The molecular structure of monomeric n-silyldimethylamine

Journal of Molecular

Structure

231

Elsevicr Publishing Company, Amsterdam. Printed in the Ncthcrlands.

THE MOLECULAR AMINE

STRUCTURE

C. GLIDEWELL,D.

W.

University

Chemical

Laboratory,

(Received

October 7th. 1969)

ABSTRACT

H. KANKIN,

OF MONOMERIC

A. (3. ROBIE’ITE

Lcnsjield

Road,

AND

Cambridge,

N-SILYLDIMETHYL-

G. M. SHELDRICK CB,’

I E W (England)

’

The molecular structure of monomeric N-silyldimethylamine, SiHaN(CHJ)z, has been determined in the vapour phase by the sector-microphotometer method of electron diffraction. The Si-N and C-N bond lengths are 1.715&-0.004 A and 1.462 + 0.004 A, respectively: the Si-N-C and C-N-C angles are 120.0 + 0.4” and 111. I f 1.2”, respectively.

INTRODUCTION

Electron diffraction studies have shown that trisilylamine’*2 and N-methyldisilylamine3 adopt planar configurations at the nitrogen atom. The Si-N bond lengths found are 1.736 f0.002 A2 and 1.726 +0.003 A3, respectively, considerably shorter than expected. “Short” Si-0 bond lengths and “wide” angles at oxygen have been found in phenyl silyl ether* and methyl silyl ether’. The molecular structure of N-silyldimethylamine, SiH ,N(CH,), , where there is a single’silyl group is thcreforc of interest: an X-ray study6 of this compound in the crystalline state showed that it consisted of cyclic pentamers, whereas its molecular weight in the vapour phase is that expected for the monomer ‘. We have undertaken an electron diffraction study of monomeric N-silyldimethylamine in the vapour phase, the results of which are reported here.

EXPERIMENTAL

Samples of the amine were prepared by the gas-phase reaction of silyl bromide with dimcthylaminc’ and by the reaction of silyl bromide with lithium tctrakis(dimethylamino) aluminate ‘. The compound was purified by repeated fractional condensation in the vacuum system and the purity checked by IR J. Mol.

Structure, 6 (1970)

231-239

C. GLIDEWELL,

232

D. W.

H. RANKIN,

A. G. ROBIETTE,

G. M. SHELDRICK

spectroscopy. Intensities were recorded photographically on the Bakers KD.G2 gas diffraction apparatus at the University of Manchester Institute of Science and Technology9, and transferred to punched paper tapes using an automated JoyceLoebl microdensitometer. The wavelength, determined from the diffraction pattern of powdered thallium(I) chloride, was 0.05569f0.00003 A, and data from 100, 50 and 25 cm nozzle-to-plate distances were used, giving an overall range of I-30 A- ’ in the scattering variable, s. Computations were carried out using the Cambridge University “Titan” computer, and programmes that have been described elsewhere“p I’. The weighting functions, correlation parameters and scale factors are given in Table 1 and the elements of the weight matrix in Table 2. The complex scattering factors of Cox and Bonham” wcrc employed; all distances are ro( 1)’ 3 and no corrections for shrinkage have been applied. TABLE

I

WEIGHTING

FUNCTIONS.

CORRELATION

Height del s (cm) ----___--_100 0.100 --

50

0.200

25

0.400

TABLE

WI) ==

s1

AND

SCALE

s2

SrnSX

.-_~-.

_-

FACTORS

k

P/h

-

-~

1.00

2.50

6.00

8.00

0.4999

0.7 I 5 kO.029

2.80

5.20

12.00

16.00

0.4925

0.984 f0.026

6.80

IO.00

26.00

30.00

0.3350 _--

0.814f0.033 __.-

-_-.

2

ELE.MENTS

WI1 -

Lra

PARAMETERS

OF

THE

WEIGHT

61 -ad/(s1

-%d

I

MATRIX

5

%I”

WII = knJ, -Sd/(h.

-s2)

s2

5

W(j

=

0

igj*tr

WI)

_

--o.s(w,,+W,,)(P/lt),

i-j&l

MOLECULAR

51

s

s1

St is St s sz Sl 5:ma.

