Synthesis, electrochemical, catalytic and antimicrobial activities of novel unsymmetrical macrocyclic dicompartmental binuclear nickel(II) complexes

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Synthesis, electrochemical, catalytic and antimicrobial activities of novel unsymmetrical macrocyclic dicompartmental binuclear nickel(II) complexes S. Sreedaran a, K. Shanmuga Bharathi a, A. Kalilur Rahiman a, K. Rajesh a, G. Nirmala a, L. Jagadish b, V. Kaviyarasan b, V. Narayanan a,* a

Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai 600 025, India b Centre for Advanced Studies in Botany, University of Madras, Guindy Campus, Chennai 600 025, India Received 27 January 2008; accepted 20 February 2008

Abstract A series of novel unsymmetrical dicompartmental binuclear nickel(II) complexes have been prepared by simple Schiff base condensation of the compound 1,8-[bis(3-formyl-2-hydroxy-5-methyl)benzyl]-l,4,8,11-tetraazacyclotetradecane L with appropriate aliphatic or aromatic diamine, nickel(II) perchlorate and triethylamine. All the complexes were characterized by elemental and spectral analysis. Positive ion FAB mass spectra show the presence of dinickel core in the complexes. The electronic spectra of the complexes show the d–d transition in the range of 550–1040 nm. Electrochemical studies of the complexes show two irreversible one electron reduction process around E1pc ¼ 0:79 to  1:27 V and E2pc ¼ 1:28 to  1:43 V. The reduction potential of the binuclear nickel(II) complexes shifts towards anodically upon increasing chain length of the macrocyclic ring. All the nickel(II) complexes show two irreversible oxidation waves around 0.72 to +1.52 V. The observed rate constant values for catalysis of the hydrolysis of 4-nitrophenyl phosphate are in the range of 9.20  103–16.81  103 min1. The rate constant values for the complexes containing aliphatic diimines are found to be higher than that of the complexes containing aromatic diimines. Spectral, electrochemical and catalytic studies of the complexes were compared on the basis of increasing chain length of the imine compartment. All the complexes were screened for antifungal and anti bacterial activity. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Compartmental ligand; Macrocycle; Cyclam; Nickel(II) complexes; Hydrolysis of nitrophenylphosphate; Cyclic voltammetry

1. Introduction Interest in the design and synthesis of coordination compounds of azamacrocyclic ligands with pendant substituents are increasing, due to their potential applications in the areas such as MRI, imaging with isotopes and radiotherapy, luminescent probes and DNA cleaver [1–6]. Pendant arms bearing additional potential ligating groups have been introduced at nitrogen atoms of macrocyclic

*

Corresponding author. Tel./fax: +91 44 2230 0488. E-mail address: [email protected] (V. Narayanan).

ring. The advantage of this area derives from the concept that the presence of two pendant arms, bearing aldehyde or ketone gives raise to quite sophisticated unsymmetrical dicompartmental macrocyclic ligands on simple Schiff base condensation with aliphatic or aromatic diamines [7–9]. These unsymmetrical macrocyclic ligands provide adjacent, dissimilar binding sites which can each accommodate a metal and so produce dinuclear complexes with coordination environment resembling the active sites in urease [10–12]. The chemistry of metal complexes with dicompartmental ligands has become a rapidly growing area of research [13], because of their importance in biomimetic studies of

0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.02.022

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binuclear metalloproteins [14], their interesting catalytic properties [15] and their ability to stabilize unusual oxidation states and mixed-valance compounds. In particular, phenol based macrocyclic binucleating ligands containing two different compartments have particularly received great attention due to their capability to bind two metal centers in close proximity [16–21]. The influence of the donor atoms and their relative position, the number and size of the chelate ring formed, shape of the coordination moiety play important role in the biological and catalytic activity of the complexes formed [22–29]. Hence, synthesis of model compounds that mimic the physical and chemical properties of the active sites present in metalloenzymes is very essential and the studies on such compounds is becoming increasingly important in understanding biological functions of the bimetallic cores [30]. Various types of compartmental ligands and their complexes have been prepared, and their structures and reactivities were investigated by Bosnich [31–33] and Busch [34]. Such ligands contain two compartments: one includes six coordination atoms (N4O2) and the other with tetradentate N2O2 donor set. Our research focuses on the synthesis of cyclam based dissimilar dicompartmental binuclear nickel(II) complexes containing hexa (amine compartment) and tetra (imine compartment) coordination sites. The amine compartment consists of two secondary nitrogens, two tertiary nitrogens and two phenolic oxygens (N4O2), whilst the imine compartment comprises of two imine nitrogens and two phenolic oxygens (N2O2). In these systems, one metal is in six-coordinate site and the other is in four-coordinate site, if a ligand like ClO4  is coordinated, this site can be made five-coordinate [34]. Spectral, electrochemical, catalytic and hydrolysis of 4-nitrophenyl phosphate of the complexes were discussed. Antifungal and antibacterial activities of the complexes were also discussed.

were carried out under oxygen free condition using a threeelectrode cell in which a glassy carbon electrode was the working electrode, a saturated Ag/AgCl electrode was the reference electrode and platinum wire was used as the auxiliary electrode. A ferrocene/ferrocenium (1+) couple was used as an internal standard and E1/2 of the ferrocene/ferrocenium (Fc/Fc+) couple under the experimental condition is 470 mV. Tetra(n-butyl)ammonium perchlorate (TBAP) was used as the supporting electrolyte. Room temperature magnetic moment was measured on a PAR vibrating sample magnetometer Model-155. The hydrolysis of 4-nitrophenylphosphate by the nickel(II) complexes were studied in a 103 M dimethylformamide solution. The reaction was followed spectrophotometrically and the hydrolysis of p-nitrophenylphosphate was monitored by following the UV absorbance change at 420 nm (assigned to the 4-nitrophenolate anion) as a function of time. A plot of log (Aa/Aa  At) versus time was made for each complex and the rate constant for the hydrolysis of 4-nitrophenylphosphate was calculated. 2.1.1. Chemicals and reagents 5-Methyl salicylaldehyde [35], 3-chloromethyl-5-methyl salicylaldehyde [36], 1,4,8,11-tetraazatricyclo[9.3.1.1[4,8]]hexadecane [37] and 1,8-[bis(3-formyl-2-hydroxy-5-methyl)benzyl]-4,11-diazaniatricyclo[9.3.1.1[4,8]]hexadecane dichloride [38] were prepared by following the literature methods. Analytical grade methanol, acetonitrile and dimethylformamide were purchased from Qualigens and used as such. TBAP used as supporting electrolyte in electrochemical measurement was purchased from Fluka and recrystallised from hot methanol. (Caution! TBAP is potentially explosive; hence care should be taken in handling the compound). All other chemicals and solvents were of analytical grade and were used as received without any further purification. 2.2. Synthesis of ligand (L)

2. Experimental 2.1. Analytical and physical measurements Elemental analysis of the complexes was obtained using Haereus CHN rapid analyzer. 1H NMR spectra were recorded using JEOL GSX 400 MHz NMR spectrometer. Electronic spectral studies were carried out on a Hitachi 320 spectrophotometer in the range 200–1100 nm. IR spectra were recorded on a Shimadzu FTIR 8300 series spectrophotometer on KBr disks in the range 4000–400 cm1. Molar conductivity was measured by using an Elico digital conductivity bridge model CM-88 using freshly prepared solution of the complex in dimethylformamide. The atomic absorption spectral data were recorded using Varian spectra AA-200 model atomic absorption spectrophotometer. Mass spectra were obtained on a JEOL SX-102 (FAB) mass spectrometer. Cyclic voltammograms were obtained on CHI-600A electrochemical analyzer. The measurements

2.2.1. Synthesis of 1,8-[bis(3-formyl-2-hydroxy-5-methyl)benzyl]-l,4,8,11-tetraazacyclotetradecane (L) The compound 1,8-[bis(3-formyl-2-hydroxy-5-methyl)benzyl]-4,11-diazaniatricyclo[9.3.1.1[4,8]]hexadecane dichloride (1 g, 0.0017 mol) was dissolved in 200 ml of an aqueous NaOH solution (0.3 M) with stirring. After stirring for 4 h, the solution was extracted with CHCl3 (5  30 ml). The combined CHCl3 extracts were dried with anhydrous MgSO4, and concentrated under vacuum to give the expected compound 1,8-[bis(3-formyl-2-hydroxy-5-methyl)benzyl]-l,4,8,11-tetraazacyclotetradecane. Yield: 72%. m.p.: 282 °C (dec). Anal. Calc. for C28H40N4O4: C, 67.72; H, 8.12; N, 11.28. Found: C, 67.64; H, 8.02; N, 11.17%. IR data (KBr disc): 3389 cm1, 3285 cm1, 1680 cm1; 1H NMR: d (ppm in CDCl3), 1.5 (q, 4H, b-CH2), 2.32 (t, 16H, a-CH2), 4.1 (s, 4H, N–CH2–Ar), 7.26 (d, 4H, Ar–H), 9.92 (s, 2H, Ar– CHO), 12.5 4 (br, s, Ar–OH). 13C NMR: d (ppm in CDCl3), 28.3, 49.4, 54.8, 55.2, 55.7, 60.5, 122.4, 126.2, 126.8, 131.0, 136.8, 157.6, 196.0.

