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Synthesis and carbanion reactions of methylphosphazenes Oakley, Richard Thomas 1976

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SYNTHESIS AND CARBANION REACTIONS OF METHYLPHOSPHAZENES  by  RICHARD THOMAS OAKLEY B.Sc. (Hons.), University of B r i t i s h Columbia, 1969 M . S c , University of British Columbia, 1970  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of Chemistry  We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA June, 1976 0  Richard Thomas Oakley, 1976  In p r e s e n t i n g t h i s t h e s i s  in p a r t i a l  f u l f i l m e n t o f the requirements  an advanced degree at the U n i v e r s i t y of B r i t i s h C o l u m b i a , the L i b r a r y I further  s h a l l make i t f r e e l y a v a i l a b l e f o r  agree t h a t p e r m i s s i o n f o r e x t e n s i v e  I agree  r e f e r e n c e and copying of  this  It  i s understood that c o p y i n g o r  thesis  permission.  Department  of  The U n i v e r s i t y o f B r i t i s h Columbia  2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5  Date  •Z3./JZ  or  publication  o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my written  that  study.  f o r s c h o l a r l y purposes may be g r a n t e d by the Head of my Department by h i s r e p r e s e n t a t i v e s .  for  ABSTRACT  The reaction of dimethyltrichlorophosphorane with methylamine hydrochloride provides a novel preparative route to the methylphosphazenes (NPMe ) 4 , 2 3  the separation of the two compounds being aided by the different  s o l u b i l i t i e s of their salts (NPMe ) .RCl and (NPMe ) .2RCl (R=H, Me) in 2  acetonitrile.  3  2 4  A general method for the synthesis of methylphosphazenes of  large ring size ( i . e . (NPMe ) , n ^ 6) by the methylation of the appropriate 2  n  fluorophosphazene (NPF ) with methylmagnesium bromide has now been achieved. 2  n  A variety of quaternary salts (NPMe ) .RI and a number of trimethylphosphinimine 2  derivatives have also been prepared.  n  The differences in the properties of the  cyclic and monomeric molecules have been interpreted in terms of their electronic structures. The methylphosphazenes (NPMe ) _iQ are c r y s t a l l i n e solids at room 2 3  temperature, and the molecular structures of (NPMe ) (n=4,5,7 and 8) suggest 2 n  that their molecular f l e x i b i l i t y is not suppressed by iT-bonding; their conformations can be accounted for in terms of non-bonded and electrostatic interactions, which favour the staggered orientations (GT and CT) of successive ring bonds. A study has been made of the acidic properties of the P-methyl protons of methyl phosphazenes.  The multiply-charged carbanions N^P^Meg^CH,,"^  (x=2,4) and the monocarbanion N^Ph^MetCHg") can be prepared by the deprotonation of the neutral methylphosphazenes with alkyllithiums.  Their  reactions with a variety of electrophiles (e.g. Mel, Me SiCl, C0 , PhCOOEt, 3  Br ) establish 2  2  that they are useful intermediates for the synthesis of novel  - ii -  organophosphazenes.  The orientation of the ethyl groups in N ^ M e g E t ^ H C l  shows that the second deprotonation of N^P^Meg occurs trans-antipodally to the first. The N-methyl quaternary salts (NPMe ) -MeI and N P Ph Me .MeI 2 3  can also be deprotonated.  4  3  3  4  2  The products are not the expected exocyclic ylids  but the novel azaphosphorins Me _-j (NHMe)P N _-|CH (n=3,4) and 2n  n  n  Me(NHMe)Ph P N CH, formed by a rearrangement in which the methylated nitrogen 4  3  2  atom is displaced from the PN ring by the i n i t i a l l y produced exocyclic methylene group (Equation 1).  The driving force of the reaction is l i k e l y to H  »3\  CH3  H \ 3  1  hhC  / 2 H  3  ,CH  N—CH \ /  3  •  3  N  N  Base$  C  .. 1 R—R  R-R. R  N  R  F R  R  be the attainment of c y c l i c aromaticity.  The  R  R  n.m.r. spectra of the  azaphosphorins indicate a rapid proton exchange between the endocyclic carbon and the exocyclic nitrogen, which can be slowed by the addition of an auxiliary base (e.g. pyridine).  When KOtBu is used to deprotonate the  quaternary s a l t s , nucleophilic attack competes with proton removal, and the linear oxides (NHMe)(PMe N) PMe 0 (n=2-4) have been isolated from these 2  n  2  reactions. The azaphosphorins Me _-| (NHMe)P N _-|CH (n=3,4) are hydrolysed in 2n  aqueous ethanol to give the c y c l i c oxides  n  M e  n  2 -l ^) n n-l 2' n  P  N  C H  a n d  r e a c t  w i t h  methyl iodide by a transylidation reaction to give the hydroiodides Me _-| (NHMe)P N _^CH.HI. 2n  n  n  Their reaction with benzoyl chloride leads to the  benzoylated derivatives Me (NHMe)P N CC0Ph and Me (NMeCC0Ph)P N CC0Ph, the 7  4  3  5  3  2  i n i t i a l substitution on carbon indicating that i t i s the primary basic centre. The molecular structures of azaphosphorins are consistent with the delocalization of charge from the endocyclic carbon into the c y c l i c PN skeleton.  Their structural parameters can be accounted for by considering the  effect of the replacement of a nitrogen atom with a =CR- group on the two equivalent components of the cyclic n-system.  - iv -  TABLE OF CONTENTS Page ABSTRACT  i  TABLE OF CONTENTS  iv  LIST OF TABLES  ix  LIST OF FIGURES  :  ACKNOWLEDGEMENTS CHAPTER I 1.1  1.2  1.3  1.4  x i  xiii  GENERAL INTRODUCTION  1  Preparation of Phosphazenes  3  1.1.1  Direct Methods  3  1.1.2  Substitution Reactions  4  Electronic Structure of Phosphazenes  9  1.2.1  Endocyclic iT-bonding  9  1.2.2  Exocyclic u-bonding  11  1.2.3  Chemical Consequences of Exocyclic Tr-bonding  12  Carbanion and Ylid Formation in Organophosphorus Chemistry.  15  1.3.1  Phosphorus Ylids  17  1.3.2  Phosphorus Carbanions  20  Summary  21  - V -  Page CHAPTER II 2.1  PREPARATION OF METHYLPHOSPHAZENES  23  Preparation of Methylphosphazenes  24  2.1.1  Preparation of (NPMe ) .(n=3,4)  24  2.1.2  Preparation of (NPMe )  26  2.1.3  Preparation of Methylphosphazenium Quaternary Salts.  29  2.1.4  Structure and Spectra of Methylphosphazenes  31  2.1.5  Structure and Spectra of Methylphosphazenium  2 n  (n>4)  2 n  Quaternary Salts 2.2  Preparation of gem-Me Ph P N 2  2.2.1 2.3  2.4  2.5  39 4  3  42  3  Preparation of Me Ph P N .Mel 2  4  3  44  3  Preparation and Chemistry of Tetramethylphosphinimine  45  2.3.1  Reaction of Me P and MeN  45  2.3.2  Preparation of [Me PNMe ] I~  2.3.3  Preparation and Deprotonation of [Me PNHMe] Cl"  46  2.3.4  Reaction of Dihalophosphoranes with (Me Si ) NMe . . . .  48  2.3.5  Structure and Spectra of Phosphinimine Derivatives..  48  3  3  3  2  46  +  +  3  3  Chemistry of gem-Cl Ph P N 2  4  3  2  52  3  2.4.1  Attempted Methylation of C l P h P N with MeMgBr  52  2.4.2  Dimethylamination of CI Ph P N  52  2.4.3  Fluoridation of C l P h P N  2.4.4  Preparation of (NMe )FPh P N  2.4.5  Attempted Preparation of a Phosphazenium Cation  2  2  2  4  2  4  3  4  4  3  3  3  3  53  3  3  3  Experimental 2.5.1 Preparation of Methylphosphazenes  54 54 56 56  - vi Page  CHAPTER III  2.5.2  Preparation of Methylphosphazene Quaternary Salts . . .  63  2.5.3  Preparation of Methylphosphinimine Derivatives  66  2.5.4  Preparation of XYPh P N Derivatives  70  4  3  3  REACTIONS OF METHYLPHOSPHAZENES WITH BASES  73  3.1  Preparation and Reactions of N P Me _ (CH Li)  3.2  Preparation and Reactions of N P Ph Me(CH Li)  3.3  Spectra and Structure of N P.Me (CH~R) (x=2,4)  4  4  3  8  3  A  s  x  2  4  x  (x=2,4)  75  2  V  73  V  Derivatives  76  3.3.1  Disubstftuted Derivatives  77  3.3.2  Tetrasubstituted Derivatives  79  3.4  Structure and Spectra of N P Ph Me(CH R) Derivatives  80  3.5  Carbanion Stabilization by Phosphazenes 31 P Chemical Shifts of Heterogeneously Substituted Phosphazenes 3.6.1 P Chemical Shifts of N P Ph Me(CH R) Derivatives . . 3.6.2 P Chemical Shifts of N-P-Meo „(CH R) Derivatives .  82  3.6  3  3  4  3 1  3  2  3  4  2  3 1  0  H *f  3.7  o X  C  _  85 89 89  X  Experimental  90  3.7.1  Preparation of Methyllithium  91  3.7.2  Preparation of N P Me Et  92  3.7.3  Preparation of N P (CH SiMe )  3.7.4  Preparation of N P Me (CH SiMe )  3.7.5  Attempted Preparation of N P Me (CH SiMe )  96  3.7.6  Preparation of N P Me _ (CH MMe )  97  3.7.7  Preparation and Reactions of N P Ph Me(CH Li)  4  4  4  4  4  4  4  4  2  3  6  2  3  4  4  4  8  94  4  x  4  95  2  7  2  2  3  3  3  x  3  (x=2,4, M=Ge,Sn) . 4  2  97  - vii Page CHAPTER IV 4.1  4.2  PREPARATION AND REACTIONS OF AZAPHOSPHORIN DERIVATIVES Reaction of N-methyl Methylphosphazenium Salts with Bases Reaction of NaN(SiMeo) with N-methyl Methylphosphazenium Salts  105  4.1.2  Reaction of KOtBu with N-methyl Methylphosphazenium Salts  107  4.1.3  Reaction of Methyllithium with N-methyl Methylphosphazenium Salts  109  4.1.4  Discussion  110  ?  ^H n.m.r. and Infrared Spectra of Azaphosphorins  116  4.2.1  H n.m.r. Spectra  116  Infrared Spectra  124  ]  4.3  Linear Phosphazene Oxides X(PR N) PR 0: Structure and Spectra  4.4  Reactions of Azaphosphorins  2  n  2  Their Preparation,.  125 128  4.4.1  Hydrolysis of Me _ (NHMe)P N _ CH (n=3,4)  129  4.4.2  Reaction of Me ,(NHMe)P N ,CH (n=3,4) with Methyl Iodide f ?:!  132  2n  1  n  n  1  9  1  4.4.3  Reaction of Me _i (NHMe)P N -,CH (n=3,4) with 2n  Benzoyl Chloride  ..}'.\  n  r  134  Spectra and Structure of Azaphosphorin Derivatives  136  4.5.1 4.5.2  136  ^H n.m.r. Spectra of Azaphosphorin Hydrohalides n.m.r. Spectra of Phosphazene Oxides Me _ (0)P N _ CH (n=3,4)  140  4.5.3  ^H n.m.r. Spectra of Benzoylated Azaphosphorins  143  4.5.4  Infrared Spectra of Benzoylated Azaphosphorins  144  2n  4.6  105  4.1.1  4.2.2  4.5  104  1  n  n  1  2  Experimental 4.6.1  Reaction of NaN(SiMe3)2 with Methylphosphazenium Quaternary Salts  145 146  - viii Pa^e 4.6.2  Reaction of ( N P I ^ ^ . M e l with KOtBu  148  4.6.3  Reaction of (NPMe ) .MeI with KOtBu  149  4.6.4  Reaction of (NPMe ) -2MeS0 F with KOtBu  149  4.6.5  Reaction of (NPMe ) .MeI with Methyllithium  149  4.6.6  Reaction of N P Ph Me .MeI with Methyllithium  150  4.6.7  Hydrolysis of M e g ^ N H M e j P N ^ H (n=3,4)  150  4.6.8  Reaction of Me  2 3  2 4  3  2 4  3  3  9  4  2  ,(NHMe)P N , (n=3,4) with  Methyl Iodide ? ? : ! g  ...7!  4.6.9 Reaction of Me (NHMe)P N CH with Benzoyl Chloride . .  152  4.6.10 Reaction of Me (NHMe)P N CH with Benzoyl Chloride . .  152  7  5  CHAPTER V  4  3  3  2  MOLECULAR STRUCTURES OF METHYLPHOSPHAZENES AND METHYLAZAPHOSPHORINS  g  151  154  5.1  Conformations of Methylphosphazenes  154  5.2  Structures of Azaphosphorin Derivatives  164  5.3  Summary  174  REFERENCES  176  APPENDIX  189  - ix -  LIST OF TABLES Table 1.1  2.1  Page Relative rates for the deuteroxide catalysed hydrogendeuterium exchange between ammonium, phosphonium and sulphonium ions and deuteri urn oxide  16  N.m.r. parameters, (P=N) stretching frequencies and melting points of the methyl phosphazenes (NPMe ) (n 3-10) . . . .  32  31P n.m.r. parameters of N-methyl methylphosphazeniurn iodides  39  =  2 n  2.2 2.3  n.m.r. parameters, (P=N) and (C-N) stretching frequencies of methylphosphazenium quaternary salts  2.4 2.5  n.m.r. parameters of Me PNMe and i t s derivatives  3.2  Yields and analytical data for the reaction (NPF ) + 2nMeMgBr —> (NPMe ) (n = 6-10) and ^P n.m.r. parameters of N P Me _ (CH R) derivatives .  61 78  Infrared and Raman absorption frequencies of Me P N and Me Et P N  81  n  2 n  3  4  3.3  4  4  4.1 4.2  8  x  2  x  and  3 1  4  4  4  P n.m.r. parameters of N P Ph MeCH R derivatives 3  3  4  2  31p chemical shifts of geminally substituted phenylhalophosphazenes X P h _ P N 2n  3.5  4  8  4  3.4  49  3  2  3.1  40  3 1  6  2n  3  3  P chemical shifts of XYPh P N derivatives 4  3  90 92  3  ^H n.m.r. parameters and selected vibrational frequencies of azaphosphorin derivatives  118  31p and n.m.r. parameters, and selected infrared frequencies of phosphazene oxides X(PMe2N)PMe 0  127  2  4.3  n.m.r. parameters of azaphosphorin hydrohalides  4.4  n.m.r. parameters of phosphazene oxides Me2 _i (0)P N _-|CH2-  4.5  83  n  137 n  n  n.m.r. parameters and carbonyl stretching frequencies of Me (NMeC0Ph)P N CC0Ph and Me (NHMe)P N CC0Ph 5  3  2  7  4  3  141 144  -  X  -  Table 5.1 5.2 5.3 5.4  |  Page Mean values of principal structural parameters in methylphosphazenes  155  Interatomic potential constants used for the calculation of van der Waals interactions in a (PN)2PMe2 unit  161  Observed torsion angles and local conformations in methylphosphazenes  163  Values of P=C and P-C (mean) bond lengths in a selection of phosphorus ylids  166  5.5  Bond lengths and torsion angles in C-benzoylated y l i d s  167  5.6  Mean bond lengths in azaphosphorin and protonated phosphazene structures  169  - xi -  LIST OF FIGURES Figure 1.1  Page Activation energies for the reaction CI" + (NPC1 ) — * (NPCl ) + CI" ...  10  Survey of typical reactions of the methyldiphenylphosphine oxide carbanion Ph P(0)CH "M  21  (A) 31P chemical shifts and (B) v(P=N) frequencies of phosphazenes (NPX ) (X = F, CI, OMe, Me)  34  2.2  The mass spectra of the methyl phosphazenes (NPMe )g_-jQ  37  2.3  The ordinary 100 MHz ^H n.m.r. spectrum (A) and P decoupled 'H n.m.r. spectrum (B) of (NPMe )y.MeI  41  General views of (a) the N^P^MegEto.2HC1 structure, and (b) the chair conformation of the N P MegEt H cation  79  Relative charge densities on phosphorus atom in a perturbed P N ring  89  0  2  1.2  n  2  2.1  2  +  2  n  2  3 1  2  3.1  4  3.2  3  4  2  2  2+  3  4.1  Deprotonation of N-methyl methylphosphazenium salts  106  4.2  31p decoupled 100 MHz ^H n.m.r. spectrum (A) and ordinary 100 MHz 'H n.m.r. spectrum (B) of Me (NHMe)P N CH (in benzene-dg solution)  117  4.3  100 MHz H n.m.r. spectrum of Me(NHMe)Ph P N CH in C D solution  119  4.4  220 MHz ^ n.m.r. spectrum of Me(NHMe)Ph P N CH in pyridined solution, (A) at 20°C and (B) at 60°C 123  5  3  ]  4  4  3  3  2  2  6  6  2  5  4.5  The 100 MHz ^H n.m.r. spectra of the azaphosphorin hydroiodide Me (NHMe)P N CH.HI, in CDC1 solution  138  Possible rotational isomers of the CH -P(NHMe)MeN unit in the cations [Me _i (NHMe)P N _-,CH ] (n = 3,4)  139  The 100 MHz ^H n.m.r. spectra of the phosphazene oxide Me (0)P N CH  142  5.1  Overall views of some methylphosphazene structures  156  5.2  Three possible conformations for 8-membered phosphazene rings (NPX )  157  5  4.6  3  3  2  2n  4.7  2  y  4  3  n  n  2  +  2  2  4  - xi i -  Figure 5.3  Page Angular conventions for the measurement of torsion angles in a (PN) PMe unit n L Potential energy (a) and electrostatic energy (b) contours of (PN)2PMe2 unit as a function of ^ and ^ Overall geometries and principal structural parameters of methylazaphosphorins  165  5.6  Nomenclature system for ring bonds in azaphosphorin and protonated phosphazene structures  169  5.7  Calculated bond orders and observed bond length inequalities in a protonated tetrameric phosphazene ring . .  171  Calculated bond orders in the out-of-plane iT and in-plane ir components of the iT-system in a P 4 N 0 C azaphosphorin ring 7.7  173  Energy level diagrams and Tr-electron energies for a number of phosphazene and azaphosphorin structures  174  0  5.4 5.5  5.8  162  a  s  5.9  160  - xiii -  ACKNOWLEDGEMENT  I wish to express my sincere thanks to my supervisor, Professor N.L. Paddock, for his advice and guidance during the past eight years.  Working with him has been the most valuable and rewarding experience  of my university education, and one for which I w i l l always be grateful. This work has benefited greatly from the many contributions of the various members of my research group.  To a l l of them I express my gratitude,  especially to Dr. M.R. Le Geyt for his many practical ideas and suggestions, and to Drs. H.P. Calhoun and S. Rettig for their crystallographic work.  I  also am indebted to the technical staff of the Chemistry Department for their efforts on my behalf.  In this regard, my special thanks go to Messrs. P. Borda,  B. C l i f f o r d and W. Lee, whose patience and care surpassed the extraordinary. I also wish to thank Ms. Louise Hon for her diligence and perseverance in the typing of this thesis. F i n a l l y , I would l i k e to thank my family and friends for their constant help and encouragement during my studies at U.B.C.  To, for, and with Marion  - 1 -  CHAPTER 1 GENERAL INTRODUCTION  The fundamental chemical difference between the elements of the f i r s t and second row of the periodic table is the property of the latter to possess more than one valence state.  Phosphorus, for example, is found  in both t r i - and pentavalent forms, and sulphur in d i - , t e t r a - and hexavalent states.  By contrast, their congeners in the f i r s t row,  nitrogen and oxygen, are restricted generally to the lowest valency. The interpretation of this difference in terms of molecular binding has long been a central concern of inorganic chemistry, and the potential participation of d-orbitals in bonding has often been suggested to account 1 2 for the existence of higher valence states ' .  In recent years, there-  fore, much experimental evidence has been gathered with the purpose of determining the r o l e , i f any, of d-orbitals in bond formation. One area of research that has been particularly useful in this regard is the study of phosphazene derivatives.  These compounds may be  broadly classed as linear or cyclic polymers containing the repeating unit (N=PX2).  The importance of phosphazenes to molecular bonding theory  l i e s not so much in their basic chemistry, which is similar in many ways to that of mononuclear phosphoryl compounds, as in their a v a i l a b i l i t y for study over a large range of ring sizes and ligand types; the variations in the physical and chemical properties of (NPX ) , as a function of X and n, 2  n  provide a sensitive and specific tool for examining the binding properties of pentavalent phosphorus.  The formal structural resemblance of (NPX )^ 9  - 2 -  (I)  to benzene (II)  has been recognised for many years , and much work has  been devoted to comparing the two types of aromaticity found in these molecules^.  I  . I I  From an historical point of view, chemical knowledge of phosphazenes spans more than a century.  The best known phosphazene, the trimeric 56  chloride (NPC12)3>  w a s  f i r s t isolated in 1834 ' , but i t was not until  1896, when Stokes prepared and characterized the chlorophosphazenes (NPC12)3,7» that a cyclic structure was suggested for them''.  During the  f i r s t half of this century, l i t t l e attention was paid to their physical and chemical properties, and i t was not until the 1950's, when the requirements of the aerospace industry stimulated research in inorganic polymer chemistry, that attention was focused on the synthesis and study of phosphazenes. The results of work done in this area in the last 35 years 8-18 have been the subject of many review a r t i c l e s and several books ~ . In the past, research in phosphazene chemistry has generally been directed towards the influence of different ligands on the physical and  - 3 -  chemical properties of the cyclic PN skeleton.  The s p i r i t of the work  reported in this thesis is somewhat different, in that i t is concerned with the effect of the phosphazene ring i t s e l f on the chemistry of the attached ligands.  More s p e c i f i c a l l y , the thesis deals with the preparation and  chemistry of methylphosphazenes (NPMe ) , focusing attention on the 2  n  relationship between the acceptor properties of the phosphazene ring and the chemical reactivity of the methyl groups attached to i t .  The purpose  of this introductory chapter is to summarize certain aspects of phosphorus(V) chemistry in general, and phosphazene chemistry in particular, which relate directly to the results presented in later chapters.  1.1 1.1.1  Preparation of Phosphazenes Direct Methods '  :  4  There exist several methods for preparing phosphazenes, by far the best known of which is the ammonolysis of a phosphorus chloride, according to Equation 1.  Usually, the reaction is carried out in a suitably inert  X,PC1c o + NH-Cl 4  —•  -n( N P XLJ n + 4HC1  .... 1 19  solvent, tetrachloroethane or chlorobenzene being the most commonly used 7 lq ?o As well as the chlorophosphazenes (NPC1 ) ' , the bromo (NPBr ) , 2  alkyl ((NPMe ) 26 2  (NPPhBr)  n  n  21  and ( N P E t ) 2  n  22  2  ) and aryl ( ( N P P h ) 2  n  2 3  ) derivatives can be prepared in this way.  n  ' , (NPPhCl) , 2 4  n  25  Usually the c y c l i c  trimer and tetramer are formed, with smaller amounts of the higher ring +  sizes and linear species of the type [ClR P=N-(PR =N) PR Cl] CI 2  2  n  2  71 7Q  .  -  1.1.2  4  -  Substitution Reactions The second major synthetic route to phosphazenes is by ligand  substitution, starting, usually, from a halophosphazene (Equation 2).  ... 2  The importance of this type of reaction stems from the fact that the ammonolysis reaction (Equation 1) provides the higher members of the series (NPX2) (n > 5) only n  in the case of the chlorophosphazenes.  Other large  ring size phosphazenes must, therefore, be prepared by ligand substitution of the appropriate chlorophosphazene. The scope and v e r s a t i l i t y of substitution reactions may be conveniently illustrated by classifying the products according to the periodic group to which the element bound to phosphorus belongs. 1.1.2.1  Group VII  Derivatives  The reaction of chlorophosphazenes with anionic fluorinating agents such as sodium fluoride 30 and potassium fluorosulphite 31 ' 32 to give mixed fluorochloro- and f u l l y fluorinated phosphazenes has been extensively studied (see reference 18, p. 206).  The latter compounds are formed  preferentially, since the introduction of one fluorine atom into the PN  -5  -  ring enhances the reactivity of the ring to further substitution.  By  contrast, antimony t r i f l u o r i d e reacts with chlorophosphazenes to give 33 34 p a r t i a l l y substituted products  '  , the reaction depending on the  preliminary coordination of antimony to a ring nitrogen atom, the basicity of which is diminished by successive substitution.  The effectiveness of  both types of reaction appears to be independent of ring s i z e ; the fluorophosphazenes (NPF^n (n=3-20), for example, have a l l been prepared by the action of KSC^F on the appropriate chlorophosphazene. 1.1.2.2  Group VI Derivatives As in the case of the fluorination of chlorophosphazenes, the  reaction of halophosphazenes with a variety of alkoxides and aryloxides (and their mercapto analogues) to y i e l d organosubstituted derivatives (reference 18, p. 150) is successful for a large range of ring sizes. The reaction is carried out using the alcohol i t s e l f and an auxiliary base (e.g. triethylamine or sodium carbonate), or the alcoholate anion, which is more reactive.  In this way, the phenoxy [NPtOPh),,]^ g  methoxy [ N P ^ M e ^ l ^ g '  phosphazenes have been prepared in good y i e l d .  1.1.2.3  '  and  Group V Derivatives Aminophosphazenes, formed by the action of primary and secondary  amines, or their anions, on halophosphazenes, form the largest single class of phosphazene derivatives (reference 18, p. 175).  Both f u l l y and  p a r t i a l l y aminated products can be prepared, the isolation of the l a t t e r being aided by the fact that successive amination reduces the reactivity of  - 6 -  the ring to further substitution.  The a p p l i c a b i l i t y of this reaction, l i k e  those outlined above, appears to be independent of ring s i z e ; the f u l l y 36 38 dimethylaminated phosphazenes [NP(NMe ) ] are known for n = 3 to 9 2  1.1.2.4  2  .  n  Group IV Derivatives The scarcity of aryl and alkyl phosphazenes contrasts sharply with the  abundance of amino, alkoxy and aryloxy derivatives, and reflects the greater d i f f i c u l t y of their preparation by substitution reactions. The introduction of a carbon based ligand into a phosphazene ring can be achieved in two ways.  Syntheses of the Friedel and Crafts type can 39 be used to prepare p a r t i a l l y phenylated derivatives (Equation 3), but the Ph Ph CI CI \ /  >/  N •  N  N  benzene  ... 3 CI—P  P—CI  N  / CI  AlCh  CI CI  CI  N  Pv ci  formation of f u l l y phenylated derivatives requires extreme c o n d i t i o n s ' , 40  41  Potentially more productive is the action of organometallic reagents on halophosphazenes, according to Equations 4 and 5. 2nRLi  +  (NPX ) 2  n  (NPR )  n  + 2nLiX  ... 4  (NPR )  n  + 2nMgXBr  ... 5  2  2nRMgBr +' (NPX ) 2  n  However, the results of  2  (X = halogen, R = aryl or alkyl)  - 7 -  such reactions d i f f e r markedly from those obtained from the nucleophilic substitutions already discussed, and i t was with the intention of understanding the cause(s) of these differences that the present work was begun. 39 Early work on the reaction of phenyllithium bromide " 42  47  and phenylmagnesium  with (NPC1 ) has shown that the expected reactions 2  3  (Equations 4 and 5) are, at best, an alternative to the primary reaction path.  Although (NPPh ) can be isolated (in ^5% yield) from such reactions, 2  3  the principal products are open chain phenylphosphazenes such as Ph P=N-Ph P=N=Ph P=N-MgX, formed, presumably,by the addition of a mole of 3  2  2  nucleophile across a PN bond and cleavage of the cyclic phosphazene skeleton (Equation 6) 47  CI CI  Cl  \/  s  CI i >R MgX  RMgX  a-, • CI  Cl-Pv N  a  a'  ,MgX  N Cl-  W  CI  •CI  \  Cl  CI  When fluorophosphazenes, rather than chlorophosphazenes, are employed, the substitution reaction is more successful, probably because the PN ring system i s strengthened and stabilized by the more electronegative fluorine ligands.  Thus, the phenylphosphazenes (NPPh ) 2  n  (n=4-6)  have a l l been prepared in good yield by the action of phenyllithium on the 48 appropriate fluorophosphazene  .  The alkylation of halophosphazenes has, to the present time, met with only limited, success.  The chlorophosphazenes appear to be unsuited to  - 8 -  this type of reaction" ", and the few reports of the reaction of alkylmetals 1  with chlorophosphazenes that have appeared suggest that addition  followed  39 49 by ring cleavage dominates the course of the reaction  '  .  As in the  case of phenylation reactions, substitution is favoured over addition when fluorophosphazenes are employed.  Thus, the reaction of methyl lithium with  the tetrameric fluoride (NPF ) affords the completely methylated 2  4  phosphazene (NPMe ) in 70% y i e l d . 5 1  2 4  However, in contrast to t h i s , the  complete methylation of (NPF ) , to yield (NPMe ) , is not possible, and 2  3  2  3  only the monomethyl (MeFj-P^) and gem-dimethyl (Me F P N ) derivatives 2  have been isolated from partial substitutions.  4  3  3  Complementing the results  of the previous two reactions, the pentameric methylphosphazene (NPMe )  2 5  can be prepared in 20% y i e l d by the reaction of methyl 1ithium and (NPF ) . 2  5  The o s c i l l a t i n g yields in these methylation reactions have been interpreted as being consistent with the competing addition reaction, the influence of which is predicted to be dependent on ring size (vide i n f r a ) The original purpose of the present work was to examine the nature of the addition reaction which competes with substitution during the methylation of fluorophosphazenes, and to find general preparative routes to methylphosphazenes, especially the larger ring sizes. work done in this area are described in Chapter  The results of  II.  By contrast, the alkylation of phosphoryl chlorides leads to moderate yields of t r i a l k y l phosphine oxides (e.g. C1-P0 + 3MeMgBr — — • 3Me P0; y i e l d = 52% ). 3  50  6  5  - 9 -  1.2  Electronic Structure of Phosphazenes  1.2.1  Endocyclic ir-Bonding The relationship between the molecular structure of phosphazenes  and their chemistry has been extensively studied . 4  The s t a b i l i t y of  halophosphazenes towards hydrolysis (unlike phosphoryl halides) and the fact that, in a l l phosphazenes, the ring angles at both nitrogen and phosphorus are much greater than the tetrahedral value, both indicate that the simple ionic formulation (III)  i s inadequate in rationalizing  their basic chemical behaviour and structural features. The existence of delocalized n-system in phosphazenes, based on the 3 52 use of nitrogen 2p and phosphorus 3d orbitals '•  is suggested by many  physical and chemical properties, especially the equality and shortness of the ring bonds.  In hexachlorotriphosphazene  CI CI \/  for example, the PN 1769 A  1-593 A  +  (IV),  N'  0. 0 — P — — N—H Rr-Cl  CI—  Cl—P.  ^P.—Cl  Cl III  IV  V  °53 9 bond length i s ! . 5 9 3 A , considerably shorter than the value of 1.769 A 54 found in the phosphoramidate ion (V) , in which ir-bonding is not possible. Evidence for the existence of n-bonding is also found in the 55 ionization potentials of fluorophosphazenes , which indicate the presence  - 10 -  of both heteromorphic '  and homomorphic '  ir-systems.  The former is of  lower energy and accounts for the general s t a b i l i t y of the molecules, whilst the latter is more important chemically.  Thus, the dependence on ring size  of the relative rates of the f i r s t and second substitution of chloride with fluoride ion for the series ( N P C 1 ) _ g ' , and of the measured activation 2  3  57  58  energy for the exchange process between ( N P C 1 g and chloride i o n ^ , is believed to originate from homomorphic -rr-effects . 4  3  U  5 n-in  Fig. 1.1  (PNCl ) 2  Figure 1.1 shows, for  6  n  Activation energies for the reaction CI" + (NPCl ) • (NPCl ) •+ CI" [from D.B. Sowerby, J . Chem. S o c , 1396 (1965)]. The upper curve shows calculated (HMO) Tr-electron densities at phosphorus [from D.P. Craig and N.L. Paddock, in "Nonbenzenoid Aromatics" Vol. 2, p. 273. Academic Press, 1971]. 2  2  n  n  - 11 -  ekampleV'how the variation of this latter parameter with ring size is consistent with relative n-electron densities on phosphorus (as calculated for a homomorphic iT-system).  The o s c i l l a t i o n in value of these parameters,  with change in ring s i z e , suggests that the actual nature of the reaction of phosphazenes with nucleophiles (e.g. substitution versus addition) may also be dependent on ring size.  Such an effect has already been noted for the 51  methylation of the fluorophosphazenes ( N P F , , ) ^ with methyllithium (see Sec. 1.1.4).  During the present work, a similar effect was observed  in the reactions of the N-methyl methylphosphazeniurn salts (Me^PN)^ ^-Mel with potassium t-butoxide.  The details of these reactions are discussed  in Chapter IV. 1.2.2  Exocyclic ir-Bonding The presence of 7r-bonding in phosphazenes is not limited to the  cyclic framework.  Conjugation between the endocyclic ir-system and the  exocyclic ligands is also possible.  Unlike the conjugation found in  aromatic hydrocarbons, the interaction between a ligand on a phosphazene ring and the cyclic Tr-system is not restricted to a particular orientation of the ligand about the phosphorus-1igand bond.  This is because of the  potential a v a i l a b i l i t y of more than one phosphorus 3d orbital (rather than the one carbon 2p orbital in benzene) through which conjugation can occur. In phosphine oxides R^PO, conjugation is strongest when R is an electron releasing group.  Consistent with t h i s , the phosphazenes which  show structural evidence of exocyclic n-bonding are those in which the ligand can most readily donate electrons onto phosphorus, i . e . , alkoxy and  - 12 -  .alky^l.aipi'rto ^derivatives,..' /In. t.he -methoxyphosphazenes [NP(0Me) ]  r  2  4  g  g  6 0 - 6 2  ,  CO al] the P-O-.C angles are close to 120° , as in phosphate esters the C-O-C angle in organic esters i s about 112° .  , whereas  Also, the P-0 bonds are  shorter than expected for a P-0 single bond (1.71 A ) .  The same effects  6 4  are seen in the dimethylamino phosphazenes [NP(NMe ) ] 5 3 2  2  4  "  > in which  the near planarity of the three bonds from the exocyclic nitrogen atoms (e bond angles at nitrogen -350°) indicates substantial electron release to phosphorus. . . Exocyclic conjugation has also been studied through i t s effect 19 :  the  on  F n.m.r. spectrum of a fluorophenyl group attached to a phosphazene  ring.  The value of & ^ - 6p (the difference in the chemical m  shifts of the para and meta fluorine atoms) for the series CO N  h' h-2ri-1 * 6^5 ( P  C  n = 3 _ 8  )  shows that, as a group, the fluorophosphazenes  are strong iT-acceptors, comparable to a cyano group. 1.2.3  Chemical .Consequences of Exocyclic ir-Bonding In contrast to the structural and spectroscopic manifestations of  conjugative interactions, the influence of such interactions on the chemistry of phosphazenes is less well documented.  