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Ultraviolet absorption studies and analytical applications of the ether complexes of boron trifluoride and phosporus pentafluoride
Ar~ulyriccr Chitnica Acta, 67 ( 1973) 448452
448 Q
SHORT
Elscvicr Scicntilk
Publishing
Company.
Amsterdam
of the ether complexes
of
POBINER
Americcrn Cm (Rcccivcd
in The Ncthcrlands
COMMUNICATION
Ultraviolet absorption studies and analytical applications boron trifluoride and phosphorus pentafluoride
HARVEY
- Printed
Conrpu~~y, Priricctott
9th March
Laboratory.
P.O. Bos 50, Primetort,
N;J. 08540 (U.S.A.)
1973)
It has been observed in this laboratory that a new ultraviolet absorption spectrum accompanies complexation between ethers and certain volatile Lewis acids, such as boron trifluoride and phosphorus pentafluoride. This new ultraviolet spectrum, not shown by the individual reactants, is useful as a confirmatory test for these two Lewis acids. It has also been developed into a rapid quantitative analysis for the boron trifluoride, which may be used as a polymerization catalyst. Boron trifluoride and phosphorus pentafluoride readily form complexes with oxygen’compounds, such as ethers, which have unshared electron pairs to donate to available orbitals of the boron and phosphorus atoms’. For example, composition studies of the addition compounds of phosphorus pentafluoride with different ethers2 and of boron trifluoride with simple ethers3 have been reported. In the analytical study discussed here, a spectrally pure ether, such as p-dioxane, can act both as the electron donor for the complex with the Lewis acid fluoride, and, in excess, as the solvent in which to measure the ultraviolet spectrum. Experimental Sources of the Lewis acid halides were commercial boron trifluoride-ether complexes, pure anhydrous gases, and certain commercial compounds4 which act as iu situ generators of phosphorus pentafluoride on thermal decomposition, such as Phosfluorogen A (p-chlorobenzenediazonium hexafluorophosphate) and potassium hexafluorophosphate. Calibration with boron trifluoride. The following procedure permits the catalyst composition of a boron trifluoride-etherate stock solution used in polymerization to be monitored. In a nitrogen-purged glove box, prepare a solution containing 5 ml of boron trifluoride ethyl ether (Eastman, purified, BF, *O(C,H,),, No. 4272) made up to volume in a 250-ml volumetric flask with p-dioxane (Matheson, Coleman and Bell, Spectroquality, .No. DX 2095). Prepare a series of dilutions, containing aliquots from 1 ml to 20 ml of the master solution, in a series of 25-ml volumetric flasks, made up to volume with p-dioxane. The p-dioxane stabilizes the BF,etherate so that these dilutions can be made outside the glove box. Record the ultraviolet absorption spectrum of each dilution vs. p-dioxane in a l-cm cell in a double-beam spectrophotometer (e.g. Beckman DK-2A). Determine the absorp-
SHORT
449
COMMUNICATION
tion at the maximum, 275 nm, relative to a zero reading at 335 nm. Use density and chemical conversion factors to calculate the BF3 concentration in each dilution and prepare a calibration curve. Beer’s law is obeyed in the range O-10 g BF,. 1: I. Complexation with phosphorus pentafluoricle gas. The following procedure ‘is used for determining the ultraviolet spectrum of the phosphorus pentafluoridep-dioxane complex, followed by hydrolysis and determination of the fluoride ion as a measure of the PF, in the system. In a nitrogen-purged glove box. connect a lecture bottle of phosphorus pentafluoride (Matheson Company, East Rutherford, N.J.) via polyethylene tubing to a 500-ml gas washing bottle (Kontes No. K-65700) containing about 180 ml of dioxane. Bubble in the phosphorus penta~uoride gas until the solution turns red-brown. Then filter, in the glove .box, into a 250-mi volumetric flask and dilute to the mark with dioxane. Dilute as necessary in the glove box with dioxane to obtain an U.V. absorption maximum at 312 nm. : Into another gas washing bottle in the glove box, bubble phosphorus pentafluoride into the dioxane until red brown. At this point, add 5 ml of distilled water and dilute to 250 ml with dioxane. This hydrolyzed solution is stable and can be diluted for the spectral measurement. Dilute as necessary to obtain the absorbance of the band at 225 nm relative to a zero reading at 360 nm. Remove an aliquot, dilute in distilled water and determine fluoride ion colorimetrically5. Use the appropriate conversion factor to calculate the PI;, originahy in the sample. 0.9
0.6 .
