Ruthenium(III) complexes of amine-bis(phenolate) ligands as catalysts for transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.poly.2007.10.024

Twelve ruthenium(III) complexes bearing amine-bis(phenolate) tripodal ligands of general formula [Ru(L¹-L³)(X)(EPh₃)₂] (where L¹-L³ are dianionic tridentate chelator) have been synthesized by the reaction of ruthenium(III) precursors [RuX₃(EPh₃)₃] (where E = P, X = Cl; E = As, X = Cl or Br) and [RuBr₃(PPh₃)₂(CH₃OH)] with the tripodal tridentate ligands H₂L¹, H₂L² and H₂L³ in benzene in 1:1 molar ratio. The newly synthesized complexes have been characterized by analytical (elemental and magnetic susceptibility) and spectral methods. The complexes are one electron paramagnetic (low-spin, d⁵) in nature. The EPR spectra of the powdered samples at RT and the liquid samples at LNT shows the presence of three different 'g' values (g_x ≠ g_y ≠ g_z) indicate a rhombic distortion around the ruthenium ion. The redox potentials indicate that all the complexes undergo one electron transfer process. The catalytic activity of one of the complexes [Ru(pcr-chx)Br(AsPh₃)₂] was examined in the transfer hydrogenation of ketones and was found to be efficient with conversion up to 99% in the presence of isopropanol/KOH.

Full text of DOI:10.1016/j.poly.2007.10.024

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 701–708
www.elsevier.com/locate/poly
Ruthenium(III) complexes of amine-bis(phenolate) ligands
as catalysts for transfer hydrogenation of ketones
*
Sethuraman Kannan, K. Naresh Kumar, Rengan Ramesh
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
Received 15 October 2007; accepted 30 October 2007
Available online 3 December 2007
Abstract
Twelve ruthenium(III) complexes bearing amine-bis(phenolate) tripodal ligands of general formula [Ru(L1–L3)(X)(EPh3)2] (where
L1–L3 are dianionic tridentate chelator) have been synthesized by the reaction of ruthenium(III) precursors [RuX3(EPh3)3] (where
E = P, X = Cl; E = As, X = Cl or Br) and [RuBr3(PPh3)2(CH3OH)] with the tripodal tridentate ligands H2L1, H2L2 and H2L3 in benzene
in 1:1 molar ratio. The newly synthesized complexes have been characterized by analytical (elemental and magnetic susceptibility) and
spectral methods. The complexes are one electron paramagnetic (low-spin, d5) in nature. The EPR spectra of the powdered samples at
RT and the liquid samples at LNT shows the presence of three different ‘g’ values (gx ¼ gy ¼ gz) indicate a rhombic distortion around the
ruthenium ion. The redox potentials indicate that all the complexes undergo one electron transfer process. The catalytic activity of one of
the complexes [Ru(pcr-chx)Br(AsPh3)2] was examined in the transfer hydrogenation of ketones and was found to be efficient with
conversion up to 99% in the presence of isopropanol/KOH.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Synthesis; Ruthenium(III); Symmetrical tripodal chelator; Characterization; Catalytic transfer hydrogenation
1. Introduction
merization, nucleophilic addition to multiple bonds and
carbon–carbon bond formation [6]. The ability of transi-
tion metal catalysts to add or remove hydrogen from
organic substrates by either direct or transfer hydrogena-
tion process is a valuable synthetic tool [7]. Ruthenium
complexes have a long pedigree as catalysts for transfer
hydrogenation reactions in the presence of 2-propanol as
hydrogen source [8,9]. Several recent examples of ruthe-
nium N-heterocyclic carbene complexes [10–12], Ru(arene)-
(diamine) [13] and Ru(BINAP)(diamine) [14] catalysts have
become the most prominent members for the reduction of
ketones in high yields. Pincer-type arylruthenium(II) com-
plexes containing the monoanionic terdentate NCN and
PCP ligands have been reported by van Koten as active
catalysts for the transfer hydrogenation of ketones in the
presence of i-PrOH and KOH [15]. A number of other
groups have followed Noyori’s work using XylBinap [16]
and have demonstrated the use of other diphosphines that
give rise to high activities and selectivities when used in this
catalyst system [17]. Ruthenium(II) complex bearing the
Even though highly efficient catalytic systems for selec-
tive transformations are known [1–3], the design of efficient
catalysts for large-scale applications is still a challenging
problem. Noteworthy, a prerequisite for achieving high
activity and selectivity is the fine tuning of the metal by
introduction of ligands. With the high activity, ruthenium
complexes play a central role in many organic transforma-
tions as versatile catalyst due to its reversible and accessible
