Neutral 1,3-diindolylureas for nerve agent remediation

Article abstract of DOI:10.1002/chem.201202028

Efficient neutralization of nerve-agent simulants by 1,3-diindolylureas in a neutral medium was investigated (see scheme; DCP=diethylchlorophosphate, DCNP=diethylcyanophosphonate). The rate of hydrolysis of the simulants was found to increase by as much as 45 % in the presence of these compounds. A mechanism based on the simulant complexation was established. Copyright

Full text of DOI:10.1002/chem.201202028

DOI: 10.1002/chem.201202028
Neutral 1,3-Diindolylureas for Nerve Agent Remediation
Andrea Barba-Bon,[a, b] Ana M. Costero,*[a] Margarita Parra,[a] Salvador Gil,[a]
Ramꢀn Martꢁnez-MꢂÇez,*[b] Fꢃlix Sancenꢀn,[b] Philip A. Gale,*[c] and
Jennifer R. Hiscock[c]
Nerve gases are a class of warfare agent. Chemically, they
can be described as organophosphonates that contain good
leaving groups. These compounds are highly toxic to both
humans and animals by inhibition of acetylcholinesterase.[1]
As chemical weapons, they are classified as weapons of
mass destruction by the United Nations. Even though pro-
duction and stockpiling of these agents was outlawed by the
Chemical Weapons Convention of 1993, their use in terrorist
attacks makes the development of new sensing systems and
the design of new remediation products of prime impor-
tance.[2] In relation to the latter, there has been an increased
interest in the chemistry of nerve agents and in the develop-
ment of methods of neutralization of these chemicals. Stock-
piles of nerve agents should be destroyed and the safety as
well as the environmental impact of the destruction process
is of crucial concern. Remediation procedures involve the
conversion of nerve agents to less toxic products. Examples
under development include nerve-agent oxidation through
the use of supercritical water,[3] caustic bleaching, incinera-
tion, and bioremediation.[4] However these procedures show
certain limitations. For instance, supercritical water has
proved effective in oxidizing organophosphorous-based
nerve agents. However, this procedure is associated with
high energy costs. The use of bleach requires solubilization
of nerve agents in large quantities of solvent, which must be
dealt with later in the process. In the case of bioremediation,
free or immobilized enzymes are used. But the stability of
these enzymes requires cold-chain handling and the exten-
sion of small lab-scale procedures to pilot-scale or field-
scale procedures has not been widely employed. This makes
high-temperature incineration the only currently approved
technique for the destruction of stored nerve agents. There-
fore efforts to develop new remediation methods are under
investigation[5] and chemical processes that can effectively
detoxify stored nerve agents are in great demand. Alterna-
tive approaches tested involve the use of catalysts able to
enhance the hydrolysis of nerve agents. Even though some
metal-catalyzed hydrolysis strategies have been developed,[6]
examples of organocatalysts in this context are very rare
and include, for instance, iodosylcarboxylates promoting the
hydrolysis of G-series nerve agents.[7]
Taking the above issues into account, and as a part of our
interest in these chemicals, it was in our aim to test the pos-
sible use of supramolecular interactions as a route to the
design of supramolecular-based organocatalysts for nerve-
agent remediation. In fact, host-guest chemistry has already
found many applications in catalysis[8] and the construction
of catalysts that employ supramolecular forces has recently
become a powerful tool.[9] In particular, we wanted to study
the use of supramolecular receptors for organophosphorous
derivatives that may enhance the hydrolysis rate of these
compounds through a suitable combination of coordinative
forces and enhancement of the electrophilic character of the
phosphorous atom. As a proof-of-concept we report herein
[a] A. Barba-Bon, Prof. A. M. Costero, Prof. M. Parra, Prof. S. Gil
Centro de Reconocimiento Molecular
y Desarrollo Tecnolꢀgico (IDM)
Unidad Mixta Universidad Politꢁcnica
de Valencia-Universidad de Valencia
Universidad de Valencia, Doctor Moliner 50
46100, Burjassot, Valencia (Spain)
Fax : (+34)963543831
E-mail: Ana.Costero@uv.es
[b] A. Barba-Bon, Prof. R. Martꢂnez-MꢃÇez, Dr. F. Sancenꢀn
Centro de Reconocimiento Molecular
y Desarrollo Tecnolꢀgico (IDM)
the catalytic remediation effect of a family of 1,3-diindol
ureas and thioureas (compounds 1–4, Figure 1) as potential
receptors for nerve-agent destruction.
ACHTUNGTRENNUNGyl-
AHCTUNGTRENNUNG
Unidad Mixta Universidad Politꢁcnica
de Valencia-Universidad de Valencia
The choice of neutral 1,3-diindolylureas and thioureas was
based in their known properties to selectively coordinate
phosphate anions in polar solvent mixtures and in the solid
state through NH···O hydrogen-bonding interactions.[10] We
expected that the corresponding formation of complexes
with nerve agents might lead to an enhanced polarization of
the P=O bond, which might result in an enhancement of the
rate of hydrolysis. In fact, while this research was under de-
velopment, the interaction between 1,3-diindolylureas and
the nerve agent Soman was demonstrated.[11] However, stud-
Departamento de Quꢂmica and CIBER de Bioingenierꢂa
Biomateriales y Nanomedicina (CIBER-BBN)
Universidad Politꢁcnica de Valencia
Camino de Vera s/n, 46022 Valencia (Spain)
E-mail: rmaez@qim.upv.es
[c] Prof. P. A. Gale, Dr. J. R. Hiscock
Chemistry, University of Southampton
Southampton SO17 1BJ (UK)
E-mail: philip.gale@soton.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202028.
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Chem. Eur. J. 2013, 19, 1586 – 1590

COMMUNICATION
Figure 1. Catalysts 1–4 used for nerve-agent destruction and structure of
indictor 5.
ies on the possible use of this interactions in remediation
has not been explored. Remediation studies have been car-
ried out here with the safer-to-handle simulants diethylchlo-
ACHTUNGTRENNUNGroACHTUNGTRENNUNGphosphate (DCP) and diethylcyanophosphonate (DCNP,
Figure 3. a) 1H NMR (aromatic region) in [D6]acetone corresponding to
free compound 1 (bottom) and compound 1 in the presence of 2 equiva-
lents of DCNP (top). b) 1H NMR (aromatic region) corresponding to
free compound 4 (bottom) and compound 4 in the presence of 1 equiva-
lent of DCNP (top).
Figure 2). These compounds have similar reactivity, but re-
duced toxicity compared with the corresponding chemical
warfare agents Sarin, Soman, and Tabun.
WinEQNMR[12] the corresponding association constants
were determined (see Table 1).
Table 1. Stoichiometry and association constants of compounds 1–4 with
DCNP in [D6]acetone.
Compound
Complex stoichiometry
Association Constant
1
2
3
4
1:2
1:1
1:2
1:1
881Æ90
4.4Æ0.3
Figure 2. Chemical structures of nerve agents and the simulants DCP and
DCNP.
1743Æ35
10.1Æ0.9
1
As a first step, H NMR experiments were carried out to
verify the formation of complexes between the receptors
and the simulants. Studies in [D6]acetone with receptors 1–4
and DCNP clearly demonstrated the existence of interac-
tions. This was reflected in shifts of the NH resonances of
the urea and indole groups and of the CH resonance at the
C6 position of the indole when 1–4 were mixed with DCNP.
Figure 3 shows the aromatic region of the NMR spectra cor-
responding to receptors 1 and 4 (see the Supporting Infor-
mation for spectra of compounds 2 and 3). Titration experi-
ments demonstrated that under these conditions 1:2 and 1:1
(receptor/DCNP) complexes were formed with receptors 1
and 3 or 2 and 4, respectively (see Figure 4). Considering
these stoichiometries and by using the software package
The stability constants revealed a clear relation between
the number of possible interactions for each receptor, the
stoichiometry of the complexes, and the strength of the in-
teraction. For instance, for receptors 1 and 3, which have a
larger number of binding sites, the formation of 1:2 (recep-
tor/DCNP) complexes was favored, whereas receptors 2 and
4, with a lower number of coordination sites, were only able
to form 1:1 species. Moreover, as expected 1 and 3 showed
larger association constants with DCNP than 2 and 4. In ad-
dition when the coordination constant of DCNP with 1 was
compared with that of 3, the latter was to a greater extent
due to the presence of more NH groups. Additionally the
higher value of the stability constant determined for 4 com-
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A. M. Costero, R. Martꢂnez-MꢃÇez, P. A. Gale et al.
1
with the naked eye. Control UV and H NMR experiments
demonstrated that there was no interaction between the in-
dicator and compounds 1–4.
In a typical experiment, two series of samples (3 mL
each) of indicator 5 (10À5 m in 1:3 v/v acetonitrile/2-morpho-
linoethanesulfonic acid (10-1 m) solution at pH 5.6) were
used. In addition, two DCNP (or DCP) solutions were also
prepared. An uncatalyzed solution contained 10% (v/v)
DCNP (or 5% (v/v) DCP) in acetone/water (80:20 v/v),
whereas the analogous catalyzed solution, had the corre-
sponding catalyst plus 10% (v/v) of DCNP (or 5% (v/v)
DCP) in acetone/water (80:20 v/v). The time at which these
solutions were prepared was noted and considered t=0. The
temperature during these experiments was maintained at
258C. At regular intervals (1 min), uncatalyzed and cata-
lyzed solutions (100 mL) were added to the corresponding
indicator solutions and the color was observed. Changes in
color for the different aliquots could eventually be related
to the hydrolysis of the nerve-gas simulants. Although this
procedure does not allow determination of the absolute hy-
drolysis rate, it permits calculation of the enhancement in
the hydrolysis for the catalyzed reaction in relation to the
uncatalyzed one. The variation of the simulant hydrolysis
rate was determined as Dn=100À
ACTHGNUTERN[UNNG (ncatꢅ100)/nuncat] for which
Figure 4. a) 1H NMR titration binding curve for compound 1 plus DCNP
in [D6]acetone for which [1]initial =5.80ꢅ10À2 m; 293 K. b) Job plot for the
DCNP–4 system in [D6]acetone for which [4]+[DCNP]=4.23ꢅ10À2 m;
293 K.
n is the time at which the color change occurred. Studies
were performed with two different concentrations of cata-
lysts (0.001 and 0.005 equiv). The results are summarized in
Table 2.
pared with 2 can be attributed to the presence of more
acidic protons in the thiourea group in 4 than for the same
moiety in 2. In addition, we also tested the interaction of re-
ceptor 4 and DCNP as a model simulant in [D6]acetone/
D2O (85:15), which is similar to the medium in which the
hydrolysis experiments were carried out (see below). In par-
ticular, NMR studies in this solvent mixture clearly demon-
strated that formation of complexes still occurred in this
competitive medium (see the Supporting Information).
After the demonstration of the formation of complexes of
1–4 with DCNP, remediation experiments were carried out
in buffered acetone/water (80:20 v/v) solutions. A potential
problem with hydrolysis of these agents is that they can
form particles that might, to some extent, protect them from
hydrolysis and may complicate the analysis of the results.
Therefore light-scattering studies in the presence and ab-
sence of compounds 1–4 were performed, however no for-
mation of such particles was observed under the conditions
used in this study.
The detection of the hydrolysis of the nerve-gas simulants
(DCP or DCNP) in the presence of the corresponding orga-
nocatalyst (1–4) was based on detecting acid generated
upon hydrolysis of the simulant with an appropriate indica-
tor.[13] When the hydrolyzed simulant generates enough pro-
tons to outstrip the leveling effect of the buffer, indicator 5
(2(E)-5-(2-(4-cyanophenyl)diazenyl)2-(dimethylamino)phe-
noxy)ethanol, Figure 1) changes the color of the solution
from yellow to colorless. These color changes were detected
Table 2. Variation of the hydrolysis rate of the simulants DCP and
DCNP in acetone/water (80:20 v/v) in the presence of organocatalysts 1–
4. Data is given as the percentage of enhancement of the hydrolysis rate
compared with the hydrolysis of the simulants in the absence of catalyst.
Catalyst
0.001 equiv[a]
DCP
0.005 equiv[a]
DCP
DCNP
DCNP
1
2
3
4
44.6Æ0.4%
13Æ1%
17Æ2%
30Æ1%
6.3Æ0.2%
6.3Æ0.4%
6.4Æ0.2%
11.3Æ0.4%
6.4Æ0.3%
19.5Æ0.6%
6.7Æ0.3%
14.8Æ0.2%
29.5Æ0.5%
12.7Æ0.2%
27Æ2%
9.8Æ0.5%
[a] Equivalents of catalyst relative to the amount of simulant.
The data clearly shows that there is an enhancement in
the hydrolysis rate of the nerve-agent simulants in the pres-
ence of the corresponding supramolecular organocatalysts.
Additional conclusions can also be drawn from these data.
For instance, for all four catalysts, the enhancement in the
rate for DCP hydrolysis was higher than that for DCNP.
