A novel cationic dinuclear ruthenium complex: Synthesis, characterization and catalytic activity in the transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.inoche.2011.09.003

A novel cationic dinuclear ruthenium complex [RuCl(HL)(TFTPP)]₂ (H₂L = 2,6-bis(5-phenyl-1H-pyrazol-3-yl)pyridine; TFTPP = tri(p-trifluoromethylphenyl)phosphine) has been synthesized and characterized by ³¹P{¹H} NMR, ¹H NMR, elemental analysis and X-ray crystallography. This complex is the first cationic dinuclear ruthenium complex bearing N₄ ligand characterized by single crystal X-ray analysis. It exhibits good catalytic activity for the transfer hydrogenation of ketones in refluxing 2-propanol.

Full text of DOI:10.1016/j.inoche.2011.09.003

Inorganic Chemistry Communications 14 (2011) 1884–1888
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A novel cationic dinuclear ruthenium complex: Synthesis, characterization and
catalytic activity in the transfer hydrogenation of ketones
Lei Wang a,b, Qin Yang a, Hua Chen a, Rui-Xiang Li a,
⁎
a
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China
College of Chemistry and Chemical Engineering, Xuchang University, Xuchang, Henan 461000, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2011
Accepted 7 September 2011
Available online 14 September 2011
A novel cationic dinuclear ruthenium complex [RuCl(HL)(TFTPP)]2 (H2L=2,6-bis(5-phenyl-1H-pyrazol-3-yl)
pyridine; TFTPP=tri(p-trifluoromethylphenyl)phosphine) has been synthesized and characterized by 31P{1H}
NMR, 1H NMR, elemental analysis and X-ray crystallography. This complex is the first cationic dinuclear ruthe-
nium complex bearing N4 ligand characterized by single crystal X-ray analysis. It exhibits good catalytic activity
for the transfer hydrogenation of ketones in refluxing 2-propanol.
Keywords:
Ruthenium
© 2011 Elsevier B.V. All rights reserved.
N4 ligand
Dinuclear complex
Transfer hydrogenation
Ketone
Transfer hydrogenation in 2-propanol is a potentially useful protocol
for the reduction of ketones to their corresponding alcohols [1-4]. It has
emerged as an efficient alternative to hydrogenation of ketones for it is
safe, highly selective and ecofriend [5-7]. Ru(II) complexes bearing
arene are usually the most potential catalysts for transfer hydrogena-
tion of ketones [6-20]. This kind of complexes shows the high catalytic
activity and a short lifetime of active species because the arene is easily
slipped from central Ru(II) in course of catalytic reaction [21]. Recently,
Ru(II) complexes bearing tridentate pincer ligands have been success-
fully developed and explored to construct an effective catalyst system
for transfer hydrogenation of ketones [22-27]. Compared with Ru(II)
complexes bearing arene, Ru(II) complexes bearing tridentate pincer
ligands show the long lifetime of active species due to the chelating
effect of tridentate ligands for the active metal center. Especially,
Ru ONO-type pincer complex shows excellent catalytic activity in
transfer hydrogenations with turnover numbers up to 590,000 [28].
In the same time, some researcher's attention is focused on dinuclear
ruthenium complex because of their special structures and poten-
tially catalytic activities. Several dinuclear ruthenium complexes
have been documented for transfer hydrogenation of ketones, but
all of them bear arene ligands [29-31]. Therefore, to develop some
new dinuclear ruthenium pincer complexes as the efficient catalysts
are still strongly desired in this area. Bispyrazolyl ligands, as a kind of
anionic multidentate linkers, have been successfully used to construct
multinuclear transition metal complexes [32]. So we have been inter-
ested in the synthesis and application of multinuclear ruthenium com-
plex bearing H2L. In this paper, we synthesized and characterized a
novel cationic dinuclear ruthenium complex, [RuCl(HL)(TFTPP)]2 (1)
(Structures of H2L and TFTPP are shown in Scheme 1), and investigated
its catalytic performance for the transfer hydrogenation of ketones.
Reagent-grade RuCl3·3H2O was used as received. Other materials
TFTPP [33] and H2L [34] were prepared with the reported methods.
According to the reported result [35], [RuCl2(PPh3)(Me4BPP)]
(Me4BPP=2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine) could be syn-
thesized by reacting RuCl3(Me4BPP) with PPh3 (1: 1) in the presence
of Et3N. Hence, we tried to synthesize complex [RuCl2(TFTPP)(H2L)]
by reacting RuCl3·3H2O with H2L and TFTPP (1: 1: 1) in alcohol.
Under this condition, some air-stable red crystals were isolated
[36]. The single crystal of complex 1 was obtained and identified by
X-ray crystallography [37]. Its crystal data and structure refinement
⁎
Corresponding author: Tel./fax: +86 28 8541 2904.
E-mail address: liruixiang@scu.edu.cn (R.-X. Li).
Scheme 1. Structures of ligands.
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.09.003

