Synthesis and Characterization of Co(II), Ni(II), Cu(II), and Zn(II) Complexes with 2-Furaldehyde-2- Thenoylhydrazone

Three types of Metal(II) complexes with hydrazone derived from condensation of thiophene-2-carboxylic acid hydrazide and 2-furaldehyde namely 2-furaldehyde-2thenoylhydrazone (fth) of the general formulas [M (fth-H)2],[M(fth)2 Cl2] and [M(fth)2] (NO3)2, where M= Co(II), Ni(II), Cu(II), and Zn(II), were synthesized and characterized on the basis of elemental analysis, molar conductance, magnetic moment, infrared and electronic spectral data. The infrared and molar conductance data of the complexes reveal the monobasic tridentate ONO nature of the ligand and coordination through the enolate oxygen, furan ring oxygen and azomethine nitrogen atoms, in the complexes [M(fth-H2)] upon deprotonation of the ligand. The same ligand acts as neutral bidentate ON in the chloro-complexes coordinated through the carbonyl oxygen and azomethine nitrogen atoms and neutral tridentate ONO in the nitrate complexes coordination through carbonyl oxygen and furan ring oxygen atoms in addition to the azomethine nitrogen atom. The magnetic and electronic spectral data suggested an octahedral geometry for the complexes. ــــــــــــــــــــــــــــــــــــــــــــــــــــ تادقعم صيخشت و ريضحت Zn(II), Cu(II), Ni(II), Co(II) عم 2 دياهيدلارويف 2 نوزارديه لينويث


INTRODUCTION
There has been continuing interest in coordination chemistry of the hydrazones that derived from heterocyclic acid hydrazides and aldehydes or ketones (Dianu et al., 2010 ;Nair and Thankamani, 2011).The remarkable biological activity of acid hydrazides and their corresponding aroylhydrazones and the dependence of their activity on the mode of coordination with transition metal ions present in the living system have been also of significant interest by many workers.(Rollas et al., 2007 ;Vicini et al., 2006).
The coordination compounds of aroylhydrazones have been reported to be useful due to their pharmacological and analytical applications (Singh et al., 1982 ;Kumar et al., 2004Aguelles et al., 2004;Vallinath et al., 2010).
An important feature in the chemistry of aroylhydrazones and their complexes is the acid character of the hydrogen of the NH group of the hydrazide moity; this allows for either neutral or anionic ligands.When coordinating as anionic ligands the conjugation is extended to include the carbonyl group (Fig. 1).It has been proposed that such conjugation system enhanced the biological activity of such compounds (Latheef et al., 2009 ;Martinez et al., 1996).Therefore, the reaction of aroylhydrazones with transition metal ions can proceed according to two pathways attaining the ketonic (Fig. 1) or enolic structure for the hydrazide part of the molecule (Seleem et al., 2006) .
The mode of bonding depends on the nature of both the ligand and metal ions, the anion of the metal salt, the pH of the reaction medium and the solvent used (Iskander et al., 2003).
As little work has appeared on the isolation and characterization of metal chelates with hydrazones derived from thiophone-2-carboxylic acid hydrazide and mainly concerned with the synthesis and structural studies of organotin adducts with 2,6-diacetylpylpyridine bis(2thenoyl hydrazone (Carini et al., 1989 ;de Sausa et al., 1999) and di-2-pyridylketone-2thenoyl hydrazone (Carcelli et al., 1995).The present investigation reports the synthesis and characterization of some divalent transition metal complexes of the hydrazone derived from thiophene-2-carboxylic acid hydrazide and 2-furaldehyde (fth): All chemical used were reagent grade from B.D.H or Fluka chemical companies, used as supplied, except for 2-furaldehyde which was purified by vacuum distillation according to literature (Perrin et al., 1980).
IR spectra of the ligand and its complexes were recorded as KBr pellets in the region 4000-400 cm -1 on Tensor 27 Brucker FT-IR spectrophotometer.Electronic spectra were recorded on Shimadzu UV-1650 PC-spectrophotometer for 10 -2 M solution of the complexes in DMF at room temperature using 1cm cells.The metal contents of the complexes were determined spectrophotometrically using Shimadzu AA670 atomic absorption spectrophotometer, after decomposition with concentrated nitric acid.Conductivity measurements were performed on 10 -3 M solutions of the complexes in DMF at room temperature, using PMC3 Jeneway and Terminal 740 wtw conductivity meters.Magnetic susceptibility measurements of the complexes were carried out using Bruker BM6 instrument at ambient temperature.Melting points were determined using Electrothermal 9300 apparatus.

