Aluminate-mediated assembly of Keggin-type thorium-oxo clusters with a substitutable central octahedron and tunable shell ligands

Aluminate-mediated assembly of Keggin-type thorium-oxo clusters with a substitutable central octahedron and tunable shell ligands

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Chunhui Wang^a, Zhe Han^a, Yuan Gao^a, Chaoyue Fan^a, Puyong He^a, Bojun Li^a, Dong Ma^a, Yang Zhang^a, Lei Zhang^b, Jie Qiu^a

Chinese Chemical Letters | 2026, 37(10) : 111563

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Chinese Chemical Letters | 2026, 37(10): 111563

• Communication •

Aluminate-mediated assembly of Keggin-type thorium-oxo clusters with a substitutable central octahedron and tunable shell ligands

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Chunhui Wang^a, Zhe Han^a, Yuan Gao^a, Chaoyue Fan^a, Puyong He^a, Bojun Li^a, Dong Ma^a, Yang Zhang^a, Lei Zhang^b, Jie Qiu^a

Affiliations

^aSchool of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China

^bNingbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

About Author:

CRediT authorship contribution statement

Chunhui Wang: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation.

Zhe Han: Data curation.

Yuan Gao: Data curation.

Chaoyue Fan: Data curation.

Puyong He: Data curation.

Bojun Li: Data curation.

Dong Ma: Data curation.

Yang Zhang: Investigation.

Lei Zhang: Conceptualization.

Jie Qiu: Writing – review & editing, Visualization, Project administration, Methodology, Funding acquisition, Formal analysis, Data curation, Conceptualization.

Published: 2026-10-15 doi: 10.1016/j.cclet.2025.111563

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Abstract

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Metal-oxo clusters (MOCs), particularly those with renowned Keggin-type architecture, have been extensively studied for their facile synthesis, robust stability, and diverse applications. However, the isolation of thorium-oxo clusters with novel structural motifs remains elusive due to the distinctive hydrolytic behaviour of Th⁴⁺ ions. Herein, we report nine electropositive α-Keggin-type thorium-oxo clusters, M@Th₁₂@L₈ (where M is fully or co-occupied by Al³⁺, W⁶⁺, Mo⁶⁺, V⁵⁺, Cr³⁺, or Mn⁴⁺; L = Al or Ga), synthesized via aluminate-regulated Th⁴⁺ hydrolysis. Unlike Th₆(O/OH)₈-based structures for most reported thorium-oxo clusters, M@Th₁₂@L₈ features an unprecedented triple-layered Platonic architecture: a central MO₆ octahedron, a Th₁₂ icosahedron, and an outer cube of eight LO₅ ligands. Structural rigidity maintains invariant M–O bond lengths despite variable MO₆ and LO₅ units, while the central M ion dictates solution pH during cluster formation, crystallization kinetics, crystal yield, and tunable optical properties. Time-resolved small-angle X-ray scattering (SAXS) and ²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) unveil the partial decomposition of aluminate precursors and the synchronized self-assembly of M@Th₁₂@L₈. This work advances our understanding of thorium hydrolysis in heterometallic systems and establishes a modular strategy to prepare MOCs with tailored architectures and functionalities.

Key words

Metal-oxo cluster / Keggin-type architecture / Thorium-oxo cluster / Self-assembly / Hydrolysis / Heterometallic ions

Cite this Article

Chunhui Wang, Zhe Han, Yuan Gao, Chaoyue Fan, Puyong He, Bojun Li, Dong Ma, Yang Zhang, Lei Zhang, Jie Qiu. Aluminate-mediated assembly of Keggin-type thorium-oxo clusters with a substitutable central octahedron and tunable shell ligands[J]. Chinese Chemical Letters, 2026 , 37 (10) : 111563 - . DOI: 10.1016/j.cclet.2025.111563

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Since their initial discovery by Berzelius in 1826 [1], Keggin-type metal-oxo clusters and related materials have attracted considerable interest due to their convenient synthetic methods, remarkable chemical stability, and versatile applications in many fields such as catalysis, energy storage and conversion, biomedicine, and materials science [2–5]. The classical Keggin cluster contains a central XO₄ (X = P, Si, Ge, As, etc.) tetrahedron surrounded by twelve MO₆ octahedra (M = W and Mo) [6]. Recent advances in synthetic chemistry have propelled the exploration of Keggin-type clusters beyond traditional confines, extending to main group metals (Al₁₃ [7]), other transition metals (Fe₁₃ [8], Mn₁₃ [9], Zr₁₃ [10], and Hf₁₃ [11]), and lanthanides (Eu₁₃ [12]). However, Keggin-type actinide-oxo clusters remain largely unexplored.

Research on actinide-oxo clusters lags far behind other metals' clusters, primarily due to the radioactivity and complex chemical behaviors of actinides [13]. Among actinides, thorium is particularly noteworthy as the most abundant in Earth's crust [14] and the fuel for thorium nuclear reactors [15]. Therefore, a thorough understanding of thorium chemistry is crucial for its practical applications and addressing associated challenges, including mineral extraction, nuclear fuel preparation, radioactive waste processing, and the decorporation of internalized radionuclides [15–17]. The most common oxidation state of thorium is +4. Owing to the relatively high charge and small radius, Th⁴⁺ ions readily undergo hydrolysis and condensation at pH > 2, forming various products such as monomers, oligomers, colloids, and precipitates [18]. When suitable ligands are present to coordinate with thorium ions on the surface of hydrolysis products, thorium-oxo clusters form [19]. The presence of these hydrolysis products can not only complicate thorium extraction but also impact its solubility and migration in the natural environment [20]. Despite its significance, research on thorium-oxo clusters remains limited compared to non-radioactive metals or even more radioactive neptunium and plutonium [21–24].

