
base. Over a period of 1 h, conversion of acetophenone was reached
96%, 90%, and 93%, respectively, revealing an order of the catalytic
activity for TH of acetophenone: 8a > 9 > 8b. Next, the reduction of
a variety of ketones was carried out by using 8a or 9 as the catalyst,
producing the desired alcohol products in decent yields within
5 min – 5 h, reaching up to 99% conversion for the substrates with
final TOF values up to 5940 hꢁ1 (Table 4, entries 4–5, and 23–24).
Pyridyl ketones showed relatively low reactivity (Table 4, entries
Fig. 1. Molecular structure of 8a incorporated with an Et2O molecule.

M. Zhao et al. / Journal of Organometallic Chemistry 694 (2009) 3068–3075
3071
Table 4
TH of ketones catalyzed by complex 8a.
O
OH
R2
O
OH
Ru(II) cat.
iPrOK
+
+
R1
R2
R1
Entry
1
Ketone
Time (h)
Yielda (%)
Final TOF (hꢁ1
)
O
480
1
96
Me
O
2
3
4
5
1
96
99
99
99
480
990
Me
Cl
O
1/2
1/12
1/12
Me
Br
O
O
Fig. 2. Molecular structure of 11.
5940
5940
Me
Me
21–22), presumably due to the competing coordination of the N-
donor atoms in the substrates to the Ru(II) metal center which thus
decreased the catalyst activity. Basically, complex 8a exhibited an
excellent catalytic activity comparable to that of complex 1 for TH
of ketones in refluxing 2-propanol [23]. However, 8a only demon-
strated a catalytic activity much lower than complex catalysts 2
and 3 in TH of ketones [24,26] because no 16-electron Ru(II) spe-
cies could be formed during the catalytic reactions for 8a and the
less efficient coplanarity of the pyrazolyl-imino-pyridine ligands
of type 6 than pyrazolyl-imidazolyl-pyridine ligands in 2 and 3.
The catalytic activity of complex 8a was also explored at room
temperature (Table 5). With 0.2 mol% 8a as the catalyst, TH of ace-
tophenone at 28 °C under nitrogen atmosphere reached 87% con-
version for the substrate within 7 h, while the conversion was
obviously improved by increasing the catalyst loading to
0.5 mol% (Table 5, entry 1). Thus, TH of chloroacetophenones,
cyclohexanone and 2-heptanone was carried out at room temper-
ature for 4–5 h, forming the corresponding alcohol products in
89–95% yields and revealing a moderate catalytic activity for
complex 8a at room temperature (Table 5). As shown in Table 6,
complex 9 demonstrated a catalytic activity lower than 8a in most
cases, showing no N–H effect of the ligand during the reaction. It is
plausible that ligand 7 may not exist as a planar NNN ligand in
complex 9, resulting in steric/electronic effects which lessen the
catalyst activity. However, complexes 8a and 9 showed no catalytic
Me
OMe O
6
7
8
9
1/6
1/2
1/2
1
97
98
98
97
2910
980
980
485
Me
O
Cl
Me
O
Br
Me
O
Me
Me
O
90b
225
MeO
10
2
Me
O
11
12
13
14
15
1/2
1
98
98
97
94
98
980
490
485
940
245
Me
O
F
Me
O
Cl
Me
1
O
Br
Table 3
Screening of catalysts for TH of acetophenone.
Me
1/2
2
OH
O
OH
O
Ru(II) cat.
iPrOK
O
Me
MeO
+
+
Ph
Ph
Ketone
Me
Entry
1
Cat.
Time (h)
1
Yielda (%)
96
O
O
O
O
O
16
17
3
5
96b
160
99
8a
Me
Me
Me
99b,c
2
3
8b
9
1
1
90
93
O
18
5
60c
60
Reaction conditions: ketone/cat./iPrOK = 500/1/25. Ketone, 2.0 mmol (0.1 M in
20 mL iPrOH); 0.1 MPa, 82 °C.
a
GC yield of the corresponding alcohol.

