Synthesis and characterisation of ruthenium(III) complexes containing monobasic bidentate Schiff bases and their biological activities

Article abstract of DOI:10.1081/SIM-120003211

New hexa-coordinated ruthenium(III) complexes of the type [RuX₂(L)(EPh₃)₂] (X = Cl or Br; L = monobasic bidentate Schiff base ligand; E = P or As) have been synthesised by the reactions of [RuCl₃(PPh₃

Full text of DOI:10.1081/SIM-120003211

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SYNTHESIS AND CHARACTERISATION OF RUTHENIUM(III)
COMPLEXES CONTAINING MONOBASIC BIDENTATE SCHIFF
BASES AND THEIR BIOLOGICAL ACTIVITIES
T. Daniel Thangadurai a , M. Gowri a & K. Natarajan b
a Department of Chemistry , Bharathiar University , Coimbatore, 641 046, India
b Department of Chemistry , Bharathiar University , Coimbatore, 641 046, India
Published online: 15 Feb 2007.
To cite this article: T. Daniel Thangadurai , M. Gowri & K. Natarajan (2002) SYNTHESIS AND CHARACTERISATION OF
RUTHENIUM(III) COMPLEXES CONTAINING MONOBASIC BIDENTATE SCHIFF BASES AND THEIR BIOLOGICAL ACTIVITIES, Synthesis
and Reactivity in Inorganic and Metal-Organic Chemistry, 32:2, 329-343, DOI: 10.1081/SIM-120003211
To link to this article: http://dx.doi.org/10.1081/SIM-120003211
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SYNTH. REACT. INORG. MET.-ORG. CHEM., 32(2), 329–343 (2002)
SYNTHESIS AND CHARACTERISATION
OF RUTHENIUM(III) COMPLEXES
CONTAINING MONOBASIC BIDENTATE
SCHIFF BASES AND THEIR BIOLOGICAL
ACTIVITIES
T. Daniel Thangadurai, M. Gowri, and K. Natarajan*
Department of Chemistry, Bharathiar University,
Coimbatore-641 046, India
ABSTRACT
New hexa-coordinated ruthenium(III) complexes of the type
[RuX2(L)(EPh3)2] (X ¼ Cl or Br; L ¼ monobasicbidentate
Schiff base ligand; E ¼ P or As) have been synthesised by the
reactions of [RuCl3(PPh3)3], [RuCl3(AsPh3)3], [RuBr3-
(AsPh3)3] or [RuBr3(PPh3)2(MeOH)] with the appropriate
ligands such as anthranilic acid cinnamaldehyde (Hanthcin),
anthranilicacid p-tolualdehyde (Hanth-p-tol) and anthranilic
acid p-anisaldehyde (Hanth-p-ans). All of these new com-
plexes were characterised using various physico-chemical
methods such as elemental analyses, spectral (IR, electronic
and EPR), magnetic moment and cyclic voltammetric data.
The antibacterial activities of ligands and their complexes
have also been determined. An octahedral structure has been
tentatively assigned to all of the new complexes.
*Corresponding author. E-mail: karuppannannatarajan@yahoo.com
329
Copyright # 2002 by Marcel Dekker, Inc.
www.dekker.com

330
THANGADURAI, GOWRI, AND NATARAJAN
INTRODUCTION
In recent years there has been a great deal of interest in the synthesis
and characterisation of transition metal complexes containing Schiff bases
as ligands, due to their importance as catalysts for many reactions1. Fur-
thermore, tetradentate Schiff base complexes are important for designing
metal complexes related to synthetic and natural oxygen carriers2. Although
there is a wealth of information available on transition metal complexes of
Schiff bases, only very little attention has been given to their ruthenium
analogues375. Besides, some carbonyl complexes of ruthenium play an
important role in homogeneous catalysis6. Ruthenium(III) complexes
showed a positive result in antibacterial activity studies1.
In view of growing interest in oxygenation and carbonylation with
ruthenium(III) complexes as new catalysts, we describe in this paper the
synthesis and characterisation of stable ruthenium(III) complexes of the
type [RuX2(L)(EPh3)2] (X ¼ Cl or Br; HL ¼ monobasicbidentate Schiff base
ligand; E ¼ P or As). The ligands used in this study are of the type shown in
Fig. 1.
Figure 1. Structure of the ligand.

