arsine as co-ligands

Spectrochimica Acta Part A 74 (2009) 591–596

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Synthesis, spectral characterization, catalytic and antibacterial studies of new Ru(III) Schiff base complexes containing chloride/bromide and triphenylphosphine/arsine as co-ligands S. Arunachalam, N. Padma Priya, C. Jayabalakrishnan, V. Chinnusamy ∗ Post Graduate and Research Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, India

a r t i c l e

i n f o

Article history: Received 6 September 2008 Received in revised form 22 June 2009 Accepted 30 June 2009 Keywords: Schiff bases 1,4-Diformylbenzene Electrochemical Catalytic oxidation Antibacterial

a b s t r a c t A new Ru(III) Schiff base complexes of the type [RuX(EPh3 )L] (X = Cl/Br; E = P/As; L = dianion of the Schiff bases were derived by the condensation of 1,4-diformylbenzene with o-aminobenzoic acid/oaminophenol/o-aminothiophenol in the 1:2 stoichiometric ratio) have been synthesized from the reactions of [RuX3 (EPh3 )3 ] with appropriate Schiff base ligands in benzene in the 2:1 stoichiometric ratio. The new complexes have been characterized by analytical, spectral (IR, electronic, 1 H, 13 C NMR and ESR), magnetic moment and electrochemical studies. An octahedral structure has been tentatively proposed for all these new complexes. All the new complexes have been found to be better catalyst for the oxidation of alcohols using molecular oxygen as co-oxidant at ambient temperature and aryl–aryl coupling reactions. These complexes were also subjected to antibacterial activity studies against Escherichia coli, Aeromonas hydrophilla and Salmonella typhi. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Schiff base complexes of transition metals containing ligands with N, S or N, S, O donors are known to exhibit interesting stereochemical, electrochemical and electronic properties [1–3]. This attention is still growing, so that a considerable research effort is today devoted to the synthesis of new Schiff base complexes with transition metal ions, to further develop applications in the area of material and pharmaceutical chemistry [4–6]. In particular, triphenylphosphine/arsine complexes of ruthenium [7–9] have been employed as catalysts for various organic transformations such as oxidation [10], hydrogenation [11], C–C couplings [12], hydroformylation [13], isomerization [14], polymerization [15], racemization [16], etc. Among these, catalytic oxidation of alcohols to carbonyl compounds is a pivotal reaction due to their utility in fine chemicals and pharmaceutical industries. Ruthenium complexes are known to mediate oxidation of alcohol using variety of co-oxidants such as PhIO [17], NMO [18], BrO3 – [19], 2SO8 − [20], t-BuOOH [21], TEMPO [9] and O2 or air [22]. In this paper, we discuss the synthesis, spectral characterization, redox behaviour, catalytic and antibacterial activities of new complexes of ruthenium(III) containing tetradentate Schiff base ligands. The Schiff bases were derived

∗ Corresponding author. Tel.: +91 422 2692461. E-mail addresses: [email protected], [email protected] (V. Chinnusamy). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.06.061

by the condensation of 1,4-diformylbenzene with o-aminobenzoic acid/o-aminophenol/o-aminothiophenol in the 1:2 stoichiometric ratio. The general structure of the Schiff base ligands are given in Scheme 1. 2. Experimental 2.1. Physical measurements All the reagents used were of analar grade. RuCl3 ·3H2 O was purchased from Loba Chemie and was used without further purification. The starting complexes [RuCl3 (PPh3 )3 ] [23], [RuCl3 (AsPh3 )3 ] [24], [RuBr3 (PPh3 )3 ], [RuBr3 (AsPh3 )3 ] [25] were prepared by reported methods. Catalytic oxidation [26] and aryl–aryl coupling [27] reactions have been carried out by reported literature methods. The C, H, and N analysis were performed using CarloEraba 1106 instrument. IR spectra were recorded in KBr pellets in the region 400–4000 cm−1 using Shimadzu instrument. 1 H and 13 C NMR spectra for the ligands were recorded in Indian Institute of Science, Bangalore. Electronic spectra were recorded in CH2 Cl2 with a Systronics Double beam UV–Vis Spectrophotometer2202 in the range 200–800 nm. The X-band EPR spectra of the powdered samples were recorded on JEOL JESFA200 EPR spectrometer using diphenylpicryl hydrazyl as reference. Electrochemical studies were recorded in acetonitrile solution using a glassy carbon working electrode and [NBu4 ]ClO4 was used as supporting electrolyte. Catalytic and antimicrobial studies were carried out.

