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Metallomicelles Made of Ni(II) and Zn(II) Complexes of 2-Pyridinealdoxime-Based Ligands as Catalyst of the Cleavage of Carboxylic Acid Esters† Fabrizio Mancin,‡ Paolo Tecilla,*,§ and Umberto Tonellato*,‡ University of Padova, Department of Organic Chemistry and Centro CNR Meccanismi di Reazione Organiche, Via Marzolo, 1 I-35131 Padova, Italy, and University of Trieste, Department of Chemical Sciences, Via L. Giorgieri 1, I-34127 Trieste, Italy Received July 19, 1999. In Final Form: November 8, 1999 Ligands featuring a 6-alkylaminomethyl-2-pyridinealdoxime moiety (alkyl ) CH3, 2a, or n-C12H25, 2b) have been synthesized, and the reactivity of their Ni(II) and Zn(II) complexes in the cleavage of p-nitrophenyl acetate (PNPA) and hexanoate (PNPH) has been investigated in the absence (2a) or in the presence (2b) of CTABr micelles. The micellar complexes are effective in promoting the cleavage of the substrate with accelerations strongly dependent on the pH, being larger in moderately acidic than in neutral solutions. At pH 5 the Ni(II)/2b/CTABr micelles increase the rate of the cleavage of PNPH by 3 orders of magnitude as compared to CTABr only and by 2 orders of magnitude as compared to the nonmicellar complex of 2a. Moreover the system is truly catalytic with a turnover rate approaching that of the cleavage. Analysis of the second-order rate constants allows the conclusions that the increased reactivity of the micellar system is due to concentration and local-pH effects and not to the activation of the nucleophile. A comparison of the reactivity of the systems made of complexes of 2 with that of the analogues of the 2-pyridineketoxime ligands (1) previously investigated indicates that the insertion of the new chelating atom in the binding subsite, on one hand, increases the formation constant of the metal ion complexes and, on the other hand, decreases the nucleophilicity of the complexed oximate ion. The balance of these two effects, in the operative conditions, favors ligands 2 over ligands 1 due to the higher fraction of complexed oximes.
Introduction Transition metal ions play a key role in the hydrolytic activity of a variety of enzymes.1 In the active site the metal ion, most often Zn(II), activates available nucleophiles, such as hydroxyls or water molecules, by promoting their deprotonation at physiological pHs. To reproduce such mode of action and to realize effective hydrolytic catalysts, much effort has been devoted to the design of metallocomplexes containing nucleophilic groups covalently linked to the ligand subsite or capable of activating added nucleophilic species.2 Most of the metallocomplexes so far investigated feature an hydroxy function, properly located on the ligand structure to allow coordination with the metal ion. Recently, in the search of new and more efficient catalytic systems, we turned our attention to oxime-functionalized ligands. Oximes are known to be very effective R-nucleophiles and, when compared with common oxygen nucleophiles of similar basicities, display enhanced reactivity toward cleavable substrates, such as carboxylic or phosphoric acid esters.3 Furthermore, their reactivity is little * Corresponding author. † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry”. ‡ University of Padova. § University of Trieste. (1) Lippard, S. J. Principles of Bioinorganic Chemistry; University Science Books: Mill Valley, CA, 1994. (2) (a) Chin, J. Acc. Chem. Res. 1991, 24, 145. (b) Fife, T. H. In Perspectives on Bioinorganic Chemistry; Hay, R. W., Dilworth, J. R., Nolan, K. B., Eds.; JAI Press: London, Vol. 1, 1991; p 43. (c) Koike, T.; Kimura, E. J. Am. Chem. Soc. 1991, 113, 8935. (d) Suh, J. Acc. Chem. Res. 1992, 25, 273. (e) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Phys. Org. Chem. 1992, 5, 619 and references therein. (3) Terrier, F.; MacCormack, P.; Kizilian, E.; Halle`, J.-C.; Demerseman, P.; Guir, F.; Lion, C. J. Chem. Soc., Perkin Trans. 2 1991, 153 and references therein.
influenced by the basicity of the oximate function, at least in the pKa range 7-12, and the use of relatively acidic oximes at pHs close to neutrality ensures a large fraction of their highly reactive conjugated bases. Thus, for instance 2-PAM, 2-[(hydroxyimino)methyl]-1-methylpyridinium iodide, a well-known reactivator4 of the enzyme acethylcholinesterase poisoned by organophosphorus (nerve gas) inhibitors, features a rather acidic function (pKa ) 7.75) due to the presence of the positively charged pyridinium residue. In the active site of the enzyme the weakly basic oximate anion acts as a very effective nucleophile in the cleavage of the P-O bond of the phosphorylated serine and reactivates the enzyme. Lowering of the pKa of the oximic function may be achieved also through coordination of a transition metal ion,5 and literature data indicate that, at least with Zn(II) and Ni(II), the nucleophilicity of the oximate ion is maintained within the complex.5df,6 More recently, the use of lipophilic ligands allowed the incorporation of such active complexes in surfactant aggregates.7 The resulting metalloaggregates present several advantages and, in particular, they add to the system a hydrophobic recognition site for the substrate, thus refining the mimicking of the metalloenzymes. (4) (a) Bedford, C. D.; Harris, R. W.; Howd, R. A.; Miller, A.; Nolen, H. W., III; Kenley, R. A. J. Med. Chem. 1984, 27, 1431. (b) Kenley, R. A.; Bedford, C. D.; Dailey, O. D.; Howd, R. A.; Miller, A. J. Med. Chem. 1984, 27, 1201. (c) Bedford, C. D.; Miura, M.; Bottaro, J. C.; Howd, R. A.; Nolen, H. W., III. J. Med. Chem. 1986, 29, 1689. (5) (a) Orama, M.; Saarinen, H. Acta Chem. Scand. 1998, 52, 1209. (b) Orama, M.; Saarinen, H.; Korvenranta, J. Acta Chem. Scand. 1989, 43, 407. (c) Suh, J.; Cheong, M.; Suh, M. P. J. Am. Chem. Soc. 1982, 104, 1645. (d) Breslow, R.; Chipman, D. J. Am. Chem. Soc. 1965, 87, 4195. (e) Suh, J.; Suh, M. P.; Lee, J. D. Inorg. Chem. 1985, 24, 3088. (f) Suh, J.; Cheong, M.; Han, H. Bioorg. Chem. 1984, 12, 188. (6) Yatsimirsky, A. K.; Gomez-Tagle, P.; Escalante-Tovar, S.; RuizRamirez, L. Inorg. Chim. Acta 1998, 273, 167.
