Polynuclear zinc(II) complexes of phenol–imine and –amine macrocycles

Abstract

The template condensation of 2,6-diformyl-4-methylphenol with 1,11-diamino-3,6,9-trioxaundecane using zinc(II) acetate as the metal source, followed by addition of an excess of NaClO₄, gave a tetranuclear zinc(II) complex [Zn₄L¹(µ-O₂CMe)₄][ClO₄]₂1, where H₂L¹ is a [2 + 2] tetra-Schiff-base macrocycle. A single-crystal X-ray analysis of the dichloromethane adduct, 1·2CH₂Cl₂, revealed that this complex cation is centrosymmetric and contains two pairs of zinc ions bridged by a phenolic oxygen and two acetate ligands. Recrystallisation of this complex from a hydrous methanolic solution gave another tetranuclear complex, [Zn₄L¹(µ-OH)₂(µ-O₂CMe)₂][ClO₄]₂·2H₂O 2, which was also structurally characterised. While the four zinc ions are encapsulated in the macrocycle as observed for 1, in complex 2 the two dimeric units formed by phenoxo- and acetato-bridges are further connected by two hydroxyo bridges. Proton NMR spectroscopy has shown that this structural conversion is reversible and depends on the presence of water or acetic acid in solution. Treatment of complex 1(or 2) with NaBH₄ in methanol gave a dinuclear zinc complex, [Zn₂L²][ClO₄]₂·H₂O3, together with a metal-free phenol–amine macrocycle, H₂L², which is a reduced form of H₂L¹. Both complex 3 and H₂L², have been characterised on the basis of spectroscopic data.