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Tuning Anion-Binding Properties of 22-Membered Unclosed Cryptands by Structural Modification of the Lariat Arm
Kajetan Dąbrowa1, Patryk Niedbała1, Marcin Pawlak1
1Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland.
Abstract:
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-d 6 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.
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