The molecular structure of gaseous, monomeric bis(trimethylsilyl)methyl-lithium, LiCH(Si(CH3)3)2, as determined by electron diffraction

Journal Elsevier

Structure. 127 (1985) Publishers B.V.. Amsterdam

ofMolecular Science

TEE MOLE(xIIAR

STRUCTURE

BIS(~ILYL)METHYL-LITHIUM, DETERMINED BY ELECXRON

TGRGNY

95-105 -Printed

in Tbe

Netherlands

OF GASEOUS, MONOMERIC LiCH(Si(CH,),),, DIFFRACTION

FJELDBERG”

Department of Chemishy. College of Ark and Science. The Uniuersity N-7055 DnquoU (Norway) MICHAJXL

F. WPERT

and ANDRJZW

of Trondheim.

J. THORNE

of Chemistry and MoLecubr Sciences, Uniuersity of Suez, (Gt Britain)

School

(Received

AS

Brighton BNl 9QJ

3 July 1984)

Gaseous bis(trimethykilyl)methyl-lithium, LiCH(SiMe,),, has been studied by electron diffraction at a nozzle temperature of ca 413 K. The diffraction data are consistent with a model consisting of monomers only, although small percentages of dimers or higher oligomen cannot be excluded Assuming C, symmetry for the C(SiMe,), moiety affords the following principal bond lengths (rd and valence angles: Li-C 2.03(6) A, central C-Si (C-SiMe,) l-892(2) A, terminal C-Si (Si-CH,) l-877(2) A. C-H l-108(6) A; &iCSi 119.0(1.5)‘, LCSiCH, 111.0(1.2)0, cSiCH(CH,) 112.3(1.4)” and LSiCLi 115(2)O. The SiMe, groups are twisted g(3)” away from the reference position typified by eclipsed SMe, groups when viewe along the Si- - -Si axis and one Si-C bond of each SiMe, group is anti to the further central C-Si bond. The methyl groups are twisted 14(5)’ away from their staggered referenov positions_ The least-squares fit was not significantly improved by introducing a non-xero tilt angle of the SiMe, groups. Due to the low scabring power of the lithium at3m and to the Li-C bond distance not being very dirferent from the FSi bond distances. the position of the lithium atom could not be determined with high accuracy_ INTRODUCTION

In a recent study it was found (by gasphase electron diffraction (GED)) that the vapour of bis(trimethylsilyl)methyl-lithium, LiCH(SiMe,), consists of monomeric species. A brief report on its structure has been published elsewhere [l] .The diffraction data are the first for a gaseous, organic alkali-metal complex, and the structures a monomeric lithium alI@ with a mono-coordinate environment at Li is without precedent. In the same communication, the structure of crystalline LiCH(SiMe3)2 determined by X-ray mction is *Author

for correspondence.

0022-2860/85/$03.30

Q 1985

-vies

Science

Publishers

B.V.

96

reported. Solid LiCH(SiMe3)2 consists of nearly linear, infiite polymers, having three “molecules” of monomer in the unit cell. The lithium atoms are two-coordinate, and the C, atom of the bridging CH(SiMq), group is thus five-coordinate. Such an arrangement has not previously been found for any other alkali-metal complex At the present moment eleven types of monohapto lithium alkyl skeletal arrangements may be recognized. These are shown in Fig. 1, and are denoted (I)-(XI). Lithium alkyls where the ligand is polyhapto, e.g. as in benzyllithium [2], in which Li and the alkyl group are best regarded as ion pairs with the negative charge of the alkyl group delocalised, are thus excluded. Structural types (I) to (IV) represent monomeric lithium alkyls; (I) is the one derived in the present study. Other Panmples of monomeric lithium alhyls include the crystalline LiCH(SilMe3)2- (pmdeta) @ mdeta = Me,Nand fijC(Fh)S(CH,):S) - (tmeda). (thf) ICW INWe)[~25~~e21 131 [tmeda = MelN(CH1)2NMel, thf = O(CH=),] 141, (II), both with fourcoordinate lithium, the anion [.Li(C(SiMe&),] - of iLi(tbf),] [Li(C(SiMq),),] in whkh Ii 13 two-coordinate and fiiy cqstalhne LiC[51, um, (SiMezPh), -(thf) [S], (IV), in which the lithium atom is probably three coordinate because of a strong interaction between this atom and the ipso carbon atom of one of the phenyl groups. I. .

