Antimicrobial Screening of Co(Ii), Ni(Ii) and Cu(Ii) Complexes With Nitrogen and Oxygen Containing Schiff Base

Introduction

Schiff Base ligands and their application have aroused considerable interest mainly due to preparative accessibility, diverse reactivity and structural variability. Metal complexes of Schiff base have played a vital role in the development of coordination chemistry and many applications in various fields³. Schiff base ligands and their coordination compounds have several applications in biology and industry due to their role in catalysis and organic synthesis. They are also useful in catalysis in medicine as antibiotics and to treat industrial wastes. Schiff bases are versatile ligands having several biocidal importance as plant growth regulators antibacterial, antiviral, antileukemic and enzymatic reaction inhibitors. The coordination compounds containing Schiff base ligand can be utilized to unfold the mechanism of various biocidal activities occurring in nature. In view of above importance of Schiff and their biocidal activity and our earlier recent work in this field we have taken up antimicrobial screening of Co(II), Ni(II) and Cu(II) complexes.

 

TABLE 1: Analytical and Physical Data of Schiff Base, Etqs and their Metal Complexes

 

Compounds	Molar mass	Yield %	Analysis % found (calculated)				D T oC	Wm ohm-1 cm2 mol-1	meff B. M.	l max electronic cm-1
			M	C	N	H
ETQS Light yellow	72	60		57.26 (57.35)	30.79 (30.88)	5.80 (5.88)
[Co(ETQS)2Cl2] Brown	673.93	64	8.66 (8.74)	46.12 (46.29)	24.82 (24.92)	4.69 (4.74)	227	4.5	4.96	13600, 19680, 22400
[Co(ETQS)2Br2]Brownish red	762.75	62	7.64  (7.72)	40.77 (40.90)	21.89 (22.02)	4.13 (4.19)	217	4.7	4.94	13800, 19320, 22100
[Co(ETQS)2I2]     Deep brown	856.73	61	6.79 (6.87)	34.28 (36.41)	19.51 (19.60)	3.67 (3.75)	209	4.1	4.99	12400, 19200, 22640
[Co(ETQS)2(NO3)2]Deep brown	726.93	62	8.01 (8.10)	42.80 (42.9)	22.98 (23.11)	4.33 (4.40)	201	4.9	5.09	12600, 19700, 22580
[Ni(ETQS)2Cl2]Green	673.71	65	8.62 (8.71)	46.20 (46.31)	24.81 (24.23)	4.69 (4.74)	204	5.9	3.16	11900, 15400, 24200
[Ni(ETQS)2Br2]Green	762.52	64	7.60 (7.69)	40.79 (40.91)	21.86 (22.03)	4.12 (4.19)	198	5.4	3.14	11700, 15100, 24360
[Ni(ETQS)2I2]       Deep green	856.52	60	6.79 (6.85)	36.30 (36.42)	19.49 (19.61)	3.67 (3.73)	192	5.6	3.10	12000, 15700, 24780
[Ni(ETQS)2(NO3)2]Greenish red	726.71	60	7.98 (8.07)	42.86 (42.93)	23.01 (23.11)	4.32 (4.40)	197	5.4	3.13	11980, 16000, 24800
[Cu(ETQS)2Cl2]    Blue	678.54	62	9.27 (9.36)	45.84 (45.98)	24.64 (24.75)	4.63 (4.71)	224	6.9	1.86	10700, 14670
[Cu(ETQS)2Br2]   Blue	767.36	63	8.19 (8.28)	40.53 (40.65)	21.78 (21.89)	4.09 (4.17)	208	6.7	1.81	10500, 14460
[Cu(ETQS)2(NO3)2]Blue	731.54	64	8.59 (8.68)	42.52 (42.64)	22.88 (22.96)	4.32 (4.37)	213	6.5	1.89	11100, 14680

 

Preparation of the ligand (ETQS)

A mixture of 1-ethyl 1, 2, 3, 5 tetrahydroimidazo (2, 1-b) quinazolin 5-one (0.02 mol) and semicarbazide hydrochloride dissolved in ethanol was refluxed for 3 h using air condensor with frequent shaking. After cooling the precipitate was collected, washed with tetrahydrofuran, treated with dilute sodium carbonate solution and filtered. The solid was washed throughly with water and crystallised with ethanol to give 1-ethyl-1, 2, 3, 5- tetrahydroimidazo. (2-1, b) quinazolin- 5- one semicarbazone as light yellow needles m. p 209+ 1⁰C yield – 60%.

 Prepration of the complexes

Metal complexes were synthesized by the reaction of 20ml ethanolic solution of ligand ETQS with ethanolic solution of respective metal halides/nitrate in the molar ration 2:1. This mixture was then refluxed for 2-3 h on water bath. The procedure carried out in each case of similar nature with slight variation of timing of reflux. On cooling, coloured precipitates were obtained. Products were filtered and washed with ethanol followed by ether and dried in oven. Yield in all cases 60-65%.

Result and Discussion

I.R.Spectra

It is established from the literature¹³ that semicarbazone ligand can coordinate in a bidentate manner in most cases through azomethine nitrogen and carbonyl oxygen of semicarbazone moiety.

IR spectrum of the ligand ETQS exhibits a medium broad band at 3220 cm^‑1 assigned^14,15 to n_N—H. In the spectra of the complexes this band appears without any change indicating that either primary amino or secondary amino groups has not taken part in the coordination with metal ion.

