Catalytic transfer hydrogenation of ketones catalyzed by orthometalated ruthenium(III) 2-(arylazo)phenolate complexes containing triphenylarsine

Article abstract of DOI:10.1016/j.tetlet.2005.05.116

Air-stable, mononuclear orthometalated ruthenium(III) 2-(arylazo)phenolate complexes of the general composition [RuX(AsPh₃)₂(L)] (X = Cl or Br; L = CNO donor of the 2-(arylazo)phenolate ligands) have been synthesized and characterized by IR, UV-vis, and EPR as well as by elemental analysis. One of the complexes [RuBr(AsPh₃)₂(azo-OMe)] was structurally characterized by X-ray analysis and was found to be an efficient catalyst for the transfer hydrogenation of ketones with excellent conversion in the presence of isopropanol at 80°C in 1 h.

Full text of DOI:10.1016/j.tetlet.2005.05.116

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5215–5218
Catalytic transfer hydrogenation of ketones catalyzed by
orthometalated ruthenium(III) 2-(arylazo)phenolate
complexes containing triphenylarsine
Galmari Venkatachalam and Rengan Ramesh*
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
Received 13 April 2005; revised 18 May 2005; accepted 25 May 2005
Available online 15 June 2005
Abstract—Air-stable, mononuclear orthometalated ruthenium(III) 2-(arylazo)phenolate complexes of the general composition
[RuX(AsPh3)2(L)] (X = Cl or Br; L = CNO donor of the 2-(arylazo)phenolate ligands) have been synthesized and characterized
by IR, UV–vis, and EPR as well as by elemental analysis. One of the complexes [RuBr(AsPh3)2(azo–OMe)] was structurally
characterized by X-ray analysis and was found to be an efficient catalyst for the transfer hydrogenation of ketones with excellent
conversion in the presence of isopropanol at 80 °C in 1 h.
Ó 2005 Elsevier Ltd. All rights reserved.
The intramolecular C–H activation, or cyclometalation/
orthometalation reaction, which is a major achievement
of organometallic chemistry, provides access to metala-
cyclic derivatives of the transition metals.1 Many papers
dealing with this subject have been published in the lite-
rature over the last 20 years demonstrating the enormous
interest generated particularly by the cycloruthenated
compounds.2 More recently, transition metal catalyzed
transfer hydrogenation reactions using isopropanol as
the hydrogen source have become efficient methods in
organic synthesis as illustrated by several useful applica-
tions reported in recent years.3 Among the different
metal catalyzed reactions, ruthenium based catalytic
systems are effective in the transfer hydrogenation of
ketones.4 Noyori and Ohkuma discovered that ruth-
enium(II) complexes containing diphosphine and 1,2-di-
amine ligands, in the presence of a base and 2-propanol,
are excellent catalysts for the hydrogenation of ketones
under mild conditions.5 Although the well known
BINAP–Ru(II) complex catalysts [BINAP = 2,20-bis-
(diphenylphosphine)-1,10-binaphthyl] have proved to be
extremely efficient for the asymmetric hydrogenation of
functionalized ketones, they are unable to catalyze the
asymmetric hydrogenation of simple ketones because
of the lack of heteroatoms to anchor the ruthenium
metal.6 Furthermore the N–N- and O–N- donors of the
ligands play an important role in the catalytic reactions.
Recently, Chakravarty et al.3b reported a ruthenium-
(II)p-cymene system containing arylazo phenolate
ligands which in the presence of base and isopropanol,
proved to be a good catalysts for the transfer hydrogena-
tion of acetophenone. In addition, large numbers of tran-
sition metal phosphine/arsine complexes have been used
in catalysis due to their characteristic steric and electron-
ic properties.7 Although a number of cyclometalated
transition metal complexes are known, to the best of
our knowledge, there are no reports available on synth-
esis and catalytic transfer hydrogenation of orthometa-
lated ruthenium(III) 2-(arylazo)phenolate complexes
incorporating C-, N-, and O- donors and triphenylarsine.
Herein, we describe a convenient synthesis of a series of
new orthometalated ruthenium(III) 2-(arylazo)phenol-
ate complexes along with a study of their catalysis of
transfer hydrogenation of a variety of ketones.
The direct reaction of the ruthenium(III) precursors,
[RuCl3(AsPh3)3] and [RuBr3(AsPh3)3] with 2-(arylazo)-
phenol ligands8 in dry benzene under reflux yielded a
series of mononuclear orthometalated ruthenium com-
plexes of the type [RuX(AsPh3)2(L)] (X = Cl or Br;
L = CNO donor of the ligands).9 The choice of these
molecules was due to their versatility as ligands, since
they exhibit various modes of bonding to the metallic
center and also because of the potential biological and
Keywords: Synthesis; Ruthenium(III); Orthometalation; CNO donor;
Characterization; Transfer hydrogenation.
*
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045,
2407020; e-mail: ramesh_bdu@yahoo.com
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.05.116

