Catalysis of Aryl Ester Hydrolysis in the Presence of Metallomicelles

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Catalysis of Aryl Ester Hydrolysis in the Presence of Metallomicelles Containing a Copper(II) Diethylenetriamine Derivative Anastasios Polyzos,† Andrew B. Hughes,* and John R. Christie Department of Chemistry, La Trobe UniVersity, Victoria 3086, Australia ReceiVed September 10, 2006. In Final Form: NoVember 8, 2006 The novel metallosurfactant Cu(II)-1-tetradecyldiethylenetriamine (Cu(II)TDET) was prepared, and the hydrolyses of 2-acetoxy-5-nitrobenzoic acid (1), 4-acetoxy-3-nitrobenzoic acid (2), 4-nitrophenyl acetate (3), and 2-nitrophenyl acetate (4) in the presence of micellar Cu(II)TDET were examined. The rate of ester hydrolysis for the series followed the order 1 ≈ 2 > 3 > 4. The larger observed rate (kψ) for 1 and 2 was attributed to (i) electrostatic interaction between the carboxylate anion and the cationic metallomicelle surface and (ii) the formation of a ternary complex metal: surfactant ligand:substrate (MLnS). The position of the carboxylate anion in the substrate did not significantly affect catalysis. Similar rates were observed when the carboxylate anion was ortho to the acyl ester 1 or para to the reaction center 2. The absence of a significant difference may be associated with the ternary complex coordination geometry, which unfavorably aligned the ligated substrate and the metal-bound hydroxyl. Mixed micellar solutions containing Cu(II)TDET and MTAB or Triton X-100 were examined. Added cosurfactants have a pronounced effect on the catalytic activity of Cu(II)TDET. At a low concentration of Cu(II)TDET the addition of MTAB or Triton X-100 increased the pseudo-first-order rate constant (kψ) for the hydrolysis of 1 and 3 relative to the rate in pure Cu(II)TDET. The addition of a cosurfactant increased the total micellar volume (VM), promoting substrate incorporation within the pseudophase. At higher metallosurfactant concentration, the rate enhancement was smaller due to the dilution of the substrate within the co-micellar pseudophase.

Introduction Within the continuum of amphiphilic self-assembly structures, metallomicelles exhibit great utility as catalysts of organic transformations.1-10 Metallomicelles are micelles comprised of functionalized amhiphiles that contain one or more metal atoms. Those useful for catalysis usually contain a transition metal chelated to a lipophilic ligand. In aqueous solution, the metal ion complex forms the headgroup that interfaces at the Stern region.11 Early interest12,13 in metallomicellar catalysis stemmed from the need to detoxify stockpiles of phosphorus(V)-containing pesticides and chemical warfare agents such as GD 8 and Soman. Menger et al. established the catalytic potential of metallomicelles by reporting that micellar Cu(II)-N,N,N′-trimethyl-N′-tetradecylethylenediamine (Cu(II)TTMED) catalyzed and accelerated the hydrolysis of 4-nitrophenyl diphenyl phosphate (PNPDPP), * Corresponding author. E-mail: [email protected]. † Present address: Ian Wark Laboratory, Molecular and Health Technologies, CSIRO, Bag 10, Clayton South, Victoria 3169, Australia. (1) Bhattacharya, S.; Snehalatha, K.; George, S. K. J. Org. Chem. 1998, 63, 27. (2) Bhattacharya, S.; Snehalatha, K.; Kumar, V. P. J. Org. Chem. 2003, 68, 2741. (3) Hampl, F.; Liska, F.; Mancin, F.; Tecilla, P.; Tonellato, U. Langmuir 1999, 15, 405. (4) Rispens, T.; Engberts, J. B. F. N. Org. Lett. 2001, 3, 941. (5) Weijnen, J. G. J.; Engbersen, J. F. J. Recl. TraV. Chim. Pays-Bas 1993, 112, 351. (6) Weijnen, J. G. J.; Koudijs, A.; Schellekens, G. A.; Engbersen, J. F. J. J. Chem. Soc., Perkin Trans. 2 1992, 829. (7) You, J.-S.; Yu, X.-Q.; Su, X.-Y.; Wang, T.; Xiang, Q.-X.; Yang, M.; Xie, R.-G. J. Mol. Catal. A: Chem. 2003, 202, 17. (8) You, J. S.; Yu, X. Q.; Zhang, X. L.; Xie, R. G. Chin. Chem. Lett. 1998, 9, 335. (9) Hafiz, A. A. J. Surfactants Deterg. 2005, 8, 359. (10) Hafiz, A. A.; El Awadi, M. Y.; Badawi, A. M.; Mokhtar, S. M. J. Surfactants Deterg. 2005, 8, 203. (11) Griffiths, P. C.; Fallis, I. A.; Willock, D. J.; Paul, A.; Barrie, C. L.; Griffiths, P. M.; Williams, G. M.; King, S. M.; Heenan, R. K.; Goergl, R. Chem.s Eur. J. 2004, 10, 2022. (12) Gellman, S. H.; Petter, R.; Breslow, R. J. Am. Chem. Soc. 1986, 108, 2388. (13) Menger, F. M.; Gan, L. H.; Johnson, E.; Durst, D. H. J. Am. Chem. Soc. 1987, 109, 2800.

