[7783-77-9] · F6Mo · Molybdenum(VI) Fluoride · (MW 209.94)
Alternate Name: molybdenum hexafluoride.
Physical Data: mp 17.5 °C; bp 35 °C; d 2.543 g cm-3 (l, 19 °C).
Solubility: sol saturated hydrocarbons, chlorinated and fluorinated hydrocarbons. The rate of decomposition of the solution of MoF6 in n-hexane is 1% h-1. The reaction of MoF6 with CCl4 takes place at ca. 150 °C. MoF6 reacts violently with H2O and protic solvents.
Form Supplied in: colorless liquid.
Analysis of Reagent Purity: the purity of a MoF6 solution can be checked by 19F NMR: d19F (MoF6) +290 (MeCN/CFCl3, d +ve for downfield shifts); d19F (MoOF4) +148.
Handling, Storage, and Precautions: can be handled in normal glassware at or below room temperature. It attacks silica at 120-150 °C to give MoOF45 and must be handled in stainless steel autoclaves for reactions at high temperature. Any leak can be detected by the deep blue color which results from the reaction of MoF6 with moisture. Normal precautions when working with MoF6 include gloves, glasses, and a good fume hood. Contact of a CH2Cl2 solution with the skin leads to superficial burnings, readily detected by their blue color. Molybdenum is reputed to be of low toxicity.
The difference between the formation enthalpies of MoF6(g) and MoOF4(g) is ca. 55 kcal mol-1,6 which may be compared with ca. 2 × 50 kcal mol-1 for the SF4/SO2 system. Hence, MoF6 is expected to reproduce the oxygen to fluorine exchange reactions performed by Sulfur Tetrafluoride. Indeed, in methylene chloride at room temperature with Boron Trifluoride as a catalyst, MoF6 readily reacts with ketones and aldehydes to give the corresponding gem-difluoro compounds (eq 1).2,7
The transformation is compatible with organic functionalities such as Cl, Br, CO2R, C(O)NR2, CN, NO2, and P(O)R2. Alcohols, amines, ethers, and double bonds are attacked. MoF6 is significantly stronger than SF4 as a Lewis acid. This technique has been used inter alia for the preparation of gem-difluoro-substituted fatty acids.8
At high temperature and without a catalyst, MoF6 is also able to convert aliphatic9 and aromatic10 carboxylic acids into gem-trifluoro derivatives (eqs 2 and 3).
Aryl trifluoromethyl derivatives can also be obtained under less drastic conditions by fluorination of aromatic acid chlorides (eq 4).11
Ortho substitution of the arene ring blocks the reaction. In a related transformation, aryl chlorothioformates can be converted into aryl trifluoromethyl ethers (eq 5).12
Owing to the presence of the oxygen spacer, the interconversion is no longer sensitive to ortho substitution and the yields are consistently high. Finally, two results worthy of note: MoF6 converts Ethylene Oxide and trifluoroethanol, respectively, into 1,1-difluoroethane (42%) and 1,1,1,2-tetrafluoroethane (85%).9
Electrochemical studies in acetonitrile have shown that MoF6 is a rather powerful oxidizing agent. The order UF6 > MoF6 > NO+ > Cu2+ &egt; WF6 has been proposed.13
This property of MoF6 can be used in organic synthesis. For example, Olah et al. have described the oxidative cleavage of hydrazones (eq 6).3
MoF6 has also been used for the oxidative fluorination of phosphines into fluorophosphoranes (eqs 7 and 8).4
Ecole Polytechnique, Palaiseau, France