RECEPTORS FOR OXO ACIDS - EFFECTS OF INTRA-ION-PAIR HYDROGEN-BONDING ON ACID-BASE EQUILIBRIA

Receptor molecules 1-3 containing a quinoline unit and hydroxyl groups were synthesized, and their properties in salt formation with oxo acids such as p-toluenesulfonic acid (TsOH) and methyl phenylphosphonate (MPP) were studied by using H-1 NMR spectroscopy. In CDCl3 solution the salt formation of the receptors with TsOH or MPP is enhanced by hydrogen bonds between the hydroxyl groups and the counteranion. The salt-formation enhancement of 1 over 2 suggests the presence of multiple interactions. The X-ray structure of 1.MPP.(acetone)2 confirms that three hydrogen-bonding interactions stabilize the salt. In the salt formation of 5-(dimethylamino)quinoline derivative 3, the hydroxyl groups also affect the relative basicity of the basic nitrogens in 3. The results described here emphasize that acid-base equilibria in apolar solvents are significantly affected by intra-ion-pair hydrogen bonding (i.e., hydrogen bonding within a contact ion pair).