Rapid degradation of cyclic peroxides by titanium and antimony chlorides

Article abstract of DOI:10.1039/c5dt00200a

First responders face extraordinary risks when dealing with organic peroxides such as TATP due to the extreme sensitivity of this class of explosives, and to a lack of a robust chemical means of safe and rapid neutralisation. The Lewis acids TiCl₄ and SbCl₃ have been found to demonstrate a novel degradation mechanism, with TiCl₄ degrading a model cyclic peroxide in minutes when used in a two-fold excess, or ～3 hours at half equivalence. The products cannot re-form peroxide compounds as is the case with acid degradation, suggesting the two mechanisms are fundamentally different. The Lewis acids mediate a rearrangement reaction in the cyclic peroxide backbone leading to relatively innocuous products through deactivation of oxidising O. Sub-stoichiometric TiCl₄ reactions highlight a secondary reaction pathway that also leads to some oxidative chlorination products, possibly mediated by an unconfirmed titanium-oxychloride species. SbCl₃ was found to exhibit similar reactivity to TiCl₄, although at a slower rate. A mechanism is proposed, consistent with the observations for both stoichiometric and sub-stoichiometric quantities of TiCl₄.

Full text of DOI:10.1039/c5dt00200a

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Rapid degradation of cyclic peroxides by titanium
and antimony chlorides†
Cite this: Dalton Trans., 2015, 44,
6775
a
b
a
a
Mark S. Bali, David Armitt, Lynne Wallace and Anthony I. Day*
First responders face extraordinary risks when dealing with organic peroxides such as TATP due to the
extreme sensitivity of this class of explosives, and to a lack of a robust chemical means of safe and rapid
neutralisation. The Lewis acids TiCl
mechanism, with TiCl degrading a model cyclic peroxide in minutes when used in a two-fold excess, or
3 hours at half equivalence. The products cannot re-form peroxide compounds as is the case with acid
4 3
and SbCl have been found to demonstrate a novel degradation
4
∼
degradation, suggesting the two mechanisms are fundamentally different. The Lewis acids mediate a
rearrangement reaction in the cyclic peroxide backbone leading to relatively innocuous products through
deactivation of oxidising O. Sub-stoichiometric TiCl
that also leads to some oxidative chlorination products, possibly mediated by an unconfirmed titanium-
oxychloride species. SbCl was found to exhibit similar reactivity to TiCl , although at a slower rate. A
mechanism is proposed, consistent with the observations for both stoichiometric and sub-stoichiometric
quantities of TiCl
4
reactions highlight a secondary reaction pathway
Received 16th January 2015,
Accepted 24th February 2015
3
4
DOI: 10.1039/c5dt00200a
www.rsc.org/dalton
4
.
Alternative methods have been investigated, such as the use
Introduction
of tin chloride, which destroys TATP within 2 h but requires
4
Triacetone triperoxide (TATP) is an organic peroxide explosive heating under reflux. The heating of explosives samples is
readily manufactured from easily sourced chemical reagents neither acceptable nor practical in a field environment. More
and with relatively simple equipment. As a consequence it has recently, Oxley et al. have investigated the use of zinc and
found widespread use in the terrorist community. Its friction copper salts and found that TATP solutions could be neutralised
sensitivity of <0.1 N and impact sensitivity of 0.3 Nm make it within 24 h at room temperature in the presence of a three-fold
dangerous to handle and unpredictable in an explosive excess of either salt in combination with their respective pure
1
3
device. Consequently, it has been a stated aim of several metals. While this represents a promising alternative to acid
research groups to find a rapid and safe chemical method of there are still disadvantages in the time and the quantities
2
–4
degrading TATP.
Much work on TATP degradation has of deactivating agents required. Hexamethylene triperoxide
centered on acid-catalysed degradation. As an example, hydro- diamine (HMTD), another common organic peroxide explosive
2
chloric acid will degrade TATP, forming mainly acetone, in is also reported to be degraded under these conditions.
4
+
3+
3+
essentially a reversal of the equilibrium for the synthesis of
Theoretical studies on the metal ions Ti , Sb and Sc in
TATP. Accordingly there is the potential for peroxides to re- the gas phase have indicated a significant binding affinity with
5
,6
form. Furthermore, it has been reported that treating solid TATP and were predicted to trigger its complete destruction.
4
+
3+
TATP with concentrated sulfuric acid causes initiation of This work encouraged us to examine reactivity of Ti and Sb
3
explosion in >mg quantities. This observation makes the use compounds in solution with a cyclic peroxide model.
of a protic acid problematic as a means of safely disposing of
significant quantities of organic peroxide.
We report that both TiCl and SbCl react with cyclic per-
4 3
oxides via a rearrangement mechanism to give ester and
ketone as primary products, with no peroxides formed. The
reaction rate can be extremely fast or moderate depending
upon the stoichiometric ratio. The identification of this reac-
tion presents a substantial contribution to the chemistry of
organic peroxide degradation, and may also provide insight
into a potential catalytic approach to the problem.
