Silver Ion-Induced Formation of Unprecedented Thorium Nonamer Clusters via Lacuna-Construction Strategy

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE10 Jun 2022Silver Ion-Induced Formation of Unprecedented Thorium Nonamer Clusters via Lacuna-Construction Strategy Xiang-He Kong†, Qun-Yan Wu†, Lei Mei, Li-Wen Zeng, Zhi-Wei Huang, Ji-Pan Yu, Chang-Ming Nie, John K. Gibson, Zhi-Fang Chai, Kong-Qiu Hu and Wei-Qun Shi Xiang-He Kong† Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001 †X.-H. Kong and Q.-Y. Wu contributed equally to this wok.Google Scholar More articles by this author , Qun-Yan Wu† Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 †X.-H. Kong and Q.-Y. Wu contributed equally to this wok.Google Scholar More articles by this author , Lei Mei Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Li-Wen Zeng Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Zhi-Wei Huang Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Ji-Pan Yu Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Chang-Ming Nie School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001 Google Scholar More articles by this author , John K. Gibson Chemical Sciences Division, Lawrence Berkeley National Laboratory (LBNL), Berkeley, California 94720 Google Scholar More articles by this author , Zhi-Fang Chai Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Kong-Qiu Hu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author and Wei-Qun Shi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202054 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Herein, we report the synthesis and structures of two novel mixed-metal clusters denoted as Th9Ag6 and Th9Ag12. Both clusters feature unprecedented Th9 cores. The cores are tricapped trigonal prism moieties that are novel among actinides. Attempted alternative synthesis routes indicate that the Th9 clusters are accessible only through slow introduction of Ag+ into a solution containing a Th6 cluster modified with 2-picolinic acid. Alternative rapid addition of Ag+ leads to dissociation of the Th6 cluster with formation of a high-purity (ThAg)∞ two-dimensional layered structure material. A mechanism for cluster dissociation and reassembly to yield Th9 from Th6 is proposed that is consistent with spectroscopic observations and computational results. Because of Ag⋯Ag and π–π interactions, the Th9Ag12 cluster exhibits high stability in air, at elevated temperature, under γ-irradiation, and in common solvents. Download figure Download PowerPoint Introduction Polynuclear metal clusters are of interest due to intriguing structures1–4 and numerous applications in fields such as molecular magnetic materials,5,6 luminescence sensing,7,8 catalysis,9 and optics.10 In comparison with many known clusters based on transition, main group, and lanthanide metals,11–16 the realm of actinide clusters is relatively underdeveloped.17 The tendency of actinides to hydrolyze results in soluble polynuclear species that play critical roles in environmental transport and migration of radionuclides.18–20 Actinide clusters that have been synthesized and characterized include U(IV)/U(V)/U(VI)-containing clusters U4, U6, U9, U13, U24, and U38,21–25 transuranic Pu3826,27 and Np38,28 and several based on uranyl-peroxide.29–36 Thorium is the most abundant naturally occurring actinide and is a leading candidate for next generation nuclear fuel in liquid fluoride thorium reactors.37,38 Despite recent advances in synthesis of thorium complexes,39–45 reports of thorium clusters prepared under ambient conditions remain less developed. As a bridge between metal ions and bulk materials, clusters provide models for understanding more complex systems. In particular, syntheses and characterization of stable polynuclear thorium clusters can provide insights into materials such as thorium-based colloids and nuclear fuels. Reported thorium clusters have been prepared from mononuclear thorium ions or complexes via ammonolysis,46 hydrogenolysis,40 and hydrolysis17 with those from the first two approaches containing bridging NH3 or H moieties that are generally unstable under ambient conditions. Approaches that take advantage of the propensity for Th4+ to hydrolyze typically result in clusters with low-nuclearity or ThO2 nanoparticles.47–57 Therefore, the design and synthesis of stable high-nuclearity thorium clusters is