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Complexes of disilicon hexachloride with trimethylamine
J. Inorg. Nuc]. Chem., 1964. Vol. 26, pp. 415 to 420. Pergamon
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Ireland
COMPLEXES OF DISILICON HEXACHLORIDE TRI M E T H Y L A M I N E ~a,
WITH
A . KACZMARCZYK and G. URRY* Richard Benbridge Wetherill Laboratory, Purdue University, Lafayette, Indiana
(Received 17 June 1963) Abstract--Disilicon bexachloride forms a well defined complex with trimethylamine, Si,CI6'2N(CHs)8, stable below--45 °. For the reaction Si~CI¢2N(CHs)3 = Si~CI6'N(CH3)s + N(CHa)3a A H o f a p p r o x i mately 10 kcal per mole can be obtained. The complex Si2C1B'N(CHs)s is stable below --45 ° and exhibits no dissociation pressure below that temperature. At --45 ° Si~C16"N(CHa)s disproportionates slowly, obeying zero order kinetics, to silicon tetrachloride, trimethylamine, and [SiCl2.a6.0 21N
(CHs)s],~ where m is probably 6. When [SiCI2.38.0"21N(CHa)a],~is warmed to room temperature, the disproportionation continues until the residual composition is [SiCla.78'0'l N(CHs)3]~. This material appears to be a mixture. As a result of the described disproportionations, no AH could be obtained for the reaction: Si2CI~'N(CHa)s = Si2Cle + N(CHs)v MOST o f the tetravalent halogenated silanes are known to form complexes with tertiary amines. Several workers have investigated these complexes.tLs, 4) BURG has determined the dissociation pressures and heats of dissociation for SiC14.N(CHa) a and SiH2CI2.N(CHa)a .tS) Presumably these complexes involve the formation o f a coordinate b o n d utilizing the " n o n b o n d i n g " electron pair on the nitrogen and the vacant 3d orbitals o f the silicon. Apparently the penta co-ordinated 1 : 1 complex is stable at lower temperatures, but the 1 : 2 complex which would require the silicon to be in a hexa-co-ordinated state has no unique stability, although BURG does report unstable solids, varying in composition from SiCI4.2N(CHs) a to SiC14.1.5N(CHs)3, which he attributes to solid solution behaviour o f SiC14.N(CHa) a and N(CHs) a. We u n d e r t o o k the examination o f the disilicon hexachloride-trimethylamine system i n order to determine whether both silicon atoms would participate in the formation o f a complex with the trimethylamine. RESULTS
AND
DISCUSSION
I f a solution o f disilicon hexachloride in trimethylamine, prepared at --79 ° and maintained at that temperature with vigorous stirring for 24 hr, is held at -- 79 ° and equilibrated with a trap maintained at --95 °, the solvent trimethylamine distills into the --95 ° trap, leaving a solid o f the composition, Si~CIs'2N(CHs)a, in the --79 ° vessel. This solid exhibits a dissociation pressure of 0.5 m m at --79 °. If trimethylamine is removed from the 1 : 2 complex, this equilibrium dissociation pressure can be observed as constant until the composition of the complex becomes Si2CI6"N(CHs):~, * To whom inquiries concerning this publication should be addressed. t~ Recent Developments in the Chemistry of the Perchloropolysilanes-II. For a discussion of the results described in this paper in relation to the results reported in other papers of this series see paper I, G. URRV,J. Inorg. Nucl. Chem. 26, 409 (1964). ~2~W. R. TROST, Canad. J. Chem. 29, 877-84; Nature. Lend. 169, 289 (1952). ~s~j. E. FERGUSSON,D. K. GRANT, R. H. HICKFORDand C. J. WILKINS,J. Chem. Soc., 99, (1959). t4, U. WANNAGATand F. V/ELaERt3,Z. Anop;g. Chem. 291,310-13 (1952). 15~A. B. BURG.J. Amer. Chem. Soc. 76, 2674-5 (1954). 415
