Ionic liquids of chelated orthoborates as model ionic glassformers

Ionic liquids based on various chelated orthoborate anions of different N-containing onium cations have been synthesized using an economic synthesis strategy. Most orthoborates do not crystallize. They are found to have much higher glass transition temperatures and room-temperature viscosities than those with perfluorinated anions such as TFSI^-, BF₄^-, and CF₃SO₃^- (Tf^-), as predicted from anion polarizability arguments. The ambient conductivities of the new ionic liquids are low relative to those with perfluorinated anions. The transport properties all show that cohesion in these liquids increases, and ionic mobilities decrease, as anion size increases, implying that van der Waals interactions, not Coulomb interactions, have become the controlling influence. In view of their resistance to crystallization, the large range of temperature over which these liquids can be studied, their hydrophobic properties, and their high fragilities, these liquids may provide good model systems for fundamental liquid state investigations and interesting solvents for large-molecule dissolution.