The recognition of n -alkyl phosphonic or carboxylic acid by mono-squaramide-functionalised pillar[5]arenes

The results of 1 H NMR titration, Job Plot, ESI-MS and 2D NMR experiments reveal that 1a forms a 1:1 stoichiometric complex with HPA and HA, respectively, and its association constants (log K /M − 1 ) with HPA and HA are determined to be 3.83 ± 0.05 and 1.98 ± 0.04, respectively, suggesting much stronger binding ability of 1a with HPA over HA.

To compare with 1a , 1b with the electron-withdrawing group (CF 3 ) on the phenyl ring of squaramide moiety is synthesised as well, which shows the similar selective binding ability for HPA and HA but with a little stronger binding ability than 1a .