Bromo-di(silyl)methanes - Precursors to Disilylcarbanionic LigandsSebastian Bommers, Holger Beruda, Martin Paul, Hubert Schmidbaur*Anorganisch-chemisches Institut der Technischen Universität München,Lichtenbergstraße 4, D-85747 GarchingZ. Naturforsch. 49b, 821-827 (1995); received July 5, 1994Chlorosilanes, Disilylmethanes, Bromo-di(silyl)methanes

The reaction of the series of methyl/phenylchlorosilanes Me2PhSiCl, MePh2SiCl and Ph3SiCl with CHBr3/n-BuLi to give the corresponding bromo-di(silyl)methanes has been investigated. The selectivity of the reaction proved to be strongly influenced by the number of phenyl groups bound to silicon. As already established for Me3SiCl, S i-C coupling readily occurs with Me2PhSiCl to give (Me2PhSi)2CHBr (1) in good yields, whereas MePh2SiCl af­fords (MePh2Si)2CHBr (2) in lower yields. The molecular structures of 2 and the by-product (MePh2Si)2CBr2 have been determined by single crystal X-ray diffraction. In the case of the fully arylated species, Ph3SiCl, only the monosilylated compound (Ph3Si)CH2Br (3) is generated. (HM e2Si)2CHBr (5) can be obtained starting from 1 by treatment with triflic acid to give (CF3S 0 3Me2Si)2CHBr (4), followed by reduction of 4 with LiAlH4 in diethyl ether. In situ metalation of 5 with n-BuLi affords (HM e2)2CHLi, which reacts instantaneously with a second equivalent of 5 to give the halogen-free dimer [(HMe2Si)2CH]SiMe2CH9SiMe2H (6).

Introduction

Silylated organolithium compounds are valuable synthons in organometallic chemistry. This is also true for the syntheses of organogold compounds, the majority of which is prepared via organometal­lic reagents [1-4]. Recent interest has focussed on the aggregation of gold at interstitial carbon atoms, owing to the unusual molecular geometry and chemical bonding in the resulting clusters[2,3]. In the course of these studies the specific effects of silyl substituents at the carbanionic cen­ters of aurated hypercoordinate carbon species on the stoichiometry and molecular structure have also been studied [3-5]. From the disilylated pre­cursor (Me3Si)2CHLi a novel type of compound with hypervalent, pentacoordinate carbon could be synthesized [5]. The cation of the product has a trigonal-bipyramidal structure with the two tri- methylsilyl groups in equatorial positions (Scheme 1).

The auration proceeds in steps, and both inter­mediate mono- and bis-aurated compounds can be isolated. These findings have suggested experi­ments to synthesize other methanium compounds

* Reprint requests to Prof. Dr. H. Schmidbaur.

J Z — AuPPh3 X m --------- (Me3Si)2C(AuPPh3)2Me3Sr |

AuPPh3

Scheme 1

with silyl groups of different electronic and steric effects, which could further improve the stability of these unique structures. For these studies bromo-di(silyl)methanes are required. The results of attempts to synthesize the most obvious precur­sors are presented in this report.

Results

Synthesis o f the compoundsFollowing the example of the published synthe­

ses of the compounds (Me3Si)2CHBr or (Me3Si)2CHCl [6], the members of the series of chlorosilanes Me„Ph3_„SiCl were each reacted with n-BuLi and CHBr3 in tetrahydrofuran at low temperature. The course of the reaction is strongly dependent on the substitution pattern at silicon. The selectivity of the reaction decreases with

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822 S. Bommers et al. ■ Bromo-di(silyl)methanes

increasing space required by the silyl groups in the order:

SiMe3 < SiMe2Ph < SiMePhb < SiPh3

Whereas the sterically least hindered Me3SiCl affords (Me3Si)2CHBr in 90% yield [6], the reac­tion of Me2PhSiCl yields 63% (Me2PhSi)2CHBr (1) (eq. (1)) aside from various by-products, mainly Me2PhSi(«-Bu) and (Me2PhSi)2CH2, as observed in GC-MS studies.

