Nickel Ion Induced Multistage Assembly of Th_13 Cluster

Domain: earth_science
Published: 2025-04-29
Editorial outcome: major_revision

Abstract
Clusters have proven useful models to elucidate the correlation between macroscopic properties and microstructures. However, the actinide clusters still remain much less developed compared to the splendid and systematic works of other members of this family. Here we show the synthesis of a tetra-shell mixed-metal cluster [ThO8@Th12(OH)24@Ni6(H2O)18@(sba)12] (IHEP-25, H2sba: 2-sulfobenzoic acid) with the strategic introduction of competitive Ni2+ ions. A cluster-based framework (IHEP-28), featuring a 2D honeycomb network and demonstrating excellent stability, has been constructed through the multistage assembly of IHEP-25, facilitated by the introduction of cationic cluster [Na4(OH)3(H2O)3]+. IHEP-28 acts as a highly effective visible light-driven photocatalyst for CO2 reduction. This study demonstrates that the incorporation of heterometal ions not only alters the hydrolysis patterns of actinide metal ions, resulting in the formation of new actinide clusters, but also broadens their potential applications.

Introduction
Clusters, serving as intermediaries between discrete metal ions and bulk materials, present perfect models to elucidate the correlation between macroscopic properties and microstructures, leading to the synthesis and comprehensive characterization of numerous polynuclear metal clusters involving transition, main-group, and lanthanide metals1–12. In contrast, the exploration of actinide clusters lags behind13. In recent years, with the development of the nuclear industry, actinide clusters have gradually attracted attention due to their potential applications in managing nuclear waste and mitigating radioactive pollution. For instance, under neutral and alkaline conditions, the pronounced propensity of actinides, especially their hypervalent ions, for hydrolysis can lead to the genesis of soluble polynuclear species. These species are of paramount importance in understanding the environmental transport and migration of radionuclides14–16. The methodical investigation of actinide clusters thus offers critical insights for precisely depicting the mobility of radionuclides in environmental matrices. Against this backdrop, several notable actinide clusters have been synthesized and detailed, including U(IV)-based clusters such as [U16], [U24], [U38]17, an assortment of uranyl-peroxide clusters18–25, the mixed-valent cluster [UV(UVIO2)28]26, and transuranic clusters [Pu38]27,28 and [Np38]29, showcasing significant strides in the field.

Thorium is gaining attention as a potential new source of nuclear energy, especially with the development of Molten Salt Reactors (MSR)30–32. Consequently, the chemistry of thorium has become increasingly significant in recent years33–37. The synthesis and structural characterization of polynuclear thorium clusters, in particular, play a crucial role in enhancing our understanding of the structures and properties of thorium-based materials, including thorium colloids and thorium-based nuclear fuels. While there have been recent advances in the chemistry of uranium clusters, the study of thorium (Th) clusters remains comparatively underdeveloped13,34–42. Due to the acidity and high charge density, the propensity of Th4+ ions to undergo hydrolysis typically leads to the formation of lower-nuclearity clusters or ThO2 nanoparticles23,43–53. A reasonable strategy for synthesizing large-nuclearity thorium clusters involves the introduction of competitive metal ions. These ions, alongside organic or inorganic ligands, can serve as terminating agents that prevent the aggregation of actinide polynuclear species into stable colloids54,55. By varying the types of competitive metal ions introduced, it’s possible to tailor the hydrolysis and condensation behaviors of actinides, potentially leading to the formation of mixed-metal clusters with greater nuclearity and size. Furthermore, considering the complexity of natural environmental systems, understanding the formation processes and structural traits of various mixed-metal clusters can aid in predicting the occurrence, migration pathways, and concentration mechanisms of actinide elements under diverse environmental conditions56–61. This insight is crucial for advancing research in environmental science and geology. Meanwhile, it can expand the potential applications of actinide materials62.

As a proof-of-concept, herein we report the synthesis and characterization of a mixed-metal cluster, [Th13Ni6O8(OH)24(H2O)18(sba)12] (IHEP-25, H2sba: 2-sulfobenzoic acid). At its core, IHEP-25 features a [Th13O8(OH)24] structure, analogous to the classical α-Keggin type molecule. By modifying the reaction conditions, some of the surface solvent ligands can be exchanged, resulting in the formation of three additional [Th13Ni6] clusters: [Th13Ni6(O)8(OH)24(sba)12(H2O)12(CH3CN)6] (IHEP-26), [Th13Ni6(O)8(OH)24(sba)12(H2O)12(CH3CN)6][Th13Ni6(O)8(OH)24(sba)12(H2O)18] (IHEP-27) and [Th13Ni6(O)8(OH)24(sba)12(H2O)10(CH3CN)2] (IHEP-31). Furthermore, the stable [Th13Ni6] cluster can also assemble into a 2D honeycomb net structure, IHEP-28 {[Th13Ni6(O)8(OH)24(sba)14(H2O)12(CH3COO)3(CH3CN)2] [Na4(OH)3(H2O)3]2}n, induced by the cation cluster [Na4(OH)3(H2O)3]+. IHEP-28 demonstrates exceptional stability in air and various solvents and serves as an effective CO2 reduction photocatalyst in the visible light region.

Results
Structural characterization
Single-crystal X-ray diffraction reveals that IHEP-25 crystallizes in the cubic space group Pn\bar{3}n (Supplementary Table S1). As shown in Fig. 1a and Supplementary Fig. S1, the asymmetric unit of IHEP-25 contains three crystallographically independent Th atoms, one Ni atom, six O2−/OH− anions, two deprotonated 2-sulfobenzoic acid anion ligands (sba2−) and three coordinated H2O. In this unit, Th3 is located at the special position of the symmetry axis with a position occupancy of 1/6, while the other two Th atoms have full occupancies 1.0. Through an axisymmetric operation the complete [Th13Ni6(O)8(OH)24(OH)18(sba)12(H2O)18] structure of IHEP-25 is obtained (Fig. 1b). The entire structure of IHEP-25 is organized into four concentric shells (Supplementary Fig. S2). In the innermost shell, a Th atom (Th2) resides at the center, coordinated by eight μ4-O2−, forming an approximate cube with O···O distances ranging from 2.7532 Å to 2.7908 Å (Fig. 1c, f and Supplementary Figs. S3 and S4a). The cube is enveloped by 12 Th atoms as the second shell, shown in Fig. 1d. These 12 Th atoms are interconnected by 30 oxygen atoms, stemming from 18 μ3-OH−, 6 μ2-OH− and 6 sba2− ligands (Supplementary Fig. S3), constructing an approximate Archimedean Solid icosahedron structure (Fig. 1g), with Th···Th distances varying from 3.8140 Å to 4.7679 Å (Supplementary Fig. S4b). The central [ThO8@Th12(OH)24]12+ core is reminiscent of an α-Keggin type molecule. While such cores have been noted in various metal clusters9, this report marks the first appearance of such a core structure in thorium clusters. Furthermore, six Ni atoms are uniformly distributed around the Th cluster, forming a six-membered ring with a chair conformation as the third shell, with Ni···Ni distance of 6.7329 Å (Supplementary Fig. S4c and S4d). Each Ni atom is linked to three Th atoms via three μ3-OH− ligands (Fig. 1e) to form a compressed tetrahedral NiTh3 structure. These six NiTh3 units interconnect through a vertex-sharing mode and integrated onto the [Th13] icosahedron surface via a face-sharing approach (Fig. 1h). The fourth shell of IHEP-25 is consisting of 12 sba2− ligands, which serve to shield the heterometal clusters from further hydrolysis and prevent polymer formation, resulting in the matryoshka nanocluster [ThO8@Th12(OH)24@Ni6(H2O)18@(sba)12] (Fig. 1b and Supplementary Fig. S2d). Based on the differing coordination interactions between the sba2− ligands and metal ions, the twelve coordinating ligands of IHEP-25 are categorized into two groups with coordination modes of μ-η1η2η1 and μ-η1η1 (Supplementary Fig. S5). The coordination modes of all the ligands involved in IHEP-25 have been collected in Table 1. As shown in Supplementary Fig. S6, each [Th13Ni6] cluster is connected to six adjacent equivalent clusters through hydrogen bonding (C-H···O-S, C-H···O-C, O-H···O-S), forming a 3D supramolecular structure.

Theoretical calculation of mixed-metal [Th13Ni6] clusters
The structures of the inner cores [Th13(O)8(OH)24]12+ cluster and [Th13(O)8(OH)24Ni6(sba)12(H2O)18] molecule were explored using the scalar-relativistic density functional theory (DFT) to explore the electronic structures. The detail computational methods are provided in SI, and the optimized structures are shown in Supplementary Fig. S15. The front molecular orbitals (MOs) and the corresponding energy levels of [Th13(O)8(OH)24]12+ are provided in Fig. 2. The energy gap between the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) of [Th13(O)8(OH)24]12+ is 4.03 eV, which is significant larger than that of Au20 (1.82 eV)63 and C60 (1.57 eV)64 clusters, which reflects that the [Th13(O)8(OH)24]12+ cluster is highly stability. The energy level of MOs obviously shows three parts in Fig. 2, the lowest band is Th 6d and O 2p orbitals, the middle band is mainly O 2p orbitals, while for the highest band is dominated by Th 6d orbitals. Moreover, the six lower HOMOs (HOMO-66-HOMO-71) appear obvious interactions between Th and O atoms. The analysis of the electron localization function (ELF) of the [Th13(O)8(OH)24]12+ cluster with three planes, including different Th-O bonds, is displayed in Supplementary Fig. S16. There is clear interaction between the Th and O atom with more electron density, which also implies the stable electronic structure. In addition, the electronic structure of [Th13(O)8(OH)24Ni6(H2O)18(sba)12] is also explored at the same level of theory, and the front MOs are showed in Supplementary Fig. S17. The HOMO-LUMO gap (2.61 eV) of [Th13(O)8(OH)24Ni6(H2O)18(sba)12] is smaller than that of [Th13(O)8(OH)24]12+ core because the HOMOs of former are contributed by the Ni 3d orbitals, which suggest that Ni2+ has the potential to serve as the catalytic active center of [Th13(O)8(OH)24Ni6(H2O)18(sba)12] cluster.

Controlling the Secondary Assembly of [Th13Ni6] Cluster
Similar to other stable clusters, the surface ligands of the [Th13Ni6] cluster are exchangeable65. By modifying the reaction conditions, three additional [Th13Ni6] clusters, IHEP-26, IHEP-27, and IHEP-31, have been synthesized. As shown in Supplementary Figs. S7 and S32, the asymmetric unit of IHEP-26 constitutes half of a [Th13Ni6] cluster, presenting three types of Ni2+ cations characterized by various coordination modes. Ni1 is bonded to three OH−, two sba2− ligands, and one H2O molecule. Ni2 is coordinated by three OH−, two H2O, and one CH3CN molecule. The coordination sphere of Ni3 is occupied by three OH−, two CH3CN molecules, and one sba2− ligand. Consequently, the six coordinated H2O molecules in IHEP-25 are substituted by six CH3CN molecules, resulting in the comprehensive [Th13Ni6(O)8(OH)24(sba)12(H2O)12(CH3CN)6] cluster structure of IHEP-26 (Fig. 3a). As shown in Fig. 3b, the entire structure of IHEP-27 contains two independent Th13Ni6 clusters with molecule formula of [Th13Ni6(O)8(OH)24(sba)12(H2O)18][Th13Ni6(O)8(OH)24(sba)12(H2O)12(CH3CN)6]. One of the Th13Ni6 clusters matches that of IHEP-25. The other, while akin to the IHEP-25’ cluster, features one of the Ni2+ cation coordinating H2O molecules replaced by CH3CN (Supplementary Fig. S8). As shown in Supplementary Fig. S9a, the asymmetric unit of IHEP-31 constitutes half of a [Th13Ni6] cluster, presenting two types of Ni2+ cations characterized by various coordination modes. The coordination modes of Ni1 and Ni2 are similar, both are bonded to three OH−, one sba2− ligand and one H2O molecule. Ni3 is coordinated by three OH−, one sba2− ligand, one H2O, and one CH3CN molecule (Supplementary Fig. S9c). The comprehensive [Th13Ni6(O)8(OH)24(sba)12(H2O)10(CH3CN)2] cluster structure of IHEP-31 is shown in Supplementary Fig. S9b.

The adaptability of the surface ligands enables the Th13Ni6 cluster to function as a versatile node for constructing cluster-based materials, akin to other stable clusters such as [Fe6], [Th13], [Ag112], and others. Illustrating its potential, the [Th13Ni6] cluster can be interlinked with the [Na4(OH)3(H2O)3]+ cationic cluster to forge a 2D honeycomb network, forming the foundational structure of IHEP-28 (Fig. 3c). The asymmetric unit of IHEP-28 comprises half a [Th13Ni6] cluster core and a third of a [Na4] cluster core, as depicted in Supplementary Fig. S10a and S10b. The coordination environments of the metal ions and the coordination patterns of sba2− ligands are detailed in Supplementary Figs. S11 and S12, respectively. While the [Th13Ni6] cluster core of IHEP-28 remains consistent with those in IHEP-25, 26, 27, and 31, its surface ligands exhibit significant differences. Some of the coordinated H2O molecules are substituted with CH3CN molecules, CH3COO− ligands, and sba2− ligands, culminating in the comprehensive [Th13Ni6(O)8(OH)24(sba)14(H2O)12(CH3COO)3(CH3CN)2] cluster structure (Supplementary Fig. S10c and S10d). Each [Th13Ni6] cluster is tethered to four adjacent units via two [Na4] clusters, creating an extensive heterotrimetal honeycomb (6,3) network (Fig. 3c and Supplementary Fig. S13) with a [Th13Ni6] cluster separation of 19.1534(3) Å. The honeycomb network’s minimal structural unit is composed of six [Th13Ni6] clusters and six [Na4] clusters, boasting a diameter of 24.0932(4) Å (Supplementary Fig. S13b). Moreover, the adjacent 2D networks exhibit an ABCABC stacking pattern (Supplementary Fig. S13d), evolving into a 3D porous framework (Supplementary Fig. S14).

