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Synthesis and carbanion reactions of methylphosphazenes Oakley, Richard Thomas 1976-02-11

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SYNTHESIS AND CARBANION REACTIONS OF METHYLPHOSPHAZENES by RICHARD THOMAS OAKLEY B.Sc. (Hons.), University of British Columbia, 1969 M.Sc, 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 presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date •Z3./JZ ABSTRACT The reaction of dimethyltrichlorophosphorane with methylamine hydrochloride provides a novel preparative route to the methylphosphazenes (NPMe2)3 4, the separation of the two compounds being aided by the different solubilities of their salts (NPMe2)3.RCl and (NPMe2)4.2RCl (R=H, Me) in acetonitrile. A general method for the synthesis of methylphosphazenes of large ring size (i.e. (NPMe2)n, n ^ 6) by the methylation of the appropriate fluorophosphazene (NPF2)n with methylmagnesium bromide has now been achieved. A variety of quaternary salts (NPMe2)n.RI and a number of trimethylphosphinimine derivatives have also been prepared. The differences in the properties of the cyclic and monomeric molecules have been interpreted in terms of their electronic structures. The methylphosphazenes (NPMe2)3_iQ are crystalline solids at room temperature, and the molecular structures of (NPMe2)n (n=4,5,7 and 8) suggest that their molecular flexibility 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, Me3SiCl, C02, PhCOOEt, Br2) establish that they are useful intermediates for the synthesis of novel - ii -organophosphazenes. The orientation of the ethyl groups in N^MegEt^HCl shows that the second deprotonation of N^P^Meg occurs trans-antipodally to the first. The N-methyl quaternary salts (NPMe2)3 4-MeI and N3P3Ph4Me2.MeI can also be deprotonated. The products are not the expected exocyclic ylids but the novel azaphosphorins Me2n_-j (NHMe)PnNn_-|CH (n=3,4) and Me(NHMe)Ph4P3N2CH, formed by a rearrangement in which the methylated nitrogen atom is displaced from the PN ring by the initially produced exocyclic methylene group (Equation 1). The driving force of the reaction is likely to »3\ CH3 N R N R Base $ H3\ /H2 ,CH3 R F R H 1 N R-R. R hhC N —CH3  3 \ / • C .. 1 R—R R be the attainment of cyclic aromaticity. The 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 salts, nucleophilic attack competes with proton removal, and the linear oxides (NHMe)(PMe2N)nPMe20 (n=2-4) have been isolated from these reactions. The azaphosphorins Me2n_-| (NHMe)PnNn_-|CH (n=3,4) are hydrolysed in aqueous ethanol to give the cyclic oxides Me2n-l ^)PnNn-lCH2' and react with methyl iodide by a transylidation reaction to give the hydroiodides Me2n_-| (NHMe)PnNn_^CH.HI. Their reaction with benzoyl chloride leads to the benzoylated derivatives Me7(NHMe)P4N3CC0Ph and Me5(NMeCC0Ph)P3N2CC0Ph, the initial substitution on carbon indicating that it is the primary basic centre. The molecular structures of azaphosphorins are consistent with the delocalization of charge from the endocyclic carbon into the cyclic 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 x LIST OF FIGURES : xi ACKNOWLEDGEMENTS xiiCHAPTER I GENERAL INTRODUCTION 1 1.1 Preparation of Phosphazenes 3 1.1.1 Direct Methods1.1.2 Substitution Reactions 4 1.2 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 1.3 Carbanion and Ylid Formation in Organophosphorus Chemistry. 15 1.3.1 Phosphorus Ylids 17 1.3.2 Phosphorus Carbanions 20 1.4 Summary 21 - V -Page CHAPTER II PREPARATION OF METHYLPHOSPHAZENES 23 2.1 Preparation of Methylphosphazenes 4 2.1.1 Preparation of (NPMe2)n .(n=3,4) 22.1.2 Preparation of (NPMe2)n (n>4) 6 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 Quaternary Salts 39 2.2 Preparation of gem-Me2Ph4P3N3 42 2.2.1 Preparation of Me2Ph4P3N3.Mel 44 2.3 Preparation and Chemistry of Tetramethylphosphinimine 45 2.3.1 Reaction of Me3P and MeN3 42.3.2 Preparation of [Me3PNMe2]+I~ 6 2.3.3 Preparation and Deprotonation of [Me3PNHMe]+Cl" 46 2.3.4 Reaction of Dihalophosphoranes with (Me3Si )2NMe .... 48 2.3.5 Structure and Spectra of Phosphinimine Derivatives.. 48 2.4 Chemistry of gem-Cl2Ph4P3N3 52 2.4.1 Attempted Methylation of Cl2Ph4P3N3 with MeMgBr 52 2.4.2 Dimethylamination of CI2Ph4P3N3 52.4.3 Fluoridation of Cl2Ph4P3N3 3 2.4.4 Preparation of (NMe2)FPh4P3N3 54 2.4.5 Attempted Preparation of a Phosphazenium Cation 54 2.5 Experimental 56 2.5.1 Preparation of Methylphosphazenes 5- vi -Page 2.5.2 Preparation of Methylphosphazene Quaternary Salts ... 63 2.5.3 Preparation of Methylphosphinimine Derivatives 66 2.5.4 Preparation of XYPh4P3N3 Derivatives 70 CHAPTER III REACTIONS OF METHYLPHOSPHAZENES WITH BASES 73 3.1 Preparation and Reactions of N4P4Me8_x(CH2Li)x (x=2,4) 73 3.2 Preparation and Reactions of N3P3Ph4Me(CH2Li) 75 3.3 Spectra and Structure of NAP.Mes V(CH~R)V (x=2,4) Derivatives 76 3.3.1 Disubstftuted Derivatives 77 3.3.2 Tetrasubstituted Derivatives 9 3.4 Structure and Spectra of N3P3Ph4Me(CH2R) Derivatives 80 3.5 Carbanion Stabilization by Phosphazenes 82 31 3.6 P Chemical Shifts of Heterogeneously Substituted Phosphazenes 85 3.6.1 31P Chemical Shifts of N3P3Ph4Me(CH2R) Derivatives .. 89 3.6.2 31P Chemical Shifts of N-P-Meo „(CH0R) Derivatives . 89 H *f o_X C X 3.7 Experimental 90 3.7.1 Preparation of Methyllithium 91 3.7.2 Preparation of N4P4Me4Et4 2 3.7.3 Preparation of N4P4(CH2SiMe3)4 94 3.7.4 Preparation of N4P4Me6(CH2SiMe3)2 5 3.7.5 Attempted Preparation of N4P4Me7(CH2SiMe3) 96 3.7.6 Preparation of N4P4Me8_x(CH2MMe3)x (x=2,4, M=Ge,Sn) . 97 3.7.7 Preparation and Reactions of N3P3Ph4Me(CH2Li) 97 - vii -Page CHAPTER IV PREPARATION AND REACTIONS OF AZAPHOSPHORIN DERIVATIVES 104 4.1 Reaction of N-methyl Methylphosphazenium Salts with Bases 105 4.1.1 Reaction of NaN(SiMeo)? with N-methyl Methylphos-phazenium Salts 104.1.2 Reaction of KOtBu with N-methyl Methylphosphazenium Salts 107 4.1.3 Reaction of Methyllithium with N-methyl Methylphos phazenium Salts 109 4.1.4 Discussion 110 4.2 ^H n.m.r. and Infrared Spectra of Azaphosphorins 116 4.2.1 ]H n.m.r. Spectra 114.2.2 Infrared Spectra 124 4.3 Linear Phosphazene Oxides X(PR2N)nPR20: Their Preparation,. Structure and Spectra 125 4.4 Reactions of Azaphosphorins 128 4.4.1 Hydrolysis of Me2n_1(NHMe)PnNn_1CH (n=3,4) 129 4.4.2 Reaction of Me9 ,(NHMe)P N ,CH (n=3,4) with Methyl Iodide f1?:! 132 4.4.3 Reaction of Me2n_i (NHMe)P N -,CH (n=3,4) with Benzoyl Chloride n..r}'.\ 134 4.5 Spectra and Structure of Azaphosphorin Derivatives 136 4.5.1 ^H n.m.r. Spectra of Azaphosphorin Hydrohalides 136 4.5.2 n.m.r. Spectra of Phosphazene Oxides Me2n_1(0)PnNn_1CH2 (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 4.6 Experimental 145 4.6.1 Reaction of NaN(SiMe3)2 with Methylphosphazenium Quaternary Salts 146 - viii -Pa^e 4.6.2 Reaction of (NPI^^.Mel with KOtBu 148 4.6.3 Reaction of (NPMe2)3.MeI with KOtBu 149 4.6.4 Reaction of (NPMe2)4-2MeS03F with KOtBu 149 4.6.5 Reaction of (NPMe2)4.MeI with Methyllithium 149 4.6.6 Reaction of N3P3Ph4Me2.MeI with Methyllithium 150 4.6.7 Hydrolysis of Meg^NHMejPN^H (n=3,4) 150 4.6.8 Reaction of Me9 ,(NHMe)P N , (n=3,4) with Methyl Iodide ??:! ...7! 151 g 4.6.9 Reaction of Me7(NHMe)P4N3CH with Benzoyl Chloride .. 152 4.6.10 Reaction of Me5(NHMe)P3N2CH with Benzoyl Chloride .. 152 CHAPTER V MOLECULAR STRUCTURES OF METHYLPHOSPHAZENES AND METHYLAZAPHOSPHORINS 154 5.1 Conformations of Methylphosphazenes 155.2 Structures of Azaphosphorin Derivatives 164 5.3 Summary 17g REFERENCES 6 APPENDIX 189 - ix -LIST OF TABLES Table Page 1.1 Relative rates for the deuteroxide catalysed hydrogen-deuterium exchange between ammonium, phosphonium and sulphonium ions and deuteri urn oxide 16 2.1 N.m.r. parameters, (P=N) stretching frequencies and melting points of the methyl phosphazenes (NPMe2)n (n = 3-10) .... 32 2.2 31P n.m.r. parameters of N-methyl methylphosphazeniurn iodides 39 2.3 n.m.r. parameters, (P=N) and (C-N) stretching frequencies of methylphosphazenium quaternary salts 40 2.4 n.m.r. parameters of Me3PNMe and its derivatives 49 2.5 Yields and analytical data for the reaction (NPF2)n + 2nMeMgBr —> (NPMe2)n (n = 6-10) 61 3.1 and 3^P n.m.r. parameters of N4P4Me8_x(CH2R)x derivatives . 78 3.2 Infrared and Raman absorption frequencies of Me8P4N4 and Me4Et4P4N4 83.3 and 31P n.m.r. parameters of N3P3Ph4MeCH2R derivatives 83 3.4 31p chemical shifts of geminally substituted phenylhalophos-phazenes X2nPh6_2nP3N3 90 3.5 31P chemical shifts of XYPh4P3N3 derivatives 92 4.1 ^H n.m.r. parameters and selected vibrational frequencies of azaphosphorin derivatives 118 4.2 31p and n.m.r. parameters, and selected infrared frequencies of phosphazene oxides X(PMe2N)PMe20 127 4.3 n.m.r. parameters of azaphosphorin hydrohalides 137 4.4 n.m.r. parameters of phosphazene oxides Me2n_i (0)PnNn_-|CH2- 141 4.5 n.m.r. parameters and carbonyl stretching frequencies of Me5(NMeC0Ph)P3N2CC0Ph and Me7(NHMe)P4N3CC0Ph 144 - X -Table Page 5.1 Mean values of principal structural parameters in methylphosphazenes 155 5.2 Interatomic potential constants used for the calculation of van der Waals interactions in a (PN)2PMe2 unit 161 5.3 Observed torsion angles and local conformations in methylphosphazenes 163 5.4 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 ylids 167 | 5.6 Mean bond lengths in azaphosphorin and protonated phosphazene structures 169 - xi -LIST OF FIGURES Figure Page 1.1 Activation energies for the reaction CI" + (NPC10) —* (NPCl2)n + CI" ... 10 1.2 Survey of typical reactions of the methyldiphenylphosphine oxide carbanion Ph2P(0)CH2"M+ 21 2.1 (A) 31P chemical shifts and (B) v(P=N) frequencies of phosphazenes (NPX2)n (X = F, CI, OMe, Me) 34 2.2 The mass spectra of the methyl phosphazenes (NPMe2)g_-jQ 37 2.3 The ordinary 100 MHz ^H n.m.r. spectrum (A) and 31P decoupled 'H n.m.r. spectrum (B) of (NPMe2)y.MeI 41 3.1 General views of (a) the N^P^MegEto.2HC1 structure, and (b) the chair conformation of the N4P4MegEt2H22+ cation 79 3.2 Relative charge densities on phosphorus atom in a perturbed P3N3 ring 84.1 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 Me5(NHMe)P3N2CH (in benzene-dg solution) 117 4.3 100 MHz ]H n.m.r. spectrum of Me(NHMe)Ph4P3N2CH in C6D6 solution 119 4.4 220 MHz ^ n.m.r. spectrum of Me(NHMe)Ph4P3N2CH in pyridine-d5 solution, (A) at 20°C and (B) at 60°C 123 4.5 The 100 MHz ^H n.m.r. spectra of the azaphosphorin hydro-iodide Me5(NHMe)P3N2CH.HI, in CDC13 solution 138 4.6 Possible rotational isomers of the CH2-P(NHMe)MeN unit in the cations [Me2n_i (NHMe)PnNn_-,CH2]+ (n = 3,4) 139 4.7 The 100 MHz ^H n.m.r. spectra of the phosphazene oxide Mey(0)P4N3CH2 142 5.1 Overall views of some methylphosphazene structures 156 5.2 Three possible conformations for 8-membered phosphazene rings (NPX2)4 157 - xi i -Figure Page 5.3 Angular conventions for the measurement of torsion angles in a (PN) PMe0 unit 160 n L 5.4 Potential energy (a) and electrostatic energy (b) contours of (PN)2PMe2 unit as a function of ^ and ^ 162 5.5 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 5.8 Calculated bond orders in the out-of-plane iTa and in-plane irs components of the iT-system in a P4N0C azaphosphorin ring 7.7 173 5.9 Energy level diagrams and Tr-electron energies for a number of phosphazene and azaphosphorin structures 174 - 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 will always be grateful. This work has benefited greatly from the many contributions of the various members of my research group. To all 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. Clifford 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. Finally, I would like 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 first 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 tri- and pentavalent forms, and sulphur in di-, tetra- and hexavalent states. By contrast, their congeners in the first 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 role, if 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 lies not so much in their basic chemistry, which is similar in many ways to that of mononuclear phosphoryl compounds, as in their availability for study over a large range of ring sizes and ligand types; the variations in the physical and chemical properties of (NPX2)n, as a function of X and n, provide a sensitive and specific tool for examining the binding properties of pentavalent phosphorus. The formal structural resemblance of (NPX9)^ - 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 .II From an historical point of view, chemical knowledge of phosphazenes spans more than a century. The best known phosphazene, the trimeric 5 6 chloride (NPC12)3> was first isolated in 1834 ' , but it was not until 1896, when Stokes prepared and characterized the chlorophosphazenes (NPC12)3,7» that a cyclic structure was suggested for them''. During the first half of this century, little attention was paid to their physical and chemical properties, and it 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 articles 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 spirit of the work reported in this thesis is somewhat different, in that it is concerned with the effect of the phosphazene ring itself on the chemistry of the attached ligands. More specifically, the thesis deals with the preparation and chemistry of methylphosphazenes (NPMe2)n, focusing attention on the relationship between the acceptor properties of the phosphazene ring and the chemical reactivity of the methyl groups attached to it. 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 Preparation of Phosphazenes 1.1.1 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,PC1- + NH-Cl —• -(NPXJ + 4HC1 .... 1 c o 4 n L n 19 solvent, tetrachloroethane or chlorobenzene being the most commonly used . 7 l q ?o As well as the chlorophosphazenes (NPC12) ' , the bromo (NPBr2)n , alkyl ((NPMe2)n21 and (NPEt2)n22) and aryl ((NPPh2)n23'24, (NPPhCl)n25, 26 (NPPhBr)n ) derivatives can be prepared in this way. Usually the cyclic trimer and tetramer are formed, with smaller amounts of the higher ring + 71 7Q sizes and linear species of the type [ClR2P=N-(PR2=N)nPR2Cl] CI - 4 -1.1.2 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 (n > 5) only 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 versatility 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 30 31 32 such as sodium fluoride and potassium fluorosulphite ' to give mixed fluorochloro- and fully 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 trifluoride reacts with chlorophosphazenes to give 33 34 partially 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 size; the fluorophosphazenes (NPF^n (n=3-20), for example, have all 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 yield organosubstituted derivatives (reference 18, p. 150) is successful for a large range of ring sizes. The reaction is carried out using the alcohol itself 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 ' and methoxy [NP^Me^l^g' phosphazenes have been prepared in good yield. 1.1.2.3 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 fully and partially aminated products can be prepared, the isolation of the latter being aided by the fact that successive amination reduces the reactivity of - 6 -the ring to further substitution. The applicability of this reaction, like those outlined above, appears to be independent of ring size; the fully 36 38 dimethylaminated phosphazenes [NP(NMe2)2]n are known for n = 3 to 9 . 1.1.2.4 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 difficulty 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 partially phenylated derivatives (Equation 3), but the CI CI >/ N • CI—PN / CI benzene P—CI AlCh N CI N CI Pv CI Ph Ph \/ N ci ... 3 formation of fully phenylated derivatives requires extreme conditions40'41, Potentially more productive is the action of organometallic reagents on halophosphazenes, according to Equations 4 and 5. However, the results of 2nRLi + (NPX2)n 2nRMgBr +' (NPX2)n (NPR2)n + 2nLiX (NPR2)n + 2nMgXBr ... 4 ... 5 (X = halogen, R = aryl or alkyl) - 7 -such reactions differ markedly from those obtained from the nucleophilic substitutions already discussed, and it was with the intention of under standing the cause(s) of these differences that the present work was begun. 39 Early work on the reaction of phenyllithium and phenylmagnesium bromide42"47 with (NPC12)3 has shown that the expected reactions (Equations 4 and 5) are, at best, an alternative to the primary reaction path. Although (NPPh2)3 can be isolated (in ^5% yield) from such reactions, the principal products are open chain phenylphosphazenes such as Ph3P=N-Ph2P=N=Ph2P=N-MgX, formed, presumably,by the addition of a mole of nucleophile across a PN bond and cleavage of the cyclic phosphazene skeleton (Equation 6) 47 a-, • CI CI CI \/ N a RMgX CI Cls i >R MgX Cl-Pv a' W CI N Cl-Cl ,MgX \ •CI CI When fluorophosphazenes, rather than chlorophosphazenes, are employed, the substitution reaction is more successful, probably because the PN ring system is strengthened and stabilized by the more electro negative fluorine ligands. Thus, the phenylphosphazenes (NPPh2)n (n=4-6) have all 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"1", and the few reports of the reaction of alkylmetals 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 (NPF2)4 affords the completely methylated phosphazene (NPMe2)4 in 70% yield51. However, in contrast to this, the complete methylation of (NPF2)3, to yield (NPMe2)3, is not possible, and only the monomethyl (MeFj-P^) and gem-dimethyl (Me2F4P3N3) derivatives have been isolated from partial substitutions. Complementing the results of the previous two reactions, the pentameric methylphosphazene (NPMe2)5 can be prepared in 20% yield by the reaction of methyl 1ithium and (NPF2)5. The oscillating 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 infra)5 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. The results of work done in this area are described in Chapter II. By contrast, the alkylation of phosphoryl chlorides leads to moderate yields of trialkyl phosphine oxides (e.g. C1-P0 + 3MeMgBr ——• 3Me3P0; yield = 52%50). 6 - 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 studied4. The stability of halophosphazenes towards hydrolysis (unlike phosphoryl halides) and the fact that, in all phosphazenes, the ring angles at both nitrogen and phosphorus are much greater than the tetrahedral value, both indicate that the simple ionic formulation (III) is 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 (IV), for example, the PN CI CI \/+ N' CI— Rr-Cl Cl 1-593 A Cl—P. ^P.—Cl 1769 A 0. 0—P—— N—H III IV V °53 9 bond length is!.593 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 stability of the molecules, whilst the latter is more important chemically. Thus, the dependence on ring size of the relative rates of the first and second substitution of chloride with fluoride ion for the series (NPC12)3_g57'58, and of the measured activation energy for the exchange process between (NPC1g and chloride ion^, is believed to originate from homomorphic -rr-effects4. Figure 1.1 shows, for 3 U 5 6 n-in (PNCl2)n Fig. 1.1 Activation energies for the reaction CI" + (NPCl2)n • (NPCl2)n•+ CI" [from D.B. Sowerby, J. Chem. Soc, 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]. - 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 oscillation in value of these parameters, with change in ring size, 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 (NPF,,)^ 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 availability 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 this, 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 -r.alky^l.aipi'rto ^derivatives,..' /In. t.he -methoxyphosphazenes [NP(0Me)2]4 g g60-62, CO al] the P-O-.C angles are close to 120° , as in phosphate esters , whereas the C-O-C angle in organic esters is about 112° . Also, the P-0 bonds are shorter than expected for a P-0 single bond (1.71 A64). The same effects are seen in the dimethylamino phosphazenes [NP(NMe2)2]4 53 " > 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 its effect on 19 the F n.m.r. spectrum of a fluorophenyl group attached to a phosphazene ring. The value of &^ - 6pm (the difference in the chemical shifts of the para and meta fluorine atoms) for the series CO Nh'Ph-2ri-1 *C6^5 (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 nitrogen69,70, indicative of the lower base strength of the exocyclic nitrogen caused by the drainage of iT-charge from it into the ring. However, there is at least one example of the coordination of an exocyclic amino group to a metal71 (in [NP(NMe2)2]4W(C0)4), which suggests that the reduction in base strength is not Targe. Consistent with this, the reaction of [NP(NMe2)2]3 with Me30+BF4~ results in the methylation of both an endocyclic and an exocyclic - 13 -17 nitrogen (although the former is probably quaternized first). In aminophosphazenes, therefore, the chemical effect of exocyclic conjugation is small, but, to the extent that it occurs, is such as to reduce the basic properties of the ligand on phosphorus. This conclusion is crucial, and provides the link between past work in phosphazene chemistry and the results reported in this thesis. During the course of this work, it has been found that the ability of the phosphazene ring to accept u-charge from an exocyclic ligand can be effectively utilized in the synthesis of novel phosphazene derivatives. The initial experiments that lead to this discovery were concerned with the reaction of methyllithium with octamethyltetraphosphazene (NPMe2)4- By analogy with the addition of methylmagnesium iodide across the Si-0 bond of 73 (0SiMe2)4 (Equation 7), and of alkyllithiums across the C=N bond of pyridine74 (Equation 8), it was believed that methyllithium might add across 75 the P^N bond of methyl phosphazenes (Equation 9a). Early work showed that, H3C CH2 0' 0 H3n : GH3 MeMgl 6 \ / >• Si / V H20 H3G OMgl (CH3)3SiOH Li BuLi r 14 -CH N N N + CH4 H20, s> (CH3)3PO 9b 9a although a reaction did take place, its nature was uncertain. Trimethyl-phosphine oxide, for example, could not be isolated via the hydrolysis of the reaction mixture. In fact, it 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. The chemistry of these carbanions is fully discussed in Chapter III. The acidity of protons in methylphosphazenes complements the reported behaviour of the compounds as donors. Both (NPMe2)3 and (NPMe2)4 7fi form a variety of simple salts , as well as reacting to give complexes with metal carbonyls77'78 and metal ions79'80. They also react with alkyl iodides (e.g. Equation 10) to give N-alkyl quaternary salts (NPMe9)~ «.RI 10 - 15 -(R=Me, Et)/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 methylphos phazenes towards deprotonation, but by the formal structural resemblance of quaternary phosphazenium salts to more conventional phosphonium salts, whose reaction with bases, to give phosphorus ylids, is well known (vide infra). It was believed that, if the analogy were justified, then the phosphazenium;salts;(NPMe2)n.MeI could also be deprotonated with a suitably 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 final product of the reaction is quite different from the expected exocyclic ylid. 