Synthesis, Characterization and Microbiological evaluation of Co(II), Cu(II), Zn(II) Cr(III) and Fe(III) complexes of 2,4-dimethylaniline salicylaldimine

All the chemicals used for the synthesis of the ligands & their metal complexes were chemically pure. Solvents were purified & dried before use by literature method [6]. The ligand used in the present work was not commercially available hence it was synthesized in our laboratory by conventional method. All the metal salts were obtained from SD’s fine chemicals.

Synthesis 2,4-dimethylanilinesalicylaldimine(LH)

A hot ethanolic solution of salicylaldehyde (0.01 mol in 20 ml ethanol) was added to a hot ethanolic solution of 2.4-dimethyl aniline (0.01 mol in 20 ml ethanol) and the reaction mixture was refluxed on a water-bath for about 5-6h. On cooling two layers were formed in the solutions which were separated by separating funnel. After two or three days yellow coloured crystals were appeared. The product was filtered under suction and re-crystallized from 50% ethanol.

The ligand was obtained as yellow coloured crystalline solids which were filtered, washed with ethanol, dried at room temperature and finally recrystallized from hot ethanol (yield; 70 %, m. p. 170^oC).

Synthesis of the Complexes

A hot ethanolic solution (10 ml) of corresponding metal salt [Co(OAc)_2.4H₂O; Cu(OAc)_2.H₂O; ZnCl₂ ; CrCl₃.6H₂O and anhydrous FeCl₃] was added to a warm ethanolic (10 ml) solution of ligand LH. The reaction mixture was heated under reflux with stirring for about 4-5 h. The isolated coloured solid obtained was filtered, and washed thoroughly with ethanol and finally with petroether and dried at room temperature.

Yield ~ 65 %.

The metal and chloride content of complexes were analyzed by standard methods.⁶ The ¹H

NMR spectra of ligand was recorded and obtained from RSIC Chandigarh. IR spectra of the compounds were recorded on Perkin Elmer 842 spectrophotometer in the region 400-4000cm^-1, Carbon, Hydrogen and Nitrogen analysis were carried out at RSIC, Punjab University, Chandigarh. The molar conductance of the complexes at 10^-3 M dilution in DMF were determined using equiptronic digital conductivity meter EQ-660 with a cell constant  1.00 cm^-1 at room temperature. The magnetic moment measurement were made on a Gouy balance at room temperature using [HgCo(SCN)₄] as the calibrant.

Table 1:Analytical data, magnetic moment and molar conductance of the compound

Compounds	Colour	Mol.wt.	Analysis % Found (calc.)					µeff   B.M.	LM (Ω-1 cm2 mol-1)
			M	C	H	N	Cl
C15H15NO (LH)	Yellow Crystalline	225	—	80.34 (80.00)	06.87 (06.66)	06.41 (06.22)	—	—	—
[CoL2(H2O)2]H20	Pale Yellow	560.93	9.01 (9.18)	64.12 (64.25)	5.25 (5.34)	4.83 (4.99)	_	4.58	12.8
[CuL2]2H20	Black	547.54	11.48 (11.60)	65.69 (65.80)	5.35     (5.47)	5.09 (5.15)	_	1.85	6.28
[ZnL2 (H2O) 2]	Yellow	549.37	11.74 (11.89)	65.49 (65.52)	5.30 (5.46)	5.00 (5.09)	4.52 (4.61)	—	15.2
[CrL2 (H2O)Cl] H2O	Terracota	571.49	9.02 (9.09)	62.85 (62.99)	5.15 (5.24)	4.75 (4.89)	6.14 (6.21)	3.45	12.7
[FeL2 (H2O)Cl] H2O	Black	575.35	9.67 (9.70)	62.54 (62.62)	5.10 (5.21)	4.72 (4.86)	6.11 (6.17)	5.46	8.5

The complexes prepared are coloured solids and air stable and are non-hygroscopic. The elemental analyses of the complexes suggest 1:2 (metal : ligand) stoichiometry.

