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Spectral studies on pyridinium hexafluorophosphate
Spafrochimica
Acta.
Vol. 4lA,
No. 5, pp. 125-728,
1985
0584m8539185
Printedin Great Britain.
S3.00 + 0.00
0 1985 PergamonPressLtd.
Spectral studies on pyridinium hexafluorophosphate K. SYED Department
of Inorganic
and Physical
MOHAMED
Chemistry,
(Received 27 August
and D. K. Indian
PADMA*
Institute
1984; inJina/Jorm
of Science, Bangalore
5 Nouember
560012,
India
1984)
Abstract-Pure of pyridinium
samples of pyridinium hexafluorophosphate (C,H,&HPF;) were prepared by the reaction poly(hydrogen fluoride) and phosphorus(V) halides. The i.r. spectral data in the range 40&4000 cm-’ and ‘H, 19F and ” C NMR spectra for this compound are reported.
INTRODUCTION
Pyridinium hexafluorophosphate (C,H,hHPF,) is the starting material for the preparation of several metal hexafluorophosphates [l] as well as alkyl ammonium hexafluorophosphates [2]. These hexafluorophosphates have varied applications, such as corrosion inhibitors [3], catalysts for polymerization [4, 51, flame retardants [6], electrolytes in lithium anode batteries and the diazonium hexafluorophosphates have found extensive uses as catalysts for photopolymerization [7]. Spectral data for this compound have not been reported so far except for a short range i.r. spectrum (200&4000cm-‘) by NUTTAL et al. [S]. This paper reports the spectral data along with their assignments for pyridinium hexafluorophosphate. EXPERIMENTAL
Pyridinium hexafluorophosphate was prepared by reacting phosphorus(V) halide with pyridinium poly(hydrogen fluoride) in excess [ 11. Recrystallized samples (m.p. 175180°C) were used for the study. The i.r. spectrum was recorded with a Perkin-Elmer Model 599 spectrometer on the solid compound in KBr discs. The ‘Hand 13C NMR spectra were recorded on Varian T60 MHz and Brucker WH-270 MHz F-T NMR spectrometers, respectively, with DMSO-ds as solvent and TMS as internal standard. The 19F NMR spectrum was recorded on a Varian FT 80A (74.84 MHz) spectrometer in DMSO-d, as solvent (CFCI, internal standard). RESULTS
AND DISCUSSION
(i) Infrared spectrum The i.r. spectrum in the range 40&4000cm-1 is shown in Fig. 1. The probable assignments of various bands are presented in Table 1. In Table 1 the four absorption peaks at 3330,3210,3150 and 3120 cm-‘, appearing in the region 350&1650cm-‘, have been assigned to the N-H and C-H stretching vibrations. Thenext fourpeaksat 1640,1610,1540and 1490cm-’ in the region 1650_14OOcm-’ are assigned to the ring vibrations. In the region 1400-lOOOcm_’ there are six peaks, 1335, 1252, 1208, 1175, 1056 and 1002cm-‘, which are assigned to the six in-plane hydrogen
*Author to whom correspondence
should
be addressed. 725
deformation vibrations. These assignments for various absorptions are based on the vibrational spectral studies on other pyridinium salts by COOK [9]. In the region 90@4OOcm- ’ there are four peaks. The very strong peak (broad) at 830cm-’ and another sharp strong peak at 558 cm- ’ are assigned to the +(F,,) (P-F stretching) and v,(F,,) (P-F bending) vibrations, respectively, of the octahedrally symmetric (0,) PF; ion based on earlier reports for the hexafluorophosphate ion [lo]. The other two peaks at 740 and 670cm- ’ could be assigned to the ring breathing and bending modes of the pyridinium cation. For pyridinium hexafluorophosphate, NUTTAL et al. [8] report five peaks at 3230(s), 3190(s), 3110(s), 2920(s) and 2870(w) cm-‘. These authors assigned the first three peaks and the peak at 2870cm- ’ to the N-H stretching vibrations. The peak at 2920cm-’ was assigned to the C-H stretching vibration. It is rather surprising that these authors have not reported such a prominent peak at 3330cm-’ (Fig. 1) as recorded in the present study. On the other hand the two peaks at 2970 and 2870cm-’ reported by them have not been observed with any of the prepared samples. This strong peak at 3330 cm- ’ has been assigned to one of the modes of N-H