Effect of N-substituents on protonation chemistry of trichlorophosphazenes

Article abstract of DOI:10.1016/S0020-1693(99)00385-0

The protonation chemistry of trichlorophosphazene (R¹-N=PCl₃) with sulfonic acids (R²SO₃H) was found to be affected by the N-substituents R¹, yielding bis(sulfonyl)imides containing both R¹ and R², and mixed sulfonylphosphonyl imides containing either R¹ or R². In the formation of the latter a hitherto unobserved chemistry occurred. An intramolecular 'imine SN₂' mechanism was proposed to rationalize the reactions observed. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00385-0

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Inorganica Chimica Acta 298 (2000) 16–23
Effect of N-substituents on protonation chemistry of
trichlorophosphazenes
Kang Xu *, C. Austen Angell
Department of Chemistry, Arizona State Uni6ersity, Tempe, AZ 85287-1604, USA
Received 10 May 1999; accepted 3 August 1999
Abstract
The protonation chemistry of trichlorophosphazene (R1ꢀNꢁPCl3) with sulfonic acids (R2SO3H) was found to be affected by the
N-substituents R1, yielding bis(sulfonyl)imides containing both R1 and R2, and mixed sulfonylphosphonyl imides containing either
R1 or R2. In the formation of the latter a hitherto unobserved chemistry occurred. An intramolecular ‘imine SN2’ mechanism was
proposed to rationalize the reactions observed. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Protonation chemistry; Trichlorophosphazenes; Bis(sulfonyl)imides
sively carboxylic acids, especially formic acid [24–26].
With those weak proton-donors the protonation chem-
istry was exhaustively investigated using almost every
known member of the phosphazene family, and it was
found that all of the protonation reactions conform to
a so-called ‘chlorine–oxygen exchange’ rule [22], where
the phosphorus dechlorinates and acquires oxygen, leav-
ing imidic nitrogen protonated as shown in Eq. (1).
1. Introduction
The first member of the trichlorophosphazene family,
trichlorophosphazosulfonyl chloride, ClSO2NꢁPCl3,
was made by Ephraim and Gurewitsch [1], but it was
mistakenly identified as the adduct of NH2SO2Cl and
PCl3. Not until four decades later did Kirsanov [2]
correct the error and embark on a three decade period
of fruitful research on phosphazene chemistry, during
which a large number of P- or N-substituted phos-
phazenes were synthesized [3–9]. Much effort has been
made to understand the toxicology of these due to their
importance as microbiocides [3–11]. Extensive knowl-
edge of their structure and physical properties was also
obtained using a variety of spectroscopic means includ-
ing IR [12], 31P, 15N and 35Cl NMR [13,14], photoelec-
tron spectrum [15] and dielectric studies [16] as well as
molecular orbital computation approaches [17].
(1)
Between the two protonated products shown in Eq.
(1), the O-protonated tautomer (phosphoric acid) was
found to be disfavored in the equilibrium with the
N-protonated tautomer (imidic acid) in most cases,
while phosphazenes with not so electron-deficient phos-
phorus centers, e.g. certain substituted ClnR23−nPꢁNꢀR1
[24] (R1=alkyl or acyl, R2=alkyl or phenoxide; n=1–
2), yield the O-protonated tautomer as the dominant
species.
The understanding of the phosphazene chemistry in-
cluded cyclo-dimerization [4,7,18,19], hydrolysis
[18,20,21], solvolysis with protic solvents like alcohols
and phenols [22,23], and acidolysis with almost exclu-
* Corresponding author. Present address: Electrochemistry Branch,
Sensor and Electronic Device Directorate, Mail Stop: AMSRL-SE-
DC, Army Research Laboratory, Adelphi, MD 20783-1197, USA.
Tel.: +1-301-721 3452; fax: +1-301-721 3402.
The only known exception to the ‘chlorine–oxygen
exchange’ rule was found in the acidolysis of
trichlorophosphazene with acids stronger than car-
E-mail address: kangxu@yahoo.com (K. Xu)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00385-0