a formidable challenge. In particular, exploring the assembly process facilitates further development and application of actinide functional materials at the later stage. In this work, we employ a lacuna-construction strategy to prepare novel large-nuclearity thorium clusters. In this approach, competitive metal ions, here Ag+, are introduced into a solution of stable precursor thorium clusters such as Th6(μ3-O)4(μ3-OH)4(HCOO)12 modified with organic ligands to induce partial dissociation of precursor thorium clusters to yield lacunary thorium clusters that can assemble to form larger clusters. Dissociation of the precursor thorium clusters, and specifically avoidance of ThO2 nanoparticles, is controlled by the rate and amount of added competitive metal ions, which may be incorporated during assembly to yield mixed metal clusters. We employed this strategy by adding Ag+ to a solution of Th6(μ3-O)4(μ3-OH)4(HCOO)12 cluster modified by 2-picolinic acid, which resulted in novel mixed-metal clusters denoted as Th9Ag6 (complex 1) and Th9Ag12 (complex 2). Both new Th-Ag clusters feature a tricapped trigonal prism Th9 unit. To the best of our knowledge, complex 2, Th9Ag12, is the largest reported actinide-silver cluster. Based on structural considerations and theoretical calculations, a Ag+-induced dissociation and reassembly mechanism is proposed for the observed conversion of cluster Th6 to Th9. Experimental Methods Caution! Thorium nitrate is radioactive; suitable precautions for safety and protection must be taken. All chemicals of reagent grade were commercially available and used without further purification. See the Supporting Information for details of the determination of single-crystal structure, powder X-ray diffraction (PXRD), thermogravimetric analysis, and the spectroscopic measurements. In the synthesis of Th9 clusters, two methods, single-tube diffusion and solvent evaporation, were adopted, and the detailed experimental procedures can be found in the Supporting Information. Theoretical Methods The models of clusters [Th9(μ3-O)7(μ3-OH)8Ag6(NO3)2(PyCOO)12]6+ and [Th9(μ3-O)8(μ3-OH)6Ag12(PyCOO)20]6+ are cut from the crystal structures of 1 and 2, respectively. Full structural optimization employed the Perdew–Burke–Ernzerhof (PBE) functional using the ADF 2013.01 package.58 The triple-ζ plus one polarization (TZP) basis set was used for Th and Ag atoms, while the double-ζ plus one polarization (DZP) basis set was used for H, C, N, and O atoms. Scalar relativistic effects were taken into account using the zero-order regular approximation approach.59 Analysis of the electron localization function (ELF) was carried out with the Multiwfn program.60 Results and Discussion Crystal structures of mixed-metal clusters Single-crystal X-ray diffraction reveals that complex 1 crystallizes in a monoclinic space group C2/c ( Supporting Information Tables S1 and S2). As shown in Supporting Information Figure S1a, the asymmetric unit of 1 consists of five Th and three Ag atoms that are crystallographically independent, eight μ3-O2−/OH− anions, six deprotonated 2-picolinic acid anion ligands (PyCOO−), four NO3− anions, and five-coordinated H2O molecules. In this unit, Th4 is located at the special position of the symmetry axis with position occupancy of 0.5, whereas occupancy of the other metal atoms is 1.0 ( Supporting Information Figure S1a). Through an axisymmetric operation the whole Th9Ag6 cluster of 1 was obtained (Figure 1a), with a metal core consisting of three shells (Figure 1b,e). In the first shell, six Th atoms lie at the vertices of a triangular prism with Th⋯Th distances ranging from 3.781 to 3.970 Å (Figure 1c and Supporting Information Figure S2). The remaining three Th atoms in the second shell are each connected to four Th atoms of the first shell through two μ3-O2− and two μ3-OH− forming three tetragonal pyramid units connected with the triangular prism by the face-sharing mode to generate a Th9 tricapped trigonal prism (Figure 1d). This unprecedented Johnson-type (J51) nonanuclear moiety is the first reported for an actinide compound.61 Each Th atom of the Th6 first shell is connected with an Ag atom through two PyCOO− ligands ( Supporting Information Figure S3) to create the final Th9Ag6 cluster. Coordination of the cluster core by 12 PyCOO−, two NO3−, and nine H2O leads to Th atom environments with