The novel U(VI) and Np(VI) mixed-ligand complexes based on quinaldic acid zwitterion

Interaction of U(VI) and Np(VI) with quinaldic acid in the presence of various inorganic (nitrate, chloride, bromide, perrhenate, selenate, sulfate) or organic (dipicolinic acid) counter-ions allowed obtaining a series of complexes where quinaldic acid presents in the zwitterionic form. In all compounds, except one complex, qui-naldic acid acts as a monodentate ligand and coordinates metal centers via one of the carboxylic oxygen atoms. In all these cases, isolated molecular complexes are formed. In contrast, uranyl sulfate exhibits 2D layered structure where both sulfate-ions and molecules of quinaldic acid function as linkers. Determination of stability constants for 1:1 complexes has shown that the complexation of U(VI) and Np(VI) with quinaldic acid in solution likely occurs via coordination of the carboxylic group. Prevalence of the zwitterionic form and the coordination via carboxylic group both in solution and solid-state is unexpected taking into account similarity in structures of quinaldic and picolinic acids.