10.1021/la9909594 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/04/2000
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Langmuir, Vol. 16, No. 1, 2000 Chart 1
On these premises, we turned our attention to the esterolytic reactivity in water of complexes of a variety of transition metal ions with lipophilic 2-pyridineketoximes comicellized with hexadecyltrimethylammonium bromide (CTABr, see Chart 1). Previous work8 indicated that complexes with Zn(II) and especially Ni(II) are very effective in promoting the cleavage of acetate (PNPA) and hexanoate (PNPH) p-nitrophenyl esters. The case of Ni(II) metalloaggregates is quite impressive insofar as its apparent pKa is approximately 3.7 and its reactivity is virtually unaffected by the pH in a rather broad interval (3.5-8). At pH 4, in the presence of Ni(II)/1b/ CTABr the observed rate of cleavage of PNPH is more than 5 orders of magnitude larger than that in its absence and the system is truly catalytic with a sizable turnover rate. This paper presents an extended study of the properties of this type of metalloaggregates aimed at better defining their esterolytic reactivity and catalytic properties. The ligands previously investigated do not bind Ni(II) and Zn(II) ions strongly enough to allow estimation of the fraction of complexed species in metallomicelles, and the whole picture was further complicated by the presence of complexes of different ligand/metal ion stoichiometries. As a consequence, many parameters, such as the secondorder rate constant for the reaction, could not be evaluated with the accuracy needed to elucidate the origins of the observed acceleration and, particularly, the role of the aggregates. To overcome this limitation, we planned to introduce a third chelating nitrogen on the ligand in order to have stronger complexes with better defined stoichiometries and synthesized the 2-pyridinealdoxime derivatives 2 functionalized in the 6 position with a alkylaminomethyl substituent (see Chart 1). The reactivity of the Zn(II) and Ni(II) complexes of the ligands was studied (7) (a) Gellman, S. H.; Petter, R.; Breslow, R. J. Am. Chem. Soc. 1986, 108, 2388. (b) Menger, F. M.; Gan, L. H.; Johnson, E.; Durst, D. H. J. Am. Chem. Soc. 1987, 109, 2800. (c) Fornasier, R.; Scrimin, P.; Tecilla, P.; Tonellato, U. J. Am. Chem. Soc. 1989, 111, 224. (d) Weijnen, J. G. J.; Koudijs, A.; Engbersen, J. F. J. J. Org. Chem. 1992, 57, 7258. (e) Ogino, K.; Kashihara, N.; Ueda, T.; Isaka, T.; Yoshida, T.; Tagaki, W. Bull. Chem. Soc. Jpn. 1992, 65, 373. (f) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1991, 56, 161. (g) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Am. Chem. Soc. 1992, 114, 5086. (h) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1994, 59, 18. (i) Bunton, C. A.; Scrimin, P.; Tecilla, P. J. Chem. Soc., Perkin Trans. 2 1996, 419. (j) Kriste, A. G.; Vizitiu, D.; Thatcher, R. J. Chem. Commun. 1996, 913. (k) Scrimin, P.; Tecilla, P.; Moss, R. A.; Bracken, K. J. Am. Chem. Soc. 1998, 120, 1179. (8) (a) Budka, J.; Hampl, F.; Liska, F.; Scrimin, P.; Tecilla, P.; Tonellato, U. J. Mol. Catal. A: Chem. 1996, 104 (3), 201. (b) Hampl, F.; Liska, F.; Mancin, F.; Tecilla, P.; Tonellato, U. Langmuir 1999, 15, 405.
Mancin et al. Scheme 1. Synthesis of Oximes 2
(i) Di-tert-butyl dicarbonate, triethylamine, dioxane, r.t.; (ii) selenium dioxide, dioxane, reflux; (iii) hydroxylamine hydrocloride, Na2CO3, EtOH, 60 °C; (iv) trifluoroacetic acid, r.t.
