METHOD OF SELECTIVE REDUCTION OF POLYHALOSILANES WITH ALKYLTIN HYDRIDES

Abstract: The invention relates to the selective and stepwise reduction of polyhalosilanes by reacting at room temperature or below with alkyltin hydrides without the use of free radical intermediates. Alkyltin hydrides selectively and stepwise reduce the Si-Br, Si-Cl, or Si-I bonds while leaving intact any Si-F bonds. When two or more different halogens are present on the polyhalosilane, the halogen with the highest atomic weight is preferentially reduced.

Description: The present invention relates to the discovery that alkyltin hydrides display selectivity with respect to both (a) the preferential reduction, under mild conditions, of one halogen to the exclusion of another halogen in a mixed halosilane and to (b), the stepwise reduction of halosilanes. Substitution of hydrogen for chlorine in polychlorosilanes such as (CH₃)SiCl₃ is known and can be effected via reaction with trialkylsilanes in the presence of catalytic quantities of aluminum chloride. However, this catalyst is notoriously effective in promoting skeletal rearrangements (i.e., redistributions involving Si-C and/or Si-Si bonds). Organotin hydrides have long been known as effective reducing agents in organic chemistry and their reactivity patterns have been well documented. Perhaps the most common use of tin hydrides is in the reduction of organic halides. (4-n)RX + R' n SnH 4-n ------> (4-n)RH + R' n SnX 4-n The above reaction has a wide scope: R can be alkyl, aryl, acyl, propargyl, etc. The reaction conditions will vary with the R group (eg., aryl halides are more difficult to reduce than alkyl halides) and with the halide (for alkyl halides the order of reactivity is RI > RBr > RCl > RF). These reactions proceed through free radical mechanisms and require the addition of free radical initiators. U.S. Patent No. 4,617,357, issued October 14, 1986 to Pallie et al., teaches the reduction of residual chlorine in glycidyl compounds with certain organo-tin hydrides without the glycidyl group thereby being attacked. Aono et al., U.S. Patent No. 4,639,361, issued January 27, 1987, teaches a process of preparing disilane Si₂H₆ from the reduction of hexachlorodisilane Si₂Cl₆ with a mixture of LiH and LiAlH₄. Hiiro et al., U.S. Patent No. 4,115,426, issued September 19, 1978, teaches a method of preparing dialkylchlorosilanes by the selective dechlorination reduction of a dialkyldichlorosilane. The reducing agents in the preparation are sodium borohydride and sodium hydride. Grady et al., J. Org. Chem., vol. 34, p. 2014-2016, 1969, teaches the use of organotin hydrides for the stepwise free radical reduction of organic halides, such as geminal polyhalides, and simple carbonyl compounds. Grady et al. does not teach the selective stepwise or other reduction of halosilanes, nor does it teach reduction methods in the absence of free radical mechanisms. Similarly, Wilt et al., J. Am. Chem. Soc., vol. 105, p. 5665-5675, 1983, teaches the reduction of silicon-containing organic halides by organotin hydrides, but is limited to the reduction of alpha-halosilanes, i.e., the halogen atom is bonded to a carbon atom adjacent to a silicon atom but is not bonded directly to a silicon atom. Organotin hydrides typically react with organohalides and unsaturated bonds via a free radical mechanism. These reactions either require heating or initiation by a free radical source. United States Patent No. 3,439,008, issued April 15, 1969 to Berger, teaches and claims a method of reacting an organostannane and a halogenated Group IVb elemental material selected from the group consisting of a halosilane, a halogermane, and a halosiloxane. However, Berger requires the presence of a Group III metal compound promoter and does not teach or claim the selective, stepwise reduction of halosilanes. There exists a need for a preferential reduction of one halogen species to the exclusion of another in molecules such as SiF₂Br₂ and other halomonosilanes. The present invention relates to the selective and stepwise reduction of polyhalosilanes by reacting at room temperature or below with alkyltin hydrides without the use of free radical intermediates. Alkyltinhydrides