S donor ligands

Spectrochimica Acta Part A 65 (2006) 678–683

Synthesis, characterization, electro chemistry, catalytic and biological activities of ruthenium(III) complexes with bidentate N, O/S donor ligands K.P. Balasubramanian a , K. Parameswari a , V. Chinnusamy a , R. Prabhakaran b , K. Natarajan b,∗ a

Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641 020, India b Department of Chemistry, Bharathiar University, Coimbatore 641 046, India Received 3 August 2005; accepted 17 December 2005

Abstract New hexa-coordinated ruthenium(III) complexes of the type [RuX2 (EPh3 )2 (L)] (E = P or As; X = Cl or Br; L = monobasic bidentate Schiff base derived from the condensation of benzhydrazide with furfuraldehyde, 2-acetylfuran and 2-acetylthiophene) have been synthesized from the equimolar amounts of [RuX3 (EPh3 )3 ] or [RuBr3 (PPh3 )2 (MeOH)] and Schiff bases in benzene. The new complexes have been characterized by analytical, spectral (IR, electronic and EPR), magnetic moment, and cyclic voltammetry. An octahedral structure has been tentatively proposed. All the complexes have exhibited catalytic activity for the oxidation of benzyl alcohol, cyclohexanol and cinnamylalcohol in the presence of N-methylmorpholine-N-oxide as co-oxidant. All the new complexes were found to be active against the bacteria such as E. coli, Pseudomonas, Salmonella typhi and Staphylococcus aureus. The activity was compared with standard Streptomycin. © 2005 Elsevier B.V. All rights reserved. Keywords: Ruthenium(III) complexes; EPR; Cyclic voltammetry; Catalytic activity; Antibacterial studies

1. Introduction In the recent years, there has been considerable interest in the chemistry of transition metal complexes containing Schiff bases, mainly due to the fact that they offering opportunities for inducing substrate chirality, turning metal centered electronic factor, enhancing stability of either homogeneous or heterogeneous catalyst [1–9]. The interest in the study of compounds containing sulphur and nitrogen donor atoms arises from their significant antifungal [10], antibacterial, anticancer and catalytic activities [11–15]. Moreover, it is well known that some drugs have increased activity when administered as metal complexes [16,17] and their interaction with DNA have been reported [18]. Several metal chelates have also been shown to inhibit tumor growth [19]. The recent reports have been shown that the ruthenium based complexes are not only having catalytic activity and also very good medicinal properties [20–25]. The biological

∗

Corresponding author. Tel.: +91 422 2424655; fax: +91 422 2422387. E-mail address: k [email protected] (K. Natarajan).

1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.12.029

activities of chemical and industrial versatility of metal-Schiff base hydrazones of pyidoxal phosphate and its analogs have been studied [26,27]. Having these all in our mind, we have interested to prepare a single complex having very good catalytic and medicinal properties. Here, we disclose the synthesis, spectral characterization, electrochemistry, catalytic activity and biological activities of ruthenium(III) Schiff base complexes containing triphenylphosphine/arsine. The general structure of Schiff base ligands used in the present work is given in Fig. 1. 2. Experimental 2.1. Materials and methods All the reagents used were of analar or chemically pure grade. Solvents were purified and dried according to the standard procedures [28]. RuCl3 ·3H2 O purchased from Loba chemie, was used without further purification. The analyses of carbon, hydrogen and nitrogen were performed at the Central Drug Research Institute, Lucknow, India. IR spectra of the complexes were recorded in KBr pellets with a Shimadzu 8000 FT-IR spectrophotome-

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2.2. Recommended procedures 2.2.1. Synthesis of new ruthenium(III) complexes All the new complexes were prepared by following general procedure. To a solution of [RuCl3 (PPh3 )3 ], [RuCl3 (AsPh3 )3 ], [RuBr3 (AsPh3 )3 ] or [RuBr3 (PPh3 )2 (MeOH)] (0.099–0.125 g, 0.1 mmol) in benzene (25 cm3 ) the appropriate Schiff base ligands (0.1 mmol) were added. The mixture was then heated under reflux for 6 h. The resulting solution was concentrated to ca. 3 cm3 and cooled. Light petroleum (60–80 ◦ C) was then added, whereupon the product complex separated. The solid was filtered off, washed and recrystalized from CH2 Cl2 /light petroleum (60–80 ◦ C) and dried in vacuo. 3. Results and discussion

Fig. 1. General structure of the ligands.

ter in the 4000–400 cm−1 range. The electronic spectra were recorded in CH2 C12 solution with Perkin-Elmer 20/200 spectrophotometer in the 800–200 nm range. EPR spectra of the powdered samples were recorded on a Bruker E-112 Varian model instrument in X-band frequencies at room temperature. Magnetic susceptibilities were recorded on EG and G-PARC vibrating sample magnetometer. The cyclic voltammetric studies were carried out with BAS CV-27 model electro chemical analyzer in acetonitrile solution using a glassy carbon working electrode and the potentials were referenced to saturated calomel electrode. The starting complexes [RuCl3 (PPh3 )3 ] [29], [RuCl3 (AsPh3 )3 ] [30], [RuBr3 (AsPh3 )3 ] [31], [RuBr3 (PPh3 )2 (MeOH)] [32], and the ligands [33–36] were prepared according to the reported procedures. The procedure for catalytic activity and antibacterial study are similar to that reported in our earlier publication [37].