Our investigations have largely utilized the model
compound tripentanone triperoxide (TPTP), which we have
previously demonstrated as an effective model for TATP in
a
School of Physical, Environmental and Mathematical Sciences, The University of
New South Wales, UNSW Canberra, PO Box 7916, Canberra BC 2610, Australia.
E-mail: a.day@adfa.edu.au; Fax: +61 262688017; Tel: +61 262688230
b
Weapons & Combat Systems Division, Defence Science and Technology
Organisation, Edinburgh, Adelaide, SA 5111, Australia
1
†
Electronic supplementary information (ESI) available: NMR spectra of cata-
lysed degradation of TPTP reaction mixtures under varying conditions of temp-
erature time and catalyst ratios. Included are FTIR spectra of reaction solutions
and solid by-products. See DOI: 10.1039/c5dt00200a
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7
degradation reactions. Detailed experiments were conducted followed by a slower phase (discussed later). While the same
using this compound as a safer and readily accessible major products, EP and 3P, were formed in all cases, their rela-
cyclic peroxide.
4
tive proportions varied slightly as the ratio of TiCl was
lowered, and a number of other minor products were formed.
The reactions were therefore examined in more detail.
Effect of reagent stoichiometry. Samples with sub-stoichio-
Results and discussion
Titanium tetrachloride
4
metric quantities of TiCl , were used to study the slower phase
of the reaction which was most pronounced under these con-
ditions. Although EP and 3P were the major products, as
observed in (R1) above, the H NMR spectra of the reaction
mixture also showed the formation of various chlorinated pro-
ducts derived from both EP and 3P (Fig. 1). However, under
these conditions the ketone 3P (including its chloro-derivatives)
was formed as the major solution product. In addition, sub-stoi-
Degradation of TPTP. The treatment of TPTP with excess
TiCl (>2 : 1) at room temperature in dry CDCl resulted in the
4 3
1
instantaneous generation of a pale yellow/green coloured
1
solution. H NMR spectra taken immediately after addition
(
<2 min) showed no TPTP remaining (see ESI†). The primary
products were ethyl propanoate (EP) and 3-pentanone (3P),
with the proton signals strongly shifted downfield from those
of the free ligands, indicating they are bound to titanium
4
chiometric amounts of TiCl led to a pale yellow precipitate that
began to form after ∼1 h, and accumulated over several days.
This two-stage reaction model is supported by observing
the dependence of the degradation rate on the ratio of added
(
3
these shifts are further discussed below). The addition of pure
P and EP (either individually or as a mixture) directly to a
solution of TiCl resulted in the same immediate colour
4
TiCl
0 minutes. In these experiments, the rate of the slower step
appeared to follow pseudo first-order kinetics. Where TiCl
4
. Fig. 2 shows the progress of the reactions over
change and downfield shifts, which appear to be indicative of
the formation of a Ti–(OvC) complex. Thus the degradation of
TPTP under these conditions is extremely fast.
3
4
was used in a two-fold excess (Fig. 2d), the large difference
between the known moles of TPTP at t and the extrapolated t
In experiments where TiCl was in excess, the initial ratio of
4
0
0
products was extremely consistent regardless of the ratio of
intercept indicates that most degradation occurred by the
much faster reaction (R1). The extrapolated t values for TPTP
4
TiCl used. Each molecule of TPTP that reacted resulted in two
0
molecules of EP and one of 3P:
can therefore be used to approximate the proportion of the
TPTP þ TiCl4 ! TiA þ 2ðEPÞ þ ð3PÞ
A
represents a new (unknown) titanium species.
ðR1Þ reaction that took place at faster kinetics (i.e. (R1)): 1TPTP : 0.5
4
TiCl – 33%; 1 : 1 – 52%; 1 : 1.5 – 60%; and 1 : 2 –81%.
where Ti
These observations are consistent with a two-stage general
reaction of TiCl with TPTP:
With lower ratios of TiCl , the rate of degradation of TPTP
4
4
decreased, but even when sub-stoichiometric amounts (0.5
equivalent) were used the degradation was still complete
within ∼3 h (Fig. 1). However, these experiments also showed
that with lower amounts of TiCl4 the degradation seems to
occur in two general phases: a fast initial stage (i.e. (R1))
2
ðTPTPÞ þ TiCl4 ! TPTP þ TiA þ 2ðEPÞ þ ð3PÞ
ðR2Þ
ðR3Þ
A
Ti þ TPTP ! xðEPÞ þ yð3PÞ þ Cl* þ TiB
1
Fig. 1 H NMR spectra (CDCl
3
) following the reaction of TPTP with 0.5
Fig. 2 Reaction of TPTP and TiCl
4
at varying molar ratios. Solid line is
equiv. TiCl
4
over time. ■ – ethyl propanoate, ▲ – 3-pentanone, ● –
based on a single phase exponential decay fitted using GraphPad PRISM
TPTP. The minor peaks are chlorinated organic products, due to side
reactions.
software, dashed line indicated extrapolation. Blue circle on y axis indi-
0
cates actual nTPTP at t .
