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CHAPTER IV
New series of Ru(III) Schiff base complexes of N,O- donors:
Synthesis, spectral, electrochemical, antibacterial and
DNA binding properties
Abstract
A series of new ruthenium(III) complexes of the type [Ru(Cl)2(DMAPIMP-
X)(EPh3)2]; DMAPIMP={2-[(4-N,N’-dimethylaminophenylimino)-methyl]-4-X-Phenol};
X = Cl, Br or I; E = P or As have been synthesized by reacting [Ru(Cl)3(EPh3)3] (Where
E = P or As) with bidentate Schiff base ligand DMAPIMP in 1:1 molar ratio. The
complexes have been characterized by spectral (UV-Vis, IR and ESR), magnetic and
cyclic voltammetric studies. All complexes show strong metal to ligand charge transfer
(MLCT) transition in the visible region. The coordination of imine nitrogen and phenolic
oxygen of ligands to ruthenium metal was confirmed with the change in the IR
stretching frequency values. The three line e.s.r spectrum of RuIII complexes with gx ≠ gy
≠ gz indicates magnetic anisotropy and an asymmetry in the electronic environment
around the Ru atom. Based on the above datas, tetragonally distorted octahedral structure
has been confirmed for the new complexes. Cyclic voltammogram of the complexes in
nitrogen atmosphere show an irreversible RuIII/RuII reduction couple. The oxidation
potential observed in the range –0.567 to –0.667 V and reduction around –1.076 to
–1.296 V versus Ag/AgCl electrode. The peak-to-peak separation value (∆Ep) fell in the
range 505 to 650 mV suggesting that the irreversible metal centered electron transfer
79
process. The representative Schiff bases and their complexes were tested in vitro to their
antibacterial activity against Gram-Positive bacteria Staphylococcus aureus and Gram-
negative bacteria Proteus mirabilis. Further more, the DNA binding experiment of the
complex [Ru(Cl)2(DMAPIMP-Br)(AsPh3)2] (5) was carried out by UV-Vis absorption
spectral titration and the binding constant Kb = 2.9 ± 0.4 × 104 M−1 have been found.
Introduction
Molecules containing donor-acceptors such as Schiff bases are the subjects of
current interest. Due to the increasing potential as versatile catalysts for organic synthesis
and polymer chemistry, in particular polymeric ultraviolet stabilizers, as laser dyes and as
molecular switches in logic or memory circuits . Ru-complexes witnessed a spectacular
development during the last decade [1-10]. Several families of ruthenium compounds
have been prepared and extensively used in a variety of chemical transformations such as
hydrogenation [11], hydration [12], oxidation [5], epoxidation [13], isomerization [14],
decarbonylation [15], cyclopropanation [16], olefin metathesis [17], Diels-Alder reaction
[18], Kharasch addition [19], enol-ester synthesis [19], atom transfer radical
polymerization [20] and other related catalytic processes [21]. To achieve an appropriate
balance between the electronic and steric environment around the metal and in order to
control their activity, stability and chemoselectivity, many of these novel ruthenium
complexes have been endowed with specific ligands (eg. hydride, halide, hydrate,
carboxylate, phosphine, amine, oxygen or nitrogen chelating groups, Schiff bases, arenes,
carbenes, etc.) [22-27]. Some of the novel ruthenium complexes are chiral [28] or
immobilized on solid supports [29]. As result of their particular structure, these
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ruthenium complexes display an enhanced activity and selectivity in a multitude of
organic transformations [30]. Significantly, some of the above ligands impart to the
catalyst a good tolerance towards organic functionalities, air and moisture, in this way
widening the area of their application [31,32]. For several reasons, Schiff bases have been
found to be among the most convenient and attractive ligands for ruthenium complexes.
