J. Chem. Sci. (2018) 130:9 © Indian Academy of Sciences https://doi.org/10.1007/s12039-017-1409-9 REGULAR ARTICLE Physico-chemical characterization and biological studies of newly synthesized metal complexes of an Ionic liquid-supported Schiff base: 1-{2-[(2-hydroxy-5-bromobenzylidene)amino]ethyl}-3- ethylimidazolium tetrafluoroborate SANJOY SAHAa,∗, GOUTAM BASAKb and BISWAJIT SINHAc aDepartment of Chemistry, Kalimpong College, Kalimpong, West Bengal 734 301, India bDepartment of Microbiology, Raiganj University, Raiganj, West Bengal 733 134, India cDepartment of Chemistry, University of North Bengal, Darjeeling, West Bengal 734 013, India E-mail: [email protected] MS received 31 August 2017; revised 22 November 2017; accepted 22 November 2017; published online 1 February 2018 Abstract. Co(II), Ni(II) and Cu(II) complexes of an ionic liquid-supported Schiff base 1-{2-[(2-hydroxy-5- bromobenzylidene)amino]ethyl}-3-ethylimidazolium tetrafluoroborate were synthesized and characterized by various analytical and spectroscopic methods such as elemental analysis, UV-Visible, FT-IR,1H NMR, ESI MS, molar conductance and magnetic susceptibility measurements. Based on the spectral studies, tetra coordinated geometry was proposed for the complexes and molar conductance of the complexes revealed their electrolytic nature. The synthesized Schiff base and its complexes were evaluated for in vitro antibacterial activities against Gram positive and Gram negative bacteria. The complexes along with the Schiff base showed very significant biological activity against the tested bacteria. Keywords. Ionic liquid-supported Schiff base; Co (II)complex; Ni (II)complex; Cu (II)complex; antibacterial activity. 1. Introduction the field of inorganic and material chemistry. 8,9 The con- cept of functionalized ionic liquid (FILs), by introducing Ionic liquids (ILs) are organic salts which have low melt- additional a functional group as a part of cation or anion, ing points below the boiling point of water and are stable has presently become a subject of interest. 10–15 There ◦ in a liquid state at 100 C, even at room temperature. is a huge possibility of chemical structure modifica- They can exhibit numerous desirable properties such as tions through the incorporation of specific functionality. negligible vapor pressure, 1 ability to dissolve various Such FILs are able to interact with a metal centre substrates, high electrical conductivity 2 and thermal sta- and contribute to enhanced stability of metal salts. 16 bility. 3–5 ILs are touted as alternatives to volatile organic Metal-containing ILs are considered as promising new solvents (VOC) in various organic transformations. Due materials that combine the feature of ILs with additional to low toxicity and biodegradability, they have been intrinsic magnetic, catalytic and spectroscopic proper- termed as green solvents. 6 An unusual feature of ILs ties depending on the incorporated metal ion. 17 is the tenability of their physical and chemical proper- Schiff bases, usually formed by the condensation of a ties by variation of cations and anions. Usually, large primary amine with an aldehyde are one of the most organic cations and smaller anions are designed to carry prevalent ligands in coordination chemistry. 18 Schiff on required functions. 7 Although most of the works on bases containing hetero-atoms such as nitrogen, oxygen ILs highlight their use as reaction media in organic syn- and sulphur are of special interest due to their different thesis, these liquids are gradually drawing attention in ways of bonding with transition metal ions and unusual configuration. 19 They have been reported to exhibit a variety of biological actions due to the presence of *For correspondence azomethine linkage, which is responsible for different Electronic supplementary material: The online version of this article (https:// doi.org/ 10.1007/ s12039-017-1409-9) contains supplementary material, which is available to authorized users. 1 9 Page 2 of 9 J. Chem. Sci. (2018) 130:9 types of antibacterial, herbicidal and antifungal activ- were determined by the open capillary method. Antibacte- ities. 20,21 Transition metal complexes of Schiff bases rial activities (in vitro) of the synthesized compounds were carrying nitrogen and other donor sites have a variety tested by disc diffusion method. All the bacteria strains were of applications including biological, medicinal analyt- procured from MTCC, Chandigarh, and were cultured at the ical in addition to their vital role in organic synthesis Department of Microbiology, Raiganj University, Raiganj, and catalysis. 