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Donor-acceptor properties of methylphosphonitriles Mah, Timothy W. J. 1974

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DONOR-ACCEPTOR PROPERTIES OF METHYLPHOSPHONITRILES  by  TIMOTHY W. J . MAH B.Sc.  •  (Hons.) U n i v e r s i t y o f B r i t i s h Columbia, 1972,  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE  REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  i n t h e Department of Chemistry  We a c c e p t t h i s required  THE  t h e s i s as conforming t o t h e  standard  UNIVERSITY OF BRITISH COLUMBIA June, 1974  In presenting  t h i s thesis in p a r t i a l f u l f i l m e n t of the requirements f o r  an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make i t freely a v a i l a b l e f o r reference and study. I further agree that permission for extensive  copying of t h i s thesis  for scholarly purposes may be granted by the Head of my Department or by his representatives.  It i s understood that copying or p u b l i c a t i o n  of t h i s thesis f o r f i n a n c i a l gain shall not be allowed without my written permission.  Department of  o  ~~Jby,^  The University of B r i t i s h Columbia Vancouver 8, Canada  ABSTRACT  The m e t h y l p h o s p h o n i t r i l e s method o f B e n e s i with I  2  .  and H i l d e b r a n d  w e r e shown b y t h e  t o form outer  The e l e c t r o n i c s p e c t r a o f t h e c o m p l e x e s a s w e l l  as t h e d e t e r m i n e d e q u i l i b r i u m c o n s t a n t s use  of themethylphosphonitriles  This  complexes  i s confirmed  indicatethe  as n-donors toward I  by t h e m o l e c u l a r  structure of N P Me -I , 3  i n w h i c h t h e N - - - I - I u n i t was f o u n d t o be l i n e a r . i n t e r p r e t a t i o n o f both t h e e l e c t r o n i c and p r o t o n of these  complexes i n d i c a t e t h e r e l a t i v e base  of t h e m e t h y l p h o s p h o n i t r i l e s  as being  N P Me  This order  4  4  g  >N P Me 5  5  1 Q  >N P Me . 3  3  6  .  2  3  g  2  The spectra  strengths  i n the order i sexplicable i n  terms o f t h e u - e l e c t r o n d e n s i t i e s a t t h e r i n g  nitrogen  a t o m s , a s a f f e c t e d by t h e homomorphic Tr-system, a n d t h e effects of c-hybridization. The s y n t h e s i s o f i n n e r c o m p l e x e s f o r m e d b y t h e interaction of iodine with the methylphosphonitriles f u r t h e r analogies  show  of pyridine with the methylphosphonitriles.  The s t r u c t u r e o f N ^ M e g l j  was shown t o be ( N ^ M e g l ) I  by t h e e l e c t r o n i c , p r o t o n ,  and v i b r a t i o n a l  +  3  s p e c t r a as  w e l l a s by c o n d u c t i v i t y m e a s u r e m e n t s . The e l e c t r o n i c s p e c t r a and p o l a r o g r a p h s  of the  methylphosphonitrilium  and d i m e t h y l p y r i d i n i u m  show t h a t t h e a c c e p t o r  levels of the phosphonitrilic  rings l i e at higher ring.  energies  than that o f the p y r i d i n e  I n CHC1 , t h e s i m i l a r i t i e s 3  methylphosphonitrilium,  iodides  i n the spectra of the  dimethylpyridinium,  and t e t r a -  alkylammonium i o d i d e s i n d i c a t e t h a t a l l t h e charget r a n s f e r t o c a t i o n processes  are similar, the r e l a t i v e  energies  affected only to a small  of t r a n s i t i o n being  degree by t h e s p e c i f i c c a t i o n i n v o l v e d .  TABLE OF  CONTENTS Page  ABSTRACT  i  TABLE OF CONTENTS  ,  i  L I S T OF TABLES  i  i  V  L I S T OF FIGURES  v i i  ACKNOWLEDGEMENTS  ix  CHAPTER I . INTRODUCTION  1  CHAPTER I I . CHARGE-TRANSFER SPECTRA OF THE IODIDE ION.. 18 II.A. Introduction  18  II.B. Charge-Transfer-to-Solvent II.C. Charge-Transfer  Spectra  19  to Cation Spectra  25  II.D. Charge-Transfer Spectra of the Methylphosphonitrilium Iodides I I . D . l . Experimental  32 34  a. P r e p a r a t i o n o f t h e D i m e t h y l p y r i d i n i u m Iodides 34 II.D.2.  Spectroscopic  II.D.3.  Discussion  Results  37 41  I I . E. P o l a r o g r a p h i c R e d u c t i o n o f M e t h y l p h o s p h o n i t r i l i u m and D i m e t h y l p y r i d i n i u m Iodides  47  CHAPTER I I I . DONOR-ACCEPTOR CHARGE-TRANSFER COMPLEXES.. 51 I I I . A. I n t r o d u c t i o n III.A.l.  Determination  51 of the Positions of  Equilibrium III.A.2. Charge-Transfer  57 Complexes o f I o d i n e  I I I . B . C h a r g e - T r a n s f e r Complexes o f I o d i n e Methylphosphonitriles I I I . B . l . Experimental  ... 62  with 65 69  Page A. D e t e r m i n a t i o n  of E q u i l i b r i u m Constants..  70  B. A s s i g n m e n t o f A b s o r p t i o n B a n d s  72  C. N u c l e a r M a g n e t i c R e s o n a n c e S p e c t r a o f Donor-Acceptor M o l e c u l a r Complexes  78  D. T e m p e r a t u r e E f f e c t s o n t h e C h e m i c a l Shift of N P Me -I 3  E. I I I . B.2.  3  6  2  79  Synthesis of N ^ M e ^ ^  80  Discussion  82  CHAPTER I V . INNER CHARGE-TRANSFER I V . A.  Introduction  IV.B.  Experimental  COMPLEXES  99 99  and R e s u l t s  102  I V . B . l . S y n t h e s i s o f Complexes  102  IV.B.2. N u c l e a r Magnetic  103  IV.B.3. E l e c t r o n i c  Resonance S p e c t r a  Spectra  104  IV.B.4. V i b r a t i o n a l S p e c t r a o f N ^ M e ^  104  IV.B.5. C o n d u c t i v i t y Measurements  106  IV.C.  107  REFERENCES  Discussion  114  L I S T OF TABLES  Table  Page  1  S t r u c t u r a l i n f o r m a t i o n o f p h o s p h o n i t r i l e s ...  4  2  I n f o r m a t i o n o n t h e e f f e c t s o f t h e homomorphic and h e t e r o m o r p h i c i n t e r a c t i o n w i t h c h a n g e s i n ring size  10  3 4 5  6  X a n d dE /dT f o r K I i n v a r i o u s m e d i a ... max max P o s i t i o n s o f t h e c h a r g e - t r a n s f e r bands f o r some 1 - a l k y l - p y r i d i n i u m i o d i d e s  30  S p e c t r o s c o p i c R e s u l t s on t h e d i m e t h y l p y r i dinium, m e t h y l p h o s p h o n i t r i l i u m , and t e t r a n-heptylammonium i o d i d e s .  38  X  and e  21  f o r the dimethylpyridinium  iodides m  CHLl^  4J  -  7  Half-wave p o t e n t i a l s o f d i m e t h y l p y r i d i n i u m iodides  50  8  p K ' v a l u e s f o r some p h o s p h o n i t r i l e s i n *>» "i An i t r o bV *e n z e^"nv v\e  66  D o n o r - a c c e p t o r r a t i o s and o p t i c a l d e n s i t i e s f o r N-P-Me. a n d I -  70  D o n o r - a c c e p t o r r a t i o s and o p t i c a l d e n s i t i e s f o r N P M e and I  71  D o n o r - a c c e p t o r r a t i o s and o p t i c a l d e n s i t i e s for N P Me and I  71  New a b s o r p t i o n b a n d s o f s o l u t i o n s o f I the methylphosphonitriles  77  a  1  9  v\ rw  0  10  4  11  5  12 13 14  4  5  8  1 Q  2  2  with  Summary o f r e s u l t s f o r t h e c o m p l e x e s o f I with the methylphosphonitriles ....  77  Proton chemical complexes  79  s h i f t s of the donor-acceptor  Table  15  Page  Temperature e f f e c t s on t h e c h e m i c a l  shifts of  W ^ " ^ 16  X  8  o f t h e i o d i n e band f o r N - P M e ' I  m a x  3  various  3  6  2  0  in  solvents  81  17  S t r u c t u r a l i n f o r m a t i o n o f some I  complexes  18  Magnitudes o f t h e b l u e - s h i f t f o r t h e complexes of t h e methylphosphonitriles with I  2  ..  85 9  2  2  19 20  Calculated association shifts complexes  f o rthe outer 9 4  Values o f t h e i n t e r a c t i o n parameter o u t e r complexes  f o r the 9  21  M i c r o a n a l y s i s r e s u l t s o f t h e i n n e r complexes  22  Proton chemical and N P ^ M e I 3 3 6 2 0  23  24  c  5 pE ? n  r  e  S  p  e  shifts of the inner  .  5  102  complexes 1  0  3  0  C  t  r  a  l  d  a  t  a  f  °  rN  4 4 P  M e  8  a  n  d  Molar c o n d u c t i v i t i e s o f t h e complexes  1  0  5  l  i 0 7  L I S T OF FIGURES Figure  Page  1  Structures  2  Modes o f i n t e r a c t i o n i n t h e Tr a n d TV systems  3  o f (NPF ) . and ( N P C l ) 2  3  2  2  4  a  7  Schematic arrangement o f t h e TT-electron f o r homomorphic a n d h e t e r o m o r p h i c interactions  4  V a l e n c e b o n d s t r u c t u r e s o f CgH-N a n d  5  Structures  6  S  of N ^ M e g  levels 9 11 12  and N ^ t t e g H  Thermodynamic c y c l e f o r t h e e v a l u a t i o n o f max  23  7  Models o f t h e c h a r g e - t r a n s f e r  24  8  Description of the charge-transfer to cation process of 1-methylpyridinium iodide  29  8a)  Spectrum o f 1-methylpyridinium i o d i d e  30  9  Charge-transfer  E  10.  process  spectrum o f ( N P M e ) I ~ +  3  3  7  D e s c r i p t i o n o f t h e ground and e x c i t e d of ( N P M e ) l " i n CHC1  states  +  3  11  12  13  14  3  7  40  3  46  S t r u c t u r e s o f p h o s p h o n i t r i l e s c o n t a i n i n g 2,3d i o x y n a p t h y l a n d 1,8 d i o x y n a p t h y l side groups  47  P l o t o f t h e c o r r e l a t i o n between t h e i o n i z a t i o n p o t e n t i a l o f t h e donor and t h e energy o f charge-transfer  55  P l o t o f t h e c o r r e l a t i o n between t h e o x i d a t i o n p o t e n t i a l o f t h e donor and t h e energy o f charge-transfer  56  Benesi-Hildebrand triethylamine«I  59  2  p l o t f o r t h e system  Figure >15 16  Page V a r i a t i o n o f AS° w i t h AH° f o r i o d i n e c o m p l e x e s in various solvents  61  C o r r e l a t i o n between t h e b l u e - s h i f t o f t h e i o d i n e band and t h e e n t h a l p y o f f o r m a t i o n f o r some I c o m p l e x e s  63  P l o t of -S_L versus  73  2  17  C  Ac  18  Plot of  P A  f o r N.P.Meo a n d I . . .. 74 4 4 8 2 Q 01 1 "2 Plot of — v e r s u s (^-^ Tfor N P M e a n d I ... 75 C A E l e c t r o n i c spectrum o f N P M e and I i n CH C1 76 1  versus  0  C  c  19 20  f o r N_P~Me, a n d I _ 3 3 6 2  A  A  5  c  c  5  1 Q  2  i n  0  2  2  21  2  P l o t o f energy o f c h a r g e - t r a n s f e r ionization potential  versus 83  22  Structure of N P Me 'I  23  N u m b e r i n g scheme o f N ^ M e g  87  24  T T - l e v e l s o f N P M e 6 a n d t h e 5-membered segment w h i c h r e s u l t s upon r e m o v a l o f one n i t r o g e n f r o m t h e r i n g TT-system  90  25  P l o t o f l o c a l i z a t i o n energy versus  91  26  Plot of AE  27  Structure of the P y I  28  P o t e n t i a l energy diagram o f complex f o r m a t i o n . .  108  29  Structure of N P Me I  110  30  Proposed s t r u c t u r e  3  3  t  3  6  84  2  3  and 8 v e r s u s +  2  4  4  g  ring size  ..  ring size  cation  4  of N ^ M e g l g  97 100  110  ACKNOWLEDGEMENTS  The  a u t h o r would  l i k e t o t h a n k P r o f e s s o r N. L.  Paddock f o r h i s s u p e r v i s i o n and e n c o u r a g i n g  interest  t h r o u g h o u t t h e c o u r s e o f t h i s work. I am g r a t e f u l group  t o t h e o t h e r members o f t h e  f o r t h e i r comments a n d a d v i c e .  g o e s t o M r . M. L e G e y t  A special  thanks  f o r h e l p i n t h e i l l u s t r a t i o n s and  t o Ms. L. Hon f o r t y p i n g t h e o r i g i n a l m a n u s c r i p t . Financial Columbia  support from t h e U n i v e r s i t y  i s acknowledged.  of B r i t i s h  CHAPTER I INTRODUCTION  Phosphonitrilic derivatives, characterized the repeating and  by  u n i t NPX , e x i s t i n a s e r i e s o f c y c l i c  l i n e a r polymers.  2  The c y c l i c m o l e c u l e s  X c a n be a v a r i e t y o f s u b s t i t u e n t s  (NPX ). , 2  n  where  i n c l u d i n g F, C I , B r ,  N R , N , OR, OAr, R a n d A r , o c c u r i n a l a r g e r a n g e o f 2  3  ring sizes  ( i n the (NPF ) 2  n  s e r i e s n ranges  Though a l a r g e v a r i e t y o f p h o s p h o n i t r i l i c are  from 3 t o 1 7 ) . derivatives  known ( f o r r e v i e w s s e e r e f e r e n c e s 1 - 5 ) , f o r t h e  homogeneously s u b s t i t u t e d preparation notation  derivatives  studied  of t h i s thesis the simple " p h o s p h o n i t r i l i c "  c a n be u s e d .  F o r example,  (NPF ) 2  3  w i l l be c a l l e d t r i m e r i c f l u o r o p h o s p h o n i t r i l e meric chlorophosphonitrile are  i nthe  shown i n F i g u r e 1.  respectively.  and  (NPC1 ) 2  4  and t e t r a -  T h e s e compounds  ci  Figure  1.  As polymers are  Structures  of  seen from t h e i r  (NPF ) 2  TT-bonds i s b e l i e v e d  t o be  s i n g l y - o c c u p i e d 2p  o r b i t a l on  o r b i t a l on  the  due  phosphorus.  and  (NPC1 ) . 2  s t r u c t u r e s , the  formally unsaturated.  z  3  The  The  cyclic  formation  of  between  the  to overlap nitrogen  4  with  a  3d  occurrence of such  a  heteromorphic ,-system i n p h o s p h o n i t r i l i c d e r i v a t i v e s is  s u p p o r t e d by  spectroscopic  structural,  thermochemical,  measurements, " 2 0  2 6  m a i n l y on  g e n e o u s l y s u b s t i t u t e d compounds.  The  P-N  homogeneously s u b s t i t u t e d p h o s p h o n i t r i l e s equal i n length, on  the  the  bond d i s t a n c e  s u b s t i t u e n t and  r a n g e 1.50  - 1.-60  A,  on  the  ring  considerably  being  and homo-  bonds i n are  dependent  s i z e , and shorter  normally  are  than  in the  only the  s i n g l e P-N a bond d i s t a n c e  o f 1.77 - 1.78 A /  2 7 - 2 9  Some  s t r u c t u r a l data of p h o s p h o n i t r i l i c d e r i v a t i v e s are presented i n Table I . Later,  i t was f o u n d t h a t some p r o p e r t i e s ,  , , 30-32 _ . , ... . such as base s t r e n g t h , alternated with increase i n nj  ring the  size,  and t h e r i r - b o n d i n g  theory  i n c l u s i o n o f an ( i n - p l a n e )  utilizing  was c o m p l e t e d by  homomorphic 7r-system,  the lone p a i r s o f e l e c t r o n s o f the nitrogens  to form a second The  (in-plane)  coordinate  ir-system.  u s e ,of 3d o r b i t a l s o f p h o s p h o r u s i n  f o r m i n g iT-bonds i s made p o s s i b l e b y t h e e l e c t r o n e g a t i v e s u b s t i t u e n t s on t h e phosphorus w h i c h w i l l the  bring  c o n t r a c t i o n o f t h e 3d o r b i t a l s a n d l o w e r  r- • • energy s u f f i c i e n t l y experimental  action i s very  their  • 33-37 f o r bonding t o occur.  evidence:'in strong.  favour  about  The  of t h i s type of i n t e r -  Those h i g h l y  electronegative  s u b s t i t u e n t s w h i c h w o u l d be e x p e c t e d t o c o n t r a c t t h e m o s t t h e d o r b i t a l s do i n d e e d s h o r t e n the  P-N b o n d s t h e m o s t .  ring nitrogens the with  and  strengthen  Moreover, t h e b a s i c i t y o f t h e  decreases as t h e e l e c t r o n e g a t i v i t y o f  s u b s t i t u e n t s on t h e phosphorus i n c r e a s e , the increased  toward phosphorus.  drawing o f t h e lone p a i r  consistent electrons  Measurements o f i o n i z a t i o n p o t e n t i a l s  Table I Structural  information Ref.  Conformation  Compound  P-N(A)  P-X(A)  PNP(deg)  NPN(deg)"  XPX(deg)  (NPPh )  1.597 (.6)  1.804(7)  122.1(4)  117.8(3)  103.8(3)  (NP ( N M e ) ) 3  •1.588(3)  1.652 (4)  123.0(4)  116.7(4)  101.5(8)  (NPCl )  1.581(3)  .11993.(2)  121.4(4)  118.4(3) ' ,101.4(2)  1.575(2)  1.582(2)  121.9(3)  117.3(3)  .  1.560 (10)  1.521(10)  120.6(8)  119.4(9)  99.3(6)  1.596(5)  1.805 (8)  132.0(3)  119.8(2)  1 0 4 . 1 (2)  Saddle, S  4  ~  D  2 d  11  1.578(10)  1.678(10)  133.0(6)  120.0(5)  103.8 (5)  Saddle, S  4  ~  D  2 d  12  (NPC1 ) IK)  1.570(9)  1.989(4)  131.3 (6)  121.2 (5)  102.8(2)  Tub,  (NPC1 ) (T)  1.559(12)  1.989(4)  135.6(8)  1 2 0 . 5 (7)  103.1(2)  Chair,  (NPF )  4  1.507(16)  1.515(15)  147.2(14)  122.8(10)  (NPC1 )  5  1.521(13)  1.961(8)  148.6(11)  2  3  2  2  2  3  (NP(OPh) ) 2  (NPF ) 2  (NPMe ) 2  3  3  4  (NP(NMe ) ) 2  2  2  4  4  2  4  2  2  98  Flat chair,  C  3  v  ~D  D  9  ~ 2 C  Planar,  S  10  D . 3h  13  4  99.9(9)  Planar,  118.4(8)  102.0 (4)  Planar,  C  14  2 h  D q  15  4 h  16  2  1.584(6)  134.4(5)  1113.6(4)  103.3 (3)  D o u b l e Tub,  (NP(NMe ) )  1.563.(10)  1.669(10)  147.5(7)  120.1(5)  102.9(5)  Related  t o Tub, Sg  6  (NP(OMe) )  1.561(14)  1.576(13)  136.7(10)  116.7(7)  101.3(7)  Chair, C  i  8  2  -  C  17  1.567(8)  2 (  6  8  ~ 3h  6  2  h  7  (NP(OMe) ) 2  3  i  .  18 19  of the (NPFj s e r i e s i n d i c a t e t h a t t h e h i g h e s t ir-system 2 n i s o f t h e homomorphic t y p e .  2 1  I t has a l s o been p o i n t e d  o u t t h a t t h e 4s a n d p a r t i c u l a r l y t h e 4p o r b i t a l s be  lowered  i n energy as w e l l ,  a n d t h a t t h e 4p o r b i t a l 38 3 9  c o u l d a l s o p a r t i c i p a t e i n -rr-bonding. use  o f o n l y 3d o r b i t a l s  '  In fact, the  i n ^-bonding i s probably  o v e r s i m p l i f i c a t i o n s i n c e 3d-4s m i x i n g concept  would  i s likely,  an and t h e  o f d i s c r e t e H i i c k e l - t y p e a - TT b o n d s may be  invalid.  N e v e r t h e l e s s , t h e u s e o f a 3dTr - 2pTT b o n d i n g  model i n e x p l a i n i n g p h o s p h o n i t r i l i c c h e m i s t r y has been extremely use  s u c c e s s f u l and t h i s i s t h e model a d o p t e d f o r  i n this thesis.  A b r i e f o u t l i n e of the bonding  t h e o r y i n v o l v i n g diT - p^ i n t e r a c t i o n f i r s t C r a i g and P a d d o c k  4 0  p r o p o s e d by  i s now p r e s e n t e d , w i t h t h e a s p e c t s  r e l e v a n t t o t h i s t h e s i s being emphasized. comprehensive treatment  F o r a more  o f t h e t h e o r y s e e r e f . 5.  Bonding i n P h o s p h o n i t r i l e s  In p h o s p h o n i t r i l i c d e r i v a t i v e s ,  phosphorus  u s e s 3 s p - h y b r i d o r b i t a l s t o f o r m a-bonds t o t h e 3  neighbouring  r i n g n i t r o g e n s a n d t h e two s u b s t i t u e n t s ,  t h e 3d o r b i t a l s b e i n g u t i l i z e d  f o r Tr-bonding.  