(3)

CH3

Si— Cl Ic h 3

CHBrj/n BuLi

n BuBr LiCl

C H , H CH ,

C H , Br C H ,

(1)

(HMe2Si)2CHBr (5) has been obtained by methods successfully employed already for the preparation of other highly silylated hydrosilane molecules [7-9]. From (Me2PhSi)2CHBr (1) the phenyl groups can be removed quantitatively by selective protodesilylation [10,11] with triflic acid (eq. (4)).

CbSi— ClCHBrj/n BuLi

n BuBr LiCl

0 L O

0-H4-© »C H ,B r CH

CF3SO3HCH H C H ,I I I

CF3SO 3— Si— C— Si— O 3SCF,

M r U

The synthesis of (MePh2Si)2CHBr (2) (eq. (2)) from MePh2SiCl proceeds in a yield of only 20%. Concurring side reactions lead to numerous by­products (MePh2SiCH2Br > MePh2SiSiPh2Me > MePh2SiH > (MePh2Si)2CBr2).

Attempts to apply the same synthetic procedure for the synthesis of the triphenylated compound, starting with Ph3SiCl, were not successful. The re­sults of spectroscopic investigations and work-up of the reaction mixture suggest that after a first S i-C coupling, followed by metallation, the elec- trophilic attack of a second Ph3SiCl molecule meets with sterical hindrance. Even after extended reaction times no (Ph3Si)2CHBr could be ob­tained. As derived from NMR and 13C NMR spectroscopic and GC-MS data, the highly reactive intermediate (Ph3Si)CHBrLi undergoes a Wurtz type reaction with the /7-BuBr released in the reac­tion to give small amounts of (Ph3Si)CHBr(A7-Bu), but is stabilized mainly by proton abstraction from the solvent THF (eq. (3)). (Ph3Si)CH2Br, 3, was isolated as a microcrystalline solid (m.p. 132 °C) and has been characterized by spectroscopic data (see Experimental).

(4)

The resulting silyl triflate [(CF3S 0 3)Me2Si]2- CHBr (4), a colourless liquid fuming in air, can be converted into the target molecule 5 without any further purification. The H/CF3S 0 3 substitution is accomplished with LiAlH4 in diethyl ether (eq.(5)) and affords (HM e2Si)2CHBr 5 in 77% yield over the two steps (b.p. 66 °C/20 Torr).

CH H C H ,

CF3SO3— l i — i i — O3SCF3 - ^ A1H4

(^H, Br d:H,

CH H CH

H— i i — d:— i i — H (5)

(^H, Br d:H,

Compound 5 is a bromomethane species with smaller silyl groups than (Me3Si)2CHBr. It thus appeared to be a suitable starting material for ste­rically less crowded polynuclear gold compounds. For the synthesis of these, the formation of a lithiomethane precursor would be the crucial step, preferably by transmetallation using alkyllithium reagents. (HMe2Si)2CHBr is offering two possible reaction sites (C -B r, S i-H ) for the attack of al­kyllithium compounds. The reactivity of S i-H groups is known to increase with the number of haloalkyl groups bound to the silicon atom, and

S. Bommers et al. • Bromo-di(silyl)methanes 823

alkylation is therefore likely to occur predomi­nantly at one of the S i-H bonds of compound 5. G. Fritz et al. have investigated the reactivity of similar systems, e.g. of C-chlorinated disilapropane (H3Si)2CCl2, towards MeLi. These studies have shown that Si-alkylation is prevailing, whereas methylation of the carbon atom has never been observed. The reaction does not proceed in a uni­form way, however. Only if an excess of MeLi is applied, the reaction selectively affords the chlo­rine-free species (Me3Si)2CH2 [12].

The results of our investigations of the metalla- tion of 5 with one equivalent of n-BuLi have con­firmed the conclusions of this earlier work, in that C-butylated compounds have never been detected. The major product formed in the reaction was the bromine-free species [(HMe2Si)2CH]SiMe2CH2Si Me2H (6) (50% as estimated from GC-MS data), accompanied by several by-products, mainly (n- Bu)SiMe2CH2SiMe2H. The formation of 6 is indi­cating that in a first step (HMe2Si)2CHLi is formed in situ by metal halogen exchange at the bridging CHBr unit, followed by S i-C coupling with a second molecule of (HMe2Si)2CHBr (eqs (6), (7)).

