Capture and Activation of Carbon Dioxide Using Guanidine Superbases

Abstract

Due to its abundance and low-cost, carbon dioxide is a desirable C₁-building block within organic transformations. However, the thermodynamic and kinetic stability of CO₂ often necessitates preliminary activation before it can be inserted into organic molecules. This prompts the need for compounds that can effectively promote the activation of CO₂. This research investigates the capture and activation of carbon dioxide using a class of superbases that incorporate the bicyclic guanidine unit, 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]-pyrimidine (hppH, 1). A series of compounds containing multiple hpp-units assembled around a phenyl ring scaffold were synthesized and investigated in the functionalization of CO₂. The work presented in this study has demonstrated the ability of protonated superbasic hppH derivatives to efficiently and effectively capture and activate carbon dioxide from ambient air to form the corresponding guanidinium bicarbonate salts. A series of optimization reactions was carried out, and showed that addition of substoichiometric concentrations of a proton source activates these guanidine compounds to their fully protonated cationic forms, and results in CO₂ capture through bicarbonate formation.

A series of protonation studies were employed to fully characterize the cationic species. The tetraphenylborate and hydrochloride guanidinium salts were synthesized, isolated, and characterized by ¹H NMR and ¹³C NMR spectroscopic analysis. Molecular structures of relevant crystals were obtained through single crystal X-ray diffraction. These structures revealed a complex hydrogen-bonding network within these ionic species, and showed efficient delocalization of the formal positive charge within the protonated guanidinium units.

The guanidine superbases were implemented in a series of reactions attempting the functionalization of CO₂ and an alcohol to form corresponding alkylcarbonate products. However, the synthesis of these carbonate products was not achieved under the reaction conditions employed. This lack of success has been attributed to the hygroscopic nature of this class of compounds, resulting in the preferential capture of ambient water.