RUTHENIUM(III) COMPLEXES
EXPERIMENTAL
331
All reagents used were chemically pure or analytical reagent grade.
Solvents were purified and dried according to standard procedures1.
RuCl3Á3H2O was purchased from Loba Chemie Pvt. Ltd., Bombay and was
used without further purification. [RuCl3(PPh3)3]7, [RuCl3(AsPh3)3]8,
[RuBr3(AsPh3)3]9 and [RuBr3(PPh3)2(MeOH)]10 and the ligands11 were
prepared by reported literature methods.
The analyses of carbon, hydrogen and nitrogen were performed on
Carlo Erba 1106 and Perkin-Elmer Model 240 CHN analyzers at the
Central Drug Research Institute, Lucknow, India. IR spectra were recorded
in KBr pelletes in the 40007500 cmÀ1 region using a Shimadzu FTIR-8200
spectrophotometer. Electronic spectra were recorded in dichloromethane
solution with a Hitachi Perkin-Elmer 20=200 spectrophotometer in the
range 8007200 nm. EPR spectra of the powdered samples were recorded
with a Bruker Model ER 200-D spectrometer at X-band frequencies.
Magnetic susceptibilities were recorded on an EG and G-PARC vibrating
sample magnetometer. Cyclic voltammetric studies were carried out in
acetonitrile using a glassy-carbon working electrode and the potentials were
referenced to a saturated calomel electrode. Melting points were recorded
with a Boetius micro-heating table and are uncorrected.
Preparation of the Ligands
The monobasicbidentate ligands have been prepared by the ocn-
densation reaction of substituted aldehydes [cinnamaldehyde (6.30 mL,
0.05 mol) or p-tolualdehyde (5.89 mL, 0.05 mol) or p-anisaldehyde (6.08 mL,
0.05 mol)] with anthranilicacid (6.85 g, 0.05 mol) in 1 : 1 molar ratio in ethyl
alcohol (20 mL). The solution was heated under reflux for 7 h and then
concentrated to 5 mL. On cooling the solutions the products separated out.
The crude products were purified by column chromatography on silica gel
using petroleum ether=ethyl acetate (95=5%) as eluant. The purity of the
compounds was checked by TLC. They were dried under vacuum.
Characterisation of the Ligands
The ligands, namely Hanthcin, Hanth-p-tol and Hanth-p-anz, have
been characterised on the basis of elemental analysis and IR spectra. The
elemental analyses (Table I) agree very well with the molecular formulas.
The IR spectra of all the ligands showed a strong band in the region

332
THANGADURAI, GOWRI, AND NATARAJAN

RUTHENIUM(III) COMPLEXES
333

334
THANGADURAI, GOWRI, AND NATARAJAN
162071600 cmÀ1 characteristic of the azomethine n(C¼N) group. The
ligands showed the n(OH) stretching frequency in the region
350073300 cmÀ1 and a strong band around 1670 cmÀ1 due to n(C¼O).
Preparation of the Complexes
All the preparations were carried out under strictly anhydrous con-
ditions. The Schiff bases (0.2170.28 g, 0.01 mmol) were added to a solution
of [RuX3(EPh3)3] (0.9971.13 g, 0.01 mmol) (E ¼ P, X ¼ Cl; E ¼ As, X ¼ Cl,
Br) or [RuBr3(PPh3)2(MeOH)] (0.90 g, 0.01 mmol) in 1 : 1 molar ratio in
benzene (25 mL) and the mixtures were refluxed for 7 h. The resulting dark
coloured solutions were concentrated to about 3 mL. The complexes were
precipitated by the addition of a small quantity of petroleum ether (b.p.
ꢀ
60780 C). The complexes were then filtered, washed with petroleum ether
and recrystallised from CH2Cl2=petroleum ether (1 : 4 n=n) and dried under
vacuum. All of the complexes were prepared using the above procedure.
RESULTS AND DISCUSSION
The ligands used in this work were synthesised by the condensation of
anthranilicacid with the appropriate substituted aldehydes in 1 : 1 molar
ratio. These ligands react with [RuCl3(PPh3)3], [RuCl3(AsPh3)3],
[RuBr3(AsPh3)3] or [RuBr3(PPh3)2(MeOH)] in 1 : 1 molar ratio to give hexa-
coordinated ruthenium(III) complexes of the type [RuX2(L)(EPh3)2] (X ¼ Cl
or Br; L ¼ monobasicbidentate Schiff base ligand; E ¼ P or As). All of the
complexes are dark coloured and quite stable in the air and light. The
analytical data given in Table I for the complexes are in good agreement
with the formulae proposed.
IR Spectra
The characteristic infrared frequencies for all the complexes listed in
Table II. The spectra of the free Schiff base ligands have been compared
with those of the complexes formed by the corresponding Schiff bases in
order to confirm the coordination of the ligands to the metal atom. The
n(OH) absorption observed around 350073300 cmÀ1 in the free Schiff bases
is absent in the complexes, indicating that deprotonation occurs prior to the
coordination through the oxygen atom. The n(C¼O) (carboxyl group)
showed a band around 1670 cmÀ1 in the free Schiff bases. In the complexes,

