[organophosphorus chemistry] organophosphorus chemistry volume 37 || phosphazenes

Phosphazenes

Gabino A. Carriedo

DOI: 10.1039/b704720g

Introduction

This review covers the literature over the period January 2006–December 2006. Thephosphazene chemistry is discussed in three sections. Section 1 (Linear phospha-zenes) corresponds to the l5-phosphazene compounds of the type R3PQNR [thenitrogen analogues of the phosphorus ylides (aza-ylides)], frequently called imino-phosphoranes (phosphoranimines) or iminophosphines (phosphinimines), togetherwith other non-cyclic species having one or more PQN bonds and their derivatives.Section 2 deals with cyclic oligomers of the general type [NPR2]n (cyclophospha-zenes), and Section 3 with the high molecular weight phosphazene polymers [NPR2]n(polyphosphazenes).

1. Linear phosphazenes

Quantum Mechanical ab initio calculations on the prototrope equilibriumbetween the aminophosphines (phosphazane form) (1) and the iminophosphorane(phosphazene form) (2) (normally shifted to the left), have confirmed that theenergy difference is in favour of the former but decreases as the electronegativityof the R substituent is increased from 1.5 to 3, in accord with experimentalobservations. The calculations also support the mechanism for the formationof polyphosphazene (3) from tris(amino)phosphine (1a), via its phosphazenetautomer (2a) (see also Section 3).1

Numerous new iminophosphoranes have been synthesized. Although the mostcommon methods are based on the Staudinger reaction between azides andphosphines, other procedures have been developed.As an alternative, the Kirsanov reaction, (i.e., the bromination of a phosphine,

followed by treatment with a primary amine and a base, see also ref. 62 in Section 2)has been used to synthesize the di-cationic phosphonium species (4) from bis(di-phenylphosphino)-methane. The reaction of (4) with NBu3 or LiMe, in differentproportions, led to the mono-cation (5) (in tautomeric equilibrium), to thephosphazenes (6, R = Pri or (S)-MeCHPri) or (7), and to their anionic derivatives(8)–(9). The structures of the compounds (4, R = (S)-MeC*HPri), (5, R = Pri),(7, 8, R = Ph), and (9, R = (S)–MeC*HPri), were determined by X-ray diffraction.The PN distances varied from ca. 1.61 A in compounds (4), (5) and (8) to 1.56 A in 7.The associated structure of the carbanionic species (9) in the solid state is similar to

Facultad de Quimica, Universidad de Oviedo, C/Julian Claveria S/N, Oviedo 33071, Spain.E-mail: [email protected]

262 | Organophosphorus Chem., 2008, 37, 262–322

This journal is �c The Royal Society of Chemistry 2008

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those of (32) and (33) (see later). The atom charges were also calculated by densityfunctional theory.2

The multicomponent reaction of PPh3, urea (or N-methylurea) and dialkylacetylenedicarboxylates gave the stabilized phosphorus ylides (10) that underwentan interesting thermal transformation (beginning with the H migration to the ylide-carbon) to give the iminophosphoranes (11a) (already known) or (11b) and thecorresponding olefins as E/Z isomeric mixtures.3

The new N-(dichlorophosphino)phosphoranimines R3PQNPCl2 (12) (R = Bun

or Ph) have been obtained from the unusual reaction of Cl3PQNSiMe3 with tertiaryphosphines. The proposed mechanism, shown in Scheme 1, involves a sequence ofdechlorination (i and iii)/elimination (ii and iv) with formation of the transient

Organophosphorus Chem., 2008, 37, 262–322 | 263


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http://dx.doi.org/10.1039/b704720g

l3-phosphazene ClPQNSiMe3. To support this conclusion, the iminium salt (13, R= Ph) was prepared from Cl3PQNSiMe3 and Ph3P, and new compound (14) wasobtained by reacting Ph3SiNH2 directly with PCl5 and NEt3 (or by an indirect routeusing PCl3 as shown). Reacting (14) with R3P generated the more stablel3-phosphazene ClPQNSiPh3 that could be clearly detected. The structure of(14), has a significantly short P–N bond (1.490 A), and a wide PNSi angle(145.71), and the iminiun salt (13, R = Ph) has Ph3P–N = 1.610 A and NQPCl3= 1.52 A, as determined by X-ray diffraction.4

The Staudinger reactions of the tris-azide SP(N3)3 with PPh3 or (SiMe3)2N–(SiMe3)N–PPh2 under different conditions, gave the iminophosphoranes (15)–(17),that are neither heat nor shock-sensitive. Heating (16) in solution gave the eight-membered cylic dimer (18) by an intermolecular elimination of (SiMe3)N3. Thestructures of (16) and (18) were studied by X-ray diffraction and theoretical B3LYPcalculations, and showed that (18) has a short P–N bond (1.59 A) and long P–Nbond (1.71 A) indicating that it is not a cyclophosphazene but rather a tetraazate-traphosphocine.5

Scheme 1



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In a study of the photophysical properties regarding the possibilities of makingorganic light emiting diodes, various new phosphorus dendrimers (19) have beenprepared. The route involved successive Staudinger reactions between the azide (20)and the corresponding lower generation phosphine terminated precursors. Thereaction was first tested using (20) and PPh3 to give (21) (see also Section 2).6

Phosphazides (R3PNNNR) are intermediates in the Staudinger reaction. Thus,heating chiral macrobicyclic triphosphazides (22, R1 to R6 = H, Cl, Br or Me),induced a stepwise triple extrusion of N2 to afford the tri-l5-phosphazenes (23) withthe same chiral propeller-like topology of their precursors. Starting compounds (22)were formed by the self-assembly (via tripod–tripod coupling) of the correspondingtetra-substituted tris(3-azidobenzyl)amines with 1,1,1-tris[(diphenylphosphino)-methyl]ethane. The synthetic strategy was also suitable for other macrobicyclictriphosphazides. Fluxionality associated with E/Z isomerization of the PN3 unitswas observed for (22) as shown in Fig. 1.7 Protonation of the phosphazene nitrogensof the previously reported compounds (23, R1 to R6 = H or Br), with HCl orCF3COOH in dichloromethane, led to the hydrolytically resistant salts (24) wherethe three counter anions (Cl� or CF3COO�) are situated outside the internal cavityalthough intercalated within the grooves of its macrobicyclic skeleton. The X-ray

Fig. 1



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structure of salt (24; R = H, X = CF3COO�) showed, among other features, astabilizing CH� � �p interaction of the pivotal C–CH3 with the closer phenyl rings.8

The reactivity of iminophosphoranes towards a variety of reagents has beenexplored. In particular their hydrolysis is very important with many consequences inorganic synthesis (see later). Usually, phosphazenes react with water with P–Ncleavage and formation of OPR3 and RNH2. However the hydrolysis of thephosphoranes (25) and (26a–d) (that were obtained by the Staudinger reactionsbetween the corresponding phosphines with the azide N3(CH2)9CHCH2) dependedon the substituents and on the acidity of the media. In acidic media (MeCN/H2O/HCl), all the hydrolyses followed the classical route of PN cleavage forming thephosphine oxides and NH2(CH2)9CHCH2 (or its protonated form). However, inneutral media (MeCN/H2O) or basic media (MeCN/H2O/NaOH), whereas (25) alsofollowed the classical route, (26a), in contrast, underwent an unprecedented processwith loss of pyridine and formation of the compound (27) (X-ray analysis showedP–N= 1.627 A). The mixed derivatives (26b–d) behaved similarly to (26a) in neutralor basic MeCN giving the corresponding (27)-type analogues.9

The monodeprotonation of the bis(iminophosphorano)methane (28), analogousto (7), with calcium bis(trimethylsilyl)amide led to complex (29), characterized byX-ray diffraction, while the 2-fold deprotonation using the dibenzylcalcium complex[Ca(CH2C6H4Bu

t)2(THF)4] gave the stabilized calcium carbene derivative that



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crystallized as the dimer (30), also characterized by X-ray diffraction. Analysis of thecalculated atomic and group charges showed a considerable increase in the electrondensity at the central carbon reaching a negative charge of �1.8 in (30) (i.e., it can bedescribed as a methanediide). The stabilized calcium carbene complex reacted poorlywith ketones and cyanides, giving only the stable intermediate 1:1 adduct withbenzophenone (30 �OCPh2), the 1:2 adduct with adamantylcyanide (30 � 2NC-adam), and a clean [2 + 2]-cycloaddition with cyclohexyl isocyanate to give (31),all of them characterized by X-ray diffraction (31 as the 3C6H6 solvate).

10

The disodium geminal organodimetallic compound (32) was prepared by thedouble deprotonation of a neutral bis(phosphinimine) ligand using NaBun, whereasthe mixed Li/Na derivative (33) could be prepared by controlled sequential depro-tonation, by transmetalation, or by mixing the homometallic analogues. X-raycrystal diffraction, revealed a dimeric structure (mean PN bond = 1.61 A) withthe ligands capping an Na4 square or a Li2Na2 rhomboid and is strongly related tothat of 9 (ref. 2).11

The reactions of the iminophosphorane HNP(NMe2)3 with the icosahedralcarboranes 1,2-C2B10H12, 1,7-C2B10H12 and 1,12-C2B10H12 led to the formation ofa variety of products. X-ray diffraction showed them to include simple C–H� � �Nhydrogen bonded adducts (34) to (36). The iminophosphorane could remove aboron atom from the ortho-carborane 1,2-C2B10H12 and less readily from themeta-carborane 1,7-C2B10H12, forming the nido anions [7,8-C2B9H12]

� and



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[7,9-C2B9H12]�, respectively (isolated as salts of the cations [H2NP(NMe2)3]

+ or[{(Me2N)3PN}2BNHP(NMe2)3]

+. Other unexpected products were isolated fromreactions carried out in moist air.12

The solvothermal reaction of Cl3PQNR (R = n-propyl, p-tolyl) withCuCl2 � 2H2O, led to P–N and P–Cl bond cleavage with the in situ formation ofthe templating salt [RNH3]Cl, producing the new anionic copper(II) chlorideoligomers [PrnNH3]2[Cu6Cl14] and [4-CH3C6H4NH3][2-Cl-4-CH3C6H3NH3]-[Cu4Cl10] (characterized by X-ray crystallography). They are different from thecomplex [PrnNH3]2[CuCl4] obtained by the direct addition of [RNH3]Cl to a solutionof CuCl2 � 2H2O under the same conditions.13

An increasingly important aspect of the chemistry of phosphazenes such as But-P1(37), But-P2 (38) and But-P4 (39), is the basicity of the iminic N atoms that makesthem very strong neutral or non-ionic bases (Schwesinger’s bases).14 The rates ofproton transfer from 1-nitro-1-(4-nitro-phenyl)alkanes NO2C6H4-CHRNO2 (R =H, Me, Et, Pri) to the phosphazene (37) are unexpectedly slow and the kineticparameters of the process indicate the importance of steric effects on the hinderedreaction centre.15

The basicity, expressed as acidity constants of the conjugated acids (pKa) of theR–P1(pyrrolidino) iminophosphoranes (40) have been determined in tetrahydrofur-an by potentiometry complemented by conductometric measurements. Comparisonwith other bases on an absolute scale of basicity established for this solvent covered arange from 7.4 for dimethylaniline to 21.7 for (40e).16

The basicities in water and in aqueous surfactant solution of another 15aryl-P1(pyrrolidino) [(NC4H8)3PQN–Ar] and aryl-P1(dimethylamino)[(NMe2)3PQN–Ar] phosphazenes (Ar = mono or di substituted phenyl rings)have been measured and correlated with previous data using acetonitrileand THF. A comparison with other bases such as guanidines, amines andpyridines showed that, in all cases, the phosphazenes were the strongestbases. The results confirmed earlier conclusions about the partly ylidic characterof the NP double bond.17 Density functional theory calculations have shown thatthe phosphazenes (41) and (42) are very powerful neutral organic superbases at thesite marked by an asterisc (the calculated pKa values in acetonitrile were 44.8 and37.8, respectively).18



