229
Acta Chim. Slov. 1999, 46(2), pp. 229-238
NOVEL SYNTHESES OF SOME BINARY FLUORIDES: THE ROLE OF ANHYDROUS HYDROGEN FLUORIDE *
Zoran Mazej, Karel Lutar and Boris Zemva
Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
(Received 3.4.1999)
Abstract
The novel syntheses of MnF3, CoF3, BiF3, Pd2F6, PdF4, LaF3 and SmF3, using the approach in liquid anhydrous hydrogen fluoride as a solvent at room temperature are described. Some advantages, like mild reaction conditions and purity of the reaction products, are discussed in comparison to the methods described so far for the syntheses of these binary fluorides.
Introduction
Anhydrous hydrogen fluoride (aHF) has a very important and versatile role in the syntheses of binary fluorides. It can be used as fluorinating agent, solvent, catalyst, protecting atmosphere, etc. Only a few of the binary fluorides can be prepared just by dissolving corresponding element in liquid aHF, in majority of cases elevated temperature and use of gaseous aHF are necessary. A lot of fluorination reactions of different starting compounds (e.g. oxides, halides, carbonates, etc.) with either pure aHF or its concentrated water solutions are described in the literature. Most of them proceed at higher temperatures.
Anhydrous HF has a very long usable redox potential range of about 4.5 V. This
'Dedicated to the memory of Prof. Dr. Jože Šiftar
230
unique property makes it an excellent solvent for oxidative or reductive syntheses of binary fluorides at room and even lower temperatures. E.g., in liquid aHF elemental fluorine oxidizes AgF to AgF2 [1], I2 to IF5 or IF7 [2] and with KrF2 tetrafluorides of terbium and praseodymium can be obtained from Tb4O7 and Pr6O11 [3]. The reduction of hexafluorides of Mo, Re, Os and Ir in aHF by hydrogen or silicon leads to the corresponding pentafluorides or tetrafluorides [4,5]. Another approach for the preparation of binary fluorides in liquid aHF is their precipitation from corresponding anions with strong fluoride ion acceptors [6]. In this way some previously unknown compounds were prepared, e.g. AgF3 [7], NiF4 and NiF3 [8] beside some already known compounds, like pentafluorides of Nb, Mo, W, and Os [9], and PdF4 [10]. With the addition of fluoro bases to the aHF solutions of ternary compounds binary fluorides from corresponding cationic part are precipitated, e.g. PdF2 from Pd(SbF6)2 solution [11]. The oxidation of MF6- anions (M = Pt, Ru) by solvated cationic NiIV and AgIII species in aHF gives corresponding hexafluorides [12]. Solvolysis of some ternary compounds in aHF is also a route to binary fluorides, e.g. dissolutions of XeF2ACrF4 or Na2PrF6 in aHF yield CrF4 [13] and PrF4 [14] respectively. Thermal decomposition of complex compounds which are stable in liquid aHF only at lower temperatures leads to very pure binary fluorides, e.g. MnF4 [15]. Metathetic reactions could also be useful for the preparation of binary fluorides, e.g. mixing of solutions of K2NiF6 and Ni(AsF6)2 yields black precipitate of NiF3 [8].
Catalytic influence of aHF in many syntheses is well known, e.g. at photochemical preparation of XeF2 and XeF4 [16]. Anhydrous HF is sometimes used to prevent the hydrolysis, e.g. in the preparation of pure anhydrous binary fluorides via removal of hydrated water at high temperatures from corresponding hydrated fluorides in the flow of gaseous aHF.
In this paper a few novel syntheses of already known compounds in aHF as a solvent are described.
