Construction of highly active ruthenium(ii) nnn complex catalysts bearing a pyridyl-supported pyrazolyl-imidazolyl ligand for transfer hydrogenation of ketones

Article abstract of DOI:10.1021/om801080p

A family of hemilabile ruthenium(II) NNN complexes bearing a unsymmetrical 2-(benzoimidazol-2-yl)-6-(pyrazol-l-yl)pyridine ligand has been synthesized and exhibited good to excellent catalytic activity in transfer hydrogenation of ketones in refhixing 2-p

Full text of DOI:10.1021/om801080p

Organometallics 2009, 28, 1855–1862
1855
Construction of Highly Active Ruthenium(II) NNN Complex
Catalysts Bearing a Pyridyl-Supported Pyrazolyl-Imidazolyl Ligand
for Transfer Hydrogenation of Ketones
Fanlong Zeng† and Zhengkun Yu*,†,‡
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian,
Liaoning 116023, P. R. China, and State Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, P. R. China
ReceiVed NoVember 12, 2008
A family of hemilabile ruthenium(II) NNN complexes bearing a unsymmetrical 2-(benzoimidazol-2-
yl)-6-(pyrazol-1-yl)pyridine ligand has been synthesized and exhibited good to excellent catalytic activity
in transfer hydrogenation of ketones in refluxing 2-propanol, reaching final TOFs up to 7.2 × 105 h-1
with 0.05 mol % loading. The γ-NH effect of the benzoimidazol-2-yl moiety in the ligand and coordination
modes of the metal center in a Ru(II) NNN complex has great influence on the catalytic activity of the
complex catalyst in transfer hydrogenation of ketones. It has been demonstrated that one of the structural
prerequisites for an active Ru(II) complex catalyst is the coordinatively unsaturated environment around
the metal center in the complex or the precatalyst, and the catalytic activity of a complex catalyst can be
enhanced by making its metal center cationic. This paper presents a methodology to construct new types
of efficient Ru(II) complex catalysts for transfer hydrogenation of ketones.
a few active Ru(II) catalysts which do not feature an ancillary
N-H functionality have also been documented for TH of
ketones,5 the need to develop new efficient catalysts is still
strongly desired in this area. Planar tridentate N3 ligands have
recently been successfully developed,6 but little attention has
been paid to unsymmetrical planar tridentate ligands due to their
complicated synthetic schemes. We have been interested in
construction of new types of phosphine-free pyridyl-based 2,6-
Introduction
Hydrogen transfer (HT) catalysis is an attractive protocol for
reduction of ketones to alcohols and has been extensively
studied.1 Ruthenium(II) complexes are usually applied as the
most useful catalysts for transfer hydrogenation (TH) of ketones.
The most important and significant catalysts are ruthenium(II)
complexes containing monotosylated 1,2-diamines or aminoal-
cohols, discovered by Noyori and co-workers, which offer high
catalytic activity and selectivity due to the presence of a N-H
functionality (bifunctional catalysis).2 A great number of related
ligands and transition metal complex catalysts have been
developed.3 Recently, Baratta et al. reported a series of
ruthenium(II) 2-(aminomethyl)pyridine (ampy) phosphane com-
plex catalysts for TH of ketones which have demonstrated a
remarkable acceleration effect by the ampy ligand.4 In all the
systems containing amine ligands, the presence of such a N-H
functionality in the complex catalysts has been proven to be
crucial for achieving highly efficient TH of ketones. Although
(3) (a) Schlattera, A.; Woggon, W.-D. AdV. Synth. Catal. 2008, 350,
995. (b) Huang, X. H.; Ying, J. Y. Chem. Commun. 2007, 1825. (c) Canivet,
J.; Su¨ss-Fink, G. Green Chem. 2007, 9, 391. (d) Ma, G. B.; McDonald, R.;
Ferguson, M.; Cavell, R. G.; Patrick, B. O.; James, B. R.; Hu, T. Q.
Organometallics 2007, 26, 846. (e) Cheung, F. K.; Lin, C. X.; Minissi, F.;
Criville, A. L.; Graham, M. A.; Fox, D. J.; Wills, M. Org. Lett. 2007, 9,
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metallics 2007, 26, 5346. (g) Morris, D. J.; Hayes, A. M.; Wills, M. J.
Org. Chem. 2006, 71, 7035. (h) Zaitsev, A. B.; Adolfsson, H. Org. Lett.
2006, 8, 5129. (i) Vastila, P.; Zaitsev, A. B.; Wettergren, J.; Privalov, T.;
Adolsson, H. Chem.-Eur. J. 2006, 12, 3218. (j) Schiffers, I.; Rantanen,
T.; Schmidt, F.; Bergmans, W.; Zani, L.; Bolm, C. J. Org. Chem. 2006,
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2008, 14, 5588. (b) Baratta, W.; Chelucci, G.; Herdtweck, E.; Magnolia,
S.; Siega, K.; Rigo, P. Angew. Chem., Int. Ed. 2007, 46, 7651. (c) Del
Zotto, A.; Baratta, W.; Ballico, M.; Herdtweck, E.; Rigo, P. Organometallics
2007, 26, 5636. (d) Baratta, W.; Siega, K.; Rigo, P. Chem.-Eur. J. 2007,
13, 7479. (e) Baratta, W.; Chelucci, G.; Gladiali, S.; Siega, K.; Toniutti,
M.; Zanette, M.; Zangrando, E.; Rigo, P. Angew. Chem., Int. Ed. 2005, 44,
6214. (f) Baratta, W.; Da Ros, P.; Del Zotto, A.; Sechi, A.; Zangrando, E.;
Rigo, P. Angew. Chem., Int. Ed. 2004, 43, 3584.
(5) (a) Liu, D. L.; Xie, F.; Zhao, X. H.; Zhang, W. B. Tetrahedron 2008,
64, 3561. (b) Lundgren, R. J.; Rankin, M. A.; McDonald, R.; Schatte, G.;
Stradiotto, M. Angew. Chem., Int. Ed. 2007, 46, 4732. (c) Reetz, M. T.; Li,
X. G. J. Am. Chem. Soc. 2006, 128, 1044. (d) Leijondahl, K.; Fransson,
A.-B. L.; Bajckvall, J.-E. J. Org. Chem. 2006, 71, 8622.
