Syntheses, characterizations, and antimicrobial activities of binuclear ruthenium(III) complexes containing 2-substituted benzimidazole derivatives

Article abstract of DOI:10.1080/00958972.2012.731050

Binuclear ruthenium(III) complexes [RuX₃L]₂· nH₂O (X=Cl, L=L¹, L², L³; n=1, L⁴ and L⁵, X=Br; L=L³), [RuX₃L _1.5]₂·nH₂O (X=Br, L=L¹; n=0, L⁴; n=6 and L⁵; n=10), and [RuX₃L ₂]₂ (X=Br, L=L²) have been isolated by treatment of hydrated RuX₃ (X=Cl/Br) in acetone with 2-(2′-aminophenylbenzimidazole) (L¹), 2-(3′- aminophenylbenzimidazole) (L²), 2-[(3′-N-salicylidinephenyl) benzimidazole] (L³), 2-(3′-pyridylbenzimidazole) (L ⁴), and 2-(4′-pyridylbenzimidazole) (L⁵) in acetone. The complexes were characterized by elemental analysis, conductivity and magnetic susceptibility measurements, IR, electronic, EPR, and mass spectral studies. The complexes were dimeric; based on analytical and spectral studies, an octahedral geometry was proposed for the complexes. The synthesized complexes were screened against Gram-positive and Gram-negative bacteria and fungi.

Full text of DOI:10.1080/00958972.2012.731050

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Syntheses, characterizations, and
antimicrobial activities of binuclear
ruthenium(III) complexes containing 2-
substituted benzimidazole derivatives
B. Roopashree a , V. Gayathri a , A. Gopi b & K.S. Devaraju b
a Department of Chemistry , Central College Campus, Bangalore
University , Bangalore 560 001 , India
b Department of Biotechnology , Jnanabharathi Campus,
Bangalore University , Bangalore 560 056 , India
Accepted author version posted online: 18 Sep 2012.Published
online: 01 Oct 2012.
To cite this article: B. Roopashree , V. Gayathri , A. Gopi & K.S. Devaraju (2012) Syntheses,
characterizations, and antimicrobial activities of binuclear ruthenium(III) complexes containing 2-
substituted benzimidazole derivatives, Journal of Coordination Chemistry, 65:22, 4023-4040, DOI:
10.1080/00958972.2012.731050
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Journal of Coordination Chemistry
Vol. 65, No. 22, 20 November 2012, 4023–4040
Syntheses, characterizations, and antimicrobial activities of
binuclear ruthenium(III) complexes containing 2-substituted
benzimidazole derivatives
B. ROOPASHREEy, V. GAYATHRI*y, A. GOPIz and K.S. DEVARAJUz
yDepartment of Chemistry, Central College Campus, Bangalore University,
Bangalore 560 001, India
zDepartment of Biotechnology, Jnanabharathi Campus, Bangalore University,
Bangalore 560 056, India
(Received 11 June 2012; in final form 27 August 2012)
Binuclear ruthenium(III) complexes [RuX3L]2 ꢀ nH2O (X ¼ Cl, L ¼ L1, L2, L3; n ¼ 1, L4 and L5,
X ¼ Br; L ¼ L3), [RuX3L1.5]2 ꢀ nH2O (X ¼ Br, L ¼ L1; n ¼ 0, L4; n ¼ 6 and L5; n ¼ 10), and
[RuX3L2]2 (X ¼ Br, L ¼ L2) have been isolated by treatment of hydrated RuX3 (X ¼ Cl/Br) in
acetone with 2-(20-aminophenylbenzimidazole) (L1), 2-(30-aminophenylbenzimidazole) (L2),
2-[(30-N-salicylidinephenyl)benzimidazole] (L3), 2-(30-pyridylbenzimidazole) (L4), and
2-(40-pyridylbenzimidazole) (L5) in acetone. The complexes were characterized by elemental
analysis, conductivity and magnetic susceptibility measurements, IR, electronic, EPR, and mass
spectral studies. The complexes were dimeric; based on analytical and spectral studies, an
octahedral geometry was proposed for the complexes. The synthesized complexes were screened
against Gram-positive and Gram-negative bacteria and fungi.
Keywords: Ruthenium(III) complexes; 2-Substituted benzimidazoles; Spectral studies;
Antimicrobial activity
1. Introduction
The benzimidazole ring is an important pharmacophore. A variety of benzimidazoles
such as thiabendazole and flubendazole (anthelmintic), omeprazole and lansoprazole
(antiulcerative) and astemizole (antihistaminic) are in use. The chemistry and
pharmacology of benzimidazoles are of interest to medicinal chemistry [1, 2] because
of their ability to interact with a range of different enzymes and receptors. Substituted
benzimidazoles exhibit pharmacological activities, such as anti-inflammatory, antiox-
idant, gastroprotective, and antiparasitic activities [3]. Transition metal complexes of
biologically important ligands are more effective than uncoordinated heterocycles.
Ruthenium(III) complexes represent a new family of promising metal-based anticancer
drugs that offer the potential of reduced toxicity compared to antitumor platinum(II)
complexes used currently. Ruthenium compounds offer medicinal applications as an
*Corresponding author. Email: gayathritvr@yahoo.co.in
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.731050

