Josef Tomeček

In recent times all-metal aromaticity has arisen, challenging classical views of aromaticity. All-metal aromaticity invokes metal-metal bonding, but this remains rare for the actinides. Recently, one-electron trithorium superatom clusters exhibiting exalted diamagnetism and hence open-shell Jellium aromaticity have been reported. Those clusters complement closed-shell two-electron congeners, but computational assessments advanced conflicting interpretations. Here, we report one- and two-electron trithorium clusters that all exhibit exalted diamagnetism, demonstrating open- and closed-shell Jellium aromaticities. We show that these Jellium aromats exhibit non-linear magnetic responses, so a foundational assumption of ring current calculations is not met, accounting for the experimental-computational disagreement. This work suggests that while π-aromaticity results from preorganized coherent wave functions, for σ-aromats there may be an electronic reorganization barrier to cohesive wave functions and Jellium aromatic ring currents. This work demonstrates the importance of caution when using ring current calculations to assign aromatic character if the experimental magnetic response is not understood.

The synthesis, characterization, and electronic structure of Cp*₂Am(^tBu₂bipy^•–) (1-Am) (Cp* = pentamethylcyclopentadienide; ^tBu₂bipy = 4,4′-di-tert-butyl-2,2′-bipyridine), the first isolated and structurally verified americium complex containing a radical ligand, is reported. Theoretical calculations (DFT and multiconfigurational methods) alongside experimental comparisons of M–N distances reveal a greater degree of metal–ligand covalency in 1-Am compared to near-isoradial lanthanide counterparts 1-Nd and 1-Sm. Electrochemical studies suggest that 1-Am may behave as an “Am(II) synthon”, and a preliminary attempt to probe the chemical reactivity of 1-Am is briefly discussed.

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 1S ¹ magic number, the valence delocalization enables exalted diamagnetism, which is aromaticity, via superatom ring currents.

The actinide–actinide bonding in tri‐actinide clusters [An₃Cl₆]^z (An=Ac–Pu, z=1–6) and [An₃Cl₆Cp₃]^z (z=−2–+3; Cp=(η⁵‐C₅H₅)) is studied using density functional theory. We find 3‐centre bonding similar to the tri‐thorium cluster [{Th(η⁸‐C₈H₈)(μ₃‐Cl)₂}₃{K(THF)₂}₂]_∞, as we previously reported (Nature 2021, 598, 72–75). The population of 3‐centre molecular orbitals (3c‐MOs) by zero, one or two electrons correlates with shortening of the An−An bond lengths, which also decrease with increasing actinide atomic number, consistent with the contraction of the actinide valence atomic orbitals. Mulliken analyses indicate that these 3c‐MOs predominantly involve An 6d and 5 f orbitals. Various methods evidence the presence of An−An bonding in most systems with populated 3c‐MOs, including bond orders (Mayer and Wiberg), quantum theory of atoms in molecules metrics (ρ, ∇²ρ, −G/V, H, delocalization indices), electron localization function, and electron density assessments. Additionally, we explore the effect of Cp ligand substitution on uranium complexes, finding that bulkier Cp ligands can induce U−U bond distortions and result in slightly longer U−U bonds. Overall, this study advances our understanding of metal–metal bonding in tri‐actinide clusters, highlighting its effects on geometric and electronic structures.

The tri‐thorium cluster [{Th(η⁸‐C₈H₈)(μ₃‐Cl)₂}₃{K(THF)₂}₂]_∞ (Nature 2021, 598, 72–75) was reported to feature intriguing σ‐aromatic bonding between the thorium atoms, a mode of metal–metal bonding unique in the actinide series. However, the presence of this bonding motif has since been challenged by others. Here, we computationally explore electron delocalisation in a molecular cluster fragment of [{Th(η⁸‐C₈H₈)(μ₃‐Cl)₂}₃{K(THF)₂}₂]_∞ and examine its responses to an applied magnetic field using a variety of methods. We also discuss the importance of the choice of basis set for the Th atoms and issues regarding locating QTAIM bond critical points. When taken together, the computed data consistently suggest the presence of delocalised Th−Th bonding and Th₃ σ‐aromaticity.

In the current study, the coordination chemistry of nine‐coordinate Ac(III) complexes with 35 monodentate and bidentate ligands was investigated using density functional theory (DFT) in terms of their geometries, charges, reaction energies, and bonding interactions. The energy decomposition analysis with naturals orbitals for chemical valence (EDA‐NOCV) and the quantum theory of atoms in molecules (QTAIM) were employed as analysis methods. Trivalent Ac exhibits the highest affinities toward hard acids (such as charged oxophilic donors, fluoride), so its classification as a hard acid is justified. Natural population analysis quantified the involvement of 5f orbitals on Ac to be about 30% of total valence electron natural configuration indicating that Ac is a member of the actinide series. Pearson correlation coefficients were used to study the pairwise correlations among the bond lengths, ΔGreaction energies, charges on Ac and donor atoms, and data from EDA‐NOCV and QTAIM. Strong correlations and anticorrelations were found between Voronoi charges on donor atoms with ΔG, EDA‐NOCV interaction energies and QTAIM bond critical point densities.

Multitopic supramolecular guests with finely tuned affinities toward widely explored cucurbit[n]urils (CBs) and cyclodextrins (CDs) have been recently designed and tested as functional components of advanced supramolecular systems. We employed various spacers between the adamantane cage and a cationic moiety as a tool for tuning the binding strength toward CB7 to prepare a set of model guests with K_CB7 and K_β-CD values of (0.6–5.0) × 10¹⁰ M^–1 and (0.6–2.6) × 10⁶ M^–1, respectively. These accessible adamantylphenyl-based binding motifs open a way toward supramolecular components with an outstanding affinity toward β-cyclodextrin. ¹H NMR experiments performed in 30% CaCl₂/D₂O at 273 K along with molecular dynamics simulations allowed us to identify two arrangements of the [email protected]β-CD complexes. The approach, joining experimental and theoretical methods, provided a better understanding of the structure of cyclodextrin complexes and related molecular recognition, which is highly important for the rational design of drug delivery systems, molecular sensors and switches.