a GD analogue, by more than 100000-fold at 25 °C and pH 8.0.13 Since these early reports, metallomicellar catalysis of the hydrolytic cleavage of phosphate1,5,14-16 and carboxylate17-27 esters has been described. It is generally accepted1,2,5,14-27 that ester hydrolysis in metallomicellar systems is facilitated initially by the coordination of the substrate to the metal center to form a ternary complex, followed by the pseudo-intramolecular nucleophilic attack of a metal-bound hydroxyl. Water bound to a metal ion at near-neutral pH at the micellar surface has enhanced acid dissociation due to the electronic distortion of the O-H bond by the Lewis acidity of the metal ion. Factors to be considered in accounting for rate enhancements in metallomicelles thus include changes in acid-base equilibria of nucleophiles, associated with micellization and with electrophilic activation by the metal ion, as well as local concentration effects in the micellar pseudophase. Substrate structure is an important factor in the rate of ester hydrolysis in the presence of metallomicelles.5,17,28,29 The position (14) Scrimin, P.; Ghirlanda, G.; Tecilla, P.; Moss, R. A. Langmuir 1996, 12, 6235. (15) Bunton, C. A.; Scrimin, P.; Tecilla, P. J. Chem. Soc., Perkin Trans. 2 1996, 419. (16) Kimura, E.; Hashimoto, H.; Koike, T. J. Am. Chem. Soc. 1996, 118, 10963. (17) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1991, 56, 161. (18) Weijnen, J. G. J.; Koudijs, A.; Engbersen, J. F. J. J. Chem. Soc., Perkin Trans. 2 1991, 1121. (19) Tagaki, W.; Ogino, K.; Fujita, T.; Yoshida, T.; Nishi, K.; Inaba, Y. Bull. Chem. Soc. Jpn. 1993, 66, 140. (20) Fornasier, R.; Scrimin, P.; Tecilla, P.; Tonellato, U. J. Am. Chem. Soc. 1989, 111, 224. (21) Fujita, T.; Ogino, K.; Tagaki, W. Chem. Lett. 1988, 981. (22) Ogino, K.; Kashihara, N.; Fujita, T.; Ueda, T.; Isaka, T.; Tagaki, W. Chem. Lett. 1987, 1303. (23) Fornasier, R.; Milani, D.; Scrimin, P.; Tonellato, U. J. Chem. Soc., Perkin Trans. 2 1986, 233. (24) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Am. Chem. Soc. 1992, 114, 5086. (25) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1994, 59, 18. (26) De Santi, G.; Scrimin, P.; Tonellato, U. Tetrahedron Lett. 1990, 31, 4791. (27) Weijnen, J. G. J.; Koudijs, A.; Engbersen, J. F. J. J. Org. Chem. 1992, 57, 7258. (28) Scrimin, P.; Tecilla, P.; Tonellato, U.; Bunton, C. A. Colloids Surf., A 1998, 144, 71.

10.1021/la0626454 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/09/2007

Catalysis of Aryl Ester Hydrolysis Chart 1. Surfactants and Substrates Studied in This Work

of groups capable of metal ligation becomes important in addition to the electrostatic and hydrophobic forces that determine orientation in nonfunctionalized surfactants.30 Many studies of ester hydrolyses in metallomicelles have examined the hydrolysis of activated carboxylic esters such as 4-nitrophenylpicolinate (PNPP).18,19,23,31-34 This substrate and its isomer, 4-nitrophenylisonicotinate (PNPIN), are particularly suitable for studying these effects. The high binding affinity of the pyridine nitrogen for the transition metal ensures strong binding of these substrates to the metallomicelles, and known and contrasting presentation geometry.35,36 In our previous studies,37 we have investigated substrates which coordinate to metallosurfactants through a carboxylate anion. These included the isomeric pair 2-acetoxy5-nitrobenzoic acid (1) and 4-acetoxy-3-nitrobenzoic acid (2) (Chart 1). We reported that a carboxylate anion ortho to the reaction center increased the rate of ester hydrolysis in the presence of the copper metallomicelle. A 7100-fold increase in rate was observed for the hydrolysis of 1 in the presence of Cu(II)TTMED compared to the buffered aqueous rate. A much smaller rate enhancement was reported for the hydrolysis of 2 in similar conditions. Full kinetic investigation of these systems was limited, though, by the low stability of Cu(II)TTMED at high pH (>7.5),38 and by the degradation of the complex at mildly elevated temperatures (>31 °C). On this basis, the surfactant Cu(II)-1-tetradecyldiethylenetriamine (Cu(II)TDET) was designed to overcome these limitations. The tridentate ligation was expected to provide superior pH and thermal stability over the bidentate coordination in the first-generation metallosurfactant. Furthermore, it was of interest to compare the catalytic efficiency of the bidentate (29) Moss, R. A.; Ragunathan, K. G. Langmuir 1999, 15, 107. (30) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (31) Tagaki, W.; Ogino, K.; Tanaka, O.; Machiya, K.; Kashihara, N.; Yoshida, T. Bull. Chem. Soc. Jpn. 1991, 64, 74. (32) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1994, 59, 4194. (33) Fubin, J.; Bingying, J.; Xiaoqi, Y.; Xiancheng, Z. Langmuir 2002, 18, 6769. (34) Ogino, K.; Kashihara, N.; Ueda, T.; Isaka, T.; Yoshida, T.; Tagaki, W. Bull. Chem. Soc. Jpn. 1992, 65, 373. (35) Fife, T. H.; Przystas, T. J. J. Am. Chem. Soc. 1985, 107, 1041. (36) Sigman, D. S.; Jorgensen, C. T. J. Am. Chem. Soc. 1972, 94, 1724. (37) Broxton, T. J.; Nasser, A. Can. J. Chem. 1997, 75, 202. (38) Broxton, T. J.; Cox, R. A. Can. J. Chem. 1993, 71, 670.

