Metal-organic Anion Receptors: Arranging Urea Hydrogen-bond Donors to Encapsulate Sulfate Ions

A new class of synthetic receptors for anions can be prepared by arranging urea hydrogen-bond donor groups on a simple metal-organic scaffold. The complex cation [PtL4]2+ (L = 8-(n-butylurea)iso-quinoline) can adopt four conformations reminiscent of calix[4]arene-based receptors; "cone", "partial cone", "1,2-alternate", or "1,3-alternate". 1H NMR solution data and solid-state X-ray structures show that a "1,2-alternate" conformation is used to bind spherical halide ions while a "cone" conformation is involved in strong binding with the tetrahedral oxy-anions such as the sulfate ion; even in a strongly competitive solvent such as DMSO.