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 ylids from mononuclear phosphorus compounds. Because of their greater structural simplicity and ease of preparation, these 11 - 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 facilitating 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 vividly 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) . Me3ft-CH3 + OD" <—» Me3N"-CH2 + HOD .... 12a Me3P"-CH3 + OD' < • Me3?-CH2 + HOD .... 12b Me2S-CH3 + OD" 5=1* Me3S-CH2 + HOD 12c Compound Relative Rate C-X Bond Length (A) -41 Me,N+ 1 1.47 Me4P+ 2.4 x 106 1.87 Me3S+ 2.0 x 107 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. Soc, 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 stabilization 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 106 times greater than for the 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). Me3P+ - CH2 « » Me3P = CH2 ....13a Me2S+ - CH2 4 * Me2S = CH2 .... 13b 1.3.1 Phosphorus Ylids This pronounced acidity of protons on carbon atoms adjacent to phosphorus and sulphur has many applications in synthesis. Phosphonium salts [R3PCH2R]+X~ can be deprotonated with relative ease to produce phosphorus ylids. R3PCHR, the use of which, in the Wittig reaction , is well known . By contrast, nitrogen ylids can only be isolated when a sufficiently electron withdrawing group is attached to the a-carbon to stabilize the negative charge that it acquires upon deprotonation86. The ylids 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 -[R3IP-CHR2R3]+X" rJp-CR2R3 + HX .... 14 2 3 groups on carbon (R and R ) enhance the acidity of the phosphonium salt, the polar bond of the corresponding ylid being more effectively stabilized, either by resonance or inductive effects. Thus, [Ph3PCH2C00Me]+Br~ can be 87 88 deprotonated by a relatively weak base such as sodium carbonate ' , the polar P=C bond so obtained being sufficiently stabilized by resonance (Equation 15) so as to render it stable to water. The ligands R1 attached OMe -/ H Ph3P-CN 'C=0 OMe Ph2P-C _ 9~° OMe ... 15 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 [Ph3PCH2C00Me]+Br~ are replaced with cyclohexyl groups, sodium hydroxide, rather than sodium carbonate, must be used to induce deprotonation, and 87 the resulting ylid is decomposed on contact with water . In the limit, unsubstituted alkylphosphonium salts are sufficiently 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 salts, by means of a reaction which is generally referred to as "transylidation .89 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 -Ph3P-CHR1 + [Ph3P-CH2R2]+ [Ph^CH^Y + Ph3P=CHR2 .... 16 acidic phosphonium salt. For example, the reaction of Ph3P=CH2 with [Ph3PCH2C0Ph]+Br" results in the formation of [Ph3PCH3]+Br" and 'PhgPCHCOPh in nearly 90% yield89. 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 pka values of phosphonium salts [Ph3PCH2R]+X~, the acceptor 89-91 ability of various R groups increases in the following order : CH3 < H < Ph < P(0)Ph2 = P(S)Ph2 < COOR < CN < C(0)Ph < PPh3- According to this series, the treatment of a phosphonium salt [Ph3PCH2R1]+X~ with a phosphorus 2 1 2 ylid Ph3PCHR results in transylidation if R is a better acceptor than R . Consistent with their basic character, ylids are methylated on carbon by methyl iodide91"95 (Equation 17). In agreement with the above R^CHR2 + Mel —• [R^P-CHMeR2]4^" ....17 series, the phosphonium salts [R13PCHMeR2]V (R]=Ph; R2=H92'93, -P(0)Ph291, 94 1 2 95 -COOMe ; R =Me, R =H ) so formed are unreactive to a second mole of ylid. By contrast, the acylation of ylids with acid chlorides results in transylidation of the initially formed salt (Equation (18), the strong acceptor properties of the acyl group making it sufficiently acidic to react with a second mole of starting ylid . This difference in behaviour of ylids upon methylation and acylation is an important concept. Its relevance to the present work will be fully discussed in a later chapter. - 20 -R3P=CHR1 + AcCl * [R3P-CHR1Ac]Cl [R3P-CHR1Ac]+CT + R3P=CHR].—» R3P=CRAc + [R3P-CH2R]+C1" .... 18 (Ac = acyl) 1.3.2 Phosphorus Carbanions The acidity of phosphonium ions [R3PCH2R]+ is enhanced by their charge. However, in formally neutral phosphoryl compounds, the acidity of methyl and methylene protons adjacent to phosphorus is still appreciable. The removal of a proton from such compounds produces a carbanion, the stability of which is 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 (R0)2P(0)CH2R99'100, and phosphine oxides R2P(0)CH2R100"108, have been extensively studied. Figure 1.2 shows, for example, a selection of the reactions that the phosphine oxide carbanion Ph2P(0)CH2" undergoes. Its chemical behaviour appears to depend on the nature of the counter cation. With carbonyl compounds, for example, it forms an intermediate betaine Ph,,P(0)CH2C(0)R2, which is stable when the 107 108 cation is lithium ' . However, when a cation other than lithium is present, decomposition of the betaine can occur (as in the Wittig reaction), - 21 -Ph ^,CH2SiR3 Ph N0 •A R Ph CHo-C-R V v Ph .CH9CR0OH V Ph 0 (M=Li) (M=Na) R3SICI (M=Li) Ph R2C0 LjJ Ph^ CH2C(0)R P Ph CH2 Rcoggt^ /\ \ / 2 ^ (M=U) \ / \/ G02<H+ \R2P(o)Cl /4v (M=Li) \(M=Li) Ph X0 0 Ph Ph 0~ \ • R2C=CH2 0 Ph CrbCOOH A, Ph 0 Ph Ph CH9 Ph x 0 0 Ph Fig. 1.2 Survey of typical reactions of the methyldiphenylphosphine oxide carbanion Ph2P(0)2CH2-M+. i 0? with elimination of an alkene and the formation of a phosphinate anion . The chemical similarity between these mononuclear phosphoryl carbanions and the phosphazene carbanions reported in this thesis is discussed in detail in Chapter III. 1.4 Summary As is indicated in the foregoing introduction, this thesis is 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 (NPMe2)n and their quaternary salts (NPMe2)n.MeI. Such topics will be discussed in later chapters. 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 partially methylated phosphazene Me2Ph4P3N3 and its methiodide salt Me2Ph4P3N3.MeI is 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 fully methylated phosphazenes for the study of the chemistry of the -N=PMe2- unit. Section 3 deals with the phosphinimine Me3PNMe and a number of its derivatives. This molecule constitutes the monomeric counterpart of the cyclic methylphosphazenes, and its preparation and study was undertaken to allow a comparison of the properties of the -N=PMe2- unit in monomeric and polymeric systems. The final section of the chapter is concerned with the chemical reactivity of the trimeric phosphazene CI2Ph4P3N3• Although it 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 fully halogenated phosphazenes described in the introduction (Chapter 1, Section 1). - 24 -2.1 Preparation of Methylphosphazenes 2.1.1 Preparation of (NPMe2)n (n=3,4) To date, the most common method for the preparation of the methyl-phosphazenes (NPMe2)3 4 has been by the ammonolysis of dimethyltrichloro-phosphorane21'76'77'109, according to Equation 1. The phosphazenes are Me2PCl3 + NH4C1 —+ ^(NPMe2)n + 4HC1 (n=3,4) .... 1 actually produced as their hydrochlorides, which can be converted into the neutral compounds either by a tertiary amine76'77 or aqueous alkali109. 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 yield is diminished77. Mixtures of (NPMe2)3 and (NPMe2)4 are also formed, in low yield, by heating Me2P(NH2)2Cl77,110, but, however the mixture is produced, the separation of the individual components by fractional crystallization is tedious and time consuming. The tetramer is more easily obtained than the trimer because it is the major product of such reactions and forms much better crystals. These difficulties prompted the search, in this work, for a new synthetic route to (NPMe2)3. As a resultof this investigation, two useful facts have come to light. (1) By substitution of methylamine hydrochloride for ammonium chloride in reaction 1, the corresponding methyl chloride salts (NPMe2)3.MeCl and (NPMe2)4.2MeCl are formed instead of the hydrochlorides. 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 (NPMe2)4.2RCl (R=Me or H) are insoluble in acetonitrile, whereas the quaternary chlorides of the trimer (NPMe2)3.RCl (R=Me or H) are appreciably soluble in this solvent. 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 (NPMe2)3 ^ can easily be separated. The procedure for the preparation of (NPMe2)3 reported here111 uses chlorobenzene as solvent. An oily intermediate is formed, which has not 112 been characterized but, by analogy with the reactions of PCI5 and 113 + PhPCl^ , probably contains the dimeric phosphazane (MeNPMe2Cl)2T. Pyrolysis of this oil in vacuo then gives a mixture of the quaternary salts (NPMe2)3.MeCl and (NPMe2)4.2MeCl. It is important to note that the ratio of (NPMe2)3:(NPMe2)4 in the product is very sensitive to the length of time for which the oily 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 latter, and also lowered the overall yield. This may be the result of the polymerization of the dimeric phosphazane, known to occur on prolonged heating of the analogous (MeNPF3)2115. When the reaction conditions outlined above are used in the reaction of Me2PCl3 with NH^Cl, a high yield of the tetramer (73% compared to . By contrast, the reaction of Ph2PCl3 with alkylamine hydrochlorides RNH2.HC1 gives simple salts [Ph2ClP-NHR]+Cl_ 114. - 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 salts, can then be separated by virtue of the different solubilities of their salts in acetonitrile. The neutral (NPMe2)3 is obtained from (NPMe2)3.MeCl by heating it in vacuo. There appears to be a similarity, at least in principle, to the formation of (NPPh2)3 by the dehydrohalogenation of 116 (HNPPh2F)2 with cesium fluoride . It is also interesting to note a difference from the behaviour of (MeNPCl^, prepared from PCI5 and 112 methylamine hydrochloride . Upon pyrolysis, this compound does not give quaternary salts, presumably because the chlorophosphazenes are too weakly basic; a high polymer of composition (NPC19) is formed instead117. 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 (NPMe2)n (n>4) from such reactions, 2.1.2 Preparation of (NPMe2)n (n>4) Although the methylphosphazenes (NPMe2)3 4 have been known for 21 many years , studies on the chemical and physical effects of ring size variations within the homologous series (NPMe2)n (n=3,4,5...) have always been limited by the difficulty in preparation of the different (more especially the larger) ring sizes. Unlike most phosphazenes (NPX2)n (e.g. X=NMe2, OPh, OMe), which have been prepared in a wide variety of 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, 20%)51'76, via Equation 2, have been interpreted in terms of a competing addition reaction (see Chapter 1, Section 1.1). (NPF2)n + 2nMeLi • (NPMe2)n + 2nLiF .... 2 Accordingly, unsuccessful attempts to prepare methylphosphazenes of larger ring sizes (n^6) have been attributed to the increased statistical difficulty of effectively substituting all the P-F bonds of the appropriate fluorophosphazene. During the course of this work, the possibility of using methylmag-nesium bromide as a methylating agent for the large ring size phosphazenes 118 was considered. Early attempts to methylate (NPF2)4 g with MeMgBr were unsuccessful, and the greater success obtained when methyllithium was used was attributed to the greater nucleophilicity of the latter, which would favour complete substitution of all the P-F bonds, and to the lower Lewis acidity of lithium, which would reduce the possibility of ring cleavage via coordination of the metal to a ring nitrogen atom. However, upon re-examination of the reaction of MeMgBr with (NPF2)g, it was discovered that a good yield of (NPMe2)g could be achieved by suitably modifying the experimental procedure from that used previously. Consequently, it has been determined that the reaction of methylmagnesium bromide with the appropriate fluorophosphazene, according to Equation 3, offers an efficient and facile preparative route to large ring size methylphosphazenes, and the derivatives (NPMe9) (n=6-10) have all been - 28 -prepared by this method. 2nMeMgBr + (NPF2)n (NPMe2)n + 2nMgBrF 3 In practice, the reaction of MeMgBr with (NPF2)n is slow. Unlike the immediate exothermic reaction that occurs between methyllithium and fluorophosphazenes, it 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 its greater charge, the magnesium ion is more prone to complex formation than is lithium, and, in the present case, methylphosphazenes appear to be sufficiently strong donors to complex with magnesium bromide, but not with lithium bromide^. The difference in the coordinating ability 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 and 121 PhLi with (NPF2)3, which give, respectively, the geminally and non-geminally disubstituted derivatives PI^F^P^N^. However, regardless of the actual mechanism of substitution, it is apparent that the difficulties 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 (NPMe2)n- This conclusion is consistent with many other properties of phosphazenes, which also indicate that the initially 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 (NPMe2)n.MeI (n=3,4,5)76'81. The synthetic 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 (NPMe2)n By following the procedure reported for the preparation of the salts (NPMe2)n.MeI (n=3-5)76'81, the analogous salts of the hexameric and heptameric methylphosphazenes have now been prepared (Equation 4). In (NPMe„) + Mel • (NPMeJ .Mel (n=3-7) 4 2 n 2 n - 30 -practice, the neutral methyl phosphazene is heated under reflux for several hours in neat methyl iodide. The product, (NPMe2)n.MeI (n=3-7) is precipitated from the solution in nearly quantitative yield. 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 [(NPMe2)n.CH2C00Et]+I" (n=3,4) have been obtained by heating the appropriate phosphazene in ethyl iodoacetate (Equation 5). As with the methiodide salts the product is precipitated from the solution and separated by filtration. 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 (NPMe2)4.2MeCl can be prepared via the reaction of Me2PCl3 and methyl amine hydrochloride (see Section 2.1.1). However, the dication [(NPMe2).2Me]++ can be more conveniently prepared by the diquaternization of neutral (NPMe2)4 with a sufficiently powerful 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 ' , and its ease of handling make it a more convenient choice than Me30+BF4". Accordingly, it has been found that MeS03F reacts rapidly with 5 - 31 -(NPMe2)4, in acetonitrile solution, to give the diquaternary salt (NPMe^^MeSOgF. The di-iodide salt (NPMe,,)^. 2MeI was prepared from it by ion exchange. It is probable that methyl fluorosulphate is capable of forming polycations from the methylphosphazenes of large ring size (e.g. [(NPMe2)6.3Me]3+ and [(NPMe2)8.4Me]4+), The preparation of (NPMe^g^g* extends the series of the known 2.1.4 Structure and Spectra of Methylphosphazenes * ) methylphosphazenes from the trimeric to the decameric derivative, and thereby creates the largest single family of phosphazenes other than the halo-derivatives (NPX)2 (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, and a simple doublet in their n.m.r. spectra^. Unlike dimethylsiloxanes, and many other phosphazenes of large ring sizes, all the methylphosphazenes (NPMe2)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)2J4 g ^60-62 65-67 and [NP(NMe2)234 53" a^ snow 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 (NPMe2)8_10. ^ Because of conformational changes, the exact magnetic equivalence of phosphorus and hydrogen nuclei is probably attained statistically. - 32 -Table 2.1: N.m.r. parameters9, (P=N) stretching frequenciesb, and melting points of the methylphosphazenes (NPMeJ (n=3-10). n M.pt. (°c). v (P=N) asym v • •• ' (cm-1) 6H (ppm) 6P (ppm) 3 187-188 (1185)c 1.46 (13.0) 90.7 4 161-162 (1122)c 1.51 (11.5) 100.3 5 64-65c (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 (a) From dilute solutions, 6u (ppm) in CDCI3, reference internal TMS; P-H coupling constants (in Hertz) in parenthesis. 6P (ppm) in CgDg, reference external P4O6. (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). - 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 first 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. The value of L(P=N) in (NPMe2)p (n=4, 1.60A126; n=5, 1.59^127; n=7, 1.59K125; n=8, 1.58A125) is longer than in (NPF2)4 (1.51A028), (NPC12)4 (1.57A129a and 1.56A129b) and [NP(0Me)2]4 (1.57A130), and indicates the extent to which lone pair delocalization into the ring Tr-system is suppressed. A more detailed analysis of the structures and conformations of methylphosphazenes is given in Chapter V. 2.1.4.1 31P Chemical Shifts of (NPMe2)p (n=3-10) 31 The localization of charge on nitrogen is also observed in the P n.m.r. chemical shifts of methylphosphazenes. Figure 2.1(A) shows how the value of 6p in the series (NPX2)n depends upon the nature of X and the value of n. The phosphorus atoms are effectively deshielded by methyl ligands; not only are the individual values of <5 • for (NPMe2)n lower than when more 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, is least for the methylphosphazenes. - 34 -1160 1 — — 3 I* 5 6 7 8 9 10 n in (PNX2)n Figure 2.1 (A) J,P chemical shifts (ppm, relative to ext. Pd06) and (B) v(P=N) frequencies (cm-1) of phosphazenes (NPX?)n (X=Fa, CP, OMeC, Me) as a function of ring size (n). (a) A.C. Chapman, N.L. Paddock, D.H. Paine, H.T. Searle and D.R. Smith, J. Chem. Soc, 3608(1960). (b) L.G. Lund, N.L. Paddock, J.E. Proctor and H.T. Searle, J. Chem. Soc, 2542 (1960). (c) R.A. Shaw, Chem. and Ind., 54 (1959); G. Allen, 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 (NPMeJn (n=3-10) Although the infrared spectra of the methyl phosphazenes (NPMe,,^-^ are, broadly speaking, similar, the detailed analysis and correlation of the spectra of different ring sizes is much more difficult than it is, for instance, in the case of halophosphazenes. The reason for this difficulty lies in the ambiguity in assignment of skeletal vibrations and those due to the CH^ ligands. Certain features however, are common to all the spectra. The antisymmetric ring vibration, v(P=N), is easily identified by its 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 is not thought to be related directly to bond strength. The same type of variation is observed for the ring stretching frequency of dimethyl siloxanes131 '132. In both systems, (X2P=N)n and (X2Si-0)n, the 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 for, all ring sizes, in the region as o of 1410-1430 cm-1. The symmetric deformation, <sSym(CH3)» 1S similarly invariant, and is observed between 1290 and 1305 cm-1. - 36 -2.1.4.3 Mass Spectra of (NPMeJn (n=6-10) Unlike the series of halophosphazenes (NPC12)3_g134 and (NPF2)3_.j2^35, whose mass spectra can be analysed in reasonable detail, the methylphos phazenes (NPMe2)2_iQ are less easily studied by mass spectrometry. The near equivalence in mass of an -NP- unit and three -CH^ units, and the possibility of proton abstractions (which create a problem in the distinction between a -CH2~ group and a nitrogen atom) both make the detailed interpretation of the fragmentation patterns of the methylphosphazenes ambiguous. Thus, the classification of fragments into a cyclic and linear species cannot be made with certainty (as it 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 (NPMe2)g_-|Q are relatively simple. Only a few fragments are observed, nearly all of which correspond to ions of formulae [NxPxMe2X-v^+ •J (y=0,l). 