Determination of the gas-phase molecular structure of tris(trimethylsilyl)(trichlorosilyl) methane by electron diffraction

Journal of Molecular Structure, 196 (1989) 21-29 Elsevier Science Publishers B.V., Amsterdam - Printed

21 in The Netherlands

DETERMINATION OF THE GAS-PHASE MOLECULAR STRUCTURE OF TRIS(TRIMETHYLSILYL)(TRICHLOROSILYL) METHANE BY ELECTRON DIFFRACTION

DAVID G. ANDERSON,

DAVID W.H. RANKIN

and HEATHER

E. ROBERTSON

Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ (Gt. Britain) ALAN H. COWLEY and MAREK

PAKULSKI

Department of Chemistry, University of Texas, Austin, TX 78712 (U.S.A.) (Received 2 August 1988)

ABSTRACT The molecular structure of tris (trimethylsilyl ) (trichlorosilyl ) methane in the gas phase has been determined by electron diffraction. The mean inner Si-C distance (r,) is 190.9(&3) pm, with the difference between Me,Si-C and Cl,Si-C distances fixed at 2.3 pm. The outer Si-CH, bonds are 187.8(6) pm long, and r(Si-Cl) is 203.3 (6) pm. The SiCSi angles are close to tetrahedral, being 108.1(6) ’ between the Me,Si groups and 110.9 (6) ’ between Me,Si and Cl,!% groups. Effects of steric crowding are seen mainly in the angles at the silicon atoms, with C (Me) SIC (Me) 107.0 (11) ’ and ClSiC 114.6 (11) O. All the Six, groups are twisted by about 23” from the fully eclipsed configuration, in which the central C(SiX,), skeleton approximates to Td symmetry, reducing this to approximate T symmetry. This twisting minimises long-range interactions between the silyl groups.

INTRODUCTION

Since the preparation of (Me,Si)3CLi was first reported [ 11, a wide range of tris (trimethylsilyl) methyl (“trisyl”) compounds has been described. This bulky group is of particular importance because its presence is often associated with unusually low reactivity. This subject has been reviewed [ 21. Tris (trimethylsilyl) (trichlorosilyl)methane, the subject of the present study, is remarkable in that it does not undergo nucleophilic reactions with water, methanol, alcoholic silver nitrate, phenyl lithium or Grignard reagents [3]. This resistance of the Si-Cl bonds to attack is believed to be attributable to the protecting influence of the trimethylsilyl groups, which in such a sterically crowded molecule must hinder the approach of reagent molecules. We have therefore studied the structure of this molecule in the gas phase, and present our results here. To the best of our knowledge, the only previous structural

0022-2860/89/$03.50

0 1989 Elsevier Science Publishers

B.V.

22

study of a trisyl-substituted silicon compound (Me,Si),CSiMe,Ph [4].

is an X-ray

analysis of

EXPERIMENTAL

A sample of (MesSi),CSiC1, was prepared by the reaction of (Me,Si),CLi with SiCl, under reflux in tetrahydrofuran/diethyl ether, and purified as described earlier [ 31. Its purity was checked by NMR spectroscopy. Electron diffraction scattering intensities were recorded photographically on Kodak Electron image plates using the Edinburgh electron diffraction apparatus [ 51, with nozzle-to-plate distances of 96 and 256 mm and an accelerating voltage of ca. 44.5 kV. During the exposure periods the nozzle temperature was held at 295 K and the sample temperature was maintained at 455 K. Two plates obtained at each camera distance were used. The intensity data were converted to digital form for subsequent analysis using a computer-controlled Joyce-Loebl MDM6 microdensitometer at the S.E.R.C. Laboratory, Daresbury [ 61. The electron wavelengths were determined from the scattering patterns of gaseous benzene, recorded immediately after those of the sample. Data reduction [ 61 and subsequent least-squares refinement [ 71 were carried out using programs and scattering factors [ 81 described previously. The weighting points used to set up the off-diagonal weight matrices are given in Table 1, as are the s ranges and intervals, electron wavelengths, correlation parameters, scale factors and camera distances for each compound. STRUCTURAL ANALYSIS

Molecular model The SiCHB groups were assumed to have local CBvsymmetry, and were described by Si-C and C-H distances and the SiCH angle. The Si (CH, ) 3 groups were assumed to have local C, symmetry, with all three CH, units twisted by the same amount and in the s,ame direction from the fully staggered arrangement. The CSiC angle within the group completed the definition of the strucTABLE 1 Weighting functions, correlation parameters (CP) and scale factors Camera height (mm)

As

255.86 95.86

2 4

&in

SW1

20 80

40 100

(nm-‘)

SW2

%lax

CP

Scale factor

Electron wavelength (pm)

134 240

156 300

0.481 0.409

0.716( 15) 0.387 (24)

5.690 5.690

23

TABLE 2 Geometrical parameters” No.