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(Scheme 2). All the complexes are soluble in most of the common organic solvents. Their purity was checked by TLC on silica gel. The analytical data obtained for the new complexes agree well with the proposed molecular formulae (Table 1). In all of the above reactions, the Schiff bases behave as binegative tetradentate ligands. Magnetic measurements show that all the complexes are paramagnetic which confirms that ruthenium is in +3 oxidation state. 3.1. Spectroscopic studies

Scheme 1. General structure of the new Schiff base ligands.

Melting points were recorded on a Raaga apparatus and are uncorrected. 2.2. Recommended procedures 2.2.1. Synthesis of new Schiff base ligands The Schiff base ligands were derived by condensing 1,4-diformylbenzene (0.1 mmol) with o-aminobenzoicacid (0.2 mmol)/oaminophenol (0.2 mmol)/o-aminothiophenol (0.2 mmol) in 1:2 molar ratio in ethanolic medium and refluxed for 6 h. The resultant product was washed with ethanol and the purity of the ligands were checked by TLC. 2.2.2. Synthesis of new Ru(III) Schiff base complexes All the new complexes were prepared by the following general procedure described below (Scheme 2). To a solution of [RuX3 (EPh3 )3 ] (0.2 mmol) in benzene (20 cm3 ) the appropriate Schiff base (0.1 mmol) was added in 1:1 molar ratio and heated under reflux for 6 h. The resulting solution was then concentrated to 3 cm3 and cooled. The complex was precipitated by the addition of small quantity of petroleum ether (60–80 ◦ C), recrystallized from CH2 Cl2 /petroleum ether and dried in vacuo. 3. Results and discussion Stable ruthenium(III) Schiff base complexes of the general formula [RuX(EPh3 )(L)] (X = Cl/Br; E = P/As; L = dianion tetradentate Schiff base) have been prepared by reacting [RuX3 (EPh3 )3 ] (E = P/As) with the respective Schiff bases in a 1:1 molar ratio in benzene

3.1.1. Infrared spectral analysis The IR spectra of the ligands were compared with those of the ruthenium complexes in order to confirm the binding mode of the Schiff base ligands to the ruthenium ion in the complexes. The free Schiff base ligands showed a strong band in the region 1603–1627 cm−1 , which is the characteristic frequency of the azomethine (C N) group [28–30]. In all the complexes, the (C N) band is shifted to lower frequency 1595–1605 cm−1 , indicating coordination of the Schiff bases through the azomethine nitrogen atom. For the anthranilic acid moiety, the (OH) absorption observed at 3300 cm−1 in the free Schiff base ligand H2 L1 and the (C O) frequency of the carbonyl was seen as a band at 1680 cm−1 and also shows the absorption band at 1648 cm−1 and 1491 cm−1 for asymmetric (asyCOO− ) and symmetric (syCOO− ) stretching of the carboxylato group. In the

complexes the bands were observed in the 1615–1665 cm−1 and 1409–1430 cm−1 regions arising from asymmetric (asyCOO− ) and symmetric (syCOO− ) stretching of the carboxylato group [29]. This indicates the coordination of the carboxyl group to the ruthenium metal ion in the complexes [RuCl(PPh3 )(L1 )], [RuCl(AsPh3 )(L1 )], [RuBr(PPh3 )(L1 )] and [RuBr(AsPh3 )(L1 )]. The differences between the asymmetric and symmetric stretching frequencies of the coordinated carboxyl group lie in the 206–235 cm−1 range, a clear indication of the monodentate coordination of the carboxyl group with the free carbonyl group [29,31]. A medium intensity band which appeared at 3000 cm−1 due to (OH) in the free Schiff base ligand H2 L2 disappeared upon complexation showing deprotonation prior to coordination through the oxygen atom in the complexes [RuCl(PPh3 )(L2 )], [RuCl(AsPh3 )(L2 )], [RuBr(PPh3 )(L2 )] and [RuBr(AsPh3 )(L2 )]. A band that appeared at 1355 cm−1 due to phenolic C–O stretching in the free Schiff base ligand H2 L2 has been shifted to higher frequency in the range 1385–1430 cm−1 indicating the coordination through the phenolic oxygen atom in the complexes [32–34]. The band corresponding to thiophenolic S–H also disappears in the complexes containing H2 L3 ligand. Moreover the absorption due to (C–S) of H2 L3 at 1240 cm−1 is shifted to 1254–1269 cm−1 in the these com-