The air and light stable complexes of general formula [RuX2 (EPh3 )2 (L)] (E = P or As; X = Cl or Br; L = monobasic bidentate Schiff base anion) have been prepared by reacting [RuX3 (EPh3 )3 ] or [RuBr3 (PPh3 )2 (MeOH)] with the respective Schiff bases in a 1:1 molar ratio in benzene. [RuX3 (EPh3 )3 ]

or

[RuBr3 (PPh3 )2 (MeOH)]

+ HL

Benzene

−→ [RuX2 (EPh3 )2 (L)] + HX + EPh3

Reflux, 6 h

All the complexes are green and soluble in common organic solvents. The analytical data obtained for the new complexes (Table 1) agree very well with proposed molecular formulae. In all of the above reactions, the Schiff bases behave as mononegative bidentate ligands. 3.1. Infra red spectra The IR spectra of the free ligands when compared with that of the new complexes confirm the coordination of hydrazones to the ruthenium metal (Table 1). The IR spectra of the free ligands showed bands in the region 3250, 1660, 1640 and 1015 cm−1

Table 1 Analytical data of new ruthenium(III) complexes Complexa

(1) [RuCl2 (PPh3 )2 (BFMH)] (2) [RuCl2 (PPh3 )2 (BFEH)] (3) [RuCl2 (PPh3 )2 (BTEH)] (4) [RuBr2 (PPh3 )2 (BFMH)] (5) [RuBr2 (PPh3 )2 (BFEH)] (6) [RuBr2 (PPh3 )2 (BTEH)] (7) [RuCl2 (AsPh3 )2 (BFMH)] (8) [RuCl2 (AsPh3 )2 (BFEH)] (9) [RuCl2 (AsPh3 )2 (BTEH)] (10) [RuBr2 (AsPh3 )2 (BFMH)] (11) [RuBr2 (AsPh3 )2 (BFEH)] (12) [RuBr2 (AsPh3 )2 (BTEH)] a

Green.

Melting point (◦ C)

163 167 181 175 187 190 169 171 176 178 172 176

Found (calculated) (%) C

H

N

62.15 (62.13) 62.73 (62.75) 61.68 (61.69) 57.00 (56.91) 56.12 (56.14) 55.23 (55.27) 57.00 (57.02) 57.40 (57.42) 56.52 (56.53) 57.56 (57.60) 57.95 (57.98) 57.03 (57.08)

4.18 (4.20) 4.25 (4.37) 4.28 (4.30) 3.83 (3.85) 4.06 (4.09) 4.50 (4.51) 3.92 (3.93) 4.0 (4.0) 3.96 (3.94) 3.68 (3.90) 4.03 (4.04) 3.89 (3.98)

4.52 (4.53) 4.48 (4.48) 4.38 (4.40) 4.17 (4.15) 4.10 (4.09) 4.02 (4.03) 4.10 (4.15) 4.08 (4.10) 4.01 (4.03) 4.19 (4.20) 4.10 (4.14) 4.02 (4.07)

680

K.P. Balasubramanian et al. / Spectrochimica Acta Part A 65 (2006) 678–683 Table 2 IR and electronic spectral data of new ruthenium(III) complexes

Fig. 2. Electronic spectra of (7).