stretching vibrations based on the report by COOK[~] that the N-H stretching vibration varies in frequency from 2439cm- ’ in pyridinium chloride to 3302cm-’ in pyridinium reineckate. The former lower value has been attributed to+the strong hydrogen bonding involved between the -N-H group and the chloride ion and the latter higher frequency being due to the absence of such hydrogen bonding. Crystallographic studies by COPELAND et a[. [ll] have shown that there is little hydrogen bonding present in pyridinium hexafluorophosphate. Thus, it is reasonable to expect a higher value for the N-H stretching vibration as found at 3330cm- ‘. This is further confirmed by deuteration of the samples by recrystallization from D,O, when the intensities of the peak at 3330 and 3280cm-’ decrease and two new peaks appear at 2475 (vH/vD = 1.34) and 2420cm:’ (vJv,, = 1.33). The intensities of the other peaks are also slightly affected, probably due to partial deuteration of the pyridine ring protons (Fig. 1b). The ratio of (v”/vo) reported for assigning the N-H vibrations is in the range 1.33-1.35 [9].
K. SYED MOHAMEDand D. K. PADMA
126
(b)
80 -
40000
3500
3000
2500
2000
1800
1600
1400
WAVENUMBER
Fig. 1. Infrared spectra of (a) pyridinium hexafiuorophosphate hexafluorophosphate. Table 1. Infrared spectral data for pyridinium hexafluorophosphate
1200
1000
@c 800
600
(on-‘)
and (b) partially deuterated pyridinium
The ‘H NMR spectrum of pyridinium hexafluorophosphate, reported for the first time, is given in Fig. 2
along with the spectrum of pyridine. The spectrum shows considerable deshielding of the pyridine ring protons. The characteristic complex multiplet of pyridine occurs at a lower field than that in pyridine. It cnxurs~~8.v^~lI’irrpyrn_nnetiar &?45o’foI+trIe salt This deshielding is due to the draining of electron density from the ring upon salt formation. In addition to the complex pattern, a new singlet appears at 13.15 6, which is shifted to 6.0 S 09 deuteration. This signal may be attributed to the -N-H proton of the pyridinium cation. The wide line nuclear magnetic resonZSSX SW+ 07 p~kV%Sm lSC?XaB#~~%a~e has
Ring vibrations 1640 s, 1610 vs, 1540 vs, 1490 vs In-plane hydrogen deformation vibrations 1335rt& ‘Q-ccct& XZII tn. ‘II% tt$.‘t-If&tn. ‘1WLm V#I”) 830 vs Ring vibrations 740 s, 670 s
V‘dFl”) 558s
(b)
I
I
13
: I
(ii) ‘H NMR spectrum
vcNH)and vccH)vibrations 333Ovs, 3210s 315Os, 3120s
14
400
I,
I
“9
I
I
I
I
I
8
7
9
8
7
PPM (S)
Fig. 2. ‘H NMR spectra of (a) pyridinium hexafluorophosphate
and (b) pyridine.
727
Pyridinium hexafluorophosphate
been reported by MATTHEWSet al. [ 121. However, they do not report the ‘H NMR spectrum of this compound. (iii) 13C NMR spectrum The 13C NMR spectrum of pyridine hexafluorophosphate has been recorded (DMSO-d6 solvent; TMS internal standard) and is reported for the first time. The spectrum, along with that of pyridine, is given in Fig. 3. The “C chemical shifts of pyridine, the pyridinium cation and pyridinium hexafluorophosphate are given in Table 2. T’he values observed for the pyridinium ion in pyridinium hexafluorophosphate are in good agreement with those reported
for the pyridinium cation. The chemical shifts for the pyridinium cation of the pyridinium hexafluorophosphate with respect to the j?- and ycarbon atoms move downfield: 123.94-127.25 and 136.14-146.43 ppm, respectively, while the a-carbon atoms show an upfield shift from 149.7 to 141.97 ppm. This upfield chemical shift of the a-carbon atoms can be expected when pyridine is converted to the pyridinium cation as there is a change in bond order (decreased bonding between N-C,) during salt formation. The downfield shift arises due to charge transfer polarization effects. A theoretical analysis of the chemical shift expression has been dealt with by PUGMIRE and GRANT[~~] who indicate that 13C
(a)
I
I
150
1.40
(b)
I
I
130
I
120 160
I
I
I
I
150
140
130
120
PPM (6)
Fig. 3. “C NMR spectra of (a) pyridinium hexafluorophosphate
and (b) pyridine.
