Studies on the synthesis, spectra, catalytic and antibacterial activities of binuclear ruthenium(II) complexes

Article abstract of DOI:10.1016/j.saa.2010.05.019

A new series of stable binuclear ruthenium(II) carbonyl complexes of the general formula [{RuX(CO)(EPh₃)₂}₂L] (where X = H or Cl; E = P or As and L = dibasic tetradentate diacetyl resorcinol (H ₂-DAR)) have been synthesised by reacting ruthenium(II) starting complexes [RuHX(CO)(EPh₃)₃] (where X = H or Cl; E = P or As) and 4,6-diacetylresorcinol (H₂-DAR) ligand in benzene medium. The structure of the new binuclear ruthenium(II) carbonyl complexes was established using elemental analysis, spectra (FT-IR, UV-vis and ¹H NMR), electrochemical and thermal studies. In these reactions, the 4,6-diacetylresorcinol (H₂-DAR) ligand behaves as a binegative tetradentate chelating ligand coordinating through O,O atoms of both the carbonyl and phenolic C-O groups by replacing a molecule of PPh ₃/AsPh₃ and a hydride ion from the starting complexes. Further, all these complexes were also employed as new catalysts for the oxidation of primary and secondary alcohols in the presence of N-methylmorpholine-N-oxide (NMO) as a more viable co-oxidant. The free ligand and their metal complexes have also been screened for their antibacterial activity against the growth of gram +ve and gram -ve bacterial cultures.

Full text of DOI:10.1016/j.saa.2010.05.019

Spectrochimica Acta Part A 77 (2010) 258–263
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Studies on the synthesis, spectra, catalytic and antibacterial activities of
binuclear ruthenium(II) complexes
P. Krishnamoorthy, P. Sathyadevi, K. Deepa, N. Dharmaraj∗
Department of Chemistry, Bharathiar University, Coimbatore 641 046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A
new series of stable binuclear ruthenium(II) carbonyl complexes of the general formula
Received 6 March 2010
Received in revised form 28 April 2010
Accepted 15 May 2010
[{RuX(CO)(EPh3)2}2L] (where X = H or Cl; E = P or As and L = dibasic tetradentate diacetyl resorcinol (H2-
DAR)) have been synthesised by reacting ruthenium(II) starting complexes [RuHX(CO)(EPh3)3] (where
X = H or Cl; E = P or As) and 4,6-diacetylresorcinol (H2-DAR) ligand in benzene medium. The struc-
ture of the new binuclear ruthenium(II) carbonyl complexes was established using elemental analysis,
spectra (FT-IR, UV–vis and 1H NMR), electrochemical and thermal studies. In these reactions, the 4,6-
diacetylresorcinol (H2-DAR) ligand behaves as a binegative tetradentate chelating ligand coordinating
through O,O atoms of both the carbonyl and phenolic C–O groups by replacing a molecule of PPh3/AsPh3
and a hydride ion from the starting complexes. Further, all these complexes were also employed as new
catalysts for the oxidation of primary and secondary alcohols in the presence of N-methylmorpholine-
N-oxide (NMO) as a more viable co-oxidant. The free ligand and their metal complexes have also been
screened for their antibacterial activity against the growth of gram +ve and gram −ve bacterial cultures.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Binuclear ruthenium(II) complex
Spectroscopic characterisation
Thermal behaviour
Electrochemistry
Catalytic and antibacterial activity
1. Introduction
the quest for effective catalytic systems that use clean, inexpensive
primary oxidants such as molecular oxygen or hydrogen peroxide,
Studies on the coordination chemistry of binuclear transition
metal complexes have received much attention in recent years
[1,2]. Bimetallic coordination compounds serve as model systems
for variety of biological reactions such as oxygen transport [3], oxy-
gen activation, photocatalytic water splitting [4], electron transfer
process [5], metal–metal interaction, etc. The presence of two metal
ions in close proximity can lead to a spin-exchange and unusual
magnetic properties. The proximity of another metal ion in the
same molecule can modify the redox properties of each metal cen-
ters and hence affect the catalytic activity of the metal complexes
[6]. In the use of transition metal carbonyls as reactive species in
the homogeneous catalytic reactions such as hydrogenation, hydro-
formylation and carbonylation, carbon monoxide serves simply as
ligand providing the complex with the necessary reactivity and sta-
bility to allow reaction [7]. The accessibility of ruthenium higher
oxidation states [8,9] converts them into excellent candidates for
catalytic redox reactions.
