Alkali Metal Ions As Probes of Structure and Recognition Properties of Macrocyclic Pyridyl Urea Hosts

We report the one-pot synthesis of three symmetrical macrocyclic pyridyl urea hosts. X-ray crystal studies were used to confirm the structures of the free hosts and their host·guest complexes with alkali metal ions. These solid-state studies revealed the interactions that were important for binding cations (Li+, Na+, and K+). The affinity of these hosts for alkali metal salts were evaluated in solution (CD3CN), and the stoichiometries of the solution complexes were compared with their solid-state structures. Two of these hosts showed high affinity for LiBF4, which was primarily due to strong interactions between the urea oxygens and the cations with pyridine nitrogens contributing additional stabilizing interactions.