Synthesis, Spectroscopic Characterization and Antibacterial Properties of some Metal (II) Complexes of 2-(6-methoxybenzothiazol-2- ylimino)methyl)-4-nitrophenol.

Abstract

Mn(II), Co(II), Ni(II), Cu(II), Zn(II), and Pd(II) complexes of the Schiff base 2-(6-methoxybenzothiazol-2-ylimino)methyl)-4-nitrophenol (HL) derived from condensation of 2-hydroxy-5-nitrobenzaldehyde and 2-amino-6-methoxybenzothiazole have been synthesized and characterized by percentage metal, solubility, infrared and electronic spectra measurements. Their in-vitro anti-bacterial activities against Bacillus subtilis, Staphylococcus aureus, Proteus mirabilis, Klebsiella oxytoca, Pseudomonas aeruginosa, and Escherichia coli were also evaluated. The metal complexes formed as [ML(NO3)].bH2O, where M = Mn, Co, Cu, Zn; b = 0.5 - 4, with the exceptions of the Pd(II) and Ni(II) complexes that analyzed as [Pd(L)2 ].4H2O and [Ni(L)Cl(H2O) ]. The infrared spectra confirmed that coordination by the Schiff base is via the imine nitrogen and phenol oxygen atoms respectively, resulting in a NO chromophores around the metal ion. The electronic spectra measurement was corroborative of a 4-coordinate tetrahedral/square planar geometry for the complexes. The complexes mostly exhibit good in-vitro antibacterial activities against S. aureus, E. coli and P. mirablilis with inhibitory zones range of 16.0-27.0 mm, 13.0-15.0 mm and 17.0-20.0 mm respectively. Its noteworthy that the Pd(II) and Zn(II) complexes had broad spectrum anti-bacteria activity against the bacteria used with the exception of B. subtilis with inhibitory zones range of 13.0-22.0 and 13.0-27.0 mm respectively.

Introduction

Benzothiazole and its derivatives are one of the most important heterocyclic compounds that have received overwhelming response owing to its biological applications e.g. they have been reported as inhibitors of mycobacterium tuberculosis ATP phosphoribosyl tranferase [1], antidiabetic agents [2] , antimicrobial agents [3,4,5], anti-inflammatory agents [6], analgesic agents[7], inhibitors of human cox-2 and exhibits hypoglycemic activity [8] . The combination of different heterocyclic compounds is a well-known approach for drug-like molecules build-up, which allows achieving compounds with new pharmacological profile, action strengthening or toxicity lowering abilities [9]. Schiff base formation affords the opportunity of such combinations, and has largely been reported as possessing wide range of biological applications [10]. Specifically, Schiff bases of benzothiazole derivatives have shown positive activity towards Parkinson’s disease [11], and many are useful as antitumour/anticancer, anticandidous, antihistaminic, antiflammatory anti bacterial agents [12,13,14,15,16,17,18,19]. A thorough literature search shows that the Schiff base prepared from the condensation reaction of 2-amino-5,6-dimethylbenzothiazole and 2-hydroxy-5-nitrobenzaldehyde (2-(6-methoxybenzothiazol-2-ylimino)methyl)-4-nitrophenol) has not been reported.

Therefore, we are reporting the synthesis and characterization of metal (II) complexes of the Schiff base, of 2-(6-methoxybenzothiazol-2-ylimino)methyl)-4-nitrophenol for the first time as a continuation of the research activities of our group [20-25].The aims of this study is to investigation the physicochemical properties of these metal complexes as well as their potency as in-vitro antibacterial agents

Experimental

Chemicals

Reagent grade 2-hydroxy-5-nitrobenzaldehyde, 2-amino-6-methoxybenzothiazole, manganese(II) nitrate hexahydrate,
copper(II) nitrate hexahydrate, zinc(II) nitrate hexahydrate, palladium(II) chloride, nickel(II) chloride dihydrate and cobalt(II) nitrate
hexahydrate were purchased from Aldrich and BDH chemicals, and were used as received. Solvents were purified by distillation.