i.e., a green method for converting alcohols to carbonyl compounds
on an industrial scale remains an important challenge [12]. Most
studies of alcohol oxidation using both homogeneous and hetero-
geneous catalysts involve the use of group VIII metal complexes.
Ruthenium compounds such as RuCl3 and some other high valent
oxoruthenium complexes have been extensively investigated as
catalyst for alcohol oxidation using variety of primary oxidants
like iodobenzene, NMO [13], tert-hydroperoxide [14], hypochloride
[15], bromate or a combination of oxygen and an aldehyde.
Reports on the oxidation of cholesterol, geraniol, etc., catal-
ysed by ruthenium complexes in the presence of NMO and
N,N-dimethylaniline-N-oxide were reported in the literature [16]
and the catalytic activities of ruthenium complexes containing ter-
tiary phosphine and arsine ligands are well established [17,18].
In addition, the presence of ruthenium–halogen bonds in several
ruthenium complexes exhibiting anticancer activity suggests that
these bonds may also play some important role [19]. Further, there
has been an upsurge of interest in the chemistry of transition metal
chelates containing O,O; O,N; N,S and S,S donor ligands due to
their potential carcinostatic, antitumour, antifungal, antiviral and
antibacterial activities [20–22].
Among the different catalytic processes, the one involving the
oxidation of primary and secondary alcohols into their correspond-
ing aldehydes and ketones plays a central role in organic synthesis
[10,11]. From both an economic and environmental point of view,
This article deals with the study of coordination behaviour of
4,6-diacetylresorcinol (H2-DAR) towards the ruthenium(II) com-
plexes containing triphenylphosphine and triphenylarsine. The
structure of new binuclear ruthenium(II) carbonyl complexes
∗
Corresponding author. Tel.: +91 422 2428316; fax: +91 422 2422387.
E-mail address: dharmaraj@buc.edu.in (N. Dharmaraj).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.05.019

P. Krishnamoorthy et al. / Spectrochimica Acta Part A 77 (2010) 258–263
259
8 h. Then the resulting solution was concentrated to 5 ml and the
product precipitated by the addition of petroleum ether (60–80 ◦C)
was recrystallised using CH2Cl2. The compounds were dried under
vacuum and the purity of the complexes was checked by TLC.
Our sincere effort to obtain single crystals of the complexes went
unsuccessful.
Fig. 1. Structure of 4,6-diacetylresorcinol (H2-DAR) ligand.
2.5. Catalytic oxidation
obtained has been proposed on the basis of elemental analysis,
UV–visible, IR, 1H NMR, electrochemical and thermal studies. From
the application point of view, catalytic and antibacterial activities
of the new binuclear ruthenium(II) carbonyl complexes have also
been carried out.
The catalytic activity of the above complexes for the oxidation
of benzyl alcohol and cyclohexanol to benzaldehyde and cyclohex-
anone was tested in the presence of NMO as a co-oxidant. In a
typical reaction, binuclear ruthenium(II) complexes were used as a
catalyst and primary and secondary alcohols namely benzyl alco-
hol, cyclohexanol were used as substrates at 1:200 molar ratio as
described bellow.