1H NMR Spectral data of the ligand

The coordinating modes of the ligand were confirmed by ¹H NMR spectra (CDCl₃ + DMSO). The observed signals (δ in ppm) are 10.95 (1H, s, phenolic OH); 9.11 (1H, s, benzylidenimin); 7.95, 7.55, 7.35 7.10 (4H, m, phenyl) 7.00, 6.85(3H, m phenyl) and 2.44 ppm (6H, S, Ar-methyl).^7,8

Infrared Spectra of the compounds

The coordination sites participating in bonding with metal ion have been determined by comparing the infrared spectra of complexes to that with the free ligands. The IR spectra of complexes show peculiar changes as compared to the free ligand.

Important assignments of the ligand and its metal complexes are given in the Table 3.6. The peak observed at 3250 cm^-1 in the spectrum of ligand may be assigned due to phenolic –O-H stretching frequency.⁹ The peak observed at 1630 cm^-1 and 1540 cm^-1may be assigned due to C=N (azomethine) stretching frequency and phenolic C-O stretching frequency respectively.¹⁰ The peak observed at 3020 cm^-1  may be assigned due to Ar-H stretching vibration.The peaks observed in the range 1500 – 1300 cm^-1 were characteristics of aromatic ring stretching frequency. In the spectra of all the complexes, the n(C=N) (Azomethine) shifts to lower frequency by 10-13 cm^-1 indicating the coordination of azomethine nitrogen atom to the metal ions.¹¹ The n(C –O) band shifts in the complexes to higher frequency be 15 cm^-1 indicating the coordination of phenolic oxygen atom to the metal ion.¹² This reveals that the ligand coordinated with all the metal ions through nitrogen and phenolic oxygen and behaves as monobasic bidentate. The lower frequency region of the spectra also confirms the participation of the phenolic oxygen & azomethine nitrogen by the appearance of band in the region 580-500 cm^-1& 490-410 cm^-1 assignable to n(M-O) &  n(M-N) respectively.^13,14 The coordination of H₂O in the Co(II), Zn(II), Cr(III) & Fe(III) is indicated by the appearances of band at 3500-3300, 1640-1610 and 850-830 cm^-1 indicate the presence of coordinated water molecule.¹⁵

Magnetic moment and Electronic Spectra

The cobalt(II)  complexes exhibited ligand field bands at about 8955, 18225, 20800 and 32500 cm^-1. The transition can be attributed to ⁴T_1g(F)® ⁴T_2g(F)(n₁), ⁴T_1g(F)® ⁴A_2g(F)(n₂) and ⁴T_1g(F)® ⁴T_1g(P)(n₃), whereas the forth band may be assigned to CT band. The ligand field parameter like Dq (917.0, 877.5cm^-1) B(809.3, 705.1cm^-1) n₂/n₁ (2.02,2.0) B_15, (0.83,0.81) and Q(20.48, 23.45) suggest an octahedral geometry.¹⁶ The electronic spectrum of copper(II) complex shows a broad band at 15307 & 15384 cm^-1 in solution and solid state respectively, which assigned to ²B_1g ®²A_1g, ²B_2g ®²E_g, in a square-planar geometry        with CuO₂N₂ chromophore.¹⁷  The electronic spectra of Cr(III) complex shows bands at

16051cm^-1(n₁) corresponds to ⁴A_2g(F)® ⁴T_2g(F) transition, 21276 cm^-1(n₂) corresponds to ⁴T_2g (F)®⁴T_1g(F) transition and 25706 cm^-1(n₃) corresponds to ⁴A_2g(F) ®⁴T_1g(P) transition.¹⁸ The electronic spectra of Fe(III) complexes showed three bands at 14930, 25590 , and 30150 cm^-1 which may be assigned to ⁶A_2g®⁴T_1g(G), ⁶A_1g ® ⁴T_2g and charge transfer  transition respectively suggesting an octahedral geometry.¹⁹ The Zn(II) complexes are found to be diamagnetic in nature. The octahedral  geometry has been suggested on the basis of analytical and IR data.

