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.