* To whom correspondence should be addressed. E-mail: zkyu@
dicp.ac.cn.
† Dalian Institute of Chemical Physics.
‡ Shanghai Institute of Organic Chemistry.
(1) For selected recent reviews, see: (a) Ikariya, T.; Blacker, A. J. Acc.
Chem. Res. 2007, 40, 1300. (b) Wu, X.; Xiao, J. L. Chem. Commun. 2007,
2449. (c) Gladiali, S.; Alberico, E. Chem. Soc. ReV. 2006, 35, 226. (d)
Samec, J. S. M.; Bajckvall, J.-E.; Andersson, P. G.; Brandt, P. Chem. Soc.
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Chem. ReV. 2004, 248, 2201. (f) Everaere, K.; Mortreux, A.; Carpentier,
J.-F. AdV. Synth. Catal. 2003, 345, 67. (g) Palmer, M. J.; Wills, M.
Tetrahedron: Asymmetry 1999, 10, 2045. (h) Noyori, R.; Hashiguchi, S.
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Chem., Int. Ed. 2002, 41, 2008. (c) Haack, K.-J.; Hashiguchi, S.; Fujii, A.;
Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. 1997, 36, 285. (d) Uematsu,
N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc.
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10.1021/om801080p CCC: $40.75
2009 American Chemical Society
Publication on Web 02/23/2009

1856 Organometallics, Vol. 28, No. 6, 2009
Scheme 1. Highly Active Ru(II) NNN Complex Catalysts for Transfer Hydrogenation of Ketones
Zeng and Yu
Scheme 2. Synthesis of Complexes 2-6a
a
Legend: (i) RuCl2(PPh3)3, PhMe, 110 °C, 2 h, 91%. (ii) NaHCO3, CH2Cl2/MeOH (v/v ) 5:1), r.t., 5 h, 96%. (iii) RuCl2(PPh3)3, Et3N, PhMe,
110 °C, 2 h, 65%. (iv) DMSO/DMF (v/v ) 5:1), 100 °C, 15 min, 77%. (v) DMSO/DMF (v/v ) 5:1), 100 °C, 15 min, 60%. (vi) PPh3, CDCl3, r.t.
(vii) NaHCO3, CH2Cl2/MeOH (v/v ) 5:1), r.t., 15 h, 96%. (viii) DMSO-d6 or DMSO in CDCl3, r.t.
Scheme 3. Synthesis of Ligand 9a
(mixed N-heterocycles) ligands that can potentially provide a
dynamic “on and off” chelating effect for the metal center during
catalysis.7,8 In a recent preliminary comunication, we reported
a new class of highly active robust ruthenium(II) NNN
complexes bearing a hemilabile unsymmetrical pyridyl-based
pyrazolyl-imidazolyl ligand featuring no N-H functionality.9
a
Legend: (i) 1,2-phenylenediamine, nitrobenzene, 150 °C, 12 h, 53%.
These Ru(II) complex catalysts showed exceptionally high
(ii) MeI, Cs2CO3, DMSO, rt, 3.0 h, 98%.
catalytic activity in TH of ketones in 2-propanol, reaching 100%
conversion for the ketone substrates and final TOFs up to 7.2
× 105 h-1 with 0.05 mol % catalyst at 82 °C and 55 800 h-1
3 in 96% yield by extrusion of 1 equiv of hydrogen chloride
with 0.1 mol % catalyst at room temperature. In our case,9
formation of a coordinatively unsaturated ruthenium(II) center
in the complex catalyst is the key factor in constructing a highly
active Ru(II) NNN complex catalyst (Scheme 1). Herein, we
report the detailed methodology for synthesis of these unsym-
metrical and the related Ru(II) NNN complexes and explora-
tion of their catalytic activity in transfer hydrogenation of ke-
tones.
with base NaHCO3. Complex 3 was easily converted to 4 by
reacting with DMSO.9 In the presence of triethylamine, treat-
ment of 1 with RuCl2(PPh3)3 in refluxing toluene formed
complex 5, whose structure was confirmed by X-ray crystal-
lographic determinations. In polar solvent DMSO, 2 was
transformed to cationic Ru(II) complex 6, which could also be
converted to the neutral complex 4 by using NaHCO3 as the
base. Dissolution of 5 in DMSO-d6 or treatment of 5 with
DMSO in CDCl3 formed complex 4 (L ) DMSO-d6 or DMSO,
Scheme 2), which was monitored and confirmed by 31P{1H}
NMR determinations.
Synthesis of Ligand 9 and Complexes 11-14. In a fashion
similar to the synthesis of 1,9 pyrazolyl-imidazolyl pyridine 8
and its N-methyl form 9 were prepared (Scheme 3). Treatment
of ligands 9 and 109 with 1.0 equiv of RuCl2(PPh3)3 in refluxing
toluene afforded the neutral 18-electron Ru(II) complexes 11
and 12,9 and heating of 11 and 12 in their DMSO/DMF solutions
produced cationic Ru(II) complexes 13 and 14, respectively
(Scheme 4). It should be noted that complex 14 is hygroscopic
in air.
Results and Discussion
Synthesis of Complexes 2-6. Pyrazolyl-imidazolyl pyridine
1 was synthesized from the oxidative condensation of 1,2-
phenylenediamine with 6-(3,5-dimethylpyrazol-1-yl)pyridine-
2-carbaldehyde, which was prepared from the reaction of
2-bromo-6-(3,5-dimethylpyrazol-1-yl)pyridine and DMF in the
presence of n-BuLi.9 Reaction of 1 with 1.0 equiv of
RuCl2(PPh3)3 in refluxing toluene produced Ru(II) complex 2
in 91% yield; 2 was further transformed to 16-electron complex
(7) Zeng, F. L.; Yu, Z. K. J. Org. Chem. 2006, 71, 5274.
(8) (a) Sun, X. J.; Yu, Z. K.; Wu, S. Z.; Xiao, W.-J. Organometallics
2005, 24, 2959. (b) Deng, H. X.; Yu, Z. K.; Dong, J. H.; Wu, S. Z.
