Investigations on a series of novel ionic liquids containing the [closo-B12Cl12]2− dianion

Abstract

A series of novel imidazolium, ammonium, phosphonium and pyridinium based salts with the [closo-B₁₂Cl₁₂]²⁻ dianion were synthesized by straight forward metathetic reactions and characterized by physical methods. The melting point of each salt with [closo-B₁₂Cl₁₂]²⁻ dianion was measured to investigate if it can be classified in the field of ionic liquids. Thermodynamic results showed observed higher melting points, which can be illustrated both from the higher symmetry of the dianion and the enhanced electrostatic forces of the salts synthesized. Given the above mentioned reasons, the [closo-B₁₂Cl₁₂]²⁻ dianion containing ionic liquids can be designed either by optimizing the structure of cation, especially of the alkylation pattern which naturally affected the cations' packing efficiency, or by shielding the cation–dianion interactions with appropriate hydrogen bond (H-bond) donors to form ‘quasi-eutectic ILs’. This can be rationalized by the increased interionic separation and the consequent weakening of the electrostatic forces. Additionally, influence of H-bond interactions on the phase transition temperatures of [closo-B₁₂Cl₁₂]²⁻ dianion containing salts was also investigated and related changes were explained by ab initio calculations, illustrating weak H-bond interactions between the hydroxy protons and the chlorine atoms which showed modest effects on the melting points of the salts with [closo-B₁₂Cl₁₂]²⁻ dianion.