Tetrafluoroammonium

The tetrafluoroammonium cation (also known as perfluoroammonium) is a positively-charged polyatomic ion with chemical formula NF+
4. It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine.[1]. Tetrafluoroammonium ion is isoelectronic with tetrafluoromethane CF4 and tetrafluoroborate BF−
4 ion.

The tetrafluoroammonium ion forms salts with a large variety of fluorine-bearing anions. These include the bifluoride anion (HF−
2), tetrafluorobromate (BrF−
4), metal pentafluorides (XF−
5 where X is Ge, Sn, or Ti), hexafluorides (XF−
6 where X is P, As, Sb, Bi, or Pt), heptafluorides (XF−
7 where X is W, U, or Xe), octafluorides (XeF2−
8),[2] various oxyfluorides (XF5O− where X is W or U; FSO−
3, BrF4O−), and perchlorate (ClO−
4).[3] Attempts to make the nitrate salt, NF4NO3, were unsuccessful because of quick fluorination: NF+
4 + NO−
3 → NF3 + FONO2.[4]

Contents

Structure

The geometry of the tetrafluoroammonium ion is tetrahedral, with an estimated nitrogen-fluorine bond length of 124 pm. All fluorine atoms are in equivalent positions.[5]

Synthesis

Tetrafluoroammonium salts are prepared by oxidising nitrogen trifluoride with fluorine in the presence of a strong Lewis acid which acts as a fluoride ion acceptor. The original synthesis by Tolberg, Rewick, Stringham, and Hill in 1966 employs antimony pentafluoride as the Lewis acid:[5]

NF3 + F2 + SbF5 → NF4SbF6

The hexafluoroarsenate salt was also prepared by a similar reaction with arsenic pentafluoride at 120 °C:[5]

NF3 + F2 + AsF5 → NF4AsF6

The reaction of nitrogen trifluoride with fluorine and boron trifluoride at 800 °C yields the tetrafluoroborate salt:[6]

NF3 + F2 + BF3 → NF4BF4

NF+
4 salts can also be prepared by fluorination of NF3 with krypton difluoride (KrF2) and fluorides of the form MFn, where M is Sb, Nb, Pt, Ti, or B. For example, reaction of NF3 with KrF2 and TiF4 yields [NF+
4]2TiF2−
6.[7]

Many tetrafluoroammonium salts can be prepared with methatesis reactions. More information: http://books.google.hu/books?id=xLP_KevavOkC&lpg=PA47&ots=f2nv-FsKKs&dq=tetrafluoroammonium&pg=PA39#v=onepage&q&f=false.

Reactions

Tetrafluoroammonium salts are extremely hygroscopic. The NF+
4 ion is readily hydrolysed into nitrogen trifluoride, H2F+, and oxygen gas:

2 NF+
4 + 2 H2O → 2 NF3 + 2 H2F+ + O2

Some hydrogen peroxide (H2O2) is also formed during this process.[5]

Reaction of NF+
4SbF−
6 with alkali metal nitrates yields fluorine nitrate, FONO2.[4]

Applications

NF+
4 salts are important for solid propellant NF3–F2 gas generators. They are also used as reagents for electrophilic fluorination of aromatic compounds in organic chemistry.[5]

See also

References

  1. ^ Nikitin, I. V.; Rosolovskii, V. Y. (1985). "Tetrafluoroammonium Salts". Russian Chemical Reviews 54 (5): 426. Bibcode 1985RuCRv..54..426N. doi:10.1070/RC1985v054n05ABEH003068.  edit
  2. ^ Christe, K. O.; Wilson, W. W. (1982). "Perfluoroammonium and alkali-metal salts of the heptafluoroxenon(VI) and octafluoroxenon(VI) anions". Inorganic Chemistry 21 (12): 4113–4117. doi:10.1021/ic00142a001.  edit
  3. ^ Christe, K. O.; Wilson, W. W. (1986). "Synthesis and characterization of tetrafluoroammonium(1+) tetrafluorobromate(1-) and tetrafluoroammonium(1+) tetrafluorooxobromate(1-)". Inorganic Chemistry 25 (11): 1904–1906. doi:10.1021/ic00231a038.  edit
  4. ^ a b Hoge, B. (2001). "On the stability of NF+
    4    NO−
    3     and a new synthesis of fluorine nitrate". Journal of Fluorine Chemistry 110 (2): 87–88. doi:10.1016/S0022-1139(01)00415-8.      edit
  5. ^ a b c d e Sykes, A. G. (1989). Advances in Inorganic Chemistry. Academic Press. ISBN 0120236338. 
  6. ^ Patnaik, Pradyot (2002). Handbook of inorganic chemicals. McGraw-Hill Professional. ISBN 0070494398. 
  7. ^ John H. Holloway; Eric G. Hope (1998). A. G. Sykes. ed. Advances in Inorganic Chemistry. Academic Press. pp. 60–61. ISBN 012023646X.