The mechanism of the Cu²âº[12-MCCu(Alaha)-4] metallacrown formation and lanthanum(III) encapsulation.

A kinetic, calorimetric, mass spectrometry and EPR study has been performed on the formation of the metallacrown Cu(2+)[12-MCCu(Alaha)-4] from Cu(ii) and α-alaninehydroxamic acid (H2L). The acidity range where Cu(2+)[12-MCCu(Alaha)-4] is stable lies between pH 3.5 and 6.0. For pH values below that range the complex CuHL(+) is the prevailing species. This species plays a fundamental role in the formation of Cu(2+)[12-MCCu(Alaha)-4]. Actually, depending on the Cu(II)/H2L ratio and on pH, it can originate a dimer Cu2(HL)2(2+) or a dinuclear complex Cu2L(2+). Both species constitute the nuclei necessary for a further oligomerisation reaction which ends when the crown is formed. The kinetics of Cu(2+)[12-MCCu(Alaha)-4] formation is biphasic. Under conditions of Cu(II) excess the fast phase leads to formation of Cu2L(2+). The slow phase is interpreted in terms of a sequential addition of monomers (CuHL(+)) to the Cu2L(2+) nucleus to form the crown. The interaction of La(III) with Cu(2+)[12-MCCu(Alaha)-4] has also been investigated. The system displays a biphasic behaviour; in the first phase the intermediate complex Cu[12-MCCu(Alaha)-4]La is formed which, in excess of ligand, evolves towards the larger metallacrown La(3+)[15-MCCu(Alaha)-5]. The reaction mechanisms of the two investigated systems are discussed.

Mechanism of Ni2+ and NiOH+ interaction with hydroxamic acids in SDS: evaluation of the contributions to the equilibrium and rate parameters in the aqueous and micellar phase.

The equilibria and kinetics (stopped-flow) of the binding of Ni(II) to salicylhydroxamic acid (SHA) and phenylbenzohydroxamic acid (PBHA) have been investigated in aqueous solutions containing SDS micelles. The two ligands are fairly distributed between the two pseudophases present, so the binding reaction occurs in both phases. The contributions to the total reaction from each phase has been evaluated, following a procedure where use is made of the experimentally determined partition coefficients of the reactants involved. The mechanism of the reaction occurring on the micelle surface has been derived and comparison with the mechanism in water shows that the step Ni(2+) + HL ⇄ NiHL(2+) is operative in both pseudophases, whereas the step Ni(2+) + L(-)⇄ NiL(+), which is operative in water, is replaced in SDS by the step NiOH(+) + HL ⇄ NiL(+). The analysis of the equilibrium and of the kinetic data enabled the evaluation of the equilibrium and the rate constants of the individual steps taking part in the binding process over the micelle surface. Interestingly, the first hydrolysis constant of the Ni(H(2)O)(6)(2+) ion in SDS is more than two orders of magnitude higher than in water. The agreement between the equilibrium constants derived from kinetics and those obtained by static measurements confirms the validity of the proposed mechanism.

Route to metallacrowns: the mechanism of formation of a dinuclear iron(III)-salicylhydroxamate complex.

The equilibria and the kinetics of the binding of Iron(III) to salicylhydroxamic (SHA) and benzohydroxamic (BHA) acids have been investigated in aqueous solution (I = 1 M (HClO(4)/NaClO(4)), T = 298 K) using spectrophotometric and stopped-flow methods. Whereas Iron(III) forms a 1:1 complex (ML) with BHA, it forms both ML and M(2)L complexes with SHA. The presence of M(2)L in aqueous medium is corroborated by FTIR measurements. The reactive form of Iron(III) is the hydrolyzed species FeOH(2+), which binds to the O,O site in ML and to the O,O and O(P),N (P = phenolate) sites in M(2)L, inducing full deprotonation of the latter. The reaction pathway is discussed in terms of a multistep mechanistic scheme in which the metal-ligand interaction is coupled to hydrolysis and self-aggregation steps of Iron(III). The observation and characterization of M(2)L as a stable species is important because it contains the -Fe-O-N-Fe- sequence, which constitutes the repetitive motif of the SHA-based metallacrown ring and provides the rationale for 12-MC-4 metallacrowns. In the framework of this study, the kinetics of the Iron(III) dimerization and trimerization have also been investigated using the stopped-flow method to perform dilution jumps. The reaction scheme put forward involves two parallel steps (FeOH(2+) + FeOH(2+) and Fe(3+) + FeOH(2+)) that lead to formation of the Fe(2)(OH)(2)(4+) dimer and a slower step (FeOH(2+) + Fe(2)(OH)(2)(4+)) to form the trimer species. The kinetics of the last step have been investigated here for the first time, and the results deduced indicate that, of the two possible trimer structures reported in the literature, Fe(3)(OH)(3)(6+) and Fe(3)(OH)(4)(5+), the latter prevails by far.