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 stability of the parent ion increases with increasing ring size, so that, in (NPMe2)g_-jg, the parent ion is the most abundant species. When it occurs, fragmentation of the parent ion is a very specific process. The stepwise removal of ligands is 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 -(PNMe2)10 (PNMe2)g (PNMe2)8 (PNMe2)7 (PNMe^6 m/e 300 450 600 750 p3NsJ F^N^J P5N5J PeNgJ P7N^J P8NgJ PgNgJp^oJ Figure 2.2 The mass spectra of the methylphosphazenes (NPt^^.-jn- Tne scales are such that the base peaks of each compound have the same relative intensity. All fragments with an abundance of less than 5% of the base peak are ignored. - 38 -In the fluorophosphazenes, cleavage of the ring occurs in two ways. Abstraction of an N3?3 unit is the principal reaction path, but, as far as (NPF,,)^, significant quantities of fragments indicating the loss of one and two NP units are still observed. In (NPMe2)g_-|0» this latter type of degradation is almost non-existent, and the removal of an N3P3 unit and, to a lesser extent, an N^P^ unit, from the parent ion is the only route through which fragmentation proceeds; e.g. NgPgMe-j2 gives only N3P3 fragments, N^P^Me^ gives mainly N^P^ fragments, and NgPgMe-jg mainly N^P^ fragments. In the spectrum of (NPMe2)g, a significantly large quantity of N3P3 species is observed, suggesting that two N3P3 units are easily removed from the NgPg molecule. As in the case of (NPF2)10, the spectrum of (NPMe2)1Q shows little difference in yield between the smaller fragments. The mass spectra of (NPMe2)g_-|0 are, therefore, distinguished by the propensity of the phosphazene ring to lose one or more N3P3 units. 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 likely 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 (NPMe2)n.MeI (n=3-7) reflect the effects of the perturbation of the cyclic charge distribution by the quaternization of a ring nitrogen atom, and allow the classification 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 shifts indicating their proximity to the positively charged nitrogen atom. The P-methyl protons are more removed from the asymmetric charge distribution in the ring, and their magnetic inequivalence is consequently less marked. Accordingly, as Figure 2.3 shows, the n.m.r. Table 2.2: 31P n.m.r. parameters3 of N-methyl methylphosphazenium iodides. Compound 6PA 6PB 5Pc 6PD (NPMe2)3.MeIb 54.2 76.6 -(NPMe2)4.MeIb 70.0 83.5 - -(NPMe2)5.MeIb 74.1 90.4 90.4 -(NPMe2)g.MeI 76.4 94.5 95.7 - . (NPMe2)7.MeI 77.0 95.8 97.2 97.8 (NPMe2)4.2MeI 65.7 (a) Dilute solutions in DoO, 6(ppm) reference external P406. (b) Data from H.T. Searle, J. Dyson, T.N. Ranganthan and N.L. Paddock, J. Chem Soc Dalton, 203 (1975). - 40 -Table 2.3: H n.m.r. parameters9, (P=N) and (C-N) stretching frequencies'3 of methylphosphazenium quaternary salts. Compound v(P=N) cm- • 'v(C-N). cm"1 6(MeN) 6(MePA)c 6(MePB)C 6(MePc) (NPMe2)3.MeId 1196 1242 1072 3.20 (10.8) 2.02 (12.8) 1.64 (13.9)e -(NPMe2)4.MeId 1220 1240 1075 3.14 (11.2) 2.05 (13.5) 1.55 (12.6) -(NPMe2)5.MeId 1230 1250 1071 3.05 (11.5) 2.04 (13.5) 1.54 (12.4) 1.43 (11.5) (NPMe2)6.MeI 1263 1075 3.08 (11.5) 2.08 (13.5) 1.61 (13.5) 1 .51 (12.5) (NPMe2)7.MeI 1230 1270 1040 2.99 (11.0) 2.01 (13.0) 1.61 (12.5) 1.51f (12.5) (NPMe2)4.2MeS03F 1275 1330 1070 2.84 (11.5) 1.95 (13.7) - -(NPMe2)3.CH2C00EtI+ 1200 1240 g 4.12h (14.5) 1.81 (13.0) 1.581' (14.5) -(NPMe2)4.CH2C00EtI^ 1190 1220 g 4.08J (15.0) 1.85 (13.5) 1.51 (13.5) -(a) Dilute solutions in CDCU, except =(=, in CD3CN. 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(HPA)=1.5Hz. (f) 6(HCPCH(HCPD). (g) Assignment unclear, v(C0) in trimer = 1740 cm-1, and in tetramer = 1737 cm"l. (h) For ethyl group, 6(CH3)=1.37 ppm, S(CH2)=4.22 ppm, J(HH)=7.0Hz. (i) J(HPA)=1 .0Hz. (j) For ethyl group, 5(CH3)=1.37 ppm, 6(CH2)=4.22 ppm, 0(HH)=7.0Hz. - 41 -(CH3)2PD,C (CH3)2PB <CH3>2PA—^ (CH3)N I CH, H3C 'PCH, A H3C—PD-CH3 N H4C / CH V Pa' •CH, H,C /C^N=PB^' H3C \\ 'PCH, CH, PNCH3 ^P'NCH, 3.5 3.0 1.5 1.0 2.5 2.0 S(ppm) Figure 2.3 Ordinary 100 MHz 'H n.m.r. spectrum (A) and 31P decoupled ]H n.m.r. spectrum (B) of (NPMe9)7.MeI (both on samples in CDC10 solution). c 1 6 - 42 -spectrum of (NPMe2)7.MeI does not differentiate between the two types of proton (PcMe2 and PpMe,,) most remote from the N-methyl group. The single resonances of the P^Me2 and PgMe,, protons, however, are easily distinguished, as is the characteristic triplet corresponding to the N-methyl protons. The n.m.r. spectra of the N-ethylacetato salts C(NPMe2)3 4CH2C00Et]+I" are qualitatively similar to those of the N-methyl salts, the N-methylene resonance being split into a triplet by coupling with the two nearest ring phosphorus atoms. The diquaternary cation 2+ [(NPMe2)42Me] has a centrosymmetric structure, as expected on a simple 31 1 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 salts, 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 salts, but the v(C=0) band in them is easily observable near 1740 cm-1. 2.2 Preparation of gem-MepPh^P^N^ "IOC "I o o To date, three different methods , all involving ring closure reactions, have been reported for the preparation of Me2Ph4P3N3 (although 137 138 in two cases ' it was only isolated as its hydrohalide salt Me2Ph4P3N3HX (X=C1, I)). - 43 -The method used in this work is a modification of that first ,136 , and employs the reaction between dimethyltrichlorophosphorane reported and the linear phosphazene hydrochloride [NH2(Ph2)PNP(Ph2)NH2]"rCl (Equation 6). The product is isolated as its hydrochloride, which is cr NH2 Ph—/P—Ph / \ kJ n., -3HCI Me2PCl3 • Ph Ph CI" P—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 still not good) yield of ^20%. It is apparent from this reaction, and similar ring closure reactions involving [NH2(Ph2)PNP(Ph2)NH2]+Cl" and phosphorus(V) chlorides (PClg139'140, PhPCl^ and Ph^Cl^40) that competing reactions play a significant role in lowering the yield of the desired product (usually <35%). The reaction of [NH2(Ph2)PNP(Ph2)NH2]+Cl" and PCIg in the absence of solvent gives140 not only the expected product CI3Ph3P3N3 (in 25% yield) but also the tetrameric derivative Cl^Ph^P^N^ (in 6% yield). Similarly, when the ring closure is attempted using Ph2PCl3, 24% of the tetrameric phosphazene (NPPh2)4 is - 44 -formed along with the expected trimer (NPPh2)3140. Consistently, it has been found in the present work that the reaction of Me^PCl^ and [NH2(Ph2)PNP(Ph2)NH2]+Cl" gives significant quantities of (NPPh2)4 and (NPMe2)4 as well as the desired product NgP-jPh^Me^ Although the formation of these anomalous products has been 140 previously recognised , its explanation has remained in doubt. The isolation of (NPMe2)4 from the reaction of [NH2P(Ph2)NP(Ph2)NH2]+Cl and Me2PCl.j does, however, provide a useful insight into the reaction mechanism. It is postulated that the chlorophosphorane R2PC13 can act as a chlorinating agent (Equation 7), producing the linear phosphorane [NH2-(Ph2)P=N=P(Ph2)NH2]+Cl" + R2pCl3 -»• NH2-(Ph2)P=N-P(Ph2)-Cl2 + [R9P(NH9)C1]+C1" / * ....7 Ph8P4N4 Ph4R2P3N3 Ph4R4P4N4 R8P4N4 NH2P(PH2)NP(Ph2)Cl2 and the amino salt [R2P(NH2)C1]+Cl" Cyclization can then occur in a number of ways, producing the observed range of cyclic 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 [R9P(NH9)]+C1 from chloramine and R2PNH2110»141, * * - 45 -salts (although (NPPh2)3 can be quaternized by Me^OBF^ ). Dimethyltetra-phenylcyclotriphosphazene provides a compromise between the two extremes and can be quaternized by methyl iodide to give the salt Me2Ph4P3N3.MeI. 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 PMe2 group (Equation 8). Me Me \/ N N Ph— / Ph Ph Mel N Ph— / Ph Me Me \/ .Me •N Ph 2.3 Preparation and Chemistry of Tetramethylphosphinimine 2.3.1 Reaction of Trimethylphosphine and Methyl Azide The classical preparative route to phosphinimines is via the 142 Accordingly, 143 reaction of a tertiary phosphine and an organic azide the preparation of Me3PNMe by this method has recently been reported1 (Equation 9). However, during the present work, the same reaction has been Me3P + MeN3 —• Me3P=NMe + N2+ studied independently, and slightly 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% yield 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 salt-like compound [Me3P=N=PMe3]+N3~. The isolation of this latter compound is unexpected, but not without precedent. Similar bis(trialkyl-phosphine)iminium salts have been isolated during the methanolysis of 144 145 trialkylphosphinimines and the pyrolysis of aminophosphonium salts (Equations 10 and 11). R3PNH Me0H» [R3P=N=PR3]+0CH3" .... 10 [R3PNH2] CI" —A> [R3P=N=PR3]+C1" .... 11 2.3.2 Preparation of [MegPNMeJ+I-Consistently with its basic character, Me3PNMe reacts rapidly with methyl iodide to give the expected N-methyl salt [Me3PNMe2]+l". The isolation of such a compound, via the reaction of methyl iodide and dimethyl ami no-dimethylphosphine, has already been described146. However, its reported melting point of 315-320°C is much higher than that found in the present work (d> 220°C), and its 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 all 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 -R3PC12 ^jrNH2 [R3PNHR']+Cr ~HX • R3P=NR' .... 12 R p 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 salt, and varies from triethylamine147 to 149 I5n sodium amide and sodium in liquid ammonia. In the present work, the aminophosphonium salt [Me3PNHMe]+Cl~ was prepared by the reaction of methylamine and Me3PCl2, and its subsequent deprotonation attempted with (1) sodium amide in liquid ammonia, and (2) potassium t-butoxide (Equation 13). The reaction between [Me3PNHMe]+Cl" and NaNH2 yielded not the expected NaNhL [Me3P=N=PMe3]+cr NH?Me , , Me.PClA [Me-PNHMe] CI" ^ ; .... 13 L 6 \K0tBu Me3PNMe product, but the linear salt [Me3P=N=PMe3]+Cl~. The isolation of the [ P=N=P ]+ cation from this reaction is not easily rationalized. This type of behaviour is not encountered in the reaction of sodium amide with aminotriarylphosphonium salts, but its occurrence is reminiscent of the difficulties alluded to in other work on the preparation of trialkylphos-phinimines144'150. The reaction of potassium t-butoxide and [Me3PNHMe]+Cl" was more successful, and a good yield of Me3PNMe was obtained. However, repeated fractional distillation was required to purify the product from the side-product, t-butanol. - 48 -2.3.4 Reaction of Dihalophosphoranes with (Me3Si)2NMe The reaction between fluorophosphoranes RPF^ (R=Me^"1, Ph16\ 151 152 F ' ) and heptamethyldisilazane, to give dimeric phosphazanes (Equation 14), is well known. In this work, the reactions of the RPF4 + (Me3Si)2NMe • 7j-(RF2PNMe)2 + 2Me3SiF 14 dihalophosphoranes Me3PCl2 and Me3PF2 with (Me3Si)2NMe have been investigated to see if the phosphinimine Me3PNMe could be produced. However, in the case of Me3PF2, it was found that no reaction took place. At first sight, this result is somewhat surprising, but it is consistent IM l^l 1^? with the observed low reactivity of Me2PF3 , Ph2PF3 1 and PF3 towards N-alkyl hexamethyldisilazanes. To account for this, it has been suggested that the reactivity of phosphorus fluorides towards silazanes depends on 151 their Lewis acidity In the case of Me3PCl2, a reaction did occur, but the expected 2+ product was not isolated. Instead, the linear salt [Me3P=N(Me)-PMe3] 2C1 was formed, according to Equation 15. This behaviour parallels that of 2Me3PCl2 + (Me3Si)2NMe —• [(Me3P)2NMe]2+2Cl" + 2Me3SiCl .... 15 phosphorus trichloride, which, with both MeNH2 and MeN(SiMe3)2> gives not ICO I CA the cyclic molecule (MeNPCl)2 , but the linear species (Cl2P)2NMe . 2.3.5 Structure and Spectra of Phosphinimine Derivatives Although there exists a formal structural resemblance between the phosphinimine Me3PNMe and the methylphosphazenes (NPMe2)n, the chemical - 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 is, for example, capable of decomposing chloroform (presumably by proton removal). Consistently, its hydro chloride [MePNHMe]+Cl~ is a much weaker acid than are the hydrochlorides of methylphosphazenes; whereas (NPMe2)3.HCl can be deprotonated with triethyl amine, [Me3PNHMe]+Cl~ requires potassium t-butoxide. The highly polar nature of the PN bond in Me3PNMe is also indicated by its rapid hydrolysis in moist air. By contrast, methylphosphazenes are stable in hot aqueous alkali. 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 Me3PNMe is found to high field Table 2.4: n.m.r. parameters9 of Me^PNMe and its derivatives Compound Solvent 6H(MeP) 6H(MeN) Me3PNMe C6D6 . 0.98(12.5) 3.01(27.5)b Me3PNMe.HCl CDC13 2.11(14.0) . 2.69(14.5)° Me3PNMe.MeI CDC13 2.23(13.3) 2.73(10.6)b [(Me3P)2NMe]2+2Ci; 2.36(13.0) 3.24(10.0) [(Me3P)2N]+CT CDC13 1.93(13.1) -[(Me3P)2N]+N3 CD3CN 1.70(13.7) -(a) Dilute solutions. <5(ppm) reference internal TMS, except reference internal DSS. PH coupling constants (in Hertz) in oarenthesis. (b) 6(CH~N) in Ph3PNMe=3.01 ppm (24.7Hz), <$(CH3N) in Ph3PNMe.HBr=2.78 ppm (13.8Hz), 6 F. Kaplan, G. Singh and H. Zimmer, J. Phys. Chem., 6]_, 2509 (1963). (c) J(H-H)=5.8Hz. - 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(CH3P) in [Me3PNMe2]+l" is 2.23 ppm, and in (NPMe2).MeI it is 2.02 ppm (see Table 2.3). Consistent with this order, the N-methyl protons in [Me3PNMe2]+I~(6=2.73 ppm) are more effectively shielded than they are in (NPMe2)3.MeI (6=3.20 ppm). The trends in these chemical shifts suggest that, in the phosphinimine salt [Me3PNMe2]+I~, the positive charge of the cation is located almost completely on phosphorus (Equation 16), with little or no double bond character associated with the PN bond. Consistently, no v(P=N) is observed in the infrared spectrum of [Me3PNMe2]+I~. By contrast, the ^H chemical shifts of the [(NPMe2)3>Me]+ cation indicate that (in 7 F> agreement with calculated ir-charge densities ) much of the positive charge of the ion is localized on the quaternized nitrogen atom (Equation 17), thereby preserving cyclic aromaticity. Me \ • / Me — P-—N / \ Me Me Me Me Me—P: / Me +/ \ Me Me 16 Me Me N N Me—P. ^P—Me Me N Me Me Me \/ Me Me—P. . \ /P —Me Me Me 17 - 51 -The localization of charge on phosphorus, rather than on nitrogen 2+ is again observed in the doubly charged cation [(Me3P)2NMe] , the methyl protons of which resonate at* 6 = 2.36 ppm. In the cyclic dication 2+ [(NPMe2)4.2Me] , the analogous signal is observed at 6 = 1.95 ppm. Charge delocalization, and the preservation of iT-character for the PN bond, is only possible in the cation [Me3P=N=Me3]+; accordingly, the P-methyl protons in [(Me3P)2N]+Cl~ are more shielded (6=1.93 ppm) than they are in, for instance, [Me3PNMe2]+I~ (6=2.23 ppm). Consistently, a strong v(P=N) band (at 1246 cm"1) is observed in the i.r. spectrum of [(Me3P)2N]+Cl~. As expected, the infrared spectra of the phosphinimine derivatives described here show many similarities. The vibrational spectra of 143 Me3PNMe itself are described in detail elsewhere (see also Chapter IV). The ionic compounds [Me3PNMeR]+ (R=H, Me, PMe3+) and [(Me3P)2N]+ all display bands corresponding to 5symCH3 (1300-1330 cm-1), CH3 rocking (960-980 cm"1), CH3 wagging (870-890 cm"1) and P-CH3 stretching (720-770 cm"1). In addition, the derivatives [Me3PNMeR]+ (R=H, Me, PMe3+) all 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. These topics will be fully discussed in the following chapters. - 52 -2.4 Chemistry of gem-Dichlorotetraphenylcyclotriphosphazene 2.4.1 Attempted Methylation of CI2Ph4P3N3 witn MeMgBr During the course of work on the preparation of Me^Ph^P^N^, the 139 140 phosphazene C^Ph^N^II) was prepared ' to see if its reaction with MeMgBr, according to Equation 18, would afford a convenient preparative route N CI CI \/ N Ph—P. J?— Ph Ph * Ph II Cl2Ph4P3N3 + 2MeMgBr • Me2Ph4P3N3 + 2MgC1Br ... 18 to Me2Ph4P3N3> However, it was found that, unlike ClgP3N3, Cl2Ph4P3N3 does not react at all with MeMgBr. After boiling a solution of CI2Ph4P3N3 in ether with an excess of MeMgBr for four days, only the starting material was recovered (almost quantitatively). Because of its surprisingly low reactivity towards MeMgBr, a number of other substitution reactions of CI2P'14P3N3 were attempted, in order to compare its reactivity with that of CUP-N,. 5 3 3 2.4.2 Dimethylamination of ClpPh^Nj Consistent with its low reactivity towards MeMgBr, and in contrast to 1 55 the behaviour of ClgP^1 , CI2Ph4P3N3 reacts only in the presence of an 53 excess of dimethylamine and/or under extreme conditions (high temperature). The reaction of CI2PH4P3N3 and dimethylamine, in the molar proportion 1:2, 156 according to Equation 19, has been reported to give a very low yield (^9%) of (NMe2)ClPh4P3N3. In the present work, the yield has been improved el2PH4P3N3 + HNMe2 * (NMe2)C1Ph4P3N3 + HNMe2-HC1 •••• 19 to a.nearly quantitative value by bubbling an excess of dimethylamine through an ethereal solution of cl2PH4P3N3 for several nours- Substitution of the second chlorine, however, cannot be achieved under such mild conditions, and (NMe2)2Ph4P3N3 can only be prepared by reacting CI2PH4P3N3 1 ETC with an excess of HNMe2 at 180°C . 2.4.3 Fluorination of C12Ph4P3N3 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 32 fluorosulphite ' , is most useful for the preparation of fully fluorinated phosphazenes, since the introduction of one fluorine into the ring activates it to further substitution. Consecutive substitution is, 57 58 as expected, predominantly geminal ' . BY contrast, the fluoridation of chlorophosphazenes using antimony trifluoride usually takes place non-geminally ' ... Because the effect of SbF3 depends on.its preliminary coordination to a ring nitrogen atom, progressive fluoridation is inhibited by the decreasing donor strength of the phosphazene ring. By itself, SbF3 is too weak an acceptor to react with (NPC12)3157. 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^ , (NMe^Cl^Ng 157 and (NI^^C^P^N^ ) or when antimony pentachloride is added to the reaction mixture34'158. 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 its 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^Ph^N^. Similarly, sodium fluoride, and an (NaF+HF) 1 CO mixture , both failed to fluorinate it. However, the reaction of an SbF^/SbClg mixture with C^Ph^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 (Nf^KlPh^P.^. Consistent with the activating effect of the amine ligand, it was found that the fluorination of (NMe2)ClPh4P3N3 could be effected using SbF^ alone, without the presence of SbClg. 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 electro negative chlorine atoms being capable of withdrawing ir-electron density from the P(Ph2) phosphorus atoms onto the P(C12) atom. In the phosphazene ClPhgP^, the reactivity of the P(PhCl) atom towards nucleophiles is substantial160'161. 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 of-.(NMe^ClP^ (e.g. Equation 20). Consistent with this, silver perchlorate reacts with ClPhgP3N3 in pyridine, yielding the salt-like compound PhgP3N3.CIO^.py. Me Me ' • V . \ / Me2N CI Jj+ CI" P P II I II I During the present work, an attempt was made to prepare a similar salt to be the reaction of BF3.Et20 with (NMe2)FPh4P3N3, according to Equation 21. Unfortunately, the product that was isolated was not the BF3.Et20 + (NMe2)FPh4P3N.3—• [(NMe2)Ph4P3N3]+BF4" + Et20 .... 