Synthesis , Spectral , Magnetic and Thermal Studies of the Complexes of Co II and Ni II With Some Bidentate and Tridentate Hydrazone Ligands

The reaction of Co(NO3)2.6H2O and Ni(NO3)2.6H2O with hydrazones derived from 1-phenyl-3-methyl-4-acyl-5-pyrazolone (where acyl = acetyl, propionyl, butyryl and benzoyl) with 2-picolinic acid hydrazide have been studied and characterized on the basis of elemental analysis, magnetic moments, molar conductivity measurements, IR and electronic spectral studies and thermogravimetric analysis. Various ligand field parameters have been calculated. Electronic spectral data and the magnetic moment values suggest an octahedral structure for all cobalt(II) and nickel(II) complexes.


Introduction
In the last two decades, much interest has been focused on compounds containing hydrazide and hydrazone moieties and their complexes with first row transition metals [1][2][3][4][5][6] .Such interest has been growing due to their use in medicine 7 (for treatment of tuberculosis), biological systems 8 and analytical chemistry 9 .
The present study is concerned with the preparation of hydrazone ligands (Scheme 1) from 1-phenyl-3-methyl-4acyl-5-pyrazolone (where acyl = acetyl, propionyl, butyryl and benzoyl) with 2-picolinic acid hydrazide, which are capable of multidentate behavior by virtue of having a large number of donor atoms.Different modes of chelation are proposed.In view of this and the cobalt(II) and nickel(II) complexes of these hydrazone ligands have been synthesized and characterized.The results of these studies are presented in this paper.

Synthesis of hydrazone ligands
A quantity of 3.42 g (0.025 M) of 2-picolinic acid hydrazide was dissolved in ethanol (150 ml) and then 0.025 M of PMAcP [where Ac = acetyl (5.40 g), propionyl (5.75 g), bytyryl (6.10 g) and benzoyl (6.95 g)] was added in it.The reaction mixture was refluxed on a water bath for 1-2 h.The colour of the initial solution was changed.After cooling to room temperature a large amount of yellowish or yellowish orange precipitate was obtained.Vidyanagar.The infrared spectra were recorded as KBr pellets using a Perkin-Elmer 3010 spectrophotometer.The 1 H NMR spectra were recorded at RSIC, IIT, Mumbai on a Perkin-Elmer 300 MHz instrument using TMS as internal standard.Electronic spectra of the complexes were recorded on a Shimadzu UV-160 A spectrophotometer using quartz cell of 1 cm 3 optical path.

Results and Discussion
The hydrazone ligands used in the present investigation are the condensation products of 2picolinic acid hydrazide and respective PMAcP ligands (Where Ac=acetyl, propionyl, butyryl and benzoyl) in 1:1 molar ratio.The reaction leading to the formation of the hydrazone ligands can be represented according to the scheme-1 as shown above.The hydrazone ligands have been characterized by elemental analysis, i.r. and 1 H NMR spectral studies.Analytical and physical data are collected in Table 1.
The 1 H n.m.r.spectra of PHE, PHP, PHBy and PHBz in CDCl 3 and DMSO-d 6 show low field signals respectively at δ12.48, 12.56, 12.68 and 12.38 ppm for the -OH proton of pyrazoline ring.The -NH proton of hydrazide residue observed at δ11.57, 10.20, 10.00 and 10.03 ppm in PHE, PHP, PHBy and PHBz ligand respectively.In the n.m.r.spectra of PHE ligand, two methyl (-CH 3 ) protons are observed at δ 2.34 and 1.26 ppm for pyrazoline ring -CH 3 and 4-ethyl group -CH 3 respectively.In the spectra of PHP, PHBy and PHBz, pyrazoline ring -CH 3 proton is observed at δ 2.40, 2.44 and 1.62 ppm, respectively.Signals due to pyridine ring (C 5 H 4 N) protons occur as multiplets between δ 8.03 -9.10 ppm.

Characterization of metal complexes
Reaction of corresponding hydrazone ligands with metal(II) salts yielded complexes having 1:2 metalligand stoichiometries.Formation of the complexes has been shown below.
Where M = Co II and Ni II HL 1 = PHE, PHP and PHBy HL 2 = PHBz All the complexes are microcrystalline solids, stable at room temperature, non-hygroscopic.They are insoluble in water, sparingly soluble in common organic solvents but completely soluble in coordinating solvents like DMF and DMSO. 10 -3 M DMF solutions were subjected to conductivity measurements.The molar conductance values for Co II and Ni II complexes lie in the range 11.79-13.64 -1 cm 2 mol -1 and 11.89-14.58 - cm 2 mol -1 , respectively.These values suggest the non-electrolytic nature of all the complexes.