First, steric and electronic effects around the ‘Ru’ core can be finely tuned by an
appropriate selection of bulky and/or electron withdrawing or donating substituents
incorporated into the Schiff bases. Secondly, the two donor atoms, N and O, of the
chelated Schiff base exert two opposite electronic effects: the phenolate oxygen is a hard
donor known to stabilize the higher oxidation state of the ruthenium atom whereas the
imine nitrogen is a softer donor and, accordingly, will stabilize the lower oxidation state
of the ruthenium. Thirdly, Schiff bases are currently prepared in high yield through one-
step procedures via condensation of common aldehydes with amines, in practically
quantitative yields [33]. Taking into account the highly desirable attributes of this type of
ligands, vast families of bidentate, tridentate and tetradentate Schiff base-ligated
ruthenium complexes, of wide applicability as catalysts in numerous organic reactions,
have been designed and prepared. Some of them, e.g. tetradentate Ru–salen [34] and Ru–
porphyrin [35,36] complexes, boost good to excellent activity and remarkably high dia-
and enantioselectivity in catalyzing a variety of organic processes. The so-called
“dangling-ligands”, particularly those of salicylaldiminato-type, have been recently
introduced in arene, alkylidene, vinylidene and diene ruthenium complexes [37,38]. In
association with other commonly used ligands like chloride, phosphine, imidazol-2-
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ylidene, cyclodienes, they provided a novel class of ruthenium catalysts of versatile
application and utility in organic synthesis and polymer chemistry.
A number of metal chelates have been used as probes of DNA structures in
solution as agents for mediation of duplex DNA and as chemotherapeutic agents [39].
A large number of Schiff bases and their complexes are significant interest and
attention because of their biological activity including anti-tumor, antibacterial,
fungicidal and anti-carcinogenic properties [41,42].
Scope of the present work
Schiff base complexes of transition metals [42] having O and N donor atoms have
shown an exponential increase as inorganic catalysts for various organic transformations.
Among the second row transition metal ions, ruthenium mediated oxidations are finding
application due to the unique properties of this extremely versatile transition metal,
whose oxidation state can vary from _2 to +8 [43,44]. The transition metal
phosphine/arsine complexes, especially ruthenium complexes, find application in
classical catalytic processes such as hydrogenation, isomerisation, decarbonylation,
reductive elimination, oxidative addition and in making C-C bonds [45-49].
Unquestionably, transition metal based catalytic systems have established themselves as
the most employed ‘work-horses’ in modern oxidation chemistry for preparative
purposes. Studies pertaining the metal ion-DNA intercalation have been utilized for
developing novel chemotherapeutic agents, foot printing agents and for gene
manipulation in biotechnology and medicine. In addition, new kind of chemotherapeutic
82
Schiff bases are now attracting the attention of biochemists [50]. Earlier work reported
that some drugs showed increased activity when administered as metal complexes rather
than as organic compounds and their intercalations with DNA have been reported [51].
Chen et al [52] have proposed that intercalation, hydrogen bonding and p–p interactions
are some of the features implicated in the mode of action of ruthenium complexes as
antitumour and antimetastatic agents and some compounds are in advance stages of
preclinical studies. Although the mechanism of the action of antitumour-active ruthenium
compounds is not fully understood yet, it is thought that, similar to platinum drugs [53],
the chloride complexes can hydrolyze in vivo, allowing the Ru to bind to the nucleobases
of the DNA [54].
The crucial role of Schiff bases in the biological function of bacteriorhodopsin has
also been proven [55]. The retinal chromophore is bound covalently to the protein via a
protonated Schiff base [56].
In view of recent interest in the energetic of metal ligand binding in metal chelates
involving N, O donor ligands [57-60] we started to study the course of the reaction of
Schiff base 2-[(4-N,N’-dimethylaminophenylimino)-methyl]-4-X-Phenol with Ru(III),
and the products were characterized. The antibacterial screening of these complexes and
the free ligands against the bacterial species Staphylococcus aureus and Proteus
mirabilis as well as DNA binding properties have been reported.
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Experimental
The instruments employed for recording the UV-Vis, IR & NMR spectra and
Cyclic Votammetry are described in Chapter II.
Synthesis of Schiff base ligands
Synthesis of Schiff bases employed to prepare Ru(III) complexes have
been described in Chapter III (Scheme 3.1).
