Mechanism of Electron Transfer Reaction of Cr(III) Iminodiacetate Ternary Complex Involving Malonate with N-Bromosuccinimide

The kinetics of oxidation of the chromium(III) complexes, [Cr III (IDA)(Ma)(H2O)] [IDA=iminodiacetate, Ma=malonic] by N-bromosuccinimide in aqueous solution to yield chromium(IV) has been studied spectrometrically at 25-45°C. The reaction rate increased gradually with increasing pH in the range of (5.75–6.86). The reaction proceeds by one firstorder pathway in each reactant. Conformation of the formation of the ternary complex has been done using IR spectrum, TGA, uv-visible spectroscopic and cyclic voltammetry measurements. Thermodynamic activation parameters have been calculated. It is proposed that electron transfer proceeds through an innersphere mechanism via coordination of NBS to chromium(III). A common mechanism for this reaction is proposed and supported by an excellent isokinetic relationship between ∆H* and ∆S* values for some ternary chromium(III) complexes.


INTRODUCTION
Ternary metal complexes might appear in biological fluids creating specific structure [1][2][3], most frequently manifesting themselves as enzyme-metal ion-substrate complexes [3][4][5][6]. This explains why ternary system has recently received increasing attention. Chromium(III) complexes with nucleotides are used as enzymatic labels by substitution of the activator or inhibitor [7,8] and finding the role of chromium(III) in transcription processes and RNA and DNA interaction [9]. The oxidation of chromium from +3 to +6 is an important environmental process because chromium(VI) is easily taken up by cells and is subsequently reduced to the trivalent form, the formation of chromium(III) or other intermediate oxidation states such as chromium(V) and (IV) is believed to play a role in the adverse biological effects of chromium(VI) compounds [10].
Iminodiacetic acid is the backbone of hydroxamic acid derivatives that could be used to treat cancer. Also it is useful in the prevention and treatment of TRX-mediated diseases, such as autoimmune, allergic and inflammatory diseases [11]. Transition metal complexes of iminodiacetate have been widely adopted in biology and were gaining increasing use in biotechnology, particularly in the protein purification technique known as immobilized metal-ion [12].
Succinimide and its derivatives are biologically and industrially useful compounds. Pharmaceutically, they are used as analgesics, nephrotoxic, anticonvulsant, ionic inhibitors of human leukocyte, etc. It has been reported that sulfonated derivatives ofsuccinimide are more effective than aspirin and paracetamol. They are also used in industry as antifoaming agent, lubricating tackifires, emulsion explosive, and corrosion inhibitors. N-bromosuccinimide(NBS) serves as an oxidizing agent in the synthesis of drugs and hormones.
N-bromosuccinimide(NBS) has been used widely as a brominating and oxidizing agent for organic compounds. It was reported that the oxidation process proceeds via bromonium ion Br + [13] in a polar medium or, alternatively by a free radical path involving the homolytic dissociation of NBS with reducing metal ions yields useful intermediates; The initiation is considered to be effected by one of both succinimidyl and bromine free radicals [14,15].
Here, preparation and kinetics of oxidation of [Cr III (IDA)(Ma)(H2O)2] are reported. Aim of this study is attributed to some considerations. Firstly, transition metal ternary complexes have been received particular focus and employed in mapping protein surfaces as probes for biological redox centers and in protein capture for both purification and study. Secondly, due to the probability of formation and oxidation of this complex in vivo, study the same system has been done as a model in vitro. Finally, to know effect of malonic ligand on stability of Cr(III) towards oxidation process.
Cyclic voltammetry measurements are collected using potentiostat/Galvan state wenking PGS 95 with singlecompartment voltammetric cell equipped with a platinum working electrode (area= 0.5cm 2 ), a Pt wire counter electrode, and a SCE as reference electrode. The uv-vis absorption spectra of Cr III complex and its oxidation product were recorded on Schimadzu UV-1601PC spectrophotometer equipped by automatic circulating water bath.

Reagents
All reagent grade or analar chemicals were used. Freshly prepared solutions of NBS were used. Solutions of Na2HPO4, NaH2PO4 and NaNO3 were prepared by weighing. Na2HPO4/NaH2PO4 buffers of known pH were used, and the ionic strength was adjusted with NaNO3 solution. Doubly distilled H2O was used in all kinetic runs.

