Successive Crystallization of Organically Templated Uranyl Sulfates: Synthesis and Crystal Structures of [pyH](H O)[(UO ) (SO (H 2 (SO 2 and

: Three new uranyl sulfates, [pyH](H 3 O)[(UO 2 ) 3 (SO 4 ) 4 (H 2 O) 2 ] ( 1 ), [pyH] 2 [(UO 2 ) 6 (SO 4 ) 7 (H 2 O)] ( 2 ), and [pyH] 2 [(UO 2 ) 2 (SO 4 ) 3 ] ( 3 ), were produced upon hydrothermal treatment and successive isothermal evaporation. 1 is monoclinic, P 2 1 / c , a = 14.3640(13), b = 10.0910(9), c = 18.8690(17) Å, β = 107.795(2), V = 2604.2(4) Å 3 , R 1 = 0.038; 2 is orthorhombic, C 222 1 , a = 10.1992(8), b = 18.5215(14), c = 22.7187(17) Å, V = 4291.7(6) Å 3 , R 1 = 0.030; 3 is orthorhombic, Pccn , a = 9.7998(8), b = 10.0768(8), c = 20.947(2) Å, V = 2068.5(3) Å 3 , R 1 = 0.055. In the structures of 1 and 2 , the uranium polyhedra and SO 4 tetrahedra share vertices to form 3 ∞ [(UO 2 ) 3 (SO 4 ) 4 (H 2 O) 2 ] 2 − and 3 ∞ [(UO 2 ) 6 (SO 4 ) 7 (H 2 O)] 2 − frameworks featuring channels (12.2 × 6.7 Å in 1 and 12.9 × 6.5 Å in 2 ), which are occupied by pyridinium cations. The structure of 3 is comprised of 2 ∞ [(UO 2 ) 2 (SO 4 ) 3 ] 2 − layers linked by hydrogen bonds donated by pyridinium cations. The compounds 1 – 3 are formed during recrystallization processes, in which the evaporation of mother liquor leads to a stepwise loss of hydration water.

A variety of positively charged species, both organic and inorganic, have been employed in templating the uranyl-based inorganic frameworks, as the latter are generally negatively charged. As yet, the relationships between the nature of the cationic species and structure and composition of inorganic counterparts and total outcome are rather vague, so new structures are expected to help further understanding and subsequent targeted synthesis. Their composition and structures are likely to depend not only on the initial ratio of the reactants but also on their absolute and relative concentrations 2 of 8 which are expected to vary also in the case of sequential crystallization. These phenomena have been monitored in a relatively small number of systems studied. Hereby, we report on the synthesis and crystal structures of three new uranyl sulfates templated by pyridinium cations, [ (3), which were successively produced by hydrothermal treatment with subsequent isothermal evaporation. We discuss the peculiarities of their crystal structures as well as the templating effect of pyridinium cations.

Synthesis
Caution! While the compounds of depleted uranium are only weakly radioactive, their chemical toxicity is essential. All safety regulations should be followed strictly.
Crystals of 1-3 were grown from a solution containing 0.5 g of (UO 2 )(NO 3 ) 2 ·6H 2 O (Vekton, 99.7%), 2 mL of H 2 SO 4 (Vekton, 99.7%), 0.01 mL of pyridine (Aldrich, 99.5%), and 5ml of H 2 O. It was heated at 220 • C in a Teflon-lined autoclave under autogeneous pressure for two weeks. The transparent yellow solution was allowed to slowly evaporate in air in a fume hood. The crystals formed within 3-10 days. Those of 1 appear at pH ≈ 2.5 as spherical aggregates (Figure 1a,b). Further evaporation leads to the dissolution of surface crystals; at pH = 2 to 1.5, they transform into crystals of 2 (Figure 1c), so that both 1 and 2 coexist on the surface of the spherical aggregates until pH ≈1. The structural studies (vide infra) indicate that the conversion of 1 into 2 leads to the loss of one molecule of hydration water.
ChemEngineering 2020, 4, x FOR PEER REVIEW 2 of 9 and structure and composition of inorganic counterparts and total outcome are rather vague, so new structures are expected to help further understanding and subsequent targeted synthesis. Their composition and structures are likely to depend not only on the initial ratio of the reactants but also on their absolute and relative concentrations which are expected to vary also in the case of sequential crystallization.  (3), which were successively produced by hydrothermal treatment with subsequent isothermal evaporation. We discuss the peculiarities of their crystal structures as well as the templating effect of pyridinium cations.

Synthesis
Caution! While the compounds of depleted uranium are only weakly radioactive, their chemical toxicity is essential. All safety regulations should be followed strictly.
Crystals of 1-3 were grown from a solution containing 0.5 g of (UO2)(NO3)2·6H2O (Vekton, 99.7%), 2 mL of H2SO4 (Vekton, 99.7%), 0.01 mL of pyridine (Aldrich, 99.5%), and 5ml of H2O. It was heated at 220 °C in a Teflon-lined autoclave under autogeneous pressure for two weeks. The transparent yellow solution was allowed to slowly evaporate in air in a fume hood. The crystals formed within 3-10 days. Those of 1 appear at pH ≈ 2.5 as spherical aggregates (Figure 1a,b). Further evaporation leads to the dissolution of surface crystals; at pH = 2 to 1.5, they transform into crystals of 2 (Figure 1c), so that both 1 and 2 coexist on the surface of the spherical aggregates until pH ≈1. The structural studies (vide infra) indicate that the conversion of 1 into 2 leads to the loss of one molecule of hydration water. At pH ≈ 1, the crystals of 1 and 2 completely dissolve, after which the platelets of 3 ( Figure 1d) are formed within 12 h. Further evaporation leads to complete drying without any other transformations. Qualitative electron microprobe analysis of 1-3 (LINK AN-10000 EDS system) revealed no other elements, except U and S, with an atomic number greater than 11 (Na). At pH ≈ 1, the crystals of 1 and 2 completely dissolve, after which the platelets of 3 (Figure 1d) are formed within 12 h. Further evaporation leads to complete drying without any other transformations. Qualitative electron microprobe analysis of 1-3 (LINK AN-10000 EDS system) revealed no other elements, except U and S, with an atomic number greater than 11 (Na).

