Metallomicellar Catalysis. Cleavage of p-NitrophenylPicolinate in Copper(II) Coordinating

N-Myristoyl-N-(â-hydroxyethyl)ethylenediamine in CTABMicelles

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-(â-hydroxyethyl)ethylenediamine in CTAB micelles wasstudied in aqueous buffer of pH ranging from 5.0 to 7.0 at 25 °C. The effect of pH on the reactivity isdiscussed. The result indicates that the 1:1 complex of the ligand and Cu2+ is an active nucleophile in 0.01mol‚dm-3 CTAB micellar solution. The pKa of the hydroxyl group of the ternary complex was determinedto be 6.52 in aqueous CTAB micelles. The rate constant, which shows the nucleophilic reactivity of ionizedhydroxyl anion of the ternary complex toward PNPP, was determined to be kNmax ) 0.625 s-1.

Introduction

Because functional micelles can mimic some aspects ofenzyme, such as a hydrophobic microenvironment andactive site, the investigations of functional micellarcatalysis have been very active in recent years.1-15

One of the functional micellar catalysis is the metal-lomicellar catalysis that can mimic some aspects ofhydrolytic metalloenzyme catalysis. Scrimin and co-workers6-9 studied the effect of the metallomicelles formedfrom pyridine derivatives with Cu2+ or Zn2+on the cleav-ages of PNPP and R-amino acid esters. Tagaki and co-workers13-15 reported on the metallomicelles formed fromlipophilic imidazole derivatives with Cu2+or Zn2+ showinggood catalytic properties in the cleavage of PNPP.

It should be noted that the reports on the quantitativetreatment of metallomicellar catalysis are very few byfar. Tagaki and his co-worker13 have established thequantitative treatment of metallomicellar catalysis. How-ever, it is only applicable to the case that a substrate moietyshould be weak as a ligand base in the ground state in

aqueous buffers because it is not involved with a ternarycomplex containing ligand, metal ion and substrate.

In the present paper, it is our purpose to investigatethe effect of the matallomicelles made of ligand surfactantN-myristoyl-N-(â-hydroxyethyl)ethylenediamine in thepresence of Cu2+on the cleavage of PNPP and to establishan approximate quantitative treatment of metallomicellecatalysis involving a ternary complex.

Approximate Quantitative Treatment ofMetallomicellar Catalysis

The process of a metallomicelle-catalyzed reaction isdescribed as follows: A metal ion (M) forms a binarycomplex (MLn) with n ligands (L) with an associationconstant (KM), and the binary complex forms a ternarycomplex (MLnS) with a substrate (S) with an associationconstant (KS). Then a intracomplex nucleophilic substitu-tion reaction in the rate-limiting step takes place in theternary complex with an apparent first-order rate constant(kn) to afford the products (P). The products are also formedthrough a k0′ process without involving a ternary complex.The process can be expressed as

where k0 is the rate constant due to the buffer, kL and kMare the second-order rate constants due to the ligand andCu 2+ alone, [L]T, [M]T, and [S] are the total concentrationsof the ligand, Cu2+, and S at reaction time t, respectively,[MLn]T is the MLn concentration, and [MLnS] is theconcentration of MLnS.

(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.

M + nL y\zKM

MLn; KM ) [MLn]T/[M][L]n (1)

MLn + S y\zKS

MLnS 98kN

P (2)

KS ) [MLnS]/{([MLn]T - [MLnS])([S] - [MLnS])} (3)

S 98k0′

P (4)

k0′ ) k0 + kL[L]T + kM[M]T (5)

1621Langmuir 1999, 15, 1621-1624

10.1021/la9713917 CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 02/12/1999

The difference of the process of a metallomicelle-catalyzed reaction in the present paper from that in ref9 is that a ternary complex (MLnS) is involved (eq 2).

