*3.2. Synthetic Uranyl Compounds with Selenite Ions*

The first synthetic and the simplest uranyl selenite, [(UO2)(SeO3)], was obtained in 1978 [33] (and its neptunyl analog has been recently reported [34] as well). Further, the research undertaken by V. E. Mistryukov and Yu. N. Mikhailov from the Kurnakov Institute of General and Inorganic Chemistry RAS (Russian Federation), and by V.N. Serezhkin and L.B. Serezhkina from the Samara State University (Russian Federation) should be mentioned, who studied uranyl selenites with electroneutral ligands and the first Na-bearing synthetic uranyl selenite compounds. Nearly half of the synthetic compounds described within this review were synthesized and characterized by T.E. Albrecht-Schmitt and co-workers (Table 2). The significant impact of their works on the development of uranyl selenites' structural chemistry should be especially noted.

Synthetic compounds, whose structures are based on inorganic units with the linkage of *Ur* to selenite oxyanions (Table 2), could be divided into two groups: Pure inorganic and organically templated phases.




Most of the inorganic uranyl selenites were obtained during low or medium temperature hydrothermal experiments in the temperature range of 100 to 220 ◦C using Teflon-lined steel autoclaves. Various reagents were used as the source of uranium (U(VI) oxide, uranyl hydroxide, uranyl nitrate hexahydrate, uranyl acetate dihydrate), whereas selenous acid (H2SeO3) was the only source of Se(IV). To be precise, either acid itself or SeO2 were used in the reactions, but Se(IV) dioxide reacts with water to form selenous acid. H2SeO3 is a very weak acid and it hardly dissociates at room temperature, which explains the required heating for the reaction. Several compounds obtained in different ways should be mentioned separately. Compounds **22** [44] and **23** [45] were obtained during evaporation at room temperature. The first compound was obtained from the reaction of UO2(NO3)2·6H2O with selenic acid (H2SeO4) in aqueous medium for 1 year, which could be explained by the reduction of Se(VI) to Se(IV) in the solution during the experiment. Moreover, as it was recently shown, the hydronium ions usually enter the structure at the very latest crystallization stages, when there are no more other cations in the solution [52,59,60]. The Na-bearing compound was obtained in the presence of sodium oxalate, which probably could be regarded as a catalyst of the uranyl selenite crystallization process. Another five compounds, **11** [38], **21** [43], **27** [33], **28** [38], and **32** [47], were obtained in the gas–solid or hydrothermal reactions using sealed tubes. In the case of the last three compounds, the temperature reached over 425 ◦C.

The majority of the organically templated compounds are actually uranyl selenites-selenates. The selenite anions that are arranged in the structures of such compounds are in minor amounts with respect to the selenate groups. Such a tendency comes from the experimental conditions, in which the source of Se was the selenic acid. Selenic acid is less stable in environmental conditions than selenous acid, and it reduces to the latter during storage. Initially, pure H2SeO4 reagent after a few months of storage contains significant amount of [SeO3] <sup>2</sup><sup>−</sup> and [HSeO3] − ions, which participate in the structure formation along with [SeO4] <sup>2</sup><sup>−</sup> groups. There are only five structures of organically templated uranyl selenites known without [SeO4] <sup>2</sup><sup>−</sup> oxyanions (Table 2), three of which (**34** [49], **35** [49], and **46** [58]) were obtained during mild hydrothermal experiments (130–150 ◦C) when the source of Se was again selenic acid. Here, the temperature and amine molecules or ionic liquids [58] acted as a reduction agent for H2SeO4, since it is known that the selenic acid is easily reduced to H2Se4<sup>+</sup>O3 and oxygen upon heating above 160 ◦C [61]. The other two organically templated uranyl selenites (**33** [48] and **36** [50]) were obtained during evaporation at room temperature from the aqueous solution of UO2(NO3)2·6H2O, SeO2, and respective amine. Since SeO2 transforms to weak selenous acid in water, the low dissociation ability of the latter [62] and the presence of [NO3] − groups in the structures of both compounds explains the long crystallization process of 1 to 2 months. It is likely that dissociation of uranyl nitrate and the presence of amine finally helped to create an environment sufficient for the selenous acid dissociation, and thus to start the crystallization of uranyl selenites. Nitrate groups, in these cases, act as additional oxyanions involved in structure formation with a shortage of [SeO3] <sup>2</sup><sup>−</sup> groups.
