The molecular structure of gaseous octamethyltrisilane, (CH3)3SiSi(CH3)2Si(CH3)3, AS determined by electron diffraction

Journal ofMolecular Elsevier

Science

Structure, 112 (1984)

Publishers

B.V.,

Amsterdam

239-245 -

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THE MOLECULAR STRUCTURE OF GASEOUS OCTAMETHYLTRISILANE, (CH3)$Si-Si(CH3)2-Si(CH3)s, DETER.MINED BY ELECTRON DIFFRACTION

ARNE

AS

ALMENNINGEN

University of Oslo, Department TORGNY

of Chemistry, Blindern, Oslo 3 (Norway}

FJBLDBERG

University of Trondheim,~College Dragvoll (Norway) EDWIN

of Arts and Science, Department

of Chemistry, N-705Fi

HIZNGGE

Institut fiir Anorganische Chemie der Technischen Universittit in Graz, A-8010 (Au&k) (Received

Graz

13 June 1983)

ABSTRACT The molecular structure of gaseous octamethyltriiiiane, Si,Me,, has been determined by eIectron diffraction at a nozzle temperature of 339 K. The diffraction data are consistent with a model with only minor distortions from C,,. symmetry, with approximately staggered SiMe, groups relative to the central SiMe, group when viewed along the Si-Si bonds, and with one C-Si bond (SiMe,) nearly antf to the remote Si-Si bond. Principal bond lengths (r-a) and valence angles are: Si-Si 2.325(12) X, C-Si (average) l-887(3) -9, C-II l.lOO(5) A; SiSiSi 118.0(2.5)O, SiSiC (SiMe,) 108.7(0.9)O, CSiC (Sihle:) 109.1(1.5)” and SiCH 111.4(0.8)‘. The parenthesized values are one standard deviation, corrected for experimental errors. INTRODUCXTON

When bonded to a carbon atom, the large and bulky trimethylsilyl (SiMe,) groups and also their carbon counterparts (t-butyl groups) will in general give rise to considerable steric strain, resulting in bond lengths, valence angles and torsional angles often significantly different from those found in unstrained reference molecules. This has been demonstrated in several gas-phase electron diffraction (GED) studies [ 1-53 . As the Si-Si bond is much longer than the C-Si bond, considerably less strain would be thought to be introduced by the SiMe3 groups when bonded to a silicon atom. A GED study of Me$Si-SiMe3 [6] gave Si-Si 2.34(3(g) -A, not significamly different from Si-Si 2.331(3) A in the unstrained reference molecule HaSi-SiH3 171, and C-Si l-877(3) HLsimilar to the C-Si bond distance in SiMe, (l-875(2)) A [6]. Some steric strain was however indicated by 0022-2860/84/$03.00

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a small torsional displacement (10”) from the reference position characterized by staggered SiMe3 groups. Even with four SiMe3 groups bonded to a silicon atom as in Si(SiMe~), [S] , st-eric deformations are comparatively modest. The Si-Si bonds and the .C-Si bonds are somewhat stretched (2.361(3) A and 1.889(3) A, respectively), and the SiMe, groups are twisted about 14” away from Td symmetry to relieve skeric interactions. Molecular-mechanics calculations (MM) on the tris-trimethylsilyl silane SiH(SiMe& [9] predict nearly unstrained bonds,. hut somewhat compressed HSiSi angles (108.2”) and SiMe3 groups twisted 14.7” away from staggered reference positions. Foliowing this line, it would be of interest to investigate the effects of SiMe3 groups in a his-trimethylsilyl silane. Because of the lower congestion about the central silicon atom in such a compound than in the tris and tetrakis substituted compounds, eventual steric strain would effectively be minimalized by an opening of the SiSiSi angle. The title compound, (Me,Si),SiMe,, was chosen for this reason. It also presents another interesting feature, namely the presence of interactions between SiMe, and SiMe, groups. EXPERIMENTAL

PROCEDURE

Octamethyltrisilane was made by a one step reaction of a mixture (2 : 1) of trimethylchlorosilane and dimethylchlorosilane with lithium [ 10 J . The product was purified by distillation (67”C/5 torr) and identified by NMR. Electron scattering patterns were recorded in the Oslo apparatus unit [ll] with a nozzle temperature of 66°C. The wavelength of the electron beam calibrated using diffraction patterns of benzene, was 0.06466 A. Exposures were made with nozzle-to-plate distances of 48 and 20 cm. In the structure analysis, five 48 cm and four 20 cm plates were used. The data were processed by standard methods 1121. The complex scattering factors, f’(s), were calculated from analytical representations of the atomic potential [13], using a program written by Yates [14]. Molecular intensities were modified by multiplying them by s/lfAi(s)lI&(S)!. Intensity curves obtained for each camera distance were averaged and then connected to derive a final intensity curve with s-limits: .smin= 2.00 A-‘, smax = 35.50 A-’ with As = 0.125 A-’ below 15.00 Ad1 and As = 0.250 A-’ above. CHOICE

OF MODEL

_&ND REFINEMENTS

-4 molecular model of Si,Me, is shown in Fig. 1. Some assumptions concerning the geometry of the molecule were made: (i) The plane formed by the central silicon atom and its two carbon atoms is perpendicular to the plane formed by the L&ee silicon atoms, and the line bisecting the SiSiSi angle also bisects the CSiC angle.

