Ruthenium(III) mediated C-H activation of azonaphthol: Synthesis, structural characterization and transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.jorganchem.2007.04.042

Treatment of [RuCl₃(PPh₃)₃] with 1-(arylazo)naphthol ligands in benzene under reflux afford air-stable new organoruthenium(III) complexes with general composition [Ru(an-R)Cl(PPh₃)₂] (where, R = H, Cl, CH₃, OCH₃, OC₂H₅) in fairly good yield. The 1-(arylazo)naphtholate ligands behave as dianionic tridentate C, N, O donors and coordinates to ruthenium through phenolic oxygen, azo nitrogen and ortho carbon generate two five-membered chelate rings. The composition of the complexes have been established by analytical (elemental analysis and magnetic susceptibility measurement) and spectral (FT-IR, UV-Vis, EPR) methods. The complexes are paramagnetic (low-spin, d⁵) in nature and in dichloromethane solution show intense d-d transitions and ligand-to-metal charge transfer (LMCT) transitions in the visible region. The solution EPR spectrum of complex [Ru(an-CH₃)Cl(PPh₃)₂] (3) in dichloromethane at 77 K shows rhombic distortion around the ruthenium ion with three different 'g' values (g_x ≠ g_y ≠ g_z). The single crystal structure of the complex [Ru(an-OCH₃)Cl(PPh₃)₂] (4) has been characterised by X-ray crystallography, indicates the presence of a distorted octahedral geometry in these complexes. All the complexes exhibit one quasi-reversible oxidative response in the range 0.60-0.79 V (Ru^IV/Ru^III) and two quasi-reversible reductive responses (Ru^III/Ru^II; Ru^II/Ru^I) within the range -0.50 to -0.62 V and -0.93 to -0.98 V respectively. The formal potential of all the couples correlate linearly with the Hammett constant of the para substituent in arylazo fragment of the 1-(arylazo)naphtholate ligand. Further, the catalytic efficiency of one of the ruthenium complexes (4) was determined for the transfer hydrogenation of ketones with an excellent yield up to 99% in the presence of isopropanol/KOH.

Full text of DOI:10.1016/j.jorganchem.2007.04.042

Journal of Organometallic Chemistry 692 (2007) 3380–3391
www.elsevier.com/locate/jorganchem
Ruthenium(III) mediated C–H activation of azonaphthol:
Synthesis, structural characterization and transfer
hydrogenation of ketones
a
a,
b
Sethuraman Kannan , Rengan Ramesh *, Yu Liu
a
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
Institute of Inorganic Chemistry, University of Zu¨rich, Winterthurerstrasse 190, CH-8057 Zu¨rich, Switzerland
b
Received 9 February 2007; received in revised form 28 March 2007; accepted 2 April 2007
Available online 6 May 2007
Abstract
Treatment of [RuCl3(PPh3)3] with 1-(arylazo)naphthol ligands in benzene under reflux afford air-stable new organoruthenium(III)
complexes with general composition [Ru(an-R)Cl(PPh3)2] (where, R = H, Cl, CH3, OCH3, OC2H5) in fairly good yield. The 1-(ary-
lazo)naphtholate ligands behave as dianionic tridentate C, N, O donors and coordinates to ruthenium through phenolic oxygen, azo
nitrogen and ortho carbon generate two five-membered chelate rings. The composition of the complexes have been established by ana-
lytical (elemental analysis and magnetic susceptibility measurement) and spectral (FT-IR, UV–Vis, EPR) methods. The complexes are
paramagnetic (low-spin, d5) in nature and in dichloromethane solution show intense d–d transitions and ligand-to-metal charge transfer
(LMCT) transitions in the visible region. The solution EPR spectrum of complex [Ru(an-CH3)Cl(PPh3)2] (3) in dichloromethane at 77 K
shows rhombic distortion around the ruthenium ion with three different ‘g’ values (gx ¼ gy ¼ gz). The single crystal structure of the com-
plex [Ru(an-OCH3)Cl(PPh3)2] (4) has been characterised by X-ray crystallography, indicates the presence of a distorted octahedral geom-
etry in these complexes. All the complexes exhibit one quasi-reversible oxidative response in the range 0.60–0.79 V (RuIV/RuIII) and two
quasi-reversible reductive responses (RuIII/RuII; RuII/RuI) within the range ꢀ0.50 to ꢀ0.62 V and ꢀ0.93 to ꢀ0.98 V respectively. The
formal potential of all the couples correlate linearly with the Hammett constant of the para substituent in arylazo fragment of the 1-(ary-
lazo)naphtholate ligand. Further, the catalytic efficiency of one of the ruthenium complexes (4) was determined for the transfer hydro-
genation of ketones with an excellent yield up to 99% in the presence of isopropanol/KOH.
ꢀ 2007 Published by Elsevier B.V.
Keywords: Ru(III); C–H activation; Crystal structure; Electrochemistry; Catalytic transfer hydrogenation
1. Introduction
lacycles due to p-acidic nature of the azo function [4]. Par-
ticularly the azo (–N@N–) group due to its strong p-acid
character stabilizes ruthenium in lower oxidation states
while phenolate oxygen being a hard base stabilizes the
higher oxidation states of the metal ion [5]. It has been
found in the literature that the arylazo phenol ligands are
known to coordinate metal ions usually in a bidentate fash-
ion with N, O donor forming a six-membered chelate ring.
