Calcium Fluoride Goes Soluble

The mineral fluorspar, CaF₂, plays a unique role in the chemical industry as the only raw material source for fluorine. That's a significant distinction given the importance of fluoropolymers and the growing use of fluorine as a substituent to stabilize active pharmaceutical ingredients.

Nonetheless, there are no direct fluorination processes that use CaF₂ as a reagent. Instead, CaF₂ is treated with sulfuric acid to generate hydrogen fluoride, which is used as a fluorinating reagent and as a key precursor leading to myriad other fluorinating reagents.??

Herbert W. Roesky, a chemistry professor at the University of G??ttingen, in Germany, decided a dozen years ago to try to remedy this situation. And this week, Roesky described his group's progress in making CaF₂ derivatives that are soluble in organic solvents and, as a result, are potentially more useful for some applications than CaF₂ itself. He spoke before the Division of Fluorine Chemistry at the American Chemical Society's national meeting in Boston.

"To our surprise, there is hardly any chemistry known of CaF₂, and no derivatives of CaF₂ have ever been reported," Roesky said. One reason is that the compound is not very soluble in water or organic solvents, has a high melting point (1,418 ??C), and in its mineral form is very stable, he explained. But when heated, CaF₂ gives rise to fluorescence, a term that historically derives its name from fluorspar.

Aside from preparing HF, these properties make CaF₂ suitable as a coating material for infrared and ultraviolet wavelengths, which is useful for a variety of applications, Roesky noted. And when applied to the surface of glass, silicon, or metal, CaF₂ makes the material resistant to chemical attack. But because of the low solubility, particularly in organic solvents, coating applications must be carried out by vapor deposition at high temperatures, Roesky noted.

Seeking to address this issue, Roesky's group came up with the first organic-soluble CaF₂ complexes a few years ago. But in these molecules, CaF₂ is trapped in a soluble titanocene matrix, which is not particularly useful, he noted. Nevertheless, the researchers pressed ahead.

Earlier this year, Roesky and coworkers finally reported the first well-defined organic-soluble calcium fluoride complex, [LCaF(thf)]₂, where L is a β-diketiminate ligand bearing bulky diisopropylphenyl groups and thf is tetrahydrofuran (Angew. Chem. Int. Ed. 2007, 46, 2512). They prepare the complex by reacting LCaN[Si(CH₃)₃]₂(thf) with the tin fluorinating reagent (CH₃)₃SnF, another Roesky invention.

The isolated calcium monofluoride product???which is a white solid that's soluble in benzene, n—hexane, and tetrahydrofuran???exists as a dimer. The ligands form a cage around the calcium atoms, which are bridged by a pair of fluorine atoms and further stabilized by coordinated tetrahydrofuran molecules.

The team has tested solutions of the new complex by using a dip-coating procedure to deposit thin CaF₂ coatings on a silicon substrate at room temperature. "With this compound, or any organic-soluble calcium derivative and a fluoride source, it is now possible to generate room-temperature coatings of CaF₂ by dip or spraying techniques," Roesky said.

[LCaF(thf)]₂ is "the first example of a calcium monofluoride that shows exceptional solubility in organic solvents," according to Roesky. On the basis of these results, he anticipates broad applications for CaF₂ coatings.??????