Ortho-metallated ruthenium(III) complexes with some acid hydrazide based Schiff bases

Article abstract of DOI:10.1016/j.jorganchem.2006.10.026

Ortho-metallated ruthenium(III) complexes with Schiff bases (H₂L) derived from one mole equivalent each of benzaldehyde and acid hydrazides are described. Reactions of H₂L with [Ru(PPh₃)₃Cl₂] in presence of NEt₃ (1:1:2 mole ratio) under aerobic conditions in methanol provide the complexes having the general formula trans-[Ru(L)(PPh₃)₂Cl] in 55-60% yields. The complexes have been characterized with the help of elemental analysis, magnetic susceptibility, electrochemical and various spectroscopic (infrared, electronic and EPR) measurements. The +3 oxidation state of the metal centre in these complexes is confirmed by their one-electron paramagnetic nature. Molecular structures of two representative complexes have been determined by X-ray crystallography. In each complex, the metal ion is in a distorted octahedral CNOClP₂ coordination sphere. The dianionic C,N,O-donor ligand (L^2-) together with the chloride form a CNOCl square-plane and the P-atoms of the two PPh₃ molecules occupy the two axial sites. The electronic spectra of the complexes in dichloromethane solutions display several absorptions due to ligand-to-metal charge transfer and ligand centred transitions. In dichloromethane solutions, the complexes display a ruthenium(III) → ruthenium(IV) oxidation in the potential range 0.35-0.98 V (vs. Ag/AgCl). All the complexes in frozen (110 K) dichloromethane-toluene (1:1) solutions display rhombic EPR spectra.

Full text of DOI:10.1016/j.jorganchem.2006.10.026

Journal of Organometallic Chemistry 692 (2007) 824–830
www.elsevier.com/locate/jorganchem
Ortho-metallated ruthenium(III) complexes with some acid
hydrazide based Schiff bases
*
Raji Raveendran, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
Received 16 September 2006; received in revised form 12 October 2006; accepted 12 October 2006
Available online 19 October 2006
Abstract
Ortho-metallated ruthenium(III) complexes with Schiff bases (H2L) derived from one mole equivalent each of benzaldehyde and acid
hydrazides are described. Reactions of H2L with [Ru(PPh3)3Cl2] in presence of NEt3 (1:1:2 mole ratio) under aerobic conditions in meth-
anol provide the complexes having the general formula trans-[Ru(L)(PPh3)2Cl] in 55–60% yields. The complexes have been characterized
with the help of elemental analysis, magnetic susceptibility, electrochemical and various spectroscopic (infrared, electronic and EPR)
measurements. The +3 oxidation state of the metal centre in these complexes is confirmed by their one-electron paramagnetic nature.
Molecular structures of two representative complexes have been determined by X-ray crystallography. In each complex, the metal
ion is in a distorted octahedral CNOClP2 coordination sphere. The dianionic C,N,O-donor ligand (L2ꢀ) together with the chloride form
a CNOCl square-plane and the P-atoms of the two PPh3 molecules occupy the two axial sites. The electronic spectra of the complexes in
dichloromethane solutions display several absorptions due to ligand-to-metal charge transfer and ligand centred transitions. In dichlo-
romethane solutions, the complexes display a ruthenium(III) ! ruthenium(IV) oxidation in the potential range 0.35–0.98 V (vs. Ag/
AgCl). All the complexes in frozen (110 K) dichloromethane–toluene (1:1) solutions display rhombic EPR spectra.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Ruthenium(III); Schiff bases; Acid hydrazides; Cyclometallation; Crystal structures
1. Introduction
1
[Ru(L )(PPh ) Cl] (1) (R = CH )
3 2
3
PPh
3
2
Cl
[Ru(L )(PPh ) Cl] (2) (R = C H )
Organometallic complexes of ruthenium are essentially
limited to its diamagnetic +2 oxidation state [1,2]. Only a
few examples of authentic ruthenium(III)–C r-bond con-
taining species with carbonyl or non-carbonyl ligand sys-
tems are known [3–13]. Among the non-carbonylic
systems cyclometallated species are very rare. The few
examples known are with tridentate aromatic-C, azo- or
imine-N and phenolate-O coordinating ligands [10–13].
These complexes are one-electron paramagnetic and EPR
active. The X-ray crystal structure of one of these com-
plexes with the C,N,O-donor azophenolate has been
reported [10].
