ISSN: ; CODEN ECJHAO E-Journal of Chemistry , 8(3),

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1 ISSN: ; CODEN ECJHAO E- Chemistry , 8(3), Kinetic Study on Induced Electron Transfer Reaction in Pentaamminecobalt(III) Complexes of α-hydroxy Acids by Pyridinium Fluorochromate in Micellar Medium B. MOHAMMED NAWAZ *, K.SUBRAMANI and MANSUR AHMED PG and Research Department of Chemistry Islamiah College, Vaniyambadi , Tamilnadu, India Received 24 November 2010; Accepted 22 January 2011 Abstract: Pyridinium fluorochromate (PFC) oxidation of pentaamminecobalt(iii) complexes of α-hydroxy acids in micellar medium yielding nearly 100% of carbonyl compounds are ultimate products. The decrease in UV-visible absorbance at λ=502 nm for Co(III) complex corresponds to nearly 100% of the initial absorbance. The stoichiometry of unbound ligand and cobalt(iii) complex is accounting for about 100% reduction at the cobalt(iii) centre. The kinetic and stoichiometric results have been accounted by a suitable mechanism. Keywords: Pentaamminecobalt(III) complexes, Induced electron transfer reaction, Triol, PFC. Introduction Pyridinium fluorochromate (PFC) is an efficient reagent for oxidation of primary and secondary alcohols to carbonyl compounds. A large class of organic compounds were oxidized by PFC has been reported 1-5. Since induced electron transfer in pentaamminecobalt(iii) complexes of α-hydroxy acids with various oxidants have been studied The extent of PFC oxidation of pentaamminecobalt(iii) complexes of α-hydroxy acids in micellar medium as an oxidisable hydroxyl group is separated from carboxyl bound to Co(III) centre by a saturated fragment namely C-C bond 12.The cation radical is formed due to the oxidation of hydroxyl group by PFC is nearly in a synchronous fashion of electron transfer resulting in a carbon-carbon, oxygen-hydrogen bonds scission and reduction at cobalt(iii) center. Experimental 184 ml of 6 M HF was added to 100 g of CrO 3 with stirring, the resulting solution was cooled and 79.1 g of pyridine was added over 100 min. The solid pyridinium fluorochromate was filtered and recrystallised with suitable solvent. α-hydroxy acids employed as ligands (Aldrich products) were used as obtained.

2 1103 B. M. NAWAZ et al. The monomeric cobalt(iii) complexes of α-hydroxy acids were prepared as their perchlorates by the method of Fan and Gould 13. The tris (µ-hydroxo) complex; (NH 3 ) 3 Co (OH) 3 Co (NH 3 ) 3 (ClO 4 ) 3 (triol) has been prepared by the procedure of Siebert and Co workers 14,15. And unbound ligands in the presence of micelles were carried out at 35±0.2 0 C in an electrically operated thermostat bath. The concentrations of unreacted PFC was determined iodometrically. The disappearance of Co(III) was followed spectrophotometrically by following the decrease in absorbance at 502 nm. (For the monomeric Co(III) complex). Ionic strength was maintained by the addition of suitable quantities of NaClO 4. The specific rates estimated from the optical density measurements agree with the values from the volumetric procedure within ±7% curiously, the change in absorbance observed at 502 nm Co(III) complexes of α-hydroxy acids corresponds to nearly 100% of the initial concentration of Co(III), while the change in optical density at 374 nm for PFC corresponds to nearly 100% of [Co(III)] initial. Co(II) was estimated after completion of reaction, by diluting the reaction mixture 10-fold with concentrated HCl, Allowing the evolution of chlorine gas to cease and then measuring the absorbance of blue fluoro complex of Co(II) at 69 2 nm (ε =560 dm 3 mol -1 cm -1 ) 16,17. The amount of Co(II) estimated in all these cases corresponds to nearly 100% of [Co(II)] initial. After 48 h, the product was extracted with diethyl ether and analyzed iodometrically for the amount of benzaldehyde formed was determined by measuring absorbance at 250 nm [ε=11,400 dm 3 mol -1 cm -1 ] 18,19. The yield of benzaldehyde in all these cases was nearly 100% [Co(III)] initial Table 1 & 2. Table 1. Stoichiometric data for PFC oxidation of Co(III) bound and unbound α-hydroxy acids in presence of NaLS at 35±0.2 0 C 10 3 [Compound] 10 2 [PFC] initial 10 2 [PFC] final 10 3 [PFC] [Compound]: [PFC] Mandelic acid : : : 0.66 Lactic acid : : : 0.65 Glycolic acid : : : 0.66 Co(III)-Mandelato : : : 0.33 Co(III)-Lactato : : : 0.34 Co(III)-Glycolato : : : 0.35 [HClO 4 ]=1.00, [NaLS]=1.00x10-4, Temperature=35±0.2 o C

