Calix[ n ]bispyrrolylbenzenes: Synthesis, Characterization, and Preliminary Anion Binding Studies

A series of novel calixpyrrole‐like macrocycles, calix[ n ]bis(pyrrol‐2‐yl)benzene (calix[ n ]BPBs, n =2–4) 9 a – 11 a , have been synthesized by means of the TFA‐catalyzed condensation reaction of bis(pyrrol‐2‐yl)benzene 8 a with acetone. Calix[2]BPB 9 a represents an expanded version of calix[4]pyrrole in which two of the four meso bridges are replaced by benzene rings. By contrast, systems 10 a and 11 a , which bear great considerable to calixbipyrroles 2 and 3 , represent higher homologues of the basic calix[ n ]BPB motif. Solution‐phase anion binding studies, carried out by means of 1 H NMR spectroscopic titrations in [D 2 ]dichloromethane and isothermal titration calorimetry (ITC) in 1,2‐dichloroethane, reveal that 9 a 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 9 a , benzene dipyrrole 8 a , also displays a higher affinity for anions than the building block for 1 , dimethyldipyrromethane 16 . Structural studies, carried out by single‐crystal X‐ray diffraction analyses, are consistent with the solution‐phase results and reveal that 9 a is able to stabilize complexes with chloride and nitrate in the solid state. Structures of the PF 6 − and NO 3 − complexes of 10 a were also solved as were those of the acetone adduct of 9 a and the ethyl acetate adduct of 11 a .