MODEL

The molecular model employed, of C, symmetry, was exactly the same as for N-methyldisilylamine3, except that the silyl and methyl groups were interchanged. The model assumes that the CH,- and SiHJ- groups lie on local axes of three-fold symmetry along the C-N and Si-N bonds, respectively, and that there is a plane of symmetry bisecting the C-N-C angle with four close and two distant (C)H . - - H(C) atom pairs. Eight parameters define all the interatomic distances: these were chosen as the C-H, Si-H, C-N and Si-N distances and the H-C-H, H-%-H, C-N-C and C-N-Si angles; the SiH, group was assumed to bc staggered about the Si-N bond with respect to the C-N bonds. J. Mol. Smcfwe,

6 (1970) 231-239

STRUCTUREOFMONOMERIC

N-SILYLDIMETHYLAMINE

233

KEFINEMENT

The experimental radial distribution curve, P(r)/r, is given in Fig. 1. By analogy with N-methyldisilylamine the strong peaks at about 1.09, 1.72 and 2.76 A can be assigned to the C-H, Si-N and Si - - . C distances, respectively. The very strong maximum at about 1.48 A must contain contributions from both the Si-H and

Fig. !. Observed and differcncc radial distribution curve, P(r)/r. Bcforc the Fourier inversion the wcrc multiplied by S. cxp( -0.0025 s~)/(z~,--/~~)(P~-/N).

data

C-N distances in the approximate intensity ratio 1:2. Weaker shoulders at about 2.1 and 2.4 A must then be assigned to the N . - - H(C) and C - - . C distances, respectively. This results in an approximately tetrahedral N-C-H angle and a C-N-C angle of 111”. Since the C-N-Si angle is dctermincd by the relatively well defined C-N, Si-N and Si * * - C distances, the question of planarity at nitrogen rests mainly on the precision with which the C - - - C distance can be determined. The possible independent parameters consisted of the four bonded interatomic distances and their associated amplitudes, four independent angles, the amplitudes of vibration associated with twenty-seven dependent distances, and the scale factors for the three camera distances. The C-H and Si-N distances refined satisfactorily, but it was not found possible to refine both the C-N and Si-H distances simultaneously, probably bccause these two distances are very similar. Consequently, since reported values of r,(l) (Si-H) fall in the range 1.47-1.50 A, the StH distance was fixed at 1.485 A in most refinements. Attempts to refine the H-C-H and H-Si-H angles showed that these parameters were poorly defined as a result of the small contributions of the N - - * H(C) and N - - - H(Si) non-bonded atom pairs to the total molecular J. Mol. Srrucfurc, 6 (1970) 231-239

234

c.

GLIDEWELL,

D. W.

H. RANKIN,

A. G. ROBIETTE,

G. M. SHELDRICK

scattering; these angles were fixed at the tetrahedral value in subsequent refinemcnts. All four bonded amplitudes were refined; the value for u(C-H) is lower than expected. The vibrational amplitudes associated with the C - * - C and C - - - Si non-bonded distances refined satisfactorily as did that of the four-fold C - - - H(C) distance. The N - - - H(C) and N - - - H(Si) amplitudes were constrained to be equal and the three Si - - - H(C) amplitudes were similarly constrained; under these conditions, both sets refined satisfactorily. The amplitudes of the other C - - - H(C) distances could not be satisfactorily refined and they were fixed at typical values14. Amplitudes of H - - - H distances through one angle were fixed at 0.120 A and all other H - - - H amplitudes were fixed at 0.200 A. Finally, a series of refinements was carried out with the Si-H distances fixed at 1.475 A or 1.495 A in order to estimate the effect of fixing this distance on the values of the other parameters. Only very minor changes were observed, the most significant alterations in the independent parameters being 0.002 A in r(C-N) and 0.2” in the C-N-C angle. The final R factors are R, = (U’WU/I’WI)* = 0.15 and R, = (XWj,Uj’/ C,VjjI/2)4 = 0.11, where I is the vector of intensities, U the vector of residuals and W the weight matrix with elements tvjr.