Table 2. 13C Chemical shifts (ppm) of pyridine, pyridinium hexafluorophosphate and the pyridinium ion Pyridine C-Atom
Observed
Y
149.70 123.94 136.14
Reported[13] 150.20 123.90 135.95
CSH5NHPF,* 141.97 127.25 146.43
CSH,kHt 142.45 128.95 148.35
*Observed values for the pyridinium ion in pyridinium hexafluorophosphate. t Reported values for pyridinium ion [ 133.
K. SYEDMOHAMEDand D. K. PADMA
728
chemical shifts are critically dependent on both charge transfer features as well as variation in bond order parameters. (iv) 19F NMR spectrum The 19F NMR spectrum of pyridinium hexafluorophosphate in CH3CN (CFC13 internal standard) has been recorded for the first time and is given in Fig. 4. It shows a doublet of equal intensity at 71.66 ppm upfield
with respect to CFC13. This doublet is due to the coupling of the “P nucleus with spin l/2 with flourine in the PF, anion. The coupling constant Jp_F is found to be 702 Hz. These results are in good agreement with those reported by REDDYand SCHMUTZLER[14] for other compounds containing a hexafluorophosphate anion.
Acknowledgements-The
authors thank Professor A. R. VASUDEVA MURTHYfor his interest and one of the authors (K.S.M.) thank the UGC for the fellowship under the Faculty Improvement Programme.
REFERENCES [l]
[2]
CFC13
[3] [4]
[5] [6]
K. SYED MOHAMED,D. K. PADMA,R. G. KALBANDKERI and A. R. VASUDEVA MURTHY,J. Fluor. Chem. 23,509 (1983). K. SYED MOHAMED,D. K. PADMA~~~A. R. VASUDEVA MURTHY, paper presented at the International Conference on Phosphorus Chemistry, Nice, France, September 1983;abstract in Phosphorus-sulphur 18,428 (1983). H. P. HEUBUSCH,U.S. Nat. Tech. Inform. Serv. AD. Rep 1973 No. 17332/2GA, p. 121. M. C.THROCKMORTON, U.S. Pat. No. 3624OOO,CA.,76, 142 117 (30 November 1971). P. DREYFUSS and J. P. KENNEDY,J. Polym. Sci., Polym. Symp. (4th Int. Symp. Cationic. Polym.) 56, 129 ( 1976). P. J. PECK,P. G. GORDONand P. E. INGHAM,Text. Res. J. 46, 478 (1976).
r71_ KIRK-OTHMER,Encyclopedia of Chemical Technology, _
Vol. 10, pp. 785. Wiley, New York (1980).
r81 R. H. NUTTAL. D. W. A. SHARP and T. C.
I
I
I
I
0
60
70
80
PPM (6)
Fig. 4. 19FNMR
spectrum of pyridinium . . pnospnate.
hexafluoro-
L A WADDINGTON, J: them. Sot. 4965 (1960). [9] D. COOK, Can. J. Chem. 39, 2009 (1961). [lo] H. F. SHURVELL, Can. Spectrosc. 12, 156 (1967). [ 1 l] R. F. COPELAND,S. H. CONNERand E. A. MEYERS,J. phys. Chem. 70, 1288 (1968). [ 121 C. H. MAl-rHEWSandD. F. R. GILSON,Can. J. Chem. 48, 2625 (1970). [ 131 R. J. PUGMIREand D. M. GRANT,J. Am. them. Sot. 90, 697 (1968). [14] G. S. REDDYand R. SCHMUTZLER, Z. Naturforsch. 25b, 1199 (1970).
