Crystalline Radicals Derived from Classical N-Heterocyclic Carbenes.

One-electron reduction of C2-arylated 1,3-imidazoli(ni)um salts (IPrAr )Br (Ar=Ph, 3 a; 4-DMP, 3 b; 4-DMP=4-Me2 NC6 H4 ) and (SIPrAr )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 [(IPrAr )]. (Ar=Ph, 5 a; 4-DMP, 5 b) and [(SIPrAr )]. (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.