INTRODUCTION:

Azo dyes are characterized by the presence of one or more azo group (-N=N-) in association with two or more aromatic or heterocyclic system [1-6]. In recent years many transition metal coloured complexes with azo dyes as ligand were prepared and widely studied in different application such as biological studies [7,8] ,coordination polymers [9,10] ,textile industry [11,12] , solvent extraction [13] ,and spectrophotometric determination [14-16] .Azo dyes usually react with metal ions as bidentat ligand [17] or as tridentate [18] in the field of coordination chemistry . In this work, the synthesis and characterization of Co (II) and Cu(II) complexes with a new bisazo dye derived from 4,5-diphenyl imidazole.

EXPERIMENTAL:

Materials and Measurements:-

All chemicals used in the present investigation were obtained from commercial sources expect of 4,5- diphenyl imidazole was prepared as reported procedure [19].

Elemental analysis were carried out by Vario Elemental Apparatus Shimadzu.IR spectra were recorded on a Shimadzu 8000 FT-IR spectrophotometer in the(4000-400) cm-1 range using KBr discs . Electronic spectra were obtained on a Shimadzu 1700 UV-spectrometer using ethanol as solvent in the (1000-200) nm range. The NMR spectra (¹H-NMR and ¹³C-NMR) were taken in Bruker Biospis 600 MHz Auance III and 400 MHz (Germany) using DMSO-d₆ as the solvent.

Synthesis of bis azo ligand [(2-(4- methoxy-2-(4-methoxy phenyl)diazenyl)phenyl)diazenyl)-4,5-diphenylimidazol:

Bisazo dye was synthesized according to the following general procedure [20](Scheme1). There are two diazotization and two coupling reaction in the synthesis of bisazo dye.

Step (1):-Synthesis 4-methoxy-2-(4-methoxy phenyl azo) aniline

p-methoxy aniline (1.23 gm , 0.01 mole) was dissolved in 30 ml of water and 2 ml concentrated hydrochloric acid. This solution was diazotized below 5°C with 15 ml of aqueous (0.69 gm ,0.01 mole) sodium nitrite . The resulting diazonium chloride solution was mixed with p- methoxy aniline ( 1.23 gm , 0.01 mole ) dissolved in 30 ml alkaline ethanol cooled below 5°C . The orange precipitate was filtered off , recrystallized from ethanol and dried .( m.p: 125°, Anal. Calc. for C₁₄H₁₅N₃O₂):C(65.35); H(5.88); N(16.33); O(12.44). Found C(65.15) ; H (5.63); N(16.12); O(12.34% ).

Step (2):- Synthesis bisazo ligand [2-(4- methoxy-2-(4-methoxy phenyl)diazenyl)phenyl)diazenyl)-4,5-dipheny limidazole].

Monoazo dye (prepared in step (1)) (2.57 gm, 0.01 mole) was dissolved in 30 ml ethanol and 2 ml concentrated hydrochloric acid. This solution was diazotized below 5°C with 15 ml of aqueous solution (0.69 gm, 0.01 mole) sodium nitrite .The resulting diazonium chloride solution was mixed with 4,5- diphenyl imidazole (2.2 gm ,0.01 mole) dissolved in 150 ml alkaline ethanol cooled below 5°C .After leaving for 24 hour ,the mixture was acidified with (0.1 mole L-1) hydrochloric acid until pH=6 . The precipitate brown was filtered off , recrystallized from ethanol and dried . (m.p :145°, Anal. Calc. for C₂₉H₂₆N₆O₂: C(71.0); H(5.34), N(17.13), O(6.52). Found:C(71.4), H(5.14), N(17.1) , O(6.54%) .

The complexes were synthesized by addition of ethanolic solution of bisazo ligand to an aqueous solution of the metal chloride Co(II) and Cu(II) in 1:2(Metal:Ligand) molar ratios . After stirring for 15 min., the colored precipitates were filtered off, washed with 5 ml hot ethanol, recrystallized from ethanol and dried. [red brown precipitate of [CoL₂]CI₂.H₂O (m.p: 300° , Anal.Calc. for C₅₈H₅₄N₁₂O₅Co :C(65.84) ,H(5.14) , N(15.89) ,O(7.56) ,Co(5.57) ; Found :C(65.67) ,H(5.11) , N(15.78), O(7.46), Co(5.43%).