416
A. KACZMA~CZYKand G. U~Y.
which exhibits no dissociation pressure at any temperature below --45 °. Thus, the observed dissociation pressure of 0.5 mm at --79 ° is a true equilibrium pressure for the reversible equilibrium: Si~CI6"2N(CHa)a ~ Si2Cle'N(CHa)3 + N(CHa)a
(1)
and there appears to be no evidence for solid solution behaviour of the different amine complexes. On the basis of two dissociation pressures, an approximate AH for Equation (1) can be calculated as 10 kcals per mole. This compares with the 11.9 kcal per mole for the dissociation of SiC14"N(CHa)a, reported by BURG.tS) The dissociation of the 1 : 1 complex could not be studied, since, as previously mentioned, there is no observable dissociation pressure below --45 ° and at this temperature a slow disproportionation occurs. This disproportionation occurs in two distinct stages. During the first stage equimolar amounts of silicon tetraehloride and trimethylamine are evolved and the kinetics with respect to silicon tetrachloride formation are zero order. The second stage ensues when the residual solid has attained the average composition, [SiC12.57.0.28N(CHa)3]~. From this point the reaction proceeds at a slower rate. During the second stage approximately first order kinetics are observed and the ratio of silicon tetrachloride to amine evolved is greater than one. When the solid residue has achieved the composition, [SiCla.aa'0"21N (CHa)3],~, the evolution of silicon tetrachloride and trimethylamine ceases entirely. The evolution of silicon tetrachloride and trimethylamine was observed at a low (10 --a mm) constant pressure. If [SiC12.ae'0"21N(CHa)a],~ is warmed from --45 ° to room temperature silicon tetrachloride and amine again are evolved at an even slower rate than the second stage of the --45 ° disproportionation. This reaction ceases when the solid has reached the composition, [SiCll.~s'0" 1N(CHa)a]~. The amounts of silicon tetrachloride and trimethylamine evolved during the course of disproportionation of SizCIe'N(CH3)3 at --45 ° are presented in Table 1. Similar data for the room temperature disproportionation of [SiClz.30"0"21N(CHs)8],n are presented in Table 2. The following equations are suggested as a possible explanation of the course of this disproportionation and are consistent with the observations. Si~Cle'N(CH3)8 ~-Si2C16 + N(CHa)3
(2)
Si2C10"N(CH3)3 + Si~C16---~SisCIs'N(CH3)a + SiCla
(3)
SiaCIs.N(CHs)8 + Si2CIe -~ Si4C11o.N(CH3)3 + SiCI4
(4)
Si4Cll0.N(CHs) 3 + Si~C1e ~ SisCI12.N(CH3)3.SiC14
(5)
N(CHa)3 + SisCI=N(CH3)s'SiC14 + Si~C10~ SieClx4"2N(CHs)3"SiCI~ + SIC14 (6) SisCI~.N(CHs) 3 + Si2CI6.N(CH3) 3 --~ SieCI~4"2N(CHz)s'SiCla
(7)
Si~CI~'2N(CH~)z'SiCI~ ~ Si~Clla.2N(CHa)a + SiCI~
(8)
Si~Cll~.2N(CHs)z ~ SioClx~'N(CHz) ~ + N(CHa) z
(9)
Complexes of disilicon hexachloride with trimethylamine
417
Equation (2) is presented as the rate determining equilibrium since a small excess o f trimethylamine pressure was sufficient to inhibit the disproportionation at --45 °. I f the uncomplexed disilicon hexachloride is assumed to be used up rapidly in reactions (3), (4) and (5), the evolution o f equimolar a m o u n t s o f silicon tetrachloride and trimethylamine can be accounted for. The change in rate observed when the residual composition is [SilC12.5~.0.28N(CHa)a],, taken in conjunction with the fact that during the second stage o f the reaction non-equivalent amounts o f silicon tetrachloride and trimethylamine are evolved, leads us to postulate a reaction represented by either Equation (6) or (7). The first order kinetics observed after this change in rate, can be explained by assuming stepwise dissociations as represented by Equations (8) and (9). In this case, the rate o f silicon tetrachloride evolution could be different from the rate o f evolution o f trimethylamine, as observed, if the various complexes formed a solid solution, with the rate o f evolution o f either silicon tetrachloride or trimethylamine governed by the activities o f the various complex species undergoing dissociation. The zeroorder kinetics o f the first stage is to be expected, since all o f the postulated and k n o w n reactants would be solids at --45 ° . The r o o m temperature disproportionation is o f interest because it results in a material that could be related to a c o m p o u n d reported by SCHWARZ, (11) Sil0Cl~s. The composition o f the end p r o d u c t of the r o o m temperature disproportionation is [SiCI1.7s'0"IN(CHz)zL. A determination o f the average molecular weight in methylene chloride indicated a value o f 10 for x. The material is not homogeneous, however, and Si5Cl12 has been identified as one c o m p o n e n t o f this mixture. It is clear, therefore, that the composition o f the residue must possess a lower chlorine content than Si~0Clls. This possibility thus is ruled out by the k n o w n facts. It is possible that the (SiC12)~ reported by WILKINS(7) is a similar mixture. APPARATUS AND EXPERIMENTAL METHODS Unless otherwise stated, all manipulations necessary in the present work were carried out in the absence of air and moisture. The techniques used were generally similar to those described in the published literatureJS, g) Analytical methods
All compounds with appreciable vapour-tensions at room temperature were measured as gases in calibrated volumes in the vacuum apparatus. Disilicon hexachloride was transferred in vacuo and weighed. Mixtures of silicon tetrachloride and trimethylamine, which proved impossible to separate using standard fractional condensation techniques, were treated as follows: The total volume of the mixture was determined with all material in the vapour phase. To this mixture was added an equal volume of anhydrous hydrogen chloride. The mixture, thus obtained, was allowed to stand for 12 hr at room temperature, during which time all of the trimethylamine present had precipitated as trimethylamine hydrochloride. There remained in the vapour phase, at this point, a mixture of silicon tetrachloride and anhydrous hydrogen chloride. This mixture was separated into its constituents by repeated distillation through a trap maintained at - 112o until the fraction retained at that temperature was pure silicon tetrachloride. The amount of silicon tetrachloride was determined by measurement of its total volume in the vapour phase. The amount of trimethylamine was determined both by difference and by a standard chloride determination, carried out on the trimethylamine hydrochloride precipitated during the treatment of the initial mixture with anhydrous hydrogen chloride. ee) R. SCUWARZand A. KOSTER,Z. Anorg. Chem. 270, 2-15 (1952). c7) C. J. WILKINS,J. Chem. Soc. 3409-12 (1953). (8) A. STOCK,Hydrides of Boron and Silicon, Cayuga Press, Ithaca, New York (1933). (9) R. T. SANDERSON,Vacuum Manipulation of Volatile Compounds, Wiley, New York (1949).
