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ISSN: 1948-5948
Journal of Microbial & Biochemical Technology
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Evaluation and Rearrangement of Novel Supramolecular 7-(2,3-dihydro- 1,3-benzothiazol-2-yl)quinolin-8-ol Complexes and their Biological Effect

El-Sonbati AZ1*, Diab MA1, El-Bindary AA1, Nozha SG1 and Nassar MI2

1Chemistry Department, Faculty of Science, Damietta University, Egypt

2Entomology Department, Faculty of Science, Cairo University, Giza, Egypt

*Corresponding Author:
El-Sonbati AZ
Chemistry Department, Faculty of Science
Damietta University, Damietta 34517, Egypt
Tel: +201060081581
Fax: +20 572403867
E-mail: [email protected]

Received date: July 28, 2014; Accepted date: August 12, 2014; Published date: August 19, 2014

Citation: El-Sonbati AZ, Diab MA, El-Bindary AA, Nozha SG, Nassar MI (2014) Evaluation and Rearrangement of Novel Supramolecular 7-(2,3-Dihydro-1,3-benzothiazol-2-yl)quinolin-8-ol Complexes and their Biological Effect. J Microbial Biochem Technol S4:001. doi: 10.4172/1948-5948.S4-001

Copyright: © 2014 El-Sonbati AZ, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Novel tridentate Schiff base ligand derived from the 2-aminobenzenthiol and 8-hydroxy-7-quinolinecarboxaldehyde (oxine) and its metal complexes have been prepared. The reaction of a solution of 7-(2,3-dihydro-1,3-benzothiazol- 2-yl)quinolin-8-ol (H2L) with di-and tetravalent transition metals have been studied chemically and biologically. The optimized bond lengths, bond angles and calculated the quantum chemical parameters for the ligand (H2L) [the cyclic form (A) and the Schiff base form (B)] were investigated. The complexes have been characterized by elemental analyses, IR spectra, thermal analyses and 1H-NMR spectroscopy. The spectral studies of the isolated complexes showed that the rearrangement of the benzothiazoline to the Schiff base had occurred. The data suggest that the Co(II) and Ni(II) complexes are binuclear and octahedral dimmers, while the rest are monomeric with square planar/ tetrahedral geometries. The ESR spectrum of the Cu(II) complex show axial type symmetry with gll > g┴>2.0023, indicating dx2-y2 ground state with significant covalent bond character. Five doses were topically applied against the newly emerged adults of the red palm weevil Rhynchophorus ferrugineus. Percent of adult mortality was 98.2%, which occurred after adult treatment with the highest dose (2.2 mg/ml) of Ni(II) complex. The lowest mortality was 5.6% which obtained due to effect of oxine after adult was treated with dose of 0.25 mg/ml. Five doses were topically applied against the newly emerged adults of the stored product weevil, Sitophilus granaria. While the lower mortality, 5% caused due to effect of least dose, 0.15 mg/ml of the compound oxine.

Keywords

2-Aminobenzenethiol; Biological effect; Quantum chemical parameters; Schiff base; Supramolecular structure; ESR; Molecular structures

Introduction

Schiff bases continue to occupy an important position as ligands in metal coordination chemistry even after almost a century since their discovery. Sulphur and nitrogen have long been used to increase the biological activity of organic moiety [1,2] and quinoline compounds have also found applications in medicinal chemistry [3]. However, no systematic study has been performed on such complexes.

It has been well established that condensation of 2-aminobenzenthiol with carbonyl compound does not normally lead to the isolation of the corresponding Schiff base, but thiazoline or benzothiazoline is obtained [4]. However, metal ions may act as a template and favor the formation of Schiff base.

Schiff bases are some of the most widely used organic compounds. They are used as pigments and dyes, catalysts, intermediates in organic synthesis and as polymer stabilizers [5,6]. Schiff bases have also been shown to exhibit a broad range of biological activities, including antifungal, antibacterial, antimalarial, antiproliferative, antiinflammatory, antiviral, and antipyretic properties [2,5]. It has been suggested that azomethine linkage (C=N) might be responsible for the biological activities of Schiff bases.

Studies of the complexing ability and analytical applications of heterocyclic azomethines derived from thiazoline compounds are very important [7-9]. It is well known that some drugs have increased activity when administered as metal complexes and a number of metal chelates inhibit tumor growth [10].

The number and diversity of nitrogen and oxygen chelating agents used to prepare new coordination and organometallic compounds has increased rapidly recently [11-13].

The behavior of the C=N bond is strongly dependent on the structure of the amine moiety, which in turn controls the efficiency of the conjugation and may incorporate structural elements able to modulate the steric crowding around the coordination [14].

