Synthesis, spectral, and antifungal activity of Ru(II) mixed-ligand complexes

Article abstract of DOI:10.1081/SIM-120021656

Several new hexa-coordinated ruthenium(II) complexes of the type [Ru(CS)(Cl)(Py)(PPh₃)(L)], [Ru(CS)(Cl)(Bipy)(L)] and [Ru(Cl)(Phen)-(PPh₃)(L)], (where Py = pyridine, Bipy = 2,2′-bipyridyl, Phen = 1,10-phenanthroline, L = monobasic bidentate Schiff base viz salicylaldehy-demethylamine (HSalmet), salicylaldehydecyclohexylamine (HSalchx) and salicylaldehyde-2-aminopyridine (HSalampy)) have been synthesized. All of the new complexes have been characterised on the basis of analytical and spectral data. These new complexes showed a reasonable amount of antifungal activity.

Full text of DOI:10.1081/SIM-120021656

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Synthesis, Spectral and Antifungal Activity of Ru(II)
Mixed‐Ligand Complexes
R. Ramesh a & M. Sivagamasundari a
a Department of Chemistry, Bharathidasan University, Tiruchirappalli, 620 024, India
Version of record first published: 20 Aug 2006
To cite this article: R. Ramesh & M. Sivagamasundari (2003): Synthesis, Spectral and Antifungal Activity of Ru(II) Mixed‐Ligand
Complexes, Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry, 33:5, 899-910
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 5, pp. 899–910, 2003
Synthesis, Spectral, and Antifungal Activity of
Ru(II) Mixed-Ligand Complexes
*
R. Ramesh and M. Sivagamasundari
Department of Chemistry, Bharathidasan University,
Tiruchirappalli, India
ABSTRACT
Several new hexa-coordinated ruthenium(II) complexes of the type
[Ru(CS)(Cl)(Py)(PPh3)(L)], [Ru(CS)(Cl)(Bipy)(L)] and [Ru(Cl)(Phen)-
(PPh3)(L)], (where Py = pyridine, Bipy = 2,2’-bipyridyl, Phen = 1,10-
phenanthroline, L = monobasic bidentate Schiff base viz salicylaldehy-
demethylamine (HSalmet), salicylaldehydecyclohexylamine (HSalchx)
and salicylaldehyde-2-aminopyridine (HSalampy)) have been synthe-
sized. All of the new complexes have been characterised on the basis of
analytical and spectral data. These new complexes showed a reasonable
amount of antifungal activity.
Key Words: Schiff base ligands; Ruthenium(II) mixed-ligand
complexes; Spectra; Antifungal activity.
*
Correspondence: R. Ramesh, Department of Chemistry, Bharathidasan Univer-
sity, Tiruchirappalli 620 024, India; Fax: 0091-431-2407045,2407020; E-mail:
ramesh_bdu@yahoo.com.
899
DOI: 10.1081/SIM-120021656
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

900
Ramesh and Sivagamasundari
INTRODUCTION
There is considerable interest in the chemistry of transition metal
complexes of ligands containing oxygen, nitrogen and sulfur donor atoms
due to the carcinostatic, antitumour, antiviral, antifungal and antibacterial
activity and their industrial uses of complexes derived from them.[1–4] In
addition, the presence of nitrogen and oxygen donor atoms in the com-
plexes makes these compound effective and stereospecific catalysts for oxi-
dation,[5] reduction,[6] hydrolysis[7] and they also show biological activity[8]
and other transformations of organic and inorganic chemistry. It is well
known that some drugs have higher activity when administered as metal
complexes than as free ligands.[9–11] Although a large number of reports
are available on the chemistry of transition metal complexes with various
donor sites, only very few reports have been available on Ru(II) complexes
containing thiocarbonyl and nitrogen heterocycles.[12,13] In the present work
we report the synthesis, characterization and antifungal activity of such
ruthenium(II) mixed ligand complexes. The Schiff base ligands used in this
study are as shown in Figure 1.
EXPERIMENTAL
All reagents used were Analar grade. All the solvents were freshly dis-
tilled before use. C, H and N analyses were performed at the Regional
Sophisticated Instrumentation Centre, Central Drug Research Institute, Luck-
now. IR spectra were recorded in KBr pellets on a Perkin-Elmer 597 IR
grating spectrophotometer in the range 4000–200 cm–1. Electronic spectra
were recorded in CHCl3 with a Hitachi model UV 3410 UV-Visible
1
spectrophotometer in the range 800–200 nm. The H NMR spectra were
Figure 1. Structure of Schiff base ligands.