418
A. KACZMARCZYKand G. Urtrty
Reagents Disilicon hexachloride. Disilicon hexachloride used in the present work, was required to have a high degree of purity. Since the product of the standard preparation c1°~ is usually contaminated by small amounts of both hexachiorosiloxane and titanium tetrachloride, we devised a mercury arc reduction of silicon tetrachioride which although an inefficient method, produces disilicon hexachloride of exceptional purity. In this preparation, silicon tetrachloride vapour, at a pressure of approximately 4-5 mm, is passed through a mercury arc similar to that already described in the literatt~re, m~ The product disilicon hexachloride was separated from unchanged silicon tetrachioride by repeated distillations through a trap maintained at --23*. Anhydrous trimethylamine. This compound was obtained from the Matheson Company, Joliet, Illinois, was purified by fractional condensation in traps maintained at --126 ° and --196 °. The fraction retained at --126 ° exhibited a 0 ° vapour tension of 686 mm, in good agreement with the literature value of 683 mm.
Procedure Trimethylamine complexes ofdisilicon hexachloride. A sample of disilicon hexachloride, weighing 1.0562 g (3.91 mmoles), was distilled into a stirred vessel and maintained at a temperature of --79 °. Into an adjoining U-tube 20.33 mmoles (455 ml at S.T.P.) of trirnethylamine were condensed. The amine then was allowed to warm to room temperature and distill over into the stirred vessel containing disilicon hexachloride at --79*. The mixture was stirred for 24 lax and the excess amine then was removed, against a pressure of 1.6 mm, by distillation into a trap maintained at --95*. After several hours, 12.34 mmoles of trimethylamine had condensed in the trap maintained at --95 °, and no more amine could be removed, even when the distillation was continued for 12 hr. A 5 rain distillation of the remaining amine vapour, against a 0.3 mm pressure into a trap maintained at -- 112 °, collected 0.19 mmole more of trimethylamine. All of the amine distilled away was returned into the stirred vessel, still maintained at --79 °, and the entire procedure repeated. After several repetitions, with variations in the times of distillation, it was found that, on the average, a total of 12.53 mmoles of trimethylamine readily could be removed from the mixture maintained at --79* (vapour-tension of pure trimethylamine at --79 ° is 6 mm). This indicates the formation of a 2 : I complex containing 7.80 and 3"91 mmoles of N(CHs)8 and SisCl6, respectively. Upon standing at --79 °, this complex exhibits a reversible dissociation pressure of 0.5 4- 0'1 mm. The reversibility of this dissociation pressure was demonstrated by removal of trimethylamine vapour until less than the stoichiometric amount of amine was present. The pressure above this mixture was still observed to be 0.5 mm at - 7 9 °. The dissociation pressures of several compositions below the stoichiometric composition were noted to be identical. By using the same procedure the dissociation pressure at --63 ° was found to be 3'5 4- 0.1 mm. The heat of dissociation for the reversible reaction: Si2CI¢2N(CHs)3 ~ Si2Cle'N(CHs)3 + N(CH3)8 was calculated from the equation: 2270 log K . = l l . 0 2 4 - - - - - ~ , and found to be approximately 10 kcal per mole. This can be compared with the 11.9 kcal per mole for the dissociation of SiCI~-N(CHs)s. t6~ The equation is, of necessity, approximate, since only two dissociation pressures were available for its derivation. Prolonged distillation, at a pressure of 10-s mm and a temperature of --63% reduced the 2 : 1 complex to the composition, SisCle'0"996 N(CH,)3, which contained 3.91 mmoles of Si~Cle and 3"90 mmoles of N(CHs)s. No further an'fine could be removed at - 6 3 ° from this 1 : 1 complex, even by pumping for about 8 hr. The latter complex had no measurable dissociation pressure at --63 °, while measurements at --45 ° were rendered impossible by the disproportionation of disilicon hexa~10~W. C. SCnUMB, Inorganic Synthesis, Vol. 1, p. 42. J. Wiley, New York (1939). ~xl) G. URRy, T. WAR, K, K. E. MOOREand H. I. SCHLESINGER,J. Am. Chem. Soc. 76~ 5293 (1954).
Complexes of disilieon hexaehloride with trimethylamine x
419
chloride and the accompanying irreversible evolution of silicon tetrachloride which ensues at that temperature. Disproportionation of the 1 : 1 trimethylamine complex of disilicon hexachloride at - 4 5 °. The sample containing 3"91 mmoles of the Si=C16"N(CH3)8complex was allowed to stand for 26 hr at --45 °. After this period of time all the products volatile at --45 ° were removed and treated with anhydrous HC1. This treatment liberated 0.19 mmole (4.25 ml at S.T.P.) of silicon tetrachloride and indicated that 0.19 mmole of trimethylamine constituted the remainder of the volatile reaction products. Since the disproportionation proceeded very slowly under conditions where the volatile reaction products were allowed to accumulate, the reaction was subsequently carried out in a manner such that all the products volatile at --45 ° would be condensed in a trap maintained at -~196 °. The cumulative amounts of silicon tetrachloride and trimethylamine, recovered at various times over a period of 75 hr, are presented in Table 1. The apparent rate, with respect to mmole of SiCI4 evolved, is plotted in Fig. 1.
RESIDUE: SiClz.36.0.21 N(CH3)3\ 3.00 Q tl.J
0> 2.36 1,1.1 2.00
RESIDUE'. SIC12.5.t 0.28 N(CH3)3
.m
o~
./
0
E =E
I.O0
/ 0
/
o/
I
I
20
I
I
~
30 40 0 TIME IN HOURS
I
60
I
70
80
FIG. 1.--Rate of evolution of SiCI4 from Si=C18'N(CH~)s. The data indicate that after approximately 2'4 mmoles of silicon tetrachloride had been evolved the rate changed from a zero order essentially to first order. When the composition of the residue became [SilC12.8e'0'21 N(CHa)a]m, the disproportionation at - 4 5 ° became so slow as to be unobservable. Warming this white solid to room temperature resuited in a further evolution of silicon tetrachloride and trimethylamine. Under conditions where volatile products were immediately removed, the cumulative amounts shown in Table 2 were recovered at several times during a period of 204 hr. The table includes amounts recovered at room temperature after 0.52 mmole of SiCI~ and 0.33 mmole of N(CH3)3 had been collected in 10 hr at 0 °. The disproportionation at room temperature ceased after the amounts of SiCI, and N(CHs)8 evolved indicated that the yellow solid remaining in the reaction vessel had the composition, [Si1Cll.78" 0.1 N(CH3)3]~. The latter yellow solid was stable indefinitely at room temperature in vacuo, and underwent no change when heated to 100 ° in vacuo.
A. KACZMARCZYKand G. URRY
420
TABLE 1 Total hr. at --45 °
0
6
9
12
18
27
33.6
45.6
54.6
63.6
75.0
Mmoles of SiCl4.
0.19
0-62
0.86
1.08
1.46
2.07
2.46
2.75
2.92
3.00
3.05
Mmoles of N(CHa)s.
0.19
0"65
0.89
1.11
1.49
2.11
2.50
2.65
2.81
2.86
2.89
Apparent average molecular weightof the yellow subchloride. A sample of the yellow subchloride, weighing 0.4286 g, was dissolved in 45.01 mmoles of methylene chloride. The resulting solution exhibited a vapour-tension of 360.65 mm at 20.7 ° compared with the 364.15 mm vapour-tension of the pure solvent at that same temperature. Assuming ideal behaviour, a depression of 3.50 mm would be produced by 0"437 mmole of solute in 45.01 mmoles of solvent. Thus the apparent average molecular weight is 982 for the yellow solid. TABLE 2 Total time (hr)
48
96
144
204
Mmoles of SiCI,
0.37
0.55
0.65
0.72
Mmoles of N(CHa)s
0.10
0.19
0.31
0.35
When the solvent was removed from the solution, just described, it was noted that a crystalline solid precipitated as the solution became more concentrated. Furthermore, when all of the solvent had been removed, there remained a yellow solid identical in appearance to the original yellow subchloride. Heating of this residue at 70 ° resulted in the sublimation of a volatile crystalline solid, which appeared to be Si5Cl12. Thus, the yellow subehloride [SiClv¢0"IN(CHs)8]® is either a mixture of Si+Clxt and some polysilane more complex than Si~oCl~8,in which case it is difficult to understand why the Si~Clxt did not sublime when the material was heated to 100 ° before dissolution in methylene chloride, or it is possibly Six0ClaeN(CHs)8, which in methylene chloride disproportionates into SisClx~ and a polysilane more complex than Si~0Clxs. Since the evidence does not indicate homogeneity of [SiClbs'O'IN(CHs)s]=, the molecular weight obtained in this fashion is useful only as an average.
