Spectroscopic, redox and biological activities of transition metal complexes with ons donor macrocyclic ligand derived from semicarbazide and thiodiglycolic acid

Spectrochimica Acta Part A 60 (2004) 2153–2162

Spectroscopic, redox and biological activities of transition metal complexes with ons donor macrocyclic ligand derived from semicarbazide and thiodiglycolic acid Sulekh Chandra∗ , Sangeetika Department of Chemistry, Zakir Husain College University of Delhi, New Delhi 110002, India Received 1 September 2003; accepted 8 September 2003

Abstract A novel macrocyclic Schiff base ligand (2,5,9,12,14,18-hexaoxo-7,16-dithia-1,3,4,10,11,13-hexaazacycloocta-decane (H6 L) with N4 S2 coordinating sites was prepared by the reaction of the semicarbazide and thiodiglycolic acid. The transition metal complexes with macrocyclic ligand were synthesized and characterized by elemental analyses, magnetic susceptibility measurements, molar conductance, IR, electronic, and EPR spectral studies. Mass, 1 H NMR and IR spectral techniques suggest the structural features of macrocyclic ligand. Magnetic and electronic spectral studies suggest an octahedral geometry of complexes. Electrochemical behaviour of cobalt, nickel and copper complexes were determined by cyclic voltammetry. The cyclic voltammogram of the copper complex at room temperature shows a quasi-reversible peaks for Cu(III) → Cu(II) and Cu(II) → Cu(I) couples. The macrocyclic ligand and its complexes show growth inhibitory activity against pathogenic bacteria and plant pathogenic fungi A. niger, A. alternata and P. variotii. Most of the complexes have higher activities than that of free ligand. © 2003 Elsevier B.V. All rights reserved. Keywords: Semicarbazide; Magnetic; Electronic; EPR; Cyclic voltammetry

1. Introduction Macrocyclic ligands derived from semicarbazide and precursor diketone are of considerable interest because of their potentially beneficial pharmacological properties such as antibacterial, antifungal, antiviral, anticancer, [1–3] etc. and a wide variation in their modes of bonding and stereochemistry. This type of compounds also form a strong chelate ring, giving possible electron delocalization associated with extended conjugation that may affect the nature of the complex formed [4]. The number and relative proportion of donor atoms and the cavity size of the macrocyclic compounds gave special reactivity to these molecules. The bacterial and fungicidal activities of transition metal complexes are due to their ability to form chelates with the essential metal ions bond through nitrogen as well as sulphur donor atom of ligand [5]. However, we know that structural factors which hinder the semicarbazone ability to function as a chelating agent diminish its biological activity [6,7]. ∗

Corresponding author. E-mail address: schandra [email protected] (S. Chandra).

1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2003.09.027

Electrochemical studies have been performed on their metal complexes because their biological activities may be related to their redox behavoiur [8,9]. In the present paper, we report the synthesis and characterization of chromium(III), manganese(II), iron(IIII), cobalt(II), nickel(II), copper(II), ruthenium(III) and iridium(III) complexes with 2,5,9,12,14,18-hexaoxo-7,16dithia-1,3,4,10,11,13-hexaazacycloocta decane. We have also studied the electrochemical behaviour of Co(II), Ni(II) and Cu(II) complex by cyclic voltammetry.

2. Experimental All the chemicals used were of AnalaR grade. Solvents were purified before use according to standard procedures. 2.1. Preparation of ligand A warm ethanolic solution (50 ml) of semicarbazide hydrochloride (0.01 mol, 1.12 g) was added to a warm ethanolic solution (50 ml) of thiodiglycolic acid (0.01 mol, 1.50 g).

