Acta Chim. Slov. 2002, 49, 209-220. 209 HIGHER FLUORIDES OF NICKEL: SYNTHESES AND SOME PROPERTIES OF Ni2F5† Melita Tramšek and Boris Žemva* Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia † Dedicated to the memory of fluorine chemist and friend dr. Karel Lutar (d. September 2, 2000) Received 02-07-2001 Abstract Ni2F5 was prepared by thermal decomposition of R-NiF3 at 373 K or by reduction of R-NiF3 with xenon or XeF2. Reaction of Ni2F5 in anhydrous HF (aHF) acidified with AsF5 yielded Ni(AsF6)2 and F2 while Ni2F5 disproportionated in aHF made basic with KF yielding NiF2 and K2NiF6. Ni2F5 is able to oxidize xenon to XeF2 and fluorinate C3F6 to C3F8. Ni2F5 is most probably playing an important role in the electrochemical fluorination. Introduction Higher binary fluorides of nickel have been a subject of investigation since the development of electrochemical fluorination of organic compounds (Simon´s process ECF).1,2 This process is particularly efficient if nickel is used as an anode.2 Several authors3,4 were convinced that higher nickel fluorides are formed at the anode during electrochemical fluorination and that these nickel fluorides have the major role in the fluorination of organic compounds by Simon´s process. Even more, some authors5,6 tried to prepare higher nickel fluorides using electrochemical fluorination. Court and Dove7,8 tried to prepare higher nickel fluorides with the reaction between K2NiF6 and Lewis acid (e.g. AsF5, BF3) in anhydrous hydrogen fluoride (aHF) but their material was always heavily contaminated with co-produced potassium salts (KBF4, KAsF6). The oxidation state of their mostly brown precipitates was always lower than +3. It is surprising that they did not observe NiF4 or isolate the relatively long-lived NiF3, which is thermally stable at 293 K as dry solid. It was not until 19899 that in joint efforts of researchers from the University of California, Berkeley and “Jožef Stefan” Institute, Ljubljana the evidence for the existence of NiF4 was provided. In the next years10 three forms of NiF3 have been M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5... 210 Acta Chim. Slov. 2002, 49, 209-220. prepared, their structures identified and the oxidizing properties described. The existence of Ni2F5 was also mentioned and the X-ray powder diffraction patterns of Ni2F5 prepared by different synthetic routes were given.10 In this paper the efforts to synthesize pure Ni2F5 are described together with its characterization and some of its properties. Results and discussion 1. Synthetic routes to Ni2F5 Ni2F5 is red brown solid with negligible vapour pressure at room temperature. It is stable when dry and in an inert atmosphere. It slowly decomposes at room temperature in the suspension of aHF to NiF2 and fluorine. Among all higher nickel fluorides it is the most stable binary fluoride of nickel. Ni2F5 could be prepared from R-NiF3 by thermal decomposition or by reduction with xenon or xenon(II) fluoride. R-NiF3 is the most thermodynamically unstable of all three forms of NiF3 and it loses F2 at temperatures higher than 312 K.10 Thermal decomposition at 312 K is very slow process therefore for the preparation of Ni2F5 higher temperatures between 353 to 373 K are used. The mass balance is in accordance with the reaction: 373 K 2R-NiF3 ---------------> Ni2F5 + 1/2F2 The chemical analysis of the obtained product is giving the molar ratio Ni:F=1:2.47. X-ray powder diffraction pattern matches X-ray powder diffraction pattern of the product NiFx (2 2Ni2F5 + XeF2 + (n-1)Xe 295 K 4R- NiF3 + nXeF2 -----------> 2Ni2F5 + XeF4 + (n-1)XeF2 The obtained products were even less crystalline as the product obtained by thermal decomposition. Their X-ray powder diffraction patterns were the same and they match the X-ray powder diffraction pattern of NiFx (2 4R-NiF3 + (1+z)Kr + zF2 + (n-1-z)KrF2 n is excess of KrF2 z is the amount of KrF2 which decomposes in aHF ? M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5... Acta Chim. Slov. 2002, 49, 209-220. 