2. Experimental
To a solution of the respective substrates (1 mmol) in CH2Cl2,
N-methylmorpholine-N-oxide (3 mmol) and a solution of ruthe-
nium complex (0.005 mmol) in the same solvent were added and
refluxed for 3 h. The solvent was then evaporated to dryness under
reduced pressure and the residue thus obtained was then extracted
thrice with petroleum ether (60–80 ◦C) (20 ml) and the combined
extract was evaporated to yield corresponding aldehyde or ketone
which was then quantified as 2,4-dinitrophenylhydrazone deriva-
tive [28]. The catalytic activity of the complex was determined from
the percent yield (Y %) and the turnover (T.O.) conversion of alcohol
to aldehyde or ketone is as follows:
2.1. Materials and reagents
Resorcinol, acetic anhydride and zinc chloride were Merck
chemicals. RuCl3·3H2O was purchased from Loba Chemie Pvt Ltd.
N-methylmorpholine-N-oxide was purchased from Aldrich. All the
reagents used were chemically pure and analar grade. Solvents
were purified and dried according to standard procedures [23].
2.2. Physical measurements
Microanalysis of carbon and hydrogen was carried out using
Vario EL III CHNS analyser. The IR spectra of the ligand and their
complexes were recorded as KBr pellets on a Nicolet Avatar model
in 4000–400 cm−1 range. Electronic spectra of the metal com-
plexes were recorded on a Jasco UV–vis spectrophotometer using
dichloromethane as a solvent. The 1H NMR spectra of the lig-
and (H2-DAR) and ruthenium(II) complexes were recorded on a
Bruker 400 MHz instrument at room temperature using CDCl3 as
a solvent and TMS as an internal standard. Thermal analysis of
the complexes has been carried out by using NETZSCH Gerateban
Bestell-Nr 348472C. Electrochemical measurements were made
using a CH Instruments electrochemical analyser. Melting points
were recorded using Raaga apparatus and were uncorrected.
weight of alcohol oxidised to aldehyde or ketone
Y (%) =
T.O. =
× 100
total weight of alcohol
millimoles of product
millimoles of catalyst
2.6. Antibacterial activity
The in vitro antibacterial screening of the ligand (H2-DAR) and
the corresponding binuclear ruthenium(II) carbonyl complexes
have been carried out against human pathogenic bacteria such
gram positive (Staphylococcus aureus) and gram negative (Klebsiella
pneumoniae and Escherichia coli) using disc diffusion method. The
test organisms were grown on nutrient agar medium in petriplates.
Triplicate plates were maintained in the antibacterial assays. The
compounds to be tested were dissolved in DMSO to a final concen-
tration of 0.10%, 0.25%, 0.50% and 1.0% and soaked in filter paper
disc of 5 mm diameter and 1 mm thickness. These discs were placed
on the previously seeded plates and incubated at 37 ◦C for 24 h. The
diameter (mm) of inhibition zone around each disc was determined
after 24 h. Streptomycin was used as a standard.
The
starting
complexes
[RuHCl(CO)(PPh3)3]
[25]
[24],
and
[RuHCl(CO)(AsPh3)3]
[25], [RuH2(CO)(PPh3)3]
[RuH2(CO)(AsPh3)3] [26] were prepared according to the literature
reports.
2.3. Synthesis of 4,6-diacetylresorcinol ligand (H2-DAR) [27]
Acetylation of resorcinol (5 g; 45.5 mmol) with acetic anhydride
(9.28 g; 91.0 mmol) using excess zinc chloride (10.0 g; 73.4 mmol)
was carried out at 140 ◦C in a paraffin oil bath. After 5 h, the hot mix-
ture was cooled to room temperature and poured onto 140 ml of
50% dil. HCl and the orange precipitate formed was allowed to stand
for an hour. The crude product was filtered, washed several times
with distilled water and further purified by column chromatogra-
phy using petroleum ether and ethyl acetate mixture (95:5) which
yielded colourless compound (Fig. 1) (yield: 78%, M.P.: 179 ◦C).
3. Results and discussion
Binuclear ruthenium(II) carbonyl complexes of the composition
[{RuX(CO)(EPh3)2}2L] (where X = H or Cl; E = P or As and L = dibasic
tetradentate 4,6-diacetylresorcinol ligand (H2-DAR)) were synthe-
sised in quantitative yield from the reactions of [RuHX(CO)(EPh3)3]
(where X = H or Cl; E = P or As) with H2-DAR ligand in 2:1 molar ratio
in dry benzene as shown in Scheme 1.
2.4. Synthesis of the new metal complexes
The prepared complexes are listed in Table 1 together with their
elemental analysis and colour. The complexes are green or brown
in colour and the proposed structure of the complexes are in good
agreement with the stoichiometries concluded from their analyt-
ical data. All the complexes are stable in air at room temperature,
non-hygroscopic in nature and are soluble in most common organic
solvents such as CH2Cl2, CHCl3, C6H6, CH3CN, DMSO and DMF. In
all the reactions, the H2-DAR behaves as a binegative tetradentate
chelating ligand through O,O atoms of both the carbonyl C O and
All the new metal complexes were prepared according to
the following general procedure. To a boiling solution of 4,6-
diacetylresorcinol (H2-DAR) (0.0194 g; 0.1 mmol) (L) in benzene,
few drops of triethylamine was added, followed by the addition
of [RuHCl(CO)(PPh3)3] (0.190 g; 0.2 mmol), [RuHCl(CO)(AsPh3)3]
(0.216 g; 0.2 mmol), [RuH2(CO)(PPh3)3] (0.183 g; 0.2 mmol),
[RuH2(CO)(AsPh3)3] (0.202 g; 0.2 mmol) in benzene refluxed for

260
P. Krishnamoorthy et al. / Spectrochimica Acta Part A 77 (2010) 258–263
intensity band appeared around 3415 cm−1 in the spectra of free
ligand due to ꢀ(OH) was absent in the spectra of the complexes indi-
cating the deprotonation of H2-DAR ligand prior to coordination
[30].
This fact is further supported by the increase in pheno-
lic C–O stretching frequency from 1251 cm−1 in the ligand to
1322–1316 cm−1 in the complexes which indicates the other coor-
dination site of H2-DAR ligand is phenolic oxygen [28,31]. Also, the
existence of a strong band in the region of 1939–1936 cm−1 in the
spectra of all the complexes reveals that the presence of terminally
coordinated carbon monoxide [32]. In addition to the above, other
characteristic bands due to PPh3/AsPh3 were also present in the
expected regions for the complexes.
3.2. Electronic spectra
All the new binuclear ruthenium(II) carbonyl complexes have
been found to be diamagnetic indicating the presence of ruthe-
6
nium in the +2 oxidation state (t2g configuration). The ground
1
state of ruthenium(II) in an octahedral environment is A1g aris-
6
ing from the t2g configuration. The excited states corresponding
3T2g
,
1T1g
,
1T2g. Hence, four
5
1
3
to t2g eg configuration are T1g
,
Scheme 1.
1
3
3
bands corresponding to the transitions A1g → T1g
,
1A1g → T2g
,
1
1
1A1g → T1g and 1A1g → T2g are possible in the order of increasing
energy. Electronic spectra of all the complexes in dichloromethane
showed two bands in the region of 340–275 nm (Table 2). The band
in the 330–340 nm range has been assigned to the spin allowed
phenolic C–O by replacing one molecule of PPh3/AsPh3 and one
molecule of hydride ion from the starting monomeric complexes.
1
1A1g → T1g transition which is in confirmity with the assignments
3.1. Infrared spectra
made earlier for other similar octahedral ruthenium(II) complexes
[33,34]. The other high intensity bands in the region 270–280 nm
have been assigned to the charge transfer transition arising from
the excitation of an electron from the metal t2g level to the unfilled
molecular orbitals derived from the ␲* level of the ligands [33–36].