However, under forced conditions these ligands undergo
C–H activation by transition metal ions and leads to the
formation of cyclometalated compounds. Since, the first
cyclometalated complex was synthesized [6], cyclometala-
Coordination of ruthenium by ligand of different types
is of significant importance [1–3] because of the fascinating
reactivities exhibited by the resultant complexes and the
nature of the ligand that dictates the property of those
complexes. Among the various ligand systems, arylazo
ligands I are potentially able to form M–C bonds or metal-
*
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045/
20.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
0022-328X/$ - see front matter ꢀ 2007 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2007.04.042

S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
3381
tion has become an important part of organometallic
chemistry and several reviews covering the subject have
appeared [7,8].
catalysts [27]. Pioneering work by Noyori, in the mid
1990s, showed that ruthenium complexes of the type
[(diphosphine)-RuCl2-(diamine)] used in 2-propanol in
the presence of a base are very efficient and selective cata-
lysts for the asymmetric reduction of unfunctionalised
ketones [28]. Since Noyori’s work using XylBinap [29] a
number of other groups have demonstrated the use of other
diphosphines that give rise to high activities and selectivi-
ties when used in this catalyst system [30]. Van Koten
reported pincer-type arylruthenium(II) complexes contain-
ing the monoanionic terdentate NCN and PCP ligands and
are used as active catalysts for the transfer hydrogenation
of ketones in the presence of i-PrOH and KOH [31]. Fur-
ther, transfer hydrogenation of ketones was efficiently car-
ried out with a ruthenium(II) complex bearing the ligand
2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine in isopropanol
[32]. Hence, it is well known from the literature that most
of the cyclometalated ruthenium(II) complexes [33] con-
taining different types of ligand including chiral versions
have been successfully used as effective catalysts for trans-
fer hydrogenation of ketones and the ruthenium(III) com-
plexes are completely neglected for this purpose.
Therefore we focused our interest on the synthesis, char-
acterization and catalytic application of ruthenium(III)
complexes and we have observed that the ruthenium(III)
complexes show excellent performance in catalytic transfer
hydrogenation of ketones than classical ruthenium(II)
complexes. Herein, we describe the synthesis of a series of
five cyclometalated ruthenium(III) complexes via C–H acti-
vation of 1-(arylazo)naphthol ligands. The molecular and
electronic structure of the complexes are probed with the
help of X-ray crystal structure, in combination with FT-
IR, electronic and EPR spectra. The relative stabilities of
oxidation and reduction states are monitored electrochem-
ically. One of the complexes is employed as catalyst for
ketone hydrogenation in the presence of 2-propanol and
KOH.
H3CO
H3C
OH
O
OH
N
N
N
RuL2X
N
N
N
R
R
R
I
III
II L = PPh3 or AsPh3
X = Cl or Br
Cyclometalated compounds show important applica-
tions, such as their use in regiospecific organic and organo-
metallic reactions [9], insertion reactions [10], the synthesis
of new metal mesogenic compounds [11] and in biologically
active compounds [12]. Even though a number of reports
are known for cyclometalated ruthenium(II) complexes
[13–15], only very little has been covered on cyclometalated
ruthenium(III) complexes [16–19] II in the literature includ-
ing our recent reports.
Further 1-(arylazo)naphthols III are also potential
ligands that coordinate to metal ion normally through oxy-
gen and nitrogen in a bidentate manner. The naphthyl
group in this ligand is more reactive, a better electron
donating agent and provides greater steric crowding com-
pared to the phenyl ring. So far two reports are available
on the reaction of these ligands with ruthenium(II) [15]
and ruthenium(III) [5] precursors and in both the above
said reactions the arylazo naphthols behaved as a monoan-
ionic bidentate N, O donor ligands. In the present work
we have used a set of 1-(arylazo)naphthol as targeted
ligands 3 to synthesize new cyclometalated ruthenium(III)
complexes via C–H activation. The interest behind the
choice of this azonaphthol is that it can also act as dian-
ionic tridentate C,N,O donor which generates a five-mem-
bered metallacycles.
Recent advances in hydrogenation catalysts are revolu-
tionizing carbonyl reduction chemistry [20]. The hydroge-
nation of carbonyl compounds using iso-propanol as a
hydrogen source to alcohols is one of the most applied pro-
cesses in organic chemistry [21] as illustrated by several use-
ful applications in recent years [22]. Among the different
metal catalyzed hydrogenation reactions, ruthenium-based
catalytic systems are found to be effective in the transfer
hydrogenation of ketones [23] and imines [24,25]. The abil-
ity of ruthenium complexes to dehydrogenate alcohols and
deliver the hydrides to a ketone [26] or an a,b-unsaturated
ketone has made them useful as transfer hydrogenation
2. Experimental
2.1. Materials and instrumentation
Commercial RuCl3 Æ 3H2O was purchased from Loba-
Chemie Pvt. Ltd. All the reagents used were chemically
pure and are of analytical reagent grade. The solvents were
purified and dried according to standard procedures. The
primary amines were purchased from Aldrich. The sup-
porting electrolyte tetrabutyl ammonium perchlorate
(n-Bu4NClO4) was dried in vacuum prior to use. The pre-
cursor complex [RuCl3(PPh3)3] [34] was prepared by
reported literature method. The azonaphthols were pre-
pared by coupling diazotized aniline and p-substituted ani-
lines with b-naphthol.
The microanalysis of carbon, hydrogen and nitrogen
were recorded by analytic function testing Vario EL III
CHNS elemental analyzer at STIC, Cochin University of
Science and Technology, Cochin, India. Infrared spectra

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S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
were taken on a Jasco 400 Plus, FTIR Spectrophotometer
in the range 4000–400 cmꢀ1 using KBr pellets. Electronic
spectra of the complexes in dichloromethane were recorded
on a Cary 300 Bio UV–Vis Varian spectrophotometer. The
room temperature magnetic susceptibilities were measured
on an EG and G model 155 vibrating sample magnetome-
ter at SAIC, IIT, Chennai, India. EPR measurements were
made with JEOL JES-FA200 X-band EPR spectrometer
for powder samples at room temperature and solution at
77 K (liquid nitrogen) at Pondicherry University, Pondi-
cherry, India. All spectra were calibrated with the help of
DPPH (g = 2.0037). Cyclic voltammetric measurements
were carried out using a Princeton EG and G-PARC model
potentiostat. Results were obtained using dichloromethane
solutions of [(n-C4H9)4N](ClO4) (TBAP) as supporting
electrolyte under nitrogen atmosphere. A three electrode
cell was employed with glassy carbon working electrode,
a platinum wire counter electrode and an Ag/AgCl refer-
ence electrode. The catalytic yields were 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.