Scheme 2. Preparation of new Ru(III) Schiff base complexes.

S. Arunachalam et al. / Spectrochimica Acta Part A 74 (2009) 591–596

593

Table 1 Elemental, IR and UV–vis spectral data of ruthenium(III) tetradentate Schiff base complexes. Complexes

Melting point (◦ C)

H2 L1 H2 L2 H2 L3 [RuCl(PPh3 )(L1 )] [RuCl(PPh3 )(L2 )] [RuCl(PPh3 )(L3 )] [RuCl(AsPh3 )(L1 )] [RuCl(AsPh3 )(L2 )] [RuCl(AsPh3 )(L3 )] [RuBr(AsPh3 )(L1 )] [RuBr(AsPh3 )(L2 )] [RuBr(AsPh3 )(L3 )] [RuBr(PPh3 )(L1 )] [RuBr(PPh3 )(L2 )] [RuBr(PPh3 )(L3 )]

121 146 129 211 247 187 218 198 157 219 182 151 179 227 203

FT-IR (cm−1 )

Found (calculated) (%) C

H

N

S

C

70.9 (69.8) 75.9 (74.8) 68.9 (68.7) 62.4 (62.3) 63.9 (62.7) 31.2 (60.9) 59.1 (58.9) 60.2 (59.7) 57.8 (57.0) 55.9 (55.2) 56.9 (56.1) 54.7 (53.9) 59.1 (58.8) 60.3 (59.2) 57.8 (57.2)

4.3 (4.1) 5.1 (4.9) 4.6 (4.3) 3.7 (3.6) 4.1 (4.0) 3.9 (3.7) 3.59 (3.51) 3.8 (3.58) 3.7 (3.61) 3.4 (3.31) 3.6 (3.48) 3.5 (3.29) 3.6 (3.51) 3.9 (3.71) 3.7 (3.58)

7.5 (7.2) 8.8 (8.6) 8.0 (7.9) 3.6 (3.4) 3.92 (3.9) 3.7 (3.6) 3.4 (3.1) 3.69 (3.61) 3.5 (3.5) 3.3 (3.1) 3.5 (3.3) 3.4 (3.1) 3.4 (3.1) 3.7 (3.5) 3.5 (3.4)

– – 18.4 (18.2) – – 8.6 (8.5) – – 8.13 (8.0) – – 7.6 (7.4) – – 8.1 (8.07)

1603 1614 1627 1595 1598 1602 1605 1598 1595 1598 1595 1602 1605 1598 1595

plexes [RuCl(PPh3 )(L3 )], [RuCl(AsPh3 )(L3 )], [RuBr(PPh3 )(L3 )] and [RuBr(AsPh3 )(L3 )], indicating that the another coordination site is a thiophenolic sulfur atom [3]. The characteristic bands due to triphenylphosphine/arsine were observed in the expected regions.

3.1.2. Electronic spectral analysis The electronic spectra of all the ligands and complexes in dichloromethane showed three bands in the 275–559 nm regions (Table 2). The electronic spectra of all the free ligands showed two types of transitions, the first one appeared at range 275–291 nm which can be assigned to ␲–␲* transition due to transitions involving molecular orbitals located on the phenolic, thiophenolic and carboxylic chromophore. These peaks have been shifted in the spectra of the complexes. This shifting may be due to the donation of a lone pair of electrons from the oxygen of the phenolic and carboxylic and thiophenolic sulfur group to the central metal atom, respectively. This reveals that one of the coordination site is oxygen of the phenolic and carboxylic and sulfur of the thiophenolic groups, respectively. The second type of transitions appeared at range 352–559 nm assigned to n → ␲* transition due to azomethine groups and benzene ring of the ligands. These bands have also been shifted in the spectra of the new complexes indicating the involvement of imine group nitrogens in coordination with central metal atom. The spectra of the all the complexes showed another types of transitions which is different from the free ligands. All the complexes showed absorption in the range 248–472 nm which can be assigned to ligand to metal charge transfer followed by intra-ligand transitions, respectively. The other type of bands in the visible