The protonation of  aminophosphazenes occurs, in general, on an endocyclic n i t r o g e n  6 9 , 7 0  ,  indicative of the lower base strength of the exocyclic nitrogen caused by the drainage of iT-charge from i t into the r i n g .  However, there i s at least  one example of the coordination of an exocyclic amino group to a metal  71  (in [NP(NMe ) ] W(C0) ), which suggests that the reduction in base strength 2  i s not Targe.  2  4  4  Consistent with t h i s , the reaction of [NP(NMe ) ] with 2  2  3  Me 0 BF ~ results in the methylation of both an endocyclic and an exocyclic 3  +  4  - 13 -  nitrogen  17  (although the former is probably quaternized f i r s t ) .  In  aminophosphazenes, therefore, the chemical effect of exocyclic conjugation i s small, but, to the extent that i t occurs, i s such as to reduce the basic properties of the ligand on phosphorus.  This conclusion is c r u c i a l , and  provides the link between past work in phosphazene chemistry and the results reported in this thesis. During the course of this work, i t has been found that the a b i l i t y of the phosphazene ring to accept u-charge from an exocyclic ligand can be effectively u t i l i z e d in the synthesis of novel phosphazene derivatives.  The  i n i t i a l experiments that lead to this discovery were concerned with the reaction of methyllithium with octamethyltetraphosphazene (NPMe ) 2  4  By  analogy with the addition of methylmagnesium iodide across the S i - 0 bond of 73 (0SiMe ) (Equation 7), and of alkyllithiums across the C=N bond of p y r i d i n e (Equation 8), i t was believed that methyllithium might add across 2  4  74  75 the P^N bond of methyl phosphazenes (Equation 9a).  H3C  0'  CH  MeMgl  2  H n 3  6  >•  \  : Si  / H3G  0  GH  3  H0 2  (CH ) SiOH 3 3  V  OMgl  Li  BuLi  /  Early work  showed that,  r 14 CH H 0, s> 2  (CH ) PO 3  9a  3  N N  N  +  CH  9b  4  although a reaction did take place, i t s nature was uncertain.  Trimethyl-  phosphine oxide, for example, could not be isolated via the hydrolysis of the reaction mixture.  In f a c t , i t has now been established that  methyllithium reacts with methylphosphazenes as a base (Equation 9b), rather than as a nucleophile, and deprotonates a methyl group attached to phosphorus, producing novel phosphazene carbanions. carbanions is f u l l y discussed in Chapter  The chemistry of these  III.  The acidity of protons in methylphosphazenes complements the reported behaviour of the compounds as donors. 7fi form a variety of simple salts with metal c a r b o n y l s ' 77  78  Both (NPMe ) and (NPMe ) 2 3  2 4  , as well as reacting to give complexes  and metal i o n s ' . 7 9  8 0  They also react with alkyl  10  iodides (e.g. Equation 10) to give N-alkyl quaternary salts (NPMe )~ «.RI 9  - 15 -  (R=Me, E t ) ' ° ' . / D  During this work, the potential applications to  synthesis of these latter compounds has been explored.  This study was  encouraged not only by the observed reactivity of the neutral methylphosphazenes towards deprotonation, but by the formal structural resemblance of quaternary  phosphazenium salts to more conventional phosphonium s a l t s ,  whose reaction with bases, to give phosphorus y l i d s , is well known (vide infra).  It was believed that, i f the analogy were j u s t i f i e d , then the  phosphazenium;salts;(NPMe ) .MeI could also be deprotonated with a suitably 2  n  11  strong base* to yield phosphazene derivatives containing an exocyclic double bond (Equation 11), the physical and chemical properties of which would reflect the extent of conjugation between the ring and ligand.  However,  the results of this study, as described in Chapter IV, show that although deprotonation of the N-methyl quaternary salts can be effected, the f i n a l product of the reaction is quite different from the expected exocyclic y l i d .  1.3  Carbanion and Ylid Formation in Organophosphorus Chemistry Although they present many novel features, the reactions of methyl-  phosphazenes and methylphosphazenium ions with bases are related to the formation of carbanions and y l i d s from mononuclear phosphorus compounds. Because of their greater structural simplicity and ease of preparation, these  - 16 -  latter compounds have been studied extensively, and an outline of their chemistry is helpful in understanding the behaviour of phosphazenes.  The  present section of this chapter is designed to afford the reader some background information on this topic, thereby f a c i l i t a t i n g the later comparison of the chemistry of the phosphazene compounds studied in this work with that of the analogous, but more well known, mononuclear phosphorus derivatives. The acidity of methyl and methylene protons attached to second row elements is well known, and is generally attributed to the influence of 82 d-orbitals  , the effects of which are v i v i d l y demonstrated by the  exchange rates (Table 1.1) of the deuteroxide catalysed exchange of tetramethylammonium, tetramethylphosphonium and trimethylsulphonium ions with deuterium oxide (Equations 12a-12c) Me ft-CH  .  + OD"  <—»  Me P"-CH  3  + OD'  <  Me S-CH  3  + OD"  5=1* Me S-CH  3  3  2  3  Me N"-CH  2  + HOD  . . . . 12a  • Me ?-CH  2  + HOD  . . . . 12b  2  + HOD  12c  3  3  3  Compound  Relative Rate  C-X Bond Length (A)  Me,N -4  1  1.47  +  1  Me P  +  2.4 x 10  6  Me S  +  2.0 x 10  7  4  3  1.87 1.81  Table 1.1: Relative rates for the deuteroxide catalysed hydrogen-deuterium exchange between ammonium, phosphonium and sulphonium ions and deuterium oxide. From W. von E. Doering and A.K. Hoffman, J . Amer, Chem. S o c , 77, 521 (1955).  - 17 -  In the absence of other factors, the relatively short N-C bond should lead to a more stable intermediate (simple Coulombic s t a b i l i z a t i o n energy being inversely proportional to the charge separation) and hence a faster exchange rate for the Me^N ion. +  The fact that the observed rates  for phosphonium and sulphonium ions are 10 times greater than for the 6  ammonium ion suggests that the dipolar transition state for the former ions is stabilized by the formation of a 3d-2p n-bond between the two polar centres (e.g. Equation 13). Me P - CH  2  Me S - CH  2  +  3  +  2  1.3.1  «  »  Me P = CH  4  *  Me S = CH  3  2  2  ....13a  2  . . . . 13b  Phosphorus Ylids This pronounced acidity of protons on carbon atoms adjacent to  phosphorus and sulphur has many applications in synthesis.  Phosphonium  salts [R PCH R] X~ can be deprotonated with relative ease to produce 3  2  +  phosphorus ylids. R PCHR, the use of which, in the Wittig reaction  , is well  3  known  .  By contrast, nitrogen ylids can only be isolated when a  s u f f i c i e n t l y electron withdrawing group is attached to the a-carbon to stabilize the negative charge that i t acquires upon deprotonation . 86  The y l i d s obtainable by the deprotonation of a phosphonium salt are also capable of adding HX (X=halide), thereby regenerating a phosphonium salt.  The two compounds may therefore be considered as a conjugate acid-  base pair, the equilibrium of which is controlled by the nature of the substituents on phosphorus and carbon (Equation 14).  Electron-attracting  - 18 -  [R P-CHR R ] X" 3  I  2  2 groups on carbon (R  3  rJp-CR2R  +  3  . . . . 14  + HX  3 and R ) enhance the acidity of the phosphonium s a l t ,  the polar bond of the corresponding y l i d being more effectively s t a b i l i z e d , either by resonance or inductive effects.  Thus, [Ph PCH C00Me] Br~ can be 87 88 3  +  2  deprotonated by a relatively weak base such as sodium carbonate  '  , the  polar P=C bond so obtained being s u f f i c i e n t l y stabilized by resonance (Equation 15) so as to render i t stable to water. Ph P-C 3  -/  The ligands R attached 1  H Ph P-C 2  N  'C=0 OMe  _  . . . 15  9~°  OMe  OMe  to phosphorus can also influence the acid^-base equilibrium.  The inductive  donating effect of alkyl groups makes alkylphosphonium ions weaker acids than their aryl counterparts.  If the three phenyl groups of  [Ph PCH C00Me] Br~ are replaced with cyclohexyl groups, sodium hydroxide, 3  2  +  rather than sodium carbonate, must be used to induce deprotonation, and 87 the resulting y l i d is decomposed on contact with water  .  In the l i m i t ,  unsubstituted alkylphosphonium salts are s u f f i c i e n t l y weak acids to require alkyl metals to effect proton removal. The basic nature of phosphorus ylids has led to their use as deprotonating agents for other phosphonium s a l t s , .89 by means of a reaction which is generally referred to as "transylidation In Equation 16, the position'of the equilibrium is determined by the nature of the ligands R and R , the direction of the reaction being such as to afford the least  - 19 -  Ph P-CHR + [Ph P-CH R ] 1  3  3  2  acidic phosphonium s a l t .  2  [ P h ^ C H ^ Y + Ph P=CHR  +  . . . . 16  2  3  For example, the reaction of Ph P=CH with 3  2  [Ph PCH C0Ph] Br" results in the formation of [Ph PCH ] Br" and 3  +  2  3  +  3  'PhgPCHCOPh in nearly 90% y i e l d . 8 9  The transylidation reaction renders possible the comparison of the net influence of the inductive and resonance effects exercised by the substituents on carbon on the acidity of phosphonium ions.  In agreement  with the pk values of phosphonium salts [Ph PCH R] X~, the acceptor a  3  2  +  89-91 a b i l i t y of various R groups increases in the following order CH < H < Ph < P(0)Ph = P(S)Ph < COOR < CN < C(0)Ph < PPh 3  2  2  3  : According to this  series, the treatment of a phosphonium salt [Ph PCH R ] X~ with a phosphorus 2 1 2 y l i d Ph PCHR results in transylidation i f R is a better acceptor than R . Consistent with their basic character, y l i d s are methylated on 3  2  1  +  3  carbon by methyl i o d i d e " 9 1  R^CHR  2  9 5  (Equation 17).  + Mel — •  [R^P-CHMeR ] ^" 2  series, the phosphonium salts [R PCHMeR ]V 94 1 2 95 1  -COOMe  ; R =Me, R =H  In agreement with the above  3  2  ....17  4  (R =Ph; R = H ' , - P ( 0 ) P h ]  2  92  93  2  91  ,  ) so formed are unreactive to a second mole of y l i d .  By contrast, the acylation of y l i d s with acid chlorides results in transylidation of the i n i t i a l l y formed salt (Equation (18), the strong acceptor properties of the acyl group making i t s u f f i c i e n t l y acidic to react with a second mole of starting y l i d  .  This difference in behaviour  of y l i d s upon methylation and acylation is an important concept.  Its  relevance to the present work w i l l be f u l l y discussed in a later chapter.  - 20 -  R P=CHR + AcCl  *  1  3  [R P-CHR Ac]Cl 1  3  [R P-CHR Ac] CT + R P=CHR .—» R P=CRAc + [R P-CH R] C1" 1  3  +  3  ]  3  3  2  +  . . . . 18  (Ac = acyl)  1.3.2  Phosphorus Carbanions The acidity of phosphonium ions [R PCH R] is enhanced by their 3  charge.  +  2  However, in formally neutral phosphoryl compounds, the acidity of  methyl and methylene protons adjacent to phosphorus is s t i l l appreciable. The removal of a proton from such compounds produces a carbanion, the s t a b i l i t y of which i s attributed to resonance with the phosphoryl group (Equation 19).  The potential applications to synthesis of such carbanions  are many; accordingly, the preparation and chemistry of the carbanion  A  ,  . / ' \ .  derivatives of phosphonate esters ( R 0 ) P ( 0 ) C H R ' 2  R P(0)CH R 2  2  100  "  108  2  , have been extensively studied.  99  100  , and phosphine oxides  Figure 1.2 shows, for  example, a selection of the reactions that the phosphine oxide carbanion Ph P(0)CH " 2  2  undergoes.  Its chemical behaviour appears to depend on the  nature of the counter cation.  With carbonyl compounds, for example, i t  forms an intermediate betaine Ph,,P(0)CH C(0)R , which is stable when the 107 108 2  cation is lithium  '  .  2  However, when a cation other than lithium is  present, decomposition of the betaine can occur (as in the Wittig reaction),  - 21 -  Ph  ^,CH SiR 2  Ph  3  V Ph  N  CH  /  (M=Na) Ph^  L j J  2  ^  2  2<H  +  CH C(0)R  0~  \  2  P /  Rcoggt^  • R C=CH 2  Ph  2  0  \  (M=U)  2  \ / / 4 v  \(M=Li)  Ph  \R P(o)Cl  Ph  CrbCOOH  A , Ph  0  Ph  2  (M=Li) Ph  Ph  v  R C0  (M=Li)  G0  V  0  R3SICI  \  .CH9CR0OH  (M=Li)  •A  Ph  Ph  R CHo-C-R  Ph  x  X  \  0  0  CH9  Ph  00  Ph  /  Ph  0  Fig. 1.2 Survey of typical reactions of the methyldiphenylphosphine oxide carbanion Ph P(0) CH -M . 2  2  2  +  i 0? with elimination of an alkene and the formation of a phosphinate anion The chemical s i m i l a r i t y between these mononuclear phosphoryl carbanions and the phosphazene carbanions reported i n this thesis i s discussed in detail in Chapter  III.  1.4 Summary As i s indicated in the foregoing introduction, this thesis i s composed of three related areas of research:  (1) the preparation of  .  - 22 -  methylphosphazenes, (2) the synthesis and reactions of methylphosphazene carbanions, and (3) the reactions of N-methyl methylphosphazenium quaternary salts with bases.  The results obtained from these areas are  described and discussed in the following three chapters.  The final  chapter of the thesis is concerned with the structural aspects of some of the compounds reported in earlier chapters, and relates, where possible, the chemistry of the compounds to their molecular and electronic structures.  - 23 -  CHAPTER II PREPARATION OF METHYLPHOSPHAZENES  As was pointed out in the introductory chapter, the principal concern of this work has been the study of the reactivity of the methyl protons in methylphosphazenes (NPMe ) and their quaternary salts (NPMe ) .MeI. 2 n  2  topics w i l l be discussed in later chapters.  Such  n  The purpose of this present  chapter (Section 1) is to report the advances that have been made in the preparation of the methylphosphazenes themselves.  The preparation of the  p a r t i a l l y methylated phosphazene Me Ph P N and i t s methiodide salt 2  Me Ph P N .MeI 2  4  3  3  4  3  3  i s also described in this chapter.  The usefulness of these  latter compounds stems from the fact that each contains only one potentially reactive P-methyl group; they therefore provide a simpler model than do the f u l l y methylated phosphazenes for the study of the chemistry of the -N=PMe - unit. 2  Section 3 deals with the phosphinimine Me PNMe and a  number of i t s derivatives.  3  This molecule constitutes the monomeric  counterpart of the cyclic methylphosphazenes, and  i t s preparation and study  was undertaken to allow a comparison of the properties of the -N=PMe - unit 2  in monomeric and polymeric systems.  The f i n a l section of the chapter is  concerned with the chemical reactivity of the trimeric phosphazene CI2 4 3 3• Ph  P  N  Although i t is not directly related to the other topics  covered in the chapter, the reactivity of this compound towards nucleophiles offers an interesting contrast to that of the f u l l y halogenated phosphazenes described in the introduction (Chapter 1, Section 1).  - 24 -  2.1  Preparation of Methylphosphazenes  2.1.1  Preparation of (NPMe )  (n=3,4)  2 n  To date, the most common method for the preparation of the methylphosphazenes (NPMe )  2 3  phosphorane ' ' ' 2 1  7 6  7 7  4  1 0 9  has been by the ammonolysis of dimethyltrichloro, according to Equation 1.  Me PCl + NH C1 — + 2  3  4  ^(NPMe ) + 4HC1 2  n  The phosphazenes are  (n=3,4)  .... 1  actually produced as their hydrochlorides, which can be converted into the neutral compounds either by a tertiary a m i n e ' 76  77  or aqueous a l k a l i  1 0 9  .  When the reaction is carried out in tetrachloroethane, the molar ratio of trimer:tetramer is about 1:4  .  If the solvent is omitted, the tetramer  is the sole product, and the overall y i e l d is diminished . 77  Mixtures of  (NPMe ) and (NPMe ) are also formed, in low y i e l d , by heating 2 3  2 4  Me P(NH ) Cl 2  2  2  77,110  , but, however the mixture i s produced, the separation of  the individual components by fractional c r y s t a l l i z a t i o n is tedious and time consuming.  The tetramer is more easily obtained than the trimer  because i t is the major product of such reactions and forms much better crystals. These d i f f i c u l t i e s prompted the search, in this work, for a new synthetic route to (NPMe ) . 2  facts have come to l i g h t .  3  As a resultof this investigation, two useful  (1) By substitution of methylamine hydrochloride  for ammonium chloride in reaction 1, the corresponding methyl  chloride  salts (NPMe ) .MeCl and (NPMe ) .2MeCl are formed instead of the 2 3  hydrochlorides.  2 4  Not only is the overall yield of the methylphosphazenes  greatly improved, but the ratio of the yields of the two ring sizes favours  - 25 -  the trimeric derivative.  (2) The diquaternary chlorides of the tetramer  (NPMe ) .2RCl (R=Me or H) are insoluble in a c e t o n i t r i l e , whereas the 2 4  quaternary chlorides of the trimer (NPMe ) .RCl (R=Me or H) are 2  appreciably soluble in this solvent.  3  Therefore, since the hydrochlorides  of the trimer and the tetramer are easily produced from the neutral compounds by their reaction with hydrogen chloride, any mixture of (NPMe ) ^ can easily be separated. 2 3  The procedure for the preparation of (NPMe ) reported here 2 3  111  uses  chlorobenzene as solvent.  An o i l y intermediate is formed, which has not 112 been characterized but, by analogy with the reactions of PCI and 113 + PhPCl^ , probably contains the dimeric phosphazane (MeNPMe Cl) . 5  2  2  T  Pyrolysis of this o i l in vacuo then gives a mixture of the quaternary salts (NPMe ) .MeCl and (NPMe ) .2MeCl. 2 3  2  of (NPMe ) :(NPMe ) 2  3  2  4  It is important to note that the ratio  in the product is very sensitive to the length of  4  time for which the o i l y intermediate is heated in the presence of solvent. Several preliminary experiments using prolonged heating periods shifted the relative yields of the trimer and tetramer in favour of the l a t t e r , and also lowered the overall y i e l d .  This may be the result of the polymerization  of the dimeric phosphazane, known to occur on prolonged heating of the analogous (MeNPF ) 3  2  115  .  When the reaction conditions outlined above are used in the reaction of Me PCl with NH^Cl, a high yield of the tetramer (73% compared to . 2  3  By contrast, the reaction of Ph2PCl with alkylamine hydrochlorides RNH .HC1 gives simple salts [Ph ClP-NHR] Cl 114. 3  2  2  +  _  - 26 -  7fi  77  literature values of 56%  and 60% ) is produced with almost complete  exclusion of the trimer.  Hence, by the appropriate choice of starting  materials and reaction conditions, good yields of trimer or tetramer can be achieved.  The two compounds, which are always formed as their s a l t s , can  then be separated by virtue of the different s o l u b i l i t i e s of their salts in acetonitrile.  The neutral (NPMe ) is obtained from (NPMe ) .MeCl by  heating i t in vacuo.  2 3  2 3  There appears to be a s i m i l a r i t y , at least in  principle, to the formation of (NPPh ) by the dehydrohalogenation of 116 (HNPPh F) with cesium fluoride . It is also interesting to note a difference from the behaviour of ( M e N P C l ^ , prepared from PCI and 112 2  2  3  2  5  methylamine hydrochloride  .  Upon pyrolysis, this compound does not give  quaternary , presumably the chlorophosphazenes weakly basic;s aal t shigh polymer of because composition (NPC1 ) is formed are i n s too tead 9  117  .  The positive charge on the quaternized methylphosphazenes would tend to prevent such polymerization, an inference which provides a rationale for the absence of larger methyl phosphazenes (NPMe ) (n>4) from such reactions, 2 n  2.1.2  Preparation of (NPMe )  2 n  (n>4)  Although the methylphosphazenes (NPMe ) 21  2 3  many years  4  have been known for  , studies on the chemical and physical effects of ring size  variations within the homologous series (NPMe ) (n=3,4,5...) have always 2 n  been limited by the d i f f i c u l t y in preparation of the different (more especially the larger) ring sizes.  Unlike most phosphazenes (NPX ) 2  n  (e.g. X=NMe , OPh, OMe), which have been prepared in a wide variety of 2  ring sizes (from n=3 to 8) by the substitution of a halophosphazene,  - 27 -  alkylphosphazenes have been less easily obtained.  The varying yields  observed for the methylation of the fluorophosphazenes (NPF^)^  5  (yields,  for n=3, 0%; n=4, 70%; n=5, 2 0 % ) ' , via Equation 2, have been interpreted 5 1  7 6  in terms of a competing addition reaction (see Chapter 1, Section 1.1). (NPF ) + 2nMeLi 2  n  • (NPMe ) + 2nLiF  .... 2  2 n  Accordingly, unsuccessful attempts to prepare methylphosphazenes of larger ring sizes (n^6) have been attributed to the increased s t a t i s t i c a l d i f f i c u l t y of effectively substituting a l l the P-F bonds of the appropriate fluorophosphazene. During the course of this work, the p o s s i b i l i t y of using methylmagnesium bromide as a methylating agent for the large ring size phosphazenes 118 was considered.  Early attempts  to methylate (NPF ) 2  4  with MeMgBr were  g  unsuccessful, and the greater success obtained when methyllithium was used was attributed to the greater nucleophilicity of the l a t t e r , which would favour complete substitution of a l l the P-F bonds, and to the lower Lewis acidity of lithium, which would reduce the p o s s i b i l i t y of ring cleavage via coordination of the metal to a ring nitrogen atom.  However,  upon re-examination of the reaction of MeMgBr with (NPF ) , i t was 2  g  discovered that a good yield of (NPMe ) could be achieved by suitably 2 g  modifying the experimental procedure from that used previously. Consequently, i t has been determined that the reaction of methylmagnesium bromide with the appropriate fluorophosphazene, according to Equation 3, offers an e f f i c i e n t and f a c i l e preparative route to large ring size methylphosphazenes, and the derivatives (NPMe ) 9  (n=6-10) have a l l been  - 28 -  prepared by this method. 2nMeMgBr + (NPF ) 2  (NPMe ) + 2nMgBrF  n  2  3  n  In practice, the reaction of MeMgBr with (NPF ) is slow. 2  n  Unlike  the immediate exothermic reaction that occurs between methyllithium and fluorophosphazenes, i t requires an induction time of 1-2 hours before the precipitation of magnesium halides commences (from ethereal solution).  The  key to the successful isolation of the product is the work-up procedure, which is similar to that required for the preparation of trimethylphosphine 50 oxide  ; the reaction mixture is quenched with water, and the magnesium  precipitated from the solution by the addition of a base (sodium carbonate and hydroxide have both been used).  Evaporation of the solvent from the  aqueous extract, and re-extraction of the residue with chloroform, then yields the desired product. The necessity of an aqueous work-up when methylmagnesium bromide is used is consistent with the extent to which complexation of magnesium and 119 lithium ions is known to occur  .  Because of i t s greater charge, the  magnesium ion is more prone to complex formation than is lithium, and, in the present case, methylphosphazenes appear to be s u f f i c i e n t l y strong donors to complex with magnesium bromide, but not with lithium bromide^.  The  difference in the coordinating a b i l i t y of the two metals may also affect the mechanism of the reactions of MeMgBr and MeLi with (NPF^,)^.  Such a  This assumes that the methyllithium is prepared by the action of methyl bromide on metallic lithium.  - 29 -  belief is supported, for example, by the reactions of PhMgBr 121 PhLi  and  with (NPF ) , which give, respectively, the geminally and 2  3  non-geminally disubstituted derivatives PI^F^P^N^. However, regardless of the actual mechanism of substitution, i t is apparent that the d i f f i c u l t i e s encountered in the preparation of the small ring size methylphosphazenes (especially the trimer), via the substitution of a fluorophosphazene, are not observed during the preparation of the higher members of the series (NPMe ) 2  This conclusion is consistent with many  n  other properties of phosphazenes, which also indicate that the i n i t i a l l y significant effects of ring size variation disappear for n>6. 2.1.3  Preparation of Methylphosphazenium Quaternary Salts In agreement with their expected behaviour as donor molecules,  the methylphosphazenes (NPMe,,)^ react with methyl iodide to give simple N-methyl quaternary salts (NPMe ) .MeI ( n = 3 , 4 , 5 ) ' . 2  n  76  The synthetic  81  uses of these compounds are discussed in Chapter IV; the purpose of this present section is to report the preparation of several new quaternary salts of this type. 2.1.3.1  Preparation of Monoquaternary Salts of (NPMe ) 2  n  By following the procedure reported for the preparation of the salts (NPMe ) .MeI ( n = 3 - 5 ) ' , the analogous salts of the hexameric 2  n  76  81  and heptameric methylphosphazenes have now been prepared (Equation 4). (NPMe„) + Mel 2 n  • (NPMeJ .Mel 2n  (n=3-7)  In 4  - 30 -  practice, the neutral methyl phosphazene is heated under reflux for several hours in neat methyl iodide.  The product, (NPMe ) .MeI (n=3-7) 2 n  is precipitated from the solution in nearly quantitative y i e l d .  To date,  apart from the simple N-ethyl and N-methyl derivatives, no other quaternary salts of methylphosphazenes have been prepared.  However,  during the course of this work, the novel N-ethylacetato quaternary salts [(NPMe ) .CH C00Et] I" (n=3,4) have been obtained by heating the 2  n  +  2  appropriate phosphazene in ethyl iodoacetate (Equation 5).  As with the 5  methiodide salts the product is precipitated from the solution and separated by f i l t r a t i o n .  The reaction is much slower than with methyl  iodide, and the yields are only moderate (60-70%). 2.1.3.2  Preparation of Diquaternary Phosphazenium Salts The diquaternary salt (NPMe ) .2MeCl can be prepared via the reaction 2 4  of Me PCl and methyl amine hydrochloride (see Section 2.1.1). 2  3  dication [(NPMe ).2Me] 2  ++  However, the  can be more conveniently prepared by the  diquaternization of neutral (NPMe ) with a s u f f i c i e n t l y powerful 2 4  72 methylating agent. Similar diquaternizations, of aminophosphazenes and 122 pyrazmes , have been carried out using trialkyloxonium tetrafluoroborate. However, in this work, the use of methylfluorosulphate ("Magic Methyl") was considered to be preferable. Its strength as an electrophile is well 123 124 known  '  Me 0 BF ". 3  +  4  , and i t s ease of handling make i t a more convenient choice than Accordingly, i t has been found that MeS0 F reacts rapidly with 3  - 31 -  (NPMe ) , in acetonitrile 2  solution, to give the diquaternary salt  4  (NPMe^^MeSOgF.  The di-iodide salt (NPMe,,)^. 2MeI was prepared from i t  by ion exchange.  It is probable that methyl fluorosulphate is capable of  forming polycations from the methylphosphazenes of large ring size (e.g. [(NPMe ) .3Me] 2  2.1.4  6  and [(NPMe ) .4Me] ),  3+  2  4+  8  Structure and Spectra of Methylphosphazenes The preparation of (NPMe^g^g** extends the series of the known )  methylphosphazenes from the trimeric to the decameric derivative, and thereby creates  the largest single family of phosphazenes other than the  halo-derivatives (NPX) (X=C1,F) (see Table 2.1).  Their cyclic nature is 31 confirmed by the presence of a single resonance in their P n.m.r. spectra, 2  and a simple doublet in their  n.m.r. s p e c t r a ^ .  Unlike dimethylsiloxanes, and many other phosphazenes of large ring sizes, a l l the methylphosphazenes (NPMe )  2 n  (n=3-10) are crystalline solids  at room temperature and, as such, are potentially useful sources of precise structural information. The molecular structures of [NP(0Me) J ^60-62 65-67 2  and [NP(NMe ) 3 2  2  4  5 3 "  a  ^  s n o w  4  g  interesting conformational effects, but  their interpretation in terms of non-bonded interactions is complicated by the size and shape of the ligands.  In the case of the methylphosphazenes  however, the more nearly spherical nature of the methyl ligands and their ^The author is grateful to Mr. K.D. Gallicano for the preparation of the compounds (NPMe ) _ . 2  ^  8  10  Because of conformational changes, the exact magnetic equivalence of phosphorus and hydrogen nuclei is probably attained s t a t i s t i c a l l y .  - 32 -  Table 2.1:  n  N.m.r. parameters , (P=N) stretching frequencies , and melting points of the methylphosphazenes (NPMeJ (n=3-10). 9  M.pt.  b  (°c).  v asym (P=N) • •• ' (cm-1)  3  187-188  (1185)  c  4  161-162  (1122)  c  5  64-65  (1255)°  1 .50 (12.0)  106.7  6  163-165  1213, 1255 (1250)  1.50 (12.3)  109.3  7  128-130  1210, 1255 (1242)  1.52 (12.5)  109.7  8  171-173  1210, 1268 (1225)  1 .53 (13.0)  110.4  9  123-125  1220 (1220)  1.54 (13.0)  109.9  10  100-102  1165,1210,1236 (1216)  1.53 (13.2)  109.6  v  c  H (ppm)  P (ppm)  1.46 (13.0)  90.7  1.51 (11.5)  100.3  6  6  (a) From dilute solutions, 6 u (ppm) in C D C I 3 , reference internal TMS; P-H coupling constants (in Hertz) in parenthesis. 6 (ppm) in CgDg, reference external P 4 O 6 . (b) From nujol mull spectra; values in parenthesis from CC14 solutions, (c) From H.T. Searle, J . Dyson, N.L. Paddock and T.N. Ranganathan, J . Chem. Soc. Dalton, 203 (1975). P  - 33 -  smaller size greatly reduces the number of non-bonded interactions, and conformational trends are therefore more easily understood.  Consequently, the  crystal structure determinations of several methylphosphazenes of large ring size are currently being carried out; those of the heptamer and octamer have recently been completed (that of (NPMe,,)^ being the f i r s t structure analysis of a heptameric phosphazene).  The principal structural parameter of these  molecules, the P=N bond length, is consistent with the presence of the inductive donating methyl groups on phosphorus. (NPMe )  (n=4, 1 . 6 0 A  2 p  126  ; n=5, 1 . 5 9 ^  ; n=7, 1 . 5 9 K  ) , and indicates the extent to which lone pair  2  4  (1.57A  130  4  and 1 . 5 6 A  125  [NP(0Me) ]  2  (1.57A  ; n=8, 1.58A  4  028  ) , (NPC1 )  125  longer than in (NPF ) 2  (1.51A  1 2 7  The value of L(P=N) in  129a  delocalization into the ring Tr-system i s suppressed.  129b  )  is  ) and  A more detailed analysis  of the structures and conformations of methylphosphazenes i s given in Chapter V. 2.1.4.1  3 1  P Chemical Shifts of (NPMe ) 2  p  (n=3-10) 31  The l o c a l i z a t i o n of charge on nitrogen is also observed in the n.m.r. chemical s h i f t s of methylphosphazenes.  P  Figure 2.1(A) shows how the  value of 6p in the series (NPX ) depends upon the nature of X and the 2  value of n.  n  The phosphorus atoms are effectively deshielded by methyl ligands;  not only are the individual values of <5 • for (NPMe ) lower than when more 2 n  electronegative groups are present on phosphorus, but the increase in 6^ along the series n = 3-10, which is usually related to a widening of the ring angle at nitrogen, i s least for the methylphosphazenes.  - 34 -  1160  1  —  — 3  I*  5  6  7  n in (PNX ) 2  8  9  10  n  Figure 2.1 (A) P chemical shifts (ppm, relative to ext. P 0 ) and (B) v(P=N) frequencies (cm-1) of phosphazenes (NPX?) (X=F , C P , OMeC, Me) as a function of ring size (n). J ,  d  6  n  a  (a) A.C. Chapman, N.L. Paddock, D.H. Paine, H.T. Searle and D.R. Smith, J . Chem. S o c , 3608(1960). (b) L.G. Lund, N.L. Paddock, J . E . Proctor and H.T. Searle, J . Chem. S o c , 2542 (1960). (c) R.A. Shaw, Chem. and Ind., 54 (1959); G. A l l e n , D.J. Oldfield, N.L. Paddock, F. Rallo, J . Serreqi and S.M. Todd, Chem. and Ind., 1032 (1965); N.L. Paddock, unpublished results.  - 35 -  2.1.4.2  Infrared Spectra of (NPMeJ  n  (n=3-10)  Although the infrared spectra of the methyl phosphazenes (NPMe,,^-^ are, broadly speaking, s i m i l a r , the detailed analysis and correlation of the spectra of different ring sizes is much more d i f f i c u l t than i t i s , for instance, in the case of halophosphazenes.  The reason for this d i f f i c u l t y  l i e s in the ambiguity in assignment of skeletal vibrations and those due to the CH^ ligands.  Certain features however, are common to a l l the spectra.  The antisymmetric ring vibration, v(P=N), is easily identified by i t s position and intensity.  The extent of lone pair delocalization from  nitrogen into the ring ir-system, and hence the strength of the P=N bond, is reflected in the magnitude of v(P=N); as a group, the values of v(P=N) for the series (NPMe,,^.^ are lower than for phosphazenes containing more electronegative ligands (Figure 2.IB).  