Transition anionic complex in trihexyl(tetradecyl)phosphonium-bis(oxalato)borate ionic liquid - revisited

It was found that Li[BOB]center dot nH(2)O salts were not readily suitable for the synthesis of high-purity orthoborate-based tetraalkylphosphonium ionic liquids, as exemplified here for trihexyl(tetradecyl)phosphonium bis(oxalato)borate, [P-6,P-6,P-6,P-14][BOB]; along with [BOB](-), a metastable transition anionic complex (TAC) of dihydroxy(oxalato)borate with oxalic acid, [B(C2O4)(OH)(2)center dot(HOOC-COOH)](-), was also formed and passed into the ionic liquid in the course of the metathesis reaction with trihexyl(tetradecyl)phosphonium chloride. On the contrary, Na[BOB] was found to be a more suitable reagent for the synthesis of this IL, because [BOB](-) anions safely passed into the final IL without hydrolysis, when metathesis reactions were performed using aqueous-free media. Since ultra-pure Na[BOB] is not commercially available, in this work, a preparation protocol for ultra-pure (>99%) Na[BOB] was developed: (i) molar ratios of boric and oxalic acids were optimised to minimise boron-containing impurities, (ii) the Na[BOB] product was thoroughly purified by sequential washing of a fine powder product in hot acetonitrile and ethanol and (iii) characterised using powder X-ray diffraction and solid-state B-11 MAS NMR spectroscopy. The physico-chemical properties of the prepared boron-impurity-free IL, i.e., its density, viscosity, electric conductivity, glass-transition temperature and thermal stability, were found to be significantly different from those of the previously reported [P-6,P-6,P-6,P-14][BOB], containing ca. 45 mol% of TAC, [B(C2O4)(OH)(2)center dot(HOOC-COOH)](-). It was found that a high-purity [P-6,P-6,P-6,P-14][BOB] prepared in this work has a considerably lower viscosity, a higher viscosity index and a wider electro-chemical window (ECW) compared to those of the sample of [P-6,P-6,P-6,P-14][BOB] with ca. 45 mol% of TAC. Interestingly, [B(C2O4)(OH)(2)center dot(HOOC-COOH)](-) in the latter sample almost completely transformed into [BOB](-)
QC 20210422