Calix[6]pyrrole and Hybrid Calix[n]furan[m]pyrroles (n+m=6): Syntheses and Host-Guest Chemistry

Calix[6]pyrrole 2 and the “hybrid systems” calix[3]furan[3]pyrrole 12, calix[2]furan[4]pyrrole 13, and calix[1]furan[5]pyrrole 14, have been synthesized by increasing conversion of the furan units present in the readily accessible calix[6]furan 3 to pyrroles. The host–guest chemistry of these novel macrocycles towards a number of anions, including halogen ions, dihydrogen phosphate, hydrogen sulfate, nitrate, and cyanide has been investigated in solution by 1H NMR titration techniques and/or phase transfer experiments. The solid‐state structures of the free receptors 2, 12, and 13, the 1:1 complexes of calix[6]pyrrole 2 with chloride and bromide, and the 1:1 complex of 14 with chloride are also reported. A number of “hybrid” calix[n]furan[m]pyrrole analogues (m + n=6) and calix[6]pyrrole (1) have been synthesized from calix[6]furan 2 (see picture). Their host–guest chemistry towards a number of anions (F−, Cl−, Br−, I−, H2PO4−, HSO4−, NO3−, and CN−) has been investigated in solution by NMR spectroscopy and, where possible, in the solid‐state by X‐ray crystallography. The association constants for many of the macrocycles with the screened anions have also been determined.