Abstract
Proton spectra of 1,3 dimethyluramildiacetic acid and its complexes with several lanthanide ions as well as titration curves of uramildiacetic and barbituric acids where 13C chemical shifts were followed as a function of pH, have enabled the establishment of the conformation in solution of the complexes of the uramildiacetate-type ligands and of the most probable mechanism of ionization of these compounds. It is definitely shown that uramildiacetic acid is tetradentate and that it coordinates to metal ions through the iminodiacetate group and one of the carboxyl groups in position 4,6 of the molecule. The pyrimidinic ring and one of the chelator rings are coplanar and the complexes have axial symmetry. As a side result it is shown that barbituric acid is diprotic and its second ionization constant, p k 2, is approx. 12; the same happens with 5,5 barbiturates.
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