The Red palm weevil Rhynchophorus ferruginous oliv. Coleoptera- Curculionidae is a devastating insect pest of date palm in the Arabian Gulf region [15]. It was reported on date palm, for the first time, from the United Arab Emirates in the mid-1980s then its reported distributed expanded its range westwards till reached the entire Gulf area [16,17].

A survey of literature reveals that no work has been carried out on the synthesis of 7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin-8-ol (H2L) ligand and its Cu(II), Co(II), Ni(II), Zn(II), Cd(II) and Zr(IV) complexes. The present work aims at highlighting the synthesis and characterization of novel supramolecular chemistry of complexes with H2L and studies their spectral properties. Molecular and electronic structures of the investigated ligand (H2L) have been discussed. The effect of the ligand and its complexes on the red palm weevil Rhynchophorus ferrugineus (Oliv.) and on the stored food weevils Sitophilus granary were studied.

Experimental

All chemicals were reagent grade (BDH Chemical Ltd.) and were used as supplied. The experimental techniques were as described previously [18-20]. 8-Hydroxy-7-quinoline carboxaldehyde (oxine) was previously prepared by El-Sonbati [21].

Preparation of 7-(2,3-dihydro-1,3-benzothiazol-2-yl) quinolin-8-ol (H2L)

A solution of 8-hydroxy-7-quinolinecarboxaldehyde (oxine) (0.06 mol) in DMF:Ethanol (15 ml) was treated with 2-aminobenzenethiol (0.06 mol) in ethanol (15 ml) (Scheme 1). The reaction was refluxed for 3.5 hrs. The pale brown precipitate formed was purified by recrystallization from hot ethanol. Yield 55%, (Found: C 68.4, H 4.1, N 9.6, S 11.1. Calculated for C16H12N2SO: C 68.6, H 4.3, N 10.0, S 11.4%). The purity was checked by elemental analyses, IR and 1H NMR spectroscopies.

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Scheme 1: Synthetic routes for the preparation of tautomeric forms of 7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin-8-ol (H2L).

Oxidation of 7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin- 8-ol (H2L)

Oxidation was carried out by bubbling air for four hrs in benzene solution of 7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin-8-ol (H2L). The oxidation product was benzothiazol. Found C, 68.87; H, 3.50; N, 9.77; S, 11.51%. Calculated for C16H10N2SO; C, 69.07; H, 3.61; N, 10.07; S, 11.37%. No band was observed in the IR spectrum above 3130 cm-1, indicating the absence of NH.

General method for preparing complexes

Reaction of 7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin-8- ol with metal ions: The complexes were prepared by methods similar to the reported in earlier publications [14,22-24] according to the following procedures:

1) Mono(7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin-8-ol)Cd and Zn [(I) and (II)] (Scheme 2)

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Scheme 2: The structure of complexes (I,II).

M(OAc)2.2H2O (0.5 mmol) in ethanol (15 ml), was added to ligand (H2L) (0.5 mmol) in DMF:Ethanol (1:2 v/v) (25 ml) and the resulting mixture was stirred at room temperature for 3 h. The formed precipitate was filtered off, washed with ethanol and ether, respectively, and dried in vacuum over P2O5.

2) Chloro(7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin-8-ol) zirconium [(III)] (Scheme 3)

microbial-biochemical-technology-structure

Scheme 3: The structure of complex (III).

Solution of ZrCl4 (1 mmol) and the ligand (H2L) (1 mmol) in DMF:Methanol (1:2 v/v) (45 ml) was mixed and stirred for 4 h. The obtained precipitate was filtered, washed with MeOH and dried in vacuum over P2O5.

3) Bi(7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin-8-ol)Cu/Ni/ Co [IV-VI)] (Scheme 4)

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Scheme 4: The structure of complexes (IV-VI).

The reaction of the ligand with MX2.nH2O was carried out by stirring a mixture of DMF:Ethanol solutions of the ligand and the MX2. nH2O for four hours at room temperature. The obtained product was filtered off and washed with ethanol, then dried in vacuum over P2O5.

Insect rearing

Colonies of Rhynchophorus ferrugineus are serious pest of coconut causing damage and often killing the plam in its prime of life. The hatched grubs burrow into the trunk and feed on tissue of the stem. In the present study, the adult, larvae and pupal stages were collected from large cavities of infested date trees from Nagran region in KSA particularly for those received no chemical insecticides. Laboratory culture of Rhynchophorus ferrugineus was established under 26 ± 2ºC and 60 ± 5% R.H. Insect was feed on the freshly shred sugar-cane stem tissues. The colony was checked daily for the deposited eggs between sugar-cane tissues and changed when becoming moldy. The sugar-cane pieces on which the adults laid their eggs were transferred to small plastic cages until hatching. The newly hatched larvae were cultured on a medium consisting of sugar-cane shreds.

Bioassay of inorganic chemical compounds

Five concentrations of oxine, H2L (Scheme 1B) and complexes (IV,V) (Scheme 4) were prepared by using Dimethylsulfoxide (DMSO) as solvent: 2.2, 1.12, 0.56, 0.25, and 0.125 mg/ml. Four replicates contained (5 last larval instars each) for each concentration. The adults of Rhynchophorus ferrugineus were topically received 3μ for each concentration using Transferrepette-micropipette for the treatments. Also the control experiments were conducted by four replicates and the adults were topically treated with Dimethylsulfoxide (DMSO) as solvent only. All treated and control insects were kept at 26 ± 2ºC and 60 ± 5% R.H.

Criteria studied and statistical analysis

The adult mortalities were observed during 24 hrs. Data obtained were analyzed using t-distribution and refined by Bessel correction [17].

Measurements

Microanalysis of all samples was carried out at King Khalid University Analytical Center, Saudi Arabia, using a Perkin-Elmer 2400 Series II Analyzer. The metal content in these complexes were estimated by standard methods [25]. IR spectra were recorded on a Perkin-Elmer 1340 spectrophotometer. Ultraviolet-Visible (UV-Vis) spectra of the compounds were recorded in Nuzol solution using a Unicom SP 8800 spectrophotometer. Magnetic measurements were carried out at room temperature using Gouy’s method, employing Hg[Co(SCN)4] for calibration purposes, and were corrected for diamagnetism by using Pascal’s constant. Magnetic moments were calculated using the equation: μeff.=2.84 [TχMcoor]1/2. 1H NMR spectra obtained on JEOL FX900 Q Fourier transform spectrometer with d6-DMSO as solvent and TMS as internal reference. Thermogravimetric analysis (TGA) measurements were made using a DuPont 950 thermobalance. Ten milligram samples were heated at 10°/min in a dynamic nitrogen atmosphere (70 ml/ min); the sample holder was boat-shaped, 10×5×2.5 mm deep; the temperature measuring thermocouple was placed within 1 mm of the holder. The halogen content was determined by combustion of the solid complex (30 mg) in an oxygen flask in the presence of a KOHH 2O2 mixture. The halide content was then determined by titration with a standard Hg(NO3)2 solution using diphenyl carbazone as an indicator. The molecular structures of the investigated compounds were optimized by HF method with 3-21G basis set. The molecules were built with the Perkin Elmer ChemBio Draw and optimized using Perkin Elmer ChemBio3D software [26]. Quantum chemical parameters such as the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO) and HOMO–LUMO energy gap (ΔE) for the investigated molecules were calculated.

Results and Discussion

The ligand 7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin-8- ol (H2L) is shown in Scheme 1. The optimized structures of cyclic form (A) and Schiff base form (B) are given in Figure 1. The selected geometrical structures of the investigated ligand (H2L) [cyclic form (A) and the Schiff base form (B)] were calculated by optimizing their bond lengths and bond angles and listed in Tables 1 and 2. From Table 3 the computed net charges on active centers, it is found that the most negative charges in Schiff base form (B) and cyclic form (A) are N3 and S20.

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Figure 1: Geometry optimized structures of H2L.

Bond lengths (Å)
Cyclic form (A) Schiff base form (B)
C19-H32 1.113 S20-H32 1.346
N18-H31 1.047 N18-H31 1.104
C16-H30 1.102 C17-H30 1.103
C15-H29 1.103 C16-H29 1.103
C14-H28 1.103 C15-H28 1.106
C13-H27 1.104 C12-H27 1.093
O11-H26 0.97 O11-H26 0.97
C8-H25 1.103 C8-H25 1.104
C7-H24 1.103 C7-H24 1.103
C6-H23 1.102 C6-H23 1.102
C5-H22 1.102 C5-H22 1.102
C4-H21 1.103 C4-H21 1.103
C4-N3 1.264 C14-C19 1.354
C2-N3 1.269 C18-C19 1.347
C1-C2 1.347 C17-C18 1.34
C10-C2 1.35 C16-C17 1.338
C9-C10 1.351 C15-C16 1.341
C8-C9 1.345 C14-C15 1.354
C7-C8 1.34 C4-N3 1.264
C1-C7 1.342 C2-N3 1.269
C6-C1 1.344 C1-C2 1.347
C5-C6 1.34 C10-C2 1.351
C4-C5 1.34 C9-C10 1.353
C12-C17 1.346 C8-C9 1.348
N18-C17 1.268 C7-C8 1.34
C16-C17 1.34 C1-C7 1.341
C15-C16 1.343 C6-C1 1.344
C14-C15 1.344 C5-C6 1.34
C13-C14 1.344 C4-C5 1.34
C12-C13 1.342 C19-S20 1.832
S20-C12 1.474 C14-N13 1.273
C19-S20 1.792 C12-N13 1.264
N18-C19 1.473 C9-C12 1.353
C19-C9 1.514    

Table 1: Bond lengths for the cyclic form and Schiff base form.

Cyclic form (A) Schiff base form (B)
Bond angles (o) Dihedral angles (o) Bond angles (o) Dihedral angles (o)
H29-C15-C16 119.758 C17-N18-C19-C9 118.396 H32-S20-C19 110.219 N13-C14-C19-C18 179.992
H29-C15-C14 119.848 N18-C19-C9-C8 133.06 H31-C18-C19 121.254 N13-C14-C15-C16 179.965
C16- C15-C14 120.394 C2-C10-O11-H26 101.723 H31-C18-C17 117.264 C14-C19-S20-H32 72.327
H28-C14-C15 119.635 C13-C12-C17-N18 179.608 C19-C18-C17 121.482 C15-C14-N13-C12 179.713
H28-C14-C13 119.694 C19-N18-C17-C16 178.568 H30-C17-C18 120.731 C9-C12-N13-C14 179.955
C15-C14-C13 120.67     H30-C17-C16 120.43 C8-C9-C12-N13 179.933
H30-C16-C17 120.688     C18-C17-C16 118.839    
H30-C16-C15 120.399     H29-C16-C17 120.014    
C17-C16-C15 118.912     H29-C16-C15 120.512    
C12-C17-N18 115.06     C17-C16-C15 119.474    
C12-C17-C16 120.83     C14-C19-C18 120.752    
N18-C17-C16 124.11     C14-C19-S20 127.244    
H27-C13-C14 119.24     C18-C19-S20 112.005    
H27-C13-C12 121.781     H28-C15-C16 116.104    
C14-C13-C12 118.979     H28-C15-C14 120.762    
C17-C12-C13 120.214     C16- C15-C14 123.134    
C17-C12-S20 114.008     C19-C14-C15 116.32    
C13-C12-S20 125.777     C19-C14-N13 132.385    
C12-S20-C19 97.185     C15-C14-N13 111.295    
H31-N18-C17 122.202     C14-N13-C12 131.33    
H31-N18-C19 123.005     H27-C12-N13 117.554    
C17-N18-C19 114.764     H27-C12-C9 116.044    
H32-C19-S20 109.889     N13-C12-C9 126.402    
H32-C19-N18 107.338     H26-O11-C10 109.471    
H32-C19-C9 113.911            
S20-C19-N18 98.942            
S20-C19-C9 113.811            
N18-C19-C9 111.791            
H26-O11-C10 109.399            

Table 2: Bond angles and dihydral angles for the cyclic form and Schiff base form.

 
Atom Charges
  Cyclic form (A) Schiff base form (B)
N3 -0.62 -0.62
N18 -0.8691 -
O11 -0.5325 -0.5325
N 13 - -0.629
S20 -0.3315 -0.2815

Table 3: Net charges on active centers of the cyclic form and Schiff base form.

Both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are the main orbital takes part in chemical stability. The HOMO represents the ability to donate an electron, LUMO as an electron acceptor represents the ability to obtain an electron. The HOMO and LUMO for cyclic form (A) and Schiff base form (B) are shown in Figure 2. The calculated quantum chemical parameters are given in Table 4. Additional parameters such as ΔE, absolute electronegativities, χ, chemical potentials, Pi, absolute hardness, η, absolute softness, σ, global electrophilicity, ω, global softness, S, and additional electronic charge, ΔNmax, have been calculated according to the following equations [27-29]:

microbial-biochemical-technology-molecular

Figure 2: HOMO and LUMO molecular orbital of H2L [cyclic form (A) and Schiff base form (B)].

Compound EHOMO (a.u.) ELUMO (a.u.) ΔE (a.u.) χ (a.u.) η (a.u.) σ (a.u.)-1 Pi (a.u.) S (a.u.)-1 ω (a.u.) ∆Nmax
Cyclic form (A) -0.211 -0.092 0.119 0.151 0.059 16.731 -0.151 8.365 0.192 2.532
Schiff base form (B) -0.231 -0.148 0.083 0.189 0.042 24.041 -0.189 12.021 0.095 4.558

Table 4: The calculated quantum chemical parameters for H2L.

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The HOMO–LUMO energy gap, ΔE, which is an important stability index, is applied to develop theoretical models for explaining the structure and conformation barriers in many molecular systems [26-29]. The value of ΔE for Schiff base form (B) and cyclic form (A) was found 0.038 and 0.119 a.u., respectively, so the Schiff base form (B) more stable and highly reactive than cyclic form (A).

Structural of the ligand and complexes

The reaction of oxine with 2-aminobenzenethiol afforded a product whose IR spectrum shows a band at 3300 cm-1 (assigned to νNH) but no absorption at 2620 cm-1 ν(SH). Thus the Schiff base (7-(2,3-dihydro- 1,3-benzothiazol-2-yl)quinolin-8-ol)(H2L) (B) is ruled out. The sulphur atom of the thiol group tends to form another S–C bond.

