Cycloplatinated aryl ketoximes as efficient biomimicking catalysts for hydrolysis of esters of phosphorothioic acid

Article abstract of DOI:10.1023/A:1014334129893

Cyclometallated aryl ketoximes are introduced as catalysts for hydrolysis of organophosphorus neurotoxins. Platinum-containing catalysts exhibit the highest activity and selectivity with respect to O-alkyl phosphorothioates (parathion, methyl parathion, coumaphos) and efficiently promote the hydrolysis of S-alkyl phosphorothioates and -dithioates (demeton-S, malathion) at the P - S bonds.

Full text of DOI:10.1023/A:1014334129893

«
G. M. Kazankov, V. S. Sergeeva, A. A. Borisenko, A. I. Zatsman, and A. D. Ryabov
1
844
Russian Chemical Bulletin, International Edition, Vol. 50, No. 10, pp. 1844—1848, October, 2001
Cycloplatinated aryl ketoximes as efficient
biomimicking catalysts for hydrolysis
of esters of phosphorothioic acid
Department of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie gory, 119899 Moscow, Russian Federation
Fax: (095) 939 5417. E-mail: g_kazankov@hotmail.com
Cyclometallated aryl ketoximes are introduced as catalysts for hydrolysis of organophos-
phorus neurotoxins. Platinum-containing catalysts exhibit the highest activity and selectivity
with respect to O-alkyl phosphorothioates (parathion, methyl parathion, coumaphos) and
efficiently promote the hydrolysis of S-alkyl phosphorothioates and -dithioates (demeton-S,
malathion) at the P—S bonds.
Key words: metal rings, neurotoxins; hydrolysis, catalysis, biomimicking, phosphorothioates,
platinum and palladium complexes.
Study of metal complexes as models of the active
Enzymatic hydrolysis based on the use of organo-
phosphate hydrolase (OPH), which is a metal-depen-
dent phosphotriesterase, is among the most promising
methods for this treatment.11—14 On the basis of the
mechanism of substrate transformation in the OPH
active site,15 in this work, we propose highly efficient
metal complex catalysts 7—9 for the hydrolysis of sul-
fur-containing organophosphorus compounds prepared
from cyclometallated complexes of aryl ketoximes. These
catalysts provide nearly enzymatic rates of hydrolysis,
possess high selectivity with respect to sulfur-containing
esters, and they are much more stable than the enzyme.
These complexes were chosen due to several reasons.
Previously,16—18 it has been shown that cyclometallated
aryl ketoximes mimic metal-dependent esterases and
accelerate hydrolysis of esters including amino acid
esters with high regio- and stereoselectivity. Since PtII is
a "soft" Pearson´s acid, it exhibits a high affinity toward
a "soft" base, i.e., the electron-donating sulfur atom.
sites of enzymes, in particular, of various phosphomono-
and diesterases is one of the most vigorously devel-
oped lines of research in bioinorganic chemistry and
biomimetics.1—7
Hundreds of thousand tons of organophosphorus
pesticides and toxic chemical agents corresponding to
the class of O-alkyl phosphorothiates (parathion (1),
methyl parathion (2), and coumaphos (3)) and S-alkyl
phosphorothioates and -dithioates (demeton-S (4) and
malathion (5)) have been produced during the last
several decades. This fact has entailed grave environ-
mental problems and calls for the development of meth-
ods for annihilation of these substances.8—10
X
RO
P
O
NO2
RO
1
: R = Et, X = S
: R = Me, X = S
: R = Et, X = O
2
6
R1
Me
S
P
EtO
EtO
O
O
Me
OH
Me
OH
O
Cl
Z
M
N
Cl Pt
N
Cl
Me S
Cl
2
Cl
Me
O
3
EtO
EtO
P
S
7a—f, 8
9
S
Et
7
8
: M = Pt (a—f), Z = dmso (a—e), py (f),
R1 = H (a, f), MeO (b), Me (c), F (d), Cl (e)
: M = Pd, Z = py, R1 = H
4
S
P
MeO
MeO
H
O
S
C
OEt
O
Study of the catalytic activity of these complexes with
respect to substrates 1—6 is the object of the present
study.
