dTr-P~ r INTERACTION WITH

NITROGEN

IN BONDS

OF

SILICON

AND PHOSPHORUS

A. N. E g o r o c h k i n , N. a n d S. Y a . K h o r s h e v

S.

Vyazankin,

UDC 541.57+546.28+546.17-18

P r e v i o u s l y we had p r o p o s e d a method for estimating the capacity of substituents to undergo d~r-p7r interaction, which was b a s e d on an analysis of the stretching vibration frequencies (v) of the S i - H bond in substituted silanes [1, 2]. In t r i - , di- and monosubstituted silanes (the substituents of which do not f o r m d r - P T r bonds with silicon) the values of v are related to the induction constants of the substituents by the l i n e a r equations: v =23.1Zcr* + 2112, v = 2 3 . 1 Z a * + 2121 and v =23.1Zcr* + 2132. In silanes with substituents, entering into d~r- p v interaction, the Vexp have s m a l l e r values when c o m p a r e d with the vcalc f r o m these equations. The differences Av a r i s e as a r e s u l t of a d e c r e a s e in the induction effect under the influence of the effect of the d~ - p ~ interaction, which is aimed in the opposite direction. The values of Av are a m e a s u r e of the effect of the d R -PTr interaction. On this b a s i s we analyzed the d T r - P r conjugation in the bonds of silicon with halogens [2] and elements of the VIB group [3]. In the p r e s e n t paper the study was extended to compounds containing s i l i c o n - n i t r o g e n and s i l i c o n - p h o s p h o r u s bonds (Table 1). The values of Av in compounds 1-3, with one S i - N bond, are close to the Av values in the monochlot o - and monofluorosilanes [2]. (Here AV, i.e., theeffect: of d ~ - p ~ r conjugation, has a tendency to i n c r e a s e with i n c r e a s e in the e l e c t r o n - d o n o r p r o p e r t i e s of the substituents R in the H2N group.) The same as in the case of the f l u o r o - and chlorosilanes [2], the positive charge on the silicon atom d e c r e a s e s with i n c r e a s e in the number of S i - N bonds in the molecule and the diffusivity of its d - o r b i t a l s i n c r e a s e s . This causes a d e c r e a s e in the overlapping between the dTr-orbitals of silicon and the p~r-orbital of nitrogen in the f r a g m e n t S i - N , and a reduction in the value of the d r - P r conjugation, falling to each S i - N bond, with accumulation of these bonds in the molecule. Smaller values of Av in the methylsilanes when c o m p a r e d with the ethyl derivatives are c h a r a c t e r i s t i c (compounds 5, 6 and 4, Table 1). The same as in the case of the alkylchlorosilanes [14], this is caused by the effect of a,(~-conjugation, which is g r e a t e r in the methyl derivatives. H /

~C'~-Si~-X

On i n c r e a s i n g the - I - e f f e c t of a substituent, having a nitrogen atom ~ to the silicon, the positive charge on the silicon atom and the degree of d ~ - p~ interaction should both i n c r e a s e . This is actually obs e r v e d in the silyiazide H3SiN3, in which the a* constant of the N3 group is equal to 2.62, while Av is equal to 45 cm -1. The --NCS group has a l a r g e r - I - e f f e c t than N3, but the d~ - p ~ interaction in H3SiNCS is s m a l l e r than in the silylazide. The r e a s o n for this is a d e c r e a s e in the electron density on the nitrogen atom due to the conjugation ~s~--~=c=~, which competes with the d~r - p u interaction.

The latter p r o c e s s is e x p r e s s e d

/

to a g r e a t e r degree in the compound H3SiN(H){CGHs), where the d r - P r

interaction in the S i - N bond is c o m -

pletely s u p p r e s s e d by the conjugation effect of nitrogen with the phenyl group --~s~--~%~=~n~ . The absence of a / ~ - c h a r a c t e r for the S i - N bond in (CH3)3SiN(H)(C~Hs) was also shown by the NMR method [15]. A s i m i l a r phenomenon is observed in the aminoboranes R1R2B- NR3R4. The B - N bond in the ease where R = Alk has Institute of C h e m i s t r y , Academy of Sciences of the USSR. Translated f r o m Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2609-2611, November, 1970. Original article submitted March 5, 1970.

C 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West I7th 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.

2454

T A B L E 1. S t r e t c h i n g V i b r a t i o n F r e q u e n c i e s of S t - H and V a l u e s of A u , C a u s e d b y d~r - P T r I n t e r a c t i o n No.

