Mass spectroscopy of organosilicon compounds

Journal of Organometalhc Chemtstry, 63 (1973) 79 83

79

Elsevier Sequoia S.A, Lausanne - Printed in The Netherlands

MASS S P E C T R O S C O P Y OF O R G A N O S I L I C O N C O M P O U N D S . F R A G M E N T A T I O N PATTERNS O F LINEAR O R G A N O P O L Y S I L A N E S

Y A S U H I R O N A K A D A I R A , Y O S H I T E R U K O B A Y A S H I and H I D E K I S A K U R A I

Department of Chemistry, Faculty of Scwnce, Tohoku Umverslty, Aobayama, Sendat (Japan) (Received May 15th, 1973)

SUMMARY

Mass-spectral fragmentation patterns of some lower homologues of hnear polysdanes are discussed. Two main types of fragmentation were observed, type A fragmentation involves elimination of a trimethylsilyl group from a molecular ion followed by a successive loss of a dimethylsilylene unit, while type B fragmentation involves elimination of a neutral fragment through a four-membered transition state to give a "disilene" radical cation.

INTRODUCTION

The mass spectra of various types of organosllicon compounds have been reported recently, and novel lntramolecular migrations of silyl groups appear in the fragmentation processes 1- s. However no investigations on the mass spectroscopic properties of organopolysilanes have been reported to date except for some disilane derivatives 6'7. In contrast to the carbon analogues, some remarkable features have been reported recently for organopolysilanes on thermolysis 8 and photolysis 9-12. In connection with the studies, it seemed desirable to elucidate the mass-spectroscopic behavior of some lower homologues of organopolysilanes, and this paper describes the general fragmentation patterns of linear polysilanes. RESULTS AND DISCUSSION

Fragmentation processes for a hnear polysilane can be classified into two types. Type A fragmentation involves elimination of a trimethylsilyl group from a molecular ion followed by a successive loss ofa dimethylsdylene unit, as shown in Scheme 1. In the case of a straight chain hydrocarbon, this type of fra~gnentation process apparently constitutes a main fragmentation path and makes its mass spectrum a useful tool for structural determination. Type B fragmentation involves the elimination of a neutral fragment through a four-membered transition state as depicted in Scheme 2 In contrast to type A process, this second fragmentation pattern as unique to linear organopolysilanes. A relationship between mass spectroscopic properties and photochemical

Y NAKADAIRA, Y KOBAYASHI, H SAKURA1

80 SCHEME 1 TYPE A FRAGMENTATION Me(SIMe2)n Me.+

~

Me(SiMe2)n.l

)- Me(SiMe2)n.2 +

÷

Example Me(SIMe2)s M e .+

:

role 320 (6.2)

Me(SiMe2)4 +

m/e 247 (3.1)

m* 111.

/

Me(SiMe2)a +

= Me(SiMe2)2+

m/e 189 (16.1) (base peak)

m* 69.6

m/e 131 (6.7)

~) MeSiMe2÷

m/e 73 (4.2)

SCHEME 2 TYPE B FRAGMENTATION Figures m parenthesis are the Z4o values of the corresponding peaks R

R

I

I

R m*

R \+

MeSi--S1Me ~

/

Si--Si

I

I

/

Me3Si

X

Me

\

Me

(v) (I) R = M e , X = M e (II) R = Me, X = SiMe 3 (III) R = Me, X = Si2Me s (IV) R = - (CHz) z, X = Me

Oga)m/e 116 (13.1) (Vb) m/e 116 ( 8 . 7 ) (Vc) m/e 116 ( 8 . 7 ) (Vd)m/e 142 ( 3 . 3 )

reactions has been suggested in recent years 13- 15. Irradiation of an organopolysilane with ultraviolet light has been s h o w n to evolve a silylene 9- t 2, but no significant peaks corresponding to this type of photochemical process were detected. Therefore, direct loss of a dimethylsilylene unit from a molecular ion of a polysilane does not occur, at least as a main path.

Type A fragmentation The fragments p r o d u c e d by the type A path are apparently very abundant, and constitute m a j o r peaks in mass spectra of linear polysilanes, as illustrated by Fig. 1. These processes are confirmed by the presence of some appropriate metastable peaks. A l t h o u g h the mechanisms of these fragmentation steps can be expected to be quite similar to one another, the corresponding metastable transitions are not always observed in all cases F o r example, even in what is formally the same process as the fragmentation of the pentamethyldisilanyl to the trimethylsilyl cation, with oc-

MASS SPECTRA OF ORGANOPOLYSILANES

81

tamethyltrisilane (I), decamethyltetrasilane (II) and dodecamethylpentasilane (III), the corresponding metastable peak is not detected in the case of (I). This may be attributed to either different energy distribution in the fragments or different ion structures. In contrast, for the step from the heptamethyltrisilanyl to the pentamethyldisilanyl cation, the corresponding metastable peak is observed in both (II) and (III). In addition to the fragmentation path A, elimination ofa pentamethyldisilanyl group from a molecular ion (shown by a dotted line in Scheme 1) is indicated by the presence of an appropriate metastable peak. This process gives the peak at (M-131) which can also arise from the path A, involving the elimination of a trimethylsilyl unit (M-73) followed by the ejection of a dimethylsilylene unit (M-73-58). These peaks then constitute the base peaks in the mass spectra of (III) and (II) measured at 25 eV. This suggests that the loss of the disilanyl unit may be rather important in the fragmentation process of a linear permethylated organopolysilane. This may be rationalized on the basis of the fact that a radical or a radical cation derived from a disilane is more stabilized than that from a monosilane 16

