Synthesis and Structural Elucidation of Complexes of Some 3d-Series Divalent Transition Metals with 2-hydroxy-4-nitro acetophenone hydrazone, Schiff Base Ligand

Introduction

Schiff’s bases are organic compounds having an azomethine group ( >C=N).Various studies^1-2 have shown that >C = N group has considerable biological importance^3-5. A broad spectrum of biological potential is reported to be associated with a number of Schiff’s bases^6-8. Many evidences show that biological activity of a Schiff’s base enhances many fold on its co-ordination with suitable metal ions^9-17. Due to this interesting structural features and important applications we have undertaken the study of synthesis and characterization of complexes of Co(II), Ni(II) and Cu(II) with Schiff’s base.

Experimental

Preparation of ligand

3 gms of hydrazine hydrochloride was dissolved in dil. HCl and  6 gms of 2 – Hydroxy – 4 – nitroacetophenone was dissolved in methanol. Both the solution were mixed together and refluxed for an hour. Yellowish coloured precipitate appeared on cooling the  solution. The crude product was recrystallised from alcohol and melting point was recorded (M.P = 480 K). The pure dried compound was chemically analysed (Table – 1). The compound is soluble in alcohol but insoluble in acetone and water.

Preparation of complexes

Aqueous solution of chlorides of divalent metals (cobalt, nickel and copper) was prepared. Ligand was dissolved in methanol. Aqueous solution of metal chloride was mixed with methonolic solution of ligand and the mixture was refluxed on water bath for about three hours. The cold mixture was again refluxed with 2N ammonium hydroxide for an hour.On cooling the mixture metal complex is precipitated. The precipitate was washed dried and analysed chemically (Table – 1).

Table 1: Elemental Analysis Data of Ligand and Complexes

Name of compound	SYMBOL OF ELEMENTS
	C	H	N	O	M
Ligand (HNAPH)	49.32(49.23)	4.48(4.61)	21.46(21.53)	(24.74(24.61)	—
[Co(HNAPH)3]	44.48(44.51)	3.65(3.71)	20.42(20.40)	22.25(22.26)	9.20(9.12)
[Ni(HNAPH)3]	44.83(44.92)	3.65(3.74)	19.38(19.65)	22.94(22.46)	9.20(9.23)
[Cu(HNAPH)2]	42.45(42.52)	3.50(3.54)	18.42(18.60)	21.53(21.25)	14.10(14.06)

Results

Solubility

All the complexes of cobalt, nickel and copper are soluble in DMF but insoluble in water. Cu(II) complex dissolves slightly in ethanol whereas complexes of cobalt and nickel are insoluble in methanol, ethanol and acetone.

Theoretical value is given in parenthesis

Table 2: Decomposition temperature, magnetic moment and electrical conductivity

Complex	Decomposition temperature	Magnetic moment in (BM)	Molar electrical conductivity (Ohm-1)
[Co(HNAPH)3]	573 K	4.9	17.4
[Ni(HNAPH)3]	523 K	3.01	18.9
[Cu(HNAPH)2]	513 K	1.82	13.8

Table 3: Electronic spectral data of complexes 

Complex	Electronic spectra,  d-d transition bands (in cm-1)	Assignment
[Co(HNAPH)3]	9500	4T1g – 4T2g (F)
	19495	4T1g – 4A2g (F)
	22315	4T1g – 4T1g (P)
[Ni(HNAPH)3]	11240	3A2g (F) – 3T2g (F)
	15995	3A2g (F) – 3T1g (F)
	24685	3A2g (F) – 3T1g (P)
	10750	Spin forbidden transition
[Cu(HNAPH)2]	13845 – 14000	3Eg – 3T2g

Table 4:

Position of absorption bands (cm-1)				Assignment
Ligand(HNAPH)	[Co(HNAPH)3]	[Ni(HNAPH)3]	[Cu(HNAPH)2]
3420	3390	3400	3400	NH2 stretching
3195				O – H (Hydrogen bonded)
3140	3140	3140	3140	C – H stretching
3075	3080	3080	3080	> N – H stretching
2980			2980	C – H (Aromatic) stretching
2750				C – H stretching
2460	2460	2460	2460	Skeletal vibrations
2200	2200	2200	2200
1910	1900	1900	1900
16501620	16751630	16701625	16651635	NH2, > N – H bending
1590	1570	1565	1555	C = N stretching
1405	1395	1395	1395	C – N stretching
1260	1300	1290	1295	C – O (Phenolic)
920	920	920	920	Characteristic vibrationsof  1,2,4 – tri – substitutedbenzene ring
840	840	840	810
780	780	780	785
—	580	510	510	M – N bond
	490	475	470	M – O bond

Discussion

Structure of ligand

The interpretation of I. R. spectra is quite complicated due to the presence of various similar groups and hence many absorption bands. However, comparison of the spectral bands of the ligand (HNAPH), with those of its complexes (Table – 4), gives some important information regarding the nature of the ligand as well as the co-ordination sites through which metal ion co-ordinate with the ligand.

The broad band at 3195 cm^-1 in the ligand assignable to phenolic O – H (hydrogen bonded) stretching frequency disappears in the Ni (II), Co(II) complexes showing deprotonation of phenolic protons through complexation¹⁸. The ligand also shows strong bands near 1260 cm^-1 which may be attributed to the phenolic vibration. A shift of this band to higher frequency (1300,1290&1295 cm^-1) in the complexes indicated chelation of the ligand to metal ion through phenolic oxygen.