Experimental part 1. Apparatus and reagents. A nickel vacuum line with a mechanical pump, a mercury diffusion pump, and soda lime scrubbers were used for the manipulation of volatile fluorides. The part of the vacuum line used for the transfer of aHF, AsF5, and KrF2 was
231
made entirely from Teflon or FEP (tetrafluoroethylene-hexafluoropropylene copolymer) in order to diminish the corrosion and to avoid the formation of hydrogen. This part of the line was equipped with a Monel Helicoid pressure gauge (0-1500 Torr, " 0.3%, Bristol Babcock. Inc.) connected to the line via Teflon valve. The manipulation of the nonvolatile materials was done in a drybox (MBraun). The residual water in the atmosphere within the drybox never exceeded 1 ppm. PFA (tetrafluoroethylene-perfluoroalkylvinylether copolymer) reaction vessels (16 mm i.d. x 19 mm o.d., V . 40 ml) equipped with Teflon valves and Teflon coated stirring bars were used for all experiments. Prior to their use the reaction vessels were passivated with elemental fluorine.
Anhydrous hydrogen fluoride (aHF) (Praxair, 99.9%) was treated with K2NiF6 for several days prior to use. Fluorine was used as supplied (Solvay, 99.98%). KrF2 was prepared by the irradiation of a liquified mixture of fluorine and krypton with near-UV light at 77 K [17]. AsF5 was prepared from As2O3 and fluorine as described for the syntheses of PF5 [18]. K2MnF6 was prepared from a mixture of 9.00 mmol KF (0.523 g) (Merck, 99.9%) and 4.50 mmol MnF2 (0.418 g) (Riedel - De Haën) which was loaded in a PFA reaction vessel in a drybox. After evacuation of the reaction vessel on a vacuum line, aHF (6 ml) and KrF2 2.800 g (23 mmol) were condensed onto a mixture at 77 K. After warming to the room temperature the reaction was completed and clear orange solution was obtained. After the removal of aHF and gaseous decomposition products Kr and F2, a yellow solid K2MnF6 was obtained (calcd. 1.112 g; found 1.109 g), what was confirmed by X-ray powder diffraction pattern. Mn(AsF6)2 was prepared from MnF2 and AsF5 as described previously [19]. CoF2 was prepared from cobalt powder (Aldrich, 99.9%) and aHF in a flow reaction at 523 K [20]. Pd powder (Aldrich, 99.9%), and Bi powder (Alfa Johnson Matthey, 99.999%) were used as supplied. La2O3 (Koch Light Laboratories Ltd., 99.9 % (REO)) and Sm2O3 (Ventron, Alfa Products, 99.9 % (REO)) were heated prior to use at 1373 K for two hours in air to remove absorbed H 2O and CO2 [21]. Chemical analysis of lanthanoid oxides on metals after the heating gave: La2O3: La, calcd: 85.27%, found: 85.4%; Sm2O3: Sm, calcd: 86.23%, found: 86.1%. Both oxides were characterized also by X-ray powder diffraction patterns.
232
2.  Instrumentation. X-ray powder diffraction patterns were obtained by the Debye-Sch errer meth od using CuKa radiation on Seifert apparatus. Raman spectra were recorded on Renishaw Raman Imaging Microscope System 1000, with He-Ne laser with wavelength 632.8 nm.
3. Chemical analysis. After complete decomposition of the sample in alkaline melt total fluoride ion content was determined with an ion-selective electrode using ORION 960 Autochemistry System Analyser. Metals were determined by complexometric titrations.
4.   Preparation of binary fluorides. Synthesis of MnF3. For this experiment the reaction vessel was combined from two PFA tubes in a T-shape manner. In the dry-box 4.18 mmol (1.034 g) of K2MnF6 was loaded in one arm and 4.18 mmol (1.810 g) of Mn(AsF6)2 in the other arm. Then aHF was condensed onto both compounds at 77 K. After warming to room temperature colourless solution of Mn(AsF6)2 and red solution of K2MnF6 were obtained, both without any undissolved material. Then the solution of Mn(AsF6)2 was slowly poured onto the solution of K2MnF6 during simultaneous stirring. Red precipitate formed instantly and at the end of the reaction the solution was colourless. The precipitate was purified by decanting of the solution with KAsF6 and back distilling of aHF. This procedure was repeated several times. During this separation always some losses of MnF3 were observed due to incomplete sedimentation of MnF3. Therefore, the weight of the obtained product did not correspond to calculated one. The purity of obtained MnF3 was checked by X-ray powder diffraction pattern (the strongest lines of MnF3 were only observed) and by chemical analysis (Table 2). The product in another arm was KAsF6, as shown by X-ray powder diffraction pattern.