The  r i n g n i t r o g e n u s e s 2 s p - h y b r i d o r b i t a l s t o f o r m a-bonds 2  mm  to the adjacent  mm  p h o s p h o r u s atoms and  p a i r of e l e c t r o n s , the remaining used f o r ir-bonding.  The  2-p  t o house the orbital  being  h y b r i d i z a t i o n at phosphorus  n i t r o g e n i s o n l y approximate s i n c e the angles appreciably atoms. i n one  f r o m 109°28' and  There are  two  at the  1  system).  The  i n the other  are c l a s s i f i e d according  t h e N-P^N  t h i s being  they are  i n the  TT  2  2.)  symmetric  the  local  (TTs  2  orbitals  symmetry  of  the  of  of the p l a n a r i t y of the i n the  ^  ring.  system  r e p r e s e n t a t i o n s , whereas t h o s e  s y s t e m b e l o n g t o t h e A, o r B, . 1 1  system used i n c l a s s i f y i n g Figure  plane  n i t r o g e n atomic  p a r t i c i p a t i n g atomic o r b i t a l s or B  antisymmetric  to the r e p r e s e n t a t i o n s  unit, regardless  belong t o the  differ  respective  i n the molecular  p h o s p h o r u s and  p o i n t g r o u p C^,  and  types of d e r e a l i z a t i o n p o s s i b l e :  to r e f l e c t i o n  (TT a s y* s t e m ) ', and  The  120°  the p a r t i c i p a t i n g o r b i t a l s are  with respect  lone  the o r b i t a l s  (The  axis  i s shown i n  Overlap c a l c u l a t i o n s suggest t h a t the  two  d - o r b i t a l s m a i n l y i n v o l v e d i n t h e PTT - dTr i n t e r a c t i o n a r e do o (TT ) and d (TT ) . The TT system i s of the x'-y' s xz a s J  homomorphic t y p e ,  as  i n e.g.  b e n z e n e , w h e r e PTT  i n t e r a c t i o n occurs,  s i n c e i n t e r a c t i o n of the  o r b i t a l w i t h the  2  nitrogen  2sp -hybrid  - pir 3d  x 2  orbitals containing  lone p a i r s are of the  same s i g n .  On  the  _  y 2  the other  (b)  Figure  2. ( r e f . 5)  ._  (a) i n t e r a c t i o n o f t h e d with of  t h e p„ o r b i t a l  an e x o c y c l i c  orbital  at nitrogen.  group i s a l s o  (b) i n t e r a c t i o n o f t h e with  t h e s-p  •r o r b i t a l  a t phosphorus  orbital  y  2  a  orbital  shown.  orbital  at nitrogen.  o f an e x o c y c l i c  A p*  a t phosphorus A 7T  group i s a l s o  S  P  shown.  hand,  t h e 7i system i s o f t h e heteromorphic a  type,  s i n c e i n t e r a c t i o n s o f t h e 3d o r b i t a l w i t h t h e 2p xz z o r b i t a l s of the adjacent  nitrogens  are of the opposite  sign. Using equations  the Hiickel approximation,  secular  f o r t h e two , - s y s t e r n s c a n be f o r m e d , s o l u t i o n s  o f w h i c h g i v e s t h e e n e r g y l e v e l s o f t h e two , - s y s t e m s . If  t h e e l e c t r o n e g a t i v i t i e s o f phosphorus  are r e l a t e d by  = a  t h e Coulomb i n t e g r a l integral the  + 6, w h e r e , f o r e x a m p l e ,  p  is  f o r n i t r o g e n and B i s t h e r e s o n a n c e  f o r phosphorus  interacting with  schematic arrangement  levels  and n i t r o g e n  for various ring  nitrogen,  o f t h e , - e l e c t r o n energy  s i z e s i s a s shown i n F i g u r e  3.  01 he i* d i f f e r e n c e s ID3twesn t h e two TT Sys16ms 3.ire i l l u s t r a t e d i n T a b l e 2. Because (see F i g u r e  of the s i m i l a r valence  bond s t r u c t u r e s  4 ) , many c o m p a r i s o n s h a v e b e e n made b e t w e e n  the m e t h y l p h o s p h o n i t r i l e s  and t h e m e t h y l p y r i d i n e s .  to the e l e c t r o n - r e l e a s i n g e f f e c t s of the methyl  Due  sub-  s t i t u e n t s on t h e phosphorus,, e l e c t r o n d e n s i t y i s concentrated  enough on t h e r i n g n i t r o g e n s  methylphosphonitriles clox^ors •  so t h a t t h e y  of the  a.ct a.s s t r o n g  electron  •2/5  a Av  -6<—©-G—O -@—G-  2/3  +  6  3 (o)  -2/5  °5A v  -C—On =  3  4  2/5  6  5 (b)  Figure  3.  Schematic arrangement o f the TT-electron for  (a) h o m o m o r p h i c a n d  actions.  (b) h e t e r o m o r p h i c  H. M. 0. c a l c u l a t i o n s ,  ( a f t e r C r a i g and  Paddock ). 5  oC = N  levels inter-  CC~ + P  5  Table 2  Symmetric n  Interactions  Antisymmetric Interactions  3  Energy of highest Delocalisation TT-charge energy/electron on N occ. o r b i t a l  3  Energy of highest Delocalisation TT-charge energy/electron on N occ. orbital  3  1.117  0.314  1.379  0.500  0.250  1.518  4  0.500  0.272  1.477  0.500  0.272  1.477  5  0.795  0.289  1.418  0.500  0.280  .1.458  6  0.500,  0.282  1.449  0.500  0.282  1.449  7  0.669  0.286  1.431 |  0.500  0.284  1.444  8  0.500  0.284  1.440  0.500  0.284  1.440 '.  :  i  a)  The delocalisation energies are i n units of S; the orbital energies are  in units of 6, relative to the average Coulomb integral a.  CH  Figure  4.  On  N P Me 3  3  g  the other  (5.03)  respects i s not  42  P  K  are  values  a  of p y r i d i n e  are  s i n c e the  expected to d i f f e r nitrogen  formally involved  lone  i n the  the  homomorphic T r - s y s t e m . '  the  changes i n the  2 1  the  pyridine.  case of  4 0  a t l e a s t i n some  Tr-system are  This  pyridine  whereas  involved  in  is illustrated  r i n g s t r u c t u r e of N ^ t t e g  4 4  41  methyl-  p a i r i n the  those i n the m e t h y l p h o s p h o n i t r i l e s  protonation,  (5.21)  comparable i n aqueous s o l u t i o n .  h a n d , t h e m e t h y l p y r i d m e s and  phosphonitriles  3  V a l e n c e bond s t r u c t u r e s .  ... i n . f a c t , t h e and  / H  3  by  upon  whereas such changes are n e g l i g i b l e i n  CH,  CH,  \  CHg-  N  •CH,  //  132  / \\  CH<  CH,  N  \l-61A N  NI  1.60A CHo  /l.54A  1.70A P  /  CHo  — C H  C H  3  5.  ^ P  7  CH,  CH<  Figure  3  \  S t r u c t u r e s o f N,P.Me a n d N P M e H . 4 4 8 +  R  4  P h o s p h o n i t r i l e s a r e known t o f o r m w i t h hydrogen  •CH,  4  R  adducts  h a l i d e s i n much t h e same way a s t h e  derivatives of pyridine, with protonation occurring a t the r i n g nitrogens  instead of a t the ligands,  for aminophosphonitriles. methylpyridines iodides.  4  2  ,  4  6  ,  4 5  M e t h y l p h o s p h o n i t r i l e s and  form q u a t e r n a r y s a l t s w i t h 4  7  These a l k y l  even  iodides  4 7 , 5 0  alkyl react i n a  s i m i l a r manner a s t h e i o d i d e s o f t h e p y r i d i n i u m t e t r a a l k y l a m m o n i u m 49 . ions with t r a n s i t i o n carbonyls,  48  and  metal  r e s u l t i n g i n d i s p l a c e m e n t o f c a r b o n monoxide.  B I  +  +  M(CO)  > B [M(CO) I]  +  +  6  5  M = Cr,  CO  Mo  P h o s p h o n i t r i l i c d e r i v a t i v e s h a v e b e e n shown t o a c t as  l i g a n d s i n metal complexes, mainly  donors to the metal through donation e l e c t r o n s of the r i n g n i t r o g e n s . and  (NPBr ) 2  (NPMe ) 2  f o r m 1:1  3  of the  For  a-  lone  example,  pair (NPC1 ) 2  3  a d d u c t s w i t h A l B r ^ i n CS,,. "^ 5  reacts with CuCl  4  as  2  i n methyl e t h y l k e t o n e  5 2  to  53 form a complex of  structure  (N^P^MegH)CuCl^,  in  which c o o r d i n a t i o n to the metal occurs  by  through a r i n g n i t r o g e n , w i t h a proton  covalently  bonded t o t h e o p p o s i t e  (NPMe )  (NP(NMe ) ) 2  carbonyl and  2  ring nitrogen.  r e a c t w i t h molybdenum and  4  to y i e l d complexes of the  a-donation  2  tungsten  type  (NPMe ) 2  N P (NMe )g-W(CO) , i n f r a r e d spectra  again  4  4  2  through the  ir-donation. ^ 5  ring nitrogens  T h e r e has  by  rather than  o n l y b e e n one  ,- and hexa-Mo(CO)  3  suggesting  4  t h a t c o o r d i n a t i o n to the metal occurs  donation  4  athrough  brief report  of  54 a 7r-complex,  a compound r e p o r t e d  t o be  T T - ( N P C 1 ) Mo (CO) 2  3  but d i r e c t s t r u c t u r a l evidence i s l a c k i n g . Charge-transfer-to-cation methylpyridinium  i o d i d e s has  spectra of  been r e p o r t e d  by  the  3  Kosower e t . a l .  5  S i m i l a r s t u d i e s were u n d e r t a k e n  5  the methylphosphonitrilium  I n a d d i t i o n , s i n c e no  absorptions  p h o s p h o n i t r i l i c r i n g are observable ultraviolet was  h o p e d t h a t t r a n s i t i o n s due  be  observable  these  processes  occur  p y r i d i n i u m as w e l l as iodides i n order  w i t h any  the  No  spectra  compound w h e r e  simultaneously.  Polarographic  the  methyl-  methylphosphonitrilium  to confirm the r e s u l t s  spectroscopically  charge-  solvent.  r e d u c t i o n e x p e r i m e n t s w e r e p e r f o r m e d on  i n regards  obtained  to the acceptor  levels  inner donor-acceptor  complexes w i t h 1" -  andJ  56  2  phase diagrams suggest t h a t s o l i d occur  in  d e l o c a l i z e d systems. P y r i d i n e i s known as w e l l t o f o r m b o t h  and  the  visible-  not o n l y to  i n an a p p r o p r i a t e  f a r been o b t a i n e d  t h e two  to  also charge-transfer-to-solvent  have thus two  i n the  due  spectra of the m e t h y l p h o s p h o n i t r i l e s , i t  t r a n s f e r - t o - c a t i o n but may  to a s c e r t a i n  i r - l e v e l s o f t h e two d e l o c a l i z e d  the d i f f e r e n c e s i n the systems.  iodides i n order  with  outer Although  state interactions  between a v a r i e t y o f p h o s p h o n i t r i l i c d e r i v a t i v e s  x. t e ta.r a c y a n o e t4h-yu l-i ene  or u h e x a m e t4.1. h y l- bi ew n z e n e , 57-59  t h a t t h e more b a s i c a m i n o p h o s p h o n i t r i l e s  interact  and, with  60,61 xodine their  i n nonpolar  solvents,  i n t e r a c t i o n s i s not  fully  the exact nature clear.  From  their  of  s i m i l a r i t i e s w i t h the d e r i v a t i v e s of p y r i d i n e , the methylphosphonitriles  are expected t o form charge-  t r a n s f e r complexes w i t h a c c e p t o r s .  W i t h the use o f  the m e t h y l p h o s p h o n i t r i l e s  as d o n o r s , i n t e r a c t i o n s can  be d e t e c t e d w i t h s t a n d a r d  c h e m i c a l t e c h n i q u e s s u c h as  p r o t o n n u c l e a r m a g n e t i c r e s o n a n c e as w e l l a s scopically.  F u r t h e r , from the well-known  of complexes  i n v o l v i n g i o d i n e as a c c e p t o r ,  a c t i o n s of the m e t h y l p h o s p h o n i t r i l e s ,  spectro-  properties the  inter-  (NPMe ) , 2  n  n = 3,4,5, w i t h i o d i n e a l l o w a n e l u c i d a t i o n o f t h e r e l a t i v e base s t r e n g t h s o f the s e r i e s o f methylphosphonitriles,  leading to a c l e a r e r view of the e f f e c t s  on b a s e s t r e n g t h o f a - h y b r i d i z a t i o n a s w e l l as electron density.  T h i s method has i t s a d v a n t a g e s i n  that the s p e c i f i c e f f e c t of s o l v a t i o n energies very  TT-  are  s m a l l i f n o t n e g l i g i b l e due t o t h e s m a l l amounts  of charge t r a n s f e r r e d i n the ground s t a t e of the complexes.  In the determination  o f pK  values,  protonation  leads t o l a r g e s o l v a t i o n e f f e c t s the magni-  t u d e s o f w h i c h c a n n o t be d e t e r m i n e d q u a n t i t a t i v e l y T h i s work i s t h e r e f o r e c o n c e r n e d w i t h complexes  of the methylpyridines  phonitriles.  the  and t h e m e t h y l p h o s -  I t s o b j e c t i s t o compare t h e  characteristics  o f t h e b e t t e r known d e l o c a l i z e d s y s t e m w i t h t h a t o f  t h e p h o s p h o n i t r i l e s t o s e e how f a r t h e same  qualitative  c o n c e p t s a r e a p p l i c a b l e , a n d t o draw q u a n t i t a t i v e c o n c l u s i o n s where p o s s i b l e . The  results,  as o b t a i n e d ,  show t h e s i m i l a r i t y  between t h e m e t h y l p h o s p h o n i t r i l e s and t h e d e r i v a t i v e s of p y r i d i n e i n t h a t both form outer  as w e l l as i n n e r  complexes w i t h i o d i n e , though t h e s t r u c t u r e s o f t h e i n n e r complexes a r e d i f f e r e n t .  R e s u l t s from t h e o u t e r  c o m p l e x e s show t h e r e l a t i v e b a s e s t r e n g t h s o f t h e m e t h y l p h o s p h o n i t r i l e s as N ^ M e g explicable  > N^Me^  >  N^Me^  i n t e r m s o f t h e c o m b i n e d e f f e c t s o f a-  h y b r i d i z a t i o n and T r - e l e c t r o n d e n s i t y a t t h e n i t r o g e n s due  t o t h e homomorphic Tr-system.  s p e c t r a and p o l a r o g r a p h i c acceptor lie ion.  The c h a r g e - t r a n s f e r  e x p e r i m e n t s show t h a t t h e  l e v e l s of the methylphosphonitrilium  at higher  energies  ions  than those o f the p y r i d i n i u m  Careful interpretation of the charge-transfer-  to-cation spectra of the methylphosphonitrilium tetra-n-heptylammonium i o d i d e s , i n which t r a n s f e r — t o - s o l v e n t s p e c t r a were obtained,  show i n t h i s  iodides  charge-  simultaneously  instance a greater  between t h e m e t h y l p h o s p h o n i t r i l i u m ammonium  and  The s i m i l a r i t i e s  similarity  and t h e t e t r a a l k y l — i n the energies  of c h a r g e - t r a n s f e r - t o - c a t i o n i n the acceptors  i n d i c a t e that a l l the  processes are s i t i o n being cation.  similar,  three  types  charge-transfer  the r e l a t i v e energies  affected only  of  of  t o a s m a l l d e g r e e by  tranthe  CHAPTER I I  CHARGE-TRANSFER SPECTRA OF THE  I I . A.  IODIDE  ION  Introduction  In s t u d i e s  of the e l e c t r o n i c spectra  of the  i o d i d e i o n i n s o l u t i o n , two p r i m a r y t y p e s o f t r a n s i t i o n s h a v e t h u s f a r b e e n d e t e r m i n e d , t h a t o f (1) c h a r g e ¬ * *. o . u i ..62-67 , transfer to the solvent and (2) c h a r g e - t r a n s f e r to the c a t i o n . separation  f b H  These t r a n s i t i o n s i n v o l v e t h e  o f an e l e c t r o n f r o m t h e a n i o n ,  the excited  s t a t e s c o n s i s t i n g o f an i o d i n e atom a n d a n  electron  which i s t r a n s f e r r e d r e s p e c t i v e l y to a c a v i t y  surrounded  by s o l v e n t m o l e c u l e s o r t o t h e l o w e s t u n o c c u p i e d o r b i t a l of the c a t i o n .  Evidence that the t r a n s i t i o n s  r e s u l t i n an i o d i n e atom i s f o u n d i n t h e of  observation  two s i m i l a r b a n d s s e p a r a t e d by an e n e r g y  o f a p p r o x i m a t e l y 21.8 k c a l m o l e " , t h a t 1  f o r t h e energy d i f f e r e n c e between t h e P 2  difference  calculated and  2  P  4-v, 4. 69 s t a t e s o r* trie i• o d i n e atom.  ..fry.  the 2  P  „ i -+  i o d i n e atom p r o d u c e d may states.  The  than the  e  the  "  be  be  with a 5p —>5p 6s 6  e i t h e r i n the  5  r u l e d out x  c h a r a c t e r i s t i c s of the  t r a n s i t i o n s are  presented before  2  P^  any  or  internal  since the  transition is  i o n i z a t i o n p o t e n t i a l of the The  process  p o s s i b i l i t y of observing  e l e c t r o n i c t r a n s i t i o n s can associated  F^o r  iodide two  energy  greater ion.  types  of  d i s c u s s i o n of  the  p r e s e n t work.  II.  B.  Charge-Transfer-to-Solvent  Spectra  Charge-transfer-to-solvent (abbreviated intense  as CTTS) a r e  absorption  Solvated  200  t r a n s i t i o n s are Law.  The  my  region.  i n aqueous s o l u t i o n , f o r example, region of the spectrum.  i n d e p e n d e n t o f c a t i o n and  CTTS s p e c t r a  have r e c e i v e d  smooth, s t r u c t u r e l e s s ,  bands i n the u l t r a v i o l e t  iodide ions  absorb i n the  spectra  of the  These  obey B e e r ' s  iodide ion, i n p a r t i c u l a r ,  d e t a i l e d a t t e n t i o n because of the  r e s o l u t i o n of the  iodide doublet  (work p r i o r t o  good 1942  is  summarized i n r e f . 7 0 ) .  a r e c h a r a c t e r i z e d by t h e i r  I n g e n e r a l , CTTS s p e c t r a s e n s i t i v i t y t o changes i n  solvent, temperature, pressure, o f added s a l t s . The  and t h e c o n c e n t r a t i o n  ( F o r a r e c e n t r e v i e w s e e r e f . 71.)  energies ^  o f a b s o r p t i o n maxima, E ^ ' max  f o r i o d i d e i n a r a n g e o f s o l v e n t s show e x t r e m e sensitivity.  solvent  T h i s i s t o be e x p e c t e d , s i n c e t h e e x c i t e d  s t a t e o f t h e t r a n s i t i o n i s l e s s p o l a r than t h e ground state.  The o b s e r v e d e f f e c t , h o w e v e r , i s v a r i a b l e i n  magnitude  and d i r e c t i o n , u n d e r g o i n g s m a l l  blue-shifts  i n hydroxylic solvents, but l a r g e r red s h i f t s  i n non-  h y d r o x y l i c s o l v e n t s , a s t h e p o l a r i t y o f t h e medium i s reduced ' 6 3  E  6 4  f o r a CTTS a b s o r p t i o n d e c r e a s e s  max  c  an i n c r e a s e i n t e m p e r a t u r e . dependence o f E  m  a  x  6 3  "  6 6  '  on t e m p e r a t u r e  CTTS s p e c t r a f o r a g i v e n  7 2  ~  with  The l i n e a r  7 5  i s such t h a t t h e  ion i n a particular  c a n be c h a r a c t e r i z e d by t h e s l o p e ,  d  E  m  a  .  x  6  3  solvent -  6  5 A  dT w i t h temperature i s d i a g n o s t i c  marked s h i f t o f E max of t h i s type o f t r a n s i t i o n s i n c e , i n systems i o d i d e i s p a r t o f a complex, i  -i  energy band i s v e r y The  t h e s h i f t o f the: l o w -  -., , s m a l l and s o m e t i m e s  dependence o f E  where  m  a  x  zero.  on p r e s s u r e  i n water i s such t h a t E increases with max  65 f o r iodide  pressure,  Table 3 A  m a x  ( A ) ' AND d E  solvent  m a x  /dT  ( C a l . / d e g . ) FOR K I I N VARIOUS MEDIA  added s a l t (M)  20  H 0 ?  