213 Ag3F8 is an other example of mixed valence compound (Ag(II)Ag(III)2F8) and it cannot be oxidized to AgF3 by KrF2 in aHF at room temperature. Pure AgF3 can be synthesized only by the method for the preparation of thermodynamically unstable binary fluorides.9,14 From these results it could be concluded that Ni2F5 is less stable against oxidation than Ag3F8. 3.2. The reaction of Ni2F5 in acidic aHF Ni2F5 is slowly decomposing already in aHF yielding NiF2 and elemental fluorine. In acidified aHF e.g. when AsF5 is present in the aHF, the decomposition is finished in 10 minutes transforming red-brown solid Ni2F5 into yellow solution Ni(AsF6)2 and elemental fluorine. 273 K Ni2F5 + 4AsF5 ------------> 2Ni(AsF6)2 + 1/2F2 aHF The same reaction proceeds also with R-NiF3. Fluoride ion affinity of AsF5 is high enough (481 kJ/mol)15 to remove F- from Ni(III) in Ni2F5 generating NiF2+ cation which is highly electronegative and electron capture and release of elemental fluorine is expected. Cationic species in acidic aHF (e.g. AgF2+, NiF3+, NiF2+) are the strongest oxidizers known today.16 3.3. The reactions of Ni2F5 in basic aHF Ni2F5 slowly reacts with excess of Lewis base (e.g. KF) in aHF at 273 K according to the following reaction: 273 K 2Ni2F5 + 2KF ~ * 3NiF2 + K2NiF6 The decomposition of Ni2F5 in aHF at the reaction temperature (273 K) is practically negligible and only disproportionation is taking place. NiF2 and K2NiF6 were characterized by X-ray powder diffraction patterns of the solids when aHF was pumped M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5... 214 Acta Chim. Slov. 2002, 49, 209-220. away. K2NiF6 was shown also by red colour of the aHF solution and Raman spectrum showing its characteristic bands at 560 cm-1, 518 cm-1 and 308 cm-1. The reaction between R-NiF3 and good fluoride ion donor (e.g. KF, XeF6) at room temperature was very quick but there besides of disproportionation also some decomposition of R-NiF3 in aHF was taking place.10 4. Oxidizing properties of Ni2F5 Reaction of Ni2F5 with excess of Xe in aHF at room temperature shows that the fluorinating and oxidizing abilities of the compound are still high. The mass balance of the reaction is in accordance with the following equation: 295 K 2Ni2F5 + nXe ------------> 4NiF2 + XeF2 + (n-1)Xe aHF Ni2F5 is the least potent oxidizer among all higher binary fluorides of nickel. This is understandable because the amount of Ni(III) is the smallest among all higher Ni fluorides. Ni2F5 is still enough strong oxidizer to fluorinate some organic compounds. The reaction between C3F6 vapour and excess of solid Ni2F5 is very rapid and exothermic although there is no carbon-carbon bond cleavage. Infrared spectrum of the obtained gaseous product is showing perfluoropropane without even traces of CF4. 295 K C3F6 + 2Ni2F5 ------------> C3F8 + 4NiF2 According to Sartori and his coworkers17,18 Simon´s process proceeds in two stages. Fluorination of organic molecules takes place by chemical reaction between organic molecules and higher nickel fluorides formed on the anode. Higher nickel fluorides on the anode are formed by electrochemical process. Bartlett and his coworkers19 suggest that nickel fluoride formed on the anode is not R-NiF3 but less potent form of nickel fluoride. They explain this statement by the fact that MeCOF is efficiently perfluorinated in the Simon´s process while the reaction between MeCOF and R-NiF3 is giving very little CF3COF but mostly decomposition products CF4 and COF2. M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5... Acta Chim. Slov. 2002, 49, 209-220. 