Calix[n]bispyrrolylbenzenes: Synthesis, characterization, and preliminary anion binding studies

Abstract

A series of novel calixpyrrole-like macrocycles, calix[n]bis(pyrrol-2-yl) benzene (calix[n]BPBs, n = 2-4) 9a-11a, have been synthesized by means of the TFA-catalyzed condensation reaction of bis(pyrrol-2-yl)benzene 8 a with acetone. Calix[2]BPB 9a represents an expanded version of calix[4]-pyrrole in which two of the four meso bridges are replaced by benzene rings. By contrast, systems 10a and 11a, which bear great considerable to calix-bipyrroles 2 and 3, represent higher homologues of the basic calix[n]BPB motif. Solution-phase anion binding studies, carried out by means of 1H NMR spectroscopic titrations in [D2]dichloromethane and isothermal titration calorimetry (ITC) in 1,2-dichloroethane, reveal that 9a binds typical small anions with substantially higher affinities than 1, even though the same number of hydrogen bonding donor groups are found in both compounds. The basic building block for 9a, benzene dipyrrole 8a, also displays a higher affinity for anions than the building block for 1, dimethyldipyrro-methane 16. Structural studies, carried out by single-crystal X-ray diffraction analyses, are consistent with the solution-phase results and reveal that 9a is able to stabilize complexes with chloride and nitrate in the solid state. Structures of the PF6- and NO3- complexes of 10a were also solved as were those of the acetone adduct of 9a and the ethyl acetate adduct of 11a. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA.