Group 13 chelates in nerve gas agent and pesticide dealkylation

Article abstract of DOI:10.1039/b717041f

Schiff base boron and aluminium bromides have been used to cleave organophosphate nerve agents and pesticides and their simulants: salben( ^tBu)[BBr₂]₂ was very effective in cleaving the VX simulants EMPPT and DEPPT and ner

Full text of DOI:10.1039/b717041f

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LETTER
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Group 13 chelates in nerve gas agent and pesticide dealkylationw
Amitabha Mitra,a David A. Atwood,*a Jeffrey Struss,a Daniel J. Williams,*bc
Bradley J. McKinney,b William R. Creasy,c David J. McGarvey,c
H. Dupont Durstd and Roderick Fryc
Received (in Durham, UK) 5th November 2007, Accepted 31st January 2008
First published as an Advance Article on the web 5th March 2008
DOI: 10.1039/b717041f
Schiff base boron and aluminium bromides have been used to
cleave organophosphate nerve agents and pesticides and their
simulants: salben(tBu)[BBr2]2 was very effective in cleaving the
VX simulants EMPPT and DEPPT and nerve agent VX;
salen(tBu)AlBr was effective in cleaving the nerve agents VX
and Soman and the pesticide Diazinon.
ers,13–15 and more recently, studies conducted at the US Army
Edgewood Chemical and Biological Center at Aberdeen Prov-
ing Ground, Maryland, have shown aqueous solutions of
aluminium sulfate and sodium aluminate buffered to pH 4
to be very promising for destroying large quantities of VX, GB
and GD.16 The search for alternative agents for decontamina-
tion of organophosphate nerve agents and pesticides still
continues.
Decontamination of nerve gases is required in battlefields,
laboratories, storage and destruction sites. Organophosphate
pesticides are the most widely used type of pesticide and have
replaced the environmentally persistent organochlorine re-
agents. Despite their potential environmental problems the
use of organophosphate pesticides has been increasing and is
predicted to increase in the future because of the lack of
suitable substitutes.1 Long-lived pesticides pose a threat when
they spread beyond their intended application.
Many of the toxic nerve agents and pesticides possess a
P–O–C linkage in their structure. Cleaving the P–O–C bond
could be a means of deactivating these toxic agents.2,3 This is
convenient due to the nucleophilicity of the phosphorus and
polar nature of the P–O and C–O bonds. Previously, it was
shown that binuclear boron halide chelate compounds can
dealkylate a wide range of phosphates at ambient tempera-
ture.17–19 For example, salpen(tBu)[BBr2]2 dealkylates (MeO)3-
P(O) by 89% and (nBuO)3P(O) by 99% in only 30 min.17 This
is significant considering the fact that BCl3 or BBr3 alone are
ineffective for phosphate dealkylation. In the dealkylation
reaction MeCl or MeBr is produced along with several poten-
tial phosphate-containing chelate compounds. The reaction
can be monitored by comparing the 1H NMR peak integration
of methyl halide to that of the original phosphate. It was also
shown that mononuclear Schiff base aluminium compounds
such as salen(tBu)AlBr could dealkylate organophosphate
esters under mild conditions.20 Herein are reported the results
obtained from dealkylation studies on actual nerve agents and
pesticides and their simulants with boron and aluminium salen
chelate compounds combined in a 1 : 1 stoichiometry.
Two Salen(tBu) compounds of boron and aluminium
salben(tBu)[BBr2]2 (1) and salen(tBu)AlBr (2) (Fig. 1) were
used to study the the P–O–C bond cleavage in a series of
organophosphate nerve agents and pesticides and their simu-
lants. The nerve agents used were VX, Sarin (O-isopropyl
methylphosphonofluoridate, GB) and Soman (3,3-dimethyl-2-
Nerve gases can be destroyed by bleach through oxidation
to less toxic inorganic phosphates, and alkali through hydro-
lysis of the P–O or P–S bond.2,3 One disadvantage of using
bleach is that a large excess is required. Also the active
chlorine content of bleach solutions decreases with time.
Moreover, bleach is indiscriminately corrosive to any surface
or compound it comes into contact with. Base hydrolysis also
has some limitations, such as the requirement for large quan-
tities to maintain a high pH level. Also VX (O-ethyl-S-[2-
(diisopropylamino)ethyl] methylphosphonothiolate) has low
solubility in alkaline solution and reacts with base very slowly.
Catalytic hydrolyses involving metal ions (e.g., Cu2+, Ag+,
Hg2+).2,4–6 and enzymes (e.g., organophosphorus acid anhy-
drases).2,7–11 have been proposed but they have limitations
too. For example, Cu(II) has limited capability for the hydro-
lysis of VX whereas Ag+ is too expensive and Hg2+ is too
toxic to be used as a decontamination agent. The enzyme
organophosphorus hydrolase (OPH) is highly efficient for the
hydrolysis of organophosphorus pesticides but has much low-
er efficiency for the hydrolysis of VX (kcat = 0.3 sÀ1).12
Nanocrystalline metal oxides as catalysts for nerve agent
destruction have also been investigated by various research-
a Department of Chemistry, University of Kentucky, Lexington, KY
40506-0055, USA. E-mail: datwood@uky.edu
b Department of Chemistry and Biochemistry, Kennesaw State