Single-Crystal Studies
Selected single crystals were attached to glass fibers using an epoxy resin and mounted on a Bruker SMART APEX II DUO diffractometer equipped with a micro-focus X-ray tube utilizing MoK radiation. The experimental datasets were collected at 100K. Unit cell parameters were calculated using least-squares fits. Structure factors were derived using APEX 2 after introducing the required corrections. Crystal structures were solved using direct methods. The final model includes site coordinates and anisotropic thermal parameters for all atoms except hydrogens, which were localized using AFIX command in calculated positions (d(A-H) = 1.00(1) Å). Hydrogen atoms of the water molecules could not be localized. Further details are collected in Table 1. Crystals of 3 were found to be unstable under the X-ray beam and decompose within 1.5 h. Crystal system Monoclinic orthorhombic 14.3640 (13) 10.0910 (9) 18.8690 (17) 10.1992 (8) 18.5215 (14) 22.7187 (17) 9.7998 (8)

Discussion
In all three structures, the pyridinium cations form hydrogen bonds to the inorganic backbones ( Figure 6). In 1 (Figure 6a), a bifurcated hydrogen bond is formed to the termi-

Discussion
In all three structures, the pyridinium cations form hydrogen bonds to the inorganic backbones ( Figure 6). In 1 (Figure 6a), a bifurcated hydrogen bond is formed to the terminal (d(H1⋯O9) = 1.793 Å) and bridging (d(H1⋯O2) = 2.447 Å) oxygen atoms of the sulfate tetrahedra. In 2 (Figure 6b), these bonds are longer (d(H1⋯O6) = 2.523 Å, d(H1⋯O13) = 2.707 Å, and d(H1⋯O18) = 2.992 Å) and formed only to the equatorial atoms of the uranyl polyhedra (Figure 6b). In contrast, in 3, two symmetrically independent pyridinium cations form different systems of hydrogen bonds (Figure 6c)  A commonly addressed question is the effect of the organic species on the composition and topology of inorganic backbones where two opposite opinions have been expressed. On the one hand, the relatively small set of dominating inorganic structures suggests that the role of the positively charged organic matter is just to compensate the negative charge of the inorganic part [40]. On the other hand, the layers are sometimes strongly twisted or even rolled into nanotubes [24] due to the formation of organic micelles or other stable aggregates. In the structures of 1-3, the pyridinium cations contribute to already known inorganic architectures; hence, their structure-driving role therein can be considered as modest. The least common is the 6:7 architecture wherein the size of the pyridinium cation most likely fits well the size of channels. This suggestion is tentative; to corroborate it, additional experiments seem to be desirable using the chemical analogs of pyridine. Yet, this structure is not produced when just slightly different 2-and 4-aminopyridimium cations are employed. Therefore, the shape requirements for this framework may be essential for rigid (e.g., aromatic) templates. The pyH + cations exhibit the common parallel or perpendicular stacking of aromatic rings, which is clearly seen in Figure 6a,c. The same stacking is suggested to be responsible for templating a much more complex nanotubular structure in Na(C8H10NO2)7[(UO2)6(SO4)10]·3.5H2O [35]. It is possible A commonly addressed question is the effect of the organic species on the composition and topology of inorganic backbones where two opposite opinions have been expressed. On the one hand, the relatively small set of dominating inorganic structures suggests that the role of the positively charged organic matter is just to compensate the negative charge of the inorganic part [40]. On the other hand, the layers are sometimes strongly twisted or even rolled into nanotubes [24] due to the formation of organic micelles or other stable aggregates. In the structures of 1-3, the pyridinium cations contribute to already known inorganic architectures; hence, their structure-driving role therein can be considered as modest. The least common is the 6:7 architecture wherein the size of the pyridinium cation most likely fits well the size of channels. This suggestion is tentative; to corroborate it, additional experiments seem to be desirable using the chemical analogs of pyridine. Yet, this structure is not produced when just slightly different 2-and 4-aminopyridimium cations are employed. Therefore, the shape requirements for this framework may be essential for rigid (e.g., aromatic) templates. The pyH + cations exhibit the common parallel or perpendicular stacking of aromatic rings, which is clearly seen in Figure 6a,c. The same stacking is suggested to be responsible for templating a much more complex nanotubular structure in Na(C 8 H 10 NO 2 ) 7 [(UO 2 ) 6 (SO 4 ) 10 ]·3.5H 2 O [35]. It is possible that electrostatic repulsion between the positively charged pyridinium cations may counteract the stacking. This repulsion between pyridinium rings is probably stronger in comparison to that between the anilinium parts of the phenylglycinium species in Na(C 8 H 10 NO 2 ) 7 [(UO 2 ) 6 (SO 4 ) 10 ]·3.5H 2 O, which in neighbor cations point into different directions.