3072
M. Zhao et al. / Journal of Organometallic Chemistry 694 (2009) 3068–3075
Table 4 (continued)
by the reaction of 8 with iPrOK base to form a Ru(II)–alkoxide spe-
Entry
Ketone
Time (h)
1
Yielda (%)
67c
Final TOF (hꢁ1
)
cies which then undergoes b-H elimination to form a RuH species
and acetone. Such a RuH species is presumably considered as the
catalytically active species although it was not successfully
isolated by reacting 8 or 9 with EtONa or iPrOK in refluxing ethanol
or 2-propanol. Formation of RuH complexes from Ru–Cl precursors
has been reported [33], and the in situ generated RuH species have
been known to act as the active catalysts for TH of ketones
[34,35]. The ketone substrate is then reduced to alcohol on the
RuH species.
O
19
335
O
20
1/2
95
950
N
N
3. Summary
21
22
24
24
65
70
14
15
O
O
In summary, ruthenium(II) NNN complexes bearing a unsym-
metrical pyrazolyl-imino-pyridine ligand have been synthesized
and exhibited good to excellent catalytic activity in transfer hydro-
genation of ketones in refluxing 2-propanol. These results have
demonstrated rare examples of active ruthenium(II) NNN complex
catalysts that do not feature a N–H functionality for efficient TH of
ketones.
N
O
23
24
25
1/12
1/12
1
99
99
99
5940
5940
495
O
4. Experimental
O
4.1. General considerations
Me
5
Unless otherwise noted, all the starting materials were com-
mercially available and used without further purification. The cat-
alytic reactions were carried out under a nitrogen atmosphere. All
Reaction conditions: ketone/cat./iPrOK = 500/1/25. Ketone, 2.0 mmol (0.1 M in
20 mL iPrOH); 0.1 MPa, 82 °C.
a
GC yield of the corresponding alcohol.
Isolated yields.
b
C
cat. = 0.5 mol%.
Table 6
activity for the TH of ketimines such as butylimine and phenylim-
ine of acetophenone under the conditions shown in Tables 4 and 6.
Although no catalytically active species has been isolated, a
possible mechanism is proposed to explain the TH of ketones
catalyzed by 8 and 9. The present TH reactions may follow an
inner-sphere mechanism [26,32]. Thus, TH of a ketone is initiated
TH of ketones catalyzed by complex 9.
Entry
1
Ketone
Time (h)
Yielda (%)
93
Final TOF (hꢁ1
)
O
1
465
Me
Table 5
Br
O
O
TH of ketones catalyzed by complex 8a at room temperature.
2
3
4
5
6
1/12
1/12
1/3
1/3
2
98
98
99
96
97
5880
5880
1485
1440
243
Me
Me
Entry
Ketone
Time (h)
Yielda (%)
93 (94)b
Me
O
O
1
5
Me
OMe O
Cl
Me
O
2
4
92
Me
O
Cl
3
4
4
4
93
91
Me
Me
Me
Me
O
Br
O
Cl
Me
O
O
5
6
4
4
95
89
7
8
1
1
92
95
460
475
Me
MeO
O
O
Me
5
Me
5
Reaction conditions: ketone/cat./iPrOK = 200/1/25. Ketone, 2.0 mmol (0.1 M in
20 mL iPrOH); 0.1 MPa, 28 °C.
Reaction conditions: ketone/cat./iPrOK = 500/1/25. Ketone, 2.0 mmol (0.1 M in
20 mL iPrOH); 0.1 MPa, 82 °C.
a
GC yield of the corresponding alcohol.
a
b
GC yield of the corresponding alcohol.
7 h.