22–26 We reported in previous articles the West Bengal, India. synthesis, characterization and biological influence of 2.3 Synthesis of 1-(2-aminoethyl)-3-ethylimidazolium Cu(II), Mn(II) and Co(II) complexes of analogous ionic tetrafluoroborate, [2-aeeim]B F (1) liquid-supported Schiff bases. 27,28 This paper reports 4 on the synthesis of transition metal Co(II), Ni(II) and The amino functionalized ionic liquid [2-aeeim]BF4 was syn- Cu(II) complexes of an ionic liquid-supported Schiff thesized by following a literature procedure. 29 Yield: 79%; base and their characterization using spectroscopic, ana- C7H14F4N3B : Anal. Found: C, 37.02; H, 6.12; N, 18.38% lytical and magnetic data. Furthermore, the applications Calc.: C, 37.04; H, 6.22; N, 18.51%. IR (KBr, υ/cm−1): of the Schiff base and its complexes as potential antibac- (υO−H) 3447, 3086, 2896, 1626, 1452, (υBF4) 1084. ESI-MS + + terial agents have also been demonstrated. (m/z): Calc.: 140: Found: 140 ([M-BF4] ,M=[C7H14N3] ). 1 H NMR (400 MHz, D2O, TMS): δ3.63 (2H, m, NH2-CH2), 4.16 (3H, s, CH3),4.49(1H,t,N-CH2),4.56(1H,t,N-CH2), 7.40 (1H, s, NCH), 7.50 (1H, s, NCH), 8.61 (2H, s, NH2), 2. Experimental 13 8.87 (1H, s, N(H)CN); C NMR (400 MHz, D2O, TMSO) δ: 135.95, 123, 122.50, 50.81, 45.54, 45.3, 14.57. 2.1 Materials 2.4 Synthesis of imidazolium ionic liquid-tagged All the reagents used were of analytical grade and used with- Schiff base, LH (2) out further purification. 1-ethylimidazole, 2-bromoethylamine hydrobromide and sodium tetrafluoroborate were procured The ionic liquid-tagged Schiff base (LH) was synthesized by from Sigma Aldrich, Germany. 5-bromo-2-hydroxy ben- a slight modification of a literature procedure. 30 A mixture zaldehyde, Co(II), Ni(II) and Cu(II) acetates and all other of 5-bromo-2-hydroxy benzaldehyde (2.01 g, 10 mmol) and chemicals were used as received from SD Fine Chemicals, [2-aeeim]BF (2.27 g, 10 mmol) in methanol was stirred at India. The solvents methanol, petroleum ether, chloroform, 4 room temperature for 12 h. After completion of the reaction, DMF and DMSO were used after purification by the standard as indicated by TLC, the reaction mixture was diluted with methods described in the literature. EtOH. The precipitate was filtered, washed with cold ethanol and dried to afford the expected ligand as a light yellow 2.2 Instrumentation solid. ◦ IR spectra were recorded in KBr pellets with a Perkin- 2.4a LH(2): M.p.: 98–100 C; Yield: 65–70%; C14H17 Elmer Spectrum FT-IR spectrometer (RX-1) operating in the N3OBBrF4Anal. Found: C, 40.91; H, 4.11; N, 10.21%. Calc.: −1 1 −1 region 4000 to 400 cm . H-NMR was recorded at room C, 41.01; H, 4.18; N, 10.25(%). IR (KBr, υ /cm ):(υO−H) temperature on an FT-NMR (Bruker Avance-II 400 MHz) 3449, (υCH=N) 1673, (υC−O) 1276, (υBF4) 1114. UV-Vis spectrometer using DMSO-d6 and D2O as solvents. Chemi- (Methanol) λmax/nm: 218, 250, 336. ESI-MS (m/z): Calc. + + 1 cal shifts are mentioned in ppm downfield of internal standard 323: Found: 323 ([M-BF4] ,M=[C14H17N3O] ). HNMR: tetramethylsilane (TMS). Elemental microanalyses (C, H and (400 MHz, DMSO-d6,TMS):δ 3.32 (3H, s, CH3),3.82(1H, N) were conducted by using Perkin–Elmer (Model 240C) ana- t, N-CH2),3.99(1H,t,N-CH2),4.52(1H,t,N-CH2), 6.91– lyzer. Metal content was determined with the aid of AAS 6.85 (3H, m, Ar-H), 7.33 (1H, s, NCH), 7.42 (1H, s, NCH), (Varian, SpectrAA 50B) by using standard metal solutions 8.50 (1H, s, N=CH), 7.73 (1H, s, N(H)CN), 9.10 (1H, s, OH). 13 from Sigma-Aldrich, Germany. Mass spectra were recorded C NMR (400 MHz, DMSO-d6,TMSO):δ 137.31, 135.59, on a JMS-T100LC spectrometer. The purity of the prepared 123.76, 123.09, 122.41, 122.25, 119.63, 53.91, 48.52, 48.14, compounds was confirmed by thin layer chromatography 44.99, 43.71, 41.15, 35.90. (TLC) on silica gel plates and the plates were visualized with UV-light and iodine as and when required. The UV-Visible 2.5 Synthesis of metal complexes(3, 4 and 5) spectra were recorded in methanol with a JascoV-530 dou- ble beam Spectrophotometer at ambient temperature. Molar To a solution of ligand, LH (0.410 g, 1 mmol), in EtOH (20 conductances were measured at (298.15 ± 0.01) K with a mL) solution of ethanolic metal acetate salt Co(II), Ni(II) and Systronic conductivity meter, TDS-308. Magnetic suscepti- Cu(II)), viz., (0.5 mmol) was added and the reaction mix- bilities were measured at room temperature using a magnetic ture was refluxed for 4 h until the starting materials were susceptibility balance (Magway MSB Mk1, Sherwood Scien- completely consumed as monitored by TLC. On completion tific Ltd). The melting point of the ligand and its complexes of the reaction, solvents were evaporated and the reaction J. Chem. Sci. (2018) 130:9 Page 3 of 9 9 Scheme 1.