Synthesis, Spectral Characterization and Antimicrobial Screening of Co(II) and Ni(II) Complexes derived from

(E)-1-((p-tolylimino)methyl)naphthalene-2-ol

 

V. Gomathi, R. Selvameena, R. Subbalakshmi and G. Valarmathy

PG and Research Department of Chemistry, Seethalakshmi Ramaswami College,

Tiruchirappalli-620 002, Tamil Nadu, India.

*Corresponding Author E-mail: vemathi@gmail.com and rsmchemsrc@gmail.com

 

The new coordination complexes of Co(II) and Ni(II) have been synthesized from Schiff base derived from 2-hydroxy-1-naphthaldehyde and p-toluidine. The nature of bonding and the structural features of the Schiff base and its complexes have been deduced from elemental analysis, molar conductance, magnetic susceptibility measurements, IR, ¹H NMR, UV-Vis, and cyclic voltammetry studies. The spectral data of the complexes have revealed bidentate complexing nature of the Schiff base through azomethine nitrogen atoms and phenolic oxygen. The Schiff base and its metal complexes were screened for their antimicrobial activity. The complexes showed enhanced antimicrobial activity as compared to uncomplexed Schiff base.

 

 

INTRODUCTION:

Schiff bases are versatile ligands which are synthesized by the condensation of primary amines with carbonyl groups under different conditions and in different solvents with the elimination of water molecules¹. R – NH₂ + R – CHO à R – N = CH – R + H₂O. Schiff base ligands are able to coordinate with the metal through imine nitrogen. Several scientists showed² that the presence of a lone pair of electrons in sp² hybridized orbital of nitrogen atom of the azomethine group is of considerable chemical and biological importance. Nowadays Schiff bases are attracting the biochemists. They are known to be medicinally important and are used to design medicinal compounds³. The Schiff base metal complexes are therapeutically active and possessing analgesic⁴, cytotoxic⁵, antiinflammatory⁶, antitumorous⁷, antiviral⁸, antipyretic⁹, and antitubercular activity besides their applications as antimicrobial agent¹⁰. 

 

EXPERIMENTAL:

All the reagents used were of AR grade. Solvents were purified and dried according to the standard procedures. Conductivity measurements for the complexes were carried out using Elico conductivity bridge and dip type conductivity cell. Magnetic susceptibilities were determined using electromagnet/Gouy method-PICO make.

 

Melting points were determined using Elico melting point apparatus. Elemental analysis (C,H,N) were performed using elemental analyser. IR spectra of the ligand and its complexes were recorded in KBr pellets with Perkin Elmer IR RXI Spectrometer in the 4000-400 cm^-1 range.  The ¹H NMR spectra was recorded on a Bruker 400 MHz FT- PMR Spectrometer (DMSO-d₆). The electronic spectra were recorded in Perkin Elmer Lambda 35 spectrometer in the 190-1100 nm range. Cyclic Voltammetric measurements for the complexes were measured using Princeton applied Research –Multichannel Versastat-II.

 

Synthesis of Schiff base - (E)-1-((p-tolylimino)methyl) naphthalene-2-ol(L):

The Schiff base was prepared by the condensation of equimolar amounts of p-toluidine (0.004mol) and 2-hydroxy-1-naphthaldehyde (0.004 mol) in minimum quantity of ethanol. The resulting mixture was then refluxed for 4 hours. A yellow coloured solid mass separated out on cooling is filtered, washed and dried over anhydrous CaCl₂ in a desiccator. The purity of the ligand was checked by TLC and spectral data.

 

Synthesis of complexes:

Complexes of (E)-1-((p-tolylimino)methyl)naphthalene-2-ol were synthesized by mixing a hot ethanolic solution of Schiff base(L) (0.004 mol) in minimum amount of dimethylformamide, and ethanolic solution of corresponding M(II) acetate (0.002mol).

Table -1 Physical Characteristics of the Schiff base and

its complexes

 

Table-2 Analytical data of Schiff base and its complexes

 

 

The mixture was refluxed for 6 hours. The complexes obtained in each case were cooled, filtered and washed with ethanol several times to remove any excess ligand. Finally the complexes were washed with anhydrous diethylether and dried in a desiccators.

 

RESULTS AND DISCUSSION:

The physical characteristics of the Schiff base and its complexes are given in Table 1. The analytical data (Table 2) of the Schiff base and its complexes agree very well with the proposed molecular formulae and also indicates the formation of 1:2 (M:L) complexes of general formula of [M(L-H)₂(H₂O)₂] [L=Mn²⁺ and Zn²⁺ ]. 

 

Molar Conductance:

The molar conductance of metal complexes are measured using  10^-3M DMF solutions and are found within the range 2.6 and 4 ohm^-1 cm² mol^-1 suggesting the  non-electrolytic nature¹¹ and indicate that no anions are present outside the coordination sphere (Table 2).