‘5
0.7
0.2
0. I
0 210
235
260
310
285 WAVELENGTH
335
360
(nm)
Fig. I. Absorption spectra of the BF,-diethyl cthcrntc in dioxunc. Con~ntr~~tion, 5.0 g BF, I-‘. (A) In dioxanc, spcctroquality, run immediately. (B) Same as A. after 16 h. (C) In dioxanc containing 2% water, run immediately. (D) Same as C, after 16 h.
SHORT
450
COMMUNICATION
discussion
The spectrophotometric measurement of the Lewis acid-etherate complex is particularly useful as a quick monitor of the available catalyst (e.g. BF,) level in stock solutions of the etherate used in polymerization work. It is also useful as a confirmatory test for the detection of BF, or PFS that is liberated into a reaction vessel by the radiation of an adduct or compound containing the bound catalyst. For example, the PF, generated from thermal decomposition of the hexalluorophosphates, such as NaPF,, and the diazonium compounds, RN=NPFG4, can be detected by the U.V. measurement of the complex formed with ethers. Effect of moisture on the boron trifluoride-etherate U.U.spectrum. The dioxane has an apparent stabilizing effect on the boron trifluoride-diethylether commercial reagent. The latter reagent normally undergoes instantaneous hydrolysis, marked by strong fuming, outside a nitrogen-purged glove box, but in the presence of dioxane, as. in the calibration procedure, the visible evidence of hydrolysis is eliminated. The ultraviolet spectra in Fig. 1 demonstrate this stability. Curves A and B show that the boron trifluoride-diethyl etherate is reasonably stable after 16 h in dioxane, the flask being opened under normal conditions for periodic sampling. Curve C shows the decreased absorbance caused by instability in the presence of 2% water. Curve D shows complete disappearance of the absorption maximum, after 16 h, in the presence of 2% water, probably because of complete decomposition of the complex. ’ Use of other solvents for complexation with boron trifluoride. It must be emphasized that the absorption maximum at 275 nm is specifically a function of the boron tri~uoride~iethyi etherate, This maximum will be observed if the*dilution is in a non-ether solvent, such as isopropanol, but the stabilizing property of dioxane as the solvent should be noted. One can bubble anhydrous boron trifluoride gas, exclusive of its diethylether complex, into other ether solvents, and obtain different ultraviolet spectra. For example, the complex between boron trifluoride gas and tetrahydrofuran, as a cyclic ether donor-solvent, shows a U.V. maximum at 283 nm, and that between boron trifluoride gas and dioxane, will absorb at 298 nm. The latter two complexations have been used only qualitatively here. Analytical recovery and precision data. Table I shows that a 1.6% average TABLE
I
ANALYSES Sample
1 2 3 4 5 6
OF SYNTHETIC
BLENDS
OF p-DIOXANE
BF, (B !-‘I
CONTAINiNG
BF,-ETHERATE
“TODeaiation from theory
Theory
Found
1.25 2.13 2.61 4.27 5.10 10.46
1.25 2.18 2.54 4.25 5.23 10.30
0.0 -+ 2.3 -2.8 -0.5 +2.5 - 1.6 Average
1.6
-
SHORT
451
COMMUNICATION
deviation from theory is to be expected when analyzing for the boron trifluoridediethyl etherate, corresponding to concentrations of l-10 g BF, l- I, in dioxane. In a series of ten determinations at a concentration level of 5.0 g BF,. l-l, the relative standard deviation for the variance from the mean was 0.7%. Ejyect of moisture on the phosphorus pentafluoride-etherate U.V. kpectrum.. The absorbance band produced by the complexation of phosphorus pentafluoride with dioxane is a broad band with a maximum at 310-312 nm. The complex is susceptible to hydrolysis while in the absorption cell, and should only be used qualitatively to detect phosphorus pentafluoride liberated from catalysts. This hydrolytic instability is in agreement with the literature’** which indicates that the organic complexes of PFS are less stable than those of BF, and are rapidly decomposed by water. In the presence of 2% moisture, the absorption maximum at 312 nm is eliminated, and a new broad maximum appears at 223 nm. The phosphorus pentafluoride-dioxane solutions containing ?% moisture are spectrally stable for at least 2 h, do not exhibit any hydrolysis in the air, and can be quantitatively diluted. Synthetic mixtures of possible hydrolysis products, such as orthophosphoric acid with hydrofluoric acid in dioxane, do not produce this spectrum. This absorption at 225 nm is possibly due to some intermediate hydrolysis product of the phosphorus pentafluoride-etherate. These spectra are shown in Fig. 2.