N

max (nm)

asyCOO−

syCOO−

C–O−

S–O−

1648 – – 1615 – – 1640 – – 1665 – – 1650 – –

1491 – – 1409 – – 1415 – – 1429 – – 1430 – –

– 1355 – – 1385 – – 1397 – – 1421 – – 1430 –

– – 1240 – – 1254 – – 1259 – – 1265 – – 1269

291,358,476 275,379,559 287,352,473 333,602 288,554,466 351,336 248,333,436,614 288,549,462 332,452 312,472 300,468,574 333,366 338,426 332,461,572 330,346,368

region 572–614 nm can be attributed to d–d transitions involving the metal orbitals [35]. 3.1.3. 1 H and 13 C NMR spectra of the Schiff base ligands The formation of Schiff base ligands were conveniently monitored by peak ratios in the 1 H and 13 C NMR spectra of all the ligands were taken in DMSO·d6 solvent. The aromatic region is a set of multiplets in the range 6.8–8.2 ppm for all the Schiff base ligands. The carboxylic OH proton of the H2 L1 ligand appears as a singlet at 10 ppm while, the Ph-OH proton of the ligand H2 L2 appears as a singlet at 9.1 ppm and the Ph-SH proton of the ligand H2 L3 appears as a singlet at 3.4 ppm. The 1 H NMR spectra of the CH proton appears as a singlet in the region 8.1–8.31 ppm for all the Schiff base ligands and a representative figure is depicted (Fig. 1). The 13 C NMR spectra of the ligands have been recorded and the spectrum is shown in Fig. 2. The chemical shifts for the carbon atoms of the phenyl rings were recorded in the region 115–139 ppm. In all the ligands –CH N carbon appears in the range 154–155 ppm. 3.1.4. EPR spectral analysis The solid-state EPR spectra of powdered samples of all the complexes were recorded at room temperature and the ‘g’ values are listed in Table 2 and depicted in Fig. 3. The EPR spectra of ruthenium(III) complexes [RuCl(PPh3 )(L2 )] (Fig. 1), [RuCl(AsPh3 )(L2 )] and [RuBr(AsPh3 )(L3 )] in room temperature exhibited three lines with different ‘g’ values, indicating the presence of magnetic anisotropy. The presence of three ‘g’ values is indicative of a rhombic distortion in these complexes [36]. The complexes

Table 2 EPR, magnetic moment and electrochemical data of new ruthenium(III) Schiff base complexes. Complexes

RuIII /RuII Epa (V)

Epc (V)

Ep (V)

E1/2 (V)

[RuCl(PPh3 )(L1 )] [RuCl(PPh3 )(L1 )] LNT [RuCl(PPh3 )(L2 )] [RuCl(PPh3 )(L3 )] [RuCl(AsPh3 )(L1 )] [RuCl(AsPh3 )(L2 )] [RuCl(AsPh3 )(L3 )] [RuBr(AsPh3 )(L1 )] [RuBr(AsPh3 )(L2 )] [RuBr(AsPh3 )(L3 )] [RuBr(PPh3 )(L1 )] [RuBr(PPh3 )(L2 )] [RuBr(PPh3 )(L3 )]

−1091 – −1255 −1326 −1292 −564 −1165 −1108 −387 −1182 −844 −1173 −1216

−481 – −546 −556 −581 −277 −539 −489 −167 −513 −522 −649 −522

610 – 709 770 711 287 626 619 220 669 322 524 694

0.786 – 0.9 0.941 0.936 0.42 0.852 0.798 0.277 0.847 0.683 0.911 0.869

Supporting electrolyte [NBu4 ]ClO4 (0.1 M); Ep = Epa − Epc ; ga = [1/3gx2 + 1/3gy2 + 1/3gz2 ]

