Anion-templated formation of supramolecular multinuclear assemblies

Lewis acidic zinc(II)-based salphen complexes have been found to interact strongly with various anions both in solution as well as in the solid state. A biologically relevant model anion (acetate, shown in red) has been exploited to serve as a molecular template for the synthesis of multinuclear Zn assemblies (Zn shown in green) with a short metal-metal distance, and consequently, resembles in part the metal-ion proximity found in a range of multimetallic enzymes.
The use of anions as supramolecular building motifs is reported herein. The interaction of various anions with [Zn(salphen)] complexes (salphen=N,N’-phenylenebis(salicylideneimine)) has been studied, with a particular focus on assembly formation in the presence of acetate anions. Strong association of the latter with the Zn metal centre of the salphen complex was quantified by UV/Vis titration studies. The [Zn(salphen)]-OAc assemblies were further characterized by NMR spectroscopy and X-ray analysis and revealed that the acetate anions can function as bridging modules between two [Zn(salphen)] units. The binding strength of a second complex to the acetate anion may be regulated by the substitution pattern of the salphen ligand. The anion-templated assembly formation was further demonstrated by the supramolecular synthesis of a trinuclear Zn₃ assembly. These results hold promise for the directed assembly formation of [Zn(salphen)] complexes with biologically relevant carboxylates, such as amino acids, and potentially new bio-sensing materials.