21 expected phosphazenium salt(III) but the protonated salt [(NMe2)FPh4P3N3H]+[BF30H]". Although it was not the desired product, this compound is, nonetheless, informative, since it 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 NMe2 group sufficiently 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 its structure is V rather than IV. Protonation of the F(NMe2)Ph4P3N3 molecule greatly affects the electron distribution in the PN ring. In relation to the neutral molecule (6P(Ph2)=94.1 ppm, 6PFNMe2=95.6 ppm), the P(Ph2) atoms are considerably deshielded (6=87.2 ppm), with a consequent transfer of charge onto the PFNMe2 atom (6=100.6 ppm). Ms Me \/ N* BF/" N Ph-/ Ph N Ph Me?N F 1 \ / N J>—Ph Ph-P Ph + N ^VPH ^ >h Me9N F 2 \/ N N Ph—R / Ph + N I H •Ph Ph III IV 2.5 Experimental Section 2.5.1 Preparation of Methylphosphazenes l fi^ Dimethyltrichlorophosphorane was prepared by the chlorination of tetramethyldiphosphine disulphide in carbon tetrachloride, and dried in vacuo over P20g. Methylamine hydrochloride and ammonium chloride were - 57 -commercial products, and were oven dried at 120°C for 24 hours before use. The fluorophosphazenes (NPF2)g_-jQ were obtained from Prof. N.L. Paddock. They had been prepared by the fluorination, with KSC^F, of a mixture of 133 chlorophosphazenes (NPC12)n (n>5), and separated by v.p.c. . Methyl 165 magnesium bromide was prepared using standard procedures from commercially obtained magnesium turnings and methyl bromide. Diethyl ether was dried before use by distillation 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 oil which was immiscible with the solvent. After the mixture had been heated for one hour under reflux, the solvent was distilled off to leave a viscous oil. (N.B. Prolonged heating under reflux of this mixture reduces the relative yield of the trimer; it also lowers the overall yield of methylphosphazenes). The oil was then heated at 165°C/0.05 Torr for four hours, during which time it gradually solidified into an off-white mass. This solid was then extracted with 3 x 200 ml of hot acetonitrile. The extracts were combined and concentrated, whereupon a white, slightly hygroscopic solid (11.94g) crystallized out. A further 0.84g of the same solid was obtained by evaporation, under nitrogen, of the mother liquor. The whole was recrystallized from acetonitrile and identified by its analysis and n.m.r. spectrum as the quaternary salt 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, triplet, JpH=10.5Hz), 1.87 (12H, doublet, JpH=13.5Hz), 1.57 (6H, doublet, JpH=14.5Hz, long range JpH=1.5Hz); i.r. v(P=N) 1190, 1245 cm"1. Anal, calcd. for C7H21C1N3P3: C, 30.50; H, 7.68; CI, 12.86; N, 15.24. Found: 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 salt, and N^Me^ was collected, in nearly quantitative yield, by sublimation onto a water-cooled cold-finger. The compound was identified 21 by its infrared spectrum , analysis and melting point. Anal, calcd. for C6H18N3P3: C, 31.98; H, 8.06; N, 18.66. Found: C, 32.13; H, 7.99; N, 18.80. M.pt. 187-188°C, lit. 195-196°C21 and 187-190°C (dec)77. The insoluble residue from the acetonitrile extraction (2.76g) was recrystallized from hot methanol/chloroform and dried in vacuo. This slightly hygroscopic solid was identified by its analysis and n.m.r. spectrum as the diquaternary salt N4P4Meg.2MeCl (6.9 mmol, 15%), m.pt. >275°C. ]H n.m.r. (6, D20, internal DSS), 2.93 (6H, triplet, JPH=11.5Hz), 2.03 (24H, doublet, JPH=13.5Hz); i.r. v(P=N) 1350 cm"1. Anal, calcd. for C10H30C12N4P4: C' 29-94' H' 7-54' CI, 17.67; N, 13.97. Found: C, 29.68; H, 7.70; CI, 18.00, N, 13.79. 2.5.1.2 Preparation of (NPMe2)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 acetonitrile, 11.2g of an insoluble solid, which was identified by its infrared spectrum77'109 as the dihydrochloride (NPMe2)4.2HCl. This salt was converted quantitatively into the neutral compound by treating it 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 (NPMe2)4, which was identified by its infrared spectrum21 and melting point, 161-162°C (lit. 162-163°C21, 155-156°C77 and 157-158°C110). Evaporation of the acetonitrile extract yielded a yellow oil which, upon treatment with NEt^ and extraction with hexane, yielded a small quantity (^lOOmg) of (NPMe2)3, which was identified by its infrared spectrum and melting point (see Section 2.5.1.1), 2.5.1.3 Separation of (NPMeJj and (NPMeJ4 This experiment demonstrates how a mixture of (NPMe2)3 and (NPMe2)4 109 can be separated via their hydrochlorides . A mixture of (NPMe2)3 (1.34g, 5.94 mmol) and (NPMe2)4 (1.41g, 4.71 mmol) was dissolved in 200 ml of diethyl ether. Hydrogen chloride was bubbled through the solution until precipitation of the methylphosphazene hydrochlorides was complete. The resulting white precipitate was filtered off and dried in vacuo (3.36g). This solid was shaken with 100 ml of hot acetonitrile and the mixture filtered. The insoluble N4P4Meg.2HCl (1.78g, quantitative) was identified by its infrared spectrum77'109. Anal, calcd. for C8H26C12N4P4: C' 25>75; H, 7.02, CI, 19.00; N, 15.02. Found: C, 25.61; H, 6.78; CI, 18.90; N, 15.05. M.pt. >260°C. Evaporation of the acetonitrile extract gave N3P3Me6.HCl (1.51g, 97%), m.pt. 247-249°C, i.r. v(P=N) 1170, 1230 cm"1.. Anal, calcd. for C6HlgClN3P3: 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 V. 2.5.1.4 Preparation of (NPMe2)n (n=6-10) The experimental procedure for the preparation of the methylphos phazenes (NPMe2)g_-]Q is identical. Therefore, only the preparation of one of them (the hexamer) will be described in detail. A solution of (NPF2)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 left at reflux temperature, under nitrogen, for a 48 hour period. The solvent was then distilled 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 all the magnesium as its carbonate. The solution was filtered and the solvent evaporated from the filtrate to leave a white solid, which was repeatedly extracted with hot chloroform. The solvent was distilled 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 refiltered, the solvent removed from the filtrate, 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 (NPMe2)( m.pt. 163-165°C. Anal, calcd. for C12H36N6P6: C' 32-01» H' 8-06; N, 18.66. Found: C, 32.00; H, 8.06; N, 18.74. The phosphazenes (NPM^^-IO were PreParecl using a procedure analogous to that just described^. 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 (NPF2).h + 2nMeMgBr->(NPMe2) . (n=6-10)-. n in Yield M.pt. Composition3 (NPMe2)n % °C %C %\ 6 67 163-165 32.00 8.06 18. .74 7 ., 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 Anal, calcd. for (C2HgNP) n: C 32.01; H, 8.06; N, 18.66 The compounds (NPMe?)R_ln were prepared by Mr. K.D. Gallicano. - 62 -2.5.1.5 Preparation of Me2Ph4P3N3 The acyclic compound [NH2(Ph2)PNP(Ph2)NH2]+Cl~ was prepared by the ammonolysis of diphenyltrichlorphosphorane, and purified by . 24 139 recrystallization from acetonitrile ' . Dimethyltrichlorophosphorane (17.14g, 102.3 mmol) and [NH2(Ph2)PNP(Ph2)NH2]+CT (46.20g, 102.3 mmol) were heated together, under reflux, in a slurry of 200 ml of chlorobenzene. After one hour, the . solvent was removed by distillation, and the residual light brown oil 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 solid, the i.r. spectrum of which indicated the presence of the hydrochloride (NPMe2)4.2HCl. This solid was then heated with NEt3 (as described in section 2.5.1.2) and the product extracted with hot hexane to yield, upon evaporation of the solvent, 2.96g (10.0 mmole, 40%) of (NPMe2)4, which was identified by its infrared spectrum and melting 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 it to be the hydrochloride 138 Me^h^P^N-j.HCl . This compound was dissolved in chloroform and the mixture treated with aqueous triethyl amine. The organic layer was washed with water, dried over Na2S04, and evaporated to leave a white solid, which was purified by recrystallization from ether/hexane to give colourless blocks of Me2Ph4P3N3 (7.2g, 15.2 mmol, 15%), melting point 142-144°C - 63 -(lit.IJD 140-142°C). Anal, calcd. for C^gN^: C, 65.96; H, 5.54; N, 8.88. Found: C, 65.89; H, 5.66; N, 8.81. 2.5.2 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 redistilled, under nitrogen, before use. Acetonitrile was dried by distillation from calcium hydride. The methylphosphazenes (NPMe2)n were all dried by sublimation in vacuo before use. Me2Ph^P3N3 was oven dried at 100°C for 3 hours. 2.5.2.1 Preparation of (NPMeJg.Mel A sample of (NPMe2)g (0.366g, 0.813 mmol) was dissolved in 10 ml of methyl iodide and the mixture gently heated under reflux for two hours. Subsequent filtration of the mixture, and recrystallization of the filtered precipitate (0.480g, 0.81 mmol, 100%) from acetonitrile/toluene yielded colourless, slightly hygroscopic crystals of the adduct (MPMe2)6.MeI.(CgH5.CH3) (m.pt. 171-173°C). Heating the powdered crystals at 90°C/0.01 Torr for seven hours freed the product of solvated toluene. Anal, calcd. for C'13H3gIN6P6: C, 26.38; H, 6.64; I, 21.44; N, 14.20. Found: C, 26.08; H, 6.60; I. 21.20; N, 14.13. M.pt. 175-178°C. 2.5.2.2 Preparation of (NPMeJ7.MeI A sample of (NPMe2)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 solid, which was filtered off and recrystallized from a mixture of hot toluene and a few drops of acetonitrile to give air stable crystals of (NPMe2)y.MeI (0.270g, 0.405 mmol, 100%). Anal, calcd. for c-|5H45IN7P7: C, 27.00; H, 6.80; I, 19.02; N, 14.69. Found: C, 26.95; H, 6.87; I, 18.91; N, 14.69. M.pt. 153-155°C. 2.5.2.3 Preparation of Me,,Ph4P3N3.MeI A sample of Me2Ph4P3N3 (1.296g, 2.73 mmol) was dissolved in 20 ml of methyl iodide and the mixture heated gently under reflux for 72 hours. The mixture was then filtered 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 Me2Ph4P3N3.MeI. Anal. calcd. for C27H29IN3P3: C, 52.70; H, 4.75; I, 20.62; N, 6.83. Found: C, 52.90; H, 5.00; I. 20.40; N, 6.83. M.pt. 226-227°C (dec). 31P n.m.r. spectrum (s, CDC13, external P40g); 64.1 (IP, PMe2 atom), 81.0 (IP, PPh2 atom adjacent to N(Me) atom), 93.5 (IP, PPh2 atom remote from N(Me) atom). "'H n.m.r. spectrum (6, CDC13, internal TMS); 3.07 (3H, triplet, JpH=11.0Hz), 2.22 (6H, doublet, JpH= 14.0Hz), 7.0-8.0 (20H, broad unresolved multiplet). i.r. spectrum; v(P=N), 1220, 1260 cm"1; v(C-N), 1070 cm"1. 2.5.2.4 Preparation of [(NPMe3)3.CH2C00Et]+I" A sample of (NPMe2)3 (0.633g, 2.81 mmol) was dissolved in 10 ml of ethyl iodoacetate and the mixture stirred, under a nitrogen atmosphere, at - 65 -room temperature, for 48 hours. The mixture was then filtered, and the precipitate recrystallized from acetonitrile/benzene to give colourless blocks of [(NPMe2)3.CH2C00Et]+l" (0.943g, 2.14 mmol, 76%). Anal, calcd. for C10H25IN302P3: C, 27.35; H, 5.74; I. 28.90; N, 9.56. Found: C, 27.63; H, 5.76; I, 28.70; N, 9.64. Dec. >220°C. 2.5.2.5 Preparation of [(NPMe2)4-CHoCOOEt]"1"!" A sample of (NPMe2)4 (0.982g, 3.27 mmol) was dissolved in a solution of 8 ml of ICH2C00Et and 30 ml of diethyl ether, and the mixture heated gently under reflux for 96 hours. The white precipitate was then filtered from the solution and recrystallized from acetonitrile/benzene to give small, colourless crystals of [(NPMe2)4.CH2C00Et]+I" (1.073g, 2.09 mmol, 64%). Anal, calcd. for C12H31IN402P4: C, 28.14, H, 5.71; I, 24.78; N, 10.94. Found: C, 28.20; H, 5.80; I, 24.52; N, 10.71. M.pt. 176-178°C. 2.5.2.6 Preparation of [(NPMe2)4-2Me12+ 2X~ (X=FS03>I) A solution of MeS03F (1.20g, 10.5 mmol) in 10 ml of acetonitrile was added to a stirred solution of (NPMe2)4 (1.545g, 5.15 mmol) in 50 ml of acetonitrile, and the mixture heated under reflux, under an atmosphere of nitrogen, for two hours. Upon cooling the solution, colourless needles of (NPMe2)4-2(MeS03F) appeared. 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 (NPMe2)4.2MeS03F. Anal, calcd. for C-|0H30F2N4°6S2P4: - 66 -C, 22.73; H, 5.72; N, 10.60. Found: C, 22.90; H, 5.91; N, 10.56. M.pt. 231-233°C. The di-iodide salt (NPMe2)4.2MeI was prepared, in quantitative yield, by eluting an aqueous solution of (NPMe2)4.2MeS03F through a column 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 acetonitrile, gave colourless blocks of (NPMe2)4.2MeI. Anal, calcd. for c-|q^^SO1 2N4P4: C' 20-56; H, 5.18; I, 43.45; N, 9.59. Found: 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 methods1^-1^. Methyl azide was prepared by the action of dimethylsulphate on sodium azide170'171, and purified (from Me20) by trap-to-trap fractionation. Trimethyldichlorophosphorane was prepared by the chlorination of Me3P050 with thionyl chloride172. Similarly, trimethyldi-fluorophosphorane was prepared by the fluoridation of Me3P0 with sulphur tetrafluoride (yield, 73%). Me3PF2 has not previously been made by this method; it was identified by its infrared spectrum and boiling point 173 (76°C, lit. 76°C). Heptamethyldisilazane was prepared by the reaction of methylamine with chlorotrimethylsilane. Methylamine, sodium amide and methyl iodide were commercial products and used without purification. 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 MeN^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 originally formed precipitate redissolved. The mixture was then filtered under nitrogen, and the residue recrystallized from acetonitrile to give air stable crystals of (Me3P)2N+ N3~ (0.661g, 3.17 mmol). Anal, calcd. for C^gN^: C, 34.60; H, 8.72, N, 26.92; P, 29.77. Found: C, 34.42; H, 9.00; N, 22.26*; P, 30.20. M.pt. 211-214°C. The solvent was distilled from the filtrate to leave a colourless, extremely hygroscopic liquid, which was distilled in vacuo to give Me^PNMe (1.51g, 14.4 mmol'). Anal, calcd. for C4H12NP: C, 45.68; H, 11.51; N, 13.33. Found: C, 45.38; H, 11.58; N, 13.30. B.pt. 73°C/18 Torr. 2.5.3.2 Preparation of [Me3PNMeJ+r 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 Me3PNMe (0.219g, 2.08 mmol) in 5 ml of ether. A white precipitate was No satisfactory nitrogen analysis could be obtained for this compound. - 68 -immediately formed. After j hour, the solution was filtered and the residue recrystal1ized from acetonitrile to give air stable crystals of [Me3PNMe2]+l"(0.512g, 2.06 mmol, 99%). Anal, calcd. for C^INP: C, 24.29; H, 6.12; N, 5.67. Found: C, 24.46; H, 6.20; N, 5.44. Dec >220°C. 2.5.3.3 Preparation of [Me3PNHMe]+Cl" Methylamine was bubbled through a suspension of Me3PCl2 (8.46g, 57.6 mmol) in 50 ml of chloroform for ^ hour. The mixture was then heated under reflux for ^ hour to remove the excess methylamine. Filtration of the solution and removal of solvent from the filtrate yielded a white hygroscopic solid, which was recrystallized from acetonitrile/toluene to give [Me3PNHMe]+cr (8.06g, 57.0 mmol, 99%). Anal, calcd. for C^CINP: C, 33.91; H, 9.26; CI, 25.05; N, 9.90. Found: C, 32.67*; H, 9.57; CI, 25.03; N, 9.80. M.pt. 180-185°C (dec). 2.5.3.4 Reaction of [Me3PNHMe]+Cl with Sodamide in Liquid Ammonia A sample of [Me3PNHMe]+Cl~ (3.43g, 24.2 mmol) was added to a slurry of sodamide (l.Olg, 25.9 mmol) in 25 ml of liquid ammonia, and the mixture allowed to react for one hour. The ammonia was then removed and the residual solid extracted with hexane. Evaporation of this extract yielded no product. 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 -(Me3P)2N+Cl" (1.15g, 5.71 mmol). Anal, calcd. for CgH^ClNP^ C, 35.72; H, 9.00; CI, 17.59; N, 6.95. Found: C, 35.37; H, 9.20; CI, 17.90; N, 6.65. Dec > 210°C. 2.5.3.5 Reaction of [Me3PNHMe]+Cl~ with Potassium t-Butoxide A sample of [Me3PNHMe]+Cl was added under an atmosphere of nitrogen, 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 filtered and the solvent distilled off to leave a colourless liquid which was then distilled in vacuo. The infrared and n.m.r. spectrum of this product (2.075g) showed it 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 distilled with the heptane, and the careful fractional distillation in vacuo of the residual liquid yielded, as a final fraction, a small quantity of pure Me^PNMe (as shown by its boiling point, 73°C/18 Torr, and n.m.r. spectrum). 2.5.3.6 Reaction of Me3PF2 with (Me3Si)2NMe Heptamethyldisilazane (2.13g, 12.2 mmol) was sealed in a Carius tube with Me3PF2 (1.39g, 12.2 mmol) and the resultant mixture heated at 70°C for 48 hours. No visible change had occurred after this time, and the starting materials were recovered by fractional distillation of the reaction mixture. - 70 -2.5.3.7 Reaction of Me3PC12 with (Me3Si)2NMe A solution of (Me^i^NMe (5.70g, 32.6 mmol) in 10 ml of chloroform was added to a slurry of Me3PCl2 (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 filtered from the solution and recrystallized from a methanol/chloroform mixture to give colourless, hygroscopic crystals of [(Me3P)2NMe]2+2Cl~ (2.97g, 11.8 mmol, 73%). Anal, calcd. for C7H21CI2NP2: C, 33.32; H, 8.40; CI, 28.14; N, 5.56. Found: C, 33.18; H, 8.42; CI, 27.74; N, 5.46. Dec >220°C. 2.5.4 Preparation of XYPh4PoN3 Derivatives Dichlorotetraphenylcyclotriphosphazene was prepared by the reaction of phosphorus pentachloride with [NH2(Ph2)PNP(Ph2)NH2] CI in benzene . Dimethylamine (lecture bottle), antimony trifluoride and antimony pentachloride were all commercial products and used without further purification. Boron trifluoride etherate, BF3.Et20, was also obtained commercially, but was distilled, under nitrogen, before use. 2.5.4.1 Preparation of (NMeJClPh^No Dimethylamine was bubbled through a solution of CI2P'n4P3N3 C -292g, 2.51 mmol) in 150 ml of diethyl ether for four hours. The reaction mixture was then gently heated under reflux to expel the excess dimethylamine. Filtration of the mixture and evaporation of the filtrate yielded a crystalline solid, which was recrystallized from hot hexane and identified by its analysis and melting point (163-165°C, lit.156 164-165°C) as (NMe2)ClPh4P3N3 (2.251g, 2.39 mmol, 95%). Anal, calcd. for C^H^CIN^: - 71 -C, 59.72; H, 5.01; CI, 6.78; N, 10.71. Found: C, 59.61; H, 5.07; CI, 6.58; N, 10.91. ]W n.m.r. spectrum (6, CDC13, internal TMS); 2.68 (6H, doublet, JpH=16.5Hz), 7.0-8.0 (20H, unresolved multiplet). 2.5.4.2 Preparation of FoPh4P3N3 A sample of CI2Ph4P3N3 (3.024g, 5.79 mmol) was added to a slurry of an excess of SbF3 (^4g) in 25 ml of sym-C2H2Cl4 and the mixture brought to reflux temperature under a nitrogen atmosphere. After 1^ hours at this temperature, no visible 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 filtered, the precipitate washed with benzene, and the solvent distilled from the filtrate to leave a dark oil. A few drops of water were added to this oil (to destroy excess antimony halides), and the whole extracted with dichloromethane. This extract was dried over Na2S04, and then evaporated to leave a white solid, which was recrystal1ized from hot octane to give colourless blocks of F2P'14P3N3 (°-7339' ^-52 mmol, 26%), which was identified by its analysis, icq Icq melting point (135-136°C, lit. 137-138°C) and infrared spectrum . Anal, calcd. for. C24H20F2N3P3: C' 59-88'> H> 4-19^ N,. 8.73. Found: C, 59.65; H, 4.28; N, 8.54. 2.5.4.3 Preparation of (NMejFPh^Ng A sample of (NMe2)ClPh4P3N3 (4.31g, 8.26 mmol) was added to a slurry of SbF3 (^4g) in 100 ml of sym-C2H2Cl4 and the mixture heated under reflux for 4 hours. The mixture was then cooled and filtered, and the precipitate. - 72 -washed with a little benzene. The solvent was then distilled from the filtrate 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,,S04. Evaporation of the solvent from this extract then yielded a pasty solid, which was recrystallized from hot octane to give colourless flakes of F(NMe2)Ph4P3N3 (2.753g, 5.43 mmol, 67%). Anal, calcd. for C?6H26FN4P3: C, 61.66; H, 5.18; N, 11.06. Found: C, 61.49; H, 5.20; N, 11.10. M.pt. 154-156°C. ]H n.m.r. spectrum (6, CDC13, internal TMS); 2.64 (6H, doublet, JpH=12.0Hz); 7.0-8.0 (20H, unresolved multiplet). 2.5.4.4 Preparation of [NMe2)FPh4P3N3.H]+[BF30H]~ An excess (1 mmol) of BF3.Et20 was added, via a syringe and septum, to a solution of (Me2N)FPh4P3N3 (0.312g, .60 mmol) in 100 ml of diethyl ether. Precipitation slowly commenced after about h hour. The mixture was stirred, under nitrogen, for a further 48 hours, and then filtered, to give a white solid, which was recrystallized from chloroform/toluene as the protonated salt [(Me2N)FPh4P3N3.H]+[BF30H]_ (0.249g, 0.44 mmol). Anal, calcd. for C26H28F4N40P3: C' 52-73; H> 4-76; N, 9.46. Found: C, 52.96; H, 4.80; N, 9.18. M.pt. 184-188°C. 31P n.m.r. spectrum (6, CDC13, external P40g); 87.2 ppm (2P, P(Ph2) atoms, doublet, Jpp=15Hz), 100.6 (IP, P(NMe2)F atom, doublet, JpF=935Hz). ^ n.m.r. spectrum (6, CDC13, internal TMS); 2.77 (6H, doublet, JpH=12.0Hz), 7.0-8.0 (20H, unresolved multiplet). 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 little work has been directed towards the influence of the phosphazene ring itself 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. H3GV /H3 H3C CHf H3C. CH2R N N N N N "' N III The principal conclusions of this study stem from reactions carried out using two methylphosphazenes, (1) the fully methylated tetrameric phosphazene (NPMe^^, and (2) the partially methylated trimeric phosphazene gem-Me^Ph^PgNg. The details of these reactions are given in the following sections. 3.1 Preparation and Reactions of N4P4Me4(CHpLi )4 and N4P4Me6(CHol_i )2 In diethyl ether solution, methyllithium reacts rapidly and - 74 -exothermically with octamethylcyclotetraphosphazene, N^P^Meg, to produce the dicarbanion N^Me^Cr^"^ and the tetracarbanion N^Me^CH,,")^ (equations 2 and 3). 2MeLi + N4P4Me8 —• N^MegtCHpLi )g + 2CH4 ...2 4MeLi + N4P4Meg —•+ N4P4Me4(CH2Li)4 + 4CH4 ... 3 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 di- and tetra-substituted phosphazene derivatives of the type N4P4Meg_x(CH2MMe3)x (x=2,4; M=Si, Ge, Sn). They also react with methyl iodide to give the tetraethyltetramethylphosphazene N4P4Me4Et4 and the diethylhexamethylphosphazene N4P4MegEt2 (isolated as its hydrochloride lyyiegEtg.2HC1 )176. The intermediacy of a mono- and tricarbanion N4P4Meg_x(CH2Lii )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 di- 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 N4P4Me7(CH2") indicated that the removal of a second proton from the already deprotonated species is favoured over the primary deprotonation of unreacted N4P4Meg; i.e. reaction 4b is favoured over reaction 4a. - 75 -N4P4Me7(CH2Li) + CH4 ... 4a N4P4Me8 + MeLi \ l*N4P4Me8 + 1sN4P4Me6(CH2Li)2 + CH4 ... 