Ruthenium(II) carbonyl complexes of dehydroacetic acid thiosemicarbazone: Synthesis, structure, light emission and biological activity

Article abstract of DOI:10.1016/j.jorganchem.2008.03.023

The reaction of [RuHCl(CO)(B)(EPh₃)₂] (where E = As, B = AsPh₃; E = P, B = PPh₃, py, pip, or mor) and dehydroacetic acid thiosemicarbazone (abbreviated as H₂dhatsc where H₂ stands for the two dissociable protons) in benzene under reflux afford a series of new ruthenium(II) carbonyl complexes containing dehydroacetic acid thiosemicarbazone of general formula [Ru(dhatsc)(CO)(B)(EPh₃)] (where E = As, B = AsPh₃; E = P, B = PPh₃, py, pip or mor; dhatsc = dibasic tridentate dehydroacetic acid thiosemicarbazone). All the complexes have been characterized by elemental analyses, FT-IR, UV-Vis, and ¹H NMR spectral methods. The thiosemicarbazone of dehydroacetic acid behaves as dianionic tridentate O, N, S donor and coordinates to ruthenium via phenolic oxygen of dehydroacetic acid, the imine nitrogen of thiosemicarbazone and thiol sulfur. In chloroform solution, all the complexes exhibit metal-to-ligand charge transfer transitions (MLCT). The crystal structure of one of the complexes [Ru(dhatsc)(CO)(PPh₃)₂] (1) has been determined by single crystal X-ray diffraction which reveals the presence of a distorted octahedral geometry in the complexes. All the complexes exhibit an irreversible oxidation (Ru^III/Ru^II) in the range 0.76-0.89 V and an irreversible reduction (Ru^II/Ru^I) in the range -0.87 to -0.97 V. Further, the free ligand and its ruthenium complexes have been screened for their antibacterial and antifungal activities. The complexes show better activity in inhibiting the growth of bacteria Staphylococcus aureus and Escherichia coli and fungus Candida albicans and Aspergillus niger. These results made it desirable to delineate a comparison between free ligand and its ruthenium complexes.

Full text of DOI:10.1016/j.jorganchem.2008.03.023

Journal of Organometallic Chemistry 693 (2008) 2251–2257
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Ruthenium(II) carbonyl complexes of dehydroacetic acid thiosemicarbazone:
Synthesis, structure, light emission and biological activity
b
Sethuraman Kannan a, M. Sivagamasundari a, R. Ramesh a, , Yu Liu
*
a School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
b Institute of Inorganic Chemistry, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of [RuHCl(CO)(B)(EPh3)2] (where E = As, B = AsPh3; E = P, B = PPh3, py, pip, or mor) and
dehydroacetic acid thiosemicarbazone (abbreviated as H2dhatsc where H2 stands for the two dissociable
protons) in benzene under reflux afford a series of new ruthenium(II) carbonyl complexes containing
dehydroacetic acid thiosemicarbazone of general formula [Ru(dhatsc)(CO)(B)(EPh3)] (where E = As,
B = AsPh3; E = P, B = PPh3, py, pip or mor; dhatsc = dibasic tridentate dehydroacetic acid thiosemicarba-
zone). All the complexes have been characterized by elemental analyses, FT-IR, UV–Vis, and 1H NMR
spectral methods. The thiosemicarbazone of dehydroacetic acid behaves as dianionic tridentate O, N, S
donor and coordinates to ruthenium via phenolic oxygen of dehydroacetic acid, the imine nitrogen of thi-
osemicarbazone and thiol sulfur. In chloroform solution, all the complexes exhibit metal-to-ligand charge
transfer transitions (MLCT). The crystal structure of one of the complexes [Ru(dhatsc)(CO)(PPh3)2] (1) has
been determined by single crystal X-ray diffraction which reveals the presence of a distorted octahedral
geometry in the complexes. All the complexes exhibit an irreversible oxidation (RuIII/RuII) in the range
0.76–0.89 V and an irreversible reduction (RuII/RuI) in the range ꢀ0.87 to ꢀ0.97 V. Further, the free ligand
and its ruthenium complexes have been screened for their antibacterial and antifungal activities. The
complexes show better activity in inhibiting the growth of bacteria Staphylococcus aureus and Escherichia
coli and fungus Candida albicans and Aspergillus niger. These results made it desirable to delineate a com-
parison between free ligand and its ruthenium complexes.
