Steric/π-electronic insulation of the carbo-benzene ring: Dramatic effects of tert-butyl versus phenyl crowns on geometric, chromophoric, redox, and magnetic properties
Résumé
Hexa-tert-butyl-carbo-benzene (C18tBu6) and three phenylated counterparts (C18tBumPh6-m; m=4, 2) have been synthesized. The peralkylated version (m=6) provides exptl. access to intrinsic features of the insulated C18 core independently from the influence of π-conjugated substituent. Over the series, structural, spectroscopic, and electrochem. properties are compared with those of the hexaphenylated ref. (m=0). Anchoring tBu substituents at the C18 macrocycle is shown to enhance stability and soly., 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 solns. of C18tBu6 happen to be yellow (λmax=379 vs. 472 nm for C18Ph6). In comparison to C18Ph6, the redn. of C18tBu6 remains reversible, but occurs at twice as high an abs. potential (E1/2=-1.36 vs. -0.72 V). Systematic XRD analyses and DFT calcns. show that the C18 ring symmetry is the nearest to D6h for m=6, which indicates a max. geometric aromaticity. According to calcd. nucleus-independent chem. shifts (NICS), the macrocyclic magnetic aromaticity is also max. for C18tBu6: NICS(0)=-17.2 ppm vs. (-18.0±0.1) ppm for the theor. refs. C18H6 and C18F6, and -13.5 ppm for C18Ph6. Accurate correlations of NICS(0) with exptl. recorded or calcd. max. UV/Vis absorption wavelengths, λmax, and chem. hardness, η=ELUMO-EHOMO, are evidenced.