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A
where Cl* represents an active chlorinating agent, and Ti and
Ti refer to the first and second titanium products respectively.
B
It is also possible that Ti
(
(
B
is itself the active chlorinating agent
Cl*). Arbitrary product stoichiometry is shown in equation
R3) above, as the exact ratio of products was not as clear as
that in (R2) (see later discussion). Overall, one mole of TiCl
4
can degrade two equivalents of TPTP within ∼3 h under the
conditions described.
Binding of reaction products to titanium species. Under
anhydrous conditions, the products EP and 3P coordinate with
the TiA species, as indicated by a strong de-shielding effect
1
visible by H NMR (see ESI†). The Ti
A
-bound ligands are
immediately released upon moisture entering the solution,
and a white precipitate forms, which is reasonably assumed to
comprise titanium hydroxide and oxide products. Table 1
shows the chemical shift of the reaction products at various
ratios of TiCl to TPTP and thereby Ti to ligand (Ti : L, where
4
A
A
L = EP or 3P).
Deshielding of a similar magnitude was found when EP
and 3P were added to TiCl4 under identical conditions
(
Table 1). The size of the shift is correlated with the amount of
titanium species present, with Δδ increasing to a maximum at
: 1 TiCl to L ratio, indicating fast intermolecular exchange
1
4
between bound and free L and the formation of a strongly
associated 1 : 1 TiCl : L complex. This behaviour is consistent
4
8
with that observed for other reported TiCl -ketone adducts.
4
Variable temperature experiments showed that at higher temp-
eratures the chemical shifts returned closer to those of the free
ligand in both TiCl : L and Ti : L samples, again indicating
4
A
very fast intermolecular exchange (see ESI†).
These observations were supported by FTIR studies of the
reaction progress. Table 2 summarises the data for reaction
mixtures together with comparative spectral data of key reac-
tion participants (spectra are provided in ESI†).
1
a
Table 1 Chemical Shift of H NMR of EP and 3P at given titanium ratio
Ti : TPTP ratio (calc’d.
Ti : L ratio)
Ti : L ratio
b
c
Free
2.37
0.99
0.5 (0.33 )
+0.10
+0.08
1 (0.33)
+0.33
+0.18
3 (1)
+0.60
+0.30
1
α
β
+0.56
+0.31
α′
α
β′
β
4.08
2.26
1.21
1.09
+0.13
+0.14
+0.06
+0.07
+0.29
+0.40
+0.12
+0.14
+0.49
+0.63
+0.22
+0.21
+0.48
+0.60
+0.22
+0.21
a
Ti : L refers to titanium to ligand ratio, which in the case of TPTP
reaction mixtures is calculated as Ti : (EP + 3P). Shifts of bound ligands
are given in ppm as a shift from free ligand (+ indicates downfield
b
shift). Value at close to t
0
when 3 ligands are formed per titanium
centre. This ratio changes over time as the reacted titanium continues
c
to slowly degrade TPTP. Free ligand added directly to TiCl
4
in the
absence of TPTP.
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Upon 1 : 1 reaction of EP or 3P with TiCl
4
(analogous to the
3
: 1 degradation reaction), the carbonyl stretch was red-shifted
−
1
by 112 or 62 cm respectively, consistent with the bonding of
the CvO moiety to the titanium centre. For EP the C–O stretch
was also shifted, but in the opposite direction (135 cm− ).
Strong peaks appeared in the low frequency region where the
Ti–Cl stretches would be expected (at ∼400, 455, 495 cm− ).
A similarly red-shifted carbonyl peak was seen for the pro-
1
1
ducts (EP and 3P) of TPTP reactions where excess TiCl
4
was
used. At a 1 : 1 ratio of TPTP to TiCl , peaks for both free and
4
bound carbonyl could be seen, consistent with the NMR experi-
ments that showed resonances intermediate between free and
bound ligands under the same conditions. However, no strong
bands were evident in the low frequency region in this case,
Fig. 3 The reaction of TPTP and TiCl
single phase exponential decay fitted using GraphPad PRISM software,
dashed line indicates extrapolation. Blue circle on y axis indicates actual
4
L
2
/TiCl
4
. Solid line is based on a
A 4
confirming Ti as a different species to TiCl . The general broad
0
nTPTP at t .
−1
absorption at ∼350–500 cm seen at a 1 : 2 ratio of TPTP to
TiCl could indicate that TiCl : L complexes still occur in the
4
4
presence of Ti
A
: L complexes, but loss of the clear TiCl
4
: L cussed later in the context of a side chlorination reaction. The
slower reaction of TiCl (EP) may be linked to the stronger car-
In reactions of TPTP with 0.5 equivalent of TiCl , carbonyl bonyl binding indicated in the IR results.
pattern indicates that the complex geometry has been affected.