The variation in v(P=N) along  the series n = 3-10 i s not thought to be related directly to bond strength. The same type of variation is observed for the ring stretching frequency of dimethyl siloxanes  131  '  1 3 2  .  In both systems, (X P=N) and ( X S i - 0 ) , the 2  n  2  n  primary effect of an increase in n is to decrease the phase difference between the vibrational motion of ring bonds, and hence to lower the value 4 13 133 of the asymmetric ring stretching frequency ' ' . The deformational modes of the CH^ groups are also easily discernible, since they occur in a region which is free of possible interference from skeletal vibrations.  <5,(CI-L) is found f o r , a l l ring sizes, in the region as  of 1410-1430 c m . -1  o  The symmetric deformation,  < s  S  y ( 3)» m  C H  invariant, and is observed between 1290 and 1305 c m . -1  1 S  similarly  - 36 -  2.1.4.3  Mass Spectra of (NPMeJ  n  (n=6-10)  Unlike the series of halophosphazenes ( N P C 1 ) _ 2  3  g  134  and (NPF ) _.j ^ , 2  3  2  35  whose mass spectra can be analysed in reasonable d e t a i l , the methylphosphazenes (NPMe )2_iQ are less easily studied by mass spectrometry.  The near  2  equivalence in mass of an -NP- unit and three -CH^ units, and the p o s s i b i l i t y of proton abstractions (which create a problem in the distinction between a -CH ~ group and a nitrogen atom) 2  both make the detailed interpretation  of the fragmentation patterns of the methylphosphazenes ambiguous.  Thus,  the c l a s s i f i c a t i o n of fragments into a cyclic and linear species cannot be made with certainty (as i t can be for the halophosphazenes), and the assignment of formulae to small (m/e<200) fragments is speculative. Nonetheless, some useful information can be obtained, especially from the higher members of the series, since, above the region m/e^ 200, the spectra of (NPMe )g_-|Q are relatively simple.  Only a few fragments are  2  observed, nearly a l l of which correspond to ions of formulae [ N P x  (y=0,l).  x  Me  2 -v^ X  +  •J  The relative abundances of these fragments, which are shown in  schematic form in Figure 2.3, display several interesting features. 135 As in the case of the fluorophosphazenes  , the s t a b i l i t y of the  parent ion increases with increasing ring size, so that, in (NPMe )g_-jg, 2  the parent ion i s the most abundant species. of the parent ion is a very specific process.  When i t occurs, fragmentation The stepwise removal of  ligands i s not observed; instead, ring cleavage predominates. The same 135 effect is seen in the fluorophosphazenes , but in the chlorophosphazenes, where the phosphorus-ligand bond is weaker, ligand abstraction is more 134 common  - 37 -  (PNMe ) 2  10  (PNMe )g 2  (PNMe )  2 8  (PNMe )  2 7  (PNMe^  6  m/e  p  3 sJ N  300  450  600  750  F^N^J P5N5J PeNgJ P N ^ J P N g J P g N g J p ^ o J 7  8  Figure 2.2 The mass spectra of the methylphosphazenes ( N P t ^ ^ . - j n scales are such that the base peaks of each compound have the same relative intensity. A l l fragments with an abundance of less than 5% of the base peak are ignored. T  n  e  - 38 -  In the fluorophosphazenes, cleavage of the ring occurs in two ways. Abstraction of an N ? unit i s the principal reaction path, but, as far as 3  3  (NPF,,)^, significant quantities of fragments indicating the loss of one and two NP units are s t i l l observed.  In (NPMe )g_-| » this l a t t e r type of 2  0  degradation is almost non-existent, and the removal of an N P unit and, 3  3  to a lesser extent, an N^P^ unit, from the parent ion is the only route through which fragmentation proceeds; e.g. NgPgMe-j gives only N P 2  3  3  fragments, N^P^Me^ gives mainly N^P^ fragments, and NgPgMe-jg mainly N^P^ fragments.  In the spectrum of (NPMe ) , a significantly large quantity of 2  g  N P species is observed, suggesting that two N P units are easily 3  3  3  removed from the N P molecule. g  of (NPMe ) 2  1Q  g  3  As in the case of ( N P F ) , the spectrum 2  10  shows l i t t l e difference in yield between the smaller fragments.  The mass spectra of (NPMe )g_-| are, therefore, distinguished by the 2  0  propensity of the phosphazene ring to lose one or more N P units. 3  3  The  potential importance of transannular interactions as a means to ring 135 cleavage has already been noted for the fluorophosphazenes  , and their  influence in the present system is l i k e l y to be augmented by the increased basicity of,the nitrogen atoms, which would favour such transition states as I.  Me  Me  - 39 -  2.1.5  Structure and Spectra of Methylphosphazenium Quaternary Salts The details of the n.m.r. and infrared spectra of the methylphos-  phazenium quaternary salts prepared in this work are given in Tables 2.2 31 1 and 2.3.  Both the  P and the  H n.m.r. spectra of the methiodides  (NPMe ) .MeI (n=3-7) r e f l e c t the effects of the perturbation of the cyclic 2 n  charge d i s t r i b u t i o n by the quaternization of a ring nitrogen atom, and allow the c l a s s i f i c a t i o n of the various phosphorus atoms and P-methyl protons according to their location with respect to this nitrogen. In the 31 P n.m.r. spectrum of (NPMe,,)^ .Mel, for example, the four formally inequivalent types of phosphorus nuclei are differentiated magnetically, their chemical s h i f t s indicating their proximity to the positively charged nitrogen atom.  The P-methyl protons are more removed from the asymmetric  charge d i s t r i b u t i o n in the r i n g , and their magnetic inequivalence is consequently less marked. Table 2.2:  3 1  Accordingly, as Figure 2.3 shows, the  n.m.r.  P n.m.r. parameters of N-methyl methylphosphazenium iodides. 3  Compound  A 54.2  B 76.6  6P  6 P  5 P  c  6P  D  (NPMe ) .MeI  b  (NPMe ) .MeI  b  70.0  83.5  -  -  (NPMe ) .MeI  b  74.1  90.4  90.4  -  (NPMe ) .MeI  76.4  94.5  95.7  -  (NPMe ) .MeI  77.0  95.8  97.2  (NPMe ) .2MeI  65.7  2  3  2  4  2  2  2  5  g  7  2 4  -  .  97.8  (a) Dilute solutions in DoO, 6(ppm) reference external P 0 . (b) Data from H.T. Searle, J . Dyson, T.N. Ranganthan and N.L. Paddock, J . Chem Soc Dalton, 203 (1975). 4  6  - 40 -  Table 2.3:  H n.m.r. parameters , (P=N) and (C-N) stretching frequencies' of methylphosphazenium quaternary s a l t s . 9  Compound  v(P=N) cm •  'v(C-N).  1196 1242  1072  1220 1240  3  6(MeN)  6(MeP )  3.20 (10.8)  2.02 (12.8)  1.64 (13.9)  1075  3.14 (11.2)  2.05 (13.5)  1.55 (12.6)  -  1230 1250  1071  3.05 (11.5)  2.04 (13.5)  1.54 (12.4)  1.43 (11.5)  (NPMe ) .MeI  1263  1075  3.08 (11.5)  2.08 (13.5)  1.61 (13.5)  1 .51 (12.5)  (NPMe ) .MeI  1230 1270  1040  2.99 (11.0)  2.01 (13.0)  1.61 (12.5)  1.51 (12.5)  (NPMe ) .2MeS0 F  1275 1330  1070  2.84 (11.5)  1.95 (13.7)  -  -  (NPMe ) .CH C00EtI+  1200 1240  g  4.12 (14.5)  1.81 (13.0)  1.58 ' (14.5)  -  (NPMe ) .CH C00EtI^  1190 1220  g  4.08J (15.0)  1.85 (13.5)  1.51 (13.5)  -  -  (NPMe ) .MeI 2  3  d  (NPMe ) .MeI  d  (NPMe ) .MeI  d  2  4  2  5  2 6  2 7  2 4  3  2 3  2  4  2  2  cm"  A  c  6(MeP ) B  C  6(MeP ) c  1  h  e  1  f  (a) Dilute solutions in CDCU, except =(=, in C D 3 C N . 6(ppm) reference internal TMS. PH coupling constants (in Hertz) in parenthesis, (b) From nujol mull spectra, assignments tentative, (c) Phosphorus atoms in alphabetical order from quaternized nitrogen atom, (d) Data from H.T. Searle, J . Dyson, T.N. Ranganathan and N.L. Paddock; J . Chem. Soc. Dalton, 203 (1975). (e) J(HP )=1.5Hz. (f) 6 ( H C P C H ( H C P D ) . (g) Assignment unclear, v(C0) in trimer = 1740 cm-1, and in tetramer = 1737 cm"l. (h) For ethyl group, A  6(CH )=1.37 ppm, S(CH )=4.22 ppm, J(HH)=7.0Hz. 3  2  (i)  J(HP )=1 .0Hz. A  (j) For ethyl group, 5(CH3)=1.37 ppm, 6(CH )=4.22 ppm, 0(HH)=7.0Hz. 2  - 41 (  C  H  3  )  2  P  D,C  (CH ) P 3 2  B  < 3> P —^ CH  2  A  (CH )N 3  I  CH, H C 3  'PCH,  /  A H C—P -CH 3  D  N  H  •CH, 'PCH,  3  4  C  / ^N=P ^' C  H,C  CH VPa' H 3 C \\  B  CH,  PNCH  3  ^P'NCH,  3.5 Figure 2.3  3.0  2.5  2.0  S(ppm)  1.5  1.0  Ordinary 100 MHz 'H n.m.r. spectrum (A) and P decoupled H n.m.r. spectrum (B) of (NPMe ) .MeI (both on samples in CDC1 solution). 3 1  ]  9 7  c  1  0  6  - 42 -  spectrum of (NPMe ) .MeI does not differentiate between the two types of 2 7  proton (P Me and PpMe,,) most remote from the N-methyl group. c  The single  2  resonances of the P^Me and PgMe,, protons, however, are easily distinguished, 2  as is the characteristic t r i p l e t corresponding to the N-methyl protons. The  n.m.r. spectra of the N-ethylacetato salts  C(NPMe ) CH C00Et] I" are qualitatively similar to those of the N-methyl 2 3  4  +  2  s a l t s , the N-methylene resonance being s p l i t into a t r i p l e t by coupling with the two nearest ring phosphorus atoms. The diquaternary cation 2+ [(NPMe ) 2Me] has a centrosymmetric structure, as expected on a simple 31 1 2 4  electrostatic basis.  Its  P and H n.m.r. spectra exhibit, therefore,  single resonances for both the phosphorus atoms and P-methyl protons. O  ;  Apart from the easily identified v(P=N) vibration, the infrared spectra of the methylphosphazenium salts are largely uninterpretable.  In  the case of the N-methyl s a l t s , the spectra are characterized by the appearance of a v(C-N) band at 1040-1075 cm  -1  (this region is completely  clear of absorptions in the neutral compounds).  Such a band is less  easily assigned in the case of the N-ethylacetato s a l t s , but the v(C=0) band in them is easily observable near 1740 c m . - 1  2.2  Preparation of gem-MepPh^P^N^ "IOC  To date, three different methods  "I  oo  , a l l involving ring closure  reactions, have been reported for the preparation of Me Ph P N 137 138 2  in two cases  '  4  3  3  3  3  i t was only isolated as i t s hydrohalide salt  Me Ph P N HX (X=C1, I)). 2  4  (although  - 43 -  The method used in this work is a modification of that f i r s t ,136 reported , and employs the reaction between dimethyltrichlorophosphorane and the linear phosphazene hydrochloride (Equation 6).  [NH (Ph )PNP(Ph )NH ]" Cl 2  2  2  2  r  The product is isolated as i t s hydrochloride, which is  cr NH  2  .,  Me PCl k J  n  2  -3HCI 3  •  P h — / P — P h  /  \  Ph  CI" P—Ph  Ph  easily converted into the neutral compound by treatment with base (e.g. triethylamine).  The reported procedure used benzene as a reaction  solvent and the low yield of 4% (calculated in terms of the consumption of Me^Clg) can be attributed, in part, to the boiling point of the solvent being insufficiently high to ensure complete reaction.  In the  present work, a higher boiling point solvent, chlorobenzene, has been used, thereby affording a better (but s t i l l not good) yield of ^20%. It i s apparent from this reaction, and similar ring closure reactions involving [NH (Ph )PNP(Ph )NH ] Cl" 2  2  2  2  and phosphorus(V) chlorides ( P C l g  +  1 3 9  '  1 4 0  PhPCl^ and P h ^ C l ^ ) that competing reactions play a significant role in 4 0  lowering the yield of the desired product (usually <35%). [NH (Ph )PNP(Ph )NH ] Cl" 2  2  2  2  +  The reaction of  and PCI in the absence of solvent g i v e s g  only the expected product CI3 3 3 3 ( Ph  P  N  i n  derivative Cl^Ph^P^N^ (in 6% y i e l d ) .  25  3  not  % yield) but also the tetrameric  Similarly, when the ring closure is  attempted using P h P C l , 24% of the tetrameric phosphazene (NPPh ) 2  140  2  4  is  ,  - 44 -  formed along with the expected trimer ( N P P h ) 2  3  140  .  Consistently, i t has  been found in the present work that the reaction of Me^PCl^ and [NH (Ph )PNP(Ph )NH ] Cl" gives significant quantities of (NPPh ) and 2  2  2  2  +  2  4  (NPMe ) as well as the desired product NgP-jPh^Me^ 2 4  Although the formation of these anomalous products has been 140 previously recognised  , i t s explanation has remained in doubt.  The  isolation of (NPMe ) from the reaction of [NH P(Ph )NP(Ph )NH ] Cl and 2 4  2  2  2  +  2  Me PCl.j does, however, provide a useful insight into the reaction 2  mechanism.  It is postulated that the chlorophosphorane R PC1 can act as 2  3  a chlorinating agent (Equation 7), producing the linear phosphorane [NH -(Ph )P=N=P(Ph )NH ] Cl" + R C l -»• NH -(Ph )P=N-P(Ph )-Cl 2  2  2  2  +  2  p  3  2  2  2  2  + [R P(NH )C1] C1" 9  / Ph P N 8  NH P(PH )NP(Ph )Cl 2  2  2  2  4  4  Ph R P N 4  2  3  3  4  ....  *  Ph R P N 4  4  4  RPN 8  4  2  7  4  and the amino salt [R P(NH )C1] Cl" 2  +  9  Cyclization can  +  then occur in a number of ways, producing the observed range of c y c l i c products. 2.2.1  Preparation of MepPh^PjNg.Mel' Phenylphosphazenes are much weaker bases than are methylphosphazenes,  and do not, for example,react with methyl iodide to give N-methyl quaternary  ^ Such a reaction is reminiscent of the preparation of [R P(NH )] C1 from chloramine and R PNH 0»141, * * 9  2  2  11  9  +  - 45 -  salts (although (NPPh ) can be quaternized by Me^OBF^ 2  3  ).  Dimethyltetra-  phenylcyclotriphosphazene provides a compromise between the two extremes and can be quaternized by methyl iodide to give the salt Me Ph P N .MeI. 2  4  3  3  31  The presence of three distinct phosphorus environments in the  P n.m.r.  spectrum of this salt (see Section 2.5.3 of this chapter for details) confirms that, as expected, quaternization takes places on the nitrogen atom adjacent to the PMe group (Equation 8). 2  Me  \/  N  Me Me \/  Me  .Me N  Mel  Ph— / Ph  Ph— / Ph  2.3 2.3.1  N  Ph  •N  Ph  Preparation and Chemistry of Tetramethylphosphinimine Reaction of Trimethylphosphine and Methyl Azide  The classical preparative route to phosphinimines is via the 142 reaction of a tertiary phosphine and an organic azide Accordingly, the preparation of Me PNMe by this method has recently been reported 143 1  3  (Equation 9).  However, during the present work, the same reaction has been  Me P + MeN — • 3  3  MeP=NMe + N + 3  2  studied independently, and s l i g h t l y different results were obtained. In the 143 procedure outlined in the literature , the reactants were mixed, in the  - 46 -  absence of solvent, at -78°C, producing a 70% y i e l d of Me^PNMe.  In this  work, the reaction was carried out in pentane solution at 0°C, thereby affording not only the desired product, but also, as a side product, the s a l t - l i k e compound [Me P=N=PMe ] N ~. 3  3  +  3  The isolation of this latter  compound is unexpected, but not without precedent.  Similar b i s ( t r i a l k y l -  phosphine)iminium salts have been isolated during the methanolysis of 144 145 trialkylphosphinimines and the pyrolysis of aminophosphonium salts (Equations 10 and 11). R PNH  M e 0 H  3  »  [R P=N=PR ] 0CH " 3  3  +  . . . . 10  3  [R PNH ] CI" — A > [R P=N=PR ] C1" 3  2.3.2  2  3  3  . . . . 11  +  Preparation of [MegPNMeJ+IConsistently with i t s basic character, Me PNMe reacts rapidly with 3  methyl iodide to give the expected N-methyl salt [Me PNMe ] l". 3  2  +  The  isolation of such a compound, via the reaction of methyl iodide and dimethyl ami no-dimethylphosphine, has already been d e s c r i b e d  146  .  However,  i t s reported melting point of 315-320°C is much higher than that found in the present work (d> 220°C), and i t s true nature is therefore believed to be uncertain.  In the present case, the ^H n.m.r., infrared and mass  spectra of the product are a l l consistent with the proposed structure. 2.3.3  Preparation and Deprotonation of [Me.,PNHMe]"'"Cl~ The second most common method for the preparation of phosphinimines  involves the deprotonation of an aminophosphonium salt (Equation 12), which  - 47 -  R  3  P C 1  2 ^jrNH  2  [R PNHR'] Cr R  p  ~  +  3  HX  •  R P=NR'  . . . . 12  3  R'NHCl 147  can be prepared by the action of a primary amine on a dihalophosphorane 148 or of a chloramine on a tertiary phosphine . The type of base required for the second step depends (as in the preparation of phosphorus ylids) on the acidity of the phosphonium s a l t , and varies from t r i e t h y l a m i n e 149 I5n sodium amide  and sodium  in liquid ammonia.  147  to  In the present work, the  aminophosphonium salt [Me PNHMe] Cl~ was prepared by the reaction of +  3  methylamine and Me PCl , and i t s subsequent deprotonation attempted with 3  2  (1) sodium amide in liquid ammonia, and (2) potassium t-butoxide (Equation 13).  The reaction between [Me PNHMe] Cl" and NaNH yielded not the expected NaNhL [Me P=N=PMe ] cr NHMe , , Me.PClA [Me-PNHMe] CI" ^ . . . . 13 \K0tBu Me PNMe +  3  2  +  3  3  ?  ;  L  6  3  product, but the linear salt [Me P=N=PMe ] Cl~. 3  [ P=N=P ]  +  3  +  The isolation of the  cation from this reaction is not easily rationalized.  This type  of behaviour is not encountered in the reaction of sodium amide with aminotriarylphosphonium s a l t s , but i t s occurrence is reminiscent of the d i f f i c u l t i e s alluded to in other work on the preparation of trialkylphosphinimines  144  '  150  .  The reaction of potassium t-butoxide and [Me PNHMe] Cl" 3  was more successful, and a good yield of Me PNMe was obtained. 3  However,  repeated fractional d i s t i l l a t i o n was required to purify the product from the side-product, t-butanol.  +  - 48 -  2.3.4  Reaction of Dihalophosphoranes with (Me Si) NMe 3  2  The reaction between fluorophosphoranes RPF^ (R=Me^" , P h 1  1 6  \  151 152 F  '  ) and heptamethyldisilazane, to give dimeric phosphazanes  (Equation 14), is well known.  In this work, the reactions of the  RPF + (Me Si) NMe 4  3  • 7j-(RF PNMe) + 2Me SiF  2  2  2  14  3  dihalophosphoranes Me PCl and Me PF with (Me Si) NMe have been 3  2  3  2  3  2  investigated to see i f the phosphinimine Me PNMe could be produced. 3  However, in the case of Me PF , i t was found that no reaction took place. 3  2  At f i r s t sight, this result is somewhat surprising, but i t i s consistent IM l^l 1^? with the observed low r e a c t i v i t y of Me PF , Ph PF and PF towards 2  N-alkyl hexamethyldisilazanes.  3  2  3  1  3  To account for t h i s , i t has been suggested  that the reactivity of phosphorus fluorides towards silazanes depends on 151 their Lewis acidity In the case of Me PCl , a reaction did occur, but the expected 3  2  2+ product was not isolated.  Instead, the linear salt [Me P=N(Me)-PMe ] 3  was formed, according to Equation 15. 2Me PCl + (Me Si) NMe — • 3  2  3  2  3  2C1  This behaviour parallels that of  [(Me P) NMe] 2Cl" + 2Me SiCl 3  2  2+  . . . . 15  3  phosphorus t r i c h l o r i d e , which, with both MeNH and MeN(SiMe ) gives not ICO I CA the cyclic molecule (MeNPCl) , but the linear species (Cl P) NMe . 2  3  2  2.3.5  2  2>  2  Structure and Spectra of Phosphinimine Derivatives Although there exists a formal structural resemblance between the  phosphinimine Me PNMe and the methylphosphazenes (NPMe ) , the chemical 3  2  n  - 49 -  behaviour of the two types of compound reflect subtle differences in their bonding.  Tetramethylphosphinimine is a very strong base, much stronger than  are the methylphosphazenes, and i s , for example, capable of decomposing chloroform (presumably by proton removal).  Consistently, i t s hydro-  chloride [MePNHMe] Cl~ is a much weaker acid than are the hydrochlorides of +  methylphosphazenes; whereas (NPMe ) .HCl can be deprotonated with 2  3  triethyl amine, [Me PNHMe] Cl~ requires potassium t-butoxide. 3  The highly  +  polar nature of the PN bond in Me PNMe is also indicated by i t s rapid 3  hydrolysis in moist a i r .  By contrast, methylphosphazenes are stable in  hot aqueous a l k a l i . The  n.m.r. parameters (Table 2.4) of the phosphinimine derivatives p  described in this section also provide useful information for the comparison of the bonding in phosphinimine and phosphazene derivatives.  For example,  the resonance of the P-methyl protons of Me PNMe is found to high f i e l d 3  Table 2.4:  n.m.r. parameters of Me^PNMe and i t s derivatives 9  Compound  Solvent  6 (MeP)  6 (MeN)  0.98(12.5)  3.01(27.5)  H  H  Me PNMe  C  Me PNMe.HCl  CDC1  2.11(14.0)  . 2.69(14.5)°  Me PNMe.MeI  CDC1  2.23(13.3)  2.73(10.6)  2.36(13.0)  3.24(10.0)  -  3  3  6 6  .  D  3  3  3  [(Me P) NMe] 2Ci; 3  2+  2  [(Me P) N] CT  CDC1  3  1.93(13.1)  [(Me P) N] N  CD CN  1.70(13.7)  3  3  2  2  +  +  3  3  b  b  (a) Dilute solutions. <5(ppm) reference internal TMS, except reference internal DSS. PH coupling constants (in Hertz) in oarenthesis. (b) 6(CH~N) in Ph PNMe=3.01 ppm (24.7Hz), <$(CHN) in Ph PNMe.HBr=2.78 ppm (13.8Hz), F. Kaplan, G. Singh and H. Zimmer, J . Phys. Chem., 6]_, 2509 (1963). (c) J(H-H)=5.8Hz. 3  3  3  6  - 50 -  of the equivalent signal in (NPMe^)^ (6=1.46 ppm, Table 2.1).  However,  upon quaternization of the two molecules, the order of these chemical shifts is reversed; 6(CH P) in [Me PNMe ] l" is 2.23 ppm, and in (NPMe ).MeI 3  3  i t is 2.02 ppm (see Table 2.3).  2  +  2  Consistent with this order, the N-methyl  protons in [Me PNMe ] I~(6=2.73 ppm) are more effectively shielded than 3  2  +  they are in (NPMe ) .MeI (6=3.20 ppm). 2 3  The trends  in these chemical shifts suggest that, in the  phosphinimine salt [Me PNMe ] I~, the positive charge of the cation i s 3  2  +  located almost completely on phosphorus (Equation 16), with l i t t l e or no double bond character associated with the PN bond.  Consistently, no  v(P=N) is observed in the infrared spectrum of [Me PNMe ] I~. 3  the ^H chemical s h i f t s of the [(NPMe ) Me] 2  3>  2  By contrast,  +  cation indicate that (in  +  7 F>  agreement with calculated ir-charge densities  ) much of the positive charge  of the ion i s localized on the quaternized nitrogen atom (Equation 17), thereby preserving cyclic aromaticity.  Me Me \ • / Me — P-—N  /  \ Me  Me Me  Me Me—P: / Me  Me  N  Me +/  \  Me  \/  16  Me  Me Me  N 17  Me—P. Me  ^P—Me N  Me  Me—P. .\ Me  /P —Me Me  - 51 -  The localization of charge on phosphorus, rather than on nitrogen 2+ is again observed in the doubly charged cation [(Me P) NMe] , the methyl protons of which resonate at* 6 = 2.36 ppm. In the cyclic dication 2+ 3  [(NPMe ) .2Me]  2  , the analogous signal is observed at 6 = 1.95 ppm. Charge  2 4  delocalization, and the preservation of iT-character for the PN bond, is only possible in the cation [Me P=N=Me ] ; accordingly, the P-methyl protons 3  +  3  in [(Me P) N] Cl~ are more shielded (6=1.93 ppm) than they are i n , for 3  2  +  instance, [Me PNMe ] I~ (6=2.23 ppm). Consistently, a strong v(P=N) band 3  2  +  (at 1246 cm" ) i s observed in the i . r . spectrum of [(Me P) N] Cl~. 1  3  +  2  As expected, the infrared spectra of the phosphinimine derivatives described here show many s i m i l a r i t i e s .  The vibrational spectra of 143  Me PNMe i t s e l f are described in detail elsewhere  (see also Chapter IV).  3  The ionic compounds [Me PNMeR] (R=H, Me, PMe ) and [(Me P) N] a l l 3  +  display bands corresponding to 5  3  sym  CH  +  3  2  +  (1300-1330 c m ) , CH rocking -1  3  3  (960-980 cm" ), CH wagging (870-890 cm" ) and P-CH stretching (720-770 1  cm" ). 1  1  3  3  In addition, the derivatives [Me PNMeR] (R=H, Me, PMe ) a l l 3  +  3  +  contain a v(C-N) band at 1050-1095 cm" . 1  Summary -  The chemical and physical properties of methylphosphinimines  and methylphosphazenes indicate a marked difference in the nature of the PN bond in the two systems.  The importance of cyclic aromaticity in the  latter compounds is clearly evident, the PN bond polarity in them being greatly reduced from that expected from a series of isolated PN units.  The  presence of a delocalized iT-system within the PN ring of methyl phosphazenes has important consequences for the chemistry of the attached ligands. topics w i l l be f u l l y discussed in the following chapters.  These  - 52 -  2.4  Chemistry of gem-Dichlorotetraphenylcyclotriphosphazene  2.4.1  Attempted Methylation of CI2 4 3 3 Ph  P  N  w i t n  MeMgBr  During the course of work on the preparation of Me^Ph^P^N^, the 139 140 phosphazene C ^ P h ^ N ^ I I ) was prepared  '  to see i f i t s reaction with  MeMgBr, according to Equation 18, would afford a convenient preparative route CI CI \/ N  N  Ph—P.  J?—  Ph  *  Ph Ph  II C l P h P N + 2MeMgBr 2  to Me Ph P N 2  4  3  4  3>  3  • Me Ph P N + 2MgC1Br  3  2  4  3  . . . 18  3  However, i t was found that, unlike ClgP N , C l P h P N does 3  not react at a l l with MeMgBr.  3  2  4  3  3  After boiling a solution of CI2 4 3 3 Ph  P  N  i n  ether with an excess of MeMgBr for four days, only the starting material was recovered (almost quantitatively).  Because of i t s surprisingly low  reactivity towards MeMgBr, a number of other substitution reactions of CI2 ' 4 3 3 P 1  P  N  w e r e  attempted, in order to compare i t s reactivity with that of  CUP-N,. 5 33 2.4.2  Dimethylamination of C l p P h ^ N j  Consistent with i t s low reactivity towards MeMgBr, and in contrast to 1 55 the behaviour of C l g P ^ , CI Ph P N reacts only in the presence of an 1  2  4  3  3  53  excess of dimethylamine and/or under extreme conditions (high temperature). The reaction of  CI2PH4P3N3  dimethylamine, in the molar proportion 1:2, 156  a n d  according to Equation 19, has been reported (^9%) of (NMe )ClPh P N . 2  e l  4  2PH4 3 3 P  N  +  3  H N M e  to give a very low yield  In the present work, the yield has been improved  3  2  * (  N M e  2)  C 1 P h  4 3 3 P  N  +  H N M e  2-  ••••  H C 1  1 9  to a.nearly quantitative value by bubbling an excess of dimethylamine through an ethereal solution of 2 P H 4 3 3 c l  P  N  f  o  r  s e v e r a l  n o u r s  -  Substitution  of the second chlorine, however, cannot be achieved under such mild conditions, and (NMe ) Ph P N can only be prepared by reacting 1 ETC 2  2  4  3  with an excess of HNMe at 180°C  .  2  2.4.3  Fluorination of C1 Ph P N 2  CI2PH4P3N3  3  4  3  3  As was mentioned in the previous chapter, there exist two principal methods for the preparation of fluorophosphazenes from chlorophosphazenes. 30 The use of anionic reagents, such as sodium fluoride and potassium 31  fluorosulphite  32  '  , i s most useful for the preparation of f u l l y  fluorinated phosphazenes, since the introduction of one fluorine into the ring activates i t to further substitution.  Consecutive substitution i s ,  57 58  as expected, predominantly geminal  '  .  B  Y contrast, the fluoridation  of chlorophosphazenes using antimony t r i f l u o r i d e usually takes place non-geminally  '  ...  Because the effect of SbF depends on.its 3  preliminary coordination to a ring nitrogen atom, progressive fluoridation is inhibited by the decreasing donor strength of the phosphazene ring. By i t s e l f , SbF i s too weak an acceptor to react with ( N P C 1 ) 3  2  3  157  .  Reaction  - 54 -  occurs when the base strength of the phosphazene is increased by the 33 presence of dimethylamine ligands (e.g. in (NMeg^Cl^P-jN^ , ( N M e ^ C l ^ N g 157 and (NI^^C^P^N^  ) or when antimony pentachloride is added to the  reaction m i x t u r e ' 3 4  1 5 8  .  The fluorophenylphosphazene gem-F^^h^Ng has previously been 159 prepared by the phenylation of gem-F^PI^P^Ng with phenyllithium  .In  the present work i t s preparation has been attempted by the fluorination of C^Ph^P^N-j.  As expected from the results of the previous experiments, the  use of anionic reagents was ineffectual.  Potassium fluorosulphite, in a  variety of solvents (boiling heptane, acetonitrile and nitrobenzene) did not react with C ^ P h ^ N ^ . 1 CO mixture  Similarly, sodium fluoride, and an (NaF+HF)  , both failed to fluorinate i t .  However, the reaction of an  SbF^/SbClg mixture with C ^ P h ^ N ^ was more successful, and, in this way, a 27% yield of-FgPh^PjNg was obtained. 2.4.4 Preparation of (NMejFPh^Ng The comparative ease of fluorination of Cl^Ph^P^N^ with SbF^/SbCl^ prompted an experiment on the fluorination of the monodimethylamino derivative ( N f ^ K l P h ^ P . ^ .  Consistent with the activating effect of the  amine ligand, i t was found that the fluorination of (NMe )ClPh P N could 2  be effected using SbF^ alone, without the presence of SbClg.  4  3  3  A high  yield of (NMegjFPh^P^N^ was obtained using this method. 2.4.5 Attempted Preparation of a Phosphazenium Cation The reactions described above indicate an unusually low reactivity of C^Ph^PgNg towards nucleophiles.  The cause of this behaviour is thought  - 55 -  to be based upon electronic rather than steric factors, the more electronegative chlorine atoms being capable of withdrawing ir-electron density from the P(Ph ) phosphorus atoms onto the P(C1 ) atom. 2  2  In the phosphazene C l P h g P ^ , the reactivity of the P(PhCl) atom towards nucleophiles is s u b s t a n t i a l  1 6 0  '  1 6 1  .  It is believed that, in this  compound, the PCI bond may be weakened by conjugation of the geminal phenyl group to phosphorus, in a fashion similar to that postulated to account for the apparent reactivity o f - . ( N M e ^ C l P ^  (e.g. Equation 20).  Consistent  with t h i s , silver perchlorate reacts with ClPhgP N in pyridine, yielding 3  3  the s a l t - l i k e compound PhgP N .CIO^.py. 3  3  Me '  •  V  .  Me N  /  Jj +  CI  2  Me  \  P  CI"  P  I  II  I  II  During the present work, an attempt was made to prepare a similar salt to be the reaction of BF .Et 0 with (NMe )FPh P N , according to 3  Equation 21.  2  2  4  3  3  Unfortunately, the product that was isolated was not the  BF .Et 0 + (NMe )FPh P N. —• [(NMe )Ph P N ] BF " + Et 0 3  2  2  4  3  3  2  4  3  3  +  4  2  . . . . 21  expected phosphazenium s a l t ( I I I ) but the protonated salt [(NMe )FPh P N H] [BF 0H]". 2  4  3  3  +  3  Although i t was not the desired product, this  compound i s , nonetheless, informative, since i t indicates the relative effects of dimethylamino and fluorine ligands on the base strengths of the  - 56 -  molecules, i . e . , the presence of the electron donating NMe group 2  s u f f i c i e n t l y offsets the electron withdrawing effect of the fluorine ligand to allow the protonation of the weakly basic ring nitrogen atoms. 31 The  P n.m.r. spectrum of the salt shows that i t s structure is V rather  than IV.  Protonation of the F(NMe )Ph P N molecule greatly affects the 2  4  electron distribution in the PN ring.  3  3  In relation to the neutral  molecule (6P(Ph )=94.1 ppm, 6PFNMe =95.6 ppm), the P(Ph ) atoms are 2  2  2  considerably deshielded (6=87.2 ppm), with a consequent transfer of charge onto the PFNMe atom (6=100.6 ppm). 2  Ms Me \ / N* BF/"  Me N ?  1  N  Ph-  /  Ph  Ph  +N  Ph-P  ^V  Ph  2.5.1  \ /  N  P H  >h  Ph—R / Ph  N  + N I H  •Ph Ph  IV  III  2.5  ^  F  9  2  N  N  J>—Ph  \ /  Me N  F  Experimental Section Preparation of Methylphosphazenes l fi^ Dimethyltrichlorophosphorane  tetramethyldiphosphine disulphide in vacuo over P 0 . 2  g  was prepared by the chlorination of in carbon tetrachloride, and dried  Methylamine hydrochloride and ammonium chloride were  - 57 -  commercial products, and were oven dried at 120°C for 24 hours before use. The fluorophosphazenes (NPF )g_-j were obtained from Prof. N.L. Paddock. 2  Q  They had been prepared by the f l u o r i n a t i o n , with KSC^F, of a mixture of 133 chlorophosphazenes (NPC1 ) (n>5), and separated by v . p . c . . Methyl 165 2  n  magnesium bromide was prepared using standard procedures commercially obtained magnesium turnings and methyl bromide.  from Diethyl ether  was dried before use by d i s t i l l a t i o n from lithium aluminium hydride. 2.5.1.1  Preparation of (NPMeJo Methylamine hydrochloride (12.Og, 191 mmol) was added to a slurry  of dimethyltrichlorphosphorane (32.Og, 191 mmol) in 125 ml of chlorobenzene, and the mixture heated to reflux temperature.  The evolution of hydrogen  chloride then commenced, and the reactants rapidly fused into a pale yellow o i l which was immiscible with the solvent.  After the mixture had been  heated for one hour under reflux, the solvent was d i s t i l l e d off to leave a viscous o i l .  (N.B. Prolonged heating under reflux of this mixture  reduces the r e l a t i v e y i e l d of the trimer; i t also lowers the overall yield of methylphosphazenes).  The o i l was then heated at 165°C/0.05 Torr for  four hours, during which time i t gradually s o l i d i f i e d into an off-white mass.  This solid was then extracted with 3 x 200 ml of hot a c e t o n i t r i l e .  The extracts were combined and concentrated, whereupon a white, s l i g h t l y hygroscopic solid (11.94g) c r y s t a l l i z e d out.  A further 0.84g of the same  solid was obtained by evaporation, under nitrogen, of the mother liquor. The whole was recrystallized from a c e t o n i t r i l e and identified by i t s analysis and  n.m.r. spectrum as the quaternary s a l t N^P^Meg.MeCl;  m.pt. 195-200°C (dec) (yield 12.8g, 46.5 mmol, 73%); H n.m.r. (<5, CD^CN, ]  - 58 -  internal TMS), 2.94 (3H, t r i p l e t , J =10.5Hz), 1.87 (12H, doublet, pH  Jp =13.5Hz), 1.57 (6H, doublet, J =14.5Hz, long range J =1.5Hz); H  pH  i . r . v(P=N) 1190, 1245 cm" .  