Steric/π‐Electronic Insulation of the carbo ‐Benzene Ring: Dramatic Effects of tert ‐Butyl versus Phenyl Crowns on Geometric, Chromophoric, Redox, and Magnetic Properties

Hexa‐ tert ‐butyl‐ carbo ‐benzene (C 18 t Bu 6 ) and three phenylated counterparts (C 18 t Bu m Ph 6− m ; m =4, 2) have been synthesized. The peralkylated version ( m =6) provides experimental access to intrinsic features of the insulated C 18 core independently from the influence of π‐conjugated substituent. Over the series, structural, spectroscopic, and electrochemical properties are compared with those of the hexaphenylated reference ( m =0). Anchoring t Bu substituents at the C 18 macrocycle is shown to enhance stability and solubility, and to dramatically modify UV/Vis absorption and redox properties. Whereas all carbo ‐benzenes reported previously were obtained as dark‐reddish/greenish solids, crystals and solutions of C 18 t Bu 6 happen to be yellow ( λ max =379 vs. 472 nm for C 18 Ph 6 ). In comparison to C 18 Ph 6 , the reduction of C 18 t Bu 6 remains reversible, but occurs at twice as high an absolute potential ( E 1/2 =−1.36 vs. −0.72 V). Systematic XRD analyses and DFT calculations show that the C 18 ring symmetry is the nearest to D 6 h for m =6, which indicates a maximum geometric aromaticity. According to calculated nucleus‐independent chemical shifts (NICS), the macrocyclic magnetic aromaticity is also maximum for C 18 t Bu 6 : NICS(0)=−17.2 ppm versus (−18.0±0.1) ppm for the theoretical references C 18 H 6 and C 18 F 6 , and −13.5 ppm for C 18 Ph 6 . Accurate correlations of NICS(0) with experimentally recorded or calculated maximum UV/Vis absorption wavelengths, λ max , and chemical hardness, η = E LUMO − E HOMO , are evidenced.