Structure and Thermal Stability of Carbohydrazide Complexes with Uranium(VI) Nitrate, Perrhenate, Perchlorate, and Chloride

Abstract

Uranyl complexes with carbohydrazide (CHZ) and nitrate, perchlorate, perrhenate, or chloride counterions have been synthesized; their crystal structures, along with that of the CHZ condensation product, have been determined by X-ray diffraction. The complexes are identified as [UO₂(CHZ)₃](NO₃)₂, [UO₂(CHZ)₃](ClO₄)₂, [UO₂(CHZ)₃](ReO₄)₂•H₂O, and [UO₂(CHZ)₃]Cl₂•3H₂O. All synthesized compounds feature a complex cation, [UO₂(CHZ)₃]²⁺, where the molecular ligand CHZ coordinates with the uranium atom through its carbonyl O and amino group N atoms. These complexes adopt a distorted hexagonal bipyramidal structure, with CHZ acting as a chelating ligand. Thermal stability has been investigated using thermogravimetry and differential scanning calorimetry, revealing that these compounds are stable up to 190-220 °C. Further heating leads to multiple-stage decomposition under both air and inert gas atmospheres. The influence of the counteranion on decomposition intensity has been established, with complexes containing strongly oxidizing anions (ClO₄^- and NO₃^-) exhibiting more vigorous decomposition and a more subtle degree of uranium reduction. The decomposition mechanisms and resulting products are characterized, offering insights into the thermal behavior of these compounds. The findings have implications for industrial radiochemical processes, as CHZ demonstrates potential as an alternative to hydrazine in spent nuclear fuel aqueous reprocessing.