THE

d~r-PTr

AND

GROUP

INTERACTION VIB

BONDS

SILICON

BETWEEN

ELEMENTS

A. N. Egorochkin, and S. Ya. Khorshev

N.

S.

UDC 541,124+541.57+546.28

Vyazankin,

The f r e q u e n c i e s of the v a l e n c e v i b r a t i o n s (v) of the S i - H b o n d in the i n f r a r e d s p e c t r a of s u b s t i t u t e d s i l a n e s a r e a s s o c i a t e d b y the l i n e a r e q u a t i o n s v =23.1 E(r* + 2112, u =23.1 Eo-* + 2 1 2 1 , a n d v =23.1 No-* + 2132 with the i n d u c t i o n c o n s t a n t s of the s u b s t i t u e n t s w h i c h c a n n o t f o r m dzr - P ~ r c o n j u g a t i o n in t r i - , d i - , and m o n o - s u b s t i t u t e d s i l a n e s [11. In s i l a n e s w i t h s u b s t i t u e n t s c a p a b l e of a d r -PTr i n t e r a c t i o n with the s i l i c o n a t o m , the Vex p h a v e l o w e r v a l u e s than the u c a l e d e r i v e d f r o m t h e s e e q u a t i o n s . The d i f f e r e n c e A,; = Uealc - Uex_o a r i s e s f r o m t h e d e c r e a s e i n the i n d u c t i o n I - e f f e c t u n d e r the i n f l u e n c e , d i r e c t e d t o w a r d s the p o s i t i v e s i d e , of the dTr-PTr i n t e r a c t i o n . T h u s , the v a l u e s of Au q u a n t i t a t i v e l y c h a r a c t e r i z e the e f f e c t of the dTr - p n i n t e r a c t i o n . The s t r e n g t h of the d~r - P T r b o n d s i s d e t e r m i n e d b y the o v e r l a p i n t e g r a l of the d o r b i t a l of the s i l i c o n a t o m a n d the p - o r b i t a l of i t s p a r t n e r in the b o n d w h i c h is c a p a b l e of dTr - P ~ r c o n j u g a t i o n . T h e o v e r l a p i s o n l y e f f e c t i v e when the d - and p - o r b i t a l e n e r g i e s a r e c l o s e in v a l u e ; the d - o r b i t a t s , h o w e v e r , n o r m a l l y h a v e a h i g h e r e n e r g y . T h e p r e s e n c e of a p o s i t i v e c h a r g e on the s i l i c o n a t o m which c a n l o w e r the e n e r g y of i t s d - o r b i t a l i s e s s e n t i a l [2]. We t h e r e f o r e c o n c l u d e t h a t the e x t e n t of dTr - P z r c o n j u g a t i o n in a s i l a n e m o l e c u l e i s d e t e r m i n e d b y the c h a n g e in m a g n i t u d e and s i g n , u n d e r the i n f l u e n c e of the s u b s t i t u e n t s , of the c h a r g e on the s i l i c o n a t o m . I t w a s on t h i s b a s i s t h a t we p r e v i o u s l y c o n s i d e r e d the dTr -PTr c o n j u g a t i o n in h a l o g e n a t e d s i l a n e s [1]. I t w a s shown t h a t the m a g n i t u d e of the c h a r g e on the s i l i c o n a t o m i s a f u n c t i o n of the i n d u c t i o n e f f e c t of the h a l o g e n s and of the d T r - P z r b o n d s in the m o l e c u l e . In t h i s p a p e r the s t u d y h a s b e e n e x t e n d e d to c o v e r c o m p o u n d s w i t h S i - X b o n d s w h e r e X =O, S, and Se ( T a b l e 1). T A B L E 1. The F r e q u e n c i e s of V a l e n c e V i b r a t i o n s of Si - H B o n d s and the V a l u e s of /x~ P r o d u c e d b y dTr-P~r I n t e r a c t i o n s in S i l a n e s w i t h S i - X B o n d s (X = O , S, and Se) Compound No.

[ Vcalc'

formula

em'l

I my. The value of Av Vexp, cm'li em-t bondf~ a single S i - X

i

I CHsOSiH3 2t88 II (C=HaOe)=(C2Ha)SiH 2172 lII (CHsO)sSiH 22t2 2t82 IV CH~SSiH8 V (AIkS)sSiH 2195 t 2209 $ VI CsHTSeSiH3 22t0 VII CGHsOSiH8 VIII (C6HsO)=(CeHs)SiH 22~0 2287 IX (Cc,H~O)aSiH 2t98 X C6HsSSiH~

21[o [31 2t57 * 22o3 [4] 2160 [51 --2t50 [61 2~09 [71 2193 [81 2185 * 2243 [91 2~00181

38 22 N4~ t7 35

38 8 3 22 ~15 9 t7 17 t5 0

* This wor'k. I"The value of o* was calculated from the average value of o I for the CH38- group of 0.19 [10]. $ Calculated, taking o* of the CsHrSe- group as the same as o* of the cyelo CdI~sSe- group i.e. 2.37 [11].

Institute of Chemistry, SSSR, Seriya Khimicheskaya, i0, 1970.

