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

Dymytrii Listunov; Carine Duhayon; Albert Poater; Serge Mazères; Alix Saquet; Valérie Maraval; Remi Chauvin

doi:10.1002/chem.201800835

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

Chemistry. 2018 Jul 25;24(42):10699-10710. doi: 10.1002/chem.201800835. Epub 2018 Jun 25.

Authors

Dymytrii Listunov^{1

2}, Carine Duhayon^{1

2}, Albert Poater³, Serge Mazères⁴, Alix Saquet^{1

2}, Valérie Maraval^{1

2}, Remi Chauvin^{1

2}

Affiliations

¹ CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, 31077, Toulouse Cedex 4, France.
² Université de Toulouse, UPS, ICT-FR 2599, 118 route de Narbonne, 31062, Toulouse Cedex 9, France.
³ Institut de Química Computacional i Catàlisi, Departament de Química, Universitat de Girona, Campus Montilivi, 17071, Girona, Catalonia, Spain.
⁴ UMR CNRS 5089, IPBS (Institut de Pharmacologie et de Biologie Structurale), 205 route de Narbonne, 31077, Toulouse Cedex, France.

PMID: 29653465
DOI: 10.1002/chem.201800835

Abstract

Hexa-tert-butyl-carbo-benzene (C₁₈ tBu₆ ) and three phenylated counterparts (C₁₈ tBu_m Ph_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 C₁₈ tBu₆ happen to be yellow (λ_max =379 vs. 472 nm for C₁₈ Ph₆ ). In comparison to C₁₈ Ph₆ , the reduction of C₁₈ tBu₆ 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 C₁₈ tBu₆ : NICS(0)=-17.2 ppm versus (-18.0±0.1) ppm for the theoretical references C₁₈ H₆ and C₁₈ F₆ , and -13.5 ppm for C₁₈ Ph₆ . 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.

Keywords: aromaticity; carbo-benzenes; chromophores; steric hindrance; substituent effects.