Knowledge Bank

Photoelectron spectroscopy in combination with computational studies over the past decade has shown that boron clusters possess planar or quasi-planar structures, in contrast to that of bulk boron, which is dominated by three-dimensional cage-like building blocks. All planar or quasi-planar boron clusters are observed to consist of a monocyclic circumference with one or more interior atoms. The propensity for planarity has been found to be due to both $\sigma$ and $\pi$ electron delocalization throughout the molecular plane, giving rise to concepts of $\sigma$ and $\pi$ double aromaticity. We have found further that the central boron atoms can be substituted by transition metal atoms to form a new class of aromatic compounds, which consist of a central metal atom and a monocyclic boron ring (Mopyright B). Eight-, nine-, and ten-membered rings of boron have been observed, giving rise to octa-, ennea-, and deca-coordinated aromatic transition metal compounds [1-3]. References: [1]"Aromatic Metal-Centered Monocyclic Boron Rings: Coopyright B$\mathrm{9-}$ and Ruopyright B$\mathrm{9-}$" (Constantin Romanescu, Timur R. Galeev, Wei-Li Li, A. I. Boldyrev, and L. S. Wang), Angew. Chem. Int. Ed. \textbf{50}, 9334-9337 (2011). [2]"Transition-Metal-Centered Nine-Membered Boron Rings: Mopyright B$9$ and Mopyright B$\mathrm{9-}$ (M = Rh, Ir)" (Wei-Li Li, Constantin Romanescu, Timur R. Galeev, Zachary Piazza, A. I. Boldyrev, and L. S. Wang), J. Am. Chem. Soc. \textbf{134}, 165-168 (2012). [3]"Observation of the Highest Coordination Number in Planar Species: Decacoordinated Taopyright B$\mathrm{<mrow\; data-mjx-texclass="ORD">10</mrow>-}$ and Nbopyright B$\mathrm{9-}$ Anions" (Timur R. Galeev, Constantin Romanescu, Wei-Li Li, L. S. Wang, and A. I. Boldyrev), Angew. Chem. Int. Ed. \textbf{51}, 2101-2105 (2012).