Fluorine Chemistry Meets Liquid Ammonia

Home , Beryllium fluoride, Fluoride, Uranium hexafluoride, Uranium tetrafluoride

DOI 10.1515/irm-2012-0003 BioInorg React Mech 2012; 8(1-2): 29–39

Review

Florian Kraus* Fluorine chemistry meets liquid ammonia

Abstract : Chemistry of metal and non-metal fluorides in Secco, 1972). X-ray structural studies were carried out liquid ammonia is often severely hampered, due to the low and compounds like [BF3 (NH3 )], [SiF4 (NH3 )2 ], [Zr(NH3 ) solubility of inorganic fluorides. This can be overcome by F 4 ], [Hf(NH3 )F4 ], [Ga(NH3 )F3 ], and [Ga(NH3 )2 F3 ] were char- applying either strongly oxidizing fluorides, appropriate acterized (Plitzko and Meyer, 1996; Plitzko et al., 1997; fluoride ion acceptors, or by the reduction or conversion of Roos and Meyer, 1999; G ö bbels et al., 2002). Ammoniates fluorides using solvated electrons. The article summarizes like [Sn(NH 3 )4 ]F2 have been considered as precursors for the state-of-the-art of the chemistry of inorganic fluorides nitride fluorides (Woodward et al., 1998), of which only in liquid ammonia, with special emphasis on compounds M NF (M = Ti, Zr, Hf, U) (Juza and Sievers, 1968; Yoshihara M M of beryllium, silver and uranium. et al., 1969; Jung and Juza, 1973a,b), 2 NF ( = Mg, Sr) (Andersson, 1970; Wagner, 2002; Brogan et al., 2012), Keywords: beryllium; inorganic fluorides; liquid ammonia; one iron (Menil et al., 1975), and some rare earth metal silver; uranium. (Tanguy et al., 1971, 1972; Pezat et al., 1976; Vecher et al., 1984; Vogt et al., 1989) and zinc nitride fluorides (March- and and Lang, 1971) are known. Only two anionic nitride *Corresponding author: Florian Kraus, Arbeitsgruppe fluorides, [TcNF ] − and [ReNF ] − , have been described Fluorchemie, Department Chemie, Technische Universität München, 4 4 Lichtenbergstrasse 4, D-85747 Garching, Germany, (Baldas et al., 1997; Voigt et al., 1998). Recently the first e-mail: [email protected] quaternary nitride fluoride Ce2 MnN3 F2 − δ was obtained by

direct fluorination of Ce2 MnN3 (Headspith et al., 2009).

Nitrides such as Li3 N, Be3 N2 , BN, Si3 N4 , TiN and VN react

with F 2, if at all, only under formation of N 2 and the pure Introduction fluoride (Schumb and O ’ Malley, 1964). Nitride fluorides, however, are an interesting class of compounds The knowledge about reactions of fluorides in liquid and should – due to the analogy N3 − + F − ≈ 2O2 − – feature ammonia is very limited. In 1898, Kraus and Franklin properties similar to oxides (Headspith and Francesconi, commented on the solubility of metal fluorides in 2009). ammonia that “ the fluorides are insoluble ” (Franklin The information about amide and imide fluorides and Kraus, 1898). Due to the high lattice energies of many is also very limited. The existence of amide and imide fluorides and to the resulting low solubilities, reactions fluorides of boron has been shown by mass spectroscopy of fluorides in ammonia remained largely unexplored. (G ö bbels et al., 2002) and the crystalline amide fluo-

Fluorides like NH 4F, alkali and alkaline earth metal rides Ga(NH3 )(NH 2 )F 2 and Sn(NH2 ) 2F 2 were obtained in fluorides, many transition metal fluorides, rare earth solid-state reactions (Weber et al., 1998; Roos and Meyer, and actinide fluorides are insoluble in liquid ammonia 1999b). (Jander et al., 1966; Dougal et al., 1988; Lagowski, 2007). The number of reactions of fluorides with liquid or

