Uranyl Nitrates: By-Products of the Synthetic Experiments or Key Indicators of the Reaction Progress?

Abstract

Six novel uranyl nitrate compounds K3[(UO2)(NO3)Cl3](NO3) (1, 2), α-Cs2[(UO2)(NO3)Cl3] (3), [(UO2)(NO3)2(H2O)2][(CH3NH3)2(NO3)2] (4), Cs2[(UO2)(NO3)4] (5), and [(UO2)2(OH)2(NO3)2(H2O)3](H2O) (6) have been prepared from aqueous solutions. Their structures were analyzed using single-crystal X-ray diffraction technique. Structural studies have shown that the crystals of 1 and 2 are isotypic but differ in the distortion at the counter ion’s sites. The crystal of 3 is a low-temperature polymorph modification of the recently studied compound. The crystal structure of 4 is composed of uranyl-dinitrate-dihydrate and methylamine-nitrate electroneutral complexes linked through the system of H-bonds. The crystal structure of 5 is based on the finite [(UO2)(NO3)4]2− clusters that are arranged in pseudo-chained complexes extended along [100] and are arranged according to a hexagonal packing or rods. The crystal of 6 is also a novel polymorph modification of previously studied compound, the structure of which is based on the very rare topological type of the finite clusters. Nowadays, uranyl nitrate finite clusters of nine various topological types are known. We give herein a short review of their topological features and relationships. Crystallization of uranyl nitrates usually occurs when all other competitive anions in the system have already formed crystalline phases, or the reaction of reagents have slowed down or even stopped. Thus it is suggested that crystallization of uranyl nitrates can be used as a key indicator of the reaction progress, which points to the necessity of the initial concentrations of reagents correction, or to the replacement of reagents and adjustment of the thermodynamic (P,T) parameters of the synthesis.

The Role of Acidity in the Synthesis of Novel Uranyl Selenate and Selenite Compounds and Their Structures

“…In the present study and supported by observations in recent investigations [17][18][19][20]22,23], the use of low pH conditions acts as a facile path towards novel structure generation and obtaining topology architectures that would otherwise require more intricate synthesis methods, such as the use of organic templating methods. Consequently, it is suggested then, from the observations in this investigation, that the use and presence of acid during synthesis provides a facile pathway to acquiring uranyl selenite/selenate structures with functional groups or topologies that are either novel or require more intricate methods.…”

“…The role of acidity during synthesis, as described previously, has been noted in several recent studies to act as a key determinant in controlling such chemical properties as structural motif, isomerism and also redox processes [17][18][19][20]22,23]. Nazarchuk et al [22] described that, during the evaporation of acidic solutions, the successive reduction in pH accompanied by a loss of solution resulted in distinctive organically templated uranyl sulfate structures forming which were structural dependent and proportional to the pH at which crystallisation occurs.…”

“…This theme of utilizing acidic media in synthesis control, although a relatively recent innovation, has seen further promising applications in a variety of other uranyl/actinyl structural chemical investigations. Examples can be seen in uranium and neptunium nitrate studies, including the identification acid controlled geometric isomerism in neptunium and uranium nitrates [17], and also uncovering the crystallization of uranium nitrates as an indicator of reaction competition [18]. The salient effect solution acidity has upon controlling structure formation in uranyl systems has also been recently observed in uranyl chromates and chromate-nitrates [19][20][21].…”