- Email: [email protected]
Reaction of hexachlorodisilane with bases and alkyl halides
J. inorg, nucL Chem., 1967, Vol. 29, pp. 2081 to 2084. Pergamon Press Ltd. Printed in Northern Ireland
REACTION OF HEXACHLORODISILANE WITH BASES A N D ALKYL HALIDES H. J. EMELI~USand MUHAMMAD TUFAIL University Chemical Laboratory, Cambridge (Received 9 September 1966)
Abstract--Hexachlorodisilane decomposes according to the equation nSi2Cle = nSiCl4 + (SiCl~)n when heated with catalytic quantities of triphenylphosphine or triphenylarsine. No reaction occurs with tristrifluoromethylphosphine. Hexabromodisilane is similarly decomposed by trimethylamine. Hexachlorodisilane reacts with hydrogen chloride in presence of pyridine according to the equation Si2Cle + 2HCI = 2SiC14+ Hv With excess of hydrogen bromide and hexachlorodisilane a mixture of hydrogen chloride and bromide is recovered. Trichlorosilane reacts with hydrogen chloride in presence of pyridine according to the equation SiHC13 + HC1 = SIC14+ Ha. Hexachlorodisilane reacts with methyl chloride in presence of triphenylphosphine to give a mixture of methyltrichlorosilane and silicon tetrachloride. When hexachiorodisilane and methyl bromide are heated together in presence of triphenylphosphine a mixture of methyl chloride and bromide is recovered. WILKINStl) first observed the cleavage of hexachlorodisilane when heated with ammonium halides, which leads to the formation of silicon tetrachloride and trichlorosilane in roughly equimolar proportions, together with silicon-nitrogen condensation products of low volatility. Trimethylamine in catalytic amounts also brought about a reaction: nSi~C16~ nSiC14 + (SIC12)n. It was suggested that the reaction was initiated by free base so that, when ammonium salts were used, it would depend on the initial thermal dissociation of the latter. Formation of trichlorosilane would then result from a reaction involving free hydrogen chloride. Base-catalysed decomposition of hexachlorodisilane has been studied in detail subsequently, especially by URRY and his co-workers, t~ who have used it to prepare a range of polysilicon halides. COOPER et al. tz~ have also shown, inter alia, that tetraethylphosphonium iodide acts in the same way as nitrogenous bases, the trialkylphosphine being the active species. We have shown that triphenylphosphine and triphenylarsine are also effective, even though these bases do not form stable adducts with silicon tetrachloride. On the other hand tristrifluoromethylphosphine, which is devoid of donor properties, was unreactive. When hexachlorodisilane was heated to 200 ° with hydrogen chloride in presence of a small amount of pyridine the reaction that occurred was: Si2C16 + HC1 ----2SIC14 + H~. Hydrogen formation in this reaction was not observed in the work of Wilkins or of Cooper and Gilbert, though their experiments were made at lower temperatures. It is doubtful, also, if their experimental arrangements would have allowed its detection. Trichlorosilane was also found to yield silicon tetrachloride and hydrogen when heated to 150° with a small amount of base. When hexachlorodisilane and methyl chloride were heated together in presence of base the normal decomposition reaction of the disilicon halide was replaced by one giving methyltrichlorosilane and silicon tetrachloride in approximately equimolar chem. Soc. 3409 (1953). tz~ For a summary see G. URRY,J. inorg, nucl. Chem. 26, 409 (1964). cs~G. D. COOPERand A. R. GILBERT,J. Am. chem. Soe. 82, 5042 (1960). 17 2081 (11 C . J. WILKINS, J .