5216
G. Venkatachalam, R. Ramesh / Tetrahedron Letters 46 (2005) 5215–5218
H3C
H3C
OH
AsPh3
[RuCl3(AsPh3)3]
O
Benzene
reflux
N
(OR)
N N
Ru
AsPh3
X
N
R
[RuBr3(AsPh3)3]
R
Where X=Cl (or)Br: R=H; azo-H (HL1), R=Cl; azo-Cl (HL2), R=Me; azo-Me (HL3),
R=OMe; azo-OMe (HL4), R=OEt; azo-OEt (HL5)
Figure 1. Structures of orthometalated ruthenium(III) 2-(arylazo)phenolate complexes.
catalytic importance of their complexes. The 2-(aryl-
azo)phenolate ligands were observed to undergo C–H
activation at one ortho position of the phenyl ring in
the arylazo fragment leading to the formation of five-
membered cyclometalated complexes (Fig. 1). Elemental
analyses (C, H, and N) were consistent with the compo-
sition proposed for all complexes.
The IR spectra of the complexes confirmed the coordi-
nation of the azo groups (mN@N = 1376–1388 cmꢀ1
)
and phenolic oxygens (mC–O = 1286–1310 cmꢀ1) to the
metal atoms. In addition, these complexes showed new
bands near 540, 690, 740, and 1554 cmꢀ1 due to the tri-
phenylarsine ligands. The electronic spectra of the
complexes in the visible region showed intense absorp-
tion at 665–630 nm (e = 708–1280 dm3/mol/cm) due to
d–d transitions and an absorption at 419–409 nm
(e = 1668–3268 dm3/mol/cm) probably due to ligand-
to-metal charge transfer (LMCT) transitions in addition
to the p–p* and n–p* ligand centered transitions. Room
temperature magnetic susceptibility measurements
showed that these complexes (1.92–1.96 BM) are one
electron paramagnetic which supports the trivalent state
of the ruthenium (low spin d5; S = 1/2) in an octahedral
environment. The EPR spectra of all the complexes in
the solid state exhibited well-defined single isotropic fea-
tures near giso = 2. However, the complex [RuBr(As-
Ph3)2(azo–OMe)] in dichloromethane at 77 K showed
a rhombic spectrum with three distinct ÔgÕ values
(gx 5 gy 5 gz, gx = 2.212, gy = 2.019, gz = 1.920) indi-
cating rhombic distortion in these complexes.
Figure 2. The X-ray crystal structure of the complex [RuBr(As-
˚
Ph3)2(azo–OMe)]; selected bond lengths [A] and bond angles [°]:
Ru(1)–N(1) 2.011(5), Ru(1)–C(8) 2.038(6), Ru(1)–O(1) 2.129(4),
Ru(1)–As(2) 2.4727(8), Ru(1)–As(1) 2.4727(8), Ru(1)–Br(1)
2.4882(8), N(1)–N(2) 1.265(6), N(1)–C(7) 1.413(7), N(2)–C(9)
1.391(7), O(1)–C(1) 1.312(7), N(1)–Ru(1)–C(8) 75.5(2), N(1)–Ru(1)–
O(1) 78.55(18), C(8)–Ru(1)–O(1) 152.5(2), C(8)–Ru(1)–As(2)
94.46(16), O(1)–Ru(1)–As(2) 93.87(11), N(1)–Ru(1)–As(1) 92.96(13),
N(1)–Ru(1)–As(2) 88.55(13), C(8)–Ru(1)–As(1) 85.37(16), O(1)–
Ru(1)–As(1) 87.00(11), As(2)–Ru(1)–As(1) 178.38(3), N(1)–Ru(1)–
Br(1) 176.38(13), C(8)–Ru(1)–Br(1) 107.35(16), O(1)–Ru(1)–Br(1)
98.95(12), As(2)–Ru(1)–Br(1) 89.01.
the metal center. The overall CNOAs2Br coordination
sphere around the ruthenium is distorted octahedral in
nature, which is reflected in all the bond parameters
around ruthenium.
To understand the coordination mode of the 2-(arylazo)-
phenolate ligands and the stereochemistry of the ruth-
enium(III) complexes, the X-ray crystal structure of
the complex [RuBr(AsPh3)2(azo–OMe)] was determined
and is shown in Figure 2. The 2-(arylazo)phenolate
ligand is coordinated in a tridentate fashion binding the
metal center at O(1), N(1), C(1) so forming two five-
membered chelate rings with bite angles of 78.55(18)
for N(1)–Ru(1)–O(1) and 75.5(2) for N(1)–Ru(1)–C(8).
The entire tridentate ligand skeleton together with the
ruthenium and bromide constitute the equatorial plane
while the bromide is in the trans position with respect
to the azo nitrogen. The two AsPh3 ligands are mutually
trans to each other, the Ru(1)–As(1) [2.472(8)] and
Ru(1)–As(2) [2.472(8)] bond distances being equal from
Catalytic transfer hydrogenation in which hydrogen is
transferred from one organic molecule to another by
ruthenium complexes is well known and this prompted
us to carry out this type of reaction. The present experi-
mental procedure for catalytic transfer hydrogenation
involving a variety of aliphatic and aromatic ketones
using [RuBr(AsPh3)2(azo–OMe)] was carried out as fol-
lows; under an inert atmosphere a mixture containing
the ketone (3.75 mmol), ruthenium catalyst (0.0125
mmol) and 0.03 mmol of KOH was heated to reflux in
5 ml of isopropanol at 80 °C for the appropriate period
as quoted in Table 1. The catalyst was removed from the
reaction mixture by the addition of light petroleum
followed by filtration and subsequent neutralization

