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Boron Radical Cations from the Facile Oxidation of Electron‐Rich Diborenes
Author(s) -
Bissinger Philipp,
Braunschweig Holger,
Damme Alexander,
Kupfer Thomas,
Krummenacher Ivo,
Vargas Alfredo
Publication year - 2014
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201311110
Subject(s) - electronegativity , chemistry , homo/lumo , boron , absorption spectroscopy , benzene , absorption (acoustics) , phosphine , lewis acids and bases , ligand (biochemistry) , crystallography , photochemistry , medicinal chemistry , materials science , molecule , organic chemistry , physics , catalysis , biochemistry , receptor , quantum mechanics , composite material
The realization of a phosphine‐stabilized diborene, Et 3 P⋅(Mes)BB(Mes)⋅PEt 3 ( 4 ), by KC 8 reduction of Et 3 P⋅B 2 Mes 2 Br 2 in benzene enabled the evaluation and comparison of its electronic structure to the previously described NHC‐stabilized diborene IMe⋅(Dur)BB(Dur)⋅IMe ( 1 ). 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 BB double bond system. Thus, the stronger σ‐donor strength and larger electronegativity of the NHC ligand results in an energetically higher‐lying HOMO, making 1 a stronger neutral reductant as 4 ( 1 : E 1/2 =−1.55 V; 4 : −1.05 V), and a smaller HOMO–LUMO gap of 1 accompanied by a noticeable red‐shift of its lowest‐energy absorption band with respect to 4 . Owing to the highly negative reduction potentials, 1 and 4 were easily oxidized to afford rare boron‐centered radical cations ( 5 and 6 ).