Exceptions are AgF, LiF, NaF, BeF2 , BF3 , SiF4 , and UF6 gaseous ammonia is very small compared to the corre- (Jander et al., 1966). Biltz and coworkers obtained the sponding reactions of the other halides, pseudo halides, ammoniates of some of the insoluble fluorides by extract- Zintl anions and related compounds. In some reviews of ing hydrates of the fluorides with liquid ammonia. This “ liquid ammonia chemistry ” the fluorides are not men- way, they obtained the ammoniates of AlF 3 , GaF3 , InF3 , tioned at all (Dougal et al., 1988; Lagowski 2007). However,

MnF 2 , FeF2 , CoF2 , NiF2 , AgF, and ZnF2 and character- the reactions of BF3 , SiF4 and SF4 with NH3 have been inves- ized them by tensieudiometric (vapour pressure and gas tigated in great detail (Gay-Lussac and Thenard, 1809; volume) measurements or by elemental analysis (Biltz Davy, 1812; Brown and Johnson, 1945; Cohen et al., 1966; et al., 1927). Patil and Secco carried out thermal analy- Plitzko and Meyer, 1996; G ö bbels et al., 2002; Kraus and sis on ammoniates of bivalent metal fluorides (Patil and Baer, 2010).

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Mixed anionic compounds of other halides with nitro- as usual, to the aqueous system. However, due to differ- gen-containing anions are much more common. These ences in solvation enthalpies of the two solvent systems, compounds have very different physical and chemical standard potentials do differ, leading to pronounced properties compared to the fluorides (Jung and Juza, 1973b). differ ences. For example, Cu and Cu(II) should spontane- Recently we have shown that if appropriate reaction ously form Cu(I) in liquid ammonia; in water, dispropor- conditions are employed, liquid ammonia is a much better tionation is thermodynamically favored. However, pure solvent for inorganic fluorides than expected (Meng 2008; thermodynamic reasoning is also improper; NH3 has a very Kraus and Baer, 2009, 2010, 2011a,b; Kraus et al., 2009a,b, narrow window of electrochemical stability of only 0.04 V, 2011, 2012; Baer and Kraus, 2010). Subsequently we inves- but it is kinetically very stable, as large overpotentials can tigated the chemistry of inorganic fluorides in liquid be present (Jander et al., 1966). So, redox potentials may ammonia solution in more detail. only serve as an aid for the choice of proper systems, but other effects have to be taken into account, which must be established experimentally. Discussion Two new compounds containing cesium fluoride have been obtained as side-products from the reactions of Cs CuF and Cs KDyF , which are both strong oxidiz- For successful fluorine chemistry in liquid ammonia, the 2 6 2 6 ers, with liquid ammonia. To the best of our knowledge, solvation problem has to be “ solved ” . Biltz and Rahlfs used no redox potentials have been determined for compounds the extraction of fluoride hydrates with liquid ammonia containing Cu(IV). Cs CuF reacts under the formation (Biltz and Rahlfs, 1927). These hydrates dissolve reasonably 2 6 of the blue compound [Cu(NH ) ]F · NH , and the color- well in liquid ammonia, because they