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)₂BP] (3-NHC) (NHC = SIPr = C{N(Dipp)CH₂}₂; IPr = C{N(Dipp)CH}₂, Me-IPr = C{N(Dipp)CMe}₂; Dipp = 2,6-iPr₂C₆H₃; BP = 1,1′-C₆H₄C₆H₄) 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 (ΔE_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 %.