Metal Complexes Derived from Mixed Azo-linked Schiff-base Ligand with Dithiocarbamate Derivative : Formation , Spectral Characterization and Biological Study

The study includes preparation and characterisation of mixed azo-linked Schiff-base and DTCs ligands and their complexes. The starting material was isolated from the mixing of naphthyl amine diazonium salt with 2-aminophenolein a 1:1 mole ratio in water. In this work, the formation of azo-linked Schiff-base and DTCs ligands are reported. Ligand of the azo-linked Schiff-base was achieved by the reaction of starting material with 4-(dimethylamino)benzaldehyde) (HL1). The DTCs was isolated by the reaction of (C6H5)2NH with carbon disulphide in potassium hydroxide (L 2). The complexes were prepared by mixing the azo-linked Schiff-base ligand and DTCs ligand with the metal salts; CoII, NiII, ZnII and CdII in a 1:1:1 mole ratio. Ligands and complexes were characterised by analytical and spectroscopic analyses including; microanalysis, chloride content, thermal analysis, magnetic susceptibility for complexes, conductance, FTIR, UV-Vis and 1H-NMR spectroscopy. Physico-chemical techniques indicated complexes demonstrated four and six coordinate structures in the solid and solution state. Biological activity of the ligands and their metal complexes were screened for their antimicrobial activity against four bacterial species (Escherichia coli and Enterobacter Gram ve, (Bacillus stubtilis and Staphylococcus aureus Garam + ve.


INTRODUCTION
Azo compounds are an interesting materials that have shown a range of applications including; food technology, analytical chemistry, pharmaceutical application and dyeing or textile industry.Their role in coloring approach has been widely investigated and a range of compounds are fabricated 1 .The biological activity of azo-compounds allowed them to be used in the treatment of textile materials, also azo-compounds are well known for their medicinal importance and have shown a variety of applications as antitumor, antibacterial, antiseptics and antineoplastics 2 .Variety of ligands type Schiff-base and their metal complexes have been isolated, these compounds have very flexible and diverse structures, therefore their properties have been studied 3 .Dithiocarbamates (DTCs) are class of organic compounds that are capable to chelate to metal ions 4,5 .DTCs compounds have a significant role in coordination chemistry.This may due to the metal ion stabilization ability in many oxidation states, and permitting the metal ion to implement its desirable structure 6 .DTCs have shown a significant biological activity including their role as antibacterial, antitumor and antifungal agents.They have other potential applications in materials science and supramolecular chemistry 7,8 .In our research, we report here the preparation of mixed azo-linked Schiff-base and DTCs ligands and their metal(II) dithiocarbamate complexes.

ExpERIMENTAL
All chemicals in this work are commercially available and used as received.Solvents were distilled using appropriate protocol before use.

physical measurements
Elemental micro-analyses (carbon, hydrogen, nitrogen and sulphur) for ligand and its metal complexes were conducted on a Euro EA 3000.Electrothermal Stuart SMP40 apparatus was used to record melting points.FT-IR spectra were recorded as potassium bromide discs with a Shimadzu 8300s in the range 4000-400 cm -1 and as CsI discs in the range 400-200 cm -1 .UV-Vis spectra were obtained with 10-3 M solutions between 200-1100 nm in dimethylsulfoxide (DMSO) spectroscopic grade solvent at 25 °C using a Perkin-Elmer spectrophotometer Lambda.TGA was carried out using a STA PT-1000 Linseis.NMR spectra ( 1 H-NMR) were acquired in DMSOd 6 and CDCl 3 using a Brucker-400 MHz and a Brucker-300 with tetramethy lsilane (TMS).A Shimadzu (A.A) 680 G atomic absorption spectrophotometer was implemented to determine metal content in complexes.Conductivity measurements were performed using a Jenway 4071 digital conductivity meter with DMSO solutions at room temperature.Chloride was determined using potentiometer titration method on a (686-Titro processor-665 Dosimat-Metrohm Swiss).A magnetic susceptibility balance (Sherwood Scientific) was used to determine magnetic moments of complexes.

