Gas-phase on-line generation and infrared spectroscopic investigations of polyphosphazenes, (NPX2)3 where X = F, Cl and Br

Article abstract of DOI:10.1016/j.saa.2004.11.058

Gas-phase infrared spectra of polyphosphazenes (phosphonitrilic halides trimer), (NPX₂)₃ where X = F, Cl and Br have been recorded. The molecules were generated for the first time by an on-line process using solid (NPCl₂)₃ as a precursor passed over heated sodium fluoride and potassium bromide at about 550 and 700 °C for (NPF ₂)₃ and (NPBr₂)₃ production, respectively. The products were characterized by the infrared spectra of their vapors. The low-resolution gas-phase Fourier transform infrared spectra reported for the first time show strong bands centered at 1295, 1215 and 1200 cm ^-1, assigned to ν₇(E′), in plane PN stretching mode of (NPX₂)₃, where X = F, Cl and Br, respectively.

Full text of DOI:10.1016/j.saa.2004.11.058

Spectrochimica Acta Part A 61 (2005) 1499–1503
Gas-phase on-line generation and infrared spectroscopic investigations of
polyphosphazenes, (NPX2)3 where X = F, Cl and Br
Abdul W. Allaf∗
Atomic Energy Commission of Syria, Department of Chemistry, P.O. Box 6091, Damascus, Syria
Received 24 June 2004; received in revised form 26 October 2004; accepted 5 November 2004
Abstract
Gas-phase infrared spectra of polyphosphazenes (phosphonitrilic halides trimer), (NPX2)3 where X = F, Cl and Br have been recorded. The
molecules were generated for the first time by an on-line process using solid (NPCl2)3 as a precursor passed over heated sodium fluoride and
potassium bromide at about 550 and 700 ◦C for (NPF2)3 and (NPBr2)3 production, respectively.
The products were characterized by the infrared spectra of their vapors. The low-resolution gas-phase Fourier transform infrared spectra
reported for the first time show strong bands centered at 1295, 1215 and 1200 cm−1, assigned to ν7(Eꢀ), in plane P N stretching mode of
(NPX2)3, where X = F, Cl and Br, respectively.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Gas-phase; On-line generation; Polyphosphazenes; Infrared; Phosphonitrilic halides trimer
1. Introduction
The cyclic trimer (NPF2)3 has an accurately planar six-
membered ring (D3h symmetry) in which all six P N dis-
tances are equal. Most other trimers, (NPCl2)3 and (NPBr2)3,
are almost planar (pseudo chair structure). The planarity of
the ring, the equal P N bond distances, the shortness of the
P N bonds and the stability of the compounds suggest the
delocalization behavior [1,2].
Polyphosphazenes or phosphonitrilic halides trimer,
(NPX2)3 where X = F, Cl and Br were prepared long time
ago. The synthesis of the fluoro analogues, (NPF2)3 was first
made in 1956 and the bromo compound (NPBr2)3 was pre-
pared in 1960, where the chloro analogues was made much
earlier in 1924. The basic preparation for the laboratory and
industrial scales of the chloro analogue was applied under the
following reaction: nPCl5 + nNH4Cl → (NPCl2)n + 4nHCl
using the appropriate solvents at about 120–150 ◦C [1]. By
varying the conditions, yields the cyclic trimer or tetramer
and the other oligomers can be optimized and the compounds
can be separated by the fractionation.
Analogues bromo compounds may be prepared in the
same manner, except that bromine must be added to suppress
the decomposition of the phosphorus penta-bromide. The
analogues fluoro can be prepared indirectly by fluorination
of the chloro analogues [1].
˚
The geometry for (N◦PF2)3 are as◦follows: P N◦1.56 A,
˚
P F 1. 51 A, ∠NPN 121 , ∠PNP 119 and ∠FPF 99 where
˚
the geometry for (NPCl2)3 are as foll◦ows: P N 1.58 A, P Cl
(Scheme 1).
◦
◦
˚
1.97 A, ∠NPN 118.4 , ∠PNP 121.4 and ∠ClPCl 102 [2]
In the late sixties of the last century, two spectroscopic
papers were published regarding the phosphonitrilic halides
trimer, (NPX2)3 where X = F, Cl and Br but in the liquid and
crystal states [3,4]. The first paper was published by Manley
and Williams who recorded the infrared spectra of (NPBr2)3
and (NPCl2)4 in the range 40–3000 cm−1 and the materials
were either presented in CCl4 solutions or in solid state [3].
Their Raman data indicated that (NPBr2)3 in solution has D3h
symmetry, but in the crystal form showed a slight deviation
from D3h to C3v symmetry, as in (NPCl2)3. The fundamental
frequencies of both (NPCl2)3 and (NPBr2)3 in comparison
∗
Tel.: +963 11 6111926; fax: +963 11 6112289.
E-mail addresses: aallaf@aec.org.sy, scientific@aec.org.sy
(A.W. Allaf).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.11.058

