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with I > 2.0sI) (28 848 independent reflections, Rint 0.0922), for 2q in the range 1.56 to 50.08 (GOF 0.900). Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-140825. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax : ( 44) 1223-336-033 ; e-mail : deposit@ ccdc.cam.ac.uk). [9] L. Zhao, C. J. Matthews, L. K. Thompson, S. L. Heath, Chem. Commun. 2000, 265 ± 266. [10] Variable temperature magnetic measurements were carried out in the range 2 ± 300 K using a SQUID magnetometer (Quantum Design MPMS5S) in fields of 0.1 ± 5.0 T. Background corrections for the sample holder and diamagnetic portions of the complex were applied. [11] A. M. Garcia, F. J. Romero-Salguero, D. M. Bassani, J.-M. Lehn, G. Baum, D. Fenske, Chem. Eur. J. 1999, 5, 1803 ± 1808.

Needless to say, creation of ªgreenº catalytic processes for degradation of thiophosphoric acid esters is an urgent task of contemporary chemical technology and biotechnology. Significant progress has recently been made by the use of organophosphate hydrolases (OPH), a class of metal-dependent enzymes whose vital virtue is in the detoxification of organophosphoric species by reaction (1).[3] These enzymes
RR'P(X)Z H2O
OPH

þ3 RR'P(X)OH HZ

(1)

Highly Efficient Degradation of Thiophosphate Pesticides Catalyzed by Platinum and Palladium Aryl Oxime Metallacycles**
Gregory M. Kazankov, Victoria S. Sergeeva, Elena N. Efremenko, Larissa Alexandrova, Sergey D. Varfolomeev, and Alexander D. Ryabov*
Thiophosphoric acid esters such as 1 and 4, pesticides with an annual world production of hundred thousands of tons, are strong pollutants and their accumulation in the environment is a recognized ecological threat.[1] Some neurotoxins and V-type chemical warfare agents are chemically similar, and a potential danger is emerging from their aging stockpiles.[2]

are superior to their low molecular weight analogues in terms of catalyst specificity and activity, but the latter are often also efficient, more stable, and less subject to inactivation. Chemical catalysts with the features of enzyme active sites are believed to be particularly promising.[4] Recently, we introduced orthometalated complexes of PdII and PtII as mimetics of metal-dependent esterases. The catalysts display regio- and stereoselectivity as well as reasonable rate accelerations at neutral pH values.[5] However, the rates of hydrolysis of amino acid esters were not very striking, although the catalysts had features typical of metallohydrolases.[5b] It could be argued that a square-planar metal configuration, which is not reported in enzymes, is less favorable for catalysis, and that PdII and PtII are ªsoftº acids which require ªsofterº bases for efficient catalysis. The thiophosphoric acid esters 1 ± 5 are examples of such bases, and therefore metallacycles 6 and 7 (dmso dimethyl sulfoxide, py pyridine) have been tested as catalysts for the hydrolysis of pesticides 1 ± 5. Here we show that 6 and 7[6] as well as the PtIV complex 8[6] are excellent catalysts for the degradation of thiophosphate pesticides and neurotoxins. Parathion (1) is spontaneously hydrolyzed at pH 8.5 at an immeasurably slow rate. Addition of a catalytic amount of the PtII complex 6a enhances the rate of hydrolysis of 1 (by reaction (1)), which proceeds to completion and follows firstorder kinetics.[7] Methyl parathion (2) and coumaphos (4) behave similarly (Figure 1). No intercept is observed in the

[*] Prof. Dr. A. D. Ryabov, Dr. G. M. Kazankov, V. S. Sergeeva, Dr. E. N. Efremenko, Dr. L. Alexandrova,[] Prof. Dr. S. D. Varfolomeev Department of Chemistry Moscow State University 119899 Moscow (Russia) Fax : ( 7) 95-939-5417 E-mail : ryabov@enz.chem.msu.ru [] Permanent address : Instituto de Investigaciones en Materiales UNAM, Circuito Exterior s/n, Ciudad Universitaria (Mexico) [**] Financial support from the Russian Foundation for Basic Research (grant no. 98-03-33023a) and INTAS (project 97-0166) is gratefully acknowledged.
Angew. Chem. Int. Ed. 2000, 39, No. 17