CH H CH

H—i i — h—i i — H

<!:h ,~

rvBuLi

, Br C H ,

5

n BuBr

CH,H CH

H—ii —(j:—ii— H

d:H,ii d H,(6)

C H , H C H ,

H—i i — i i — H

i!:h , u (!:h ,

C H ,H CH ,

H—i i — h—i i — H

d :H , i r <1:h ,

pound has been fully characterised by analytical and spectroscopic data. As a consequence of the diastereotopic nature of the methyl groups bound to the silicon atom at a prochiral carbon center, two different sets of Me signals can be observed in the 'H NMR and 13C NMR spectra (see Experi­mental). In compound 2 the phenyl groups at the silicon atoms are diastereotopic (see Experi­mental).

Compound 2 is obtained by tedious fractional crystallization as white plates (m.p. 84 °C), suit­able for an X-ray diffraction study.

C35

C34 C25

C24

C33

)C31

C43

C32

C42

C2 C1

)C41 C16,

C44

C11 Y

B r 1

C45 jT

\ C46C12

C15

Fig. 1. Molecular structure of (M ePh2Si)2CHBr 2 (50% probability level for non-hydrogen atoms, arbitrary radii for hydrogen atoms). Selected distances [A] and angles [°]: C -S il 1.884(4), C -S i2 1.890(4), C - B r l 1.986(4); S i l - C - S i2 123.0(2), S i l - C - B r 107.5(2), S i2 -C -B r 108.6(2), C 1 2 -C 1 1 -C 1 6 115.4(4).

CH,— Si,

LiBr

CH3CH,\ I

HC— Si— CH2- S i-HCH,

CH,— Si J / \

H CH,CH, CH,

(7)

When (HM e2Si)2CHBr was treated with n- BuLi in a 2:1 molar ratio at temperatures around -9 0 °C, S i-H butylation was suppressed and the yield of [(HMe2Si)2CH]SiMe2CH2SiMe2H was observed to rise to about 80%.

Properties, spectroscopic data and structures

Compound 1 is a colourless liquid (b.p. 135 °C/0.01 Torr), stable to moisture and air. The com-

The orthorhombic crystals of 2 belong to the space group Pbca with Z = 8 molecules in the unit cell. Bond distances and angles of the molecule, which has no crystallographic symmetry, are within the expected range. The configuration at the central carbon atom is strongly distorted from a tetrahedral geometry due to the bulky MePh2Si groups (angle S i l - C - S i2 = 123°, S il-C /S i2 -C bond distances 1.884(4) Ä/1.890(4) A). The methyl and phenyl groups of the silyl substituents are in staggered conformations, with the phenyl rings ex­hibiting the well known distortion from an ideal hexagon [13,14] (Fig. 1). The molecular structure in the solid state is in accordance with the specros- copic data for solutions.

824 S. Bommers et al. ■ Bromo-di(silyl)methanes

Fig. 2. Molecular structure of (MePh2- Si)2CBr2 (50% probability level for non­hydrogen atoms, arbitrary radii for hy­drogen atoms). Selected distances [A] and angles [°]: C -S il 1.923(2), C - B r l 1.984(2), S i l a - C l - S i l 118.0(2), B r l - C - S ila 108.7(1), B r l - C - S i l 107.5(1), B r l - C - B r la 106.0(2), C 2 2 -C 2 1 -C 2 6 116.7(2).

The crystal structure of (MePh2Si)2CBr2, a mi­nor by-product of the synthesis of 2, has also been determined. The crystal system of the dibromo compound is monoclinic (space group C2/c) with Z = 4 formula units in the unit cell. The molecule has crystallographic C2 symmetry with the twofold axis passing through the central carbon atom and the middle of the B r-B r edge of the isosceles tri­angle B r -C -B r (Fig. 2).

Due to the presence of two bromine atoms at the central carbon atom ( B r l - C - B r l a = 106.0(2)°) a smaller S i-C -S i angle is observed ( S i l - C - S i l a = 118.0(2)°), as compared to the monobromo species 1 (S i l -C -S i2 = 123.0(2)°). Significant steric strain is evident from the unusu­ally long S i-C distances of 1.993(2) A, and from these angle deformations.

