Phosphonitrilic bromides

J. Inorg.Nucl.Chem.. 1961.Vol.22. pp. 199 to 204. PergamonPress Ltd. Printedin NorthernIreland

PHOSPHONITRI

LIC

BROMIDES

K. JOHN and T. MOELLER Noyes Chemical Laboratory, University of Illinois, Urbana, Illinois (Received 28 April 1961 ; in revised form 5 June 1961)

A~traet--The reaction between phosphorus(Ill) bromide, bromine, and ammonium bromide in either sym-tetrachloroethane or sym-tetrabromoethane yields, depending upon the temperature and the time of reaction, either the compound NPBrzPBr,, or members of the homologous series, (NPBr~),,. Conditions for the synthesis and separation of these compounds in good yields and certain of their properties are described. ALTHOUGH a phosphonitrilic bromide was isolated at the time H. N. STOKES lirst recognized the phosphonitrilic chlorides as an homologous series, very little information dealing with the bromides has appeared in subsequent years. This is undoubtedly a reflexion o f the difficulties o f synthesis and separation that characterize these compounds. A few crystals of the trimer, (NPBr2) 3, were obtained by BESSONCa~as a result o f treatment o f the ammonolysis product of phosphorus(V) bromide with excess phosphorus(V) bromide in a sealed tube at elevated temperature. A somewhat better yield o f the trimer and the first synthesis o f the tetramer resulted from an adaptation o f SCHENK and ROMER'S procedure (2) for the phosphonitrilic chlorides to the reaction of phosphorus(V) bromide with a m m o n i u m bromide in s y m - t e t r a c h l o r o e t h a n e . (31 Unfortunately, the thermal decomposition o f phosphorus(V) bromide and the consequent loss o f bromine at the o p t i m u m reaction temperatures seriously reduce the yield. In a measure, this has been compensated for either by addition of bromine during the reactionl3, 4~ or at the beginning o f the reaction. (5~ It has been shown, (6~ however, that by allowing phosphorus(Ill) bromide to react with a m m o n i u m bromide and excess bromine at above 130°C in either s v m - t e t r a chloroethane or s y m - t e t r a b r o m o e t h a n e one can obtain the phosphonitrilic bromides in yields comparable to the best reported for the chlorides.(7, 8) The best yields (ca. 50-60 per cent) o f trimeric and tetrameric phosphonitrilic bromide are obtained when the temperature is maintained at 140-145~C for a period o f 11 days. Troublesome sublimation o f phosphorus(V) bromide in the initial stages o f the reaction is avoided by raising the temperature to this range over a 3 day period. Under these conditions, formation o f certain amounts o f the oily higher homologues is unavoidable. The percentage o f higher homologues in the reaction product increases with increasing reaction temperature. At 143°C, the reaction product was 58 per cent trimer and tetramer with ca. 8 per cent o f tetramer in this mixture. At 175°C, the product ~'~ A. BESSON,C.R. Acad Sci. Paris 114, 1479 (1892). (2~ R. SClt~NKand G. R6MER,Ber. Dtsch. Chem. Ger. 57 B, 1343 (1924). caJ H. BODE,7.. Anorg. Chem. 252, 113 (1943). ,4~W. GR1MMV.,Dissertation, Mfinster (1926). ,b~N. E. BEANand R. A. SHAW,Chem. and lndustr. 1189 (1960). ~e~K. JOHNand T. MOr.LLER,J. Amer. Chem. Soc. 82, 2647 (1960). tT~L. G. LUND,N. L. PADDOCK,J. E. PROCTORand H. T. SEARLE,d. Chem. Soc. 2542 (1960). '~ M. L. NXELSENand G. CKANFORD,Inorganic S),ntheses, Vol. 6 pp. 94-97, (Edited by E. G. Rocttow). McGraw-Hill, New York (1960). 199