oxidation states [4]. Many research groups are developing
the synthetic chemistry related to ruthenium complexes
because of their multiple applications in many different
scientific fields [5]. Ruthenium complexes have been used
as catalysts or catalyst precursors for a variety of purposes
including hydrogenation, oxidation, isomerisation, poly-
*
Corresponding author. Tel.: +91 431 2407053; fax: + 91 431 2407045/
2407020.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.10.024

702
S. Kannan et al. / Polyhedron 27 (2008) 701–708
ligand 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine in isopro-
panol [18] has been proved to be an efficient catalyst for
the transfer hydrogenation of ketones. The diversity of ter-
tiary phosphines in terms of their Lewis basicity and bulk-
iness render them excellent candidates to tune the reactivity
of octahedral complexes towards a variety of chemical pro-
cesses, such as oxidative addition and substitution reac-
tions [19]. Even though there are number of reports
available on the transfer hydrogenation of ketones, to the
best of our knowledge, there are no reports available for
catalytic transfer hydrogenation of ketones by ruthenium
tripodal complexes.
Tripodal ligands are in general facially coordinating tri-
dentate chelate ligands which have received considerable
attention in both coordination and organometallic chemis-
try. Over the past 20 years, there have been hundreds of
published reports concerning various aspects of metal com-
plexes which contain tripod ligands. Although the coordi-
nation properties of tripodal ligand have been extensively
investigated using first row transition metals [20], few sec-
ond and third row transition metals have been the target
of studies with this kind of ligand. Tripodal complexes
are of considerable interest as they find application in cat-
alytic processes such as polymerization [21], epoxidation
[22], enantioselective hydrogenation of olefins [23], Diels–
Alder reaction [24], addition of TMS-CN to carbonyl com-
pounds [25].
In view of the growing interest in the catalytic activity of
ruthenium complexes to act as an efficient catalyst in trans-
fer hydrogenation of ketones, we herein report the synthe-
sis of a series of hexacoordinated ruthenium(III) tripodal
complexes containing PPh3/AsPh3. The characterization
of the complexes were accomplished by analytical and
spectral (FT-IR, UV–Vis and EPR) methods. The redox
properties of the complexes have been studied by cyclic vol-
tammetric technique. Further, one of the synthesized com-
plexes has been effectively used as catalyst in transfer
hydrogenation of ketones in the presence of isopropanol
and KOH as base. The following symmetrical amine-
bis(phenolate) tripodal ligands have been used to synthesis
the ruthenium(III) complexes (Scheme 1).
fication. The primary amines and p-cresol were purchased
from Aldrich. The supporting electrolyte tetrabutyl ammo-
nium perchlorate (TBAP) was dried in vacuum prior to
use. The ketones and their corresponding alcohols, which
were used in the catalytic studies, were purchased from
Merck and Aldrich.
2.2. Physical measurements
The analysis of carbon, hydrogen and nitrogen were per-
formed by analytic function testing VarioEL III CHNS ele-
mental analyzer at STIC, Cochin University of Science and
Technology, Cochin, India. FT-IR spectra were recorded
in KBr pellets with a JASCO 460 plus spectrophotometer
in the range 4000–400 cmꢀ1. A Cary 300 Bio UV–Vis Var-
ian spectrophotometer was used to record the electronic
spectra. Room temperature solid-state magnetic suscepti-
bility measurements were carried out by EG and G model
155 vibrating sample magnetometer at IIT, Chennai. EPR
spectra were recorded on JEOL JES-FA 200 EPR spec-
trometer at X-band frequencies for powder samples at
278 K and solution at 77 K (liquid nitrogen) with micro-
wave power 1.00 mW, modulation amplitude 125 and the
field being calibrated with diphenylpicryl hydrazyl (DPPH,
g = 2.0037) at Pondicherry University, India. Electrochem-
ical studies were performed using a Princeton EG and G-
PARC model potentiostat with 0.05 M dichloromethane
solutions of [(n-C4H9)4N]ClO4 (TBAP) as supporting elec-
trolyte under a nitrogen atmosphere. A three-electrode cell
was employed with glassy carbon working electrode, a plat-
inum wire auxiliary electrode and an Ag/AgCl reference
electrode. Melting points were recorded with a Boetius
micro-heating table and are uncorrected. The conversion
of products from the catalytic reactions was determined
using HP 6890 series GC-FID with a DP-5 column of
30 m length, 0.32 mm diameter and 0.25 lm film thickness.