using PNPA and PNPH as substrates in the absence (2a) and in the presence (2b) of CTABr micelles. The results here reported confirm the efficacy of such metallomicellar systems in catalyzing the cleavage of carboxylic acid esters and indicate that an important role is played by the structure of the ligand. Results and Discussion Syntheses and Complexation. Oximes 2 were prepared as outlined in Scheme 1. The proper 2-hydroxymethyl-6-alkylaminomethylpyridine was first Boc protected at the amino nitrogen and then oxidized to the corresponding aldehyde. Reaction with hydroxylamine and deprotection gave the target oximes of E-configuration, as confirmed by the observed nuclear Overhauser effect between the hydroxyl proton and the hydroxyimino CH.9 While 2a is reasonably soluble in water (up to 3 mM) at any pH value, oxime 2b is so only in acidic water. In neutral or basic water stable solutions can be obtained by addition of CTABr in at least a 10-fold molar excess. The formation of metalloaggregates with Ni(II) and Zn(II) ions, highlighted by the diagnostic changes in the UV-vis spectra (decrease of the free ligand absorbance in the 240-290 nm region and appearance of a new band at 310-330 nm, depending on the metal ion), occurs instantaneously at any pH value for the Zn(II) ion and more slowly with Ni(II), especially in acidic solutions and in the presence of comicelles. At any rate, the formation of the Ni(II) complexes of oximes 2 is faster than that of oximes 1: at pH 4 full complexation is achieved in about 2 h using 2b in comicelles with CTABr and in about 2 days in the case of ligand 1c under the same conditions. In view of that, all the measurements reported in this work have been performed after complete equilibration of the system. The formation constants of the complexes and the pKa values of the oximic functions were obtained from UV-vis titrations of the complexes and, in some cases with a better degree of confidence, from the kinetic analysis of the systems (see below and Table 1). Effective Stoichiometries. In the kinetic study, we used two different sets of conditions: (a) a 1:1 molar ratio of metal ion and ligand to allow a coherent comparison with the data previously reported for ligands 1 and (b) an excess of metal ion concentration over that of the ligand, to ensure the presence of only 1:1 ligand/metal ion species, to evaluate the second-order rate constant for the reaction in aggregate and in bulk water. (9) Holzer, W.; Heinisch, G. Tetrahedron Lett. 1990, 31, 3109.
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Table 1. Acidity Dissociation Constants (pKa) for the Oximic Hydroxyl of Ligands 2 and Their Metal Complexes entry
ligand
1 2 3 4 5 6
2a 2a 2b 2b 2b 2b
additive
CTABr CTABr CTABr CTABr
metal ion
pKa
Ni(II) Zn(II)
6.5a (6.5)b,c (7.7)b,c (10)b,d 5.5a (5.6)b-d (6.0)b,d,e 7.3a
Ni(II) Ni(II) Zn(II)
a Determined from the log k obs/pH profiles under the conditions of Figure 3. b Determined from UV-vis titration at [ligand] ) 8 × 10-5 M. c [metal ion] ) 8 × 10-5 M. d [CTABr]/ligand] ) 20. e [Ni(II)] ) 4 × 10-5 M.
Figure 2. Kinetic profiles for the cleavage of PNPH as a function of the [Ni(II)]/[2b] (O) and [Zn(II)]/[2b] ([) ratios in MES buffer, pH 6.3, 25 °C. [2b] ) 0.4 mM; [CTABr] ) 0.8 mM.
Figure 1. Kinetic Job’s plot for the cleavage of PNPH by 2b/ CTABr 1:20 in the presence of Ni(II) ([, pH 6.3, MES buffer, 0.05 M) and Zn(II) (b, pH 6.5, MES buffer, 0.05 M). The sum of the concentrations of ligand and metal ion was kept constant at 0.75 mM. The k0 value was taken as the rate constant measured in the presence of the corresponding concentration of free ligand.
The Zn(II) and Ni(II) ions coordinate up to six ligand atoms and, therefore, may form complexes with 1:1 and 1:2 metal ion/ligand stoichiometries with oximes 2. Preliminary experiments were carried out to evaluate the most reactive complex with the method of continuous variations.10 The apparent first-order rate constants, kψ, for cleavage of PNPH were measured for a series of solutions in which the sum of the concentrations of ligand and metal ion was kept constant while their ratio was varied. The results obtained using PNPH (2 × 10-5 M), [metal ion + 2b] ) 7.5 × 10-4 M, and [CTABr] ) 20[2b] in MES buffer at pH 6.3 in the case of Ni(II) and at pH 6.5 in that of Zn(II) are reported in Figure 1. The plots clearly indicate that in the case of Zn(II) the 1:1 stoichiometry is the most active, whereas in the case of Ni(II) the rather flat maximum between 0.5 and 0.7 suggests that the two possible complexes (1:1 and 1:2 Ni(II)/2b) are characterized by very similar reactivities. To confirm these indications, we titrated with metal ion comicelles made of 2b and CTABr using solutions where their concentration ([2b] ) 0.4 mM, [CTABr] ) 8.0 mM) was kept constant. The results obtained using PNPH as substrate in MES buffer at pH 6.3 are shown in Figure 2. In the case of Ni(II), as its concentration increases, the kψ constants increase linearly up to a ratio [Ni(II)]/[2b] of 0.5 and then remain constant even for a concentration of metal ion 5 times larger than that of the ligand. Clearly a strong 1:2 complex is formed and its reactivity is virtually (10) Connors, K. A. Binding Constant; Wiley: New York, 1987.
Figure 3. pH Dependence of the rate constant for the cleavage of PNPH by Ni(II)/2a (3), Ni(II)/2b/CTABr (b), Zn(II)/2b/CTABr (0), 2b/CTABr (O), or CTABr (4). Conditions: [metal ion]/ [ligand]/[CTABr] ) 1:1:20; [Ni(II)] ) 0.83 mM; [Zn(II)] ) 0.40 mM; [buffer] ) 5 mM; for the buffers used, see Experimental Section.
the same as that of the 1:1 complex which prevails at high Ni(II) concentration. The case of Zn(II) is different: the rate constants increase smoothly with the metal ion concentration, reaching a plateau only for a metal to ligand molar ratio larger than 5. This indicates a smaller formation constant for the complex than for that with Ni(II); the fitting of data for a 1:1 complexation model gives an apparent binding constant of 1200 M-1 and a limiting rate kψ ) 1.8 × 10-2 s-1. Acidity of the Complexes. To evaluate the acidity of the complexed oximic ligands, we defined the effect of pH on the apparent reactivity. Figure 3 reports the pH-rate profiles for the cleavage of PNPH in the presence of the nonmicellar complex Ni(II)/2a, of micelles of CTABr only, and of comicelles of Ni(II)/2b/CTABr (1:1:20) and 2b/ CTABr (1:20). On moving from low to high pH values, the observed rate constants for the micellar complex Ni(II)/ 2b increase linearly with a slope close to unity up to a plateau region. The change of slope from unity to zero,
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Mancin et al.
