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 1 H NMR titrations in DMSO- d 6 with 0.5% water reveal enhanced selectivity for H 2 PO 4 - vs Cl - and PhCO 2 - . The para -attachment of the electron-withdrawing nitro group (-NO 2 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 H 2 PO 4 - is associated with minimal conformational change in the lariat arm moiety and involve four hydrogen bond acceptor and one donor sites of host.