Syntheses of CoF3, BiF3, Pd2F6, PdF4, LaF3 and SmF3. Particular starting materials were loaded in PFA reaction vessels in a drybox. Then aHF was condensed onto the solid at 77 K and reaction mixture was warmed to room temperature. The detailes of reaction conditions are given in Table 1. For the preparation of CoF3, BiF3 and Pd2F6 fluorine was slowly added at the room temperature to the final pressure in reaction vessel as given in Table 1. The excess of fluorine was cca 1.5 to 4. After reactions were completed, aHF and F2 were removed and products of typical colours (light brown CoF3, black Pd2F6 , grey-white BiF3) were obtained. In the case of La2O3 or Sm2O3 reactions were carried out
233
without the addition of elemental fluorine. After reactions were completed white LaF3 and SmF3 were isolated. For the preparation of PdF4 krypton difluoride was added to the mixture of palladium and aHF immediately at 77 K. Already at 253 K yellow solution occurred and close to room temperature red solid started to precipitate. After one day
Table 1: Reaction conditions for the syntheses of CoF3, BiF3, Pd2F6, PdF4, LaF3
and SmF3
Starting compound	Mass		aHF (ml)	Pf2 (torr)	KrF2 (g)	Time of reaction (days)	Final product
	(g)	(mmol)					
CoF2	0.253	2.61	6	2000	-	4	CoF3
Bi	1.040	4.98	10	6000	-	1	BiF3
Pd	0.242	2.27	6	6500	-	7	Pd2F6
Pd	0.042	0.39	4	-	1.4	1	PdF4
La2O3	0.206	0.63	7	-	-	4	LaF3
Sm2O3	0.216	0.62	7	-	-	4	SmF3
Table 2: Mass balances and chemical analyses of obtained binary fluorides
Compound	Mass balance		Chemical analyses			
	Calculated	Obtained	Calculated		Obtained	
	(g)	(g)	%M	%F	%M	%F
MnF3	-	-	49.1	50.9	47.1	50.9
CoF3	0.303	0.306	50.8	49.2	50.4	48.7
BiF3	1.325	1.324	78.6	21.4	78.3	20.4
Pd2F6	0.371	0.401	65.1	34.9	63.9	34.6
PdF4*	0.071	0.083	-	-	-	-
LaF3	0.247	0.247	70.9	29.1	70.6	29.0
SmF3	0.257	0.257	72.5	27.5	72.2	27.5
* Not enough sample for chemical analysis.
234
aHF and gaseous decomposition products Kr and F2 were removed. A brick-red solid PdF4 was obtained.
Mass balances and chemical analyses of MnF3, CoF3, BiF3, Pd2F6, PdF4, LaF3 and SmF3 are collected in Table 2. All products with the exception of PdF4 were confirmed by X-ray powder diffraction patterns. PdF4 was poorly crystallised and, therefore, it was not possible to obtain X-ray powder diffraction pattern. The Raman spectra of PdF4 and Pd2F6 were recorded.
