Divalent Metal Ion-Catalyzed Hydrolysis of ... - ACS Publications

Environ. Sci. Technol. 1997, 31, 1664-1673

Divalent Metal Ion-Catalyzed Hydrolysis of Phosphorothionate Ester Pesticides and Their Corresponding Oxonates JEAN M. SMOLEN† AND ALAN T. STONE* Department of Geography and Environmental Engineering, G. W. C. Whiting School of Engineering, The Johns Hopkins University, Baltimore, Maryland 21218

The divalent metal ion-catalyzed hydrolysis of thionate (PdS) and oxonate (PdO) organophosphorus pesticides has been examined in light of three possible catalysis mechanisms: (1) metal ion coordination of the thionate sulfur or oxonate oxygen to enhance the electrophilicity of the phosphorus electrophilic site; (2) metal ion coordination and induced deprotonation of water to create a reactive nucleophile; and (3) metal ion coordination of the leaving group to facilitate its exit. The effect of the following metals at a concentration of 1 mM was examined: CoII, NiII, CuII, ZnII, and PbII. These metal ions were chosen for their ability to complex organic ligands and inorganic nucleophiles. Of these metal ions, CuII possesses properties most suitable for all three catalytic mechanisms and serves as the most effective catalyst for the five thionate esters (chlorpyrifosmethyl, zinophos, diazinon, parathion-methyl, and ronnel) and the two oxonate esters (chlorpyrifos-methyl oxon and paraoxon) included in this study. A decrease in the degree of CuII catalysis at high pH arises from solubility limitations. PbII nearly matches CuII as a catalyst for oxonate esters, but is a less effective catalyst for thionate esters. Catalysis by CoII, NiII, and ZnII is negligible. Phenolate product analysis indicates that metal catalysis in some instances shifts hydrolysis from alkyl carbon-centered pathways to phosphoruscentered pathways.

Introduction Phosphorothionate triesters (which will be referred to as thionate esters) contain a PdS moiety and are among the most widely used insecticides in the United States. One thionate ester, chlorpyrifos, is used in amounts in excess of 14 million lb/year in U.S. agriculture (1). Thionate esters exhibit a lower mamalian toxicity than corresponding oxonate esters, which contain a PdO moiety and, for this reason, are more widely used (2). Thionate esters are however converted to oxonates by oxidative processes both inside and outside of organisms (3). Thus, both thionate and oxonate esters must be considered when addressing pesticide transformation and fate. Hydrolysis is a major degradation pathway for both thionate ester pesticides and their corresponding oxonates. Many thionate and oxonate esters hydrolyze quickly in alkaline solution (4-6), but persist under the neutral and * Corresponding author phone: 410-516-8476; fax: 410-516-8996; e-mail: DOG [email protected]. † Present address: United States Environmental Protection Agency, 960 College Station Road, Athens, GA 30605; e-mail: SMOLEN.JEAN@ EPAMAIL.EPA.GOV.

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slightly acidic conditions typical of surface waters, soils, and aquifer sediments. There is growing evidence that pollutants may interact with dissolved and particulate chemical constituents of aquatic environments, thereby altering pathways and rates of degradation (7, 8). In particular, the role of dissolved metal ions and metal-containing mineral surfaces in catalyzing hydrolysis reactions merits consideration (9, 10). The pesticides examined here can hydrolyze via two pathways: attack by OH- and H2O at the phosphorus atom, which is a “hard” electrophilic site, and attack by H2O at the carbon atoms within the alcoholate ester linkages, which are “soft” electrophilic sites (5). Although both thionate and oxonate esters are believed to hydrolyze in this way, the ratio of phosphorus-centered attack to carbon-centered attack may differ. The same chemical properties that impart biological activity to thionate and oxonate ester pesticides also determine their susceptibility toward hydrolysis. The pKa of the most facile leaving group must be below 8.0 in order for reaction with the enzyme acetylcholinesterase to occur, which kills the target organism, but higher than 6.0 so that the pesticide persists long enough to contact the target organism (11). Many commercial pesticides accomplish this with a combination of one phenolate leaving group and two alcoholate leaving groups (Figure 1). The pKa of the most facile leaving group also determines the site where the initial hydrolysis step will occur. Pathways of ester hydrolysis are presented in Figure 2. Each hydrolysis step may involve loss of either the phenolate leaving group or one of the alcoholate leaving groups. Measurements of phenolate product concentrations during the hydrolysis reaction can provide information regarding mechanism. If the phenolate product is less than the amount of parent lost, then a significant fraction of the triester hydrolysis must occur through the alcoholate leaving group. If the phenolate product is equal to the amount of parent lost, then two possibilities exist: triester hydrolysis occurs exclusively via the phenolate group or a significant fraction of triester hydrolysis occurs via an alcoholate leaving group, but subsequent hydrolysis steps are fast enough to yield stoichiometric amounts of phenolate product. This work is part of a broader study that seeks to (i) identify thionate ester pesticides subject to metal catalysis; (ii) compare the susceptibility of thionate esters and oxonate esters toward catalysis; (iii) examine how catalysis depends upon the properties of the metal catalyst; (iv) examine how catalysis depends upon the structure of the ester; (v) examine how pH affects catalysis; and (vi) interpret these findings in light of possible mechanisms of catalysis. The divalent transition metal ions selected for study in the present work (CoII, NiII, CuII, ZnII, and PbII) are not likely to be significant catalysts for the hydrolysis of organophosphorus triesters in most soils because their concentrations are too low. [CuII may be an exception to this due to its high reactivity as a catalyst and its use as a fungicide on some agricultural lands (2).] These metal ions do, however, possess well-characterized and distinctive chemical properties, which can be used to shed light on mechanisms of metal ion catalysis. Our objective is to obtain fundamental information that can direct future work examining catalysis by more environmentally relevant dissolved metal ions and metal-containing surfaces.

