Synthesis, solution thermodynamics, and x-ray study of Cu-II [12]metallacrown-4 with GABA hydroxamic acid: An unprecedented crystal structure of a [12]MC-4 with a gamma-aminohydroxamate

The solution equilibria of gamma-aminobutanehydroxamic acid (GABAha) with H+ and Cu2+ were investigated by potentiometry, titration calorimetry, spectrophotometry, NMR spectroscopy, and ESI-MS. The thermodynamic parameters of the Cu(II) [12]metallacrown-4 obtained for GABAha were compared with those of the corresponding complexes of (S)-alpha-Alaha and beta-Alaha. The stability sequence was beta-Alaha >>alpha-Alaha > GABAha, whereas the order of formation enthalpies was beta-Alaha >> GABAha >alpha-Alaha. These data were interpreted on the basis of the dimensions of the chelate rings and the planarity of the metallamacrocycles. The Cu-II [12]metallacrown-4 ([12]MC-4) complex of GABAha was isolated and its crystal structure, which is the first reported for a [12]MC-4 of a gamma-aminohydroxamic acid, fully supports the structural features interpreted from the thermodynamic data.