The ruthenium(III) complexes [RuCl3(PPh3)3], [RuCl3-
(AsPh3)3], [RuBr3(AsPh3)3], [RuBr3(PPh3)2(CH3OH)] [27–
30] were prepared according to the literature reports.
2.3. Preparation of amine-bis(phenolate) tripodal ligands
2. Experimental
A solution of p-cresol (20.0 mmol, 2.16 g), primary
amines such as methylamine, cyclohexylamine and 2-ami-
nopyridine (10.0 mmol, 0.31–0.99 g) and aqueous formal-
dehyde (1.15 g, 38.4 mmol) in methanol (5 mL) was
stirred and refluxed for 18 h. The mixture was cooled
in freezer overnight and the supernatant solution was
decanted. The solid residue was washed with ice cold
methanol, filtered, and washed thoroughly with cold
methanol. The solid residue was then again washed with
acetonitrile for recrystallization to give a colorless prod-
uct [31].
2.1. Reagents and materials
All the reagents used were chemically pure and AR
grade. Solvents were purified and dried according to stan-
dard procedures [26]. RuCl3 Æ 3H2O was purchased from
Loba Chemie Pvt. Ltd. and was used without further puri-
R
Where R = -CH3 : H2L1 (pcr-me);
CH3
H3C
-C6H11 : H2L2 (pcr-chx);
-C5H4N : H2L3 (pcr-ampy)
Spectral and analytical data for ligands: H2L1 (pcr-me)
N
1
(white solid): M.p.: 58 ꢁC. IR (KBr, cmꢀ1): 3442, 1260. H
HO
OH
NMR (400 MHz, CDCl3): d 10.6(s, 2H, OH), d 6.9–8.0 (m,
6H, Ar–H), d 3.3 (s, 4H, benzylic protons), d 2.3 (s, 9H,
Scheme 1. Structure of tripodal ligands.

S. Kannan et al. / Polyhedron 27 (2008) 701–708
703
R
N
CH3). Anal. Calc. for C17H21NO2: C, 75.24; H, 7.80; N,
5.16. Found: C, 74.95; H, 7.68; N, 4.98%.
CH3
[RuX3(EPh3)3]
or
[RuBr3(PPh3)2(CH3OH)]
H3C
reflux/ 7 h
EPh3
+
H2L2 (pcr-chx) (white solid): M.p.: 157 ꢁC. IR (KBr,
HO
OH
1
cmꢀ1): 3440, 1274. H NMR (400 MHz, CDCl3): d 10.5
(s, 2H, OH), d 7.0–7.8 (m, 6H, Ar–H), d 3.6 (s, 4H, benzylic
protons), d 2.2 (s, 6H, CH3), d 1.0–2.0 (br multiplets, 11 H).
Anal. Calc. for C22H29NO2: C, 77.83; H, 8.61; N, 4.12.
Found: C, 77.61; H, 8.46; N, 3.88%.
1:1
Benzene
H3C
H2L3 (pcr-ampy) (white solid): M.p.: 150 ꢁC. IR (KBr,
1
cmꢀ1): 3445, 1251. H NMR (400 MHz, CDCl3): d 9.8 (s,
O
2H, OH), d 6.9–7.5 (m, 10H, Ar–H), d 3.7 (s, 4H, benzylic
Where, E = P or As, X = Cl or Br;
R = -CH3, -C6H11, -C5H4N
Ru
X
N
R
protons),
d 2.2 (s, 6H, Ar–CH3). Anal. Calc. for
O
EPh3
C21H22N2O2: C, 75.42; H, 6.53; N, 8.37. Found: C, 75.20;
H, 6.34; N, 8.04%.
H3C
2.4. Synthesis of ruthenium(III) tripodal complexes
Scheme 2. Structure of Ru(III) tripodal complexes.