These results are most likely related to the fact that ClÀ is a
better leaving group than CNÀ. In addition, it can be also
concluded that the catalytic effect was higher when there
were a larger number of hydrogen-bond donors present in
the receptor. Thus, 1 and 3 catalyzed the hydrolysis more ef-
fectively than 2 and 4. These data strongly support the hy-
pothesis that the catalytic mechanism includes simulant
complexation by increasing the electrophilic character of the
P atom and enhancing the final nucleophilic attack of water,
which results in the formation of the corresponding organo-
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Nerve Agent Remediation
COMMUNICATION
phosphate derivative and HCl or HCN. After hydrolysis, the
new generated species, which are less hydrophobic, are re-
leased from the catalyst, which is now able to start the cata-
lytic process again. This influence of the hydrophobic effect
in the binding site can be also observed if we compare the
results obtained with receptors 1 and 3. Thus, even though 3
showed higher association constants than 1, its catalytic
effect was poorer due to its lower hydrophobic character in
the catalytic center. Moreover, the presence of 1:2 com-
plexes of 1 and 3 may also favor the existence of some auto-
catalytic mechanisms similar to those shown in hydrolysis
studies with V-type nerve agents.[14] This suggestion is sup-
ported by the experiments carried out at different concen-
trations. Thus, experiments at other concentrations (from
0.0005 to 0.01 equiv of catalyst) showed that the increase in
catalyst concentration did not always lead to better results
and, in some cases, lower hydrolysis rates were observed.
This effect was more pronounced with 1 and 3 for which a
larger amount of receptor would give rise to lower concen-
trations of the 1:2 complex and as a consequence the auto-
catalytic effect decreases. With receptors 2 and 4, which
form 1:1 complexes, this effect was not observed (see the
Supporting Information).
Scheme 1. The proposed catalytic cycle involving catalysts 1–4 and
DCNP. The cycle involves 1) complexation or the nerve-gas simulants
with catalysts 1–4, 2) enhancement of the electrophilic character of the P
atom due to coordination, 3) nucleophilic attack of water, and 4) forma-
tion of the corresponding less toxic organophosphate derivative.
Finally, the influence of the temperature was also consid-
ered and studies with 1 and DCNP were carried out to mon-
itor the hydrolysis enhancement from 10 to 508C. An in-
crease in the hydrolysis rate was observed. However, a simi-
lar increase of the hydrolysis was also found in the corre-
sponding uncatalyzed samples (see the Supporting Informa-
tion).
concepts for new effective remediation procedures of nerve
agents.
Acknowledgements
DGICYT (projects MAT2009-14564-C04-1 and MAT2009-14564-C04-3)
and GeneralitatValenciana (project PROMETEO/2009/016) are grateful-
ly acknowledged. A.B.B. thanks MICINN for a pre-doctoral FPI fellow-
ship. SCSIE (Universidad de Valencia) is gratefully acknowledged for all
the equipment employed. PAG thanks the University of Southampton
for a post-doctoral fellowship (JRH).
In conclusion, we have observed that neutral 1,3-diindol-
ACHTUNGTRENNUNGylACHTUNGTRENNUNGureas and thioureas (1–4) are useful supramolecular orga-
nocatalysts in remediation of nerve-agent simulants. To the
best of our knowledge, this is the first time that supramolec-
ular-based organocatalysts have shown catalytic effect in re-
mediation processes for nerve agents, such as organophos-
phorous derivatives. In addition this method shows several
advantages compared with other previously described proce-
dures for the elimination of organophosphorous hazards
(see the Supporting Information). An enhancement of the
hydrolysis rate to near 45% in some cases was observed in
the presence of submolar concentrations of the receptors.
The formation of complexes between the receptors and
DCP and DCNP suggests that the catalytic mechanism in-
volves complexation of the nerve-gas simulants in a hydro-
phobic environment provided by the receptors, enhance-
ment of the electrophilic character of the P atom, and the
final nucleophilic attack of water that results in the forma-
tion of the corresponding less toxic organophosphate deriva-
tives (see Scheme 1). Moreover our findings suggest that
other hydrogen-bond-donor derivatives that are able form
complexes with organophosphorous compounds can also en-
hance the hydrolysis rates of these hazardous compounds.
By considering the large number of hydrogen-bond-donor
derivatives known in the supramolecular field, this observa-
tion opens new routes towards the possibility of designing
other organocatalysts based on supramolecular coordinative
Keywords: catalysts · complexation · diindolylureas · nerve
agents · remediation
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Received: June 7, 2012
Revised: November 7, 2012
Published online: December 23, 2012
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Chem. Eur. J. 2013, 19, 1586 – 1590