Crystalline Radicals Derived from Classical N-Heterocyclic Carbenes

摘要

One-electron reduction of C2-arylated 1,3-imidazoli(ni)um salts (IPrAr)Br (Ar=Ph, 3a; 4-DMP, 3b; 4-DMP=4-Me2NC6H4) and (SIPrAr)I (Ar=Ph, 4a; 4-Tol, 4b) derived from classical NHCs (IPr=:C{N(2,6-iPr(2)C(6)H(3))}(2)CHCH, 1; SIPr=:C{N(2,6-iPr(2)C(6)H(3))}(2)CH2CH2, 2) gave radicals [(IPrAr)](center dot) (Ar=Ph, 5a; 4-DMP, 5b) and [(SIPrAr)](center dot) (Ar=Ph, 6a; 4-Tol, 6b). Each of 5a,b and 6a,b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid-state characterization of NHC-derived carbon-centered radicals 6a,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.