3 2
6 5
3
Ru
[Ru(L )(PPh ) Cl] (3) (R = 4-CH -C H )
3 2
3
6 5
HC
4
[Ru(L )(PPh ) Cl] (4) (R = 4-OCH -C H )
O
3 2
3
6 5
N
PPh
3
5
[Ru(L )(PPh ) Cl] (5) (R = 4-Cl-C H )
3 2
6 5
N
C
6
[Ru(L )(PPh ) Cl] (6) (R = 4-NO -C H )
3 2
2
6 5
R
We have been working on the Schiff bases (H2L) derived
from acid hydrazides and aromatic aldehydes, which can
act as monoanionic or bianionic tridentate aromatic-C,
imine-N and amide-O donor ligands to provide cyclometal-
lated complexes. Recently, we have reported some ortho-
palladated species with H2L [14,15]. These results
prompted us to explore the possibility of using this Schiff
*
Corresponding author. Tel.: +91 40 2313 4756; fax: +91 40 2301 2460.
E-mail address: spsc@uohyd.ernet.in (S. Pal).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.10.026

R. Raveendran, S. Pal / Journal of Organometallic Chemistry 692 (2007) 824–830
825
base system for the synthesis of new ortho-metallated
ruthenium species using the 16-electron starting material
[Ru(PPh3)3Cl2]. In this effort, we have isolated a series of
paramagnetic ruthenium(III) organometallic complexes of
C,N,O-coordinating L2ꢀ. In the following account, we
have described the synthesis, structure and physical proper-
ties of these complexes having the general molecular for-
mula trans-[Ru(Ln)(PPh3)2Cl] (n = 1–6).
complex separated in about 2 days was collected by filtra-
tion and dried in air. Yield, 150 mg (59%). A single crystal
suitable for X-ray structure determination was collected
from this material.
2.4. Synthesis of [Ru(L2)(PPh3)2Cl] (2)
Solid [Ru(PPh3)3Cl2] (300 mg, 0.31 mmol) was added to
a methanol solution (35 ml) of H2L2 (70 mg, 0.31 mmol)
and N(C2H5)3 (0.09 ml, 65 mg, 0.65 mmol) and the mixture
was stirred at room temperature for 2 h. The orange-red
solid precipitated was collected by filtration, washed with
hexane and finally dried in air. Yield, 165 mg (60%).
The other complexes (3–6) reported in this work were
synthesized in 55–60% yields by following the same general
procedure as described above for 2. In powder form, com-
plexes 3–5 are orange-red in colour as 2, while complex 6 is
dark-brown in colour. Among complexes 2–6, single crys-
tals of only 3 could be obtained by the same way as
described for 1.
2. Experimental
2.1. Materials
The Schiff bases H2Ln (n = 1–6) were prepared in 60–
80% yields by condensation reactions of 1 mole equiv. of
benzaldehyde with 1 mol equiv. of the corresponding acid
hydrazide in methanol. [Ru(PPh3)3Cl2] was prepared by
following a reported procedure [16]. All other chemicals
and solvents were of analytical grade available commer-
cially and were used without further purification.
2.2. Physical measurements
2.5. X-ray crystallography
Microanalytical (C, H, N) data were obtained with a
Thermo Finnigon Flash EA1112 series elemental analyzer.
Infrared spectra were collected by using KBr pellets on a
Jasco-5300 FT-IR spectrophotometer. A Shimadzu 3101-
PC UV/vis/NIR spectrophotometer was used to record
the electronic spectra. EPR spectra were recorded on a Jeol
JES-FA200 spectrometer. Magnetic susceptibilities were
measured using a Sherwood Scientific balance. Diamag-
netic corrections calculated from Pascal’s constants [17]
were used to obtain the molar paramagnetic susceptibili-
ties. Solution electrical conductivities were measured with
a Digisun DI-909 conductivity meter. A CH-Instruments
model 620A electrochemical analyzer was used for cyclic
voltammetric experiments with dichloromethane solutions
of the complexes containing tetrabutylammonium perchlo-
rate (TBAP) as the supporting electrolyte. The three elec-
trode measurements were carried out at 298 K under
dinitrogen atmosphere with a platinum disk working elec-
trode, a platinum wire auxiliary electrode and an Ag/AgCl
reference electrode. Under identical condition the Fc+/Fc
couple was observed at 0.65 V. The potentials reported in
this work are uncorrected for junction contributions.