Magnetic moments and electronic spectra
The magnetic moments of cobalt(II) complexes with PHE, PHP, PHBy and PHBz ligands (Table 1) are respectively found to be 4.86, 4.90, 4.92 and 4.88 B.M. suggesting a high spin octahedral geometry with a very high orbital contribution attributable to the three fold degeneracy of the 4 T 1g (F) ground state term 13,14 .The electronic spectra of cobalt(II) complexes were recorded in DMF.Two distinct bands in the range 9327-9381 and 18761-18867 cm -1 were observed in the electronic spectra of cobalt(II) complexes (Table 3) attributable to the 4 T 1g (F) → 4 T 2g (ν 1 ) and 4 T 1g (F) → 4 T 2g (P) (ν 3 ) transition respectively in an octahedral field.Besides the d-d transitions, the band also observed at 25573-37735 cm -1 in the UV region, attributable to charge transfer transitions.
Important ligand field parameters are presented in Table 3. Racah interelectronic repulsion parameter (B), covalent factor (β 35 ) and 10Dq values were calculated using standard equation 15 .The ratio ν 3 (obs.)/ν 1 (obs.)and ν 3 (calc.)/ν 1 (calc.)were found to be in the range 2.00-2.02as required for octahedral cobalt(II) complexes.The reduction of the Racah parameter from the free ion value 971 cm -1 to 700.33 cm -1 and β value of 28% are taken as evidence of considerable covalence in the complex.
The electronic spectral data of nickel(II) complexes are given in Table 3.The electronic spectra of these paramagnetic complexes show two bands in visible region in the range 9832 -9861 cm -1 and 16528 -16806 cm -1 attributable to the 3 A 2g → 3 T 2g (ν 1 ) and 3 A 2g → 3 T 1g (F) (ν 2 ) transition respectively are in conformity with octahedral arrangements for Ni II ion and also show the bands in the UV region in the range 25641-26315, 28571-29411 and 32258-32786 cm -1 , attributable to charge transfer transitions.
The spectral bands are utilized to compute important ligand field parameter using the ligand field theory of spin allowed transition in d 8 configuration 16 .The value of 10 Dq and B are employed to calculate ν 2 and ν 3 (Table 3) leading to the following conclusion.Comparison of 10 Dq and B values for these Ni II complexes indicates that the ligands give reasonably strong fields and form strong covalent bonds.The high values of Dq and B are also consistent with coordination of azomethine nitrogen.The ratio of ν 2 /ν 1 , lies in the range 1.68-1.70expected for octahedral geometry for nickel (II) complexes 17 .Also, the present nickel(II) complexes exhibit magnetic moments in the range 2.84-2.87B.M. (Table 1) indicating an octahedral geometry 18 .

Infrared spectra
Some important bands observed (Table 2) in the infrared spectra of PHE, PHP, PHBy and PHBz and their metal complexes are considered for the identification of donor sites of the ligands.The IR spectra of the free ligands viz.PHE, PHP and PHBy, exhibit ν(N-H) absorbance bands at ca.3190 cm -1 and ν(C=0) bands at ca.1695 cm -1 indicating that the ligands exist in keto form in the solid state.However in solution, the ligands probably exist in equilibrium with tautomeric enol form.By the loss of proton, the enolic form may act as a uninegative ligand.The bands appearing in the spectra of ligands ~ 1695, 1640, 1535, 1015 cm -1 are attributed 19 to amide I [ν(C=O)], ν(C=N), amide II [β (N-H)] + (C-N)] modes respectively.