D  temp.  2°  NaCl(0.67)  2° 3 MeOH H  X  . max  20  2261 2251  20  2254  dE  m a x  /dT  32  20  2198  65 74  30  65 132  2240  N H  ref.  11  65  Me OH  2196  133  MeOH  2210  131  2190  65  2170  133  LiCl(O.l)  MeOH  20  EtOH 20  EtOH  2185  10  65  EtOH  2192  131  n-PrOH  2156  133  n-BuOH  2180  131  20  2190  65  iso-PrOH iso-PrOH  LiCl(0.24)  20  2185  65  iso-PrOH  LiCl(l.O)  20  2180  65  20  2458  MeCN  57  65  MeCN  2444  133  MeCN  2461  131  EtCN  20  2494  47-57  65  H  2°  +  MeCN(55%)  20  2308  50  65  H  2° 2°  +  MeCN(84.2%)  20  2372  60  65  +  EtOH(39%)  20  2212  43  65  H  H  2°  +  EtOH(70%)  20  2194  27  65  H  2°  +  EtOH(83.5%)  20  2192  26  65  the p r e s s u r e s e n s i t i v i t y being g e n e r a l l y g r e a t e r that for intramolecular  transitions.  e f f e c t i s observed w i t h the and  A  7 6 , 7 7  similar  a d d i t i o n of a l k a l i  o t h e r m e t a l s a l t s t o aqueous s o l u t i o n s o f  w i t h an  increase  in E  Theoretical  salt  ^ These s h i f t s are 63  d i a g n o s t i c o f CTTS t r a n s i t i o n s .  halides iodide,  upon i n c r e a s i n g the  , ,. 63,64,70*73,78,79 concentration. ' ' ' ' '  than  also  64 '  Treatment  Quantitative  treatments of the process  charge t r a n s f e r to solvent  of  have a t t e m p t e d m a i n l y  to  c o r r e l a t e the v a r i a t i o n s of E solvent  and  temperature.  w i t h changes i n max ^ E a r l i e r t h e o r i e s had  a t t e m p t e d t o c o r r e l a t e dE  /dT  with  entropies  max electrode current the  reactions  theories  a n i o n but  64  t h a t the  and  and  they d i f f e r  71  of s o l v a t i o n .  i n v o l v e the  d e s t i n a t i o n of the refs.  8 0  7 5  All  l o s s of a p e l e c t r o n  by  i n t h e i r d e s c r i p t i o n of  the  e l e c t r o n upon p h o t o e x c i t a t i o n  for detailed discussion).  e j e c t e d e l e c t r o n becomes s o l v a t e d  same s e n s e as  of  c  for electrons  in liquid  The 8 1  (see  concept  i n the 82  ammonia  has  been r e j e c t e d , however, s i n c e m a j o r r e o r g a n i z a t i o n the  s o l v e n t m o l e c u l e s w o u l d be  required  during  the  of  act of l i g h t  absorption.  By f a r t h e m o s t i m p o r t a n t t h e o r e t i c a l  model  was  that f i r s t  all  s u b s e q u e n t m o d e l s b a s e d on i t s m a i n f e a t u r e s .  The  e x c i t e d e l e c t r o n was  discrete,  p r o p o s e d by P l a t z m a n and F r a n c k ,  with  6 7  d e s c r i b e d as m o v i n g i n a  centrosymmetric o r b i t a l defined l a r g e l y  the p o t e n t i a l f i e l d  of those p o l a r i z e d solvent  c u l e s which are o r i e n t e d around the anion. expression  for E  m  a  x  was  by  mole-  The  derived using the f o l l o w i n g  cycle.  X  +  X aq(aq)  aq  B  Emax  X •  ->  Figure  On  X'-  +  e  (aq)  6.  the b a s i s o f t h i s model, which c o u l d  h o w e v e r p r e d i c t a t e m cp e r a t u r e d e pc e n d e n t c  and Symons qualitatively  + aq  proposed a model which  not  E max , S m i t h  (1) p r e d i c t e d  t h e t e m p e r a t u r e dependence o f E  m  a  x  (2) c o r r e l a t e d t h e e n e r g y o f t r a n s i t i o n w i t h t h e of the primary s o l v e n t s h e l l around the anion,  and radius  the  m o d i f i c a t i o n made b e i n g b a s e d o n t a k i n g  into  the  In the  e f f e c t o f the  first  solvent  layer.  account following  f i g u r e t h i s p r o c e s s i s r e p r e s e n t e d b y (a) w h e r e a s (b) r e p r e s eennttss tthhee m o d e l o f C o n n i c k e t . a l .  81  8  G3 i-G3 -ft.Itctron  lot txrt cawrry  \~"/  i n f a n t cgvlt]  8 Q  Elflctron inn** cavity.  Iodine otoat In oriqioal ecitj  Figure  7.  Photochemical studies tant role i n supplying transfer process.  have p l a y e d an impor-  evidence f o r the  Flash photolysis  aqueous s o l u t i o n s o f h a l i d e i o n s  studies  ion.  No a b s o r p t i o n s  as the  i n the  900  my r e g i o n  L a t e r work o n the  i o d i d e i n n e u t r a l and i n the  on tentative dihalide  .  were o b s e r v e d ,  h o w e v e r , w h i c h m i g h t be expected i f s o l v a t e d were formed.  8 3  have l e d t o  i d e n t i f i c a t i o n o fa transient species •  o v e r a l l charge-  photolysis  electrons o f aqueous  alkaline solutions yielded  iodine  presence o f s o l u t e s which scavenged hydrogen o r •  solvated  electrons.  considered  8 5  '  The l a t t e r  8 6  species  was  f o l l o w i n g a d v a n c e s made i n t h e i n t e r p r e ¬  t a t i o n o f t h e r a d i o l y s i s o f aqueous s o l u t i o n s . A series of studies act  involves  9 4  "  formation  1 0 0  have e s t a b l i s h e d  o f an e x c i t e d  '  y J  •  that the i n i t i a l  state which  then  decays e i t h e r v i a a r a d i a t i o n l e s s t r a n s i t i o n t o t h e ground s t a t e o r i n t o a r a d i c a l and s o l v a t e d in close  proximity.  —X aq  aq  ground state  This  II.  > (X ' • + e ) aq aq  excited state  d e s c r i p t i o n i s i n general  the model o f t r a n s i t i o n f i r s t Franck.  Spectra  The  due t o s o l v a t e d  ions, i n p a r t i c u l a r iodide, spectra  t r a n s f e r t o t h e c a t i o n may be o b t a i n e d  with  with  6 7  I n a d d i t i o n t o CTTS s p e c t r a  low  agreement  p r o p o s e d by P l a t z m a n and  C. C h a r g e - T r a n s f e r t o C a t i o n  halide  electron  due t o c h a r g e  i n solvents of  p o l a r i t y where t h e a n i o n a n d t h e c a t i o n one a n o t h e r , r e s u l t i n g i n t h e f o r m a t i o n absorption  spectra  interact of ion-pairs.  i n t h e two c a s e s d i f f e r  environment around t h e h a l i d e  ions  since the  are d i f f e r e n t .  The forward.  concept of i o n - p a i r i n g i s not s t r a i g h t -  The a n a l y s i s o f c o n d u c t i v i t y  data,  1  0  1  ,  1  0  2  . . , 103,104 . k i n e t i c data, e l e c t r o n spin resonance spectra of r a d i c a l a n i o n s , and u l t r a s o n i c r e l a x a t i o n d a t a 1 0 5  made i t n e c e s s a r y t o d e f i n e in solution. Griffiths (1)  a n a Symons  6  -  1  of ion-pairs  Contact ion-pairs  : - Ions, i n contact, solvent  Solvent-shared ion-pairs electrostatically  with  no  molecules. - Pairs of ions  linked  through a solvent  t h i s molecule being part s o l v a t i o n s h e l l of both (3)  0  The f o l l o w i n g d e f i n i t i o n s a r e t h o s e o f  intervening (2)  three classes  1  Solvent-separated ion-pairs  molecule,  of the primary ions.  - Pairs of ions,  linked  e l e c t r o s t a t i c a l l y b u t s e p a r a t e d b y more t h a n one The  molecule.  p o s s i b i l i t y of the formation of d i f f e r e n t types of  ion-pairs  a r e n e c e s s a r y t o r e c o n c i l e e v i d e n c e , e.g.,  conductivity X.-  solvent  4.  4=  and s p e c t r o s c o p y , w h i c h g i v e 4-U  J  4T  •  •  different  4-'  1  1  5  estimates o f the degree o f i o n a s s o c i a t i o n . characterized  T  Ion-pairs  by c o n d u c t i v i t y may be s e p a r a t e d by one  o r more s o l v e n t  m o l e c u l e s , and, w h i l e  t h e r e may be a  c o n s e q u e n t c h a n g e i n i o n i c m o b i l i t y , one may o b s e r v e no changes i n t h e a b s o r p t i o n  spectra.  I t i s only  through  1  4  the formation  of contact or p o s s i b l y solvent-shared  p a i r s t h a t one  i s a b l e t o o b s e r v e changes i n the  t i o n spectra other there  i s a p o s s i b i l i t y of the  simultaneously, i o n s and types  than small s h i f t s .  contact  In a  ion-  absorp-  solution,  four species being  present  w i t h e q u i l i b r i a e x i s t i n g between f r e e i o n - p a i r s as w e l l as t h e  of i o n - p a i r s .  intermediate  I t i s f o r t h i s reason that  a t l i n k i n g changes i n the a b s o r p t i o n  attempts  s p e c t r a of a s o l u t e  w i t h the changes i n a s s o c i a t i o n c o n s t a n t  derived  from  c o n d u c t i v i t y d a t a have sometimes been d i s a p p o i n t i n g . For contact  i o n - p a i r s are  likely  t o f o r m , one  g i v i n g s p e c t r a comparable to those phase.  b e n z e n e a r e due  being one  might  expect  of a l k a l i  halides in  However, s u c h s p e c t r a h a v e n o t  a b s o r p t i o n b a n d a t 290  withdrawn,  1 2 0  to the c a t i o n ,  o b s e r v e d f o r a l k y l a m m o n i u m i o d i d e s , and  and  "  i o d i d e s i n l o w - p o l a r i t y s o l v e n t s , where  to observe a s p e c i f i c charge-transfer  t h e gas  1 1 6  my  been  claims that  an  i n both carbon t e t r a c h l o r i d e  to such t r a n s i t i o n s  6 5  have been  the t r a n s i t i o n i n carbon t e t r a c h l o r i d e  reassigned  as a c h a r g e - t r a n s f e r  s o l v e n t m o l e c u l e as a c c e p t o r .  1 2 2  process  involving  i n s p i t e of  these  l i m i t a t i o n s , the s p e c t r a of i o d i d e s i n l o w - p o l a r i t y s o l v e n t s i • 121,123 show g o o d e v i d e n c e f o r i o n - p a i r s , ' though u  necessarily contact pairing  i n these  ion-pairs.  . not  Evidence f o r i o n -  s o l v e n t s i s b a s e d on  the  observation  that E  i s s o l v e n t and c a t i o n d e p e n d e n t , ^ '  max  a given  solvent generally decreasing  cation size, salts  being  max  in  with increase i n  s m a l l e r f o r s u b s t i t u t e d alkylammonium  than a l k a l i - m e t a l i o d i d e s .  the g r e a t e r  E  1 2 1  Therefore,  i n general,  t h e s t a b i l i z a t i o n o f t h e i o n - p a i r due t o  electrostatic  interaction,  the greater  isE  .  m  For  max alkylammonium i o n s c o n t a i n i n g a m e t h y l group, e.g., R-jN —Me, E i s n o t d e p e n d e n t o n R, w h i c h i n d i c a t e s 3 max +  c  that these  i o n - p a i r s have s t r u c t u r e s where t h e c h a r g e -  c e n t e r s a r e c l o s e s t , e.g. R N - M e I - . +  1 2 4  3  '  Th  1 2 5  i n f l u e n c e o f s i z e and shape o f t h e c a t i o n on E  and max  the r e l a t i v e unimportance o f the nature  of the cation  shows t h a t t h e c a t i o n i s n o t a c t i n g a s an e l e c t r o n acceptor.  The g o o d c o r r e l a t i o n  iodide i n tetrahydrofuran o plus  1 2 1  between E  for  max a n d t h e sum o f i o n i c r a d i i  3 A points t o the presence of solvent-shared i o n -  pairs . The o n l y e v i d e n c e p r e s e n t e d charge-transfer  thus f a r of d i r e c t  to the cation i s that of the spectra of  pyridinium iodides, the transferred electron entering the lowest unoccupied i r - l e v e l . ' This process should yield E v a l u e s s e n s i t i v e t o t h e s u b s t i t u t i o n on t h e max J  ring,  s i n c e s u b s t i t u t i o n would a f f e c t markedly t h e  acceptor  abilities  of the ring.  The h i g h  sensitivity  o f the band p o s i t i o n s to the nature o f t h e s u b s t i t u e n t on the r i n g i m p l i e s  s t r o n g l y t h a t an e l e c t r o n t r a n s f e r  process i s responsible  f o r the a b s o r p t i o n  bands.  As  expected f o r a p r o c e s s whereby an e l e c t r o n i s t r a n s f e r r e d from an i o d i d e i o n , two s i m i l a r bands o f 21.8 K c a l s e p a r a t i o n were observed f o r 1-methylpyridinium in chloroform. iodide  5 5  5 5  mole  iodide  The spectrum of 1-methylpyridinium  and t h e p o s i t i o n s o f the c h a r g e - t r a n s f e r  bands  126 f o r some 1 - a l k y l - p y r i d i n i u m Table 4 and F i g u r e  CH  8a.  iodides  a r e shown i n  The t r a n s i t i o n f o r 1-methylpyri  3  Figure  8.  - 1  Table 4  Substituent  x  a m  a  x  (  e m  a  x  )  E  b T  4-CH3  3590 (1230)  79.64  3-CH2  3700 (1310)  77.27  H  3738 (1200)  76.49  3-COOCH2  4070 (1850)  70.25  4-COOCH  4489 (1230)  63.69  4-CN  4912 (922)  58.20  3  F i g u r e 8a.  Evidence f o r t h i s type of process the  f l a s h photolysis of  iodide, the  l-ethyl-4-carbomethoxypyridinium  i n w h i c h t h e s o l u t i o n was  1 2 7  l - e2tt h y l - 4 - c a r b o m e t h o x y p y r i d i n y l Since  charge-transfer  excited s t a t e , blue absorption occur solvent.  was f o u n d i n  found t o c o n t a i n radical.  results i n a less polar  s h i f t s of the charge-transfer  o n c h a n g i n g f r o m a l e s s t o a more p o l a r  F o r e x a m p l e , a s h i f t f r o m 4489 A i n c h l o r o f o r m 12 8  t o 3311 A i n 7:3 e t h a n o l - w a t e r charge-transfer ethiodide.  band o f  i s reported  f o r the  4-methoxycarbonylpyridine  A d i r e c t r e l a t i o n s h i p between E  Winstein-Grunwald Y values ionizing power)  1 2 9  and t h e max (a k i n e t i c m e a s u r e o f s o l v e n t  seems t o e x i s t f o r a l k y l p y r i d i n i u m  128 iodides,  whereas t h e e f f e c t o f change o f s o l v e n t i n  63 64 t h e c a s e o f CTTS i s v a r i a b l e i n m a g n i t u d e and d i r e c t i o n . ' In general,  i t is a fairly  d i s t i n g u i s h between t h e p r o c e s s e s  simple  procedure to  of charge t r a n s f e r t o  t h e c a t i o n a n d CTTS, s i n c e CTTS s p e c t r a a r e f o u n d u s u a l l y at higher  energies,  coefficients.  and a l s o h a v e l a r g e r m o l a r e x t i n c t i o n  A c o m p a r i s o n c a n be made o f  inium  1-methylpyrid-  i o d i d e i n CHC1-. (X : 3796, 2945 A; e : 1210, 3 max max 1550-) and t h e i o d i d e i o n i n a q u e o u s o r a l c o h o l i c solutions  55  1 3 0  (X : 2 2 6 0 , 1940 A; e : 12,600, max ' max  x  12,600),  though i t should  be n o t e d t h a t t h e  values  E  i n the  max case of c h a r g e - t r a n s f e r values  since these  to the c a t i o n are only  absorptions  apparent  do n o t o b e y B e e r ' s  Law.  I n h i g h l y p o l a r s o l v e n t s , w h e r e i o n - p a i r i n g o f any f o r m i s not  favoured,  spectra.  one w o u l d e x p e c t t o o b s e r v e o n l y CTTS  I n l e s s p o l a r s o l v e n t s , where i o n - p a i r s a r e  f o r m e d i n s o l u t i o n , one w o u l d e x p e c t t o o b s e r v e absorptions as CTTS.  due t o c h a r g e - t r a n s f e r  t o t h e c a t i o n as w e l l  Thus f a r , h o w e v e r , i n no p u r e s o l v e n t  simultaneous  b a n d s b e e n o b s e r v e d w h i c h c a n be  have  separately  i d e n t i f i e d w i t h i o d i d e i n an i o n - p a i r and i n t h e f r e e state.  II.  D. C h a r g e - T r a n s f e r trilium  Spectra  of the Methylphosphoni-  Iodides.  Methylphosphonitriles  are s u f f i c i e n t l y  strong  b a s e s t o a l l o w q u a t e r n i z a t i o n by m e t h y l i o d i d e o r o t h e r 11  i  alkyl  • x-j,  iodides,  42,46  _  .  forming  _  .  ^  _  s a l t s analgous t o those  of  55 the p y r i d i n i u m i o d i d e s .  Kosower e t . a l .  a b s o r p t i o n b a n d s due t o c h a r g e - t r a n s f e r to  the p y r i d i n i u m c a t i o n .  chloroform,  observed  from the i o d i d e  Except f o r 1-methylpyridinium  t h e second band e x p e c t e d a t s h o r t e r  lengths  for transitions  visible  f o r the other  f r o m t h e i o d i d e was o n l y  iodide i n  wavepartially  alkylpyridinium iodides studied,  o w i n g t o o c c u l t a t i o n by ring.  the absorptions  of the  pyridinium  In the case of the m e t h y l p h o s p h o n i t r i l e s ,  n = 3,4,5, no observable  absorptions  down t o 190  my,  due  to the parent  a l l o w i n g f o r the  Therefore,  the m e t h y l p h o s p h o n i t r i l i u m w o u l d be  expected  the pyridinium  are  the  the  to  transitions those  for  c a t i o n s , assuming t h a t the t r a n s f e r r e d  phosphonitrilium ring.  lowest  unoccupied l e v e l of  Further,  i f the charge  t o a homomorphic i r - l e v e l , one  + (N-.P Me_) I to  + (Nj.Pj.Me,,) I .  3  Viewing  as c o m p e t i t i v e a c c e p t o r s  the  was  would  an a l t e r n a t i o n i n t h e t r a n s i t i o n e n e r g i e s  molecules  at  i f charge-transfer  cations occur,  n  observation  a t s h o r t e r wavelengths than  charge would enter the  transferred  2  compound  of charge t r a n s f e r a b s o r p t i o n bands, i f p r e s e n t , shorter wavelengths.  (NPMe ) ,  expect  on g o i n g  the  from  solvent  of the t r a n s f e r r e d  •  c h a r g e , more f r e q u e n t o c c u r r e n c e t r a n s f e r t o t h e c a t i o n w o u l d be  o f CTTS t h a n expected  p h o s p h o n i t r i l i u m i o d i d e s compared t o t h e i o d i d e s , as t h e a c c e p t o r - l e v e l e n e r g i e s i n the  former  f o r the  p h o n i t r i l e s a t the  pyridinium a r e more c o m p a r a b l e  of the  methylphos-  s h o r t e r w a v e l e n g t h s , i t was  hoped  spectra of the methylphosphonitrilium  determined i n v a r i o u s s o l v e n t s would y i e l d to both  methyl-  case.  Owing t o t h e t r a n s p a r e n c y  the e l e c t r o n i c  charge-  CTTS and  spectra  c h a r g e - t r a n s f e r to the c a t i o n .  that iodides  due  The  r e l a t i v e t r a n s i t i o n energies to  (N P Me 5  5  ) I ~ may y i e l d +  i ; L  f o r the series  ( N ^ M e ^ I  information concerning the  acceptor l e v e l s i n the p h o s p h o n i t r i l i c r i n g s . study  i n c l u d e d 1,2-, 1,3-, a n d  This  1,4-dimethylpyridinium  i o d i d e s as w e l l as t e t r a - n - h e p t y l a m m o n i u m  iodide,  p r o v i d i n g a b a s i s f o r comparison w i t h the s p e c t r a of t h e m e t h y l p h o s p h o n i t r i l i u m i o d i d e s as w e l l as  extending  v. K o s o w e r e4t .- Ta l 55,134 p r e v i o u s w o r k! by . v  II.  D.l.  