Aquatic interaction of uranium with two naturally ubiquitous pyrazine compounds: Speciation studies by experiment and theory

Abstract

The present studies interpret the speciation of uranyl (UO22+) with the most ubiquitous class of natural species named pyrazines in terms of stability, speciation and its identification, thermodynamics, spectral properties determined by a range of experimental techniques and further evidenced by theoretical insights. UO22+ forms ML and ML2 kind of species with a qualitative detection of ML3 species, while the ESI-MS identified the formation of all the complexes including ML3. Both the ligands act as bidentate chelators with a difference in ring size and coordinating atoms in the complex formed. The ML3 complexes involve the third ligand participation as monodentate via carboxylate only due to the restricted coordination number and space around the UO22+ ion to accommodate three ligand molecules in its primary coordination sphere. All the complexes are found to be endothermic and purely entropy driven formations. The complex formations showed redshift in the absorption spectra and the shift was further enhanced from ML to ML2 formation. The UO22+ ion redox properties are used to explore the redox potential and heterogeneous electron-transfer kinetic parameters as a function of pH and concentration of UO22+ in presence of pyrazine carboxylates. Interestingly, the cyclic voltammograms identified the ligands also as redox sensitive. The theoretical calculation gave inputs to understand the complex formation at the molecular level with major emphasis on geometry optimization, energetics, bonding parameters, molecular orbital diagrams and bond critical point analyses. The experimental observations in combination with theoretical addendum provided detailed knowledge on the interaction of UO22+ with pyrazine-2-carboxylate and pyrazine-2,3-dicarboxylates.