Chemists developed a new form of Lewis acid capable of effective hydrodefluorination. Source: http://www.flickr.com/photos/horiavarlan/
Caputo et al. recently developed a Lewis acid, [(C6F5)3 PF]+, which has proven highly effective in hydrodefluorination, the conversion of a carbon-fluorine bond into a carbon-hydrogen bond. A Lewis acid is any substance which can accept an electron pair, while a Lewis base is any substance which can donate an electron pair (1).
[(C6F5)3 PF]+ is much improved over PF5, a strong Lewis acid useful in organic synthesis as a fluorination agent. PF5’s usefulness is mitigated by its corrosive nature—it tends to react with water and release hydrofluoric acid. Caputo et al. reduces this corrosiveness with the addition of three C6F5 substituents; instead of the unstable P-F bond in PF5, the molecule would now possess a stable C-F bond that would not break in the presence of water (1,2).
Apart from the three C6F5 substituents on [(C6F5)3 PF]+, the fourth substituent is a fluorine atom that forms a polar bond with the phosphorus. This bond is associated with a phosphorus-centered site characterized as an antibonding σ* orbital. Due to the reciprocal relationship between anti-bonding and bonding orbitals, the electronegative fluorine, by drawing electron from the P-F bond toward it, causes most of the volume of the phosphorus-centered σ* orbital to protrude in the direction opposite to the fluorine atom.
Lewis bases are attracted to this site, and they will readily donate a pair of electrons to it and thus form a bond. Fluoride anions have an especially strong tendency to react at this site, forming (C6F5)3 PF2. This reaction is so thermodynamically favorable that [(C6F5)3 PF]+ is able to steal the fluoride anion from relatively unreactive fluorocarbons during hydrodefluorination.
[(C6F5)3 PF]+ is not as effective as a silylium-based molecule developed by Douvris and Ozerov. This molecule is based on SiR3+, which is a reactive silicon-containing cation with three ligands attached to it. However, [(C6F5)3 PF]+ has a full octet and is therefore more stable than the silylium-based molecule, which is a cation that has only three ligands and which does not possess a full octet.
Furthermore, the P and F nuclei make [(C6F5)3 PF]+’s chemistry easy to follow with nuclear magnetic resonance, a technique in which nuclei absorb and reemit electromagnetic radiation, thereby allowing a scientist to trace them (1,3).
The utility of this discovery extends beyond the creation of a new Lewis acid; it will allow for further development of Lewis acid cations involving group 15 elements. Additionally, the tetrahedral geometry of such cations could be useful in chiral synthesis, which is a method of synthesis for a molecule which favors one type of stereoisomer over another.
References:
1. Gabbaï, François P., “Lewis Acids with a Difference” (20 September 2013). Available at http://www.sciencemag.org/content/341/6152/1348.full.pdf (27 September 2013).
2. “Phosphorus Pentafluoride.” Air Liquide. Air Liquide, n.d. Web. 01 Nov. 2013. <http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=49>.
3. Hornak, Joseph P., “The Basics of NMR” (1997). Available at http://www.cis.rit.edu/htbooks/nmr/inside.htm (27 September 2013).
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