Synthesis of the receptor L and anion complexes 1 and 2.
A neutral tris(ferrocenylurea) receptor (L; Scheme 1) was synthesized as an anion sensor through optical and electrochemical responses by researchers from Northwest University and Lanzhou Institute of Chemical Physics of the CAS.
The CV studies revealed significant selectivity of the receptor for sulfate and phosphate ions, which induced distinct cathodic shifts of the ferrocene/ferrocenium redox couple in chloroform. UV/Vis and NMR spectroscopic results indicated selectivity for sulfate ions as proved by the increase in the d–d transition band of the receptor L and very large downfield shifts of the urea NH groups upon addition of the sulfate anions. In addition, the crystal structures of two anion half-capsules (TBA)2·[SO4L]·H2O (1) and TBA· [FL] (2) have also been obtained, which facilitated the understanding of the anion-binding properties of L.
There has been growing interest in anion recognition because anions play ubiquitous roles in chemical and biochemical processes. In particular, the binding of sulfate and phosphate ions represents a very important part in this field. However, due to its large standard Gibbs energy of hydration (△G=-1080 kJmol-1) and high charge density according to the Hofmeister effect, sulfate separation remains a challenge. Many efforts have been put into the design of synthetic sulfate receptors, among which tripodal receptors that bear amide, urea, or other functional groups can efficiently bind the sulfate anion because of the favorable conformation for the formation of multiple hydrogen bonds.
Moreover, various signaling groups, such as optically, fluorescence, or electrochemically sensitive units, have been installed on anion receptors to realize the convenient detection of anions. The design of redox-responsive receptors for anions is a lively and attractive topic in this regard. Among the redox-active fragments, ferrocene represents an ideal component because of its unique properties, such strong p-electron donation and reversible oxidation to the radical cation (ferrocenium) in a desirable range.
The researchers are interested in anion binding and have synthesized a series of oligo(pyridylurea)-based receptors. They have reported a tripodal tris (urea) receptor that can selectively encapsulate a sulfate ion with one or two ligands in different second-sphere, hydrogen-bonded networks recently. To enhance the reporting functions of such receptors, they have designed the new anion sensor N,N’,N’’-(nitrilotri-2,1-ethanediyl)tris-(N’-ferrocenylurea) by combining the well defined tripodal tris (urea) scaffold and the redox-active ferrocenyl group for complementary anion binding and signaling purposes, respectively.
The detailed report has been published in Chem. Eur. J. (Chem. Eur. J. 2011, 17, 2272 – 2280).