Scheme 2
which can be taken as the operative pKa of the nucleophile, occurs at pH 5.5. In the case of the nonmicellar complex Ni(II)/2a the profile is similar and the pKa of the oximic function is approximately 6.5. As shown in Table 1, where comparison is possible, very similar pKa values are obtained by kinetic and UV-vis titration. Thus, micellization of the complex lowers the basicity of the oximic function by 1 order of magnitude but increases its reactivity by 2 orders of magnitude in the whole pH range considered. When compared to the noncatalyzed reaction in the presence of only CTABr, the acceleration strongly depends on the pH, being higher in moderately acidic media: metallomicelles of Ni(II)/2b/CTABr accelerate the cleavage of PNPH by over 3 orders of magnitude at pH 5 and by only 50 times at pH 8. On the other hand, the uncomplexed micellar 2b is by itself very effective in promoting the cleavage of the substrate. In this case the observed rate increases linearly with a unity slope in the whole pH range explored, so that, whereas at acidic pHs the ligand is less effective than its Ni(II) complex, at pH > 6.5 the opposite is true. The absence of any break point in the pH-rate profile indicates a pKa larger than 9, and indeed, a value of approximately 10 was obtained by UVvis titration. Figure 3 reports also the pH-rate profile obtained in the case of the Zn(II)/2b/CTABr system, which is very similar to that observed with the Ni(II) complex. Changing Ni(II) for Zn(II) increases the pKa values of the oximic function to 7.3, thus providing a less acidic but more nucleophilic oximate anion. Catalytic Cycle. The results illustrated in the previous section, together with the spectrophotometric titration, clearly indicate that the active nucleophile is the oximate anion coordinated to the metal ion. As a result of the cleavage of the substrate, an acylated oxime should be formed, and if this is rapidly hydrolyzed, the complex behaves as a catalyst with turnover. To elucidate this point, we performed a series of kinetic experiments using [ligand] ) 6 × 10-5 M and a large (>6 times) excess of substrate with respect to the metal complex. For solubility reasons, the substrate of choice was PNPA, and to ensure the presence of a fully formed and deprotonated complex with a 1:1 stoichiometry, we used a Ni(II)/2b ratio of 20 at pH ) 7. In each case a “burst” kinetic is obtained in which, after an initial fast (kinetically first-order) release of p-nitrophenol, a slower (zeroth-order) process follows. The “burst” process accounts for the release of 1 equiv of p-nitrophenol with respect to the metal complex, whereas the rest of the substrate is cleaved in the steady-state portion of the curve. This kinetic profile fits nicely a reaction scheme in which the oximate anion reacts fast with PNPA, releasing the p-nitrophenol and forming an acylated oxime which, in turn, is deacylated in a slower process, possibly mediated by a metal ion-coordinated water molecule (ka > kd in Scheme 2). From the analysis11 of the data obtained at different concentrations of PNPA, (11) Fornasier, R.; Tonellato, U. J. Chem. Soc., Faraday Trans. 1980, 76, 1301.
Figure 4. Rate constant versus 2b concentration profiles for the cleavage of PNPH in the presence of an excess of Ni(II) (b) and Zn(II) (O) ions. Conditions: [CTABr]/[2b] ) 20; [Ni(II)] ) 1.4 × 10-2 M, pH 7.0, HEPES buffer, 0.05 M; [Zn(II)] ) 2.5 × 10-3 M, pH 6.5, MES buffer, 0.05 M.