selectively and stepwise reduce the Si-Br, Si-Cl, or Si-I bonds while leaving intact any Si-F bonds. When two or more different halogens are present on the polyhalosilane, the halogen with the highest atomic weight is preferentially reduced. The present invention provides a method for the selective or stepwise reduction of at least one of the silicon-X bonds of a halosilane to a silicon-hydrogen bond, wherein the halosilane is of the formula X m SiH n F y R¹ z , where m is 1, 2, 3, or 4; n is 0, 1, or 2; y is 0, 1, 2, or 3; z is 0, 1, or 2; n + y + z is less than or equal to 3; m + n + y + z is equal to 4; m + y is equal to 2, 3 or 4; X is independently selected from the group consisting of Cl, Br, or I; and R¹ is an alkyl group having one to twenty carbon atoms, said method comprising the step of contacting said halosilane in the absence of a promoter with an alkyltin hydride of the formula R² x SnH (4-x) , wherein x is 3, 2, or 1, and wherein R² is an alkyl group containing 1 to 10 carbon atoms, or an aryl group of 6 to 10 carbon atoms, whereby at least one of the silicon-X bonds of the halosilane is reduced to a silicon-hydrogen bond, the halogen or halogens other than fluorine being selectively reduced, substantially without reduction of fluorine when fluorine is present, and, where two or more halogen atoms other than fluorine are present, in the presence or absence of fluorine, said reduction being stepwise where only the same halogen other than fluorine is present, and preferentially stepwise where two or more different halogens other than fluorine are present, the halogen with the higher or highest atomic weight being preferentially reduced. By "halosilane" in the present invention is meant dihalosilanes and polyhalosilanes of the formula X m SiH n F y R¹ z , where m is 1, 2, 3, or 4; n is 0, 1, or 2; y is 0, 1, 2, or 3; z is 0, 1, or 2; n + y + z is less than or equal to 3; m + n + y + z is equal to 4; m + y is equal to 2, 3 or 4; X is independently selected from the group consisting of Cl, Br, or I; and R¹ is an alkyl group having one to twenty carbon atoms. To be included in the group "halosilane" are the halosilanes, as defined above, in which two or more halogens are independently selected from the group consisting of fluorine, chlorine, bromine, and iodine. By "stepwise reduction" in the present invention is meant the essentially complete conversion of, for example, X¹ a SiX² b into X¹ (a-1) SiHX² b , where X¹ has a higher atomic weight than does X². The organotin hydrides used in the method of the present invention were allowed to react with a variety of halosilanes, including fluorohalo monosilanes. The fluorohalomonosilanes can be obtained, for example, from the redistribution reaction shown below Organotin hydrides were allowed to react with SiF₃Br in the gas phase to produce a group of reduction products: where x = 2, R = CH₃, and where x = 3, R = CH₃ or CH₃CH₂. According to the present invention, alkyltin hydrides selectively reduce the Si-Br bond of SiF₃Br to form SiF₃H together with traces of SiF₂H₂ and SiH₄ detected only at very low levels. When an excess of the tin hydride is added to the reaction mixture, there is no further reduction of the SiF₃H observed after several hours. The same results were observed regardless of the tin hydride employed. However, a comparable reduction with Vitride (NaAlH₂(OCH₂CH₂OCH₃)₂) as the reducing agent resulted in both the Si-Br and Si-F bonds being reduced, producing SiH₄ in >90% yield. This illustrates the selectivity and mildness of the alkyltin hydride reductions of the present invention. Tin hydrides were allowed to react with SiF₂Br₂ using a 2:1 molar ratio of Si-Br to Sn-H. There was very little SiF₂H₂ observed in the reaction mixture. The major product was the stepwise reduction product SiF₂HBr. This was characterized by the infrared (Si-H stretch at 2283 cm⁻¹) ¹H and ¹⁹F nmr spectroscopy. Addition of a second equivalent of tin hydride resulted in the disappearance of the band at 2283 cm⁻¹ of the infrared spectrum due to SiF₂HBr and the appearance of bands at 2251 cm⁻¹ for SiF₂H₂. (See Example 2.) To the best knowledge of the inventors, this represents the first time