1/2

.

gx

gy

gz

ga

eff (BM)

1.58 1.67 0.96 1.64 1.63 1.67 0.93 1.61 1.70 0.92 1.70 0.90 1.70

1.58 1.67 1.37 1.64 1.63 1.37 0.93 1.61 1.70 1.60 1.79 0.90 –

1.78 1.87 1.73 1.93 1.72 1.73 1.64 1.81 1.90 1.80 1.90 1.53 –

1.64 1.74 1.62 1.74 1.66 1.60 1.22 1.68 1.77 1.77 1.76 1.31 0.98

1.71 – – 1.76 – – 1.74 – – 1.72 – –

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Fig. 1.

1

H NMR spectrum of the Schiff base ligand H2 L2 .

Fig. 4. Cyclic voltammogram of the complex [RuCl(PPh3 )L2 ].

[RuCl(PPh3 )(L1 )], [RuCl(PPh3 )(L3 )], [RuCl(AsPh3 )(L3 )], [RuBr(AsPh3 )(L1 )], [RuBr(AsPh3 )(L2 )], [RuBr(AsPh3 )(L3 )], [RuBr(PPh3 )(L1 )] / gz . The and [RuBr(PPh3 )(L2 )] exhibited the spectra with gx = gy = two different ‘g’ values (gx = gy = / gz ) are indicative of a tetragonal distortion in these octahedral complexes [37]. Moreover, the complex [RuBr(PPh3 )(L3 )], exhibit a single isotropic resonance with ‘g’ value at 0.98, indicating a very high symmetry around the ruthenium ions. Such isotropic lines are usually observed either due to the intermolecular spin exchange which can broaden the lines or due to occupancy of the unpaired electrons in a degenerate orbital. The spectrum of the complex [RuCl(PPh3 )(L1 )] at LNT show improved resolution with the ‘g’ value and there is no much variation observed when compared with that observed that of RT. In addition, the nature and position of the lines in the spectra of these complexes are similar to those of the other octahedral complexes [38].

Fig. 2.

13

C NMR spectrum of the Schiff base ligand H2 L2 .

3.1.5. Magnetic moments The magnetic moments for a few complexes have been measured at room temperature using a vibration sample magnetometer. The values obtained lie in the 1.71–1.79 BM range corresponding to one unpaired electron, suggesting a low spin t5 configuration for ruthenium(III) ion (Table 2). 3.1.6. Electrochemistry Complexes were electrochemically examined at a glassy carbon working electrode in acetonitrile solution. A representative voltammogram has been depicted in Fig. 4 and the potential data are listed in Table 2. All the complexes display irreversible oxidation potential (Ru(IV) –Ru(III) ). In all the complexes, the Ru(III)–Ru(II) redox couple is quasi-reversible in nature, with a peak-to-peak separation (Ep ) of 220–770 mV [39]. Coordination of the sulfur atom makes the metal center more electron-rich and shifts the oxidation potential towards more negative values [40]. 3.2. Catalytic studies

Fig. 3. EPR specta of the complex [RuCl(PPh3 )(L1 )] at (a) RT and (b) LNT.

3.2.1. Catalytic oxidation The oxidation of benzyl alcohol, cyclohexanol, cinnamyl alcohol, isopropyl alcohol and butan-2-ol was carried out with the different ruthenium complexes as catalysts using molecular oxygen as co-oxidant at ambient temperature in dichloromethane. In no case was there any detectable oxidation of alcohols in the presence of molecular oxygen alone without the ruthenium complexes. The results of the catalytic oxidation by ruthenium(III) com-