200

K. Joan and T. MOELLER

contained only 33 per cent of trimer and tetramer, but ca. 50 per cent of this mixture was tetramer. Separation of the trimer-tetramer fraction from the higher polymers can be effected by either solvent extraction with benzene or n-heptane or direct vacuum sublimation. During sublimation, the higher oily homologues are polymerized to greater or lesser degree to phosphonitrilic bromide rubber. (9) This rubber resembles the corresponding chloride rubber in hydrolyzing in moist air and in losing its plasticity at higher temperatures as a result of depolymerization. Separation of the trimer from the tetramer is best effected by crystallization from solvents such as n-heptane or petroleum ether (b.p. 95-110°C). Although solubility determinations indicate the bromides to be less soluble than their chlorine analogues, they demonstrate substantial useful differences between trimer and tetramer (Table 1). TABLE I . - - S o L u B I L I T Y DATA FOR

CNPBr2)3AND (NPBr~), AT 25°C Solubility (g/100 g of solvent)

Solvent (NPBr~)~ Carbon disulphide Benzene Chloroform Diethyl ether Carbon tetrachloride Cyclohexane Petroleum ether (90-110°C) n-Heptane

(NPBrz)4 7.40 3'85 1 "85 0.90 0"95 0'80 0'27 0.15

24'50 18.60 12.10 6"65 5.50 4.00 2"30 1-45

TABLE 2 . - - N U C L E A R MAGNETIC RESONANCE SPECTRA OF

Compound (NPBr2)3 (NPBr~)4

(NPBrz)a AND (NPBrz)~

Chemical Shift (p.p.m., reference 85~o HsPO4)

Solvent

r.f. frequency (Mc/s)

&49.5 --'71'8

CS2 CS2

16.2 16"2

,

TABLE3.--INVRA-REDSPECTRAOr PHOSPHONrrRILICBROMIDES NaC1 Region Compound

CsBr Region

P-N Frequency (cm-O

Medium

1276 1256 1175,1126 1272 ca. 1302

Nujol Nujol CS2 CS, CS,

NPBrsPBr5 NPBr~PBr7 ('NPBrs)s (NPBr2), (NPBr~), (oil) * All very strong (v.s.). t0) D. L. HERRING, Chem. and.

lndustr.

717 (1960).

Frequency* (cm-1)

Medium

509 (broad) 494 (broad) 526, 437 517, 447, 415

Nujol Nujol Nujol Nujol

Phosphonitrilic bromides

201

The nuclear magnetic resonance spectra indicate the same positive shift with the tetramer as compared with the trimer noted with other phosphonitrilic halides (Table 2). ~7~ Similarly, the infra-red spectra show the same general differences in P-N stretching frequency (NaCI region) characteristic of other trimeric and tetrameric phosphonitrilic compounds (Table 3). ~°) A general increase in this frequency parallels increase in degree of polymerization. Both trimer and tetramer give characteristic and different X-ray diffraction patterns (Table 4). TABt.E 4 . - - X - R A Y I)IFI'RACTION POWDER I)ATA ( 2 D IN c m . ) (NPBr:)~

(NPBr~),

NPBr.,PBr5

i

N PBr.,PBr:

3.64 vw

5.48 m

4'95 w

4"71 s

4'48 vw

6"14 s

5'56 s

5"52 s

5'60 v,"

6'50

5"81 w

5"78

5-74 s

6"78 m

7"21 m

6"79 s

6"06 m

7"04 m

7.57 s

7.14 s

6.64 s

7.22 m

8"21 w

7.81

6.80s

7.81 s

8.71 w

8"30 m

6.94 s

8"22 s

9.32 w

8.73 m

7.45 m

8-50 w

9'68 w

9.06 m

m

m

8.31 s

8.74 w

9.97 m

8"41 s

9.36 w

11"02 w

10"11 m

9.48 s

9-96 m

11"34 w

10"73 m

9-85 w

10"40 w

12'24 w

11"43 m

I0.17 w

10"66 w

12.52 m

I l.gl w

10"60 w

11'18 vw

12.84 w

12.23 m

10"93 m

11.28 w

13.22 w

12.67 w

11-31 vw

11"70w

13-76 w

13.18 w

11"61 m

12"08

14.22 m

13"64 w

11"81 m

12'52m

14'57 m

14'05 vw

12"14 m

12"62 w

12'34 m

12'84 m

13"64 w

13"22 w

w

9 - 7 0 t,w

14'10 w 15"30 m rw-:vcryweak;