Received 31 January 2008
Received in revised form 15 March 2008
Accepted 21 March 2008
Available online 28 March 2008
Keywords:
Ru(II) carbonyl
Dehydroacetic acid thiosemicarbazone
Crystal structure
Redox property
Antimicrobial activity
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
D, N, S coordination usually takes place. Such donor systems are
able to generate novel stereochemical, electrochemical and elec-
Derivatives of semicarbazones and thiosemicarbazones are
amongst the most widely studied nitrogen and oxygen/sulphur do-
nor ligands [1–4]. Particularly, thiosemicarbazones have emerged
as an important class of sulphur donor ligands for transition metal
ions because of their mixed hard–soft donor character and versa-
tile coordination behaviour. In particular, transition metal com-
plexes of thiosemicarbazones have been receiving considerable
interest largely because of their pharmacological property.
Complexation of the thiosemicarbazone usually occurs via dis-
sociation of the acidic proton, resulting in the formation of a
five-membered chelate ring. Such studies received a new impetus
with the discovery of significant antibacterial, antiviral, antimalar-
ial, antileprotic, and even anticancer activities of such ligands and
some of their metal complexes, both in vitro and in vivo [5–11].
When an additional donor site D is incorporated in such ligands,
linked to the carbonylic carbon via one or two intervening atoms,
tronic properties [12,13]. Moreover, the –N@CH– (imine group)
imparts in elucidating the mechanism of transaminations and
rasemisation reactions [14,15].
Carbon monoxide is an important building block for the synthe-
sis for many compounds [16]. The interaction of small molecules
such as ‘‘CO” and ‘‘O2” with transition metal complexes particularly
those containing a ruthenium metal centre coordinated to nitrogen
and oxygen donor ligands have attracted a great deal in recent
years. Kenny et al. have recently reported the in vitro anticancer
activity of some N-ortho-ferrocenyl benzoyl dipeptide esters [17].
Organometallic technetium and rhenium complexes of a 50-carbox-
amide 5-ethyl-20-deoxyuridine derivative are able to selectively in-
hibit Herpes simplex virus thymidine kinase type 1 (HSV1-TK)
[18]. The use of organometallic compounds in the treatment of
cancer has also been an active field of research [19].
Chelation causes drastic changes in the biological properties of
a ligand and also the metal moiety. Several metal chelates have
also been shown to inhibit tumour growth [20] and their interac-
tions with DNA have been reported [21]. The chemistry of ruthe-
nium is currently receiving a lot of attention, primarily because
* 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 Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.03.023

2252
S. Kannan et al. / Journal of Organometallic Chemistry 693 (2008) 2251–2257
of the fascinating electron transfer and energy transfer properties
displayed by the complexes of this metal [22]. Ruthenium com-
plexes have appeared to be promising candidates after cis-platin
in several areas of chemistry from synthetic molecular electronic
devices to their interactions with biomolecules [23–25]. A direct
correlation between cytotoxicity and DNA binding property of
ruthenium complex has been reported by Clarke et al. [26].
Over the past few years, our group has been working on the
coordination chemistry of ruthenium with different ligand systems
and on their characterization [27–30]. In the present work, we re-
port the results of our study on ruthenium(II) carbonyl complexes
of dehydroacetic acid thiosemicarbazone incorporating PPh3/AsPh3
and nitrogen heterocycles. The complexes have been characterized
by physiochemical methods. The structure of one of the complexes
has been probed with the help of single crystal X-ray diffraction
analysis. Further, the electrochemical behaviour has been exam-
ined by cyclic voltammetry along with the antimicorbial activity
of these complexes in terms of their growth-inhibition capacity
against Gram +ve and Gram ꢀve bacteria Staphylococcus aureus
(209 P) and E. coli (2231), respectively and fungus Candida albicans
and Aspergillus niger.
