Recognition, Sensing, and Trapping of Bicarbonate Anions with a Dicationic meso-Bis(benzimidazolium) Calix[4]pyrrole

Abstract

A meso-aryl calix[4]pyrrole derivative bearing benzimidazolium groups at the two diametrical meso positions was synthesized. The receptor acts as an effective host for the bicarbonate anion at concentrations as low as 4 nM. A change in fluorescence response was observed under conditions of fluorescence dye displacement when this functionalized calix[4]pyrrole was exposed to the HCO3– anion. Tests with various carbonated drinks revealed its ability to function as an easy-to-use sensor for dissolved CO2, as monitored through the bicarbonate anion concentration. Depending on the specific choice of conditions, recrystallization of the calix[4]pyrrole receptor in the presence of Cs2CO3 and methanol yielded crystals of either the HCO3– anion complex or methyl carbonate (CH3OCO2–), a normally difficult-to-access species whose formation under non-forcing equilibrium conditions is ascribed to trapping by the calix[4]pyrrole receptor. The present system thus provides a recognition-based approach to the chemical capture of hydrated, anionic forms of CO2. CO2 is intimately associated with life, both as a building block for carbohydrate synthesis and as the end product of aerobic respiration. It is also key to maintaining the pH of waterways and remains a topic of intense ongoing debate involving climate change. The central nature of CO2 in the chemical world and beyond provides an incentive to develop easy-to-use “tools” to capture, sense, and chemically “fix” CO2 and its hydrated forms (e.g., HCO3–). Here, we report a dicationic meso-bis(benzimidazolium) calix[4]pyrrole that works to capture CO2 by modulating the HCO3–/CO2 balance (including in carbonated beverages) and acts as a HCO3– sensor (at concentrations as low as 4 nM) under solution-phase indicator displacement assay conditions. It also promotes the conversion of hydrated CO2 into methyl carbonate (CH3OCO2–) under near-neutral conditions by stabilizing this normally unstable mono-ester species in bound form. The chemistry appears generalizable to other related host systems. A synthetic dicationic meso-bis(benzimidazolium) calix[4]pyrrole permits the sensing and chemical capture of CO2 in its hydrated forms.