Investigation of electron-transfer reaction between alkaline hexacyanoferrate(III) and ranitidine hydrochloride – a histamine H2 receptor antagonist, in the presence of homogenous ruthenium(III) catalyst

The ruthenium(III)-catalyzed electron-transfer reaction between hexacyanoferrate(III) and ranitidine hydrochloride is studied in alkaline medium at 25°C and at an ionic strength of 1.10 mol/dm3. The reaction stoichiometry is established and is found to be 1:4, that is, for the oxidation of one mole of ranitidine, four moles of hexacyanoferrate(III) are consumed. The reaction products were characterized by spectral studies such as IR, GC-MS, 1H-NMR and 13C-NMR. The reaction rate shows a less than unit order in substrate and alkali and a first-order dependence in oxidant, [Fe(CN)6]3− and the catalyst, ruthenium(III) concentrations. The active species of ruthenium(III), [Ru(H2O)5OH]2+, forms an intermediate complex with the substrate. The attack of complex by hexacyanoferrate(III) in the rate determining step produces a radical cation, which is further oxidized in the subsequent step to form the oxidation product. The effect of the reaction environment on the rate constant upon adding varying concentrations of KNO3 and t-butanol was studied. The initially added products did not have any significant effect on the reaction rate. A plausible mechanism is proposed based on the experimental results. The effect of varying temperature on the reaction rate was also studied. The activation parameters for the slow step and the thermodynamic quantities for the equilibrium steps were evaluated. The mechanism of title reaction has been studied and one mole of ranitidine consumes four moles of [Fe(CN)6]3−, as shown in the following equation: GRAPHICAL ABSTRACT


Introduction
Oxidation capacity depends upon the redox potential of the oxidant, for instance, the pH of the medium is governing the redox potential of [Fe(CN) 6 ] 3− / [Fe(CN) 6 ] 4− couple in acid ( + 0.36 V) and basic medium ( + 0.40 V). [1] This indicates that hexacyanoferrate(III), [Fe(CN) 6 ] 3− , is a good oxidant in basic medium and it reduces to the stable product, hexacyanoferrate(II). In most of the oxidation reactions, [Fe(CN) 6 ] 3− is mainly used as hydrogen atom abstractor. [2,3] and/or free radical generator. [4] Due to the unique properties of the cyanide ligand (strong σ -donor, good π-acceptor, and weak π -donor) and very high formation constants for both [Fe(CN) 6 ] 3− and [Fe(CN) 6 ] 4− , the hexacyanoferrate(III) complex is often used in model mechanistic studies of ligand substitution and electron-transfer processes. [5] Moreover, its reactions with biological compounds are very interesting from a medicinal point of view. Unless a catalyst is employed these reactions are either extremely slow or do not proceed at all.
The ability of H 2 -antagonists to block histamine-stimulated gastric secretion was of special clinical significance. Two H 2 -receptor antagonists have been developed and used for the treatment of peptic ulcers. The first of these is metiamide and the other is cimetidine. Ranitidine (RNH) belongs to the pharmacologic and therapeutic classes of histamine H 2 -receptor antagonist and antiulcer drug. Ranitidine is a newly developed H 2 -receptor antagonist that lacks the imidazole ring common to histamine and these other two H 2 -antagonist drugs. [6] The infection of Helicobacter pylori is a key factor in the occurrence and reoccurrence of peptic ulcer. Helicobacter pylori is a gram negative and microaerophilic organism which can wreck the mucosa, disturb the secretion of gastric acid, and induce inflammation. [7] Ranitidine is highly effective in the treatment of duodenal and stomach ulcers and Zollingen-Ellison syndrome. Ranitidine has been shown to be five to eight times more potent as an inhibitor of gastric secretion than other histamine H 2 receptor antagonists. [8] It is highly susceptible to oxidation and this property has been used for development of analytical methods for quality control. [9] It undergoes protonation in aqueous solutions with generation of different ionic forms depending on the pH of the solution. [10] Ranitidine undergoes degradation in the presence of oxidation agents [11,12] with production of mainly N-and S-oxides and desmethyl ranitidine. [13] It is believed that under the influence of high temperature and mild oxidative agent, only the S-oxide is generated. [12] Nitro ketene aminal group is optimum for activity. The skeletal formula of ranitidine is Transition metal ions can function as homogenous catalysts because they can move between different oxidation states. The metal-catalyzed oxidation of organic substrates is a topic of great interest, especially for reactions in which the substrates are not easily oxidized by employing the oxidants. The redox potential of the couple, Ru(IV)/(Ru(III) in acetonitrile medium using saturated calomel electrode [14] is + 1.3 V. [1] Ruthenium(III) catalysis in redox reactions involves different degrees of complexity, due to the formation of different intermediate complexes, free radicals and different oxidation states of ruthenium. Ruthenium was chosen based on its unique characteristics as an extremely active catalyst for the oxidation reactions.
The literature survey reveals that kinetic studies on the oxidation of ranitidine were limited, using diperiodatocuprate(III) (DPC), [15] chloramines-T, [16] and KMnO 4 [17] as oxidants. To have a further insight into it, we have carried out the present work. The uncatalyzed reaction of ranitidine by [Fe(CN) 6 ] 3− in alkaline medium is slow, so ruthenium(III) in catalytic concentration is employed, as it allows the kinetics to be studied over a reasonable time. The understanding of mechanism of redox reaction is important, as it helps in the synthesis of specific reaction products. Hence in the present study, the reaction rates were measured, the empirical power-rate law equation from experimental results over a wide range of conditions is performed, and also the nature of the product resulting from the chemical reaction by employing [Fe(CN) 6 ] 3− is studied.

