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Chemistry of nitrilohexaphosphonitrilic chloride Oakley, Richard Thomas 1970-12-31

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THE CHEMISTRY OF  NITRILOHEXAPHOSPHONITRILIC CHLORIDE  by  RICHARD THOMAS OAKLEY Sc.(Hons.), U n i v e r s i t y o f B r i t i s h Columbia, 1  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 accept t h i s  t h e s i s as conforming t o t h e  required standard  THE UNIVERSITY OF BRITISH COLUMBIA August, 1970  In p r e s e n t i n g an the  this  thesis  advanced degree at Library  shall  the  in p a r t i a l  fulfilment of  University  of  make i t f r e e l y  I f u r t h e r agree t h a t permission for  s c h o l a r l y p u r p o s e s may  by  his  of  this  written  representatives.  be  available  for  for extensive  granted  by  the  It i s understood  thesis for financial  gain  permission.  Department The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, Canada  British  Columbia  shall  requirements  Columbia,  Head o f my  be  I agree  r e f e r e n c e and copying of  that  not  the  that  study.  this  thesis  Department  copying or  for  or  publication  allowed without  my  ABSTRACT  • ^  The i s o l a t i o n of n i t r i l o h e x a p h o s p h o n i t r i i i c c h l o r i d e i s reported  i n t h i s work, and i t s s t r u c t u r e shown to be a t r i c y c l i c condensed r i n g . The p h y s i c a l and chemical properties of t h i s molecule are u n l i k e the monocyclic p h o s p h o n i t r i l i c c h l o r i d e s .  The reasons for t h i s are not  f u l l y understood, but are thought to involve the weakness of the c e n t r a l P - N bond; the c r y s t a l structure and mass spectrum of the molecule show that t h i s c e n t r a l bond i s long and weak.  S u b s t i t u t i o n r e a c t i o n s , common  i n the monocyclic c h l o r i d e s , b r i n g about e i t h e r a p a r t i a l or t o t a l break-down of the t r i c y c l i c P^Ny skeleton.  In the case of e l e c t r o n -  withdrawing substituents C -g- -F,OMe), complete loss of the phosphoe  n i t r i l i c skeleton i s achieved, and, i n the case of electron-donating groups  (-NIVi^)  > s u b s t i t u t i o n onto the r i n g brings about cleavage of  one of the i n t e r n a l P-N bonds.  The molecule that i s formed during the  dimethylamination of P^N^CA^ i s unique i n p h o s p h o n i t r i l i c chemistry and i t s suggested structure v i s u a l l y emphasizes the weakness of the central P-N bond i n PJsLCJL.. o  /  y  A series of molecular o r b i t a l c a l c u l a t i o n s on the P - N - C i l - molecule o  /  y  were c a r r i e d out i n t h i s study, and i t i s suggested that the observed s t r u c t u r a l trends i n the molecule can be a t t r i b u t e d to the properties of the phosphorus 3 d ^  z  o r b i t a l on the bridgehead atoms.  - ii -  TABLE OF CONTENTS Page ABSTRACT  i  TABLE OF CONTENTS  . .  LIST OF TABLES  i i  .  LIST OF FIGURES  ••••••  ACKNOWLEDGEMENTS  _  v vi> viii  CHAPTER 1.  INTRODUCTION  1  CHAPTER 2.  PREPARATION OF NITRILOHEXAPHOSPHONITRILIC CHLORIDE  10  I.  Condensation R e a c t i o n s  10  II.  High Temperature Rearrangements  11  EXPERIMENTAL  •.  (a)-(d) Thermal Decomposition o f Phosphon i t r i l i c High Polymer  15  15  (e) R e a c t i o n o f L i t h i u m N i t r i d e w i t h P ^ 6 ^ 1 2 (f) R e a c t i o n o f ( M e S n ) N w i t h ^ ^ ^ ^ 3  3  6  6  1 2  •••  (g) R e a c t i o n o f ( M e S i ) N w i t h P^^Cl^ 3  CHAPTER 3.  ^ 2  1  21  3  PROPERTIES OF NITRILOHEXAPHOSPHONITRILIC CHLORIDE  23  31 I. II.  P n.m.r. Spectrum  23  Mass Spectrum  25  I I I . U l t r a - v i o l e t Spectrum  31  IV.  V i b r a t i o n a l Spectra  32  V.  Base S t r e n g t h Measurements  VI.  Lewis A c i d Adduct Formation  ...  38 42  - iii  -  Page CHAPTER 4.  SUBSTITUTION REACTIONS OF NITRILOHEXAPHOSPHONITRILIC CHLORIDE  43  I. (a)  43  Fluorination of P N C £ 6  (i  (ii  ?  g  j" R e a c t i o n o f P ^ C ^ Cyclohexane  w i t h KS0 F i n . ... 2  ) R e a c t i o n o f P,N,C£.„ w i t h  KF  1  47  2  Civ ) R e a c t i o n o f P N C £ 6  II.  Activated  ....tt. . ..  C i i i ) R e a c t i o n o f PJvLCfl,. w i t h KF  I. (b)  y  Methoxylation o f P N C £ 6  y  g  Activated  /  w i t h AgF  49  9  52  o f P ^ (NH) ( N M e ) C £ 2  (a) I n f r a - r e d Spectrum  (c)  8  2  •  (b) Mass Spectrum 31  56 57  •••••  P n.m.r. Spectrum  57  58  (d) '''H n.m;r. Spectrum  62  (e) C o n f o r m a t i o n a l A n a l y s i s Summary  69 72  THE MOLECULAR AND ELECTRONIC STRUCTURE OF HEXAPHOSPHONITRILIC CHLORIDE  NITRILO-  I n t e r a c t i o n s i n V,N.Cln  I.  Orbital  II.  Symmetry-based  C a l c u l a t i o n s on P^N^Citg ....  I I I . The M o l e c u l a r S t r u c t u r e IV.  48  y  I I I . The M o l e c u l a r S t r u c t u r e  CHAPTER 5.  48  Amination o f VMnCln o  IV.  46  D e t a i l e d Hiickel M.O.  of P ^ C ^  C a l c u l a t i o n s on P^N^C£„  74 74 78 83 89  - iv -  Page  APPENDIX I.  EVALUATION OF OVERLAP INTEGRALS IN P.N_C£ ... 6 7 9  APPENDIX I I .  DETAILS OF INSTRUMENTAL  REFERENCES  n  TECHNIQUES  97  101  •  103  - V -  LIST OF TABLES  Table 3:1.  3 1  P chemical s h i f t s f o r (PNCXJ- , and P.N.,CJL,.  3:2.  Fragmentation pattern of P^N^Cilg and P ^ 6 ^ 1 2  3:3.  R e l a t i v e abundances of a l l ions i n c y c l i c s e r i e s i n fragmentation pattern of PgN^C&g  3:4.  V i b r a t i o n a l spectra of  3:5.  V i b r a t i o n a l spectra of P,.N.CJL „ .... • o o 1z Assignments for the v (PCJ^) in-phase mode i n phosphon i t r i l i c chlorides . . f ^ ? >  3:6.  P^N^Cl^  3:7.  Average  4:1.  Data from  4:2.  Data from H n.m.r. spectra of P ^ (NH) (NMe ) C£  4:3.  R e l a t i o n s h i p between 6 ^ and K for some secondary amines .. Charge density and bond order differences between PgN^ and P N systems as derived from symmetry based calculations ............................  5:1.  L  PNP 3 1  i n (PNC£ ) _ 2  3  .........  5  P n.m.r. .spectrum of P ^ (NH) (NMe ) C£ 2  g  2  ..  1  2  g  2  &  fi  ?  5:2.  Bond angles and bond lengths i n P^N^CJlg  5:3.  Observed and g-adjusted Tr-contractions i n P^N^C£g . . . .  5:4.  Calculated charge d e n s i t i e s i n P^N^CX^ using a l l -3do r b i t a l s on phosphorus  5:5.  Calculated bond orders i n P^N^C£g using a l l 3 d - o r b i t a l s on phosphorus .  - vi -  LIST OF FIGURES Page Figure 1.1.  Structures o f some condensed r i n g inorganic molecules .  2  1.2.  T r i c y c l i c structure proposed for P^NjClg  6  1.3.  P o s s i b l e structures o f parent molecules o f ions i d e n t i f i e d i n mass spectra  6  2.1.  Furnaces used i n preparation o f P^N C£g  3.1.  P o s s i b l e structure o f parent molecule of ions based on P^N^ nucleus Variation i n p a r t i t i o n r a t i o of phosphonitrilic chlorides between hexane and cone. H^SO^  40  4.1.  Apparatus used i n attempted f l u o r i n a t i o n of P^N C£  50  4.2.  Suggested structure o f P ^ ( N H ) (NMe ) CJi  4.3.  Gross structure o f P,N (NH)(NMe„) C£. i n d i c a t e d by i t s 31 z a z P n . m . r . spectrum  3.2.  ..  y  y  2  £  8  g  ....  18  27  56  2  0  59  31 4.4. 4.5.  P chemical s h i f t s o f some t r i m e r i c dimethylaminochlorophosphonitriles  63  Two p o s s i b l e conformations for the P,N,(NH)(NMe ) C£„ i i bo z o z molecule ...  _ 70  Suggested stereochemistry o f n u c l e o p h i l i c attack at a bridgehead phosphorus atom i n P^N^CJig • ....  71  Overlap schemes for Tr-bonding at a non-bridgehead phosphorus atom i n P^N C£ •  76  Overlap schemes for out-of-plane TT-bonding at a b r i d g e head phosphorus atom i n P^N C£g ....  77  5.3.  Energy l e v e l s for the PgN^ and ? N  80  5.4.  O r b i t a l nomenclature used i n symmetry based c a l c u l a t i o n s on P ^ C J L ,  4.6. 4.7. 5.1.  H n.m.r. spectra of P N (NH) (NMe ) C&  61 •. . .  :  fi  6  g  2  0  y  5.2.  2  0  g  y  6  y  systems  A  81  - vii -  Page Figure 5.5.  Nomenclature for bonds and atoms i n P^N_C£ o  5.6.  Molecular structure of P,N_C£_  82  rt  y  .........  84  Rotated coordinate scheme used i n the evaluation of overlap i n t e g r a l s at the P atoms i n the IT system . . .  98  o  A.l.  /  /  R  y  - viii -  ACKNOWLEDGEMENTS Professor N . L . Paddock has been my supervisor during the course of t h i s work. remember.  The debt of gratitude that I owe him I w i l l always  For h i s h e l p , advice, and patience, I can only offer my  thanks. I would also l i k e to thank Dr. R.D. Spratley for many h e l p f u l discussions and for h i s useful advice. I am very grateful to the following people for t h e i r constructive comments and assistance i n the course of t h i s Thesis:  Mr. E. B i c h l e r ,  Mr. R.W. H a r r i s o n , Mr. T . N . Raganathan, Dr. J . S e r r e q i , and Mr. C . J . Stewart.  My thanks also go to Miss V. Ormerod for typing t h i s  t h e s i s , and to the National Research Council of Canada, for f i n a n c i a l assistance. F i n a l l y , but not l e a s t , I would l i k e to thank my friends and my family for t h e i r help and encouragement during the l a s t year.  To Mum and Dad  - 1 -  CHAPTER 1  INTRODUCTION  Inorganic r i n g systems, of the general type ( A B ) , have been n  prepared between many elements of the f i r s t two rows of the P e r i o d i c Table.  However, p o l y c y c l i c r i n g systems are much r a r e r .  These p o l y -  c y c l i c compounds are u s u a l l y unstable and are formed i n small y i e l d s as decomposition products of reactions i n v o l v i n g monocyclic systems. In most instances where the i s o l a t i o n of a p o l y c y c l i c condensed r i n g species has been reported, no d i s c u s s i o n of the chemistry of these compounds has been given. Condensed r i n g species i n inorganic chemistry are of i n t e r e s t not only because of t h e i r n o v e l t y , but because t h e i r chemistry may be very d i f f e r e n t from that of ordinary c y c l i c systems, the properties of bridgehead atoms often being quite unusual i n comparison to that of non-bridgehead atoms. In siloxane chemistry, l i n e a r , c y c l i c and c r o s s l i n k e d species are a l l known.  In borazine chemistry, the p y r o l y s i s of c y c l o t r i b o r a z i n e  at 380°C gives hydrogen and a mixture of condensed species,^ one of which i s b e l i e v e d to have the structure shown i n F i g . 1.1a. B - t r i c h l o r o c y c l o t r i b o r a z i n e reacts with methylmagnesium bromide to give the expected B-trimethyl d e r i v a t i v e , CMeB-NH)^. • The other product, 2 which i s not expected, i s again a naphthalene analogue (see F i g . 1.1b).  - 2 -  Condensed r i n g silazanes have been prepared, and the compound shown i n F i g . 1.1c has been i s o l a t e d as a minor product during the r e a c t i o n of cyclohexamethyltrisilazane with potassium hydroxide at ~180°C. The ammonolysis of methyldichlorosilane y i e l d s , as a minor product, 4 the compound i l l u s t r a t e d i n F i g . l . l d .  In carbon-nitrogen r i n g  systems, the p y r o l y s i s of melamine (NH2-C=N)g at 350°C y i e l d s a compound whose s t r u c t u r e i s suggested to be a condensed r i n g ^ ( F i g . L i e ) . Thus, w i t h i n any h e t e r o c y c l i c r i n g system ( A B ) , t h e r e , e x i s t ,  .at  n  most, only a few condensed r i n g species,- and very l i t t l e i s known of the p h y s i c a l and chemical properties of these species i n comparison to t h e i r monocyclic congeners. The family of compounds known as the c y c l i c phosphonitriles contain the same basic repeating u n i t -£ P^2=N  The c l a s s i c a l  r e a c t i o n used i n the preparation of c y c l i c phosphonitriles i s that between phosphorus pentachloride and ammonium c h l o r i d e , and was f i r s t 6 c a r r i e d out by Stokes i n 1897:PCZ  C  5  + NH.C£ —*• -(NPCA„) + 4HC£ 4 n 2'n  Unlike a l l other inorganic r i n g systems, and a l l organic  L l  annulenes,  r i n g s i z e s corresponding to n = 3,4,5 . . . are formed i n t h i s r e a c t i o n . This fundamental property of these compounds, namely the ease of preparation of d i f f e r e n t r i n g s i z e s , i l l u s t r a t e s the unique nature of the bonding i n these compounds.  The o r i g i n a l theories  7 8 ' on the  - 3-  H  H  H  H  H  H  Me  Me  a).  1e-Si  b).  Me H ^  Si- Me  .~  N  .Me  H  ,,/ ^,/ S i  /  M^Mrf?*  Me  x  ^  H  0  - \ . ^ / M e  H  k  Me Me Me Me  H  H  >-  C  H  / -Me  Me  >-  N  N  fi  i  i  .^  D  r  H  X  !  '  e).  F i g . 1.1.  Suggested structures of some inorganic molecules that are believed to be condensed r i n g compounds.  6  - 4 -  bonding i n phosphonitriles invoked the use of d - o r b i t a l s to e x p l a i n t h i s unique type o f bonding, the difference i n symmetry properties of d- and p - o r b i t a l s being used to account for the general s t a b i l i t y of a l l ring sizes. The chemistry of the phosphonitriles has been studied extensively and ligand replacement reactions are numerous.  However, up to the  present time, a l l experimental and t h e o r e t i c a l studies have been r e s t r i c t e d to monocyclic d e r i v a t i v e s .  In the course of the work  c a r r i e d out by Stokes, a compound was i s o l a t e d with the molecular formula P^NjCHg.  This compound was made as a minor product i n the  r e a c t i o n between phosphorus pentachloride and ammonium c h l o r i d e (equation 1.1).  This r e a c t i o n , as c a r r i e d out by Stokes, was done i n  sealed glass tubes at a temperature of 200°C.  More recent workers  have done t h i s experiment i n r e f l u x i n g solvents at temperatures less than 140°C, and the compound P^N^CZ^ has never been i s o l a t e d during these r e a c t i o n s .  The chemistry of t h i s compound has, up u n t i l now,  never been studied.  I t s s t r u c t u r e , both molecular and e l e c t r o n i c ,  has not been known. In 1955, Krause apparently obtained the compound P^N^C&g by the 9 thermal depolymerization of a p h o s p h o n i t r i l i c c h l o r i d e high polymer. However, few d e t a i l s were given by him on the c h a r a c t e r i z a t i o n of t h i s compound and no studies were made of i t s chemistry.  At t h i s time, the  structure that was suggested for t h i s molecule was a t r i c y c l i c condensed r i n g ( F i g . 1.2) but no evidence was given i n support of t h i s  - 5 -  structure. Studies on the mass spectra of the p h o s p h o n i t r i l i c c h l o r i d e s ^ have shown evidence of the existence of i o n s , the parent molecules of which are thought to have condensed r i n g s t r u c t u r e s .  One of these  structures corresponds to that proposed for P^N^CAg ( F i g . 1.2).  The  proposed structures of the other condensed r i n g molecules are given i n Fig.  1.3. Thus, at the beginning of t h i s work, there was only a small  amount of scattered information on condensed r i n g p h o s p h o n i t r i l i c chemistry.  Because of the lack of information i n t h i s f i e l d ,  the  purpose of t h i s present work has been to study the chemistry of condensed r i n g p h o s p h o n i t r i l i c c h l o r i d e s .  During t h i s work, the  i s o l a t i o n of P^N^CJig has been c a r r i e d out and the compound i s now 31 f u l l y characterized.  V i b r a t i o n a l , mass and  P n.m.r. spectra con-  firmed the condensed r i n g structure of the molecule, the proper name of which i s 3,7,11 ,nitr'ilo-nonachlorocyclohexaphosphonitrile.  The  name assigned to the compound by Stokes was n i t r i l o h e x a p h o s p h o n i t r i l i c c h l o r i d e , and t h i s nomenclature w i l l be used throughout t h i s work. Even though the compound was prepared i n only very small q u a n t i t i e s , the chemical properties  of the molecule were studied and several  ligand s u b s t i t u t i o n reactions were attempted, i n order that the chemistry of P^ijCZ^ might be r e l a t e d to that of the ordinary c y c l i c p h o s p h o n i t r i l i c chlorides (PNC£„) . Ligand s u b s t i t u t i o n reactions with the ordinary p h o s p h o n i t r i l i c chlorides are common.  F l u o r i n a t i o n of the chlorides with potassium  Cl  Cl  \  /  Cl  N Cl  Cl  N'  Jk—ci  P  /  Cl  Cl  Cl  F i g . . 1.2.  T r i c y c l i c condensed r i n g structure proposed hexaphosphonitrilic c h l o r i d e .  for n i t r i l o -  F i g . 1.3.  Possible structures of parent molecules'corresponding ions i d e n t i f i e d i n mass spectra.  to  - 7 -  f l u o r o s u l p h i t e produces the p h o s p h o n i t r i l i c f l u o r i d e s  (PNF ) 2  amination of both the chlorides and the f l u o r i d e s y i e l d s p a r t i a l l y and fully^'"*"^ aminated p h o s p h o n i t r i l e s .  12  Aryl''""' and a l k y l " ^  d e r i v a t i v e s can be prepared by r e a c t i n g a p h o s p h o n i t r i l i c f l u o r i d e with the appropriate Grignard reagent. The preparation 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 with d i f f e r e n t s u b s t i t u t i o n patterns has also received a t t e n t i o n .  For example,  s u b s t i t u t i o n of a c h l o r i n e l i g a n d i n (PNCi!^^ by a f l u o r i n e atom increases the e l e c t r o p h i l i c i t y of the phosphorus atom at which the s u b s t i t u t i o n has occurred.  Further s u b s t i t u t i o n i s therefore  favoured,  and i n the case of f l u o r i n a t i o n , t h i s s u b s t i t u t i o n i s always geminal. On the other hand, amination of the p h o s p h o n i t r i l i c r i n g deactivates the r i n g to further s u b s t i t u t i o n , e s p e c i a l l y at the  substituted  phosphorus atom.  found to take  Further s u b s t i t u t i o n i s therefore 12  place i n a non-geminal fashion. Unlike that of the ordinary c y c l i c c h l o r i d e s , the chemistry of nitrilohexaphosphonitrilic chloride i s limited.  The i n t r o d u c t i o n of  the c e n t r a l nitrogen i n t o the P^N^ r i n g system has a d r a s t i c on the properties of the molecule.  effect  The fragmentation pattern and  c r y s t a l structure o f the compound i n d i c a t e that the i n t e r n a l PN bond i s weak.  