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Because of their special characteristics, many commercially available phosphazenebases such as (37)–(39), are good alternatives to strong ionic bases such as ButOK,NaH, LiBu, that are regularly used in the synthesis of organic or inorganiccompounds.19 Thus, the phosphazene base But-P4 (39) was found to be an excellentcatalyst for the condensation of various carbonyl compounds (including formani-lides) with trimethylsilylalkyl substrates carrying electron withdrawing groups (Ew)(Scheme 2) where other bases including But-P2 (38) or tetrabutylammoniun fluoridewere ineffective. The two proposed mechanisms involved the formation of thephosphazenium cation of But-P4 either with SiMe3 or with H.19a The phosphazene(39) also catalyses halogen–zinc exchange in the reaction of ZnEt2 with aryl iodides,19b

the selective functionalizations of aryltrimethylsilanes in the absence of strong electronwithdrawing groups on the aromatic rings,19c and modified Julia olefination reactionsof 3,5-bis(trifluoromethyl)phenylsulfones with carbonyl compounds at room tempera-ture to afford tri- and tetrasubstituted olefins in good yields.19d

Other synthetic applications of the phosphazene bases include the use of But-P1(37) in the alkylation of adenine in solution or solid phase,19e and thecatalytic esterification of various glycerol derivatives with fatty methyl estersat room temperature.19f On occasions, the action of the phosphazene basesmay promote processes that other proton abstractors do not, such as theobserved rearrangement of (43) to the flavone (44) in the presence of (40e)(OBn = benzoyloxy).19g Patented process,20 include the purification20a andassembly20b of polyhedral oligomeric silsesquioxane monomers, and the catalyticactivation of silylated nucleophiles.20c

Scheme 2



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Due to their potential in organic synthesis and other fields (see later), thepreparation of phosphazenes as Schwesinger strong bases is a field of great interest.21

Thus, the Px (x = 2, 3, 4) phosphazenes (45)–(48), were prepared by deprotonationwith NaH or NaNH2 of the corresponding phosphazenium cations [Px–H]+, thatwere obtained as chloride or iodide salts by the efficient and simple method based ona sequential route starting from lithium phosphonium azayldiide Ph3PNLi as aprecursor, as shown in Scheme 3 for making (P2–H)+ and (P3–H)+. The cations of(47) and (48) were similarly formed from Ph3PNLi but using 0.5 mol of PhPCl2 and0.25 mol of PCl5, respectively in the first step.22

The HBF4 salts of the new chiral phosphazene bases (49) were synthesizedenantiomerically pure and in high yields by treatment of the corresponding (S)-2-(dialkylaminomethyl)pyrrolidines with phosphorus pentachloride in the presence ofNEt3 in CH2Cl2 and subsequent addition of gaseous ammonia to give (49), followedby protonation with aqueous HBF4. The molecular structure and the absoluteconfiguration of the salts were determined by X-ray analysis. Density functiontheory calculations indicated that (49a) is more basic than the R–P1 Schwesingerbases [(R2N)3PQNR] by ca. nine pKa units.

23

Continuing efforts to identify or develop phosphazenium cations with maximumstability under basic conditions, and to know their potential paths of decay, have ledto the study of various (some already known) peralkylated polyaminophosphaze-nium cations of the types (50, R = Me or R2 = –(CH2)4–); (51, R = cyclohexyl,Pri); (52, R =Me, or R2 = –(CH2)4–, –(CH2)3, -cis-CHMe(CH2)3CHMe-); (53) and

Scheme 3



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(54, R =Me or R2 = –(CH2)4–), that were isolated as their BF4� or PF6

� salts. Thestructures of (51) with R= cyclohexyl (both BF4

� and PF6� salts) and with R= Pri

(BF4� salt) were determined by X-ray diffraction. It was found that the cations

exhibited extraordinary base resistance under phase-transfer conditions, with halflives exceeding those of the most stable conventional organic cations by factors of upto 3000. Other advantages as counter ions included greatly enhanced stabilitytowards aqueous base and maximized cation–anion separation in the ion pairsimproving the anion reactivities.24

Novel anhydrous fluorides of cations (51, R = cyclohexyl), (52, R = Me andR2 = –(CH2)4–), (53), (54, R =Me or R2 = –(CH2)4–) and (55), have been obtainedby the exchange reaction of the BF4

� salts with KF in methanol. The fluorides of(52, R = Me) and (54, R = Me) were characterized by X-ray analysis. It was foundthat the fluorides of the permethylated derivatives (51, R = Me) and (54, R = Me)are the most convenient fluoride sources among the phosphazenium fluorides,exhibiting unprecedented reactivity and selectivity in E2 elimination reactions. InTHF, the fluoride of cation (54) is by far the best approximation to a ‘‘naked’’fluoride and probably the strongest stable metal-free base known to date (with anestimated pKa of 37.6, it readily deprotonates 4-phenyltoluene).25

The technological importance of the phosphazeniun salts, mainly in phase orsupported catalysis is demonstrated by numerous patents,26 e.g., as reusablecatalysts for the polymerization of cyclic monomers, for the replacement ofsubstituents, for carbon–carbon bond formation, and other reactions.26a They arealso used as catalysts for the ring opening polymerization of alkylene oxides in themanufacture of purified polyethers without using mass-volume adsorbents,26b forthe preparation of aromatic ethers or polyethers,26c for producing flexible polyur-ethane foam with low resonance transmissibility and good resilience,26d in themanufacture of polyamide foams,26e as catalysts for polymerization,26f in the synthesisof aromatic dianhydrides,26g in the formation of ion-conducting polymers, for mem-branes in fuel cells and electrolyzers,26h in a process to prepare a heptazine-phosphi-nimine as a flame retardant,26i and as ionic liquids for electrochemical devices.26j

Another important aspect of the chemistry of iminophosphoranes is their poten-tial as reagents in organic and inorganic synthesis. In particular the N-silylpho-sphoranimines are useful reagents as precursors for cyclic and poly-meric phosphazenes (see also Sections 2 and 3). The thermolysis of variousknown mixed-substituted P-alkyl-N-silylphosphoranimines of general formula



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XYRPQNSi(CH3)3 (X, Y = Br, OCH2CF3 or OPh, R = Prn, Bun or Pri) was studiedunder sealed-ampule or dynamic vacuum conditions. In the presence of two differentleaving groups (Y = Br, X = OCH2CF3 or OPh) the dynamic vacuum technique wasmore selective (only bromosilane was eliminated) giving the cyclotriphosphazenes offormula [NPXR]3 as cis and trans isomers, while the sealed-ampule thermolysis gavemixed trimers [NPXmBrnR]3 and, in the case of the phosphoranimines (PhO)2(Pr

n)PQNSi(CH3)3, it gave the new polymer [NP(OPh)(Prn)]n.

27a Also investigated were thereactions of the phosphoranimines (CF3CH2O)2RPQNSi(CH3)3 (R = Prn, Bun, Pri, Ph)with trifluoroethanol that gave the expected cyclotriphosphazenes [NP(OCH2CF3)(R)]3except in the case of R= Ph that led to the formation of polymer [NP(OCH2CF3)Ph]n.

27a

The dynamic vacuum thermolysis was the preferred route for the production of [NP(Prn)2]3from silylphosphoranimine (Prn)2(OPh)PQNSi(CH3)3

27b (see Section 2).The reaction of the N-silyl(P-bromo)organophosphoranimines (56, R1 = R2 =

Me or OCH2CF3) with phosphines PR30 (R0 = Bun or Me) has led to the first

P-donor stabilized phosphoranimine cations (57). X-ray diffraction of [Me3P–P(Me2)QN-SiMe3]Br gave P–P = 2.223 A, PN = 1.533 A. The extension of thereaction (56; R1 or R2 = Me or C6H5) to phosphites (P(OR)3; R = Me, Et, Ph),however, gave a new more advantageous method to the generation of the highmolecular weight poly(alkyl/aryl)phosphazenes (58; R1, R2 = Me, Ph) at ambienttemperature (the mechanism has not yet been elucidated).28

The phosphoranimine terminated poly(propyleneglycol) (59), that was used as anintermediate in the synthesis of amphiphilic polyphosphazene–polyethyleneglycol–polyphosphazene triblock copolymers (see Section 3), was prepared using commer-cially available poly(propyleneglycol) (Mw ca. 4000) with NH2 terminal groups.29

The phosphoranimines are also important precursors for a variety ofreagents useful in organic synthesis. Thus, the metalation of the phosphazenesPh2RPQNX (R = Me, Et, X = P(O)(OPh)2, CO2Me) by LiBu gave carbanionicnucleophiles suitable for the aromatic substitution of hydrogen in variousnitrobenzenes providing a convenient alternative for the synthesis of benzylicphosphorus derivatives.30 Reviews have appeared on the synthesis and reactivityof the N-vinylic phosphazenes and their potential as precursors for acyclicand heterocyclic organic compounds,31 as well as a wide range of applications ofthe Ca-lithium phosphazenes as polyfunctional synthons (combining a nucleophiliccentre adjacent to the phosphorus atom with a tunable reactivity through thefunctional groups linked to the nitrogen atom).32

Because of their reactivity, the iminophosphoranes are very frequently used as inter-mediates in multistep organic synthesis. The formation of the iminophosphoraneR30PQNR by the Staudinger reaction of an azide and phosphine can be followed either

by an aza-Wittig process (i.e. iminophosphorane with carbonyl reagents) or by otherreactions such as the hydrolysis of the iminophosphorane. In the latter case the originalR–N3 group is transformed into a R–NH2 group (Staudinger reduction).33 For examplethe aza-Wittig reaction of phosphazenes Me3PQNR with b-g-unsaturated a-ketoesters isan efficient preparation of a-aminoesters-derived 1-azadienes as intermediates in the



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formation of a-amino acids,34 and the Staudinger reduction is one of the eleven steps in theenantioselective synthesis of two diastereomeric enantiopure 4-fluoro-4,5-dihydroceramideswith the natural D-erythro configuration at the 2- and 3-carbon atoms.35 A one-potStaudinger reduction followed by an intramolecular aza-Wittig reaction-imine capture,gave a derivative of the levorotatory C28–C40 azaspiracid domain that was synthesized forthe development of a general, sensitive, portable, and quantitative assay for the azaspiracidclass of marine toxins.36 Other intramolecular Staudinger-aza-Wittig steps have beeninvolved in the synthesis of 2-alkylidenepyrrolidines and pyrroles after the condensation of1,3-dicarbonyl dianions with a-azido ketones,37 as well as a domino Staudinger aza-Wittigreaction for a short and highly efficient route to sugar-aza-crown ethers.38 Recent advancesin the aza-Wittig reaction of phosphazenes with carbonyl compounds such as aldehydes,ketones, esters, thioesters, amides, anhydrides, and sulfimides, and their applications in thesynthesis of acyclic, heterocyclic and macrocyclic systems, have been reviewed.39

A computational and experimental study of the mechanism of the aza-Wittigreaction between phosphazenes and aldehydes showed it to consists of two con-secutive asynchronous thermally allowed (because they do not correspond to the p-systems) [2 + 2] cycloaddition (i)–cycloreversion (ii) processes (the second of whichcontrols the stereochemical outcome of the whole reaction) via the relatively stableintermediate I1 (Scheme 4). The results indicate that P-trimethyl-l5-phosphazenesare more reactive than their P-triphenyl analogues, and that the formation of thecorresponding (E)-imines is preferential or exclusive.40

