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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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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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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RECEIVED July 7, 1981.
Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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