Kekulé diradicaloids derived from a classical N-heterocyclic carbene

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-iPr(2)C(6)H(3))}(2)CH(2)CH(2)), by means of nickel catalysis gives rise to 1,3-imidazolinium salts [(SIPr)(C(6)H(4))(SIPr)](I)(2) (2) and [(SIPr)(C(6)H(4))(2)(SIPr)](Br)(2) (3) as off-white solids. Two-electron reduction of 2 and 3 with KC(8) cleanly yields Kekulé diradicaloid compounds [(SIPr)(C(6)H(4))(SIPr)] (4) and [(SIPr)(C(6)H(4))(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.