Metal-Catalyzed Hydrolysis: Theory More than six distinct mechanisms of metal-catalyzed nucleophilic substitution can be postulated (12). For the

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hydrolysis requires coordination to a neutral oxygen atom. Metal ion coordination is expected to be quite low (15), although sufficient for catalysis. Complex formation constants for these groups are not available in the literature, and therefore we will have to rely upon the analogy to another neutral ligand, ammonia (NH3):

Me2+ + NH3 ) MeNH32+

FIGURE 1. Thionate esters included in this study. pKa values for auxiliary ligand donor groups and for leaving groups (denoted as LG) are provided. Calculated values (designated in figure by a superscript 1) were determined by the computer program SPARC (40).

KNH3 )

[MeNH32+] [Me2+][NH3]

Values of KNH3 for the metal ions considered in this study have been obtained from the compilation by Smith et al. (16) and are shown in Figure 3. Log KNH3 for CuII is 1.3 log units higher than for the next highest metal ion, NiII. ZnII, CoII, PbII, FeII, and especially MnII exhibit significantly lower values of log KNH3. Thus, it can be expected that the ability of CuII, and to a lesser extent NiII, to coordinate the thionate sulfur or the oxonate oxygen exceeds that of the other five metals considered. In principle, a suitably placed auxiliary ligand donor group within the organophosphorus triester structure could facilitate coordination to the thionate sulfur or oxonate oxygen via chelate ring formation. KMe would increase in magnitude, yielding a higher concentration of the metalester complex. The metal ion catalyst would be held in a position that makes possible withdrawal of electron density from the phosphorus atom, thereby facilitating nucleophilic attack. Mechanism 2. In this mechanism, the metal ion coordinates the nucleophile rather than the parent ester. Nucleophilic attack must be rate-limiting in order for this mechanism to occur. Hydroxide ion (OH-) is a stronger nucleophile than H2O, but its concentration in neutral and acidic solutions is low. Divalent metal ions can induce the deprotonation of coordinated water molecules, thereby generating metal hydroxo species, e.g., MeOH+, which may serve as nucleophiles. The conventional rate equation for ester hydrolysis (17) can be modified to include catalysis via mechanism 2:

-d[parent] ) (ka[H+] + kn + kb[OH-] + dt + kMe(OH)n[Me(OH)n(2-n) ])[parent] (2)

∑

FIGURE 2. Conversion of a parent triester into diesters, monoesters, and final hydrolysis products. The thionate core group is denoted by X, the phenolate leaving group is denoted by P, and two alcoholate leaving groups are denoted by A.

In order for catalysis involving a hydroxo species to be significant, the product of the rate constant (kMe(OH)n) multiplied by its concentration ([Me(OH)n(2-n)+]) must be equal to or greater than the other terms in the rate equation. Information is available that can be used to calculate the concentration of metal hydroxo species as a function of pH. Consider the equilibrium expression written below for the formation of the first hydroxo species:

Me2+ + H2O ) MeOH+ + H+ thionate and oxonate ester pesticides under consideration here and for the divalent metal ions under consideration as catalysts, the following three mechanisms are believed to be pertinent: Mechanism 1. The metal ion coordinates the thionate sulfur or oxonate oxygen, withdrawing electron density away from the phosphorus atom and generating a more reactive electrophile (13, 14). If complex formation occurs more quickly than subsequent nucleophilic attack, it can be treated as a pre-equilibrium step. Overall rates of metal-catalyzed hydrolysis should be proportional to the concentration of the metal-ester complex, which is a function of the free metal ion concentration and the magnitude of the metal-ester complex formation constant, KMe. Thionate ester hydrolysis via mechanism 1 requires metal ion coordination to a neutral sulfur atom, while oxonate ester

*K1 )

[MeOH+][H+] [Me2+]

(3)

Taking the log of both sides of this equation, it can be seen that when the pH is greater than -log *K1, the concentration of the first hydroxo species MeOH+ is greater than the concentration of the hexaaquo species, Me2+(aq). As the pH is increased, additional hydroxo species grow in importance and eventually predominate. Figure 3 lists values of log *K1 reported in Smith et al. (16) for the divalent metal ions under consideration. CuII and PbII possess log *K1 values of -7.5 and -7.6, which are at least 1.5 log units above values for the other metal ions listed. Thus CuII and PbII yield considerably higher concentrations of metal hydroxo species near neutral pH and thus merit special examination. MeOH+ is only the first of a series of hydroxo species that can form as the solution pH is increased; complex formation constants for monomeric

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FIGURE 3. Log *K1, log KNH3, and log KL values (16) for divalent metal ions considered in this study. and in some instances polymeric hydroxo species are compiled in Smith et al. (16). Dissolved metal hydroxo species concentrations are ultimately limited by the solubility of (hydr)oxide solids. Mechanism 3. The prior two mechanisms are applicable to hydrolysis reactions that are limited by the rate of nucleophilic attack. A third mechanism may occur when the exit of the leaving group from the pentacoordinate intermediate is rate-limiting. Mechanism 3 involves metal ion coordination of the leaving group, which weakens the bond to the phosphorus center, facilitating the leaving group exit. This mechanism is most likely to occur with parent esters that possess leaving groups with high pKa values; the metal ion is required to assist in the breakdown of relatively strong leaving group-phosphorus bonds. In order to make comparisons among divalent metal ion catalysts, 8-hydroxyquinoline has been selected as an analogous ligand. Complex formation constants for this ligand, obtained from Smith et al. (16), are presented in Figure 3. CuII exhibits the highest complex formation constant; PbII and the other divalent metal ions considered exhibit values that are at least 3 log units lower in magnitude. If this analogy is a reasonable one, then CuII should be considerably more effective than the other divalent metal ions in catalyzing hydrolysis via mechanism 3. Note that all three mechanisms are sensitive to the magnitude of metal ion complex formation constants (to either the parent ester, hydroxide ion, or leaving group) and to the polarizability of the metal ion (the ability of the metal ion to alter the electronic structure of the coordinated species). Metal ion complex formation with OH- has already been discussed; CuII and PbII far exceed the other divalent metal ions shown in Figure 3 in their ability to coordinate this hard, anionic ligand.