G. Venkatachalam, R. Ramesh / Tetrahedron Letters 46 (2005) 5215–5218
5217
Table 1. Catalytic activity in transfer hydrogenation of ketones by [RuBr(AsPh3)2(azo–OMe)]/i-PrOH/KOHa
Entry
Substrate
Product
Time (h)
Conversion (%)
TONb (TOF)c
O
OH
1
1
97
291 (291)
258 (129)
O
OH
2
3
4
2
2
1
86
79
H3CO
H3CO
OH
O
237 (119)
297 (297)
Cl
Cl
OH
O
>99
OH
OH
O
O
5
6
1
1
>99
>99
297 (297)
297 (297)
O
OH
7
8
2
2
>99
85
297 (149)
255 (128)
O
OH
OH
O
9
2
98
294 (147)
a Conditions: reactions were carried out at 80 °C using 3.75 mmol of ketone (5 ml isopropanol); catalyst/ketone/KOH ratio 1:300:2.5.
b Yield of product was determined using a HP 6890 series GC-FID with a DP-5 column of 30 m length, 0.32 mm diameter and 0.25 lm film thickness
and by comparison with authentic samples. TON = ratio of moles of product obtained to the moles of the catalyst used.
c TOF (TON hꢀ1).
with 1 M HCl. The ether layer was filtered through a
short path of silica gel. The filtrate was subjected to
GC analysis and the hydrogenated product was identi-
fied by comparison with authentic samples in all cases.
The catalyst performed efficiently for both aliphatic
and aromatic ketones with high conversion and turn-
over. However, the reduction of p-methoxy and p-chloro-
acetophenone was slower and led to a lower yields when
compared to the reduction of acetophenone. It was gen-
erally observed from the literature, that the catalytic
transfer hydrogenation of aliphatic ketones was found
to be difficult and takes a longer time for maximum con-
version. Interestingly, the complex catalyzed efficiently
the reduction of diethyl ketone, methyl propyl ketone,
and isobutyl methyl ketone with a high percentage of
conversion. Moreover, the complex showed good activ-
ity, >99%, for the transfer hydrogenation of both five-
and six-membered cyclic ketones. In the absence of a
base no transfer hydrogenation of ketones was ob-
served. The base facilitated the formation of the ruth-
enium alkoxide by abstracting the proton of the alcohol
and subsequently, the alkoxide underwent b-elimination
to give a ruthenium hydride which is the active species in
this reaction. This mechanism has been proposed by sev-
eral workers on the studies of ruthenium catalyzed
transfer hydrogenation reactions involving metal hy-
dride intermediates.10 Although several catalytic systems
have been reported to support transfer hydrogenation
reactions of ketones, a catalyst of this type is novel for
its CNO donor, AsPh3 and Cl or Br ligand environment.
In conclusion, a series of new orthometalated ruth-
enium(III) 2-(arylazo)phenolate complexes have been
synthesized, characterized, and employed as new and
efficient catalysts for transfer hydrogenation of ketones.
Acknowledgements
Financial support from the Council of Scientific and
Industrial Research (CSIR), New Delhi [Grant No. 01
(1725)/02/EMR-II] is gratefully acknowledged. One of
the authors (G.V.) thanks CSIR for the award of Senior
Research Fellowship (SRF). We thank the National

5218
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Single Crystal X-ray Diffraction Facility, IIT Bombay,
Mumbai for providing X-ray data collection.
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