Actinide 2-metallabiphenylenes that satisfy Hückel’s rule

Aromaticity and antiaromaticity, as defined by Huckel’s rule, are key ideas in organic chemistry, and are both exemplified in biphenylene1–3—a molecule that consists of two benzene rings joined by a four-membered ring at its core. Biphenylene analogues in which one of the benzene rings has been replaced by a different (4n + 2) π-electron system have so far been associated only with organic compounds4,5. In addition, efforts to prepare a zirconabiphenylene compound resulted in the isolation of a bis(alkyne) zirconocene complex instead6. Here we report the synthesis and characterization of, to our knowledge, the first 2-metallabiphenylene compounds. Single-crystal X-ray diffraction studies reveal that these complexes have nearly planar, 11-membered metallatricycles with metrical parameters that compare well with those reported for biphenylene. Nuclear magnetic resonance spectroscopy, in addition to nucleus-independent chemical shift calculations, provides evidence that these complexes contain an antiaromatic cyclobutadiene ring and an aromatic benzene ring. Furthermore, spectroscopic evidence, Kohn–Sham molecular orbital compositions and natural bond orbital calculations suggest covalency and delocalization of the uranium f2 electrons with the carbon-containing ligand. The synthesis of uranium- and thorium-containing metallabiphenylenes demonstrates the ability of the actinides to stabilize aromatic/antiaromatic structures where transition metals have failed.