H C
2
OEt
5
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1761—1765, October, 2001.
066-5285/01/5010-1844 $25.00 © 2001 Plenum Publishing Corporation
1

Cycloplatinated aryl ketoximes
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 10, October, 2001
1845
Experimental
The kinetics of hydrolysis of esters 4 and 5 in the presence
of complexes 7—9 was studied by spectrophotometry. For this
purpose, a solution of the Pt or Pd complex (1•10– to
5
1H and 31P NMR spectra were recorded on Varian VXR
00 and Bruker instruments operating at 400 and 300 MHz,
•10–5 mol L–1) in the corresponding buffer was prepared
5
4
from an acetonitrile solution of the complex (1•10 mol L–1).
–2
respectively. Deuterated acetonitrile, chloroform, methanol,
and water were used as solvents. Spectrophotometric studies
were carried out on a Hitachi 150-20 spectrophotometer (Japan)
equipped with a cell maintained at a constant temperature.
Paraoxon 6 (Sigma) was purified prior to use in the follow-
ing way: 1 mL of the sample was dissolved in 250 mL of
dichloromethane, the products of hydrolysis were extracted
with water, the organic phase was separated, and MgSO4 was
added and filtered off. Dichloromethane was evaporated using a
rotary evaporator. The purified paraoxon was dissolved in dou-
bly distilled water (c 1•10–2 mol L–1).
An aliquot portion of an organophosphorus compound in MeCN
1•10–2 mol L–1) was added to the buffered solution of the
complex in such a way that the organophosphate concentration
(
–6
–5
–1
in the buffer solution was 1•10 —5•10
mol L . The
mixture was kept in the thermostated spectrophotometer cell
holder, the variation of the absorption spectrum of the reaction
mixture with time was recorded, and the wavelength corre-
sponding to the maximum change of absorption was deter-
mined. Then the kinetic data were obtained at this wavelength.
The observed rate constants were determined from analysis of
full kinetic curves, as described above.
Parathion 1 (Sigma), methyl parathion 2 (Fluka), couma-
phos 3 (Fluka), demeton-S (4) (ChemService, USA), and
malathion 5 (Fluka) and organic solvents, HPLC grade aceto-
nitrile and dichloromethane (Reakhim), were used as received.
The cyclometallated complexes were synthesized by known
procedures.16,17 The kinetics of hydrolysis of esters 1—4 in the
presence of compounds 7—9 was studied by spectrophotometry
at 25 °C and an ionic strength of 0.01 Ì in sodium acetate and
sodium 5,5´-diethyl barbiturate buffer solutions with a total
Results and Discussion
Parathion (1) is hydrolyzed very slowly in aqueous
solutions at neutral pH values (Table 1). The addition of
catalytic amounts (10–6—10–5 mol L–1) of complexes
—9 markedly accelerates the hydrolysis.
Analysis of the resulting full kinetic curves in the
7
concentration of 5•10–3 mol L–1
.