Cyclization takes place with the resulting formation of a thiazoline: Complexes of the Schiff base with Cu(II), Co(II), Ni(II), Zn(II), Cd(II) and Zr(IV) have been isolated and characterized. The complexes formed are characterized with respect to its composition by elemental analysis and showed agreement with theoretical data. The elemental analysis data listed in Table 5.

Complexa Found (Calc.)%
C H N S
[Cd(L)(OH2)] 7 7.2 3 47.1
(I) -7.8 -6.9 -2.9 -47
[Zn(L)(OH2)] 9.1 8.1 3.2 53.1
(II) -8.9 -7.8 -3.3 -53.1
[Zr(L)Cl2] 6.8 5.8 2.6 -41.7
(III) -7 -6.1 -2.7 41.8
[Cu(L)(OH2)] 8.7 8.3 3.5 53.7
(IV) -9 -7.9 -3.4 -53.8
[Ni(L)(OH2)2] 7.8 6.9 3.6 48.8
(V) -8.1 -7.1 -3.6 -48.9
[Co(L)(OH2)2] 8.4 7.4 3.5 48.9
(VI) -8.1 -7.1 -3.6 -48.9

Table 5: Microanalytical data of the complexes. aMicroanalytical data as well as metal estimations are in good agreement with the stoichiometry of the proposed complexes [for molecule structures see Schemes 2-4].

The analytical data of the novel complexes prepared agree very well with the compositions [M(L)(OH2)n] (where M=Cd(II), Zn(II), Cu(II) at n=1; M=Co(II), Ni(II) at n=2) and [Zr(L)(Cl)2]. This can be represented by the following equations:

H2L + M(OAc)2.2H2O → [M(L)(OH2)] + 2CH3COOH {M=Cd(II) (I), Zn(II) (II)}

H2L + ZrCl4.H2O → [Zr(L)Cl2] + 2HCl (III)

H2L + Cu(OAc)2.2H2O → [Cu(L)(OH2)] + 2CH3COOH (IV)

H2L + MCl2.2H2O → [M(L)(OH2)2] + 2HCl {M=Ni(II) (V), Co(II) (VI)}

The effervescence of HCl and evolution of ethanolic acid during complex formation was assessed quantitatively by pH titration, a spot test, and by its characteristic odor.

Spectral and thermal properties of the ligand and complexes

The IR spectrum of H2L exhibits the absence of the bands at ν(SH) (2500–2600 cm-1) and ν(C=N)(1600–1660 cm-1) and the appearance of a broad and strong band which is observed in the region, 3360–3060 cm-1. This band is attributed to ν(OH) and/or ν(NH). Also, an intense band at 828 cm-1 due to the C–S–C linkage and not Schiff base form [7]. Two bands of medium intensity at 1580 and 1505 cm-1 are attributed to thiazoline ring vibration (Table 6).

Compound ν(C=N) ν(Αροματιχ ϖιβ.)) ν(χ-O) ν(M-Σ) ν(M-O) ν(M-N)
H2Lb c - 1272 - - -
(I) 1585 15081464 1274 360 510 450
(II) 1583 15081456 1270 365 515 455
(III) 1577 15091458 1273 363 518 460
(IV) 1575 15101460 1275 1370 520 410
(V) 1572 15121462 1340 1375 525 465
(VI) 1574 15121459 1344 1377 528 463

Table 6: Characteristic IR bands (cm-1) of the Schiff base and its metal complexesa. aThe serial number corresponds to that used in Table 5. bν( C–S–C) link. vib.= 828 cm-1, cCyclic form.

The 1H NMR data of uncomplex and diamagnetic complexes [18-20] have been listed in Table 7. The ΔOH (~12.02 ppm) and ΔNH (4.18 ppm) proton signals are present in the ligand. These signals disappear upon addition of D2O.

Compound -OH -C6H4
Scheme 1 A 12.02 6.53 – 7.24
(I) 12.12 6.33 – 7.44
(II) 11.92 6.30 – 7.46

Table 7: 1H NMR data of ligand* and its complexesa. aThe serial number corresponds to that used in Table 5. *No signals even weak, of the open ring tautomer (Scheme 1B) can be detected. Note: NH proton signal is observed at 4.18 ppm (Scheme 1A).

The phenyl protons are observed in the region Δ6.53–7.24 ppm and proton due to –CH shows a marked downfield shift on complex formation.

By heating the cyclic form (A) (Scheme 1) at ≥210°C an intenseorange yellow crystalline compound is formed, that was supposed to be due to the transformation in 7-(2,3-dihydro-1,3-benzothiazol-2-yl) quinolin-8-ol (D) (Scheme 1) by the loss of a hydrogen molecule from it.