Li-_C \

II)

N N/

(II) N

lx--c’

-‘-N

‘\

/ N-0

N---yi_C... ---&-G_.=‘_.. /

.

‘I hrllll

W

/

0’1)

\

97

If the alkyl or aryl ligands attached to the lithium atom are less bulky than the ligands of the compounds mentioned above, molecular association generally will take place, even if donor-molecules may be coordinated to lithiurns. Dimers may be electron-precise, i.e., with fourcoordinate C, of the alkyl or aryl group, or they may be electrondeficient, i.e., with five coordinate C,. Examples of the former include the crystalline compounds [Li(CH,PMe*)(tmeda)]* 173, E(CH,SPh)(tmeda)] 2 [S] and m{C(Me)S(CH&S)(tmeda)] I! [4], (V), as well as [Li{C(SiMe,),(C,H,N)}] 2 [9], (‘TU). Examples of the latter include the crystalline compounds [Li(CHISMe)(tmeda)], 181, [Li{&H(&Z&H,) - (tmeda)lz [2] and Efi(CHCH&HAIALi-Br)AOBtA

WI,

‘W-

With even less bulky alkyl groups bonded to the lithium atom, tetramers (e.g., solid LiCH3 [2] and LiCH&H3 [ll], (IX), as well as LiJMe(tmeda),,, 1121 and Li{CH(Me)(CH,),OMe} 1133, (VIII)) or hexamers (e.g., solid Li(C&,) [14], (X)) may be formed Tetramers and hexamers are deltahedral, being based on a tetzahedzal or octahedral assembly of lithium atoms, respectively, which are electron-deficient, with triangular face-bridging alkyl groupsIn this paper we present fu!l details of the GED-structure of the donorfree bis(trimethylsilyl)methyl-lithium, and important geometrical parameters relating to this structure are compared to corresponding ones in related structures studied both experimenUy and theoretically. EXPERIMENTAL

PROCEDURE

The sample of bis(trimethylsilyl)methyl-lithium was prepared from (~Me$i)&HCl and lithium metal in diethyl ether, using the procedure described previously [ 15). The etheral solution was evaporated to dryness and was recrystallized twice from cold the residue of crude LiCH(SWe3)2 (-30°C) n-pentane. It was then distilled, subliming at ca 120°C at lo3 Torr,

ir,to the sample tubes from which the vapour of LiCH(SiMe3)2 could be directly introduced into the GED apparatus. The eleclzon scattering patterns were recorded on Balzers Eldigraph KDG-2 with a nozzle temperature of ca. 140°C. To keep the temperature at a minimum and thus avoid thermal decomposition a nozzle with a wide opening and convergent beam geometry was used TIzis nozzle has been described in some detail in a paper by Ashby et al [ 163 dealing with the structure of his(neopenQl)magnesium. The reservoir pressure may be reduced to 1 Torr using this nozzle, compared ti about 10 Torr for a nozzle with conventional design. The wavelength of the electron beam, calibrated using mction patterns of benzene, was 0.05869 A. Exposures were made with nozzle--plate distances of 50 and 25 cm. In the structure analysis, four 50 cm and five 25 cm plates were used, The data were processed by standard methods [ 171. The complex scattering factors, f’(s), were calculated from analytical representations of the atomic potential [18], using a program written by

98

Yates

[19].

Mole&

kknsities

were modified by multiflying

then by

S/lfLi(S)llfG(S)IIntensity curves obtained for each camera distance were averaged to derive two final average curves with s limits: sti = 2.625 A-‘, s,, = 13.750 A+, 3s = 0.125 A” (50 cm), .s,~ = 4.5 A+, smH = 25 A-‘, As = 0.250 A-l (25 cm).

