Hydroxamic acids interactions with metals in aqueous and micellar media: a mechanistic study of complexation reactions and metallacrown formation

This thesis is focused on the different aspects of hydroxamic acids coordination, in particular on their ability to give rise to metal complexes of different stoichiometry and geometry. An in depth analysis of the binding mechanisms is carried out with the help of complementary approaches, ranging from classical spectrophotometry to a more peculiar kinetic analysis by fast techniques. Hydroxamic acids have a high binding affinity to a range of transition metal ions, particularly Fe(III), and are ligands ubiquitous in coordination chemistry and chemical biology. They have been widely employed as chelating agents for a long time and studies of their metal complexes still attract great attention. Since the possible biological activity of hydroxamic acids must be correlated with the formation of their metal complexes, interest in these systems has increased extensively over the past decade. Furthermore, hydroxamic acids can be used in extractive metallurgy as flotation agents, because of their ability to form very stable chelates with various metal ions. Hence, studies on hydroxamic acids metal complexes formation and dissociation reactions are object of vast interest, as they conduct to the comprehension of the involved microscopic mechanisms and to the evaluation of their physical and chemical requirements. In this context this thesis focuses on the equilibria and mechanisms involved in this class of complexation reactions mainly in aqueous solution, but also in micellar media. Reactions of some hydroxamic acids with three relevant and common cations in biological, industrial and environmental fields are investigated. In particular, examples of three different modes of coordination of hydroxamic acids with metal ions are reported and analyzed: from mononuclear complexes (Ni(II)) to supramolecular compounds (Cu(II)), passing through a dinuclear complex (Fe(III)). The hydroxamic acids taken into account are salicylhydroxamic acid (SHA), benzohydroxamic acid (BHA) and (S)-α-alaninehydroxamic acid (Alaha). This study is mainly based on a combination of kinetic and thermodynamic methods in order to analyze the investigated systems in different perspectives. In this thesis a new method is developed that enables the evaluation of the separate contributions to the equilibrium and rate parameters in the aqueous and micellar phase (Nickel-SHA system). Furthermore, the formation of a dinuclear complex (M2L) in aqueous solution, formed by the reaction of Fe(III) with SHA, is demonstrated for the first time. Such a complex provides the rationale for the building of supramolecular structures as metallacrowns, whose self-assembly mechanism is also worked out in this thesis work, together with metallacrowns host-guest solution equilibria (Copper-Alaha and Copper-Lanthanum-Alaha systems). Special attention is also paid to the complex speciation of the Fe(III) ion in aqueous solution. In particular, the non negligible contribution of the Fe(III) trimer is proved and its structure is clarified. This thesis, together with putting into light the richness of hydroxamic acids coordination mechanisms, reveals the importance of a complementary approach, based on both kinetics and thermodynamics, for the treatment of complex systems involved in multiple equilibria.