B R I E F C O M M U N I C A T I O N S

CONFORMATION OF CYANOMETHYLPHOSPHINE

g

E. A. Ishmaeva, A. N. Pudovik, and A. N. Vereshchagin

O X I D E S

UDC 539,143.43 + 661,718.1

In a p r e v i o u s p a p e r [1] we d i s c u s s e d the p r o b l e m of r o t a t i o n a round the Csp2 - P bond in o~-eyano- s t y r y l phosphona tes and benzoy l phosphona tes . It was found that the e l e c t r o s t a t i c i n t e r a c t i o n of two s t r o n g d ipoles c an e x e r t an i m p o r t a n t effect on the r e l a t i v e s t a b i l i t y of the c o n f o r m e r s . It s e e m e d i n t e r e s t i n g to us to s tudy the in f luence of this effect i n the p u r e s t a t e . We e x a m i n e d the dipole m o m e n t s of the d i e thy l - and d ipheny l cyanome thy lphosph ine oxides , in which the subs t i t u t i ng C - CN group has a s u b s t a n t i a l d ipole m o m e n t , and at the s a m e t ime does not c r e a t e m a s s i v e s t e r i c h i n d r a n c e , which f inds e x p r e s s i o n in i t s s m a l l " c o n f o r m a t i o n ene rgy" [2]. It is known that s t a g g e r e d c o n f o r m a t i o n s a r e s tab le d u r i n g ro t a t i on a round the C - P bond [3]. F o r this r e a s o n , the m o l e c u l e s of the d i s c u s s e d compounds can ex i s t as the spa t i a l s t r u c t u r e s dep ic ted in F ig . 1.

The v a l e n c e ang les at the t e t r a h e d r a l phosphorus a tom were taken the s a m e as g iven in [1], while in the c y a n o m e t h y l group they were c o n s i d e r e d to be t e t r a h e d r a l . The dipole m o m e n t s w e r e ca l cu l a t ed by the addi t ive s c h e m e , ba sed on the m o m e n t s of the Csp3 - H 0.28 and Csp2 - H 0.70 D bonds [4]. The m o m e n t s of the bonds of the phospho rus a tom ca l cu l a t ed f r o m these da ta a r e g iven in [1]. The m o m e n t of the C - C bond (3.28) was ca l cu l a t ed f r o m the e x p e r i m e n t a l d ipole m o m e n t of p r o p i o n i t r i l e (3.56 D) [5]. When c a l c u - l a t ing the dipole m o m e n t s the m o l e c u l e s a r e o r i e n t e d in space as is shown in F ig . 1. The bonds of the p h o s - p h o r u s a tom make a c o n t r i b u t i o n to the m o m e n t s that i s i ndependen t of the i n t e r n a l r o t a t i on which is e x p r e s - sed by the equa t i ons :

m'~ = m (P = 0) cos 115 ~ -~- 2 [m (C--H) -[- m (C--P)]cos52~ 67~ ' -- m (Csp, -- P) (la)

my' = 0. (lb)

m z' = m (P = 0)sintl5 ~ -}- 2 [m (C--H) -}- m (C--P)] cos52~ 67%0 ' (le)

The c o n t r i b u t i o n made by the d ipo les of the c ya nome t hy l group depends on the r o t a t i o n a round the C - P bond:

rex" = [m (C--CN) -]- 2m (C--H)] cos t09 ~ 28' (2a)

my" = [m (C--CN)sini09~ ' i 2m (C--II) sin t09~ ' cos 60~ ~ (2b)

mz" = [m (C--CN) sin109~ ' -- 2m (C--H) sin t09~ , cos 60 ~ cos (p (2C)

where ~ is the angle between the P - C - CN and C - P = O planes. For the trans-conformation q~=180 ~ and for the gauche it is • ~ Substituting the above-given bond moments in Eqs. (I) and (2), followed by adding, gives the components along the coordinate axes and the resulting moments given in Table I.

R" "Y CN ~' ~y h" 2

Fig. I. Possible spatial structures of the diethyl- and diphenylcyan0methyl- phosphine oxides: i) trans, 2) gauche.

TABLE 1~ Dipole Moments of Studied Compounds (D)

Confor- % 1 Ixz l*calc mation ~x

~ans i 87 0,9 t 0,59 1,96 gauche 11:87 5,61 6,59

V. I. U l ' y a n o v - L e n i n Kazan State U n i v e r s i t y . T r a n s l a t e d f r o m I z v e s t i y a Akademi i Nauk SSSR, S e r i y a K h i m i c h e s k a y a , No. 12, pp. 2790-2792, D e c e m b e r , 1970. O r i g i n a l a r t i c l e submi t t e d J a n u a r y 22, 1970.

