NHC‐Terphenyl Radicals and Anions: Tuning Stability and Redox Properties via Substituent Patterning

Herein, we report the influence of C2-terphenyl substitution patterns (i.e., p-terphenyl versus m-terphenyl) on the redox behavior and stability of the corresponding radicals and anions derived from N-heterocyclic carbenes (NHCs). Three well-known NHCs; SIPr (1a), IPr (1b), and Me-IPr (1c) (SIPr = C{N(Dipp)CH₂}₂, IPr = C{N(Dipp)CH}₂; Me-IPr = C{N(Dipp)CCH₃}₂; Dipp = 2,6-iPr₂C₆H₃); were functionalized at the C2 position using 4-bromo-p-terphenyl (p-TerBr) and 5'-bromo-m-terphenyl (m-TerBr) under nickel catalysis, yielding the corresponding cations [(NHC)p-Ter]Br (2a-c) and [(NHC)m-Ter]Br (3a-c), respectively. Cyclic voltammetry (CV) measurements of 2a-c reveal two distinct reversible redox events, while 3a-c exhibit one reversible and one irreversible or quasi-reversible wave. Reduction of 2a-c and 3a-c with KC₈ readily affords stable radicals [(NHC)p-Ter]^● (4a-c) and [(NHC-m-Ter]^● (5a-c), isolated as crystalline solids. Further reduction of 4a-c produces diamagnetic anions [(NHC)p-Ter]K (6a-c-K), consistent with the electrochemical data. In contrast, 5b and 5c are unreactive toward KC₈ under similar conditions, while 5a (derived from the more electrophilic NHC 1a) can be reduced to the corresponding anion [(SIPr)m-Ter]K (7a-K). Selected compounds have been characterized by spectroscopic techniques and single-crystal X-ray diffraction, with computational studies supporting the experimental findings. The results highlight how the NHC and the C2-terphenyl substituent influence the properties and stability of the resulting species.