**2.2 Synthesis of imidazolium ionic liquid-supported Schiff base, LH (2a)**

5-nitro-2-hydroxybenzaldehyde (1.67 g, 10 mmol) and [2-aeeim]BF4 (2.27 g, 10 mmol) were taken in methanol and stirred at 25°C for 4 h. After completion of reaction, the product was diluted using ethanol. The precipitate was filtered, washed with cold EtOH and dried properly to collect the expected ligand as a yellowish brown solid; (282 mg, 75%). mp. 95–97°C. <sup>1</sup> H NMR: (400 MHz, DMSO-*d*6, TMS): *δ* = 3.36 (q, 2H, N▬CH2), 3.60 (s, 3H, CH3), 3.92 (t, 2H, N▬CH2), 4.60 (t, 2H, N▬CH2), 7.44 (s, 1H, NCH), 7.52 (s, 1H, NCH), 7.53 (s, 1H, N═CH), 7.61–7.59 (m, 3H, Ar-H), 8.65 (s, 1H, N(H)CN), 8.88 (s, 1H, OH). 13C NMR: (400 MHz, DMSO-*d*6, TMS): *δ* = 159.76, 138.43, 134.08, 130.47, 130.31, 123.89, 119.80, 118.65, 110.65, 39.86, 39.65, 39.24, 39.03 and 38.82. IR (KBr): ʋ = 3448 (O▬H), 3071, 1664 (C═N), 1343 (N▬O), 1293 (C▬O), 1095 (B▬F). UV/vis (methanol, λmax): 206, 234, 306 nm; ESI-MS (CH3OH, *m/z*) [M-BF4] + calcd. for [C14H17N4O3] + : 289, found; 289. Anal. calcd. for C14H17N4O3BF4 (376): C 44.71, H 4.56, N 14.90. found: C 44.64, H 4.49, N 14.83.

#### **2.3 Synthesis of the metal complexes (3a, 4a and 5a)**

To an ethanolic solution of ligand, LH (2c) (0.376 g, 1 mmol) in round bottomed flask, metal acetate salt Co(II), Ni(II) and Cu(II), *viz.*, (0.5 mmol) dissolved in ethanol was added and the reaction mixture was refluxed for 12 h until the starting materials were completely consumed as monitored by TLC. On completion of the reaction, solvents were evaporated and the reaction mixture was cooled to room temperature. The precipitate was collected by filtration, washed successively with cold ethanol (10 mL × 3). Finally it was dried in vacuum desiccators to obtain the solid product. The complexes were soluble in *N*,*N*-dimethylformamide, dimethylsulfoxide, acetonitrile, methanol and water. A schematic representation of the syntheses is given in **Figure 1**.

### *2.3.1 Co(II)complex (3a)*

Brown solid; (0.54 g, 67%), decomposes at ~293°C. IR (KBr): ʋ = 3386 (O▬H), 1648 (C═N), 1332 (N▬O), 1177 (C▬O), 1106 (B▬F), 651 (M▬O), 510 (M▬N). UV/vis (methanol, λmax/nm): 227, 246, 358. ESI-MS (CH3OH, *m/z*) [M-2BF4] + calcd. for [C28H32CoN8O6] + : 635, found: 635; anal. calcd. for C28H36CoB2F8N8O8 (809): C 41.56, H 3.99, N 13.85, Co 7.28, found: C 41.36, H 3.71, N 13.55, Co 7.12.

#### *2.3.2 Ni(II)complex (4a)*

Light green solid; (0.56 g, 69%), decomposes at ~293°C. IR (KBr): ʋ = 3396 (O▬H), 1637 (C═N), 1330 (N▬O), 1172 (C▬O), 1102 (B▬F), 646 (M▬O), 526 (M▬N).

*Solvents, Ionic Liquids and Solvent Effects*

Gram-negative/positive bacteria.