K. Xu, C.A. Angell / Inorganica Chimica Acta 298 (2000) 16–23
17
boxylic acids, where PꢁN bond cleaves followed by the
formation of a bis(sulfonyl)imides [27–29] as shown in
Eq. (2):
procedures were carried out under the protection of N2
from a vacuum manifold. Trichlorophosphazenes with
general formula of R1NꢁPCl3 were prepared according
to Kirsanov with or without solvents [2,3]. Protonation
was conducted by adding sulfonic acids to
trichlorophosphazenes with vigorous stirring. Some pro-
tonation reactions were very exothermic and the reac-
tion mixture required cooling, using salt/ice bath. Other
protonations proceeded so sluggishly that heating was
required for the reaction to reach completion. In all
cases a cold trap cooled by acetone/dry ice was con-
nected to the reaction vessel so that the volatile products
could be collected and subjected to analysis. The imidic
acids were then fractionated under vacuum (between 0.1
and 5 mmHg) and collected at 1–2°C boiling range
(boiling points were not corrected). Recrystallization in
appropriate solvents was conducted when the proto-
nated products were solid at room temperature, and the
melting points were determined using differential scan-
ning calorimetry (Perkin–Elmer DSC-7).
(2)
This exception to ‘chlorine–oxygen exchange’ reac-
tion presents a potentially valuable route to the synthe-
sis of bis(sulfonyl)imide, which has been a challenge
because it is usually difficult to introduce the second
sulfonyl group onto the already non-nucleophilic sulf-
amide nitrogen center1 [30]. For example, a well-known
synthesis of bis(sulfonyl)imide involved laborious com-
bination of N-deprotonation/silylation as means of acti-
vating the nucleophilicity of nitrogen center, and
subsequent SiꢀF bond formation as additional driving
force, leading to a series of bis(perfluoroalkylsulfonyl)
imidic acids [31,32]. This procedure is far more compli-
cated compared with the acidolysis of phosphazene (Eq.
(2)), in which the tricholorophospho moiety acts as
activating agent, and relatively mild sulfonic acids are
used instead of their corresponding acid halides. How-
ever, to the best of our knowledge, no serious effort has
been made to explore the possibility of applying the
chemistry in Eq. (2) to the synthesis of bis(sul-
fonyl)imides, following the first reports by Appel et al.
[28] and Ruff [30].
In recent years, lithium salts based on bis(sul-
fonyl)imide anions have been given much attention due
to the discovery of their promising properties as elec-
trolyte solutes [33]. Because of the large anion size and
low melting point, this class of lithium salts has high
solubility and ion conductivity in non-aqueous solvents.
It is the interest of this research group in seeking new
low melting, stable lithium salts [34–37] that has led us
to explore an economical synthetic route to imide an-
ions with unsymmetrical structure. A serendipitous re-
sult of this synthetic endeavor is the further
understanding about the little-known protonation
chemistry of phosphazenes, which we report in this
paper.
For the liquid imidic acids the elemental analysis was
conducted on their lithium salts, which were made either
by metathesis with LiCl or by neutralization with LiH,
followed by purification through recrystallization from
appropriate solvents.
1H and 13C NMR spectra, using Si(CH3)4 as internal
reference, were collected on a Varian Gemini 300, while
31P and 19F NMR spectra with 85% H3PO4 as external
reference and CFCl3 as internal reference, respectively,
were obtained with an Oxford 400. IR spectra were
recorded with a Galaxy Series FT-IR model 2020 using
liquid film on KBr despite the fact that most of the
synthesized imidic acids react with KBr, rendering it
difficult to observe the characteristic NꢀH vibration.
Mass spectra (EI, Source temp. 250°C) were carried out
with a Finnigan MAT Model 312 using either ether or
acetone as carrier.
3. Results
The acidolysis of phosphazene with sulfonic acids was
found to strongly depend on N-substituent R1. Among
the substituents studied in this work, the protonation
reaction can be classified into three distinct categories as
summarized by Tables 1–3, in which the major struc-
tural identification data of the protonated products are
also listed along with the synthesis and physical proper-
ties.
2. Experimental
All sulfonic acids were obtained from commercial
sources. All solvents were distilled before use, while
other chemicals were used as received. All the synthetic
(1) R1 is alkyl, where ‘chlorine–oxygen exchange’ is
observed as shown by Eq. (3) in Table 1. The main
backbone of the molecule R1ꢀNꢁP remains intact while
N is protonated and P-oxygenated.
Fig. 1 shows the 31P NMR spectra of CH3ꢀNꢁPCl3
(a) and its protonated product CH3ꢀN(H)P(O)Cl2 (b). It
has been known that when R1 is alkyl, trichlorophos-
1 The basicity of nitrogen is much reduced by the strongly electron-
withdrawing sulfonyl group, e.g. compare pKa=3.8 for
RꢀNHꢀSO2R% with pKa=20–30 for RꢀNHꢀR% in aqueous media.

