1- INTRODUCTION:

Quinoxaline-based Schiff bases were synthesized and characterized by several workers1-2] reported the synthesis of some Schiff bases from 6-benzoyl-3-amino-2- imino-2,3-dihydrothiazolo[4,5-b] quinoxaline. Varghese et al. reported [3] the synthesis and crystal structure of N,N′-bis[(E)-quinoxalin-2-ylmethylidene]ethane-1,2-diamine.The Schiff base was prepared by condensation of quinoxaline-2-carboxaldehyde and ethylenediamine. The synthesis and crystal structure of two bis-azomethines derived from quinoxaline-2-carboxaldehyde were reported [4]. The crystal structures of the Schiff bases, N,N′-bis[(E)-quinoxalin-2-ylmethylidene]propane-1,3-diamine and N,N′-bis[(E)-quinoxalin-2-ylmethylidene]butane-1,4-diamine revealed that conformations of the crystal structures were stabilized by aromatic π–π stacking interactions.

A bis-azomethine. The Schiff base synthesized by condensation of 3-hydroxyquinoxaline-2- carboxaldehyde and 2,3-diaminomaleonitrile[5]was characterized by IR, ¹H and ¹³C NMR, cyclic voltammetry and themal analyses(TG-DTA).The studies revealed that the Schiff base exists in two major tautomeric forms, i.e. minor enol and major keto (5b) forms [6-7]. Synthesis, characterization, and catalytic activity studies of metal complexes of Schiff bases, N,N′-bis-(3-3hydroxyquinoxaline-2-carboxalidene)-ethylenediamine, N,N-bis-(3-hydroxyquinoxaline-2-carboxilidene)-o-phenylenediamine-3-hydroxyquinoxalinehydrazone and N,N′-bis-(3-hydroxyquinoxaline-2carboxilidene)diethylene triamine, were reported. The present work described the synthesis and characterization of new Co, Ni and Cu(II) complexes with new Schiff base derived from condensation of 2-amino-phenol and 5-methoxy-2-hydroxy-carboxaldehyde.

2-EXPERIMENTAL:

2-1-Materials

The anhydrous metal chlorides CoCl2,NiCl₂,and CuCl₂.2H₂O were purchased from Sigma-Aldrich Company and used without purification. Oxalic acid and solvents were supplied from Fluka Company in 99% purity. 1,2-phenylenediamine and1,2-2-amino-phenol were purchased from Fluka, England. All other chemicals used were of AnalR grade. The 5-methyl-2-hydroxy-quinoxalin-carbixaldehyde has prepared according to the method published in literature [8].

Methods:

2-2-Synthesis of HL Schiff base:

The new Schiff base HL have prepared according to the modified method published in literature[9]. An equimolar amounts of 5-methoxy-2-hydroxy-quinoxaline-2-carboxaldehyde in 10ml of methanol was mixed with 10ml ethanolic solution of 2-amino-phenol and refluxed the mixture on water bath with constant stirring for 5hours.The crude yellow precipitate was formed up on cooling at room temperature, thus recrystallization from hot ethanol afforded 80%yield of HL ligand, scheme(1).

Scheme(1)-structure of HL ligand

2-3-Instrumens:

C.H.N-elemental analyses of the new solid complexes were determined using Carlo-Erba 1106 Elemental analyzer .Electronic spectra were recorded for solutions of quinoxalin-2,3-dione and its metal template complexes using Shimadzu spectrometer in the range200-800nm in DMF solvent . Magnetic susceptibility measurements were carried out using Bruker 14K-Magnetic balance at room temperature via Gouy method using Hg[Co(SCN)4] as calibrate. The ¹Hand ¹³C NMR spectra were carried at Al-bait -University on Bruker 300 MHZ spectrometer in DMSO-d6 solvent. The molar conductivity measurements of complexes solutions in DMF were done on Philips digital conductivity meter of Pt-electrode with 1cm-1 cell constant.

3-RESULTS AND DISCUSSION:

The analytical and physical data of the Schiff base and its metal complexes are listed in Table(1). The percent of C,H,N and M obtained from elemental analyses and atomic absorption spectroscopy are in good agreement with the general molecular formula proposed for the complexes [MLCl] M=Ni(II), Co(II),and Cu(II) respectively. The values of molar conductance of the complexes solutions in DMF lie in the range 10-30 ohm^-1.mol-1.cm2 indicating the neutral character of all complexes ,thus agree with the proposed formula[10].

3-1-IR spectra

The preliminary identification of the Schiff base ligand has been compared with the its metal complexes IR spectra, which shows the absence of uncondensed functional groups –NH2 of 2-aminophenol and CH=O of pyrazine-carboxaldehyde starting materials suggesting the formation of the proposed Schiff base involving imine moiety. The IR spectral data show that the new Schiff base HL is a uninegative tridentate depending on the metal salt used and the medium of the reaction. The ligand exhibits bands at 1655 and 1610 cm^_1 due to υ(C=O) of quinoxaline ring and υ(H-C=N), respectively. In spectra of all the complexes, υ(C=O) is at 1630-1645 cm^_1, this red shift clearly suggesting involvement of carbonyl oxygen in bonding to metal. In the spectrum of the free ligand[10]. The appearance of strong to medium absorptions bands in the region 3280 and 1650 cm-¹ corresponds to υ NH and –C=O stretching frequency[11].All the metal complexes spectra showed downshift in the wave numbers of –C=O and –C=N- groups in the region 1630-1645cm^-1 confirming the donation of lone pairs of nitrogen and oxygen atoms to the empty orbital of metal(II)ions[12-13].Beside the changes in the positions and intensity some bands, the appearance of Far-IR spectrum of M-N,M-O and M-Cl in the regions 433-477,465-511 and 278-384 cm-1 respectively [12-14].However, the absence of broad band in the spectra of metal complexes could be considered to deprotonation of –OH phenolic group up on covalence bonding with the metal(II)ions., Table(2).