we evaluated an acylation rate constant (ka) of 0.06 M-1 s-1 and a deacylation constant (kd) of 9.4 × 10-4 s-1. Using eq 1 (see below), it is possible to evaluate that the firstorder rate constant for the acylation step, under the above conditions, is 8.1 × 10-3 s-1. Therefore, the turnover rate is less than 1 order of magnitude smaller than that of the acylation process. Nucleophilicity of the Metallomicellar Systems. To gain insight into the reactivity effects observed, we performed kinetic experiments designed to evaluate the second-order rate constants for the cleavage of PNPA and PNPH by the metal complexes of ligands 2. As said above, in these experiments we used a large excess of metal ion over that of the ligand to ensure the presence of a 1:1 ligand/metal ion complex as the only and most effective species. Moreover, we chose the highest pH value compatible with the solubility of the uncomplexed metal ion to ensure the presence of a fully dissociated oximic function in the case of micellar Ni(II)/2b and of a large fraction of oximate for the other complexes. In the latter case, the rate constants were corrected for such a fraction using the pKa values of Table 1. The evaluation of the overall second-order rate constants for the nonmicellar complexes with 2a was straightforward. Not so simple was the treatment of the kinetic data in the case of the micellar complex with ligands 2b. We measured the kψ for solutions increasingly concentrated in metal ion complexes and CTABr (the ratio being 1:20). The resulting rate-concentration profiles, showing saturation behavior, are reported in Figure 4. Analysis of the curves, using a Michaelis-Menten type equation,11 allows us to estimate the limiting rate constant value (kψmax), which could be observed when all the substrate is bound to the aggregate. The second-order rate constant for the reaction in the micellar pseudophase is then calculated by dividing the kψmax value by the concentration of the nucleophile in the “effective” micellar pseudophase. Since we are interested in comparing second-order rate constants in the aqueous phase and the micellar pseudophase, it is convenient to express the concentration in the latter region as local molarity. This is a debated problem in micellar reactivity, but it is generally accepted that in the case of
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Table 2. Second-Order Rate Constant (k2, M-1 s-1) for the Cleavage of PNPA and PNPH by Ni(II) and Zn(II) Complexes of Ligands 2 under Micellar and Bulk Water Conditions complex 2a/Ni(II) 2b/Ni(II) 2a/Zn(II) 2b/Zn(II)
additive
k2(PNPH), M-1 s-1
k2(PNPA), M-1 s-1
CTABr
0.066 0.10
0.10 0.11 2.5 1.1
CTABr
functional micelles, where every head group is in the reactive form, the local molarity is the reciprocal of the molar volume VM of the reaction region at the micellewater interface.12 This approach can be extended to comicelles of functional surfactants, provided that the dilution of the reactive function in the micelle is taken in account.11,13 The most frequently used VM value for micelles with trimethylammonium head groups is 0.37 dm3 mol-1. Thus, the second-order rate constant in the micellar pseudophase k2 was calculated from eq 1 where [Dt]m is the total concentration of micellized surfactant, [Df]m is the concentration of functionalized ligand, and Ka is the dissociation constant for the complexed oxime. The term [Dt]m/[Df]m takes into account the dilution of the reactive complex in the CTABr aggregate, and the term (1 + [H+]/ Ka) gives the fraction of dissociated oxime. The k2 values are in Table 2.
(
)
[Dt]m [H+] k2 ) kψmaxVM f 1 + Ka [D ]m
(1)
Analysis of the data of Table 2 shows that (i) the reactivities of the complex Ni(II)/2 toward both substrates are virtually the same; (ii) the Zn(II) complexes are about 1 order of magnitude more reactive than the Ni(II) complexes; and (iii) the second-order rate constants for the reaction in bulk water and in the micellar region are within a factor of 2. The last observation comes as no surprise: a wealth of data concerning bimolecular processes occurring in nonfunctionalized and functionalized micelles and, more recently, in Cu(II)-containing metallomicelles indicate that micellization generally does not affecttheintrinsicreactivityofthenucleophilesinvolved.7i,12,14 The observed accelerations in solutions of micelles and metallomicelles are mainly due to the concentration of the reagents in the relatively small aggregate volume. Moreover, the apparent pKa of the oximic function in micelles is about 1 order of magnitude (see Table 1) lower than that in bulk water, thus making available a larger fraction of oximate ion than expected at the operational pH. Such a decrease in pKa is due to an enhanced local pH at the cationic surface of the aggregate, and this is not expected to modify the nucleophilicity of the micellar oximate. On the other hand, the higher reactivity of the Zn(II) complexes than that of the Ni(II) analogues is probably related to their higher basicity. As mentioned above, the Brønsted plot relating rate and acidity, for the cleavage of PNPA by different oximate anions, shows a nonlinear behavior: the second-order rate constants increase linearly with a Brønsted βnuc of about 0.75 up to pKa values of ≈7 and then level off up to pKa ≈ 12 (βnuc (12) (a) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (b) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. Acc. Chem. Res. 1991, 24, 357. (13) (a) Bunton, C. A. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 2, p 519. (b) Bunton, C. A.; Hamed, F. H.; Romsted, L. S. J. Phys. Chem. 1982, 86, 2103. (14) Scrimin, P.; Tecilla, P.; Tonellato, U.; Bunton, C. A. Colloids Surf., A 1998, 144, 71.
Table 3. Pseudo-First-Order Rate Constants (kψ, s-1) and Relative Rates (krel) for the Cleavage of PNPH in the Presence of Different Micellar Catalysts at pH 6.3a entry
micelles
1 2 3 4 5 6 7 8 9
CTABr 1c/CTABr 2b/CTABr 1c/CTABr 1c/CTABrb 2b/CTABr 2b/CTABrb 1c/CTABr 2b/CTABr
metal ion
kψ, s-1
krel
Ni(II) Ni(II) Ni(II) Ni(II) Zn(II) Zn(II)
3.7 × 10-6 6.8 × 10-5 2.01 × 10-3 1.82 × 10-3 3.16 × 10-3 4.32 × 10-3 4.30 × 10-3 7.86 × 10-4 7.41 × 10-3
1 18 534 486 854 1167 1162 210 2002
a [CTABr] ) 8 mM, [ligand] ) 0.4 mM, [metal ion] ) 0.4 mM; MES buffer 0.05 M, 25 °C. b [ligand] ) 0.8 mM.
