Organically Templated Layered Uranyl Molybdate [C 3 H 9 NH + ] 4 [(UO 2 ) 3 (MoO 4 ) 5 ] Structurally Based on Mineral-Related Modular Units

: A new organically templated uranyl molybdate [C 3 H 9 NH + ] 4 [(UO 2 ) 3 (MoO 4 ) 5 ] was prepared by a hydrothermal method at 220 ◦ C. The compound is monoclinic, C с , a = 16.768(6), b = 20.553(8), c = 11.897(4) Å, β = 108.195(7), V = 3895(2) Å 3 , R 1 = 0.05. The crystal structure is based upon [(UO 2 ) 3 (MoO 4 ) 5 ] 4 − uranyl molybdate layers. The isopropylammonium cations are located in the interlayer. The layers in the structure of [C 3 H 9 NH + ] 4 [(UO 2 ) 3 (MoO 4 ) 5 ] are considered as modular architectures. Topological analysis of layers with UO 2 : T O 4 ratio of 3:5 ( T VI = S, Cr, Se, Mo) was performed. Modular description is employed to elucidate the relationships between di ﬀ erent structural topologies of [(UO 2 ) 3 ( T O 4 ) 5 ] 4 − layers and inorganic uranyl-based nanotubules. The possible existence of uranyl molybdate nanotubules is discussed.


Introduction
Investigations of natural and synthetic compounds of hexavalent uranium are important both for materials science and mineralogy. Uranium minerals are important constituents of oxidation zones of uranium deposits [1,2]. The development of the nuclear industry also requires further investigations in the area of safe nuclear waste management [3]. The latter requires proper understanding of phase formation processes, relationships between chemical compositions, crystal structures, and properties of the compounds formed. Of particular interest are both the chemical nature of the possible secondary phases and their interactions with the geological environment [4].
In the last two decades, particular attention has been paid to the hexavalent uranium (uranyl) compounds containing organic moieties, which are important for lanthanide/actinide separation. Common among these are the hybrid materials wherein the organic molecules are directly coordinated to the uranyl cation as ligands [5,6]. The templated structures based on weakly bonded organic and inorganic parts are less numerous [7] but exhibit essentially larger structural diversity of inorganic motifs.
The uranium compounds are formed during various steps of the nuclear cycle and present in the nuclear waste [3]. They accumulate various nuclides, thus preventing their migration into the environment [4]. The composition of nuclear waste is extremely complex and can contain both newly formed phases, as well as products of interaction between the fuel and container material, as well as with geological environment. The molybdenum isotopes ( 95 Mo, 97 Mo, 98 Mo and 100 Mo), formed The 0D complexes are observed in the sulfate minerals belakovskiite, Na7(UO2)(SO4)4(SO3OH)•3H2O [9] and bluelizardite, Na7(UO2)(SO4)4Cl•2H2O [10], which are the secondary products of uraninite weathering; both species have been found in the Blue Lizard mine, San Juan, UT, USA. Though a large number of natural uranyl sulfates has been described [11], the complexes observed in belakovskiite ( Figure 1а) and bluelizardite (Figure 1b) are yet unique.
A large number of uranium minerals with a common formula A n+ [(UO 2 )(TO 4 (Figure 1h). Therein, four out of five equatorial vertices of uranium polyhedra are occupied by oxygen atoms of sulfate tetrahedra while the fifth is occupied by a water molecule; all such vertices are similarly oriented in the layer plane. The layers in the structure of wetherillite, Na 2 Mg(UO 2 ) 2 (SO 4 ) 4 ·18H 2 O [12], differ in that the vertices occupied by water molecules exhibit two positions ( Figure 1i). Yet another layered arrangement is formed by vertex-sharing UO 6 (H 2 O) and SO 4 moieties ( Figure 1j) in the structure of beshtauite, (NH 4 ) 2 (UO 2 )(SO 4 ) 2 ·2H 2 O [27]. The topology of this layer is similar to that of goldichite, KFe(SO 4 ) 2 ·4H 2 O [28].
The structural diversity of natural and synthetic compounds is mostly governed by their formation conditions, including the acidity/basicity and the mechanism of crystallization. The majority of the minerals discussed above have been found on the walls of abandoned mines [10]. They crystallize upon evaporation under almost invariable temperature but decreasing pH. These conditions are expected to affect the structural outcome. The oxidation zones are formed under conditions changing from basic to weakly acidic [1]. Interestingly, in most structures of these minerals, UO n bipyramids and TO 4 tetrahedra preferably involve corner sharing only.
Essentially richer structural chemistry with similar architecture is observed for synthetic uranyl compounds, particularly for those that are organically templated and formed in acidic environments. Hydrogen-bonded motifs are particularly common in organically templated synthetic uranyl compounds. Most synthetic protocols, using evaporation techniques, for uranyl compounds with tetrahedral anions employ essentially acidic media (pH = 2-4 and below) [29][30][31][32].
Hence, the minerals formed upon solution evaporation on the mine walls, as well as synthetic compounds, also formed by evaporation of acidic solutions, demonstrate topologically similar crystal structures. This indicated that studies of synthetic compounds, their topological variability and structural peculiarities are of essential importance.
Among both natural and synthetic uranium compounds, the largest topological diversity is exhibited by the structures of compounds containing tetrahedral TO 4 2− anions (T VI = S, Cr, Se, Mo) [14]. Despite chemical differences, they commonly demonstrate topologically identical structural motifs. Note that crystal chemistry of Mo VI compounds is more complex due to its flexible coordination environments.

