Binuclear ruthenium(II) pyridazine complex catalyzed transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.tetlet.2012.06.119

A convenient and general method of synthesis of binuclear ruthenium(II) pyridazine complex was reported. The synthesized complex was characterized by analytical and spectral methods. The structure of the complex was confirmed by X-ray diffraction technique and was found to be an efficient catalyst for the transfer hydrogenation of ketones with excellent conversions in the presence of isopropanol/KOH at 82°C. The effect of solvents, bases, and different catalyst/substrate ratio for the reaction was also investigated.

Full text of DOI:10.1016/j.tetlet.2012.06.119

Tetrahedron Letters 53 (2012) 4770–4774
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Tetrahedron Letters
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Binuclear ruthenium(II) pyridazine complex catalyzed transfer
hydrogenation of ketones
⇑
Nandhagopal Raja, Rengan Ramesh
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient and general method of synthesis of binuclear ruthenium(II) pyridazine complex was
reported. The synthesized complex was characterized by analytical and spectral methods. The structure
of the complex was confirmed by X-ray diffraction technique and was found to be an efficient catalyst for
the transfer hydrogenation of ketones with excellent conversions in the presence of isopropanol/KOH at
82 °C. The effect of solvents, bases, and different catalyst/substrate ratio for the reaction was also
investigated.
Received 2 June 2012
Revised 25 June 2012
Accepted 27 June 2012
Available online 2 July 2012
Keywords:
Ó 2012 Elsevier Ltd. All rights reserved.
Binucleating pyridazine
Ruthenium(II) complex
Molecular structure
Transfer hydrogenation
Nitrogen-containing ligands have been extensively studied in
catalysis and have been shown to have important applications in
the fields of homogeneous catalysis and organic synthesis due to
the easy manipulations and high reactivity of their transition metal
complexes.1 Ruthenium homogeneous hydrogenation catalysts
have been known for the past years2,3 and they are proving to be
the most useful catalysts for these reactions. The transfer hydroge-
nation of carbonyl compounds to alcohols is an important transfor-
mation for the preparation of fine chemicals, perfumes,
agrochemicals, and pharmaceuticals.4 Compared with the com-
monly used reduction processes, which involve high hydrogen
pressure or hazardous reducing reagents (e.g., NaBH4 and
LiAlH4),5–9 transfer hydrogenation has emerged as a safe, eco-
friendly, and versatile tool for the reduction of carbonyl com-
pounds. Transition metal catalyzed transfer hydrogenation reaction
using isopropanol as a hydrogen source, has become an efficient
method in organic synthesis.8–11 In addition, large numbers of tran-
sition metal phosphine/arsine complexes have been used in cataly-
sis due to their characteristic steric and electronic properties.12
Transition metal complexes with pyridazine and related ligands
have been of great interest due to their importance in structural
properties,13 electrochemical application,14 proton transfer reac-
tions,15 magnetic property,16 material applications,17 and biological
activities.18 Campagna and his co-workers have reported the syn-
thesis and characterization of ruthenium(II) complexes containing
ligand 3,6-bis(2-pyridyl)pyridazine.19 Further, the electro catalytic
oxidation of benzyl alcohol by ruthenium dipyridopyridazine
catalyst with good conversions has been described.20 Xu et al. have
reported the synthesis and characterization of 3,6-dipyridyl substi-
tuted pyridazine biruthenium complexes along with the catalytic
activity toward the oxidation of water to molecular oxygen evolu-
tion.21 Recently, the optical and electrochemical properties of the
ruthenium complexes containing phenyl, pyridyl, and pyrazinyl
pyridazine ligands have been described.22 Although a number of
pyridazine ligand containing transition metal complexes are
known, the synthesis of
l-halo bridged ruthenium(II) complexes
bearing pyridazine ligand and their catalytic transfer hydrogena-
tion of ketones have not been described. The ruthenium phosphine
complexes have been used as catalysts for transfer hydrogenation
of ketones under basic conditions.23 Aydemir et al. have reported
the synthesis and characterization of ruthenium(II) arene com-
plexes along with their activity in the transfer hydrogenation of car-
bonyls to the corresponding alcohols.24 Ruthenium hydride
complexes with the bis(trimethylsilyl)-substituted hydroxycyclo-
pentadienyl have been reported as efficient catalysts for the hydro-
genation of aldehydes and ketones.25 Albrecht and his co-workers
reported the use of ruthenium complexes containing N-heterocy-
clic carbene as active catalysts for transfer hydrogenation of
ketones in the presence of i-PrOH/KOH.26 In view of the promising
results obtained on the synthesis and catalytic applications of Ru,
Rh, and Pd complexes from our laboratory,27–29 we focused our
interest on binuclear ruthenium complexes for investigation. We
describe here the synthesis of new binuclear ruthenium(II) pyrida-
zine complex and its catalytic application toward transfer hydroge-
nation of ketones.
