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.