Stoichiometry and product analysis
Five different sets of reaction mixtures with 1:2, 1:4, 1:3, 1:6, 1:5 of ranitidine : [Fe(CN) 6 ] 3− were allowed to react completely in the presence of 5.0 × 10 −6 mol/dm 3 of Ru(III) concentration at 25°C in 0.2 mol/dm 3 OH − and at an ionic strength of 1.10 mol/dm 3 in a closed vessel for 8 h. The unreacted [Fe(CN) 6 ] 3− was obtained spectrophotometrically at 420 nm. The results confirmed that 1:4 reaction ratio as indicated in the following equation: After the completion of the reaction, the reaction mixture was acidified using 10% hydrochloric acid in the cold condition then concentrated and extracted with ether. The formation of single oxidation product was confirmed by thin-layer chromatography. The obtained product was characterized by physicochemical spectral studies.
In the FT-IR spectrum ( Figure 1) of N-((E)-2-((5-((dimethylamino)methyl)furan-2-yl) methylsulfinyl)vinyl)-N-methyl-2-nitroethene-1,1-diamine, the asymmetric and symmetric stretching of nitro group is found to be at 1577 and 1388 cm −1 , respectively. A band at 1041 cm −1 is due to the stretching of S=O group. A band at 1617 cm −1 is observed due to C=C stretching. Asymmetric and symmetric stretches of C-O-C gave bands at 1245 and 1015 cm −1 , respectively. A broad band for N-H and aliphatic C-H stretching was observed in the region 3219-2780 cm −1 . The GC-MS spectrum ( Figure 2) showed a molecular ion peak at 328 amu and a base peak at 44 amu, which is consistent with the product. The reduced product, [Fe(CN) 6 ] 4− is determined by titration in 1.0 mol/dm 3 H 2 SO 4 using N-phenylanthranilic acid. [18] The product was also confirmed by 1 H NMR and 13 C NMR spectra. 1 H NMR spectrum was recorded on a BRUKER 400 MHz spectrometer using DMSO-d 6 as solvent and tetramethylsilane as internal reference. The methylene protons (C 6 ) adjacent to sulfoxide appeared as a doublet at 5.07 ppm. The vinylic protons of C 7 and C 8 resonated as doublets at 6.46 and 8.45 ppm, respectively (J = 14.8 Hz). The protons of C 5 and C 4 appeared as doublets at 6.28 and  6.27 ppm, respectively. The methyl groups of tertiary amine appeared at 2.31 ppm and that of C 10 appeared at 6.09 ppm. The 1 H NMR spectrum is included as supporting information (SI figure 1).
In 13 C NMR spectrum, the C 8 and C 7 carbon atoms resonated at 133 and 124 ppm, respectively. The C 6 carbon appeared at 36 ppm. All the other 13 C nuclei resonated at their expected regions. The 13 C NMR spectrum is included as supporting information (SI figure 2).

Reaction orders
The reaction orders with respect to oxidant, substrate, alkali and catalyst were determined from the slopes of log k obs versus log (concentration) plots by varying one of these reactants at a time and keeping the concentrations of other reactants and conditions constant.

