1. INTRODUCTION:

The synthesis and characterization of coordination compounds with macrocyclic ligands has evolved during the last years as one of the main research areas in coordination chemistry [1–5]. Recent publications have shown that the rigorous elimination of humidity and the use of chromatographic separations lead to a significant increase in the yields. Additional information related to the spectroscopic and electrochemical properties of these ligands and some of their transition metal complexes is now available [6,7]. Coordination geometry and properties of most transition metal complexes with 16-membered macrocyclic ligands containing six nitrogen atoms in fully saturated macrocyclic framework have been studied [2].

Aza-type ligands appear as very promising to be used as antifertile, antibacterial, antifungal and other biological properties Metal-Schiff-base complexes are excellent coordination/organometallic compounds to construct supramolecular compounds due to their powerful applications in the field of catalysis, magnetic materials, and bioinorganic chemistry [3-11]. The macrocyclic Schiff base obtained has been versatile in forming a series of complexes with Cr(III) [12,13], Mn(II)[13], Mn(III)[14], Fe(II)[15], Fe(III)[13],Co(II)[4,15,16], Ni(II)[6,16], Cu(II)[3,15,16], Zn(II)[15,16], Ru(II)[9,10], Ru(III)[13], Rh(III)[5], Ir(III)[13], Pd(II)[8] and Pt(II)[13] ions under well defined conditions and these complexes have been investigated with particular reference to the structural aspects of the ligand moiety in the metal complexes. The coordination chemistry of macrocyclic ligands is a fascinating area of intense study for inorganic chemists [2,17-19]. The aspect of interest in macrocyclic ligands is raised from features such as the nature, number and arrangement of ligand donors as well as ligand conjugation, substitution and flexibility, which produce different types of macrocyclic molecules suitable for specific uses [2]. Transition metal complexes of tetradentate Schiff base ligands find applications in catalysis [5,6,10].

Metal template condensation reaction often provides selective routes toward products that are not obtainable in the absence of metal ion. The reactions are simple “one-pot reactions”, cheap and high yielding. We have been interested in the synthesis of various poly-aza macrocyclic complexes from one-pot metal template condensation reactions. However the ligand is prepared first and its structure is confirmed by IR and mass spectroscopy. The complexes of Cu(II), Mn(II), Co(II) and Ni(II) are first synthesized from macrocyclic ligand containing glutarimide and 2,6-diamino pyridine and structurally characterized.

2. EXPERIMENTAL:

2.1. Preparation of complexes

The complexes were prepared by template method because the yield of ligand is very low. The appropriate metal salt (1 mmol) was dissolved in 20 ml of hot ethanolic solution and added to it an ethanolic solution (20 ml) of glutariamide (1 mmol). Then 2,6-diamino pyridine was added and resultant mixture was refluxed on water both at 76 ^oC. The coloured complexes separate out. The products were filtered off, washed with alcohol and dried over P₂O₅. The reaction was attempted with nitrates and chlorates of Cu(II), Mn(II), Ni(II) and Co(II).

2.2. Physical measurements

2.2.1. IR spectra

IR spectra were recorded on a Perkin-Elmer 137 instrument as Nujol mulls/KBr pellets.

2.2.2. Elemental analysis

Microanalyses (C, H and N) of these complexes were carried out on a Carlo-Erba 1106 elemental analyzer.

2.2.3. Electronic spectra

Electronic spectra were recorded in DMSO solution on a Shimadzu UV mini-1240 spectrophotometer.

2.2.4. EPR spectra

EPR spectra of the complexes were recorded as polycrystalline sample and in the DMSO solution, at liquid nitrogen temperature for Co(II) and at room temperature for Mn(II) and Cu(II) complexes on E4-EPR spectrometer using the DPPH as the g-marker.

2.2.5. Molar conductance

The molar conductance was measured on an Elico conductivity bridge (type CM82T).

2.2.6. Magnetic moment

Magnetic moment measurements (Guoy balance) were made at room temperature using CUSO4·5H2O as callibrant.