Organometallics 2005, 24, 4110.
NMR Characterization of the Complexes. The broadened
proton resonance signals of the N-H moiety in 2 and 6 are
shifted downfield by about 3 ppm as compared to that of the
free ligand 1 (δN-H, 12.58 ppm), suggesting that the non-NH
(9) Zeng, F. L.; Yu, Z. K. Organometallics 2008, 27, 2898.

Highly ActiVe Ruthenium(II) NNN Complex Catalysts
Scheme 4. Synthesis of Complexes 11-14a
Organometallics, Vol. 28, No. 6, 2009 1857
a
Legend: (i) RuCl2(PPh3)3, PhMe, 110 °C, 2 h, 85% (11), 92% (12). (ii) DMSO/DMF (v/v ) 5:1), 100 °C, 15 min, 60% (13), 86% (14).
Figure 3. Perspective view of complex 13 with the dichloromethane
molecule and chloride anion omitted for clarity.
Figure 1. Perspective view of complex 5.
ppm in their 13C NMR spectra, and further confirmed by their
X-ray single crystal structural analysis (Figure 2 and ref. 9).
That dissolution of 5 in DMSO-d6 or treatment of 5 with DMSO
in CDCl3 at ambient temperature afforded complex 4 further
confirmed that DMSO or DMSO-d6 can be easily coordinated
to the metal center of 3 in solution. Reaction of 3 with PPh3
also produced 5 at ambient temperature by 31P NMR analysis.
The proton and 13C NMR features of complex 5 are similar to
those of 3 and 4.9 The 31P NMR signals of complexes 3 and 4
appear at 59.3 and 32.7 ppm in CDCl3, respectively, suggesting
different coordination environments around the metal center in
these two complexes. Complex 3 is 16-electron with a five-
coordinate metal center, whereas other complexes are 18-
electron with a six-coordinate metal core.
The solid-state molecular structures of complexes 4,9 12,9 5,
6, and 13 were confirmed by X-ray crystallographic studies
(Tables 1 and 2, Figures 1-3). The single crystal structure of
5 features a neutral ruthenium(II) center, and those of 6 and 13
reveal a cationic ruthenium(II) center in which the metal center
is coordinated by the imidazolyl, pyridyl, and pyrazolyl nitrogen
atoms. The ruthenium atom is in a distorted octahedral environ-
ment with one PPh3 ligand and one DMSO (or PPh3) trans to
each other on the two sides of the NNN ligand plane. The
unsymmetrical “pincer”-type NNN ligand occupies the three
meridional sites with the three N-heterocyclic rings in a quasi-
planar disposition, and the chloride ligand is positioned trans
to the pyridyl nitrogen atom. A DMSO molecule is coordinated
to the Ru(II) center through its sulfur atom in 6 or 13 and the
Ru-S bond distances are 2.320(12) Å in 6 and 2.347(18) Å in
13. The length difference between the Ru-Nimidazolyl bond
(2.111(4) Å) in 6 and those (2.088(17) and 2.084(5) Å) in 5
and 13 is attributed to the presence of a N-H functionality in
6. The average Ru-P bond lengths in 5, 6, and 13 are almost
the same (2.367(6), 2.369(13), and 2.361(19) Å), while the other
Ru-P bond (2.415(6) Å) in 5 is longer than those in other
Figure 2. Perspective view of complex 6.
nitrogen atom in the imidazolyl group is coordinated to the metal
center. The 1H NMR signals of the pyrazolyl-CH in the ligands
appear at 6.15-6.18 ppm, and those of the Ru(II) complexes
are shifted downfield to the region of 6.38-6.46 ppm, revealing
that the pyrazolyl is also coordinated to the metal center. The
31P NMR signals of complexes 2 and 6 appear at 31.5-31.9
ppm as singlets in DMSO-d6, whereas those of 3 and 4 are
shown at ca. 33.8 ppm. At ambient temperature, complex 5
exhibits one 31P NMR signal at 22.0 ppm in CDCl3, but in
DMSO-d6, it demonstrated one singlet at 33.7 ppm correspond-
ing to that of 4 and the other signal at -6.5 ppm for free PPh3,
revealing that one coordinated PPh3 ligand in 5 was dissociated
from the complex in the DMSO-d6 solution to form 4 (L )
DMSO-d6) (Figure 1). Coordination of a DMSO (or DMSO-
d6) molecule to the Ru(II) center in complexes 4 and 6 is
concluded by the presence of a 13C resonance signal at ca. 40.4

1858 Organometallics, Vol. 28, No. 6, 2009
Zeng and Yu
Table 1. Crystallographic Data and Refinement Details for 5, 6, and 13
5
6
13 · CH2Cl2
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C53H44ClN5P2Ru
949.39
293(2)
monoclinic
P2(1)/n
12.1008(7)
20.7883(11)
17.5491(10)
90
96.5180(10)
90
4386.0(4)
4
1.438
0.536
C37H36Cl2N5OPRuS
801.71
293(2)
orthorhombic
P2(1)2(1)2(1)
11.1431(6)
14.5034(8)
22.8917(13)
90
90
90
3699.6(4)
4
1.439
C37H36Cl4N5OPRuS
872.61
293(2)
triclinic
j
P1
10.3359(10)
10.9327(11)
18.5189(19)
74.200(2)
74.175(2)
85.627(2)
1937.2(3)
2
1.496
0.813
888
0.40 × 0.18 × 0.08
1.18-26.50
11170
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Dc (gcm-3
)
µ (mm-1
F(000)
)
0.705
1640
0.35 × 0.27 × 0.19
1.66-27.00
22017
1952
Crystal size (mm3)
0.42 × 0.34 × 0.23
1.95-27.00
25557
θ limits (deg)
No. of data collected
No. of unique data
9509
8022
7867
R(int)
0.0665
7795
561
0.991
0.0483/0.0382
0.0929/0.0890
0.829 (-0.509)
0.0592
6111
437
0.950
0.0582/0.0440
0.1030/0.0984
0.926 (-0.314)
0.0676
4179
454
0.858
0.1004/0.0564
0.1639/0.1331
1.819 (-0.834)
No. of data observed with I > 2σ(I)