O 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.

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T A B L E 2. E n e r g y of E l e c t r o s t a t i c I n t e r a c t i o n ( e r g s / m o l e c u l e ) i

Interacting dipoles

P----O . . . CN P = O . . . CH' P = O . . . CH" Csp s - P . . . CN

C~p~- -P . . .CH '

C~p~- -P . . .C t t "

C]p3--P �9 �9 �9 CN

Csp~ --P" . . . . CH'

t r a i l s

0,2i6 --0,036 --0,036 --0,071

0,013

0,008

--0,07t

0,008

gauche

0,283 --0,04i --0,029 --0,068

0,008

0,014

--0,06t

0,013

Interacting dipole,,

C ~ p s - - P ,

C~p, - -P .

C~p~ --P .

C~p~ --P .

C~p~ --P.

C~p, --P .

CSp,--P �9

trans

�9 CH" 0,013

�9 C N - - 0 , 0 3 6

�9 C H ' 0 , 0 0 6

�9 CH" 0,004

�9 C N --0,036

�9 CH' 0,004

�9 CH" 0,006

gauche

0,014

--0,039

0,004

0,007

--0,030

0,007

0,007

T A B L E 3. C o e f f i c i e n t s of C a l c u l a - t ion Equa t ions and E x p e r i m e n t a l D ipo le M o m e n t s of C o m p o u n d s

O

I1 R2P - CH2CN

t:[ ~ 7

I C~H5 8,484 I 0,t80 CeH5 10,064 I 0,t95

P0, cms ~. D

226,404 288,830

3,33 3,76

Due to the e q u a l i t y of the g roup m o m e n t s of C2H 5 - P and C6H 5 - P , the c a l c u l a t e d d ipo l e m o m e n t s f o r the d i e t h y l - and d i - p h e n y l c y a n o m e t h y l p h o s p h i n e ox ides a r e i d e n t i c a l . The e x p e r i - m e n t a l d i p o l e m o m e n t s of the d i e t h y l - and d i p h e n y l c y a n o m e t h y l - p h o s p h i n e o x i d e s a r e r e s p e c t i v e l y equa l to 3.33 and 3.76 D. On the b a s i s of t h e s e v a l u e s , u s i n g the equa t ion # t 2nt + #g 2ng = p2exp, we d e t e r m i n e d the r a t i o of the t r a n s - and g ~ u c h e - c o a f o r m e r s . The amount of the t r a n s - f o r m in d i e t h y l c y a n 0 m e t h y l p h o s p h i n e oxide i s 74%, and in d i p h e n y l c y a n o m e t h y l p h o s p h i n e ox ide i t is 66%, i . e . , e n e r g e t i c a l l y i t i s m u c h m o r e f a v o r a b l e .

The d i f f e r e n c e in the f r e e e n e r g i e s of the g a u c h e - and t r a n s - c o n f o r m e r s , c a l c u l a t e d f r o m the e x p e r i m e n t a l da ta , t ak ing t h e i r

s t a t i s t i c a l we igh t s into account , i s 1�9 k c a l / m o l e fo r d i e t h y l c y a n o m e t h y l p h o s p h i n e oxide and 0.80 k c a l / m o l e f o r d i p h e n y l c y a n o m e t h y l p h o s p h i n e ox ide . The d i s c r e p a n c y in t h e s e v a l u e s fo r the two compounds t e s t i f i e s to the fac t that an i n c r e a s e in the s i z e of the r a d i c a l a t t a c h e d to the p h o s p h o r u s a t o m c a u s e s s o m e s t e r i c h i n d r a n c e f o r the cyano g roup when it h a s a t r a n s • H o w e v e r , the g r e a t e r s t a b i l i t y of the t r a n s - s t r u c t u r e s p e c i f i c a l l y i n d i c a t e s that the s p a t i a l i n t e r a c t i o n in the g iven c a s e does not p l a y the d e c i d i n g r o l e .

We c a l c u l a t e d the d ipo l e - d ipo l e i n t e r a c t i o n be tw e e n the d i p o l e s of C - CN and C - H, on the one hand, and P = O and C - P , on the o t h e r , f o r the t r a n s - and g a n c h e - c o n f o r m e r s u s i n g the equa t ion

3 0~1~)(m~s) (,~1~)

w h e r e m 1 and m 2 a r e the i n t e r a c t i n g d i p o l e s , and r i s the v e c t o r c o n n e c t i n g them. The d i p o l e s of the C - H bonds w e r e l o c a l i z e d at the con t ac t p o i n t s of the a t o m s [6], whi le the C - P bonds and the C - CN g r o u p s w e r e l o c a l i z e d as d e s c r i b e d in [7]. The c a l c u l a t i o n r e s u l t s a r e g iven in T a b l e 2.