Physical measurements

Percentage manganese, cobalt, nickel, copper, zinc and palladium in the complexes were determined titrimetrically [26].
Infrared spectra were measured in deuterated dimethyl sulfoxide solvent on a Shimadzu FTIR-8400 spectrophotometer while
electronic spectra in chloroform were recorded on Unicam Helios -γ spectrophotometer, and melting points (uncorrected) were done
using a Stuart scientific melting point apparatus smp3.

Synthesis

Synthesis of 2-(6-methoxybenzothiazol-2-ylimino)methyl)-4-nitrophenol

The Schiff base was prepared by adding 5g (0.03 mole) of 2-amino-6-methoxybenzothiazole neat in bits to a stirring 5g (0.03 mole) of 2-hydroxy-5-nitrobenzaldehyde in 50 mL of hot methanol. The resulting orange-coloured homogeneous solution was then refluxed for 3 h after the addition of 4 drops of acetic acid. The products formed were filtered, washed with methanol and dried over silica gel.

Color (Yellow); IR (DMSO, cm⁻¹): νOH (3457s), νC=N (1608s 1558s) νC=C(1491s); UV (DMSO) λmax (kK, Ɛ): 40.32 (1 × 10⁶), 34.97 (1 × 10⁵), 28.25 (1 × 10⁶); Formula mass: 329.33; melting point: 264°C.

Synthesis of metal complexes

To a stirring solution of 0.28 g (9.0 x10^-4 moles) of HL in 20 mL of methanol, and 0.16 – 0.30g (9.0 x10^-4 moles) of the metal (II) nitrates and chlorides (M= Cu, Mn, Co, Ni, Zn, Pd) in 20 mL of methanol were added dropwisely. The resulting homogeneous solutions pH was adjusted to 9, with triethylamine, and then refluxed for 3 h. The precipitates formed were filtered, washed with methanol and dried under vacuum.

[Mn(L)NO₃]0.5H₂O: Color (Yellow); IR (DMSO, cm⁻¹): νOH (3434s), νC=N (1600s) νC=C(1464 s); νM-N(637 m), vM-O(549 m); Vis/UV λmax (kK, Ɛ): 15.36 (100); 23.47 (100), 35.33 (1x 104), 40.14(1x 10⁵); Formula mass 453.77; melting point: 318°C.

[Pd(L)₂]4H₂O: Color (Reddish brown); IR (DMSO, cm⁻¹): νOH (3445 s), νC=N (1601s) νC=C (1469 s); νM-N (672 m), vM-O (543 m); Vis/UV λmax (kK, Ɛ): 14.79(100), 22.37(200), 36.33 (1x 104), 42.84(1x 10⁵); Formula mass 835.12; melting point: 318°C.

[Pd(L)₂]4H₂O: Color (Reddish brown); IR (DMSO, cm⁻¹): νOH (3445 s), νC=N (1601s) νC=C (1469 s); νM-N (672 m), vM-O (543 m); Vis/UV λmax (kK, Ɛ): 14.79(100), 22.37(200), 36.33 (1x 104), 42.84(1x 10⁵); Formula mass 835.12; melting point: 318°C.

[Ni(L)Cl(H₂O)]: Color (Yellow); IR (DMSO, cm⁻¹): νOH (3440s), νC=N (1606 s 1548 s) νC=C (1469 s); νM-N(510 m), vM-O/M-Cl(370 m) Vis/UV λmax (kK, Ɛ): 23.73 (400); 35.30 (1 × 10⁵), 40.46 (1 × 10⁶)Formula mass 542.04; melting point 318°C.