One mmol pyridazine diamine ligands30 directly reacts with
2 mmol [RuHCl(CO)(AsPh3)3]31 in dry benzene under reflux
⇑
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407053.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.119

N. Raja, R. Ramesh / Tetrahedron Letters 53 (2012) 4770–4774
4771
H3C
H3C
H3C
NH
NH
OC Cl
Ru AsPh3
N
N
N
Benzene
Reflux / 6h
Cl
Cl
[RuHCl(CO)(AsPh3)3]
N
Ru AsPh3
NH
OC Cl
NH
H3C
Scheme 1. Synthesis of binuclear ruthenium(II) pyridazine complex.
Table 1
Effect of solventsa
Entry
Alcohol
Time (h)
Conversion (%)b
1
2
3
Methanol
Ethanol
Isopropanol
5/12
5/12
5/12
05/10
24/45
90/92
a
Conditions: reactions were carried out at 82 °C using substrate (1 mmol), cat-
alyst (0.002 mmol) in 5 ml of isopropanol, base (0.005 mmol).
b
Conversion of product was determined using a 1H NMR.
Table 2
Effect of basesa
Entry
Base
Time (h)
Conversion (%)b
1
2
3
4
5
6
7
8
NaOH
KOH
LiOH
5/12
5/12
5/12
5/12
5/12
5/12
5/12
5/12
90/92
90/92
12/36
21/29
11/16
0
K2CO3
Na2CO3
Et3N
tBuOK
NaO-Ac
26/30
10/14
Figure 1. X-ray crystal structure of the complex [Ru2(AsPh3)2(l-Cl)2Cl2(CO)2(L)]
a
Conditions: reactions were carried out at 82 °C using substrate (1 mmol), cat-
selected bond length (Å) and bond angle (°): As(1)–Ru(1) 2.447(1), As(2)–Ru(2)
2.444(2), N(3)–Ru(1) 2.18(1), N(2)–Ru(2) 2.19(2), Cl(3)–Ru(1) 2.455(5), Cl(2)–Ru(1)
2.387(4), Cl(4)–Ru(1) 2.425(4), C(55)–Ru(1) 1.82(2), Cl(3)–Ru(2) 2.416(3), Cl(2)–
Ru(2) 2.433(4), Cl(1)–Ru(2) 2.440(6), C(56)–Ru(2) 2.02(4), N(3)–N(2) 1.36(4), C(55)–
O(1) 1.11(3), O(2)–C(56) 0.69(5), As(1)–Ru(1)–N(3) 175.8(5), As(1)–Ru(1)–Cl(3)
97.5(1), As(1)–Ru(1)–Cl(2) 92.7(1), As(1)–Ru(1)–Cl(4) 87.9(1), As(1)–Ru(1)–C(55)
86.3(6), N(3)–Ru(1)–Cl(3) 83.8(5), As(2)–Ru(2)–N(2) 177.3(5), As(2)–Ru(2)–Cl(3)
92.5(1), As(2)–Ru(2)–Cl(2) 98.8(1), As(2)–Ru(2)–Cl(1) 87.4(1), As(2)–Ru(2)–C(56)
85(1), N(3)–Ru(1)–Cl(2) 83.5(5), N(3)–Ru(1)–Cl(4) 96.1(5), N(3)–Ru(1)–C(55)
92.2(7), Cl(3)–Ru(1)–Cl(2) 84.5(1), Cl(3)–Ru(1)–Cl(4) 86.8(1), Cl(3)–Ru(1)–C(55)
175.5(6), Cl(2)–Ru(1)–Cl(4) 171.3(1), Cl(2)–Ru(1)–C(55) 92.9(6), Cl(4)–Ru(1)–C(55)
