RESEARCH
SiF2* Expanding Silicon-Fluorine Chemistry Radical's unusual chemistry offers promise of many new Si-F compounds Silicon difluoride, a silicon analog of a carbene, is providing a way to expand the chemistry of silicon-fluorine compounds. Because the radical is easy to prepare (compared to other radicals), and has a long lifetime and high reactivity, the number of compounds containing silicon and fluorine that might be made from silicon difluoride may rival the number containing carbon and fluorine. A great deal of exploratory work on silicon-fluorine compounds is being done at Rice University by a group headed by Dr. John L. Margrave. This group is working on the chemistry of silicon difluoride and its polymer (the silicon analog of polytetrafluoroethylene). Reactions being studied include those of silicon difluoride with aromatic, unsaturated aliphatic, and oxygenated compounds, and with boron trifluoride and other inorganic materials. In addition, the chemical and physical properties of the homologous series of perfluorosilanes, various perfluoroborosilanes, and other compounds containing silicon, fluorine, and other elements are being determined. The chemistry of silicon difluoride is unusual—it is more reactive at —196° C. than it is at room temperature, Dr. Margrave says. Dr. Margrave, Dr. Peter L. Timms (now at the University of California, Berkeley), and Dr. Thomas C. Ehlert (now at; Marquette University) have found that the radical (as a gas) polymerizes spontaneously in the presence of many gaseous organic or inorganic compounds at room temperature. At —196° C , both Lewis acids and bases PRODUCTS. Dr. J. C. Thompson (left) and Dr. J. L. Margrave use a mass spectrometer to detect the many products produced from silicon difluoride 40
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react with silicon difluoride; polymerization also occurs. Diradicals may be the key, Dr. Margrave suggests. Carbenes (such as carbon difluoride) polymerize to relatively stable olefins. Silicon difluoride, however, probably first gives unstable diradicals, such as #SiF2—SiF2· and -SiF 2 -SiF 2 -SiF 2 ·. These are reactive and polymerize rapidly, Dr. Margrave, Jouette M. Bassler, and other Rice workers have found. At low temperatures, lifetimes of the diradicals are long enough for them to react with many other molecules. Final products from these reactions usually contain two or more silicon atoms in a chain. Part of the reason for this behavior is in some of the silicon difluoride's unique properties: • The silicon-fluorine bond energy is very high (averaging over 140 kcal. per mole in the di- and tetrafluoride); this means the bond is rarely broken thermally. Thus, compounds of the
type >SiF—XF decompose spontaneously or on pyrolysis to >SiF 2 *+ -X-. • Silicon difluoride has a long lifetime—about a two-minute half life in a borosilicate glass flask (room temperature and 0.2 mm. H g ) . • The silicon-fluorine bond does not seem to reduce the electron acceptor and donor properties of silicon or the general reactivity of silicon difluoride. The long lifetime permitted Dr. R. F. Curl and Dr. V. M. Rao at Rice to observe the microwave spectrum of silicon difluoride. The data, indicative of the structure, also brought surprises. The F—Si—F bond angle is slightly less than 101°, though Dr. Curl expected it to be about 125°. (For a rough comparison, isoelectronic sulfur dioxide has a bond angle of 119.5°.) The Si—F distance in silicon difluoride is almost equal to that in the tetrafluoride, suggesting little differ-
ence in the amount of p7r^d7r bonding between the two. These data indicate that the bonding involves mainly p 2 -hybrids of silicon orbitals, with the p-orbitals of the fluorine atoms directed along the bonds. Silicon difluoride is made by a method developed by Dr. D. C. Pease of Du Pont in which silicon tetrafluoride is passed over silicon at about 1050° C. (0.1 to 0.2 mm. H g ) . This gives about a 50% conversion of the tetrafluoride to the difluoride. Dr. M. Schmeisser and Dr. K. P. Ehlers of Technische Hochschule, Aachen, West Germany, have reported making silicon difluoride by disproportionation of Si 2 F 6 at 700° C. If cooled to about —80° C , gaseous silicon difluoride condenses and forms à polymer, (SiF 2 ) x . If the gaseous products are held at about 1120° C , an equilibrium mixture forms containing about 6 5 % silicon difluoride. The remainder of the mixture is mostly silicon tetrafluoride. The polymer burns in air and reacts slowly and incompletely with water. Hydrogen and silanes evolve from reaction with dilute hydrofluoric acid solutions. If heated in a vacuum to between 200° and 350° C , the polymer melts, decomposes slowly to perfluorosilanes, and leaves a solid, sili-
