Synthesis and characterization of new ruthenium(III) complexes containing tetradentate schiff bases

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SYNTHESIS AND CHARACTERIZATION OF NEW
RUTHENIUM(III) COMPLEXES CONTAINING
TETRADENTATE SCHIFF BASES
T. Daniel Thangadurai 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: 20 Aug 2006.
To cite this article: T. Daniel Thangadurai & K. Natarajan (2001) SYNTHESIS AND CHARACTERIZATION OF NEW RUTHENIUM(III)
COMPLEXES CONTAINING TETRADENTATE SCHIFF BASES, Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry,
31:4, 549-567, DOI: 10.1081/SIM-100104786
To link to this article: http://dx.doi.org/10.1081/SIM-100104786
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SYNTH. REACT. INORG. MET.-ORG. CHEM., 31(4), 549–567 (2001)
SYNTHESIS AND CHARACTERIZATION
OF NEW RUTHENIUM(III) COMPLEXES
CONTAINING TETRADENTATE
SCHIFF BASES
T. Daniel Thangadurai and K. Natarajan*
Department of Chemistry, Bharathiar University,
Coimbatore - 641 046, India
ABSTRACT
Several new hexa-coordinated ruthenium(III) complexes of
the type [RuX(EPh3)(LL0)] (X ¼ Cl, Br; E ¼ P, As; LL0 ¼
anthacac, anthdibm, 2-amtpacac or 2-amtpdibm) have been
synthesised by reacting [RuCl3(PPh3)3], [RuCl3(AsPh3)3],
[RuBr3(AsPh3)3] or [RuBr3(PPh3)2(MeOH)] with tetradentate
Schiff bases such as bis(anthranilic acid)acetylacetimine
(H2-anthacac),
bis(anthranilic
acid)dibenzoylmethimine
(H2-anthdibm), bis(2-aminothiophenol)acetylacetimine (H2-
2-amptacac) or bis(2-aminothiophenol)dibenzoylmethimine
(H2-2-amtpdibm). All of the complexes have been char-
acterised by elemental analyses, IR, electronic spectra,
EPR, magnetic moment and cyclic voltammetric data. An
octahedral structure has been tentatively proposed for the
*Corresponding author. E-mail: knatraj@bharathi.ernet.in
549
Copyright # 2001 by Marcel Dekker, Inc.
www.dekker.com

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THANGADURAI AND NATARAJAN
complexes. These complexes were also tested for their anti-
bacterial properties.
INTRODUCTION
Several low-spin d5 complexes of second and third row transition
metal ions with tertiary phosphines and arsines as ligands have been
reported1À6. The interest in the synthesis and characterisation of transition
metal complexes containing Schiff bases lies in their catalytic and biological
activity in many reactions7À10. Furthermore, tetradentate Schiff base com-
plexes are important for designing metal complexes related to synthetic and
natural oxygen carriers11. An extensive amount of information is available
on the chemistry of transition metal complexes containing Schiff bases and it
is related mostly to first row transition metals12À14. Relatively little work has
been done on ruthenium complexes containing Schiff bases15,16. Though
investigations on the reactions of O2N2 and O2S2 chromophores containing
Schiff bases with ruthenium(II) and ruthenium(III) are available17À22, no
work has been done with Schiff bases containing N2S2 chromophores.
Besides, a study of compounds containing S and N atoms is interesting due
to their significant antifungal, antibacterial and anticancer activities23. With
the view to not only investigate the coordination behaviour of Schiff bases
containing N2S2 chromophores, but also to assess the biological activity of
the new complexes, we report in the present communication the synthesis
and characterisation of some stable ruthenium(III) Schiff base complexes of
the type [RuX(EPh3)(LL0)] (X ¼ Cl or Br; E ¼ P or As; LL0 ¼ anthacac,
anthdibm, 2-amtpacac or 2-amtpdibm) and their biological activities. The
ligands used in this study are of the type as shown in Fig. 1.
EXPERIMENTAL
All reagents used were chemically pure or analytical reagent grade.
Solvents were purified and dried according to standard procedures.
RuCl3.3H2O was purchased from Loba Chemie Pvt. Ltd., Bombay,
India and was used without further purification. [RuCl3(PPh3)3]2,
[RuCl3(AsPh3)3]24, [RuBr3(AsPh3)3]25 and [RuBr3(PPh3)2(MeOH)]4 were
prepared by reported literature methods.
The analyses of carbon, hydrogen and nitrogen were performed on
Carlo Erba 1106 and model 240 Perkin-Elmer CHN analyzers at the Central
Drug Research Institute, Lucknow, India. IR spectra were recorded in KBr
pelletes in the 4000À500 cmÀ1 region using a Shimadzu FTIR-8200 spec-
trophotometer. Electronic spectra were recorded in dichloromethane