To date, only a few dozen thorium-oxo clusters have been reported [19,25–34]. The most common are hexanuclear clusters with octahedral Th₆(O/OH)₈ cores stabilized by various ligands [19,25–29,34], while the largest cluster identified is a dodecamer (Th₁₂) formed by linking two Th₆(O/OH)₈ subunits [33]. In contrast, other tetravalent metal-oxo clusters exhibit greater diversity and higher nuclearity. For example, the highest nuclearity clusters reported for Zr⁴⁺, Ce⁴⁺, U⁴⁺, Np⁴⁺, and Pu⁴⁺ are Zr₇₀ [21], Ce₁₀₀ [22], U₈₄ [35], Np₃₈ [24], and Pu₃₈ [23], respectively. The isolation of thorium-oxo clusters with higher nuclearity or novel topological architectures remains challenging. This difficulty primarily stems from the distinctive hydrolytic property and reactivity of Th⁴⁺ ions compared to other tetravalent metal ions [36,37]. To address this challenge, it is crucial to explore and regulate the hydrolysis behaviors of Th⁴⁺ ions.

The majority of reported thorium-oxo clusters have been synthesized by controlling thorium hydrolysis through adjusting the solution pH, reaction temperature, solvents, and ligands [19,25–34]. Beyond these conventional approaches, studies of transition metal and lanthanide clusters have widely applied the method by introducing heterometallic ions into the synthesis, yielding mixed-metal clusters with more diverse structures, richer properties, and broader applications [38,39]. In contrast, thorium-containing mixed-metal oxo clusters remain limited. Beyond several clusters that were synthesized using lacunary polyoxometalates (POMs) of W and Mo to stabilize thorium ions [40], the known heterometallic thorium-oxo clusters are only a few, mainly including Th₂Al₆(edta)₂ [41], Th₂Al₆(hedta)₂ [42], Th₂Al₈(hdpta) [42], Th₂Al₈(heidi) [43], Th₄Al₁₀(edta) [41], ThFe₂(dhpta) [43], Th₂Fe₂(cit) [43], Th₂Fe₁₀(PhPO₃) [44], Th₆Mn₁₀(PhCOO) [45], Th₉Ag₆(PyCOO) [46], and Th₉Ag₁₂(PyCOO) [46]. These clusters form through the synergistic effects of heterometallic ions and organic ligands on the hydrolysis and complexation of thorium ions. However, the strong chelating capacity of organic ligands in these systems often leads to Th⁴⁺ ions being primarily coordinated by the ligands themselves [41–44]. The chelated Th⁴⁺ ions are then attached to the central aluminum or iron core via bridging oxygen atoms or hydroxyl groups.

Notably, the isolated influence of heterometallic ions on the formation and structures of thorium-containing clusters has mostly been uninvestigated [43]. Al³⁺ ions readily hydrolyze to form various polynuclear species at pH > 3. Moreover, aluminum ores such as bauxite contain significantly higher thorium content compared to thorium's average crustal abundance, with most thorium occurring in grains of stable minerals like zircon and a fraction adsorbed onto alumina minerals [47,48]. Although this adsorption behavior affects thorium's environmental mobility, the underlying molecular-scale mechanisms remain poorly understood [43].

Building on the above research background, we have developed a synthetic strategy involving introducing Th⁴⁺ ions into an aluminate solution to regulate their hydrolysis and condensation. Al³⁺ ions in this solution already reach hydrolysis equilibrium and exist in the form of polynuclear species [49]. This solution thereby serves a dual role: acting as a pH buffer and supplying aluminate groups that function as ligands, promoting the assembly of Th clusters. Through this approach, we successfully synthesized six electropositive Keggin-type thorium-oxo clusters, M@Th₁₂@Al₈, with a substitutable MO₆ center (M = Al, W_0.69Al_0.31, Mo, V, Cr_0.14Al_0.86, and Mn). Given the chemical similarity between aluminum and gallium, we extended this methodology to a mixed aluminate and gallate solution. This modification enabled partial substitution of shell Al atoms by Ga atoms, yielding three electropositive M@Th₁₂@Ga_mAl_n clusters (M = W, Mo, and V; m + n = 8).

Herein, we report syntheses, structures, optical properties, and self-assembly of the nine M@Th₁₂@L₈ (L₈ = Al₈ or Ga_mAl_n with m + n = 8) clusters. This work is of great interest to us for the following reasons:

(1) This work advances our understanding of thorium hydrolysis and establishes a modular strategy for designing thorium-oxo clusters with tunable architectures and tailored properties.

(2) The obtained nine novel clusters constitute the first electropositive Keggin-type actinide-containing clusters featuring both a substitutable central MO₆ octahedron and tunable shell ligands.

(3) These clusters represent the high nuclear thorium-oxo clusters constructed from three concentric Platonic layers, as opposed to the conventional M₆(O/OH)₈ subunits found in most tetravalent metal oxo clusters.

(4) These clusters represent rare multinary heterometallic clusters incorporating main group, transition metal, and actinide elements.

(5) These clusters exhibit distinct optical properties depending on the identity of the central MO₆ octahedron.

(6) Research on thorium-oxo cluster formation remains scarce. Using time-resolved SAXS and ²⁷Al MAS NMR, we observed simultaneous partial decomposition of precursor aluminate clusters and self-assembly of M@Th₁₂@L₈ clusters, which stabilize in solution post-formation.

As shown in Scheme 1 and detailed in the Synthetic Procedures section (Supporting information), solutions of thorium nitrate and tris(hydroxymethyl)aminoethane (Tris) were added to an aluminum solution, which was prepared using a reported method [49]. The resulting mixture was heated at 80 ℃ for 16 h and then cooled to room temperature. Subsequently, disodium 2,6-naphthalenedisulfonate (2,6-NDS-Na₂) was introduced to serve as a crystallization mineralizer. Slow evaporation of the resulting solution at ambient temperature over three days yielded crystals with the formula [(AlO₆)Th₁₂(OH)₁₂(AlO(OH)₃H₂O)₈(H₂O)₁₂]·(2,6-NDS)₅·Cl·37H₂O (designated as Al@Th₁₂@Al₈). This formula was determined by single crystal X-ray diffraction, elemental analysis, oxidation state characterization, and thermogravimetric analysis (TGA).

