Fluoride Ion-Encapsulated Germoxane Cages Modified with Organosiloxane Chains as Anionic Components of Ionic Liquids

Herein, we report the preparation of a new class of inorganic-organic hybrid ionic liquids by modification of a double-four-ring (D4R)-type germoxane (Ge-O-Ge) compound, containing a fluoride ion inside and a tetraethylammonium cation outside, with organosiloxane chains. D4R-type inorganic compounds are attractive as anionic components of ionic liquids because of their ability to encapsulate a fluoride ion inside the cage and allow various functional groups to be linked at the cage corners. The crystalline D4R-type germoxanes with corner Ge-OH groups were successfully converted to clear liquids by modification with dimethylvinylsilyl groups, followed by hydrosilylation with flexible oligosiloxanes (HSi(OSiMe₃)_nMe_3-n, n = 1-3). The resulting products showed liquid behaviors below 0 °C and were stable against thermal decomposition up to ∼200 °C. Additionally, they had high affinities for both hydrophilic and hydrophobic substances, attributable to the organosiloxane chains surrounding the ionic cage. Furthermore, the differences between the D4R-type germoxane and D4R-type siloxane were investigated using density functional theory calculations. The symmetry of the optimized structure and its electrostatic nature indicated that D4R-type germoxane is advantageous for the creation of ionic liquids.