A comparison between the IR spectral data of the free ligand and
the metal complexes was made to study the binding mode of the
H2-DAR to the ruthenium ion in the new metal complexes. Some
of the important infrared absorption bands of H2-DAR ligand and
the complexes are shown in Table 2.
In the infrared spectra of the free ligand, a strong band char-
3.3. 1H NMR spectra
acteristic of carbonyl ( C O) group is observed at 1657 cm−1
.
Coordination of the H2-DAR ligand to the metal through carbonyl
oxygen atom is expected to reduce the electron density in the car-
bonyl link and lower the ꢀ(C O) absorption frequency. In the spectra
of all the new complexes, a considerable decrease in the frequency
of the above functional group was observed and appeared in the
1601–1595 cm−1 region, indicating the coordination of the car-
bonyl oxygen ( C O) to ruthenium metal [29]. Another medium
The 1H NMR spectra of the H2-DAR and the corresponding binu-
clear ruthenium(II) carbonyl complexes were recorded in CDCl3 to
confirm the presence of coordinated ligand in the complexes. The
data and their assignments are listed in Table 3.
In the 1H NMR spectrum of the ligand, a sharp singlet was
observed at 12.8 ppm due to hydroxylic –OH protons (with an
integration corresponding to two protons). But the spectra of all
Table 1
Physico-chemical and analytical data of H2-DAR and binuclear ruthenium(II) carbonyl complexes.
Ligand/complex
Empirical formula
Colour
M.P. (◦C)
Found (calculated) (%)
C
H
H2-DAR
C10H10O2
White
Green
Brown
Green
Brown
179
182
178
212
206
–
–
[{RuCl(CO)(PPh3)2}2L]
[{RuCl(CO)(AsPh3)2}2L]
[{RuH(CO)(PPh3)2}2L]
[{RuH(CO)(AsPh3)2}2L]
C84H68O6Cl2P4Ru2
C84H68O6Cl2As4Ru2
C84H70O6P4Ru2
C84H70O6As4Ru2
63.68 (64.24)
56.90 (57.78)
67.02 (67.19)
59.56 (60.15)
4.12 (4.36)
3.02 (3.92)
4.58 (4.69)
3.76 (4.20)
Table 2
Characteristic infrared frequencies and electronic spectral data of H2-DAR ligand and binuclear ruthenium(II) carbonyl complexes.
Ligand/complex
ꢀ(O–H)
ꢀ(C
ꢀ(Ph–CO)
ꢀ(Ru–CO)
ꢁmax (nm)
O)
H2-DAR
3415
1657
1601
1601
1598
1595
1251
1320
1316
1320
1322
–
1938
1937
1936
1939
–
[{RuCl(CO)(PPh3)2}2L]
[{RuCl(CO)(AsPh3)2}2L]
[{RuH(CO)(PPh3)2}2L]
[{RuH(CO)(AsPh3)2}2L]
–
–
–
–
340a, 275b
330a, 278b
335a, 280b
338a, 276b
ꢀ is in units of cm−1
.
a
1A1g
→
1T1g
.
b
Charge transfer.

P. Krishnamoorthy et al. / Spectrochimica Acta Part A 77 (2010) 258–263
261
Table 3
1H NMR data of H2-DAR ligand and binuclear ruthenium(II) carbonyl complexes.
Ligand/complex
1H NMR data (ppm)
H2-DAR
12.8 (–OH, s), 6.3 and 8.1 (Ar, s), 2.5 (–COCH3, s)
6.5–8.2 (a, m), 2.6 (–COCH3, s)
6.2–8.2 (a, m), 2.7 (–COCH3, s)
[{RuCl(CO)(PPh3)2}2L]
[{RuCl(CO)(AsPh3)2}2L]
[{RuH(CO)(PPh3)2}2L]
6.4–8.2 (a, m), 2.7 (–COCH3, s)
Scheme 2.