Various s u b s t i t u t i o n reactions were attempted on the  condensed r i n g c h l o r i d e .  When r i n g a c t i v a t i n g groups are used (-F,  -OMe), the t r i c y c l i c r i n g structure of the PN skeleton i s destroyed. Reaction with d e a c t i v a t i n g groups (-NMe2) i s less d e s t r u c t i v e , and  - 8 -  only p a r t i a l r i n g cleavage occurs, leading to the i s o l a t i o n of a compound i n which the t r i c y c l i c PN skeleton has been l o s t , but i n which some c r o s s l i n k i n g of the r i n g i s s t i l l  apparent.  The differences between the condensed r i n g c h l o r i d e and the ordinary c y c l i c p h o s p h o n i t r i l i c chlorides are therefore quite marked. The l a t t e r are stable to s u b s t i t u t i o n , whereas the former i s not. The p h y s i c a l and chemical properties of P^N^Ci^ suggest that the reason for t h i s l i e s i n the weakness of the i n t e r n a l PN bond, which appears to be weaker than an e x o c y c l i c  PCJl bond.  Simple Hiickel molecular o r b i t a l c a l c u l a t i o n s have been c a r r i e d out on an i d e a l i z e d condensed r i n g framework, using the d - o r b i t a l s on phosphorus and the out-of-plane to form Tr molecular o r b i t a l s .  out-of-plane  o r b i t a l on nitrogen  In so far as lone p a i r d e r e a l i z a t i o n  of the c e n t r a l nitrogen takes place only through the  out-of-plane  TT-system, any effect which t h i s d e r e a l i z a t i o n has on the in-plane Tr-system w i l l only be secondary.  Since i t i s thought that i t i s the  in-plane TT-system which controls the chemical r e a c t i v i t y of phosphon i t r i l e s , the effect of the c e n t r a l nitrogen on the properties of the molecule should only be minimal.  The fact that t h i s i s not the  case,  and that the r e s u l t s of base strength measurements and trends i n bond-lengths are the reverse of those predicted by the M.O. c a l c u l a t i o n s on the out-of-plane TT-system, i n d i c a t e s that more d e t a i l e d c a l c u l a t i o n s , as w e l l as more experimental information are  necessary,  before a complete understanding of t h i s system i s accomplished.  -  9 -  In the following chapters, the points o u t l i n e d above w i l l be f u l l y discussed.  In chapter (2^ a f u l l account i s given of the  various methods, both successful and unsuccessful, that were used i n the preparation of the condensed r i n g c h l o r i d e .  - 10 -  CHAPTER 2  PREPARATION OF NITRILOHEXAPHOSPHONITRILIC CHLORIDE  The o r i g i n a l i s o l a t i o n of P^N C£ by Stokes was achieved during y  a high temperature r e a c t i o n .  g  In the present work, several d i r e c t e d  syntheses from N^P^CA.^ have been attempted without success, and at present the only route to t h i s compound seems to be v i a high temperature  rearrangements.  For the purposes of t h i s t h e s i s , the d i s c u s s i o n of the types of preparation can be divided i n t o two parts , the f i r s t i n v o l v i n g attempted d i r e c t condensation r e a c t i o n s , and the second i n v o l v i n g high temperature  rearrangements.  I. Condensation Reactions Throughout the course of t h i s work, many comparisons were made between n i t r i l o h e x a p h o s p h o n i t r i l i c chloride and the open r i n g hexaphosphonitrilic chloride. experiments  Because of t h i s analogy, a series of  was c a r r i e d out on the (PNCJ^)^ molecule i n attempt to  add an extra nitrogen atom to the PN skeleton.  Lithium n i t r i d e has  17 been used to make t e r t i a r y amines,  and was used here i n an attempt  to condense the P , N , r i n g with the e l i m i n a t i o n of l i t h i u m c h l o r i d e , o o The use of amino- d e r i v a t i v e s of the group IV metals as aminating agents has been reported i n the l i t e r a t u r e , ^ ' - ^ and dimethylamino-  - 11 -  t r i m e t h y l s i l a n e and -stannane have been used e f f e c t i v e l y to prepare monofunctional dimethylamino-derivatives of p h o s p h o n i t r i l i c f l u o r i d e s .  20  In view of the strength of the M-CZ bond (M = S n , S i ) , when compared to the M-N bond, i t was hoped that the r e a c t i o n of (Me^M)^ with hexap h o s p h o n i t r i l i c c h l o r i d e might produce the condensed r i n g ,  following  the r e a c t i o n path suggested i n equation 2 . 1 .  (R M) N + CPNC£ ) 3  3  2  • P N C£  6  6  7  + 3R MC£  Q  (2,1)  3  None of these reactions produced any P^N^C£g, nor was there any i n d i c a t i o n that t h i s type of d i r e c t preparation would do so.  There i s  an obvious s t e r i c s t r a i n involved i n forming the molecule by t h i s type of r e a c t i o n , and i t i s now thought that preparations of t h i s type are unfeasible.  II.  High Temperature Rearrangements The two previous reports of the i s o l a t i o n of  the use of high temperatures. t i o n of (PNCJ^) rings.  i  n  P^N^Cl^  have involved  As has already been stated, the prepara-  r e f l u x i n g solvents does not produce any condensed  The mechanism of the r e a c t i o n between phosphorus pentachloride  and ammonium c h l o r i d e has been studied by Becke-Goehring and 21 22 coworkers.  '  E s s e n t i a l l y , the mechanism involves the formation of  l i n e a r cations of the type [C£ P=N-PCJi ] . +  3  3  These cations e i t h e r  lengthen by further r e a c t i o n or c y c l i z e i n t o the neutral p h o s p h o n i t r i l i c  - 12 -  chlorides.  I f condensed r i n g species were formed i n t h i s  / V PC  at some stage an ion of the form [=C£„P-N X  PC£ =  reaction,  +  ,]  would have to be  2  present.  That t h i s i s not the case implies t h a t t h e  [=P=N-P=]  J  linkage i s stronger than the [EP-N^  moiety.  Although r i n g rearrangements of the p h o s p h o n i t r i l i c chlorides have not been observed at low temperatures, transannular processes are noticeable i n the reactions of, e s p e c i a l l y , the larger r i n g s i z e phosphonitriles.  The f l u o r i n a t i o n of hexameric c h l o r i d e with  potassium f l u o r o s u l p h i t e always y i e l d s a small amount (-5% of a l l f l u o r i n a t e d products) of tetrameric  fluoride.  A l s o , the mass spectra  of p h o s p h o n i t r i l i c c h l o r i d e s ' ^ show that transannular processes are very important i n the fragmentation pattern of hexameric, and octameric c h l o r i d e s .  For example, 40.6% of a l l fragments i n the  mass spectrum of CPNCJ^)^ contain the (PN)^ nucleus. by  some s o r t o f t r a n s a n n u l a r  process i s therefore  / %i_ ^% p  N  II  P  i  t  N |  heptameric,  - *  A rearrangement  indicated.  /%  2 ||  12.2)  |  P y r o l y s i s of any p h o s p h o n i t r i l i c c h l o r i d e at a temperature >300°C for several hours produces a polymer, the structure of which 23 can be r a t i o n a l i z e d i n terms of a c r o s s l i n k e d matrix weight of the order o f 60,000. i n two steps.  with a molecular  The formation of the polymer proceeds  The f i r s t step i s the formation of an apparently  - 13 -  uncrosslinked long chain polymer which then c r o s s l i n k s to form the polymeric inorganic rubber. As a r e s u l t of the high temperature experiments c a r r i e d out on the monocyclic chlorides and described below, the following conclusions were drawn.  Depolymerization of p h o s p h o n i t r i l i c chloride high polymer i n  vacuo i n v a r i a b l y produces (PNCJ^^ a trace of P^N^Cilg.  a n  j 4  a n a  " sometimes, but not always,  No other products are obtained, no matter which  p h o s p h o n i t r i l i c chloride i s used to produce the high polymer. instance,  For  P^N^Cilg i s even recovered from the depolymerization of polymer  which has been prepared from t r i m e r i c and tetrameric c h l o r i d e s .  When  the c r o s s l i n k e d high polymer i s extracted i n t o benzene, no condensed r i n g chloride i s found i n the benzene.  These observations  suggest that the  P^N^Ciig i s a product of the depolymerization process, rather than being formed as a side product of the polymerization step. An X-ray d i f f r a c t i o n study of the high polymer has shown that i t 24 consists of a long chain h e l i x .  Thus the formation of n i t r i l o h e x a -  p h o s p h o n i t r i l i c chloride from the high polymer may be viewed i n the following way.  I  J>  J> N ^  N  I  St.  11  I ^  N I P  /P N ^  ^P N^  1  I] N II P  ll  I  II  il I  S, II  /  I  ^P N ^  N" N I II P P  .P N^  1  / N^ N I P  .P N^  i!  N ^ II  I  p  I  11  /  p  \  N I A  II  /P  N  11  S, II  P  W^ N ^ ^N ^ "^N*<N II  p  .... 12.3)  - 14 -  The r e s u l t s of a recent study by Todd et. a l . ' conclusion.  support t h i s  They found that the slow, c o n t r o l l e d polymerization of  ( P N C i y ^ at 250°C produces a pale yellow polymer which, upon e x t r a c t i o n with l i g h t petroleum, y i e l d s some (PNCS^^ 5 as the c r o s s l i n k e d high polymer. these d i f f e r e n t  a n c  j ^  a s  well  The mechanism of the formation of  r i n g sizes i s thought to involve a polymerization-  depolymerization e q u i l i b r i u m process of the l i n e a r polymer. In the vapour phase, p h o s p h o n i t r i l i c chlorides are stable.  remarkably)  After passing the vapour of (PNCJ?^^ through a s i l i c a  furnace  at 600°C, the condensate remained unchanged and no rearrangements occurred.  A p y r o l y s i s i n the l i q u i d phase was c a r r i e d out by passing  a l i q u i d mixture of homologues  (PNCJl^J > mean n ~5, through a v e r t i c a l  s i l i c a furnace at d u l l red heat (~600°C). l i q u i d (PNCi!^^ r a p i d l y polymerized.  Inside the furnace  the  Depolymerization q u i c k l y  followed, and the products of depolymerization were forced out of the furnace under a s l i g h t p o s i t i v e pressure of nitrogen.  Although t h i s  sort of experiment did produce a trace of P^N C£ , further  investiga-  tions along t h i s l i n e were not pursued, since considerable  breakdown  y  g  of the polymer occurred i n the furnace, with the formation of phosphorus pentachloride  and elemental phosphorus.  The most r e l i a b l e way of producing the condensed r i n g was found to be through the polymerization of the c y c l i c chlorides at 300°C to give the c r o s s l i n k e d high polymer.  The subsequent d i s t i l l a t i o n of  polymer y i e l d e d , as a f i n a l f r a c t i o n , a very small quantity of  - 15 -  P^N^C&g.  The rate of polymerization was never reproducible,  as  trace amounts of i m p u r i t i e s catalysed the r e a c t i o n , e.g. water increased the rate of polymerization and the presence of a i r decreased it.  The r i n g s i z e of the s t a r t i n g chloride also affected the rate of  polymerization.  (PNCJ^Jj takes up to 8 hours to polymerize at 300°C  whereas (PNCJ^)^ takes only h a l f an hour. It i s not known i f adaptations of these processes w i l l improve the y i e l d of the condensed r i n g compound.  In general, from a p o l y -  merization i n v o l v i n g 200 grams of p h o s p h o n i t r i l i c c h l o r i d e , only 0.2-0.5 gm o f P^N C£g could be recovered. y  The high temperature  chemistry of p h o s p h o n i t r i l i c compounds has not received d i r e c t a t t e n t i o n , and consequently,  new compounds, s i m i l a r to P^N C£ , y  g  such as the P^N^C£ and P NgC£^ found by mass spectrometry may y  y  remain to be discovered.  EXPERIMENTAL (a) Thermal Decomposition of P h o s p h o n i t r i l i c High Polymer Several hundred grams of p h o s p h o n i t r i l i c chloride o i l (PNCJ^^ were added to a large round-bottomed  f l a s k which was f i t t e d with a  s t i l l - h e a d , a short a i r condenser, and a r e c e i v e r f l a s k .  Keeping  the r e a c t i o n vessel at room pressure, the o i l was heated i n a sand bath to a bath temperature of about 300°C.  Trimeric p h o s p h o n i t r i l i c  chloride b o i l s at 257°, but at a bath temperature of 300°C and even higher, d i s t i l l a t i o n of t h i s compound was almost n e g l i g i b l e .  After  - 16 -  heating the o i l at t h i s temperature for several hours, the o i l slowlydarkened and f i n a l l y polymerized i n t o a dark, rubbery polymer.  The  length of time required for polymerization was a function of the p u r i t y and composition of the o i l .  F i l t r a t i o n of a s o l u t i o n of the  o i l i n p e t r o l ether (30-60°) through s i l i c a gel or Clearsorb columns removed a l l p o l a r i m p u r i t i e s and produced a colourless l i q u i d on evaporation of the solvent.  This o i l was more d i f f i c u l t to polymerize  since the presence of polar i m p u r i t i e s reduced the polymerization time. The actual process of p o l y m e r i z a t i o n , once s t a r t e d , i s quite rapid.  The change from a dark, but mobile, l i q u i d to a rubbery  polymer, takes less than a minute. appears, therefore,  The presence of some polymer  to catalyse further p o l y m e r i z a t i o n .  Depolymeriza-  t i o n of t h i s rubbery material was achieved by heating the polymer i n vacuo.  Depolymerization was best at a pressure of <0.1 t o r r and at  a bath temperature of 300°C.  The use of higher temperatures and  higher pressures reduced the y i e l d of the depolymerization products, which were t r i m e r i c and tetrameric p h o s p h o n i t r i l i c chlorides and a trace of P^N^CJlg.  Even under high vacuum, complete depolymerization  took about 48 hours.  The f i n a l f r a c t i o n , which was the condensed  r i n g compound, u s u a l l y s o l i d i f i e d i n the condenser arm, and was i s o l a t e d by washing the condenser w i t h p e t r o l ether (30°-60°C) to remove the monocyclic chlorides) and then with benzene, i n which P,N_C£„ i s s o l u b l e .  The r e c e i v e r f l a s k was also washed i n the same  way, i . e . f i r s t l y with p e t r o l ether, and then w i t h benzene.  - 17 -  The residue l e f t i n the d i s t i l l a t i o n f l a s k was a l i g h t f l a k y black ash, the composition of which i s unknown.  In a t y p i c a l depoly-  merization r e a c t i o n 200 grams of ( P N C i ^ ^ were polymerized and the polymer cracked to y i e l d 170 grams of a mixture of tetrameric and t r i m e r i c chlorides  ( i n a r a t i o of -2:1) and 0.30 grams of the  condensed r i n g c h l o r i d e .  The y i e l d of P^N^CJlg was v a r i a b l e , and i n  some reactions no condensed r i n g was recovered.  The compound was  i d e n t i f i e d by i t s melting p o i n t , 235°C ( l i t e r a t u r e v a l u e and by analysis  6  237.2°C),  (calculated for P N C £ ; P, 30.84; N , 16.29; CH, 52.87;  found N , 16.19; Cl, 52.81).  6  ?  g  The mass spectrum  (which w i l l be discussed  i n chapter I I I ) of the compound completed the c h a r a c t e r i z a t i o n of the compound. The next three experiments were, l i k e the f i r s t , a l l p y r o l y s i s reactions c a r r i e d out on p h o s p h o n i t r i l i c c h l o r i d e s , and although they did produce a l i t t l e P^N^Cl^, were not as r e l i a b l e nor as  efficient  as the f i r s t process. (b)  This experiment involved the use of a v e r t i c a l s i l i c a glass  column (see F i g . 2.1a) with a c o n s t r i c t i o n at the lower end to support a packing of hengar granules.  The upper end of the column was so  designed that p h o s p h o n i t r i l i c o i l could be introduced to the column under a s l i g h t p o s i t i v e pressure of n i t r o g e n .  The lower end of the  column was heated to d u l l red heat (~600°C) with an e l e c t r i c  furnace  and p h o s p h o n i t r i l i c chloride oil-was added dropwise at the top of the column.  Immediate polymerization of the o i l occurred, and a depoly-  (PNCi> ) i n 2 n  -  1!  (PNC£ ) in 2 n 0  } N, in.  a  Er b).  a),  y ,x x x * ^ x l  C) To  % *U  v k, U  Vacuum  A i r leak  P5=  L i q u i d Nitroger Cold Trap  Fig. 2 . 1 .  Diagrams of furnaces used i n the preparation of N i t r i l o hexaphosphonitrilic Chloride.  - 19 -  merization-polymerization process ensued.  Any v o l a t i l e products  passed out of the bottom of the heated zone, where they were c o l l e c t e d as l i q u i d s .  At the furnace temperature used, breakdown  of the (PNCJ?^^ u n i t was apparent, as i n t h i s type of experiment, phosphorus pentachloride was emitted from the column and red phosphorus s o l i d i f i e d i n the r e c e i v e r f l a s k .  I f lower temperatures  were used to stop t h i s breakdown, depolymerization of the high polymer could not be achieved and the column would q u i c k l y become blocked with polymer.  In a t y p i c a l experiment of t h i s type,  100 grams of (PNCJ^)^ w e r e pyrolysed to give 55 grams of ( P N C ^ ^ (n = 3,4 and 5) and 0.10 grams of P^N^C£g.  Separation and i s o l a t i o n  of the condensed r i n g compound was done using the techniques described i n experiment (a). (c)  This experiment was designed as a modification of experiment (b)  and involved the use of the copper furnace i l l u s t r a t e d i n F i g . 2.1b. The bottom of the furnace was heated to d u l l red heat with a bunsen burner.  P h o s p h o n i t r i l i c c h l o r i d e o i l was dropped onto the hot hengar  granules on the bottom of the furnace.  Polymerization, depolymeriza-  t i o n and v a p o r i z a t i o n of the o i l then occurred and any v o l a t i l e products were c o l l e c t e d through a short a i r condenser.  No P^N^CZg  was recovered from t h i s type of p y r o l y s i s , and indeed much of the p h o s p h o n i t r i l i c o i l was decomposed, with less than 50% of the s t a r t ing material being recovered.  - 20 -  (d)  A p y r o l y s i s r e a c t i o n was attempted i n the gas phase, to see i f  r i n g rearrangement occurs i n the vapour.  Using the apparatus shown  i n F i g . 2.1c, tetrameric c h l o r i d e was d i s t i l l e d through a h o r i z o n t a l s i l i c a glass furnace packed with hengar granules, and heated to ~600°C.  The products of the r e a c t i o n were c o l l e c t e d i n a dry i c e /  acetone t r a p .  