Binding of bi- and tri-valent nickel by azophenolates incorporating thioether/ether donor sites

Abstract

Nickel(II) complexes of tri- and hexa-dentate ligands in which the donors are azo nitrogen, phenolic oxygen and thioether sulfur or ether oxygen have been isolated. Structure determination of three complexes has established the presence of distorted octahedral NiN₂O₂X₂(X = S or O) co-ordination spheres. In dichloromethane solution the nickel(III)–nickel(II) couple is electrochemically observable for the thioether complexes with E_½ in the range 0.65–0.85 V vs. saturated calomel electrode. The blueviolet nickel(III) species can be quantitatively electrogenerated in solution. Upon freezing (77 K) axial EPR spectra (g_∥≈ 2.18, g_⊥≈ 2.06) compatible with the uncommon hole configuration (d_x²–y²)¹ are observed. The ether complexes are more difficult to oxidise and the nickel(III) species are not tractable. This is consistent with the higher strength of nickel–thioether compared to –ether binding.