than for N,N-dimethyl-p-phenylenediamine, due to the increased probability of electron transfer. The reaction rate decreases with increasing acidity of the ...
511
Transition Metal Chemistry 24: 511±516, 1999.
Ó 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Kinetics and mechanism of the copper(II) catalyzed oxidation of N-substituted p-phenylenediamines by 2,3,9,10-tetramethyl-1,4,5,7,8,11,12,14-octaazacyclotetradeca-1,3,8,10-tetraenecopper(II) Ibrahim A. Salem* Chemistry Department, Faculty of Science, United Arab Emirates University, Al-Ain, P. O. Box 17551, United Arab Emirates Mohamed Gaber and Diaa F. Badr-Eldeen Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt Received 29 September 1998; accepted 10 December 1998
Abstract The kinetics of oxidation of N,N-dimethyl- and N,N,N¢,N¢-tetramethyl-p-phenylenediamines, to the corresponding semiquinonediimine radical and the quinonediimine, with a macrocyclic copper(II)-complex were studied at pH £ 7. Under pseudo-®rst order conditions, the reaction rate for the N,N,N¢,N¢-tetramethyl derivative was much faster than for N,N-dimethyl-p-phenylenediamine, due to the increased probability of electron transfer. The reaction rate decreases with increasing acidity of the medium as a result of protonation of the amine nitrogen atoms. The rate constants and activation parameters were evaluated and the reaction was found to be enthalpy controlled. Furthermore, kinetic measurements revealed a remarkable superadditive eect when CuCl2 solution was added, even at concentrations lower than that of the copper complex. This observation was used for the kinetic determination of copper ions at concentrations 350 nm [10, 12]. In addition, a synproportionation takes place between the reduced, R, and the totally oxidized p-quinonediimine, T. The latter can also undergo rapid deamination in both alkaline and * Author for correspondence
Scheme 1.
acidic conditions resulting in the formation of pquinonemonoimine, M [7]. The kinetics and mechanism of the deamination have been investigated in detail [18, 20±22]. Transition metal complexes of iron(III) and cobalt(III) were used to oxidise N-substituted p-phenylenediamines [5, 8±15, 23±26]. The oxidation was very slow in homogeneous solution [5, 8±10], but occurred rapidly on noble metal surfaces as well as in the presence of their colloids [11±15, 23±26]. Experiments carried out with both noble metal colloids and large metal discs have shown that silver is a better catalyst than gold [13, 14]. The reaction at silver was strongly inhibited by the formation of silver halide [11±15, 25], whereas the
512 the reaction was followed at the wavelength of maximum absorbance of the corresponding p-semiquinonediimine (552 nm for DMPPD and 610 nm for TMPPD). The solutions of the PPDs and the oxidant were prepared immediately before use. The reaction was initiated by injecting a solution of PPD into a solution containing a complex + buer or a complex + buer + CuCl2 according to the conditions required. Results and discussion reaction on gold was inhibited by adsorption of organic compounds as well as by iodide [14, 15, 26]. This paper deals with the kinetics and mechanism of the oxidation of N,N-dimethyl- and N,N,N¢,N¢-tetramethyl-p-phenylenediamine with a macrocyclic copper(II) complex. One of the objectives of this work was to check the activity of this copper complex in oxidizing organic amines, which may found as pollutants in waste water. We hoped that this complex could be useful for oxidizing other organic substrates in heterogeneous systems, i.e. if supported on a solid surface. The reaction has also been applied to the kinetic determination of Cu+2 ions. Experimental Materials and reagents N,N-dimethyl-p-phenylenediamine dihydrochloride, DMPPP á 2HCl, and N,N,N¢,N¢-tetramethyl-p-phenylenediamine dihydrochloride, TMPPD á 2HCl, from Merck were