-[ red brown precipitate of [CuL₂]CI₂.H₂O (m.p: ˃300°, Anal.Calc. for C₅₈H₅₄N₁₂O₅Cu :C(65.55) ,H(5.12), N(15.82), O(7.53) ,Cu(5.98) ; Found :C(65.35) ,H(5.04) , N(15.76), O(7.46), Cu(5.88%).

Standard metal solutions:-

Stock solution of 10-3M Co(II) and Cu(II) were prepared by dissolving the appropriate weight of CoCI2.6H2O and CuCI2 in D. water. A series of standard of Co(II) and Cu(II) solutions were prepared by several dilutions (10-3 – 10-5) of the stock solution.

Standard ligand solutions:

Ethanolic solution of the standard ligand 10-3 M was prepared. A series of standard of dye solutions were prepared by several dilutions (10-3–10-5) of the stock solution.

Buffer solution:

Buffer solutions in (5-10) range were prepared by standard method for the pH adjustment by dissolving (0.0077 gm) of ammonium acetate in 100 ml and adjusted by acetic acid and ammonia solution and measured by pH-meter.

RESULTS AND DISCUSSION:

Spectroscopic characterization of bisazo ligand :

Nuclear Magnetic Resonance Spectra: (1H, 13C, HMBC and HSQC spectra were acquired in DMSO-d6 solvent):

-The 1H-NMR spectra of bisazo ligand (400 MHz) shows characteristic signals due to following protons [21].

-Methoxy protons: bisazo ligand shows in the region (3.89)ppm ,the hydrogens on the carbon next to oxygen are deshielded due to the electronegativity of the attached oxygen.

-Aromatic protons: the bisazo ligand exhibited multiple signals due to the aromatic hydrogen's of the phenyl and imidazole ring in the range of (7.16-7.93)ppm.

- N-H imidazole:-this hydrogen is deshielded due to the resonance that removes electron density from nitrogen and changes it hybridization, appears in the region (13.24)ppm.

-The 13C-NMR Spectra of bisazo ligand shows absorption signals for Methoxy –O-CH3 in the region (56.6)ppm .The dye exhibit absorption signals due to the carbon atom of aromatic groups of (C aromatic +C4+C5 imidazole) in the range of (115.31 – 129.32)ppm,(C2 imidazole appear in the region(136.08)ppm,(C1a+1b) in the region(139.69)ppm and the carbon attached oxygen (4'+4''(C-OMe)) appear in the region (162.61) ppm.

1H, 13C, HMBC and HSQC spectral data of the synthesized bisazo dye are shown Fig.(1-4):

The IR spectra data (KBr disk) of bisazo ligand and Co(II) and Cu(II) complexes are summarized in Table(1). The IR spectrum of the free bisazo ligand showed a medium and broad band around 3400 cm-1,which can be attributed to (-N-H) stretching vibration of imidazole moiety [22].The position of this band interacts with broad band at (3409 -3186cm-1 in the spectra of Co(II) and Cu(II) complexes due to presence of water molecule [23].The IR spectra of ligand appear band at (1604)cm-1 due to ν(C=N) of the N imidazole nitrogen, which shifted to lower frequencies (1542 – 1545 )cm-1 in the prepared complexes spectra due to present of diazo group with conjugated system and the linkage of metal ion with nitrogen imidazole ring [24].The ν(N=N) stretching vibration appears at (1490) cm-1 in the free ligand spectra ,this band shifted to relatively lower energy (1458-1465)cm-1 in the complexes spectra indicates the N=N group coordination [25,26]. New weak bands in the region (462-563) cm-1 in the complexes spectra which were not present in the spectra of the ligand may be attributed to ν(M-N) [27]. The ν(O-CH3) methoxy vibration appears at (1250) cm-1 and (1031) cm-1 due to C-O-C asymmetric and symmetric respectively, which were not change in the spectra of the complexes spectra [21].

The results presented above lead to suggest that the ligand behaves as tridentate chelating agent, coordinating with metal ions by a nitrogen of two azo group and nitrogen in imidazole ringtogive six and five –membered chelate rings Fig.(5-8)

Table (1): Characteristic IR frequencies (cm-1) of the monoazo ,bisazo dye and its metal complexes .

