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-1 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 103-106-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-1 coordination to the S atom of the thiourea derivative 2 promotes the formation of unusually stable H-bond complexes with Cl- and Br-.