S. Arunachalam et al. / Spectrochimica Acta Part A 74 (2009) 591–596

plexes are summarized in Table 3. Benzaldehyde was formed from benzyl alcohol, cyclohexanol was converted into cyclohexanone, cinnamyl alcohol was converted cinnamaldehyde and butan-2-ol was converted to butan-2-al after 3 h of stirring at room temperature. The aldehyde/ketone formed were quantified as their 2,4-dinitrophenylhydrazone derivatives. All the synthesized ruthenium complexes were found to catalyse the oxidation of alcohols to aldehydes/ketones, but the yields and the turnover were found to vary with different catalysts. The relatively higher product yield obtained for the oxidation of benzyl alcohol than for cyclohexanol was due to the fact that the ␣-CH moiety of benzyl alcohol Table 3 Catalytic oxidations of alcohols and aryl–aryl coupling of Ru(III) Schiff base complexes. Complexes

Oxidation of alcohols

Aryl–aryl coupling

Substrate

Product

Yielda

Turnover

Yield (g)

Yield (%)

[RuCl(PPh3 )(L )]

A B C D

E F G H

58 68 71 72

59 69 71 74

0.592

61.47

[RuCl(PPh3 )(L2 )]

A B C D

E F G H

56 67 74 77

57 67 75 77

0.589

61.20

[RuCl(PPh3 )(L3 )]

A B C D

E F G H

54 67 75 79

56 67 75 79

0.601

62.44

[RuCl(AsPh3 )(L1 )]

A B C D

E F G H

50 61 66 70

51 63 67 72

0.595

61.82

[RuCl(AsPh3 )(L2 )]

A B C D

E F G H

54 62 68 74

54 63 68 76

0.554

57.56

[RuCl(AsPh3 )(L3 )]

A B C D

E F G H

53 64 67 74

54 65 67 76

0.549

57.04

[RuBr(PPh3 )(L1 )]

A B C D

E F G H

52 65 70 73

54 66 71 74

0.589

61.20

[RuBr(PPh3 )(L2 )]

A B C D

E F G H

52 63 73 76

53 65 75 76

0.596

61.92

1

[RuBr(PPh3 )(L3 )]

595

is more acidic than that of cyclohexanol [39]. The relatively higher product yield obtained for the oxidation of cinamyl alcohol than benzyl alcohol is due to the presence of unsaturated aliphatic group in the cinnamyl alcohol [5]. It has also been found that triphenylphosphine complexes possess higher catalytic activity than the triphenylarsine complexes [3,40]. This may be due to the higher donor ability of the arsine ligand compared with that of phosphine [33]. 3.2.2. Aryl–aryl coupling Magnesium turnings (0.320 g) were placed in a two-necked round-bottomed flask with a CaCl2 guard tube. A crystal of iodine was added. PhBr [0.75 cm3 of total 1.88 cm3 ] in anhydrous Et2 O (5 cm3 ) was added with stirring and the mixture was heated under reflux. The remaining PhBr in Et2 O (5 cm3 ) was added drop wise and the mixture was refluxed for 40 min. To this mixture, 1.03 cm3 (0.01 mol) of PhBr in anhydrous Et2 O (5 cm3 ) and the ruthenium complex (0.05 mmol) chosen for investigation were added and heated under reflux for 6 h. The reaction mixture was cooled and hydrolysed with a saturated solution of aqueous NH4 Cl and the precipitated biphenyl was chromatographed to get pure sample and compared well with an authentic sample (69–72 ◦ C) [41]. 3.3. Antibacterial activity studies Three pathogenic microbials were used to test the biological potential of the Schiff bases and their ruthenium(III) complexes. They were (i) Escherichia coli, (ii) Aeromonas hydrophilla and (iii) Salmonella typhi. The antibacterial activity of the compound was determined by disc diffusion method [42]. The results showed (Table 4) that the ruthenium chelates are more toxic compared to their parent ligands against the same microorganisms under identical conditions. The toxicity of ruthenium chelates increases on increasing the concentration [6]. The increase in the antibacterial activity of metal chelates may be due to the effect of the metal ion on the normal cell process. A possible mode of the toxicity increase may be considered in light of Tweedy’s chelation theory [43]. Chelation considerably reduces the polarity of the metal ion because of partial sharing of its positive charge with the donor groups and possible ␲-electron delocalization over the whole chelate ring. Such chelation could enhance the lipophilic character of central metal atom, which subsequently favours its permeation through the lipid layers of cell membrane. Furthermore the mode of action of the Table 4 Antibacterial activity data of ligands and Ru(III) complexes. Compound

