Tuning Anion-Binding Properties of 22-Membered Unclosed Cryptands by Structural Modification of the Lariat Arm

A series of 22-membered unclosed cryptands end-capped with intra-annular methyl (1), phenyl (2), p-tert-butylphenyl (3), and p-nitrophenyl (4) amide substituents (lariat arm) were synthesized to elucidate the effect of steric and electronic factors on their anion recognition behavior. The 1H NMR titrations in DMSO-d6 with 0.5% water reveal enhanced selectivity for H2PO4– vs Cl– and PhCO2–. The para-attachment of the electron-withdrawing nitro group (−NO2 vs −H and −t-Bu) was found to increase anion-binding affinity, whereas the steric bulkiness of lariat arm (methyl vs aryl) has a marginal effect. DFT calculations reveal that binding of H2PO4– is associated with minimal conformational change in the lariat arm moiety and involve four hydrogen bond acceptor and one donor sites of host.