4
2
4
peaks were observed at frequencies corresponding to free EP
Proposed primary degradation mechanism. The experi-
and 3P. This observation, together with the significant broad- mental observations point toward a near instantaneous reac-
1
ening effect and very small downfield shifts seen by H NMR tion between TiCl
4
and TPTP (R2), followed by a further (but
for the equivalent reaction sample, suggests weaker binding of slower) reaction (R3), which is also associated with oxidative
the ligands with Ti
cular geometry.
A
compared to TiCl
4
, and/or altered mole- chlorination. Since the chlorination reaction becomes signifi-
cant only at low molar ratios of TiCl , where a precipitate is
: L complexes also formed, these side reactions are not included here but are
4
1
Interestingly, the H NMR spectra of TiCl
4
were sharp at ambient temperatures whereas TPTP degra- discussed separately below.
dation mixtures exhibited broadening that was most evident at Scheme 1 provides an interpretation of the reaction pro-
low initial ratios of TiCl
(see ESI†). Dynamic behaviour is fre- gression that is consistent with the experimental results. Com-
4
1
0
quently observed for titanium complexes in solution, and plexation of organic products to titanium species has been
can arise from various phenomena including ligand exchange omitted in this scheme for clarity. A concerted mechanism is
and aggregation of titanium species to form dimers and postulated, based on the very fast rate of (R2); and an analo-
higher oligomers. Fluxional behaviour is common in penta- gous mechanism is proposed for (R3), which gives identical
1
1
coordinate metal centres and has also been reported for hexa- initial product ratios (EP : 3P 2 : 1 respectively). The slower rate
1
2
coordinate titanium centres. In the present case, the slight of (R3) can be accounted for by a lower reactivity of TPTP
broadening of peaks observed in reactions of TPTP could be toward Ti , produced during (R2). Rapid stepwise reactions
caused by a somewhat slower rate of ligand exchange with the cannot be ruled out; however no intermediate peroxy com-
Ti centre than with the TiCl
centre, or by altered geometry, pounds were observed by NMR or IR even in the slower (R3)
A
A
4
or both. The very broad peaks initially observed in the tita- reactions. Cyclic peroxide breakdown intermediates under
nium-limited reactions seem to be related to the presence of acidic conditions have been proposed to be peroxy/hydroperoxy
2
,7,13
excess TPTP, which showed reactivity towards the Ti
after consumption of TiCl
. The broadening effect could not reaction is fundamentally different to the broadly accepted
be investigated by varying temperature, as increasing tempera- mechanism of acidic degradation.
A
species oligomers or dimers.
It seems therefore, that the present
4
ture expedited further reaction in titanium-limited samples
The reaction products EP and 3P account for only 5 of the 6
where the broadening was most evident, and lower tempera- oxygen atoms contained in 1 mole of TPTP. Given the well-
tures precipitated the analytes.
Since the products 3P and EP bond to titanium, it was ing oxygen atom is bonded to titanium. The species rep-
anticipated that this may have an effect on the TPTP degra- resented as Ti and Ti have yet to be characterised
dation reaction. To better understand this each of the potential conclusively; however we found no IR evidence for TiOCl (as
), for which a strong absorption at ∼820 cm could be
established oxophilicity of titanium, it is assumed the remain-
A
B
2
−
1
ligands were pre-reacted with TiCl
to the TPTP. Over a 40 min reaction period TiCl
little effect on the initial degradation, in contrast to TiCl (EP)
4
before adding this reagent Ti
(3P)
showed expected. We postulate that a Ti-OCl species is formed in
(R2) (Ti ), which is still reactive towards TPTP in (R3) where it
which slowed the reaction (Fig. 3) relative to that observed might also act as a source of chlorination (discussed below).
with TiCl alone (R1). A progressive slight increase in the
Side reactions with limited TiCl : chlorination and precipi-
degradation rate with TiCl (3P) was observed, which is dis- tation. At low molar ratios of TiCl , (R3) is linked with
A
1
4
4
2
4
2
A
4
4
4
2
4
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Scheme 1 A possible mechanism for the concerted decomposition of TPTP by TiCl
4
in anhydrous conditions. Coordinated ketone/ester products
are omitted for clarity.
1
5
oxidative chlorination (primarily forming 2-chloro-3-penta- ium propanoate. The precipitate was moisture-sensitive, and
1
none), and the slow formation of a precipitate.
hydrolysis with D O caused the release of organic ligands. H
2
The chlorination products were identified by comparison NMR analysis confirmed that the bulk of this organic com-
against NMR spectra from the literature of probable rational ponent was propanoic acid (>80 mol%), along with a complex
products, and further validated by structural information from mix of chlorinated and non-chlorinated alcohols, acids and
COSY experiments conducted on the reaction mixtures (see acid chlorides in minor quantities (these products were not
ESI†). All identifiable organic products were quantified after exhaustively elucidated). We can thus reasonably conclude that
1
chlorination reactions had stabilised (9 days) using H NMR precipitate from (R3) is largely compromised of titanium-
integration (Table 3). The results showed that ketone (or propanoate species derived from the breakdown of EP. Thus the
ketone-derived chloro-species) form a greater proportion of the higher proportion of ketone (and ketone-derived chlorinated
total solution product distribution in sub-stoichiometric reac- species) observed in solution in (R3), compared to (R2), is due
tions. The overall 51% mol equivalent of chlorine atoms (with to loss of the primary EP product within the precipitate.
