Preparation, Physicochemical Properties, and Crystal Structures of Trifluoroacetic Acid Aminium Salts

Abstract

Monoethanolaminium trifluoroacetate (HO‒CH₂CH₂‒NH\(_{3}^{ + }\) ^‒O(O)C‒CF₃), triethanolaminium trifluoroacetate ((HO‒CH₂CH₂)₃NH^{+ ‒}O(O)C‒CF₃), tris(hydroxymethyl)methanaminium trifluoroacetate ((HO‒CH₂)₃C-NH\(_{3}^{ + }\) ^‒O(O)C‒CF₃), and 2,2'-(ethylenedioxy)di(ethylaminium) bis(trifluoroacetate) (CF₃-C(O)O^{‒ +}H₃N-(CH₂CH₂O)₂CH₂CH₂-NH\(_{3}^{ + }\) ^‒O(O)C‒CF₃) were synthesized by the reactions of trifluoroacetic acid with monoethanolamine, triethanolamine, tris(hydroxymethyl)methanamine, and 2,2'-(ethylenedioxy)di(ethylamine), respectively. Ammonium trifluoroacetate (NH\(_{4}^{ + }\) ^‒OC(O)CF₃) was prepared by the reaction of trifluoroacetic acid with hexamethyldisilazane and with trimethylsilyl trifluoroacetate CF₃C(O)OSi(CH₃)₃. The synthesized compounds are low-melting crystalline salts, which are transferred into a vapor phase upon heating to 200–220°С. The reaction of CF₃C(O)OSi(CH₃)₃ with monoethanolamine afforded monoethanolaminium trifluoroacetate instead of expected 2,2,2-trifluoro-N-(2-hydroxyethyl)acetamide CF₃C(O)-NH-CH₂CH₂OH. The aminium salts were characterized by IR and NMR spectroscopy, mass spectrometry, high-performance liquid chromatography, and scanning electron microscopy. The thermal behavior and thermal stability of the salts were studied by differential scanning calorimetry and thermogravimetry, respectively. The molecular structures of monoethanolaminium trifluoroacetate, triethanolaminium trifluoroacetate, tris(hydroxymethyl)methanaminium trifluoroacetate, and ammonium trifluoroacetate were determined.