) 0). As a consequence, oximes with pKa in the range 7-12 display the same reactivity toward PNPA, and a decrease in the second-order rate constants is observed only for more acidic oximes.3 The micellar complexes of 2b with Zn(II) and Ni(II) have acidities respectively above and below the break point of the Bro¨nsted plot, and this is the source of their different reactivity. Comparison between Metallomicelles Made by Ligands 1 and 2. The reactivities of the different esterolytic systems should be correctly compared on the basis of the second-order rate constants. However, in the case of ligands 1, the low binding constants for the metal ions and the presence of complexes with different stoichiometries make difficult the evaluation of the k2 values, especially in the case of the comicellized ligands. Only in the case of ligands 1a, from experiments with a large excess of Ni(II) ions, could we obtain a k2 value of 6.5 M-1 s-1 for the cleavage of PNPA. Assuming that micellization does not affect nucleophilicity (see above), it appears that the Ni(II) complex of micellar 1c is 60 times more reactive than the analogous complex of 2b. It follows that, by changing the metal ion binding site from bi- to tridentate and on going from ketoxime to aldoxime, the nucleophilicity of the complexed oximate ion decreases although the binding constant of the complex increases. If the comparison based on the k2 constants allows a deeper insight into the mechanistic facets for an evaluation of the systems investigated, the comparison between the apparent first-order rate constants kψ better highlights the differences between ligands 1 and 2 as actual reactants under the operative conditions and may rely on a wider set of data. The kψ values strongly depend on the pH, as a consequence of the different basicities of the oximic functions in the ligand metal ion complexes. In principle, the pH of choice should be high enough to ensure the complete deprotonation of all the oximic systems considered: pH 6.3 looks like a good compromise with the data in our hands. In fact, at this pH value, there is a large fraction of deprotonated metal ion complexes, at least in the case of the Ni(II) ion, and still pronounced acceleration of the reaction relative to the uncatalyzed one is observed. The rate data obtained under these conditions for a ligand/ metal ion ratio of 1.1 and 2:1 are listed in Table 3. The data of Table 3 allow the following main observations. (a) In each case the micelles of ligand 2b and their complexes are more reactive than the corresponding micelles of oxime 1c. The rate ratios are 30 (free ligands, entries 2 and 3), 2.5 (1:1 Ni(II) complexes, entries 4 and 6), 1.3 (2:1 Ni(II) complexes, entries 5 and 7), and 9.5 (Zn(II) complexes, entries 8 and 9). (b) The metal ion complexes of ligands 1c and 2b are more reactive than the uncomplexed ligands, although to a different extent.
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Langmuir, Vol. 16, No. 1, 2000
Figure 5. Proposed structures for the 2:1 complexes of ligands 1 (a) and 2 (b) with Ni(II).
The rate constants of the 1:1 Ni(II) and Zn(II) complexes of micellar 1c are 27 and 11 times higher those of 1c/ CTABr, while the corresponding accelerations in the case of 2b are only 2 and 4 times, respectively. (c) The Zn(II) complex of 2b is more reactive than the Ni(II)/2b complex while the opposite is observed with the complexes of ligand 1c. Thus, complexation of the metal ions increases the reactivity of the micellar ligands and the metallomicelles made with 2b are more efficient that the ones made with 1c. The higher reactivity observed in the case of 2b/Zn(II) than in that of the 1c/Zn(II) complexes appears to be related, on one hand, to the larger binding constant of ligand 2b for the metal ion and, on the other hand, to the lower pKa of the complexed 2b (the pKa is 7.3 for 2b/Zn(II)/CTABr and 8 for 1c/Zn(II)/CTABr), which at pH 6.3 ensures a larger fraction of dissociated nucleophile. The effect on the basicity of the oximic function seems less important in the case of the 1:1 Ni(II) complexes. Although the pKa value for the micellar 1:1 complex 1c/Ni(II) is not available, it is reasonable to assume that it is similar to that of the 1:1 2b/Ni(II) complex (pKa ) 5.5), as observed in the case of the monomeric complexes 1a/Ni(II) and 2a/ Ni(II) (pKa ) 6.2 and 6.5, respectively), and that micellization with CTABr exerts the same effect on both complexes. Thus, the enhanced reactivity of 2b/Ni(II) may be explained with the higher formation constant of the complex and hence with the presence of a larger fraction of complexed oxime. The effect on the basicity of the oximes in the case of the Ni(II) complexes with a 2:1 ligand/metal ion stoichiometry is quite large and remarkable. In fact, the 2:1 1c/ Ni(II) complex is very acidic, its pKa being 3.7 (vis-a`-vis that of 6.0 of the analogous complex of 2b). Such a low pKa is probably related to the (first) acid dissociation constant of the micellar 2:1 complex, whose conjugate base may have the structure shown in Figure 5a (actually, it is the one defined for crystals of the Ni(II) complex with pyridineketoxime 1a obtained from aqueous solutions of pH 5).15 Such a structure is apparently stabilized by the formation of a hydrogen bond between the two juxtaposed oximate and oximic functions. Such juxtaposition and the formation of the hydrogen bond cannot be achieved in the case of a 2:1 complex with ligands 2, which, from inspection of models, may reasonably have a structure like that shown in Figure 5b. As a consequence, the pKa values of the 2:1 Ni(II) complexes with 2 are higher and similar to those measured for the 1:1 complexes, so that micellar 2b misses one of the most peculiar characteristics of ligand 1c, which is the extraordinary reactivity at moderately acidic pH. If the two systems are compared at pH 4, under the conditions indicated for the data of Table 3, the acceleration factors observed for micellar Ni(II) complexes of ligands 1c and 2b are respectively 34 000 and 700. (15) Riggle, K.; Lynde-Kernell, T.; Schlemper, E. O. J. Coord. Chem. 1992, 25, 117.
Mancin et al.
