Antibacterial cobalt (II), copper (II), nickel (II) and zinc (II) complexes of mercaptothiadiazole—derived furanyl, thienyl, pyrrolyl, salicylyl and pyridinyl Schiff bases

Abstract

A series of Co (II), Cu (II), Ni (II) and Zn (II) complexes of mercaptothiadiazole—derived furanyl, thienyl, pyrrorlyl, salicylyl and pyridinyl Schiff bases were synthesized, characterized and screened for their in vitro antibacterial activity against four Gram-negative, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi and Shigella flexneri, and two Gram-positive; Bacillus subtilis and Staphylococcus aureous bacterial strains. The results of these studies show the metal complexes to be more antibacterial as compared to the prepared un-complexed Schiff bases.

Keywords::

• Mercaptothiadiazoles
• metal complexes
• antibacterial
• organometallic compounds

Introduction

DNA is a main target for the therapeutic treatment of various disorders and diseases. It can bind/interact with many biomolecules [1] and synthetic compounds, including proteins, polyamines, metal complexes, organometallic compounds and antibacterial/antifungal/antiviral agents [2]. Several medicinally important compounds are also excellent binding agents [3,4]. Therefore, investigations of the interaction of some antibacterial compounds with transition metals can further provide and/or improve our understanding about rational metal-based drug design chemistry. This demand is driven by an emerging medical problem of bacterial drug resistance to presently available antibiotics associated with an accelerating rate at which bacteria develop resistance [5]. This resistance is spreading alarmingly among Gram-positive as well as Gram-negative organisms [6,7]. In order to address this problem, we have commenced a program 8-15 for synthesizing classes of bactericidal and fungicidal complexes of transition metals, which could potentially reduce (interfere with) the mechanism of bacterial resistance via coordination of the metal ion (s) 16-20. Mercaptothiadiazoles are amongst such classes of compounds already known to be associated with diverse pharmacological activities 21-23. In continuation of this approach the present paper describes the synthesis, characterization and in vitro evaluation of the antibacterial activity of newly synthesized Co (II), Cu (II), Ni (II) and Zn (II) complexes (1)–(24) with the Schiff bases (L¹)–(L⁶) obtained from the reaction of 5-amino-1,3,4-thiadiazole-2-thiol with furan-2-carboxaldehyde, thiophene-2-carboxaldehyde, 4-bromothiophene-2-carboxaldehyde, pyrrole-2-carboxaldehyde, salicyldehyde and pyridine-2-carboxaldehyde, respectively. The structures of all the synthesized compounds were confirmed by elemental analysis, IR, NMR, UV–Visible, magnetic moment and conductance measurements. All the compounds were screened for bactericidal activity against four Gram-negative, E. coli, P. aeruginosa, S. typhi and S. flexeneri, and two Gram-positive, B. subtilis, and S. aureus bacterial strains. Most compounds possessing significant activity against one or more bacterial strains.

Material and methods

Solvents used were analar grade. All metal (II) salts were used as chlorides. Thin-layer chromatography was performed using aluminium sheets (Merck) coated with silica gel 60 F₂₅₄. IR spectra (KBr pellets) were recorded on a Hitachi Model 200–50 IR spectrophotometer. NMR were recorded in d₆-DMSO on Bruker AM 300 and AM 400 spectrometers (Rheinstetten—Forchheim, Germany) operating at 300 and 400 MHz, respectively. CHN analyses were carried out using an Elemental Analyzer Flash EA 1112. Conductance of the metal complexes was measured on conductivity meter 4071, Jenway (USA). Magnetic susceptibility measurements of the metal complexes in the solid state were determined by a Gouy's balance at room temperature. Melting points were recorded on a Gallenkamp apparatus and are uncorrected.

Synthesis

General procedure for the preparation of ligands (L¹)–(L⁶)

To a hot stirred solution of 5-amino-1,3,4-thiadiazole-2-thiol (0.01 mol) in ethanol (30–40 ml) was added the appropriate aldehyde (0.01 mol) and 2–3 drops of concentrated sulphuric acid (see Scheme 1) and the resultant mixture heated under reflux for 2 h. The reaction mixture was reduced to 10–15 ml and kept at room temperature for 0.5 h. Addition of the same to chilled water and stirring for a few minutes resulted in formation of a solid product which was collected by suction filtration. Recrystallization from aqueous ethanol furnished the required pure product. The compounds were characterized as given below:

Scheme 1 Synthesis and structures of (L¹)–(L⁶).

5-[(2-Furylmethylidene) amino]-1, 3, 4-thiadiazole-2-thiol (L¹)

Yield 75% as yellow powder; m.p. 170–172°C; IR (KBr, cm^{− 1}): 2590 (SH), 1635 (HC = N), 1620 (C = N, thiadiazole). ¹H NMR (DMSO-d₆, δ, ppm): 3.87 (s,1H, SH), 6.29 (s, 1H,CH = N), 6.66 (dd,1H, J = 3.63, 1.82 Hz, furanyl C₄-H), 6.91 (d,1H, J = 3.63 Hz, furanyl C₃-H), 7.44 (d,1H, J = 1.82 Hz, furanyl C₅-H). Anal. Calcd. for C₇H₅N₃OS₂: C, 39.8; H, 2.4; N, 19.9. Found: C, 40.0; H, 2.0; N, 19.5%.