It was found that the tetrameric c h l o r i d e had  d i s t i l l e d unchanged through the apparatus, and no c y c l i c  rearrange-  ments had taken place i n the gas phase. (e)  Reaction of Lithium N i t r i d e with Hexaphosphpnitrilic Chloride A s o l u t i o n of (PNC& ) 2  6  (8.8 g . , 12.6 mmol.) i n 50 ml. of t e t r a -  hydrofuran was added, over 10-15 minutes, to a s t i r r e d s l u r r y of l i t h i u m n i t r i d e (0.44 g . , 12.6 mmol.) i n 50 ml. o f tetrahydrofuran. The mixture was then gently heated under r e f l u x for 2-1/2 hours, a f t e r which time the s o l u t i o n was f i l t e r e d under n i t r o g e n , and most of the tetrahydrofuran d i s t i l l e d o f f .  100 ml- of benzene were added'  and immediate p r e c i p i t a t i o n (of LiC&) followed.  The s o l u t i o n was  allowed to s e t t l e and some o f the c l e a r s o l u t i o n drawn o f f and allowed to evaporate.  The product l e f t on evaporation was i d e n t i f i e d  by i t s i n f r a - r e d spectrum as being ( P N C ^ ) ^ .  T  n  e  gummy white  p r e c i p i t a t e could not be c h a r a c t e r i z e d , but a strong O-H s t r e t c h i n g band i n i t s i n f r a - r e d spectrum: suggests that h y d r o l y s i s of the phosphon i t r i l i c r i n g had occurred.  No  P,N_C£Q  was recovered.  - 21 -  (f )  Reaction of T r i s C t r i m e t h y l s t a n n y l ) a m i n e with Hexaphosphpnitrilic Chloride A s o l u t i o n of (Me^Sn^N (3.0 g . , 5.9 mmol.) i n 30 ml. of benzene  was added over a period of 20 minutes to a s t i r r e d s o l u t i o n of the p h o s p h o n i t r i l i c c h l o r i d e (4.1 g . , 5.9 mmol.) i n 50 m l . o f benzene. The r e a c t i o n mixture was kept under an atmosphere of nitrogen and allowed to b o i l gently under r e f l u x overnight.  On d i s t i l l a t i o n of  the solvent, there remained a gum, h a l f of which was then heated to 250°C at a pressure of 0.5 t o r r .  This heating of the gum produced a  white lumpy s o l i d , which had the c h a r a c t e r i s t i c  smell of t r i m e t h y l  t i n c h l o r i d e , but which was i n s o l u b l e i n p e t r o l ether, chloroform, benzene, d i l u t e mineral a c i d and d i l u t e a l k a l i .  The mass spectrum  of the pyrolysed material showed a pattern c h a r a c t e r i s t i c (PNCJ^)^.  of  To that h a l f of the gum which was not pyrolysed, a l i t t l e  p e t r o l ether was added.  This produced an amorphous white  precipitate  which was i n s o l u b l e i n hot a l k a l i , n i t r i c a c i d , chloroform and benzene. The i n f r a - r e d spectrum was c h a r a c t e r i s t i c  of a p h o s p h o n i t r i l i c com-  pound, w i t h a broad peak at 1340 cm ^ , as w e l l as others at 955 cm ^ (med, broad), 790 cm * (w), 755 cm ^ (med), 601 cm ^ ( s t ) , 525 cm * (med) and 470 cm * (w).  However, i t was not the condensed r i n g  compound, and no evidence o f the formation of P^N^CJig was detected at any stage i n the (g)  reaction.  Reaction of T r i s ( t r i m e t h y l s i l y l ) a m i n e with Hexaphosphpnitrilic Chloride A s o l u t i o n of (Me,Si)_N (1.4 g . , 6.0 mmol.) i n 25 ml, of benzene  - 22 -  was added over a period of 10 minutes to a s o l u t i o n o f hexaphosphon i t r i l i c c h l o r i d e (4.1 g . , 5.9 mmol.) i n 50 ml-of benzene and the mixture s t i r r e d and heated under r e f l u x i n an atmosphere of dry nitrogen.  After 24 hours, the benzene was d i s t i l l e d and the neat  l i q u i d heated to 200°C at a pressure of 0 . 5 ; t o r r . f o r one hour. c o o l i n g , the r e s i d u a l l i q u i d was i d e n t i f i e d as unreacted  On  (PNOp^.  With the i s o l a t i o n of n i t r i l o h e x a p h o s p h o n i t r i l i c c h l o r i d e achieved, the compound was subjected to a thorough spectroscopic analysis.  The r e s u l t s of the various p h y s i c a l measurements made on  the molecule are discussed i n the following chapter. Although no PgN C£g was i s o l a t e d from the above-mentioned d i r e c t y  syntheses, enough of i t was i s o l a t e d from the p y r O l y t i c reactions to allow a thorough spectroscopic analysis to be made of the molecule. The r e s u l t s of the various p h y s i c a l measurements made on the molecule are discussed i n the following  chapter.  - 23 -  CHAPTER 3  PROPERTIES OF NITRILOHEXAPHOSPHONITRILIC CHLORIDE  Although, up to now, no s t r u c t u r a l information has been a v a i l a b l e on the P^N C£ molecule, a t r i c y c l i c structure has been proposed, y  g  mainly from simple valence considerations.  That t h i s was indeed the  31 correct structure was confirmed by  P n.m.r. spectrometry.  In t h i s  present chapter, d e t a i l s are given of the r e s u l t s obtained from the i  n . m . r . , mass, u l t r a v i o l e t , and v i b r a t i o n a l spectroscopic i n v e s t i g a t i o n s c a r r i e d out on the molecule. 31 I.  P n.m.r. Spectrum Because of the low s o l u b i l i t y of P^N^CJlg i n a l l common s o l v e n t s , 31  the  P n.m.r. spectrum proved very d i f f i c u l t to o b t a i n .  Saturated  solutions i n hot benzene and xylene f a i l e d to produce a s i g n a l . F i n a l l y , a 40% s o l u t i o n of the compound i n molten naphthalene  (at  150°C) produced a weak but d e f i n i t e spectrum, c o n s i s t i n g of two peaks of equal i n t e n s i t y .  The peaks d i d e x h i b i t f i n e structure but the  signals were too weak to allow any r e s o l u t i o n of t h i s f i n e  structure  31 to be made.  The  P chemical s h i f t s of these two peaks are given i n  Table 3:1 together with those of the monocyclic p h o s p h o n i t r i l i c 31 chlorides.  The  P n.m.r. pattern of two s i g n a l s of equal i n t e n s i t y  i s quite d e f i n i t i v e and can only be interpreted i n terms of a molecule  - 24 -  Compound  6  p  (PNC£ ) 2  CPNCil ) 2  4  CPNC£ ) 2  5  ' (PNC£ ) 2  P N C£  6  6  7  g  p.p.m.  r e l a t i v e to 85% H P0 3  3  -20.0  +7.0  +17.0  +16.0  -20.2,+3.5  4  TABLE 3.T.  P n.m.r. chemical s h i f t s for the p h o s p h o n i t r i l i c c h l o r i d e s (PNC£ ) _ 2  3  6  and for P N C £ . 6  y  g  Values for ( P N C £ ) _ 2  3  6  are from  reference 26.  having two sets of three equivalent phosphorus atoms.  The t r i c y c l i c  condensed r i n g structure i s unique i n t h i s regard, with one s i g n a l corresponding to the bridgehead (=P-C£) phosphorus atoms (see F i g . 1:2), and the other corresponding to the non-bridgehead (=PC£ type) 2  phosphorus atoms.  The i n t e r p r e t a t i o n of phosphorus chemical s h i f t s  is d i f f i c u l t ,  as angular as w e l l as e l e c t r o n i c factors can contribute  to the s h i f t .  However, the environment of the =PC& phosphorus atoms 2  i n the t r i c y c l i c condensed r i n g structure i s very s i m i l a r to that of the phosphorus atoms i n ( P N C £ ) 2  are expected to be s i m i l a r .  3>  and the phosphorus chemical s h i f t s  Thus the resonance at 6 =-20.2 p.p.m.  i s assigned to the =PC£ type phosphorus atoms of the condensed r i n g . 2  The s i g n a l at 6 = +3.5 p.p.m. i s therefore assigned to the b r i d g e head phosphorus atoms.  On a simple f i r s t order b a s i s , lone p a i r  - 25 -  d e l o c a l i z a t i o n from the c e n t r a l nitrogen atom would be expected to increase the chemical s h i f t of the bridgehead phosphorus atoms with respect to the =PC&2  t y  P > e  a n  d t h i s idea i s i n complete agreement with  the above assignment. The t r i c y c l i c condensed r i n g structure shown i n F i g . 1.2 was thus established for P^N C£g, and the subsequent X-ray c r y s t a l structure y  a n a l y s i s (discussed i n chapter 5) confirmed t h i s assignment.  I I . Mass Spectrum The fragmentation pattern of t h i s compound i s of i n t e r e s t for several reasons.  Not only does the mass spectrum serve as an  excellent a n a l y t i c a l t o o l for the c h a r a c t e r i z a t i o n of the compound, but i n t e r e s t i n g comparisons may be made between the fragmentation pattern of t h i s compound and those of the ordinary c y c l i c c h l o r i d e s , comparisons which may be r e l a t e d to the chemistry of the two types of compound. As i n the mass spectra of the ordinary c y c l i c c h l o r i d e s , (PNCJ^)^ fragments with an even number of electrons are more s t a b l e , and therefore more abundant, than fragments with an odd number of e l e c t r o n s . The fragments can be divided i n t o three c l a s s e s .  Those with the  formula CP N ,C£ j x x+1 y  n  of c h l o r i n e atoms.  For doubly charged ions i n t h i s s e r i e s , the  +  have stable s i n g l y charged ions for even numbers • • J  abundances of fragments are greatest for those with an odd number of chlorine atoms.  The second type of fragment that i s seen i s of the  - 26 -  form (P N C£ ) structure.  n +  , which corresponds to a fragment with a c y c l i c  For these fragments abundances alternate with v a r i a t i o n  i n y so that for singly-charged species, ions with an odd number of c h l o r i n e atoms predominate w h i l s t for doubly charged species, those with an even number of c h l o r i n e s predominate.  The t h i r d type of  fragment has the general formula (P N ^CH )  and these fragments  are thought to be l i n e a r .  n +  This type of i o n i s known i n preparative  chemistry, the ( P ^ ^ C i l g ) * i o n being an intermediate i n the preparation of p h o s p h o n i t r i l i c c h l o r i d e s .  These l i n e a r fragments, as would be  expected, have abundance c h a r a c t e r i s t i c s which are the reverse of those observed for the c y c l i c i o n s .  Namely, for s i n g l y charged species,  ions with an even number of c h l o r i n e atoms predominate, w h i l s t for doubly charged species, those with an odd number of c h l o r i n e atoms are more abundant. Thus, the important point of the above argument i s that there i s general correspondence between the chemical s t a b i l i t y of an ion and i t s abundance i n a mass spectrometer. Fragments of the type ( P  N x  x +  i^^y)  n +  a  r  e  thought to have con-  densed r i n g s t r u c t u r e s , with the c e n t r a l nitrogen atom, bound to three d i s t i n c t phosphorus atoms, being maintained i n the structure of the i o n .  The abundance r a t i o of ions with odd and even numbers  of electrons i s consistent with these fragments being completely cyclic.  Within t h i s s e r i e s of i o n s , fragments with formulae  corresponding to three values of x (= 6,5 and 4) are found.  Frag-  - 27 -  -ments based on the PgN and the P^N^ n u c l e i have already been y  i d e n t i f i e d i n mass spectra, but the ions based on the P^N^ nucleus have not been seen before.  These ions are believed to be based on  a tetrameric r i n g system (see F i g . 3 . 1 ) , with an extra nitrogen atom bound to three separate phosphorus atoms.  In the mass spectra of the  CI  F i g . 3 . 1 . ' Possible structure of parent molecule o f ions based on the P . N nucleus. r  phosphonitrilic fluorides, has been seen.  27  + the s e r i e s C T ^ ^ ^ ^ n 6^ ^  n  =  ^-12)  The smallest ion i n t h i s s e r i e s , P ^ N ^ ^ * , may be  r e l a t e d to the P^N^C£^  n+  i o n s , except that i n the former, there would  be two nitrogen atoms t r i p l y bound to the tetrameric r i n g system, one being above the plane of the r i n g , and the other being below i t . The abundance of ions based on the P.N,, and P N , n u c l e i i s much r  4 5  o o  less than that o f ions based on the parent P^N nucleus, which cleaves y  p r e f e r e n t i a l l y i n t o c y c l i c fragments based on the PgN^ and  P4N4  ring.  Table 3.2 shows the fragmentation pattern of P^N C£g, and for y  comparison, the fragmentation pattern of (PNC£„)  .  In t h i s Table,  P N Cil 6  ?  9  FRAGMENT  P  P  PN  PN  2  P N  2  3  P N  2  3  3  PN 4 3  P.N. 4 4  Condensed Ring  P  PN 5 5  19.4  2.4 1.2  14.8  18.2  7,4  9.6  P  5 6 N  P  6 6 N  4.2  1.3  Cyclic Linear  PN 4 5  6 7 N  17.8 0  1.1  P  TOTAL  23.3 32.5 43.9  2.3  6 6 12 N  C £  FRAGMENT  P  Cyclic  PN  PN 2  P N 3  2  P N 3  3  PN 4 3  9.0  0.1  2.3  3.8  PN 5 4  10.3  .40.6  0.4  4 4  PN 5 5  PN 6 6  TOTAL  15.1  12.0  78.4 21.4  Linear  6.2  TABLE 3.2.  The fragmentation pattern of P^N C£g and P^N^C£^ i n t o condensed, c y c l i c and l i n e a r species. y  2  The  number given i s the t o t a l percentage of ions of the type shown, i r r e s p e c t i v e of c h l o r i n e content or charge.  For convenience, chlorides of phosphorus are included with the l i n e a r s e r i e s , and the  PN s e r i e s with the c y c l i c compounds.  - 29 -  a l l i o n s , s i n g l y or doubly charged, belonging to a p a r t i c u l a r parent nucleus P N^ are grouped together. X  Several conclusions can be made by comparing the two patterns. The (P N C£ ) x x+1 y v  n +  n  series only accounts for 23.3% of the t o t a l frag-  ments, a high proportion of the fragments being small l i n e a r i o n s . This i n d i c a t e s a general i n s t a b i l i t y of the condensed r i n g skeleton with respect to the c y c l i c and, more e s p e c i a l l y , l i n e a r species. (PNCi^)^  breaks down i n t o mainly P^N^ fragments, which shows the  importance of a transannular effect i n cleaving the P^N^ nucleus into two P^Ng u n i t s .  In the c y c l i c fragments formed from the breakdown of  P^N^ u n i t s again predominate.  P^NjClg,  The high proportion of ? N 4  4  u n i t s does not f i n d an obvious explanation. The actual mechanisms involved i n the breakdown of the phosphon i t r i l i c c h l o r i d e s are not understood.  Nonetheless, some deductions  can be made from the fragmentation pattern of the condensed r i n g . Within the s e r i e s of c y c l i c fragments, the most abundant are ( P N C £ ) 4  4  7  +  and ( P N C £ ) 3  3  5  +  (see Table 3 . 3 ) .  fragments  Any mechanism which  i s postulated for the formation of these ions must involve the cleavage of the i n t e r n a l PN bond and the loss of the. bridgehead phosphorus atoms.  (HPC£)  I f =PC&2 type phosphorus atoms were l o s t , the  r e s u l t a n t ions would have too few c h l o r i n e atoms. In the formation of the ( P N C £ ) 4  atoms must be l o s t . from P^N-CJt,, o / y  4  7  +  i o n , two  PC£ type phosphorus  Of the three nitrogen atoms which must be l o s t  i n order to form the (P.N.C£_) 4  4 /  +  fragment, one must be  FRAGMENT FORMULA  PARENT RING PN 5 5  PN  PN 3 3  TABLE 3 3.  P  5 5 7 N  a  5  C  5  0  1.5  0  0.7  0  4  y  P  5  O  DO  2.8  P N C£ 4  P N C£  P N_CJL  CHARGE ON ION  4 4 *6 N  P  C  + ++  4 4 5 N  C £  P  4 4 4 N  C £  P  4 4 3 C A  N  23.5  1.0  9.7  0  3.7  +  0  2.2  0  2.0  0  ++  P„N_C£, J  P N C£ 3  3  P N C£ 3  5  3  4  P N C£ 3  3  3  P N C1 3  3  2  P N Cil 3  3  D  J  0  39.0  2.4  37.5  0  4.2  +  0  0  1.7  0  0  0  ++  Relative abundances of a l l ions i n the various c y c l i c s e r i e s i n the fragmentation pattern of  P,N-CJl . n  The numbers given are not percentages and represent only the  i n t e n s i t i e s of peaks r e l a t i v e to each other.  - 31 -  the c e n t r a l nitrogen atom.  It i s believed that the loss of t h i s  nitrogen atom i s achieved i n a process which involves the cleavage of the c y c l i c structure of the condensed r i n g nucleus to give a l i n e a r species which can then e i t h e r r e c y c l i z e i n t o the smaller P^N^ and PjN^. u n i t s , or form a series of l i n e a r fragments. proportion of l i n e a r fragments from P^N C£ y  (PNC£ ) 2  g  The high  (43.9%), i n comparison to  (21.4%) supports t h i s hypothesis.  6  In so far as the fragmentation pattern of P^N C£g i s a guide to y  i t s chemical p r o p e r t i e s , several i n t e r e s t i n g comparisons can be made. Loss of the condensed r i n g nucleus i s favoured i n the mass spectrum, with the formation of a large proportion of l i n e a r fragments (much more so than i n the monocyclic c h l o r i d e s ) and small "cyclic fragments. The c e n t r a l nitrogen atom, and the bridgehead phosphorus atoms, seem to be l o s t i n these processes.  These r e s u l t s i n d i c a t e a greater chemical  i n s t a b i l i t y for P^N C£ i n comparison to the monocyclic c h l o r i d e s , and, y  g  as the r e s u l t s of chapter 4 w i l l show, the chemistry of the condensed r i n g compound i s l i m i t e d by the i n s t a b i l i t y of the P^Ny nucleus.  III.  U l t r a - v i o l e t Spectra The monocyclic p h o s p h o n i t r i l i c c h l o r i d e s do not show any U.V.absorp-  tions  above 2100 X, where TT-»IT* t r a n s i t i o n s a r e n o r m a l l y expected.  Below  o  2100 A , n-*rr* t r a n s i t i o n s on the c h l o r i n e ligands do occur, but these t r a n s i t i o n s have not received any s p e c i a l a t t e n t i o n . of PJ^ Ci 7  Q  The U.V. spectrum  was recorded i n cyclohexane and a c e t o n i t r i l e s o l u t i o n s , and  - 32 -  no spectrum was observed above 2100 A.  The appearance of an absorption  o  below 2100 A was n o t i c e d , i n d i c a t i n g that the type of U.V.  transitions  i n PgN^CJlg are s i m i l a r to those observed i n the monocyclic c h l o r i d e s , and that the a d d i t i o n of the c e n t r a l nitrogen does not produce any e a s i l y observable  TT-HT*  transitions.  IV. V i b r a t i o n a l Spectra Several d e t a i l e d studies have been made on the v i b r a t i o n a l assignor «.!. + • • 28-32 , . . . 28,29,33 , , .„ ... ments of the t r i m e r i c and tetrameric phosphonitrilic chlorides but only t e n t a t i v e assignments have been made for the penta27 meric  and hexameric c h l o r i d e s .  The i n f r a - r e d (mull and s o l u t i o n )  and Raman (powder) spectra of P^N^CJlg were recorded. are given i n Table 3.4.  These spectra  The v i b r a t i o n a l spectra of (PNCl^)^,  detailed  i n Table 3.5, act as an i n t e r e s t i n g comparison. Whilst complete v i b r a t i o n a l assignments are nearly s e t t l e d for N^P^CJl^, for the larger r i n g s i z e s , assignments by d i f f e r e n t authors vary.considerably.  