215 Ni2F5 is the most stable higher nickel fluoride therefore we believe that nickel fluoride formed on the Ni anode during electrochemical fluorination in the Simon´s process is Ni2F5. Conclusions Higher binary fluorides of nickel, including Ni2F5, have been studied previously.10 In this paper we report about different synthetic routes for the preparation of Ni2F5. On the basis of the obtained results it could be concluded that the best synthetic approach for the preparation of Ni2F5 is thermal decomposition of R-NiF3 at higher temperature (373 K). According to infrared spectra two nickel species are present in the compound: Ni(II) and Ni(III). Further it was shown that the oxidizing and fluorinating abilities of Ni2F5 are still high and that it is able to oxidize xenon in aHF to XeF2. The most important feature is that by our opinion it is involved in electrochemical fluorination in the Simon's process. In comparison with R-NiF3 the advantages of Ni2F5 are its thermal stability and its fluorinating ability. Experimental 1. Apparatus, technique and reagents Apparatus - A nickel vacuum line and Teflon vacuum system were used as previously described.20 Non-volatile materials, which were very sensitive to traces of moisture, were handled in the dry argon atmosphere of a glove box with maximum content of 0.1 ppm of water vapour (Mbraun, Garching, Germany). The reactions with aHF were carried out in reaction vessels constructed from PFA (Polytetra, Germany) tubes (16 mm i.d. x 19 mm o.d.) equipped with Teflon valves or in reaction vessels constructed from two PFA tubes (16 mm i.d. x 19 mm o.d.) each drawn down to 8 mm i.d. ×10 mm o.d., joined at right angles by a Teflon Swagelok T compression fitting and joined to a Teflon valve.10 Metal reaction vessels with inner Teflon coating (volume 5-6 ml) were used for thermal decomposition of R-NiF3 at temperatures around 373 K. Vessels were constructed at the Institute Jožef Stefan and equipped with modified valve. All reaction vessels were prior use pretreated with elemental fluorine. M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5... 216 Acta Chim. Slov. 2002, 49, 209-220. Technique- X ray diffraction powder patterns (XRPP) were obtained by using the Debye-Scherrer camera (143.2 mm diameter), Ni filtered CuK? radiation. Powdered samples were filed in 0.3 mm quartz capillaries. Infrared spectra were taken on a FTIR spectrometer (Perkin Elmer 1710) on samples powdered between AgCl windows in a leak tight brass-cell. A 10 cm nickel cell with AgCl windows was used for volatile samples. Raman spectra of powder samples in sealed quartz capillaries were taken on a Renishaw Ramanscope dispersive instrument (System 1000) with the exiting line at 632.8 nm of a He-Ne laser. Reagents - Fluorine (Solvay, 99.9%), BF3 (Ucar), perfluoropropene (Aldrich Chemical Company, 99%), xenon (L'Air Liquide, 99.95%) were used as supplied. Anhydrous HF (Praxair, 99.9%) was purified by treatment with K2NiF6 for at least 24 hours. K2NiF6 (Ozark-Mahoning Pennwalt, 99.5%), which was used for the syntheses of R-NiF3, was heated in F2 (1.5·106 Pa) at 573 K for at least 24 hours. To extract KF impurity and to remove in aHF insoluble impurity a method described elsewhere was used.10 XeF2 was prepared in the photochemical reaction between Xe and F2 at room temperature.21 KrF2 was prepared by irradiation of liquefied mixture of F2 and Kr with near UV light at 77 K.22 AsF5 was synthesised by the reaction of As2O3 with elemental fluorine under high pressure at 573 K.23 KF (Merck, anhydrous, p.a.) was heated at 573 K and at the same time pumped in a dynamic vacuum. 