Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 3(4), 29-35, April (2013) Res. J. Chem. Sci. International Science Congress Association 29 Kinetics and Mechanism of (salen)MnIII Catalyzed Oxidation of Aryl phenyl Sulfides with Sodium MetaperiodateRamasamy Subramanian and Arunachalam ChellamaniDepartment of Chemistry, Manonmaniam Sundaranar University, Tirunelveli – 627012,INDIAAvailable online at: www.isca.in Received 21st January 2013, revised 2nd February 2013, accepted 5th March 2013Abstract The oxidation of 4-substituted phenyl phenyl sulfides was carried out with several oxo(salen)manganese(V) complexes in MeCN/HO 9:1. The oxidation follows an overall second-order kinetics, first-order each in sulfide and oxo(salen)manganese(V) complex. Electron-attracting substituents in the sulfide and electron-releasing substituents in salen of the oxo(salen)manganese(V) complexes reduce the rate of oxidation. A Hammett analysis of the rate constants for the oxidation of 4-substituted phenyl phenyl sulfides gives a negative value ( = -2.29) indicating an electron-deficient transition state. The log k value observed in the oxidation of each 4-substituted phenyl phenyl sulfides by substituted oxo(salen)manganese(V) complexs also correlate with Hammett constants, giving positive value. The substituent-, acid-, and solvent-effect studies indicate direct O-atom transfer from the oxidant to the substrate in the rate-determining step. Keywords: Organic sulfides, sodium metaperiodate, (salen) MnIII complexes, catalyzed oxidation, reaction mechanism. Introduction The periodate oxidation is undoubtedly one of the most widely used reactions in organic chemistry1,2. Transition metal complexes with Schiff base and porphyrin ligands have been extensively used as models for the heme containing cytochrome P-450. As biomimetic models, transition metal Schiff base complexes have been found to be good catalysts in a variety of oxygenation reactions including epoxidation of olefins and the hydroxylation of alkenes. Many efficient biomimetic oxidation systems using iron and manganese porphyrins as catalysts, and various single oxygen atom donors, such as PhIO, ClO, H, ROOH or IO have been reported4,6. Organic sulfides, which are strong nucleophiles, are oxidized to sulfoxides exclusively by electrophilic oxygen transfer reagents7,8. Mirkhani and co-workers studied the (salen)MnIII-catalyzed oxidation of sulfides, primary aromatic amines, alkene epoxidation and oxidative decarboxylation with NaIO. Mohajer and co-workers10 reported that sodium periodate can efficiently epoxidize various alkenes in the presence of chlorotetraphenyl-porphinatomanganese(III). The periodate oxidation of sulfides to sulfoxides under mild conditions is widely used in syntheses11. Ruff and co-workers studied the oxidation of sulfides with sodium periodate in the absence of catalyst12. Recently, Chellamani et al, reported the results on the [MnIII(salen)]-catalyzed NaOCl and PhIO oxidation of organic sulfides and sulfoxides13. A survey of the literature showed that there are only a few reports on the oxidation of organic sulfides with NaIO9-12 and there is no report on the mechanistic study of metal complexes catalyzed oxidation of organic sulfides with NaIO. In this paper, we report the kinetics and mechanism of the oxidation of aryl phenyl sulfides with NaIO catalyzed by (salen)MnIII complexes 1a-1f. The active species in the present reaction is considered to be the oxo(salen)manganese(V) complex, as proposed by Mirkhani and co-workers and others10 in the (salen)MnIII-catalyzed NaIO oxidation of achiral derivatives. The oxo(salen)manganese(V) complexes 2a-2f are generated in situ from the corresponding [(salen)MnIIIPF complexes and NaIO (scheme 1). Scheme-1 Material and MethodMaterials: Sodium metaperiodate purchased from