M. Zhao et al. / Journal of Organometallic Chemistry 694 (2009) 3068–3075
3073
the solvents were dried prior to use according to the standard pro-
cedures. Ligand 5a was synthesized according to the literature pro-
cedure [24]. 1H, 13C{1H} and 31P{1H}NMR spectra were obtained
with a 400 MHz NMR spectrometer.
p-toluenesulfonic acid (0.02 g, 0.1 mmol), and 4A MS (3.00 g) in
30 mL toluene was refluxed with stirring under N2 atmosphere
for 2 days. After filtered, the volatiles were removed under reduced
pressure and the resultant residue was recrystallized from metha-
nol at ꢁ20 °C to give 6a as a yellow solid (0.52 g. 90%). M.p.:
111–112 °C. 1H NMR (CDCl3, 23 °C) d 8.55 (s, 1 H, CH@N), 8.07
and 7.94 (d each, J = 7.4 and 7.9 Hz, 1:1 H, 3-H and 5-H), 7.88
(t, J = 7.6 Hz, 1 H, 4-H), 7.23 (s, 4 H, C6H4), 6.02 (s, 1 H, 40-H),
2.71, 2.39, and 2.32 (s each, 3:3:3 H, 3 ꢂ CH3). 13C{1H} NMR (CDCl3,
23 °C) d 159.5 (Cq, C@NAr), 153.5 and 148.4 (Cq each, C2 and C6),
153.0 and 150.2 (Cq each, C30 and C50), 141.8 and 136.9 (Cq each,
C100 and C400, C6H4), 138.9 (C4), 130.0 and 121.2 (C300 and C200),
118.1 and 117.0 (C3 and C5), 109.4 (C40), 21.2 (C400–CH3), 14.9
and 13.8 (C30–CH3 and C50–CH3). HRMS (EI) Anal. Calc. for
C18H18N4: 290.1531. Found: 290.1525.
4.2. X-ray crystallographic studies
Single crystal X-ray crystallographic studies for compounds 8a
and 11 were carried out on a SMART APEX diffractometer with
graphite-monochromated Mo Ka radiation (k = 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 anisotropi-
cally. 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 are listed in Table 1 and the selected bond lengths and an-
gles are given in Table 2.
4
3
5
Me
5' N
N
2
Me
1"
6
6''
4'
N
N
5''
4''
3'
Me
2''
Me
3''
4
3
5
Me
4.5. Synthesis of {1-[6-(3,5-dimethyl-pyrazol-1-yl)pyridin-2-
yl]ethylidene}-p-tolylamine (6b)
5' N
N
2
Me
6
4'
N
O
3'
In a fashion similar to the synthesis of 6a, reaction of 5b (1.08 g,
5.0 mmol) and p-methylaniline (0.52 g, 5.0 mmol) in refluxing
toluene (40 mL) afforded 6b as a yellow solid (1.26 g, 83%). M.p.:
89–91 °C. 1H NMR (CDCl3, 23 °C) d 8.13 and 7.98 (d each, J = 7.6
and 8.0 Hz, 1:1 H, 3-H and 5-H), 7.87 (t, J = 7.6 Hz, 1 H, 4-H), 7.19
and 6.75 (d each, J = 7.8 Hz, 2:2 H, C6H4), 6.03 (s, 1 H, 40-H), 2.73
(s, 3 H, C400–CH3), 2.37 (s, 3 H, C30–CH3), 2.34 (s, 3 H, N@CCH3),
2.33 (s, 3 H, C50–CH3). 13C{1H} NMR (CDCl3, 23 °C) d 166.9 (Cq,
C@NAr), 154.9 and 150.1 (Cq each, C2 and C6), 152.6 and 148.6
(Cq each, C30 and C50), 141.5 and 133.2 (Cq each, C6H4), 138.8
(C4), 129.7 and 119.4 (2:2 CH, C6H4), 118.1 and 116.5 (C3 and
C5), 109.4 (C40), 21.0 (C400–CH3), 16.7 (N@CCH3), 15.2 and 13.8
(C30–CH3 and C50–CH3). HRMS (EI) Anal. Calc. for C19H20N4:
304.1688. Found: 304.1689.
Me
4.3. Synthesis of 6-(3,5-dimethylpyrazol-1-yl)-2-acetylpyridine (5b)
To a stirred mixture of 4.0 mL n-BuLi (2.5 M in hexanes,
10.0 mmol) and 30 mL THF was added dropwise a solution of
2-bromo-6-(3,5-dimethylpyrazol-1-yl)pyridine (2.51 g, 10.0 mmol)
in 20 mL THF at ꢁ78 °C over a period of 30 min. After the mixture
was further stirred at ꢁ78 °C for 30 min, anhydrous DMA (0.87 g,
10.0 mmol) was added. The mixture was allowed to warm up to
0 °C within one hour and the reaction was then quenched with sat-
urated aqueous NH4Cl (20 mL), and extracted with CH2Cl2
(3 ꢂ 20 mL). The combined organic phase was dried over anhy-
drous MgSO4 and filtered. All the volatiles were removed under re-
duced pressure and the resultant residue was subject to
purification by flash silica gel column chromatography (petroleum
ether (60–90 °C)/ethyl acetate, v/v = 100:1), affording compound
5b as a white solid (1.07 g, 50%). M.p.: 104–106 °C. 1H NMR (CDCl3,
23 °C) d 8.12 and 7.91 (d each, J = 7.6 and 7.8 Hz, 1:1 H, 3-H and
5-H), 7.87 (t, J = 7.6 Hz, 1 H, 4-H), 6.03 (s, 1 H, 40-H), 2.74, 2.69,
and 2.30 (s each, 3:3:3 H, 3 ꢂ CH3). 13C{1H} NMR (CDCl3, 23 °C) d
199.6 (Cq, C@O), 152.9 and 141.7 (Cq each, C2 and C6), 151.3
and 150.5 (Cq each, C30 and C50), 139.3 (C4), 119.0 and 118.4 (C3
and C5), 109.9 (C40), 26.4 (CH3CO), 15.4 and 13.8 (C30–CH3 and
C50–CH3). HRMS (EI) Anal. Calc. for C12H13N3O: 215.1059. Found:
215.1062.
4
3
5
Me
5' N
N
2
Me
iPr
6
4'
6''
1"
N
N
5''
4''
3'
2''
Me
iPr
3''
4.6. Synthesis of (2,6-Diisopropylphenyl)-{1-[6-(3,5-dimethyl-
pyrazol-1-yl)pyridin-2-yl]ethylidene} amine (6c)
In a fashion similar to the synthesis of 6a, reaction of 5b (0.86 g,
4.0 mmol) and 2,6-diisopropylaniline (0.71 g, 4.0 mmol) in reflux-
ing toluene (30 mL) afforded 6c as a yellow solid (1.08 g, 72%) after
purification by flash silica gel column chromatography (petroleum
ether (60–90 °C)/ethyl acetate, v/v = 100:1). M.p.: 151–152 °C. 1H
NMR (CDCl3, 23 °C) d 8.26 and 8.05 (d each, J = 7.6 and 8.1 Hz,
1:1 H, 3-H and 5-H), 7.91 (t, J = 8.1 Hz, 1 H, 4-H), 7.21 (d,
J = 7.3 Hz, 2 H, 300-H and 500-H), 7.14 (t, J = 6.8 Hz, 1 H, 400-H), 6.06
(s, 1 H, 40-H), 2.79 (m, 2 H, 2 ꢂ CH(CH3)2), 2.78 (s, 3 H, N@CCH3),
2.36 (s, 3 H, C30–CH3), 2.23 (s, 3 H, C50–CH3), 1.20 and 1.19 (d each,
