Manganese(IV) fluoride

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Manganese(IV) fluoride
Alpha-MnF4-from-xtal-1987-CM-3D-polyhedra.png
Identifiers
CAS number 15195-58-1
ChemSpider 14941034
Properties[2][3]
Molecular formula MnF4
Molar mass 130.93 g mol−1
Appearance blue solid
Density 3.61 g cm−1 (calc.)[1]
Melting point 70 °C decomp.
Solubility in water reacts violently
Structure
Crystal structure tetragonal, tI80[1][4]
Space group I41/a (No. 88)[Note 1]
Lattice constant a = 1263 pm, c = 604.9 pm
Related compounds
Other cations Manganese(II) fluoride
Manganese(III) fluoride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Manganese tetrafluoride, MnF4, is the highest fluoride of manganese. It is a powerful oxidizing agent and is used as a means of purifying elemental fluorine.[3][5]

Preparation[edit]

Manganese tetrafluoride was first unequivocally prepared in 1961[Note 2] by the reaction of manganese(II) fluoride (or other MnII compounds) with a stream of fluorine gas at 550 °C: the MnF4 sublimes into the gas stream and condenses onto a cold finger.[2][7] This is still the commonest method of preparation, although the sublimation can be avoided by operating at increased fluorine pressure (4.5–6 bar at 180–320 °C) and mechanically agitating the powder to avoid sintering of the grains.[3][8] The reaction can also be carried out starting from manganese powder in a fluidized bed.[9][10]

Other preparations of MnF4 include the fluorination of MnF2 with krypton difluoride,[11] or with F2 in liquid hydrogen fluoride solution under ultraviolet light.[12] Manganese tetrafluoride has also been prepared (but not isolated) in an acid–base reaction between antimony pentafluoride and K2MnF6 as part of a chemical synthesis of elemental fluorine.[13]

K2MnF6 + 2 SbF5 → MnF4 + 2 KSbF6

Chemistry[edit]

Decomposition[edit]

Manganese tetrafluoride is in equilibrium with manganese(III) fluoride and elemental fluorine:

MnF4 is in equilibrium with MnF3 + ½ F2

Decomposition is favoured by increasing temperature, and disfavoured by the presence of fluorine gas, but the exact parameters of the equilibrium are unclear, with some sources saying that MnF4 will decompose slowly at room temperature,[14][15] others placing a practical lower temperature limit of 70 °C,[3][16] and another claiming that MnF4 is essentially stable up to 320 °C.[17] The equilibrium pressure of fluorine above MnF4 at room temperature has been estimated at about 10−4 Pa (10−9 bar), and the enthalpy change of reaction at +44(8) kJ mol−1.[18][Note 3]

Other reactions[edit]

Manganese tetrafluoride reacts violently with water and even with sodium-dried petroleum ether. It immediately decomposes on contact with moist air.[2]

Reaction with alkali metal fluorides or concentrated hydrofluoric acid gives the yellow hexafluoromanganate(IV) anion [MnF6]2−.[17]

Applications[edit]

The main application of manganese tetrafluoride is in the purification of elemental fluorine. Fluorine gas is produced by electrolysis of anhydrous hydrogen fluoride (with a small amount of potassium fluoride added as a support electrolyte) in a Moissan cell. The technical product is contaminated with HF, much of which can be removed by passing the gas over solid KF, but also with oxygen (from traces of water) and possibly heavy-metal fluorides such as arsenic pentafluoride (from contamination of the HF). These contaminants are particularly problematic for the semiconductor industry, which uses high-purity fluorine for etching silicon wafers. Further impurities, such as iron, nickel, gallium and tungsten compounds, can be introduced if unreacted fluorine is recycled.[5]

The technical-grade fluorine is purified by reacting it with MnF3 to form manganese tetrafluoride. As this stage, and heavy metals present will form involatile complex fluorides, while the HF and O2 are unreactive. Once the MnF3 has been converted, the excess gas is vented for recycling, carrying the remaining gaseous impurities with it. The MnF4 is then heated to 380 °C to release fluorine at purities of up to 99.95%, reforming MnF3, which can be reused.[3][5] By placing two reactors in parallel, the purification process can be made continuous, with one reactor taking in technical fluorine while the other delivers high-grade fluorine.[5] Alternatively, the manganese tetrafluoride can be isolated and transported to where the fluorine is needed, at lower cost and greater safety than pressurized fluorine gas.[3][8]

