The mechanism of the Cu2+[12-MCCu(Alaha)-4] metallacrown formation and lanthanum(iii) encapsulation

Abstract

A kinetic, calorimetric, mass spectrometry and EPR study has been performed on the formation of the metallacrown Cu²⁺[12-MC_Cu(Alaha)-4] from Cu(II) and α-alaninehydroxamic acid (H₂L). The acidity range where Cu²⁺[12-MC_Cu(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 Cu²⁺[12-MCCu(Alaha)-4]. Actually, depending on the Cu(II)/H₂L ratio and on pH, it can originate a dimer Cu₂(HL)₂²⁺ or a dinuclear complex Cu₂L²⁺. Both species constitute the nuclei necessary for a further oligomerisation reaction which ends when the crown is formed. The kinetics of Cu²⁺[12-MC_Cu(Alaha)-4] formation is biphasic. Under conditions of Cu(II) excess the fast phase leads to formation of Cu₂L²⁺. The slow phase is interpreted in terms of a sequential addition of monomers (CuHL⁺) to the Cu₂L²⁺ nucleus to form the crown. The interaction of La(III) with Cu²⁺[12-MC_Cu(Alaha)-4] has also been investigated. The system displays a biphasic behaviour; in the first phase the intermediate complex Cu[12-MC_Cu(Alaha)-4]La is formed which, in excess of ligand, evolves towards the larger metallacrown La³⁺[15-MC_Cu(Alaha)-5]. The reaction mechanisms of the two investigated systems are discussed.