No. of refined parameters
Goodness-of-fit on F2
R (all data/obsd. data)
wR2(all data/obsd. data)
Residual Fmax (e Å-3
)
Table 2. Selected Bond Distances (Å) and Angles (deg) for 5, 6 and 13
complex 5
Ru-N(1)
2.088(17)
2.415(6)
156.88(8)
177.40(6)
Ru-N(3)
1.981(19)
2.367(6)
178.76(2)
92.00(2)
Ru-N(5)
Ru-Cl(1)
2.094(18)
2.470(6)
Ru-P(1)
Ru-P(2)
N(1)-Ru-N(5)
N(3)-Ru-Cl(1)
P(1)-Ru-P(2)
P(1)-Ru-Cl(1)
complex 6
Ru-N(1)
1.999(3)
Ru-N(2)
2.111(4)
2.369(13)
91.34(11)
Ru-N(4)
N(2)-Ru-N(4)
S(1)-Ru-P(1)
2.077(3)
156.55(15)
174.84(5)
Ru-S(1)
2.320(12)
87.42(11)
173.97(11)
Ru-P(1)
N(1)-Ru-S(1)
N(1)-Ru-Cl(1)
N(1)-Ru-P(1)
complex 13
Ru-N(1)
1.975(5)
Ru-N(2)
2.084(5)
2.361(19)
93.26(14)
Ru-N(4)
N(4)-Ru-N(2)
S(1)-Ru-P(1)
2.092(5)
156.84(19)
177.14(6)
Ru-S(1)
2.347(18)
89.09(14)
177.49(16)
Ru-P(1)
N(1)-Ru-S(1)
N(1)-Ru-Cl(1)
N(1)-Ru-P(1)
complexes, suggesting that the two PPh3 ligands in 5 have
great trans influences to each other. The Ru-Nimidazolyl
Ru-Npyridyl, and Ru-P bonds in 13 are shorter than those in
other complexes, implying that complex 13 is structurally
more stable.
a higher loading, e.g., 0.1 mol %, yielding the reduction product
in 97% yield with a final TOF of 3880 h-1 over a period of 15
min (Table 3, entry 4). With 0.3 mol % loading, complexes
11-13 exhibited much lower catalytic activity than complexes
2-6 (Table 3, entries 6-8), and acetophenone was reduced to
,
Catalytic Transfer Hydrogenation of Ketones (eq 1).
Reduction of acetophenone to 1-phenylethanol by 2-propanol
was chosen as the model reaction to test the catalytic activity
of the Ru(II) NNN complexes (eq 2, Table 3). The catalytic
reactions were carried out using a 0.1 M solution of acetophe-
none in refluxing 2-propanol with 0.05-0.3 mol % of the
complex as catalyst, and freshly prepared iPrOK as the base
under nitrogen atmosphere. Complexes 2 and 3 exhibited the
same exceptionally high catalytic activity, achieving 98%
conversion for acetophenone and a final TOF of 705 600 h-1
within 10 s (Table 3, entries 1 and 2). Complex 6 can be
considered as a precursor to complex 4 that they demonstrated
the same high catalytic activity, reaching 98% conversion for
the ketone and a final TOF of 117 600 h-1 within one minute
(Table 3, entries 3 and 5), presumably due to the instant
transformation of 6 to 4 under the basic conditions. However,
complex 5 only showed a moderate catalytic activity even with
Table 3. Transfer Hydrogenation of Acetophenone Catalyzed by
Complexes 2-6 and 11-14a
entry complex (mol %) time (min) conversion (%)b final TOF (h-1
)
19
2
2 (0.05)
3 (0.05)
4 (0.05)
5 (0.1)
6 (0.05)
11 (0.3)
12 (0.3)
13 (0.3)
14 (0.1)
1/6
1/6
1
15
1
10
10
10
5
98
98
98
97
98
98
98
96
98
705 600
705 600
117 600
3880
117 600
1960
1960
1920
11 760
39
4
5
6
7
8
9
a Conditions: acetophenone, 2.0 mmol (0.1 M in 20 mL i PrOH);
iPrOK/cat ) 20/1; 0.1 MPa, 82 °C. b GC yield of the corresponding
alcohol.

Highly ActiVe Ruthenium(II) NNN Complex Catalysts
Organometallics, Vol. 28, No. 6, 2009 1859
Table 4. Transfer Hydrogenation of Ketones Catalyzed by 3a
Table 5. Transfer Hydrogenation of Ketones Catalyzed by
Complexes 4-6 and 14a
a Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); catalyst: 4
and 6, 0.05 mol%; 5 and 14, 0.1 mol%; iPrOK:cat. ) 20:1; 0.1 MPa, 82
°C. b GC yield of the corresponding alcohol. c By 1H NMR. d Catalyst
(0.2 mol %).
activity as complex 2 did in the TH of ketones, reaching >98%
conversion for most of the ketone substrates with final TOFs
up to 720 000 h-1 within 10 s (Table 4, entries 3, 4, 21, and
22). Complexes 4-6 and 14 demonstrated good to excellent
catalytic activities in TH of ketones (Table 5). 4 and 6 almost
exhibited the same high catalytic activity and reached 95-100%
conversion for the ketones and final TOFs up to 240 000 h-1
within 0.5-5 min, presumably due to the instant conversion of
6 to 4 under the reaction conditions (Table 5, entries 1, 2, 8, 9,
and 11). Complex 5 only exhibited good catalytic activity as
compared with those of complexes 2-4 and 6 but showed better
catalytic activity than Ru(II) complex A bearing a symmetrical
NNN ligand8 (Scheme 1). Although the neutral Ru(II) complex
12 only showed fairly good catalytic activity in TH of
acetophenone (entry 7, Table 3), its cationic form, that is,
complex 14, exhibited good to excellent catalytic activity in
the TH of various ketone substrates (Table 5) and achieved final
TOFs up to 29 400 h-1. To date, the highest TOF value in TH
a Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); complex 3,
0.05 mol%; ketone/iPrOK/cat.) 2000:20:1; 0.1 MPa, 82 °C. b By GC
c
1
analysis. By H NMR.
the corresponding alcohol in 96-98% yields over a period of
10 min. Unexpectedly, the cationic form of the neutral complex
12, that is, 14, exhibited much higher catalytic activity than its
parent complex 12, achieving 97% conversion for the ketone
with a final TOF of 11 760 h-1 within 5 min using 0.1 mol %
of the complex as catalyst (Table 3, entry 9).