have lower lattice 3 5 2 3 less ammoniate Cs F (NH ) ( Figure 1 ) (Baer and Kraus, energies compared to those of the anhydrous fluorides, 3 3 3 4 2010). The latter is also obtained from the reaction of BeF and water dissolves excellently in liquid ammonia. By this 2 with CsNH in liquid ammonia. Cs F (NH ) represents method, they synthesized several ammoniates of fluorides 2 3 3 3 4 the first ammoniate of the “ insoluble ” alkali metal fluo- (Biltz et al., 1913; Biltz, 1914a,b, 1925a,b; Biltz and Stollen- rides. The reaction of Cs KDyF leads to colorless crystals werk, 1920, 1921; Biltz and H ü ttig, 1921; Biltz and Fischer, 2 6 of NH CsF ( Figure 1 ) (Baer and Kraus, 2010), which could 1922, 1923; Biltz and Hansen, 1923; Biltz and Messerknecht, 4 2 not be obtained from solutions of NH F and CsF in liquid 1923, 1925; Biltz and Mau, 1925; Biltz and Wein, 1925). 4 ammonia, or from aqueous solutions. We came up with three different approaches in order to perform fluorine chemistry in liquid ammonia: 1. Usage of compounds with atoms in high oxidation states. Chemistry of silver in high oxidation states 2. Usage of strong fluoride ion acceptors. 3. Usage of ammonia-soluble metals for the conversion Silver compounds with the element in the higher oxidation of fluorides. states Ag(II) or even Ag(III) are accessible via fluorine chemistry. These highly oxidized silver cations are strong oxidizing agents, and thus the “ insoluble fluorides ” (Franklin and Usage of compounds with atoms in high Kraus, 1898) are expected to react with ammonia, and the resulting Ag(I) cations should be solvated. It is well known oxidation states that Ag+ cations can easily be converted into the diammine silver(I) complex with aqueous ammonia. We prepared com- The employment of elemental fluorine or other strong pounds with the compositions Ag M F (M = Zr, Hf), AgF and fluorooxidants (ClF ) in general leads to high oxidation 3 2 14 2 3 AgF and reacted them with liquid ammonia. The intense states. If metal fluorides with elements in a high oxidation colors of the silver(II) compounds vanished immediately state are brought into contact with liquid ammonia at low upon contact with ammonia, and the evolution of nitro- temperatures, the NH molecule is oxidized and nitrogen is 3 gen was observed. From these reactions, we obtained com- produced. At the same time, the metal fluoride is reduced pounds like [Ag(NH ) ]F · 2 NH ( Figure 2 ), N H F ( Figure 3 ), and dissolved in NH . This method of “ dissolving insolu- 3 2 3 2 7 3 [M F (NH ) ] · NH (isotypic to the uranium compound, see ble fluorides” has been called the “ reactive fluoride route” 4 3 4 3 Figure 4 ), and [Ag(NH ) ( μ -NH )Ag(NH ) ][M F ] (M = Zr, Hf) (Meng and Kraus, 2008). As the liquid ammonia solvent 3 3 3 3 4 6 (Figure 3 ) (Meng and Kraus, 2008; Kraus et al., 2009a). As system shows some similarities to the aqueous system, an example, the reduction of AgF is given in Eq. (1): one may think that a correlation of redox potentials should 2 exist. The redox potentials available for oxidizers refer, 6AgF2 +38NH3 6[Ag(NH3 )2 ] F · 2 N H3 +N2 +6N2 H7 F (1)