Figure 1. Dependencies of methyl parathion (2, &), and at different concentrations. B4O7/NaOH buffer, pH 8.5,

kobs for the hydrolysis of parathion (1, *), coumaphos (4, ~) catalyzed by PtII complex 6a Conditions : [ester] 1 á 10þ4 m , 0.005 m Na20.01m NaClO4 , 25 8C.

plot of the observed rate constant versus the concentration of 6a, indicating good catalysis according to the rate law (2).
k
obs

k2 [catalyst]
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(2)

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Kinetic data were obtained for all the esters 1 ± 5 in combination with complex 6a, and for parathion (1) with complexes 6 ± 8. The rate law (2) holds throughout. The values The data in Table 1 are indicative of several remarkable features. First, the catalysis of paraoxon (3) hydrolysis by 6a is 30 times less efficient than that of parathion (1) hydrolysis. Thus, a soft sulfur donor center is more favorable than the harder oxygen. Interestingly, the base hydrolysis of paraoxon is about 1000 times faster than that of parathion. Second, the square-planar PtII complexes 6 are 30-fold more catalytically active than the structurally similar octahedral PtIV species 8, presumably due to steric effects imposed by a more crowded coordination sphere. Third, the catalytic activity of 6 can be increased threefold by introducing substituents at the benzene ring of the cyclometalated ligand. Remarkably, both electrondonating and electron-withdrawing groups favor the catalysis. Fourth, comparison of the ªtwinº PtII and PdII complexes 6f and 7 shows that the former is about five times more catalytically active. The hard ± soft principle seems to hold here as well. The hydrolysis of 1 catalyzed by 6a is pH-dependent in the range 6 ± 10 (Figure 2). The profile suggests two catalytically

of k2 are summarized in Table 1; the rates of the hydroxidecatalyzed hydrolysis of several esters were also measured at 0.001 ± 1.0 m NaOH, and the k2 values are included for comparison. The catalytic activity of 6 in the hydrolysis of 1
Table 1. Observed second-order rate constants k2 [m þ1 sþ1] for the catalyzed hydrolysis of phosphoric acid triesters 1 ± 5.[a] Catalyst 6a 6b 6c 6d 6e 6f 7 8 OH 1 310 ô 17 914 ô 21 773 ô 29 429 ô 8 452 ô 17 230 ô 8 54 ô 3 10.9 ô 0.4 (9.5 ô 0.6) á 10 2 175 ô 7 Ester 3 10.2 ô 0.1 4 5

141 ô 5 27.8 ô 0.3

þ

þ5

(2.6 ô 0.4) á 10

þ4

7.5 á 10

þ2[b]

[a] Conditions : pH 8.5, 25 8C, 0.01m NaClO4 . [b] Reference [3].

is 106 ±107 times higher than that of hydroxide. Assuming that only the OHþ-catalyzed pathway is operative in the absence of a metal catalyst at pH 8.0, the metal-catalyzed hydrolysis is 109 times more efficient than the spontaneous reaction for a 6b concentration as low as 10þ4 m ! This is in fact a significant number. In other systems where the hydrolysis of nitrophenyl phosphates catalyzed by biomimetically relevant transition metal catalysts was investigated,[8] the catalytic effects were usually in the range of 104 ±106. However, these estimates should be treated with care, since they often refer to the promoted hydrolysis rather than the catalyzed reaction, and the kinetic data are sometimes not used carefully enough. For example, the estimate of 1011 reported by Williams et al. corresponds to the promoted cleavage of 4-nitrophenyl methyl phosphate coordinated to a dinuclear CoIII complex.[9] A comparison of the catalytic activity of complexes 6 and OPH in the hydrolysis of 1 shows that the low molecular weight catalyst is fairly competitive, although OPH is a very reactive enzyme (kcat 600 sþ1 and KM 2.8 á 10þ4 m ).[3] Recalculation for a gram of catalyst provides k2 % 1 and kcat/KM % 60 gþ1 sþ1, which nicely advertises for the PtII catalysts taking into account that the rate of the enzymatic reaction levels off at high concentrations of 1. 3118
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Figure 2. The pH profile for 6a-catalyzed hydrolysis of parathion (1). Conditions : [1] 1 á 10þ4 m , 0.01m NaClO4 , 25 8C. The buffer components were 0.005 m Na2B4O7 (pH 8 ± 10) and 5,5'-diethylbarbituric acid (pH 6 ± 7.8).