**2. Materials and physical measurements**

by changing their cation and anion pairing [8]. IL-based solvent system typically exhibits enhanced reaction kinetics resulting in the efficient use of time and energy [1]. Due to these properties, ILs are treated as a new generation of solvents for catalysis, ecofriendly reaction media for organic synthesis and a successful replacement for conventional media in chemical processes [1, 9]. Recently, many researchers have focused on the synthesis of new ionic liquids called functionalized ionic liquids (FILs) with different functional groups in the cationic moiety [10–15]. Such functionalization of the cation can easily be done in a single reaction step and thus both the cationic and anionic moieties of the FILs can be altered as required for specific applications like

Of note Schiff base being a salient class of multidentate ligand has played a key role in coordination chemistry. They exhibit varied denticities, chelating capability [18–20], functionalities [21] and diverse range of biological, pharmacological and antitumor activities. Schiff-bases containing hetero-atom such as N, O, and S are drawn special interest for their varied ways of coordination with different transition metal ions and having unusual configurations [22–24]. The present chapter describes the syntheses and physicochemical characterizations of an IL-supported Schiff base, 1-{2-(2-hydroxy-5-nitrobenzylideneamino)ethyl}-3-ethylimidazolium tetrafluoroborate and its Co(II), Ni(II) and Cu(II) complexes. The ligand and its metal complexes were screened for their *in vitro* antibacterial activities against Gram-negative bacteria *Escherichia coli*, *Pseudomonas aeruginosa*, *Proteus vulgaris*, *Enterobacter aerogenes* and Gram-positive bacteria *Staphylococcus aureus* and *Bacillus cereus*. The complexes and the ligand were found most effective against the tested

Analytical grade chemicals were used for synthesis without further purification. 1-ethyl imidazole, 2-bromoethylamine hydrobromide, 5-nitro-2-hydroxybenzaldehyde and NaBF4 (sodium tetrafluoroborate) were purchased from Sigma Aldrich, Germany. Metal acetates and other reagents were used as obtained from SD Fine Chemicals, India. CH3OH, petroleum ether, CHCl3, DMF and DMSO were used after purification by standard methods described in the literature. FT-IR spectra were recorded by KBr pellets on a Perkin-Elmer Spectrum FT-IR spectrometer (RX-1).

H NMR and 13C NMR spectra were recorded on a FT-NMR (Bruker Avance-II 400 MHz) spectrometer by using D2O and DMSO-*d*6 as solvents. Powder X-ray diffraction (XRD) data were obtained on INEL XRD Model Equinox 1000 using Cu Kα radiation (2*θ* = 0–90°). Elemental microanalysis (CHN analysis) was performed on Perkin-Elmer (Model 240C) analyzer. Metal content was obtained from AAS (Varian, SpectrAA 50B) by using standard metal solutions procured from Sigma-Aldrich, Germany. ESI-MS spectra were obtained on a JMS-T100LC spectrometer. The purity of the synthesized products was confirmed by thin layer chromatography (TLC) Merck 60 F254 silica gel plates (layer thickness 0.25 mm) and the spots were visualized using UV-light. The UV-visible spectra were obtained from JascoV-530 double beam spectrophotometer using CH3OH as solvent. Specific conductance was measured at (298.15 ± 0.01) K with a Systronic conductivity TDS-308 metre. Magnetic susceptibility was measured with a Sherwood Scientific Ltd. magnetic susceptibility balance (Magway MSB Mk1) at ambient temperature. The melting point of synthesized compounds was determined by open capillary method. Antibacterial activity (*in vitro*) of the synthesized ligand and complexes were evaluated by well diffusion method against six bacterial strains (two Gram-positive and four Gramnegative). The bacterial strains were obtained from MTCC, Chandigarh, India.

increased catalytic stability and reduced catalyst leaching, etc. [16, 17].

**82**

1

#### **Figure 1.**

*Synthesis of Ionic liquid supported Schiff base (2a) and its metal complexes 3a, 4a and 5a from 2a.*

UV/vis (methanol, λmax/nm): 220, 340, 400. ESI-MS (CH3OH, *m/z*) [M-2BF4] + calcd. for [C28H32NiN8O6] + : 634, found: 634; anal. calcd. for C28H36NiB2F8N8O8 (809): C 41.57, H 3.99, N 13.85, Ni 7.26, found: C 41.22, H 3.63, N 13.46, Ni 7.11.

#### *2.3.3 Cu(II) complex (5a)*

Dark green solid; (0.57 g, 70%), decomposes at ~295°C. IR (KBr): ʋ = 3429 (O▬H), 1656 (C═N), 1334 (N▬O), 1175 (C▬O), 1103 (B▬F), 633 (M▬O), 471 (M▬N). UV/vis (methanol, λmax/nm): 226, 244, 354. ESI-MS (CH3OH, *m/z*) [M-2BF4] + calcd. for [C28H32CuN8O6] + : 634, found: 634. Anal. calcd. for C28H36CuB2F8N8O8 (813.76): C 41.33, H 3.96, N 13.77, Cu 7.81, found: C 41.12, H 3.61, N 13.46, Cu 7.61.

