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

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)), U2H2 and U2H4 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 ThHn (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 sigma-, pi-, and delta-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 sigma-aromatic bond(16,17) that is counter to the focus of previous theoretical predictions. The experimental discovery of actinide sigma-aromatic bonding adds to main group and d transition metal analogues, extending delocalized sigma-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.

Automated summary

Actinide-actinide bonding is typically predicted to be weak and difficult to observe under normal conditions, but the experimental discovery of thorium-thorium bonding in a crystalline cluster has revealed a new type of sigma-aromatic bond that challenges previous theoretical predictions. This discovery extends delocalized sigma-aromatic bonding to the heaviest elements in the periodic table and offers a new approach to studying actinide-actinide bonding.