Hückel and Möbius Expanded para-Benziporphyrins: Synthesis and Aromaticity Switching

Szyszko, B.; Sprutta, N.; Chwalisz, P.; Stępień, M.; Latos-Grażyński, L.
Chem. Eur. J. 2014, 20, 1985-1997 | view at publisher's site

Four expanded p-benziporphyrins: A,C-di-p-benzi[24]­pen­ta­phy­rin(, N-fused A-p-benzi[24]pentaphyrin, A,D-di-p-benzi­[28]­hexaphyrin(, and A,C-di-p-benzi[28]hexaphyrin( were obtained in three component Lindsey-type macrocyclizations. These compounds were explored as macrocyclic ligands and as potential aromaticity switches. A BODIPY-like difluoroboron complex was obtained from A,C-di-p-benzi[24]pentaphyrin, whereas A,C-di-p-benzi[28]hexaphyrin yielded a Möbius-aromatic Pd(II) complex containing fused pyrrole and phenylene subunits. Conformational behaviour, tautomerism, and acid-base chemistry of new macrocycles were characterized by means of nuclear magnetic resonance spectroscopy and DFT calculations. Free base N-fused A-p-benzi[24]pentaphyrin showed temperature dependent Hückel-Möbius aromaticity switching, whereas A,C-di-p-benzi[28]hexaphyrin formed a Möbius aromatic dication.