N-Heterocyclic Carbene Stabilized Boryl Radicals.

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 σp 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.