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A.W. Allaf / Spectrochimica Acta Part A 61 (2005) 1499–1503
vestigation reported in the literature before for the polyphos-
phazenes compounds, (NPX2)3 where X = F, Cl and Br. This
paper investigates the first attempt of gas-phase on-line pro-
duction of polyphosphazenes, (NPX2)3 where X = F and Br
using (NPCl2)3 as a starting material passed over heated
sodium fluoride and potassium bromide at about 550 ◦C and
700 ◦C for (NPF2)3 and (NPBr2)3 production, respectively
and the products were characterized by the infrared spec-
tra of their vapors. Also, semi empirical calculations have
been performed on (NPBr2)3 based on the D3h symme-
try.
2. Experimental
Scheme 1. The schematic resonance structure of (NPX2)3 where X = F, Cl
and Br showing the delocalisation of ␲-bonding.
The first step was to optimize the production conditions of
the chloro analogue, (NPCl2)3, before attempting to observe
the following analogues, (NPF2)3 and (NPBr2)3.
with (NPF2)3 were assigned on the basis of D3h symmetry
[3]. The second paper was published by Stahlberg and Steger
who reported the infrared spectra of triphosphonitrilic chlo-
ride bromide, P3N3ClnBr6 − n where n = 0–6, between 200
and 4000 cm−1 in the solid state [4]. The Raman spectra were
recorded for the previous species with solid samples and for
P3N3Cl5Br only in solution including the polarization mea-
surements. The authors reported that the decrease of symme-
try has little effect on the spectra because the P Cl and P Br
bonds are very similar. But the effects surpass those of the
crystal symmetry, C3v as known for P3N3Cl6 and P3N3Br6
[4].
In 1970, Emsley reported the infrared and Raman spec-
tra of the triphosphonitrilic fluoride chloride, P3N3FnCl6 − n
where n = 0–6, between 300 and 2500 cm−1 including the
related P3N3Cl6 and P3N3F6 compounds [5]. The solids
P3N3Cl6 and P3N3FCl5 were studied as Nujol mulls and in
the liquid state as liquid films; P3N3F6 was studied in CS2
and CCl4 solutions above 650 cm−1 and as the vapors below
650 cm−1 [5].
The (NPCl2)3 solid material (Fluka, 99.99%) was heated
alone in the vacuum line to about 106 ◦C using heating tape
in order to obtain enough vapor pressure and recording the
gas-phase infrared spectrum of this starting material. Then,
this vapor was used and passed directly via a vacuum line
over the chosen heated salt by an on-line process into an IR
cell in order to produce (NPF2)3 and (NPBr2)3. The remain-
ing conditions were slow flow, 2 cm−1 resolution and un-
der 0.4–0.5 Torr pressure. The synthetic reaction routes for
the preparation of (NPF2)3 and (NPBr2)3 can be written as
follows:
550 ◦C
(NPCl2)3(g) + 6NaF(s) −→ 6NaCl(g) + (NPF2)3(g)
700 ◦C
(NPCl2)3(g) + 6KBr(s) −→ 6KCl(g) + (NPBr2)3(g)
The salts containing halogens used in these experiments
were obtained from Aldrich with GR purity. The products
from these reactions were passed directly into an IR cell
(15 cm path length) fitted with two KBr windows.
The KBr optics is used to cover the conventional medium
region of the FTIR Spectrometers (400–4000 cm−1) due to
the noninterference of KBr bond with the recorded spectra of
that region.
¨
Schnockel and co-workers recorded the matrix isolation
infrared spectrum of P
O species (C∞v) in the Argon
atmosphere [6,7]. Both fundamental stretching, N O and
N, vibrations were observed at 1754.7 and 865.2 cm−1
respectively.
N
P
,
Atkins and Timms recorded the matrix isolation infrared
spectrum of (PN)3 species (D3h symmetry) in the Krypton
atmosphere [8]. Both eꢀ (P N stretch) and eꢀ (P N stretch)
frequencies were observed at 1137 and 718 cm−1, respec-
The distance between the heated furnace and the IR cell
was about 3 cm.
In order to avoid any disproportionation or polymeriza-
tion, the IR cell was heated to about 200 ◦C with heating
tape. The vacuum line system was pumped out via a liq-
uid nitrogen trap with a rotary pump (ILMVAC) displacing
23 m3 h−1. The infrared spectra were recorded on a JASCO
300E FTIR spectrometer with a resolution of 2 cm−1. Similar
experimental setup can be seen in our previous work which
was focused on the gas-phase on-line preparation and the
infrared spectroscopic investigation of phosphoryl halides,
POX3 where X = F, Cl, Br and I [11].
¨
tively. The work was extended and confirmed by Schnockel
and co-worker who recorded the matrix isolation infrared
spectrum of only one fundamental eꢀ (P N stretch) in the
Argon atmosphere at 1141 cm−1 [9]. The matrix isolation
infrared frequency of the elusive species P N, which was
produced from the trimer, has been observed at 1323 cm−1
[8,10].
It can be noticed from the literature survey that, there is no
gas-phase on-line generation and infrared spectroscopic in-