CHOICE

OF MODEL

Figure 2 shows the experimental radial distribution (RD) curve obtained by Fourier inversion of the experimental, modified intensities. As may be seen from this figure, there seem to be no significant contributiofis from interatomic d?l;tances longer than ca 6 A in the experimental RD curve. A theoretical model consisting of ciimers involving electrorrdeficient Li-C-Li bonding will give rise t-r, an RD curve extending to about 7.5 A before it goes to zero. l+rthermorr-. the paa in the area 3.5-6 A WiLl be larger and much less resolved in the tf,eoreticaJ RD curve calculated for such dimers than the peaks in the same am& of the experimental RD curve. Models involving both monomers and dime% were esamined, and the discrepancy between theoretical and experimenti RD curves became smaller as the proportion of dimers was reduced. The best fit was obtained by not introducing dimers into the theoretical model at aR, and it was thus decided to interpret the experiiental diffraction data in terms of a model consisting of only monomers. A molecular model of monomeric LiCH(SiMe,), is shown in Fig. 3. The following assumptions concerning the geometry of the molecule were made. (i) The projections of the lithium atom and the unique hydrogen atom fall on the bisector of the SiCSi angle. (ii) The C(Sil’Me3)2 moiety has C2 symmetry. (iii) The SiMe, groups have local C,, symmetry, but their threefold 5-y CIc

I

0

1

2

3

4

5

6

7

0

RN)

9

Fig. 2. Experimental radkl distribution (RD) curYe for bis(trimethylsilyl)methyl-lithium, LiCH(SiMe,),, (above), and the residual curve (diff. = exp. RD - theo. RD) corrzsponding to the b& theoretical, monomeric model (below). Artificial damping factor k = 0.0025 62.

99

axes do not necessarily coincide with the central C-Si bonds. (iv) The methyl and their threefold axes are assumed to groups have local Cs, symmetry, coincide with the terminal C-Si bonds. The model may therefore be described by four bond distances (Li-C, terminal CZ-Si, central C,-Si, and C-H), five valence angles (Sic,& C,SiC, SiCLHL, Sic&i, and LiCJ-I?), one tilt angle (T~~~~,) and two torsional ar&es (@l(C,SiC$i’) and &(H&SiC&))_ The tilt angle IS defined as being equal to zero when the threefold axes of the SiMe, groups coincide with the central C-Si bonds. It is taken as positive when the SiMe3 groups are tilted away tirn each other, and both groups are assumed to be tilted an equal angle in the same direction. The SiMe3 torsional angle (I$~) is defined as being equal to zero when one C-Si bond of each SiMe3 group is anti to the remote central C--Si bond. The SiMe, groups are then eclipsed relative to each other when viewed along the Si--- Si axis_ The angle & is taken as positive in the counter-clockwise direction when viewed along the central C-Si bonds from the carbon atom. The methyl tosional angle (I&) is defined as being equal to zero when the methyl group is staggered relative to the SiC(Me):! moiety of SiC(Me)B when viewed along the terminal C-Si bonds from the silicon atom. The sign of this torsional angle is defined analogously to @,_ Refinements The molecular structure was refined by least-squares calculations using a diagonal weight matrix [ 203. Perpendicular amplitude correction coefficients (Kij) were not used during the refinements. Root mean square amplitudes of vibration (Iii values) for distances not involving the lithium atom were either refined or fixed ti values similar to those we previously calculated for CH,(SiMe& [21]_ The I, values for dktance~ involving the lithium atom were either refined or fixed to typical values_ It proved very difficult to refine the two types of C-Si bond distances as dependent param eters, and likewise to refine one of them and the difference between them. The Li< bond distance would then converge vey poorly, and an unreasonably large difference (0.05 A) between terminal and central C-Si bond lengths was obtained with the terminal bonds being the longest ones. Steric considerations indicate that the ce&zal C-Si bonds are the longer ones, as demonstrated in GED studies of CH2(SiMe3)2 [21] and CH~SiMe~)~ [22] and in an X-ray study of Hg(C(SiMe,)& 1231. In the X-ray studies of LiCH(SiMe 3) z - (pmdeta) 131 and CUthfM CWW~edM [S] the terminal C-Si bonds were, however, found to be the longer ones. The difference between the cenixal and terminal CSi bond lengths, A = R(C,-Si) - R(C,-Si), was therefore fixed to the value found for CH,(SiMe& [21] (0,015 A), and the terminal C-Si bond lengths were refmed as independent parameters. The value of the Li-C bond length then converged satisfactorily, but due to the low scattering power of the lithium

