5,20-Di(pyridin-2-yl)-[28]hexaphyrin(1.1.1.1.1.1): A Stable H眉ckel Antiaromatic Hexaphyrin Stabilized by Intramolecular Hydrogen Bonding and Protonation-Induced Conformational Twist To Gain M枚bius Aro
文摘
5,20-Di(pyridin-2-yl)-[28]hexaphyrin(1.1.1.1.1.1) 7 was prepared and characterized as a stable H眉ckel 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 H眉ckel 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 M枚bius 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.