7 8 9 t0 tl t2 t3 t4

Compound (CeHs)2NSiHs (CHa)~NSiH3 (i-CaltT)2(HeN)SiH (CH@e[(CH3)2N]SiH (C2H5) ['(C~H~)2NI2SiH (CHs) [(CH@~.NI2SiH [(CH3)~N]sSiH [(C~H~)2Nl~SiIt (C~Ks) (H)NSiH3 N~SiH~ SCNSiHa (C1M(Cft@~N]SiH (C1) [(CH@eN]~SiIt H~PSiH~

Ycalo em-~'~exp, 2t69 ? 2t69 ~ 2t17 ? 2t26 2i37 2t40 2t54

~2180?? 2215~ 22325~ 2260

2207 2165~

Crfl *1

2t40 [4] 2t42 [4]

2005 [51

21t7 [6] 2095 T 21t0 [6] 2tt2 [51

2110!;I

a190 2170 2205 [t01 2240 [~11

2175 [ill 2t68 [~3]

AU, cm-~ 29 27 22 9 42 30 42 44 --t0 45 27 20 32 --3

4fThe o* values of the groups (CHa)2N, (C2Hs)2N and H2N are taken equal to 0.6. Present paper. "~The approximate o* value of the group -N(H)(C6H5), equalto ~1.0, was obtained by us from affanalysis of the function: o* of the -NHR group vs, the - I-effect of R. ~:$ Thee* values of the groups Na(2.62) and SCN- (3.37) are taken from [s]. tJ'J'/It is assumed that the o* of the PH2 group (0.44) is equal to the o* of the P(CH~)2 group [12].

a p a r t i a l double b o n d c h a r a c t e r .

The a d d i t i o n of a r y l g r o u p s to the n i t r o g e n a t o m r e d u c e s the o r d e r of m a g -

n i t u d e of t h i s b o n d due to the c o n j u g a t i o n )B--~%C,H~ [16]. P r e v i o u s l y we h a d o b t a i n e d t h e e q u a t i o n A ~ = - 6 5 . 6 ( ~ p - cr ') + 3, w h i c h l i n k s the A p v a l u e s ( c h a r a c t e r i z i n g the e f f e c t of dTr -PTr c o n j u g a t i o n of the s i l i c o n a t o m with c e r t a i n s u b s t i t u e n t s ) and ~ p - ~ ' , w h i c h c h a r a c t e r i z e the a b i l i t y of t h e s e s a m e s u b s t i t u e n t s to u n d e r g o c o n j u g a t i o n w i t h the b e n z e n e r i n g [17]. T h i s e q u a t i o n , o b e y e d b y a w i d e g a m u t of s u b s t i t u e n t s [3, 17], i s not f u l f i l l e d f o r the d i a l k y l a m i n o g r o u p . The Alk2N g r o u p s h a v e a p - C r ' e q u a l to a p p r o x i m a t e l y - 0 . 8 . C o n s e q u e n t l y , in c o m p o u n d s of the H3SiN(Alk)2 t y p e , f r o m the e q u a t i o n i t c o u l d b e e x p e c t e d that the v a l u e s of A~ w i l l b e a p p r o x i m a t e l y 50 c m -1, w h e r e a s the e x p e r i m e n t a l l y o b s e r v e d v a l u e s of A~ a r e h a l f of t h i s . (We w i l l m e n t i o n t h a t an Alk2N g r o u p , a t t a c h e d to a p h o s p h o r u s a t o m , a l s o t a k e s a c o m p a r a t i v e l y w e a k p a r t i n the dTr - P ~ r c o n j u g a t i o n [18].) T h e s e f a c t s h a v e the f o l l o w i n g e x p l a n a t i o n . The d i a l k y l a m i n o g r o u p , a s a s u b s t i t u e n t in the a r o m a t i c r i n g , h a s a s m a l l - Z - e f f e c t (~* ~ 0.6), b u t an e x c l u s i v e l y h i g h c a p a c i t y f o r c o n j u g a t i o n . C o n s e q u e n t l y , in s i l a n e s of the t y p e Alk2NSiH 3 a n d (Alk2N)Si(Alk)2H the r o l e of the p , c r - c o n j u g a t i o n e f f e c t A,k~N~--Si@ii s i . m p e r t a n t (which c a n a l s o b e e x p r e s s e d b y the s t r u c t u r e Alk2N + = SiH2H- ). The e f f e c t of d ~ - p ~ c o n j u g a t i o n , w h i c h l o w e r s the - I - e f f e c t of the Alk2N g r o u p , f a c i l i t a t e s p , a - c o n j u g a t i o n . The p , a - c o n j u g a t i o n e f f e c t i s f o r m a l l y e q u i v a l e n t to the i n c r e a s e in the e l e c t r o n - a c c e p t o r c h a r a c t e r of h y d r i d e h y d r o g e n . W h e n c a l c u l a t i n g A ~ t h i s w a s not t a k e n into a c c o u n t , and f o r t h i s r e a s o n the ~ c a l c v a l u e s and, c o n s e q u e n t l y , the A~ v a l u e s w i l l p r o v e to b e low. The v a l u e s of A ~ in c o m p o u n d s 12 and 13 a r e s h a r p l y l o w e r t h a n c o u l d b e e x p e c t e d on the a s s u m p t i o n of a n a d d i t i v e c o n t r i b u t i o n b y c h l o r i n e [2] and the N(CH3) 2 g r o u p in Av. The p , a - c e n jugation~c~)~S~--s~@-c, i s a l s o r e s p o n s i b l e f o r this. The Si- P bond does not exhibit a v-character, which could be expected if the low capacity of the P(CH3) 2 group to undergo p,~-conjugation [12] is taken into account. This is apparently caused by the low p-character of the unshared electron pair of trivalent phosphorus [19]. CONCLUSIONS depend

The stretching vibration frequencies of the Si - H bond in substituted silanes containing an Si - N bond both on the inductive effect of the substituents and the dTr- p~, p,o-- and cr,~-conjugation effects.

2455

LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

2456

CITED

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