Diuranium arene complexes with an inverse-sandwich structure are well known. However, despite the similarities between  thorium and uranium, there had been no known examples of inverse-sandwich dithorium arene complexes so far. Interactions between the f-elements and arenes are useful in catalysis and can stabilize unusual oxidation states of the metals.

Paula L. Diaconescu, University of California, Los Angeles, USA, Wenliang Huang, Peking University, Beijing, China, and colleagues have synthesized a series of inverse-sandwich, arene-bridged dithorium complexes (general structure pictured). The team reduced precursors of the type (NN^TBS)ThCl₂(THF) (NN^TBS = 1,1′-ferrocenediyl-(NSi^tBuMe₂)₂, THF = tetrahydrofuran) with potassium graphite in the presence of arenes to obtain the desired complexes. The arenes used were benzene, toluene, biphenyl, and naphthalene.

The researchers found that the complexes feature a bridged tetraanionic arene and two thorium(IV) centers. Density functional theory (DFT) calculations indicate a δ bonding interaction in these compounds, analogous to similar to uranium and rare-earth metal complexes. According to the team, further studies of the reactivity and electronic structures of the dithorium complexes are ongoing.

• Arene-Bridged Dithorium Complexes: Inverse Sandwiches Supported by a δ Bonding Interaction,
  Chao Yu, Jiefeng Liang, Chong Deng, Guillaume Lefèvre, Thibault Cantat, Paula L. Diaconescu, Wenliang Huang,
  J. Am. Chem. Soc. 2020.
  https://doi.org/10.1021/jacs.0c11215