Synthesis and structural characterization of anion complexes with azacalix[2]dipyrrolylmethane: effect of anion charge on the conformation of the macrocycle

Abstract

Tetrahomodiazacalix[2]dipyrrolylmethane 1, [–CH₂(C₄H₃N)CR₂(C₄H₃N)CH₂N(Me)–]₂ an expanded version of the calix[4]pyrrole system, has the tendency to change its ring conformation (1,3-alternate) upon anion binding analogous to calix[4]pyrrole. However, owing to its tertiary amine nitrogen atoms in the ring, it can be protonated and becomes a powerful cationic receptor for anions, besides its inherent hydrogen bonding features. Macrocycle 1 binds with a series of monoanions BF₄⁻, Cl⁻, PhCOO⁻ and ClO₄⁻, and their X-ray structures showed that the ring conformation constitutes the 1,2-alternate form. Upon binding with dianions SO₄²⁻, CrO₄²⁻, SiF₆²⁻ and S₂O₃²⁻, the ring conformation changes to the cone conformation. The intermediate partial cone conformation results for complexes with NO₃⁻ and Cr₂O₇²⁻ ions. The change in the orientation of the pyrrole NH groups depending on the charge of the anion demonstrates the flexibility of the macrocycle and the dicationic macrocycle stabilizes the anions via both hydrogen bonding and electrostatic interactions.