Academy of Sciences of the USSR. Translated from Izvestiya Akademii Nauk No. i0, pp. 2394-2395, October, 1970. Original article submitted February

9 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

2255

The values of Av in the compounds (I), (IV), and (VI) with one substituent capable of d~ -PTr conjugation indicate that there is a d e c r e a s e in the capacity to f o r m such conjugation through the s e r i e s O > S > Se. We r e a c h e d this conclusion p r e v i o u s l y in studies of the i n f r a r e d [12] and p a r a m a g n e t i c resonance [13] s p e c t r a of other compounds of this type. The accumulation of alkoxy groups in alkoxysilanes (I) to (III) is a c companied by a sharp d e c r e a s e of both the double-bond c h a r a c t e r of a single Si - O bond and of the degree of d v - P r conjugation in the molecule in general. In the sulfur-containing analogs of the alkoxysilanes the accumulation of a n u m b e r of AlkS- groups is accompanied by an insignificant d e c r e a s e in the double-bond c h a r a c t e r of each S i - S bond. In this case the total d ~ - P r conjugation effect in the molecule i n c r e a s e s . These facts are explained as follows: the AlkS- (cr*~ 1.2) and AlkO- (a*~ 1.5) groups have closely s i m i l a r values of the I - e f f e c t and t h e r e f o r e reduce, to approximately the same extent, the electron density at the silicon atom by the induction m e c h a n i s m . The v e r y strong d r - p ~ interaction in CH3OSiH 3 is accompanied by a d e c r e a s e in the positive (or induction of a negative) charge on the silicon atom. This p r o c e s s p r o g r e s sively i n c r e a s e s in di- and trialkoxysilanes. In this c a s e , however, there is an i n c r e a s e in the diffusiveness of the silicon d - o r b i t a l s and a d e c r e a s e in their overlap with the oxygen p - o r b i t a l s which is apparent as a drop in the dTr -P~r conjugation in the molecule. In CH3SSiH 3 the d~ - P r conjugation is a l m o s t half that in CH3OSiH 3. T h e r e f o r e , with an accumulation of AlkS- groups in the molecule the basic effect is found to be the I - e f f e c t of these groups. The positive charge on the silicon atom and the d r -PTr conjugation in the molecule i n c r e a s e as a result. We should note that a c o m p a r i s o n of these r e s u l t s with those for halogenated silane in [1] indicates a close analogy in the c h a r a c t e r of the Av change in alkoxy- and fluorosilanes and in thioalkoxy- and chlorosilanes. The values of Av in CcHsXSiH 3 compounds are lower than in the corresponding CH3XSiH 3 silanes, where X =O, S. We have p r e v i o u s l y d e r i v e d an equation Av = - 6 5 . 6 ( a n - - a ' ) + 3, which a s s o c i a t e s the values of Av c h a r a c t e r i z i n g the effect of the d r -PTr conjugation of a silicon atom with certain substituents with (an a ' ) , which c h a r a c t e r i z e s the capacity of these substituents to couple with a benzene ring. The equation is valid for methoxy- and thiomethoxy- groups ( a n - a ' = - 0 . 2 4 ) in CH3OSiH 3 and CH3SSiH 3 but not for the pheno x y - g r o u p (C~n- ~r' = - 0 . 4 1 ) in phenoxysilane. This indicates the partial inhibition of the d~ - p ~ interaction by the m o r e effective conjugation with the phenyl group -

CsHs~X--SiH a where X =O, S. In contradistinction to the alkoxysilanes, the accumulation of a number of C6H50- groups in phenoxysilanes r e s u l t s in an i n c r e a s e in the total effect of the d r -PTr interaction in the molecule. (The situation is analogous in f o r m to that o b s e r v e d in thioalkoxy derivatives.) Two f a c t o r s a r e responsible for this: the d r - P r conjugation in monophenoxysilane is less than in monoalkoxysilanes and the I - e f f e c t of the phenoxy- group is g r e a t e r (a* = 2.38) than of the alkoxy- group (~* ~ 1.5). CONCLUSIONS 1. In AlkXSiH 3 compounds where X =O, S, Se, the capacity of the X substituents to develop d ~ - p ~ interaction with a silicon atom d e c r e a s e s in the o r d e r O > S > Se. 2. In silanes with a few S i - O and S i - S bonds, the dTr - P r the silicon atom. LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

2256

interaction is determined by the charge on

CITED

A . N . Egorochkin, N. S. Vyazankin, S. Ya. Khorshev, T. I. Chernyshevai and O. V. Kuz'min, Izv. Akad. Nauk SSSR, Ser. Khim., 1651 (1970). H . H . Jaffe, J. Phys. Chem., 5_~8, 185 (1954). B. Sternbach and A. G. MacDiarmid, J. A m e r . Chem. Soc., 8__33,3384 (1961). H . W . Thompson, Spectrochim. Acta, 1__66,238 (1960). B. Sternbach and A. G. MacDiarmid, J. Inorg. and Nucl. Chem., 23, 225 (1961). M. S c h m e i s s e r and H. F r o u z a n f a r , Z. Chemic, 7, 254 (1968). E.A.V. Ebsworth, H. J. E m e l e u s , and N. Welchman, J. Chem. Soc., 2290 (1962). C. Glidewell and D. W. H. Rankin, J. Chem. Soc., A.753 (1969). H. K r i e g s m a n , Allgem. u n d P r a k t . Chem., 1_)9, 5 (1968). Yu. A. Zhdanov and V. I. Minkin, C o r r e l a t i o n Analysis in Organic C h e m i s t r y [in Russian], I z d - v o Rostovsk. Un-ta. (1966). M. Charton, J. Organ. Chem., 2_.99, 1222 (1964).

12. 13.

A.N. Egorochkin, S. Ya. Khorshev, A.N. Egorochkin, N. S. Vyazankin, ch. Khim., 38, 396 (1968).

and N. S. Vyazankin, DokI. Akad. Nauk SSSR, 18___55,353 (1969). M. N. Bochkarev, V. T. Bychkov, and A. I. Burov, Zh. Obsh-

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