4024
B. Roopashree et al.
alternative to platinum-based antitumor drugs for treatment of cancer cells that are
resistant to cisplatin and its analogs since ruthenium has the ability to mimic iron in
binding to certain biological molecules, rate of ligand exchange, and the range of
accessible oxidation states [4]. Ruthenium complexes with bisbenzimidazole derivatives
display potent cytotoxicity against ovarian carcinoma cells [5, 6]. Ruthenium(III)
complexes of Schiff-base ligands are useful catalysts in reactions such as hydrogenation,
oxidation, carbonylation and hydroformylation [7–12]. Transition metal systems in
which metal ions are linked by a bridging ligand can differ significantly with regard to
the nature and/or extent of metal–metal electronic interactions [13]. Here, we report the
syntheses, spectroscopic characterizations, and antimicrobial activities of dimeric
ruthenium(III) complexes of biologically important 2-substituted benzimidazoles
(figure 1).
2. Experimental
2.1. Materials, methods, and equipment
All reagents used were of analytical grade. N-heterocycles, 2-(20-aminophenyl
benzimidazole), 2-(30-aminophenyl benzimidazole), 2-(30-pyridyl benzimidazole), and
2-(40-pyridyl benzimidazole) (L1, L2, L4, and L5) were prepared as per literature
methods [14]. Hydrated RuCl3 and RuBr3 were procured from Johnson Matthey
Chemicals limited. The solvents used were purified according to standard procedure
[15]. Ampicillin and fluconazole were procured from Ranbaxy, India.
Microanalyses were carried out on a Finnegan Eager 300 elemental analyzer. IR
(nujol mull) spectra were recorded on a Shimadzu FTIR 8400s and far-IR spectra were
recorded on a Thermo Nicolet 6700 model. Electronic spectra were recorded on a
Shimadzu UV 3101PC spectrophotometer. FAB mass spectra were recorded on a JEOL
SX102 Mass Spectrometer using argon/xenon as the FAB gas and m-nitrobenzyl
alcohol as the matrix and ESI-MS on a Thermo LCQ Deca XP MAX. Molar
conductivity measurements were made on a Systronic conductivity meter 304-cell type
CD-10. Determination of magnetic susceptibilities were carried out by Faraday’s
method and variable temperature magnetic susceptibility measurements were carried
out on a vibrating sample magnetometer, Lakeshore VSM 7410. Biological activities of
the N-heterocycles and complexes were carried out by the agar diffusion method.
2.2. Syntheses of ligands
2.2.1. L1, L2, L4, and L5. L1, L2, L4, and L5 were prepared by condensation of
orthophenylene diamine with corresponding organic acid using polyphosphoric acid as
a condensing agent at 240ꢁC for 6 h. The resulting mixture was poured into ice cold
water and neutralized with 15% NaOH/Na2CO3 solution. The solids obtained were
filtered and recrystallized in alcohol.
2.2.2. L3, 2-[(30-N-salicylidinephenyl)benzimidazole]. A new Schiff base, 2-[(30-N-
salicylidinephenyl)benzimidazole] was prepared by condensation of methanolic solution
of L2 with salicylaldehyde at refluxing temperature for 6 h. The reaction mixture was