Single crystal X-ray diffraction data revealed that Al@Th₁₂@Al₈ crystallizes in the triclinic space group P1 (Table 1 and Table S1 in Supporting information). Its structure features a novel hybrid Al-Th cluster comprising three concentric Platonic-like layers (Figs. 1a-c), with a diameter of 14.838 Å (Fig. S1 in Supporting information). The center is an AlO₆ octahedron (Fig. 1d). The middle layer is an icosahedron (Th₁₂) formed by 12 thorium polyhedra (Fig. 1e), with an average Th···Th distance of 3.8984 Å (Table S2 in Supporting information). The outer layer is a cube (Al₈) constructed from eight AlO₅ polyhedra (Fig. 1f), with an average Al···Al distance of 6.3458 Å (Table S3 in Supporting information). Such a nested Platonic structure in Al@Th₁₂@Al₈ is unprecedented in actinide-oxo clusters. The pursuit of metal-oxo clusters with Platonic structure is a goal sought after by synthetic chemists due to the high symmetry and aesthetically pleasing appearance of such structures [50], and our work greatly enriches this family. More interestingly, Th₁₂ represents not only the first thorium cluster with icosahedral configuration, but also the highest-nuclearity thorium-oxo cluster reported to date. Furthermore, the structure of Al@Th₁₂ could be envisioned as four thorium trimers connected to the central AlO₆ octahedron (Fig. S2 in Supporting information). This arrangement of thorium polyhedra is very similar to that in α-Keggin-type clusters like Al₁₃ (Fig. S2) [51,52]. Therefore, Al@Th₁₂@Al₈ represents the first α-Keggin-type actinide-oxo cluster, stabilized by AlO₅ groups.

A closer examination of the structure of Al@Th₁₂@Al₈ and bond valence sum (BVS) analysis of the coordinated O atoms reveals that the Al³⁺ ion in the central symmetric octahedron is coordinated by six µ₃-O atoms, with Al-O bond lengths ranging from 1.8645 Å to 1.8707 Å (Tables S4 and S5, Fig. S23 in Supporting information). In addition to this Al³⁺ ion, each of the six µ₃-O atoms is also coordinated with two Th⁴⁺ ions, connecting the central octahedron and middle icosahedron (Fig. 1g). The remaining coordination sites of each Th⁴⁺ ions are occupied by two µ₂-OH groups and two µ₄-O atoms from two AlO₅ groups in the cube, three µ₃-OH groups, and one H₂O molecule (Fig. 1i), with an average Th-O bond length of 2.4726 Å (Table S2). Compared to the symmetric octahedral coordination geometry of central AlO₆, the AlO₅ is asymmetric. Its five O atoms are assigned as one µ₄-O atom, three µ₂-OH group, and one H₂O molecule (Fig. 1j), with average bond lengths of Al-(µ₄-O) = 1.8919 Å, Al-(µ₂-OH) = 1.7940 Å, Al-(OH₂) = 1.9134 Å (Table S3). Each AlO₅ group is connected to three Th⁴⁺ ions through one µ₄-O atom and three µ₂-OH groups (Fig. 1h), passivating the surface of Th₁₂.

A significant characteristic of classical transition metal (e.g., W and Mo) Keggin-type POMs is that their central polyhedron serves as a template for their formation and is substitutable [53]. In addition to Al₁₃ [51], limited research has been conducted on the role and properties of central polyhedra in recently reported Keggin-type clusters (e.g., Fe₁₃ [8], Mn₁₃ [9], Zr₁₃ [10], Hf₁₃ [11], and Eu₁₃ [12]). In this study, to elucidate the role of the central AlO₆ octahedron in cluster Al@Th₁₂@Al₈, we systematically introduced nearly all accessible transition metal salts into its reaction solution (Synthetic Procedures section in Supporting information). As shown in Fig. 2, we ultimately obtained crystals of five new clusters, designated as M@Th₁₂@Al₈ (M = W_0.69Al_0.31, Mo, V, Cr_0.14Al_0.86, and Mn). These crystals have similar formulas (Table 1 and Table S1). Notably, they exhibit topological structures similar to Al@Th₁₂@Al₈, primarily differing in their central MO₆ octahedra (Fig. 2 and Figs. S3-S7 in Supporting information). This demonstrates that the central MO₆ octahedra in M@Th₁₂@Al₈ are substitutable.

Since the ligands of some tetravalent metal-oxo clusters like Th₆ are substitutable, we also conducted syntheses using mixed solutions containing aluminate-gallate or aluminate-germanate. As shown in Table 1 and Table S1, only the aluminate-gallate solution proved effective, yielding crystals of three new clusters: M@Th₁₂@Ga_mAl_n (M = W, Mo, and V; m + n = 8). These clusters exhibit structures similar to M@Th₁₂@Al₈, with the key difference being the substitution of some AlO₅ groups by GaO₅ groups (Fig. 2 and Figs. S8-S10 in Supporting information). This suggests that the AlO₅ (or GaO₅) groups in the series of M@Th₁₂@L₈ (M = Al, W_0.69Al_0.31, Mo, V, Cr_0.14Al_0.86, and Mn; L₈ = Al₈ or Ga_mAl_n with m + n = 8) clusters act as tunable ligands.