L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1884–1888
1885
Table 2
results are presented in Table 1. The selected bond distances and
Selected bond lengths (Å) and bond angles (°) for complex 1.
bond angles are compiled in Table 2. Its structure is shown in Fig. 1.
The single crystal structure of complex 1 is quite unusual and it exhibits
a Ru(II) bimetallic species with the (HL)− ligand and chloride ligand,
which acts as a bridge between the two ruthenium centers. The ge-
ometry around the two ruthenium centers can be defined as slightly
distorted octahedral. Each ruthenium center is coordinated with one
phosphorus atom of TFTPP, one chloride, three nitrogen atoms of
(HL)−, and one nitrogen atom of another (HL)−. Another chloride
anion, which is located in the outside of the coordination sphere,
forms a hydrogen bond with H(10) in N–H bond. To the best of our
knowledge, this is the first crystallographic determination of bimetallic
ruthenium complex with this sort of ligand. From Table 2, we can find
that the two bond distances between ruthenium and phosphine are
almost the same. It is indicated that they are in the same chemical
environment. Subsequently, the red crystals were identified by NMR.
A singlet at 50.98 ppm in 31P{1H} NMR spectrum suggested that the
chemical environment of two phosphorus atoms was equivalent,
which is consisted with the results of X-ray crystallography. The pro-
tons on the pyridine ring in complex 1 appeared as multi-signal at
7.87–7.96 ppm in 1H NMR spectrum and protons of N–H as a singlet
at 14.56 ppm. The downfield shift of the N–H proton in complex 1
should be caused by the low electronic density and the electronic
shielding effect due to the formation of hydrogen bond. The ratio of
the protons of N–H to that of pyridine ring is 1: 3, which is also consisted
with the results of X-ray crystallography. The elemental analysis results
further confirmed the structure of complex 1.
As a special dinuclear ruthenium complex, is the complex 1 an ef-
ficient catalyst for the transfer hydrogenation of ketones? So the cat-
alytic activity of complex 1 was investigated in the optimum reaction
condition (Table 3) [38]. This complex exhibited a good catalytic ac-
tivity for the transfer hydrogenation of ketones (Entries 1–16). The
nature and position of substituents in the aromatic ketones result in
significant effects on the catalyst activity [39, 40]. Ketones with elec-
tron-withdrawing substituents can be reduced smoothly, while the
introduction of electron-donating substituent in the substrate tends
to decrease the reaction rate. The substituted acetophenones with
methyl group are reduced more slowly than acetophenone (Entries
1, 6–8). Notably with an electron-donating methyl group in the o-po-
sition the reaction is accelerated, but when the p-position of substrate
is substituted the rate is lowered (Entries 6, 8). Furthermore, the
Bond lengths (Å)
Bond angles (°)
Ru1–Cl1
Ru1–P2
Ru1–N5
Ru1–N7
Ru1–N8
Ru1–N9
Ru2–Cl1
Ru2–P1
Ru2–N2
Ru2–N3
Ru2–N4
Ru2–N6
2.4746(11)
2.2695(12)
2.237(4)
2.008(3)
1.980(4)
2.125(3)
2.4538(10)
2.2592(12)
2.096(4)
1.980(4)
2.015(4)
2.220(3)
Ru2–Cl1–Ru1
P1–Ru2–Cl1
P2–Ru1–Cl1
N4–Ru2–N2
N5–Ru1–P2
N5–Ru1–Cl1
N6–Ru2–Cl1
N6–Ru2–P1
N7–Ru1–N9
92.57(4)
92.06(4)
87.82(4)
157.73(15)
175.81(10)
90.24(10)
86.11(9)
172.14(10)
155.69(14)
transfer hydrogenation of aromatic ketones with an electron-with-
drawing group, such as chloro and bromo, led to the high conversions
(up to 98%) (Entries 3–5, 9–10). Interestingly, this catalyst also shows
excellent activity for the conversions of five and six membered cyclic
ketones to their corresponding alcohols with 97 and 100% conver-
sions (Entries 12–13). Furthermore, complex 1 was also an efficient
catalyst for the transfer hydrogenation alkyl ketones and alpha-
tetralone (Entries 14–16). With a reduced loading of catalyst (1 of
0.05 mol%), phenylethanol of 71% was obtained with a high TON of
710 per Ru (Entry 2). This result is much higher than [C6H4-1,3-
(OPPh2{Ru(η6-p-cymene)Cl2})2] (TON is 42 per Ru) [29], [C10H6N2
{NHPPh2-Ru(η6-p-cymene)Cl2}2] (TON is 95.2 per Ru) [30] and