Parameter

Value

Pl P2

r( Si-C ) outer r ( Si-C ) inner, mean Ar(SiMe:,-C) minus (Sic&-C) r(Si-Cl) r(C-H) L ClSiC SiCl, twist LC(Me)SiC(Me) L SiCH CH, twist SiMe, tilt LSi(Cl,)CSi(Me,,) SiMezltwist

187.8(6) 190.9 (8) 2.3 (fixed) 203.3(6) 114.5(12) 114.6(11) 23.5(26) 107.0(11)h 112.0(fixed) 0.0 (fixed) 3.0(fixed) 110.9(6)b 22.9(30)

P3 P4 PS PS P7 PS

P9 PI0 Pll P12 PIti

“Units: ra (pm), L B (degrees). bError may be underestimated; see text.

ture of this fragment. Similarly, the whole (Me,Si),C (“trisyl”) group was assumed to have C, symmetry. The three Me,Si components were allowed to twist away from the fully staggered arrangement and to tilt their local C3 axes away from each other so that they no longer coincided with the central Si-C bonds. The angle between Cl,Si-C and Me,Si-C bonds was used to define the cone angle of the trisyl groups. Finally, the CSiC& group was described by the lengths of the Si-C and SiCl bonds, the ClSiC angle, and the twist angle of the Sic& groups relative to the staggered position. Because the Me,Si-C and Cl.$i-C distances were very similar to one another, and consequently likely to be strongly correlated, the refinable parameters were chosen to be the weighted mean and the difference (see Table 2 ) . Refinement of structure The radial distribution curve for (Me,Si),CSiCl, (Fig. 1) shows only two clearly resolved peaks in the region corresponding to bonded interatomic distances, and two more in the region associated with atom pairs separated by two bonds. As there are five different bond lengths and ten contributions to the two-bond region, it is inevitable that there are some major correlations between structural parameters (Table 3 ). Nevertheless, the region of the radial distribution curve above 350 pm contains a wealth of information about long range distances, and the structural analysis is not so difficult as might at first be supposed. Of the thirteen geometrical parameters needed to describe the structure, listed in Table 2, nine could be refined without difficulty. Two of the

24

Fig. 1. Observed and final weighted difference radial distribution curves, P(r)/r, for (Me,Si),CSiCl,. Before Fourier inversion the data were multiplied by s*exp( -0.00002s*)/ (.G,-fsi)(.G-fc). TABLE 3 Least squares correlation

matrix X 100

Pl

P4

PS

PS

PI2

-70

-53

68

-65 -61

-51 -69

PI3

43

U20

-60 -60

-95 -67

63 60

P2 PS P7 PB 47

“Only elements with absolute values > 50% are included.

others related to the positions of the hydrogen atoms. When the CH, twist parameter was freed to refine it remained close to O”, but with an esd of ca. 20’. This twist angle was then fixed at zero, and the SiCH angle was also fixed, at 112’. The remaining two parameters were the tilt of the Me,Si groups and the difference between the Me,Si-C and Cl,Si-C bondlengths, and these needed to be investigated carefully, When the tilt angle was allowed to refine, it tended to go to a value (ca. 7” ) which seemed to be unreasonably large, and it was strongly correlated with other parameters, notably the angles SiCSi and CSiC. It was therefore fixed at 3”, which is a typical value found in this sort of situation. Fixing this parameter means that quoted errors for other parameters, particularly the angles SiCSi and CSiC, are likely to be underestimated. Although there can be little doubt that the mean length of the inner Si-C bonds is substantially greater than the Si-C(methy1) distance, it was unfortunately impossible to derive information about the difference between the two types of inner Si-C distance. If the difference parameter was allowed to refine, its estimated standard deviation became very large. When a series of refine-

25 TABLE 4 Interaction distances Distance, r, (pm) r1 r2 r3 r4 r5 r6 r7

r8 r9

r10 rll

r12 r13 r14 r15

rl6 r17 r18 r19

rzo rzl rz2 rz3 rz4 r25

r26 rz7

rz8 rz9 r90 r3] rc12 r33 rR4 r:35

Si(6)-C(7) Si(G)-C(1) Si(B)-C(I) Si-Cl C-H C(7)***C(ll) C(l).*.C(7) C(l)**.C(ll) C(l)***C(15) Si(2)***Si(6) Si(6)a**Si(19) C1(3)***C(l) C1(3)***C1(4) Si(G).**H Si(6)***C1(4) Si(6)***C1(5) Si(6).**Ci(3) Si(B)**.C(ll) Si(2)*.*C(15) Si(2).**C(7) Si(6)**.C(28) Si(6)**.C(37) Si(6)..*C(ZO) Si(6).**C(24) Si(6)*.*C(33) Si(6).**C(41) C1(3)***C(7) C1(3)**.C(24) C1(3)...C(28) C1(3).**C(37) C1(3)*..C(ll) C1(3)***C(15) C1(3)***C(20) C1(3)**.C(33) C1(3)***C(41)

187.8(6) 191.4(8) 189.1(8) 203.3 (6) 114.5( 12) 301.8( 16) 309.0(11) 318.5( 10) 314.8(11) 313.5(11) 309.7(19) 330.4(18) 320.1(22) 253.9(10) 360.8( 18) 419.3 (38) 493.9(12) 354.7(23) 405.0(21) 472.6(10) 473.0(15) 477.8(11) 331.0(25) 403.9(23) 389.2(29) 341.5(23) 638.9( 14) 358.3 (40) 371.1(32) 378.1(53) 557.7(26) 557.0(33) 547.9( 16) 553.0(46) 550.9 (26)