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After addition the mixture was refluxed for 7–8 h. On keeping it overnight at 0 ◦ C, a white coloured crystalline solid was formed, which was filtered, washed with ethanol and dried in vacuo over P4 O10 (yield 60%), mp 160 ◦ C (found: C-43.45; H-4.89; N-30.02; calculated for C10 H14 N6 S2 O2 : C-43.16; H-5.07; N-30.20%). 2.2. Preparation of complexes Complexes of Cr(III), Mn(II) and Fe(III) were prepared by adding a warm ethanolic (10 ml) solution of corresponding metal salt (CrCl3 ·6H2 O, MnCl2 ·4H2 O and FeCl3 ) (0.002 mol) to a warm ethanolic (10 ml) suspension of the macrocyclic ligand (0.001 mol, 0.34 g). The mixture was heated under reflux with stirring for 5–6 h and then reduced to half volume. On cooling, a coloured complex precipitated out, which was filtered, and washed with ethanol and dried in vacuo over P4 O10 (yield 50–62%). The complexes of Co(II), Ni(II), Cu(II), Ru(III) and Ir(III) was obtained by mixing a warm ethanolic (10 ml) solution of corresponding metal salt(CoCl2 ·2H2 O, NiCl2 ·6H2 O, CuCl2 ·2H2 O, RuCl3 ·XH2 O, IrCl3 ·XH2 O) (0.001 mol) to a warm ethanolic (10 ml) suspension of the macrocyclic ligand (0.001 mol, 0.34 g). The mixture was heated under reflux with stirring for 2–3 h. On cooling a coloured complex precipitated out, which was filtered, washed with ethanol and dried in vacuo over P4 O10 (yield 60–65%).

each solution for 5 min. This procedure was done at room temperature.

3. Result and discussion The reaction of the transition metal ions with the macrocyclic ligand led to isolation of different macrocyclic complexes. Colours, elemental analyses, molar conductivity values, melting point and yield of the complexes are included in Table 1. All of the complexes are stable in air and gave different stoichiometries ratio of compounds. The analytical data of the complexes correspond well with the general formula [M2 LCl4 ]Cl2 , [M2  LCl4 ], [M L]Cl2 and [M L]Cl3 where L: macrocyclic ligand; M: Cr(III), Fe(II), M : Mn(II); M : Co(II), Ni(II), Cu(II) and M : Ru(III) or Ir(III). The mass spectrum of macrocyclic ligand confirms the proposed formula for ligand. The molecular ion peak for the ligand was observed at a 314 amu (M+ − 1) [C10 H14 N6 S2 O2 ]+ . It also shows various peaks for different fragments (Fig. 1). These data suggests the 2 + 2 condensation of thiodiglycolic acid and semicarbazide.

2.3. Physical measurements Microanalysis (C, H and N) of these complexes were carried out on a Carlo-Erba 1106 elemental analyzer. IR spectra were recorded on a Perkin Elmer 137 instrument as nujol mulls/KBr pellets. Electronic spectra were recorded in DMSO solution on a Shimadzu UV mini-1240 spectrophotometer. Molar conductance is measured on an ELICO conductivity bridge (Type C M 82 T). Magnetic susceptibility measurements were made on Gouy balance at room temperature using CuSO4 ·5H2 O as calibrant. Electron impact mass spectra were recorded on JEOL, JMS, DX-303 mass spectrometer. 1 H NMR spectra were recorded on Hitachi FT-NMR, Model R-600 spectrometer using deuteriated DMSO as solvent. Chemical shifts are given in ppm relative to tetramethylsilane. EPR spectra of the complexes were recorded as powder samples at room temperature on an E-4 EPR spectrometer using DPPH as the g-marker and in DMSO solution. Molecular weight of the complexes was determined in benzene (freezing point). Electrochemical properties were performed using platinum wire as auxiliary, Ag/AgCl as reference electrode and glassy carbon as working electrode. Cyclic voltammetry studies of complexes were carried out in 0.01 M solutions in dimethylsulfoxide containing [NBu4 ][PF6 ] as supporting electrolyte. The range of potential was studied in between +1 and −1.5 V. Before plotting graph, nitrogen gas was passed in

Fig. 1. Mass spectrum of H2 L, on the basis of elemental analysis, molar conductance measurements, magnetic susceptibility, IR, electronic, EPR data and the subsequent discussion given above the following structures may be proposed for all the complexes. [M2 LCl4 ]Cl2 , M: Cr (III), Fe(II),[Mn2 LCl4 ][M L]Cl2 ; M : Co (II), Ni(II), Cu(II)[M L]Cl2 and M : Ru (III) or Ir(III).