Fluoromanganate(IV) complexes[edit]

The yellow hexafluoromanganate(2−) of alkali metal and alkaline earth metal cations have been known since 1899, and can be prepared by the fluorination of MnF2 in the presence of the fluoride of the appropriate cation.[12][20][21][22] They are much more stable than manganese tetrafluoride.[13] Potassium hexafluoromanganate(IV), K2MnF6, can also be prepared by the controlled reduction of potassium permanganate in 50% aqueous hydrofluoric acid.[23][24]

2 KMnO4 + 2 KF + 10 HF + 3 H2O2 → 2 K2MnF6 + 8 H2O + 3 O2

The pentafluoromanganate(1−) salts of potassium, rubidium and caesium, MMnF5, can be prepared by fluorination of MMnF3 or by the reaction of [MnF4(py)(H2O)] with MF.[22][24] The lemon-yellow heptafluoromanganate(3−) salts of the same metals, M3MnF7, have also been prepared.[25]

Notes and references[edit]

Notes[edit]

  1. ^ The space group has also been given as R3c (No. 161) or R3c (No. 167); a β-form appears to crystallize in the rhombohedral system.[1]
  2. ^ Reports of the preparation of MnF4 date back to the nineteenth century,[6] but are inconsistent with the now-known chemistry of the genuine compound.
  3. ^ These two results are inconsistent with one another, as ΔrHo would have to be about +80 kJ mol−1 for peq(F2) ≈ 10−9 bar at 298 K, given that the overwhelming contribution to ΔrSo is So(F2) = 202.791(5) J K−1 mol−1.[19] The quoted value of ΔrHo is consistent with most reported decomposition temperatures.

References[edit]