Cobalt(II) and nickel(II) complexes of PHE, PHP and PHBy
Bands due to ν(N-H) and ν(C=O) stretching vibrations are not observed in the spectra of complexes.Instead, they show new bands characteristic of ν(NCO) in the spectra of these complexes.The appearance of ν(NCO) stretching vibration in the spectra of these complexes suggest the presence of >C=N-N=C< residues of the stoichiometry and hence destruction of keto group via enolisation and bonding through resulting enolate oxygen.A strong band observed in the spectra of these ligands at 1640 cm -1 is shifted to lower wave number suggesting the participation of azomethine nitrogen in coordination 21 .In the spectra of free ligand, the band observed at ca.3440 and ca.1590 cm -1 are attributed to ν(O-H) and ν(C=N) 22 of pyrazoline ring, respectively.Which were remain unaltered in the spectra of complexes, indicating that non-participation of this groups.The appearance of bands at 875-870 and 688-680 cm -1 in spectra of the complexes attributed to ρ r (H 2 O) and ρ w (H 2 O), indicates the presence of coordinated water 23 .The non-ligand bands observed in the 530-470 and 460-420 cm -1 regions are tentatively assigned to ν(M-O) and (M-N) vibrational modes respectively.

Cobalt(II) and nickel(II) complexes of PHBz
The presence of ν(N-H) in the spectra of these complexes suggests that PHBz remain protonated in chelation.A band observed in the spectra of PHBz at 1628 cm -1 is shifted to lower wave number in the spectra of complexes suggesting the participation of azomethine nitrogen in bonding.Similarly, the red shift of the ν(C=O) band in the IR spectra of these complexes suggest the participation of carbonyl oxygen in complex formation.A broad band observed at 3410 cm -1 in the spectra of PHBz is due to ν(O-H) of the pyrazoline ring.This band was disappeared in the spectra of these complexes suggesting enolisation exist at -OH group of the pyrazoline ring by replacing hydrogen forming covalent bond with the metal ions.This may be due to loss of two H 2 O molecules.In the temperature range 226-310 0 C shows an exothermic peak was observed at 292 0 C showing a 4.75% loss in weight of the compound.This may be due to loss of the -OH group of the pyrazoline ring.On increasing the temperature further, the residue decomposes, resulting in a loss of 46.08% in the temperature range 310-545 0 C with exothermic peak at 365 0 C, due to the loss of pyrazoline ring moiety and a loss of 34.12% in the temperature range 545-787 0 C with exothermic peak at 695 0 C due to loss of pyridine ring moiety and methyl group (Table 4).No loss in weight was observed beyond 740 0 C, indicating the formation of stable metal oxide.
The DTA thermogram of [Ni(PHBy) 2 .(H 2 O) 2 )] in the temperature range 57-193 0 C shows an endothermic peak at 174 0 C accompanied by a weight loss of 4.42%.This may be due to loss of two water molecules.In the temperature range 193-358 0 C with endothermic peak at 321 0 C showing a 34.94% loss in weight of the compound.This may be due to loss of the pyrazoline ring moiety.Two exothermic peaks at 387 and 605 0 C shows loss in weight of 27.86 and 23.36% which may be accounted for by considering the loss of pyridine ring moiety and (CH 3 -CH 2 -CH 2 -C=N-, -CH 3 and -OH groups), respectively (Table 4).On increasing the temperature further, no loss in weight occurred up to 800 0 C which indicates the formation of a stable metal oxide.

2 ( 5 Figure 2
Pyrazoline ring + >C=N-) 2 (C 5 H 4 NCON + -CH 3 ) a= endothermic, b= exothermic peak On the basis of analytical and physical data, the probable structure of CO II and Ni II complexes as shown in Figure.1.Where M = Co II or Ni II R = -CH 3 , -CH 2 -CH 3 , -CH 2 -CH 2 -CH 3 The probable structure of the complexes The precipitate was filtered and washed with excess of ethanol and dried over fused CaCI 2 .

Table 1 .
Physical and analytical data of hydrazone ligands and its Co II and Ni II complexes

Table 2 .
Important IR spectral data (cm -1 ) of hydrazone ligands and its Co II and Ni II complexes The DTA thermogram of [Co(PHE) 2 .(H 2 O) 2 ] in the temperature range 53-226 0 C shows an endothermic peak at 110 0 C accompanied by a weight loss of 4.35% as shown by the TG thermogram.
a cyclic; b azomethine stretching Thermal analysis

Table 3 .
Electronic spectral data and ligandss field parameters of Co II and Ni II hydrazone complexes Ratio of the complex and free ion, B 0 (%) ** = Percentage covalency and δv *** = Difference in observed and the calculated values Synthesis, spectral, magnetic and thermal studies * =

Table 4 .
Thermo analytical data of some Co II and Ni II complexes