New core-expanded calix[4]pyrroles and N-donor-extended dipyrromethanes: syntheses, anion binding and conformational...

A series of dipyrromethane-based N-donor-extended dipodal receptors (2a-c) and meso-(1,3)-modified expanded calix[4]pyrroles (3a-c) were synthesized via Mannich reactions. While the two non-equivalent meso-substituents at the dipyrromethane unit(s) in both the acyclic and macrocyclic Mannich bases induced diastereotopicity of the methylene (-CH₂-) protons, the two equivalent meso-substituents made them homotopic. The anion binding studies of a representative macrocycle (3c) and an acyclic Mannich base (2c), carried out via UV-vis titration experiments in acetonitrile, showed that the azamacrocycle binds halides (F^-, Cl^-, Br^-, and I^-) and two other oxyanions (HSO₄^- and ClO₄^-) in a 1 : 1 (H : G) stoichiometric ratio, whereas the acyclic receptor binds the anions in a 2 : 1 (H : G) model, except for HSO₄^-, which follows a 1 : 2 stoichiometry in solution. The X-ray structures indicated an anion-induced conformational change upon binding monoanionic PhCOO^- ions with the diprotonated azamacrocycle ([3aH₂]²⁺), in which the 1,3-alternate form of the pyrrole-ring-conformation converts to the 1,2-alternate orientation. Conversely, the cone conformation of the aryl-extended azamacrocycle ([3bH₂]²⁺) remained unchanged upon binding with the dianionic SiF₆^2- ion. Interestingly, the conformational preferences between 1,3-alternate, 1,2-alternate, cone, and partial cone arrangements of the azamacrocycles depend on the meso-substituents at the dipyrromethane unit(s) and were studied by DFT calculations.