Under the typical conditions for TH of ketones (82 °C, 0.1
M ketone in 2-propanol), transfer hydrogenation of various
ketones was explored by means of complexes 3-6 and 14 as
the catalysts (Tables 4 and 5). As reported in the preliminary
communication,9 complex 3 nearly exhibited the same catalytic

1860 Organometallics, Vol. 28, No. 6, 2009
Zeng and Yu
Scheme 5. Proposed Mechanism
Conclusions
In summary, hemilabile ruthenium(II) NNN complexes bear-
ing a unsymmetrical pyridyl-supported pyrazolyl-imidazolyl
ligand have been synthesized and exhibited exceptionally high
catalytic activity in transfer hydrogenation of ketones at 82 °C,
demonstrating rare examples of highly active Ru(II) NNN
complex catalysts that do not feature a N-H functionality.13
The hemilability feature of the unsymmetrical NNN ligand and
the coordinatively unsaturated environment around the Ru(II)
center in the complex or precatalyst provide the complex catalyst
with a remarkable acceleration effect during the TH reactions
of ketones.
Experimental Section
General Considerations. Unless otherwise noted, all the starting
materials were commercially available and used without further
purification. The catalytic reactions were carried out under a
nitrogen atmosphere. All the solvents were dried prior to use
according to the standard procedures. 1H, 13C{1H} and 31P{1H}NMR
spectra were obtained with a 400 MHz NMR spectrometer.
X-Ray Crystallographic Studies. Single crystal X-ray diffrac-
tion studies for compounds 5, 6, and 13 were carried out on a
SMART APEX diffractometer with graphite-monochromated Mo
KR 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 crystal-
lographic data and refinement details for 5, 6, and 13 are listed in
Table 1, and the selected bond lengths and angles are given in Table
2.
of ketones, that is, 2.5 × 106 h-1 (at 50% conversion of the
ketone, 82 °C), has been reported in TH of 3-chloroacetophe-
none by using 0.005 mol% Baratta’s Ru(II) CNN catalyst
featuring a N-H functionality.4e Stradiotto’s cationic Ru(II)
catalyst featuring no N-H functionality has also shown very
high catalytic activity in TH of ketones with 0.05 mol % loading
(TOFs up to 2.2 × 105 h-1).5b In our cases, both the neutral
complexes 2 and 3 exhibited exceptionally high catalytic
activity, complex 4 and its cationic form 6, and the cationic
complex 14 also demonstrated excellent catalytic activity in TH
of ketones. Complexes 2 and 3 are among the three most
efficient complex catalysts for transfer hydrogenation of ketones
to date.4e,5b
On the basis of the behaviors of complexes 2-6 and 11-14 in
solution and the transformations as shown in Scheme 2, an inner-
sphere mechanism10 is proposed for TH of ketones catalyzed
by 2 or 3 (Scheme 5). Thus, TH of a ketone can be initiated
directly from 3 or in situ generated 3 by extrusion of one
equivalent of hydrogen chloride from 2 with the base, that is,
iPrOK. Complex 3 interacts with the base to form Ru(II)-
alkoxide D which undergoes ꢀ-H elimination to result in a
Ru-H intermediate E and releases of acetone. Coordination of
a ketone substrate to E produces species F and is followed by
insertion of the coordinated ketone carbonyl into the Ru-H bond
to form Ru(II)-alkoxide G which is then reacted with 2-propanol
to afford the alcohol product and regenerate species D. Ru(II)
hydride E is presumably considered as the catalytically active
species although it was not successfully isolated by reacting 3
with EtONa or iPrOK in refluxing ethanol (or 2-propanol).
Formation of Ru-H complexes from Ru-Cl precursors has
been well-documented,11 and such in situ formed Ru-H species
can act as the active catalysts for TH of ketones.1,5c,11,12 In the
present case, the presence of a Ru-Nimidazolyl bond and the
coordinatively unsaturated 16-electron environment around
the ruthenium center in 3 are crucial to the exceptionally high
catalytic activity of complex 3.
Synthesis of Complex 49 from its Cationic Form 6. A mixture
of complex 6 (0.26 g, 0.32 mmol) and NaHCO3 (0.27 g 3.20 mmol)
in dichloromethane/methanol (20 mL, v/v ) 5:1) was stirred at
ambient temperature for 15 h. The resultant mixture was filtered
through a short pad of celite and the filtrate was collected. All the
volatiles were removed under reduced pressure to afford complex
4 (0.24 g, 96%).
Synthesis of Complex 5. 2.1 mL of triethylamine (1.52 g, 15.0
mmol) was added to a slurry of RuCl2(PPh3)3 (1.44 g, 1.5 mmol)
and benzoimidazole 19 (0.43 g, 1.5 mmol) in toluene (50 mL) with
stirring and the mixture was refluxed for 2 h. The mixture was
cooled to ambient temperature and the resultant solid was filtered
off, washed with iPrOH (2 × 50 mL) and diethyl ether (50 mL),
and dried in vacuum to afford compound 5 as an orange microc-
(10) Comas-Vives, A.; Ujaque, G.; Lledo´s, A. Organometallics 2007,
26, 4135.