3.5. Electron transfer properties
Dichloromethane solutions of the complexes have been used
to study the electron transfer properties with the help of cyclic
voltammetry and the data are presented in Table 4. All the
complexes are redox active and in each case, the one electron stoi-
chiometry of the observed responses is confirmed by comparing the
current heights with reported values of one electron redox process
[40–42]. All the diruthenium complexes showed two successive
responses for quasi-reversible oxidation and the well defined half
wave potentials E1/2 in the range 1.05–1.07 V correspond to the first
oxidation response and the second oxidation response in the range
0.44–0.55 V. The responses observed for complexes are assigned to
the metal centered Ru(II)Ru(II) to Ru(III)Ru(II) and Ru(III)Ru(II) to
Ru(III)Ru(III) oxidation process.
Fig. 2. 1H NMR spectrum of [{RuCl(CO)(PPh3)2}2L] complex.
−
−
−e
−e
Ru(II)–Ru(II)
ꢀ
Ru(III)–Ru(II)
ꢀ
(oxd2)
1/2
Ru(III)–Ru(III)
E
(oxd1)
E
the metal complexes (Fig. 2) showed no signal at around 12 ppm
due to the hydroxylic proton, which revealed that the hydroxyl
group underwent deprotonation prior to the coordination with
ruthenium ion [34,37]. The signals observed at 8.1 and 6.3 ppm
in the spectrum of the free ligand were due to aromatic protons
and another sharp singlet at 2.5 ppm in the ligand is assigned for
the methyl protons of –COCH3 group. In the spectra of the com-
plexes, the signal due to methyl protons of –COCH3 group has
been observed around 2.6–2.7 ppm as a singlet and the character-
istic multiplets of the PPh3/AsPh3 and the aromatic protons were
appeared in the range of 6.2–8.2 ppm.
1/2
From all the above facts, it is concluded that all the diruthenium
complexes didnot showanyreduction responses atnegative poten-
tials [43]. The redox potentials are independent of the various scan
rates and support the quasi-reversible responses. It has also been
observed that there is no much variation in the redox potentials
of the complexes due to the substitution of triphenylphosphine by
triphenylarsine.
3.6. Catalytic oxidation
The applications of diruthenium complexes as an inexpensive
and easy to handle co-oxidant for selective oxidation of alcohols
are well known in the literature [44,45]. No oxidation of alcohols
to their corresponding aldehydes and ketones was achieved in our
case by employing NMO only. Thus, the catalytic oxidation of ben-
zyl alcohol to benzaldehyde and cyclohexanol to cyclohexanone
by the precursor catalyst [{RuX(CO)(EPh3)2}2L] (where X = H or
Cl; E = P or As and L = H2-DAR) in the presence of NMO in 1:200
molar ratio of catalyst to substrate at room temperature in dry
CH2Cl2 was carried out (Scheme 2). Good experimental yield with
appreciable turnover of benzyl alcohol to benzaldehyde and cyclo-
hexanol to cyclohexanone (Table 5) was achieved using the newly
synthesised ruthenium(II) complexes similar to that of other ruthe-
nium(II) complexes reported elsewhere [46,47]. The formation of
benzaldehyde and cyclohexanone was confirmed by the formation
of 2,4-dinitrophenylhydrazone.
3.4. Thermogravimetric analysis
Thermal gravimetric analysis (TGA) was used as a probe to
prove the presence of associated water or solvent molecules in
the coordination sphere or in the crystal lattice [38]. The ther-
mal gravimetric analysis was carried out for two of the complexes
[{RuCl(CO)(PPh3)2}2L] and [{RuCl(CO)(AsPh3)2}2L] in the range of
about 50–800 ◦C and it showed that the thermal decomposition of
the complexes takes place in several steps. The thermal analysis
data of the ruthenium(II) complexes indicates that they are stable
up to 200 ◦C owing to their anhydrous nature. DTA curves show
no endothermic peaks up to 200 ◦C confirming the absence of lat-
tice or coordinated water molecules in the complexes [14,39]. A
sharp decomposition corresponding to the loss of organic moiety
(85–86.5%) was observed in the temperature range of 350–400 ◦C
beyond which no decomposition was observed indicating the for-
mation of stable ruthenium oxide.