Unit cell parameters and the intensity data for 1 and 3
were obtained on a Bruker-Nonius SMART APEX CCD
single crystal diffractometer, equipped with a graphite
monochromator and a Mo Ka fine-focus sealed tube
˚
(k = 0.71073 A) operated at 2.0 kW. The detector was
placed at a distance of 6.0 cm from the crystal and the data
were collected at 298 K with a scan width of 0.3ꢁ in x and
an exposure time of 15 s/frame. The SMART software was
used for data acquisition and the SAINT-PLUS software was
Table 1
Crystallographic data for trans-[Ru(L1)(PPh3)2Cl] (1) and trans-
[Ru(L3)(PPh3)2Cl] (3)
Complex
1
3
Chemical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
RuC45H38N2OClP2 RuC51H42N2OClP2
821.23
897.33
Orthorhombic
Pbca
Triclinic
ꢀ
P1
˚
a (A)
17.9809(9)
17.7702(9)
24.2867(12)
90
90
90
7760.2(7)
8
1.406
9.9424(5)
12.1929(6)
18.9142(9)
94.610(1)
104.140(1)
94.449(1)
2205.0(2)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
2.3. Synthesis of [Ru(L1)(PPh3)2Cl] (1)
3
˚
V (A )
Z
q (g cmꢀ3
)
1.352
0.528
Solid [Ru(PPh3)3Cl2] (300 mg, 0.31 mmol) was added to
a methanol solution (30 ml) of H2L1 (50 mg, 0.31 mmol)
and N(C2H5)3 (0.09 ml, 65 mg, 0.65 mmol) and the mixture
was heated under reflux for 2 h. On cooling to room tem-
perature the complex was precipitated as a yellow solid.
It was collected by filtration and washed with hexane.
The solid was dissolved in dichloromethane–acetonitrile
(1:6) mixture (7 ml). Hexane (4 ml) was added to this solu-
tion and left in air for slow evaporation. The crystalline
l (mmꢀ1
)
0.593
Reflections collected
Reflections unique
Reflections [I P 2r(I)]
Parameters
45509
9295
5190
470
24430
9903
7777
524
R1, wR2 (I P 2r(I))
R1, wR2 (all data)
GOF on F2
0.0623, 0.1003
0.1301, 0.1190
0.972
0.931 and ꢀ0.522
0.0458, 0.0920
0.0628, 0.0982
0.981
1.247 and ꢀ0.466
ꢀ3
˚
Largest peak and hole (e A
)

826
R. Raveendran, S. Pal / Journal of Organometallic Chemistry 692 (2007) 824–830
used for data extraction [18]. The data were corrected for
absorption with the help of SADABS program [19]. Com-
plexes 1 and 3 crystallize in the space groups Pbca and
moments of 1–6 in powder phase are in the range 1.89–
2.09 lB (Table 2). These values are consistent with the +3
oxidation state and low-spin character (S = 1/2 ground
state) of the metal ions. It is very likely that the aerial oxy-
gen acts as the oxidizing agent in the oxidation of the metal
ion during the synthesis of these complexes from the ruthe-
nium(II) starting material [Ru(PPh3)3Cl2].
ꢀ
P1, respectively. The structures were solved by direct meth-
ods and refined on F2 by full-matrix least-squares proce-
dures. In each case, the asymmetric unit contains one
complete complex molecule. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were included
in the structure factor calculation at idealized positions by
using a riding model. The SHELX-97 programs [20] were
used for structure solution and refinement. The ORTEX6a
package [21] was used for molecular graphics. Selected
crystallographic data are listed in Table 1.
3.2. Spectroscopic properties
The infrared spectra of 1–6 do not display the character-
istic bands associated with the N–H and the C@O bonds of
the amide functionality [24,25] present in the free Schiff
bases. Thus the amide functionality is in the enolate form
and the ligand coordinates the metal ion through the
amide-O and the imine-N to form a five-membered ring
as commonly observed in the complexes with Schiff bases
derived from acid hydrazides [14,15,26–29]. A medium to
strong band observed in the range 1564–1605 cmꢀ1 is
assigned to the conjugated AC@NAN@CA fragment of
the ligand [14,15,26–29]. Three strong bands observed in
the ranges 741–747, 693–696 and 515–521 cmꢀ1 indicate
the presence of ruthenium bound PPh3. Similar bands are
reported for complexes containing the trans-{Ru(PPh3)2}
unit [12,28–30].
