Diborabutatriene: An Electron-Deficient Cumulene

• Published in: Angewandte Chemie International Edition 2014-08, Vol.53 (34), p.9082-9085
• Main Authors: Böhnke, Julian, Braunschweig, Holger, Ewing, William C., Hörl, Christian, Kramer, Thomas, Krummenacher, Ivo, Mies, Jan, Vargas, Alfredo
• Format: Article
• Language: English
• Subjects: Bonding; boron; Boron Compounds - chemistry; carbenes; Complexation; Crystallography, X-Ray; cumulenes; Density; Electrons; Equivalence; Ligands; main-group elements; Models, Molecular; multiple bonds; Polyenes - chemistry; Sodium; Spectrophotometry, Ultraviolet; Structural analysis; Synthesis; Voltammetry
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201403888

The complexation of two equivalents of a cyclic (alkyl)(amino)carbene (CAAC) to tetrabromodiborane, followed by reduction with four equivalents of sodium naphthalide, led to the formation of the CAAC‐stabilized linear diboracumulene (CAAC)2B2. The capacity of the CAAC ligand to facilitate B2→CAAC donation of π‐electron density resulted in important differences between this species and a previously reported complex featuring a BB triple bond stabilized by cyclic di(amino)carbenes, including a longer BB bond and shorter BC bonds. Frontier orbital analysis indicated sharing of valence electrons across the entire linear C‐B‐B‐C unit in (CAAC)2B2, which is supported by natural population analysis and cyclic voltammetry. The use of π‐acidic carbenes as stabilizing ligands on a central diboron unit has enabled the synthesis and characterization of a diborabutatriene, an inorganic/organic electron‐deficient cumulene with a linear CBBC core. Structural characterization, theoretical analysis, and electrochemical investigation each indicated even sharing of electrons across the four‐membered center.