kindly donated by Dr U. Nickel, Germany. All other chemicals were analytical reagent grade (Merck and Aldrich), and were used without further puri®cation. Stock solutions were prepared with doubly distilled H2O purged with N2 gas for ca. 30 mins. Phosphate buer solutions were used throughout. The 2,3,9,10-tetramethyl-1,4,5,7,8,11,12,14-octaazacyclotetradeca-1,3,8,10-tetraenecopper(II) complex was prepared according to the literature method [27]. Two equivalents of 2,3-butandione dihydrazone were mixed with two equivalents of CH2O in the presence of CuCl2 (one equivalent) in H2O. The cyclization process is acidcatalyzed (HClO4). The resulting shiny green complex was characterized by elemental analysis, electronic and i.r. absorption spectra. All values were in agreement with those reported [27]. Measurements pH measurements were performed with a Crison pHmeter digital 501 which was calibrated before use. Kinetic and spectrophotometric measurements were carried out on a Shimadzu 2100s uv/vis. spectrometer, operating via a Shimadzu data acquisition system and equipped with an electronic temperature controller TCC260. The optical path length was 1 cm and the course of
It is known that p-semiquinonediimine, S+, is the ®rst oxidation product of N-substituted-p-phenylenediamine, R [5, 7, 12], a compound which is very stable in aqueous solution [7]. It absorbs strongly in the visible region whereas the parent compound, PPD, as well as the totally oxidized form, p-quinonediimine, T+, do not absorb above 350 nm [5, 7]. Figure 1 shows time resolved spectra for the reaction of the macrocyclic copper complex with an excess of N,N,N¢,N¢-tetramethylp-phenylenediamine at pH=5.59. N,N-dimethyl-p-semiquinonediimine exhibits two equivalent bands at 510 and 552 nm, whereas N,N,N¢,N¢-tetramethyl-p-semiquinonediimine exhibits two equivalent bands at 560 and 610 nm. Therefore, the formation rate for p-semiquinonediimine can easily be followed at 552 nm for dimethyl- and at 610 nm for tetramethyl- derivatives. At these wavelengths, the measured absorbance change is directly proportional to the change in concentration of the corresponding p-semiquinonediimine [9]. In addition to the two-step oxidation, according to equations (1) and (2), a synproportionation takes place between the reduced and the totally oxidized form, equation (3) [5, 7]. k1
CuII AL R ÿ! S CuI AL k2
S CuII AL ÿ! T CuI AL H k3
R T H ) * 2S kÿ3
1
2
3
The relative values of k3 and k)3 are strongly dependent on the substituents on the p-phenylenediamine [5, 7]. Under the conditions chosen, R0 CuII AL0 , the formation of p-quinonediimine according to equation (2) is suppressed throughout the entire oxidation [5, 9]. This can be proved by running analogous measurements at 299 nm, the maximum absorbance of p-quinonediimine. As long as the concentration of T is very low compared to that of S+, the deamination can be neglected [7]. Experiments carried out at a constant pH=5.95, keeping the concentration of either one of the reactants constant whilst varying that of the other, showed that this redox reaction is ®rst order in both reactants. Therefore, the irreversible formation rate of p-semiquinonediimine, S+, is given by:
513
Fig. 1. Time resolved spectra for the reaction between 2 ´ 10)4 M copper(II)-complex and 10)3 M TMPPD at pH 5.95 and 25 °C.
dS =dt kRCuII AL
4
The actual concentrations of both R and the complex can be expressed by the dierence between their initial concentrations and the actual concentration of p-semiquinonediimine, S+; R R0 ÿ S and CuII AL CuII ÿ L0 ÿ S
5 Substituting into equation (4);
Fig. 2. Dependence of the formation rate of p-semiquinonediimine (dA/dt) on its corresponding concentration (measured as absorbance at 610 nm) according to equation 8 for the reaction of 2 ´ 10)4 M copper(II)-complex with dierent concentrations of TMPPD at pH 5.95 and 25 °C.