[Ru(an-OMe)(Cl)(PPh3)2] (4). Yield: 60%; Anal.
Calc. for C53H42ClN2O2P2Ru: C, 67.86; H, 4.48; N, 2.98.
Found: C, 67.54; H, 4.19; N, 2.75%; IR (KBr pellets):
1312 (mC–O), 1378 (mN@N) cmꢀ1
.
[Ru(an-OEt)(Cl)(PPh3)2] (5). Yield: 62%; Anal. Calc.
for C54H44ClN2O2P2Ru: C, 68.12; H, 4.62; N, 2.94. Found:
C, 67.98; H, 4.32; N%, 2.67; IR (KBr pellets): 1288 (mC–O),
1388 (mN@N) cmꢀ1
.
2.3. X-ray crystallography
Single crystals of [Ru(L4)(Cl)(PPh3)2] (4) are grown by
slow evaporation from a dichloromethane–methanol solu-
tion at room temperature. Significant crystal data collec-
tion and refinement parameters are listed in Table 3. A
single crystal of suitable size was covered with Paratone
oil, mounted on the top of a glass fiber, and transferred
to a Stoe IPDS diffractometer using monochromated
MoKa radiation (kI = 0.71073). Data were collected at
183 K. Corrections were made for Lorentz and polariza-
tion effects as well as for absorption (numerical). The struc-
ture was solved with direct method using SIR-97 [35] and
was refined by full matrix least-squares method [36] on F2
with SHELXL-97. Non-hydrogen atoms were refined with
anisotropy thermal parameters. All hydrogen atoms were
geometrically fixed and allowed to refine using a riding
model.
2.2. Synthesis of ruthenium(III)1-(arylazo)naphtholate
complexes
The complexes [Ru(L1–5)(Cl)(PPh3)2] 1–5 were synthe-
sized from the reactions of [RuCl3(PPh3)3] with the ligands
L1–L5 by following a general procedure strictly under
anhydrous conditions.
2.4. Typical procedure for catalytic transfer hydrogenation
[Ru(an-R)(Cl)(PPh3)2]. The ruthenium precursor
[RuCl3(PPh3)3] (0.15 mmol; 0.135 g) and 1-(arylazo)naph-
tholate ligands (H2an–R) (where R = –H:H2an–H; –Cl:
H2an–Cl; –Me:H2an–Me; –OMe:H2an–OMe; –OEt:H2-
an–OEt) (0.15 mmol; 0.037–0.044 g) were taken in 20 ml
of benzene. The progress of the reaction was monitored
periodically by TLC while the mixture was heated under
reflux. After 12 h, the reaction mixtures were cooled to
room temperature and solids, if any were filtered off. Fil-
trate was then evaporated under reduced pressure, which
was then subjected to purification by using column chro-
matography using silica gel (100–200 mesh) and chloro-
form as eluent. A green solid complex was obtained on
complete evaporation of the solvent. The elemental
(CHN) analysis is consistent with the composition pro-
posed for all the complexes.
Under an inert atmosphere a mixture containing ketones
(3.75 mmol), the ruthenium catalyst (0.0125 mmol) and
0.0625 mmol of 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.
3. Results and discussion
A series of organoruthenium(III) derivatives was con-
veniently accessed from the direct cyclometalation of
azonaphthols with [RuCl3(PPh3)3]. The cyclometalated
compounds of the general formula [Ru(L1–5)Cl(PPh3)2]
(where L = 1-(arylazo)naphtholate ligands) described in
this paper were synthesized in good yield via C–H activa-
tion of five 1-(arylazo)naphthol ligands abbreviated as
(H2an–R) (where R = –H, –Cl, –Me, –OMe, –OEt) by
[RuCl3(PPh3)3] in dry benzene in equimolar ratio. It has
been observed that the 1-(arylazo)naphthol ligands behave
as dianionic tridentate and replace two chlorides and
one triphenylphosphine from the ruthenium(III) precur-
sor. Since the Ru(III) center is a strong Lewis acid, the
[Ru(an-H)(Cl)(PPh3)2] (1). Yield: 45%; Anal. Calc.
for C52H40ClN2OP2Ru: C, 68.78; H, 4.41; N, 3.08. Found:
C, 68.62; H, 4.34; N, 2.96%; IR (KBr pellets): 1315 (mC–O),
1376 (mN@N) cmꢀ1
.
[Ru(an-Cl)(Cl)(PPh3)2] (2). Yield: 50%; Anal. Calc.
for C52H39Cl2N2OP2Ru: C, 66.26; H, 4.14; N, 2.97. Found:
C, 66.10; H, 3.88; N, 2.84%; IR (KBr pellets): 1302 (mC–O),
1384 (mN@N) cmꢀ1
.
[Ru(an-Me)(Cl)(PPh3)2] (3). Yield: 54%; Anal. Calc.
for C53H42ClN2OP2Ru: C, 69.04; H, 4.56; N, 3.04. Found:
C, 68.88; H, 4.22; N, 2.89%; IR (KBr pellets): 1298 (mC–O),
1380 (mN@N) cmꢀ1
.