4b Although these multiply charged carbanions formed from N4P4Meg provide a useful insight into the reactivity of the methylphosphazene ring, their application to the synthesis of novel phosphazenes is somewhat restricted. Their reaction with electrophiles is complicated by the possibility of competing coupling reactions, and a mixture of products is often formed, the successful separation of which depends on the relative solubilities of the different components. Preliminary attempts to form a monocarbanion from the reaction of ^3P3Me6 w1'^*1 metnyllithium showed that, for this system also, multiple deprotonation is favoured over the removal of a single proton. Attention was therefore focused on the partially substituted methylphosphazene Me^h^N-j, the deprotonation of which leads exclusively to the formation of a monocarbanion. 3.2 Preparation and Reactions of N3P3Ph4Me(CHoLi) Like N4P4Meg, gem-dimethyltetraphenylcyclotriphosphazene reacts with methyl- or butyl lithium in ethereal solution, resulting in the precipitation of the monocarbanion N3P3Ph4Me(CH2Li). This intermediate is particularly useful in studying the versatility of phosphazene carbanions in synthesis, since its reaction with electrophiles (Equation 5) leads to a single product. The chemistry of this carbanion is related to that of phosphine oxide carbanion Ph2P(0)CH2Li (see Chapter 1) and the a-picolyl anion - 76 -Me Me \/ N RLi Me CHf \/ N N RX Me CH2R \/ X Ph~~7P\ ^\—Ph Ph—p\ ^\—Ph Ph—p\ — N Vh Pr> N \h P/ N Ph N .... 5 .P Ph R= Br, Me^Sn, COOH, PhCO NC -H4(CH2Li). Like the latter two anions104,177'178, N3P3Ph4Me(CH2Li) reacts smoothly with carbon dioxide and esters, to give phosphazene derivatives containing carbonyl and carboxyl substituents. Also like phosphine oxide carbanions , and the polycarbanions of N^Meg, N3P3Ph4Me(CH2Li) reacts 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, N3P3Ph4Me(CH2Li) reacts with bromine to give the bromomethyl phosphazene N3P3Ph4Me(CH2Br). The behaviour of the a-picolyl anion is different; it 180 undergoes a coupling reaction with bromine to give 1,2-dipyridylethane 3.3 Spectra and Structure of N4P4Mep_x(CH2R)x (x=2,4) Derivatives Assuming that the phosphazene carbanions N4P4Me6(CH2")2 and N4P4Me4(CH2~)4 do not undergo a charge rearrangement during their reaction with electrophiles, the structures of the di- and tetra-substituted derivatives N4P4Meg_x(CH2R)x (x=2,4) reflect 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 triplet fine 31 1 structure (P-P coupling) observed for both of the 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 CH2R groups, showing, typically, the 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 3 31 b Table 3.1 H and P n.m.r. parameters of N4P4Me8_x^CH2R^x derivatives 5p(PMe2) <5p(PMe) «H(PMe2) 6H(PMe) 6H(PCH2R) <sH(R) N4P4Me6Et2.2H'Cl^ 75.8 (12) 71.1 (12) 2.41 (14.0) 2.38 (13.0) 2.62 (12.0) 1-66 r (22.0)C N4P4Me4Et4.2HCl* - 71.2 - 2.39 (12.0) 2.62 (M2.0) 1.68 , (21.0)d N4P4Me4Et4 - 89.4 - 1.41 (11.5) 1.65 H2.0) 1.10 (17.0)e N4P4Meg(CH2SiMe3)2 95.5 (12) 99.6 (12) 1.42 (13.5) 1.45 (13.0) 1.18 (18.0) 0.11 N4P4Me4(CH2SiMe3).4 - 100.1 - 1.46 (12.5) 1.19 (17.0) 0.24 N4P4Me6(CH2GeMe3)2 94.4 (12) 99.6 (12) 1.41 (13.5) 1.42 (13.5) 1.23 (16.5) 0.25 N4P4Me4(CH2GeMe3)4 - 98.1 - 1.42 (12.0) 1 .23 (17.5) 0.19 N4P4Me6(CH2SnMe3)2 92.8 (12) 100.0 (12) 1.45 (13.0) 1.48 (12.0) 1.27 (14.0) 0.14 N4P4Me4(CH2SnMe3)4 - 96.5 • - 1.40 (12.5) 1.23 (12.3) 0.16 (a) 6(ppm), in CDCIq, reference internal TMS, except f in D20, reference external TMS. J(PH), in Hertz, in parenthesis, (b) 6(ppm) reference external P4O6; all in CDCI3, except =f in D2O. J(PP), in Hertz, in parenthesis. 6p(N4P4Me8) in CDCT3 is 94.4 ppm. (c) J(HH) = 7.5 Hz. (d) J(HH) = 7.1 Hz. (e) 3(HH) = 7.3 Hz. - 79 -determination of the dihydrochloride of N4P4MegEt2iyi. 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 is not uncommon in phosphazene chemistry. Its occurrence ©CI C(2) (a) (b) Figure 3.1. General views of (a) the N4P4Me5Et2.2HCl structure, and (b) the chair conformation of the N4P4Me5Et2H22+ cation (from H.P. Calhoun, R.T. Oakley, N.L. Paddock and J. Trotter, Can. J. Chem., 53, 2413 (1975). in this molecule can be understood in terms of electrostatic repulsion between the CH2~ groups of the carbanion N4P4Me6(CH2~)2. Model calculations on molecules with the conformations found in 8-membered phosphazene rings (saddle, D2d; tub, S4; chair, C2h; crown, C4y) show that, for normal values of bond angles, repulsion in the dicarbanion is 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 P n.m.r. spectra, and of a single P-methyl resonance in their n.m.r. spectra, indicates that the substitution pattern found in the tetrasubstituted derivatives N^P^Me^CCH^R)^ is 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 cis,cis,cis,trans (VII), and the cis,cis,trans,trans (VIII). Of the two, the latter is favoured electrostatically, but its existence cannot be confirmed by n.m.r. spectrometry. However, the near mutual exclusion of the Raman and infrared spectra of N4P4Me4Et4 (Table 3.2) is consistent with the centrosymmetric structure VIII, but, because of possible accidental degeneracies of non-skeletal modes, the cis,cis,cis,trans configuration cannot be entirely excluded from consideration. In general, the infrared spectra of the tetra-substituted derivatives N4P4Me4(CH2R)4 are, as expected, similar to that of the parent N4P4Me8 ecu!e. All contain a characteristically strong band in the region of 1215-1250 cm"1, which is easily assigned to the v c (P=N) - 81 -Table 3.2 Infrared9 and Raman absorption frequencies (in cm ) of MeoP.N-0 and Me4Et4P4N4 844 Me8 P4N4 Me4Et4P4N4 Infrared Raman Infrared Raman 280 w -s 251 (6.9) 435 m (broad) 185 (1.5) 390 s 284 (2.4) 625 m 256 (6.0) 433 s -v 361 (0.6) 670 w 355 (0.5) 460 sh "N- 433 (0.7) 710 w - 500 (10.0) 505 w --- 507 (10.0) 750 w v 580 (5.0) 630 s 588 (5.2) 767 w x 675 (0.4) 736 m -x 698 (1.0) 790 s 712 (1.0) 760 m -x X 736 (5.2) 860 w X 790 (0.6) 787' w 761 (1.7) 881 v.s. 860 s -v 767 (3.8) 940 w 990 (1.0) 870 s X 860 (0.9) 1010 w 920 s 1041 m 1045 (1.2) 980 sh 1230 v.s. (broad) 995 m 1270 v.s. 1040 w 1300 v.s. 1222 v.s. (broad) 1379 v.w. 1240 sh 1418 w 1425 (0.8) 1290 s 1461 w 1465 (0.5) 1300 s 2880 w 1415 w - - -- 1414 (2.8) 2910 w 2915 (3-0) 1422 w -. - - 1426 (1.9) 2940 m 1430 w 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., Brit. Col., 1971. - 82 -RCH2 -R \\-\-/ P -CH2R / \ RCH2—p. CH2R N-P\—-N CHoR P CHoR VIII RCH2 P, CH2R / / -CH2R VII vibration. The 6=r. (CH-) vibration at 1414-1427 cm"1 is complemented by 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 N4P4Me4Et4, 6(CH2) is found at 1461 cm"1. For the N4P4Me4(CH2MMe3)4 (M=Si, Ge, Sn.) derivatives, it occurs in an Otherwise clear region of 1040-1110 cm"1, within the limits found for this vibration in a variety of XCH2SiMe3 molecules (X=SiMe3182; CI, Cr, Sn, Pb183). 3.4 Structure and Spectra of N3P3Ph4Me(CH2R) Derivatives The details of the 31P and ]H n.m.r. spectra of the N3P3Ph4Me(CH2R) derivatives are given in Table 3.3. The 31P n.m.r. spectra all consist, as expected, of two signals of relative intensity 1:2 - 83 -(PMe(CHoR):2PPh2). The proton n.m.r. spectra are also consistent with 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 a 31 b Table 3.3 H and P n.m.r. parameters of N^P^Ph^MeCHpR derivatives R= 6p(PMeCH2R) Sp(PPh2) <5H(MeP) <5H(CH2P) Hc 85.2 98.2 1.58(14.0)f 1.58(14.0)f SnMe.^ 80.8 99.3 1.52(13.5) 1.37(14.0)g COPh 87.5 98.3 1.75(15.0) 3.61(18.0) C00He 86.6 96.8 1.76(15.0) 2.88(17.0) Br 89.0 98.2 1.72(15.0)h 3.20(6.5) (a) <5(ppm) in CDCI3, reference internal TMS. J(PH), in Hertz, in paren thesis. S(Phenyl) ^7.0-8.0 ppm. (b) 6(ppm), in CDCI3 reference external P4O5. (c) See also R. Appel and 6. Saleh, Chem. Ber., 106, 3455 (1973). (d) 6H(Me3Sn) = 0.02 ppm. (e) 6H(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. 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 N3P3Ph4Me2 and its derivatives N3P3Ph4Me(CH2R). In all the compounds, ^aSym (P=N) occurs at 1160-1200 cm-1, near to the value found for (NPPh2)3 (1190 cm"1 24) and (NPMe2)3 (1180 cm"1 21). In N-jP^Ph^Me,,, 6g (CH3) appears as a sharp doublet (in-phase and out-of-phase) at 1298 cm"1 and 1310 cm"1 (in Me3P0184'185 this band appears at ^1300 cm"1). 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-1 and 866 cm"1 are assigned, respectively, to the out-of-phase rocking and wagging motions 184 185 -1 of the P-methyl groups ' .On this basis, the two bands at 955 cm" and 871 cm"1 in NgP^Ph^l^ may reasonably be assigned to equivalent 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 ^3P3P*14^e<"^2000^"' in *'ie former it occurs at 1666 cm-1 and, in the latter, at 1695 cm"1. For NgPgPh^eCHgCOOH, the v(0H) band is not visible. 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 is 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 is demonstrated by equations 6a-6c. In all three cases, the carbanion is stabilized by conjugation. In the oxide, resonance is 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 „CH2" \ / /\ R 0 K 0 6a 3 \/ 2 N N R—P. / R 3\// 2 R N R-P. R N" H3C CH? • P>v N' N R R— / R N .P-R R 6b . 6c The formation of a tetracarbanion is not unique. Tetrathiane, (SCH,,)^, is deprotonated by butyllithium to yield the tetracarbanion - 186 (SCH")4 , the stability of which is attributable to the well known acceptor properties of divalent sulphur. In the present case, the ability of the P^N4 phosphazene ring to stabilize the negative charge of the M4P4Me4(CH2~U 10n 1S consistent with the structural features described in chapter 1; viz the apparent loss of electron density from oxygen and nitrogen in [NP(0Me)2]n and [NP(NMe2)2]n into the cyclic u-system. In fact, in the 1imit of complete delocalization of charge from the methylene groups into the ring, the structure of the N^P^Me^CHg")^ anion (IX) becomes isoelectronic with the phosphazene [MeNP(0)0Me]4 (X) (formed by the thermal 107 rearrangement of [NP(0Me)2]4 ), and the tetrametaphosphate ion [p03"]4 (XI). - 86 -The analogy between the tetracarbanion N^P^Me^Cr^-)^ and the tetrametaphosphate ion is important, since it allows a comparison between Me CH9 MeC- 0 "0 0 \// 2 Me \// Me \// N N" N N 0^ ^0 H2G%/ \/Me °'%/ \/GMe Oj \^0" Me^\ /NH2 MeO^\ /S) - .0^\ /%> /\ Me /\ XMe /\ H2C Me 0 OMe 0 CL IX X XI the acidic properties of N^P^Meg and tetrametaphosphoric acid, about which more is known. In general, the acidities of the cyclic metaphosphoric acids differ from those of linear phosphoric acids. The tri- and tetra metaphosphoric acids [HPOglg 4, for instance, behave as strong acids (their ka is greater than k^ of orthophosphoric acid), and dissociate in dilute solution according to equation 7 " . By contrast, the [HP03]n * nH+ + [P03]nn" (n=3,4) ...7 titration curve of the linear tripolyphosphoric acid [(H0)3P0P(0H)20P(0H)3] exhibits three inflexion points. Not only are the terminal protons less acidic than the central ones, but they display different acidities 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 all display approximately^ the same acidity, 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 difficulty in the preparation of mono- and tricarbanions from N^P^Meg has already been noted, the removal of the first proton apparently enhancing, or at least not inhibiting, 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 is a useful tool for establishing the orientation patterns of 31 partially 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 is not always successful. The importance of angular and steric 31 factors, which have a profound influence on the P chemical shifts of T The apparent weakness, as observed in titration curves, of the fourth proton of [HP03]4188>'9', and of successive chain protons in linear acids, is attributed to ion association with the cation192. - 88 -193 mononuclear phosphorus compounds , is simply not known for phosphazenes. It is generally accepted that, for homogeneously substituted 31 phosphazenes (NPX2)p, there is an approximate correlation between their P chemical shifts and the ability 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 is also observed in phosphine 194 195 196 oxides and sulphides ' , phosphorus ylids and bis(biphenylene)-197 phosphoranes . In all these compounds the primary influence of the ligand is 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 directly related to the electronegativity of the ligand. The origin of the effect is illustrated 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 P chemical shifts of some heterogeneously substituted phosphazenes. 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 < OPr1, this order being broadly consistent with change in electro negativity of the R group198. - 89 -c CD CD CD > a CD c o 10 c CD 02n 00 -0-2 J -1 0 +1 g (units of yS} Figure 3.2. Relative charge densities (P*-P) on phosphorus atoms in a perturbed P3N3 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 ap* = ap - 6. (-3 < 6 < 3). The influence of different ligands on remote phosphorus atoms is effectively demonstrated by the series of geminally substituted phenylfluo-rophosphazenes F2nPh6-2nP3N3 (n=0>l»2.>3) (Table 3.4). Shielding of the P(Ph2) atoms decreases steadily with increasing fluorination of the ring, 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(F2) atoms. An equivalent effect is observed in the chlorophosphazenes ^2.i\^&-2x\ "^1 (n~0,l,2,3) - 90 -(Table 3.4); it is less marked than in the previous case, the phenyl and chlorine ligands having more nearly equal electronegativities. Table 3.4 31P chemical shifts3 P chemical shifts of geminally substituted phenylhalophos phazenes X2nPh6_2nP3N3. n= 0 12 X=F P(PhJ 98.2b 85.2C 82.1° P(FJ - 106.3C 101.lc 98.6C '2 2 X=C1 P(Ph2) 98.2b 95.9d 94.2d P(C12) - 93.4d 91.9d 92.5e (a) 6(ppm), relative to external P4O6. (b) H.P. Latscha, Z. Anorg. Allgem. Chem., 382, 7 (1968). (c) C.W. Allen, 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. Soc, 2542 (1960). Simple explanations such as these are not always possible. The anomalous chemical shift of (NPBr2)3 (<5p = 157.9 ppm , but 6p in (NPF2)3 is 98.6 ppm159 and in (NPC12)3 is 92.7 ppm200) shows that angular effects may be important. Similarly, the decrease in the chemical shift of the P(F2) atoms on moving from FgP^ to F^e^P^N^ finds no explanation in 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 31P Chemical Shifts of XYPh^P^ Derivatives 31 The P n.m.r. parameters of the geminally substituted tetraphenyl-phosphazenes XYPh^P^ prepared in this work are given in Table 5. For the series Me(CH2R)Ph4P3N3 (R=H, SnMe3, Br, COPh, COOH), it is evident that, as expected, the alkyl groups are more efficient donors than phenyl groups, the greater shielding of the P(Ph2) atoms indicating a polarization of u-charge onto them. The effect of a change in the nature of R on the remote phosphorus atom is small, but on the P(CH2R) atom, it is more substantial. The total effect is most easily followed by use of the parameter A =6(PXY) - 6(PPh2), which provides a measure of the polarization of ir-charge from the P(Ph2) atoms towards the P(XY) atom. For the alkyl derivatives, A is negative, but its value increases along the series R=SnMe3 < H < COOPh * COOH < Br. This order is broadly consistent with that i 179 observed for the chemical shifts of phosphonate esters RCH(P03Pr 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 NMe2), the variation in A with change in X or Y is more marked than it is in the Me(CH2R)Ph4P3N3 derivatives (with change in R). 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 + NMe2) > E(C1 + CI)). 3-6.2 31P Chemical Shifts of N^P.Mep „(CH0MMe,) Derivatives 4—4—o-X ' C o-^-X 31 The details of the P n.m.r. spectra of these compounds (XII and XIII) are presented in Table 1. As might be expected from the similarity - 92 -Table 3.5 J,P chemical shifts3 of XYPh„P0N0 derivatives. X Y 6(PXY) 6(PPh2) F Fc 106.3 85.2 21.1 F NMe2d 95.6 94.1 1.5 Ph Phe 98.2 98.2 0 CI Clf 93.4 95.9 - 2.5 CI NMe29 85.2 94.4 - 9.2 Me CH2Br 89.0 98.2 - 9.2 Me CH2C00H 86.6 96.8 -10.2 Me CH2C0Ph 87.5 98.3 -10.8 Me Me 85.2 98.2 -13.0 Me CH2SnMe3 80.8 99.3 -19.3 (a) <5(ppm), reference external P4O6. All values from CDC13 solutions. (b) A = 6(PXY) - 6(PPh2). (c) C.W. Allen, 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. - 93 -RCH '2—k_i—' RCH2-4 CHgR I i CH2R CH2R XII XIII R = Me, MMe3 (M = Si, Ge, Sn) of the ligands, the differences in chemical shifts between the various compounds is small, and, possibly because of this, the interpretation of the chemical shift variations in terms of a ir-inductive effect is not particularly successful. shielded than in N^P^Meg, consistent with the expected greater donating ability of the ethyl groups. However, the chemical shifts of the N4P4Me4(CH2MMe3U derivatives indicate an unexpectedly high electro negativity for the (Me3MCH2) group. Moreover, whereas the Sp values in N4P4Me4(CH2MMe3)4 suggest the order of electronegativities of the different metals to be E(Sn) < E(Ge) < E(Si), the chemical shifts of the PMe2 atoms in N4P4Me4(CH2MeMe3)4 indicate a reverse order*. 3.7 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 (H3M)3P (M=C,Si,Ge,Sn). The chemical shifts For example, the phosphorus atoms in N-P-Me-Et. are less E(Sn) - 94 -use; (NPMe2)4 by sublimation in vacuo and fi^Ph^Me,, by recrystallization from ether/hexane and heating it at 100°C/24 hours. The various electrophiles (e.g. Mel, Me3SiCl, Me3GeCl, Me3SnCl, C02, PhC02Et and Br2) which were reacted with the phosphazene carbanions were obtained from commercial sources. Methyl iodide and trimethylsilyl chloride were redistilled, and trimethylstannyl chloride sublimed in vacuo before use. Bromine, available commercially in 1 ml vials, was used without further purification, as were carbon dioxide (lecture bottle, "Bone Dry" grade) and ethyl benzoate. Because of the sensitivity of organolithium reagents to oxygen and moisture, the preparations and reactions of the phosphazene carbanions were all carried out under an atmosphere of dry nitrogen. 3.7.1 Preparation of Methyl!ithium All the reactions described in this section involve the use of methyllithium. Although solutions of it (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 relies on the insolubility 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 -202 The method is similar to one that has already been reported , and involves the reaction of methyl iodide and butyl lithium in hexane at reduced temperatures (equation 9). In a typical preparation, methyl Hexane BuLi + Mel <Q°C* 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 filtered (under nitrogen!) from the solution and dissolved in diethyl ether (previously dried by distillation from lithium aluminium hydride). The solution so formed was filtered (if necessary) and standardized by quenching an aliquot of it in water and titrating it 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 N4P4Me4Et4 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 N4P4^e8 (2-712g, 9.04 mmol) in 50 ml of ether. 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 distilled off to leave a white paste (10.747 g), which was dissolved in water and neutralized with sulphuric acid (residual alkalinity = 2.15 mmol OH"). Potassium fluoride (2.9 g) was added to the solution to precipitate lithium as its fluoride. After filtration, the solution was distilled 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 left a crystalline mass which was extracted with 3 x 50 ml of hot hexane. Evaporation of these extracts yielded colourless crystals of N4P4Me4Et4 (2.58 g, 7.2 mmol, 80%), which was purified by sublimation in vacuo and recrystall ization from hexane. Anal, calcd. for c-| 2H32N4P4: C, 40.45; H, 9.05; N, 15.72. Found: C, 40.21; H, 8.89; N, 16.00. Melting point 65-67°C. 3.7.2.1 Isolation of N4P4Me4Et4.2HI The treatment of the CHC13 extract in the above experiment with base 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 complex is converted into the dihydroiodide N4P4Me4Et4.2HI. This hydroiodide was isolated in a preliminary experiment in which triethylamine was not used. It was recrystallized from acetonitrile as colourless cubes. Anal, calcd. for C12H34I2N4P4: C' 23'53; H' 5'60; L 41'47; N' 9'16, Found: C' 23-15; H, 5.68; I, 41.20; N, 9.19. Dec. 188-190°C. - 97 -3.7.2.2 Isolation of N4P4Me6Et2.2HC1 In another preliminary experiment in which the use of triethyl-amine was omitted, the chloroform extract was evaporated to leave an oil, 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 oil which, upon standing, yielded a few crystals, which were recrystallized, under nitrogen, from acetonitrile/benzene as small hygroscopic cubes of N4P4MegEt2.2HCl. Anal, calcd. for c-|QH30C1' 2N4P4: C, 29.94; H, 7.54; CI, 17.67; N, 13.97. Found: C, 29.64; H, 7.55; CI, 18.05; N, 14.06. M.pt. 231-234°C. 3.7.2.3 Preparation of N4P4Me4Et4.2HCl.HqO This compound was prepared by saturating a solution of N4P4Me4E1:4 (°-l07 g, 0.30 mmol) in carbon tetrachloride with hydrogen chloride gas. The resulting precipitate (0.112g) was filtered from the solution and recrystallized, under nitrogen, from an acetonitrile/benzene solution as colourless hygroscopic blocks. Anal, calcd. for C|2H36C12N40P4: C, 32.22; H, 8.11; CI, 15.85; N, 12.53. Found: C, 32.29; H, 8.30; CI, 15.90; N, 12.68. M.pt. 180-185°C. 3.7.3 Preparation of N4P4Me4(CHoSiMe3)4 A solution of methyllithium (15.0 ml, 12.0 mmol of MeLi) in ether was added, via a syringe, into a solution of N4P4Me8(0.758g, 2.52 mmol) in 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 stir overnight under a nitrogen atmosphere, and then filtered. The solvent was evaporated from the filtrate to leave an oil which was distilled at M60°C/0.01 Torr to give a colourless liquid which solidified on standing at room temperature. This solid was recrystallized by cooling a concentrated solution of it in hexane to -23°C to give colourless blocks of N^Me^Ch^SiMe.