Binuclear ruthenium(III) complexes
4025
Figure 1. Structures of ligands.
concentrated to get a yellow compound. The solid was filtered, dried, and recrystallized
in methanol. Yield: 76%; m.p. 235ꢁC.
2.3. Syntheses of the complexes
[RuX3L]2 ꢀ nH2O (X ¼ Cl, L ¼ L1, L2, L4,or L5, n ¼ 0; L ¼ L3, X ¼ Cl or Br, n ¼ 2),
[RuBr3L1.5]2 ꢀ nH2O (L ¼ L1, n ¼ 0; L ¼ L4, n ¼ 6; L ¼ L5, n ¼ 10), and [RuBr3L22]2.

4026
B. Roopashree et al.
About 1 mmol of RuX3 (X ¼ Cl/Br) in acetone (10 mL) was refluxed with 2 mmol of
ligand (L ¼ L1, L2, L3, L4 and L5) in 10 mL acetone for 6 h. Green/brown/reddish brown
compounds were formed. The solids were filtered, washed with methanol/ether, and
dried in vacuo. Yield: 50–70%.
3. Results and discussion
RuX3 (X ¼ Cl/Br) reacts with L1, L2, L3, L4, and L5 in 1 : 2 mole ratio in acetone to yield
green/brown/reddish brown complexes, [RuX3L]2 ꢀ nH2O (X ¼ Cl, L ¼ L1, L2, L4, or L5,
n ¼ 0; X ¼ Cl or Br, L ¼ L3, n ¼ 2), [RuBr3L1.5]2 ꢀ nH2O (L ¼ L1; n ¼ 0, L ¼ L4; n ¼ 6 and
L5; n ¼ 10), and [RuX3L2]2 (X ¼ Br, L ¼ L2). [RuBr3L41:5]2 and [RuBr3L15:5]2 were soluble
in common organic solvents such as acetone and alcohol whereas other complexes were
sparingly soluble in common organic solvents but soluble in DMF and DMSO. The
conductivity measurements were carried out in 10ꢂ3 mol Lꢂ1 solution in DMF/DMSO.
The molar conductance values were 24–49 ꢀꢂ1 cm2 molꢂ1. The observed values were
not high enough to be characterized as 1:1 electrolytes, but these values fall within the
acceptable range for non-electrolytes [16]. The higher conductivity values may be due to
partial replacement of Clꢂ/Brꢂ in the complex by strong donor solvents.
Thermogravimetric analyses for [RuX3L3]2 ꢀ 2H2O (X ¼ Cl/Br), [RuBr3L14:5]2 ꢀ 6H2O,
and [RuBr3L51:5] ꢀ 10H2O were carried out to 300ꢁC in nitrogen at a heating rate of 15ꢁC
per minute to analyze the nature of water associated with these complexes. All four
complexes lost water within 100ꢁC, indicating the presence of lattice water.
Thermogravimetric analysis was repeated after drying the complexes in vacuum by
heating for 48 h. The results are in accord with the composition of the complexes as
determined by elemental analyses. The physical properties and analytical data of the
compounds are listed in table 1.
Table 1. Physical properties and analytical data of the complexes.
Found (Calcd) (%)
H
ꢁ
m.p./d.p. (ꢀꢂ1 cm2
ꢀ
(B.M.)
Complex
Color
(ꢁC)
molꢂ1
)
C
N
[RuCl3L1]
[RuBr3L11:5
[RuCl3L2]
[RuBr3L22]
Green
Green
Green
Green
Brown
Brown
Green
235
>280
>280
>280
>280
240
34
23
49
24
38
49
27
39
0.8
1.9
0.9
1.4
1.6
1.8
1.6
1.7
37.45 (37.38) 2.66 (3.07)
36.61 (35.70) 2.76 (2.52)
37.05 (37.50) 2.45 (3.07)
41.82 (41.29) 3.44 (2.92) 11.63 (11.11)
44.70 (44.58) 3.60 (3.80)
35.20 (35.74) 2.40 (2.55)
9.09 (8.80)
9.94 (9.63)
9.05 (8.80)
]
[RuCl3L3] ꢀ H2O
[RuBr3L3] ꢀ H2O
[RuCl3L4]
8.40 (7.78)
6.10 (6.25)
>280
235
35.70 (35.79) 2.89 (2.25) 10.80 (10.44)
31.32 (31.45) 3.63 (2.86) 8.78 (9.17)
[RuBr3L41:5] ꢀ 3H2O Reddish
brown
Green
[RuCl3L5]
>280
220
29
43
1.2
1.9
35.70 (35.79) 2.62 (2.25) 10.25 (10.44)
29.75 (29.88) 3.15 (3.27) 8.03 (8.71)
[RuBr3L51:5] ꢀ 5H2O Reddish
brown