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coordinated Cu(II)TTMED with the tridentate Cu(II)TDET, since it is well-established that the structure of the lipophilic surfactant ligand in metallomicelles is known to influence the rate and mechanism of ester hydrolysis.2,15,19,26,31-34 Structural variation in the ligand is known to affect the critical micelle concentration (cmc),15,37 the pKa of metal-bound water or alcohol,38 and the coordination geometry of the metal ion.28 There have been few studies of ester hydrolyses in pure metallomicellar solutions.13,18,20 The low solubility of many metallodetergents has limited study to micellar systems containing cationic or nonionic cosurfactants. The presence and type of cosurfactant can influence the catalytic efficiency of metallomicelles.5,19,23,31,33,39 Inhibition can occur due to the dilution effect, or acceleration resulting from the assisted solvation of substrate with added cosurfactant. Tagaki et al.19 studied the hydrolysis of PNPP in the presence of imidazole and Cu(II) or Zn(II) metallosurfactants with cationic (CTAB), anionic [sodium dodecyl sulfate (SDS)], or nonionic (Triton X-100) cosurfactants. They reported that the addition of SDS cosurfactant promoted the formation of a more catalytically active species, with greater hydrolytic efficiency. The variety of effects observed in comicellar systems underlines the need to find pure metallomicellar systems to provide a better model for examining the effects of metallomicelles on hydrolysis. Accordingly, a central feature of the present study has been to examine ester hydrolysis in the presence of pure Cu(II)TDET micelles. Moreover, no detailed study that directly compares carboxylic ester hydrolysis in pure metallomicelle and mixed systems has previously been reported. We therefore extended our studies to determine the influence of added cosurfactants (Triton X-100 and MTAB) on the rates of hydrolysis of esters 1, 3, and 5 in the presence of the copper micelle Cu(II)TDET. Kinetic effects can be elucidated from rate variations with metallosurfactant and cosurfactant concentrations, and electrostatic effects can be elucidated from comparison of anionic and neutral carboxylic esters. Experimental Section 1H

13C

and NMR spectra were recorded on a Bruker Avance 300 spectrometer (300 and 75 MHz for 1H and 13C, respectively) in deuterated chloroform (CDCl3) (Cambridge Isotope Laboratories) unless otherwise noted. ESI-MS (electrospray ionization mass spectrometry) spectra were obtained on a VG Bio-Q triple quadrupole mass spectrometer (VG Bio Tech, Altringham, Cheshire, U.K.) using a water/methanol/acetic acid (50:50:1) mobile phase or on a PerkinElmer API-300 triple quadrupole mass spectrometer using a water/ acetonitrile/formic acid (50:50:1) mobile phase. Infrared spectra were recorded on a Bruker Vector 22 spectrophotometer. Conductivity measurements were used to determine the critical micelle concentration (cmc) of Cu(II)TDET. Measurements were recorded at 30.0 °C using a jacketed beaker. An Activon PT1-10 conductivity meter, with a platinum electrode, was used to measure the conductivity of pure aqueous solutions of the metallosurfactant. Materials. Diethylenetriamine (Aldrich) and 1-bromotetradecane (Aldrich) were used without further purification. Copper(II) chloride (BDH) and copper(II) acetate (BDH) were of analytical grade and used without further purification. 4-Acetoxy-3-nitrobenzoic acid (2) and 2-nitrophenyl acetate (4) were available from previous studies.37 4-Nitrophenyl acetate (3) was commercially available from Aldrich and was used without further purification. 2-Acetoxy-5-nitrobenzoic Acid (1). 2-Hydroxy-5-nitrobenzoic acid (4.6 g, 26 mmol) was dissolved in acetic anhydride (4.9 cm3, 52 mmol) and the mixture was heated to 125 °C, followed by the addition of three drops of H2SO4 (concentrated). The reaction was stirred at 125 °C for 15 min and then cooled to room temperature. (39) Cibulka, R.; Dvorak, D.; Hampl, F.; Liska, F. Collect. Czech. Chem. Commun. 1997, 62, 1342.

1874 Langmuir, Vol. 23, No. 4, 2007 Water (10 cm3) was added to crystallize the title compound 1. The acetate 1 was recrystallized from benzene and was isolated as offwhite crystals (0.8 g, 17%), (64% lit.37), mp 152-154 °C (lit.37 153-154 °C). 4-Nitrophenyl Benzoate (5). Compound 5 was prepared by the procedure of Cilento;40 mp 140-141 °C (lit.40 145-146 °C). 1-Tetradecyldiethylenetriamine (6). 1-Bromotetradecane (10.0 g, 30 mmol) was added to diethylenetriamine (37.2 g, 360 mmol) in absolute ethanol (110 cm3). The mixture was heated at reflux for 1 h and then cooled to room temperature. When cool, a solution of NaOH was added (2.0 g in 2 cm3 of water) and the mixture was then heated to reflux for a further 16 h. After cooling to room temperature, water (100 cm3) was added and the solution was extracted with diethyl ether (3 × 80 cm3). The ether extract was washed with brine (2 × 110 cm3) and then dried (MgSO4), and the ether was removed under reduced pressure to give a yellow semisolid, which solidified on standing. The crude reaction solid (14.4 g) was purified using vacuum distillation (bp 180-200 °C at 0.3 mmHg) (bp lit.41,42 149 °C at 0.05 mmHg), to give a clear oil (6.9 g, 64%), which solidified on standing; mp 45-47 °C. m/z (LSIMS) [Found: 300.3378 (M + H); C18H41N3, requires 300.3379 (M + H)]. Kinetic Studies. A 20 mM metallosurfactant stock solution was prepared in Millipore (MilliQ) water. The water was purified from distilled water by treatment with a Waters Millipore system to achieve a resistivity of at least 12 MΩ cm-1. Copper(II) chloride (0.18 g, 10 mmol) was dissolved in water (10 cm3) and added to 1-tetradecyldiethylenetriamine in a 50 cm3 volumetric flask. Water was added to bring the solution to the required volume and achieve a final concentration of 20 mM. The solution was sonicated for 0.5 h to ensure complete dissolution of the complex. A stock solution of the complex was prepared in MilliQ water. The appropriate volume of the metallosurfactant stock solution (20 mM) was added to a volumetric flask, followed by the required volume of HEPES (pH 7.0) buffer. MilliQ water was then used to dilute to the mark. Stock solutions (10 mM) of 1-5 were prepared in HPLC-grade acetonitrile. All reactions with these solutions were performed in a 1 cm quartz cuvette, which was placed within a thermally equilibrated cell compartment (40.0 ( 0.1 °C) of a Varian Cary 50 UV/vis spectrophotometer, coupled to a thermostatted water bath. A 3 cm3 aliquot of the stock solution was placed in the quartz cuvette and thermally equilibrated for at least 15 min. The reaction was initiated by the addition of a stock solution of the required substrate by microsyringe, with stirring. A 20 µL sample of substrates 2, 3, 4, and 5 was injected to give a final concentration of 6.6 × 10-6 M. A 30 µL sample of substrate 1 was injected to give a final concentration of 1.0 × 10-5 M. The concentration of each substrate was chosen to achieve an absorbance change of approximately 0-1 absorbance units. Nitrophenolate anion formation was followed spectrophotometrically at a wavelength of 415 nm for structures 1 and 2, 400 nm for structures 3 and 5, and 415 nm for structure 4, respectively. The reactions were followed for at least 4 half-lives for the slower reactions and up to 11 half-lives for the rapid reactions. At the completion of each reaction, the temperature and pH were measured within the cuvette. The rate constant was calculated using the Varian “Kinetics” software program (Cary WinUV) contained within the Varian Cary 50 UV/vis spectrophotometer workstation. A Marquardt nonlinear fit technique was used to obtain a first-order rate constant. A minimum of at least three similar runs were conducted for each reaction, and the rate constant and standard deviation (s.d.) were reproducible within (5%. cmc Measurements. Conductivity measurements were used to determine the cmc of Cu(II)TDET. Measurements were recorded at 30 °C using a jacketed beaker. The solutions were not buffered. An Activon PT1-10 conductivity meter, utilizing a platinum electrode, measured the conductivity of a pure solution of the metallosurfactant subsequent to the addition of 0.5 mL aliquots of the surfactant stock (40) Cilento, G. J. Am. Chem. Soc. 1953, 75, 3748. (41) Jacobelli-Turi, C.; Palmera, M.; Maracci, F.; Margani, A. Ann. Chim. (Rome) 1970, 60, 674. (42) Kanetani, F.; Negoro, K.; Matsue, K. Nippon Kagaku Kaishi 1985, 5, 938.