2082
H . J . EMELI~USand MOHAMMADTUr^tL
amounts, with no higher silicon chlorides. Preliminary experiments on the reaction of hexachlorodisilane with excess of methyl bromide in presence of base also led to recovery of a mixture of methyl chloride and methyl bromide, together with less volatile alkylated products containing chlorine and bromine. Reaction of hexachlorodisilane with excess of hydrogen bromide in presence of base also gave a volatile fraction in the product containing both hydrogen chloride and hydrogen bromide. Cooper and Gilbert put forward the following tentative reaction scheme for the decomposition of hexachlorodisilane by catalytic amounts of base (L). The last reaction accounts for the formation of trichlorosilane, the hydrogen chloride arising from thermal dissociation of the base hydrochloride. ClaSiSiC13 + L ~- L+SiCIa + SiC13SiCl a- + ClaSiSiC1a -* ClaSiSiCI2SiCIa + C1C1- + L+SiCla --~ SiC14 + L SiC1a- + HC1 -* HSiCIa + Cl-. To explain the formation of hydrogen the following steps are suggested: ClaSiH + L --~ L+SiCls + H HC1 + L--* LH + + C1Cl- + L+SiC13 -+ SiCI 4 + L H- + LH + --~ H9 + L. Reaction of hexachlorodisilane with methyl chloride in presence of base to form SiC14 and CHaSiC1a, with no higher chlorosilanes, can be accounted for by assuming the first of Cooper and Gilbert's reactions as the initial step. Since no higher chlorosilanes are formed there must be some preferred reaction of SiCla-. If this is with CHaC1, the probable products would be CH3SiC18 and CI-, the latter then reacting with L+SiCla to give SiCI t and L. In the reaction of CHzBr with SizC1e in presence of base the one clearly established fact is that, with excess of CHzBr, a mixture of unreacted CHzBr with CHzC1 remains. Exchange leading to CH3CI would be likely to occur if at some stage C1- were present, but there is insufficient evidence as to the nature of the solid products to justify speculation on the reaction mechanism. EXPERIMENTAL
Materials Hexachlorodisilane was prepared by a literature method. ~4~ Hexabromodisilane was prepared similarly, tS~ Alternatively, finely powdered calcium silicide (40 g) was stirred vigorously under reflux with carbon tetrachloride (250 ml), and bromine (30 ml) was added 1 ml at a time as the colour was discharged. The residue was filtered rapidly, washed with warm carbon tetrachloride and the filtrate fractionated. Hexabromodisilane (m.p. 95 °) distilled at 45°/1 mm as white crystals into an air condenser. A typical run gave: SiBr4, 150 g; SiaBre, 20 g; higher bromosilanes (as residue), 30 g. The proportion of residue was greater when ethylene dibromide was used as the refluxing solvent and with pentachloroethane (b.p. 161 °) as solvent 30 g of silicide and 50 ml of bromine gave 80 g of higher bromosilanes but no Si2Brs was isolated.
Decomposition of polysilicon halides by bases Reactions were studied in a series of sealed tubes (25-30 ml), each fitted with a tap lubricated with KelF grease and having a standard taper for attachment to the vacuum system. In examining volatile products it was shown that hexachlorodisilane could be separated from silicon tetrachloride
~4~Inorganic Syntheses, Vol. I, p. 42. McGraw-Hill, New York (1939). ~ Inorganic Syntheses, Vol. II, p. 98. McGraw-Hill, New York (1946).
Reaction of hexachlorodisilane with bases and alkyl halides
2083
by fractional condensation in the vacuum system. The former was condensed at --50 ° and the latter passed to a trap cooled in liquid nitrogen. Silicon tetrabromide was separated from hexabromodisilane by distillation in high vacuum at --15 °, though the recovery was somewhat less quantitative. Typical results for the decomposition of hexaehlorodisilane by bases are shown below (Table 1). The theoretical weights referred to are those calculated from the equation nSi2Cl6 = nSiCl4 + (SiCI2),. TABLE l
Si~CI~ (g) 1.596 1.607 1.598 2.472
Base
Wt. of base (g)
Time (fir)
Temp (°C)
Wt. of SIC14 found (g)
Wt. of SiCI4 theor. (g)
(C6Hs)3P (CeHs)aP (CeHs)3P (CnHs)3As
0-177 0.074 0.074 0.222
40 I00 100 72
50 50 100 100
0.986 1.012 1.010 1.335
1.008 1.012 1.009 1.579
Silicon tetrachloride was recovered quantitatively from either triphenylphosphine or triphenylarsine by distillation at --45 °, i.e. no stable adduct was formed. When Si~CI6 (1.575 g) and (CFs)sP (0.120 g), were heated at 200 ° (24 hr), 0.107 g of the pure phosphine was recovered by fractionation and the residue was unchanged hexachlorodisilane (m.p. --1°). In a typical experiment SisBr, (8"992 g) was heated in a sealed tube with (CH3)sN (0.029 g) at 50 ° (20 hr). Silicon tetrabromide (5"040 g) was formed (theoretical for NSi2Br~ = nSiBr4 + (SiBr2),, 5'837 g), together with an involatile solid residue.