Influence of varying concentrations of hexacyanoferrate(III)
The rate constant, k obs was determined by varying the concentration of HCF(III) in the range 0.50 × 10 −4 -5.0 × 10 −4 mol/dm 3 as a function of time, while keeping all other reactant concentrations and conditions constant ( Table 1). The non-variation of the pseudo-first-order rate constants at varying concentrations of [Fe(CN) 6 ] 3− indicates that the order in [Fe(CN) 6 ] 3− concentration is unity (Table 1). Further the linearity of the plot of log (absorbance) versus time also confirms the observed order in [Fe(CN) 6 ] 3− concentration.

Influence of varying concentrations of ranitidine
The effect of ranitidine on the reaction rate was studied over a wide concentration range of 0.50 × 10 −3 -5.0 × 10 −3 mol/dm 3 while keeping all the reactant concentrations and conditions constant. It was observed that the rate constant, k obs increased with the increase in concentration of ranitidine. The slope of the plot of log k obs versus log [RNH] shows less than unit order dependence in RNH concentration (0.705).

Influence of varying concentrations of alkali
The effect of alkali was studied by varying the concentrations of KOH, over a concentration range of 0.05-0.5 mol/dm 3 while keeping all the conditions and reactant concentrations constant. The increase in concentration of alkali increases the rate and the slope of the plot of log k obs versus log [OH − ], shows less than unit order dependence in alkali concentration (0.341).  Figure 3. Plot of log k obs versus √ I for the ruthenium(III)-catalyzed oxidation of ranitidine by hexacyanoferrate(III) in alkaline medium.

Influence of varying concentrations of ruthenium(III)
The effect of ruthenium(III) was studied by varying its concentration, over a range of 1.0 × 10 −6 -10.0 × 10 −6 mol/dm 3 while keeping all the conditions and reactant concentrations constant. As the ruthenium(III) concentration increases, the rate of reaction also increases. The order with respect to ruthenium(III) concentration was found to be unity.

Influence of ionic strength and dielectric constant on the rate
The effect of ionic strength was studied by varying KNO 3 in the concentration range of 0.1-1.0 mol/dm 3 while keeping all the conditions and reactant concentrations constant. The rate was found to decrease with the increase in the concentration of KNO 3 . The plot of log k obs versus I 1/2 was linear with negative slope (Figure 3). The effect of dielectric constant was studied by varying the percentage of t-butanol-water (v/v) content in the reaction mixture with all the conditions and reactant concentrations being constant. It was found that there was no effect on the rate.

Influence of added product
The effect of initially added product, [Fe(CN) 6 ] 4− was studied in the concentration range of 1.0 × 10 −4 -5.0 × 10 −4 mol/dm 3 while keeping all the conditions and reactant concentrations constant. It was found that the added product did not alter the rate of the reaction, thus having no significant effect.

Polymerization study
To test the intervention of free radicals, the reaction mixture was mixed with 2 cm 3 of acrylonitrile monomer and was kept at room temperature for 2 h under inert atmosphere. And upon dilution with methanol, a white precipitate of polymer was formed confirming the intervention of free radicals in the reaction. The blank experiments of either hexacyanoferrate(III) or ranitidine or ruthenium alone with acrylonitrile did not induce polymerization under the same condition as those induced with reaction mixtures. Initially added acrylonitrile decreases the rate which further supports the intervention of free radicals.  were calculated and are given in Table 2.

