Anion binding to a tetracopper resorcinarene-based complex

UV–vis and electron paramagnetic resonance (EPR) spectroscopic studies have been carried out on the multinuclearcopper(II) complex Cu4BpaRes. The copper atoms are in a tetragonal distorted geometry with nitrogens frombispicolylamine (Bpa) coordinating in the equatorial plane and water molecules or anions completing four coordinationsites. The interaction of anions with this polynuclear copper complex in aqueous solution supports the formation of differentcomplex species, which depend on the type and the concentration of the anions. In the presence of excess anions, frozensolution EPR parameters show the formation of species in which the in-plane coordination is characterised by the presenceof three nitrogen atoms coming from the ligand and a donor atom from the specific anion. For the bidentate anion ligands andespecially for malonate, UV–vis titrations indicate the formation of a 1:4 (Cu4BpaRes:anion) species. EPR experimentssupport the formation of such a species and indicate that the four copper centres are equivalent and reach penta-coordinationvia the coordination of both oxygens from the bidentate ligand.