Aromaticity and antiaromaticity, as defined by Hückel’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 Biphenylene analogues in which one of the benzene rings has been replaced by a different (4n + 2) π-electron system have so far been associated only with organic compounds4,5.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.
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Acknowledgements For financial support of this work, we acknowledge the US Department of Energy (DOE) through the Los Alamos National Laboratory (LANL) Laboratory Directed Research and Development Program; the LANL G. T. Seaborg Institute for Transactinium Science (Postdoctoral Fellowships to J.K.P., K.A.E. and S.K.C.); the Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program (GRA Fellowship to J.K.P.); and the Office of Basic Energy Sciences, Heavy Element Chemistry program (to J.L.K., P.Y. and B.L.S., for materials and supplies). We thank R. Michalczyk and L. A. Silks (LANL) for assistance with two-dimensional NMR experiments, and J. Thompson and P. Rosa (LANL) for assistance in collecting magnetometry data. This work was in part funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the US DOE through grant USDOE/ DESC002183 (to J.X. and L.G.). We also acknowledge the US National Science Foundation (grants CHE-1265608 and CHE-1565658 to R. Waterman). The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE (contract DE-AC05-06OR23100). LANL is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the US DOE (contract DE-AC52-06NA25396).
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