As a final comment, in the whole set of kinetic data the reactivity of micellar 2b in the absence of metal ions is surprisingly high, as it is not only 30-fold higher than that of 1c but also, at the pH of choice, very close to that of its Ni(II) and Zn(II) complexes. The high reactivity of 2b is probably due to both structural features: the different positions of the alkyl chain in the two ligands which may favor a better insertion in the aggregate, and the presence of the 6-aminomethyl group. As a possible explanation for the latter effect, we suggest that, at the operative pH, the protonated amino nitrogen may act as a hydrogen bond donor toward the negative oxygen of the tetrahedral intermediate which develops in the transition state and hence stabilize it. If these are factors at play, it is not surprising that the advantage of 2b as a nucleophilic reactant fades out in the metal complexes. Clearly, we cannot provide evidence for such a hypothesis, and further investigation is probably worthwhile in view of the large difference observed. Conclusions The present study allows definition of the mechanistic factors at play in the esterolytic reactivity of the transition metal ion complexes with oximic ligands and the effect of micellization in the corresponding metalloaggregates. Comparison of the second-order rate constants indicates that the intrinsic reactivity of the oximate function is virtually the same inside and outside the cationic aggregates, thus providing further evidence that the microenvironment experienced by a negatively charged nucleophilic function in micellar aggregates is just as that in bulk water. Thus, the kinetic benefits of the aggregates mainly derive from concentration effects due to the transfer of the substrate into the pseudophase and from the effect of bringing into proximity the reagents, as in most cases of functionalized micelles or vesicles. However, there is something specific in the case of the cationic metalloaggregates of the oximic ligands. Complexation with Zn(II) or Ni(II) (and other transition metal ions) combined with the electrostatic effects of a cationic micellar surface sharply decreases the apparent pKa of the function and favors its deprotonation to such an extent that also in slightly acidic solutions a large fraction of oximate nucleophiles is available. The key point is that the nucleophilicity of the oximate functions does not depend, up to a point, on its basicity: thus, cationic micelles provide a large amount of dissociated and very reactive oxime functions at neutral or slightly acidic pHs. A second point of interest, stemming from the comparison of ligands 1 and 2, is the strong dependence of the reactivity of these metalloaggregates on the ligand’s structure and strength. Tridentate ligands 2 form a 1:1 complex with much higher formation constants than the bidentate 1, and this may be beneficial, particularly with Zn(II), insofar as it ensures a larger fraction of reactive complex under conditions of comparable concentration of ligand and metal ion. On the other hand, ligands 1 more easily than 2 can form complexes with stoichiometries higher than 1:1, and the 2:1 ones, at least in the case of Ni(II), are exceptionally reactive in moderately acidic solutions, probably due to ingenious geometries, such as that of Figure 5e, that are strongly favored in micellar aggregates. Experimental Section General Methods and Materials. Melting points are uncorrected. 1H NMR spectra were recorded on a Bruker AC 250F operating at 250 MHz, and chemical shifts are reported relative to internal Me4Si. Elemental analyses were performed by the Laboratorio di Microanalisi of the Organic Chemistry Department
Catalyst of the Cleavage of Carboxylic Acid Esters of the University of Padova. Surface tension measurements were performed with a Kruss type 8451 tensiometer. UV-vis spectra and kinetic traces were recorded on a Perkin-Elmer Lambda 16 spectrophotometer equipped with a thermostated cell holder. Zn(NO3)2 and Ni(NO3)2 were analytical grade products. Metal ion stock solutions were titrated against EDTA following standard procedures. The buffer components were used as supplied by the manufacturers: acetic acid (Aldrich), 2-morpholinoethanesulfonic acid (MES, Fluka), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES, Sigma), 4-(2-hydroxyethyl)-1-piperazinepropanesulfonic acid (EPPS, Aldrich), 2-(cyclohexylamino)ethanesulfonic acid (CHES, Aldrich), 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS, Aldrich). n-Hexadecyltrimethylammonium bromide (CTABr, Fluka) was an analytical grade commercial product. The p-nitrophenyl esters of acetic (PNPA) and hexanoic acid (PNPH) were Sigma products used as received. 