No.	Compound	υ(O─H) H₂O	υ(NH₂) + υ(N-H) imidazole	υ(C=N)	υ(N=N)	υ (O-CH₃)	υ (M─N)
1	Monoazo dye	ـــــــــــ	3200	1602	1506s	1250s	─
2	Bisazo dye (L )	ــــ=ــــــــ	3425m	1604s	1490s	1250s	ـــ
3	[CoL₂]CI₂.H₂O	3409br.	3186br	1542w	1465w	1257m	462m
4	[Cu L₂]CI₂ .H₂O	3317br	3163br	1545w	1458w	1265m	563m

Fig (5): FT-IR spectra of the monoazo dye.

Fig (6):FT-IR spectra of bisazo ligand.

Fig (7): FT-IR spectra of the [CoL₂]Cl₂.H₂O complex.

Fig (8):FT-IR spectra of the [CuL₂] Cl₂.H₂Ocomplex.

The electronic spectra data of the bisazo ligand and its complexes were shown Fig(9-11). The ligand is characterized by three bands in UV-Visible .These bands are appearing at the position 213 nm(46948 cm-1) and 291 nm(34364 cm-1) , the first can be attributed to a π-π* transition with in heterocyclic imidazole ,while the second UV band observed at longer wave length can be ascribed to the excitation of the π-electrons of the phenyl system [28]. The characterization third visible band occurred at 413 nm (24213 cm-1) may be to π-π* transition due to presence of conjugation in the ligand molecule [7]. The position of ligand bands are shifted which may be due to chelation between metal ions and bisazo ligand , which were assign to d-d transition ( weak in intensity ) and ligand field.

Fig(9):Electronic spectra of bisazo ligand

Fig(10):Electronic spectra of Co(II)-complex.

Fig(11): Electronic spectra of Cu(II)-complex.

Effect of pH and Time:

Optimal pH values effected on the absorbance for metal complexes solution were shown in fig.(12) , Table (2). The ligand formed stable complexes with metal ions ( Co(II) and Cu(II) at pH (7-9). Also the reaction is complete moment at room temperature and remains stable for long time, this show the ligand strong coordination with Co(II) and Cu(II) complexes ,the results are shown in fig.(13).

Fig (12):Effect of pH on the absorbance of Co(II) Co(II) and Cu(II) complexes.

Fig (13): Effect of time on the absorbance of Co(II) and Cu(II) complexes.

Metal : Ligand ratio :

The metal-ligand ratios of complexes were determined by molar ratio method at fixed concentration of metal ion and increasing concentration of ligand at optimum pH and λmax. The mole ratio of complexes (Metal:Ligand) was (1:2) for the Co(II) and Cu(II) ions ,these results are in agreement with values reported for some aryl azo imidazole complexes [29,30] The results are given in table (2), fig(14).

Fig(14): Mole ratio method (M:L) of Co(II) and Cu(II) chelates at optimum and concentration.

Calculation of stability constant of the metal complexes:

Stability constant (B) values are obtained spectrophotometrically by measuring the absorbance of solutions of ligand and metal mixture at optimum condition [31].The calculated B and logB values for the Co(II) and Cu(II) complexes are given in table(2).

Conductivity measurement:

The data obtained from measurement of molar conductance of the complexes are shown in table (2 ) ,were carried out in DMF ( 10-3 ) at room temperature .The high value of molar conductivity for Co(II) and Cu(II) complexes indicating that ( 1:2 ) electrolyte and suggest that Cl- anion present outside the coordination spheres [32].

According to these results the following structure of these complexes may be proposed:

Fig(15): The suggest structural formula of Co(II) and Cu(II) chelate complexes.

Table (2): Optimum condition, conductivity and stability constant (B) values of Co (II) and Cu (II) complexes.

Conductivity

(S.mol^-1.cm²)

logB

Metal: Ligand

λ_max

Berr's law validityX10^-5 M

Compound

143.7

12.23

1.7 X10¹²

1:2

466

1-10

[CoL₂]CI₂.H₂O

135.9

13.25

1.8 X10¹³

1:2

469

1-9

[CuL₂]CI₂.H₂O

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Received on 18.01.2014 Modified on 25.02.2014

Asian J. Research Chem. 7(3): March 2014; Page 262-268