2. Syntheses of Ni2F5: Rombohedral NiF3 (R-NiF3) was a starting material for the preparation of Ni2F5. It was synthesised by the reaction between K2NiF6 and BF3 in aHF as described previously.10 Thermal decomposition of R-NiF3: (5.53 mmol) R-NiF3 was heated at 395 K and thermal decomposition was completed in several days. During decomposition elemental fluorine was pumped away several times. Ni2F5 (2.73 mmol) was characterized by XRPP (Table 1) and chemical analysis: calculated for Ni2F5: F: 55.28%; Ni: 44.72%; found: F: 53.7%, Ni: 42.9%, K: 1.5% and B: 1.0%. Infrared spectra of Ni2F5, R-NiF3 and NiF2 were recorded (Fig. 1) M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5... Acta Chim. Slov. 2002, 49, 209-220. 217 Reaction of R-NiF3 with Xe: solid, dry R-NiF3 (3.51 mmol) was put into PFA reaction vessel, excess of gaseous Xe (2.79 mmol) at 293 K was added tensiometricaly. The change in colour from black R-NiF3 to red-brown Ni2F5 was noticeable in approximately 24 hours. The excess of Xe was pumped away at 193 K. XeF2, which was formed (0.45 mmol, calc.: 0.88 mmol), was pumped away at room temperature through the infrared cell for volatile samples and checked by IR spectroscopy. Ni2F5 (2.15 mmol, calc.: 1.755 mmol) was determined by XRPP (Table 1). Reaction of R-NiF3 with XeF2: R-NiF3 (2.31 mmol) and excess of solid XeF2 (3.56 mmol) were put into PFA reaction vessel in a glove box. Reaction at 293 K was finished in several days. Formed XeF4 and the unreacted XeF2 were pumped out at room temperature and checked by recording their infrared spectra. Red-brown Ni2F5 (1.40 mmol, calc.: 1.155 mmol) was determined by XRPP (Table 1). Results of chemical analyses were: F: 40.6%, Ni: 42.9%, together: 83.5%. Table 1: X-ray Powder Diffraction Data for Ni2F5 d (pm) a I/I0 d (pm) b I/I0 d (pm) c I/I0 355 w 261 w 249 w 248 w 245 s 214 vs 217 s 216 s 164 vs 166 s 166 s 141 s 143 vw 143 vw a.) Ni2F5 synthesised with thermal decomposition of R-NiF3 b.) Ni2F5 synthesised with reaction of R-NiF3 with Xe c.) Ni2F5 synthesised with reaction of R-NiF3 with XeF2 Intensities of the lines were estimated visually (vs-very strong, s-strong, w-weak, vw-very weak). M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5... 218 Acta Chim. Slov. 2002, 49, 209-220. 3. The reactions of Ni2F5: Oxidation of Ni2F5 with KrF2: Ni2F5 (1.13 mmol) reacted with large excess of KrF2 (approximately 8 mmol) in aHF at 273 K in PFA reaction vessel. Reaction was finished after four days. Fluorine, which is the product of thermal decomposition of KrF2 had to be pumped away during the reaction several times at 77 K. Kr, product of the reaction and from thermal decomposition of KrF2, was also pumped away at 213 K. Black R-NiF3 (2.16 mmol) was characterised by XRPP. Reaction of Ni2F5 with AsF5 in aHF: AsF5 (4.7 mmol) was added to the suspension of Ni2F5 (0.87 mmol) in aHF. Product of the reaction at 273 K was yellow solution, from which Ni(AsF6)2 (1.64 mmol, calc.: 1.74 mmol) was isolated (determined by XRPP). The reaction was very quick even at 273K (10 minutes). Reaction of Ni2F5 with KF in aHF: Ni2F5 (1.16 mmol) was weight in one arm of PFA vessel and KF (1.88 mmol) into another one. Anhydrous HF was condensed in the arm with Ni2F5 and thermostated for several hours at 273 K. The solution above solid Ni2F5 was completely colourless. Than KF dissolved in aHF which was decanted from the arm with Ni2F5 was added to the Ni2F5. Reaction between soluble KF and insoluble Ni2F5 was running for three days at 273 K. We noticed presence of K2NiF6 by red colour of the solution. Soluble KHF2 and K2NiF6 (mass of the soluble products was 0.199 g, calc.: 0.202 g) were separated from NiF2 (2.20 mmol, calc.: 1.74 mmol) by decantation of aHF solution. Washing procedure was repeated several times