Synthesis of starting complexes
Starting complexes for the synthesis of the ruthenium(III) complexes were
prepared by the following reported procedure [61].
Synthesis of [RuCl3(EPh3)3] (E = P or As)
Solutions of commercial RuCl3.3H2O (1.0 mmol) in ethanol (20 ml) and con. HCl
(50 ml) were added quickly and successively to a boiling solution of triphenylphosphine
or triphenylarsine (6.0 mmol) in ethanol (50 ml). The resultant solution was heated under
reflux for 10 minutes and allowed to cool to room temperature. The reddish brown solid
that obtained was filtered and washed with ether and dried in vacuo.
Synthesis of Schiff base Ruthenium(III) complexes (Scheme 4.1)
A solution of [Ru(Cl)3(PPh3)3] (0.1 mmol, 99.342 mg) or [Ru(Cl)3(AsPh3)3]
(0.1 mmol, 112.542 mg) in C6H6 ( 20 cm3 ) and a solution of the appropriate Schiff base
(0.1 mmol, 27.4-36.6 mg) in CHCl3( 10 cm3 ) was added, colour change was noticed on
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the addition of the ligand due to the formation of complexes. The contents were refluxed
for 4-5h. The resulting solution was concentrated to small volume (3 cm3) on a rotary
evaporator and the product was separated by the addition of small amount of pet-ether
(60-80◦C). The compound that separated was filtered, washed with benzene followed by
ether, dried in vacuo over anhydrous CaCl2, then recrystallised from 1:2 (v:v)
chloroform-pet ether (60-80◦C) mixture.
85
Benzene
CN
N
HO
CN
N
1 : 1
[ Ru Cl3 (EPh3)3 ] +
( X = Cl / Br / I )
( E = P / As )
4 - 5 h
CH3
H
H3C
X
RuCl
Cl OEPh3
EPh3
X
H
H3C CH3
( E = P / As )
( X = Cl / Br / I )
Reflux
Scheme 4.1 Synthesis of Schiff base Ruthenium(III) complexes.
86
Antibacterial Screening
The in vitro antibacterial screening effects of the investigated compounds were
tested against the bacteria Staphylococcus aureus and Proteus mirabilis by well diffusion
method and followed the procedure described in chapter III.
Results and discussion
All the complexes are amorphous powder, insoluble in water and ether, but
soluble in solvents such as CHCl3, CH2Cl2, MeCN, DMF and DMSO.
Electronic spectra
The electronic absorption spectral bands of the complexes (Fig 4.1-4.4) were
recorded over the range 200-800 nm in MeCN together with tentative assignments [62]
(Table 4.1) are discussed in detail.
The low spin paramagnetic ruthenium(III), a d5 system with 2T2g as the ground
term and the first excited doublet levels in the order of increasing energy are 2A2g and
2T1g , which arise from 4T2g → 1Eg configuration.
All complexes exhibit three bands in the range 203-630 nm. The band between
616-630 nm is assigned to the d-d transition [63,64] (2T2g → 2A2g) [30]. The spectral
profiles below 400 nm are very similar and are ligand centered transitions. These bands
have been designated as π-π * and n-π * transitions of non-bonding electrons present on
the imine group in the Schiff base complexes [65,66].
87
Thus the band positions of electronic spectra of all the complexes are
characteristic and similar of those observed for other octahedral ruthenium(III)
complexes.
FT-IR spectra
The IR spectra (Table 4.2) of the free Schiff bases (Fig 3.9&3.10) were compared
with their respective ruthenium complexes (Fig 4.5-4.6) in order to determine the
coordination mode of the ligands [67]. The free ligands show a broad band of medium
intensity observed at ca. 3430-3464 cm-1 is assigned to υ (O–H) of the phenolic group. In
complexes the two intense bands centered at ca.1605-1619 cm-1 and 1355-1467 cm-1
assigned to υ (C=N) and υ (C–O) respectively [68]. The bands observed in the region
407-474 cm-1 has been assigned to υ (Ru–O) andυ (Ru–N) [63]. The disappearance of υ (O–H),
the downward shift of υ (C–O) and the lower frequency of υ (C=N) [69] on complexation
proves the bonding of ligands through imine nitrogen [66] and deprotonated phenolic
oxygen [70,71]. Also the characteristic bands due to triphenylphosphine/arsine were
observed in the expected regions [5].
