Synthesis, Characterisation and Antimicrobial Screening of Co(II), Mn(II) Ni(II), Cu(II) and Zn(II) Complexes of Schiff Base Ligand

Introduction

Schiff bases derived from aromatic amines and aromatic aldehydes have a wide variety of applications in many fields eg., biological, inorganic and analytical chemistry.^1-5 They are known to exhibit potent antibacterial, anticonvulsant and anti inflammatory activities⁶.  In addition some Schiff bases show pharmacologically useful activities like anticancer⁷, anti-hypertensive and hypnotic⁸ activities. Schiff bases are important  class of compounds due to their flexibility, structural similarities with natural biological substances and also due to the  presence of imine moiety (-N=CH-) which is potential  in elucidating the mechanism of transformation and resamination reaction in biological system.  These novel compounds could also act as valuable ligands whose biological activity has been shown to increase on complexation.  Schiff bases of isatin derivatives have been used to demonstrate a variety of biological activities, such as anti-inflammatory⁹ , anti HIV¹⁰ and anti-depressant activities.

The present paper describes the synthesis and characterization of     Co(II), Mn(II), Ni(II), Cu(II) and Zn(II) complexes derived from 3-aminophenol and 2-hydroxy-3-methoxy benzaldehyde.  The ligand and its metal complexes are characterized by elemental analysis, IR, ¹H NMR, UV and molar conductance measurements. The biological activities are also studied against gram positive and gram negative  bacterial and fungi organisms for Schiff base ligand and their complexes.  The  structure of Schiff base ligand  is given in Figure 1.

Experimental

Materials and Methods

All chemicals used were of analytical reagent grade (AR) and of highest purity available .  Solvents were purified and dried according to the standard procedures.  All metal (II) compounds were used as acetate salts.  IR spectra of the complexes were recorded in KBr pellets with a Perkin Elmer RX1 FT-IR Spectrophotometer in the 4000-400cm^-1 range.  The electronic spectra were recorded in DMF on a Perkin Elmer Lambda 35 spectrophotometer in the 190-1100 nm range.  The ¹H NMR spectra were recorded on a Bruker 400MHz FT-PMR spectrometer(DMSO-d₆).  Melting points were determined using  melting point apparatus (Elico) and were uncorrected. Conductivity measurements for the complexes were carried out using Elico conductivity bridge and a dip conducitivity cell in dimethyl formamide as solvent.

Synthesis of Schiff base ligand

(L)  The Schiff base was prepared by the condensation of equimolar amounts of  3-aminophenol (0.002mol) and 2-hydroxy-3-methoxy benzaldehyde

(0.002mol)  in minimum quantity of ethanol.  The resulting mixture was then refluxed on a water bath for 4 hours.  An orange coloured solid mass separated out on cooling was filtered, washed and dried.The purity of the ligand was checked by melting point, TLC and spectral data. The ligand is insoluble in some common organic solvents viz.acetone, benzene and soluble in polar solvents viz.DMF, DMSO.

Synthesis of metal complexes

Metal complexes were synthesized by mixing the hot solution of ligand (0.004 mole) in minimum quantity of dimethyl formamide and ethanolic solution of metal  acetates (0.002 mole). The resulting mixture was then refluxed in a water bath for 6 hours.  The complexes obtained in each case were cooled, filtered and washed with ethanol several times to remove any excess of the ligand.  Finally complexes were washed  and dried.

The micro analytical data, melting point, colour and other physical properties of the ligand and its metal complexes are given in Table 1. The probable structure of the complexes proposed in the present work is given in Figure 2.

Figure 1: Click here to View figure

 

Figure 2 Click here to View figure

Table 1: Physical Characteristics and Microanalytical data of Schiff base ligand and their complexes.