Synthetic comments
In solvothermal reactions, some organic functional groups may undergo in situ reactions due to high temperature and pressure conditions. Typical examples of such reactions include the hydrolysis of cyano and ester groups66, as well as the oxidation of halogen groups67. Notably, during the synthesis of coordination polymers, it is quite common for cyano groups to hydrolyze and then coordinate with metal ions, a phenomenon that also accounts for the source of acetate ligands in IHEP-28. In addition to the aforementioned reactions, all the compounds reported in this work involve the in situ oxidation of thiol groups to sulfonate groups, a process that has been documented in several literatures68. In our study, we conducted a solvothermal reaction by dissolving nickel nitrate, thiosalicylic acid, and sodium hydroxide in a mixed solution of water and acetonitrile, obtaining a thiol-coupled product, 2, 2’-dithiosalicylic acid. However, when thorium nitrate was added to the reaction system, thiosalicylic acid was oxidized to 2-sulfobenzoic acid. This oxidation phenomenon may stem from the strong hydrolysis of Th4+ under high-temperature and high-pressure conditions, leading to the generation of hydrogen ions from water decomposition. This process significantly enhances the oxidizing ability of nitrate anions, thereby facilitating the oxidation of thiosalicylic acid to generate 2-sulfobenzoic acid69.

To explore the potential formation mechanism of [Th13Ni6] clusters, a series of comparative experiments were conducted. Based on the results of these experiments and the structural comparison of thorium complexes reported in this work, a possible formation mechanism for [Th13Ni6] clusters was proposed (Supplementary Fig. S18). Reported thorium monomers include Th(OH)3+ and Th(OH)22+, as well as dimers e.g., Th2(OH)26+, tetramers e.g., Th4(OH)84+, and hexamers e.g., Th6(μ3-O)4(μ3-OH)412+, all of which are important species of thorium ions in solution23. In addition, the presence of carboxylate anion ligands can promote the formation of hexamer [Th6] clusters70–72, which is consistent with our experimental observations: in the absence of Ni2+ ions, Th4+ ions preferentially assemble with the sba2− ligand to form [Th6(O)4(OH)4(sba)6(H2O)12] cluster (IHEP-29, Supplementary Fig. S19). As the concentration of NaOH increases, the morphology of the [Th6] cluster changes, accompanied by the generation of amorphous thorium hydroxide, which eventually becomes the main product. After the introduction of Ni2+ ions, [Th6] clusters were no longer observed in the reaction products; instead, [Th13Ni6] clusters and 1D chain structure (IHEP-30) with mononuclear Th4+ as nodes formed. This is due to the competition between nickel ions and the [Th6(sba)12] cluster for the sba2− ligands, leading to its dissociation into units such as [Th5], [Th4], and mononuclear [Th]. In a low-alkalinity environment, the dissociation products are primarily mononuclear [Th], which further form a 1D chain structure IHEP-30, {Th(sba)(H2O)2}n·2nH2O (Supplementary Fig. S20). As the alkalinity of the reaction system increases, the stability of the lacuna oligomers [Th5] and [Th4] is enhanced. This enhancement facilitates their secondary assembly in a co-vertex manner, resulting in the formation of an intermediate product, [Th9], characterized by a sandwich structure. This product continues to capture mononuclear [Th] in the reaction system, ultimately leading to the formation of the [Th13] cluster. A portion of the O2−/OH− coordination sites on the surface of the [Th13] cluster is occupied by nickel ions, resulting in the formation of the heterometallic cluster [Th13Ni6]. Furthermore, under the induction of different alkali metal cations, the [Th13Ni6] clusters can further assemble to form 3D frameworks with various packing patterns and topological structures (IHEP-25, IHEP-26, IHEP-27, IHEP-28, and IHEP-31). Their core structure [Th13Ni6(O)8(OH)24]24+ remains stable (Table 2). Therefore, the introduction of nickel ions plays a critical role in the formation of [Th13] clusters. On the one hand, the lacuna clusters formed by the partial disassembly of [Th6(sba)12] clusters induced by Ni2+ subsequently reorganize in the solution to form [Th13] clusters. On the other hand, Ni2+ and sba2− ligands function as capping units, halting further aggregation of the clusters into polymers e.g., thorium hydroxide.

CO2 photoreduction using IHEP-28
In recent years, solar-driven artificial carbon dioxide reduction reactions (CO2RR) have gradually gained widespread attention for their potential to reduce greenhouse gas emissions and lower energy demand. To achieve this goal, synthesizing highly efficient and selective photocatalysts to lower the thermodynamic energy barrier has become a critical issue that need to be addressed. Among the various photocatalysts developed for CO2 reduction, nickel compounds are considered one of the promising candidates73–75. A significant advantage of these photocatalysts is that CO2 can effectively bind to and be activated by the nickel center, thereby promoting its conversion efficiency under solar irradiation. In this work, the [Th13Ni6] cluster exhibits a local [NiTh3O3] structural unit, which can be regarded as a discrete molecular equivalent of the nickel-supported catalyst Ni@thoria54,76,77 (Supplementary Fig. S21). Therefore, the [Th13Ni6] cluster holds promise for application in the photocatalytic reduction of CO2. However, as mentioned previously, due to the substitutability of the surface ligands on the [Th13Ni6] clusters, the discrete [Th13Ni6] clusters (IHEP-25, IHEP-26, IHEP-27, and IHEP-31) we obtained are difficult to simultaneously meet the requirements of high purity and high yield in experimental applications, primarily because these clusters often mix with each other. Once the [Th13Ni6] clusters are interlinked with the [Na4(OH)3(H2O)3]+ clusters to form a 2D coordination polymer (IHEP-28), both purity and yield are significantly enhanced, and this 2D structure also contributes to improving the stability of the materials. Furthermore, the porous structure of IHEP-28 can expose more active sites, thereby further enhancing its photocatalytic performance. In addition, IHEP-28 features excellent stability in both air and common solvents (Supplementary Fig. S22). Based on these factors, in the subsequent research, we will focus on IHEP-28 as our primary subject to explore the photocatalytic reduction performance of the [Th13Ni6] cluster for CO2.

Photocatalytic CO2 reduction experiments utilizing IHEP-28 as the catalyst were conducted in a CH3CN:H2O mixture (4:1 volume ratio), employing [Ru(bpy)3]Cl2·6H2O as the photosensitizer (10 mg) without the need for an additional sacrificial agent. The CO2RR results, presented in Fig. 4a under 1 atm of pressure and visible light irradiation (λ > 420 nm), reveal an nearly linear growth in CO production over 3 h, amounting to a rate of 112.7 µmol·h−1·g−1. The photocatalytic process displayed an exceedingly high selectivity for CO production, nearly 100%, with no other gas or liquid phase products detected (Supplementary Fig. S23a). This rate is comparable to reported Ni-based cluster or MOF photocatalysts without additional sacrificial agent (Supplementary Table S2). The CO2 photoreduction system manifests an apparent quantum yield (AQY) of 0.74% under monochromatic light irradiation of 525 nm. Control experiments, as depicted in Fig. 4b and Supplementary Table S3, yielded a negligible amount of CO. Notably, when CO2 was substituted with Ar, no CO production was observed, confirming that the CO is derived from CO2 reduction rather than the decomposition of IHEP-28, which can also be confirmed by the isotopic test (Supplementary Fig. S23b). Remarkably, IHEP-28 efficiently catalyzes the photocatalytic reduction of CO2 even in the absence of photosensitizers and sacrificial agents, achieving a CO production rate of 10.1 µmol·h-1·g-1. Post-photocatalysis, PXRD analysis confirmed that IHEP-28 retained its crystallinity, indicating its excellent chemical stability (Supplementary Fig. S24). Intriguingly, after three catalytic cycles, IHEP-28 maintained similar catalytic activity and high crystallinity, demonstrating its excellent stability during the photocatalytic process (Supplementary Fig. S25).

The band gap energy (Eg) of IHEP-28 is determined to be 2.43 eV, based on the UV-Vis-NIR spectrum and Tauc plots (Supplementary Fig. S26a, b). The Mott-Schottky plots result indicates that IHEP-28 behaves as a typical n-type semiconductor, evidenced by a positive slope with the flat band potential of −1.12 V (vs. Ag/AgCl), as shown in Supplementary Fig. S26c. Consequently, the conduction band (CB) potential of IHEP-28 is estimated to be −0.92 V vs NHE. Therefore, the valence band (EVB) of IHEP-28 is determined to be 1.51 V (vs NHE), according to the empirical formula EVB = ECB + Eg (1) (Supplementary Fig. S26d)78. To confirm the products of the reaction between the photoinduced holes and H2O, we first examined the gaseous products and did not detect the presence of oxygen. Subsequently, based on the classic Ti-oxalate method reported in the literature, we tested the solution after the photocatalytic reaction and detected the formation of H2O2 (Supplementary Fig. S27)79. Therefore, under visible light irradiation, photoinduced electrons move to catalytically active sites for CO2 reduction while corresponding holes are consumed by H2O with the formation of H2O2.

A set of electrochemical techniques were used to check the electrochemical behaviors in IHEP-2880,81. The electrochemical impedance spectroscopy (EIS) test was performed to investigate the interfacial charge transfer and recombination (Fig. 4c). Nyquist plots acquired under illumination conditions show that the charge transfer impedance of the IHEP-28 + [Ru(bpy)3]2+ mixture is smaller than that of the IHEP-28 alone, suggesting the better separated photoexcited charged pairs by introduction of Ru(bpy)32+, ultimately improving the photocatalytic performance. As shown in Fig. 4d, there is a much higher transient photocurrent response signal observed on the IHEP-28 + [Ru(bpy)3]2+ mixture, while the absence of Ru(bpy)32+ yields a much weaker photocurrent. These test results indicate that the introduction of Ru(bpy)32+ significantly improved the transfer and separation efficiency of photogenerated electrons and holes in IHEP-28, which aligns with the experimental results showing that Ru(bpy)32+ enhances the photocatalytic reduction efficiency of CO2 by IHEP-28.

Mechanism of photocatalytic CO2 reduction
By employing in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), it is possible to monitor in real-time the surface species adsorbed during the CO2RR process and the intermediates generated from CO2 conversion. As shown in Fig. 5, the time-resolved spectra of IHEP-28, after introducing moist CO2 in the dark, reveal characteristic infrared peaks of the active •CO2− intermediate (1632 cm−1), bidentate carbonate (b-CO32−, 1540 cm-1), monodentate carbonate (m-CO32−, 1514 cm-1)82. Under light irradiation, several characteristic infrared peaks can be observed around 1698 cm-1, indicating the generation of *CO2, which is attributed to the activation of adsorbed CO2 by photogenerated electron83. Meanwhile, new peaks at 1226 and 1558 cm−1 can also be observed, and their peak intensities increase as time go on. Both peaks should be assigned to the COOH* group, which is a key intermediate during CO2 photoreduction into CO84. In addition, the detection of the CO* absorption band at 2077 cm−1 and the gas CO absorption band around 2162 cm−1 further confirms the formation of CO. Moreover, the CO2RR process detects the formation of HCO3− (1426 and 1462 cm−1)85.

DFT calculations were conducted to further elucidate the CO2RR mechanism of IHEP-28. As shown in Supplementary Fig. S28, considering the similar coordination mode of Ni2+ ions in IHEP-28, we extracted the Th3NiII(H2O)2 structural unit from the Th13Ni6 cluster as a model for DFT calculations. In this model structure, the Th3 unit bridged by three hydroxyl groups serves as the support for the catalytically active center Ni2+. Under the excitation of visible light, Th3NiII(H2O)2 enriched a photogenerated electron from the photosensitizer while simultaneously losing two coordinated water molecules, thereby forming Th3NiI with unsaturated coordination. As shown in Fig. 6, the Th3NiI unite combines with a CO2 molecule to form the intermediate complex Th3NiII(CO2), accompanied by an exothermic process of −0.29 eV, which indicates that this reaction is thermodynamically feasible. In the Th3NiII(CO2) complex, the CO2 molecule is coordinated to the Ni atom with the η2(C, O) coordination mode, where the Ni-C and Ni-O bond distance is 1.993 and 2.063 Å, respectively. The reaction from Th3NiII(CO2) to Th3NiII(COOH) is endothermic by 0.08 eV. In this process, an O-H bond was formed in the Th3NiII(COOH) complex, resulting in a change in the coordination mode between the Ni atom and CO2 molecule from η2(C, O) to η1(C), with the Ni-C bond distance of 1.991 Å. Subsequently, the Th3NiII(COOH) complex captured an additional photogenerated electron from the photosensitizer while simultaneously acquiring a proton (H+) to form the intermediate complex Th3NiII(CO)(H2O). This process was accompanied by a significant exothermic release of −3.58 eV. In the transformation of Th3NiII(COOH) to Th3NiII(CO)(H2O), one C=O bond of the CO2 molecule is broken, resulting in the formation of a coordinated H2O molecule and a coordinated CO molecule. In the Th3NiII(CO)(H2O) complex, the bond lengths are 2.015 Å for the Ni-O bond and 2.565 Å for the Ni-Owater bond. The elongated Ni-Owater bond distance facilitates cleavage in the presence of water molecule, ultimately resulting in the formation of the Th3NiII(H2O)2 complex while concurrently releasing a CO molecule, through an exothermic process with the energy release of −0.73 eV.

Based on previous related reports86,87, the aforesaid in situ IR analysis, and the DFT calculations, the photocatalytic CO2RR mechanism of IHEP-28 is proposed as follows. Under visible light irradiation, electrons photoinduced by the photosensitizer [Ru(bpy)3]2+ transferred to the Th13Ni6 cluster of IHEP-28, and then the CO2 molecules absorbed on IHEP-28 surface were reduced into CO undergo a two-electron process (Fig. 7).