[Co(L)NO₃]3H₂O: Color (Orange); IR (DMSO, cm⁻¹): νOH (3440 s), νC=N (1603 s) νC=C(1442 s); νM-N(657 m), vM-O(492 m) UV λmax (kK, Ɛ): 15.36(100), 23.47 (300); 27.47 (1 × 10⁵), 40.00 (1 × 10⁵), 40.00 (1 × 10⁵); Formula mass 503.34; melting point 320°C.

[Zn(L)NO₃]1.5H₂O : Color (Yellow); IR (DMSO, cm⁻¹): νOH (3399s), νC=N (1616 s) νC=C (1447 s); νM-N(501 m), vM-O(364 m); UV λmax (kK, Ɛ): 22.0 (100); 27.0(1000), 39.30 (1 × 10⁵), 42.0 (1 × 10⁶)Formula mass 482.33; melting point 342°C.

Antibacterial assay

The assay was carried out on the metal-free Schiff base and their metal(II) complexes using Agar diffusion technique. The surface of the agar in a Petri dish was uniformly inoculated with 0.3 mL of 18 hours old bacteria culture of Bacillus subtilis, Staphylococcus aureus, Proteus mirabilis, Klebsiella oxytoca, Pseudomonas aeruginosa, and Escherichia coli. Using a sterile cork borer, 6 mm wells were bored into agar. Then 0.06 mL of 10 mg/mL concentration of each metal complex in DMSO was introduced into the wells and the plates were allowed to stand on the bench for 30 min before incubation at 370C for 24 h. Inhibitory zones (in mm) were taken as a measure of antimicrobial activity. The experiments were conducted in duplicates and streptomycin was used as the reference drug [27].

Results and Discussion

The equations for the formation of the complexes are given below;

M(NO₃)2 .6H₂O+ HL → [M(L)NO₃]bH₂O + 2HNO₃+ xH₂O ------------------------------ (1)

(where M = Zn(II), Cu(II),Mn(II), Co(II), b= 0.5 – 4, x = 2- 5.5 )

PdCl₂ + 2HL → [Pd(L)₂] + 2HCl--------------------------------------(2)

NiCl₂ .2H₂O+ HL → [Ni(L)Cl(H₂O)] + HCl +H₂O -------------------------------------(3)

The metal complexes formed as [ML(NO₃)].bH₂O, where M = Mn, Co, Cu, Zn; b = 0.5 - 4, with the exceptions of the Pd(II) and Ni(II) complexes that analyzed as [Pd(L)₂ ].4H₂O and [Ni(L)Cl(H₂O) ]. Proposed structures for the ligand is shown in Figure 1. The formation of this ligand is confirmed by melting point, IR and electronic spectra measurements.

Figure 1: The structural formula of the Schiff base

Electronic spectra

The Mn(II) complex showed two bands at 15.36 and 23.47 kK respectively, typical of a 4-coordinate tetrahedral geometry and were assigned as ⁶A₁ → 4E1 and ⁶A₁ →⁴T₂ transitions [20]. Similarly, the Co(II) complexes displayed two bands at 18.97 and 22.37 kK assigned to ⁴A₂ → 4T1(F), (v₂), and ⁴A₂ → 4T1(P), (v₃) transitions of a 4-coordinate tetrahedral geometry [9]. In addition, the Pd(II) complex exhibited two bands at 14.70 and 22.37 kK indicative of 4-coordinate square-planar geometry and were assigned to ¹A₁g→¹B_1g and ¹A₁g→¹E_2g transitions [23]. Literature confirmed that square-planar Ni(II) complexes absorb in the range 22.0-25.0 kK, while tetrahedral complexes Cu(II) complexes have single absorption band below 10.0 kK and square-planar Cu(II) complexes absorb between 10.0-20.0 kK [10]. Thus, the Ni(II) and Cu(II) complexes in this study displayed lone bands each at 23.73 and 23.81 kK typical of square planar geometry and were assigned to ¹A₁g→¹A₂g and 2B_1g → ²E_1g transitions respectively [21]. The Zn(II) complex was expectedly diamagnetic with M →L CT band at 22.20 kK and since it’s CFSE is zero, the assumed geometry was tetrahedral [28,29].

