Diradical Character Enhancement by Spacing: N‐Heterocyclic Carbene Analogues of Müller's Hydrocarbon

Two‐fold C−C cross‐coupling of N‐heterocyclic carbenes [NHCs; SIPr=C(NArCH 2 ) 2 , 1 ; IPr=C(NArCH) 2 , 2 ; Me‐IPr=C(NArCMe) 2 , 3 ; Ar=2,6‐ i Pr 2 C 6 H 3 ] with 4,4′′‐diiodo‐ p ‐terphenyl under Ni catalysis furnished [(SIPr)(C 6 H 4 ) 3 (SIPr)](I) 2 ( 4 ), [(IPr)(C 6 H 4 ) 3 (IPr)](I) 2 ( 5 ), and [(Me‐IPr)(C 6 H 4 ) 3 (Me‐IPr)](I) 2 ( 6 ). Two‐electron reduction of 4 – 6 with KC 8 readily afforded NHC analogues of Müller's hydrocarbon (MH), [(SIPr)(C 6 H 4 ) 3 (SIPr)] ( 7 ), [(IPr)(C 6 H 4 ) 3 (IPr)] ( 8 ), and [(Me‐IPr)(C 6 H 4 ) 3 (Me‐IPr)] ( 9 ), respectively, as highly colored crystalline solids. Quantum chemical calculations suggested that the singlet ground state for 7 – 9 possesses a vertical singlet–triplet energy gap Δ E S‐T of −7.24 to −7.60 kcal mol −1 , which is significantly lower compared to that of the NHC analogues of Thiele's (TH) and Chichibabin's (CH) (18–38 kcal mol −1 ) hydrocarbons. Importantly, the calculated diradical character ( y ) of 7 – 9 ( y ≈0.6) is considerably higher compared to that of the related TH and CH ( y =0.1–0.4), suggesting the open‐shell singlet character of 7 – 9 .