Boron Radical Cations from the Facile Oxidation of Electron‐Rich Diborenes1. Issue 22 (7th April 2014)

• Record Type: Journal Article
• Authors: Bissinger, Philipp; Braunschweig, Holger; Damme, Alexander; Kupfer, Thomas; Krummenacher, Ivo; Vargas, Alfredo
• Is Part Of: Angewandte Chemie international edition. Volume 53:Issue 22(2014)
• Page Start: 5689
• Page End: 5693
• Publication Date: 2014-04-07
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/anie.201311110
• Languages: English
• ISSNs: 1433-7851

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The realization of a phosphine‐stabilized diborene, Et₃P⋅(Mes)BB(Mes)⋅PEt₃ (<bold>4</bold>), by KC₈ reduction of Et₃P⋅B₂Mes₂Br₂ in benzene enabled the evaluation and comparison of its electronic structure to the previously described NHC‐stabilized diborene IMe⋅(Dur)BB(Dur)⋅IMe (<bold>1</bold>). Importantly, both species feature unusual electron‐rich boron centers. However, cyclic voltammetry, UV/Vis spectroscopy, and DFT calculations revealed a significant influence of the Lewis base on the reduction potential and absorption behavior of the BB double bond system. Thus, the stronger σ‐donor strength and larger electronegativity of the NHC ligand results in an energetically higher‐lying HOMO, making <bold>1</bold> a stronger neutral reductant as <bold>4</bold> (<bold>1</bold>: <italic>E</italic>_1/2=−1.55 V; <bold>4</bold>: −1.05 V), and a smaller HOMO–LUMO gap of <bold>1</bold> accompanied by a noticeable red‐shift of its lowest‐energy absorption band with respect to <bold>4</bold>. Owing to the highly negative reduction potentials, <bold>1</bold> and <bold>4</bold> were easily oxidized to afford rare boron‐centered radical cations (<bold>5</bold> and <bold>6</bold>).</p> </abstract>