w.:

weak;

m .... m e d i u m ;

s..=strong

If the reaction temperature is decreased below 130°C, the yield of cyclic phosphonitrilic bromides is decreased sharply. At between 115 and 120°C, no cyclic compounds are obtained. Rather, large quantities of reddish, crystalline NPBr2PBr.~.... (n :.~- O)result, 2PBrs-I (2 !-n)Br 2 ! NH4Br---NPBr4PBrs+,,-!-4HBr This rather indefinite species loses bromine reversibly in steps, as NPBrzPBr.~ ~" red

room '[emp,'rature~ Br.,.

' , .\ . . . . .

NPBrzPBr 7 \~(,.-a0o~t:"\ NPBrzPBr ~ \ i(r.,. r~'d

yelh)w

Both NPBr2PBr 7 and NPBr2PBr 5 appear to be true compounds and not mixtures containing the cyclic phosphonitrilic bromides. Thus, each gives a distinctive X-ray diffraction pattern which contains no lines of the trimer or tetramer (Table 4), both ~10) N . L. PADDOCK a n d H . I ' . SEARLE, Adeances in Inorganic ChemiMrr and Radiochemistry, ( E d i t e d b y 11. J. E M E L / : ( J s a n d A. ( i . SHARm9 Vol. I, pp. 347 383. A c a d e m i c P r e s s . N e w Y o r k (1959).

202

K. JOHN and T. MOELLER

are insoluble in o r d e c o m p o s e d b y o r g a n i c solvents, each hydrolyses m u c h m o r e r a p i d l y t h a n the t r i m e r o r t e t r a m e r , a n d e x t r a c t i o n o f either with organic solvents yields no t r i m e r o r tetramer. T h e two c o m p o u n d s dissolve readily in liquid b r o m i n e a n d a d d b r o m i n e b y reversal o f the a b o v e equilibria. A l l o f these m a t e r i a l s p o l y m e r i z e a b o v e 120°C to small quantities o f t r i m e r a n d t e t r a m e r a n d s o m e w h a t larger quantities o f the higher h o m o l o g u e s . T h e infra-red spectra o f the c o m p o u n d s where n = 0 a n d 2 are s o m e w h a t less sharply defined than those o f the t r i m e r a n d tetramer, b u t they d o indicate the presence o f the P - N g r o u p (NaC1 region). N u c l e a r magnetic resonance d a t a c o u l d be o b t a i n e d only with liquid b r o m i n e solutions o f the samples. A l t h o u g h well-defined four-line a n d one-line spectra were o b t a i n e d for the n ==0 a n d 2 samples, respectively, they c a n n o t be considered definitive because the reversible reaction with b r o m i n e previously i n d i c a t e d u n d o u b t e d l y leads to mixtures. These c o m p o u n d s resemble closely similar c o m p o u n d s isolated in the c h l o r i d e series ~7~ a n d p r o p o s e d to have linear structures.* It is r e a s o n a b l e to r e g a r d t h e m as intermediates in the f o r m a t i o n o f the p h o s p h o n i t r i l i c halides f r o m the p h o s p h o r u s ( V ) halides by a n a l o g y to the c o m p o u n d CI~2[PNP(NH2)2~ z isolated from the reaction o f d i p h e n y l p h o s p h o r u s trichloride a n d a m m o n i a a n d p o l y m e r i z a b l e to d i p h e n y l p h o s phonitrile, ca1) EXPERIMENTAL Materials All chemicals were of the highest purity commercially available. Phosphorus(Ill) bromide and bromine were used without further purification. Ammonium bromide was dried over phosphorus(V) oxide and ground. Sym-tetrachloroethane and sym-tetrabromoethane were dried over phosphorus(V) oxide and distilled. The n-heptane used for fractional crystallization and the benzene used for extraction were dried with sodium. Solvents used for solubility studies were distilled.