N-Vinylic phosphazenes, are useful building blocks, that have been used in Aza-Wittig reactions with unsaturated aldehydes to form 3-azatrienes through a [2 + 2]-cycloaddition-cycloreversion sequence.41,42 The presence of an alkyl substituent inposition 3 of N-vinylic phosphazenes increases the steric interactions, and [4 + 2]periselectivity (1,4 addition) is observed.41 Other Aza-Wittig reactions include thereaction of iminophosphorane (60) with aromatic isocyanates to obtain, inter alia,useful carbodiimides for the selective synthesis of pyrimidones.43 Also the iminopho-sphorane (61) was reacted with furan-2-carbaldehyde, thiophene-2-carbaldehyde,furan-3-carbaldehyde, and thiophene-3-carbaldehyde to give, depending on tem-perature and aldehyde, trans imines or mixtures of trans and cis imines.44 The

Scheme 4



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iminophosphorane (62) was used in an efficient one-pot method for the synthesis ofvarious pyrazino[20,30:4,5]thieno[3,2-d]pyrimidinone derivatives via a tandem aza-Wittig/heterocumulene-mediated annulation process. Compound (62) was alsoreacted with aryl isocyanates and subsequently with secondary amines to give thecorresponding guanidine intermediates that were transformed into tricyclic com-pounds in the presence of a catalytic amounts of potassium carbonate.45

The iminophosphoranes (63), that are readily synthesized from 2-chloro-5-cyano-6-ethoxy-4-phenylpyridine-3-carboxaldehyde, were reacted with aromatic isocya-nates to obtain directly several new 1,8-naphthyridines in an aza-Wittig/electrocyclicring-closure process.46 Phosphazene (64) was reacted with phenyl isocyanate (or 4-chlorophenyl isocyanate) to give a carbodiimide which was cyclized to afford a seriesof new, 2-substituted 3-aryl-8,9,10,11-tetrahydro-5-methyl[1]benzothieno[3,2:5,6]-pyrido[4,3-d]pyrimidin-4(3H)-ones.47

In the reactions of iminophosphoranes with carbonyl and unsaturated compounds, bothnormal and abnormal aza-Wittig processes can be observed. Thus, the tandem aza-Wittigreaction of the iminophosphorane (65) with isocyanate or CS2 generated the 3,5-dihydro-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones in satisfactory yields, and the vinyl iminophosphorane (66)was transformed by normal aza-Wittig reactions to carbodiimides (as part of the syntheticroute to 3,5-dihydro-6H-imidazo[1,2-b]-1,2,4-triazol-6-ones). By contrast, a re-examinationof the reaction of the iminophosphorane (67) with the aromatic isocyanate PhNCO showedthat it gives an unexpected mixture of carbodiimides.48 In another example (Scheme 5), the

Scheme 5



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reaction of g-azido-b-hydroxyketones with triphenylphosphine resulted in a domino Stau-dinger/semi-aza-Wittig/fragmentation rather than in the normal aza-Wittig reaction.49

Other works involving iminophosphoranes in organic synthesis,50 include the use ofasymmetric aza-Wittig reactions mediated by chiral phosphorus(III) reagents for theenantioselective synthesis of b-quaternary azacycles,50a the synthesis of 2-substituted5,8,9-trimethyl-3-phenylthieno[30,20-5,6]pyrido[4,3-d]pyrimidin-4(3H)-ones,50b a studyshowing that the tandem Staudinger/aza-Wittig reactions for the synthesis of isothio-cyanates gave better results that the conventional stepwise method,50c novel approachesto fused phospha-pyrimidines based on heterocyclization of pyrazolylamidines withphosphorus(III) halides followed by oxidation, sulfuration or imination,50d the regiose-lective reduction of one azido group in glycopyranoside and mannitol derivativescontaining two azido functions with triphenylphosphine,50e the first example of aStaudinger-aza-Wittig-type reaction on a substituted furanoside in which a b-azidoglyco-a-aminonitrile was converted into fused iminopyrrolidines in good yields,50f andthe synthesis of fluorinated imines and carbodiimides from azides.50g The use ofdiphosphine bis(diphenylphosphino)ethane (DPPE) (compared with other phosphines)has the advantage that the phosphine oxide formed as byproduct is more easily removed.This has been applied to the Staudinguer type synthesis of glycopyranosyl amides50h andN-glycoside neoglycotrimers.50i The Staudinger reduction-aza-Wittig process has alsobeen used as one of the new methods for facile biomimetic spiroaminal syntheses,50j theregioselective Staudinger reduction for the synthesis of a broad-spectrum aminoglycosideantibiotic,50k a simple regiospecific synthesis of 4-alkoxy(amino)-2-trifluoromethyl pyr-roles from 5-azido-4-alkoxy(amino)-1,1,1-trifluoro-pent-3-en-2-ones,50l and the synthesisof various bridged nicotinates having >n](2,5)pyridinophane skeletons (n = 8–14).50m

An improved syntheses of bis(b-cyclodextrin) derivatives by a microwave-promotedaza-Wittig reaction involved the use of polymer-bound triphenylphosphine.50n

The Staudinger reaction has also been employed for synthetic purposes other thanthe preparation of iminophosphoranes. Thus, reactions between appropriate l5-phosphorus azides and phosphines gave the amine functionalized dendron (68)that could be grafted onto a tetraphosphorus macrocycle with four pendantP–NQP(C6H5)2(p-C6H4-CHO) groups in order to obtain the expected topologicalyamplified compound as a mixture of diastereoisomers.51



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One very important aspect of phosphazene chemistry is Staudinger ligation.It can be non-traceless or traceless (i.e., depending whether or not the OPR3

is eliminated leaving residual atoms).52 The two routes have been comparedin a study of bioorthogonal reactions with azides to provide a guide for biologistsin choosing a suitable ligation chemistry.53 The reaction mechanism of the tracelessStaudinger ligation between various phosphinothioesters (or phosphinoesteres)and azides has been investigated in detail. The use of [18O]H2O showed that thereaction mediated by (diphenylphosphino)methanethiol proceeds by S–Nacyl transfer of the iminophosphorane intermediate to form an amidophosphoniumsalt (path A) (i.e., the 18O was only in the phosphine oxide as shown in Scheme 6),rather than by an aza-Wittig reaction and subsequent hydrolysis of theresulting thioimidate (path B). The rate-determining step (i) involves the formationof the initial phosphazine intermediate.54

Another study has shown that the Staudinger ligation of peptides at non-glycylresidues, mediated by (diphenylphosphino)methanethiol, may be directed away fromthe aza-Wittig process that leads to the phosphonamide by-product by increasing theelectron density at the phosphorus, either by installing functional groups on the Phsubstituents (e.g. p-methoxy) or by using low polarity solvents (toluene or dioxane),without affecting the chemoselectivity and the rate of the reaction.55

An important use of the Staudinger ligation is the immobilization of proteins.Thus, various works have been published describing the Staudinger ligation betweenazide functionalized proteins and phosphine modified surfaces,56 and a generalapproach has been developed for the regio- and chemoselective covalent immobi-lization of derivatized proteins on glass surfaces bearing linkers containing azide(‘‘click’’ chemistry) or phosphine (Staudinger ligation) groups.57 Other applica-tions58 include the stereoselective synthesis of a-glycosyl amides with potential use assugar mimics,58a a simple and efficient synthesis of N-linked glycoamino acids andglycopeptides from deprotected sugars,58b a convergent glycopeptide synthesis,58c

the facile condensation of small peptide fragments on a novel core-shell-type resin(an example of solid-phase Staudinger ligation),58d for the synthesis of 15 and 16

Scheme 6



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membered biaryl-type lactams,58e and to generate O-alkyl imidate links between 6-7-dihydrogeranyl azide-containing peptides and phosphine based reagents.58f Medicalapplications of Staudinger ligation include its use as a trigger for drug-release,58g andtargeted imaging and therapeutic uses.58h

The coordinating ability of the N-imine atoms of the phosphazenes and itsderivatives makes them useful ligands in coordination and organometallic chemistry.Thus phosphazene (69) was used to make various 1,3-dimetallacyclobutanes (70–74)that were characterised by X-ray crystallography. R and S enantiomers of (70) and(73) were obtained using different solvents. The new type of compound (74) wasconverted to (72) after re-crystallization in diethyl ether and the crystallization of(70a) in THF gave its isomeric form (70b). The latter structure suggests that it is anintermediate in the interconversion of the enantiomers R-(70a) and S-(70a) insolution accounting for the absence of optical activity. It was proposed that thecompounds 70a, 71, 72 and 73 are formed from ‘‘head-to-tail’’ cycloaddition of themetallavinylidene intermediates [:MQC(Pri2PQNSiMe3)(2-Py)] (M=Ge, Sn, Pb).59

The fluorenyl-phosphazene ligand (75), prepared using the Staudinger reaction,was deprotonated with n-buthyl lithium and subsequently reacted with [Rh(m-Cl)(nbd)]2 (nbd = norbornadiene) to give the complex (76), that according toDFT calulations, is best described by the phosphazene structure (76a) with onlyminor contribution of the ylide form (76b). Both (75) and (76) were characterized byX-ray diffraction. The P–N bond changed from 1.57 A in (75) to 1.62 in (76)according to the different contributions of the ylide form.60

The new heterotridentate (P,N,N0) ligand (77), featuring phosphine, imino-phosphorane, and amine groups, was reacted with [RuCl2(PPh3)4] to give the



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dichlororuthenium(II) complex (78). It was subsequently reacted with ButOK in iso-propanol to give quantitatively (although isolated in 60% yield) the stable (but veryreactive towards oxygen) hydrido amido complex (79) that was characterized byX-ray diffraction.61

The Kirsanov methodology (see also ref. 2) starting from Ph3PBr2 was used toobtain the iminophosphonium salt (80), that could be treated successively with (i)BunLi, (ii) BunLi followed by R2PCl and (iii) aqueous HCl to produce, in one pot,the salt (83; R = Ph, Pri). This route avoided the need to isolate the very sensitiveiminophosphoranes (81) (already known) and (82). The latter compound is a newtype of bidentate ligand having an iminophosphorane and a phosphino group, thatcould be generated in situ from (83) and BunLi and reacted with [Pd(COD)Cl2] toform the expected six-membered palladacycle complex (84), which was structurallycharacterized by X-ray diffraction. The same synthetic strategy was used toprepare the tetradentate ligands (85) isolated enantiomerically pure as their hydro-chloride salts.62

The iron–iminophosphorane–carbonyl complex (86) reacted with the activatedalkyne dimethyl acetylenedicarboxylate to give the acyl–amino–phosphine complex(87), that was characterized by X-ray diffraction (PN distance 1.727 A). In contrast,the reaction with CO2 afforded the aza-Wittig-type metathesisproducts, PhNQCQNPh and the complex (88), probably via a four-memberedaza-phosphacycle as an intermediate. Although free acetonitrile did not react with(86), the UV-irradiation of [Cp*(CO)2Fe(NCMe)]PF6, with P(OMe)2[N(Ph)(SiMe3)]gave, after hydrolysis, the cationic complex (89a) that was transformed into the five-membered metallacycle (89b) through a base-catalyzed rearrangement consisting ofthree steps: the formation of the neutral intermediate complex (90a), the rapidnucleophilic attack of the resulting imino nitrogen to the carbon atom of the



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coordinated acetonitrile to form (90b), and the protonation of the latter as shownin Scheme 7.63

The regioisomeric iminophosphorane–phosphine ligands (91) and (92) (preparedby selective mono- and di-imination of 2-diphenylphosphino-1-phenylphospholanewith the corresponding N3R azides) reacted with the dimer [RuCl2(p-cym)]2 (p-cym= para-cymene) to give the neutral and cationic mono- and di-nuclear (6-arene)–ruthenium(II) complexes (93)–(95). The related complexes (96) and (97) wereobtained by similar methods starting from (98), which was formed by directimination of the free PPh2 groups in the corresponding phospholane complex. Theproducts (92, R = P(O)(OPh)2), (95) and (97) were characterized by X-raydiffraction. The catalytic activities of the ruthenium complexes, both in racemic