RESULTS

AND

DISCUSSION

The results of the final least-squares cycle are given in Table 3, the observed and difference molecular intensity data are in Fig. 2, and the least-squares correlation matrix is set out in Table 4. The uncertainties quoted in Table 3 arc estimated standard deviations obtained from the analysis of errors in the least-squares refinement, increased in order to include estimated contributions from the uncertainty in the wavelength and from the constraints applied to certain of the parameters. The Si-N distance (1.7 I5 +O.CKMA) may be compared with the corresponding distances in disilylaminc’ ’ (1.725 +0.003 A), N-methyldisilylamine3 (1.726 +_0.003 A) and trisilylamine2 (1.736+0.002 A): it is possible that these differences reflect the enhancement of (p 3 d) n-bonding between nitrogen and silicon as the number of silyl groups decreases. The C-N distance ( 1.462 f 0.004 A) is very similar to that found in dimcthylamine” (r,, 1.466 f 0.005 A; I;( I), 1.455&-0.002 A), N-methyldisilyiamine3 (r,( 1), 1.465 + 0.005 A), and methylamine16 (t,( 1), I.465 &0.002 A). The presence of silyl groups has no significant effect on the value of this parameter. The C-N-C angle (lll.lf1.2”) is also similar to the value in dimethylamine” (rs, 112.2kO.2”; r,(l), 110.6+0.6”). It is interesting to note that in SiH30R compounds, e.g. SiH30CH3’, SiH30C6Hs4 and SiHJOCHO”, the bond lengths and angles of the organic 1. Mol. Sfr&crure.6

(1970)

23 l-239

OF MONOMEKIC

STRUCTURE TABLE

235

N-SILYLDIMETIKYLAMINE

3

MOLECULAR

PARAMETERS:

DISTANCES,

AMPLITUDES

AND

ANGLES _--._

---__-(a)

_

Independent distances

(C-N) (Si-N) I 3 (C-H) r 4 (Si-H) ..--.----__p--

r I I 2

Distance (d)

Amplitude (A )

a (anharmonic constant)

I.462 :t0.004

0.046*0.014

2.00

1.715*0.004 1.084 f0.008 I .485 (fixed)

0.036&0.012 0.064 f0.008 0.086 f0.040 _.__ _ ._

2.00 2.00 2.00 --

(b) Independent angles -.- --.--_ -_-_ L 1 (C-N-C) Li. 2 (C-N-Si) L 3 (H-C-H) L 4 (H-Si-H) -- ------.

-

.____

111.1*1.2 120.0 *0.4 109.5 (fixed) 109.5 (fixed) -.-_-_.--.

--.-_---

(c) Dependent distances -

Distance (/I ) ---.--.---.-.-___----_-.-.-_-_

d I d2 d3 d4 dS d6 d7 d8 d9 d IO

Amplitude (.4 )

(C - -.C) (C.**Si) (N-.-H) (N.--H) (C..+H) (C-*-H) (C-.-H)

2.412*0.016 2.755 f0.004 2.091 *o.O06 2.616rtO.002 3.350*0.014 2.653 rtO.022 3.338 *to.014

0.058 *0.014 0.074 _++0.006 0.124f0.018 0. I24 (fixed) 1 0. I60 (tixcd) 0.222~0.120 0.190 (fixed)

(C-..?I)

3.136*0.010

0.190 (fixed)

(C+-*H) (Si- . - 1.1) dll (Sic-*H) d 12 (Si - - - H) d13 (H.--H) d 14 (H * * * H) d IS (H .. -Ii) d 16 (H . -. H) d 17 (H -. . H) d 18 (H . . . H) d 19 (H - - - H) d20 (H-*-H) d 21 (H . . . H) d22 (H * * * H) d23 (H.--H) d24 (H -a 1 H) d25 (14 . . - H) d26 (H.--H) d27 (H---H)

3.960~~0.004

0.190

2.808 *to.012

0.155*0.044

3.625 &O.OlO 3.21 I k-o.022 1.771 *to.012 2.425 (fixed) 4.165&0.014 3.414kO.018 2.430 *0.028 3.414f0.018