Highly ActiVe Ruthenium(II) NNN Complex Catalysts
Organometallics, Vol. 28, No. 6, 2009 1861
rystalline solid (0.93 g, 65%). Red single crystals suitable for X-ray
crystallographic study were grown by diffusion of diethyl ether
vapor into a saturated solution of the complex in CH2Cl2/MeOH
g, 20.00 mmol), pyrazole (1.77 g, 26.00 mmol), 1,10-phenanthroline
monohydrate (0.79 g, 4.00 mmol), CuI (0.38 g, 2.00 mmol, 10 mol
%), and K2CO3 (3.04 g, 20.00 mmol) in toluene (80 mL) was stirred
at 120 °C for 24 h. After cooling to ambient temperature, the
mixture was filtered through celite, and all the volatiles were
removed under reduced pressure. Isolation by silica gel column
chromatography (petroleum ether (60-90 °C)/ethyl acetate, v/v )
1
(v/v ) 4:1) at ambient temperature. mp: dec > 240 °C. H NMR
(CD2Cl2, 23 °C): δ 8.29 and 7.39 (d each, J ) 7.1 and 7.1 Hz, 1:1
H, 3-H and 5-H), 8.08 (s, 1 H, 4-H), 7.11 and 6.88 (m each, 33 H,
Ph in PPh3, 6′′-H, 7′′-H, and 8′′-H), 6.48 (d, J ) 8.3 Hz, 1 H,
5′′-H), 5.90 (s, 1 H, 4′-H), 2.36 (s, 3 H, C3′-CH3), 2.30 (s, 3 H,
C5′-CH3). 13C{1H} NMR (CD2Cl2, 23 °C): δ 157.2 and 151.2 (s
and Cq each, C2 and C6), 151.9 and 143.4 (s and Cq each, C3′
and C5′), 143.1, 131.8 and 131.5 (s and Cq each, C2′′, C4′′, and
C9′′), 132.8, 132.8, 128.7 and 127.2 (s each, Phenyl CH in PPh3),
131.7 (s, C4), 122.9, 121.6, 114.4 and 112.4 (s each, Phenyl CH),
120.4 and 118.7 (s each, 1:1 CH, C3 and C5), 107.6 (s, C4′), 14.6
(s, C3′-CH3), 14.4 (s, C5′-CH3).31P{1H} NMR (CD2Cl2, 23 °C): δ
21.4. 31P{1H} NMR (CDCl3, 23 °C): δ 22.0. Anal. calcd for
C53H44ClN5P2Ru: C, 67.05; H, 4.67; N, 7.38. Found: C, 66.73; H,
4.71; N, 7.42.
1
40:1) afforded 7 as white solid (2.50 g, 72%). H NMR (CDCl3,
23 °C) δ 10.05 (s, 1 H, CHO), 8.68 (d, J ) 2.4 Hz, 1 H, 3′-H),
8.23 and 7.84 (d each, J ) 8.0 and 7.6 Hz, 2 H, 3-H and 5-H),
7.99 (t, J ) 15.6 Hz, 1 H, 4-H), 7.78 (s, 1 H, 5′-H), 6.52 (s, 1 H,
4′-H). 13C{1H} NMR (CDCl3, 23 °C) δ 192.7 (s, CHO), 151.9 and
151.2 (s and Cq each, C2 and C6), 142.8, 139.9 and 108.5 (s each,
3 × CH, C3′, C4′, and C5′), 127.4 (s, C4), 119.2 and 117.0 (s
each, 2 × CH, C3 and C5).
Synthesis of 2-(Benzoimidazol-2-yl)-6-(pyrazol-1-yl)pyridine
(8). A mixture of aldehyde 7 (0.50 g, 2.89 mmol) and 1,2-
phenylenediamine (0.31 g, 2.89 mmol) in nitrobenzene (50 mL)
was stirred at 150 °C for 12 h. All the volatiles were removed under
reduced pressure and the resultant residue was subject to purification
by flash silica gel column chromatography (CH2Cl2/ethyl acetate,
v/v ) 3:1), affording compound 8 as a pale yellow solid (0.40 g,
Synthesis of Complex 6. A mixture of complex 29 (0.35 g, 0.47
mmol) and DMF/DMSO (20 mL, v/v ) 5:1) was stirred at 100 °C
for 15 min. After cooling to ambient temperature, the resultant
solution was layered with diethyl ether and recrystallized at -20
°C to afford complex 6 as red crystals (0.30 g, 77%). mp: dec >
210 °C.1H NMR (DMSO-d6, 23 °C) δ 15.49 (s, 1 H, NH), 8.40
and 8.23 (d each, J ) 8.0 and 8.0 Hz, 1:1 H, 3-H and 5-H), 7.93
(t, J ) 16.4 Hz, 1 H, 4-H), 7.64 (m, 2 H, 5′′-H and 8′′-H), 7.48 (m,
2 H, 6′′-H and 7′′-H), 7.25 and 7.11 (m each, 9:6 H, Ph in PPh3),
6.45 (s, 1 H, 4′-H), 2.72 (s, 3 H, C3′-CH3), 2.58 (s, 3 H, C5′-CH3),
2.54 (s, 6 H, DMSO). 13C{1H} NMR (DMSO-d6, 23 °C) δ 158.0
and 151.9 (s and Cq each, C2 and C6), 151.8 and 142.1 (s and Cq
each, C3′ and C5′), 149.5, 146.0 and 134.1 (s and Cq each, C2′′,
C4′′, and C9′′), 137.0 (s, C4), 132.7 (d, o-C of PPh3), 130.8 (d, i-C
of PPh3), 129.8 (s, p-C of PPh3), 128.1 (d, m-C of PPh3), 125.5,
124.3, 113.5 and 113.2 (s each, C5”, C6′′, C7′′, and C8′′), 119.3
and 119.1 (s each, C3 and C5), 111.2 (s, C4′), 40.4 (s, CH3 of
DMSO), 14.4 (s, C3′-CH3), 14.2 (s, C5′-CH3). 31P{1H} NMR
(DMSO-d6, 23 °C) δ 31.9. Anal. calcd for C37H36Cl2N5OPRuS: C,
55.43; H, 4.53; N, 8.74. Found: C, 55.40; H, 4.50; N, 8.77.
1
53%). mp: 208-209 °C. H NMR (DMSO-d6, 23 °C) δ 13.10 (s,
1 H, NH), 9.26 (s, 1 H, 3′-H), 8.22 (d, J ) 7.6 Hz, 1 H, 3-H), 8.15
(t, J ) 16.0 Hz, 1 H, 4-H), 7.99 (d, J ) 8.4 Hz, 1 H, 5-H), 7.88 (s,
1 H, 5′-H), 7.75 and 7.64 (d each, J ) 8.0 and 9.2 Hz, 1:1 H, 5′′-H
and 8′′-H), 7.31 and 7.25 (m each, 2 H, 6′′-H and 7′′-H), 6.70 (d,
J ) 1.2 Hz, 1 H, 4′-H). 13C{1H} NMR (DMSO-d6, 23 °C) δ 150.5
and 149.9 (s and Cq each, C2 and C6), 147.0, 144.0 and 134.7 (s
and Cq each, C2′′, C4′′, and C9′′), 142.6 and 140.9 (s each, C3′
and C5′), 128.2 (s, C4), 123.6, 122.1, 112.2 and 111.9 (s each,
phenyl CH), 119.6 and 118.7 (s each, C3 and C5), 108.2 (s, C4′).