3 Kinetic Study on Induced Electron Transfer Reaction 1104 Table 2. Stoichiometric data for PFC oxidation of Co(III) bound and unbound α-hydroxy acids in presence of CTAB at 35±0.2 0 C 10 3 [Compound] 10 2 [PFC] initial 10 2 [PFC] final 10 3 [PFC] [Compound]: [PFC] Mandelic acid : : : 0.66 Lactic acid : : : 0.67 Glycolic acid : : : 0.66 Co(III)-Mandelato : : : 0.35 Co(III)-Lactato : : : 0.33 Co(III)-Glycolato : : : 0.35 [HClO 4 ] = 1.00, [CTAB] =1.00x10-4, Temperature=35±0.2 o C After neutralization of the reaction mixture with sodium bicarbonate, the ph of the aqueous layer was adjusted to about 6.0 and the aqueous layer was separated by filtration in the case of both free ligands and corresponding complexes. On evaporation of water under reduced pressure, the product separated and the percentage yield was calculated. Though the yield of cobalt(ii) was 100%, the estimation of cobalt(ii), Cr(V) and carbonyl compounds were quantitative, In both the cases the IR spectra of the product agreed with IR spectra of authentic samples. Results and Discussion Table 3 summarizes the kinetic data for the PFC oxidation of free α-hydroxy acids with 1 N HClO 4 in presence of anionic and cationic micelles at 35±0.2 0 C. The reaction exhibits total second order dependence on [Cobalt(III)] as well as [α-hydroxy acids]. Based on the oxidation of PFC with α-hydroxy acids the following rate law has been deduced. Rate=k[α-hydroxy acid] [PFC] Table 4 lists the formation constants for PFC co-complexes of α-hydroxy acid along with the specific rates. Such complex formation seems to be absent when the carboxyl and it is tied up by Co(III) and the reaction between PFC and Co(III) complexes of α-hydroxy acids exhibit uncomplicated second order kinetics.

4 1105 B. M. NAWAZ et al. From a comparison, the specific rates for PFC oxidation of the respective Co(III) complexes and the dimeric cobalt(iii) glyoxalato complex, one can infer that the oxidation rates of α-hydroxy acids are not significantly affected by complex formation. This may be due to the point of attack lies away from the Co(III) centre so that its electrostatic influence is less felt. There is, however a considerable change in the specific rate of PFC oxidation of the Co (III) keto acid complex as the two Co(III) centers can exert greater electrostatic influence over the reacting centre. This suggests that PFC attacks the O-H centre in the slow step of the reaction, leading to ligand oxidation takes place. The rate of the reaction is increased by the addition of both NaLS and CTAB. A plot of specific rate constant versus micellar concentration is sigmoidal in shape the catalytic effect is more in CTAB than NaLS. The specific rate of the lactato complex is more when compared to both the rate of unbound ligand and mandelato complexis due to the ligation of lactic acid to cobalt(iii) centre has probably increased its reactivity towards PFC and this effect seems to be more specific for this ligands only. In NMR spectrum of lactato complex the alpha methine proton has undergone considerable downfield shift compared to the alpha C-H proton of the unbound ligand [δ C-H=1.73 ppm in lactic acid and δ=c-h 2.30 ppm in lactato complex whereas δ= C-H 4.75 ppm in mandelic acid δc-h=3.85 ppm in the respective complex]. Suggesting an increase in acidic nature of methine proton of lactic acid is due to ligation to metal centre. If the reaction proceeds through a performed chromate ester, then the rate of alpha C-H will be enhanced, resulting in an increased rate of oxidation of lactato complex such a precursor complex may be sterically hindered in the case of mandelato and glycolato complexes. The stoichiometric results indicate that for 1 mole of cobalt(iii) complex, about 0.65 mole of PFC is consumed, whereas with the unbound ligands for 1 mole of α-hydroxy acids about 0.92 mole of PFC is consumed (Table 3 & 4). The stoichiometric results coupled with kinetic data and product analysis can be accounted for by the following the reaction Scheme 1. Table 3. First order rate constants for PFC oxidation of α-hydroxy acids at 35±0.2 0 C 10 2 [α-hydoxy 10 4 k 1, s k 2 dm 3 acids] mol -1 s k 1, s -1 NaLS CTAB NaLS 10 2 k 2 dm 3 mol -1 s -1 CTAB Lactic acid Mandelic acid Glycollic acid [PFC]=2.00x10-3, [HClO 4 ] =1M, [NaLS] =1.00x10-4, [CTAB] =1.00x10-4 Temperature=35±0.2 0 C