To our knowledge, M@Th₁₂@L₈ represents the first series of Keggin-type actinide-containing clusters with both a substitutable central polyhedron and tunable shell ligands. Among the series of M@Th₁₂@L₈ clusters, Mo@Th₁₂@Al₈ exhibits the fastest crystallization rate (within hours) and highest yields (up to 70.2%). In contrast, Mn@Th₁₂@Al₈ formed at the highest solution pH of 5.19, where thorium hydroxides precipitate [54]. These results indicate that the identity of the central metal ions affects the formation and yield of M@Th₁₂@L₈ clusters.

It is noteworthy that the nine clusters exhibit a co-occupation phenomenon. Specifically, the central M ion in W_0.69Al_0.31@Th₁₂@Al₈ is co-occupied by W and Al ions, and that in Cr_0.14Al_0.86@Th₁₂@Al₈ is co-occupied by Cr and Al ions. Additionally, several shell L ions in M@Th₁₂@Ga_mAl_n are co-occupied by Al and Ga ions. Similar co-occupation phenomena are also common in mineral structures [55].

More remarkably, we found that the Al-O, W-O, Mo-O, V-O, Cr-O, and Mo-O bond lengths for the central octahedra in M@Th₁₂@L₈ clusters consistently measure ~1.87 Å, despite significant variations reported for these bonds in other structures (Table S4 in Supporting information). For example, V-O bonds in literature span 1.67–2.49 Å [56]. Furthermore, the diameters of M@Th₁₂@Al₈ remain constant (~14.85 Å) regardless of the central MO₆ octahedron type (Figs. S1 and S8-S10 in Supporting information). Similarly, M@Th₁₂@Ga_mAl_n clusters maintain slightly larger fixed diameters (~15.00 Å), due to the larger radius of Ga³⁺ compared to Al³⁺ (Figs. S8-S10 in Supporting information). These results demonstrate the structural rigidity of M@Th₁₂@L₈ clusters, which imposes strict geometric constraints on central metal ion coordination. By contrast, in classic Keggin-type clusters XW₁₂, the X-O bond lengths within their central XO₄ tetrahedra vary significantly depending on the identity of the heteroatom X [57].

Structural analysis of the nine M@Th₁₂@L₈ clusters reveals that they adopt a general formula of [(MO₆)Th₁₂(OH)₁₂(LO(OH)₃H₂O)₈(H₂O)₁₂]⁽⁸⁺^x⁾⁺, where x represents the charge of the central metal ion. According to BVS analysis, the central metal ions were identified as Al³⁺, Cr³⁺, Mn⁴⁺, V⁵⁺, Mo⁶⁺, and W⁶⁺, resulting in the cluster charges ranging from +11 to +14 (Tables S25 and S26 in Supporting information). This is in stark contrast to most POMs, which are typically negatively charged [2]. The unusual positive charges of these clusters are balanced by Cl^- and 2,6-NDS (or 2,7-NDS) anions dispersed among them (Figs. S11-S20 in Supporting information). The periodic arrangement of 2,6-NDS (or 2,7-NDS) anions, along with the 3.5–3.8 Å spacing between their naphthalene rings, indicates the presence of π−π stacking [58].

To complement the single crystal X-ray diffraction data, multiple techniques were employed to further characterize the structure, composition, and properties of M@Th₁₂@L₈ crystals. The powder X-ray diffraction (PXRD) patterns were used to investigate the purity and crystallinity of these crystals (Figs. S21-S29 in Supporting information). Solid-state ²⁷Al MAS NMR spectroscopy was used to confirm the coordination environments of Al³⁺. As shown in Fig. 3a, the spectrum of Al@Th₁₂@Al₈ exhibits a broad peak at ~26 ppm and a sharp peak at −6 ppm. This broad peak is assigned to shell AlO₅ groups [59,60], with its line broadening originating from the asymmetric coordination geometry of the shell Al ions. By contrast, the sharp peak is attributed to the central AlO₆ groups [59]. The narrow linewidth and high intensity of this resonance reflect the highly symmetric octahedral coordination environment of the central Al³⁺ ion, a rare feature among aluminum-based clusters. AlO₅ groups in Al clusters typically adopt asymmetric configurations due to mixed coordination with O atoms, OH groups, and H₂O molecules, resulting in broad NMR peaks [51]. The unusually sharp resonance observed here for the AlO₆ group in Al@Th₁₂@Al₈ is thus distinctive and can serve as a diagnostic marker for this cluster. Notably, the spectrum of Mo@Th₁₂@Al₈ exhibits only the AlO₅ resonance, consistent with its lack of central AlO₆.

Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) data qualitatively confirmed the existence of C, N, O, S, Al, Ga, Th, and central transition metal elements (e.g., W, Mo, V, Cr, and Mn) in these crystals (Figs. S30-S38 in Supporting information). For quantitative characterization of the metal content, inductively coupled plasma-mass spectrometry (ICP-MS) was performed. As summarized in Table 2 and Tables S27 and S28 (Supporting information), the measured molar ratios of metal elements in M@Th₁₂@L₈ were consistent with the calculated values, thereby verifying the accuracy of the chemical formulas presented in Table 1.

In addition to BVS analysis (Tables S25 and S26 in Supporting information), X-ray photoemission spectroscopy (XPS) was employed to confirm the oxidation states of metal ions in M@Th₁₂@L₈ crystals. The obtained full-scan and high-resolution XPS spectra (Figs. 3b-g and Figs. S39-S47 in Supporting information) demonstrate the presence of Al³⁺, Ga³⁺, Th⁴⁺, W⁶⁺, Mo⁶⁺, V⁵⁺, Mn⁴⁺, and S⁶⁺, with detailed peak assignments summarized in Table 3. Due to the low Cr content in Cr_0.14Al_0.86@Th₁₂@Al₈, collecting a reliable high-resolution XPS spectrum was challenging. Therefore, electron paramagnetic resonance (EPR) was used to further characterize this compound. The obtained EPR spectrum exhibits a broad signal centered at g = 1.9624 (Fig. 3h). This signal is consistent with the g value of standard CrCl₃ [61], confirming the presence of Cr³⁺. Similarly, the six-line hyperfine splitting in the EPR spectrum of Mn@Th₁₂@Al₈ further verifies the presence of Mn⁴⁺ (Fig. 3i) [62]. All the above experimental data align well with the BVS analysis results, providing consistent evidence for the assigned oxidation states.

