Valence-delocalized trithorium nanocluster superatoms with open-shell exalted diamagnetism

nature chemistry Article https://doi.org/10.1038/s41557-025-01790-3 Valence-delocalized trithorium nanocluster superatoms with open-shell exalted diamagnetism Received: 1 October 2024 Accepted: 28 February 2025 Published online: xx xx xxxx John A. Seed 1,2, Xinglan Deng1,2, Josef Tomeček 1,2, Adam Brookfield1,3, David Collison1,3, Floriana Tuna 1,3, Ashley J. Wooles 1,2, George F. S. Whitehead 1, Nikolas Kaltsoyannis 1,2 & Stephen T. Liddle 1,2 Check for updates Quantum-confined nanoclusters can be described by the jellium model, which emphasizes closed-shell electron configurations, but an open-shell variation with jellium aromaticity has been proposed. Such clusters are termed superatoms because they behave like an atom, and they exhibit unusual properties. Superatoms feature metal–metal bonding; hence, since their discovery 40 years ago, superatoms have exclusively involved main group or transition metals, with actinides only considered computationally as dopants owing to actinide–actinide bonding being exceedingly rare. Here we report trithorium nanoclusters exhibiting three-centre-one-electron actinide–actinide bonding. Experimental and computational analysis demonstrates Robin–Day Class III 6d-orbital valence delocalization in these clusters. These S = 1/2 clusters are paramagnetic, but in external applied magnetic fields they exhibit exalted diamagnetism, evidencing actinide open-shell jellium aromaticity superatom character. Exalted diamagnetism is not normally associated with a single unpaired electron, but with a 1S1 magic number, the valence delocalization enables exalted diamagnetism, which is aromaticity, via superatom ring currents. The properties of metal nanoclusters can deviate substantially from bulk metals owing to quantum confinement effects1. While bulk metals are described by band theory, nanoclusters with free-electron metals can be described by the jellium model, where valence electrons in the nanocluster are subject to a uniform potential2–5. The jellium model emphasizes closed-shell electron configurations 1S21P61D102S2 1F142P61G182D10… with electron magic numbers 2, 8, 18, 20, 34, 40, 58, 68… but an open-shell variation with magic numbers 1, 5, 13, 19, 27, 37, 49, 63… and jellium aromaticity has been proposed6. Such clusters have been termed superatoms because collectively they behave like an atom, extending the periodic table into a ‘third dimension’1,5. Superatoms have demonstrated altered properties, such as increased electron affinity, that is, superhalogens7,8, and anomalous magnetic susceptibilities9, and since they offer atom-level modifications to engineer superatom properties, they promise numerous opportunities in catalysis, materials and devices10,11. Superatom clusters inherently feature metal–metal bonding, which requires sufficiently diffuse metal valence orbitals. Thus, for the decades that superatoms have been known, they have been based on main group or transition metals1–5,7–11. By contrast, although f elements, and in particular actinides, have been considered computationally as dopants into superatoms12–14, the relatively limited radial distribution Department of Chemistry, The University of Manchester, Manchester, UK. 2Centre for Radiochemistry Research, The University of Manchester, Manchester, UK. 3Department of Chemistry and Photon Science Institute, The University of Manchester, Manchester, UK. e-mail: ; 1 Nature Chemistry Article https://doi.org/10.1038/s41557-025-01790-3 Previous work This work Me3Si 0.5 K* Cl Cl 0.33 Th K* Th K* Cl Cl Cl Th Cl K* = [K(THF)2] 3 Me3Si SiMe3 22 K+ 2 SiMe3 K* n Me3Si SiMe3 –0.5 Me3Si THF Th Cl Cl THF 1 2.2.2-crypt MC8 -'C8' Cl Cl Th 0.33 SiMe3 Th Cl Cl Cl Th Cl N O O M O O O O N 4K (M = K), 4Rb (M = Rb), 4Cs (M = Cs) Fig. 1 | Synthesis of 3 and 4M (M = K, Rb and Cs). In previous work (ref. 21), treatment of 1 with 2 afforded 3, which contains a three-centre-two-electron trithorium bonding interaction. In this work, treatment of 1 and 2.2.2-cryptand with MC8 reducing agents affords complexes 4M (M = K, Rb, Cs). The fate of the excess M and 2.2.2-cryptand component was not determined. of actinide 5f valence orbitals has restricted the number of compounds featuring actinide–actinide bonding15–22. Notably, endohedral fullerene-encapsulated Th2, U2 and Th2F have been prepared19,20,22, but those actinide–actinide bonds cannot be readily translated to synthetic molecular chemistry. Previously, we discovered that reduction of the thorium complex [Th(η8-C8H8)(Cl)2(THF)2] (1)23 by the cyclobutadienyl reagent [K2{C4(SiMe3)4}] (2)24 produced the diamagnetic two-electron trithorium nanocluster [{Th(η8-C8H8)(μ3-Cl)2}3{K(THF)2}2]∞ (3)21, which features three-centre-two-electron Th 6d-based metal–metal bonding. Conspicuously, while one-electron U–U and Th–Th bonds have been proposed in endohedral fullerenes19,22, experimental spectroscopic and magnetic verification has been lacking. Experimental isolation and confirmation of one-electron actinide–actinide bonding beyond trapped dimetal units would thus be anticipated to afford unusual properties. The isolation of 3 suggested to us that one-electron mixed-valence actinide–actinide nanocluster bonding might be accessible utilizing the 6d-orbitals of thorium. In this Article, we report the preparation and isolation of the crystalline mixed-valence trithorium nanoclusters [M(2.2.2-cryptand)] [{(η8-C8H8)Th(μ-Cl)2}3] (4M, M = K, 4K; Rb, 4Rb; Cs, 4Cs) that exhibit three-centre-one-electron actinide–actinide bonding. Experimental and computational analysis demonstrates Robin–Day Class III formalism 6d-orbital valence delocalization in these clusters. These clusters are paramagnetic, which should be the dominant physicochemical behaviour, but in external applied magnetic fields, they instead exhibit exalted diamagnetic responses, experimentally evidencing actinide superatom and open-shell jellium aromaticity. Results and discussion Synthesis Previously, we reported that reduction of the thorium complex [Th(η 8-C 8H 8)(Cl) 2(THF) 2] (1) 23 by the cyclobutadienyl reagent [K2{C4(SiMe3)4}] (2)24 produced the diamagnetic two-electron reduced trithorium nanocluster [{Th(η8-C8H8)(μ3-Cl)2}3{K(THF)2}2]∞ (3)21, which features three-centre-two-electron Th 6d-based metal–metal bonding (Fig. 1). In our report of 3, its high-yield isolation (89%) resulted from rapid formation and precipitation enabled by the strongly reducing and soluble nature of 2. Indeed, reduction of 1 to give 3 using the heterogeneous reductant KC8 was low yielding (10%). Targeting one-electron reduction, we modulated the slow reaction of 1 with MC8 (M = K, Rb, Cs) by addition of 2.2.2-cryptand to increase efficacy without over-reduction. Hence, careful addition of a 1:1 solution of 1 and 2.2.2-cryptand in THF to one equivalent of MC8 (M = K, Rb, Cs) in benzene produces, after work-up, analytically pure blue [M(2.2.2-cryptand)] [{(η8-C8H8)Th(μ-Cl)2}3] (4M, M = K, (...truncated)