Short communication
Kinetics and mechanism of the metal exchange reaction between N,N'-bis(ß-carbamoylethyl) ethylendiamine
nickel(II) and copper(II)
Rasoul Vafazadeh and Farzaneh Gholami
Department of Chemistry, Yazd University, yazd, Iran
* Corresponding author: E-mail: rvafazadeh@yazduni.ac.ir Tel: +98 351 8122661; Fax: +98 351 7250110
Received: 04-08-2009
Abstract
The kinetics of the [Ni N,N'-bis(P-carbamoylethyl) ethylendiamine)]2+/Cu2+, Ni(bcen)2+/Cu2+ metal exchange reaction in aqueous solution has been studied using spectrophotometry to follow Cu(bcen)2+ formation at 640 nm (bcen is N,N'-bis(P-carbamoylethyl) ethylendiamine). The reaction was studied at pH 6.9, ionic strength 0.5 M and in the temperature range 15 to 35 °C. The kinetics was studied under pseudo-order conditions with [Ni(bcen)2+] >> [Cu2+]. The reaction this under the condition has been found to be a two-step process mechanism is proposed involving: (1) partially dissociation of the Ni(bcen)2+ complex, following with formation of a dinuclear intermediate and (2) metal exchange. Step 1 is rapid and depends on [Ni(bcen)2+] while step 2 is rate determining and independent of [Ni(bcen)2+]. The rate constants k1 (step 1) and k2 (step 2) are 4.40 + 0.14 M-1s-1 and (11.25 + 0.94) x 10-2 s-1 respectively at 25 °C. Activation parameters are also reported.
Keyword: Kinetics, Mechanism, Metal exchange, Diamine diamide ligand, Biphasic reaction
1. Introduction
Metal exchange reactions of complexes with multi-dentate ligands as represented by equation 1 have been the subject of very extensive studies for a variety of metal ions and ligands under different conditions using various methods.1-7
ML + M' ^ M'L + M	(1)
Metal exchange reactions have also been used for analytical measurement of the concentration of metal ions.8 Metal exchange reactions have been shown to proceed through two general pathways differentiated by the nature of the intermediate species involved. The mechanism involves dissociation of the initial multidentate li-gand-metal, followed by reaction of the entering metal with the free ligand. The other mechanism is a direct attack of the entering metal on the complex to form a binuc-lear intermediate species, in which the multidentate ligand is partially cleaved from the initial metal ion complex and partially bound to the entering metal ion. These two reac-
tion pathways may be influenced by the nature of both the metal and the ligand. Although many metal exchange reactions proceed predominantly by forming binuclear intermediates,1-3, 9 10 the mechanism of metal exchange reaction reported by Hay et al11 proceeds via the first pathway while the mechanism of metal exchange reaction reported by Rorabacher et al4 and Mentasti et al12 proceeds via both pathways. From the thermodynamic point of view, one would expect that there is a driving force for replacing the initial metal ion with the incoming metal ion to form more stable complexes.
Scheme 1. Structural of complexes in the present study.
In this investigation, we have studied the kinetics of the metal exchange reaction between N,N'-bis(P-carba-moylethyl) ethylendiamine nickel(II) nitrate complex, [Ni(bcen)](NO3)2 and copper(II), scheme 1.
An important aspect of chemistry of these complexes is the ability of the amide groups coordinate to metal through oxygen or nitrogen. We have chosen, therefore, to investigate the rate and mechanisms of metal exchange reaction involving such as ligands are interest to study.
2. Experimental
2. 1. Syntheses
The diamine diamide ligand N,N'-bis(P-carbamoy-lethyl) ethylendiamine (bcen) was prepared as described by Chung13 from ethylendiamine and acrylamide in aceto-nitrile by heating the mixture under reflux. The white powder re-crystallized from CHCl3.
The metal complexes (scheme 1) were prepared as described by Hay14 using the reaction between a solution of metal nitrate salt in EtOH and a solution of the bcen ligand in H2O (1:1 ratio). The blue solution was heated with constant stirring for 1 h. The hot solution was then filtered and the filtrate allowed standing overnight.
[Ni(bcen)](NO3)2: Yield 57%. Anal. Calculated for C8H18NiN6O8 : C, 24.96; H, 4.71; N, 21.83. Found : C, 258.09; H, 4.63; N, 21.66. A,max / nm (e, M-1cm-1) in H2O, 910 (22), 600 (8), 363 (15).""
