Structure, Conformation, and Basicity in ... - ACS Publications

R. A. SHAW and S. Ν. NABI. Department of Chemistry .... -Francois; Nabi, Syed Nurun; de Ruiter, Barteld; Shaw, Robert. A. J.Chem. Research, 1977, (S)...
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64 Structure, Conformation, and Basicity in Cyclophosphazenes and Related Compounds R. A. SHAW and S. Ν. NABI

Downloaded by UNIV OF SYDNEY on April 3, 2018 | https://pubs.acs.org Publication Date: November 11, 1981 | doi: 10.1021/bk-1981-0171.ch064

Department of Chemistry, Birkbeck College, University of London, Malet Street, London, WC1E 7 H X , England

The results so far achieved by means of basicity studies on cyclophosphazenes can be summarised as follows (1): (i) estab­ lishment of a basicity series in homogeneously substituted compounds, e.g., for the six-membered ring system, N P X , X = NHAlk > Alk > Ph ~NHPh > OAlk > SAlk > SPh > OPh > OCH CF > Cl. A similar, slightly less comprehensive, series was established for the eight-membered ring system, NP4X (ii) proof that in all of the above systems protonation occurred at a ring nitrogen atom; (iii) distinction between positional (but not geometric) isomers in partially substituted derivatives, N3P3XnX'6-n. If N P X is more basic than N P X' , then the geminal isomers NPXnX'6-n (n = 3,4) are more basic than the nongeminal ones; (iv) evaluation of substituent constants αx , αx,, γx and γx, for N3P3XnX'6-n; these represent the basicity contributions to a ring nitrogen atom by substituents X and X' on a phosphorus atom αor γ to this ring nitrogen atari. Selected Substituent Constants (in pK'a units). X αX γX NMe 5.6 2.8 Ph 4.2 2.3 OMe 3.6 1.8 SEt 3.6 1.8 OPh 3.1 1.3 (v) observation that with phosphazenylcyclophosphazenes, e.g., N3P3X4X' (NPPh) two sets of differing, but self-consistent, basicity data are obtained. These were related to the conform­ ations of the NPPh group relative to the local NPN ring segment and an explanation in terms of endo or exo protonation was ten­ tatively offered. The conformational changes have been amply demonstrated. The endo versus exo-protonation is still a hypo­ thesis; (vi) the triphenylphosphazenyl group, Ph PN, was shown to be the most electron-releasing substituent hitherto observed in basicity studies on cyclophosphazenes. 3

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0097-6156/81/0171-0307$05.00/0 © 1981 American Chemical Society Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Hie present paper reports progress i n four areas o f stnacture, conformation and b a s i c i t y : (1) the e f f e c t of the cxuformation o f amino groups on the values of the substituent constants? (2) further studies of phœphazenylc^clophosphazenes ; (3) the e f f e c t of replacing chlorine by f l u o r i n e i n s t r u c t u r a l l y related phosph­ azenes ; (4) the r e l a t i v e b a s i c i t i e s o f r i n g segments P-N-P, S-N-S, C-N-C and As-N-As, and the preferred s i t e s of protonation i n mixed r i n g systems œntaining nitrogen and riiosphorus and/or sulphur, carbon, arsenic. (1) In e a r l i e r studies the substituent constants a and γ were derived by averaging the data from a number of catpounds. Conform­ a t i o n a l e f f e c t s were not considered. The r e s u l t s obtained from b a s i c i t y and X-ray c r i s t a l l o g r a p h i e studies on phosphazenylcyclophosphazenes (ν), together with an increasing number o f c r y s t a l l o graphic studies on other ooqpounds, where i t was sham that i n many cases the conformations of the s u b s t i t u a i t X varied consider­ ably f o r the environments ΞΡ01Χ (nongeminal) and =PX (geminal) focussed attention on the e f f e c t of œnformation on the values o f the substituent constants. Whilst those averaged values had given f a i r to excellent agreement between calculated and observed values f o r most caipounds investigated, there remained seme obvious d i s ­ crepancies. Perhaps the most widely investigated group i n these studies i s the dimethylamino group. For t h i s , the substituent constants α and γ were found to be 5 . 6 and 2.8 respectiv­ ely. Hcwever, on passing from structure (I) to structure (II) (asterisks indicate points of protonation), the value deduced f o r the additional a substituent i s only 4.4. S i m i l a r l y from averaged r e s u l t s α | = 5 . 6 , ΎΌΙΌ = 2.8, a = 5.9 and y = 3.0, (pip = piperidino, pyr - p y r r o l i d i n e ) , but f o r the additional group added on passing from structure (I) t o ( I I ) , the values are lower, viz. a = 4 . 5 and a = 4.9. I t i s known from numerous crystallographic investigations that NMe groups i n the moieties =PCl(NMe ) and = P 0 ^ e ) adopt type I and type I I I conformations respectively. Other secondary amines no doubt behave s i m i l a r l y , and i t can be deduced that i n œnformation (III) the contribution to b a s i c i t y i s less than i n ( I ) .

