Cross-bridging reaction of 5,20-diethynyl substituted hexaphyrins to vinylene-bridged hexaphyrins

5,20-Diethynyl substituted [26]hexaphyrins(1.1.1.1.1.1) undergo a thermal or spontaneous crossbridging eaction to provide trans-vinylene-bridged [26]hexaphyrins, probably through strongly folded conformations. Facile aromatic-antiaromatic switching of vinylene-bridged hexaphyrins has been demonstrated upon two-electron redox processes. Curiously, a resonance contributor of the vinylenebridged hexaphyrins is regarded as [16]diazaannuleno[16]diazaannulene but its contribution should be minor because of the perpendicular geometry of the bridge. Vinylene-bridged hexaphyrin serves as an effective ligand that coordinates two zinc(II) ions using the central vinylene double bond in a 2-fashion along with the three pyrrolic nitrogen atoms.