N-Heterocyclic Carbene Stabilized Boryl Radicals

The reaction of the 2-(trimethylsilyl) imidazolium triflate 9 with diarylboron halides (4-R-C6H4)(2)BX (R = H, X = Br; R = CH3, X = Cl; R = CF3, X = Cl) afforded the NHCstabilized borenium cations 10a-c. Cyclic voltammetry revealed a linear correlation between the Hammett parameter sp of the para substituent and the half-wave potential. Chemical reduction with decamethylcobaltocene, [(C5Me5)(2)Co], furnished the corresponding radicals 11a-c; their characterization by EPR spectroscopy confirmed the paramagnetic character of 11a-c, with large hyperfine coupling constants to the boron isotopes B-11 and B-10, 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 11a-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 11c was established by Xray diffraction analysis.