Aldrich was used as such. Acetonitrile (GR, Merck) was first refluxed over for 5h and then distilled. Diphenyl sulfide (DPS), para-substituted aryl phenyl sulfides were prepared by known methods13. The [(salen)MnIIIPF complexes 1a-1f were synthesized according to reported procedures13,14. Kinetic measurements: The oxo(salen)manganese(V) complexes 2a-2f were obtained by mixing equimolar quantities of (salen)MnIII complex and sodium periodate (scheme 2). The solutions were prepared freshly for each kinetic run. The kinetic Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(4), 29-35, April (2013) Res. J. Chem. Sci. International Science Congress Association 30 measurements were carried out in 90%CHCN-10%HO at 20±0.1°C under pseudo-first-order conditions using 20-100 fold excess of the substrate in a Perkin-Elmer UV-visible spectrophotometer (Lambda 25) fitted with thermo-stated cell compartments. Reaction mixtures for kinetic runs were prepared by quickly mixing the solutions of the oxo complex and sulfide in varying volumes so that in each run the total volume was 5ml. The progress of the reaction was monitored by following the decay of oxo complex at 680nm. The rate constants were evaluated from slopes of linear plots of log(A-A) vs. time, where A is the absorbance at time and A is the experimentally determined infinity point. The values of k were obtained from =k1(obs)-k1(dec). Where k1(obs) is the pseudo-first-order rate constants in the presence of sulfide and k1(dec) is the self-decomposition rate constants of oxo(salen)manganese(V) complexes. The second-order rate constants were obtained from =k/[sulfide]. Scheme-2 Data analysis: Data analysis was performed using Microcal Origin (version 6.0) computer software. The goodness of the fit is discussed using the correlation coefficients and standard deviations. Results and DiscussionStoichiometry and reaction product: The stoichiometry of the reaction between the (salen)Mn=O complex and sulfide was studied under the experimental conditions [2a] = 0.0028M and [PhSPh] = 0.20M. Gas chromatographic analysis of the samples showed that the yield of sulfoxides is 72% and that of MnIIIcomplex is 95% with negligible amount of sulfone. In view of that, the stoichiometry of the reaction can be represented by equation 1. O=Mn(salen)+PhSPh MnIII(salen)+PhSOPh (1) Effect of reactant concentrations: At constant initial concentration of DPS, constant values of k were obtained upon varying the initial concentration of 2a table-1, this, coupled with the observation of linear log (At-) versus time plots (r � 0.994), ensures that the order in 2a is one. The unit slope observed from log-log plots of k and [DPS] (figure 1; r � 0.996) and the linear plots of k versus [sulfide] which pass through origin (figure 2; r � 0. 996) establish that the reaction is first-order in sulfide. Hence, the reaction is overall second-order, first-order in each reactant. Therefore the rate law can be given by equation 2. (-d[])/dt =kk2][sulfide] (2) Figure-1 Double log plots of k and [DPS] Figure-2 Plots of k1 vs. [sulfide] for the oxidation of a)DPS with 2b, b) DPS with 2a, c) DPS with 2c, d) 4-MeCSPh with 2a, e) DPS with 2d, and f) 4-MeOCSPh with 2a Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(4), 29-35, April (2013) Res. J. Chem. Sci. International Science Congress Association 31 Effect of pyridine N-oxide (pyo): The effect of PyO on the reaction rate has been studied and the rate data are listed in table-2. The rate of the reaction is not affected appreciable by the addition of PyO. This indicates that PyO is not binding with oxo(salen)manganese(V) complex and has no catalytic effect during the oxidation process6,15. Similar results are observed in the (salen)MnIII-catalyzed reactions13. Effect of trichloroacetic acid and solvent composition: To understand