Indian Journal of Chemistry Vol. 49A, February 2010, pp. 185-189
A kinetic study on Ru(III)–catalysed oxidation of mercaptosuccinic acid by methylene blue in acidic medium K K Mishra*, Ranu Chaturvedi & M Shukla Department of Postgraduate Studies and Research in Chemistry, Rani Durgavati University, Jabalpur 482 001, India Email:
[email protected]/
[email protected] Received 22 July 2009; revised and accepted 13 January 2010 Mercaptosuccinic acid and methylene blue interact in a molar ratio of 2:1 in presence of hydrochloric acid and Ru(III) as a catalyst in aqueous acetone medium (40% v/v) forming dithiodimalic acid and dihydromethylene blue. The reaction follows a half order kinetics in methylene blue. The order in mercaptosuccinic acid is unity. The average value of half order rate constant remains practically unaffected on varying [H+] up to 2.0 × 10-2 M. The order in methylene blue changes from 1/2 to 1 at lower [H+] (1.0 × 10-2 M) as well as at lower concentrations of catalyst (1.0 × 10-5 M), while at higher [Ru (III)] (≥ 2.0 × 10-5 M) the order in the oxidant becomes zero. This indicates that the relationship between the rate constant and [Ru(III)] is not simple. The rate constant decreases on increasing the time of equilibration of Ru(III) with other ingredients of the reaction system. SEM images suggest the formation of square shaped crystals with size ranging between 50-200 µm. The addition of reaction products does not influence the rate. Activation parameters have been evaluated and a plausible reaction scheme, presuming the reaction between the transient complex (Ru(III)SR’) and MBH+ as the rate determining step, has been proposed. Keywords: Kinetics, Reaction mechanisms, Oxidations, Dyes, Methylene blue, Ruthenium IPC Code: Int. Cl.9 C07B33/00
Sulfur plays a major role in energy transduction, enzyme action, and as a necessary constituent in certain biochemicals such as vitamins, cofactors and hormones1. The main reservoir of free energy in biological processes is electron-excited states of complex molecular systems. Delocalized excited π -electrons in protein macromolecules are the basis of this energy reservoir2. This highlights the importance of electron transfer reactions with regard to sulphur compounds and metal ions. The substrate chosen presently is mercaptosuccinic acid (thiomalic acid, TMA) which has been widely used as a chelating agent for the treatment of metal related diseases3. Recently, mercaptosuccinic acid has been used in the formation of non-cadmium-based quantum dots (QDs) which are found to be highly efficient and
nontoxic optical probes for imaging live pancreatic cancer cells4. Methylene blue (MB), a thiazine dye has been employed as a model oxidant to probe the electron transfer reactions involved in mitochondria. Metal ion Ru(III), used as a catalyst presently, also shows a greater resistance to hydrolysis and more selective action on tumors unlike traditional platinum complexes5, in addition to its catalytic role6. A comparison of Ru (III)-catalyzed oxidation of TMA with that of cysteine7 and glutathione8 by MB reveals that besides the difference in order of reaction in the principal reactants, the system highlights specificity in the time dependent metal–substrate interaction along with a transition in order in methylene blue at higher concentrations of Ru(III). The kinetic findings suggest that Ru(III) and TMA (RSH) interact to form an outer-sphere complex while Ru(III)– catalysed oxidation of cysteine hydrochloride by MB involves the participation of inner-sphere mechanism. Experimental Mercaptosuccinic acid, (TMA, RSH; Evans Chemetics Inc., USA; assay 99.3%) was used as supplied and its solution was prepared in acetone (Merck GR). Solution of the oxidant, methylene blue (E. Merck, Germany) was prepared in doubly distilled water. The solutions were prepared afresh for each run. The oxidation product, i.e., the corresponding disulphide (dithiodimalic acid) was prepared by oxidizing TMA with hydrogen peroxide and extracting the product with ether. Dihydromethylene blue (leuco base) was prepared by reducing MB solution with tin-hydrochloric acid couple9. Excess of hydrogen gas was boiled off and the solution was stored under a nitrogen atmosphere. The reaction was carried out in presence of acetone (40% v/v) and hydrochloric acid (ca. 2.5 × 10-2 M) in vessels (Pyrex, England) coated black from outside. Methylene blue and acetone do not interact under the prevailing conditions. For isolating the reaction products, the reaction mixture was allowed to evaporate after completion of the reaction. The residue was then washed and extracted with ether and the solid mass was subsequently dissolved in methanol (Merck, GR). Addition of acetone was avoided due to the interference in the UV region. The alcoholic solution of the products was then subjected to spectral analysis.