5-[(2-Thienylmethylidene) amino]-1, 3, 4-thiadiazole-2-thiol (L²)

Yield 80% as bright yellow powder; m.p. 184–186°C; IR (KBr, cm^{− 1}): 2585 (SH),1635 (HC = N), 1620 (C = N, thiadiazole). ¹H NMR (DMSO-d₆, δ, ppm): 3.88 (s,1H, SH), 6.33 (s, 1H,CH = N), 7.16 (dd,1H, J = 4.76, 3.83 Hz, thienyl C₄-H),7.18 (d,1H, J = 4.76 Hz, thienyl C₅-H), 7.25 (d,1H, J = 3.83 Hz, thienyl C₃-H). Anal. Calcd. for C₇H₅N₃S₃: C, 37.0; H, 2.2; N, 18.5. Found: C, 37.3; H, 2.4; N, 18.9%.

5-{[(4-Bromo-2-thienyl)methylidene]amino}-1,3,4-thiadiazole-2-thiol (L³)

Yield 80% as yellow powder; m.p. 194–196°C; IR (KBr, cm^{− 1}): 2585 (SH), 1640 (HC = N), 1615 (C = N, thiadiazole). ¹H NMR (DMSO-d₆, δ, ppm): 3.89 (s,1H, SH), 6.31 (s, 1H,CH = N), 7.51 (s, thienyl C₅-H), 7.63 (s,1H, thienyl C₃-H). Anal. Calcd. for C₇H₄BrN₃S₃: C, 27.5; H, 1.3; N, 13.7. Found: C, 27.9; H, 1.0; N, 13.9%.

5-[(1H-Pyrrol-2-yl methylidene) amino]-1, 3, 4-thiadiazole-2-thiol (L⁴)

Yield 60% as yellow powder; m.p. 218–220°C; IR (KBr, cm^{− 1}): 3215 (NH, pyrrole), 2585 (SH),1635 (HC = N), 1620 (C = N, thiadiazole). ¹H NMR (DMSO-d₆, δ, ppm): 3.86 (s,1H, SH), 5.92 (dd,1H, J = 3.96, 2.89 Hz, pyrrolyl, C₄-H), 6.26 (s, 1H,CH = N), 6.61 (d,1H, J = 2.89 Hz, pyrrolyl C₅-H), 6.94 (d,1H, J = 3.96 Hz, pyrrolyl C₃-H), 9.77 (s, 1H, NH). Anal. Calcd. for C₇H₆N₄S₂: C, 40.0; H, 2.9; N, 26.6. Found. C, 40.4; H, 2.5; N, 26.9%.

5-{[(2-Hydroxyphenyl) methylidene]amino}-1,3,4-thiadiazole-2-thiol (L⁵)

Yield 80% as bright yellow powder; m.p. 248–250°C; IR (KBr, cm^{− 1}): 3455(OH), 2580 (SH), 1640 (HC = N), 1615 (C = N, thiadiazole). ¹H NMR (DMSO-d₆, δ, ppm): 3.88 (s,1H, SH), 6.88 (ddd, 1H, J = 7.72, 7.57, 2.12 Hz, phenol C₅-H), 6.98 (dd, 1H, J = 7.42, 2.12 Hz, phenol C₃-H), 7.18 (ddd, 1H, J = 7.72, 7.42, 2.14 Hz, phenol C₄-H), 7.55 (dd, 1H, J = 7.57, 2.14 Hz, phenol C₆-H), 7.75 (s, 1H, CH = N), 9.55 (s, 1H, OH). Anal. Calcd. for C₉H₇N₃OS₂: C, 45.6; H, 3.0; N,17.7. Found: C, 45.9; H, 2.8; N, 17.4%.

5-[(2-Pyridinylmethylidene) amino]-1, 3, 4-thiadiazole-2-thiol (L⁶)

Yield 63% as yellow powder; m.p. 146–148°C; IR (KBr, cm^{− 1}): 2590 (SH), 1635 (HC = N), 1615 (C = N mercaptothiadiazole), 1610 (C = N, pyridine). ¹H NMR (DMSO-d₆, δ, ppm): 3.93 (s,1H, SH), 7.29 (ddd, 1H, J = 7.71, 5.18, 1.21 Hz, pyridinyl C₅-H), 7.75 (dd, 1H, J = 7.88, 1.21 Hz, pyridinyl C₃-H), 7.92 (ddd, 1H, J = 7.88, 7.71, 1.92 Hz, pyridinyl C₄-H), 8.35 (dd, 1H, J = 5.18, 1.92 Hz, pyridinyl C₆-H), 8.87 (s, 1H, CH = N). Anal. Calcd. for C₈H₆N₄S₂: C, 43.2; H, 2.7; N, 25.2. Found: C, 43.6; H, 2.3; N, 24.9%.

General procedure for the preparation of metal complexes of ligands (L¹)–(L⁶)

To a hot stirred solution of an appropriate ligand (0.02 mol) in ethanol (15–20 ml) was added the solution of metal (II) chloride (0.01 mol) made in warm ethanol (10 ml) and the resultant mixture refluxed for 1 h. The solid formed during refluxing or upon cooling was collected by suction filtration. Thorough washing with hot ethanol followed by ether or recrystallization from aqueous ethanol gave purified products.

Cobalt (II) 5-[(2-furylmethylidene) amino]-1,3,4-thiadiazole-2-thiol (1)

Yield 60% as bluish-purple powder; m.p. (decomp.) 215–217°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1625 (HC = N), 1615 (C = N, thiadiazole), 525 (M–N), 455 (M–O); UV (DMSO): λ_max (cm^{− 1}) 30210, 17590 and 8620; B.M. (μ_eff) 4.2; Molar conductance (90 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₁₀CoCl₂N₆O₂S₄: C, 30.4; H, 1.8; N, 15.2. Found: C, 30.8; H, 1.5; N, 15.7%.

Copper (II) 5-[(2-furylmethylidene) amino]-1,3,4-thiadiazole-2-thiol (2)

Yield 58% as dirty green powder; m.p. (decomp.) 223–225°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1625 (HC = N), 1610 (C = N, thiadiazole), 520 (M–N), 460 (M–O); UV (DMSO): λ_max (cm^{− 1}) 30515 and 22280; B.M. (μ_eff) 1.4; Molar conductance (92 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₁₀CuCl₂N₆O₂S₄: C, 30.2; H, 1.8; N, 15.1. Found: C, 30.5; H, 1.5; N, 15.5%.

Nickel (II) 5-[(2-furylmethylidene) amino]-1,3,4-thiadiazole-2-thiol (3)

Yield 60% as green powder; m.p. (decomp.) 218–220°C; IR (KBr): ν _max cm^{− 1} 2580 (SH), 1620 (HC = N), 1610 (C = N, mercaptothiadiazole), 525 (M–N), 455 (M–O); UV (DMSO): λ_max (cm^{− 1}) 29130, 15820 and 9740; B.M. (μ_eff) 3.3; Molar conductance (97 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₁₀NiCl₂N₆O₂S₄: C, 30.5; H, 1.8; N, 15.2. Found: C, 30.2; H, 1.7; N, 15.4%.

Zinc (II) 5-[(2-furylmethylidene) amino]-1,3,4-thiadiazole-2-thiol (4)

Yield 58% as light yellow powder; m.p. (decomp.) 213–215°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1625 (HC = N), 1610 (C = N, thiadiazole), 525 (M–N), 455 (M–O); UV (DMSO):λ_max (cm^{− 1}) 28590; B.M. (μ_eff) Diamagnetic; Molar conductance (97 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₁₀ ZnCl₂N₆O₂S₄: C, 30.1; H, 1.8; N, 15.0. Found: C, 30.5; H, 1.6; N, 14.7%.

Cobalt (II) 5-[(2-thienylmethylidene) amino]-1,3,4-thiadiazole-2-thiol (5)

Yield 58% as light pink powder; m.p. (decomp.) 216–218°C; IR (KBr): ν _max cm^{− 1} 2580 (SH), 1625 (HC = N), 1610 (C = N, thiadiazole), 525 (M–N), 360 (M–S); UV (DMSO):λ_max (cm^{− 1}) 30480, 18225 and 8760; B.M. (μ_eff) 4.6; Molar conductance (92 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₁₀CoCl₂N₆S₆: C, 28.8; H, 1.7; N, 14.4. Found: C, 28.4; H, 1.4; N, 14.8%.

Copper (II) 5-[(2-thienylmethylidene) amino]-1,3,4-thiadiazole-2-thiol (6)

Yield 57% as greenish powder; m.p. (decomp.) 212–214°C; IR (KBr): ν _max cm^{− 1} 2580 (SH), 1620 (HC = N), 1605 (C = N, thiadiazole), 525 (M–N), 365 (M–S); UV (DMSO): λ_max (cm^{− 1}) 30620 and 22370; B.M. (μ_eff) 1.5; Molar conductance (88 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₁₀CuCl₂N₆S₆: C, 28.5; H, 1.7; N, 14.3. Found: C, 28.8; H, 1.6; N, 14.8%.

Nickel (II) 5-[(2-thienylmethylidene) amino]-1,3,4-thiadiazole-2-thiol (7)

Yield 58% as light green powder; m.p. (decomp.) 220–222°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1625 (HC = N), 1610 (C = N, thiadiazole), 530 (M–N), 365 (M–S); UV (DMSO): λ_max (cm^{− 1}) 28420, 16225 and 10130; B.M. (μ_eff) 3.2; Molar conductance (96 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₁₀NiCl₂N₆S₆: C, 28.8; H, 1.7; N, 14.4. Found: C, 28.6; H, 2.1; N, 14.7%.

Zinc (II) 5-[(2-thienylmethylidene) amino]-1,3,4-thiadiazole-2-thiol (8)

Yield 60% as light yellow powder; m.p. (decomp.) 214–216°C; IR (KBr): ν _max cm^{− 1} 2580 (SH), 1620 (HC = N), 1610 (C = N, mercaptothiadiazole), 525 (M–N), 360 (M–S); UV (DMSO): λ_max (cm^{− 1}) 28570; B.M. (μ_eff) Diamagnetic; Molar conductance (95 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₁₀ZnCl₂N₆S₆: C, 28.5; H, 1.7; N, 14.2. Found: C, 28.4; H, 1.9; N, 14.5%.

Cobalt (II) 5-{[(4-bromo-2-thienyl)methylidene] amino}-1,3,4-thiadiazole-2-thiol (9)

Yield 58% as light pink powder; m.p. (decomp.) 220–222°C; IR (KBr): ν _max cm^{− 1} 2580 (SH), 1625 (HC = N), 1605 (C = N, thiadiazole), 525 (M–N), 360 (M–S); UV (DMSO): λ_max (cm^{− 1}) 30420, 18190 and 8755; B.M. (μ_eff) 4.5; Molar conductance (94 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₈Co Br₂Cl₂N₆S₆: C, 22.6; H, 1.1; N, 11.3. Found: C, 22.9; H, 1.5; N, 11.0%.

Copper (II) 5-{[(4-bromo-2-thienyl)methylidene] amino}-1,3,4-thiadiazole-2-thiol (10)

Yield 63% as dark brown powder; m.p. (decomp.) 222–224°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1625 (HC = N), 1610 (C = N, thiadiazole), 530 (M–N), 365 (M–S); UV (DMSO): λ_max (cm^{− 1}) 30610 and 22360; B.M. (μ_eff) 1.4; Molar conductance (96 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₈Cu Br₂Cl₂N₆S₆: C, 22.5; H, 1.1; N, 11.3. Found: C, 22.9; H, 1.4; N, 11.0%.

Nickel (II) 5-{[(4-bromo-2-thienyl)methylidene] amino}-1,3,4-thiadiazole-2-thiol (11)

Yield 58% as light green powder; m.p. (decomp.) 217–219°C; IR (KBr): ν _max cm^{− 1} 2580 (SH), 1625 (HC = N), 1605 (C = N, mercaptothiadiazole), 525 (M–N), 360 (M–S); UV (DMSO): λ_max (cm^{− 1}) 28430, 16235 and 10155; B.M. (μ_eff) 3.4; Molar conductance (90 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₈Ni Br₂Cl₂N₆S₆: C, 22.7; H, 1.1; N, 11.3. Found: C, 22.3; H, 1.5; N, 11.1%.

Zinc (II) 5-{[(4-bromo-2-thienyl)methylidene]amino}-1,3,4-thiadiazole-2-thiol (12)

Yield 60% as off white powder m.p. (decomp.) 218–220°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1620 (HC = N), 1605 (C = N, thiadiazole), 530 (M–N), 360 (M–S); UV (DMSO): λ_max (cm^{− 1}) 28580; B.M. (μ_eff) Diamagnetic; Molar conductance (98 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₄H₈Zn Br₂Cl₂N₆S₆: C, 22.5; H, 1.1; N, 11.2. Found: C, 22.7; H, 1.0; N, 11.0%.

Cobalt (II) 5-[(1H-pyrrol-2-yl methylidene) amino]-1,3,4-thiadiazole-2-thiol (13)

Yield 59% as orange powder m.p. (decomp.) 232–234°C; IR (KBr): ν _max cm^{− 1} 2580 (SH), 1620 (HC = N), 1605 (C = N, thiadiazole), 525 (M–N), 540 (M–N); UV (DMSO): λ_max (cm^{− 1}) 30280, 18175 and 8865; B.M. (μ_eff) 4.5; non-electrolyte. Anal. Calcd. for C₁₄H₁₀CoN₈S₄: C, 35.2; H, 2.1; N, 23.5. Found: C, 35.0; H, 2.5; N, 23.1%.

Copper (II) 5-[(1H-pyrrol-2-yl methylidene) amino]-1,3,4-thiadiazole-2-thiol (14)

Yield 65% as dark green powder; m.p. (decomp.) 258–260°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1625 (HC = N), 1610 (C = N, mercaptothiadiazole), 520 (M–N), 540 (M–N), UV (DMSO): λ_max (cm^{− 1}) 30630 and 22340; B.M. (μ_eff) 1.3; non-electrolyte. Anal. Calcd. for C₁₄H₁₀CuN₈S₄: C, 34.8; H, 2.1; N, 23.2. Found: C, 35.3; H, 2.0; N, 23.6%.

Nickle (II) 5-[(1H-pyrrol-2-yl methylidene) amino]-1,3,4-thiadiazole-2-thiol (15)

Yield 60% as light green powder; m.p. (decomp.) 228–230°C; IR (KBr): ν _max cm^{− 1} 2580 (SH), 1620 (HC = N), 1605 (C = N, thiadiazole), 525 (M–N), 535 (M–N); UV (DMSO): λ_max (cm^{− 1}) 28585 and, 16180 and 9860; B.M. (μ_eff) 3.4; non-electrolyte. Anal. Calcd. for C₁₄H₁₀NiN₈S₄: C, 35.2; H, 2.1; N, 23.5. Found: C, 35.6; H, 2.0; N, 23.8%.

Zinc (II) 5-[(1H-pyrrol-2-yl methylidene) amino]-1,3,4-thiadiazole-2-thiol (16)

Yield 61% as off white powder; m.p. (decomp.) 240–242°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1620 (HC = N), 1610 (C = N, thiadiazole), 525 (M–N), 535 (M–N), UV (DMSO): λ_max (cm^{− 1}) 28535; B.M. (μ_eff) Diamagnetic; non-electrolyte. Anal. Calcd. for C₁₄H₁₀ZnN₈S₄: C, 34.7; H, 2.1; N, 23.1. Found: C, 34.5; H, 2.6; N, 23.4%.

Cobalt (II) 5-{[(2-Hydroxyphenyl)methylidene] amino}-1,3,4-thiadiazole-2-thiol (17)

Yield 59% as orange powder; m.p. (decomp.) 268–270°C; IR (KBr): ν _max cm^{− 1} 2575 (SH), 1630 (HC = N), 1605 (C = N, thiadiazole), 1380 (C–O), 520 (M–N), 445 (M–O); UV (DMSO): λ_max (cm^{− 1}) 30355, 17890 and 8895; B.M. (μ_eff) 4.4; non-electrolyte. Anal. Calcd. for C₁₈H₁₂CoN₆ O₂ S₄: C, 40.7; H, 2.3; N, 15.8. Found: C, 40.3; H, 2.5; N, 16.1%.

Copper (II) 5-{[(2-Hydroxyphenyl)methylidene] amino}-1,3,4-thiadiazole-2-thiol (18)

Yield 61% as dirty green powder; m.p. (decomp.) 265–267°C; IR (KBr): ν _max cm^{− 1} 2575 (SH), 1625 (HC = N), 1610 (C = N, thiadiazole), 1380 (C–O), 520 (M–N), 440 (M–O); UV (DMSO): λ_max (cm^{− 1}) 30580 and 22325; B.M. (μ_eff) 1.4; non-electrolyte. Anal. Calcd. for C₁₈H₁₂CuN₆ O₂ S₄: C, 40.3; H, 2.2; N, 15.7. Found: C, 40.6; H, 2.5; N, 15.4%.

Nickle (II) 5-{[(2-Hydroxyphenyl)methylidene] amino}-1,3,4-thiadiazole-2-thiol (19)

Yield 60% as light green powder; m.p. (decomp.) 266–268°C; IR (KBr): ν _max cm^{− 1} 2580 (SH),1620 (HC = N), 1610 (C = N, thiadiazole), 1380 (C–O), 525 (M–N), 450 (M–O); UV (DMSO): λ_max (cm^{− 1}) 28570, 16220 and 9870; B.M. (μ_eff) 3.3; non-electrolyte. Anal. Calcd. for C₁₈H₁₂NiN₆ O₂ S₄: C, 40.7; H, 2.3; N, 15.8. Found: C, 41.0; H, 2.2; N, 15.4%.

Zinc (II) 5-{[(2-Hydroxyphenyl)methylidene]amino}-1,3,4-thiadiazole-2-thiol (20)

Yield 61% as light yellow powder; m.p. (decomp.) 263–265°C; IR (KBr): ν _max cm^{− 1} 2580 (SH), 1630 (HC = N), 1605 (C = N, thiadiazole), 1380 (C–O), 525 (M–N), 445 (M–O); UV (DMSO): λ_max (cm^{− 1}) 28565; B.M. (μ_eff) Diamagnetic; non-electrolyte. Anal. Calcd. for C₁₈H₁₂ZnN₆ O₂ S₄: C, 40.2; H, 2.2; N, 15.6. Found: C, 39.9; H, 2.5; N, 15.9%.

Cobalt (II) 5-[(2-pyridinylmethylidene) amino]-1, 3,4-thiadiazole-2-thiol (21)

Yield 59% as orange red powder; m.p. (decomp.) 202–204°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1620 (HC = N), 1610 (C = N, thiadiazole), 1605 (C = N, pyridine), 525 (M–N), 515 (M–N); UV (DMSO): λ_max (cm^{− 1}) 30380, 17825 and 8875; B.M. (μ_eff) 4.4; Molar conductance (97 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₆H₁₂CoCl₂N₈S₄: C, 33.5; H, 2.1; N, 19.5. Found: C, 33.8; H, 2.6; N, 19.7%.