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Excellent reviews (18-20) discuss the metal-catalyzed hydrolysis of carboxylic acid esters, amides, nitriles, phosphate monoesters, phosphate diesters, phosphate triesters, phosphonate esters, and other compound classes. Determining the contributions of mechanisms 1-3 toward overall metal ion catalysis is an arduous task. The most definitive conclusions are obtained when a substitution inert catalyst is used (e.g., CoIII) and placed within an organic chelate structure that controls (i) the number of positions available for coordinating H2O, OH-, and the substrate and (ii) the orientation of the metal ion catalyst and metal ioncoordinated nucleophile relative to the electrophilic site of the substrate. Conclusions regarding mechanisms of catalysis by aquo and hydroxo forms of substitution labile metal ions [all of the metal ions considered here with the possible exception of NiII (21)] are inherently more difficult. In addition, thionate triesters have not been the subject of mechanistic studies in the past, and only limited studies of mixed phosphate triesters have been made (22-25). Mechanistic studies have focused particular attention toward mechanism 2, nucleophilic attack by metal ioncoordinated hydroxide ion. Conclusions from these studies are pertinent here. In the case of CuII, comparison of -log *K1 with -log Kw indicates that CuOH+ is more than 106 times less basic than OH-. Although the relations between nucleophilicity and basicity is complex, it is likely that CuOH+ is a much poorer nucleophile than OH-; direct attack by CuOH+ probably occurs with only the most electrophilic substrates (18, 26). On the other hand, CuII and other metal ions may simultaneously coordinate both hydroxide ion and substrate. Intramolecular nucleophilic attack within such a complex may be efficient enough to make mechanism 2 important. If the site is bound to the metal ion via the ester

linkage, mechanisms 1 and 2 may both be operative. Such “push-pull” mechanisms are frequently invoked to explain metal ion catalysis (18, 19, 23, 27).

Metal-Catalyzed Hydrolysis: Practice The susceptibility of parathion-ethyl and paraoxon-ethyl toward CuII-catalyzed hydrolysis (7.8 < pH < 8.6) was first reported in 1956 (28). An important observation from this work is that the oxonate ester hydrolyzes more rapidly than the thionate ester in the absence of metal catalysts, while the opposite is true in the presence of CuII. It has been reported that chlorpyrifos, diazinon, and ronnel (5 < pH < 6) are all susceptible toward Cu-catalyzed hydrolysis but are slightly or negligibly affected by comparable concentrations of MgII, CaII, CoII, NiII, ZnII, and AlIII (29). It has been reported that if the CuII concentration is increased (pH 4.8), rates of chlorpyrifos-methyl and chlorpyrifos-ethyl hydrolysis increase proportionally until a plateau is reached at a CuII concentration slightly greater than 10 mM (30). More recently, it has been reported that HgII (pH 5.5) yields a 2 or 3 order of magnitude increase in the hydrolysis rate of four thionate esters (malathion, fenitrothion, parathion-methyl, and fenthion) and one oxonate ester (dichlorvos) (31). Mortland and Raman (29) and more recent researchers (30, 32) have hypothesized that the susceptibility of chlorpyrifos toward CuII-catalyzed hydrolysis arises from the ability of the pyridyl nitrogen to coordinate CuII, thereby making a five-membered chelate ring involving the thionate sulfur atom possible. It should be noted, however, that the electronwithdrawing nature of the three chloro substituents substantially lowers the basicity of the pyridyl nitrogen, making it an unlikely participant in chelate ring formation. The observation that pesticides without auxiliary ligand donor groups (e.g., ronnel and parathion) are also susceptible toward metal-catalyzed hydrolysis (29) provides additional evidence that chelate ring formation is not necessarily responsible for CuII-catalyzed chlorpyrifos hydrolysis. Clay minerals and divalent metal ion-treated clays are also capable of catalyzing the hydrolysis of a wide range of thionate ester pesticides (33-39). The difficulties in interpreting results from clay experiments are discussed in depth by El-Amamy and Mill (39), who studied the hydrolysis of ethyl acetate, cyclohexene oxide, isopropyl bromide, 1-(4-methoxyphenyl)2,3-epoxypropane, and N-methyl,p-tolyl carbamate in the presence of montmorillonite and kaolinite. Clay water content, the identity of exchangeable cations within the clay, and the extent of ester adsorption all influence the degree of catalysis observed. Hydrous oxides that do not possess interior cavities and do not contain exchangeable cations are inherently easier to study. Surface-catalyzed hydrolysis (5 < pH < 7) of chlorpyrifos-methyl, parathion-methyl, and ronnel by Al2O3, TiO2, and FeOOH has been recently reported (14). In agreement with metal-catalyzed hydrolysis reactions in solution, auxiliary ligand donor groups are apparently not necessary in order for surface catalysis to occur.

Materials and Methods Chemical Reagents. Phosphorothionate and phosphate esters were obtained from Chem Service (West Chester, PA) at purities greater than 97%: chlorpyrifos-methyl (O,Odimethyl-O,3,5,6-trichloro-2-pyridyl phosphorothioate), chlorpyrifos-methyl oxon (O,O-dimethyl-O,3,5,6-trichloro-2pyridyl phosphate), diazinon (O,O-diethyl-O-[2-isopropyl4-methyl-6-pyrimidyl]), parathion-methyl (O,O-dimethyl-Op-nitrophenyl phosphorothioate), paraoxon (O,O-diethyl-Op-nitrophenyl phosphate), ronnel (O,O-dimethyl-O-(2,4,5trichlorophenyl) phosphorothioate), and zinophos (O,Odiethyl-O-2-pyrazinyl phosphorothioate). All esters are ethyl esters unless otherwise designated. CuCl2‚2H2O, NiCl2‚6H2O,