Ru(II) Mixed-Ligand Complexes
901
recorded in CDCl3 with a Bruker 300 MHz instrument using TMS as
reference. Melting points were recorded with a Boetius micro heating table
and are uncorrected.
The antifungal activity studies of the ligands and their complexes have
been carried out against the fungus Aspergillus flavus in Sabour dextrose
agar medium by the diffusion method with an incubation period of 72 h at
30 °C. The test solutions are prepared in DMSO. The percentage inhibition
of fungal growth is calculated by the Vincent equation.[14]
% inhibiton ¼ 100ðC À TÞ=C
where C =diameter of fungal growth on the control plate T =diameter of
fungal growth of the test plate.
The starting complexes, [Ru(Cl)2(Py)(CS)(PPh3)2], [Ru(CS)(Cl)2(Bi-
py)(PPh3)], [Ru(Cl)2(Phen)(PPh3)2][15] and the Schiff base ligands[16] were
prepared by literature methods.
Preparation of Complexes
[Ru(Cl)(CS)(Py)(PPh3)(L)]
To a solution of [Ru(Cl)2(CS)(Py)(PPh3)2] (0.082 g, 0.100 mmol) in dry
benzene (20 mL) the appropriate Schiff base ligand (0.014–0.020 g, 0.100
mmol) (1:1 mole ratio) was added. The solution was heated under reflux for
6 h. After concentrating the solution to about 2 mL, the product was separated
by the addition of a small quantity of petroleum ether (60–80 °C). The
complex was then filtered, washed with petroleum ether and recrystallised
from a CH2Cl2/petroleum ether mixture in 1:3 volume ratio, respectively.
[Ru(Cl)(CS)(Bipy)(L)]
To a solution of [Ru(Cl)2(CS)(Bipy)(PPh3)] (0.063 g, 0.100 mmol) in dry
benzene (20 mL) the appropriate Schiff base ligand (0.014–0.020 g, 0.100
mmol) (1:1 mole ratio) was added. The solution was heated under reflux for
6 h. After concentrating the solution to about 2 mL, the product was separated
by the addition of a small quantity of petroleum ether (60–80 °C). The
complex was then filtered, washed with petroleum ether and recrystallised
from a CH2Cl2/petroleum ether mixture in 1:3 volume ratio, respectively.
[Ru(Cl)2 (Phen)(PPh3)2 (L)]
To a solution of [Ru(Cl)2(Phen)(PPh3)2] (0.088 g, 0.100 mmol) in dry
benzene (20 mL) the appropriate Schiff base ligand (0.014–0.020 g, 0.100