Experimental  The m e t h y l p h o s p h o n i t r i l i u m i o d i d e s c a n be prepared and  by a p r e v i o u s l y r e p o r t e d m e t h o d .  (N P Me 5  5  ) I ~ were p r e v i o u s l y p r e p a r e d +  i : L  workers i n the l a b o r a t o r y .  4 6  (N P Me ) I~ +  3  3  7  by o t h e r  The compounds 1,2-,  and  1,4-dimethylpyridinium  4-u the  4-w A f* o lnl o w i•n g m e t hod . 47  II.  D.l.a. Preparation of the Dimethylpyridinium  1,3-,  i o d i d e s w e r e s y n t h e s i z e d by  Iodides  The m e t h y l p y r i d i n e s u s e d i n t h e p r e p a r a t i o n s w e r e p u r c h a s e d f r o m B r i t i s h D r u g House L t d . a n d t h e methyl i o d i d e from F i s h e r S c i e n t i f i c  Co.  Before use,  t h e m e t h y l p y r i d i n e s w e r e p u r i f i e d by d i s t i l l i n g  from  calcium  hydride  nitrogen. sieves.  The Dry  while  u n d e r an  i n e r t atmosphere of  m e t h y l i o d i d e was  d i e t h y l ether  was  from l i t h i u m aluminum h y d r i d e  dried with  obtained u n d e r an  by  dry  molecular distilling i t  atmosphere of  dry  nitrogen. 1.  1,4-Dimethylpyridinium  Iodide  Excess methyl i o d i d e 4-methylpyridine in  (20 ml.)  was  (1.1755 g) t h r o u g h t h e  added  top of a  through the  s y s t e m was  f l a s k was  maintained during  eously,  almost  a w h i t e c r y s t a l l i n e compound b e i n g  f l a s k was being  the  then wrapped i n t i n - f o i l ,  the  The  e x c e s s Mel  was  p r o d u c t washed c o p i o u s l y pressure  of dry  reaction.  nitrogen.  in a vial  A  instantanThe  formed  orange-coloured  removed by Vacuum and  with dry d i e t h y l ether A f t e r d r y i n g , the  c r y s t a l s of 1,4-dimethylpyridinium stored  nitrogen  formed.  salt  l i g h t - s e n s i t i v e , d e c o m p o s i n g t o an  solid.  of  shaken a f t e r each a d d i t i o n of Mel.  s l i g h t l y exothermic r e a c t i o n occurred  and  condenser  s m a l l p o r t i o n s , a l l a i r h a v i n g been f l u s h e d o u t  the r e a c t i o n f l a s k w i t h dry n i t r o g e n ; A f l o w of  The  to  the under  a  colourless  i o d i d e were c o l l e c t e d  wrapped i n t i n f o i l .  The r e s u l t s o f t h e m i c r o a n a l y s i s w e r e a s f o l l o w s : Element Expected Found  C (%)  (%)  D e t e r m i n e d m.p.: Lit.  values:  H  35.74  4.25  5.96 "  35.51  4.08  5.73  155-157.5°C  153.3-154.3°C 157-158°C  2.  N  135  1 3 6  1,2-Dimethylpyridinium  Iodide  The same p r o c e d u r e a s t h a t u s e d i n t h e p r e p a r a t i o n o f t h e 1,4-compound was u t i l i z e d . obtained  was a l s o c o l o u r l e s s b u t l e s s  The  product  crystalline.  The r e s u l t s o f t h e m i c r o a n a l y s i s w e r e a s f o l l o w s :  Element Expected  C  N  35.74  4.25  5.96  (%)  35.84  4.45  5.77  D e t e r m i n e d m.p.:  228-232°C  Found  Lit.  (%)  H  value:  229-231°C  1 3 5  3. . 1 , 3 - D i m e t h y l p y r i d i n i u m I o d i d e The  same p r o c e d u r e a s t h a t u s e d i n t h e p r e v i o u s  c a s e s was f o u n d t o be - i n a p p l i c a b l e t o 3 - m e t h y l p y r i d i n e . The  r e a c t i o n was r e p e a t e d , t h i s t i m e w i t h t h e  3 - m e t h y l p y r i d i n e d i s s o l v e d i n 20 m l o f d r y d i e t h y l before the addition of Mel.  The p r o d u c t o b t a i n e d was  c o l o u r l e s s and r e s e m b l e d t h e o t h e r The  Found  C (%)  (%•)  Determined  II.  D.2.  iodides.  r e s u l t s o f t h e m i c r o a n a l y s i s were as f o l l o w s :  Element Expected  ether  m.p.:  H  N  35.74  4.25  5.96  35.53  4.09  5.7 9  97-99°C  Spectroscopic Results  All  s o l v e n t s used  i n obtaining the electronic  s p e c t r a were o f S p e c t r o g r a d e q u a l i t y  (Mallinckrodt)  f o r w a t e r , w h i c h was p u r i f i e d by a s p e c i a l  except  procedure  d e v i s e d by D r . J . B . F a r m e r o f t h i s D e p a r t m e n t .  The  s p e c t r a w e r e r e c o r d e d o n a C a r y 14 S p e c t r o p h o t o m e t e r a p a i r o f m a t c h e d 1-cm q u a r t z c e l l s . ammonium I o d i d e was p u r c h a s e d and u s e d w i t h o u t f u r t h e r  using  Tetra-n-heptyl-  from Eastman O r g a n i c  purification.  Chemicals  The r e s u l t s o f t h e s p e c t r o s c o p i c m e a s u r e m e n t s a r e t a b u l a t e d i n T a b l e 5, a l o n g w i t h t h e a s s i g n m e n t o f the observed a b s o r p t i o n bands.  T a b l e 5.  Compound 1,2-dimethylpyridinium iodide  Solvent  x  max  a  (£  max»  Assignment  363.5 (990) 292.5 (1530)  CT t o c a t i o n  1 , 3 - d i m e t h y l p y r i d i n i u m CH2 C12 iodide  367. 0 (1735)  CT t o c a t i o n  1,4-dimethylpyridinium CH C1 iodide  359.5 (1469) 304. 0 (1670)  CT t o c a t i o n  C H  2  C 1  2  2  (N P Me ) I~  2  226.0 194.0  CTTS  246.0  CTTS  CHgCN  246 .0  CTTS  CHC1  245.0  CTTS  +  3  3  ?  H  2°  C H  2  C 1  2  3  292.0 363 .0 (N P Me ) I+  4  4  9  H  2°  C H  2  C 1  CHC1  2 3  CT t o c a t i o n  226.0 193.0  CTTS  246.0  CTTS  243. 0  CTTS  361. 0 shoulder a t 290-300  CT t o c a t i o n  Table 5 Compound (N P Me 5  5  ) l" +  i : L  (cont'd)  Solvent  A  CHC1  242.0  3  max  a  ( e  max  Assignment  ) b  CTTS  370.0 shoulder a t 290-300 [CH (CH )g] N I~ +  3  2  4  C H  3  OH  CHC1  3  a  I n my.  b  Molar e x t i n c t i o n c o e f f i c i e n t s .  CT t o c a t i o n CTTS  220.5  CTTS CT t o c a t i o n  244.0 364. 0  (±5)  The a b s o r p t i o n s p e c t r u m o f ( N P M e ) I ~ +  3  is  shown i n F i g u r e 9.  3  7  i n CHC1  3  FIG.9: Charge-transfor spectrum of ( N P M e ) I 3  3  ?  Concentration- saturated Solvent: C H C l 3  O  II.  D.3.  Discussion  The  charge-transfer  dimethylpyridinium those  of  the  i o d i d e s i n CH C1 •are comparable to 2  o f K o s o w e r and  r e s u l t s are given  absorptions  Skorcz  i n Table  2  measured i n CHCl^.  Their  6.  Table 6 Dimethylpyridinium  Substituent  X  a  ;  Iodides  b  A  max 1-Me  e max  3738  1200  1.2- Me  2  3640  860  1.3- Me  2  3700  1310  1.4- Me  2  3590  1230  a  S e c o n d maxima n o t  given.  b  Determined i n CHC1 . 3  The  r e p l a c e m e n t o f a n u c l e a r h y d r o g e n atom  by  a m e t h y l g r o u p s h i f t s t h e maximum t o s h o r t e r w a v e l e n g t h s from t h a t of 1-methylpyridinium  i o d i d e , but  the  extent  o f t h e change depends m a r k e d l y upon t h e p o s i t i o n o f  the  a d d i t i o n a l methyl group. energies CH C1 2  2  The same o r d e r o f t r a n s i t i o n  f o r the dimethylpyridinium  as i n C H C 1 .  i o d i d e s was f o u n d i n  1 3 4  3  1,4-dimethyl > 1,2-dimethyl > 1,3-dimethyl  The upon a d d i n g  general  increase i n transition  a methyl group t o t h e r i n g  energies  i s explicable i n  t e r m s o f s t a b i l i z a t i o n o f t h e c a t i o n by t h e e l e c t r o n r e l e a s i n g e f f e c t s o f an a l k y l energies  group.  The  relative  of transition i n the dimethylpyridinium  are approximately  iodides  e x p l i c a b l e i n terms o f c a l c u l a t e d  r r - e l e c t r o n d e n s i t i e s a t t h e 2-, 3-, a n d 4 - p o s i t i o n s i n pyridine  ( 0 . 9 4 3 , 0.991, a n d 0.950 r e s p e c t i v e l y )  1 3 7  , the  i n c r e a s e d p o s i t i v e c h a r g e a t t h e 2- a n d 4- p o s i t i o n s resulting  i n greater i n t e r a c t i o n with the methyl  substituent,  (though t h e simple  r e v e r s e t h e o r d e r o f t h e 1,2- a n d iodides).  A likely  s t r a i n generated  rr-electron e f f e c t  would  1,4-dimethylpyridinium  additional effect i s that  steric  by t h e i n t e r a c t i o n o f t h e 2 - a l k y l  group w i t h t h e i o d i d e i o n d e s t a b i l i z e s t h e c a t i o n and hence reduces t h e t r a n s i t i o n energy f o r pyridinium iodide. d e n s i t i e s and o f s t e r i c qualitatively  1,2-dimethyl-  The c o m b i n e d e f f e c t s o f T r - e l e c t r o s t r a i n could therefore explain  the observed r e l a t i v e  transition  energies  of the dimethylpyridinium The  iodides.  r e l a t i v e t r a n s i t i o n energies  p y r i d i n i u m i o d i d e s a r e i n t h e same o r d e r reduction p o t e n t i a l s i n water factor of steric  of the dimethyl-  as t h e i r  (see p a r t I I . E ) .  s t r a i n due t o i n t e r a c t i o n o f t h e 2 - a l k y l  g r o u p w i t h t h e i o d i d e i o n c a n n o t be c o n s i d e r e d case o f t h e p o l a r o g r a p h i c  experiments,  t h e i o n s a r e e x p e c t e d t o be f u l l y The  reduction p o t e n t i a l s provide charge t o the lowest  i n the  however,  since  s o l v a t e d i n water.  c o r r e l a t i o n between t h e e n e r g i e s  rings.  The  o f t r a n s i t i o n and  evidence f o r the t r a n s f e r of  unoccupied -rr-levels o f t h e p y r i d i n i u m  The r e l a t i v e e n e r g i e s  the t h r e e d i m e t h y l p y r i d i n i u m  of the acceptor  levels i n  ions are therefore w e l l  e s t a b l i s h e d , even though q u a n t i t a t i v e agreement i s s t i l l incomplete. None o f t h e i o d i d e s s t u d i e d w e r e s o l u b l e enough i n CC1  fortheir  4  solvent.  s p e c t r a t o be d e t e r m i n e d i n t h i s  I n t h e more p o l a r s o l v e n t s , H 0 , CH OH, a n d 2  3  CH^CN, o n l y CTTS s p e c t r a w e r e o b t a i n a b l e , t h e A J max values  determined being  i n agreement w i t h  previous  r e s u l t s f o u n d f o r s o l v a t e d —i o d i d e 1 n s (s e e T a ID 1 e 3 ) In C H ^ C l o n l y for  one a b s o r p t i o n a t 246 my was o b s e r v e d  (N^P^Me^) I  a n d (N^P^Me^) I , d e f i n i t e l y due t o  CTTS by c o n s i d e r a t i o n o f t h e e n e r g y o f t r a n s i t i o n , indicating  little  o r no i o n — p a i r i n g i n t h i s  thus  solvent.  In  the s l i g h t l y p o l a r s o l v e n t CHC1 trated  3  a n d u s i n g more c o n c e n -  s o l u t i o n s , h o w e v e r , weak a b s o r p t i o n s a t l o n g e r  wavelengths i n a d d i t i o n t o those  due t o CTTS a p p e a r i n  the s p e c t r a o f t h e m e t h y l p h o s p h o n i t r i l i u m  and t e t r a - n -  h e p t y l a m m o n i u m i o d i d e s , t h e s e new a b s o r p t i o n s t h e same r e g i o n a s t h o s e  being i n  due t o c h a r g e - t r a n s f e r i n . t h e  case o f the p y r i d i n i u m i o d i d e s .  The s p e c t r u m o f t e t r a -  n - b u t y l a m m o n i u m i o d i d e i n C H C l ^ a s r e p o r t e d by G r i f f i t h s 65 and  Symons  showed o n l y t h e 245 my b a n d due t o CTTS, no  absorptions regions.  being  reported i n the longer  wavelength  They r e p o r t e d , h o w e v e r , a b s o r p t i o n a t 290 my  i n t h e s o l v e n t s C H,, E t 0 , and t e t r a - h y d r o f u r a n , a s c r i b e d bo A r  o  to the formation of i o n - p a i r s . observed i n CHC1 expected  i n the spectrum of tetra-n-heptylammonium i o d i d e 3  t h e r e f o r e a p p e a r s t o be t h e o t h e r h a l f o f t h e doublet  f o r i o d i d e i n an i o n - p a i r , t h e 290 my  band b e i n g o b s c u r e d partial  The band a t 364 my  by t h e i n t e n s e CTTS a b s o r p t i o n .  The  f o r m a t i o n o f i o n - p a i r s i n t h e s o l v e n t CHC1 f o r 3  tetra-alkylammonium iodides i s therefore postulated, a new i n t e r p r e t a t i o n b e i n g g i v e n t o t h e r e s u l t s o f G r i f f i t h s and  Symons.  292  a n d 363 my g i v e an e n e r g y d i f f e r e n c e o f 19.2 K c a l m o l e  6 5  F o r ( N P M e ) I ~ , t h e two a b s o r p t i o n s a t +  3  3  7  i d e n t i f y i n g p o s i t i v e l y t h e i o d i d e i o n as t h e o r i g i n o f  1  ,  the  transition.  For  (N P Me ) I +  4  4  s e c o n d band e x p e c t e d i n t h e on  the  _  9  and  (N^Me^) I~,  2 90-3 00 my  o n l y as  shoulders  CTTS b a n d .  derived  from the p o s i t i o n s of the  the  +  region  Since  first  are  the  A  seen  energies  do  not  show  max any  ir-systems of the  c o r r e l a t i o n w i t h the  methylphospho-  n i t r i l e s f charcje t r . a n s f e r t o t h e u n o c c u p i e d TT l e \ r e l s c a n n o t be  postulated.  The  s i m i l a r energies  of  transition  i n t h e c a s e o f t e t r a n h e p t y 1 amnionium i o d i d e / i n w h i c h no  TT l e v e l s e x i s t s a l s o sucjc^est t h e  shared i o n - p a i r s , i n which the are  only  slightly  bands a s c r i b e d solvent  a f f e c t e d by  energies the  to charge-transfer  shared i o n p a i r are the  nitrogen  corresponding  pyridinium being  The  confirmation  transition Since  w i t h the  iodide i n a  or  a l l the  saine r e c j i o n ^ one  5 5  absorptions  may  on  294.5  pyridinium  cations.  The  pyridinium  i o d i d e s i n C H C l ^ and  give  p r o p e r t i e s of  the  i s r a i s e d of  the  possibility CB^C^  e x i s t i n g as  solvent-  shared i o n — p a i r s . As  shown by  the  my,  methyl-  tetra-n-heptylammonium i o d i d e s ,  of the b e t t e r acceptor  the  f o r 1—methyl—  o c c u r a t 379.6 and  wavelengths than those of the  phosphonitrilium  solvent  c a t i o n s which i s centered  iodide in CHCl^  at longer  of  of  cation.  i n the  e x p a n d e d o r b i t a l on atom.  formation  s i m u l t a n e o u s measurement  of  s p e c t r a due t o b o t h CTTS a n d c h a r g e - t r a n s f e r  t o the  c a t i o n f o r t e t r a a l k y l a m m o n i u m as w e l l as t h e m e t h y l p h o s p h o n i t r i l i u m i o d i d e s , i o n - p a i r i n g i s not complete i n CHC1 , as p r e v i o u s l y thought f o r t h e p y r i d i n i u m i o d i d e s .  1 2 8  3  This i s the f i r s t  report of charge-transfer  absorptions i n  a p u r e s o l v e n t w h i c h c a n be s e p a r a t e l y i d e n t i f i e d i o d i d e i n an i o n - p a i r and i n t h e f r e e s t a t e . u n c e r t a i n why no a b s o r p t i o n s and  Symons  w e r e o b s e r v e d by G r i f f i t h s  were r e c o r d e d The  and  i n t h e 290 my  when  absorptions  region.  f o l l o w i n g type  of e q u i l i b r i u m process  would  t o d e s c r i b e t h e ground and e x c i t e d s t a t e s o f ,  example,  (N P Me ) l +  3  3  y  ™„,™-,„  i n C H C 1 , b a s e d on m o d e l s 3  63,64,128  + I solvent-shared  ion pair  [(N P Me -)CHCl (I.)] 3  4  i n t h e 360 my r e g i o n , t h e p o s i t i o n o f  the other h a l f o f the expected doublet,  for  It is  f o r (n-Bu).NI i n such s o l v e n t s as C C 1  6 5  tetrahydrofuran  seem b e s t  with  3  7  3  free  s o l v  ions  solv  (I-e  )  solv  II.  E. P o l a r o g r a p h i c R e d u c t i o n and  Dimethylpyridinium  of Methylphosphonitrilium Iodides  A t t e m p t s h a v e b e e n made t o r e d u c e  phosphonitrilic  compounds p o l a r o g r a p h i c a l l y i n n o n a q u e o u s m e d i a t o y i e l d Ai 138,139 , • . , .' ' ' r a d i c a l anions, w h i c h c a n t h e n be s t u d i e d by e l e c t r o n s p i n resonance techniques.  I t has been  found  t h a t t r i m e r i c and t e t r a m e r i c p h o s p h o n i t r i l e s w h i c h c o n t a i n o n l y f l u o r o , c h l o r o , bromo, phenoxy, g - d i o x y p h e n y l , c a n n o t be r e d u c e d However,  trifluoroethoxy,  o r phenylamino s u b s t i t u e n t s  a t p o t e n t i a l s more p o s i t i v e t h a n  species c o n t a i n i n g phenyl,.  2,3-dioxynapthyl  -3V.  p-nitrophenoxy,  ( 1 ) , and 1 , 8 - d i o x y n a p t h y l  g r o u p s c a n be r e d u c e d s p e c t r a c a n be  methoxy,  (2) s i d e  i n t h e -1 t o -3V r a n g e ,  and e s r  detected.  J  3  In the  l a t t e r three  the s u b s t i t u e n t s hyperfine  w h i c h become r e d u c e d , w i t h t h e  s p l i t t i n g s observed being  the o r g a n i c  r a d i c a l anion.  b e n z e n e and and  toluene  an u n r e s o l v e d  c a n n o t be  V  (NPPh ) 2  t h a t the  and  3  (even though  reduced at t h i s  potential), It  electron spin d e r e a l i z a t i o n  or t h a t r a p i d e l e c t r o n exchange takes  equivalent  of  s i n g l e t esr spectrum i s observed.  i s assumed t h a t e x t e n s i v e occurs  a t -2.65  4  esr  characteristic  In the case of  (NPPh ) , reduction occurs 2  s p e c i e s , however, i t i s  place.  The  r e d u c t i o n p o t e n t i a l o f b o t h compounds s u g g e s t s  lowest  antibonding  orbital  p h o s p h o n i t r i l i c r i n g are u n a f f e c t e d conjugation  of the  ir-system of the  levels i n by  ring  the  size.  The  pentafluorophenyl  g r o u p w i t h a homomorphic Tr-system i n t h e p h o s p h o n i t r i l i c irincf has tooen shown by actions being  F ninr s p e c t r a ,  1 4 0  these  possibly responsible for altering  i n the p h o s p h o n i t r i l i c r i n g ,  a p o s i t i v e c h a r g e on  the r i n g  made t o r e d u c e p o l a r o g r a p h i c a l l y t h e  methylphosphonitrilium Mainly  levels  phenylphosphonitriles. i s expected  to lower the r e d u c t i o n p o t e n t i a l s r e q u i r e d , the was  the  resulting therefore i n a  lower r e d u c t i o n p o t e n t i a l i n the Since  inter-  attempt  s e r i e s of  i o d i d e s i n aqueous s o l u t i o n .  f o r comparison w i t h the p h o s p h o n i t r i l i u m  the r e d u c t i o n p o t e n t i a l s of the i o d i d e s were a l s o d e t e r m i n e d .  s e r i e s of  iodides,  dimethylpyridinium  Experimental  Reduction  p o t e n t i a l s w e r e d e t e r m i n e d on  a  H e a t h D r o p p i n g M e r c u r y E l e c t r o d e M o d e l EU A-19-6 i n w a t e r w h i c h was may  be  specially purified.  