Preparation of the Ligand Preparation of thiophene-2-carboxylic acid hydrazide:
This was prepared by the reaction of ethylthiophene-2-carboxylate with hydrazine hydrate in absolute ethanol by the procedure reported earlier(Al-Saady and Al-Daher, 2000), m.p. 136 C º .

Preparation of 2-Furaldehyde-2-thenoylhydrazone (fth):
This was prepared following a general procedure (Sacconi, 1953).as follows: Thiophene-2-carboxylic acid hydrazide (2.84g, 0.02 moles) was dissolved in 20 ml of absolute ethanol.To this solution, 2-furaldehyde (1.92g, 0.02 mole) in 20 ml ethanol was added.The reaction mixture was refluxed on a water bath for ca.4h.The partial removal of solvent followed by cooling on ice produced a yellow crystalline product, which was filtered, washed with cold alcohol and dried under vacuum.m.p. 161 C º .
For complexes (2, 8, 10) after completion of the reaction, the reaction mixture was concentrated and left to stand overnight at room temperature which resulted in the formation of solid products.The products, thus obtained were filtered, washed and dried as mentioned above.

RESULTS AND DISCUSSION
The reaction of appropriate metal (II) acetate with the hydrazone, derived from thiophene-2-carboxylic acid hydrazide and 2-furaldehyde (fth), in the presence of KOH in a molar ratio of 1:2:2 using absolute ethanol as solvent gave a monomeric complexes of the type [M(fth-H) 2 ] [M=Co(II), Ni(II), Cu(II) and Zn(II)].The formulation were in accordance with the data of elemental analysis and physicochemical measurements (Tables 1-3).The molar conductances, determined at concentration of ca.1×10 -3 M, in DMF of the complexes (1,4,7 and 10) show small values in the range 6.2-16.5 ohm -1 cm 2 mol -1 , indicating that the complexes are non-electrolytes (Geary, 1971) and that the ligand is deprotonated acting as monobasic ligand due to enolisation (Fig. 1) .
All the prepared complexes are air and moisture stable solids.They are soluble in DMF and DMSO, sparingly soluble in methanol, ethanol and acetone and insoluble in ether and chloroform.Their melting temperatures, solubility and crystalline nature suggest that they are non-polymeric (Chohan et al., 2000) .10.39 10.57 • 10 -3 M solution .

Infrared spectra and mode of chelation :
The bonding mode of the ligand was elucidated by comparison of IR spectra of fth and its complexes.The main vibrational bands of the ligand and its complexes are shown in (Table 2) .
The IR spectrum of the ligand exhibit two bands at 3222 cm -1 and 1638 cm -1 due to υ (NH) and υ (C=O) stretches.The absence of these bands in the spectra of the complexes (1,4,7 and 10), is consistent with enolisation of the amide functionality and subsequent coordination of the enolic oxygen to the metal ion.A new band appearing in the 1028-1038 cm -1 range assigned to the υ (C-O) (enolic) mode (Lee and Koo, 2005).This is further supported by the formation of new strong intensity band υ (NCO -) at 1525-1537 cm -1 (Affan et al., 2009) .
On the other hand, the spectra of the complexes (2,3,5,6,8,9,10,11 and 12) in this region are different, showing that the υ (C=O) band shifts to lower frequencies in these complexes, which is indicative of bonding of the carbonyl oxygen to the metal ion.The position of the υ (NH) absorption are almost unchanged or shifted slightly to smaller frequencies in these complexes probably due to the hydrogen bond in which the N-H group is involved being weaker than in free ligand (de Sousa et al., 1999) .
The strong intensity absorption at 1614 cm -1 in the spectrum of the uncomplexed hydrazone is ascribed to the υ (C=N) vibration, which is specific for the azomethine group.This was satiated at lower wave numbers, 1580-1608 cm -1 , in all the complexes (1-12), consequently confirming the coordination of azomethine nitrogen atom to the metal ions (Agarwal et al., 2006 ;Chandra et al., 2009 ;Dianu et al., 2010).The band due to υ (N-N) mode appears at 933 cm -1 in the spectrum of the free ligand was shifted to higher frequency by 12-39 cm -1 which further supports the coordination of the azomethine nitrogen atom.The high frequency shift is expected because of diminished repulsion between the lone pairs of adjacent nitrogen atoms (Maurya et al., 2002) The band at 1290 cm -1 in the IR spectrum of the free fth assigned to furan ring υ (C-O-C) stretching vibration was shifted to lower values in the spectra of the complexes (1,3,4,6,7,9,10 and 12) suggesting the involvement of the oxygen atom of the furan ring moity in coordination in these complexes (Sallomi, 2000;Taher, 2000).While this band remains almost unchanged in the spectra of the complexes (2,5,8 and 11) indicating the non-participation of the furan ring oxygen atom in coordination of the chloro-complexes.Conclusive evidence of the bonding is also shown by observation that new bands appear in the spectra of all the complexes at 468-553 cm -1 and 430-472 cm -1 which are assigned to υ (M-O) and υ (M-N) stretching vibration, respectively (Agarwal et al., 2006 ;Chohan et al., 2000;Chandra et al., 2009).
The Bands observed in the IR spectrum of the free ligand at 841,712 and 652 cm -1 were assigned to υ (C-S-C) thiophene ring stretching vibration, υ CS asymmetric and, υ CS symmetric vibration, respectively (Spinu and Kriza, 2000).These bands were found almost unchanged in the spectra of all complexes indicating that the sulfur atom of the thenoyl moity is not taking part in the coordination of the ligand fth to the metal ions.This behavior was also reported in the X-ray structural studies of organotin complexes with 2,6diacetylpyridine bis (2-thenoylhydrazone) and di-2-pyridylketone-2-thenoylhydrazone (Carini et al., 1989;Carcelli et al., 1995).
The nitrate complexes (3,6,9 and 12) exhibits a very strong bands at the region 1371-1385 cm -1 and medium intensity band at the region 825-833, which are attributed to the υ 3 and υ 2 vibrations, respectively of uncoordinated nitrate ion of D 3 h symmetry (Chandra et al., 2009).
Table 2 : Some selected infrared frequencies (cm -1 ) of the ligand and its complexes .s= strong , m= medium , w= weak