Binuclear ruthenium(III) complexes
4027
3.1. IR spectral studies
The IR spectral data of the ligands and the complexes were recorded as nujol mulls
(table 2). The spectra of the complexes are similar to that of uncoordinated
N-heterocycles except for minor shifts in the position of the peaks. Free L1, L2, L3,
L4, and L5 display ꢁN–H of benzimidazole at 3379, 3174, 3157, 3165, and 3169 cmꢂ1
,
respectively; ꢁN–H of NH2 in L1 were twin peaks at 3170 and 3143 and L2 at 3350 cmꢂ1
[17]. In complexes of L1, ꢁN–H of benzimidazole and NH2 merged and appeared as a
broad peak at 3177–3188 cmꢂ1. Complexes of L2 display a broad peak at 3109–3166
and 3170 cmꢂ1 assigned to ꢁN–H of benzimidazole and ꢁN–H of NH2. The ruthenium
complexes of L4 and L5 display ꢁN–H of benzimidazole at 3103–3188 cmꢂ1 and ꢁC¼N of
benzimidazole and pyridine at 1609–1630 cmꢂ1. The shift in ꢁC¼N in spectra of the
Table 2. Electronic, ESR, and far-IR spectral data of the complexes.
ESR
Far-IR (cmꢂ1
)
gb
ꢃRu–X(b)
ꢃRu–X(t)
Electronic
spectral
data ꢂnm
a
Compound
L1
d!d Transitions
g
k
g
?
X ¼ Cl/Br
X ¼ Cl/Br
316(3407),
–
–
–
–
–
275
–
–
354(25,575)
301(43,243),
336(22,244)
302(54,414),
310(46,983)
300(11,760),
309(10,942),
332(3808)
[RuCl3L1]
537(5257)
675(1804)
712(3108)
–
–
2.08
–
1.89
1.99
–
312
245
–
[RuBr3L11:5
L2
]
512(1938),
589(1729)
–
–
[RuCl3L2]
300(52,253),
322(36,813),
345(16,171)
307(41,255),
325(28,065)
309(31,747),
325(22,499),
353(11,092)
308(71,632),
321(54,731),
365(18,516)
309(77,704),
323(51,267),
347(21,003)
308(19,299),
320(14,508)
312(53,321),
329(38,511)
313(76,897),
332(53,471)
311(18,085),
325(13,390)
312(44,576),
329(32,379)
325(76,152),
358(46,439)
537(4952)
664(1859)
gav ¼ 2.4
249
[RuBr3L22]
L3
555(3016)
–
675(1879)
–
2.14
1.99
–
–
–
241
–
–
2.04
2.25
–
[RuCl3L3] ꢀ H2O
[RuBr3L3] ꢀ H2O
560(1609)
547(4355)
670(4953)
688(1697)
–
258
211
288,
294
2.02
253
L4
–
–
–
–
–
–
[RuCl3L4]
[RuBr3L41:5] ꢀ 3H2O
L5
461(7842)
471(6981)
–
671(3105)
680(468)
–
2.14,
2.05
gav ¼ 2.02
257
309
210
–
–
–
–
–
–
[RuCl3L5]
[RuBr3L51:5] ꢀ 5H2O
460(8157)
435(11,129)
680(1471)
700(1226)
2.05
250
214
303
238
2.05,
1.80
a" values in parentheses (dm3 molꢂ1), bat 77 K.

