Non‐Kekulé meta‐Quinodimethane Singlet Diradicals Based on Classical N‐Heterocyclic Carbenes

Diradicals based on a meta ‐quinodimethane ( m ‐QDM ) scaffold generally have a triplet ground state and are rather scarce. Herein, m ‐QDM ‐based non‐Kekulé diradicals [3,3′‐(NHC) 2 BP] ( 3 ‐NHC) (NHC = SIPr = C{N(Dipp)CH 2 } 2 ; IPr = C{N(Dipp)CH} 2 , Me‐IPr = C{N(Dipp)CMe} 2 ; Dipp = 2,6‐ i Pr 2 C 6 H 3 ; BP = 1,1′‐C 6 H 4 C 6 H 4 ) featuring N ‐heterocyclic carbene (NHC) pendants are reported as crystalline solids. The EPR spectra of 3 ‐NHC show both allowed (Δm s = 1) and forbidden (Δm s = 2; ‘half‐field’) transitions characteristic for triplet diradicals. Variable temperature EPR studies however reveal a singlet ground state for 3 ‐SIPr. Consistent with the EPR spectra, calculations predict a remarkably small singlet‐triplet energy gap (Δ E ST ≤ 0.26 kcal/mol) for the 3 ‐NHC compounds. The calculated singlet diradical character for the ground states of the 3 ‐NHC compounds amounts to ~99 %.