J = 6.8 Hz, 12 H, 4 ꢂ CH3). 13C{1H} NMR (CDCl3, 23 °C) d 166.6 (Cq,
C@NAr), 154.3 and 150.0 (Cq each, C2 and C6), 152.8 and 146.3
(Cq each, C30 and C50), 141.4 (Cq, C100), 138.9 (C4), 135.9 (Cq, C200
4
3
5
Me
5' N
N
2
6
6''
4'
1"
N
N
5''
4''
Me
3'
Me
2''
3''
4.4. Synthesis of {6-[(3,5-dimethyl-pyrazol-1-yl)pyridin-2-
yl]methylene}-p-tolyl-amine (6a)
A mixture of 6-(3,5-dimethylpyrazol-1-yl)pyridine-2-carbalde-
hyde (5a) (0.40 g, 2.0 mmol), p-methylaniline (0.22 g, 2.0 mmol),

3074
M. Zhao et al. / Journal of Organometallic Chemistry 694 (2009) 3068–3075
and C600), 123.8 (C400), 123.1 (C300 and C500), 118.0 and 116.5 (C3 and
C5), 109.4 (C40), 28.4 (C(CH3)2), 23.3 and 23.0 (CH3 of C(CH3)2), 17.7
(CH3C@NAr), 15.2 and 13.8 (C30–CH3 and C50–CH3). HRMS (EI) Anal.
Calc. for C24H30N4: 374.2470. Found: 374.2480.
4
5
3
2
Me
4'
5' N
N
Me
6
6''
3''
Cl
1"
N
N
5''
4''
Me
Ru
3'
4
Me
Ph3P
2''
5
Cl
3
Me
5' N
N
2
6
6''
3''
4'
4.9. Synthesis of complex 8b
HN
N
1"
5''
4''
Me
3'
Me
2''
In a fashion similar to the synthesis of 8a, reaction of 6b
(0.20 g, 0.66 mmol) and RuCl2(PPh3)3 (0.63 g, 0.66 mmol) in
refluxing toluene (40 mL) for 24 h afforded complex 8b as a pur-
ple solid (0.10 g, 20%) after purification by flash silica gel column
4.7. Synthesis of [6-(3,5-dimethyl-pyrazol-1-yl)pyridin-2-ylmethyl]-p-
tolylamine (7)
chromatography (CH2Cl2/MeOH, v/v = 30:1). M.p.: >300 °C, dec. 1
H
NMR (CDCl3, 23 °C) d 8.25, 6.83 and 6.05 (br each, 1:1:1 H, 4-H,
3-H and 5-H), 7.43 and 7.39 (d each, J = 8.0 Hz, 1:1 H), and 7.31
and 7.28 (d each, J = 8.0 and 7.8 Hz, 1:1 H) (aromatic CH of
C6H4), 5.98 (s, 1 H, 40-H), 2.68 (s, 3 H, C400–CH3), 2.64 (s, 3 H,
C30–CH3), 2.42 (s, 3 H, CH3C@NAr), 2.30 (s, 3 H, C50–CH3).
13C{1H} NMR (CDCl3, 23 °C) d 170.0 (Cq, C@NAr), 162.4 and
154.2 (Cq each, C2 and C6), 157.5 and 145.1 (Cq each, C30 and
C50), 143.0 (Cq, C100), 136.6 (C4”), 133.3, 129.0 and 127.7 (s each,
CH of 3 ꢂ Ph), 133.2 and 127.6 (2:2 CH of C6H4), 132.7 and 132.2
(Cq and d, PPh3), 130.5 (C4), 121.5 and 113.5 (C3 and C5), 107.9
(C40), 21.4 (C400–CH3), 18.4 (CH3C@NAr), 15.2 and 14.7. (C30–CH3
and C50–CH3). 31P{1H} NMR (CDCl3, 23 °C) d 42.7 (s, PPh3). Anal.
Calc. for C37H35Cl2N4PRu: C, 60.16; H, 4.78; N, 7.59. Found: C,
59.60; H, 4.82; N, 7.54%.