241

Fig. 1. Mokular model of Si,(CH,),. Atoms denoted t are referred to as terminal in text. Atoms denoted c are referred to as central.

(ii) Both central C-Si bonds are equal, and so are all terminal C--S1 bonds and all C-H bonds. (iii) All SiSiC (SiMe,) are equal, and all SiCH angles are assumed equal. (iv) Both terminal SiMe, groups are twisted an equal angle in the same direction when viewed from the central silicon to the terminal silicon atoms, all terminal methyl groups are twisted equally and so are both central methyl groups.

These assumptions imply that the SiMe, groups and the methyl groups have all local CxV symmetry, and that the model of the molecule belongs to point group C2, the bisector of the SiSiSi angle being the twofold ases. The model is therefore described by four bond distances (Si-Si, central C-Si, terminal C-Si, C--H), four valence angles (SiSiSi, CSiC (SiMe2), SiSiC (SiiMe,), SiCH) and three torsional angles (Q1(C,Si,Si,C,), +2(IltCtSitSi,), @,(H,C,Si,C,)). During the refinements the central C-Si bond distances were actually chosen as dependent distances, the terminal ones and the difference A = R(C,-Si,) - R(C,-Si,) being refined independently. The SiIIe3 torsional angle, @I, is defined as being equal to zero when the SiMe, group is staggered relative to the central SiMe, group and one C-Si bond is anti to the further Si-Si bond, it is taken as positive in the counter-clockwise direction when viewed along the Si-Si bond from the central silicon atom to the terminal one. The terminal methyl torsional angle, 02, is defined as being equal to zero when the methyl group is staggered relative to the SiSi(Me), moiety of SiSi(Me), and the central methyl torsional angle, &, is defined as being equal

242

to zero &hen the methyl group is staggered relative to the SiSi(Me)Si moiety of SiSi(Me)zSi. The sigm of these two torsional angles is defined in an analogous manner as described for @I. The molecular structure was refined by least-squares calculations using a diagonal weight matrix under the constraints of a geometrically consistent r,-structure [15] . Root mean square amplitudes of vibration (iii) and perpendicular amplitude correction coefficients (Klj) included in the analysis were calculated. from a non-diagonal force-field-by a program written by Hilderbrandt and Wieser fl6] _ Several torsional positions of the SiMe, groups were tried during the refinements. A model with SiMeJ groups eclipsed relative to the central SiMe, group and with one C-Si bond of each SiMe3 group syn to the further Si-Si bond (& = 60”) gave large discrepancies in the parts of the intenskjr curve corresponding to the longest C - - - Si and C - - - C distances. Also, some of the amplitudes of vibration refined to quite unrealistic values. Intermediate values of this torsional angle (between 60” and 0”) were also tried, but the best fit was obtained for 0”. In the final refinements the SiMes torsional angle was refined (starting at 0”) as were all independent bond distances, all independent valence angles and both methyl torsional angles. In addition, I, values for the bond distances, for some of the C - - - Si. and C - - - C distances, for the non-bonded Si - - - Si distance and for the Si - - - H (SiMe,, SiMe2) distances were refined. The experimental radial distribution (RD) curve calculated by Fourier inversion of the experimental, modified molecular intensities together with the residual curve corresponding to the final model, is given in Fig. 2. The agreement between observed and calculated RD curves may be seen to be quite good.

Fig. 2. Experiment+ radial distrhution (RD) c&e for Si,(CH,), (above), and theresidual cume diff. = RDob”- - RDC3’= (below). Artificial damping factor k = 0.002 A’.

