Synthesis and photophysical properties of chiral, binuclear metal complexes

The synthesis of isomerically pure multicentred species is an important task for preparative chemists. Only stereochemically well-defined structures of polynuclear metal complexes lead to photophysical results that are clearly interpretable. We have developed several strategies to synthesise such compounds that have a predetermined chirality (Δ- or Λ-helix) at each metal center. The approximate local symmetry of the described metal complexes is D3 (octahedral, tris-bidentate). The methods are the following: (a) resolving a racemic building block followed by substitution of the two labile monodentate ligands by a bidentate diimine ligand under total retention of configuration. (b) Synthesis of an optically pure precursor complex in which two chelating bipyridine ligands are bridged and each contain an optically active pinene unit. The remaining coordination sites are then replaced by a bidentate diimine ligand. (c) Use of ligands that have sterically demanding substituents perpendicular to the molecular plane. The use of such ligands leads direct to optically active compounds. Examples of these methods will be given, while considering their photophysical application.