Di- and poly-nuclear zinc(II) Schiff base complexes: synthesis, structural studies and reaction with an α-amino acid ester

Abstract

In order to model peptide hydrolysis by dimetallic aminopeptidases, the dinuclear zinc(II) Schiff base complex [Zn₂L¹(CH₃CO₂)₂]ClO₄1a [HL¹ = 2,6-bis{N-[2-(dimethylamino)ethyl]iminomethyl}-4-methylphenol] has been prepared and characterised by X-ray crystallography. The zinc ions are bridged by the deprotonated phenolic oxygen of L¹ and two acetate groups, the Zn⋯Zn distance being 3.234(1) Å. Complex 1a was shown to promote hydrolysis of glycine ethyl ester under mild conditions. Reaction of the hydrolysis product glycine with 1a led to the conversion of the ligand into L² (H₃L² = corresponding Schiff base derived from glycine). The X-ray analysis of the resulting zinc complex 2 revealed the novel pentanuclear complex [{Zn₂L²(CH₃CO₂)₂}₂Zn(H₂O)₄]·4.5H₂O, and the crystal structure of polymeric {[Zn(HL²)]·3H₂O}_∞ was determined. Complex 2 is built up of two dinuclear acetate-bridged Zn₂/ligand moieties that are linked through bridging carboxylate functions of the ligand to the fifth Zn atom. In the polymer one co-ordination site of the binucleating ligand is occupied by a Zn, while the second one is protonated.