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AgF2 is reduced to Ag(I), thereby forming the first Chemistry of uranium in high oxidation states structurally characterized halide salt of the well-known diammine silver(I) cation as an ammoniate. Nitrogen and From the available literature, it has become obvious that those

N2 H7 F are produced as byproducts. reactions of uranium which have been explored, are mainly

b c

a b

Figure 1 The unit cells of Cs3 F3 (NH3 )4 and NH4 CsF2 . Displacement parameters at 70% at -150°C.

b a

+ Figure 2 The unit cell of [Ag(NH3 )2 ]F · 2 NH3 and the hydrogen bonding of the [Ag(NH3 )2 ] cation to adjacent fluoride ions and ammonia molecules. Displacement parameters at 70% at -150°C.

a c

Zr/Hf F Ag N

Figure 3 The unit cell of [N2 H7 ]F and the molecular structures of [Ag(NH3 ) 3 ( µ -NH3 )Ag(NH3 )4 ][M F6 ] (M = Zr, Hf). Displacement parameters at 70% at -150°C.

- 10.1515/irm-2012-0003 Downloaded from De Gruyter Online at 09/28/2016 09:24:33PM via Technische Universität München 32 F. Kraus: Fluorine chemistry meets liquid ammonia relevant for mining, purification and isotope separation. (Berthold and Hein, 1969). Juza and coworkers where However, in the last couple of years, uranium chemistry has able to obtain the mixed anionic compounds UNCl, UNBr been revisited, especially in the field of organometallic chem- und UNI from the tetrahalide and ammonia at high tem- istry. Molecular uranium compounds are used for the cata- peratures (Juza and Meyer, 1969). Molecular NUF 3 was lytic activation of nitrogen and carbon dioxide (Ephritikhine, observed spectroscopically at 4 K (Andrews et al., 2008). 2006; Bart and Meyer, 2008; Fox et al., 2008); organometal- These results prompted us to attempt the synthesis of lic hexakisamido complexes of uranium have been prepared such compounds in liquid ammonia at low temperatures. (Meyer and Minidola, 2000; Meyer et al., 2000), as well as Redox reactions of uranium compounds in liquid organometallic imido complexes, (Berthet et al., 2008) and ammonia seem to be limited. Galkin and coworkers observed rings of uranium with alternating nitride and azide bridges the reduction of UF 6 by NH 3 leading to a uranium(V) com- (Evans et al., 2005). Recently, an organometallic nitrido pound (Galkin et al., 1960). In our own studies, we found that cluster of uranium was isolated (Nocton et al., 2008a,b). More this reduction leads to a uranium(IV) compound, namely uncommon oxidation states of uranium, like U(V), have been (NH4 ) 2 (N 2 H 7)[UF 7 (NH 3 )] (Figure 5 ) (Kraus and Baer, 2009). stabilized using appropriate ligands (Graves and Kiplinger, It is interesting to note that Kline and Kerschner used

2009), and we have focused on purely inorganic systems silver(I) acetate in liquid NH3 for the oxidation of a green considering the choice of ligands and the solvents used. uranium(IV) to a yellow U(VI) compound under deposit- Berthold and coworkers investigated some ammonoly- ion of Ag (Kline and Kershner, 1966). We, however, were sis reactions of uranium halides and were able to prepare an not able to reproduce this result. 238 imide chloride of uranium (Berthold and Knecht, 1965a,b,c,d, Depleted uranium hexafluoride UF6 is available 1966a,b, 1969; Berthold and Delliehausen, 1966; Berthold et worldwide in enormous amounts; the production of 1ton al., 1966; Berthold and Hein, 1969). At the same time, Burk of enriched uranium (3 – 5% 235 U) for nuclear power sta- considered the formation of amide halides in reactions of tions leads to about 7 tons of depleted uranium. However,

UCl3 and UBr3 with ammonia as unlikely; UCl4 and UBr4 , the long-term storage of UF6 is problematic due to its cor- however, form a series of mixed anionic compounds, and rosivity. Thus, the conversion into less-reactive substances also the pure nitrides could be obtained (Burk and Naumann, would be desirable. UF6 reacts with NH 3 already at -78 ° C. 1966, 1969; Burk, 1967, 1969). Both Berthold and Burk used The products of this reaction have only been characterized temperatures between 20 and 1000° C for their experiments. by elemental analysis (Galkin et al., 1960). We studied the

The reactions of uranium halides with organo-substituted reaction of UF6 with liquid NH3 in more detail and charac- − amides like [N(SiMe 3 )2 ] led to uranium amide complexes terized all reaction products, which are N2 , emerald green

(Drozdzynski, 2005). However, information on the reactions [UF 4 (NH3 )4 ] · NH3 , and signal green (NH4 )2 (N2 H7 )[UF7 (NH3 )] with pure amides remained elusive. In reactions of uranium (see above) (Kraus, 2009). tetrafluoride, Berthold and coworkers obtained emerald- green crystals (Berthold and Delliehausen, 1966b), however,