Press Ltd. Printed in Northern
Ireland
COMPLEXES OF DISILICON HEXACHLORIDE TRI M E T H Y L A M I N E ~a,
WITH
A . KACZMARCZYK and G. URRY* Richard Benbridge Wetherill Laboratory, Purdue University, Lafayette, Indiana
(Received 17 June 1963) Abstract--Disilicon bexachloride forms a well defined complex with trimethylamine, Si,CI6'2N(CHs)8, stable below--45 °. For the reaction Si~CI¢2N(CHs)3 = Si~CI6'N(CH3)s + N(CHa)3a A H o f a p p r o x i mately 10 kcal per mole can be obtained. The complex Si2C1B'N(CHs)s is stable below --45 ° and exhibits no dissociation pressure below that temperature. At --45 ° Si~C16"N(CHa)s disproportionates slowly, obeying zero order kinetics, to silicon tetrachloride, trimethylamine, and [SiCl2.a6.0 21N
(CHs)s],~ where m is probably 6. When [SiCI2.38.0"21N(CHa)a],~is warmed to room temperature, the disproportionation continues until the residual composition is [SiCla.78'0'l N(CHs)3]~. This material appears to be a mixture. As a result of the described disproportionations, no AH could be obtained for the reaction: Si2CI~'N(CHa)s = Si2Cle + N(CHs)v MOST o f the tetravalent halogenated silanes are known to form complexes with tertiary amines. Several workers have investigated these complexes.tLs, 4) BURG has determined the dissociation pressures and heats of dissociation for SiC14.N(CHa) a and SiH2CI2.N(CHa)a .tS) Presumably these complexes involve the formation o f a coordinate b o n d utilizing the " n o n b o n d i n g " electron pair on the nitrogen and the vacant 3d orbitals o f the silicon. Apparently the penta co-ordinated 1 : 1 complex is stable at lower temperatures, but the 1 : 2 complex which would require the silicon to be in a hexa-co-ordinated state has no unique stability, although BURG does report unstable solids, varying in composition from SiCI4.2N(CHs) a to SiC14.1.5N(CHs)3, which he attributes to solid solution behaviour o f SiC14.N(CHa) a and N(CHs) a. We u n d e r t o o k the examination o f the disilicon hexachloride-trimethylamine system i n order to determine whether both silicon atoms would participate in the formation o f a complex with the trimethylamine. RESULTS
AND
DISCUSSION
I f a solution o f disilicon hexachloride in trimethylamine, prepared at --79 ° and maintained at that temperature with vigorous stirring for 24 hr, is held at -- 79 ° and equilibrated with a trap maintained at --95 °, the solvent trimethylamine distills into the --95 ° trap, leaving a solid o f the composition, Si~CIs'2N(CHs)a, in the --79 ° vessel. This solid exhibits a dissociation pressure of 0.5 m m at --79 °. If trimethylamine is removed from the 1 : 2 complex, this equilibrium dissociation pressure can be observed as constant until the composition of the complex becomes Si2CI6"N(CHs):~, * To whom inquiries concerning this publication should be addressed. t~ Recent Developments in the Chemistry of the Perchloropolysilanes-II. For a discussion of the results described in this paper in relation to the results reported in other papers of this series see paper I, G. URRV,J. Inorg. Nucl. Chem. 26, 409 (1964). ~2~W. R. TROST, Canad. J. Chem. 29, 877-84; Nature. Lend. 169, 289 (1952). ~s~j. E. FERGUSSON,D. K. GRANT, R. H. HICKFORDand C. J. WILKINS,J. Chem. Soc., 99, (1959). t4, U. WANNAGATand F. V/ELaERt3,Z. Anop;g. Chem. 291,310-13 (1952). 15~A. B. BURG.J. Amer. Chem. Soc. 76, 2674-5 (1954). 415
