Uranyl Sulfate Nanotubules Templated by N-phenylglycine

The synthesis, structure, and infrared spectroscopy properties of the new organically templated uranyl sulfate Na(phgH+)7[(UO2)6(SO4)10](H2O)3.5 (1), obtained at room temperature by evaporation from aqueous solution, are reported. Its structure contains unique uranyl sulfate [(UO2)6(SO4)10]8− nanotubules templated by protonated N-phenylglycine (C6H5NH2CH2COOH)+. Their internal diameter is 1.4 nm. Each of the nanotubules is built from uranyl sulfate rings sharing common SO4 tetrahedra. The template plays an important role in the formation of the complex structure of 1. The aromatic rings are stacked parallel to each other due to the effect of π–π interaction with their side chains extending into the gaps between the nanotubules.


Introduction
From the point of view of environmental chemistry and mineralogy, uranyl sulfates are of great interest as possible alteration products of nuclear waste [1][2][3]. Uranyl oxysalt structures often contain large open spaces like channels or cavities and are therefore materials with potential practical importance, e.g., for sorption, separation, and catalysis processes in the actinide industry, or for the management of spent nuclear fuel [4]. The crystal chemistry of U(VI) compounds containing various tetrahedral oxo-anions (e.g., SO 4 , SeO 4 , VO 4 , PO 4 etc.) is dominated by two-dimensional (2D) structural motifs, resulting from the well-known and strong directional anisotropy of the bond distribution in the uranyl coordination geometry [5]. In contrast, framework structures are less common for uranyl oxysalts [6], and the current knowledge concerning purely inorganic nanotubules is limited to three uranyl selenate based species [7][8][9], which differ in their inner diameters, viz., two of them are 0.7 nm [7,8] and the other one is 1.5 nm [9]. In other, not purely inorganic, U(VI) compounds, such as metal-organic or hybrid systems, the number of structures containing nanotubules is higher [10][11][12].

Single Crystal X-ray Diffraction Studies
Single crystals of 1 were mounted on thin glass fibers for X-ray diffraction (XRD) analysis and tested using a X8 APEX II X-ray diffractometer (Bruker, Karlsruhe, Germany) with a fine-focus X-ray tube delivering MoKα radiation, λ = 0.71073 Å at 50 kV and 40 mA. More than a hemisphere of three-dimensional data was collected with a frame width of 0.5 • in ω, and 30 s exposition time per frame. The diffraction data were integrated and corrected for absorption using a multi-scan type model integrated in the APEX2 and SADABS programs (Bruker, Madison, WI, USA). The unit cell parameters of 1 (a = 44.001(10) Å, c = 10.367(2) Å, V = 17382(9) Å 3 ) were determined and refined by least-squares techniques on the basis of 37,916 reflections. The value of the|E 2 -1|parameter, 0.738, indicated a high probability of a non-centrosymmetric space group, which was confirmed by the subsequent structure solution and refinement. The refined Flack parameter (x = 0.180(9)) indicated that the studied crystal of 1 has almost pure absolute configuration. The structure was solved in space group R3m by direct methods and refined to R 1 = 0.028 (wR 2 = 0.076) for 8212 reflections with |F o | ≥ 4σF by using the SHELXL-2013 program implemented in the WinGX program package (University of Glasgow, Glasgow, Great Britain). The final model included coordinates and anisotropic displacement parameters for all atoms, except hydrogen atoms which could not be localized. Data collection refinement parameters and detailed crystallographic information are provided in Table 1. CCDC 1579352 contains the supplementary crystallographic information for 1.

High-Temperature Powder X-ray Diffraction Studies
Some of the obtained single crystals were ground using an agate mortar and then subjected to a high-temperature powder X-ray diffraction analysis in air by means of a Ultima X-ray diffractometer (CuKα radiation) (Rigaku, Tokyo, Japan) equipped with a high-temperature camera HTA 1600 (Rigaku, Tokyo, Japan). The samples were prepared from heptane suspension on Pt-Rh plates. Temperature steps were 5 K in the range 20-110 • C. The evolution of the powder diffraction patterns with increasing temperature is shown in Figure 2.

Infrared Spectroscopy
In order to obtain infrared (IR) absorption spectra, a powdered sample of 1 was mixed with dried KBr, pelletized, and the spectra recorded using an ALPHA FTIR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) with a resolution of 4 cm −1 . In total, 16 scans were accumulated. The IR spectrum of an analogously prepared pellet of pure KBr was used as a reference.

Crystal Surface Microtopography
To determine a possible correlation of the presence of nanotubules in the structure with the micromorphology of the crystal faces, the surface of the crystals of 1 was studied by means of an atomic force microscopy (AFM) (NT-MDT, Ntegra Prima, Zelenograd, Russia) in a contact mode with the measurement of the cantilever (ETALON HA-C, NT-MDT, Zelenograd, Russia) DFL signal. Crystals were studied in air and without additional surface treatment. Results are reported in the caption of Figure S2.