4028
B. Roopashree et al.
complexes containing pyridine-based ligands, L4 and L5, suggests coordination of
Ru(III) through tertiary nitrogen of benzimidazole and pyridine. L3 exhibited ꢁN–H
of benzimidazole and ꢁO–H of salicylidine moiety at 3157 cmꢂ1. Ruthenium(III)
complexes of L3 display ꢁN–H of benzimidazole at 3170 and ꢁO–H of salicylidine moiety
and lattice water together as a broad peak at 3400 cmꢂ1 [18]. ꢁC–N and ꢄN–H in all the
complexes were at 3110 cmꢂ1. The IR spectral data of the complexes suggest that the
ligands bind to ruthenium(III) via tertiary nitrogen of benzimidazole. In complexes of
L1 and L2, shift in the position of ꢁN–H of NH2 and in complexes of L3, shift in
azomethine peak suggest coordination of these groups to ruthenium(III).
The far-IR spectra of the chloro complexes display peaks due to bridging ꢁRu–Cl at
250–280 cmꢂ1 and terminal ꢁRu–Cl at 230–290 cmꢂ1, indicating the presence of both
bridging and terminal chlorides. Bromo complexes of L3, L4, and L5 exhibited ꢁRu–Br(t)
around 250 and ꢁRu–Cl(b) around 210 cmꢂ1 [19] whereas bromo complexes of L1 and L2
displayed peaks corresponding only to ꢁRu–Br(t), indicating the absence of bridging
bromides.
3.2. Electronic spectral studies
Electronic spectra of the ligands and their ruthenium(III) complexes were recorded in
DMF (table 2). Low-spin ruthenium(III) is a d5 system with a ground state 2T2g and the
first excited doublet levels in order of increasing energy are 2A2g and 2A1g arising from a
t42g e1g configuration. The spectra of free ligands displayed intense bands around 300 and
400 nm which were assigned to ꢅ!ꢅ* and n!ꢅ* transitions, respectively. Spectra of
the ruthenium(III) halo complexes containing L1, L2 and L3 also exhibited moderately
intense bands at 660–715 nm and around 512–560 nm which were assigned to
4
2
4
2T2g! T1g and T2g! T2g, respectively.
Spectra of the ruthenium complexes containing L4 and L5 in addition to ligand bands
showed another prominent band at 460–560 nm due to ligand-to-metal charge transfer
and a weaker d–d band appeared as a shoulder at 530 nm characteristic of low-spin
Ru(III) with octahedral geometry [20–25].
3.3. Magnetic moment and EPR spectral studies
The room temperature magnetic moment values of the ruthenium(III) complexes
containing L3, L4, and L5 were 1.4–1.9 B.M., consistent with low-spin d5 octahedral
ruthenium(III) with one unpaired electron. The magnetic moment values of chloro
complexes of ruthenium(III) containing L1 and L2 at room temperature were less than 1
B.M. Hence variable temperature magnetic susceptibility measurements have been
carried out for these two chloro complexes from 0 to 300 K (figures 2 and 3). The
effective magnetic moment values for these complexes were low at 50 K and increased
with rise in temperature, revealing quenching of the unpaired spins. The lowering of
magnetic moment with temperature may be attributed to antiferromagnetic coupling
between two Ru(III) centers through bridging chloride or due to lower symmetry ligand
fields [26]. Similar values were observed for binuclear ruthenium(III) complexes
reported by others [27–29]. Variable temperature magnetic moment measurements of
binuclear copper(II) complexes of m-aminophenyl benzimidazole (L2) reported

