Organically Templated Uranyl Sulfates and Selenates: Structural Complexity and Crystal Chemical Restrictions for Isotypic Compounds Formation

This paper reviews the state of the art in the structural chemistry of organically templated uranyl sulfates and selenates, which are considered as the most representative groups of U-bearing synthetic compounds. In total, there are 194 compounds known for both groups, the crystal structures of which include 84 various organic molecules. Structural studies and topological analysis clearly indicate complex crystal chemical limitations in terms of the isomorphic substitution implementation, since the existence of isotypic phases has to date been confirmed only for 24 compounds out of 194, which is slightly above 12%. The structural architecture of the entire compound depends on the combination of the organic and oxyanion parts, changes in which are sometimes realized even while maintaining the topology of the U-bearing complex. An increase in the size of the hydrocarbon part and number of charge functional groups of the organic cation leads to the formation of rare and more complex topologies. In addition, the crystal structures of two novel uranyl sulfates and one uranyl selenate, templated by isopropylammonium cations, are reported.


Introduction
Crystal chemical studies of uranium compounds began to develop actively in the middle of the last century; however, the most rapid growth of structural research occurred at the turn of the century and continues to this day. Of particular interest from the structural chemistry point of view is the study of hexavalent uranium compounds. The unique structural diversity cannot leave indifferent researchers in the field of crystallography, thereby generating new discovered substances and new published papers every year. Two of the most abundant groups of synthetic U-bearing compounds are uranyl selenates and sulfates, and a significant portion of them are hybrid organic-inorganic compounds. Their study is of genuine interest, since such complexes inherit the properties of both structural components: a solid inorganic uranium-bearing structure and a flexible organic one.
At present, almost 200 organically templated compounds within both named groups are known (Table 1). In this review, we evaluate the possibility of isostructural compounds' existence among uranyl sulfates and selenates, as well as involve a recently developed analytical approach to calculating the structural complexity parameters, which allows the comparison of crystal structures in terms of the information content. In addition, we report on a description of the crystal structures of three novel uranyl compounds, [C 3 H 10 (2) to twice-better convergence parameters and interatomic bonds precision, all of which are templated by isopropylammonium cations, which are reported herein.

Structural Topology
The layered complexes in the structures of 2-4 belong to one of the most common topological types (cc2-2:3-4) among uranyl compounds of both pure inorganic or organically templated origin. The topology of the layer can be represented by a black-and-white graph where Ur polyhedra are denoted by black vertices, SO 4 or SeO 4 coordination polyhedra are denoted by white vertices, and two vertices are connected by a line if the corresponding polyhedra have a common O atom (Figure 1c). Within the current review, the structures of 24 organically templated uranyl sulfates and selenates are based on the layers of this topology, including compounds 2-4. Being tridentate bridging, sulfate and selenate tetrahedra have their fourth non-shared O atom arranged up or down relative to the plane of the layer. This variability can generate the formation of geometrical isomers with various orientations of tetrahedral groups that can be described by the orientation matrices [90]. Symbols u (up), d (down), or (tetrahedra missing in the graph) are assigned to each tetrahedral site (white vertex) at the graph of the layer (Figure 1c). The aforementioned change in the interlayer space filling results, however, does not entail differences in the geometric isomerism of the layers. Thus, the orientation matrix for the U-bearing layers in the structures of 2-4 can be written as (uud )/( udd). Moreover, the degree of layer distortion is also the same. Layer undulation (Figure 2a,b) can be calculated as the shortest interatomic distance between the neighbor wave crests, and the thickness can be calculated as the normal distance between the mean planes that pass through the most protruding parts of the layer (i.e., terminal O atoms of the tetrahedra). The layer undulation and thickness parameters are 7.5 and 5.9 Å, 7.

Isotypic Uranyl Sulfates and Selenates
An aforementioned example demonstrates the rather high resistance of the U-bearing structural type to substitutions in the oxyanion substructural complex. However, this case in the total amount of known structural data is not so frequent. Only 11 pairs of isotypic sulfate-selenate compounds, excluding those reported here, are known. Most of them account for the uranyl compounds templated by various amino acid molecules (174-185, 188, 189 [76]). Two pairs correspond to quite rare piperazine (122 [47], It is known that hydrophilic amine groups of organic cations in the structures of organically templated uranyl compounds prefer to associate with dense fragments of Ubearing substructural complexes (four-membered rings of the graph), while hydrocarbon components of the molecules, which do not play a charge-compensating role, are usually arranged in front of rarefied zones (six-membered rings of the graph). It is of interest that the arrangement of the isopropylammonium cation in the structure of 4 fully corresponds to that in the structures of 2 and 3 (Figure 2c,d). The arrangement of the selenous acid molecule in 4 plays a role of the hydrocarbon part of the second isopropylammonium cation in 2 and 3, so that the H 3 O + molecule occupies a position different from H 2 O in the structures of 2 and 3, and functions as an amino group.