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Figure 1. Rate-detergent concentration profiles for hydrolysis of compounds 1-4 in the presence of Cu(II)TDET at pH 7.0 (HEPES buffer, 70 mM). Lines are drawn to show the trend. Inset: expanded view of the sub-cmc rates. solution (20 mM). The cmc was obtained by plotting the conductivity (G) as a function of metallosurfactant concentration and extrapolating the two slopes to an intersection. The intersection of the two slopes was taken to be the cmc.

Results Metallosurfactant and Substrates. The lipophilic ligand 6 was obtained by the alkylation of diethylenetriamine with 1-bromotetradecane. Details of the synthetic procedure are given in the Experimental Section. The corresponding Cu(II) complex (Cu(II)TDET) was prepared by the addition of mole equivalents of 6 to cupric chloride in water. Although the ligand 6 was highly insoluble in water, the resulting copper complex was quite soluble in aqueous solution at pH 7. No cosurfactant was needed to solubilize the complex. The formation of (Cu(II)TDET) was confirmed by ESI-MS analysis [Cu(II)TDET + HCOO- m/z 407.2], and the formation of metallomicelles was confirmed by the detection of a cmc (0.8 mM) from conductivity measurements. Kinetics. (a) Hydrolysis in the Presence of Cu(II)TDET. Pseudo-first-order rate constants, kψ, for the hydrolyses of 1-4 in the presence of Cu(II)TDET were determined at pH 7.0 using a HEPES buffer (70 mM) at a temperature of 40 ( 0.1 °C. The rate of ester hydrolysis, which was monitored spectrophotometrically by UV absorption of the product phenolate ion, was measured in the presence of 0.2-6 mM Cu(II)TDET. The rate constant for ester hydrolysis in buffered water (k0) without detergent was similarly determined for each substrate. The variation of kψ with metallosurfactant concentration for the hydrolyses of esters 1-4 is shown in Figure 1. Above the kinetically determined cmc (ca. 0.9 mM) that prevails in the reaction conditions, the surfactant catalyzed the hydrolysis of all esters. A pronounced acceleration was observed for the acidic substrates 1 and 2, which plateaued above 4 mM metallosurfactant concentration. A small acceleration was observed for the neutral substrates 3 and 4 up to 6 mM Cu(II)TDET. For substrates 1 and 2, saturation behavior was displayed above 5 and 3 mM detergent concentration, respectively. For model compounds 3 and 4, the moderate catalysis by the micelle was reflected in a linear increase in kψ, without observable saturation kinetics, up to 6 mM Cu(II)TDET concentration. (b) Estimated Micellar Parameters: (KS) and (kn). Table 1 summarizes the estimated kinetic parameters for the hydrolysis

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Table 1. Kinetic Parameters for the Hydrolysis of Compounds 1-4 in the Presence of Cu(II)TDETa entry

substrate

cmc/mM

(kψ/k0)b,c

KS/M-1

kn/s-1

1 2 3 4

1 2 3 4

0.90 0.90 1.60 1.60

232 183 38 16

174 1070 155 30.2

177 64.9 16.9 20.4

a Conditions: 70 mM buffer, pH 7.0; 40.0 °C. b kψ ) klim × 10-4 s-1. c k0(1) ) 0.32 × 10-4 s-1; k0(2) ) 0.29 × 10-4 s-1; k0(3) ) 0.16 × 10-4 s-1; k0(4) ) 0.15 × 10-4 s-1.

Figure 2. Observed rate constants kψ for hydrolysis of esters 1 and 3 in 1 mM Cu(II)TDET and in the presence of MTAB [conditions: at 40.0 °C, pH 7.0 (HEPES buffer, 70 mM)]. Lines are drawn to show the trend.