The Fourier transform infrared (FT-IR) spectra of M@Th₁₂@L₈ crystals exhibit similar vibrational peaks in the 4000–400 cm^-1 range, confirming the presence of solvent water molecules, coordinated hydroxyl groups, and 2,6-NDS/2,7-NDS (Figs. S48 and S49 in Supporting information). Specifically, the broad envelope at 3385 cm^-1 corresponds to vibrations from solvent water molecules [69]. The peak at 1635 cm^-1 arises from bending modes of coordinated hydroxyl groups [69]. Peaks in the 1200–1000 cm^-1 range are assigned to sulfonate groups of 2,6-NDS or 2,7-NDS [69]. Peaks in the 900–750 cm⁻¹ range are attributed to the C–H in-plane or out-of-plane bending, ring deformation, and ring breathing of naphthalene in 2,6-NDS or 2,7-NDS [70]. The peak at 700 cm^-1 is assigned to the stretching vibrations of Al(Ga)-O bonds; the peaks at 658 and 618 cm^-1 are attributed to the stretching vibrations of Al-OH bonds; the weak peak around 511 cm^-1 corresponds to the stretching vibrations of Al-OH₂ bonds [71].

Thermogravimetric analysis (TGA) was employed to quantify the solvent water content and evaluate the thermal stability of M@Th₁₂@L₈ crystals. As shown in Fig. 3j and Figs. S50-S57 (Supporting information), these crystals exhibit similar TGA plots. Using Al@Th₁₂@Al₈ as an example, a weight loss of 10.67% between 33 and 200 ℃ corresponds to the removal of 37 solvent water molecules per formula unit [35]. Further weight loss in the range of 200–400 ℃ is attributed to the release of coordinated water molecules and hydroxyl groups [72]. The sharp weight loss at 520 ℃ is related to the decomposition of 2,6-NDS, particularly the combustion of naphthalene rings [72]. The calculated number of solvent water molecules for each crystal is summarized in Table 1. Additionally, the residues remaining after TGA were analyzed using PXRD (Fig. S58 in Supporting information). The obtained patterns confirm the presence of crystalline ThO₂, while previous studies suggest that the residues may also contain amorphous Al₂O₃ [72].

The solid-state ultraviolet-visible (UV–vis) spectra of the nine M@Th₁₂@L₈ clusters reveal that clusters with AlO₆, MoO₆, or WO₆ centers exhibit strong absorption exclusively in the UV region, attributed to oxygen-to-metal charge transfer [73]. In contrast, those with VO₆, CrO₆, or MnO₆ centers show additional absorption in the visible region (Figs. 3k and l). For V@Th₁₂@L₈ and Cr_0.14Al_0.86@Th₁₂@Al₈, the absorption peaks at 390 and 450 nm are attributed to the d → d electronic transitions of V⁵⁺ and Cr³⁺ [74], while the absorption at 470 nm for Mn@Th₁₂@Al₈ is assigned to the charge transfer transition of ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g of Mn⁴⁺ ions [75]. This difference highlights the significant influence of the central metal ions on the optical properties of these clusters. Furthermore, the optical band gaps of these clusters, calculated using the Kubelka–Munk function [76], range from 2.37 eV to 3.60 eV (Figs. S59-S67 in Supporting information). These values suggest their semiconducting properties [77].

Furthermore, the photoluminescence properties of Mn@Th₁₂@Al₈ crystals were investigated using steady-state transient fluorescence spectroscopy, as they exhibit prominent pink fluorescence under illumination. As shown in Fig. 4a and Fig. S68 (Supporting information), these crystals exhibit multiple intense peaks in the region of 625–750 nm upon excitation at 470 nm. This distinct photoluminescence behavior originates from the ²E_g → ⁴A_2g transition of Mn⁴⁺ center [78], with a fluorescence lifetime of 1.08 milliseconds and the quantum yield of 39.77% (Fig. 4b and Fig. S69 in Supporting information). Furthermore, temperature-dependent luminescence spectra of this cluster show a gradual decrease in emission intensity as the temperature increases from 77 K to 437 K (Fig. 4c and Fig. S70 in Supporting information). Notably, when measured Mn@Th₁₂@Al₈ crystals using PXRD after thermal cycling, the obtained data demonstrate their good thermal stability (Fig. S71 in Supporting information).

The above results suggest that the optical properties of M@Th₁₂@L₈ clusters can be effectively modulated by varying the central metal ion. On the other hand, these clusters with distinct optical properties show promising potential in many applications, such as the preparation of stable Mn⁴⁺-doped phosphors, as the Th₁₂ layer provides excellent protection for the Mn⁴⁺ luminescence centers [79].

The stability of the representative M@Th₁₂@Al₈ cluster was investigated using time-resolved SAXS (Fig. 5) and ²⁷Al NMR (Fig. 6). Guinier analysis of the SAXS data provided the radius of gyration (R_g) for each sample [80]. For the solution obtained by dissolving the Al@Th₁₂@Al₈ crystals, the R_g value is 4.9 Å. This value agrees well with the theoretical R_g value (4.6 Å) derived from the fitting of the crystallographic data, confirming that the Al@Th₁₂@Al₈ cluster remains intact in solution [81]. The NMR spectrum of this solution exhibits a narrow peak at −2.4 ppm, which is attributed to the central symmetric AlO₆ octahedron, further demonstrating the stability of Al@Th₁₂@Al₈ in solution.