that SiF₂HBr has been synthesized. According to the present invention, (CH₃)₃SnH was allowed to react with SiCl₄ whereby the predominant reduction product was HSiCl₃. (CH₃)₃SnH was then allowed to react with HSiCl₃ to produce H₂SiCl₂ (75% yield) and SiH₄ (25%). Some of the HSiCl₃ did not react; what did react went 75% to H₂SiCl₂. This indicates the stepwise reductive properties of the organotin hydrides as compared to conventional reducing agents such as LiAlH₄, where reaction of either SiCl₄, or HSiCl₃ with LiAlH₄ would proceed to SiH₄. The methods of the present invention can reduce in a stepwise manner silicon- and halogen-containing materials which contain two or more halogens independently selected from the group consisting of chlorine, bromine, and iodine. Fluorine may also be present but is not reduced. The halogen with the highest atomic weight is preferentially reduced by the methods of the present invention. The methods of the present invention could be conducted by those skilled in the art under static or dynamic conditions, at elevated or cooled reaction temperatures, using reactants in the liquid (i.e., condensed) phase or gas phase. The methods of the present invention are useful in preparing partially reduced halosilanes which possess utility in a wide variety of applications, such as forming crystalline silicon and in RSiCl₃ reactions. SiF₂HBr is produced for the first time by the method of the present invention. The materials produced by the method of the present invention are useful as precursors for the chemical vapor deposition (CVD) and plasma enhanced CVD of various halosilane compounds in the generation of amorphous silicon semiconducting films, and films for the protection of surfaces. The materials are also useful as precursors for disilane formation by Wurtz coupling reaction of a fluorobromosilane, for example, or as precursors in the photolysis of a fluorohydridiosilane. The materials produced by the method of the present invention are also useful as precursors for the hydrosilylation reaction with an unsaturated hydrocarbon. Examples The following examples were performed on a high vacuum line at room temperature. Each reactant was allowed to expand into a constant volume to a specific pressure, typically 10-20 torr (1,3-2,7 kPa). The progress of the reactions was monitored by the appearance of bands in the Si-H stretching region of the infrared spectrum (2400-2000 cm⁻¹), as well as by the disappearance of the Sn-H stretches (1900-1750 cm⁻¹). Both of these regions are free of interferences such as combination and overtone bands. The frequencies cited refer to the SiH stretch. The predominant Sn by-products of these reactions were shown to be R x SnBr (4-x) and R x SnCl (4-x) by their ¹H nmr and IR spectra. 1. Reduction of SiF₃Br to SiF₃H with (CH₃)₃SnH, (CH₃CH₂)₃SnH or (CH₃)₂SnH₂. One equivalent of (CH₃)₃SnH was allowed to react with SiF₃Br at room temperature for approximately 7 hours in a sealed flask attached to a high vacuum line. The reaction mixture was analyzed by Fourier transform spectroscopy. The predominant Si-containing product (>90% yield) was SiF₃H (2315 cm⁻¹) together with traces of SiF₂H₂ (2251 cm⁻¹) and SiH₄ (2186 cm⁻¹). The addition of excess (CH₃)₃SnH did not result in the further reduction of SiF₃H. The reduction of SiF₃H was repeated using (CH₃CH₂)₃SnH and (CH₃)₂SnH₂ and the same results were obtained. 2. Reduction of SiF₂Br₂ to SiF₂HBr with (CH₃)₃SnH, (CH₃CH₂)₃SnH, or (CH₃)₂SnH₂. Synthesis of SiF₂HBr. One equivalent of (CH₃)₃SnH was allowed to react with SiF₂Br₂ in a 1:1, Sn-H:SiF₂Br₂ molar ratio at room temperature for approximately 12 hours in a sealed flask attached to a high vacuum line. The reaction mixture was then passed through a trap cooled to -131 degrees Centigrade into a trap cooled to -196 degrees Centigrade. The material trapped at -196 degrees Centigrade was analyzed by infrared, ¹H, and ¹⁹F nmr spectroscopies. The major component (>90% yield) exhibited the following spectral data: IR (cm⁻¹) 2283 (m), 979 (s), 969 (s), 888 (vs), 832 (s), 826 (s), 469 (m). ¹H nmr (200 MHz, D₈-toluene): triplet centered at 4.99 ppm, J H-F = 80.5 Hz, J F-29Si = 378.8 Hz. ¹⁹F nmr (188 MHz, D₈-toluene): doublet