Copper (II) 5-[(2-pyridinylmethylidene) amino]-1, 3,4-thiadiazole-2-thiol (22)

Yield 60% as dark green powder; m.p. (decomp.) 206–208°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1615 (HC = N), 1610 (C = N, thiadiazole), 1600 (C = N, pyridine), 520 (M–N), 510 (M–N); UV (DMSO): λ_max (cm^{− 1}) 30595 and, 22350; B.M. (μ_eff) 1.4; Molar conductance (94 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₆H₁₂CuCl₂N₈S₄: C, 33.2; H, 2.1; N, 19.4. Found: C, 33.6; H, 2.4; N, 19.8%.

Nickle (II) 5-[(2-pyridinylmethylidene) amino]-1, 3,4-thiadiazole-2-thiol (23)

Yield 60% as light green powder; m.p. (decomp.) 201–203°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1620 (HC = N), 1610 (C = N, thiadiazole), 1605 (C = N, pyridine), 525 (M–N), 510 (M–N); UV (DMSO): λ_max (cm^{− 1}) 28555, 16195 and 9865; B.M. (μ_eff) 3.3; Molar conductance (97 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₆H₁₂NiCl₂N₈S₄: C, 33.5; H, 2.1; N, 19.5. Found: C, 33.1; H, 2.5; N, 19.1%.

Zinc (II) 5-[(2-pyridinylmethylidene) amino]-1, 3,4-thiadiazole-2-thiol (24)

Yield 61% as pale yellow powder; m.p. (decomp.) 198–200°C; IR (KBr): ν _max cm^{− 1} 2585 (SH), 1620 (HC = N), 1610 (C = N, thiadiazole), 1605 (C = N, pyridine), 525 (M–N), 515 (M–N); UV (DMSO): λ_max (cm^{− 1}) 28575; B.M. (μ_eff) Diamagnetic; Molar conductance (95 ohm^{− 1} cm^{− 2} mol^{− 1}). Anal. Calcd. for C₁₆H₁₂ZnCl₂N₈S₄: C, 33.1; H, 2.1; N, 19.3. Found: C, 33.3; H, 2.5; N, 19.0%.

Antibacterial bioassay

Preliminary screening

The synthesized compounds (L¹)–(L⁶) and their corresponding metal (II) complexes (1)–(24) were screened in vitro for their antibacterial activity against four Gram-negative (E. coli, P. aeruginosa, S. typhi and S. flexneri) and two Gram-positive (Bacillus subtilis and S. aureus) bacterial strains by the agar-well diffusion method [24]. The wells (6 mm in diameter) were dug in the media with the help of a sterile metallic borer with centres at least 24 mm apart. Two to eight hours old bacterial inocula containing approximately 10⁴–10⁶ colony-forming units (CFU/ml) were spread on the surface of the nutrient agar using a sterile cotton swab. The recommended concentration of the test sample (1 mM/ml in DMSO) was introduced in the respective wells. Other wells supplemented with DMSO and reference antibacterial drug, imipenum, served as negative and positive controls, respectively. The plates were incubated immediately at 37°C for 24 h. Activity was determined by measuring the diameter (mm) of zones showing complete inhibition. Growth inhibition was calculated with reference to a positive control.

Minimum inhibitory concentration (MIC)

Compounds containing high antibacterial activity (over 80%) were selected for minimum inhibitory concentration (MIC) studies. The minimum inhibitory concentration was determined using the disc diffusion technique by preparing discs containing 10, 25, 50 and 100 mM/ml of the compounds and applying the reported protocol [25].

Results and discussion

Chemistry

The Schiff bases (L¹)–(L⁶), were prepared by refluxing equimolar (0.01 mol) quantities of 5-amino-1,3,4-thiadiazole-2-thiol and the respective aldehydes for 2 hours in ethanol (30–40 ml) containing a few drops of conc. H₂SO₄ (Scheme 1). All the synthesized Schiff bases were characterized by spectroscopic techniques (IR and ¹H-NMR) and their elemental analyses.

The metal complexes (1)–(24) of the Schiff bases (L¹)–(L⁶) were air stable and prepared by the stoichiometric reaction of the corresponding metal (II) chlorides with the Schiff base ligands in a molar ratio (M:L) of 1:2. These complexes are intensely coloured amorphous solids except those of zinc (which are light yellow in colour) and decompose without melting. They are insoluble in common organic solvents such as ethanol, methanol, chloroform or acetone but are soluble in DMSO and DMF. Molar conductance values (88–98 ohm^{− 1} cm^{− 2} mol^{− 1}) of the complexes (1)–(12) and (21)–(24) of cobalt (II), copper (II), nickel (II) and zinc (II), soluble in DMF (10^{− 3} M solution at 25°C), indicated that they are electrolytic in nature; however, the metal complexes (13)–(20), are found to be non electrolytic in nature [26]. The elemental analysis data agree well with the proposed formulae for the ligands and the [M(L)₂]Cl₂ and M(L)₂ composition of the metal (II) complexes.