r e d u c e d was  through the  stages  removed by b u b b l i n g  A constant  nitrogen hour p r i o r  stream of dry n i t r o g e n  to was  over the s o l u t i o n throughout the e n t i r e  of the experiment.  at a concentration of 10 M _ 1  The  supporting  was  KC1,  R e s u l t s and  electrolyte  w i t h the  a t an a p p r o x i m a t e c o n c e n t r a t i o n o f 10  3  iodides  M.  Discussion  I t was nitrilium  dry  s o l u t i o n f o r at l e a s t one-half  experimentation. maintained  D i s s o l v e d oxygen which  f o u n d t h a t none o f t h e  methylphospho-  i o d i d e s were r e d u c i b l e w i t h i n t h e  capabilities  •  of t h i s polarograph  (^-2.3 V)  whereas the  p y r i d i n i u m i o d i d e s were f o u n d t o r e d u c e The  h a l f - w a v e p o t e n t i a l s a r e as f o l l o w s :  dimethyl-  irreversibly.  Table 7 Half-Wave P o t e n t i a l s  Compound  1.2- d i m e t h y l p y r i d i n i u m i o d i d e  -1.96 V  1.3- d i m e t h y l p y r i d i n i u m i o d i d e  -1.93 V  1.4- d i m e t h y l p y r i d i n i u m i o d i d e  -2.13 V  From t h e s e  i t may be c o n c l u d e d  results,  t h e empty a n t i b o n d i n g energies  that  levels i n the phosphonitrilic rings  lie  at higher  than those  The  slight variation i n  values  i n the pyridine ring. i n the three  dimethyl-  p y r i d i n i u m i o d i d e s c a n be q u a l i t a t i v e l y r a t i o n a l i z e d i n terms o f t h e i n d u c t i v e e f f e c t s o f t h e m e t h y l group a t t h e 2-, 3-, a n d 4 - p o s i t i o n s r e s p e c t i v e l y , t h e same arguments as were used i n t h e d i s c u s s i o n o f t h e r e l a t i v e energies being  of charge-transfer  still  valid.  t o t h e c a t i o n ( s e e I I . D)  The c o r r e l a t i o n b e t w e e n t h e e n e r g i e s  of charge-transfer  and t h e r e d u c t i o n p o t e n t i a l s o f t h e  dimethylpyridinium  i o d i d e s , being  dimethyl  i n the order  > 1,2-dimethyl > 1,3-dimethyl,  1,4-  shows t h a t ,  r e g a r d l e s s o f the exact d e s c r i p t i o n , the charge i s transferred either electrolytically or photolytically t o t h e same t y p e  of vacant  orbital.  CHAPTER I I I  DONOR-ACCEPTOR CHARGE-TRANSFER  III.  A. I n t r o d u c t i o n  I n 1949, B e n e s i solutions an  COMPLEXES  containing  and Hildebrand ''" 14  an a r o m a t i c  electronic absorption  reported  that  h y d r o c a r b o n and i o d i n e had  band n o t p r e s e n t  i n e i t h e r component  142 alone.  Mulliken  characteristic  was t h e f i r s t  absorption  to recognize  of such s o l u t i o n s c o u l d  t h r o u g h an i n t e r m o l e c u l a r c h a r g e - t r a n s f e r c o m p l e x e s were d e s c r i b e d  that the  i n general  arise  transition.  as b e i n g  The  f o r m e d by t h e  weak i n t e r a c t i o n between e l e c t r o n d o n o r s and e l e c t r o n acceptors, components.  i n v o l v i n g u s u a l l y simple The t e r m complex  mean a s u b s t a n c e  integral  i n this  ratios  of the  case i s taken to  f o r m e d by t h e i n t e r a c t i o n o f two o r more  components w h i c h c a n r e v e r s i b l y d i s s o c i a t e i n t o i t s compon e n t s u n d e r s u c h c o n d i t i o n s as b e i n g  i n t h e vapor phase o r  on  dissolution.  A  (g)  A  (solv)  +  n B  (g) +  (solv)'  n B  This d e f i n i t i o n  The  ^  suggests  f o r m a t i o n i s v e r y s m a l l and play a strong role  (g)  A B r i  ^'"(solv)  t h a t AH o f  complex  t h a t c o v a l e n t bonding does  i n the ground s t a t e of the  types of donors used i n the  c o m p l e x e s c a n be p l a c e i n two  not  complex.  studies of  these  general categories:  (1) TT d o n o r s a r e compounds i n w h i c h t h e e l e c t r o n s d o n a t e d are those  contained  adducts they  form are c a l l e d  include unsaturated and  aromatic  i n TT m o l e c u l a r  ring  orbitals  Tr-complexes.  and  the  They  1 4 3  compounds s u c h as a l k e n e s ,  alkynes,  systems.  (2) n d o n o r s a r e compounds i n w h i c h n o n - b o n d e d e l e c t r o n s are a v a i l a b l e  for donation.  as a l c o h o l s , s u l f i d e s , heterocyclic ring aromatic  the  They i n c l u d e s u c h compounds  i o d i d e s , n i t r o g e n bases,  s y s t e m s , where i f t h e y  are  and  also  lone p a i r s are donated i n preference  to  the  iT-electrons. ^The halogens,  a c c e p t o r s used i n the s t u d i e s are m a i n l y especially  i o d i n e , but  also include a  number o f more uncommon compounds s u c h as  the  great  tetracyanoethylene.  1 4 5  In the  study of  these complexes, the  acceptor are u s u a l l y simply colour It  mixed i n s o l u t i o n , the  i s now  generally  accepted t h a t the  e l e c t r o n from the ,,  M  associated  donor t o the  142,146  Mulliken  quantum m e c h a n i c a l l a n g u a g e . of complex f o r m a t i o n ,  he  -A  ), where D i s the  ^  the  the  colour  = f(D,A)  Q  w h e r e X and  y will  The  m  +  of  first  t  to put  this  considered  the  a polar  d o n o r and  into  treatment  i n t e r a c t i o n of excited  A the  a  state  acceptor,  p r o d u c e a s t a b i l i z e d g r o u n d s t a t e h a v i n g a wave ¥  of  transfer  I n h i s v a l e n c e bond  n o - b o n d g r o u n d s t a t e T(D,A) and + m(D  with  formation.  acceptor.  .. was  intense  and  resultant  change b e i n g u s u a l l y i n d i c a t i v e of complex  t h e s e c o m p l e x e s i s t o be an  donor  to  function  AY(D -A )  (1)  +  m o s t c a s e s be  small  compared w i t h  unity,  no-bond g r o u n d s t a t e wave f u n c t i o n d e s c r i b e s .  a structure i n which binding  r e s u l t s only  from  such  •  'physical'  forces  as v a n  der Waals i n t e r a c t i o n s .  s t a t e wave f u n c t i o n d e s c r i b e s e l e c t r o n has  a structure  The  i n which  been t r a n s f e r r e d from the donor t o the  excited  one acceptor.  In the  c a s e where D and  involve and  D.  ~* l ' f  the  large  internuclear  band i s . a s s o c i a t e d o  c  c  u  r  r  i  h i s Planck's As transfer, hv D  t o an  n  g  a t  with  w h e r e A,  C T  ,  from a donor w i t h an  assumed t o be  it  has  the  can  be  an  electron  difference  charge-  ( E - E ) / h , where 1  Q  energy of  d o n o r s and  the  b e e n shown t h a t  frequency of  affinity,  seen i n the  E^,  between the  g r o u n d and  the  related  binding excited  energies states.  involving  a c c e p t o r , s u c h as  there i s a reasonable  I , 2  linear  i o n i z a t i o n p o t e n t i a l of charge-transfer b a n d .  following  is  • • •  same e l e c t r o n  the  charge-  ionization potential,  a s e r i e s of m o l e c u l a r complexes  c o r r e l a t i o n between the and  The  c o n s t a n t i n a s e r i e s of  components i n the  electron  A~  electronic transition  a f i r s t a p p r o x i m a t i o n , the  acceptor with  For  in  weak h o w e v e r  distance. the  will  constant.  compounds, i s t h e the  this  electrons  be  frequency v =  t h e  •  of  odd  This bonding i n general w i l l  +  transfer  I ,  both n e u t r a l ,  c o v a l e n t bonding between the  because of  ^0  A are  the 1 4 7  "  donor This  1 5 0  f i g u r e f o r complexes of  I . 0  2.51  7.0  :  I 7.5  I 8.0  I 8.5  1— 9.0  / (eV)  i 9.5  i 10.0  i 10.5  i 11.0  115  D  F i g u r e 12 (From r e f . 1 5 4 ( a ) ) .  For compounds whose d i r e c t i o n i z a t i o n are not y e t a v a i l a b l e , p o l a r o g r a p h i c o x i d a t i o n  potentials may p r o v i d e  a measure o f t h e i o n i z a t i o n p o t e n t i a l o f a compound. a s e r i e s o f c h e m i c a l l y r e l a t e d compounds, v a l u e s o f f o r donor m o l e c u l e s , o b t a i n e d  lines.  e  °  X  from p o l a r o g r a p h i c measure¬  ments o f a c e t o n i t r i l e s o l u t i o n s , values  For  when p l o t t e d  f o r complexes o f a common a c c e p t o r ,  a g a i n s t v_, CT  yield  m  straight  2-8  2-4  20  0-8  10  13  !-1  IS  £ ? / V (donor) X  5  F i g u r e 13  (From r e f . 151).  S i m i l a r l y , f o r a p a r t i c u l a r donor, the frequency  of the c h a r g e - t r a n s f e r band i s p r o p o r t i o n a l to  the e l e c t r o n a f f i n i t y of the a c c e p t o r i n a r e l a t e d  s e r i e s , 68  but the p r o p o r t i o n a l i t y c o n s t a n t s , however, do not always have the expected  value of u n i t y .  For example,  Av/AI  D  f o r the case o f i o d i n e and a s e r i e s of s a t u r a t e d , o l e f m i c , and aromatic  A  donors  148  . ' , i s 0.67, and f o r t e t r a c y a n o e t h y l e n e n  c n  with a s e r i e s of m e t h y l b e n z e n e s ,  1 4 9  the v a l u e i s  0.49.  Apart from the u n c e r t a i n t i e s i n the v a l u e s of the  ionization  potentials, steric  In the  f a c t o r s seem to be important.  s e r i e s of u n s u b s t i t u t e d p o l y c y c l i c aromatic where s t e r i c f a c t o r s are l e s s important,  hydrocarbons,  s l o p e s c l o s e to  u n i t y are o b t a i n e d w i t h a number o f a c c e p t o r s .  III.  A . l . Determination  of the P o s i t i o n s of E q u i l i b r i u m  Many s t u d i e s h a v e b e e n made o f t h e between c h a r g e - t r a n s f e r  c o m p l e x e s and  molecules i n solution.  A v a r i e t y of methods such  u l t r a v i o l e t and  spectrophotometry,  s c o p y , and  visible  n u c l e a r m a g n e t i c r e s o n a n c e has  r e s u l t s obtained enthalpy  and  f r e e component  changes of complex  as  infrared  spectro-  been used.  The  are then used to determine the  entropy  By  corresponding  formation.  f a r t h e most o f t e n u t i l i z e d method o f  brium constant  determination  of the donor or acceptor.  equili-  r e l i e s upon the c h a n g i n g i n  i n t e n s i t y of the c h a r g e - t r a n s f e r  is  their  equilibria  band w i t h  concentration  A b r i e f o u t l i n e of t h i s  method  given.  Method of B e n e s i  For  the r e a c t i o n  K  —=r-=  =  C  where C  c  and'Hildebrand  1 4 1  D +  A r ^ C ,  (3)  D A C  f o r example i s the c o n c e n t r a t i o n of the  complex  at equilibrium. Since  the absorbance A  t h e c o m p l e x a b s o r b s i s g i v e n by  due t o C i n a r e g i o n w h e r e c B e e r ' s Law,  A  ic  _  c  (4)  c  c  A  c  must a l s o i n c r e a s e as C  A  c  a t a s e t wavelength f o r a s e r i e s of concentrations C ,  with C  N  A be o b t a i n e d Benesi  From t h e a b s o r b a n c e  f i x e d a n d s m a l l r e l a t i v e t o C , K a n d e, c a n D A  kept  A  increases.  q  f o r the e q u i l i b r i u m process  by t h e m e t h o d o f  and H i l d e b r a n d . The B e n e s i - H i l d e b r a n d  analysis  starts  1 4 1  t h e a s s u m p t i o n t h a t o n l y one e q u i l i b r i u m e x i s t s solution.  from  i nthe  Then, u s i n g a z e r o s u b s c r i p t t o d e n o t e t h e  t o t a l c o n c e n t r a t i o n , we h a v e  \ c \~^2 \ c  Cc c  K  /J /  -D  "  (C  A°- c»  ™  C  As s t a t e d , C ° i s much g r e a t e r t h a n C ° , f o r ' D  A  s m a l l K, t h e amount o f c o m p l e x f o r m e d , i s (i.e.  C ° >> C ° > c ) . D  A  c  the equation then  A  c  Q  f r o m B e e r ' s Law,  becomes  A "AT" D ° A ° " D ° E  =  (C °-C ) can t h e r e f o r e  Using the expression f o r C  be n e g l e c t e d .  1 K  The t e r m  also small  1  C  C  C  D i v i d i n g through  by  obtains the Benesi-Hildebrand  •  ( 6 )  e  a n d r e a r r a n g i n g , one  equation  °  IC  Ke.  +  V  (7)  In a given experiment, the values of l , C °, A  C ° D  a r e known; A  i s measured f o r a s e r i e s o f s o l u t i o n s w i t  q  v a r y i n g C ° , E^L. A  values  i s then p l o t t e d against ^  to obtain D  C  o f K and e . A  triethylamine-I  and  2  0  An e x a m p l e o f a p l o t f o r t h e c o m p l e x  is given.  4  8  12  1 5 2  16  20  24  28  32  10 /CD. _ ,  Figure  14.  I l l u s t r a t i o n of t h e use o f the B e n e s i - H i l d e b r a n d e q u a t i o n t o o b t a i n K and e a t f o u r w a v e l e n g t h s f o r t h e t r i e t h y l a m i n e - I c o m p l e x . (From r e f . 152) 2  K the  and e  c  acceptor i  respectively, a  n  d  £  max  v  a  l  u  e  2  m a x  v a l u e s determined f o r n-donors  r a n g e f r o m 0.5 - 7,500 a n d 3000 - 30,000  whereas complexes o f .-donors w i t h l s  o  f  with  o n l  Y  0-  1  have  2  K  Q  " 2.0 a n d 5000 - 15,000  respectively. I f K c a n be m e a s u r e d , t h e o t h e r  thermodynamic  p r o p e r t i e s t h a t a r e a s s o c i a t e d w i t h complex f o r m a t i o n c a n be o b t a i n e d b y u s i n g w e l l - k n o w n t h e r m o d y n a m i c If  the a c t i v i t i e s  relationships.  a r e approximated as c o n c e n t r a t i o n s ,  then  AG°  =  -RT  (8)  £n K  c and AH° a n d AS° c a n be o b t a i n e d b y t h e u s e o f t h e e q u a t i o n  Assuming t h a t  AH° w i l l  temperature range s t u d i e d ,  be c o n s t a n t  a p l o t o f Jin K  over the versus  ijj-  w  i  l  1  — AH ° yield  R. *  a straight line  of slope  R  and whose  intercept i s  AS°  When o n e r e f e r s t o t h e " s t a b i l i t y " w h a t i s meant i s t h e m a g n i t u d e o f A H ° . From  o f t h e complex, results  o b t a i n e d i t i s e v i d e n t t h a t a s t h e c o m p l e x becomes more stable,  t h e d e c r e a s e i n e n t r o p y due t o t h e l o s s o f f r e e d o m  as D and A combine to form C a l s o tends to become l a r g e r . I t has been found e m p i r i c a l l y t h a t -AS° i s a l i n e a r of -AH° f o r most complexes.  The two  function  f u n c t i o n s AH° and  have opposing e f f e c t s on the v a l u e of AG so t h a t as AH 0  AS° 0  becomes more n e g a t i v e the c o r r e s p o n d i n g decrease i n entropy prevents AG° from becoming n e g a t i v e as r a p i d l y as does AH mu  *  11  •  1  4-154b  The f o l l o w i n g p l o t s h i p between the two  . • , 1 4.1, 4. A expected shows c l e a r l y the expected  19  15.  1 4 - -  relation-  functions.  20  Figure  0  - i—i—I—I—I—I—I—I—I—I—t~jr, .  •  V a r i a t i o n of AS° w i t h AH f o r i o d i n e complexes i n various solvents. (From r e f . 154b). 0  III.  A.2. C h a r g e - T r a n s f e r  In  Complexes o f  Iodine  a d d i t i o n t o the appearance of a  charge-  t r a n s f e r b a n d upon c o m p l e x f o r m a t i o n , t h e e l e c t r o n i c s p e c t r a o f complexes w i t h i o d i n e as a c c e p t o r  show a  shift  o f t h e v i s i b l e a b s o r p t i o n band o f i o d i n e t o h i g h e r a c c o m p a n i e d by an i n c r e a s e i n i t s i n t e n s i t y .  This  energies s h i f t of  t h e v i s i b l e band i s o f t e n l a r g e enough t o make p o s s i b l e spectrophotometric  determinations  o f thermodynamic  constants  o f complexes f o r w h i c h t h e c h a r g e - t r a n s f e r band i s n o t observed  e i t h e r because i t i s too f a r i n the  or i t i s overlapped  by a d o n o r b a n d .  ultraviolet  The same  techniques  as a r e u s e d f o r t h e c h a r g e - t r a n s f e r band c a n be a p p l i e d t o the s h i f t e d  band.  The s h i f t i s due t o t h e r a i s i n g  energy of the lowest unoccupied  of the  o r b i t a l o f i o d i n e on c o m p l e x  f o r m a t i o n , t h i s o r b i t a l s e r v i n g as both  the upper l e v e l of  t h e i o d i n e a b s o r p t i o n i n t h e v i s i b l e and t h e a c c e p t o r of the c h a r g e - t r a n s f e r a b s o r p t i o n i n the u l t r a v i o l e t  level region.  I t seems e v i d e n t t h a t t h e r e i s a c o r r e l a t i o n b e t w e e n t h e s t a b i l i t y o f t h e c o m p l e x , a s d e f i n e d by t h e e n t h a l p y f o r m a t i o n , and t h e b l u e - s h i f t o f t h e i o d i n e b a n d .  of  The  c o r r e l a t i o n b e t w e e n t h e m a g n i t u d e o f t h e b l u e s h i f t and t h e enthalpy  of formation  i s shown i n t h e f o l l o w i n g g r a p h .  1 5 4 c  1 5 5 , 1 5 6  16.0 112.13  2.0  0  F i g u r e 16.  4.0  6.0 8.0 -Atf°(kcal/mo!e)  10.0  12.0  14.0  C o r r e l a t i o n between t h e b l u e s h i f t o f t h e v i s i b l e I band ( A W i = h v p - hvf i 2  v  s  c o m  r e e  )  2  and t h e e n t h a l p y o f f o r m a t i o n , - A H ° , f o r some I 2 complexes. (From r e f . 1 5 4 c ) .  Mulliken b o n d i n g M.O.,  u  anti-  the i o d i n e molecule complex, and t h e n  i n the v i s i b l e  band, must be l a r g e r  M.O.'s i n t h e n o r m a l s t a t e o f 1^-  the outer occupied  u  has p o i n t e d o u t t h a t t h e o  w h i c h c o n t a i n s t h e e l e c t r o n e x c i t e d by t h e  absorption of l i g h t  (a -  1 5 7  i s t h e n p a i r e d up w i t h a p a r t n e r  than  When m a  i s excited by'absorption of l i g h t  , ) , i t s suddenly g  swollen size increases the repulsion  energy between i t and t h e donor.  This r e p u l s i o n energy,  w h i c h s h o u l d be g r e a t e r t h e more i n t i m a t e t h e c o m p l e x , i s added t o t h e u s u a l energy o f t h e e x c i t e d i o d i n e giving a blue s h i f t  i n the absorption frequency.  molecule, I fthe  stability  o f a c o m p l e x c a n be i n t e r p r e t e d i n t e r m s o f t h e  donor s t r e n g t h o f t h e donor m o l e c u l e ,  then, from t h e  m a g n i t u d e s o f t h e b l u e - s h i f t , one may d e d u c e t h e r e l a t i v e d o n o r s t r e n g t h s f o r a s e r i e s o f compounds. Simple molecular complexes i n v o l v i n g  o r b i t a l treatment  i o d i n e as a c c e p t o r  the complex P y r i d i n e • I .  