The magnetic moments of the complexes were measured at room temperature. The magnetic moments of Co(II) complex is 4.58 B.M. suggest the presence of three unpaired electrons. The observed values for the Cu(II) complex is 1.85 B.M. in corresponding to one unpaired electron. The Cr(III) complex has a magnetic moment of 3.45 B.M., indicating the presence of three unpaired electrons, and that of Fe(III) complex, 5.46 B.M. towards the high-spin octahedral configuration of Fe(III) ion. The Zn(II) complex is diamagnetic as expected.

Thermogravimetric Analysis

Thermogravimetric analysis have been carried out to estimate the contribution of the weight losses during ignition and for determination of lattice and coordinated water molecules present in the complexes and to know their decomposition pattern. The perusal of thermograms of ligand and its metal complexes indicates that the ligand decomposes in two steps while all the complexes in three stages. All the complexes are stable upto 60-70⁰C. Elimination of one water molecule form Co(II) , Cr(III), & Fe(III) complexes up to 120⁰C and two water molecule from Cu(II) & Zr(IV), complexes up to 140⁰C have been observed. The % weight loss (observed /calculated) are depicted in the Table 2.

 

Table 2: Percentage weight loss

Complexes	% Weight loss upto 140oC		% Weight loss upto 220oC
	Observed	Calculated	Observed	Calculated
Co(II)	3.30	3.20	6.56	3.20
Cu(II)	6.65	6.58	—	—
Zn(II)	—	—	6.60	6.55
Cr(III)	3.20	3.15	3.28	3.15
Fe(III)	3.18	3.13	3.26	3.13

 

Table 3: Half decomposition temperature and kinetic parameters of the compound

 

Complexes	Half Decomposition Temparature(0C)	Activation energy kJmol-1			Z S-1	ΔS Jk-1mol-1	ΔF kJ mol-1
		FC-Method	HM-Method	Broido-Method
LH	300	3.88	5.55	4.44	88.85	212.48	118.86
Co(II)	420	6.51	9.30	8.37	167.49	209.35	154.53
Cu(II)	462	11.28	11.28	10.16	203.31	208.54	170.28
Zn(II)	409	6.35	9.07	9.07	181.50	208.58	152.66
Cr(III)	499	11.28	11.28	10.16	203.31	208.54	170.28
Fe(III)	485	8.82	8.82	9.80	196.11	208.26	158.79

 

Antimicrobial activity

The zones of inhibition effect of the Schiff bases ligand and its metal complexes on the growth of various bacteria is summarized in Table 4. The compounds are found to show low bacteriociadal behavior against most of the bacterial culture and are resistance towards the other. In general the results reveal that the activity of the ligand was found to enhance on complexation with metal.

Table 4: Antimicrobial activity²³

Ligand  and its complexes	Zone of inhibition (in mm)
	B. subtilis (mm)	P. vulgaris (mm)	S. aureus (mm)	E. coli (mm)	P. fluorescen (mm)	A. aerogenes (mm)	B. megatherium (mm)
LH	S15	S7	R	R	R	R	S11
Co-LH	S11	S13	S10	R	S17	S16	R
Cu-LH	R	S17	R	R	R	S11	R
Zn-LH	S13	R	S11	S14	R	S12	R
Cr-LH	S13	R	S15	S9	R	S8	S9
Fe-LH	R	S9	S14	R	R	S13	R

 

Acknowledgement

The authors wish to thank SAIF Chandigarh, SAIF, IIT Chennai, CDRI Lucknow for recording elemental analyses, IR, NMR and electrical  spectral facilities and Microbiology department, Shri Shivaji Science College, Amravati for antimicrobial screening of compounds

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