Metal-enhanced H-bond donor tendencies of urea and thiourea toward anions: ditopic receptors for silver(I) salts.

The heteroditopic receptors 1 and 2 firmly include an Ag(I) ion into the NS2O2 crown and establish selective H-bond interactions with anions at the covalently linked urea/thiourea (U/T) subunit. Studies in MeCN solution showed that the metal center induces a 10(3)-10(6)-fold enhancement of the anion association constant, due to a coordinative interaction with the O/S atom of the U/T moiety. In the presence of F-, the Ag-O interaction promotes the release of a proton from one of the urea N-H fragments. Ag(I) coordination to the S atom of the thiourea derivative 2 promotes the formation of unusually stable H-bond complexes with Cl- and Br-.