The electronic spectroscopic data of 1–6 in dichloro-
methane solutions are listed in Table 2. The spectral pro-
files are quite similar. The moderately intense absorptions
in the visible region are perhaps due to the ligand-to-metal
charge transfer transitions [10–13,27,29,30]. The intense
absorptions in the higher energy region are likely to be
due to the ligand based transitions.
The EPR spectral profiles of 1–6 in frozen (120 K)
dichloromethane–toluene (1:1) solutions are very similar.
A representative spectrum is shown in Fig. 1. Each com-
plex displays three distinct signals (Table 3) indicating the
distortion of the CNOClP2 coordination sphere around
the metal centre from octahedral symmetry. This distortion
can be divided into axial (D) and rhombic (V) components.
The axial distortion splits the t2 level into ‘‘a’’ and ‘‘e’’ and
the rhombic component again splits ‘‘e’’ into non-degener-
ate components (Fig. 1). In addition to D and V, spin–orbit
coupling also affects the extent of energy gaps between
3. Results and discussion
3.1. Synthesis and characterization
Reactions of [Ru(PPh3)3Cl2], the Schiff bases and
N(C2H5)3 (1:1:2 mole ratio) in methanol under aerobic
conditions produce the complexes 1–6 in moderate yields.
For 1, boiling of the reaction mixture under reflux condi-
tion was necessary, while for 2–6 stirring at room temper-
ature was sufficient. In the latter cases, boiling causes
contamination of the products by colourless mer-
[Ru(CO)(H)2(PPh3)3] [22,23]. We have confirmed the iden-
tity of this impurity by determining its X-ray structure
which has been already published [22]. Formation of this
carbonyl containing complex from [Ru(PPh3)3Cl2] in the
presence of potassium aryloxide in methanol–dichloro-
methane mixture has been reported recently [23]. The ele-
mental analysis data for the complexes are satisfactory
with the general molecular formula [Ru(Ln)(PPh3)2Cl]
(Table 2). The complexes are highly soluble in dichloro-
methane and chloroform, affording yellow (1) to brown
(2–6) solutions. However, except 1 the other complexes
are not stable in solution. After about 1/2 h the colour
changes to green. Despite our best efforts, we have not been
able to isolate and characterize these green species. For this
reason all the spectroscopic studies (vide infra) have been
performed within 5 min after the preparation of the solu-
tions. In solutions, all the complexes are electrically non-
conducting. The room temperature (298 K) magnetic
Table 2
Elemental analysis, electronic spectroscopica and magnetic susceptibilityb data
Complex
Found (Calc.) (%)
C
kmax (nm) (10ꢀ4 · e (Mꢀ1 cmꢀ1))
leff (lB)
H
N
1
2
3
4
5
6
a
65.72 (65.81)
67.75 (67.99)
68.13 (68.26)
66.89 (67.07)
65.26 (65.43)
64.41 (64.69)
4.58 (4.66)
4.49 (4.56)
4.61 (4.72)
4.50 (4.63)
4.14 (4.28)
4.05 (4.23)
3.34 (3.41)
3.05 (3.17)
3.10 (3.12)
2.98 (3.07)
2.87 (3.05)
4.32 (4.53)
450 (0.34), 380c (1.1), 353 (1.8), 270 (7.3)
495c (0.31), 410 (0.79), 308 (3.5), 260c (5.9)
482c (0.27), 407c (0.58), 323c (1.8), 245c (6.6)
480c (0.35), 415c (0.67), 315c (3.1), 247 (8.3)
490c (0.33), 418 (0.65), 310c (2.5), 245 (5.6)
505c (0.36), 445c (0.62), 350c (2.4), 250 (4.9)
2.06
2.05
1.92
2.09
1.94
1.89
In dichloromethane.
b
c
At 298 K.
Shoulder.