spherical capped square antiprism (Th1, Th3, and Th5), augmented sphenocorona (Th2) or spherical tricapped trigonal prism (Th4) geometries, while the Ag sites have quadrilateral (Ag1 and Ag2) or pyramid (Ag3) geometries ( Supporting Information Figure S4). Figure 1 | (a) Structure of Th9Ag6 (complex 1). (b) Core structure of 1. (c) Triangular prism structure of the Th6 first shells of 1 and 2. (d) Tricapped trigonal prism structure of Th9 cores of 1 and 2. (e and f) Simplified geometries of Th9Ag6, 1 in (e) and Th9Ag12, 2 in (f). Color scheme: Th, green; Ag, purple; C, gray; O, red; N, blue. H atoms are omitted for clarity. Download figure Download PowerPoint Complex 2, like 1, crystallizes in the monoclinic space group C2/c, as determined by single-crystal X-ray diffraction ( Supporting Information Tables S1 and S3). The asymmetric structural unit shown in Supporting Information Figure S1b, contains five Th and six Ag atoms that are crystallographically independent, seven μ3-O2−/OH− anions, 10 PyCOO− anions, three NO3− anions, and three coordinated H2O. The complete structure of 2 features a Th9 cluster similar to that of 1 ( Supporting Information Figure S2). However, in 2 each Th atom of the inner shell Th6 is connected with two Ag atoms through three PyCOO− ligands ( Supporting Information Figures S5 and S6), resulting in a Th9Ag12 cluster (Figure 2a,b). This difference between 1 and 2 indicates that four H2O and two NO3− anions in the inner shell Th6 of 1 are replaced by six PyCOO− in 2, which capture six Ag atoms that are further coordinated by two PyCOO− and six NO3− to complete the coordination shell and stabilize the overall structure. From the simplified perspective of the Th9Ag12 cluster of 2 shown in Figure 1f, the additional six silver ions are each coordinated by two PyCOO− ligands to create two Ag5 chains with Ag⋯Ag distances ranging from 2.872 to 3.304 Å ( Supporting Information Figure S7), a range similar to distance in other reported Ag clusters that suggests Ag⋯Ag interactions.62–67 Complex 2 is particularly notable as it is the largest actinide-silver cluster yet reported. Figure 2 | (a) Structure of Th9Ag12 (complex 2). (b) Core structure of 2. Download figure Download PowerPoint Some important physicochemical properties of complexes 1 and 2 are shown in the Supporting Information Figures S22–S35. After storage of a powder sample of complex 2 under ambient conditions for almost 1 year, PXRD patterns indicate retention of the original structure ( Supporting Information Figure S13). Unlike other Th or Ag clusters,46,68–70 complex 2 is practically insoluble in common solvents like H2O, dimethylformamide, CH3OH, CH3CN, and CHCl3. As shown in Supporting Information Figures S14–S16, powder samples of 2 were stable for several days in these solvents and in water over a wide pH range, as well as for several hours under UV light exposure. More impressively, the PXRD patterns of the irradiated samples confirm that complex 2 fully retains its crystallinity after γ-irradiation from a 60Co source ( Supporting Information Figure S17). We attribute the extraordinary stability of complex 2 to the formation of a one-dimensional supermolecular chain structure via Ag⋯Ag and π⋯π interactions between Th9Ag12 clusters ( Supporting Information Figure S8). The X-ray photoelectron spectroscopy spectrum of complex 2 displays two peaks at binding energies 367.6 and 373.6 eV, indicating that all silver atoms are in oxidation state Ag(I) ( Supporting Information Figure S21).71,72 Theoretical calculation of mixed-metal clusters To understand the novel structures of complexes 1 and 2, calculations were performed using relativistic density functional theory. The following clusters extracted from crystal structures of 1 and 2 serve as models for electronic structures and other properties: [Th9(μ3-O)7(μ3-OH)8Ag6(NO3)2(PyCOO)12]6+ (model 1) and [Th9(μ3-O)8(μ3-OH)6Ag12(PyCOO)20]6+ (model 2). As shown in Supporting Information Figure S36, these representative clusters contain cores [Th9(μ3-O)7(μ3-OH)8]14+ and [Th9(μ3-O)8(μ3-OH)6]14+, respectively, for which full geometry optimizations were performed. The calculated Th–O bond distances ( Supporting Information Table S4 and Figure S37) are in good agreement with those from the crystal structures, supporting the assignments of bridging μ3-O2− and μ3-OH−. The occupied molecular orbitals (MOs) for the model clusters shown in Supporting Information Figure S38 have energies in the ranges −28.73 to −29.22 eV for 1 and −27.05 to −27.83 eV for 2. For further insight into the electronic structure of the core [Th9(μ3-O)8(μ3-OH)6]14+ of model 2, MOs and corresponding energy levels are shown in Figure 3. The large energy gap between the lowest unoccupied MO (LUMO) and highest occupied MO (HOMO) of 3.95 eV is close to that of a highly stable Hf13 cluster, and larger than the energy gap for stable C60 and Au20,7,73 which indicates high electronic stability of the Th9 cluster. The MO energy levels in Figure 3 exhibit three distinct bands, with the lowest energy band comprised of Th 6p orbitals, the middle comprised mainly of O 2p with some Th 5f and 6d contributions, and the highest dominated by Th 5f. The ELF for the core [Th9(μ3-O)8(μ3-OH)6]14+ performed for four planes ( Supporting Information Figure S39) reveals high electron density between Th and O consistent with the large HOMO–LUMO energy gap and high stability. To assess relative stabilities of model 1 and 2 clusters, the following interconversion reaction was considered: [Th9(μ3-O)7(μ3-OH)8Ag6(NO3)2(PyCOO)12(H2O)7]6+ (model 1) + 6Ag+ + 8(PyCOO−) → [Th9(μ3-O)8(μ3-OH)6Ag12(PyCOO)20(H2O)3]6+ (model 2) + 2NO3− + 5H2O. The computed reaction energy of −400.81 kcal/mol suggests a higher stability of model 2, consistent with the smaller HOMO–LUMO gap of 3.52 eV for the model 1 core [Th9(μ3-O)7(μ3-OH)8]14+, versus 3.95 eV noted above for [Th9(μ3-O)8(μ3-OH)6]14+ (Figure 3 and Supporting Information Figure S40). Figure 3 | Energy level (eV) of MOs for the core [Th9(μ3-O)8(μ3-OH)6]14+ from complex 2 and representative MOs for each energy level band. (The isosurface value of MOs is set to be 0.03 au.) Download figure Download PowerPoint Possible dissociation and reassembly mechanism for conversion of cluster Th6 to Th9 induced by Ag+ It was found that direct addition of silver nitrate solution to the solution containing Th6(μ3-O)4(μ3-OH)4(HCOO)12 clusters and organic ligands resulted after a few days in crystals of complex 3, (ThAg)∞, with a two-dimensional (2D) planar structure ( Supporting Information Figures S9 and S20). In contrast, slow addition of Ag+ by the diffusion method resulted after several weeks in a solid mixture of complex 1 and complex 2 at the solution interface. Notably, the complex 1 component gradually transformed, eventually yielding pure 2 over time ( Supporting Information Figures S18 and S19). Given that the structures of the Th6 and Th9 cores of 1 and 2 both feature a Th5 unit ( Supporting Information Figure S11), we suggest a possible dissociation and reassembly mechanism (Figure 4) for conversion of the Th6 cluster to Th9 clusters. Thorium monomers including Th(OH)3+ and Th(OH)22+; dimers, for example, Th2(OH)26+; tetramers, for example, Th4(OH)124+; and hexamers, for example, Th6(μ3-O)4(μ3-OH)412+, have been reported as significant solution species.34 Carboxylate anion ligands can promote formation of stable neutral hexamers like Th6(μ3-O)4(μ3-OH)4(RCOO)12.47,52,74 In the synthesis reported here, addition of PyCOOH to a solution containing Th6(μ3-O)4(μ3-OH)4(HCOO)12 should produce Th6(μ3-O)4(μ3-OH)4(PyCOO)x(HCOO)12-x (Th6Lx) (Figure 4a,b). Such ligand replacement is indicated by a shift in the UV–vis absorption peaks from 264.3 nm (the absorption peak of the ligand) to 267.5 nm ( Supporting Information Figure S24a) coincident with appearance of a peak at 274.5 nm ( Supporting Information Figure S24c). As shown in Supporting Information Figure S24d, when silver ions are added to the aqueous mixture of ligand and Th6, the intensity of the peak at ∼275 nm gradually diminishes, while that of peak at 264.3 nm increases, and the peak at 267.5 nm undergoes a slight blue shift. The results suggest that silver ions compete with Th6Lx clusters for carboxylate ligands, with possible cluster breakdown upon depletion of ligands. Such dissociation of Th6 clusters was found to depend on the rate of introduction of silver ions, with rapid addition producing 2D planar complex 3 ( Supporting Information Figure S9). Solution species from dissociation of the Th hexamer likely include monomers Th(PyCOO)22+ (Figure 4d), two of which can combine with two metalloligands cis-(Ag(PyCOO)2)− ( Supporting Information Figure S10a) to form a ring structure (Figure 4e). Such rings can further interconnect by metalloligands trans-(Ag(PyCOO)2)− ( Supporting Information Figure S10b) to form the 2D planar structure of complex 3 (Figure 4f and Supporting Information Figure S9). As suggested by the route indicated by the red arrow below Figure 4c, gradual introduction of silver ions could lead to partial decomposition of Th6Lx clusters with formation of Th5 units that could capture Ag+ and PyCOO− ligands to form Th5Ag4 clusters (Figure 4g,h). Some Th4+ cations detached from Th6Lx clusters could condense via hydrolysis to form tetramer species with approximate planar structure that could associate with a Th5Ag4 cluster to form a Th9Ag4 cluster (Figure 4i and Supporting Information Figure S12). Further coordination of Th9Ag4 by metalloligand cis-(Ag(PyCOO)2)−, NO3−, and H2O would yield the Th9Ag6 cluster of complex 1 (Figure 4j). Coordinated H2O and NO3− anions in the inner shell Th6 assembly of 1 could be replaced by six PyCOO− (Figure 4k) that in turn capture six Ag+. Further coordination by two PyCOO− and six NO3− would yield the final Th9Ag12 cluster of complex 2 (Figure 4l). Exothermic transformation of the Th9Ag6 cluster to Th9Ag12 cluster by such a route is consistent with the calculated energy for the interconversion reaction considered above. The observed synthesis and proposed mechanism for formation of Th9Ag6 and Th9Ag12 clusters require the Th6(μ3-O)4(μ3-OH)4(HCOO)12 precursor, rather than smaller species like monomeric Th(NO3)4. Although some details of this proposed mechanism may differ from actual growth, the suggested process is consistent with observations and provides insight and guidance. Figure 4 | (a) Th6(μ3-O)4(μ3-OH)4(HCOO)12 core structural unit. (b) Th6(PyCOO)x(HCOO)12-x unit from replacing HCOOH with PyCOOH. (c) Structure of Th5 unit. (d) Mononuclear Th(PyCOO)2)2+ unit from dissociation of Th6(PyCOO)x(HCOO)12-x cluster. (e) Ring structure found in complex 3. (f) 2D planar structure of 3. (g and h) Th5 unit captures Ag+ ions and carboxylate ligands to form Th5Ag4 cluster. (i) Th9Ag4 cluster formed by assembly of Th5Ag4 and Th tetramer. (j) Structure of Th9Ag6 cluster, complex 2. (k and l) Transformation of complex 1, Th9Ag6, to complex 2, Th9Ag12. Color scheme: Th, green; Ag, purple; C, gray; O, red; N, blue. H atoms omitted for clarity. Download figure Download PowerPoint Conclusion Two novel mixed-metal clusters, Th9Ag6(O)7(OH)8L12(NO3)8(H2O)9 (Th9Ag6, complex 1) and Th9Ag12(O)8(OH)6L20(NO3)6(H2O)5 (Th9Ag12, complex 2), where ligand L is 2-picolinate, were synthesized and characterized. Both clusters feature a rare Th9 tricapped trigonal prism structure, which is a new hydrolytic species of tetravalent actinides. Experiments indicate Ag+ plays a critical role in formation of the Th9 clusters, which are accessible only through a modulated approach of slowly introducing Ag+ into a solution of Th6 cluster and 2-picolinic acid. A Ag+-induced Th6 cluster dissociation and reassembly mechanism is proposed to elaborate evolution from Th6 to Th9 clusters. Intermolecular and intramolecular Ag⋯Ag interactions, and intermolecular π–π interactions impart excellent air, γ-irradiation, thermal, and solvent stability to the Th9Ag12 cluster. This silver-induced dissociation, polymerization, and assembly of stable thorium clusters is pertinent to environmental speciation and migration of actinides in the environment. In contrast to previous studies of Th–Fe/Ag and other heterometallic clusters, this work focuses, for the first time, on using a heterometallic ion, Ag+, to control chemistry of stable Th clusters. Supporting Information Supporting Information is available and includes detailed descriptions of synthetic procedures and calculation method, single-crystal structure, spectral characterization, thermal analysis, stability test, and micrographs. Crystallographic data for complexes 1– 3 (CCDC 2062120–2062122) are available. Conflict of Interest There are no conflicts to declare. Acknowledgments We thank the National Science Fund for Distinguished Young Scholars (grant no. 21925603). We also acknowledge the support of the National Natural Science Foundation of China (grant nos. 22076187, 11975152, and 11875057). The work of J.K.G. was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Heavy Element Chemistry program at LBNL under Contract No. DE-AC02-05CH11231.