Metal-Organic Anion Receptors: Arranging Urea Hydrogen-Bond Donors to Encapsulate Sulfate Ions

摘要

Selective synthetic receptors for the sulfate anion are relatively rare. Pflugrath and Quiocho's crystal structure of sulfate bound within the sulfate binding protein revealed that Nature uses seven hydrogen bonds from neutral donor groups to selectively complex the anion within the protein. Synthetic receptors for anions frequently incorporate a combination of (i) hydrogen bond donor groups, (ii) a positively charged component for effective electrostatic interactions, and (iii) a suitable framework onto which these structural components can be assembled. Strategies to bind sulfate have followed these general principles and have involved the formation of multiple hydrogen bonds between the anion and receptor, resulting in strong complexation either by neutral receptors or by receptors that employ hydrogen bonds in combination with electrostatic interactions.