Kinetics and Mechanism of Electron Transfer to Manganese(VII) by Fluorene and Its Halogenated Derivatives in Neutral Organic Medium

The kinetics of oxidation of fluorene (Fl) and its halogenated derivatives, namely, 2,7-dichlorofluorene (Fl-Cl), 2,7-dibromofluorene (Fl-Br) and 2,7-diiodofluorene (Fl-I), by permanganate ion in neutral organic medium in the presence of phosphate buffer solution has been investigated at a constant temperature of 25°C. The progresses of the reactions were followed spectrophotometrically. The stoichiometry of the reactions was found to be 3:4 (fluorene: permanganate). The oxidation reactions exhibited a first order dependence in [permanganate] and less than unit order dependences with respect to fluorenes concentrations. Under comparable experimental conditions, the order of the oxidation rate of the fluorene derivatives was: Fl-Cl > Fl > Fl-I > Fl-Br. The final oxidation products of fluorene derivatives were identified by GC/MS, FT-IR and chemical tools as the corresponding ketones (9H-fluorenone derivatives). The proposed oxidation mechanism involves formation of 1:1 intermediate complexes between fluorene derivatives and permanganate ion in pre-equilibrium step. The ratelaw expression was deduced. The reactions constants involved in the different steps of the mechanism were evaluated. The activation parameters associated with the second order rate constants were calculated and discussed.

Permanganate ion is an efficient oxidant in acid, neutral and alkaline media [15][16][17][18][19][20][21][22][23][24][25][26][27][28] which still remains as one of the most important, eco-friendly and powerful multi-electron oxidants employed in the kinetic studies [29]. The mechanism of oxidation by this multivalent oxidant depends not only on the substrate but also on the medium used for the study. During oxidation by permanganate ion, it is evident that the Mn(VII) species in permanganate is reduced to various oxidation states in different media. The present study deals with the oxidative behavior of permanganate ion with fluorene and its halogenated derivatives in neutral organic medium to establish the optimum conditions affecting oxidation of such organic Its Halogenated Derivatives in Neutral Medium substrates and to elucidate a plausible oxidation mechanism on the basis of the obtained kinetic and spectral results.

Materials
All chemicals employed in this investigation were of Aldrich grades. Fluorene and its derivatives, 2,7dichloroflourene, 2,7-dibromoflourene and 2,7-diiodoflourene were prepared according to the described procedures with some modifications [30,31]. The synthesized fluorene derivatives were confirmed by both spectroscopic and analytical tools. All solvents used were of spectroscopic grade and used without further purifications. The solvents used were checked for the absence of absorbing or any fluorescent impurities. A fresh solution of potassium permanganate was prepared and standardized as reported [32]. Phosphate buffer solution was used to maintain the neutral medium.

Kinetic Measurements
The kinetic runs were followed under pseudo-first order conditions where fluorene substrates (abbreviated by F) were exist in large excess over that of potassium permanganate. Initiation of the reactions were done by mixing the formerly thermostatted solutions of permanganate and substrate that also contained the required amounts of phosphate buffer solution. The courses of the reactions were followed up to not less than two half-lives by monitoring the diminish in the absorbance of permanganate as a function of time at its absorption maximum (λ = 525 nm), whereas the other constituents of the reaction mixtures did not absorb considerably at the determined wavelength. Fluorene derivatives were confirmed by both spectroscopic and analytical tools. The melting points of fluorene derivatives were recorded using a Gallenkamp melting point apparatus. NMR was recorded on a Bruker Advance 400 MHz with CDCl 3 and DMSO as solvents with tetramethylsilane (TMS) as the internal reference. GC-Mass spectra were recorded on a Shimadzu GCMS-QP1000 EX mass spectrometer at 70 eV. The absorption measurements were done in a temperature-controlled Shimadzu UV-VIS-NIR-3600 double-beam spectrophotometer.
First order plots of ln(absorbance) versus time were recorded to be straight lines up to at least 70% of the reactions completion and the observed first order rate constants (k obs ) were calculated as the gradients of such plots. Ordinary values of at least two independent determinations of the rate constants were taken for the analysis. The rate constants were reproducible to within 4%. The orders of the reactions with respect to the reactants were determined from the slopes of the log k obs versus log(concentration) plots by varying the concentrations of substrates while keeping other conditions constant.

