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

Abstract

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.