The self-assembly process of Al@Th₁₂@Al₈ clusters in the reaction solution was also investigated using SAXS and NMR (Figs. 5 and 6). For the aluminate solution, the R_g is approximately 5.7 Å, consistent with the presence of nano-sized aluminate clusters in solution [49]. The corresponding NMR spectrum exhibited peaks at 3.7 and 66.5 ppm, which are attributed to asymmetric AlO₆ and symmetric AlO₄ groups in Al clusters, respectively [49]. After adding thorium nitrate solution and Tris to the aluminate solution and heating the mixture, the SAXS profile changes significantly, and the R_g decreases to 4.8 Å, which matches the theoretical value (4.6 Å) derived from the fitting of the crystallographic data of Al@Th₁₂@Al₈. Concurrently, a new NMR peak at −2.4 ppm appears, characteristic of the central AlO₆ in Al@Th₁₂@Al₈. Meanwhile, the relative intensity of the peak at 66.5 ppm decreases. These results confirm the formation of Al@Th₁₂@Al₈ clusters and the partial decomposition of the original aluminate clusters in solution. Subsequent addition of the mineralizer (2,6-NDS) caused no further changes in SAXS or NMR, demonstrating the clusters' stability in solution. Similarly, after crystallization, the SAXS and NMR profiles remained unchanged, further supporting the persistence of Al@Th₁₂@Al₈ clusters in solution. According to the SAXS and NMR data, a schematic was prepared to illustrate the assembly process of Al@Th₁₂@Al₈ clusters (Fig. S72 in Supporting information).

A striking structural feature of the nine M@Th₁₂@L₈ clusters is their tri-layered architecture, in which the central MO₆ octahedron and the middle Th₁₂ icosahedron adopt a Keggin-type arrangement. The outer cubic layer consists of eight non-interconnected LO₅ polyhedra that act as inorganic ligands. Although both the central MO₆ and outer LO₅ polyhedra are chemically variable, the M–O bond lengths remain invariant across different M ions. This invariance stems from the clusters' inherent structural rigidity, which imposes strict constraints on the coordination geometry of the central M ion. In contrast, Keggin-type XW₁₂, XMo₁₂, and XAl₁₂ clusters feature a variable central XO₄ tetrahedron, where X–O bond lengths depend on the identity of X [51,57]. Other metal-based Keggin clusters, primarily including Fe₁₃ [8], Mn₁₃ [9], Zr₁₃ [10], Hf₁₃ [11], and Eu₁₃ [12], are stabilized by organic ligands, but the substitutability of their central polyhedra has not been explored (Fig. S73 in Supporting information).

From a compositional perspective, the nine M@Th₁₂@L₈ clusters represent rare multinary heterometallic clusters incorporating main group, transition metal, and actinide elements. To our knowledge, only a limited number of thorium-containing heterometallic clusters have been reported. Apart from a few clusters stabilized by lacunary POMs of W and Mo to stabilize thorium ions [40], the known heterometallic clusters primarily include Th₂Al₆(edta)₂ [41], Th₂Al₆(hedta)₂ [42], Th₂Al₈(hdpta) [42], Th₂Al₈(heidi) [43], Th₄Al₁₀(edta) [41], ThFe₂(dhpta) [43], Th₂Fe₁₀(PhPO₃) [44], Th₉Ag₆(PyCOO) [46], and Th₉Ag₁₂(PyCOO) [46] (Fig. S74 in Supporting information). Among these binary clusters, the Th-Al and Th-Fe clusters form within a pH range of 2.35–5.95 and exhibit structures with a planar Al or Fe core, constructed via bridging O atoms and OH groups [41–44]. Th⁴⁺ ions are primarily chelated by organic ligands and then individually attached to the core. These structural features suggest that the hydrolysis and condensation of Al³⁺ or Fe³⁺ ions drive cluster formation, while chelated Th⁴⁺ ions function as ligands.

Although the M@Th₁₂@L₈ clusters form in a similar pH range of 3.02–5.19, only their middle Th₁₂ layer assembles via bridging O atoms and OH groups. This structural distinction implies that the hydrolysis and condensation of Th⁴⁺ ions direct the assembly of the M@Th₁₂@L₈ clusters, with AlO₅ groups passivating the Th₁₂ layer. This conclusion aligns with our SAXS and NMR data, which reveal the simultaneous decomposition of precursor Al clusters and self-assembly of M@Th₁₂@L₈ clusters. The stark structural contrast between reported Th-Al/Fe clusters and our M@Th₁₂@L₈ clusters indicates divergent hydrolysis behaviors and roles of Th⁴⁺ and Al³⁺ ions in the formation of heterometallic clusters, depending on the presence or absence of organic ligands. Notably, the two Th-Ag clusters contain Th cores with Ag⁺ ions attached peripherally. This structural motif is attributed to the identity of Ag⁺ ions as soft Lewis acids and their weaker hydrolysis ability [46].

Since M@Th₁₂@L₈ clusters are primarily formed through the hydrolysis and condensation reactions of thorium ions, they can be regarded as thorium-oxo clusters stabilized by AlO₅ ligands. In contrast, most other tetravalent metal-oxo clusters typically exhibit fluorite-like topological structures [23,24], and/or architectures built from octahedral M₆(O/OH)₈ subunits (Fig. S75 in Supporting information) [21,33,35]. The distinct tri-layered architecture of M@Th₁₂@L₈ clusters arises from the synergistic effects of the central heterometal (M) and the aluminate ligands. These components collectively modulate the hydrolysis and condensation pathways of Th⁴⁺ ions, enabling this unprecedented structural motif.

