Exploring anion-induced conformational flexibility and molecular switching in a series of heteroaryl-urea receptors

Anion binding studies of 1,10-phenanthroline- and 2-pyridyl-substituted urea-based receptors reveal that guest-dependent conformations exist in structural variants related to a previously investigated bipyridyl-based receptor. Dynamic conformational switching persists in a monofunctional pyridyl-urea receptor, and the preorganization provided by a phenanthroline-based analogue promotes convergence of anion coordinating groups to a single guest. Despite this predisposition for anion coordination, the conformational flexibility of the bipyridyl-based receptor provides the most selective motif for H(2)PO(4) (–) coordination. Furthermore, the two new phenanthrolyl- and pyridyl-receptors serve as models of the bipyridyl-based receptor, elucidating accurate stepwise association constants for 1 : 2 host/guest binding by this receptor, and suggest that oxoanions prefer the embrace of a “U” conformation in 1 : 1 complexes.