Diversity of binding of sulfate and nitrate anions with laterally asymmetric aza cryptands

Abstract

An X-ray crystallographic study of binding of nitrate and sulfate anions with two laterally asymmetric aza cryptands (L_o and L_m) having different cavity dimensions is reported. A variety of binding modes for these anions have been observed. Cryptand L_m having a larger cavity than that of L_o, prefers to encapsulate a planar nitrate over Td sulfate in its cavity when both anions are available for complexation. A discrete (H₂O)₂₉ cluster can be identified as surrounded by six sulfate ions attached to two L_m units. ¹H NMR titration results indicate that either [H₃L_o]³⁺ or [H₃L_m]³⁺ binds strongly with sulfate rather than nitrate in solution.