100

atom and to the low multiplicity of the Li-C

bond length parameter, this

parameter could only be determined with relatively low arc~ra~y. In the fmal calculations the following refinement scheme was used: Li-C,

C-H,

and terminal C-Si

the difference 0.015 A. All valence angles, exempt the LiCW angle (which was arbitrarily fixed to 109.5”) were refined_ The tilt angle (rs=,,) was refined in some calculations. However, it converged poorly, and being strongly negative correlated to the SiCSi angle, unusually low values for this valence angle were achieved. As there was no significant improvement of the least-squares fit when r&ning the tilt angle or fixing it to positive values, it was tied equal to zero in the final re5nements. Furthermore, both to-.sional angles (Q, and &) were refined. Initial calculations indicated a value of & close to O”, and a value of & close to ZO”, and these were taken as start-values in the final refinements. A number of rookmean-square amplitudes of vibration (lo values) were also re5ned. including those of the bond distances, the lsi.. . si value, the between

the

central

bond distances were refined. fa

and terminal

C--Si

bond

Lsi...c values, most of the 1, . . . c values, some mLf value, and some of the Zsi. . . H V~U~S.

distances

tn

of the Ic . . . Li V~US,

the

ISi- s

In addition to the experimental RD curve, Fig. 2 also shows the residual curve corresponding to the best monomeric model. The agreement between experimental and theoretical RD curves is judged to be reasonably good.

RESULTS

AND

DlSCUSSION

Final independent parameter values together with some of the more important dependent parameter values, and also refined and assumed iii values, are given in Table 1. The neglect of data correlation and the assumptions concerning local C, syrmneky for the Si?vle3groups and the methyl groups and C2 symmetry for the C(SiMe3)2 moiety, imply unrealistically small values for the leastcsquares standard dciriations (us) of the geometrical param eters Because of this feature and also in order to account for the uncertainty in the electron wavelength, these standard deviations have been multiplied by a factor of thIee to obtain “corrected” standard deviations, which are shown in parentheses. With reference to Fig. 2, it may be seen that a model consisting only of monomeric LiCH(SiMe3)z fits the observed diBaction data quite well It seems very unlikely that significant amounts of dimers or higher oligomers are present, as the experimental RD curve does not exhibit significant contributions from distances longer than ca 6 A. Furthermore, several of the refined amplitudes of vibration for long, nonbonded distances would have been unusually large, as a compensation for absent close-lying distances only ‘SDbe produced by a model including dimers or higher oligomers as weJl. As can be seen from Table 1, I, values for such distances are unexceptional. Although there are some small discrepancig

101 TABLE

1

Geometrical

parameters

for monomeric

Parameter

R, (AI

1~

Li-C

2.03(6) l-877(2) 0.015 (ass.) l-892(2) l-108(6) 119_9(1_5)” 111_0(1.2)” 112.3(1-4)’ 115(2)” 109.5” (ass.)

C,-Si Ab C,-Si C-H LSiCSi LCcSiC, rSiGHt LSiCLi LLiCW c 01 o:= %Me, Si- - -Si C; - - -Si (anti) Ci - - - Si (guuche) c;-__ Si (gauche)

9(3)O 14(5)’ 0’ (zzs.) 3_26( 1) 4.86(l) 3.75(3) 3.97(5)

LiCH(SiMe,), Parameter

R,

0.098(S) O-064(3)

c.=. . .c, ct=-• G

O-065(3) 0.084(6)

C,--.c;(AG) c,- - -c; (AG) ct.--c;(GG) C,- - -C; (GG’) CL- --c; (GG’) Si.--Lj

3.11(l) 3_04(1) 6_20(2) 5.57(2) 5.54(2) 3_66(6) 4.27(12) 4_99(12) 3.31(5) 3.72(12) 4_95(8) 4.02(27) 3_54(10) 4.26(30) 4.88(4) 2_51(2)

