BARTELL,CLIPPARD, AND BOATES
2436 Inorganic Chemistry, Vol. 9, No. 11, 1970 carbonyl has been the subject of electrophilic attack (either by SO2 itself or by the SOz-BF3adduct, which is stable below ambient temperature^^^). The product is probably best considered as an intermediate (2) in the
v--__-__-
=%
(OC),Fe
1
vy: (OC)3Fe-.
\O-BF3
,
yrJ
-BF,_
/
(OChFe-0
2
No: ..
3
(45) H. S. Booth and D. R. Martin, J . Amev. Chem. Sac., 64, 2198 (1942).
hypothetical Friedel-Crafts sulfination reaction 1 + 3. [It should be emphasized that complexes of type 3 have yet to be reported. However, a complex of type 2 containing the Lewis acid SbFj, rather than BFa, has been prepared by Kaesz and coworker^.^^] Acknowledgments.-We thank Professor H. D. Kaesz and Dr. D. A. T. Young for supplying samples of the material investigated. This work has been generously supported by the National Science Foundation (Grant No. GP-8077) and the Advanced Research Projects Agency (Contract No. SD-88). J. W. acknowledges, with gratitude, the receipt of a Graduate National Fellowship for 1967-1970 from Harvard University.
CONTRIBUTION FROM THE DEPARTMENT OF CHEMISTRY, UNIVERSITY O F MICHIGAN, ANS ARBOR, i x I C H I G A N 48104
An Electron Diffraction Study of the Molecular Structure of Tetrakis(trimethy1silyl)silanel BY L. S.BARTELL,*a F. B. CLIPPARD,
JR., AND
T. L. BOATES
Received December 24, 1969 Gas-phase Si[Si(CHa)a]r possesses S i S i and Si-C bond lengths of 2.361 =t0.003 and 1889 i 0.003 8, including estimated standard errors, respectively, and SiSi-C and Si-C-H angles of 110.9 i 0.6 and 109.3 zt 1.7". The (CH3)sSi groups appear t o undergo cooperative torsional displacements of about 14" from Td symmetry in order to relieve nonbonded interactions between methyl groups.
Introduction The first successful preparation of tetrakis(trimethy1silyl)silane, Si[Si(CH3)3]4,was reported in 1964 by Gilman and Smith.3 Quite stable and unreactive, the yellow solid melts a t 261-263". The proton nmr exhibits a sharp singlet a t r 9.79, indicating that all protons are equivalent. Tetrakis(trimethylsily1) silane presented a favorable opportunity to extend studies of the influence of nonbonded repulsions on molecular structure4 to an analog of tetrakis(t-buty1)methane. Experimental Section The electron diffraction apparatus employed has been described elsewhere.6 Except for the special heated nozzle assembly necessitated by the relatively nonvolatile sample, diffraction patterns were recorded and measured by the usual techniques.6 Leastsquares structure refinements upon the experimental leveled intensity were based on the elastic scattering factors of Cox and Banham' and the inelastic scattering factors of Tavard* and did (1) This work was supported by a grant from the National Science Foundation. (2) Author to whom correspondence should be addressed. (3) H. Gilman and C. L. Smith, J . Amev. Chem. Soc., 86, 1454 (1964). (4) E. J. Jacob, H. B. Thompson, and L, S. Bartell, J. Chem. Phys., 47, 3736 (1967); L. S. Bartell, J. Chem. Educ., 46, 754 (1968); L. S. Bartell and H. K. Higginbotham, Inovg. Chem., 4, 1346 (1965); L. S. Bartell, J . Chem. ~ h y s . sa, , 827 (1960). (5) L. S. Bartell, R.Kuchitsu, and R. J . DeNeui, ibid., 96, 1211 (1961). (6) R. A. Bonham and L. S. Bartell, ibid., 91, 702 (1959). (7) H. L. Cox, Jr., and R. A. Bonham, ibid., 47, 2599 (1967). (8) C. Tavard, D. Nicolas, and M. Rouault, J . Chim. Phys., 64, 540 (1967).
not differ significantly from procedures described elsewhere.9 Comparison between experimental and theoretical points was carried out utilizing a weighting function proportional to the scattering variable for the composite reduced molecular intensity curve, M ( s ) . Asymmetry constants a were estimated10 to be 2.5 b-l for C-H distances and 2.0 b-l for Si-Si and Si-C distances and were taken to be 1.0 k-l for all nonbonded distances. No correction was made for shrinkage effects."