Burk did not observe any reaction with UF3 or UF4 in NH3 Usage of strong fluoride ion acceptors (Burk, 1969). We confirmed the existence of the emerald- green crystals and elucidated their composition and struc- Fluoride ion abstraction in liquid ammonia may afford ture to be UF4 (NH3 )4 · NH3 ( Figure 4 ) (Kraus and Baer, 2009). amide, imide and nitride fluorides and may very well also At high temperatures, Berthold and coworkers obtained crystalline amide and imide fluorides of U(III), which were said to crystallize in the fluorite-type structure N

c F a b

U U

N F c

Figure 5 The unit cell of (NH4 )2 (N2 H7 )[UF7 (NH3 )] and the molecular 3 − Figure 4 The unit cell of [UF4(NH3)4] · NH3 and molecular structure of structure of the [UF7 (NH3 )] anion. Displacement parameters at 70%

[UF4(NH3)4]. Displacement parameters at 70% at -150°C. at -150°C.

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lead to pure nitrides as the final products. Since the abun- According to a Guinier photograph, Be(NH3 )4 Cl2 should be dantly used strong fluoride ion acceptors SbF 5 and AsF5 isotypic to K2 BeCl4 (Semenenko, 1965) – a finding which we cannot be used in liquid ammonia (due to decomposition were not able to reproduce (Kraus et al., 2012). to yet unidentified orange products), we set out to explore Finally, we succeeded in the synthesis and unambigu- other fluoride ion acceptors such as BF 3 and SiF4 . We also ous characterization of the tetraammine beryllium cation 2+ 2+ studied Be and UO2 as fluoride ion acceptors which are in the compound [Be(NH3 )4 ]2 Cl4 · 17 NH3 ( Figure 6 ) (Kraus et much easier to handle compared to BF3 and SiF4 . al., 2012). This, which was characterized by single-crystal X-ray analysis, IR and NMR spectroscopy, should be a very versatile fluoride ion acceptor in liquid ammonia. Fluoride ion acceptors based on beryllium Reactions of beryllium fluorides in ammonia have been studied even less than those of the other halides or

Compared to its neighbors in the periodic table of elements, pseudohalides: Grigor’ ev and coworkers reported that BeF 2 the chemistry of beryllium is poorly developed (Metz et reacts with liquid ammonia under formation of [Be(NH3 )4 ] al., 2006; Dehnicke and Neum ü ller, 2008), and the sheer [BeF4 ] (Grigor’ ev et al., 1967), but we have shown that this number of theoretical studies published on beryllium reaction leads to [BeF2 (NH3 )2 ] ( Figure 7 ). The compound chemistry is by far larger than the number of experimen- was characterized by X-ray structural analyses on single tal investigations. However, the chemistry of beryllium in crystals, IR and Raman spectroscopy and by thermogravi- aqueous solutions (Schmidt and Schmidbaur, 1998; Holle- metric analyses (Kraus et al., 2009b; Kraus et al., 2012). man and Wiberg, 2007) in glasses and in organic solvents With BeF2 as a fluoride ion acceptor from UF4 in liquid is very well explored (Schmidbaur et al., 1991; Han and ammonia, [N2 H7 ][BeF3 (NH3 )] was obtained as a product Parkin, 1993; Ruhlandt-Senge et al., 1993; Sohrin et al., (see below) (Kraus et al., 2012), which was probably 1993; Dressel et al., 2003; Puchta et al., 2005; Neumüller et formed via the reactions in Eqs. (2) –(4). al., 2008; Puchta and van Eldik, 2008a,b,c; Puchta et al., 2009a,b) and comprehensive studies in the field of beryllium chemistry have been performed and summarized in a review by Dehnicke (Dehnicke and Neum ü ller, 2008). However, reactions of beryllium compounds with and in N liquid ammonia have not been greatly investigated. Beryllium is insoluble in liquid ammonia (Jander et Be al., 1966). The addition of NH4 Cl or electrolysis leads to faintly bluish solutions. In 1927, Bergstrom reacted Be with c NH4 Cl in liquid ammonia and obtained two products of the composition BeCl2 · 2 NH3 and BeCl 2 · 4 NH3 which were stable b a at ambient temperature. The same products had been obtained in 1913 by Mieleitner and Steinmetz from BeCl2 Figure 6 Left: The unit cell of [Be(NH ) ] Cl · 17 NH .[Be(NH ) ]2+ cations and NH3 (Mieleither and Steinmetz, 1913; Bergstrom, 1928). 3 4 2 4 3 3 4 are depicted as small tetrahedra, [Cl (NH ) ]4 − anions are shown BeCl · 4 NH presumably contains the tetraammine beryl- 4 3 4 2 3 as large tetrahedra. Right: Molecular structure of the tetra ammine lium cation for which, to our knowledge, no structural evi- beryllium dication. Displacement parameters at 70% at -150°C. dence is available so far, although it is mentioned in many text books on inorganic chemistry (Cotton et al., 1999; Hol- 9 leman and Wiberg, 2007). A Be NMR spectrum of BeCl2 c has been recorded in liquid ammonia, and the signal has b 2+ a F been assigned to the Be(NH3 )4 cation, (Kovar and Morgan, 1970). We could, however, show that the reported signal was Be μ 3+ due to the oxygen-containing species [Be3 ( -OH)3 (NH3 )6 ] (Kraus, 2012). IR analyses gave evidence of the existence N 2+ of [Be(NH3 )4 ] (Grigor’ ev et al., 1969; Sipachev et al., 1969), but these findings were heavily doubted (Schmidt and Mü ller, 1976). In tensieudiometric measurements, Biltz and coworkers have found adducts of two, four, six and twelve Figure 7 The unit cell of [BeF2 (NH3 )2 ] and its molecular structure. molecules of ammonia (Biltz and Messerknecht, 1925). Displacement parameters at 70% at -150°C.