Isotypic Uranyl Sulfates and Selenates
An aforementioned example demonstrates the rather high resistance of the U-bearing structural type to substitutions in the oxyanion substructural complex. However, this case in the total amount of known structural data is not so frequent. Only 11 pairs of isotypic sulfate-selenate compounds, excluding those reported here, are known. Most of them account for the uranyl compounds templated by various amino acid molecules (174-185, 188, 189 [76]). Two pairs correspond to quite rare piperazine (122 [47], 123 [48]) and 3-Aminotropane (154, 155 [64]) molecules. Additionally, only two pairs of compounds represent more common organic molecules that were used in the synthetic experiments: 1-butylamine (26 [11], 30 [14]) and tetramethylammonium (71 [33], 72 [34]). There are also several examples of a very close structural architecture, for example, compounds templated by 1,4-diaminobutane (47 [12,13], 48 [26]) and N.N-dimethylethylenediamine (89 [43], 91 [36]). Those pairs of compounds have the same topology of the U-bearing layers, and even close unit cell parameters; however, an arrangement of the respective organic and additional H 2 O molecules in the interlayer space differs, which leads to the impossibility of classifying them as isotypic compounds.

Topology of U-Bearing Complexes
An analysis of Table 1 (Figure 3f-k). Moreover, the cc2-2:3-10 topology of the layered U-bearing complexes was observed in the structures of the compounds templated by 11 various organic molecules; the cc2-1:2-2 topology was described for 11 molecules of various shapes and sizes, and the most common topological type, cc2-2:3-4, was observed in the structures with 17 various amine molecules. There were five compounds, including compound 1, of which the structures were based on microporous frameworks. Additionally, the crystal structures of 31, 166, and 191 contained nanotubules, formed by vertex-sharing of Ur bipyramids and sulfate or selenate tetrahedra. It is of interest that nanotubules in all three compounds can be unfolded into the planar fragments of the cc2-3:5-2 topological type.  ((b,e), respectively); layers of cc2-1:2-2 (f), cc2-2:3-10 (h), and cc2-2:3-4 (j) topologies and their respective graphs (g,i,k). Legend: see Figure 1; blue triangles = NO3 groups; gray nodes and double line = edge-shared TO4 tetrahedra or NO3 group.

Structural Complexity
The method of calculating and analyzing structural complexity parameters has been quite successfully used in the study of mineral associations [94][95][96][97], as well as in the analysis of various groups of inorganic compounds, including uranyl compounds [98][99][100][101].