Results Figure 1 shows the molecular intensity curve determined for Si[Si(CH3)a]4by blending together data from the individual camera distances. The index of resolution was 0.95 for each of the three experimental camera ranges. Fourier inversion of the molecular intensity produced the radial distribution function illustrated in Figure 2. The results of our structural refinement are summarized in Table I. Experimental data were analyzed by least-squares fittings of the molecular intensity, and a representation of the error matrix determined during the final runs is reproduced in Table 11. Listings of the experimental leveled intensity and the background used in data analysis a t regular intervals of the scattering variable are given in Table 111. Table (9) L. S. Bartell, D. A. Kohl, B. L. Carroll, and R . M. Gavin, Jr., J . Chem. Phys., 42, 3079 (1965). (10) D. R. Herschbach and V. W. Laurie, ibid., 35, 438 (1961). (11) Y.Morino, S. J. Cyvin, K . Kuchitsu, and T Iijima, ibid., 36, 1109 (1962).
Inorganic Chemistry, Vol. 9, No. 11,1970 2437
TETRAKIS (TRIMETHYLSILYL) SILANE
n
I
I
I
I
1
20
IO
0
I
I
30
I
---*
A.
”
-3
I
-
r,
1
40
S A
Figure 1.-Molecular
intensity curves determined for Si[% (CHs)al4.
I V compares parameters determined in this investigation with parameters for related molecules.
TABLE I PARAMETERS FOR ( CH3)&iba Parameter
Discussion The Si-Si and Si-C distances in Si[Si(CH3)3]4 are nearly the same as the corresponding distances in elemental silicon12 and in silicon carbide, l 3 respectively. The C-Si-C bond angle determined is slightly less (-1 .So) than the tetrahedral angle, in accordance with simple Gillespie-Nyholm considerations. Two somewhat approximate assessments, one experimental and one theoreticai, were made of the possible consequences of intermethyl repulsions in the molecule. I n the first, least-squares analyses of the ex-
yg,
A
I,,
A
Angle, deg
Si-Si 2.3614Z 0.003 0 , 0 6 5 3 ~ 0.003 Si-C 1 . 8 8 9 4 ~0.003 0 . 0 5 5 f 0 . 0 0 3 C-H 1 . 1 1 7 3 ~0.004 0.0713Z 0.004 L Si-Si-C 110.9 rt 0 . 6 L Si-C-H 109.3 3Z 1 . 7 (CH3)aSi torsionb 11 f 3 . 6 s i . . .sic 3.855 0 . 1 3 5 4 ~0.015 3.054 0 . 1 1 6 f 0.018 C . . . C( 1,3)c Uncertainties are estimated standard errors including the effects of known systematic errors. See L. S. Bartell in “Physical Methods in Chemistry,” A . Weissberger and B. W. Rossiter, Ed., 4th ed, Interscience, New York, N. Y . ,in press. * See text for meaning. Distance which is a dependent parameter.