Figure 3. Observed rate constants kψ for hydrolysis of esters 1, 3 and 5 in the presence of 4 mM Cu(II)TDET and in the presence of MTAB [conditions: at 40.0 °C, pH 7.0 (HEPES buffer, 70 mM)]. Lines are drawn to show the trend.

of compounds 1-4 in the presence of Cu(II)TDET. A nonlinear regression analysis of the rate-concentration data was undertaken using a modified ternary complex kinetic model5,6,19,23,32-34 to obtain values for the binding constant, KS, and the overall firstorder rate constant, kn. The best fit to data was obtained when the cmc of Cu(II)TDET was fixed at 0.90 mM for 1 and 2, respectively, and 1.6 mM for 3 and 4, respectively. Actual cmc values would not usually be expected to vary with substrate. This point will be considered in the Discussion. The relative rate enhancement (kψ/k0) observed is an order of magnitude greater when a carboxylate anion is present on the substrate (entries 1 and 2) as compared with the neutral model compounds (entries 3 and 4). The analysis suggests a binding constant (KS) for 1 somewhat lower than that for isomer 2, despite a larger rate enhancement. (c) Cosurfactants. To examine reactivity of Cu(II)TDET in the presence of a cosurfactant, the rates of hydrolysis of compounds 1, 3, and 5 were investigated in co-micellar solutions where Cu(II)TDET was mixed with either the cationic surfactant, MTAB, or the nonionic surfactant, Triton X-100. Addition of cosurfactant significantly affected the catalytic activity of the Cu(II)TDET. Reaction rate varied with both type and concentration of the added cosurfactant, as well as with metallosurfactant concentration and substrate structure. The results are summarized below. Addition of MTAB. Figures 2 and 3 show the effects on hydrolysis rates of adding MTAB to form co-micelles. For the anionic substrate 1, at 1 mM Cu(II)TDET, a rate enhancement reached at maximum an approximate 3-fold increase relative to pure Cu(II)TDET with 1 mM added MTAB. The rate then decreased with further addition of cosurfactant up to 10 mM. For 4 mM Cu(II)TDET, the addition of MTAB resulted in only a

slight increase in kψ, again with a decrease at higher concentrations of MTAB. The hydrolysis of model substrate 3 was insensitive to small additions of MTAB to 1 mM Cu(II)TDET. Enhanced catalysis was observed for larger additions, rising to an approximate 8-fold increase in kψ when 10 mM MTAB was present. For the higher fixed concentration of Cu(II)TDET (4 mM), the addition of MTAB resulted in increasing kψ, up to about 2-fold for 4 mM MTAB, where it reached a plateau that continued to 10 mM MTAB. For the hydrophobic benzoate ester 5, rate studies were unreliable in the presence of 1 mM Cu(II)TDET due to the low solubility of the substrate, even with the addition of cosurfactant. However, the substrate was solubilized within the pseudophase at 4 mM Cu(II)TDET. There was no clear evidence in this case that addition of MTAB produced an increase in kψ above that of the pure metallomicelle. Addition of Triton X-100. The influence of added Triton X-100 on the metallomicelle catalyzed hydrolysis of these compounds is shown in Figures 4 and 5. The effects are qualitatively similar to those obtained with MTAB co-micelles, and differ only in detail. For 1, with 1 mM metallomicelle, a maximum 3-fold rate enhancement was seen, with a falloff at higher Triton X-100 concentrations. The maximum occurred at lower cosurfactant concentration. At the higher 4 mM metallomicelle concentration, the slight enhancement that was observed for small additions of MTAB was not seen. Addition of Triton X-100 produced inhibition at all concentrations. Substrate 3 again exhibited rate enhancement with added cosurfactant which rose to a plateau at about 9-fold enhancement for 1 mM metallodetergent and 2-fold for 4 mM. Substrate 5, which could only be studied at the higher metallomicelle concentration, showed only inhibition for all concentrations of added Triton-X100.

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Figure 4. Observed rate constants kψ for hydrolysis of substrates 1 and 3 in 1 mM Cu(II)TDET and in the presence of Triton X-100 [conditions: at 40.0 °C, pH 7.0 (HEPES buffer, 70 mM)]. Lines are drawn to show the trend.

Figure 5. Observed rate constants kψ for hydrolysis of esters 1, 3, and 5 in the presence of 4 mM Cu(II)TDET and in the presence of Triton X-100 [conditions: at 40.0 °C, pH 7.0 (HEPES buffer, 70 mM)]. Lines are drawn to show the trend.

Discussion Hydrolysis in the Presence of Cu(II)TDET. The novel catalyst, Cu(II)TDET, catalyzes the hydrolysis of the esters 1-4. The rate-concentration profiles of esters 1-4 show that the rate of ester cleavage for all substrates is dependent on catalyst concentration (Figure 1). Below the cmc that prevails under the reaction conditions (ca. 0.9 mM), a near-linear increase in rate is observed (Figure 1, inset). Above the cmc, the rate increases rapidly with concentration. This is a clear indication of micellar catalysis in the systems studied. For the neutral substrates 3 and 4, the addition of higher concentrations of metallosurfactant (greater than 4.0 mM) results in a slight reduction in rate and is indicative of the onset of saturation kinetics. There are three possible bases for catalytic rate increases in reaction systems of this type: changes in the environment of