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2.3. Synthesis of the macrobicyclic binuclear nickel(II) complexes 2.3.1. [Ni2L[1a](ClO4)](ClO4) A methanolic solution of nickel(II) perchlorate hexahydrate (0.74 g, 0.002 mol) was added to a hot solution of ligand L (1.00 g, 0.002 mol) in methanol, followed by the addition of 1,2-diaminoethane (0.12 g, 0.002 mol) and triethylamine (0.41 g, 0.004 mol) in methanol. After an hour, another 1 equiv. of nickel(II) perchlorate (0.74 g, 0.002 mol) was added and the solution was refluxed in a water bath for 24 h. The resulting solution was filtered whilst hot and allowed to stand at room temperature. After slow evaporation of the solvent at 25 °C, dark green compound was collected by filtration, which was recrystallised in acetonitrile, and dried in vacuum. Yield: 0.96 g (58%). Anal. Calc. for C30H42N6O10Cl2Ni2: C, 43.15; H, 5.07; N, 10.07; Ni, 14.06. Found: C, 43.09; H, 4.97; N, 10.01; Ni, 14.00%. FAB mass (m/z (%)): [Ni2L[1a] (ClO4)–ClO4]+ 736. Conductance (Km, S cm2 mol1) in DMF: 73. Selected IR data (KBr) (m, cm1): 3385, 1630, 1100, 1104, 1093, 626. The complexes [Ni2L[1b](ClO4)](ClO4), [Ni2L[1c](ClO4)](ClO4), [Ni2L[1d](ClO4)](ClO4), and [Ni2L[1e](ClO4)](ClO4) were synthesized by following the above procedure using 1,3diaminopropane (0.15 g, 0.002 mol), 1,4-diaminobutane (0.18 g, 0.002 mol), 1,2-diaminobenzene (0.22 g, 0.002 mol) and 1,8-diaminonaphthalene (0.32 g, 0.002 mol) respectively, instead of using 1,2-diaminoethane. 2.3.2. [Ni2L[1b](ClO4)](ClO4) Dark green compound. Yield: 1.07 g (63%). Anal. Calc. for C31H44N6O10Cl2Ni2: C, 43.86; H, 5.22; N, 9.89; Ni, 13.83. Found: C, 43.80; H, 5.19; N, 9.81; Ni, 13.78%. FAB mass (m/z (%)): [Ni2L[1b]–2ClO4]+ 650. Conductance (Km, S cm2 mol1) in DMF: 78. Selected IR data (KBr) (m, cm1): 3389, 1634, 1105, 1095, 1089, 626. 2.3.3. [Ni2L[1c](ClO4)](ClO4) Dark green compound. Yield: 0.89 g (52%). Anal. Calc. for C32H46N6O10Cl2Ni2: C, 44.51; H, 5.37; N, 9.74; Ni, 13.61. Found: C, 44.47; H, 5.33; N, 9.69; Ni, 13.58%. FAB mass (m/z (%)): [Ni2L[1c]–2ClO4]+ 664. Conductance (Km, S cm2 mol1) in DMF: 85. Selected IR data (KBr) (m, cm1): 3385, 1637, 1103, 1093, 1087, 624. 2.3.4. [Ni2L[1d](ClO4)](ClO4) Dark green compound. Yield: 1.0 g (57%). Anal. Calc. for C34H42N6O10Cl2Ni2: C, 46.25; H, 4.97; N, 9.52; Ni, 13.29. Found: C, 46.21; H, 4.93; N, 9.48; Ni, 13.26%. FAB mass (m/z (%)): [Ni2L[1d]ClO4–ClO4]+ 783. Conductance (Km, S cm2 mol1) in DMF: 91. Selected IR data (KBr) (m, cm1): 3392, 1643, 1107, 1090, 1083, 626. 2.3.5. [Ni2L[1e](ClO4)](ClO4) Dark green compound. Yield: 1.1 g (59%). Anal. Calc. for C38H44N6O10Cl2Ni2: C, 48.91; H, 4.75; N, 9.01; Ni,