centered at 81.7 ppm (upfield from external CFCl₃), J F-H = 80.4 Hz, J F-29Si = 320.0 Hz. These data indicate the structural formula is SiF₂HBr. There were also peaks in the IR and nmr spectra of SiF₂HBr due to small amounts of SiF₃H probably formed from the reduction of small amounts of SiF₃Br present as an impurity in the starting material. Addition of a second equivalent of (CH₃)₃SnH resulted in the disappearance of the SiF₂HBr (2283 cm⁻¹) and the formation of SiF₂H₂ (2251 cm⁻¹). There was no further reduction of SiF₂H₂ upon addition of excess organotin hydride. The above example was repeated with (CH₃CH₂)₃SnH and (CH₃)₂SnH₂, separately, and the results obtained were the same as those described above for the reduction with (CH₃)₃SnH. 3. Reduction of SiFBr₃ with (CH₃)₃SnH, (CH₃CH₂)₃SnH, or (CH₃)₂SnH₂). Using the procedure of Example 2, three equivalents of (CH₃)₃SnH were allowed to react with one equivalent of SiFBr₃ at room temperature. After 30 minutes, an infrared spectrum indicated the presence of SiH₄ (2186 cm⁻¹), SiF₂H₂ (2251 cm⁻¹), and a small amount of SiF₃H (2315 cm⁻¹). There was no SiFH₃ observed in the reaction mixture. The SiH₄ is present due to the high propensity of SiFH₃ to redistribute F and H to produce SiH₄ and SiF₂H₂. In a similar manner, the SiF₃H is believed to be from SiF₂H₂/SiFH₃ redistribution. Three equivalents of (CH₃)₃SnH and one equivalent of SiFBr₃ were condensed into a trap held at -196 degrees Centigrade. A -78 degrees Centigrade bath was then placed around the trap and an infrared spectrum of the reaction mixture was obtained after 90 minutes. The spectrum contained bands due to SiFH₃ (2206, 2196, 991, 873, 729 cm⁻¹). 4. Reduction of SiCl₄ with Organotin Hydrides. Equimolar quantities of SiCl₄ and (CH₃)₃SnH were allowed to react in a sealed container at room temperature. After 40 minutes, an infrared spectrum was obtained which indicated that no reaction had occurred. An infrared spectrum obtained after 2.5 hours indicated the presence of a major product, SiCl₃H, (2260 cm⁻¹) together with substantially lesser amounts of SiCl₂H₂ (2236 cm⁻¹) and SiH₄ (2186 cm⁻¹). The reaction was allowed to proceed for 36 hours, at which point the (CH₃)₃SnH had completely reacted. The Si-containing products included unreacted SiCl₄, SiCl₃H (70-75% of the reduced species), and small amounts of SiCl₂H₂ and SiH₄. Repeating the reduction with (CH₃CH₂)₃SnH and (CH₃)₂SnH₂, separately, replacing the (CH₃)₃SnH did not change the results. 5. Reduction of SiCl₃H with Organotin Hydrides (condensed phase) Equimolar quantities of SiCl₃H and (CH₃)₃SnH were allowed to react at room temperature. Approximately 12 hours were required for the Sn-H material to be consumed, at which point unreacted SiCl₃H (2260 cm⁻¹) was still present in the mixture. The major products of the reaction were SiCl₂H₂ (43% total yield, representing 70-75% of the reduced species) (2236 cm⁻¹) and SiH₄ (18% of total yield, 25% of the reduced species) (2186 cm⁻¹). There was approximately 38% unreacted SiCl₃H in the product mixture. There was no evidence of SiClH₃ (2212 cm⁻¹) in the mixture. The same results were obtained regardless of the organotin hydride employed. 6. Reduction of (CH₃)SiCl₃ with Organotin Hydrides. Equimolar quantities of (CH₃)SiCl₃ and (CH₃)₃SnH were allowed to react to room temperature over night. An infrared spectrum of the mixture indicated that most of the starting materials remained unreacted, with only a trace of (CH₃)SiCl₂H (2218 cm⁻¹) being formed. The amount of (CH₃)SiCl₂H did not increase significantly, even after warming the mixture to 60-65 degrees Centigrade for 12 hours. There was also no further reduction of the (CH₃)SiCl₂H. The same results were obtained regardless of the organotin hydride employed. 7. Reactions of (CH₃)₂GeH₂ with SiCl₃H or SiFBr₃. Equimolar amounts of (CH₃)₂GeH₂ and SiCl₃H (or (SiFBr₃) were allowed to react together in the gas phase at room temperature. In each case, no reaction was observed (IR spectroscopy) after 12 hours. 8. Reactions of (CH₃)₃SiH with SiF₂Br₂ or SiF₃Br. Equimolar amounts of (CH₃)₃SiH and SiF₂Br₂ (or SiF₃Br) were allowed to react at room temperature for 12 hours. In each case, no reaction was observed (IR spectroscopy) after 12 hours. 