Langmuir 1999, 15, 1621-1624
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Metallomicellar Catalysis. Cleavage of p-Nitrophenyl Picolinate in Copper(II) Coordinating N-Myristoyl-N-(β-hydroxyethyl)ethylenediamine in CTAB Micelles Zeng Xiancheng,* Zhang Yuanqin, Yu Xiaoqi, and Tian Anmin Department of Chemistry, Sichuan University, Chengdu 610064, China Received December 18, 1997. In Final Form: November 23, 1998 The quantitative treatment of metallomicellar catalysis involving a ternary complex containing ligand, metal ion, and substrate is proposed in this paper. The catalysis of the cleavage of p-nitrophenyl picolinate (PNPP) by the Cu2+-coordinating N-myristoyl-N-(β-hydroxyethyl)ethylenediamine in CTAB micelles was studied in aqueous buffer of pH ranging from 5.0 to 7.0 at 25 °C. The effect of pH on the reactivity is discussed. The result indicates that the 1:1 complex of the ligand and Cu2+ is an active nucleophile in 0.01 mol‚dm-3 CTAB micellar solution. The pKa of the hydroxyl group of the ternary complex was determined to be 6.52 in aqueous CTAB micelles. The rate constant, which shows the nucleophilic reactivity of ionized hydroxyl anion of the ternary complex toward PNPP, was determined to be kNmax ) 0.625 s-1.
Introduction Because functional micelles can mimic some aspects of enzyme, such as a hydrophobic microenvironment and active site, the investigations of functional micellar catalysis have been very active in recent years.1-15 One of the functional micellar catalysis is the metallomicellar catalysis that can mimic some aspects of hydrolytic metalloenzyme catalysis. Scrimin and coworkers6-9 studied the effect of the metallomicelles formed from pyridine derivatives with Cu2+ or Zn2+on the cleavages of PNPP and R-amino acid esters. Tagaki and coworkers13-15 reported on the metallomicelles formed from lipophilic imidazole derivatives with Cu2+or Zn2+ showing good catalytic properties in the cleavage of PNPP. It should be noted that the reports on the quantitative treatment of metallomicellar catalysis are very few by far. Tagaki and his co-worker13 have established the quantitative treatment of metallomicellar catalysis. However, it is only applicable to the case that a substrate moiety should be weak as a ligand base in the ground state in (1) Moss, R. A.; Bizzigotti, G. O.; Huang, C. W. J. Am. Chem. Soc. 1980, 102, 754. (2) Kunitake, T.; Okahata, Y.; Sakamoto,T. J. Am. Chem. Soc. 1976, 98, 7799. (3) Anoardi, L.; Fornasier, R.; Tonellato, V. J. Chem. Soc., Perkin. Trans 2, 1980, 260. (4) Fornasier, R.; Tonellato, V. J. Chem. Soc., Perkin Trans. 2, 1982, 899. (5) Menger, F. M.; Gan, L. H.; Durst, D. H. J. Am. Chem. Soc. 1987, 109, 2800. (6) Fornasier, R.; Scrimin, P.; Tecilla, P.; Tonellato, V. J. Am. Chem. Soc. 1989, 111, 224. (7) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1991, 56, 161. (8) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1994, 59, 4194. (9) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1994, 59, 18. (10) Weijnen, J. G. J.; Koudjis, A.; Engberson, J. F. J. J. Org. Chem., 1992, 57, 7258. (11) Lim, Y. Y.; Tan, E. H.; Gan, L. H. J. Colloid Interface Sci. 1993, 157, 442. (12) Faivre, V.; Brembilla, A.; Lochon, P. J. Mol. Catal. 1993, 85, 45. (13) Tagaki, W.; Ogino, K.; Tanaka, O.; Machiya, K. Bull. Chem. Soc. Jpn. 1991, 64, 74. (14) Ogino, K.; Kashihara, N.; Ueda, T.; Isaka, T. Bull. Chem. Soc. Jpn. 1992, 65, 373. (15) Tagaki, W.; Ogino, K.; Fujita, T.; Yosbida, T. Bull. Chem. Soc. Jpn. 1993, 66, 140.
aqueous buffers because it is not involved with a ternary complex containing ligand, metal ion and substrate. In the present paper, it is our purpose to investigate the effect of the matallomicelles made of ligand surfactant N-myristoyl-N-(β-hydroxyethyl)ethylenediamine in the presence of Cu2+on the cleavage of PNPP and to establish an approximate quantitative treatment of metallomicelle catalysis involving a ternary complex. Approximate Quantitative Treatment of Metallomicellar Catalysis The process of a metallomicelle-catalyzed reaction is described as follows: A metal ion (M) forms a binary complex (MLn) with n ligands (L) with an association constant (KM), and the binary complex forms a ternary complex (MLnS) with a substrate (S) with an association constant (KS). Then a intracomplex nucleophilic substitution reaction in the rate-limiting step takes place in the ternary complex with an apparent first-order rate constant (kn) to afford the products (P). The products are also formed through a k0′ process without involving a ternary complex. The process can be expressed as KM
KM ) [MLn]T/[M][L]n
M + nL y\z MLn; KS
kN
MLn + S y\z MLnS 98 P
(1) (2)
KS ) [MLnS]/{([MLn]T - [MLnS])([S] - [MLnS])} (3) k0′
S 98 P
(4)
k0′ ) k0 + kL[L]T + kM[M]T
(5)
where k0 is the rate constant due to the buffer, kL and kM are the second-order rate constants due to the ligand and Cu 2+ alone, [L]T, [M]T, and [S] are the total concentrations of the ligand, Cu2+, and S at reaction time t, respectively, [MLn]T is the MLn concentration, and [MLnS] is the concentration of MLnS.