HRMS calcd for C15H11N5: 261.1014. Found: 261.1019.
Synthesis of 2-(N-Methyl-benzoimidazol-2-yl)-6-(pyrazol-1-
yl)pyridine (9). A mixture of benzoimidazole 8 (0.68 g, 2.60 mmol)
and Cs2CO3 (1.69 g, 5.20 mmol) in 50 mL DMSO was stirred at
80 °C for 30 min and cooled to ambient temperature. Iodomethane
(0.55 g, 3.90 mmol) was then added, and the reaction was continued
at ambient temperature for 3 h. Dichloromethane (100 mL) was
added, and the resultant mixture was washed with water (3 × 100
mL). The organic phase was separated, dried over anhydrous
MgSO4, and filtered. All the volatiles were evaporated from the
filtrate under reduced pressure, and the resultant residue was subject
to purification by flash silica gel column chromatography (CH2Cl2/
ethyl acetate, v/v ) 3:1), affording 9 as a white solid (0.70 g, 98%).
Synthesis of 6-(Pyrazol-1-yl)pyridine-2-carbaldehyde (7). Com-
pound 7 was prepared using a procedure different from the reported
method.14 A mixture of 6-bromopyridine-2-carbaldedyhyde (3.72
(11) Li, T.; Churlaud, R.; Lough, A. J.; Abdur-Rashid, K.; Morris, R. H.
Organometallics 2004, 23, 6239.
1
(12) (a) Casey, C. P.; Clark, T. B.; Guzei, I. A. J. Am. Chem. Soc. 2007,
129, 11821. (b) Guan, H.; Iimura, M.; Magee, M. P.; Norton, J. R.; Zhu,
G. J. Am. Chem. Soc. 2005, 127, 7805. (c) Chowdhury, R. L.; Bajckvall,
J.-E. J. Chem. Soc., Chem. Commun. 1991, 1063. (d) Aranyos, A.; Csjernyik,
G.; Szabo´, K. J.; Bajckvall, J.-E. Chem. Commun. 1999, 351.
(13) Enthaler, S.; Hagemann, B.; Bhor, S.; Anilkumar, G.; Tse, M. K.;
Bitterlich, B.; Junge, K.; Erre, G.; Beller, M. AdV. Synth. Catal. 2007, 349,
853.
M.p.: 134-135 °C. H NMR (DMSO-d6, 23 °C) δ 8.73 (d, J )
2.1 Hz, 1 H, 3′-H), 8.25 (d, J ) 7.7 Hz, 1 H, 3-H), 8.16 (t, J )
15.8 Hz, 1 H, 4-H), 8.02 (d, J ) 8.1 Hz, 1 H, 5-H), 7.89 (s, 1 H,
5′-H), 7.75 and 7.69 (d each, J ) 8.0 and 8.0 Hz, 1:1 H, 5′′-H and
8′′-H), 7.35 and 7.29 (m each, 2 H, 6′′-H and 7′′-H), 6.65 (d, J )
1.6 Hz, 1 H, 4′-H), 4.29 (s, 3 H, N-CH3). 13C{1H} NMR (DMSO-
d6, 23 °C) δ 150.0 and 148.5 (s and Cq each, C2 and C6), 148.8,
142.0 and 137.1 (s and Cq each, C2′′, C4′′, and C9′′), 142.6 and
(14) Vacher, B.; Bonnaud, B.; Funes, P.; Jubault, N.; Koek, W.; Assie,
M.-B.; Cosi, C. J. Med. Chem. 1998, 41, 5070.

1862 Organometallics, Vol. 28, No. 6, 2009
Zeng and Yu
140.6 (s each, C3′ and C5′), 127.5 (s, C4), 123.4, 122.5, 112.2 and
110.9 (s each, phenyl CH), 120.0 and 119.6 (s each, C3 and C5),
108.7 (s, C4′), 33.0 (s, N-CH3). HRMS calcd for C16H13N5:
275.1171. Found: 275.1171.
of DMSO), 32.8 (s, N-CH3). 31P{1H} NMR (DMSO-d6, 23 °C) δ
32.8. C36H34Cl2N5OSPRu · 0.5CH2Cl2: C, 52.81; H, 4.25; N, 8.44.
Found: C, 52.53; H, 4.30; N, 8.55.
Synthesis of Complex 11. A mixture of RuCl2(PPh3)3 (0.34 g,
0.36 mmol) and N-methyl-benzoimidazole 9 (0.10 g, 0.36 mmol)
in toluene (8 mL) was refluxed for 2 h, forming a red-brown
microcrystalline solid. The mixture was cooled to ambient tem-
perature and the solid was filtered off, washed with diethyl ether
(3 × 30 mL), and dried in vacuum to afford complex 11 (0.22 g,
85%) as a red-brown crystalline solid. mp>280 °C. 1H NMR
(DMSO-d6, 23 °C) δ 9.19 and 8.32 (d each, J ) 3.2 and 1.8 Hz,
1:1 H, 3′-H and 5′-H), 8.39 (d, J ) 9.0 Hz,1 H 5-H), 8.03 (m, 3 H,
3-H, 4-H and 5′′-H), 7.88 (d, J ) 9.0 Hz,1 H, 8′′-H), 7.55 (m, 2 H,
6′′-H and 8′′-H), 7.26 and 7.12 (m each, 9:6 H, Ph in PPh3), 6.88
(t, J ) 5.0 Hz,1 H, 4′-H), 4.26 (s, 3 H, N-CH3). 13C{1H} NMR
(DMSO-d6, 23 °C) δ 151.3 and 150.9 (s and Cq each, C2 and C6),
148.5, 141.0 and 136.0 (s and Cq each, C2′′, C4′′, and C9′′), 147.1
and 136.8 (s each, C3′ and C5′), 133.2 (s, C4), 132.7 (d, o-C of
PPh3), 130.2 (s, p-C of PPh3), 129.8 (d, i-C of PPh3), 128.2 (d,
m-C of PPh3), 125.6, 124.5, 112.3 and 111.2 (s each, phenyl CH),
121.0 and 119.6 (s each, C3 and C5), 110.9 (s, C4′), 32.8 (s,
N-CH3). 31P{1H} NMR (DMSO-d6, 23 °C) δ 32.8. Anal. calcd for
C34H28Cl2N5PRu: C, 57.55; H, 3.98; N, 9.87. Found: C, 57.30; H,
4.06; N, 10.00.