Complexes (molecular formula)

[Cr2 H6 LCl4 ]Cl2 Cr2 C10 H14 N6 S2 O6 Cl6 [Mn2 H6 LCl4 ] Mn2 C10 H14 N6 S2 O2 Cl4 [Fe2 H6 LCl4 ]Cl2 Fe2 C10 H14 N6 S2 O2 Cl6 [CoH6 L]Cl2 CoC10 H14 N6 S2 O6 Cl2 [NiH6 L]Cl2 NiC10 H14 N6 S2 O6 Cl2 [CuH6 L]Cl2 CuC10 H14 N6 S2 O6 Cl2 [RuH6 L]Cl3 RuC10 H14 N6 S2 O6 Cl3 [IrH6 L]Cl3 IrC10 H14 N6 S2 O6 Cl3 a

Decomposition temperature.

Colour

Green Cream Yellow Pink Light blue Green Black Brown

Molar condensation (−1 cm2 mol−1 ) 245 12 225 255 195 255 390 385

Melting pointa (◦ C) 240 200 270 265 225 245 260 240

Molecular weight found (calculated)

Found (calculated)%

688 623 691 500 500 520 580 670

14.59 17.14 15.29 11.21 11.23 12.67 17.57 28.56

(695.09) (630.07) (700.97) (508.22) (507.98) (512.83) (585.82) (676.96)

M

C (14.96) (17.44) (15.64) (11.59) (11.55) (12.39) (17.25) (28.39)

17.58 19.29 16.92 23.28 23.18 23.13 20.11 17.35

H (17.27) (19.06) (17.12) (23.63) (23.64) (23.42) (20.50) (17.74)

1.86 2.06 2.48 2.36 2.52 2.98 2.01 2.31

N (2.03) (2.23) (2.01) (2.77) (2.77) (2.75) (2.40) (2.08)

12.38 13.03 11.64 16.24 16.26 16.10 14.09 12.12

(12.09) (13.34) (11.98) (16.53) (16.54) (16.38) (14.34) (12.41)

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Table 1 Colour, melting point, analytical and molar conductance data of complexes

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Table 2 Important IR spectral bands (cm−1 ) and their assignments Complexes

ν(N–H)

␯(C=N)

Amide or thioamide bands I

II

III

IV

[Cr2 H6 L10 Cl4 ]Cl2 [Mn2 H6 LCl4 ] [Fe2 H6 L10 Cl4 ]Cl2 [CoH6 L]Cl2 [NiH6 L]Cl2 [CuH6 L]Cl2 [RuH6 L]Cl3 [IrH6 L]Cl3

3267, 3180 3270 3250 3195 3180 3182 3220 3190

1624 1602 1589 1610 1615 1606 1600 1601

1637 1637 1640 1642 1635 1623 1645 1642

1530 1578 1545 1532 1530 1542 1542 1540

1237 1237 1247 1240 1250 1247 1240 1241

650 680 675 666 670 656 680 680

1H

NMR spectral data of macrocyclic ligand in deuteriated DMSO solution does not show any signal corresponding to free –NH and –OH groups. It gives multiplet in the region of 9.21–8.54 ppm of amide protons [10] (CO–NH) (6H). The low field position of NH protons could be attributed to the deshielding due to amide group. It also gives a singlet in the region of 2.32–3.10 ppm assignable to methylene protons (CO–CH2 –S) (8H). The principal IR bands of macrocyclic ligand and its metal complexes are listed in Table 2. The infrared spectrum of this ligand do not exhibit any bands characteristics for primary amine group, and alcoholic group confirms the complete condensation of precursor molecule. The four new bands appeared, characteristics of amide groups at 1687 ν(C=O)amide 1, 1585 ν(CO–NH), 1530 ν(C–N) + δ(N–H) amide II, 1218 δ(N–H) amide III and 724 cm−1 φ(C=O) amide IV which support macrocyclic species. Sharp bands observed in the region 3256–3067 cm−1 may be assigned to ν(N–H) of the secondary amino group. The proposed structure of the ligand is given in Fig. 1. In the IR spectra of complexes a new band at 408–465 cm−1 is appeared. It may be formation of ν(M–N) bond. It supports the involvement of nitrogen in coordination [11]. A strong to medium intensity band in the region of 650–680 cm−1 is due to ν(C–S). The position of ν(C–S) band is shifted by ca. 30 cm−1 in the complexes. This clearly indicates that the coordination [12] takes place through S of C–S group.

ν(M–N)

ν(M–Cl)

408 465 456 450 452 450 456 458

355 360 365 – – – – –

ing from the lifting of the degeneracy of the orbital triplet(in octahedral symmetry) in the order of increasing energy and assuming D4h symmetry [13]. The C4v symmetry has been ruled out because of higher splitting of the first band. This suggests it possess distorted octahedral geometry. Thus ‘B’ has been evaluated from the relation: B=

2ν12 + ν22 − 3ν1 ν2 15ν2 − 27ν1

where ν1 and ν2 are energies of the transitions 4 B1g → 4 Ega , 4 B → 4 B transition, respectively. The value of Dq, B and 1g 2g β are presented in Table 3. The nephelauxetic parameter [14] β, is obtained by using the relation β = Bcomplex /Bfrre ion , where Bfree ion = 918 cm−1 . The value of β indicates that the complex has low covalent character. The EPR spectrum of the complex has been recorded as polycrystalline sample at room temperature. No hyperfine interaction was observed in the EPR spectra of the complex at room temperature. The g-values are calculated by using the expression, g = 2.0023(1−4λ/10 Dq) where λ is the spin–orbit coupling constant for the metal ion in the complex. Owen [15] gives the reduction of the spin–orbit coupling constant from the free ion value, 90 cm−1 for chromium(III) can be employed as a measure of metal–ligand covalency (Fig. 3a). It is possible to define a covalency parameter analogues to the nephelauxetic parameter which is the ratio of the spin–orbit coupling constant for the complex and the free Cr(III) ions.

3.1. Chromium(III) complex The complex shows magnetic moment corresponding to three unpaired electrons (i.e. 3.74 BM), which is approximately equal to spin only value. Six coordinated Cr(III) complexes with Oh symmetry show three spin allowed bands in the range of 18,000–30,000 cm−1 . While the complex under study shows four bands in the range of 16,949–38,461 cm−1 . This type of complex may have either C4v or D4h symmetry. The Cr(III) complex display bands at 16,949, 24,560, 27,472 and 38461 cm−1 , respectively (Fig. 2a). These bands may be assigned to 4 B1g → 4 Ega (ν1 ), 4 B1g → 4 B2g (ν2 ), 4 B1g → 4 Eb (ν ) and 4 B → 4 A (ν ) transitions, respectively, aris1g 1g 4 g 3

Table 3 Ligand field parameters of complexes Complexes

Dq (cm−1 )

B (cm−1 )

β

LFSE (kJ mol−1 )

[Cr2 H6 L10 Cl4 ]Cl2 [Mn2 H6 LCl4 ] [Fe2 H6 L10 Cl4 ]Cl2 [CoH6 L]Cl2 [NiH6 L]Cl2 [CuH6 L]Cl2 [RuH6 L]Cl3 [IrH6 L]Cl3

1689 1845 – 1180 1496 – – –

796 635 – 803 880 – 491 274

0.86 0.80 – 0.71 0.77 – 0.78 0.41

242 – – 112 214 – – –

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Fig. 2. Electronic spectra of complexes: (a) [Cr2 H6 LCl4 ]Cl2 , (b) [MnH6 LCl4 ], (c) [CoH6 L]Cl2 , (d) [NiH6 L]Cl2 , (e) [CuH6 L]Cl2 , (f) [RuH6 L]Cl3 .