Results and discussion
Three binary fluorides of manganese are known, MnF2, MnF3 and MnF4. Preparations of MnF3 may involve contaminations with MnF2 or MnF4, depending on the synthetic route. MnF3 is usually prepared by flow fluorination of MnX2 (X = F, Cl, I) at higher temperatures [22]. Other preparations include reactions between Mn(IO3)2 and BrF3, where it is necessary to remove BrF3 at high temperature (773 K) [22], and reactions of MnO or Mn3O4 with fluorine [22]. The metathetic reaction between Mn(II) and Mn(IV) ternary fluorides is based on the method used for synthesis of NiF3 [8], and proceeds according to the equation:
Mn ( A sF 6 ) 2 + K 2 Mn F 6 ľ aRľH. TFľ.   ľ? 2MnF3 + 2KAsF6
This reaction offers MnF3 without MnF2 and MnF4 as impurities. However, possible impurity here is KAsF6. Therefore thorough washing of MnF3 precipitate with aHF, in which KAsF6 is rather well soluble, is necessary. (XeF5)2MnF6 is even better starting compound as K2MnF6, because XeF5AsF6 is much more soluble in aHF than KAsF6. Chemical analysis of MnF3 in Table 2 shows that some KAsF6 can still be present in the sample, although X-ray powder diffraction pattern showed only the strong lines of MnF3. Reactions between CoF2, Bi and Pd in aHF with excess of elemental fluorine proceed smoothly and the course of reactions was followed by changing of the colours of the solids where possible (starting compounds and products are not soluble in aHF), and by consumption of fluorine followed by the drop of its pressure. The mass balances and chemical analyses showed that obtained products are CoF3, BiF3 and Pd2F6 (Table 2). X-ray powder diffraction photographs showed only the lines attributed to these compounds.
235
The literature methods for the syntheses of above mentioned binary fluorides in all cases quote more severe conditions, e.g. temperatures above 473 K. Fluorination in liquid aHF is especially appropriate for the synthesis of BiF3. With direct reaction of elements without the presence of aHF, BiF5 is formed [23]. BiF3 is usually prepared from Bi2O3 or BiOCl in aqueous HF [24, 25]. In this case water is a by-product and must be removed. Another problem can be the formation of bismuth oxide-fluorides [25]. The suggested methods for the preparation of Pd2F6 are reactions between PdCl2 or PdBr2 with BrF3 and subsequent decomposition of PdF3/BrF3 adducts [26], or fluorination of palladium metal [27]. In the first case there is a problem of removing last traces of BrF3. For the second reaction high temperature is needed and there is also a possibility that some PdF4 will form.
Synthesis of PdF4 requires fluorination of Pd2F6. For this reaction it is important that the starting compound is nearly amorphous [28], otherwise fluorination process is slow. The contamination with the starting compound may be avoided only by fluorination over a period of several days. More efficient preparation is the precipitation of PdF4 with Lewis acids from PdF62- salts [10]. In this case, similarly to the case of MnF3, it is necessary to remove by-product. The course of the reaction between palladium and KrF2 in aHF is similar to the one in the system MnF2/KrF2/aHF [15]. The reaction started to proceed already at lower temperatures (~253 K) with the formation of yellow-coloured solution, indicating that the adduct between PdF4 and KrF2 exists (PdF62- ions are yellow). With further reaction the yellow soft-lumped material is formed. This intermediate adduct is thermally unstable, and close to room temperature, without the excess of KrF2 decomposes to PdF4, krypton and fluorine. At the end of reaction homogeneous brick-red PdF4 and colourless solution are obtained. It was not possible to obtain X-ray powder diffraction pattern of isolated reaction product. In the Raman spectrum beside all characteristic bands of PdF4 [29], the strongest band of Pd2F6 (565 cm-1) was always present. This is in accordance with Bartlett [29] noticing that it is very difficult to prepare PdF4 without Pd2F6 as an impurity. In the described method the reason for the partial formation of Pd2F6 can be the presence of insoluble soft-lumps, specifically lighter material then aHF, which covered some Pd2F6 and hindered further reaction. With the purpose of obtaining completely soluble intermediate products the reaction between Pd and KrF2 in aHF was carried out with the addition of AsF5. First
236
completely clear green-blue solution of Pd(AsF6)2 was obtained [30], already below ambient temperature. The colour of this solution quickly turned to yellow which for a short period contained no solid residues. Obviously KrF2 was able to oxidize cationic Pd2+ to PdF62-. From this solution then brick-red solid started to precipitate owing to thermal decomposition of KrF2/PdF4 adduct. The formed PdF4 is strong enough fluoro acid that it does not react with AsF5 in aHF. The X-ray powder diffraction pattern of the isolated PdF4 showed only the lines attributable to PdF4. The Raman spectrum of the red solid material in aHF solution was without the strongest band for the Pd2F6, while the isolated PdF4 showed this band again. Therefore, one can take into account also the possibility that PdF4 decomposes in the laser beam.