2.4. Synthesis of ruthenium(II) carbonyl complexes of DHATSC
To a solution of 0.1 mmol of [RuHCl(CO)(B)(EPh3)2] (where
E = As, B = AsPh3; E = P, B = PPh3, py, pip (or) mor) in benzene
(20 ml) was added 0.1 mmol ligand H2dhatsc (mole ratio of ruthe-
nium complex and ligand is 1:1) and the mixture was refluxed for
10 h. The solution was concentrated to about 3 ml and petroleum
ether was added where by the ruthenium(II) thiosemicarbazones
were separated. The resulting complexes were recrystallized from
CH2Cl2/petroleum ether and dried under vacuo. The purity of the
complexes was checked by TLC. Yield: ꢂ68%.
2.5. X-ray crystallography
Single crystals of [Ru(dhatsc)(CO)(PPh3)2] (1) are grown by slow
evaporation of chloroform solution at room temperature. 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 diffrac-
tometer using monochromated Mo Ka radiation (kI = 0.71073).
Data were collected at 183 K. Corrections were made for Lorentz
and polarization effects as well as for absorption (numerical). The
structure was solved with direct method using SIR-97 [36,37] and
was refined by full matrix least-squares method [38] on F2 with
SHELXL-97. Non-hydrogen atoms were refined with anisotropy ther-
mal parameters. All hydrogen atoms were geometrically fixed and
allowed to refine using a riding model.
2. Experimental
2.1. Reagents and materials
Commercially available RuCl3 ꢁ 3H2O was used as supplied from
Loba Chemie. All the reagents used were chemically pure and ana-
lytical grade. The solvents were freshly distilled using the standard
procedures [31]. Dehydroacetic acid (DHA) and thiosemicarbazide
were purchased from SRL. The supporting electrolyte tetrabutyl
ammonium perchlorate (TBAP) was dried in vacuum prior to use.
2.6. Microbial assay
The in vitro antimicrobial screenings of the free ligand and its
ruthenium(II) complexes containing dhatsc were tested for their
effect on certain human pathogenic bacteria and fungus by disc dif-
fusion method. The ligand and their ruthenium(II) complexes were
stored dry at room temperature and dissolved in 10% dimethyl
sulphoxide in methanol. Both the Gram +ve (S. aureus) and Gram
ꢀve (E. coli) bacteria were grown in nutrient agar medium and
incubated at 37 °C for 48 h followed by frequent subculture to
fresh medium and were used as test bacteria. C. albicans and A. ni-
ger grown as Sabourard Dextrose Agar medium were incubated at
27 °C for 72 h followed by periodic sub culturing to fresh medium
and were used as test fungus. Then the petriplates were inoculated
with a loop full of bacterial and fungal culture and spread through-
out the petriplates uniformly with a sterile glass spreader. To each
disc the test samples and reference antibiotic (ciproflaxin 5 lg/clo-
trimazole 10 lg) were added with a sterile micropipette. The plates
were then incubated at 35 2 °C for 24–48 h and at 27 1 °C for
bacteria and fungus, respectively. Plates with disc containing
respective solvents served as control. Inhibition was recorded by
measuring the diameter of the inhibitory zone after the period of
incubation. The experiment was repeated thrice and the average
values are presented.
2.2. Physical measurements
The analysis of carbon, hydrogen, nitrogen and sulphur were
performed at Sophisticated Test and Instrumentation Centre (STIC)
Cochin University, Kochi. Infrared spectra of complexes were re-
corded in KBr pellets with a Perkin-Elmer 597 spectrophotometer
in the range 4000–400 cmꢀ1. Electronic spectra of the complexes
were recorded in CHCl3 solution with a Cary 300 Bio UV–Vis Varian
spectrophotometer in the range 800–200 nm. Emission intensity
measurements were carried out by using a Jasco FP-6500 spectro-
fluorimeter with 5 nm exit slit. The 1H NMR spectra were recorded
in CDCl3 and DMSO with Bruker 400 MHz instrument using TMS as
internal reference. Melting Points were recorded in the Boetius mi-
cro heating table and are uncorrected. Electrochemical measure-
ments were made using
a Princeton EG and G-Parc model
potentiostat using a glassy carbon-working electrode and [(n-
C4H9)4N](ClO4) (TBAP) as supporting electrolyte. All the potentials
were referenced to Ag/AgCl electrode and the solutions were
purged with N2 before each set of experiments. The bacterial and
fungal species were obtained from National Chemical Laboratory
(NCL), Pune, India. The precursor complexes [RuHCl(CO)(PPh3)3]
[32], [RuHCl(CO)(B)(PPh3)2] [33] (where B = py, pip (or) mor) and
[RuHCl(CO)(AsPh3)3] [34] and the ligands [35] were prepared by
reported literature methods.