In this study, we report the synthesis of M@Th₁₂@L₈, the first series of electropositive α-Keggin-type thorium-oxo clusters featuring unprecedented triple-layered Platonic architectures: a central MO₆ octahedron, a Th₁₂ icosahedron, and an outer LO₅ cube. These clusters represent rare high-nuclearity thorium species and heterometallic systems integrating main-group, transition-metal, and actinide elements. Their structural rigidity ensures constant M–O bond lengths despite variability in central MO₆ octahedra, while the central M ion governs the solution pH during cluster formation, crystallization kinetics, crystal yield, and tunable photoluminescence. Time-resolved SAXS and ²⁷Al MAS NMR reveal the synchronized partial decomposition of precursor Al clusters and concurrent self-assembly of M@Th₁₂@L₈ clusters. This work establishes a platform for designing novel thorium clusters via heterometallic ion-regulated hydrolysis, with central/shell substitution enabling tailored properties.

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doi: 10.1016/j.cclet.2025.111563

Receive Date：2025-05-26
Online Date：2026-10-27
Published：2026-10-15

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Received：2025-05-26
Revised：2025-07-02
Accepted：2025-07-08

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^aSchool of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China

^bNingbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

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E-mail addresses: qiujie1228@xjtu.edu.cn (J. Qiu)

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Table 1 Crystallographic data and formulas for M@Th₁₂@L₈ crystals.

Compounds	Formula	Space group	a/b/c [Å]	α/β/γ [°]	CCDC
Al@Th₁₂@Al₈	[(AlO₆)Th₁₂(OH)₁₂(AlO(OH)₃H₂O)₈(H₂O)₁₂]·(2,6-NDS)₅·Cl·37H₂O	P $1 ¯$	15.7913(6) 16.2416(7) 18.5791(8)	108.961(1) 96.252(1) 112.251(1)	2440, 934
W_0.69Al_0.31@Th₁₂@Al₈	[(W_0.69Al_0.31O₆)Th₁₂(OH)₁₂(AlO(OH)₃H₂O)₈(H₂O)₁₂]·(2,6-NDS)₆·1.07Cl·38H₂O	P $1 ¯$	15.6216(5) 18.5440(6) 20.1398(7)	116.2980(10) 100.4570(10) 94.2680(10)	2440, 935
Mo@Th₁₂@Al₈	[(MoO₆)Th₁₂(OH)₁₂(AlO(OH)₃H₂O)₈(H₂O)₁₂]·(2,6-NDS)₆·2Cl·43H₂O	P $1 ¯$	15.6113(4) 18.5198(5) 20.1694(6)	116.2710(10) 100.4920(10) 94.2700(10)	2440, 936
V@Th₁₂@Al₈	[(VO₆)Th₁₂(OH)₁₂(AlO(OH)₃H₂O)₈ (H₂O)₁₂]·(2,6-NDS)₆·Cl·39H₂O	P $1 ¯$	15.5993(5) 18.5446(6) 20.2190(6)	115.0740(10) 100.5770(10) 94.4910(10)	2440, 937
Cr_0.14Al_0.86@Th₁₂@Al₈	[(Cr_0.14Al_0.86O₆)Th₁₂(OH)₁₂ (AlO(OH)₃H₂O)₈(H₂O)₁₂]·(2,6-NDS)₅·Cl·35H₂O	P $1 ¯$	15.7066(7) 16.1913(9) 18.7098(10)	108.865(2) 96.271(2) 112.122(2)	2440, 938
Mn@Th₁₂@Al₈	[(MnO₆)Th₁₂(OH)₁₂(AlO(OH)₃H₂O)₈(H₂O)₁₂]·(2,6-NDS)₅·2Cl·35H₂O	P $1 ¯$	17.5551(9) 20.7465(11) 23.2198(12)	74.509(2) 85.344(2) 82.050(2)	2440, 939
W@Th₁₂@Ga_3.69Al_4.31	[(WO₆)Th₁₂(OH)₁₂(Ga_3.69Al_4.31(O(OH)₃H₂O)₈)(H₂O)₁₂]·(2,7-NDS)₅·4Cl·38H₂O	P $1 ¯$	15.6236(11) 17.3098(14) 18.8235(14)	67.512(2) 79.724(2) 69.668(2)	2440, 940
Mo@Th₁₂@Ga_4.43Al_3.57	[(MoO₆)Th₁₂(OH)₁₂(Ga_4.43Al_3.57 (O(OH)₃H₂O)₈)(H₂O)₁₂]·(2,7-NDS)₅·4Cl·38H₂O	P $1 ¯$	15.6787(7) 17.0746(8) 18.8293(8)	65.714(2) 79.270(2) 70.639(2)	2440, 941
V@Th₁₂@Ga_4.85Al_3.15	[(VO₆)Th₁₂(OH)₁₂(Ga_4.85Al_3.15 (O(OH)₃H₂O)₈)(H₂O)₁₂]·(2,7-NDS)₆·Cl·37H₂O	P2₁/c	16.0864(2) 30.0580(4) 20.8001(3)	90 93.7110(10) 90	2440, 942

Table 2 The summary of ICP-MS results for M@Th₁₂@L₈ crystals.

Sample	Elements	Experimental ratios	Calculated ratios
Al@Th₁₂@Al₈	Al:Th	10.80:12.00	9:12
W_0.69Al_0.31@Th₁₂@Al₈	W:Al:Th	0.41:7.07:12.00	0.69:8.31:12
Mo@Th₁₂@Al₈	Mo:Al:Th	1.07:8.63:12.00	1:8:12
V@Th₁₂@Al₈	V:Al:Th	0.96:7.89:12.00	1:8:12
Cr_0.14Al_0.86@Th₁₂@Al₈	Cr:Al:Th	0.34:7.72:12.00	0.14:8.86:12
Mn@Th₁₂@Al₈	Mn:Al:Th	0.88:7.17:12.00	1:8:12
W@Th₁₂@Ga_3.69Al_4.31	W:Al:Ga:Th	1.00:4.88:3.41:12.00	1:4.31:3.69:12
Mo@Th₁₂@Ga_4.43Al_3.57	Mo:Al:Ga:Th	0.99:4.65:3.56:12.00	1:3.57:4.43:12
V@Th₁₂@Ga_4.85Al_3.15	V:Al:Ga:Th	0.57:3.66:3.63:12.00	1:3.15:4.85:12

Table 3 XPS peak positions and assignments for Al [63], Th [29], Ga [64], W [65], Mo [66], V [67], Cr [68], and Mn [62] ions in M@Th₁₂@L₈ crystals.