S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
3383
activation of C–H of the ligand proceeded effectively and
the oxidation state of ruthenium remains unchanged dur-
ing the formation of cyclometalated species. All the com-
plexes are found to be air stable in both the solid and the
liquid states at room temperature and are non-hygroscopic
in nature. The synthesized ruthenium(III) complexes are
soluble in common solvents such as chloroform, dichloro-
methane, toluene and benzene producing intense green
solution.
3
2
1
0
3.1. Characterization
220
320
420
520
620
720
Infrared spectra of the complexes [Ru(L1–5)Cl(PPh3)2]
are very similar and they exhibit many sharp and strong
vibrations within 1600–400 cmꢀ1. However, the infra-
red spectra of all the ligands exhibit bands around
1425–1434 cmꢀ1 and 1270–1278 cmꢀ1 corresponding to
azo m(–N@N–) and phenolic m(C–O) stretching frequencies
respectively. On complexation m–(N@N–) appears at lower
frequency in the range 1376–1388 cmꢀ1 and this red shift
supports the coordination of N(azo) to ruthenium ion
[37]. The band corresponding to phenolic m(C–O) stretching
is shifted to higher frequency in the range 1288–1315 cmꢀ1
in all the complexes confirming that the other coordination
site is the phenolic oxygen [38]. This was further supported
by the disappearance of m(OH) band in the range
3439–3450 cmꢀ1 in all the complexes. In addition, the
bands near 535, 695, 740, 1556 cmꢀ1 in the spectra of all
the complexes are attributed to the presence of PPh3
ligands [37,39]. The confirmation for the cyclometalation
is shown by extra bands in the IR spectrum of the com-
plexes in the 700–800 cmꢀ1 and 1100 cmꢀ1 regions charac-
teristic of an ortho-disubstituted benzene nucleus [40].
Electronic spectra of all the complexes have been
recorded in dichloromethane solution in the range 800–
200 nm. Spectral data are displayed in Table 1 and a
selected spectrum is shown in Fig. 1. The ground state of
ruthenium(III) in an octahedral environment is 2T2g arising
from the t52g configuration and the first excited doublet lev-
els in the order of increasing energy are 2A2g and 2T1g, aris-
ing from t42ge1g configuration. Hence, two bands
Wavelength (nm)
Fig. 1. Electronic spectrum of [Ru(an-OCH3)Cl(PPh3)2].
2
2
2
2
corresponding to T2g! A2g and T2g! T1g are possible.
All the complexes show four to five intense absorptions
in the region 633–242 nm. The intense absorptions in the
region of 633–620 nm are attributed to d–d transitions
and the absorptions observed in the region 430–413 nm
are probably due to charge transfer transitions (LMCT)
taking place from the filled ligand (HOMO) orbital to
the singly-occupied ruthenium t2 orbital (HOMO). The
absorptions in the ultraviolet region below 380 nm are very
similar and are attributable to the transitions within the
ligand orbitals (n–p*, p–p*) taking place in the 1-(ary-
lazo)naphtholate ligands. The pattern of the electronic
spectra of all the complexes indicated the presence of an
octahedral environment around ruthenium(III) ion similar
to that of other ruthenium(III) octahedral complexes [19].
The room temperature magnetic susceptibility measure-
ment of the complexes in the range 1.81–1.92 BM, shows
that these complexes are one electron paramagnetic, which
is in accordance with the +3 state of ruthenium (low spin
d5; S = 1/2) in an octahedral environment in these com-
plexes. The solid state EPR spectra of all the complexes
were recorded in X-band frequencies at room temperature.
The ‘g’ values are given in Table 2 and a representative
spectrum is shown in Fig. 2a. 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
spectra of all the complexes exhibit an anisotropy spectrum
with g^ around 2.03–2.60 and gi around 1.90–1.99. For an
octahedral field with tetragonal distortion gx = gy ¼ gz and
hence, two ‘g’ values also indicate an axial symmetry for
these complexes and hence, trans position are assigned
Table 1
Electronic spectral data of the ruthenium(III)1-(arylazo)naphtholate
complexes
Complexes
kmax (nm) e (dm3/mol/cm)
[Ru(an-H)Cl(PPh3)2] (1)
620a (1098), 430b (2564), 344 (8674),
275 (15685), 235 (18945)
[Ru(an-Cl)Cl(PPh3)2] (2)
[Ru(an-Me)Cl(PPh3)2] (3)
[Ru(an-OMe)Cl(PPh3)2] (4)
[Ru(an-OEt)Cl(PPh3)2] (5)
633a (1480), 479c (2010), 420b (3470),
374 (4230), 269 (12513), 245 (15800)
626a (730), 422b (1905), 365 (8542),
242 (10465)
Table 2
EPR data of ruthenium(III)1-(aryalazo)naphtholate complexes
*
Complexes
gx
gy
gz
Ægæ
625a (2220), 413b (4135), 314 (8715),
269 (16365), 250 (17170)
1
2
3
4
5
2.12
2.10
2.03
2.04
2.60
2.12
2.10
2.03
2.04
2.60
1.91
1.94
1.90
1.99
1.93
2.05
2.04
1.98
2.02
2.37
622a (2245), 472c (3975), 415b (6430),
366 (7665), 320 (9975), 260 (21630)
a
d–d transition.
b
Charge transfer (LMCT) transition.
Shoulder.
hgiꢁ ¼ ½1=3gx2 þ 1=3gy2 þ 1=3g2z ꢂ
.