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12.58. Found: C, 48.89; H, 4.73; N, 8.97; Ni, 12.54%. FAB mass (m/z (%)): [Ni2L[1e]ClO4–ClO4]+ 843. Conductance (Km, S cm2 mol1) in DMF: 97. Selected IR data (KBr) (m, cm1): 3395, 1650, 1102, 1094, 1085, 626. 3. Results and discussion A series of macrobicyclic binuclear nickel(II) complexes were synthesized by Schiff’s base condensation of the precursor compounds with diamines in presence of metal ion. The synthetic pathway of binuclear complexes is shown in Scheme 1. In all the prepared nickel(II) complexes, one of the nickel(II) ion in the amine compartment is six coordinated, and the other in the imine compartment is five coordinated. Conductivity measurements of (Km, 73– 97 S cm2 mol1) all the complexes show that they are 1:1 conductors in DMF solution [39]. The effective magnetic moments per mole of nickel for the nickel(II) complexes fall in the range of 2.94–3.05 lB at room temperature. This may be caused by a strong nickel–nickel interaction. The magnetic moment value for the nickel(II) complex fall in the range usually observed for octahedral nickel(II) complexes. All attempts to grow single crystals of the complexes (e.g. by the diffusion of diethyl ether vapor into DMF solutions or recrystallization of the complexes from acetonitrile) have failed and only green powder or micro crystals were obtained. Spectral, electrochemical, catalytic and antimicrobial studies of the complexes were carried out. 3.1. Spectral studies The IR spectra of all the binuclear nickel(II) complexes show bands in the region 3245–3320 cm1, indicating the presence of NH groups in the complexes. The IR spectra of ligand L shows a band at 1680 cm1 due to the presence of C@O (–CHO) group. All the complexes show a sharp band in the region of 1630–1650 cm1 due to the presence of C@N in the complexes. This shows that the aldehyde groups had been completely converted into imine groups, by the disappearance of the aldehyde C@O stretching band at around 1680 cm1 and the appearance of a strong band at around 1630–1650 cm1 assigned to C@N stretching vibration mode (Schiff base condensation) [40,41,33]. All the binuclear nickel(II) complexes showed two sharp peaks at near 1100 cm1 and 620 cm1, which are assigned to perchlorate ions [42]. The peak around 1100 cm1 is split, which clearly explains the presence of a coordinated perchlorate ion [43], while the other peak does not show any splitting, indicating the presence of an uncoordinated perchlorate ion. The IR spectral data of the complexes, clearly shows that the presence of two different types of perchlorate ions. Further, the appearance of new bands in the 1530–1560 cm1 region in all the complexes suggests phenoxide bridging with the metal ions [44]. Electronic spectra of all the complexes were obtained in DMF medium. The electronic spectra of all the complexes

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CH3

N OH

O

NH NH

O

N

N

Ni(ClO4)2.6H2O H2N-R-NH2

OH

+

CH3

O NH

Triethylamine Ni(ClO4)2.6H2O

NH

N Ni

Ni O N

CH3

R N

ClO4

CH3 R L1a = -(CH2)2L1b = -(CH2)3L1c = -(CH2)4L1d =

L1e = Scheme 1.

exhibit three main features. One or two peaks in the range of 250–300 nm is assigned to the intra ligand charge transfer transition (p–p*). An intense peak in the range of 380–420 nm is due to ligand-to-metal charge transfer transition, and the d–d transition for the nickel(II) complexes show three main bands in the range of 550–1040 nm, which is characteristic of Ni2+ in the 5/6 coordination environment [45]. Although the appearance of the spectra bear similarities with those for octahedrally coordinated Ni(II) ions, the assignment of the three most intense bands to the spin allowed transitions from the 3A2g ground state of an octahedrally coordinated d8 ion to the next higher excited triplet states (3T1g(P), 3T1g(F) and 3T2g(F)) would be a misinterpretation. Furthermore, the molar extinction coefficients of the absorption band maxima of are unusually high. The spectra in fact indicate a square pyramidal or trigonal bipyramidal coordination environment of at least one of the Nickel(II) ions [45].

The electronic spectral studies inferred that an increase in kmax (red shift) of the d–d transition of nickel(II) ion in the ligand L[1a] to L[1c] and L[1d] to L[1e]. This is due to the flexibility of the macrocyclic ring that is imparted by the distortion of the geometry of the complexes due to the increase in macrocyclic ring size, which causes more distortion of the geometry [46,47]. The electronic spectral data of the binuclear nickel(II) complexes are given in Table 1. 3.2. Electrochemical properties of the complexes The electrochemical properties of all the binuclear nickel(II) complexes were studied by Cyclic voltammetry in dimethylformamide containing 101 M tetra(n-butyl)ammonium perchlorate. The electrochemical data are summarized in Table 2. Cyclic voltammograms for the nickel(II) complexes are shown in Fig. 1.

Table 1 Electronic spectral data of binuclear nickel(II) complexes No.

Complexes

1 2 3 4 5

[Ni2L[1a](ClO4)](ClO4) [Ni2L[1b](ClO4)](ClO4) [Ni2L[1c](ClO4)](ClO4) [Ni2L[1d](ClO4)](ClO4) [Ni2L[1e](ClO4)](ClO4)

kmax, nm (e, M1 cm1) d–d

Charge transfer

987 (48), 705 (85), 552 (367) 997 (78), 717 (96), 564 (347) 1017 (53), 723 (94), 581 (318) 1030 (67), 742 (98), 605 (180) 1037 (43), 754 (69), 613 (247)

357 363 370 375 383

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(10 800), (11 400), (12 100), (12 800), (13 600),

269 271 273 277 277

(14 400) (14 840) (15 000) (17 000) (17 800)

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Table 2 Electrochemicala and hydrolysis of 4-nitrophenylphosphateb data for the complexes No.

1 2 3 4 5

Complexes

[Ni2L[1a](ClO4)](ClO4) [Ni2L[1b](ClO4)](ClO4) [Ni2L[1c](ClO4)](ClO4) [Ni2L[1d](ClO4)](ClO4) [Ni2L[1e](ClO4)](ClO4)

Reduction (at cathodic)

Oxidation (at anodic)

E1pc (V)

E2pc (V)

E1pc (V)

E1pc (V)

1.00 0.91 0.79 1.27 1.21

1.43 1.31 1.28 1.37 1.31

0.72 0.80 0.98 1.10 1.18

1.23 1.32 1.39 1.43 1.52

Rate constant (k) (103) min1

10.36 13.13 16.81 9.20 9.87

Concentration of the complexes: 1  103 M. Concentration of 4-nitrophenylphosphate: 1  101 M. The rate constant values the are average of three experiments. a Measured by CV at 50 mV/s. E vs. Ag/AgCl conditions: GC working and Ag/AgCl reference electrodes; supporting electrolyte TBAP; concentration of complex 1  103 M, concentration of TBAP 1  101 M. b Measured spectrophotometrically in DMF. Concentration of the complexes: 1  103 M. Concentration of 4-nitrophenylphosphate: 1  101 M. The rate constant values are average of three experiments.

Fig. 1. Cyclic voltammogram of the binuclear nickel(II) complexes: (a) [Ni2L[1a](ClO4)](ClO4), (b) [Ni2L[1b](ClO4)](ClO4) and (c) [Ni2L[1c](ClO4)](ClO4) (reduction process).

3.3. Reduction process at negative potential The cyclic voltammograms of the nickel(II) complexes were recorded in the potential range 0 to 1.6 V in DMF. Cyclic voltammogram for nickel(II) complexes are shown in Fig. 1. The electrochemical data are summarized in Table 2. The macrocyclic doubly phenoxo-bridged dinickel complexes typically undergo two well-separated one electron reductions [48,49a]. It is observed that all the binuclear complexes show two irreversible reduction waves in the cathodic potential region. The first reduction potential ranges from 0.79 to 1.27 V and the second reduction potential lies in the range of 1.19 to 1.43 V. Controlled potential electrolysis was also carried out and the experiment reports that each couple corresponds to one-electron transfer process. The two reduction processes are assigned as follows: NiII NiII ! NiII NiI ! NiI NiI The first reduction potential of the Ni2+/Ni+ couple of [NiL[1a-c](ClO4)]ClO4 complexes in the range of 0.79 to