Table – 2: Ir Spectral Measurements Of Ligand Etqs And Its Metal Complexes

Compounds	nC=O	nC=N	nM-O	nM-N	nM-X
ETQS	1690 s,b	1450 s,m
[Co(ETQS)2Cl2]	1660 m,b	1420 m,b	505 m	455 m	310 m
[Co(ETQS)2Br2]	1660 m,b	1430 m,b	500 m	445 m	280 m
[Co(ETQS)2I2]	1655 m,b	1425 m,b	515 m	440 m	260 m
[Co(ETQS)2(NO3)2]	1650 m,b	1425 m,b	510 m	455 m
[Ni(ETQS)2Cl2]	1650 m,b	1430 m,b	515 m	460 m	320 m
[Ni(ETQS)2Br2]	1655 m,b	1420 m,b	525 m	465 m	300 m
[Ni(ETQS)2I2]	1650 m,b	1415 m,b	520 m	475 m	295 m
[Ni(ETQS)2(NO3)2]	1650 m,b	1410 m,b	530 m	470 m
[Cu(ETQS)2Cl2]	1650 m,b	1410 m,b	540 m	455 m	310 m
[Cu(ETQS)2Br2]	1650 m,b	1410 m,b	535 m	450 m	290 m
[Cu(ETQS)2(NO3)2]	1650 m,b	1410 m,b	540 m	445 m	285 m

m = medium, b = broad,   s = strong,    s b = strong and broad

IR spectrum of the ligand exhibits sharp and strong band at 1690 cm^-1 assigned^14,16 to n_C=O. In the spectra of the complexes this band is shifted to lower frequency region and appear at 1660 cm^-1 with slightly reduced intensity. The shift of the band and change in intensity suggests coordination through carbonyl oxygen of semicarbazone moiety. The linkage with oxygen atom is further supported by the appearance of a new band in the far ir region at 540-500 cm^-1 in the complexes assignable^14,17 to n_M–O. The IR spectrum of the ligand exhibit a band at 1450 cm^-1 assigned^14,18 to n_C=N. In the spectra of the complexes this band shows red shift with slightly reduced intensity. The shift of the band and change in intensity proposes coordination of azomethine N with metal ion. The linkage with nitrogen atom is further supported by the appearance of a band in the far ir region at 475-440 cm^-1 assigned¹⁴ to n_M–N. The evidence of metal halogen is confirmed by the low molar conductance of the complexes in the range 6.9-4.1 ohm^-1 cm² mol^-1. This statement is further confirmed by the appearance of a band in far ir region at 320-260 cm^-1 which assigned to n_M–X. Nitrate complexes shows characteristic medium intensity bands at 1375 and 1255 cm^‑1 with a separation of about 120 cm^‑1 due to monodentately coordinated nitrate group^19,20.

Electronic spectra and magnetic susceptibility of the complexes

The electronic spectra of all the complexes have been recorded in the region 10,000-25,000 cm^‑1. The copper complexes exhibits two broad bands in the region 11500-11100 cm^-1 and 14800-14400 cm^-1 assigned to ²T_2g (F) ¬ ⁴E_g and charge transfer bands respectively indicating distorted octahedral^21,22 geometry for the copper(II) complexes. The magnetic moment^23,24 values for copper(II) complexes i.e. in the range 1.81 to 1.94 B.M. The cobalt(II) complexes exhibit tthree bands in the regions 13800-12400 cm^-1, 19700-19200 cm^‑1 and 22640-22100 cm^-1 assignable to the transitions ⁴T_2g (F) ¬ ⁴T_1g (F), ⁴A_2g (F) ¬ ⁴T_1g (F) and ⁴T_1g (P) ¬ ⁴T_1g (F), respectively. The above mentioned spectral bands indicate octahedral^21,25 geometry for the cobalt(II) complexes. The octahedral geometry of cobalt(II) complexes is also supported^23,24 by magnetic susceptibility values in the range  4.90 to 5.09 B.M. The nickel(II) complexes exhibits three absorption bands in the region 11700-12,000 cm^-1, 16000-15100 cm^‑1 and 24800-24200 cm^‑1 respectively assignable to ³T_2g (F) ¬ ³A_2g (F), ³T_1g (F) ¬ ³A_2g (F) and ³T_1g (P) ¬ ³A_2g (F) transitions respectively. The suggesting octahedral^21,26 geometry for Ni(II) complexes is further supported^23,24 by magnetic susceptibility values of the complexes in the range 3.10 B.M. to 3.16 B.M.

Conductivity measurement

Molar conductance values of the complexes of cobalt(II), nickel(II) and copper(II) were found to be in the range 6.9-4.1 ohm^-1 mol^-1 in DMF indicating their non electrolytic³⁰ nature. The values of conductance also support the structure assigned on the basis of elemental analysis, IR spectra and magnetic susceptibility values.

Biocidal screening

Schiff base ETQS and their metal complexes have been evaluated for their antimicrobial activity against E. coli and B. subtilis by disc diffusion method²⁸ at concentrations 50 and 25 m_g ml^-1 using streptomycin as a control. On comparison with reference to antibiotic, the complexes were found to be more effective than free ligand. Further it is also observed that the order of activity for complexes are Cu(II) > Ni(II) > Co(II) and also nitrate complexes were effective biocidal effect than metal-halide complexes, which is supported by literature^29-31.

Conclusions

Based on the stoichiometrics and spectroscopic studies it is proposes that ETQS behaves as neutral bidentate ligand and coordination takes place through azomethine N and carbonyl oxygen. The remaining position of metal ions are satisfied by negative ions such as Cl^–, Br^–, I^– or NO₃. Hence on the basis of above studies the geometry of the complexes of Co(II), Ni(II) and Cu(II) of the type (M(ETQS)₂X₂] may be proposed to have octahedral geometry as shown in Fig-1.

Figure 1: [M(ETQS)2X2] M = Co(II) and Ni(II); X = Cl–, Br–, I– or NO3–; M = Cu(II); X = Cl–, Br– or NO3 Click here to View figure

 

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