RUTHENIUM(III) COMPLEXES
335

336
THANGADURAI, GOWRI, AND NATARAJAN
the absorptions due to carbonyl group were observed at around
165071620 cmÀ1 and around 143071410 cmÀ1 arising from asymmetricand
symmetric stretchings, respectively. The difference between nasym and nsym
vibrations of the carbonyl group has been found to be around 210 cmÀ1
.
This is a clear indication of the monodentate coordination of the carboxyl
group with the free carbonyl group12. A strong band was observed at
162071600 in the free ligand which is characteristic of the azomethine
(HC¼N) group. It is expected that coordination of the nitrogen to the metal
atom would reduce the electron density in the azomethine link and thus
lower the n(C¼N) stretching frequency. In the complexes this frequency was
observed in the region 159571575 cmÀ1 indicating coordination of the Schiff
bases through the azomethine nitrogen13. In addition to the above, the other
characteristic bands due to triphenylphosphine and triphenylarsine were
also present in the expected region.
Electronic Spectra
Electronic spectra of all the complexes in CH2Cl2 showed one or two
bands in the region 5207305 nm (Table II). The ground state of rutheniu-
m(III) (t2g configuration) is T2g and the first excited doublet levels in the
5
2
2
order of increasing energy are A2g and T1g which arise from the t2g eg
2
4
1
configuration. In most of the ruthenium(III) complexes the electronic
spectra show only charge transfer bands15. It has been observed that the
charge transfer bands of the type Lp*!t2g are prominent in the low energy
region which obscure the weaker dÀÀd transition. Hence, it is difficult to
assign conclusively the bands of ruthenium(III) complexes in the visible
region. However, the bands around 5207480 nm have been assigned to the
2
2T2g! A2g transition based on the low extinction coefficient value ðEÞ as
compared to charge transfer bands. This is in conformity with assignments
made for similar other octahedral ruthenium(III) complexes14,15. The other
bands in the region 4007305 nm have been assigned to charge transfer
bands.
Cyclic Voltammetry
Cyclic voltammetric studies have been carried out for the complexes in
acetonitrile solution at a glassy-carbon working electrode. The oxidations
and reductions of some complexes are achieved by well defined waves with
Ef values in the range from 0.62 V to 0.76 V (oxidation) and from À0.36 V to
À0.76 V (reduction) against saturated calomel electrode (Table III). The

RUTHENIUM(III) COMPLEXES
337

338
THANGADURAI, GOWRI, AND NATARAJAN
redox process observed from these complexes are metal-centered only. Most
of the complexes showed reversible redox couples with peak-to-peak
separation values (DEp) ranging from 60 to 100 mV, indicating a single step,
one-electron transfer process16,17. The complexes (10) and (11) showed only
a reduction potential and the complex (15) showed only an oxidation pro-
cess. The reason for the irreversibility for these complexes may be due to the
short-lived oxidation state of the metal ion18 or due to oxidative degradation
of the ligands19.
Magnetic Moment Studies
The magneticmoments for some of the complexes have been measured
at room temperature using a vibrating sample magnetometer and diamag-
netic corrections have been applied. The values obtained were from 1.82 to
1.95 BM indicating the presence of one unpaired electron, suggesting a low-
5
spin t2g configuration for the ruthenium(III) ion in an octahedral envir-
onment in all of these complexes.
EPR Spectra
EPR spectra of powdered samples were recorded at room temperature.
The spectra of the complexes (4), (8) and (12) exhibit an axially symmetric
system with g r around 2.2072.34 and g11el around 2.0572.29 (Table IV).
?
For an octahedral field with tetragonal distortion gx ¼ gy ¼ gz and hence,
two ‘g’ values indicate tetragonal distortion in these complexes. The other
Table IV. EPR Spectral Dataa Ruthenium(III) Complexes
S. No
Complex
gx
gy
gz
hgi*
(4)
(5)
[RuCl2(anthcin)(PPh3)2]
[RuCl2(anthcin)(AsPh3)2]
[RuBr2(anthcin)(PPh3)2]
[RuCl2(anth-p-tol)(PPh3)2]
[RuBr2(anth-p-tol)(AsPh3)2]
[RuBr2(anth-p-tol)(PPh3)2]
[RuCl2(anth-p-ans)(PPh3)2]
[RuBr2(anth-p-ans)(AsPh3)2]
[RuBr2(anth-p-ans)(PPh3)2]
2.34
7
7
2.34
7
7
2.20
2.34
2.12
2.11
2.34
2.13
2.14
2.20
2.07
2.13
2.27
7
7
2.29
7
7
2.05
2.24
7
7
2.32
7
7
2.15
(7)
(8)
(9)
(11)
(12)
(14)
(15)
7
7
7
7
7
7
a
2
2
2 1=2
hgi* ¼ (1=3 gx þ1=3 gy ¼ 1=3 gz )
.

