engleski

Thermodynamic study of anion complexation by urea-, thiourea- and amide- based receptors in solution

Although chemistry of anion recognition and complexation has developed into an important field of supramolecular chemistry, peak selectivity and high binding affinity are rarely reported, especially in competitive media. In order to accomplish these goals it is necessary to expand the understanding of the anion binding reaction by identifying all interactions that contribute to the stabilization of the complexes and gathering information regarding the structure of the complexes. In this work, the complexation of anions by receptors based on dehydroacetic acid bearing urea (1), thiourea (2a and 2b), and amide (3a–c) moieties (scheme 1) was investigated in acetonitrile (MeCN) and dimethyl sulfoxide (DMSO). Dihydrogen phosphate complexes of 1:1 and 2:1 (anion:receptor) stoichiometry were formed in MeCN, whereas in all other cases only the 1:1 complexes were detected. 1H NMR was proven to be a very effective technique for the study of anion complexation of the synthesized compounds due to pronounced changes in chemical shift of the (thio)urea and amide NH proton signals. Further on, the subtle changes regarding the signal of the amine proton during the titrations with H2PO4– strongly indicated that the carbonyl oxygen acts as a proton acceptor allowing the formation of additional hydrogen bond. Recent microcalorimetric investigations of H2PO4– dimerisation were expanded using 31P NMR spectroscopy. Gathered data was used in the course of the anion binding investigations. Acetate and dihydrogen phosphate complexes were found to be the most stable ones. which can be attributed to the shape and size complementarity of these anions and the urea moiety.1 Among the series of investigated receptors, the thiourea derivatives showed the highest anion-binding affinity which is most likely a consequence of the highest proton donating affinity / acidity of the thiourea group. It was found that the arrangement of the amide groups in the structure of receptors 3a and 3b plays an important role in the stabilisation of the anion complexes.

anion receptors ; thermodynamics ; stability constant ; solvation ; hydrogen bond

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