S.No	Ligand/Complexes	Colour	Molecular Formula	M.P ◦C	Yield  %	Elemental Analysis (%) Calcd(found) C              H              N			CN	%M cald(found)	Λmohm-1m2 mol-1
1	L	orange	C14H13NO3	140	75	57.73(58.03)	4.46(4.40)	4.81(4.80)	–	–	–
2	[CoL2]	Pale yellow	C28H22N2O6Co	250	60	62.11(62.01)	4.06(4.03)	5.18(5.05)	6	10.89(10.5)	2.60
3	[MnL2]	Dark brown	C28H22N2O6Mn	220	65	62.57(62.37)	4.09(3.91)	5.21(5.20)	6	10.22(9.58)	2.37
4	[NiL2]	Yellow	C28H22N2O6Ni	221	70	62.14(62.12)	4.06(4.01)	5.18(5.12)	6	10.85(10.2)	2.94
5	[CuL2]	Dark brown	C28H22N2O6Cu	160	55	61.59(61.52)	4.00(4.12)	5.13(5.09)	6	11.65(11.0)	2.37
6	[ZnL2]	Pale yellow	C28H22N2O6Zn	150	55	61.38(61.31)	4.01(4.02)	5.11(5.15)	6	11.95(14.3)	6.02

Results and Discussion

The Schiff base ligand is synthesized by using  equimolar quantities of  3-aminophenol and

2-hydroxy-3-methoxybenzaldehyde and is complexed with metal acetates.  The metal complexes derived vary in their colour.  All the complexes are stable, non-hygroscopic and coloured solids.  The low molar conductance values in the range of 2.3 – 6.0 ohm^-1cm²mol^-1 in Co(II), Mn(II) , Ni(II), Cu(II), and Zn(II) chelates indicate that they are non-electrolytic  in nature ¹².

Infrared Spectra

The infrared spectral data of the Schiff base and its metal complexes are recorded in Table-2.  Schiff base showed a strong absorption band at 1603 cm^-1 characteristic of ν (C=N) whereas the broad band at 3474 cm^-1 characteristic of hydrogen bonded  ν(O-H) stretching vibration ¹³.  The azomethine   ν(>C=N) band at 1603 cm^-1 in Schiff base is shifted to lower frequency in Co(II), Mn(II),Ni(II),Cu(II), and Zn(II) by 6,11,5,109 and 3cm^-1  respectively which indicated the co-ordination of azomethine nitrogen on complexation ¹⁴. The disappearance of phenolic ν(OH) at 3474 cm^-1 in all the complexes suggests the coordination of phenolic oxygen after deprotonation¹⁵.  The linkage with oxygen atom is further supported by the appearance of a band in the region around  702 cm^-1  which may be assigned to ν (M-O)¹⁶. A further evidence of the coordination of the Schiff base with the metal atom was shown by the appearance of a new weak frequency band at 533-560cm^-1 assigned to the metal nitrogen ν(M-N)¹⁷. These new bands were observed only in the spectra of the metal complexes and not in Schiff base  confirming the participation of the donor groups.

Table 2: IR and Electronic spectral data.

Ligand/ Complexes	IR spectral data , cm-1    ν(O-H)           ν(C=N)     ν(M-N)        ν(M-O)				Electronic spectral data, cm-1
L	3474	1603	–	–	47431,43767,32235
[CoL2]	3423	1609	559	741	32130,27392
[MnL2]	3421	1614	558	740	30032,36182
[NiL2]	3427	1608	468	771	36212,30521,26126
[CuL2]	3418	1712	533	702	26130
[ZnL2]	3410	1606	541	732	31225

Electronic Spectra

Electronic spectrum of the ligand shows two high intensity bands at 47431 and 32235  cm^-1 indicating  n  →    n*   and π   →    π*  transitions respectively of the ligand moiety ¹⁸.  The electronic spectra of Co(II) complex displays bands at 32130 and 27392 cm^-1. The two bands corresponds to  ⁴T_1g(F)    →      ⁴A_2g(F) ,   ⁴T_1g (F)    →    ⁴ T_1g(P) suggesting octahedral geometry of this complex¹⁹.  Ni(II) complex shows  absorption  bands at 36212, 30521 and 26126 cm^-1.  The high intensity band at 36212cm^-1 is  relatively attributed toL  →  M charge transfer transitions ²⁰ whereas at 30521 and 26126cm^-1 may be due to³A_2g ³T_1g and