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+ − [UF 4 (NH 3 ) 4 ]+[BeF 2 (NH 3 ) 2 ] UF 3(NH 3 ) 5 ] +[BeF 3 (NH 3 )] ( 2 ) et al., 1969a,b; Myers, 1990). It is interesting to note that M only some complex fluorides of the uranyl cation 3 UO2 F5 + + M [UF3 (NH3 )5 ] +NH3 [UF3 (NH2 )(NH3 )4 ]+NH4 (3) or 3 (UO2 )2 F7 are known (Olsson, 1930), of which the physical properties have been determined. + − 2+ NH 4 +[BeF 3 (NH 3 )] +NH 3 [N 2H 7 ][BeF 3 (NH 3 )] (4) The UO 2 cation is said to abstract a fluoride ion from BF − (John et al., 2004). However, no compounds It is possible that the amide fluoride [UF (NH )(NH ) ] 4 3 2 3 4 with BF (or SiF ) are known (Ahrland and Larsson, is produced in solution, although it could not be isolated. 3 4 1954). In aqueous solution, UO 2+ accepts a maximum of Probably this amide either condensates or reacts with 2 four fluoride anions, leading to the formation of UO F 2 − further BeF following the routes shown in Eqs. (5) and (6). 2 4 2 (Johnson and Kraus, 1952; Johnson et al., 1954; Ahrland Deprotonation of the ammine ligand et al., 1956). Compounds with SbF5 are known with com-

[UF 3(NH 2 )(NH 3 ) 4 ]+NH 3 [UF 2(NH 2 ) 2 (NH 3 ) 4 ]+ “ NH 4 F ’ ’ (5) positions of UO2 F2 · nSbF5 (n = 2, 3, 4). However, they have not been characterized in detail (Fawcett et al., 1982; Deprotonation of the amide ligand 2+ Holloway et al., 1982). This fluoride ion affinity of UO2