2.2.7. Electrochemical procedures

The experiments were carried on a PAR 173 potentiostat/ galvanostat instrument with a PAR 175 model universal programmer coupled to a PAR 179 digital coulometer. The voltammograms and the simultaneous current intensity-time plots for electrolysis were registered in a X–Y Houston-Ommigraphic 2000 recorder. Cyclic voltammetry and controlled potential electrolysis coulometry at room temperature (RT) (approximately 25^oC) under either inert (N₂) or saturated CO₂ atmosphere were performed in a specially designed air-tight electrochemical cell with a double three-electrode system, to obtain cyclic voltammograms ‘in situ’ for the electrolytically generated products. All electrodes were introduced into the cell body by means of a ST glass joint and caps with Teflon-faced-silicon septa were used for the needles that bubble and exit gases. Coulometric measurements were performed using a reticulated vitreous carbon (ERG) and saturated calomel as working and reference electrodes respectively. The last one was supplied by a vycor tip glass-end. Platinum spiral wire counter electrode was separated by means of a salting bridge through sinterized glass ends. For cyclic voltammetry experiments a vitreous carbon-polished teflon-coated button was used as a working electrode.

2.2.8. Antibacterial activity

Antibacterial activities of the ligand and their complexes were evaluated by the disc diffusion technique [20]. Filter paper (Whatman No. 4) discs (5 mm diameter) were dipped in the solution of the test compound of 0.2–0.8 mg/cm3 concentration in DMSO and placed over seeded plates, after drying to remove the solvent, and incubated at 37^oC for 2 h. The compounds diffuse on nutrient agar plates and prevent the growth of bacteria in the zone around the disc. DMSO was used as the control and gentamicin as the standard drug. The results indicate that the compounds inhibit the growth of bacteria to a greater extent as concentration is increased.

3. RESULTS AND DISCUSSION:

A new series of mononuclear macrocyclic complexes [ML]X₂ (M = Cu, Mn, Ni, Co and X = NO₃, Cl) have been prepared. The complexes were stable in atmosphere and were polycrystalline. All the complexes have high melting point (>250⁰C). The result of elemental analysis (Table 1) support the proposed macrocyclic structure (Fig.5-8) .

3.1. Molar conductance

The molar conductance values of all the complexes in DMSO solution suggest that the complexes are 1:2 electrolyte [21].

3.2. IR spectra

Infrared spectra of the ligand under study do not exhibit any band due to –NH₂ group of diamine in 3200-3350 cm^-1 region and >C = O group of carbonyl compounds above 1700 cm^-1.The appearance of main characteristic, strong band >C = N in the range of 1599-1628 cm^-1 confirms the reaction between primary amino group –NH₂ group of diamine and carbonyl group of diketone or any other carbonyl compounds. These confirm the elimination of water molecule, complete condensation and supports macrocyclic structure. Further two extra medium to strong bands at 1593-1591 cm^-1and 1454-1452 cm^-1appears for the two highest energy pyridine ring vibrations. [Fig. 1]. Mass spectra of ligand shows a peak 371amu corresponding to molecular ion (M⁺+ 1) and different peaks corresponding various fragments [Fig. 2]

(Ligand – 1) → (Molecular ion) M⁺+ 1

C₂₀H₂₀N₈→ [C₂₀H₁₉N₈ ]⁺ + 1

(372.43) (Mass = 371)

Fig. 1 Structure of ligand

The IR spectra of the complexes show moderate intensity absorptions at 1680 cm^-1 attributable to the imine v (C=N) [22] but no bands are observed for free C=O or primary diamine indicating that complete condensation has been occurred and support the macrocyclic structure. A shift in absorption of the v (C=N) frequency in case of the complexes suggests the coordination through the nitrogen of (>C=N) group. The presence of a broad band at 1384 cm^-1 indicates the uncoordinated behaviour of nitrate groups.

3.3. Electronic spectra

These macrocyclic complexes containing glutarimide are soluble in DMSO. Electronic spectra data for Cu(II), Mn(II), Co(II), Ni (II) complexes are shown in Table 2.

