Structure of the {U13} polyoxo cluster U13O8Cl x (MeO)38–x (x = 2.3, MeO = methoxide)

A new type of uranium polyoxo cluster complex consisting of thirteen uranium atoms, [U13(μ4-Ooxo)8Cl x (MeO)38-x ] (x = 2.3, MeO: methoxide), was synthesized and structurally characterized by single crystal X-ray diffraction.

The structure of a new type of polyoxo cluster complex that contains thirteen uranium atoms, {U 13 }, is reported. The complex crystallized from methanol containing tetravalent uranium (U IV ) with a basic organic ligand, and was characterized as dichloridooctacosa-2 -methanolato-octakis(methanolato)octa-4 -oxido-tridecauranium, [U 13 (CH 3 O) 35.7 Cl 2.3 O 8 ] or [U 13 ( 4 -O oxo ) 8 Cl x -(MeO) 38-x ] (x = 2.3, MeO = methoxide) (I), by single-crystal X-ray diffraction. The characterized {U 13 } polyoxo cluster complex (I) possesses a single cubic uranium polyhedron at the centre of the cluster core. To the best of our knowledge, this is the very first example of a polyoxo actinide complex that bears a single cubic polyhedron in its structure. The cubic polyhedron in I is well comparable in shape with those in bulk UO 2 . The U-O bonds in the cubic polyhedron of I are, however, significantly shorter than those not only in bulk UO 2 but also in another analogue in the {U 38 } cluster. This shortening of U-O bonds, together with BVS calculations and the overall negative charge (2À) of I, suggests that the central uranium atom in I, which forms the single cubic coordination polyhedron, is presumably oxidized to the pentavalent state (U V ) from the original tetravalent state (U IV ). Complex I is, hence, the first example of a polyoxo cluster possessing a single cubic coordination polyhedron of U V .

Chemical context
Hydrolysis is one of the most fundamental reactions in aqueous chemistry. The strong hydrolysis of highly charged metal cations (M +n ) induces olation (to form hydroxobridging: M-OH-M) and oxolation (to form oxo-bridging: M-O-M), which eventually results in the formation of hydroxo/oxo-bridged oligomer and cluster complexes in an aqueous solution (Henry et al., 1992). Amongst the hydroxo/ oxo-based oligomer/cluster complexes of metal cations, the polyoxo cluster complexes of f-block elements (i.e. lanthanides and actinides) have been extensively investigated over the last few decades, not only for their engineering applications and environmental impact associated with nuclear industry, but also for the fundamental chemical science of fblock elements (Knope & Soderholm, 2013;Qiu & Burns, 2013). As a discrete polyoxo cluster complex (i.e. not a chainor wheel-shaped cluster) of f-block elements, the largest cluster complex reported thus far is the cluster containing 100 metal cations ({M 100 }) (Russell-Webster et al., 2021), within which a large variety of nuclearity was reported. Based on this background, the present work contributes to further development of the polyoxo cluster chemistry of f-block metals by ISSN 2056-9890 reporting a new member of the polyoxo cluster family of tetravalent uranium (U IV ) that contains thirteen metal centres: {U 13 }.

Structural commentary
The best refinement for the SC-XRD data of the dark-black crystals resulted in the chemical formula C 35.7 H 107.1 Cl 2.3 O 43.7 U 13 , which corresponds to the molecular formula [U 13 ( 4 -O oxo ) 8 ( 4 -O MeO ) 2 ( 2 -O MeO ) 24 Cl 2.3 (O MeO ) 9.7 ] (I). The molecular structure of I (i.e. the {U 13 } cluster) contains seven distinct crystallographically independent uranium centres (U1-U7), which are bridged by eight 4 -O oxo , two 4 -O MeO , and twenty-four 2 -O MeO oxygen donors to form the {U 13 } core. The exterior of the {U 13 } core is further decorated with monodentate chloro and methoxide (MeO) ligands to complete the uranium centres' coordination spheres in terminal positions, eventually forming the {U 13 } cluster compound (I) (Figs. 1 and 2a). In