RUTHENIUM(III) COMPLEXES
339
complexes (5), (7), (9), (11), (14) and (15) showed single isotropicresonances
with ‘g’ values in the range 2.0772.14. Such isotropic lines are usually
observed either due to intermolecular spin exchange which can broaden the
lines or due to occupancy of the unpaired electron in a degenerate orbital.
The position of the lines and the nature of the EPR spectra for all of the
complexes suggest an almost perfect octahedral environment around
ruthenium ion in the complexes20722
.
Antibacterial Activity Studies
Antibacterial activity studies of the complexes and the ligands have
been carried out against Enterobacteria feacalis and Aeromonas hydrophilla
using nutrient agar (NA) medium by the agar-well diffusion method23 with
ꢀ
an incubation period of 24748 h at 35 C. The test solutions were prepared
in DMF. Among the complexes and ligands tested, the ligand Hanth-p-ans
and the appropriate complexes have been found to be more toxic against
both species of bacteria than the other ligands and complexes under similar
conditions. It has also been observed from these studies that the ruthenium
chelates have a higher activity than the respective free ligands against the
same microorganism under identical experimental conditions (Table V)
which is consistent with the earlier reports24. The mode of action of the
Table V. Antibacterial Activity Data of Ligands and Ruthenium(III) Complexes
Diameter of Inhibition Zone (mm)a
E. feacalis
1% 2%
A. hydrophilla
1%
2%
S. No
Compound
Colistin (standard bacteriocide)
Hanthcin
[RuCl2(anthcin)(AsPh3)2]
[RuBr2(anthcin)PPh3)2]
Hanth-p-tol
[RuCl2(anth-p-tol)PPh3)2]
[RuBr2(anth-p-tol)(AsPh3)2]
Hanth-p-anz
[RuCl2(anth-p-ans)(AsPh3)2]
[RuBr2(anth-p-ans)(PPh3)2]
7
þ3
11
þ1
8
10
þ1
0
þ7
(5)
(7)
þ5
þ8
þ9
þ6
þ8
þ8
þ9
þ5
þ7
þ8
þ10
þ12
þ15
þ14
þ18
þ24
(8)
(10)
þ7
þ9
þ10
þ14
þ12
þ15
þ21
þ10
þ8
þ12
þ11
þ13
þ17
(13)
(15)
þ11
þ13
a1% and 2% indicate 1 g and 27 g of the compound in 100 mL of test solution.

340
THANGADURAI, GOWRI, AND NATARAJAN
Complex No.
X
E
R
(4)
(5)
(6)
(7)
Cl
Cl
Br
Br
Cl
Cl
Br
Br
Cl
Cl
Br
Br
P
As
As
P
CH¼CHC6H5
CH¼CHC6H5
CH¼CHC6H5
CH¼CHC6H5
C6H4CH3
C6H4CH3
C6H4CH3
C6H4CH3
C6H4OCH3
C6H4OCH3
C6H4OCH3
C6H4OCH3
(8)
(9)
P
As
As
P
(10)
(11)
(12)
(13)
(14)
(15)
P
As
As
P
Figure 2. Suggested structure of the ruthenium(III) complexes.
compounds may involve the formation of hydrogen bonds through the
azomethine ( > C¼N) group with the active centres of cell constituents
resulting in the interference with normal cell processes25. The complexes
containing an electron donating group (-OCH3) showed more activity than
the other complexes26. On comparision with the standard antibiotic Colis-
tin, the ruthenium(III) complexes showed higher biological activity. Nor-
mally, the inhibitive action of the complexes increases with increase in the
concentration of the solution.
Based on the analytical, spectral (IR, UV-visible and EPR) and elec-
trochemical data, the following octahedral structure (Fig. 2) has been ten-
tatively proposed for all the ruthenium(III) complexes.

RUTHENIUM(III) COMPLEXES
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Received April 30, 2001
Accepted November 21, 2001
Referee I: K. Moedritzer
Referee II: D. A. Freedman