A.W. Allaf / Spectrochimica Acta Part A 61 (2005) 1499–1503
1501
signed to ν14(Aꢀ2ꢀ), the ring deformation, out of ring plane.
This band can be compared with the same mode observed
in solid (NPCl2)3 which has a value of 601 cm−1 [13]. It is
well known that there could be a shift in the band position
between the gas-phase and the solid states. The shift of about
20 cm−1 is justified and acceptable.
3. Results and discussion
It should be pointed out here first, that there are 20 funda-
mental modes of vibrations regarding the (NPX2)3 molecules
based on the D3h symmetry.
The fundamental symmetry modes of vibration can be
divided as follows:
The third band at 540 cm−1 has been assigned to the
ν15(Aꢀ2ꢀ), the P Cl2 asymmetric stretch in plane vibration of
(NPCl2)3.
4A1ꢀ + 2A2ꢀ + 1Aꢀ2ꢀ + 3Aꢀ2ꢀ + 6Eꢀ + 4Eꢀꢀ
where the Aꢀ1 and Eꢀꢀ are Raman active only, Aꢀ2ꢀ is infrared
active only, Eꢀ is active in both infrared and Raman and A2ꢀ
and Aꢀ1ꢀ are in active in both effects. The assignments of the
recordedgas-phaseinfraredspectraarebasedontheD3h sym-
metry.
This band can be compared also with the similar modes
observed in solid and liquid form of (NPCl2)3 which has a
frequency of 540 and 544 cm−1, respectively [13,12]. Our
recorded results are consistent and very close and some fre-
quencies are identical with those reported earlier [12,13].
The remaining small bands at 671, 562 and 410 cm−1
are not assigned and could be due to the combination
frequencies.
Having confirmed the existence of gas-phase infrared
spectrum of (NPCl2)3, the next step was to record the gas-
phase infrared spectrum of (NPF2)3 using (NPCl2)3 as a start-
ing material by an on-line process of preparation.
Fig. 2 shows the gas-phase infrared spectrum of (NPF2)3.
This spectrum was the result of passing a controlled vapor
pressure of (NPCl2)3 precursor over heated sodium fluoride
at about 550 ◦C. Three characteristic bands were observed at
1295, 980 and 868 cm−1 in the range 600–1500 cm−1. The
first band centered at 1295 cm-1 is assigned to ν7(Eꢀ), in plane
P N stretching mode of (NPF2)3.
Second, it should be mentioned also that a complete halo-
gen exchange reaction between (NPCl2)3 gas and the solid
NaF or KBr by an on-line process to produce N3P3F6 and
N3P3Br6, respectively, is in general far from being trivial
due to the appearance of the precursor bands under the ap-
plied gas-phase on-line production conditions. Nevertheless,
we could observe some of the most characterized fundamen-
tal modes of N3P3F6 and N3P3Br6 in the gas-phase within
the range of the spectrometer used.
Fig. 1 shows the gas-phase infrared spectrum of (NPCl2)3
in the range (400–1300 cm−1). This spectrum was the re-
sult of heating (NPCl2)3 solid material only in the vacuum
line to about 106 ◦C in order to record the gas-phase infrared
spectrum of this starting material before generating (NPF2)3
and (NPBr2)3 compounds by an on-line process. Six bands
centered at 1215, 671, 621, 562, 540 and 410 cm−1 were
observed. The first band centered at 1215 cm−1 is assigned
to ν7(Eꢀ), in plane P N stretching mode of (NPCl2)3. This
band can be compared with the similar mode observed in
the liquid and solid (NPCl2)3 which has a frequency of 1226
and 1210 cm−1, respectively [3,12,13]. The band has a PR
type structure and the two sides frequencies are at 1232 and
1206 cm−1. The observed frequency at 1215 cm−1 lies within
the acceptable range. The second band at 621 cm−1 is as-
This band can be compared with the similar mode ob-
served in the solid state of (NPF2)3 which has a value
of 1305 and 1300 cm−1, respectively [14,15]. The band
has a PR type structure as in the previous chloro ana-
logues and the two sides frequencies are at 1310 and
1271 cm−1
.
This results is consistent with the expectation and the band
is shifted to higher frequency when fluorine replaces the chlo-
rine in (NPCl2)3 in order to form (NPF2)3.
Fig. 1. Gas-phase infrared spectrum of (NPCl2)3. Three characteristic bands
Fig. 2. Gas-phase infrared spectrum of (NPF2)3. Three characteristic bands
were observed at 1295, 980 and 868 cm−1 in the range 600–1500 cm−1
.
were observed at 1215, 621 and 540 cm−1 in the range 400–1300 cm−1
.