respect to all soluble organic products) quantitatively accounts
The freshly prepared precipitate was analysed for C, H, N,
for ∼3 chlorine atoms from TiCl , with ∼2 of those being Ti and Cl. Table 4 shows the results compared against a poss-
4
related to oxidative chlorination processes (2-chloro-3-penta- ible titanium propanoate species indicated by the stoichio-
none, 2-chloropropanoyl chloride).
metry of the reaction. Since the NMR spectrum indicates the
The yellow precipitate from (R3) after 9 days was isolated precipitate is not a pure substance, this elemental analysis is
under N , dried and examined by FTIR (KBr disk). A strong not intended to provide a definitive proof of characterisation.
2
−1
broad peak at 3250–3400 cm and broad absorption from Instead the results confirm an oxygen– and chlorine-rich
1
4
00–900 cm− were strongly indicative of a propanoate salt content with a clear 1 : 1 : 4.5 Ti–Cl–O ratio in the precipitate.
with the overall spectrum being very similar to that of potass- The 1 : 1 Ti : Cl ratio agrees well with the earlier chlorinated
organics analysis, as it infers that the remaining chlorine of
TiCl4 remains bound to the Ti-centre. The inferred oxygen
Table 3 Organic products from 0.5 : 1 TiCl
4
–TPTP reaction (9 days)
ratio of 4.5 per titanium accounts for the two oxygen atoms
in propanoate, as well as the two oxygen atoms “missing”
from the two units of degraded TPTP by each titanium centre
Product
mol%
Sum
57
1
2
3
4
Ethyl propanoate
1-Chloroethanol
Chloroethane
44
5
2
6
Table 4 (R3) Precipitate elemental analysis
2-Chloropropanoyl chloride
Measured
(R3) Precipitate
Calculated
C
12.00
C
H
2.76
H
Ti
25.9
Ti
Cl
20.2
Cl
O
39.11
O
a
5
6
7
3-Pentanone
3-Chloro-3-pentanol
2-Chloro-3-pentanone
11
6
26
43
(C H O ) Ti H O Cl
3
3
5
2 2
3
5
8
13.62
2.86
27.14
20.12
36.28
a
a
Total mol% of chlorine
51
The balance required to make 100% was used to estimate O content,
a reasonable assumption as no other elements were present in the
sample. N was not detected in the analysis.
a
2
-Chloropropanoyl chloride contains 2 mole equivalents of chlorine.
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2
0
(
i.e. not accounted for in the 2(EP) : 1(3P) product distri- of the mechanism of manganese catalysts. Direct spectro-
bution). The “excess” 0.5 oxygen might indicate further hydro- scopic evidence for the OCl ligand is troublesome, with the
lysis or formation of oxo-bridged species, which may be only potential reference source being the O–Cl stretch of hypo-
2
1
−1
explained by the chlorination mechanism proposed in the next chlorous acid at 739 cm in the IR spectrum, which is
section. The precipitate produced in this reaction may within the absorption band of the solvent CH Cl . However,
2
2
resemble sol–gel products derived from the reaction of Ti-alk- the stringent anhydrous conditions make it difficult to suggest
oxides and acetic acid, esters and water. These reactions have alternative explanations that account for the fate of the
been shown to form highly ordered products such as hexa- missing organic oxygen and the observed chlorination pro-
1
6,17
meric ring structures,
which would account for the con- cesses. The much smaller amounts of non-oxidative chlori-
sistent microanalysis results between reaction samples in the nation products seen in the late stages of reaction suggest the
−
present case.
presence of trace levels of free Cl such as HCl.
Implication of chlorination reactions. Broadly speaking,
where a half equivalent of TiCl is used, two chlorines from et al. suggested that the Sb ion would be similarly active in
4
Antimony trichloride. The theoretical work by Dubnikova
3
+
4
+ 6
TiCl are accounted for in oxidative chlorination products. Oxi- facilitating the ring-opening of TATP, but less so than Ti . In
4
dative chlorination of a degradation product (mainly 3P) may initial experiments, aliquots taken directly from anhydrous
occur via its reaction with a Ti–OCl species. Thus the presence reactions of SbCl
of chloro- adducts in the reaction mixtures may support the workup. While this gave near-complete degradation within
proposed mechanisms for (R2) and (R3) (Scheme 1) where the 10 minutes, even at 0.5 mol equivalent of SbCl (Fig. 4), it was
postulated Ti–OCl species as Ti and Ti , respectively, effects soon identified that this reaction was largely taking place at
chlorination as outlined in the reaction (R4) (Scheme 2). the GCMS injection liner at 130 °C. Considering the absence
R4) relies on 3P forming a titanium enolate, which is of broad or tailing peaks in the chromatogram, it also
known to be a highly favourable process due to the oxophilicity appeared that the reaction was complete before the sample
3
and TPTP were analysed by GCMS without
3
A
B
(
1
8
of titanium. A similar mechanism has been used to explain entered the column for separation (i.e., within seconds). These
an analogous α-chlorination of ketones using urea-hydrogen initial experiments still provided some useful information,
peroxide with SiCl
be the active chlorinating agent. Hypochlorous acid could least two equivalents of TPTP. Modification of the sampling to
then readily chlorinate 3P at the α-carbon position, breaking quench and extract SbCl before analysis showed that the reac-
down the now-unstable titanium-adduct to leave a hydroxo tion was sluggish at room temperature.