IR spectra

The IR spectra of all the Schiff bases (L¹)–(L⁶) showed the absence of bands at 1735 and 3420 cm^{− 1} due to carbonyl v(C = O) and v (NH₂) stretching vibrations, respectively (present in the starting materials) and instead, a strong new band at ∼1635–1640^{− 1} assigned to azomethine v(HC = N) vibrations appeared indicating condensation of the starting carbonyl moieties with the amino group containing heteroaromatic systems [27]. The IR spectra of all the ligands exhibited the azomethine (HC = N) and thiadiazole stretchings in the regions 1635–1640 and 1615–1620 cm^{− 1}, respectively. In addition, the spectrum of (L⁴) showed a band resulting from the NH stretching of the pyrrole moiety at 3215 cm^{− 1}. The SH stretchings appeared in the 2580–2590 cm^{− 1} region. The IR spectrum of the Schiff base (L⁵) showed bands resulting from the SH and OH stretchings of the thiol and phenol functions at 2580 and 3455 cm^{− 1}, respectively. The azomethine (HC = N) and the thiadiazole (C = N) stretchings were observed respectively at 1640 and 1615 cm^{− 1}. In the case of the Schiff base (L⁶), the SH stretching appeared at 2590 cm^{− 1}, whereas the azomethine (HC = N) and thiadiazole (C = N) stretchings were observed at 1635 and 1615 cm^{− 1}, respectively [28]. The pyridine (C = N) stretching appeared at 1610 cm^{− 1}.

The comparison of the infrared spectra of the Schiff bases (L¹)–(L⁶) with their metal complexes (1)–(24) principally revealed that the ligands are tridentately coordinated to the metal ions. The band appearing at ∼1635–1640 cm^{− 1} due to the azomethine stretching vibration is shifted to lower frequency by ∼10–15 cm^{− 1} indicating participation of the azomethine nitrogen in complexation. The band at ∼1615–1620 cm^{− 1} assigned to the thiadiazole ring v(C = N) vibrations is also shifted to lower frequency by ∼10–15 cm^{− 1}, which is indicative of the involvement of the thiadiazole ring in chelation. Also, a band at ∼3215 cm^{− 1} attributed to νNH in the ligand (L⁴) disappeared in its metal complexes (13)–(16) indicating deprotonation of the NH moiety during coordination [29]. Also, a band at ∼3455 cm^{− 1} attributed to νOH in the ligand (L⁵) disappeared in its metal complexes (17)–(20) and instead appearance of a (C–O) band at ∼1380 cm^{− 1} indicated deprotonation of the OH moiety during coordination. The band at ∼1610 cm^{− 1} assigned to the pyridine ring v(C = N) vibrations in the ligand (L⁶) is also shifted to lower frequency by ∼5 cm^{− 1} in its metal complexes (21)–(24), which is indicative of the involvement of the N atom of the pyridine ring in chelation. However, further conclusive evidence of the coordination of the ligands with the metal atoms was established by the far IR spectra in which new bands at 360–365, 440–450, 455–460, 510–515, 520–530 and 535–540 cm^{− 1} assigned to (M-S (thienyl), (M-O (deprotonated salicylyl), (M-O (furanyl), M–N (pyridyl), (M–N (thiadiazole) and (M–N (pyrrolyl) in the spectra of the metal complexes were observed, which are not present in the spectra of their corresponding ligands [29].

Magnetic moment

The room temperature magnetic moments of the solid cobalt (II) complexes were found to lie in the range 4.2–4.6 B.M indicative of three unpaired electrons per Co (II) ion in an octahedral environment. The Cu(II) complexes showed μ_eff values in the range 1.3–1.5 B.M indicative of one unpaired electron per Cu(II) ion, suggesting [30] that these complexes had structures within the range consistent to spin-free distorted octahedral geometry. Similarly, the Ni (II) complexes showed μ_eff values in the range 3.2–3.4 B.M corresponding to two unpaired electrons per Ni (II) ion for their ideal six-coordinated configuration. The Zn (II) complexes were all found to be diamagnetic.