the effect of acid and solvent composition on the kinetics of this reaction, the rates at different concentrations of trichloroacetic acid and solvent composition were measured, and the data are collected in table-3. The rate data show that the increase in [acid] as well as the increase in the polarity of the medium favors the rate of oxidation. Effect of substituents: The effect of substituents present at the 4-position of the substituted phenyl phenyl sulfides on the reaction rate has been studied and the rate data presented in table-4. Electron-attracting substituents in the phenyl ring retard the rate while electron-releasing substituents produce the opposite effect. The log k values are better correlated with constants (figure 3; r = 0.997, = -2.29±0.09 s = 0.077) than with constants (figure 4; r = 0.990, = -1.27±0.11, s = 0.164). The negative value indicates an accumulation of positive charge at S-atom, and the magnitude of the value indicate the extent of charge development at the S-atom in the transition state of the rate-limiting step16. Figure-3 Hammett plot for the oxidation of 4-substituted phenyl phenyl sulfides by 2a Figure-4 Hammett plot for the oxidation of 4-substituted phenyl phenyl sulfides by 2a The effect of changes in the electronic nature of the oxidant on the rate of oxidation of the 4-substituted phenyl phenyl sulfides with oxo(salen)manganese(V) complexes 2a-2d, and the second-order rate constants are included in table-4. It is seen that electron-releasing substituents at the 5,5'-positions of salen ligand retard the rate while electron-withdrawing substituents accelerate it. Hammett correlation of log k with 2 is excellent with the value of 0.38 ± 0.02 (figure 5; r = 0.996, s = 0.038). The positive value indicates the build-up of negative charge at the metal centre in the transition state of the rate-determining step. Figure-5 Hammett plot for the oxidation of DPS by substituted oxo(salen)manganese(V) complex Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(4), 29-35, April (2013) Res. J. Chem. Sci. International Science Congress Association 32 Table-1 Rate constants for the oxidation of DPS by 2a-f in 90 % acetonitrile-10%water(v/v) mixture at 2010 2 [DPS], M 10 3 [2], M 10 4 k 1(obs) , b s - 1 10 4 k 1(dec) , c s - 1 10 4 k 1 , d s - 1 10 3 k 2 , e M - 1 s - 1 - 2a - - - - 10.0 1.60 8.25±0.16 5.52±0.05 2.73±0.11 2.73±0.11 10.0 1.80 8.42±0.22 5.49±0.07 2.93±0.15 2.93±0.15 10.0 2.00 8.02±0.09 5.56±0.03 2.46±0.06 2.46±0.06 10.0 2.50 8.16±0.29 5.61±0.04 2.55±0.25 2.55±0.25 10.0 2.80 8.32±0.12 5.78±0.06 2.54±0.06 2.54±0.06 10.0 3.00 8.12±0.15 5.77±0.05 2.35±0.10 2.35±0.10 20.0 2.80 11.7±0.12 5.78±0.06 5.92±0.06 2.96±0.03 40.0 2.80 16.4±0.42 5.78±0.06 10.6±0.36 2.66±0.09 50.0 2.80 20.6±0.46 5.78±0.06 14.8±0.40 2.96±0.08 100.0 2.80 32.1±0.86 5.78±0.06 26.3±0.80 2.63±0.08 - 2b - - - - 10.0 2.80 6.81±0.21 4.85±0.03 1.96±0.18 1.96±0.18 20.0 2.80 8.49±0.15 4.85±0.03 3.64±0.12 1.82±0.06 50.0 2.80 14.8±0.38 4.85±0.03 9.95±0.35 1.99±0.07 100.0 2.80 24.6±0.72 4.85±0.03 19.8±0.69 1.98±0.07 - 2c - - - - 5.0 2.80 8.28±0.36 5.65±0.05 2.63±0.31 5.26±0.06 10.0 2.80 10.8±0.16 5.65±0.05 5.15±0.11 5.15±0.11 20.0 2.80 15.7±0.38 5.65±0.05 10.1±0.33 5.05±0.02 50.0 2.80 31.1±0.72 5.65±0.05 25.5±0.67 5.10±0.01 - 2d - - - - 2.5 2.80 9.00±0.12 6.12±0.05 2.88±0.07 11.5±0.03 10.0 2.80 17.3±0.45 6.12±0.05 11.2±0.40 11.2±0.40 20.0 2.80 28.9±0.87 6.12±0.05 22.8±0.82 11.4±0.41 25.0 2.80 35.4±1.02 6.12±0.05 29.3±0.97 11.7±0.39 - 2e - - - - 10.0 2.80 8.23±0.15 5.63±0.06 2.60±0.09 2.60±0.09 20.0 2.80 10.2±0.54 5.63±0.06 4.57±0.48 2.29±0.24 50.0 2.80 17.8±0.92 5.63±0.06 12.2±0.86 2.44±0.17 100.0 2.80 28.9±1.02 5.63±0.06 23.3±0.96 2.33±0.96 - 2f - - - - 10.0 2.80 7.89±0.23 5.56±0.08 2.33±0.15 2.33±0.15 20.0 2.80 9.83±0.48 5.56±0.08 4.27±0.42 2.13±0.21 50.0 2.80 16.8±0.69 5.56±0.08 