3. Results and discussion
The new ruthenium(II) carbonyl complexes of dehydroacetic
acid thiosemicarbazone of the type [Ru(dhatsc)(CO)(B)(EPh3)]
(Scheme 1) have been obtained from the reaction of [RuHCl(CO)(-
B)(EPh3)2] (where E = P, B = PPh3, py, pip (or) mor; E = As,
B = AsPh3) with tridentate Schiff base H2dhatsc in dry benzene in
1:1 molar ratio.
2.3. Preparation of H2dhatsc
To the hot solution of thiosemicarbazide (0.182 g; 0.002 mol),
hot ethanolic solution of dehydroacetic acid (0.336 g; 0.002 mol)
was added and refluxed for 3 h. On reducing the solvent followed
by subsequent cooling, the solid product was separated out and
was recrystallized from ethanol.
All the new tridentate ruthenium(II) carbonyl complexes are
colored, stable to air and light and soluble in chloroform, dichloro-
methane, benzene, DMF, DMSO. They are found to be diamagnetic,
characteristic of the low spin d6 ruthenium(II) acceptor center. The
analytical data are given in Table 1 and are in good agreement with

S. Kannan et al. / Journal of Organometallic Chemistry 693 (2008) 2251–2257
2253
H3C
H3C
O
O
C
N-NH-C-NH2
*
B
Benzene
S
O
O
O
N
CO
*
Reflux/10 h
*
[RuHCl(CO)(B)(EPh3)2]
+
OH
Ru
1:1
S
C
EPh3
H3C
N
C
NH2
H3C
(where, E = P, B = PPh3, py, pip, mor;
E = As, B = AsPh3)
Scheme 1. Structure of ruthenium(II) carbonyl complexes of dhatsc.
Table 1
Analytical data of ruthenium(II) carbonyl complexes of DHATSC
S. no.
Complexes
Calculated(Found) (%)
C
H
N
S
1
2
3
4
5
[Ru(dhatsc)(CO)(PPh3)2]
61.80(61.40)
55.75(55.45)
60.29(60.71)
53.63(53.29)
56.24(55.88)
4.42(4.20)
4.22(3.98)
4.85(4.93)
4.47(4.13)
4.07(3.78)
4.75(4.38)
7.88(7.48)
7.84(7.45)
7.82(7.44)
4.27(3.96)
3.56(3.75)
4.55(4.10)
4.45(4.22)
4.47(4.17)
3.26(2.98)
[Ru(dhatsc)(CO)(py)(PPh3)]
[Ru(dhatsc)(CO)(pip)(PPh3)]
[Ru(dhatsc)(CO)(mor)(PPh3)]
[Ru(dhatsc)(CO)(AsPh3)2]
the general molecular formula proposed. In all the reactions, the li-
gand behaves as dibasic tridentate ligand replacing one hydride,
one chloride and one triphenylphosphine or triphenylarsine mole-
cule from the precursors.
complexes, a strong band in the region 1952–1954 cmꢀ1 is due
to the terminally coordinated carbonyl group and is observed at
higher frequency than in the precursor complexes. The spectra of
the free dhatsc ligand show strong bands at 3380 cmꢀ1 due to
the NH2 group [43] and at 1682 cmꢀ1 for (mC@O) lactone carbonyl
groups. These absorption remains same even after the coordination
indicates that the groups are not involved in coordination. In addi-
tion, other characteristic bands due to triphenylphosphine/tri-
phenylarsine are also present around 1434 cmꢀ1 in the spectra of
all the complexes. A medium intensity band is observed in the
1092 cmꢀ1 region characteristic of the co-ordinated pyridine and
piperidine [44]. Bands in the region 410–400 cmꢀ1 due to M–O,
M–N [42,44] and M–S [44], respectively.
3.1. Characterization
In principle, the ligand H2dhatsc exhibit thione–thiol tautomer-
ism, since it contains a thioamide –NH–C@S functional group [39].