To a stirred solution of 6a (1.17 g, 4 mmol) in 40 mL anhydrous
methanol was added NaBH4 (0.45 g, 12 mmol) in several portions
at 0–5 °C within 0.5 h. After the addition, the mixture was further
stirred for 1 h at room temperature and then refluxed for 2 h. The
mixture was cooled to ambient temperature and the reaction was
quenched with 10 mL aqueous 10% NH4Cl solution. Most of the
methanol was removed under reduced pressure and the resulting
mixture was extracted with CH2Cl2 (3 ꢂ 20 mL), dried over anhy-
drous sodium sulfate, and filtered. All the volatiles were removed
under reduced pressure and the resultant residue was subject to
purification by flash silica gel column chromatography (petroleum
ether (60–90 °C)/ethyl acetate = 50:1), affording compound 7 as a
white solid (0.61 g, 54%). M.p.: 72–74 °C. 1H NMR (CDCl3, 23 °C) d
7.72 (d, J = 4.3 Hz, 2 H, 3-H and 5-H), 7.18 (t, J = 7.9 Hz, 1 H, 4-H),
7.00 and 6.59 (d each, J = 8.0 Hz, 2:2 H, C6H4), 6.01 (s, 1 H, 40-H),
4.59 (s and br, 1 H, NH), 4.43 (s, 2 H, CH2NH), 2.67 (s, 3 H, C400–CH3),
2.31 (s, 3 H, C30–CH3), 2.24 (s, 3 H, C50–CH3). 13C{1H} NMR (CDCl3,
23 °C) d 157.2 and 150.0 (Cq each, C2 and C6), 153.2 and 145.7 (Cq
each, C30 and C50), 141.5 and 127.0 (Cq each, C6H4), 139.0 (C4),
129.9 and 113.3 (2:2 CH, C6H4), 118.4 and 114.0 (C3 and C5),
109.2 (C40), 49.4 (NHCH2), 20.5 (C400–CH3), 14.9 and 13.8
(C30–CH3 and C50–CH3). HRMS (EI) Anal. Calc. for C18H20N4:
292.1688. Found: 292.1682.
4
5
3
2
Me
5' N
N
6
H
6''
3''
4'
Cl
1"
N
N
5''
4''
Me
Ru
3'
Me
Ph3P
2''
Cl
4.10. Synthesis of complex 9
In a fashion similar to the synthesis of complex 8a, reaction of
7 (0.15 g, 0.5 mmol) and Ru(PPh3)3Cl2 (0.48 g, 0.5 mmol) in reflux-
ing toluene (30 mL) for 2 h afforded complex 9 as a yellow solid
(0.26 g, 72%). Complex 9 always coexisted with 8a due to the easy
oxidation of 9 to 8a in solution by air during the work-up process.
M.p.: >260 °C, dec. 1H NMR (CDCl3, 23 °C) d 8.02 (d, J = 20.1 Hz, 1
H, 3-H), 7.82 and 7.67 (m each, 1:1 H, 5-H and 4-H), 7.42 and
6.86 (d each, J = 7.4 Hz, 2:2 H, C6H4), 7.25–7.12 and 7.05–6.93
(m each, 9:6 H, 3 ꢂ Ph), 5.89 (s, 1 H, 40-H), 4.02 (d, J = 15.0 Hz)
and 3.72 (t, J = 13.0 Hz) (1:1 H, CH2NH), 2.82, 2.72 and 2.29
(s each, 3:3:3 H, 3 ꢂ CH3). 31P{1H} NMR (CDCl3, 23 °C) d 53.0
(s, PPh3).
