Manganese(IV) fluoride

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.[2][5]

Manganese(IV) fluoride

Names
IUPAC name manganese tetrafluoride
Other names manganese(IV) fluoride
Identifiers
CAS Number	15195-58-1
3D model (JSmol)	Interactive image
ChemSpider	14941034
PubChem CID	10129971
InChI InChI=1S/4FH.Mn/ h4*1H;/q;;;;+4/p-4 Key: KWKYNMDHPVYLQQ-UHFFFAOYSA-J InChI=1/4FH.Mn/h4*1H;/q;;;;+4/p-4 Key: KWKYNMDHPVYLQQ-XBHQNQODAK
SMILES [F-].[F-].[F-].[F-].[Mn]
Properties[1][2]
Chemical formula	MnF4
Molar mass	130.93 g mol^−1
Appearance	blue solid
Density	3.61 g cm^−3 (calc.)[3]
Melting point	70 °C (158 °F; 343 K) decomposes
Solubility in water	reacts violently
Structure
Crystal structure	tetragonal, tI80[3][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 otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Preparation

Manganese tetrafluoride was first unequivocally prepared in 1961[Note 2] by the reaction of manganese(II) fluoride (or other Mn^II compounds) with a stream of fluorine gas at 550 °C: the MnF₄ sublimes into the gas stream and condenses onto a cold finger.[1][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.[2][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

Decomposition

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

MnF₄ ⇌ 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,[2][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

Manganese tetrafluoride reacts violently with water and even with sodium-dried petroleum ether. It immediately decomposes on contact with moist air.[1]

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

Applications

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.[2][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.[2][8]

Fluoromanganate(IV) complexes

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]

When potassium hexafluoromanganate is doped into Potassium fluorosilicate it forms a narrow band red phosphor.[26]

Notes and references

Notes

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.[3]
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 Δ_rH^o would have to be about +80 kJ mol⁻¹ for p_eq(F₂) ≈ 10⁻⁹ bar at 298 K, given that the overwhelming contribution to Δ_rS^o is S^o(F₂) = 202.791(5) J K⁻¹ mol⁻¹.[19] The quoted value of Δ_rH^o is consistent with most reported decomposition temperatures.

References

1. Hoppe, Rudolf; Dähne, Wolfgang; Klemm, Wilhelm (1961), "Mangantetrafluorid, MnF₄", Naturwissenschaften, 48 (11): 429, Bibcode:1961NW.....48..429H, doi:10.1007/BF00621676, S2CID 30724467.
2. WO, "Method of manufacturing manganese tetrafluoride", published 2006-03-30, assigned to Astor Electronics/Showa Denko.
3. Müller, B. G.; Serafin, M. (1987), "Die Kristallstruktur von Mangantetrafluorid", Z. Naturforsch. B, 42 (9): 1102–6, doi:10.1515/znb-1987-0908, S2CID 95703093.
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. WO, "Process for the purification of elemental fluorine", published 2009-06-18, assigned to Solvay Fluor.
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. WO, "Method for preparing manganese tetrafluoride", published 2009-06-18, assigned to Solvay Fluor.
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. 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. 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 978-0131755536.
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. 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), "Über Fluormanganite", Z. Anorg. Allg. Chem., 20: 40, doi:10.1002/zaac.620200106.
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. 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. 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.
26. Verstraete, Reinert; Sijbom, Heleen F.; Joos, Jonas J.; Korthout, Katleen; Poelman, Dirk; Detavernier, Christophe; Smet, Philippe F. (2018), "Red Mn4+-Doped Fluoride Phosphors: Why Purity Matters" (PDF), ACS Applied Materials & Interfaces, 10 (22): 18845–18856, doi:10.1021/acsami.8b01269, PMID 29750494

Further reading

• 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, Bibcode:1966RuCRv..35..930G, 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.