All the reactions were carried out under anhydrous
conditions and were prepared by the following general
procedure. To
a methanol molecule from the precursor complexes (see
Table 1).
a
benzene (20 cm3) solution of the
ruthenium(III) precursors [RuX3(EPh3)3] (0.120–0.157 g;
0.125 mmol) (E = P, X = Cl; E = As, X = Cl or Br) or
[RuBr3(PPh3)2(CH3OH)] (0.112 g; 0.125 mmol) was added
the appropriate tripodal ligands (0.030–0.032 g;
0.125 mmol) (H2L1–H2L3). The solution was allowed to
heat under reflux for 7–8 h. The reactions were monitored
by TLC. The reaction mixture gradually changed to deep
color and the solvent was removed under reduced pressure.
The residue was precipitated by addition of light petroleum
ether (60–80 ꢁC) and recrystallized from CH2Cl2/pet. ether
(60–80 ꢁC). The compounds were dried under vacuum
(yield: 58–72%).
3.1. Spectroscopic characterization
Infrared spectra of the complexes show many sharp and
strong vibrations within 400–1600 cmꢀ1 and the assignment
of all these vibrations has not been attempted. However, the
ligands exhibit bands in the region 1251–1274 cmꢀ1 corre-
sponding to phenolic mC–O stretching vibration. The coordi-
nation through phenolic oxygen is confirmed by the increase
of mC–O stretching at higher frequencies in the region 1258–
1304 cmꢀ1 in all the complexes (Table 2). This was further
supported by the disappearance of mOH band in the range
3440–3445 cmꢀ1 in all the complexes. Further, the com-
plexes show a strong band near 530, 690, 740 and
1556 cmꢀ1, which are due to PPh3/AsPh3 ligands. Besides,
all the complexes show strong bands corresponding to
mM–N and mM–O in the region 400–450 cmꢀ1 and 500–
550 cmꢀ1, respectively [32,33].
2.5. Procedure for catalytic transfer hydrogenation
Under an inert atmosphere a mixture containing ketones
(3.75 mmol), the ruthenium catalyst (0.0125 mmol) and
(0.0625 mmol) KOH was heated to reflux in 10 ml of
i-PrOH for appropriate period of time as mentioned. The
catalyst was removed as precipitate from the reaction mix-
ture by the addition of diethyl ether followed by filtration
and subsequent neutralization with 5 ml of 1 M HCl. Then
the ether layer was passed through a short path of silica gel
and the filtrate was subjected to GC analysis. The hydroge-
nated product was identified and was determined with
authentic sample.
The electronic spectra of all the complexes were
recorded in dichloromethane solution in the range 800–
200 nm. All the complexes exhibit three to four absorption
bands in the region 754–231 nm and are listed in Table 2.
2
The ground state of ruthenium(III) is T2g and the first
excited doublet levels in the order of increasing energy
2
2
1
are A2g and A1g, both arise from t2g4eg configuration.
The bands observed in the region of 754–600 nm are attrib-
utable to d–d transitions and the absorptions observed in
the region of 575–403 nm are probably due to charge
transfer transitions taking place from the filled ligand
(HOMO) orbital to the singly-occupied ruthenium t2 orbi-
tal (LUMO). In most of the ruthenium(III) complexes the
charge transfer bands of the type Lpy ! T2g are prominent
in the low energy region, which obscures the weaker bands
due to d–d transitions [34]. It is therefore difficult to assign
conclusively the bands of ruthenium(III) complexes that
appear in the visible region. However, the extinction
3. Results and discussions
Ruthenium(III) precursors of the type [RuX3(EPh3)3]
(E = P, X = Cl; E = As, X = Cl or Br) or [RuBr3(PPh3)2-
(CH3OH)] were allowed to react with the tripodal ligands
(L1–L3) in 1:1 molar ratio in dry benzene resulting in the
formation of low-spin ruthenium(III) complexes (Scheme
2). In all the reactions, the ligands replace two chlorides/
bromides, one triphenylphosphine or triphenylarsine and