Acta.
Vol. 4lA,
No. 5, pp. 125-728,
1985
0584m8539185
Printedin Great Britain.
S3.00 + 0.00
0 1985 PergamonPressLtd.
Spectral studies on pyridinium hexafluorophosphate K. SYED Department
of Inorganic
and Physical
MOHAMED
Chemistry,
(Received 27 August
and D. K. Indian
PADMA*
Institute
1984; inJina/Jorm
of Science, Bangalore
5 Nouember
560012,
India
1984)
Abstract-Pure of pyridinium
samples of pyridinium hexafluorophosphate (C,H,&HPF;) were prepared by the reaction poly(hydrogen fluoride) and phosphorus(V) halides. The i.r. spectral data in the range 40&4000 cm-’ and ‘H, 19F and ” C NMR spectra for this compound are reported.
INTRODUCTION
Pyridinium hexafluorophosphate (C,H,hHPF,) is the starting material for the preparation of several metal hexafluorophosphates [l] as well as alkyl ammonium hexafluorophosphates [2]. These hexafluorophosphates have varied applications, such as corrosion inhibitors [3], catalysts for polymerization [4, 51, flame retardants [6], electrolytes in lithium anode batteries and the diazonium hexafluorophosphates have found extensive uses as catalysts for photopolymerization [7]. Spectral data for this compound have not been reported so far except for a short range i.r. spectrum (200&4000cm-‘) by NUTTAL et al. [S]. This paper reports the spectral data along with their assignments for pyridinium hexafluorophosphate. EXPERIMENTAL
Pyridinium hexafluorophosphate was prepared by reacting phosphorus(V) halide with pyridinium poly(hydrogen fluoride) in excess [ 11. Recrystallized samples (m.p. 175180°C) were used for the study. The i.r. spectrum was recorded with a Perkin-Elmer Model 599 spectrometer on the solid compound in KBr discs. The ‘Hand 13C NMR spectra were recorded on Varian T60 MHz and Brucker WH-270 MHz F-T NMR spectrometers, respectively, with DMSO-ds as solvent and TMS as internal standard. The 19F NMR spectrum was recorded on a Varian FT 80A (74.84 MHz) spectrometer in DMSO-d, as solvent (CFCI, internal standard). RESULTS
AND DISCUSSION
(i) Infrared spectrum The i.r. spectrum in the range 40&4000cm-1 is shown in Fig. 1. The probable assignments of various bands are presented in Table 1. In Table 1 the four absorption peaks at 3330,3210,3150 and 3120 cm-‘, appearing in the region 350&1650cm-‘, have been assigned to the N-H and C-H stretching vibrations. Thenext fourpeaksat 1640,1610,1540and 1490cm-’ in the region 1650_14OOcm-’ are assigned to the ring vibrations. In the region 1400-lOOOcm_’ there are six peaks, 1335, 1252, 1208, 1175, 1056 and 1002cm-‘, which are assigned to the six in-plane hydrogen
*Author to whom correspondence
should
be addressed. 725
deformation vibrations. These assignments for various absorptions are based on the vibrational spectral studies on other pyridinium salts by COOK [9]. In the region 90@4OOcm- ’ there are four peaks. The very strong peak (broad) at 830cm-’ and another sharp strong peak at 558 cm- ’ are assigned to the +(F,,) (P-F stretching) and v,(F,,) (P-F bending) vibrations, respectively, of the octahedrally symmetric (0,) PF; ion based on earlier reports for the hexafluorophosphate ion [lo]. The other two peaks at 740 and 670cm- ’ could be assigned to the ring breathing and bending modes of the pyridinium cation. For pyridinium hexafluorophosphate, NUTTAL et al. [8] report five peaks at 3230(s), 3190(s), 3110(s), 2920(s) and 2870(w) cm-‘. These authors assigned the first three peaks and the peak at 2870cm- ’ to the N-H stretching vibrations. The peak at 2920cm-’ was assigned to the C-H stretching vibration. It is rather surprising that these authors have not reported such a prominent peak at 3330cm-’ (Fig. 1) as recorded in the present study. On the other hand the two peaks at 2970 and 2870cm-’ reported by them have not been observed with any of the prepared samples. This strong peak at 3330 cm- ’ has been assigned to one of the modes of N-H stretching vibrations based on the report by COOK[~] that