Iron(III)- and copper(II) complexes of an asymmetric, pentadentate salen-like ligand bearing a pendant carboxylate group

Abstract

The equilibrium and solution structural properties of the iron(III) and copper(II) complexes of an asymmetric salen-like ligand (N,N′-bis(2-hydroxybenzyl)-2,3-diamino-propionic acid, H3bhbdpa) bearing a pendant carboxylate group were characterized in aqueous solution by potentiometric, pH-dependent electron paramagnetic resonance (EPR) and UV–Vis (UV–Visible) measurements. In the equimolar systems the pentadentate ligand forms very stable, differently protonated mononuclear complexes with both metal ions. In the presence of iron(III) {NH, PhO−, COO−}, {2NH, 2PhO−, COO−} and {2NH, 2PhO−, COO−, OH−} coordinated complexes are dominant. The EPR titrations reflected the presence of microscopic complex formation pathways, leading to the formation of binding isomers in case of Cu(H2bhbdpa)+, Cu(Hbhbdpa) and Cu(bhbdpa)−. The {2NH, 2PhO−+COO−/H2O} coordinated Cu(bhbdpa) is the only species between pH 6–11. At twofold excess of metal ion dinuclear complexes were detected with both iron(III) and copper(II). In presence of iron(III) a μ-carboxylato-μ-hydroxo-bridged dinuclear complex (Fe2(bhbdpa)(OH)3) is formed from Fe(H2bhbdpa)2+ through overlapping proton release processes, providing one of the rare examples for the stabilization of an endogenous carboxylate bridged diiron core in aqueous solution. The complex Cu2(bhbdpa)+ detected in the presence of copper(II) is a paramagnetic (S=1) species with relatively weakly coupled metal ions.