The ligand exhibit a band near 3420 cm^-1 and 3075 cm^-1 which are assigned to NH₂ and > N – H stretching¹⁹ respectively, does not change appreciably in the complexes indicating non – involvement of > N – H and NH₂ nitrogen atom in chelate formation. Bands at 2460 cm^-1, 2200 cm^-1, 1900 cm^-1, 920 cm^-1, 840 cm^-1 and 780 cm^-1 are the characteristic bands of the compound which remain unaffected in complex formation.

In these cases of metal complexes, the most interesting feature noted is about the doublet band²⁰ observed in the I. R. spectrum of the ligand near 1620 cm^-1. The higher one goes upto 1650 cm^-1 to 1675  cm^-1 probably due to the bending mode of the NH₂ group which remain practically unaffected and shows that the NH₂ and > N – H mostly does not take part in the co-ordination. The other component which is due to C=NH group stretching frequency shifts to the lower frequency about (~1570 cm^-1, 1565 & 1555 cm^-1) in complexes, indicating the co-ordination through C = N (imine) group. In the case of Ni (II) and Cu(II) it has been observed that as the reaction is carried out at higher p^H, co-ordination of Ni(II) and Cu(II) through C = N group, takes place by deprotonation. Martin and co-workers²¹ have also reported the same results.

A sharp band at 1590 cm^-1 observable in the free ligand is assigned to C = N vibration of Schiff’s base residue. This band shifts to the lower frequencies in the complexes, which indicates that azomethine nitrogen of C = N takes part in co-ordination.

The due to nitro group, appearing in the region 1405 cm^-1 in the free ligand, remains unaltered in all the complexes, suggesting non-participation of nitro group in co-ordination.

Thus 2 – Hydroxy – 4 – nitro acetophenone hydrazone (HNAPH) behaves as a bidentate ligand co-ordinating through phenolic oxygen ( C – O), nitrogen atom of  C = N, of azomethine group and nitrogen atom of C=N, of imine group. It is further confirmed with experimental report.

It is noteworthy that the ligand behaves as monoprotonic for all the metal ions under investigation except for Ni (II) and Cu (II) when complexation is carried out at higher p^H (p^H = 9). In the later cases ligand behaves like biprotonic involving the deprotonation of C = NH (imine) proton. The structure of ligand may be represented as

Figure 1 : 2 – Hydroxy – 4 – nitroacetophenone . Click here to View figure

 

Structure of complexes

The diffuse reflectance spectra of cobalt complex gives three d-d transition bands which have been assigned to ⁴T_1g (F) ⁴T_2g (F), ⁴T_1g (F) ⁴A_2g (F) and ⁴T_1g (F) ⁴T_1g (P) and suggest an octahedral geometry²². Linkage of metal ions with the ligand through oxygen and nitrogen is further confirmed by the presence of I.R. bands. The room temperature magnetic moments of these Co(II) complexes around 4.9 BM suggest presence of  three unpaired electrons and outer orbital octahedral structure for its complex. Molar conductance value suggest non-electrolytic nature of the complex. Taking all these facts into account an octahedral structure has been given to this complex.

Ni(II) complex show four bands in their reflectance spectra favouring an octahedral stereochemistry. Out of these four bands, three can be assigned to spin allowed d-d transition eg ³A_2g (F) – ³T_2g(F) ,³A_2g (F) ³T_1g (F) and ³A_2g (F) – ³T_1g (P).

The fourth band near 10750 cm^-1 is probably spin forbidden and may be assigned to ² B_1g² Eg transition.

The effective magnetic moment value (2.94 BM) are in good agreement with an octahedral geometry having two unpaired electrons.

Electrical conductivity data indicates non-electrolytic nature of nickel complex.

Copper (II) has  configuration which makes Cu (II) subject to John – Teller distortion, if placed in an environment of cubic like regular octahedral or tetrahedral symmetry and this has a profound effect on all its stereochemistry^{23 -24}.

The typical distortion is an elongation along one four fold axis, show that there is planar array of four short Cu-L bonds with two trans long ones. In the limit of course, the elongation leads to a situation indistinguishable  from square co – ordination as found in many discrete complexes of Cu (II) . Thus the cases of tetragonally distorted “Octahedral” co – ordination and square co – ordination cannot be sharply differentiated.

Figure 2 : Tris ( 2 – hydroxy – 4 – nitro acetophenone hydrazone) cobalt (II) Click here to View figure

 

Figure 3 : Bis – ( 2 – hydroxy – 4 – nitro acetophenone hydrazone) Cu (II). Click here to View figure

 

Because of the relatively low symmetry of the environments in which the Cu²⁺ ion is characteristically found, detailed interpretation of spectra and magnetic properties are somewhat complicated even though one is dealing with the equivalent of a one – electron case^25-28.

The complex [Cu (HNAPH)₂],  show a band at 2000 cm^-1 in their electronic spectra, which suggest their square planar geometry, assigned to the overlap of  ²B_1g®²A_1g, and ²B_2g²E_g transitions²⁹ .

The magnetic moment of these complexes correspond well with the presence of one unpaired electron and gives a specific information about their stereochemistries.

The molar conductance values suggest non – electrolytic nature of these complexes.Taking all these facts into consideration, along with their elemental analysis the most probable structures of these complexes are given below. These results are in conformity with our observations reported earlier.

Graph 1Click here to View graph

 

Graph 2 : 2 – Hydroxy – 4 – nitroacetophenone. Click here to View table

 

Graph 3:  [ Co(HNAPH)3],  (b) : [Ni(HNAPH)3] ,   (c) : [Cu(HNAPH)2].   Click here to View garph

 

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