R. Raveendran, S. Pal / Journal of Organometallic Chemistry 692 (2007) 824–830
827
Table 4. In each molecule, the metal centre is in a distorted
octahedral CNOClP2 coordination sphere. The meridional
tridentate ligand ((L1)2ꢀ in 1 and (L3)2ꢀ in 3) coordinates
the metal centre via the aromatic-C, the imine-N and the
deprotonated amide-O atoms forming two five-membered
chelate rings. The chloride completes a CNOCl square-
˚
˚
plane (maximum deviation 0.05 A for 1 and 0.04 A for 3)
around the metal centre. As commonly observed for hexa-
coordinated complexes containing the {Ru(PPh3)2} unit,
the two bulky PPh3 molecules occupy the remaining two
axial sites [11,28–30]. The N(2)–C(8) and C(8)–O bond
lengths are consistent with the enolate (AN@C(Oꢀ)A)
form of the amide functionality in the tridentate ligands
[14,15,26–29]. The five-membered chelate rings are highly
planar. In the chelate ring formed by Ru, N(1), N(2),
˚
C(8) and O, the maximum deviation is 0.03 A for 1 and
˚
˚
0.01 A for 3. All deviations are below 0.02 A for 1 and
˚
0.006 A for 3 in the chelate ring formed by Ru, C(1),
C(6), C(7) and N(1). Both chelate ring planes are essentially
Fig. 1. X-band EPR spectrum of trans-[Ru(L1)(PPh3)2Cl] (1) in
dichloromethane–toluene (1:1) glass (120 K) and the t2 splitting pattern.
C42
C38
C39
C34
C37
C36
C43
C41
these levels. Thus, two ligand-field transitions of energies
DE1 and DE2 (DE1 < DE2) are possible within these three
levels. The distortion parameters and the transition ener-
gies have been calculated using the EPR g-values and the
g tensor theory for low-spin d5 metal ion complexes [31].
The calculated values are listed in Table 3. In all the com-
plexes, the axial and the rhombic distortions are compara-
ble. The DE1 transition is expected to occur in the range
4644–6359 cmꢀ1 (2144–1573 nm), while the DE2 transition
is expected to occur within 13,535–18,146 cmꢀ1 (739–
551 nm) (Table 3). However, no absorption could be
detected in the DE1 region, perhaps due to the low intensity
of this absorption [10–13,31] and the poor transparency of
the solvent. No band could be observed in the DE2 region
also as it falls on the onset of the moderately intense
absorption observed in the range 505–450 nm (Table 2).
C32
C31
C44
C40
Cl
C33
C45
C2
P2
C30
C29
O
Ru
C8
N2
C1
C3
C9
N1
C6
P1
C7
C27
C4
C5
C26
C11
C25
C12
C13
C22
C10
C15
C16
C17
C21
C24
C23
C14
C20
C18
C19
3.3. X-ray structures of 1 and 3
Fig. 2. Molecular structure of trans-[Ru(L1)(PPh3)2Cl] (1) with the atom
labeling scheme. All atoms are represented by their 30% probability
thermal ellipsoids. Hydrogen atoms are omitted for clarity.
The molecular structures of 1 and 3 are depicted in Figs.
2 and 3, respectively. Selected bond parameters are listed in
Table 3
EPR g-values,a distortion parameters and near-IR transitionsb
Complex
g1
g2
g3
D/k
V/k
DE1/k
DE2/k
1
2
3
4
5
6
a
2.34
2.35
2.32
2.33
2.35
2.34
2.10
2.10
2.07
2.11
2.11
2.09
1.96
1.95
1.93
1.95
1.95
1.95
12.169
10.711
8.984
10.051
10.381
10.895
ꢀ11.774
ꢀ10.239
ꢀ8.858
ꢀ8.622
ꢀ9.373
ꢀ10.771
6.359
5.768
4.664
5.822
5.779
5.598
18.146
15.931
13.535
14.469
15.172
16.380
In 1:1 dichloromethane–toluene at 120 K.
The calculations are based on the spin–orbit coupling constant (k) = 1000 cmꢀ1
b
.

828
R. Raveendran, S. Pal / Journal of Organometallic Chemistry 692 (2007) 824–830
and 3, respectively. The P(1)–Ru–P(2) is the largest
C48
C47
C49
C50
(178.50(4)ꢁ for 1 and 172.95(3)ꢁ for 3), while O–Ru–C(1)
is the smallest (151.09(14)ꢁ for 1 and 150.85(10)ꢁ for 3)
trans angle in both cases. The trans Cl–Ru–N(1) angle is
173.92(10) and 170.42(7)ꢁ for 1 and 3, respectively. The
bond lengths associated with the metal ion in 1 and 3 are
very similar. The Ru–N(imine) and the Ru–O(amide) bond
lengths are comparable with the bond lengths observed in
ruthenium(III) complexes containing the same coordinat-
ing atoms [27,29]. The Ru–P bond lengths are similar to
those reported for ruthenium(III) complexes containing
the trans-{Ru(PPh3)2} moiety [11,29,30]. The Ru–Cl bond
lengths in 1 and 3 are unexceptional [29,30]. All the com-
plexes (1–6) have the general molecular formula
[Ru(Ln)(PPh3)2Cl] and similar spectroscopic and electro-
chemical properties (vide infra). Hence, they are assumed
to have similar molecular structures as observed for 1
and 3.
