M.N. Patel et al. / Inorganic Chemistry Communications 13 (2010) 1480–1484
1481
Table 1
water molecule. Both the ligands decompose in one step in the
Electronic spectral data for the ruthenium(II) complexes.
max/nm (ε/dm3 mol−1 cm−1
π→π*
temperature range 360–600 °C. Magnetic moment value of all
complexes was found to be zero, which suggests low-spin configu-
ration with d2sp3 hybridization for octahedral RuII complexes.
Perchlorate as a counterion is confirmed by a very strong, broad peak
at around 1086 cm−1 and the strong, sharp peak at around 625 cm−1
[8]. In the spectra of complexes 3 and 4, band at ~1230 cm−1 can be
attributed to the asymmetric stretching of aromatic ether. A weak, broad
peak at around 3070 cm−1, characteristic of aromatic C–H stretching as
well as a sharp peak at around 2930 cm−1, characteristic of C–H
stretching of methyl. A sharp peak of medium intensity at around 1600
and 1498 cm−1, characteristic of aromatic ring stretching. An intense,
sharp peak at around 760 cm−1, characteristic of C–H out-of-plane
deformations, appears as expected from a structure including aromatic
rings. A weak, sharp band in the range 490–520 cm−1, characteristic of
Ru–N stretching mode. A Ru–Cl stretching mode would be expected in
the region less than 400 cm−1 [9]. The suggested structure of complex 1
is shown in Fig. 1.
The electronic spectra of the ruthenium complexes were recorded in
dimethylsulfoxide medium and the band maxima and molar extinction
coefficients are listed in Table 1. The UV–Vis spectrum of RuII complexes
exhibits a typical metal to ligand charge transfer (MLCT) transition due
to Ru(dπ)→terpy(π*) with λmax from 491 to 497 nm (ε=18 670–22
210 dm3 mol−1 cm−1). The spin allowed metal to ligand charge
transfer (MLCT) band in the visible spectral region undergoes an
increase in intensity and a red shift, regardless of the electron-donor or
electron-acceptor nature of the substituent. Complexes show two bands
at around 283 nm (ε=45 714–57 283 dm3 mol−1 cm−1) and 312 nm
(ε=42 356–56 132 dm3 mol−1 cm−1) due to the overlap of ligand
centered transitions dmphen(π)→dmphen(π*) and terpy(π)→terpy
(π*) [10].
Complexes
λ
)
MLCT
[RuII(2‴-pytpy)(dmphen)(Cl)]ClO4 (1) 281 (45 714), 309 (46 190) 491 (18 670)
[RuII(3‴-pytpy)(dmphen)(Cl)]ClO4 (2) 280 (46 545), 311 (42 356) 491 (18 750)
[RuII(3-boptpy)(dmphen)(Cl)]ClO4 (3) 282 (56 386), 312 (52 740) 495 (21 340)
[RuII(4-boptpy)(dmphen)(Cl)]ClO4 (4) 285 (57 283), 314 (56 132) 497 (22 210)
and E. coli but found less active in the case of P. aeruginosa. All the
complexes found less active than pefloxacin.
Binding of the compound with DNA through intercalation
generally results in hypochromism and bathochromism due to the
intercalative mode involving a strong stacking interaction of the
planar aromatic chromophore of the compound with the base pairs of
DNA [12]. Fig. 2 shows the absorption spectra of complex 1 in the
presence of increasing amounts of DNA at room temperature. With
increasing DNA concentration to complex 1, hypochromism of the
MLCT band at 493 nm was observed along with a 2 nm red shift. The
red shift observed in complexes 2, 3 and 4 is 2 nm, 1 nm and 1 nm,
respectively. Similar red shift of the MLCT band reported for [Ru
(phen)2NMIP]2+ and [Ru(tpy)(PHNI)]2+ complexes [13,14]. So, small
red shifting of the MLCT band suggests that the complexes may bind
with DNA via partial intercalative mode.
In order to further investigate the binding strength of the
complexes, the intrinsic binding constants (Kb) were determined by
monitoring change in absorbance of the MLCT band with increasing
concentration of DNA. The intrinsic binding constants (Kb) of
complexes 1, 2, 3, 4 were found to be 2.47×104, 2.64×104,
4.56×103, 5.38×103 M−1, respectively. The binding constants of
ligands were found in the range of 1.35×103 to 4.43×103 M−1, which
are lower than intrinsic binding constants of the complexes. So,
complexation increases the DNA binding affinity. The binding
constants of complexes are smaller than that observed for [Ru(bpy)2
(dppz)] (N106) [15]. The reason for the low binding constant is
the presence of methyl groups on the 2nd and 9th positions of
phenanthroline causing severe steric constraints near the RuII core
when the complex intercalates to the DNA [16]. Another reason for
low Kb is the nonplanarity of terpyridines.
FAB-mass spectra of all complexes were obtained using m-nitro
benzyl alcohol as matrix. Peaks at 136, 137, 154, 289 and 307 m/z
values are due to usage of matrix. For complex 1, the peak at 793 m/z
value is assigned as a molecular ion peak associated with matrix (136)
and two H+ ions without perchlorate ion [11]. The base peak observed
at m/z=209 is due to the dmphen ligand associated with one proton.
Several other fragments at m/z 758, 448, 413 and 310 values are
observed, attributed to fragments associated with matrix or H+ ions.
The isotopic pattern for ruthenium and chlorine atoms is observed in
the spectrum.
Fig. 3 shows the plots of [DNA]/(εa −εf) versus [DNA] for [RuII(2‴-
pytpy)(dmphen)Cl](ClO4) (20 μM) with increasing amount of DNA
(0–16.4 μM) in phosphate buffer (pH 7.2), containing various
concentrations of NaCl and incubated for 10 min at 37 °C. The non-
All the ligands and their ruthenium complexes were screened in-
vitro for their growth inhibitory activity against two Gram(+ve)
,
,
Staphylococcus aureus and Bacillus subtilis, and three Gram(−ve)
Serratia marcescens, Pseudomonas aeruginosa and Escherichia coli,
microorganisms. No zone of inhibition was observed for any of the
ligands with 100 μL solution having a concentration of 1 μg/μL. MIC
data denotes that all the ligands show poor antibacterial activity than
complexes. All the complexes found more active against S. marcescens
Fig. 2. Electronic absorption spectra of [RuII(2‴-pytpy)(dmphen)Cl](ClO4) with
increasing amount of [DNA] in phosphate buffer (Na2HPO4/NaH2PO4, pH 7.2),
[complex]=20 μM, [DNA]=0–16.4 μM with an incubation period of 10 min at 37 °C,
inset: plot of [DNA]/(εa −εf) versus [DNA]. Arrow shows the absorbance change upon
increasing DNA concentrations.
Fig. 1. Suggested structure of the complex [RuII(2‴-pytpy)(dmphen)Cl](ClO4) (1).