Compound 4 is an oily liquid, which has not been isolated (above), and was only characterized by its spectral data. Surprisingly, and in contrast to the spectra for compounds 1 and 5, [(CF3S 0 3)Me2Si]2CHBr shows only one methyl signal in the ‘H NMR, as well as in the 13C NMR spectrum (see Experimental). This phenomenon may be explained by a rapid triflate exchange be­tween the silicon atoms via pentacoordinate tran­sition states, rendering the methyl groups equiva­lent on the NMR time scale. This exchange can occur both intra- and intermolecularly.

Compound 5 is easily identified by standard spectroscopic methods. Solutions in C6D 6 show four ‘H NMR signals revealing two magnetically

inequivalent methyl groups, (3 = 4.2 ppm [xp-oct. V(HSiCH) - 3 Hz, SiH], (3 = 2.0 ppm [t. V(HCSiH) = 3 Hz, CHBr], (3 = 0.15 ppm [d. V(HCSiH) = 3.7 Hz, CH3], (3 = 0.10 ppm [d. 3/(HCSiH ) = 3.7 Hz, CH3'j. The 29Si NMR reso­nance appears as a doublet of ^-octets of doublets at (3 = 11.7 ppm (‘/(SiH) = 191, 2/(S iC H ) = 6.7. V(SiCSiH) = 1.7 Hz). In agreement with the pro­posed formula, three 13C resonances can be de­tected at c3 — 20.9 ppm [d tr sept, V(CSi) = 56. ‘/(C H ) = 127, 2/(CSiH ) = 11, 3/(CSiCH ) = 23 Hz. CHBr], <3 = -3 .7 and -4.2 ppm [q d ^-quin. ‘/(CSi) = 53, ‘/(C H ) = 120, 2/(C SiH ) = 7.8. 3/(CSiCH) = 2.3 Hz, CH3/CH3'].

Conclusion

Three new bromo-di(silyl)methanes could be obtained from modifications of established syn­thetic routes. One of these (HMe2Si)2CHBr (5) has smaller, the other two, (Me2PhSi)2CHBr (1) and (MePh2Si)2CHBr (2), have larger silyl groups than the reference compound (M e3Si)2CHBr. (Ph3Si)2CHBr could not be prepared via the stan­dard synthetic procedures, probably owing to ste­ric effects and modified leaving group properties of the halogen atoms.

(HM e2Si)2CHBr (5) could not be lithiated in a transmetallation reaction using n-butyllithium. but instead underwent a SiC coupling reaction with hydride elimination to give a branched carbon/sili­con frame (6).

S. Bommers et al. • Bromo-di(silyl)methanes 825

M etallation of (Me2PhSi)2CHBr (1) was found to proceed normally, however, and the resulting (Me2PhSi)2CHLi reagent could finally be used for step-wise auration with Ph3PAu+ reagents [15].

ExperimentalAll experiments were carried out under pure

dry nitrogen. Glassware and solvents were puri­fied, dried and kept under nitrogen. C6D 6 and CDC13 were used as solvents for NMR spectros­copy, tetramethylsilane was employed as the refer­ence compound (JEOL GX 270, GX 400 and Bruker WT 100 SY spectrometer). For the GC- MS measurements a gaschromatograph (Hewlett Packard HP GC 5890 Series II, 12 m column HP 1) and mass-selective detector (Hewlett Pack­ard HP MS 5971 A) were applied.

(Me2PhSi)2CHBr (1)A solution (250 ml) of 1.6 M «-BuLi in hexane

(0.4 mol) was added to a mixture of 68.3 g (0.4 mol) of Me2PhSiCl, 50.5 g (0.2 mol) of CHBr3, and 500 ml of TH F at -78 °C within 6 h. To achieve complete precipitation of the lithium salts, hexane (300 ml) was added at room temperature to the crude product mixture. After separation of the lithium salts and distillation of the filtrate in a vac­uum, 48.5 g (63%) of an oily liquid were isolated (b.p. 125 °C/0.01 Torr).