C42
C38
C37
C41
C43
C44
C46
C40
C36
C13
P2
Cl
C1
C45
C12
C11
C14
C35
Ru
O
C15
C8
C2
C9
C10
C3
N2
N1
C7
C6
P1
C4
C30
C31
C17
C18
C5
C23
C24
C25
C22
C33
C21
C27
C26
C20
C19
Fig. 3. Molecular structure of trans-[Ru(L3)(PPh3)2Cl] (3) with the atom
labeling scheme. All atoms are represented by their 30% probability
thermal ellipsoids. Hydrogen atoms are omitted for clarity.
3.4. Electron transfer properties
Electron transfer properties of 1–6 in dichloromethane
solutions have been investigated by cyclic voltammetry.
None of the complexes shows any response on the cathodic
side of Ag/AgCl reference electrode. Complex 1 displays an
irreversible oxidation at 0.98 V, while complexes 2–6 dis-
play a reversible oxidation in the potential range 0.35–
0.58 V. In the latter cases, the peak-to-peak separations
are within 80–90 mV and the cathodic and the anodic peak
currents are essentially identical. A representative cyclic
voltammogram is shown in Fig. 4 and the potential data
are listed in Table 5. The one-electron stoichiometry of this
oxidation response is confirmed by comparing the current
heights with known one-electron redox processes under
identical conditions [12,26–30,32]. The free ligands do not
Table 4
Selected bond parameters for trans-[Ru(L1)(PPh3)2Cl] (1) and trans-
[Ru(L3)(PPh3)2Cl] (3)
Complex
1
3
˚
Bond lengths (A)
Ru–O
Ru–Cl
Ru–N(1)
Ru–C(1)
Ru–P(1)
Ru–P(2)
N(2)–C(8)
C(8)–O
2.153(3)
2.3588(12)
2.030(3)
2.1455(18)
2.3598(7)
2.028(2)
2.051(3)
2.3992(7)
2.3930(7)
1.307(4)
1.288(3)
2.041(4)
2.4050(11)
2.3835(11)
1.326(5)
1.272(5)
Bond angles (ꢁ)
O–Ru–Cl
O–Ru–N(1)
O–Ru–C(1)
O–Ru–P(1)
100.20(8)
74.16(12)
151.09(14)
91.66(8)
96.57(5)
73.87(8)
150.85(10)
96.16(6)
90.26(6)
170.42(7)
112.41(8)
87.98(2)
88.40(2)
77.17(10)
92.41(7)
92.17(7)
88.26(8)
87.53(8)
172.95(3)
O–Ru–P(2)
86.88(8)
0.6
Cl–Ru–N(1)
Cl–Ru–C(1)
Cl–Ru–P(1)
Cl–Ru–P(2)
N(1)–Ru–C(1)
N(1)–Ru–P(1)
N(1)–Ru–P(2)
C(1)–Ru–P(1)
C(1)–Ru–P(2)
P(1)–Ru–P(2)
173.92(10)
108.66(13)
89.40(4)
NO2
1∝A
90.51(4)
77.08(16)
88.52(10)
91.42(10)
90.51(12)
90.94(12)
178.50(4)
0.5
0.2
0.4
0.6
E (V) vs. Ag/AgCl
Cl
H
0.4
0.3
CH3
OCH3
coplanar. The dihedral angle between the two chelate ring
planes is 1.23(2)ꢁ and 3.69(8)ꢁ for 1 and 3, respectively. The
chelate bite angle formed by the imine-N and the amide-O
(74.16(12)ꢁ for 1 and 73.87(8)ꢁ for 3) is slightly smaller than
that formed by the aromatic-C and the imine-N (77.08(16)ꢁ
for 1 and 77.17(10)ꢁ for 3). All other cis angles are in the
ranges 86.88(8)–108.66(13)ꢁ and 87.53(8)–112.41(8)ꢁ for 1
-0.3
0.0
0.3
0.6
0.9
p
Fig. 4. Correlation between the E1/2 values for the ruthenium(IV)–
ruthenium(III) couple and the Hammett substituent constants. The
straight line represents a linear least-squares fit. Inset: Cyclic voltammo-
gram (scan rate 50 mV sꢀ1) of trans-[Ru(L2)(PPh3)2Cl] (2) in dichloro-
methane solution (0.1 M TBAP).