ESR Spectra
The solid state e.s.r spectrum of ruthenium(III) complexes recorded at liquid
nitrogen temperature (77 K) shows a simple three line spectra indicating magnetic
anisotropy [5,72] in these systems and the complexes are in a low spin octahedral
geometry. The average ‘g’ values (gx, gy, gz) with gx ≠ gy ≠ gz lie in the 2.219-2.226 BM
88
range (Table 4.3) and these values fit very well with the values obtained for other similar
octahedral ruthenium (III) complexes [73-75].
Magnetic properties
The effective magnetic moments µeff (Table 4.3) lie in the range 1.916-1.923 also
suggesting that these complexes are one electron paramagnetic low spin 5t2g (s =1/2)
configuration of the ruthenium(III) ion in an octahedral environment which is similar to
reported O,N – coordinated Ru(III) complexes [65].
Cyclic Voltammetry
The redox properties of the mixed ligand complexes of ruthenium in MeCN were
studied by cyclic voltammetry under nitrogen atmosphere and the relevant
electrochemical data are given in Table 4.4. All Ru(III) complexes (Fig 4.7-4.10)exhibit
redox peak (E1/2 values) in the range – 0.826 to – 0.971 V has been assigned to metal
based reduction [Ru(III)/Ru(II)] with peak-to-peak separation value (∆Ep) ranging from
505 to 650 mV suggesting that an irreversible one electron transfer process. The presence
of such redox waves seems to be typical for ruthenium salicyliminato complexes
[65,68&76]. The Schiff bases, when co-ordinated through N and O, stabilise
ruthenium(III) rather than ruthenium(II). That is, the hard oxygen atom stabilises the
higher oxidation state of ruthenium and lower valencies are stabilised by π-acid ligands
like triphenyl phosphine and triphenyl arsine.
89
Antibacterial investigation
The Schiff base ligands and ruthenium(III) complexes were screened in vitro for
their microbial activity against two pathogenic bacterial species S. aureus and
P. mirabilis using the well diffusion method (Fig 4.11 & 4.12). The test solutions were
prepared in MeCN and the results are summarized in Table 4.5. Blank experiments with
RuCl3.3H2O and the Ru(III) precursors were carried out under identical experimental
conditions and show the inability of these complexes to inhibit the bacterial growth. The
effectiveness of an antimicrobial agent in sensitivity is based on the zones of inhibition.
The diameter of the zone is measured to the nearest millimeter (mm). These compounds
were found to exhibit moderate activity against both the organisms. The complexes are
more active than their parent ligands, which is consistent with earlier reports [68,77&78].
Such an increased activity for the metal chelates as compared to the free ligands can be
explained on the basis of Tweedy’s chelation theory [79]. The activity increases with
increase in concentration of test solution containing the new complexes. The different
compounds exhibit microbial activity with small variations against the bacterial species
and this difference in activity could be attributed to the impermeability of the cell of the
microbes or differences in the ribosomes of the microbial cells [80]. Although the
complexes are active, they did not reach the effectiveness of the conventional bactericide
ampicillin.
DNA binding Experiment
Electronic absorption spectroscopy is one of the most useful experimental
techniques for probing metal ion-DNA interactions. The electronic spectra of Ru(III)
90
complex [Ru(Cl)2(DMAPIMP-Br)(AsPh3)2] (5) was monitored in both the presence and
absence of Herring sperm DNA. The ultraviolet band centered at 262 nm arises from
inter ligand π-π* transition is prominent than the charge transfer band. The binding
constant for the interaction of complex (5) with DNA was obtained from absorption
titration data. A fixed concentration (5µM) of the complex (5) was titrated with
increasing amounts of HS DNA over a range of 0 — 60 µM. The binding constant was
determined using
[ DNA ] / ( εA − εf ) = [ DNA ] / ( εb − εf ) + 1 / Kb ( εb − εf )
Where εA, εf and εb correspond to Aobsd / [Ru], the extinction coefficient for the
free ruthenium complex (5) and the extinction coefficient for the complex in the fully
bound form respectively.