0.155 (fixed) 0.155 (fixed) 0.120 (fixed) 0.120 (flxcd) 0.200 (fixed) 0.200 (fixed) 0.200 (fixed) 0.200 (fixed) 0.200 (fixed) 0.200 (fixed) 0.200 (fixed) 0.200 (fixed) 0.200 (fixed) 0.200 (flxcd) 0.200 (fixed) 0.200 (fixed) 0.200 (fixed)

3.422*0.006 2.651 fO.020 4.138-j=O.O12 4.362*0.014 3.953 kO.024 4.639rtO.016 3.315*0.040 3.765&0.016 4.480fO.016

.-

--.

-----

(fixed)

(d) Dependent angles L 5 (tilt) L 6 (H-C-N) L 7 (H-SCN)

27.8 f2.0 109.5 (fixed)

109.S(fixcd) J. Mol.

Sfructwe.

6 (1970)

231-239

i

0

loo0 -73 -95 -331 -417 -497 371 -5 548 -43 24 -182 -117 81 26 35 113

rl

-73 1000 92 -I -423 403 -227 97 -283 -97 52 -180 1 82 71 109 64

r2

-95 92 1000 176 -103 338 -II3 216 -278 32 161 IS4 56 -16 -41 -22 229

r3

---

E

LEAST SQUARES CORRELATION

v, ;:

m

h

-331 -I 176 IOCO -148 216 -5 152 -152 183 60 431 659 -245 95 157 220


<2

--

-497 403 338 216 -5 1000 -672 253 -890 29 56 270 -57 -56 - 191 -209 30

II 2 371 -227 -113 -5 -I70 -672 1000 147 844 56 351 -150 285 88 372 476 585

DY 1m

I1 I -.-

MATRIX btULTlPLIED

-5 97 216 I52 -174 253 147 1000 -55 94 348 148 151 24 86 I65 532

Ii 3 548 -283 -278 -152 -145 -890 844 -55 1000 -2 170 -283 184 98 331 402 322

Ii 4 -43 -97 32 183 7 29 56 94 -2 1000 9 368 149 49 -5 -2 149

II 5

-

24 52 161 60 - I55 56 351 348 170 9 1000 67 78 -283 184 292 642

I1 6 --.-182 -180 154 431 -121 270 -I50 148 -283 368 67 Icoo 84 -216 -224 -221 109

U7 10

I 56 659 -331 -57 285 ISI 184 149 78 84 1000 -123 486 607 346

-117

II

-I6 -245 I05 - 56 88 24 98 49 -283 -216 -123 1000 II3 I31 77

81 82

III4 --.

26 71 -41 95 -156 -191 372 86 331 -5 184 -224 486 II3 1000 675 322

kl

459

35 109 -22 157 -237 -209 476 I65 402 -2 292 -221 607 131 675 loo0

k2

4 -2 F

459 322 1000

r

m

4 ;

.? ? 9

xz

3 x ; *

p

532 322 149 642 109 346 77

64 229 220 -304 30 585

II3

k3 -

-

237

STRUCTUREOFMONOMERICN-SILYLDIMETHYLAMINE

1OOcm

SOcm

Fig.2. Observed and difference molecularintensity data for camera distances of 100,SOand 25 cm. J. Mol. Sfructwe.

6 (1970) 231-239

C. GLIDEWELL,

238

D. W. H. RANKIN,

A. G.

ROBIETTE,

G.

M. SHELDRICK

moiety are essentially the same as those in corresponding CH,ORcompounds’ 8-20. The Sia-C angle, however, is consistently in the range 120f2”; we find in Nsilyldimethylamine an Si-N-C angle of 120.0 f 0.4”. By contrast with compounds containing (SiHJ)2Ngroups, which seem always to adopt planar configurations at nitrogen’*3*11~2’*22, the heavy-atom skeleton shows an appreciable deviation from planarity. The sum of the angles at nitrogen is 351.1-&2.0”, and the Si-N bond is inclined at an angle of 27.8f2.0” (the tilt angle in Table 2) to the C-N-C plane. The configuration at nitrogen is consistent with the greater basicity of N-silyldimethylamine when compared with disilylamino derivatives’.