Scheme 7



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and enantiomerically pure forms, in Diels–Alder cycloaddition reactions werestudied.64

Starting with the iminophosphorane–phosphine ligands (99, R = Et, Ph), thenovel Pd(II) derivatives (100), (101, L = CNBut, CN-2,6-C6H3Me2, Py, P(OMe)3,P(OEt)3), (102), (103) and (104), were obtained and used as catalysts for thequantitative cyclo-isomerization of (Z)-3-methylpent-2-en-4-yn-1-ol into 2,3-di-methylfuran. The structures of (100, R = Ph), (101, R = Et, L = CN-2,6-C6H3Me2), (102) and (103) were determined by X-ray diffraction.65



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The reaction of the 1,10-bis(phosphoranylidenamino)ferrocenes (105; R = Ph, Cy)with [Pd(CNMe)2Cl2] gave the complexes (106a) and (106b), that exhibited differentspectroscopic data showing a difference in the coordination geometry depending on R.The X-ray crystal structure (106a) confirmed that the Pd(II) centre was cationic and hada nearly square-planar (NPdN angle of 1621) coordination with a relatively short Fe–Pd distance (2.67 A). The formation of the dative Fe–Pd bond was attributed to thelower electron-rich character of the nitrogen atoms induced by the Ph rings.66

The cationic Rh and Ir complexes with chiral (iminophosphoranyl)ferroceneligands (107) and (108) were found to be very powerful catalysts for asymmetrichydrogenation of a series of unfunctionalized di- and trisubstituted olefins with almostperfect enantiomeric excesses (up to 99% ee) under mild conditions. In some cases therhodium complexes were even better catalysts than their iridium counterparts.67



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The one electron reduction of complex (PNP)NbCl3 (PNP = N[2-P(CHMe2)2-4-methylphenyl]2) or the reaction of NbCl3(DME) (DME = dimethoxyethane) withLi(PNP), generated the transient Nb(III) intermediate (PNP)NbCl2 (capable ofatmospheric N2 activation) that reacted with diazobenzene cleaving the NQN bondgiving directly the phosphoranimine complex (109) the structure of which wasdetermined by X-ray diffraction (PQN bond length 1.637 A).68

2. Cyclophosphazenes

The bonding in cyclotriphosphazenes has been revisited. A new topological analysis byRHF methods for the molecules [PnNnX2n] (X = H, F, Cl; n = 2, 3, 4) focussed on thechemical bond and local electronic properties at the electron density critical points. Itincluded their dependence on cycle size and phosphorus substituents. The results showedthat the PN distances decrease and the PNP angles increase with the ring size; that theP–Cl interactions are covalent-polar whereas the P–F and P–N bonds are intermediatepolar, and that the distribution of the electron density Laplacian and electron pairlocalization function at the N and P atoms help the formation of noncovalentintermolecular P–N interactions along the symmetry axis, that can explain the frequentparticipation of cyclophosphazenes in self assembling supramolecular aggregates.69 AnIR spectroscopic study and a structural optimization and normal mode analysis by abinitio density functional theory, have been carried out on hexaphenoxycyclotriphos-phazene [NP(OPh)2]3 and its completely deuterated isotopomer. Taking into account thegeometry of the compound, formed by a cyclophosphazene core with terminal phenoxygroups, it was considered to be a zero generation phosphorus dendrimer.70

Structural investigations on cyclophosphazenes continue to be of interest. Thus,the X-Ray crystal structures of [N3P3Cl5(NHBut)] and [N3P3Cl2(NHBut)4] havebeen determined at 120 K, and those of [N3P3Cl6] and [N3P3Cl4(NHBut)2] have beenre-determined at this temperature. A comparison with homoleptic derivatives[N3P3Cl6] (PN = 1.577 A) and [N3P3(NHBut)6] (PN = 1.578 A) (already knownat low temperature) showed various effects of the substitution on the PN bondlengths and a general increase of the basicities. The structures of the mixedderivatives (110–112) showed eight-membered-ring H-bonded dimers that are absent



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in the hexasubstituted [N3P3(NHBut)6], probably due to steric reasons. Basedon the generally accepted mechanism for nucleophilic substitution at a phosphorussite bearing PCl(NHBut) groups, it was proposed that the non-existenceof tris derivatives [N3P3Cl3(NHBut)3] is due to the deprotonation of (113) that isreversible when X = Cl, but irreversible if X is the stronger donor butyl aminogroup (Scheme 8).71

Many new cyclic phosphazenes have been obtained, mainly by the substitution ofchlorines in [N3P3Cl6] by the appropriate nucleophiles in the presence of a base, orby chemical modification of the existing groups in a substituted precursor. Thesynthesis of cyclic phosphazenes by chemical derivatization after a lithiation step hasbeen reviewed.32 Other synthetic methods are based on the reactivity of phosphor-animines (Section 1). Thus, the mixed substituted cyclophosphazenes (114) wereobtained as cis and trans isomeric mixtures by thermolysis (sealed ampoule or, moreselectively, under dynamic vacuum conditions) of the appropriate trimethylsilylphosphoranimines. In the case of [NP(Pri)(OPh)]3 the pure trans isomer wasobtained and characterized by X-ray diffraction (PN in the range 1.569–1.610 A).The cyclophosphazenes [NPR(OCH2CF3)]3 (R = Prn, Bun, Pri) were also preparedby the reaction of (CF3CH2O)2RPQNSiMe3 with trifluoroethanol.27a The dynamicvacuum thermolysis of (PhO)(Prn)2PQNSiMe3 was preferred for the synthesis ofthe rather insoluble (when very pure) hexa-n-propylcyclotriphosphazene (NPPrn2)3in 47% yield. X-ray diffraction showed mean PN = 1.60 A).27b

Scheme 8



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The enantiomers of the new dibenzylamino cyclotriphosphazenes (115) (twostereogenic centers) and (116) (one stereogenic center), were prepared by thesubstitution methods already described for the similar derivatives. They wereseparated by high-performance liquid chromatography using a reproducible andselective method based on the chiral column Whelk-01. It was found that both theseparation factor and resolution factor of molecules with two equivalent stereogeniccentres are greater than those for analogues with only one centre.72

The reaction of [N3P3Cl6] with appropriate amounts the sodium salts of2-mercapto-1-methylimidazole gave the new mono or multi-geminally substitutedcyclophosphazenes with thiolate groups (117) and (118), while the reaction with



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2-mercaptopyrimidine in the presence of K2CO3 gave a mixture of (119) and (120)that was separated by chromatography.73

The reaction of the known achiral spermine-bridged cyclophosphazene (121) withvarious molar ratios of 1,3-propanediol and NaH in THF, gave mixtures of thespiro- and ansa-cyclophosphazenes (122a–g) with different stereogenic propertiesand with the expected preference for the spiro forms. The structures of the di-monospiro (122c) (meso) and tetra-spiro (122g) had been previously characterized.The mono-ansa (122b) was observed by NMR spectroscopy in solution but noevidence was found for the monospiro/monoansa (CP2, CP3 combination), which isa necessary precursor of compound (122f). The structures of the mono-spiro (122a),di-mono-ansa (122d) and di-spiro/mono-ansa (122f) were determined by X-raydiffraction. The stereogenic properties of many of the products were confirmed byX-ray crystallography and/or by 31P NMR spectroscopy using the chiral solvatingagent, (S)-(+)-2,2,2-trifluoro-1-(9-anthryl)ethanol.74

The novel phosphazenes spiro–bino–spiro (124), (125a), (125b) and (125e) havebeen synthesized via the condensation reactions of [N3P3Cl6] with the correspondingbiphenolic aminopodand reagents or dibenzo–diaza–crown ethers. The new fullysubstituted spiro–ansa–spiro phosphazenes (123b) and (123c) were prepared byreacting the corresponding partially substituted spiro–ansa–spiro-phosphazene(123a) with pyrrolidine and 1,4-dioxa-8-azaspiro[4,5]decane, respectively. Unexpect-edly, the reactions of the tetrachloro derivatives (125c) and (125d) with pyrrolidineresulted in the corresponding geminal crypta derivatives (125f) and (125g). Thesolid-state structures of (123c) and (125b) were determined by X-ray diffraction.75

The reactions of the previously known bicyclic phosphazene crown ether (126)with ethanethiol, thiophenol, dimercaptoethylene and mercaptoethanol are regio-selective affecting only to the non-macrocyclic phosphorus atoms giving the



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corresponding geminally disubstituted derivatives (127) and (128). The process isrepeatable opening new synthetic possibilities for substituted derivatives of (126). X-ray diffraction of (128b) showed the N3P3 ring to be planar with equal PN bondlengths (mean 1.578 A), and the 16-membered PNP-crown-5-ether ring exists in theso far unobserved conformation ac� ap ac� sc+ sc+ ap sc+ ap ap sc� ap sc� sc� ac+

ap ac+. All the ether oxygen atoms are directed into the interior of the ring. The P–Sbond properties were characterized in terms of natural bond orbital analysis. Amechanism for the general spirocyclization reaction was proposed in the case ofcompound (128b) based on the preference of the S-terminal atoms of the enteringnucleophile for the non-macrocyclic phosphorus atoms.76

Known compound (129) has increase water solubility due to the presence of thecrown ether, compared to other tetrakis-azidirine cyclophosphazenes. It has been testedin the in vitro disease-oriented anti-tumour screen showing remarkable cytostaticactivity. X-Ray diffraction showed macrocyclic C–C shortening effect and all crownether oxygen atoms directed toward the interior of the ring. The mean P–N bond lengthwas 1.59 A. The results were consistent with a carbocation-like carbon in the aziridinylgroup and the lone electron pairs of their nitrogen atoms directed outside the moleculemaking them the primary places for the interaction of the molecule with acid residuesand explaining the detected cyclostatic activity.77 Other derivatives (130), bearing,among other R groups, 2-chloroethylamino or 2-oxybenzaldehyde (or its Schiff basewith 2-chloroethylamine) have been tested for in vitro antileukemic activity exhibitingantiproliferative activity against the MOLT4, L 1210, HL-60, and P388 cell lines.78

New stereogenic spirophosphazenes with spiro–spiro (131), spiro–ansa (132) oransa–ansa (133) structures have been prepared from the reaction of [N3P3Cl6] orgem-[N3P3Cl4R2] with pentaerythritol. X-ray crystallography showed that the spiro–spiro and spiro–ansa bridged gem-disubstituted cyclophosphazenes are chiral andexist as racemates, while the ansa–ansa bridged cyclophosphazenes (133a) and(133b) have meso configurations.79



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The sequential substitution of chlorines with different nucleophiles allows thesynthesis of mixed cyclophosphazenes, with combined properties in the design ofnovel materials. Thus, as an extension of earlier works, the [N3P3Cl6] was reactedfirst with the sodium salt of a methoxypolyethylene glycol (MPEG), such asmethoxytetraethylene glycol (MTEG), to give the half substituted intermediate I,and subsequently with an oligopeptide (such as the tripeptide GlyPheGlyEt), used asthe ethyl ester (NH2R

0), to obtain amphiphilic grafted-cyclotriphosphazenes ofgeneral type (134) (that are yellow viscoelastic liquids) with low critical solutiontemperatures (LCST) between 34 and 60 1C, increasing with the chain length of theoligopeptide. Their self assembly in water resulted in the formation of sphericalmicelles.80

Other syntheses based on chlorine substitution include the preparation of hexa-substituted cyclotriphosphazenes of general formula [N3P3(OC6H4–NQN–Ar)6] (Ar= C6H4–X-4 where X=H, F, Cl, Br, I, COMe, C6H4–Cl-2 and C6H4–Cl-3) bearingdiazo chromophores, which were studied by UV-Vis, IR and 1H NMR spectro-scopy.81 Various mono and di-substituted cyclophosphazenes were prepared by thereaction of [N3P3Cl6] with 2-amino-3-methylpyridine, 2-amino-4-methylpyridineand 2-amino-5-methylpyridine.82

The reactivity of the groups attached to a cyclophosphazene precursor offers avariety of alternative synthetic paths ways to new derivatives. Thus, the 36 and 42-membered macrocycles (135) and (136) were prepared by the [2 + 2] condensation ofthe aldehyde cyclophosphazene [N3P3(O2C12H8)2(OC6H4CHO)2] with PhP(O)-[N(Me)NH2]2 or 1,6-diaminohexane, respectively.83



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Other examples include the preparation of the new hexaaryloxycyclophosphazenederivatives84 (138a–b)84a and (138c–f),84b as mixtures of syn and anti isomers, byreacting the known oxime derivatives (137a–b) with various alkyl and acyl chloridesin the presence of K2CO3 as a proton abstractor. With some alkyl chlorides theconversion of the oxime groups was incomplete giving mixed derivatives (e.g.,[N3P3(OC6H4–CHQNOH)2(OC6H4–CHQNOMe)4])

84a and [N3P3(OC6H4–CMeQNOH)2(OC6H4–CMeQNOCH2Ph)4]

84b) or impure poorly defined products(e.g. in the reactions of (137) with propyl chloride, monochloroacetone, 1,4-dichlorobutane84a and 4-methoxybenzoyl or 2-chlorbenzoyl chlorides84b). In thereactions of (137a, R1 = H) with benzene or naphthalene sulfanoyl chloridesdehydration of the oxime occurred to give the known [N3P3(OC6H4-CN)6].

