A crystalline tri-thorium cluster with σ-aromatic metal–metal bonding

• Published in: Nature (London) 2021-10, Vol.598 (7879), p.72-75
• Main Authors: Boronski, Josef T., Seed, John A., Hunger, David, Woodward, Adam W., van Slageren, Joris, Wooles, Ashley J., Natrajan, Louise S., Kaltsoyannis, Nikolas, Liddle, Stephen T.
• Format: Article
• Language: English
• Subjects: 119/118; 140/131; 140/133; 639/638/263/910; 639/638/911/406/910; Bonding strength; Clusters; Computer applications; Crystal structure; Crystallinity; Diamagnetism; Fullerenes; Geometry; Humanities and Social Sciences; Hydrocarbons; Ligands; Lymphocytes T; Metal bonding; Metal-metal bonding; Metals; multidisciplinary; NMR; Nuclear magnetic resonance; Periodic table; Science; Science (multidisciplinary); Solvents; Spectrum analysis; Thorium; Transition metals
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-021-03888-3

Metal–metal bonding is a widely studied area of chemistry 1 – 3 , and has become a mature field spanning numerous d transition metal and main group complexes 4 – 7 . By contrast, actinide–actinide bonding, which is predicted to be weak 8 , is currently restricted to spectroscopically detected gas-phase U 2 and Th 2 (refs. 9 , 10 ), U 2 H 2 and U 2 H 4 in frozen matrices at 6–7 K (refs. 11 , 12 ), or fullerene-encapsulated U 2 (ref. 13 ). Furthermore, attempts to prepare thorium–thorium bonds in frozen matrices have produced only ThH n ( n  = 1–4) 14 . Thus, there are no isolable actinide–actinide bonds under normal conditions. Computational investigations have explored the probable nature of actinide–actinide bonding 15 , concentrating on localized σ-, π-, and δ-bonding models paralleling d transition metal analogues, but predictions in relativistic regimes are challenging and have remained experimentally unverified. Here, we report thorium–thorium bonding in a crystalline cluster, prepared and isolated under normal experimental conditions. The cluster exhibits a diamagnetic, closed-shell singlet ground state with a valence-delocalized three-centre-two-electron σ-aromatic bond 16 , 17 that is counter to the focus of previous theoretical predictions. The experimental discovery of actinide σ-aromatic bonding adds to main group and d transition metal analogues, extending delocalized σ-aromatic bonding to the heaviest elements in the periodic table and to principal quantum number six, and constitutes a new approach to elaborate actinide–actinide bonding. A crystalline cluster exhibits thorium–thorium bonding, adding to our knowledge of actinide–actinide bonding.