Weakly coordinating anions, and the exceptional conductivity of their nonaqueous solutions

We describe the synthesis and properties of a new class of lithium salts in which the anions are dicarboxylic acid derivatives of orthoborate. The single negative charge of these anions is shared by eight oxygen atoms (more accurately four oxygen atoms and four carbonyl groups), hence it is very spread out, and the coordinating ability of the anions is consequently very weak. Judged by comparison of the high-temperature conductivities of 0.5 M lithium salt solutions with those of other weakly basic anions in a low dielectric constant solvent 1,2-dimethoxyethane, the first member of the new family, lithium bis(oxalato)borate, LiBOB, is dissociated much more than are LiClO₄ or LiCF₃SO₃ and even more than lithium bis(trifluoromethane)sulfonylimide, LiTFSI. Conductivities of solutions in acetonitrile approach those of aqueous solutions. Solutions of LiBOB in propylene carbonate have high electrochemical stabilities, up to approx. 4.5 V vs. Li⁺/Li. Higher members of the series exhibit lower solubilities, but are also highly dissociated in dilute solution.