9. Reaction of (CH₃)₃SnH with SiF₃Cl. Equimolar quantities of SiF₃Cl and (CH₃)₃SnH were condensed together and allowed to react for 60 hours at room temperature. The only product observed from this reaction was SiF₃H. There was no further reaction observed upon the addition of excess (CH₃)₃SnH. 10. Reaction of (CH₃)₃SnH with SiBr₄. (Condensed phase) Equimolar quantities of SiBr₄ and (CH₃)₃SnH were condensed together and allowed to react for 1 hour at room temperature. An infrared spectrum of the reaction mixture indicated that all of the (CH₃)₃SnH had been consumed. Aside from some unreacted SiBr₄, the products of the reaction consisted of SiH₄ and SiBr₃H (ca. 60:40 molar ratio). There was no evidence of SiBr₂H₂ in the infrared spectrum of the mixture. 11. Reaction of (CH₃)₃SnH with SiBr₄. (gas phase) Equimolar quantities of SiBr₄ and (CH₃)₃SnH were allowed to react for 12 hours at room temperature in the gas phase. An infrared spectrum of the reaction mixture indicated that the only reduced species present was SiBr₃H; there was no evidence of SiBr₂H₂ or SiH₄. 12. Reaction of (CH₃)₃SnH with SiBr₄. (gas phase)(2 equiv.) The procedure in Example 11 was repeated and the resulting SiBr₃H was allowed to react with a second equivalent of (CH₃)₃SnH. After 12 hours at room temperature, an infrared spectrum of the reaction mixture indicated the presence of unreacted SiBr₃H, SiH₄, and a trace of SiBr₂H₂. 13. Reaction of (CH₃)₃SnH with SiBrCl₃. Equimolar quantities of (CH₃)₃SnH and SiBrCl₃ were condensed together in a vacuum line trap and allowed to warm to room temperature. An infrared spectrum of the reaction mixture was obtained after three hours at room temperature. The only Si-H containing products were SiCl₃H and SiH₄ (due to redistribution) (ca: 4:1 molar ratio). There was also some SiCl₄ observed in the reaction mixture. This reaction shows the preferential reduction of the halogen with the highest atomic weight before the reduction of any of the lower atomic weight halogen atoms.

Claims: A method for the selective or stepwise reduction, of at least one of the silicon-X bonds of a halosilane to a silicon-hydrogen bond, wherein the halosilane is of the formula X m SiH n F y R¹ z , where m is 1, 2, 3, or 4; n is 0, 1, or 2; y is 0, 1, 2, or 3; z is 0, 1, or 2; n + y + Z is less than or equal to 3; m + n + y + z is equal to 4; m + y is equal to 2, 3 or 4; X is independently selected from the group consisting of Cl, Br, or I; and R¹ is an alkyl group having one to twenty carbon atoms, said method comprising the step of contacting said halosilane in the absence of a promoter with an alkyltin hydride of the formula R² x SnH (4-x) , wherein x is 3, 2, or 1, and wherein R² is an alkyl group containing 1 to 10 carbon atoms, or an aryl group of 6 to 10 carbon atoms, whereby at least one of the silicon-X bonds of the halosilane is reduced to a silicon-hydrogen bond, the halogen or halogens other than fluorine being selectively reduced, substantially without reduction of fluorine when fluorine is present, and, where two or more halogen atoms other than fluorine are present, in the presence or absence of fluorine, said reduction being stepwise where only the same halogen other than fluorine is present, and preferentially stepwise where two or more different halogens other than fluorine are present, the halogen with the higher or highest atomic weight being preferentially reduced. A method as claimed in Claim 1, wherein the alkyltin hydride is (CH₃)₃SnH. A method as claimed in Claim 1, wherein the alkyltin hydride is (CH₃CH₂)₃SnH. A method as claimed in Claim 1, wherein the alkyltin hydride is (CH₃)₂SnH₂. A method as claimed in Claim 1, wherein the halosilane is SiF₂Br₂. A method as claimed in Claim 1, wherein the halosilane is SiF₃Br. A method as claimed in Claim 1, wherein the halosilane is SiFBr₃. A method according to Claim 1, wherein the halosilane is SiF₂Br₂ and the product of the selective, stepwise reduction is a compound of the formula SiF₂HBr.

Inventors: D'ERRICO, JOHN JOSEPH, SHARP, KENNETH GEORGE
Applicants: DOW CORNING CORPORATION
Application year: 1988
Main classification: C01B33
IPC codes: C01B33, C07F7