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(NArCH2)2, 1; IPr=C(NArCH)2, 2; Me‐IPr=C(NArCMe)2, 3; Ar=2,6‐iPr2C6H3] with 4,4′′‐diiodo‐p‐terphenyl under Ni catalysis furnished [(SIPr)(C6H4)3(SIPr)](I)2 (4), [(IPr)(C6H4)3(IPr)](I)2 (5), and [(Me‐IPr)(C6H4)3(Me‐IPr)](I)2 (6). Two‐electron reduction of 4–6 with KC8 readily afforded NHC analogues of Müller's hydrocarbon (MH), [(SIPr)(C6H4)3(SIPr)] (7), [(IPr)(C6H4)3(IPr)] (8), and [(Me‐IPr)(C6H4)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 ΔES‐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. Radical taming by NHCs. Crystalline N‐heterocyclic carbene (NHC) analogues of the highly reactive Müller's hydrocarbon (A), [(NHC)(C6H4)3(NHC)] (B) are reported [NHC=C(NArCH2)2, SIPr; C(NArCH)2, IPr; C(NArCMe)2, Me‐IPr; Ar=2,6‐iPr2C6H3]. They feature a singlet ground state with small singlet–triplet energy gaps (ΔES‐T≈7.4 kcal mol−1) along with an intermediate diradical character (y=65 %).