[Cu(bcen)](NO3)2: Yield 72%. Anal. Calculated for C8H18CuN6O8 : C, 24.65; H, 4.65; N, 21.56. Found : C, 248.52; H, 46.53; N, 21.61. A,max / nm (e, M-1cm-1) in H2O, 654 (154).
2. 2. Reagents
All chemical reagents and solvents used in this study were obtained from Merck and Fluka and used without further purification. The twice distilled water was used in the preparation of all the solutions.
2. 3. Kinetic Measurements
The metal exchange reaction of [Ni(bcen)]2+ is demonstrated by reaction 2:
Ni(bcen) + Cu2+ ^ Cu(bcen)2+ + Ni2
(2)
To measure reaction rate, visible spectra of reaction mixtures were recorded as a function of time using a Cintra 101 spectrophotometer. The greatest changes in molar absorptivity, due to formation of Cu(bcen)2+, were observed at 640 nm, and therefore all kinetic measurements of absorbance versus time were made at this wavelength. All runs were made at pH 6.9 and ionic strength 0.50 M (adjusted with NaNO3). No buffer was used since no protons
were involved in the reaction. The Cu(II) concentration was held constant at 2.5 x 10-3 M while [Ni(bcen)2+] varied up 30 times that of [Cu2+] so that pseudo-first-order kinetic behavior was exhibited. The reaction was studied in the temperature range (15-35) + 0.1 °C.
To initiate reaction, equal volumes of Cu(NO3)2+ and Ni(bcen)2+ were mixed and the absorbance of product Cu(bcen)2+ at 640 nm was followed as a function of time. At least three runs at each concentration and each temperature were recorded. The absorbance versus time data were computer fitted by sigmaplot 9.0 software with equations 3 (irreversible first-order reaction), 4 (biphasic reaction), and 5 (triphasic reaction) to find the best fit and
A = a1 exp[-kobs(1)t]
(3)
A = a1 exp[-kobs(1)t] + a2 exp[-koJ2)t] + A^ (4)
A = a1 exp[-kobs(1)t] + a2 exp[-kobs(2)t] +	(5)
a3 exp[-kobs(3)t] + A^	(5)
where a1 and a2 are comprised of rate constants and molar absorptivities.
3. Results and Discussions
3. 1. Absorption Spectra
As shown in Fig. 1, the visible absorption spectra of copper and nickel complexes in H2O show maximum absorption due to a d-d transition at 650, 913, 597 and 365 nm. The spectra indicate that Cu(bcen)2+ and Ni(bcen)2+ complexes exist in solution, and that there are no deproto-nated species of these complexes (i.e. Cu(H^bcen)+, Cu(H-2bcen), Ni(H-1bcen)+ and Ni(H-2bcen)).13, 16-18 It is clear from the visible spectrum of the complexes that the metal exchange reaction can be followed spectrophotome-trically. Figure 2 shows a set of spectra recorded in H2O for the Ni(bcen)2+/Cu2+ system at 25 ± 0.1oC and is representative of an increasing observed shift, with respect to the Ni(bcen)2+ spectrum. The spectrum produced by mixing corresponding amounts of the Cu(bcen)2+ complex and Ni2+ ions in H2O is similar to the latter spectrum shown in Fig. 2. These observations indicate that the Ni(bcen)2+ complex has been converted to the Cu(bcen)2+ complex (reaction 2).
In fact, the observed changes in the visible spectrum due to the conversion of Ni(bcen)2+ to Cu(bcen)2+ indicates that the Cu(bcen)2+ is thermodynamically more stable than the Ni(bcen)2+ complex. To confirm this conclusion, the conversion of Cu(bcen)2+ to the Ni(bcen)2+ complex has been examined. It has been found that the opposite direction to reaction 2 does not take place. Therefore, under the studied conditions, the Ni(bcen)2+ complex can be only converted to the Cu(bcen)2+ complex irreversibly. This
Figure 1. UV-Visible spectra of [Ni(bcen)](NO3)2, (5.0 X 10-3 M) and [Cu(bcen)](NO3)2, (5.0 X 10-3 M) complexes in H2O
Figure 2. Changes in the absorption spectrum of a [Ni(bcen)] (NO3)2, (2.5 X 10-3 M) with increasing Cu(NO3)2, (2.5 X 10-3 M) in H2O within 20 min at 25 ± 0.1 °C
observation is in agreement with a common feature of the greater stability of copper complexes over nickel complexes. The formation constant values (log Kf) for Cu(bcen)2+ and Ni(bcen)2+ in aqueous solution at 25 °C of 12.58 ± 0.07 and 7.90 ± 0.06, respectively,17' 18 are in agreement with this conclusion.