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substrate and/or nucleophile that lead to lowering of an activation energy barrier, the concentration effect of bringing substrate and nucleophile together from the total solution volume into a much smaller pseudophase volume, and orientation effects which can ensure that the substrate is presented to the nucleophile in the correct position for reaction to proceed. With a metallomicellar system each of these factors can arise either from ligation of substrate and/or nucleophile to the metal, or from the creation of an amphipathic environment at the surface of the micellar pseudophase. For this system it is clear that both ligation and micellization play important and somewhat synergistic roles. The effect of micellization is clearly seen in the change in the rate-concentration profiles at the cmc. Ligation is the most plausible explanation of the accelerated rate observed for the anionic substrates in this metallodetergent system over similar simple cationic detergent systems. Saturation kinetics in this type of system would usually be expected to show as a plateau in the rate-concentration profile after the rapid increase in rate that occurs with the onset of micelle formation. This is indeed observed for substrates 1-4, although the effect is more pronounced for the anionic substrates. No falloff in rate at higher detergent concentrations is observed, except possibly for substrate 1sthe small observed rate reduction in the figure is not outside the variance range of the individual data points. Such a falloff in rate with higher detergent concentrations is commonly observed in micellar catalysis, but it is usually absent for functionalized detergents where substrate or nucleophile is directly bound to the detergent. It typically arises from competition between an anionic nucleophile and inert anionic counterions for sites in the Stern layer accessible to the substrate. The presence of a carboxylate group on the substrate resulted in larger observed kψ for the metallomicellar catalyzed hydrolysis. The ratio of relevant maximum rate enhancement for the acidic substrates 1 and 2 to the neutral substrates 3 and 4, (kψ/k0)acidic/ (kψ/k0)neutral, is 6 for esters 1 and 3 and 11 for 2 and 4, respectively. We attribute the higher reactivity of 1 and 2 to the coordination of these substrates to the Cu(II) metal. 2-Acetyloxybenzoic acid, the non-nitro-activated analogue of 1, readily forms stable mononuclear and dinuclear coordination complexes with Cu(II).43-48 Ternary complex formation leads to greater partitioning of each substrate into the micellar pseudophase from the bulk aqueous phase, in turn leading to the localization of the substrate within the small volume of the Stern region. Within this region, the close proximity of a high concentration of nucleophilic species increases the rate of bimolecular reaction. Position of the Carboxylate Group. The regioisomeric pair, compounds 1 and 2, were studied to examine the influence of the position of the carboxylate anion relative to the acyl reaction center. The carboxylate anion is ortho and para to the acyl group for compounds 1 and 2, respectively. A comparison of kψ for 1 and 2 at 5 mM catalyst concentration indicates a rate of hydrolysis of 1 only 1.4 times greater than that of 2. Thus the presence of a carboxylate anion ortho to the reaction center does not lead to favorable catalysis in the presence of Cu(II)TDET. This result contrasts with that from a previous study37 where a 14-fold increase in kψ was observed for 1 with respect to 2, in the presence (43) Manojlovic-Muir, L. Chem. Commun. 1967, 20, 1057. (44) Meier, J. L.; Coughenour, C. E.; Carlisle, J. A.; Carlisle, G. O. Inorg. Chim. Acta 1985, 106, 159. (45) Hijleh, A. L. A. Tetrahedron 1989, 8, 2777. (46) Hijleh, A. L. A.; Woods, C.; Bogas, E.; Guenniou, G. L. Inorg. Chim. Acta 1992, 195, 67. (47) Melnik, M.; Macaskova, L.; Mrozinski, J. Polyhedron 1988, 7, 1745. (48) Greenaway, F. T.; Pezeshk, A.; Cordes, A. W.; Noble, M. C.; Sorenson, J. R. J. Inorg. Chim. Acta 1984, 93, 67.

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relative to the acyl group. Further work is required to distinguish these effects.

Figure 6. Schematic representation of the ternary complex between ester 1 and Cu(II)TDET (left) or ester 2 and Cu(II)TDET (right).

Figure 7. Schematic representation of the activated complex formed by the intramolecular attack of a Cu(II)-bound hydroxyl in the ternary complex of ester 1 and Cu(II)TDET.

of bidentate Cu(II)TTMED. The result was unexpected because larger rates were anticipated for the ortho isomer, where the metallomicellar hydrolysis can proceed by an intramolecular nucleophilic attack of a metal-bound hydroxyl. Along these lines, the small difference in observed kψ for 1 and 2 suggests that the mechanism of hydrolysis for 1 does not proceed by an intramolecular pathway. The very similar rate suggests an intermolecular mechanism for 1, since the hydrolysis of 2 must necessarily involve an intermolecular mechanism. A consideration of the geometry of ternary complexes of 1 and 2 with Cu(II)TDET supports this postulate. Figure 6 gives a schematic representation of the ternary complex between compound 1 or 2 and Cu(II)TDET. In the ternary complex of 2 and Cu(II)TDET, the acyl group arranges to enforce a relatively large distance between the nucleophile and the ester. Accordingly, the intramolecular delivery of the Cu(II)-bound hydroxide is inhibited and a less effective intermolecular mechanism proceeds, which almost certainly involves a hydroxide ion associated with the Stern region. Further consideration of the ternary complex between Cu(II)TDET and 1 (Figure 6) reveals steric crowding in the coordination sphere of the Cu(II) ion. Within the ternary complex, the diethylenetriamine subunit presumably lies in the same coordination plane as the ligated substrate, again leading to an increased distance between the coordinated ester and the metal-bound hydroxyl, thereby disfavoring the intramolecular nucleophilic pathway and inducing the intermolecular delivery of a hydroxyl from the Stern region. Moreover, an intramolecular mechanism would involve an activated complex with two fivemembered- and one eight-membered-ring chelates as represented in Figure 7. The eight-membered chelate ring in this context is probably highly disfavored, because of high levels of strain and crowding in the coordination sphere of any tricyclic complex. Thus the rate of tetrahedral intermediate formation, which is the rate-determining step in ester hydrolysis,49,50 is reduced and an intermolecular nucleophilic attack by a hydroxide ensues. It is likely that these two factors operate to mitigate the kinetic advantage of the ortho substitution of the carboxylate anion (49) Bender, M. L. Chem. ReV. 1960, 60, 53. (50) Johnson, S. L. General Base and Nucleophilic Catalysis of Ester Hydrolysis and Related Reactions; Academic Press: New York, 1967; Vol. 5, pp 237-325.

