METHOD OF SELECTIVE REDUCTION OF HALODISILANES WITH ALKYLTIN HYDRIDES

Abstract: The invention relates to the selective and sequential reduction of halodisilanes by reacting these compounds at room temperature or below with trialkyltin hydrides or dialkyltin dihydrides without the use of free radical intermediates. The alkyltin hydrides selectively and sequentially reduce the Si-Cl, Si-Br or Si-I bonds while leaving intact the Si-Si and Si-F bonds present.

Description: The present invention relates to the discovery that trialkyltin hydrides and dialkyltin dihydrides 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 halogen with preservation of the silicon-silicon bond and (b) the sequential reduction of halogen atoms in halodisilanes having two or more atoms of the same halogen. The products formed by reducing the halodisilanes in this manner are suitable as precursors for preparing amorphous silicon films for use as semiconductors and protective coatings. Substitution of hydrogen for chlorine in chlorosilanes 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 notorious in promoting skeletal rearrangements (i.e., redistributions involving Si-C and/or Si-Si bonds). The latter behavior makes this reaction unsuitable for reduction of halodisilanes when one wants to avoid skeletal rearrangements. Organotin hydrides are known as effective reducing agents in organic chemistry and their reactivity patterns have been well documented. A common use of organotin 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 wide scope since R can be alkyl, aryl, acyl, propargyl, etc. The reaction conditions and efficiencies will vary with the R group (e.g., 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). However, these reactions are known to proceed through free radical mechanisms and require the addition of free radical initiators. This state of the art is revealed by US-A-3 439 008 and J. Am. Chem. Soc. 1983, 105, 5665. Thus, there exists no known method to effect preferential reduction of one halogen species to the exclusion of another in halodisilanes, such as SiF₃SiHBr₂, to enable preparation of fluorodisilanes which are suitable as precursors for preparing amorphous silicon films for use as semiconductors and protective coatings. Further, this method of preferential reduction should not include free radicals or acidic or basic catalysts which are known to attack Si-Si bonds. The present invention relates to the selective and sequential reduction of halodisilanes by reacting these compounds at or below room temperature with trialkyltin hydrides or dialkyltin dihydrides without the use of free radical intermediates. Reactions may be conducted with or without solvents for the reactants. Trialkyltin hydrides or dialkyltin dihydrides selectively and sequentially reduce the Si-I, Si-Br or Si-Cl bonds while leaving intact the Si-F and Si-Si bonds. The subject invention may be used for the synthesis of fluorodisilanes of the formulae Si₂F x H 6-x , especially where x is 3, 4, or 5. The fluorodisilanes are suitable as precursors for preparing amorphous silicon films for use as semiconductors and protective coatings. The method of the present invention includes the steps of contacting the halodisilane with a trialkyltin hydride of the formula R₃SnH, or dialkyltin dihydride of the formula R₂SnH₂, wherein R is an alkyl group containing 1 to 10 carbon atoms, whereby at least one silicon-chlorine, silicon-bromine or silicon-iodine bond of the halodisilane is reduced to a silicon-hydrogen bond. The present invention relates to a method of selective or sequential reduction of at least one silicon-chlorine, silicon-bromine or silicon-iodine bond of a halodisilane to a silicon-hydrogen bond. The method