Synthesis and anion binding properties of m-diethynylbenzene expanded calix[4]pyrrole

A new expanded calix[4]pyrrole, calix[2]diethynylbenzene[4]pyrrole (3), was synthesized from a bispyrrole intermediate (1,3-bis(3-methyl-3-(1H-pyrrol-2-yl)but-1-ynyl)benzene). The anion binding properties of calixpyrrole 3 were also investigated. Calixpyrrole 3 was found to bind dihydrogen phosphate (H2PO4-) anions better than others. The structure of 3H2PO4- was optimized using semiempirical PM3 calculations. The optimized structure indicates that the weak anion binding properties of calixpyrrole 3 could be attributed to the presence of dimethylmethylene (meso position) and rigid ethynyl groups that block the facile approach of other parts of hydrogen-bonding motifs.