1.27 (Table 2) is similar to that observed for [Ni(salen)]2+ complexes, and may suggest that the reduction wave observed for these bimetallic complexes refer to reduction of the nickel(II) in the four-coordinate site. Similarly, the second reduction wave observed for [NiL[1a-c](ClO4)]ClO4 in the range 1.28 to 1.43 V, is attributed reduction of nickel(II) ion in the amine compartment (N4O2). Hence the second reduction potential suggests that the reduction wave observed for these bimetallic complexes refers to the reduction of the nickel(II) in the six-coordinate site [49b,49c]. It seems, interesting to compare the reduction potential of the nickel(II) complexes. It is apparent to say that in the binuclear nickel(II) complexes, both first and second reduction potentials shift towards anodically, from 0.79 to 1.27 V and from 1.28 to 1.43 V, respectively, as the number of methylene groups is increased [50–53]. For example, the complex [Ni2L[1c](ClO4)](ClO4) (E1pc 0.79 V and E2pc 1.28 V), which are less negative in comparison to those of the complex [Ni2L[1b](ClO4)](ClO4) (E1pc 0.91 V and E2pc 1.31 V), which, in turn, are less negative in comparison to those of the complex [Ni2L[1a](ClO4)](ClO4) has the values of (E1pc 1.00 V and E2pc 1.43 V). This shows that as the number of methylene groups between the imine nitrogen (chain length) increases, the entire macrocyclic ring becomes more flexible, which causes easy reduction. Thus, the large size of the cavity easily holds the reduced cation and stabilizes the formation of Ni(I) in both compartments. The observed reduction potential for [Ni2L[1d](ClO4)](ClO4) (E1pc 1.06 V and E2pc 1.28 V) and [Ni2L[1e](ClO4)](ClO4) (E1pc 1.31 V and E2pc 1.47 V), shows that the reduction of complexes of the ligands L[1d] and L[1e] is rather difficult when compared to the reduction of the complexes of the ligands L[1a–1c] due to the planarity induced by the aromatic ring, which makes the system more rigid. 3.4. Oxidation process at anodic potential All the nickel complexes show two oxidation processes in the range 0.72–1.52 V. The cyclic voltammogram of the complexes is shown in Fig. 2 and the data are

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summarized in Table 2. The oxidation process is irreversible in nature. Controlled potential electrolysis experiment indicates that the two oxidation peaks are associated with stepwise oxidation process at nickel(II) center NiII NiII ! NiII NiIII ! NiIII NiIII The first and second oxidation potential of the complexes [Ni2L[1a–c](ClO4)](ClO4)] shifts towards more positive value [53] (from E1pc 0.72 to 0.98 V and E2pc 1.23 to 1.39 V) as the number of methylene groups (chain length) increases in the imine compartment. This is because as the ring size increases due to flexibility, the planarity of the complex decreases and the electrochemical oxidation process occurs with difficult. For example the first oxidation potential of the complex [NiL[1c](ClO4)]ClO4 is 0.91 V (E1pc ) and the second oxidation potential is 1.39 V (E2pc ) which are more positive when compared [NiL[1b](ClO4)]ClO4 (E1pc 0.80 V and E2pc 1.32 V) which is more positive than [NiL[1a](ClO4)]ClO4 (E1pc 0.72 V and E2pc 1.23 V). Similar trend has been observed for complexes [NiL[1d–e](ClO4)]ClO4. This is because for complexes with aromatic diimines, an increase in unsaturation will decrease the electron on the metal through delocalization, on to the ligand and this increases the difficulty to oxidize the metal ion. 3.5. Kinetic studies of hydrolysis of 4-nitrophenylphosphate The catalytic activity of the nickel(II) complexes on the hydrolysis of 4-nitrophenylphosphate was determined spectrophotometrically by monitoring the increase in the characteristic absorbance of the 4-nitrophenolate anion at 420 nm over the time in dimethylformamide at 25 °C. For this purpose, 103 mol dm3 solutions of complexes in dimethylformamide were treated with 100 equivalents of 4-nitrophenyl phosphate in the presence of air. The course of the reaction was followed at 420 nm for nearly 45 min at regular time intervals. The slope was

