Anion and Cation Complexes of Expanded Porphyrins

The successful isolation of fully conjugated polypyrrolic macrocycles with peripheries containing more than 18 pi-electrons led to a new class of azaannulenes. These systems, referred to as expanded porphyrins, have been found to possess unexpected electronic features, display multiple redox states, and display dynamic aromaticity and antiaromaticity characteristics, as well as interesting reactivity. The large cavities found in many expanded porphyrins have made them excellent systems for both metal cation coordination and anion binding. Often, they display recognition chemistry that differs dramatic from their smaller porphyrin congeners. This has made expanded porphyrins of both academic interest and potential utility in the development of more efficient extractants, sensors, and smart electronic devices. Here, the cation complexation chemistry of representative expanded porphyrins containing more than four pyrrole (or pyrrole-like) subunits is presented. Also reviewed are the anion binding properties of several well-studied protonated oligopyrrolic macrocycles.