[UF 3 (NH 2 )(NH 3 ) 4 ]+2NH 3 [UF 2(NH)(NH 3 ) 5 ]+ “ NH 4 F ” (6) might be of great importance for the synthesis of new compounds in liquid ammonia. So, further reactions can take place which may finally Ammoniates of UO Cl are known (von Unruh, 1909; lead to [U(NH ) (NH ) ], to oligonuclear complexes or to 2 2 2 4 3 4 Peters, 1912; Spacu, 1936), but those of uranyl fluorides have polymers. Upon deprotonation of the amide, imides and not been published so far. In 1909, von Unruh provided evi- nitride fluorides like [UFN(NH ) ] may become available 3 6 dence for the compounds UO F ·n (NH ) (n = 2, 3, 4) by vapor and the removal of NH should lead to pure UNF. 2 2 3 3 pressure measurements (von Unruh, 1909). Only two hydra-

zine adducts of UO2 F 2 are known which are convertible to

Fluoride ion acceptors based on uranium – uranyl UF4 at higher temperatures (Sahoo and Satapathy, 1964). fluoride and chloride Recently, we obtained [UO2 F2 (NH3 )3 ]2 · 2 NH3 , which was formed in the reaction of uranyl fluoride in liquid Uranyl fluoride is a well known and explored compound, ammonia, the structure of which is shown in Figure 8 . as it was suggested it might serve as a nuclear fuel (Myers, Uranyl chloride UO2 Cl2 reacts with liquid ammonia to

1990). Its solubility in water and organic solvents and its give [UO 2 (NH3 )5 ]Cl2 · NH3 (Figure 9 ), and the fluoride-accep- 2+ phase diagrams have been determined, and its properties tor-properties of the [UO 2 (NH3 )5 ] are currently under in nuclear power generation have been evaluated (Knacke investigation (Kraus et al., 2012; Woidy et al., 2012).

F U

N b

Figure 8 The unit cell of [UO2 F2 (NH3 )3 ]· NH3 and molecular structure of [UO2 F2 (NH3 )3 ]. Displacement parameters at 70% at -150°C.

O N U

c a b

2+ Figure 9 The unit cell of [UO2 (NH3 )5 ]Cl2 · NH3 and molecular structure of [UO2 (NH3 )5 ] . Displacement parameters at 70% at -150°C.

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It is interesting to note that TaF5 does not seem to be an appropriate fluoride ion acceptor in liquid ammonia.

In a reaction of Sr(TaF6 )2 with liquid NH3 , we obtained

[TaF 5 (NH3 )3 ] and SrF 2 as the only products (F. Kraus, unpublished results).

From the reaction of BF 3 and SiF4 in liquid ammonia, we obtained the low-temperature ammoniates of the

long-known compounds [BF3 (NH3 )] and [SiF4 (NH3 )2 ] (Figure 11 ) (Kraus and Baer, 2010), and the usage of these two compounds as fluoride ion acceptors in NH is under b 3 investigation. c

Figure 10 Depiction of the unit cell of (N2 H7 )[BeF3 (NH3 )]. Thermal displacement parametersc at 70% at -150°C. Usage of ammonia-soluble metals for the conversion of fluorides Reactions with fluoride ion acceptors in liquid ammonia It is well known that metal fluorides can be reduced at Silicon atoms of certain silanes are well known as fluoride higher temperatures to the respective elements using ion acceptors. Uranium hexafluoride was co-condensated an alkali or alkaline earth metal. By this method, e.g., by Jacob with Si(OCH3 )4 yielding U(OCH3 )6 and SiF(OCH3 )3 depleted uranium metal can be obtained in chunks up to (Jacob, 1982). The reaction is driven by the thermodynamic 500 kg. If, however, metal fluorides are reacted with, for stability of the Si − F bond. To the best of our knowledge, example, alkali metals dissolved in liquid ammonia, the Be2+ has not been used as an acceptor for fluoride ions, at reactions rarely lead to the respective elements, but yield least not in liquid ammonia, and the same is true for the mostly grayish or greenish residues which have not been uranyl cation which is said to be the strongest fluoride ion characterized yet (Jander et al., 1966). acceptor. It is possible that these residues are low-valent metal

In the reaction of UF4 with BeF2 , we found [N2 H7 ] fluorides with oligonuclear or polymeric structures, and

