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 γ‐Aminohydroxamate

The solution equilibria of γ‐aminobutanehydroxamic acid (GABAha) with H + and Cu 2+ 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 )‐α‐Alaha and β‐Alaha. The stability (−Δ G 0 ) sequence was β‐Alaha≫α‐Alaha>GABAha, whereas the order of formation enthalpies (−Δ H 0 ) was β‐Alaha≫GABAha>α‐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 γ‐aminohydroxamic acid, fully supports the structural features interpreted from the thermodynamic data.