5,20-Di(pyridin-2-yl)-[28]hexaphyrin(1.1.1.1.1.1): A Stable Hückel Antiaromatic Hexaphyrin Stabilized by Intramolecular Hydrogen Bonding and Protonation-Induced Conformational Twist To Gain Möbius Aromaticity

5,20-Di­(pyridin-2-yl)-[28]­hexaphyrin­(1.1.1.1.1.1) 7 was prepared and characterized as a stable Hückel
antiaromatic
molecule with a dumbbell-like structure stabilized by effective intramolecular
hydrogen bonding interactions involving the 2-pyridyl nitrogen atoms.
Pd­(II) metalation of 7 afforded two bis-Pd­(II) complexes, 9-syn and 9-anti, whose structures are rigidly
held by Pd­(II) coordination, rendering 9-syn to be nonaromatic
because of its highly distorted structure and 9-anti to
be Hückel antiaromatic because of its enforced planar dumbbell
structure. In contrast, protonation of 7 with methanesulfonic
acid (MSA) led to the formation of its triprotonated species 7H3, which has been shown to take
on twisted conformations with Möbius aromaticity in CH2Cl2, while the structure was held to be a planar
rectangular conformation in the crystal. Excited-state dynamics were
measured for 7, 7H3, 9-syn, and 9-anti, which indicated their
electronic nature to be antiaromatic, aromatic, nonaromatic, and antiaromatic,
respectively.