Because of the great importance in nuclear fuel cycle and environmental decontamination, actinide chemistry has gained increasing attentions from chemists, geologists and materials specialists^[1,2]. As a naturally occurring actinide element, uranium, which is mostly featured by the form of hexavalent uranyl cation (UO₂²⁺) in ambient atmosphere, has been utilized as metal node in many uranium-based metal-organic compounds^[3,4,5,6]. In close relation with the strong hydrolysis tendency of uranyl cation at varying aqueous pH values^[7,8,9], a large variety of uranyl building units with different uranium nuclearities and/or connecting modes could be found in the large library of uranyl compounds^[6]. Therefore, structural identification of uranyl compounds with diversified uranyl compositions in solid state can provide an alternative route to explore the solution chemistry of uranyl cation in terms of hydrolysis and speciation under different conditions. Most importantly, the well-defined structural and chemical features of these uranyl compounds with high-nuclear units could be helpful to get more insight on precise control of actinide hydrolysis and effective isolation of certain high-nuclear species from the actinide aqueous solution.

While many oligomeric uranyl units as well as infinite chain-like polynuclear uranyl motifs have been characterized in uranyl-organic compounds, few cases of uranyl-organic compounds with oligomeric octa-nuclear uranyl motifs^[10,11] have been reported so far, though there are several octa-nuclear uranyl cage species constructed through coordination-driven assembly^[12,13]. The scarcity of higher nuclearity might be attributed to harsh forming conditions for them as well as their instability in molecular structure.

Herein, a uranyl-oxalate coordination polymer containing a new type of oligomeric octa-nuclear uranyl motif, [(UO₂)₈O₄-(μ₃-OH)₂(μ₂-OH)₂]⁴⁺, was isolated successfully and identified with the aid of a flexible U-shaped dicarboxylate linker (m-Xyl-BPy4CA) with an extra-long xylylene chain.

(UO₂)₈O₄(μ₃-OH)₂(μ₂-OH)₂(C₂O₄)₂(C₂₀H₁₆N₂O₄)₂ (1) UO₂(NO₃)₂·6H₂O (200 μL, 0.10 mmol), [m-Xyl- BPy4CEt]Br₂ (28.0 mg, 0.05 mmol), NaOH (3.2 mg, 0.08 mmol), ultrapure water (1.0 mL) was loaded into a 15 mL autoclave (the initial pH is 2.72), sealed and heated to 150 ℃ in an oven for 2 d, then automatically cooled to ambient temperature. Dark yellow crystals of 1 (Fig. S1) were produced, filtered off, and rinsed with ultrapure water and subjected to air-drying at room temperature to give 1 in pure phase in spite of preferred alignment of some crystal indices (Fig. S2). Yield: 14 mg (35% based on uranium).

[(UO₂)₂(μ₂-OH)₂(C₂₀H₁₆N₂O₄)₂(H₂O)₂](NO₃)₂ (2) compound 2 was synthesized using the same protocol except a reducing amount of NaOH (2.4 mg, 0.06 mmol). Prism yellow-green crystals were filtered off, and rinsed with ultrapure water and subjected to air-drying at room temperature to give 2 in pure phase (Fig. S1 and S3). Yield: 22 mg (31% based on uranium).

(UO₂)(C₂O₄)(C₆H₅NO₂) (3) compound 3 was synthesized using the same protocol like 1 and 2 but without NaOH added. Stick-like yellow-green crystals of 3 can be obtained as accompanied by considerable amount of 2 and other unidentified impurities (Fig. S1 and S4).

¹H-NMR spectra were recorded on a Bruker AVANCE III (500 MHz, Bruker, Switzerland) with deuterium oxide (D₂O) as solvent. ESI-MS spectra were obtained with a Bruker AmaZon SL ion trap mass spectrometer (Bruker, USA). Powder X-ray diffraction (PXRD) measurements were recorded on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=0.15406 nm) in the range 5°-60° (step size: 0.02°). Thermogravimetric analysis (TGA) was performed on a TA Q500 analyzer over the temperature range of 25-800 ℃ in air with a heating rate of 10 ℃/min. Solid-state fluorescence spectra were measured on a Hitachi F-4600 fluorescence spectrophotometer. The Fourier transform infrared (IR) spectra were recorded from KBr pellets in the range of 4000-400 cm^-1 at Bruker Tensor 27 spectrometer. Raman spectra were recorded on a Thermo Fisher Scientific DXRxi Micro Raman imaging spectrometer excited at 780 nm.