Binuclear ruthenium(III) complexes
4029
Figure 2. Variation in magnetic susceptibility with temperature (K) for [RuCl3L1]2.
Figure 3. Variation in magnetic susceptibility with temperature (K) for [RuCl3L2]2.
earlier [30] revealed the presence of antiferromagnetic coupling between two Cu(II) ions
through bridged halides.
X-band ESR spectra of powdered samples of ruthenium(III) complexes have been
recorded at ambient temperature and 77 K (table 2). The spectral profiles were typical

4030
B. Roopashree et al.
of axial type (g > g ) implying a dx2–y2 ground state. The spectral features resemble
k
those of six-coordinate octahedral ruthenium(III) complexes [31]. ESR spectra of
?
Ru(III) chloro complexes of L1 and L3 exhibited only one peak with g of 1.89 and 2.04,
k
respectively. Ruthenium bromo complexes of L1, L2, and L3 exhibited two peaks with
g-values of 2.08–2.25 (g ) and g (1.99–2.02). Ruthenium complexes of L5 and chloro
k
?
complex of L4 exhibited g -values of 1.80–2.05. ESR signals corresponding to g were
centered around 1.95. The g -values less than 2.3 (table 2) for the complexes indicated
k
?
k
that the complexes possessed considerable covalent character in the metal–ligand bond.
The ESR spectra of [RuCl3L2]2 and [RuBr3L4 ]2 showed a g hyperfine structure
?
1:5
with six-line splitting arising due to the nuclear spin of ruthenium (I ¼ 5/2), the
hyperfine splitting constant (A) being 88 G. The splitting of signal has been attributed to
interaction of the unpaired electron with the metal nuclei.
3.4. Mass spectral studies
Mass spectra of the ruthenium(III) complexes support the binuclear nature of the
complexes [32]. The ESI mass spectra of the ruthenium(III) complexes exhibited peaks
corresponding to Ru2Cl6L21 (m/z ¼ 834), Ru2Br6L31 (m/z ¼ 1309), Ru2Cl6L22 (m/z ¼ 834),
Ru2Br6L24 (m/z ¼ 1518), Ru2Cl6L23 (m/z ¼ 1042), Ru2Br6L32 (m/z ¼ 1308), Ru2Cl6L24
(m/z ¼ 805), Ru2Br6L34 (m/z ¼ 1267), Ru2Cl6L52 (m/z ¼ 805), and Ru2Br6L53 (m/z ¼ 1266)
which supported the binuclear nature of all the complexes. Isotope pattern (figure 4) for
representative complexes such as Ru2Cl6L12, Ru2Br6L42, Ru2Cl6L32, and Ru2Br6L23 had
been carried out and they displayed intensity ratios consistent with the calculated
isotopic distributions for such binuclear complexes (isopro 3.0), supporting formation
of binuclear complexes [33, 34]. The fragmentation patterns of Ru2Br6L13, Ru2Cl6L24,
and Ru2Br6L41:5 are depicted in ‘‘Supplementary material’’ section, which showed that
the decomposition of these complexes finally led to ligand peak m/z ¼ 210 and
m/z ¼ 195 corresponding to 2-aminophenyl benzimidazole and 3-pyridyl benzimidazole,
respectively.
4. Stereochemistry
The elemental analyses and IR spectral studies indicate that ligand is coordinated to
ruthenium(III). TGA data indicate the presence of lattice water in some complexes.
Mass spectral studies support binuclear ruthenium complexes. Electronic spectra of the
complexes were similar to those of six-coordinate octahedral ruthenium(III) complexes.
Magnetic susceptibility measurements indicate that the complexes are low-spin d5 with
one unpaired electron. Based on these studies, distorted octahedral geometry is
proposed for the complexes (figure 5).
L1 is chelating bidentate in chloro complex and both bridging and chelating in bromo
complex. The far-IR spectrum of the chloro complex indicates the presence of both
bridging and terminal chloride whereas bromo complex exhibited peaks due to terminal
bromides. Hence for these complexes, binuclear structure with chloro bridges for
[RuCl3L1]2 and bridging L1 for bromo complex are suggested (figure 5a and b).