84b

The reactivity of the groups placed on the substituents of the P3N3 cycle has alsobeen exploited to synthesize polymeric materials. Thus, it has been found that,despite the presence of five potentially interfering P–Cl bonds, the ring openingmethathesis polymerization (ROM) of the mono(5-norbornenyl-2-methoxy)penta-chloro cyclotriphosphazene (139, endo/exo mixture), with the Grubb’s first genera-tion catalyst trans-cis-[Ru(QCHPh)(PCy3)2Cl2], gave the corresponding polymer(140) in a reproducible manner, allowing predictable chain lengths to be obtained.The substitution of the chlorides in (140) with the appropriate NaOR salts (e.g.NaOCH2CF3) gave the corresponding substituted derivatives (141). This methodopened a new advantageous synthetic route to inorganic-organic hybrid polymerswith potential interest for lithium ion and proton conductors, new separationmembranes, and novel photonic materials.85



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In fact, the copolymers (143), from the ROM co-polymerization of mixtures of theendo-exo norbornenemethoxy-cyclotriphosphazenes (142a) and (142b) (both pre-pared from 139), were successively reacted with KOBut, aqueous HCl, and LiOH totransform the 4-(propylcarboxalato)phenoxy side groups (R2) first into the –COOHand finally into the COO�Li+ to synthesize novel lithium-ion conductive polymersas prospective membranes for Lithium-Seawater Batteries. The dependence of iontransport and hydrophobic properties on the polymer composition were discussed.86

As in the synthesis mentioned above, many cyclophosphazene derivatives can beprepared by a combination of chlorine-substitution and chemical modificationmethods.Thus, supermolecular liquid crystals with a cyclotriphosphazene dendritic core

and polycatenar mesogenic units (144) were obtained in three steps by the conven-tional sequence of substitution (i), derivatization (ii and iii) methods from [N3P3Cl6](Scheme 9). Due to the microsegregation of the alkyl chains and the aromatic centralcores and the space-filling properties, compounds (144) adopt a discotic conforma-tion assembled in a columnar mesophase and illustrate the possibilities of usingcyclotriphosphazenes for the design of columnar assemblies at room temperature, inthe mesophase or in a vitrified solid state with interest for applications in materialscience.87 Similarly, the new family of solution processable, photoluminescent,monodisperse nanocomposite dendrimers (145) (Tg 4 165 1C, Tdec. 4 465 1C)based on cyclic phosphazene cores, were prepared in high yields from [N3P3Cl6] andfunctionalized bromophenols (Scheme 10) to give bromophenoxy derivatives that

Scheme 9



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were subsequently reacted with N-p-tolylpyren-1-amine using Buchwald–Hartwigamination.88

Scheme 10



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The new phosphazene dendrimers with a cyclo phosphazene central core andnaphthalene or pyrene chromophoric end groups (146a) and (146b), have beenprepared by a combination of substitution reactions of terminal P–Cl bonds bysodium 2-naphthoxide, or condensation reactions of the secondary amine (147) withthe terminal OC6H4-CHO groups in the corresponding cyclophosphazene precur-sors. The compounds were tested as organic light emitting diodes. The study of thephotophysical properties of these and other phosphorus dendrimers without PQNbonds revealed that, in order to preserve the fluorescence, the fluorophore must notbe linked to the dendrimer through a heteroelement (oxygen or nitrogen) butthrough an alkyl linkage (see also Section 1).6

The synthesis of cyclophosphazenes based on conventional methods (formation ofprecursors followed by chemical modifications) can be designed to achieve a specificpurpose with technological interest. Thus, the hexasubstituted cyclotriphosphazenescarrying both vinyloxyethoxyethoxy and methoxyethoxyethoxy groups (148) havebeen thermally cross-linked with 2,20-azo-bis-isobutyl nitrile (AIBN) in the presenceof LiSO3CF3 or LiN(SO2CF3)2 to obtain lithium ion conducting networks with goodmechanical properties (Scheme 11). Their conductivities are at least the same asMEEP polymer based polyelectrolytes, [NP(OCH2CH2OCH2CH2OCH3)2]n and theease and flexibility of their synthesis with controllable substitution patterns, makethem an attractive alternative to conventional salt-in-polymer electrolytes (see alsoSection 3).89 The technological interest of lithium conductive materials based oncyclophosphazenes is reflected in various patents.90 Other examples include, thepolycondensation of hexachlorocyclophosphazene with 4,40-sulfonyldiphenol in anultrasonic bath in the presence of triethylamine to give cross-linked poly(cyclotri-phosphazene-co-4,40-sulfonyldiphenol). Rod-like fibres of end-open nano-tubes (5–10 or 10–20 nm in diameter) are formed by the template action of high surface energynano-crystals of triethylammonium chloride produced in situ during the polymer-ization that are easily removed with water (Scheme 12). The size and inner diameterof the nano-tubes could be modified by adjusting the synthetic parameters.91a Micro-tubes (1–3 mm in width and about 100 mm in length) containing hexagon-shapedchannels, were prepared via one-pot synthesis using acetone at room temperature.91b

Nano-fibre matrices (20–50 nm in diameter and 500 nm in length) of highly cross-linked poly(cyclotriphosphazene-co-sulfonyldiphenol) have also been synthesized.91c



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Reactions at pendant groups have also been used to attach polymeric chains tocyclophosphazenes. Thus, the polymerisation of aminoacids (such as L-a-alanineand g-esters of L-a-glutamic acid N-carboxyanhydrides) initiated on the terminalNH groups of the known hexakis(p-aminophenoxy)cyclotriphosphazene and tetra-phenyltetraamino cyclotetraphosphazene gave polypeptide chains of various lengthsattached to a central inorganic core,92a and the amphiphilic star copolymer ofpoly(L-lactic acid)–poly(ethylene glycol) has been formed on the terminal hydroxylgroups of the cyclotriphosphazene [N3P3(OC6H4-p-CH2OH)]6, obtained by thereduction of the known aldehyde precursor.92b

Other aspects of the reactivity of the cyclophosphazenes have been explored withsynthetic purposes. Thus, in spite of its low basicity, hexachloro-cyclo-triphospha-zene reacted with the carborane electrophiles [H(Mes)]Carb, CH3(Carb) andSiR3

0(Carb) [Mes = mesitylene, R0= Me, Et, Carb = (CHB11R5X6)� with R =

H, CH3, X = Cl, Br)] to give the corresponding N-protonated, N-methylated, andN-silylated cations (149–151), isolated as carborane salts. The salts[H(N3P3Cl6)][CHB11H5Br6] (149a), [CH3(N3P3Cl6)][CHB11Me5Br6] (150a), [Me3-Si(N3P3Cl6)][CHB11Cl11] (151a) and [Et3Si(N3P3Cl6)][CHB11Cl11] (151b) werestudied by X-ray crystallography.93

The reaction of [N3P3Cl6] with sodium disulfide gave the cross-linked phospha-zene material of idealized formula [(NPS2)3]n (152), which is thermally stable to

Scheme 11

Scheme 12



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200 1C. Because of its initial discharge capacity (almost 93.5% of the theoreticalspecific capacity) and excellent cyclic ability, it may have great potential as a cathodefor secondary lithium batteries.94

By analogy with most acetylenes, the (b-phenylethynyl)pentafluoro cyclotriphos-phazene [F5P3N3CCPh] reacted with the cobalt cyclopentadienyl complex Z5-(MeOC(O)C5H4)Co(PPh3)2 (generated in situ) to give a mixture of products fromwhich the cobaltacyclopentadienylmetallacycles (153) and (154) and the sandwichcompound (155) were isolated, although in low yields (11–20%). The reaction of(153) with diphenylacetylene or phenylacetylene gave the novel aryl-bridged penta-fluorocyclotriphosphazenes (156) in 54% yield. X-Ray diffraction of compounds(153–155) and (156, R = Ph) showed them to have the structures shown below.95

The potential use of cyclophosphazenes as guests for inclusion compoundscontinues to attract attention. Thus, the molecular dynamics and ordering ofpyridine in cyclophosphazene inclusion compounds has been evaluated by variabletemperature 2H NMR experiments carried out on pyridine-d5–tris-(1,2-dioxyphe-nyl)cyclotriphosphazene in the temperature range 110–300 K, to show that thepyridine guests are highly mobile.96 The thermal, optical and electronic properties ofan inclusion adduct of polyaniline in a channel constructed from tris(2,3-naphthy-lenedioxy)cyclotriphosphazene have been investigated.97

The use of theoretical methologies lead to the prediction that many tricyclopho-sphazenes structurally related to the tris(ortho-phenylenedioxy)cyclotriphosphazene(TPCP), have the ‘‘paddle wheel’’ shape responsible for inclusion adducts formationmaking them potential candidates for organic superconductors with conductiveproperties that can be modulated.98 Micrometer-sized crystallites of (TPCP) showingzeolite-like reversible sorption of I2 and CH3I have been formed by ultrasound andball milling. The thermal stability of open-pore TPCP could be improved by partial



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loading with pyrazine. The sorption properties of open-pore TPCP, were investi-gated by the 131I radioactive tracer method and compared with activated charcoal.TPCP exhibited higher sorption efficiency for I2 dissolved in water and also in thecase of a humid gaseous source of methyl iodide.99 Reversible sorption of N2 and Xehas been observed for guest-free zeolite TPCP.100

Another important aspect of the chemistry of cyclophosphazenes is their use asadditives in materials science. For example, hexaanilinotriphosphazene has beenused as a curing agent for ortho-chloro substituted epoxy resins based on tetra-glycidyldiaminodiphenylmethane and the effect of structure on cure and mechanicalproperties studied.101 The presence of the cyclophosphazenes (157) and (158a)during the melt process of different blends of poly(butylene terephthalate) (PBT)with the polyamides 6(PA6) or 6,6(PA66) causes an increase of the rupture proper-ties and in the viscosity, especially in the PA6 rich blends containing (158a). Theresults were attributed to a chain extension effect on the polyamide phase and also tothe in situ formation of polyamide/PBT copolymers promoted by the presence of thecyclophosphazenes as indicated by NMR and MALDI-TOF analyses.102a Similarly,an increase in the rupturtensile properties, impact strength and the viscosity wereobserved in high density polyethylene-polyamide-6 blends melt processed in thepresence of (158a) or (158b) and ethylene/acrylic acid copolymers.102b