pH  Anal, calcd. for C H C1N P :  1  7  H, 7.68; CI, 12.86; N, 15.24.  Found:  21  3  3  C, 30.50;  C, 30.94; H, 7.75; CI, 13.00;  N, 14.84. Slow pyrolysis of NgPgMeg.MeCl at 200°C/0.1 Torr decomposed the s a l t , and N^Me^ was collected, in nearly quantitative y i e l d , by sublimation onto a water-cooled cold-finger. 21 by i t s infrared spectrum C  6 18 3 3 H  N  P  N, 18.80.  , analysis and melting point.  C, 31.98; H, 8.06; N, 18.66.  :  The compound was identified  Found:  M.pt. 187-188°C, l i t . 195-196°C  21  Anal, calcd. for  C, 32.13; H, 7.99;  and 187-190°C ( d e c ) . 77  The insoluble residue from the acetonitrile extraction (2.76g) was recrystallized from hot methanol/chloroform and dried in vacuo.  This  s l i g h t l y hygroscopic solid was identified by i t s analysis and  n.m.r.  spectrum as the diquaternary salt N P Meg.2MeCl (6.9 mmol, 15%), m.pt. 4  >275°C.  ]  H n.m.r. (6, D 0, internal DSS), 2.93 (6H, t r i p l e t , J =11.5Hz), 2  PH  2.03 (24H, doublet, J =13.5Hz); i . r . PH  C  10 30 2 4 4 H  C1  4  N  P  :  C  '  2 9  -  9 4  '  H  '  7  -  5 4  v(P=N) 1350 cm" .  ' CI, 17.67; N, 13.97.  1  Anal, calcd. for Found: C, 29.68;  H, 7.70; CI, 18.00, N, 13.79. 2.5.1.2  Preparation of (NPMe )  2 4  Ammonium chloride (8.8g, 164 mmol) was added to a slurry of dimethyltrichlorphosphorane (27.5g, 164 mmol) in 100ml of chlorobenzene, and the mixture subjected to the same heating cycle as described above (Section 2.5.1.2).  The powdery solid so obtained yielded, after extraction  - 59 -  with a c e t o n i t r i l e , 11.2g of an insoluble s o l i d , which was identified by i t s infrared s p e c t r u m ' 77  as the dihydrochloride (NPMe ) .2HCl.  109  2  This  4  salt was converted quantitatively into the neutral compound by treating i t with an excess of triethyl amine in chloroform.  Evaporation of the  solvent and extraction of the crystalline residue with hot hexane afforded, upon removal of the solvent, the neutral (NPMe ) , which was identified by 2  i t s infrared spectrum 155-156°C  77  21  4  and melting point, 161-162°C ( l i t . 162-163°C , 21  and 157-158°C ).  Evaporation of the acetonitrile extract  110  yielded a yellow o i l which, upon treatment with NEt^ and extraction with hexane, yielded a small quantity (^lOOmg) of (NPMe ) , which was 2  3  identified by i t s infrared spectrum and melting point (see Section 2.5.1.1), 2.5.1.3  Separation of (NPMeJj and (NPMeJ  4  This experiment demonstrates how a mixture of (NPMe ) and (NPMe ) 109 2 3  can be separated via their hydrochlorides  .  2 4  A mixture of (NPMe )  2 3  (1.34g, 5.94 mmol) and (NPMe ) (1.41g, 4.71 mmol) was dissolved in 200 ml 2 4  of diethyl ether.  Hydrogen chloride was bubbled through the solution until  precipitation of the methylphosphazene hydrochlorides was complete.  The  resulting white precipitate was f i l t e r e d off and dried in vacuo (3.36g). This solid was shaken with 100 ml of hot acetonitrile and the mixture filtered.  The insoluble N P Me .2HCl (1.78g, quantitative) was identified 4  4  by i t s infrared s p e c t r u m ' 77  109  H, 7.02, CI, 19.00; N, 15.02. N, 15.05.  M.pt. >260°C.  g  .  Anal, calcd. for Found:  C  8  H  26 2 4 4 C 1  N  P  :  C  '  2 5 > 7 5 ;  C, 25.61; H, 6.78; CI, 18.90;  Evaporation of the  acetonitrile extract gave  N P Me .HCl (1.51g, 97%), m.pt. 247-249°C, i . r . v(P=N) 1170, 1230 cm" .. 3  3  1  6  Anal, calcd. for C H C l N P : 6  lg  3  3  C, 27.53; H, 7.32; CI, 13.55; N, 16.07.  - 60 -  Found:  C, 37.30; H, 7.39; CI, 13.27; N, 16.20.  The neutral methyl-  phosphazenes can be regenerated from their hydrochlorides by treatment 2176  with triethylamine 2.5.1.4  V.  Preparation of (NPMe )  (n=6-10)  2 n  The experimental procedure for the preparation of the methylphosphazenes (NPMe )g_-]Q is i d e n t i c a l . 2  Therefore, only the preparation of one  of them (the hexamer) w i l l be described in d e t a i l .  A solution of (NPF ) 2  g  (6.77g, 13.6 mmol) in 50 ml of diethyl ether was added, under an atmosphere of nitrogen, to a stirred solution of MeMgBr (from 5.61g, 231 mmol, of magnesium) in 250 ml of ether.  No immediate reaction occurred, but, after  an induction period of approximately two hours, precipitation of magnesium fluoride slowly commenced. To ensure the completion of the reaction, the mixture was l e f t at reflux temperature, under nitrogen, for a 48 hour period.  The solvent was then d i s t i l l e d from the reaction vessel, and the  reaction mixture dissolved (carefully!) in 200 ml of water.  300 ml of a  1 molar solution of sodium carbonate was then added to precipitate a l l the magnesium as i t s carbonate.  The solution was f i l t e r e d and the solvent  evaporated from the f i l t r a t e to leave a white s o l i d , which was repeatedly extracted with hot chloroform.  The solvent was d i s t i l l e d from the combined  chloroform extracts to leave a white crystalline solid (2.52 g).  The  original water-insoluble precipitate was recombined with the water-soluble residue and the mixture boiled in 300 ml of ^1M NaOH for one hour.  This  mixture was then r e f i l t e r e d , the solvent removed from the f i l t r a t e , and the residue extracted with hot chloroform, thereby affording a further 1.62g  of material soluble in chloroform.  The two fractions of the product  (4.14g) were combined and purified by sublimation in vacuo and recrystallization from hot hexane to give colourless blocks of (NPMe )  2 (  m.pt. 163-165°C. N, 18.66.  Anal, calcd. for C  Found:  12  H  36 6 6 N  P  :  C  '  3 2  -  0 1  »  H  '  8  -  0 6 ;  C, 32.00; H, 8.06; N, 18.74.  The phosphazenes (NPM^^-IO analogous to that just described^.  P P  w e r e  r e  a r e c l  using a procedure  The yields and analytical data  obtained from these reactions are given in Table 2.5.  Table 2.5:  Yields and analytical data for the reaction (NPF ). + 2nMeMgBr->(NPMe ) . (n=6-10)-. 2  h  2  n in (NPMe )  Yield %  M.pt. °C  6  67  163-165  32.00  8.06  18..74  40  128-130  31.90  7.90  18 .80  8  40  171-173  32.38  7.90  18 .77  9  28  123-125  32.28  8.22  18 .33  TO  ^5  100-102  31.73  8.11  18 .60  2 n  7  .,  Anal, calcd. for (C H NP) : 2  g  The compounds (NPMe ) _ ?  R  n  ln  Composition  3  %C  %\  C 32.01; H, 8.06; N, 18.66  were prepared by Mr. K.D. Gallicano.  - 62 -  2.5.1.5  Preparation of Me Ph P N 2  4  3  3  The acyclic compound [NH (Ph )PNP(Ph )NH ] Cl~ 2  2  2  2  +  was prepared by the  ammonolysis of  diphenyltrichlorphosphorane, and purified by . 24 139 recrystallization from acetonitrile ' . Dimethyltrichlorophosphorane (17.14g, 102.3 mmol) and [NH (Ph )PNP(Ph )NH ] CT (46.20g, 102.3 mmol) were heated together, 2  2  2  2  +  under reflux, in a slurry of 200 ml of chlorobenzene.  After one hour, the .  solvent was removed by d i s t i l l a t i o n , and the residual light brown o i l heated at ^165°C/0.1 Torr for a further three hours.  On cooling the reaction  mixture to room temperature, a glassy off-white solid was obtained. Extraction of this solid with 3 x 300 ml of hot acetonitrile gave 6.26g of a white insoluble s o l i d , the i . r .  spectrum of which indicated the  presence of the hydrochloride (NPMe ) .2HCl. 2  with NEt  3  4  This solid was then heated  (as described in section 2.5.1.2) and the product extracted with  hot hexane to y i e l d , upon evaporation of the solvent, 2.96g (10.0 mmole, 40%) of (NPMe ) , which was identified by i t s infrared spectrum and melting 2  4  point (161-162°C).  The acetonitrile extracts were concentrated and cooled,  whereupon a white crystalline solid precipitated from the solution.  This  solid was extracted with hot benzene, leaving, as the insoluble part, a white solid whose infrared spectrum showed i t to be the hydrochloride 138 Me^h^P^N-j.HCl  .  This compound was dissolved in chloroform and the  mixture treated with aqueous t r i e t h y l amine.  The organic layer was washed  with water, dried over Na S0 , and evaporated to leave a white s o l i d , which 2  4  was purified by recrystallization from ether/hexane to give colourless blocks of Me Ph P N 2  4  3  3  (7.2g, 15.2 mmol, 15%), melting point 142-144°C  - 63 -  (lit.  140-142°C).  I J D  N, 8.88. 2.5.2  Found:  Anal, calcd. for C ^ g N ^ :  C, 65.96; H, 5.54;  C, 65.89; H, 5.66; N, 8.81.  Preparation of Methylphosphazenium Quaternary Salts Methyl iodide and ethyl iodoacetate were obtained from commercial  sources, and used without further purification.  Methylfluorosulphate was  also a commercial product, but was r e d i s t i l l e d , under nitrogen, before use. Acetonitrile was dried by d i s t i l l a t i o n from calcium hydride.  The  methylphosphazenes (NPMe ) were a l l dried by sublimation in vacuo before 2 n  use.  Me Ph^P N was oven dried at 100°C for 3 hours. 2  2.5.2.1  3  3  Preparation of (NPMeJg.Mel A sample of (NPMe )  (0.366g, 0.813 mmol) was dissolved in 10 ml of  2 g  methyl iodide and the mixture gently heated under reflux for two hours. Subsequent f i l t r a t i o n of the mixture, and recrystallization of the f i l t e r e d precipitate (0.480g, 0.81 mmol, 100%) from  acetonitrile/toluene yielded  colourless, s l i g h t l y hygroscopic crystals of the adduct (MPMe ) .MeI.(CgH .CH ) (m.pt. 171-173°C). 2 6  5  3  Heating the powdered crystals  at 90°C/0.01 Torr for seven hours freed the product of solvated toluene. Anal, calcd. for C' H IN P : 13  Found: 2.5.2.2  3g  6  6  C, 26.38; H, 6.64; I, 21.44; N, 14.20.  C, 26.08; H, 6.60; I. 21.20; N, 14.13.  M.pt. 175-178°C.  Preparation of (NPMeJ .MeI 7  A sample of (NPMe )  2 7  (0.215g, 0.409 mmol) was dissolved in 10 ml of  methyl iodide and the solution gently refluxed for two hours.  The addition  of a large volume (^250 ml) of diethyl ether to this solution precipitated a  - 64 -  white s o l i d , which was f i l t e r e d off and recrystallized from a mixture of hot toluene and a few drops of acetonitrile to give a i r stable crystals of (NPMe ) .MeI (0.270g, 0.405 mmol, 100%).  Anal, calcd. for  2 y  C, 27.00; H, 6.80; I, 19.02; N, 14.69. N, 14.69. 2.5.2.3  Found:  c  -|  5  H  4 5  I N  7 7  C, 26.95; H, 6.87; I,  P  :  18.91;  M.pt. 153-155°C. Preparation of Me,,Ph P N .MeI 4  3  3  A sample of Me Ph P N (1.296g, 2.73 mmol) was dissolved in 20 ml of 2  4  3  3  methyl iodide and the mixture heated gently under reflux for 72 hours.  The  mixture was then f i l t e r e d and the residue (1.65g, 2.68 mmol, 98%) recrystallized from hot ethanol.  The needle-like crystals so obtained  contained one mole of solvent per mole of product.  Heating the powdered  crystals at 60°C/0.01 Torr for seven hours produced an analytically pure sample of Me Ph P N .MeI. 2  4  3  Anal. calcd. for C H I N P :  3  27  H, 4.75; I, 20.62; N, 6.83. M.pt. 226-227°C (dec).  3 1  Found:  29  3  3  C, 52.70;  C, 52.90; H, 5.00; I. 20.40; N, 6.83.  P n.m.r. spectrum (s, CDC1 , external P 0 ) ; 3  4  g  64.1 ( I P , PMe atom), 81.0 ( I P , PPh atom adjacent to N(Me) atom), 93.5 2  2  ( I P , PPh atom remote from N(Me) atom). 2  "'H n.m.r. spectrum (6, CDC1 , 3  internal TMS); 3.07 (3H, t r i p l e t , J =11.0Hz), 2.22 (6H, doublet, J = pH  pH  14.0Hz), 7.0-8.0 (20H, broad unresolved multiplet).  i.r.  spectrum;  v(P=N), 1220, 1260 cm" ; v(C-N), 1070 cm" . 1  2.5.2.4  Preparation of  1  [(NPMe ) .CH C00Et] I" 3  A sample of (NPMe )  2 3  3  2  +  (0.633g, 2.81 mmol) was dissolved in 10 ml of  ethyl iodoacetate and the mixture s t i r r e d , under a nitrogen atmosphere, at  - 65 -  room temperature, for 48 hours.  The mixture was then f i l t e r e d , and the  precipitate recrystallized from acetonitrile/benzene to give colourless blocks of [(NPMe ) .CH C00Et] l" 2  3  for C H I N 0 P : 10  25  3  2  (0.943g, 2.14 mmol, 76%).  +  2  C, 27.35; H, 5.74; I. 28.90; N, 9.56.  3  C, 27.63; H, 5.76; I, 28.70; N, 9.64. 2.5.2.5  Anal, calcd. Found:  Dec. >220°C.  Preparation of [(NPMe ) -CHoCOOEt]" "!" 1  2 4  A sample of (NPMe ) 2  4  (0.982g, 3.27 mmol) was dissolved in a  solution of 8 ml of ICH C00Et and 30 ml of diethyl ether, and the mixture 2  heated gently  under reflux for 96 hours.  The white precipitate was then  f i l t e r e d from the solution and recrystallized from  acetonitrile/benzene  to give small, colourless crystals of [(NPMe ) .CH C00Et] I" (1.073g, 2.09 2  mmol, 64%). N, 10.94. 2.5.2.6  Anal, calcd. for C H I N 0 P : 12  Found:  31  4  2  4  4  +  2  C, 28.14, H, 5.71; I,  C, 28.20; H, 5.80; I, 24.52; N, 10.71.  Preparation of [(NPMe ) -2Me1 2  4  2+  24.78;  M.pt. 176-178°C.  2X~ (X=FS0 I) 3>  A solution of MeS0 F (1.20g, 10.5 mmol) in 10 ml of acetonitrile was 3  added to a stirred solution of (NPMe ) 2  4  (1.545g, 5.15 mmol) in 50 ml of  a c e t o n i t r i l e , and the mixture heated under r e f l u x , under an atmosphere of nitrogen, for two hours.  Upon cooling the solution, colourless needles  of (NPMe ) -2(MeS0 F) appeared. 2 4  3  These crystals were removed from the  solution and a further quantity of the product obtained by the addition of a large volume of ether to the mother liquor.  The combined product  (2.703g, 5.12 mmol, 99%) was recrystal1ized from hot acetonitrile to give colourless needles of (NPMe ) .2MeS0 F. 2  4  3  Anal, calcd. for -| 30 2 4°6 2 4 C  0  H  F  N  S  P  :  - 66 -  C, 22.73; H, 5.72; N, 10.60. 231-233°C.  Found:  C, 22.90; H, 5.91; N, 10.56.  M.pt.  The di-iodide salt (NPMe ) .2MeI was prepared, in quantitative 2 4  y i e l d , by eluting an aqueous solution of (NPMe ) .2MeS0 F through a column 2 4  3  packed with Amberlite CG400 anion exchange resin (previously charged with iodide ion).  Evaporation of the solvent from the eluant, and  recrystallization of the residual solid from a c e t o n i t r i l e , gave colourless blocks of (NPMe ) .2MeI. 2  4  Anal, calcd. for  H, 5.18; I, 43.45; N, 9.59.  Found:  c  -|q^^SO 1 2 N 4 P 4  :  C  '  2 0  -  5 6 ;  C, 20.26; H, 5.11; I, 43.29; N, 9.58.  M.pt. >250°C. 2.5.3  Preparation of Methylphosphinimine Derivatives Trimethylphosphine was prepared by the methylation of phosphorus  trichloride with MeMgBr, using a combination of several literature methods ^ ^. 1  Methyl azide was prepared by the action of dimethylsulphate  - 1  on sodium a z i d e fractionation.  1 7 0  '  1 7 1  , and purified (from Me 0) by trap-to-trap 2  Trimethyldichlorophosphorane was prepared by the  chlorination of Me P0 3  50  with thionyl c h l o r i d e  172  .  S imilar ly , trimethyldi-  fluorophosphorane was prepared by the fluoridation of Me P0 with sulphur 3  tetrafluoride (yield, 73%).  Me PF has not previously been made by this 3  2  method; i t was identified by i t s infrared spectrum and boiling point 173 (76°C, l i t .  76°C).  Heptamethyldisilazane was prepared by the reaction  of methylamine with chlorotrimethylsilane.  Methylamine, sodium amide and  methyl iodide were commercial products and used without p u r i f i c a t i o n . Potassium t-butoxide was also obtained commercially, but was purified 175 before use by sublimation in vacuo .  - 67 -  2.5.3.1  Reaction of Me,,P and MeN^ A cold solution (0°C) of M e N ^ l ^ g , 31.3 mmol) in 10 ml of pentane  was slowly added, under an atmosphere of nitrogen, to a stirred solution of trimethylphosphine (2.34g, 30.8 mmol) in 20 ml of pentane at 0°C. Effervescence was immediate, and was accompanied by the formation of a fine white precipitate.  When the addition was complete, the mixture was allowed  to equilibrate to room temperature, and then stirred overnight under nitrogen.  The following day, the mixture was gently heated under reflux,  during which time much of the o r i g i n a l l y formed precipitate redissolved. The mixture was then f i l t e r e d under nitrogen, and the residue recrystallized from acetonitrile to give a i r stable crystals of (Me P) N N ~ (0.661g, 3  3.17 mmol). P, 29.77.  Anal, calcd. for C ^ g N ^ : Found:  2  +  3  C, 34.60; H, 8.72, N, 26.92;  C, 34.42; H, 9.00; N, 22.26*; P, 30.20.  M.pt. 211-214°C.  The solvent was d i s t i l l e d from the f i l t r a t e to leave a colourless, extremely hygroscopic l i q u i d , which was d i s t i l l e d in vacuo to give Me^PNMe (1.51g, 14.4 mmol'). N, 13.33. 2.5.3.2  Found:  Anal, calcd. for C H NP: 4  C, 45.68; H, 11.51;  12  C, 45.38; H, 11.58; N, 13.30.  B.pt. 73°C/18 Torr.  Preparation of [Me PNMeJ r +  3  A solution of methyl iodide (0.296g, 3.08 mmol) in 10 ml of diethyl ether was added, dropwise, under nitrogen, to a stirred solution of Me PNMe (0.219g, 2.08 mmol) in 5 ml of ether. 3  A white precipitate was  No satisfactory nitrogen analysis could be obtained for this compound.  - 68 -  immediately formed.  After j hour, the solution was f i l t e r e d and the  residue recrystal1ized from acetonitrile to give a i r stable crystals of [Me PNMe ] l"(0.512g, 2.06 mmol, 99%). 3  2  +  C, 24.29; H, 6.12; N, 5.67.  Found:  Anal, calcd. for C ^ I N P :  C, 24.46; H, 6.20; N, 5.44.  Dec >220°C. 2.5.3.3  Preparation of [Me PNHMe] Cl" +  3  Methylamine was bubbled through a suspension of Me PCl (8.46g, 3  57.6 mmol) in 50 ml of chloroform for ^ hour.  2  The mixture was then heated  under reflux for ^ hour to remove the excess methylamine.  F i l t r a t i o n of  the solution and removal of solvent from the f i l t r a t e yielded a white hygroscopic s o l i d , which was recrystallized from acetonitrile/toluene to give [Me PNHMe] cr (8.06g, 57.0 mmol, 99%). +  3  C, 33.91; H, 9.26; CI, 25.05; N, 9.90.  Found:  Anal, calcd. for C ^ C I N P : C, 32.67*; H, 9.57;  CI, 25.03; N, 9.80. M.pt. 180-185°C (dec). 2.5.3.4  Reaction of [Me PNHMe] Cl with Sodamide in Liquid Ammonia +  3  A sample of [Me PNHMe] Cl~ (3.43g, 24.2 mmol) was added to a slurry 3  +  of sodamide ( l . O l g , 25.9 mmol) in 25 ml of l i q u i d ammonia, and the mixture allowed to react for one hour.  The ammonia was then removed and the  residual solid extracted with hexane. no product.  Evaporation of this extract yielded  The reaction residue was then re-extracted with chloroform,  and upon slow evaporation, this extract yielded colourless needles of  No satisfactory carbon analysis could be obtained for this compound.  - 69 -  (Me P) N Cl" (1.15g, 5.71 mmol). 3  2  +  H, 9.00; CI, 17.59; N, 6.95.  Anal, calcd. for CgH^ClNP^  Found:  C, 35.72;  C, 35.37; H, 9.20; CI, 17.90;  N, 6.65. Dec > 210°C. 2.5.3.5  Reaction of [Me PNHMe] Cl~ with Potassium t-Butoxide +  3  A sample of [Me PNHMe] Cl was added under an atmosphere of nitrogen, +  3  to a slurry of KOtBu (3.68g, 32.9 mmol) in 100 ml of hexane, and the mixture heated, under reflux, for 16 hours.  The mixture was then f i l t e r e d  and the solvent d i s t i l l e d off to leave a colourless liquid which was then d i s t i l l e d in vacuo.  The infrared and  n.m.r. spectrum of this product  (2.075g) showed i t to be Me^PNMe plus a small quantity (^20%) of t-BuOH. This crude product was dissolved in 20 ml of heptane and the mixture redistilled.  Most of the t-BuOH d i s t i l l e d with the heptane, and the careful  fractional d i s t i l l a t i o n in vacuo of the residual liquid yielded, as a final f r a c t i o n , a small quantity of pure Me^PNMe (as shown by i t s boiling point, 73°C/18 Torr, and 2.5.3.6  n.m.r. spectrum).  Reaction of Me PF with (Me Si) NMe 3  2  3  2  Heptamethyldisilazane (2.13g, 12.2 mmol) was sealed in a Carius tube with Me PF (1.39g, 12.2 mmol) and the resultant mixture heated at 3  2  70°C for 48 hours.  No v i s i b l e change had occurred after this time, and  the starting materials were recovered by fractional d i s t i l l a t i o n of the reaction mixture.  - 70 -  2.5.3.7  Reaction of Me PC1 with (Me Si) NMe 3  2  3  2  A solution of (Me^i^NMe (5.70g, 32.6 mmol) in 10 ml of chloroform was added to a slurry of Me PCl 3  2  (4.79g, 32.6 mmol) in 50 ml of chloroform,  and the mixture heated under reflux for 24 hours, under an atmosphere of nitrogen.  The chloroform insoluble part was f i l t e r e d from the solution and  recrystallized from a methanol/chloroform mixture to give colourless, hygroscopic crystals of [(Me P) NMe] 2Cl~ (2.97g, 11.8 mmol, 73%). 3  Anal, calcd. for C H CI NP : 7  Found: 2.5.4  21  2  2  2+  2  C, 33.32; H, 8.40; CI, 28.14; N, 5.56.  C, 33.18; H, 8.42; CI, 27.74; N, 5.46. Preparation of  Dec >220°C.  XYPh PoN Derivatives 4  3  Dichlorotetraphenylcyclotriphosphazene was prepared by the reaction of phosphorus pentachloride with [NH (Ph )PNP(Ph )NH ] CI 2  2  2  2  in benzene  .  Dimethylamine (lecture bottle), antimony t r i f l u o r i d e and antimony pentachloride were a l l commercial products and used without further purification.  Boron t r i f l u o r i d e etherate, B F . E t 0 , was also obtained 3  2  commercially, but was d i s t i l l e d , under nitrogen, before use. 2.5.4.1  Preparation of (NMeJClPh^No Dimethylamine was bubbled through a solution of CI2 ' 4 3 3 C -292g, P  2.51 mmol) in 150 ml of diethyl ether for four hours.  n  P  N  The reaction mixture  was then gently heated under reflux to expel the excess dimethylamine. F i l t r a t i o n of the mixture and evaporation of the f i l t r a t e yielded a crystalline s o l i d , which was recrystallized from hot hexane and identified by i t s analysis and melting point (163-165°C, l i t . (NMe )ClPh P N (2.251g, 2.39 mmol, 95%). 2  4  3  3  1 5 6  164-165°C) as  Anal, calcd. for C ^ H ^ C I N ^ :  - 71 -  C, 59.72; H, 5.01; CI, 6.78; N, 10.71. N, 10.91.  ]  Found:  C, 59.61; H, 5.07; CI, 6.58;  W n.m.r. spectrum (6, CDC1 , internal TMS); 2.68 (6H, doublet, 3  J =16.5Hz), 7.0-8.0 (20H, unresolved multiplet). pH  2.5.4.2  Preparation of FoPh P N 4  A sample of CI Ph P N 2  4  3  3  3  (3.024g, 5.79 mmol) was added to a slurry of  3  an excess of SbF (^4g) in 25 ml of sym-C H Cl and the mixture brought to 3  2  2  4  reflux temperature under a nitrogen atmosphere.  After 1^ hours at this  temperature, no v i s i b l e change was apparent in the reaction mixture.  A  solution of 2 ml of SbCl^ in 10 ml C^H^Cl^ was added to the mixture, producing an immediate exothermic reaction.  After 3 hours at the reflux  temperature, the mixture was cooled and f i l t e r e d , the precipitate washed with benzene, and the solvent d i s t i l l e d from the f i l t r a t e to leave a dark oil.  A few drops of water were added to this o i l (to destroy excess  antimony halides), and the whole extracted with dichloromethane.  This  extract was dried over Na S0 , and then evaporated to leave a white s o l i d , 2  4  which was recrystal1ized from hot octane to give colourless blocks of F  2 ' 4 3 3 (°P  1  P  N  7 3 3  9' ^-  mmol, 26%), which was identified by i t s analysis,  5 2  melting point (135-136°C, l i t . Anal, calcd. for. 4 2 0 2 3 3 C  2  H  F  N  P  icq  :  C  137-138°C) and infrared spectrum  '  59  - '> > 88  H  4  1 9  ^ N,. 8.73.  Icq  .  Found:  C, 59.65; H, 4.28; N, 8.54. 2.5.4.3  Preparation of (NMejFPh^Ng A sample of (NMe )ClPh P N 2  4  3  (4.31g, 8.26 mmol) was added to a slurry  3  of SbF (^4g) in 100 ml of s y m - C H C l and the mixture heated under reflux 3  for 4 hours.  2  2  4  The mixture was then cooled and f i l t e r e d , and the precipitate.  - 72 -  washed with a l i t t l e benzene.  The solvent was then d i s t i l l e d from the  f i l t r a t e to leave a brown paste, which was treated with a few drops of water to destroy the excess of antimony halides.  The whole was then  extracted with 3 x 50 ml of dichloromethane, and the extract dried over Na,,S0 .  Evaporation of the solvent from this extract then yielded a pasty  4  s o l i d , which was recrystallized from hot octane to give colourless flakes of F(NMe )Ph P N (2.753g, 5.43 mmol, 67%). 2  4  3  3  C, 61.66; H, 5.18; N, 11.06. 154-156°C.  ]  Found:  Anal, calcd. for C  ? 6  H  C, 61.49; H, 5.20; N, 11.10.  2  6 4 3 F N  P  :  M.pt.  H n.m.r. spectrum ( 6 , CDC1 , internal TMS); 2.64 (6H, doublet, 3  J =12.0Hz); 7.0-8.0 (20H, unresolved multiplet). pH  2.5.4.4  Preparation of  [NMe )FPh P N .H] [BF 0H]~ 2  4  3  3  +  3  An excess (1 mmol) of BF .Et 0 was added, via a syringe and septum, 3  2  to a solution of (Me N)FPh P N (0.312g, .60 mmol) in 100 ml of diethyl ether. 2  4  3  3  Precipitation slowly commenced after about h hour.  The mixture was s t i r r e d ,  under nitrogen, for a further 48 hours, and then f i l t e r e d , to give a white s o l i d , which was recrystallized from chloroform/toluene as the protonated salt [(Me N)FPh P N .H] [BF 0H] 2  C  4  26 28 4 4 3 H  F  N  0P  :  C  3  '  +  3  5 2  -  7 3 ;  3  >  H  N, 9.18. M.pt. 184-188°C.  4  -  3 1  _  7 6 ;  (0.249g, 0.44 mmol). N, 9.46.  Found:  Anal, calcd. for  C, 52.96; H, 4.80;  P n.m.r. spectrum ( 6 , CDC1 , external P 0g); 3  4  87.2 ppm (2P, P(Ph ) atoms, doublet, J =15Hz), 100.6 (IP, P(NMe )F atom, 2  doublet, J =935Hz). pF  pp  2  ^ n.m.r. spectrum ( 6 , CDC1 , internal TMS); 2.77 (6H, 3  doublet, J =12.0Hz), 7.0-8.0 (20H, unresolved m u l t i p l e t ) . pH  73  CHAPTER  III  REACTIONS OF METHYLPHOSPHAZENES WITH BASES  Although there have been many investigations of the effect of different substituents on the behaviour of cyclic phosphazenes, relatively l i t t l e work has been directed towards the influence of the phosphazene ring i t s e l f on the properties of the substituents.  The results reported in this  chapter are from a study of the latter type, and are concerned with the reaction of methylphosphazenes(I) with strong bases (e.g. alkyllithiums) to give carbanion derivatives (II)  (Equation 1).  These in turn can react  with a wide variety of electrophiles, yielding novel phosphazenes (III)  of  synthetic interest. H  3  G  V  / 3 H  N  N  HC  CHf  3  N  H C.  CH R  3  N  2  N  " '  N  III  The principal conclusions of this study stem from reactions carried out using two methylphosphazenes, (1) the f u l l y methylated tetrameric phosphazene (NPMe^^, and (2) the p a r t i a l l y methylated trimeric phosphazene gem-Me^Ph^PgNg.  The details of these reactions are given in the following  sections. 3.1  Preparation and Reactions of N P Me (CHpLi ) 4  4  4  4  and N P Me (CHol_i ) 4  4  6  2  In diethyl ether solution, methyllithium reacts rapidly and  - 74 -  exothermically  with octamethylcyclotetraphosphazene, N^P^Meg, to produce  the dicarbanion N ^ M e ^ C r ^ " ^ and the tetracarbanion N^Me^CH,,")^ (equations 2 and 3). 2MeLi  + N P Me — • N^MegtCHpLi )  g  + 2CH  ...2  4MeLi  + N P Me —•+ N P Me (CH Li)  4  + 4CH  ... 3  4  4  4  8  4  g  4  4  4  2  4  4  Although these species have not been isolated, their existence, in solution, is confirmed by their reaction with a number of group IV metal chlorides Me^MCl (M=Si, Ge and Sn) to form d i - and tetra-substituted phosphazene derivatives of the type N P Me _ (CH MMe ) (x=2,4; M=Si, Ge, Sn). 4  4  g  x  2  3  They  x  also react with methyl iodide to give the tetraethyltetramethylphosphazene N  4 4 4 4 P  M e  E t  a n d  the diethylhexamethylphosphazene N P Me Et 4  hydrochloride lyyiegEtg.2HC1 )  1 7 6  4  g  (isolated as i t s  2  .  The intermediacy of a mono- and tricarbanion N P Meg_ (CH Lii ) 4  4  x  2  x  (x=l and 3) is necessary in the formation of the above mentioned species, but their presence, in ethereal solution, has not been established. Attempts to isolate mono- and tri-substituted derivatives similar to the d i - and tetra-substituted phosphazenes already mentioned were unsuccessful. On a simple elestrostatic basis, the introduction of a negative charge onto the phosphazene ring by the primary deprotonation step should retard the removal of a second proton.  However, experiments designed to prepare the  monocarbanion N P Me (CH ") indicated that the removal of a second proton 4  4  7  2  from the already deprotonated species is favoured over the primary deprotonation of unreacted N P Meg; i . e . reaction 4b is favoured over 4  reaction 4a.  4  - 75 N P Me (CH Li) + CH 4  N  4 4 8 P  Me  +  4  7  2  . . . 4a  4  \  M e L i  l  * 4 4 8 N  P  Me  +  1  sN P Me (CH Li) + CH 4  4  6  2  2  . . . 4b  4  Although these multiply charged carbanions formed from N P Meg 4  4  provide a useful insight into the reactivity of the methylphosphazene r i n g , their application to the synthesis of novel phosphazenes i s somewhat restricted.  Their reaction with electrophiles is complicated by the  p o s s i b i l i t y of competing coupling reactions, and a mixture of products is often formed, the successful separation of which depends on the relative s o l u b i l i t i e s of the different components. Preliminary attempts to form a monocarbanion from the reaction of ^3 3 6 '^* P  Me  w1  1  m e t n  y l l i t h i u m showed that, for this system also, multiple  deprotonation i s favoured over the removal of a single proton.  Attention  was therefore focused on the p a r t i a l l y substituted methylphosphazene M e ^ h ^ N - j , the deprotonation of which leads exclusively to the formation of a monocarbanion. 3.2  Preparation and Reactions of N P Ph Me(CHoLi) 3  3  4  Like N P Meg, gem-dimethyltetraphenylcyclotriphosphazene reacts 4  4  with methyl- or butyl lithium in ethereal solution, resulting in the precipitation of the monocarbanion N P Ph Me(CH Li). 3  3  4  2  This intermediate is  particularly useful in studying the v e r s a t i l i t y of phosphazene carbanions in synthesis, since i t s reaction with electrophiles (Equation 5) leads to a single product. The chemistry of this carbanion is related to that of phosphine oxide carbanion Ph P(0)CH Li (see Chapter 1) and the a - p i c o l y l anion 2  2  - 76 Me  Me  Me  \/  \/  CHf  N  ~~7 \ ^ \ — P  P  h  P  h  Vh  N  \/  N  RLi P h  Me  —  p  \  Pr>  ^\—  P  h  P  NC -H (CH Li). 4  Like the latter two a n i o n s  2  h  .P  — \ p  P/  R= Br,  1 0 4 , 1 7 7  '  N  .... 5  \ h  N  2  X  N  RX  CH R  PhCO  , N P Ph Me(CH Li) reacts 3  3  4  2  smoothly with carbon dioxide and esters, to give phosphazene derivatives containing carbonyl and carboxyl substituents. carbanions  Also like phosphine oxide  , and the polycarbanions of N ^ M e g , N P Ph Me(CH Li) reacts 3  3  4  2  with group IV monohalides to give organometal1ic phosphazene derivatives. The reaction of halogens with phosphine oxide carbanions has not been reported, but their reaction with methylene diphosphonate carbanions, to 179 give halomethylene derivatives, is well known  .  Consistently,  N P Ph Me(CH Li) reacts with bromine to give the bromomethyl phosphazene 3  3  4  2  N P Ph Me(CH Br). 3  3  4  The behaviour of the a-picolyl anion is different; i t 180  2  undergoes a coupling reaction with bromine to give 1,2-dipyridylethane 3.3  Spectra and Structure of 4 4 p _ ( 2 ) N  P  M e  x  C H  R  x  ( 2 , 4 ) Derivatives x=  Assuming that the phosphazene carbanions N P Me (CH ") and 4  N P Me (CH ~) 4  4  4  2  4  4  6  2  2  do not undergo a charge rearrangement during their reaction  with electrophiles, the structures of the d i - and tetra-substituted derivatives N P Me _ (CH R) 4  4  g  x  2  x  Ph  Ph  N  Me^Sn, COOH,  1 7 8  —  (x=2,4) r e f l e c t the structuresof their parent  - 77 -  carbanions, and hence the orientation of successive deprotonation of  3.3.1  Pi substituted Derivatives The determination of the mode of substitution in these compounds  has relied on the use of both spectroscopic and crystal!ographicmethods. 31  The presence of two signals, of equal intensity, in their  P n.m.r.  spectra is in agreement with both the vicinal (IV) and antipodal (V) arrangements, but  only the latter (V) is consistent with the t r i p l e t fine 31  structure (P-P coupling) observed for both of the  1  P signals.  The  H n.m.r.  spectra (Table 3.1) of the disubstituted compounds are also consistent with the non-geminal arrangement of the CH R groups, showing, t y p i c a l l y , the 2  distinct resonances of four equivalent geminally substituted P-methyl groups, two equivalent  PfCr^R)-methyl groups, and two equivalent P-methylene groups. 31 1  However, although the  P and  H n.m.r. spectra indicate the phosphorus  atoms at which successive deprotonations occur, they do not establish the relative orientation of the -CH^R groups. This problem has been resolved by the recent crystal structure  - 78 -  1 Table 3.1  31  3  H and  b  P n.m.r. parameters of 4 4 8 _ ^ 2 ^ x N  5 (PMe ) p  N P Me Et .2H'Cl^ 4  4  6  2  N P Me Et .2HCl* 4  4  4  4  N P Me Et 4  4  4  2  N P Me (CH SiMe ).  