Crystalline Radicals Derived from Classical N-Heterocyclic Carbenes

Abstract

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.

Keywords: EPR spectroscopy; N-heterocyclic carbenes; X-ray diffraction; density functional calculations; radicals.