Uranyl-oxalate coordination polymer (namely compound 1, Fig. 1(a)) containing a new type of oligomeric octa- nuclear uranyl motif, [(UO₂)₈O₄-(μ₃-OH)₂(μ₂-OH)₂]⁴⁺ (Fig. 1(b)), was isolated successfully and identified with the aid of a flexible U-shaped dicarboxylate linker (m-Xyl-BPy4CA) with an extra-long xylylene chain (Fig. 1(c-d)). By a direct comparison with another uranyl compounds (namely compound 2 and compound 3) with monomeric or dimeric uranyl nodes from the same organic ligand, factors affecting the formation of compound 1 such as aqueous pH and hydrothermal stability of organic ligand have been discussed. The physicochemical properties of compound 1 were also characterized and compared in detail.

Crystallographic analysis shows that compound 1 crystallizes in the monoclinic crystal system with a space group of P2₁/c, and its asymmetric unit contains four unique uranium centers, a linker of m-Xyl- BPy4CA ([C₂₀H₁₆N₂O₄]) and an in situ-formed oxalate ligand ([C₂O₄]^2-) (Fig. 2(a)). The formation of oxalate ion is more or less a little surprising in spite of several similar cases found in other heterocycle-based or labile organic ligands^[13]. Viewing from the extending structure of the asymmetric unit, an elongated U8 motif can be found (Fig. 2(b)), among which every uranium center is in a pentagonal bipyramid (P-type) geometry with equatorial U-O bond distances in the range of 0.2193(14)-0.2578(14) nm (Table S1), and links to each other through sharing equatorial edges with other neighboring polyhedrons. Based on the bond valence sum analysis^[14], the bond valence value of O(11) is 1.393, which should be assigned to μ₃-hydroxo group, while the atoms of O(9) and O(10) are μ₃-oxo groups (bond valence values of 1.943 and 2.20, respectively), and O(12) is μ₂-hydroxo group (bond valence values of 1.137). The feature of μ₃- hydroxo group for O(11) is ascribed to weak binding affinity to U(3) and U(4). Compared with the cation- cation interactions (CCIs) directed U8 motif reported previously^[11], the [(UO₂)₈O₄(μ₃-OH)₂(μ₂-OH)₂]^4- motif found here represents a new type of uranyl oligomer with a nearly planar geometry (Fig. S5).

Each U8 motif is eight-connected with four oxalate and four m-Xyl-BPy4CA moieties surrounded (Fig S6(a)). The oxalate ligands, always go together with a U-shaped bidentate m-Xyl-BPy4CA, promote the extension of U8 motif from four directions through connecting four adjacent ones in a bridging mode (one side is in μ^2-η² : η¹ mode and the other side is in η² mode) (Fig. S6(b)), and subsequently lead to a 2D network with the minimum rhombic loop in size of 1.193 nm×1.077 nm (Fig. S6(c-d)). Detailed analysis reveals that, every U8 motif displays a different overall orientation from that of its adjacent U8 with an angle of inclination of 36.6(4)°, which is contributed to the distortion of the above rhombic loop (Fig. S7).

When the amount of sodium hydroxide added to the hydrothermal system of uranyl and [m-Xyl- BPy4CEt]Br₂ decreased gradually, another two uranyl compounds 2 and 3 emerge. Compound 2 is a molecular compound with a dimeric uranyl motif containing two equivalent uranyl centers (Fig. 3(a-b)). Each uranyl is in a pentagonal bipyramid geometry with equatorial U-O bond distances in the range of 0.2325(2)-0.2576(2) nm (Table S1). Besides two bridging hydroxyl groups and two water molecules, the uranyl dimer is coordinated by two bidentate η^1-mode m-Xyl-BPy4CA linkers both in a head-to-tail way, and finally gives a pattern of twinned double loops (Fig. 3(b)). The simple double loops interact with two nitrate anions by hydrogen bonds (Fig. S8 and Table S2) and stack with each other to form the final crystal lattice of molecular compound (Fig. 3(c-d)).