95.8(6), N(2)–Ru(2)–Cl(3) 86.2(6), N(2)–Ru(2)–Cl(2) 83.4(6), N(2)–Ru(2)–Cl(1)
94.3(6), N(2)–Ru(2)–C(56) 93(1), Cl(3)–Ru(2)–Cl(2) 84.4(1), Cl(3)–Ru(2)–Cl(1)
170.8(2), Cl(3)–Ru(2)–C(56) 90(1), Cl(2)–Ru(2)–Cl(1) 86.6(2), Ru(2)–N(2)–N(3)
115(1).
alyst (0.002 mmol) in 5 ml of isopropanol, base (0.005 mmol).
b
Conversion of product was determined using 1H NMR.
Table 3
Effect of catalyst/substrate ratioa
Entry
Catalyst/substrate
Time (h)
Conversion (%)b
1
2
3
4
1:2000
1:1500
1:1000
1:500
24
24
24
5
35
54
62
90
a
Conditions: reactions were carried out at 82 °C using substrate (1 mmol), cat-
alyst (0.002–0.0005 mmol) in 5 ml of isopropanol, KOH (0.005 mmol).
b
Conversion of product was determined using a 1H NMR.
O
OH
R'
Catalyst: 0.002 - 0.0005 mmol
Solvent / Base
(C, H, and N) of the synthesized complex was consistent with com-
position proposed for the complex.
R
R'
R
IR spectrum of the complex confirmed the coordination of the
pyridazine nitrogen (1603 cmÀ1) and terminally coordinated car-
bonyl group (1956 cmÀ1) to the metal atom. In addition, the com-
plex shows new bands near 530, 695, 740, and 1557 cmÀ1 due to
the triphenyl arsine ligand. The electronic spectrum of the complex
in chloroform showed intense absorption at 346 nm in the ultravi-
olet region due to Metal to Ligand Charge Transfer (MLCT) transi-
R = R' = alkyl or aryl
Scheme 2. Transfer hydrogenation of ketones.
condition yielding a
l-halo bridged binuclear ruthenium(II) car-
bonyl complex [Ru2(AsPh3)2(CO)2(
l-Cl)2Cl2(L)] (L = N,N-disubsti-
tions in addition to the
p–
p⁄ and n–p⁄ ligand centered
tuted pyridazine-3,6-diamine) (Scheme 1). The complex is stable
and is soluble in common organic solvents such as chloroform,
methanol, ethanol, and acetonitrile. Elemental analysis (Calcd: C,
51.63; H, 3.71, and N, 4.30; Found: C, 51.63; H, 3.70, and N, 4.30)
transitions (262–212 nm). In 1H NMR of the complex, the aromatic
protons appeared as a multiplet in the region of 5.9–8.2 ppm.
Methyl proton appears as a singlet at 2.3 ppm.