Synthesis of compounds All syntheses were carried out with standardized quantities of reactants; namely 300 g (1 mole) of phosphorus(Ill) bromide, 350 g (2"19 mole) of bromine, 300 g (3-06 mole) of ammonium bromide, and ca. 600 ml of solvent. The temperature recorded for each synthesis is that of the oil bath used for heating. All yields are based upon the quantity of phosphorus(liD bromide used. 1. Reaction at low temperatures--synthesis of NPBrzPBrb and NPBr~PBr, (n > 5). To the standard mixture of phosphorus(Ill) bromide and ammonium bromide in sym-tetrachloroethane, approximately 150 g of bromine was added. The mixture was shaken thoroughly and then heated slowly to 115-118°C. The remaining bromine was then added. After the mixture had been held at this temperature range for 12-14 hr, the excess ammonium bromide was removed without cooling. The solution was then cooled slowly to 0°C to yield 290 g of dark red, crystalline "NPBr2PBrn (n > 5). Upon evaporation of excess bromine from the filtrate and concentration of the resulting solution, an additional 50 g of the product was obtained. The compound is insoluble in cold organic solvents, decomposed by hot organic solvents, readily soluble in liquid bromine, and very easily hydrolysed. On standing in a dry atmosphere at room temperature, it loses bromine slowly and reversibly to give red crystalline NPBr~PBrT. (Found: N, 1'81; P, 8.09; Br, 90'70. Calcd. for P2NBr,: N, 1.76; P, 7'79; Br, 90"45~/o). Seventeen and twenty-one hundredths grammes of NPBr~PBr7 was heated at 85°C for 2 days under a vacuum of 0"25 mm with the loss of 3.44 g (3.46 g theoretical) of bromine. The resulting bright orange compound is similar in properties to NPBr2PBr7 but has a different X-ray diffraction * The notations NPBrtPBr s and NPBrsPBr ~ are not intended to suggest structures. Although it is possible to formulate a number of simple or polymeric structural arrangements that preserve the indicated P-N bonding and do not violate the accepted concepts of electronic configuration and covalency, the available experimental data do not permit one to distinguish among them. It is apparent that a detailed structural analysis by X-ray methods is essential.
Phosphonitrilic bromides