Structural Complexity
The method of calculating and analyzing structural complexity parameters has been quite successfully used in the study of mineral associations [94][95][96][97], as well as in the analysis of various groups of inorganic compounds, including uranyl compounds [98][99][100][101].
Considering the full set of available structural data, the only obvious correlation was observed between complexities of the U-bearing structural unit and entire structure (Figure 4a). On the one hand, this trend is rather obvious: the more complex the structural unit, the more complex the structure is. However, one should keep in mind that the most accurate comparison and analysis of the calculated complexity values are possible for compounds with similar chemical compositions (polymorph modifications). Deviations in the chemical composition or, to be more precise, in a number of atoms in the crystal structure automatically create certain allowances, since the complexity parameters directly depend on the number and multiplicity of atomic sites. For example, a single H2O molecule introduces three atomic sites into the calculation. Therefore, organic molecules should contribute to the overall complexity due to the large number of atomic sites compared to the inorganic substructural unit. However, there is no such tendency observed in the graphs, if complexity values per unit cell are taken into account (Figure  4b,c). The situation becomes somewhat better when using complexity parameters per atom (Figure 4d,e). However, even here, there were no real trends, minor tendencies. This was mainly due to the fact that organic molecules with similar numbers of atoms had completely different functionalities (size, shape, number of amino groups, etc.), which presented different effects on the U-bearing structural complexes. Therefore, it made sense to consider some groups of molecules separately.
Thus, the most representative groups were the rows of chained amine and diamine molecules. For these groups, firstly, there was a long-term trend towards an increase in the hydrocarbon part of the molecule, and secondly, there were relatively large numbers of representatives to obtain better statistics. Both of these statements are more relevant to the group of diamines; however, in comparison with the other types of molecules, the statistics are, unfortunately, less obvious. As it can be seen from the graph (Figure 5), an increase in the length of the hydrocarbon moiety of the chain amine correlates both with an increase in the complexity of the entire structure (which is expected) and with an in- On the one hand, this trend is rather obvious: the more complex the structural unit, the more complex the structure is. However, one should keep in mind that the most accurate comparison and analysis of the calculated complexity values are possible for compounds with similar chemical compositions (polymorph modifications). Deviations in the chemical composition or, to be more precise, in a number of atoms in the crystal structure automatically create certain allowances, since the complexity parameters directly depend on the number and multiplicity of atomic sites. For example, a single H 2 O molecule introduces three atomic sites into the calculation. Therefore, organic molecules should contribute to the overall complexity due to the large number of atomic sites compared to the inorganic substructural unit. However, there is no such tendency observed in the graphs, if complexity values per unit cell are taken into account (Figure 4b,c). The situation becomes somewhat better when using complexity parameters per atom (Figure 4d,e). However, even here, there were no real trends, minor tendencies. This was mainly due to the fact that organic molecules with similar numbers of atoms had completely different functionalities (size, shape, number of amino groups, etc.), which presented different effects on the U-bearing structural complexes. Therefore, it made sense to consider some groups of molecules separately.
Thus, the most representative groups were the rows of chained amine and diamine molecules. For these groups, firstly, there was a long-term trend towards an increase in the hydrocarbon part of the molecule, and secondly, there were relatively large numbers of representatives to obtain better statistics. Both of these statements are more relevant to the group of diamines; however, in comparison with the other types of molecules, the statistics are, unfortunately, less obvious. As it can be seen from the graph (Figure 5), an increase in the length of the hydrocarbon moiety of the chain amine correlates both with an increase in the complexity of the entire structure (which is expected) and with an increase in the complexity of the uranyl-bearing substructural complex. Of course, the trend line cannot be called absolute, but rather a trend of the average complexity values for each of the molecules. crease in the complexity of the uranyl-bearing substructural complex. Of course, the trend line cannot be called absolute, but rather a trend of the average complexity values for each of the molecules. A rather good agreement with this tendency can also be observed for compounds with amino acid molecules ( Figure 6). A rather good agreement with this tendency can also be observed for compounds with amino acid molecules ( Figure 6). crease in the complexity of the uranyl-bearing substructural complex. Of course, the trend line cannot be called absolute, but rather a trend of the average complexity values for each of the molecules. A rather good agreement with this tendency can also be observed for compounds with amino acid molecules ( Figure 6). Most of the remaining groups of molecules did not have a large number of compounds available; therefore, it was rather difficult to analyze them. However, several interesting trends could be observed as well. Considering the features of cyclic molecules, one can notice that small strained molecules, such as azetidine, pyridine, imidazole, etc., are located at the beginning of the graph (Figure 7a,b). Those points correspond to rather complex U-bearing structural units, as well as structures in general. As the cycle increases and multiple bonds disappear, the complexity of the substructural building units decrease. Additionally, they begin to increase again as branches from the cyclic base appear. Most of the remaining groups of molecules did not have a large number of compounds available; therefore, it was rather difficult to analyze them. However, several interesting trends could be observed as well. Considering the features of cyclic molecules, one can notice that small strained molecules, such as azetidine, pyridine, imidazole, etc., are located at the beginning of the graph (Figure 7a,b). Those points correspond to rather complex U-bearing structural units, as well as structures in general. As the cycle increases and multiple bonds disappear, the complexity of the substructural building units decrease. Additionally, they begin to increase again as branches from the cyclic base appear. The importance of the number of atoms is well illustrated in the calculation of complexity parameters by the example of crown molecules (Figure 7c,d). Crown ether molecules do not contain amino groups and are electrically neutral within the structures of the corresponding compounds. Thus, the role of their size in the formation of more complex structures is not clearly traced. This is all the more obvious if one compares the molecules of 12-crown-4 ether and cyclene, which are nearly identical in size and shape. The presence of four amino groups in the structure of the latter, instead of four O atoms, firstly affects the complexity of the molecule itself (eight additional atoms), and secondly increases the complexity of substructural units due to the active participation of amino groups in a particular topology templating process. The importance of the number of atoms is well illustrated in the calculation of complexity parameters by the example of crown molecules (Figure 7c,d). Crown ether molecules do not contain amino groups and are electrically neutral within the structures of the corresponding compounds. Thus, the role of their size in the formation of more complex structures is not clearly traced. This is all the more obvious if one compares the molecules of 12-crown-4 ether and cyclene, which are nearly identical in size and shape. The presence of four amino groups in the structure of the latter, instead of four O atoms, firstly affects the complexity of the molecule itself (eight additional atoms), and secondly increases the complexity of substructural units due to the active participation of amino groups in a particular topology templating process.