15

73

0

116

.,

loo

189 M'204

I

16o

LlS9

,

J

~o

Me3SiSiMe2SiMe3 25 eV

I 260I

1go 116 [131

12 73 0

113~

Me3Si(SiMe2)zSiMe3 M£262 25 eV

if

z6o

2~o

Me3Si(SiMe2)3SiMe 3

15 lo 5 73

oi

16o

~go

260

2go

3bo

Fig 1 Mass spectra of octamethyltnsllane, decamethyltetrasdane and dodecamethylpentasdane

Type B fragmentation The occurrence of formally four-center fragmentation processes (Type B) is supported by the presence of the appropriate metastable peaks, as shown in Scheme 2 The fragments (Va-d) formally correspond to radical cations of compounds having a Si-Si double bond. The rather high abundance of these fragments suggest that these degradation processes are important in the mass spectra of linear polysilanes. Their considerable stabilities m a mass spectrometer, in contrast to their properties in the ground state 17, are interesting. Finally the fragmentation pattern of mass spectrum of dodecamethylpentasilane (III) is shown in Scheme 3, which may be taken as typical of the mass spectra of homologous series of linear organopolysilanes. Related work on cyclic polysilanes and carbopolysilanes will be described in a forthcoming paper.

82

Y NAKADAIRA, Y KOBAYASHI, H SAKURAI

SCHEME 3 Me(SiMe2)s

Me(SiMea )s ÷

Me'+

m/e 320 (7.1)

role 305 (1.3) I I i

t

m* 121.0

Me(SiMe2)4 +

m/e 231 (1.9) Me I

Me(SiMe2 )3 ÷

-

rn/e 189(16.1)

(SiMe2)2 Si=CH2 +

--m,(Me2 S1)3+.

m/e 174 (8.4)

m/e 173 (8.4)

m* 69.6

m* 1 90.9 Me(SiMe2)2 ÷ - . . . .

m/e 131 (6.7) -=

Mea Sl+

i

(Me2 S03 Si=CH2 ÷

m/e 247 (3.1)

1

Me

-

-P- Me2 SISIMe2+"

m/e 116 (8.7)

m*

MeS1S1Me2÷

88.0

m/e 101 (0.3)

~II

role 73 (4.2)

m* 42.0

EXPERIMENTAL

Organopolysilanes (I), (II) and (III) were prepared by methods described in the literature is. A new compound (IV) was prepared during the course of photochemical investigations ~2. Details will be published elsewhere. Mass spectra were taken by Mr. T. Sato with a Hitachi RMU-60 mass spectrometer. ACKNOWLEDGMENT

We are indebted to the Toshiba Silicone C o , Ltd. for a gift of chlorosilanes REFERENCES 1 2 3 4 5 6 7 8

W P Weber, R A Fehx and A K Wdlard, J Amer Chem Soc, 92 (1970) 1420 T H Klnstle, P J Ihrig and E J Goettert, J Amer Chem Soc, 92 (1970) 1780 W P Weber, R A Fehx and A K Wlllard, Tetrahedron Lett, (1970) 907 W P Weber, A K Wlllard and H G Boettger, J Or# Chem, 31 (1971) 1620 E White, V S Tsuboyama and J A McCloskey, J Amer Chem 3o~., 93 (1971) 6340 D B Chambers and F Glockhng, J Chem Soc, A, (1968) 735 H Sakurai, M Kira and T Sato, J Organometal Chem, 42 (1972) C24 H Sakural and A Hosoml, J Orqanometal Chem, 36 (1972) C15, and references oted thereto

MASS SPECTRA OF ORGANOPOLYSILANES 9 10 11 12 13 14 15 16 17 18

M lshxkawa and M Kumada. J Chem Soc D, (1970) 612. M Ishlkawa and M Kumada, J Organometal Chem, 42 (1972) 325 M Ishxkawa, T Takaoka and M Kumada, J Organometal Chem, 42 (1972) 338 H Sakural, Y Kobayashl and Y Nakadalra, J Amer Chem Soc, 93 (1971) 5272 T W Bently and R A W Johnstone, Advan Phys Or9 Chem, 8 (1970) 151 M Ohasht. K Tsullmoto, A Yoshmo and Y Yonezawa. Or9 Mass Spectrom, 4 (1971) 203 M K Hoffman. M M Bursey and R E K Winter, J Amer Chem Soc, 92 (1970)727 H SakuraL m J K Kochl (Ed). Free Radwals. Vol 2, Wiley. New York. 1973 D N Roark and G J D Peddle. J Amer Chem Soc. 94 (1972)5837 H Gilman and R L Harrell. J Organometal Chem, 5 (1966) 201

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