243 RESULTS

AND DISCUSSION

of the most important dependent parameter values, and also calculated and refined I, values are given in Table 1. To account for the error introduced by the donnection of intensity curves before refining, and also to account for data correlation and for the uncertainty in the electron wavelength, the least-squares standard deviations (Us) Final

TABLE

independent

param eter

values

together

with

some

1

Geometrical parameters for S&(CH,),. theses

One corrected standard deviation is given in paren-

Parameter

ra W)

Si-Si

2.325(12) 1.887(3) l.SSO(S) 1.9@(l) l-100(5) 118.0(2.5)” 108.7(0.9)” 109.1(1.5)” 111.4(0.8)0 2.5(4.O)O 5(6S= 25(9)” 3.99(6) 3.40(l) 3.43(l) 4.48(B) 5.57(5) 3.096(11) 3.07(3) 3.94(2) 3.91(3) 5.01( 1) 5.15(10) 4.X2(16) 6.15(6) 6.86(2) 2.47(l) Z-49(2)

C-Sic C-Si (SiMe,) C-Si(SiMe,) C-H SiSiSi SiSiC (SiMe,) CSiC (SiMez) SiCH %d oh” 0, Si---Si &---Sit Ct.*.Si, C;. - . Sit (gauche)e C;- - - Si, (nr~ti)~ Ct- - - c, c,.--c, Cc- - - Ct (gauche)f C, - -. Ct (gauche)f c,. - - ct (anti)’ Ct - - - C; (GG)g Ct . - - C; (GG’)g Ct- . . C; (AGIg Ct- - * C; (AA)” Sit-- -Ht Si,.-.H,

0.062

0.058(8)

0.056 0.058 0.078

0.056(S) 0.059(6) 0.077(6)

0.095 0.119

0.103(10) 0.115(20) 0.115(20)

0.117 0.264 0.123 0.098 0.097 0.250 0.250 0.125 0.336 0.315 0.256 0.162 0.123 0.123

O.OSS( 15) 0.103(15) 0.276(28) 0.2’73(38) O-121( 30) 0.385( 30)

0.132(30) 0.125(30)

a*blij values calculated from the force field and determined from the GED data, respectively. =This is the average C-Si bond length in the molecule. dSee text for definition. “Ci... Si, is denoted gauche (anti) when C;-Si; is gauche (anti) relative to Sit-Si, fDistances C, - - - C, are denoted gauche (anti) when C,-Si, isgauche (anti) relative to Ct,+it. gA distance Ct - - - C; is denoted AG when Ct-Sit is anti to Sic-Si; and Ci-Si; is gauche to Sic-Sit. The explanation for k4, GG and GC’ are similar. (GG’: C-SI bonds point in the same direction, GG: C-Si bonds point in opposite directions.)

244

have been mukiplied by a factor of three to produce a corrected (T.These corrected s’adard deviations are given in parentheses. The assumptions concerning local C 3vsymmetry for the SiMe, groups and methyI groups seem to be justified as significant deviations from this would have resulted in I, values for distances such as Ct - - - Ct, Si, - - - Ct, Sic - - - Ht and Si, - - -H, being abnormally large. As can be seen from Table 1, the I, values for these distances are all quite normal and not significantly different from the caIcuIaked values. The relatively good fit in the long distance range of the RD curves, also supports the assumption concerning twofold symmetry of the molecule. The GED data of Si3Mes reveal no significant strain in the Si-Si bond. Its value (2.325(12) A) is not significantly different from the value of the Si-Si bond length in Si& (2.331(3) A) [7] which may be regarded as approximat.ely unstrained; it is slightly smaller than the value of the Si-Si bond distance in Si,Me, (2.340(g) A) [C;]. The Si-Si bond length of S&Me, is furthermore comparable to the Si-Si bond Iength in the trisilane SiJCIs (2.329(7) A) [ 171, but definitely shorter than the same distance in Si(SiMe,), (2.361(3) A) [S], and it is also somewhat shorter than the Si-Si bond lengths in the two cyclic polysilanes SisI-I10(2.342(3) A) [ 183 and Si6HII (2.342(5) A) [191Some steric strain can be detected in the C-Si bonds (mean value l-887(3) A)_ However, this value is unexceptional, and it is only slightly larger than the value of the C-Si bond distances in Si2Me6 (l-877(2) BL)161, and also somewhat larger than the values for the bond distances in the nearly unstrained SiMe4 (1.875(2) ipi) [63 and Me&H (l-873(6) .&) [20]. Somewhat longer central C-Si bonds (1.90(l) -8) were found than terminal ones (1.880(S) A). This might well be due to steric interactions between central and terminal methyl groups. It is further a point of interest that interactions between the two Safe, groups do not lead to a significant twist of these groups away from their staggered reference positions relative to the central SiIMez group (2.5(4)‘). This conformation is highly effective in reducing interactions between central and terminal methyl groups, but it leads to parallel 1,3-methyl interactions which in general are regarded as unfavourable. Obviously, the rather long Si-Si bonds reduce much of the SiMe, interactions, but some strain is revealed in the SiSiSi angle, which is opened to the value of 118.0(2.5)“. Hummel- et al. [21] give a value of 111.7” for a strainfree SiSiSi valence angle. Empirical force field calculations (EFF) [21] on SiJHs predict a compressed SiSiSi angle of 109.9”, while the SiSiSi angles in S&HI0 are calculated to be less compressed jlli3.5°). In Si4Me10,‘however, the EFF calculations yield expanded SiSiSi valence angles both for the anti form (113-4”) and for the gauche form (116.8”). Expanding of the SiSiSi angle was also found in the GED study of SisCls (118.7(1.6)“) [17]. Although the SiSiSi valence angle in the title compound can not be determined with high precision, it is definitely significantly larger than an unstrained SiSiSi valence angte.