R. Raveendran, S. Pal / Journal of Organometallic Chemistry 692 (2007) 824–830
829
Table 5
examples of ruthenium mediated activation of ortho-C–H
bond of the pendant phenyl group of H2L. All the com-
plexes are one-electron paramagnetic and display rhombic
EPR spectra typical of ruthenium(III) complexes contain-
ing distorted octahedral low-spin metal centres. The com-
plexes are redox active and display a metal centred
oxidation. The potential of this ruthenium(IV)–ruthe-
nium(III) couple is sensitive to the polar effect of the sub-
stituent on the aroyl moiety of the ligand.
Cyclic voltammetrica,b data
Complex
E1/2 (V)
DEp (mV)
trans-[Ru(L1)(PPh3)2Cl] (1)
trans-[Ru(L2)(PPh3)2Cl] (2)
trans-[Ru(L3)(PPh3)2Cl] (3)
trans-[Ru(L4)(PPh3)2Cl] (4)
trans-[Ru(L5)(PPh3)2Cl] (5)
trans-[Ru(L6)(PPh3)2Cl] (6)
a
0.98c
0.40
0.38
0.35
0.45
0.58
–
80
90
80
80
90
In dichloromethane solution (298 K) at a scan rate of 50 mV sꢀ1
E1/2 = (Epa + Epc)/2, where Epa and Epc are anodic and cathodic peak
.
b
5. Supplementary material
potentials, respectively; DEp = Epa ꢀ Epc
.
c
Epa value.
CCDC 620905 and 620906 contain the supplementary
crystallographic data for trans-[Ru(L1)(PPh3)2Cl] (1) and
trans-[Ru(L3)(PPh3)2Cl] (3), respectively. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.
cam.ac.uk.
display any response in the above potential range. Thus the
observed oxidation is assigned to ruthenium(III) to ruthe-
nium(IV) process. It may be noted that the previously
reported cyclometallated ruthenium(III) complexes of aro-
matic-C, azo- or imine-N and phenolate-O donor ligands
also display similar ruthenium(III) to ruthenium(IV) oxida-
tion responses [10–13]. In these complexes, the stability of
the +3 oxidation state and the accessibility of the +4 state
are primarily attributed to the phenolate-O coordination
[10–13]. In the present series of complexes (1–6), a similar
situation prevails due to the deprotonated amide-O coordi-
nation. The significantly higher potential for 1 compared to
the potentials of 2–6 indicates easy accessibility of the +4
oxidation state of the metal centre due to aroyl amide-O
coordination than acetyl amide-O coordination. The irre-
versible nature of the oxidation in the case of 1 suggests
that the oxidized species is unstable in the cyclic voltamme-
try time scale. In the cases of 2–6, the trend in the E1/2 val-
ues of the ruthenium(IV)–ruthenium(III) couple reflects the
effect of the electronic nature of the substituents (R) on the
aroyl fragment of the tridentate ligands. For the most elec-
tron withdrawing substituent (R = NO2) the oxidation of
the metal ion occurs at the highest potential while for the
most electron releasing substituent (R = OCH3) it occurs
at the lowest potential (Table 5). A satisfactory linear rela-
tionship is observed (Fig. 4) when the E1/2 values are plot-
ted against the Hammett substituent constants (rp) [33].
Thus, as the r-bonding ability of the amide-O of the tri-
dentate ligand decreases with the increasing electron with-
drawing effect of the substituent, the ruthenium(III) to
ruthenium(IV) oxidation becomes more difficult.
Acknowledgements
Financial support for this work was provided by the
Council of Scientific and Industrial Research, New Delhi
(Grant No. 01(1880)/03/EMR-II). X-ray crystal structures
were determined at the National Single Crystal Diffractom-
eter Facility, School of Chemistry, University of Hydera-
bad (funded by the Department of Science and
Technology, New Delhi). We thank the University Grants
Commission, New Delhi for the facilities provided under
the UPE and CAS programs.
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