For any molecule, v i b r a t i o n a l assignments can be  given which f i t the observed frequencies, but the reasons behind the assignments are d i f f i c u l t to j u s t i f y completely.  J u s t i f i a b l e assign-  ments can r e a l l y only be given when the spectra of a large number of d e r i v a t i v e s of a p a r t i c u l a r compound have been studied, and p o l a r i z a t i o n measurements are a v a i l a b l e . In the case of P,N_C£ , no substituted d e r i v a t i v e s have been made, o / y A  Only a powder Raman spectrum has been obtained, so that no p o l a r i z a t i o n  - 33 -  RAMAN SOLID  -1 cm  INFRA-RED NUJOL MULL cm  INFRA-RED SOLUTION cm  ASSIGNMENT  149 (3.5) 169 (7.0) 288 (0.4)  280 w  332 (7.0)  330 w  411 (0.6)  402 v.w  442 (4.0)  435 m  507 (0.8)  503 v.w  6(PCil ) in-phase 2  430 w  v  sym  (PC£„) in-phase 2 r  525 w 548 sh 564 St  562 m  592 v. St  603 m  661 (1.5)  660 St  662  807 (1.5)  804 m  805 w  870 St  858 sh  557 (1.0)  v(P-C£) in-phase V S  ym^ V C  °" ° t_  -phase  f  A^ r i n g v i b ' n (see t e x t )  880 m 903 v.w 1188 sh -1280 v.St  1279 s t  degen r i n g v i b ' n s  -1320 sh  TABLE 3.4.  V i b r a t i o n a l spectra of n i t r i l o h e x a p h o s p h o n i t r i l i c c h l o r i d e . For the Raman spectrum, the figures i n parenthesis  indicate  the r e l a t i v e i n t e n s i t i e s of peaks with respect to each other. t  - 34 -  RAMAN -1 SOLID cm  INFRA--RED NUJOL MULL cm  INFRA-RED SOLUTION cm  ASSIGNMENT  128 CL5) 154 Ci.o) 167 CO.8) 198 (0.4) 275 (1.5)  SCPCl^)  in-phase  343 w 430 m v  451 (10.0)  578 (1.0)  463 st  465 w  518 m  518 w  584 sh  -580 sh  602 st  602 st  748 st  748 m  830 m  853 m  1432 v. st  TABLE 3.5.  1427 st  (PC£„) in-phase 2 v (PC£_) 1st degen. sym 2 sym  r  6  V  sym J  (PC£_) out-of2 , phase  degen. r i n g v i b ' n  V i b r a t i o n a l spectra of hexaphosphonitrilic c h l o r i d e . For the Raman spectrum, the figures i n parenthesis i n d i c a t e the r e l a t i v e i n t e n s i t i e s of peaks with respect to each other.  - 35 -  measurements are a v a i l a b l e .  The low s o l u b i l i t y of the compound i n a l l  solvents has made the detection of a Raman spectrum i n s o l u t i o n unfeasible.  Attempts to take Raman spectra of the melt also f a i l e d .  Because of t h i s dearth of information, rather than quote v i b r a t i o n a l modes and then match frequencies to them, i t i s the purpose of t h i s study to take the most prominent frequencies from the Raman and i n f r a - r e d spectra, and to c o r r e l a t e them with the generally accepted assignments i n the smaller c y c l i c p h o s p h o n i t r i l i c c h l o r i d e s . The appearance of the i n f r a - r e d spectrum i s s i m i l a r to that observed i n the monocyclic c h l o r i d e s , the strong broad band at 1280 cm being c h a r a c t e r i s t i c of a l l p h o s p h o n i t r i l i c compounds and assigned to a degenerate r i n g v i b r a t i o n .  The large number of coincidences i n the  Raman and i n f r a - r e d spectra suggest that the molecule has low symmetry. 31 I f one assumes the presence of the 3-fold axis i n d i c a t e d by  P n.m.r.  spectrometry, the symmetries D^, D^^ and D^^ can be excluded since these do not lead to Raman and i n f r a - r e d coincidences of t o t a l l y symmetric v i b r a t i o n s .  This leaves  symmetries the molecule can possess.  or C^ as being the only v  That the bridgehead c h l o r i n e  atoms are a l l c i s to one another i s also i n d i c a t e d s i n c e , i f not,  the  r e s u l t a n t C symmetry would produce a much more complex spectrum. s  The two most prominent features of the Raman spectrum are the two intense peaks at 332 cm * and 442 cm ^.  In the monocyclic  c h l o r i d e s , the most intense peak i n the Raman spectrum i s the p o l a r i z e d band corresponding to the PC£„ in-phase symmetric s t r e t c h i n g v i b r a t i o n .  - 36 -  Table 3.6 shows the frequencies of these peaks i n the Raman spectrum of the monocyclic c h l o r i d e s .  Compound  (PNC£ ) 2  On the basis of these assignments,  (PNC£ )  3  S  v  J  References  TABLE 3:6.  387 391  365  sym 2 in-phase cm  4  2  4  C  2h  '  413  2  5  439  29,33 29; 33  29,30 31,32  (PNC£ )  29  (PNC£ ) 2  the  P N C£  6  6  ?  451  442  This work  This work  9  Assignments for the symmetrical in-phase PC& s t r e t c h i n g vibration in phosphonitrilic chlorides. 2  band at 442 cm  i n the Raman spectrum of P^N^CJlg i s assigned to the  V (PC£„) in-rphase mode. - sym 2 32 r  In ( P N C £ ) , 2  3  the  v S  y ( P G £ ) out-of-phase mode occurs as a very m  2  strong band i n the i n f r a - r e d spectrum at 530 cm  Although i t i s not  forbidden i n the Raman spectrum, i t does not appear there.  On these  grounds the analogous mode i n P N CJ£ i s assigned to the absorption 6  at 592 c m  - 1  7  g  i n the i n f r a - r e d spectrum.  S i m i l a r l y , the band at 602 cm"  i n the i n f r a - r e d spectrum of (PNCZ^)^ i s assigned to the  v S  y CP0^ ^ m  2  out-of-phase mode.in that molecule. The strong peak at 332 cm * i n the Raman spectrum i s assigned to the in-phase PC& s c i s s o r s v i b r a t i o n . 2  t h i s mode i n the monocyclic s e r i e s i s u n c e r t a i n .  The assignment of It i s expected to  - 37 -  occur at a lower frequency than the symmetric s t r e t c h i n g motion.  In  32 the t r i m e r i c c h l o r i d e , Emsley d i d not assign t h i s mode to any 30 -1 frequency. Califano has assigned i t to a peak, at 100 cm and Hisatsune  31  to a peak at ~304 cm  -1  .  In (PNF,,)^ , the in-phase P F 4  2  s c i s s o r s v i b r a t i o n l i e s about 100 cm * below the symmetric in-phase s t r e t c h i n g mode.  The i n t e n s i t y of the peak at 332 cm  spectrum of P^N C£g suggests that i t i s an y  i n the Raman  mode and that the above  assignment i s v a l i d . The next assignment that can be made concerns the strong i n f r a - r e d band at 564 cm ^.  This i s thought to be the symmetric P-CZ s t r e t c h i n g  motion of the bridgehead c h l o r i n e atoms.  In P^N^P^CH, the P-CZ v i b r a -  -1 32 t i o n i s assigned to a peak at 637 cm . The corresponding v i b r a t i o n -1 34 i n PgN^Br^CJi occurs at 563 cm . In mononuclear phosphorus (V-J com-1 -1 35 pounds, v(P-C£) v a r i e s between 513 cm and 567 cm . The assignment of v  sym  (P-CJlj at 564 cm * seems l o g i c a l , 6  A recent review of the i n f r a - r e d absorption frequencies of PN bonds i n d i c a t e s a broad range of frequencies between 700 cm ^ and 950 cm ^ as being the s t r e t c h i n g frequency of the PN s i n g l e bond. P_N_C£ NMe„, o 3 b I r  37  v(P-N) occurs at 711 c m . - 1  In  In P,N^C£ the i n f r a - r e d 6 / 9 rt  peak at 804 cm * weakens i n s o l u t i o n and i s a strong peak i n the Raman spectrum at 807 cm ^.  The condensed r i n g PN nucleus has one more A^  r i n g s t r e t c h i n g mode than does the open r i n g P^N^ nucleus.  This  e s s e n t i a l l y involves the symmetric s t r e t c h i n g motion of the i n t e r n a l PN bonds.  On the basis of the above information, the i n f r a - r e d  - 38 -  absorption at 804 cm  i s assigned to t h i s v i b r a t i o n .  Although further assignments could be made here, t h e i r v a l i d i t y would be questionable.  The prominent features of the spectra have  been given p r e l i m i n a r y assignments, and the r e s u l t s are consistent with the P N C £ molecule having C 6  7  g  3 v  symmetry.  The shape of the molecule i s  therefore completely defined, and the bridgehead P-C£ bonds must a l l be c i s to one another..  V. Base Strength Measurements Although base strength i s a chemical property, the i n t e r p r e t a t i o n of t h i s property i s i n terms o f a p h y s i c a l phenomenon, and therefore t h i s d i s c u s s i o n o f the base strength of P^N^CJlg has been grouped with the other p h y s i c a l properties o f the molecule. The p h o s p h o n i t r i l i c c h l o r i d e s are weak bases, the r i n g nitrogen atoms a c t i n g as the b a s i c centres.  To the extent that lone p a i r d e r e a l -  i z a t i o n from the r i n g nitrogen atoms affects the base strength o f these molecules, then the measurement of the r e l a t i v e base strength o f the condensed r i n g compound and the open r i n g (PNCi^Dg molecule gives some i n s i g h t i n t o the difference i n the bonding o f the two compounds. The r e l a t i v e values o f the base strengths of the p h o s p h o n i t r i l i c c h l o r i d e s (PNCJ^)  (n = 3-6) have been obtained by the measurement' o f  the p a r t i t i o n r a t i o s o f the c h l o r i d e s between hexane and concentrated 38 sulphuric a c i d .  A s i m i l a r study was c a r r i e d out on the condensed  r i n g c h l o r i d e , using (PNC£ ), to standardize the sulphuric a c i d . 2  By  - 39 -  varying the concentration of the sulphuric a c i d , the change i n the p a r t i t i o n r a t i o was measured as a function of the concentration o f the a c i d .  The r e s u l t s of these measurements,  together with those found  for the monocyclic c h l o r i d e s , are shown i n F i g . 3 . 2 , where the p a r t i t i o n r a t i o i s shown Con the v e r t i c a l a x i s ) , p l o t t e d against the Hammett a c i d i t y function HQ.  The HQ function represents the a b i l i t y o f a  solvent to donate a proton to a neutral base and i s defined by equation 3 . 1 .  The p h y s i c a l chemistry behind t h i s separation i s not  H  = pK + U  fflJ-  - log  -(3.1)  [BJ  M  f u l l y understood, and at present there i s no explanation for the TBH 1 +  v a r i a t i o n of the slopes i n F i g . 3 . 2 .  When  IB]  — = 1, i . e . along the  x - a x i s , HQ = pKgj_| and thus the intercept of the various l i n e s with +  the x - a x i s gives an approximate measure of the d i s s o c i a t i o n constant for a p a r t i c u l a r compound.  Although the quotation of absolute values  of Kg^+ have l i t t l e meaning, the r e s u l t s show that the base strength of the condensed r i n g c h l o r i d e i s less than that of the open r i n g hexameric c h l o r i d e by a value of ApKg^+ - 0 . 1 . The i n c r e a s i n g r i n g angle at nitrogen with increasing r i n g s i z e i n p h o s p h o n i t r i l i c c h l o r i d e s (see Table 3.7) i s symptomatic of increased d e r e a l i z a t i o n of the nitrogen lone p a i r e l e c t r o n s , with a consequent weakening of t h e i r b a s i c character.  In planar molecules,  2 bonding of the sp  lone p a i r hybrid on nitrogen i s c h i e f l y with the  F i g . 3.2. •;  V a r i a t i o n i n the p a r t i t i o n r a t i o of p h o s p h o n i t r i l i c c h l o r i d e s between hexane and concentrated sulphuric a c i d with change i n concentration of the s u l p h u r i c a c i d .  - 41 -  Compound  (PNC£ ) 2  (PNC£ ) 2  3  (PNC£ ) 2  4  Ring angle at nitrogen  120°  132°  149°  Reference  39  40  41  TABLE 3:7.  5  Average r i n g angle at nitrogen i n the monocyclic phosphon i t r i l i c chlorides (PNC£ ) _^. 2  3  in-plane dxo -y 2 o r b i t a l on phosphorus..  In non-planar systems, however,  overlap can also occur i n t o the out-of-plane d - o r b i t a l s on phosphorus. Ring nitrogen l o n e - p a i r d e r e a l i z a t i o n i s therefore c o n t r o l l e d by several f a c t o r s , and the extent to which i t occurs i s a balance of these f a c t o r s .  A f u l l e r i n t e r p r e t a t i o n of these r e s u l t s i s given i n  Chapter 5. _3 Method:-  10  molar s o l u t i o n s of P N C £  cyclohexane s o l v e n t .  6  7  g  and (PNC£ ) were made up i n 2  6  The p a r t i t i o n r a t i o s were measured by mixing 30 ml  of the cyclohexane s o l u t i o n with 30 ml of concentrated sulphuric a c i d , shaking the mixture for 2 minutes, and then f i n d i n g the  resultant  concentration of c h l o r i d e i n the cyclohexane layer by evaporating a known volume of s o l u t i o n to dryness and weighing the residue. c e n t r a t i o n i n the a c i d layer was determined by d i f f e r e n c e . ments were made i n t r i p l i c a t e .  The con-  A l l measure-  Each concentration of acid was standard-  ized by f i n d i n g the p a r t i t i o n r a t i o of hexameric c h l o r i d e i n the a c i d  - 42 -  and f i t t i n g the point so obtained onto the o r i g i n a l p l o t for hexaraeric chloride. ** 3  V I . Lewis Acid Adduct Formation The a b i l i t y of the p h o s p h o n i t r i l i c c h l o r i d e s to form Lewis a c i d : adducts i s r e a l l y a measure of t h e i r base strengths, and to t h i s extent they are only weak bases, since only a few such adducts have ever been prepared.  (PNCK^g forms a 1:2 complex with aluminium t r i c h l o r i d e 43  and a 1:1 complex with aluminium t r i b r o m i d e .  The larger r i n g s i z e  c h l o r i d e s do not form complexes and an attempt to react P^N^C&g with aluminium tribromide d i d not produce any complex.  The d e r e a l i z a t i o n  of the lone p a i r on the c e n t r a l nitrogen i s therefore  considerable,  and the molecule as a whole must be viewed as being less basic than CPNC£ ) . 2  3  Method:-  A s o l u t i o n of resublimed aluminium tribromide (0.039 g.  0.145 mmol) i n 9.0 ml of carbon disulphide was added, under an atmosphere of dry n i t r o g e n , to a s t i r r e d s o l u t i o n of P^N^Cilg (0.087 g . , 0.145 mmol) i n 10 ml of carbon d i s u l p h i d e . was no v i s i b l e sign of a r e a c t i o n .  After h a l f an hour, there  The carbon disulphide was  d i s t i l l e d o f f and 0.083 g. of the P, N_C£_ recovered from the residue • 6 7 9 by e x t r a c t i o n i n t o benzene.  - 43 -  CHAPTER 4  SUBSTITUTION REACTIONS OF NITRILOHEXAPHOSPHONITRILIC CHLORIDE  A series of s u b s t i t u t i o n reactions was attempted i n t h i s study. The r e s u l t s of these reactions show t h a t , u n l i k e the monocyclic phosphon i t r i l i c c h l o r i d e s , the condensed r i n g compound i s not stable to substitution.  The weakness of the i n t e r n a l PN bonds, i n d i c a t e d i n the  mass spectrum, i s apparent i n the chemistry of the compound.  Attempted  i n t r o d u c t i o n of electron-withdrawing groups (-F, -OMe) i n t o P^N^Ci^ brings about a c o l l a p s e of the P^N skeleton, and only when e l e c t r o n y  r e l e a s i n g groups are introduced does the condensed r i n g compound form stable products.  The following d i s c u s s i o n i s divided i n t o several p a r t s .  The f i r s t part describes the r e s u l t s of the f l u o r i n a t i o n and methoxylat i o n r e a c t i o n s , and the second part describes the r e s u l t s of dimethylamination of the condensed r i n g compound.  In the t h i r d s e c t i o n , a  discussion i s given of the spectra and structure of PgN^(NH) QWk^igCi^-  I ( a ) . F l u o r i n a t i o n of N i t r i l o h e x a p h o s p h o n i t r i l i c C h l o r i d e . Monocyclic p h o s p h o n i t r i l i c f l u o r i d e s are v o l a t i l e l i q u i d s , ^ and i t was consequently expected that P^N^F^ would be s i m i l a r , and hence easy to separate from other r e a c t i o n products.  I t s properties were  19 also expected to be informative, since  F has a nuclear s p i n , and more  s t r u c t u r a l information would be obtainable s p e c t r o s c o p i c a l l y from the  - 44 -  f l u o r i d e than the c h l o r i d e of the condensed r i n g p h o s p h o n i t r i l e . Because of these f a c t s , many attempts were made to f l u o r i n a t e the condensed r i n g c h l o r i d e . t i n g agents were used. fully,  11  A v a r i e t y of reaction, conditions and f l u o r i n a Potassium f l u o r o s u l p h i t e has been used success-  as has argentous f l u o r i d e ,  43  to f l u o r i n a t e p h o s p h o n i t r i l i c 44  chlorides.  A c t i v a t e d potassium f l u o r i d e  phosphorus c h l o r i d e s .  has been used to f l u o r i n a t e  A l l these reagents were used, but none produced  any compound which could be properly characterized.  Some of the  f l u o r i n a t i o n reactions were done using open systems.  In these cases,  although no f l u o r i d e s were i s o l a t e d , the condensed r i n g c h l o r i d e was l o s t i n the r e a c t i o n , with the formation of i o n i c c h l o r i n e . no explanation could be given for these f a i l u r e s .  At f i r s t  F i n a l l y , an e x p e r i -  ment was c a r r i e d out using a closed system w i t h a l i q u i d nitrogen trap to condense out any gaseous products. The products of t h i s r e a c t i o n , which was c a r r i e d out using s i l v e r f l u o r i d e as the f l u o r i n a t i n g agent, were a l l e i t h e r gases or v o l a t i l e l i q u i d s at room temperature.  It i s thought that there are several  products of t h i s r e a c t i o n , a l l of which are f u l l y f l u o r i n a t e d , and a l l of which appear to have l o s t the condensed r i n g PgN^ nucleus. number of products made e f f i c i e n t separation impossible. f r a c t i o n a t i o n was attempted using dry ice/acetone  The  Trap to trap  i-97°C), dry i c e / C C ^  (-23°C), dry ice/chlorobenzene (-45°C) cold t r a p s , but no f r a c t i o n a t i o n was achieved.  The appearance of PQR patterns i n the vapour-phase  infra-  red spectra of the v o l a t i l e products i n d i c a t e d that some of the products  - 45 -  had low molecular weights, i . e . low enough to allow r o t a t i o n a l fine structure to be observed.  The magnitude of the s p l i t t i n g of the PQR  branches was such that the distance between the maxima of the P and R branches was, i n a l l cases, about 20 cm \  For l i n e a r molecules, the  separation, Av, between the maxima of the two branches i s given by the formula'**':  Av = /8kTB  =  2.358 yflT  (4.1)  he  where T i s the absolute temperature and B i s the r o t a t i o n a l of the molecule (B =  ).  constant  This formula only holds r i g o r o u s l y for  8TT IC 2  l i n e a r molecules, but may be used as an approximate guide for nonl i n e a r molecules.  Using equation 4 . 1 , the value of B corresponding  to a Av of 20 cm" i s B = 0.24 c m . 1  - 1  The B value for PF_ i s B = o 3 o  46 0.26084.  