U(VI) Coordination Modes in Complex Uranium Silicates: Cs[(UO₆)₂(UO₂)₉(Si₂O₇)F] and Rb₂[(PtO₄)(UO₂)₅(Si₂O₇)]

Abstract

Crystals of two new inorganic uranyl silicates, Cs[(UO₆)₂(UO₂)₉(Si₂O₇)F] (1) and Rb₂[(PtO₄)(UO₂)₅(Si₂O₇)] (2), were produced from melts in evacuated silica tubes. Their structures have been solved by direct methods: 1 is trigonal, P-31c, a = 10.2040(3), c = 17.1278(5) Å, V = 1544.45(10) ų, R₁ = 0.042; 2 is tetragonal, P4/mbm, a = 16.0400(24), c = 3.9231(6) Å, V = 1009.34(10) ų, R₁ = 0.045. 1 is the first example of cation–cation interactions between the uranyl polyhedra in uranyl silicates. Therein, U^VI adopts three coordination modes, UO₆ octahedra, UO₆F, and UO₇ pentagonal bipyramids, with the latter sharing common edges to form U₂O₁₂ dimers. Three dimers associate into six-membered rings via cation–cation interactions. The structure of 1 can be described as a complex uranyl fluoride silicate framework with channels filled by the U1 atoms and disordered Cs⁺ cations. 2 represents a new type of topology never observed before among the structures of uranyl compounds; it is also a first complex uranium platinum oxide. Therein, the UO₆ tetragonal bipyramids share edges to form chains. Five such chains are stitched into a complex ribbon via the silicon polyhedra. The ribbons are connected into a framework by the PtO₄ squares; rubidium atoms are located in the channels of the framework.

1. Introduction

Silicates of hexavalent uranium are commonly observed in the oxidation zones of uranium deposits [1]; they are assumed to contribute to the migration of uranium in geological media [2]. To date, there are as many as 21 mineral species containing tetra- and/or hexavalent uranium cations and silicate anions [3]. Strong metamictization of mineral samples essentially hinders their studies by diffraction methods [4]. During model experiments on uraninite oxidation, the KNa₃[(UO₂)₂(Si₄O₁₀)₂](H₂O)₄ compound was found [5]. Thus, it was demonstrated that uranium silicates can form upon the interaction of uranium-containing wastes with the walls of geological repository. An important property of uranium silicates is the possibility of cation exchange, demonstrated on the samples of boltwoodite [6,7] and cuprosklodowskite [8]. It can play an important role in immobilization or, reversely, the release and subsequent migration of fission products, e.g., ¹³⁷Cs.

In contrast to minerals, synthetic uranium silicates exhibit more diverse structural chemistry due to the larger ranges of experimental conditions and compositions studied; last but not the least, note the ability of the SiO₄ tetrahedra to polymerize into disilicate groups, chains, layers, and frameworks [8,9,10,11,12,13,14,15]. Microporous uranyl silicates are of particular interest as potential absorbents and ion exchangers, as well as molecular sieves [13]. Many of these intriguing compounds have been produced using flux techniques [16,17].

The topological diversity of uranium compounds is underpinned by the relatively non-rigid coordination of hexavalent uranium, as well as varied modes of polyhedra linkage. In the majority of its inorganic compounds, hexavalent uranium forms a uranyl cation coordinated, in its equatorial plane, by four to six ligands [18]. However, cases are known when the coordination of U^VI is closer to regular octahedral [19] or a so called “tetraoxide core” is formed [20]. Most commonly, the uranium polyhedra share their equatorial vertices or edges to each other. The cases interactions via the axial oxygen atoms of uranyl cations are very rare [19,21,22].

Among the uranyl compounds with tetrahedral TO₄ cations, silicates form a relatively small but structurally diverse group. Silicates are often isostructural with reported germanates [17], yet the structural chemistry of the former is richer due to the size difference between the GeO₄ and SiO₄ species, the larger condensation degree of silicate tetrahedra, and better size agreement between the Si₂O₇ group and the equatorial edge of the uranium polyhedron [11].

In the current paper, we report two new complex uranyl silicates: Cs[(UO₆)₂(UO₂)₉(Si₂O₇)F] (1) and Rb₂[(PtO₄)(UO₂)₅(Si₂O₇)] (2).