Ternary complex preparation
Ternary complex, Na[Cr III (IDA)(Ma)(H2O)2].H2O was prepared by heating an aqueous solution containing equal moles of chromium(III) nitrate, disodium iminodiacetate and malonic acid for 3 hours at 70ºC. A deep pink crystal was separated, on cooling, from the concentrated solution. After washing with alcohol and drying the crystals, the elemental analysis data of the obtained complex are Found; C, 23.52; H, 3.50; N, 3.98٪, Calcd: C, 23.21; H, 3.59; N, 3.87٪ J a n u a r y 25, 2 0 1 4 required amount of separately thermostated NBS stock solution was rapidly mixed, and then the recording of absorbance was started.
The pH of the reaction mixture was measured using a Chertsey Surrey, 7065 pH-meter. Pseudo-order conditions were maintained in all runs by the presence of a large excess (>10-fold) of [NBS] over that of Cr III complex concentration. The error limits for results are calculated using Microcal ™ Origin ® (version 6.0).  To confirm the formula of the complex, IR spectrum and TGA are recorded. In the infrared spectrum, bands in the (3515-3363) Cm -1 region are attributed to γ (OH) of the water molecules. The OH of the carboxylic group disappeared and a new γ (COO -) appeared in the region (1400-1300) Cm -1 , indicating that the carboxylic group of the ligands participates in the coordination with metal ion through deprotonation. Band at 3228Cm -

Oxidation products.
During the oxidation, the deep pink aqueous solution of [Cr III (IDA)(Ma)(H2O)2] changed gradually to yellow and original absorption maxima, at 345 and 385nm, were replaced by a single peak at 370 nm with absorption coefficient ε370 = 193.43 dm -3 mol -1 cm -1 (Figure 4), as the experimental value for Cr VI at the same pH. Addition of AgNO3 solution to the reaction mixture resulted in a yellowish white precipitate of AgBr, suggesting the presence of Br in the product. AgNO3 solution had no effect on NBS under the experimental conditions. J a n u a r y 25, 2 0 1 4 Plots of ln(A∞-At) versus time, were linear up to 85 % of reaction where At and A∞ are the absorbance at time t and time infinity, respectively. Pseudo-first order rate constants, kobs, obtained from the slopes of these plots, are collected in Table 1   Plots of kobs against [NBS] at different temperatures were found to be linear with intercept, Figure 5. The dependence of kobs on [NBS], Table 1, is described by Values of k1 and k2 were calculated at different temperatures and are listed in Table 2. The thermodynamic activation parameters, associated with k1 and k2, were calculated by using the transition state theory equation. The enthalpies of activation, ∆Η1* and ∆Η2* were obtained as 25.08 ±0.11 and 22.12±0.02 kJ mol -1 , respectively. , respectively. These activation parameters are composite values.
The kinetics of the reaction was studied over pH range of (5.75-6.86) at different temperatures. Table 3 lists the variation of k2 with pH at different temperatures which indicate that the reaction rate increased gradually with increasing pH. Plots of k2 versus [H + ] -1 were linear, with Intercepts as shown in Figure 6. This behavior can be described by equation (4).  Values of k3 and k4 were calculated at different temperatures, and were listed in Table 4. The enthalpies of activation, ∆Η3* and ∆Η4*, associated with k3 and k4, were found to be 8.36±0.02 and 6.32±0.28kJ mol -1 , respectively. The corresponding entropies of activation, ∆S3* and ∆S4*, were calculated as -396.   From equation 3, it is surprising to note that one reaction path is independent of NBS. It is also observe that kobs value wasn't reproducible when different sources of reagent were used. This observation has drawn our attention to the possibility of catalysis by trace amount of metal ions originating in the reagents and in the solvent, especially iron(II) and manganese(II) [27].
The effect of iron(II) on the reaction rate was investigated over the (3.