Sample	XPS peak positions (eV) and assignments
Al@Th₁₂@Al₈	74.77 (Al 2p)	344.47 (Th 4f_5/2)	335.17 (Th 4f_7/2)
W_0.69Al_0.31@Th₁₂@Al₈	74.97 (Al 2p)	344.67 (Th 4f_5/2)	335.37 (Th 4f_7/2)	38.77 (W 4f_5/2)	36.57 (W 4f_7/2)
Mo@Th₁₂@Al₈	74.77 (Al 2p)	344.37 (Th 4f_5/2)	335.17 (Th 4f_7/2)	236.37 (Mo 3d_3/2)	232.87 (Mo 3d_5/2)
V@Th₁₂@Al₈	74.77 (Al 2p)	344.47 (Th 4f_5/2)	335.07 (Th 4f_7/2)	525.37 (V 2p_1/2)	517.67 (V 2p_3/2)
Cr_0.14Al_0.86@Th₁₂@Al₈	74.57 (Al 2p)	344.27 (Th 4f_5/2)	334.97 (Th 4f_7/2)	581.67 (Cr 2p_1/2)	575.87 (Cr 2p_3/2)
Mn@Th₁₂@Al₈	74.57 (Al 2p)	344.27 (Th 4f_5/2)	334.97 (Th 4f_7/2)	653.87 (Mn 2p_1/2)	624.77 (Mn 2p_3/2)
W@Th₁₂@Ga_3.69Al_4.31	74.50 (Al 2p)	344.50 (Th 4f_5/2)	335.20 (Th 4f_7/2)	38.20 (W 4f_5/2)	36.10 (W 4f_7/2)	1145.2 (Ga 2p_1/2)	1118.4 (Ga 2p_3/2)
Mo@Th₁₂@Ga_4.43Al_3.57	75.20 (Al 2p)	345.10 (Th 4f_5/2)	335.70 (Th 4f_7/2)	236.17 (Mo 3d_3/2)	232.67 (Mo 3d_5/2)	1145.9 (Ga 2p_1/2)	1119.0 (Ga 2p_3/2)
V@Th₁₂@Ga_4.85Al_3.15	74.50 (Al 2p)	344.30 (Th 4f_5/2)	335.00 (Th 4f_7/2)	525.87 (V 2p_1/2)	518.47 (V 2p_3/2)	1145.3 (Ga 2p_1/2)	1118.50 (Ga 2p_3/2)

Scheme 1 Synthetic route of M@Th₁₂@L₈ clusters: solutions containing thorium salt and templated ions were mixed with an Al or Al/Ga stock solution; then a mineralizer was added to the solution; crystals of M@Th₁₂@L₈ were obtained after a certain period.

Fig.1 (a) Ball-stick, (b) graphical, and (c) polyhedral representations of the crystal structure of Al@Th₁₂@Al₈, and its three concentric Platonic layers: (d) Central octahedron, (e) thorium icosahedron, and (f) aluminum cubic. (g) Connection of the central AlO₆ group to twelve thorium ions via six µ₃-O. (h) Coordination of each AlO₅ group to three thorium ions via three µ₂-OH and one µ₄-O. (i) Nine-coordination mode of each Th⁴⁺ ion. (j) Peripheral AlO₅ group. Color code: Th, green; central Al, blue; peripheral Al, cyan; O, red.

Fig.2 Crystals and structures of six M@Th₁₂@Al₈ (M = Al, W_0.69Al_0.31, Mo, V, Cr_0.14Al_0.86, or Mn) clusters and three M@Th₁₂@Ga_mAl_n (M = W, Mo, or V) clusters with a variable central MO₆ octahedron. Color code: Th, green; central Al, blue; peripheral Al, cyan; W, grey; Mo, grey pink; V, light blue; Cr, purple; Mn, brown; Ga, pink; O, red.

Fig.3 (a) Solid state ²⁷Al MAS NMR spectra of Al@Th₁₂@Al₈ and Mo@Th₁₂@Al₈. (b-g) Representative XPS high-resolution spectra of Al, Th, W, Mo, V, and Ga in M@Th₁₂@L₈. (h) EPR spectra of Cr in Cr_0.14³⁺Al_0.86³⁺@Th₁₂@Al₈ and (i) Mn in Mn@Th₁₂@Al₈. (j) TGA curves of Al@Th₁₂@Al₈. UV–vis spectra of (k) M@Th₁₂@Al₈ and (l) M@Th₁₂@Ga_mAl_n.

Fig.4 (a) Photoluminescence emission spectrum, (b) fluorescence lifetime, and (c) temperature-dependent photoluminescence emission spectra of Mn@Th₁₂@Al₈.

Fig.5 (a) SAXS and (b) Guinier plots for solutions related to synthesis and crystals of M@Th₁₂@Al₈: (A) Aluminate stock solution; (B) mixture of aluminate stock solution, thorium nitrate solution, and tris(hydroxymethyl)aminoethane after heating at 80 ℃); (C) aforementioned mixed solution with the addition of 2,6-NDS-Na₂; (D) centrifuged solution after the formation of Al@Th₁₂@Al₈ crystals; (E) solution obtained by dissolving Al@Th₁₂@Al₈ crystals into tetrabutylammonium bromide solution.

Fig.6 ²⁷Al solution NMR plots for solutions related to synthesis and crystals of Al@Th₁₂@Al₈. The sample labels are the same as in Fig. 5.

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