The free ligand displays m(C@S) and m(N–H) absorptions at 788 cmꢀ1
and 3181 cmꢀ1 respectively and were disappeared upon complex-
ation. These observations may be attributed to the enolization of
–NH–C@S and subsequent coordination through the deprotonated
sulfur [40]. The ligand showed a strong band at 1627 cmꢀ1 which is
characteristic of the azomethine group (C@N). Co-ordination of the
Schiff base to the ruthenium ion through azomethine nitrogen
atom is expected to reduce the electron density in the azomethine
link and thus lowers the mC@N absorption frequency in the region
1587–1609 cmꢀ1 (Table 2) after complexation indicating the
co-ordination of azomethine nitrogen [41] to ruthenium ion. The
strong band observed at 1247 cmꢀ1 in the ligand has been assigned
to phenolic mC–O stretching. On co-ordination, this band shifts to
higher frequency in the range 1258–1262 cmꢀ1 showing that the
other co-ordination is through the phenolic oxygen atom [42]. This
fact is further supported by the disappearance of broad band mOH
around 3350–3450 cmꢀ1 in all the complexes indicating deproto-
nation of the phenolic proton prior to coordination. For all
The electronic spectra of the complexes in CHCl3 showed two
bands in the region 366–261 nm (Table 2). The ground state of
ruthenium(II) is 1A1g, arising from the t62g configuration in an octa-
hedral environment. Excited state corresponding to the t52ge1g con-
figuration are 3T1g, 3T2g, 1T1g, 1T2g. Hence, four bands corresponding
1
to the transitions 1A1g ? 3T1g
,
1A1g ? 3T2g
,
1A1g ? 1T1g and A1g
?
1T2g are possible, in order of increasing energy. The bands in the
366–311 nm region present in all the complexes may be assigned
to the Ru(4dp) ? p* (imine) (MLCT) transition [45–47]. The charge
transfer bands are prominent in the low energy region, which ob-
scures d–d bands in these complexes due to the presence of the
unsaturated amine ligands for which a very large body of CT exists
[48]. The other lower wavelength bands below 275 nm are due to
intra ligand transitions occurring within ligand orbitals. The
Table 2
IR and electronic spectral data of ruthenium(II) carbonyl complexes of DHATSC
Complexes
m(C„O) (cmꢀ1
)
m(C@O) (cmꢀ1
)
m(C@N) (cmꢀ1
)
m(C–O) + m(C–S) (cmꢀ1
)
kmax (nm) e (dm3/mol/cm)
Fluorescence data kmax (nm)
1
2
3
4
5
1953
1952
1952
1954
1952
1682
1685
1684
1680
1682
1609
1594
1587
1604
1601
1258
1262
1258
1260
1262
366(24612)a, 267(84709)b
365(21967)a, 272(55258)b
312(18677)a, 269(49225)b
330(20486)a, 264(40256)b
311(34709)a, 261(74612)b
530
495
500
485
450
a
MLCT.
b
Intra-ligand charge transfer.

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S. Kannan et al. / Journal of Organometallic Chemistry 693 (2008) 2251–2257
Table 3
pattern of the electronic spectra of all the complexes indicate the
presence of an octahedral geometry around ruthenium(II) ion [49].
The light emitting property of all the complexes was investi-
gated in DMSO at ambient temperature (298 K). The excitation
was made at the charge transfer band for all the complexes. The
emission maxima of all the complexes have experienced a positive
shift of the order of 130–188 nm and are listed in Table 2. The
emission maximum fall in the range 450–530 nm and a represen-
tative spectrum is shown in Fig. 1. The observed CT luminescence
in these complexes may be due to the presence of imine functional
group. It is likely that the emission originates from the lowest en-
ergy metal to ligand charge transfer (MLCT) state, probably derived
from the excitation involving dp(Ru) ? p* (imine) MLCT transitions
and similar MLCT observed in ruthenium(II) bipyridyl complexes
[50]. The present result shows that the Ru(II) carbonyl Schiff base
complexes have decreased emission intensity when compared to
ruthenium bipyridyl complexes [51]. Among triphenylphosphine
and triphenylarsine complexes, the emission maxima is reduced
in the arsine complex where the heavier atom As induces the spin
forbidden transition [30].