the crystal structure, there is some disorder between the chloro and methoxide ligands at the terminal positions (i.e. Cl1-Cl3). This means that partial chloro ligands and partial methoxide groups occupy the same coordination sites in the average structure and that they can be found on either of three out of the seven refined uranium centres of the asymmetric unit. Given this fact, it is more appropriate to describe the molecular formula of I as [U 13 ( 4 -O oxo ) 8 Cl x (MeO) 38-x ], where x was determined to be 2.3 by SC-XRD. The uranium centres in I are mostly eightfold coordinated, whilst only U3 and U4 are sevenfold coordinated (pink polyhedra in Fig. 2). One uranium centre (U6), which is positioned at the centre of the {U 13 } core unit, forms a nearly ideal cubic polyhedron (dark-purple polyhedra in Fig. 2), whilst the rest of the eightfold coordinated uranium centres (U1, U2, U5 and U7) define distorted square-antiprismatic polyhedra (green polyhedra in Fig. 2). The central cubic uranium polyhedron (U6) is sandwiched with two {U 3 } subunits (pink and green polyhedra in Fig. 2b) along the c-axis direction, and it is further surrounded by a {U 6 } ring (pink and green polyhedra in Fig. 2c). Hence, one cubic uranium polyhedron, two {U 3 } subunits, and one {U 6 } ring assemble to the {U 13 } core [i.e. 1 + (2 Â 3) + 6 = 13]. The sevenfold-coordinated uranium centres (U3 and U4, pink polyhedra in Fig. 2), from a different perspective, form the corners of a square around the central U6, the edges of which are open to allow for a direct view of the central cubic uranium centre as in Fig. 2a Molecular structure of the {U 13 } cluster I. Uranium atoms are illustrated with coloured polyhedra. The structure is drawn as [U 13 ( 4 -O oxo ) 8 -Cl 6 (MeO) 32 ] in order to omit the disorder between chloride and methoxide anions for clarity. Colour code: hydrogen, white; carbon, black; oxygen, red; chlorine, light green. Hydrogen and carbon atoms are also omitted for clarity in (b) and (c).    (Yuan et al., 2017), in which a single cubic polyhedron of the central cerium centre is surrounded by two {Ce 3 } and one {Ce 6 } ring subunits with distorted square-antiprismatic polyhedra.
When assuming the formal oxidation numbers of +4 for U IV , À2 for oxo groups, and À1 for chloride ions and methoxides, the overall charge of the molecule [U 13 ( 4 -O oxo ) 8 Cl x (MeO) 38x ] is calculated to be À2, which is not neutral. Such an unbalanced charge is often observed for the polyoxo cluster complexes of f-block elements (e.g. Takao et al., 2009;Falaise et al., 2013c). In fact, the bond-valence-sum (BVS) calculation (Brown, 1978) (v.u.) for the average charge of thirteen uranium atoms in I (Table 1), which is higher than the formal charge of U IV (i.e. > +4). The results of BVS calculations further indicate that the BVS charge of the U6 atom, which is the central uranium atom in the {U 13 } cluster (dark-purple polyhedra in Fig. 2), is comparable to the pentavalent state (5.16 v.u.), whilst the rest of the uranium atoms (U1-U5 and U7) exhibit BVS charges close to 4 v.u. (i.e. the original tetravalent state) (Table 1). Hence, the central uranium atom U6 in I is presumably oxidized to U V , partly compensating the negative charge of the oxo, chloride and methoxide anions to neutralize the whole molecule. Similar partial oxidation of U IV was also presumed for a {U 38 } polyoxo cluster (Falaise et al., 2013c). Another possible charge compensation to keep the neutrality of I is the replacement of oxo (À2) by hydroxo ligands (À1). That is, the bridging oxo ions (or methoxide groups) in I could be partly protonated, which was also proposed in the {U 38 } polyoxo cluster (Falaise et al., 2013c). Hence, a partial protonation and the oxidation of U IV to U V presumably compensate the negative charges of oxo, chloride and methoxide anions and result in a neutral molecule of I.