704
S. Kannan et al. / Polyhedron 27 (2008) 701–708
Table 1
Analytical data of ruthenium(III) tripodal complexes
S. no.
Complexes
Empirical formula
Color
Melting point (ꢁC)
Elemental analysis
Found (calculated) (%)
C
H
N
1
2
[Ru(pcr-me)Cl(PPh3)2]
[Ru(pcr-me)Cl(AsPh3)2]
[Ru(pcr-me)Br(AsPh3)2]
[Ru(pcr-me)Br(PPh3)2]
[Ru(pcr-chx)Cl(PPh3)2]
[Ru(pcr-chx)Cl(AsPh3)2]
[Ru(pcr-chx)Br(AsPh3)2]
[Ru(pcr-chx)Br(PPh3)2]
[Ru(pcr-ampy)Cl(PPh3)2]
[Ru(pcr-ampy)Cl(AsPh3)2]
[Ru(pcr-ampy)Br(AsPh3)2]
[Ru(pcr-ampy)Br(PPh3)2]
C53H49NO2ClP2Ru
C53H49NO2ClAs2Ru
C53H49NO2BrAs2Ru
C53H49NO2BrP2Ru
C58H57NO2ClP2Ru
C58H57NO2ClAs2Ru
C58H57NO2BrAs2Ru
C58H57NO2BrP2Ru
C57H50NO2ClP2Ru
C57H50NO2ClAs2Ru
C57H50NO2BrAs2Ru
C57H50NO2BrP2Ru
brown
brown
brown
brown
green
brown
brown
brown
brown
brown
brown
brown
172
164
186
178
192
182
165
192
188
189
178
182
68.13 (68.41)
62.16 (62.50)
59.58 (59.88)
65.65 (65.30)
69.44 (69.77)
63.82 (64.11)
61.22 (61.59)
66.39 (66.79)
68.64 (68.91)
63.03 (63.30)
60.46 (60.79)
65.64 (65.95)
4.88 (5.27)
4.68 (4.81)
4.46 (4.61)
4.88 (5.03)
5.52 (5.71)
5.05 (5.25)
4.98 (5.04)
5.45 (5.47)
4.89 (5.03)
4.72 (4.62)
4.41 (4.44)
4.72 (4.82)
1.79 (1.50)
1.25 (1.37)
1.19 (1.32)
1.20 (1.44)
1.43 (1.40)
1.60 (1.29)
1.04 (1.24)
1.08 (1.34)
2.69 (2.82)
2.48 (2.59)
2.22 (2.48)
2.34 (2.69)
3
4
5
6
7
8
9
10
11
12
co-efficient for the bands in the 754–600 nm region are
found to be very low as compared to that of charge transfer
bands. Hence, the band around 754–600 nm have been
alone it is not possible to accurately determine the symme-
try or the magnitude of the low-symmetry ligand-field
component.
2
2
assigned to T2g ! A2g transition. The absorptions in
the ultraviolet region below 400 nm are due to the intrali-
gand transitions (n–p*, p–p*). The pattern of the electronic
spectra of all the complexes indicate the presence of an
octahedral environment around ruthenium(III) ion similar
to that of other ruthenium(III) octahedral complexes
[35–37].
However, epr measurements clearly demonstrate, at
least in frozen solution, the presence of a rhombic ligand-
field component, as one might expect from the nature of
the ligands around the metal ion. The distortion of the
pseudooctahedral complexes can be expressed as the sum
of axial (D) and rhombic (V) components. The theory of
EPR spectra of distorted octahedral low-spin d5 (idealized
2
5
t2g ground term from T2g) complexes is documented in
the literature [40]. The gyromagnetic tensor ‘g’ values are
given in Table 3. The low-spin d5 configuration is a good
probe of molecular structure and bonding since the
observed g values are very sensitive to small changes in
the structure and to metal-ligand covalency. The EPR spec-
tra of the powdered ruthenium(III) complexes have been
recorded at room temperature which shows the presence
of three different ‘g’ (gx = 2.29–2.43, gy = 2.00–2.16, gz =
1.84–1.92) values indicate a rhombic distortion [41,42] in
the complexes and the trans position are assigned for tri-
3.2. Magnetic moments and EPR spectra
The complexes are uniformly paramagnetic with mag-
netic moments in the range of 1.81–1.93 BM, corresponds
to one unpaired electron at 298 K (low-spin RuIII, t2g5) in
an octahedral environment. In a low-spin d5 system the
2T2g (octahedral) ground state is orbitally degenerate and
will be affected by spin–orbit coupling and low-symmetry
ligand-field effects, both of which will remove this degener-
acy and influence the temperature dependence of the
moment. The theory appropriate to such a situation has
been described [38,39]. From powder susceptibility data
phenylphosphine/arsine.