assigned to ␯NH , ␯C O , ␯C N and ␯N N , respectively in view of earlier report [38]. The bands due to ␯NH and ␯C O stretching vibrations are not observed in the complexes. This indicates the enolisation of >C O followed by deprotonation and complexation with metal. In the IR spectra of ruthenium(III) complexes, the absorption due to ␯C N have been shifted to lower frequency [39] suggesting the presence of >C N–N C residues of the stoichiometry and the destruction of keto group presumably viz., enolisation and bonding of the ligand through the resulting enolate oxygen. The two strong bands observed at 710–690 and 610–605 cm−1 regions in the complexes are due to furan/thiophene modes [40,41]. There was no change in the position of the furan/thiophene bands in the spectra of the ruthenium complexes owing to non-coordination of the furan/thiophene ring. In addition to the above absorptions, bands due to triphenyphosphine/arsine were also present in the spectra of all the complexes. 3.2. Electronic spectra The electronic spectra showed one to two bands in the 231–680 nm region (Fig. 2). The ground state of ruthenium(III) is 2 T2g and the first excited doublet levels, in the order of increasing energy are 2 A2g and 2 T1g which arises from t42g e1g configuration [42]. In the most of the ruthenium(III) complexes the electronic spectra showed only charge transfer bands [43]. The band in the 680–592 nm region have been assigned to the d–d transition, which is in conformity with assignments made for the similar ruthenium(III) complexes [44,45]. Other bands in the 371–231 nm region have been assigned to the charge transfer transitions. In general the electronic spectra of all the complexes are characteristic of an octahedral environment around ruthenium(III) ions. 3.3. Magnetic moments The magnetic moments for some of the complexes have been measured at room temperature using a vibration sample magnetometer. The values obtained lie in the 1.87–1.97 B.M. range corresponding to one unpaired electron, suggesting a low spin t52g configuration for ruthenium(III) ion in pseudo-octahedral environment [46] (Table 2).

Complex

␯C

N

␯N–N

␯C N–N

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

1587 1585 1581 1582 1535 1521 1587 1533 1517 1587 1515 1548

1002 1010 1010 1012 1001 1003 1005 1020 1004 1002 1002 1000

1649 1649 1649 1649 1649 1649 1646 1647 1649 1625 1640 1647

C

PPh3 /AsPh3

λmax

1434, 1085, 692 1433, 1085, 692 1433, 1078, 692 1433, 1078, 688 1433, 1085, 692 1434, 1093, 694 1433, 1078, 688 1433, 1080, 690 1433, 1078, 670 1434, 1085, 696 1433, 1078, 686 1436, 1074, 690

231, 263, 371 230, 260, 328 240, 258, 341 233, 266, 341 234, 263, 361 237, 263, 321 237, 274, 592 235, 371, 680 235, 372, 680 239, 371, 595 231, 257, 598 234, 253, 596

ν, cm−1 ; λ, nm.

3.4. EPR spectra All the ruthenium(III) complexes are paramagnetic, showing a +3 oxidation state for ruthenium ion (Table 3). The solid state of EPR spectra at X-band frequencies for several of the new ruthenium(III) complexes have been recorded at room temperature. All the complexes exhibit three lines with different ‘g’ values indicating the presence of magnetic anisotropy (Fig. 3). The average ‘g’ values lies in the 2.15–2.35 range. These values are in the range that are obtained for similar other ruthenium(III) complexes [45,47,48]. The presence of three ‘g’ values is indicative of a rhombic distortion in these complexes (Tables 4–6). 3.5. Electrochemistry Since, the ligands used in this study are not reversibly oxidized or reduced in the applied potential range, it is assumed that all redox processes are metal centered only. The cyclic voltammograms of five complexes exhibit a reversible reduction peak and a reversible oxidation peak at a scan rage of 100 mV s−1 . The oxidation and reduction of each complexes are characterized by well-defined waves with Ef values in the range −0.06 to −1.95 V and +0.86 to +7.0 V, respectively (Fig. 4). The redox processes [Ru(IV)–Ru(III)/Ru(III)–Ru(II) couples] are reversible (or quasi-reversible) with peak to peak separation (Ep ) values ranging from 132 to 389 mV, suggestive of a single step one electron transfer process [49–51]. Furthermore, a close examTable 3 EPR spectral data of new ruthenium(III) complexes Complex

gx

gy

gz

ga

(1) (2) (3) (4) (7) (8) (10) (11) (12)

2.31 2.31 2.13 2.32 2.22 2.15 2.21 2.17 2.22

2.36 2.32 2.16 2.36 2.25 2.18 2.16 2.14 2.26

2.38 2.29 2.17 2.38 2.27 2.20 2.20 2.20 2.29

2.35 2.31 2.15 2.34 2.22 2.17 2.18 2.16 2.25

a

2

g = [1/3(gx2 + gy2 + gz2 )] .

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Table 4 Cyclic voltammetric dataa of some ruthenium(III) complexes Complex

(1) (3) (9) (10) (12)

Ru(IV)–Ru(III)

Ru(III)–Ru(II)

Epa (V)

Epc (V)

Ef (V)

Ep (mV)

Epa (V)

Epc (V)

Ef (V)

Ep (mV)

−96 −111 −76 −60 −400

−213 −500 −213 −72 −700

−0.029 −0.195 −0.144 −0.006 −0.150

250 389 289 132 300

797 – 117 758 129

547 – 55 645 85

0.672 – 0.086 0.701 0.107

250 – 62 113 44

a Working electrode, glassy carbon electrode; Reference electrode, silver/silver chloride electrode; Supporting electrode, [NBu ][CLO ] (0.05 m); Scan rate, 4 4 100 mV s−1 ; Ef , 0.5 (Epa + Epc ), where Epa and Epc are anodic and cathodic potentials, respectively.

