Nickel(II) and cobalt(II) 3-hydroxy-4-pyridinone complexes: Synthesis, characterization and speciation studies in aqueous solution

The interaction of nickel(II) and cobalt(II) with a set of 3-hydroxy-4-pyridinone ligands (Hmpp, Hmepp, Hetpp, Hempp, Hdepp and Hdmpp) was investigated in aqueous solution. The family of chelators under study is pharmaceutically relevant in the treatment of iron-overloaded patients and it is foreseen to be of use in occupational health situations associated with accumulation of nickel and cobalt such as nasal/pulmonary and skin hazards and allergies. Ligand dissociation constants and stability constants for the cobalt(II) and nickel(II) complexes have been determined by potentiometric methods allowing the establishment of the corresponding speciation plots as a function of pH. The values of the stability constants are consistent with the Irving-Williams series and indicate the formation of stable ML₂ complexes which were isolated and characterized. Following the trend observed for 3-hydroxy-4-pyridinone ligands the solids were obtained as hydrates whose composition was characterized as [ML₂(H₂O) ₂]A·xH₂O complexes. DFT calculations allowed geometry optimization and provided information about bond lengths and angles. The affinity of 3,4-HPO ligands towards Co^II and Ni^II allows their precipitation as neutral ML₂ complexes. Although not suitable for oral use due to their higher affinity to Fe^III, Cu ^II and Zn^II, 3,4-HPO ligands may be of use in smart materials to prevent inhalation of nickel(II) or cobalt(II) particles in industrial environments as it is possible to vary the substituents without altering their chelating properties.