Preparation and crystal structure of tetraoctylphosphonium tetrakis(pentafluorophenyl)borate ionic liquid for electrochemistry at its interface with water

•Facilitated K+ transfer at water|ionic liquid micro-interfaces was investigated.•Highly electrochemically stable alkylphosphonium ionic liquid was discovered.•Novel crystal structure of tetraoctylphosphonium tetrakis(pentafluorophenyl)borate was determined.•TRUEX spent nuclear fuel ligand octyl(phenyl)-N,N’-diisobutylcarbamoylphosphine oxide was employed.

With ongoing and expending global energy needs, nuclear power is likely to be a key energy source for many more generations. A key question is still the fate of spent nuclear fuel (SNF). SNF contains a great deal of valuable material. In order to improve safety and handling, herein is proposed relatively high melting point ionic liquid (IL) or organic ionic plastic crystals (OIPCs) for biphasic water|IL metal ion extraction. The proposed IL, tetraoctylphosphonium tetrakis(pentafluorophenyl)borate, P8888(C6F5)4, was discovered to have a melting point of ∼55 °C and a high degree of order by differential scanning calorimetry and X-ray diffraction (CCDC 1550126), respectively. In this way, the IL/OIPC could be used in ligand assisted metal ion extraction from water solutions at elevated temperatures and then ‘frozen’ at ambient conditions for facile manipulation and transport of the SNF. To test the feasibility of this, electrochemistry such as cyclic voltammetry at a water|IL micro-interface (25 mm in diameter) was employed to assess the thermodynamics of K+ ligand assisted transfer. It was revealed that the TRUEX (Transuranium Extraction) ligand octyl(phenyl)-N,N’-diisobutylcarbamoylphosphine oxide (CMPO) has a high to moderate coordination to K+ at the w| P8888(C6F5)4, micro-interface. Two ligand to K+ stoichiometries were determined to be 3:1 and 2:1, respectively.

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