0.9
0.6
0.7
0.6 = t E u
0.5
_ s
0.4
2 3 g
0.3
0
_._
ZIO
_-_
235
Fig. 2. Absorption spectra containing 2% water.
- -_
260
of the PF,-dioxanc
265
310
WAVELENGTH
tnm) (A) In dioxane,
complex.
335
spectroquality.
360
(B) In dioxane
452
SHORT
COMMUNICATION
Interferences are not a major consideration because the Interferences. analytical samples generally contain only one Lewis acid, either boron trifluoride or phosphorus pentafluoride, in the form of a compound or adduct. Phosphorus pentabromide and phosphorus pentachloride did not produce a new U.V. absorption spectrum in dioxane, nor did phosphorus trichloride and phosphorus oxychloride. l
REFERENCES F. A. Cotton and G. Wilkinson, Adouttced Inorganic Chemistry, Wiley, New York, 1962, pp. 155, 381. I. K. Gregor, Aust. J. C/Jew., 20 (1967) 775. T. A. Shchegoleva, V. D. Sheludyakov nnd B. M. h;likhnilov, Dokl. Akad. Nauk SSSR, 152 (1963) 888; Chern. Abstr., 60 (1964) 6455~. R. E. Kirk and D. F. Othmer, Ew~&yedia of Chernicul Tcci~nology, Vol. 9. Interscicncc-Wiley. New York, 2nd Ed., 1966, pp. 635. 644. J. M. Icken and B. M. Blank. Attar. C/rem, 25 (1953) 1741.
448 Q
SHORT
Elscvicr Scicntilk
Publishing
Company.
Amsterdam
of the ether complexes
of
POBINER
Americcrn Cm (Rcccivcd
in The Ncthcrlands
COMMUNICATION
Ultraviolet absorption studies and analytical applications boron trifluoride and phosphorus pentafluoride
HARVEY
- Printed
Conrpu~~y, Priricctott
9th March
Laboratory.
P.O. Bos 50, Primetort,
N;J. 08540 (U.S.A.)
1973)
It has been observed in this laboratory that a new ultraviolet absorption spectrum accompanies complexation between ethers and certain volatile Lewis acids, such as boron trifluoride and phosphorus pentafluoride. This new ultraviolet spectrum, not shown by the individual reactants, is useful as a confirmatory test for these two Lewis acids. It has also been developed into a rapid quantitative analysis for the boron trifluoride, which may be used as a polymerization catalyst. Boron trifluoride and phosphorus pentafluoride readily form complexes with oxygen’compounds, such as ethers, which have unshared electron pairs to donate to available orbitals of the boron and phosphorus atoms’. For example, composition studies of the addition compounds of phosphorus pentafluoride with different ethers2 and of boron trifluoride with simple ethers3 have been reported. In the analytical study discussed here, a spectrally pure ether, such as p-dioxane, can act both as the electron donor for the complex with the Lewis acid fluoride, and, in excess, as the solvent in which to measure the ultraviolet spectrum. Experimental Sources of the Lewis acid halides were commercial boron trifluoride-ether complexes, pure anhydrous gases, and certain commercial compounds4 which act as iu situ generators of phosphorus pentafluoride on thermal decomposition, such as Phosfluorogen A (p-chlorobenzenediazonium hexafluorophosphate) and potassium hexafluorophosphate. Calibration with boron trifluoride. The following procedure permits the catalyst composition of a boron trifluoride-etherate stock solution used in polymerization to be monitored. In a nitrogen-purged glove box, prepare a solution containing 5 ml of boron trifluoride ethyl ether (Eastman, purified, BF, *O(C,H,),, No. 4272) made up to volume in a 250-ml volumetric flask with p-dioxane (Matheson, Coleman and Bell, Spectroquality, .No. DX 2095). Prepare a series of dilutions, containing aliquots from 1 ml to 20 ml of the master solution, in a series of 25-ml volumetric flasks, made up to volume with p-dioxane. The p-dioxane stabilizes the BF,etherate so that these dilutions can be made outside the glove box. Record the ultraviolet absorption spectrum of each dilution vs. p-dioxane in a l-cm cell in a double-beam spectrophotometer (e.g. Beckman DK-2A). Determine the absorp-