Reaction of Si~CI~ and SiHCI3 with HC1 and HBr in presence of bases Experiments were done in Carius tubes, into which the weighed reactants were distilled in vacuum before sealing. Silicon tetrachloride formed in the reactions with HCI was separated by fractional condensation at -- 130 °, HCI passing to a trap cooled in liquid nitrogen. The identity of both fractions was checked by molecular weight determinations. Typical results with Si~C16 or SiHC1, and HCI are shown below (Table 2). The quantities are expressed in grammes with millimolar quantities in brackets. The base was pyridine. The experiments with Si~Cle were done at 200 ° (40 hr) and those with SiHCI3 at 150 ° (40 hr). TABLE 2 Si2C16 SiHCIa HC1 taken Pyridine SiCI4 formed H2 formed
4.04(I 5.0) -2.21(65.3) 0"40 4-88(28.6) 0-0306(15"3)
4.01 (14-9) -2.18(59.9) 0"40 4"88(28"6) 0"0296(14"8)
-2.71(20.0) 1.615(44.2) 0"20 3.34(19.6) 0"040(20"0)
-2.65(19-6) 1.41(38.6) 0" 17 3.31(19-4) 0"039(19"5)
In preliminary experiments with Si2Cln and HBr heated in presence of triphenylphosphine at 100 ° (40 hr), the most volatile product was a mixture of HCI and HBr, as was shown by its molecular weight and i.r. spectrum. Hydrogen was also formed. The other volatile products were chlorobromosilanes; a typical analysis: Si, 14.6; Br, 66"1; CI, 18.6~. No separation was attempted.
Reaction of Si2CI~ with CH3C1 and CHsBr in presence of bases The same techniques were used. With SizC16, CH3C1 and triphenylphosphine, fractional condensation of the products gave a condensate at --112 ° which was shown by its i.r. spectrum to be free of CH3CI. It showed bands at 2990, 2923, 1417 and 1271 em -1 associated with the CH3 group, an Si-C frequency at 764 em -1 and Si-C1 frequencies at 577 and 458 em -x. This is consistent with a mixture of CH3SiC18 and SiCI4. The presence of these components was confirmed by comparing the vapour phase chromatogram with that of synthetic mixtures of the two components. Unchanged CHsCI passed the trap at --112 ° and was condensed at --196 °. The identity was confirmed by the
2084
H . J . EMELI~USand M
~
TOFAm
i.r. spectrum and molecular weight. Results with triphenylphosphine as base are shown below (Table 3). The first experiment was made at 100 ° (40 hr) and the second at 200 ° (40 hr). Millimolar quantities are shown in brackets. The higher molar reaction ratio of CH3C1 to Si2Cle at 200 ° may indicate further methylation. The weight of the --112 ° fraction was less than the combined weights of Si2C16 and CHsC1 used. This is consistent with small losses due to absorption on stopcock grease. TABLE 3 Si~Cl0 (g) CHsCI (g) CHsC1 used (g) Wt. of base (g) CHsSiC13 + SiCl, (--112 °) (g) Residue (g)
1.714 1.784 0.304 0.160 1.948 0.232
(6"36) (35.3) (6.02) (M, 164)
2-193 2-196 0.850 0-206 2.956 0.278
(8.14) (43.5) (10-6) (M, 157)
A preliminary experiment at 150 ° (40 hr) with Si2C16 (2.464 g) and CH3Br (4.179 g) in presence of triphenylphosphine (0"213 g) gave a most volatile fraction of product with M, 79.7, intermediate between the value for CHsCI (M, 50.47) and CH3Br (M, 94"9). It showed i.r. bands at 1355, 1020 and 732 cm -1 due to CHsCIand at 1306, 957 and 610 cm -x due to CHsBr. A less volatile fraction retained at --112 ° could not be separated into its components. It was probably a mixture of silicon chlorobromides with some methylated material. The proton N M R spectrum showed one group of peaks between ~- = 8.95 and r = 8.50 and a much weaker group between z = 7-8 and ~- = 7.2, but these results were not interpreted.