4  4  4  g  4  2  4  2  3  3  N P Me (CH GeMe )  2  N P Me (CH GeMe )  4  4  4  4  6  4  2  4  2  3  3  N P Me (CH SnMe ) 4  4  6  2  3  2  N P Me (CH SnMe ) 4  4  4  2  3  4  <5 (PMe) p  M e  x  C H  R  d  e  r  i  v  a  t  i  « (PMe ) 2  6 (PMe)  6 (PCH R)  H  H  H  2  v  e  s  <s (R) H  75.8 (12)  71.1 (12)  2.41 (14.0)  2.38 (13.0)  2.62 (12.0)  1-66 (22.0)  -  71.2  -  2.39 (12.0)  2.62 (M2.0)  1.68 , (21.0)  1.41 (11.5)  1.65 H2.0)  1.10 (17.0)  1.42 (13.5)  1.45 (13.0)  1.18 (18.0)  0.11  -  1.46 (12.5)  1.19 (17.0)  0.24  1.41 (13.5)  1.42 (13.5)  1.23 (16.5)  0.25  -  4  N P Me (CH SiMe ) 4  2  P  95.5 (12)  94.4 (12)  89.4 99.6 (12) 100.1 99.6 (12)  -  98.1  -  1.42 (12.0)  1 .23 (17.5)  0.19  92.8 (12)  100.0 (12)  1.45 (13.0)  1.48 (12.0)  1.27 (14.0)  0.14  -  96.5  • -  1.40 (12.5)  1.23 (12.3)  0.16  (a) 6(ppm), in C D C I q , reference internal TMS, except f in D 0, reference external TMS. J(PH), in Hertz, in parenthesis, (b) 6(ppm) reference external P 4 O 6 ; a l l in C D C I 3 , except =f in D2O. J(PP), in Hertz, in parenthesis. 6p(N P Me ) in CDCT3 is 94.4 ppm. (c) J(HH) = 7.5 Hz. (d) J(HH) = 7.1 Hz. (e) 3(HH) = 7.3 Hz. 4  4  8  C  d  -  2  r  e  - 79 -  determination of the dihydrochloride of N P M e E t 4  4  g  2  i y i  .  The molecular  geometry of this compound (Figure 3.1) shows that the two ethyl groups are arranged in a trans-antipodal fashion.  The centrosymmetrical "chair"  shaped structure i s not uncommon in phosphazene chemistry.  Its occurrence  © C I  C(2)  (a) Figure 3.1.  (b)  General views of (a) the N4P Me5Et2.2HCl structure, and (b) the chair conformation of the N4P4Me5Et2H2 cation (from H.P. Calhoun, R.T. Oakley, N.L. Paddock and J . Trotter, Can. J . Chem., 53, 2413 (1975). 4  2+  in this molecule can be understood in terms of electrostatic repulsion between the CH ~ groups of the carbanion N P Me (CH ~) . 2  4  4  6  2  2  Model calculations  on molecules with the conformations found in 8-membered phosphazene rings (saddle, D ; tub, S ; chair, C ; crown, C ) show that, for normal values 2d  4  2h  4y  of bond angles, repulsion in the dicarbanion i s least in the trans-antipodal chair conformation, as is found in the diethyl derivatives.  Coordination of  lithium ions to both the methylene groups and the ring nitrogen atoms would also be especially easy for the chair conformation (e.g.  VI).  - 80 -  3.3.2  Tetrasubstituted Derivatives 31 The appearance of a single resonance in their  and of a single P-methyl resonance in their  P n.m.r. spectra,  n.m.r. spectra, indicates  that the substitution pattern found in the tetrasubstituted derivatives N^P^Me^CCH^R)^ i s non-geminal, as expected on a simple electrostatic basis. Theoretically, the further reaction of the trans-antipodal dicarbanion to give the non-geminal tetracarbanion can give rise to two possible isomers, the c i s , c i s , c i s , t r a n s (VII), and the c i s , c i s , t r a n s , t r a n s (VIII).  Of the two,  the latter i s favoured e l e c t r o s t a t i c a l l y , but i t s existence cannot be confirmed by n.m.r. spectrometry.  However, the near mutual exclusion of  the Raman and infrared spectra of N P Me Et (Table 3.2) i s consistent with 4  4  4  4  the centrosymmetric structure VIII, but, because of possible accidental degeneracies of non-skeletal modes, the c i s , c i s , c i s , t r a n s configuration cannot be entirely excluded from consideration. In general, the infrared spectra of the tetra-substituted derivatives N P Me (CH R) are, as expected, similar to that of the parent 4  N  4 4 8 P  Me  4  ecu!e.  4  2  4  A l l contain a characteristically strong band in the  region of 1215-1250 cm" , which i s easily assigned to the v 1  c  (P=N)  - 81 -  Table 3.2  Infrared and Raman absorption frequencies (in cm ) of MeoP.Nand Me Et P N 9  4  Me 4 4 Infrared 8 P  280 w 390 s  4  4  4  Me Et P N 4  N  s  Raman 251 (6.9)  Infrared  284 (2.4) 361 (0.6)  625 m 670 w  433 s 460 sh " - 433 (0.7) 505 w - - - 507 (10.0) 630 s 588 (5.2) 736 m 698 (1.0) 760 m - X 736 (5.2) 787' w 761 (1.7) 860 s 767 (3.8) v  N  x  x  v  870 s 920 s  8  X  860 (0.9)  980 sh 995 m 1040 w 1222 v.s. (broad) 1240 sh 1290 s 1300 s 1415 w - - - - 1414 (2.8) 1422 w - . - - 1426 (1.9) 1430 w  4  790 s 860 w  4  4  Raman  435 m (broad)  710 w 750 w 767 w  4  4  x  185 (1.5) 256 (6.0) 355 (0.5) 500 (10.0) 580 (5.0) 675 (0.4)  X  712 (1.0) 790 (0.6)  v  881 v . s . 940 w 990 (1.0) 1010 w 1041 m 1045 (1.2) 1230 v.s. (broad) 1270 v.s. 1300 v . s . 1379 v.w. 1418 w 1425 (0.8) 1461 w 1465 (0.5) 2880 w 2910 w 2940 m  2915 (3-0)  2970 m  2985 (1.0)  (a) Spectra recorded on nujol and hexachlorobutadiene mulls, (b) Spectra recorded on powdered solid (relative intensities in parenthesis), (c) From T.N. Ranganathan, Ph.D., B r i t . C o l . , 1971.  0  - 82 CH R 2  RCH —p. 2  P  N-  \—-N  P  CHoR RCH  CHoR  -R  2  / \  \\-\/  P  -CH R 2  RCH  VIII  CH R 2  P,  2  / /  -CH R 2  VII  vibration.  The 6 . (CH-) vibration at 1414-1427 cm" i s complemented by 1  =r  a S  J  the deformational mode of the methylene group, the frequency of which is dependent on the mass of the atom bonded to the methylene carbon atom. In N P Me Et , 6(CH ) i s found at 1461 cm" . 4  4  4  4  1  2  For the N P Me (CH MMe ) 4  4  4  2  3  4  (M=Si, Ge, Sn.) derivatives, i t occurs in an Otherwise clear region of 1040-1110 cm" , within the limits found for this vibration in a variety of 1  XCH SiMe molecules (X=SiMe 2  3.4  3  3  182  ; CI, Cr, Sn, P b  1 8 3  ).  Structure and Spectra of N P Ph Me(CH R) Derivatives 3  The details of the  3 1  3  4  2  P and H n.m.r. spectra of the ]  N P Ph Me(CH R) derivatives are given in Table 3.3. 3  3  4  2  The  3 1  P n.m.r. spectra  a l l consist, as expected, of two signals of relative intensity 1:2  - 83 -  (PMe(CHoR):2PPh ). The proton n.m.r. spectra are also consistent with 2  expectation, and show the effect of substitution of an R group onto one of the P-methyl groups, the chemical shifts of the resulting methylene protons being broadly consistent with the electronegativity of R. T Table 3.3  31  a  H and H  R=  b  P n.m.r. parameters of N^P^Ph^MeCHpR derivatives SnMe.^  c  C00H  COPh  Br  e  6 (PMeCH R)  85.2  80.8  87.5  86.6  89.0  S (PPh )  98.2  99.3  98.3  96.8  98.2  <5 (MeP)  1.58(14.0)  f  1.52(13.5)  1.75(15.0)  1.76(15.0)  1.72(15.0)  <5 (CH P)  1.58(14.0)  f  1.37(14.0)  3.61(18.0)  2.88(17.0)  3.20(6.5)  p  2  p  2  H  H  2  g  h  (a) <5(ppm) in C D C I 3 , reference internal TMS. J(PH), in Hertz, in parenthesis. S(Phenyl) ^7.0-8.0 ppm. (b) 6(ppm), in C D C I 3 reference external P4O5. (c) See also R. Appel and 6. Saleh, Chem. Ber., 106, 3455 (1973). (d) 6 (Me Sn) = 0.02 ppm. (e) 6 (0H) = 9.38 ppm. (f) JlPH) (long range) = 1.5 Hz. (g) J(PH) (long range) = 4.0 Hz. (h) J(PH) (long range) = 2.0 Hz. H  3  H  The infrared spectra of these compounds are largely uninterpretable, the superposition of phenyl vibrations, and those of the R group, with skeletal modes making detailed assignments speculative.  However, a few  definite observations can be made regarding the difference between unsubstituted N P Ph Me and i t s derivatives N P Ph Me(CH R). 3  3  4  2  3  In a l l the compounds, to the value found for (NPPh ) 2  In N-jP^Ph^Me,,, 6  3  ^ y aS  m  1  2  - 1  (1190 cm"  1  2 4  )  and (NPMe ) 2  3  (1180 cm"  1  2 1  ).  (CH ) appears as a sharp doublet (in-phase and out-of-  g  3  1  cm" ).  4  (P=N) occurs at 1160-1200 c m , near  phase) at 1298 cm" and 1310 cm" (in M e P 0 ^1300  3  1  3  1 8 4  '  1 8 5  this band appears at  Substitution on one of the methyl groups reduces this  doublet to a sharp singlet near 1300 cm" . 1  - 84 -  In Me^PO, the infrared bands at 950 cm and 866 cm" are -1  1  assigned, respectively, to the out-of-phase rocking and wagging motions 184 185 -1 of the P-methyl groups ' . O n this basis, the two bands at 955 cm" and 871 cm" in NgP^Ph^l^ may reasonably be assigned to equivalent 1  vibrations.  Corroborating this assignment, both these bands disappear upon  substitution on one of the methyl groups. Regarding the R groups themselves, assignments can only be made with certainty for the v(C=0) frequency in N^Ph^MeCr^COPh and ^3 3 * 4^ "^2 ^"' P  P  1  e<  000  at 1695 cm" . 1  i n  *' f ie  o r m e r  it  o c c u r s  a t  1666 cm and, in the l a t t e r , -1  For NgPgPh^eCHgCOOH, the v(0H) band is not v i s i b l e .  Intra-  molecular, as well as intermolecular, hydrogen bonding, to both oxygen and nitrogen, probably accounts for the lack of resolution of this band. 3.5  Carbanion Stabilization by Phosphazenes The formation of carbanions from methylphosphazenes provides a  new method for the synthesis of phosphazene derivatives.  The ease of  successive deprotonation i s an unusual and interesting feature of this process, and indicates the importance of conjugative interactions between the ligands and the ring. The structural similarity between the carbanions formed from methylphosphazenes, methylphosphine oxides, and a-picoline i s demonstrated by equations 6a-6c. conjugation.  In a l l three cases, the carbanion i s stabilized by  In the oxide, resonance i s restricted to a single phosphoryl  group, but in the cyclic phosphazenes and pyridines, the negative charge of the anion can be diffused over the entire aromatic system.  - 85 R  „CH " / 2  \  /\  R  \ /  3  6a K  0  2  N  \//  3  0  H C 3  2  CH  ?  • P>v  N  N  N"  N'  N  6b  R—P.  R - P .  /  R  R  R  N  R— / R  R  . P - R R  . 6c  The formation of a tetracarbanion is not unique.  Tetrathiane,  (SCH,,)^, is deprotonated by butyllithium to yield the tetracarbanion - 186 (SCH")  , the s t a b i l i t y of which is attributable to the well known  4  acceptor properties of divalent sulphur.  In the present case, the a b i l i t y  of the P^N phosphazene ring to s t a b i l i z e the negative charge of the 4  M  4 4 4( 2~U P  Me  CH  1 0 n  1 S  consistent with the structural features described in  chapter 1; viz the apparent loss of electron density from oxygen and nitrogen in [NP(0Me) ] and [NP(NMe ) ] into the cyclic u-system. 2  n  2  2  n  In f a c t , in the  1imit of complete delocalization of charge from the methylene groups into the r i n g , the structure of the N^P^Me^CHg")^ anion (IX) becomes isoelectronic with the phosphazene [MeNP(0)0Me] (X) (formed by the thermal 4  107  rearrangement of [NP(0Me) ] 2  (XI).  4  ), and the tetrametaphosphate ion [ 0 " ] p  3  4  - 86 -  The analogy between the tetracarbanion N^P^Me^Cr^ )^ and the -  tetrametaphosphate ion is important, since i t allows a comparison between Me CH \//  MeC0 Me \//  9  2  H%/ G 2  N  N" \ /  M  N °'%/  e  /NH  Me^\ / \  HC 2  Me  Me  Me N \/GMe  0^  -.0^\  / \ Me 0 OMe  /%> / \ 0 CL  X  IX  ^0 \^0"  O j  /S)  MeO^\  2  "0 0 \//  X  XI  the acidic properties of N^P^Meg and tetrametaphosphoric a c i d , about which more is known.  In general, the a c i d i t i e s of the cyclic metaphosphoric  acids d i f f e r from those of linear phosphoric acids. metaphosphoric acids [HPOglg , 4  The t r i - and t e t r a -  for instance, behave as strong acids  (their k i s greater than k^ of orthophosphoric acid), and dissociate a  in dilute solution according to equation 7 [HP0 ] 3  n  *  nH + [ P 0 ] " +  3  n  n  "  .  (n=3,4)  t i t r a t i o n curve of the linear tripolyphosphoric acid exhibits three inflexion points.  By contrast, the ...7 [(H0) P0P(0H)20P(0H) ] 3  3  Not only are the terminal protons less  acidic than the central ones, but they display different a c i d i t i e s at each end, the removal of one terminal proton reducing the acidity of those at the other end, presumably by a simple charge accumulation effect.  In long  chain phosphoric acids, the terminal hydrogen atoms are too greatly separated to have any influence on each other, and their dissociation  - 87 -  constants are the same.  The non-terminal protons a l l display approximately^  the same a c i d i t y , but are, nonetheless, less acidic than the hydrogen atoms in the cyclic acids. In the light of this evidence, the behaviour of N^P^Meg as an acid is more easily rationalized.  The d i f f i c u l t y in the preparation of  mono- and tricarbanions from N^P^Meg has already been noted, the removal of the f i r s t proton apparently enhancing, or at least not i n h i b i t i n g , further deprotonation.  This behaviour is entirely consistent with the mono-  functionality of metaphosphoric acids, and indicates the effectiveness of the cyclic Tr-systems in both the (P-0) and (P=N) rings in accepting charge density from the ligands. 31 3.6  P Chemical Shifts of Heterogeneously Substituted Phosphazenes  31 When applied to the study of phosphazene chemistry, P n.m.r. spectroscopy i s a useful tool for establishing the orientation patterns of 31 p a r t i a l l y substituted derivatives.  However, the interpretation of the  P  chemical shifts of phosphazenes, in terms of the charge distribution within them, is not so straightforward, and although there exist simple empirical explanations of the shifts observed in homogeneously substituted phosphazenes, the application of the sample principles to heterogeneously substituted derivatives i s not always successful.  The importance of angular and steric 31 factors, which have a profound influence on the P chemical shifts of  The apparent weakness, as observed in t i t r a t i o n curves, of the fourth proton of [ H P 0 ] 4 > ' ' , and of successive chain protons in linear acids, is attributed to ion association with the c a t i o n . T  3  188  9  1 9 2  - 88 -  193 mononuclear phosphorus compounds  , i s simply not known for phosphazenes.  It is generally accepted that, for homogeneously substituted 31 phosphazenes (NPX2) , there i s an approximate correlation between their  P  p  chemical shifts and the a b i l i t y of the ligand X to attract electron density from the PN ring onto phosphorus (see Chapter 2, section 2.1.4.1).  This  trend is not unique to phosphazenes, but i s also observed in phosphine 194 195 196 oxides and sulphides ' , phosphorus y l i d s and bis(biphenylene)197 phosphoranes  .  In a l l these compounds the primary influence of the  ligand i s upon the ir-system within the molecule, rather than upon the 193 1igand-phosphorus bond, as is found in tervalent phosphorus compounds  ,  and results in the shielding of phosphorus being d i r e c t l y related to the electronegativity of the ligand. The origin of the effect is i l l u s t r a t e d in Figure 3 . 2 , which shows how the perturbation of one phosphorus atom (P*) in a P^N^ ring by substituents of different electronegativities affects the Tr-charge density on that phosphorus.  The variation in ligand electronegativity is simulated  by a change in the Coulomb parameter of the 3d orbital on the P* atom; accordingly, more electronegative substituents give rise to a lower Coulomb parameter, and result in an increase in the ir-charge density on the substituted (P*) atom. 31 This same argument can also be used to rationalize the shifts of some heterogeneously  substituted phosphazenes.  P chemical  In the series  RClgPgN^, for example, the shielding of the substituted phosphorus atom has been shown to increase along the series R = NMe^ < CI < OMe < OCH^CF^ < F < OEt < OPr , this order being broadly consistent with change in e l e c t r o 1  negativity of the R g r o u p  198  .  - 89 -  02n c CD CD  c o  CD >  10  a  CD  00  c  CD  -0-2 J  -1  0 g  Figure 3.2.  +1 (units of yS}  Relative charge densities (P*-P) on phosphorus atoms in a perturbed P 3 N 3 ring (assuming homomorphic interactions between 3d orbitals on phosphorus and 2p orbitals on nitrogen). Coulomb parameters (a) expressed as a function of the resonance parameter (g) such that ap = - 23. Coulomb parameter of the perturbed phosphorus atom (P*)'varied such that a * = a - 6 . p  (-3  < 6 <  p  3).  The influence of different ligands on remote phosphorus atoms is effectively demonstrated by the series of geminally substituted phenylfluorophosphazenes 2 n 6 - 2 n 3 3 ( >l»2.>3) (Table 3.4). F  Ph  P  N  n=0  Shielding of the  P(Ph ) atoms decreases steadily with increasing fluorination of the r i n g , 2  indicative of an increasing flow of u-charge.from the phenylated to the fluorinated phosphorus atoms.  Consistently, progressive phenylation of the  ring causes an increase in the shielding of the P(F ) atoms.  An equivalent  2  effect is observed in the chlorophosphazenes ^2.i\^&-2x\ "^1 ( ~ 0 , l , 2 , 3 ) n  - 90 -  (Table 3.4); i t is less marked than in the previous case, the phenyl and chlorine ligands having more nearly equal electronegativities.  Table 3.4  PP chemical h i f t s of geminally substituted phenylhalophoschemical sshifts phazenes X P h _ P N .  3 1  3  2 n  n=  6  2 n  3  3  0  X=F X=C1  1  P(PhJ '2 P ( F2J  98.2  P(Ph )  98.2  2  P(C1 ) 2  b  b  -  2  85.2  C  82.1°  106.3  C  95.9  d  94.2  93.4  d  91.9  101.l  c  98.6  C  92.5  e  d  d  (a) 6(ppm), relative to external P 4 O 6 . (b) H.P. Latscha, Z. Anorg. Allgem. Chem., 382, 7 (1968). (c) C.W. A l l e n , F.Y. Tsang and T. Moeller, Inorg. Chem., 7_, 2183 (1968). (d) B. Grushkin, M.G. Sanchez, M.V. Ernest, J . L . McClanahan, G.E. Ashby and R.G. Rice, Inorg. Chem., 4, 1538.(1965). (e) L.G. Lund, N.L. Paddock, J . E . Proctor and H.T. Searle, J . Chem. S o c , 2542 (1960).  Simple explanations such as these are not always possible. anomalous chemical shift of (NPBr ) 2  is 98.6 ppm  159  and in (NPC1 )  may be important.  2  3  3  (<5 = 157.9 ppm p  The  , but 6 in (NPF ) p  2  3  is 92.7 ppm ) shows that angular effects 200  Similarly, the decrease in the chemical shift of the  P(F ) atoms on moving from F g P ^ to F^e^P^N^ finds no explanation in 2  terms of a n-inductive effect.  Caution is therefore required in assessing  the origins of chemical shift variations between different phosphazenes. Bearing in mind the above caveat, the purpose of the present 31 section is to report the  P chemical shifts of a variety of heterogeneously  substituted phosphazenes prepared during this work, and where possible, to relate these shifts to the Tr-electron distribution within the cyclic PN skeleton.  - 91 -  3.6.1  3 1  P Chemical Shifts of XYPh^P^ Derivatives 31 The  P n.m.r. parameters of the geminally substituted tetraphenyl-  phosphazenes X Y P h ^ P ^ prepared in this work are given in Table 5.  For  the series Me(CH R)Ph P N (R=H, SnMe , Br, COPh, COOH), i t is evident 2  4  3  3  3  that, as expected, the alkyl groups are more e f f i c i e n t donors than phenyl groups, the greater shielding of the P(Ph ) atoms indicating a polarization 2  of u-charge onto them.  The effect of a change in the nature of R on the  remote phosphorus atom i s small, but on the P(CH R) atom, i t is more 2  substantial.  The total effect is most easily followed by use of the  parameter A =6(PXY) - 6(PPh ), which provides a measure of the polarization 2  of ir-charge from the P(Ph ) atoms towards the P(XY) atom. 2  For the alkyl  derivatives, A is negative, but i t s value increases along the series R=SnMe < H < COOPh * COOH < Br. 3  This order is broadly consistent with that i 179  observed for the chemical shifts of phosphonate esters RCH(P0 Pr ) 3  2  (in which 6p increases in the order R = Na < Me < H < Br < COOMe), and corresponds approximately to the order of electronegativities of the R group. For the series of compounds XYPh^P^ (X=F, CI or NMe ), the variation in 2  A with change in X or Y is more marked than i t is in the Me(CH R)Ph P N 2  derivatives (with change in R).  4  3  3  The sense and magnitude of the variation  are such as to provide a useful measure of the combined electronegativities of X and Y (e.g. E(F + NMe ) > E(C1 + CI)). 3-6.2 P Chemical Shifts of N^P.Mep „(CH MMe,) 2  3 1  0  4—4—o-X '  31 The details of the XIII) are presented in Table 1.  C  Derivatives  o-^-X  P n.m.r. spectra of these compounds (XII and As might be expected from the similarity  - 92 -  Table 3.5  J ,  P chemical s h i f t s  X  Y  3  of XYPh„P N 0  0  derivatives.  6(PXY)  6(PPh )  106.3  85.2  21.1  95.6  94.1  1.5  2  F  F  F  NMe  Ph  Ph  e  98.2  98.2  0  CI  Cl  f  93.4  95.9  - 2.5  CI  NMe  9  85.2  94.4  - 9.2  Me  CH Br  89.0  98.2  - 9.2  Me  CH C00H  86.6  96.8  -10.2  Me  CH C0Ph  87.5  98.3  -10.8  Me  Me  85.2  98.2  -13.0  Me  CH SnMe  80.8  99.3  -19.3  c  2  2  d  2  2  2  2  3  (a) <5(ppm), reference external P 4 O 6 . A l l values from CDC1 solutions. (b) A = 6(PXY) - 6(PPh ). (c) C.W. A l l e n , E.Y. Tsang and T. Moeller, Inorg. Chem., 7, 2183 (1968). (d) J(PF) = 893 Hz, J(PP) = 22 Hz. (e) H.P. Latscha, Z. Anorg. Allgem. Chem., 382, 7 (1968). (f) B. Grushkin, M.G. Sanchez, M.V. Ernest, J . L . McClanahan, G.E. Ashby and R.G. Rice, Inorg. Chem., 4, 1538 (1965). (g) J(PP) = 12 Hz. 3  2  - 93 -  RCH  ' k_i—'  RCH -4  2 —  2  I  CHgR  CH R  i  CH R 2  2  XII  XIII  R = Me, MMe (M = S i , Ge, Sn) 3  of the ligands, the differences in chemical shifts between the various compounds is small, and, possibly because of t h i s , the interpretation of the chemical shift variations in terms of a ir-inductive effect is not particularly successful. For example, the phosphorus atoms in N-P-Me-Et. are less shielded than in N^P^Meg, consistent with the expected greater donating a b i l i t y of the ethyl groups. N  4 4 4( 2 P  M e  C H  M M e  However, the chemical shifts of the  3 U derivatives indicate an unexpectedly high electro-  negativity for the (Me MCH ) group. 3  Moreover, whereas the Sp values in  2  N P Me (CH MMe ) suggest the order of electronegativities of the different 4  4  4  2  3  4  metals to be E(Sn) < E(Ge) < E(Si), the chemical shifts of the PMe atoms 2  in N P Me (CH MeMe ) indicate a reverse order*. 4  3.7  4  4  2  3  4  Experimental . The preparation of the methylphosphazenes used in the following  experiments was described in Chapter 2.  Each compound was dried before  For the series of phosphines (H M) P (M=C,Si,Ge,Sn). 3  E(Sn)  3  The chemical shifts  - 94 -  use; (NPMe ) by sublimation in vacuo and fi^Ph^Me,, by recrystallization 2 4  from ether/hexane and heating i t at 100°C/24 hours.  The various  electrophiles (e.g. Mel, Me SiCl, Me GeCl, Me SnCl, C0 , PhC0 Et and Br ) 3  3  3  2  2  2  which were reacted with the phosphazene carbanions were obtained from commercial sources.  Methyl iodide and trimethylsilyl chloride were  r e d i s t i l l e d , and trimethylstannyl chloride sublimed in vacuo before use. Bromine, available commercially in 1 ml v i a l s , was used without further p u r i f i c a t i o n , as were carbon dioxide and ethyl benzoate.  (lecture bottle, "Bone Dry" grade)  Because of the sensitivity of organolithium reagents  to oxygen and moisture, the preparations and reactions of the phosphazene carbanions were a l l carried out under an atmosphere of dry nitrogen. 3.7.1  Preparation of Methyl!ithium A l l the reactions described in this section involve the use of  methyllithium.  Although solutions of i t (in ether and THF) are commercially  available, accurate work on a small scale requires that such solutions be prepared and used immediately, since decomposition of the solvent occurs on standing.  The usual method of preparing methyl lithium is by the reaction  of methyl bromide and lithium metal (equation 8).  However, the different  method employed in this work r e l i e s on the i n s o l u b i l i t y of methyl lithium in MeBr + 2Li  *>  LiMe + Li Br  ... 8  hexane, and provides a source of methyl!ithium that is free from the potential interference of lithium halides.  - 95 -  The method i s similar to one that has already been reported 202 , and involves the reaction of methyl iodide and butyl lithium in hexane at reduced temperatures (equation 9). BuLi  In a typical preparation, methyl  Hexane <Q°C*  + Mel  Bui  + MeLi4-  ... 9  iodide was added, at -78°C, to a hexane solution containing a slight excess of n-butyllithium (prepared by the dilution  with hexane of a commercially  available solution from about 2.0 M to 0.5 M).  The mixture was stirred  under an atmosphere of nitrogen, and allowed to warm slowly to 0°C, whereupon a white precipitate of methyllithium was produced.  This  precipitate was f i l t e r e d (under nitrogen!) from the solution and dissolved in diethyl ether (previously dried by d i s t i l l a t i o n from lithium aluminium hydride).  The solution so formed was f i l t e r e d ( i f necessary) and  standardized by quenching an aliquot of i t in water and t i t r a t i n g i t against normal sulphuric acid.  The solutions of methyllithium prepared in  this way were used within 24 hours of their preparation. 3.7.2  Preparation of N P Me Et 4  4  4  4  A solution of methyllithium in ether (56.0 ml, equivalent to 39.2 mmol of MeLi) was added dropwise from a syringe, to a solution of N  4 4 ^ 8 (2-712g, 9.04 mmol) in 50 ml of ether. P  e  Effervescence was observed  to occur for about h hour following the addition of methyllithium.  The  mixture was then heated under reflux for 2 hours, when a slight turbidity was apparent in the solution.  A large excess (^25 ml) of methyl iodide was  then added to the reaction mixture, and a rapid exothermic reaction ensued.  - 96 -  The mixture was heated under reflux for a further 4 hours, and the solvent and excess methyl iodide d i s t i l l e d off to leave a white paste (10.747 g), which was dissolved in water and neutralized with sulphuric acid (residual a l k a l i n i t y = 2.15 mmol OH").  Potassium fluoride (2.9 g)  was added to the solution to precipitate lithium as i t s fluoride.  After  f i l t r a t i o n , the solution was d i s t i l l e d to dryness and the residue extracted with 3 x 30 ml of hot chloroform.  Triethylamine (10 ml) was added to the  chloroform extract, which was then shaken for 5 minutes.  Evaporation of the  solvent l e f t a crystalline mass which was extracted with 3 x 50 ml of hot hexane. N  Evaporation of these extracts yielded colourless crystals of  4 4 4 4 (2.58 g, 7.2 mmol, 80%), which was purified by sublimation in P  M e  E t  vacuo and recrystall ization from hexane. C, 40.45; H, 9.05; N, 15.72.  Found:  Anal, calcd. for -| 2 32 4 4 c  H  N  P  :  C, 40.21; H, 8.89; N, 16.00.  Melting point 65-67°C. 3.7.2.1  Isolation of N P Me Et .2HI 4  4  4  4  The treatment of the CHC1 extract in the above experiment with base 3  is important.  If this step is omitted from the procedure, the neutral  phosphazene is not obtained.  Presumably, the immediate product of the  reaction is a complex between the neutral phosphazene and lithium iodide and, upon precipitation of the lithium, the dihydroiodide N P Me Et .2HI. 4  4  4  complex is converted into the  This hydroiodide was isolated in a  4  preliminary experiment in which triethylamine was not used. recrystallized from acetonitrile as colourless cubes. 12 34 2 4 4 ' ' ' ' ' ' ' H, 5.68; I, 41.20; N, 9.19. Dec. 188-190°C.  C  H  I  N  P  :  C  2 3  5 3 ;  H  5  6 0 ;  L  4 1  4 7 ;  N  9  1 6 ,  It was  Anal, calcd. for  F o u n d :  C  '  2 3  -  1 5  ;  - 97 -  3.7.2.2  Isolation of N P Me Et .2HC1 4  4  6  2  In another preliminary experiment in which the use of t r i e t h y l amine was omitted, the chloroform extract was evaporated to leave an o i l , which was dissolved in water and eluted through an Amberlite CG400 anion exchange column (previously charged with chloride ion).  Evaporation of the  eluant gave an o i l which, upon standing, yielded a few c r y s t a l s , which were r e c r y s t a l l i z e d , under nitrogen, from acetonitrile/benzene as small hygroscopic cubes of N P MegEt2.2HCl. 4  Anal, calcd. for -|Q 30 ' 2 4 4 c  4  C, 29.94; H, 7.54; CI, 17.67; N, 13.97. CI, 18.05; N, 14.06. 3.7.2.3  Found:  H  C1  N  P  :  C, 29.64; H, 7.55;  M.pt. 231-234°C.  Preparation of N P Me Et .2HCl.HqO 4  4  4  4  This compound was prepared by saturating a solution of N  4 4 4 P  Me  E1:  4 (°-l  chloride gas.  0 7  g, 0.30 mmol) in carbon tetrachloride with hydrogen  The resulting precipitate (0.112g) was f i l t e r e d from the  solution and r e c r y s t a l l i z e d , under nitrogen, from an acetonitrile/benzene solution as colourless hygroscopic blocks. C, 32.22; H, 8.11; CI, 15.85; N, 12.53. CI, 15.90; N, 12.68. 3.7.3  Anal, calcd. for C | 3 6 2 4 4 2  Found:  H  C1  N  0P  C, 32.29; H, 8.30;  M.pt. 180-185°C.  Preparation of N P Me (CHoSiMe ) 4  4  4  3  4  A solution of methyllithium (15.0 ml, 12.0 mmol of MeLi) in ether was added, via a syringe, into a solution of N P Me (0.758g, 2.52 mmol) in 4  4  8  50 ml of ether, and the mixture heated under reflux for 2 hours under an atmosphere of nitrogen.  The mixture was then cooled to room temperature,  :  - 98 -  and an excess of Me^SiCl (2.0 ml, 15.9 mmol) added slowly to the solution. A vigorous exothermic reaction occurred, and a white precipitate was formed. The mixture was allowed to s t i r overnight under a nitrogen atmosphere, and then f i l t e r e d .  The solvent was evaporated from the f i l t r a t e to leave an  o i l which was d i s t i l l e d at M60°C/0.01 Torr to give a colourless liquid which s o l i d i f i e d on standing at room temperature.  This solid was  recrystallized by cooling a concentrated solution of i t in hexane to -23°C to give colourless blocks of N ^ M e ^ C h ^ S i M e . ^ . C  20 56 4 4 4 H  N  N, 9.50. 3.7.4  P  Si  :  C  '  4 0  '  7 9 ;  H  '  9 > 5 9 ;  N  '  9  -  5 1  -  Anal, calcd. for  F o u n d :  >  C  4 0  -  4 9  » > H  9  -  5 6  ;  M . p t . 97-102°C.  Preparation of  N P Me (CHoSiMe ) 4  4  c  3  2  A solution of methyllithium in ether (28.0 ml, 18.2 mmol MeLi) was reacted, as described above, with a solution of in 150 ml of ether.  N^Meg  (2.348 g, 7.83 mmol)  The mixture was then reacted (as described in Expt.  3.7.2) with a solution of Me SiCl (2.06 g, 19.0 mmol) in 20 ml of ether. 3  Subsequent f i l t r a t i o n of the reaction mixture and evaporation of the f i l t r a t e yielded a colourless o i l (3.54 g), which was d i s t i l l e d in vacuo (^160°C/0.01 Torr).  The resulting o i l was dissolved in a minimum of  pentane and, on cooling i t to 0°C, 0 . 5 g of unreacted N P Meg c r y s t a l l i z e d 4  from the solution.  4  The remaining solution was then cooled to -23°G and,  after several days, a colourless crystalline solid was obtained, which was recrystallized from pentane at -23°C as needles of N ^ M e g ^ h ^ S i M e ^ . Anal, calcd. for C H N P S i : ] 4  4 Q  4  4  C, 38.15; H , 8.95; N, 12.36.  2  C, 37.82; H , 9.07; N, 12.60. M.pt. 63-65°C.  Found:  - 99 -  3.7.5  Attempted Preparation of N P Me (CHpSiMe ) 4  4  7  3  A solution of methyllithium in ether (3.2 mis, 1.85 mmol MeLi) was reacted, as described above, with a solution of N P Meg (0.530 g, 4  1.76 mmol) in 50 ml of ether.  4  A solution of Me SiCl (0.232 g, 2.15 mmol) 3  in 5 ml of ether was then added, as described in Expt. 3 . 7 . 3 , to the reaction mixture.  After f i l t r a t i o n of the mixture and evaporation of the solvent  from the f i l t r a t e , a white pasty  solid was obtained.  This solid was  extracted into hexane, and the solution f i l t e r e d and allowed to slowly evaporate.  A total of 0.321 g of unreacted N P Meg (identified by i t s 4  4  infrared spectrum and melting point, 161-162°C) crystallized from the solution, leaving an o i l y residue (0.244 g).  No monosubstituted derivative  could be isolated from this o i l ; in fact, i t s  n.m.r. spectrum showed  i t to be a mixture of polysubstituted products of formulaeN P Meg_ (CH SiMe ) 4  (x=2<3).  4  x  2  The following mass balances demonstrate the probable course of  the reaction (see Text). (a) Observed:  N  Mass balance  4 4 8 P  M e  >  N  4 4 8 P  M e  N P Me . (CH S1Me )  +  4  4  8  0.530g,l.76mmol 0.321g,l.07mmol  x  2  3  x  0.244g  (b) Calculated for the reaction: N  Mass balance  4 4 8 P  M e  0.530g,l.76mmol  >  N  4 4 8 P  M e  +  0.265g,0.88mmol  ^ P Me (CH SiMe ) 4  4  6  2  3  0.390g,0.88mmol  2  3  x  - 100 -  (c) Calculated for the reaction:  >  W8 e  Mass balance 3.7.6  0.530g,l.76mmol  Preparation of N P Me 4  4  W8 e  ' *N P Me (CH S1Me )  +  4  0.396g, 1.32mmol -(CH MMe )  fl  g  4  4  2  3  4  0.259g,0.44mmol  (x=2,4, M=Ge, Sn)  3  The preparation of these compounds^ is similar to that of the derivatives N ^ M e g ^ C C r ^ S i M e ^ (x=2,4) and has been described elsewhere . 203  3.7.7 3.7.7.1  Preparation and Reactions of N P Ph Me(CH,,Li) 3  Preparation of  3  4  N^PtyiefCHoLi)  In a typical preparation, a slight molar excess (^10%) of a freshly prepared solution of methyllithium in e t h e r ^ was added, via a syringe, to a solution of N P Ph Me in ether. 3  3  4  After an induction period of  2  a few minutes, a heavy white precipitate of N P Ph Me(CH Li) was formed. 3  3  4  2  To ensure completion of the reaction, the mixture was gently heated under reflux for 2 hours before reacting i t with an electrophile. 