SHORT
449
COMMUNICATION
tion at the maximum, 275 nm, relative to a zero reading at 335 nm. Use density and chemical conversion factors to calculate the BF3 concentration in each dilution and prepare a calibration curve. Beer’s law is obeyed in the range O-10 g BF,. 1: I. Complexation with phosphorus pentafluoricle gas. The following procedure ‘is used for determining the ultraviolet spectrum of the phosphorus pentafluoridep-dioxane complex, followed by hydrolysis and determination of the fluoride ion as a measure of the PF, in the system. In a nitrogen-purged glove box. connect a lecture bottle of phosphorus pentafluoride (Matheson Company, East Rutherford, N.J.) via polyethylene tubing to a 500-ml gas washing bottle (Kontes No. K-65700) containing about 180 ml of dioxane. Bubble in the phosphorus penta~uoride gas until the solution turns red-brown. Then filter, in the glove .box, into a 250-mi volumetric flask and dilute to the mark with dioxane. Dilute as necessary in the glove box with dioxane to obtain an U.V. absorption maximum at 312 nm. : Into another gas washing bottle in the glove box, bubble phosphorus pentafluoride into the dioxane until red brown. At this point, add 5 ml of distilled water and dilute to 250 ml with dioxane. This hydrolyzed solution is stable and can be diluted for the spectral measurement. Dilute as necessary to obtain the absorbance of the band at 225 nm relative to a zero reading at 360 nm. Remove an aliquot, dilute in distilled water and determine fluoride ion colorimetrically5. Use the appropriate conversion factor to calculate the PI;, originahy in the sample. 0.9
0.6 .
‘5
0.7
0.2
0. I
0 210
235
260
310
285 WAVELENGTH
335
360
(nm)
Fig. I. Absorption spectra of the BF,-diethyl cthcrntc in dioxunc. Con~ntr~~tion, 5.0 g BF, I-‘. (A) In dioxanc, spcctroquality, run immediately. (B) Same as A. after 16 h. (C) In dioxanc containing 2% water, run immediately. (D) Same as C, after 16 h.
SHORT
450
COMMUNICATION
discussion
The spectrophotometric measurement of the Lewis acid-etherate complex is particularly useful as a quick monitor of the available catalyst (e.g. BF,) level in stock solutions of the etherate used in polymerization work. It is also useful as a confirmatory test for the detection of BF, or PFS that is liberated into a reaction vessel by the radiation of an adduct or compound containing the bound catalyst. For example, the PF, generated from thermal decomposition of the hexalluorophosphates, such as NaPF,, and the diazonium compounds, RN=NPFG4, can be detected by the U.V. measurement of the complex formed with ethers. Effect of moisture on the boron trifluoride-etherate U.U.spectrum. The dioxane has an apparent stabilizing effect on the boron trifluoride-diethylether commercial reagent. The latter reagent normally undergoes instantaneous hydrolysis, marked by strong fuming, outside a nitrogen-purged glove box, but in the presence of dioxane, as. in the calibration procedure, the visible evidence of hydrolysis is eliminated. The ultraviolet spectra in Fig. 1 demonstrate this stability. Curves A and B show that the boron trifluoride-diethyl etherate is reasonably stable after 16 h in dioxane, the flask being opened under normal conditions for periodic sampling. Curve C shows the decreased absorbance caused by instability in the presence of 2% water. Curve D shows complete disappearance of the absorption maximum, after 16 h, in the presence of 2% water, probably because of complete decomposition of the complex. ’ Use of other solvents for complexation with boron trifluoride. It