!H NMR: d = 7.4 (m, ArH), 7.1 (m, ArH), 2.5 (s, CHBr), 0.23 (s, CH3), 0.20 (s, CH3'). - 13C NMR: d = 137.7 (m, Q ), 134.4 [d, ip -1 , V(CH) = 156, 3/(C C C H ) = 7 Hz, C2/6], 129.6 [d t d, V(CH) = 159, 2/(C C H ) = 1, -V(CCCH) = 7 Hz, C4], 127.9 [d m, '/(C H ) = 162 Hz, C3/5], 25.6 [d sept, J/(CSi = 43, lJ(CH) = 124, 3/(CSiCH) = 2 Hz, CHBr], -2 .0 [q t/^-qui, V(CSi) = 56, V(CH) = 120, 37(CSiCH) =1.4 Hz, CH3], -2 .7 [q ty-qui, ^/(CSi) = 56, V(CH) = 120, 3/(CSiCH) = 1.4 Hz, CH3']. - 29Si NMR (3 = -2 .4 [oct, 2/(SiCH) = 7 Hz]. - MS (El, 70 eV): m/z = 347-349 (M-15, C16H 20BrSi2), 148 (C9H 12Si), 135 (C8H„Si).

(MePh2Si)2CHBr (2)A solution (269 ml) of 1.6 M rc-BuLi in pentane

(0.43 mol) was added to a mixture of 100 g (0.43 mol) of MePh2SiCl, 54.3 g (0.22 mol) of CHBr3, and 500 ml of THF at -7 8 °C within 6 h. To achieve complete precipitation of the lithium salts, hexane (300 ml) was added at room temp, to the crude product mixture. After separation of the

lithium salts all solvents were removed under re­duced pressure. From the oily residue, 24 g (22.5%) of 2 have been obtained by fractionated crystallization (EtOH/ethylacetate) as a white solid (m.p. 84 °C). From the mother liquor small amounts of (MePh2Si)2CBr2 have also been ob­tained as single crystals.

lH NMR: Ö = 7.1, 7.4, 7.6 (m, ArH, A rH '), 3.3 (s, CHBr), 0.3 (s, CH3). - 13C NMR: d = 136.8 (m, CO, 136.6 (m, Q ') , 135.5 [d m, V(CH) = 157.6 Hz, C3/5], 135.1 [d m, V(CH) = 157.6 Hz, C3/5'], 129.8 [d t, 7 (C H ) = 159, 2J(CCH) = 6.9 Hz, C4], 129.6 [d t, ^ (C H ) - 159, V(CCH) = 6.9 Hz, C4'j, 128.0 [d m, V(CH) = 160 Hz, C2/6], 128.0 [d m, 7(C H ) = 160 Hz, C2/6'], 20.0 [d m, V(CSi) - 46, V(CH) =123 Hz, CHBr], -3 .5 [q d, ^(C Si) = 57, V(CH) = 121, V(CSiCH) = 1.8 Hz, CH3]. - 29Si{JH} NMR: d = -7.8. - MS (Cl): m/z = 471-473 (M-15, C26H 24BrSi2), 407 409 (C21H22BrSi2)5 197 (C13H 13Si).

C27H27BrSi2 (485.59)Calcd C 66.5 H 5.5 Br 16.4 Si 11.5%, Found C 65.7 H 5.5 Br 16.7 Si 11.7%.

Reaction o f Ph3SiCl with CHBr3 and n-BuLi; bromo(triphenylsilyl)methane (3)

To a solution of 25.0 g (85 mmol) of Ph3SiCl and10.7 g (42.5 mmol) of CHBr3 in 500 ml of THF,42.4 ml of 2 M n-BuLi in pentane (85 mmol) was added at -7 8 °C. The mixture was allowed to warm up to room temp, and stirred for 56 h. GC-MS measurements showed that this crude pro­duct mixture consisted of equal amounts of Ph3SiCH2Br (3) and Ph3SiCl. Hydrolysis with i- PrOH followed by fractionated crystallisation from EtO H yielded 5.4 g (36%) of Ph3SiCH2Br (m.p. 132°C).

lH NMR (C6D 6): <5 = 7.3-7.5 (m, ArH), 3.17 (s, CH2Br). - 13C NMR (C6D 6): (5 = 135.8 [d ^-tr, V(CH) = 158, 2/(C C H ) = 8 Hz, C3/5], 132.5 [s, Q ], 130.1 [d tr, V(CH) = 160, 27(CCH) = 8 Hz, C4], 128 [d d, V(CH) = 159, 27(CCH) - 7.5 Hz, C2/6],13.0 [t, ^ (C H ) = 139 Hz, CH2], - 29Si{!H} NMR (C6D 6): (3 = -14.1. - MS (El, 70 eV): 352-354 (C19H 17BrSi), 259 (C18H 15Si), 181 (C12H 10Si), 105 (C6H 5Si).