Materials and Methods
Synthesis. temperature. The cooling rate was 2 • C/h. The crystals occur as aggregates of greenish-yellow transparent plates up to 0.2 mm in maximum dimension.
Single-crystal studies. A single crystal was attached to glass fiber using an epoxy resin and mounted on a Bruker SMART APEX II DUO diffractometer (Bruker, Karlsruhe, Germany) equipped with a micro-focus X-ray tube utilizing MoKα radiation. The experimental data set was collected at 100 K. Unit cell parameters were calculated using least-squares fits. Structure factors were derived using APEX 2 after introducing the required corrections. The structure was solved using direct methods. Further details are collected in Table 1. 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(C-H, N-H) = 1.00 Å). The data are deposited in CCDC under Entry No. 2014144. Selected interatomic distances are given in Table 2.  5 ]. Experiments were carried out at 100 K with Mo K α radiation on a Bruker Smart DUO, CCD.

Results
The structure of 1 (Figure 2a) contains three symmetrically independent uranium atoms each forming the uranyl cation (<U-O> = 1.780, 1.745, 1.760 Å, for U1, U2, U3, respectively). (UO2) 2+ cations (Ur) are each coordinated by five oxygen atoms in the equatorial plane (<Ur-Oeq> = 2.376, 2.391, 2.375 Å, respectively). Five symmetrically unique molybdenum atoms form (MoO4) 2− tetrahedra (<Mo-O> = 1.742, 1.749, 1.760, 1.755, 1.725 Å, respectively). The bond valence sums for U and Mo were calculated using parameters given in [33]: 5.98, 6.17 and 6.14 for U1, U2 and U3, and 6.12, 6.02, 5.86, 5. layer topology has been described earlier in the structures of uranyl molybdates [34], selenates [35] and chromates [36]. The orientation matrix for the terminal oxygen atoms of the molybdate tetrahedra in 1 is given in Figure 3b. A compound with the same chemical composition, same graph of the layers (Figure 3c), but with the different orientation matrix for the terminal oxygen atoms of the molybdenum tetrahedra was found in the structure of [C3N2H12](H3O)2[(UO2)3(MoO4)5] [34]. According to the definition given in [37], such layers are orientation geometric isomers. Thus, the orientation matrix for the [(UO2)3(MoO4)5] 4− layer in 1 has not been observed before. The bond valence sums for U and Mo were calculated using parameters given in [33]: 5.98, 6.17 and 6.14 for U1, U2 and U3, and 6.12, 6.02, 5.86, 5.  [34], selenates [35] and chromates [36]. The orientation matrix for the terminal oxygen atoms of the molybdate tetrahedra in 1 is given in Figure 3b. A compound with the same chemical composition, same graph of the layers (Figure 3c), but with the different orientation matrix for the terminal oxygen atoms of the molybdenum tetrahedra was found in the structure of [C 3 N 2 H 12 ](H 3 O) 2 [(UO 2 ) 3 (MoO 4 ) 5 ] [34]. According to the definition given in [37], such layers are orientation geometric isomers. Thus, the orientation matrix for the [(UO 2 ) 3 (MoO 4 ) 5 ] 4− layer in 1 has not been observed before. There are three common approaches to the description of the structural topology of uranium minerals and synthetic compounds: anionic topologies, graphs, and modular description. In [38], the latter approach was applied to