Crystalline Anions Based on Classical N‐Heterocyclic Carbenes

Herein, the first stable anions K[SIPr(Bp)] (4 a‐K) and K[IPr(Bp)] (4 b‐K) (SIPr(Bp)=BpC{N(Dipp)CH(2)}(2), IPr(Bp)=BpC{N(Dipp)CH}(2); Bp=4‐PhC(6)H(4); Dipp=2,6‐iPr(2)C(6)H(3)) derived from classical N‐heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a‐K and 4 b‐K are prepared by KC(8) reduction of the neutral radicals [SIPr(Bp)] (3 a) and [IPr(Bp)] (3 b), respectively. The radicals 3 a and 3 b as well as [Me‐IPr(Bp)] 3 c (Me‐IPr(Bp)=BpC{N(Dipp)CMe}(2)) are accessible as crystalline solids on treatment of the respective 1,3‐imidazoli(ni)um bromides (SIPr(Bp))Br (2 a), (IPr(Bp))Br (2 b), and (Me−IPr(Bp))Br (2 c) with KC(8). The cyclic voltammograms of 2 a–2 c exhibit two one‐electron reversible redox processes in −0.5 to −2.5 V region that correspond to the radicals 3 a–3 c and the anions (4 a–4 c)(−). Computational calculations suggest a closed‐shell singlet ground state for (4 a–4 c)(−) with the singlet‐triplet energy gap of 17–24 kcal mol(−1).