(A)’

0_087( 24) O-146(39) O-174(66) O-174(66)

ct ---Ci(AA)

Ki --Ct---Li CL-.-Li CL---Li Ct---Li Si---Ht

i

(A)

ILs (W 0_090(24)

O-095(24) 0.131 (z&i O-171(45) O-179(45) 0.347 (an) 0.317(39) 0.222 (ass) O-100(24) 0.253 (an) O-188(39) 0.200 (ass.) O-250 (ass.) 0.200 (a) O-188(39) O-143(15

=Thes.e values were either found by least-squares refinements or fixed to values similar to those calculated for CH,(SiMe,),. be = R(C,-Si) - R(C,-Si) was Tied at C-015 -4, as was found for CH,(SiMe,):. R(C,-Si) was refined as an independent parameter_ =See tent for definition_ The terms anti. gauche, AA, AG. GG or GG’ are explained in ref. 20, for example.

between 4.5 and 6.0 A, we consider that our assumption of monomers only being in the gas jet are justified_ We cannot, however, exclude the possibilie that very small percentages of dimers or higher oligomers were present in the gas jet As far as the geometry of the monomer is concerned (see Fig. 3), the assumptions of local C3 symmetry of the SiMe3 groups and the methyl groups seem justified, as significant deviations from this would have resulted in some of the amplitudes of vibration being abnormally large. As can be seen from Table 1. the I,- values for distances like C, - - -G, C, - - -Cc, and Si- -- H, are unexc~~ptional, and they do not differ significantly from the corresponding values calculated in the study of CH2(SiMe~)2 [21]. As refined I, values for distances between atoms in different SiMe, groups are also similar to conesponding values calculated in the study of CH2(SiMe3)2 (in which the force-field analysis was based upon a model of C, symmeky), the asfllmption concerning C, symmetry of the C(SiMe& moiety seems reasonable. The L+C bond distance of gaseous LiCH(SiMe3)2, 2.03(6) A, is ca 0.33 A longer than the Be-C bond distance in Be(CH& 1243 and ca 0.45 A longer than the B-C bond distance in B(CH& [253, illustrating the effect of

102

Fig_ 3_ Molecular model of monomeric bis(trimethylsilyl)methyi-lithium -Atoms denoted c and t are referred to in test as cent& and terminal,

LiCH(Si(CH,),)z. respectively.

increasing the atomic radiuswith decreasing atomic number in the first period of the Periodic Table_ The Li-C bond length of monomeric LiCH(SiMq jz is, furthermore, not significantly different from the value of the corresponding parameter in monomeric LiCH, as found by moleclllar orbital calculations, 2-00 A (3-21G/3-21G, ab initio) [26]_ Some MNDO-calculations 1261 yielded however a lower value for the Li-G bond distance, 1.82 A. In Table 2, Li-C bond distances in some monohapto lithium alkyls are coliected. As can be seen from the table, the Li-C bond distance found in the present study is significantly smaller than in lithium alkyls where the lithium coordination sphere is expanded relative to the unique monocoordinate environment at Li in LiCH(Si(CH,)& Thus, on formation of the complex with pmdeta, the coordination number of Li increases from 1 in the unsaturated monomeric LiCH(SiMe& to 4 in the complex, and the Li-C bond distance increases by 0.1 A_ A similar effect has been found for Be(CH& 1241, Al(CH& [27], and Mg(CH& 1283 on formation of complexes with electron donors. -4 point of interest is the elongation of the L+C bonds when increasing the coordination number of C, from 4 to 5, the coordination number of Li being fixed, Another noteworthy feature is the longer L&C bonds of solid polymeric LiCH(SiMe& than of the monomeric vapour-

103 TABLE

2

Ls bond distances in some monobapto lithium alkyls (pmdeta = Me,N[CH,l.N(Me)CCH,l.NMe,, tmeda = Me,N[CH, Compound

LiCII(siMe~l. c~i~,
U.-i=1 . . =F

CLiC6Hul,. CX) CLiCEWZBfe3)J -.