3. 2. Kinetic Study
The study of the metal exchange reaction of the Ni(bcen)2+/Cu2+ system is only possible at pH = 6.8-7.0 at which the predominant form of Ni(II) is the Ni(bcen)2+ complex. On the one hand, as in the reaction solution pH is raised, the protons of the amide group in the Ni(bcen)2+ complex are ionized, and the Ni-O bond is rearranged to form a Ni-N bond. On the other hand, with a decrease in the pH of the reaction solution mixture, the predominant form of the reactant species becomes Ni2+.17 All the experiments in this work were therefore carried out at pH = 6.9. Under these conditions, nickel exists only as the Ni(bcen)2+ complex.
Under [Ni(bcen)2+] >> [Cu2+] conditions, the conversion of Ni(bcen)2+ to Cu(bcen)2+ according to equation 2, turned out to be described best as a biphasic reaction. Also, the reactions at different temperatures were found to be a two-step process, biphasic (Fig. 3). Therefore, two rate constants kobs(1) and kobs(2) were obtained by fitting the absorbance versus time data to equation 4 (Table 1). The first-step reaction was fast while the subsequent step reaction was slow (i.e. kobs(1) > kobs(2)). The dependence of the rate constants on the excess concentration of the Ni(bcen)2+ complex has also been studied. Fig. 4 shows the effect of changes in [Ni(bcen)2+] on the rate constants kobs(1) and kobs(2). This figure indicates that kobs(1) is dependent on the concentration of the Ni(bcen)2+ complex, while kobs(2) is independent of the concentration of the Ni(bcen)2+ complex.
Figure 3. Plot of absorbance vs. time for reaction of [Ni(bcen)] (NO3)2, (2.5 X 10-3 M) with Cu(NO3)2, (2.5 X 10-3 M) in
Figure 4. Plot of the rate constants kobs(1), ■ and kobs(2), • vs. [Ni(bcen)(ClO4)2] for reaction 2 ([Cu2+] = 2.5 X 10-3 Mol, I = 0.5 M NaNO3 and 254±2 0.1 °C)
H2O at 25 ± 0.1 °C
The value of kobs(1) varies linearly as a function of [Ni(bcen)2+] with a proportionality constant kr Therefore, it can be written
kobsd) = ki [Ni(bcen)2+]	(6)
where k1 = 4.40 ± 0.14 M1 s-1 at 25 ± 0.1 °C. The rate constant of the second-step reaction can be expressed as
kobs(2) = k2	(7)
which is ~ (11.3 ± 0.94) 10-2 s1 at 25 ± 0.1 °C. Table 1 gives a summary of the rate constants at different temperatures.
tions are in agreement with this conclusion in which the second-step is rate determining. The entropies of activation will be markedly affected by specific solvation effect, both for metal ion and complex, the precise origin of these values is difficult to define. However, the negative values of AS" support associative mechanism (formation of binuclear intermediate) presented in scheme 2.
Chung et al18 in a kinetic study of the formation and dissociation of the Ni(bcen)2+ complex, reported that dissociation involves firstly the fast pre-equilibration of an intermediately bonded species, and finally, second nickel-nitrogen bond breakage, the latter being the rate-determining step. Chung et al,19 in an additional kinetic study of the formation and dissociation of the Cu(bcen)2+ complex,
Table 1. Rate constants and activation parameters for the metal exchange reaction [Ni(bcen)]2+ with Cu2+
t oC	K1, M-1 s-1	102k2, s-1	AH/,kJ mol-1	AS/, J mol-1 K-1	AH/, kJ mol-1	AS/, J mol-1 K-1
15	2.91 ± 0.13	5.83 ± 0.66				
20	3.36 ± 0.18	9.89 ± 0.94				
25	4.40 ± 0.14	11.25 ± 0.94	35.4 ± 3.8	-113.8 ±1 2.9	42.1 ± 4.5	-121.7 ± 24.3
30	5.56 ± 0.43	15.09 ± 1.47				
35	8.20 ± 0.55	21.37 ± 2.55				
3. 3. Proposed Mechanism
The metal exchange reaction in the Ni(bcen)2+/Cu2+ system can be carried out via two different pathways:
(1) Dissociation of the Ni(bcen)2+ complex followed by an attack of copper on the dissociated bcen li-gand
Ni(bcen)2+ ^ Ni2+ + bcen Cu2+ + bcen ^ Cu(bcen)2+
(8) (9)
(2) Attack by copper on partially dissociated Ni(bcen)2+ to form a binuclear intermediate.