902
Ramesh and Sivagamasundari

Ru(II) Mixed-Ligand Complexes
903
mmol) (1:1 mole ratio) was added. The solution was heated under reflux
for 6 h. After concentrating the solution to about 2 mL, the product was
separated by the addition of a small quantity of petroleum ether (60–
80 °C). The complex was then filtered, washed with petroleum ether
and recrystallised from CH2Cl2/petroleum ether mixture in 1:3 volume
ratio, respectively.
RESULTS AND DISCUSSION
All the new complexes of the type [Ru(CS)(Cl)(Py)(PPh3)(L)],
[Ru(CS)(Cl)(Bipy)(L)] and [Ru(Cl)(PPh3)(Phen)(L)] have been obtained
from the reaction of [Ru(CS)(Cl)2(Py)(PPh3)2], [Ru(CS)(Cl)2(Bipy)(PPh3)],
[Ru(Cl)2(Phen)(PPh3)2] with various bidentate Schiff bases (L) viz., HSalmet,
Hsalchx and Hsalampy in dry benzene in the molar ratio 1:1, respectively.
All of the new Schiff base ruthenium(II) complexes are highly co-
loured, stable to air and light and soluble in chloroform, methylene di-
chloride, benzene, DMF and DMSO. The analytical data (Table 1) are in
good agreement with the general molecular formula proposed Figure 2.
IR Spectra
In order to study the binding mode of the Schiff bases to ruthenium in
the new complexes, IR spectra of the free Schiff bases were compared with
the spectra of known ruthenium(II) complexes. A strong band is observed
in the free ligand at around 1604–1629 cm–1, characteristic of the azo-
methine (>C=N) group.[17] Coordination of the Schiff bases to the ruthe-
nium ion through the azomethine nitrogen atom is expected to reduce the
electron density in the azomethine link and, thus, lower the n(C=N)
absorption frequency. Hence, this band undergoes a shift to lower frequency
to 1596–1606 cm–1 after complexation indicating coordination ofthe azo-
methine nitrogen[18,19] to ruthenium (Table 2).
A medium intensity band around 3313–3417 cm–1 in the spectra of the
free ligands due to n(OH) was absent in the spectra of all the complexes,
indicating the deprotonation of Schiff bases prior to coordination[17] through
its oxygen atom. This is further supported by an increase in the absorption
frequency of the phenolic C–O band from 1249–1280 cm–1 in the spectra
of the free ligands to 1282–1361 cm–1 in the spectra of the complexes,
indicating that the other coordination site of the Schiff base is the phenolic
oxygen.[18–20] A strong band observed around 1260–1280 cm–1 due to
n(C=S) in the starting complexes containing a thiocarbonyl group is merged
with the n(C–O) absorptions in the complexes.

904
Ramesh and Sivagamasundari
Figure 2. Formation of the new ruthenium(II) complexes.

Ru(II) Mixed-Ligand Complexes
905

906
Ramesh and Sivagamasundari
In the spectra of the complexes containing coordinated pyridine, a
weak band is observed[21,22] around 1027 cm–1. The n(Ru-Cl) absorption
lies around 320–335 cm–1 in the spectra of the complexes. In addition to
the above absorption, other characteristic bands due to coordinated tri-
phenylphosphine were also present around 1432–1436 cm–1 in the spectra
of ruthenium(II) Schiff base complexes.
Electronic Spectra
Electronic spectra of all the complexes in chloroform showed two to
three bands in the region 266–609 nm. The ground state of ruthenium(II) in
1
an octahedral environment is A1g arising from the t2g configuration. The
6
5
1
3
3
excited states corresponding to the t2g eg configuration are T1g, T2g,1T1g,
1
and T2g. Hence, four bands corresponding to the transition A1g
1
3
!
T1g,
3
1
1
1
1
1A1g ! T2g, A1g ! T1g. and A1g ! T2g are possible in order of increas-
1
1
ing energy. The bands around 585–609 nm are assigned to A1g ! T1g and
1
1
the bands around 443–479 nm to the transition A1g ! T2g which con-
firms the assignment made for similar octahedral complexes.[23–25] The
other high-intensity band observed around 273–325 nm has been assigned
to charge transfer transition arising from the excitation of electrons from the
metal t2g level to the unfilled molecular orbitals derived from the p level of
the ligands.
1H NMR Spectra
Further evidence for the coordinating mode of the ligands is obtained
1
from H NMR spectra of the complexes (Figure 3). In the spectra of the
complexes [Ru(CS)(Cl)(Bipy)(salmet)] and [Ru(CS)(Cl)(Bipy)(salchx)] a
sharp singlet appeared at d 9.1 and has been assigned to the azomethine
proton (>C=N). The positions of azomethine signal in the complexes are
downfield in comparison with that of the free ligands, suggesting de-
shielding of the azomethine proton due to its coordination to ruthenium
through the azomethine nitrogen.[25] The aromatic protons of the Schiff
base and 2,2’-bipyridyl appeared as a multiplet in the region d 7.16–8.57.
The methyl protons showed a singlet in the region d 1.65. The cyclohexyl
CH2 protons showed a singlet at d 1.56. The absence of a resonance in the
region d 12.50–12.92 in the complexes indicates deprotonation of the
phenolic group of the Schiff bases on complexation and coordination to
ruthenium through the phenolic oxygen[17] (Table 3).
It is difficult to assign conclusively the relative position of CS and Cl
in the bipyridyl complex in the absence of a crystal structure. However, the
starting bipyridyl compound, [Ru(CS)(Cl)2(Bipy)(PPh3)] was prepared from
the dimer [Ru(CS)(Cl2)(PPh3)2]2 where the CS and Cl are oriented cis to