h a s b e e n done w i t h  Qualitatively  1 5 8  2  of these  t h e same  i n t e r p r e t a t i o n s c a n be p l a c e d o n t h e f r e q u e n c i e s o f t h e s h i f t e d b a n d a n d t h e c h a r g e - t r a n s f e r b a n d a s was d e d u c e d by M u l l i k e n .  The r e s u l t s o b t a i n e d  show t h a t , f o r  i n c r e a s i n g i n t e r a c t i o n between t h e n i t r o g e n o f t h e p y r i d i n e r i n g and t h e i o d i n e m o l e c u l e , band i n c r e a s e s .  the frequency  of the s h i f t e d  A l s o , f o r a l l degrees o f i n t e r a c t i o n  between t h e donor and a c c e p t o r , i n c r e a s e i n the frequency  the treatment  p r e d i c t s an  o f t h e charge t r a n s f e r band, a  small decrease i n the frequency a decrease i n the enthalpy  o f t h e v i s i b l e band, and  o f f o r m a t i o n w i t h i n c r e a s i n g base  electronegativity. The  g e n e r a l r e s u l t s q u o t e d above o u t l i n e s i m p l y  some o f t h e f e a t u r e s o f d o n o r - a c c e p t o r to  t h e work u n d e r t a k e n i n t h i s t h e s i s .  review  For a general  o f t h e work i n t h e f i e l d o f c h a r g e - t r a n s f e r  the books l i s t e d reviews  complexes r e l e v a n t  listed  complete  complexes,  i n r e f . 159 a r e a v a i l a b l e a l o n g w i t h some  i n r e f . 160.  III.  B. C h a r g e - T r a n s f e r C o m p l e x e s o f I o d i n e  with  Methylphosphonitriles  The  r e l a t i v e base s t r e n g t h s  molecules are usually considered density The  a t the nitrogen,  of  heteroaromatic  i n terms o f ir-charge  a s a f f e c t e d by s u b s t i t u e n t s .  p h o s p h o n i t r i l i c r i n g i s c h a r a c t e r i z e d by t h e p r e s e n c e  of a l o n e p a i r o f e l e c t r o n s on each n i t r o g e n ,  though  f o r m a l l y t h e s e a r e i n v o l v e d i n t h e homomorphic rr-system. Owing t o t h e g r e a t  d i f f e r e n c e s i n e l e c t r o n e g a t i v i t y between  phosphorus and n i t r o g e n Pauling  ( 2 . 1 a n d 3.0 r e s p e c t i v e l y o n t h e  _ , 161. . , _ ^' ^ , Scale ) , h o w e v e r , much o f t h e e l e c t r o n d e n s i t y u  i s concentrated  on t h e n i t r o g e n  atoms and b a s e s t r e n g t h s o f  u s i•g n i f i c a n t . t h e s e m o ln e c u lie s s huo u lIA d be h a v e m e a s u r e d t h e P-K  a  phosphonitriles  values  o f a l a r g e number o f d i f f e r e n t  i n nitrobenzene with  a c i d as a reagent.  Shaw a n A d c o w o ri k e r s 162,169  I n some c a s e s ,  the use of p e r c h l o r i c  b o t h P a-^ K  a n <  ^ P  K a  2  v a  l  u e s  c o u l d be d e t e r m i n e d f o r t h e i n t r o d u c t i o n of. t h e f i r s t and second protons. Their with of  Some o f t h e r e s u l t s a r e p r e s e n t e d i n T a b l e 8. results illustrate  the electron-withdrawing  the substituents.  stronger a general  how b a s e s t r e n g t h s  or -supplying c h a r a c t e r i s t i c s  Some c y c l i c  tetramers are s l i g h t l y  bases than the analogous t r i m e r s , but t h i s rule,  the reverse  electron-supplying  vary  being  often  i s not  t r u e when s t r o n g l y  substituents are present.  T a b l e 8' R e p r e s e n t a t i v e pK ' V a l u e s f o r P h o s p h o n i t r i l e s i n N i t r o b e n z e n e a t 25°.  Compound (  N P C 1  P a/ K  2 3 or 4 )  (NPPh ) (NPPh )  4  (NPEt )  3  (NPEt )  4  2  2  -6.0  169  1.50  2  2  Ref  :  2.20 .  164 -5.80  6.40 7.60  164 164  0.20  164  [NP(OMe) ]  3  -1.9  164  [NP(OMe) ]  4  -1.0  164  2  2  [NP(NHMe) ]  8.8 ± 0.6  [NP(NHMe) ] [NP(NHEt) ]  3  [NP(NHEt) ]  4  [NP(NMe ) ]  3  2  2  2  2  [NP(NMe ) ] 2  -2.0 ±0.4  163  8.2  3.4  163  8.2  -1.3  163  8.1  3.8  163  7.6  -3.3  163  8.3  0.6  163  When c o n s i d e r i n g t h e r e l a t i o n s h i p b e t w e e n b a s e strengths  and  u - e l e c t r o n d e n s i t i e s , t h e homomorphic  h e t e r o m o r p h i c ir-systems  are e x p e c t e d t o have d i f f e r e n t  e f f e c t s as a f u n c t i o n o f r i n g s i z e .  The  heteromorphic  system decreases charge d e n s i t y a t the r i n g  nitrogen  atoms as r i n g  s i z e i n c r e a s e s , an e f f e c t w h i c h w o u l d  to decreasing  base s t r e n g t h w i t h  For  and  increasing ring  lead  size.  t h e h o m o m o r p h i c s y s t e m , h o w e v e r , an a l t e r n a t i o n i n  the  c h a r g e d e n s i t i e s a t n i t r o g e n i s c a l c u l a t e d (see C h a p t e r I ) , resulting  i n the  expected order  pentamer > t r i m e r .  of base s t r e n g t h s  tetramer  Along w i t h the c o n s i d e r a t i o n of  the  c h a r g e d e n s i t i e s , t h e e f f e c t s of h y b r i d i z a t i o n must a l s o considered  as r i n g  s i z e s are v a r i e d .  mean a n g l e s a t n i t r o g e n (N P C1 3  3  8 6  121.4°; N P C l g 4  suggests t h a t the size. are  i n the  I f the  1 3  4  A comparison of  131.3°; N ^ C l ^  hence  donation.  making the The  two  and  148.6°)  sp h y b r i d o r b i t a l ,  ring  increasing p  atom the  character,  e l e c t r o n s , more r e a d i l y a v a i l a b l e f o r  sp  cases.  tertiary Inqold  4 1  amine  (e.g.  and  trimethylamine)  had  t o t h e d i f f e r e n c e s i n hnvy bi orrirda ix -  r e s p e c t i v e l y , of the A d i f f e r e n c e o f 4.5  lone p a i r  i n t h e pK  orbital,  's w e r e a.  observed.  the  d i f f e r e n c e i n b a s i c i t y between p y r i d i n e  b e e n a t t r i 2b u t e d3 by  i n the  6  lone p a i r of e l e c t r o n s a t the n i t r o g e n  a typical aliphatic  z a t i o n , sp  1  increase with  angle at n i t r o g e n would r e s u l t i n i n c r e a s e d and  be  chlorophosphonitriles  angles at nitrogen  t a k e n .to o c c u p y an  >  Basicities by  Ranganathan  with  showed a s t e a d y  4 2  increasing ring  N P Me 4  of the methylphosphonitriles  4  pK * 2  g  pK  a  size  increase  (N P Me 3  3  = 5.72 ± .01; N P M e 5  = 3.97 ± . 0 1 ) .  5  6  1 Q  p'K pK  determined  i n base  = 5.03 ±  strength  .01);  = 6.69 ± .01,  a  T h i s was i n t e r p r e t e d a s b e i n g  due t o  a  the  dominating e f f e c t s of a - h y b r i d i z a t i o n over that of  electron  density. I n t h e measurement o f b a s e s t r e n g t h s  determination absolutely «  o f pK  values,  c l e a r trends  •  i 1  '  _  I 1  i t seems t h e r e f o r e t h a t no  c a n be o b s e r v e d . •  "1  1  by t h e  1  ^  1  The i n h e r e n t i 1  "1  rf—  y—  a  p r o b l e m i n t h i s method i s t h a t , as w e l l a s t h e e f f e c t s o f ^  I  "1  B  I  1  1  1  '  T  •  I  T  1  t  '  e l e c t r o n d e n s i t y and a - h y b r i d i z a t i o n , s o l v a t i o n e f f e c t s also play  a major r o l e  strengths. •  1  the T  T  I  If this  •  I  '  i n determining  solvation effect l  1  l  i  I  *1  evident.  1  I  "  1  1  c a n be e l i m i n a t e d , I 1  1  T  i  l  i  •  —  /~  - i  and h y b r i d i z a t i o n w o u l d be more  clearly  The u s e o f d o n o r - a c c e p t o r c o m p l e x e s , w i t h t h e  •  f  "1 "I  1  T  s i n c e no f o r m a l l y c h a r g e d effects  i  - l - i  6 1 hi^^ 3_ tio s 1"T^) n I t !C i 1 s s SL S clo noi* s  are therefore In u s i n g  attempt  involving the  base  r e l a t i o n s h i p between b a s e s t r e n g t h a n d t h e e f f e c t s o f  electron density  an  the r e l a t i v e  species  i  t ti 1 s  —  *1  T  I  •  a r e formed a n d s o l v a t i o n  the methylphosphonitriles  to detect charge-transfer i o d i n e as a c c e p t o r ,  i n greater  GCJ U. i r cm on t "1  negligible.  complex  the methyl  p h o s p h o r u s atoms a r e e x p e c t e d  resulting  s SL t i s f  •  as donors i n formation  s u b s t i t u e n t s on  t o be e l e c t r o n r e l e a s i n g ,  charge d e n s i t i e s a t the r i n g  nitrogen  atoms and  t h e r e f o r e s i g n i f i c a n t donor p r o p e r t i e s .  e x i s t e n c e of the complex P y r i d i n e • I  1 5 6 2  w o u l d l e a d one  expect a s i m i l a r type of i n t e r a c t i o n to occur methylphosphonitriles  and  iodine.  n u c l e a r m a g n e t i c r e s o n a n c e and spectroscopy  The  The to  between  methods of  the  proton  visible-ultraviolet  a r e used t o determine the e x t e n t of the  inter-  actions .  III.  B.l.  Experimental  The p r e v i o u s l y and  methylcyclophosphonitriles  c o m m e r c i a l i o d i n e used were s u b l i m e d  vacuo immediately complexes  p r i o r t o use.  (NPMe ) -xI 2  n  were o b t a i n e d  2  s o l u t i o n s o f i o d i n e and  u s i n g matched q u a r t z  over s i l i c a  on a C a r y 14  g e l ) by Spectra  mixing of  spectrophotometer  cells.  d e t e c t i o n o f new  o f the components a l o n e Hildebrand  the  i n dichloromethane  the p h o s p h o n i t r i l e s .  t h e complexes were r e c o r d e d  in  E l e c t r o n i c spectra of  (Mallinckrodt Spectrograde'dried  The  synthesized  absorptions  l e d t o the use  n o t due  of the  to e i t h e r  Benesi-  method t o d e t e r m i n e the e q u i l i b r i u m c o n s t a n t  complex f o r m a t i o n .  Solutions containing varying ratios  methylphosphonitrile absorbance values  t o i o d i n e were p r e p a r e d  and  S o l u t i o n s of the pentameric  of  the  a t s p e c i f i c w a v e l e n g t h s d e t e r m i n e d on  Turner Spectrometer.  of  a  methyl-  phosphonitrile nitrogen as A.  were p r e p a r e d  u n d e r an a t m o s p h e r e o f d r y  due t o i t s h y d r o s c o p i c p r o p e r t y .  The r e s u l t s a r e  follows: Determination of Equilibrium (1) N P M e 3  3  + I  6  2  [ N P M e ] = 5.44 x 1 0 ~ 3  3  Constants  M  4  6  Temp.  _  room temp.  Table 9  Molarity  of I  1.130 ± 0.050  5. 64  1.040 ± 0.050  4. 51  0.960 + 0.025  3.38  0.880 ± 0.025  2.25  0.725 ± 0.010  1.13  0.500 ± 0.005  4  +. I  g  2  [ N P M e ] = 4.27 x 1 0 ~ 4  O p t i c a l D e n s i t y a t 360 my  3  6.76  (2)' N P M e 4  x 10  4  g  4  M  Temp. = room temp.  T a b l e 10  Molarity of I  5  x 10  3  Optical Density  a t 380 my  6.12  1.190 ± 0.050  5.10  1.060 ± 0.050  4. 08  0.900 ± 0.025  3. 06  0.755 ± 0.010  2. 04  0.570 ± 0.005  1. 02  0.321 ± 0.005  ;(3) N P M e 5  2  1 Q  + 1"  2  [N P Me 5 5 10  _  L  4.02 x 1 0 "  4  M  Temp.  room temp.  Table IT  Molarity of I  2  x 10-  Optical Density  a t 420 my  7.13  1.370 ± 0.050  5.94  0.970 ± 0.025  4.75  0.672 ± 0.010  3. 56  0.396 ± 0.0050  2.38  0.125 ± 0.0025  °'l  C  Plots of  —  versus  yield  i n t e r a c t i o n of N ^ M e g  straight lines  and N ^ M e g  for the  with iodine,  indicative  o f t h e f o r m a t i o n i n s o l u t i o n o f 1:1 d o n o r - a c c e p t o r c o m p l e x e s . c °t Plots of ~ — versus -^r yield a straight line f o r the c A D  1  A  C  interaction of N P Me 5  5  and I .  1 Q  2  A modified Benesi-Hildebrand  equation C °l  ,  n  ~~A  K7-  =  c  (  CT°-2)  +  do)  A  X  x  c a n be o b t a i n e d f o r t h e e q u i l i b r i u m N P Me 5  5  by m a k i n g a f u r t h e r  1 0  B.  2I v 2  assumption  Tentatively, then, t h i s donor-acceptor  +  process  —  N P Me 5  5  1 0  -2l  2  of C ° being equal to C . D  i n d i c a t e s t h e f o r m a t i o n o f a 1:2  complex.  A s s i g n m e n t o f A b s o r p t i o n Bands For donor-acceptor  complexes i n v o l v i n g  l , two 2  a b s o r p t i o n s n o t due t o e i t h e r o f t h e components a l o n e a r e expected. shifted  The i o d i n e v i s i b l e a b s o r p t i o n i s e x p e c t e d  t o s h o r t e r wavelengths,  to i o d i n e i n a complex.  this  t o be  a b s o r p t i o n b e i n g due  An i n t e n s e b a n d , a t s h o r t e r wave-  l e n g t h s t h e n t h e i o d i n e band, i s due t o t h e i n t e r m o l e c u l a r  charge-transfer (NPMe ) 2  n  transition.  The e l e c t r o n i c s p e c t r a o f  n = 3,4,5, i n s o l u t i o n w i t h I  new a b s o r p t i o n uncomplexed  show the f o l l o w i n g  bands as w e l l as the band a t 503 my due t o  I :  Table 12  Compound  New A b s o r p t i o n Band Maxima (my)  N P Me  6  390,257.5  N P Me  g  360,285.0  N P Me  1()  363,292.0  3  4  5  3  4  5  Analogous t o the s p e c t r a o f complexes i o d i n e and other o-donors, the longer  involving  wavelength  i n each case i s a s s i g n e d as t h e i o d i n e a b s o r p t i o n s h o r t e r wavelength a b s o r p t i o n  the  following  and the  as t h a t due to  from the donor t o i o d i n e .  absorption  charge-transfer  The r e s u l t s a r e summarized i n  table: Tablee 13  Donor CH C1  • K c  3  3  N  4 4  N  5 5  P  P  M e  M e  6  8 lO  _CT "max  3  257.5  10.3 x l 0  7.23xl0  3  285.0  3.90 x l 0  3.85 x l 0  4  292.0  2.20 x, 1 0  2' max 897  390  3.74 x l 0  i n 14 j Jim  360  5.39xloVm" ^  363  2  N P Me  ^CT max  A max 503  477 Sim'  1  „ -1  I  e  4  5  4  C.  Nuclear Magnetic  Resonance S p e c t r a o f  Donor-Acceptor  Mo:lecular Complexes The  f o r m a t i o n of c h a r g e - t r a n s f e r complexes  h a v i n g b e e n e s t a b l i s h e d by t h e e l e c t r o n i c  s p e c t r a , the  c h e m i c a l s h i f t s o f t h e p r o t o n s on t h e p h o s p h o n i t r i l i c  rings  r e l a t i v e t o i n t e r n a l t e t r a m e t h y l s i l a n e were d e t e r m i n e d a s c e r t a i n the magnitude of charge molecule.  to  donation to the i o d i n e  Upon c o m p l e x f o r m a t i o n , i t i s e x p e c t e d  t h a t the  p r o t o n s become more d e s h i e l d e d a s e l e c t r o n d e n s i t y i s removed from the p h o s p h o n i t r i l i c  ring.  Proton chemical s h i f t s of v a r y i n g r a t i o s phosphonitrile to i  2  were o b t a i n e d i n C H C 1  HA-60 s p e c t r o m e t e r  at a temperature  2  o b t a i n e d showed a d o u b l e t due  2  o f 35°C.  on a V a r i a n A l l spectra  to coupling w i t h the  n u c l e i , t h e c o u p l i n g c o n s t a n t b e i n g 12 c . p . s .  of  phosphorus  Though t h e  c h e m i c a l s h i f t s move i n c r e a s i n g l y d o w n f i e l d - w i t h an i n c r e a s e i n the r a t i o of l , 2  the equivalence of the protons  a r a p i d e q u i l i b r i u m p r o c e s s whereby l among t h e d o n o r s i t e s . following  The  2  molecules  indicate  are  exchanged  r e s u l t s are summarized i n the  table:  A =•• 6 , , - 6 obsd parent  T a b l e 14  Compound  6  A  a  N-.P-.Me, 3 3 6  1.40  0  N P Me  1.41  0  1.40  0  1. 56  0.16  1. 63  0.23  1.67  0.27  1.56  0.15  1. 63  0.22  1.52  0.12  1. 62  0.22  4  4  g  N P Me 5  5  N P Me 3  3  1 Q  6  NgPgMe N P Me 3  N  3  4 4 P  M e  6  8  +  X  +  2 I  2  +  3 I  2  +  I  N.P.Me_ + 4 4 8  2  2 2  2 I  N P Me  1 Q  +  X  N P Me  1 Q  +  2 I  5  5  D.  obsd  5  5  2  2  I n p.p.m. Temperature E f f e c t s on t h e C h e m i c a l S h i f t o f N - ^ M e ^ ^ P r o t o n c h e m i c a l s h i f t s o f N-P-Me .I„ w e r e o b t a i n e d c  a t a s e r i e s o f l o w t e m p e r a t u r e s i n an e f f o r t t o s l o w t h e exchange o f I  2  among t h e d o n o r s i t e s a n d h e n c e a l l o w a  d i f f e r e n t i a t i o n of the protons closest  t o t h e donor  However, t h e d e c r e a s e i n t h e t e m p e r a t u r e r e s u l t e d the  site.  only i n  broadening o f t h e resonances b u t t h e spectra s t i l l  showed  31 a single doublet.  Decoupling of the  P nuclei  resulted  i n t h e o b s e r v a t i o n o f a s i n g l e o e a k w h i c h became b r o a d e r a s the  t e m p e r a t u r e was d e c r e a s e d .  A t t h e extremely low temperatures  the chemical s h i f t s appear t o move i n c r e a s i n g l y d o w n f i e l d . The r e s u l t s determined  i n CDC1  3  r e l a t i v e to i n t e r n a l  t e t r a m e t h y l s i l a n e on a V a r i a n HA-100 spectrometer a r e as follows:  Table 15  Compound  N  3 3 P  M e  6  N P Me -I 3  3  Temperature  g  2  N P->Me, • I» 0  N P Me -I 3  g  2  35°C  1.50  35°C  1.55 -  0°C  1.55  -30°C  1.60  -80°C  1.65  In p.p.m.  a  E.  3  obsd  Synthesis of N - ^ M e ^ ] ^ N P Me -I 3  3  solutions of I heptane,  g  2  .  can be prepared by m i x i n g t o g e t h e r  and N ^ M e g  2  ':'  i n a nonpolar s o l v e n t such as  the q u a n t i t a t i v e f o r m a t i o n o f an orange  o c c u r r i n g almost immediately upon m i x i n g .  precipitate  The s o l i d was  found t o be s o l u b l e i n the more p o l a r s o l v e n t s such as CHC1  3  and C H C 1 2  2  but i t i s i n s o l u b l e i n nonpolar  solvents.  The  s o l i d c a n a l s o be s u b l i m e d  heating without nature.  decomposition,  i n vacuo w i t h  i n d i c a t i v e of i t s molecular  C r y s t a l s o f t h e compound s u i t a b l e f o r c r y s t a l a n d  molecular  structural determination  were o b t a i n e d  d i s s o l v i n g t h e powder i n a m i x t u r e  o f CHC1  then a l l o w i n g the solvent t o evaporate The  C  Expected  by  and C C 1  3  4  and  slowly.  m i c r o a n a l y s i s r e s u l t s a r e as f o l l o w s :  Element  Found  slight  (%)  (%)  H  N  15.12  3.81  8.82  15.33  3.91  8.58  E l e c t r o n i c s p e c t r a o f t h e compound i n C H C 1 2  the  same a b s o r p t i o n s  N P Me 3  3  6  and I  as were o b s e r v e d f o r .mixtures  c o m p l e x a s d e t e r m i n e d by t h e m e t h o d o f B e n e s i spectra of the s o l i d  of  and H i l d e b r a n d .  i n v a r i o u s s o l v e n t s show  s m a l l v a r i a t i o n s i n a b s o r p t i o n maxima.  