Acknowledgements--One of
the authors (M. T.) thanks the University of the Panjab, Lahore, for granting Study Leave and I.C.I. (Mond Division) for a grant.
REACTION OF HEXACHLORODISILANE WITH BASES A N D ALKYL HALIDES H. J. EMELI~USand MUHAMMAD TUFAIL University Chemical Laboratory, Cambridge (Received 9 September 1966)
Abstract--Hexachlorodisilane decomposes according to the equation nSi2Cle = nSiCl4 + (SiCl~)n when heated with catalytic quantities of triphenylphosphine or triphenylarsine. No reaction occurs with tristrifluoromethylphosphine. Hexabromodisilane is similarly decomposed by trimethylamine. Hexachlorodisilane reacts with hydrogen chloride in presence of pyridine according to the equation Si2Cle + 2HCI = 2SiC14+ Hv With excess of hydrogen bromide and hexachlorodisilane a mixture of hydrogen chloride and bromide is recovered. Trichlorosilane reacts with hydrogen chloride in presence of pyridine according to the equation SiHC13 + HC1 = SIC14+ Ha. Hexachlorodisilane reacts with methyl chloride in presence of triphenylphosphine to give a mixture of methyltrichlorosilane and silicon tetrachloride. When hexachiorodisilane and methyl bromide are heated together in presence of triphenylphosphine a mixture of methyl chloride and bromide is recovered. WILKINStl) first observed the cleavage of hexachlorodisilane when heated with ammonium halides, which leads to the formation of silicon tetrachloride and trichlorosilane in roughly equimolar proportions, together with silicon-nitrogen condensation products of low volatility. Trimethylamine in catalytic amounts also brought about a reaction: nSi~C16~ nSiC14 + (SIC12)n. It was suggested that the reaction was initiated by free base so that, when ammonium salts were used, it would depend on the initial thermal dissociation of the latter. Formation of trichlorosilane would then result from a reaction involving free hydrogen chloride. Base-catalysed decomposition of hexachlorodisilane has been studied in detail subsequently, especially by URRY and his co-workers, t~ who have used it to prepare a range of polysilicon halides. COOPER et al. tz~ have also shown, inter alia, that tetraethylphosphonium iodide acts in the same way as nitrogenous bases, the trialkylphosphine being the active species. We have shown that triphenylphosphine and triphenylarsine are also effective, even though these bases do not form stable adducts with silicon tetrachloride. On the other hand tristrifluoromethylphosphine, which is devoid of donor properties, was unreactive. When hexachlorodisilane was heated to 200 ° with hydrogen chloride in presence of a small amount of pyridine the reaction that occurred was: Si2C16 + HC1 ----2SIC14 + H~. Hydrogen formation in this reaction was not observed in the work of Wilkins or of Cooper and Gilbert, though their experiments were made at lower temperatures. It is doubtful, also, if their experimental arrangements would have allowed its detection. Trichlorosilane was also found to yield silicon tetrachloride and hydrogen when heated to 150° with a small amount of base. When hexachlorodisilane and methyl chloride were heated together in presence of base the normal decomposition reaction of the disilicon halide was replaced by one giving methyltrichlorosilane and silicon tetrachloride in approximately equimolar chem. Soc. 3409 (1953). tz~ For a summary see G. URRY,J. inorg, nucl. Chem. 26, 409 (1964). cs~G. D. COOPERand A. R. GILBERT,J. Am. chem. Soe. 82, 5042 (1960). 17 2081 (11 C . J. WILKINS, J .