Methods
Materials availability
Caution! Thorium nitrate (Th(NO3)4·4H2O, 98%, Aladdin) is radioactive and chemically toxic; precautions with suitable care and protection for handling such substances must be followed. Ni(NO3)2·6H2O (98%, Xilong Chemical Industry Incorporated Co. LTD), thiosalicylic acid (99%, Aladdin), [Ru(bpy)3]Cl2·6H2O (98%, Aladdin), LiOH·H2O (98%, Energy Chemical), NaOH (99%, Innochem), KOH (95%, Shanghai Titan Scientific Co., Ltd), CH3CN (99%, Shanghai Titan Scientific Co., Ltd) and other reagents were analytical grade and used without further purification.

Synthesis of IHEP-25 [Th13Ni6(O)8(OH)24(sba)12(H2O)18]
Th(NO3)4 (0.5 M, 200 µL), Ni(NO3)2 (0.5 M, 200 µL), thiosalicylic acid (0.4 mmol), CH3CN (3 mL), KOH (1 M, 75 µL) were loaded into a 25 mL autoclave. The autoclave was sealed, heated to 150 °C in an oven for 12 h, and then cooled to ambient temperature. A small quantity of almost colorless square block crystals of IHEP-25 could be obtained, as well as a lot of amorphous solids.

Synthesis of IHEP-26 [Th13Ni6(O)8(OH)24(sba)12(H2O)12(CH3CN)6]
Th(NO3)4 (0.5 M, 200 µL), Ni(NO3)2 (0.5 M, 200 µL), thiosalicylic acid (0.4 mmol), CH3CN (3 mL), KOH (1 M, 125 µL) were loaded into a 25 mL autoclave. The autoclave was sealed, heated to 150 °C in an oven for 12 h, and then cooled to ambient temperature. Almost colorless rhomboid crystals of IHEP-26 could be obtained, as well as a lot of amorphous solids.

Synthesis of IHEP-27 [Th13Ni6(O)8(OH)24(sba)12(H2O)18]·[Th13Ni6(O)8(OH)24(sba)12(H2O)12(CH3CN)6]
Th(NO3)4 (0.5 M, 200 µL), Ni(NO3)2 (0.5 M, 200 µL), thiosalicylic acid (0.4 mmol), CH3CN (3 mL), NaOH (1 M, 210 µL) were loaded into a 25 mL autoclave. The autoclave was sealed, heated to 150 °C in an oven for 12 h, and then cooled to ambient temperature. A small quantity of light green polygonal crystal crystals of IHEP-27 could be obtained, as well as a lot of green square block crystals of IHEP-28 and amorphous solids.

Synthesis of IHEP-28 {[Th13Ni6(O)8(OH)24(sba)14(H2O)12(CH3COO)3(CH3CN)2] [Na4(OH)3(H2O)3]2}n
Th(NO3)4 (0.5 M, 200 µL), Ni(NO3)2 (0.5 M, 200 µL), thiosalicylic acid (0.4 mmol), CH3CN (3 mL), NaOH (1 M, 270 µL) were loaded into a 25 mL autoclave. The autoclave was sealed, heated to 150 °C in an oven for 12 h, and then cooled to ambient temperature. The green square block crystals of IHEP-28 were produced and washed three times with fresh CH3OH.

Synthesis of IHEP-29 Na6[Th6(O)4(OH)4(sba)6(H2O)12]
Th(NO3)4 (0.5 M, 200 µL), thiosalicylic acid (0.2 mmol), CH3CN (3 mL), NaOH (1 M, 185 µL) were loaded into a 25 mL autoclave. The autoclave was sealed, heated to 150 °C in an oven for 12 h, and then cooled to ambient temperature. The colorless fusiform crystals of IHEP-29 were produced, as well as a small quantity of big block crystals of IHEP-30.

Synthesis of IHEP-30 [Th(sba)2(H2O)2]n·2nH2O
Th(NO3)4 (0.5 M, 200 µL), thiosalicylic acid (0.2 mmol), CH3CN (3 mL), NaOH (1 M, 120 µL) were loaded into a 25 mL autoclave. The autoclave was sealed, heated to 150 °C in an oven for 12 h, and then cooled to ambient temperature. The block crystals of IHEP-30 were produced.

Synthesis of IHEP-31 [Th13Ni6(O)8(OH)24(sba)12(H2O)10(CH3CN)2]·20H2O
Th(NO3)4 (0.5 M, 200 µL), Ni(NO3)2 (0.5 M, 200 µL), thiosalicylic acid (0.4 mmol), CH3CN (3 mL), LiOH (1 M, 270 µL) were loaded into a 25 mL autoclave. The autoclave was sealed, heated to 150 °C in an oven for 12 h, and then cooled to ambient temperature. The yellow-green block crystals of IHEP-31 were produced and washed three times with fresh CH3OH.

Physical properties
IR measurements were obtained on a Bruker Tensor 27 infrared spectrometer. The sample was diluted with spectroscopic KBr and pressed into a pellet. The measured wavenumber is between 400 and 4000 cm−1. Powder X-ray diffraction measurements were made using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the range 5-50° (step size: 0.02°). Thermogravimetric analysis (TGA) was performed on a TA Q500 analyzer over the temperature range of 30–800 °C in an air atmosphere with a heating rate of 5 °C·min−1. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was conducted on a Nicolet iS50 (Thermo Fisher Scientific, USA) FT-IR spectrometer. The UV-vis diffuse-reflectance spectroscopy (UV-vis DRS) were obtained on a UV-visible spectrophotometer (HITACHI UH4150), and BaSO4 was used as a reference. The photocurrent responses, Mott-Schottky plots, and the electrochemical impedance spectroscopy (EIS) were measured at an electrochemical workstation (PGSTAT302N Autolab, Metrohm). Normally, the Ag/AgCl electrode as the reference electrode, the platinum sheet electrode as the counter electrode, and the glassy carbon electrode as the working electrode formed a three-electrode system with 0.1 M sodium sulfate as the electrolyte. In the test of photocurrent responses, the F-doped tin oxide (FTO) conductive glass electrode was selected as the working electrode.

Photocatalytic performance measurements
The photocatalyst (15 mg) and photosensitizer [Ru(bpy)3]Cl2·6H2O (10 mg) was added to 4 mL of acetonitrile and 1 mL H2O in a custom-made 20 mL photocatalytic reaction cell, which was degassed and saturated with ultra-pure CO2 before sealing. The photocatalytic reactions were carried out under irradiation of a 300 W Xe lamp (CEL-PE300L-3A) equipped with a 420 nm filter. After the reaction, potential gaseous products (H2, CO, CH4) were determined by gas chromatography (GC, 8890, Agilent Technologies, Inc.). Potential liquid products were determined by 1H NMR spectra using 500 μL of the liquid after reaction mixed with 100 μL of D2O and 0.05 μL dimethyl sulfoxide as internal standard. In the isotope labeling experiment, 13CO2 was used as the reactant to analyze the carbon source of photocatalytic CO2 reduction to produce CO. The 13CO was analyzed by gas chromatography-mass spectrometry (7890B, Agilent Technologies).