The p o s i t i o n o f t h e  s h i f t e d i o d i n e band i n v a r i o u s s o l v e n t s a r e a s f o l l o w s :  T a b l e 16 Solvent c  show  , c o n f i r m i n g t h e r e s u l t s o f a 1:1 d o n o r - a c c e p t o r  2  The  2  c  4  395  3  CH C1 2  1 2  405  l  CHC1  A ' max  2  390  (my)  III.  B.2.  Discussion  From t h e v a l u e s  of K  and t h e p o s i t i o n s o f t h e c  charge-transfer certain  and b l u e - s h i f t e d i o d i n e b a n d s , i t i s  that the methylphosphonitnles  toward i o d i n e , u t i l i z i n q  a c t as n - d o n o r s  t h e n i t r o q e n l o n e p a i r s i n much  t h e same way a s do a m i n e s i n t h e i r Q1G C110 O IT 9. C C 6]3 t o 10 S •  interaction  The d e p e n d e n c e o f t h e e n e r g y o f upon t h e i o n i z a t i o n P ° ^ i a l b e e n p r e v i o u s l y shown. of N P M e 3  7.99 is  3  6  and N ^ M e  eV r e s p e c t i v e l y .  o  f  a  s  e  r  i  e  s  o  f  with  charge-transfer a  m  i  n  e  s  h  a  s  The i o n i z a t i o n p o t e n t i a l s h a v e b e e n d e t e r m i n e d as 8.35 Though  the value  and  f o r Nj-Pj-Me^Q  n o t known, i t c a n be a p p r o x i m a t e d a s 8 eV by  comparison  with the observed trend of i o n i z a t i o n p o t e n t i a l s f o r the series of chlorophosphonitriles.  2 1  P l a c i n g the  p o i n t s on t h e p l o t o f e n e r g y o f c h a r q e - t r a n s f e r i o n i z a t i o n p o t e n t i a l f o r some t y p i c a l it  i s evident  corresponding versus  amines  (Fiq. 21), g t h a t o n l y N_,P.,Me, a c t s a s a t y p i c a l a m i n e ,  w h e r e a s t h e p o i n t s f o r N.P.Me,, a n d N P_Me,_ f a l l 4 4 8 5 5 10  well off  the  energies  r  c  line.  The o b s e r v e d c o r r e l a t i o n b e t w e e n t h e  of charge-transfer  and t h e i o n i z a t i o n p o t e n t i a l s f o r t h e  series of methylphosphonitriles  does c o n f i r m ,  however,  that ionization  i n v o l v e s t h e l o s s o f an e l e c t r o n f r o m  t h e homomorphic  -rr-system, o f w h i c h t h e n i t r o g e n  lone-pairs  T  Z /eV D  F i g . 21.  P l o t o f energy o f c h a r g e - t r a n s f e r v e r s u s i o n i z a t i o n p o t e n t i a l from r e f . 160(e). Added p o i n t s a r e 11, 12, 13. The p o i n t s correspond to (1) ammonia; (2) p y r i d i n e ; (3) methylamine; (4) ethylamine; (5) n-butylamine; (6) dimethy1amine; (7) d i e t h y l a m i n e ; (8) t r i m e t h y l a m i n e ;  (lD^P^Meg^lI)  N P Megt 4  4  U3) N ^ M e ^ . '  are an i n t e g r a l  part.  The assignment  o f the m e t h y l p h o s p h o n i t r i l e s as  n-donors i s confirmed by c r y s t a l l o g r a p h i c s t u d i e s o f the complex N P M e • I . 3  3  6  The s t r u c t u r e was found t o be  1 7 0  2  s i m i l a r t o t h a t o f a l l the amine-iodine complexes d e t e r ¬ mined thus f a r , w i t h the N••••I-I CH  ^ , C H  3  group  found t o be l i n e a r ,  3  N CH-  •CH,  /  CH  F i g u r e 22.  N  \  3  S t r u c t u r e o f N^P^Me 'I . c  0  The N-I d i s t a n c e (2.42 A) i s s h o r t e r than the sum o f the Van der Waals r a d i i and the I - I d i s t a n c e (2.82 A) i s i n c r e a s e d over t h a t o f uncomplexed I  2  (2.67 A ) ,  being c o n s i s t e n t w i t h the d o n a t i o n o f charge i n t o the o a n t i b o n d i n g o r b i t a l on i o d i n e . The s i m i l a r i t i e s between N P M e * I 3  3  6  2  and the  amine-iodine complexes a r e summarized i n Table 17.  u  T a b l e "IT Structural  Compound 4-methylpyridine • I (CH ) N'I 3  3  2  N P Me -I 3  3  6  171  1 7 0 2  Information  N-I(A)  I-I(A)  2.31  2.83  2.27  2.83  N• • • I - I  2.42  2 . 82  N• • - I - I  The two P-N b o n d s i n v o l v i n g bonded t o i o d i n e a r e s l i g h t l y the other  Configuration • •«N•••I-I l i n e a r  the nitrogen  of only  s l i g h t electron withdrawal  smaller  angle a t the i n t e r a c t i n g  as c o m p a r e d t o t h a t a t t h e o t h e r mean 124.4°, i s c o n s i s t e n t w i t h  atom  indicative  from t h e r i n g .  The  n i t r o g e n a t o m , 123.4°, two r i n g n i t r o g e n less  atoms,  electron density i n  t h e P-N b o n d s m e e t i n g a t t h e i n t e r a c t i n g i n weaker i n t e r b o n d  linear  l o n g e r , mean 1.64 A, t h a n  f o u r P-N b o n d s , mean 1.60 A, b e i n g  resulting  linear  repulsions.  effect  nitrogen  atom,  The o b s e r v e d  i n t h e s t r u c t u r e s o f [ (NPMe_) .H] C u C l / and 175 2 4 3 [(NPMe ) H] CoCl a r e much l a r g e r , a s e x p e c t e d , s i n c e 7 4  2  4  2  4  more e l e c t r o n d e n s i t y i s r e m o v e d f r o m t h e r i n g upon ation.  The f o r m a t i o n  o f a d o n o r - a c c e p t o r complex can  t h e r e f o r e be v i e w e d a s an i n t e r m e d i a t e formation and  proton-  stage i n the  o f a a-bond b e t w e e n t h e n i t r o g e n atom o f t h e r i n g  the nearest  i o d i n e atom o f t h e l  2  molecule.  Continuing energies  w i t h t h i s t r a i n of thought,  o f c h a r g e - t r a n s f e r m i g h t be  l a t e w i t h the l o c a l i z a t i o n energies  expected  the  to corre-  required for  the  r e m o v a l o f a p a i r o f e l e c t r o n s f r o m t h e h o m o m o r p h i c TTsystem of the r i n g ,  s u c h as o c c u r s  upon p r o t o n a t i o n  a l k y l a t i o n o f a r i n g n i t r o g e n atom. donor-acceptor  should The  Though i n t h e c a s e  of  c o m p l e x f o r m a t i o n , o n l y a s m a l l amount o f  c h a r g e i s removed from the expected  or  still  ring,  the q u a l i t a t i v e  results  hold.  l o c a l i z a t i o n energies  methylphosphonitriles  can  be  f o r the  s e r i e s of  c a l c u l a t e d as f o l l o w s .  L o c a l i z a t i o n Energy C a l c u l a t i o n s B a s e d on treatment  of the  the  readily obtained. 23 and  being  f o r example N  on  3  3  g  the  x  represent  orbitals.  such t h a t the will  orbitals,  nature, The  molecule i s used i n t h i s case i n  secular determinants  be  the  same a t o m , t h e a t o m i c  s p e c i f i e d as t o t h e i r e x a c t  intermediate o r b i t a l s  the  t h e n u m b e r i n g scheme shown i n  combined t o form the m o l e c u l a r the N P M e  Orbital  s e r i e s of p h o s p h o n i t r i l e s can  Using  letting  appropriate o r b i t a l without  Hiickel Molecular  ir-levels i n phosphonitrilic rings,  Tr-energy l e v e l s o f any  Figure  simple  symmetry forming  s o l v i n g of  become s i m p l i f i e d .  are  the For  the  of  CH o CH 3  CH  x  |2 N  r  homomorphic symmetric  23.  CH 3  6  N,  CH  Figure  Q  CH  3  3  Numbering scheme of r i n g  atoms.  system of N ^ M e g , the symmetric  and a n t i -  combinations of atomic o r b i t a l s a r e a s f o l l o w s  (symmetric)  A  '  :  N  l '  /f  ( P  2  + P  6 '  /f ^ 3  )  N  + N  5 ' < )  P  (P -Pg) , J=- (N "N ) .  (antisymmetric) A":  2  3  5  S e t t i n g the r e l a t i v e e l e c t r o n e g a t i v i t i e s a  N  = a  p  by u s i n g  + 23 and u s i n g a = ^ ( a + a ) , the s e c u l a r d e t e r N  p  minants are as f o l l o w s :  a + 3 - E /2~B  ...  /2  e  a - 3  0  3  0  0  E  0  0  3  0  a + 3 - E /2  3 •  a - 3  - 3 - E  a  A"  3  6  Substituting  x -  a  and s o l v i n g ,  E g  o b t a i n e d a r e as  E = a ± 2 . 2 3 6 0 3,  A" :  E = a ± 1.4142 3 Filling  the  the eigenvalues  follows:  A':  pairs  = 0  a + 3 - E  a ± 1.4142 3  the three lowest l e v e l s with the lone  of electrons  on t h e n i t r o g e n atoms, t h e energy o f  Tr-system i s f o u n d t o be 10 .1288  3 r e l a t i v e t o a.  L o c a l i z a t i o n of a pair of electrons the  n i t r o g e n atoms r e s u l t s  the  r e m a i n i n g f i v e a t o m s , t h e e n e r g y o f w h i c h c a n be  similarly  i n a ir-system  o n one o f  consisting of  calculated.  (symmetric)  (P^Pg) , ^  A':  ( a n t i s y m m e t r i c ) A":  i (  P  -  P  ),  v2 The c a l c u l a t e d A': A" :  e i g e n v a l u e s a r e as  ( N  i  /T  2  + N  4  ' 3  )  P  (N„-N„) z  4  follows:  E = a ± 2 3 , E = a - 3 E = a ± vT 3  The two l o w e s t l e v e l s a r e o c c u p i e d by t h e f o u r lone-pair  electrons  o f t h e n i t r o g e n atoms i n t h e s e g m e n t ,  with the l o c a l i z e d  p a i r o f e l e c t r o n s now o c c u p y i n g t h e  l e v e l o f e n e r g y a + 3, e q u i v a l e n t as d e f i n e d electron  f o r n i t r o g e n atoms.  system i s t h e r e f o r e  t o t h e Coulomb  integral  The e n e r g y o f t h e s i x -  f o u n d t o b e 8.8284 3 r e l a t i v e  t o a. The  localization  energy r e q u i r e d f o r t h e removal  of a p a i r o f e l e c t r o n s from t h e N P M e 3  therefore  (10.1288  3  .-system i s  6  - 8.8284)3 = 1.3004 3.  S i m i l a r c a l c u l a t i o n s c a r r i e d through f o r N ^ M e g and N P M e 5  5  yield - localization  1 Q  energies  o f 1.2157 3 a n d  1.2429 3 r e s p e c t i v e l y . Figure  24 shows t h e c h a n g e s  levels of N P Me 3  electrons.  3  upon l o c a l i z a t i o n  6  Figure  of a lone-pair of  2 5 shows t h e r e l a t i o n s h i p b e t w e e n  s i z e and l o c a l i z a t i o n The  i n the o r b i t a l  ring  energies.  consideration of localization  energies  t h e r e f o r e p r e d i c t a l a r g e r energy o f c h a r g e - t r a n s f e r f o r N  3 3 p  M e  6  t  h  a  n  N  4 4 P  M e  8  o  r  N  give the correct order lowering  5 5 P  M e  10'  s  f  o  u  n  d  for the latter  of. the energy r e q u i r e d  to reasons o f geometry,  a  '  b u t does n o t  two c a s e s .  for N P Me  whereby t h e I  5  5  1 Q  may be due  molecule  t o a p p r o a c h more c l o s e l y t h e i n t e r a c t i n g n i t r o g e n because o f t h e l a r g e r angles and g r e a t e r m o l e c u l a r  i n t h e ten-membered  flexibility.  The  i s able atom ring  6-membered r i n g  F i g . 24 .  5-raerabered  Tr-levels o f N P M e 3  3  6  segment  a n d t h e 5-membered  segment  w h i c h r e s u l t s u p o n l o c a l i z a t i o n o f one l o n e pair of electrons.  Fig.  25.  A p l o t of l o c a l i z a t i o n energy versus size for phosphonitrilic rings.  ring  The m a g n i t u d e o f t h e b l u e - s h i f t o f t h e i o d i n e visible  band, as i n t e r p r e t e d by M u l l i k e n  1 5 7  i n d i c a t i o n o f t h e r e l a t i v e base s t r e n g t h s phosphonitriles, i n each case. the  h  e  * ^ o f  of the methyl-  The c o r r e l a t i o n o f t h e b l u e - s h i f t w i t h o f t h e complexes has a l r e a d y  The m a g n i t u d e o f t h e s h i f t s  methylphosphonitriles, are  g i v e s an  i f t h e mode o f i n t e r a c t i o n i s t h e same  formation  shown.  ,  been  f o r the series of  e x p r e s s e d as energy  differences,  a s shown i n T a b l e 1 8 .  T a b l e 18 • Magnitudes o f t h e B l u e - s h i f t s  Compound  AE  — hv  N P Me •! 3  3  4  4  N P Me 5  5  g  1 ( )  —2  0.980 eV  2  -I  0.94 9 eV  2  From t h e v a l u e s o f A E , i t seems t h e r e f o r e t  of base s t r e n g t h s  free  f  0.712 eV  g  N P Me •I  hv  comp  i s N„P,.Me 4  4  0  o  > N P Me c  c  n n  o  b  1 (J  that the order  > N P Me,. 0  3  0  3  6  The t r a n s f e r o f c h a r g e f r o m t h e d o n o r t o t h e a c c e p t o r even i n t h e ground s t a t e o f t h e complex have a m e a s u r a b l e donor m o l e c u l e s .  should  e f f e c t on t h e e l e c t r o n d e n s i t i e s o f t h e I n t h e i n t e r a c t i o n s o f i o d i n e and o f  iodine monochloride with various methylpyridines, shifts  downfield  o f t h e m e t h y l p r o t o n s i n t h e donor a r e o b s e r v e d  when t h e a c c e p t o r i s a d d e d .  '  Such s h i f t s a r e  e x p l i c a b l e i n terms o f a t r a n s f e r - o f - c h a r g e l o w e r i n g t h e methyl-proton s h i e l d i n g constants. shifts  Similar  downfield  o f t h e m e t h y l protons a r e d e t e c t e d i n t h e m e t h y l -  p h o s p h o n i t r i l e s when i o d i n e i s a d d e d . From t h e d i f f e r e n t i a l  s h i f t s measured,  i t is  possible to calculate A , the association s h i f t , f o r n  each m e t h y l p h o s p h o n i t r i l e i n t e r a c t i n g w i t h i o d i n e . P  ^  For  num. p r o c s s s t h e f o l l o w i n g e c j u a t i o n i s  applicable. A = A P Q  p  c  _ no. o f m o l e s o f c o m p l e x e d b a s e t o t a l no. o f moles o f base  A - differential A 6  Q  c  =• 6 . , - 6 obsd free = association shift = 6 - 6  free  shift  £  q  =  c h e m i c a  of ^complex  =  l  f r e e  s h i f t of t h e f r e e donor i n t h e absence  exchange c h e m i c a  l  s h i f t of t h e complexed donor i n t h e  absence o f exchange  Since the chemical s h i f t s  observed are not  extremely s e n s i t i v e to temperature over a small range, the  K  values determined  Benesi  and  a t room t e m p e r a t u r e  by  t h e method o f  H i l d e b r a n d are used t o c a l c u l a t e P  , hence c  e n a b l i n g the e v a l u a t i o n of The  A. Q  r e s u l t s a r e as  follows:  Table  19.--  Calculated Association Shifts  3. f  Compound N P Me -I  2  0.96  0.17  N P Me -I  2  0. 92  0.16  0.986  0.12  3  3  4  N  a  b  6  4  5 5 p  g  M e  In  io  , : c  3D  -0-  :  CH C1 2  2  I n u n i t s o f p.p.m.  Assuming t h a t the d o w n f i e l d s h i f t s of the r e s o n a n c e s a r e an i n d i c a t i o n o f t h e d e g r e e s o f b e t w e e n t h e d o n o r and  proton  interaction  the acceptor, then i n order  to  compare t h e r e l a t i v e d e g r e e s o f c h a r g e - t r a n s f e r , i t i s necessary  to take i n t o account  t h e number o f d o n o r  sites  a v a i l a b l e p e r m e t h y l p h o s p h o n i t r i l i c m o l e c u l e . . . The c a l c u l a t e d a s s o c i a t i o n s h i f t s a r e f o r t h e e f f e c t s o f one I molecule  i n t e r a c t i n g w i t h one donor m o l e c u l e .  2  Sincea l l  t h e p r o t o n s o f o n e d o n o r m o l e c u l e have t h e same c h e m i c a l shift,  e a c h n i t r o g e n atom e f f e c t i v e l y  interacts with ^  m o l e c u l e o f l , w h e r e n i s t h e number o f n i t r o g e n atoms 2  per donor m o l e c u l e . of  interaction,  9 = nA . Q  As a m e a s u r e  of the relative  degrees  an i n t e r a c t i o n p a r a m e t e r 6 i s d e f i n e d a s  Calculated values of 0 f o rthe series of methyl-  phosphonitriles i s listed  i n Table 20.  T a b l e 20 Values of the I n t e r a c t i o n  Parameter  Compound  e(p.p.m.)  N P Me -I  2  0.51  N P Me -I  2  0.64  3  4  3  4  N P Me 5  5  6  8  1 ( )  .I  0.60  2  B a s e d o n t h e c a l c u l a t e d v a l u e s o f 9, t h e o r d e r of  b a s e s t r e n g t h s i s o n c e more N P M e 4  4  g  > N P Me 5  5  1 Q  >  N^Me,,  b e i n g i n q u a l i t a t i v e agreement w i t h t h e c o n c l u s i o n s r e a c h e d by a c o n s i d e r a t i o n o f t h e b l u e - s h i f t s o f t h e i o d i n e  band.  Figure between  26 shows t h e r e a s o n a b l e q u a n t i t a t i v e a g r e e m e n t the r e l a t i v e magnitudes of AE  t  and e f o r t h e  series of methylphosphonitriles. The o r d e r i n g o f t h e m e t h y l p h o s p h o n i t r i l e s regards to  donor s t r e n g t h s  in  as d e d u c e d f r o m t h e c h a r g e -  t r a n s f e r c o m p l e x e s f o r m e d w i t h i o d i n e seems t o be w e l l established.  The p r i m a r y e f f e c t o f i - e l e c t r o n d e n s i t i e s  a t t h e r i n g n i t r o g e n atoms due t o t h e homomorphic s y s t e m o f ^ - i n t e r a c t i o n w o u l d g i v e t h e same o r d e r i n g o f b a s e strengths  as was  found but w i t h a greater d i f f e r e n c e  b e t w e e n t h e t e t r a m e r and t h e p e n t a m e r .  The i n c l u s i o n o f  a s e c o n d a r y e f f e c t due t o t h e d i f f e r e n c e s i n t h e s t a t e s o f h y b r i d i z a t i o n i n t h e two c a s e s w o u l d r e s u l t i n a g r e a t e r similarity  i n the donor s t r e n g t h s .  t r e n d of base s t r e n g t h s assumed  A general  upward  a s a f f e c t e d by h y b r i d i z a t i o n was  on g o i n g t o l a r g e r r i n g s because o f t h e i n c r e a s e d  p-orbital character  i n the hybrid  R e i d and M u l l i k e n  1 5 6  orbitals.  had found t h a t , f o r t h e  s h i f t e d i o d i n e b a n d , a c h a n g e i n p o s i t i o n t o s h o r t e r wavelengths  and a n i n c r e a s e i n i n t e n s i t y o c c u r on a d d i n g more  p y r i d i n e to the s o l u t i o n s of P y l  2  i n n-heptane.  s o l v e n t e f f e c t was o b s e r v e d f o r t h e c o m p l e x  A  similar  N^P Me,-I . ?  9  The d e c r e a s e i n w a v e l e n g t h o f t h e s h i f t e d i o d i n e band going from CC1  4  t o t h e more p o l a r s o l v e n t C H C 1 , 2  2  on  indicative  Fig.  26.  A p l o t o f A E a n d 6 v e r s u s r i n g s i z e t o show t h e a g r e e m e n t i n t h e two p a r a m e t e r s . t  of  increased  stability  of the complexes formed, i s  a t t r i b u t a b l e to the increased of t h e d i p o l a r complex the  formed.  stabilization  by t h e  The e f f e c t i s s m a l l  solvent since  amount o f c h a r g e t r a n s f e r r e d i n t h e g r o u n d s t a t e o f t h e  complex i s s m a l l .  CHAPTER I V  INNER- CHARGE-TRANSFER COMPLEXES  IV.  A.  Introduction  I t i s w e l l known t h a t a l o n g acceptor  with  forming donor-  c o m p l e x e s , where t h e t r a n s f e r r e d c h a r g e goes  the donor t o the a  from  m o l e c u l a r o r b i t a l of I , i o d i n e  u  i n t e r a c t s s t r o n g l y w i t h c e r t a i n bases, r e s u l t i n g i n the breaking  o f t h e i o d i n e - i o d i n e bond and t h e f o r m a t i o n  i o n i c compounds.  