Of particular note is that the calculated KS value for the ester 1 (173 M-1) was 6-fold smaller than that of the ester 2 (1070 M-1), suggesting a weaker binding to the metallomicelle for 1. There is no obvious basis for dissimilarity in the distribution of the two substrates in the pseudophase. Similar pKa values are expected for the respective carboxylic acids and, consequently, a similar propensity to form ion pairs with the cationic metallosurfactant. Furthermore, the coordinate bonds between the carboxylate ion and Cu(II) in 1 or 2 are also of similar strength. Steric crowding in the 1-Cu(II)TDET ternary complex might provide a rationalization for the small KS value. A large part of this difference could be artifactual since the mathematical treatment of the ternary complex kinetic model is an oversimplification of the very complex kinetic scheme that would actually apply. Nonetheless, the difference is so large that it must be regarded as significant. Estimated Micellar Parameters: (KS) and (kn). The quantitative treatment of micellar catalysis in the presence of metallomicelles can be described by the ternary complex kinetic (TCK) model. The TCK model has been used successfully to account for the catalysis of ester hydrolysis,5,18,19,23,32-34 The model is based on the principles of the pseudophase model.51 Unlike the pseudophase models, though, the TCK model does not make allowance for the degree of counterion binding (β) or the ion-exchange constant (KXY). The latter describes the selectivity of reactive counterions for the micellar surface when bimolecular reactions involve reactive counterion species. In metallomicelles, the ester hydrolysis mechanism does not generally rely on reactive counterions as a source of nucleophilic species. The nucleophilic species in these systems are usually derived from a metal-bound water molecule or an alkoxide ion covalently bound to the lipophilic ligand, so β and KXY are not required in the kinetic treatment. The TCK model assumes the formation of a binary complex (MLn) between the metal (M) and a number (n) of surfactant ligands (L), with an association constant KM (eq 1). The binary complex then rapidly forms a ternary complex (MLnS) with the substrate (S) with an equilibrium association constant (KS), followed by a rate-determining pseudo-intramolecular or intermolecular hydrolysis with an overall first-order rate constant (kn), to afford the products (P) (eq 2). The substrate can also undergo hydrolysis (k0′) in the bulk aqueous phase and also in association with individual surfactant monomers or submicellar aggregates (MLnS)cmc below the cmc (kcmc) (eq 5). Allowance is also made, k0, for the concurrent rate of reaction in buffered aqueous solution. KM

M + nL 98 MLn

[MLn]T KM ) [M][L]

KS

kn

MLn + S 98 MLnS 98 P

KS )

(1)

n

[MLnS] ([MLn]T - [MLnS])([S] - [MLnS])

(51) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698.

(2)

(3)

1878 Langmuir, Vol. 23, No. 4, 2007 k0′

S 98 P kcmc

(MLnS)cmc 98 P

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(4) (5)

The observed rate constant kobs is then given by the expression

kobs )

k0′ + kcmc + knKS([MLn] - cmc) 1 + KS([MLn] - cmc)

(6)

This modified approach differs from previous reports adopting the TCK model by taking into account the monotonic increase in rate at detergent concentrations below the cmc. This increase is attributed to individual detergent-substrate interaction and micellar aggregates at concentrations below the cmc. Based on the rate-detergent profiles from Figure 1, a cmc of approximately 0.9 mM was assumed for Cu(II)TDET in the presence of all substrates under the given reaction conditions. A linear regression analysis was used to obtain the optimum fit for kobs from 0 to 0.5 mM Cu(II)TDET for all substrates for the contribution from kcmc. A nonlinear regression analysis was used to obtain the optimum fit for kn and KS to kobs above the cmc, for the micellar catalyzed component. We note that there is some insensitivity in the regression toward countervariation of rate constant and KS, in which the data fitting showed a strong inverse correlation between KS and kn. Despite this, a good fit to data was obtained for the ionic substrates. However, the optimal fit was unsatisfactory for the neutral regioisomers 3 and 4 with the cmc fixed at 0.9 mM. An improved fit was obtained for 3 and 4 when the cmc was included as a variable parameter in the nonlinear regression (data not shown). The optimal cmc values for Cu(II)TDET were significantly larger for 3 and 4: 1.62 and 1.56 mM, respectively. The analysis proceeded using a cmc for Cu(II)TDET fixed at 1.60 mM for the optimization of KS and kn for 3 and 4. The anionic substrates 1 and 2 may well promote formation of micelles by the cationic detergent at concentrations below those where micellization might usually prevail. The actual cmc of Cu(II)TDET may be higher in the presence of the neutral substrates, and the estimation of the cmc value from the rate-concentration profiles in Figure 1 may underestimate the actual cmc value. There is a broad concentration range over which the kinetic cmc appears for Cu(II)TDET in the presence of 3 and 4, and a precise value is difficult to discern. From the data in Table 1, the larger estimated KS values for the anionic substrate pair, compounds 1 and 2, supports the hypothesis that the accelerated rate of hydrolysis for these compounds results from the stronger binding to the aggregate. The observed catalysis for the model substrates (3 and 4) would then be attributed to a nonspecific hydrophobic interaction with the metallomicelle, which localizes the substrates within the metallomicellar pseudophase. Since it is unlikely that the neutral esters 3 and 4 bind efficiently to the Cu(II) in Cu(II)TDET, the ternary complex required for effective catalysis is not formed.17 A comparison of the data in Table 1 for neutral and anionic substrates shows that the trend in the magnitude of the value KS is not reflected in the overall first-order rate constant (kn). This is further evidence that the lower rates for the neutral esters are a result of the weaker binding to the micelle rather than less effective catalysis in the given reaction context. Cosurfactants. Addition of cosurfactants greatly affects the reactivity of Cu(II)TDET micelles. There are several dimensions where comparisons can be made in the results of the studies of mixed micellar systems: (i) whether the added surfactant was an ionic (MTAB) or nonionic cosurfactant (Triton X-100), (ii)