ln(A /(A – A(t)) – t coordinates, where A is the optical
General procedure of investigations. A solution of a complex
in the appropriate buffer solution was prepared from an acetoni-
trile solution of the complex (c 1•10–2 mol L–1) in such a way
that the concentration of the complex in the buffer solution was
∞
∞
density, shows that the reaction obeys exactly the first
order and proceeds to completion even with a 100-fold
excess of the substrate, i.e., the process is catalytic. This
is also confirmed by the fact that the second-order rate
constant, which determines the efficiency of catalysis,
remains invariable in the range of parathion (1) concen-
trations of 5•10–6—1•10–4 mol L–1, i.e., it does not
depend on the substrate concentration. Hydrolysis of
esters 2 and 3 in the presence of complexes 7—9 obeys
the same kinetics. The observed first-order rate con-
stants are described satisfactorily by the equation
5
•10–7—5•10–5 mol L–1. An aliquot portion of an acetonitrile
–
2
–1
solution of an organophosphorus compound (c 1•10 mol L
)
was added to the buffered solution of the complex in such a way
that the concentration of the organophosphorus compound in
the reaction mixture was 1•10–4 mol L–1. The mixture was kept
in the spectrophotometer cell at a constant temperature. The
hydrolysis kinetics of substrates 1—4 was studied by monitoring
the optical density of the solution at wavelengths corresponding
to the hydrolysis products, namely, 4-nitrophenol (pKà = 7.20,
λ = 405 nm, ε = 17000 L mol–1 cm–1 for the deprotonated
form, λ = 320 nm for the protonated form) or 7-chloro-8-
kobs = k /ccat,
(1)
2
–
1
–1
hydroxybenzopyrone (λ = 384 nm, ε = 9000 L mol cm ).
The observed rate constants were determined either from analy-
sis of full kinetic curves in the first-order reaction coordinates,
ln(A /(A – A(t)) = kobst, or using the Guggenheim20 method.
where ccat is the catalyst concentration.
The kinetics of hydrolysis of substrates 1—4 was
studied in the presence of complex 7à, while that for
substrate 1 was measured in the presence of each of
∞
∞
The rate constants determined by different methods were in
satisfactory agreement with one another. The catalytic rate
constants at a given pH were calculated as the slope ratio of the
plot of kobs vs. complex concentration using the least-squares
method.
The kinetic isotope effect of the solvent (kH2O/kD2O) in the
hydrolysis of 1 catalyzed by complex 7c was found as the ratio
of second-order rate constants in a borate buffer solution at
–1
s–1) for the
Table 1. Catalytic rate constants (k /mol L
2
hydrolysis of esters 1—4 and 6 in the presence of complexes
7—9 at pH 8.5, 25 °Ñ, 0.01 M NaClO4
Com-
plex
1
2
3
4
6
2
5 °C. The pH value for the buffer solution was calculated as
pD = pH + 0.4, where pH is the reading of a pH-meter.
The activation parameters of the hydrolysis of 1 in the
presence of complex 7à were determined from the data ob-
tained at 20—50 °C; the pH of the buffer solution was brought
to 9.5 at each temperature. The resulting catalytic rate con-
stants were analyzed in the lnk2 – T –1 coordinates. The
activation energy was found from the slope of this plot, while
7
7
7
7
a
b
c
d
310±17
914±21
773±29
429±8
452±17
230±8
175±7 141±5 22.4±2.2 10.2±0.1
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
24.4±2.8
—
27.2±2.6
7e
7
8
9
f
the activation enthalpy was calculated from the relation
54±4
10.9±0.4
10.1±1.8
—
≠
∆
H
= Ea – RT. The activation entropy was determined from
≠
≠
the equation k = k T/h•exp(–(∆H – T∆S /RT )) at 25 °C,
–5
–4
–4
–2
B
ÎÍ* (9.5±0.6)•10 (2.6±0.4)•10
6.67•10 7.5•10
where kB is the Boltzmann constant and h is the Planck
constant.
* The rate of alkaline hydrolysis.

1
846
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 10, October, 2001
Kazankov et al.
complexes 7à—f, 8, and 9. Equation (1) was found to
hold in all cases. The second-order rate constants for
hydrolysis of compounds 1—4 are presented in Table 1;
the rate constants for alkaline hydrolysis are presented
in the same Table for comparison. Comparison of the
rate constants for alkaline and 7à-catalyzed hydrolyses
of parathion (1) demonstrates that the rate constant for
the catalytic process exceeds the rate constant for the
plex related by an acid-base equilibrium are present in
the solution.
+
+
k2 = (kAH[H ] + k K )/([H ] + K ).