Four significant changes in the IR spectrum of the complexes were observed: (i) the band due to ν(NH) mode is absent; (ii) three strong bands at 1504, 1436 and 1397 cm-1, and two bands of medium intensity at 1387 and 1530 cm-1 (benzene and thiazol ring vibrations) [30,31]; (iii) a strong band at 828 cm-1 assignable to the C–S–C linkage; (iv) the νC=N band is observed in the oxidized product at 1595 cm-1. In the electronic spectrum of the ligand, two bands are observed at ~ 39850 and 28580 cm-1. They are fully support the typical spectrum of cyclic from of benzothiszoline and may be attributed to σ-σ* and π-π* benzenoid transitions. The position of this band remains unchanged with pH. On the other hand, in the corresponding complexes, an additional band around ~ 24400 cm-1 is also observed. This new band may be assigned to either n-π* transitions [7,32] or π-π* transition [7,32] of the double of the azomethine group.

Rearrangement of the ligand in the presence of metal ions

The reaction of 7-(2,3-dihydro-1,3-benzothiazol-2-yl)quinolin-8-ol (H2L) with the metal ions investigated in this work are invariably accompanied with the opening of the thiazoline ring and the rearrangement of the ligand to the Schiff base 7-(2,3-dihydro-1,3- benzothiazol-2-yl)quinolin-8-ol (B) (Scheme 1). Thus the complexes obtained were of the form [M(L)(OH2)n] (where M=Cd, Zn, Cu at n=1; M=Co, Ni at n=2) and [Zr(L)(Cl)2]. L is stands for the conjugate anion of the Schiff base (H2L) (B).

Reaction of a solution of (A) in DMF:Ethanol (2:1) with M(CH3COO)2 (M=Zn or Cd), ZrCl4 and MCl2 (M=Cu, Co, Ni) with stirring at room temperature yields a dark crystalline complexes (Schemes 2-4). The IR spectra of this complexes show absorptions at ~1590, 1510 and 1460 cm-1 (Table 6) which can be ascribed to imine and aromatic ring vibrations. On the other hand, no C–S–C linkage band (characteristic of the closed ring structure) appears. These results indicate that the metal complexes of the tautomeric Schiff base were obtained.

Important IR bands of the ligand and the complexes have been compared in order to find out bonding sites of the ligand. A broad and strong band at 3360–3060 cm-1, attributable to νOH and/or νNH disappears altogether in the complexes, indicating bond formation between metal and phenolic oxygen atom after deprotonation. This is also supported by the disappearance of a series of bands in the vicinity of 2960 cm-1 due to intramolecular hydrogen bonding (Scheme 1) [11,13,33]. However, new bands appear at ~1595, ~3430, and 795–815 cm-1, possibly due to νC=N, ν(OH) and Δ(H2O) of the coordinated azomethine and water molecules in the complexes. The disappearance, in the spectra of all complexes of a peak due to ν(S–H), indicates the deprotonation of thiolic proton on complex formation [34]. The participation of phenolic oxygen and thiolic sulphur in coordination to the metal ion is further supported by an upward shift [34] in ν(C–O) (phenolic) to extent of 20–30 cm-1 and a downward shift [17-19,34] in ν(C–S) in all the complexes. The major shift of ν(C–O) (phenolic) to higher energy by ~30 cm-1 in the case of Ni(II) and Co(II) complexes certainly indicates the presence of phenoxo bridge [35]. The new medium to strong absorption bands at 510–530, 440–470, 360–385 and ~300 cm-1 are assigned to ν(M-O), ν(M-N), ν(M-S) and ν(Zr-Cl) modes [36], respectively.

Taking into consideration the previous reports [37] we have attributed a band of medium intensity appearing around 290 cm-1 to ν(Zr-Cl) vibration. The appearing of only one medium intensity band in the above region makes us to corroborate that chlorine atoms in the complex occupy the Trans position. The ligand acts as tridentate and dibasic coordinating to the metal atom via the nitrogen atom of the azomethine group, sulphur and oxygen atoms of thiazoline as evidenced by their spectral data.

Magnetic moments

The magnetic moments of the complexes (Table 8) were measured at room temperature. On the basis of the magnetic and spectral evidence the Cu(II) complex shows the presence of one unpaired electron (1.92 B.M.). The relatively higher 1.92 B.M. value seems to suggest the relatively high tetrahedral distortion from square planar geometry.