The possible mechanism for the catalytic oxidation of alcohols
to carbonyl compounds in the presence of co-oxidant could be pro-
Table 4
Cyclic voltammetric data of binuclear ruthenium(II) carbonyl complexes.
II,II
Complex
Ru2III,III–Ru2
Epa (V)
Epc (V)
E1/2 (V)
ꢂEp (mV)
Epa (V)
Epc (V)
E1/2 (V)
ꢂEp (mV)
[{RuCl(CO)(PPh3)2}2L]
[{RuCl(CO)(AsPh3)2}2L]
[{RuH(CO)(PPh3)2}2L]
[{RuH(CO)(AsPh3)2}2L]
1.12
1.13
1.12
1.14
1.01
0.98
1.02
1.00
1.06
1.05
1.07
1.07
110
150
100
140
0.64
0.51
0.56
0.57
0.45
0.37
0.41
0.39
0.55
0.44
0.49
0.48
190
140
150
180

262
P. Krishnamoorthy et al. / Spectrochimica Acta Part A 77 (2010) 258–263
Table 5
Data for catalytic oxidation of alcohols using binuclear ruthenium(II) carbonyl complexes in the presence of NMO.
Complex
Substrate
Product
Yielda (%)
TONb
[{RuCl(CO)(PPh3)2}2L]
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
71.93c
45.60c
70.30c
43.03c
69.95c
42.78c
71.17c
43.49c
148.92
95.25
145.80
89.88
145.07
89.36
147.60
90.84
[{RuCl(CO)(AsPh3)2}2L]
[{RuH(CO)(PPh3)2}2L]
[{RuH(CO)(AsPh3)2}2L]
a
Yield based on substrate: alcohol (1 mmol); NMO (3 mmol); catalyst (0.005 mmol).
Ratio of moles of product obtained to the moles of the catalyst used.
Estimated as 2,4-dinitrophenylhydrazone derivative.
b
c
Table 6
Antibacterial activity data of binuclear ruthenium(II) carbonyl complexes.
Ligand/complex Diameter of inhibition zone (mm)
S. aureus
Klebsiella pneumoniae
E. coli
0.10%
0.25%
0.50%
1.0%
0.10%
0.25%
0.50%
1.0%
0.10%
0.25%
0.50%
1.0%
H2-DAR
2 ( 0.3)
6 ( 0.8)
8 ( 0.8)
8 ( 2.0)
9 ( 0.5)
13 ( 0.4)
5 ( 1.8)
9 ( 0.8)
10 ( 0.5)
10 ( 0.8)
12 ( 0.8)
18 ( 0.9)
6 ( 0.5)
15 ( 0.5)
14 ( 0.8)
15 ( 0.5)
13 ( 0.5)
21 ( 1.3)
7 ( 1.1)
20 ( 0.8)
20 ( 0.5)
18 ( 0.6)
16 ( 1.4)
25 ( 0.5)
4 ( 1.1)
7 ( 0.8)
8 ( 0.8)
10 ( 0.8)
9 ( 0.5)
11 ( 0.6)
5 ( 0.6)
14 ( 1.4)
10 ( 0.8)
15 ( 0.5)
14 ( 0.5)
17 ( 1.5)
7 ( 0.8)
18 ( 0.6)
14 ( 1.2)
19 ( 0.5)
18 ( 0.5)
22 ( 0.8)
7 ( 0.8)
20 ( 0.5)
17 ( 1.4)
20 ( 0.5)
22 ( 0.8)
26 ( 2.0)
4 ( 0.5)
9 ( 0.5)
7 ( 0.8)
7 ( 0.5)
9 ( 0.3)
12 ( 0.5)
4 ( 0.8)
10 ( 0.5)
11 ( 1.7)
8 ( 0.5)
12 ( 0.3)
17 ( 1.1)
5 ( 0.8)
13 ( 0.8)
18 ( 1.5)
14 ( 1.1)
15 ( 0.3)
23 ( 1.8)
7 ( 0.8)
16 ( 1.2)
20 ( 0.5)
20 ( 0.8)
19 ( 0.5)
26 ( 0.7)
1
2
3
4
S
1: [{RuCl(CO)(PPh3)2}2L]; 2: [{RuCl(CO)(AsPh3)2}2L]; 3: [{RuH(CO)(PPh3)2}2L]; 4: [{RuH(CO)(AsPh3)2}2L]; S: streptomycin.