(2)
A
a
a
Fitting the experimental data to Eq. (2) made it
possible to determine the kAH and kA constants corre-
sponding to aqua and hydroxo forms of complexes:
4
0±20 and 340±20 M–1 s–1, respectively.
The acid dissociation constant K for complex 7à is
6
7
alkaline hydrolysis by a factor of 10 —10 .
At pH 8.0, the rate of the catalytic reaction in the
presence of 1•10–4 mol L–1 of complex 7b is 109 times
a
16
(
1.2±0.5)•10–8 mol–1 L (pK 7.9). Previously, it was
a
shown that Kà refers to deprotonation of the hydroxy
group of the oxime ligand.
as high as the rate of hydrolysis of 1 without a catalyst.
The catalytic effect attained exceeds substantially the
values found previously for acceleration of hydrolysis of
a series of 4-nitrophenyl phosphates in the presence of
The temperature dependence of the rate constant for
parathion (1) hydrolysis in the presence of complex 7à
was used to calculate the activation enthalpy and en-
1
—7,19
various transition metal complexes,
which are at
tropy, 25.0±0.5 kJ mol–1 and –113±7 J (mol K)–1
,
4
6
most 10 —10 . Analysis of the rate constants deter-
mined, which are listed in Table 1, shows that substrate
coordination to the softer PtII ion is more efficient for
the catalysis of hydrolysis than the coordination to PdII
and, similarly, the coordination of metal ion to the soft
S atom of the P=S group is more favorable than the
coordination to the "hard" O atom of the P=O group.
The square planar PtII complexes (7) proved to be
respectively. Since the first step of the process is very
fast and cannot be detected by any experimental meth-
ods, it can be considered that the measured activation
parameters refer to the rate-determining step of the
reaction, namely, to the decomposition of the sub-
strate—catalyst primary complex. The large negative
entropy reflects, apparently, the necessity of spatial
orientation of the reactant molecules for the intramo-
lecular nucleophilic attack on the coordinated substrate
by the oximate ion, i.e., it attests to a highly ordered
transition state in the bimolecular reaction in question.
3
0 times more active in the reaction in question than
IV
the octahedral Pt complex (9). It also follows from
Table 1 that cycloplatinated aryl ketoximes exhibit the
highest catalytic activity when the aromatic ring of
complex 7 contains electron-donating substituents (MeO
in 7b or Me in 7c).
The kinetic isotope effect k
/k
for the catalysis
H O D O
2
2
of hydrolysis of ester 1 by complex 7b is 1.2±0.1.
Therefore, it appears that no proton transfer takes place
in the rate-determining step of the process, i.e., the free
OH– ion does not participate in the reaction, and the
nucleophilic attack occurs intramolecularly under
the action of the coordinated oximate ion, i.e.,
nucleophilic rather than specific basic catalysis is
involved.
The data presented above suggest the following
mechanism for the catalysis of hydrolysis of phosphoro-
thioates by cycloplatinated and cyclopalladated aryl
ketoximes. The substrate can be coordinated to the
metal ion through the S atom upon substitution of one
ligand in the coordination sphere. The formation of this
complex decreases the electrophilicity of the attacked
phosphorus center of the substrate. This is followed by
intramolecular attack on the phosphorus atom by the
coordinated oximate ion.
To elucidate the mechanism of hydrolysis of
phosphorothioates in the presence of complexes 7—9,
the dependence of the catalytic constant for hydrolysis
of ester 1 on the medium pH was studied. Figure 1
shows the dependence measured for complex 7à. It can
be seen from the Figure that the curve contains two
horizontal sections at 8 < pH and pH < 7. The resulting
pH profile containing two pH-independent sections in-
dicates that two catalytically active forms of the com-
k2/L mol–1 s–1
3
3
2
2
1
1
50
00
50
00
50
00
Compounds 4 and 5 are barely hydrolyzed in water
and are exceptionally stable even against enzymatic
hydrolysis. The kcat/Km ratio (kcat, Km are parameters of
the Michaelis—Menthen equation for a successive two-
step reaction mechanism) for hydrolysis of paraoxon,
demeton-S, and malathion in the presence of OPH are
5
0
8
3
–1
L s–1, respectively,
1
.27•10 , 1.6•10 , and 0.8 mol
6
7
8
9
pH
Fig. 1. Rate constant of parathion (1) hydrolysis catalyzed by
complex 7à (parathion concentration 1•10–4 mol L–1
whereas the constants of alkaline hydrolysis differ by
only 2 or 3 orders of magnitude.12,14 In addi-
tion, demeton-S exerts a substantial inhibitory effect
on OPH.