1n2
c

3384
S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
of the complex [Ru(an-CH3)Cl(PPh3)2] (3) (Fig. 2b)
recorded in dichloromethane solution at 77 K shows rhom-
bic spectrum with three different ‘g’ values (gx ¼ gy ¼ gz;
gx = 2.60, gy = 2.03, gz = 1.98) in decreasing order of mag-
nitude. Of these three major resonances, two are lower and
one is higher than the field (ꢃ3200 G). From the spectra it
is revealed that the t5g configuration of the complexes is a
positive hole in the t2g subshell as g^ > gi. The observed
rhombicity of the EPR spectrum is understandable in terms
of the gross molecular symmetry of these complexes con-
taining non-equivalent axes. The presence of rhombic dis-
tortion is apparent in the splitting of the perpendicular
resonance into two spaced components (gx and gy). This
is indicative of a gross change in the geometry of the com-
plexes in solution. The rhombicity of the spectra reflects the
asymmetry of electronic environments around ruthenium
in these complexes [42]. Hence, the results from EPR spec-
tral analysis indicate that these ruthenium(III)1-(ary-
lazo)naphtholate complexes are significantly distorted
from ideal octahedral geometry as observed in the crystal
structure of [Ru(an-OCH3)Cl(PPh3)2] (4).
Fig. 2. EPR spectra of (a) [Ru(an-OC2H5)Cl(PPh3)2]; (b) [Ru(an-
CH3)Cl(PPh3)2] in dichloromethane in solution at 77 K.
3.2. X-ray structure
for triphenylphosphine groups. Over all the position of
lines and nature of the EPR spectra of the complexes are
characteristic of low spin ruthenium(III) octahedral com-
plexes [41]. However, the EPR measurements in frozen
solution, clearly demonstrates the presence of a rhombic
ligand field component, as one might expect from the nat-
ure of the ligands around the metal ion. The EPR spectrum
To find out the coordination mode of the 1-(arylazo)-
naphtholate ligands in the complexes as well as the stereo-
chemistry of the complexes, the molecular structure of one
of the complexes [Ru(an-OCH3)Cl(PPh3)2] (4) has been
determined by single crystal X-ray diffraction. The ortep
view is shown in Fig. 3. [Ru(an-OCH3)Cl(PPh3)2] was
Fig. 3. The ORTEP diagram of the complex [Ru(an-OCH3)Cl(PPh3)2] (4) showing 30% probability. For reasons of clarity, hydrogen atoms have been
omitted.

S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
3385
Table 4
crystallized with methanol as a solvent for crystallization.
˚
Selected bond lengths (A) and angles (ꢁ) for complex 4
The summary of single crystal X-ray structures and refine-
ment is shown in Table 3 and selected bond lengths and
bond angles are given in Table 4. The molecular structure
shows that the 1-(arylazo)naphtholate ligands are coordi-
nated to ruthenium ion via dissociation of the phenolic
proton as well as another proton from one ortho position
of the phenyl ring in the arylazo fragment as a tridentate
C, N, O-donor ligands at O, N, C with bite angles of
76.9(3)ꢁ O(1)–Ru(1)–N(1) and 76.3(3)ꢁ N(1)–Ru(1) C(16)
˚
Bond lengths (A)
Bond angles (ꢁ)
Ru1–Cl
Ru1–P1
Ru1–O1
Ru1–N1
Ru1–C16
Ru1–P1a
P1–C18
P1–C24
P1–C30
O1–C1
O2–C14
O2–C17
O3–C36
N1–C10
N1–N2
1 2.369 (3)
2.4015 (13)
2.149 (6)
2.033 (8)
2.050 (8)
2.4015 (13)
1.834 (6)
1.833 (6)
1.823 (7)
1.304 (11)
1.366 (14)
1.36 (2)
Cl1–Ru1–P1
Cl1–Ru1–O1
Cl1–Ru1–N1
Cl1–Ru1–C16
Cl1–Ru1–P1
P1–Ru1–O1
P1–Ru1–N1
P1–Ru1–C16
P1–Ru1–P1i
O1–Ru1–N1
O1–Ru1–C16
P1–Ru1–O1
N1–Ru1–C16
P1–Ru1–N1
P1–Ru1–C16
Ru1–P1–C18
Ru1–P1–C24
Ru1–P1–C30
C18–P1–C24
C24–P1–C30
Ru1–O1–C1
Ru1–N1–C10
Ru1–N1–N2
Ru1–C16–C11
Ru1–C16–C15
87.69 (5)
108.88 (18)
174.2 (2)
97.9 (3)
87.69 (5)
86.62 (4)
92.74 (5)
94.64 (4)
170.12 (7)
76.9 (3)
˚
˚
and bond lengths of 2.149(6) A Ru(1)–O(1), 2.033(8) A
˚
Ru(1)–N(1) and 2.050(8) A Ru(1)–C(16). The coordinated
153.2 (3)
86.62 (4)
76.3 (3)
1-(arylazo)naphtholate ligand, chloride ion constitute one
equatorial plane with the metal at the center where the
chloride ion is trans to the coordinated azo (–N@N–)
1.47 (5)
1.405 (11)
1.290 (10)
1.379 (12)
92.74 (5)
94.64 (4)
117.96 (19)
111.64 (19)
115.15 (19)
103.7 (3)
103.2 (3)
111.7 (5)
117.8 (6)
121.6 (6)
112.1 (6)
130.2 (8)
˚
nitrogen. The N–N distance is 1.290(10) A and is longer
N2–C11
˚
than the free azo bond length (1.25 A) [43]. The increase
in bond length is undoubtedly because of coordination
of N(azo) that can lead to a decrease in the N–N bond
order due to both r- and p-acceptor character of the
ligands – the latter character having a more pronounced
effect and may be the reason for elongation [44]. This is
an indication of metal–ligand p-interaction localised in
the M-azo fragment. There are no H bonding interactions,
ESD in parenthesis.