Synthesis
Caution: While isotopically depleted U was used in these experiments, precautions for handling radioactive materials should be followed.
To reveal the features of the isotypic uranyl compounds' crystallization upon substitution in cationic and anionic substructural complexes, a series of synthetic experiments were conducted. Uranyl sulfate with a microporous structure [C 4 H 12 N] 2 [(UO 2 ) 6 (SO 4 ) 7 (H 2 O) 2 ] (28) [11], in the channels of which small-chained molecules of 1-butylamine were arranged, was chosen as the starting point. A similar ratio of initial reagents was taken; however, another small amine with a branched aliphatic part, isopropylamine, was chosen as an organic template.
An aqueous solution of 0.1720 g (0.34 mmol) of uranyl nitrate was dissolved in 4 mL of deionized distilled water. Then, 0.500 mL (9.38 mmol) of H 2 SO 4 and 0.012 mL (0.14 mmol) of isopropylamine were added to the solution, which was stirred until all solid material dissolved. The resulting yellowish transparent solution was left to evaporate in a watch glass at room temperature. Individual, single, flat, rhombic crystals of 1 (Figure 8a) began crystallizing after 3 days. It should be noted that compound 1 was also obtained using another protocol as follows. An aqueous solution of 0.6400 g (1.51 mmol) of uranyl acetate was dissolved in 1 mL of deionized distilled water. Then, 0.200 mL (3.75 mmol) of H 2 SO 4 (98%) and 0.012 mL (0.14 mmol) of isopropylamine were added to the solution, which was stirred until all solid material dissolved. The resulting yellowish transparent solution was placed in a steel autoclave with a Teflon capsule, which was kept in an oven at a temperature of 180 • C for 24 h. After cooling, the solution was poured onto a watch glass, where individual crystals of 1 began crystallizing after 30 min.