186
INDIAN J CHEM, SEC A, FEBRUARY 2010
The progress of the reaction was followed spectrophotometrically by measuring the decrease in concentration of MB employing an ATI-Unicam UV 2-100 spectrophotometer set at wavelength 664 nm, since MB shows maximum absorbance at 664 nm (εmax = 6.76 × 104 M-1 cm-1). Results and discussion The stoichiometric investigations revealed that two moles of the substrate were oxidized by one mole of MB to form the corresponding disulphide, 2RSH + MB RSSR + H2MB. The formation of the corresponding disulphide was confirmed spectrophotometrically since the peak obtained at 257 nm is in good agreement with the value of λmax obtained for a known sample of the disulphide. It was further confirmed by recording FTIR spectrum (Shimadzu FTIR Spectrometer, model No. 8400 S) of the disulphide which shows a characteristic band of the disulphide linkage in the region 400-600 cm-1. The formation of the corresponding disulphide could have been further authenticated by recording mass and NMR spectra of the reaction system. However, mass spectroscopic studies were difficult in the present case due to the presence of non-volatile inorganic components of the system. Similarly, NMR studies could not be carried out due to the presence of non-magnetic nuclei of sulphur in the –S-S-group. Under such conditions, the presence of the disulphide group could be verified by IR only. An attempt was made to elucidate the mode of interaction of Ru(III) with TMA by recording the FTIR spectra of TMA in absence and presence of Ru(III). Appreciable changes were noticed in the stretching modes (2400 - 2700 cm-1) of –SH group of TMA as well as in the bending modes (near 700 cm-1) which suggests that TMA interacts with Ru(III) to form a complex. Moreover, the spectrum of the reaction mixture was substantially different from Ru(III)-TMA system and showed the presence of dithiodimalic acid (disulphide). Thus, it may be concluded that the metal ion is not permanently bound to the substrate and the in situ, interaction is responsible for the catalytic role. The order in methylene blue is 1/2, which was confirmed by van’t Hoff differential method. The rate constant increased from 0.52 M1/2s-1 to 5.45 M1/2s-1 on increasing [TMA] from 0.5-3.0 M , indicating a first order kinetics in the substrate. The rate constant increases on decreasing the initial concentration of MB which indicates the formation of
some inhibiting species during the course of reaction. The rate constant remained unaffected on varying the ionic strength of the system. However, the reaction shows a transition in order in MB on adding KNO3 from 1/2 to 1 which may be attributed to environmental effects on the chromophoric nature of MB. The rate constant increased on decreasing the dielectric constant of the medium and the double log plot gives a straight line with a negative slope. The order in MB changes from 1/2 to 1 at lower concentrations of HCl (~ 1.0 × 10 5 M). However, the half order rate constant remained practically unaffected (variation between 3.19 × 10-6 to 4.02 × 10-6 M1/2s-1 ) on varying [H+] from 3.5 × 10-2 M to 2.0 × 10-2 M. It may be emphasized that the rate and character of oxidation of –SH groups depends on the pH of these reaction systems10. The rate of reaction increases on increasing [Ru(III)]. The system, however, shows a remarkable feature vis–a–vis transition in order in methylene blue at higher concentrations of Ru(III). The order in MB changes from 1/2 to 1 at lower concentrations of the catalyst (~ 1.0 × 10-5 M) and beyond 2.0 × 10-5 M, the order in the oxidant becomes zero with zero order rate constant varying between 2.12 × 10-8 to 2.83 × 10-8 M s-1 on increasing [Ru(III)] from 3.0 × 10-5 to 5.0 × 10-5 M. A linear plot was obtained between log k0 and log [Ru(III)] with a slope of 0.55. This shows that the relationship between the rate constant and [Ru(III)] is not simple. On increasing the time of equilibration of Ru(III) with other ingredients of the reaction system from 0 – 30 min., the rate constant decreased from 3.18 – 1.74 M1/2s-1. This indicates that the interaction of the metal ion with the substrate is time-dependent. The reaction system involves the participation of nanoparticles, as is evident from SEM images (Fig. 1). The size of the nanoparticle is usually in the range of 1 nm to 1 µm and due to this small size of particles, exhibit unusual chemical, electrical, optical and mechanical properties. In the present reaction system, the particle size varied between 1 and 200 µm. A larger particle size results in a strong binding of substrates on surface of the nanoparticles which in the case of proteins, is called opsonization. For efficient drug delivery, opsonization should be minimum11. In the light of this fact, a larger value of enthalpy of activation obtained presently as compared to Ru(III)-catalyzed oxidation of cysteine by MB7 may be attributed to a larger adherence of TMA on nanocrystals making the process substrate–specific.