^. Anal, calcd. for C20H56N4P4Si4: C' 40'79; H' 9>59; N' 9-51- Found: C> 40-49» H> 9-56; N, 9.50. M.pt. 97-102°C. 3.7.4 Preparation of N4P4Mec(CHoSiMe3)2 A solution of methyllithium in ether (28.0 ml, 18.2 mmol MeLi) was reacted, as described above, with a solution of N^Meg (2.348 g, 7.83 mmol) in 150 ml of ether. The mixture was then reacted (as described in Expt. 3.7.2) with a solution of Me3SiCl (2.06 g, 19.0 mmol) in 20 ml of ether. Subsequent filtration of the reaction mixture and evaporation of the filtrate yielded a colourless oil (3.54 g), which was distilled in vacuo (^160°C/0.01 Torr). The resulting oil was dissolved in a minimum of pentane and, on cooling it to 0°C, 0.5 g of unreacted N4P4Meg crystallized from the solution. 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^Meg^h^SiMe^. Anal, calcd. for C]4H4QN4P4Si2: C, 37.82; H, 9.07; N, 12.60. Found: C, 38.15; H, 8.95; N, 12.36. M.pt. 63-65°C. - 99 -3.7.5 Attempted Preparation of N4P4Me7(CHpSiMe3) A solution of methyllithium in ether (3.2 mis, 1.85 mmol MeLi) was reacted, as described above, with a solution of N4P4Meg (0.530 g, 1.76 mmol) in 50 ml of ether. A solution of Me3SiCl (0.232 g, 2.15 mmol) in 5 ml of ether was then added, as described in Expt. 3.7.3, to the reaction mixture. After filtration of the mixture and evaporation of the solvent from the filtrate, a white pasty solid was obtained. This solid was extracted into hexane, and the solution filtered and allowed to slowly evaporate. A total of 0.321 g of unreacted N4P4Meg (identified by its infrared spectrum and melting point, 161-162°C) crystallized from the solution, leaving an oily residue (0.244 g). No monosubstituted derivative could be isolated from this oil; in fact, its n.m.r. spectrum showed it to be a mixture of polysubstituted products of formulaeN4P4Meg_x(CH2SiMe3)x (x=2<3). The following mass balances demonstrate the probable course of the reaction (see Text). (a) Observed: N4P4Me8 > N4P4Me8 + N4P4Me8.x(CH2S1Me3)x Mass balance 0.530g,l.76mmol 0.321g,l.07mmol 0.244g (b) Calculated for the reaction: N4P4Me8 > N4P4Me8 + ^4P4Me6(CH2SiMe3)2 Mass balance 0.530g,l.76mmol 0.265g,0.88mmol 0.390g,0.88mmol - 100 -(c) Calculated for the reaction: We8 > We8 + ' *N4P4Me4(CH2S1Me3)4 Mass balance 0.530g,l.76mmol 0.396g, 1.32mmol 0.259g,0.44mmol 3.7.6 Preparation of N4P4Mefl -(CHgMMe3) (x=2,4, M=Ge, Sn) The preparation of these compounds^ is similar to that of the derivatives N^Meg^CCr^SiMe^ (x=2,4) and has been described elsewhere203. 3.7.7 Preparation and Reactions of N3P3Ph4Me(CH,,Li) 3.7.7.1 Preparation of N^PtyiefCHoLi) In a typical preparation, a slight molar excess (^10%) of a freshly prepared solution of methyllithium in ether^ was added, via a syringe, to a solution of N3P3Ph4Me2 in ether. After an induction period of a few minutes, a heavy white precipitate of N3P3Ph4Me(CH2Li) was formed. To ensure completion of the reaction, the mixture was gently heated under reflux for 2 hours before reacting it with an electrophile. 3.7.7.2 Preparation of N3P3Ph4Me(CHgSnMe3) A solution of Me3SnCl (0.300g, 1.50 mmol) in 20 ml of ether was ^ 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 N3P3Ph4Me2 (0.651 , 1.38 mmol) and methyl lithium (1.5 mmol) in 100 ml of ether. The turbidity of the mixture became less intense following the addition. The mixture was then left to stir overnight in order to ensure completion of the reaction. Subsequent filtration of the mixture and evaporation of the filtrate yielded a colourless crystalline solid, 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. Anal, calcd. for C29H34N3P3Sn: C' 54-75' H> 5-39; N> 6-60- Found: C, 54.88; H, 5.47; N, 6.46. M.pt. 146-148°C. 3.7.7.3 Preparation of N3P3Ph4MeCH2C00H Anhydrous carbon dioxide was bubbled for 15 minutes through a slurry of N^PI^MeCHgLi prepared from the reaction of N3P3Ph4Me2 (0.408 g, 0.863 mmol) and methyl 1ithium (1.00 mmol) in 100 ml of ether. All turbidity rapidly disappeared. 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 solid, which was recrystallized from acetone as white flakes of N3P3Ph4Me(CH2C00H) (0.390 g, 0.754 mmol, 87%). Anal, calcd. for C28H26N3°2P3: C, 62.67; H, 5.06; N, 8.12. Found: C, 62.85; H, 5.15; N, 7.98. M.pt. 185-186°C (dec). - 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 N3P3Ph4Me(CH2Li) prepared from 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 left to stir 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. The organic layer was extracted and dried over anhydrous sodium sulphate. Subsequent evaporation of the solvent yielded a white solid, which was recrystallized from benzene/toluene and ether/hexane to give a microcrystal1ine mass of N3P3Ph4Me(CH2C0Ph). Anal, calcd. for C31H3QN30P: C, 68.48; H, 5.26; N, 7.31. Found: C, 68.66; H, 5.39; N, 7.13. M.pt. 144-146°C. 3.7.7.5 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 N3P3Ph4Me(CH2Li) prepared from the reaction N3P3Ph4Me2 (0.787 g, 1.66 mmol) with a solution of methyllithium (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 it. The organic layer was separated and the aqueous layer washed with 50 ml of CH2C12 (this step 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 it to be a ^4:1 mixture - 103 -of product (N3P3Ph4MeCH2Br) and starting material (NgPgPh^Meg)'. Repeated recrystal1ization of this solid from benzene/octane and chloroform/octane produced analytically pure crystals of N3P3Ph4MeCH2Br. Anal, calcd. for C26H25BrN3P3: C, 56.54; H, 4.56; Br, 14.47; N, 7.61. Found: C, 56.88; H, 4.45; Br, 14.10; N, 7.70. M.pt. 176-178°C. 104 -CHAPTER IV PREPARATION AND REACTIONS OF AZAPHOSPHORIN DERIVATIVES The acidic properties of methylphosphazenes described in the previous chapter reflect the similarity 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 if they could be deprotonated, like phosphonium salts, 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 final products of such reactions are not the expected ylids (I), but novel azaphosphorin derivatives (II). The structural rearrangement required for the formation of these compounds is hitherto unknown in phosphazene chemistry, and its occurrence here is significant, since it illustrates 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 H3C CH2 \// 'N ,CH3 H3C ' N—CH3 \/ . 1 II - 105 -4.1 Reaction of N-methyl Methylphosphazenium Salts with Bases The formal similarity of methylphosphazenium quaternary salts to phosphonium salts led initially to the use of a variety of bases commonly used for the preparation of phosphorus ylids as reagents for the deprotonation of the phosphazenium salts. Because of their sensitivity to oxygen and water, phosphorus ylids are usually prepared and used in situ (e.g. in the Wittig reaction). When the ylid itself is to be isolated organolithium bases, usually the reagents of choice, cannot be used, since lithium salts form complexes with ylids. 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 (KOtBu204-206 and NaN(SiMe3)2207) and have an appreciable solubility in hydrocarbon solvents, which are the most convenient media for achieving a clean separation of the products. However, as will 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(SiMe3)2 with N-methyl Methylphosphazenium Salts In boiling octane or toluene, NaN(SiMe3)2 reacts smoothly with the monoquaternary phosphazenium iodides (NPMe,,)^ ^.Mel and N^P^Ph^Me^Mel, removing a proton from an exocyclic P-methyl group. However, the final products isolated from the reactions are not the expected ylids III and V (Figure 4.1); - 106 -H3CX CH3 CH3 H3\ ^CH2 Base N R N R R 'N ,CH3 N H H3G N—CH3 R-P. R R—R 'N R R = Me. Ph III IV // H3G GH3 P \/GH3 // ^GH3 H H3G N—CH3 ^P^ N' H3G^\ N: / \ ^Base .. _ _. J? / \  vBas  y H3G GH3 \^H+ H3G^ ^GH2 / H3G H // 3 .N GH-: N' P^ /GH3 H3G\/ \ XH3 VI H3<\ /XCH3 N A H3G CH3 Figure 4.1. Deprotonation of N-methyl methylphosphazenium quaternary salts to give exocyclic ylids, which then rearrange in situ to give azaphosphorin derivatives containing an endocyclic carbon atom. - 107 -instead, the initially formed ylids rearrange in situ displacing the methylated nitrogen atom from the ring, and forming the novel azaphosphorin derivatives IV and VI (Figure 4.1). The analogous reaction between the diquaternary salt (NPMe2)4.2MeX (X=FS03, I) and NaN(SiMe3)2 is less successful. Although a reaction does take 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 directly at a phosphorus atom adjacent to the quaternized nitrogen. The products obtained from the reactions of (NPMe2)3 4-MeI and (NPMe2)4.2MeS03F with KOtBu demonstrate the occurrence of both possibilities, and indicate that their relative importance is strongly dependent on the size and charge of the phosphazenium cation. The reaction of (NPMe2)3.MeI with KOtBu illustrates one possible 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). The subsequent H3C CH3 GH3 \ / H3G. | tfBu N ; N N ' KOtBu. H3C—P. 4> CH3 H3G—P 3u. XCH3 fW^N' (NHMe)(PMe2N)2PMe20 + ... 2 H^. N tHo H3G" ^N' CH3 \ eH3 0(tBu)2 or M^CC^ - 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)(PMe2N)2PMe20 in high yield (92%). 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 still occurs, to give the linear oxide (NHMe)(PMe2N)3PMe20, but the yield of this product is low (%5%). The principal product (80%) is the cyclic triazatetraphosphorin Me7(NHMe)P4N3CH, formed H HqG N—GH3 V/ H N H3G GH3 H3G\/ \ .CH-. N // •N H3C^\ H3G- \ N V^tSHs KOtBu A H3G GH3 // 3 (80%) H3G GH3 (NHMe)(PMe2N)3PMe20 (-5%) + 0(tBu)2- or Me2CCH2 presumably by a deprotonation reaction and a phosphazene-phosphorin rearrangement (Figure 4.1). The reaction of the diquaternary ion [NPMe2)4.2Me]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)PMe2NPMe,,0, indicating that ring cleavage has occurred in two places (Equation 4). 4-1-3 Reaction of Methyllithium with N-methyl Methylphosphazenium Salts ylids is well known, and for this reason salt-free solutions of ylids cannot be obtained when alkyllithiums are used to deprotonate phosphonium salts. On the basis of n.m.r. measurements, a tetrahedral coordination of the ylid 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 ylidic carbon atom. Accordingly, two deprotonation experiments using methyllithium as base have been carried out in order to determine if the presence of lithium ions, complexed to the initial product of the reaction (e.g. Ill and V, Figure 1), will sufficiently stabilize the latter to prevent its rearrangement into an 4 H3G GH3 The formation of complexes between lithium halides and phosphorus - no -azaphosphorin. However, the results of these two reactions (see below) indicate that the presence of lithium does not interfere with the rearrangement step, since the azaphosphorins (e.g. IV and VI, Figure 4.1) are still produced. The two experiments, involving the reaction of methyllithium with (a) NgPgPh^Meg.Mel and (b) (NPMe2)4.Mel, both required an aqueous work-up in order to remove lithium ions from the reaction mixture. In experiment (a), the azaphosphorin (IV, R=Ph, Figure 1) itself was not obtained; instead, its hydroiodide Me(NHMe)Ph4P3N2CH.HI was isolated^. In experiment (b), hydrolysis of the initial product (VI, Figure 4.1) occurred during the aqueous extraction, yielding the linear phosphazene oxide HO(PMe2N)3PMe20. This hydrolysis reaction will be discussed in more detail in a later section. 4.1.4 Discussion Simple Huckel M.O. calculations have shown7^ that, in quaternary phosphazenium salts, 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 salt, rather than the neutral compound, parallels the reaction of N^Me^CH^i )4 and methyl iodide to give the dihydroiodide N4P4Me4Et4.2HI, and not the neutral N4P4Me4Et4. - Ill -212 aromaticity) . (2) When an electron withdrawing substituent (e.g. a halogen) is 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 ylid IX ' •"N" "CH2 I CH2Ph, VII N I CH3 VIII ^0=0 I Ph IX 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 N3P3Me7+ ion is apparently more susceptible to nucleophilic attack and ring opening than is N^P^Me^, and it is believed that this tendency of the trimeric ring to cleavage may be related to the difficulties encountered in preparing trimeric alkylphosphazenes by substitution reactions. - 112 -In the case of N-methyl methylphosphazenium salts, 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 Ag20 [(NPMe2)4Et]V —-> (NPMe2)4 + C2H4 ... 5 nitrogen ylids from methylphosphazenium salts, using the N-ethylacetato derivatives [(NPMe,,)^ 4CH2C00Et]+I~, have been unsuccessful. 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 earlier, it seems likely that the initial step in the reaction (Equation 6) is the formation of an exocyclic ylid (X). The : subsequent rearrangement of this intermediate into the final 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) or methylamino (XIII) group. 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 -3U\ //GH2 \// N Me X RBBJ>P i R N XI H3C N—Me H N R — / R N R XII t H rhCv N—Me \ / H RT/p\N^rR R R XIII supported by the structures of azaphospholes214, which, in solution, exist 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) R1 I R R—e H / -C—R-\ / N=P—Me Me XIV R2-G \ li C—R3 H •P—Me Me XV - 114 -over the P=N bonded structure (XII), since it is only in that form that cyclic aromaticity is attained. However, in solution, proton transfer from the endocyclic carbon to the exocyclic nitrogen is sufficiently rapid to cause the collapse of 1H-1H coupling (see the following section). Rearrangements of this type are not uncommon in phosphorus chemistry. For example, the ylid 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 initiated by catalytic amounts of base, which produce the carbanion XVII. Nucleophilic Me Me Me Me ...... 7 - 115 -attack of this carbanion on a silicon atom, followed by ring cleavage, then generates a new carbanion XVIII, which yields the final 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 , to yield not the expected product [NH2P(Ph2)=CH-P(Ph2)NH2]+Cl" but [NH2P(Ph2)=N=P(Ph2)CH3]+Cl" (Equation 8). This same mechanism may also have a /fe Cfe / • \ NH3 / \ + Ph2P PPh2 —^ Ph2P PPh2 CI" 8 NH2 \NH3 Ph2P' PPh2 Ph2P ,PPh, «h Ph2P pph2 NH2 CH3 NH2 KH cclA NH bearing on the reaction of bis(diphenylphosphino)methylamine with chloramine 217 and ammonia (Equation 9) . Here also, 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 it is the methyl group alone that migrates (as in the Stevens 218 rearrangement ), or the entire -NMe- unit, has not been established. NH MeN(PPh0L ^ [(NHMe)P(Ph9)NP(Ph,)NH,] CI" + NH.Cl -..9 L L NH2C1 <L <L d. 4 - 116 -In summary, the novel phosphazene-azaphosphorin rearrangement reported here is unique in phosphazene chemistry. The presence of sterically bulky groups on phosphorus (e.g. R=Ph, Equation 6), and the torsional restrictions 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 ylid (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 Me2n_1 (NHMe)PnNn_1CH (n=3,4) and Me^HMeJPh^P^N^CH are given in Table 4.1. The deceptively simple appearance of these spectra, run at ambient temperature on samples in deuterobenzene solution, reveals little 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 Meg(NHMe)P3N2CH, however, three closely spaced doublets, corresponding to the methyl groups on the three different phosphorus atoms, can be distinguished (Figure 4.2). The chemical shift of the N-methyl protons (6=2.28-2.47 ppm) is pi q similar to that found in N3P3(NHMe)g (6H(NMe)=2.57 ppm ) and N-methylaniline 220 1 1 (6H(NMe)=2.67 ppm ). In all three types of molecule, H- H coupling between - 117 -(CH3)2P—»-| (CH3)2P— H*C~/PX H3C 3.5 3.0 2.5 2.0 S(ppm) 1.5 1.0 31 1 Figure 4.2. P decoupled 100 MHz H n.m.r. spectrum (A) and ordinary 100 MHz >H n.m.r, spectrum (B) of Me5(NHMe)P.N?CH. Spectra run on samples in benzehe-dg solution. - 118 -Table 4.1 "'H n.m.r. parameters3 and selected vibrational frequencies'3 of azaphosphorin derivatives. Me5(NHMe)P3N2CH Me^(NHMejP^N^CH Me(NHMe)Ph4P3N2CHf 6(MeN) 2.47 (14.0)C 2.46 (13.0)C 2.28 (14.0)C 5(MeP) 1.45 (14.0) M.42e 1.43 (14.5) 6(MepP) 1.32 (12.5) ^1.42e - d L 1.39 (13.5) v(N-H) 3180 3210 3195 v(C-N) 1081 1082 1067 v (P=N) 1159, 1187 1153, 1185 1144, 1170 1209 1191 (a) s(ppm), in C6D6, 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) 6H(CH) = 1.23 ppm, 6H(NH) = 1.76 ppm. the N-methyl and NH protons is not observed, but in the case of the azaphos phorins Me2n_.| (NHMe)PnNn_1CH (n=3,4), the actual resonance of the NH proton is also absent. For N3P3(NHMe)g and (NHMe)CgHg, this phenomenon is similar, in origin, 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 its explanation is more complicated. The chemical shift of the protons attached to ylidic carbon atoms varies with the nature of the other groups on carbon. In simple ylids - 119 -210 221 (e.g. Me3P=CHR and Ph3P=CH2 ) the anionic nature of the carbon atom has a large shielding effect on the CH proton (6H(CH)=-0.5 to -1.0 ppm). When an electron withdrawing group is present on carbon the CH proton is less 999 shielded (e.g. 6H(CH) in Ph3P=CHC(0)Me is 6.32 ppm ). In phosphorins, electron withdrawal also occurs, but to a lesser extent, and the CH resonance ?lfi 999 is usually found in the region 6 = 1.5-2.0 ppm ' . Accordingly, the CH signal of Me(NHMe)Ph4P3N2CH (in benzene solution) appears as a broadened singlet (not the expected triplet) at 6 = 1.23 ppm (Figure 4.3). For Me2 -| (NHMe)P N -|CH (n=3,4), no CH resonance is observed (although the possibility that it is obscured by the P-methyl resonances cannot be entirely excluded). H I H3C N-CH3 \/ 3 kPh (CHOP (CHON PNCH V 4.0 3.5 3.0 2.5 8 (ppm) 2.0 1.5 1.0 Figure 4.3. 100 MHz ]H n.m.r. spectrum of Me(NHMe)Ph-PoN„CH (CgD6 solution) Phenyl region is not shown - 120 -The absence of the CH (and NH) resonance from the ^H n.m.r. spectra of Me2n_-] (NHMe)PnNn_^CH (n=3,4), its presence (as a broadened singlet) in the spectrum of Me(NHMe)Ph4P3N2CH, and the lack of 1H-1H coupling from the NH proton in all three molecules, can all 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). H H I Ma i N—Me \ / % / Me—P —Ma Me \ Me—P H H \ / N —Me . / P—Me .. 10 This equilibrium is similar to that proposed for the sulphur ylid 3-dimethylsulphonioindole (XX), which rapidly incorporates deuterium into the S-methyl group when dissolved in deuterochloroform . It is suggested that such an exchange requires (a) the intermediacy of the tautomer XXI (Equation 11), XX CH3 CH3 S CH3 S=CH2 . 11 XXI - 121 -and (b) a sufficiently strong basic character to enable the compound to deprotonate chloroform*. The azaphosphorin MeS(N2P2Ph4)CH222'225 (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, is similar to that found in Me(NHMe)Ph4P3N2CH, and its resolution into the expected triplet cannot be achieved even at -30°C. As before, proton exchange (Equation 12) between the two possible tautomers XXII and XXIII is believed to account for this lack of resolution. l/Ph l/Ph N—— P N=P / \ / \/H HqC—S G-H H2G=S C; ... 12 \ / \ / H N=PV N = P ' Ph I Ph PhXXII XXIII In simple phosphorus ylids, the sensitivity of P- H coupling to 210 99"i 99f\ traces of acid is well known ' ' . For example, a rapid reversible proton exchange, caused by the addition of methanol to a solution of 31 1 Me3P=CH2 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 * The azaphosphorins Mepn_i (NHMe)PnNp_-|CH (n=3,4) decompose chloroform on contact. Me(NHMe)Ph4P3N2CH is less basic, and is stable in chloroform solution (but proton exchange may still occur). - 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 ylids suppresses such exchanges and aids in the enhancement of the otherwise unobserved fine structure of the resonances of ylidic protons. This effect has also been observed in the present work. Specifically, it has been found that the n.m.r. spectrum of Me^HMeJPh^N^CH in 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 triplet structure of the CH proton is clearly visible. As in 210 the case of Me-^CHp^ , 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 ylidic 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)PnNn_-|CH (n=3,4). These derivatives are much more basic than pyridine, and will compete successfully with it 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 H I H3C N-CH3 3 / 1ST C-H Ph NPh NH I NH I r 123 -(CHJN 'PNCH, (CH3)P-(CHJN ;PNCH2 JHNCH, (CH3)P A' 'PCH, 'PCH, H-C ^P 'PCH X. 4.5 4.0 3.5 3.0 2.5 2.0 S(ppm) 1.5 1.0 Figure 4.4. 