Double tailed scorpiand-type calix[10]phyrin: Synthesis and proton-driven anion recognition features

Abstract

Reported here is a large oligopyrrole macrocycle, a calix[10]phyrin bearing two diformyl substituents. The combination of conjugation provided by two pentapyrrolic subunits and flexibility resulting from the presence of sp 3 hybridized meso bridges allows the system to adopt a double tailed, scorpiand-like structure with two near-symmetric binding pockets. In methanol solution, this macrocycle was found to interact with a number of test acids, including sulfuric acid, trifluoroacetic acid, phosphoric acid, hydrochloric acid, and fluoroboric acid, via a combination of protonation and counter anion binding. Species specific behavior was seen, allowing for the colorimetric-based discrimination between the acids. Efforts to model the underlying binding equilibria led to the conclusion that upon treatment with excess H 2 SO 4 , up to four protons and two counteranions are bound, whereas in the case of HCl the dominant species is a monoanionic complex. The responses of several other acids were rationalized in terms of these limiting scenarios.