3.7.7.2  Preparation of N P Ph Me(CH SnMe ) 3  3  4  g  3  A solution of Me SnCl (0.300g, 1.50 mmol) in 20 ml of ether was 3  ^ The author is grateful to Mr. R.H. Lindstrom for the preparation of these compounds. ^  n-Butyllithium in hexane has also been used.  - 101 -  added to a slurry of NgP^Ph^MelCh^Li) prepared from the reaction of N P Ph Me 3  3  4  ether.  2  (0.651 , 1.38 mmol) and methyl lithium (1.5 mmol) in 100 ml of  The turbidity of the mixture became less intense following the  addition.  The mixture was then l e f t to s t i r overnight in order to ensure  completion of the reaction.  Subsequent f i l t r a t i o n of the mixture and  evaporation of the f i l t r a t e yielded a colourless crystalline s o l i d , which was recrystallized from ether/hexane to give needles of N^Ph^NMCr^SnMe-j) (0.861g, 1.35 mmol, 97%).  An analytically pure sample was obtained by  recrystallization three times from hot hexane. C  29 34 3 3 H  N  N, 6.46. 3.7.7.3  P  S n :  C  '  5 4  -  ' >  7 5  H  5  -  3 9 ;  > -  N  6  6 0  -  Found:  Anal, calcd. for C, 54.88; H, 5.47;  M.pt. 146-148°C. Preparation of N P Ph MeCH C00H 3  3  4  2  Anhydrous carbon dioxide was bubbled for 15 minutes through a slurry of N^PI^MeCHgLi prepared from the reaction of N P Ph Me (0.408 g, 3  3  0.863 mmol) and methyl 1ithium (1.00 mmol) in 100 ml of ether. rapidly disappeared.  4  2  A l l turbidity  50 ml of 0.1N sulphuric acid was then added to the  mixture and the whole thoroughly shaken. The organic layer was extracted and dried over anhydrous sodium sulphate.  Subsequent evaporation of  the solvent yielded a white s o l i d , which was recrystallized from acetone as white flakes of N P Ph Me(CH C00H) (0.390 g, 0.754 mmol, 87%). 3  3  calcd. for C 8 26 3°2 3 2  H  H, 5.15; N, 7.98.  N  P  4  :  2  C, 62.67; H, 5.06; N, 8.12.  M.pt. 185-186°C (dec).  Found:  Anal,  C, 62.85;  - 102 -  •3.7.7.4  Preparation of N-faPh^MeCHpCOPh A solution of ethyl benzoate (0.525 g, 3.5 mmol) in 30 ml of  ether was added, dropwise, to a slurry of N P Ph Me(CH Li) prepared from 3  3  4  2  the reaction of N^Ph^Me,, (1.50 g, 3.17 mmol) and methyllithium (3.5 mmol) in 100 ml of ether.  An immediate lessening of turbidity was noted in the  reaction mixture, which was then l e f t to s t i r at room temperature until the following day.  50 ml of 0.1N sulphuric acid was then added to the mixture,  and the whole thoroughly shaken. over anhydrous sodium sulphate.  The organic layer was extracted and dried Subsequent evaporation of the solvent  yielded a white s o l i d , which was recrystallized from benzene/toluene and ether/hexane to give a microcrystal1ine mass of N P Ph Me(CH C0Ph). 3  calcd. for C H N 0P: 31  3Q  3  H, 5.39; N, 7.13. 3.7.7.5  C, 68.48; H, 5.26; N, 7.31.  3  4  Anal,  2  Found:  C, 68.66;  M.pt. 144-146°C.  Preparation of N^PiyieCHgBr An excess of bromine ( 0 . 5 g, 3.0 mmol) in 10 ml of ether was  added to a cold slurry (-78°C) of N P Ph Me(CH Li) prepared from the 3  3  4  2  reaction N P Ph Me (0.787 g, 1.66 mmol) with a solution of methyllithium 3  3  4  2  (1.75 mmol) in 100 ml of ether.  The mixture was then allowed to warm to  room temperature and 50 ml of dilute NaOH added to i t .  The organic layer  was separated and the aqueous layer washed with 50 ml of CH C1 (this step 2  2  was necessary because the product was not overly soluble in ether).  The  organic extracts were combined and dried over anhydrous sodium sulphate. Evaporation of the solvent then yielded a white crystalline solid (0.92 g). The  n.m.r. spectrum of this crude product showed i t to be a ^4:1 mixture  - 103 -  of product (N P Ph MeCH Br) and starting material (NgPgPh^Meg)'. 3  3  4  2  Repeated  recrystal1ization of this solid from benzene/octane and chloroform/octane produced analytically pure crystals of N P Ph MeCH Br. 3  C H BrN P : 26  25  3  3  3  4  2  C, 56.54; H, 4.56; Br, 14.47; N, 7.61.  H, 4.45; Br, 14.10; N, 7.70.  M.pt. 176-178°C.  Anal, calcd. for Found:  C, 56.88;  104 -  CHAPTER IV  PREPARATION AND REACTIONS OF AZAPHOSPHORIN DERIVATIVES  The acidic properties of methylphosphazenes described in the previous chapter reflect the s i m i l a r i t y in behaviour between cyclic phosphazenes and simple phosphoryl compounds.  In view of this analogy, and the structural  resemblance of mononuclear phosphonium salts to the quaternary salts of methylphosphazenes, a study of the reactions of bases with the latter compounds was undertaken during the present work to determine i f they could be deprotonated, l i k e phosphonium s a l t s , to give phosphazene derivatives containing a formal exocyclic double bond (I)  (Equation 1).  However, whilst the results of this study, as reported in the present chapter, confirm that deprotonation can be achieved, they also show that the f i n a l products of such reactions are not the expected ylids (I), azaphosphorin derivatives (II).  but novel  The structural rearrangement required for the  formation of these compounds is hitherto unknown in phosphazene chemistry, and i t s occurrence here is significant, since i t i l l u s t r a t e s the fact that although many properties of phosphazenes do correspond to those of mononuclear phosphorus compounds, certain features of their chemistry are unique to them alone. H H C 3  \//  CH  HC  2  3  ,CH  '  N—CH  3  \ / 3  'N  .1  II  - 105 -  4.1  Reaction of N-methyl Methylphosphazenium Salts with Bases The formal similarity of methylphosphazenium quaternary salts to  phosphonium salts led i n i t i a l l y to the use of a variety of bases commonly used for the preparation of phosphorus y l i d s as reagents for the deprotonation of the phosphazenium s a l t s .  Because of their s e n s i t i v i t y to oxygen and water,  phosphorus y l i d s are usually prepared and used in situ (e.g. in the Wittig reaction).  When the y l i d i t s e l f is to be isolated organolithium bases,  usually the reagents of choice, cannot be used, since lithium salts form complexes with y l i d s .  In spite of this complication, a few deprotonation  experiments using methyllithium have been carried out. The two bases most extensively used in this work have been potassium t-butoxide and sodium bis-(trimethylsilyl)amide.  Both have been used  successfully in the Wittig reaction ( K O t B u  and NaN(SiMe )  2 0 4 - 2 0 6  3  2  207  ) and have  an appreciable s o l u b i l i t y in hydrocarbon solvents, which are the most convenient media for achieving a clean  separation of the products.  However,  as w i l l be described in the following sections, the reactions of these two bases with N-methyl methylphosphazenium salts indicates that there is a profound difference in the behaviour of oxygen and non-oxygen bases with phosphazenium compounds. 4.1.1  Reaction of NaN(SiMe ) with N-methyl Methylphosphazenium Salts 3  2  In boiling octane or toluene, NaN(SiMe ) reacts smoothly with the 3  2  monoquaternary phosphazenium iodides (NPMe,,)^ ^.Mel and N^P^Ph^Me^Mel, removing a proton from an exocyclic P-methyl group.  However, the f i n a l products  isolated from the reactions are not the expected y l i d s III and V (Figure 4.1);  - 106 H H C 3  CH  X  H \  3  3  CH  ^CH  H G ,CH  3  3  R-P. R  N  R  3  'N  N  Base  N—CH  3  2  N  R  R = Me. Ph  R—R  'N  R  R  IV  III  H HG  GH  3  H G  3  N—CH  3  //  \/GH  H  ^ P ^  N'  3  3  P  // ^  G  H  H  3  3  G^\  // .N  N: HG 3  / \  GH  vBase ^Base \ ^  3  H  +  .. _ _. H G^ ^GH 3  /  2  /  P^  G H  J?y  H  3  G \ /  \  3 < \  /  XH X  C H  \ GH-: VI  N' H  / H G 3  3  3  3  3  N A H G 3  Figure 4 . 1 .  CH  3  Deprotonation of N-methyl methylphosphazenium quaternary salts to give exocyclic y l i d s , which then rearrange in situ to give azaphosphorin derivatives containing an endocyclic carbon atom.  - 107 -  instead, the i n i t i a l l y formed y l i d s rearrange in situ displacing the methylated nitrogen atom from the r i n g , and forming the novel azaphosphorin derivatives IV and VI (Figure 4.1). The analogous reaction between the diquaternary salt (NPMe ) .2MeX 2 4  (X=FS0 , I) and NaN(SiMe ) is less successful. 3  3  Although a reaction does take  2  place, no characterizable materials have been isolated. 4.1.2  Reaction of KOtBu with N-methyl Methylphosphazenium Salts Potassium t-butoxide can react with methylphosphazenium salts in at  least two ways.  It can act (a) as a base, removing a proton from an exocyclic  methyl group, resulting, as above, in the formation of an azaphosphorin ( i . e . IV (R=Me), VI), or (b) as a nucleophile, attacking d i r e c t l y at a phosphorus atom adjacent to the quaternized nitrogen.  The products obtained from the  reactions of (NPMe ) -MeI and (NPMe ) .2MeS0 F with KOtBu demonstrate the 2 3  4  2 4  3  occurrence of both p o s s i b i l i t i e s , and indicate that their relative importance is strongly dependent on the size and charge of the phosphazenium cation. The reaction of (NPMe ) .MeI with KOtBu i l l u s t r a t e s one possible 2 3  reaction pathway.  Abstraction of a proton does not occur; instead, t-butoxide  ion effects a nucleophilic attack on the phosphorus atom adjacent to the quaternized nitrogen cleaving the PN skeleton (Equation 2). H C  CH  3  3  H G. 3  N  N  ;  ' H C—P. N  X  N  (NHMe)(PMe N) PMe 0 2  KOtBu.  +  4>  CH  3  3  GH | tfBu 3  \ /  H^.u .  The subsequent  C t HHo  3  3  H G—P 3  HfG"W ^ ^NN ' 3  \  e  CH  H  3  3  0(tBu)  2  2  2  ... 2  or M ^ C C ^  - 108 -  elimination of isobutene (as in the reaction of chlorophosphazenes with 208 NaOtBu ) or di-(t-butyl)ether (as in the reaction of phosphonium salts with 209 t-butoxide ion  ) then produces the novel linear phosphine oxide  (NHMe)(PMe N) PMe 0 in high y i el d (92%). 2  2  2  The reaction of the tetrameric quaternary salt (NPMe,,)^.Mel with KOtBu (Equation 3) shows a marked contrast to the one just described. Nucleophilic attack of t-butoxide ion s t i l l occurs, to give the linear oxide (NHMe)(PMe N) PMe 0, but the y i e l d of this product i s low (%5%). 2  3  The principal  2  product (80%) is the c y c l i c triazatetraphosphorin Me (NHMe)P N CH, formed 7  4  3  H HqG  N—GH  V /  H  N  \ /  \  .CH-.  H C^\  //  3  H  HG 3  GH  3  G  3 3  N  •N  // H G3  3  V^tSHs  KOtBu  HG  A  (80%) GH  3  \  3  N HG 3  GH  (NHMe)(PMe N) PMe 0 (-5%) + 0(tBu) - or Me2CCH2 2  3  3  2  2  presumably by a deprotonation reaction and a phosphazene-phosphorin rearrangement (Figure 4.1). The reaction of the diquaternary ion [NPMe ) .2Me] 2  4  2+  with KOtBu  - 109 -  reflects the effect of an increase in charge on the tetrameric ring. Nucleophilic attack at phosphorus now predominates, the only product of the reaction, the linear oxide (NHMe)PMe NPMe,,0, indicating that ring cleavage has 2  occurred in two places (Equation 4).  4  HG 3  4-1-3  GH  3  Reaction of Methyllithium with N-methyl Methylphosphazenium Salts The formation of complexes between lithium halides and phosphorus  y l i d s is well known, and for this reason s a l t - f r e e solutions of ylids cannot be obtained when alkyllithiums are used to deprotonate phosphonium s a l t s .  On  the basis of n.m.r. measurements, a tetrahedral coordination of the y l i d carbon atom has been proposed, the carbon-lithium bond being substantially 210 covalent  .  It can be inferred, therefore, that lithium salts can  effectively reduce the nucleophilic character of an y l i d i c carbon atom. Accordingly, two deprotonation experiments using methyllithium as base have been carried out in order to determine i f the presence of lithium ions, complexed to the i n i t i a l product of the reaction (e.g. I l l and V, Figure 1), w i l l sufficiently s t a b i l i z e the latter to prevent i t s rearrangement into an  - no -  azaphosphorin.  However, the results of these two reactions (see below)  indicate that the presence of lithium does not interfere with the rearrangement step, s i n c e the  azaphosphorins (e.g. IV and VI, Figure 4.1) are s t i l l  produced. The two experiments, involving the reaction of methyllithium with (a) NgPgPh^Meg.Mel and (b) (NPMe ) .Mel, both required an aqueous work-up in 2  4  order to remove lithium ions from the reaction mixture.  In experiment (a), the  azaphosphorin (IV, R=Ph, Figure 1) i t s e l f was not obtained; instead, i t s hydroiodide Me(NHMe)Ph P N CH.HI was isolated^. 4  3  2  In experiment (b),  hydrolysis  of the i n i t i a l product (VI, Figure 4.1) occurred during the aqueous extraction, yielding the linear phosphazene oxide HO(PMe N) PMe 0. 2  3  2  This  hydrolysis reaction w i l l be discussed in more detail in a later section. 4.1.4  Discussion Simple Huckel M.O. calculations have shown ^ that, in quaternary 7  phosphazenium s a l t s , much of the positive charge of the cation is localized on nitrogen, as in N-alkyl pyridinium ions.  However, for both types of system,  their reaction with anionic reagents also depends on other factors. For example, the reaction of N-alkyl pyridinium salts with base can occur in different ways  '  .  (1) When a methyl group is present in the  2-position, deprotonation of that group takes place, to give a methide VII (which alkylates on carbon rather than on nitrogen, thereby conserving ring  " The formation of this s a l t , rather than the neutral compound, parallels the reaction of N ^ M e ^ C H ^ i ) and methyl iodide to give the dihydroiodide N P Me Et .2HI, and not the neutral N P Me Et . 4  4  4  4  4  4  4  4  4  - Ill  aromaticity)  212  .  -  (2) When an electron withdrawing substituent (e.g. a halogen)  i s present in the 2- (or 4-) position, the susceptibility of the molecule to nucleophilic attack at these positions is increased, and, in basic solution, 211 such salts are hydrolysed to pyridones VIII  .  If the alkyl group on  nitrogen is such that the acidity of the a-protons is enhanced by an electron withdrawal (as in the phenacyl salt [CgHgNCf^COPh^Br"), then deprotonation of the a-carbon can occur, yielding a nitrogen y l i d IX  •"N"  "  I  C H  2  '  N  I  CH  3  ^ 0 = 0 I Ph  CH Ph, 2  VIII  IX  VII  In the reactions of N-alkyl phosphazenium salts with bases, similar variations are observed.  Deprotonation of a methyl group in the 2-position can  occur, but so, also, can nucleophilic attack at phosphorus (when oxygen containing bases, such as KOtBu, are used).  The dependence of these two  alternatives on ring size is an interesting feature of these reactions, and reflects the importance of ring size variations in influencing the chemistry of phosphazenes (see Chapter 1).  The N P Me 3  3  7  +  ion is apparently more susceptible  to nucleophilic attack and ring opening than i s N^P^Me^, and i t i s believed that this tendency of the trimeric ring to cleavage may be related to the d i f f i c u l t i e s encountered in preparing trimeric alkylphosphazenes by substitution reactions.  - 112 -  In the case of N-methyl methylphosphazenium s a l t s , deprotonation of the N-methyl group does not occur.  When an N-ethyl group is present,  reaction of the salt with base (Equation 5) eliminates ethylene and yields the neutral methylphosphazene (a Hofmann elimination).  Attempts to prepare  Ag 0 2  [(NPMe ) Et]V 2  4  — - >  (NPMe )  2 4  + CH 2  ... 5  4  nitrogen ylids from methylphosphazenium s a l t s , using the N-ethylacetato derivatives [(NPMe,,)^ CH C00Et] I~, have been unsuccessful. 4  2  +  A reaction does  take place, but no characterizable materials have been isolated. The mechanism of the formation of azaphosphorins via the deprotonation of methylphosphazenium quaternary salts is not completely understood.  As mentioned e a r l i e r , i t seems l i k e l y that the i n i t i a l step in  the reaction (Equation 6) is the formation of an exocyclic y l i d (X).  The  :  subsequent rearrangement of this intermediate into the f i n a l structure probably involves the nucleophilic attack of the exocyclic methylene group on an endocyclic phosphorus atom (XI), with the displacement of the quaternized nitrogen from the ring^. The resulting azaphosphorin can then exist in two possible tautomeric forms, containing either an exocyclic methylimino (XII) methylamino (XIII) group.  or  In the absence of other effects, a P=N double bond  can be considered stronger than a P=C double bond.  This statement is  Such a 4-centre mechanism is reminiscent of that proposed for the Wittig reaction° . 5  - 113 H3C  N—Me H  N  3\ U  /  \//  /  G H  R — / R  2  N  R XII  N Me  t  N  H  RBBJ>P  rhC  i R X  N—Me  v  \  / H  XI  R  T / R  p  \  N  ^ r  R  R XIII  supported by the structures of azaphospholes , which, in solution, exist 214  completely in the P=N form (XIV) rather than the P=C form (XV).  However,  in the present system, the P=C bonded structure (XIII) is favoured (vide infra)  R  R  1  I  R—e  H  C—R-/  \  /  N=P—Me Me XIV  R2-G  C—R  \ •P—Me li  H  Me XV  3  - 114 -  over the P=N bonded structure (XII), since i t i s only in that form that c y c l i c aromaticity is attained.  However, in solution, proton transfer from the  endocyclic carbon to the exocyclic nitrogen is s u f f i c i e n t l y rapid to cause the collapse of H- H coupling (see the following section). 1  1  Rearrangements of this type are not uncommon in phosphorus chemistry.  For example, the y l i d XVI (Equation 7) isomerizes (slowly at 25°C, 215  rapidly at 120°C) into the six membered ring XIX  .  The conversion  is similar to that of the present reaction, and is suggested to be i n i t i a t e d by catalytic amounts of base, which produce the carbanion XVII. Me  Me  Me  Nucleophilic  Me  ...... 7  - 115 -  attack of this carbanion on a s i l i c o n atom, followed by ring cleavage, then generates a new carbanion XVIII, which yields the f i n a l product XIX by reprotonation. A 4-centre mechanism has also been suggested for the reaction of p  "I  c  ammonia with bis-(diphenylphosphino)methane in carbon tetrachloride yield  not the expected product [NH P(Ph )=CH-P(Ph )NH ] Cl" 2  2  2  [NH P(Ph )=N=P(Ph )CH ] Cl" (Equation 8). 2  2  2  3  /fe  Ph P  /  • \  2  PPh  3  2  /  Ph P  Cfe  2  \  + PPh NH  PPh  2  NH  Ph P  2  CH3  2  ,PPh,  2  NH  2  KH  but  CI"  2  2  \NH  Ph P'  +  This same mechanism may also have a  +  NH — ^  2  , to  « h c c l  A  P  h  2  3  P  8  p p h  2  NH  bearing on the reaction of bis(diphenylphosphino)methylamine with chloramine 217 and ammonia (Equation 9)  .  Here a l s o , a rearrangement occurs, the N-methyl  group ostensibly migrating from the central nitrogen to a terminal nitrogen atom.  As before, the driving force of the reaction is the formation of the  resonance stabilized [P=N=P] cation (cf. Chapter II, +  section 2.3), but  whether i t i s the methyl group alone that migrates (as in the Stevens 218 rearrangement ), or the entire -NMe- unit, has not been established. NH MeN(PPh L ^ [(NHMe)P(Ph )NP(Ph,)NH,] CI" + NH.Cl NH C1 <L <L d. 4 0  L  9  L  2  -..9  - 116 -  In summary, the novel phosphazene-azaphosphorin rearrangement reported here is unique in phosphazene chemistry.  The presence of s t e r i c a l l y  bulky groups on phosphorus (e.g. R=Ph, Equation 6 ) , and the torsional r e s t r i c t i o n s of a six-membered PN ring do not impede the conversion, which is thought to occur in order to reestablish an aromatic n-system in the cyclic framework.  The results of simple Huckel M.O. calculations (see Chapter V) on  the exocyclic y l i d (X) and azaphosphorin (XIII) structures support this argument. 4.2  n.m.r. and Infrared Spectra of Azaphosphorins  4.2.1  n.m.r. Spectra The numerical details of the  n.m.r. spectra of the azaphosphorins  Me _ (NHMe)P N _ CH (n=3,4) and Me^HMeJPh^P^N^CH are given in Table 4 . 1 . 2n  1  n  n  1  The deceptively simple appearance of these spectra, run at ambient temperature on samples in deuterobenzene solution, reveals l i t t l e resolution of the P-methyl signals, indicating that the perturbation of the charge distribution within the phosphazene skeleton, caused by the replacement of a nitrogen atom with a carbon atom, is small.  In Me^(NHMeJP^N^CH, for example, the P-methyl  signals appear as an unresolved multiplet.  In the case of Me (NHMe)P N CH, g  3  2  however, three closely spaced doublets, corresponding to the methyl groups on the three different phosphorus atoms, can be distinguished (Figure 4.2). The chemical s h i f t of the N-methyl protons (6=2.28-2.47 ppm) is pi q  similar to that found in N P (NHMe) (6 (NMe)=2.57 ppm ) and N-methylaniline 220 1 1 (6 (NMe)=2.67 ppm ). In a l l three types of molecule, H- H coupling between 3  H  3  g  H  - 117 (CH ) P—»-| 3  2  (CH ) P— 3  H  HC  2  * ~/ X C  P  3  3.5  3.0  2.5  2.0  1.5  1.0  S(ppm)  Figure 4.2.  31  1 P decoupled 100 MHz H n.m.r. spectrum (A) and ordinary 100 MHz >H n.m.r, spectrum (B) of Me (NHMe)P.N CH. Spectra run on samples in benzehe-dg solution. 5  ?  - 118 -  Table 4.1  "'H n.m.r. parameters and selected vibrational frequencies' of azaphosphorin derivatives. 3  3  Me (NHMe)P N CH 5  3  Me^(NHMejP^N^CH  2  6(MeN)  2.47 (14.0)  5(MeP)  1.45 (14.0)  M.42  e  6(Me P)  1.32 (12.5) 1.39 (13.5)  ^1.42  e  p  L  2.46 (13.0)  C  Me(NHMe)Ph P N CH 4  3  2.28 (14.0)  C  2  f  C  1.43 (14.5) -  d  v(N-H)  3180  3210  3195  v(C-N)  1081  1082  1067  v (P=N)  1159, 1187  1153, 1185 1209  1144, 1170 1191  (a) s(ppm), in C6D , reference internal TMS. J(PH) (in Hertz) in parenthesis. (b) v(cm"') from nujol mull spectra, assignments tentative, (c) No 'H-'H coupling observed (at ambient temperature), (d) s(Phenyl) ^ 7.0-8.0 ppm. (e) Unresolved multiplet. (f) 6 (CH) = 1.23 ppm, 6 (NH) = 1.76 ppm. 6  H  H  the N-methyl and NH protons is not observed, but in the case of the azaphosphorins Me _.| (NHMe)P N _ CH (n=3,4), the actual resonance of the NH proton is 2n  also absent.  n  n  1  For N P (NHMe)g and (NHMe)CgHg, this phenomenon is s i m i l a r , in 3  3  o r i g i n , to the lack of coupling of OH protons in alcohols, and is accounted for by assuming a rapid intermolecular exchange of the NH protons.  For the present  azaphosphorin system an exchange process is also postulated, but i t s explanation is more complicated. The chemical shift of the protons attached to y l i d i c carbon atoms varies with the nature of the other groups on carbon.  In simple ylids  - 119 -  210 (e.g. Me P=CHR  221 and Ph P=CH  3  3  ) the anionic nature of the carbon atom has  2  a large shielding effect on the CH proton (6 (CH)=-0.5 to - 1 . 0 ppm). H  When  an electron withdrawing group is present on carbon the CH proton is less 999  shielded (e.g. 6 (CH) in Ph P=CHC(0)Me i s 6.32 ppm ). In phosphorins, electron withdrawal also occurs, but to a lesser extent, and the CH resonance ?lfi 999 H  3  is usually found in the region 6 = 1.5-2.0 ppm  '  .  Accordingly, the CH  signal of Me(NHMe)Ph P N CH (in benzene solution) appears as a broadened singlet 4  3  2  (not the expected t r i p l e t ) at 6 = 1.23 ppm (Figure 4.3).  For Me  2  -| (NHMe)P N -|CH  (n=3,4), no CH resonance is observed (although the p o s s i b i l i t y that i t is obscured by the P-methyl resonances cannot be entirely excluded).  (CHOP  (CHON PNCH  HC 3  H I N-CH  \/  3  3  k  Ph  V 4.0  Figure 4.3.  3.5  3.0  2.5 8 (ppm)  2.0  1.5  1.0  100 MHz H n.m.r. spectrum of Me(NHMe)Ph-PoN„CH (C D Phenyl region is not shown ]  g  6  solution)  - 120 -  The absence of the CH (and NH) resonance from the ^H n.m.r. spectra of Me _-] (NHMe)P N _^CH (n=3,4), i t s presence (as a broadened 2n  n  n  singlet) in the spectrum of Me(NHMe)Ph P N CH, and the lack of H - H 4  3  1  2  1  coupling from the NH proton in a l l three molecules, can a l l be rationalized in terms of an intramolecular proton exchange between the exocyclic nitrogen and the endocyclic carbon (Equation 10). Proton exchange between nitrogen and carbon has already been reported for methylene diphosphine dioxides in the 223 presence of aniline  , and, in the present system, corresponds to the  equilibrium between the amino and imino tautomeric forms referred to earlier ( i . e . XII and XIII, Equation 6). Ma  \  Me—P  H I  H i  /  %  H H \ /  Me \  N—Me  /  —Ma  N —Me  . /  Me—P  P—Me  . . 10  This equilibrium i s similar to that proposed for the sulphur y l i d 3-dimethylsulphonioindole (XX), which rapidly incorporates deuterium into the S-methyl group when dissolved in deuterochloroform  .  It i s suggested that  such an exchange requires (a) the intermediacy of the tautomer XXI (Equation 11), CH S  CH  3  CH  3  S=CH  3  2  . 11  XX  XXI  - 121 -  and (b) a s u f f i c i e n t l y strong basic character to enable the compound to deprotonate chloroform*. The azaphosphorin M e S ( N P P h ) C H 2  2  4  222  '  225  (XXII) provides a  combination of the structural features found in the previous two systems. The CH resonance of this compound, as a broad singlet at 6 = 1.6 ppm, i s similar to that found in Me(NHMe)Ph P N CH, and i t s resolution into the expected 4  3  2  t r i p l e t cannot be achieved even at -30°C.  As before, proton exchange  (Equation 12) between the two possible tautomers XXII and XXIII i s believed to account for this lack of resolution.  l / N—— P  HqC—S  /  \  \  P  l / N = P  h  G-H  H G=S 2  /  N = P  V  ' Ph Ph  P  h  /  \ / H  \  /  C;  N=  . . . 12  H  P  I Ph Ph  XXII  XXIII  In simple phosphorus y l i d s , the sensitivity of P- H coupling to 210 99"i 99f\ traces of acid i s well known ' ' . For example, a rapid reversible proton exchange, caused by the addition of methanol to a solution of 31 1 Me P=CH in benzene results in the collapse of P- H coupling to the methyl 21 n groups, and the complete disappearance of the methylene signal . A similar 3  2  * The azaphosphorins Mep _i (NHMe)P Np_-|CH (n=3,4) decompose chloroform on contact. Me(NHMe)Ph4P N2CH is less basic, and i s stable in chloroform solution (but proton exchange may s t i l l occur). n  3  n  - 122 -  effect can be produced, in the absence of acid, by warming the solution to 100°C. 221 ?7f> 227  Several workers  '  '  have found that the addition of base  (e.g. aluminium oxide, butyllithium) to solutions of y l i d s suppresses such exchanges and aids in the enhancement of the otherwise unobserved fine structure of the resonances of y l i d i c protons. observed in the present work.  This effect has also been  S p e c i f i c a l l y , i t has been found that the  n.m.r. spectrum of Me^HMeJPh^N^CH i  n  pyridine (Figure 4.4) displays much  more resolution of fine structure than does the corresponding spectrum in benzene (Figure 4.3); ^H-^H coupling is observed between the NH and N-methyl protons, and the t r i p l e t structure of the CH proton is clearly v i s i b l e . 210 the case of Me-^CHp^  As in  , an increase in temperature accelerates proton  transfer; at 60°C, the spectrum of Me^HMeJPh^P-^CH shows the collapse of ^H-^H coupling for the N-methyl group and of "^P-^H coupling for the y l i d i c hydrogen.  The temperature and solvent dependence of 6(NH) is noteworthy; more  detailed study of this phenomenon may provide more quantitative information on the exchange process. The use of pyridine as solvent has no significant effect on the ^H n.m.r. spectra of methylated azaphosphorins Me^^-j (NHMe)P N _-|CH (n=3,4). n  n  These derivatives are much more basic than pyridine, and w i l l compete successfully with i t for traces of acid.  Proton exchange between the two  tautomeric forms XII and XIII is not, therefore, inhibited. In summary, the ^H n.m.r. spectra of the azaphosphorins prepared in this work are consistent with the existence, in solution, of an acid-catalysed proton exchange between two tautomeric forms.  In the strongly basic methylated  r 123 -  H3C 3  1ST Ph  H I N-CH  /  (CH )P 3  A' 'PCH,  (CHJN 3  'PNCH,  C-H  N  Ph  NH I (CH )P3  'PCH,  (CHJN ;  PNCH  2  JHNCH,  H-C  ^P 'PCH  NH I  X.  4.5  4.0  3.5  3.0  2.5  2.0  S(ppm) Figure 4.4.  1.5  1.0  220 MHz ' H n.m.r. spectrum of Me(NHMe)Ph P N CH in pyridine-d,solution, (A) at 20°C and (B) at 60°C. 4  3  2  5  - 124 -  compounds Me _-| (NHMe)P N _^CH (N=3,4), the exchange is fast enough to 2n  n  n  prevent the observation of the NH and CH resonances.  The phenylated  derivative MetNHMejPh^P^N^CH is less basic, and the exchange is consequently slower; the NH and CH resonances are therefore observed as broad unresolved singlets (in benzene).  The use of pyridine as solvent further slows the  exchange, and allows the resolution of a l l the expected coupling of the CH and NH protons. 4.2.2.  Infrared Spectra Because of the low molecular symmetry of the azaphosphorin  derivatives, their infrared spectra (on samples in nujol mulls) are complex, and the detailed assignment of vibrational modes is d i f f i c u l t and speculative. However, some vibrations (Table 4.1) can be assigned with reasonable certainty. For a l l the compounds, the degeneracy of the v(P=N) vibration is removed by the replacement of a ring nitrogen atom with a carbon atom. Consequently, the single v(P=N) vibration observed in the parent phosphazene appears as a number of bands in the corresponding azaphosphorin. The observation of a v(N-H) vibration is an important feature of a l l the spectra.  The frequency of this band at ^3180 cm  in N-alkyl phosphoramidates R P(0)NHR  -1  is similar to that found  , and indicates the existence, in the  2  solid state, of the amino tautomer (P-NHMe) rather than the imino structure (P=NMe).  This conclusion is also supported by the frequency of the v(N-C)  vibration, which appears, in a l l the azaphosphorin derivatives, in the region 1067-1082 cm" . The v a l i d i t y of this assignment is confirmed by the infrared 1  spectrum of the deuterated compound Me (NHCD^)P N^CH, in which the v(N-C) 7  A  - 125 -  -1 4 frequency is lowered to 1037 cm '. In N-alkyl phosphoramidates, the v(N-C) vibration is observed between 1020-1220  228 231 ' , but in N-methyl phosphinimines, i t occurs at much -1 232  lower frequencies ( e . g . , 848 cm  in Ph PNMe  ), because of coupling with  3  Hence the high value of v(N-C) found for the azaphosphorins confirms  6(P=N).  the existence of an exocyclic methyl ami no group (e.g. XIII) rather than an exocyclic methylimine. 4.3  Linear Phosphazene Oxides  X(PRQN^PRQO:  Their Preparation, Structure and  Spectra Phosphazenes of formula X(Ph~PN) PPh 0 have been known for some time. 2 n 2 o  The compound ClPI^PNPPhgO was f i r s t prepared by the oxidation of diphenylchlorophosphine with diphenylphosphinyl azide  .  Various attempts to lengthen the  chain in this molecule by the inclusion of more (PI^PN) units have led not to the next member, CI (PPh^N^PPh^O, but to the s a l t - l i k e compound. CI(PPh N) PPh 0 2  3  2  236  '  237  ,  The  3 1  P n.m.r. spectrum of this molecule displays  only two signals, suggestive of the c y c l i c (XXIV) rather than the linear (XXV) structure.  Its ionic nature i s also apparent from the fact that the chlorine  atom can be replaced by another anion without changing the i . r . ^ v(C-N) in MeNH is at 1044 c m - " deuteration of the methyl group. 2  ^  1 2 2 9 , 2 3 0  spectrum  , and is reduced to 973 cm  -1  upon  This is equivalent to the coupling found in simpler molecules, e.g. v(N-C)  in MeNH is at 1044 c m " 2  921 cm  -1  in M e N 0  2  234  1 2 2 9  '  2 3 0  , but is reduced to 910 cm  , and 928 cm" in MeNCO . 1  235  -1  in M e N  3  233  ,  - 126 *  f  R  R  R p_R  \  //  "  \ / R  \  //  N  N  N  R \ R4  //  R  // R /\  Yi  K  XXIV  X  XXV  237 of the cation  .  However, the reaction of CI (PI^PN^PPi^O with amines and  phenoxide ion leads to cleavage of the c y c l i c skeleton and the formation of the covalent linear oxides X(PPh N) PPh 0 (X=NH 2  3  2  2>  NHMe, NMe , OPh), the 2  structures of which are confirmed by the presence of four d i s t i n c t resonances 31 237 in their P n.m.r. spectra . Hydrolysis of the chloro-compound leads to the corresponding hydroxy derivative HO(PPh N) PPh 0 2  3  2  '  .  This compound,  and i t s congeners HO(PPh N) PPh 0 (n=l,2) have also been isolated (in low 2  yield) from other reactions  n  2  '  .  Linear structures have also been proposed  for these compounds, but no spectroscopic data has been reported to support this belief. In the present work, the preparation of the oxides MeHN(PMe N) PMe 0 2  n  (n=l,2,3), by the ring cleavage of N-methyl methylphosphazenium s a l t s , is described.  The hydroxy derivative H0(PMe N) PMe 0 has also been isolated. 2  3  2  This is the f i r s t report concerning f u l l y methylated oxides of this type, possibly because, unlike their phenylated analogues, the compounds X(PMe N) PMe 0 are a l l extremely hygroscopic, and therefore more d i f f i c u l t to 2  handle.  