TABLE I1
ERROR MATRIXFOR ( CHa)l&isa r(Si-Si)
r(Si-Si) 5.7 r(Si-C) r( C-H) L SiSi-C L Si-C-H Rotnb I( Si-Si) Z(Si-C) Z(C-H) Z(Si. .Si) Z(C. * .C)c
r(Si-C)
-0.5 3.1
r(C-H)
LSi-Si-C
LSi-C-H
-1.1 -1.1 4.7
-4.8 -2.2 1.8 6.2
-5.4 -4.6 -4.0 5.4 18.9
Rot$
-2.5 3.5 -4.1 3.3 -8.5 42.3
R
l(Si-Si)
0.8 -1.8 0.9 2.0 -2.9 -4.9 3.9
1(Si-C)
-1.0 -1.3 1.2 2.0 3.2 -4.7 1.6 2.5
1(C-H)
-1.3 -2.4 0.4 2.9 5.5 -6.6 2.2 2.2 6.1
$ ( S i . .,Si)
1(C. .C)c
R
3.3 -2.3 0.5 -3.7 7.1 -23.8 1.8 2.1 3.1 18.8
3.5 -1.9 1.9 4.7 -5.1 -14.0 3.0 1.9 2.7 9.3 20.0
-4.6 -8.7 4.4 10.0 19.2 -22.7 7.7 7.5 11.1 11.0 9.4 38.8
a Values are x 103. Based on 110 intensity values interpolated from 297 data points. Units for the distances and amplitudes are Pngstroms; those for the angles are radians; the index of resolution R is dimensionless. Matrix elements are given by u%j = sign[B,, -‘I { lBz,-llv’wv/(n - m ) ) ’ I 2 ,where the notation corresponds to that of 0. Bastiansen, L. Hedberg, and K. Hedberg, J . Chem. Phys., 27, 1311 (1957). Since the elements are based on a diagonal (nonoptimum) weight matrix, they do not represent bonafide standard errors. Rotation of the ( CH3)3Sigroups from tetrahedral symmetry. See text. I( C * . .C) for (CH&Si group.
perimental intensities were run a t various fixed torsional displacements from T, symmetry. These displacements moved all corresponding groups equally, preserving T symmetry. For sake of calculation, an(12) M .E.Straumanis and E. 2. Aka, J. A g g l . Phys., 23,330 (1952). (13) N. W. Thibault, Ameu. Mineral., a9, 249 (1944); L. S. Ramsdell, ibid., 29, 431 (1944); 30, 519 (1945).
gular displacements about the Si-C bonds were taken t o be equal to the displacements about the Si-Si bonds and were in a sense to augment the interhydrogen avoidance. This constraint for the Si-C rotations to follow the Si-Si rotations surely far too great a methyl - imposes twist, as we shall see, but has only a secondary influence on the diffraction analyses. The methyl twists in-
2438 Inorganic Chemistry, Vol. 9, No. 11, 1970
BARTELL,CLIPPARD,AND BOATES
TABLE111
TABLE IV STRUCTURAL PARAMETERS FOR SILICON DERIVATIVES
EXPERIMENTAL LEVELED INTENSITY AND BACKGROUND DAT.4 USED FOR Si[Si(CHa)a]4"
Distance, A, or angle, deg Si-Si Si-C C-H LC-Si-C LSi-C-H
Si[Si(CHa)alP
Sib
2.361 f 0.003
2.352
1.889AO.003 1.117 & 0.004 107.9 5 0 . 5 109.3 + 1 . 7
...
... .,. ., .
Sicc ,
Si(CHa)ld
...
..
1.885
.. , .. . ..
,
Si%Hae 2.32 & 0.03
1.888f0.02 1.10 zt 0.05 109 5 110 i 3
... ... I
.
.
...
This work. I, M. E. Straumanis and E. Z. Aka, J . Appl. Phys., 23, 330 (1952). N. W. Thibault, Amer. Mineral., 2 9 , 249, 327 (1944); L. S. Ramsdell, $ b i d , 29, 431 (1944); 30, 519 (1945). W. F. Sheehan, Jr., and 1'. Schomaker, J . Amer. Chem. Sac., 74, 3956 (1952). e L. 0. Brockway and J. Y. Beach, ibid.,60, 1836 (1938); G. W. Bethke and M. K. Wilson, J . Chem. Pltys., 26, 1107 (1957).