1502
A.W. Allaf / Spectrochimica Acta Part A 61 (2005) 1499–1503
These results is consistent also with the expectation and
the band is shifted to lower frequency when bromine replaces
the chlorine in (NPCl2)3 in order to form (NPBr2)3.
The second band observed at 560 cm−1 is assigned is as-
signed to ν14(Aꢀꢀ), the ring deformation out of ring plane. This
band can be co2mpared with the same mode observed in the
solid (NPBr2)3 which has a value of 544 cm−1 [3]. The shift
between the solid and the gas-phase results is about 16 cm−1
.
The third band at 532 cm−1 has been assigned to ν15(Aꢀꢀ),
the P Br2 asymmetric stretch in plane vibration and the mo2de
can be compared with the similar mode observed in the liquid
form which has a value of 532 cm−1 [3].
The remaining bands shown in Fig. 3 are due to the starting
material or to the impurities.
Semi empirical calculations for (NPBr2)3 based on the
D3h symmetry have been performed on (NPBr2)3 using
Gamess-UK package in order to estimate the geometry of
Fig. 3. Gas-phase infrared spectrum of (NPBr2)3. Three characteristic bands
were observed at 1200, 560 and 532 cm−1 in the range 500–1300 cm−1
.
˚
this molecule. The geometry is as follows: P N 1.58 A, P Br
◦
◦
2.12 A, ∠NPN 117.3 , ∠PNP 123.2 and ∠BPBr 103.2◦
˚
The second band at 980 cm−1 is assigned to ν14(Aꢀ2ꢀ), the
ring deformation, out of ring plane. This band can be com-
pared with the same mode observed in the solid (NPF2)3
which has a value of 975 cm−1 [14].
(Scheme 1).
All the observed frequencies (cm−1) of (NPX2)3 where
X = F, Cl and Br determined by this gas-phase infrared spec-
troscopy work along the assignments are listed in Table 1.
The third band at 868 cm−1 has been assigned to ν15(Aꢀ2ꢀ),
the P F2 asymmetric stretch in plane vibration of (NPF2)3.
This band can be compared with the similar mode observed
in the solid state of (NPF2)3 which has a value of 860 and
865 cm−1, respectively [14,15]. It can be deduced that the
reportedresultsareveryconsistentwiththepreviousrecorded
results.
4. Conclusion
The present work reports for the first time, the gas-phase
on-line production and detection of (NPX2)3 where X = F,
Cl and Br using FTIR technique. Our recorded results are
consistent with the solid and liquid states of the investigated
compounds which were reported earlier. Satisfactory trend
was observed from fluoro to bromo analogues.
Fig. 3 shows the gas-phase infrared spectrum of (NPBr2)3
in the range 500–1300 cm−1 This spectrum was the result of
passing a controlled vapor pressure of (NPCl2)3 precursor
over heated potassium bromide at about 700 ◦C.
The first small band centered at about 1200 cm−1 is ten-
tatively assigned to ␯7(Eꢀ), in plane P N stretching mode of
(NPBr2)3. This band can be compared with the similar mode
observed in the liquid and solid form of (NPBr2)3 which has
a frequency of 1173 and 1180 cm−1, respectively [3,4]. The
shift range between the previous reported and the gas-phase
results are about 20–27 cm−1. This band has also a PR type
structure and can be compared with the previous fluoro and
Acknowledgments
The author would like to thank Professor I. Othman,
the Director General and Professor G. Zayzafoon, Heads of
Chemistry Department, for their encouragement and support.
I am very grateful to Professors H. Kellawi, Ahmad Hajj Said,
F. Kandeel and A.M. Sheikh Hussein for reviewing the work
and Miss D. Naima and Mr. M.N. Odeh for setting up the
experiment.
chloro analogues, which have values of 1295 and 1215 cm−1
,
respectively. Therefore, a nice trend has been seen starting
from (NPF2)3 to (NPBr2)3.
Table 1
The gas-phase observed frequencies (cm−1) of (NPX2)3 where X = F, Cl and
Br
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Molecule
ν7(Eꢀ), in plane
␯
14(Aꢀ2ꢀ), the ring
␯
15(Aꢀ2ꢀ), the P X2
asymmetric stretch
in plane vibration
(cm−1
)
P
N stretching
deformation, out
of ring plane
(cm−1
vibration (cm−1
)
)
(NPF2)3
(NPCl2)3
(NPBr2)3
1295
1215
1200 (T)
980
625
560
868
540
532
(T): Tentative assignment.

A.W. Allaf / Spectrochimica Acta Part A 61 (2005) 1499–1503
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