A similar pattern has been
observed in the solution spectra of the complexes
[Ru(pcr-me)Br(AsPh3)2] (3) and [Ru(pcr-ampy)Br(PPh3)2]
Table 2
IR and electronic spectral data of ruthenium(III) tripodal complexesA
Table 3
Complexes
m(C–O)
kmax (e)
EPR data of ruthenium(III) tripodal complexes
1
2
1258
1264
1270
1274
1304
1294
1308
1292
1265
1274
1272
1286
600a (367), 350 (13444), 240 (17220)
Complexes
gx
gy
gz
Ægæa
611a (337), 427b (1684), 263 (21492)
740a (207), 502b (1505), 286 (12276), 237 (16412)
540b (2356), 270 (12160), 245 (16544)
1
2
2.34
2.43
2.36
2.40
2.36
2.37
2.40
2.42
2.30
2.34
2.29
2.35
2.06
2.14
2.02
2.01
2.02
2.01
2.16
2.10
2.04
2.00
2.03
2.10
1.87
1.86
1.86
1.88
1.92
1.84
1.90
1.89
1.90
1.87
1.86
1.90
2.09
2.14
2.08
2.09
2.10
2.07
2.15
2.13
2.08
2.07
2.06
2.11
3
4
635a (344), 426b (2140), 353 (13476), 265 (18256)
754a (640), 562b (2684), 394 (12342), 252 (18234)
667b (129), 403b (2690), 248 (13248)
3
5
4
6
5
7
575b (1245), 274 (9160), 235 (14288)
6
8
647a (486), 439b (4452), 356 (11652), 235 (17084)
667a (245), 445b (3936), 307 (14756), 231 (14668)
625a (554), 446b (3154), 275 (16436)
7
9
8
10
11
12
A
9
604 (586)a, 564 (2562)b, 235 (20132)
10
11
12
a
Where m in cmꢀ1, kmax in nm, e in dm3 molꢀ1 cmꢀ1
.
a
2
2T2g ! A2g
.
1=2
½1=3g2x þ 1=3gy2 þ 1=3g2z ꢁ
.
b
Charge transfer (LMCT).

S. Kannan et al. / Polyhedron 27 (2008) 701–708
705
Table 4
(12) at LNT and the ‘g’ values gx = 2.47, gy = 2.30, gz =
Electrochemical data of ruthenium(III) tripodal complexes
1.99 and gx = 2.40, gy = 2.06, gz = 1.92 for the complexes
(3) and (12), respectively. These values fit very well with
the values obtained for other similar ruthenium(III) com-
plexes [42–44]. Further, we have simulated the EPR spectra
of both powder (Fig. 1a) and liquid (Fig. 1b) samples. It
has been observed that both the experimental and the sim-
ulated ‘g’ values are very close to each other.
Complexes
RuIV/RuIII
RuIII/RuII
Epa
Epa
DEP (mV)
E1/2 (V)
Epc
1
2
0.60
0.64
0.56
0.75
1.14
0.98
0.86
0.90
1.08
0.63
0.93
0.91
0.67
0.72
0.64
0.83
1.06
0.90
0.80
0.98
1.16
0.71
0.87
0.84
70
80
80
80
80
80
60
80
80
80
60
70
0.63
0.68
0.60
0.79
1.10
0.94
0.83
0.94
1.12
0.67
0.90
0.87
ꢀ0.77
ꢀ0.70
ꢀ0.72
ꢀ0.73
ꢀ0.76
ꢀ0.72
ꢀ0.75
ꢀ0.67
ꢀ0.72
ꢀ0.74
ꢀ0.75
ꢀ0.77
3
4
5
6
3.3. Redox properties
7
8
9
The electrochemical properties of all the complexes have
been examined cyclic voltametrically under N2 atmosphere
at glassy carbon working electrode in dichloromethane
solution (0.05 M: NBu4ClO4) and the redox potentials
are expressed with reference to Ag/AgCl. All the complexes
(1 · 10ꢀ3 M) are electroactive with respect to the metal
centers and exhibited two redox processes in the potential
range +1.5 V to ꢀ1.5 V. The complexes display a reversible
10
11
12
Supporting electrolyte: NBu4ClO4 (0.05 M); complex: 0.001 M; solvent:
CH2Cl2 where Epa and Epc are anodic and cathodic potentials, respec-
tively; scan rate: 100 mV sꢀ1
.
oxidative (RuIV/RuIII
) and an irreversible reductive
tative voltamogram of a complex 6 is shown in Fig. 2.
responses (RuIII/RuII) at the scan rate of 100 mV sꢀ1
.