Binuclear ruthenium(III) complexes
4031
Figure 4. Isotope patterns for (a) [RuCl3L1]2, (b) [RuBr3L22]2, (c) [RuCl3L3]2, and (d) [RuBr3L3]2.

4032
B. Roopashree et al.
Figure 4. Continued.

Binuclear ruthenium(III) complexes
4033
Figure 5. Proposed structures for (a) [RuCl3L1]2, (b) [RuBr3L11:5]2, (c) [RuCl3L2]2, (d) [RuBr3L22]2,
(e) [RuX3L3]2; X ¼ Cl/Br, (f) [RuCl3L4]2, (g) [RuBr3L41:5]2, (h) [RuCl3L5]2, and (i) [RuBr3L1.5]2.

4034
B. Roopashree et al.
Figure 5. Continued.

Binuclear ruthenium(III) complexes
4035
Figure 5. Continued.

4036
B. Roopashree et al.
Far-IR spectrum of ruthenium(III) chloro complex of L2 exhibited the presence of
both bridging and terminal chlorides. Hence, a binuclear structure with two chloro
bridges and L2 as bridging bidentate are suggested for [RuCl3L2]2 (figure 5c). The far-
IR spectrum of the bromo complex of L2 exhibited peaks only due to terminal bromides
analyzing as [RuBr3L22]2. Hence a binuclear structure with two L2 as bridging bidentate
ligands and the other two as monodentate are suggested (figure 5d).
Far-IR spectra of ruthenium complexes with L3 exhibited peaks due to bridging and
terminal halides. Hence a binuclear structure with bridging bidentate ligand coordi-
nating through tertiary nitrogen of benzimidazole and azomethine nitrogen with
halogen bridges were assigned (figure 5e).
The far-IR spectra of chloro complexes of L4 and L5 exhibited peaks due to bridging
and terminal chlorides. Hence, a binuclear structure with bridging bidentate ligands
coordinating through pyridine nitrogen and tertiary nitrogen of benzimidazole with two
chloro bridges were assigned (figure 5f and h).
Far-IR spectra of bromo complexes of L4 and L5 exhibited peaks due to bridging and
terminal bromides, analyzing as [RuBr3L1.5 2
structure with two ligands monodentate and one bridging bidentate with two bromo
bridges were suggested (figure 5g and i).
]
(L ¼ L4 or L5). Hence a binuclear
5. Biological activity
Free ligands, their metal complexes, metal salts, control (DMSO), and the standard
drugs ampicillin and fluconazole were screened against bacteria Staphylococcus aureus,
Bacillus aureus, Escherichia coli, and Salmonella typhi and fungi, Candida albicans and
Aspergillus niger by the agar diffusion method (table 3). The stock solution was
prepared by dissolving 10 mg mLꢂ1 of the compounds in DMSO. Stock solution was
diluted to get 50, 150, 200, and 250 ppm concentrations. The nutrient agar was prepared
by dissolving peptone, beef extract, sodium chloride, dextrose, and agar in 1000 mL of
distilled water. The mixture was autoclaved at 121ꢁC for 15 min at pH 7.4 and 15 lb
pressure per in2. The mixture was cooled to 45ꢁC and then dispended into sterilized
petri dishes, allowed to solidify and then used for inoculation. Fresh cultures of
microorganisms were prepared separately by adding 6 g of peptone, 1.5 g of beef
extract, 1 g of dextrose, 15 g of agar, and 5 g of sodium chloride in 1000 mL of distilled
water and allowed to solidify. 50, 150, 200, and 250 ppm of sterilized solutions were
used for the application in the well. Standard drug solution and DMSO control were
also poured into separate wells in each plate. The petri dishes were covered and
incubated at 37ꢁC for 24 h for antibacterial activity and 28ꢁC for 48 h for antifungal
activity. Zone inhibitions were observed and average three readings were recorded. The
percentage activity index data for the ligand and metal complexes were calculated as
follows:
Zone of inhibition by test compound ðdiameterÞ
%Activity index ¼
ꢃ 100:
Zone of inhibition by standard ðdiameterÞ