must be emphasized that the absorption maximum at 275 nm is specifically a function of the boron tri~uoride~iethyi etherate, This maximum will be observed if the*dilution is in a non-ether solvent, such as isopropanol, but the stabilizing property of dioxane as the solvent should be noted. One can bubble anhydrous boron trifluoride gas, exclusive of its diethylether complex, into other ether solvents, and obtain different ultraviolet spectra. For example, the complex between boron trifluoride gas and tetrahydrofuran, as a cyclic ether donor-solvent, shows a U.V. maximum at 283 nm, and that between boron trifluoride gas and dioxane, will absorb at 298 nm. The latter two complexations have been used only qualitatively here. Analytical recovery and precision data. Table I shows that a 1.6% average TABLE
I
ANALYSES Sample
1 2 3 4 5 6
OF SYNTHETIC
BLENDS
OF p-DIOXANE
BF, (B !-‘I
CONTAINiNG
BF,-ETHERATE
“TODeaiation from theory
Theory
Found
1.25 2.13 2.61 4.27 5.10 10.46
1.25 2.18 2.54 4.25 5.23 10.30
0.0 -+ 2.3 -2.8 -0.5 +2.5 - 1.6 Average
1.6
-
SHORT
451
COMMUNICATION
deviation from theory is to be expected when analyzing for the boron trifluoridediethyl etherate, corresponding to concentrations of l-10 g BF, l- I, in dioxane. In a series of ten determinations at a concentration level of 5.0 g BF,. l-l, the relative standard deviation for the variance from the mean was 0.7%. Ejyect of moisture on the phosphorus pentafluoride-etherate U.V. kpectrum.. The absorbance band produced by the complexation of phosphorus pentafluoride with dioxane is a broad band with a maximum at 310-312 nm. The complex is susceptible to hydrolysis while in the absorption cell, and should only be used qualitatively to detect phosphorus pentafluoride liberated from catalysts. This hydrolytic instability is in agreement with the literature’** which indicates that the organic complexes of PFS are less stable than those of BF, and are rapidly decomposed by water. In the presence of 2% moisture, the absorption maximum at 312 nm is eliminated, and a new broad maximum appears at 223 nm. The phosphorus pentafluoride-dioxane solutions containing ?% moisture are spectrally stable for at least 2 h, do not exhibit any hydrolysis in the air, and can be quantitatively diluted. Synthetic mixtures of possible hydrolysis products, such as orthophosphoric acid with hydrofluoric acid in dioxane, do not produce this spectrum. This absorption at 225 nm is possibly due to some intermediate hydrolysis product of the phosphorus pentafluoride-etherate. These spectra are shown in Fig. 2.
0.9
0.6
0.7
0.6 = t E u
0.5
_ s
0.4
2 3 g
0.3
0
_._
ZIO
_-_
235
Fig. 2. Absorption spectra containing 2% water.
- -_
260
of the PF,-dioxanc
265
310
WAVELENGTH
tnm) (A) In dioxane,
complex.
335
spectroquality.
360
(B) In dioxane
452
SHORT
COMMUNICATION
Interferences are not a major consideration because the Interferences. analytical samples generally contain only one Lewis acid, either boron trifluoride or phosphorus pentafluoride, in the form of a compound or adduct. Phosphorus pentabromide and phosphorus pentachloride did not produce a new U.V. absorption spectrum in dioxane, nor did phosphorus trichloride and phosphorus oxychloride. l
REFERENCES F. A. Cotton and G. Wilkinson, Adouttced Inorganic Chemistry, Wiley, New York, 1962, pp. 155, 381. I. K. Gregor, Aust. J. C/Jew., 20 (1967) 775. T. A. Shchegoleva, V. D. Sheludyakov nnd B. M. h;likhnilov, Dokl. Akad. Nauk SSSR, 152 (1963) 888; Chern. Abstr., 60 (1964) 6455~. R. E. Kirk and D. F. Othmer, Ew~&yedia of Chernicul Tcci~nology, Vol. 9. Interscicncc-Wiley. New York, 2nd Ed., 1966, pp. 635. 644. J. M. Icken and B. M. Blank. Attar. C/rem, 25 (1953) 1741.