Table 1
R1=alkyl a: ‘chlorine–oxygen exchange’ rule
(3)
/
c
R1
R2
B.p.
(°C/mmHg)
Yield
(%) b
NMR (ppm, CDCl3)
1H
Major m/z
13C
31P
CH3
Cl
98/5
38
81
73
2.82 (d, JP(H 19.80 Hz, 3H),
5.75 (br, 1H)
2.80 (d, JP(H 19.80 Hz, 3H),
5.69 (br, 1H)
0.96 (t, 3H), 3.04 (m, 2H),
5.40 (br, 1H)
28.23 (s)
28.23 (s)
28.22 (s)
19.60 (q, JP(H
19.80 Hz)
19.61 (q, JP(H
19.80 Hz)
19.58 (q, JP(H
20.11 Hz)
147 (M+), 131 (NPOCl2+) 117 (POCl2+), 101 (PCl2+),
83 (POHCl+), 66 (PCl+
)
CH3
CF3
CH3
140/10
110/3
298
CH3CH2
161 (M+), 117 (POCl2+), 101 (PCl2+), 83 (POHCl+),
66 (PCl+), 44 (C2H5NH+
)
)
1
a The phosphazenes are in the form of dimer (Cl3PꢀNR1)2 as confirmed by their 31P NMR spectra. For example, in spectra for (Cl3PꢀNCH3)2 a heptet (l −78.30 ppm, JPꢀH 21.30 Hz) was
observed due to the PꢀH coupling with protons on two neighboring methyl groups.
b Isolated yields.
c Only the first 4–6 peaks in the relative abundance order are shown.

Table 2
R1=alkyl- or halosulfonyl: formation of bis(sulfonyl)imides
(4)
R1
R2
B.p.
(°C/mmHg)
Yield
(%)
NMR (ppm, CDCl3 or acetone-d6)
Major m/z
1H
13C
19F
ClSO2
ClSO2
ClSO2
F
115/2.0 a
95/1.0 b
110/8.0
70
88
72
9.88 (br)
9.83 (br)
11.25 (br)
58.56 (s)
197 (M+), 162 (FSO2NHSO2+), 113
/
(ClSO2N+), 99 (ClSO2+), 83 (SO2F+
)
Cl
213 (M+), 178 (Cl SO2NHSO2+), 113
(ClSO2N+), 99 (ClSO2+
)
CF3
118.38 (q, JCꢀF 317.20 Hz)
73.50 (s)
74.20 (s)
70.12 (s)
247 (M+), 211 (CF3SO2NSO2+), 177
(ClSO2NSO2+) 147 (CF3SO2N+), 133
(CF3SO2+), 69 (CF3+
)
CH3SO2
C6H5SO2
C6H5SO2
CF3
CF3
CH3
87/0.5
60
3.70 (s, 3H), 11.2 (br, 1H)
48.76 (s), 114.68 (q, JCꢀF 317.20 Hz)
227 (M+), 212 (CF3SO2NHSO2+), 157
(CH3SO2NSO2+), 147 (CF3SO2N+), 93
(CH3SO2N+), 69 (CF3+
)
298
85/0.1
39
7.5 (m, 3H), 7.7 (m, 2H),
11.67 (br, 1H)
48.70 (s), 114.71 (q, JCꢀF 318.30 Hz),
123.2 (s), 126.19 (s), 132.04 (s),
140.36 (s)
37.66 (s), 123.88 (q, JCꢀF 318.30 Hz),
125.1 (s), 130.2 (s), 136.1 (s)
289 (M+), 155 (C6H5SO2N+),
77 (C6H5+), 69 (CF3+
(
)
150(dec.)
ꢀ30 c
7.4 (m, 3H), 3.68 (s, 3H), 7.7 (m, 2H),
8.67 (br, 1H)
155 (C6H5SO2N+), 141 (C6H5SO2+), 93
(CH3SO2N+), 79 (CH3SO2+), 14 (CH2+
1
)
a White needle with m.p. 15.5°C.
b White needle with m.p. 35.5°C.
c The attempt to isolate this acid failed due to the decomposition upon distillation and the difficulty associated with recrystallization. The listed yield was estimated from that of the isolated
Li-salt. The instability of the acid is also reflected in the absence of M+ in its mass spectra.

