title: Cross-conjugated hexaphyrins and their bis-rhodium complexes.
abstract: A cross-conjugated hexaphyrin that carries two meso-oxacyclohexadienylidenyl (OCH) groups 9 was synthesized from the condensation of 5,10-bis(pentafluorophenyl)tripyrrane with 3,5-di-tert-butyl-4-hydroxybenzaldehyde. The reduction of 9 with NaBH4 afforded the Möbius aromatic [28]hexaphyrin 10. Bis-rhodium complex 11, prepared from the reaction of 10 with [{RhCl(CO)2}2], displays strong Hückel antiaromatic character because of the 28 π electrons that occupy the conjugated circuit on the enforced planar structure. The oxidation of 11 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) yielded complexes 12 and 13 depending upon the reaction conditions. Both 12 and 13 are planar owing to bis-rhodium metalation. Although complex 12 bears two meso-OCH groups at the long sides and is quinonoidal and nonaromatic in nature, complex 13 bears 3,5-di-tert-butyl-4-hydroxyphenyl and OCH groups and exhibits a moderate diatropic ring current despite its cross-conjugated electronic circuit. The diatropic ring current increases upon increasing the solvent polarity, most likely due to an increased contribution of an aromatic zwitterionic resonance hybrid.
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan).
authors.forename: Koji
authors.initials: K
authors.lastname: Naoda
authors.forename: Young Mo
authors.initials: YM
authors.lastname: Sung
authors.forename: Jong Min
authors.initials: JM
authors.lastname: Lim
authors.forename: Dongho
authors.initials: D
authors.lastname: Kim
authors.forename: Atsuhiro
authors.initials: A
authors.lastname: Osuka
keywords: aromaticity; cross conjugation; expanded porphyrins; hexaphyrin; porphyrinoids