These  are termed i n n e r  complexes  as o p p o s e d  described  i n Chapter I I I . Glusker  to the outer  and M i l l e r  1 7 8  charge-transfer  charge-transfer  have examined  6  and h a s a m.p.  7  2  2  f u n c t i o n s worked  iodine.  i s water s o l u b l e , a l c o h o l i n s o l u b l e ,  o f 244°C.  i n s o l u b l e and i t s m.p.  complexes  two d i f f e r e n t  - i t i . , . . . 1 i* dn compounds f o r m e d by 4 - m e t h y l p y r i d x n e and  Compound I , ( C H N ) I  of  Compound I I , C g H N I , i s w a t e r  i s 83°C.  7  Radial  2  distribution  o u t b a s e d on i n t e n s i t i e s  c o l l e c t e d from  X - r a y powder e x p e r i m e n t s l e d t o t h e c o n c l u s i o n compound I has  an  ionic  compound i s an o u t e r c a r r y i n g out product  structure while  complex.  Hassel  the  and  a structural determination  o f p y r i d i n e and  second  Hope  1 7 9  (CJ-HJ-N) I 2  . cation.  compound I m e n t i o n e d a b o v e p r o b a b l y  has  by  of a r e a c t i o n  i o d i n e demonstrated the +  of a centrosymmetric  that  . c a t i o n of  The a  existence  similar  structure.  I-  N—  Figure  The  27.  The  +  2  cation.  s y s t e m P h A s and 3  a t t r i b u t e d by B h a s k e r e t . a l . complex t o the  i n n e r complex i s a p p a r e n t l y case.  Py I  v a r i a t i o n of the charge-transfer  i n t e n s i t y w i t h time of the  of the outer  N  . . . .  to the  I  2  band has  been  transformation  i n n e r complex, where  the  energetically stable i n  They p r o p o s e d t h e m e c h a n i s m o f t r a n s f o r m a t i o n  Ph As + I 3  i  2  x  '  [Ph As 3  outer [Ph As 3  • I ] 2  —>  •  I ] 2  complex  [Ph AsI] l" +  3  inner  complex  this as  The c h a r g e - t r a n s f e r band o f P h S b - I 3  d i d n o t show much  2  v a r i a t i o n w i t h t i m e as i n t h e c a s e o f P h A s - I , 3  of l i t t l e  i n n e r complex f o r m a t i o n .  the P h P - I  2  3  2  indicative  The i n n e r c o m p l e x i n  s y s t e m i s so much more s t a b l e t h a t t h e o u t e r  complex i s n o t even o b s e r v e d i n t h e s p e c t r u m , t h e h i g h b a s i c i t y of triphenylphosphine greater s t a b i l i t y outer  being  the cause of t h e  o f t h e i n n e r complex as compared t o t h e  complex. The e x i s t e n c e o f b o t h t y p e s  o f complexes has l e d  to a great d e a l of i n t e r e s t i n t h i s area of study, the main problem h a v i n g  been t o d e t e r m i n e whether i n t e r a c t i o n s  b e t w e e n t h e b a s e and f o r e x a m p l e i o d i n e r e s u l t i n i n n e r complexes being  formed i n t h e case o f t h e use o f s t r o n g  donors. In the l i g h t of the high b a s i c i t i e s of the methylp h o s p h o n i t r i l e s , i t w o u l d n o t be s u r p r i s i n g t h a t should  they  form h i g h l y s t a b l e i n n e r complexes w i t h i o d i n e .  However, i t was t h e a c c i d e n t a l s y n t h e s i s o f t h e compound N P Me I , 4  4  g  4  later  identified  t h a t l e d t o the study  as t h e i o n i c compound  synthesizing a diquaternary 4  was  4  g  +  4  4  g  o f i n n e r c o m p l e x e s f o r m e d by i o d i n e  with the methylphosphonitriles.  N P Me  (N P Me I) I ~,  I t was i n t h e a t t e m p t o f  p h o s p h o n i t r i l e by r e f l u x i n g  i n n e a t 1 , 4 - d i i o d o b u t a n e t h a t t h e compound N P Me I  formed.  3  IV. B. Experimental  and R e s u l t s  IV. B . l . S y n t h e s i s o f Complexes  Compounds o f formula N P Me I , 4  4  8  6  N^Me^,  N^Me^,  and NgPgMe^Ig can be prepared  by h e a t i n g  together the a p p r o p r i a t e m e t h y l p h o s p h o n i t r i l e with an excess  o f i o d i n e i n a s e a l e d g l a s s tube a t 140°C f o r  s e v e r a l hours. or C H C 1 2  The s o l i d s were e x t r a c t e d w i t h e i t h e r  a f t e r t h e removal o f excess  2  4  4  4  8  6  3  i o d i n e by washing  w i t h CC1 and s u b l i m a t i o n i n vacuo, t h e e x c e p t i o n N P Me I  CHC1  which r e q u i r e d e x t r a c t i o n w i t h  being  acetonitrile.  These compounds a r e s o l u b l e o n l y i n t h e more p o l a r s o l v e n t s such as C H C 1 2  and  2  and do n o t sublime.  CH3CN  M i c r o a n a l y s i s r e s u l t s o f t h e compounds a r e as  Table 21  Compound  C  N  3 3 P  M e  6 4  N  4 4  M e  8 4  P  I  I  N P Me I 4  N  4  5 5 P  8  M e  1  I  H  9  ,  8  4 2  1  >  8  8 2  ,  -  4  9  5  -  7  3 1 2  9 6  -  9  3 1  8  -'  5  -  (%) P  69.27  6 8  3  I  4 6  2  8  4  C  1  0  -  5  72  -  6  6 6  -  1  6  13.62 66.99.  Found (%) H N P  9.65 2.40 5. 50 H'  9.05 2.28 5.28 11.67 71.72  g  10 6  Req'd N  6 9  I 69.00  3.15 6.79 15.07 62.81 9.16 2.35 4.95  10.80 2.68 6.20  71.58 66.67  I V . B.2. N u c l e a r M a g n e t i c  Resonance S p e c t r a  The p r o t o n m a g n e t i c  resonance  spectra of the  i n n e r complexes a r e o b t a i n e d f o r comparison of the o u t e r  with  those  complexes.  Results T e m p e r a t u r e : 35°C S o l v e n t : CDC1  3  (+ f e w d r o p s o f DMSO-dg)  Standard: i n t e r n a l  TMS  T a b l e 22  Compound  6  obs P-P- -> (  m  A  =  6  o b s ~ p a r e n t donor 6  N P Me  6  1.50  0  N P Me  g  1.50  0  1.51  0  3  3  4  4  N P Me 5  5  1 Q  N  3 3 P  M e  6 2 I  1  ,  5  5  °-  0 5  N  3 3 P  M e  6 4 I  1  -  7  3  °-  2 3  N  4 4  M e  8 4  1  '  °-  3 0  P  I  N P Me I 4  4  N P Me 5  5  g  1 Q  6  I  6  All mately  8 0  1.96  0.46  1.73  0.22  s p e c t r a o b t a i n e d show a d o u b l e t o f a p p r o x i -  t h e same c o u p l i n g c o n s t a n t s  (12 Hz) as t h e p a r e n t  m e t h y l p h o s p h o n i t r i l e s , w i t h no  d i f f e r e n t i a t i o n of  methyl groups being  shown upon c o m p l e x  I V . B.3.  Spectra  Electronic  the  formation.  UV-VIS s p e c t r a o f a l l t h e i n n e r c o m p l e x e s show two  intense absorptions  higher  a t 293  and  e n e r g y band a p p r o x i m a t e l y  362  my,  with  the  t w i c e as i n t e n s e as  the  other.  IV. B.4.  V i b r a t i o n a l Spectra  of  Raman s p e c t r a o f t h e b a n d s , one t h e two  a t 104  polarized.  and  4  4  144 cm"  1  cm" , 1  b a n d was  two  CH C1 , 2  with a  2  relative  found to  on a C a r y  equipped w i t h a Spectra-Physics  be  81  helium-neon  source. 4  N u j o l m u l l b e t w e e n C s l p l a t e s on spectrometer N P Me . 4  showed In  -1  I n f r a r e d s p e c t r a of N P M e I  4  4  cm .  These s p e c t r a were o b t a i n e d  spectrometer  4  4  a t 143  and  t h e 109  4  solid N P MegI  the other  b a n d s a p p e a r a t 109  i n t e n s t i y o f 10:2,  laser  and  N P MegI  g  4  g  4  obtained  a Perkin-Elmer  showed s i g n i f i c a n t c h a n g e s f r o m t h a t  S h i f t s i n b a n d p o s i t i o n s as w e l l as t h e  a n c e o f new  bands a r e  shown i n T a b l e  23.  as  a  457 of appear-  T a b l e 23 Infrared  Spectra of  N P Me 4  4  1306  4  4  g  and  N P Me I 4  4  3  N P Me I  8  3160 c m "  N P Me  4  1  4  g  m  3160 m  s  1314 s 1301  a, b 4  4  s  1287  s  1290 s  1220  s  1266  s  1240 s 1180  s  1197 s  920  m  922 s 915  s  890 m 880 s 868  s  870 m  860  s  853  m  775  w  765  w  754  m  753  m  733  m  739  m  647  m  648 m  629  m  612 m 502 w  430 384  s  433  m  395  m  380 w  A b s o r p t i o n s due t o t h e N u j o l a r e n o t i n c l u d e d .  a  b  s  T  -1 I n cm  IV. B.5. C o n d u c t i v i t y Measurements  Molar c o n d u c t i v i t i e s o f both the i n n e r and the outer complexes were determined i n spectrograde  acetonitrile  which was f u r t h e r p u r i f i e d by d i s t i l l a t i o n from  calcium  h y d r i d e w h i l e under an atmosphere o f dry n i t r o g e n . Measurements were o b t a i n e d Bridge,  on a Wayne K e r r  Universal  the temperature b e i n g r e g u l a t e d by a Sargent Model  ST Thermoniter.  The c e l l c o n s t a n t of the c o n d u c t i v i t y  c e l l employing p l a t i n u m  e l e c t r o d e s was 0.176 + 0.001  as determined w i t h a standard  s o l u t i o n of KC1.  cm" , 1  Conductivity  measurements i n each case were c o r r e c t e d f o r t h a t due t o the  solvent. For each compound e i t h e r s p e c i f i c r a t i o s of  methylphosphonitrile  t o i o d i n e o r the s o l i d complexes were  weighed out and the s o l u t i o n s prepared  i n volumetric  flasks.  Since the v a l u e s o f Ag, the molar c o n d u c t i v i t y a t i n f i n i t e d i l u t i o n , c o u l d not be determined by e x t r a p o l a t i o n with a Kohlrausch of 1 0 M _3  p l o t , molar c o n d u c t i v i t i e s a t the c o n c e n t r a t i o n  were determined f o r a b a s i s of comparison.  T a b l e 24 Molar C o n d u c t i v i t i e s Compound  A  N P Me T "3*3 6 4  ^150  N P Me I  4  136  N P Me I  6  337  4  4  4  g  4  g  N^P.Me, + 3 3 6 2  29  N P Me  g  +  44  N P Me  g  +  N P Me  g  +  J  3  3  4  4  4  a  4  A t 10  3  2 I  57  2.  M concentration,  k I n l i t e r mole  IV.  38  "^2  1  ohm  cm .  1  1  C. D i s c u s s i o n  The occurs  observation  almost immediately  that the formation upon m i x i n g  o f N„P„Me - I 3 3 6 2  the reactants  the i n n e r complexes a r e formed s l o w l y a t h i g h e r confirms  i n general  ine  stability  temperatures  t h e p o t e n t i a l energy diagram f o r complex  t i o n p r o p o s e d by B h a s k a r e t . a l . a c t i v a t i o n being  whereas  1  8  0  ,  a g r e a t e r energy o f  r e q u i r e d f o r i n n e r complex  o r an o u t e r  formation,  complex d e c r e a s e s t h e tendency  forma-  toward  f o r m a t i o n o f t h e i n n e r complex by i n c r e a s i n g t h e  value of Ea".  D,A  c  nner Complex  Reaction Coordinate  Figure  28. P o t e n t i a l  energy diagram o f complex  formation.  Compared t o t h e o u t e r c o m p l e x e s , t h e f o r m a t i o n of t h ei n n e r complexes should r e s u l t i n g r e a t e r d e s h i e l d i n g of the methyl  protons.  the d i f f e r e n t i a l  I n comparing t h echemical  s h i f t s o f t h e i n n e r complexes a r e i n  general about f o u r t o f i v e times the o u t e r complexes.  as great as those o f  F o r t h e system  new  peaks observed  had  been a t t r i b u t e d ^ " t o t h e i o n i c  lower  i ntheproton 1  (TMP)2  1  1  i whereas t h e s l i g h t  field  shifts,  trimethylpyridine,  s p e c t r a upon a d d i n g I  2  species o f formula  s h i f t o f t h e p a r e n t peak t o  had been a t t r i b u t e d  t o f o r m a t i o n of t h e o u t e r  complex TMP-I .  Differential  2  s h i f t s observed  m e t h y l groups were a p p r o x i m a t e l y respectively  o f t h e a-  0.1 a n d 0.33 p.p.m.  f o r t h e o u t e r and i n n e r complexes o f t r i m e t h y l -  p y r i d i n e and i o d i n e .  The d i f f e r e n t i a l  s h i f t s of the  complexes o f t h e m e t h y l p h o s p h o n i t r i l e s w i t h i o d i n e a r e o f t h e same m a g n i t u d e a s t h o s e It  observed  for trimethylpyridine.  i s s u r p r i s i n g t h a t , even w i t h i n n e r complex  f o r m a t i o n , no d i f f e r e n t i a t i o n o f t h e m e t h y l g r o u p s a s t o their positions relative can  be d e t e c t e d .  t o t h e i n t e r a c t i n g n i t r o g e n atom  I t isdifficult,  i n view of the high  a c t i v a t i o n energy r e q u i r e d , t o conceive breaking  a n d r e f o r m i n g o f t h e N-I b o n d , t h e i o d i n e  alternatively resulting The  i n t e r a c t i n g w i t h e a c h n i t r o g e n atom,  i n t h e methyl groups being  proton  s h i f t t o lower 'of t h e e x c h a n g e 4  4  the s o l i d  T h i s may be i n d i c a t i v e o f a s l o w i n g  process. g  4  i s assigned  the structure ( N ^ M e g l ) I +  s t a t e as w e l l as i n s o l u t i o n based on  f r o m many s o u r c e s . as v  ±  • • 182 iodide ion. relative  o f t h e resonances as w e l l as a s l i g h t  field.  N P Me I  assigned  equivalently deshielded.  s p e c t r a o f t h e i n n e r complexes a t low tempera-  t u r e s show b r o a d e n i n g  in  of the rapid  The Raman b a n d s o b s e r v e d  (109 cm" ) a n d v 1  (144 c m ) - 1  3  3  ~  evidence  c a n be of the t r i -  , ^ ^ , , The e l e c t r o n i c s p e c t r u m shows b a n d s a n d  i n t e n s i t i e s characteristic of the t r i i o d i d e  ion,  1 1 8  c o n f i r m i n g the assignments o f the v i b r a t i o n s .  The p r o t o n  s p e c t r a i n d i c a t e s a p o s i t i v e charge on the r i n g , the molar c o n d u c t i v i t y o f 136 being i n the range o f 120-160 f o r 1:1 e l e c t r o l y t e s i n a c e t o n i t r i l e .  expected  1 8 3  CH  \  CH. 3' CH,3'  ,|  \  /V  ^ p = ^  F i g u r e 29.  By analogy,  \  N  .  CHg \  H  Structure of  N.P,Me I,. 4 4 8 4 Q  along w i t h the c o n s i d e r a t i o n o f the  A v a l u e o f ^150, the s t r u c t u r e o f N ^ M e ^ as  (N3P3Me I) I -. +  6  3  can be a s s i g n e d  The A v a l u e o f N ^ M e ^  i n d i c a t e t h i s as b e i n g , a 2:1 e l e c t r o l y t e .  seems t o A values f o r  2:1 e l e c t r o l y t e s i n a c e t o n i t r i l e a r e i n the range 2 2 0 - 2 8 0 . A p o s s i b l e s t r u c t u r e f o r the compound as a d i q u a t e r n a r y i s postulated.  r  C H  1  CH  3  II  « .---CH,  \ = =CH =NC I 3  CH  3  II  3""~i  F i g u r e 30.  ,CH  3\ \  I;\3 H  3  s t r u c t u r e of N ^ M e  I  183  The  structure of N ^ M e ^ I g  t h a t o f N.P.Me-l^. 4  4  of N P M e - I 3  the  3  6  spectrum 4  showed o n l y of N P M e g I 4  of N P Me . 4  In general,  o b  2  4  phosphonitrile the r i n g .  at  12 6 6 a n d 12 4 0 cm  in  the spectra  phonitnlic is  8  that  1  the  from  1  6  that  t o t h e P=N s t r e t c h i n g band by two bands  The same v  from  3  methyl-  show a change s i m i l a r t o t h a t  hydrochlorides.  of the outer  1180 c m  - 1  a  g  3  3  triphos-  P=N v i b r a t i o n  i n N^Me'g  complex N P M e - I 6  1 8 4  upon  I t i s evident  2  of the "lone-pair"  found  at a ring  nitrogen  s i m i l a r e f f e c t s i n the v i b r a t i o n a l spectra of  phosphonitriles.  ductivities  complexes  than the i o n i c  inner  show f a r l o w e r  complexes,  their molecular structure, the fact  tivities lous in  changes  band o f t h e t e t r a m e r i c  - 1  Though t h e o u t e r  of  3  o f a s e r i e s of hexa-n-alkylamino  the l o c a l i z a t i o n  produces  showed l a r g e  spectrum  from t h a t o f N P M e ,  The r e p l a c e m e n t o f t h i s  4  s h i f t e d t o 1170 cm"  formation  changes  h a s been a s s i g n e d  of  1  whereas t h e i n f r a r e d  small  4  The 1220 c m  8  i s e x p e c t e d t o be s i m i l a r t o  are s t i l l  appreciable  that  con-  indicative their  i s surprising.  conduc-  This  anoma-  b e h a v i o u r had a l s o b e e n o b s e r v e d f o r s o l u t i o n s o f I  pyridine.  conductivity  and B i r r  to the f o l l o w i n g  Py + l * - * P y l 2  Audrieth  1 8 5  2  Pyl  +  2[Pyl ] v = *Pyl 2  a t t r i b u t e d the  equilibrium:  + i " +  2  Py  + I-Pyl " 2  + +  + 2l"  (1) (2)  I n l i g h t o f t h e known s t r u c t u r e seems more l i k e l y a truer  that  [Py I] I ", i t +  2  3  the following equilibrium  represents  view.  r i  ,_ 2 P y l  2  _^  (3)  2  [Py I] I "  .  +  2  3  (4)  Analogously, the c o n d u c t i v i t y of the outer complexes o f the m e t h y l p h o s p h o n i t r i l e s  with.I  •i s postulated  as being  due t o t h e f o l l o w i n g e q u i l i b r i u m , i n v i e w o f t h e e v i d e n c e presented f o r the structure N P Me 4  N  4 4 P  M e  8  , i ;  2  +  4  X  g  2  + l  [ N  2  +  4  .  4 4 P  [N P Me I] I -.  M e  4  8  3  N P Me -I 4  8  Popov and D e s k i n  I ] + I  1 8  3~  4  ; F = =  8  ^  t N  (5)  2  4 4 P  M e  8  I ] +  * • 3" I  ( 6 )  ^ had found e v i d e n c e f o r t h e  CH^CN-I. t o t h e i n n e r t r a n ssiitt ii oo n o f t h e o u t e r c o m p l e x CH^CN-Icomplex  [CH CNI] I 3  from c o n d u c t i v i t y measurements.  t h i s work, however, l i t t l e  evidence f o r t h i s  was f o u n d i n t h e d e t e r m i n a t i o n in  e q uuiilliibbrriiu um m  of the conductivity of I ~  acetonitrile. It  seems e v i d e n t  form both i n n e r  types o f donors l i e s formed.  that the  methylphosphonitriles  and o u t e r c o m p l e x e s w i t h  same way a s d o e s p y r i d i n e .  are  In  The d i f f e r e n c e b e t w e e n t h e two  i n the structures  The e x i s t e n c e  I _ , much i n t h e  of the cations  of the cation  (Py-I-Py)  that  has  158 b e e n r a t i o n a l i z e d by a 3 - c e n t e r M.O.  treatment  which •  .  indicates a substantial stability to the separated ( P y l )  and P y .  +  for this cation The  relative  same p r o b a b l y h o l d s  t r u e f o r t h e m e t h y l p h o s p h o n i t r i l e s as w e l l , w i t h t h e o f s t e r i c h i n d r a n c e due t o t h e a d j a c e n t m e t h y l g r o u p s  factor on  the phosphorus p l a y i n g a r o l e i n f a v o u r i n g the f o r m a t i o n o f c a t i o n s s u c h as  (N P Me I) . +  4  4  g  I t w o u l d be o f  interest  t o d e t e r m i n e t h e t y p e s o f c a t i o n s w h i c h w o u l d be by t r i m e t h y l p y r i d i n e w i t h I steric  2  t o a s c e r t a i n whether  f a c t o r does i n d e e d p l a y a r o l e .  s t r u c t u r e f o r N P M e I g may 4  structure  4  g  determination.  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