2082
H . J . EMELI~USand MOHAMMADTUr^tL
amounts, with no higher silicon chlorides. Preliminary experiments on the reaction of hexachlorodisilane with excess of methyl bromide in presence of base also led to recovery of a mixture of methyl chloride and methyl bromide, together with less volatile alkylated products containing chlorine and bromine. Reaction of hexachlorodisilane with excess of hydrogen bromide in presence of base also gave a volatile fraction in the product containing both hydrogen chloride and hydrogen bromide. Cooper and Gilbert put forward the following tentative reaction scheme for the decomposition of hexachlorodisilane by catalytic amounts of base (L). The last reaction accounts for the formation of trichlorosilane, the hydrogen chloride arising from thermal dissociation of the base hydrochloride. ClaSiSiC13 + L ~- L+SiCIa + SiC13SiCl a- + ClaSiSiC1a -* ClaSiSiCI2SiCIa + C1C1- + L+SiCla --~ SiC14 + L SiC1a- + HC1 -* HSiCIa + Cl-. To explain the formation of hydrogen the following steps are suggested: ClaSiH + L --~ L+SiCls + H HC1 + L--* LH + + C1Cl- + L+SiC13 -+ SiCI 4 + L H- + LH + --~ H9 + L. Reaction of hexachlorodisilane with methyl chloride in presence of base to form SiC14 and CHaSiC1a, with no higher chlorosilanes, can be accounted for by assuming the first of Cooper and Gilbert's reactions as the initial step. Since no higher chlorosilanes are formed there must be some preferred reaction of SiCla-. If this is with CHaC1, the probable products would be CH3SiC18 and CI-, the latter then reacting with L+SiCla to give SiCI t and L. In the reaction of CHzBr with SizC1e in presence of base the one clearly established fact is that, with excess of CHzBr, a mixture of unreacted CHzBr with CHzC1 remains. Exchange leading to CH3CI would be likely to occur if at some stage C1- were present, but there is insufficient evidence as to the nature of the solid products to justify speculation on the reaction mechanism. EXPERIMENTAL
Materials Hexachlorodisilane was prepared by a literature method. ~4~ Hexabromodisilane was prepared similarly, tS~ Alternatively, finely powdered calcium silicide (40 g) was stirred vigorously under reflux with carbon tetrachloride (250 ml), and bromine (30 ml) was added 1 ml at a time as the colour was discharged. The residue was filtered rapidly, washed with warm carbon tetrachloride and the filtrate fractionated. Hexabromodisilane (m.p. 95 °) distilled at 45°/1 mm as white crystals into an air condenser. A typical run gave: SiBr4, 150 g; SiaBre, 20 g; higher bromosilanes (as residue), 30 g. The proportion of residue was greater when ethylene dibromide was used as the refluxing solvent and with pentachloroethane (b.p. 161 °) as solvent 30 g of silicide and 50 ml of bromine gave 80 g of higher bromosilanes but no Si2Brs was isolated.
Decomposition of polysilicon halides by bases Reactions were studied in a series of sealed tubes (25-30 ml), each fitted with a tap lubricated with KelF grease and having a standard taper for attachment to the vacuum system. In examining volatile products it was shown that hexachlorodisilane could be separated from silicon tetrachloride
~4~Inorganic Syntheses, Vol. I, p. 42. McGraw-Hill, New York (1939). ~ Inorganic Syntheses, Vol. II, p. 98. McGraw-Hill, New York (1946).
Reaction of hexachlorodisilane with bases and alkyl halides
2083
by fractional condensation in the vacuum system. The former was condensed at --50 ° and the latter passed to a trap cooled in liquid nitrogen. Silicon tetrabromide was separated from hexabromodisilane by distillation in high vacuum at --15 °, though the recovery was somewhat less quantitative. Typical results for the decomposition of hexaehlorodisilane by bases are shown below (Table 1). The theoretical weights referred to are those calculated from the equation nSi2Cl6 = nSiCl4 + (SiCI2),. TABLE l
Si~CI~ (g) 1.596 1.607 1.598 2.472
Base
Wt. of base (g)
Time (fir)
Temp (°C)
Wt. of SIC14 found (g)
Wt. of SiCI4 theor. (g)
(C6Hs)3P (CeHs)aP (CeHs)3P (CnHs)3As
0-177 0.074 0.074 0.222
40 I00 100 72
50 50 100 100
0.986 1.012 1.010 1.335
1.008 1.012 1.009 1.579
Silicon tetrachloride was recovered quantitatively from either triphenylphosphine or triphenylarsine by distillation at --45 °, i.e. no stable adduct was formed. When Si~CI6 (1.575 g) and (CFs)sP (0.120 g), were heated at 200 ° (24 hr), 0.107 g of the pure phosphine was recovered by fractionation and the residue was unchanged hexachlorodisilane (m.p. --1°). In a typical experiment SisBr, (8"992 g) was heated in a sealed tube with (CH3)sN (0.029 g) at 50 ° (20 hr). Silicon tetrabromide (5"040 g) was formed (theoretical for NSi2Br~ = nSiBr4 + (SiBr2),, 5'837 g), together with an involatile solid residue.