The possibility of surface functionalization of the hydroxylated surface of siliconbased materials with phosphazene substrates has been explored103 by a combinationof experimental XPS analysis and theoretical ab initio calculations it has been shownthat, in the interaction of [N3P3Cl6] with the Si(100)–OH surface, water plays acrucial role and a solvent such as THF is essential.103a Also, the specific surfacemodifications of silicon-based materials such as silica gel beads and crystallineSi(100) wafers, have been achieved by reacting the residual –OH with the pendantNHCH2CH2CH2Si(OMe)3 groups of cyclotriphosphazenes carrying an equimole-cular proportion –OC6H4-p-CN substituents used as markers through its IR band at2230 cm�1.103b

The flame-retardant properties of cyclophosphazenes is another topic of researchthat continues to generate interest. Thus, useful parameters for designing flame-retardant phosphazenes with respect to the char forming tendency of the N3P3 group(35.04) were estimated from the Van Krevelen’s theory and the experimental TGAresidue left in air for a phosphazene cyclomatrix material obtained from the reactionof tri(4-nitrophenoxy)tri(phenoxy) cyclotriphosphazene with bisphenol-A.104 Thefire-retardant properties of the viscose rayon containing alkoxycyclotriphospha-zenes, prepared by the method of the blending spinning, have been evaluated. It is



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suggested that the flame retardant phosphazene additive plays a multiple role of heatabsorption, catalytic dehydration and carbonization, condensation-phase and gas-phase flame retardation.105

The use of cyclophosphazene in the formulation of flame retardant materials hasgenerated numerous patents.106 Various known cyclo-matrices based on phospha-zene bismaleimide or triazine and, especially, the styrene polymer (159) having cyclo-alkoxy or aryloxy-phosphazenes as pendant groups, have been found to be resistantto atomic oxygen, probably by the formation of a protecting layer of phosphate.107

Known compound (160) has six lipophylic L-glutamide pendant groups, and wasfound to be a self-assembling organogelator that, compared with the correspondingfree L-glutamide, exhibited enhanced gelation ability and chirality, and an unusualability to self-restore to a gel state (thixotropy).108 Other studies include, thepreparation of ultra-filtration membranes with functionalized cyclotriphospha-zenes,109a or nano-filtration phosphazene membranes.109b Patents related withother potential applications of cyclophosphazene include lubricants,110a-c energystorage devices,110d and organic electroluminescent display devices.110e

The donor properties of the endocyclic N atoms and/or the presence of susbti-tuents carrying groups with coordinating ability to form metal-complexes is anotherclassical aspect of the chemistry of cyclophosphazenes with wide implications incoordination and organometallic chemistry. Thus, reaction of the aminocyclophos-phazene (161, R = Prn) with 3 equivalents of ZnEt2 gave a compound having anstructure (as determined by X-ray diffraction) consisting of a dimer of the phos-phazenate segment (162, R = Prn). The reaction of the hydrates of (161, R = Prn,Cy) with 4.5 or 8 equivalents, respectively of ZnEt2 in hexane gave phosphazenate-zinc oxide clusters that were also characterized by X-ray diffraction. It was foundthat the structures consisted of trimeric ZnO or hexameric ZnO clusters sandwichedbetween phosphazenate segments (162) (PN distances in the range 1.63–1,65 A)showing a template effect.111

The reaction of P3N3Cl6 with silver(I) tetraalcoxyaluminates Ag[Al(OR)4] in CH2Cl2/CS2 solution led, depending on R and the conditions, to tetraalkoxyaluminate salts (163–164, R=C(CF3)3), and to the adduct (165, R=CMe(CF3)2), all of them thermally verystable (no deposition of AgCl was observed at 60 1C in an ultrasonic bath). X-Ray



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diffraction and solution and solid-state 31P NMR studies in combination withquantum mechanical calculations indicate that the cycle [P3N3Cl6] is weaklybound to the silver atoms and is only a slightly stronger ligand than P4 and CH2Cl2,and far weaker ligand towards the Ag cation than S8, P4S3, toluene, and 1,2-Cl2C2H4.The formation of [P3N3Cl5]

+ cations was studied by quantum mechanicalcalculations.112

A new cyclophosphazene carrying phosphine ligands (166) has beenprepared from bis(2,20-dioxy-1,10-biphenyl)dichlorocyclotriphosphazene andortho-diphenylphosphine–phenol, and has been used to form the Au(I) and Pt(II)complexes (167–169) as shown by X-ray crystallography.113 The sequentialreaction of the potentially polydentate ligand hexakis(2-pyridyloxy)cyclotriphos-phazene (OPy) with the anhydrous dichlorides MaCl2 and MbCl2 (M

a = Cu or Co,Mb = Co or Zn) gave the heteropolynuclear complexes (170–172),that were characterized by X-ray diffraction. The formation of the cationic complex(172) is the result of an unusual P–O bond cleavage of one oxypyridines, themechanism of which was not elucidated. In the structure (170), the anionic MbCl3fragment and the cationic MaCl fragment are maintained in solution (i.e. noconductivity was observed).114



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Very similar structures were obtained by reacting ligand OPy or the related ligandhexakis(4-methyl-2-pyridyloxy)cyclotriphosphazene (Me–OPy) with the dihalidesCoX2 (X = Cl or Br), NiCl2 or ZnCl2 under different conditions, to give complexes[CoLX2], [(CoLX)(CoCl3)] (X = Cl or Br, L = OPy or MeOPy), [CoLX]PF6 (X =Cl, Br, L = OPy), [Ni(OPy)Cl2] (green and red isomers), [Ni(OPy)Cl2]PF6 and[(ZnCl2)2(OPy)], the latter being identical to compound (171) mentioned above. X-ray diffraction of (171) and (173–177) showed that the orientation of the non-coordinating OPy (or Me–OPy) groups differ depending on the MX2 moiety. In theneutral and zwitterionic structure (174) there are differing interactions between the Xligands of the anionic MX3 fragment and the OPy rings of the cationic MXfragments. Complexes (174a) and (170a) have similar structures.115

Other cyclophosphazenes have been used as ligands in coordination chemistry.Thus the complexes (178) (already known), (179) and (180), were prepared by



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reaction of the corresponding cyclophosphazene ligands with [Pt(CNR)2Cl2] or[Pt(CNR)2Cl4] [in the later case, the reaction involved the reduction of Pt(IV) toPt(II)].116 Oxypyridine groups have also been used as ligands in cyclic phosphazenessuch as (181), to form complexes such as (182) with a tungsten pentacarbonylfragment.117

3. Polyphosphazenes

Aspects of phosphazene research118a and of the history of phosphazenes havebeen examined.118b The prototrope equilibrium (Scheme 13) between thepolyaminophosphines (NH form) and the polyiminophosphines (polyphosphazeneor PH form) has been studied by Quantum Mechanical ab initio calculations.When R = H, the energy difference is in favour of the NH tautomer, but ifR = NH2 the more stable is the polyphosphazene form. In fact, the preference forthe later increases with the electronegativity of the R substituent, and inagreement with the experimental facts, the calculations showed that thepolymerization of the monophosphazanes (1) (see Section 1) should be favourablewhen the electronegativity of R is about 3. The polyhydrido phosphazenehas an helical structure with small bond alternation. The calculations alsosupport the mechanisms for the formation of the polyhydridophosphazene (3)from tris(amino)phosphine P(NH2)3 (1a) via its phosphazene (NH2)2HPQNH (2a)tautomer.1

A complete morphological study by solid state 1H, 19F, 31P, and 13C NMRmethods of the well known semi-crystalline polymer [NP(OCH2CF3)2] has beenpublished showing, among other things, that above 90 1C the only mobile phaseobserved is probably the 2D mesophase.119 The secondary structure of the chiralpolyphosphazene random copolymers containing phenoxy and (R)-binaphthoxychromophoric groups (183) has been studied by a combination of steady-state andtime-resolved fluorescence techniques with theoretical molecular dynamics calcula-tions. The analysis of the data (excitation and emission spectra, fluorescencedepolarization and lifetimes) suggested that 100% of the excitation energy of thephenoxy groups is transferred to the binaphthoxy groups and subsequently, thisenergy migrates among binaphthoxy groups along the polymer chain with anoticeable quenching of the binaphthoxy fluorescence. The efficiency of this energymigration process increases with the number of binaphthoxy groups and is favoured



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by the presence of helical sequences along the chain. The results of the moleculardynamics simulations on several polymer fragments were in good agreement with theexperimental measurements.120

Running parallel to that of the cyclic phosphazenes (Section 2), the synthesis oflong chain lineal polyphosphazenes has been carried out following well establishedmethods. The most frequently used are based on the so- called macromolecularsubstitution of chlorines of the parent polydichlorophosphazene, [NPCl2]n, by theappropriate nucleophiles (OR or NHR). However, the alternative methods based onthe polycondensation of phosphoranimines are also commonly used. The chemicalderivatization of a polyphosphazene precursor carrying the appropriate functionalgroups, which may present more difficulties, is another possibility. The particularcase of the deprotonation of a polyphosphazene with organolithium reagents,followed by a substitution reaction has been reviewed.32 The lithiation route ofpoly(alkyl/aryl phosphazenes) led to the well defined graft-poly(methylmetacrylate)copolymers (185) with various chain lengths, starting from poly(methylphenyl)phos-phazene [NPPhMe]n (PMPP) in a two step approach that involved the hydroxyl

Scheme 13



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derivative PMPP-OH. The new precursor polymer (184) could be obtained free ofvinyl terminal groups by avoiding the very facile elimination of HBr (base and heatpromoted) from the terminal bromoalkyls. These terminal bromoalkyls acted asatom transfer radical polymerization initiators in the reaction of (184) withmethylmethacrylate in the presence of CuCl/bipyridine catalyst.121

The synthesis of hydrido-amino phosphazenes can be carried out directly fromaminophosphines. Thus, the dialkylamino(amino)phosphine (Me2N)2PNH2 (gener-ated in situ by amminolysis of the corresponding chlorophosphine) underwent a fastkinetically controlled polycondensation process without cross-linking in solution atlow temperature to form the low P-hydrido(dimethylamino)polyphosphazene[N P(H)(NMe2)]n (186) with absolute Mw = 41.000 (PDI = 1.5) in a planar cis-trans or twisted helical conformation. The proposed mechanism for the polymeriza-tion was supported by ab initio calculations on the model (H2N)2PNH2 reported inref. 1 (see above).122

The preparation of the polymer [NP(OPh)(Prn)]n (Mw, by SEC = 132 000 withIPD= 1.1 and a minor peak at 33 000 with IPD= 1.1) during the thermolysis of thephosphoranimine (PhO)2(Pr

n)PQNSiMe3, and formation of the polymer[NP(OCH2CF3)Ph]n (Mw by SEC = 44 000 with IPD = 1.1) in the reaction of(CF3CH2O)2PhP = NSiMe3 with trifluoroethanol27a were already mentioned inSection 1.