Pb(NO3)2, ZnCl2, 2,4,5-trichlorophenol, 4-nitrophenol (purity >99%), and MOPS [3-(N-morpholino)propanesulfonic acid] were purchased from Aldrich Chemical Co. CoCl2‚6H2O, sodium acetate, NaCl, Tris [tris(hydroxymethyl)aminomethane], phosphoric acid (analytical grade), acetonitrile, and methanol (HPLC grade) were purchased from J. T. Baker. 3,5,6-trichloropyridinol (TCHP) was provided by Dow Chemical Company. Experimental Setup. Solutions were prepared from 18 MΩ‚cm resistivity water (DDW, Millipore Corp., Milford, MA). All glassware was soaked in 6 M HNO3 and rinsed several times with DDW before use. Experiments were performed in amber glass vials, and solutions were stirred with Tefloncoated stir bars. NaCl was added to all solutions to yield a concentration of 10 mM. The ionic strength was calculated by accounting for the concentrations of all ionic species in the system. The ionic strength ranged from 15 to 17 mM. Unless otherwise stated, the pH was maintained from pH 3.5 to pH 7.0 using 5 mM acetate or MOPS buffers. pH was measured using a Fisher Accumet pH meter. Before adding the ester stock solution, 200 µL of a 0.1 M metal salt stock solution (pH 3.0) was added to the buffered solution, yielding a 1.0 mM metal ion solution. The stock ester solution was prepared by dissolving the authentic standards in varying amounts of methanol (50-100%) and water. This stock solution was added to the buffered solution, for a total volume of 20 mL. Experiments conducted at methanol concentrations from 0 to 25% indicate that the methanol concentration does not affect rates of hydrolysis in solutions in either the presence or the absence of metal ions. Solutions were filtered with Nuclepore 0.2-µm membrane filters (Nuclepore Corp.), and reaction vials were autoclaved to inhibit biological growth during the experiments. Analytical Methods. The extent of pesticide ester hydrolysis was determined by following loss of the parent compound using reversed-phase HPLC (µ-Bondapac-C18 column, Waters Corp., Milford, MA) with a variable wavelength UV detector. Isocratic eluents consisted of varying ratios of acetonitrile and 1 mM H3PO4. The hydrolysis products of chlorpyrifos-methyl, chlorpyrifos-methyl oxon, parathion-methyl, paraoxon, and ronnel were detected and quantified using authentic standards. Column separation of parent compound and product was achieved by adjusting the phosphoric acid/acetonitrile ratio. Hydrolysis experiments were monitored for 2 months or through 3 half-lives, whichever was shorter. Plots of ln [parent compound] versus time are linear, indicating that the hydrolysis reactions in the presence and absence of metal ion catalysts are first order with respect to parent compound. Hydrolysis rate constants (kobs, in min-1) correspond to the slope of these plots. Centrifugation at 5000 rpm for 30 min using a Sorvall SS34 rotor was used to separate supernatant solution from (hydr)oxide or carbonate precipitated solids that may have formed. Sufficient 0.1 M HNO3 (redistilled) was added to the collected supernatant solution to yield a pH of 3.0. Dissolved metal ion concentration was measured in this solution by flame atomic absorption spectrophotometry (Perkin Elmer 4000). pKa Determination. The program SPARC (40) was used to calculate pKa values of ester leaving groups and nitrogen auxiliary donor atoms. SPARC is a computer program that estimates reactivity parameters from chemical structure. pKa values are shown in Figure 1 with the corresponding esters.

Results Metal Ion Speciation. Total dissolved metal concentrations were measured in solutions of the same chemical composition as the hydrolysis experiments (1.0 mM MeT, 10.0 mM NaCl, 5.0 mM acetate). Raising the solution pH to 8.0 does not result in a significant decrease in the total dissolved nickel


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FIGURE 4. Nickel, copper, zinc, and lead speciation calculated using HYDRAQL: 1.0 mM MeII and 10.0 mM NaCl. concentration. The total dissolved copper concentration and the total dissolved lead concentration, in contrast, decrease substantially at pH values above 6.0. The computer program HYDRAQL (41), employing equilibrium constants from Smith et al. (16), was used to calculate

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metal ion speciation (Figure 4) in the solutions described in the preceding paragraph. As the pH increases, dissolved metal ion-hydroxo species grow in importance, and saturation with respect to metal hydroxide solids [Me(OH)2(s)] is eventually reached. According to HYDRAQL calculations, CuII and PbII

FIGURE 5. Rate constants for thionate ester hydrolysis as a function of pH in the presence and absence of divalent metal ion catalysts. Reaction conditions: 5.0 mM acetate or MOPS buffer, 10.0 mM NaCl, and 1.0 mM MeII. should begin to precipitate at approximately pH 6.0 and 7.0, respectively, as observed experimentally. The model predicts that NiII precipitation should begin at a higher pH, at approximately pH 8.1. The equilibrium constants for formation of the first hydroxo speices MeOH+ (*K1) decrease in the order: CuII ∼ PbII > ZnII > CoII ∼ NiII (Figure 3). Equilibrium constants for the formation of higher hydroxo species and multinuclear species in solution and for the solid-phase Me(OH)2(s) also decrease in the same order. These differences in affinity for OH- should be kept in mind in subsequent sections where the catalytic ability of the metal ions is compared. Equilibrium constants from Smith et al. (16) and the computer program HYDRAQL (41) were also used to calculate the speciation of 1.0 mM CuII in the presence of 5.0 mM acetate and 5.0 mM Tris (0.10 mM NaCl). Only Tris complexed a significant amount of added CuII (50% at pH 6.5) and raised the pH required for Cu(OH)2(s) precipitation. Thionate Ester Hydrolysis. Figure 5A-E depicts hydrolysis rate constants for five thionate esters as a function of pH in the presence and absence of divalent metal ion catalysts. In metal-free solutions, significant loss of the parent compound was observed with ronnel, parathion-methyl, and chlorpyrifos-methyl throughout the pH range examined. Rate constants (kobs) for these three thionate esters varied little with pH: 1 × 10-5 min-1 for chlorpyrifos-methyl, 2.8 × 10-6 min-1 for parathion-methyl, and 3.6 × 10-6 min-1 for ronnel. In metal-free solutions, loss of zinophos was only observed above pH 6.0, and loss of diazinon was only observed below pH 4.5. A set of experiments examined the loss of 250 µM zinophos in solutions containing increasing amounts of the buffers acetate (pH 5.0), MOPS (pH 6.5), and Tris (pH 7.5). In the absence of CuII, increasing buffer concentrations had no effect on the hydrolysis rate constant kobs. In the presence of 1.0 mM CuII, concentrations of acetate and MOPS buffers up to