[BeF3 (NH3 )] as a product (Figure 10 ). This result shows thus this process could turn out to be a very simple way that BeF2 indeed acts as a fluoride ion acceptor in liquid to obtain these otherwise hardly accessible compounds. It ammonia and confirms our postulate (Kraus et al., 2012). is also possible that the residues contain amides, imides The usage of fluoride ion acceptors opens a novel syn- or nitrides. Thus, the reactions of metal fluorides with thesis route toward ammine fluorides in liquid ammonia, metals dissolved in ammonia are well worth exploring. It since the formation of nitrogen containing fluorides is known that Li, Na, K, Rb, Cs, Ca, Sr, Ba, Eu, and Yb dis- should be quite simple this way. If a fluoride ion is, e.g., solve in ammonia (Jander et al., 1966). The solutions are abstracted from [UF4 (NH3 )4 ], it is very likely that the result- intensively blue, bronze or golden in color, depending on + ing [UF3 (NH3 )4 ] cation is deprotonated, leading to an the concentration of the metal. It has been reported that amide fluoride. by electrochemical reduction of beryllium compounds in

b c

a a b

Figure 11 The unit cells of [BF3 (NH3 )]· 3NH3 and [SiF4 (NH3 )2 ]· 2NH3 . Displacement parameters at 70% at -150°C.

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Figure 12 The unit cell of K2[Zr(NH2)6] and molecular structure of the hexamido zirconate(IV) anion. Displacement parameters at 70% at -150°C. liquid ammonia, faintly blue solutions were generated Summary (Jander et al., 1966).

So far, we studied the reaction of ZrF 4 with an excess For a long time, preparative fluorine chemistry in liquid of potassium in liquid ammonia. After several weeks, the ammonia seemed to be a futile goal, but we have shown bluish solution became colorless and contained colorless that fluorine chemistry in liquid ammonia is possible. crystals which were subjected to a single-crystal X-ray anal- By three methods, the usage of high oxidation states, of ysis at low temperatures. The composition of the very mois- strong fluoride ion acceptors and of ammonia-soluble ture-sensitive compound was established to be K2 [Zr(NH 2 ) 6 ] metals and amides, we have established novel accesses to ( Figure 12 ) (Kraus et al., 2011). Thus, zirconium(IV) is not nitrogen- and fluorine-containing compounds via activa- reduced by solvated electrons but is converted into the hex- tion of NH3 in liquid ammonia. amido zirconate(IV). The amide ions are generated by the The interest in nitrogen- and fluorine-containing reduction of ammonia protons to hydrogen under forma- species has initiated a rich chemistry in such different tion of KNH2 , as illustrated by Eqs. (7) and (8). fields as beryllium and uranium chemistry. We were able

to synthesize fundamental compounds like BeF2 (NH3 )2 , or 2 K+2 NH3 2 KNH2 +H2 (7) 2+ the cation [Be(NH3 )4 ] , which can be considered as promising educts for many further exploratory syntheses. We [ZrF 4 (NH 3 ) 4 ]+6 KNH 2 K 2[Zr(NH 2 ) 6]+4 KF+4 NH 3 ( 8 ) synthesized [UF4 (NH3 )4 ] · NH3 and compounds containing 3− This result is very promising in many respects: amides, the [UF7 (NH3 )] anion as convenient starting materials for imides or nitrides of strong fluoride ion acceptors like Be further syntheses in uranium chemistry. or even Zr act, on the one hand, as fluoride ion accep- We have prepared ammines of so called “ insolu- tors and on the other, they may transfer amide or imide ble ” fluorides like NH4 F or the alkali metal fluorides and or even nitride ions to another metal, and pure ternary studied their reactions in liquid ammonia. We elaborated amides have often only been obtained by high-pressure, preparative syntheses for fluorides, ammine fluorides and high-temperature reactions with ammonia or using metal- amides from these reactions, and it can be expected that organic educts. With the synthesis of K2 [Zr(NH2 )6 ], we have other classes of compounds will follow. shown that such compounds are also accessible at low temperatures from the fluorides. Received April 16, 2012; accepted May 29, 2012

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