Normally, the molar extinction coefficient (Ɛ) allows a distinction of tetrahedral geometry from octahedral geometry. Tetrahedral complexes are expected to have a higher molar extinction coefficient in the range 10²–10³ cm² mol^-1 and octahedral complexes should have Ɛ between 1–50 cm² mol^-1. None of the complexes in this study had Ɛ below 10² cm² mol^-1, this is indicative of a tetrahedral/square-planar geometry [20].

Infrared spectra

The strong bands at 1608 – 1558 cm⁻¹ in the metal free Schiff base was assigned to the vC=N of azomethine and thiazole stretching vibrations. These bands were mostly shifted to lower wave numbers in the range 1606 – 1548 cm⁻¹, with the exception of [Zn(L)NO₃] that experienced an increased shift to 1616 cm⁻¹. This indicated the coordination of the imine nitrogen to metal ions. Similarly, the band for vC=C at 1491 cm⁻¹ in the ligand was shifted to 1469-1462 cm⁻¹ in the metal complexes due to coordination [9-10]. The phenol band at 3457 cm⁻¹ in the spectrum of the Schiff base was absent in the spectra of the metal complexes due to the coordination of the enol oxygen atom [19]. In the spectra of all the metal complexes, a broad band at 3500 cm⁻¹ was observed due to presence of coordinated /hydration water. The bands due to M-N and M-O/M-Cl were observed in the range 672-501 cm-1 and 549-364 cm-1 respectively. These bands were absent in the spectra of the Schiff base [29].

Antibacterial activities

The antibacterial activities are presented in Figure 2. The Schiff base was active against all the bacteria used S. aureus, B. subtilis , K. oxytoca, E. coli and P. mirabilis with the exception of P. aeruginosa with inhibitory zones range of 13.0-26.0mm. All the metal complexes were active against P. mirabilis with inhibitory zones range of 17.0-21.0mm. Similarly, all the complexes were active against S. aureus and E. coli with inhibitory zones range of 16.0-27.0 mm and 13.0-15.0 mm with the exceptions of Mn(II) and Cu(II) complexes. On the contrary, P. aeruginosa was sensitive to only Zn(II) and Pd(II) complexes with inhibitory zones of 13.0 mm and 15.0 mm respectively. In addition, K. oxytoca was sensitive to Zn(II), Mn(II) and Pd(II) complexes with inhibitory zones range of 13.0-15.0 mm while B. subtilis was sensitive only to the Ni(II) complex with inhibitory zone of 14.0 mm. Generally, the metal complexes were less active than the Schiff base. This is contrary to chelation theory (which states that chelation increases antimicrobial activity, because of partial sharing of its positive charge with donor groups of the ligand and possible π-electron delocalization which increases the lipophilic character) [9]. The lower activities of the metal complexes may be attributed to lower lipophilicity of the complexes, which decrease the penetration of the complexes through the lipid membrane [10]. Streptomycin activities (30.0–44.0 mm) against the various isolates relative to the metal complexes (13.0–27.0 mm) show that the activities of the latter are much lower, with the optimum activity being about 75 percent that of streptomycin in Zn(II) complex against Staphylococcus aureus. Its noteworthy that the Pd(II) and Zn(II) complexes had broad spectrum anti-bacteria activity against the bacteria used with the exception of B. subtilis with inhibitory zones range of 13.0-22.0 mm and 13.0-27.0 mm respectively, proving their potentials as broad-spectrum antibacterial agents [25,26,27,28,29]./p>

Figure 2: Histogram of the antibacterial activities of ligand and its metal complexes

Mn(II), Co(II), Ni(II), Cu(II), Zn(II), and Pd(II) complexes of the Schiff base 2-(6-methoxybenzothiazol-2-ylimino)methyl)-4-nitrophenol coordinated to the metal ions via the imine nitrogen and phenol oxygen atoms respectively. The electronic spectra measurement was corroborative of a 4-coordinate tetrahedral/square planar geometry for the complexes. The complexes exhibited good in-vitro antibacterial activities against S. aureus, E. coli and P. mirablilis , while Pd(II) and Zn(II) complexes had broad spectrum anti-bacteria activity against all the bacteria used with the exception of B. subtilis proving their potentials as broad-spectrum antibacterial agents.