6-[(Methylamino)methyl]-2-(hydroxymethyl)pyridine and 6-[(n-dodecylamino)methyl]-2-(hydroxymethyl)pyridine were prepared as reported.7h General Procedure for the Synthesis of Ligands 2. To a solution of the proper 6-[(alkylamino)methyl]-2-hydroymethylpyridine (3.8 mmol, alkyl ) CH3 for 2a and n-C12H25 for 2b) in 40 mL of dioxane were added di-tert-butyl dicarbonate (0.87 g, 4.2 mmol) and triethylamine (0.60 mL, 4.3 mmol). The reaction mixture was stirred for 4 h, and then a second portion of ditert-butyl dicarbonate (0.10 g, 0.5 mmol) and triethylamine (0.10 mL, 0.5 mmol) was added. After 2 h the solvent was removed under vacuum. The residue was treated with 80 mL of a 5% solution of NaHCO3 and extracted with CHCl3 (3 × 70 mL). The organic phase was dried (Na2SO4) and the solvent was evaporated to give the proper 6-[N-tert-butoxycarbonyl(alkylamino)methyl]2-hydroymethylpyridine as a yellowish oil in 85-92% yield. To a solution of this Boc-protected derivative (3.5 mmol) in 40 mL of dioxane was added selenium dioxide (0.40 g, 3.6 mmol). After it was stirred and heated at 70 °C for 6 h, the mixture was filtered through a short Celite pad and the solvent was evaporated. The crude product obtained was purified by column chromatography (silica gel, CHCl3/CH3OH 50:1 or 80:1 (v/v) for the methyl or dodecyl derivative, respectively) to give the pure aldehyde as a pale yellow oil. 6-[N-tert-Butoxycarbonyl(methylamino)methyl]-2-formylpyridine: yield 97%. 1H NMR (CDCl3) δ: 1.40 and 1.47 (bd, 9H, C(CH3)3); 2.94 (s, 3H, NCH3); 4.65 (s, 2H, NCH2Py); 7.4 (m, 1H, H5Py), 7.83 (m, 2H, H3Py and H4Py); 10.04 (s, 1H, CHO). 6-[N-tert-Butoxycarbonyl(n-dodecylamino)methyl]-2-formylpyridine: yield 85%. 1H NMR (CDCl3) δ: 0.87 (t, J ) 6.6 Hz, 3H, (CH2)11CH3); 1.2-1.6 (bm, 29H, CH2(CH2)10CH3 and CC(CH3)3); 3.25 (m, 2H, CH2(CH2)10CH3); 4.61 (bs, 2H, NCH2Py); 7.45 (m, 1H, H5Py); 7.84 (m, 2H, H3Py and H4Py); 10.04 (s, 1H, CHO). To a solution of the above aldehyde (3.4 mmol) in 15 mL of EtOH was added first a solution of hydroxylamine hydrochloride (0.30 g, 4.3 mmol) in 10 mL of water and then a solution of Na2CO3‚10H2O (0.63 g, 2.2 mmol) in 10 mL of water. The reaction mixture was stirred and heated at 60 °C for 90 min; during this time a white precipitate is formed. After the mixture was cooled to room temperature, 30 mL of water was added and the aqueous phase was extracted with CHCl3 (3 × 50 mL). The organic phase was dried (Na2SO4), and the solvent was evaporated, leaving the oxime, which was purified as indicated below. 6-[N-tert-Butoxycarbonyl(methylamino)methyl]-2-[(hydroxyimino)methyl]pyridine: purified by crystallization from CH2Cl2, yield 97% (white solid). 1H NMR (CDCl3) δ: 1.42 and 1.49 (bd, 9H, C(CH3)3); 2.93 (s, 3H, NCH3); 4.53 (s, 2H, NCH2Py); 7.2 (m, 1H, H5Py); 7.6-7.7 (m, 2H, H3Py and H4Py); 8.12 (s, 1H, CHNO); 8.22 (s, 1H, NOH).
Langmuir, Vol. 16, No. 1, 2000 233 6-[N-tert-Butoxycarbonyl(n-dodecylamino)methyl]-2-[(hydroxyimino)methyl]pyridine: purified by column chromatography (silica gel, CHCl3/CH3OH 80:1 (v/v)), yield 33% (oil). 1H NMR (CDCl3) δ: 0.86 (t, J ) 6.7 Hz, 3H, (CH2)11CH3); 1.2-1.6 (bm, 29H, CH2(CH2)10CH3 and CC(CH3)3); 3.25 (m, 2H, CH2(CH2)10CH3); 4.56 (bs, 2H, NCH2Py); 7.25 (m, 1H, H5Py); 7.65 (m, 2H, H3Py and H4Py); 8.20 (s, 1H, CHNO); 8.25 (s, 1H, NOH). To a solution of the above oxime (3.3 mmol) in 30 mL of CH2Cl2 was added 4.5 mL of trifluoroacetic acid. The reaction mixture was stirred for 1 h; then the pH was adjusted to 8 with a 20% solution of Na2CO3. The aqueous phase was extracted continuosly with CHCl3, the organic phase was dried (Na2SO4), and the solvent was removed under vacuum, giving a crude material which was purified by crystallization. 6-[(Methylamino)methyl]-2-[(hydroxyimino)methyl]pyridine (2a): crystallized from CH2Cl2, yield 79%, mp 119-120 °C. 1H NMR (CDCl3) δ: 2.50 (s, 3H, NCH3); 3.91 (s, 2H, NCH2Py); 7.27 (dd, J ) 6.5 and 2.2 Hz, 1H, H5Py); 7.62 (m, 2H, H3Py and H4Py); 8.21 (s, 1H, CHNO); 8.22 (s, 1H, NOH). Anal. Calcd for C8H11N30: C, 58.17; H, 6.71; N, 25.44. Found: C, 58.02; H, 6.72; N, 25.13. 6-[(n-Dodecylamino)methyl]-2-[(hydroxyimino)methyl]pyridine (2b): crystallized from CH2Cl2/hexane, yield 89%, mp 8283 °C. 1H NMR (CDCl3) δ: 0.87 (t, J ) 6.6 Hz, 3H, (CH2)11CH3); 1.30 (m, 20H, CH2(CH2)10CH3); 2.66 (t, J ) 7.2 Hz, 2H, CH2(CH2)10CH3); 3.93 (s, 2H, NCH2Py); 7.29 (dd, J ) 6.7 and 1.5 Hz, 1H, H5Py); 7.65 (m, 2H, H3Py and H4Py); 8.21 (s, 1H, CHNO); 8.24 (s, 1H, NOH). Anal. Calcd for C19H33N30: C, 71.43; H, 10.41; N, 13.15. Found: C, 71.67; H, 10.44; N, 12.92. Kinetic Studies. The reactions were followed on a PerkinElmer Lambda 16 spectrophotometer equipped with a thermostated cell holder. Solutions of the ligands, metal ions, and additives were prepared in the proper buffer. Reaction temperature was maintained at 25 ( 1 °C. Reactions were started by addition of 20 µL of a solution of (1-2) × 10-3 M substrate in CH3CN to 2 mL of a solution of ligand and additives, and no changes in pH were observed during the kinetic runs. The final concentration of substrate was (1-2) × 10-5 M, and the kinetics follow in each case a first-order rate law up to 90% reaction. The rate constants were obtained by nonlinear regression analysis of the absorbance versus time data,16 and the fit error on the rate constant was always
