title: N-Heterocyclic Carbene Stabilized Boryl Radicals.
abstract: The reaction of the 2-(trimethylsilyl)imidazolium triflate 9 with diarylboron halides (4-R-C6H4)2BX (R=H, X=Br; R=CH3, X=Cl; R=CF3, X=Cl) afforded the NHC-stabilized borenium cations 10 a-c. Cyclic voltammetry revealed a linear correlation between the Hammett parameter σpof the para substituent and the half-wave potential. Chemical reduction with decamethylcobaltocene, [(C5Me5)2Co], furnished the corresponding radicals 11 a-c; their characterization by EPR spectroscopy confirmed the paramagnetic character of 11 a-c, with large hyperfine coupling constants to the boron isotopes11B and10B, while delocalization of the unpaired electron into the NHC is negligible. DFT calculations of the percentage of spin density distribution between the carbene (NHC) and the boryl fragments (BR2) revealed for 11 a-c a spin density ratio (BR2/NHC) of ca. 9:1, which underlines their distinct boryl radical character. The molecular structure of the most stable species 11 c was established by X-ray diffraction analysis.
authors.affiliation: Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106, Braunschweig, Germany.
authors.forename: Miguel F
authors.initials: MF
authors.lastname: Silva Valverde
authors.affiliation: Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106, Braunschweig, Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Schweyen
authors.affiliation: Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106, Braunschweig, Germany.
authors.forename: Daria
authors.initials: D
authors.lastname: Gisinger
authors.affiliation: Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106, Braunschweig, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Bannenberg
authors.affiliation: Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106, Braunschweig, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Freytag
authors.affiliation: Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106, Braunschweig, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Kleeberg
authors.affiliation: Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106, Braunschweig, Germany.
authors.forename: Matthias
authors.identifier: 0000-0002-5364-0357
authors.initials: M
authors.lastname: Tamm
keywords: EPR spectroscopy; N-heterocyclic carbenes; borenium cations; boron; boryl radicals