Reactions Stoichiometry and Products Analysis
The stoichiometry of the reactions was analyzed periodically by spectrophotometric technique. The results indicate consumption of four permanganate ions for three molecules of fluorene derivatives to yield the final oxidation products as shown in the following equation, The above stoichiometric equation is in good consistent with the results of products analysis as confirmed by the head-space GC/MS, FT-IR spectra and chemical tools as reported earlier [33,34].

Time-Resolved Spectra
Time-resolved spectra throughout the oxidation of fluorene and its halogenated derivatives (F) by permanganate ion in neutral organic medium in the presence of phosphate buffer solution are shown in Figure 1(a-d). The main characteristic feature observed from the figure was the gradual disappearance of permanganate band at λ = 525 nm. In some cases, there were changes in the absorption spectra of permanganate ion with appearance of isosbestic points in the spectra.

Rates Dependence on Permanganate Concentration
The effect of permanganate concentration on the oxidation rates of fluorene and its halogenated derivatives was studied by varying its concentration in the range of (1.0 -10.0) x 10 -4 mol dm -3 at fixed fluorenes concentration and temperature. The order with respect to [MnO 4 -] was found to be unity, as plots of ln(absorbance) versus time were linear up to about 70% of the reactions completion. The first order dependence of the reactions on [MnO 4 -] was also confirmed by the nonvariation of the observed first order rate constant (k obs ) at various [MnO 4 -] while keeping others constant.

Rates Dependence on Fluorenes Concentrations
The oxidation rates were examined at different initial concentrations of the fluorenes reductants keeping other conditions constant. It was found that the oxidation rates increased with increasing the concentration of fluorenes as listed in Table 1. The plots of k obs versus [F] were found to be linear with positive intercepts on the k obs axes as shown in Figure 2 suggesting that the orders with respect to fluorenes concentrations were less than unity.

Effect of Temperature
The rates of oxidation were measured at four different temperatures namely, 15, 25, 35 and 45°C at constant concentrations of the reactants and other conditions being The results indicate that the rate constants increased with rise in temperature. The activation parameters of the second order rate constant (k 2 ) are calculated using Eyring and Arrhenius equations and are listed in Table 2.

Free Radical Test
The involvement of free radicals in the oxidation reactions was examined by addition of known quantities of acrylonitrile monomer to the reactions mixtures and was kept in inert atmosphere for about 6 hours at room temperature. When the reactions mixtures were diluted with methanol, heavy white precipitates were formed suggesting that there was participation of free radicals in the present oxidation reactions. When the experiments were repeated in the absence of the fluorene derivatives under similar conditions, the tests were negative. This indicates that the reactions were routed through free radical paths.