NEW RUTHENIUM(III) COMPLEXES
551
Figure 1. Structure of the Schiff base ligands.
solution with a Hitachi Perkin-Elmer 20=200 spectrophotometer in the
range 800À200 nm. EPR spectra of powdered samples at room tempreature
were recorded with a model ER 200-D Bruker spectrometer at X-band
frequencies. Magnetic susceptibilities were recorded on an EG and G-PARC
vibrating sample magnetometer. Cyclic voltammetric studies were carried
out with a BAS CV-27 cyclic voltammeter in acetonitrile using a glassy-
carbon electrode as 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 Schiff Bases
The dibasic tetradentate Schiff base ligands have been prepared by the
condensation reaction of b-diketones [acetylacetone (2.6 mL, 0.025 mol) or

552
THANGADURAI AND NATARAJAN

NEW RUTHENIUM(III) COMPLEXES
553

554
THANGADURAI AND NATARAJAN
dibenzoylmethane (5.55 g, 0.025 mol)] with anthranilic acid (6.85 g,
0.050 mol) or 2-aminothiophenol (5.34 mL, 0.050 mol) in 1:2 molar ratio in
ethyl alcohol (25 mL). The solution was heated under refluxfor 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 Schiff Bases
The Schiff bases namely, H2-anthacac, H2-anthdibm, H2-2-amtpacac
and H2-2-amtpdipm, have been characterised on the basis of elemental
analyses and IR spectra. The elemental analyses (Table I) agree very well
with the molecular formulas.
The IR spectra of all the Schiff bases showed a strong band in the region
1620À1590 cmÀ1 characteristic of the azomethine n(C¼N)27. The Schiff bases
H2-anthacac and H2-anthdibm showed the n(OH) stretching frequency in the
region 3500À3300 cmÀ1 and a strong band around 1670 cmÀ1 due to
n(C¼O)26. The Schiff bases H2-2-amtpacac and H2-2-amtpdibm showed a
weak absorption band in the region 2600À2500 cmÀ1 due to n(SH).
Preparation of the Complexes
All the preparations were carried out under strictly anhydrous con-
ditions. The Schiff bases (0.012À0.026 g, 0.01 mmol) were added to a solu-
tion of [RuX3(EPh3)3] (0.10À0.13 g, 0.01 mmol) (where E ¼ P, X ¼ Cl;
E ¼ As, X ¼ Cl, Br) or [RuBr3(PPh3)2(MeOH)] (0.09 g, 0.01 mmol) in 1:1
molar ratio in benzene (20 mL) and the mixtures were refluxed for 5 h. The
dark coloured solutions obtained were concentrated to about 3 mL. The
complexes were precipitated by the addition of a small quantity of petro-
leum ether (60À80 ꢀC). The complexes were then filtered, washed with
petroleum ether and recrystallised from CH2Cl2=petroleum ether (1:4 v=v)
and dried under vacuum. All of the complexes were prepared using the
above procedure.
Experimental Procedure for Antibacterial Activity Studies
The nutrient agar (NA) medium was prepared by adding 3 g of beef
extract to 5 g of peptone. Then 5 g of sodium hydroxide was added to the

NEW RUTHENIUM(III) COMPLEXES
555
solution and stirred well to get a clear solution. The resulting solution was
made up to 1000 mL by adding distilled water and 15 g of agar was added to
it. The required pH was adjusted to 7.0 by adding sufficient acid or base.
A quantity of 10 mL of the medium was poured into the sterilised Petri plates
and allowed to solidify. The plates were inoculated with spore suspensions of
Eenterobacteria feacalis and Aeromonas hydrophilla. By using a sterilised
cork borer (9 cm diameter), wells were dug in the centre of the culture plates.
The test complexsolutions in DMF were added (0.5 mL) to these wells, and
the plates were incubated at 35 ꢀC for 24 h. Then, the inhibition zone
appearing around the wells in each plate was measured and recorded as the
cytotoxic effect of the appropriate complexes. A solvent only treated plate
was maintained separately in order to avoid the activity due to solvent if any.
RESULTS AND DISCUSSION
The tetradentate Schiff bases react with the ruthenium(III) complexes
of the type of [RuX3(EPh3)3] (X ¼ Cl, E ¼ P; X ¼ Cl or Br, E ¼ As) or
[RuBr3(PPh3)2(MeOH)] to yield complexes of the type [RuX(EPh3)(LL0)]
(X ¼ Cl or Br; E ¼ P or As, LL0 ¼ dianion of the tetradentate Schiff bases).
The analytical data (Table I ) confirm the molecular formula arrived at for
the new complexes. It has been found that the Schiff bases replace two of the
phosphines=arsines and two halides from the starting complexes indicating
binegative tetradentate behaviour of the Schiff bases.
IR Spectra
The IR spectra of the free Schiff base ligands have been compared with
those of the complexes formed with the corresponding Schiff bases in order
to confirm the coordination of the ligands to the metal atom.
The n(OH) absorption observed around 3500À3300 cmÀ1 in the free
Schiff bases H2-anthacac and H2-anthdibm disappeared in the corres-
ponding complexes indicating deprotonation prior to the coordination
through the oxygen atom. The n(C¼O) frequency of the carboxyl group was
seen as a band around 1670 cmÀ1 in the free Schiff bases. In the complexes,
the absorptions due to the carbonyl group were observed at around
1630À1610 cmÀ1 and around 1430À1390 cmÀ1 arising from asymmetric and
symmetric stretchings, respectively. The difference between nas and nsym
vibrations of the carbonyl group has been found to be around 220 cmÀ1.
This is a clear indication of the monodentate coordination of the carboxyl
group with the free carbonyl group26.

