Formation of axial phenolate–metal bonds in square-pyramidal complexes

Abstract

Copper(II) complexes derived from tripodal ligands capable of forming 5,6,6- or 5,5,6-membered chelate ring sequences have been synthesised and characterised. The crystal structures of two complexes [CuL-(O₂CBu^t)]·MeCN [HL =(2-hydroxy-5-nitrophenylmethyl)(pyridin-2-ylethyl)(pyridin-2-ylmethyl)amine] and [CuL(O₂CMe)]·H₂O [HL =(2-hydroxy-5-nitrophenylmethyl)bis(pyridin-2-ylmethyl)amine] have been solved. They are neutral, mononuclear copper(II) species in the solid state. In contrast to copper(II) complexes derived from related tripodal ligands forming 6,6,6-membered chelate rings, the present complexes have an axial phenolate–copper(II) bond in their square-pyramidal structures. The formation of this bond is related to the steric factors arising from the flexibility of the ligand pendant arms. The complex [CuL(O₂CMe)] exhibits structural features related to the biosite in galactose oxidase; an acetate co-ordinates to copper equatorially and a phenolate oxygen atom occupies the axial position.