Kinetics and Mechanism of Palladium(II)-Catalyzed Oxidation of Inositol by Hexachloroplatinate(IV) in Perchlorate Solutions

Oxidation of inositol (INOS) by hexachloroplatinate(IV) complex (HCP) in perchlorate solutions in the presence of palladium(II) catalyst was studied spectrophotometrically. The reaction rate was very slow in the absence of the catalyst. The reaction exhibited a first order dependence on [HCP], and fractional-first order dependences with respect to [INOS], [H + ] and [Pd(II)]. Increasing ionic strength and dielectric constant was found to increase the oxidation rate. A probable oxidation mechanism has been suggested and the rate-law expression has been derived. Both spectral and kinetic evidences revealed formation of a 1:1 intermediate complex between INOS and Pd(II) prior to the rate-controlling step. The final oxidation product of inositol was identified by both spectral and chemical analyses as the corresponding monoketone derivative, namely inosose. The activation parameters of the second order rate constant were evaluated and discussed.

Inositol is a water soluble six membered cyclic polyol, which is synthesized by both eukaryotes and prokaryote [16]. In humans most inositol is synthesized in the kidneys. Inositol and some of its mono and polyphosphates function as the basis for a number of signaling and secondary messenger molecules [17,18]. These are involved in insulin signal transduction, gene expression, breakdown of fats and reducing blood cholesterol [19]. Oxidation of inositol was studied by different oxidants like alkaline potassium periodate [20], alkaline diperiodatocuprate(III) [21], alkalinepermanganate [22], V(V) in acid medium [23] and chromic acid [24].
Kinetic studies on the oxidation of organic compounds catalyzed by various metal ions are considered as a significant field of chemistry because of the role of metals in biological systems [25][26][27][28][29][30][31][32]. One of the important metal catalysts is palladium which is a rare and lustrous silverywhite metal referred to platinum group metals. Catalysis by Palladium is very significant in organic chemistry [33] e.g. medicinal chemistry [34] and the preparation of fine chemicals [35]. It is a versatile metal applied inhomogeneous catalysis. Most studies using palladium as catalyst have employed it in the form of palladium(II) chloride [31,[36][37][38] which exists as [PdCl 4 ] 2− in acidic media.
The present title deals with the kinetics of oxidation of inositol by hexachloroplatinate(IV) complex in perchlorate solutions in the presence of palladium(II) catalyst. This work aims to study the selectivity of inositol towards HCP in acidic medium, to check the catalytic efficiency of Pd(II) catalyst and to elucidate a probable oxidation mechanism.

Materials
Reagent grade chemicals and bidistilled water have been employed in the present work. A stock solution of inositol was prepared by dissolving the sample (E. Merck) in bidistilled water. Chloroplatinic acid solution (Johnson Matthey) was freshly prepared by dilution of the original solution with bidistilled water and standardized spectrophotometrically [39]. Sodium perchlorate and acetic acid solutions have been used to study the effects of ionic strength and dielectric constant of the medium, respectively.

Kinetic Measurements
All kinetic measurements were performed under pseudofirst order conditions, where [INOS] >> [HCP]. The ionic strength, I, of the reaction mixture was adjusted to 1.0 mol dm -3 . The course of the reaction was followed spectrophotometrically by monitoring the decrease in the absorbance of HCP at λ = 263 nm, using a temperaturecontrolled Shimadzu UV-VIS-NIR-3600 double-beam spectrophotometer.
It was observed that the oxidation reaction was proceeded very slowly in the absence of Pd(II) catalyst. The observed rate constants of the catalyzed reaction (k C ) were obtained from the linear portion of ln(Abs.) -time plots. These values were the average of at least two independent kinetics runs and were reproducible to within ±3%. Reactions orders were calculated from the slope of log k C versus log(Conc.) plots by varying the concentrations of INOS, HClO 4 and Pd(II) in turn and keeping all other concentrations and conditions constant.
The spectral changes throughout palladium(II)-catalyzed oxidation of inositol by HCP in perchlorate solutions are illustrated in Figure 1. The scanned spectra indicate gradual disappearance of the HCP absorption band with time. Two isosbestic points were also observed in the the spectra.

Stoichiometry and Product Analysis
Different sets of the reaction mixture containing various amounts of HCP and INOS at fixed acidity, [Pd(II)], ionic strength, and temperature were allowed to react for about 24 h. After completion of the reaction, the unreacted [HCP] was determined spectrophotometrically. The obtained results showed that the reaction stoichiometry is 1:1, as represented by the following stoichiometric equation: The oxidation product of inositol was identified by both spectral and chemical analyses [40,41] as the corresponding monoketone derivative, namely inosose. Also, the formed [PtCl 4 ] 2was confirmed as reported elsewhere [9][10][11][12][13][14][15]. Hexachloroplatinate(IV) in Perchlorate Solutions

Effect of [HCP]
The effect of the hexachloroplatinate(IV) oxidant was investigated by varying its concentration in the range of (2.0 -10.0) x 10 -5 mol dm − (Table 1) indicates that the order with respect to the oxidant is confirmed to be one.