comprises contacting a halodisilane with a trialkyltin hydride of the formula R₃SnH, or dialkyltin dihydrides of the formula R₂SnH₂ wherein R is an alkyl group containing 1 to 10 carbon atoms, whereby at least one silicon-chlorine, silicon-bromine or silicon-iodine bond of the halodisilane is reduced to a silicon-hydrogen bond, and wherein the silicon-silicon and silicon-fluorine bonds of the halodisilane remain intact. Either trialkyltin hydrides or dialkyltin hydrides may be used as the reducing agent in the method of this invention. The alkyl substituents of the tin hydrides may contain 1 to 10 carbon atoms so that useful tin hydrides include, for example, trimethyltin hydride, dimethyltin dihydride, tributyltin hydride, dioctyltin dihydride, diisobutyltin dihydride and tridecyltin hydride. Generally trimethyltin hydride is the preferred reducing agent. The compounds reduced by the method of the present invention are described generally as halodisilanes. Halodisilanes contain two silicon atoms bonded together with each silicon atom bearing three other substituents. In general, the substituents may be all halogen atoms or any mixture of halogen and hydrogen atoms. The term "fluorohalodisilane" is used in the present invention to designate molecules of the formula X m Si₂H n F y , where m is an integer from 1 to 5 inclusive; n is an integer from 0 to 4 inclusive; y is an integer from 0 to 5 inclusive; and the sum of m + n + y is equal to 6. Each X designates a halogen independently selected from the group consisting of Cl, Br, or I. Fluorohalodisilanes which may be reduced in the method of this invention include, for example completely halogenated disilanes such as SiF₃SiBr₃, SiFBr₂SiBr₃, SiF₂ClSiCl₃, SiF₃SiI₃, SiF₂BrSiF₂Br, SiF₂BrSiF₂Cl and SiF₂BrSiF₃, and partially halogenated disilanes which contain 1 to 4 hydrogen substituents such as SiFHBrSiBr₃, SiF₂HSiCl₃, SiF₃SiHI₂, SiF₂HSiF₂Br, SiFHBrSiF₂Cl and SiH₃SiHFBr. A selective reduction for the purposes of the present invention is a reaction where one halogen is preferentially reduced to the exclusion of another halogen in the same molecule. In general, selective reduction is illustrated, for example, by the conversion of Si₂X′ a X″ b to Si₂X′ (a-1) HX″ b , where the atomic weight of X′ > X″. The conversion of SiF₃SiF₂Br to SiF₃SiF₂H is a specific example of selective reduction. Sequential reduction on the other hand refers to the stepwise reduction of one halogen at a time in a molecule having two or more of the same halogen atoms, for example, the conversion of Si₂X a H 6-a to Si₂X a-1 H 6-a+1 , then optionally to Si₂X a-2 H 6-a+2 in a second step. In a sequential reduction, the product isolated still retains at least one halogen substituent which is the same as the halogen removed in the reduction reaction. The conversion of SiH₃SiBr₃ to SiH₃SiHBr₂ is a specific example of a sequential reduction. It should be apparent that the present invention may also encompass conversions which are both selective and sequential, for example, the conversion of SiF₃SiBr₃ to SiF₃SiHBr₂. Organotin hydrides may be employed to convert silicon-halogen bonds to silicon-hydrogen bonds in disilane compounds where the conversion is performed with materials or in a manner that is not selective or sequential. For instance, the conversion of SiBr₃SiBr₃ directly to SiH₃SiH₃ is neither selective nor sequential. Reduction of halodisilanes with organotin hydrides, however, is especially useful and preferred when a selective or sequential conversion of halogen substituents to hydrogen is desired. The method of the present invention is applicable to a variety of halodisilanes, and particularly fluorohalodisilanes. The reductions can be carried out in the liquid or vapor (gas) phases, with or without solvent present. Solvents compatible with the reduction chemistry include aromatic hydrocarbons, alkanes, ethers and chlorofluorocarbons. Protic solvents such as alcohols or highly basic solvents such as amines are incompatible. Reactions can be conducted over a temperature range of from -80°C. to 100°C., but it is preferable to use temperatures from -80°C. to 25°C. to obtain increased selectivity and control over the extent of sequential reduction desired. The fluorohalodisilanes which are preferred materials for selective reduction by the method of this invention can be obtained, for example, from the reactions of difluorosilylene with a hydrogen halide, shown below specifically for reaction with HBr: Reaction (3) corresponds to the redistribution of the principal product of the reaction, SiF₂HSiF₂Br. Further illustrating the reduction of preferred fluorohalodisilanes, trialkyltin hydride or dialkyltin dihydride reacts with, for example, SiF₃SiHBr₂ in the liquid phase to produce a fluorodisilane of the formula Si₂F x H 6-x , where x is 3, for example: Thus, according to the present invention, the trialkyltinhydride or dialkyltin dihydride reduces the Si-Br bonds of SiF₃SiHBr₂ to form essentially SiF₃SiH₃ without affecting the Si-Si or Si-F bonds. Additionally, when the reaction is conducted at low temperatures, the product of the first reduction step, SiF₃SiH₂Br, can readily be detected. A comparable reaction with a commonly used reducing agent, Vitride™ (NaAlH₂(OCH₂CH₂OCH₃)₂), conducted under similar conditions resulted in both the Si-Br and Si-F bonds being reduced, producing SiH₄, SiF₃H and SiF₄. Silicon-silicon bonds were destroyed, since there were no disilanes observed in the mixture of products. Similarly, a comparable reduction using DIBAL™ (i-Bu₂AlH) reducing agent also resulted in both the Si-Br and Si-F bonds being reduced, producing SiF₄ and SiF₃H. There also were no disilanes observed in the product mixture. This example illustrates the mildness, selectivity and sequential nature of the alkyltin hydride reduction of silicon- and halogen-containing materials which contain fluorine and another halogen selected from the group consisting of chlorine, bromine or iodine. 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 vapor, or liquid phases with or without the presence of solvents. The methods of the present invention are useful in preparing partially reduced halodisilanes which are useful for the generation of amorphous silicon semi-conducting films and films for the protection of surfaces by chemical vapor deposition (CVD) and plasma enhanced CVD. The majority of the following examples was 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 to 20 torr (1.3-2.6×10³ Pa). 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 in the examples refer to the Si-H stretch. The predominant tin-containing by-products of these reactions were shown to be (CH₃)₃SnBr and (CH₃)₃SnCl by their ¹H nmr and IR spectra. EXAMPLE 1 - REDUCTION OF SiF₃SiHBr₂ WITH (C₃)₃SnH One equivalent (typically 0.2 mmoles) of SiF₃SiHBr₂ was condensed in a trap with two equivalents of (CH₃)₃SnH. The mixture was allowed to warm to room temperature for 30 minutes. An IR spectrum indicated that all of the SiF₃SiHBr₂ had been consumed but a small amount of (CH₃)₃SnH remained unreacted. SiBr₄ was added to react with the excess (CH₃)₃SnH (room temperature, 1 hr). The mixture was then passed through a trap cooled to -95°C. (toluene) into a trap cooled to -196°C. The material trapped at -196°C. was pure SiF₃SiH₃ (90% yield). This compound was identified by its IR and ¹H and ¹⁹F nmr spectra. This example illustrates a selective reduction using the method of the present invention. EXAMPLE 2 - REDUCTION OF SiF₃SiFHBr WITH (CH₃)₃SnH Equimolar quantities of SiF₃SiFHBr (a small amount of SiF₃SiF₂H was present in the SiF₃SiFHBr) and (CH₃)₃SnH were allowed to react at room temperature for 10 minutes. An IR of the mixture indicated that all of the SiF₃SiFHBr had been consumed. The products consisted of a mixture of SiF₃SiF₂H (Si-H, 2230 cm⁻¹) and SiF₃SiH₃ (Si-H, 2189, 2178 cm⁻¹) in an approximately 1:1 molar ratio. Equimolar quantities of SiF₃SiFHBr and (CH₃)₃SnH were condensed into a 5 mm medium wall nmr tube which contained toluene-d8. The nmr tube was flame sealed and stored at -196°C. ¹⁹F nmr spectra of the reaction mixture were recorded as the tube was allowed to gradually warm up. There was a small amount of SiF₂HSiF₂Br present as an impurity. The temperature was maintained at -80°C. for 3 hours, -60°C. for 1 hour, -30°C. for 45 minutes and 10°C. for 10 minutes. After the mixture was held at -80°C. for 30 minutes, the nmr spectrum contained peaks due to SiF₃SiFHBr (major), SiF₃SiH₃, SiF₃SiF₂H, SiF₃SiHBr₂, and a small amount of the new compound SiF₃SiH₂Br. After 2 hours at -80°C., there was much less SiF₃SiFHBr present, while the amounts of SiF₃SiH₃, SiF₃SiF₂H and SiF₃SiHBr₂ increased. The peaks due to SiF₃SiH₂Br were still evident. This same pattern was seen to continue after the mixture had been held at -60°C. for 25 minutes. Finally, after the mixture was held at 10°C. for 10 minutes, the major products were SiF₃SiH₃ and SiF₃SiF₂H, with small amounts of SiF₃SiFHBr and SiF₃SiHBr₂ remaining. This example illustrates a selective reduction accompanied by redistribution of the reduction product. EXAMPLE 3 - THE REDUCTION OF SiF₂HSiF₂Br WITH (CH₃)₃SnH Equimolar amounts of SiF₂HSiF₂Br and (CH₃)₃SnH were allowed to react at room temperature for 10 minutes. An IR spectrum of the mixture indicated that all of the SiF₂HSiF₂Br had been consumed. The major products of the reaction were SiF₃SiF₂H (Si-H, 2230 cm⁻¹) and SiF₃SiH₃(Si-H, 2189, 2178 cm⁻¹). Equimolar quantities of SiF₂HSiF₂Br and (CH₃)₃SnH were condensed into a 5 mm medium wall nmr tube with toluene-d8. The nmr tube was flame sealed and stored at -196°C. ¹⁹F nmr spectra of the reaction mixture were recorded as the tube was allowed to gradually warm up. After the tube was allowed to warm to -60°C., the only peaks observed were those due to SiF₂HSiF₂Br and a small amount of SiF₃SiF₂H present as an impurity. The sample was then warmed to -40°C. for 20 minutes. The peaks due to SiF₂HSiF₂Br decreased while those due to SiF₃SiF₂H increased. There were also small amounts of SiF₂HSiF₂H, SiF₃SiFHBr, SiF₃SiHBr₂ and SiF₂BrSiFHBr observed in the mixture. There was a quartet centered at -114.7 ppm (relative to external CFCl₃) with J FSiSiH =6.9 Hz. Both the chemical shift and coupling constant are consistent with the new compound SiF₂BrSiH₃. After the mixture was held at -25°C. for 35 minutes, the peaks due to SiF₂HSiF₂Br continued to decrease while those due to SiF₃SiF₂H increased. There were now significant amounts of SiF₃SiFHBr and SiF₂BrSiFHBr present in the mixture, while SiF₂BrSiH₃, SiF₂HSiF₂H and SiF₃SiHBr₂ also increased. A new compound (triplet centered at -122.9 ppm with J FSiSiH =7.7 Hz) also appeared at this point. This chemical shift and coupling constant are consistent with the molecule SiF₃SiH₂Br. Most of the SiF₂HSiF₂Br was gone after the mixture had been held at 10°C. for 20 minutes. The main constituents at this point are SiF₃SiF₂H and SiF₃SiFHBr. The amounts of SiF₂HSiF₂H and SiF₂BrSiFHBr decreased, while SiF₂BrSiH₃, SiF₃SiHBr₂ and SiF₃SiH₂Br increased. After 20°C. for 10 minutes, the main products of the reaction were SiF₃SiF₂H and SiF₃SiH₃. The starting material was completely consumed, but there were still small amounts