active species. Fitting the data to Equation (3) leads to the rate constants kAH 40 ô 20 m þ1 sþ1 and kA 340 ô 20 m þ1 sþ1, and a Ka value of (1.2 ô 0.5) á 10þ8 m þ1 (pKa 7.9). As in our previous work,[10] the acid ± base equilibrium is ascribed to the deprotonation of the NOH group of the orthometalated oxime, a key nucleophilic center in the reaction.
k2 k
AH

H kA K
a

a

H K

(3)

It is well documented by us[10] and others[11] that the coordinated halides trans to the s-bound phenyl carbon in 6 or 7 are rapidly and completely hydrolyzed in water. Therefore, the reaction mechanism shown in Scheme 1 can be envisioned. The esters form sulfur-bonded intermediates B with aqua/hydroxo species A. Subsequently intramolecular nucleophilc attack of the oximate oxygen at PV takes place.[12] Thus, PdII or PtII play a dual role in the catalysis. The metal centers are binding sites for the substrate in close proximity to the coordinated oxime, the pKa of which is substantially lowered
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[3] D. P. Dumas, S. R. Caldwell, J. R. Wild, F. M. Raushel, J. Biol. Chem. 1989, 33, 19 659 ± 19 665. [4] a) J. Chin, Acc. Chem. Res. 1991, 24, 145 ± 152 ; b) J. Suh, Acc. Chem. Res. 1992, 25, 273 ± 279. [5] a) A. K. Yatsimirsky, G. M. Kazankov, A. D. Ryabov, J. Chem. Soc. Perkin Trans. 2 1992, 1295 ± 1300 ; b) E. V. Krooglyak, G. M. Kazankov, S. A. Kurzeev, V. A. Polyakov, A. N. Semenov, A. D. Ryabov, Inorg. Chem. 1996, 35, 4804 ± 4806 ; c) A. D. Ryabov, G. M. Kazankov, S. A. Kurzeev, P. V. Samuleev, V. A. Polyakov, Inorg. Chim. Acta 1998, 280, 57 ± 61; d) G. M. Kazankov, O. G. Dyachenko, A. V. Nemukhin, Scheme 1. Plausible reaction mechanism for the hyrolyisis of 1 ± 5 catalyzed by complexes 6 and 7. A. D. Ryabov, Mendeleev Commun. 1997, 159 ± 162 ; e) S. A. Kurzeev, G. M. Kazankov, A. D. Ryabov, Zh. Org. Khim. 2000, in press ; f) S. A. Kurzeev, due to coordination with the metal. As a result, a strong G. M. Kazankov, A. D. Ryabov, Inorg. Chim. Acta 2000, 305, 1±6. nucleophile is generated at neutral pH values. [6] Complexes 6b ± f were prepared as described for 6a : A. D. Ryabov, Coordination of the esters and the metals should also cause G. M. Kazankov, I. M. Panyashkina, O. V. Grozovsky, O. G. Dyachenan increase in the effective positive charge at PV, and thus ko, V. A. Polyakov, L. G. Kuzmina, J. Chem. Soc. Dalton Trans. 1997, facilitate the intramolecular nucleophilic attack by the 4385 ± 4391. Their full characterization has been reported : L. Alexandrova, O. G. Dyachenko, G. M. Kazankov, V. A. Polyakov, P. V. oximate. The replacement of DMSO or pyridine ligands at Samuleev, E. Sansores, A. D. Ryabov, J. Am. Chem. Soc. 2000, 122, PtII in 6 and 7 is unlikely since no characteristic bands for free 5189 ± 5200. The PdII complex 7 was obtained as described in ref. [10] . pyridine were observed in the UV/Vis spectrum of a mixture [7] Stock solutions of 1 (Sigma) or 2 ± 4 (Fluka) in MeCN were added to a of 5 and 7. The mechanism in Scheme 1 is supported by the UV/Vis cuvette (d 1 cm) witha solution of 6 or 7 in a buffer, and the progress of reaction (1) was monitored at l 405 (e 17000 m þ1 cmþ1) UV/Vis and 31P