Table 3
Crystal data and structure refinement for complex 4
however there is an aromatic p–p stacking interaction [45]
Empirical formula
Formula weight
Temperature (K)
C54H46ClN2O3P2Ru
969.39
183(2)
0.71073
Monoclinic
Cm
between rings (C1–C4/C9–C10) and (C30–C35) with a
˚
centroid–centroid distance of 3.977(4) A. The two PPh3
˚
Wavelength (A)
ligands have taken up the remaining two axial positions,
and hence, they are mutually trans, the Ru(1)–P(1)
Crystal system
Space group
Unit cell dimensions
˚
˚
2.4015(13) A and Ru(1)–P(1a) 2.4015(13) A bond dis-
tances are equal from the metal center. However, the
Ru–N distance is found to be significantly longer than
usual, and this elongation may be attributed to the strong
trans effect of the coordinated chloride [46]. The RuIII–O
˚
a (A)
18.298(4)
15.361(3)
9.6987(19)
90
120.63(3)
90
2345.7(8)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
bond distance in these complexes are longer than reported
3+
and trans-Ru(bpy)2–(OH)(OH2)2+ of
3
˚
for Ru(OH2)6
Volume (A )
˚
˚
Z
bond distances 2.029(7) A and 2.007(3) A, respectively
[47]. The lengthening of this RuIII–O bond may be due
to the trans effect of the C(16). Ruthenium is therefore sit-
ting in a CNOP2Cl coordination environment, which is
distorted octahedral in nature as reflected in all the bond
parameters around ruthenium. As all the complexes
display similar spectral properties, the other four com-
plexes are assumed to have similar structure to that of
complex 4.
Dcalc (Mg/m3)
1.373
0.505
998
Absorption coefficient (mmꢀ1
F(000)
)
Crystal size (mm3)
0.36 · 0.32 · 0.22
3.61–24.00
ꢀ20 6 h 6 20, ꢀ17 6 k 6 17,
ꢀ11 6 l 6 11
7581
3639 [0.0298]
96.9
Numerical
h Range for data collection (ꢁ)
Index ranges
Reflections collected
Independent reflections [R(int)
Completeness to h = 24.00ꢁ (%)
Absorption correction
Maximum and minimum
transmission
]
0.9165 and 0.8424
3.3. Electrochemistry of the complexes
Refinement method
Full-matrix least-squares on F2
3639/2/325
The electron transfer property of all the complexes were
studied in dichloromethane solution by cyclic voltammetry
under N2 atmosphere using a glassy-carbon working elec-
trode and the redox potentials are expressed with reference
to SCE. All the complexes (1 · 10ꢀ3 M) are electroactive
with respect to the metal centers and exhibited three redox
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
1.150
R1 = 0.0488, wR2 = 0.1351
R1 = 0.0495, wR2 = 0.1357
Largest difference in peak and hole 1.471 and ꢀ0.486
ꢀ3
˚
(e A
)

3386
S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
Table 5
Electrochemical data of the ruthenium(III)1-(arylazo)naphtholate complexes
Complexes
RuIV/RuIII
RuIII/RuII
RuII/RuI
Epa (V)
Epc (V)
E1/2 (V)
DEp (mV)
Epa (V)
Epc (V)
E1/2 (V)
DEp (mV)
Epa (V)
Epc (V)
E1/2 (V)
DEp (mV)
1
2
3
4
5
0.92
0.89
0.80
0.75
0.70
0.62
0.70
0.52
0.48
0.49
0.77
0.79
0.66
0.61
0.60
300
190
280
270
210
ꢀ0.42
ꢀ0.41
ꢀ0.47
ꢀ0.49
ꢀ0.50
ꢀ0.62
ꢀ0.59
ꢀ0.79
ꢀ0.75
ꢀ0.72
ꢀ0.52
ꢀ0.50
ꢀ0.63
ꢀ0.62
ꢀ0.61
200
180
320
260
220
ꢀ0.90
ꢀ0.80
ꢀ0.86
ꢀ0.80
ꢀ0.81
ꢀ1.03
ꢀ1.07
ꢀ1.02
ꢀ1.17
ꢀ1.09
ꢀ0.96
ꢀ0.93
ꢀ0.94
ꢀ0.98
ꢀ0.95
130
270
160
370
280
Supporting electrolyte: NBu4ClO4 (0.005 M); complex: 0.001 M; solvent: CH2Cl2; DEp = Epa ꢀ Epc where Epa and Epc are anodic and cathodic potentials
respectively; E1/2 = 0.5(Epa + Epc); scan rate: 100 mV sꢀ1
.
processes in the potential range +1.5 V to ꢀ1.5 V (0.05 M
tetrabutyl ammonium perchlorate as supporting electrolyte
at 298 K). The complexes display a quasi-reversible oxida-
The potentials of both the oxidation (RuIV/RuIII) and
reduction (RuIII/RuII; RuII/RuI) have been found to be
sensitive to the nature of the substituent (R) in the 1-(ary-
lazo)naphtholate ligands which perturbs both the metal
oxidation and reduction. The potential increases linearly
with increasing electron-withdrawing character of R. The
half wave potentials (E1/2) moves to negative value with
electron donating substituent(s) in the aryl ring and reverse
movement is observed for electron withdrawing group. The
plot of formal potentials E1/2 versus r (r – Hammett para
tive (RuIV/RuIII) and two quasi-reversible reductive (RuIII
/
RuII; RuII/RuI) responses on the positive and negative side
of SCE, respectively at the scan rate of 100 mV sꢀ1. The
potentials are summarized in Table 5 and a representative
voltammogram is shown in Fig. 4. All the complexes
showed well-defined waves E1/2 lies in the range 0.61–
0.79 V (RuIV/RuIII) and ꢀ0.50 to ꢀ0.63 V (RuIII/RuII)
and ꢀ0.93 to ꢀ0.98 V (RuII/RuI). The redox processes
are quasi-reversible in nature, characterized by a rather
large peak-to-peak separation (DEp) of 130–370 mV [48].