B(S) 4.354 4.478 4.S$ 4.725 4.653 4.974 5.399 5.223 5.347 5.471 2.595 -e119 5.843 5.961 6.091 5.215 6.339 6.463 6.587 5.711 6.135 6.959 1.082 1.205 7.330 7.454 7.578 7.701 1.825 7.9d9
8.371. 8.195 8.319 8.443 8.567 8.690 8.814 8.937 9.060 9,184 8.307 9.431 9.554 9.677
3.1835 3.8488 3,9285 3.P593 4.0465 4.0541 4.0222 3.9653 3.9264 3.9194 3,9250 3.9675
3.9980 4.0012 5.9565 3.8711 3.7351 3.6591 3.5693 3.4962 3.4519 3.4178 3.3949 3.3841 3.3892
II.fI4 II.C43 12.272 12.531 12.731 12.059 13.188 13.416 13.645 13.P13 14.10l 14.329
14.551 Id.781
15.012 15.239 15.465 15.693 15.920 15.147 15.313 16.600 16.F26 17.052 17.271
17.503 1?.72P 17.953
10.90P lI.031 11,154 I 1.271 11.100 11.523
11.645 I I. 7 5 9 11.591 12.013 12.136 12.259 12.381 12.534 12.626 12.748 12.871 12.993 13.115 13.238 13.360 13.482 13.604
13.725 13.848 13.970 14.092 14.214 14.336 14.458 14.580 14.701
14.623 14.945 15.066
3.4083 3.4583 3.5274 5.6067 3.6990 3.7841 3.8605 3.9198 3.9645 3.988? 4.0002
3.9986 3.9844 3.9567 3.9111 3.2463 3.8085 3.1423 3.6139 3.SIPP 3.577.4 3.5606 3.5734 3.6078 3.6604 3.7140 3.'674 3.8124 3.8493 3.8809 3.9083 3.9323 3.9488 3.9641 3.9194 3.9652
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4.2313 4.1015 4.1575
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3.1772
15.188 15.309 15.431 15.552 15.674 15.795 15.911 16.031 16.159
I6.2PO 15.101 16.522 15.645
15.764 15.884 17.C05 17.126 17.747 17.367 17.488 17.609 17.129
3.1198
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3.7816 3.7828 3.7840 3.7853 3 ,7361 3.7885 3.1308 3.7950 3.7996 3.8044 3.8094 3,8145 1.8194 3,8235 3,8210 3.8303 3.8334 3.8365 3.8394 3.8424 3.8455
17.970 15.090 IR.ZI! 18.331 18.451 18.571 18.601 18.811
18.931 19.S51 19.171 19.291 19.410 19.530 19.650 19.769
19.P89 20.C08 20.127 20.247 20.366
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3t.903 33,415 3C.678 30.?35 !I.047 31.751 31.467 31.576 3 I .F?5
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4.2147 4.2144
5.971
7.897
9.833
3.3165 3 . 54 0 59 50 3.524? 3.5138 3.4537 3.3745 3.3516 3.3844 3.4298 5.5173 3.5906 3.6415 d ,6532 3.6714 3.6912 3.6652 3.5985 3.7609
1.2116
A.40Q3 "4P17 i.494q .?.46!6 t.3688 4.?2?6 4.0537
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i.0740 4.1820 4.2730 4.3297 4.36P3 4.3547
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4.P4P3 4.2305 4.?328
4.2279 4.2242 4.2113 4.2189 L.1172 4.?162 4.2155 4.2151 4.2151 *,2153 4.2158 4.2163 4.2169
21.072
22.194 Qa.415 7?.635 99.456 23.C76 23.296 23.516 33 .i,!S 23.P55 ?6.174
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r.2111 4.21 70
24.830 25.041 25.265 25.484 25.101 25.918 25.135 25.351 26.563
4.2159
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4,2168 4,2167 4.2165 4.2154 4.2162 4.2160 4.2157 4.2153 4.215c
27.C05 $7.715