con-rich polymer. Perfluorosilanes appear to be the only solvents for the polymer, which reacts with alcohols, ketones, ethers, and amines. The properties of silicon difluoride and its polymers suggest experimental difficulties. Inert atmospheres, low temperatures, and dry boxes are used in most work. Disproportionation occurring on the stationary phase in gas chromatography makes many separations difficult. Mass spectrometry is an indispensable analytical tool. A new direct inlet system which requires no expansion bulb or valves was developed for rapid examination of new products. Recently completed work with simple aromatics—benzene, toluene, and mono- and difluoro derivativesturned up unusual bridged compounds with two or three silicon difluoride groups linked across a benzene ring in the para position. A silicon difluoride aromatic polymer of varying compositions is made by vaporizing the aromatic into the stream of gases coming from the reaction between silicon and the tetrafluoride, and condensing the mixture at liquid nitrogen temperatures. When the polymer is pyrolyzed at 100° to 140° C , the most abundant product obtained is C G H 6 (SiF 2 ) 3 , accounting for about 20% of the reacted benzene. C G H G (SiF 2 ) 3 is a colorless
SiFr and Aromatics Give Unusual Bridged Compounds
The structure of the most abundant compound obtained from the reaction of SiF2* and benzene is believed to be the one shown here, it is a colorless crystalline material. Its molecular formula is CeHe(SiF2)»-2,2,3t3,4,4-hexafluoro-2,3,4-trisiiabicyclo [3.2.2]nona-6,7-diene
A similar compound has been made by Dow Coming's Dr. D. R. Weyenberg and Dr. L H. Topocer; this one is 2r2,4Λ*tetramethyl·3-oxa·2,4*disiiabicyclo[3.2.2]nona-6f8• diene
crystalline material melting at 72.5° C. A similar compound forms with toluene. Dr. Margrave, Dr. Timms, and Dr. R. A. Kent (now at Los Alamos Scientific Laboratory) found that a conjugated C = C system doesn't exist for this compound. From the nuclear magnetic resonance spectrum, they describe the structure of the compound as 2,2,3,3,4,4-hexafluoro-2,3,4trisilabicyclo[3.2.2]nona - 6,8 - diene. The spectrum is very similar to that obtained by Dr. D. R. Weyenberg and Dr. L. H. Popocer of Dow Corning for a somewhat similar bridged compound, 2,2,4,4-tetramethyl-3-oxa-2,4disilabicyclo[3.2.2]nona-6,8-diene. Dr. Margrave points out that the 1,4-addition is unusual for carbenes. However, it can be explained for silicon difluoride by the attack of diradical intermediates on a benzene ring. Similar reactions for carbenes don't occur because they dimerize, lose energy, and form stable olefins. Silicon difluoride abstracts fluorine from perfluorobenzene and gives mono-, bis-, and trisfluorosilylfluorobenzenes as well as disilyl and higher derivatives. In contrast with the benzene reaction, the mono-SiF 2 product probably involves a three-membered ring with a hexadiene system. (A mono-SiF 2 bridged compound may form with benzene, but apparently it is very unstable and decomposes so rapidly that it has not been detected.) The system then rearranges exothermically to regain aromaticity in the ring, rather than adding more silicon difluoride to form bridged species. The spontaneous rearrangement, accompanied by visible flashes of light, can be explained by the difference between silicon-fluorine and carbon-fluorine bond energies, he says. Not all of silicon difluoride's reactions can be easily explained. Recently, some of the Rice group reacted the compound with acetylene and found the main product to be an unsymmetrical compound, RC = CSiF 2 SiF 2 CH=CH 2 . Besides getting more details of reaction mechanisms and properties of compounds made so far, Dr. Margrave, Dr. Gottfried Besenbruch, Dr. James C. Thompson, and others at Rice have begun studies on reactions of silicon difluoride with oxygen, oxygen difluoride, and organic compounds containing oxygen. This work has resulted in their making several silicon oxyfluorides. NOV.
2 9, 1965
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