Data availability
The authors declare that the data supporting the findings of this study are provided in the article/Supplementary Information/Source Data file. All data are available from the corresponding author upon request. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Center (CCDC), under deposition numbers 2324079-2324084 and 2412223. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif. Source data are provided in this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors reported the synthesis and characterization of an α-Keggin type heterometal cluster,\nTh13Ni6O8(OH)24(H2O)18(sba)12 (named IHEP-25). This report marks the first appearance of such a core structure in\nthorium clusters. Furthermore, by exchanging the surface solvent ligands or introducing cationic cluster [Na4(OH)3(H2O)3],\nthey can obtain additional three clusters, named as IHEP-26, IHEP-27, and IHEP-28. What’s more, they found that, without\nrequiring extra sacrificial agents, IHEP-28 afforded a CO production rate of 112.7 µmol·h-1·g-1 under light-driven conditions\nfor CO2 reduction. However, in the present form, the paper is still too preliminary to be publish, it can be only accepted after\na major revision, as details shown below:\n1. As their structures shows, some of the 2-sulfobenzoic acids are in monodentate coordination, it’s not reasonable to\nassume all sba ligand are in -2 oxidation state, instead, some of the ligands should be Hsba. With that, it’s required to check\ntheir structure analysis again, especially for the confirmation of the structure formula.\n2. In the section of Abstract, they claimed that IHEP-28 demonstrates excellent stability in both air and solvents, however, no\ndata or even description about this was found in the main text.\n3. Charge should be added for the cation cluster [Na4(OH)3(H2O)3] throughout the manuscript.\n4. The authors mentioned the importance of the introduction of competitive metal ions to obtain the large-nuclearity thorium\nclusters, and claimed that their study can help understand the formation process to aid in predicting the occurrence,\nmigration pathways, and concentration mechanisms of actinide elements under diverse environmental conditions. However,\nas a key point of this work, limited discussion was found about the cluster formation mechanisms in the main text. For\nexample, how will the Th13Ni6O8(OH)24(H2O)18(sba)12 be synthesis and also how [Na4(OH)3(H2O)3] plays a critical role\nin the formation of Th13 clusters? It must be further explained.\n5. It felt sudden that IHEP-28 was chosen for application as no reasons given, so as the CO2 reduction application. The\nauthors should enhance their logistic writing throughout the paper.\n6. What’s more, are there any similarities and differences among those reported clusters, regarding their physical properties\nand chemical properties, given their structural differences are not significant and one would claim that they are actually\nisostructural in core but with only slight differences to their outer surface coordination.\n7. The catalysis part is also too preliminary, for example, they claimed that the enhanced catalytic performance post cycles\nwas attributed to the photosensitizer [Ru(bpy)3]2+ being adsorbed, but it’s very difficult to believe or understand this\nassumption as their interactions between the photosensitizer and the framework are hardly investigated, and what’s real\nunderlying mechanisms for this phenomenon? More discussion and characterizations are requested.\n8. For the photocatalysis: As stated by the authors under different conditions, the photocatalytic CO production rate of 112.7\nµmol·h-1·g-1 with photosensitizers, which was lower by approximately 90% in the absence of photosensitizers. As a result,\nIHEP-28 shows a lack of obvious advantages in the field of photocatalytic CO2 reduction. The author should emphasize that\nthe excellent performance of the catalyst comes from the [Ru(bpy)3]2+, especially electrons photoinduced by the\nphotosensitizer.\n9.For the mechanism: Under light irradiation, the activation of photoelectrons is important for the adsorption of CO2 and the\ngeneration of *CO2. So the detail electrochemical tests should utilized to check the electrochemical behaviors in IHEP-28.\nThe authors can refer to the literature: Nature energy, 1, 16184 (2016).\n10. Figure S21: It’s totally not OK to only have a result description for a Figure with formal introduction, not to mention their\ndescription is also wrong: “the photocatalytic activity of IHEP-28 has not only decreased, but has even slightly improved.”\n11. Figure S23: the speculative mechanism is not correct, there is no way to generate O2 by combination of H2O with hole in\nthis system, not to mention they don’t have any proofs to support this.\nSome other minor comments:\n1. For the material preparation: In the synthesis of IHEP-27 and IHEP-28, why NaOH was used instead of proceeding to\nKOH? Please explain the effect of different reagents on cluster formation.\n2. In regards of “photocatalytic CO2 reduction experiments utilizing IHEP-28 as the catalyst were conducted in a\nCH3CN:H2O mixture (4:1 volume ratio), employing [Ru(bpy)3]2+ as the photosensitizer without the need for an additional\nsacrificial agent.”, the description on the amount of [Ru(bpy)3]2+ needs to be described in the manuscript.\n3.For the manuscript content: There are some handwriting problems, such as “according to the empirical formula EVB =\nECB + Eg (Figure S21d)” in Page 10 etc., please check the whole MS.", "reviewer_2": "The scientific content of the ms is very interesting and this work thus deserves – according to my opinion – acceptance and\npublication in NATURE COMMUNICATIONS. I have really enjoyed studying the ms, which is an interdisciplinary work\ncombining synthetic, structural and theoretical chemistry as well as photocatalytic studies. Thus, it is appropriate for the\nbroad and heterogeneous readership of this highly respected journal. I am sure that the paper will attract the intense interest\nof scientists working in the general area of actinide cluster chemistry, with emphasis on the 3d/5f coordination clusters. Also,\nI do believe that the article will receive a respectable number of citations in the near and far future. The research theme of the\nms is currently “hot”; the chemistry of Th has recently attract great attention because of its use as new nuclear energy source.\nThorium(IV) clusters are also interesting because contribute to the understanding of Th(IV) materials; such polynuclear\nclusters are rare.\nSalient features of this work – which support my proposal for acceptance – are: (a) The initially synthesized {Th13Ni6}\ncluster presents novel structural characteristics; it is the largest 3d/5f cluster which has been structurally characterized and\nits {Th13O8(OH)24} subcore is a a-Keggin type unit being the largest Th(IV) cluster reported to date. (b) The role of the NiII\nion for the formation of the cluster is decisive competing with the ThIV units for the doubly deprotonated ligands. (c) The\nauthors have studied in detail the reactivity chemistry of the {Th13Ni6} clusters which has led to variants with other solvate\nmolecules and, more remarkably, to the isolation of a structurally similar 2D coordination polymer with a honeycomb motif;\nthus the heterometallic cluster can act as a functional node for the construction of a cluster-based molecular material. (d) The\nelectronic structures of the inner {Th13O8(OH)24}12+ subcore and the whole cluster molecules have been investigated by\nDFT methods which reveal their stability; and (e) The 2D compound serves as an effective VIS-light-driven photocatalyst for\nCO2 reduction with CO selectivity approaching 100%. The ms is well organized and the quality of figures is high. The\nreferences list covers the whole topic satisfactorily. The Supporting Info is detailed, valuable and diagnostic, a model for\nauthors.\nRevision points/comments/suggestions to be taken into account:\n(1) The English of the ms should be polished.\n(2) “Abstract”: The ligand sba should be defined.\n(3) The coordination modes of all the ligands involved (sba, acetates, water, oxo, hydroxo) should be collected in a chart/scheme in the main text (Chem Draw); this will be helpful for the non-familiar readers. This material exists in Figures S3 and S5, but it is my belief that it should be in the main body of the ms.\n(4) A “Synthetic comments” part is necessary after “Results and Discussion”. The authors can briefly describe their synthetic\nphilosophy and comment on the formation of sba and acetates which were not present in the reaction mixtures; are their\nsimilar precedents in the literature? Can the authors provide a mechanism for this transformation? Is the transformation metal\n– ion promoted? If yes, are both metal – ions responsible for this?\n(5) Square brackets should be used in the formulae of the clusters to indicate the coordination entity and the symbol { }n in\nthe formula of the 2D net. Thus, the nature and number of lattice solvent molecules should become apparent.\n(6) In the “Structural characterization” part of “Results and Discussion”, a table is necessary with the formulae of the highly-\nnuclearity Th(IV) and footnotes with information about the main ligand; the formula of the core is also desirable.\n(7) The authors should mention briefly the rationale behind the use of the 2D coordination polymer for the VIS and near IR\nphotoreduction of CO2. Their explanations are not so clear (at least to me); I would like to say that the in-situ IR spectroscopy\nstudies are excellent.\n(8) The authors should mention other molecular compounds that can photoreduce CO2 and compare the literature results\nwith their findings; and\n(9) The text for TGA should be deleted from the Supplementary Info because it is probably wrong ( I doubt that coordinated\nwater molecules are eliminated above 300 oC); retainment of Figure S25 is enough.", "reviewer_3": "This work successfully synthesized the mixed metal cluster Th₁₃Ni₆ (IHEP-25) by introducing competitive Ni²⁺ ions.\nFurthermore, through the incorporation of the cationic cluster [Na₄(OH)₃(H₂O)₃], a cluster-based framework (IHEP-28) was\nconstructed during the multi-stage assembly process of IHEP-25. This framework features a 2D honeycomb network and\nexhibits exceptional stability in both air and solvent environments. IHEP-28 can function as a visible-light-driven\nphotocatalyst for CO₂ reduction without the need for additional sacrificial agents.\nWhile this work demonstrates a degree of novelty in its synthetic approach, the elucidation of the underlying mechanism\nremains vague and is accompanied by a lack of critical scientific data. Additionally, significant progress has been made in\nthe field of photocatalytic CO₂ reduction to CO, with numerous research teams contributing valuable insights into catalyst\nperformance, reaction mechanisms, and potential applications. However, the performance and mechanistic explanations\npresented in this work do not demonstrate any innovation. Therefore, I contend that this manuscript is not suitable for\npublication in Nature Communications for the following reasons:\nIn the absence of the photosensitiser [Ru(bpy)₃]²⁺, the CO₂ photoreduction performance of IHEP-28 was significantly poorer,\na phenomenon that suggests that [Ru(bpy)₃]²⁺ plays a key role in this reaction process. However, many comparative\nexperiments have not yet been carried out, e.g., what exactly is the photoreduction performance of CO₂ in the presence of\nthe photosensitiser only.\nIt is widely acknowledged that the use of photosensitizers is not only costly and challenging to recycle, but also poses\nenvironmental hazards. Therefore, what effective strategies can be employed to eliminate the reliance on photosensitizers?\nThe explanation of IHEP-28 in terms of photocatalytic CO₂ reduction mechanism is still insufficient. The compound is\nassembled from Th₁₃Ni₆ and [Na₄(OH)₃(H₂O)₃] clusters, but exactly which part of them plays a role in the CO₂ reduction\nprocess, or the exact location of the active site is still unclear. I think a lot of controlled experiments as well as DFT\ncalculations need to be performed to elaborate on this.\nDoes Th play a significant role in the process of photocatalytic CO₂ reduction? Is it necessary to synthesize the Th₁₃ cluster\nin order to clarify the catalytic function of thorium?\nWhat advantages does this catalyst's quantum yield possess in comparison to other catalysts, and is it feasible for\napplication under sunlight?\nFor Th₁₃Ni₆ clusters, are the positions of the energy levels derived from theoretical calculations consistent with\nexperimental?\nThe authors propose utilizing IHEP-28 for CO₂ reduction under visible and near-infrared light; however, they have not\nconducted an investigation into the CO₂ reduction process under near-infrared illumination. What might account for this\nomission?"}, "author_response": "Response to Reviewers' Comments\nReviewer #1 (Remarks to the Author):\nThe authors reported the synthesis and characterization of an α-Keggin type\nheterometal cluster, Th Ni O (OH) (H O) (sba) (named IHEP-25). This report\n13 6 8 24 2 18 12\nmarks the first appearance of such a core structure in thorium clusters. Furthermore, by\nexchanging the surface solvent ligands or introducing cationic cluster\n[Na (OH) (H O) ], they can obtain additional three clusters, named as IHEP-26, IHEP-\n4 3 2 3\n27, and IHEP-28. What’s more, they found that, without requiring extra sacrificial\nagents, IHEP-28 afforded a CO production rate of 112.7 µmol·h-1·g-1 under light-driven\nconditions for CO reduction. However, in the present form, the paper is still too\n2\npreliminary to be publish, it can be only accepted after a major revision, as details\nshown below:\nResponse: Thank you very much for your positive comments.\n1. As their structures shows, some of the 2-sulfobenzoic acids are in monodentate\ncoordination, it’s not reasonable to assume all sba ligand are in -2 oxidation state,\ninstead, some of the ligands should be Hsba. With that, it’s required to check their\nstructure analysis again, especially for the confirmation of the structure formula.\nResponse: Thank you for your insightful question. In high-nuclearity cluster structures,\nit is challenging to analyze local details such as lattice solvents and uncoordinated\ngroups. By comparing the structural data of the clusters we reported with similar crystal\nstructures in the literature, we tend to believe that the monodentate ligands are in -2\noxidation state.\nFirst, the S-O single bond in the benzenesulfonate acid is significantly longer than\nthe S=O double bond, while after deprotonation, the three S-O bonds in the\nbenzenesulfonate anion tend to be more consistent in length. For example, in the\ncompound C H O S •C H O S •2C H N O •2H O (CCDC number 1506978), both\n10 6 6 2 10 8 6 2 8 11 4 2 2\nprotonated and deprotonated benzenesulfonate structural units are present. The bond\nlengths of the protonated benzenesulfonate unit are 1.434 Å, 1.435 Å, and 1.498 Å,\nwhile those of the deprotonated benzenesulfonate unit are 1.451 Å, 1.453 Å, and 1.460\nÅ (Figure R1). In IHEP-25, the three S-O bonds of the uncoordinated sulfonate group\nare similar in length (1.455 Å, 1.464 Å, and 1.465 Å), suggesting that it has likely lost\na proton.