3.2. Manganese(II) complex

3.3. Iron(III) complex

The electronic spectrum of Mn(II) complex displays weak bands at 13,157,14,925, 23,809 and 31,250 cm−1 (Fig. 2b), characteristics of an octahedral geometry. The assignments are obtained by fitting the observed spectrum to the Tanabe–Sugano diagram. Thus these bands may be assigned to following transitions 6 A1g → 4 T (4 G) (10B + 5C) (ν ), 6 A → 4 E , 4 A (4 G) (10B + 1g 1 1g g 1g 5C) (ν2 ), 6 A1g → 4 Eg (4 D) (17B + 5C) (ν3 ) and 6 A1g → 4 T (4 P)(ν ), respectively. Various ligand field parameters 1g 4 have been calculated [14,16]. The calculated value of β indicates that the complex has appreciable ionic character. The EPR spectrum of a polycrystalline sample at room temperature gives a broad isotropic signal centered at approximately the free ion value. The broadening of spectrum is due to spin relaxation. In DMSO solution the EPR spectrum of the complex shows splitting into six equally spaced absorption peaks arising due to hyperfine interaction between the unpaired quantum number (Fig. 3b). Ms corresponding to these lines are −5/2, −3/2, −1/2, 1/2, 3/2, 5/2 from low to high field, having Aiso value in the range 90– 100 G.

The magnetic moment of Fe(III) complex corresponds to five unpaired electrons, indicating the presence of the high-spin Fe(III) ion. The high spin of Fe(III) state is also confirmed by Mossbauer spectra. The isomer shift value at 0.6521 mm s−1 indicates that iron exist in (III) state with S = 5/2. In iron(III) complex there is no valence contribution to quadrupole splitting [17]. The only source of QS therefore remains the lattice contribution arising mainly from the asymmetry of the ligand field, which is further supported by the temperature independence of the QS value. Iron(III) is isoelectronic with Mn(II) but the electronic spectral characterization of Fe(III) is less, because of much greater tendency of the trivalent ion to have charge transfer bands in the near UV region with strong low energy wings in the visible that obscure the very weak, spin-forbidden d–d bands. The spectra of Fe(III) complexes generally exhibit a small number of fairly broad bands rather than the series of narrow bands expected. Electronic spectrum of Fe(III) complex exhibit three bands of varying intensities at 22,418 (ν1 ), 25,360 (ν2 )

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Fig. 3. (a) EPR spectrum of [Cr2 H6 LCl4 ]Cl2 in polycrystalline state. (b) EPR spectrum of [MnH6 LCl4 ] in solution state. (c) EPR spectrum of [CuH6 L]Cl2 in polycrystalline state.

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and 27,450 (ν3 ) cm−1 , but difficult. It is very unlikely derivatives, which are more ganese(II), can be described B.

the assignment of bands is that the spectra of iron(III) covalent than those of manin terms of single values of

3.4. Cobalt(II) complex Co(II) complex shows magnetic moment at 5.08 BM corresponding to three unpaired electrons. Electronic spectrum shows bands at 13,650 (ν1 ), 15,151 (ν2 ) and 25,000 cm−1 (ν3 ) (Fig. 2c). These bands may be assigned to following transitions 4 T1g → 4 T2g (F), 4 T1g → 4 A2g (F) and 4 T1g → 4 T (P), respectively. The position of bands suggest octahe1g dral geometry of Co(II) complex [18]. The various ligand field parameters were calculated for the cobalt(II) complex. The value of Dq has been calculated from Orgel energy level diagrams using the ν3 /ν1 ratio. The value for B (free ion) is 1120 cm−1 . The value of β indicates that the covalent character of metal ligand ␴-bond is low(Table 3). The EPR spectrum of Co(II) complex was recorded as polycrystalline sample. No EPR signal was observed at room temperature because the rapid spin lattice relaxation of Co(II) broadens the lines at higher temperature. It shows a very broad signal at liquid nitrogen temperature. The deviation of ‘g’ values from the free electron value (2.0023) may be due to angular momentum contribution in the complexes [18,19] (Table 4). 3.5. Nickel(II) complex The nickel(II) complex shows magnetic moment 2.93 BM corresponding to two unpaired electrons. Electronic spectrum displays bands at 10,362, 15,797 and 21,450 cm−1 (Fig. 2d) These bands may be assigned to 3 A (F) → 3 T (F)(ν ), 3 A (F) → 3 T (F)(ν ) and 2g 2g 1 2g 1g 2 3 A (F) → 3 T (P)(ν ) transitions, respectively. It sug2g 1g 3 gests octahedral geometry of Ni(II) complex. The various ligand field parameters are calculated by Orgel diagrams and compared [17]. The value of Dq has been calculated by using Orgel energy level diagrams using the ν3 /ν1 ratio. The value for B (free ion) is 1041 cm−1 . The value of β indicates that the covalent character of metal ligand ␴ bond is low (Table 3). Table 4 EPR spectral data of complexes at RT(as polycrysatalline) Complexes L10 Cl