before aHF was pumped away and products of he reaction were checked by XRPP. K2NiF6 was also determined by Raman spectroscopy. Oxidation of Xe in aHF: excess of gaseous Xe (approximately 1.2 mmol) was added in the suspension of Ni2F5 (0.75 mmol) in aHF at room temperature. After 17 hours of the reaction all Ni2F5 was reduced to NiF2 (1.61 mmol, calc.: 1.50 mmol) and Xe was oxidized to XeF2 (0.35 mmol, calc.: 0.375 mmol). Unreacted Xe and aHF were pumped out at 238 K and XeF2 was pumped out at 293 K and checked by infrared spectroscopy. Ni2F5 like fluorinating agent: excess of solid and dry Ni2F5 (0.78 mmol) was exposed to perfluoropropen vapor (1.2.104 Pa). Rapid reaction at room temperature was M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5... Acta Chim. Slov. 2002, 49, 209-220. 219 exothermic and yellow solid NiF2 and gaseous C3F8, as was shown by infrared spectroscopy, were the only products. Acknowledgements This work was supported by the Ministry of Education, Science and Sport of the Republic of Slovenia. References 1. Simons J.H., J. Electrochem. Soc, 1949, 95, 47-67. 2. Simons J.H., Fluorine Chemistry; Academic Press, New York, 1950, vol. I, pp. 414-422. 3. Gramstead T.; Haszeldine R.N., J. Chem. Soc. 1956, 173-180. 4. Burden J.; Tatlow J. C., Adv. Fluorine Chem. 1960, 1, 129-165. 5. Page M., C.R. Acad. Sc. 1967, 246C, 2094-2097. 6. Stein L.; Neil J. M.; Alms, G. R., Inorg. Chem. 1969, 11, 2472-2476. 7. Court T. L.; Dove M. F. A., J. Chem. Soc. Chem. Commun. 1971, 726. 8. Court T. L.; Dove M. F. A., J. Chem. Soc. Dalton Trans. 1973, 1995-1997. 9. Žemva B.; Lutar K.; Jesih A.; Casteel W. J. Jr.; Bartlett N., J. Chem. Soc. Chem. Commun. 1989, 346-347. 10. Žemva B.; Lutar K.; Chacón L.; Fele-Beuerman M.; Allman J.; Shen C.; Bartlett N., J. Am. Chem. Soc. 1995, 117, 10025-10034. 11. Hector, A. L.; Hope E. G.; Levason, W.; Weller, M.T. Z. anorg. allg. Chem. 1998, 624, 1982-1988. 12. Shen, C.; Chacón L. C.; Rosov, N.; Elder, S. H.; Allman, J. C.; Barlett. N., C. R. Acad. Sci. Paris 1999, 2 Serie IIc, 557-563. 13. Baur W.H., Acta. Cryst. 1958, 11, 488-490. 14. Žemva B.; Lutar K.; Jesih A.; Casteel W.J. Jr.; Wilkinson A.P.; Cox D.E.; Von Dreele, Borrmann H., Bartlett N., J. Am. Chem. Soc. 1991, 113, 4192-4198. 15. T.E. Mallouk, Ph.D. Thesis, University of California, Berkely, 1983 16. Lucier G.; Shen C.; Casteel W.J. Jr.; Chacón L.; Bartlett N., J. Fluorine Chem. 1995, 72, 157-163. 17. Sartori P.; Ignat'ev N.; Datsenko S., J. Fluorine Chem. 1995, 75, 157-161. 18. Sartori P.; Ignat'ev N., J. Fluorine Chem. 1998, 75, 157-162. 19. Bartlett N.; Chambers R.D.; Roche A.J.; Spink R.C.H.; Chacón L.; Whalen J.M., Chem. Commun. 1996, 1049-1050. 20. Borrmann H.; Lutar K.; Žemva B., Inorg. Chem. 1997, 36, 880-882. 21. Šmalc A.; Lutar K., Inorganic Syntheses; Edited by R. N. Grimes, John Wiley&Sons, New York, USA, 1992, 29, pp. 1-4. 22. Šmalc A.; Lutar K.; Žemva B., Inorganic Syntheses; Edited by R. N. Grimes, John Wiley&Sons, New York, USA, 1992, 29, pp. 11-15. 23. Jesih A.; Žemva B., Acta. Chim. Slov. 1986, 33, 25-28. Povzetek Ni2F5 lahko pripravimo s termičnim razkrojem R-NiF3 pri 373 K in z redukcijo R-NiF3 s Xe oziroma XeF2. Termični razkroj je najprimernejša metoda sinteze. Produkta reakcje v kislem HF (AsF5) sta Ni(AsF6)2 in F2. Pri reakciji v bazičnem HF (KF) pa Ni(III) v Ni2F5 disproporcionira in dobimo NiF2 in K2NiF6. Ni2F5 je še vedno dovolj močan oksidant in M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5... 220 Acta Chim. Slov. 2002, 49, 209-220. fluorirno sredstvo da lahko oksidira ksenon do XeF2 in fluorira C3F6 do C3F8. Zaradi tega je njegova vloga pomembna v procesu elektrokemijskega fluoriranja. M. Tramšek, B. Žemva: Higher Fluorides of Nickel: Syntheses and Some Properties of Ni2F5...