203

pattern (Table 4). When heated above 110°C, it polymerizes. (Found: N, 2.28; P, 9.94; Br, 87.53. Calcd. for P2NBrT: N, 2"21; P, 9.75; Br, 88.04~). The compound NPBr,.PBr5 was treated with bromine, either as such or in an inert solvent, to yield NPBr2.PBr, (n ~ 7 or >7). Extraction with benzene or carbon disulphide removed no cyclic trimer or tetramer. Attempts to determine molecular weights in bromine by cryoscopic methods gave indeterminate results because of equilibrium addition and loss of bromine. An analogous behaviour has been observed for PBr5 itself in bromine. "2,;:" 2. Reactions at medium temperatures--synthesis of (NPBr2)3 and (NPBr2)~. As a normal procedure, the standard mixture of phosphorus(Ill) bromide and ammonium bromide in sym-tetrachloroethane with 200 g of bromine was heated to 140-142~C over a period of 5 days. The temperature was maintained at ca. 142°C for two days, and the remaining 150 g of bromine was added. The reaction was then stopped and the unreacted ammonium bromide removed by filtration. Evaporation of the solvent permitted recovery of 255.2 g of crude product. This was extracted with hot benzene to remove the trimeric and tetrameric phosphonitrilic bromides. After evaporating the benzene and subliming twice, 119-0 g (52"5 per cent yield) of trimer-tetramer mixture was obtained. In addition, 126.2 g (35.9 per cent yield) of NPBr2PBr5 resulted. A similar experiment employing the same initial conditions but maintaining the reaction mixture at 142'C for 6 days, yielded no NPBr2PBrs, 131.8 g (58-1 per cent) of trimer-tetramer mixture, and 73-5 g (32.4 per cent) of a mixture of higher homologous. In each instance, about 8 per cent of the tetramer was found in the trimer-tetramer mixture. A similar experiment carried out at 150-157~C for 160-167'C) for 6 days decreased the yield of trimertetramer mixture to 49'6 per cent (or 45'7 per cent), but increased the tetramer content to 14 per cent (or 20 per cent) while increasing the yield of higher homologues to 40 per cent (or 45 per cent). As an alternative, 300 g of ammonium bromide in 600 ml of.sTm-tetrachloroethane was heated to 132--134'C, and a mixture of 300 g of phosphorus(liD bromide and 250 e of bromine was added dropwise over a period of 14 hr. The mixture was then maintained at this temperature for 9 days. during which period the remaining 100 g of bromine was added. The procedure outlined above yielded 89.5 g (39-5 per cent yield) of trimer-tetramer mixture containing 5 per cent of the tetramer. The major product of the reaction was NPBrzPBr~. A similar experiment, differing only in the fact that the mixture was heated initially to 142~C and then maintained at 142-144~C for ten days, yielded 89.9 g (39-6% yield) of trimer-tetramer mixture containing ca. 14% of the telramer. A large quantity of higher homologues resulted. 3. Reactions at hti(her temperatures in sym-tetrabromoethane. Tile normal procedure outlined under (2) was repeated with substitution of sym-tetrabromoethane as solvent, increase of the temperature to 160'C over a period of 6 days, and maintenance of this temperature for an additional 5~ days. After removal of unreacted ammonium bromide and evaporation of the solvent tit 9 0 C (2 ram), a black, buttery residue was obtained. Penta-bromoethane resulting in the reaction was removed from this residue with small quantities of benzene, leaving 215 g (94"5 per cent yield) of crude product. Two sublimations yielded 72.9 g (32.1 per cent) of trimer-tetramer mixture containing 32-.35 per cent of the tetramer. A similar experiment, differing only in that the temperature was raised to 175'C and maintained there for 6 days, yielded 74.5 g (32'8 per cent) of trimer-tetramer mixture containing 4 3 ~ 6 per cent of the tetramcr. Another experiment at 180:C (10 days) gave 58"1 g (25.7 percent) of trimertetramer mixture containing about 50 per cent of the tetramer. Extensive bromination and decomposition of the solvent prevent the use of still higher temperatures and the probable further enhancement of the tetramer content.

Separation of (NPBr,,)3 from (NPBr2)4 By fractional sublimation, mixtures of trimeric and tetramcric phosphonitrilic bromides can be scparated into trimer-rich (90-100'C at 0.025 mm) and tetramer-rich (130-150 C at 0.025 mm) fractions, but the method is too time-consuming to effect complete separation. The same can be said of chromatographic separation on alumina with benzene or diethyl ether as solvent. Fractional crystallization from n-heptane or petroleum ether, b.p. 90--110'C (Table 1), gives the most efficient separation. The usual technique of fractional crystallization is employed. (Found: N, 6 8 7 ; P, 15.29; Br, 78.37 (n - 3). N, 6'79; P, 15.40; Br, 78.20 (n 4). Calcd. for (PNBr._,),,: ~;z~W. PLOr>J[KOW. Z. Phys. Chem. 48, 220 (1904). (;31 W. FINKELS'rEIN,Z. Phys. Chem. 105, 10 (1923).

204

K. JOHN and T. MOELLER

N, 6'84; P, 15'13; Br, 78.03~. Mol. wt. (cryoscopic in benzene). Found: 598. Calcd. for n ~ 3: 614.46. Found: 824. Calcd. for n ~ 4: 819'28).