11.2±0.61 2.24±0.01 100.0 2.80 26.3±1.02 5.56±0.08 20.7±0.94 2.07±0.94 As determined by a spectrophotometric technique following the disappearance of oxomanganse(V) at 680nm; the error quoted in k values is the 95% confidence limit of Student’s t-test. Estimated from pseudo-first-order plots over 40-45% reaction. Estimated from first-order plots over 50-60% reaction in the absence of sulfide. Obtained as k=k1(obs)-k1(dec). Individual k values estimated as k/[DPS]. Table-2 Effect of pyridine N-Oxide on the rate of oxidation of DPS by 2a (0.0028 M) in 90% acetonitrile-10%water (v/v) mixture at 20°Ca 10 2 [DPS], M 10 2 [PyO], M 10 4 k 1(obs) , b s - 1 10 4 k 1(dec), c s - 1 10 4 k 1 , d s - 1 10 3 k 2 , e M - 1 s - 1 10.0 0.0 8.32±0.12 5.78±0.06 2.54±0.06 2.54±0.06 10.0 2.5 8.35±0.21 5.54±0.05 2.81±0.16 2.81±0.16 10.0 5.0 8.62±0.23 5.38±0.08 2.74±0.17 2.74±0.17 10.0 10.0 8.23±0.16 5.50±0.07 2.73±0.10 2.73±0.10 10.0 20.0 8.72±0.35 5.79±0.06 2.93±0.29 2.93±0.29 10.0 25.0 8.64±0.19 5.62±0.06 3.02±0.13 3.02±0.13 As determined by a spectrophotometric technique following the disappearance of oxomanganese(V) at 680nm; the error quoted in k values is the 95% confidence limit of Student’s t-test. Estimated from pseudo-first-order plots over 40-45% reaction. Estimated from first-order plots over 50-60% reaction in the absence of sulfide. Obtained as k = k1(obs) –k1(dec). Individual k values estimated as k/[DPS]. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(4), 29-35, April (2013) Res. J. Chem. Sci. International Science Congress Association 33 Table-3 Effect of adding acid and changing the solvent composition on the rate of oxidation of DPS by 2a at 20a,b10 3 [Acid] c, M 10 4 k 1, d s - 1 CH 3 CN%-H 2 O% (v/v) 10 4 k 1 , e s - 1 0.0 2.54±0.06 90:10 5.92±0.06 0.5 3.65±0.22 85:15 9.02±0.45 1.0 6.61±0.20 80:20 12.2±0.27 5.0 25.6±0.59 75:25 15.7±0.64 10.0 43.7±1.70 70:30 18.4±1.04 20.0 99.3±3.6 - - General conditions: [2a = 0.0028 M. In the evaluation of rate constants, the self-decomposition of 2a at different [acid] and solvent composition taken into account. CClCOOH. [DPS] = 0.10 M; Solvent: 90%CHCN – 10%HO (v/v), [DPS] = 0.20 M. Table-4 Second-order rate constants for value for the reactions of 4-XCSPh with 2a-d in 90%acetonitrile-10%water(v/v) mixture at 20a,bOxo(salen)manganese(V)complex 10 3 k 2 , M - 1 s - 1 X 2b 2a 2c 2d d (r) OMe 9.55±0.48 15.5±0.35 24.6±0.80 35.4±0.97 0.266±0.05 (0.963) Me 3.47±0.03 5.95±0.11 10.7±0.40 18.6±0.88 0.345±0.05 (0.977) H 1.82±0.06 2.96±0.03 5.05±0.02 11.4±0.41 0.380±0.02 (0.996) Cl 0.62±0.12 0.94±0.10 2.13±0.03 3.29±0.12 0.353±0.07 (0.957) NO 2 c 0.03±0.02 0.05±0.02 0.08±0.02 0.13±0.01 0.299±0.04 (0.977)  e -2.29±0.11 -2.29±0.09 -2.29±0.12 -2.30±0.11 - - (r) (0.997) (0.997) (0.996) (0.996) - - The error quoted in k2 is the 95% confidence limit of Student’s t-test. General conditions: [0 = 0.0028M; [sulfide] = 0.50M. The values were obtained by correlating log k with 2 for the reaction of various oxo(salen)manganese(V) complexes with a given sulfide. The values were obtained by correlating log k with for the reaction of various sulfides with a given oxo(salen)manganese(V) complex. Table-5 Second-order rate constants and activation parameters for the oxidation of p-XCSPh by 2a in 90%acetonitrile-10%water(v/v) mixture at four different temperatures.a 10 3 . k 2 [M - 1 s - 1 ] X 293K 298K 303K 313K  [kJ mol-1]  [JK-1 mol-1]  [kJ mol-1 OMe b 14.7±0.22 21.4±0.91 29.4±1.35 58.4±2.22 49.8 141 92.5 Me 5.95±0.11 9.25±0.42 11.9±0.46 23.0±0.95 47.9 279 132.5 H 2.54±0.06 3.82±0.23 5.65±0.17 10.8±0.27 52.6 191 110.5 Cl 0.94±0.10 1.57±0.11 2.18±0.18 4.60±0.59 56.9 124 94.5 NO 2 c 0.05±0.02 0.15±0.01 0.21±0.06 0.70±0.20 93.3 72.9 115.4 General condition: [2a = 0.0028M; [sulfide] = 0.20 M, unless otherwise noted. [sulfide]= 0.10M. [sulfide] = 0.50M. at303k