Calixphyrins. Hybrid macrocycles at the structural crossroads between porphyrins and calixpyrroles

Calixphyrins are a class of hybrid molecules that lie at the structural cro ssroads between porphyrins and calixpyrroles. Porphyrins, long known for th eir versatile metal cation coordination chemistry, are macrocycles that con tain only sp(2)-hybridized bridging meso carbon atoms within their framewor k. Calix[n]pyrroles, on the other hand, are porphyrin analogs that contain pyrroles bridged exclusively by sp(3) meso carbon centers, and in recent ye ars have been shown to display remarkable anion-binding properties. Calix[n ]phyrins bear analogy to both the porphyrins and calixpyrroles and are macr ocyclic analogs that contain a mixture of sp(2)- and sp(3)-hybridized meso carbon bridges. This leads to partial interruptions in the conjugation path way of the molecule, introduces novel structural features, and leads to int eresting anion and cation recognition properties. It also allows for modula r syntheses. In the present paper, the chemistry of calix[n]phyrins, still at an early stage of exploration, is reviewed.