Synthesis of Complex 14. A mixture of complex 129 (0.74 g,
1.00 mmol) and 20 mL DMSO was stirred at 100 °C for 15 min.
After cooling to ambient temperature, diethyl ether (300 mL) was
added to precipitate the crude product. The resultant brown solid
was dried in vacuum to afford compound 14 (0.70 g, 86%).
1
Compound 14 is hydroscopic in air. mp: dec > 210 °C. H NMR
(DMSO-d6, 23 °C) δ 8.54 and 8.08 (d each, J ) 7.0 and 8.1 Hz,
1:1 H, 3-H and 5-H), 7.88 (m, 2 H, 4-H and 5′′-H), 7.75 (d, J )
8.6 Hz, 1 H, 8′′-H), 7.53 (m, 2 H, 6′′-H and 7′′-H), 7.24 and 7.12
(m each, 9:6 H, Ph in PPh3), 6.45 (s, 1 H, 4′-H), 4.25 (s, 3 H,
N-CH3), 2.76 (s, 3 H, C3′-CH3), 2.57 (s, 3 H, C5′-CH3), 2.54 (s, 6
H, CH3 of DMSO). 13C{1H} NMR (DMSO-d6, 23 °C) δ 157.9 and
152.3 (s and Cq each, C2 and C6), 150.9 and 141.2 (s and Cq
each, C3′ and C5′), 149.1, 146.1 and 136.1 (s and Cq each, C2′′,
C4′′, and C9′′), 136.7 (s, C4), 132.7 (d, o-C of PPh3), 130.8 (d, i-C
of PPh3), 129.8 (s, p-C of PPh3), 128.0 (d, m-C of PPh3), 125.5,
124.5, 113.4, and 112.3 (s each, phenyl CH), 120.6 and 119.7 (s
each, C3 and C5), 111.6 (s, C4′), 40.4 (s, CH3 of DMSO), 32.9
(N-CH3), 14.6 (s, C3′-CH3), 14.0 (s, C5′-CH3). 31P{1H} NMR
(DMSO-d6, 23 °C): δ 31.5 (s, PPh3). C38H38Cl2N5OSPRu · H2O: C,
54.74; H, 4.84; N, 8.40. Found: C, 54.48; H, 4.91; N, 8.58.
General Procedure for Catalytic Transfer Hydrogenation
of Ketones. The catalyst solution was prepared by dissolving
complex 14 (16.3 mg, 20.0 µmol) in 2-propanol (40.0 mL). Under
nitrogen atmosphere, the mixture of a ketone substrate (2.0 mmol),
4.0 mL of the catalyst solution (0.1 mol % catalyst 14), and
2-propanol (15.6 mL) was stirred at 82 °C for 5 min. Then, 0.4
mL of 0.1 M iPrOK (0.04 mmol) solution in 2-propanol was
introduced to initiate the transfer hydrogenation of the ketone. At
the stated time, 0.1 mL of the reaction mixture was sampled and
diluted with 0.5 mL of 2-propanol precooled at 0 °C for immediate
GC analysis. After the reaction was complete, the reaction mixture
was condensed 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.
Synthesis of Complex 13. A mixture of complex 11 (0.30 g,
0.42 mmol) and DMF/DMSO (15 mL, v/v ) 5:1) was stirred at
100 °C for 15 min. After cooling to ambient temperature, all the
volatiles were removed under reduced pressure and the resultant
residue was dissolved in 20 mL CH2Cl2. The resultant solution was
layered with diethyl ether and recrystallized at -20 °C to afford
complex 11 as red crystals (0.20 g, 60%). M.p.: dec > 210 °C
°C.1H NMR (DMSO-d6, 23 °C) δ 9.21 and 8.32 (d each, J ) 3.0
and 1.8 Hz, 1:1 H, 3′-H and 5′-H), 8.39 (d, J ) 9.2 Hz, 1 H, 5-H),
8.07 (m, 2 H, 3-H, and 5′′-H), 8.00 (t, J ) 16.0 Hz, 1 H, 4-H),
7.88 (d, J ) 6.0 Hz, 1 H, 8′′-H), 7.54 (m, 2 H, 6′′-H and 8′′-H),
7.26 and 7.12 (m each, 9:6 H, PPh3), 6.88 (t, J ) 5.1 Hz, 1 H,
4′-H), 4.26 (s, 3 H, N-CH3), 2.53 (s, 6 H, DMSO). 13C{1H} NMR
(DMSO-d6, 23 °C) δ 151.3 and 150.9 (s and Cq each, C2 and C6),
148.5, 141.0 and 136.0 (s and Cq each, C2′′, C4′′, and C9′′), 147.1
and 136.8 (s each, C3′ and C5′), 133.3 (s, C4), 132.7 (d, o-C of
PPh3), 130.2 (s, p-C of PPh3), 129.8 (d, i-C of PPh3), 128.2 (d,
m-C of PPh3), 125.6, 124.5, 112.4 and 111.2 (s each, phenyl CH),
121.1 and 119.6 (s each, C3 and C5), 111.0 (s, C4′), 40.4 (s, CH3
Acknowledgment. We are grateful to the National Natural
Science Foundation of China (20772124) and the National
Basic Research Program of China (2009CB825300) for
support of this research.
Supporting Information Available: X-ray crystallographic data
of 5, 6, and 13, also in cif format. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM801080P