Synthesis and characterization of nickel(II) complexes with cyclic and linear Schiff-base ligands incorporating sulphur, nitrogen, and oxygen donor atoms

Abstract

A series of nickel(II) complexes containing macrocyclic and linear Schiff-base ligands has been prepared by condensation of 1,2-diaminoethane, 1,2-diaminopropane, 1,3-diaminopropane, diethylenetriamine, or triethylenetetramine with the thiodiketone, 4-thiaheptane-2,6-dione (D) in the presence of nickel(II) salts. The ligands could not be synthesized in the absence of nickel(II) ions. In the case of 1,2-diaminoethane (en) a purple product and a reddish brown product are formed depending on the reaction conditions. A single-crystal X-ray analysis of the purple product [NiL(en)][NO₃]₂[L is the linear Schiff base, NH₂(CH₂)₂NC(CH₃)CH₂SCH₂COCH₃] indicates a highly distorted octahedral stereochemistry involving co-ordination of N, N, S, and O atoms of the new quadridentate ligand L. The remaining two positions of the octahedron are occupied by en. The nitrate groups are non-co-ordinating. The reddish brown product is [NiL¹]X₂(L¹ is the macrocyclic sexidentate ligand formed by cyclocondensation of L, X = NO₃^– or Cl^–). Complexes of macrocyclic ligands L² and L³, [NiL²]X₂ and [NiL³(CH₃OH)][NO₃]₂, were obtained by 2 + 2 (L²) and 1 + 1 (L³) cyclocondensations of 1,3-diaminopropane and triethylenetetramine respectively with D in the presence of nickel(II) nitrate or chloride. Diethylenetriamine and 1,2-diaminopropane undergo 1 + 1 (L⁴) and 2 + 2 (L⁵) condensations with D in the presence of nickel(II) salts to form complexes of open-chain Schiff-base ligands, [NiL⁴(C₂H₅OH)][NO₃]₂ and [NiL⁵][NO₃]₂ respectively. The failure of ring closure in these cases may be attributed to steric hindrance and/or mismatch of the sizes of the cavities and the nickel(II) ions. From their i.r. and electronic spectra, and the results of electrical conductivity and magnetic susceptibility measurements, these complexes have been assigned distorted six-co-ordinate structures.