Effect of [INOS]
The rate constant (k C ) was determined at various inositol concentrations keeping others constant. The results showed that k C increases with increasing [INOS] as listed in Table 1.

Effect of [H + ]
The oxidation rate was measured at constant concentration of INOS, HCP, Pd(II) but with various [H + ] (0.1 -0.9 mol dm -3 ). The reaction rate was found to increase as [H + ] increased with less than unit order as found from the plot of log k C versus log [H + ] (Figure 3).

Effect of [Pd(II)]
The rate of reaction was measured with various concentration of palladium(II) catalyst in the concentration range of (2.0 -10.0) x 10 -5 mol dm -3 at constant other variables. The oxidation rate was found to increase with increasing [Pd(II)] as listed in Table 1. The order with respect to [Pd(II)] was less than unity as found from the plot of log k C versus log [Pd(II)] as illustrated in Figure 4.

Effect of Ionic Strength and Dielectric Constant
The effect of ionic strength on the oxidation rate was investigated by addition of NaClO 4 to the reaction medium at constant other variables. The results showed that the observed rate constant increases with increasing ionic strength, and the Debye-Hückel plot was found to be linear with a positive slope as shown in Figure 5. Also, the effect of dielectric constant (D) of the reaction medium on the oxidation rate was examined by measuring the oxidation rate at different solvent compositions (v/v) of acetic acid and water. The rate constant decreased with the decrease in dielectric constant of the solvent mixture, i.e., increase in acetic acid content. The plot of log k C versus 1/D was found to be linear with a negative slope as shown in Figure 6.

Effect of Temperature
The oxidation rate was were performed at five different temperatures in the range of 288 -308 K, at constant concentrations of the reactants. The activation parameters of the second order rate constants (k 2 ) are calculated using Arrhenius and Eyring plots and are listed in Table 2.

Polymerization Test
The involvement of free radicals in the oxidation reaction was examined by the polymerization test. The reaction mixture to which a known quantity of acrylonitrile monomer was added initially and was kept for about 6 h. Upon diluting the reaction mixture with methanol, there was no white precipitate formed, suggesting absence of free radical intervention during the oxidation reaction. This indicates that the reaction was not routed through free radical path.

Discussion
The present reaction between inositol and hexachloroplatinate(IV) in perchlorate solution have a stoichiometry of 1:1. The reaction exhibited a first order dependence with respect to [HCP], less than unit order dependences with respect to [INOS], [H + ] and [Pd(II)]. The observed enhancement of the oxidation rate upon increasing acid concentration with less than unit order dependence suggests that the protonated form of inositol substrate may be considered as the kinetically reactive species in the ratedetermining step, which play the main role in the reaction kinetics. The less than unit order dependences with respect to both inositol and palladium(II) concentrations suggests formation of an intermediate complex between inositol and the active species of Pd(II) catalyst, [PdCl 4 ] 2-, as reported earlier [31,[36][37][38]. Complex formation was proved kinetically by the non-zero intercept of the plot of [Pd II ]/k C versus 1/[INOS] (Figure 7). Spectroscopic evidence to support complex formation was obtained from the UV-Vis spectra as the appearance of two isosbestic points as shown in Figure 1.
Owing to the experimental results and the abovementioned arguments, the oxidation mechanism illustrated in Scheme 1 suggests that the protonated inositol combines with [PdCl 4 ] 2to form an intermediate complex (C). Such complex slowly reacts with one molecule of HCP to give the oxidation product of inositol with regeneration of the catalyst Pd(II). Increasing the oxidation rate with increasing both ionic strength and dielectric constant of the reaction medium suggests that the reaction in the rate-determining step occur between two similarly charged ions [42,43], i.e. between the negatively charged complex (C) and [PtCl 6 ] 2-.

Scheme 1. Mechanism of palladium(II)-catalyzed oxidations of inositol by hexachloroplatinate(IV) in perchlorate solutions.
According to the suggested mechanistic Scheme 1, the relationship between the rate of oxidation and the concentrations of inositol, hydrogen ion, palladium(II) catalyst and hexachloroplatinate(IV) oxidant may be represented by the following rate-law expression, Under pseudo-first-order conditions, the rate law can be expressed as, Comparing Eqs. (5) and (6) leads to Eq. (7), According to equations (8) and (9)   The activation parameters listed in Table 2 may be interpreted as follows. The obtained negative values of ∆S ≠ suggest that the reactions point towards the inner-sphere pathway [44]. The positive values of both ∆H ≠ and ∆G ≠ confirm endothermic formation of the intermediate complexes and their non-spontaneities, respectively.

Conclusions
The kinetics of oxidation of inositol by hexachloroplatinate(IV) has been investigated in perchlorate solutions in the presence of palladium(II) catalyst. The reaction was proceeded very slowly in the absence of the catalyst. A probable oxidation mechanism has been suggested. The oxidation product of inositol substrate identified as the corresponding mono ketone, namely inosose.