220 MHz 'H n.m.r. spectrum of Me(NHMe)Ph4P3N2CH in pyridine-d,-solution, (A) at 20°C and (B) at 60°C. 5 - 124 -compounds Me2n_-| (NHMe)PnNn_^CH (N=3,4), the exchange is fast enough to 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 all 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 difficult and speculative. However, some vibrations (Table 4.1) can be assigned with reasonable certainty. For all 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 all the spectra. The frequency of this band at ^3180 cm-1 is similar to that found in N-alkyl phosphoramidates R2P(0)NHR , and indicates the existence, in the 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 all the azaphosphorin derivatives, in the region 1067-1082 cm"1. The validity of this assignment is confirmed by the infrared spectrum of the deuterated compound Me7(NHCD^)PAN^CH, in which the v(N-C) - 125 --1 4 frequency is lowered to 1037 cm '. In N-alkyl phosphoramidates, the v(N-C) vibration is observed 228 231 between 1020-1220 ' , but in N-methyl phosphinimines, it occurs at much -1 232 lower frequencies (e.g., 848 cm in Ph3PNMe ), because of coupling with 6(P=N). Hence the high value of v(N-C) found for the azaphosphorins confirms 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) PPho0 have been known for some time. 2 n 2 The compound ClPI^PNPPhgO was first prepared by the oxidation of diphenylchloro-phosphine 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 salt-like compound. CI(PPh2N)3PPh20236'237, The 31P n.m.r. spectrum of this molecule displays only two signals, suggestive of the cyclic (XXIV) rather than the linear (XXV) structure. Its ionic nature is also apparent from the fact that the chlorine atom can be replaced by another anion without changing the i.r. spectrum ^ v(C-N) in MeNH2 is at 1044 cm-"1229,230, and is reduced to 973 cm-1 upon deuteration of the methyl group. ^ This is equivalent to the coupling found in simpler molecules, e.g. v(N-C) in MeNH2 is at 1044 cm"1229'230, but is reduced to 910 cm-1 in MeN3233, 921 cm-1 in MeN02234, and 928 cm"1 in MeNCO235. - 126 * R f R R p_R // \ // \ " N N N \ // R \ // R / R4 /\ R R K Yi X XXIV XXV 237 of the cation . However, the reaction of CI (PI^PN^PPi^O with amines and phenoxide ion leads to cleavage of the cyclic skeleton and the formation of the covalent linear oxides X(PPh2N)3PPh20 (X=NH2> NHMe, NMe2, OPh), the structures of which are confirmed by the presence of four distinct resonances 31 237 in their P n.m.r. spectra . Hydrolysis of the chloro-compound leads to the corresponding hydroxy derivative HO(PPh2N)3PPh20 ' . This compound, and its congeners HO(PPh2N)nPPh20 (n=l,2) have also been isolated (in low yield) from other reactions ' . 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(PMe2N)nPMe20 (n=l,2,3), by the ring cleavage of N-methyl methylphosphazenium salts, is described. The hydroxy derivative H0(PMe2N)3PMe20 has also been isolated. This is the first report concerning fully methylated oxides of this type, possibly because, unlike their phenylated analogues, the compounds X(PMe2N)nPMe20 are all extremely hygroscopic, and therefore more difficult to handle. 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. - 127 -31 a 1 b Table 4.2. P and H n.m.r. parameters, and selected infrared frequencies0 of phosphazene oxides X(PMe2N)nPMe20. X- HNMe HNMe HNMe OH^ 1 2 3 3 6pPA 81.7 98.0 101.8 d 6pPB 81.2 85.3 98.3 d 6pPc - 80.9 87.28 <5pPD - - 80.7 6H(Me2PA) 1-22(13.5)e 1.20(13.5)e 1.18(13.0)e 1.50(14.0) 6H(Me2PB) 1.48(14.0) 1.37(12.0)e 1.36(13.0)e 1.66(13.5) 6H(Me2Pc) - 1.51(12.5) 1.41(15.0)' 6H(Me2PD) - - 1.50(15.0) 6H(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 \e (a) 6(ppm), in CDC13, reference external P4O5. Phosphorus atoms lettered alphabetically from the oxygen atom, (b) 6(ppm), in CD3CN, except | in CDC13, reference internal TMS. J(PH), in Hertz, in parenthesis, (c) From nujol mulls, assignments tentative, (d) <Sp values unavailable, (e) J(PH) (long range) = 1.0 Hz. (f) v(0H) = 2500 cm"1 (broad). - 128 -For the compounds NHMe(PMe2N)nPMe20 (n=l,2,3) a linear structure is indicated, since all the phosphorus atoms, and the PMe2 protons, are inequivalent (^H n.m.r. assignments are consistent with the results of a 31 single frequency P decoupling experiment on NHMe(PMe2N)2PMe20). The 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 shift variations in the 237 phenylated oxides X(PPh2N)3PPh20 . The <5p values of the methylated oxides NHMe(PMe2N)nPMe20 are therefore assigned by analogy with the above trends. The ^H n.m.r. spectrum of HO(PMe2N)3PMe20 differs from those of the other oxides. The presence of only two P-methyl signals indicates a rapid inter- or intramolecular hydrogen transfer between the terminal oxygens. The effect of such hydrogen bonding is also observed in NH2P(Ph2)NP(Ph2)NH, 31 239 which exhibits only one P n.m.r. signal , and in NHMePMe2NPMe20, where 31 the two P n.m.r. signals are almost coincident. Hydrogen bonding is also indicated, for all the compounds listed in Table 4.2, by the low value of v(P=0) (1160 cm"1 in Me3P0184'185), the effect being greatest in the hydroxy derivative (v(P=0)=1108 cm-1), as expected. 4.4 Reactions of Azaphosphorins 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 ylids R3PCH2 and phosphorins (e.g. XXVI), it also displays features which are reminiscent of the behaviour of phosphinimines. Ph Ph XXVI For example, unlike simple phosphorus ylids, which are easily 240 241 oxidized to phosphine oxides ' , the azaphosphorins do not react with molecular oxygen. Also in contrast to simple ylids, but like other 242 phosphorins (e.g. XXVI) , they do not undergo the Wittig reaction, the P=C bond being insufficiently polar. A number of reactions of the azaphosphorins have been studied in detail, and these are discussed in the following sections. 4.4.1 Hydrolysis of Me2 ^ (NHMe)P N ^CH (n=3,4) The hydrolysis of the azaphosphorins Me2n_^(NHMe)PnNn_^CH (n=3,4) 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 stability of aminophosphazenes, and indicates the increased susceptibility of the aminated phosphorus atom in the 130 H3C ,0 y H H H3C CH3 CH3 XXVII X/ H / N* H \/CH3 H3C /A CH3 /\ H3C GH3 XXVIII azaphosphorins to nucleophilic attack (e,g, by hydroxide ion). By analogy 240 241 with the hydrolysis of simple phosphinimines ' , 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 H I Me i N—Ma \X% / Ma—P ^P—Me Me \ Me—P H H N—Me . / P—Me 13 H H Me \ / 0 Me—P —Me - MeNhb =-H20 H H Me \ /• \ -° Me—P N—Me + P—Me OH" It is 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 cyclic 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(PMe2N)3PMe20 (i.e. XXV, R=Me, X=0H). Ph-Ph \ N . Ph XP—Ph N C i Ms XXIX H20 Ph Ph H e i Me NH2 MePh2P0 .. 14 It is apparent that, in the azaphosphorin system, hydrolysis can occur in two ways, (1) by loss of the exocyclic methylamino group, and (2) initial 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(Ph2)CH2P(Ph2)=NPh. 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 MePh2P0 and (NHPh)Ph2P0 (Equation 15b) . 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)Ph4P3N2CH (Section 4.2.1), and it is likely that the influence of pH on the position of this equilibrium will have profound effects on the chemical behaviour of the compounds. - 132 -0=P(Ph2)CH2P(Ph2)=0 • PhN=P(Ph2)CH2P(Ph2)=NPh (NHPh)Ph2P=0 + MePh2P=0 15a 15b 4.4.2 Reaction of Me£ (NHMe)P N ^CH (n=3,4) with Methyl Iodide As was mentioned in Chapter 1, the reaction of phosphorus ylids R3P=CHR with methyl iodide yields simple C-methylated phosphonium salts [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 i Ph—P ^P —Ph N N C Me XXIX Ph Mel £> N H Me I_ V ph P —Ph i Me XXX N MJ3 HI Ph-Ph V N Me rL ph C Me XXXI 16 In the reaction of the azaphosphorins Me2n_-j (NHMe)PnNn_^CH (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 Me-H N—Me —Me Me Me V Me V I N—Me v -Me XXXIII H3C N—Me y H H H3C 7N^r H3C CH3 CH3 XXXIV Me Me—P' Me Me—P' H Me \ / H N —Me . / + P—Me XXXII H H \ / N—Me + P—Me \ H i H3G N—CH3 N" \/ -P H e'-H H3C // \ \/CH3 II CH3 .N A H3C CH3 XXXV 17 (see Chapter 1) of C-methylated and C-protonated phosphonium salts is reversed. However, such behaviour is consistent with the acidities of aminophosphonium salts, e.g. the alkylation of simple phosphinimines is immediately followed by a proton transfer reaction with the starting material ,244 245 (Equation 18) ' . This similarity 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. R3P=NH + RX • [R3PNHR]+X-[R3PNHR]V + R3P=NH —• R3P=NR + [RgPNH^V ... 18 R3P=NR + RX • [R3PNR2]V 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 a second transylidation reaction with the starting material. The observed stoichiometry of the overall reaction is as shown in Equation 19. 3Me2n_1 (NHMe)PnNn_1CH + 2MeI 2Me2n_1(NHMe)PnNn_1CH.HI ... 19 (n=3,4) + unisolated neutral products 4.4.3 Reaction of Me,, •) (NHMe)P N ^CH (n=3,4) with Benzoyl Chloride In the reaction of the azaphosphorins Me2n_-| (NHMe)PnNn_-|CH (n=3,4) with methyl iodide, the reaction cannot be halted after the first 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 Me2n_1(NHMe)PnNn_1CH (n=3,4) and benzoyl chloride, in the belief that the initially 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 Me7(NHMe)P4N3CCOPh (XXXVI). 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 first stage. Instead, the initially 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 Me5(NMeC0Ph)Pj-N2CC0Ph (XXXVII). 2Mey(NHMe)P4N3CH + PhCOCl —• Me7(NHMe)P3N2CCOPh (XXXVI) ... 20 + Me7(NHMe)P4N3CH.HCl 3Me5(NHMe)P3N2CH + 2PhC0Cl —> Me5(NMeC0Ph)P3N2CC0Ph (XXXVII) + 2Me5(NHMe)P3N2CH.HCl 21 0 V Ph H i CH3 H3G XXXVI XXXVII - 136 -4.5 Spectra and Structure of Azaphosphorin Derivatives 4.5.1 n.m.r. Spectra of Azaphosphorin Hydrohalides The ]H n.m.r. spectra of the hydrohal ides Me2n_] (NHMe)P N .,CH.HX (n=3,4; X=I,C1) and Me(NHMe)Ph4P3N2CH.HI (Table 4.3) do not exhibit the effects 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 [Me2n-1(NHMe)PnNn_-|CH2]+ (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 six- 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; firstly because it 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 tri- and penta-249 250 coordinate ' phosphorus. In the present case, the "intrinsic - 137 -Table 4.3 n.m.r. parameters3 of the azaphosphorin hydrohalides Me2n_] (NHMe)PnN -,-CH.HX and Me(NHMe)Ph4P3N2CH.HI Me5(NHMe)P3N2CH.HX Me7(NHMe)P4N3CH.HX Me(NHMe)Ph4P3N2CH.HI X= I CI I CI Ib S(MeN) 2.65(13.5) 2.63(13.5) 2.62(13.0) 2.57(13.5) 2.19(14.5) J(H-H) 5.5 5.5 5.5 6.0 5.5 6(HN) 5.85 6.03 4.24 5.37 4.51 6(MeP) 1.99(13.0) 1.99(13.5) 1.96(13.5) 2.04(13.5) 1.83(14.5) 6(Me?P)c 1.86(13.5) 1.79(14.0)D 1.79(13.5) 1.78(13.5) 1.73(14.0) 6(Me9P')C 1.58(14.0) 1.53(14.0) 1.53(13.5)D 1.58(15.0)D C 1.49(13.5) 1.51(15.0) 6(Me2P")c - - 1.49(13.5) 1.51(15.0) H. 3.74 4.09 3.75 3.96 3.77 6(CH„) rt (15.0,11.0) (14.0,11.0) (15.5,12.5) (15.0,12.0) (f) L HR 2.78 2.27 3.05 2.86 3.55 B (13.5,13.5) (14.0,13.5) (15.5,12.5) (15.0,13.0) (f) 6(HAHB) 15.5 15.0 15.5 15.0 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 Me2P, MegP' and Me2P" protons are uncertain, (d) Methyl groups on the same phosphorus atom are magnetically inequivalent. (e) ABXY pattern, partially obscured by other signals; J(PH) values are approximate, (f) J(PH) values not assigned. In CD3CN, HA=HB, 6 = 3.44 ppm, J(PH) = 13.0 Hz. 1 - 138 (CH3)2P —*-H H,C (j—CH, II I H3C N ™3 (CH3)N 1 H-N I (CH^P (CH^JP-A JHNCH, JHAHu . . LX 'PCH, I— 4.5 5.0 4.0 3.5 3.0 8 (ppm) 2.5 —i— 2.0 1.5 1.0 igure 4.5. The 100 MHz H n.m.r. spectra of the azaphosphorin hydroiodide Me5(NHMe)P3N2CH.HI, in CDC13 solution. (A) The ordinary high resolution spectrum, (B) the same spectrum with 1 drop of D?0 added, and (C) the 31p decoupled spectrum. - 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, theoretically, three rotational isomers for the -CH2-P(NHMe)MeN= unit, but, because of the constraints of the cyclic skeleton only two (A and C) are possible. It is readily apparent that, even allowing for equal participation of both these structures, the average environmentsof H and H, are different, a b H Me N H 1 H, N Me (A) H Me \ • \ • N it I D Hb . N Me H a IB) H Me V p. ; a i N Me (0 Figure 4.6. Possible rotational isomers of the CH2-P(NHMe)MeN unit in the cations [Me2n-1(NHMe)PnNn_]CH2]+ (n=3,4). In these structures, the methylene carbon atom is located above the aminated phosphorus atom (not shown). 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 31P-1H and ^H-^H coupling. However, the addition of one drop of D20 to a solution of Me5(NHMe)P3N2CH.HI in CDC13 brings about the 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 is rapid. - 140 -The methylene protons are expected to be less acidic, and their exchange with protic solvents is consequently slower, but the effect is nonetheless observable. For example, when Me5(NHMe)P3N2CH.HI is dissolved in D^O, the methylene resonance appears as a simple triplet which, upon allowing the solution to stand, becomes less intense and finally vanishes (after about 2 hours). 4.5.2 ]H n.m.r. Spectra of Phosphazene Oxides Me2n-1(°)PnNn-1CH2 ^n=3'4^ As in the case of their parent azaphosphorins, the structures of the oxides Me2n_-| (0)pnNn_-]CH2 (n=3,4) can be represented by either of two 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). H H H Mve I 0-H Me \ / 0 Me—P ^P — Me •«-». Me—P^ ^P—Me ... 22 xxxvin 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 triplet (due to P- H coupling). - 141 -Table 4.4 ^ n.m.r. parameters3 of phosphazene oxides Me„ ,(0)P N ,CH„ (n=3,4). 2n_1 n n-1 2 6(CH2) 6(MeP) 6(Me2P)b <5(Me2P')b 6(Me2P")b n = 3 1.95(13.5)C 1.96(13.5) 1.61(14.0) 1.48(15.0)d 2.11(13.5) 1.53(15.0) n=4 2.31(12.5) 1.94(13.0) 1.63(14.0)e 1.54(13.0)e 1.47(14.0) 1.56(14.0) 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» Me2P' and Me2P" protons are uncertain, (c) ABXY pattern partially 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. 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 cyclic delocalization, as in XXXVIII^. Such is not the case in the phosphazenes Phg(0H)2P4N4 (non-geminal) and Ph5(0H)P3N3, for 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 its greater ability ^ D(P=0) in Me3P=0 is 139.3 KcaV/mole, whereas D(P=N) in MeoP=NEt is 97 Kcal/mole252. ° J - 142 7,. 30 Z5 20 15 ID 8 (ppm) Figure 4.7. The 100 MHz H n.m.r. spectrum of the phosphazene oxide Me7(0)P4N3CH2, in CDC13 solution. (A) The ordinary high resolution spectrum, and (B) the 31p decoupled spectrum. - 143 -in forming ir-bonds to phosphorus, is thought to account for the stability of the hydroxy form. P\h N /°-H Ph I 0 Phw- ^ ph-vN^//-Ph ...» XL XLI 4.5.3 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 Me7(NHMe)P4N3CC0Ph (Table 4.5) reveal very little resolution of the P-methyl resonances, thereby indicating the uniformity of the charge distribution within the ring. For the tetrameric compound ^H-^H coupling between the NH and N-methyl protons is observed (as it is in simple N-methyl amides); proton exchange between nitrogen and the benzoylated carbon is obviously suppressed by the low basicity of the latter. The shielding of the N-methyl protons in Me5(NMeC0Ph)P3N2CC0Ph (6=2.50 ppm) is greater than in N-methyl benzanilide (6=3.40 ppm^), indicating that the diffusion of lone pair density from nitrogen to phosphorus is limited. ^ On a sample in CDC13 solution. The compound was prepared by the benzoylation of N-methyl aniline254. - 144 -Table 4.5 n.m.r. parameters3 and carbonyl stretching frequencies*5 of Me5(NMeCOPh)P3N2CCOPh and Me7(NHMe)P4N3CC0Ph. 6(MeN) 6(MeP) 6(Me9P)C v(C=0) Me5(NMeCOPh)P3N2CC0Ph 2.50(9.0) 1.70(15.0) 1.64(13.5) 1.60(13.0) 1502 1.55(13.5) 1642 1.48(13.0) Me7(NHMe)P4N3CC0Ph 2.46(14.0)d e e 1530 (a) 6(ppm) in CDC13 solution, reference internal TMS. J(PH), in Hertz, in parenthesis, (b) v(C=0) in cm-1, from nujol mull spectra, (c) Absolute assignments are uncertain, (d) J(HH) = 5.5 Hz, 5(NH) = 2.70 ppm. (e) All P-methyl resonances occur as an unresolved multiplet near 6 = 1.55 ppm. 4.5.4 Infrared Spectra of Benzoylated Azaphosphorins As with the other azaphosphorin derivatives, the infrared spectra of Me5(NMeC0Ph)P3N2CC0Ph and Me?(NHMe)P4N3CC0Ph are too complex to allow a detailed interpretation. However, their carbonyl stretching frequencies (Table 4.5) are well isolated and can be easily identified. For both the trimeric and tetrameric derivatives, the betaine (XLII) carbonyl frequency 84 255 256 is similar to that found in acylated ylids ' ' , 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 V -p ^p XLII Ph 0-V -V / XLIII The amide carbonyl frequency of Me5(NMeCOPh)P3N2CCOPh (1642 cm"1) is similar to that found in simple tertiary amides (v(C=0) in PhC0NMe2 is 1640 cm"1 257, 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)P3N2CCOPh suggests that similar conjugative interactions, between phosphorus and nitrogen, are limited. 4.6 Experimental Nearly all 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 purification. From the present work; the value quoted is from a sample in CC14 solution (in nujol mull, v(C=0) is 1650 cnH). An earlier report258 gave a lower value of 1641 cm-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 distillation from sodium, and diethyl ether by distillation from lithium aluminium hydride. 4.6.1 Reaction of NaN(SiMe.,)o with Methylphosphazenium Quaternary Salts The following reactions all involve the same experimental procedure. Therefore, only one reaction (4.6.1.1) will be described in detail. 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(SiMe3)2 with (NPMe2)4.MeI Freshly cut sodium (0.200 g, 8.69 mmol) was added to a slurry of (NPMe2)4.MeI (3.54 g, 8.01 mmol) in 200 ml of octane, and the mixture heated to the reflux temperature, whereupon % h ml of HN(SiMe3) was injected, via a syringe and septum, into the reaction vessel. In practice, it was not necessary to add the correct stoichiometric amount of HN(SiMe3)9, since it was regenerated in the course of the reaction. However, to avoid possible evaporation losses (from boiling octane), further small additions (^%ml) of HN(SiMe3)2 were made every 24 hours, in order to ensure an adequate supply. After 72 hours, the reaction mixture was cooled and filtered, and the solvent distilled from the filtrate to leave a white solid, which was purified by sublimation at ^130°/0.01 Torr and recrystal1ization from hot hexane to give colourless hygroscopic prisms of Mey(NHMeJP^N^CH (2.21 g, 7.04 mmol, 88%). - 147 -Anal, calcd. for CgH^N^: C, 34.29; H, 8.63; N, 17.77. Found: C, 34.44; H, 8.54; N, 17.57. M.pt. 104-105°C. 4.6.1.2 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 filtered (while still hot, to avoid recrystallization of the product) and the solvent distilled from the filtrate to leave a white solid, which was purified by sublimation at/vl30°C/0.01 Torr and recrystal1ization from hot hexane to give colourless hygroscopic plates of Me5(NHMe)P3N2CH (2.68 g, 11.2 mmol, 80%). Anal, calcd. for CyH^N^: C, 35.15; H, 8.43; N, 17.57. Found: C, 35.40; H, 8.50; N, 17.58. M.pt. 140-142°C. 4.6.1.3 Reaction of MepPh^N^Mel with NaN(SiMe3)2 A sample of Me2Ph4P3N3.MeI (recrystallized from ethanol and powdered 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 filtered and the solvent distilled from the filtrate to leave a white solid which was purified by recrystallization from benzene/octane to give colourless prisms of Me(NHMe)Ph4P3N2CH (0.953 g, 2.01 mmol, 93%). Anal, calcd. for C27H28N4P3: C' 66-10; H' 5-33' N' 8-89-Found: C, 65.93; H, 5.55; N, 8.65. M.pt. 147-149°C. 4.6.1.4 Reaction of (NPMe2)r2MeX with NaN(SiMe3)2 In two separate experiments (NPMe2)4.2MeS03F (a) and (NPMe2)4.2MeI (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 left in the reaction flask, 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 oil. 4.6.1.5 Reaction of [(NPMeJ3 4.CH2C00EtlV with NaNfSiMeJg-Equimolar quantities of [NPMe2)3 4.CH2C00Et]+I~ and sodium were 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 (NPMeJ4.MeI with KOtBu Finely powdered (NPMe2)4.MeI (3.00 g, 6.80 mmol) was added to a 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 filtered and the solvent distilled from the filtrate 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)P4N3CH (1.71 g, 5.44 mmol, 80%), which was identified by comparison of its infrared spectrum with that of an authentic sample, and by its melting point, 104-105°C. The residual oil remaining in the sublimation vessel solidified into a white solid, which was purified by recrystal1ization from hot hexane to give colourless hygro scopic cubes of NHMe(PMe2N)3PMe20 (0.11 g, 0.33 mmol, 5%). Anal, calcd. for C9H28N40P4: C' 32-50; H> 8-49; N> 16-86- Found: C, 32.62; H, 8.57; N, 16.89. M.pt. 72-74°C. - 149 -4.6.3 Reaction of (NPMeJj.Mel with KOtBu Similarly, (NPMe2)3.MeI (1.38 g, 3.76 rnrnol) was allowed to react with a slurry of KOtBu (0.50 g, 4.46 mmol) in 50 ml of hexane. After 24 hours, the mixture was filtered and the solvent removed from the filtrate to leave a colourless oil. Sublimation of this oil at ^130°/0.01 Torr onto a cold finger at -78°C yielded a white hygroscopic solid, NHMe(PMe2N)2PMe20 (0.886 g, 3,44 mmol. 92%), which was recrystallized as colourless plates by cooling a concentrated solution of it in pentane to -23°C. Anal, calcd. for C^H^N^OPg: 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 (NPMe2)4.2MeS03F with KOtBu In a similar experiment, (NPMe2)4.2MeS03F (2.70 g, 5.12 mmol) was 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 distilled from the solution to yield a white solid, which was recrystallized from hot hexane to give colourless feather-like crystals of NHMe(PMe2N)PMe20 (0.927 g, 5.02 mmol, 50%). Anal, calcd. for C5H]6N20P2: C, 39.97; H, 8.85; N, 15.38. Found: C, 33.28; H, 8.94; N, 15.25. M.pt. 73-75°C. 4.6.5 Reaction of (NPMe2)4-MeI with Methyllithium A solution of methyllithium in diethyl ether (8.3 ml, 5.5 rnrnol of MeLi) was added to a slurry of (NPMe2)4.MeI (2.225 g, 5.03 mmol) in 125 ml 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 filtration of the solution from the precipitated lithium fluoride, the filtrate was evaporated to dryness (in vacuo). The residual solid was extracted with benzene to yield as the benzene soluble part a colourless, hygroscopic crystalline solid, which was recrystallized from benzene/octane as H0(PMe2N)3PMe20 (0.50 g, 1.5 mmol, 30%). Anal, calcd. for C8H25N302P4: C, 30.10; H, 7.89; N, 13.16. Found: C, 29.87; H, 7.94; N, 13.08. M.pt. 96-100°C. 4.6.6 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 Me2Ph4P3N3>MeI (0.784 g, 1.40 mmol) in 50 ml of ether. 