ligand. At 50 °C under anhydrous conditions using 1 mol equi-
The latter species could immediately form Ti–O–Ti bridges valent of SbCl , TPTP degradation was complete within
a significant driving force in similar sol–gel reactions) and 20 minutes (Fig. 5). As with the direct injection results, EP was
4 3
, where a silyl-OCl species was postulated to with the 1 : 0.1 reaction suggesting that SbCl can degrade at
1
8
3
3
1
6
(
may explain the precipitation that occurs with (R3).
formed in only slight excess, relative to 3P, rather than the 2 : 1
reaction. Colorimetric speciation indi-
chlori- cated that some Sb was being consumed in the reaction, pre-
The suggestion of an OCl ligand in Ti
A 4
is unusual, though ratio seen with the TiCl
3
+
this moiety has been proposed previously, in the SiCl
4
1
9
5+
3+
nation reaction referred to above, and in a theoretical study sumably by oxidation to Sb . This loss of Sb however,
2
Scheme 2 Proposed mechanism (R4) – the α-chlorination of 3P by TiCl X(OCl), where X = Cl or OCl.
Fig. 4 Reaction products of dry samples of SbCl
3
and TPTP injected directly into GC/MS inlet liner. The reaction was found to be occurring at the
elevated temperature of the inlet liner.
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analyses. The method utilised an initial column temperature
−
1
of 40 °C for 1 min, 20 °C min ramp to 100 °C, held for
min, followed by 30 °C min− finishing at 190 °C and held
1
3
for 2 min. Injector temperature was 130 °C, interface 200 °C,
and linear column flow at 2 mL min− of He gas. Split ratio
was set at 40. Concentrations were quantified using external
standards.
1
NMR spectra were recorded on a Varian Unityplus-400
spectrometer. All NMR experiments were conducted at 25 °C
1
unless otherwise stated. H NMR spectra were referenced to
1
TMS (0 ppm) at 25 °C using the residual H signal of the
3
Fig. 5 Reaction progress of sealed ampoules of 1 : 1 SbCl and TPTP
reacting in CH
mean result.
2
Cl
2
at 50 °C. Error bars reflect standard deviation from respective deuterated solvent. 1D spectra were recorded with
between 16 and 1024 transients with a 6 s relaxation delay.
COSY experiments were conducted with 1 scan per increment,
5
12 t
neither corresponds with the number of moles of EP/3P pro- was processed and graphed using Graphpad Prism.
duced, nor the moles of TPTP consumed, suggesting that this
IR Spectra were recorded on a Shimadzu IRPrestige-21
1
increments and 2 s relaxation delay. Integration data
oxidation process is not directly linked to the central degra- spectrometer. Samples were typically analysed between 350
and 3500 cm− with 16 scans. CH Cl solutions were analysed
1
dation mechanism.
2
2
The significant levels of ester production in the SbCl reac- in a 0.25 mm pathlength demountable liquid cell with KBr
3
tion, and the ability to degrade ∼2 equivalents of TPTP per windows, which was oven dried and stored over phosphorous
unit of SbCl , suggest a similar mechanism to the TiCl reac- pentoxide. The cell filled with dry CH Cl was used for the
3 4
2 2
tion, albeit with the requirement for heat to drive the reaction background measurement. The cell was rinsed with dry
at comparable rates. The lower reactivity corresponds with the CH Cl and purged with dry argon between samples. The
2
2
3
+
4+
predicted relative rates of reaction of TATP with Sb and Ti
liquid film spectrum of DPDP was collected with a drop of
ions in the gas-phase. Potential oxidation of Sb adds signifi- sample between two KBr discs.
cant complexity to the reaction system, which is not a factor
UV/Vis spectra were recorded on a Cary 50 spectrometer.
6
3+
3
+
with TiCl . As a result, this reaction was not explored to the Sb standard solutions for calibration curves were analysed
4
same extent as TiCl , where further oxidation of the metal is within 2 h of preparation.
4
not possible.