1H NMR spectral data of Ru(II) carbonyl complexes of DHATSC
Complexes 1H NMR data (ppm)
Ar–H
(m)
NH2
(s)
@CH
(s)
CNCH3
(s)
CH3 (ring)
(s)
NH
(s)
CH2
(s)
1
2
3
4
5
7.2–7.9
7.1–7.9
7.0–7.9
7.0–7.8
7.2–7.9
3.4
3.3
3.2
3.1
3.1
5.2
5.3
5.2
5.1
5.4
2.0
2.1
2.1
2.0
2.1
1.8
1.8
1.9
1.8
1.8
9.3
9.6
1.4
3.8 2.8
acid thiosemicarbazone in the complexes and stereochemistry of
the complexes. The ORTEP view is shown in Fig. 3. The summary
of the data collection and refinement parameters are shown in Ta-
ble 4 and selected bond lengths and bond angles are given in Table
5. The thiosemicarbazone ligand coordinates in a tridentate man-
ner to ruthenium ion via the phenolic oxygen, azomethine nitrogen
and thiolate sulfur in addition to two PPh3 and one carbonyl
groups. Ruthenium is therefore sitting in a CNSOP2 coordination
environment, which is distorted octahedral in nature as reflected
in all the bond parameters around ruthenium. The bite angles
around Ru(II) are N(1)–Ru(1)–O(2) = 87.29(19)°; N(1)–Ru(1)–
S(1) = 81.18(15)°; O(2)–Ru(1)–C(28) = 99.2(2)° and S(1)–Ru(1)–
C(28) = 92.3(2)°, summing up the in-plane angle to be exactly
360°. This shows the high planarity of the CO and O, N, S donor
atoms of dehydroacetic acid thiosemicarbazone. Thus the coordi-
nated thiosemicarbazone ligand (dhatsc) and carbonyl constitute
one equatorial plane with the metal at the center and the CO group
is trans to the azomethine (–C@N–) nitrogen. The thiosemicarba-
zone ligand binds the metal center at O, N and S forming one five
membered and one six membered chelate ring with bite angles
of 87.29(19)0 O(2)–Ru(1)–N(1) and 81.18(15)0 N(1)–Ru(1)–S(1)
and bond lengths of 2.096(5) Å Ru(1)–O(2), 2.114(5) Å Ru(1)–
N(1) and 2.3613(17) Å Ru(1)–S(1). The two PPh3 ligands are mutu-
ally trans to each other. Usually the PPh3 ligands prefer to occupy
mutually cis positions for better p-interaction [52]. However, in
these complexes the presence of CO, which is stronger p-acidic
ligand, has probably forced the bulky PPh3 to take up mutually
trans position for steric reasons. The Ru–C bond length
1.867(7) Å in Ru(1)–C(28) fragment is quite normal like that in
the structurally characterized carbonyl complexes of ruthenium
[53]. Coordination causes C(41)–O(2), 1.283(8) Å to be substan-
tially longer than C(45)–O(4), 1.214(8) Å, which is formally a dou-
The bonding arrangement is further confirmed by 1H NMR spec-
tra (Table 3). In the spectra of all the complexes, a multiplet ob-
served around 7.0–7.9 ppm is assigned to aromatic protons of the
phenyl group of triphenylphosphine/arsine and pyridine. The
methine proton appeared as a singlet around 5.2–5.3 ppm and
for NH2 protons a singlet occurs in the 3.0–3.3 ppm region. The
CH3 protons of azomethine group appeared as
a singlet at
2.1 ppm and the ring CH3 proton as singlet at 1.8–1.9 ppm. The sig-
nal for methylene proton appeared at 1.4 ppm in 3 and a represen-
tative spectrum of 5 is shown in Fig. 2. The signal at d 3.8 ppm is
due to methylene proton nearer to oxygen and at d 2.8 ppm is
due to methylene proton nearer to nitrogen in 4. The spectra of 3
and 4 showed a singlet at d 9.3 ppm and at d 9.6 ppm which have
been assigned to the NH protons of piperidine and morpholine. The
absence of resonance for OH/SH protons in the complexes indicate
deprotonation of the phenolic/thiophenolic group of the Schiff base
on complexation and coordination to ruthenium through phenolic
oxygen and thiophenolic sulphur atom.