Discussion
In the present work, in the absence of ruthenium(III) the oxidation of ranitidine by [Fe(CN) 6 ] 3− is not facile, while the reaction is smooth with a measurable speed in aqueous alkaline media in   and hence the hydroxylated species of ruthenium [Ru(H 2 O) 5 OH] 2+ is presumed to be the reactive species of ruthenium(III). This species is well documented in the literature. [19] Less than unit order and also the increase in the rate with an increase in concentration of OH − indicates the presence of the highly stable trivalent hydroxylated species of ruthenium(III), [Ru(H 2 O) 5 OH] 2+ as reactive species, as shown in the first equilibrium step of Scheme 1 which is also in accordance with the earlier work. [20] Inner-sphere electron-transfer reactions involve the formation of a bridged complex in which the two metal ions are connected by a bridging ligand that helps to promote the electron transfer. Often, but not always, the bridging ligand itself is transferred from one metal center to the other. In the sequence of steps, we note that, in the first equilibrium step, the labile hydroxo species of ruthenium, [Ru(H 2 O) 6 ] 3+ undergoes ligand replacement with the negatively charged less labile ligand, OH − through a dissociative mechanism. During this process a coordination site on the metal is created and also there is a decrease in the overall charge on [Ru(H 2 O) 5 6 ] 3+ which is also in this support. In the second equilibrium step, the formation of a complex(C) between the substrate, RNH, and catalyst, ruthenium(III) is supported by the observed less than unit order in RNH concentration. The substrate is most likely attached through the sulfur atom rather than through tertiary amino groups because the dipositively charged ruthenium ion in [Ru(H 2 O) 5 OH] 2+ is considered to be nearer to class "b" metal ions. And also the decrease in the overall charge on [Ru(H 2 O) 5 OH] 2+ due to the formation of hydroxyl species increases its tendency for complexation through sulfur which is also a class "b" base. Such a complexation has also been observed in earlier studies. [21,22] There is less scope for an effective electron transfer from sulfur to ruthenium(III) in the complex and an equilibrium in electron distribution may exist thus decreasing the rate of the reaction. In the slow step, [Fe(CN) 6 ] 3− attaches to ruthenium(III) in complex via cyanide bridge to form a radical cation derived from RNH. Such a type of cationic free radical has been reported in the literature. [23] There is a regeneration of the catalyst, ruthenium(III). Ruthenium(III) facilitates the electron transfer from RNH to [Fe(CN) 6  The results of the study suggest the formation of a complex between ranitidine and ruthenium(III). The spectral evidence for it was obtained from the UV-VIS spectra of ranitidine and a mixture of ruthenium(III) and ranitidine. A bathochromic shift of 5 nm from 341 to 346 nm is observed. Further kinetic support for the complex formation is obtained from the non-zero intercept of the plot of [Ru(III)]/k obs versus 1/[RNH]. The probable structure of the complex formed between RNH and ruthenium(III) is as follows: From Scheme 1, the following rate law (7) can be derived as follows: The total concentration of RNH is given by, (subscripts t and f stand for total and free, respectively).
Therefore, the free [RNH] f is given by, Similarly, Similarly, Substituting Equations (2)-(4) in Equation (1) and omitting the subscripts, we get In view of low concentration of ruthenium(III) used, the terms {1 + K 1 K 2 [OH − ] [Ru(III)]} and {1 + K 1 [Ru(III)]} in the denominator of Equation (6) are approximately equal to unity. Therefore, Equation (6) can be written asthe following equation, and we get Furthermore, Equation (7) can be rearranged to the following equation, which is suitable for verification.
[Ru(III)] k obs = 1 The effect of ionic strength on the rate is understood on the basis of ionic species involved. The rate decreased with increasing ionic strength which implies that there is an involvement of opposite charges in the reaction which lead to a negative salt effect, as seen in Scheme 1. Amis has earlier described the effect of solvent on the rate of reaction. [24] The negligible effect of dielectric constant on the rate of reaction indicates the involvement of a neutral species, as seen in Scheme 1.
The positive values of G = and H = indicate that the transition state is highly solvated, while negative value of entropy of activation ( S = ) suggests the formation of an activated complex with a reduction in the degree of freedom of molecules. The values of H = and S = were both favorable for electron-transfer processes. The observed modest enthalpy of activation and higher rate constant of the slow step indicate that the oxidation presumably occurs via an innersphere mechanism. This conclusion is supported by earlier observations. [25] In Scheme 1, one equivalent oxidant interacts with four equivalent substrate in accordance with the generally wellaccepted principle of non-complementary oxidations taking place in sequences of one-electron steps. The possibility of electron transfer in non-complementary reaction is dependent on the nature of both the oxidant and substrate.
The values of H, S, and G were calculated for the first and second equilibrium steps of the reaction and are given in Table 2. A comparison of enthalpy of reaction ( H) of the first equilibrium step with H = of the slow step indicates that the reaction before the rate determining step is fairly slow and involves a high activation energy. [26]

Conclusions
The order in [Fe(CN) 6 ] 3− and ruthenium(III) concentrations is unity, whereas the order in ranitidine and alkali concentrations is less than unity. The stoichiometry is found to be 1:4 in reductant to oxidant. The reactive species of catalyst is [Ru(H 2 O) 5 OH] 2+ , which is formed from the [Ru(H 2 O) 6 ] 3+ through ligand replacement with OH − ion. A micro amount of ruthenium(III) is sufficient to catalyze the reaction between [Fe(CN) 6 ] 3− and ranitidine in alkaline medium. The results obtained for the present study are essentially different from the earlier reports, as ranitidine, has led to a different oxidation product. There is intervention of free radicals in the reaction. The reaction proceeds through inner-sphere mechanism and also it is in accordance with the non-complementary reaction. The mechanism is consistent with the experimental results. The rate constant and activation parameters for the slow step are evaluated. Similarly, the equilibrium constants and thermodynamic quantities for the equilibrium steps are also evaluated.