Synthesis preparation of the precursor
The compound was prepared according to the literature 9,10 1-naphthyl amine (1 g, 6.98 mmol) was dissolved in H 2 O(10 ml) and hydrochloric acid conc.(2.14 ml, 60.98 mmol) mixture with stirring, a clear solution was obtained.Temperature of (0-5) o C have been kept, then aqueous solution of sodium nitrite (0.48g, 6.98 mmol) dissolved in (5 ml) water, was dropwise slowlyadded, keeping the temperature below 5 o C, followed by mixture stirring for 1 h, using ice bath, then little amount of urea was added, the pH was adjusted to 6-7 using sodium acetate [solution (1)].H 2 NC 6 H 4 OH (0.762 g, 6.98 mmol) was dissolved in 12 ml KOH 10 mmol aqueous solution, cooled by ice bathto (0-5) o C solution (2).Gradually the last solution was mixed with cooling with (solution1), the mixture resulted was stir redat (0-5) o C continually for 2h, the precipitate resulted was then filtered using acidification, cold H 2 O used to wash several times after drying, a brown solid precipitate was obtained, Yield:1.35 g (73.7%), melting point132 o C.

Synthesis of complexes
A one pot approach reported in 14 was used to prepare the mixed ligand metal complexes.

preparation of [CoII(HL 1 )(L 2 )(H 2 O) 2 ]
To a mixture of HL1 (0.2 g, 0.506 mmol) dissolved in (10 ml) was added (0.0568 g, 1.0139 mmol) of potassium hydroxide dissolved in 10 ml ethanol.While, the solution was allowed to stirring, a mixture of CoII salt (0.120 g, 0.506 mmol) in 10 ml ethanol with (0.143 g, 0.506 mmol) of dithocarbamte ligand (L 2 ) was added to the above solution.The reaction mixture was kept stirring for two h, during which time agreen product was filtered off.Washed with absolute ethanol and recrystallized from ethanolto give the pure product.Yield: 0.187 g, 50.33%, (Dec.over320).

preparation of Ni(II), Zn(II) and Cd(II) complexes
An analogues method to that reported for the synthesis of CoII complex was implemented to prepare Ni(II), Zn(II), and Cd(II) mixed ligands complexes.Table 1 displays the physical properties of the complexes and their reactant amount.

Synthesis
The precursor was obtained using a standard azo dye approach.The reaction of naphthyl amine diazonium salt with 2-amino-phenole in a 1:1 mole ratio in water solvent gave the required compound, see Scheme 1. Two sorts of ligands were prepared; (i) Azo-linked Schiff-base ligand, which isolated by reaction of the precursor with (CH 3 ) 2 NC 6 H 4 CHO and (ii) DTCs ligand that obtained from the reaction of (C 6 H 5 ) 2 NH with carbon disulphide using KOH base (Schemes 2 and 3).The complexes were prepared by mixing the ligands with the metal salts in a 1:1:1 mole ratio (Scheme 4).A range of analytical and spectroscopic techniques were used to confirm the entity of compounds including; CHNS, FT-IR, UV-Vis, magnetic susceptibility and , which designated to u s (CS 2 ) and uas(CS 2 ) respectively 22 .