245

The SiSiC (SiMe3) valence angle of 108.7(0.9)” is quite unesceptional, being similar to that of SizMeB (108.4(0.4)‘) [6] and somewhat smaller than the SiSiC valence angle in ClMe*Si-SiMe2C1 (109.8(0.7)“) 1221, and the CSiC (SiMez) valence angle of 109.1(1.5)” is comparable to the CSiC angle in Me$iH (llO(2)O) [20] and to that of Si2Me6 (110.5(0.4)“) [6]. The SiCH valence angle (111.4(0.8)“) is somewhat larger than the same angles in SiMeJ (109.2(0.8)“) and SitMe, (108.7(0.8)“) [6]. This might be due to steric effects, as large values of this angle will reduce interactions between hydrogen atoms bonded to terminal carbon atoms and also interactions between hydrogen atoms bonded to one terminal and one central carbon atom will be reduced. Interactions between such central and terminal hydrogen atoms are also relieved by a twist of the central methyl groups of 25(g)” away from their staggered reference positions, whereas the terminal methyl groups are essentially staggered (5(6)“). As a concluding remark, we note that steric strain in Si,Mes is relieved predominantly by an opening of the central SiSiSi valence angle, while bond stretches are very modest. Although less pronounced, this opening of the central angle is comparable to the opening of the corresponding angles in the bis(trimethylsily1) and bis(t-butyl) substituted methanes, CH,(SiMe,)2 [3] and CH2(CMe3)z [5] (SiCSi 123.2(0.9)“, CCC 128.0(6.0)“), whereas bond stretches are much more pronounced in the bis-substituted methane CH,(CMe,),. REFERENCES 1 B. Beagley, R. G. Pritchard, C. Eaborn and S. S. Washburne J. Chem. Sot., Chem. Commun., (1$81) il0. 2 B. Beagley ana R. G. Pritchard, J. Mol. Struct., 84 (1982) 129. 3 T. Fjeldberg, M. F. Lappert, R. Seip and A. J. Thorne, J_ Mol. Struct., 99 (1983) 295_ 4 H. B. Burgi and L. S. Bartell, J. Am. Chem. Sot., 94 (1972) 5236. 5 L. S; Bar-tell and W. F. Bradford, J. Mol. Struct., 37 (1977) 113. 6 B. Beagley, J. J. Monaghan and T. G. Hewitt, J. Mol. Struct., 8 (19il) 401. 7 B. Beagley, A. R. Conrad, J. M. Freeman, J. J. Monaghan, B. G. Norton and G. C. Holywell, J. Mol. Struct., 11 (1972) 371. 8 L. S. Bartell, F. B. Clippard and T. L. Boates, Inorg. Chem., 9 (1970) 2436. 9 S. K. Doun and L. S. Bartell, J. Mol. Struct., 63 (1980) 249. 10 H. Gilman and R. L. Hareil, J. Organomet. Chem., 5 (1966) 201. 110. Bastiansen, 0. Hansel and E. Risberg, Acta Chem. Stand., 9 (1955) 232. 12 B. Andersen, H. M. Seip, T. G. Strand and R. Stdlevik, Acts Chem. Stand., 23 (1969) 3224. 13 T. G. Strand and R. Bonham, J. Chem. Phys., 40 (1964) 1686. 14 A. C. Yates, Comput. Phys. Commun., 2 (1971) 175. 15 H. M. Seip, T. G. Strand and R. Stdlevik, Chem. Phys. L&t., 3 (1969) 617; C. Gundersen, Annual Report of the Norwegian Electron Diffraction Group, Oslo, 1977, available on request from the authors (A. A. and T. F.). 16 R. L. Hilderbrandt and J. D. Wieser, J. Chem. Phys., 55 (1971) 4648. 17 A. Almenningen and T. Fjeldberg, J. Mol. Struct., 77 (1981) 315. 18 Z. Smith, H. M. Seip,E. Henggeand G. Bauer, ActaChem.Scand., Ser. A, 30 (1976) 697. 19 Z. Smith, -4.Almenningen E. Henggeand D. Kovar, J-Am. Chem. Sot., 104 (1982) 4362. 20 A. C. Bond and L. 0. Brockway, J. Am. Chem. Sot., 76 (1954) 3312. 21 J. P. Hummel, J. Stackhouse and K Mislow, Tetrahedron, 33 (1977) 3312. 22 K. Kveseth, Acta Chem. Stand. Ser., A, 33 (1979) 453.