 

IR Spectra:

The Schiff base shows a strong absorption band at 1627cm^-1 characteristic of n_(>C=N–)¹² whereas the broad band at 3448cm^-1 is  characteristic of hydrogen bonded n_(O–H)¹³ stretching vibration. The azomethine peak of ligand at 1627cm^-1 is shifted to 1600cm^-1 for both  Co(II) and Ni(II) complexes suggesting the coordination through >C=N– group. The disappearance of phenolic (OH) at 3448cm^-1 in Co(II) complex suggest the coordination of phenolic oxygen to metal atom after deprotonation. The linkage with oxygen atom is further supported by the appearance of a band in the region at 651 cm^-1 and 653cm^-1 may be assigned to n_(M–O) and the appearance of a band in the region at 449cm^-1 and 464cm^-1 may be assigned to n_(M–N). The band in the region 3400-3600cm^-1 indicates the presence of water ¹⁴molecule.

 

Electronic spectra and magnetic susceptibility measurements:

The ligand  is yellow in colour shows three absorption bands at 347.02nm (28816 cm^-1), 417.2nm (23968 cm^-1), 478.80nm (20885 cm^-1) accounts for nàn* and p à p* transitions. The Co(II) complex is brown in colour and it shows absorption band around 319.95nm (31254 cm^-1). This is due to ligand-metal charge transfer and the magnetic moment at 3.8 BM confirms the octahedral geometry. The Ni(II) complex is light green in colour and exhibits three absorption bands at 271.23nm (36869cm^-1), 322.22nm (31034cm^-1), 394.13nm (25372 cm^-1)¹⁵. The first two bands may be attributed to L à M charge – transfer transitions. The band in the region 25372cm^-1 can be assigned to ³A_2g(F) à ³T_1g(P) d-d transition and the magnetic moment of 2.3 BM confirms the octahedral geometry.

 

¹H NMR:

¹H NMR Spectra of free ligand and its Zn(II) complex were recorded in DMSO-d₆.The multiplet which extents from d 7.2 to d 8.4 corresponds to the 10 protons of the aromatic rings¹⁶. The singlet peak at d 9.6 ppm suggest the presence of –CH = N – linkage¹⁷. The peak at d15.9 ppm confirms the presence of the hydroxyl proton of the ligand. The CH₃ protons appear at d 2.3ppm.

 

Cyclic voltammetry measurements:

Electrochemical cyclic voltammetry measurements were performed at room temperature in an airtight three electrode cell by using glassy carbon electrode with 0.071 cm² surface area as a working electrode, a platinum wire served as the counter electrode and a Ag/AgCl in a saturated KCl solution as reference electrode.

 

Table 3 Antimicrobial activity of Schiff base and its complexes

Sl. No.	Schiff base and its complexes	L	Co(L-H)­2 (H2O)2	Ni(L-H)­2 (H2O)2
1	Staphylococcus aureus	++	++	+++
2	E.coli	+++	++	+++
3	Pseudomonas aeruginosa	+++	+++	+++
4	Klebsiella sp.	+++	+++	+++
5	Aspergillus Niger	+++	+++	+++
6	Mucor	+++	+++	+++

Standard-Ciprofloxacin 5mg/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).

 

Figure 1 Cyclic voltammagram of Co(L-H)­₂ (H₂O)₂

 

 

Figure 2 Antifungal Activity of L, Antibacterial Activity of Ni(L-H)₂ (H₂O)₂,  Antifungal Activity of Co(L-H)₂ (H₂O)₂

 

 

 

The electrochemical reaction was charged with 10 ml DMF solution of all the complexes (1x10^-4M) and n-Bu₄NClO₄ [18] (0.1M) as supporting electrolyte. From the cyclic voltammogram obtained for Co(II) complex (Figure 1) and Ni(II) complex (Figure 1), it is inferred that the redox potential value is metal centered, one electron quasi reversible, since DEp value is greater than 100 mV.

 

Antimicrobial activities of the complexes:

Antibacterial and antifungal activity of Schiff base and its complexes (Table 3) have been tested by disc diffusion technique^19,20. The various gram positive and gram negative bacterial organisms such as gram positive bacteria (Staphylococcus aureus and Klebsiella Sp.), gram negative bacteria (E.coli and Pseudomonas aeruginosa) and fungi (Aspergillus Niger and Mucor) are used to find out the antimicrobial activity. The results were compared with standard drug ciprofloxacin 5mg/disc for bacteria and Nystatin 100 mg/disc for fungi. Here it is evident that overall potency of an uncoordinated compound is enhanced on coordination with metal ion. However, Ni(II) complex is observed to be the most active against various species.

Proposed structure of the complexes

 

Figure 3 M=Co(II) and Ni(II)

 

CONCLUSION:

Two new complexes of Co(II) and Ni(II) with (E)-1-((p-tolylimino)methyl)naphthalene-2-ol have been synthesized. On the basis of the spectral data obtained it is found that there is a formation of 1:2 (metal : ligand) complex and also indicates an octahedral environment around the metal ion and the probable structure of the complexes proposed in the present work is given in Figure 3.The complexes have been screened against gram positive and gram negative bacteria and found to have considerable therapeutic activity compared to the ligand.

 

ACKNOWLEDGEMENTS:

The authors are thankful to the University Grants Commission, Hyderabad, India, for the financial support. Laboratory facilities provided by the Management and the timely help rendered by faculty members, PG and Research Department of Chemistry, Seethalakshmi Ramaswami College, Tiruchirappalli are gratefully acknowledged.

 

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Accepted on 07.07.2013         © RJPT All right reserved