Downloaded by UNIV OF SYDNEY on April 3, 2018 | https://pubs.acs.org Publication Date: November 11, 1981 | doi: 10.1021/bk-1981-0171.ch064

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(2) Further c r i s t a l l o g r a p h i e and b a s i c i t y data have been obtained for phosphazenylcyclophosphazenes. Both NPPh groups i n g.-N P CU (NPPh ) have been shown by X-ray crystallography t o have type I I conformations (_2). This i s i n l i n e with evidence from *J (PP) œupling constants and disproves the deduction made 3

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Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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SHAW A N D NABI

Cyclophosphazenes

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Compounds

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e a r l i e r from b a s i c i t y d a t a . I t thus l e a v e s open t h e q u e s t i o n whether so f a r u n e x p l a i n e d d i f f e r e n c e s i n b a s i c i t y s h o u l d be a t t r i b u t e d t o d i f f e r e n t s i t e s o f p r o t o n a t i o n ( e x o v e r s u s endo) o r t o d i f f e r e n t conformations o f substituents c o n t r i b u t i n g d i f f e r e n t ­ l y t o t h e p K ' v a l u e s m e a s u r e d [ s e e (3) b e l o w ] . n g . - £ r a n s - N P F (NPMe ) h a s b e e n shown t o h a v e t w o o f i t s s u b ­ s t i t u e n t s i n t y p e I , t h e t h i r d i n t y p e I I c o n f o r m a t i o n (3) · N P M e may h a v e d i f f e r e n t s u b s t i t u e n t c o n s t a n t s i n d i f f é r a i t e n v i r o n m e n t s , but these are always equal t o o r greater than t h a t o f NPPh . (3) A g r e a t d e a l o f b a s i c i t y d a t a i s a v a i l a b l e o n s u b s t i t u t e d c h l o r o p h o s p h a z e n e s . None h a s h i t h e r t o b e e n p u b l i s h e d o n f l u o r o phosphazenes. A p r i o r i , o n e s u r m i s e s a g r e a t e r elecrt^con-^withdrawi n g i n d u c t i v e e f f e c t f o r f l u o r i n e , w h i c h c o u l d be compensated f o r by t h e g r e a t e r p o t e n t i a l o f f l u o r i n e t o b a c k - d o n a t e b y a mesomeric effect. D a t a i s new c o m p a r e d o n s t r u c t u r a l l y r e l a t e d c h l o r o - a n d fluoro-derivatives, e.g., a

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Downloaded by UNIV OF SYDNEY on April 3, 2018 | https://pubs.acs.org Publication Date: November 11, 1981 | doi: 10.1021/bk-1981-0171.ch064