FTIR and NMR spectra for complexes
The FTIR spectra of complexes show bands at rang (1606-1639) cm -1 were assigned to of imine u(C=N) group, with alower frequency shift.This may be related to the engagement of the nitrogen atom of the iminic moiety in the coordination reaction [26][27][28] .The shift also may explained by delocalisation process of the d-10 (metal electron density) to the ligand (usystem) 29,30 .Spectra for complexes appeared bands at rang (1500-1510) cm -1 that attributed to (N-CS 2 ) 21 .This confirms that the u(N-C) double bond character may increase as a consequence of the moving of electrons to the metal centre as a result of coordination to the DTCs 22 .Band located in the range (1454-1469) cm -1 is assigned to u(N=N) azo.Finally, the spectra showed new bands in the range (609-673) and (416-489) cm -1 that attributed to u(M-N) and u(M-O), respectively.The appearance of these bands supported the involvement of the nitrogen of imin and oxygen phenolic atoms in the coordination of the ligand to the metal centre.These results are in accordance with that reported in literature 31,32 .Bands detected at (1043-1053) and (948-997) cm -1 are due to uas(CS 2 ) asymmetric and us(CS 2 ) symmetric mode of the DTCsmoiety, respectively.This is in agreement with an anisobidentate chelation mode of the ligand to the metal ion 22 .The anisobidentate mode of chelation of the ligand, may confirmed by thebands observed in the range (302-393) cm -1 which may attributed to u(M-S) bond 33 .The spectra of Co, see Fig. (3), and Zn complexes exhibited a broad band that assigned to u(OH) of the hydrated water molecule 16 .Table (3) includes the prominent FTIR bands of complexes.The 1 H-NMR spectrum of K 2 [Cd(L 1 )(L 2 )Cl 2 ], (Fig. 9), displays peak at d=10.9ppm related to the proton of the iminicmoiety (1H, s, N=C-H) 23 .The chemical shift at d= 2.2 ppm that equivalent to six protons assigned to the methyl groups (2CH 3 ,s, 6H ).

UV-Vis Spectral data and magnetic susceptibility of the complexes
The electronic spectra of the complexes exhibited peaks in the range 262-305 nm attributed to p→p* transition of the aromatic rings 34 .The spectra of complexes revealed peaks around 349 nm related to (n→p*) transition of azo moiety.The blue shift recorded may attribute to the energy   change of the conjugated chromophore (n→p*) and (p→p*electronic transitions, due to the chelation between metal ions and azo ligand 35 .The spectrum of CoII complex, see (Fig. 6), reveals peaks at 886 and 528 nm assignable to 4 T 1 g(F)→ 4 T 2 g(F) 4 T 1 g(F)→ 4 T 1 g(P) transitions respectively whichmay confirm a distorted octahedral structure for Co(II) complex [36][37][38][39] , this result was confirmed by the magnetic moment value µeff of 5.07 B.M for the Co(II)-complex 40 .The Ni(II) complex shows a peak at 617 nm attributed to 1 A 1 g(f)→ 1 A 2 g(F), revealing a distorted square planar arrangement about Ni atom.This result was confirmed by the diamagnetic moment behavior of the Ni(II)-complex 40 .The spectra of Zn(II) and Cd(II) compounds revealed peaks attributed to ligand field p→p* and M→L charge transfer 41 .These data along with other analytical results indicated that the Zn(II) and Cd(II) complexes adopt octahedral arrangement about metal centre 42 .
The six-coordinate number for the Zn(II) and Cd(II) compounds may be due to sort of ligands that surrounding metal centre and their steric and electronic interaction that occurred upon complex formation 43 .The electronic data of the complexes are tabulated in (Table 5).

Biological activity
The synthesized ligands and its complexes were screened for their biological activity against some bacterial strains (Escherichia coli and Enterobacter (G−) Bacillus stubtilis and Staphylococcus aureus (G+).The involvement of dimethylsulphoxide in the bacterial test was confirmed by individual tests that conducted with the DMSO alone that indicated no activity towards any bacterial species 46 .The measured size of inhibition zones against growth of different microorganisms are summarised in that displays the effect of the prepared compounds on bacterial species.From collected data, it is clear that, compared with the free ligands,the ligands (HL 1 and L 2 ) showed no antimicrobial activity against Escherichia coli and Enterobacter.Hence, formation of complexes enhances the antimicrobial activity.
Such increased activity of complexes may be related to the chelation theory 33 .Therefore, the chelation decreases the polarity of the metal atom that resulted in the partial sharing of its positive charge with donor group and possible p-electron delocalisation over the whole ring.Zinc and cadmium complexes showed almost the higher antibacterial activity, compared with other compounds.This due to to their molecular weight and their electronic configuration (d 10 system), compared with other metal complexes (Figures.11-14) 47,48