Crystalline Anions Based on Classical N‐Heterocyclic Carbenes

Herein, the first stable anions K[SIPrBp] (4 a‐K) and K[IPrBp] (4 b‐K) (SIPrBp=BpC{N(Dipp)CH2}2, IPrBp=BpC{N(Dipp)CH}2; Bp=4‐PhC6H4; Dipp=2,6‐iPr2C6H3) 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 KC8 reduction of the neutral radicals [SIPrBp] (3 a) and [IPrBp] (3 b), respectively. The radicals 3 a and 3 b as well as [Me‐IPrBp] 3 c (Me‐IPrBp=BpC{N(Dipp)CMe}2) are accessible as crystalline solids on treatment of the respective 1,3‐imidazoli(ni)um bromides (SIPrBp)Br (2 a), (IPrBp)Br (2 b), and (Me−IPrBp)Br (2 c) with KC8. 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. One‐electron reduction of the C2‐biphenylated 1,3‐imidazolinium cation (2 a+) affords the radical 3 a, which undergoes further 1e‐reduction to yield the anion 4 a−. In the solid‐state, 3 a is monomeric while 4 a‐K has a hexameric tubular structure. In addition to 3 a and 4 a‐K, synthesis, structures, and reactivity of stable radicals and anions based on unsaturated N‐heterocyclic carbene frameworks have been presented.