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Chichibabin's hydrocarbons; diradicaloids; diradicals; N-heterocyclic carbenes; Thiele's hydrocarbons
Abstract:
Stable N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C6H4)(IPr)] and [(IPr)(C6H4)2(IPr)] (4 and 5, respectively; IPr=C{N(2,6‐iPr2C6H3)}2CHCH), are reported. In a nickel‐catalyzed double carbenylation of 1,4‐Br2C6H4 and 4,4′‐Br2(C6H4)2 with IPr (1), [(IPr)(C6H4)(IPr)](Br)2 (2) and [(IPr)(C6H4)2(IPr)](Br)2 (3) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC8. Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet−triplet energy gap ΔES−T of 10.7 kcal mol−1, whereas 4 features more quinoidal character with a rather large ΔES−T of 25.6 kcal mol−1. In view of the low ΔES−T, 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character.
