Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) Complexes with Multidentate Schiff Bases

Complexes of the type [M(L)2.2H2O]X2 (where M=Co(II) or Ni(II), X=Cl or NO3), [M(L)2Cl2] (where M=Cu(II),Zn(II) or Hg(II)) and [Cd(L)2](NO3)2 where L= multidentate Schiff bases ligands (resulted from the condensation of 2,3-butandione or benzil with 2-aminothiazole) 2-oxo-3-imino-(N-thiazole-2-yl) butane (atc) and 1-oxo-1,2-diphenyl-2imino-(N-thiazole-2-yl) ethane (atb), have been prepared and characterized by elemental analysis, spectral (Infrared, and U.V./Visible), magnetic data and conductivity measurements. The ligands have been acted as bidentate with Co(II), Ni(II), Cu(II), Zn(II), Hg(II) and tridentate with Cd (II) ions. The prepared complexes had hexacoordinated with octahedral geometries. Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) Complexes ... 35 Introduction Although the subject of Schiff bases has been extensively studied, their transition and non-transition metal complexes have been still found to be of great interest in coordination chemistry, since they exhibited interesting variety of sterochemical behaviors and biochemically relevant studies of metal complexes and found a wide range of applications. Schiff bases of hetrocyclic moiety like trizine, pyridine, thiazole have great interest of many authors , because they possess interesting biological activity due to the strong aromaticity of their ring system, which leads to great in vivo stability. The present study describes the synthesis and characterization of six metal complexes (Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II)) with Schiff’s bases derived from 2-aminothiazole with 2,3-butanedione and benzil in methanol. Materials and Methods Analysis and physical measurements: The complexes and the ligands were analyzed for carbon, hydrogen and nitrogen using an 1106 (Carlo Erba) micro analyzer. Conductance measurements of the complexes were carried out at room temperature (~ 300k) in DMSO solution (10M) with a conductivity meter model 4070 (JENWAY). Magnetic measurements were made at room temperature by the Faraday method using Bruker B.M.6. Infrared absorption spectra have been recorded using a Unicam SP-2000 spectrophotometer as KBr discs in the range (200-4000 cm). The electronic spectral measurements were recorded using Shimadzu U.V./visible spectrophotometer (200-1100nm), model 160 Shimadzu (Japan), using DMSO as a solvent. All the chemicals were of Analar grade (B.D.H, Aldrich or Fluka) used as supplied Preparation of Schiff base ligands 1) 2-oxo-3-imino-(N-thiazole-2-yl) butane (atc): It has been prepared by mixing (2.0 gm,0.02 mole) 2-aminothiazole with (1.72 gm, 0.02 mole) 2,3-butanedione in methanol and the mixture was refluxed for 1 hr. on a water bath. The hot solution was poured into ice cold water, brown crystalline precipitate of the Schiff base was separated out. It has been filtered off, washed with methanol and dried in vacuum.


Introduction
Although the subject of Schiff bases has been extensively studied (1) , their transition and non-transition metal complexes have been still found to be of great interest in coordination chemistry (2,3) , since they exhibited interesting variety of sterochemical behaviors and biochemically relevant studies of metal complexes and found a wide range of applications (4,7) .
Schiff bases of hetrocyclic moiety like trizine, pyridine, thiazole have great interest of many authors (8,9) , because they possess interesting biological activity due to the strong aromaticity of their ring system, which leads to great in vivo stability (10) .

Analysis and physical measurements:
The complexes and the ligands were analyzed for carbon, hydrogen and nitrogen using an 1106 (Carlo Erba) micro analyzer.Conductance measurements of the complexes were carried out at room temperature (~ 300k) in DMSO solution (10 -3 M) with a conductivity meter model 4070 (JENWAY).Magnetic measurements were made at room temperature by the Faraday method using Bruker B.M.6.Infrared absorption spectra have been recorded using a Unicam SP-2000 spectrophotometer as KBr discs in the range (200-4000 cm -1 ).The electronic spectral measurements were recorded using Shimadzu U.V./visible spectrophotometer (200-1100nm), model 160 Shimadzu (Japan), using DMSO as a solvent.
All the chemicals were of Analar grade (B.D.H, Aldrich or Fluka) used as supplied

Preparation of Schiff base ligands 1) 2-oxo-3-imino-(N-thiazole-2-yl) butane (atc):
It has been prepared by mixing (2.0 gm,0.02mole) 2-aminothiazole with (1.72 gm, 0.02 mole) 2,3-butanedione in methanol and the mixture was refluxed for 1 hr. on a water bath.The hot solution was poured into ice cold water, brown crystalline precipitate of the Schiff base was separated out.It has been filtered off, washed with methanol and dried in vacuum.

Preparation of the complexes:
The metal salt was dissolved in a minimum amount of methanol.The ligand was also dissolved in a minimum amount of methanol (Table 1).The two solutions were mixed with continuous stirring until the product appears.The resulting mixture was filtered, washed with petroleum spirit b.p.= (60-80 o ) and dried under vacuum for several hours.
Thus these metals were prefered the more base nitrogen although 7membered ring is formed. (20)or Zn (II), Hg (II) complexes the υ C=N and υ C=Nring were shifted to lower frequencies by (45-60 cm -1 for atc complexes and by 50cm -1 for atb complexes) (55-60 cm -1 for atc complexes and by 45-50cm -1 for atb complexes) respectively, indicated that the coordination took place through two nitrogen, while υ C=O band was not shared in the coordination. (21)24)(25)(26) The absorption bands of M-N ring group for atc complexes were in the range (265-285 cm -1 ) (16,27) , whereas the expected regions for υM-N ring of the atb complexes with their metals appeared at a point lower than (220 cm -1 ) (28,29) .These bands could not be noticed practically because they are near the end of the used instrument range.
One broad band was observed in case of Cu (II) complexes at (15347 and 15198 cm -1 ) region.This broad band belongs to 2 Eg 2 T 2 g transition, indicating octahedral configuration for these complexes. (24,25)

Conclusions:
Different modes of interactions of metal ions with the donor sites of the ligands would occur, which may be depends on the ionic size of the metals.Zinc and mercury complexes coordinated through the two nitrogen forming 4-membered rings (Figure 5).Cadmium coordinated through the three active sites of the ligands forming 5 and 4-membered rings (Figure 6).While in Co (II), Ni (II), and Cu (II) complexes, the metal ions coordinated through carbonyl oxygen and thiazole nitrogen as supported by IR spectroscopic data (Table 3), but this result in 7membered ring (Figure 3, 4) which are rarely occurred.This may be depends on the basicity of the donor atoms in which the imine nitrogen seems to be the lowest one (20) .The thiazole nitrogen is more basic than imine nitrogen since it belongs to the thiazole group (21) , because the coordination through the thiazole nitrogen pulls the electronic density from C=N bond of the ring as well as the azomethine nitrogin (38) as explained bellow: For Co (II) and Ni (II) complexes the molar absorptivity values, position of absorption band and magnetic moment values indicated high spin octahedral geometry.
All metals complexes that have been studied indicated six coordinated and had an octahedral environment around the metal ions.