Studies on Heterobinuclear Complexes Derived from Rubidium and Caesium Salts of Acetyl Salicylic Acid (ASPIRIN) with N, N’-1,2-ethylenebis(5-nitrosalicylaldiminato) Nickel(II)

Hetero binuclear complexes, [NiE5NSRbAp] and [NiE5NSCsAp], have been synthesized from Rubidium(I) and Caesium(I) salt of Acetyl salicylic acid (ASPIRIN) and N,N’-1,2-ethylenebis (5-nitrosalicylaldiminato) nickel(II) as ligand, respectively. This ligand [NiE5NS] has been prepared from Nickel(II) acetate and salen type dibasic Schiff base derived from 5-nitrosalicylaldehyde and Ethylenediamine. These novel hetero binuclear complexes obtained in pure solid state and their structures have been characterized with the help of Elemental analysis, Solubility, Magnetic property, Molar conductance measurement, FT-IR and Electronic absorption spectra. The results suggested molecular formula [NiLRbL’].3H2O and [NiL(H2O)2CsL’] with Square planar and Distorted octahedral geometry about Ni2+, respectively for the two hetero binuclear complexes [NiE5NSRbAp] and [NiE5NSCsAp] where L = Deprotonated Schiff base ligand N,N’-1,2-ethylenebis (5-nitrosalicylaldiminato) nickel(II), L’ = Acetyl salicylate.


INTRODUCTION
Deprotonated salen type Schiff bases act as tetradentate N 2 O 2 type ligand and form complexes with transition metals. It is well investigated that donor oxygen atoms of these N 2 O 2 type ligands have ability to co-ordinate with two metal ions. [1][2][3] This ability of the phenolic donor oxygen atoms of such ligands, has been used to synthesize hetero binuclear complexes holding two metal ions in close proximity-one of transition metal and other alkali metal. 4,5 Schiff base transition metal complexes have a great medicinal values and industrial applications 6 , and metal ions play an important role in biological systems 7 . Greater permeability of metal complexes than metal ions further augments their importance. Numerous hetero binuclear complexes have been reported in past several years but a very little work on hetero binuclear complex with alkali metal as one of the metal centres has been published. Acetyl salicylic acid (ASPIRIN) is a common drug and Rubidium or Caesium ions are handled by living organisms similar to Potassium. In the present paper synthesis and characterisation of hetero binuclear complexes derived from Rubidium(I) and Caesium(I) salt of Acetyl salicylic acid (aspirin) and N,N'-1,2ethylenebis(5-nitrosalicylaldiminato) nickel(II) as ligand, have been discussed. These studies may be useful for carrying out further research to design and develop drug from these synthesized complexes or their modified forms for therapeutic use in order to minimize side effects of Aspirin.

Synthesis of Hetero binuclear complexes from Rubidium and Caesium salt of Acetyl salicylic acid and Nickel(II) Schiff base
Ethanolic solutions of Rubidium or Caesium acetyl salicylate and Nickel(II) Schiff base in 1:1 stoichiometric proportion were mixed together, and the resulting solution was refluxed for 25 min at about 40 o C and cooled to ice temperature when coloured hetero binuclear complexes [NiE5NSRbAp] and [NiE5NSCsAp] were separated out. The complexes were filtered, washed with little ethanol and dried.

Physical properties
The synthesized hetero binuclear complexes are crystalline solid, coloured and nonhygroscopic. The complex [NiE5NSRbAp] is soluble in Petroleum ether, DMF and DMSO whereas [NiE5NSCsAp] is soluble in Acetone, DMF and DMSO. However, both the complexes are insoluble in water. Both the complexes do not decompose up to 250 o C Table 1.

Elemental Analysis
The estimated elemental data of the complexes are in good agreement with calculated value Table 1.

Molar Conductance Measurement
Molar conductance of 10 -3 M solution of the synthesized complexes in DMF was measured at 20(±0.5) o C, and is found between 7.6-7.90 S.cm 2 . mole -1 Table 1. The low molar conductance value (up to 10.1 S.cm 2 .mole -1 ) of the synthesized complexes suggests their non-electrolytic nature 9-13 .  [15][16][17] . Presence of new band at 663 cm -1 is for ν M-N stretching vibrations and sharp, medium to weak bands in the range 500-700 cm -1 is for nitrogen-metal (ν N-M ) stretching vibrations 18,19 . The absence of sharp characteristic absorption band in the region 3600-3700 cm -1 due to phenolic O-H stretching vibrations and also the absence of broad band in the region 3200-3300 cm -1 due to intermolecular hydrogen bond, indicates deprotonation during formation of nickel complex of Schiff base   Table 3.
A very strong band at 1649 cm -1 in [NiE5NS] spectrum is for ν C=N stretching 14 . The shift of the absorption band of ν C=N stretching towards lower wave number indicates coordination of the

Electronic Absorption Spectra
Band positions in UV-Visible spectra of the hetero binuclear complexes are summarised in Table 2. Study of complex [NiE5NSRbAp] spectrum excludes possibility of square pyramidal and octahedral geometry around Ni 2+ ion as no bands are present between 500-1000nm. Further appearance of no band above 1000nm excludes possibility of tetrahedral geometry around Ni 2+ ion. For a square planar geometry around Ni 2+ ion there must be appearance of two bands in region 420-500 nm assignable to 1 A 1g → 1 A 2g and 1 A 1g → 1 B 1g transitions 25 . In this case these two bands have been obscured by the CT band at that region. Hence, in the complex [NiE5NSRbAp] there is square planar geometry around Ni 2+ ion. This is further supported by zero magnetic moment of complex [NiE5NSRbAp] which indicates its diamagnetic nature and low spin complex.
In the [NiE5NSCsAp] spectrum, appearance of a band at 824nm excludes possibility of square pyramidal and square planar geometry around Ni 2+ ion. Further appearance of no band above 1000nm excludes possibility of tetrahedral geometry around Ni 2+ ion. Presence of an absorption band in the region 700-1000nm confirms that the coordination number of Ni 2+ in the adduct [NiE5NSCsAp] is 6 with octahedral geometry. For an octahedral geometry around Ni 2+ ion there must be appearance of three bands in region 300-450nm, 450-700nm and 700-1000nm assignable to 3 A 2g (F) → 3 T 2g (F), 3 A 2g (F) → 3 T 1g (F) and 3 A 2g (F) → 3 T 1g (P) transitions, respectively. Shoulder at 353nm is due to 3 A 2g (F) → 3 T 2g (F) d-d transition and band at 824nm is due to 3 A 2g (F) → 3 T 1g (P) d-d transition 26,27 . In this case one absorption band corresponding to 3 A 2g (F) → 3 T 1g (F) d-d transition has been obscured by the CT band at that region 28 Fig.1 and Fig. 2, respectively.