50 mM had no effect on kobs. In contrast, increasing the Tris buffer concentration from 1 to 100 mM decreased kobs in the presence of CuII by 95%. For this reason, Tris was not employed in the experiments reported in this paper. 1.0 mM CuII yields significant catalysis of all five thionate esters over a wide pH range, with the maximum in kobs occurring at slightly acidic pH values. The maximum values of kobs decreases in the order: chlorpyrifos-methyl ∼ zinophos ∼ parathion-methyl > diazinon ∼ ronnel. kobs is directly proportional to the total concentration of CuII at pH 5.8 and pH 5.9. Returning to Figure 5, 0.1 mM CuII yields kobs values lower than observed with 1.0 mM CuII at pH values less than 5.8. Above this pH, kobs values for 0.1 mM and 1.0 mM CuII are approximately the same. Catalysis by a number of other divalent metal ions was examined. 1.0 mM FeII, CoII, ZnII (data not shown), and NiII had a negligible effect on the hydrolysis of the five thionate esters. 1.0 mM PbII catalyzed the hydrolysis of all thionate esters examined (chlorpyrifos-methyl, parathion-methyl, and ronnel) although at a level substantially lower than CuII. For all three thionate esters, catalysis by PbII increases as the pH is increased until a plateau is reached (Figure 5). Oxonate Ester Hydrolysis. In the absence of metal ion catalysts, chlorpyrifos-methyl oxon hydrolyzes four times more quickly than the corresponding thionate. kobs for the oxonate in the absence of metal ion catalysts varies little with pH (Figure 6A). In the presence of CuII, the oxonate yields catalysis over a wide pH range with a maximum in kobs occurring at slightly acidic pH values (Figure 6A). The oxonate yields a maximum in kobs that is lower than that of the thionate, however, indicating that it is less sensitive to catalysis by CuII than the thionate. In the presence of 1.0 mM PbII, kobs is actually higher for the oxonate than for the thionate form of chlorpyrifos-methyl (Figure 6A). Thus, the oxonate is more sensitive to PbII catalysis than the thionate. PbII-catalyzed oxonate hydrolysis


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FIGURE 6. Rate constants for oxonate ester hydrolysis as a function of pH in the presence and absence of divalent metal ion catalysts. Reaction conditions: 5.0 mM acetate or MOPS buffer, 10.0 mM NaCl, and 1.0 mM MeII.

FIGURE 7. Amount of parent ester lost and phenolate product formed as a function of time. Filled symbols represent experiments performed in the presence of 1.0 mM MeT while open symbols represent experiments performed in metal-free solution (5 mM acetate or MOPS buffer, 10 mM NaCl). increases as the pH is increased until a plateau is reached, a pattern also observed with the thionate. 1.0 mM NiII, CoII, and ZnII (Figure 6B) had little effect on the hydrolysis of chlorpyrifos-methyl oxon above pH 6.0. NiII exhibited only a slight catalytic effect at the highest pH examined. Figure 6C shows the effect of 1 mM CuII on the hydrolysis of paraoxon, the oxon ester of parathion. At the lowest pH examined, kobs in the presence of CuII is nearly equal to the value measured in its absence. At pH 5.7, in contrast, kobs in the presence of CuII is nearly 10 times higher than the value measured in metal-free solution. Hydrolysis Product Formation. As discussed earlier, the amount of phenolate hydrolysis product formed in the early stages of the hydrolysis reaction will be less than the amount of parent compound lost if hydrolytic attack on the triester

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occurs at the alkyl carbon rather than the phosphorus electrophilic site. Conversion of the monoester to final hydrolysis products in the late stages of reaction should yield amounts of phenolate hydrolysis product that match the amount of parent compound lost, regardless of the initial site of attack. Results obtained with the thionate form of chlorpyrifosmethyl are presented in Figure 7A. Hydrolysis in the presence of 1.0 mM CuII is fast, and production of the phenolate product (3,5,6-trichloropyridinol, abbreviated as TCHP) closely matches parent lost (Figure 7A). In the absence of metal catalyst, hydrolysis of the thionate is slow. Uncertainties in mass balance calculations are significant when the hydrolysis rates are this low; calculated amounts of TCHP actually slightly exceed amounts of parent compound lost.

The oxonate form of chlorpyrifos-methyl also yields a close match between parent lost and product formed in the presence of 1.0 mM CuII. With 1.0 mM PbII, the match between TCHP production and parent lost is better than in metal-free solution, but poorer than in the presence of CuII (Figure 7B). With parathion-methyl, the phenolate product corresponds to 4-nitrophenol; with ronnel, it corresponds to 2,4,5trichlorophenol (abbreviated as TCP). In the absence of metal ion catalysts, both thionate esters yield substantially less phenolate product formed in comparison to parent compound lost. Catalysis by 1.0 mM PbII lessens this discrepancy, and catalysis by 1.0 mM CuII yields a near match between these two quantities (Figure 7C,D).

