Synthesis, reactions and copper(II), nickel(II), and zinc(II) complexes of N-(2-pyridinyl)ketothioacetamides

Abstract

The synthesis and properties of two N-(2-pyridinyl)ketothioacetamide ligands, HL [(4)], and their complexes with nickel(II), copper(II), and zinc(II) are described. The ligands are potential O,S,N donors and are sulphur analogues of the O,O,N donor N-(2-pyridinyl)ketoacetamide ligands (1). The failure of certain thionation methods in the preparation of (4) from (1) is demonstrated. Ligands (4) show analogous spectroscopic properties to those of (1) and display thioketo–enol tautomerism in solution but readily oxidize to the 2H-pyrido[2,3-a][1,2,4]thiadiazoles (5). This redox activity is shown to be decreased at low temperatures and eliminated in the N-benzyl-N-(2-pyridinyl)ketothioacetamides (6), the N-benzylated derivatives of (4). Complexes of the type [ML₂](M = Ni^II, Cu^II or Zn^II) display spectral and magnetic properties typical of four-co-ordinate complexes in which co-ordination of the ligands is via the S,N donor sets with the O donor remaining unco-ordinated. E.s.r. spectra of a [CuL₂] complex doped into [NiL₂] and [ZnL₂] lattices show marked differences depending on the host, with the former being typical of planar co-ordination while the latter gives a rather unusual lineshape of distorted tetrahedral geometry.