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Known polyphosphazenes [NPR1R2]n (58; R1 = R2 = Me; R1 = Me, R2 = Ph)

were formed quantitatively by the reaction of the corresponding N-silylphosphor-animines with P(OMe)3, the mechanism of which is not well understood, as discussedin Section 1. The polymers were isolated in high yield with Mw ca. 105. This newroute to poly(alkyl/aryl)polyphosphazenes has significant potential advantages overthe usual thermal (190 1C) polycondensation.28

The amphiphilic triblock copolymers (187), based on a poly(propylene glycol)(PPG) with Mw ca. 4000 as the central block flanked by hydrophobic polyphos-phazene blocks, having molar composition ratios of the repeating units of PPG topolyphosphazene (PN)x–PPG1.0–(PN)x (x = 0.2–0.7), have been synthesized (seeScheme 14) by the addition at room temperature of the N-silylphosphoranimine (59)(as a macromolecular terminator) to the living polyphosphazene (LP). This resultedin the controlled cationic-induced polymerization of Cl3PQN–SiMe3 with PCl5 and(CF3CH2O)3PQN–SiMe3 (as the end-capper reagent), followed by the final sub-stitution of all the –NQPCl2– chorines with an excess of NaOCH2CF3. Thepolymers formed spherically shaped hydrophobic micelles that self-organized inan aqueous phase.29

The direct derivatization of the chiral brominated precursor shownbelow (x = 0.2) gave the corresponding silyl phosphazene copolymers (188) wherethe phosphine ligands sit inside wide and sterically demanding chiral pockets,and therefore, with potential interest to support catalysts for enantioselectivesynthesis.123

Scheme 14



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As with the cyclic analogues, many of the reported preparations of new polyphos-phazenes are based on sequential combinations of substitution-derivatization reac-tions most frequently aiming for designed technologically relevant materials withpre-determined properties. For example, the reaction of [NPCl2]n first with N-hydroxyhexylcarbazole (substitution), followed by treatment of the resulting poly-mer with 4-nitrobenzene diazonium chloride in nitrobenzene/aqueous system in thepresence of sodium dodecylbenzene sulfonate (regioselective azo-coupling), gave thecarbazole-based film forming photorefractive polyphosphazene (189) (y = 0.29, Tg

= 50 1C). The nonlinear optical effects were studied at 633 nm at room tempera-ture.124 In another example of substitution reactions of [NPCl2]n the sodium salt of7-(2-hydroxyethoxy)-4-methylcoumarin was used to incorporate photosensitivecoumarin groups on the main phosphazene chain to obtain the cross-linkablepolymer (190) (average Mw ca. 105, Tg = 67 1C) that is stable up to 280 1C andthat, under UV irradiation, undergoes a [2 + 2] cycloaddition to form insolublecross-linked curable films.125 The related poly[(bis(3-acetylcoumarin-o-aminoben-zoylhydrazone)phosphazene] has also been reported.126

In a sequential macromolecular substitution reaction of [NPCl2]n with NaOCH2-

CH2OCH2CH2OCH3 and subsequent reaction with NaOCH2CHQCHPh randomcopolymers [NP(OR)x(OR0)y]n (191) were produced bearing a combination ofcinnamyl side groups as cross-linkable units and hydrophilic 2-(2-methoxyethoxy)-ethoxy phosphazene groups (i.e. a copolymeric derivative of the poly(bis-methoxyethoxyethoxy)phosphazene called MEEP). Their cross-linking under UV



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(320–480 nm) irradiation in the presence of a photoinitiator allowed the fabricationof three dimensional hydrogel microstructures, in the size range 50–500 mm, thatwere used to encapsulate enzymes for biosensor applications. For example horse-radish peroxidase (HRP) was used as a model system to catalyze the reaction ofH2O2 and Amplex Red to produce a fluorescent product, resorufin).127

The design of polyphosphazenes as polymeric ion conducting materials withapplication in fuel cells is an important and active area of research that has beenreviewed.128 The synthesis and characterization, transport phenomena and experi-mental techniques used to evaluate polymer electrolyte membranes for the directmethanol fuel cell, as potential replacement of lithium-ion rechargeable batteries inportable electronic devices, have also been reviewed.129 Thus, a designed sequentialmacromolecular substitution reaction of [NPCl2]n first with NaOCH2CH2OCH2-

CH2OCH3 and subsequently with [NaOC6H4SO2NSO2CF3]Na/LiCl, or NaOC6H5,gave the single ion conducting MEEP copolymers (192), with 5 to 22 mol% oflithium sulfonimide substituents, and the structurally related 2-(2-methoxyethoxy)-ethoxy phosphazene (193) carrying 5 to 20 mol% of non substituted phenoxygroups, respectively. The ambient temperature ionic conductivity of (192) was lowerfor the polymers with a greater content of lithium sulfonimide, probably due to adecrease in macromolecular motion and the steric effects of the bulky aryloxygroups. They were also lower than that of the unbound model system formed from(193) and dissolved lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The in-crease of the ionic conductivity observed at elevated temperatures was attributed toan increase in the macromolecular motion.130

Novel MEEP-type polyphosphazene–silicate hybrid network membranes (Tg �38to �67 1C), exhibiting high ionic conductivities with lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI) as the salt, have been prepared as candidates for dimen-sionally stable solid polymer electrolytes by a designed sequence of steps startingfrom [NPCl2]n and involving the incorporation and hydrolysis of triethoxysilanegroups (Scheme 15).131

The sequential aminolysis of [NPCl2]n gave the bis(2-methoxyethyl)amino and n-propylamino random copolymers (194) with x in the range 0.8–1.0 (m.p. 190 1C,decomp. o300 1C). Solution casting of (194) with variable amounts of dissolvedlithium triflate (LiSO3CF3) and NaI, allowed the preparation of transparent salt-in-polymer electrolyte membranes (Tg �50 to �36 1C) with good mechanical propertiesbelow 100 1C. The dispersion of 4 wt% Al2O3 nano-particles (o40 nm) in polyphos-phazene membranes having 10 wt% LiSO3CF3 led to an increase of the conductiv-ities by 2 orders of magnitude. The observed non-Arrhenius temperature dependenceof the conductivity was rationalised in terms of a migration model.132 The ionicconductivity of the composites was found comparable to that reported for polymerelectrolytes based on poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) but



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having far superior mechanical stability at ambient temperatures than those of thenon-cross-linked (MEEP).133

Other ionic conductive mechanically stable polymer electrolyte membranes havebeen formed by the UV-induced cross-linking of mixtures of the poly(amino)phos-phazene with poly(propylene oxide) side chains [NP(NHR)2]n (195) (R = –[CH(CH3)–CH2O]m–CH3 (m = 6–10, Mw ca. 105) and lithium triflate (LiSO3CF3)in the presence of benzophenone as photoinitiator. The ionic conductivities foundwere almost as high as those in classic MEEP-based polymer electrolytes. It wasconcluded that the separation into a flexible polymer serving as a backbone and ahigh concentration of dangling side chains as solvating units could be a goodapproach capable of improvements.134 There have been several patents on potentialapplications of polyphosphazenes as conductive materials for fuel cells,135a–d orelectrolyte solution additives for nonaqueous electrolyte batteries.135e

The design of amphiphilic polyphosphazenes with self assembled morphologiesfor thermosensitive and micelle forming materials is another topic of current interest

Scheme 15



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with wide potential applications. A study, including association temperature andviscosity measurements, has shown that the thermosensitive gelation of the poly-(organophosphazenes) (196) (x = 0.89–1.43) carrying hydrophilic a-amino-o-meth-oxy-poly(ethylene glycol) and hydrophobic amino acid ester side groups is facilitatedby the presence of the salts NaCl, KCl, NaBr and dramatically suppressed by NaIand the organic salts Et4NBr, Prn4NBr and Bun4NBr. In the case of the inorganicsalts, the salting-out effects (salting-in in the case of KI) were explained on the basisof stronger interactions of the smaller ions with the water molecules that decrease theH-bond ordering increasing the number of hydrophobic association points. In thecase of the organic salts, the salting-in effects were attributed to the interactionsbetween the tetraalkyl groups of the cations and the non-polar side groups of thepolymers that make their inter-association more difficult.136

The self-assembly morphologies of the known amphiphilic graft polyphospha-zenes (197) (Mw 12 000 to 22 000), containing various compositions of oligopoly-(N-isopropylacrylamide) and ethyl 4-aminoethylbenzoate as co-side groups, i.e.,having different hydrophobic/hydrophilic balance in aqueous solution, has beenreported. Physically cross-linked networks and the coexistence of sphere shaped andnetwork structures aggregates were observed for (197a) and (197b), respectively,while for (197c) the morphology of the aggregates were solvent dependent and insome cases, low polydispersity nano-spheres or high-genus particles were ob-tained.137

The self-assembly of new graft polyphosphazenes (198, x + y = 2) bearingpoly(N-isopropylacrylamide) as a hydrophilic segment and ethyl glycinate as ahydrophobic group (lower critical solution temperature in the range 18.5–33 1C)gave controlled nano-particles 80 to 900 nm in size at 25 1C. Their use as injectabledrug carriers for the delivery of hydrophobic compounds was discussed.138 The self-assembly properties of these polyphosphazenes, along with aspects of their chemicalreactivity such as hydrolytic degradation and biocompatibility, are crucial propertiesto possible biomedical applications.139a The most recent advances on the synthesis ofwater-soluble polyphosphazenes for biomedical applications139b and the uniqueopportunities of polyphosphazenes for the tuning surface properties from highlyhydrophilic to hydrophobic, and changing the polymer architectures that make themuseful for biomedical and commercial applications, have been reviewed.139c Ad-vances on the design of new biomedical materials with targeted properties have alsobeen reviewed.139d,e Polyphosphazenes were also included in a review on the recentdevelopments in biodegradable synthetic polymers focusing on tailoring polymerstructures to meet material specification for emerging applications such as tissue



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engineered products and therapies139f and in a book.139g The practical aspects oftheir use as biodegradable materials, the kinetics and the effects of pH, buffercomposition, temperature, casting solvents, and film thickness of the hydrolyticdegradation of the poly[di(ethyl-glycinato)phosphazene], and poly[di(ethyl-alaninato)-phosphazene] (free of residual chlorine atoms) have been investigated.140

The water-soluble polyphosphazene polyelectrolyte (199), carrying sodium car-boxylatoethylphenoxy pendant groups, synthesized via macromolecular substitutionfrom [NPCl2]n, is hydrolytically degraded in aqueous solutions with a decrease in themolecular weight and the release of side groups. For similar polymers containingdifferent amounts of residual P–Cl bonds, obtained by incomplete substitution, thedegradation is faster and increases with the chorine content. In vivo studies showedthat (199) is an efficient vaccine immune-adjuvant, and has the capability of formingmicro-spheres in aqueous solutions via ionic complexation with physiologicallyoccurring amines, such as spermine.141 A simple method, for the preparation ofmicro-spheres and nano-spheres using related poly[di(carboxylatophenoxy)phos-phazene] has been reported. The method can be applied to polyphosphazenescontaining carboxylic acid and sulfonic acid functionalities and can be used forprotein encapsulation, and possibly in vaccine delivery applications.142

The biodegradable high Mw film-forming polyphosphazenes (200–203), contain-ing amino acid ester side groups, were synthesized by the macromolecular substitu-tion route. Their glass transition temperatures, water contact angles, hydrolyticdegradation, surface wet ability, tensile strength, and modulus of elasticity variedover a wide range, making then suitable for many biomedical applications.143

Phosphazenes carrying amino acid ester substituents are biocompatible andhydrolytically controllable and are important candidates for various biomedicalapplications, including, inter alia, the design of drugs delivery systems. Thus,excellent tissue compatibility and in vivo biodegradability have been observed in asubcutaneous rat model for polyphosphazenes functionalized with L-alanine ethylester of composition poly[bis(ethylalanato)phosphazene], poly[(50% ethylalana-to)(50% methylphenoxy)phosphazene], and poly[(50% ethylalanato)(50% phenyl-phenoxy)phosphazene].144 New thermosensitive poly(amino)phosphazenes (204),with compositions given by the ratios x:y:z (cf. polymer 196)136 bearing hydrophobicside groups of l-isoleucine ethyl ester (IleOEt) and ca. 40% of hydrophilic groups ofa-amino-o-methoxy-polyethyleneglycol (AMPEG), together with ethyl-2-(O-glycyl)-lactate groups (GlyLacOEt) to increase their biodegradability, were synthesized forcontrolled release of hydrophilic polymeric model drugs.145 Another study showedthe efficacy of the analogous polymers having slightly different contents of ILeOEtand GlyLacOEt, in the release of the antitumor drug doxorubicin, making these