416
A. KACZMA~CZYKand G. U~Y.
which exhibits no dissociation pressure at any temperature below --45 °. Thus, the observed dissociation pressure of 0.5 mm at --79 ° is a true equilibrium pressure for the reversible equilibrium: Si~CI6"2N(CHa)a ~ Si2Cle'N(CHa)3 + N(CHa)a
(1)
and there appears to be no evidence for solid solution behaviour of the different amine complexes. On the basis of two dissociation pressures, an approximate AH for Equation (1) can be calculated as 10 kcals per mole. This compares with the 11.9 kcal per mole for the dissociation of SiC14"N(CHa)a, reported by BURG.tS) The dissociation of the 1 : 1 complex could not be studied, since, as previously mentioned, there is no observable dissociation pressure below --45 ° and at this temperature a slow disproportionation occurs. This disproportionation occurs in two distinct stages. During the first stage equimolar amounts of silicon tetraehloride and trimethylamine are evolved and the kinetics with respect to silicon tetrachloride formation are zero order. The second stage ensues when the residual solid has attained the average composition, [SiC12.57.0.28N(CHa)3]~. From this point the reaction proceeds at a slower rate. During the second stage approximately first order kinetics are observed and the ratio of silicon tetrachloride to amine evolved is greater than one. When the solid residue has achieved the composition, [SiCla.aa'0"21N (CHa)3],~, the evolution of silicon tetrachloride and trimethylamine ceases entirely. The evolution of silicon tetrachloride and trimethylamine was observed at a low (10 --a mm) constant pressure. If [SiC12.ae'0"21N(CHa)a],~ is warmed from --45 ° to room temperature silicon tetrachloride and amine again are evolved at an even slower rate than the second stage of the --45 ° disproportionation. This reaction ceases when the solid has reached the composition, [SiCll.~s'0" 1N(CHa)a]~. The amounts of silicon tetrachloride and trimethylamine evolved during the course of disproportionation of SizCIe'N(CH3)3 at --45 ° are presented in Table 1. Similar data for the room temperature disproportionation of [SiClz.30"0"21N(CHs)8],n are presented in Table 2. The following equations are suggested as a possible explanation of the course of this disproportionation and are consistent with the observations. Si~Cle'N(CH3)8 ~-Si2C16 + N(CHa)3
(2)
Si2C10"N(CH3)3 + Si~C16---~SisCIs'N(CH3)a + SiCla
(3)
SiaCIs.N(CHs)8 + Si2CIe -~ Si4C11o.N(CH3)3 + SiCI4
(4)
Si4Cll0.N(CHs) 3 + Si~C1e ~ SisCI12.N(CH3)3.SiC14
(5)
N(CHa)3 + SisCI=N(CH3)s'SiC14 + Si~C10~ SieClx4"2N(CHs)3"SiCI~ + SIC14 (6) SisCI~.N(CHs) 3 + Si2CI6.N(CH3) 3 --~ SieCI~4"2N(CHz)s'SiCla
(7)
Si~CI~'2N(CH~)z'SiCI~ ~ Si~Clla.2N(CHa)a + SiCI~
(8)
Si~Cll~.2N(CHs)z ~ SioClx~'N(CHz) ~ + N(CHa) z
(9)
Complexes of disilicon hexachloride with trimethylamine
417
Equation (2) is presented as the rate determining equilibrium since a small excess o f trimethylamine pressure was sufficient to inhibit the disproportionation at --45 °. I f the uncomplexed disilicon hexachloride is assumed to be used up rapidly in reactions (3), (4) and (5), the evolution o f equimolar a m o u n t s o f silicon tetrachloride and trimethylamine can be accounted for. The change in rate observed when the residual composition is [SilC12.5~.0.28N(CHa)a],, taken in conjunction with the fact that during the second stage o f the reaction non-equivalent amounts o f silicon tetrachloride and trimethylamine are evolved, leads us to postulate a reaction represented by either Equation (6) or (7). The first order kinetics observed after this change in rate, can be explained by assuming stepwise dissociations as represented by Equations (8) and (9). In this case, the rate o f silicon tetrachloride evolution could be different from the rate o f evolution o f trimethylamine, as observed, if the various complexes formed a solid solution, with the rate o f evolution o f either silicon tetrachloride or trimethylamine governed by the activities o f the various complex species undergoing dissociation. The zeroorder kinetics o f the first stage is to be expected, since all o f the postulated and k n o w n reactants would be solids at --45 ° . The r o o m temperature disproportionation is o f interest because it results in a material that could be related to a c o m p o u n d reported by SCHWARZ, (11) Sil0Cl~s. The composition o f the end p r o d u c t of the r o o m temperature disproportionation is [SiCI1.7s'0"IN(CHz)zL. A determination o f the average molecular weight in methylene chloride indicated a value o f 10 for x. The material is not homogeneous, however, and Si5Cl12 has been identified as one c o m p o n e n t o f this mixture. It is clear, therefore, that the composition o f the residue must possess a lower chlorine content than Si~0Clls. This possibility thus is ruled out by the k n o w n facts. It is possible that the (SiC12)~ reported by WILKINS(7) is a similar mixture. APPARATUS AND EXPERIMENTAL METHODS Unless otherwise stated, all manipulations necessary in the present work were carried out in the absence of air and moisture. The techniques used were generally similar to those described in the published literatureJS, g) Analytical methods
All compounds with appreciable vapour-tensions at room temperature were measured as gases in calibrated volumes in the vacuum apparatus. Disilicon hexachloride was transferred in vacuo and weighed. Mixtures of silicon tetrachloride and trimethylamine, which proved impossible to separate using standard fractional condensation techniques, were treated as follows: The total volume of the mixture was determined with all material in the vapour phase. To this mixture was added an equal volume of anhydrous hydrogen chloride. The mixture, thus obtained, was allowed to stand for 12 hr at room temperature, during which time all of the trimethylamine present had precipitated as trimethylamine hydrochloride. There remained in the vapour phase, at this point, a mixture of silicon tetrachloride and anhydrous hydrogen chloride. This mixture was separated into its constituents by repeated distillation through a trap maintained at - 112o until the fraction retained at that temperature was pure silicon tetrachloride. The amount of silicon tetrachloride was determined by measurement of its total volume in the vapour phase. The amount of trimethylamine was determined both by difference and by a standard chloride determination, carried out on the trimethylamine hydrochloride precipitated during the treatment of the initial mixture with anhydrous hydrogen chloride. ee) R. SCUWARZand A. KOSTER,Z. Anorg. Chem. 270, 2-15 (1952). c7) C. J. WILKINS,J. Chem. Soc. 3409-12 (1953). (8) A. STOCK,Hydrides of Boron and Silicon, Cayuga Press, Ithaca, New York (1933). (9) R. T. SANDERSON,Vacuum Manipulation of Volatile Compounds, Wiley, New York (1949).