Ni(bcen)2+ + Cu2+ =
Cu(bcen)2++ Ni2+
: Ni(bcen)2+.Cu2+ ^
(10)
As mentioned earlier, the metal exchange of copper with nickel in Ni(bcen)2+ has been found to be a two-step process (biphasic reaction) involving two rate constants kobs(1) and kobs(2). The rate constant of the first step reaction depends on the concentration of Ni(bcen)2+. These results show that the mechanism of this metal exchange reaction proceeds predominantly through a binuclear intermediate pathway (the second pathway, reaction 10).
Previous studies of metal exchange reactions have shown that the entry of the incoming metal depends on the reaction rate and proceeds through a binuclear intermediate species.1-3, 9 10
The activation parameters for the metal exchange reaction are shown in table 1. The AH" values for the reac-
indicated that the formation of the first copper(II)-nitro-gen bond was the rate determining step while the formation of subsequent bonds was rapid.
The observed kinetic behavior of the studied complexes in the present work together with the results of the previous study of the dissociation of Ni(bcen)2+ and formation of Cu(bcen)2+ leads us to suggest the metal exchange reaction mechanism given by scheme 2.18, 19
The first-step of this metal exchange reaction is a fast pre-equilibration step (K) along with the rupture of two nickel-oxygen bonds and the formation of the first
Scheme 2. Reaction mechanism for metal exchange reaction of [Ni(bcen)]2+ with Cu(NO3)2.
nickel-nitrogen bond.18 This is followed by the coordination the unwrapping of oxygen of the been ligand to cop-per(II) with formation of binuclear intermediate (scheme 2). It is clear from the first-step reaction that the rate constant should increase linearly with the Ni(bcen)2+ complex concentration. A kinetic expression for this process is shown in equation 6.
As indicated in scheme 2, the formation of the intermediate binuclear species is dependent on the concentration of both reactant species. The second-step of reaction is an intramolecular reaction and is independent on the [Ni(bcen)2+] concentration. The second step is the rupture of the second Ni-N bond and the formation of the first Cu-N bond simultaneously in binuclear intermediate. Since this step is slow compared with the first step, it controls the overall rate of reaction and can be regarded as rate determining step. This conclusion is in agreement with previous studies.18, 19
4. Conclusion
In this paper we studied the kinetics of metal exchange reaction in aqueous solution under pseudo-first-order conditions. Experimental data were computer-fitted to appropriate exponential functions using a non-linear least-square iterative method. It was found that the reaction rate was a two-step process, the step 1 was rapid and dependent on the concentration of Ni(bcen)2+ complex and the step 2 was slow and independent of [Ni(bcen)2+]. The metal exchange reaction in the system Ni(bcen)2+/Cu2+ showed that a Ni(bcen)2+ complex should be thermodynami-cally less stable than Cu(bcen)2+ complex.
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5. Acknowledgments
This study was supported by the yazd university graduate school. We thank Dr. Mansoor Namazian for reading the manuscript and his valuable comments.
Povzetek
Proučevali smo kinetiko reakcije izmenjave Ni(N,N'-bis(P-karbamoiletil) ethilendiamin)2+/Cu2+ ionov v vodni raztopini (okrajšano Ni(bcen)2+/Cu2+).
V temperaturnem območju med 15 in 35 °C smo zasledovali nastanek Cu(bcen)2+ kompleksa spektrofotometrično pri pH 6.9 ter ionski moči 0.5 M. Pri pogoju [Ni(bcen)2+] >> [Cu2+] lahko kinetiko procesa opišemo kot reakcijo psevdo reda, kjer smo predpostavili mehanizem v dveh stopnjah: (1) delna disociacija Ni(bcen)2+ kompleksa, ki ji sledi nastanek intermediata in (2) izmenjava kovinskih kationov. Prva stopnja je hitrejša in je odvisna kod koncentracije Ni(bcen)2+, medtem ko druga, odločujoča stopnja, od koncentracije Ni(bcen)2+, ni odvisna. Za obe stopnji smo določili konstanti reakcijske hitrosti: k1 in k2 pri 25 °C znašata 2.38 ± 0.16 M-1s-1 in (11.25 ± 0.94) x 10-2 s-1.