(CF3SO3Me2Si)2CHBr (4)

4.8 ml (0.06 mol) of trifluoromethanesulfonic acid was added dropwise to 10.0 g (0.03 mol) of 1 diluted with 10 ml of CHC13 at -2 0 °C. After 30 min the mixture was allowed to warm to room temp. After evaporation of all volatile compounds

826 S. Bommers et al. • Bromo-di(silyl)methanes

under reduced pressure, 4 was obtained in almost quantitative yield.

'H NMR: (3 = 0.3 (s, CH 3), 1.7 (s, CHBr). 13C NMR: (3 = 118.8 [q, '/(C F ) = 317 Hz, CF,], 17.3 [d, '/(C H ) = 128 Hz, CHBr], -1 .7 [q, '/(C H ) =124 Hz, CH3], - 29Si{'H} NMR: (3 = 30.7.

(HMe2Si)2CHBr (5)

A solution of freshly prepared (CF3S 0 3Me2Si)2CHBr (from 74.8 g (0.21 mol) of (Me2PhSi)2CHBr) was added dropwise to a sus­pension of 4.0 g (0.13 mol) of LiAlH4 in diethyl ether at temperatures around -15 °C. The mixture was stirred under reflux conditions for 1 h. A que­ous work-up of the product mixture and distilla­tion yielded 34 g (77% over the two steps) of 5 (b.p. 66 °C/20 Torr).

'H NMR: d = 4.2 [oct, 3/(H SiCH ) = 3 Hz, SiH].2.0 [t, 3/(H CSiH ) = 3 Hz, CHBr], 0.15 [d, 3/(HCSiH ) = 3.7 Hz, CH3], 0.1 [d, 3/(H CSiH ) =3.7 Hz, CH3']. - 13C NMR: 6 = 20.9 [d tr sept, ' / ( CSi) = 56, '/(C H ) = 127, 2/(C SiH ) = 11, 3/(CSiCH) = 2.3Hz, CHBr], -3 .7 [q d qui, '/(CSi) = 53, ’/(C H ) = 120, 2/(CSiH ) = 7.8, 3/(CSiCH) = 2.3 Hz, CH3], -4 .2 [q d qui, '/(C Si) = 53, '/(C H ) = 120, 2/(CSiH) = 7.8, 3/(CSiCH) - 2.3 Hz, CH3']. - 29Si NMR: (3 = -11.7 [d oct d, '/(S iH ) = 191, 2/(SiCH) = 6.7, 3/(SiCSiH) = 1.7 Hz], - MS (El, 70 eV) m/z = 210-212 (C5H 15BrSi2), 195-197 (C4H 12BrSi2), 150-152 (CH3BrSi2), 73 (C3H 8Si), 59 (C2H 7Si).

C5H lsBrSi2 (211.25)Calcd C 28.40 H7.15% ,Found C 29.12 H 7.24% .

[(HMe2Si)2CH]SiMe2CH2SiMe2H (6)

To a solution of 5.0 g (23.7 mmol) of compound5 in 15 ml of THF, 7.9 ml of a 1.6 M rc-BuLi solu­tion in hexane (11.8 mmol) was added at a temp, of -7 8 °C. After 1 h the crude product mixture was allowed to warm to ambient temp. To separate the LiBr formed in the reaction, all volatile com­pounds were evaporated and the product distilled under high vacuum conditions (yield 2.45 g, 79%), b.p. 37 °C/0.01 Torr.

'H NMR (C6D6): (3 = 4.2 [d V'-sept, '/(H Si) = 183, 3/(H SiC H 3) = 3.7, 3/(H SiCH ) = 1.7 Hz, HSiCH], 4.1 [non, '/(H Si) = 183, 3/(H SiCH ) = 3.7 Hz, HSiCH?], 0.16 (s, H 3CSiCH7), 0.15 [d, 3/(HCSiH ) = 3.7 Hz, H 3CSiHCH], 0.145 [d. 3/(HCSiH ) = 3.7 Hz, H 3CSiHCH'], 0.08 [d. 3/(HCSiH ) = 3.7 Hz, H 3CSiHCH2], -0.14 [d, 3/(HCSiH ) = 3.8 Hz, CH2], -9 .2 [t, 3/(H CSiH ) =

Table I. Crystal and structure solution data.