[Cr2 H6 4 ]Cl2 [Mn2 H6 LCl4 ] [Fe2 H6 L10S Cl4 ]Cl2 [CoH6 L]Cl2 [CuH6 L]Cl2 giso = g + 2g⊥ /3.

g

g⊥

giso

– – – 4.05 2.11

– – – 2.12 2.05

2.00 2.03 2.01 – 2.07

2159

3.6. Copper(II) complex The Cu(II) complex shows magnetic moment at room temperature 2.02 BM (Table 1) corresponding to one unpaired electron which is higher than spin-only value of 1.73 BM for one unpaired electron. This reveals that the complex is monomeric in nature. The electronic spectrum of Cu(II) complex shows three bands at 12,987, 14,285 and 23,809 cm−1 (Fig. 2e) assignable to 2 B1g → 2 A1g , 2 B1g → 2 B2g and 2 B1g → 2 Eg transitions respectively. This suggests that these complexes have tetragonal geometry [14]. Tetragonal Cu(II) complexes give anisotropic EPR spectra. The anisotropic g-values have been calculated using the following expressions: 

 1 − 4λ 1−λ g = 2 and g⊥ = 2 E2 E3 where λ = 823 cm−1 and E2 = 2 B1g → 2 B2g , E3 = 2 1g → Bg The EPR spectrum of the complex under study recorded as polycrystalline sample at room temperature as well in solution. Spectrum for polycrystalline sample exhibits absorptions typical for the mononuclear species with axial symmetry. A representative spectra of the copper(II) complexes as polycrystalline sample at RT is shown in (Fig. 3c). The g value obtained from the spectrum is presented in Table 4. In tetragonal complexes the unpaired electron lies in the dx2 −y2 orbital giving 2 B1g as the ground state with g > g⊥ . The g and g⊥ values were computed from the spectrum using DPPH free radical as ‘g’ marker. The ‘g’ values and spin Halmiltonian parameters are summarized in Table 4. Kivelson and Neiman [20] have reported the g|| value <2.3 for covalent character of the metal–ligand bond and >2.3 for ionic character. Applying this criterion the covalent character of the metal-ligand bond in the complex under study can be predicted. The trend g > g⊥ > ge (2.0023) observed for the complex show that the unpaired electron is localized in dx2 −y2 orbital of the Cu(II) ions and the spectral features are characterstics of axial symmetry [21]. 2B

3.7. Ruthenium(III) complex On the basis of elemental analyses, the complex was found to have general composition RuLCl3 . The molar conductivity measurement indicates that the complex is 1:3 electrolyte in nature. Thus, the complex may be formulated as [RuL]Cl3 . The Ru(III) complex show magnetic moments at room temperature 1.76 BM, which is lower than the predicted value of 2.10 BM The lowering in µeff values may be due to lower symmetry ligand fields, metal-metal interaction or extensive electron delocalization [22]. The electronic spectrum of ruthenium(III) complex displays three bands at 15,152(ν1 ), 17,345 (ν2 ) and 25,142 cm−1 (ν3 ) (Fig. 2f). These bands may be assigned to

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→ 4 T1g , 2 T2g → 4 T2g and 2 T2g → 4 A1g transitions in order of increasing energy. The position of bands is in tune with the prediction for octahedral complexes of the metal ions. The ligand field parameters ∆, B and β have been calculated by using the relation.

