Aromaticity and antiaromaticity, as defined by Hückel’s rule, are key ideas in organic chemistry, and are both exemplified in biphenylene1,2,3—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 Biphenylene analogues in which one of the benzene rings has been replaced by a different (4n + 2) π-electron system have so far been associated only with organic compounds4,5.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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All data are available upon request from J.L.K. (synthetic and spectroscopic data) or L.G. (computational data). Crystallographic data of 4 and 5 have been uploaded to the Cambridge Crystallographic Data Centre under CCDC numbers 1526648 and 1526649.
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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).
The authors declare no competing interests.
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Extended data figures and tables
NIR spectra of U(iv)-containing compounds 5 and 7 collected in toluene at 298 K. Band energies and relative intensities are strongly correlated in these spectra, but the absolute intensities in the bands for 5 are around two to four times those in the bands for 7, which suggests a greater degree of U–C covalency.
A, NICS and Bindz values were computed in the axis perpendicular to the four-membered ring and six-membered ring planes of compounds 4 and 5. The points at which the shielding tensors were calculated are represented here by small blue spheres; the points pass through the corresponding non-mass-weighted centre of each ring. B–D, NICS and Bindz profiles of the four-membered ring and six-membered ring for compounds 4 (B; An = Th), 5 (C; An = U), and cyclobutadiene and benzene (D). The PBE0/6-31G(d,p)&SDD level of theory was used. R is the distance from the centre of each ring along the axis, in Å.
A–D, NICS and Bindz profiles of the four-membered ring and six-membered ring for biphenylene (1) (A), 2-thianorbiphenylene (2) (B), 2-thianorbiphenylene sulfone (3) (C), and benzo[3,4]cyclobuta[1,2]furan (D).
Molecular orbitals for triplet (C5Me5)2U(2,5-Ph2-cyclopenta[3,4]cyclobuta[1,2]benzene) (5-triplet) as computed with PBE0/631G(d,p)&SDD. An isocontour value of 0.04 was used. The left column shows the α-spin molecular orbitals and the right column shows the β-spin molecular orbitals. For both spins, the orbitals are labelled according to their energetic order among those of the same spin. Orbitals are paired here with the closest corresponding orbital of the opposite spin. The positive and negative phases of the Kohn–Sham molecular orbitals are in red and blue, respectively.
A–F, Active molecular orbitals for the CAS(6,6) model of 5a-triplet (An = U) in its triplet ground spin state, with the occupation number given in parentheses. The positive and negative phases of the active molecular orbitals are in blue and white. G–I, Representative natural bond orbitals for 5-triplet (An = U) in the triplet state. The positive and negative phases of the natural bond orbitals are in red and yellow. U, green; C, silver. H atoms are omitted for clarity.
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Pagano, J.K., Xie, J., Erickson, K.A. et al. Actinide 2-metallabiphenylenes that satisfy Hückel’s rule. Nature 578, 563–567 (2020). https://doi.org/10.1038/s41586-020-2004-7
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