Baghdad Science Journal | 2021
Synthesis and Characterization of New Benzothiazole-derived Schiff Bases Metal Complexes
Abstract
Nitrogen-comprising heterocyclic compounds and their derivatives have empirically been invaluable as therapeutic agents. Fundamentally, 4-chloro-6-nitro-2-amino-1,3-benzothiazole 1 was synthesized via bromination of 2-chloro-4-nitro aniline with ammonium thiocyanate. This new heterocyclic haloorganoamino-1,3-benzothiazole derivative, was a starting material, which condensed and tethered with three different aromatic aldehyde pendant arm in presence of ethanol and glacial acetic acid isolating an interesting sequence of tridentate Schiff bases 2-4. These compounds were used for complexation reactions in 1:1 (metal: ligand) stoichiometry to obtain heteroleptic Al(III), Ni (II) and K(I) benzothiazole chelates 57(a-c) of the type [Al(L)Cl2, Ni(L)Cl, K(L) {L = Schiff base derivatives}]. The newly synthesized complexes were characterized by the melting points, IR and some of them by 1 H-NMR spectroscopy and only one by X-ray techniques. The structures of complexes were anticipated from the spectroscopic studies. Key Word: aniline, 1,3-benzothiazole, ligand, metal complexes, Schiff bases. Introduction: Benzothiazole is an organic heterobicyclic compound consists of a five-membered 1,3-thiazole ring comprising nitrogen and Sulphur atoms fused to a benzene ring. The benzothiazole ring is weak base, has various positions and is directed according to Sulphur, which is in No.1 position. The nine atoms of the bicycle together with the attached substituents are coplanar. One of the most important parts of scaffolds for the preparation of dyes that are used in the identification of lanthanide metal ions in aqueous media is benzothiazole ring 1 . Benzothiazole derivatives are industrially identified as antioxidant 2 , corrosion inhibitors and surface-active chelating agents for mineral processing 3 . Benzothiazole nucleus are valuable for their biological activities and found as anticancer, antimicrobial, antidiabetic, anti-inflammatory, antiviral, antileishmanial, and antiviral 4 . It is known as a plant metabolite, a xenobiotic and an environmental contaminant 5 . Benzothiazole is found as well rarely in marine and terrestrial natural compounds with significant pharmacological properties, as they act as aroma components in tea leaves and cranberries that are formed from Aspergillus fungi 6 . Copper complexes of 2aminobenzothiazole have been used as a versatile material for different derivatives 7 . It has also been found that the selective functionalization of benzothiazole with diverse substituents grows their range of action in many fields. Various recent synthetic processes have been developed for the preparation of benzothiazole compounds associated with green chemistry 8 . Benzothiazole derivatives linked with some heterocyclic ring such as thiadiazole have a high biological activity against some bacterial 9 . Benzothiazole Schiff s base is a nitrogen analogue of an aldehyde/ketone in which the carbonyl group has been switched by an imine or azomethine group. They have been used widely as antioxidant, antimicrobial, antifungal, antiinflammatory, anticancer and cytotoxic activity 10-13 . In this study we planned to prepare a new Open Access Baghdad Science Journal P-ISSN: 2078-8665 Published Online First: September 2021 E-ISSN: 2411-7986 379 substituted Schiff s base ligands derived from benzothiazole, and combined them with some metal ions [Al(III), Ni(II), K(I)] and characterizing them by spectral methods. We expect this work would grow to design better molecules, which can improve biological specificity, and will be further investigated in the future. Materials and Methods: All chemicals are provided by B.D.H and Sigma-Aldrich and used without further purification. With the Stuart Melting point apparatus, melting points were confirmed and were uncorrected. The key functional groups were identified via Fouriertransform infrared spectroscopy analysis, and recorded in the scanning range of 400−4000 cm -1 at room temperature on Shimadzu (FT-IR-8300S) spectrophotometer by KBr disc in Ibn Sina State Company (ISSC). 1 H-NMR spectra have been used to confirm the placement of protons stating signals as δ-values in ppm for the achieved compounds, and the values are recorded on a BRUKER (400 MHz) instrument operating at 300 MHZ with tetra methyl silane as an internal standard in CDCl3 and DMSOd6 as solvent, measurements were made at the Chemistry Department, Al Baath University-Syria. One of the resulted complexes was experienced for the first time with X-ray device type Shimadzu LabX-XRD-6000 X-RAY Diffractometer. Results and Discussion: 1. Synthesis and Characterization of 4-chloro-6nitro-2-amino-1,3-benzothiazole 1. The titled compound 1 presented in this work was combined from 2-chloro,4-nitroaniline and ammonium thiocyanate via bromination, Fig.1, according to published procedures 14 . The structural examination of the complex was based on its melting point and spectral (FT-IR and 1 HNMR) records, and the conformation of 4-chloro-6-nitro-2-amino-1,3benzothiazole 1 has been confirmed. The FT-IR spectrum showed significant two bands at 3390 and 3460 cm -1 that could be attributed to asymmetric and symmetric stretching vibrations of the NH2 group 15 . Also, there is band observed at around 1620 cm -1 owing to cyclic (C=N) stretching. Bands obtained at 3062-3053 are due to the aromatic (CH) stretching and bending. Besides, the appearance of two bands at 1504 and 1539 cm -1 belongs to aromatic (C=C) stretching. A Band noticed at about 1157 cm -1 is owing to (CS-C) stretching. A Band observed at 1431 cm -1 is due to the (C-N) stretching vibration 16 . (C-Cl) stretching is observed at 1080 cm -1 . 1 H-NMR spectrum of compound 1 showed the following characteristic chemical changes (DMSO-d6, ppm). Five aromatic protons were located at δ 7.36-7.76, which was approximately in agreement with the literature review 17 . Singlet signal of amino protons NH2 has been located at δ 4.50. Besides, a particular peak at δ 2.5 that was owing to DMSO.