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

Abstract: Hexa‐tert‐butyl‐carbo‐benzene (C₁₈tBu₆) and three phenylated counterparts (C₁₈tBu_mPh_6−m; m=4, 2) have been synthesized. The peralkylated version (m=6) provides experimental access to intrinsic features of the insulated C₁₈ 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 tBu substituents at the C₁₈ 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 C₁₈tBu₆ happen to be yellow (λ_max=379 vs. 472 nm for C₁₈Ph₆). In comparison to C₁₈Ph₆, the reduction of C₁₈tBu₆ 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 C₁₈ ring symmetry is the nearest to D_6h 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 C₁₈tBu₆: NICS(0)=−17.2 ppm versus (−18.0±0.1) ppm for the theoretical references C₁₈H₆ and C₁₈F₆, and −13.5 ppm for C₁₈Ph₆. 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. [ABSTRACT FROM AUTHOR]