Bicarbonate Recognition Features of a Naphthobipyrrole-strapped Calix[4]pyrrole

A calix[4]pyrrole (1) strapped with a naphthobipyrrole was prepared and its ability to bind the bicarbonate anion (HCO3-) was investigated via H-1 NMR and UV/Vis spectroscopic analyses. The resulting spectral data revealed that receptor 1 was capable of recognizing HCO3- with high affinity in solutions as well as in the solid state. In CD2Cl2, it was inferred from H-1 NMR spectroscopic titration experiment that HCO3- interacts with receptor 1 via two different modes with the first involving HCO3- being hydrogen bonded to all six NH protons of the receptor. This interaction is followed by a deprotonation process of the bipyrrole NHs upon the subsequent addition of HCO3- while the anion remains hydrogen-bonded to the calix[4]pyrrole NHs. In the presence of methanol, the bound bicarbonate anion was converted into an unstable monomethyl carbonate anion (CH3OCO2-) that was then stabilized by forming a complex with receptor 1.