20
K. Xu, C.A. Angell / Inorganica Chimica Acta 298 (2000) 16–23
phazenes tend to dimerize [4,7,18,19], forming a dative
bond between the electron-rich center N and electron-
deficient center P, as shown by the structure in Fig. 1(a).
The dimeric structure has been supported by molecular
weight measurement by ebullioscopy [7], while the heptet
P signal observed in the PꢀH coupling experiment (inset
of Fig. 1(a)) is a more direct evidence confirming the four
member-ring structure of this molecule, i.e. each P
nucleus is split by two neighboring methyl groups instead
of one as in a monomer. On the other hand, the
protonated compound, with a reduced electron donicity
on N, is shown to be an apparent monomer (inset of Fig.
1(b)), where only one methyl is adjacent to the P nucleus.
(2) R1 is sulfonyl, where bis(sulfonyl)imides are
formed as shown by Eq. (4) in Table 2. During the
reaction, R1ꢀN remains unchanged but the P-containing
moiety is replaced by the sulfonyl group bearing R2. We
think that reactions in this category deserve to be
exploited as simple synthetic routes to new imidic acids,
with yields ranging from fair to good.
(3) R1 is acyl, where a hitherto unobserved reaction
occurs with the formation of a mixed sulfonylphospho-
nyl imide, as shown by Eq. (5) above Table 3. The
process is a reversal of Eq. (3) above Table 1, i.e. R1 is
replaced by sulfonyl group bearing R2 while the P-con-
taining moiety is oxygenated.
4. Discussion
4.1. Protonation mechanism: weak acids
The Kirsanovian rule of protonation has been strictly
followed when the proton donor is a weak acid [18,20–
23], as shown in Eq. (1). Although this type of reaction
has been systematically studied, there has not been any
attempt to rationalize its possible mechanism.
Considering the much reduced basicity of N in
trichlorophosphazenes compared with ordinary amine
nitrogens, we believe that, instead of direct proton
addition to N, more likely the solvolysis/acidolysis with
weak proton donors (e.g. alcohol or carboxylic acids)
starts with an initial nucleophilic attack by oxygen in the
proton donor on the electrophilic P, which is then
followed by protonation on N:
(6)
In other words, the actual protonating agent is HCl, no
matter what protic agent (acid) is used. This seems