References

1. Cho Y, Ioerger TR, Sacchettini JC. Discovery of novel nitrobenzothiazole inhibitors for mycobacterium tuberculosis ATP phosphoribosyl transferase(HIS G) through virtual screening. J Med Chem. 2008;51(19):5984-5992.
2. Pattan SR, Suresh Ch, Pooja VD, Reddy VVK, Rasal VP, Koti BC. Synthesis and antidibetic activity of 2-amino[5‟(4-sulfonylbenzylidine)-2,4-thiazolidindione]-7-chloro-6-fluorobenzothiazole. Indian J of Chem. 2005;44B:2404-08.
3. Yadav P, Chauhan D, Sharma NK, Singhal S. 2-substituted hydrazine-6-fluro-1,3-benzothiazole:Synthesis and characterization of new novel antimicrobial agents. International journal of Chemistry Tech Res. 2010;2(2):1209-13.
4. Lion CJ, Matthews CS, Wells G, Westwel ADI.Antitumour properties of fluorinated benzothiazole-substituted hydroxycyclohexa-2,5-dienones(quinols). Bioorg Med Chem Lett. 2006;16: 5005.
5. L Cressier D et al. Synthesis, antioxidant properties and radioprotective effects of new benzothiazole and thiadiazoles. Bioorg Med Chem Lett.2009;17:5275-5284.
6. Kaur H, Kumar S, Singh I, Saxena KK, Kumar A. Synthesis, characterization and biological activity of various substituted benzothiazole derivatives. Digest J Nanomat Biostruct. 2010;5(1): 67-76.
7. Mohran MA, Ramzy F, Sembet AM. Synthesis and evaluation of new benzothiazole derivatives as schistosomocidal agents. Molecules. 2007;12: 622-633.
8. Paramashivappa R, Phani Kumar P, Subba Rao PV, Srinivasa Rao A. Design, synthesis and biological evaluation of benzimidazole/benzothiazole and benzoxazole derivatives as cyclooxygenase inhibitors. Bioorg Med Chem Lett. 2003;13:657-660.
9. Arali VKH, Revankar VK, Mahale VB, Kulkarni PJ. Cobalt(II), Nickel(II) and Copper(II) with 2-substituted Benzimidazole complexes. Transition Met Chem. 1994;19: 57 – 60.
10. Chohan, ZH. Biologically active transition metal complexes of Ni(II) Cu(II) and Zn(II) with 2-aminothiazole Schiff bases. Met-Based Drugs. 1999;6: 187-192
11. Alain B, Christinsonie DM, Assunta I. 1997 Pharmaceutical compositions containing 2-aminobenzothiazoles for treating Parkinson's disease and Parkinson Ian syndromes. Patent CA pharmaceuticals, 1: 63
12. Malik WU, Rajeev J. Quantitative Estimation of the Electronic Effects of Substituents in Five-membered Nitrogencontaining Aromatic Heterocycles. J Indian Chem Soc. 1992;LIX: 558.
13. Karia FD, Parsania PH. Biologically active Co(II) and Ni(II) complexes of N-(2-thienylmethylene)-2-aminothiadiazole. Asian J Chem. 1999;11: 991.
14. Loaizaperez AI. Preclinical Evaluation of Amino Acid Prodrugs of Novel Antitumor2-(4-Amino-3-Methylphenyl)Benzothiazoles. Mol Pharmacol. 2002;61:13.
15. Racane L. Antitumour efficiency of novel fluoro substituted 6-amino-2-phenylbenzothiazole hydrochloride salts in-vitro and in-vivo. Molecules. 2006;11:328.