Synthesis
Caution: While isotopically depleted U was used in these experiments, precautions for handling radioactive materials should be followed.
To reveal the features of the isotypic uranyl compounds' crystallization upon substitution in cationic and anionic substructural complexes, a series of synthetic experiments were conducted. Uranyl sulfate with a microporous structure [C4H12N]2[(UO2)6(SO4)7(H2O)2] (28) [11], in the channels of which small-chained molecules of 1-butylamine were arranged, was chosen as the starting point. A similar ratio of initial reagents was taken; however, another small amine with a branched aliphatic part, isopropylamine, was chosen as an organic template.
An aqueous solution of 0.1720 g (0.34 mmol) of uranyl nitrate was dissolved in 4 mL of deionized distilled water. Then, 0.500 mL (9.38 mmol) of H2SO4 and 0.012 mL (0.14 mmol) of isopropylamine were added to the solution, which was stirred until all solid material dissolved. The resulting yellowish transparent solution was left to evaporate in a watch glass at room temperature. Individual, single, flat, rhombic crystals of 1 (Figure 8a) began crystallizing after 3 days. It should be noted that compound 1 was also obtained using another protocol as follows. An aqueous solution of 0.6400 g (1.51 mmol) of uranyl acetate was dissolved in 1 mL of deionized distilled water. Then, 0.200 mL (3.75 mmol) of H2SO4 (98%) and 0.012 mL (0.14 mmol) of isopropylamine were added to the solution, which was stirred until all solid material dissolved. The resulting yellowish transparent solution was placed in a steel autoclave with a Teflon capsule, which was kept in an oven at a temperature of 180 °C for 24 h. After cooling, the solution was poured onto a watch glass, where individual crystals of 1 began crystallizing after 30 min. An attempt to crystalize the selenate compound isotypic to 1 was unsuccessful. An analysis of the crystalline precipitate showed that a [C3H10N]2[(UO2)2(SeO4)3(H2O)](H2O) (2) phase was formed, which was previously reported in [12,13]. To avoid the accidental crystallization of the compound 2, several experiments were performed in an extended range of initial reagent concentrations with approximately the same molar ratios. The best-quality single crystals of 2 were formed under the following conditions. An aqueous solution of 0.0880 g (0.18 mmol) of uranyl nitrate was dissolved in 2 mL of deionized distilled water. Then, 0.220 mL (1.79 mmol) of H2SeO4 (40%) and 0.006 mL (0.07 mmol) of isopropylamine were added to the solution, which was stirred until all solid material dissolved. The resulting yellowish transparent solution was left to evaporate in a watch glass at room temperature. The formation of crystals started in 2 days (Figure 8b). Although the crystal structure of 2 was previously described [12,13], we reported here on the refinement of its structural model with better precision.
To obtain a sulfate compound isotypic to 2, the following experiment was conducted. An aqueous solution of 0.0880 g (0.18 mmol) of uranyl nitrate was dissolved in 2 mL An attempt to crystalize the selenate compound isotypic to 1 was unsuccessful. An analysis of the crystalline precipitate showed that a [C 3 H 10 N] 2 [(UO 2 ) 2 (SeO 4 ) 3 (H 2 O)](H 2 O) (2) phase was formed, which was previously reported in [12,13]. To avoid the accidental crystallization of the compound 2, several experiments were performed in an extended range of initial reagent concentrations with approximately the same molar ratios. The best-quality single crystals of 2 were formed under the following conditions. An aqueous solution of 0.0880 g (0.18 mmol) of uranyl nitrate was dissolved in 2 mL of deionized distilled water. Then, 0.220 mL (1.79 mmol) of H 2 SeO 4 (40%) and 0.006 mL (0.07 mmol) of isopropylamine were added to the solution, which was stirred until all solid material dissolved. The resulting yellowish transparent solution was left to evaporate in a watch glass at room temperature. The formation of crystals started in 2 days (Figure 8b). Although the crystal structure of 2 was previously described [12,13], we reported here on the refinement of its structural model with better precision.
To obtain a sulfate compound isotypic to 2, the following experiment was conducted. An aqueous solution of 0.0880 g (0.18 mmol) of uranyl nitrate was dissolved in 2 mL of deionized distilled water. Then, 0.103 mL (1.92 mmol) of H 2 SO 4 (98%) and 0.006 mL (0.07 mmol) of isopropylamine were added to the solution, which was stirred until all solid material dissolved. The resulting yellowish transparent solution was left to evaporate in a watch glass at room temperature. The formation of individual, flat, octagonal crystals of 3 started in 3 days (Figure 8c).
The final attempt to substitute isopropylamine in the synthetic protocol of 2 with 1butylamine molecules was unsuccessful and resulted in the formation of a [C 4 (29) compound, where the structure was based on the layered complexes with another topology [12,13].
It is of interest that, for the synthesis of 2, a newly obtained selenic acid was used, while compound 4 was synthesized using a selenic acid reagent stored for~2 years (Figure 8d). This resulted in the incorporation of electroneutral H 2 SeO 3 molecules in the interlayer space of 4 (see Chapter 2 for details). The Se(VI) reduction to the 4+ oxidation state during the long-term storage of the selenic acid reagent is a rather frequent process, which was repeatedly noted previously [27,30,100].

Chemical Analysis
The chemical analyses of small pieces of individual single crystals of 1-4, preliminary checked using a single-crystal X-ray diffractometer, were performed using a Hitachi TM 3000 scanning electron microscope equipped with an Oxford EDX spectrometer, with an acquisition time of 30 s per point in an energy dispersive mode (acceleration voltage: 15 kV). The following standards and X-ray lines were used: S-pyrite (FeS 2 ), K α ; Se-PbSe, K α ; and U-U 3 O 8 , M β .

Single-Crystal X-ray Diffraction
Single crystals of 1-4 were selected under an optical microscope in polarized light, immersed in an oil-based cryoprotectant, and fixed on cryoloops. Diffraction data were collected at 100 K using a Rigaku XtaLAB Synergy S X-ray diffractometer operated with a monochromated microfocus MoKα PhotonJet-S (λ = 0.71073 Å) source at 50 kV and 1.0 mA, and equipped with a CCD HyPix 6000HE hybrid photon-counting detector [102]. The frame width was 0.5 or 1.0 • in ω, and there was a 1 to 16 s count time for each frame. Diffraction data were integrated and corrected for polarization, background, and Lorentz effects using the CrysAlisPro program [103]. An empirical absorption correction was applied based on the spherical harmonics (SCALE3 ABSPACK algorithm). The unit-cell parameters ( Table 2) were refined using least-squares techniques. The structures were solved by a dual-space algorithm and refined using SHELX programs [104,105] incorporated in the OLEX2 program package [106]. The final models included coordinates and anisotropic displacement parameters for all non-H atoms. The carbon-, nitrogen-and oxygen-bound H atoms were placed in calculated positions and were included in the refinement in the 'riding' model approximation, with U iso (H) set to 1.5U eq (C) and C-H 0.98 Å for CH 3 groups, U iso (H) set to 1.2U eq (C) and C-H 1.00 Å for tertiary CH groups, U iso (H) set to 1.2U eq (N) and N-H 0.91 Å for NH 3