Complex Band Position (cm-1) Assignment Dq (cm-1) B (cm-1) β υ2/υ1 LFSF (KJmol-1) µeff.b (B.M) Geometry
IV - - 1960.6 - - - 140.71 1.92 Tetrahedral
V 9520 3A2g3T2g(F) 952.2 966 0.92 1.72 136.7 3.1 Octahedral
  16390 3A2g3T1g(F)              
  26665 3A2g3T1g(P)              
VI 8695 4T1g4T2g(F) 869.5 819.3 0.84 1.97 83.2 4.46 Octahedral
  17100 4T1g4A2g(F)              
  2127 4T1g4T1g(P)              

Table 8: Electronic spectral bands, assignments ligand field parameters for complexesa. aThe serial number corresponds to that used in Table 5. bPer metal ion and measured at room temperature

Electron spin resonance

The ESR spectrum of solid Cu(II) complex at room temperature is characteristic of a monomer, d9, configuration. The ESR spectrum shows gll>g->2.0023, suggesting a dx2-y2 ground state, which is characteristic of a tetrahedral geometry [24]. The g-values are related by the expression [24], G=(gll-2)/(g--2). If G<4.0, significant exchange is present. Copper(II) complex shows G>4.0, indicating the presence of distribution from square planar towards tetrahedral configuration.

The g-values of Cu(II) complex with a 2B1g ground state (gll>g-) may be expressed [24,38] by:

image (9)

image (10)

where Kll and K- are the parallel and perpendicular components respectively of the orbital reduction factor (K). λo is the spin-orbit coupling constant for the free copper. ΔExy and ΔExz are the electron transition energies of 2B1g2B2g and 2B1g2Eg. From the above relations, the orbital reduction factor (Kll, K-, K), which are a measure of covalency [24,38] can be calculated. For an ionic environment, K=1 and for a covalent environment K<1, the lower the value of K, the greater is the covalent character. Kivelson and Neiman [39] noted that, for an ionic environment, gll >2.3 and for a covalent environment gll <2.3. Approximate metal-ligand σ-bond coefficients (α2), which is defined as the fraction of unpaired electron density located on the copper ion, for this complex was, calculated (Table 9). The empirical factor f =gll/All cm-1 value was considered as a diagnostic of stereochemistry. The gll/ All cm-1 value (Table 9) shows that the complex (IV) has a tetrahedral geometry, and this is further confirmed by the Symons plot [40].

Complex gll g^ giso Allb A^ G f Kll K^ α2
IV 2.41 2.07 2.19 168 25 5.7 143 0.64 0.53 0.98

Table 9: ESR parameter of Cu(II) complexa. aThe serial number corresponds to that used in Table 5.

The electronic spectrum of Co(II) complex showed three absorption bands which may be attributed to the three spin-allowed transitions 8695 cm-1 [4T1g(F) → 4T2g(F)(υ1)], 17100 cm-1 [4T1g(F) → 4A2g(F)(υ2)] and 2127 cm-1 [4T1g(F) → 4T1g3)]. Ni(II) complex exhibited three intense bands which are assigned to 9520 cm-1 [3A2g(F) → 3T2g1)], 16390 cm-1 [3A2g(F) → 3T1g(F)(υ2)] and 26665 cm-1[3A2g(F) → 3T1g(P)(υ3)] transition [41].

The ligand field parameters like ligand field splitting energy (10Dq), Racah inter-electronic repulsion parameter (B), covalent factor (β) and ligand field stabilization energy (LFSE) has been calculated [42] and values compiled in Table 8. The calculated Dq and CFSE values of all complexes except Zr(III), Zn(II) and Cd(II) complexes, as well as the B and β-values have been calculated only for Co(II) and Ni(II) complexes following standard equation [42,43]. The B-values are lower than the free ion values, thereby indicating the orbital overlap and delocalization of d-orbitals. The β-values obtained are less than unity commensurate considerable amount of covalent character of the metal-ligand bands [42,43].

Stereochemistry of zirconyl complex

All these observation were taken together with the wide range of coordination numbers ranging from four to eight, envisages that in the complex (III) exhibit coordination number five.

However, it is known from the available data that the complex (III) belonging to d0, d8 and d10 systems have trigonal bipyramidal configuration and only a few have square pyramidal configuration. In view of this finding for the zirconium complex it may be suggested that this complex may exist in trigonal bipyramidal structure.

Thermal analysis

The temperature range for the dehydration process shows a strong relationship with the binding mode of the water molecules of the respective metal complexes. The elimination of water occurs in two steps. The first step can be attributed to the release of the hydration water molecules in the temperature range 45-55°C [44]. In the second step, except for the complex (III), all complexes lose coordinated water at temperature 130-145°C [27,44]. The thermal decomposition of the anhydrous complexes starts in the temperature range 330-370°C and is complete at temperature 560-750°C. The final decomposition products are metal oxides [13,27]. As results of the dehydration and decomposition, the observed weight losses for all complexes are in good agreement with the calculated values.

Effect of compounds on the red palm weevil Rhynchophorus ferrugineus (Oliv.)