University, Kennesaw, GA 30144, USA. E-mail:
dwilliam@kennesaw.edu
c SAIC, Aberdeen Proving Ground, MD 21010, USA
d US Army Edgewood Chemical and Biological Center, Aberdeen
Proving Ground, MD 21010, USA
Fig. 1 The compounds used as dealkylating agents: salben(tBu)-
[BBr2]2 (1) and salen(tBu)AlBr (2).
w Dedicated to Jerry Atwood on the occasion of his 65th birthday.
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Fig. 3 1H-Spectrum of VX 0.7 h after treatment with compound 1.
Integration area assessed shown in blocks.
pounds 1 or 2 showed any activity toward cleaving P–N–C,
which neither compound did. However, for the three nerve
agents, 1 was effective in cleaving only VX whereas GB and
GD remained practically uncleaved. Headspace GC/MS
further supported these observations by showing no isopropyl
bromide in the GB reaction mixture with 1, but it was detected
in the GB reaction mixture with 2. Ethyl bromide was detected
in the VX mixtures with both 1 and 2. GD was not tested. The
reduced reactivity with Sarin and Soman could be explained
by the presence of a highly electronegative fluorine atom
attached to the phosphorus in either GB or GD. The fluorine
reduces the nucleophilicity of the phosphoryl oxygen, thus
inhibiting the formation of a phosphate coordinated cation.
Cation formation was found to be required as the first step in
phosphate cleaving by boron19 and aluminium20 Salen com-
pounds. Neither of the pesticides Diazinon and Malathion
were cleaved effectively with 1. Both of the pesticides have
PQS instead of PQO bonds. The weaker coordination with
boron to sulfur compared to oxygen might be responsible for
the lack of cleavage by boron compound 1. The salen alumi-
nium compound 2 showed dealkylation for all of the reagents
but malathion. It showed significant dealkylation for VX and
Soman. It is also interesting to note that 1 showed the highest
activity for VX but could not cleave the isopropoxy group in
GB or the 3,3-dimethylbutyl group in GD, whereas 2 showed
moderate cleaving activity (24.5% in 1.2 h for Sarin and
62.6% in 6.2 h for Soman) in spite of the unfavorable
electronic factor of the fluorine attached to the phosphorus.
Although Diazinon was not effectively cleaved by the boron
compound (1) (only 10.3%) it was moderately cleaved (46.7%
in 1 h) with the aluminium compound 2. This is a reflection of
the better coordination of the sulfur in the PQS group with
aluminium than with boron. That the salen aluminium com-
pound was not effective in cleaving the model compounds
DEPPT and EMPPT could not be explained based on the
present experimental data.
Fig. 2 Structures of the compounds studied.
butyl methylphosphonofluoridate, GD). The nerve agent
simulants used were EMPPT (O-ethyl-S-methyl phenylpho-
sphonothioate), DEPPT (O,S-diethyl phenylphosphonothio-
ate) and HMPA (hexamethylphosphoramide). The pesticides
were Diazinon and Malathion (Fig. 2).
VX, GB, and GD were obtained from Aberdeen Proving
Ground (APG), MD. Reactions with chemical warfare agents
were conducted on site at Edgewood Chemical and Biological
Center at APG by trained personnel using applicable safety
precautions. Studies on nerve gas agents must be conducted in
government approved laboratories.
Proton NMR spectra were collected on samples prepared
directly in NMR tubes (507-PP Wilmad Glass, Inc.) using a
Bruker AVANCE 300 MHz NMR spectrometer fitted with a
5 mm broadband probe. Simulants were combined in equi-
molar quantities with salben(tBu)[BBr2]2 or salen(tBu)AlBr in
5 mm NMR tubes at ambient temperature in CDCl3 and
allowed to stand for at least 30 min as described in the
literature.17–20 Similar experiments conducted on VX, GD,
and GB in CDCl3 were done in 4 mm sleeves which were
flame-sealed and placed in 5 mm NMR tubes which were also
flame sealed prior to placing in the probe.
The reaction was monitored by the disappearance of the
alkoxy peak of the phosphate and appearance of the alkyl
1
peak of the alkyl bromide in the H NMR spectrum and the
percent conversion was calculated from the integration values.
For example, with VX the a-ethoxy proton peak appears as a
multiplet centered near d 4.15 ppm prior to reaction with
compound 1. After 0.7 h following treatment with compound
1, this peak is observed along with the a-methylene proton
peak of ethyl bromide at d 3.45 ppm (See Fig. 3).
Binuclear boron compound 1 was very effective in cleaving
the phosphate ester bond of the simulants EMPPT (100%
conversion in 1.5 h) and DEPPT (70.8% in 1 h) although it did
not cleave HMPA. HMPA was investigated to see if com-
Tracking the reactions using 31P NMR spectroscopy had
limited value in that some of the reaction products may be
precipitating from solution. This was noted especially for the
VX mixtures which showed proton resonances corresponding
to ethyl bromide, the presence of which was supported by
headspace GC/MS, but showed no 31P resonance correspond-
ing to the well-known dealkylated version of VX also known
as S-[2-(diisopropylamino)ethyl] methylphosphonothioic acid
or EA-2192. Sarin, on the other hand, did show a fluorinated
species of unknown identity in the reaction mixture with 2
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784 | New J. Chem., 2008, 32, 783–785