Ru(II) Mixed-Ligand Complexes
907
Figure 3. 1H NMR spectrum of [Ru(CS)(CI)(Bipy)(salmet)] (4).
each other[15] Therefore, it is reasonable to assume the same geometrical
disposition of CS and Cl in the resulting Schiff base complexes.
1
On the basis of the elemental analyses, IR, electronic and H NMR
spectral data an octahedral structure has been tentatively proposed for all
the complexes.
Antifungal Activity
These Schiff base ligands and their ruthenium chelates were screened
in vitro in order to evaluate their antifungal activity against Aspergillus
flavus at four different concentrations. The results showed that the ruthe-
nium chelates are more toxic compared to their parent ligands against the
same microorganisms under identical experimental conditions. The toxicity
of ruthenium chelates increases on increasing the concentration. The
Table 3. 1H NMr data of Ru(II) Schiff base complexes (d, ppm).
Complexes
–C6H5
–HC=N–
–CH3
–CH2
(1)
(2)
(3)
[Ru(CS)(Cl)(Bipy)(salmet)]
[Ru(CS)(Cl)(Bipy)(salchx)]
[Ru(CS)(Cl)(Bipy)(salampy)]
7.16–8.57
7.20–7.49
7.18–8.6
9.14
8.63
8.91
1.65
–
–
1.56
–
–

908
Ramesh and Sivagamasundari
Table 4. Antifungal activity data of Ru(II) complexes and ligands.
% Inhibition
50
mgL–1
100
mgL–1
200
mgL–1
400
mgL–1
S. No.
Compound
HSalmet
(1)
(2)
12
25
21
38
32
50
44
58
[Ru(CS)(Cl)(PPh3)-
(Py)(salmet)]
Hsalchx
(3)
(4)
17
29
23
40
33
53
42
63
[Ru(CS)(Cl)-
(Bipy)(salchx)]
HSalampy
(5)
(6)
11
30
23
46
37
63
48
73
[Ru(Cl)(PPh3)-
(Phen)(salampy)
Bavistin
(7)
–
92
100
–
increase in the angifungal activity of metal chelates may be due to the effect
of the metal ion on the normal cell process. A possible mode of the toxi-
city increase may be considered in light of Tweedy’s chelation theory.[26]
Chelation considerably reduces the polarity of the metal ion because of
partial sharing of its positive charge with the donor groups and possible p-
electron delocalisation over the whole chelate ring. Such chelation could
enhance the lipophilic character of the central metal atom, which sub-
sequently favours its permeation through the lipid layers of cell membrane.
Furthermore, the mode of action of the compounds may involve formation
of a hydrogen bond through the azomethine ( >C=N) group with the active
centers of cell constituents, resulting in interference with the normal cell
processes.[27] Although there is a significant increase in the fungicidal ac-
tivity of the ruthenium complexes as compared to the free ligands, this does
not reach the effectiveness of the conventional fungicide Bavistin. But these
compounds show higher activity than other ruthenium(II) complexes con-
taining similar bidentate Schiff base ligands against Aspergillus flavus[28]
Table 4.
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Received June 21, 2002
Accepted February 18, 2003
Referee II: J. Boucher
Referee III: E. H. Wong