Thermally labile mono-alkyl phosphates and their alkali metal derivatives: synthesis and solid-state supramolecular aggregation

Abstract

New organic-soluble mono-alkyl phosphate esters [(RO)P(O)(OH)₂] have been synthesized by esterification of phosphoric acid with the corresponding aliphatic alcohols in the presence of acetic anhydride. The products were initially isolated as cyclohexylamine salts, viz. [CyNH₃]₂[(CF₃CH₂O)PO₃]·H₂O (1) and [CyNH₃]₂[((CH₃)₃CCH₂O)PO₃]·H₂O·MeOH (3). Neutralization of these cyclohexylamine salts using a suitable ion exchange resin gave the corresponding phosphate esters [(RO)P(O)(OH)₂] (R = CF₃CH₂ (2); (CH₃)₃CCH₂ (4)). Reactivities of the trifluoroethyl phosphates 1 and 2 towards lithium and sodium ions have been explored and new alkali metal phosphate complexes [(CF₃CH₂OPO₃HLi)₃]_n (5), [(CF₃CH₂OPO₃HNa)₂]_n (6) and [CF₃CH₂OPO₃(Na_0.5)₂(CyNH₃)·(H₂O)₆]_n (7) have been isolated. Compounds 1–7 were characterized by different spectroscopic and analytical methods and their molecular structures were established by single-crystal X-ray diffraction studies. These alkyl phosphates and their complexes undergo diverse secondary interactions due to the presence of P–OH and PO groups, resulting in the formation of layered structures. The new alkali metal phosphates have been shown to be efficient single-source precursors to produce useful ceramic phosphates, such as LiPO₃, NaPO₃, and MCoPO₄ (M = Li and Na), through facile decomposition involving only gaseous by-products. This chemistry offers possibilities to generate other transition metal phosphates incorporating alkali metal ions, such as LiFePO₄, etc., which are used in energy storage devices and as electrocatalysts.