Polymerization ofNPBr2PBr, (n ~_ 5) All polymerization reactions were carried out at temperatures between 130~C and 170°C. Because of the bromination~lebromination equilibrium, results obtained with all compounds of this type were comparable. 1. In the presence of ammonium bromide in sym-tetrachloroethane. A mixture of 103.2 g (0.13 mole) of NPBr2PBr7 and 50"7 g (0'51 mole) of ammonium bromide in 400 ml of ~Tm-tetrachloroethane was heated slowly to 140-145°C and maintained at this temperature for 4-5 days. Filtration permitted recovery of nearly all of the added ammonium bromide. Evaporation of the solvent showed the presence of some phosphorus(Ill) bromide. These observations ~ndicate that whatever the nature of the additional phosphorus in the compound, it is not present as adducted phosphorus(V) bromide. The dark brown, viscous liquid (87 g) remaining after evaporation of the solvent was shown by infrared examination to contain small quantities of trimeric and tetrameric phosphonitrilic bromides as well as the higher homologues in the series. The material was heated in a sublimation apparatus at 200°C and 0-25 mm. Some 9-9 g of trimer-tetramer mixture containing about 10 ~ of the tetramer together with small quantities of phosphorus(V) bromide was collected as a volatile fraction. The higher homologues were converted to phosphonitrilic bromide rubber. 2. In the presence or absence of ammonium bromide in a sealed tube. An intimate mixture of 5 g of NPBr2PBrb with 1 g of ammonium bromide was covered with a thick layer of ammonium bromide and sealed in a thick-walled tube. The tube was heated to 170°C over a period of 24 hr. The mixture turned brown and liquefied partially. The tube was opened and the lower homologues were extracted with carbon disulphide. Repeated sublimation gave 0.95 g of trimer-tetramer mixture containing 7-8 per cent of the tetramer. A similar experiment with the same quantity of NPBr2PBr7 yielded 0'93 g of trimer-tetramer mixture of the same composition. No trimer or tetramer could be detected in experiments carried out at 130-140°C. Comparable experiments at 170°C using 10 g quantities of NPBr2PBr5 and NPBraPBr7 in the absence of ammonium bromide gave 0"91 g and 0-92 g, respectively, of trimer-tetramer mixture, containing 7-8 per cent of tetramer, together with some phosphorus(V) bromide.

Solubility determinations Saturated solutions of the phosphonitrilic bromides were prepared by allowing the required solvent to equilibrate for two days at 25°C with an excess of the bromide. Tared aliquots of the solution were withdrawn, and the solvent was removed under vacuum. The residue was then weighed directly.

Instrumental studies The infra-red spectra in the sodium chloride region were obtained with a Model 21 Perkin-Elmer spectrometer, using a sodium chloride prism. The infra-red spectra in the cesium bromide region were obtained with a Perkin-Elmer Model 12 spectrometer modified to double pass action, using CsBr optics. The nuclear magnetic resonance spectra of phosphorus-31 were measured with a Model V-4300B Varian instrument, employing a Model V--4012-HR Varian magnet with a 16.2 Mc/s radiofrequency oscillator and a field of 9395 G. The X-ray diffraction powder patterns were obtained with a Hayes unit using nickel-filtered copper K~C radiation and a camera of 7'0 cm radius.

Analyses Microanalytical determinations of phosphorus, nitrogen, and bromine were carried out by the Clark Microanalytical Laboratory, Urbana, Illinois. Trimer-tetramer compositions were established either by infra-red spectroscopy or by molecular weight-determination. Acknowledgements--The authors are particularly indebted to Dr. C. F. CALLIS,Mr. J. J. YODER II1, and Mr. M. M. CRtrrCHF~ELDof the Inorganic Chemicals Division, Monsanto Chemical Company, St. Louis, Missouri, for the measurement of the nuclear magnetic resonance spectra. This investigation was supported by Contract AF 33(616)-5486 with the Materials Laboratory of Wright Air Development Command, Wright-Patterson Air Force Base, Ohio. Reproduction of this communication in whole or in part is permitted for any purpose of the United States Government.