The structures of polyoxo clusters of metal cations are often compared with those of their corresponding oxide compounds, as the polyoxo clusters can be potential precursors, which evolve into bulk oxides (Ikeda-Ohno et al., 2013). In the case of U IV polyoxo clusters, the corresponding oxide is uranium dioxide (UO 2 ). The coordination polyhedron of uranium in UO 2 is cubic, as shown in Fig. 3a (dark-purple polyhedron). Amongst the reported polyoxo oligomer and cluster complexes of U IV [i.e. dimers (Le Borgne et al., 2002;Salmon et al., 2006;Schmidt et al., 2014), trimers (Berthet et al., 1993;Duval et al., 2015;Lin et al., 2018), tetramer (Falaise et al., 2013a), hexamers (Mokry et al., 1996;Takao et al., 2009;Mougel et al., 2010;Falaise et al., 2013b), octamer (Salmon et   Cubic coordination polyhedra of uranium (dark purple polyhedra) in different compounds: (a) bulk UO 2 (Cooper, 1982), (b) the {U 38 } cluster (Falaise et al., 2013c), and (c) the {U 13 } cluster (I). The structure of I is drawn as [U 13 ( 4 -O oxo ) 8 Cl 6 (MeO) 32 ] in order to omit the disorder between chloride and methoxide anions for clarity. Colour code: carbon, black; oxygen, red; chlorine, light green; uranium, dark purple. Hydrogen atoms are omitted for clarity in (b) and (c).  (Biswas et al., 2011), and 38-mers (Falaise et al., 2013c;Martin et al., 2018)], only the 38-mers {U 38 } contain cubic coordination polyhedra of uranium, which are comparable to those in bulk UO 2 . That is, the {U 14 } core unit in the {U 38 } cluster consists of fourteen cubic uranium polyhedra, corresponding to a small fraction of face-centred cubic UO 2 ( Fig. 3a and b). The central uranium (U6) in the {U 13 } cluster (I) (dark-purple polyhedron in Fig. 3c) also defines a cubic coordination polyhedron. The cubic polyhedron in the {U 13 } cluster is, however, not surrounded by other cubic uranium polyhedra to evolve into a fraction of fcc-based UO 2 structure. Hence, the {U 13 } cluster contains the smallest unit of cubic uranium polyhedron that is comparable to that in UO 2 . Geometrical parameters of the cubic uranium polyhedra in bulk UO 2 , the {U 38 } cluster, and the {U 13 } cluster are summarized in Table 2. The average O-U-O angle in the cubic uranium polyhedra is 70.5 for all three species, indicating that the shape of the uranium polyhedron is an ideal cube even in the polyoxo clusters. The average U-O distance, however, shortens with decreasing size of the polyhedral cluster. That is, the average U-O distance shortens from 2.368 to 2.357 Å when the size of the polyhedral cluster reduces from bulk UO 2 (infinite cluster) to {U 14 } (sub-unit in the {U 38 } cluster). The U-O distance further shortens to 2.267 Å , which is $5% shorter compared with that in bulk UO 2 , in the case of the single cubic uranium polyhedron in the {U 13 } cluster. This 5% shortening of the U-O distance in the single cubic uranium polyhedron of I is rather remarkable. As a matter of fact, such drastic shortening of M-O distances is not observed in the {Ce 13 } cluster (Yuan et al., 2017), the chemical analogue of the {U 13 } cluster. That is, the Ce-O distances (average: 2.35 Å ) in the central cubic polyhedron of the {Ce 13 } cluster are well comparable with those in bulk CeO 2 (2.34 Å ) (Wyckoff, 1963). Given these facts, it is reasonable to consider that the oxidation state of the uranium ion in the single cubic polyhedron (i.e. U6) is higher than the original tetravalent state of U IV , strengthening (and thereby shortening) the U-O bonds. This also supports the BVS results suggesting a pentavalent state for uranium centre U6 (U V ). Hence, the central uranium polyhedron in I (dark-purple polyhedra in Fig. 2) should be considered an exceptionally rare example of a U V polyhedron with a cubic structure, which is comparable with the cubic U IV polyhedron as in UO 2 . Amongst the polyoxo/hydroxo metal clusters comprising thirteen metal centres ({M 13 }), the Keggin-type {Al 13 } cluster is probably the most famous complex of this type (Johansson et al., 1960;Rowsell & Nazar, 2000

Supramolecular features
Compound I crystallizes in the space group P1. The chemically analogous {U 38 } cluster crystallizes in a more symmetric crystal system in tetragonal setting (I4/m) (Falaise et al., 2013c). This symmetrical difference in crystal structure between the {U 13 } and {U 38 } clusters may stem from the symmetrical difference in their original molecular structures. That is, as shown in Fig. 2a, the molecular structure of I (i.e. the {U 13 } cluster) is slightly oval along the c axis, whilst the shape of the {U 38 } cluster molecule is rather close to a sphere (Falaise et al., 2013c). In the crystal structure of I, there are two sets of intermolecular short contacts that help the molecules to assemble into the crystal structure. These intermolecular short contacts are indicated with light blue lines in Fig. 4. One set of intermolecular short contact (SC1) is found between a hydrogen atom of one bridging methoxide group and a carbon atom of another bridging methoxide group from the adjacent molecule [C7-H8A i = 2.87 Å ; symmetry code: (i) 1 À x, 2 À y, 1 À z, Fig. 4a]. There are two such (bi-directional) SC1 between adjacent molecules, facilitating the molecules being lined up along the b-axis direction. A similar C-H intermolecular short contact (SC2) is formed between a bridging methoxide group and its analogue in an adjacent molecule [C10-H10B ii = 2.89 Å ; symmetry code: (ii) 2 À x, 2 À y, 2 À z, Fig. 4b]. Again pairs of this H-C short contact are found between adjacent bridging methoxide molecules (Fig. 4b), supporting the assembly of molecules of I more or less along a  Table 2 Geometrical parameters (Å , ) of cubic uranium polyhedra in different compounds. diagonal through the cell's origin. The engaged bridging methoxide groups are not affected by the disorders between chloride and methoxide groups. These two types of intermolecular short contacts are, hence, presumably key to assembling the molecules for crystallization. This renders the exterior methoxide groups of I, therefore, important not only for stabilizing the discrete {U 13 } core, but also for supporting the assembly and crystallization of the {U 13 } molecules and the stability of the resulting crystal lattice.
