Crystalline Radicals Derived from Classical N-Heterocyclic Carbenes

One-electron reduction of C2-arylated 1,3-imidazoli(ni)um salts (IPr^Ar )Br (Ar=Ph, 3 a; 4-DMP, 3 b; 4-DMP=4-Me₂ NC₆ H₄ ) and (SIPr^Ar )I (Ar=Ph, 4 a; 4-Tol, 4 b) derived from classical NHCs (IPr=:C{N(2,6-iPr₂ C₆ H₃ )}₂ CHCH, 1; SIPr=:C{N(2,6-iPr₂ C₆ H₃ )}₂ CH₂ CH₂ , 2) gave radicals [(IPr^Ar )]^. (Ar=Ph, 5 a; 4-DMP, 5 b) and [(SIPr^Ar )]^. (Ar=Ph, 6 a; 4-Tol, 6 b). Each of 5 a,b and 6 a,b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid-state characterization of NHC-derived carbon-centered radicals 6 a,b by single-crystal X-ray diffraction is reported. DFT calculations indicate that the unpaired electron is mainly located at the original carbene carbon atom and stabilized by partial delocalization over the adjacent aryl group.