n  2  However, the presence in them of methyl rather than phenyl ligands  makes them potentially more informative from a spectroscopic point of view. The details of their n.m.r. and infrared spectra are given in Table 4.2.  2  - 127 -  Table 4.2.  31  1  a  b  P and H n.m.r. parameters, and selected infrared frequencies of phosphazene oxides X(PMe N) PMe 0. 0  X-  2  HNMe  HNMe  1  2  n  2  HNMe  OH^  3  3  6 P p  A  81.7  98.0  101.8  d  6 P p  B  81.2  85.3  98.3  d  6 P  c  -  80.9  87.28  <5 P  -  -  p  p  6  H  D  2 A  ( M e  P  )  6 (Me P ) H  2  B  1-22(13.5) 1.48(14.0)  6 (Me P )  -  6 (Me P )  -  H  2  H  2  c  D  e  80.7  1.20(13.5)  e  1.18(13.0)  e  1.50(14.0)  1.37(12.0)  e  1.36(13.0)  e  1.66(13.5)  \e 1.41(15.0)'  1.51(12.5) -  1.50(15.0)  6 (MeN)  2.51(13.0)  2.47(14.0)  2.49(14.0)  J(HH)  7.0  8.5  6.0  v(P=N)  1205  1220  1190,1220,1260  1202,1260  v(P=0)  1150  1135  1135  1108  v(NH)  3110  3130  3103  f  H  (a) 6(ppm), in C D C 1 , reference external P 4 O 5 . Phosphorus atoms lettered alphabetically from the oxygen atom, (b) 6(ppm), in C D 3 C N , except | in C D C 1 , reference internal TMS. J(PH), in Hertz, in parenthesis, (c) From nujol mulls, assignments tentative, (d) <S values unavailable, (e) J(PH) (long range) = 1.0 Hz. (f) v(0H) = 2500 cm" (broad). 3  3  p  1  - 128 -  For the compounds NHMe(PMe N) PMe 0 (n=l,2,3) a linear structure 2  n  2  is indicated, since a l l the phosphorus atoms, and the PMe protons, are 2  inequivalent (^H n.m.r. assignments are consistent with the results of a 31 single frequency  P decoupling experiment on NHMe(PMe N) PMe 0). 2  2  The  2  shielding of the P-methyl protons increases with their proximity to the terminal oxygen atom, consistent with a polarization of charge through the molecule towards the more electronegative end, an effect which has already 31 been suggested to account for the P chemical s h i f t variations in the 237 phenylated oxides X(PPh N) PPh 0 2  3  2  .  The <5 values of the methylated oxides p  NHMe(PMe N) PMe 0 are therefore assigned by analogy with the above trends. 2  n  2  The ^H n.m.r. spectrum of HO(PMe N) PMe 0 d i f f e r s from those of the 2  other oxides.  3  2  The presence of only two P-methyl signals indicates a rapid  i n t e r - or intramolecular hydrogen transfer between the terminal oxygens. The effect of such hydrogen bonding i s also observed in NH P(Ph )NP(Ph )NH, 31 239 which exhibits only one P n.m.r. signal , and in NHMePMe NPMe 0, where 31 2  2  2  the two  P n.m.r. signals are almost coincident.  2  2  Hydrogen bonding i s also  indicated, for a l l the compounds l i s t e d in Table 4 . 2 , by the low value of v(P=0) (1160 cm" in M e P 0 1  3  1 8 4  '  1 8 5  ) , the effect being greatest in the hydroxy  derivative (v(P=0)=1108 c m ) , as expected. 4.4 Reactions of Azaphosphorins -1  The chemical properties of the azaphosphorins prepared in this work are primarily those of a strong base.  However, in order to understand the  details of their behaviour as bases, the equilibrium between their imine and amine tautomeric forms must be considered, since, depending on the position  - 129 -  of this equilibrium, the reaction of the compounds can take place using either carbon or nitrogen as the primary basic centre.  Hence, although their  chemistry is related to that of simple y l i d s R PCH and phosphorins 3  2  (e.g. XXVI), i t also displays features which are reminiscent of the behaviour of phosphinimines. Ph  Ph  XXVI  For example, unlike simple phosphorus y l i d s , which are easily 240 241 oxidized to phosphine oxides ' , the azaphosphorins do not react with molecular oxygen. Also in contrast to simple y l i d s , but l i k e other 242 phosphorins (e.g. XXVI)  , they do not undergo the Wittig reaction, the P=C  bond being i n s u f f i c i e n t l y polar. A number of reactions of the azaphosphorins have been studied in d e t a i l , and these are discussed in the following sections. 4.4.1  Hydrolysis of Me ^ (NHMe)P N ^CH (n=3,4) 2  The hydrolysis of the azaphosphorins Me _^(NHMe)P N _^CH (n=3,4) 2n  n  n  in aqueous ethanol produces the corresponding cyclic phosphazene oxides XXVII and XXVIII.  The ready loss of the exocyclic methylamino group from the  molecules is in contrast to the hydrolytic s t a b i l i t y of aminophosphazenes, and indicates the increased susceptibility of the aminated phosphorus atom in the  130 H C 3  y  X /  ,0  H  /  N*  H CH  H H  \/CH3  / A CH  H C  3  3  3  H C  CH  3  3  / \ HC 3  XXVII  GH  3  XXVIII  azaphosphorins to nucleophilic attack (e,g, by hydroxide ion). 240 241 with the hydrolysis of simple phosphinimines  '  By analogy  , the reaction (Equation  13) probably proceeds by the preliminary protonation of either the imine or amine form.  The intermediate hydroxide then decomposes, with the elimination  of methylamine, to yield the exocyclic phosphoryl group. H I  H  Me  i  \ X %  Ma—P  Me \  N—Ma  /  H  H  . /  Me—P  ^P—Me  N—Me  P—Me  13  H 0 2  H H \ /  Me  Me—P  0  —Me  - MeNhb =-  Me \ Me—P  H  \ /• -° H  N—Me + P—Me  OH"  It i s interesting that the hydrolysis of these azaphosphorins leads to the rupture of the exocyclic P-N bond rather than the endocyclic P=C bond. By contrast, the hydrolysis of the phosphorin XXIX (Equation 14) leads to  - 131 -  pi c cleavage of the c y c l i c skeleton, both a P=C and P=N bond being broken  .  A similar mechanism may account for the anomalous hydrolysis of the lithium iodide complex of Me^NHMe^N^CH (see Section 4 . 1 . 3 ) , which gives the linear phosphazene oxide- H0(PMe N) PMe 0 ( i . e . XXV, R=Me, X=0H). 2  Ph \ Ph-  .  3  2  Ph X  Ph  P — P h  Ph H 0  MePh P0  2  . . 14  2  N  H  N  NH  e i  C i  2  Me  Ms XXIX  It is apparent that, in the azaphosphorin system, hydrolysis can occur in two ways, (1) by loss of the exocyclic methylamino group, and (2) i n i t i a l cleavage of the endocyclic P=C bond, leading to ring opening.  A  similar situation has been reported for the hydrolysis of the di-imine PhN=P(Ph )CH P(Ph )=NPh. 2  2  2  In basic solution, the expected dipho.sphine dioxide  is produced (Equation 15a), but under acidic conditions, hydrolysis results in cleavage of a central P-C bond, and the formation of the simple oxides MePh P0 and (NHPh)Ph P0 (Equation 15b) 2  2  .  In the present case also, the  course of the hydrolysis reaction may be dependent on pH.  The sensitivity of  the amine-imine equilibrium to the nature of the solvent has already been noted in the ^ n.m.r. spectrum of Me(NHMe)Ph P N CH (Section 4.2.1), and i t 4  3  2  is l i k e l y that the influence of pH on the position of this equilibrium w i l l have profound effects on the chemical behaviour of the compounds.  - 132 -  0=P(Ph )CH P(Ph )=0 • 2  2  15a  2  PhN=P(Ph )CH P(Ph )=NPh 2  2  2  (NHPh)Ph P=0 + MePh P=0 2  4.4.2  Reaction of Me  15b  2  (NHMe)P N ^CH (n=3,4) with Methyl Iodide  £  As was mentioned in Chapter 1, the reaction of phosphorus ylids R P=CHR with methyl iodide yields simple C-methylated phosphonium salts 3  [R^PCHRMe] I  .  Similarly, the azaphosphorin XXIX can be methylated  (Equation 16) to give the expected phosphonium salt XXX.  Deprotonation of this  salt with ammonia then yields the neutral C-methyl phosphorin XXXI 216 H  H i Ph—P  V  Ph  Me  ^ P —Ph Mel  N  N  I _  h  Me  p  rL  Ph V Ph-  P —Ph  h  p  MJ3  £>  C Me XXIX  N  N  HI  16  N C  i  Me  Me  XXXI  XXX  In the reaction of the azaphosphorins Me _-j (NHMe)P N _^CH 2n  n  n  (n=3,4) with methyl iodide, simple methylation does occur, but the expected phosphonium salt XXXII is not isolated.  It immediately undergoes a  transylidation reaction (Equation 17) with a second mole of starting material to give the protonated salts XXXTV and XXXV, protonation taking place on carbon rather than on nitrogen.  The occurrence of this transylidation reaction  is unexpected, and indicates that, in this system, the normal order of acidities  - 133  Me-  H  H  Me \  N—Me  H N —Me  H Me \ /  Me  . /  Me—P'  + P—Me  —Me  XXXI Me  V  Me I  Me  V  N—Me  v  -Me  H\ /H  Me  3  HG\ / N—CH -P H e'-H //N" \/CH 3  H  y  HC 7 HC  \ H i  N—Me  3  N ^  3  H r CH CH 3  3  3  3  HC 3  \  II  .N  C H  3  A  HC CH XXXV 3  XXXIV  N—Me  +P—Me  Me—P'  XXXII HC  17  3  (see Chapter 1) of C-methylated and C-protonated phosphonium salts is reversed.  However, such behaviour i s consistent with the a c i d i t i e s of  aminophosphonium s a l t s , e.g. the alkylation of simple phosphinimines is immediately followed by a proton transfer reaction with the starting material ,244 245 (Equation 18)  '  .  This s i m i l a r i t y between the protonated salts XXXIV and  XXXV and simple aminophosphonium salts is interesting, and suggests that  - 134 -  although the protonation of the parent azaphosphorins occurs on carbon, the deprotonation of the resulting salts effects the removal of an NH proton rather than a CH proton. R P=NH + RX  • [R PNHR] X  3  +  3  -  [R PNHR]V + R P=NH — • R P=NR + [RgPNH^V 3  3  . . . 18  3  R P=NR + RX  • [R PNR ]V  3  3  2  The neutral products of the reaction of Equation 17 (XXXIII) have not been isolated.  They also react with methyl iodide, and the phosphonium  salts so formed then undergo starting material.  a second transylidation reaction with the  The observed stoichiometry of the overall reaction i s as  shown in Equation 19. 3Me _ (NHMe)P N _ CH + 2MeI 2n  1  n  n  1  2Me _ (NHMe)P N _ CH.HI 2n  1  n  n  1  . . . 19 (n=3,4) 4.4.3  + unisolated neutral products  Reaction of Me,, •) (NHMe)P N ^CH (n=3,4) with Benzoyl Chloride In the reaction of the azaphosphorins Me _-| (NHMe)P N _-|CH (n=3,4) 2n  n  n  with methyl iodide, the reaction cannot be halted after the f i r s t stage ( i . e . Equation 17), and the neutral C-methyl derivatives (XXXIII) have not been isolated.  In order to confirm that that substitution does occur on  carbon rather than nitrogen, a number of reactions have been carried out between Me _ (NHMe)P N _ CH (n=3,4) and benzoyl chloride, in the belief that 2n  1  n  n  1  the i n i t i a l l y formed C-benzoylated derivatives would be less susceptible than their C-methylated analogues to further reaction.  - 135 -  In agreement with expectation, the reaction of benzoyl chloride and the tetrameric azaphosphorin, according to the stoichiometry shown in Equation 20, yields the C-benzoyl derivative Me (NHMe)P N CCOPh (XXXVI). 7  4  3  The  reaction of the trimeric derivative with benzoyl chloride is more vigorous and, as with the reaction of the azaphosphorins with methyl iodide, cannot be halted at the f i r s t stage.  Instead, the i n i t i a l l y formed C-benzoyl  derivative reacts with a second mole of benzoyl chloride (Equation 21), introducing a benzoyl group onto the exocyclic nitrogen, to give the N-benzoyl C-benzoyl azaphosphorin Me (NMeC0Ph)Pj-N CC0Ph (XXXVII). 5  2  2Me (NHMe)P N CH + PhCOCl — • Me (NHMe)P N CCOPh (XXXVI) y  4  3  7  3  . . . 20  2  + Me (NHMe)P N CH.HCl 7  4  3  3Me (NHMe)P N CH + 2PhC0Cl — > Me (NMeC0Ph)P N CC0Ph (XXXVII) 5  3  2  5  3  2  + 2Me (NHMe)P N CH.HCl 5  3  0  V  H  i  HG 3  CH XXXVI  3  XXXVII  2  Ph  21  - 136 -  4.5 4.5.1  Spectra and Structure of Azaphosphorin Derivatives n.m.r. Spectra of Azaphosphorin Hydrohalides The H n.m.r. spectra of the hydrohal ides M e _ (NHMe)P N .,CH.HX ]  2n  ]  (n=3,4; X=I,C1) and Me(NHMe)Ph P N CH.HI (Table 4.3) do not exhibit the effects 4  3  2  of proton exchange which are observed for the neutral azaphosphorins, and are  therefore  more easily interpreted.  Coupled with this simplification is  the advantage that, in the cations [Me (NHMe)P N _-|CH ] 2n-1  n  n  2  +  (n=3,4), the  cyclic charge distribution is less uniform than in the neutral compounds. The differences in the magnetic environments of the formally distinct protons are therefore greater, and allow the clear observation of individual resonances. Although the absolute assignment^of some of these resonances cannot be made with complete certainty, their interpretation in terms of the proposed structures is unambiguous. The spectra of both the s i x - and eight-membered rings show the same basic features (see Figure 4.5).  The appearance of a complex multiplet at  6 ^ 2 . 8 ppm, corresponding to the resonances of the methylene protons, is important for two reasons; f i r s t l y because i t indicates that protonation of the neutral azaphosphorins occurs on the ring carbon atom, and secondly 31 because the AB coupling pattern (as revealed in the  P decoupled spectra)  provides an absolute confirmation of the structure of the present azaphosphorin system. The magnetic inequivalence of seemingly identical nuclei has been a subject of much interest , and is commonly observed in substituted 247 248 ethanes , but is also found in compounds of t r i and penta249 250 coordinate ' phosphorus. In the present case, the " i n t r i n s i c  -  Table 4 . 3  -  n.m.r. parameters of the azaphosphorin hydrohalides Me _ (NHMe)P N -,-CH.HX and Me(NHMe)Ph P N CH.HI 3  2n  ]  n  4  Me (NHMe)P N CH.HX 5  X=  137  I  3  3  2  Me (NHMe)P N CH.HX  2  7  4  Me(NHMe)Ph P N CH.HI  3  4  3  CI  I  CI  I  2.63(13.5)  2.62(13.0)  2.57(13.5)  2.19(14.5)  b  S(MeN)  2.65(13.5)  J(H-H)  5.5  5.5  5.5  6.0  6(HN)  5.85  6.03  4.24  5.37  4.51  1.99(13.0)  1.99(13.5)  1.96(13.5)  2.04(13.5)  1.83(14.5)  1.86(13.5)  1.79(14.0)  6(MeP) 6(Me P) ?  c  D  1.79(13.5)  5.5  1.78(13.5)  1.73(14.0)  6(Me P') 9  C  1.58(14.0)  1.53(14.0)  1.53(13.5)  C  6(Me P") 2  6(CH„) L  H. rt  H B  6(H H ) A  B  c  3.74 (15.0,11.0)  R  2.78 (13.5,13.5)  15.5  -  4.09  1.51(15.0)  1.49(13.5)  1.51(15.0)  (15.5,12.5)  2.27  3.05  (14.0,13.5)  (15.5,12.5)  15.0  1.58(15.0)  1.49(13.5)  3.75  (14.0,11.0)  D  15.5  3.96 (15.0,12.0) 2.86 (15.0,13.0)  15.0  D  3.77  (f)  3.55 (f)  15.0  (a) 6(ppm), in CDCI3 solution, reference internal TMS. J(PH), in Hertz, in parenthesis, (b) Phenyl region omitted, (c) Absolute assignments of M e 2 P , MegP' and M e 2 P " protons are uncertain, (d) Methyl groups on the same phosphorus atom are magnetically inequivalent. (e) ABXY pattern, p a r t i a l l y obscured by other signals; J(PH) values are approximate, (f) J(PH) values not assigned. In CD3CN, H A = H B , 6 = 3 . 4 4 ppm, J(PH) = 1 3 . 0 Hz.  1  2  - 138 (CH ) P — * 3  H (j—CH,  H,C  ™  N  3  (CH^P  I  II HC  2  (CH^JP-  3  (CH )N 1 3  A JHNCH,  H-N  I  J  H Hu  .  A  .  L X 'PCH,  5.0  I—  4.5  4.0  3.5  3.0  2.5  —i— 2.0  1.5  1.0  8 (ppm)  igure 4.5.  The 100 MHz H n.m.r. spectra of the azaphosphorin hydroiodide Me (NHMe)P N2CH.HI, in CDC1 solution. (A) The ordinary high resolution spectrum, (B) the same spectrum with 1 drop of D?0 added, and (C) the 31p decoupled spectrum. 5  3  3  - 139 -  251 asymmetry"  of the methylene protons is caused by their interaction with  a phosphorus atom (the aminated atom) of low symmetry.  As is demonstrated by  Figure 4 . 6 , there are, t h e o r e t i c a l l y , three rotational isomers for the -CH -P(NHMe)MeN= u n i t , but, because of the constraints of the c y c l i c 2  skeleton only two (A and C) are possible.  It is readily apparent t h a t , even  allowing for equal participation of both these structures, the average environmentsof Ha and H,b are different, H  Me  H N H  it  H,  1  H  \ • N  b  H  Me  Me  V D  .  p.  ;  a  i  Me  N  Me  H a  (0  IB)  (A)  Figure 4.6.  •  I  N  Me  N  \  Possible rotational isomers of the CH -P(NHMe)MeN unit in the cations [Me -1(NHMe)P N _]CH ] (n=3,4). In these structures, the methylene carbon atom is located above the aminated phosphorus atom (not shown). 2  2n  n  n  2  +  The appearance and position of the resonance of the N-methyl protons is similar to that found in simple phosphinimine salts (see Table 2 . 4 ) , exhibiting both  3 1  P - H and ^H-^H coupling. 1  However, the addition of one  drop of D 0 to a solution of Me (NHMe)P N CH.HI in CDC1 brings about the 2  5  3  2  3  immediate collapse of ^H-^H coupling, and the loss of the NH resonance (Figure 4.5B), indicating that proton exchange between the solvent and the exocyclic nitrogen i s rapid.  - 140 -  The methylene protons are expected to be less a c i d i c , and their exchange with protic solvents i s consequently slower, but the effect is nonetheless  observable.  For example, when Me (NHMe)P N CH.HI is dissolved 5  3  2  in D^O, the methylene resonance appears as a simple t r i p l e t which, upon allowing the solution to stand, becomes less intense and f i n a l l y vanishes (after about 2 hours). 4.5.2  ]  H n.m.r. Spectra of Phosphazene Oxides  M e  2 n  -1(°) n n-1 2 ^ P  N  C H  n = 3  ' ^ 4  As in the case of their parent azaphosphorins, the structures of the oxides Me2 _-| ( 0 ) n  p  n  N  n  _ - ] 2 (n 3,4) can be represented by either of two CH  =  tautomeric forms (XXXVIII or XXXIX).  However, the appearance of  characteristic methylene signals in their ^H n.m.r. spectra (Table 4 . 4 ) , and the absence of a v(0H) band in their infrared spectra, clearly indicates that the equilibrium of Equation 22 favours the oxide structure (XXXIX).  M  v  e  Me—P  H I  0-H ^ P — Me  H H \ /  Me •«-».  xxxvin  Me—P^  0  ^P—Me  . . . 22  XXXIX  In the trimeric derivative, the methylene protons are magnetically inequivalent (as expected from structure XXXIX), and give rise to an AB coupling pattern. In the tetrameric oxide, some interaction of the methylene protons with oxygen is indicated, their equivalent resonances appearing (Figure 4.6) as 31 1  a single t r i p l e t (due to  P- H coupling).  - 141 -  Table 4.4  ^ n.m.r. parameters of phosphazene oxides Me„ (n=3,4). 3  2 n _ 1  6(CH ) 2  n = 3  1.95(13.5)  C  6(MeP)  6(Me P)  1.96(13.5)  1.61(14.0)  2  2.31(12.5)  n  <5(Me P')  b  2  1  2  6(Me P")  b  1.48(15.0)  2.11(13.5) n=4  ,(0)P N ,CH„ n-  2  b  d  1.53(15.0) 1.94(13.0)  1.63(14.0)  1.54(13.0)  e  1.56(14.0)  1.47(14.0)  e  1.47(13.0)  (a) <5(ppm) in CDCl-j solution, reference internal TMS. J(PH), in Hertz, in parenthesis, (b) Absolute assignments of Me2P» Me P' and Me2P" protons are uncertain, (c) ABXY pattern p a r t i a l l y obscured by P-methyl resonances; J(PH) values are approximate; J(H/\Hg) = 13.5 Hz. (d) Inequivalent geminal methyl groups displaying second order coupling, (e) Inequivalent geminal methyl groups. 2  The position of the equilibrium of Equation 22 is in contrast to the imine-amine tautomerism of the azaphosphorin system, and indicates that the strength of the P=0 bond in XXXXIX is sufficient to offset any increase in iT-energy achieved by c y c l i c delocalization, as in XXXVIII^. case in the phosphazenes Phg(0H) P N 2  4  4  Such is not the  (non-geminal) and Ph (0H)P N , for 5  3  3  which the hydroxy (XL) rather than the oxide (XLI) structure has been proposed  ' .  (Equation 23).  For these latter compounds, the higher  electronegativity of nitrogen (as compared to carbon), and i t s greater a b i l i t y  ^ D(P=0) in Me P=0 is 139.3 KcaV/mole, whereas D(P=N) in MeoP=NEt is 97 Kcal/mole 5 . ° 3  2  2  J  - 142  30  Z5  7,.  20  15  ID  8 (ppm)  Figure 4.7.  The 100 MHz H n.m.r. spectrum of the phosphazene oxide Me (0)P N CH , in CDC1 solution. (A) The ordinary high resolution spectrum, and (B) the 31p decoupled spectrum. 7  4  3  2  3  - 143 -  in forming ir-bonds to phosphorus, i s thought to account for the s t a b i l i t y of the hydroxy form.  P  \  h  P h  wN  / ° -  H  ^ p-v^//- ...» Ph  h  XL  4.5.3  I  N  0  P h  XLI  n.m.r. Spectra of Benzoylated Azaphosphorins As in the case of their parent azaphosphorins, the  n.m.r.  spectra of the benzoylated derivatives Me^NMeCOPhjP^CCOPh and Me (NHMe)P N CC0Ph (Table 4.5) reveal very l i t t l e resolution of the P-methyl 7  4  3  resonances, thereby indicating the uniformity of the charge distribution within the r i n g .  For the tetrameric compound ^H-^H coupling between the NH  and N-methyl protons i s observed (as i t i s in simple N-methyl amides); proton exchange between nitrogen and the benzoylated carbon i s obviously suppressed by the low basicity of the l a t t e r .  The shielding of the N-methyl  protons in Me (NMeC0Ph)P N CC0Ph (6=2.50 ppm) i s greater than in N-methyl 5  3  2  benzanilide (6=3.40 ppm^), indicating that the diffusion of lone pair density from nitrogen to phosphorus i s limited.  ^ On a sample in CDC13 solution. of N-methyl a n i l i n e . 2 5 4  The compound was prepared by the benzoylation  - 144 -  Table 4.5  n.m.r. parameters and carbonyl stretching frequencies* of Me (NMeCOPh)P N CCOPh and Me (NHMe)P N CC0Ph. 3  5  3  5  2  7  4  6(MeN)  3  6(MeP)  6(Me P) 9  C  v(C=0)  1.64(13.5) Me (NMeCOPh)P N CC0Ph 5  3  2  2.50(9.0)  1.70(15.0)  1.60(13.0)  1502  1.55(13.5)  1642  1.48(13.0) Me (NHMe)P N CC0Ph 7  4  2.46(14.0)  3  e  d  e  1530  (a) 6(ppm) in CDC1 solution, reference internal TMS. J(PH), in Hertz, in parenthesis, (b) v(C=0) in c m , from nujol mull spectra, (c) Absolute assignments are uncertain, (d) J(HH) = 5.5 Hz, 5(NH) = 2.70 ppm. (e) A l l P-methyl resonances occur as an unresolved multiplet near 6 = 1.55 ppm. 3  - 1  4.5.4  Infrared Spectra of Benzoylated Azaphosphorins As with the other azaphosphorin derivatives, the infrared spectra  of Me (NMeC0Ph)P N CC0Ph and Me (NHMe)P N CC0Ph are too complex to allow a 5  3  2  detailed interpretation.  ?  4  3  However, their carbonyl stretching frequencies  (Table 4.5) are well isolated and can be easily i d e n t i f i e d .  For both the  trimeric and tetrameric derivatives, the betaine (XLII) carbonyl frequency 84 255 256 is similar to that found in acylated y l i d s  '  '  , and reflects the  extent to which resonance structures such as XL 111 (Equation 24) weaken the C=0 bond.  The lower frequency found in the trimeric compound suggests that  electron release onto the C-benzoyl group is greater for the six-membered ring  - 145 Ph  0  Ph  V - p  0-  V ^p  /  - V  XLII  XLIII  The amide carbonyl frequency of Me (NMeCOPh)P N CCOPh (1642 cm" ) 5  3  1  2  is similar to that found in simple tertiary amides (v(C=0) in PhC0NMe is 2  1640 cm"  1  2 5 7  , and, in PhCONMePh, is 1653 cm" ^). 1  In these latter compounds,  the presence of an N-phenyl group produces an increase in v(C=0), the competitive effect of the aromatic ring for the nitrogen lone pair reducing '+  1  - 259  the influence of the ionic form Ph-N =C-0  .  The relatively low value of  the amide carbonyl frequency in Meg(NMeCOPh)P N CCOPh suggests that similar 3  2  conjugative interactions, between phosphorus and nitrogen, are limited. 4.6  Experimental Nearly a l l of the products and/or reactants in the following  reactions were found to be sensitive to moisture and, in some cases, to oxygen.  They were therefore manipulated in an oxygen-free dry box, and the  reactions themselves were carried out under an atmosphere of dry nitrogen. The methylphosphazenium quaternary salts were prepared as described in Chapter II.  Sodium metal, hexamethyldisilazane, methyl iodide and benzoyl  chloride were commercial products, and were used without further p u r i f i c a t i o n .  From the present work; the value quoted is from a sample in CC1 solution (in nujol mull, v(C=0) is 1650 c n H ) . An earlier r e p o r t gave a lower value of 1641 cm 4  258  -1  - 146 -  Potassium t-butoxide was also obtained commercially, but was purified by 165 sublimation in vacuo before use  .  The solvents hexane and octane were  reagent grade and were not subjected to any drying procedures.  Toluene,  however, was dried by d i s t i l l a t i o n from sodium, and diethyl ether by d i s t i l l a t i o n from lithium aluminium hydride. 4.6.1  Reaction of NaN(SiMe.,)o with Methylphosphazenium Quaternary Salts The following reactions a l l involve the same experimental  procedure. detail.  Therefore, only one reaction (4.6.1.1) w i l l be described in  The later experiments (4.6.1.1 to 4.6.1.5) were performed in an  analogous fashion. 4.6.1.1 Reaction of NaN(SiMe ) with (NPMe ) .MeI 3  2  2 4  Freshly cut sodium (0.200 g, 8.69 mmol) was added to a slurry of (NPMe ) .MeI (3.54 g, 8.01 mmol) in 200 ml of octane, and the mixture 2 4  heated to the reflux temperature, whereupon % h ml of HN(SiMe ) was injected, 3  via a syringe and septum, into the reaction vessel.  In practice, i t was not  necessary to add the correct stoichiometric amount of HN(SiMe ) , since i t was 3  regenerated in the course of the reaction.  9  However, to avoid possible  evaporation losses (from boiling octane), further small additions (^%ml) of HN(SiMe ) were made every 24 hours, in order to ensure an adequate supply. 3  2  After 72 hours, the reaction mixture was cooled and f i l t e r e d , and the solvent d i s t i l l e d from the f i l t r a t e to leave a white s o l i d , which was purified by sublimation at ^130°/0.01 Torr and recrystal1ization  from hot hexane to give  colourless hygroscopic prisms of Me (NHMeJP^N^CH (2.21 g, 7.04 mmol, 88%). y  - 147 -  Anal, calcd. for C g H ^ N ^ : H, 8.54; N, 17.57. 4.6.1.2  C, 34.29; H, 8.63; N, 17.77.  Found:  C, 34.44;  M.pt. 104-105°C.  Reaction of (NPMeJj.Mel with NaNfSiMeJp Powdered (NPMeg)^.Mel (5.107 g, 13.9 mmol) was allowed to react with  sodium (0.352 g, 15.3 mmol) in 200 ml of octane.  After 96 hours, the mixture  was f i l t e r e d (while s t i l l hot, to avoid recrystallization of the product) and the solvent d i s t i l l e d from the f i l t r a t e to leave a white s o l i d , which was purified by sublimation at/vl30°C/0.01 Torr and recrystal1ization from hot hexane to give colourless hygroscopic plates of Me (NHMe)P N CH (2.68 g, 5  11.2 mmol, 80%). Anal, calcd. for C y H ^ N ^ : Found: 4.6.1.3  C, 35.40; H, 8.50; N, 17.58.  3  2  C, 35.15; H, 8.43; N, 17.57.  M.pt. 140-142°C.  Reaction of MepPh^N^Mel with NaN(SiMe ) 3  2  A sample of Me Ph P N .MeI (recrystallized from ethanol and powdered 2  4  3  3  and dried at 100°C/760 Torr for 24 hours) (1.335 g , 2.17 mmol) and sodium (0.060 g, 2.61 mmol) were allowed to react together in 60 ml of boiling toluene.  After 24 hours the mixture was f i l t e r e d and the solvent d i s t i l l e d  from the f i l t r a t e to leave a white solid which was purified by recrystallization from benzene/octane to give colourless prisms of Me(NHMe)Ph P N CH (0.953 g, 4  2.01 mmol, 93%). Anal, calcd. for 2 7 2 8 4 3 C  Found: 4.6.1.4  C, 65.93; H, 5.55; N, 8.65.  H  N  P  :  C  '  6 6  -  1 0 ;  H  '  5  3  -  3 3  2  ' ' N  8  -  8 9  -  M.pt. 147-149°C.  Reaction of (NPMe ) 2MeX with NaN(SiMe ) 2 r  3  2  In two separate experiments (NPMe ) .2MeS0 F (a) and (NPMe ) .2MeI 2 4  3  2 4  (b) were allowed to react with 2 molar equivalents of sodium in boiling octane  - 148 -  In experiment (a), after heating the mixture under reflux for 72 hours, a brown octane insoluble paste was l e f t in the reaction f l a s k , and no product was isolated upon removal of the solvent from the solution.  In experiment  (b), an octane soluble extract was isolated, but this consisted of an uncharacterizable o i l . 4.6.1.5  Reaction of [(NPMeJ  3  4  . C H C 0 0 E t l V with NaNfSiMeJg2  Equimolar quantities of [NPMe ) .CH C00Et] I~ and sodium were 2  3  4  2  allowed to react in a slurry of boiling toluene.  +  After 96 hours, the solution  was separated from the residues, but evaporation of the solvent yielded only a small quantity of a brown tar. 4.6.2  Reaction of (NPMeJ .MeI with KOtBu 4  Finely powdered (NPMe ) .MeI (3.00 g, 6.80 mmol) was added to a 2 4  slurry of KOtBu (0.839, 7.39 mmol) in 100 ml of hexane and the mixture heated under reflux for 24 hours.  The mixture was f i l t e r e d and the solvent  d i s t i l l e d from the f i l t r a t e to leave a white solid (1.82 g) which was sublimed at M30°C/0.01 Torr onto a water cooled cold finger.  The sublimate  was recrystallized from hot hexane to give Me7(NHMe)P N CH (1.71 g, 5.44 mmol, 4  3  80%), which was identified by comparison of i t s infrared spectrum with that of an authentic sample, and by i t s melting point, 104-105°C.  The residual  o i l remaining in the sublimation vessel s o l i d i f i e d into a white s o l i d , which was purified by recrystal1ization from hot hexane to give colourless hygroscopic cubes of NHMe(PMe N) PMe 0 (0.11 g, 0.33 mmol, 5%). 2  C  9 28 4 4 H  N  0P  N, 16.89.  :  C  '  3 2  -  5 0 ;  > -  H  8  M.pt. 72-74°C.  3  4 9 ;  2  >  N  1 6  - 68  Found:  Anal, calcd. for  C, 32.62; H , 8.57;  - 149 -  4.6.3  Reaction of (NPMeJj.Mel with KOtBu Similarly, (NPMe ) .MeI (1.38 g, 3.76 rnrnol) was allowed to react with 2 3  a slurry of KOtBu (0.50 g, 4.46 mmol) in 50 ml of hexane.  After 24 hours, the  mixture was f i l t e r e d and the solvent removed from the f i l t r a t e to leave a colourless o i l .  Sublimation of this o i l at ^130°/0.01 Torr onto a cold  finger at -78°C yielded a white hygroscopic s o l i d , NHMe(PMe N) PMe 0 (0.886 g, 2  2  2  3,44 mmol. 92%), which was recrystallized as colourless plates by cooling a concentrated solution of i t in pentane to -23°C. C, 32.69; H, 8.62; N, 16.34.  Found:  C, 32.58; H, 8.52; N, 16.0.  M.pt.  35-37°C.  4.6.4  Reactions of (NPMe ) .2MeS0 F with KOtBu 2 4  Anal, calcd. for C^H^N^OPg:  3  In a similar experiment, (NPMe ) .2MeS0 F (2.70 g, 5.12 mmol) was 2 4  3  allowed to react with a slurry of KOtBu (1.30 g, 11.6 mmol) in 60 ml of hexane.  After 24 hours, the solution was decanted from the pasty residue,  and the solvent d i s t i l l e d from the solution to yield a white s o l i d , which was recrystallized from hot hexane to give colourless feather-like crystals of NHMe(PMe N)PMe 0 (0.927 g, 5.02 mmol, 50%). 2  2  C, 39.97; H, 8.85; N, 15.38.  Found:  Anal, calcd. for C H N 0 P : 5  ]6  2  2  C, 33.28; H, 8.94; N, 15.25.  M.pt.  73-75°C.  4.6.5  Reaction of (NPMe ) -MeI with Methyllithium 2 4  A solution of methyllithium in diethyl ether (8.3 ml, 5.5 rnrnol of MeLi) was added to a slurry of (NPMe ) .MeI (2.225 g, 5.03 mmol) in 125 ml 2 4  of ether.  After a few moments, effervescence was observed in the reaction  - 150 -  mixture.  The mixture was heated under reflux for 2 hours, and then quenched  by the addition of 20 ml of water.  Potassium fluoride (250 mg) was added to  the aqueous layer, and after f i l t r a t i o n of the solution from the precipitated lithium fluoride, the f i l t r a t e was evaporated to dryness (in vacuo).  The residual solid was extracted with benzene to yield as the  benzene soluble part a colourless, hygroscopic crystalline s o l i d , which was recrystallized from benzene/octane as H0(PMe N) PMe 0 (0.50 g, 1.5 mmol, 30%). 2  3  2  Anal, calcd. for C H N 0 P :  C, 30.10; H, 7.89; N, 13.16.  C, 29.87; H, 7.94; N, 13.08.  M.pt. 96-100°C.  8  4.6.6  25  3  2  4  Found:  Reaction of MeoPh^Nj.Mel with Methyllithium A solution of methyllithium in ether (4.4 ml, 1.54 mmol) was added  to a slurry of Me Ph P N MeI (0.784 g, 1.40 mmol) in 50 ml of ether. 2  4  3  3>  After  24 hours, the mixture was quenched with 50 ml of water, producing a white solid which was insoluble in both solvents.  This solid was washed with  benzene to leave, as a benzene insoluble part, Me(NHMe)Ph P N CH.HI (0.319 g, 4  3  2  0. 52 mmol, 34%), which was recrystallized from acetonitrile/toluene as colourless blocks.  Anal, calcd. for 2 7 3 o 3 3 C  1, 20.62; N, 6.83. Found:  H  I N  P  :  c  »  5 2  - > 7 0  H  »  C, 52.57; H, 4.80; I, 20.45; N, 6.67.  Dec. 248-252°C. 4.6.7  Hydrolysis of Me  g  ^ (NHMe)P N ^CH (n=3,4)  A sample (300-400 mg) of ^ 2 n - l (NHMe)P N -jCH (n=3,4) was dissolved e  in 25 ml of a 50/50 ethanol-water mixture and the solution l e f t to s t i r overnight.  The following day, the solvent was removed at room temperature  - 151 -  in vacuo to leave a white s o l i d .  For n=3, recrystallization of this solid  from benzene gave colourless hygroscopic blocks of Me (0)P N CH . 5  calcd. for C g H ^ O P ^ . H, 7.59; N, 12.28.  C, 31.87; H, 7.58; N, 12.39.  M. pt. 184-186°C.  3  2  Found:  Anal,  2  C, 32.20;  For n = 4, recrystallization of the  solid from hot hexane gave colourless feather-like crystals of Me (0)P N CH . 7  Anal, calcd. for C H N 0 P : g  H, 7.91; N, 13.98. 4.6.8  23  3  4  C, 31.90; H, 7.70; N, 13.95.  Found:  4  3  C, 32.12;  M.pt. 140-143°C.  Reaction of Me  ^NHMejP N ifti (n=3,4) with Methyl Iodide  2n  The addition of an excess of methyl iodide to a solution of Me _i (NHMe)P N _-|CH (n=3,4) in ether resulted in the precipitation of the 2n  n  n  hydroiodide salts Me _ (Nltfle)P N .,CH.HI (n=3,4). 2n  For n = 3, 4.33 mmol  1  (1.04 g) of the azaphosphorin was converted into 2.70 mmol (0.990 g) of the hydroiodide.  Similarly for n = 4, 1.39 rnrnol (0.438 g) of the azaphosphorin  yielded 0.973 mmol (0.430 g) of the hydroiodide. part of reacted azaphosphorin (approximately  Attempts to isolate that of the quantity of starting  material) (see Text, Section 4.4.2) which remained in solution were unsuccessful.  