\nSecond, the pKa of benzenesulfonic acid is approximately -0.7, while that of benzoic\nacid is about 4.2, indicating that the acidity of benzenesulfonic acid is much stronger\nthan that of benzoic acid. Therefore, for 2-sulfobenzoic acid, the sulfonic acid group is\nmore likely to lose a proton compared to the carboxylic acid group. For example, in the\nderivatives (NH )(C H O S) and (C H N )(C H O S) (CCDC numbers 1101759 and\n4 7 5 5 12 11 2 7 5 5\n659141, respectively), the sulfonic acid group is deprotonated while the carboxylic acid\ngroup is not. In IHEP-25, since the carboxylate group of 2-sulfobenzoic acid is already\nconfirmed to be deprotonated, the sulfonate group should also have lost a proton.\nFinally, from the perspective of charge balance, when all 2-sulfobenzoic acid\nmolecules lose two protons, the cluster Th Ni O (OH) (H O) (sba) as a whole\n13 6 8 24 2 18 12\nremains electrically neutral, eliminating the need for additional counter anions. This is\nconsistent with the results of single-crystal structure analysis.\nIn summary, we assume that all sba ligand are in -2 oxidation state.\nFigure R1. The structure of C H O S •C H O S •2C H N O •2H O (CCDC number\n10 6 6 2 10 8 6 2 8 11 4 2 2\n1506978).\n2. In the section of Abstract, they claimed that IHEP-28 demonstrates excellent stability\nin both air and solvents, however, no data or even description about this was found in\nthe main text.\nResponse: Thank you for your careful check. The data and description about the\nstability of IHEP-28 in both air and solvents have been added in the resubmitted\nmanuscript and SI (Figure R2; Figure S22 in the resubmitted SI).\nFigure R2. (a) PXRD patterns of IHEP-28 stored in air for one year. (b) PXRD patterns\nof IHEP-28 after soaking in different solvents.\n3. Charge should be added for the cation cluster [Na (OH) (H O) ] throughout the\n4 3 2 3\nmanuscript.\nResponse: Thank you for your suggestion. The charge of the cation cluster\n[Na\n4\n(OH)\n3\n(H\n3\n] has been added in the revised manuscript. The formula\n[Na\n4\n(OH)\n3\n(H\n3\n] has been exchanged to [Na\n4\n(OH)\n3\n(H\n3\n]+ throughout the\nmanuscript.\n4. The authors mentioned the importance of the introduction of competitive metal ions\nto obtain the large-nuclearity thorium clusters, and claimed that their study can help\nunderstand the formation process to aid in predicting the occurrence, migration\npathways, and concentration mechanisms of actinide elements under diverse\nenvironmental conditions. However, as a key point of this work, limited discussion was\nfound about the cluster formation mechanisms in the main text. For example, how will\nthe Th Ni O (OH) (H O) (sba) be synthesis and also how [Na (OH) (H O) ] plays\n13 6 8 24 2 18 12 4 3 2 3\na critical role in the formation of Th clusters? It must be further explained.\nreassembly mechanism for the formation [Th Ni ] clusters in the resubmitted\n13 6\nmanuscript. Under the induction of different alkali metal cations, the [Th Ni ] clusters\n13 6\ncan assemble to form 3D frameworks with various packing patterns and topological\nstructures (IHEP-25, IHEP-26, IHEP-27, IHEP-28, and IHEP-31). They all feature the\ncore structure [Th Ni (O) (OH) ]24+ (Table R1; Table 1 in the resubmitted\n13 6 8 24\nmanuscript). The details are as follows:\nTable R1. The Formulae of Compounds IHEP-25 - IHEP-31 and Their Core Structures.\nCompounds Formulae of compounds Formulae of cores\nIHEP-25 [Th Ni (O) (OH) (OH) (sba) (H O) ] [Th Ni (O) (OH) ]24+\n13 6 8 18 6 12 2 18 13 6 8 24\nIHEP-26 [Th Ni (O) (OH) (sba) (H O) (CH CN) ] [Th Ni (O) (OH) ]24+\n13 6 8 24 12 2 12 3 6 13 6 8 24\n[Th Ni (O) (OH) (sba) (H O) ]•[Th Ni\nIHEP-27 13 6 8 24 12 2 18 13 6 [Th Ni (O) (OH) ]24+\n(O) (OH) (sba) (H O) (CH CN) ] 13 6 8 24\n8 24 12 2 12 3 6\n{[Th Ni (O) (OH) (sba) (H O)\nIHEP-28 13 6 8 24 14 2 12 [Th Ni (O) (OH) ]24+\n(CH COO) (CH CN) ] [Na (OH) (H O) ] } 13 6 8 24\n3 2 3 2 3 4 3 2 3 2 n\nIHEP-29 Na [Th (O) (OH) (sba) (H O) ] [Th (O) (OH) ]12+\n6 6 4 4 12 2 6 6 4 4\nIHEP-30 [Th(sba) (H O) ] •2nH O Th4+\n2 2 3 n 2\n[Th Ni (O) (OH) (sba) (H O) (CH CN) ]\nIHEP-31 13 6 8 24 12 2 10 3 2 [Th Ni (O) (OH) ]24+\n13 6 8 24\n•20H O\n2\nsba: 2-sulfobenzoic acid\nTo explore the potential formation mechanism of [Th Ni ] clusters, a series of\n13 6\ncomparative experiments were conducted. Based on the results of these experiments\nand the structural comparison of thorium complexes reported in this work, a possible\nformation mechanism for [Th Ni ] clusters was proposed (Figure R3; Figure S18 in\n13 6\nthe resubmitted SI). Reported thorium monomers include Th(OH) + and Th(OH) 2+,\n3 2\nas\nwell as dimers e.g. Th (OH) 6+, tetramers e.g. Th (OH) 4+, and hexamers e.g. Th (µ -\n2 2 4 12 6 3\nO) (µ -OH) 12+, all of which are important species of thorium ions in solution.\n4 3 4\nAdditionally, the presence of carboxylate anion ligands can promote the formation of\nhexamer [Th ] clusters, which is consistent with our experimental observations: in the\n6\nabsence of Ni2+ ions, Th4+ ions preferentially assemble with the sba2- ligand to form\n[Th (O) (OH) (sba) (H O) ] cluster (IHEP-29). As the concentration of NaOH\n6 4 4 12 2 6\nincreases, the morphology of the [Th\n6\n] cluster changes, accompanied by the generation\nof amorphous thorium hydroxide, which eventually becomes the main product. After\nthe introduction of Ni2+ ions, [Th ] clusters were no longer observed in the reaction\n6\nproducts; instead, [Th Ni ] clusters and 1D chain structure (IHEP-30) with\n13 6\nmononuclear Th4+ as nodes formed. This is due to the competition between nickel ions\nand the [Th (sba) ] cluster for the sba2- ligands, leading to its dissociation into units\n6 12\nsuch as [Th ], [Th ], and mononuclear [Th]. In a low-alkalinity environment, the\n5 4\ndissociation products are primarily mononuclear [Th], which further form a 1D chain\nstructure IHEP-30. As the alkalinity of the reaction system increases, the stability of the\nlacuna oligomers [Th ] and [Th ] is enhanced. This enhancement facilitates their\n5 4\nsecondary assembly in a co-vertex manner, resulting in the formation of an intermediate\nproduct, [Th ], characterized by a sandwich structure. This product continues to capture\n9\nmononuclear [Th] in the reaction system, ultimately leading to the formation of the\n[Th ] cluster. A portion of the O2-/OH- coordination sites on the surface of the [Th ]\n13 13\ncluster is occupied by nickel ions, resulting in the formation of the heterometallic\ncluster [Th Ni ]. Furthermore, under the induction of different alkali metal cations, the\n13 6\n[Th Ni ] clusters can further assemble to form 3D frameworks with various packing\n13 6\npatterns and topological structures (IHEP-25, IHEP-26, IHEP-27, IHEP-28, and IHEP-\n31). Their core structure [Th Ni (O) (OH) ]24+ remains stable. Therefore, the\n13 6 8 24\nintroduction of nickel ions plays a critical role in the formation of [Th ] clusters. On\n13\nthe one hand, the lacuna clusters formed by the partial disassembly of [Th (sba) ]\n6 12\nclusters induced by Ni2+ subsequently reorganize in the solution to form [Th ] clusters.\naggregation of the clusters into polymers e.g. thorium hydroxide.\nFigure R3. Possible formation mechanism of [Th Ni ] clusters.\n13 6\n5. It felt sudden that IHEP-28 was chosen for application as no reasons given, so as the\nCO reduction application. The authors should enhance their logistic writing throughout\n2\nthe paper.\nResponse: Thank you for your helpful advice. In the resubmitted manuscript, we have\nadded the reasons for selecting IHEP-28 for the CO reduction application to enhance\n2\nthe logistic of the paper. The details are as follows:\nIn recent years, solar-driven artificial carbon dioxide reduction reactions (CO RR)\n2\nhave gradually gained widespread attention for their potential to reduce greenhouse gas\nemissions and lower energy demand. To achieve this goal, synthesizing highly efficient\nand selective photocatalysts to lower the thermodynamic energy barrier has become a\ncritical issue that needs to be addressed. Among the various photocatalysts developed\nfor CO reduction, nickel compounds are considered one of the most promising\n2\ncandidates. A significant advantage of these photocatalysts is that CO can effectively\n2\nbind to and be activated by the nickel center, thereby promoting its conversion\nefficiency under solar irradiation. In this work, the [Th Ni ] cluster exhibits a local\n13 6\n[NiTh O ] structural unit, which can be regarded as a discrete molecular equivalent of\n3 3\nthe nickel-supported catalyst Ni@thoria (Figure R4; Figure S21 in the resubmitted SI).\nTherefore, [Th Ni ] cluster holds promise for application in the photocatalytic\n13 6\nreduction of CO . However, as mentioned previously, due to the substitutability of the\n2\nsurface ligands on the [Th Ni ] clusters, the discrete [Th Ni ] clusters (IHEP-25,\n13 6 13 6\nIHEP-26, IHEP-27 and IHEP-31) we obtained are difficult to simultaneously meet the\nrequirements of high purity and high yield in experimental applications, primarily\nbecause these clusters often mix with each other. Once the [Th Ni ] clusters are\n13 6\ninterlinked with the [Na (OH) (H O) ]+ clusters to form 2D coordination polymer\n4 3 2 3\n(IHEP-28), both purity and yield are significantly enhanced, and this 2D structure also\ncontributes to improving the stability of the materials. Furthermore, the porous structure\nof IHEP-28 can expose more active sites, thereby further enhancing its photocatalytic\nperformance. Based on these factors, in the subsequent research, we will focus on\nIHEP-28 as our primary subject to explore the photocatalytic reduction performance of\n[Th Ni ] cluster for CO .\n13 6 2\nFigure R4. The [Th Ni ] cluster can be regarded as a discrete molecular equivalent of\n13 6\nthe nickel-supported catalyst Ni@thoria.\n6. What’s more, are there any similarities and differences among those reported clusters,\nregarding their physical properties and chemical properties, given their structural\ndifferences are not significant and one would claim that they are actually isostructural\nin core but with only slight differences to their outer surface coordination.\nResponse: Thank you for your helpful advice. Through single-crystal structure analysis,\nit was found that IHEP-25, IHEP-26, IHEP-27, IHEP-28, and IHEP-31 all feature an\nisostructural core cluster [Th Ni ], leading to similarities in certain properties among\n13 6\nthese cluster compounds. For example, the infrared spectra of IHEP-28 and IHEP-31\nexhibit similar characteristics (Figure R5a). Although minor differences in the surface\ncoordination environment do not alter the [Th Ni ] core structure, they significantly\n13 6\ninfluence the packing patterns of the cluster molecules, resulting in different topological\nstructures and, consequently, affecting certain macroscopic properties. For instance, the\ncluster molecules in IHEP-31 display a dense packing with a porosity of 6.9%, while\nthose in IHEP-28 form a 2D framework structure with a porosity of 38.3%. Furthermore,\nthe characteristic absorption peaks in the UV-visible-NIR spectra of IHEP-28 and\nIHEP-31 are positioned similarly, but the absorption intensity of IHEP-28 is\nsignificantly higher than that of IHEP-31 (Figure R5b).\nFigure R5. (a) IR spectra of IHEP-28 and IHEP-31. (b) UV-vis-NIR spectrum of IHEP-\n28 and IHEP-31.\n7. The catalysis part is also too preliminary, for example, they claimed that the enhanced\ncatalytic performance post cycles was attributed to the photosensitizer [Ru(bpy) ]2+\n3\nbeing adsorbed, but it’s very difficult to believe or understand this assumption as their\ninteractions between the photosensitizer and the framework are hardly investigated,\nand what’s real underlying mechanisms for this phenomenon? More discussion and\ncharacterizations are requested.\nResponse: Thank you for your careful check and helpful advice. According to your\nsuggestions, we conducted adsorption experiments of the cluster material IHEP-28 with\nphotosensitizer ([Ru(bpy) ]2+) to confirm the interactions between them. In the\n3\nexperiment, 15 mg of IHEP-28 was immersed in a mixed solution of H O/CH CN (1\n2 3\nmL/4 mL) containing 5 mg of photosensitizer, and the UV-visible spectra were tested\nat different time intervals. As shown in Figure R6a, the UV-visible spectra of the system\ndid not exhibit significant changes over time, indicating that the material did not exhibit\nnoticeable adsorption of the photosensitizer. Subsequently, we re-performed the\nphotocatalytic cycling experiments of the material. The experimental results showed\nthat after three photocatalytic cycles, IHEP-28 maintained similar catalytic activity and\nhigh crystallinity, demonstrating its excellent stability during the photocatalytic process\n(Figure R6b). Based on these experimental results, we have removed the inaccurately\ndescribed content from the previous manuscript and provided a more reasonable and\nprecise description of the experimental phenomena in the resubmitted manuscript.\nFigure R6. (a) UV-vis spectrum of [Ru(bpy) ]2+ adsorption by IHEP-28 in\n3\n[Ru(bpy)\n3\n]2+/H\nIHEP-28 for the photocatalytic reduction of CO .\n2\n8. For the photocatalysis: As stated by the authors under different conditions, the\nphotocatalytic CO production rate of 112.7 µmol·h-1·g-1 with photosensitizers, which\nwas lower by approximately 90% in the absence of photosensitizers. As a result, IHEP-\n28 shows a lack of obvious advantages in the field of photocatalytic CO reduction. The\n2\nauthor should emphasize that the excellent performance of the catalyst comes from the\n[Ru(bpy) ]2+, especially electrons photoinduced by the photosensitizer.\nCO production rate in the absence of photosensitizers decreased by approximately 90%.\nThis suggests that the photosensitizer plays a crucial role in the photocatalytic CO₂\nreduction process. Notably, many excellent studies on CO₂ photocatalytic reduction\nhave introduced [Ru(bpy)₃]²⁺ as a photosensitizer, particularly in photocatalysts based\non-existent\nclusters and MOFs (Table R2; Table S2 in the resubmitted SI). Indeed, the\nphotosensitizer, particularly the electrons photoinduced by [Ru(bpy)₃]²⁺, is a critical\nfactor contributing to the excellent performance of these catalysts. Nevertheless, the\ncatalyst itself serves as the core component of the photocatalytic reaction system. For\ninstance, in this work, the photosensitizer alone exhibited minimal photocatalytic CO₂\nreduction activity in the absence of IHEP-28. Although the photocatalytic CO₂\nreduction efficiency of IHEP-28 did not significantly exceed that of other established\ncatalysts under certain conditions, its advantage lies in the lack of requirement for\nadditional organic or inorganic sacrificial agents, which is essential in many highly\nefficient catalytic systems. For example, the [Ni₃₆Gd₁₀₂] cluster demonstrated\nremarkable photocatalytic CO₂ reduction performance when both the photosensitizer\nand the sacrificial agent (TEOA) were present, but the catalytic efficiency dropped to\nnearly zero in the absence of the TEOA (J. Am. Chem. Soc. 2020, 142, 4663).