Although a q u a n t i t a t i v e comparison i s f u t i l e , the value of  B = 0.24 cm ^ does i n d i c a t e that the s i z e of the molecule i s not v a s t l y d i f f e r e n t from PF^Mass spectra r e s u l t s again i n d i c a t e d a mixture of products, mainly of low mass number, the most intense peaks corresponding to the fragments  (P F) 2  +  and ( N F ) . +  2  The c h a r a c t e r i s t i c i s o t o p i c pattern of  c h l o r i n e d i d not appear i n any fragment,  showing that complete f l u o r i n a -  t i o n had taken p l a c e , and although the average molecular weight of the product (by vapour density) was 114, fragments of mass number >400 were observed i n the mass spectrum i n low abundance.  It i s therefore  likely  - 46 -  that a l i t t l e P y F N  6  g  (M = 455) was formed, but i t could not be  characterized because of the s c a r c i t y of peaks i n the range m/e = 200-350, which made accurate mass counting impossible. Very l i t t l e p o s i t i v e information was obtained on the types of molecules present i n the product mixture.  The important  to be drawn i s that the f l u o r i n a t i o n r e a c t i o n s ,  conclusion  as c a r r i e d out i n t h i s  work, break the condensed r i n g PN skeleton to produce v o l a t i l e products of unknown composition.  An attempt was made to f l u o r i n a t e the  condensed  r i n g compound p a r t i a l l y , using a method s i m i l a r to that used i n preparing the tetrameric c h l o r i d e f l u o r i d e s P.N.F C£ . 4 4 x 8-x r  6  n  47  This r e a c t i o n  produced only unreacted P^N^CJlg and a trace of v o l a t i l e decomposition products of the type already  discussed.  Experimental i ) Reaction of N i t r i l o h e x a p h o s p h o n i t r i l i c Chloride and Potassium Fluorosulphite i n Cyclohexane. A s l u r r y of P^N^Cilg (3.5 g . , 3.85 mmol) and potassium f l u o r o sulphite  (6.5 g . , 11.0 mmol) i n 125 ml. of cyclohexane were heated under  r e f l u x i n an anhydrous atmosphere.  The r e f l u x condenser was connected  i n s e r i e s to a cold trap immersed i n a dry ice/acetone slush bath. A f t e r 16 hours, the r e a c t i o n mixture was cooled and f i l t e r e d to remove insoluble s a l t s .  The cyclohexane solvent was d i s t i l l e d o f f to y i e l d a  c r y s t a l l i n e deposit which was i d e n t i f i e d as P^NyCJlg (1.4 g . , 2.3 mmol) by i t s i n f r a - r e d spectrum.  No other fractions were i s o l a t e d i n t h i s  - 47 -  d i s t i l l a t i o n , i n d i c a t i n g that no P^N Fg was present.  The i n s o l u b l e  y  p r e c i p i t a t e was dissolved i n water and a few p e l l e t s of caustic soda added.  The s o l u t i o n was evaporated to dryness and the  dissolved i n 250 ml. of water.  residue  A small a l i q u o t of t h i s s o l u t i o n gave a  p o s i t i v e r e s u l t when tested for c h l o r i d e i o n , i n d i c a t i n g that subs t i t u t i o n of the c h l o r i n e ligands had occurred.  Another a l i q u o t o f  t h i s s o l u t i o n was oxidized with a few drops of cone, n i t r i c a c i d , and treated with a few drops of a s o l u t i o n prepared by d i s s o l v i n g ammonium and magnesium c h l o r i d e s i n d i l u t e aqueous ammonia.  The appearance of a  white p r e c i p i t a t e confirmed the presence of the phosphate ion i n the o r i g i n a l s o l u t i o n , i n d i c a t i n g that decomposition of the p h o s p h o n i t r i l i c r i n g had occurred.  i i ) Reaction of Hexaphosphonitrilic Chloride with A c t i v a t e d 44 Potassium Fluoride Freshly prepared " a c t i v e " potassium f l u o r i d e (2.5 g . , 43 mmol) was added to a small 3-necked f l a s k containing f i n e l y ground (PNCJ^Jg (2.0 g . , 2.8 mmol).  A r e f l u x condenser was attached to the apparatus.  The condenser was then connected i n s e r i e s to a cold trap (immersed i n a dry ice/acetone  slush bath) which was i t s e l f connected to a s i l i c a  gel drying tube.  The r e a c t i o n mixture was s t i r r e d mechanically and  heated i n an o i l bath to a bath temperature of 145°C.  The reaction  mixture soon turned i n t o a paste which, a f t e r h a l f an hour at 145°C, began to bubble.  After about a minute, the bubbling lessened and the  - 48 -  r e a c t i o n was allowed to continue for 4 hours, after which time the o i l bath was removed from the r e a c t i o n vessel and the whole apparatus allowed to c o o l .  No v o l a t i l e products were present i n the cold trap.  The r e a c t i o n vessel was washed with 3 x 10 ml.of d i e t h y l ether. ether extract was f r a c t i o n a l l y d i s t i l l e d to y i e l d (PNF^)^ 48% of theory), b o i l i n g point  147°C.  The  CO.70 g . ,  U  i i i ) Reaction of N i t r i l o h e x a p h o s p h o n i t r i l i c Chloride with Activated Potassium F l u o r i d e In an analogous experiment to that described above, P^N^Cilg (1.07 g . , 1.78 mmol) and activated potassium f l u o r i d e (3.6 g . , 62 mmol) were s t i r r e d together and heated to 150°C for 48 hours.  There was no  v i s i b l e evidence of any r e a c t i o n occurring at any time during t h i s period.  At the end of t h i s time, no v o l a t i l e products had been c o l l e c t e d  i n the cold t r a p .  The r e a c t i o n mixture was extracted with d i e t h y l ether,  benzene and a c e t o n i t r i l e . products were found.  On d i s t i l l a t i o n of these solvents no other  The r e a c t i o n residue was completely water soluble  and gave a p o s i t i v e t e s t for chloride ion when treated with a c i d i f i e d silver nitrate,  i n d i c a t i n g that s u b s t i t u t i o n of the condensed r i n g  compound had taken p l a c e .  i v ) Reaction of N i t r i l o h e x a p h o s p h o n i t r i l i c Chloride with Argentous Fluoride A mixture of f i n e l y ground P^N^C£g CO.800 g . , 1.34 mmol) and f i n e l y ground s i l v e r f l u o r i d e (3.5 g . , 27.5 mmol) were added to a 50 m l .  - 49 -  one-necked f l a s k and the f l a s k connected i n series to a cold trap (immersed i n l i q u i d nitrogen) and a vacuum system (see F i g . 4 . 1 ) . a pressure of  atmosphere,  sand bath for one hour. cold t r a p .  At  the reactants were heated to ~180°C i n a  No d i s t i l l a t e was noticed on the walls of the  Over a period of 15 minutes, the temperature of the sand  bath was r a i s e d to 240°C. of the cold t r a p .  A condensate slowly c o l l e c t e d on the w a l l s  The heating was continued for 3 hours, at the end  of which time, the r e a c t i o n mixture residue was a dark grey s o l i d (pure AgF i s l i g h t y e l l o w ) .  The s i l v e r f l u o r i d e plug (see F i g . 4.1) was  heated to about 100°C with an i n f r a - r e d lamp and the v o l a t i l e products of the r e a c t i o n passed through t h i s plug ten times by a series of trap to trap vacuum d i s t i l l a t i o n s . a sealed tube.  The product  (0.290 g.) was c o l l e c t e d i n  This product was gaseous at room temperature, and i t s  average molecular weight was found to be 114 (the molecular weight of P N F 6  y  9  i s 455).  I ( b ) . Methoxylation of N i t r i l o h e x a p h o s p h o n i t r i l i c Chloride The f u l l y methoxylated c y c l i c phosphonitriles  (PNfOMe^)  48 49 (n = 3-8) have been prepared.  '  These compounds are colourless  c r y s t a l l i n e s o l i d s , except for (PNCOMe^)^ liquids.  a m  j j, which are both  They are soluble i n most organic solvents, but not i n water.  However, i t i s p o s s i b l e to extract them from an organic s o l u t i o n w i t h IN hydrochloric a c i d .  Heated under vacuum, the methoxyphosphonitriles  undergo a thermal rearrangement, to give a hygroscopic g l a s s ,  the  Fig. 4.1.  Apparatus used i n the attempted f l u o r i n a t i o n of P ^ C J l g .  - 51 -  i n f r a - r e d spectrum of which i n d i c a t e s the presence of a P=0 bond. A Kirsanov type rearrangement \  i s believed to take place during t h i s r e a c t i o n .  /  MeO N  y  N  I  OMe  J?  J) .P  OMe  (4.2)  OMe OMe  These compounds are made by a simple metathetical r e a c t i o n with sodium methoxide.  Completely anhydrous reagents and conditions must be used,  since the presence o f moisture d r a s t i c a l l y reduces the y i e l d of product. The preparation of a condensed r i n g methoxyphosphonitrile was attempted 31 i n t h i s work.  1 P and  H n.m.r. spectrometry would have been p a r t i c u l a r l y  informative i n deducing the structure of any compound i s o l a t e d , and the thermal properties of a condensed r i n g methoxide would be i n t e r e s t i n g i n comparison to those of the monocyclic methoxides.  Despite elaborate  precautions for the exclusion of moisture i n the reactants, no phosphon i t r i l i c compound was obtained from the r e a c t i o n .  The y i e l d of i o n i c  chlorine from the r e a c t i o n corresponded to complete s u b s t i t u t i o n of the r i n g c h l o r i n e atoms.  Concurrent or subsequent r i n g cleavage i s believed  to have occurred, with the loss of the P,N_ skeleton. o / Method:-  Anhydrous reagents were prepared as f o l l o w s .  "Super-dry"  methanol was produced by t r i p l y d i s t i l l i n g 100% methanol from sodium. Benzene (solvent) was d r i e d by doubly d i s t i l l i n g spectroscopic grade benzene from sodium.  Sodium methoxide was prepared*^ by d i s s o l v i n g  - 52 -  c l e a n , dry sodium (0-150 g . , 6.53 mmol) i n 10 ml of "super dry" methanol.  The excess of methanol was d i s t i l l e d o f f to leave a white  powder of sodium methoxide.  Under an atmosphere of dry n i t r o g e n ,  15 ml of dry benzene were added to the powdered sodium methoxide and the mixture s t i r r e d into a s l u r r y with a magnetic s t i r r e r .  A solution  of n i t r i l o h e x a p h o s p h o n i t r i l i c c h l o r i d e (0.300 g, 0.50 mmol) i n 10 ml of dry benzene was / s l o w l y added to the sodium methoxide s l u r r y . r e a c t i o n mixture was gently heated under r e f l u x for 4 hours,  The  after  which time the s l u r r y was cooled and 20 ml of d i e t h y l ether were added to the mixture.  The r e s u l t i n g suspension was washed with cold d i s t i l l e d  water (4 x 25 ml) and t h i s water c o l l e c t e d and analysed for Cl ion by p r e c i p i t a t i o n as AgC£.  The y i e l d of Cl ion was 96% of that amount  equivalent to complete s u b s t i t u t i o n of the r i n g . The r e s i d u a l ethereal s o l u t i o n was then extracted with IN hydroc h l o r i c acid (2 x 25 m l ) , the extract n e u t r a l i z e d with sodium bicarbonate, and re-extracted with chloroform.  This s o l u t i o n was d r i e d over  anhydrous calcium c h l o r i d e for 24 hours, decanted and allowed to evaporate.  No p h o s p h o n i t r i l i c compound was obtained from t h i s  extract.  The o r i g i n a l ethereal s o l u t i o n was also d r i e d and evaporated, and no p h o s p h o n i t r i l i c compound was i s o l a t e d from t h i s s o l u t i o n .  I I . Amination of N i t r i l o h e x a p h o s p h o n i t r i l i c Chloride The r e a c t i o n between p h o s p h o n i t r i l i c c h l o r i d e s and many d i f f e r e n t types of amines has been studied.  In most cases the r e a c t i o n i s a  - 53 -  simple metathetical hydrochloride.  s u b s t i t u t i o n i n v o l v i n g the e l i m i n a t i o n of the amine  P a r t i a l l y aminated p h o s p h o n i t r i l i c chlorides can also  be made by r e a c t i o n of the two reagents i n the appropriate metric r a t i o .  stoichio-  The successive s u b s t i t u t i o n of c h l o r i n e ligands by amine  groups i s i n a non-geminal fashion, and s u b s t i t u t i o n of the r i n g tends to deactivate the r i n g to further s u b s t i t u t i o n . aminated phosphonitriles  The f u l l y dimethyl-  [ P N C N N ^ j ^ ^ $ have been p r e p a r e d  51  and a l l  are c o l o u r l e s s c r y s t a l l i n e s o l i d s . /  Because the r e a c t i o n proceeds e a s i l y at room temperature, i s not  s e n s i t i v e to moisture, and because the substituent deactivates the r i n g to further r e a c t i o n , dimethylamination of the condensed r i n g compound was attempted.  Even with an excess of dimethylamine, complete sub-  s t i t u t i o n of the c h l o r i n e ligands could not be achieved.  The compound  that was i s o l a t e d from t h i s r e a c t i o n was f i n a l l y characterized as being 3,11,imino-3,11,dichloro-octadimethylaminocyclohexaphosphonitrile,  the  molecular formula of which i s P,N,(NH)(NMe ) C&„. The course o f the o o z o I dimethylamination r e a c t i o n may be w r i t t e n simply as: 0  0  - 54 -  A d i s c u s s i o n of the structure of P^N^ CNH) Q M ^ D g C J ^ i s given i n the following s e c t i o n . A s i m i l a r r e a c t i o n was attempted w i t h diethylamine.  No c r y s t a l l i n e  product could be i s o l a t e d , and since the r e a c t i o n was done on a small s c a l e , p u r i f i c a t i o n of the r e s i d u a l o i l could not be made.  In the  monocyclic p h o s p h o n i t r i l i c dialkylamides, increasing s i z e of the a l k y l group lowers the melting point of the compound and i t i s thought that t h i s i s the reason that no c r y s t a l l i n e product could be i s o l a t e d from the r e a c t i o n of diethylamine with the condensed r i n g c h l o r i d e .  The  course of the r e a c t i o n with diethylamine i s thought to be the same as that with dimethylamine.  The o i l gives a p o s i t i v e t e s t for c h l o r i n e ,  showing that s u b s t i t u t i o n of the r i n g i s incomplete.  The i n f r a - r e d  spectrum of the o i l was t y p i c a l of a p h o s p h o n i t r i l i c d i a l k y l a m i d e , with a "P=N" absorption at - 1 2 6 0 cm * and a m u l t i p l e t of C-H s t r e t c h i n g peaks i n the  2900-3000  cm * r e g i o n .  The ^"H n.m.r. spectrum  (60  MHz)  of the o i l i n carbon t e t r a c h l o r i d e s o l u t i o n i n d i c a t e d at least p a r t i a l substitution.  Signals from methyl and methylene protons were observed.  The former appeared as a t r i p l e t  C>J^_LJ  ~  7  c . p . s . ) at 5  =  p.p.m.  1.06  and the l a t t e r , as" a broad unresolved m u l t i p l e t (caused by H - H and 1  31  1  1  P- H coupling)and making i t impossible to d i s t i n g u i s h inequivalent methylene groups appeared at 6 = 3 . 1 p.p.m. As section I I I of t h i s chapter w i l l show, the r e s u l t s of the r e a c t i o n with dimethylamine are very important i n the of the chemistry of P,N_C£ . Q  understanding  In the course of t h i s r e a c t i o n the  - 55 -  condensed r i n g PN skeleton i s broken.  Attack by a nucleophile at the  bridgehead phosphorus atoms leads e i t h e r to the loss of a c h l o r i n e ligand or to the cleavage of the i n t e r n a l P-N bond.  That the i n t e r n a l  P-N bond i s broken i n preference to the P-C£ bond indicates the chemical weakness of the P^N_ nucleus to chemical s u b s t i t u t i o n . b /  Method:-  Reaction of Dimethylamine with N i t r i l o h e x a p h o s p h o n i t r i l i c  Chloride.  12 ml of a cold (0°C) 10% s o l u t i o n o f dimethylamine  i n benzene (equiv. to 0.804 g . , 17.9 mmol of HNMe2) were slowly added to a cold (0°C), s t i r r e d s o l u t i o n of P ^ C J l g CO.509 g . , 0.845 mmol) i n 15 ml of benzene.  Immediate t u r b i d i t y was n o t i c e d .  After 3 hours,  the  r e a c t i o n mixture was f i l t e r e d , and the r e a c t i o n mixture and p r e c i p i t a t e washed with 3 x 5 ml of benzene.  The weight of the  precipitate  (0.550 g.) was equivalent to 0.68 mmol of dimethylamine hydrochloride. The benzene s o l u t i o n was d i s t i l l e d to leave a pale yellow o i l , which did not c r y s t a l l i z e on c o o l i n g .  The o i l was dissolved i n p e t r o l ether  (30°-60°C), f i l t e r e d , a n d the s o l u t i o n evaporated to leave a s o l i d product  ( y i e l d 0.460 g.) which could be r e c r y s t a l l i z e d from a c e t o n i t r i l e  to produce c o l o u r l e s s , f l a k y c r y s t a l s (m-pt. 161-163°C) of 3,11,imino-3,ll,dichloro-octadimethylaminocyclohexaphosphonitrile (calculated for P ^ (NH) (NMe^gCJ^:H, 6.94; Cl, 10.01.  Found:-  P, 26.32; N, 29.75; C, 27.18;  N , 29.71; C, 27.17; H, 6.91; Cl, 9.97.)  Unlike the parent condensed r i n g c h l o r i d e , t h i s compound i s soluble almost to the point of m i s c i b i l i t y i n benzene, chloroform,  - 56 -  carbon t e t r a c h l o r i d e and p e t r o l ether ( 3 0 ° - 6 0 ° ) . i z a t i o n can only be made from a c e t o n i t r i l e .  Efficient  recrystall-  Twinning of the  crystals  i s evident, and the c r y s t a l s are also t r i b o - e l e c t r i c . The i n f r a - r e d spectrum of the compound was recorded on samples i n a nujol mull and a carbon t e t r a c h l o r i d e i n the mull spectrum are:  solution.  The prominent peaks  504 ( s t ) , 540 (m), 658 (w), 682 (w), 730 (m),  771 (m), 781 (sh), 838 (w), 863 (w), 891 (w), 929 (m), 984 1062 (m), 1188 ( v . s t ) , 1231 ( s t ) ,  (v.st),  1280 ( v . s t ) , 1456 (m), 2790 (w),  2880 (w), 2920 (m).  I l l . The Molecular Structure of 3,11, i m i n o - 3 , 1 1 , d i c h l o r o - o c t a d i m e t h y l aminocyclohexaphosphonitrile The structure of t h i s compound, which i s discussed i n the ing s e c t i o n , i s unique i n p h o s p h o n i t r i l i c chemistry.  follow-  Although the actual  conformation of the molecule has not been completely e s t a b l i s h e d ,  the  structure of the molecule i s believed to be that shown i n F i g . 4.2.  Fig. 4.2.  Suggested structure of P N (NH) (NMe ) C£ 6  2  g  - 57 -  In the following d i s c u s s i o n of the molecule, rather than use the  full  name of the compound, the molecular formula P.N.(NH)(NMe.,) C£. w i l l be 0  Z o  DO  Z  used as a notation for the compound.. a) Infra-red Spectrum The i n f r a - r e d spectrum of t h i s compound shows a complex v i b r a t i o n a l pattern c h a r a c t e r i s t i c of p h o s p h o n i t r i l i c dimethylamides.  