Metallacrowns of copper(II) and aminohydroxamates: Thermodynamics of self assembly and host–guest equilibria

Metallacrowns (MCs) of copper(II) and aminohydroxamic acids have been extensively studied during the past few decades. Although their discovery dates back more than twenty years, systematic studies on the thermodynamics of self assembly of MCs and of their capability to act as guests for anions and cations are quite recent. This review focuses on the solution studies of these metallamacrocycles and, in particular, the following aspects are discussed: (i) the thermodynamics of self-assembly of 12-MC-4 complexes; (ii) the thermodynamics of self-assembly and core metal substitution of 15-MC-5 species; (iii) the thermodynamics of host–guest equilibria between 15-MC-5 complexes and anions. The overall thermodynamic parameters for the formation of a wide number of 12-MC-4 species of α-, β- and γ-aminohydroxamates are discussed together with the most relevant structural, spectroscopic and reactivity features reported in the literature for copper(II) metallacrowns. These data provide a thermodynamic quantitation of the “metallacrowns structural paradigm”, and show the possibility to devise new MCs with desired stabilities in different medium conditions through an appropriate choice of metal coordinating moieties and ligand dimensions. The thermodynamics of self-assembly of 15-MC-5 is discussed for Ca2+ and Ln3+ as core metals, and the overall formation constants are used to evaluate the copious literature data regarding the stability of these species in solution. The relative stability of 15-MC-5 complexes of different Ln3+ ions is also discussed, showing the extraordinary capability of these complexes to discriminate different Ln3+ ions on the basis of their dimensions. Finally, the thermodynamics of host–guest equilibria of 15-MC-5 complexes as receptors for carboxylates is presented: the binding affinities of different carboxylates for the 15-MC-5 species with Ln3+ as the core metal are discussed on the basis of guests hydrophobicity, dimension and basicity, and in terms of core metal Lewis acidity