N‐Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons

Abstract Stable N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C 6 H 4 )(IPr)] and [(IPr)(C 6 H 4 ) 2 (IPr)] ( 4 and 5 , respectively; IPr=C{N(2,6‐ i Pr 2 C 6 H 3 )} 2 CHCH), are reported. In a nickel‐catalyzed double carbenylation of 1,4‐Br 2 C 6 H 4 and 4,4′‐Br 2 (C 6 H 4 ) 2 with IPr ( 1 ), [(IPr)(C 6 H 4 )(IPr)](Br) 2 ( 2 ) and [(IPr)(C 6 H 4 ) 2 (IPr)](Br) 2 ( 3 ) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC 8 . Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet−triplet energy gap Δ E S−T of 10.7 kcal mol −1 , whereas 4 features more quinoidal character with a rather large Δ E S−T of 25.6 kcal mol −1 . In view of the low Δ E S−T , 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character.