Another uranyl compound 3 with a one-dimensional (1D) chain-like topology was produced under the more acidic condition. As the case of compound 1, the in situ-formed oxalate ligand ([C₂O₄]^2-) is present in compound 3, indicating the frequent occurrence of oxalate ion for the hydrothermal systems of pyridine derivatives^[13] such as [m-Xyl-BPy4CEt]Br₂ used here. In the crystal structure of compound 3, not only oxalate ligands that bridge monomeric uranyl groups to form a one-dimensional chain, but also protonated isonicotinate as a ligand at both sides can be found (Fig. 4). The presence of protonated isonicotinate in compound 3 suggests the higher degrees of decomposition of [m-Xyl-BPy4CEt]Br₂ under more acidic condition compared with that found in compound 1 and 2.

The distinct structural difference among compounds 1, 2 and 3 suggests the significant impact of pH on the uranyl hydrothermal systems, especially the uranyl speciation and crystallization process (Fig. 5). The strong dependence on pH of uranyl speciation is largely attributed to hydrolysis of uranyl ion in aqueous solution. Generally speaking, uranyl monomer is likely to be stable at low pH, whereas high pH of aqueous solution always induces uranyl hydrolysis and promotes its oligomerization through the olation or oxolation process. With the aid of certain organic ligands, these oligomeric uranyl species could further transfer to the solid state as poly-nuclear uranyl compounds. Resembling most uranyl systems in aqueous solution, the formation of U8 motif in compound 1 also follows a pH-regulated mechanism. As revealed by the synthesis of compound 1-3, the aqueous pH used for compound 1 (pH ~2.72) with octa-nuclear uranyl motifs is higher than that for compounds 2 (pH ~2.29) and 3 (pH ~1.35) with only dimeric or monomeric uranyl motifs. Interestingly, pH also exerts a non-negligible influence on the stabilization of organic ligands under hydrothermal conditions. In contrast to the large degree of decomposition of [m-Xyl-BPy4CEt]Br₂ to simple isonicotinate linkers found in compound 3, the original zwitterionic dicarboxylate linkers still retains its main molecular skeleton with controlled hydrolysis to its acid form of m-Xyl-BPy4CA during hydrothermal reaction with uranyl under less acidic conditions for the synthesis of compound 1. It can be speculated that the preservation of total skeleton of m-Xyl-BPy4CA with strong coordination capability toward uranyl units as demonstrated in compound 1 also contributes to the formation of U8 motifs by affording additional stabilization through coordination and cross- linkage. The role of U-shaped m-Xyl-BPy4CA linkers in the stabilization of high-nuclear U8 motifs will be discussed as followed.

It is interesting to find that most of uranyl-oxalate compounds^{[14,15,16,17,18,19,20]} only afford uranyl secondary building units with nuclearities of no more than 4. The difference of compound 1 from those reported previously in uranyl oligomerization suggests that there are other additional factors contributing to the stabilization of U8 motif with more uranyl units. Apparently, structural stabilization based on coordination bonds and/or chelating effects of organic ligands is one of the vital factors to promote the final crystallization of U8-bearing product as companied with the pH-induced formation of U8 motifs. The m-Xyl-BPy4CA linker plays dual roles in the construction of U8-based compound 1, including realizing coordination saturation of uranium centers of U8 and additional structural connectivity for rhombic loop as well as crystal packing. Specifically, two bidentate m-Xyl-BPy4CA ligands in a couple crosslink all the four U8 motifs through coordination bonds and hydrogen bonds, where one m-Xyl-BPy4CA ligand points upwards and the other points downwards from the opposite direction (Fig.S9). The coordination bonds in a bridging μ₂-η¹ : η¹ mode on both ends of m-Xyl-BPy4CA joint two adjacent U8 motifs through two binding patterns (lateral and terminal binding sites), while hydrogen bonds between the C-H groups on the ‘back’ of U-shaped linker and oxygen atoms from another two U8 motifs of the rhombic loop consolidate the location of m-Xyl-BPy4CA in the cavity. Moreover, the important role of m-Xyl-BPy4CA also reflects on its participation of hydrogen bonds between adjacent layers of 2D sheets through interacting with neighboured uranyl group from another sheet, which mainly contributes to the crystal packing of compound 1 in three-dimensional (3D) space (Fig. S10). In all, the participation of m-Xyl-BPy4CA linker increases the molecular connectivity, enhances the structural stability of high-nuclear metal clusters and thus promotes the formation of new high-nuclear uranyl motifs.