All the complexes showed well defined waves in the range
0.60–1.12 V (RuIV/RuIII) and ꢀ0.67 to ꢀ0.77 V (RuIII
/
The potentials are summarized in Table 4 and a represen-
RuII). The oxidation potentials are reversible with peak-
to-peak separation (DEp) values ranging from 60 to
80 mV corresponding to one electron process. The one elec-
tron process of the oxidation has been verified by compar-
ing its current height (ipa) with that of ferrocene/
ferrocenium couple under identical experimental condi-
tions [45]. The irreversibility observed for the reductive
response may be due to the short live reduced state of the
metal ion [46] or probably due to the fast dissociation of
chloride or bromide ligand from the ruthenium(III) com-
plexes [47]. There is not much variation in the potentials
due to replacement of chlorides by bromides and triphenyl-
phosphine by triphenylarsine.
3.4. Catalytic transfer hydrogenation of ketones
Ruthenium mediated transfer hydrogenation reactions
are found to be effective catalytic systems in which hydro-
gen is transferred from one organic molecule to another
and this made us to carry out this type of reactions. One
Fig. 1. (a) EPR spectrum of [Ru(pcr-me)Br(AsPh3)2] (3) at RT (A,
experimental and B, simulated). (b) EPR spectrum of [Ru(pcr-me)-
Br(AsPh3)2] (3) at LNT (A, experimental and B, simulated).
Fig. 2. Cyclic voltammogram of [Ru(pcr-chx)Cl(AsPh3)2].

706
S. Kannan et al. / Polyhedron 27 (2008) 701–708
Table 5
of the complexes [Ru(pcr-chx)Br(AsPh3)2] (7) is taken as
model catalyst and the catalytic activity in the transfer
hydrogenation of various aliphatic and aromatic ketones
in the presence of isopropanol and KOH as promoter has
been explored. The complex catalyzes the reduction of
ketones to corresponding alcohols with >90% conversions
in all cases and the results are listed in Table 5.
Catalytic transfer hydrogenation of ketones by [Ru(pcr-chx)Br(AsPh3)2]/
i-PrOH/KOHa
Entry Substrates
Products
OH
Time (h) Conversionb TONc
O
1
1
1
90.8
90.7
272
272
OH
O
OH
O
2
Catalyst : 0.0125mmol
H3CO
i-prOH/KOH
H3CO
R
R'
R
R'
OH
O
R, R' = alkyl (or) aryl
3
1
1
93.9
98.9
281
296
Cl
Cl
The necessity of the ruthenium complex to observe the
ensuing transfer hydrogenation was ascertained by carry-
ing out a series of blank or control experiments suggest
that none of RuCl3 Æ 3H2O, Ru(III) precursors or tripodal
ligands alone or as a mixture causes these transformations
under identical reaction conditions. The alcohols formed
after the reflux was determined by GC with authentic sam-
ples. In this method, the base facilitates the formation of a
ruthenium alkoxide by abstracting the proton of the alco-
hol, which then undergoes b-elimination to give a ruthe-
nium-hydride which is the active catalyst (Scheme 3).
This classical mechanism is proposed by several workers
on the studies of ruthenium complexes catalyzed transfer
hydrogenation reaction by metal hydrides [48,49].
Excellent conversions are obtained for both aliphatic
and aromatic ketones. Benzophenone was converted to
benzhydrol in >98%. The complex efficiently catalyzed
the reduction of aliphatic ketones such as methyl ethyl
ketone, methyl propyl ketone, isobutyl methyl ketone and
diethyl ketone to their corresponding alcohols with
99.6%, 98.8%, 97.8% and 90.5% conversions, respectively.
