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Spectroscopy and Electrochemistry of Tantalum(V) in 1-Butyl-1-Methylpyrrolidinium Trifluoromethanesulfonate

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Spectroscopy and Electrochemistry of Tantalum(V) in 1-Butyl-1-Methylpyrrolidinium Trifluoromethanesulfonate Spectroscopy and Electrochemistry of Tantalum(V) in 1-Butyl-1-methylpyrrolidinium Trifluoromethanesulfonate Olga B. Babushkina, Silvia Ekres, and Gerhard E. Nauer ECHEM Centre of Competence in Applied Electrochemistry, Viktor-Kaplan Straße 2, A-2700 Wiener Neustadt, Austria Reprint requests to Dr. O. B. B.; Fax: +43 2622 22266 50; E-mail: olga.babushkina@echem.at Z. Naturforsch. 63a, 73 – 80 (2008); received June 26, 2007 Presented at the EUCHEM Conference on Molten Salts and Ionic Liquids, Hammamet, Tunisia, September 16 – 22, 2006. FTIR spectroscopic and electrochemical characterizations of tantalum(V) in 1-butyl-1-methyl- pyrrolidinium trifluoromethanesulfonate (Pyr14TFMS)-TaCl5 ionic liquids are performed with 5, 15, and 35 mol% TaCl5 as the media for potential electrochemical application. On the basis of FTIR spec- − 2− − tra the electrochemical active species are identified as follows: [TaOCl4] ,[TaOCl5] ,[TaCl5F] , and [TaCl F ]2−. The dominant species are the oxochloride complexes of tantalum(V). 5 2 ◦ Cyclic voltammograms of Pyr14TFMS-TaCl5 mixtures at 80 C exhibit three reduction peaks: at −0.8V,−0.9V,and−2.4 V. The first reduction peak corresponds to the reduction of the oxochlo- ride 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 [CF3SO3] , pos- sibly restricting the process of tantalum metal deposition from these ionic liquids. Key words: Tantalum(V) Chloride; 1-Butyl-1-methylpyrrolidinium Trifluoromethanesulfonate; Ionic Liquids; Spectroscopy; Electrochemistry. 1. Introduction Low-temperature molten salts or ionic liquids are considered as promising alternative electrolytes for Electrochemical deposition of tantalum cannot be the electrochemical deposition of refractory metals. achieved from aqueous solutions or organic solvents. Their advantages over the high-temperature molten High-temperature molten salts (molten alkali fluorides, salts are their negligible vapour pressure, wide window alkali chlorides and mixed alkali chloride-fluoride sys- of electrochemical stability, high thermal stability (up ◦ tems) were found to be efficient baths for tanta- to 350 C), and the fact that they are often liquid at lum electrochemical deposition. Alkali chloride elec- room temperature. A few studies on the electrochem- trolytes containing chlorides of refractory metals, par- istry of tantalum(V) in different ionic liquids have been ticularly TaCl5, are highly volatile and hygroscopic. published [8 – 12]. Morimitsu et al. [9, 10] and Zein However, the operating temperatures are relatively low El Abedin et al. [11] have reported that tantalum de- (above 500 ◦C) – lower than those of the alkali fluoride position was achieved from ionic liquids. However, in electrolytes. The achievements in the reduction of tan- all cases the X-ray diffraction patterns of the deposits talum(V) to the metal state from alkali chloride elec- show the presence of oxygen impurities. trolytes were reported in [1 – 3]. The purity of the electrolyte, especially the absence Alkali fluoride electrolytes containing K2TaF7 are of oxygen impurities, is a crucial factor for tantalum not as volatile and hygroscopic as alkali chlorides, but deposition as the oxohalide complexes of tantalum(V) they are more corrosive to electrolytic baths and re- can perturb the electrochemical reaction and degrade quire operating temperatures above 700 ◦C. The most the quality of the deposited metal. A few studies have successful electrochemical deposition of tantalum was been performed to investigate the formation of tanta- achieved from alkali fluoride electrolytes [4 – 7]. lum(V) oxohalide species and their subsequent influ- 0932–0784 / 08 / 0100–0073 $ 06.00 c 2008 Verlag der Zeitschrift f¨ur Naturforschung, T¨ubingen · http://znaturforsch.com 74 O. B. Babushkina et al. · Tantalum(V) in Pyr14TFMS Fig. 1. Suggested complex formation in Pyr14TFMS- TaCl5 mixtures. ence on the electrochemical behaviour of tantalum(V) and TaCl5. The components were mixed in an MBraun in high-temperature electrolytes [5 – 7, 13]. In the case Star glove box (content of O2 and H2O below 1 ppm), of electrochemical deposition of tantalum from ionic placed in Pyrex tubes and melted in an inert atmo- liquids Barnard and Hussey used phosgene gas to re- sphere of argon (99.999%). Three different composi- move oxochloride impurities [8]. tions of Pyr14TFMS-TaCl5 at the molar ratios of the The identification of halide and oxohalide species components 95 : 5, 85 : 15, and 65 : 35 were obtained of tantalum(V) in the electrolytes by means of at 50, 80, and 120 ◦C, respectively, after ∼ 3hofin- spectroscopic methods provides important informa- tensive stirring. tion about the tantalum(V) electrochemical