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 tBu ) and three phenylated counterparts (C tBu Ph ; m=4, 2) have been synthesized. The peralkylated version (m=6) provides experimental access to intrinsic features of the insulated C 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 C 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 tBu happen to be yellow (λ =379 vs. 472 nm for C Ph ). In comparison to C Ph , the reduction of C tBu remains reversible, but occurs at twice as high an absolute potential (E =-1.36 vs. -0.72 V). Systematic XRD analyses and DFT calculations show that the C ring symmetry is the nearest to D 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 tBu : NICS(0)=-17.2 ppm versus (-18.0±0.1) ppm for the theoretical references C H and C F , and -13.5 ppm for C Ph . Accurate correlations of NICS(0) with experimentally recorded or calculated maximum UV/Vis absorption wavelengths, λ , and chemical hardness, η=E -E , are evidenced.