\nTable R2. Activity comparison of recently reported Ni-based cluster or MOF\nphotocatalysts for CO reduction.\n2\nphotocatal Light Photosen Sacrifici CO rate\nCondition ref\nyst source sitizer al agent (μmol g-1 h-1)\nIHEP-28 MeCN/ HO Xe lamp [Ru] --- 112.7\nIHEP-28 MeCN/ HO Xe lamp --- --- 10.1\n2\n66-IS-Ni MeCN/ HO Xe lamp [Ru] TEOA 1350\n2\nno detectable ACS Catal. 2023, 13, 2547\n66-IS-Ni MeCN/ HO Xe lamp [Ru] ---\n2 product\nNi Gd DMF/HO LED [Ru] TEOA 102092 J. Am. Chem. Soc. 2020, 142, 4663\n36 102 2\nno detectable\nNi Gd DMF/HO LED [Ru] ---\n36 102 2 product\nno detectable\nNi Gd DMF/HO LED --- TEOA\n36 102 2 product\nNiSAs/NiN\n6,240\nCs-POPs\nMeCN/ HO Xe Lamp [Ru] BIH Adv. Sci. 2024, 11, 2401508\nNiSAs- 2\n3,530\nPOPs\nNi-COFs HO Xe Lamp [Ru] TEOA 810 J. Am. Chem. Soc. 2019, 141 , 7615\nNC@NiCo2O4 MeCN/ H 2 O Xe Lamp [Ru] TEOA 26200 Energy Environ. Sci. 2018, 11 , 306\nm-NiAl- Angew. Chem., Int. Ed. 2019, 58,\nMeCN/ HO Xe Lamp [Ru] TEOA 90\nLDH 2 11860\nNiO MeCN/ HO LED [Ru] TEOA 3940 Nano Res. 2021, 14, 730\nNiNi MeCN LED [Ru] 813.95 J. Am. Chem. Soc. 2024, 146, 28832\nTEOA\nCB[7]-Ni MeCN/ H 2 O Xe Lamp [Ru] TEOA 7210 Angew. Chem. Int. Ed. 2024, e202417384\nNi-MOF 68.15\nMeCN/ HO Xe Lamp [Ru] TEOA DOI: 10.1039/d4qi02143f\nLa/Ni-MOF 2 334.65\nNi-BDC 885\nNi-NDC MeCN/ HO Xe Lamp [Ru] TEOA 1319 Mater. Lett. 2024, 357, 135669\nWO-Ni-g-\nx HO Xe Lamp --- --- 4.2 J. Power Sources 2021, 510, 430420\nCN 2\n3 4\nNiONPor 24700\nNiSNPor MeCN/ HO LED [Ru] TEOA 38800\n3 2 2316199\nNiNPor 6600\n30 60 MeCN/ HO Xe Lamp --- ---\nNiO NCs 2 66.67 e202213162\n[Ru]: [Ru(bpy)]2+; TEOA: triethanolamine; BIH: 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole\n3\n9. For the mechanism: Under light irradiation, the activation of photoelectrons is\nimportant for the adsorption of CO and the generation of *CO . So the detail\n2 2\nelectrochemical tests should utilized to check the electrochemical behaviors in IHEP-\n28. The authors can refer to the literature: Nature energy, 1, 16184 (2016).\nResponse: Thank you for your helpful advice. A set of electrochemical techniques were\nused to check the electrochemical behaviors in IHEP-28 (Figure R7; Figures 4c and 4d\nin the resubmitted manuscript). The electrochemical impedance spectroscopy (EIS) test\nwas performed to investigate the interfacial charge transfer and recombination (Figure\nR7a). Nyquist plots acquired under illumination conditions show that the charge\ntransfer impedance of the IHEP-28+[Ru(bpy) ]2+ mixture is smaller than that of the\nintroduction of Ru(bpy) 2+, ultimately improving the photocatalytic performance. As\n3\nshown in Figure R7b, there is a much higher transient photocurrent response signal\nobserved on the IHEP-28+[Ru(bpy) ]2+ mixture while the absence of Ru(bpy) 2+ yields\n3 3\na much weaker photocurrent. These test results indicate that the introduction of\nRu(bpy) 2+ significantly improved the transfer and separation efficiency of\n3\nphotogenerated electrons and holes in IHEP-28, which aligns with the experimental\nresults showing that Ru(bpy) 2+ enhances the photocatalytic reduction efficiency of\nIHEP-28 + [Ru(bpy)\n3\n]2+ mixture under visible light irradiation. (b) Transient\nphotocurrent response (TPR) of IHEP-28 and the IHEP-28 + [Ru(bpy) ]2+ mixture\n3\nunder visible light irradiation.\n10. Figure S21: It’s totally not OK to only have a result description for a Figure with\nformal introduction, not to mention their description is also wrong: “the photocatalytic\nactivity of IHEP-28 has not only decreased, but has even slightly improved.”\nResponse: Thank you for your careful check and helpful advice. The mistake has been\ncorrected in the resubmitted manuscript. The details are as follows:\nAfter three catalytic cycles, IHEP-28 maintained similar catalytic activity and high\ncrystallinity, demonstrating its excellent stability during the photocatalytic process\n(Figure R8; Figure S25 in the resubmitted SI).\nFigure R8. The recycle performance of IHEP-28 for the photocatalytic reduction of CO .\n2\n11. Figure S23: the speculative mechanism is not correct, there is no way to generate\nO by combination of H O with hole in this system, not to mention they don’t have any\n2 2\nproofs to support this.\nResponse: Thank you for your careful check and helpful advice. According to your\nadvice, we have conducted additional characterizations (Figures R9, R10; Figures S26,\nS27 in the resubmitted SI), the details of which are presented below.\nThe band gap energy (Eg) of IHEP-28 is determined to be 2.43 eV, based on the\nUV-Vis-NIR spectrum and Tauc plots (Figures R9a, b). The Mott-Schottky plots result\nindicates that IHEP-28 behaves as a typical n-type semiconductor, evidenced by a\npositive slope with the flat band potential of -1.12 V (vs. Ag/AgCl), as shown in Figure\nR9c. Consequently, the conduction band (CB) potential of IHEP-28 is estimated to be\n-0.92 V vs NHE. Therefore, the valence band (E ) of IHEP-28 is determined to be\nVB\n1.51 V (vs NHE), according to the empirical formula E = E + E . To confirm the\nVB CB g\nproducts of the reaction between the photoinduced holes and H O, we first examined\n2\nthe gaseous products and did not detect the presence of oxygen. Subsequently, based\non the classic \"Ti-oxalate method \" reported in the literature (Water Research, 2024,\n253,121256), we tested the solution after the photocatalytic reaction and detected the\nformation of H O (Figure R10). Therefore, under visible light irradiation,\n2 2\nphotoinduced electrons move to catalytically active sites for CO reduction while\n2\ncorresponding holes are consumed by H O with the formation of H O (Figure R9d).\n2 2 2\nFigure R9. (a) UV-vis-NIR spectrum of IHEP-28. (b) Corresponding plot of (ahν)2\nversus photon energy (hν) for IHEP-28. (c) Mott-Schottky plots for IHEP-28. (d)\nSpeculative mechanism for photocatalytic CO -to-CO conversion.\n2\nformation of H O .\n2 2\nSome other minor comments:\n12. For the material preparation: In the synthesis of IHEP-27 and IHEP-28, why NaOH\nwas used instead of proceeding to KOH? Please explain the effect of different reagents\non cluster formation.\nResponse: Thank you for your careful check and helpful advice. In the process of\nmaterial preparation, we used different alkali metal hydroxides such as LiOH, NaOH,\nand KOH to regulate the alkalinity of the reaction system, successfully synthesizing\nfive cluster compounds with [Th Ni (O) (OH) ]24+ as the core structure. The\n13 6 8 24\nexperimental results show that the selection of different alkali metals did not affect the\ncore structure of the [Th Ni (O) (OH) ]24+ cluster, but significantly altered the\n13 6 8 24\ncoordination environment on the surface of the cluster, including the types and\nquantities of coordinating solvents and their coordination modes. These changes further\ninduced different assembly modes of the [Th Ni ] clusters, forming 3D frameworks\n13 6\nwith different stacking patterns and topological structures (IHEP-25, IHEP-26, IHEP-\n27, IHEP-28, and IHEP-31).\n13. In regards of “photocatalytic CO reduction experiments utilizing IHEP-28 as the\n2\ncatalyst were conducted in a CH CN:H O mixture (4:1 volume ratio), employing\n3 2\n[Ru(bpy) ]2+ as the photosensitizer without the need for an additional sacrificial agent.”,\n3\nthe description on the amount of [Ru(bpy) ]2+ needs to be described in the manuscript.\nresubmitted manuscript: “employing [Ru(bpy) ]Cl ·6H O as the photosensitizer (10 mg)\n3 2 2\nwithout the need for an additional sacrificial agent”.\n14. For the manuscript content: There are some handwriting problems, such as\n“according to the empirical formula E = E + Eg (Figure S21d)” in Page 10 etc.,\nVB CB\nplease check the whole MS.\nResponse: Thank you for your careful check. These handwriting problems have been\ncorrected in the resubmitted manuscript and SI, such as “E = E + E ”.\nVB CB g\nReviewer #2 (Remarks to the Author):\nThe scientific content of the ms is very interesting and this work thus deserves -\naccording to my opinion - acceptance and publication in NATURE\nCOMMUNICATIONS. I have really enjoyed studying the ms, which is an\ninterdisciplinary work combining synthetic, structural and theoretical chemistry as well\nas photocatalytic studies. Thus, it is appropriate for the broad and heterogeneous\nreadership of this highly respected journal. I am sure that the paper will attract the\nintense interest of scientists working in the general area of actinide cluster chemistry,\nwith emphasis on the 3d/5f coordination clusters. Also, I do believe that the article will\nreceive a respectable number of citations in the near and far future. The research theme\nof the ms is currently “hot”; the chemistry of Th has recently attract great attention\nbecause of its use as new nuclear energy source. Thorium(IV) clusters are also\ninteresting because contribute to the understanding of Th(IV) materials; such\npolynuclear clusters are rare.\nSalient features of this work - which support my proposal for acceptance - are: (a)\nThe initially synthesized {Th Ni } cluster presents novel structural characteristics; it\n13 6\nis the largest 3d/5f cluster which has been structurally characterized and its\n{Th O (OH) } subcore is a a-Keggin type unit being the largest Th(IV) cluster\n13 8 24\nreported to date. (b) The role of the Ni(II) ion for the formation of the cluster is decisive\ncompeting with the Th(IV) units for the doubly deprotonated ligands. (c) The authors\nhave studied in detail the reactivity chemistry of the {Th Ni } clusters which has led\n13 6\nto variants with other solvate molecules and, more remarkably, to the isolation of a\nstructurally similar 2D coordination polymer with a honeycomb motif; thus the\nheterometallic cluster can act as a functional node for the construction of a cluster-based\nmolecular material. (d) The electronic structures of the inner {Th O (OH) }12+\n13 8 24\nsubcore and the whole cluster molecules have been investigated by DFT methods which\nreveal their stability; and (e) The 2D compound serves as an effective VIS-light-driven\nphotocatalyst for CO reduction with CO selectivity approaching 100%. The ms is well\n2\norganized and the quality of figures is high. The references list covers the whole topic\nsatisfactorily. The Supporting Info is detailed, valuable and diagnostic, a model for\nauthors. Revision points/comments/suggestions to be taken into account:\nResponse: Thank you very much for your good comments.\n(1) The English of the ms should be polished.\nResponse: Thank you for your careful review. The resubmitted manuscript has been\nthoroughly proofread and revised to meet the publication standards.\n(2) “Abstract”: The ligand sba should be defined.\nResponse: Thank you for your suggestion. The ligand sba2- has been defined in the\n“Abstract” of the resubmitted manuscript (H sba: 2-sulfobenzoic acid).\n2\n(3) The coordination modes of all the ligands involved (sba, acetates, water, oxo,\nhydroxo) should be collected in a chart/scheme in the main text (Chem Draw); this will\nbe helpful for the non-familiar readers. This material exists in Figures S3 and S5, but it\nis my belief that it should be in the main body of the ms.\nResponse: Thank you for your suggestion. The coordination modes of all the ligands\ninvolved in compounds reported in this work have been collected in Scheme R1\n(Scheme 1 in the resubmitted manuscript).\nScheme R1. Coordination modes of all the ligands involved in compounds IHEP-25 -\nIHEP-31.\n(4) A “Synthetic comments” part is necessary after “Results and Discussion”. The\nauthors can briefly describe their synthetic philosophy and comment on the formation\nof sba and acetates which were not present in the reaction mixtures; are their similar\nprecedents in the literature? Can the authors provide a mechanism for this\ntransformation? Is the transformation metal - ion promoted? If yes, are both metal –\nions responsible for this?\nResponse: Thank you for your suggestion. In the resubmitted manuscript, we have\nadded the“Synthetic comments” part. The details are as follows:\nIn solvothermal reactions, some organic functional groups may undergo in-situ\nreactions due to high temperature and pressure conditions. Typical examples of such\nreactions include the hydrolysis of cyano and ester groups, as well as the oxidation of\nhalogen groups. Notably, during the synthesis of coordination polymers, it is quite\ncommon for cyano groups to hydrolyze and then coordinate with metal ions, a\nphenomenon that also accounts for the source of acetate ligands in IHEP-28. In addition\nto the aforementioned reactions, all the compounds reported in this work involve the\nin-situ oxidation of thiol groups to sulfonate groups, a process that has been\ndocumented in several literatures (Journal of Solid State Chemistry, 2015, 221, 14-20).\nIn our study, we conducted a solvothermal reaction by dissolving nickel nitrate,\nthiosalicylic acid, and sodium hydroxide in a mixed solution of water and acetonitrile,\nobtaining a thiol-coupled product, 2, 2'-dithiosalicylic acid. However, when thorium\nnitrate was added to the reaction system, thiosalicylic acid was oxidized to 2-\nsulfobenzoic acid. This oxidation phenomenon may stem from the strong hydrolysis of\nTh4+ under high-temperature and high-pressure conditions, leading to the generation of\nhydrogen ions from water decomposition. This process significantly enhances the\noxidizing ability of nitrate anions, thereby facilitating the oxidation of thiosalicylic acid\nto generate 2-sulfobenzoic acid (Coordination Chemistry Reviews, 2000, 199, 159-179).\n(5) Square brackets should be used in the formulae of the clusters to indicate the\ncoordination entity and the symbol { }n in the formula of the 2D net. Thus, the nature\nand number of lattice solvent molecules should become apparent.\nResponse: Thank you for your suggestion. Square brackets have been used in the\nformulae of the clusters (IHEP-25, IHEP-26, IHEP-27, IHEP-29 and IHEP-31), and the\nsymbol { } have been used in the formulae of the 1D chain (IHEP-30) and the 2D net\nn\n(IHEP-28).\n(6) In the “Structural characterization” part of “Results and Discussion”, a table is\nnecessary with the formulae of the highly-nuclearity Th(IV) and footnotes with\ninformation about the main ligand; the formula of the core is also desirable.\nResponse: Thank you for your suggestion. The formulae of compounds IHEP-25 -\nIHEP-31 and their core structures have been collected in Table R3 (Table 1 in the\nresubmitted manuscript).\nTable R3. The Formulae of Compounds IHEP-25 - IHEP-31 and Their Core Structures.