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B a s i c i t y v a l u e s f o r o t h e r s t r u c t u r a l l y r e l a t e d compounds a r e : n g . - N P X * ( N P P h ) X = C I o r F + 0 . 2 ; n g . - N P X * (NMe ) ( N P P h ) X = C I , - 4 . 8 ; X = F , - 4 . 2 ; n g . - N P X (NM3 )„ X = C l , - 1 . 4 ; Χ = F , +0.6. I t c a n b e s e e n t h a t , i n some c a s e s , t h e f l u o r o compounds h a v e a p p r o x i m a t e l y t h e same b a s i c i t y a s t h e i r d i l o r o a n a l o g u e s , w h i l s t i n o t h e r c a s e s , t h e y a r e m a r k e d l y more b a s i c . On t h e a s y e t l i m i t e d d a t a a v a i l a b l e , no s i g n i f i c a n t c o n c l u s i o n s c a n be drawn. R e l a t e d f l u o r o - a n d c h l o r o - c o t ç o m d s h a v e t h e same b a s i c ­ i t y , e x c e p t t h a t when N M e s u b s t i t u e n t s a r e p r e s e n t , t h e y seem t o i n c r e a s e t h e b a s i c i t y o f t h e f l u o r o - a n a l o g u e b y about 0 . 5 p K ' u n i t s p e r NMe g r o u p . (4) A c o n s i d e r a b l e amount o f b a s i c i t y d a t a h a s b e e n p u b l i s h e d o n cyclophosphazenes, i . e . , systems œ n t a i n i n g o n l y P - N - P segments. A s m a l l number o f m e a s u r e m e n t s h a v e b e e n p u b l i s h e d cm m i x e d P - N - S r i n g s y s t e m s , ( 4 ) , e . g . , (NPR ) (NSOR) a n d (NPR ) (NSOR) . I h e s e a n d o t h e r d a t a show t h a t p r o t o n a t i o n o f r i n g s e g m e n t s i s preferred i n the order P-N-P > P-N-S > S-N-S. The v a l u e s o f s u b s t i t u e n t constants on phosphorus and s u l p h u r appear t o be t h e same. 3

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Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Sane b a s i c i t y d a t a on 1 , 3 , 5 - t r i a z i n e s , (NCR) , and mixed P-N-C systems, (NPR ) (NCR), i s now a v a i l a b l e . Our measurements i n d i c a t e t h a t i n t h e mixed r i n g systems p r o t o n a t i o n i s p r e f e r r e d a t P-N-C r a t h e r than a t P-N-P segments, i n l i n e w i t h n.m.r. s t u d ­ i e s by Schmidpeter and E b e l i n g (5). B a s i c i t y d a t a on t h i s mixed r i n g s y s t e n and on 1 , 3 , 5 - t r i a z i n e s has been e v a l u a t e d . α-Values o f s u b s t i t u e n t constants appear t o be s i m i l a r f o r phosphorus and carbon, b u t γ-values seem t o be h i g h e r f o r t h e l a t t e r than t h e former. Using t h e b a s i c i t y d a t a a v a i l a b l e , we can deduce t h a t t h e elœtron-withdrawing power o f t h e groupings d i s c u s s e d i s S0C1 > PC1 > CCI. Hence t h e probable o r d e r o f b a s i c i t i e s o f t h e p a r e n t c h l o r i d e s i s (with t e n t a t i v e calculated v a l u e s where a v a i l a b l e , i n b r a c k e t s ) : (NCC1) , ( -12.4); (NCC1) (NPC1 ); (NCC1)(NPC1 ) , ( -13 t o -17); ( N P C 1 ) , ( -20.4); (NPC1 ) (S0C1),( -24.5); (NPC1 ) (NS0C1) ; (NS0C1) . F i n a l l y , b a s i c i t y v a l u e s f o r two conpounds œ n t a i n i n g t h e As-N-As segments, N A s P h ( +6.0) and N^As^Phe ( +8.6) can new be compared w i t h t h e i r phosphorus analogues, N P P h ( +1.5) and N^P^Phe (+2.2). The much g r e a t e r b a s i c i t y o f t h e a r s e n i c analogues i s noteworthy. The b a s i c i t y o r d e r o f t h e segments As-N-As > P-N-P > C-N-C > S-N-S can now be t e n t a t i v e l y e s t a b l i s h e d f o r those conpounds where o n l y one h e t e r o element a p a r t frcm M t r o g e n i s p r e s e n t i n t h e r i n g system. Literature c i t e d 1. Shaw, Robert A. Z. Naturforsch. 1976, 31b, 641-667 2. Krishnaiah, M.; Ramamurthy, L.; Ramabrahman, P . ; Manohar, H. submitted for publication. 3. Bullen, G.J.; Tam, K.O. personal communication. Faucher, Jean-Paul; van de Grampel, Johan C.; Labarre, Jean-Francois; Nabi, Syed Nurun; de Ruiter, Barteld; Shaw, Robert A. J.Chem. Research, 1977, (S) 112-113; (M) 1257-1294. 5. Schmidpeter, Α . ; Ebeling, J. Chem. Ber. 1968, 101, 3883-3901. 3

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Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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