rate variation with concentration of cosurfactant, (iii) rate variation with concentration of Cu(II)TDET, and (iv) rate differences for different substrates and their relationship to substrate structure. Each of these factors is considered below. (i) Influence of the Nature of Added Surfactant. There are two obvious bases for any observed difference in behavior between mixed micelles containing MTAB and those containing Triton X-100. First, MTAB is a typical cationic detergent that will introduce a large extra concentration of anionic counterions into the Stern layer around the micelle, as well an additional cationic contribution to the already cationic metallomicelle surface. Triton X-100, as a nonionic surfactant, will not increase the anion concentration in the same way. Indeed, because of the slight negative polarity of its hydrophilic surface, due to the electronrich ethylene oxide groups, it may well cause a slight reduction in the anionic population of the Stern layer. Second, Triton X-100 has a greater density of hydrophilic groups that will locate at the micelle surface, and will therefore occupy a much larger micelle surface area per molecule. At a given concentration it will be a much more effective diluent for any process involving the micelle surface. The comparative results for the mixed micelle studies of these systems are strongly indicative of the second as the more significant factor. The qualitative results are quite similar for the two types of mixed micelle, but in the Triton X-100 based systems rate maxima occur at lower concentrations if at all, and rate falloffs at higher concentrations are more pronounced. There does not appear to be any strong evidence of significant electrostatic effects or ion competition in the observations. This result is consistent with a mechanism in which a substrate unit ligated to a metal center interacts with a nucleophiles presumably a hydroxyl ionsbound to a different metal center. The absence of an obvious ion competition or electrostatic effect may be illusory, however. It is likely that similar behavior in the MTAB and Triton X-100 mixed micelles may arise from different but related electrostatic effects. If the effective nucleophile is an unbound Stern layer hydroxyl ion, then a rate falloff with increasing Triton X-100 concentration could be a simple reflection of a lower concentration of anionic sites in the Stern layer, while a similar falloff in MTAB might reflect hydroxyl losing out to increasing amounts of bromide in the competition for an effectively steady concentration of anionic sites in those mixed micelles. The results are quite consistent with either type of mechanism, and no firm conclusion has been established by the results of this study. (ii) Variation with Amount of Cosurfactant Added to a Fixed Amount of Metallosurfactant. In the profiles shown in Figures 2-5, there are three characteristics. In Figure 2 only, for small additions of MTAB cosurfactant to 1 mM metallodetergent, there is a region of the profile where the additions have little or no influence on rate for 1 and 3. This can probably be associated with the presence of little micellar pseudophase, and a rate largely dominated by nonmicellar processes. The second characteristic is a rate rise with addition of cosurfactant, associated with an increased volume of micellar pseudophase environment where substrate and nucleophile can be incorporated to interact. This region is more pronounced at the lower metallosurfactant concentration, and is indeed absent for the strongly ligated substrate at the higher metallosurfactant concentration. The third characteristic is a falloff with increasing concentration that occurs at high cosurfactant concentrations, particularly for 1, which is extensively partitioned within the pseudophase. This is not observed for some of the more weakly bound substrate systems 3 and 5. It can only be associated with a dilution effect, which

Catalysis of Aryl Ester Hydrolysis

could be either a reduction in the local concentration of an anionic nucleophile in the Stern layer, or a reduction in the concentration of adjacent metal sites where a nucleophile might be ligated. (iii) Comparison of the Different Metallosurfactant Concentrations in the Mixed Micelle Studies. The choice of 1 and 4 mM metallodetergent for these studies represents systems close to the onset of micellization, and where there is sufficient micellar pseudophase to comfortably accommodate the substrate, respectively. The observations are as would be expected, reflecting the solubility of each substrate at the respective initial concentration of Cu(TDET). For 1, 3, and 5, rate increases with added detergent are more pronounced at the lower metallodetergent concentration, and absent for the strongly ligated substrate at the higher metallodetergent concentration. (iV) Comparison of the Substrates. It is clear in the results from the mixed micelle studies that the combination of electrostatic attraction of the carboxylate anion to the cationic metallomicelle surface and the strong ligation of substrate 1 to the metal center are the crucial factors in the very strong catalysis that is observed for this substrate. There is ample evidence of weaker binding of the more slowly reacting substrates to the micelle, the requirement for a larger pseudophase volume to achieve the optimal reaction rate, and the tendency for rates to plateau rather than show significant falloff with the addition of higher concentrations of cosurfactant.

Conclusion In summary, this paper described the preparation of the novel metallosurfactant Cu(II)TDET and the catalysis of nitro-activated aryl ester hydrolysis in the presence of micellar aggregates of Cu(II)TDET at neutral pH. Although the magnitude of catalysis was moderate compared to previous reports of the metallomicellar catalyzed hydrolysis of activated esters, striking substituent effects were observed for the aryl ester series studied. First, the presence of a carboxylate anion on the substrates (1 and 2) resulted in larger observed rates (kψ) compared to the neutral substrates studied (3 and 4). The larger rates were attributed to the extensive

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partitioning of the anionic substrates within the micellar pseudophase that resulted from the formation of a ternary complex, facilitated by the coordination of the substrate to the Cu(II) cation via the carboxylate anion and by favorable electrostatic interactions with the cationic micellar surface. Second, and unexpectedly, the location of a carboxylate anion ortho or para to the acyl ester had little effect on the magnitude of catalysis. It is likely that stereochemical constraints within the tridentate Cu(II)-containing ternary complex disfavor the pseudo-intramolecular nucleophilic attack of metal-bound hydroxide and require that the reaction proceed via some slower intermolecular pathway. These observations did not parallel the previous study of these substrates using a bidentate Cu(II) metallosurfactant,37 and these unprecedented effects in the presence of metallomicelles warrant further investigation. Furthermore, the addition of a cationic cosurfactant (MTAB) or a nonionic cosurfactant (Triton X-100) influenced the rate of metallomicellar catalyzed ester hydrolysis. The influence of cosurfactant on rate enhancement was dependent on the concentration of Cu(II)TDET or cosurfactant in addition to substrate structure. The results are consistent with two distinct possibilities for the dominant mechanism: a reaction between a substrate ligated to a metal site and an anionic nucleophile in the Stern layer, or a reaction between a ligated substrate molecule and a nucleophile ligated at a different but neighboring metal site. The results further suggest that any kinetic analysis involving mixed metallomicellar systems must account for the contribution of the cosurfactant to the observed rates. Moreover, in mixed micellar systems careful consideration should be given to the electrostatic contribution, as well as the concentration of cosurfactant, as these factors have been shown to have a marked effect on the catalytic activity of pure Cu(II)TDET micelles. Supporting Information Available: NMR and IR spectral data of 1-tetradecyldiethylenetriamine (6) and substrates 1 and 5. This material is available free of charge via the Internet at http://pubs.acs.org. LA0626454