Table 1.Physical properties and elemental analysis of the prepared metal complexes.

Compound	Colour	M.p. °C	C% Calcd. (found)	H% Calcd. (found)	N% Calcd. (found)	M % Calcd. (found)
HL	Pale yellow	180-182	65.14 (64.07)	4.65 (3.99)	14.77 (13.82)
[CoLCl]	Red	270-272	50.33 (49.89)	3.67 (2.91)	10.69 (11.902)	14.12 (14.00)
[NiL Cl]	Orange	3O4D	50.00 (48.92)	4.11 (3.72)	10.11 (10.99)	14.37 (13.90)
[CuL Cl]	Brown	299D	50.59 (49.00)	3.21 (2.80)	10.66 (11.00)	15.52 (14.97)

Table 2.FT-IR absorptions of the Schiff base HL and its metal(II) complexes in cm-1.

Complex	νNH, ν C= O	ʋ C=N- (C – N)	ʋ M – N , M-O	νM – Cl
[HL]	3280(br.), 1655(s)	1610 (w), 1152 (s)
[CoLCl]	3100(m), 1630(s)	1590 , 1230(s)	477, 507(m)	355-370(w)
[NiLCl]	3235(m) , 1637 (s)	1566 , 1135(s)	433(w), 465(m)	290, 311(w)
[CuLCl]	3400(m.), 1645(s)	1544, 11451(s)	470(m), 511(w)	278, 384(w)

s=strong, m=medium, w=weak.br,=broad.

3-3-Magnetic Moment and UV. Visible Spectra

The HL solution in absolute ethanol exhibits high intensity peaks at 288 and 350 nm, that related to electronic transitions of C=N-,C=O and C=C- groups of the types (π→π*and n→π*)[10,16-17].The electronic spectrum of the Co(II) complex in DMF displays spin allowed transitions in the regions 655,456 and 377nm,that are related to ⁴A₂→⁴T₂(F),⁴A₂→⁴T₁(P)and ⁴A₂(F)→⁴T₁(F) respectively[17-18].The electronic spectrum of the orange solution of Ni(II)complex exhibits bands at 410 and 366 nm which are assignable to transitions charge transfer of ³T₁(F)→³T1(P) and LMCT respectively, thus reveals the electronic spectra of tetrahedral symmetry for Co and Ni(II) complexes[13]. However, the brown solution of Cu(II)complex in DMF shows two low energy bands in the regions 750 and 680nm that may be ascribed to d-d and overlapped MLCT respectively. Due to the distorted tetrahedral configuration, the Cu(II) complex shows a broad band at 680 nm for ²T₂→²E transition[18].The low energy peaks of copper(II)complex formed with the new Schiff base confirms the tetrahedral environment around copper(II)ion. The magnetic moments of Co(II)complex was 4.33BM.,therefore agree well with the high spin of d7 tetrahedral geometry[16-18].As well as the values of magnetic moments for Ni(II) and Cu(II)complexes were 2.6 and 1.71BM respectivel supporting the expected magnetic properties of Ni and Cu(II) complexes positioned under low energy system donor teredentate ligands[19-20],Figures(3and4).

Table 3. The electronic spectra ν^1- (cm^-1), molar conductance and magnetic moments of the prepared complexes

Complex	ν^1- (nm)	Λ_m	µ_eff(B.M)	Geometry
HL	288,355
[CoLCl]	655,456 , 377	33	4.3	Tetrahedral
[NiLCl]	410,366,278	15	2.6	Tetrahedral
[CuLCl]	750,680,255	10	1.71	Tetrahedral

m= molar conductance in ohm-¹ cm² mol^-1.inDMF.

Figure (3)-UV-Visible spectrum of HL in ethanol

Figure (4)-Visible spectrum of Cu(II) complex [CuLCl] in DMF solution

4-CONCLUSION:

The structures of the new cobalt, nickel and copper (II) complexes with HL ligand were tetrahedral according to the data obtained from electronic spectra and magnetic moments. As well as the spectral data, elemental analyses and molar conductance reveals that the molar ratio of metal to ligand in the formed complexes was 1:1. The FTIR B spectra study support the NO2 type donor ligand via coordination the metal ions understudy to nitrogen atom of –C=N- and two oxygen atoms of –OH and –C=O in the phenolic and pyrazine moiety respectively, scheme (2).

Scheme (2)-Tetrahedral structures of Co, Ni and Cu(II)complexes.

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Received on 31.12.2013 Modified on 19.01.2014

Asian J. Research Chem. 7(2): February 2014; Page 204-208