K. Xu, C.A. Angell / Inorganica Chimica Acta 298 (2000) 16–23
21
Fig. 1. 31P NMR spectra for CH3ꢀNꢁPCl3 (a) and its protonated product CH3ꢀN(H)P(O)Cl2 (b). Inset: 31P multiplicity with PꢀH coupling.
reasonable because, irrespective of the protic agent used,
the structure of the protonated product remains the
same.
could be considered a transition state in a more con-
certed mechanism) would undergo an intra-molecular
SN2 process, resulting in the formation of a new SꢀN
bond. The neutral, stable species POCl3, is eliminated in
a subsequent rearrangement resulting in the desired
bis(sulfonyl)imide (step 2 and 3 in Eq. (7)).
The above two step hypothesis was supported by the
observation that, while treatment of CH3ꢀNꢁPCl3 with
PhONa readily afforded nucleophilic substitution
product triester, CH3ꢀNꢁP(OPh)3, only protic agent
PhOH can lead to protonated product CH3ꢀNHꢀPOCl2
[23]. More convincing evidence is that, even when a
protic agent was present, if HCl produced from the initial
nucleophilic attack was rapidly removed from the reac-
tion medium, the protonation would not occur [38].
Thus, the proposed mechanism seems to hold true for
the protonation chemistry of trichlorophosphazenes
with weak proton donating agents (weak Brønsted
acids).
(7)
Apparently, the crucial factor governing the key step
in the formation of the new SꢀN bond is the N-nucleo-
philicity. It is a logical inference that, in the hypothet-
ical intermediate amide, the N-nucleophilicity is higher
than in either starting agent R1ꢀNꢁPCl3 or the interme-
diate imine in Eq. (6), in which case the above-pro-
posed intramolecular ‘imine SN2’ reaction mechanism
seems reasonable based on current knowledge about
N-basicity [39].
4.2. Protonation mechanism: strong acids
However, when the proton source is a strong proto-
nating agent (strong acid), the rule no longer holds true
in all the cases but depends on the N-substituents as we
have shown in Eqs. (2)–(5), i.e. only when R1 is alkyl,
is the Kirsanovian type of reaction observed (Eq. (3)).
Obviously, the change is caused by the higher protonat-
ing strength of the sulfonic acids compared with alco-
hols, phenols and carboxylic acids.
For sulfonic acids, the much lower O-donicity makes
it a dubious nucleophile for the electron-deficient P-cen-
ter; rather, the strongly acidic proton tends to function
as a specific proton catalyst. Furthermore, this protona-
tion of N would in turn increase the electrophilicity of
P, thus assisting an O-attack from the sulfonate anion
(step 1 in Eq. (7)). The resultant intermediate (which
Compared with the mechanism for weak acid proto-
nation (Eq. (6)), the chemistry in Eq. (7) with strong
acids represents ‘specific proton catalysis’ by the sul-
fonic acids used, i.e. the protonating agent here is the
protic agent itself. As would be expected the protonated
product bears the structural signature (R2ꢀSO2) of the
protic agents (Table 2).

22
K. Xu, C.A. Angell / Inorganica Chimica Acta 298 (2000) 16–23
4.3. Effect of N-substituent R1 on protonation
mechanism
trifluoromethanesulfonyl or trifluoroacyl, a hitherto un-
observed reaction occurs. The above chemistry can be
summarized in the following Scheme:
It should be pointed out that the four-member cyclic
ammonium intermediate in Eq. (7) is only hypothetical.
In reality the formation of the final product bis(sul-
fonyl)imide could have involved anything between a
multi-step process with distinctive intermediates and a
complete concerted one step process.
On the other hand, if such an intermediate does exist,
then indirect evidence for such a ‘proton catalysis’
rationale would be the formation of an N-sulfonation
product other than the products observed in either of
Eqs. (3) and (4), i.e. a third possibility which will
produce an R2-containing mixed phosphonylsul-
fonylimide, as follows in Eq (8):
Acknowledgements
This work is part of this laboratory’s search for
highly ionic non-crystallizing lithium salts [34–37],
which was funded by the Department of Energy under
grant number DEFG0289ER45398-004. The authors
want to thank Dr Camil Joubran for his assistance with
NMR spectra and Dr James Lehman with mass
spectra.
(8)
This is actually observed in Eq. (5) (Table 3), when
R1 is a very good leaving group, e.g. as trifluoro-
methanesulfonyl
(CF3SO2−
)
and
trifluoroacyl
(CF3CO−), and the R1-moiety can be readily removed.
To the best of our knowledge this phosphazene-proto-
nation chemistry has not been reported before. Note
the last two entries of Table 3, where the protonation of
different phosphazenes with the same acid CH3SO3H
results in identical protonated products. This should be
viewed as strong evidence that all the protonation
proceeds via a common intermediate as suggested in
Eq. (7).
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