(MePh2Si)2CHBr (MePh2Si)2CBr2

Formula C27H27®^Si2 C27H26Br2Si2M [g/mol] 487.6" 566.5Temp. [°C] 20 -71Crystal system orthorhombic monoclinicSpace group Pbca C2/c«[A ] 12.393(1) 15.591(1)HA] 18.155(1) 12.332(1)c [A] 22.549(1) 13.976(1)<z[°] 90 90ß [ ° ] 90 113.19(1)y [°1 90 90v [A3] 5073(3) 2470.0(12)Pealed, [g/cm3] 1.277 1.523z 8 4F(000) [e] 2016 1144H (M o-K„) [cm-1] 17.26 33.92Radiation A = (0.71069 A )M o-K a,

Graphite monochromatorDiffractometer Enraf Nonius CAD 4h k l range 0-15 /0-23 /0-28 ± 19/± 15/0—17Absorption corr. 0.93/0.99 0.77/1.0

(Tmin/Tmax)Measured refl. 4354 5158Unique refl. 4352 2633

int 0.011 0.013Observed refl. 2869 2323

[F0> 4 a F 0]Refined param. 379 193Final R indices [%] 4.25 3.03Final wR indices [%] 3.89 3.85

(min/max) [eA-3] 0.30/-0.25 0.46/-0.40

1.7 Hz, CH]. - 13C NMR (C6D6): (3 = 2.9 [t m. '/(C Si) = 43.5 Hz, '/(C H ) = 110 Hz, CH2], 2.2 [q m, '/(CSi) = 50.3, '/(C H ) = 119.5 Hz, CH3SiCH2], -0 .6 [q d ip-qui, !/(CSi) = 50, '/(C H ) = 119.6, 2/(CSiH) = 7.8, 3/(CSiCH) = 2.3 Hz, CH3SiH], -0 .9 [q d tp-qui, '/(C H ) = 119.6. 2/(CSiH) = 7.8, 3/(CSiCH) = 2.7 Hz, CH3SiH'], -1.17 [q d V^-sext, '/(C H ) = 119.5, 2/(CSiH) = 7.8. 3/(CSiCH) = 2.3 Hz, CH3SiHCH,], -2 .8 [d m. '/(C H ) = 120 Hz, CH], - 29Si NMR (C6D 6): Ö =1.4 (s, SiC4), -16.0 [d m, '/(S iH ) = 184 Hz, SiCH]. -16.5 [d m, '/(S iH ) = 183 Hz, SiCH2], - MS (El. 70 eV): m/z = 261-262 (C10Si4Hx), 247-249 (C9Si4HA), 189-190 (C7Si3H v), 173-174 (C6Si3H ,), 157-159 (C5Si3H ,), 131 (C5Si2H 15), 115 (C4Si2H 12), 73 (C3SiH9), 59 (C2SiH7).

Crystal structure determination of (MePh2Si)2CHBr and (MePh2Si)2CBr2

Single crystals of (MePh2Si)2CHBr and (MePh2Si)2CBr2 were sealed under argon at dry ice temperature into capillaries. Subsequent ex­amination on the diffractometer revealed the re­spective crystal classes, the metrical symmetries of which were confirmed by reduced cell calculations.

S. Bommers et al. • Bromo-di(silyI)methanes 827

Exact cell constants were determined by least- squares refinement on the Bragg angles of 25 re­flections carefully centered on the diffractometer. Crystal data and values pertinent to data collec­tion, structure solution and refinement are col­lected in Table I. The measured integrated inten­sities were corrected for Lp effects and crystal decay. The structures were solved by direct meth­ods. After anisotropic refinement of the non-H atoms, all hydrogen atoms were located from the

difference Fourier map and refined isotropically [16, 17],

Acknowledgements

This work has been supported by Bundesmini­sterium für Forschung und Technologie, Bonn, and by Fonds der Chemischen Industrie, Frankfurt. The authors are grateful to Mr J. Riede for estab­lishing the X-ray data sets.

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