  1. ^ a b c Müller, B. G.; Serafin, M. (1987), "Die Kristallstruktur von Mangantetrafluorid", Z. Naturforsch., B: J. Chem. Sci. 42 (9): 1102–6 .
  2. ^ a b c Hoppe, Rudolf; Dähne, Wolfgang; Klemm, Wilhelm (1961), "Mangantetrafluorid, MnF4", Naturwissenschaften 48 (11): 429, doi:10.1007/BF00621676 .
  3. ^ a b c d e f WO, "Method of manufacturing manganese tetrafluoride", published 2006-03-30 .
  4. ^ Edwards, A. J. (1983), "Solid-State Structures of the Binary Fluorides of the Transition Metals", Adv. Inorg. Chem. Radiochem., Advances in Inorganic Chemistry 27: 83–112, doi:10.1016/S0898-8838(08)60105-1, ISBN 9780120236275 .
  5. ^ a b c d WO, "Process for the purification of elemental fluorine", published 2009-06-18 .
  6. ^ Melville, W. H. (1876), "Contribution towards the History of the Fluorides of Manganese", Proc. Am. Acad. Arts Sci. 12: 228–34, doi:10.2307/25138452, JSTOR 25138452 .
  7. ^ Hoppe, Rudolf; Dähne, Wolfgang; Klemm, Wilhelm (1962), "Mangantetrafluorid mit einem Anhang über LiMnF5 und LiMnF4", Justus Liebigs Ann. Chem. 658 (1): 1–5, doi:10.1002/jlac.19626580102 .
  8. ^ a b WO, "Method for preparing manganese tetrafluoride", published 2009-06-18 .
  9. ^ Roesky, H.; Glemser, O. (1963), "A New Preparation of Manganese Tetrafluoride", Angew. Chem., Int. Ed. Engl. 2 (10): 626, doi:10.1002/anie.196306262 .
  10. ^ Roesky, Herbert W.; Glemser, Oskar; Hellberg, Karl-Heinz (1965), "Darstellung von Metallfluoriden in der Wirbelschicht", Chem. Ber. 98 (6): 2046–48, doi:10.1002/cber.19650980642 .
  11. ^ Lutar, Karel; Jesih, Adolf; Žemva, Boris (1988), "KrF2/MnF4 adducts from KrF2/MnF2 interaction in HF as a route to high purity MnF4", Polyhedron 7 (13): 1217–19, doi:10.1016/S0277-5387(00)81212-7 .
  12. ^ a b Mazej, Z. (2002), "Room temperature syntheses of MnF3, MnF4 and hexafluoromanganete(IV) salts of alkali cations", J. Fluorine Chem. 114 (1): 75–80, doi:10.1016/S0022-1139(01)00566-8 .
  13. ^ a b Christe, Karl O. (1986), "Chemical synthesis of elemental fluorine", Inorg. Chem. 25 (21): 3721–24, doi:10.1021/ic00241a001 .
  14. ^ Cotton, F. Albert; Wilkinson, Geoffrey (1980), Advanced Inorganic Chemistry (4th ed.), New York: Wiley, p. 745, ISBN 0-471-02775-8 .
  15. ^ Housecroft, Catherine E.; Sharpe, Alan G. (2007), Inorganic Chemistry (3rd ed.), New York: Prentice Hall, p. 710, ISBN 0131755536 .
  16. ^ Rakov, E. G.; Khaustov, S. V.; Pomadchin, S. A. (1997), "Thermal Decomposition and Pyrohydrolysis of Manganese Tetrafluoride", Russ. J. Inorg. Chem. 42 (11): 1646–49 .
  17. ^ a b Adelhelm, M.; Jacob, E. (1991), "MnF4: preparation and properties", J. Fluorine Chem. 54 (1–3): 21, doi:10.1016/S0022-1139(00)83531-9 .
  18. ^ Ehlert, T. C.; Hsia, M. (1972), "Mass spectrometric and thermochemical studies of the manganese fluorides", J. Fluorine Chem. 2 (1): 33–51, doi:10.1016/S0022-1139(00)83113-9 .
  19. ^ Cox, J. D.; Wagman, D. D.; Medvedev, V. A. (1989), CODATA Key Values for Thermodynamics, New York: Hemisphere, ISBN 0891167587 .
  20. ^ Weinland, R. F.; Lauenstein, O. (1899), Z. Anorg. Allg. Chem. 20: 40 .
  21. ^ Hoppe, Rudolf; Blinne, Klaus (1957), "Hexafluoromanganate IV der Elemente Ba, Sr, Ca und Mg", Z. Anorg. Allg. Chem. 291 (5–6): 269–75, doi:10.1002/zaac.19572910507 .
  22. ^ a b Hoppe, Rudolf; Liebe, Werner; Dähne, Wolfgang (1961), "Über Fluoromanganate der Alkalimetalle", Z. Anorg. Allg. Chem. 307 (5–6): 276–89, doi:10.1002/zaac.19613070507 .
  23. ^ Bode, Hans; Jenssen, H.; Bandte, F. (1953), "Über eine neue Darstellung des Kalium-hexafluoromanganats(IV)", Angew. Chem. 65 (11): 304, doi:10.1002/ange.19530651108 .
  24. ^ a b Chaudhuri, M. K.; Das, J. C.; Dasgupta, H. S. (1981), "Reactions of KMnO4—A novel method of preparation of pentafluoromanganate(IV)[MnF5]", J. Inorg. Nucl. Chem. 43 (1): 85–87, doi:10.1016/0022-1902(81)80440-X .
  25. ^ Hofmann, B.; Hoppe, R. (1979), "Zur Kenntnis des (NH4)3SiF7-Typs. Neue Metallfluoride A3MF7 mit M = Si, Ti, Cr, Mn, Ni und A = Rb, Cs", Z. Anorg. Allg. Chem. 458 (1): 151–62, doi:10.1002/zaac.19794580121 .

Further reading[edit]

  • Gubkina, N. I.; Sokolov, Sergey V.; Krylov, E. I. (1966), "Fluorides of High Oxidising Power and Their Application to the Preparation of Organic Fluorine Compounds", Russ. Chem. Rev. 35 (12): 930–41, doi:10.1070/RC1966v035n12ABEH001550 .
  • Hoppe, R.; Müller, B.; Burgess, J.; Peacock, R. D.; Sherry, R. (1980), "The enthalpy of formation of manganese tetrafluoride", J. Fluorine Chem. 16 (2): 189–91, doi:10.1016/S0022-1139(00)82393-3 .