rate constant, k, was determined from the slopes of the plot, and values are given in Table 1 together with the activation parameters. The rate of oxidation of the tetramethyl-derivative is higher than that of the dimethyl-derivative, and can be attributed to the increased electron density on the nitrogen atoms as a
dS =dt k
R0 ÿ S CuII AL0 ÿ S dS =dt kR0 CuII AL0 ÿ kS
R0 CuII AL0 kS 2
6
Since A es S , path length=1 cm dA=dt kes R0 CuII AL0 ÿ kA
R0 CuII AL0 kA2 =es
7
The absorbance change, (dA/dt), was obtained by means if a mirror ruler at several points of the measured absorbance-time curves. The plot of (dA/dt), versus A gave a straight line with negative slope. The initial absorbance change (dA/dt)0, was determined from the intercept at time zero, i.e. at A Aco , where Aco is the absorbance caused by the complex (Figure 2). Therefore, equation (7) becomes:
dA=dt0 kes R0 CuII AL0
8
The plot of (dA/dt)0 versus either [R]0 (Figure 3) or CuII AL0 was linear, which means that this reaction is ®rst-order in both amine and oxidant. The second-order
Fig. 3. Dependence of the initial rate, (dA/dt)0, on the initial concentration of TMPPD for its reaction with 2 ´ 10)4 M copper(II)complex at pH 5.95 and dierent temperatures.
514 Table 1. Second order rate constants and the corresponding activation parameters for the oxidation of p-phenylenediamine derivatives by the 2,3,9,10-tetramethyl-1,4,5,7,8,11,12,14-octaazacyclotetradeca-1,3,8,10-tetraene-CuII complex, at pH = 5.95 (phosphate buer) PPD
T (°C)
k (M)1s)1)
DMPPD
20 25 30 35 40 25 30 35 40
0.26 0.37 0.46 0.57 0.76 1.47 1.98 2.43 3.09
TMPPD
E (kJ/mol)
DH à (kJ/mol)
DGà (kJ/mol)
DSà (Jmol)1 deg)1)
39.0
36.5
76.2
)131
37.9
35.4
72.9
)123
result of the positive inductive eect of the extra methyl groups in TMPPD. This in turn increases the probability of electron transfer from nitrogen atoms to the oxidant [7, 23]. The eect of pH on the rate of formation of psemiquinonediimine, S+, was studied under conditions such that the concentrations of both substituted pphenylenediamine and the oxidant were kept constant. The concentration of the p-phenylenediamine exceeded that of the oxidant more than tenfold in order to avoid the formation of p-quinonediimine [5, 9]. Both the concentration of the p-semiquinonediimine, S+, and its initial rate of formation were found to increase with increasing pH of the medium, according to the S-shaped curves (Figure 4). The decreases in the rate of reaction with increasing acidity can be simply ascribed to the protonation of nitrogen atoms, which in turn reduces the probability of electron transfer [5, 23]. Given the dierence in the ®rst and second protonation constants K1 and K2 (for DMPPD, K1=2.5 ´ 106 M)1,
K2=2.4 ´ 102 M)1 and for TMPPD, K1=3.5 ´ 106 M)1, K2=2.4 ´ 102 M)1 at ionic strength 0.1 M) [23], the pH dependence might be explained by parallel reactions of the unprotonated p-phenylenediamine, R, and its protonated form RH+. Hence in addition to equation (1), we may write; k02
CuII AL RH ÿ! S CuI AL H
9
According to this scheme, the rate equation can be written as follows: Rate k1 RCuII AL k02 RH CuII AL
10
Furthermore, the species R and its protonated form RH+ are in fast equilibrium i.e. [RH+]=K1[R][H+], therefore, the rate law is given by: Rate
k1 k02 K1 H RCuII AL
11
The total concentration of p-phenylenediamine is given by: Rt R RH
12
From equations 5,11,12 and expressing for the total initial concentration of all forms of PPD, [R]t, therefore; the rate law can be written as: Rate
k1 k02 K1 H 1 K1 H
fRt ÿ S CuII AL0 g ÿ Rt S
13
The initial reaction rate, V0, is given by the ®rst term in this equation, i.e. at zero time, S zero. Therefore; V0 Fig. 4. Variation of (dA/dt)o with pH for the reaction of 2 ´ 10)4 M copper(II)-complex with 1.33 ´ 10)3 M of p-phenylenediamine at 40 °C.