\nCompounds Formulae of compounds Formulae of cores\nIHEP-25 [Th Ni (O) (OH) (OH) (sba) (H O) ] [Th Ni (O) (OH) ]24+\n13 6 8 18 6 12 2 18 13 6 8 24\nIHEP-26 [Th Ni (O) (OH) (sba) (H O) (CH CN) ] [Th Ni (O) (OH) ]24+\n13 6 8 24 12 2 12 3 6 13 6 8 24\n[Th Ni (O) (OH) (sba) (H O) ]•[Th Ni\nIHEP-27 13 6 8 24 12 2 18 13 6 [Th Ni (O) (OH) ]24+\n(O) (OH) (sba) (H O) (CH CN) ] 13 6 8 24\n8 24 12 2 12 3 6\n{[Th Ni (O) (OH) (sba) (H O)\nIHEP-28 13 6 8 24 14 2 12 [Th Ni (O) (OH) ]24+\n(CH COO) (CH CN) ] [Na (OH) (H O) ] } 13 6 8 24\n3 2 3 2 3 4 3 2 3 2 n\nIHEP-29 Na [Th (O) (OH) (sba) (H O) ] [Th (O) (OH) ]12+\n6 6 4 4 12 2 6 6 4 4\nIHEP-30 [Th(sba) (H O) ] •2nH O Th4+\n2 2 3 n 2\n[Th Ni (O) (OH) (sba) (H O) (CH CN) ]\nIHEP-31 13 6 8 24 12 2 10 3 2 [Th Ni (O) (OH) ]24+\n•20H O 13 6 8 24\n2\nsba: 2-sulfobenzoic acid\n(7) The authors should mention briefly the rationale behind the use of the 2D\ncoordination polymer for the VIS and near IR photoreduction of CO . Their\n2\nexplanations are not so clear (at least to me); I would like to say that the in-situ IR\nspectroscopy studies are excellent.\nResponse: Thank you for your good comment and helpful advice. In the resubmitted\nmanuscript, we have added the reasons for selecting IHEP-28 for the CO reduction\n2\napplication to enhance the logistic of the paper. The details are as follows:\nIn recent years, solar-driven artificial carbon dioxide reduction reactions (CO RR)\n2\nhave gradually gained widespread attention for their potential to reduce greenhouse gas\nemissions and lower energy demand. To achieve this goal, synthesizing highly efficient\nand selective photocatalysts to lower the thermodynamic energy barrier has become a\ncritical issue that needs to be addressed. Among the various photocatalysts developed\nfor CO reduction, nickel compounds are considered one of the most promising\n2\ncandidates. A significant advantage of these photocatalysts is that CO can effectively\n2\nbind to and be activated by the nickel center, thereby promoting its conversion\nefficiency under solar irradiation. In this work, the [Th Ni ] cluster exhibits a local\n13 6\n[NiTh O ] structural unit, which can be regarded as a discrete molecular equivalent of\n3 3\nthe nickel-supported catalyst Ni@thoria (Figure R11; Figure S21 in the resubmitted SI).\nTherefore, [Th Ni ] cluster holds promise for application in the photocatalytic\n13 6\nreduction of CO . However, as mentioned previously, due to the substitutability of the\n2\nsurface ligands on the [Th Ni ] clusters, the discrete [Th Ni ] clusters (IHEP-25,\n13 6 13 6\nIHEP-26, IHEP-27 and IHEP-31) we obtained are difficult to simultaneously meet the\nrequirements of high purity and high yield in experimental applications, primarily\nbecause these clusters often mix with each other. Once the [Th Ni ] clusters are\n13 6\ninterlinked with the [Na (OH) (H O) ]+ clusters to form 2D coordination polymer\n4 3 2 3\n(IHEP-28), both purity and yield are significantly enhanced, and this 2D structure also\ncontributes to improving the stability of the materials. Furthermore, the porous structure\nof IHEP-28 can expose more active sites, thereby further enhancing its photocatalytic\nperformance. Based on these factors, in the subsequent research, we will focus on\nIHEP-28 as our primary subject to explore the photocatalytic reduction performance of\n[Th Ni ] cluster for CO .\n13 6 2\nFigure R11. The [Th Ni ] cluster can be regarded as a discrete molecular equivalent of\n13 6\nthe nickel-supported catalyst Ni@thoria.\n(8) The authors should mention other molecular compounds that can photoreduce CO\n2\nand compare the literature results with their findings.\nResponse: Thank you for your suggestion. Although the photocatalytic CO₂ reduction\nefficiency of IHEP-28 did not significantly exceed that of other established catalysts\nunder certain conditions, its advantage lies in the lack of requirement for additional\norganic or inorganic sacrificial agents, which is essential in many highly efficient\ncatalytic systems (Table R4; Figure S2 in the resubmitted SI). For example, the\n[Ni₃₆Gd₁₀₂] cluster demonstrated remarkable photocatalytic CO₂ reduction performance\nwhen both the photosensitizer and the sacrificial agent (TEOA) were present, but the\ncatalytic efficiency dropped to nearly zero in the absence of the TEOA (J. Am. Chem.\nSoc. 2020, 142, 4663).\nTable R4. Activity comparison of recently reported Ni-based cluster or MOF\nphotocatalysts for CO reduction.\n2\nphotocatal Light Photosen Sacrifici CO rate\nCondition ref\nyst source sitizer al agent (μmol g-1 h-1)\nIHEP-28 MeCN/ HO Xe lamp [Ru] --- 112.7\nIHEP-28 MeCN/ HO Xe lamp --- --- 10.1\n2\n66-IS-Ni MeCN/ HO Xe lamp [Ru] TEOA 1350\n2\nno detectable ACS Catal. 2023, 13, 2547\n66-IS-Ni MeCN/ HO Xe lamp [Ru] ---\n2 product\nNi Gd DMF/HO LED [Ru] TEOA 102092\n36 102 2\nno detectable\nNi Gd DMF/HO LED [Ru] ---\n36 102 2 product J. Am. Chem. Soc. 2020, 142, 4663\nno detectable\nNi Gd DMF/HO LED --- TEOA\n36 102 2 product\nNiSAs/NiN\n6,240\nCs-POPs\nMeCN/ HO Xe Lamp [Ru] BIH Adv. Sci. 2024, 11, 2401508\nNiSAs- 2\n3,530\nPOPs\nNi-COFs HO Xe Lamp [Ru] TEOA 810 J. Am. Chem. Soc. 2019, 141 , 7615\nNC@NiCo2O4 MeCN/ H 2 O Xe Lamp [Ru] TEOA 26200 Energy Environ. Sci. 2018, 11 , 306\nm-NiAl- Angew. Chem., Int. Ed. 2019, 58,\nMeCN/ HO Xe Lamp [Ru] TEOA 90\nLDH 2 11860\nNiO MeCN/ HO LED [Ru] TEOA 3940 Nano Res. 2021, 14, 730\nNiNi MeCN LED [Ru] 813.95 J. Am. Chem. Soc. 2024, 146, 28832\nTEOA\nCB[7]-Ni MeCN/ H 2 O Xe Lamp [Ru] TEOA 7210 Angew. Chem. Int. Ed. 2024, e202417384\nNi-MOF 68.15\nMeCN/ HO Xe Lamp [Ru] TEOA DOI: 10.1039/d4qi02143f\nLa/Ni-MOF 2 334.65\nNi-BDC 885\nNi-NDC MeCN/ HO Xe Lamp [Ru] TEOA 1319 Mater. Lett. 2024, 357, 135669\nWO-Ni-g-\nx HO Xe Lamp --- --- 4.2 J. Power Sources 2021, 510, 430420\nCN 2\n3 4\nNiONPor 24700 Adv. Funct. Mater. 2024, 34,\nNiSNPor 2 38800 2316199\n30 60 MeCN/ HO Xe Lamp --- ---\nNiO NCs 2 66.67 e202213162\n[Ru]: [Ru(bpy)]2+; TEOA: triethanolamine; BIH: 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole\n3\n(9) The text for TGA should be deleted from the Supplementary Info because it is\nprobably wrong (I doubt that coordinated water molecules are eliminated above 300\noC); retainment of Figure S25 is enough.\nResponse: Thank you for your suggestion. The text for TGA has been deleted from the\nSupplementary Info.\nReviewer #3 (Remarks to the Author):\nThis work successfully synthesized the mixed metal cluster Th₁₃Ni₆ (IHEP-25) by\nintroducing competitive Ni²⁺ ions. Furthermore, through the incorporation of the\ncationic cluster [Na₄(OH)₃(H₂O)₃], a cluster-based framework (IHEP-28) was\nconstructed during the multi-stage assembly process of IHEP-25. This framework\nfeatures a 2D honeycomb network and exhibits exceptional stability in both air and\nsolvent environments. IHEP-28 can function as a visible-light-driven photocatalyst for\nCO₂ reduction without the need for additional sacrificial agents.\nWhile this work demonstrates a degree of novelty in its synthetic approach, the\nelucidation of the underlying mechanism remains vague and is accompanied by a lack\nof critical scientific data. Additionally, significant progress has been made in the field\nof photocatalytic CO₂ reduction to CO, with numerous research teams contributing\nvaluable insights into catalyst performance, reaction mechanisms, and potential\napplications. However, the performance and mechanistic explanations presented in this\nwork do not demonstrate any innovation. Therefore, I contend that this manuscript is\nnot suitable for publication in Nature Communications for the following reasons:\nResponse: We sincerely appreciate your recognition of the novelty of the clusters and\ntheir synthetic approach reported in our work, which is an important highlight of this\nresearch. In the revised manuscript, we have integrated the results of comparative\nexperiments and the analysis of cluster structures to elaborate on the potential\nmechanism for the formation of the Th₁₃Ni₆ clusters. The introduction of nickel ions\nplays a critical role in the formation of [Th ] clusters. On the one hand, the lacuna\n13\nclusters formed by the partial disassembly of [Th (sba) ] clusters induced by Ni2+\n6 12\nsubsequently reorganize in the solution to form [Th ] clusters. On the other hand, Ni2+\n13\nand sba2- ligands function as capping units, halting further aggregation of the clusters\ninto polymers e.g. thorium hydroxide.\nAlthough there has been significant progress in the field of photocatalytic reduction\nof CO₂ to CO, especially in the study of transition metal and noble metal materials,\nresearch on the application of materials based on f-block metals, particularly 5f-block\nmetals, remains insufficient in this field. This study is the first to apply 3d-5f\nheterometallic clusters to the photocatalytic reduction of CO , representing a valuable\n2\nattempt that is expected to provide references for the application of actinide materials\nin the field of photocatalytic reduction of CO .\n2\n1. In the absence of the photosensitiser [Ru(bpy)₃]²⁺, the CO₂ photoreduction\nperformance of IHEP-28 was significantly poorer, a phenomenon that suggests that\n[Ru(bpy)₃]²⁺ plays a key role in this reaction process. However, many comparative\nexperiments have not yet been carried out, e.g., what exactly is the photoreduction\nperformance of CO₂ in the presence of the photosensitiser only.\nResponse: Thank you for your helpful advice. We indeed observed that the\nphotocatalytic CO production rate in the absence of photosensitizers decreased by\napproximately 90%. This suggests that the photosensitizer plays a key role in the\nphotocatalytic CO₂ reduction process. Nevertheless, the catalyst itself serves as the core\ncomponent of the photocatalytic reaction system. For instance, in this work, the\nphotosensitizer alone exhibited minimal photocatalytic CO₂ reduction activity in the\nabsence of IHEP-28. Several other comparative experiments have also been carried out\nin the revised manuscript. The experiment results are shown in Figure R12 (Figure 4b\nin the resubmitted manuscript) and Table R5 (Table S3 in the resubmitted SI).\nFigure R12. CO production rates of IHEP-28 under different conditions.\nTable R5. Control photocatalytic experiments using IHEP-28 as catalyst.\nEntry Control Conditions CO rate (μmol g-1 h-1)\n2 2\n3 4 2\n6\nUnder sunlight (December 20, 2024, from 1:30\n11 13.8\nPM to 3:30 PM)\n2. It is widely acknowledged that the use of photosensitizers is not only costly and\nchallenging to recycle, but also poses environmental hazards. Therefore, what effective\nstrategies can be employed to eliminate the reliance on photosensitizers?\nResponse: Thank you for your suggestion. To address the issues of high cost, difficulty\nin recovery, and environmental hazards associated with the use of photosensitizers, an\neffective strategy is to introduce photosensitive units into the framework structure of\ncatalyst materials, thereby endowing the materials with both photosensitive and\ncatalytic functions, avoiding the need to add additional photosensitizers in the catalytic\nsystem, and eliminating the reliance on photosensitizers (Inorg. Chem. Front., 2024,\n11, 3707-3730; Angew. Chem. Int. Ed. 2023, e202217565). For example, Zhang et al.\nprepared a highly active POM@MOF composite photocatalyst containing both\nsensitizing and catalytic centers by introducing a photosensitive unit (Ir-bpy) into the\nframework of MOF materials through the hydrothermal synthesis method. These\ncomposite catalysts can efficiently catalyze hydrogen production under visible light\nirradiation. By modulating the coordination environment of the sensitizing center, the\nvisible light absorption capability and interfacial electron transfer efficiency of the\ncomposite photocatalyst can be significantly improved (Angew. Chem., Int. Ed., 2024,\ne202420398).\nAdditionally, considering the fact that porphyrins and phthalocyanines are\nexceptional photosensitizers, which feature absorption bands in the visible and near-\ninfrared regions, respectively, materials based on these molecules generally exhibit\nexcellent photocatalytic performance and do not require the addition of extra\nphotosensitizers (Acc. Chem. Res., 2024, 57, 3182-3193; Coordin. Chem. Rev., 2024,\n514, 215944; Coordin. Chem. Rev., 2024, 500, 215529; Chem. Soc. Rev., 2022,51, 1124;\nJ. Mater. Chem. C., 2024, 12, 9012-9067). For instance, Jiang et al. prepared a Zr-MOF\n(PCN-222) using porphyrin tetracarboxylic acid, achieving the capture and\nphotocatalytic reduction of CO (J. Am. Chem. Soc., 2015, 137, 13440-13444). In this\n2\ncase, the porphyrin linker acts as a photosensitizing center, providing an antenna effect\nand promoting the migration of photoexcited electrons to the Zr-O cluster, where CO\n2\nis reduced to HCOOH.\nBased on the aforementioned research progress, we attempted to introduce\nphotosensitive units into the [Th Ni ] cluster to develop new photocatalytic materials\n13 6\nthat are not dependent on photosensitizers. During the synthesis of [Th Ni ], we\n13 6\nattempted to add [Ru(bpy) ]2+ to the reaction system in order to synthesize ion-pair type\n3\ncomposite photocatalyst. Additionally, we tried to introduce porphyrin carboxylate\nligands to construct cluster-based MOF materials with [Th Ni ] as the node and\n13 6\nporphyrin ligands as the connecting units. Unfortunately, due to the complexity of the\nreaction system, we were ultimately unable to successfully prepare the [Th Ni ] cluster\n13 6\nderivatives containing photosensitive unit. In subsequent studies, we will continue to\nexplore this research direction, aiming to construct a multifunctional catalytic system\nbased on [Th Ni ] cluster.\n13 6\n3. The explanation of IHEP-28 in terms of photocatalytic CO₂ reduction mechanism is\nstill insufficient. The compound is assembled from Th₁₃Ni₆ and [Na₄(OH)₃(H₂O)₃]\nclusters, but exactly which part of them plays a role in the CO₂ reduction process,\nor the exact location of the active site is still unclear. I think a lot of controlled experiments\nas well as DFT calculations need to be performed to elaborate on this.\nResponse: Thank you for your helpful advice. A considerable number of comparative\nexperiments have also been carried out in the revised manuscript. The experiment\nresults are shown in Figure R12 (Figure 4b in the resubmitted manuscript) and Table\nR5 (Table S3 in the resubmitted SI). Based on these experimental results, it can be\nconcluded that the simple ions of various metal elements comprising IHEP-28 show no\nphotocatalytic CO reduction activity when present individually. However, when these\n2\nmetal ions are assembled into IHEP-28, it exhibits significant photocatalytic CO\n2\nreduction activity. This phenomenon is consistent with the behavior of various\nheterogeneous catalysts reported in the literature. Among the various photocatalysts\ndeveloped for CO reduction, nickel-based catalysts are considered one of the\n2\npromising candidates (Table S2 in the resubmitted SI). In this study, we further\nconfirmed the role of Ni2+ as catalytic active sites in IHEP-28 through DFT calculations.\nThe details are presented below.