3.4. Cyclic voltametry and coulometry

We have studied cyclic voltammetry results in the cathodic region for Ni(II) hexa-aza macrocyclic complexes and suggested the stabilization of Ni(I) at -0.85 versus SCE, respectively. Coulometric measurements indicate that these reactions involve one-electron reduction [23]. Fig. 3 shows electrochemical results of the Ni(II) complex in nitrogen and carbon dioxide in the cathodic region. The presence of low-intensity redox process in the free ligand (in CO2) suggest that the CO2 reduction process in the Ni(II) complex is probably occurring on the metal center [24]. It also shows the clear modification of the cyclic voltammogram for nickel complex, measured in carbon dioxide media. The intensity of the cathodic peak (approximately 1.5 V versus SCE) approximately three times when nitrogen is changed to carbon dioxide. In addition, it has been found that coulometry by controlled potential electrolysis at -1.6 V in CO medium has a constant intensity for a long period of time, while the behaviour in nitrogen atmosphere is normal and shows one electron reduction process. Similar results have previously been observed in analogous systems such as Ni(cyclam)2- complexes [25,26]. The reduction of carbon dioxide might produce formic acid, -0.61 V; carbon monoxide, -0.52 V; formaldehyde, -0.48 V; methanol, -0.38 V or methane, -0.24 V (versus NHE), in multielectron transfer reactions [27]. A comparison of these electron transfers with the one-electron reduction of carbon dioxide to CO2, -1.2 V; or to COOH, -0.60 V (versus NHE), shows that the thermochemical advantage is balanced by the kinetic constrains that are imposed by the requirement of multi-electron transfers. Adducts of CO2 are probably involved in the reduction mechanism, as it has been found in radiolytic studies previously performed [28]. (fig.3.)

Fig.3 Cyclic voltammograms of the Nickel(II) complexes (0.1mM)

4. EPR spectra :

4.1. Copper(II) complexes

The complexes exhibit an anisotropic EPR spectra,recorded as polycrystalline sample at RT, which is characteristic for tetragonal Cu(II) complexes [29]. The anisotropic g-values have been calculated by Kneubuhl’s method [30] and by method reported earlier [31]. The observed g-values are reported in Table 3. G =(g|| 2)/(g.- 2), which measure the exchange interaction between the copper centers in polycrystalline samples, have been calculated. According to Hathaway [31–32],if G> 4, the exchange interaction is negligible. A value of G< 4, indicates considerable exchange interaction in solid complexes. The calculated G valus are larger than four, suggesting that there is no exchange between the copper centres.

4.2. Mn(II) complexes

EPR spectra were recorded at room temperature as polycrystalline sample and in DMSO solution. The polycrystalline spectra were isotropic and exhibit the ‘g’ value in the range 2.0178–2.0013 (Table 3). In DMSO solution Mn(II) complexes give EPR spectra containing six lines arising due to the hyperfine interaction between the unpaired electrons with the 55 Mn nucleus (I = 572). The nuclear magnetic quantum number MI, corresponding to these lines are 5/2, -3/2, -1/2, +1/2, +3/2 and +5/2 from low to high field.

4.3. Co(II) complexes

EPR spectrum of the nitrate complex recorded at RT as polycrystalline sample (g||

= 2.702, g. = 2.282), which again provide strong evidence in the favour of octahedral geometry.

4.4. Bacterial screening

Results of antibacterial screening shows in Table 4 indicate that the metal complexes inhibit higher antibacterial activity than that of free ligand and the control. The increased inhibition with increase in concentration is due to the effect of metal ion on the normal cell process (Fig. 4).

Fig.4. Antibacterial activity shown by ligand and complexes

5. CONCLUSION:

The synthesized macrocyclic complexes studied by EPR, UV–vis, magnetic, spectral and electrochemical procedures supports the macrocyclic structure and all the complexes may have octahedral geometry except Cu(II) complexes which may have tetragonal geometry. From antibacterial studies it can be seen that variation in the metal imposes an influence on the bacterial activities (Fig. 5-8).

Fig. 5. Structure of copper complex (X=NO₃,Cl)

Fig. 6. Structure of manganese complex (X=NO₃,Cl)

Fig. 7. Structure of cobalt complex (X=NO₃,Cl)

Fig. 8. Structure of nickel complex (X=NO₃,Cl)

Table1 Molar conductance and elemental analysis data of complexes

Complexes

M.W. Calc. (Found)

Molar cond. W^-¹cm²

mol^-¹

Colour

Yield %

M.Pt.

°C

Elemental analysis Calc. (Found)

[MnL₁]Cl₂

(C₂₀Cl₂H₂₀MnN₈)

498.27

(494)

234

Cream

239

11.02

(11.06)

48.21

(48.26)

4.41

(4.44)

22.48

(22.52)

[MnL₁](NO₃)₂

(C₂₀H₂₀MnN₁₀O₆)

551.37

(548)

242

Light Pink

241

9.97

(9.99)

43.56

(43.59)

3.98

(3.99)

20.31

(20.35)

[CoL₁]Cl₂

(C₂₀Cl₂CoH₂₀N₈)

502.33

(449)

210

Lotus pink

178

11.73

(11.77)

47.82

(47.86)

4.37

(4.41)

22.30

(22.35)

[CoL₁](NO₃)₂

(C₂₀CoH₂₀N₁₀O₆)