The "P=N"  s t r e t c h i n g frequency (at 1280 cm *) i s considerably lower than i n the open r i n g hexameric dimethylamide, i n which v(P=N) occurs at 1340 cm The large difference i n frequency i n d i c a t e s that P^N^(NH) (NMe )gC£ has 2  2  very d i f f e r e n t symmetry and bonding from the open r i n g dimethylamide and should not be regarded as a simple d e r i v a t i v e of the open r i n g system.  Neither the mull nor the s o l u t i o n spectra show an observable  (N-H) v i b r a t i o n .  The reason for the absence of t h i s band i s thought  to be intramolecular hydrogen bonding, which would s h i f t v(N-H) to lower frequency and also broaden the absorption peak. b) Mass Spectrum The mass spectrum of t h i s compound i s d i f f i c u l t to discuss i n d e t a i l , f i r s t l y because of the large number of peaks (caused by proton a b s t r a c t i o n s ) , which makes a d e t a i l e d assignment of fragments impossible, and secondly because there are no analogous compounds with which to compare i t . The parent peak consists of a m u l t i p l e t based at m/e 706. mass number 706 corresponds to the formula P^N^(NMe )gC£ 2  CI = 35).  The mass number of P N (NH)(NMe ) C£ fi  2  g  2  i s 707  2  The  (taking (taking CH = 35),  - 58 -  but the parent ion i s an odd e l e c t r o n . s p e c i e s .  The parent peak i t s e l f ,  at m/e 707, has an i n t e n s i t y , of 24% (± 1%) of the P - l peak. 15 13 N i s 0.38% and that of C i s 1.1%.  abundance of  On t h i s b a s i s , the  13 c a l c u l a t e d c o n t r i b u t i o n to the parent peak from [P N CNMe ) C£ ] 6  y  2  g  2  +  The natural  15  C and  fragment i s 23% that of the P - l peak.  N i n the The c o n t r i b u t i o n  of the parent i o n to the m/e 707 peak must therefore be very s m a l l , and the N-H bond must be weak and e a s i l y broken. The absence of a parent i o n i s not uncommon. 52 pattern of diphenylamine  The fragmentation  does not e x h i b i t any peak corresponding to  the i o n [C^H^-NH-C^H^] , and the peak corresponding to the i o n ( +  [C^Hj.-N-CgHj.]  +  i s the most intense peak i n the whole mass spectrum.  Presumably loss of a proton i s favoured since the r e s u l t i n g ion fragment w i l l : be s t a b i l i z e d by resonance with the two phenyl groups. 53 mass spectrum of heptasulphur imide, S NH, y  phenomenon, the i n t e n s i t y of the (S NH)  +  y  the ( S N ) 7  +  The  exhibits a similar  i o n being only 5% of that of  ion.  The higher mass range part o f the spectrum c o n s i s t s mainly of fragments of the type P ^ N ^ C ^ (NMe ) 2  x  (x = 8-2).  In t h i s part of the  spectrum, loss of dimethylamine groups i s more favoured than loss of c h l o r i n e , thereby i n d i c a t i n g a stronger P-C£ bond than P-NMe bond. 2  31 c)  P n.m.r. Spectrum 31 The  P n.m.r. spectrum i s d e t a i l e d i n Table 4 : 1 . 31 1 31 31  the 3 peaks i s poor because o f  P- H and  P-  Resolution of  P coupling.  In the case  of the l o w - f i e l d s i g n a l at 6 = -13.0 p . p . m . , the peak i s resolved i n t o  - 59 -  Type of Phosphorus Atom 6  p  P  P  P  P P 4 ' 5' 6  p.p.m.  r e l a t i v e to 85% H P0 3  P  l  -13.7  -3.7  +1.9  1  2  3  4  Intensity ratio Structure  Triplet J P  TABLE 4 : 1 .  Fig. 4.3.  Data from  _P-  =  4  0  C  - ' * p  S  broad unresolved singlet  broad unresolved singlet  P n.m.r. spectrum of P N (NH)(NMe ) C£  Gross structure of P,N (NH)(NMe ) C£ 31n P n.m.r. spectrum. 6  2  g  2  i n d i c a t e d by  - 60 -  a t r i p l e t caused by  P-  P coupling (Jp,p = 40 ± 2 c . p . s . )  coupling i s unresolved and not measurable.  P- H  The two h i g h - f i e l d s i g n a l s  are broad (half-height width - 100 c . p . s . ) unresolved peaks with no distinguishable fine structure. structure given i n F i g . 4 . 3 .  The spectrum i n d i c a t e s the gross  The lack of r e s o l u t i o n i n the h i g h - f i e l d  s i g n a l s i s most l i k e l y caused by the fact that phosphorus atoms to which these s i g n a l s are assigned can couple with more than one type of phosphorus atom, thereby causing a complex coupling pattern which i s unresolvable.  The P^ phosphorus atom, however, only couples with one type  of phosphorus atom, and the simpler coupling scheme i s , resolvable.  The chemical s h i f t s of the P^, P,- and P^ atoms are not  required to be equivalent by symmetry. unusual.  therefore,  This type of occurrence i s not  In monosubstituted hexameric f l u o r o p h o s p h o n i t r i l e s , a l l the  =PF phosphorus atoms, except those adjacent to the s u b s t i t u t e d phos2  phorus atom, have i d e n t i c a l chemical s h i f t s , even though they are not symmetrically equivalent. 31 The i n t e r p r e t a t i o n of  P chemical s h i f t s i n terms of the e l e c t r o -  n e g a t i v i t i e s of ligands i s unrewarding i n many p h o s p h o n i t r i l i c d e r i v a tives. ways.  A l i g a n d can affect the e l e c t r o n density at phosphorus i n two A conjugative effect between phosphorus and the l i g a n d can  e i t h e r donate or withdraw e l e c t r o n density from phosphorus, w h i l s t a TT-inductive effect can e i t h e r suppress or increase the d e r e a l i z a t i o n of the lone p a i r electrons on the r i n g nitrogen atoms, thereby changing the TT-electron density at phosphorus.  In f u l l y substituted phospho-  31 n i t r i l e s , the  P chemical s h i f t moves u p f i e l d with i n c r e a s i n g e l e c t r o -  - 61 -  n e g a t i v i t y of the l i g a n d . (X = F , OEt, Ome, CI,  54  S i m i l a r l y , i n the s e r i e s P^N^C^X  NMe ), the chemical s h i f t of the  substituted  2  phosphorus atom moves u p f i e l d with the i n c r e a s i n g e l e c t r o n e g a t i v i t y of X .  5 5  In these compounds, the influences of a TT-inductive effect  seem to be the most important. The c o r r e l a t i o n of the  31  P chemical s h i f t s of V.U Cl o / 31 n  P^N^(NH)(NMe )gC£ i s not understood. 2  2  At best, the  n  y  and  P n.m.r. spectrum  of P^N^(NH)(NMe )gC& serves as a way of determining the gross symmetry 2  2  of the molecule, but gives only a l i t t l e i n s i g h t i n t o the e l e c t r o n i c structure of the molecule.  The p o s i t i o n s of the three peaks i n the  spectrum r e l a t i v e to each other are d i f f i c u l t to r a t i o n a l i z e simply i n terms o f a Tr-inductive e f f e c t . S i m i l a r d i f f i c u l t i e s a r i s e i n the 31 P n.m.r. spectra of p a r t i a l l y dimethylaminated t r i m e r i c p h o s p h o n i t r i l i c chlorides. their  31  F i g . 4.4 shows three such d e r i v a t i v e s and the d e t a i l s of  P n.m.r. spectra.  56  A Tr-inductive effect which would account  B Me N 2  NMe  2  NMe,  Me N 2  C£ 6  = -22.0 p.p.m.  6  6^ = -20.4 p.p.m.  6  Fig.  4.4.  31  cis  NMe,  2  CI  = -23.9 p.p.m. R  Me N  6  = -27.6 p.p.m.  = -30.5 p.p.m.  P chemical s h i f t s of some t r i m e r i c dimethylaminochlorophosphonitriles ( a l l values r e l a t i v e to 85% H P0 ). 3  4  - 62 -  for  the observed chemical s h i f t s i n compound A , cannot account for the  s h i f t s i n compound B, where the i n t r o d u c t i o n of an extra -NMe2 group has increased the chemical s h i f t of the  atom from i t s o r i g i n a l  value i n compound G. 31 Anomalies, therefore do occur i n the  P n.m.r. parameters o f  dimethylamino-chlorophosphonitriles, and the i n t e r p r e t a t i o n o f chemical s h i f t s i n terms of Tr-electron d e n s i t y i s not always rewarding. d) *H n.m.r. Spectrum The *H n.m.r. spectra (at 60 and 100 MHz) of P N (NH) (NMe ) CJl 2  are shown i n F i g . 4 . 5 .  g  The phosphorus decoupled proton spectrum i s also  shown, together with assignments of the various peaks. At 100 MHz, the r e s o l u t i o n of the peaks i s much b e t t e r than at 60 MHz.  The s i g n a l s corresponding to the H and H^ protons have a  collapsed into a s i n g l e peak i n the 60 MHz spectrum. The r e l a t i v e 31 i n t e n s i t i e s of the signals i n both the coupled and P decoupled spectrum are i n the r a t i o of H : H, : H : H , = 1 : 2 : 3 : 2. In the P a D c d decoupled spectrum, taken using a noise band with a band-width of 3 1  31 2000 Hz at a frequency of 40.481000 MHz,  1 P- H decoupling i s completely  achieved and the ordinary high r e s o l u t i o n spectrum, c o n s i s t i n g of 4 doublets, collapses i n t o 4 s i n g l e peaks, corresponding to 4 d i f f e r e n t types of proton i n the molecule. The *H spectra have been interpreted i n terms of the molecular structure shown i n F i g . 4 . 5 .  Although there are two conformations of  the molecule which are consistent with *H n.m.r. spectra, a considerat i o n of the mechanism (which w i l l be discussed l a t e r ) of the formation  - 63 -  ab cd  H H  5  H H  ' . 1 0 1 5  F i g . 4 . 5 . • The  '  20  '  25  5  10  H n.m.r. spectra of. PgNgCNH) (NMe'^.gCJl'  15  .  20  25  Spectra B and C  are taken at f i e l d strengths of 10.0 MHz. and 60 MHz. r e s p e c t i v e l y . 31 1 Spectrum A i s the  P- H decoupled spectrum (taken using a  2000 Hz. noise band at 40.48100 MHz.).  The figure on the top  r i g h t of the diagram shows the assignments of the various peaks.  - 64 -  of the molecule has shown that the structure shown i n F i g . 4.5 i s the most probable conformation. The chemical s h i f t s and coupling constants of the various types of proton are given i n Table 4 . 2 .  It i s important to note that the  coupling constant i s J „ = 12.7 c . p . s . p  largest  Both i n mononuclear phosphorus(V)  Type of proton  H  Intensity  1  2  3  2  2 626  2.606  2.576  2.551  ratio  6 p.p.m. J _ p  H  c.p.s.  TABLE 4:2.  compounds,  A  H  10 7  Data from  B  10.7  H  H  c  10.7  D  12.7  n m.r. spectra of PgN^ (NH)(NMe ) 2  i  and i n the dimethylamino-derivatives of t r i m e r i c phospho-  n i t r i l i c c h l o r i d e , ^ Jp^ values decrease with i n c r e a s i n g aminolysis 5  and the value of Jp^ acts as an e x c e l l e n t c r i t e r i o n for d i s t i n g u i s h i n g between =PC£NMe2 and ^ ( N l ^ ^  groupings.  below 16.3 c . p . s . , and for the l a t t e r ,  For the former, J  p H  i s never  i s never above 13.9 c . p . s .  In  - 65 -  the series  (PN(NMe ) ) 2  2  n  (n = 3-6), J ^ decreases from 11.2 c . p . s . p  (for  58 n = 3) to 10.0 c . p . s .  (for n = 6 ) .  The effect of r i n g s i z e on Jp^ i s  s m a l l , and the fact that i n P N (NH) (NMe ) C£ J fi  6  2  g  <.12.7 c . p . s . proves  2  that there are no =PC£NMe groups i n the molecule, and i s a further 2  confirmation of the structure shown i n F i g . 4 . 5 . The i n t e r p r e t a t i o n of the "*"H n.m.r. spectra 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 i s i n most cases hampered by complex unresolvable c o u p l i n g . 58 For instance, the simple c y c l i c dimethylamides show a d e v i a t i o n  from  the simple doublet pattern which i s expected from a f i r s t order t r e a t ment.  The spectra consist of a sharp doublet between which l i e s a broad  peak which, i n the case of (PN(NMe ) ) accounts for 25% of the t o t a l 2  2  3  i n t e n s i t y of the s i g n a l and, i n the case of ( P N ( N M e ) ) , accounts for 2  2  g  a l l of the s i g n a l , with the outside doublet being completely unresolved. The c a l c u l a t i o n of t r a n s i t i o n energies and the r e l a t i v e i n t e n s i t i e s of each of the hydrogen n u c l e i i n p h o s p h o n i t r i l i c molecules i s very complex and has not yet been achieved.  Analysis of simpler spin  59 systems  has shown that the "odd" appearance of the spectra i n high  symmetrical spin systems such as c y c l i c p h o s p h o n i t r i l i c dimethylamides i s due e n t i r e l y to t h e i r symmetry. ^ 6  The r e s o l u t i o n of the *H n.m.r.  spectrum of P^N^(NH)(NMe ) C& into separate peaks i s i n d i c a t i v e of 2  g  2  the lack of s t r u c t u r a l symmetry i n the molecule.  Long range coupling  i s n e g l i g i b l e , and the lack of chemical, and therefore  magnetic,  equivalence of the phosphorus atoms i n the molecule eliminates the complexities which occur i n more symmetric systems.  A simple f i r s t  - 66 -  order treatment can therefore be used to interprete the  H n.m.r.  spectrum i n terms of the structure of the molecule. Another i n t e r e s t i n g feature that t h i s molecule shows i s that some p a i r s of dimethylamino-groups attached to the same phosphorus atom have d i f f e r e n t chemical s h i f t s .  This d i s t i n c t i o n o f dimethylamine  groups d i f f e r i n g only i n t h e i r conformation at a phosphorus atom i s unique i n p h o s p h o n i t r i l i c chemistry.  In (PNCNfr^J^J^ there are a x i a l  and equatorial -NMe2 groups (which have d i f f e r e n t e x o c y c l i c P-N bond lengths), but a l l protons i n the molecule resonate at the same frequency. In the monocyclic dimethylamide c h l o r i d e s , when two -NMe2 groups on a p a r t i c u l a r phosphorus atom are i n d i f f e r e n t environments, t h e i r ~~H chemical s h i f t s are s t i l l i d e n t i c a l .  The fact that there i s a d i s t i n c -  t i o n between -NMe2 groups i n P^N^(NH) (NNfc^gCJ^ must be a r e s u l t of the molecule being held i n a r i g i d conformation.  The reason for t h i s i s  thought to be intramolecular hydrogen bonding, which holds the molecule rigid.  Hydrogen bonding i s thought to occur between the (N-H) proton  and the a x i a l -NMe2 groups on the r i n g .  The (N-H) proton s i g n a l , which  i s not shown i n F i g . 4 . 5 , occurs at 6 = 6.15 p.p.m. at 60 MHz.  It i s a  weak broad peak, and although i n t e n s i t y comparisons are d i f f i c u l t ,  the  expected r a t i o of 1:48 i s not inconsistent with the observed e x p e r i mental i n t e g r a l curve. Hydrogen bonding of the (N-H) proton would tend to withdraw electron density from the dimethylamino-group and thereby lower the chemical s h i f t of the protons attached to that group.  The observed  chemical s h i f t s are consistent with t h i s theory, the extent of the hydrogen bonding depending on whether the -NMe protons are i n the ?  - 67 -  L  &  or  p o s i t i o n (see F i g . 4 . 5 ) .  The equatorial L  not i n t e r a c t with the (N-H) proton and a l l the a s i n g l e frequency.  -NMe2 groups do  protons resonate at  The H^ protons are i n d i f f e r e n t environments i n  the molecule, but as i n the case of P^H^CZ^QMe^)^ ( F i g . 4 . 4 B ) , they have i d e n t i c a l chemical s h i f t s . Some understanding of the two i n t e r n a l P-N bonds i n PgN^CNH) ( N I ^ ^ C J ^ can be gained from an examination of the p o s i t i o n of the chemical s h i f t of the CN-H) proton i n secondary amines.  Table 4:3  shows the v a r i a t i o n of 6 ^ i n a number of organic secondary amines. some of the amines, the K i s also given.  For  The r e s u l t s i n d i c a t e that the  cl  chemical s h i f t of the (N-H) proton i s a s e n s i t i v e measure of the basic character of the nitrogen atom.  Conjugation of the CN-H) group to a r y l  groups decreases the basic character of the nitrogen atom and at the same time lowers the chemical s h i f t of the CN-H) proton.  A l k y l groups,  however, suppress lone p a i r d e r e a l i z a t i o n , producing a stronger base and a high chemical s h i f t for the CN-H) proton.  That the chemical  s h i f t o f the (N-H) proton i n P N (NH) (NMe ) CJo occurs at 6 = 6.15 p.p.m. fi  6  2  g  2  indicates considerable lone p a i r d e r e a l i z a t i o n from the nitrogen atom i n t o the p h o s p h o n i t r i l i c ir-systems.  The (N-H) proton i s a c i d i c i n nature,  and t h i s information, coupled with the fact that the mass spectrum of the molecule i n d i c a t e s that t h i s proton i s e a s i l y l o s t , suggests a stronger i n t e r n a l P-N bond i n P,N (NH) (NMe )„CJl than i n PJ\LC£ . A  9  9  Q  - 68 -  NH p.p.m. 6  Compound  o-NO_C,rLNHC,H,_ zoo oo  9.39  p-NO„C,H NHC,H 2 6 5 6 5 c  8.20  CH CO-NH-CH CH  3  8.08  r  c  3  2  C,H -NH-C,H,_ 6 5 6 5  5.42  r  K a  1.62 x 10"  1  C^H -NH-CH„C,H 6 5 2 6 5  3.78  C H -NH-CH  3.34  1.41 x 10"  5  3.32  2.34 x 10"  5  3.03  7.59 x 10"  6  r  6  r  5  C H -NH 6  5  3  2  C.H -NH-CH„CH C  0 b  Z  T  o  (C H -CH ) NH 6  5  2  C H -CH -NH-CH 6  5  2  CH -NH-CH 3  1.78  2  3  3  [(CH ) CHCH ] NH 3  2  2  Reference  TABLE 4 : 3 .  1.00  2  0.79 61  1.85 x 1 0 "  1 1  1.23 x 1 0 "  1 1  62  Relationship between (N-H) proton chemical s h i f t and base strength of some secondary amines.  - 69 -  e) Conformational Analysis As was mentioned above, the  n.m.r. spectra can be interpreted  i n terms of two molecular conformations, both of which are shown i n Fig. 4.6.  The two conformations d i f f e r i n the r e l a t i v e p o s i t i o n s of  the phosphorus and nitrogen atoms i n the ten-membered r i n g . structure I , the 3 adjacent  r i n g phosphorus atoms are above the mean  molecular plane and, i n structure structures,  In  I I , they are below i t .  In both  i t i s the a x i a l dimethylamino-groups that are involved i n  hydrogen bonding, and i t i s t h i s intramolecular i n t e r a c t i o n which holds the two conformations r i g i d .  Although structures  I and I I both account  for the observed ''"H n.m.r. spectrum, the former i s b e l i e v e d , on the basis of general mechanistic considerations, to be the correct  struc-  ture . Although the stereochemical course of replacement reactions i n phosphonitriles i s not f u l l y understood, the r e s u l t s of c h l o r i n e exchange between c h l o r i d e i o n and chlorophosphonitriles suggest an 63 mechanism.  K i n e t i c studies on the dimethylamination of  CPliCH^)^  64 suggest  a penta-coordinated phosphorus atom as an intermediate, with  the attacking nucleophile and the leaving group being i n the a x i a l positions.  In general, most replacement reactions at phosphorus i n  mononuclear phosphorus compounds are accompanied by an i n v e r s i o n of configuration,^  5  and the same i n v e r s i o n i s believed to occur i n phospho-  n i t r i l e s , i n so far as s u b s t i t u t i o n i n these compounds i s effected by a bimolecular r e a c t i o n .  - 70  0  = CL  O = NMe 2  O =.N o =H  II  F i g . 4..6.  I d e a l i z e d structures for the P . N . (NH) (NMe„) C&„ molecule. ' 31 i Both structures are consistent with P and H n.m.r. data, 0  but structure I i s believed to be the correct structure (see t e x t ) .  - 71 -  The mechanism of the complete dimethylamination of P^N^CJlg i s not known.  The weakness of the i n t e r n a l PN bonds has already been  discussed, and i t i s thought that attack at a bridgehead phosphorus by a dimethylamine group leads to p r e f e r e n t i a l cleavage of the i n t e r n a l P-N bond rather than the  P-Cl bond.  I f also the mechanism o f sub-  s t i t u t i o n involves n u c l e o p h i l i c attack at these phosphorus atoms, leading to a penta-coordinated intermediate and i n v e r s i o n of c o n f i g u r a t i o n , the stereochemistry of the f i n a l product can r e a d i l y be e s t a b l i s h e d .  As  F i g . 4.7 shows, n u c l e o p h i l i c attack at a bridgehead phosphorus atom,  NMe  r  HNMer Fig. 4.7.  Suggested stereochemistry of n u c l e o p h i l i c s u b s t i t u t i o n at a bridgehead phosphorus atom i n P,N_C£_. o / y  w i t h subsequent cleavage of the i n t e r n a l P-N bond, leads to a product where the r i n g phosphorus atoms are above the mean molecular plane.  -  72 -  Thus, the i n t e r p r e t a t i o n of t h i s mechanism suggests that the conformat i o n shown i n F i g . 4.61 i s the true conformation P-N, (NH)(NMe„) C£„. 0  Ob  2  o 2.  IV. Summary The extent of t h i s study of the chemistry of P^N C£ has been y  g  l i m i t e d by the small amount of the compound that was a v a i l a b l e and by the very nature o f the compound i t s e l f .  Unlike the monocyclic  p h o s p h o n i t r i l i c c h l o r i d e s , P^N C£g i s unstable to s u b s t i t u t i o n . y  Whether  the attack of an approaching nucleophile occurs at the bridgehead or r i n g phosphorus atoms i s not known.  To the extent that lone p a i r  d e r e a l i z a t i o n from the c e n t r a l nitrogen atom deactivates the bridgehead phosphorus atoms, then to t h i s extent the r i n g phosphorus atoms w i l l be r e l a t i v e l y more e l e c t r o p h i l i c i n nature. To the present time, only one d e r i v a t i v e of the condensed r i n g compound has been i s o l a t e d .  The fact that i n t h i s compound the  condensed r i n g skeleton has been broken, v i s u a l l y emphasizes the chemi c a l weakness of the P^N skeleton. y  In the r e a c t i o n with dimethylamine,  one of the i n t e r n a l PN bonds i s broken.  That the r i n g cleavage process  stops at t h i s stage r e f l e c t s the strengthening of the two remaining i n t e r n a l P-N bonds which occurs as a r e s u l t of the loss of the f i r s t . The fact that s u b s t i t u t i o n at the two remaining bridgehead phosphorus atoms i n P^N^CNH) (NMepgCJ^ does not occur, even with an excess of dimethylamine, must be a r e s u l t of the complete d e a c t i v a t i o n of these phosphorus atoms.  - 73 -  In the case of r i n g a c t i v a t i n g nucleophiles C -g. f l u o r i n e ) e  further s u b s t i t u t i o n seems to occur, with subsequent collapse of the p h o s p h o n i t r i l i c skeleton.  In organic chemistry, s u b s t i t u t i o n at a  bridgehead carbon atom v i a an SN mechanism i s s t e r i c a l l y impossible. 2  In  P^NjClg,  the same s i t u a t i o n may also occur, and the s u b s t i t u t i o n  at bridgehead phosphorus atoms may be k i n e t i c a l l y d i s a l l o w e d .  Attack  by f l u o r i d e at the bridgehead atoms seems to break the P-N bonds rather than the P-C£ bonds. Further work may yet produce a f l u o r i n a t e d condensed r i n g molecule. The appearance of high molecular weight fragments i n the products of one of the f l u o r i n a t i o n reactions attempted i n t h i s work i n d i c a t e that t h i s may be p o s s i b l e .  The observed c h a r a c t e r i s t i c s of the condensed  r i n g molecule are e n t i r e l y unexpected, and further work i s necessary before a complete understanding of t h i s compound i s achieved. At t h i s po'int I would l i k e to thank the following people, who provided me with t e c h n i c a l assistance i n the course of t h i s work: Mr. P. Borda, for c a r r y i n g out microanalyses; Mr. G. Gunn, for taking mass spectra; Mr. R. Burton and Miss P. Watson, for running various n.m.r. spectra; and Dr. L . D . H a l l and Mr. P. S t e i n e r , for running 31 P decoupling n.m.r.  experiments.  - 74 -  CHAPTER 5  THE MOLECULAR AND ELECTRONIC STRUCTURE OF NITRILOHEXAPHOSPHONITRILIC CHLORIDE  The t r i c y c l i c condensed r i n g shape of P^NjCig  was established by  31 P n.m.r. and v i b r a t i o n a l spectroscopy and C^ for the molecule.  symmetry was suggested  The recent X-ray c r y s t a l s t r u c t u r e analysis*^ of the  molecule confirmed t h i s assignment.  In the course of t h i s study, Huckel  molecular o r b i t a l c a l c u l a t i o n s were c a r r i e d out on an i d e a l i z e d planar condensed r i n g P^N framework.  The r e s u l t s of these c a l c u l a t i o n s , and  y  the X-ray c r y s t a l structure a n a l y s i s , are discussed i n t h i s chapter. I . O r b i t a l Interactions i n P-N-CJl^ 6—7—9 Throughout the course of t h i s work, comparisons have been made between the condensed r i n g P^N C£ molecule and the open r i n g P^N^C£^ y  g  molecule, and t h i s comparison again becomes useful i n considering the bonding i n P^N C£ . y  g  In p h o s p h o n i t r i l i c compounds i n general, i f the  valencies of phosphorus and nitrogen are taken as f i v e and three r e s p e c t i v e l y , the a-framework of the c y c l i c -PX2=N- u n i t can be 3 2 described using sp h y b r i d i z a t i o n at phosphorus and sp h y b r i d i z a t i o n at n i t r o g e n .  The formation of a Tr-bond, necessary to s a t i s f y  the  valencies of phosphorus and n i t r o g e n , then involves the remaining 2p o r b i t a l on nitrogen and a 3 d - o r b i t a l on phosphorus.  - 75 -  In  P^N^CJlg, there are two types o f phosphorus atom.  non-bridgehead  At the  (SPCJ^) type atoms, the o r i e n t a t i o n s of the d - o r b i t a l s  are analogous to those that occur i n the monocyclic p h o s p h o n i t r i l e s . Fig.  5.1 shows the o r b i t a l overlap schemes for the d - o r b i t a l s o f a  EPC&2 phosphorus atom with valence o r b i t a l s o f n i t r o g e n . seen i n t h i s diagram, two types of TT-bonding can occur. it d^  that the out-of-plane system i s termed the IT  system.  As can be Convention has The d . and  o r b i t a l s on phosphorus are involved i n t h i s system, overlap o f  z  these o r b i t a l s being ( i n planar systems) with the nitrogen p  z  orbital.  The in-plane TT-system i s termed the TT system, and involves overlap S  of  ( p r i n c i p a l l y ) the d^2  y.2  a n <  l d  o r b i t a l s on phosphorus with the  2 sp  lone p a i r hybrid on n i t r o g e n . At  the bridgehead atoms of P^^Cl^,  TT-interactions of the phosphorus  d - o r b i t a l s with the r i n g nitrogen atoms are the same as the nonbridgehead atoms.  However, the a d d i t i o n of the c e n t r a l nitrogen atom  allows a further T T - i n t e r a c t i o n between the p and a phosphorus d - o r b i t a l . c e n t r a l nitrogen p selective.  z  As F i g . 5.2 shows, the overlap of t h i s  o r b i t a l with a phosphorus d - o r b i t a l i s h i g h l y  The in-plane d - o r b i t a l s  with the out-of-plane p orthogonal to the p  o r b i t a l on that nitrogen  z  orbital.  (d 2 x  2  a n c  l & y) do not X  In the TT  system, the d  interact orbital is  o r b i t a l , and only the d o r b i t a l gives a nonz yz  zero overlap with the c e n t r a l nitrogen (see F i g . 5 . 2 ) .  Therefore, any  TT-effect that t h i s atom has on the r e s t of the r i n g can only be made through i n t e r a c t i o n of t h i s atom with the d ^  z  o r b i t a l on phosphorus.  - 78 -  Since the only formal difference between the  „ system and the bo  12  system i s the i n s e r t i o n of t h i s extra n i t r o g e n , the  P^NjCig  i n the TT-bonding should not be great.  difference  The TT systems i n both molecules s  are formally the same and any differences i n molecular bonding can only occur i n the TT^ system.  The extent o f the differences depend on the  r e l a t i v e importance o f the two components of the TT TT  system.  Both i n the  and TT systems, the two separate components are not required to be  equally i n v o l v e d , and experimentally t h i s i s found to be so. calculations,  Overlap  67 68 ' have shown that i n the TT system, d -p overlap i s a xz z 2  the more important component, and i n the TT system, d^, y 2 ~ P s  s  2  overlap  i s the more important. I I . Symmetry-based M.O. C a l c u l a t i o n s on P^N^Cflg Because the only difference between the open r i n g P^N^ system and the condensed r i n g P^N^ system i s the a d d i t i o n o f the c e n t r a l nitrogen i n the l a t t e r ,  and because TT-bonding from t h i s nitrogen can only occur  i n t o one phosphorus d - o r b i t a l , a simple Hiickel M.O. c a l c u l a t i o n was c a r r i e d out on the P^N^ and P^N p h o s p h o n i t r i l i c skeletons, c a l c u l a t i o n s y  which e f f e c t i v e l y measured the difference i n bond orders and charge d e n s i t i e s between the two systems.  For both molecules, a planar  structure was assumed, where a l l r i n g angles were taken as being 120°. Overlap was only considered between the phosphorus d ^ nitrogen p For the  z  o r b i t a l and the  o r b i t a l , and a l l overlaps were taken as being equivalent. system, the energy l e v e l s that are formed are s i m i l a r to  - 79 -  those found for the phenalenyl system.^^'^^ For any p h o s p h o n i t r i l i c r i n g s i z e , the secular equations are most conveniently set up u t i l i z i n g the r o t a t i o n a l symmetry of the molecule. For i n t e r a c t i o n s i n v o l v i n g the d ^  z  o r b i t a l at phosphorus and the p  z  o r b i t a l at n i t r o g e n , the secular determinant for an open r i n g system becomes: a -E P  26cos— n  26cos— n  a -E N  D  = 0  (5.1)  XI  where £, the r i n g quantum number, takes on the values 0, ± 1 , ±2 . . . ± ( n - l ) / 2 (n odd) and 0, ± 1 , ±2 . . . n/2 (n even), and n i s the number of repeating PN u n i t i n the p h o s p h o n i t r i l i c r i n g . the two Coulomb parameters,  dp and a^, are expressed i n terms of an  e l e c t r o n e g a t i v i t y difference p, such that a r b i t r a r i l y set at p = 2.  In t h i s c a l c u l a t i o n ,  =  + p 3 ; p i s here  For the c y c l i c P^N^ system, the energy l e v e l s  deduced from the secular determinant  E = a ± \/cos ^2  (equation 5.1)  are:  + %L  where a i s the average Coulomb parameter  (5.2)  (a = (otp + a ^ ) / 2 ) .  These  energy l e v e l s are shown i n F i g . 5.3a. In the PgN condensed r i n g system, the PN skeleton has 3 - f o l d y  r o t a t i o n a l symmetry.  In the C_ point group, the c e n t r a l nitrogen p  - 80 -  - 81 -  o r b i t a l has  symmetry, and therefore can only combine with the three  A^ l e v e l s of the P^N^ system.  Using the o r b i t a l nomenclature i n d i c a t e d  i n F i g . 5.4, the three A^ intermediate o r b i t a l s with which the c e n t r a l •i *12N-^^|S| ' 2 (  cb  F i g . 5.4.  nitrogen p  (a)  N ^6  7  b  O r b i t a l nomenclature used i n symmetry based c a l c u l a t i o n s on P.N_C£_. o / y  + (j> + <J))  = ^  2  ( ) ^3 C  *  cb  o r b i t a l O f ^ ) can combine are:  z  o» ^  I , I Nr *8  y  )  = ^  =  /6  5  c*  C(j>  2  3 +  +  *  + <t> )  7  ^6  g  n  +  *10  +  *4  +  *8  +  (J)  12  )  (5.3)  Mixing of these four A^ l e v e l s produces a new 4 x 4 A^ secular determinant.  The s o l u t i o n of t h i s determinant produces the energy l e v e l s  shown i n F i g . 5.3b, the degenerate E l e v e l s and the non-bonding  - 82 -  Bond Difference between P . B . O . ' s of levels i n P , N , and P.N_ systems 6 6 6 7 Atom  PN BA  PN BB  +0.095  +0.030  +0.444  P  Difference between charge d e n s i t i e s i n A^ l e v e l s of P..N, and P.N_ systems 6 6 6 7 TABLE 5:1.  AA  A  +0.027  N  A  +0.010  P  B  N  +0.093  B  -0.426  Charge density and p a r t i a l bond order (P.B.O.) differences between P^N^ and PgN systems as derived from symmetry y  based c a l c u l a t i o n s .  A p o s i t i v e difference i n d i c a t e s P^N > y  P N . 6 6  F i g . 5.5.  Diagram showing the nomenclature used i n d e s c r i b i n g the various types o f bonds and atoms i n the P,N C£ molecule. 7  Q  - 83 -  l e v e l being unchanged from those found i n the P^N^ system.  Within t h i s  TT-system, the changes i n charge d e n s i t i e s on the r i n g atoms, and the changes i n p a r t i a l bond orders on moving from the open r i n g P^N^ system to the condensed r i n g P^N^ system were c a l c u l a t e d , and the r e s u l t s shown i n Table 5 : 1 .  are  The nomenclature used for d i f f e r e n t i a t i n g between  the various types o f atoms and bonds i n PgN^ structure i s i l l u s t r a t e d i n F i g . 5.5. The important conclusion which these r e s u l t s suggest i s a strengthening o f the P.N bond with respect to the P N . bond (see n  A A  Fig. 5.5).  D A  A considerable ir-bond order i s also i n d i c a t e d for the  PgNg bond, suggesting that t h i s bond must be quite short. As w i l l be discussed i n the next s e c t i o n , the observed bond lengths i n P^N^CJig are i n complete disagreement with the simple argument described above.  More d e t a i l e d c a l c u l a t i o n s were c a r r i e d out, as  described i n the f i n a l s e c t i o n o f t h i s chapter, and they lead to a better c o r r e l a t i o n o f theory and experiment.  I I I . The Molecular Structure of P.N_C£ , — o—/—-y 66 f  The molecular structure  o f t h i s compound i s shown i n F i g . 5.6,  with the relevant s t r u c t u r a l data given i n Table 5:2.  The molecule i s  not p l a n a r , the mean planes o f the three six-membered r i n g s (which are themselves not planar) s l o p i n g down from the c e n t r a l nitrogen atom. The  symmetry o f the molecule i s s l i g h t l y d i s t o r t e d i n the c r y s t a l ,  as one o f the  P-C£ bonds i s almost c o a x i a l with a two-fold a x i s w i t h i n  F i g . 5.6.  The molecular structure o f n i t r i l o h e x a p h o s p h o n i t r i l i c c h l o r i d e . S t r u c t u r a l data for the molecule i s given i n Table 5.2.  - 85 -  BOND ANGLES  BOND ANGLES  degrees  degrees  BOND LENGTHS (A)  C£1-P1-C£2  101.5  P5-N4-P6  123.4  Pl-Nl  1.569(6)  N1-P1-N3  116.5  P6-N3-P1  124.8  P1-N3  1.579(6)  C&3-P2-N2  104.8  P2-N2-P4  119.0  Pl-C£l  1.988(3)  N1-P2-N2  110.3  P2-N2-P6  121.0  Pl-C£2  1.970(3)  N1-P2-N7  111.8  P4-N2-P6  119.8  P2-N1  1.549(6)  N2-P2-N7  110.2  P2-N2  1.720(6)  C£4-P3-CJt5  101.3  P2-N7  1.568(6)  N7-P3-N6  118.1  P2-C£3  2.002(3)  C£6-P4-N2  104.5  P3-N7  1.557(6)  N6-P4-N2  110.1  P3-N6  1.569(6)  N6-P4-N5  113.8  P3-C&4  1.977(3)  N2-P4-N5  111.4  P3-C£5  1.990(3)  C£7-P5-C£8  101.9  N5-P5-N4  116.1  P4-N6  1.578(6)  C£9-P6-N2  102.8  P4-N2  1.733(6)  N4-P6-N2  111.4  P4-N5  1.539(6)  N4-P6-N3  111.8  N2-P6-N3  110.8  P4-C&6  2.002(3)  P1-N1-P2  125.8  P5-N5  1.574(6)  P2-N7-P3  127.3  P5-N4  1.580(6)  P3-N6-P4  123.2  P5-C£7  1.970(6)  P4-N5-P5  178.6  P5-C&8  1.981(3)  P6-N4  1.560(6)  P6-N2  1.715(5)  P6-N3  1.553(6)  P6-C£9  2.008 (3)  TABLE 5.2 .  Bond angles and bond lengths i n n i t r o l o h e x a p h o s p h o n i t r i l i c chloride (see F i g . 5 . 6 ) . Standard deviations for bond lengths are given i n parentheses.  - 86 -  the u n i t c e l l , thus causing packing forces to t w i s t the molecule out of i d e a l  symmetry.  P o s s i b l y the most important fact that i s seen i n the c r y s t a l structure i s the length of the i n t e r n a l PN bond, which i s considerably longer than the r i n g PN bonds.  As i l l u s t r a t e d i n F i g . 5.5, the r i n g  PN bonds f a l l i n t o two c l a s s e s , the P^N^ type and the PgNg type. The P-N bond lengths i n monocyclic phosphonitriles range between 1.50-1.60-A.  In the phosphoramidate  i o n , (PO^NHg)  (where there i s no  p o s s i b i l i t y of lone p a i r d e r e a l i z a t i o n r e i n f o r c i n g the a-bond),  the  o 71 P-N s i n g l e bond length i s 1.77 A.  Although the shortness of the  p h o s p h o n i t r i l i c bond i s i n d i c a t i v e of some Tr-bond character,  the bond  shortening i s also contributed to by a a - h y b r i d i z a t i o n e f f e c t . 3 s i n g l e bond i n the (PO^NH^)  i o n i s based on sp  The PN  a - h y b r i d i z a t i o n at  both centres, the bond angles being close to the tetrahedral  value.  In p h o s p h o n i t r i l i c compounds, the r i n g angle at phosphorus i s u s u a l l y close to 120°, and at nitrogen v a r i e s between 120° and 150°.  Changing  the bond angle at e i t h e r atom has the effect of changing the a - h y b r i d i z a t i o n at that atom.  The important point to consider i s that  i n p h o s p h o n i t r i l e s , up to 40% of the shortening of the p h o s p h o n i t r i l i c P-N bond w i t h respect to a P-N s i n g l e bond can be a t t r i b u t e d to such a-effects,  and any d i s c u s s i o n of the TT-bonding i n a p a r t i c u l a r molecule  can only be made after a-effects have been considered. A l l the r i n g bond lengths i n P^NjCHg  are given i n Table 5:3.  The actual T T - c o n t r a c t i o n s (with reference to the s i n g l e bond length of  - 87 -  1.77 A) are also given.  Based on a study made of the C-C bond length  i n various organic compounds,  72  Craig and Paddock have c a l c u l a t e d  73  O  adjusted values of the atomic r a d i i of phosphorus  (1.069 A) and  O  nitrogen (0.701 A) as the a - h y b r i d i z a t i o n of the two atoms i s changed. The adjustments depended on the r i n g angle at e i t h e r atom, the s i z e of the adjustment being that f r a c t i o n of the bond length by which a C-C 72 bond would change  given a s i m i l a r angle change.  