SYNTHESIS AND CHARACTERIZATION OF POTENTIAL ANTIOXIDANT AGENT OF NOVEL PYRIDYLAZO LIGAND AND ITS PALLADIUM COMPLEX

. A new azo dye ligand namely ((E)-1-(4-hydroxy-3-(pyridin-3-yldiazenyl)phenyl)ethan-1-one) (HPPE) was synthesized by the coupling reaction of diazoinium salt of 3-aminopyridine and 4-hydroxyacetophenone. The palladium complex was prepared by reacting palladium chloride with the HPPE ligand. These compounds were characterized by different techniques such as mass, 1 H-NMR, infrared, UV-Vis spectroscopies. The infrared data reveal that azo ligand reacting as bidentate via oxygen and nitrogen atom of azo group and it reveals the important functions as carbonyl group, methyl group, and azo group in the ligand and the complex such. The palladium complex is square planer. The ligand showed a visible colorimetric pH sensitive chemosensor, fast response time and is fully reversible and exhibited a large wavelength shift more than170 nm accompanied by excellent sensitivity of pH in the range of 3-9. The HPPE ligand and palladium complex showed potential antioxidant activity especially the ligand showed activity so close to the ascorbic

Azo compounds are considered one of the most important chemical compounds with a wide range of uses such as dying textile fibres, organic synthesis, liquid crystalline displays, and electro-optical devices.Azo compounds exhibit potential biological activity including antibacterial activity, pesticidal activities, antiseptic and antiprotozoal properties [1].
Recently, the spectroscopic characteristics such as color sensor , pH sensor [4,5] and metastable states [6][7][8] of the azo dyes and their complexes received great interest.The existence of a pyridine nucleus in the azo dye ligand performs a decisive role in the chemistry of azo dye systems, and these properties are present originate in a number of physiologically active compounds [9].Therefore we are focused on the synthesis azo dye of 3-aminopyridine and its palladium complex and their identification confirmations which were carried out by using numerous analytical and spectroscopic methods.Additionally, antioxidant studies were investigated for the synthesized compounds.

Materials
The used materials were supplied commercially and used without further purification, 3aminopyridive (Sigma Aldrich), hydrochloric acid, sodium hydroxide and palladium chloride(II) were supplied from Merck.Solvents were purchased from Scharlau Company.

Instrumentations
The infrared and the electronic spectra were measured using Fourier transform infrared spectrophotometer (FT-IR-8400S) (4000-400 cm -1 ) from Shimadzu and spectrophotometer CECIT CE 7200, 7000 series) from Aquaris respectively.The 1 H-NMR spectra were done by Bruker spectrometer instrument operating at (400 MHz) and the conductivity was measured using WTW SERIES, cond 722.The melting points were measured by SMP30 Stuart apparatus.

Synthesis of HPPE ligand
An ethanolic solution of 3-aminopyridine (5 mmol, 0.47 g) was mixed with 3 mL of concentrated hydrochloric acid.The acidic solution was put under cooling.Sodium nitrite (5 mmol, 0.35 g) was dissolved in distillate water (5 mL) and it was added to acidic solution of 3-aminopyridine to form diazoinium salt.The 4-hydroxyacetophenone (5 mmol, 0.68 g) was dissolved in 25 mL of ethanol and it was mixed with 10 mL of (10% sodium hydroxide).The cold solution of diazonium solution was added to cold basic solution of 4-hydroxyacetophenone.

Synthesis and characterization
The 3-(3-pyridylazo)-4-hydroxyacetophenone ligand was formed by reacting diazonium salt of 3aminopyridine with 4-hydroxyacetophenone in basic medium as shown in Scheme 1(a).The palladium complex was formed by reacting two equivalents of azo ligand with one equivalent of palladium chloride as shown in Scheme 1(b), which is in agreement with elemental analyses that showed metal-to-ligand ratio equivalent to 1:2.