On comparison of its current height (ipa) with that of the
standard ferrocene/ferrocenium couple under identical
experimental conditions reveals the one electron redox
process [18,19]. The redox potentials Epc and Epa are virtu-
ally independent of the scan rates, supporting quasi-
reversibility.
substituent constant of
R
[49] R: OCH3 = ꢀ0.27,
OC2H5 = ꢀ0.24, CH3 = ꢀ0.17, H = 0.00, Cl = 0.23) is
found to be linear for all the three couples (Fig. 5).
The slope of these lines, which is known as the reaction
a
0. 9
The first quasi-reversible response on the positive side is
due to the one electron oxidation of Eq. (1)
0. 8
0. 7
0. 6
½RuIIIðPPh3Þ2ðLÞðXÞꢂ ꢀ ½RuIVðPPh3Þ2ðLÞðXÞꢂþ þ eꢀ
ð1Þ
On further scan reversal on the negative side, two quasi-
reversible reductive responses in Eqs. (2) and (3) were
observed
½RuIIIðPPh3Þ2ðLÞðXÞꢂ þ eꢀ ꢀ ½RuIIðPPh3Þ2ðLÞðXÞꢂꢀ
ð2Þ
ð3Þ
-0 .3
-0 .2
0 .0
0 .2
0 .3
Hammet (σ)
½RuIIðPPh3Þ2ðLÞðXÞꢂ þ eꢀ ꢀ ½RuIðPPh3Þ2ðLÞðXÞꢂꢀ
0.0
b
-0.2
-0.4
-0.6
-0.8
-1.0
-0 .3
-0 .2
0 .0
0 .2
0 .3
Hammet (σ)
Fig. 5. Least-squares plot of (a) E1/2 values oxidation (RuIV/RuIII);
(b) E1/2 values reduction (RuIII/RuII; RuII/RuI) potentials versus r.
Fig. 4. Cyclic voltammogram of [Ru(an-Cl)Cl(PPh3)2].

S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
3387
constant q [50] and is a measure of the sensitivity of E1/2
with R is 0.36 for the RuIV/RuIII couple, 0.17 for the
RuIII/RuII couple and 0.12 for the RuII/RuI couple. This
shows that the nature of the para substituent R on the
1-(arylazo)naphtholate ligand, which is four bonds away
from the electroactive metal center, can still influence the
metal-centered potentials in a predictable manner. Hence,
it is inferred from the electrochemical data that the present
ligand system is ideally suitable for stabilizing the higher
oxidation state of ruthenium ion and the electron transfer
reactions take place without gross changes in the stereo-
chemistry of the complexes [5].
The catalytic reaction was carried out under a set of con-
ditions (see experimental for reaction conditions) and a ser-
ies of blank or control experiments suggest that none of
RuCl3 Æ 3H2O, Ru(III) precursors or 1-(arylazo)naphtho-
late ligands alone or as a mixture causes these transforma-
tions under identical reaction conditions. This ascertained
the necessity of ruthenium(III) complex to observe the
ensuing catalytic transformations. Complex 4 efficiently
catalyses the transfer hydrogenation of ketones with high
yield and the results are summarized in Table 7. Under
these conditions, the Ru–C r is stable and the 1-(ary-
lazo)naphtholate fragment is preserved.
The alcohols formed after the reflux for the time men-
tioned were determined by GC with authentic samples. In
this method, the base facilitates the formation of a ruthe-
nium alkoxide by abstracting the proton of the alcohol
(Scheme 1), which then undergoes b-elimination to give a
ruthenium-hydride which is the active catalyst. This classi-
cal mechanism is proposed by several workers on the
studies of ruthenium complexes catalyzed transfer hydroge-
nation reaction by metal hydrides [22,23]. Though, no
mechanistic studies have been performed, the catalytic
transformation of ketones to alcohols follow the classical
pathway in which the ketones coordinate to hydride-ruthe-
nium metal intermediate (Scheme 2).
3.4. Catalytic activity in transfer hydrogenation of ketones
Catalytic transfer hydrogenation reaction 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 reactions. Represen-
tative types of aliphatic alkyl and aryl ketones were chosen
to evaluate the performance of complex [Ru(an-OCH3)-
Cl(PPh3)2] (4) in transfer hydrogenation reaction in the
presence of iso-propanol and KOH as promoter.
OH
O
Both aliphatic and aromatic ketones are converted into
their corresponding secondary alcohols with excellent con-
versions and turnover. The complex efficiently catalyzed
the reduction of aliphatic ketones such as ethyl methyl
ketone, methyl propyl ketone and isobutyl methyl ketone
to their corresponding alcohols with 82.0%, 99.4% and
99.8% conversion, respectively. The conversion in case of
acetophenone is 88.8%. The presence of electron withdraw-
ing (Cl) and electron donating (OCH3) substituents on the
substrates (entries 2 and 3) plays a significant role in the
conversion of ketones to alcohols. 4-chloro acetophenone
is converted to corresponding alcohol in 93.7% and
4-methoxy acetophenone in 79.4% conversion. This may
be expected as reduction involves the substrate gaining
electrons and this would be more facile at an electron defi-
cient center [51]. Assuming that these catalytic transforma-
tions follow the classical pathway in which the ketone
coordinates to hydride-ruthenium intermediates [22,23],
the observed effects seem to indicate that the hydride trans-
fer from the metal to the coordinated ketone is the turn-
over-limiting step (rather than the ketone complexation)
in the catalytic cycle [52]. Interestingly, the complex effi-
ciently catalyses the reduction of five, six, seven and
eight-membered cyclic ketones to the corresponding alco-
hols with 86.5% conversion in case of cyclohexanone and
conversions >90% in case of cyclopentanone, cyclohepta-
none and cyclooctanone. The catalyst also efficiently cata-
lyzed the reduction of diethyl ketone to 3-pentanol in 86%
conversion.