NOTES
187
[RuCl 2 (H 2 O) 4 ]+ + H 2 O ⇔ [RuCl 2 (H 2 O) 3 OH ] + H 3O + … (1) It has already been mentioned that the half order rate constant remains almost unchanged on varying [H+] up to 2.0 × 10 -2 M, but the above equilibrium (Eq. 1) suggests that the rate should decrease on increasing [H+]. In fact, hydrogen ions, in this system are also involved in a diverse process, i. e., the redox behaviour of MB. The redox potential of MB increases from + 0.01 V at pH 7.0 to + 0.50 V at pH 1.0 and this may counterbalance the observed retarding influence of hydrogen ions on the rate. It thus seems that [RuCl2.(H2O)3OH] is the reactive species, and for sake of brevity, has been represented as Ru(III). Further, methylene blue is known to be protonated in acidic medium14 and sulphydryl substrates form complexes with metal ions including Ru(III)15. In the present case, FTIR spectra of TMA and TMA-Ru system show the participation of O-H and S-H stretch (3121, 3470 and 2635 cm-1 respectively) and S-H and C-H bending modes (616 and 829 cm-1 respectively) in Ru(III)–TMA interaction. Further, it has been observed that the rate decreases on increasing the time of equilibration of Ru(III) with other ingredients (excepting MB) of the reaction system. This indicates that Ru(III)-TMA interaction is time dependent and perhaps the resulting species has a distorted geometry which eludes an easy stereochemical interpretation. Fig. 1 — SEM images recorded after completion of the reaction. Nano crystals at different magnifications. [a, 2000X; b, 120X; c, 500X]. [TMA] = 2.0 × 10-3 M, [MB] = 2.0 x 10-5 M, [HCl] = 2.5 × 10-2 M, [Ru(III)] =2.0 ×10-5 M, [KCl] = 0.1 M, Acetone = 40% (v/v), µ = 0.125 M, Temp. = 35· °C].
The rate of reaction remains unaffected on external addition of the reaction products, viz., the corresponding disulphide and the leucobase. The enthalpy of activation was determined using the Arrhenius plots and was found to be 44 ± 4 kJ mol-1. The entropy (∆S*) and free energy of activation (∆G*) were evaluated and found to be -239 ± 2 J deg-1mol-1and 118 ± 3 kJmol-1 respectively. It is reported that RuCl3. 3H2O coordinates with water molecules and exists as four major species, viz., [RuCl4.(H2O)2]- , [RuCl2. (H2O)4]+ , [RuCl5. (H2O)]2and [RuCl. (H2O)5]2+ besides RuCl6 3- (refs 12, 13). Out of these, [RuCl2.(H2O)4]+ is stabilized in its hydrolysed form, [RuCl4.(H2O)3OH], according to the equilibrium Eq.(1).
The kinetic findings suggest that Ru(III) and TMA (RSH) interact to form an outer-sphere complex while Ru(III)–catalysed oxidation of cysteine hydrochloride by MB involves the participation of an inner sphere mechanism7. Incidentally, the formation of a hydrogen bonded outer–sphere complex has been reported by Banerjee and coworkers16 in the oxidation of this substrate by Mn(III) in aqueous medium. Thus,
… (2)
188
INDIAN J CHEM, SEC A, FEBRUARY 2010
The thiolate ion (RS-) may be deprotonated to facilitate the coordination of S and O atoms of the substrate with Ru(III) forming a transient species Ru(III SR′) as shown in Eq. (3). The transient complex (Ru(III)SR′) may subsequently react with MBH+ to produce another transient species C* which may in turn dissociate to give RS˙ and HM˙ (half–reduced MB) radicals17,18.
These radicals subsequently lead to the formation of the end products, viz., the disulphide and the leucobase. The participation of radicals during the course of reaction was qualitatively verified by positive polymerization test with vinyl acetate. Based on the above, the mechanism as shown in Scheme 1 is proposed.