NMR study of the 6a-promoted hydrolysis of for 1 ± 3 and at l 384 nm (e 9000 m þ1 cmþ1) for 4. Pseudo-first-order neurotoxin demeton-S (5). The 31P NMR resonance for intact rate constants were calculated from the graphs of absorbance (A) 5 is at d 30.005 in aqueous 1 á 10þ4 m NaClO4 . Addition of versus time (t) obtained by plotting ln [AI/(AI þ At) ] against t. 6a generates a new signal at d 0.655 for the reaction product Linearity was observed for three to five half-lives. Every reported rate (EtO)2P(O)OH.[13] Although the rate of hydrolysis of 5 constant is a mean value of three determinations. [8] a) P. Hendry, A. M. Sargeson, J. Am. Chem. Soc. 1989, 111, 2521 ± increases tremendously in the presence of 6a, there is no true 2527; b) Y. Chung, E. U. Akkaya, T. K. Venkatachalam, A. W. catalysis and a stoichiometric amount of 6a is necessary. Gzarnik, Tetrahedron Lett. 1990, 31, 5413 ± 5416 ; c) L. Y. Kuo, S. Cleavage of 5 liberates the potentially good ligand Kuhn, D. Ly, Inorg. Chem. 1995, 34, 5341 ± 5345 ; d) W. H. Chapman, HSCH2CH2SEt, which on interaction with the PtII C,NR. Breslow, J. Am. Chem. Soc. 1995, 117, 5462 ± 5469 ; e) I. O. Kady, B. Tan, Z. Ho, T. Scarborough, J. Chem. Soc. Chem. Commun. 1995, metallacycle blocks the substrate binding site, making intra1137 ± 1138 ; f) M. Wall, B. Linkletter, D. Williams, A.-M. Lebuis, R. C. molecular nucleophilic attack impossible. Hynes, J. Chin, J. Am. Chem. Soc. 1999, 121, 4710 ± 4711. II In conclusion, cyclometalated Pt complexes efficiently [9] N. H. Williams, W. Cheung, J. Chin, J. Am. Chem. Soc. 1998, 120, catalyze the degradation of thiophosphoric acid pesticides 8079 ± 8087. (1 ± 3). Comparison with related PdII and PtIV complexes [10] A. D. Ryabov, G. M. Kazankov, A. K. Yatsimirsky, L. G. Kuzmina, O. Y. Burtseva, N. V. Dvortsova, V. A. Polyakov, Inorg. Chem. 1992, indicates that the PtII catalysts are more active and exhibit an 31, 3083 ± 3090. increased selectivity towards sulfur-containing triesters. It has [11] M. SchmÝlling, D. M. Grove, G. van Koten, R. van Eldik, N. Veldman, been previously noticed that PdII complexes could be A. L. Spek, Organometallics 1996, 15, 1384 ± 1391. promising catalysts in reaction (1).[14] Cycloplatinated com[12] The solvent isotope effect of 1.2 ô 0.1 was determined in the reaction of 1 and 6c at pD 10.05 (borate buffer) and is consistent with the pounds with coordinated oxime could be used to create even nucleophilic mechanism ; see M. L. Bender, R. J. Bergeron, M. more specific and reactive catalysts. Komiyama, The Bioorganic Chemistry of Enzymatic Catalysis, Wiley, New York, 1984. Received : March 20, 2000 [Z 14871] [13] K. Lai, N. J. Stolowich, J. R. Wild, Arch. Biochem. Biophys. 1995, 318, 59 ± 64. [14] J. H. Mendez, R. C. Martinez, J. S. Martin, Anal. Chem. 1986, 58, 1969 ± 1972. [1] M. A. Gallo, N. J. Lawryk, Organic Phosphorous Pesticides. The Handbook of Pesticide Toxicology, Academic Press, San Diego, CA, 1991. [2] J. K. Grimsley, V. K. Rastogi, J. R. Wild, Biological Detoxification of Organophosphorus Neurotoxins, Vol. 2, Technomic Publishing Company, Inc, Lancaster, PA, 1998.
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