Measurements of FTIR spectroscopy
The FTIR method is quick and very useful even for tiny quantities of the compounds.It uses to determine the functional groups which may lead to monitor the dyes in environment and in consumer products.The chromophors of the HPPE ligand are C=O, C=N, -C=C-, -N=N-, which showed 1666, 1596, 1500 and 1491cm -1 at respectively [10,11].The OH, CH aromatic and CH aliphatic groups of HPPE dye represent auxochromes showing at 3368, 3064 and 2940 cm -1 , respectively [12,13].The Infrared spectrum of ligand is presented in Figure 2. The Pd-O and Pd-N bonds in Pd(II) complex showed at 500 cm -1 and 440 cm -1 , respectively [14].The OH peak of ligand disappeared upon complexation with Pd that means that the phenolic oxygen participates under bonding with the metal centre.The frequencies of the N=N and C-O values in the palladium complex showed a redshift which shifted to a lower frequency upon complexation to Pd(II) compared to the ligand [15].The shifting indicated that both the N and O donor atoms in the ligand were involved in the chelation to the metal centre.
The 1 H-NMR spectra of HPPE ligand and its palladium complex were recorded in DMSO-d6 solution.The signal of amino group in 3-aminopyridine disappeared in the spectrum of the HPPE ligand as shown in Figure 3, that means that the start material reacted and converted to the azo dye.The protons aromatic on the carbon atoms besides the nitrogen atom in the pyridine ring appeared at high chemical shift.These signals appeared as singlet signal at 8.73 ppm and doublet signal at 8.36 ppm.The protons of Methyl group appeared at 2.6 ppm as singlet signal.The OH group appeared at 9.23 ppm as a singlet signal while it disappears in the palladium complex which refers to deprotonate the OH group in the complex.The aromatic protons of ligand and complex appeared in the range of 6.90-7.80ppm.On complexation, the protons on the appeared chemical shifts more or less values in the complex as shown in the Figure 4 comparing to the ligand.

Electronic spectra of HPPE ligand and palladium complex
The electronic spectra of the HPPE dye and its palladium complex in DMSO solution (10 -5 M) are depicted in Figure 5.The electronic spectrum of HPPE ligand showed two types of transitions.The first electronic transition at 272 and 320 nm can be assigned to π→π* transition centered on the phenolic ring [16].The second electronic transition at 370 nm is assigned to n→π* based on the azo group and benzene ring [17,18].The third electronic transition was at 483 to 600 nm which is due to intramolecular electron transition.The palladium complex showed band at 484 nm due to charge transfer and other bands at 625, and 714 nm are assigned to the d-d transitions which are represented 1 A1g→ 1 B1g, and 1 A1g→ 1 A2g respectively [19][20][21][22].The Pd(II) complex showed bands at 282 and 327 nm of π→π* with higher intensity than that of the azo dye ligand.

Molar conductance and magnetic moment
The molar conductance value of the palladium complex was determined using 1x10 -3 M DMF solution.Molar conductivity measurements of the palladium (II) complex revealed the nonconductivity nature of the solution at room temperature where the standerd values were done at this temperature indicating the absence of any electrolytes.That means that there are no ions existing in the outer sphere coordination of the complex.The magnetic susceptibility measurements showed diamagnetic behaviour which consistent with the zero unpaired electrons of Pd(II) d 8 species having square planar geometry.The HPPE ligand and palladium complex were tested their antioxidant activity by using the DPPH.Both compounds are efficient scavengers for free radical species.The synthesized compounds showed potential scavenging for target oxidative damage which are listed in Table 1 [25,26].The antioxidant activity of the HPPE ligand and palladium complex was determined by using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity.The HPPE ligand as shown in Figure 7, showed a higher DPPH radical-scavenging activity comparing to the ascorbic acid and the palladium complex.We conclude that the azo ligand is useful source as antioxidants, which confer significant protection against free radical damage.

CONCLUSION
To sum up, we have described the design, synthesis and characterized of a visible colorimetric sensitive chemosensor namely ((E)-1-(4-hydroxy-3-(pyridin-3-yldiazenyl)phenyl)ethan-1-one) (HPPE) which was prepared by coupling diazonium salt of 3-aminopyridine and 4hydroxyacetophenone.The HPPE ligand is bidentate and it coordinates via N and O atoms to the palladium ion.Compound of Pd(III) with HPPE ligand were synthesized.The palladium complex was formulated as [Pd(HPPE)2] and its shape is square planar.The palladium complex is a neutral species and it is square planer.We conclude that the azo ligand is useful source as antioxidants, which confer significant protection against free radical damage.
Scheme 1. Preparation steps of the ligand and its palladium complex.

Figure 1 .
Figure 1.Mass spectrum of HPPE ligand.The Pd(II) complex and azo dye ligand were formed in high yields.The mass spectrum of the HPPE ligand is depicted in Figure1.Mass spectrometry can be used in the investigation the fragmentation patterns of the azo ligand and the relative abundance.The spectrum of dye revealed peaks at m/z value of 237.03 corresponding to C13H11N3O2 which represent molecular weight of

Figure 5 .
Figure 5. UV-VIS spectra of HPPE ligand and palladium complex in DMSO at room temperature.

Figure 6 .
Figure 6.pH effect on the HPPE dye in aqueous solution.

Table 1 .
The data of antioxidant activity of HPPE ligand and its palladium(II) complex.