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

80

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-

81

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.

83

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

84

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

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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