3.6. Antifungal and antimicrobial activities Antifungal and antimicrobial activities of the complexes were tested by the cup plate method using nutrient agar. The radial growth of the colony was recorded on completion of the incubation and the mean diameter for each complex at a concentration of 100 lg/ml was recorded. The average percentage inhibition of the bactericidal growth medium was compared using the Vincent (Vincent, 1947) equation: I = 100(C  T)/C, where I = percentage inhibition, T = average diameter of the bacterial growth on the tested plates and C = average diameter of the growth on the control plates. 0.35 0.3 0.25 0.2 0.15

c

absorbance(a.u)

Fig. 2. Cyclic voltammogram of the binuclear nickel(II) complexes: (a) [Ni2L[1a](ClO4)](ClO4), (b) [Ni2L[1b](ClO4)](ClO4) and (c) [Ni2L[1c](ClO4)](ClO4) (oxidation process).

determined by the method of initial rates by monitoring the growth of the 420 nm band of the product 4-nitrophenolate anion. A linear relationship for all the complexes shows a first-order dependence on the complex concentration for the systems. Plots of log (Aa/Aa  At) versus time for hydrolysis of 4-nitrophenylphosphate activity of the complexes are obtained and shown in Fig. 2. The inset in Fig. 3 shows the time dependent growth of p-nitrophenolate chromophore in the presence of [Ni2L[1a](ClO4)](ClO4). The observed initial rate constant values for all the nickel(II) complexes are given in Table 2. The catalytic activities of the binuclear complexes are found to increase as the macrocyclic ring size increases due to the intrinsic flexibility, i.e. increase in the chelate ring size enhances the rate constant of hydrolysis fairly well by producing distortion in the geometry around the metal ion that enhances the accessibility of the metal ion for the bonding of phosphate and OH groups. The catalytic activity of the complexes containing aromatic diimines [Ni2L[1d](ClO4)](ClO4) and [Ni2L[1e](ClO4)](ClO4) is found to be less than that of the complexes containing aliphatic diimines. This may be due to the planarity, which is associated with aromatic ring, imparts less catalytic efficiency due to the rigidity of the systems as observed in the case of previous literature reports [54–56].

log (Aα /Aα- At)

6

b a 500

340

Wavelength

0.1 0.05 0 0

10

20

30

40

50

Time (min) Fig. 3. Hydrolysis of 4-nitrophenylphosphate by binuclear nickel(II) complexes: (a) [Ni2L[1a](ClO4)](ClO4), (b) [Ni2L[1b](ClO4)](ClO4) and (c) [Ni2L[1c](ClO4)](ClO4) The inset is the time dependent growth of pnitrophenolate chromophore in the presence of [Ni2L[1a](ClO4)](ClO4).

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Table 3 Antibacterial and antifungal screening data of complexes No.

Complexes

Representation zone of inhibition (100 lg/ml) Antibacterial

1 2 3 4 5

[Ni2L[1a](ClO4)](ClO4) [Ni2L[1b](ClO4)](ClO4) [Ni2L[1c](ClO4)](ClO4) [Ni2L[1d](ClO4)](ClO4) [Ni2L[1e](ClO4)](ClO4)