Discussion
Permanganate ion in various media provides excellent results when used in oxidation processes. In the permanganate ion, manganese has an oxidation state of VII. It is stable in neutral or slightly alkaline media, but, in a strongly alkaline medium, [29] it disproportionates or reacts with hydroxide ion to form manganese(V) (hypomanganate) or manganese(VI) (manganate). Manganese(VII) is reduced to Mn(II) during oxidation processes via many manganese species having different oxidation states such as Mn(VI), Mn(V), Mn(IV) and Mn(III). The appearance of these intermediate oxidation states depends upon various reaction conditions and the type of substrate. In neutral or slightly alkaline solutions, permanganate used as a powerful oxidizing agent (E o = + 1.23 V) according to the equation: MnO4-+ 2H2O + 3e = MnO2 + 4OH-On the other hand, oxidation reactions involving permanganate ion as an oxidant are suggested [15][16][17][18][19][20][21][22][23] to proceed through intermediate complex formation between oxidant and substrate especially in neutral and alkaline media. The formation of manganate(VI) and/or hypomanganate(V) short-lived intermediates may be confirmed by the change in the color of the solution mixture as the reaction proceeded from purple-pink, Mn(VII), to blue, Mn(V), to green, Mn(VI). As the reactions proceed, a yellow turbidity slowly develops and on prolonged standing, the solution turns to colorless with a brown colloidal precipitate, Mn IV O 2 . The failure to detect Mn(V), absence of an absorption maximum around λ = 700 nm, may be interpreted by its extreme short lifetime and undergoing a rapid disproportionation [20,35].
The present reactions between fluorene derivatives (F) and permanganate ion in neutral organic medium have a stoichiometry of 3:4 (fluorene: permanganate) with a firstorder dependence on [MnO 4 -] and less than unit order dependences with respect to [F]. The less than unit order dependences with respect to fluorenes concentrations suggests formation of intermediate complexes between fluorenes and permanganate ion in pre-equilibrium step. Spectral evidence for complexes formation was obtained from the UV-Vis spectra (Figure 1) where there were changes in the absorption spectra of permanganate ion with appearance of isosbestic points in the spectra. Another support for complexes formation is the kinetic evidence as the plots of 1/k obs versus 1/[F] were found to be linear with positive intercepts on 1/[F] axes as shown in Figure 3, similar to the well-known Michaelis-Menten mechanism for enzyme-substrate reactions [36].
Based on the experimental results, permanganate ion is suggested to react with one mole of fluorine substrate in a pre-equilibrium step to give an intermediate complex (C). The cleavage of such complex leads to the formation of a free radical derived from fluorine substrate and an intermediate Mn(VI) species. Such intermediate is rapidly attacked by manganate(VI) ion to yield the corresponding secondary alcohol and Mn(V) species. In a further fast step, the intermediate Mn(V) being very active and unstable reacts with the formed secondary alcohol to give rise to the final oxidation product of fluorine (ketone) and an intermediate Mn(III) species. This step is followed by reactions between fluorenes and permanganate ions giving other oxidation products of fluorenes, and finally a reaction between Mn(V) and Mn(III) species was led to formation of Mn IV O 2 as the final oxidation product of permanganate, satisfying the observed reactions stoichiometry. The proposed mechanism is illustrated in Scheme 1.
The obtained large negative values of ∆S ≠ (Table 2) suggests that the more plausible mechanism is one-electron transfer of inner-sphere nature [36][37][38]. Also, the large negative values of ∆S ≠ indicate that there is a decrease in the randomness during the oxidation process. This leads to the formation of compacted intermediate complexes and such activated complexes are more ordered than the reactants due to loss of degree of freedom. On the other hand, the positive value of ∆H ≠ indicates that the complexes formation is endothermic. Also, the large positive value of ∆G ≠ suggests enhanced formation of the intermediates with increasing temperature as well as to the non-spontaneities of the complexes formation.

Scheme 1. Mechanism of oxidation of fluorenes by permanganate ion in neutral medium.
According to the mechanistic Scheme 1, the oxidation rate can be expressed as follows, According to Scheme 1, Therefore, Substituting Eq. (4) in Eq. (2) leads to the following equation, The total concentration of fluorine substrate is given by, where T and F refer to total and free concentrations, respectively. Therefore, Similarly, Under pseudo-first order conditions, the rate-law can be expressed as, 4 4 [MnO ] Rate [MnO ] Comparing Eqs (11) and (12), and with rearrangement the following relationship is obtained, According to Eq. (12), the plots of 1/k obs versus 1/[F] (for fluorene derivatives) were found to be straight lines with positive intercepts on 1/k obs axes as illustrated in Figure 3 satisfying the proposed mechanism. From the intercepts and slopes of such plots, the rate constant of the slow step, k, and the formation constant of the intermediate complexes, K, are determined and are listed in Table 3.

Conclusions
The kinetics of oxidation of fluorene and its halogenated derivatives by permanganate ion in neutral organic medium was studied spectrophotometrically. The final oxidation products of fluorene derivatives were identified by GC/MS, FT-IR and chemical tools as the corresponding ketones (9Hfluorenone derivatives). Under comparable experimental conditions, the order of the oxidation rate of the fluorene derivatives was: Fl-Cl > Fl > Fl-I > Fl-Br.