A plot (Fig 4.13) of [ DNA ] / ( εA − εf ) Vs [DNA] gives Kb as the ratio of the
slope to intercept and found to be 2.3 ± 0.4 × 104 M−1 which is lesser than the typical
intercators reported in the literature [81]. Generally, it has been shown that the
intercalators will show bathochromic shift and hypochromism. In the present case, with
the addition of DNA, hyperchromism accompanied by moderate red shift of 3 nm in the
absorbtivitty of interligand bands were observed. Such a small change in λmax is more in
keeping with groove binding, leading to small perturbations [82,83].
91
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Figure 4.1 Electronic spectra of [Ru (Cl)2 (DMAPIMP-Cl) (PPh3)2]
99
Figure 4.2 Electronic spectra of [Ru (Cl)2 (DMAPIMP-Br) (PPh3)2]
Figure 4.3 Electronic spectra of [Ru (Cl)2 (DMAPIMP-I) (PPh3)2]
100
Figure 4.4 Electronic spectra of [Ru (Cl)2 (DMAPIMP-Br) (AsPh3)2]
Figure 4.5 FT-IR spectra [Ru(Cl)2(PDMAAS-Br)(AsPh3)2]
101
Figure 4.6 FT-IR spectra [Ru(Cl)2(PDMAAS-I)(AsPh3)2]
Figure 4.7 Cyclic Voltammogram [Ru(Cl)2(PDMAAS-Cl)(PPh3)2]
102
Figure 4.8 Cyclic Voltammogram [Ru(Cl)2(PDMAAS- Br)(PPh3)2]
Figure 4.9 Cyclic Voltammogram [Ru(Cl)2(PDMAAS-Br)(AsPh3)2]
103
Figure 4.10 Cyclic Voltammogram [Ru(Cl)2(PDMAAS-I)(AsPh3)2]
104
Figure 4.11 Zone of inhibition of [Ru(Cl)2(DMAPIMP-Cl)(PPh3)2] against Staphylococcus aureus
Figure 4.12 Zone of inhibition of [Ru(Cl)2(DMAPIMP-I)(AsPh3)2] against Proteus mirabilis
105
Figure 4.13 Plot of [DNA]/(εa – εf) vs [DNA] for the absorption spectral titration of
DNA ( 10, 20, 30, 40, 50 & 60 µM ) with [Ru(Cl)2( DMAPIMP–Br)(AsPh3)2] ( 5 µM )
106
Table 4.1 Electronic spectral data
Complex
λmax* (nm)
(1) [Ru (Cl)2 (DMAPIMP-Cl) (PPh3)2] (2) [Ru (Cl)2 (DMAPIMP-Br) (PPh3)2] (3) [Ru (Cl)2 (DMAPIMP-I) (PPh3)2] (4) [Ru (Cl)2 (DMAPIMP-Cl) (AsPh3)2] (5) [Ru (Cl)2 (DMAPIMP-Br) (AsPh3)2] (6) [Ru (Cl)2 (DMAPIMP-I) (AsPh3)2]
212 a, 369 b, 630 c
213 a, 374 b, 621 c
203 a, 369 b, 618 c
214 a, 400 b, 616 c
223 a, 365 b, 619 c
211 a, 355 b, 618 c
* In acetonitrile
a π–π * transition
b n–π * transition
c d–d transition
107
Table 4.2 FT-IR spectral data (cm-1) of the ligands and RuIII complexes
Compound
υ (C=N) υ (C–O) υ (O–H) υ (Ru–O)
υ (Ru–N)
Bands for PPh3/AsPh3
DMAPIMP-Cl
DMAPIMP-Br
DMAPIMP-I