Binuclear ruthenium(III) complexes
4037

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B. Roopashree et al.
5.1. Antibacterial activity
Ligands and their complexes exhibited varying inhibitory effect toward the bacterial
strains. The complexes were not sensitive toward Gram-positive bacteria, S. aureus and
B. aureus while they were active against Gram-negative bacteria, E. coli and S. typhi.
In general, metal complexes showed better activity than the corresponding free ligands
[35]. Antibacterial activity increased with increasing concentration of the test solution.
The enhancement of activity on complexation can be explained by chelation theory and/
or Overtone’s concept [36–38]. [RuCl3L2] exhibited highest activity against E. coli while
[RuBr3L22] showed lowest activity and [RuCl3L3] ꢀ H2O was not sensitive toward E. coli.
The activity of the ligands and the complexes were lower than ampicillin. The data
indicated that the compounds were less sensitive toward S. typhi. L3, [RuCl3L4],
[RuBr3L41:5] ꢀ 3H2O, and [RuCl3L5] were not active against S. typhi whereas [RuBr3L22]
exhibited better activity compared to other complexes and uncoordinated ligands.
Chloro complexes exhibited higher activity against E. coli than bromo analogs except
for complexes of L5.
5.2. Antifungal activity
All the compounds were screened for antifungal activity against C. albicans and
A. niger. [RuCl3L5] and [RuBr3L15:5] ꢀ 5H2O showed higher activity against C. albicans
while L5 showed minimum activity. [RuBr3L22] exhibited higher activity against
C. albicans compared to all the other compounds. In general, the inhibition activity
against C. albicans was slightly augmented by bromide in comparison to chloride. The
complexes exhibited inhibition activity, but it did not reach the effectiveness of the
standard drug. L1, L3, L4, and L5 were not sensitive toward A. niger while their
complexes showed some activity. [RuCl3L2] did not inhibit the growth of A. niger while
L2 and [RuBr3L12:5] exhibited higher activity. The antifungal activity data indicated that
the ligands were less sensitive toward the antifungal activity while it enhanced upon
complexation.
The Ru(III) complexes of benzimidazole derivatives showed higher antimicrobial
activities than their corresponding ligands but less than standard drugs.
Similar behavior was found for bridged ruthenium(III) binuclear complexes by others
[27, 39–41]. Binuclear ruthenium(III) complexes of thiobis(ꢆ-diketonates), 5-nitro-o-
phenanthroline, bis-ꢆ-diketones, and 2-fluorophenyl imines showed higher antibacterial
or antifungal activity as compared to their parent ligands but did not reach the
effectiveness of the respective standard drugs. The increase in activity on complexation
was explained on the basis of chelation theory/Overtone’s concept/modern electronic
theory [36, 42–45].
Acknowledgments
The authors wish to thank VGST, Department of Science and Technology,
Government of Karnataka for the ‘‘CESEM’’ award grant No. 24 (2010-11), STIC,

Binuclear ruthenium(III) complexes
4039
Cochin University for elemental analysis and TGA, SAIF, IIT, Bombay for ESR
spectra, Central Instrumentation Facility, Pondicherry University for far-IR spectra,
Indian Institute of Science (IISc.), Bangalore for variable temperature magnetic
susceptibility measurements, and Spectroscopy/Analytical Test Facility, IISc.,
Bangalore for ESI-MS. Roopashree thanks JSSATE, Bangalore and JSSMVP,
Mysore for encouraging doctoral studies.
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