Listed data in Table 10 concluded that the different novel compounds have been produced lethal effect on the newly emerged adult of the red palm weevil Rhynchophorus ferrugineus. The highest mortality was 98.2%, which obtained due to effect of complex (V). Synchronize the lower percent mortality was 5.2% after treatment adult with the compound oxine with concentration 0.25 mg/ml. Meanwhile the higher dose (2.2 mg/ml) was produced 20.2, 66.2 and 76.3% of adult mortality after adult was treated with the novel compounds oxine, H2L and complex (IV) respectively. On the other hand the adult mortality was dose dependent, where the mortality percent was increased by increasing dose. In descending order of toxicity the novel compounds were (V), (IV) complexes, H2L, and oxine, respectively.

Doses mg/ml Mortality with different samples (%)
Oxine H2L (IV)* (V)*
2.25 20.2 66.2 76.3 98.2
1.12 15.1 45.3 64.2 77.4
0.56 8.7 37.8 42.3 53.1
0.25 5.6 18.2 17.6 22.7
0.125 - 8.4 8.5 12.6
Control - - - -

Table 10: Effect of some new biologically active chemical compounds on the red palm weevil Rhynchophorus ferrugineus (Oliv.). *The serial number corresponds to that used in Table 5.

The mode of action of these compounds was neurotoxin effect on the nerve cells of the red palm weevil, Rhynchophorus ferrugineus.

Comparative analysis for the red palm weevil Rhynchophorus ferrugineus (Oliv.) of the samples by using five doses are shown in Figure 3, it is observed that the complex (V) was more biologically active chemical than the other compounds. So it can be concluded that the some complexes exhibits higher biologically active chemical than the free ligand [45].

microbial-biochemical-technology-rhynchophorus

Figure 3: Comparative analysis for the red palm weevil Rhynchophorus ferrugineus (Oliv.) of the samples at different doses.

Effect of compounds on the stored food weevils Sitophilus granary

The recorded results in Table 11 revealed that different mortality percentage were occurred after adult treatment with other doses from the different novel compounds. Where the higher percent of adult mortality was 33.3, 75.0, 85.0, 74% after adult was treated with dose 1.6 of oxine, H2L, complex (IV) and complex (V), respectively. The lower adult mortalities were 5.0, 19.0, 42.0, 40.0 and 15% when adult of weevil was treated with the doses, 0.15 mg/ml by the same previous novel compounds, respectively. In order of toxicity effect in descending manner the chemical compounds were (IV) followed by H2L, complex (V) and finally oxine.

Doses mg/ml Mortality with different samples (%)
Oxine H2L (IV)* (V)*
1.5 33.3 75 85 74
1.1 25.6 62 79 62
0.6 18.2 54 73 54
0.3 11 31 61 27
0.15 5 19 40 15
Control - - - -

Table 11: Effect compounds on the stored food weevils Sitophilus granaria. *The serial number corresponds to that used in Table 5.

Many of the new chemical the mode of action of these compounds was neurotoxin effect on the nerve cells of the weevil, Sitophilus granaria.

Comparative analysis for the stored food weevils Sitophilus granaria of the samples by using five doses are shown in Figure 4, it is observed that the complex (IV) was more biologically active chemical than the other compounds.

microbial-biochemical-technology-Sitophilus

Figure 4: Comparative analysis for the stored food weevils Sitophilus granaria of the samples at different doses.

Conclusion

In this work, synthesis and characterization of 7-(2,3-dihydro- 1,3-benzothiazol-2-yl)quinolin-8-ol (H2L) and its Cu(II), Co(II), Ni(II), Zn(II), Cd(II) and Zr(IV) complexes have been isolated and characterized. The analytical and physicochemical analysis confirmed the composition and the structure of the newly obtained compounds. The results obtained can be summarized as follows:

i. Elemental analysis, IR and molar conductivity data are used to proof the stoichiometry and formulation of the complexes. All complexes are 1:1 (metal:ligand) stoichiometry and except (I)- (III), the H2O molecule(s) are coordinated to the metal ion. The data suggest that the Co(II) and Ni(II) complexes are binuclear and octahedral dimmers, while the rest are monomeric with square planar/tetrahedral geometries, based on the magnetic data, spectral (ESR and visible) and thermal studies.

ii. The molecular and electronic structures of the investigated ligand (H2L) [cyclic form (A) and the Schiff base form (B)] have been discussed.

iii. It was found that the Schiff base form (B) more reactive than cyclic form (A).

iv. The spectral studies of the isolated complexes showed that the rearrangement of the benzothiazoline to the Schiff base had occurred.

v. ESR calculations support the characterization of the structures of the complexes geometries.

vi. The lowest mortality was 5.6% which obtained due to effect of oxine after adult was treated with dose of 0.25 mg/ml.

The mode of action of these compounds was neurotoxin effect on the nerve cells of the red palm weevil, Rhynchophorus ferrugineus.

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