10.1021/la9713917 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/12/1999
1622 Langmuir, Vol. 15, No. 5, 1999
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The difference of the process of a metallomicellecatalyzed reaction in the present paper from that in ref 9 is that a ternary complex (MLnS) is involved (eq 2). If [S] , [MLn]T, then [MLnS] , [MLn]T. From eq 3 we have
[MLnS] ) KS[MLn]T[S]/(1 + KS[MLn])
(6)
If the equilibra between M and L, MLn, and S are reached rapidly, then the reaction rate is given by
rate ) kobsd[S] ) kN[MLnS] + k0′[S]
(7)
where kobsd is the apparent pseudo-first-order rate constant. Inserting eq 6 into eq 7 and rearranging, we obtain
1 1 1 1 ) + kobsd - k0′ kN kNKS [MLn]T
(8)
If [MLn]T , [M]T or [L]T, from ref 13, we have
n ) 1: [ML]T ) KM [L]T[M]T/{1 + KM([L]T + [M]T)} (9) n ) 2:
Figure 1. Job plots for the ligand and Cu2+ ion complexation as measured by the rates of hydrolysis of PNPP at 25 °C, pH 7.5 in 0.01 mol‚dm-3 CTAB; [L] + [M] ) 2 × 10-4 mol‚dm-3; [PNPP] ) 2 ×10-5.
[ML2]T ) KM[L]T2[M]T/{1 + KM([M]T2 + 4KM[L]T[M]T)} (10)
Inserting eqs 9 and 10 into eq 8, respectively, we have
n ) 1:
(
n ) 2:
(
)
(
)
1 1 1 ) + + kobsd - k0′ kN KSkN[L]T 1 1 1 + ‚ (11) kNKS kNKSKM[L]T [M]T 4 1 1 ) + + kobsd - k0′ kN KSkN[L]T
(
)
)
1 1 1 + ‚ (12) kNKS k K K [L] 2 [M] 2 N S M T T
Equations 11 and 12 are approximate equations for 1:1 and 2:1 complexes of the ligand and metal ion. The reasonableness of the approximation can be confirmed from the linear plots of eqs 11 and 12. From eqs 11 and 12, it can be seen that the values of kN, KM, and KS cannot be obtained from the plots of 1/ (kobsd - k0′) vs 1/[M]T when the ternary complex is involved in the reaction process. To obtain kN, KM, and KS, the consecutive graphic method should be used. From eqs 11 and 12, we can see that plots of 1/(kobsd - k0′) vs 1/[M]T should give straight lines. The intercepts, I, and slopes, Q, of these straight lines are respectively expressed as
n ) 1:
I)
1 1 1 + kN kNKS [L]T Q)
n ) 2:
I)
(13)
1 1 1 + kNKS kNKSKM [L]T
1 1 4 + kN kNKS [L]T Q)
(14) (15)
1 1 1 + kNKS kNKSKM [L]
2
T
(16)
Figure 2. Pseudo-first-order rate constants for the hydrolysis of PNPP in 0.01 mol‚dm-3 CTAB at 25 °C, pH 6.5 as the function of Cu2+concentration; a, b, c, d and e represent [L] ) 4 × 10-5, 5 × 10-5, 6 × 10-5, 8 × 10-5, and 1 × 10-4 mol‚dm-3, respectively.
According to eqs 13-16, plots of I and Q vs 1/[L]T or 1/[L]T2 should allow the estimations of kN, KM, and KS. Suppose acid dissociation takes place in the ternary complex
where kNmax is the first-order rate constant for the fully dissociated hydroxyl group of the complex and Ka is the acid dissociation constant of the ternary complex. If the active nucleophile is the dissociated complex anion as illustrated in eq 17 and the dissociation equilibrium was reached rapidly, then the reaction rate is determined by the amount of the dissociated complex anion, so that we have
kN )
Ka [H+] + Ka
kN max
(18)
Metallonicellar Catalysis
Langmuir, Vol. 15, No. 5, 1999 1623
Figure 5. Plots of 1/kN vs [H+] for the intracomplex nucleophilic substitution reaction of the ternary complex.
