Synthesis and characterisation of some cobalt(II) complexes of Schiff-base ligands and their reactions with molecular oxygen. Effects of chelate ring size

Abstract

A series of potentially tetradentate Schiff-base ligands, for example N,N′-ethylenebis(1-hydroxy-2-naphthylmethyleneimine)(H₂napen), in which the methylene linkage between the imine donors varies from C₂ to C₆, have been complexed to cobalt(II). Two types of complex are formed: CoL(H₂O)_n(n= 0, complex is planar; n= 2, complex is pseudo-octahedral) in which the ligand binds as a dianionic tetradentate N₂O₂ donor, and Co(H₂L)Cl₂, which are pseudo-tetrahedral and the ligand is neutral, binding through the imine groups only. The structure seems to be mainly dependent on the length of the methylene chain of the ligand. The reversible reaction of [Co(napen)] with dioxygen in N,N-dimethylformamide has been studied at a number of temperatures and thermodynamic data and binding curves obtained; the enthalpy of reaction (ΔH^⊖=–78.8 kJ mol^–1) is significantly different from that of the well studied [Co(salen)][H₂salen =N,N′-ethylenebis(salicylideneimine)] in dimethyl sulphoxide (ΔH^⊖=–66.9 kJ mol^–1). The reaction of the other complexes with dioxygen crucially depends upon the complex geometry and its physical state (solution or slurry). Irreversible oxidation of cobalt(II) to cobalt(II) is observed for a number of complexes.