Tetramethylammonium fluoride explained

Tetramethyl­ammonium fluoride is the quaternary ammonium salt with the formula (CH₃)₄NF. This hygroscopic white solid is a source of “naked fluoride": fluoride ions not complexed with a metal atom. Most other soluble salts of fluoride are in fact bifluorides, HF₂^–. Historically, there have been two main approaches to prepare tetramethyl­ammonium fluoride: hydrofluoric acid neutralization of tetramethyl­ammonium hydroxide, and salt metathesis between different ammonium salts and inorganic fluoride sources, such as KF or CsF.^[1] Because the fluoride anion is extremely basic, the salt slowly reacts with acetonitrile, inducing dimerization to CH₃C(NH₂)=CHCN, which co-crystallizes.^[2]

Related salts

• Tetramethyl­phosphonium fluoride (CH₃)₄PF forms stable acetonitrile solutions. It is prepared from the ylide and potassium bifluoride:

(CH₃)₃P=CH₂ + KHF₂ → (CH₃)₄PF + KF

Gaseous tetramethylphosphonium fluoride exists as the phosphorane but autoionizes in acetonitrile solution.^[3] A more elaborate phosphazenium salt ([(CH₃)₂N)₃P]₂N⁺F⁻) is also known.^[4]

• Anhydrous Tetrabutylammonium fluoride has been prepared by the reaction of hexafluorobenzene and tetrabutylammonium cyanide.^[5]

References

1. Iashin. Vladimir. Wirtanen. Tom. Perea-Buceta. Jesus E.. 2022-02-18. Tetramethylammonium Fluoride: Fundamental Properties and Applications in C-F Bond-Forming Reactions and as a Base. Catalysts. en. 12. 2. 233. 10.3390/catal12020233. 2073-4344. free. 10138/340938. free.
2. Christe, K. O. . Wilson, W. W. . Wilson, R. D. . Bau, R. . Feng, J. A. . Syntheses, Properties, and Structures of Anhydrous Tetramethylammonium Fluoride and Its 1:1 Adduct with trans-3-Amino-2-butenenitrile. Journal of the American Chemical Society. 1990. 112. 21. 7619–7625. 10.1021/ja00177a025.
3. Tetramethylphosphonium Fluoride: "Naked" Fluoride and Phosphorane. Kornath, Andreas; Neumann, F.; Oberhammer, H.. Inorganic Chemistry. 2003. 42. 9. 2894–2901. 10.1021/ic020663c. 12716181.
4. Encyclopedia: Schwesinger, Reinhard. 1,1,1,3,3,3-Hexakis(dimethylamino)-1λ5,3λ5-diphosphazenium Fluoride. e-EROS Encyclopedia of Reagents for Organic Synthesis. 2001. 1–2. 10.1002/047084289X.rh014m. 0471936235.
5. Haoran Sun . Stephen G. DiMagno . amp . Anhydrous Tetrabutylammonium Fluoride . . 2005 . 127 . 2050–1. 10.1021/ja0440497 . 15713075 . 7.