Cross-conjugated hexaphyrins and their bis-rhodium complexes

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. A good circuit: A cross-conjugated hexaphyrin that carries two meso-oxacyclohexadienyl (OCH) groups was prepared and reduced to a Möbius aromatic [28]hexaphyrin. Bis-rhodium complexes of this system can take three different electronic states, one of which bears both a phenol and an OCH group, thus displaying a moderate diatropic ring current and probably reflecting a significant contribution of an aromatic zwitterionic resonance hybrid (see figure).