4772
N. Raja, R. Ramesh / Tetrahedron Letters 53 (2012) 4770–4774
Table 4
Transfer hydrogenation of ketones using [Ru2(AsPh3)2(
l
-Cl)2Cl2(CO)2(L)]a
Products
Entry
1
Substrates
Conversion (%)b
90
TONc
450
O
OH
O
O
OH
2
3
4
95
96
83
475
480
415
Cl
Cl
OH
Br
Br
O
OH
H3C
H3C
O
OH
5
6
80
86
400
430
H3C
O
H3C
O
O
OH
O
O
O
O
OH
OH
OH
OH
7
8
9
67
71
90
335
355
450
10
11
99
91
495
455
O
OH
O
OH
OH
12
13
95
80
475
400
O
a
b
c
Conditions: reactions were carried out at 82 °C using 1 mmol of ketone, 0.002 mmol catalyst in 5 ml isopropanol; catalyst/ketone/KOH ratio 1:500:2.5.
Conversion of the product after 5 h was determined by using 1H NMR.
TON = rate of moles of product obtained to the moles of the catalyst used.
The molecular structure of the new binuclear ruthenium(II)
complex [Ru2(AsPh3)2( -Cl)2Cl2(CO)2(L) was resolved by single
Cl(3)–Ru(2)–Cl(2) is 84.4(1)° and Cl(3)–Ru(1)–Cl(2) is 84.5(1)°.
The terminal chlorine atom and carbonyl group are trans to the
bridging chlorine atoms, and the AsPh3 group coordinate through
As atoms at the trans sites of the pyridazine nitrogen. The Ru–
N(pyridazine) distances N(2)–Ru(1) 2.18(1) Å and N(3)–Ru(2)
2.19(2) Å are similar to those reported for ruthenium complexes
containing the same coordinating atoms.30 A major perturbation
of the structure is observed due to the bridging coordination nat-
ure of the pyridazine molecule. Since, the lone pairs of the two
nitrogen atoms effectively interact with each metal atom, the
two ruthenium-centered octahedral may be bent toward the bridg-
ing ligand. The Ru(II)–As distances As(1)–Ru(1) 2.447(1) Å and
As(2)–Ru(2) 2.444(2)–Å are comparatively equal. The Ru–Cl (bridg-
l
crystal X-ray crystallography and was shown inFigure 1. The struc-
tural view of the complex shows clearly that the two ruthenium
atoms are present in the complex and each ruthenium is coordi-
nated to pyridazine nitrogen and is connected by two bridged chlo-
ride ion. Each ruthenium metal contains one terminal chloride ion,
one terminal carbonyl, and one terminal AsPh3 groups. Ruthenium
is therefore sitting in a N2Cl2(l-Cl)2(CO)2As2 coordination environ-
ment, which gives rise to distorted octahedral geometry as re-
flected in all the bond parameters around each ruthenium atom.
As expected the Ru2(l-Cl)2 unit is not planar due to the presence
of the pyridazine ring. The angle between the planes containing

N. Raja, R. Ramesh / Tetrahedron Letters 53 (2012) 4770–4774
4773
ing) distances are longer than the Ru–Cl (terminal) distances,
which are comparable with the distances reported for other struc-
turally characterized binuclear ruthenium(II) complexes contain-
ing bridging and terminal chlorides.32,33
Br > Cl > H > Me > OMe. On the other hand, benzophenone is
quantitatively reduced to 1,1-diphenylmethanol and shows the
conversion of 86% (entry 6). Further, ruthenium complex was dem-
onstrated to be a very active catalyst toward a variety of alkyl ke-
tones. 2-Butanone underwent transfer hydrogenation to afford the
corresponding alcohol in 67%. The conversions of 2-pentanone and
3-pentanone into corresponding alcohols are 71% and 95%, respec-
tively. Similarly, the complex shows good activity for the transfer
hydrogenation of cyclic ketones. Particularly, 1-cyclopentanone
was reduced to 1-cyclopentanol almost quantitatively (entry 10).