The conversion in case of acetophenone is 90.8%. The pres-
ence of electron donating (OCH3) and electron withdraw-
ing (Cl) substituents on the substrates (entries 2 and 3)
has marginal effect on the catalytic transfer hydrogenation
of ketones. 4-Chloro acetophenone is converted to corre-
sponding alcohol in 93.9% and in case of 4-methoxy aceto-
phenone the conversion is 90.7%. Though the effect is
minimal, the greater conversion in case of 3 compared to
2 may be due to the fact that reduction involves the sub-
strate gaining electrons and this would be more facile at
an electron deficient center [50]. Moreover, this catalyst
efficiently catalyzes the reduction of cyclopentanone and
cyclohexanone to corresponding alcohols cyclopentanol
and cyclohexanol with 99.3% and 90.8% conversions,
respectively. The complex showed also good catalytic activ-
ity for seven and eight membered cyclic ketones cyclohep-
tanone and cyclooctanone with 99.3% and 97.9%
conversions, respectively. The presence of a catalytic
amount of base is necessary for the transfer hydrogenation
of ketones. Though no mechanistic study has been carried
out, the catalytic transformation is supposed to follow the
classical pathway in which the ketone coordinates to
hydride-ruthenium intermediates, the observed effects seem
OH
O
4
O
OH
OH
5
1
1
99.3
90.8
297
272
O
6
O
OH
7
1
1
99.3
97.9
297
293
O
OH
8
OH
OH
O
9
1
1
1
1
99.6
98.8
97.8
90.5
298
296
293
271
O
10
OH
O
11
O
OH
12
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
DP-5 column of 30 m length, 0.32 mm diameter and 0.25 lm film thick-
ness and by comparison with authentic samples.
c
Ratio of moles of product obtained to the moles of catalyst used.
base
-HCl
ß-elim
ketone
RuCl(PPh3)2L +
OH
ORu(PPh3)2L
RuH(PPh3)2L
Scheme 3. The role of base (L = arylazo ligand).
to indicate that the hydride transfer from the metal to the
coordinated ketone is the turnover-limiting step (rather
than the ketone complexation) in the catalytic cycle [51].

S. Kannan et al. / Polyhedron 27 (2008) 701–708
707
[14] (a) T. Ohkuma, H. Ooka, S. Harshiguchi, T. Ikariya, R. Noyori, J.
Am. Chem. Soc. 117 (1995) 2675;
(b) T. Okhuma, H. Takeno, R. Noyori, Adv. Synth. Catal. 343 (2001)
369.
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Organomet. Chem. 624 (2001) 271;
(a) P. Dani, T. Karlen, R.A. Gossage, S. Gladiali, G. van Koten,
Angew. Chem. 112 (2000) 759.
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Murata, E. Katayama, T. Yokozawa, T. Ikariya, R. Noyori, J. Am.
Chem. Soc. 120 (1998) 13529;
The only byproduct acetone was identified in all the
cases. As the catalyst is stable in all organic solvents and
it can be recovered and the work up process is also very
simple for this catalytic system. Although several catalytic
systems have been reported to support transfer hydrogena-
tion reactions of ketones, ruthenium(III) tripodal com-
plexes have been exploited for first time towards the
catalytic transfer hydrogenation reactions.
4. Conclusions
(b) T. Ohkuma, D. Ishii, H. Takeno, R. Noyori, J. Am. Chem. Soc.
122 (2000) 6510;
(c) T. Ohkuma, M. Koizumi, H. Ikehira, T. Yokozawa, R. Noyori,
Org. Lett. 2 (2000) 659.
[17] (a) A. Dominguez, A. Zanotti-Gerosa, W. Hems, Org. Lett. 6 (2004)
1927;
In conclusion, we have synthesized and characterized a
series of ruthenium(III) tripodal complexes. One of the
complexes (7) has been found to be an efficient catalyst in
transfer hydrogenation of both aliphatic as well as aro-
matic ketones in the presence of isopropanol/KOH. We
are currently perusing development of some new chiral
ruthenium complexes for asymmetric catalysis.
(b) J.W.H. Chen, W. Kwok, R. Guo, Z. Zhou, C.-H. Yeung, A.S.C.
Chan, J. Org. Chem. 67 (2002) 7908;
(c) J. Xie, L. Wang, Y. Fu, S. Zhu, B. Fan, H. Duan, Q. Zhou, J. Am.
Chem. Soc. 125 (2003) 4404.
[18] H. Deng, Z. Yu, J. Dong, S. Wu, Organometallics 24 (2005) 4110.
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Acknowledgements
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174 (1998) 5;
(b) A.G. Blackman, Polyhedron 24 (2005) 1.
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43 (2004) 8119.
We express our sincere thanks to Professor P. Sambasi-
va Rao, Department of Chemistry, Pondicherry University
for providing the EPR facility. We express sincere thanks
to Prof. P.R. Athappan, School of Chemistry, Madurai
Kamaraj University, for cyclic voltammetric facility.
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