Discussion Phosphorothioate Ester Properties Affecting Hydrolysis. The following properties affect hydrolysis rates of the thionate and oxonate esters employed in this study: (1) the identity of the atom bonded to the phosphorus atom: thionate versus oxonate form of the ester linkage; (2) pKa of the phenolate leaving group; (3) length of alkyl chain: methyl versus ethyl form of the alcoholate leaving groups; and (4) identity, placement, and basicity of auxiliary ligand donor groups. Replacing the thionate sulfur with the more electronegative oxygen draws electron density away from the phosphorus atom to a greater extent, making it more susceptible toward nucleophilic attack (42). As a consequence, (i) phosphoruscentered attack may grow in importance relative to carboncentered attack; (ii) rate-limiting nucleophilic attack may give way to rate-limiting leaving group exit; and (iii) overall hydrolysis rates may increase. This change may also affect metal ion catalysis, most notably mechanism 1. Like other neutral oxygen atoms (15), the oxonate oxygen is a poor Lewis base that should coordinate larger metal ions in preference to smaller ones. Like other neutral sulfur atoms (15), the thionate sulfur is expected to be larger and less strongly hydrated than neutral oxygen atoms, causing it to exhibit preference toward coordinating large, less solvated metal ions. Phosphorus-centered nucleophilic attack is followed by exit of the leaving group with the lowest pKa, which corresponds to the phenolate group for all the esters examined here. Because of its lowest pKa, the bond to the phosphorus atom is the weakest and easiest to break, and the resulting phenolate anion is far more stable in solution than alcoholate anions (43). pKa values reported in Figure 1 were either found in the literature or calculated using SPARC (40). For the esters examined in this work, leaving group pKa values decrease in the following order: parathion-methyl > ronnel > zinophos ∼ chlorpyrifos-methyl > diazinon. If the pKa of the phenolate group is sufficiently high, its exit may be rate-limiting, thereby controlling overall rates of hydrolysis. It has been reported that triesters possessing ethyl leaving groups hydrolyze more slowly than triesters possessing methyl leaving groups (4). Although both the methyl group and the ethyl group are electron-donating, the effect is stronger with a longer alkyl chain. For the compound Demeton S, for example, the methyl form yields a half-life of 3.8 h at pH 6.0 and 70 °C, while the ethyl form yields a half-life of 9.5 h (4). As mentioned earlier, auxiliary ligand donor atoms may assist metal complexation and therefore promote metalcatalyzed hydrolysis. Metal-ligand complex formation constants typically increase as the basicity (pKa) of the ligand is increased. It must be kept in mind, however, that the actual amount of metal-ligand complex formation also depends upon the degree of competition between H+ and Me2+ for the ligand donor atom (44). For the pesticides under examination here, heterocyclic nitrogen atoms are the only possible auxiliary ligand donor atoms. pKa values (calculated by SPARC) for the heterocyclic nitrogen closest to the phosphoro(thio)ate linkage decreases in the order diazinon (3.52) > zinophos (0.83) . chlorpyrifos-methyl (-4.9). The basicity

of the heterocyclic nitrogen atom of chlorpyrifos-methyl is so low that its ability to coordinate metal ions is most certainly negligible. The same conclusion probably applies to zinophos as well. Hydrolysis in the Absence of Metal Catalysts. In metalfree solution, chlorpyrifos-methyl hydrolyzes up to 10 times more quickly than parathion-methyl and ronnel, despite the fact that all three triesters possess one phenol leaving group and two methanol leaving groups. Parathion-methyl and ronnel possess phenolate leaving groups with pKa values that are more than 1 log unit higher than that of chlorpyrifos methyl. For this reason, leaving group exit is more likely to be rate-limiting for parathion-methyl and ronnel than for chlorpyrifos-methyl. In metal-free solution, parathion-methyl and ronnel yielded much greater discrepancies between parent compound lost and phenolate formed than the two other esters examined, the thionate and the oxonate esters of chlorpyrifosmethyl. The high pKa values of the phenolate leaving groups for parathion-methyl and ronnel (already discussed) may be responsible. High pKa values may slow rates of phosphoruscentered attack relative to the competing reaction, carboncentered attack. As a consequence, less phenolate anion is liberated during the initial triester hydrolysis step, thereby yielding a greater discrepancy. The oxonate form of chlorpyrifos-methyl hydrolyzes four times faster than the corresponding thionate ester. As discussed in the preceding sections, this is the expected result, arising from the greater electronegativity of the oxonate oxygen in comparison to the thionate sulfur. In agreement with prior studies (3), we observed acidcatalyzed hydrolysis of diazinon in metal-free solutions below pH 5.0. Gomaa (45), for example, reported the following halflives for diazinon hydrolysis at 20 °C: 31 days (pH 5.0), 185 days (pH 7.4), and 136 days (pH 9.0). Acid catalysis has been postulated to arise from the protonation of the heterocyclic nitrogen (pKa 3.52), which causes a slight shift in electron density away from the phosphorus atom, making it more susceptible toward nucleophilic attack (3). According to the literature values and SPARC calculations presented in Figure 1, diazinon possesses a nitrogen heteroatom with a pKa 2.7 log units higher than the corresponding group in zinophos and 8.4 log units higher than the corresponding group in chlorpyrifos-methyl. Thus, only diazinon possesses a nitrogen heteroatom sufficiently basic to affect hydrolysis rates within the pH range of our study. Effect of Metal Ion Properties on Catalysis: Overview. Dissolved metal ion measurements and HYDRAQL calculations both indicate that total dissolved CuII concentrations decrease dramatically near pH 5.5, due to the formation of a solid phase such as CuO(s), Cu(OH)2(s), or CuCO3(s). This decrease in solubility coincides with an observable, although less pronounced, decrease in kobs for CuII-catalyzed hydrolysis of the thionate esters chlorpyrifos-methyl, zinophos, diazinon, parathion-methyl, and ronnel and for the oxonate esters chlorpyrifos-methyl oxon and paraoxon-ethyl. It can be concluded, therefore, that solubility limitations are responsible for the diminished catalytic ability of CuII at high pH. Dissolved metal ion measurements and HYDRAQL calculations for PbII closely match the results with CuII. For the thionate esters chlorpyrifos-methyl, parathion-methyl, and ronnel and for the corresponding oxonate ester of chlorpyrifos-methyl, however, kobs does not decrease significantly in the pH range where PbII precipitation is believed to occur. With all thionate and oxonate triesters except diazinon, kobs values measured in the presence of CuII and PbII diminish significantly as the pH is decreased from 5.5 to 3.0. Changes in the extent of metal ion-triester complex formation are probably not significant, since the hexaaquo form of both metal ions is predominant, and parent triesters do not undergo protonation/deprotonation within this pH range. Decreases