Antifungal

Staphylococcus aureus

Bacillus cearus

Klebsiella pneumonia

pseudomonas aureginosa

16 14 16 18 19

15 14 20 24 26

14 15 15 25 25

15 14 16 20 19

3.6.1. Antifungal activity Studies on the antifungal activity of tetraazamacrocyclic ligands and their complexes are limited in the literature [57–60]. We have evaluated the antifungal activity of all the nickel(II) complexes against the phytopathogenic fungus Candida albicans. The screening data were reported in Table 3. It is observed from the results that all tested complexes show some antifungal activity. The activity reported by the present complexes was comparable with the N-substituted tetraazamacrocycles [58,61]. Generally nickel(II) complexes of N-substituted tetraazamacrocycles shows higher activity than their corresponding cobalt(III) complexes [62]. Another interesting result observed here is that the complexes of L[1d] and L[1e] shows higher activity than the complexes of the ligand L[1a] to L[1c], which contain aliphatic diimines. It seems from the results that the nature of the ligand and the coordinated metal ion plays a significant role in the inhibition activity. 3.6.2. Antibacterial activity All the prepared binuclear nickel(II) complexes have been subjected to antibacterial activity against selected five bacteria’s such as Pseudomonas aeroginosa (ATCC 2036), Staphylococcus aureus (ATCC 2079), Escherichia coli(ATCC 2567), Bacillus cereus (ATCC 11778), and Klebsiella pneumonia (ATCC 29665), respectively. The screening results are shown in Table 3. From the results it is observed that all the complexes shows higher activities against S. aureus, P. aeroginosa, K. pneumonia and B. cearus, but all the complexes were in-active against E. coli. The higher activity of the complexes may be ascribed to Tweedy’s theory, according to which chelation reduces the polarity of the central metal atom because of partial sharing of its positive charge with the ligand, which favors the permeation of the complexes through the lipid layer of the membrane [63]. However, further more studies are needed to understand the functions, which are responsible for antifungal and antibacterial activities of the tested complexes. 4. Conclusion In conclusion, it has been observed that the small variation in the chain length of the imine compartment, (i) cause red shift in the electronic spectra. (ii) Shift the reduction

Escherichia coli

Candida albicans 14 14 16 18 21

potential anodically on increasing chain length. (iii) Shift the oxidation potential to more positive potential as the chain length increases. (iv) Shows higher catalytic activity on increasing chain length. All these studies of the complexes which agree well with the established trend. Acknowledgements Financial support from University Grants Commission (RGNF), New Delhi, is gratefully acknowledged. We are thankful to CDRI-Lucknow for providing FAB mass spectral analysis. References [1] A. Binachi, L. Calabi, F. Corana, S. Fontana, P. Losi, A. Maiocchi, L. Paleari, B. Valtancoli, Coord. Chem. Rev. 204 (2000) 309. [2] O. Reany, T. Gunnlaugsson, D. Parker, Chem. Commun. (2000) 473. [3] T.A. Kaden, Top. Curr. Chem. 121 (1984) 157. [4] R.W. Hay, in: E. Kimura (Ed.), Current Topics in Macrocyclic Chemistry in Japan, Hiroshima University, 1987, p. 56. [5] N.W. Alcock, K.P. Balakrishnan, P. Moore, G.A. Pike, J. Chem. Soc., Dalton Trans. (1987) 889. [6] K.A. Arnold, L. Echegoyen, F.R. Fronczek, R.D. Gandour, V.J. Gatto, D. White, G.W. Gokel, J. Am. Chem. Soc. 109 (1987) 3716. [7] D.E. Fenton, G. Rossi, Inorg. Chim. Acta 98 (1985) L29. [8] V.J. Gatto, G.W. Gokel, J. Am. Chem. Soc. 106 (1984) 8240. [9] H. Adams, N.A. Bailey, W.D. Carlisle, D.E. Fenton, G. Rossi, J. Chem. Soc., Dalton Trans. (1990) 1271. [10] T. Koga, H. Furutachi, T. Nakamura, N. Fukita, M. Ohba, K. Takahashi, H. Okawa, Inorg. Chem. 37 (1998) 989. [11] S. Uozumi, H. Furutachi, M. Ohba, H. Okawa, D.E. Fenton, K. Shindo, S. Murata, D.J. Kitko, Inorg. Chem. 37 (1998) 6281. [12] D.E. Fenton, Inorg. Chem. Commun. 5 (2002) 537. [13] P.A. Vigato, S. Tamburini, Coord. Chem. Rev. 248 (2004) 1717. [14] S.A. Duclos, H. Stoeckli-Evans, T.R. Ward, Helv. Chim. Acta 84 (2001) 3148. [15] H. Golchoubian, A. Nemati Kharat, Polish J. Chem. 79 (2005) 825. [16] A. Niazi Rahnama, H. Golchoubian, R. Pritchard, Bull. Chem. Soc. Jpn. 78 (2005) 1047. [17] H. Golchoubian, W.L. Waltz, J.W. Quail, Can. J. Chem. 37 (1999) 77. [18] A. Ghaffarinia, H. Golchoubian, R. Hosseinzadeh, J. Chin. Chem. Soc. 52 (2005) 531. [19] H. Okawa, H. Furutachi, D.E. Fenton, Coord. Chem. Rev. 51 (1998) 174. [20] D.E. Fenton, in: A.G. Sykes (Ed.), Advances in Inorganic and Bioinorganic Mechanisms, vol. 2, Academic Press, London, 1983, p. 187.

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