] .NMe,,

thf = O)o,)

Coordixmtion

coordination

Li-c

numberof Li

numberof *lCQ

cm

1 4 2 3 4 4

4 4 4 4 4 5

2.03<6) 2.13<5) 2.16(1).2.20:1) 2.12(l) 2.150~8~.2_141<6~ 2.256(6).2.227(77)

2 4 3

4 6 6

2313<7) 2.23M6)_2.279(6?= 22052<6+2_4082(6)h 2.184<3).2.300<4) 2.14(3)-2.27(2)

:

:

ReL

Tilswork.1 3 5 6 7 8 1: il

14 1

aData for gaseous LiCH(Si.Me,),. 1 of Li{C(PhjS(CH,),S)-(tmeda)(thf) (tmeda)],

bData for solid LiCH(SiMe,),. =A similar structure is that [4]. dSimilar structures are those of [Li(CH$Phj[S ] and [Li(C(Me)S(C~),S}-(tmeda)], 141. eSim~arstructures are those of

[Li{C?H,)$X&)-(tmeda)], [2] and {Li(C~H1:~&(LiBr),(OEtz), [IO]. fA similar structure is that of [Li{CH(Me)(CH,),OMe)], [13]. aA similar structure is that of [LiMe] i [ 2]_ hTbe range, within which distances between a particular C, and its three nearest Li neighboxurs are found, is given here.

As in CH#iMe& [Zl] ,steric interactions between the SiMe, groups seem to deform several geometrical parameters which attain values significantly different from those found in unstrained reference compounds. Although certain constraints were put onto the difference between central and terminal C-Si bond iengths, it should be noted that both types of distances are longer than the C-Si bond distance in SiI&Me, l-857(7) W [29]. They are, however, similar to the corresponding bond distances in other strained organosihcon compounds, such as CH$XMe& [21] (l-889(4) and l-874(2) A) or CH(SiMe& [22] (1.888(6) and 1.87312) A). Steric stmin between the SiMe3 groups also results in a deformation of several valence angles (compared to the tetrahedral angle of 109.5”) and torsional dispIacements_ It is interesting to note that, as for CH&%Mq),, the central SiCSi angle is much more deformed (119.0(1.5)“) than the C,SiC, angIes (111.0(1.2)“) or the SiCtHt angIes (112.3(1.4)“). The SiCSi angle is, however, somewhat SnalIer than the corresponding angle in CH,(SiMe,)z (123.2(0.9)“) or in LiCH(SiMe,), - (pmdeta) 133, 124(2)“. It should aho be noted that a rather Iarge value was obtained for the LiCSi angles, lE(2)“. Such a Iarge angle would probably be required to avoid too close contacts between the Iithium atom and the nearest methyl groups This value is in good agreement with the values of the LiCSi angles found in LiCH(SiMe& (pmdeta) [3],112(2) and 117(2)“. ,A.s for CH#iMe&, the SiMe3 groups are close to being eclipsed with res-

104

pect to each other when viewed along the Si- - - Si axis, or close to being staggered to the LiCH group when viewed along the central C-Si bonds. This suggests that 1,2- are more important then 1,3-interactions. The steric. strain arising between the SiMea groups in this way, is relieved not only by an opening of the SiCSi angle, but also by a twist of the methyl groups of 14(5)” from their staggered reference positions. The vibrational amplitudes obtained for the C-Si bond disknces are very similar to those obtained iu the study of CH(SiMe& [22],0.065(2) A, but somewhat larger than those found for CH1(SiMe3)2, 0.058(Z) and O-059(2) A. Notable also is the fact that the vibrational amplitude of the Li-C bond = 0.098(S) A, is much larger than the vibrational amplitudes of distance, I,, the metal-carbon bo-A distances in Be(CH,), [24], 0.055(10) A, and in B(CH& 1251, 0.053(l) A. This may well be due to the larger atomic radius of the lithium atom, but the greater polarity of the Li-C bond than of the H or B-C bonds is probably another important reason. ACKNOWLEDGEMENTS

We thank Siv.iug_ Bagnhild Seip for recording the diffraction Mrs. Snefrid Gundersen for measuring the intensities.

data., and

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