Catalyst : 0.0125mmol
i-prOH/KOH
R
R'
R
R'
R, R' = alkyl (or) aryl
In order to optimize the reaction conditions, different
catalyst:substrate ratios were tested and the results are
summarized in Table 6. For this initial experiments cyclo-
hexanone was selected as a test-substrate and allowed it
to react in 2-propanol with catalytic quantities of orga-
noruthenium(III) 4 complex in the presence of KOH.
When increasing the C:S ratio to 1:400 or 1:500 in 2-propa-
nol, the reaction still proceeds smoothly accompanied by a
moderate drop in conversion. Thus, it was concluded that
catalyst:substrate ratio of 1:300 is the best compromise
between optimal reaction rate in 2-propanol.
Table 6
Catalytic transfer hydrogenation of cyclohexanone by [Ru(an-OCH3)
Cl(PPh3)2]/i-PrOH/KOHa
Entry
S/C
Time
Conversionb (%)
1
2
3
4
5
a
100:1
200:1
300:1
400:1
500:1
2
2
2
2
2
90.0
89.0
86.5
80.0
78.0
Conditions: reactions were carried out at 80 ꢁC using 1.25–6.255 mmol
of cyclohexanone (5 ml isopropanol).
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
In contrast to what is observed for most other catalysts, a
significant advantage of these catalysts is their insensitivity
toward air and moisture. In this way rigorous pretreatment
b
thickness and by comparison with authentic sample.

3388
S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
Table 7
Catalytic transfer hydrogenation of ketones by [Ru(an-OCH3)Cl(PPh3)2]/i-PrOH/KOHa
Entry
1
Substrates
Products
Time (h)
3
Conversionb
88.8
TONc (TOF)d
266(88)
O
OH
2
3
4
2
2
2
79.4
93.7
96.9
238(119)
281(140)
290(145)
O
OH
H3CO
H3CO
O
OH
Cl
Cl
O
OH
5
6
2
2
90.0
86.5
270(135)
259(129)
O
O
OH
OH
O
OH
7
8
2
2
96.6
92.0
289(144)
276(138)
O
OH
9
2
2
82.0
99.4
246(123)
298(149)
O
OH
10
O
OH

S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
3389
Table 7 (continued )
Entry
11
Substrates
Products
Time (h)
2
Conversion
99.8
TONc (TOF)d
299(149)
O
OH
12
2
86.0
258(129)
O
OH
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.
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 thickness and
b
by comparison with authentic samples.
c
TON = ratio of moles of product obtained to the moles of catalyst used.
TOF (TON hꢀ1).
d
base
-HCl
ß-elim
ketone
of five new organoruthenium(III) complexes are obtained.
The X-ray crystal structure of the complex (4) reveals a dis-
torted octahedral environment around ruthenium. Further,
the complex (4) has exhibited very high catalytic efficiency
in transfer hydrogenation of both purely aliphatic as well
as aromatic ketones in the presence of isopropanol/KOH.
Results of present investigation suggests promising applica-
tion of a new family of organoruthenium(III) complexes
containing 1-(arylazo)naphthol in transition-metal pro-
moted catalysis. We are currently perusing development
of chiral organoruthenium(III) complexes for asymmetric
catalysis.
RuCl(PPh3)2L
+
OH
ORu(PPh3)2L
RuH(PPh3)2L
Scheme 1. The role of base (L = arylazo ligand).
CH3
O
H3C
CH3
CH3
O
Ru
H
Ru
H
CH3
H
R'
R
(CH3)2CHO-
O
Ru
Cl
O
-
CH3
Cl
Ru
R
Acknowledgements
H
R
R'
O
R'
R
Ru
H
OH
We express sincere thanks to Dr. B. Spingler, Institute
of Inorganic Chemistry, University of Zurich, Switzer-
¨
H
R'
H
O
CH3
H3C
Ru
land for crystallographic discussion. We thank Prof. P.
Sambasiva Rao, Department of Chemistry, Pondicherry
University, for providing the EPR facility. We express
sincere thanks to Prof. PR. Athappan, School of Chem-
istry, Madurai Kamaraj University, for cyclic voltammet-
ric facility.
OH
R, R' = alkyl (or) aryl
Scheme 2. Proposed catalytic cycle for the transfer hydrogenation of
ketones by cyclometalated Ru(III)1-(arylazo)naphtholate complex (4).
of solvents and substrates is avoided, thus reaction mixtures
can be loaded into the reaction vessel in the open air and
monitoring of the reaction progress becomes very conve-
nient. Furthermore, the ease by which these catalysts are
prepared offers another important advantage. The work
up process is very simple for this catalytic system as the cat-
alyst is stable in all organic solvents and it can be recovered.
The only byproduct acetone was identified in all the cases.
Although several catalytic systems have been reported to
support transfer hydrogenation reactions of ketones, cata-
lyst like cycloruthenated(III) complex is novel for its CNO
donor, PPh3 and Cl ligand environment.
Appendix A. Supplementary material
CCDC 606695 contains the supplementary crystallo-
graphic data for this paper. The data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Complete sets of refined positional coordinates as well as
anisotropic thermal parameters and complete tabulations
of bond lengths and bond angles are presented in the online
version of this paper. Supplementary data associated with
this article can be found, in the online version, at doi:-
10.1016/j.jorganchem.2007. 04.042.
4. Conclusions
It is demonstrated that [RuCl3(PPh3)3] effectively medi-
ate C–H activation of 1-(arylazo)naphthols and a series

3390
S. Kannan et al. / Journal of Organometallic Chemistry 692 (2007) 3380–3391
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