Results and Discussion
Compound 1 is stable up to approximately 60 • C when the diffraction maxima gradually disappear (Figure 2). At 75 • C, peaks of "UO 2 SO 4 ·H 2 O" (PDF # 00-028-1418) with unknown structure appear indicating a loss of organic molecules, whereas Na is probably accumulating in an amorphous phase.
The structure of 1 contains four symmetrically independent U(VI) and six S(VI) atoms. Each uranium atom is strongly bonded to two O atoms to form a uranyl ion, UO 2 2+ (U1-O20 = 1.745 (12) [14]. UO 7 bipyramids share common corners with S1O 4 , S2O 4 , S3O 4 and S6O 4 tetrahedra, thus forming the rings shown in Figure 1b. The rings can be unfolded into single chains which were previously reported in the structures of different uranyl oxysalts with tetrahedral anions [15] (Figure 1c). Additional S4O 4 and S5O 4 tetrahedra provide the stacking of the rings into [(UO 2 ) 6 (SO 4 ) 10 ] 8− tubules extending parallel to the c axis ( Figure 3a) and packed in a hexagonal-type fashion (Figure 3b). We were unable to determine the positions of any C, N, or O atoms inside the tubes, obviously due to their heavily disordered arrangement. Various attempts using the algorithms and programs implemented in the Platon software package remained unsuccessful. The internal diameter of the tubules measured as the distance between the closest oxygen atoms across the tubule is 13.5 Å, which is close to the inner diameter of uranyl selenate nanotubules in (C 4 H 12 N) 14 [(UO 2 ) 10 (SeO 4 ) 17 (H 2 O)] [9]. It is worth noticing that for this compound, it was likewise impossible to localize the organic molecules inside the tubules. In Reference [9], the [(UO 2 ) 10 (SeO 4 ) 17   The protonated phg molecules stack in ordered piles in the gaps between the tubes probably forming NH . . . O hydrogen bonds with the apical oxygens of the SO 4 tetrahedra; additional strong links are provided by sodium cations coordinating oxygens from both sulfate and carboxyl groups. The arrangement of the ordered organic part of the structure may thus be considered as columns of phgH + cations, likely exhibiting π-π interactions between their parallel phenyl rings with their (CH 2 COOH) side chains "embracing" the tubes. There is a good size match between the size of this part and the inter-tube space which reflects the templating effect of both Na + and phgH + which can be supposed to not only assist the formation of the nanotubes, but also to determine their size.  Despite the fact that the content of the inner part of the tubes could not be localized (due to its strong disorder) by means of the single-crystal X-ray analysis, some conclusions can be drawn based on the features of the IR spectrum described above, supplemented by charge balance considerations: (i) Characteristic bands of H 3 O + groups could not be revealed in the spectrum of 1; (ii) Disordered protonated phg molecules fill the inner part of uranyl sulfate tubules; (iii) Other organic matter which was present in the synthesis has not been incorporated into the structure of 1 as characteristic bands of its molecules are not observed, in particular those expected at~3440 cm −1 , 1270-1287, 1027-1029, 867-877, and 820-830 cm −1 for aniline [26] and cyclic anilide of N,N-aniline diacetic acid.

Final Remarks
During the last 10 years, numerous attempts at obtaining nanotubular materials in a sulfate-bearing uranyl system have been without success. The present study has now demonstrated that uranyl-based nanotubes can indeed occur in such a system as well and their occurrence is not restricted to selenium containing systems. This suggests that further exploration of uranyl-based systems with other tetrahedral oxoanions is worthwhile despite the obvious predominance of layered structures in these materials. The uranyl nanotubes reported herein can notionally be unfolded into layered prototype structures in a way similar to how carbon nanotubes are related to the graphene layers of graphite [27], or how MoS 2 nanotubes form from layers in molybdenite [28]. Undiscovered, but conceivable nanotubular structures, based on uranyl polyhedra and other tetrahedral TO 4 oxoanions, promise to show great structural variety because of the well-known flexibility of the U-O-T links.
The uranyl selenate nanotubules described in References [7,8] can be described as "narrow" as their diameter is only half of that found in 1. The "wider" nanotubules of [(UO 2 ) 10 (SeO 4 ) 17 (H 2 O)] 14− [9] contain water in their walls. Compared to these two cases, the [(UO 2 ) 6 (SO 4 ) 10 ] 8− nanotubules in 1 are unique to date as they occur in a sulfate containing system and their tube walls are unhydrated. The (UO 2 ) 6 (SO 4 ) 10 ] 8− tubule in 1 has several isomorphous topologies among layered uranyl selenates and molybdates; under appropriate conditions, the formation of similar nanotubules is conceivable in these systems as well. Supposedly, the organic template with its aromatic ring and the functional side chain is essential for the formation of the structure of 1, whereby π-π interactions and hydrogen bonds probably play an important role. Unfortunately, the given limitations of the present experiment did not allow for a determination of these fine details. These limitations are mostly due to (i) inherent difficulties of studying light elements in the presence of heavy uranium using X-ray diffraction methods (ii) strong disorder of the organic part and water and (iii) possible defects of the crystals used because of their growth in a complex, little-known medium.
Recently, various mechanisms for the formation of uranyl selenate nanotubules were suggested [29]. The strong disorder in the interior of the nanotubules in Reference [9] did not allow the establishment of a well-defined model of the "large" nanotubules formation. In References [7,8], it was proposed that the K + ion was responsible for the formation of "small" nanotubules and of curved topologies in selenate systems in general. This seems to be different in 1, which contains Na and an additional organic template. It could be hypothesized that phenylglycine template forms self-assembled cylindrical micelles in solution as a result of π-π interactions between the phenyl rings, with the subsequent assemblage of uranyl cations and sulfate anions around the template. The sodium cation coordinates both the sulfate anions of the framework and the carboxylic groups. The role of organic admixtures in the crystal growth process is less well understood. We suggest that the tar formed from the cyclic anilide of the N,N-aniline diacetic acid and the unreacted aniline serves as a gel medium for the nucleation and growth of the crystals. Selenic acid, as a much more powerful oxidizing agent, would destroy both aniline and phenylglycine, thus preventing gel formation and the formation of nanotubules similar to those occurring in the sulfate system. The strong structural control of the nanotubules by the structure of the organic template suggests that any ion-exchange reactions are unlikely to occur. Further syntheses in various chemical systems, and the accumulation of data on uranium nanotubes formation, may help in predicting possible applications of uranyl-based nanotubes, e.g., the utilization of depleted uranium or separation in the actinide industry.