Unusual polarities of tetraalkylphosphonium and trialkylsulfonium ionic liquids, ylides, and carboxylated zwitterions

Abstract

Aprotic heterocyclic anions (AHAs) can abstract hydrogen atoms from the methylene groups of the trialkylsulfonium and tetraalkylphosphonium cation. The resulting trialkylsulfonium and tetraalkylphosphonium ylides readily capture CO₂ and produce unusual carboxylated zwitterions, in which the distances between the poles, S(P) and O, are 0.274–0.290 nm. The zwitterions are strongly intermolecularly stabilized thanks to the electrostatic attractions between +1.6e(+0.9e) and −0.7e charges. Herein, we report drastic polarity alterations between the states of ionic liquids, ylides, and zwitterions through electronic properties, evolutions of geometries, and interatomic charge transfers. The analysis of the unusual intermolecular attractions was performed in the framework of natural bond orbitals. We propose that a biphasic or triphasic system can be useful to boost the reactions by accounting for the polarities of the intermediates and products. The practical value of the reaction is an enhancement in CO₂ chemisorption.