Reaction of Si~CI~ and SiHCI3 with HC1 and HBr in presence of bases Experiments were done in Carius tubes, into which the weighed reactants were distilled in vacuum before sealing. Silicon tetrachloride formed in the reactions with HCI was separated by fractional condensation at -- 130 °, HCI passing to a trap cooled in liquid nitrogen. The identity of both fractions was checked by molecular weight determinations. Typical results with Si~C16 or SiHC1, and HCI are shown below (Table 2). The quantities are expressed in grammes with millimolar quantities in brackets. The base was pyridine. The experiments with Si~Cle were done at 200 ° (40 hr) and those with SiHCI3 at 150 ° (40 hr). TABLE 2 Si2C16 SiHCIa HC1 taken Pyridine SiCI4 formed H2 formed
4.04(I 5.0) -2.21(65.3) 0"40 4-88(28.6) 0-0306(15"3)
4.01 (14-9) -2.18(59.9) 0"40 4"88(28"6) 0"0296(14"8)
-2.71(20.0) 1.615(44.2) 0"20 3.34(19.6) 0"040(20"0)
-2.65(19-6) 1.41(38.6) 0" 17 3.31(19-4) 0"039(19"5)
In preliminary experiments with Si2Cln and HBr heated in presence of triphenylphosphine at 100 ° (40 hr), the most volatile product was a mixture of HCI and HBr, as was shown by its molecular weight and i.r. spectrum. Hydrogen was also formed. The other volatile products were chlorobromosilanes; a typical analysis: Si, 14.6; Br, 66"1; CI, 18.6~. No separation was attempted.
Reaction of Si2CI~ with CH3C1 and CHsBr in presence of bases The same techniques were used. With SizC16, CH3C1 and triphenylphosphine, fractional condensation of the products gave a condensate at --112 ° which was shown by its i.r. spectrum to be free of CH3CI. It showed bands at 2990, 2923, 1417 and 1271 em -1 associated with the CH3 group, an Si-C frequency at 764 em -1 and Si-C1 frequencies at 577 and 458 em -x. This is consistent with a mixture of CH3SiC18 and SiCI4. The presence of these components was confirmed by comparing the vapour phase chromatogram with that of synthetic mixtures of the two components. Unchanged CHsCI passed the trap at --112 ° and was condensed at --196 °. The identity was confirmed by the
2084
H . J . EMELI~USand M
~
TOFAm
i.r. spectrum and molecular weight. Results with triphenylphosphine as base are shown below (Table 3). The first experiment was made at 100 ° (40 hr) and the second at 200 ° (40 hr). Millimolar quantities are shown in brackets. The higher molar reaction ratio of CH3C1 to Si2Cle at 200 ° may indicate further methylation. The weight of the --112 ° fraction was less than the combined weights of Si2C16 and CHsC1 used. This is consistent with small losses due to absorption on stopcock grease. TABLE 3 Si~Cl0 (g) CHsCI (g) CHsC1 used (g) Wt. of base (g) CHsSiC13 + SiCl, (--112 °) (g) Residue (g)
1.714 1.784 0.304 0.160 1.948 0.232
(6"36) (35.3) (6.02) (M, 164)
2-193 2-196 0.850 0-206 2.956 0.278
(8.14) (43.5) (10-6) (M, 157)
A preliminary experiment at 150 ° (40 hr) with Si2C16 (2.464 g) and CH3Br (4.179 g) in presence of triphenylphosphine (0"213 g) gave a most volatile fraction of product with M, 79.7, intermediate between the value for CHsCI (M, 50.47) and CH3Br (M, 94"9). It showed i.r. bands at 1355, 1020 and 732 cm -1 due to CHsCIand at 1306, 957 and 610 cm -x due to CHsBr. A less volatile fraction retained at --112 ° could not be separated into its components. It was probably a mixture of silicon chlorobromides with some methylated material. The proton N M R spectrum showed one group of peaks between ~- = 8.95 and r = 8.50 and a much weaker group between z = 7-8 and ~- = 7.2, but these results were not interpreted.
Acknowledgements--One of
the authors (M. T.) thanks the University of the Panjab, Lahore, for granting Study Leave and I.C.I. (Mond Division) for a grant.