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Table 1 Percent dealkylation observed with boron and aluminium
bromide chelates. (Standard deviations from multiple integration
determinations are given in parentheses)a
tial support from the University of Kentucky Tracy Farmer
Center for the Environment is acknowledged.
Salben(tBu)[BBr2]2
Salen(tBu)AlBr
References
%
Time/h
%
Time/h
Alkyl group
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VX
GB
GD
Diazinon
Ethyl
74.7(7)
0
0
10.3(6)
0
100
0.7
1.0
6.5
1.7
0.5
1.5
1.0
1.3
57.1(1) 3.9
24.5(2) 1.2
62.6(2) 6.2
46.7(3) 1.0
i-Propyl
Pinacolyl
Ethyl
Malathion Methyl
0
2.0
EMPPT
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HMPA
Ethyl
Ethyl
Methyl
10.5(1) 0.7
10.1(7) 1.2
70.8(4)
0
0
1.2
a
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or GD = 3,3-dimethyl-2-butyl methylphosphonofluoridate; DEPPT
= O,S-diethyl phenylphosphonothioate; EMPPT = O-ethyl-S-methyl
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which may have been a complexed species of either Sarin itself
or of some other derivative.
In conclusion, binuclear boron Salen compound salben
(tBu)[BBr2]2 and mononuclear aluminium Salen compound
salen(tBu)AlBr have been used for the dealkylation reactions
of nerve gas agents and pesticides and their simulants under
very mild conditions. The salen boron compound was very
effective in cleaving the VX simulants EMPPT and DEPPT
and nerve agent VX. The salen aluminium compound was
effective in cleaving the nerve agents VX and GD and the
pesticide Diazinon. Neither of the two salen compounds could
cleave the P–N–C bond in HMPA. This study is the first
application of any Schiff base chelate compound to the cleav-
ing of nerve gas agents and pesticides (Table 1) Note that all of
these reactions were conducted in equimolar quantities. It
would be expected that much greater P–O–C bond cleavage
would take place in shorter periods of time using and excess of
the salen chelate. These studies are currently being conducted.
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Acknowledgements
This work was supported by the Kentucky Science and
Engineering Foundation (KSEF grant 148-502-04-100). Par-
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New J. Chem., 2008, 32, 783–785 | 785