556
THANGADURAI AND NATARAJAN
In the IR spectra of the other two Schiff bases, H2-2-amtpacac and H2-
2-amtpdibm, the n(SH) stretching frequency has been observed at around
2600À2500 cmÀ1 which disappeared in the complexes due to the coordina-
tion through the sulphur atom. In the IR spectra of the complexes, the
n(C¼N) stretching frequency was observed in the region 1595À1575 cmÀ1
indicating coordination through the azomethine nitrogen27. In addition, the
other characteristic bands due to triphenylphosphine and triphenylarsine
were also present in the expected region.
Electronic Spectra
The electronic spectra of all the complexes in CH2Cl2 showed one to
four bands in the region 660À300 nm (Table II). The ground state of Ru(III)
2
(t52g configuration) is T2g and the first excited doublet levels in the order of
increasing energy are 2A2g and 2T1g which arise from the t42g e1g configuration.
In most of the Ru(III) complexes the electronic spectra show only charge
transfer bands25. It has been observed that charge transfer bands of the type
Lpy ! t2g are prominent in the low-energy region which obscure the weaker
d-d transitions. Hence, it is difficult to assign conclusively the bands of
ruthenium(III) complexes in the visible region. However, the bands around
2
2
660À610 nm have been assigned to the T2g ! A2g transition based on the
low extinction coefficient value (e) as compared to charge transfer bands.
This is in conformity with assignments made for other similar octahedral
Ru(III) complexes28,29. The bands in the region 480À315 nm have been
assigned to charge transfer bands (Fig. 2).
Cyclic Voltammetry
Cyclic voltammetric studies were performed for some of the com-
plexes in acetonitrile solution at a glassy carbon working electrode. The
purpose of the electrochemical experiments was to investigate the suit-
ability of the present ligand system in achieving higher oxidation states of
the metal. The redoxpotentials of the compleexs are characterised by
well-defined waves in the range 0.60 to 0.80 V (oxidation) and À0.31 to
À0.74 V (reduction) versus a saturated calomel electrode. The cyclic vol-
tammogram data are given in Table III and a representative case is dis-
played in Fig. 3.
As the ligands used in this work are not reversibly reduced or
oxidised within the potential limits of þ1.20 to À1.20 V, we believe that

NEW RUTHENIUM(III) COMPLEXES
557

558
THANGADURAI AND NATARAJAN
Figure 2. Electronic spectrum of [RuCl(anthdibm)(PPh3)] in dichloromethane.
the redoxprocesses observed for these complexes are metal-centered only.
Although the ruthenium(III) complexes (12) and (16) showed only oxi-
dation potentials, the ruthenium(III) complexes (8) and (10) showed only
reduction potentials. The reason for the irreversibility observed for these
complexes may be due to short-lived oxidation states of the metal ion30 or
due to oxidative degradation31 of the ligands. The complex( 7) showed
lesser positive (0.69 V) and lesser negative potentials (À0.74 V) when
compared to complex( 18) (0.80 V and À0.31 V). For the complex( 18) the
reduction potential value indicates that the reduction takes place more
easily than for the corresponding reduction in the complex( 7). This is due
to the increased donor ability of the sulphur atom in complex( 18)
compared to the oxygen donor atom in complex (7). A similar behaviour
is shown in the oxidation potentials found for these two complexes.
Nearly the same pattern is observed in the oxidation reduction potentials
of complex( 7) when it is compared to complex( 19). The higher donor
ability of the arsine ligand compared to the phosphine ligand has been
demonstrated by a reduction potential of À0.74 V for the arsine complex
(7) compared to a reduction potential of À0.32 V for the phosphine
complex( 8). However, other complexes showed both oxidation and
reduction with peak-to-peak separation (DEp) values ranging from 60 mV
to 100 mV suggesting a single step, one-electron transfer process24,32. The
oxidation response for the complexes (14), (19) and (20) are irreversible at
50 mV sÀ1.