demonstrate that structures of some uranyl selenates may be obtained via assembly of [(UO2)2(SeO4)4(H2O)4] 4− modules. The layers in the structure of There are three common approaches to the description of the structural topology of uranium minerals and synthetic compounds: anionic topologies, graphs, and modular description. In [38], the latter approach was applied to demonstrate that structures of some uranyl selenates may be obtained via assembly of [(UO 2 ) 2 (SeO 4 ) 4 (H 2 O) 4 ] 4− modules. The layers in the structure of [Co(H 2 O) 6 ] 3 (UO 2 ) 5 (SO 4 ) 8 (H 2 O)·H 2 O can also be viewed as a modular arrangement formed by the assembly of chains [39]. A modular approach was also applied to the description of some uranyl chromates, sulfates and selenates [40,41]. It was shown [41] that layers with the ratio UO 2 :TO 4 = 2:3 can be obtained by the association of two types of fundamental chains.
The [(UO 2 ) 3 (MoO 4 ) 5 ] 4− layer in 1 (Figure 4) can be also described as consisting of several modules, i.e., fundamental chains C1, C2, C'1. The C1 and C'1 chains are related by a mirror plane. In  There are three common approaches to the description of the structural topology of uranium minerals and synthetic compounds: anionic topologies, graphs, and modular description. In [38], the latter approach was applied to demonstrate that structures of some uranyl selenates may be obtained via assembly of [(UO2)2(SeO4)4(H2O)4] 4− modules. The layers in the structure of [Co(H2O)6]3(UO2)5(SO4)8(H2O)•H2O can also be viewed as a modular arrangement formed by the assembly of chains [39]. A modular approach was also applied to the description of some uranyl chromates, sulfates and selenates [40,41]. It was shown [41] that layers with the ratio UO2:TO4 = 2:3 can be obtained by the association of two types of fundamental chains.
The  Three topologies of UO2:TO4 = 3:5 layers [14] are known to date. Two of them are represented in Figure 5. In all cases, the layers can be described as alternations of chains. Three topologies of UO 2 :TO 4 = 3:5 layers [14] are known to date. Two of them are represented in Figure 5. In all cases, the layers can be described as alternations of chains. There are three common approaches to the description of the structural topology of uranium minerals and synthetic compounds: anionic topologies, graphs, and modular description. In [38], the latter approach was applied to demonstrate that structures of some uranyl selenates may be obtained via assembly of [(UO2)2(SeO4)4(H2O)4] 4− modules. The layers in the structure of [Co(H2O)6]3(UO2)5(SO4)8(H2O)•H2O can also be viewed as a modular arrangement formed by the assembly of chains [39]. A modular approach was also applied to the description of some uranyl chromates, sulfates and selenates [40,41]. It was shown [41] that layers with the ratio UO2:TO4 = 2:3 can be obtained by the association of two types of fundamental chains.
The  Three topologies of UO2:TO4 = 3:5 layers [14] are known to date. Two of them are represented in Figure 5. In all cases, the layers can be described as alternations of chains.  Figure 5d). The C3 chain can be obtained from C2 by the removal of one link between the nodes in every second four-membered cycle.