3.2. X-ray structure
The molecular structure of one of the complexes [Ru(dhatsc)-
(CO)(PPh3)2] (1) has been determined by single crystal X-ray dif-
fraction to find out the coordination mode of the dehydroacetic
Fig. 1. Absorption (–), fluorescence (----) and excitation (. . .. . .) spectra of complex [Ru(dhatsc)(CO)(AsPh3)2] (5).

S. Kannan et al. / Journal of Organometallic Chemistry 693 (2008) 2251–2257
2255
Fig. 2. 1H NMR spectrum of complex [Ru(dhatsc)(CO)(AsPh3)2] (5).
Table 4
Crystal data and structure refinement for complex 1
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C46H39N3O4P2RuS
892.87
183(2)
0.71073
Monoclinic
P2(1)/n
14.663(3)
18.079(4)
15.616(3)
90
b (Å)
c (Å)
a (°)
b (°)
103.88(3)
90
4019.0(14)
4
1.476
0.571
c (°)
Volume (Å3
)
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
1832
Crystal size (mm3
)
0.18 ꢃ 0.15 ꢃ 0.11
h Range for data collection (°)
Index ranges
Reflections collected
1.75–23.20
ꢀ16 6 h 6 16, ꢀ20 6 k 6 19, ꢀ12 6 l 6 17
22162
Independent reflections [Rint
Completeness to h = 23.20°
Absorption correction
]
5724 [0.1592]
99.8%
Numerical
Maximum and minimum transmission 0.6076 and 0.4332
Refinement method
Full-matrix least-squares on F2
Data/restraints/parameters
5724/0/516
1.094
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
R1 = 0.0650, wR2 = 0.1641
R1 = 0.0798, wR2 = 0.1775
2.560 and ꢀ1.598
Fig. 3. The ORTEP diagram of the complex [Ru(dhatsc)(CO)(PPh3)2] (1). For reasons
of clarity, hydrogen atoms have been omitted.
Largest difference in peak and hole
(e Åꢀ3
)
ble bond, but considerably shorter than the formal single bonds
C(43)–O(3), C(45)–O(3) which are 1.378(8) Å and 1.407(8) Å,
respectively. This suggests considerable back p-bonding to the ring
system by the ruthenium(II) center.
3.3. Electrochemical study
As the ligand used in this work are not reversibly reduced or
oxidized within the potential limits employed, we believe that

2256
S. Kannan et al. / Journal of Organometallic Chemistry 693 (2008) 2251–2257
Table 5
Selected bond lengths (Å) and angles (°) for complex 1
Bond lengths (Å)
Bond angles (°)
Ru1–S1
Ru1–P1
Ru1–P2
Ru1–O2
Ru1–N1
Ru1–C28
S1–C46
P1–C2
2.3613 (18)
2.4035 (17)
2.4073 (17)
2.096 (4)
2.114 (5)
1.867 (7)
1.739 (7)
1.813 (6)
1.843 (6)
1.844 (6)
1.831 (6)
1.827 (8)
1.829 (6)
1.158 (9)
1.283 (8)
1.378 (9)
1.407 (9)
1.214 (9)
1.407 (8)
1.312 (9)
1.366 (9)
S1–Ru1–P1
S1–Ru1–P2
S1–Ru1–O2
S1–Ru1–N1
S1–Ru1–C28
P1–Ru1–P2
P1–Ru1–O2
P1–Ru1–N1
P1–Ru1–C28
P2–Ru1–O2
P2–Ru1–N1
P2–Ru1–C28
O2–Ru1–N1
O2–Ru1–C28
N1–Ru1–C28
Ru1–S1–C46
C43–O3–C45
Ru1–N1–N2
Ru1–N1–C39
N2–N1–C39
N1–N2–C46
Ru1–C28–O1
N1–C39–C38
N1–C39–C40
S1–C46–N2
S1–C46–N3
N2–C46–N3
96.92 (6)
92.33 (6)
168.27 (13)
81.17 (15)
92.3 (2)
170.19 (7)
80.79 (13)
89.95 (15)
88.8 (2)
90.79 (13)
94.70 (15)
87.5 (2)
87.31 (19)
99.2 (3)
P1–C3
P1–C5
P2–C19
P2–C22
P2–C26
O1–C28
O2–C41
O3–C43
O3–C45
O4–C45
N1–N2
N2–C46
N3–C46
173.1 (3)
95.4 (2)
121.6 (6)
118.2 (4)
127.3 (4)
114.2 (5)
114.4 (5)
175.9 (6)
119.3 (5)
120.1 (5)
126.0 (5)
116.7 (5)
117.3 (6)
Fig. 4. Cyclic voltammogram of complex [Ru(dhatsc)(CO)(py)(PPh3)] (2).