Electronic spectra

The electronic spectra of the Co(II) complexes (1), (5), (9), (13), (17) and (21) showed three bands at 8620–8895, 17590–18225 and 30210–30480 cm^{− 1}, which may be assigned to ⁴T_{1g →} ⁴T_2g(F), ⁴T_{1g →} ⁴T_2g(P) and ⁴T_{1g →} ³A_2g(F) transactions, respectively, and are suggestive of their octahedral geometry around the cobalt ions [31]. The Cu (II) complexes (2), (6), (10), (14), (18) and (22) showed absorption bands between 10Dq band for a distorted octahedral geometry corresponding to the transitions ²E_{g →} ²T_2g [32]. The Ni (II) complexes (3), (7), (11), (15), (19) and (23) exhibited three spin-allowed bands at 9740–10155, 15820–16235 and 28420–29130 cm^{− 1} assignable, respectively to the transitions ³A_2g(F) → ³T_2g(F)(v₁), ³A_2g(F) → ³T_1g(F)(v₂) and ³A_2g(F) → ³T_2g(F)(v₃), which are characteristic of Ni(II) in octahedral geometry [33]. The diamagnetic zinc (II) complexes did not show any d–d bands and their spectra are dominated only by charge transfer bands. The diamagnetic Zn (II) complexes (4), (8), (12), (16), (20) and (24) did not show any d–d bands and their spectra were dominated only by the charge transfer bands 28535–28590 cm^{− 1} [34] (Figure 1).

Figure 1 Proposed structural formulae of the metal (II) complexes.

Antibacterial bioassay

Preliminary screening

Antibacterial activity of the synthesized Schiff bases (L¹)–(L⁶) and their corresponding metal (II) complexes (1)–(24) was determined against four Gram-negative (E. coli, P. aeruginosa, S. typhi and S. flexneri) and two Gram-positive (B. subtilis and S. aureus) bacterial strains. The synthesized Schiff base compounds exhibited varying degree of inhibitory effects (low to moderate) on the growth of different tested strains (Table I). It is evident from the results collected in the Table that the potency of all the ligands was enhanced upon coordination with the metal ions. These results substantiate our own findings 35-42 and the findings of some other workers 43-46 that biologically inactive compounds become active and less biologically active compounds become more active upon coordination with the metal ions. The results given in the Table show that eight of the synthesized complexes i.e. (5)–(6), (9)–(10), (12), (14), (21)–(22) have comparatively much more activity and of these, (10) is the most active one.

Table I.  Preliminary screening of the schiff bases (L¹)–(L⁶) and their metal (II) complexes (1)–(24) for antibacterial activity (zone of inhibition in mm).

	Microbial species
	Gram-negative				Gram-positive
Schiff base/Complex	E. coli	P. aeruginosa	S. typhi	S. flexneri	B. subtilis	S. aureus
L^1	12	11	10	11	13	12
L^2	10	10	10	11	11	13
L^3	11	11	09	13	12	11
L^4	13	10	10	10	11	13
L^5	11	11	12	13	12	12
L^6	12	12	13	13	14	14
1	18	16	17	18	21	16
2	18	14	15	16	19	20
3	18	16	15	16	17	20
4	19	13	13	17	19	21
5	20	16	21	23	27	27
6	21	17	21	23	28	28
7	20	17	17	18	23	22
8	21	16	17	18	22	24
9	22	18	22	23	28	28
10	23	19	23	24	30	30
11	21	18	19	20	25	23
12	23	17	20	22	26	26
13	19	14	17	17	22	19
14	20	16	21	22	28	27
15	16	16	16	19	18	19
16	20	15	16	16	19	21
17	18	15	16	16	20	22
18	20	14	15	17	20	19
19	18	14	14	17	16	22
20	16	13	17	15	21	21
21	18	16	20	22	25	26
22	20	17	21	22	27	27
23	15	16	16	15	22	19
24	17	15	16	18	17	20
Imipenum	30	24	25	27	33	33

< 9: weak; 9–16: moderate; >16: significant.

Minimum inhibitory concentration (MIC)

The preliminary screening showed that compounds (5)–(6), (9)–(10), (14) and (22) were the most active ones against both Gram-negative and Gram-positive organisms. These six compounds were selected for minimum inhibitory concentration (MIC) studies (Table II). The MIC values of all the six active compounds ranges from 0.013–0.207 μM. Compound (10) again proved to be the most active one; it inhibited the growth of S. typhi, S. flexneri and B. subtilis at 0.013 μM concentration.

Table II.  Minimum inhibitory concentration (mM/ml) of compounds (5)–(6), (9)–(10), (14) and (22) against selected bacteria.

Compound no.	5	6	9	10	14	22
Gram-negative
S. typhi	0.017	0.042	0.013	0.013	0.021	0.017
S. flexneri	0.017	0.017	0.013	0.013	>0.207	>0.173
Gram-positive
B. subtilis	>0.171	0.042	0.034	0.013	>0.207	>0.173
S. aureus	>0.171	>0.170	0.135	0.033	0.052	0.086

The present investigation suggests that of all the metal complexes, those derived from the Schiff base (L²) bearing merceptothiadiazole and thiophene moieties have comparatively more biological activity. Further, replacement of thiophene-2- carboxaldehyde with 4- bromothiophene-2-caboxaldehyde as one of the components of the Schiff base causes enhancement of the activity of the metal complexes derived from. These studies may serve as a basis for chemical modifications directed towards the development of a new class of antibacterial agents.

Notes

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