555.43

(552)

230

Dark pink

192

10.60

(10.65)

43.24

(43.28)

3.95

(3.98)

20.16

(20.19)

[NiL₁]Cl₂

(C₂₀Cl₂H₂₀N₈Ni)

502.04

(499)

242

Dark green

182

11.69

(11.74)

47.84

(47.87)

4.37

(4.43)

22.31

(22.35)

[NiL₁](NO₃)₂

(C₂₀H₂₀N₁₀Ni O₆)

555.15

(550)

233

Leafy Green

167

10.57

(10.62)

43.27

(43.31)

3.95

(3.98)

20.17

(20.21)

[CuL₁]Cl₂

(C₂₀Cl₂CuH₂₀N₈)

506.88

(505)

231

Blue

138

12.53

(12.54)

47.39

(47.41)

4.33

(4.34)

22.09

(22.10)

[CuL₁](NO₃)₂

(C₂₀CuH₂₀N₁₀O₆)

559.98

(558)

262

Prussian

Blue

129

11.34

(11.34)

42.89

(42.90)

3.92

(3.93)

20.00

(20.01)

Table 2 Magnetic Moments and Electronic Spectral Bands of the Complexes

Complexes	µ_eff B.M	Electronic spectral bands (cm^-1) and (Lmol^-1cm^-1)	Assignment
[Mn L₁] Cl₂	5.91	17500,23590	⁴G→⁶S,⁶A₁g→⁴Eg(G),⁶A₁g→⁴Eg(D)
[Mn L₁] (NO₃)₂	6.01	17980,19440,23500	⁶A₁g→⁴Eg(G),⁶A₁g→⁴T₁g(G)
[Co L₁] Cl₂	5.02	18621,14794	⁴T₁g→A₂g, ⁴T₁g→⁴T₁g(P)
[Co L₁] (NO₃)₂	5.04	14927,19831,88103	⁴T₁g→⁴A₂g,⁴T₁g→⁴T₁g(P),⁴T₁g→⁴T₂g
[Ni L₁]Cl₂	3.01	12587,16343	³T₂g→³A₂g,³A₂g→¹Eg,³A₂g→³T₁g(P)
[Ni L₁](NO₃)₂	3.02	8500,13000,23700	³A₂g→¹Eg, ³A₂g→³T₁g
[Cu L₁] Cl₂	1.79	12500,16300,21450	²B₁g→²B₂g,2B₁g→2Eg
[Cu L₁](NO₃)₂	1.80	16000,18000	²B₁g→²B₂g,²B₁g→²A₁g,²B₁g→²Eg

Table 3 Ligand Field Parameters and EPR Spectral Data of the Complexes

Complexes	Ligand field Parameters				EPR Spectral data at RT (for Co(II) LNT)
Complexes	Dq (cm^-1)	B (cm^-1)	β	LFSE(Kjmol^-1)	g_װ	g_┴	g_iso	G
[MnL₁]Cl₂	1895	621.65	0.79	-	-	-	-	-
[MnL₁](NO₃)₂	1874	618.72	0.76	-	-	-	-	-
[Co L₁] Cl₂	946	986	0.87	91	5.861	3.077	4.002	-
[CoL₁](NO₃)₂	302	684	0.63	45	2.704	2.28	2.412	-
[Ni L₁] Cl₂	1140	661	0.64	163	-	-	-	-
[NiL₁] (NO₃)₂	955	765	0.76	134	-	-	-	-
[Cu L₁] Cl₂	-	-	-	-	2.306	2.065	2.277	4.23
[CuL₁](NO₃)₂	-	-	-	-	2.261	2.062	2.141	4.35

Table 4 Antibacterial activity of ligand and complexes (zone formation in mm)

S. No	Compounds	*S. aureus* (0.2mg/cm³)	*S. aureus* (0.8mg/cm³)	*E. coli* (0.2mg/cm³)	*E. coli* (0.8mg/cm³)
1	Control	6	7	7	9
2	Ligand	7	9	10	12
3	Mn(II)	11	13	14	15
4	Co(II)	14	16	16	18
5	Ni(II)	16	18	17	19
6	Cu(II)	9	11	12	13

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Received on 06.08.2013 Modified on 08.09.2013

Asian J. Research Chem 7(1): January 2014; Page 01-06

1. INTRODUCTION:

Table 2 Magnetic Moments and Electronic Spectral Bands of the Complexes

Dq (cm-1)

B (cm-1)

Dq (cm^-1)

B (cm^-1)