using disc diffusion method. These compounds were found to exhi-
bit considerable activity against Gram +ve (S. aureus NCIM 2079),
Gram ꢀve (E. coli NCIM 2065) bacteria and the fungus C. albicans
and A. niger. The test solutions were prepared in dimethyl sulphox-
ide and the results of the antimicrobial activities are summarized
in Table 6. In classifying the antibacterial activity as Gram +ve or
Gram ꢀve, it would generally be expected that a much greater
number would be active against Gram-positive than Gram-nega-
tive bacteria [54]. The effectiveness of an antimicrobial agent in
sensitivity testing is based on the size of the zones of inhibition.
The diameter of the zone is measured to the nearest millimeter.
In all our biological experiments, RuCl3 ꢁ 3H2O and Ru(II) carbonyl
precursors are taken as blank along with Schiff base ligand and its
complexes in identical experimental conditions. From the results it
was found that RuCl3 ꢁ 3H2O and Ru(II) carbonyl precursors does
not show any activity. It has been observed that ruthenium che-
lates possess high antimicrobial activity than that of the respective
free ligand against the same microorganism. Higher antifungal
activity has been displayed for A. niger than the C. albicans. This
would suggest that the chelation could facilitate the ability of a
complex to cross a cell membrane [55,56] and can be explained
by Tweedy’s chelation theory [57].
ESD in parentheses.
the redox processes observed for the complexes are metal centered
only. Electrochemical study was carried out for all the Ru(II)
carbonyl complexes in acetonitrile solution at a glassy carbon
working electrode. The supporting electrolyte used was 0.05 M
[Bu4N]ClO4 and the concentration of the complex is 10ꢀ3 M. A rep-
resentative voltammogram is shown in Fig. 4.
The cyclic voltammograms of all the complexes exhibit an irre-
versible oxidation and an irreversible reduction at the scan rate of
100 mV sꢀ1. The redox potential of the complexes is characterized
by well defined waves in the range 0.76–0.89 V (oxidation) and
ꢀ0.87 V to ꢀ0.97 V (reduction). The irreversible redox process ob-
served for these complexes may be due to short-lived oxidized/re-
duced state of the metal ion. It has also been observed that there is
not much variation in the redox potential due to the replacement
of triphenylphosphine/triphenylarsine by pyridine, piperidine or
morpholine.
Chelation considerably reduces the polarity of the metal ion be-
cause of partial sharing of its positive charge with donor groups
and possible p-electron delocalization over the whole chelate ring.
Such a chelation could enhance the lipophilic character of the cen-
tral metal atom, which subsequently favors its permeation through
the lipid layers of cell membrane and blocking the metal binding
sites on enzymes of microorganism. The other possible mode of ac-
3.4. Antimicrobial studies
The free ligand and its ruthenium(II) carbonyl complexes were
screened in vitro for their microbial activity against certain patho-
genic bacterial and fungal species at two different concentration
Table 6
Antimicrobial activity of ruthenium(II) carbonyl complexes of DHATSC
Complex
Antibacterial
Antifungal
Staphylococcus aureus (ppm)
Escherichia coli (ppm)
Candida albicans (ppm)
Aspergillus niger (ppm)
50
100
50
100
50
100
50
100
a
a
a
a
a
a
DHATSC
9
12
12
12
10
11
10
10
10
10
11
8
1
2
3
4
5
10
10
9
8
9
8
8
11
10
7
11
10
8
8
8
11
15
10
9
10
8
10
9
11
9
10
10
11
8
11
a
No inhibition; Standard; Solvent: DMSO (No inhibitory against the microorganisms).

S. Kannan et al. / Journal of Organometallic Chemistry 693 (2008) 2251–2257
2257
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Acknowledgements
We express sincere thanks to Dr. B. Spingler, Institute of Inor-
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graphic discussion. University Grants Commission (UGC), New
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