The effect of substituents at the 7,7'-positions of the salen ligand of oxo(salen)manganese(V) complexes on the reaction rate was studied with 2a, 2e, and 2f for the oxidation of DPS. The rate data in Table-1 show that the presence of Me or Ph groups at the 7,7'-positions slightly reduces the rate. Effect of temperature: The oxo(salen)manganese(V) oxidation of para-substituted phenyl phenyl sulfides was carried out at four different temperatures, and the thermodynamic parameters evaluated using the Eyring equation are collected along with kvalues in table-5. Mechanism of oxidation: Oxygen atom transfer to organic substrates has been proposed to proceed by two different mechanisms. The oxidants such as Cr(VI)16, Ce(IV)17 and Fe(III)-polypyridyl complexes14 oxidize sulfides by SET mechanism. Oxidation of sulfides with (salen)MnIII-catalyzed NaOCl13 follow a S2 mechanism. The observation of overall second-order kinetics, first-order each in oxo complex and sulfide, indicates that the present reaction follows simple kinetics without involving any complex mechanism. The Hammett equation has been used to analyze the oxidation of sulfides to sulfoxides. In the present study, a low value of -1.27 is obtained. The size of the value cannot be taken as evidence for the operation of a SET or S2 mechanism in a particular reaction19. For example, it has been argued that a low value for the oxidation of aryl methyl sulfides by Cr(VI) ( = - Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(4), 29-35, April (2013) Res. J. Chem. Sci. International Science Congress Association 34 2.07)16, Ce(IV)( = -3.3)17 and Fe(III)-polypyridyl complexes = -3.2)14 indicates SET mechanism and that a low value for the oxidation of sulfides with sodium periodate (-1.40)12 and (salen)MnIII-catalyzed NaOCl ( = -1.85)14 indicates S2 mechanism. Reactions that involve rate-limiting single electron transfer from sulfur to yield radical cation intermediates are known to give better Hammett correlations when substituent constants are used18-20. In the present study, as log k is better correlated with rather than , a single electron transfer is not likely the rate-limiting step of the reaction. If the transition state resembles a radical cation, as predicated by Hammond postulate21 for a SET mechanism, a better correlation should have been observed with values. Thus, the observed better correlation of log k with than may be taken as a clue for the operation of an S2 mechanism in the present reaction13, 22. In the study of electronic effect of oxomanganese(V) complexes the observed positive value of 0.38 is in favor of electrophilic attack of oxidant on the sulfide sulfur13,23. The significant increase in the rate of oxidation with increase in the concentration of trichloroacetic acid demonstrates the electrophilic nature of the oxidant and the protonated species is more electrophilic, thereby favouring the reaction13,23. Also, the rate enhancement with increase in the polarity of the medium indicates the formation of a charge-separated transition state which is in favor of the S2 mechanism for the oxo(salen)manganese(V) oxidation of organic sulfides13. Conclusion In comparison with the data reported on the oxidation of organic sulfides by various Schiff base complexes, (salen)MnIII/NaIOsystem shows the high efficiency of oxidation and follows an 2 mechanism (scheme 3). Scheme-3 The proposed mechanism envisages the formation of intermediate in the rate-limiting electrophilic attack of the oxo complex on the sulfide, which then decomposes to give (salen)MnIII and sulfoxide as the product. The (47 to 93 kJ mol-1) and (-72 to -279 JK-1 mol-1) values are in favor of two electron transfer rather than single electron transfer in the rate-limiting step of the reaction25,26. 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