NEW RUTHENIUM(III) COMPLEXES
559

560
THANGADURAI AND NATARAJAN
Figure 3. Cyclic voltammogram of [RuCl(2-amtpdibm)(AsPh3)] in acetonitrile at a
glassy carbon electrode.
EPR Spectra
EPR spectra of powdered samples were recorded at room temperature.
The complexes showed no indication for any hyperfine interaction of nuclei
with magnetic moments viz., Ru, As, P, Cl and Br. All of the complexes
exhibit a single isotropic resonances with ‘g’ values in the range 2.10À2.40
(Fig. 4). Such isotropic lines are usually observed either due to the inter-
molecular spin exchange which can broaden the lines or due to occupancy of
the unpaired electron in a degenerate orbital. However, similar chloro
complexes28 showed a single line due to the presence of one unpaired elec-
tron in a low-spin state with a degenerate dxz, dyz ground state indicating
Jahn-Teller instability28 resulting in an averaged single ‘g’ values. The
position of the lines and the nature of the EPR spectra of all the complexes

NEW RUTHENIUM(III) COMPLEXES
561
Figure 4. EPR spectrum of [RuCl(anthdibm)(PPh3)].
suggest an almost perfect octahedral environment around the ruthenium ion
in the complexes28,33
.
Magnetic Moment Studies
The magnetic moments 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 range from 1.74
to 1.81 BM indicating the presence of one unpaired electron, suggesting a
low-spin t52g configuration for the ruthenium(III) ion in an octahedral
environment in all these complexes.

562
THANGADURAI AND NATARAJAN
Antibacterial Activity Studies
The in vitro antibacterial screening of the ligands H2-anthacac,
H2-anthdibm and H2-2-amtpdibm and their ruthenium complexes have been
carried out against Eenterobacteria feacalis and Aeromonas hydrophilla using
a nutrient agar (NA) medium by the agar-well diffusion method34 (incu-
bation period 18À24 h at 35 ꢀC). The test solutions were prepared in DMF.
Among the complexes and ligands tested, the ligand H2-2-amtpdibm and the
complexes [RuCl(2-amtpdibm)(AsPh3)] and [RuBr(2-amtpdibm)(AsPh3)]
have been found to be more toxic against both species of bacteria than the
other ligands and complexes under similar conditions. The increased activity
of these compounds may possibly be due to the presence of a sulphur atom.
It has also been observed from the antibacterial screening studies that the
ruthenium chelates have higher activity than the corresponding free ligands
against the same microorganism under identical experimental conditions
(Table IV), which is consistent with earlier reports35. The possible mode of
increased toxicity of the ruthenium complexes compared to that of the free
ligands may be explained in terms of Tweedy’s chelation theory36À38
,
according to which chelation reduces the polarity of the central metal atom
because of partial sharing of its positive charge with the ligand which
Table IV. Antibacterial Activity Data of Ligand and Ru(III) Schiff Base Com-
plexes
Diameter of Minimum Inhibition
Zone (mm)a
Eenterobac-
teria feacalis
Aeromonas
hydrophilla
1%
2%
1%
2%
S. No.
Compound
(H2-anthacac)
[RuCl(anthacac)(PPh3)]
[RuCl(anthacac)(AsPh3)]
(H2-anthdibm)
[RuBr(anthdibm)(AsPh3)]
[RuBr(anthdibm)(PPh3)]
(H2-2-amtpdibm)
[RuCl(2-amtpdibm)(AsPh3)]
[RuBr(2-amtpdibm)(AsPh3)]
9
16
17
12
17
20
19
26
30
12
20
22
14
18
22
21
29
32
14
20
21
15
19
21
20
27
31
16
24
26
18
23
26
22
29
34
(5)
(6)
(11)
(12)
(18)
(19)
a
1% and 2% indicate 1 g and 2 g of the compound in 100 mL of test solution.

NEW RUTHENIUM(III) COMPLEXES
563
Figure 5. Structure of the ruthenium(III) complexes.
favours the permeation of the complexes through the lipid layer of cell
membranes. Furthermore, the mode of action of the compounds may
involve the formation of hydrogen bonds through the azomethine ( > C¼N)
group of the complexes with the active centres of cell constituents resulting
in the interference with normal cell processes39.
Suggested Structure
Based on the elemental analyses, IR, electronic and EPR spectral and
electrochemical data, the octahedral structure shown in Figure 5 has been

564
THANGADURAI AND NATARAJAN
tentatively proposed for all of the ruthenium(III) Schiff base complexes of
the present study.
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Received: February 25, 2000
Accepted: February 13, 2001
Referee I: D. P. Rillema
Referee II: W. B. Connich