Ampicillin acidity and formation constants with some metals and their thermodynamic parameters in different media. Crystal structures of two polymorphs isolated from the reaction of ampicillin with copper(II)

The dissociation constants of ampicillin and the formation constants of complexes with cobalt(II), copper(II), nickel(II), iron(III), zinc(II), lead(II), mercury(II) and cadmium(II) were evaluated at different temperatures (20-45°C) by potentiometric measurements in the presence of different percentages of ethanol-water. Two polymorphs of the degradation product 3-oxo-2-phenyl-pyrazine-5-carboxylic acid ethyl ester have been isolated from the reaction of copper chloride with ampicillin in aqueous ethanol. In the crystal structure of both polymorphs, molecules related by a center of symmetry form a dimer by coupling of the N-H⋯O hydrogen bonds resulting in the R₂²(8) supramolecular synton. The π-π contacts indicate strong stacking interactions in both polymorphs.