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 ionic liquids consisting of 1-butyl-1-methylpyrrolidinium chloride (Pyr 14 Cl) and TaCl 5 at molar ratios of Pyr 14 Cl in the range of 0.85 -0.20 was synthesized. They exhibit potential application as electrolytes for the electrochemical deposition of tantalum. The thermal behaviour of the Pyr 14 Cl-TaCl 5 mixtures was investigated, using differential scanning calorimetry (DSC) to estimate the range of thermal stability and to determine the optimal compositions for electrochemical trials. The Pyr 14 Cl-TaCl 5 mixtures formed stoichiometric 3 : 2 and 2 : 3 crystalline phases with melting points of 70 • C and 130 • C, respectively, characterized by entropies of fusion of 13.6 J K −1 mol −1 and 10.1 J K −1 mol −1 . The mixtures with molar fractions of Pyr 14 Cl equal to 0.60 and 0.40 were clear and transparent liquids and had the lowest viscosity; they were the most suitable for electrochemical applications.FTIR spectra of the Pyr 14 Cl-TaCl 5 mixtures allowed to get insight into inter-ionic interactions and the nature of formation of these ionic liquids as a function of the molar ratio of the components.
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