An e x a m i n a t i o n of the r e s u l t s d i s c l o s e s that a de f in i t e r o l e i s p l a y e d by the i n t e r a c t i o n of the s t r o n g d i p o l e s of the p h o s p h o r y l and cyano g r o u p s , which d e s t a b i l i z e the g a u c h e - c o n f o r m a t i o n to a g r e a t e r d e g r e e than the t r a n s . H o w e v e r , i t shou ld be m e n t i o n e d tha t the weak d i p o l e s of the C - H and C - P bonds a l so m a k e a n o t i c e a b l e c o n t r i b u t i o n to the d i f f e r e n c e in the e n e r g i e s , e s p e c i a l l y fo r d i e t h y l e y a n o m e t h y l p h o s p h i n e ox ide . The r e s u l t i n g d i f f e r e n c e in the e n e r g i e s of the g a u c h e - and t r a n s - f o r m s i s 1.30 k c a l / m o l e fo r d i - e t h y l c y a n o m e t h y l p h o s p h i n e ox ide and 1.08 k c a l / m o l e fo r d i p h e n y l c y a n o m e t h y l p h o s p h i n e ox ide . T h e s e v a l u e s a r e in f a i r l y good a g r e e m e n t wi th the a b o v e - g i v e n e x p e r i m e n t a l d a t a . In both c a s e s they a r e s o m e w h a t h i g h - e r (by 20-25%), but a b e t t e r a g r e e m e n t could not be e x p e c t e d in v i ew of the a p p r o x i m a t e n a t u r e of the c a l -

c u l a t i o n s .

A s a r e s u l t , i t i s p o s s i b l e to conc lude tha t the r e l a t i v e s t a b i l i t y of the c o n f o r m e r s i s d e t e r m i n e d m a i n l y by e l e c t r o s t a t i c i n t e r a c t i o n . In th i s connec t ion both the e x p e r i m e n t a l and c a l c u l a t e d d a t a i n d i c a t e a s o m e w h a t s m a l l e r d i f f e r e n c e in the e n e r g i e s fo r d i p h e n y l c y a n o m e t h y l p h o s p h i n e ox ide (by 0.23 and 0.22 k c a l / m o l e , r e s p e c t i v e l y ) � 9 A good a g r e e m e n t of t h e s e v a l u e s shows tha t not only the to ta l r a t i o of the c o n f o r m - e r s , but a l so the e f f ec t on i t o f r e p l a c i n g the r a d i c a l s a t t a c h e d to the p h o s p h o r u s a tom, a r e e x p l a i n e d by e l e c t r o s t a t i c f a c t o r s .

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EXPERIMENTAL

The studied compounds had the following constants: diethylcyanomethylphosphine oxide, bp 125-125.5 ~ (0.77 mm), n~ 1.4790; diphenylcyanomethylphosphine oxide, mp 144 ~ [8].

The dipole moments were determined in benzene solution at 25 ~ as described in [I ]. In Table 2 are giventhe coefficients of the calculation equations and the resulting data.

The authors wish to thank G. E. Yastrebov for supplying the cyanomethylphosphine oxides.

CONCLUSIONS

i. The dipole moments of the diethyl- and diphenylcyanomethylphosphine oxides were determined, and from them was found the ratio of the conformers.

2. The relative stability of the conformers is determined by electrostatic interactions.

I.

2.

3. 4. 5. 6. 7. 8.

LITERATURE CITED d E. A. Ishmaeva, A. N. Vereshchagin, N. A. Bondarenko, G. E. Yastrebova, and A. N. Pudovik, Izv. Akad. Nauk SSSR, Ser. Khim., 2695 (1970). J. A. Hirseh, in: Topics in Stereochemistry, Vol. I, New York (1968), p. 90. E. Steger, J. Rehak, and H. Faltus, Z. Physik. Chem., 22___9, II0 (1965). L. A. Gribov and E. M. Popov, Dokl. Akad. Nauk SSSR, 145, 761 (1962). O. A. Osipov and V. I. Minkin, Handbook on Dipole Moments [in Russian], Vysshaya Shkola (1965). A, N. Vereshchagin and S. G. Vul'fson, Teor. Eksp. Khim., I, 305 (1965). L. Meyer, Z. Phys. Chem., 8, 28 (1930). A. N. Pudovik, G. E. Yastrebova, L. M. Leont'eva, T. A. Zyablikova, and V. I. Nikitina, Zh. Obshch. Khim., 3__99, 1230 (1969).

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