There are previous reports about reactions between Ln2O3 and gaseous aHF, however, only at elevated temperatures (873-1073 K) [31, 32]. Our results show that reactions between some Ln2O3 (Ln = La, Sm) and liquid aHF can proceed at room temperature without the addition of fluorine, and that pure LaF3 or SmF3 form. Reactions of less basic representatives in the lanthanoid series are very slow and, therefore, the described approach is inconvenient in these cases.
Conclusions
The described syntheses of some binary fluorides (MnF3, CoF3, BiF3, Pd2F6, PdF4, LaF3 and SmF3) in aHF as a solvent at room temperature offer novel, milder approach for the formation of these compounds. All reactions were carried out in reaction vessels made of PFA, which is very resistant and inert material and therefore there is no danger of final product contamination by by-products formed between reagents and reaction vessel material. This possibility should be taken into account when metal reaction vessels and very high temperatures are involved. The advantages of reactions in solution are easier control of fluorination and homogeneous products with uniform particle sizes. Metathetic reactions in liquid aHF offer an access to intermediate oxidation states of binary fluorides which are sometimes difficult to prepare in high purity. Reactions between metals and KrF2 in liquid aHF can give binary fluorides of very high purity with metals in the highest oxidation states. The choice of starting material in this approach is sometimes also important, e.g. in the case of the synthesis of Pd2F6, the
237
reaction of palladium and fluorine in aHF proceeds rather quickly in comparison with PdO, whereas there is no reaction with PdF 2.
Therefore, the described approach deserves more frequent use in the preparation of various binary fluorides, not only for ones described here. However, this approach can be used only in laboratories properly equipped for the work with aHF.
Acknowledgements
The authors are grateful to Robert Moravec for help in preparative work, Borka Sedej for chemical analyses, and Ministry of Science and Technology of the Republic of Slovenia for financial support.
References
[I]   A.W. Jache, G.H. Cady, J. Chem. Physics 1953, 56, 1106-1109.
[2]   T.A. O’Donnell, Superacids and Acidic Melts as Inorganic Chemical Reaction Media, VCH
Publishers, Inc., New York, 1993, p. 212. [3]   J.M. Kiselev, V.B. Sokolov, Zh. Neorg. Khim. 1984, 29, 857-859. [4]   R.T. Paine, L.B. Asprey, Inorg. Chem. 1975, 14, 1111-1113. [5]   R.T. Paine, L.B. Asprey, Inorg. Chem. 1974, 13, 1529-1531. [6]   B. Ženiva, K. Lutar, A. Jesih, W.J. Casteel Jr., N. Bartlett, J. Chem. Soc. Chem. Commun. 1989,
346-347. [7]   B. Ženiva, K. Lutar, A. Jesih, W.J. Casteel Jr., A.P. Wilkinson, D.E. Cox, R.B. VonDreele, H.
Borrmann, N. Bartlett, J. Am. Chem. Soc. 1991, 113, 4192-4198. [8]   B. Ženiva, K. Lutar, L. Chac.n, M. Fele-Beuermann, J. Allman, C. Shen, N. Bartlett, J. Am. Chem.
Soc. 1995, 117,  10025-10034. [9]   T.A. O’Donnell, T.E. Peel, J. Inorg. Nucl. Chem. Supplement 1976, 61-62. [10] G.M. Lucier, C. Shen, S.H. Elder, N. Bartlett, Inorg. Chem. 1998, 37, 3829-3824.