[C10H6N2{OPPh2-Ru(η6-p-cymene)Cl2}2] (TON is 99 per Ru) [30],
but slightly lower than [C24H16N4{OPPh2-Ru(η6-benzene)Cl2}2]
(TON is 969 per Ru) [31] and [C24H16N4{OPPh2-Ru(η6-cymene)
Cl2}2] (TON is 978 per Ru) [31]. To our delight, a low concentration
1 of 0.02 mol% could give cyclohexanol of 98%. The TON was high
up to 2,450 per Ru (Entry 12). For the substituted acetophenones, cy-
clic ketones and alkyl ketones, complex 1 showed much higher TON
than other active dinuclear ruthenium complex [29-31] even Ru
ONO-type pincer complex [28].
As to the possible mechanism for the complex 1 catalyzed transfer
hydrogenation, we inferred two points for it. On the one hand, the
base facilitates the formation of ruthenium alkoxide by abstracting pro-
ton of the alcohol and subsequently alkoxide undergoes β-elimination
to give ruthenium hydride, which is an active species in this reaction.
This is the mechanism proposed by several workers on the studies of ru-
thenium catalyzed transfer hydrogenation by metal hydride intermedi-
ates [41]. On the other hand, compared with the activity of previously
reported dinuclear ruthenium complexes [30, 31], complex 1 showed
a higher TON, but it exhibited a slower reaction rate. We suggest that
the N4 ligand could efficiently stabilize the active catalytic intermediate
and extend the lifetime of active species to give high TON. Even it keeps
on some activity for more than 20 h (Entry 12). The long lifetime is very
rare in the transfer hydrogenation catalysts. Of course, the electronically
saturated precatalyst 1 might require the dissociation of a phosphine li-
gand, which could be relatively strongly ligated to the ruthenium cen-
ter, to show a low reaction rate.
Table 1
Crystal and results of structure refinement for complex 1.
Formula
C88H56Cl2F18N10P2Ru2
Formula weight [g/mol]
Temperature [K]
1930.41
120
Wavelength [Å]
0.71073
Crystal system, space group
Unit cell dimensions [Å]
Triclinic, P−1
a=14.36413(17) α=68.9392(13)
b=17.8100(2) β=74.4675(11)
c=22.5516(3) γ=85.0345(10)
5186.96(12)
2/1.236
0.448
Cell volume [Å3]
Z/calculated density [g/cm3]
Absorption coefficient [mm−1
F(000)
]
In summary, this report describes the preparation, characteriza-
tion and transfer hydrogenation activity of a novel cationic dinuclear
ruthenium complex bearing N4 ligand. The catalytic reaction results
demonstrated that this complex is highly efficient in transfer hydro-
genation of ketones, even with 0.02 mol% loading.
1936
Crystal size [mm]
θ range for data collection [°]
Limiting indices
Reflections collected/unique
Completeness to θ=26.37°
Absorption correction
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IN2σ (I)]
R indices (all data)
0.42×0.40×0.36
2.98–26.37
−17≤h≤17, −22≤k≤22, −28≤l≤28
42189/21123 [Rint =0.0489]
99.6%
Multi-scan
21123/0/1099
1.118
Acknowledgment
R1 =0.0612, wR2 =0.1578
R1 =0.0936, wR2 =0.1722
1.261 and −0.894
We thank the financial supports from the National Natural Science
Foundation of China (Nos. 20271035 and 20371032).
Largest diff. peak and hole [e/Å]

1886
L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1884–1888
Fig. 1. ORTEP representation (ellipsoids at a 50% level) and labeling for complex 1.
Table 3
Transfer hydrogenation of ketones with 2-propanol catalyzed by complex 1.
Entry
1
Ketone
S/[Ru]
250
Time (h)
2
Yielda (%)
95
TON
238
2
3
1000
250
7
2
71
98
710
245
4
250
2
98
245
5
6
250
250
2
96
92
240
230
3.5

L. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1884–1888
1887
Table 3 (continued)
Entry
7
Ketone
S/[Ru]
250
Time (h)
3.5
Yielda (%)
92
TON
230
8
9
250
250
3.5
2
88
96
220
240
10
250
2
95
238
11
12
250
2
100
98
250
2500
24
2450
13
14
250
250
2
2
97
93
242
232
15
16
250
250
3
2
90
92
225
230
Reaction condition: KOH/catalyst=40/1; KOH, 0.1 mmol; catalyst, 2.5 μmol; 2-propanol, 3 mL; 82 °C.
a
GC yield of the alcohol product.
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Appendix A. Supplementary material
CCDC 822140 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from the Cambridge
Crystallographic Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2011.09.003.
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