Actinide 2-metallabiphenylenes that satisfy Huckel's rule

Author(s): Justin K. Pagano [sup.1] [sup.2] , Jing Xie [sup.3] [sup.7] , Karla A. Erickson [sup.1] , Stephen K. Cope [sup.1] , Brian L. Scott [sup.4] , Ruilian Wu [sup.5] [...]
Aromaticity and antiaromaticity, as defined by Hückel's rule, are key ideas in organic chemistry, and are both exemplified in biphenylene.sup.1-3--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 Biphenylene analogues in which one of the benzene rings has been replaced by a different (4n + 2) [pi]-electron system have so far been associated only with organic compounds.sup.4,5. In addition, efforts to prepare a zirconabiphenylene compound resulted in the isolation of a bis(alkyne) zirconocene complex instead.sup.6. 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 f.sup.2 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.