2. Materials and Methods

2.1. Synthesis

Red crystals of 1 were obtained during a high-temperature synthesis starting from 120 mg U₃O₈ (Vecton, 99.7%), 24 mg of CsCl (Vecton, 99.7%), 24 mg of RbCl (Vecton, 99.7%), 14 mg of SiO₂ (Vecton, 99.7%), and 60 mg of PbO (Vecton, 99.7%). This mixture was transferred to a silica tube; then, 10 μL of HF was added, after which the tube was evacuated and sealed. The tube was heated to 900 °C at a rate of 100 °C/h, soaked for 50 h, and cooled to room temperature at a rate of 10 °C/h.

Red needle crystals of 2 were obtained during a high-temperature synthesis starting from 100 mg U₃O₈ (Vecton, 99.7%), 15 mg of RbCl (Vecton, 99.7%), 14 mg of SiO₂ (Vecton, 99.7%), and 30 mg of PbO (Vecton, 99.7%). This mixture was first placed in a platinum crucible and then transferred to a silica tube, evacuated, and sealed. The tube was heated to 950 °C at a rate of 100 °C/h, soaked for 100 h, and cooled to room temperature at the rate of 5 °C/h.

2.2. Single-Crystal X-Ray Experiments

Single-crystal X-ray data were collected using a Rigaku XtaLAB Synergy-S diffractometer equipped with a PhotonJet-S detector operating with MoKα radiation at 50 kV and 1 mA. A single crystal of each phase was chosen, more than a hemisphere of data was collected with a frame width of 0.5° in ω, and 10 s were spent counting for each frame. The data were integrated and corrected for absorption by applying a multi-scan type model using the Rigaku Oxford Diffraction programs CrysAlis Pro. Tiny needle crystals of 2 are unstable under an X-ray beam.

The structures of 1 and 2 were solved and successfully refined with the use of the SHELX software package [23]. The atom coordinates and thermal displacement parameters for each atom were collected in the corresponding cif files; experimental parameters are provided in

Table 1. Crystallographic parameters and structure refinement details for Cs[(UO₆)₂(UO₂)₉(Si₂O₇)F] (1) and Rb₂[(PtO₄)(UO₂)₅(Si₂O₇)] (2).

Compound	1	2
Crystal system	trigonal	tetragonal
Space group	P-31c	P4/mbm
a, Å	10.2040(3)	16.040(2)
c, Å	17.1278(5)	3.9231(6)
V, Å3	1544.45(10)	1009.3(3)
F(000)	2800	1936
Density	7.351	7.815
Radiation, wavelength, Å	MoKα, 0.71073
Ranges of h, k, l	–13 ≤ h ≤ 12, –13 ≤ k ≤ 12, –22 ≤ l ≤ 21	−18 ≤ h ≤ 18, −18 ≤ k ≤ 18, −4 ≤ l ≤ 4
Number of reflections	8347	15,047
Number of unique reflections	1093	525
2θmin–2θmax	3.313–27.987	1.79–24.63
Rint/Rsigma	0.029/0.043	0.029/0.094
R1 [F > 4σ(F)]/wR1	0.029/0.053	0.045/0.12
R2/wR2	0.039/0.091	0.064/0.13
GOF	1.162	1.151
CCDC	2,214,650	2,214,660

. Unit cell parameters are presented for a temperature of 300 K.

2.3. Elemental Analysis

Semi-quantitative elemental analyses were measured using a field emission scanning electron microprobe (LEO EVO 50) equipped with an Oxford INCA Energy Dispersive X-ray Spectrometer (EDX). EDX data were collected from several crystals of each compound and demonstrate that Pb and Cl were not incorporated, while the presence of all the elements reported has been confirmed.

3. Results

Cs[(UO₆)₂(UO₂)₉(Si₂O₇)F] (1). There are three different types of uranyl polyhedra in the structure of 1: U1O₆, U2O₆F and U3O₇. The U1 atoms (Figure 1a) are coordinated by six oxygen atoms (2.066(7) Å × 3 and 2.108(7) Å × 3), thus centering slightly distorted octahedra. Hence, U1 does not form the uranyl cation but rather contributes to the anionic part of the structure forming the [UO₆]⁶⁻ species. The U2 (Figure 1b) and U3 (Figure 1c) form typical uranyl (Ur) cations (<U-O_ap> = 1.769(7) and 1.799(7) Å, respectively). Ur(2) in the equatorial plane is coordinated by four oxygen (<U-O_eq> = 2.433 Å) and one fluorine atom to form Ur(2)O₄F polyhedra. Ur(3) is coordinated by five oxygens (<U-O_eq> = 2.359 Å).

One symmetrically independent silicon atom is tetrahedrally coordinated (Figure 1d) with <Si-O> = 1.642 Å. Two SiO₄ tetrahedra share a common O6 oxygen atom to form the Si₂O₇ disilicate group.