418
A. KACZMARCZYKand G. Urtrty
Reagents Disilicon hexachloride. Disilicon hexachloride used in the present work, was required to have a high degree of purity. Since the product of the standard preparation c1°~ is usually contaminated by small amounts of both hexachiorosiloxane and titanium tetrachloride, we devised a mercury arc reduction of silicon tetrachioride which although an inefficient method, produces disilicon hexachloride of exceptional purity. In this preparation, silicon tetrachloride vapour, at a pressure of approximately 4-5 mm, is passed through a mercury arc similar to that already described in the literatt~re, m~ The product disilicon hexachloride was separated from unchanged silicon tetrachioride by repeated distillations through a trap maintained at --23*. Anhydrous trimethylamine. This compound was obtained from the Matheson Company, Joliet, Illinois, was purified by fractional condensation in traps maintained at --126 ° and --196 °. The fraction retained at --126 ° exhibited a 0 ° vapour tension of 686 mm, in good agreement with the literature value of 683 mm.
Procedure Trimethylamine complexes ofdisilicon hexachloride. A sample of disilicon hexachloride, weighing 1.0562 g (3.91 mmoles), was distilled into a stirred vessel and maintained at a temperature of --79 °. Into an adjoining U-tube 20.33 mmoles (455 ml at S.T.P.) of trirnethylamine were condensed. The amine then was allowed to warm to room temperature and distill over into the stirred vessel containing disilicon hexachloride at --79*. The mixture was stirred for 24 lax and the excess amine then was removed, against a pressure of 1.6 mm, by distillation into a trap maintained at --95*. After several hours, 12.34 mmoles of trimethylamine had condensed in the trap maintained at --95 °, and no more amine could be removed, even when the distillation was continued for 12 hr. A 5 rain distillation of the remaining amine vapour, against a 0.3 mm pressure into a trap maintained at -- 112 °, collected 0.19 mmole more of trimethylamine. All of the amine distilled away was returned into the stirred vessel, still maintained at --79 °, and the entire procedure repeated. After several repetitions, with variations in the times of distillation, it was found that, on the average, a total of 12.53 mmoles of trimethylamine readily could be removed from the mixture maintained at --79* (vapour-tension of pure trimethylamine at --79 ° is 6 mm). This indicates the formation of a 2 : I complex containing 7.80 and 3"91 mmoles of N(CHs)8 and SisCl6, respectively. Upon standing at --79 °, this complex exhibits a reversible dissociation pressure of 0.5 4- 0'1 mm. The reversibility of this dissociation pressure was demonstrated by removal of trimethylamine vapour until less than the stoichiometric amount of amine was present. The pressure above this mixture was still observed to be 0.5 mm at - 7 9 °. The dissociation pressures of several compositions below the stoichiometric composition were noted to be identical. By using the same procedure the dissociation pressure at --63 ° was found to be 3'5 4- 0.1 mm. The heat of dissociation for the reversible reaction: Si2CI¢2N(CHs)3 ~ Si2Cle'N(CHs)3 + N(CH3)8 was calculated from the equation: 2270 log K . = l l . 0 2 4 - - - - - ~ , and found to be approximately 10 kcal per mole. This can be compared with the 11.9 kcal per mole for the dissociation of SiCI~-N(CHs)s. t6~ The equation is, of necessity, approximate, since only two dissociation pressures were available for its derivation. Prolonged distillation, at a pressure of 10-s mm and a temperature of --63% reduced the 2 : 1 complex to the composition, SisCle'0"996 N(CH,)3, which contained 3.91 mmoles of Si~Cle and 3"90 mmoles of N(CHs)s. No further an'fine could be removed at - 6 3 ° from this 1 : 1 complex, even by pumping for about 8 hr. The latter complex had no measurable dissociation pressure at --63 °, while measurements at --45 ° were rendered impossible by the disproportionation of disilicon hexa~10~W. C. SCnUMB, Inorganic Synthesis, Vol. 1, p. 42. J. Wiley, New York (1939). ~xl) G. URRy, T. WAR, K, K. E. MOOREand H. I. SCHLESINGER,J. Am. Chem. Soc. 76~ 5293 (1954).