Methods
All weighing and handling was conducted in a dry N atmos-
2
Experimental
phere using a glovebox or standard Schlenk techniques with
oven dried glassware and syringes. Any trace of moisture in the
CAUTION! The organic peroxides detailed in this study have
the characteristics of primary explosive with extreme levels of
friction, heat and impact sensitiveness. All procedures must be
carried out by properly qualified and equipped personnel
taking all relevant precautions.
presence of TiCl results in immediate cloudiness – any experi-
4
ments where this was observed were discontinued and the
samples re-prepared.
TiCl experiments. The estimated TiCl concentration from
4
4
weight/volume calculations was confirmed by a modification
2
2,23
of published titration methods.
0.1 M EDTA(aq) was stan-
Materials
dardised against 0.01 M CaCO (aq) primary standard, and
3
All chemicals and solvents were of analytical grade unless
4
0.1 M ZnSO (aq) was standardised against the EDTA solution.
otherwise noted. CDCl (99.9% D) was dried over freshly acti-
3
A sample of the TiCl4 solution was added to 5 mL of 3 M
vated 4 Å molecular sieves. TPTP, DPDP and DPDHP were syn-
thesised and purified as published previously. GC standards
H SO (aq) and heated to drive off CH Cl . After cooling,
2
4
2
2
7
3
2 2
drops of 30% H O (aq) were added to stabilise titanium in
were made with ethyl acetate and 3-pentanone using capillary
grade CH Cl . TPTP was purified by sublimation in vacuo.
2 2
solution followed by 10 mL aliquot of EDTA solution and 3.5 g
solid hexamine as buffer. The mixture was adjusted to pH 5.5
with concentrated aqueous ammonia and diluted to 80 mL.
4 drops of xylenol orange indicator were added and the
TiCl (AR) was double distilled from copper turnings under dry
4
N2 gas, before being weighed and dissolved in a measured
2 2 2 2
volume of CH Cl (AR, freshly distilled from CaH under N ).
remaining free EDTA was back-titrated with ZnSO solution to
4
TiCl solutions were used and stored under a strict N atmos-
phere, and discarded after 2 weeks.
4
2
a lasting red-orange endpoint. The concentration derived from
this titration ([Ti]) was used for all further calculations.
Analyte (TPTP, EP or 3P) was measured into a dry, sealed
Instruments
5
mL volumetric flask and diluted with dry CDCl
/CH Cl stock solution was diluted in
column (1.0 µm film, I.D. 0.25 mm, 30 m) was used for GCMS dry CDCl3 such that a 100 μL volume would provide the
3
to make a
A Shimadzu QP 2010 Ultra fitted with a SGE SolGel Wax 0.05 M solution. TiCl
4
2
2
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Dalton Transactions
desired mole ratio when added to 500 μL of the analyte stock immediately prior to inserting into the NMR spectrometer for
solution. 500 μL of analyte solution was placed in a dry 5 mm measurement.
NMR tube and vacuum degassed. 100 μL TiCl
4
/CDCl
3
was
SbCl
3
experiments. In initial experiments, 10 mM solutions
added (forming an immediate yellow/green clear solution) and of TPTP were added to solid SbCl
3
under dry conditions, and
the mixture immediately frozen in liquid N . The tube was the mixture sampled directly by GCMS. It was found that this
2
flame sealed under high vacuum. The sample was thawed and method resulted in very fast degradation within the injection
well shaken immediately prior to inserting into the NMR liner of the GCMS. The following method was then employed
spectrometer for measurement.
to study the reaction.
The sub-stoichiometric titanium reaction was conducted on
3
TPTP (61.2 mg, 0.2 mmol) and SbCl (45.6 mg, 0.2 mmol)
a larger scale in order to isolate the precipitate for further ana- were separately weighed into two dry 10 mL volumetric flasks
lysis. TPTP (135 mg, 0.44 mmol) was weighed into a dry and sealed, then diluted with dry CH Cl to make 10 mM solu-
2
2
1
0 mm NMR tube. 3 mL of dry CDCl
tents vacuum degassed. 0.87
0.25 mL, 0.22 mmol) was added and the mixture immediately sample was frozen in liquid N , vacuum degassed and flame
3
was added and the con- tions. 1 mL of each sample was added to a dry ampoule with a
M
TiCl CH Cl solution manifold attached. Upon the addition of the solutions, the
4
2
2
(
2
frozen and vacuum-sealed. On thawing, the reaction was moni- sealed under vacuum. 14 samples were prepared and stored in
1
tored by H NMR. A pale yellow precipitate formed within an liquid N
hour and continued to form over the following week. After 9 tion, 12 samples were quickly thawed in warm water with agita-
days, the tube was opened under dry N , the bulk of the super- tion, then placed in a temperature controlled bath at 50 °C
2 0
(two reserved as t controls). To commence the reac-
2
natant solution removed by syringe, and the sample dried and the stopwatch started. At 5, 10 and 20 min, 4 samples
in vacuo. The resulting sample was not weighed, as quantitative were removed and immediately frozen in liquid N . One by
2
transfer was not possible. A small sample was ground with KBr one, the samples were thawed and the ampoule opened. A
−
1
3+
in air and immediately analysed by FTIR: ν(cm ): 739(w), 500 μL aliquot was removed for Sb determination, and the
8
83(w), 1156(w), 1302(w), 1436(m), 1522(m), 1624(m), 1700(w), remaining sample quenched over CaCO with a drop of water.