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thermosensitive poly(organophosphazene)s good candidates for locally injectabledrug delivery systems.146

New biocompatible and thermosensitive poly(organophosphazenes) (205), with alower critical solution temperature (LCST) below body temperature, may be usefulfor local delivery of hydrophobic drugs such as peptide and protein drugs. They weresynthesized by introducing short chain tri- or tetraethylene glycol as a hydrophilicgroup and the dipeptide glycyl-l-glutamic diethyl ester (GlyGluEt2) as a hydro-phobic group into the polyphosphazene backbone. Reasonably good results wereobtained for loading and releasing of a human growth hormone (hGH) as a modeldrug.147 Other polyphosphazenes with potentially interest for drug delivery such as(206), have been obtained from [NPCl2]n and used to form micro-spheres incorpor-ating indomethacin (water insoluble) or 5-fluorouracil (water soluble) that weretested for in vitro release of drugs at various pH.148

The self-assembling amphiphilic polyphosphazenes, with poly(N-isopropylacryla-mide) and ethyl glycinate as side groups, were synthesized by sequential substitutionof chlorine with amino-terminated N-isopropylacrylamide oligomers and ethylglycinate (GlyEt). The polymers gave thermally responsive micelles that were usedto study the effect of temperature on in vitro drug release profiles.149a Ibuprofenloaded nano-spheres with sustained drug release in vitro were prepared with anamphiphilic graft polyphosphazene with poly(N-isopropylacrylamide) and ethyl-glycinate (0.54:1 molar ratio). They exhibited two temperature induced phasetransitions forming network micelles (below T1), narrowly dispersed nanoparticles(above T1), and inter-nanoparticle aggregation (above T2).149b The loading ofindomethacin in polymeric micelles based on amphiphilic polyphosphazenes withpoly (N-isopropylacrylamide) and ethyl tryptophan side groups and the in vitro andin vivo evaluation of the nano-carriers has been studied to determine the effects ofcopolymer composition, the chemical structure of the drug and the compatibilitybetween the later and the micellar core.149c Various strategies for mucosal delivery ofvaccines in domestic animals, including polyphosphazenes as delivery-systems, havebeen critically reviewed.150

Two novel biodegradable amino-acid-based polyphosphazenes, poly[(ethyl-alana-to)1.0(ethyl-oxybenzoate)1.0phosphazene] and poly[(ethyl-alanato)1.0(propyl oxyben-zoate)1.0phosphazene] were synthesized. Both polymers became insoluble incommon organic solvents following hydrolysis presumably due to cross-linkingreactions accompanying the degradation process. In vitro osteo-compatibility eva-luation and the enzymatic activity of the osteoblast cells cultured on the polymersdemonstrated they are promising new materials for forming self-setting bonecements.151 Bone analogue composites have been formed at 37 1C, correspondingto in vivo conditions, from poly[bis(carboxylatophenoxy)phosphazene], tetracalcium



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phosphate [Ca4(PO4)2O], and anhydrous dicalcium phosphate (CaHPO4). Theeffects of the proportion of polymer (5, 10, or 15 wt%) on the kinetics ofhydroxyapatite formation were studied.152 In a comparative study in bone, thebiocompatibility of biodegradable glycine-containing polyphosphazenes has beenexamined.153 Poly-bis(ethylalanate)phosphazene has been evaluated as a scaffold forbone tissue engineering,154 and gave very promising pressure-equalizing tubes forhealing process of tympanic membranes with no complications.155 The potentialbiomedical applications of polyphosphazenes have been reflected by various patentson drug release systems,156a,b medical implant materials,156c pharmaceutical appli-cations,156d–h and medical devices.156i–k

Another important aspect of phosphazene polymers is based on their permeabilityto gases, leading to the design of new types of gas transport membranes. Thus, aninvestigation of gas diffusion and solubility in [NP(OBu)2]n (Bu = n-, iso- and sec-Bu) and [NP(OCH2CF3)2]n (in amorphous and crystalline states) by a combinationof quantum chemistry, molecular dynamics and monte-Carlo methods and therelationship between polymer structure and gas diffusion and sorption in polypho-sphazenes, has been reviewed.157 The gas permeabilities (of CO2, CH4, O2, N2, H2,and Ar) for the polyphosphazenes (207–210), some of which were synthesized andcharacterized for the first time, have been studied. Additionally, the first gaspermeation data has been collected on hydrolytically unstable [NPCl2]n. The mostpermeable, for all the phosphazenes, was CO2 and, in agreement with other studies,for this gas (and, to a lesser degree, for all the other gases studied) the permeabilityincreased with decreasing glass transition temperature of the polymer. Except forhydrogen, the permeability data were also correlated to the gas condensability andthe gas critical pressures. While for the CO2/H2 mixture no increase in the idealseparation factors (a) was observed with decreasing Tg of the films, increases in thesefactors were noted for the CO2/CH4 mixture.158

The gas transport properties for cross-linked and non-cross-linked membranes ofpolyphosphazenes having various proportions of methoxy-ethoxy ethoxy substitu-ents have been studied in connection with the purification and re-utilization of CO2.The CO2/N2 separation factors depended on the polymer composition, the glasstransition temperature, and the temperature, being higher for the cross-linkedmembranes at low temperatures.159 The gas permeation properties to gases suchas He, H2, O2, N2 and CO2, including selectivity ratios, of cross-linked membranesprepared with mixtures of the polyphosphazene [NP(OR1)0.18(OR2)0.76(OR3)1.06]n(OR1 = 2-allylphenoxy, OR2 = 4-isobutylphenoxy, OR3 = 4-methoxyphenoxy)(Mw ca. 106) and a hydride terminated polydimethylsiloxane (Mw = 14 400) havebeen investigated.160 The permeability and solubility of benzene, cyclohexane, and n-



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hexane in poly[bis(2,2,2-trifluoroethoxy)phosphazene] membranes have been deter-mined. Both the permeability and the diffusivity, that are very dependent on themolecular size of the hydrocarbon, increase exponentially with vapour activity andwith temperature and are higher in benzene than in cyclohexane and n-hexane. Thesolubility seemed to be inversely proportional to the molecular size of the hydro-carbon.161 A patent on the use of polyphenoxyphosphazene as a membranes for gasseparation has appeared.162

There are many general studies on previously reported or slightly modified knownpolyphosphazenes. For example, the well known sulfonated poly[bis(phenoxy)phos-phazene] has been trapped in a cross-linked interpenetrating hydrophilic networkformed by cross-linking the cyclic hexakis(vinyloxyethoxyethoxy)-cyclotriphospha-zene for the preparation of proton conducting membranes.163 The water solublepolyphosphazene polyelectrolyte (211), prepared in various steps from [NPCl2]n,formed black homogeneous dispersions of single-wall carbon nano-tubes. Thesupramolecular association between (211) and the nano-tubes was investigated toshow the formation of small bundles of nano-tubes and individual nano-tubescoated with the polymer.164

The uses of polyphosphazenes in the formation of stabilized nano-particles withcontrolled size is another subject that has deserved some attention. In a comparativestudy on the efficacy of various polymeric stabilizers of modern transition-metalnano-clusters, poly(bis(ethoxy)phosphazene) was found to influence Ir(0)n nano-cluster nucleation.165

Several studies on the pyrolysis in air of transition metal complexes withpolyphosphazene ligands have been published showing the possible formationof nano-particles of different compositions.166a Thus, the complexesobtained by coordinating CpFe(dppe), CpRu(PPh2)2, (Z

5-CH3C5H4)Mn(CO)2, andW(CO)5 fragments to the already known polyphosphazene copolymer with pendantoxypyridine ligand {[NP(O2C12H8)]x[NP(OC5H4N)2]1�x}n (x = 0.7) and itsanalogue with x = 0.8, afforded metallic nano-structured materials formed by thecoexistence of metal and metal oxide in the case of tungsten and asmanganese phosphate salt in the case of manganese, as shown by transmissionelectron microscopy (TEM), scanning electron microscopy (SEM), back-electronscattered imaging (BEI), energy-dispersive X-ray microanalysis, and micro-Raman



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data. The results were proposed as a new and general method to obtainmetallic nano-structured materials.117 The pyrolysis in air at 800 1C of therelated phosphine polyphosphazene complex [{NP(OC12H8)}0.6{NP(OC6H4PPh2 �(Z5-CH3C5H4)-Mn(CO)2)2}0.4]n in the solid state afforded nano-clusters ofMn2P2O7 with sizes ranging from 50 to 90 nm and averaging about 74 nm. Thepyrolytic material showed near-infrared photoluminescence attributed to theemission of tetrahedral Mn2+ ions.166b In the case of the carborane-substitutedpolyphosphazene, {[NP({OCH2}2C2B10H10)]0.5[NP({OCH2}2C2B9H10 �NBu4)]0.5}n,the pyrolysis in air afforded BPO4 crystals of varied sizes in the micro and nanoregime, the formation of which was compared with the results observed in thepyrolysis of anchored organometallic derivatives of polyphosphazenes.166c Patentsrelated with various uses of polyphosphazenes include, binding of explosives,167a

new systems and methods for transfering fluid samples,167b and self-degrading core/shell fibers.167c

Similarly to the linear and cyclic phosphazenes, high molecular weightpolymers carrying pendant groups possess coordinating ability for transitionmetal atoms, which has led to the preparation of various types ofpolymeric coordination compounds. A general survey of the topic of metal contain-ing polyphosphazenes and their possible applications has been published.168 Thus,the reaction at room temperature of the polyspirophosphazene copolymerhaving pendant diphenylphosphine groups (212) with [Au(THT)Cl] (THT = tetra-hydrothiophene) gave the neutral polymeric complex (213), that was pyrolyzed in airat 800 1C to form gold nano-particles in the range of 90 to 130 nm.169 The newpolyphosphazenes {[NP(O2C12H8)]0.6[NP(OC6H4CO2Pr

n)(OC5H4N)]0.4}n and{[NP(O2C12H8)]0.5[NP(OC6H4CO2Pr

n)(OC6H4L)]0.5}n [L = CN or PPh2],carrying ligands were synthesized by sequential substitution from [NPCl2]n, andreacted with [W(MeOH)(CO)5] to give the corresponding tungsten carbonylcomplexes of the types (214) and (215), having high glass transition temperatures(only partial complexation was observed in the case of L = CN). The complexeswere decarbonylated at ca. 300 1C forming metal containing species with stabilizingeffects on the polymeric matrices. The residues left after heating up to 800 1C wererather high (30–50%).170

Ion-uptaking phosphazene copolymes (216) with 25 to 100% amino groupsand carrying 2-, 3- or 4-pyridine ligands, were prepared by reacting [NPCl2]n with



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NaOPh and the corresponding pyridine-amine NH2(CH2C5H4N). The2-pyridylalkyl derivatives showed selectivity for Cu(II) in the presence of Ni(II)and Co(II) that increased with the number of functional groups, whereas their3- and 4- analogues were much less active and less selective.171 One of the potentialinterests of the polymeric transition metal complexes is the possibility ofdesigning supported catalyst, a field still little explored. Poly(diaminopyridino)-phosphazenes, that were prepared by reacting poly(dichlorophosphazene)with 2-aminopyridine and 3-aminopyridine. They were then reacted withcobalt acetate, to produce polymeric materials that were used as catalystsfor the oxidation of alkenes at atmospheric pressure in the presence ofmolecular oxygen to give epoxides and ketones with very high yield andselectivity.172

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[organophosphorus chemistry] organophosphorus chemistry volume 37 || phosphazenes

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