Moreover, a comprehensive survey on the relationship between high-nuclear uranyl motifs and organic ligands used reveals that, multi-topic organic ligand with a nonlinear configuration is more likely to fit for capture, fixation and stabilization of high-nuclear uranyl motifs by coordination to uranyl units in a compact way (Fig. S11). Another important feature of m-Xyl-BPy4CA linker in molecular structure is its flexible conformation (Fig. S12). This U-shaped ligand could display varying molecular conformations in different compounds so as to be adapted to different coordination environment of uranyl motif. For example, the m-Xyl-BPy4CA linker gives a more open conformation when located at the cavity of uranyl-oxalate network in compound 1 (angle between two pyridium groups is ~47.9°, left in Fig. S12), while it becomes more compact with two dangled pyridium groups nearly paralleled to each other (angle between two pyridium groups is ~13.8°, right in Fig. S12). As shown above, the flexibility of molecular skeleton for m-Xyl-BPy4CA linker ensures it structural adaptivity and facilitates the formation of new U8 motif.

Characterizations of properties including thermogravimetric analysis (TGA), Fourier transform infrared (IR) spectra, Raman spectra, and solid-state fluorescence spectra for compounds 1 and 2 in pure phase were conducted. In terms of thermal stability, compound 1 does not undergo thermal decomposition of oxalate group on the backbone of 2D network until the temperature increases up to ~295 ℃ (Fig. S13), which is in sharp contrast to compound 2 (Fig. S14). The good stability of compound 1 is similar to the thermal behavior of uranyl oxalate^[21]. IR spectrum of compound 1 (Fig. S15) shows only a weak broad signal at ~915 nm correspondding to the characteristic vibrations of axial U=O bonds (calculated data based on the empirical equation of [d (U=O) (pm)=10650[ν₁ (cm^-1)]^-2/3+57.5]^[22]: 0.175-0.179 nm), although there are eight uranium centers in the secondary building unit of compound 1. The asymmetric stretching vibration of axial U=O bonds for compounds 2 and 3 (911 and 910 nm, respectively) is comparable to that observed for compound 1 (Fig. S15). The Raman spectrum of compound 1 (Fig. S16) also displays only two peaks assigned to asymmetric ν₃ vibrations (calculated data based on the empirical equation of [d (U=O) (pm) = 9141 [ν₃ (cm^-1)]^-2/3 + 80.4]^[22] : 0.175-0.178 nm), which is very similar to that observed for compound 3 with monomeric uranyl unit. A comparison of compound 1 with previously-reported CCI- mediated U8 motif^[11] in IR and Raman spectra shows that the latter one gives finer signals of stretching vibrations that could be resolved well and assigned to different uranyl centers among the U8 motif, while the IR and Raman signals for different uranyl centers of U8 motif in compound 1 seem to be highly overlapped with each other. This phenomenon might be, by contrasted to CCI-based U8 motif, ascribed to the nearly planar attribute of non-CCI U8 motif, which partly offsets the differences between uranyl centers of U8 motif in compound 1. In short, difference in physicochemical properties between this type of U8 motif and that mediated by CCIs reflects the particularity of U8 found here.

