Trapping of Stable [4n+1] π‐Electron Species from Peripherally Substituted, Conformationally Rigid, Antiaromatic Hexaphyrins

• Published in: Chemistry : a European journal 2019-03, Vol.25 (14), p.3525-3531
• Main Authors: Firmansyah, Dikhi, Hong, Seong‐Jin, Dutta, Ranjan, He, Qing, Bae, Jinhee, Jo, Hongil, Kim, Hakwon, Ok, Kang Min, Lynch, Vincent M., Byon, Hye Ryung, Sessler, Jonathan L., Lee, Chang‐Hee
• Format: Article
• Language: English
• Subjects: Aldehydes; antiaromatic; Aromatic compounds; Chemical synthesis; Chemistry; Crystal structure; Electrons; expanded porphyrins; Fluorine; naphthorosarin; peripheral substitution; redox potential; Species; stable radical cation (25pi); Substitutes; X-ray diffraction
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201900022

Peripherally substituted antiaromatic naphthorosarins have been synthesized for the first time. The synthesis was accomplished by acid‐catalyzed condensation of naphthobipyrrole building blocks with aromatic aldehydes. The naphthobipyrrole building blocks were synthesized by simple oxidative coupling of the corresponding pyrrole substituted aromatics. Solid‐state structural analyses of the synthesized naphthorosarins revealed that the presence of meso‐2,6‐dichlorophenyl‐ and 5,6‐difluoro‐substitution substantially alter the geometry and properties of the naphthorosarins. The substituents affect the redox potentials as well and, in turn, the proton‐coupled electron‐transfer processes leading to the formation of one‐ and two‐electron reduced forms of the corresponding naphthorosarins. One particular naphthorosarin that bears both peripheral fluorine and meso‐2,6‐dichlorophenyl substituents forms a stable 25 π‐electron species upon treating with TFA that was characterized by single‐crystal X‐ray diffraction analysis. The current study underscores how structural modifications can be used to fine‐tune the electronic features of naphthorosarins, including stabilization of odd electron species. Porphyrinoids: Peripherally substituted antiaromatic naphthorosarins were synthesized in improved yields. The effect of changes in the electronic structure were particularly manifest in the rates of proton‐coupled electron transfer leading to the corresponding reduced forms. This permitted the one‐electron reduced, [4n+1] π‐electron form of one rosarin to be isolated and characterized (see figure).