Pyridinium/urea-based anion receptor: methine formation in the presence of basic anions

Abstract

The influence of the positively charged N-methylpyridinium substituent on the anion binding tendencies of urea-based receptors has been investigated by comparing molecules 1 and 2. These receptors have been studied in acetonitrile, by performing UV-vis. and ¹H NMR titrations with several anions. UV-vis. titrations have also been performed in DMSO, MeOH and CHCl₃/CH₃CN mixture (1/1, v/v). In the case of 1, the presence of both H-donor and H-acceptor groups (urea and pyridine, respectively) favours aggregation and the formation of dimers in the solid state. In solution, this tendency to aggregate reduces affinity for anions with respect to the similar urea-based receptor 3. The methylation of the pyridyl group of 1 leads to the pyridinium-containing receptor 2. The pyridinium positive charge enhances the acidity of urea and increases anion affinity, as evidenced by the comparison of the binding constants. Both receptors (1–2) form stable adducts with all investigated anions. However, in the case of 2, the formation of 1 : 1 adducts with basic anions, such as acetate and fluoride, is followed by a proton transfer process. Quite interestingly, deprotonation does not involve the urea group, thus preserving the 1 : 1 adduct, as demonstrated by the ¹H NMR measurements. In particular, the proton transfer process takes place at the methylene group linking the pyridinium fragment to the receptor's skeleton. ¹H NMR studies indicate the formation of a stable neutral methine species, characterised by the loss of aromaticity by the pyridyl ring. These results open new perspectives in the field of anion recognition, as receptor 2 may by applied to the monitoring of both bound anion (through the urea unit) and excess anion in solution (through the development of the yellow methine species).