Phosphonium Bis(oxalato)borate Dicationic Ionic Liquids: A Platform for Halogen-Free Functional Fluids

A new class of halogen-free, nanostructured ILs (ionic liquids) with high thermal stability is introduced in the form of a series of bolaform phosphonium-based dicationic ionic liquids (DILs), [P_n,n,n-C₁₂-P_n,n,n][BOB]₂ (n = 4, 6, 8), paired with bis(oxalato)borate (BOB^–) anions. They were prepared via a straightforward two-step synthesis involving quaternization and aqueous metathesis. Comprehensive spectroscopic analysis confirmed their structure and purity. All three ILs exhibit wide thermal operating windows of nearly 300 °C, with glass transitions as low as −60 °C and decomposition temperatures exceeding 250 °C. Combined small- and wide-angle X-ray and neutron scattering (SWAXS/SANS) revealed intrinsic nanoscale structuring in the bulk, characterized by domain segregation scaling with alkyl decoration and charge ordering. Upon dispersion in d₃-acetonitrile, the ILs lose their bulk nanostructure to form well-defined ‘dication triplets’ (i.e., aggregates of one dication + two anions) with cylindrical morphology, instead of simple ion pairs. These findings position the reported phosphonium orthoborate DILs as promising, nonhalogenated alternatives for application in thermal management, energy storage, catalysis, gas separations, and lubrication.