Structural Complexity Calculations
A structural complexity approach was recently developed by S.V. Krivovichev [107][108][109][110][111][112]. This method allows estimating the information content of each particular crystal structure, as well as its substructural components. It appears to be quite useful for comparing isotypic or similar structures and quantitatively characterizing the contribution of each substructural component (uranyl sulfate or selenate complexes, interstitial organic template, etc.) to the formation of the whole structural architecture of the compound. The approach is based on the Shannon information content calculations of per atom (I G ) and per unit cell (I G,total ) using the following equations: where k is the number of different crystallographic orbits (independent sites) in the structure and p i is the random choice probability for an atom from the i-th crystallographic orbit, that is: where m i is the multiplicity of the crystallographic orbit (i.e., the number of atoms of a specific Wyckoff site in the reduced unit cell) and v is the total number of atoms in the reduced unit cell.
It should be noted that all calculations for already-studied crystal structures were based on the original cif files, which were obtained from structural databases (CCDC and ICSD) and respective publications. In addition, if H-atom sites were not reported in the original entries, they were assigned manually considering the distribution of the H-bonding system. Complexity parameters for the organic molecules and U-bearing substructural complexes were calculated manually, while the parameters for the whole structure were determined using ToposPro software [113].

Conclusions
In this paper, we reviewed the state of the art in the structural chemistry of organically templated uranyl sulfates and selenates, which were considered as the most representative groups of U-bearing synthetic compounds. In total, there were 194 compounds known for both groups, including three novel ones reported here, the crystal structures of which contained 84 various organic molecules. Such statistics illustrates both the great work already performed in the field of syntheses and structural studies, but also the obvious insufficiency of specific system studies, since it turned out that, on average, there were slightly more than two compounds per molecule. Nevertheless, quite clear regularities could be formulated for a number of groups of compounds. Thus, in accordance with the analysis, an increase in the size of the hydrocarbon part and number of charge functional groups of the organic cation led to the formation of rare and more complex topologies.
The presence, albeit in a small number, of isostructural compounds for complex molecules and the absence of such compounds for simpler ones indicated a very fine interaction between the inorganic oxyanion and organic positively charged parts of the structures. Large molecules, apparently, created a kind of a buffer due to their size and the distribution of charge-carrying amino groups, which made it possible to level the difference in the sizes of the sulfate and selenate tetrahedra. However, even in the given examples, the difficulties in obtaining isostructural sulfates and uranyl selenates were very well observed. Thus, compounds 175, 177, 179, and 189 [76] were designated as isostructural, only by the similarity of unit cell parameters, since the quality of the obtained crystals (and all of them were selenates) did not allow one to solve their structures directly. The problem of the presence of a correlation between the uranyl-bearing structural complex topology and the size and shape of the amine molecule has already been raised [12][13][14]64], and it is obvious, at present, that the structural architecture of the entire compound depends on the combination of the organic and oxyanion parts. For example, the most common layer topologies cc2-2:3-10, cc2-1:2-2, and cc2-2:3-4 (see Ch. 3.2) were described in the structures templated by amine molecules of various sizes and shapes (chained, cyclic, etc.); however, the arrangement preserved a certain position of the amino-or other charge-carrying groups. At the same time, changes in the oxyanion substructure can be sometimes realized with symmetry breaking, whilst maintaining the topology of the complex (e.g., 147, 148 [36]).
This review demonstrated the ability to form isotypic compounds, which, by analogy with recently performed studies in purely inorganic uranyl systems [98,114,115], indicated the probability of the isomorphic sulfate-selenate series' existence with substitutions in both cationic and oxyanionic moieties. At the same time, the results of the structural studies and topological analysis of all known compounds within the groups under consideration clearly indicate complex crystal chemical limitations in terms of the isomorphic substitution implementation, since the existence of isotypic phases has to date been confirmed only for 24 compounds out of 194, which is slightly above 12%.