N‐Heterocyclic Carbene Stabilized Boryl Radicals

Abstract

The reaction of the 2-(trimethylsilyl)imidazolium triflate 9 with diarylboron halides (4-R-C₆ H₄ )₂ BX (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 σ_p of the para substituent and the half-wave potential. Chemical reduction with decamethylcobaltocene, [(C₅ Me₅ )₂ Co], 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 isotopes ¹¹ B and ¹⁰ B, 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.