9

1671

in kobs with decreasing pH are probably caused by decreasing concentrations of one of the following hydroxo-containing species: (i) free hydroxide ions, (ii) monomeric metal ionhydroxo complexes, (iii) multinuclear metal ion-hydroxo complexes, or (iv) metal ion-triester-hydroxo ternary complexes. Metal Ion Properties and Thionate Ester Hydrolysis. PbII catalyzes hydrolysis of all three thionate esters examined. Other divalent metal ions that do not effectively catalyze hydrolysis (most notably NiII) surpass PbII in properties required for catalysis via mechanisms 1 and 3. By elimination, we can conclude that catalysis by PbII arises primarily from mechanism 2. CuII is a much better catalyst than any of the other divalent metal ions examined, which is not surprising since it possesses the most desirable properties pertaining to all three possible mechanisms of catalysis. Any additional conclusions regarding mechanisms of CuII catalysis are somewhat speculative. Since CuII and PbII possess comparable values of *K1, we can hypothesize that catalysis via mechanism 2 is comparable for the two metals. The much higher catalytic activity of CuII would therefore arise from additional catalysis via mechanism 1 or mechanism 3. The CuII- and PbII-catalyzed hydrolysis of parathion-methyl and ronnel yield results quite similar to chlorpyrifos-methyl. It can be concluded that the pyridyl nitrogen group of chlorpyrifos-methyl does not play a significant role in metalcatalyzed hydrolysis, in disagreement with the conclusions of Mortland and Raman (29). It is our conclusion that the low basicity of the heterocyclic nitrogen (Figure 1) makes metal coordination involving this auxiliary ligand donor atom unlikely. CuII and PbII lessen the discrepancy between parent loss and phenolate product formed in the hydrolysis of parathionmethyl and ronnel. Metal catalysis apparently increases the rate of phosphorus-centered attack relative to carboncentered attack. This observation provides no additional information regarding the relative importance of mechanisms 1-3. With regards to ethyl esters, zinophos yields results quite comparable to the other thionate esters, while diazinon yields higher hydrolysis rates under acidic conditions. The appearance of proton-catalyzed hydrolysis is made possible by the comparatively high pKa of the nitrogen heteroatom of diazinon. NiII and CoII, which require pH values above 9.7 in order for metal-hydroxo species to be predominant, did not catalyze hydrolysis near neutral pH. High pH experiments would not be expected to yield catalysis, since the formation of hydroxide and carbonate solids would limit solubility and since rates of the uncatalyzed hydrolysis reaction are already high. ZnII possesses a log *K1 value (-9.0) that is a little closer to that of CuII and PbII and has been observed to catalyze chlorpyrifosmethyl hydrolysis, but only above pH 7.0. Metal Ion Properties and Oxonate Ester Hydrolysis. The same reasoning applied to the thionate esters can be applied to the oxonate form of chlorpyrifos-methyl. PbII is only superior to CoII, NiII, and ZnII in its ability to form complexes with hydroxide ions near neutral pH. The fact that PbII is a better catalyst than these other three metal ions provides proof that oxonate esters are also subject to catalysis via mechanism 2. Compared to the corresponding thionate, the oxonate yields hydrolysis rates in the presence of 1.0 mM PbII that are much closer to those measured in the presence of 1.0 mM CuII. It can be concluded that mechanism 2 is the principal pathway for catalysis by both PbII and CuII. It is interesting that chlorpyrifos-methyl oxon and paraoxon both yield a maximum in CuII-catalyzed hydrolysis rates under slightly acidic conditions (4.5 < pH < 6). It is likely that hydrolysis mechanisms for the two oxonate triesters are similar.

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Metal-Catalyzed Hydrolysis in the Environment Aquo and hydroxo complexes of CoII, NiII, and ZnII probably do not serve as catalysts in the environment, since their reactivities and concentrations are too low. PbII exhibits catalytic properties, but concentrations encountered in most agricultural soils are also too low. Unlike the other metal ions examined in this study, CuII is added to some agricultural soils as a fungicide (e.g., Bordeaux mixture, see ref 2). Application of CuII and phosphoro(thion)ate pesticides to the same agricultural soils could result in significant metal ion catalysis. CaII and MgII are abundant divalent metal ions in most soils; FeII is abundant in some reducing soils. Their ability to form hydroxide ion complexes near neutral pH is poor, and their affinity for neutral and anionic organic ligands is even less than the affinity exhibited by CoII, NiII, and ZnII. For these reasons, catalysis by naturally-abundant divalent metal ions is also not expected to be important. +III and +IV metal ions may also serve as hydrolysis catalysts. Dissolved AlIII and FeIII aquo/hydroxo complexes are only encountered under very acidic soil conditions due to solubility constraints. Mineral surface-bound +III and +IV metal ions are abundant in soils, and their ability to catalyze hydrolysis reactions has already been established (see ref 14). Evaluating surface catalysis in light of mechanisms 1-3 is difficult. +III and +IV metal ions form complexes with hydroxide ions across a broader pH range than CuII and PbII. The nucleophilicity of metal ion-bound hydroxide ion may be diminished, however, because of the higher charge on the central metal ion. +III and +IV metal ions yield higher complex formation constants with anionic ligands than +II metal ions (see ref 16). The amount of metal ion-ligand complex that forms is diminished, however, by increased competition by hydroxide ion for coordination of the metal ion. A detailed examination of mineral surfacecatalyzed hydrolysis of phosphorothionate ester pesticides and their corresponding oxonates is found in the next work in this series (48).

Acknowledgments Support by Grant R81-8894, U.S. Environmental Protection AgencysNational Center for Environmental Research and Quality Assurance (Office of Exploratory Research), is gratefully acknowledged.