In addition, the complex exhibits better catalytic activity for six,
seven, and eight membered cyclic ketones, 1-cyclohexanone,
1-cycloheptanone, and 1-cyclooctanone, with 91%, 95%, and 80%
conversions, respectively (entries 11–13). The only byproduct ace-
tone was identified in all the cases. As the catalyst is stable in all
organic solvents it can be recovered and the work-up process is
also very simple for this catalytic system.
Ruthenium mediated transfer hydrogenation reactions are
found to be effective catalytic systems in which hydrogen is trans-
ferred from one organic molecule to another and this made us to
carry out this type of reactions.34 The complex [Ru2(AsPh3)2(
l-
Cl)2Cl2(CO)2(L)] is taken as model catalyst and the catalytic activity
in the transfer hydrogenation of various aliphatic and aromatic ke-
tones in the presence of isopropanol and base as promoter has
been explored (Scheme 2). The necessity of the ruthenium complex
to observe the ensuing transfer hydrogenation was ascertained by
carrying out a series of blank or control experiments which suggest
that none of RuCl3Á3H2O, Ru(II) arsine precursors or pyridazine li-
gand alone or as a mixture causes these transformations under
identical reaction conditions. In order to optimize the reaction con-
dition, different solvents, bases, and catalyst: substrate (C/S) ratios
were studied.
In conclusion, a new binuclear ruthenium(II) pyridazine car-
bonyl complex [Ru2(AsPh3)2(l-Cl)2Cl2(CO)2(L) has been synthe-
For overall optimization, acetophenone was taken as test sub-
strate for different conditions. We have carried out the transfer
hydrogenation in the presence of various solvents and KOH as a
base and the results are summarized in Table 1. Methanol, ethanol,
and isopropanol were taken for investigations and isopropanol is
found to be a suitable system for the maximum conversion of ace-
tophenone. As seen in Table 1, the use of methanol resulted in poor
conversion of the substrate (entry 1) when compared to ethanol
and isopropanol.
sized and characterized. X-ray diffraction study of the complex
confirms the bridging coordination of the pyridazine ligand via
two chlorine atoms and it reveals the presence of distorted octahe-
dral geometry around ruthenium. The complex is found to be an
active catalyst in the transfer hydrogenation of ketones with good
to excellent conversions.
Acknowledgments
The choice of base was chosen, as a next step for the optimiza-
tion. In transfer hydrogenation, the base facilitates the formation of
ruthenium alkoxide by abstracting proton from the alcohol and
subsequently alkoxide undergoes b-elimination to give ruthenium
hydride, which is an active species in this reaction. This mecha-
nism has been proposed by several workers for the ruthenium cat-
alyzed transfer hydrogenation reaction by metal hydride
intermediates.35 The substrate was allowed to react with a cata-
lytic amount of complex in the presence of different bases like
One of the authors (N.R.) thank University Grants Commission
(UGC), New Delhi for the award of UGC-SAP RFSMS Scholarship.
We thank DST and UGC for XRD and NMR facilities at the School
of Chemistry, Bharathidasan University
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
06.119.
t
NaOH, KOH, LiOH, K2CO3, KHCO3, Na2CO3, Et3N, and BuOK with
isopropanol (Table 2). It was found that the use of NaOH and
KOH leads to 90% of conversion (entries 1, 2), and therefore the
choice of bases for these reactions. In the absence of base, no con-
version of ketones into alcohol was observed. Similarly, the use of
triethylamine as base also leads to no conversion.
References and notes
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34. Procedure for catalytic transfer hydrogenation of ketones: Under nitrogen
atmosphere a mixture containing ketones (1 mmol), the ruthenium catalyst
(0.002 mmol), and KOH (0.005 mmol) was heated to reflux in 5 ml of
isopropanol for 5 h. The catalyst was removed from the reaction mixture by
the addition of diethyl ether followed by filtration and subsequent
neutralization with 1 M HCl. The ether layer was filtered through a short
path of silica gel by column chromatography. The filtrate was subjected to 1H
NMR analysis.
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