As shown in Fig. S17, solid-state fluorescence spectra of compounds 1 and 2 are more or less different as compared to that of uranyl nitrate (UO₂(NO₃)₂). Compound 2 gives the typical vibronic progression of uranium (VI) with the five main emission bands located at 499, 520, 543, 568 and 596 nm corresponding to the S₁₁ → S₀₀ and S₁₀ → S_0ν (ν=0-4) electronic transitions^[23]. It is very similar to that for a uranyl terephthalate compound with the same dimeric uranyl units^[24], and shows a large red-shift (7-11 nm) compared to that of uranyl nitrate. The large red-shift observed here should be attributed to markedly different coordination geometries and coordination modes for uranyl centers in compound 2 and UO₂(NO₃)₂^{[25,26,27,28]}. Interestingly, the fluorescence of compound 1 gives an unresolved broad signal with a broad peak ranging from 530 to 550 nm, and also turns to own a red-shift as well as a little attenuation in intensity compared to those of compound 2 and uranyl nitrate. Resembling the situation of IR and Raman spectra, the peak broadening might be attributed to the signal overlapping of uranium(VI) centers in the nearly planar U8 motif of compound 1^[25]. Considering the complexity for figuring out complicated interactions between different uranyl units in this large octa-nuclear uranyl cluster, the precise assignment and analysis of fluorescence spectra is still difficult here, and further study on this issue with the aid of modeling methods might be needed in the near future.

In summary, we have successfully synthesized a uranyl- oxalate coordination polymer with a new type of octa- nuclear uranyl motif, [(UO₂)₈O₄-(μ₃-OH)₂(μ₂-OH)₂]⁴⁺ by utilizing U-shaped pyridinium-based organic ligand [m-Xyl-BPy₄CEt]Br₂ via a pH-dependent regulation. The mixed ligand system of oxalate and m-Xyl-BPy4CA formed in situ plays important roles for stabilizing high-nuclear uranyl species by bridging coordination linkage and superimposed structural connectivity, respectively. The U8-bearing uranyl compound found here provides more information on uranyl hydrolysis and speciation in aqueous solution, and also facilitates the exploration of new actinide materials by employing well- designed organic linkers.

Supporting Materials related to this article can be found at https://doi.org/10.15541/jim20190118

1,1°-(1,3-phenylenebis(methylene))bis(4-(ethoxycarbonyl)pyridin-1-ium) bromide ([m-Xyl-BPy4CEt]Br₂)_. [m-Xyl-BPy4CEt]Br₂ was synthesized according to the reported procedure^[5,6,7]. A mixture of 1, 3-Bis(bromomethyl)benzene (0.65 g, 2.46 mmol) and isonicotinate (0.83 g, 5.49 mmol) were dissolved in 50 mL of acetonitrile and refluxed for 48 h. After cooling to room temperature, the solution was concentrated by evaporation in vacuum, filtered, washed with acetone, and dried under vacuum to afford the final product_. Yield: 1.287 g (92.8%). ¹H NMR (500 MHz, D₂O, δ ppm): 9.11 (d, 4H); 8.54 (d, 4H), 7.59-4.63 (m, 4H), 5.95 (s, 4H), 4.51 (q, 4H), 1.42 (t, 6H). MS (ESI): mass calculated for C₂₄H₂₆N₂O₄²⁺ (M²⁺), 406.19; m/z found, 203.11 (M²⁺/2).

X-ray diffraction data of compound 1 and 2 was performed on Bruker D8 VENTURE X-ray CMOS diffractometer with a Mo Kα X-ray source (λ=0.071073 nm) at 296 K. X-ray diffraction data of compound 3 was collected on a Agilent SuperNova X-ray CCD diffractometer with a Cu Kα X-ray source (λ=0.154184 nm) with higher diffraction capability at 293 K. Standard Agilent Crysalis software was used for the determination of the unit cells and data collection control. All the crystal structures were solved by means of direct methods and refined with full-matrix least squares on SHELXL- 2014. SIMU were used to constrain the displacement parameters of the phenyl and pyridyl groups and ISOR were used to even out the electron density associated with disordered portions of the moieties for both 1 and 3. OMIT were used to eliminate bad reflections obscured by the beamstop for all compounds. Since there is high disorder of pyridyl groups dangling aside, the pyridyl ligand was forced to be half occupied to create a chemically sensible model for 3. Solvent molecules (water) in the structure are highly disordered and impossible to be modelled as discrete atomic sites. To resolve this issue, the contribution of solvent-electron density was removed using the SQUEEZE/PLATON procedure, thereby producing a set of solvent-free diffraction intensities used for improving the structure refinements. The crystal data of both compounds are given in Table S3. Crystallographic data for the structures in this study were deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC- 1510791 (1), CCDC-1898268 (2) and CCDC-1510792 (3).

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