Halide ion effect on the ¹H NMR chemical shifts of the residual protons in commonly employed deuterated solvents with tetra-<i>n</i>-butylammonium chloride – Part 2

<div><p>Many ¹H NMR spectroscopic studies involving supramolecular binding of tetra-<i>n</i>-butylammonium halides (TBAX) with a variety of molecular receptors have been reported to date. Previously we demonstrated that the reference residual proton signal of the deuterochloroform solvent itself in TBAX solutions shifted downfield in a linear TBAX concentration-dependent relationship. We now report that a similar downfield chemical shift behaviour of the residual protons of other commonly employed deuterated solvents with TBACl can be seen for dichloromethane-<i>d</i>₂ and acetonitrile-<i>d</i>₃, but in acetone-<i>d</i>₆, methanol-<i>d</i>₄ and DMSO-<i>d</i>₆, upfield shifts are observed. A hypothesis based on Density Functional Theory (DFT) modelling is presented to account for this behaviour.</p></div

Allosteric binding properties of a 1,3-alternate thiacalix[4]arene-based receptor having phenylthiourea and 2-pyridylmethyl moieties on opposite faces

The synthesis of three new heteroditopic receptors (5a-c) which are based on thiacalix[4]arenes in the 1,3-alternate conformation is reported herein. These new receptors each have two thiourea moieties linking phenyl groups, two of which are substituted with electron-withdrawing groups at their para-positions, and at the opposite side of the thiacalix[4]arene cavity, with two 2-pyridylmethyl groups. One example (5a) was also characterized by X-ray crystallography. A limited 1H-NMR and UV-vis anion complexation study was conducted. DFT computational determinations indicated that 5c, which has strongly electron-withdrawing NO2 groups, had the most effective recognition ability towards the selected anions. The binding of Ag+ at the 2-pyridyl moieties, and the binding of the anions at the two thiourea NH groups of the p-substituted phenylthioureido moieties, respectively, was also investigated. The appearance of a positive allosteric effect with receptor 5b was also found using 1H-NMR titration experiments