Stable core-modified calixsmaragdyrins: synthesis, structure and specific sensing of the hydrogen sulfate ion

Stable calixoxa- and calixthiasmaragdyrins containing three methine bridges and two direct bonds connecting the five pyrrole/heterocycle rings were synthesized by [3 + 2] condensation of dipyrromethane with 16-oxatripyrrane and 16-thiatripyrrane respectively under mild acid-catalyzed conditions. The compounds were characterized by HR-MS, 1D & 2D NMR, absorption and electrochemical techniques and the structure of calixoxasmaragdyrin was solved by X-ray crystallography. The crystal structure analysis indicated that the calixoxasmaragdyrin macrocycle was highly distorted due to the flexibility introduced by one sp(3) meso-carbon. The compounds show ill-defined absorption bands and irreversible oxidation and reduction waves which were attributed to the disruption of conjugation of the macrocycle by incorporation of one sp(3) meso-carbon. The anion binding studies indicated that the calixoxasmaragdyrin exhibited specific sensing ability for the HSO4- ion over other anions whereas calixthiasmaragdyrins did not even show an ability to bind anions.