k1 k02 K1 H Rt CuII AL0 1 K1 H
14
At higher acidity, however, most of the amine will be in the RH+ form. Under this condition k02 K1 H k1 and K1 H 1, therefore, the rate equation becomes:
515 V0 k02 Rt CuII AL0
15
The last equation (15) demonstrates that the initial rate of the reaction becomes independent on [H+] which explains the limiting rate value at lower pH. In the pH range 4±6, a rapid increase in the V0 values was observed and k02 and K1[H+] are involved in rate equation (14). Above pH=6, however, the unprotonated form predominates and the rate constant, k1, becomes greater than both k02 and K1[H+]. Therefore, a maximum limiting V0 value is approached at pH ca. 7 according to the following equation: II
V0 k1 Rt Cu AL0
16
k2
k02
CuII AL RH ÿ! S CuI AL H k4
R Cu2 ÿ! S Cu k5
RH Cu2 ÿ! S Cu H
2
11
18
19
20
Rate fk1 CuII AL k4 Cu2 k02 K1 H CuII AL k5 K1 H Cu2 g R ÿ fk2 CuII AL k6 Cu2 gS
21
As before, using equation (5), the initial reaction rate is given by the ®rst term on the right hand side of the last equation; V0 k1 R0 CuII AL0 k02 K1 H R0 CuII AL0 k4 R0 Cu2 0 k5 K1 H R0 Cu2 0
17
Given a high value of Ke, the copper salt will remain in its oxidized form throughout almost the entire oxida-
1
CuII AL S ÿ! T CuI AL H
k6
Copper(II) ions have a marked catalytic eect on the rate of the reaction. This rate enhancement was investigated at constant concentration of both reactants, and at ®xed pH and temperature. Figure 5 reveals that the rate of reaction increased considerably upon addition of a CuCl2 solution, even at concentrations lower than that of the copper(II) complex. This is because copper(II) ions are continuously regenerated by the copper(II) complex according to the equation: Ke
k1
CuII AL R ÿ! S CuI AL
Cu2 S ÿ! T Cu H
Eect of copper(II) ions
CuII AL Cu ) * Cu2 CuI AL
tion. The catalytic mechanism can be summarized by the following equations [9]:
V0 R0 CuII AL0
k1 k02 K1 H R0 Cu2 0
k4 k5 K1 H
22
Expressing for the total initial concentration of all forms of PPD, [R]t (Equation 12); therefore; V0
Rt CuII AL0
k1 k02 K1 H Rt Cu2 0
k4 k5 K1 H 1 K1 H
23
Equation (23) describes how the rate of reaction increases with increasing [Cu2+]. This catalytic eect was used for the kinetic determination of [Cu2+] ions in solutions as represented by a calibration graph in (Figure 6). In this way we were able to determine the concentration of Cu2+ ions at concentrations in the order of 10)5 M. Conclusions
Fig. 5. Formation of p-semiquinonediimine at k=610 nm resulting from the oxidation of 5 ´ 10)4 M TMPPD with 2 ´ 10)4 M copper(II)-complex in the presence of various concentration of copper(II) chloride at pH 5.95 and 35 °C.
The copper(II)-macrocyclic complex, used in this study was found to be an eective oxidant for N-substitutedp-phenylenediamine. The rate of reaction is second order: ®rst order in both reactants. A minimum limiting rate was observed at lower pH's as a result of protonation of the amine nitrogen. The rate of reaction is accelerated by the addition of copper(II) ions. Good results were obtained for the kinetic determination of
516
Fig. 6. A calibration graph constructed from curves in Figure 5. The dashed line indicates the determination of an unknown concentration of copper(II)-chloride from the measured initial reaction rate.
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