The cesium cations are disordered over two partially occupied sites. The bond valence sums (5.70, 5.76, 5.97, 1.17, and 3.70 v.u. for U1, U2, U3, Cs1, and Si1 atoms, respectively), calculated using the parameters from [24], correlate well with the formal valences of the atoms.

Two Ur(3)O₅ bipyramids share common O2-O2 edges (Figure 1e,f) to form a Ur(3)₂O₈ dimer. Three such species are linked via O3 vertices to form six-membered rings (Figure 1f) with U1 atom in the center. The O3 vertex comprises, on the one hand, the apical (uranyl) vertex of one Ur(3)O₅ and, on the other hand, an equatorial vertex of another Ur(3)O₅ polyhedra. In other words, the dimers are linked via the cation–cation interactions. Three Ur(2)O₄F species are linked via a common fluoride vertex into a Ur(2)₃O₁₂F trimer (Figure 1g). Si₂O₇ disilicate groups are attached to Ur(2)₃O₁₂F moieties along the c axis (Figure 1h).

The structure of 1 can be described as a complex uranyl fluoride silicate framework (Figure 2a,b) with channels filled by the U1 atoms and disordered Cs⁺ cations. The framework can be split into two blocks (Figure 2c). The Ur(3)O₅ species, which are linked via the cation–cation interaction and Si₂O₇ groups, form the blocks designated here as A in Figure 2d, wherein the voids are filled by U1 atoms. The Ur(2)₃O₁₂F groups are arranged in block B with large voids occupied by Cs⁺ cations (Figure 2e).

Rb₂[(PtO₄)(UO₂)₅(Si₂O₇)] (2). The structure of 2 contains two symmetrically independent U⁶⁺ cations, which form linear (UO₂)²⁺ uranyl ions with the U⁶⁺-O bond lengths of ca. 1.76 Å (Figure 3a,b). The Ur ions are coordinated by four oxygen atoms each, arranged in the equatorial planes of the tetragonal bipyramids. The coordination environments of the Si sites are strongly distorted due to the disorder. According to the results of the stable structure solution, the Si ions are coordinated by three oxygen atoms (Figure 3c) at equatorial plane (<Si-O> = 1.548 Å) and by two half occupied oxygen atoms at vertical plane (Si-O = 1.962(1) Å). The former distances are somewhat shorter, while the latter are longer that those expected for the true “[SiO₅]⁶⁻” species (1.60 Å [25]). Most likely, the true structure contains two equiprobable “up” and “down” orientations of the SiO₄ tetrahedra, forming disordered disilicate groups (note the one-half occupancy of the O7 site); however, all attempts to split the silicon position into two tetrahedral sites did not provide a stable refinement. Compared to germanium, silicon contributes less to the overall scattering dominated by the uranium, platinum, and cesium cations, so its positioning, particularly in partially occupied sites, faces more difficulties and is less precise.

The platinum site (Figure 3d) is coordinated by four oxygen atoms (<Pt-O> = 2.046 Å) strongly preferred by Pt²⁺. The bond valence sums (2.24, 6.06, 5.77, 0.97, and 3.78 for Pt1, U1, U2, Rb1, and Si1, respectively), calculated using the parameters from [24], correlated well with the formal valences of the atoms.

The uranium and silicon polyhedra share vertices to form [(UO₂)₅(Si₂O₇)]⁴⁺ ribbons (Figure 3e). The Ur(1)O₄ species share common edges to form the Ur(1)O₂ chains, which link to the silicate tetrahedra via the O4 vertex. Similarly, the Ur(2)O₄ species share edges to form four Ur(2)O₂ chains additionally linked via O5 and O6 oxygens of the silicate units (Figure 3f). Each O5 and O6 vertex therefore belongs to two uranium and one silicon polyhedra. The O7 position is unlikely to be coordinated to the uranium cations. Therefore, each silicon polyhedron is linked to six uranium cations whose coordination mode has not been observed before; the topology of the uranyl-silicate ribbons is also new and unique. These [(UO₂)₅(Si₂O₇)]⁴⁺ ribbons in 2 are aligned alternatingly (Figure 4) and linked to the framework by the PtO₄ squares. The remaining channels host the rubidium cations.