If [S] , [MLn]T, then [MLnS] , [MLn]T. From eq 3 wehave

If the equilibra between M and L, MLn, and S are reachedrapidly, then the reaction rate is given by

where kobsd is the apparent pseudo-first-order rate con-stant.

Inserting eq 6 into eq 7 and rearranging, we obtain

If [MLn]T , [M]T or [L]T, from ref 13, we have

Inserting eqs 9 and 10 into eq 8, respectively, we have

Equations 11 and 12 are approximate equations for 1:1and 2:1 complexes of the ligand and metal ion. Thereasonableness of the approximation can be confirmedfrom the linear plots of eqs 11 and 12.

From eqs 11 and 12, it can be seen that the values ofkN, KM, and KS cannot be obtained from the plots of 1/ (kobsd- k0′) vs 1/[M]T when the ternary complex is involved inthe reaction process. To obtain kN, KM, and KS, theconsecutive graphic method should be used. From eqs 11and 12, we can see that plots of 1/(kobsd - k0′) vs 1/[M]Tshould give straight lines. The intercepts, I, and slopes,Q, of these straight lines are respectively expressed as

According to eqs 13-16, plots of I and Q vs 1/[L]T or1/[L]T

2 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 fullydissociated hydroxyl group of the complex and Ka is theacid dissociation constant of the ternary complex.

If the active nucleophile is the dissociated complex anionas illustrated in eq 17 and the dissociation equilibriumwas reached rapidly, then the reaction rate is determinedby the amount of the dissociated complex anion, so thatwe have

[MLnS] ) KS[MLn]T[S]/(1 + KS[MLn]) (6)

rate ) kobsd[S] ) kN[MLnS] + k0′[S] (7)

1kobsd - k0′ ) 1

kN+ 1

kNKS

1[MLn]T

(8)

n ) 1:[ML]T ) KM [L]T[M]T/{1 + KM([L]T + [M]T)} (9)

n ) 2: [ML2]T ) KM[L]T2[M]T/{1 + KM([M]T

2 +4KM[L]T[M]T)} (10)

n ) 1: 1kobsd - k0′ ) ( 1

kN+ 1

KSkN[L]T) +

( 1kNKS

+ 1kNKSKM[L]T

)‚ 1[M]T

(11)

n ) 2: 1kobsd - k0′ ) ( 1

kN+ 4

KSkN[L]T) +

( 1kNKS

+ 1kNKSKM[L]T

2)‚ 1[M]T

2(12)

n ) 1: I ) 1kN

+ 1kNKS

1[L]T

(13)

Q ) 1kNKS

+ 1kNKSKM

1[L]T

(14)

n ) 2: I ) 1kN

+ 4kNKS

1[L]T

(15)

Q ) 1kNKS

+ 1kNKSKM

1[L]T

2(16)

Figure 1. Job plots for the ligand and Cu2+ ion complexationas measured by the rates of hydrolysis of PNPP at 25 °C, pH7.5 in 0.01 mol‚dm-3 CTAB; [L] + [M] ) 2 × 10-4 mol‚dm-3;[PNPP] ) 2 ×10-5.

Figure 2. Pseudo-first-order rate constants for the hydrolysisof PNPP in 0.01 mol‚dm-3 CTAB at 25 °C, pH 6.5 as the functionof 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.

kN )Ka

[H+] + Ka

kN max (18)

1622 Langmuir, Vol. 15, No. 5, 1999 Xiancheng et al.

Rearrangement of eq 18 leads to

According to eq 19, the kNmax and Ka values can beobtained by a plot of 1/kN vs [H+].

Experimental SectionMaterials. Cu(NO3)2‚6H2O, CTAB, tri(hydroxymethyl)ami-

nomethane (Tris), hydrochloric acid, and acetonitrile wereanalytical grade commercial products. CTAB was recrystallizedfrom ethanol before use. p-Nitrophenyl picolinate (PNPP) andN-myristoyl-N-(â-hydroxyethyl)ethylenediamine were suppliedby the Organic Chemical Laboratory of Sichuan University. Metalion stock solutions were titrated against EDTA. PNPP stocksolution (3.00 × 10-3 mol‚dm-3) was prepared in acetonitrile.