Handling these radionuclides involves a serious risk to human health. Therefore, special precautions with appropriate lab equipment and facilities dedicated to radiation protection are required for handling these radionuclides. Single crystals of the {U 13 } cluster complex were obtained as a by-product when U IV was dissolved in methanol in the presence of a basic organic ligand. The crystals were obtained from the following two different synthetic routes: Route A: [UCl(S)-PEBA) 3 ] (S)-PEBA: (S,S)-N,N 0 -bis(1phenylethyl)benzamidinate) was prepared according to a reported procedure (Kloditz et al., 2020). A solution containing 10 mg of [UCl(S)-PEBA) 3 ] in 1 mL of methanol was transferred into a quartz cuvette and sealed doubly with a lid and Parafilm in a dry and inert glove box filled with nitrogen gas. The cuvette was then taken out of the glove box and kept under atmospheric condition. After ten days, darkblack crystals were obtained with a low yield (<1 mg).
Route B: [UCl 2 (salen) 2 (MeOH) 2 ] (H 2 salen = N,N 0 -bis-(salicylidene)ethylenediamine) was prepared according to a reported procedure (Radoske et al., 2020). A solution containing 7 mg of [UCl 2 (salen) 2 (MeOH) 2 ] in 1 mL of methanol was transferred into a quartz cuvette and sealed doubly with a lid and Parafilm in a dry and inert glove box filled with nitrogen gas. The cuvette was then taken out of the glove box and kept under atmospheric condition. After one week, dark-black crystals were obtained with a low yield (<1 mg).
Synthetic attempts in the absence of an organic ligand did not succeed in obtaining crystals of the {U 13 } polyoxo cluster. It was reported that the reaction between an alcohol molecule (methanol in the present case) and another organic molecule can generate a water molecule, which is the source to trigger the olation/oxolation reaction that could eventually result in the formation of polyoxo clusters (Martin et al., 2018). Hence, the presence of an organic ligand in an alcohol medium is presumably essential to materialize polyoxo metal cluster complexes. Another possible source of water into the synthetic route is the slow penetration of ambient moisture into the sample cuvette via the double sealing, which cannot be completely excluded. Crystals suitable for single crystal X-ray diffraction (SC-XRD) measurements were selected on a polarized light microscope and mounted on a MiTeGen MicroMount TM with mineral oil. Due to the low yield of crystals, additional characterization, such as elemental analysis, FT-IR, powder-XRD, etc., was not feasible. Chemicals (except uranium) employed in this study were commercially available from Sigma Aldrich and were used without further purification.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.  All non-hydrogen atoms were refined anisotropically. H atoms of the methoxide groups were placed in the expected geometric positions and treated in a riding mode with U iso (H) = 1.5 U eq (C). Three apical ligand positions (Cl1-Cl3) in the asymmetric unit showed pseudo-substitutional disorder between negatively charged methoxide (MeO À ) and chloride (Cl À ) ions. This disorder was modelled by constraining the sum of the site occupation factors to unity. Additional constraints (SIMU, DELU and SAME) were applied to avoid chemically unreasonable ellipsoids. Even after the completion of refinement, substantial residual electron density remained around the uranium atoms or within their ionic radii. This is not an uncommon issue in heavy atom structures and was possibly intensified by truncation errors of the Fourier series. Additionally, disorder issues between methoxide and chloride ions caused further residual electron density that could not be modelled in a chemically reasonable manner.  Data collection: SAINT (Bruker, 2016); cell refinement: APEX3 (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: shelXle (Hübschle et al., 2011).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Reflections were merged by SHELXL according to the crystal class for the calculation of statistics and refinement. _reflns_Friedel_fraction is defined as the number of unique Friedel pairs measured divided by the number that would be possible theoretically, ignoring centric projections and systematic absences.