4
5
3
2
Me
5' N
N
6
6''
3''
4'
Cl
1"
N
N
5''
4''
Me
Ru
3'
Me
Ph3P
2''
Cl
4.8. Synthesis of complex 8a
Under nitrogen atmosphere a mixture of 6a (0.32 g, 1.1 mmol)
and RuCl2(PPh3)3 (1.05 g, 1.1 mmol) in 20 mL toluene was refluxed
for 2 h. After cooling to ambient temperature, 15 mL diethyl ether
was added to precipitate the crude product. The solid was filtered
off, washed with diethyl ether and dried in vacuo, affording 8a as a
purple solid (0.70 g, 80%). Single crystals suitable for X-ray crystal-
lographic study were obtained by diffusion of diethyl ether vapor
into a saturated solution of 8a in CH3CN at ambient temperature.
M.p.: >300 °C, dec. 1H NMR (CD3OD, 23 °C) d 8.47 (s,1 H, CH@NAr),
7.79 (t, J = 7.6 Hz, 1 H, 4-H), 7.75 and 7.69 (d each, J = 8.2 and
7.4 Hz, 1:1 H, 3-H and 5-H), 7.48 and 7.24 (d each, J = 8.0 Hz, 2:2
H, C6H4), 7.25, 7.14 and 7.04 (m each, 3:6:6 H, PPh3), 6.31 (s, 1
H, 40-H), 2.83 (s, 3 H, C400–CH3), 2.63 (s, 3 H, C30–CH3), 2.48 (s, 3
H, C50–CH3). 31P{1H} NMR (CD3OD, 23 °C) d 50.4 (s, PPh3). Anal.
Calc. for C36H33Cl2N4PRuꢀ0.5O(C2H5)2: C, 59.92; H, 5.03; N, 7.36.
Found: C, 59.66; H, 5.00; N, 7.40%.
4.11. Synthesis of complex 11
A mixture of ligand 6c (0.19 g, 0.5 mmol) and NiCl2ꢀ6H2O
(0.118 g, 0.5 mmol) in 10 mL THF was stirred at room temperature
for 4 h. The resultant precipitate was filtered off, washed with
diethyl ether (3 ꢂ 15 mL) and dried in vacuo to give 11 as a brown
powder (0.227 g, 90%). Single crystals suitable for X-ray crystallo-
graphic study were grown by diffusion of diethyl ether vapor into
a saturated solution of the complex in MeOH at ambient tempera-
ture. M.p.: >300 °C, dec. IR (KBr, cmꢁ1
) m 3442 (H2O), 3128 (aro-
matic CH), 2961 and 2871 (aliphatic C–H), 1593 (C@N), 1476,
1374, 1318, 1263, 1198, 1041, 988, 808. Anal. Calc. for
C24H30Cl2N4NiꢀH2O: C, 55.21; H, 6.18; N, 10.73. Found: C, 55.11;
H, 6.15; N, 10.78%.

M. Zhao et al. / Journal of Organometallic Chemistry 694 (2009) 3068–3075
3075
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4.12. A general procedure for catalytic transfer hydrogenation of
ketones
Under nitrogen atmosphere, a mixture of ketone (2.0 mmol),
2-propanol (19 mL), and the catalyst (0.004 mmol) was stirred at
82 °C for 10 min. About 1.0 mL of 0.1 M iPrOK solution in 2-propa-
nol was then introduced to initiate the TH reaction. At the stated
time, 0.1 mL of the reaction mixture was sampled and immediately
diluted with 0.2 mL of 2-propanol for GC analysis. After the reaction
was complete, the reaction mixture was evaporated all the volatiles
under reduced pressure and subject to purification by flash silica gel
column chromatography to afford the alcohol product. The alcohol
products were identified by comparison of their GC traces with the
authentic samples and/or by proton NMR measurements.
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Acknowledgements
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We are grateful to the National Natural Science Foundation of
China (20772124) and the National Basic Research Program of Chi-
na (2009CB825300) for support of this research.
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Appendix A. Supplementary material
CCDC 724507 and 724506 contain the supplementary crystallo-
graphic data for 8a and 11, respectively. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2009.05.028.
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