The Flexibility of U-O-T Bridges
As follows from above, the inorganic uranyl NTs can be described as comprised of fundamental chains and, at a higher hierarchical level, derived from 3:5 layers or their fragments. The chain propagation axes are collinear to the axis of the NTs themselves. The NT in K5[(UO2)3(SeO4)5](NO3) 3.5H2O is comprised of six fundamental chains oriented parallel to the NT axis (Figure 6g). The scrolling of the precursor layer into the NT is probably enhanced by the flexibility of the U-O-T bridges. This essentially resembles a needle bearing wherein the rollers are replaced by the fundamental chains. The other NTs can be constructed in a similar way: the 7.4 Å-NT requires six chains (Figure 6g), 13.5 Å-NT, nine chains (Figure 6h) while ten are necessary for the 15.3 Å-NT (Figure 6i). To date, all known NTs differ merely by the number of fundamental chains they are comprised of.
In the topology of [(UO 2 ) 10 (SeO 4 ) 17 (H 2 O)] 14− nanotubules, the C'1C'1C2 combination is repeated twice. Overall, the NT is formed from ten chains. Of the remaining four, two chains correspond to the C3 described earlier, and two, to C4. The stacking sequence therefore is . . . C4C3C'1C'1C2C4C3C'1C'1C2 . . . The C4 are produced from C1 if one link is broken between the tetrahedron and bipyramid in the (UO 2 ) 2 (TO 4 ) 2 cycle. Note the essential disorder [43] of some oxygen positions in the structure of (C 4 H 12 N) 14  It was noted earlier [14] that vertex-sharing between the UO n and TO 4 polyhedra results in the formation of a flexible "hinge" as the corresponding U-O-T angles can vary in a relatively broad range, providing the flexibility of the overall polyhedral backbone sufficient to compensate, for instance, the overall thermal expansion [50,51]. Yet, this mechanism seems not to contribute essentially to the formation of the NTs. The distribution of the U-O-T angles in the UO 2 :TO 4 = 3:5 layers (Figure 7a) covers a broad range of 120 • -165 • ; it has a nearly normal character with a maximum at 135 • -140 • . Almost the same is observed for the four NTs known to date (Figure 7b

Conclusions
We described the synthesis and structure of the new uranyl molybdate templated by protonated isopropylamine molecules. The structure of modular [(UO2)3(MoO4)5] 4− layers in 1 represents a new isomer, not observed previously in uranyl molybdates.

Conclusions
We described the synthesis and structure of the new uranyl molybdate templated by protonated isopropylamine molecules. The structure of modular [(UO 2 ) 3 (MoO 4 ) 5 ] 4− layers in 1 represents a new isomer, not observed previously in uranyl molybdates.

Conclusions
We described the synthesis and structure of the new uranyl molybdate templated by protonated isopropylamine molecules. The structure of modular [(UO2)3(MoO4)5] 4− layers in 1 represents a new isomer, not observed previously in uranyl molybdates.
Four different fundamental chains (C1, C`1, C2, C3) are sufficient for constructing any known layered topology with UO2:TO4 = 3:5, whereas five chains (C1, C`1, C2, C3, C4) are required for the known architectures of uranyl-based inorganic NTs (Figure 8). The layers in the structure of 1 are topologically similar to the layers forming NTs in uranyl sulfates and selenates. In addition, the [(UO2)3(MoO4)5] 4− layers in 1 are identical to those in uranyl chromates. We can assume the formation of NTs with a similar topology in the structures of uranyl molybdates and uranyl chromates. The rarity of uranyl-based inorganic NTs, known to date, stems probably from the very narrow range of conditions which favor layer scrolling and which are only poorly understood to date.
While the majority of uranyl compounds adopt layered structures where the 1:2 and 2:3 layers are most common and exhibit rich topological diversity, these layers could not yet be scrolled. The formation of these layers requires only two types of fundamental chains [41], which is probably insufficient to form a NT. Out of ca. 1000 known uranyl compounds, only 10 contain the UO2:TO4 = The layers in the structure of 1 are topologically similar to the layers forming NTs in uranyl sulfates and selenates. In addition, the [(UO 2 ) 3 (MoO 4 ) 5 ] 4− layers in 1 are identical to those in uranyl chromates. We can assume the formation of NTs with a similar topology in the structures of uranyl molybdates and uranyl chromates. The rarity of uranyl-based inorganic NTs, known to date, stems probably from the very narrow range of conditions which favor layer scrolling and which are only poorly understood to date.
While the majority of uranyl compounds adopt layered structures where the 1:2 and 2:3 layers are most common and exhibit rich topological diversity, these layers could not yet be scrolled. The formation of these layers requires only two types of fundamental chains [41], which is probably insufficient to form a NT. Out of ca. 1000 known uranyl compounds, only 10 contain the UO 2 :TO 4 = 3:5 layers, which can be scrolled, and just a handful of 4:7 and 5:8 representatives are known. Hence, formation conditions are very specific already for the "parental" layers, to say nothing about those for the scrolling and formation of nanotubules.
The results of the topological analysis of layered UO 2 :TO 4 = 3:5 architectures and derived nanotubules suggest that the latter may also form in uranyl molybdate systems.