A B C D

E F G H

54 66 75 79

56 67 77 80

0.606

[RuBr(AsPh3 )(L1 )]

A B C D

E F G H

50 61 66 70

50 63 68 70

0.546

56.73

[RuBr(AsPh3 )(L2 )]

A B C D

E F G H

54 62 68 74

54 64 69 75

0.556

57.77

[RuBr(AsPh3 )(L3 )]

A B C D

E F G H

53 64 67 74

55 66 69 75

0.576

59.85

62.97

A: cyclohexanol; B: benzyl alcohol; C cinnamyl alcohol; D: butan-2-ol; E: cyclohexanone; F: benzaldehyde; G: cinnamaldehyde; H: butan-2-al. a Moles oE product per mole oE catalyst.

Diameter of inhibition zone (mm) Escherichia coli

Aeromonas hydrophilla

Salmonella typhi

0.25%

0.5%

1%

0.25%

0.5%

1%

0.25%

0.5%

1%

L1 [RuCl(PPh3 )(L1 )] [RuCl(AsPh3 )(L1 )] [RuBr(PPh3 )(L1 )] [RuBr(AsPh3 )(L1 )]

7 11 10 10 12

7 12 11 12 12

9 12 11 12 14

8 10 12 11 11

9 12 11 11 13

11 10 13 12 13

9 13 13 11 13

10 14 14 11 13

10 13 14 11 14

L2 [RuCl(PPh3 )(L2 )] [RuCl(AsPh3 )(L2 )] [RuBr(PPh3 )(L2 )] [RuBr(AsPh3 )(L2 )]

8 11 12 10 13

9 10 11 11 14

9 11 11 11 14

8 10 10 11 12

9 12 12 11 13

8 13 12 12 13

10 12 11 12 14

11 11 13 12 14

11 13 12 14 14

L3 [RuCl(PPh3 )(L3 )] [RuCl(AsPh3 )(L3 )] [RuBr(PPh3 )(L3 )] [RuBr(AsPh3 )(L3 )]

11 15 14 10 14

12 17 15 11 14

12 17 15 11 15

10 14 17 12 12

10 16 16 12 14

11 15 16 13 14

12 16 13 14 13

12 15 13 15 13

13 15 14 15 13

Streptomycin

22

23

28

21

23

27

29

21

25

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S. Arunachalam et al. / Spectrochimica Acta Part A 74 (2009) 591–596

Scheme 3. General structure of Ru(III) Schiff base complexes.

compounds may involve formation of a hydrogen bond through the azomethine (>C N) group with the cell constituents, resulting in interference with the normal cell processes [44]. Though the complexes possess some significant activity, it could not reach the effectiveness of the standard drug Streptomycin. The variation in the effectiveness of the different compounds against different organisms depends either on the impermeability of the cells of the microbes or differences in ribosomes of microbial cells [45,46]. 4. Conclusion The present study describes a simplistic synthesis of a new series of octahedral Ru(III) Schiff base complexes containing triphenylphosphine/triphenylarsine and chloride/bromide as coligands. An octahedral geometry (Scheme 3) has been proposed to all the complexes on the basis of analytical, IR, electronic, EPR, magnetic moment and electrochemistry spectral data. All the ruthenium(III) Schiff base complexes have been found as efficient catalysts for the oxidation of alcohols to their corresponding carbonyl compounds by molecular oxygen at ambient temperature and also for aryl–aryl coupling reactions. Further, the new complexes have been subjected to biocidal studies and the possible explanations for the mode of actions of these complexes against three different microbes are described. References [1] K.P. Balasubramanian, R. Karvembu, R. Prabhakaran, V. Chinnusamy, K. Natarajan, Spectrochim. Acta Part A 68 (2007) 50. [2] R. Prabhakaran, R. Karvembu, T. Hashimoto, K. Shimizu, K. Natarajan, Inorg. Chim. Acta 358 (2005) 2093. [3] S. Priyarega, R. Prabhakaran, K.R. Aranganayagam, R. Karvembu, K. Natarajan, Appl. Organomet. Chem. 21 (2007) 788.

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