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 7H(3), which has been shown to take on twisted conformations with Möbius aromaticity in CH(2)Cl(2), while the structure was held to be a planar rectangular conformation in the crystal. Excited-state dynamics were measured for 7, 7H(3), 9-syn, and 9-anti, which indicated their electronic nature to be antiaromatic, aromatic, nonaromatic, and antiaromatic, respectively.