Electronic Spectra and Magnetic properties:
Absorption spectra of the ligand and it's complexes were obtained in DMF and the resulting data are given in (Table3).The electronic spectrum of the fth ligand presents an absorption maxima at 31847 cm -1 and 27778 cm -1 due to π π* of the aromatic rings and to n π* transition in the C=O and C=N chromophoric groups, respectively.These transitions were found in the spectra of the complexes but shifted to lower frequencies, indicating coordination of the ligand to the metal ions.Intense absorption at the region  24041-25773 cm -1 was observed in the spectra of all complexes were assigned to a ligand to metal charge transfer (LMCT) band (Lee and Koo, 2005).
The electronic spectra of Cu(II) complexes (7-9) contain a wide band satiated at the region 13888-14492 cm -1 .This band is characterstic for a Cu(II) ion with tetragonally distorted octahedral stereochemistry and can be assigned to two or three transitions 2 B 1 g 2 Eg, 2 B 1 g 2 B 2 g, and 2 B 1 g 2 A 2 g .The magnetic moment of 1.79-1.98BM suggests a monomeric octahedral geometry (Lever, 1984 ;Dianu et al., 2010).
The electronic absorption spectra of Zn(II) complexes (9-12) show no appreciable absorptions in the region below 25000 cm -1 in DMF solution.In accordance with the filled d-orbital electronic configuration of Zn(II) (Cotton et al., 1999).The ligand fth deprotonated and acts as monobasic ONO tridentate, bonding to the metal (II) ions through the enolic oxygen, azomethine nitrogen and furan ring oxygen atoms (Fig. 2a) .The same ligand reacts with metal (II) chlorides and metal (II) nitrates in neutral medium forming also octahedral complexes of the types [M(fth) 2 Cl 2 ] and [M(fth) 2 ](NO 3 ) 2 , respectively.The ligand act as neutral bidentate ON in the chloro-complexes coordinated through carbonyl oxygen and azomethine nitrogen atoms (Fig. 2b).In case of nitrate complexes fth act as neutral tridentate ONO coordinated through carbonyl oxygen, azomethine nitrogen and furan ring oxygen atoms and the nitrate groups were uncoordinated (Fig. 2c) .

Table 1 :
Physical properties and analytical data of the ligand and its metal complexes

Table 3 :
Magnetic susceptibility and electronic spectra of the ligand and its complexes .Furaldehyde-2-thenoylhydrazone (fth) reacts with M(II) acetate in basic medium to form octahedral complexes of the type [M(fth-H) 2 ], M= Co (II), Ni (II), Cu(II) and Zn(II).