of SiF₃SiHBr₂, SiF₃SiH₂Br, SiF₃SiFHBr and SiF₂HSiF₂H present in the mixture. This example illustrates a selective reduction accompanied by redistribution of the reduction product. EXAMPLE 4 - THE REDUCTION OF SiF₃SiHCl₂ WITH (CH₃)₃SnH One equivalent of SiF₃SiHCl₂ (ca. 0.1 mmole) was condensed in a trap with two equivalents of (CH₃)₃SnH. The mixture was allowed to warm to room temperature for one hour. An IR spectrum indicated the presence of SiF₃SiH₃ (90%), together with a small amount of SiF₃H. EXAMPLE 5 - THE REDUCTION OF SiF₂HSiF₂Cl WITH (CH₃)₃SnH Equimolar quantities (ca. 0.1 mmole, each) of SiF₂HSiF₂Cl and (CH₃)₃SnH were condensed into a trap and allowed to warm to room temperature. An IR spectrum obtained of the mixture after 10 minutes at room temperature indicated that all of the products had been consumed. The products consisted of SiF₃SiF₂H and SiF₃SiH₃ (ca. 1:1 molar ratio), together with a small amount of SiF₄. This example illustrates a selective reduction using the method of the present invention. EXAMPLE 6 - REDUCTION OF SiF₃SiHBr₂ USING VITRIDE™ SiF₃SiHBr₂ (0.15 mmole) was condensed into a flask at -196°C. and then warmed to -80°C. A solution of NaAlH₂(OCH₂CH₂OCH₃)₂ (0.15 mmole, Vitride™) in 15 ml of toluene was added in portions over a 30 minute period. At this point, the mixture was pumped on dynamically in order to remove any volatile compounds which may have formed. However, none were observed. The reaction flask was sealed and the mixture was allowed to warm to room temperature for approximately 15 minutes. The mixture was again cooled to -80°C. and the volatile compounds were pumped off. An IR spectrum of the volatile material indicated the presence of SiH₄, SiF₃H and SiF₄. There were no disilanes observed in the mixture. EXAMPLE 7 - REDUCTION OF SiF₃SiHBr₂ USING DIBAL™ Approximately 0.2 mmole SiF₃SiHBr₂ were condensed into a flask at -196°C. This mixture was then allowed to warm to -80°C., at which point 0.2 mmole diisobutylaluminum hydride (DIBAL™) in toluene was added in portions over 30 minutes. After the addition was complete, the mixture was pumped on in order to remove any volatile species, but none were observed. The mixture was allowed to warm to room temperature for 15 minutes and then cooled to -80°C. The only volatile material obtained consisted of SiF₄ and SiF₃H. There was no evidence of disilanes in the mixture.

Claims: A method of reduction of a silicon-halogen bond of a halodisilane to a silicon-hydrogen bond wherein each halogen of the halodisilane is independently selected from the group consisting of fluorine, chlorine, bromine and iodine, and wherein when there are two or more different halogens present, the halogen with the highest atomic weight is preferentially reduced by contacting said halodisilane with a trialkyltin hydride or dialkyltin dihydride of the formula R₃SnH or R₂SnH₂, respectively, wherein R is an alkyl group containing 1 to 10 carbon atoms. A method of reduction of at least one of the silicon-X bonds of a fluorohalodisilane to a silicon-hydrogen bond, wherein the fluorohalodisilane is of the formula X m Si₂H n F y , where m is an integer from 1 to 5 inclusive; n is an integer from 0 to 4 inclusive; y is an integer from 1 to 5 inclusive; the sum of m + n + y is equal to 6; and each X is independently selected from the group consisting of chlorine, bromine, and iodine, said method comprising contacting the fluorohalodisilane with a trialkyltin hydride or dialkyltin dihydride of the formula R₃SnH or R₂SnH₂, respectively, wherein R is an alkyl group containing 1 to 10 carbon atoms, whereby at least one silicon-X bond of the fluorohalodisilane is reduced to a silicon-hydrogen bond.

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