ACKNOWLEDGEMEtlTS

We arc grateful to Professor D. W. the provision of experimental facilities, to Laboratory for the provision of excellent Cambridge for a Denman Baynes Research for a maintcnancc grant (to D.W.H.R.).

J. Cruickshank and Dr. B. Beaglcy for the Cambridge University Mathematical computing facilities, to Glare College, Studentship (to C.G.) and to the S.R.C.

REFERENCES 1 K. HEDBERG. /. 2 A. R. CONRAD

3 C. 4 5 6 7 8 9 IO

1I I2 I3 14 I5 16 17 I8

GLIDEWELL,

77 (19%) 6491. unpublished results. D. W. Ii. RANKIN, A. G. ROBIETTE AND G. M. SI#ELDRICK,/. Mol. Sfrucrrrre. hJ.

AND

Ch?J?J.

A.

G.

SOC..

ROBIE~,

4 (1969) 215. C. GLIDEWELL, D. W. H. RANKIN, A. G. ROLII~, G. M. SHELDRICK, B. BEAGLEYAND J. M. FREEMAN,Truns. Furuduy Sot., 65 (1969) 2621. C. GLIDEWELL, D. W. H. RANKIN. A. G. Roe~crrr. G. M. SHELDRICK.B. BEAGLEYANU J. M. F-MAN, J. Mol. Structure, 5 (1970) 417. R. RUDMAN. W. C. HAMILTON, S. NOVICK ANDT. D. GOLDFARB.J. Ant. Cheat. Sot., 89 (1967) 5157. S. Su~lsttr AND S. WITZ. /. Am. Chem. Sot., 76 (1954) 4631. C. GLIDE~ELL AND D. W. H. RANKIN, 1. Chem. Sot. (A), in press. B. BEAGLEY, A. H. CLARK AND T. G. HEWITT, J. Chem. Sot. (A), (1968) 658. B. BEAGLEY,A. G. ROBINS AND G. M. S~IELDRICK,J. Chem. Sot. (A), (1968) 3002. D. W. H. RANKIN, A. G. ROLIIETTE, G. M. SI~ELDRICK.W. S. SI~ELDRICK.B. J. AYLETT, I. A. ELLIS AND J. J. MONAGHAN. J. Chenr. Sot. (A). (1969) 1224. H. L. Cox AND R. A. BONHAM, 1. Chenr. Phys.. 47 (1967) 2599. L. S. BARTELL, J. C/tern. Phys., 23 (1955) 1219. S. J. CYVIN, Moleculur Vibrations und Mean Square Amplitudes, Univcrsitetsforlagct, Oslo, and Elscvicr, Amsterdam, 1968. J. E. WOLLRAB AND V. W. LAURIE, J. Gem. Phys., 48 (1968) 5058; B. BEAGLEY AND T. G. HEWW. 7run.s.Furuduy Sot., 64 (1968) 2561. H. K. HIGGINBOTIIAM AND L. S. BARTELL.J. Chem. Phy~., 42 (1965) 1131. C. GLIDEWELL. J. M. FREEMANAND A. G. ROBI-. unpublished results. K. KIMURA AND M. KUBO, J_ C/tern. Phys.. 30 (1959) 151.

J. Mol. Sfrucrure. 6 (1970) 23 l-239

STRUCTURE

OFMONOMERIC

N-SILYLDIMETHYLAMINE

239

AND J. M. ROBERYSON, Acru Crust., 3 (1950) 279. 30 (1959) 1529. G. M. SHELDRICK ANO W. S. SI(ELDRICK, 1. Mol. Structure, 5 (1970)423.

19 T. H. GOODWIN, M. PRZYBYUKA 20 R. F. CURL, JR., J. C/tern. fhys., 21 A. G. Roetm,

22 C. GLIDIWELL,

D. W. H. RANKIN,

A. G. ROBIFZTEAND G. M.

SHELDRICK,

1. Chem.

SW.

(A),

in press. 1. Mol. Strucrure.

6 (1970)23 l-239