4. Discussion

The framework in 1 is structurally close to those in the structures of known uranyl germanates [19]: NH₄(UO₆)₂[(UO₂)₉(GeO₄)(GeO₃(OH))], K(UO₆)₂[(UO₂)₉(GeO₄)(GeO₃(OH))], Li₃O(UO₆)₂[(UO₂)₉(GeO₄)(GeO₃(OH))], and Ba(UO₆)₂ [(UO₂)₉(GeO₄)₂]. All of these compounds, including 1, adopt the same space group, and the differences in the cell edges do not exceed 0.2 Å; given these small differences, the metrics of 1 are most close to the Ba uranyl germanate. This is not surprising given that positive differences in the radii of Rb⁺ and Ba²⁺ are compensated for by the negative differences in the radii of Si⁴⁺ and Ge⁴⁺. The differences mainly concern the coordination of the tetrahedral cation (Figure 5). In the NH₄, K, and Li compounds, the coordination of germanium is described as trigonal pyramidal so that the chains of such pyramids are stretched along [001]; in addition, one vertex is protonated. Therein, the Ge sites are disordered (Figure 5a), the mean equatorial bond length is 1.695–1.777 Å, and the axial distances range from 1.767 to 2.615 Å.

In the structure of Ba(UO₆)₂[(UO₂)₉(GeO₄)₂] (Figure 5b), the germanium sites are ordered so that [Ge₂O₇]⁶⁻ is clearly visible, and the Ge-O bond lengths range from 1.741–1.752 Å for the terminal oxygens to 2.548 Å for the bridging atoms. A similar pattern is observed for disilicate anion in 1: the coordination of silicon atoms is a nearly regular tetrahedral with a mean bond length of 1.640(7) Å and O-Si-O angles of 110.6(3) and 108.3(3)°. The F1 sites reside between the disilicate groups; the Si1-F1 separation of 2.640(7) Å (Figure 5c) can be considered nonbonding. In 2 (Figure 5d), the environment of silicon is similar to those of germanium in the structures of uranyl germanates of monovalent cations. As expected, all distances decrease when proceeding from the germanates to the silicate.

Our results illustrate once more that the use of non-ambient conditions is a powerful tool for the preparation of unusual (sometimes even unexpected) compounds. While the preparation of a silicate analog of complex germanates, 1, does not seem very much surprising, the synthesis of 2 starts a new page in the book of uranium oxide compounds.

By now, there are two compounds containing uranyl cations and platinum, yet the latter is in the state of complex anion, the most common finding in the chemistry of platinum: K₃((UO₂)₂(OH)(Pt(CN)₄)₂)₂(NO₃)(H₂O)_1.5 [26] and ((U₂(H₂O)₁₀O)(Pt(CN)₄)₃)(H₂O)₄ [27]. All of the other compounds of U and Pt are oxygen-free intermetallics. We therefore suggest that under conditions close to extreme (high temperatures and pressures, like those in synthesis of 2), even the platinum metal becomes sufficiently reactive. It is as likely that the simultaneous presence of lead and halide also enhances the reactivity of platinum, maybe via the intermediate formation of its halides: our recent study of the thermal behavior of a Pb₇O₆Br₂ oxybromide resulted in the formation of PbPt₂O₄ traces above 800 °C [28]. The formation of 2 demonstrates the possibility of preparing other, even less usual, compounds of platinum under severe conditions. On the other hand, structural analogs of 2, besides the evident replacement of Si by Ge and Rb by Cs or K, may be also obtained using other cations with strongly preferred square planar oxygen coordination, with Pd²⁺ being the most likely candidate. Square planar coordination is also common for Cu^2+, but its geometry is essentially more flexible due to the low crystal field stabilization energy (3d⁹ configuration). The structure of 2 is also a very rare, if not unique, example of incorporating square planar species into structures of uranyl derivatives with tetrahedral anions. By using appropriate heterovalent substitutions, it is also likely that related architectures can be pursued using other cations with preferred square planar oxide coordination such as Cu³⁺ or Ag³⁺ in oxidizing media or even Ni⁺ in reducing medium. On the other hand, the production of such unusual species can be further enhanced by using noble metal oxides as precursors and strongly oxidizing and basic conditions, as suggested by the successful synthesis of tetravalent platinum borate [29].

5. Conclusions

The new compound 1 is the first example of uranium silicate wherein the uranium polyhedra are linked via cation–cation interactions; it is isostructural to that series of germanium compounds prepared hydrothermally at 220 °C. In contrast, the synthesis of 1 was performed at a higher temperature (900 °C) in evacuated silica tubes, most likely from the melt. As a result, the silicon positions in 1 are fully ordered, and the hydroxide group is replaced by fluorine. Upon the synthesis of 2, fluoride was not introduced, which resulted, under similar conditions, in the formation of a compound with a poorer order of the silicate sublattice; the most exciting is the incorporation of platinum in positive oxidation state.