#### **2.4 Antibacterial assay**

The synthesized ligand (**2a**) and complexes (**3a**, **4a** and **5a**) were screened against the Gram-negative bacteria (*E. coli*, *P. aeruginosa*, *P. vulgaris* and *E. aerogenes*) and Gram-positive bacteria (*S. aureus* and *B. cereus*) strains. The tests were performed using agar disc diffusion method [26]. The nutrient agar (Hi-Media Laboratories Limited, Mumbai, India) was put in an autoclave at 121°C and 1 atm for 15–20 min. The sterile nutrient medium was kept at 45–50°C and then 100 μL of bacterial suspension containing 108 colony-forming units (CFU)/mL was mixed with sterile liquid nutrient agar and poured into the sterile Petri dishes. All the stock solutions were made by dissolving the compounds in dimethylsulfoxide (DMSO). The concentrations of the tested compounds were 10, 20, 30, 40 and 50 μg/mL. The tested microorganisms were grown on nutrient agar medium in Petri dishes. The samples were soaked in a filter paper disc of 1 mm thickness and 5 mm diameter. The discs were kept on Petri plates and incubated for 24 h at 37°C. The diameter of the inhibition zone (including disc diameter of 5 mm) was measured. Each experiment was carried out three times to minimize the error and the mean values were accepted.

**85**

**Figure 2.**

*SEM image of Co(II) complex (3a).*

*Imidazolium Ionic Liquid-Supported Schiff Base and Its Transition Metal Complexes: Synthesis…*

All the isolated compounds were stable at room temperature to be characterized by different analytical and spectroscopic methods. The complexes are soluble in *N*,*N*-dimethylformamide, dimethylsulfoxide, acetonitrile, methanol and water.

The assignments of the IR bands of the synthesized Co(II), Ni(II) and Cu(II) complexes had been made by comparing with the bands of ligand (LH) to determine the coordination sites involved in chelation. FT-IR spectra of LH (**2a**) showed

phenolic group (▬OH) with H▬C(═N) group in the ligand (OH…N═C) [27, 28].

and **5a**) suggested the presence of the solvated water molecules (probably for the presence of ▬NO2 group in the ligand and intrinsic property of the anion tetrafluoroborate) [29–31]. The band corresponding to the azomethine group (▬C═N)

complexes (**3a**, **4a** and **5a**) upon complexation. This fact established the bonding of ligand (**2a**) to the metal atoms through the N atom of azomethine and O atom of phenolic group [33]. The bands appeared in the region of 1102–1107 cm<sup>−</sup><sup>1</sup>

the metal complexes were assigned for B▬F stretching frequency. FT-IR spectra of

assigned for the NO2 group [34]. The spectra of the metal complexes exhibited

the LH (**2a**) and its complexes showed strong bands at 1330–1343 cm<sup>−</sup><sup>1</sup>

vibrations, respectively [35]. IR spectra are given in **Figures 2–8**.

 because of coordination of N atom of azomethine linkage to the Co2+, Ni2+ and Cu2+ ions respectively [32]. The band for phenolic C▬O of free ligand was

which moved to lower wave number 1172–1177 cm<sup>−</sup><sup>1</sup>

; which was due to the hydrogen bonded

. This band gets shifted in the range 1637–

were attributed to M▬O and M▬N stretching

for the metal complexes (**3a**, **4a**

for the

for

which were

*DOI: http://dx.doi.org/10.5772/intechopen.86379*

a strong broad band at 3448–3071 cm<sup>−</sup><sup>1</sup>

of the ligand was found at 1664 cm<sup>−</sup><sup>1</sup>

bands at 633–651 and 471–526 cm<sup>−</sup><sup>1</sup>

1656 cm<sup>−</sup><sup>1</sup>

observed at 1293 cm<sup>−</sup><sup>1</sup>

The broad band appeared at 3386–3429 cm<sup>−</sup><sup>1</sup>

**3. Results and discussion**

**3.1 FT-IR spectral studies**

*Imidazolium Ionic Liquid-Supported Schiff Base and Its Transition Metal Complexes: Synthesis… DOI: http://dx.doi.org/10.5772/intechopen.86379*