5 Kinetic Study on Induced Electron Transfer Reaction 1106 Table 4. First order rate constants for PFC oxidation of Co(III) complexes of α-hydroxy acids at 35±0.2 0 C 10 3 [(NH 3 ) 5 Co III -L] 10 4 k 1, s -1 NaLS 10 2 k 2 dm 3 mol -1 s -1 NaLS 10 4 k 1, s -1 CTAB 10 2 k 2 dm 3 mol -1 s -1 CTAB L = Lactato L = Mandelato L = Glycolato [PFC]=2.00x10-3, [HClO 4 ] =1M, [NaLS] =1.00x10-4, [CTAB] =1.00x10-4 Temperature=35±0.2 o C The reaction scheme proposes that PFC oxidizes OH center of the cobalt(iii) bound α- hydroxyacids at a rate of comparable to that of the unbound ligand and there is 100% reduction at the cobalt(iii) centre, forms a chromate ester with cobalt(iii) glyoxalato complex which can decompose in a slow step, proceeds through C-C bond fission leading to the formation of cobalt(ii), carbonyl compounds and carbon dioxide. As 1 mole of cobalt(iii) glyoxalato complex consumes 0.65 mole of PFC yielding nearly 100% of Co(II) and 100% carbonyl compounds. Similarly 1 mole of unbound α-hydroxy acid consumes nearly 0.92 mole of PFC, yielding 100% of carbonyl products and CO 2. Scheme 1

6 1107 B. M. NAWAZ et al. Conclusion It has been found that the rate of oxidation of Co(III) complexes of unbound and bound moieties are enhanced more in the presence of CTAB when compared to the NaLS. The micelles act as a positive catalyst in the present study. References 1. Pandurangan A, Kavitha S and Alphonse I, Indian J Chem., 2005, 44A, Bhattacherjee M N, Chaudhri M K, Dasgupta H S, Roy N and Khathing D T, Synthesis, 1982, 7, Banerji K K, J Org Chem., 1988, 53, Banerji K K, J Chem Soc Perkin Trans., 1978, 2, Corey E J and Suggs J W, Tetrahedron Lett., 1975, 16, Srinivasan V S and Gould E S, Inorg Chem., 1981, 20, Srinivasan E S and Gould E S, Inorg Chem., 1982, 21, Srinivasan V S, Inorg Chem., 1982, 21, Kalidoss P and Srinivasan V S, J Chem Soc Dalton Trans., 1984, Kalidoss P and Srinivasan V S, Indian J Chem., 1987, 26A, Kalidoss P and Srinivasan V S, Indian J Chem., 1987, 26A, Kalyan Kalisen Gupta, Amalendu Banergee and Hrishikesh Chattergee, Tetrahedron, 1992, 48, 25, 30, Fan R R F and Gould E S, Inorg Chem., 1974, 13, Sekar K G, Oxidation Communication, 2003, 26, Srinivasan V S, Singh A N, Weighardt K, Rajasekar N and Gould E S, Inorg Chem., 1982, 21, Siebert H and Anorg Z, Allg Chem., 1978, 47, Gould E S and Taube H, J Am Chem Soc., 1964, 86, Fendler J H and Fendler E J, In Catalysis in Micellar and Macromolecular Systems. Academic Press. New York, Dyer J R, Applications of Absorbtion Spectroscopy of Organic Compounds, Prentice Hall, Englewood Cliffs NJ, 1965, Streitwiser Jr A and Health Cock C H, Introduction to Organic Chemistry, Macmillan. Newyork, 1973, 596.

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