Core modified meso-aryl sapphyrins and rubyrins: structural and anion receptor properties

Abstract

Studies on structural characterisation and anion binding properties of a series of core modified meso-aryl sapphyrins and rubyrins are described. It has been shown that the sapphyrins and rubyrins bind anions such as F^-, N₃^- and CO₃^2- in their protonated form. The binding constants evaluated for a particular sapphyrin vary in the order F^- ≅ N₃^- < CO₃^2- and this has been accounted for in terms of compatibility of the cavity size of the sapphyrin, the anion size and the complete charge neutralisation. However, for the protonated rubyrins, this order is reversed because of the larger cavity sizes of the rubyrins relative to the sapphyrins. A comparison of the magnitude of binding constants with those of β-substituted N5 sapphyrins indicates a decrease of several orders of magnitude because of the availability of fewer hydrogen bonding sites for the core modified meso-aryl sapphyrins reported here. Furthermore, the single crystal X-ray structures of two sapphyrins reveal the inversion of the heterocyclic ring opposite to the bithiophene/biselenophene unit, while the rubyrins show planar structures.