Kekulé diradicaloids derived from a classical N-heterocyclic carbene

Abstract

The direct double carbenylation of 1,4-diiodobenzene and 4,4′-dibromobiphenyl with a classical N-heterocyclic carbene, SIPr (1) (SIPr = :C{N(2,6-iPr₂C₆H₃)}₂CH₂CH₂), by means of nickel catalysis gives rise to 1,3-imidazolinium salts [(SIPr)(C₆H₄)(SIPr)](I)₂ (2) and [(SIPr)(C₆H₄)₂(SIPr)](Br)₂ (3) as off-white solids. Two-electron reduction of 2 and 3 with KC₈ cleanly yields Kekulé diradicaloid compounds [(SIPr)(C₆H₄)(SIPr)] (4) and [(SIPr)(C₆H₄)₂(SIPr)] (5), respectively, as crystalline solids. Structural parameters and DFT as well as CASSCF calculations suggest the closed-shell singlet ground state for 4 and 5. Calculations reveal a very low singlet–triplet energy gap ΔE_S–T for 5 (10.7 kcal mol⁻¹), while ΔE_S–T for 4 (29.1 kcal mol⁻¹) is rather large.