the N-H stretching vibration varies in frequency from 2439cm- ’ in pyridinium chloride to 3302cm-’ in pyridinium reineckate. The former lower value has been attributed to+the strong hydrogen bonding involved between the -N-H group and the chloride ion and the latter higher frequency being due to the absence of such hydrogen bonding. Crystallographic studies by COPELAND et a[. [ll] have shown that there is little hydrogen bonding present in pyridinium hexafluorophosphate. Thus, it is reasonable to expect a higher value for the N-H stretching vibration as found at 3330cm- ‘. This is further confirmed by deuteration of the samples by recrystallization from D,O, when the intensities of the peak at 3330 and 3280cm-’ decrease and two new peaks appear at 2475 (vH/vD = 1.34) and 2420cm:’ (vJv,, = 1.33). The intensities of the other peaks are also slightly affected, probably due to partial deuteration of the pyridine ring protons (Fig. 1b). The ratio of (v”/vo) reported for assigning the N-H vibrations is in the range 1.33-1.35 [9].
K. SYED MOHAMEDand D. K. PADMA
126
(b)
80 -
40000
3500
3000
2500
2000
1800
1600
1400
WAVENUMBER
Fig. 1. Infrared spectra of (a) pyridinium hexafiuorophosphate hexafluorophosphate. Table 1. Infrared spectral data for pyridinium hexafluorophosphate
1200
1000
@c 800
600
(on-‘)
and (b) partially deuterated pyridinium
The ‘H NMR spectrum of pyridinium hexafluorophosphate, reported for the first time, is given in Fig. 2
along with the spectrum of pyridine. The spectrum shows considerable deshielding of the pyridine ring protons. The characteristic complex multiplet of pyridine occurs at a lower field than that in pyridine. It cnxurs~~8.v^~lI’irrpyrn_nnetiar &?45o’foI+trIe salt This deshielding is due to the draining of electron density from the ring upon salt formation. In addition to the complex pattern, a new singlet appears at 13.15 6, which is shifted to 6.0 S 09 deuteration. This signal may be attributed to the -N-H proton of the pyridinium cation. The wide line nuclear magnetic resonZSSX SW+ 07 p~kV%Sm lSC?XaB#~~%a~e has
Ring vibrations 1640 s, 1610 vs, 1540 vs, 1490 vs In-plane hydrogen deformation vibrations 1335rt& ‘Q-ccct& XZII tn. ‘II% tt$.‘t-If&tn. ‘1WLm V#I”) 830 vs Ring vibrations 740 s, 670 s
V‘dFl”) 558s
(b)
I
I
13
: I
(ii) ‘H NMR spectrum
vcNH)and vccH)vibrations 333Ovs, 3210s 315Os, 3120s
14
400
I,
I
“9
I
I
I
I
I
8
7
9
8
7
PPM (S)
Fig. 2. ‘H NMR spectra of (a) pyridinium hexafluorophosphate
and (b) pyridine.
727
Pyridinium hexafluorophosphate
been reported by MATTHEWSet al. [ 121. However, they do not report the ‘H NMR spectrum of this compound. (iii) 13C NMR spectrum The 13C NMR spectrum of pyridine hexafluorophosphate has been recorded (DMSO-d6 solvent; TMS internal standard) and is reported for the first time. The spectrum, along with that of pyridine, is given in Fig. 3. The “C chemical shifts of pyridine, the pyridinium cation and pyridinium hexafluorophosphate are given in Table 2. T’he values observed for the pyridinium ion in pyridinium hexafluorophosphate are in good agreement with those reported
for the pyridinium cation. The chemical shifts for the pyridinium cation of the pyridinium hexafluorophosphate with respect to the j?- and ycarbon atoms move downfield: 123.94-127.25 and 136.14-146.43 ppm, respectively, while the a-carbon atoms show an upfield shift from 149.7 to 141.97 ppm. This upfield chemical shift of the a-carbon atoms can be expected when pyridine is converted to the pyridinium cation as there is a change in bond order (decreased bonding between N-C,) during salt formation. The downfield shift arises due to charge transfer polarization effects. A theoretical analysis of the chemical shift expression has been dealt with by PUGMIRE and GRANT[~~] who indicate that 13C
(a)
I
I
150
1.40
(b)
I
I
130
I
120 160
I
I
I
I
150
140
130
120
PPM (6)
Fig. 3. “C NMR spectra of (a) pyridinium hexafluorophosphate
and (b) pyridine.