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)Ph4P3N2CH.HI (0.319 g, 0. 52 mmol, 34%), which was recrystallized from acetonitrile/toluene as colourless blocks. Anal, calcd. for C27H3oIN3P3: c» 52-70> H» 1, 20.62; N, 6.83. Found: C, 52.57; H, 4.80; I, 20.45; N, 6.67. Dec. 248-252°C. 4.6.7 Hydrolysis of Meg ^ (NHMe)P N ^CH (n=3,4) A sample (300-400 mg) of ^e2n-l (NHMe)P N -jCH (n=3,4) was dissolved in 25 ml of a 50/50 ethanol-water mixture and the solution left to stir overnight. The following day, the solvent was removed at room temperature - 151 -in vacuo to leave a white solid. For n=3, recrystallization of this solid from benzene gave colourless hygroscopic blocks of Me5(0)P3N2CH2. Anal, calcd. for CgH^OP^. C, 31.87; H, 7.58; N, 12.39. Found: C, 32.20; H, 7.59; N, 12.28. M. pt. 184-186°C. For n = 4, recrystallization of the solid from hot hexane gave colourless feather-like crystals of Me7(0)P4N3CH2. Anal, calcd. for CgH23N30P4: C, 31.90; H, 7.70; N, 13.95. Found: C, 32.12; H, 7.91; N, 13.98. M.pt. 140-143°C. 4.6.8 Reaction of Me2n ^NHMejP N ifti (n=3,4) with Methyl Iodide The addition of an excess of methyl iodide to a solution of Me2n_i (NHMe)PnNn_-|CH (n=3,4) in ether resulted in the precipitation of the hydroiodide salts Me2n_1 (Nltfle)P N .,CH.HI (n=3,4). For n = 3, 4.33 mmol (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. Attempts to isolate that part of reacted azaphosphorin (approximately 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 acetonitrile/toluene. For n = 3, anal, calcd. for C7H21IN3P3: C' 22'90; H' 5,77; l* 34'57; N' 11-45' Found: C' 22'96; H, 5.88; I, 34.20; N, 11.30. M.pt. 194-195°C. For n = 4, anal, calcd. for CgH27IN4P4: C, 24.45; H, 6.15; I, 28.70; N, 12.67. Found: C, 24.52; H, 6.30; I, 28.55; N, 12.57. M.pt. 155-156°C. - 152 -4.6.9 Reaction of Me7(NHMe)P4N3CH with Benzoyl Chloride A solution of benzoyl chloride (0.124 g, 0.88 mmol) in 10 ml of ether was added dropwise to a stirred solution of Me7(NHMe)P4N3CH (0.555 g, 1.77 mmol) in 50 ml of ether. A fine white precipitate was immediately formed. After 3 hours the mixture was filtered to yield a white hygroscopic solid, which was recrystallized from acetonitrile/benzene to give small colourless cubes of Me7(NHMe)P4N3CH.HCl (0.256 g, 0.73 mmol). Anal, calcd. for C9H27C1N4P4: c» 30.82; H, 7.76; N, 15.98. Found: C, 30.86; H, 7.66; N, 15.87. M. pt. 193-195°C. The solvent was distilled from the filtrate to leave a yellow oil which, on drying in vacuo, solidified into a yellow, air stable powder. Recrystallization of this solid from hot hexane yielded yellow mica-like plates of Me7(NHMe)P4N3CC0Ph. Anal, calcd. for C16H30N40P4: C' 45-98; H> 7-23; N' 13.39. Found: C, 45.73; H, 7.30; N, 13.10. M.pt. 133-134°C. 4.6.10 Reaction of Me^(NHMe)P3NpCH with Benzoyl Chloride 4.6.10.1 Ratio of PhCOCl:Me5(NHMe)P3NoCH = 1:2 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 filtered, and residue recrystallized from acetonitrile/benzene to give colourless hygroscopic cubes of Me5(NHMe)P3N2CH . HC1 (0.569 g, 2.07 mmol; expected yield = 1.26 mmol). Anal, calcd. for C?H21C1N3P3: C, 30.50; H, 7.68; CI, 12.86; N, 15.24. Found: C, 30.20; H, 7.80; CI, 12.55; N, 15.00. M.pt. 194-197°C. The - 153 -solvent was distilled from the filtrate 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 Me5(NMeC0Ph)P3N2CC0Ph. Anal, calcd. for C21H28N3°2P3: C' 56-38; H> 6-32' N> 9-39- Found: C, 56.13; H, 6.20; N, 9.60. Dec. 181-184°C. 4.6.10.2 Ratio of PhCOCl:Me5(NHMe)P3NoCH = 2:3 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 filtered to leave, as above, Me5(NHMe)P3N2CH.HCl (0.452 g, 1.64 mmol, expected yield = 1.49 mmol). Distillation of the solvent from the filtrate left crude Me5(NMeC0Ph)P3N2CC0Ph (0.261 g, 0.59 mmol, expected yield = 0.75 mmol), 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 (NPX2)n (e.g. F, CI), and to the isoelectronic series of siloxanes (0SiMeo) , which are noted for Z n ?fil ?fi? their extreme flexibility ' . This difference in behaviour, which can be attributed to the high polarity of the P=N bond in methylphosphazenes^, is T 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 2.2D263, whereas the P=N bond moment in phosphinimines is greater than 4D264»265. - 155 -particularly useful,from a: structural point of view, since it 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 methylphos-125-127 phazenes are known " ; their geometries are illustrated in Figures 5.1 and 5.2. As in other phosphazenes, the basic structural parameters within each ring vary little from their mean values (Table 5.1) (e.g. all 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) L(P-C) (A) PNP (deg) NPN (deg) CPC (deg) (NPMe2)4a 1.596 1.805 132.0 119.8 104.1 (NPMe2)5b 1.586 1.804 133.0 118.6 104.3 (NPMe2)7c 1.593 1.805 134.1 117.3 103.8 (NPMe2)8c 1.578 1.797 139.9 117.2 103.5 (a) M.W. Dougill, J. Chem. Soc, 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 (NPC12)^'265) rather than to iT-effects. The absence of any structural parameter which is dependent on the shape of the molecules is 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 [(Me2PN)5, M.W.. Dougill and N.L. Paddock, unpublished results; (Me?PN)7 R. S. Rettig, unpublished results]. ' - 157 -(c) Figure 5.2. Three possible conformations for 8-memberd phosphazene rings (NPX2)4: (a) D2d (saddle), (b) S4 (tub), (c) C2h (crown). The actual structure of (Me2PN)4 lies between S4 and D2cl (M.W. Dougill, J. Chem. Soc, 5471 (1951 ). - 158 -In order to understand more clearly the meaning of this conclusion, it is useful, at this point, to describe the way in which ir-bonds can be formed in phosphazene derivatives. If it is assumed that the a-skeleton of 3 phosphazenes is 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 2pz orbital on 2 nitrogen and a 3d orbital on phosphorus, and (ii) the remaining sp hybrid orbital on nitrogen and a 3d orbital on phosphorus. In planar molecules, the two types of overlap give rise to two different ir-systems, the former being termed the out-of-plane TT system and the latter the in-plane IT system. In a s non-planar molecules, the classification into u and n retains only its local a s significance and, for example, a nitrogen orbital with TT, properties will a overlap with orbitals of tts symmetry on phosphorus, and vice versa. Although the contributions to bonding of the two Tr-systems are not 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, all 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) . The "saddle" can be transformed progressively into the "tub" form, and the molecules N4P4(0Me)8 (close to "saddle")60, N4P4(NMe2)g65, N^Meg126, and N4P4CVK) (close to "tub") fall in this conformational series. The C2v "chair" conformation is represented by another polymorphic form of N4P4C18(T)129'3, and by two non-geminally substituted derivatives N^P^Ph^Cl^ - 159 -and N4P4Ph4(NHMe)4268. On the basis of the above evidence, the two components of the double 7r-system in phosphazenes do not differ greatly in strength, so that between them they provide little resistance to torsional motion about the ring bonds. It is therefore expected that nonbonded interactions will 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 sterically 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 all nonbonded interactions 269 is possible only for the 8-membered ring ; for the larger ring sizes, 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 (PN^Pf^ (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 (NPMe2)n ring. The justification for this treatment of the molecules as a set of such fragments is its success in accounting for the main qualitative features of the observed structures. - 160 r (a) (b) (c) (d) Figure 5.3. Angular conventions for the measurement of torsion angles in a (PN)2PMe2 unit. The angles i>} 2 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 illustrated-(a) GG ^ = ^=60°), (b) GT (^,=60°, ^2=180°), (c) CT (rh=0°, ^2=180°), (d) GG' (^=-60°, ^=60°). 1 The potential functions used to simulate van der Waals1 interactions were of the Lennard-Jones and Devonshire (6-12) type, and were minimized not 270 at the normal van der Waals1 distance , but at a slightly greater distance RQ (see footnote, Table 5.2 for details). This larger value RQ was used for two reasons, firstly to take into account the bulky nature of the P* "atoms" (Figure 5.2), which are in fact PMe9N groups, and secondly to allow for the - 161 -Table 5.2. Interatomic potential constants9 used for the calculation of van der Waals interactions in a (PN)9PMe9 unit. Interaction R 0 A B P...P 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)12 - (B/r)6, kcal mol-1 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 R0 (A). R0 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). 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 intuitively, the conformational energy is minimized when all 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 is 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 T Terminology from Mizushima^; the terms GT, CT, etc. refer to the approximate orientation of NP* bonds (Gauche-Trans, Cis-Trans, etc.); see Figure 5.3. r> 162 r (a) (b) Figure 5.4. Potential energy (a) and electrostatic energy (b) contours (kcal mol*"') of (PN)2PMe2 unit as a function of ^ and i|)o (energies quoted are relative). Electrostatic energy (Ej calculated using q(P) = +"s,.q(N) = -h» E = Kz(q(P).q(N))/rPN (K = 3.3207xl02 kcal.A.mol"'). The (PN)2PMe? model used has L(P=N) = 1.59A, L(P-C) = 1.80A, PNP = 135°, NPN = 120°, CPC = 105°. 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 (NPMe2)n, is probably equally important in the present system. Because of ignorance of the total charge distribution, 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 stabilize the structures in the region of GT and CT with respect to the energy of the GG conformation. - 163 -Table 5.3. Observed torsion angles3 and local conformations in methylphos-phazenes. Molecule Atom *1- *2 Conformat (symmetry) (deg) (deg) (NPMe2)4(S4). Pl,2,3,4 31.5 54.7 GG (NPMe2)5(C1) Pl 32.0 64.1 GG P2 -128.2 11.0 <J P3 67.9 16.4 ^GG P4 - 79.3 39.1 GG P5 119.1 -73.0 GG'M3T (NPMe2)7(C2) ..: Pl 36.1 68.7 GG P2,2' ±156.4 +46.3 GT p ^3,3' ± 57.4 ±72.7 GG P4,4' ±166.8 +36.1 GT (NPMe2)8(C4v) Pl,3,5,7 61.3 27.8 GG P2,4,6,8 156.5 76.8 GT (a) The signs of th 2 (see Fig. 5.3) are such that a clockwise rotation has a positive sign. In the case of (NPMe2)7, the atoms Pn n- (n=2-4) are related by a two-fold axis through Py, the values of ih 2 are'identical, but their signs are reversed. ' - 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 classified as approximately GG or GT. Only in the case of (NPMe2)5, which has a low overall molecular symmetry, are significant deviations observed. The predominance of GG in the tetramer can be attributed to cyclic constraints, i.e. it 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. (NPMe2)g 10) will show an increasing proportion of GT and CT conformations. This belief is supported 0-7C by the known structures of the high polymers (NPF,,)^ (conformer B) and ?7fi (NPC12)M , both of which exist as planar (or nearly so) CT chains. 5.2 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 of a number of the azaphosphorin derivatives described in the previous chapter. - 165 -(c) Figure 5.5. Overall geometries and principal structural parameters of methylazaphosphorins. (a) Me5(NMeC0Ph)P3N2CC0Ph. (b) Me7(NHMe)P4N3CC0Ph. (c) Me7(NHMe)P4N3CH. [S. Rettig and H.P. Calhoun, unpublished results]. - 166 -Table 5.4. Values of P=C and P-C (mean) bond lengths in a selection of phosphorus ylids. Compound L(P=C). L(P-C) Ref (A) (A) Ph3P=C=PPh3 1.629 1.832 a 1.633 1.837 Ph3P=C=C=0 1.648 1.805 b Ph3P=CH2 1.661 1.823 c Ph3P=C=C=S 1.677 1.795 d Ph3P=C=C(0Et) 1.682 1.832 e Ph3P=C-CF2-CF2-CF2 1.713 1.799 f Ph3P=C-CH=CH-CH=CH 1.718 1.806 g Ph3P=CIC0Ph 1.71 1.786 h Ph3P=CClC0Ph 1.736 1.806 h Ph(C3H7)2P=C(CN)2 1.743 1.799 i (a) A.T. Vincent and P.J. Wheatley, J. Chem. Soc, Dalton, 617 (1972). (b) J.J. Daly and P.J. Wheatley, J. Chem. Soc, A, 1703 (1966). (c) J.C.J. Bart, J. Chem. Soc, B, 350 (1969). (d) J.J. Daly, J. Chem. Soc, 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. Soc, 95, 5366 (1973). (g) H.L. Ammon, G.L.: Wheeler and P.H. Watts Jr., J. Amer. Chem. Soc, 95, 6158 (1973). (h) F.S. Stephens, J. Chem. Soc, 5640, 5658 (1965). (i) W. Dreissig, H.J. Hecht and K. Plieth, Z. Kristallogr., 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)P4N3CH (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 derealization 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 double bond is 1.23 A278) and the short central C-C)bond (L((sp2)C-C(sp2)) = 1.46 A279, 1.48-1.49 A280), but, because of the competing influence of the phosphazene ring, its extent is less than that found in simple C-benzoylated ylids281 (Table 5.5). Table 5.5. Bond lengths and torsion angles (T) in C-benzoylated ylids T(C-C) x(C-Ph) (deg) "(deg) 7.8 33.5 29.4 41.6 11 51 4.7 57.6 Compound L(P=C) (A) L(C-C) (A) L(C=0) (A) Me5(NMeC0Ph)P3N2CC0Pha 1.765° 1.431 1.244 Me7(NHMe)P4N3CC0Pha 1.760C 1.417 1.246 Ph3P=CIC0Phb 1.71 1.36 1.28 Ph?P=CClCOPhb 1.736 1.35 1.301 (a) S. Rettig, unpublished results. J. Chem. Soc, 5640, 5658 (1965). (b) Mean value, (c) F.S. Stephens, - 168 -Probably for steric reasons, the P=C-C(0)Ph unit is not planar; torsion about the central C-C bond is small enough to allow conjugation between the P=C and C=0 bonds, but, as the C(0)-Ph distances reflect, conjugation with the phenyl group is prohibited by a large torsion angle, as 281 in simple C-benzoylated ylids (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 dimethylamino-phosphazenes^-67. Substitution of a benzoyl group onto the nitrogen lengthens the P-N bond dramatically (L(P-N) in Me5(NMeCOPh)P3N2CCOPh is o 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 localized, and do not depend on a unique property of the phosphazene ring, nor do they provide any direct information on the nature of cyclic ir-bonding. In the latter 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 all the phosphorus atoms. Strictly speaking this assumption is 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 PnNn -,C rings. - 169 -Table 5.6. Mean bond lengths in azaphosphorin and protonated phosphazene structures. Compound X"P1 prNi NrP2 P -N 2 2 Me7(NHMe)P4N3CHb 1.716 1.608 1.602 1.590 ( 0.008) ( 0.002) (-0.010) Me7(NHMe)P4N3CC0PhC 1.760 1.591 1.587 1.594 ( 0.000) (-0.004) ( 0.003) Me8P4N4H+ d 1.695 1.538 1.614 1.582 (-0.040) ( 0.036) ( 0.004) Me5(NMeCOPh)P3N2CCOPhC 1.765 1.586 1.605 _ (-0.009) ( 0.010) (Me2N)6P3N3H+ e 1.669 1.561 1.598 _ (-0.018) ( 0.018) (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' ^P-| //XX N N \ II N ^P 2 "P-2 X= CR or NH+ Figure 5.6. 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 all 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 pz) is available on the X atom (Figure 5.6) for ir-bonding to phosphorus. On the other ring nitrogen 2 atoms, the sp hybrids, as well as the pz orbitals, arecapable of ir-bond formation. In protonated phosphazenes, the localization of two electrons in one 2 sp orbital on nitrogen perturbs the in-plane irs system, and the variation in 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 IT,), the deficit 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 2pz orbital on carbon will be of higher energy than its counterpart on nitrogen, (ii) Because of the lower electronegativity of carbon, the d-orbitals on the phosphorus atoms adjacent to the endocyclic carbon will be less effectively stabilized, and their Coulomb parameters will therefore - 171 -0.3 Deviation from mean bond order -o.3J -0.05 Deviation from mean bond length-A u+ 0.05 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. Soc, A, 455 (1970)). be algebraically greater than those on the phosphorus atoms bound to two nitrogens. Of the two effects the first is more important (vide infra). Figure 5.8 illustrates the calculated bond orders in the -rr and IT a s systems of a P4N3C ring. As in the case of protonated phosphazenes, the removal - 172 -of one orbital from the TTs system strengthens the P^N^ and P,,N2 '30nc's a*- *he expense of the P0N-, bond. By contrast, the reverse order is found in the IT c. i a system. The values shown for this latter case were calculated for a model in which the Coulomb parameters of all phosphorus 3d orbitals (ap) were identical, as were those of the nitrogen 2p orbitals (°^). The Coulomb parameter of the carbon 2p orbital (a^) was then set (arbitrarily) equal to the average of a^ and ap. All 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, if the TT and TT system are assumed to be equally important, as 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 P2N2 bonds. 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 its formation from the deprotonation of the N-methyl quaternary salt (NPMe2)4.MeI (see Chapter IV). Figure 5.9(a) shows the typical energy levels found for the TT or TT a s system in a tetrameric phosphazene. Figure 5.9(b) then corresponds to the r 173 -I I 1 1 P2N1 P2N2 Figure 5.8. Calculated (HMO) bond orders (relative to a P4N4 ring) in the out-of-plane ira (a) and in-plane TTs (b) components of the ir-system in a P4N3C azaphosphorin ring. Coulomb parameters (d) related to the resonance integral (e) such that = ap + 28, CIQ = (a^+ap)/2. energy levels found in the IT system of the exocyclic ylid which is formed a (presumably) as the initial product of the deprotonation of a methylphosphazene quaternary salt. As expected, the Tr-electron energy/electron of the system is lowered by the addition of the exocyclic ylid, 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 ylid and azaphosphorin structures, and is not affected by the rearrangement. 0 A ir-energy/electron (units of 3) -174 -(a) (b) ' . (c) . (d) - » XX XX XX XX XX XX . XX xx -XJL XX XX XX XX XX XX XX / P\ N N \ // //p p\ N N •V / \ 1 N""* / P\ N N \ // r \ N N \ // P^N^P 1.675 1.427 1.441 1.697 Figure 5.9. Energy level diagrams and Tr-electron energies for a number of phosphazene and azaphosphorin structures. [aM = aD + 2s, 5.3 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 earlier 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 illustrates the extent to which the molecular skeleton will rearrange to optimize such derealization. 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No depolarization measurements were taken. iii) Mass Spectra: These were all recorded at 70 e.v. on an A.E.I, type M.S. 9 mass spectrometer, samples being admitted through con ventional 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. facility 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. Chivers, R.T. Oakley and N.L. Paddock, J. Chem. Soc., A, 2324, (1970). Dimethyl-aminofluorophosphonitriles and Their Reactions With Hydrogen Halides as a Route to Mono-substituted and Non-geminal Derivatives of the Phosphonitrilic Fluorides W. Harrison, R.T. Oakley, N.L. Paddock and J. Trotter, Chem. Comm, 357, (1971). Crystal Structure and Chemistry of Nitrilo-hexaphosphonitrilic Chloride C.R. Carman and R.T. Oakley, Revista de Tec-nologia de la U.I.S., 5, 5,(1972). Un Estudio Conformacional del Polidiclorofosfonitrilo R.T. Oakley and N.L. Paddock, Can. J. Chem., 51, 520, (1973). Nitrilohexaphosphonitrilic Chloride: A Chemical and Spectroscopic Study H.P. Calhoun, R.H. Lindstrom, R.T. Oakley, N.L. Paddock and S.N. Todd, Chem. Comm., 343, (1975) . Phosphazene Carbanions as Synthetic Intermediates: Silicon, Germimium, and Tin Derivatives H.P. Calhoun, R.T. Oakley and N.L,. Paddock Chem. Comm, 454 (1975). Reaction of N-Methyl Methylphosphazenium Halides with Bases: A Phosphazene-Phosphorin Rearrangement H.P. Calhoun, R.T. Oakley and N.L. Paddock, Can. J. Chem. 53, 2413 (1975). The Crystal and Molecular Structure of the D ihydrochloride of 2, trans-6-diethyl-2,4,4,6,8,8-hexamethyl-cyclotetraphosphazene R.T. Oakley and N.L. Paddock, Can. J. Chem. 53, 3038 (1975). The Preparation of Hexa-methy1cyclotriphosphazene 

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