Literature Cited (1) Gianessi, L. P.; Anderson, J. E. Pesticide Use in U.S. Crop Production; National Summary Report; National Center for Food and Agricultural Policy: Washington, DC, 1995. (2) Hassall, K. A. The Biochemistry and Uses of Pesticides, 2nd ed.; VCH: Weinheim, 1990; pp 269-275. (3) Eto, M. Organophosphorus Pesticides: Organic and Biological Chemistry; CRC Press: Boca Raton, FL, 1979; p 93. (4) Faust, S. D.; Gomaa, H. M. Environ. Lett. 1972, 3 (3), 171-201. (5) Schmidt, K. J.; Fest, C. The Chemistry of Organophosphorus Pesticides; Springer-Verlag: New York, 1982; Chapter 2. (6) Macalady, D. L.; Wolfe, N. L. J. Agric. Food Chem. 1983, 31, 11391147. (7) Zepp, R. G.; Wolfe, N. L. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley-Interscience: New York, 1987. (8) Hoffmann, M. R. In Aquatic Chemical Kinetics; Stumm, W., Ed.; Wiley Interscience: New York, 1990. (9) Mill, T.; Mabey, W. In The Handbook of Environmental Chemistry: Reactions and Processes; Hutzinger, O., Ed.; Springer-Verlag: Berlin, 1988; Vol. 2, pp 72-111. (10) Stone, A. T.; Torrents, A. In Environmental Impact of Soil Component Interactions; Huang, P. M., Berthelin, J., Bollag, J.M., McGill, W. B., Page, A. L., Eds.; Lewis: Boca Raton, FL, 1995; Vol. 1. (11) Schrader, G. H. Kuekenthal (Bayer AG): DBP 767 153 (1938/ 1952). (12) Suh, J.; Chun, K. H. J. Am. Chem. Soc. 1986, 108, 3057-3063. (13) Hoffmann, M. R. Environ. Sci. Technol. 1980, 14, 1061. (14) Torrents, A.; Stone, A. T. Soil Sci. Soc. Am. J. 1994, 58, 738-745.

(15) Hancock, R. D.; Martell, A. E. Chem. Rev. 1989, 89, 1875-1914. (16) Smith, R. M.; Martell, A. E.; Motekaitis, R. J. NIST Critically Selected Stability Constants of Metal Complexes Database; Version 2.0; NIST Standard Reference Database 46; Plenum: New York, 1995. (17) Mabey, W.; Mill, T. J. Phys. Chem. Ref. Data 1978, 7, 383-415. (18) Sutton, P. A.; Buckingham, D. A. Acc. Chem. Res. 1987, 20, 357364. (19) Chin, J. Acc. Chem. Res. 1991, 24, 145-152. (20) Suh, J. Acc. Chem. Res. 1992, 25, 273-279. (21) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition Metal Complexes, 2nd ed.; VCH: Weinheim, 1991; Chapter 4. (22) Norman, P. R. Inorg. Chim. Acta 1987, 130, 1-4. (23) Morrow, J. R.; Trogler, W. C. Inorg. Chem. 1989, 28, 2330-2333. (24) Wagener, C. C. P.; Modro, A. M.; Modro, T. A. J. Phys. Org. Chem. 1991, 4, 516-523. (25) Kady, I. O.; Tan, B.; Ho, Z.; Scarborough, T. J. Chem. Soc. Chem. Commun. 1995, 11, 1137-1138. (26) Buckingham, D. A.; Englehardt, L. M. J. Am. Chem. Soc. 1975, 97, 5915-5917. (27) Gellman, S. H.; Petter, R.; Breslow, R. J. Am. Chem. Soc. 1986, 108, 2388-2394. (28) Ketelaar, J. A. A.; Gersmann, H. R.; Beck, M. M. Nature 1956, 177, 392-393. (29) Mortland, M. M.; Raman, K. V. J. Agric. Food Chem. 1967, 15, 163-167. (30) Blanchet, P. F.; St.-George, A. Pestic. Sci. 1982, 13, 85-91. (31) Wan, H. B.; Wong, M. K.; Mok, C. Y. Pestic. Sci. 1994, 42, 93-99. (32) Meikle, R. W.; Youngson, C. R. Arch. Environm. Contam. Toxicol. 1978, 7, 13-22. (33) Saltzman, S.; Yaron, B.; Mingelgrin, U. Soil Sci. Soc. Am. Proc. 1974, 38, 231-234. (34) Saltzman, S.; Mingelgrin, U.; Yaron, B. J. Agric. Food Chem. 1976, 24, 739-743. (35) Mingelgrin, U.; Saltzman, S.; Yaron, B. Soil Sci. Soc. Am. J. 1977, 41, 519-523. (36) Mingelgrin, U.; Saltzman, S. Clays Clay Miner. 1979, 27, 72-78.

(37) Sanchez-Camazano, M.; Sanchez-Martin, M. J. Geoderma 1983, 29, 107-118. (38) Sanchez-Camazano, M.; Sanchez-Martin, M. J. Soil Sci. 1983, 136, 89-93. (39) El-Amamy, M. M.; Mill, T. Clays Clay Miner. 1984, 32, 67-73. (40) Long, J. M. SPARC, An Expert System for Estimating Physical and Chemical Reactivity: User Manual for Calculating Ionization pKa; Ecosystems Research Division National Exposure Laboratory, U.S. Environmental Protection Agency: Athens, GA, 1996. (41) Papelis, C.; Hayes, K. F.; Leckie, J. O. HYDRAQL: A program for the computation of chemical equilibria composition of aqueous batch systems including surface-complexation modeling of ion adsorption at the oxide/solution interface; Technical Report 306; Department of Civil Engineering, Standford University: Menlo Park, CA, 1988. (42) Schmidt, K. J. In Environmental Quality and Safety, Vol. 4: Global Aspects of Chemistry, Toxicology, and Technology as Applied to the Environment: Coulston, F., Corte, F., Eds.; Academic Press: New York, 1973; p 99. (43) Solomons, T. W. G. Organic Chemistry; Wiley: New York, 1984; p 183. (44) Sigel, H.; McCormick, D. B. Acc. Chem. Res. 1970, 3, 201-208. (45) Gomaa, H. M. Residue Rev. 1969, 29, 171-190. (46) Dilling, W. K.; Lickly, L. C.; Lickly, T. D.; Murphy, P. G.; McKellar, R. L. Environ. Sci. Technol. 1984, 18, 540-543. (47) Ballinger, P.; Long, F. A. J. Am. Chem. Soc. 1960, 82, 795. (48) Smolen, J. M.; Stone, A. T. Soil Sci. Soc. Am. J. Submitted for publication.

Received for review June 11, 1996. Revised manuscript received January 25, 1997. Accepted January 25, 1997.X ES960499Q X

Abstract published in Advance ACS Abstracts, April 15, 1997.


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Divalent Metal Ion-Catalyzed Hydrolysis of ... - ACS Publications

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