Using the r e s u l t s of  these c a l c u l a t i o n s , the a-corrected TT-contractiohs for the r i n g PN bonds of P^NyC£g have been estimated, and these values are also given i n Table 5:3. Both the actual and a-corrected TV-contractions show the same trend, namely that the P^N^ type bonds have less u-character than the PgN bonds.  A  A smaller ir-bond order i s therefore i n d i c a t e d i n the former.  This experimental observation i s the reverse of that predicted by the simple t h e o r e t i c a l model described i n the previous s e c t i o n .  As has been  mentioned already, the PgNg bond i s long (1.723 X), and when corrected for a - h y b r i d i z a t i o n , i s even longer (1.748).  The length of t h i s bond  suggests only a l i t t l e Tr-character (ir-contraction =• 0.02 A) for the bond, despite the fact that the sum of the bond angles at the c e n t r a l nitrogen i s 3 5 9 . 8 ° .  In t e t r a m e r i c ^  dimethylamides, [PN(NMe ) ]4 2  t  n  e  2  are 1.68 A and 1.67 X r e s p e c t i v e l y .  4  and hexameric^ p h o s p h o n i t r i l i c average e x o c y c l i c PN bond lengths Thus the e x o c y c l i c PN bond i n  these molecules i s stronger than the PgNg bond i n P^N^CJlg, despite  the  fact that the sum of the bond angles at the e x o c y c l i c nitrogen i n the  P N A  A  Bond  P-N. Bond D 0  Bond length  Bond length  TT-contraction A Observed a-adjusted  0  A  A  A  o  Tr-contraction A Observed a-adjusted  1.569  0.20  0.14  1.578  0.19  0.15  1.574  0.20  0.13  1.539  0.23  0.19  1.580  0.19  0.13  1.560  0.21  0.18  1.579  0.19  0.13  1.553  0.22  0.18  1.569  0.19  0.14  1.5.49  0.22  0.19  1.557  0.21  0.14  1.568  0.20  0.15  1.571  0.195  0.135  1.558  0.21  0.17  TABLE 5:3.  Ring bond lengths and Tr-contractions i n P ^ N ^ C ^ . TT-contractions are tabulated  (Tr-contractions are estimated with reference to  P-N s i n g l e bond length of 1.77 A ) . value.  Both observed and a-adjusted  The l a s t figure i n each column i s an average  - 89 -  dimethylamid.es i s less than 3 5 9 . 8 ° , i n d i c a t i n g less than complete lone pair d e r e a l i z a t i o n . The e x o c y c l i c bonds i n P^N C£g also show an i n t e r e s t i n g anomaly. y  The P-C£ bonds (2.00 A) are s l i g h t l y longer than the =PC£ bonds 2  (1.98 A).  Considering the a - h y b r i d i z a t i o n at both the P. and P  D  phosphorus atoms, the HPC£ bonds have more p-character than the 2  P-C£ bonds.  Neglecting, for a moment, the Tr-character i n these bonds,  the increased s-character i n the P-C£ bonds should make them shorter than the =PC& bonds. 2  That t h i s i s not the case indicates t h a t , i n as  much as e x o c y c l i c TT-bonding does occur, i t i s much stronger at the P^ atoms than the P_ atoms. The d i s p a r i t y between the simple t h e o r e t i c a l model of the bonding and the observed s t r u c t u r a l features i s therefore s i g n i f i c a n t and consistent.  The r e v e r s a l of bond order trends between theory and e x p e r i -  ment shows that the simple theory of the bonding i s inadequate i n describing the s t r u c t u r a l features of P^ijCig.  Because of t h i s anomaly,  further, more d e t a i l e d c a l c u l a t i o n s , were c a r r i e d out, and are now described.  IV. D e t a i l e d Hiickel M.O. C a l c u l a t i o n s on——o—/—-y P^N-CJL. These molecular o r b i t a l c a l c u l a t i o n s were c a r r i e d out using a l l the phosphorus 3 d - o r b i t a l s (except the 3 d ) and the nitrogen p z 2  z  Py o r b i t a l s (see F i g s . 5.1 and 5 . 2 ) , the molecule being assumed to be planar.  No e x p l i c i t consideration was made of the c h l o r i n e  and  - 90 -  ligands and only the P^N nucleus was examined. y  The usual Hiickel  approximations were a p p l i e d , i . e . non-nearest neighbour overlap was ignored and the motion of a l l the electrons was completely uncorrelated. Suitable Coulomb parameters were chosen by reference to recent 76 c a l c u l a t i o n s on the monocyclic p h o s p h o n i t r i l i c f l u o r i d e s .  For a l l  the nitrogen atoms, values of a(Np ) = a(Np^) = -11 e.v. were used, z  and for the phosphorus atoms, values of a(Pd^ ) = z  were taken. d  a(Pd^2  yi)  = -6  Suitable Coulomb parameters for the phosphorus d ^  z  e  - v  and  o r b i t a l s were believed to be important i n the c a l c u l a t i o n s , since  t h e i r value was thought to depend on whether the p a r t i c u l a r phosphorus atom was a bridgehead or non-bridgehead atom.  The reason for t h i s  dependence of a on the type of phosphorus atom i s thought to be a r e s u l t of an exchange i n t e r a c t i o n between the c h l o r i n e ligands and the various phosphorus 3 d - o r b i t a l s .  At the bridgehead (P^) atoms, the  c h l o r i n e ligand l i e s close to the z-axis and therefore does not d e s t a b i l i z e any of the d - o r b i t a l s under consideration.  At the non-  bridgehead C ) atoms, the two (P-Cjl) bonds l i e close to the lobes of, p  A  and therefore d e s t a b i l i z e , the d o r b i t a l . Because the lobes of the ' yz d o r b i t a l are also close to the (P-CJt) bond axes at the P. atom, xy • A the effect of exchange i n t e r a c t i o n s on t h i s o r b i t a l were also considered.  The effect of exchange d e s t a b i l i z a t i o n on the above-  mentioned o r b i t a l s was introduced i n t o the c a l c u l a t i o n s by varying the value of the Coulomb parameter.  - 91 -  A l l resonance i n t e g r a l s were e s s e n t i a l l y for  pTr-d.Tr  interactions,  and the formula used for the c a l c u l a t i o n of these i n t e g r a l s was:  /VN C-6) x C - l l )  1 J  This allowed the g's to depend on the a ' s ,  as they must, and i s broadly  consistent with the conclusions obtained from a recent study of the monocyclic CPNF2) s e r i e s . n  76  The overlap i n t e g r a l s , ine  ovenaD  integrals,  . , were expressed t  t e r m s o f a p r i m i t i v e pTr-diT o v e r l a p i n t e g r a l o f S(pTr-dTr) = 0.3636, and a p r i m i t i v e pa-da overlap i n t e g r a l of SCpa-da) = 0.1277. An o u t l i n e in  of the c a l c u l a t i o n o f the various S ^ ' s  i s given i n Appendix I .  The charge d e n s i t i e s and bond orders derived from the s o l u t i o n of the 37 x 37 secular determinant are given i n Tables 5:4 and 5:5.  The  effect of exchange i n t e r a c t i o n s on the s t a b i l i t y of various phosphorus o r b i t a l s i s to produce a s i g n i f i c a n t change i n the trends i n bond orders. D e s t a b i l i z i n g the d^ aCd  z  Cand the d  ) o r b i t a l on the  atoms (by s e t t i n g  ) = a(d ) = -2 e . v . ) has the effect of decreasing the Tr-bond order yz' ^ xy a  of the P^N^ bond with respect to the PgN^ bond, which i s consistent with the trend observed i n the actual molecule. As the values o f the p a r t i a l bond orders i n Table 5:5 show, the concept o f o r b i t a l s e l e c t i v i t y i s v a l i d , and some o r b i t a l s on phosphorus (d' d  and d^ yi)  xz  xy J  2  ).  a  r  e  more important i n TT-bonding than others  (.^yZ  a  n  (  l  However, i n so far as the d o r b i t a l i s used i n TT-bonding, i t ' yz ' &  - 92 -  Atom  Orbital  Charge Density A  V  0.07480  0.02696  0.002945  yz  0.04818  0.04818  2 x -y^  0.03455  0.003796  xy E  0.18449  0.12973  0.7479  0.07607  0.07155  0.07193  0.06977  0.07126  0.01018  0.01043  0.22629  0.22969  1.899  1.910  1.919  1.933  1.860  1.860  xz  do  d  .  B  d d  xz yz  2 x -y  do  d  N  p  N  xy Z  Pz  A  y  Pz  B  TABLE 5:4.  Calculated charge d e n s i t i e s on various atoms i n P^N C£ y  using a l l phosphorus 3 d - o r b i t a l s (except d 2) • (a) Coulomb parameters for P.(d ) = P . ( d ) = - 6 . 0 e . v . A yz' A xy (b) Coulomb parameters for ( y ) A^ y^ ~ *^ (For other Coulomb parameters, see Text.) J  b  0.07480  d d  P  B  a  r  p  d  A  =  Z  P  =  X  2  e , V -  - 93 -  Orbital Overlap  Bond  d  Bond Order A  -p z d -p yz *z d x ?-y 2 Py d -p xy *y  AA  xz  r  -  r  Z  PN BA  d xz -pz r  d  -p yz *z  d x 2 -y 2 Py d -p xy *y _  PN 1 1  TABLE 5:5.  d  r  -p yz *z  B  a  b  0.2002  0.2002  0.07021  0.01348  0.1262  0.1262  0.08986  0.01733  0.4865  0.3572  0.2002  0.2024  0.06682  0.06757  0.1851  0.1877  0.02617  0.02659  0.4783  0.4843  0.2553  0.2551  Calculated bond orders for the various types of bonds i n P N C£ . 6  7  g  (a) Coulomb *parameters for P. ^ A (d ^ y z) ' =AP.^ (dx y ') = - 6.0 e.v. (b) Coulomb parameters for P^Cd^) ^A^xy-* ~ 2.0 e.v. (For other Coulomb parameters, see Text.) J  =  =  (c) The d e f i n i t i o n of p a r t i a l bond order d i f f e r e d from that used i n c a l c u l a t i o n s on monocyclic p h o s p h o n i t r i l e s . For the purposes of t h i s study, the p a r t i a l bond order, (P.B.O.) , between two atoms r and s, was defined as: ' (P.B.O.) = 2 £ c. c. 6 ^ r,s . i r i s rs ' l = a l l occupied o r b i t a l s  - 94 -  i s thought that the c o n t r i b u t i o n of t h i s o r b i t a l i s greater at the P  D  a  than at the  atom, f i r s t l y because of exchange d e s t a b i l i z a t i o n at  the P^ atom, and secondly because the d ^  z  o r b i t a l i s s t a b i l i z e d on  the Pg atom by the influence o f the c e n t r a l nitrogen atom. Another i n t e r e s t i n g point concerns e x o c y c l i c TT-bonding at the Pg atoms.  At these atoms, e x o c y c l i c TT-bonding can (by symmetry) only  occur between the c h l o r i n e l i g a n d and the d and d o r b i t a l s on • xz yz phosphorus.  The d ^  z  o r b i t a l i s s t r o n g l y involved i n forming a r i n g  TT-system, and therefore  i s less involved i n the e x o c y c l i c TT-bonds.  If  the d ^ o r b i t a l i s also s t r o n g l y involved i n r i n g TT-bonding at the Pg atom, then i t also w i l l only form.weak TT-bonds to c h l o r i n e . observed values o f the bond lengths of the than those o f the  P-C£ bonds are greater  PC&2 bonds suggests that there i s l i t t l e or no  TT-bonding i n the e x o c y c l i c  P-C£ bonds and adds credence to the above  argument i n v o l v i n g a strong i n t e r a c t i o n of the P ring  That" the  g  d^  z  o r b i t a l i n the  Tr-systems. The P^N,, bond order i s smaller than those of the P . N . and P„N. B B AA B A  bonds, but i s s t i l l quite s u b s t a n t i a l and v a r i e s l i t t l e with changes i n the a parameters o f the d - o r b i t a l s on the P^ atom.  The unusually  o  long PgNg bond length (1.723 A) cannot be properly explained by these c a l c u l a t i o n s , and the reason for i t may w e l l be s t e r i c rather than electronic. The base strength measurements, difficult  described i n Chapter 3 are also  to e x p l a i n i n terms of e l e c t r o n i c f a c t o r s .  The base strength  - 95 -  of any p a r t i c u l a r p h o s p h o n i t r i l i c compound i s contributed to by two electronic effects.  The a - h y b r i d i z a t i o n of the lone p a i r h y b r i d on  nitrogen i s a function of the r i n g angle at n i t r o g e n , so that the smaller the r i n g angle, the greater i s the s-character of the nitrogen lone p a i r hybrid.  An increase i n the s-character of t h i s lone p a i r o r b i t a l  decreases the basic character of the lone p a i r . a-effects  Thus, considering  alone, a decrease i n the r i n g angle at a nitrogen atom i s  accompanied by a decrease i n the base strength of the lone p a i r on that nitrogen atom. TT-effects also contribute to the basic character of the lone p a i r electrons.  Because of t h i s , an i n c r e a s i n g r i n g angle at n i t r o g e n , which  i s symptomatic of greater lone p a i r d e r e a l i z a t i o n i n t o the r i n g TT^ system, i s expected to be accompanied by a decrease i n the base strength of the lone p a i r . Thus, a - and Tr-effects produce trends i n the base strengths of p h o s p h o n i t r i l e s which are the opposite of each other.  Whilst  a-effects  increase base strength with increase i n ^PNP, rr-effects decrease them, and the r e s u l t a n t base strength of any p a r t i c u l a r molecule w i l l be a balance o f these two f a c t o r s . As had already been mentioned, any formal difference i n the ir-bonding of the PgNg and P^N_, system can only a r i s e i n the  system.  Since l o n e - p a i r d e r e a l i z a t i o n occurs through the TT system, the effect of s  changes i n the TT  system on the base strength can only be secondary  EL  ( i . e . through a TT /TT  interaction).  The differences i n basic properties  - 96 -  of P^N_C£_ and P.I\LC,SL„ are therefore most l i k e l y to be caused by a 6 7 9 6 6 12 a-effect rather than a if.-effect.  Since the r i n g angle at nitrogen  i n P^NyCilg i s small ( 1 2 5 . 5 ° ) , the s-character of the lone p a i r hybrid i s l i k e l y to be greater than that found i n P^N^CJi^j and thus the observed base strength of P^N C£g (which i s less than that o f y  is explainable.  However, anything more than t h i s empirical conclusion  cannot be made at present, since there for  ^^^^12^  exists, no q u a n t i t a t i v e theory  separating the a- and u - f a c t o r s which c o n t r o l the base strength o f  phosphonitriles. I would l i k e to thank Dr. K . A . R . M i t c h e l l and Mr. R. Bruce for t h e i r invaluable advice and assistance i n performing the Hiickel M.O. calculations.  - 97 -  APPENDIX I  Evaluation of Overlap Integrals for P^N^C&g In the Huckel M.O. c a l c u l a t i o n s , a l l overlap i n t e g r a l s between phosphorus and nitrogen were expressed i n terms of a l i n e a r combination of a p r i m i t i v e  piT-dTr  overlap, where  S(pTT-dTr)  pa-da overlap, where S(pa-da) = 0.1277. o r b i t a l exponents of 1.95 for the 3diT  2piT  = 0.3636 and a p r i m i t i v e  These values were based on  o r b i t a l at n i t r o g e n , 1.40 for the  o r b i t a l at phosphorus and a P-N bond length of 1.52  K  ( t h i s value of  the P-N bond length has been used for some studies on the p h o s p h o n i t r i l i c f l u o r i d e s , but the s l i g h t l y longer p h o s p h o n i t r i l i c bonds i n P^NyC£ were g  not considered to be important from the point of view of t h i s study). In the out-of-plane TT-system, overlap i n t e g r a l s were obtained quite simply using the following formulae:  At the P. atom,  S(d  -p ) xz z '  ± S(pTT-dTr) cos30°  S(d  -p ) yz z '  - S (pTT-dlT) cos60°  v  k  r  r  -p )  ±  S(pTT-dTT)  cos30°  for overlap with N . ,  S(d -p ) ^ yz *z  +  S(pTT-dTT)  cos60°  for overlap with N  S(d  +  S(pTT-dTr)  At the P  D  atom,  S(d v  D  xz z r  -p )  yz *Z  In the in-plane ir-system, the evaluation of overlap i n t e g r a l s required a r o t a t i o n of coordinates using the scheme shown below.  In  -  98 -  t h i s diagram ( F i g . A . l ) , the standard coordinate scheme at the Pg and  Fig. A . l .  Rotated coordinate scheme used i n the evaluation of overlap i n t e g r a l s at the Pg atoms i n the TT system.  atoms ( F i g . 5.7) i s rotated i n t o a new system such that:  At N ,  2 N Z  A  At Pg,  X=  /3 2  Zp  /3 2 N  +  X  1 ~ 2 P X  1  /I  2 P Z  +  2P X  Therefore, the atomic o r b i t a l s i n the new coordinate system have the form:  - 99 -  2  P x  = R  2  2  P y  = R  2 p  2  P  =  R  Z  p  2 p  xX  3d  z 2  x Y  3d  x y  x  Z  3  3d  3 d  d  f  =  R  = R  YZ  =  R  XY'  =  R  XZ  3 d  3d  x2-Y2 = 3d  C Z ^ )  3 d  Y  R  Z  3d^ -  C X Y  2  Y 2  )  ^  Using t h i s transformation and a s i m i l a r one at the  atom, the  overlap i n t e g r a l s between phosphorus and nitrogen were c a l c u l a t e d as being:  3 At the P  A  atom,  SCd _ 2 - P ) = j x2  Y  y  S(d -p ) = XY  At the Pg' atom,  y  *j  Y 2  Y  /3 S(d -p ) = j XY  y  SCpTT-d-rr) -  S(p7r-dTT) + |  3 S(d _ -p ) = - j x 2  /3 -  SCpa-da) (sign  S(pa-da)  (sign ±)  J%  S(pTr-dTr) -  3 SCp^-du) - I  g- S (pa-da) (sign •' S (pa-da)  (sign ±)  The c a l c u l a t i o n of these overlaps was a f a i r l y simple algebraic process,  e.g.  - 100 -  At N ,  2p  A  A t  P  B>  3d  Y  =- I  2p  x2_Y2 = \  i t i s easy to show that 3d overlap o f 2p  7 P  3d  2  = ' =  J  /  Y  2  - ^  r  f  +  3d _ z 2  2  2p  x 2  =  2  x  - f  /3 o" 3d  3d  x z  1 2  - y  3d  2  z —x <6 z ^ at N . and 3d at P becomes: 2  x2-Y?  2p z  z  2  and therefore x ~y 2  R  d T  + " f 2p ) ( j 3d 2 4 z  (pa-da) - j  v  z2  -4 I  Cp-iT-dTT) .  Other S . . ' s were obtained s i m i l a r l y .  3d x -y 2  z 2  - ^ 2  3d ) dx x  -  101 -  APPENDIX II  D e t a i l s o f the instruments and techniques used i n recording the various types o f spectra were not discussed i n the main text of t h i s thesis.  A summary of pertinent d e t a i l s w i l l now be given.  i)  Infra-red s p e c t r a : -  These were a l l recorded on a P e r k i n -  Elmer 457 g r a t i n g spectrophotometer, and c a l i b r a t e d against a standard polystyrene spectrum.  Nujol and hexachlorobutadiene mull spectra were  taken using cesium iodide p l a t e s .  Solution and gas phase spectra were  recorded using potassium bromide windows.  i i ) Raman s p e c t r a : -  These were a l l recorded on a Cary 81 Laser  Raman spectrophotometer.  No d e p o l a r i z a t i o n measurements were taken.  i i i ) Ultra-violet spectra:-  These were a l l recorded on a Cary 14  spectrophotometer, using quartz c e l l s and a s i n g l e beam technique.  i v ) Mass s p e c t r a : A.E.I,  These were a l l recorded at 70 e.v. on a  type M.S.9 mass spectrometer, samples being admitted through  conventional i n l e t systems.  v) N.m.r. s p e c t r a : -  *H n.m.r. spectra were run at 60 MHz on a  Varian T-60 n.m.r. spectrometer and at 100 MHz on a Varian HA-100 31 n.m.r. spectrometer.  P n.m.r. spectra were recorded at 40.1 MHz  - 102 -  on a Varian HA-100 n.m.r. spectrometer.  31  1 P- H decoupling experiments  were c a r r i e d out using a r e c e n t l y described double resonance  a  See L . D . H a l l and R. Burton; Can. J . Chem., 48, 59 (1970).  technique.  - 103 -  REFERENCES  (1)  A.W. Laubengayer, P . C . Moews, and R . F . P o r t e r , J . Amer. Chem. S o c , 83, 1337 (1961).  (2)  D.C. Carpenter and L . O . 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