Figure 3. Plots of 1/(kobsd - k0′) vs 1/[Cu2+] for the hydrolysis of PNPP in 0.01 mol‚dm-3 CTAB at 25 °C, pH 6.50; a, b, c, d, and e represent the same as those given in Figure 2.
Figure 6. Proposed structure of the ternary complex. Table 1. pH Dependencies of kN, KM, and KS in CTAB Micelles at 25 °C
Figure 4. Plots of Q (2) and I (b) respectively vs 1/[L] for the hydrolysis of PNPP at 25 °C and pH 6.5.
Rearrangement of eq 18 leads to
1/kN ) 1/kNmax + 1/(kNmax Ka) [H+]
(19)
According to eq 19, the kNmax and Ka values can be obtained by a plot of 1/kN vs [H+]. Experimental Section Materials. Cu(NO3)2‚6H2O, CTAB, tri(hydroxymethyl)aminomethane (Tris), hydrochloric acid, and acetonitrile were analytical grade commercial products. CTAB was recrystallized from ethanol before use. p-Nitrophenyl picolinate (PNPP) and N-myristoyl-N-(β-hydroxyethyl)ethylenediamine were supplied by the Organic Chemical Laboratory of Sichuan University. Metal ion stock solutions were titrated against EDTA. PNPP stock solution (3.00 × 10-3 mol‚dm-3) was prepared in acetonitrile. To avoid the influence of chemical components of different buffers, Tris-TrisH+ buffer was used in all cases and its pH was adjusted by adding analytical pure hydrochloric acid, 0.1 mol‚dm-3, in all runs. Kinetics. Kinetic measurements were made spectrophotometrically at 25 °C employing a Perkin-Combda 4B UV/VS spectrophotometer with a thermostatic cell compartment. The reactions were initiated by adding 10 µL of PNPP stock solution into 3 mL of buffer solution containing the desired reagents. The
pH
kN/s-1
10-4KM/mol-1‚dm3
10-4KM/mol-1‚dm3
7.50 6.50 6.00 5.50
0.417 0.325 0.198 0.0602
1.42 1.12 0.860 0.474
1.21 1.04 0.738 0.409
rates were followed by monitoring the release of p-nitrophenyl at 400 nm (pH ) 6.5-7.5) or at 320 nm (pH ) 5.0-6.0). Pseudofirst-order kinetics were observed for at least three half-lives in all cases. The pseudo-first-order rate constants were obtained from the spectrophotometer with a computer data processing system. Each pseudo-first-order rate constant is the average of five determinations, its average relative standard deviation is smaller than 2%.
Results and Discussions A convenient method to estimate the n value is the kinetic version of a Job plot,16 in which the rate constants are plotted as a function of the mole fraction of a ligand or metal ion, keeping their total concentration constant. The results are shown in Figure 1. Figure 1 indicates the necessity of the coexistence of the ligand and Cu2+ to attain a higher rate (kobsd). It also indicates the maximum rate is seen at r) 0.5, indicating that the 1:1 complex (n ) 1) is the active species. At pH 6.5 and different ligand concentration, the apparent first-order rate constant as a function of Cu2+ concentration is shown in Figure 2. Plots of 1/(kobsd - k0′) vs 1/[Cu2+] thus give straight lines, as shown in Figure 3; therefore, the process that we suppose involves in a ternary complex is reasonable. From these straight lines, the intercept (I) and slopes (Q) can be obtained. According to eqs 13 and 14, plots of I and Q vs 1/[L]T, respectively, also give straight lines, as shown in Figure (16) Job, P. Ann. Chim. 1928, 113, 9.
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4. The intercepts and the slopes of these new straight lines allow the calculations of kN, KM, and KS. In principle, the kN, KM and KS values at other pH can be obtained in the same way as that at pH 6.5. The kN, KM, and KS values are listed in Table 1. From Table 1, kN, KM, and KS increase with increasing pH. Therefore, the higher pH, the stronger the ability to nucleophile. Then kN, KM, and KS increase. From the fact that KM and KS are close, it can be thought that the development of the ternary complex mathematical model for metallomicellar catalysis is necessary. According to eq 19, plots of 1/kN vs [H+] give a straight line allowing the calculations of kNmax and Ka, as shown in Figure 5. The results are kNmax ) 0.625 s-1 and pKa ) 6.52. The linear relationship between 1/kN and [H+] in Figure 5 indicates it is reasonable that we suppose the dissociated
Xiancheng et al.
complex anion is the active nucleophile. It also indicates it is important to form a ternary complex in the cleavage of PNPP. The proposed structure of the ternary complex is shown in Figure 6. The hydroxyl group of the ligand is activated by Cu2+.17,18 Thus, the intracomplex nucleophilic substitution reaction can be accelerated.
Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 29873031). LA9713917 (17) Sigman, D. S.; Jorgensenm, C. T. J. Am. Chem. Soc. 1972, 94, 1724. (18) Fite, T. H. Adv. Phys. Chem. 1975, 11, 1.
