Interaction of hydroxamic acids with metals: from simple complexes to metallacrowns

The interaction of hydroxamic acids with metal ions, giving rise to bound forms of increased complexity, is described through three examples, making use of the kinetic approach. In the first example the results of an investigation on the binding of Ni(II) to salicylhydroxamic acid (SHA) and phenylbenzohydroxamic acid (PBHA) in water and in sodium dodecylsulphate (SDS) solution are presented. Besides observing the so-called “micellar catalytic effect” a change of mechanism has been detected on changing the medium from water to micelle. Actually, while in water the principal path involves reaction (1) Ni2+ + L- ⇌ NiL+ (1) in SDS the main path is represented by reaction (2) NiOH+ + HL ⇌ NiL+ + H2O (2) where HL denotes the unprotonated hydroxamic acid. The analysis of the results shows that the acid strength of the Ni(H2O)6 2+ ion increases by more than two orders of magnitude on going from water to SDS. The second example is concerned with a stopped-flow investigation of SHA and benzohydroxamic acid (BHA) with Fe(III). It is shown for the first time that SHA is able to bind two Fe(III) ions. The important feature of this complex is the binding of the second Fe(III) ion at the O,N site. This binding implies total deprotonation of SHA, a process not observable with the free acid. Moreover and more important, this complex presents the O-Fe-N-O sequence which characterizes the metallacrown structure. The complex does constitute the fundamental repeating unit of metallacrowns. In the third example are described the main reactive steps that lead to metallacrown formation. The kinetics of the Cu(II)-α-alanine hydroxamic acid metallacrown (12MC4) formation have been investigated by mixing Cu(II) and ligand (L) in the 5:4 ratio. Different stoichiometric ratios do not lead to formation of any complex structure. The kinetic behaviour shows that mononuclear ML an dinuclear M2L2 structures are rapidly formed and two dimers, reacting together, form a tetrameric structure which, upon adding further Cu(II) ion, originates the cyclic species 12MC4. In the presence of a slight excess of L and lanthanum, 12MC4 is converted to 15MC5. The binding of La(III) to the cavity of the metallacrown has been investigated as well.