Table 2. 13C Chemical shifts (ppm) of pyridine, pyridinium hexafluorophosphate and the pyridinium ion Pyridine C-Atom
Observed
Y
149.70 123.94 136.14
Reported[13] 150.20 123.90 135.95
CSH5NHPF,* 141.97 127.25 146.43
CSH,kHt 142.45 128.95 148.35
*Observed values for the pyridinium ion in pyridinium hexafluorophosphate. t Reported values for pyridinium ion [ 133.
K. SYEDMOHAMEDand D. K. PADMA
728
chemical shifts are critically dependent on both charge transfer features as well as variation in bond order parameters. (iv) 19F NMR spectrum The 19F NMR spectrum of pyridinium hexafluorophosphate in CH3CN (CFC13 internal standard) has been recorded for the first time and is given in Fig. 4. It shows a doublet of equal intensity at 71.66 ppm upfield
with respect to CFC13. This doublet is due to the coupling of the “P nucleus with spin l/2 with flourine in the PF, anion. The coupling constant Jp_F is found to be 702 Hz. These results are in good agreement with those reported by REDDYand SCHMUTZLER[14] for other compounds containing a hexafluorophosphate anion.
Acknowledgements-The
authors thank Professor A. R. VASUDEVA MURTHYfor his interest and one of the authors (K.S.M.) thank the UGC for the fellowship under the Faculty Improvement Programme.
REFERENCES [l]
[2]
CFC13
[3] [4]
[5] [6]
K. SYED MOHAMED,D. K. PADMA,R. G. KALBANDKERI and A. R. VASUDEVA MURTHY,J. Fluor. Chem. 23,509 (1983). K. SYED MOHAMED,D. K. PADMA~~~A. R. VASUDEVA MURTHY, paper presented at the International Conference on Phosphorus Chemistry, Nice, France, September 1983;abstract in Phosphorus-sulphur 18,428 (1983). H. P. HEUBUSCH,U.S. Nat. Tech. Inform. Serv. AD. Rep 1973 No. 17332/2GA, p. 121. M. C.THROCKMORTON, U.S. Pat. No. 3624OOO,CA.,76, 142 117 (30 November 1971). P. DREYFUSS and J. P. KENNEDY,J. Polym. Sci., Polym. Symp. (4th Int. Symp. Cationic. Polym.) 56, 129 ( 1976). P. J. PECK,P. G. GORDONand P. E. INGHAM,Text. Res. J. 46, 478 (1976).
r71_ KIRK-OTHMER,Encyclopedia of Chemical Technology, _
Vol. 10, pp. 785. Wiley, New York (1980).
r81 R. H. NUTTAL. D. W. A. SHARP and T. C.
I
I
I
I
0
60
70
80
PPM (6)
Fig. 4. 19FNMR
spectrum of pyridinium . . pnospnate.
hexafluoro-
L A WADDINGTON, J: them. Sot. 4965 (1960). [9] D. COOK, Can. J. Chem. 39, 2009 (1961). [lo] H. F. SHURVELL, Can. Spectrosc. 12, 156 (1967). [ 1 l] R. F. COPELAND,S. H. CONNERand E. A. MEYERS,J. phys. Chem. 70, 1288 (1968). [ 121 C. H. MAl-rHEWSandD. F. R. GILSON,Can. J. Chem. 48, 2625 (1970). [ 131 R. J. PUGMIREand D. M. GRANT,J. Am. them. Sot. 90, 697 (1968). [14] G. S. REDDYand R. SCHMUTZLER, Z. Naturforsch. 25b, 1199 (1970).