Materials and chemicals
All the chemicals were of analytical grade purity and were used as received. The stock solution of the oxidant, hexacyanoferrate(III) was prepared by dissolving potassium hexacyanoferrate(III) (SISCO-CHEM) in double-distilled water and the concentration was ascertained by iodometric titration. [18] The stock solution (0.01 mol/dm 3 ) of Ranitidine hydrochloride (Sigma-Aldrich) was prepared by accurately weighing required amount and dissolving it in double-distilled water. The ruthenium(III) solution was prepared by dissolving a known amount of RuCl 3 (s. d. finechem) in 0.20 mol/dm 3 of HCl. Mercury was added to the ruthenium(III) solution to reduce any ruthenium(IV) formed during the preparation of the ruthenium(III) stock solution. The ruthenium(III) solution was kept aside for 24 h and its concentration was assayed by EDTA titration. [27] Potassium hydroxide (BDH) was used as the source of OH − to vary the alkali concentration in the reaction medium. Potassium nitrate (Nice) was used to provide the required ionic strength. Hexacyanoferrate(II) solution was obtained by dissolving potassium hexacyanoferrate(II) (s. d. fine-chem) in water and standardizing with cerium(IV) solution. All the apparatus were of pyrex glass and there was no reaction of alkali with the glass under the conditions maintained.

Instruments used
Varian Cary 50 Bio UV-VIS spectrophotometer (Varian, Victoria, Australia) attached to a Peltier accessory (temperature controlled) was used for recording kinetic and spectral data. For product analysis, a Shimadzu 17A gas chromatograph with a Shimadzu QP-5050A mass spectrometer using the electron impact (EI) ionization technique and a Nicolet 5700 FT-IR spectrometer (Thermo Electron Corporation, Madison, WI) were used. A 400 MHz (BRUKER, Switzerland) spectrometer was used for recording 1 H NMR and 13 C NMR spectra. Elico model LI120 pH meter was used for pH measurement.

Kinetic measurements
All kinetic measurements using UV-VIS spectrophotometer were followed under pseudo-firstorder condition, where [RNH] (5.0 × 10 −3 ) > [HCF(III)] (2.0 × 10 −4 ) at 25°C ± 0.1°C and at a constant ionic strength of 1.10 mol/dm 3 in the presence of micro amounts of ruthenium(III) catalyst. The prepared stock solution of [Fe(CN) 6 ] 3− was scanned in the range of 200-800 nm (blue-violet range) to determine the wavelength of maximum absorption and was found to be 420 nm. There is no interference from other species present in the reaction mixture at this wavelength. Thus, the kinetic studies were carried out at 420 nm. The extinction coefficient was determined at 420 nm for different concentrations of [Fe(CN) 6 ] 3− and it is found to be, ε = 988 ± 10 dm 3 /mol/cm. The total volume of the reaction mixture was always kept at 10 cm 3 and these volumes of solutions are allowed to attain thermal equilibrium by suspending them in a temperature-controlled water bath. Meanwhile, the instrument was set to auto zero using water as solvent. The reaction was initiated by immediately adding the requisite amount of pre-equilibrated solution of [Fe(CN) 6 ] 3− to an equilibrated mixture of ranitidine, alkali, KNO 3 and ruthenium(III). The zero time of the reaction was noted when half of the [Fe(CN) 6 ] 3− solution was added. The progress of the reaction was followed by measuring the absorbance of [Fe(CN) 6 ] 3− solution at 420 nm in the quartz cuvettes of 1 cm length placed in the thermostated compartment of a Varian Cary 50 Bio UV-VIS spectrophotometer. The kinetics was followed for more than 75% completion of the reaction and good first-order kinetics was observed. The pseudo-first-order rate constant, k obs was obtained from the slope of the linear plot of log (absorbance) versus time. The first-order rate constants were reproducible within ± 5% and the average of at least three independent kinetic runs. The spectral changes during the oxidation reaction for the standard condition at 25°C with scanning interval of 1 minute per scan is shown in Figure 7. It is evident that [Fe(CN) 6 ] 3− decreases at 420 nm.

Disclosure statement
No potential conflict of interest was reported by the authors.

Supplemental data
Supplemental data for this article can be accessed at 10.1080/17415993.2015.1078804.