[II]  G. Lucier, S.H. Elder, L. Chac.n, N. Bartlett, Eur. J. Solid State Inorg. Chem. 1996, 33, 809-820. [12] G. Lucier, C. Shen, W.J. Casteel, Jr., L. Chac .-.n, N. Bartlett, J. Fluorine Chem. 1995, 72, 157-163. [13] K. Lutar, I. Leban, T. Ogrin, B. Ženiva, Eur. J. Solid State Inorg. Chem. 1992, 29, 713-727.
[14] J. Sorlano, M. Givon, J. Shamir, Inorg. Nucl. Chem. Letters 1966, 2, 13-14.
[15] K. Lutar, A. Jesih, B. Ženiva, Polyhedron 1988, 7, 1217-1219.
[16] K. Lutar, A. Šmalc, J. Slivnik, Vestn. Slov. Kem. Drus. 1979, 26, 435-450.
[17] A. Šmalc, K. Lutar, B. Žemva, Inorg. Synth. 1992, 29, 11-15.
[18] A. Jesih, B. Žemva, Vestn. Slov. Kem. Drus. 1986, 33, 25-28.
[19] B. Frlec, D. Gantar, J. Fluorine Chem. 1982, 19, 485-500.
238
[20] G. Brauer, Handbuch der Pr@parativen Anorganischen Chemie, Vol.1, Ferdinand Enke Verlag,
Stuttgart, 1975, p. 275. [21] B.G. Mhller, Lanthanide Fluorides, in Synthesis of Lanthanide and Actinide Compounds, Eds. G.
Meyer, L.R. Morrs, Kluwer Academic Publishers, Dordrecht, 1991. [22] R. Colton, J.H. Canterford, Halides of the First Row Transition Metals, Wiley-Interscience, 1969,
p. 214. [23] J.D. Smith, The Chemistry of Arsenic, Antimony and Bismuth, Pergamon Press, Oxford, 1975, p. 648. [24] J.D. Smith, The Chemistry of Arsenic, Antimony and Bismuth, Pergamon Press, Oxford, 1975, p. 588. [25] G. Brauer, Handbuch der Pr@parativen Anorganischen Chemie, Vol.1, Ferdinand Enke Verlag,
Stuttgart, 1975, p. 218. [26] N. Bartlett, P.R. Rao, J. Chem. Soc, 1964, 393-394. [27] G. Brauer, Handbuch der Pr@parativen Anorganischen Chemie, Vol.1, Ferdinand Enke Verlag,
Stuttgart, 1975, p. 280. [28] P.R. Rao, A. Tressaud, N. Bartlett, J. Inorg. Nucl. Chem. Supplement 1976, 23-25. [29] N. Bartlett, B. Žemva, L. Graham, J. Fluorine Chem. 1976, 7, 301-319. [30] Z. Mazej, H. Borrmann, K. Lutar, B. Žemva, J. Darriet, J. Grannec, A. Tressaud, Palladium chemistry
in anhydrous hydrogen fluoride acidified with arsenic pentafluoride, 12th European Symposium on
Fluorine Chemistry, Berlin, 1998, Abstracts, p. B20. [31] L.R. Bacanova, Uspehi Himii 1971, 40, 945-979. [32] B.G. Mhller, Lanthanide Fluorides, in Synthesis of Lanthanide and Actinide Compounds, Eds. G.
Meyer, L.R. Morss, Kluwer Academic Publishers, Dordrecht, 1991.
Povzetek
Opisane so nove sinteze MnF3, CoF3, BiF3, Pd2F6, PdF4, LaF3 in SmF3 z uporabo brezvodnega vodikovega fluorida kot topila pri sobni temperaturi. Diskutirane so nekatere prednosti, npr. blagih reakcijskih pogojev in cistote reakcijskih produktov v primerjavi z doslej opisanimi metodami za sintezo teh binarnih fluoridov.