16. Sidoova E, Loos D, Bujdakova H. New anti candidous 2-alky/thio-6-amino benzoylthiazole. Molecules. 1997;2:36-42.
17. Swarnkar PK, Kriplani P, Gupta GN, Ojaha KG. Microbial transformations of 2-substituted benzothiazoles. Indian J Chem. 2007;4:14.
18. Abignente E, De-Capariis P, Sacchi A, Marmole E. Synthesis and antibacterial activity of some new phenothiazine derivatives. Pharm Co Ed Sci. 1983;38:533.
19. Bhardwarj CN, Singh VR. Synthesis and characterization of thallium(I) complexes of biologically active benzothiazolines. Indian J Chem. 1994;33A: 423-425.
20. Osowole AA, Kempe R, Schobert R, Effenberger K. Synthesis, spectroscopic, thermal and in-vitro anticancer properties of some metal(II) complexes of 3-(-1-(4,6-dimethyl-2-pyrimidinylimino)methyl-2-napthol. Synth. React Inorg Met Org Chem & Nano-Met Chem. 2011; 41(7):825-833.
21. Osowole AA, Ott I, Ogunlana OM. Synthesis, spectroscopic, anticancer and antimicrobial properties of some Metal(II) complexes of 2-[(2,3-dihydro1H-inden-4-ylimino) methyl]-5-nitrophenol. International J Inorg Chem. 2012;Article ID 206417.
22. Osowole AA, Kempe R, Schobert R. Synthesis, spectral, thermal, in-vitro antibacterial and anticancer activities of some metal(II) complexes of 3-(-1- (4-methoxy-6-methyl)-2- pyrimidinylimino)methyl-2-napthol. Int Res J Pure App Chem. 2012;2(2):10⁵-129.
23. Osowole AA, Ott I. Synthesis, characterization, in-vitro anticancer and anti-microbial properties of some metal(II) complexes of 4-[(2,3-dihydro-1H-inden-4-ylimino) methyl] benzene-2, 4-diol. Int Res J Pure App Chem. 2012; 2(2): 156-169.
24. Osowole AA, Oni AA, Onyegbula K, Hassan AT. Synthesis, spectral magnetic and in-vitro anticancer properties of some Metal(II) complexes of 3-[2,4-dihydro-1H-inden-4-ylimino) methyl] napthalen-2-ol. Int Res J Pure App Chem. 2012;2(3): 211-220.
25. Osowole AA, Ekennia AC, Achugbu BO, Etuk GH. Synthesis, spectroscopic characterisation and structure related antibacterial activities of some metal(II) complexes of substituted triflurobutenol. Elixir Appl Chem. 2013;59:15848-15852.
26. Bassett J, Denney RC, Jeffery GH, Mendham J. 1978, Vogel’s Textbook of Quantitative Inorganic Analysis, fourth ed. ELBS, London.
27. Idrees N, Siddique M, Doshi AG, Raut AW. Synthesis of N-substituted phenyl-4-thiophenyl -2-azetidinones and its antimicrobial activity. Oriental J Chem. 2001;17(1): 143–146.
28. Han X, You ZL, Xu YT, Wang XM. Synthesis, characterization and crystal structure of a mononuclear zinc(II) complex derived from 2-methoxy-6-[(3-cyclohexylaminopropylimino)methyl]phenol. J Chem Crystallogr. 2006;36(11):743–746.
29. Chohan ZH, Pervez H, Rauf A, Scozzafava A, Supuran CT. Antibacterial Co(II), Cu(II), Ni(II) and Zn(II) complexes of thiadiazole derived furanyl, thiophenyl and pyrrolyl Schiff bases. J Enz Inh Med Chem. 2002;17(2):117–122.