To avoid the influence of chemical components of differentbuffers, Tris-TrisH+ buffer was used in all cases and its pH wasadjusted by adding analytical pure hydrochloric acid, 0.1mol‚dm-3, in all runs.

Kinetics. Kinetic measurements were made spectrophoto-metrically at 25 °C employing a Perkin-Combda 4B UV/VSspectrophotometer with a thermostatic cell compartment. Thereactions were initiated by adding 10 µL of PNPP stock solutioninto 3 mL of buffer solution containing the desired reagents. The

rates were followed by monitoring the release of p-nitrophenylat 400 nm (pH ) 6.5-7.5) or at 320 nm (pH ) 5.0-6.0). Pseudo-first-order kinetics were observed for at least three half-lives inall cases. The pseudo-first-order rate constants were obtainedfrom the spectrophotometer with a computer data processingsystem. Each pseudo-first-order rate constant is the average offive determinations, its average relative standard deviation issmaller than 2%.

Results and DiscussionsA convenient method to estimate the n value is the

kinetic version of a Job plot,16 in which the rate constantsare plotted as a function of the mole fraction of a ligandor metal ion, keeping their total concentration constant.The results are shown in Figure 1. Figure 1 indicates thenecessity of the coexistence of the ligand and Cu2+ to attaina higher rate (kobsd). It also indicates the maximum rateis 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, theapparent 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 Figure3; therefore, the process that we suppose involves in aternary 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.

Figure 3. Plots of 1/(kobsd - k0′) vs 1/[Cu2+] for the hydrolysisof 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 4. Plots of Q (2) and I (b) respectively vs 1/[L] for thehydrolysis of PNPP at 25 °C and pH 6.5.

Figure 5. Plots of 1/kN vs [H+] for the intracomplex nucleophilicsubstitution reaction of the ternary complex.

Figure 6. Proposed structure of the ternary complex.

Table 1. pH Dependencies of kN, KM, and KS in CTABMicelles at 25 °C

pH kN/s-1 10-4KM/mol-1‚dm3 10-4KM/mol-1‚dm3

7.50 0.417 1.42 1.216.50 0.325 1.12 1.046.00 0.198 0.860 0.7385.50 0.0602 0.474 0.409

1/kN ) 1/kNmax + 1/(kNmax Ka) [H+] (19)

Metallonicellar Catalysis Langmuir, Vol. 15, No. 5, 1999 1623

4. The intercepts and the slopes of these new straightlines allow the calculations of kN, KM, and KS. In principle,the kN, KM and KS values at other pH can be obtained inthe same way as that at pH 6.5. The kN, KM, and KS valuesare listed in Table 1.

From Table 1, kN, KM, and KS increase with increasingpH. Therefore, the higher pH, the stronger the ability tonucleophile. Then kN, KM, and KS increase. From the factthat KM and KS are close, it can be thought that thedevelopment of the ternary complex mathematical modelfor metallomicellar catalysis is necessary.

According to eq 19, plots of 1/kN vs [H+] give a straightline allowing the calculations of kNmax and Ka, as shownin Figure 5. The results are kNmax ) 0.625 s-1 and pKa )6.52.

The linear relationship between 1/kN and [H+] in Figure5 indicates it is reasonable that we suppose the dissociated

complex anion is the active nucleophile. It also indicatesit is important to form a ternary complex in the cleavageof PNPP. The proposed structure of the ternary complexis shown in Figure 6. The hydroxyl group of the ligand isactivated by Cu2+.17,18 Thus, the intracomplex nucleophilicsubstitution reaction can be accelerated.

Acknowledgment. This work was supported by theNational 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.

1624 Langmuir, Vol. 15, No. 5, 1999 Xiancheng et al.