Discussion
NBS exists as protonated species HNBS + In highly acidic solution, but in our pH range the predominant species is NBS [28,29]. Coordination of NBS to the metal ion is possible through the carbonyl group [30]. In weakly acidic aqueous medium the following equilibrium is shown:  [31,32]. The hydroxo-group acts in labilize a Cr-OH2 bond, so facilitates the substitution of H2O by NBS. The presence of [H + ] dependence on the rate law suggests involvement of the deprotonated form of chromium(III) complex, [Cr III (IDA)(Ma)(H2O)(OH)] -,in the rate determining step. Catalysis by iron(II) ions in several reactions is well established by the oxidation of iron(II) to iron(III), which acts as the oxidizing agent [33,34]. In view of the above considerations, kinetics of oxidation of [Cr III (IDA)(Ma)(H2O)2] by NBS proceeds by one first-order pathway in each reactant. The mechanism could be described by the following Equations.
It is known that the succinimidyl radical R . is not heavily resonance stabilized and hence is unstable with respect to succinimide or NBS [35]. Succinimide anion R may well prefer to abstract a hydrogen ion from, for example, the solvent, to form succinimide. Thus the product may be succinimide, a well known product of organic NBS oxidation. Presence of Br as a product is indicated by the formation of AgBr which, increased gradually with time up on addition of AgNO3 solution to the reaction mixture. From the above mechanism, the reaction rate is given by The reason for the term k0, being independent of the iron(II) concentration in equation 6, is not obvious. It probably arises from a catalytic pathway involving a metal ion such as copper(II) and manganese(II), being present as an impurities in the reagents used or in water [27,36].
From the above mechanism, the kinetic data may be interpreted in terms of a mechanism involving rapid preequilibrium formation of Cr III precursor complex followed by a slow intramolecular electron transfer may proceeds through on inner-sphere mechanism via coordination of NBS to Cr(III). From equation 4 and Figue 6, it can be show that k7K2= J a n u a r y 25, 2 0 1 4 6.39x10 3 mol -1 dm +3 s -1 and k8 K3 =1.08x10 -9 s -1 (using the value of K1 = 1.99x10 -6 mol dm -3 ) at the temperature used . Also, from equation 5 and Figure 8, value of kc at 35°C is 9.12x10 -2 s -1 .
In -, respectively. This behaviour is consistent with cyclic voltammetric results, E =+0.048 (mv) and E = +0.110 (mv), respectively. Which indicates that the secondary malonic ligand stabilize the complex towards oxidation. This can be explained using crystal field theory. Values of pKa for iminodiacetic and malonic acids are 2.59 and 4.18, respectively.i.e.malonic is a conjugate base more stronger, with more electrostatic field, than iminodiacetate ligand, i.e. with more electrostatic field. Thus, the crystal field stabilization energy, ∆0 , between t 3 2g and e 0 g orbitals of Cr(III) in the ternary complex is increased due to malonic secondary ligand. Therefore, loss of the three t The intramolecular electron transfer step is endothermic as indicated by the positive enthalpy of activation value and hence, the contributions of ∆H* and ∆S* to the rate constant seem to compensate each other. This suggests that the factors controlling ∆H* must be closely related to those controlling ∆S*; therefore the solvation state of the activated complex would be important in determing ∆H* [37]. Unusually small ∆H* values and large negative activation entropies reasonably could reflect a non-adiabatic contribution to the electron transfer process. Then both ∆H* and ∆S* then may be expected to systematically increases as the orientation of the oxidant in the precursor complex is altered so as to enhance overlap between donor and acceptor redox orbitals and consequently the probability of adiabatic electron transfer [38].
Enthalpies and entropies of activation for the oxidation of some complexes of chromium ( Table 7. A plot of ∆H* versus ∆S* for these complexes is shown in Figure 9.  41 J a n u a r y 25, 2 0 1 4 Similar linear plots were found for a large number of redox reactions [38,39] and for each reaction series a common rate-determing step is proposed. An excellent linear relationship is seen. This isokinetic relationship lends support to a common mechanism for the oxidation of chromium(III) complexes; reported here, by NBS. This consists of NBS coordination to the chromium(III) complexes in a step preceding the rate-determining intra-molecular electron transfer within the precursor complex. The electron transfer reactivities of these complexes with NBS are comparable, as the coordination of NBS with these complexes are identical. All of these suggest that the excellent correlation often observed between ∆H* and ∆S* mainly reflects the fact that both thermodynamic parameters are in reality two measures of the same thing, and that measuring a compensation temperature is just a rather indirect way of measuring the average temperature at which the experiments were carried out.
Our suggestion that the NBS acts as an oxidant in a way such as to follow the radical reaction mechanism is in good agreement with the theortical results [40] which indicates that the N-Br BDE (Bond Dissociation Energy) of the Br radical formation is lower than that of the Br . Or Br -.