FTIR spectroscopic and electrochemical characterizations of tantalum(V) in 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate (Pyr 14 TFMS)-TaCl 5 ionic liquids are performed with 5, 15, and 35 mol% TaCl 5 as the media for potential electrochemical application. On the basis of FTIR spectra the electrochemical active species are identified as follows: [ TaOCl 4 ] − , [TaOCl 5 ] 2− , [TaCl 5 F] − , and [TaCl 5 F 2 ] 2− . The dominant species are the oxochloride complexes of tantalum(V).Cyclic voltammograms of Pyr 14 TFMS-TaCl 5 mixtures at 80 • C exhibit three reduction peaks: at −0.8 V, −0.9 V, and −2.4 V. The first reduction peak corresponds to the reduction of the oxochloride species of tantalum(V), the second is assigned to the reduction of the mixed chloride-fluoride species of tantalum(V), and the third peak to the further reduction of tantalum.The spectroscopic data confirm the suggestion that oxochloride complexes of tantalum(V) become the dominant species in ionic liquids with the oxygen-containing functional group [CF 3 SO 3 ] − , possibly restricting the process of tantalum metal deposition from these ionic liquids.
A series of novel ionic liquids consisting of 1-butyl-1-methylpyrrolidinium chloride (Pyr14Cl) and TaCl5 were obtained in a wide range of molar compositions for electrochemical application. Raman spectroscopy was used to investigate the complex formation of tantalum(V) in the mixtures of (x)Pyr14Cl-(1 - x)TaCl5 (x = 0.80-0.30) over the temperature range 20-160 degrees C. Depending on the molar composition, different species of tantalum (V) were identified. In the basic and neutral mixtures of (x)Pyr14Cl-(1 - x)TaCl5 (x = 0.80-0.50), tantalum(V) exists in the form of octahedral [TaCl6](-) in both solid and molten states. In acidic ionic liquids (x = 0.45-0.30), [Ta2Cl10] units are the main species of tantalum(V) identified in the solid state. As the temperature rose, the gradual degradation of [Ta2Cl10] units was observed in the solid state, accompanied by the formation of [TaCl6](-) and [Ta2Cl11](-) anions. In the molten state, in the range between 130 and 160 degrees C, the latter two species exist in equilibrium and are the dominant species of tantalum(V). The formation of oxochloride species of tantalum(V) was investigated in mixtures of Pyr14Cl-TaCl5-Na2O (x = 0.65) at various O/Ta mole ratios, and the formation of the oligomeric species with Ta-O-Ta bridging bonds was determined.
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