Both hydroiodides are air-stable crystalline solids which can  be recrystallized from C  7 21 3 3 H  I N  P  :  C  '  2 2  '  9 0 ;  acetonitrile/toluene. H  '  5 , 7 7 ;  H, 5.88; I, 34.20; N, 11.30. C H IN P : g  27  4  4  l  *  3 4  '  5 7 ;  N  '  1 1  -  For n = 3, anal, calcd. for 4 5  M.pt. 194-195°C.  '  F o u n d :  M.pt. 155-156°C.  '  2 2  '  9 6 ;  For n = 4, anal, calcd. for  C, 24.45; H, 6.15; I, 28.70; N, 12.67.  H, 6.30; I, 28.55; N, 12.57.  C  Found:  2  C, 24.52;  - 152 -  4.6.9  Reaction of Me (NHMe)P N CH with Benzoyl Chloride 7  4  3  A solution of benzoyl chloride (0.124 g, 0.88 mmol) in 10 ml of ether was added dropwise to a stirred solution of Me (NHMe)P N CH (0.555 g, 7  1.77 mmol) in 50 ml of ether. formed.  4  3  A fine white precipitate was immediately  After 3 hours the mixture was f i l t e r e d to yield a white  hygroscopic s o l i d , which was recrystallized from acetonitrile/benzene to give small colourless cubes of Me (NHMe)P N CH.HCl (0.256 g, 0.73 mmol). 7  calcd. for  C  9  H  27  C 1 N  4 4 P  H, 7.66; N, 15.87.  :  c  4  Anal,  3  » 30.82; H, 7.76; N, 15.98.  M. pt. 193-195°C.  Found:  C, 30.86;  The solvent was d i s t i l l e d from the  f i l t r a t e to leave a yellow o i l which, on drying in vacuo, s o l i d i f i e d into a yellow, a i r stable powder.  Recrystallization of this solid from hot hexane  yielded yellow mica-like plates of Me (NHMe)P N CC0Ph. 7  C  16 30 4 4 H  N  0P  N, 13.10. 4.6.10 4.6.10.1  :  C  '  4 5  -  9 8 ;  >  H  7  -  2 3 ;  N  ' 13.39.  4  3  Found:  Anal, calcd. for  C, 45.73; H, 7.30;  M.pt. 133-134°C.  Reaction of Me^(NHMe)P NpCH with Benzoyl Chloride 3  Ratio of PhCOCl:Me (NHMe)P NoCH = 1:2 5  3  A solution of PhCOCl (0.178 g, 1.26 mmol) in 20 ml of ether was added dropwise to a stirred solution of Me^NHMe^^CH (0.609 g, 2.55 mmol) in 100 ml of ether.  A white precipitate was immediately produced.  After  three hours the mixture was f i l t e r e d , and residue recrystallized from acetonitrile/benzene to give colourless hygroscopic cubes of Me (NHMe)P N CH . HC1 (0.569 g, 2.07 mmol; expected y i e l d = 1.26 mmol). 5  3  2  Anal, calcd. for C H C1N P : ?  Found:  21  3  3  C, 30.50; H, 7.68; CI, 12.86; N, 15.24.  C, 30.20; H, 7.80; CI, 12.55; N , 15.00.  M.pt. 194-197°C.  The  - 153 -  solvent was d i s t i l l e d from the f i l t r a t e to leave a yellow paste (0.26 g) from which a small quantity (0.041 g) of a white air-stable solid was obtained by the addition of hexane.  This solid was recrystallized from hot benzene/  octane to give colourless blocks of Me (NMeC0Ph)P N CC0Ph. Anal, calcd. for 5  C  21 28 3°2 3 H  N  P  :  C  '  5 6  -  3 8 ;  >  H  6  -  3 2  ' > N  9  3 9  -  3  Found:  N, 9.60.  Dec. 181-184°C.  4.6.10.2  Ratio of PhCOCl:Me (NHMe)P NoCH = 2:3 5  2  C, 56.13; H, 6.20;  3  A solution of benzoyl chloride (0.211 g, 1.49 mmol) in 20 ml of ether was added dropwise to a stirred solution of Me^NHMejP^CH (0.540, 2.25 mmol) in 100 ml of ether.  After three hours the mixture was f i l t e r e d  to leave, as above, Me (NHMe)P N CH.HCl (0.452 g, 1.64 mmol, expected yield = 5  1.49 mmol).  3  2  D i s t i l l a t i o n of the solvent from the f i l t r a t e l e f t crude  Me (NMeC0Ph)P N CC0Ph (0.261 g, 0.59 mmol, expected yield = 0.75 mmol), 5  3  2  which was purified as described above (Section 4.6.10.1).  - 154 -  CHAPTER V  MOLECULAR STRUCTURES OF METHYLPHOSPHAZENES AND METHYLAZAPHOSPHORINS  The physical and chemical properties referred to in earlier chapters provide substantial evidence for the existence of a delocalized TT-system in phosphazenes.  More detailed information concerning the extent and  nature of n-bonding comes from the study of their molecular structures, and towards this end the crystal and molecular structures of a number of the compounds prepared in this work have recently been determined.  The purpose of  the present chapter is to describe and discuss the salient features of these structures, and relate them, where possible, to the current theories on bonding in phosphazene derivatives. 5.1  Conformations of Methylphosphazenes The relatively high melting points of the methylphosphazenes are in  contrast to the properties of many other phosphazenes (NPX ) 2  to the isoelectronic series of siloxanes (0SiMe ) Zn ?fil ?fi? o  their extreme f l e x i b i l i t y  '  .  n  (e.g. F, CI), and  , which are noted for  This difference in behaviour, which can be  attributed to the high polarity of the P=N bond in methylphosphazenes^, is Although a-polarity is expected to be greater in siloxanes, the added effect of n-polarity in phosphazenes makes the P=N bond more polar (e.g. the bond moment of dimethylsiloxanes has been estimated at 2 . 2 D , whereas the P=N bond moment in phosphinimines is greater than 4 D » . T  263  2 6 4  2 6 5  - 155 -  particularly useful,from a: structural point of view, since i t allows a detailed comparison of the molecular structures of different ring sizes, and an estimate of the nature of the forces which control their conformations. To the present date, the molecular structures of four methylphos125-127 phazenes are known 5.2.  "  ; their geometries are i l l u s t r a t e d in Figures 5.1 and  As in other phosphazenes, the basic structural parameters within each  ring vary l i t t l e from their mean values (Table 5.1) (e.g. a l l the bond lengths are nearly equal, even though they are not required to be so by symmetry), and Table 5.1. Mean values of principal structural parameters in methylphosphazenes L(P=N)  (A) (NPMe )  4  (NPMe )  5  (NPMe )  7  (NPMe )  8  2  2  2  2  L(P-C)  (A)  PNP (deg)  NPN (deg)  CPC (deg)  a  1.596  1.805  132.0  119.8  104.1  b  1.586  1.804  133.0  118.6  104.3  c  1.593  1.805  134.1  117.3  103.8  c  1.578  1.797  139.9  117.2  103.5  (a) M.W. Dougill, J . Chem. S o c , 5471 (1961 ). (b) M.W. Dougill and N.L. Paddock, unpublished results, (c) S. Rettig, unpublished results.  those fluctuations which are observed can be attributed to changes in a-hybridization at phosphorus and nitrogen (as in ( N P C 1 ) ^ ' 2  iT-effects.  265  ) rather than to  The absence of any structural parameter which is dependent on the  shape of the molecules i s important, and suggests that the relative orientation of successive -N=PMe - units does not affect the extent of ir-bonding. ?  Figure 5.1.  Overall views of some methylphosphazene structures [(Me PN) , M.W.. Dougill and N.L. Paddock, unpublished results; (Me PN) . S. Rettig, unpublished results]. ' 2  ?  5  7  R  - 157 -  (c)  Figure 5.2.  Three possible conformations for 8-memberd phosphazene rings (NPX ) : (a) D (saddle), (b) S (tub), (c) C (crown). The actual structure of (Me PN) l i e s between S 4 and D l (M.W. Dougill, J . Chem. S o c , 5471 (1951 ). 2  4  2d  4  2  4  2h  2c  - 158 -  In order to understand more clearly the meaning of this conclusion, i t is useful, at this point, to describe the way in which ir-bonds can be formed in phosphazene derivatives.  If i t is assumed that the a-skeleton of 3  phosphazenes i s composed of two sp hybrid orbitals on each phosphorus and two 2 sp hybrid orbitals on each nitrogen, then ir-bonds between the ring atoms can be formed in two ways, overlap occurring between (i) the 2p orbital on 2 z  nitrogen and a 3d orbital on phosphorus, and ( i i ) the remaining sp orbital  on nitrogen and a 3d orbital on phosphorus.  hybrid  In planar molecules, the  two types of overlap give r i s e to two different ir-systems, the former being termed the out-of-plane TT  system and the l a t t e r the in-plane IT system. In a s non-planar molecules, the c l a s s i f i c a t i o n into u and n retains only i t s local a s significance and, for example, a nitrogen orbital with TT, properties w i l l a overlap with orbitals of t t symmetry on phosphorus, and vice versa. Although the contributions to bonding of the two Tr-systems are not s  14 52 required to be equal  '  , their equality is indicated by the apparent lack of  conformational preferences of many phosphazene rings.  For example, for  tetrameric derivatives, several different conformations are found, a l l of which are shown (in idealized form) in Figure 5 . 1 .  The pure  "saddle" structure,  in which the phosphorus atoms are coplanar, has been observed so far only in one 2fi7  molecule, N^P^Me^F^ (the trans-geminal isomer) transformed progressively into the N P (0Me) 4  N  4  8  C  The "saddle" can be  "tub" form, and the molecules  (close to " s a d d l e " ) , N P (NMe )g , N ^ M e g 60  4 4 V ) P  .  4  4  2  65  1 2 6  , and  (close to "tub") f a l l in this conformational series.  K  The C  2 v  "chair" conformation i s represented by another polymorphic form of N P C1 (T) 4  4  8  129  ' , and by two non-geminally substituted derivatives N^P^Ph^Cl^ 3  - 159 -  and N P Ph (NHMe) 4  4  4  4  268  .  On the basis of the above evidence, the two components of the double 7r-system in phosphazenes do not d i f f e r greatly in strength, so that between them they provide l i t t l e resistance to torsional motion about the ring bonds.  It is therefore expected that nonbonded interactions w i l l play an  important role in determining the preferred conformations of phosphazene structures.  Their influence on the structures of methoxyphosphazenes has 61 62 already been examined ' , but attempts to relate the observed shapes to s t e r i c a l l y acceptable conformations in a quantitative manner were hampered by the asymmetric nature of the ligands. In the case of the methylphosphazene structures described here, the approximately spherical shape of the methyl ligands simplifies the conformational analysis.  Even so, the minimization of a l l nonbonded interactions 269  is possible only for the 8-membered ring  ; for the larger ring s i z e s ,  numerical estimates must be restricted to calculations on parts of the molecules.  Therefore, in order to study the (NPMeg)  series as a whole, an  examination has been made of the fragment ( P N ^ P f ^ (Figure 5.3).  Similar  models have been used in the analysis of methoxyphosphazene structures  '  ,  and, in the present case, the conformational energy of the (PN^PMeg unit, as a function of the two angular variables i]^ and ^  provides an insight into  the preferred local conformations at successive phosphorus atoms in an (NPMe ) ring. 2 n  The j u s t i f i c a t i o n for this treatment of the molecules as a set  of such fragments  is i t s success in accounting for the main qualitative  features of the observed structures.  - 160 r  (a)  Figure 5 . 3 .  (b)  (c)  (d)  Angular conventions for the measurement of torsion angles in a (PN) PMe unit. The angles i> have the same sign when the terminal phosphorus atoms (P*)'Tie on opposite sides of the NPN plane. A number of idealized conformations are also i l l u s t r a t e d (a) GG ^ = ^=60°), (b) GT (^,=60°, ^ =180°), (c) CT (rh=0°, ^2=180°), (d) GG' ( ^ = - 6 0 ° , ^=60°). 2  2  }  2  2  1  The potential functions used to simulate van der Waals interactions 1  were of the Lennard-Jones and Devonshire (6-12) type, and were minimized not 270 at the normal van der Waals distance 1  R  Q  , but at a s l i g h t l y greater distance  (see footnote, Table 5.2 for d e t a i l s ) .  This larger value R was used for Q  two reasons, f i r s t l y to take into account the bulky nature of the P* "atoms" (Figure 5.2), which are in fact PMe N groups, and secondly to allow for the 9  - 161 -  Table 5.2.  Interatomic potential constants used for the calculation of van der Waals interactions in a (PN) PMe unit. 9  9  Interaction  9  A  B  P...P  R 0 4.8  3.8473  3.4613  Me...P  4.4  3.6712  3.4382  N...P  3.9  3.3647  3.2584  (a) Potential V = (A/r) - (B/r) , kcal m o l for r in A . Constants by method of R.A. Scott and H.A. Scheraga (J. Chem. Phys., 42_, 2209 (1965)); B from Slater-Kirkwood expression and A by minimization of V at R (A). R equal to sum of van der Waals radii of interacting atoms; for Me and N, normal values (A. Bondi, J . Phys. Chem., 68, 441 (1964)) were used, but for phosphorus the value was increased by 0.5 A (see Text). 12  6  -1  0  0  difference between the equilibrium distances of a pair of atoms when isolated 271 and when in a condensed phase  .  The potential energy diagram for the  (PN^PMeg unit based on the use of these functions is shown in Figure 5.4(a). As expected i n t u i t i v e l y , the conformational energy is minimized when a l l the bonds are mutually staggered.  There are two minima, corresponding  approximately to the GG and CT conformations^ shown in Figure 5.3.  Using the  parameters indicated, the GG structure i s favoured over the CT by about 1% kcal/mole, and even when the model and functions are varied, GG maintains the lower  energy. However, this order can be reversed or equalized (depending on the  particular numerical choice) by the inclusion of Coulombic interactions (as Terminology from M i z u s h i m a ^ ; the terms GT, CT, etc. refer to the approximate orientation of NP* bonds (Gauche-Trans, Cis-Trans, e t c . ) ; see Figure 5.3. T  r> 162 r  (a) Figure 5.4.  (b)  Potential energy (a) and electrostatic energy (b) contours (kcal mol*"') of (PN) PMe unit as a function of ^ and i|)o (energies quoted are r e l a t i v e ) . Electrostatic energy (Ej calculated using q(P) = +"s,.q(N) = -h» E = Kz(q(P).q(N))/r (K = 3.3207xl0 kcal.A.mol"'). The (PN) PMe model used has L(P=N) = 1.59A, L(P-C) = 1.80A, PNP = 135°, NPN = 120°, CPC = 105°. 2  2  PN  2  2  ?  point charges q(P)(+) and q(N)(-) on phosphorus and nitrogen).  Such a  consideration has been found necessary to rationalize the structures of 273 274 polypeptides  '  , and in view of the substantial polarity of the P=N bond  in (NPMe ) , is probably equally important in the present system. 2  n  Because of  ignorance of the total charge d i s t r i b u t i o n , the extent of Coulombic interactions cannot be estimated as accurately as van der Waals potentials; the combined effect of the two types of interaction is therefore unknown.  Nonetheless, the  effect of electrostatic forces (Figure 5.4(b)) is such as to s t a b i l i z e the structures in the region of GT and CT with respect to the energy of the GG conformation.  - 163 -  Table 5.3.  Observed torsion angles and local conformations in methylphosphazenes. 3  Molecule (symmetry)  Atom  (NPMe ) (S ). 2  4  4  (NPMe ) (C ) 2  5  1  2  8  4v  GG  GG GG'M3T  P  2  32.0 -128.2  3 4 5  - 79.3 119.1  64.1 11.0 16.4 39.1 -73.0  36.1 ±156.4  68.7 +46.3  GG GT  ± 57.4  ±72.7  GG  ±166.8  +36.1  GT  61.3  27.8 76.8  GG  67.9  l 2,2' p ^3,3' 4,4' P  P  2  GG  l  P  (NPMe ) (C )  54.7  P  P  7  31.5  2  l,2,3,4  P  2  * (deg)  P  P  (NPMe ) (C ) ..:  Conformat  *1(deg)  P  P  l,3,5,7 2,4,6,8  156.5  <J  ^GG  GT  (a) The signs of th (see Fig. 5.3) are such that a clockwise rotation has a positive sign. In the case of (NPMe ) , the atoms P - (n=2-4) are related by a two-fold axis through Py, the values of ih a r e ' i d e n t i c a l , but their signs are reversed. ' 2  2  7  n  2  n  -  164  -  The effectiveness of the (PN^PMe^^ model in predicting the preferred local conformations is confirmed by the actual torsion angles (defined in terms of ty^ and i ) ^ ) observed in the methyl phosphazenes.  Their values (Table  5.3) show that the majority of local conformations can be c l a s s i f i e d as approximately GG or GT.  Only in the case of (NPMe ) , which has a low 2  5  overall molecular symmetry, are significant deviations observed.  The  predominance of GG in the tetramer can be attributed to c y c l i c constraints, i . e . i t is impossible to close an 8-membered ring containing a CT conformation at one phosphorus atom.  However, on progressing to larger rings,  such restrictions are removed, and an increasing abundance of GT conformations is found. It is suggested that further work (already in progress) on the structures of even larger methylphosphazene rings (e.g. (NPMe )g 2  an increasing proportion of GT and CT conformations.  10  ) w i l l show  This belief i s supported 0-7C  by the known structures of the high polymers (NPF,,)^ (conformer B)  and  ?7fi  (NPC12)M  5.2  , both of which exist as planar (or nearly so) CT chains.  Structures of Azaphosphorin Derivatives The fact that the conformational trends observed in phosphazene  structures can be adequately interpreted in terms of nonbonded interactions supports the belief, that the two Tf-systems (TT and TT_ ) within the cyclic PN a s framework are energetically equivalent. More quantitative evidence concerning their relative importance has been obtained from the recently determined 277 molecular structures in the previous chapter.  of a number of the azaphosphorin derivatives described  - 165 -  (c) Figure 5.5.  Overall geometries and principal structural parameters of methylazaphosphorins. (a) Me (NMeC0Ph)P N CC0Ph. (b) Me (NHMe)P N CC0Ph. (c) Me (NHMe)P N CH. [S. Rettig and H.P. Calhoun, unpublished results]. 5  7  4  3  3  7  4  3  2  - 166 -  Table 5.4.  Values of P=C and P-C (mean) bond lengths in a selection of phosphorus y l i d s .  Compound  L(P=C).  Ref  (A)  (A)  1.629  1.832  1.633  1.837  Ph P=C=C=0  1.648  1.805  b  Ph P=CH  1.661  1.823  c  Ph P=C=C=S  1.677  1.795  d  Ph P=C=C(0Et)  1.682  1.832  e  1.713  1.799  f  Ph P=C-CH=CH-CH=CH  1.718  1.806  g  Ph P=CIC0Ph  1.71  1.786  h  Ph P=CClC0Ph  1.736  1.806  h  1.743  1.799  i  Ph P=C=PPh 3  3  3  3  2  3  3  Ph P=C-CF -CF -CF 3  2  2  3  3  3  Ph(C H ) P=C(CN) 3  (a) (b) (c) (d)  L(P-C)  7  2  2  2  a  A.T. Vincent and P.J. Wheatley, J . Chem. S o c , Dalton, 617 (1972). J . J . Daly and P.J. Wheatley, J . Chem. S o c , A, 1703 (1966). J . C . J . Bart, J . Chem. S o c , B, 350 (1969). J . J . Daly, J . Chem. S o c , A, 1913 (1967).  (e) H. Burzlaff, U. Voll and H.J. Bestmann, Chem. Ber., 107, 1949 (1974). (f) M.A. Howells, R.D. Howells, N.C. Baenziger and D.J. Burton, J . Amer. Chem. S o c , 95, 5366 (1973). (g) H.L. Ammon, G.L. Wheeler and P.H. Watts J r . , J . Amer. Chem. S o c , 95, 6158 (1973). :  (h) F.S. Stephens, J . Chem. S o c , 5640, 5658 (1965). (i) W. Dreissig, H.J. Hecht and K. P l i e t h , Z. K r i s t a l l o g r . , 137, 132 (1973).  - 167 -  The overall geometries and principal structural parameters of the compounds to be discussed are given in Figure 5.5.  As in the structures of  phosphorus ylids containing electron withdrawing substituents on carbon (Table 5.4),  the P=C bond lengths found in the azaphosphorin Me (NHMe)P N CH ?  4  3  (mean value 1.716 K) are longer than the value of 1.66 K predicted for a P=C 278 double bond  , and indicate the extent to which electron density is  delocalized from carbon into the phosphazene ring.  ;  As expected, charge  d e r e a l i z a t i o n is enhanced, and the P=C bond lengthened, by the substitution of a benzoyl group onto the endocyclic carbon (Table 5.5).  Conjugation with  the C-benzoyl group is also indicated by the long C=0 bond (a normal C=0  A ) and the short central 1.48-1.49 A ) , but, because of the  double bond is 1.23 1.46  A  2 7 9  ,  2 7 8  2 8 0  C-C)bond (L((sp )C-C(sp )) = 2  2  competing influence of the  phosphazene r i n g , i t s extent is less than that found in simple C-benzoylated ylids  2 8 1  (Table 5.5).  Table 5.5.  Bond lengths and torsion angles (T) in C-benzoylated y l i d s  Compound  L(P=C)  L(C-C)  L(C=0)  (A)  (A)  (A)  (deg)  x(C-Ph) "(deg)  T(C-C)  Me (NMeC0Ph)P N CC0Ph  1.765°  1.431  1.244  7.8  33.5  Me (NHMe)P N CC0Ph  1.760  1.417  1.246  29.4  41.6  Ph P=CIC0Ph  1.71  1.36  1.28  11  51  1.736  1.35  1.301  5  7  3  3  4  3  b  Ph P=CClCOPh ?  b  a  2  a  C  (a) S. Rettig, unpublished results. J. Chem. S o c , 5640, 5658 (1965).  (b) Mean value,  4.7 (c) F.S. Stephens,  57.6  - 168 -  Probably for steric reasons, the P=C-C(0)Ph unit is not planar; torsion about the central C-C bond i s small enough to allow conjugation between the P=C and C=0 bonds, but, as the C(0)-Ph distances r e f l e c t , conjugation with the phenyl group is prohibited by a large torsion angle, as 281 in simple C-benzoylated y l i d s (Table 5.5). Conjugation between the ring and the exocyclic methylamino group is indicated by the shortness of the exocyclic P-N bond, which, at 1.653 A, is shorter than the average value found for the exocyclic bond in dimethylaminophosphazenes^ . -67  Substitution of a benzoyl group onto the nitrogen  lengthens the P-N bond dramatically (L(P-N) in Me (NMeCOPh)P N CCOPh is o 5  3  2  1.742 A), indicating the effectiveness of the N-benzoyl group in competing with the phosphazene ring for electron density from nitrogen.  The extent of  conjugation in the benzamide moiety was also noted in the increased shielding of the N-methyl protons and the low v(C=0) frequency of the amide carbonyl group (see Chapter IV, Sections 4.5.3 and 4.5.4). The above features are l o c a l i z e d , and do not depend on a unique property of the phosphazene r i n g , nor do they provide any direct information on the nature of cyclic ir-bonding.  In the l a t t e r regard, the P=N bond lengths  in the azaphosphorin rings are more useful (Table 5.6). The discussion of these distances is best approached by assuming that the azaphosphorin rings are homogeneously substituted on a l l the phosphorus atoms.  S t r i c t l y speaking this assumption i s incorrect, but the inhomogeneity  caused by the presence of the amine (or amide) ligand does not appear to offset significantly the twofold symmetry of the P N -,C rings. n  n  - 169 -  Table 5.6.  Mean bond lengths structures.  Compound  X  Me (NHMe)P N CH 7  4  3  4  P  1.760  C  3  " 1  1.716  b  Me (NHMe)P N CC0Ph 7  in azaphosphorin and protonated phosphazene  N P  1.608  1.602  P -N 2 2 1.590  ( 0.008)  ( 0.002)  (-0.010)  p  r  N  i  r  1.591 ( 0.000)  Me P N H 8  4  4  +  1.695  d  1.538 (-0.040)  Me (NMeCOPh)P N CCOPh 5  3  (Me N) P N H 2  6  3  3  +  C  2  1.765 1.669  e  2  1.587  1.594  (-0.004)  ( 0.003)  1.614 ( 0.036)  1.582  1.586  1.605  (-0.009)  ( 0.010)  1.561 (-0.018)  1.598 ( 0.018)  ( 0.004) _  _  (a) In Angstroms, values averaged from two bonds. Deviation from mean PN bond length in parenthesis. See Figure 5.6 for nomenclature of bonds. b H.P Calhoun, unpublished results, (c) S. Rettig, unpublished results. d) J . Trotter, S.H. Whitlow and N.L. Paddock, Chem. Commun., 695 (1969) (e) H.R. Allcock, E.C. Bissell and E.T. Shawl, Inorg. Chem., 12, 2963 (1973)  ,P'  N  \  ^ P - |  / /  X  N  II ^P  N  2  " P -2 X= CR or N H  Figure 5.6.  X  +  Nomenclature system for ring bonds in azaphosphorin and protonated phosphazene structures.  - 170 -  Using this model, the most noticeable feature of the azaphosphorin structures is the near equality of a l l the PN ring bonds (Table 5.6), which behaviour is in contrast to the alternation in bond lengths observed in 282 283 protonated phosphazenes ' . Formally, the two types of molecule are isoelectronic; in both cases only one orbital (the p ) i s available on the z  X atom (Figure 5.6) for ir-bonding to phosphorus. 2 atoms, the sp  On the other ring nitrogen  hybrids, as well as the p orbitals, arecapable of ir-bond z  formation. In protonated phosphazenes, the localization of two electrons in one 2 sp  orbital on nitrogen perturbs the in-plane i r system, and the variation in s  the calculated bond orders expected by such a perturbation is in excellent agreement with the observed alternation in bond lengths (Figure 5.7).  Moreover,  the lengths of the ring bonds to the protonated nitrogen now corresponds to the use of a single ir-component (the I T , ) , the d e f i c i t in length from the a 1.77 A characteristic of a single P-N bond  being approximately halved.  In the calculation of expected bond orders in azaphosphorin structures (using simple Huckel M.O. Theory) a second factor, quite apart from the loss of one orbital in.the'ir  system, must be taken into account.  It is  connected with the perturbation of the out-of-plane IT, system which results Q  from the replacement of a nitrogen atom with a carbon atom, and affects n-bond formation in two ways,  (i) Because of the difference in nuclear charge of  carbon and nitrogen, the 2p orbital on carbon w i l l be of higher energy than z  i t s counterpart on nitrogen,  ( i i ) Because of the lower electronegativity of  carbon, the d-orbitals on the phosphorus atoms adjacent to the endocyclic carbon w i l l be less effectively s t a b i l i z e d , and their Coulomb parameters w i l l therefore  - 171 -  -0.05 Deviation from mean bond length-A  0.3  Deviation from mean bond order  + 0.05  u  -o.3  J  Bond Number  Figure 5.7.  Calculated bond orders and observed bond length inequalities in a protonated tetrameric phosphazene ring (J. Trotter and S.H. Whitlow, J . Chem. S o c , A, 455 (1970)).  be algebraically greater than those on the phosphorus atoms bound to two nitrogens.  Of the two effects the f i r s t is more important (vide i n f r a ) .  Figure 5.8 i l l u s t r a t e s the calculated bond orders in the -rr and IT a s systems of a P N C ring. As in the case of protonated phosphazenes, the removal 4  3  - 172 -  of one orbital from the T T system strengthens the P^N^ and P,,N ' s  expense of the P N-, bond. c. i  3 0 n c  '  s  *- *he  a  By contrast, the reverse order is found in the IT a  0  system.  2  The values shown for this l a t t e r case were calculated for a model in  which the Coulomb parameters of a l l phosphorus 3d o r b i t a l s (ap) were i d e n t i c a l , as were those of the nitrogen 2p orbitals (°^).  The Coulomb parameter of the  carbon 2p orbital (a^) was then set ( a r b i t r a r i l y ) equal to the average of a^ and ap.  A l l the resonance integrals 6 were assumed to be equal, and were  related to the Coulomb parameters by the expression g = (a^-ap)/2.  Refinements  of the model, using different values of ap for the phosphorus orbitals adjacent to carbon, changed the magnitude of the variation shown in Figure 5 . 8 , but not the trend in bond orders. Hence, i f the TT  a  and TT s  system are assumed to be equally important,  the combination of the two separate trends in bond order is such as to minimize the variation in total bond order, thereby providing an explanation for the near equality of the observed lengths of the P-jN-j, P,,^ and P N bonds. 2  2  The agreement between theory and experiment is substantial, and establishes that the same theoretical model which predicts the bond length inequalities in protonated phosphazenes can be used to rationalize the equivalence of bonds in azaphosphorin structures.  The necessary link which must be used to connect the  two systems is the dual nature of the ir-bonding in the PN skeleton. The same model used above to describe the bond lengths in the azaphosphorin structure P^N^C also provides a rationale for i t s formation from the deprotonation of the N-methyl quaternary salt (NPMe ) .MeI (see Chapter 2  IV).  4  Figure 5.9(a) shows the typical energy levels found for the TT  system in a tetrameric phosphazene.  or TT  a s Figure 5.9(b) then corresponds to the  r 173 -  I  I  1  1  PN 2  1  P N 2  2  Figure 5 . 8 . Calculated (HMO) bond orders (relative to a P4N4 ring) in the out-of-plane i r (a) and in-plane T T (b) components of the ir-system in a P4N3C azaphosphorin r i n g . Coulomb parameters (d) related to the resonance integral (e) such that = ap + 28, CIQ = (a^+ap)/2. a  s  energy levels found in the IT system of the exocyclic y l i d which is formed a (presumably) as the i n i t i a l product of the deprotonation of a methylphosphazene quaternary s a l t .  As expected, the Tr-electron energy/electron of the system  is lowered by the addition of the exocyclic y l i d , but can be increased by the phosphazene-azaphosphorin rearrangement (Figure 5 . 9 ( c ) ) .  A ir system  (Figure 5.9(d)) is present in both the y l i d and azaphosphorin structures, and is not affected by the rearrangement.  -174 (a)  (b)  ' .  0 A  XX  - » XX XX  XX  XX . XX XX  xx  (d)  .  (c)  XX XX XX  XX XXXX  -XJL  XX  \  1  /  \  P  N  N //  \  ir-energy/electron (units of 3)  Figure 5.9.  1.675  //  p  N  p  \  N  •V / 1.427  / N \  N""* P  N //  N N \ // P^N^P  1.441  1.697  Energy level diagrams and Tr-electron energies for a number of phosphazene and azaphosphorin structures. [ a = a + 2s, M  5.3  r \  \  D  Summary 3  The classical model of bonding in phosphazenes , using 3d orbitals on phosphorus  and 2p orbitals in nitrogen to form a delocalized ^system,  forms a useful basis for the study of their physical and chemical properties. The acidic behaviour of methylphosphazenes, as described in e a r l i e r chapters, provides chemical evidence that charge density on the ligands can be diffused  - 175 -  into the cyclic ir-system, and the formation of azaphosphorin derivatives by the deprotonation of their quaternary salts i l l u s t r a t e s the extent to which the molecular skeleton w i l l rearrange to optimize such d e r e a l i z a t i o n .  The  structural work described in the present chapter has refined the simple concept of bonding, and stresses the presence of two components to the iT-system. The equivalence of these two components i s i l l u s t r a t e d by the f l e x i b i l i t y of the PN framework, and the conformations of methylphosphazenes appear to be controlled largely by non-bonded forces.  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S o c , Dalton, 382 (1974).  - 189 -  APPENDIX  Details of the instruments used for recording the various types of spectra were not discussed in the main text of the thesis.  A summary  of pertinent details is given below. i)  Infrared Spectra:  These were a l l recorded on a Perkin-  Elmer 457 grating spectrophotometer, and calibrated against a standard polystyrene spectrum.  Unless otherwise indicated, a l l spectra were recorded  on samples in nujol (and hexachlorobutadiene) mulls, using cesuim iodide plates.  Solution spectra were taken using c e l l s with potassium bromide  wi ndows. ii)  Raman spectra:  Raman spectrophotometer. iii)  These were recorded on a Cary 81 Laser  No depolarization measurements were taken.  Mass Spectra:  These were a l l recorded at 70 e.v. on an  A . E . I , type M.S. 9 mass spectrometer, samples being admitted through conventional inlet systems. iv)  N.m.r. Spectra:  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 n.m.r. spectrometer.  220 MHz spectra were obtained by courtesy of the 220 MHz 31 n.m.r. f a c i l i t y at the University of Toronto. P n.m.r. spectra were 31 1 recorded at 40.1 MHz on a Varian XL100 n.m.r. spectrometer.  P- H de-  coupling experiments were carried out using a recently described double resonance technique^. ^see.L.D. Hall and R. Burton; Can. J . Chem., 48, 59 (1970).  PUBLICATIONS T. C h i v e r s , R . T . O a k l e y a n d N . L . P a d d o c k , J . Chem. S o c . , A, 2 3 2 4 , ( 1 9 7 0 ) . Dimethyla m i n o f l u o r o p h o s p h o n i t r i l e s and T h e i r R e a c t i o n s W i t h H y d r o g e n H a l i d e s a s a R o u t e t o Monos u b s t i t u t e d and Non-geminal D e r i v a t i v e s o f the P h o s p h o n i t r i l i c Fluorides W. H a r r i s o n , R . T . O a k l e y , N . L . P a d d o c k a n d J . T r o t t e r , Chem. Comm, 3 5 7 , ( 1 9 7 1 ) . C r y s t a l S t r u c t u r e and C h e m i s t r y o f N i t r i l o hexaphosphonitrilic Chloride C.R. C a r m a n a n d R . T . O a k l e y , R e v i s t a d e T e c n o l o g i a d e l a U . I . S . , 5, 5 , ( 1 9 7 2 ) . Un E s t u d i o Conformacional d e l P o l i d i c l o r o f o s f o n i t r i l o R.T. O a k l e y a n d N . L . P a d d o c k , C a n . J . Chem., 5 1 , 520, ( 1 9 7 3 ) . Nitrilohexaphosphonitrilic Chloride: A Chemical and S p e c t r o s c o p i c Study H.P. C a l h o u n , R.H. L i n d s t r o m , R . T . O a k l e y , N . L . P a d d o c k a n d S.N. T o d d , Chem. Comm., 3 4 3 , (1975) . Phosphazene Carbanions as S y n t h e t i c Intermediates: S i l i c o n , Germimium, and T i n Derivatives H.P. C a l h o u n , R . T . O a k l e y a n d N.L,. P a d d o c k Chem. Comm, 454 ( 1 9 7 5 ) . R e a c t i o n of N-Methyl Methylphosphazenium Halides with Bases: A Phosphazene-Phosphorin Rearrangement H.P. C a l h o u n , R . T . O a k l e y a n d N . L . P a d d o c k , C a n . J . Chem. 5 3 , 2 4 1 3 ( 1 9 7 5 ) . The C r y s t a l and M o l e c u l a r S t r u c t u r e o f t h e D i h y d r o c h l o r i d e o f 2, t r a n s - 6 - d i e t h y l - 2 , 4 , 4 , 6 , 8 , 8 - h e x a m e t h y l cyclotetraphosphazene R . T . O a k l e y a n d N . L . P a d d o c k , C a n . J . Chem. 53, 3038 ( 1 9 7 5 ) . The P r e p a r a t i o n o f Hexamethy1cyclotriphosphazene  

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