Influence of the Number and Geometry of Binding Sites on Host–Guest Affinity: Imidazolium‐Substituted Receptor Molecules for Small Inorganic Anions

The influence of the number and relative geometry of the binding sites on the binding of spherical and tetragonal inorganic anions (Cl – , Br – , H 2 PO 4 – , and HSO 4 – ) has been studied by using imidazolium salts based on benzene and calix[4]arenes. Binding constants in DMSO were found to be in the range of 200–2000 L mol –1 . Three or four binding sites ( 2b – 4b ) lead to the nonselective binding of all anions indicating the decisive influence of the number of possible binding positions; binding constants of approximately 2000 for H 2 PO 4 – , 1000 for HSO 4 – , 900 for Cl – , and 800 L mol –1 for Br – were obtained. Benzene‐ ( 1b ) and calixarene‐based ( 5b ) bis(imidazolium) salts exhibited a high selectivity towards H 2 PO 4 – /HSO 4 – and Cl – /Br – indicating that for the complexation of H 2 PO 4 – and Cl – two binding sites are necessary, and for Br – and HSO 4 – at least three. In this case, selectivity could be obtained by simple variation of the number of identical binding positions.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)