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-iPr 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 ΔE _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.
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