2T

2g

ν1 = ∆ −

4B + 86(B)2 ∆

ν2 = ∆ +

12B + 2(B)2 σ

The value of B in free ion is 630 cm−1 . The value of β indicates that there is low covalency in the metal–ligand ␴-band. 3.8. Iridium(III) complex On the basis of elemental analyses, the complex was found to have general composition IrLCl3 . Molar conductivity measurements indicates that the complex is 1:3 electrolyte in nature. Thus, the complex may be formulated as [IrL]Cl3 . The complex of iridium(III) is diamagnetic as expected for complexes of the low-spin type having configuration t62g . The electronic spectra of the iridium(III) complex display bands at 17,550 (ν1 ), 23,094 (ν2 ) and 32,450 cm−1 . These bands may be assigned to 1 A1g → 1 T1g and 1 A1g → 1 T2g transitions in increasing order of energy The bands above 30,000 cm−1 may be due to charge transfer. The ligand field parameters are calculated and compared with those reported for other iridium(III) complexes having same chromophore [23] . The value of B free ion is 660 cm−1 .

4. Biological activities The antibacterial activity of the ligand and the soluble complex was performed in vitro growth inhibitory activity against S. aurea, E. coli and S. typhi by the disc diffusion technique. The bacteria were cultered in nutrient agar medium and is used as inoculum. Filter paper (Whatman

no. 4) discs (5 mm in diameter and 1 mm in thickness) were diped in a solution of particular complex at the concentration of 0.5 mg ml−1 in DMF and after drying placed on nutrient agar plates. Nutrient agar medium is composed of peptone 0.5 g, Buf extract 0.5 g, Agar 2.0 g, distilled water, pH 7.5 ± 0.1. The discs were placed on the already seeded plants and incubated at 30 ± 5 ◦ C for 24–48 h. The zone of inhibition was measured against S. aurea, E. coli and S. typhi, a clearing zone around the discs indicates the inhibitory activity of the compound on the organism (Fig. 4a). From table it is clear that the zone of inhibition is much larger for metal complexes than that of ligand. The increased activity of the metal chelates can be explained on the basis of chelation theory [24]. It is known that chelation tends to make the ligands act as more powerful and potent bactericidal agents, thus killing more of the bacteria than the ligand. It is observed that in complex the positive charge of the metal is partially shared with the donor atoms present in the ligands and there is may be ␲-electron delocalization over the whole chelate ring [24]. This thus increases the lipophilic character of the metal chelate and favours its permeation through the lipoid layer of the bacterial memberanes. There are other factors which also increase the activity are solubility, conductivity and bond length between the metal and ligand (Table 5). The antifungal activity of the ligand and their soluble complexes was performed in vitro against the A. niger, A. alternata and P. variotii by the poisonous food technique. Potato dextrose Agar was used as medium for text. It consists of potato: 1 g; dextrose: 2 g; agar: 2 g; distilled water: 100 cm3 ; pH: 7 ± 0.1. the compounds are to tested were dissolved in DMF to prepare 0.1% solution of each compound. The solution of compound was mixed with medium and this mixture was poured on a glass petridish. When it solidified the plates were inoculated. The inoculated plates were incubated for 48–72 h at 37 ◦ C. The percentage inhibiton of mycelial growth of the text fungus was calculated as follows (Fig. 4b) [25]. percentage inhibition = (C − T) ×

100 C

Fig. 4. (a) bacterial growth action on ligand and complexes. (b) fungal growth action on ligand and complexes.

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Table 5 Antibacterial and antifungal activity data of the ligands and complexes Compound

H6 L [Cr2 H6 L10 Cl4 ]Cl2 [Mn2 H6 LCl4 ] [Fe2 H6 L10 Cl4 ]Cl2 [CoH6 L]Cl2 [NiH6 L]Cl2 [CuH6 L]Cl2 [RuH6 L]Cl3 [IrH6 L]Cl3

Antibacterial activity (%)

Antifungal activity (%)