Amplitude of vibration, u (pm)

4.7(12) 7.7(fixed)

lO.Q(ll)

1

11.4(22) 17.6(36) 20.0 (fixed) 6.9(19) 12.O(fixed)

>

11.7(27)

4

>

12.0(fixed)

>

20.0 (fixed)

{

< ,

21.2(17)

J

ments was performed with it fixed at different values, the R factor barely changed at all, and actually reached a minimum when the Me,Si-C distance was slightly less than that for Cl,Si-C. The difference was finally set to be equal to that for the Si-C bonds in SiMe, [ 91 and Sic&Me [lo].The problem with this parameter is that it is essentially undetermined by the data, rather than too strongly correlated with other parameters, and consequently fixing it

26

Cl(3)

-C(lS)

\

Si (32) -C(33)

/'

C(37)

Fig. 2. Atom numbering scheme.

0

I

120

AdP-x/L,, V VW

tP-+---‘v

Fig. 3. Observed and final weighted difference molecular scattering (Me,Si),CSiC&, recorded at camera distances of (a) 96 and (b) 256 mm.

intensity

curves for

has little effect on the standard deviations for other parameters. The errors quoted in the tables of parameters and interatomic distance (Tables 2 and 4 respectively) are estimated standard deviations obtained in the least squares refinements, and take account of possible systematic errors in camera distances, electron wavelengths, etc. Amplitudes of vibration, of which some were refined separately or in groups, and others were fixed at reasonable values, are also listed in Table 4. The atom numbering scheme, which is needed to interpret this table, is shown in Fig. 2. Figure 3 shows the molecular scattering intensity data, and Fig. 4 is a perspective view of the molecule.

27

Fig. 4. Perspective view of a molecule of (Me,Si),CSiCI,. DISCUSSION

Tris (trimethylsilyl) (trichlorosilyl)methane is a sterically crowded molecule showing an unusually small susceptibility towards nucleophilic attack at silicon. It is therefore important to see how the crowding leads to distortions of geometrical parameters away from values normally found, and to see whether there is any direct structural explanation for the chemical behaviour of the compound. The bonds to the central carbon atom are on average some 3 or 4 pm longer than in uncrowded situations. There is a very small distortion at the carbon atom, with the angles between the C-SiMe, bonds 108.1(6) ’ and those between C-SiMe, and C-SiCl, 110.9 (6) ‘. The effect of size of the SiMe, groups on the SiC13 group is also shown in the angle ClSiC, which is 114.6( 11) ‘, corresponding to 103.9” for the angle ClSiCl. This closing of the SiCl, “umbrella” from ClSiCl angles of 109.4” in Sic&H and 108.6” in Sic&Me [lo] provides an explanation for the low susceptibility of the molecule to undergo nucleophilic substitution. If the SiC13 group is forced by neighbouring atoms to compress in this way, then formation of a transition state with five-coordinate silicon is clearly not going to be favoured. Moreover, the Si-Cl bonds, at 203.3(6) pm, are only a little longer than those in SIC&H (202.0 pm) and SiCl,Me (202.6 pm), so a facile dissociative mechanism for nucleophilic substitution is also ruled out. The rather narrow CSiC angle (107.0 (11) O) within the SiMe, groups, the presumed tilt of these groups, and their twisting to minimise interactions be-

28 TABLE 5 Average bond lengths (pm) and angles (degrees) for some trisyl compounds Compound

Si-C (inner)

Si-C (outer)

SiCSi

C (central)SiC (Me)

Ref.

(Me3SiMYH, (Me&)& (Me,Si),CSiMeaPh (Me,Si),CSiCl,

194.1(5) 193.1(3) 191.6(7) 191.4(8) 188.7(4) 188.8(6)

188.3(2) 189.6(2) 187.7(16) 187.8(6) 187.1(3) 187.3 (2)

106.9(13) 109.5 109.5(9) 108.1(6) 112.6(2) 117.5(4)

114.3(4) 113.1(2) 113.4(10) 112.0(11) 113.0(11) 112.8(3)

11 12 4 -

[ (I%Si),Cl,Hg OhSiWH

13 14

tween the groups, are all common features of compounds containing the tris (trimethylsilyl) fragment. Some parameters describing the geometry of this group in a variety of trisyl compounds are listed in Table 5. At one extreme in this table is (Me,Si),CH, in which the steric strain is relieved mainly by widening of the SiCSi angles. At the other extreme these angles are actually less than the tetrahedral angle, and long inner Si-C bonds and wide C (central) Sic (Me ) angles are observed. The trichlorosilyl derivative, which is the subject of the present study, fits well into the middle of the series. ACKNOWLEDGEMENT

We thank the Science and Engineering Research Council for financial support and for the provision of microdensitometer facilities, and the U.S. National Science Foundation for financial support. We also thank the North Atlantic Treaty Organisation for a grant which made collaboration possible between the groups in Austin and Edinburgh.

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