[Ru(Cl)2(DMAPIMP-Cl)(PPh3)2]
[Ru(Cl)2(DMAPIMP-Br)(PPh3)2]
[Ru(Cl)2(DMAPIMP-I)(PPh3)2]
[Ru(Cl)2(DMAPIMP-Cl)(AsPh3)2]
[Ru(Cl)2(DMAPIMP-Br)(AsPh3)2]
[Ru(Cl)2(DMAPIMP-I)(AsPh3)2]
1619
1619
1619
1611
1611
1611
1611
1611
1611
1369
1370
1355
1386
1361
1394
1440
1453
1440
3464
3454
3430
–
–
–
–
–
–
–
–
–
423
425
425
411
424
407
–
–
–
472
469
467
453
457
474
–
–
–
1428,1021,692
1434,1078,695
1434,1077,694
1433,1071,697
1435,1067,692
1434,1077,694
108
Table 4.3 ESR spectral and magnetic moment data of
Ruthenium(III) complexes
Complex
gx
gy
gz
<g>*
µeff
**
[Ru(Cl)2(DMAPIMP-Cl)(AsPh3)2] [Ru(Cl)2(DMAPIMP-Br)(AsPh3)2] [Ru(Cl)2(DMAPIMP-I)(AsPh3)2]
2.415
2.385
2.335
2.243
2.248
2.354
1.979
2.030
2.146
2.219
2.226
2.280
1.916
1.923
1.973
<g>*= [1/3gx2+1/3gy
2 +1/3gz2]1/2
**µ eff = gav √ s (s + 1)
109
Table 4.4 Electrochemical redox data of Ruthenium(III) complexes *
RuII/RuIII
(mV)
Potential (V)
Current (A)
Complex
Epa
Epc
∆Ep
E½
Ipa/Ipc
[Ru(Cl)2(DMAPIMP-Cl)(PPh3)2] [Ru(Cl)2(DMAPIMP-Br)(PPh3)2] [Ru(Cl)2(DMAPIMP-I)(PPh3)2] [Ru(Cl)2(DMAPIMP-Cl)(AsPh3)2] [Ru(Cl)2(DMAPIMP-Br)(AsPh3)2] [Ru(Cl)2(DMAPIMP-I)(AsPh3)2]
-0.591
-0.667
-0.576
-0.567
-0.646
-0.596
-1.147
-1.183
-1.076
-1.122
-1.296
-1.288
0.556
0.516
0.505
0.555
0.650
0.619
-0.869
-0.925
-0.826
-0.844
-0.971
-0.942
-0.20
-0.32
-0.98
-0.64
-0.20
-0.14
*Solvent – acetonitrile ; supporting electrolyte – [Bu4N]ClO4 (TBAP) 0.1M ; reference
electrode – SCE ; E½ = 0.5(Epa + Epc) where Epa and Epc are anodic and cathodic peak
potential respectively ; ∆Ep = Epa – Epc ; scan rate = 100 mVs-1
110
Table 4.5 Antibacterial activity data of Schiff base ligands and Ruthenium(III)
complexes
Diameter of inhibition zone (mm) Staphylococcus aureus Proteus mirabilis Compound
0.15% 0.2% 0.25%
0.15% 0.2% 0.25%
DMAPIMP-Cl
DMAPIMP-Br
DMAPIMP-I
[Ru(Cl)2(DMAPIMP-Cl)(PPh3)2]
[Ru(Cl)2(DMAPIMP-Br)(PPh3)2]
[Ru(Cl)2(DMAPIMP-I)(PPh3)2]
[Ru(Cl)2(DMAPIMP-Cl)(AsPh3)2]
[Ru(Cl)2(DMAPIMP-Br)(AsPh3)2]
[Ru(Cl)2(DMAPIMP-I)(AsPh3)2]
Control ( Acetonitrile )
Standard ( Ampicillin )
─ ─ ─ ─ ─ ─
─ ─ ─ ─ ─ ─
─ ─ ─ ─ ─ ─
10 12 13 15 15 16
9 10 13 14 15 16
10 11 12 13 14 16
9 12 12 16 16 17
8 9 10 14 15 17
10 12 12 16 17 18
─ ─ ─ ─ ─ ─
30 32 34 22 23 24
Symbol “─” denotes no activity.
111
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