ceeded via the formation of ␮-peroxoruthenium(IV) intermediate
species [47] that are capable to abstract hydrogen atom from the
–OH group in alcohol.
4. Conclusion
Binuclear ruthenium(II) carbonyl complexes of the composition
[{RuX(CO)(EPh3)2}2L] (where X = H or Cl; E = P or As and L = dibasic
tetradentate diacetyl resorcinol (H2-DAR)) have been prepared
by reacting ruthenium(II) starting complexes [RuHX(CO)(EPh3)3]
(where X = H or Cl; E = P or As) with 4,6-diacetylresorcinol (H2-
DAR) ligand under reflux conditions. The new complexes obtained
were characterised on the basis of elemental analysis, spectra (FT-
IR, UV–vis and 1H NMR), electrochemical and thermal studies. The
catalytic efficiency of these complexes towards the oxidation of
primary and secondary alcohols into their corresponding carbonyl
compounds showed some promising results. In vitro antibacterial
screening revealed that these complexes are effective against both
gram +ve and gram −ve pathogenic bacteria.
3.7. Antibacterial activity
The in vitro antibacterial screening of the ligand (H2-DAR)
and their binuclear ruthenium(II) carbonyl complexes has been
carried out against human pathogenic bacteria such as gram pos-
itive (S. aureus) and gram negative (K. pneumoniae and E. coli)
using disc diffusion method [48]. Triplicate plates were main-
tained in the antibacterial assays. The compounds to be tested
were dissolved in DMSO to a final concentration of 0.10%, 0.25%,
0.50% and 1.0%. The standard error for each assay is presented in
the parentheses. From the results given in Table 6, it has been
observed that the ruthenium complexes showed better activity
than the free ligand under identical experimental conditions. The
possible mode of increased toxicity of the ruthenium complexes
compared to that of free ligand may be explained in terms of
Tweedy’s theory [49], according to this, chelation reduces the polar-
ity of the metal atom because of partial sharing of its positive
charge with a donor group and possible ␲-electron delocaliza-
tion over the whole chelate ring. Such a chelation could enhance
the lipophilic character of the central metal atom, which subse-
quently favors its permeation through the lipid layers of the cell
membrane [50] and blocking the metal binding sites on enzymes
of microorganism. The variation in the effectiveness of the dif-
ferent compounds against different organisms depends on their
impermeability of the microbial cells or on the difference in the
ribosome of the microbial cells [51]. These complexes also dis-
turb the respiration process of the cell and block the synthesis
of proteins which restrict the further growth of the organism.
The inhibition of the activity of the compounds increases with
increase in the concentration. Though there is a marked increase
in the bacterial activity of ruthenium complexes as compared to
the free ligand, it could not reach the effectiveness of streptomycin
[52].
Acknowledgement
We express our sincere thanks to University Grants Commis-
sion (UGC), New Delhi for the award of Research Fellowship in
Science for Meritorious Students (RFSMS) to one of the authors (P.
Krishnamoorthy) under UGC-SAP (DRS) programme.
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