Complexes of disilieon hexaehloride with trimethylamine x
419
chloride and the accompanying irreversible evolution of silicon tetrachloride which ensues at that temperature. Disproportionation of the 1 : 1 trimethylamine complex of disilicon hexachloride at - 4 5 °. The sample containing 3"91 mmoles of the Si=C16"N(CH3)8complex was allowed to stand for 26 hr at --45 °. After this period of time all the products volatile at --45 ° were removed and treated with anhydrous HC1. This treatment liberated 0.19 mmole (4.25 ml at S.T.P.) of silicon tetrachloride and indicated that 0.19 mmole of trimethylamine constituted the remainder of the volatile reaction products. Since the disproportionation proceeded very slowly under conditions where the volatile reaction products were allowed to accumulate, the reaction was subsequently carried out in a manner such that all the products volatile at --45 ° would be condensed in a trap maintained at -~196 °. The cumulative amounts of silicon tetrachloride and trimethylamine, recovered at various times over a period of 75 hr, are presented in Table 1. The apparent rate, with respect to mmole of SiCI4 evolved, is plotted in Fig. 1.
RESIDUE: SiClz.36.0.21 N(CH3)3\ 3.00 Q tl.J
0> 2.36 1,1.1 2.00
RESIDUE'. SIC12.5.t 0.28 N(CH3)3
.m
o~
./
0
E =E
I.O0
/ 0
/
o/
I
I
20
I
I
~
30 40 0 TIME IN HOURS
I
60
I
70
80
FIG. 1.--Rate of evolution of SiCI4 from Si=C18'N(CH~)s. The data indicate that after approximately 2'4 mmoles of silicon tetrachloride had been evolved the rate changed from a zero order essentially to first order. When the composition of the residue became [SilC12.8e'0'21 N(CHa)a]m, the disproportionation at - 4 5 ° became so slow as to be unobservable. Warming this white solid to room temperature resuited in a further evolution of silicon tetrachloride and trimethylamine. Under conditions where volatile products were immediately removed, the cumulative amounts shown in Table 2 were recovered at several times during a period of 204 hr. The table includes amounts recovered at room temperature after 0.52 mmole of SiCI~ and 0.33 mmole of N(CH3)3 had been collected in 10 hr at 0 °. The disproportionation at room temperature ceased after the amounts of SiCI, and N(CHs)8 evolved indicated that the yellow solid remaining in the reaction vessel had the composition, [Si1Cll.78" 0.1 N(CH3)3]~. The latter yellow solid was stable indefinitely at room temperature in vacuo, and underwent no change when heated to 100 ° in vacuo.
A. KACZMARCZYKand G. URRY
420
TABLE 1 Total hr. at --45 °
0
6
9
12
18
27
33.6
45.6
54.6
63.6
75.0
Mmoles of SiCl4.
0.19
0-62
0.86
1.08
1.46
2.07
2.46
2.75
2.92
3.00
3.05
Mmoles of N(CHa)s.
0.19
0"65
0.89
1.11
1.49
2.11
2.50
2.65
2.81
2.86
2.89
Apparent average molecular weightof the yellow subchloride. A sample of the yellow subchloride, weighing 0.4286 g, was dissolved in 45.01 mmoles of methylene chloride. The resulting solution exhibited a vapour-tension of 360.65 mm at 20.7 ° compared with the 364.15 mm vapour-tension of the pure solvent at that same temperature. Assuming ideal behaviour, a depression of 3.50 mm would be produced by 0"437 mmole of solute in 45.01 mmoles of solvent. Thus the apparent average molecular weight is 982 for the yellow solid. TABLE 2 Total time (hr)
48
96
144
204
Mmoles of SiCI,
0.37
0.55
0.65
0.72
Mmoles of N(CHa)s
0.10
0.19
0.31
0.35
When the solvent was removed from the solution, just described, it was noted that a crystalline solid precipitated as the solution became more concentrated. Furthermore, when all of the solvent had been removed, there remained a yellow solid identical in appearance to the original yellow subchloride. Heating of this residue at 70 ° resulted in the sublimation of a volatile crystalline solid, which appeared to be Si5Cl12. Thus, the yellow subehloride [SiClv¢0"IN(CHs)8]® is either a mixture of Si+Clxt and some polysilane more complex than Si~oCl~8,in which case it is difficult to understand why the Si~Clxt did not sublime when the material was heated to 100 ° before dissolution in methylene chloride, or it is possibly Six0ClaeN(CHs)8, which in methylene chloride disproportionates into SisClx~ and a polysilane more complex than Si~0Clxs. Since the evidence does not indicate homogeneity of [SiClbs'O'IN(CHs)s]=, the molecular weight obtained in this fashion is useful only as an average.