reduction mechanism. The electrochemical and spectroscopic 2.2. Spectroscopic Measurements behaviour of TaCl in the room-temperature ionic liq- 5 FTIR spectra were recorded in the range 200 – uid 1-butyl-1-methylpyrrolidinium trifluoromethane- 4000 cm−1 using a Bruker FTIR spectrometer sulfonate (Pyr TFMS) was investigated with 5, 15, 14 Equinox 55 with an MKII Golden Gate single reflec- and 35 mol% TaCl . The structure and composition of 5 tion ATR system with a diamond top plate, KRS-5 tantalum(V) species formed in pyrrolidinium salts with lenses (Specac, England), and a MCT nitrogen-cooled the oxygen-containing functional group [CF SO ]− 3 3 detector (64 scans). was investigated, to confirm the suggestion of the for- mation of oxohalide species and to determine their in- 2.3. Electrochemical Measurements fluence on the applicability of these ionic liquids as electrolytes for the electrochemical deposition of tan- A Voltalab 40 model PGZ 301 potentiostat was talum. used for electrochemical measurements with a tightly sealed three-electrode cell with inert argon atmo- 2. Experimental sphere. The working electrode was a platinum disc (Pt wire, ∅ 0.5 mm, sealed in Duran), and the counter 2.1. Chemicals and Sample Preparation electrode was a glassy carbon rod (∅ 3 mm). A plat- inum wire was used as a quasi-reference electrode. The The ionic liquids consisted of Pyr14TFMS as the or- cyclic voltammograms were recorded at a scan rate ganic and TaCl5 as the inorganic component. The com- − ◦ of 0.1 V s 1. mercial Pyr14TFMS (m. p. 3 C, thermal stability up ◦ to 340 C) was purchased from Merck. Pyr14TFMS 3. Results and Discussion was dried under high vacuum (0.003 mbar) at 95 – ◦ 100 C with intensive stirring for 48 h prior to use; after 3.1. FTIR Spectroscopic Studies drying its water content, determined by Karl Fischer coulometric titration, was typically 6 – 7 ppm. In Pyr14TFMS-TaCl5 ionic liquids the formation of n− 2− − Ampoule-packed TaCl5 (99.999%) was purchased new [TaCl5X] (X = O ,F ) complexes is expected. from Alfa Aesar and used without preliminary purifi- One can suggest the participation of fluorine and oxy- − cation. gen from the [CF3SO3] group of the pyrrolidinium The Pyr14TFMS-TaCl5 ionic liquids were prepared salt in the formation of new tantalum(V) complexes ac- by combining stoichiometric amounts of Pyr14TFMS cording to the proposed scheme (Fig. 1). O. B. Babushkina et al. · Tantalum(V) in Pyr14TFMS 75 Fig. 2. FTIR spectra of Pyr14TFMS and Pyr Cl; * vibrations of the 14− [CF3SO3] anion. Table 1. Tentative assignment of the observed bands in the The intensities of the vibrational features of FTIR spectrum of Pyr TFMS. − 14 [CF3SO3] groups (marked with asterisks in Fig. 2) −1 a Wave number (cm ) Vibration Species were significantly higher than those of the pyrrol- 3037 m idinium ring; thus it was quite complicated to νas(CH) 2967 m ν + estimate the inter-ionic interaction in Pyr TFMS. s(CH) Pyr14 14 2880 m δ Single-crystal X-ray diffraction data of N,N-dimethyl- 1469 m (CH2) ν pyrrolidinium bis(trifluoromethanesulfonyl) amide 1254 s as(SO2) 1223 s νas(CF) − (TFSA) [21] showed this salt to consist of discrete ν [CF3SO3] 1148 s s(CF) ions with only weak C-H...O contacts between the 1028 s ν (SO ) s 2 constituent atoms of cations and anions close to 965 vw the limits of van der Waals separations. The closest 929 w + 913 vw ring mode Pyr14 approaches of the cation and anion are between the 898 vw sulfonyl oxygen atoms and C-H bonds on the cations. 825 vw FTIR spectra of Pyr14TFMS and Pyr14TFMS-TaCl5 δ 755 w s(CF3) mixtures at various molar ratios of components are 635 s δs(OSO) − δ [CF3SO3] presented in Figure 3. In the spectra of Pyr14TFMS- 572 m as(CF3) 516 m δas(CF3)orδas(OSO) TaCl5 mixtures new vibrational features were observed −1 a m, medium; s, strong; w, weak; vw, very weak. at 501, 596, 928, 969, 1203, and 1368 cm . Their in- tensities rose as the molar fraction of TaCl5 increased. The assignment of the vibrational features of the A simultaneous decrease of the peaks at 516, 572, 635, −1 pyrrolidinium ring in Pyr14TFMS was done on the 1028, 1223 and 1254 cm assigned to Pyr14TFMS basis of comparison with the FTIR spectrum of was also observed. Pyr14Cl (Fig. 2) and the results reported for liquid The SO3 groups act as electron donors, and the co- pyrrolidine [14, 15] and different alkylpyrrolidinium ordination with TaCl5 occurs through the oxygen atom salts [16, 17]. The identification of the vibrations of causing the redistribution of the electron density in − the [CF3SO3] group in Pyr14TFMS was done on the the SO3 group. This led to a decrease in the inten- −1 basis of data for CF3SO3H [18], LiCF3SO3 in acetoni- sity of the vibration δs(OSO) at 635 cm and ap- −1 trile [19] and CF3SO2OSiH3 [20]. The tentative as- pearance of a new peak at 620 cm . Simultaneously, −1 signment of the vibrations of Pyr14TFMS is summa- the intensity of the vibration νas(SO2) at 1254 cm rized in Table 1.
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