NOTES
On presuming step (4) as the rate determining step, the rate of reaction is given as Eq. (8) −
d [MB] = k 2 [(Ru(III) SR' )] MBH + − k −2 C* [RuIII] dt … (8)
[
]
[ ]
On applying steady state treatment for C* and (Ru(III)SR’) and under the conditions that [Ru (III)]2 is very small, we get Eqs. (9) and (10).
[(Ru(III) SR' )]=
k1k 3 [Ru(III)][RSH ] k -1k 3 [H + ]2 + k 2 k 3 [MBH + ] + k −1k −2 [RuIII][H + ]
… (9)
189
of hydrogen ions. Further it has already been mentioned that at higher [Ru(III)], the order in MB changes to zero. This can be explained in terms of a favoured reaction between Ru(III) and RSH (Eq. 3) to produce the transient complex, Ru(III) SR′, at higher concentrations of catalyst which will facilitate its interaction with MBH+ (Eq. 4). This will obviously result in an increase in the value of rate constants k2 and k3 and thus, on the basis of Eq. 12, the order in MB will tend to show a transition from ½ to zero as has been observed. In the light of this, the proposed reaction scheme seems justified. Acknowledgement The authors are thankful to the Department of Science and Technology (DST), Government of India for financial assistance vide project No. SR/FST/SCII017/2002 under FIST programme.
and References [C* ] =
1
k −1k 3 [ H ] + k 2 k 3 [MBH ] + 2 + + k k [Ru(III)[H ] − k −1k 3 [H ] k1 [Ru(III)][RSH] −1 − 2 + + k 3 (k −1k 3 [H ] + k 2 k 3 [MBH ] + k −1k −2 [Ru(III)[H + ] … (10) +
+
Thus, under the conditions that k3 >> k-2 [Ru(III)] , the rate of reaction is given as Eq. (11) −
2 3 4 5 6 7 8 9 10
d [MB] k1k 2 k 3 [RSH ][Ru(III)][MBH + ] = dt k −1k 3 [ H + ] + k 2 k 3 [MBH + ]
11 12
+ k −1k − 2 [Ru(III)[H + ]
… (11) 13 +
+
On substituting [MBH ] = K [MB] [H ] , the rate of reaction is as given in Eq. (12) −
14 15
k1k 2 k 3 [RSH ][Ru(III)][MB] d [MB] = dt k −1k 3 + k 2 k 3 K [MB] + k −1k −2 [Ru (III)]
16
… (12) Equation (12) explains the first order kinetics in TMA and a fractional order in MB and Ru(III). The rate expression also explains the zero order behaviour
17 18
Murahashi S I, Komiyo N, Terai H & Akae T, J Am Chem Soc, 125 (2003)15312. Korotkov K, Williams B & Wisneski L A, Altern Compl Medi, 10 (2004) 49. Ole A, Chem Rev, (1999) 2983. Yong K T, Ding H, Roy I, Law W C, Bergey E J, Maitra A & Prasad P N, ACS Nano, 3 (2009) 502 Dyson P J & Sava G, Dalton Trans, (2006)1929 Scarpellini M, Toledo J C, Neves A, Ellena J, Castellano E E & Franco D W, Inorg Chim Acta, 357 (2004) 707 Chaturvedi R & Mishra K K, Int J Chem Kinet, 40 (2008) 145. Chaturvedi Ranu & Mishra K K, Prog React Kinet Mech, 33 (2008) 253. Bhargava R & Mishra K K, Phosph Sulf Sil, 54 (1990) 39. Bagiyan G A, Koroleva I K, Soroka N V & Ufimtsev AV, Russ Chem Bull, 52 (2003)1135. Mohanraj V J & Chen Y, Trop J Pharm Res 5 (2006) 561. Taqui Khan M M, Chandraiah G R & Rao A P, Polyhedron, 11 (1992) 1821. Taqui Khan M M; Chatterjee D, Bhatt S D & Rao A P, J Mol Catal, 77 (1992) 23. Olga I, Katalias A, Kita P, Mills A, Graezyk A P & Wrzeszcz G , Dalton Trans, (2003) 348. Christian W S, Reginald F J & Wolfgang L, Inorg Chim Acta, 357 (2004) 41. Shaikh N, Panja A & Banerjee P, Int J Chem Kinet, 36 (2004) 170. Gawandi V B, Mohan H & Mittal J P, Phys Chem Chem Phys, 1 (1999)1919. Sethuram B, Some Aspects of Electron Transfer Reactions Involving Organic Molecules, (Allied Publishers,Hyderabad) 2003, p. 138.