S. aureus

E. coli

S. typhi

A. niger

A. aletrnata

P. varoitii

20–40 40–60 60–80 40–60 60–80 80–100 60–80 40–60 40–60

40–60 60–80 – 60–80 – 40–60 60–80 – 60–80

20–40 60–80 40–60 80–100 40–60 – 80–100 40–60 60–80

60–80 60–80 60–80 60–80 40–60 60–80 60–80 40–60 20–40

40–60 40–60 60–80 60–80 60–80 60–80 60–80 80–100 40–60

20–40 60–80 20–40 40–60 – 40–60 40–60 – 20–40

where C is diameter of the fungal colonies in the control, T is diameter of the fungal colonies of treated plates. 4.1. Electrochemical behaviour The electrochemical properties of metal complexes, particularly with sulphur donor atoms have been studied in order to consider spectral and structural changes accompanying electron transfer. The redox behaviour of the complexes Co(II), Ni(II) and Cu(II) complexes has been examined in DMSO at a glassy carbon as working electrode using electrochemical analyzer. All these complexes are electroactive only with respect to metal center. A well defined redox–couple is observed in the range of 1.00–1.5 V to the oxidation of the metal center.

4.2. Reduction process Complexes of Co(II) and Ni(II) show a quasi-reversible two step single electron transfer process. E1/2 values are independent of scan rate. The Ep increases with increasing scan rate and is always greater than 60 mV. Voltammetric parameters are studied in the scan range of 60–800 mV s−1 . The ratio between the cathodic peak current and square root of the scan rate (Ipc /v1/2 ) is approximately constant. The peak potential shows a small dependence with the scan rate. The ratio Ipa /Ipc is close to unity. Form these data, it can be deduced that this redox couple is related to a reversible one-electron transfer process controlled by diffusion. Cathodic peak arise at (Co, Epc = 0.68 V) and (Ni, Epc = 0.61 V). The nickel complex reduction at lower potential

Fig. 5. (a) [CoH6 L]Cl2 , (b) [NiH6 L]Cl2 , (c) [CuH6 L]Cl2 .

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S. Chandra, Sangeetika / Spectrochimica Acta Part A 60 (2004) 2153–2162

Table 6 Electrochemical data of complexes

tra and Dr Brijesh USIC, New Delhi for recording mass spectra.

Compound

Epc (V)

Epa (V)

E1/2 (V)

Ep (V)

Ipc /ipa

[CoH6 L]Cl2 [NiH6 L]Cl2 [CuH6 L]Cl2

0.68 0.61 0.62

0.48 0.40 0.71

0.58 0.50 0.66

0.47 0.47 0.19

0.94 0.86 0.88

Electrochemical data recored in millivolt, at room temperature, scan rate 100 mV s−1 , E1/2 = 1/2(Epc + Epa ), The ratio of Ipc /ipa is constant for scan in the range of 50–800 mV s−1 .

compared to cobalt complex is due to more distortion in geometry (Fig. 5a and b) (Table 6). 4.3. Oxidation process The cyclic voltammogram of cobalt and nickel complexes show two peaks corresponds to two single electron transfer process. They anodic peak Epa at 0.48 and 0.40 V of cobalt and nickel complexes. The difference between the potential of the anodic peak and cathodic peak remains constant. Also, the ratio between the cathodic peak current and square root of the scan rate is practically constant in the range studied. All the data are diagnostic for a simple quasi-reversible one-electron charge transfer controlled by diffusion method [26]. The cyclic voltammogram of the copper complex recorded at room temperature shows a quasi-reversible peak for the Cu(II) → Cu(III) couple at 0.71 V with a direct cathodic peak for Cu(III) → Cu(II) at 0.62 V. It also exhibits two irreversible peaks in cathodic region characterstics for Cu(II) → Cu(I) at −0.73 V and Cu(I) → Cu(0) at −0.91 V. These two peaks show their reduction behaviour of copper in complex. In anodic region it exhibits two peaks which correspond to oxidative behaviour of Cu(0) → Cu(I) and Cu(I) → Cu(II) is observed.( Fig. 5c) (Table 6).

Acknowledgements We are thankful to the Principal, Zakir Husain College for providing laboratory facilities and the DST, New Delhi for financial assistance, IIT Bombay for recording EPR spec-

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