3
2
362(w), 3258(br, vs), 3375(br, vs). For NMR analysis, a drop of The sample was syringe filtered, then 500 μL was placed in a
D O was added to a small sample resulting in an immediate 2 mL volumetric flask along with 20 μL of 0.2 M ethyl acetate
2
white suspension, to which MeCN-d was added and analysed in MeCN as an internal standard and made up to the mark
3
by NMR. Microanalysis of the precipitate was conducted by the with MeCN. Samples were analysed by GCMS.
Australian National University microanalysis laboratory.
In parallel, the reserved 500 μL reaction sample was used
3
+
For FTIR assays: aliquots of TiCl /CH Cl stock solution for spectrophotometric determination of Sb using a method
4
2
2
2
4
3+
were added directly to CH
2
Cl
2
solutions of TPTP, EP and 3P based on that of Christopher et al. Samples of known [Sb ]
Cl were used to confirm that adding CH Cl reaction
(50 mM) in dry, sealed vials. The samples were injected into in CH
2
2
2
2
the liquid IR cell through septa and the spectrum taken samples directly to the buffer did not affect the accuracy of the
immediately. For titanium-limited reactions, the sample was described method. The absorption at 560 nm was recorded
allowed to proceed in the vial at ambient temperature (∼20 °C) and compared against standard calibration samples. The t
0
3
+
and aliquots were observed at 30 min intervals.
For TiCl
sieves for 4 h under N
4.1 mmol) in CH Cl was cooled to 0 °C under N with a mag-
samples were excluded from the Sb determination results
4
L
2
reactions, EP and 3P were dried over molecular due to a sampling error.
. In a dry vial, 10 mL of 0.41 M TiCl
2
4
(
2
2
2
netic stir bar. 0.87 mL (8.2 mmol) 3P or 0.96 mL (8.2 mmol)
EP was added dropwise (strong exothermic reaction) to the
Conclusions
stirred TiCl , resulting in a clear orange solution once the The reactions of TiCl and SbCl described in this paper present
4
4
3
3
sample warmed to RT. The residual CHCl solvent signal of a a new direction in the search for a rapid means of degrading
batch of dry CDCl
solid aliquot of hexamethylbenzene. 50 μL of the TiCl4L2 TPTP has previously been shown to be an effective model for
CH Cl solution was dissolved in 500 μL of the same stock of TATP and by inference our current findings are expected to be
dry CDCl
sample was also subjected to the [Ti] determination method
described above, with both methods corresponding to ditions, which would limit field application in degradation
TiCl ] = 0.40 M ± 0.02 M. and neutralisation of cyclic peroxides, this does not diminish
4.5 mg TPTP (0.08 mmol) was weighed into a 2 mL dry their significance. The effectively instantaneous degradation of
volumetric flask and made up to the mark with the standar- the cyclic peroxide in solution by a molar equivalent of TiCl at
dised CDCl . 500 μL of this 0.04 M solution was added to a room temperature proceeds by a novel mechanism that
mm NMR tube, the required amount of 0.4 M TiCl L solu- deserves further investigation. In particular, we are planning
3
was quantified by integration against a cyclic peroxides like TATP. As mentioned in the introduction,
2
2
1
3
and [L] determined by H NMR integration. The applicable to the degradation of TATP and DADP.
Even though the reactions herein require anhydrous con-
[
4 2
L
2
4
3
5
4
2
tion added under a dry N
immediately frozen in liquid N
2
atmosphere and the mixture calorimetric experiments to help understand if the rearrange-
. The tube was flame sealed ment reaction itself is exothermic, and thus safe to scale up to
under high vacuum. The sample was thawed and well shaken larger reactions or on solid samples.
2
6782 | Dalton Trans., 2015, 44, 6775–6783
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Dalton Transactions
Paper
A number of key characteristics of the reaction present
useful advances in terms of peroxide degradation. Most impor-
tantly, the organic reaction products cannot re-form peroxides.
The sequestering of peroxide oxygen atoms into the organic
products is a significant step forward, giving products of low
reactivity unlike the reported acidic degradation process. In
addition, the modification of the Ti reagent as a pre-formed
ketone adduct (TiCl : EP) ) also highlights a potential for
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4
2
improved performance.
The ability of these metal compounds to induce a degree of
oxygen insertion as part of the breakdown of the cyclic per-
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Acknowledgements
1
1
1
1
2
2
This research was financially supported by the Commonwealth
of Australia through the National Security Science and Tech-
nology Centre within the Defence Science and Technology
Organisation. This support does not represent an endorsement
of the contents or conclusions of the research. This research
was also supported by the Australian Defence Force’s Chief of
Defence Force Fellowship.
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This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 6775–6783 | 6783