\nDensity Functional Theory (DFT) calculations were conducted to further elucidate\nthe CO RR mechanism of IHEP-28. As shown in Figure R13 (Figure S28 in the\n2\nresubmitted SI), considering the similar coordination mode of Ni2+ ions in IHEP-28,\nwe extracted the Th₃NiII(H O) structural unit from the Th₁₃Ni cluster as a model for DFT\n2 2 6\ncalculations. In this model structure, the Th₃ unit bridged by three hydroxyl groups\nserves as the support for the catalytically active center Ni²⁺. Under the excitation of\nvisible light, Th₃NiII(H O) enriched a photogenerated electron from the photosensitizer\n2 2\nwhile simultaneously losing two coordinated water molecules, thereby forming Th NiI\n3\nwith unsaturated coordination. As shown in Figure R14 (Figure 6 in the resubmitted\nmanuscript), the Th₃NiI unite combines with a CO₂ molecule to form the intermediate\ncomplex Th₃NiII(CO₂), accompanied by an exothermic process of -0.29 eV, which\nindicates that this reaction is thermodynamically feasible. In the Th NiII(CO ) complex,\n3 2\nthe CO molecule is coordinated to the Ni atom with the η2(C, O) coordination mode,\n2\nwhere the Ni-C and Ni-O bond distance is 1.993 and 2.063 Å, respectively. The reaction\nfrom Th NiII(CO ) to Th NiII(COOH) is endothermic by 0.08 eV. In this process, an O-\n3 2 3\nH bond was formed in Th NiII(COOH) complex, resulting in a change in the\n3\ncoordination mode between Ni atom and CO molecule from η2(C, O) to η1(C), with\n2\nthe Ni-C bond distance of 1.991 Å. Subsequently, the Th NiII(COOH) complex\n3\ncaptured an additional photogenerated electron from the photosensitizer while\nsimultaneously acquiring a proton (H+) to form the intermediate complex\nTh NiII(CO)(H O). This process was accompanied by a significant exothermic release\n3 2\nof -3.58 eV. In the transformation of Th NiII(COOH) to Th NiII(CO)(H O), one C=O\n3 3 2\nbond of the CO molecule is broken, resulting in the formation of a coordinated H O\n2 2\nmolecule and a coordinated CO molecule. In the Th NiII(CO)(H O) complex, the bond\n3 2\nlengths are 2.015 Å for the Ni-O bond and 2.565 Å for the Ni-O bond. The\nwater C=O\nelongated Ni-O bond distance facilitates cleavage in the presence of water molecule,\nC=O\nultimately resulting in the formation of the Th NiII(H O) complex while concurrently\n3 2 2\nreleasing a CO molecule, through an exothermic process with the energy release of -\n0.73 eV.\nBased on previous related reports, the in-situ IR analysis and the DFT calculations,\nthe photocatalytic CO RR mechanism of IHEP-28 is proposed as follows. Under\n2\nvisible light irradiation, electrons photoinduced by the photosensitizer [Ru(bpy) ]2+\n3\ntransferred to the Th Ni cluster of IHEP-28, and then the CO molecules absorbed on\n13 6 2\nIHEP-28 surface were reduced into CO undergo a two-electron process (Figures R15;\nFigure 7 in the resubmitted manuscript).\nFigure R13. The Th₃NiII(H O) structural unit extracted from the Th₁₃Ni cluster as a\n2 2 6\nmodel for DFT calculations.\nFigure R14. Potential energy profile for the possible mechanism of CO activation by\nFigure R15. Schematic mechanism of the photocatalytic CO reduction by IHEP-28.\n2\n4. Does Th play a significant role in the process of photocatalytic CO₂ reduction? Is it\nnecessary to synthesize the Th₁₃ cluster in order to clarify the catalytic function of\nthorium?\nResponse: Thank you for your comment and helpful advice. As we mentioned in our\nresponse to the previous question, Ni2+ acts as the catalytic center in the process of\nphotocatalytic CO₂ reduction, while the [Th₁₃] cluster structure serves as the support for\nthe catalytic center. Notably, according to relevant literature reports (J. Am. Chem. Soc.\n2020, 142, 4663) and our control experiment results, simply substituting Ni²⁺ for IHEP-\n28 did not demonstrate any activity in the photocatalytic reduction of CO₂. Therefore,\nalthough Th4+ is not the catalytic center in the photocatalytic CO₂ reduction process, as\na component of IHEP-28, it still plays a significant role in the overall catalytic system.\nWe attempted to synthesize the [Th₁₃] cluster that does not contain Ni2+, but\nunfortunately, this was unsuccessful. When Ni2+ is not introduced into the synthesis\nsystem, the main product is [Th ] cluster (IHEP-29); when Ni2+ is introduced, the main\n6\nproducts are Th4+ compound featuring 1D chains (IHEP-30) or [Th₁₃Ni₆] clusters. As\nan alternative, we tested the photocatalytic CO₂ reduction performance of Th4+ and [Th ]\n6\ncluster (IHEP-29), but neither showed any photocatalytic activity.\n5. What advantages does this catalyst's quantum yield possess in comparison to other\ncatalysts, and is it feasible for application under sunlight?\nResponse: Thank you for your comment and helpful advice. The CO\n2\nphotoreduction\nsystem manifests an apparent quantum yield (AQY) of 0.74% under monochromatic\nlight irradiation of 525 nm. This result is comparable to reported system for Ni-based\nphotocatalysts (Figure R6). Under sunlight irradiation, IHEP-28 still exhibits\nphotocatalytic activity for CO reduction, with a CO production rate of 13.8 μmol g-1\n2\nh-1 (Table R5; Table S3 in the resubmitted SI).\nTable R6. The AQY of photocatalytic CO conversion in similar systems.\nphotocatalyst Photosensitizer (AQY) ref\nsource agent products\nXe\nIHEP-28 [Ru(bpy)]2+ --- CO 525 nm/0.74% This work\nlamp 3\nXe ACS Catal. 2023, 13, 2547-\n66-IS-Ni [Ru(bpy)]2+ TEOA CO 400 nm/0.14%\nlamp 3 2554\n3D-\nNiSAs/NiNCs - Xe 450 nm/0.10% Adv. Sci. 2024, 11 (23),\n[Ru(bpy)]2+ BIH CO\nPOPs lamp 3 2401508\n3D-NiSAs-POPs ---\nNi-MOF(HO) LED 420 nm/1.28% Appl. Catal., B 2022, 309,\n2 [Ru(bpy)]2+ TEOA CO\nNi-MOF lamp 3 --- 121232\nXe J. Am. Chem. Soc. 2019, 141\nNi COFs [Ru(bpy)]2+ TEOA CO 420 nm/0.3%\nlamp 3 (18), 7615-7621\nXe Energy Environ. Sci. 2018,\nNC@NiCoO [Ru(bpy)]2+ TEOA CO 420 nm/1.07%\n2 4 lamp 3 11 (2), 306-310\nXe Angew. Chem., Int. Ed.\nm-NiAl-LDH [Ru(bpy)]2+ TEOA CO; CH 600 nm/0.95%\nlamp 3 4 2019, 58 (34), 11860-11867\nNiO LED --- Nano Res. 2021, 14 (3),\n[Ru(bpy)]2+ TEOA CO\nr-NiO lamp 3 420 nm/0.46% 730-737\nXe Angew. Chem. Int. Ed.\nCB[7]-Ni [Ru(bpy)]2+ TEOA CO 450 nm/1.34%\nlamp 3 2024, e202417384\nNi(Cl)ONPor 2.73%\nNi(Cl)SNPor [Ru(bpy)]2+ TEOA CO 4.29%\n3 lamp 3 (25), 2316199\nNiNPor 0.73%\n4\n--- --- CO\nNiO lamp 0.001% 2022, 61 (49), e202213162\n6. For Th₁₃Ni₆ clusters, are the positions of the energy levels derived from theoretical\ncalculations consistent with experimental?\nResponse: Thank you for your suggestion. The band gap energy of\n[Th (O) (OH) Ni (sba) (H O) ] from theoretical calculation (2.61 eV) is consistent\n13 8 24 6 12 2 18\nwith the experimental results (2.43 eV for IHEP-28, 2.50 for IHEP-31).\nFigure R16. (a) The energy level (eV) of MOs for [Th (O) (OH) Ni (sba) (H O) ]\n13 8 24 6 12 2 18\nand the representative MOs for each energy level band. (b) The band gap energy of\nIHEP-28 from experimental result. (c) The band gap energy of IHEP-31 from\nexperimental result.\n7. The authors propose utilizing IHEP-28 for CO₂ reduction under visible and near-\ninfrared light; however, they have not conducted an investigation into the CO₂ reduction\nprocess under near-infrared illumination. What might account for this omission?\nResponse: Thank you for your careful review. We attempted to perform CO\n2\nphotoreduction experiments using IHEP-28 under near-infrared light irradiation, but\nunfortunately, no detectable products were obtained. Therefore, in the resubmitted\nmanuscript, we have removed the description of the near-infrared light catalytic\nperformance."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The author has addressed the questions proposed by the reviewers and modified the MS in a good manner, the paper now looks more logical and systematical, but there are still some concerns that need to be aware before publication.\nFirst, their PXRD pattern for the as-synthesized IHEP-28 is not in a high-consistency throughout the paper, one has a very\ntiny peak at ~5 degree and an obvious peak at ~7 degree (Figure R2), while the other one shows the opposite (Figure R24),\ndo they think these are same clusters with same structure? Could they give some explanations about this phenomenon?\nDoes this suggest that cluster is not highly reproducible? What will this difference affect their catalytic performance?\nSecond, obviously, they use different batches of samples to do the characterization and catalysis, as we can tell the PXRD\npatterns between Figure R24 and Figure R2 are totally different. They use one batch to check the stability of the cluster in air\nand solvents, and maybe some other characterizations, but another batch of (the structure might be varied) for catalysis and\ntheir stability after catalysis. It’s suggested that they better check the data consistency of their work.", "reviewer_2": "The authors have done a great job in revising their ms. Almost all my revision points/suggestions/comments have been\nsuccessfully taken into account. Also, I can understand that the authors have addressed satisfactorily most of the points\nraised by Reviewers 1 and 3. Thus, I do believe that the ms is now ready for final acceptance in \"Nature Communications\"\nby The Editor."}, "author_response": "Response to Reviewers' Comments\nReviewer #1 (Remarks to the Author):\nThe author has addressed the questions proposed by the reviewers and modified the MS\nin a good manner, the paper now looks more logical and systematical, but there are still\nsome concerns that need to be aware before publication.\nResponse: Thank you very much for your positive comments.\n1. First, their PXRD pattern for the as-synthesized IHEP-28 is not in a high-consistency\nthroughout the paper, one has a very tiny peak at ~5 degree and an obvious peak at ~7\ndegree (Figure R2), while the other one shows the opposite (Figure R24), do they think\nthese are same clusters with same structure? Could they give some explanations about\nthis phenomenon? Does this suggest that cluster is not highly reproducible? What will\nthis difference affect their catalytic performance?\nResponse: We would like to express our sincere gratitude to the reviewer for their\nmeticulous attention to the details of the PXRD data. In addressing the observed pattern\ndiscrepancies, we have undertaken a comprehensive investigation. Through\ncomparative analysis of PXRD patterns obtained from multiple sample batches, we\nhave identified that while the diffraction peak intensities exhibit variations, the\ncharacteristic peak positions (2θ values) demonstrate complete consistency with the\nsimulated PXRD patterns. This observation confirms the structural uniformity of the\nclusters across different sample batches. Consequently, we posit that these intensity\nfluctuations do not signify fundamental structural differences in the clusters. The cluster\nofIHEP-28 is highly reproducible.\nThe observed variations in diffraction peak intensities are likely attributable to the\nphenomenon of crystal preferred orientation. During the synthesis process, external\nconditions may induce variations in particle size and morphology across different\nsample batches. Additionally, minor orientation alignments during PXRD sample\npreparation could contribute to the observed fluctuations in specific crystal plane\ndiffraction intensities. To substantiate this hypothesis, we have conducted comparative\nPXRD characterization in both the initial state and post-milling conditions. As\nillustrated in Figure R1a, the characteristic peak positions (2θ values) maintain\nconsistency before and after grinding, aligning with the simulated PXRD patterns.\nHowever, the diffraction peak intensities exhibit significant variations, demonstrating\na reduction in the preferred orientation effect following the grinding process. During\nphotocatalytic processes, the particle size and morphology of crystal samples within the\nstirred reaction system may undergo alterations, leading to further variations in PXRD\ndiffraction peak intensities post-photocatalysis (Figure R1a). Nevertheless, the\ncharacteristic peak positions (2θ values) remain consistent with the simulated PXRD\npatterns, thereby further corroborating the reproducibility and structural stability of\nIHEP-28 samples. Comparative analysis of the catalytic performance across different\nsample batches (Figure R1b) reveals that the deviation in catalytic activity remains\nwithin the experimental error margin (±5%). This finding substantiates that IHEP-28\nsynthesized through the current methodology possesses sufficient structural stability to\nmeet the requirements for photocatalytic testing.\nFigure R1. (a) The PXRD patterns of IHEP-28. (b) The catalytic performance across\ndifferent sample batches.\n2. Second, obviously, they use different batches of samples to do the characterization\nand catalysis, as we can tell the PXRD patterns between Figure R24 and Figure R2 are\ntotally different. They use one batch to check the stability of the cluster in air and\nsolvents, and maybe some other characterizations, but another batch of (the structure\nmight be varied) for catalysis and their stability after catalysis. It’s suggested that they\nbetter check the data consistency of their work.\nResponse: Thank you for your careful check. The reviewer has pointed out that the\ncharacterization and catalysis involve samples from different batches, for which we\nprovide further explanation. Due to the long duration of the research work (about 4\nyears) and sample loss caused by various structural and performance characterizations, we used\nsamples from different batches during the study. However, each batch of\nsamples was characterized by PXRD before testing to determine their purity. As\ndescribed in our response to the first question, although there are differences in the\nintensity of the PXRD diffraction peaks among samples from different batches, the\npositions of the characteristic peaks (2θ values) completely match the simulated PXRD\npatterns, indicating the consistency of the structure among the different batches. To\ndemonstrate the consistency of the experimental data, we conducted a photocatalytic\nexperiment using samples from the same batch (Batch-2#2024-11) as the stability\n-existent\nexperiment, and characterized their PXRD patterns before and after catalysis (Figure\nR2; Figure S24 in the resubmitted SI). The experimental results show that IHEP-28\npossesses good structural stability and consistent catalytic performance. In future work,\nwe will further optimize the control process for sample homogeneity (For example, by\nmechanical grinding to eliminate preferred orientation effects). We would like to\nexpress our sincere gratitude to the reviewers for their valuable comments, which are\ncrucial for enhancing the quality of our research work.\nFigure R2. PXRD patterns of IHEP-28 before and after catalysis.\nReviewer #2 (Remarks to the Author):\nThe authors have done a great job in revising their ms. Almost all my revision\npoints/suggestions/comments have been successfully taken into account. Also, I can\nunderstand that the authors have addressed satisfactorily most of the points raised by\nReviewers 1 and 3. Thus, I do believe that the ms is now ready for final acceptance in\n\"Nature Communications\" by The Editor.\nResponse: Thank you very much for your good comments."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The previous concerns have been addressed carefully by the authors, this MS can now be suggested for publication in Nature Communications."}, "author_response": ""}]

Decision letter