,
–
1
µ = 0.01 mol L NaClO , 25 ˜°C) vs. pH.
4

Cycloplatinated aryl ketoximes
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 10, October, 2001
1847
Hydrolysis of esters 4 and 5 in the presence of
in the range of 20—27 mol L–1 s–1. As shown above, the
cleavage of the ester bond in the hydrolysis of esters
1—4 depends on the pH and also on the presence of
substituents in the aromatic ring of aryl ketoxime. This
suggests that the variations observed in the UV spectrum
of the reaction mixture are due to the complex forma-
tion between the ester and cyclometallated aryl ketoxime,
and, apparently, it is complexation that is the rate-
determining step of hydrolysis of ester 5 promoted by
complexes 7 and 8, while the subsequent rupture of the
P—S bond is fast.
The results obtained suggest the following mecha-
nism of reaction between cyclometallated aryl ketoximes
and 5: the ester is coordinated to the metal ion of
complexes 7—8 through the sulfur atom attached to
phosphorus. This is followed by the nucleophilic attack
by the coordinated oximate ion, resulting in the cleav-
age of the P—S bond and abstraction of the diethyl
hydrogen phosphate. The next step is, apparently, com-
plexation between the catalyst and the chelating ligand.
The formation of the chelate complex with demeton-S
might also occur at the first, rate-determining step of
the reaction, which is followed by cleavage of the
P—S bond.
cyclometallated aryl ketoximes was studied by 31P NMR
spectroscopy and by spectrophotometry. Analysis of the
1P NMR spectra recorded before and after incubation
of ester 5 in an aqueous solution with pH 7.0 for 48 h at
5 °Ñ in the absence of a complex shows that the initial
3
2
compound is responsible for a singlet at δ 32, and no
changes take place in the spectrum during this period.
The spectrum of demeton-S recorded 15 min after the
addition of complex 7d to the reaction mixture exhibits
a signal with a chemical shift of 0.6 ppm, typical of
diethyl hydrogen phosphate, formed upon hydrolysis of
ester 5. This indicates unambiguously that the P—S
bond is cleaved in the presence of the PtII complex.
Integration of the spectrum shows that the amount of
reacted ester 5 is equivalent to the amount of complex
7
d introduced in the system, i.e., the complex acts as a
promoter rather than as a catalyst. In all probability,
II
this is due to the formation of a stable complex of Pt
with the chelating thiol HS(CH ) SEt liberated during
2
2
the reaction.
The kinetics of hydrolysis of demeton-S (4) in the
presence of metallacyclic compounds 7 and 8 was stud-
ied by UV spectroscopy. Figure 2 shows a time variation
of the spectrum of the reaction mixture containing ester
Malathion (5) is exceptionally stable against both
alkaline and enzymatic hydrolysis. Investigation of an
aqueous solution of malathion (pH 9.25, a 0.05 M
Na B O —NaOH buffer) by 31P NMR spectroscopy
4
and complex 7b. Analysis of the variation of the
absorption of the reaction mixture at 325 nm for com-
plexes 8 and 9 and demeton-S : complex ratios of 1 : 1
to 1 : 10 shows that the reaction has the exactly first
order with respect to both reactants.