Table 5 Catalytic oxidation data of alcohols by Ru(III) complexes in the presence of N-methylmorpholine–N-oxide Complexes

Substrate

Product

Yield (%)a

Turnoverb

(2)

Benzyl alcohol Cyclohexanol Cinnamyl alcohol Benzyl alcohol Cyclohexanol Cinnamyl alcohol Benzyl alcohol Cyclohexanol Cinnamyl alcohol Benzyl alcohol Cyclohexanol Cinnamyl alcohol Benzyl alcohol Cyclohexanol Cinnamyl alcohol

A K E A K E A K E A K E A K E

45.43 34.81 63.72 36.81 48.53 61.78 45.43 34.81 67.54 34.61 32.17 59.12 37.32 32.49 61.58

44.96 35.47 67.23 39.43 51.26 64.23 44.96 38.29 69.37 36.17 35.87 63.17 38.69 35.74 65.48

(3)

(4)

(8)

(10)

A, benzaldehyde; K, cyclohexanone; E, cinnamaldehyde. a Yields based on substrate. b Moles of product per mole of catalyst

ination of the data reveals that there is no remarkable change in redox potential of the complexes due to the replacement of triphenylphosphine by triphenylarsine. From the electrochemical data it is inferred that the present ligand system is highly suitable for stabilizing higher oxidation states of ruthenium. Based on the analytical, spectral and electrochemical data, an octahedral structure (Fig. 5) has been proposed for all the ruthenium(III) complexes.

Fig. 3. EPR spectrum of (3).

3.6. Catalytic activity The ruthenium(III) complexes were found to exhibit catalytic yield and turnovers comparable to those reported for similar ruthenium(III) complexes [52]. The relatively higher product

Table 6 Antibacterial activity data for the ligand and their Ru(III) complexes Complex

Diameter inhibition zone (mm) E. coli (%)

Pseudomonous aeruginosa (%)

Salmonella typhi (%)

Staphylococcus aureus (%)

0.25

0.5

0.75

1

0.25

0.5

0.75

1

0.25

0.5

0.75

1

0.25

0.5

0.75

1

H-BFMH (4) (10) H-BFEH (5) (8) H-BTEH (3) (6)

8 10 9 9 10 10 9 10 10

10 12 11 10 12 11 11 12 12

12 13 13 11 13 12 13 14 14

13 15 14 12 15 14 14 16 15

8 10 9 9 10 10 8 10 9

9 11 11 10 12 10 11 12 12

10 13 12 12 14 12 14 15 15

12 16 14 13 15 14 15 17 17

9 11 10 10 12 11 8 10 9

10 12 11 11 13 13 9 12 11

12 13 13 13 15 14 11 13 12

14 16 15 14 17 16 12 15 14

7 8 8 9 11 10 8 10 9

9 10 11 12 13 13 12 13 13

13 14 14 13 14 14 13 14 14

14 15 17 15 16 16 14 17 16

Streptomycin

18

20

21

24

18

21

23

24

18

19

21

24

19

20

23

25

682

K.P. Balasubramanian et al. / Spectrochimica Acta Part A 65 (2006) 678–683

[58]. Though the complexes possess 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 [59,60]. 4. Conclusion

Fig. 4. Cyclic voltammogram of (1).

Mononuclear complexes of the type [RuX2 (EPh3 )2 (L)] (E = P or As; X = Cl or Br; L = monobasic bidentate Schiff base derived from the condensation of benzhydrazide with furfuraldehyde, 2-acetylfuran and 2-acetylthiophene) have been synthesized. Based on the IR, electronic, EPR and electrochemical studies, an octahedral geometry has been proposed for the new complexes. The complexes exhibited considerable amount of antibacterial activity at the time of screening. Acknowledgement One of the authors (VC) thanks the University Grants Commission, SERO, Hyderabad, India for the award of minor research project. References

Fig. 5. Structure of new Ru(III) complexes.

yield obtained for the oxidation of cinnamylalcohol as compared to benzyl alcohol is due to the presence of the unsaturation in cinnamyl alcohol [53]. The relatively higher product yield obtained for the oxidation of benzyl alcohol as compared to cyclohexanol is due to the fact that the ␣-CH moiety of benzyl alcohol is more acidic [54]. It has also been found that triphenylphosphine complexes posses higher catalytic activity than triphenylarsine complexes [55]. This may be due to the higher donor ability of the arsine ligand compared to the phosphine ligand. 3.7. Antibacterial activity studies The results showed 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 [56]. 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 [57]. 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 compounds may involve formation of a hydrogen bond through the azomethine (>C N) group with the active centers of cell constituents, resulting in interference with the normal cell processes

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