⁴T_1g   →     ⁴T_2g(P). The Cu(II) complex displays a band at 26130cm^-1.  This  band corresponds to  ²E_(g)     →   ²T_2g  .The electronic spectrum of the Mn(II) complex shows two bands at 36182 and 30032 cm^-1 assignable to M   →  L charge transfer transitions and the 30032cm^-1 is due to

⁶A_1g  →   ⁴E_g(D)²¹. Zn(II)  complex displays single absorption  band at 31225cm^-1.  This is  due to Ligand → Metal charge transfer  spectra^.22,23

¹H NMR Spectra

The ¹H NMR Spectra  of Schiff base and its complexes  were recorded in DMSO (d₆).  The azomethine proton (-CH=N-) in Schiff base appeared at δ = 8.8 ppm has been shifted to  downfield   in metal complexes. This confirms the coordination by azomethine nitrogen ²⁴.  The aromatic protons in Schiff base  appeared in the  range at δ 6.93-8.0 ppm and in metal complexes  in the range δ 6.54-8.75 ppm ²⁵.  The disappearance of phenolic –OH proton signal at δ 13.2ppm confirms the coordination by phenolic oxygen to metal ion.

Antimicrobial  activity

Antibacterial and antifungal activity of Schiff base ligand and its cobalt, nickel,copper, manganese and zinc complexes have been tested by disc diffusion technique ^26,27. The various gram positive and gram negative bacterial organisms such as Gram negative bacteria pseudomonas aeruginosa, E.coli  Gram positive bacteria staphylococcus aureus, Bacillus subtilis  and fungi aspergillus niger and candida albicans were used to find out the antimicrobial and antifungal  activity. (Table 3).  Filter paper discs of diameter 6mm were used  and the diameters of zones of inhibition formed around each disc after incubating for a period of 72 hours at 25-30⁰ C were recorded .  Results were compared with standard  drug Ciprofloxacin for bacteria and Nystatin for fungi at the same concentration. All the new complexes showed a remarkable biological activity against bacteria and fungus. From the results it is clear that the  metal complexes  are found to have more biological activity than the parent ligand.

Table 3: Antimicrobial Activity of Schiff base ligand and complexes.

Antimicrobial activity of the ligand and complexes	Staphylococcus aureus	Bacillus subtilis	E.coli	Pseudomonas aeruginosa	Candida albicans	Aspergillus niger
Ligand (L)	++	++	++	++	++	++
[CoL2]	++	++	+++	+++	++	+++
[MnL2]	++	+++	++	+++	+++	+++
[NiL2]	++	+++	+++	++	+++	++
[CuL2]	+++	+++	+++	++	+++	+++
[ZnL2]	++	++	+++	+++	+++	++

Standard= ciprofloxacin 5 g/ disc for bacteria ; Nystatin= 100 units/disc for fungi.

Highly active  = +++ (inhibition zone > 15mm) ; Moderatively active = ++ (inhibition zone> 10mm) ; slightly active = + (inhibition zone >5mm); Inactive = — (inhibition zone <5mm)

Conclusion

On the basis of the results obtained from elemental analysis, infrared spectra, electronic spectra, ¹H NMR and  conductance  measurements, an octahedral structure has been proposed for all  complexes.

Acknowledgements

The authors express their gratitude to The Secretary, Seethalakshmi Ramaswami College,Tiruchirappalli, India  for providing laboratory facilities and the faculty members, PG and Research Department of  Chemistry, Seethalakshmi Ramaswami College for their timely help.

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