2
4
7
A
Study of the dependence of the second-order rate
2
000
constant (k = kobs/ccomplex) on the pH showed that the
2
A
values of these constants for complexes 7 and 8 in a pH
range of 6.8—12.0 do not depend on pH within the
experimental error. In addition, the rate constants for
complexes 7a—f, containing substituents in the benzene
ring, are close and, as can be seen from Table 1, they lie
a
b
1
100
A
2
50
300 λ/nm
1
.6
2
60
280
300
λ/nm
Fig. 3. UV spectrum of the reaction mixture containing
malathion and complex 7d: (à) the first and second steps of the
reaction, the range of recording the spectra is 1 min, (b) the
third step of reaction, the range of recording the spectra is
15 min. The arrows mark the directions of variation of
the spectra in the second and third steps of reaction;
0
2
00
Fig. 2. UV spectrum of demeton-S (4) in the presence of
complex 7b; [4] [7b]
2•10–5 mol L–1 5•10–3
Na B O —NaOH, pH 9.25, 25 °C, the range of recording the
250
300
λ/nm
=
=
,
Ì
[5] = 2•10 mol L , [7d] = 2•10 mol L–1, buffer, 5•10
–
5
–1
–5
–3
˜
2
4
7
spectra is 1 min.
mol L–1 Na B O —NaOH, pH 9.25, 20
˜
°C.
2
4
7

1
848
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 10, October, 2001
Kazankov et al.
demonstrated that dimethyl hydrogen phosphorothioate
is not formed at 25 °C even after 120 days. The NMR
spectrum is a singlet with δ 96.3, corresponding to the
initial compound 5. After the addition of complex 7b to
a solution of malathion, organophosphate is hydrolyzed
in 22 h in a concentration equivalent to the concentra-
2. M. A. De Rosch and W. C. Trogler, Inorg. Chem., 1990,
2
9, 2409.
˜
3
4
. J. Chin and X. Zou, J. Am. Chem. Soc., 1988, 110, 223.
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1
11, 2521.
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995, 117, 5462.
5
1
tion of the complex added. At 50
˜
°C, the reaction
6. N. H. Williams, A.-M. Lebuis, and J. Chin, J. Am. Chem.
Soc., 1999, 121, 3341.
7. N. H. Williams, W. Cheung, and J. Chin, J. Am. Chem.
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proceeds completely over a period of less than 3 h. The
appearance of a signal with a chemical shift of 61.8 ppm
attests to the formation of dimethyl hydrogen phos-
phorothioate and, hence, to the cleavage of the P—S
ester bond. Thus, cyclometallated aryl ketoximes are
able to increase the rate of malathion hydrolysis by
several orders of magnitude; their activity in this reac-
tion is dozens times as high as that of OPH. The time
variation of the absorption spectrum of 5 in the pres-
ence of complex 7d (Fig. 3) was also studied. Analysis
of these data indicates that hydrolysis of malathion in
the presence of PtII complexes occurs in, at least, three
steps, whose nature is currently under study.
Thus, cycloplatinated and cyclopalladated aryl
ketoximes, which are easy to synthesize, can serve as
highly efficient catalysts for hydrolysis of sulfur-con-
taining pesticides stable to degradation, such as par-
athion and coumaphos. The catalytic effect of these
complexes is comparable with the enzymatic activity of
OPH. In addition, these complexes are effective pro-
moters of hydrolysis of thiol esters 4 and 5; their activity
in these reactions markedly exceeds the activity of
organophasphatehydrolase.
8
9
. M. A. Gallo and N. J. Lawryk, Organic Phosphorous Pesti-
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1
1
1
5. V. S. Sergeeva, E. N. Efremenko, G. M. Kazankov, and
S. D. Varfolomeyev, J. Mol. Cat. B, Enz., 2000, 10, 571.
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2
80, 57.
1
1
2
8. S. A. Kurzeev, G. M. Kazankov, and A. D. Ryabov, Inorg.
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The authors wish to thank Prof. S. D. Varfolomeyev
for participation in the discussion of the results.
This work was financially supported by the INTAS
(
grant 97-0166).
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Received March 5, 20001
in revised form May 18, 2001